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\DOI{10.5802/crchim.398}
\datereceived{2024-12-05}
\daterevised{2025-03-16}
\dateaccepted{2025-04-02}
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\dateposted{2025-05-14}
\begin{document}

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\title{Hybrid catalysis: an efficient tool for biomass valorization and
for the production of new building blocks in chemistry} 

\alttitle{La catalyse hybride : un outil efficace pour la valorisation
de la biomasse et la production de nouvelles briques mol\'{e}culaires
pour la chimie}

\author{\firstname{Sara} \lastname{Arteche Echeverr\'{i}a}\CDRorcid{0009-0001-4408-0714}}
\address{Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181-UCCS-Unit\'{e} de Catalyse et Chimie du Solide, F-59000 Lille, France}
\address{Early Development Chemistry, Sanofi, 1 impasse des Ateliers, 94400 Vitry-sur-Seine Cedex, France}

\author{\firstname{R\'{e}nato} \lastname{Froidevaux}\CDRorcid{0000-0001-6007-0754}}
\address{UMR Transfrontali\`{e}re BioEcoAgro No 1158, Univ. Lille, 
INRAE, Univ. Li\`{e}ge, UPJV, JUNIA, Univ. Artois, Univ. Littoral
C\^{o}te d'Opale, Institut Charles Viollette, 59655 Villeneuve
d'Ascq, France}

\author{\firstname{Sarah} \lastname{Gaborieau}}
\addressSameAs{1}{Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181-UCCS-Unit\'{e} de Catalyse et Chimie du Solide, F-59000 Lille, France}

\author{\firstname{Anne}\nobreakauthor\lastname{Zaparucha}\CDRorcid{0000-0003-2531-4148}}
\address{G\'{e}nomique M\'{e}tabolique, Genoscope, Institut Fran\c{c}ois Jacob, CEA, CNRS, Univ Evry, Universit\'{e} Paris-Saclay, Evry, France}

\author{\firstname{Egon} \lastname{Heuson}\CDRorcid{0000-0001-6904-9214}\IsCorresp}
\addressSameAs{1}{Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181-UCCS-Unit\'{e} de Catalyse et Chimie du Solide, F-59000 Lille, France}
\email[E. Heuson]{egon.heuson@centralelille.fr}

\thanks{ANRT (CIFRE 2024/0663), French National Research Agency (ANR) (ANR-11EQPX-0037)}

\begin{abstract}
The unsustainable exploitation of finite natural resources, such as oil
reserves, has accelerated resource depletion and harmed biodiversity,
with projections indicating the exhaustion of oil reserves by
mid-century. This pressing issue highlights the necessity for
sustainable consumption and production models. Chemistry has responded
by adopting Green Chemistry principles, emphasizing renewable and
alternative raw materials over fossil-based ones. Biomass, particularly
from unexploited wastes, stands out as a renewable carbon source, but
its complex composition creates challenges for chemical
transformations. Catalysis is critical in addressing these issues.
While traditional chemical catalysts struggle with the intricate
mixtures in biomass, biological catalysts, like enzymes, excel in
processing such substrates due to their natural specificity and
efficiency. However, enzymes face limitations from biomass inhibitors,
such as plant-derived defense molecules. Innovative catalytic systems
are needed to selectively target specific molecules and mitigate
inhibitors. A promising solution lies in hybrid catalysis, which
combines biological and chemical catalysts to leverage their
complementary strengths. Indeed, hybrid systems enable multi-steps
reactions with higher yields, equilibrium shifts, and inhibitor
removals, promoting the synthesis of new molecules from challenging
substrates. Over the past decade, hybrid catalysis has expanded thanks
to its potential to transform biomass into valuable compounds. By
integrating the robustness of chemical catalysts with the selectivity
of biological ones, hybrid catalysis aligns with sustainability
objectives, offering a pathway to more efficient and innovative
chemical transformations. This emerging technology can play a pivotal
role in sustainable development and industrial innovation, especially
when it comes to biorefineries.
\end{abstract}


\begin{altabstract}
L'exploitation non durable de ressources naturelles limit\'{e}es,
telles que les r\'{e}serves de p\'{e}trole, a acc\'{e}l\'{e}r\'{e}
l'\'{e}puisement de ces ressources et largement port\'{e} atteinte
\`{a} la biodiversit\'{e} sur la plan\`{e}te, avec des projections
indiquant l'\'{e}puisement des r\'{e}serves de p\'{e}trole d'ici le
milieu du si\`{e}cle. Ce probl\`{e}me urgent met en \'{e}vidence la
n\'{e}cessit\'{e} de mod\`{e}les de consommation et de production
durables. Le domaine de la chimie a r\'{e}agi en adoptant les principes
de la chimie verte, qui privil\'{e}gient les mati\`{e}res premi\`{e}res
renouvelables et alternatives par rapport aux mati\`{e}res d'origine
fossiles. La biomasse, et en particulier les d\'{e}chets
inexploit\'{e}s qui la compose, se distingue comme source de carbone
renouvelable. Cependant, sa composition complexe pose des probl\`{e}mes
pour sa transformation chimique, tout du moins avec des approches
conventionnelles. De nouveaux types de catalyse apparaissent alors
essentiels pour r\'{e}soudre ces probl\`{e}mes. Alors que les
catalyseurs chimiques traditionnels ont du mal \`{a} traiter les
m\'{e}langes complexes de biomasse, les catalyseurs biologiques, comme
les enzymes, excellent dans le traitement de ces substrats en raison de
leur sp\'{e}cificit\'{e} et de leur efficacit\'{e} intrins\`{e}que.
Cependant, les enzymes sont souvent limit\'{e}es par les inhibiteurs
contenus dans la biomasse, tels que les mol\'{e}cules de d\'{e}fense
d\'{e}riv\'{e}es des plantes. Des syst\`{e}mes catalytiques innovants
sont ainsi n\'{e}cessaires pour cibler s\'{e}lectivement des
mol\'{e}cules sp\'{e}cifiques et att\'{e}nuer l'action de ces
inhibiteurs. Une solution prometteuse r\'{e}side dans la catalyse
hybride, qui combine des catalyseurs biologiques et chimiques afin de
tirer parti de leurs forces compl\'{e}mentaires. En effet, les
syst\`{e}mes hybrides permettent des r\'{e}actions en plusieurs
\'{e}tapes de mani\`{e}re combin\'{e}e avec des rendements plus
\'{e}lev\'{e}s, et proposant le d\'{e}placement des \'{e}quilibres
r\'{e}actionnels, ainsi que parfois l'\'{e}limination des inhibiteurs,
ce qui favorise la synth\`{e}se de nouvelles mol\'{e}cules \`{a} partir
de substrats difficiles. De par ces avantages, la catalyse hybride
s'est tr\`{e}s rapidement d\'{e}velopp\'{e}e au cours de la
derni\`{e}re d\'{e}cennie. En int\'{e}grant la robustesse des
catalyseurs chimiques \`{a} la s\'{e}lectivit\'{e} des catalyseurs
biologiques, la catalyse hybride s'aligne sur les objectifs de
durabilit\'{e}, offrant une voie vers des transformations chimiques
plus efficaces et innovantes. Cette technologie \'{e}mergente peut
ainsi jouer un r\^{o}le central dans le d\'{e}veloppement durable et
l'innovation industrielle, surtout au niveau des bioraffineries.
\end{altabstract}

\keywords{\kwd{Hybrid catalysis}\kwd{Biomass}\kwd{Platform
molecules}\kwd{Enzymes}\kwd{Chemical catalysts}\kwd{Green Chemistry}}


\altkeywords{\kwd{Catalyse hybride}\kwd{Biomasse}\kwd{Mol\'{e}cules
plateformes}\kwd{Enzymes}\kwd{Catalyseurs chimiques}\kwd{Chimie verte}}

\shortrunauthors

\maketitle

\twocolumngrid

\end{noXML}


\section{Hybrid catalysis for biomass valorization}
{\vspace*{-2pt}}

The exploitation of limited natural resources such as oil reserves has
damaged nature and biodiversity for the last decades~\cite{1}, leading
to an accelerated exhaustion of these resources. It is estimated that
natural oil reserves will be consumed by the second half of the century
if the current average exploitation is maintained~\cite{2}. With this
in mind, it is necessary to adopt sustainable consumption and
production models~\cite{3}.  In order to achieve this objective,
chemistry is shifting toward more sustainable practices, notably
following the Green Chemistry principles proposed by Anastas and Warner
at the end of the twentieth century, which include the use of
alternative and renewable prime material sources rather than limited
fossil fuels~\cite{4,5}.  Hence why, biomass, largely available in
unexploited waste, stands out as a highly potential alternative for a
new carbon source, especially as it is renewable~\cite{6}.  Due to
its complex composition, biomass presents significant challenges as a
substrate for chemical transformations. Catalysis, therefore, has a
pivotal role in addressing these challenges. However, conventional
catalysts, initially designed for transforming purified products are
ill-suited for handling complex mixtures~\cite{7}.  Biological
catalysts such as enzymes, on the contrary, are far more effective with
these types of substrates. Indeed, many organisms already use enzymes
to break down, depolymerize, and/or transform biomass
molecules~\cite{8}.  For example, cell-wall degrading enzymes (CWDE)
are used by bacteria and fungi to feed on lignocellulose, or enzymes of
the hydrolase family that are specific for degrading certain
triglycerides and fatty acids in the case of vegetable or animal oils.
Nevertheless, the use of biocatalysts remains a challenge at present,
again due to the complexity of biomass sources which may contain a
large number of inhibitors, including molecules produced by plants
themselves to defend against pathogen enzymes. Thus, to incorporate
biomass as a starting material, innovative catalysts capable of
targeting only specific molecules in complex mixtures and that could
alleviate the issue of the presence of inhibitors, need to be
developed. Rather than working on the development of new, more robust,
and more specific catalysts, an alternative solution could be to
combine existing catalysts with complementary skills. A smart design
leveraging the synergistic potential of catalysts of different
natures---biological and non-biological---could significantly enhance
chemical transformations. This approach holds particular promise for
optimizing existing chemical processes, which often face limitations
preventing their maturity and scalability at industrial levels. For
illustration, in the case of high energy consumption for heating, where
combination of catalysts logically allows to reduce the overall
energetic cost by heating just one reactor~\cite{9}.  Furthermore,
such combined orthogonal processes could pave the way for entirely new
families of molecules with valuable properties, by enabling the
coupling of reactions that are otherwise difficult to conciliate. More
specifically, the integration of chemical and biological catalysts
could harness their complementarity and/or orthogonality for
selectivity. This synergy may result in multi-step catalytic systems
with high yields, driven by reaction equilibrium shifts and, in certain
cases, the removal of inhibitors either inherent to the biomass or
generated during the reaction. This innovative catalytic approach,
which has gained significant attention over the past decade~\cite{10},
is commonly referred to as ``hybrid catalysis''
(Figure~\ref{fig1}).\looseness=-1


\begin{figure}
\includegraphics{fig01}
{\vspace*{-2pt}}
\caption{\label{fig1}Main fields of application of hybrid catalysis
(enhancement of sustainable development by reducing atom economy,
energetic and economic cost; transformation of complex mixtures through
the complementary selectivities of the combined catalysts).}
{\vspace*{-4pt}}
\end{figure}


{
Usually, the term ``hybrid catalysis'' refers to the combination of
chemical and biological catalysis. This combination illustrates the
main criteria followed when designing a hybrid catalytic system:
joining catalysis whose properties, but also limitations, are very
distinct. Although hybrid catalysis can technically be composed of as
many types and numbers of catalytic stages as desired, to our
knowledge, examples with more than two of them are rarely found
currently. The first example of a hybrid catalytic system for biomass
valorization was reported in 1980 by Makkee et~al., where
\textsc{d}-glucose was transformed into \textsc{d}-mannitol
through the combination of a biocatalytic isomerization and a
chemocatalytic hydrogenation~\cite{11}.  It is important to note that
although the starting molecule, \textsc{d}-glucose, was biosourced,
there was no question of biomass valorization when this study was
carried out. Moreover, although the value of combining these two types
of catalysts was demonstrated, particularly in a one-pot reaction,
there was no real interest, at that stage, for the treatment of complex
mixtures. Regardless of its ingenuity, this first approach only allowed
obtaining 46\% of \textsc{d}-mannitol, whereas 92\% of
\textsc{d}-glucose was converted into \textsc{d}-fructose. To
improve the efficiency of the system, the process was later optimized
by the same authors by combining the same biocatalytic isomerization
of \textsc{d}-glucose to \textsc{d}-fructose as seen before with
heterogeneous copper-based catalyst supported on silica
(Figure~\ref{fig2}). The key characteristic of this metallic catalyst
is its substrate specificity, as it prefers interacting with
\textsc{d}-fructose, and therefore here offers a solution to
selectively act on one single substrate in a mixture of isomers,
\sout{as} reflected in the increased \textsc{d}-mannitol yield
(66\%)~\cite{12}. 

}

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}One-pot/one-step hybrid process designed by Makkee
et~al.\ where a biocatalytic isomerization accesses  \textsc{d}-fructose
from \textsc{d}-glucose, preferred substrate by a copper catalyst for
its subsequent hydrogenation into \textsc{d}-mannitol. Redrawn from
Heuson et~al.~\cite{13}.}
{\vspace*{1pt}}
\end{figure*}


{

These pioneer works were a milestone in the design of new catalytic
systems and led to the exploration of new combinations of chemo- and
biocatalysts to enhance new reaction pathways with uncommon, and
sometimes unexpensive, starting materials. Biomass has emerged as an
excellent candidate to employ hybrid catalysis for several reasons. One
reason is that biomass is largely composed of low energy chemical bonds
(e.g., esters, amides, sometimes ethers with higher energy
bonds)~\cite{14}, it makes biomass suitable for reactions with
enzymes under mild conditions, unlike fossil fuel-based chemicals,
almost exclusively composed of carbon and hydrogen~\cite{15}.  The
second reason is that, biomass provides a wide variety of molecules
that are hardly discriminated against by chemical catalysts due to
their \mbox{often} low selectivity, leading to the formation of \mbox{complex}
product mixtures. In this instance, while offering access to a large
number of starting structures, the use of biomass as a starting
material also calls for selectivity, potentially offered by enzymes, in
order to limit downstream processing steps. Additionally, this complex
nature also means that pretreatment systems are needed to avoid
potentially toxic or undesirable species, or simply to make the desired
starting substrates available, especially in cases where a single
product is targeted. For instance, in lignocellulosic biomass, it is
mostly cellulose that is converted~\cite{16}.  The possibility of
exploding the entire lignocellulose matrix in a single combined process
(with the catalytic valorization steps included directly afterwards),
would therefore be advantageous both in terms of atom economy and
access to a wider range of new biosourced molecules. Hybrid catalysis
could be very helpful in coupling the single-molecule valorization step
with a pretreatment one, using a more robust and less specific
catalyst, for example.


Indeed, to better understand all the potential of hybrid catalysis, the
main attributes and resulting complementarities should be considered
when combining catalysts (Figure~\ref{fig3}). Biocatalysis, is
characterized by its high chemo-, regio- and
stereoselectivity~\cite{17}. These properties are provided by the
tridimensional structure of enzymes, which can selectively host the
substrates in their active sites, usually depending on how well they
can sterically and electronically interact with each other~\cite{13}. 
This high selectivity is of great interest in the pharmaceutical
industry since it can lead to enantiopure compounds~\cite{18}.
Besides, biocatalytic systems can be conducted in a reaction cascade
system, which provides an interesting set of chemical tools and
reactions in the same pot, leading to especially high energy- and
atom-saving processes~\cite{17}. 

}

\begin{figure}
\includegraphics{fig03}
{\vspace*{-4pt}}
\caption{\label{fig3}Complimentary catalytic properties of chemo- and
biocatalysis, leading to the synergistic properties of hybrid
catalysis.}
{\vspace*{-3pt}}
\end{figure}

{

As an example, Merck recently developed a new biocatalytic cascade
system for the synthesis of molnupiravir by combining five different
enzymes, allowing not only the production of the target antiviral, but
also the regeneration of the phosphate needed in the reaction on an
industrial scale~\cite{19}.  This system shortens the previous
reported chemical synthesis from ten to only three steps and increases
the yield from approximately 10\% to 70\%. Despite the interesting
possibilities of such elegant biocatalytic cascades, the limited
stability and high price of \mbox{enzymes} become a bottleneck in their
incorporation on an industrial scale~\cite{20}. Meanwhile,
\mbox{heterogeneous} chemocatalysts can enhance the recovery of the catalysts
from the reaction mixture, as well as providing good stability under
harsh working \mbox{conditions,} making them more suitable for industrial
applications~\cite{21}.  Regarding their properties, they are mostly
related to their electronic structure~\cite{13}, while leading to
lower selectivity due to a very limited steric control of the catalytic
center, makes them very versatile and consequently very well suited for
complex mixtures. In addition, chemocatalysts tolerate better the
exposition to toxic molecules and can then be interesting candidates
for ``depolluting'' reaction media from inhibitors, to which
biocatalysts are particularly sensitive and therefore
vulnerable~\cite{22}.  This potential use is perfectly demonstrated
by Smeets et~al.~\cite{23},  who were able to design a zeolite-based
multifunctional material, for the conversion of allylic alcohol into
glycidol, employing the toxic and in situ produced hydrogen peroxide.
Here, the TS-1 zeolite was able to both host the concerned enzyme,
e.g., glucose oxidase, and simultaneously act as an inorganic catalyst.
This hybrid material therefore allowed the production and removal, in
the same process, of a substrate that can also be a potential
inhibitor. Since the substrate here employed is \mbox{glucose,} this approach
could be extended to biomass-containing mixtures, usually rich in
polysaccharides. However, these approaches are not yet widespread.
Another example of product protection and/or inhibitor removal is the
recent development of nonionic surfactants by Novartis~\cite{24}, 
capable of hosting water nonsoluble compounds, which could be either
the product of the reaction, a chemocatalyst or any other potentially
harmful species for the biocatalyst, allowing, at last, the biocatalyst
to be protected from them. In addition, as it will be illustrated later
in this article, it is also possible to design new types of reactors
offering access to a more spatially isolated compartmentalization
system while still offering the possibility for molecules to circulate
from one compartment to another, using a liquid or solid permeable
membrane. In essence, compartmentalization techniques that could behave
as decontamination strategies enhance the cohabitat of chemo- and
biocatalysts, or avoid inhibition phenomena, and are here again very
interesting for biomass valorization. Therefore, chemo- and
biocatalysis should no longer be considered as incompatible but rather
complimentary. Their unique properties can overcome their respective
limitations, making them capable of addressing the afore-mentioned
challenging fields of biomass valorization and new compound synthesis.

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig04}
\caption{\label{fig4}General one-pot/two-steps hybrid approach for
different biomass conversion (e.g., corncob, rice stalk) into
value-added furfural derivatives by combining a tin-based heterogeneous
catalyst supported on a biobased carrier and a biocatalyst.}
{\vspace*{1pt}}
\end{figure*}

However, the number of hybrid processes designed to directly run on
biomass is still limited. In the case of lignocellulose, the few
systems available are mainly aimed at the synthesis of furfural and its
derivatives, which can be obtained by the dehydration of the sugars
present in it. Mainly derived from hemicellulose, furfural is indeed
considered as one of the top 30 platform chemicals~\cite{25}, 
as it provides access to many derivatives with very different
applications in polymer, pharmaceutical or food industries.
Nonetheless, direct and efficient furfural synthesis from untreated
biomass remains, again, mostly undeveloped, especially when it comes to
generating its functionalized derivatives such as
5-hydroxymethylfurfural (HMF), furandicarboxylic acid (FDCA), or even
furfurylamines like 5-aminomethyl-2-furancarboxylic acid (AMFC).
Indeed, the processes to reach these compounds typically need tedious
separation steps and control of the reactivity of the aldehyde moiety
of furfural, which has been found to be particularly unstable since it
can undergo multiple reactions (acetalization, \mbox{acylation,} aldolization,
amination, halogenation or redox reactions, among others)~\cite{25,26}. 
Therefore, such high reactivity not only does it make furfural a good
platform molecule, but also a particularly excellent precursor for the
above cited molecules. However, it also complexifies its related
processes and storage, requiring fully integrated systems where
furfural is present, ideally, only present in transient form before
being converted into a more stable molecule. In this context, advances
in hybrid catalysis have offered several solutions for the direct
conversion of lignocellulosic biomass into furfural and its derivatives
that otherwise would be difficultly accessible by conventional
catalysis. A good example illustrating this upgrading of
lignocellulosic biomass is the work of He et~al.~\cite{27,28,29,30,31}.
These authors first developed a one-pot/three-step process involving
the initial acidic depolymerization of various biomass sources
(chestnut shell, corncob, sorghum stems, rice straw or even bamboo
shoot shells), into sugars, typically \textsc{d}-xylose. In a
second step, a tin-based catalyst was used to convert the sugar into
furfural, which was finally selectively employed in a third step by
different biocatalysts (e.g., reductases, alcohol dehydrogenases,
transaminases) to branch a whole range of chemical groups leading to
higher value-added products (e.g., alcohols, carboxylic acids, amines).
More recently, the authors have improved their process by combining the
first two steps, where cellulose and hemicellulose are directly
chemocatalytically converted into furfural, which can then be
transformed, in a second step, by the selected biocatalyst 
(Figure~\ref{fig4}). In the case of furfurylamine production, a transaminase was
employed for example, either as the isolated enzyme or as a whole cell
system~\cite{32,33}. 
Different variants of the process have been designed, employing a
similar variety of biomass sources, adjusting it to the desired
products. In most of them, a biobased carrier (rice stalk, nutshell,
etc.) was employed for the preparation of the concerned heterogeneous
tin catalyst. The similar composition of the carrier and the
lignocellulosic substrates improves their interactions, mainly due to
their similar hydrophobicity and therefore favored adsorption on the
surface. In addition, the catalyst supported on rice stalk showed 
\mbox{592-,}
17- and more than 2-fold larger specific surface area, pore volume and
pore size, respectively, compared to the previously employed material,
which might increase the substrate load and therefore provide higher
catalytic performance. The reaction \mbox{media} appears as biphasic systems,
like in the case of the preparation of furfurylamine, which was
performed in a toluene/water solution, working at 170~{\textdegree}C
and 35~{\textdegree}C for the chemocatalytic and biocatalytic step,
respectively. The authors highlight the utility of this arrangement as
it allows the avoidance of the possible inhibition of the biocatalyst
by furfural, as it proved to be toxic at high
concentrations~\cite{32}. The use of toluene might be a drawback when
scaling up, and therefore greener alternatives should be explored. This
one-pot/two-steps system has been further explored by adding oxalic
acid as the cocatalyst in the first step, followed by the biocatalytic
step by using whole cells in deep eutectic solvent
(ethylamine/hydrochloride/glycine/water)~\cite{33}.
The motivation behind this approach was again to tackle several
problems related to furfural (e.g., low solubility in water,
degradation or cross-polymerization) by enhancing its extraction while
keeping the above-mentioned energetic advantages of working in one
single reactor. Here, adding an organic acid provides a partial
pretreatment of the mixture, adding valuable properties to the hybrid
system beyond the catalysis itself and demonstrating flexibility and
polyvalency.\looseness=-1

}

{

Overall, these examples demonstrate particularly well the versatility
of the combination of two catalysts of different nature, once the
common coupling conditions are defined. Typically, the authors were
able to combine a broad number of different biocatalysts with the same
chemical catalyst under similar conditions, giving them access to a
complete range of molecules, while simplifying the development of the
associated processes. The potential of combining a relatively universal
chemical catalyst acting first on the biomass, with variable polymer
compositions from different sources, was highlighted. The same
intermediate, in this case furfural, can then be transformed by
different enzymes.

\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig05}
{\vspace*{-1pt}}
\caption{\label{fig5}Photobiocatalytic one-pot/two-steps hybrid
approach for C--H functionalization of different alkanes, alkenes and
toluene derivatives. The system combines sodium anthraquinone sulfonate
(SAS) as a photo-organocatalyst with various enzyme families, in
aqueous medium at 30~{\textdegree}C. Redrawn from Zhang et~al.~\cite{34}.}
{\vspace*{-1pt}}
\end{figure*}

Such versatility enabled by hybrid catalysis was also reflected in the
work of Zhang et~al.~\cite{34},  who managed to functionalize
asymmetric C--H bonds on a wide alkane substrate scope through a
photobiocatalytic hybrid one-pot/two-steps reaction system. In the
first stage, sulfonate anthraquinone was employed for the
photo-oxidation of different alkanes (cyclohexane and toluene
derivatives, mainly) to ketones, which were subsequently converted, in
a second biocatalytic stage, to different functional groups (carboxylic
acids, amines, alcohol cyanides and esters) by an enzyme (e.g.,
transaminase, reductase, lyase) (Figure~\ref{fig5}). Although the
incompatibility of both catalytic steps imposed a two-step manner way
of working, a single aqueous phase at 30~{\textdegree}C could be
employed for the whole reaction, notably thanks to the sulfonate group
present in the photo-organocatalyst, which favors the latter soluble in
water. As a result, the green character of the process is favored as it
does not require undesired and often toxic organic solvents. Another
remarkable aspect relies on the fact that this coupled catalytic system
not only accepts a broad substrate scope, comprising in this case of 14
different substrates, but also leads to the formation of a large and
often chiral range of products. Hybrid catalysis provides, again, one
single process applicable to numerous substrates that leads to many
different products of higher value, notably enantiomerically enriched.
This last aspect illustrates, indeed, the additional complementarity of
the catalysts, with the association of a nonstereoselective step with a
highly stereoselective one, allowing to reach a wider product range in
a relatively simple approach.\looseness=-1

}

{

Additionally, the catalytic singularity of hybrid technologies can
also give rise to unexplored reaction pathways in organic chemistry,
allowing new and often structurally rich products to be reached. The
synthetic potential of hybrid catalysis relies on the conjunction of
the numerous reactions enhanced by chemocatalysts (e.g.,
metal-catalyzed cross-couplings, organocatalytic asymmetric synthesis,
oxidoreductions) and the high selectivity of biocatalysts, resulting,
for instance, in the delicate creation of new stereogenic centers.
Among the most targeted chemical structures by synthetic chemists,
special attention has been drawn to nitrogen-containing structures, as
they are often found in building blocks used to access pharmaceutical
compounds~\cite{17}.  As an example, a
\textit{N}-acetyl-\textsc{d}-glucosamine-derived chiral
alcohol was prepared by Hao et~al.\ employing a chemobiocatalytic
approach in a stereo- and especially chemoselective manner~\cite{35}. 
The process combined tyrosine hydrochloride for the dehydration of
\textit{N}-acetyl-\textsc{d}-glucosamine at 160~{\textdegree}C,
and a carbonyl reductase for the asymmetric reduction of a ketone at
30~{\textdegree}C (Figure~\ref{fig6}). 

}

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig06}
{\vspace*{-3pt}}
\caption{\label{fig6}One-pot/two-steps process for the asymmetric
synthesis of acetamido-5-(hydroxyethyl)furan that combines a
chemocatalytic dehydration with tyrosine chloride (TyrCl) as catalyst
and biocatalytic reduction of 
\textit{N}-acetyl-\textsc{d}-glucosamine. Redrawn from Hao 
et~al.~\cite{35}.}
{\vspace*{-3pt}}
\end{figure*}

{

Compared to the previous processes, where the intermediate is
purified, the one-pot/two-steps hybrid catalysis led to an increase in
the enantiomeric excess of the furane derivative from 91\% to over
99\%. In addition, the 99\% of the \textit{E} {factor (6025) is
related to solvent waste from product purification, which again
highlights the sustainable character of this one-pot transformation.
Nine different substrates were obtained in good to excellent yields,
showing the versatility of the system to reach multiple high
value-added building blocks from inexpensive chitin. Overall, this work
reinforces the potential of valorizing inexpensive biomass through
hybrid catalysis to access new compounds with high enantiopurity and to
reduce the environmental impact. Another interesting example in the
framework of stereocontrolled synthesis is the recent work of Gastaldi
et~al.~\cite{36},}  who prepared, from achiral alkynes, chiral
polyhydroxylated a rare analog of monosaccharides through a
one-pot/two-steps process at 60~{\textdegree}C. A gold N-heterocyclic
carbene, responsible for the hydration of the alkyne to the
corresponding ketone, was combined with an aldolase, which enhanced the
formation of a C--C bond in the presence of an \mbox{aldehyde} and led to two
new stereogenic centers. A family of five different chiral
monosaccharides were formed in excellent yields, all of them being high
value-added products with potential applications in food and
pharmaceutical industries. As in the previous example, the reaction was
performed as a one-pot/two-steps, due to the different temperatures and
pH working ranges of the catalysts, 60~{\textdegree}C and pH~3 for the
chemocatalytic step, and room temperature and pH~7 for biocatalytic
one, respectively. Striving to reach a one-step process, an attempt to
increase the pH of the chemocatalytic step was done, which
unfortunately derived in a quick decrease of the ketone production. In
addition to the synthesis of new products, the hybrid process again
shows a major advantage in terms of atom economy, as it presents a
fourfold lower value of \textit{E} factor (0.5) with respect to the
previous conventional synthesis, and also helps avoiding the isolation
of the freshly formed hydroxyacetone, which is particularly unstable
given its tendency to polymerize and/or undergo aldol
condensation~\cite{37}.  {With the same hydration step and further
alkyne transformations, Li et~al.\ have very recently reported the
hybrid regioselective conversion of alkynes to chiral alcohols also
using a gold catalyst~\cite{38}.}  {Similarly to the previous example,
the reaction occurs in a one-pot/two-steps manner and combines the
chemocatalytic hydration step with the biocatalytic reduction of the
intermediate ketone to a chiral alcohol by using a carbonic anhydrase.
Again, a step differentiation was required so the pH could be increased
from acidic to alkaline (pH~8) and so the temperature could be
decreased from 65~{\textdegree}C to room temperature, which contributed
to the energy and atom efficiency by not requiring any separation or
exogeneous energy-demanding process for the second step. This process
afforded a wide scope of differently substituted chiral alcohols with
excellent enantioselectivities (${&gt;}$99\%) and good to excellent yields
(most of products obtained in between 60\% and 90\% yield). Most
remarkably, within this work, the same authors have developed two other
parallel hybrid systems for the synthesis of chiral alcohols that allow
the enlargement of the substrate scope to alkanes and
alkenes~\cite{38}.} When alkanes were employed as starting materials, a
photocatalytic oxidation followed by a biocatalytic reduction was
achieved, while in the case of alkenes, a metal-catalyzed oxidation was
performed, followed by a biocatalytic reduction. Therefore, Li et~al.\ 
was able to design three hybrid reactions to produce chiral alcohols
from nonfunctionalized alkanes, alkenes, and alkynes, which widely
enlarged the substrate scope and led to a notable increase in the final
value of the obtained products (Figure~\ref{fig7}). In all cases, the
one-pot/two-steps setup was maintained to avoid incompatibility issues,
mainly related to pH, temperature, or light-exposure working
conditions.\looseness=-1

}


\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig07}
{\vspace*{-2pt}}
\caption{\label{fig7}Three parallel one-pot/two-steps hybrid approaches
for synthesis of chiral alcohols depending on starting from alkanes,
alkenes and alkynes, respectively. Redrawn from Li et~al.~\cite{38}.}
{\vspace*{-2pt}}
\end{figure*}

{

Among these approaches, the photobiocatalytic one is particularly
interesting from a sustainable perspective, as it allows the use of
visible light as an energy source. This case echoes the above-mentioned
work of Zhang et~al.~\cite{34},  {which already showed the potential of
photocatalysts to oxidize~C--H bonds to carbonyls for their potential
transformation by biocatalysts. Expanding this reactivity presents
therefore huge potential as different chemical functions can be
obtained from alkanes, which could also be extrapolated to
biomass-derived substrates, such as fatty acids or lignin derivatives.}

All these studies can be seen as great proof of concept regarding the
advantages that can be provided by hybrid catalysis in organic
chemistry. Firstly, it affords the possibility to combine catalysts in
one single reactor instead of sequential systems, and therefore
reducing significantly the energetic and economic cost of the process.
Secondly, as it is frequent for one-pot systems, the isolation of
\mbox{intermediates} is not necessary. This leads to more efficient atom use,
as evidenced in the afore-mentioned \textit{E} factor values, it
results in safer processes because reactive species are rapidly
transformed, minimizing {uncontrolled} side-reactions and undesired
byproducts. Over these technical aspects, the synthetic potential
should be highlighted. In all the reported works, innovative reactional
approaches were highly stereoselective. Particularly interesting for
the pharmaceutical industry, hybrid catalysis can therefore provide
high value-added structures in a relatively convenient and economic
way. Other hybrid processes, which will be detailed in the next part of
this work, alternatively focus more on introducing highly reactive and
functionalizable chemical groups on building blocks to access new
platform molecules. Whatever the final objective, hybrid catalysis
could be the key to unlocking numerous simplified, and more
importantly, efficient synthetic strategies, especially for future
biorefineries.


{Beyond all the above-mentioned synthetic and industrial interests,
technical adaptability of hybrid catalysis with the reaction should
also be mentioned. The main aspects to consider are the catalysts
involved, their allocation in the reactors and the sequence of the
reaction steps. The choice of the catalysts depends on the nature of
the reaction, the {requirements} of the process and the energy source.
The most relevant hybrid approaches for biomass conversion were
recently classified by Heuson and Dibenedetto based on the nature of
the biocatalysts involved, and the way they were combined~\cite{13}.} 
{But, as a final illustration of both the complexity and advantages of
hybrid catalysis, we finally propose to detail the constraints that can
govern the combination of catalysts of different natures and operating
conditions through a series of examples we recently \mbox{developed} around
the conversion of glucose into new platform molecules, which notably
involve the development of a completely new type of reactor
specifically designed to overcome the inhibition of the catalysts
involved in the transformation.}

}

\section{Application: from glucose to furfurylamides through hybrid
processes}

{

In line with the advantages that we have detailed previously for hybrid
catalysis, we have developed several approaches to exploit two main
advantages: the design of more economical processes and the discovery
of completely new synthetic routes to access new families of molecules.
We will illustrate these two aspects through the conversion of glucose
into HMF, a process that has already been implemented but which we have
tried to improve in a hybrid way. Then through the production of
furfurylamines, and then of furfurylamides again through a hybrid
route, families of molecules with interesting potential yet to be
identified.

}

\subsection{From \textsc{d}-glucose to 5-hydroxymethylfurfural
through a two-pots/one-step system}

{

As we have already seen from several examples cited above, one
\textsc{d}-glucose-derived platform molecule that is at the heart of
all concerns is \mbox{5-hydroxymethylfurfural} (HMF)~\cite{39}.  Indeed, HMF
can be further valorized by different reactions, including
hydrogenation, which leads to 2,5-dimethylfuran (2,5-DMF)~\cite{40},
a promising fuel additive, or to a precursor to terephthalic acid by a
dehydrative Diels--Alder reaction with ethylene to form
\textit{p}-xylene, followed by an oxidation~\cite{41,42}. 
Alternatively, HMF oxidation can also yield 2,5-furandicarboxylic acid
(FDCA), a biosourced alternative to terephthalic acid with significant
potential for polyester plastic production~\cite{43,44}.  However,
\textsc{d}-glucose, despite its abundance, is not the preferred
substrate for efficient HMF production compared to the more reactive
and costly \textsc{d}-fructose. This preference is due to the fact
that fructose, with its fructofuranose tautomer, dehydrates to a
furanic ring with lower energy barriers than
\textsc{d}-glucose~\cite{45}. Therefore, isomerizing
\textsc{d}-glucose into \textsc{d}-fructose is a crucial step
that cannot be bypassed. One solution for this additional step could be
an enzymatic \textsc{d}-glucose isomerization, which is already
widely used in industrial production of high-fructose corn syrup
(HFCS)~\cite{46}.  This enzymatic reaction remains the method of
choice due to its high efficiency and moderate cost. Although this
process requires, high-purity \textsc{d}-glucose, specialized
buffers and multiple ion-exchange resins to remove metallic impurities
in food-grade HFCS, it is still favored over chemical isomerization,
despite renewed interest in the latter~\cite{47,48,49,50}.  The
enzymatic process, however, is limited by its thermodynamic equilibrium
$(K_{\mathrm{eq}}\approx 1)$, restricting \textsc{d}-glucose
conversion~\cite{51,52}.  While this equilibrium is not an issue for
HFCS production (which tolerates
\textsc{d}-glucose/\textsc{d}-fructose mixtures), it is less
than ideal in the case of HMF production as the remaining
\textsc{d}-glucose cannot be converted into the final product. One
solution is then to shift the isomerization equilibrium by directly
coupling the first reaction with \textsc{d}-fructose dehydration, as
the latter is irreversible. Nevertheless, this requirement introduces
challenges, as catalyst compatibility between the isomerization system
and the dehydration system remains problematic. However, Huang et~al.\ 
proposed a very interesting one-pot/two-steps hybrid process \mbox{combining}
a thermophilic glucose isomerase immobilized on
aminopropyl-functionalized mesoporous silica (FMS) with a heterogeneous
Br\o{}nsted acid catalyst (propylsulfonic-FMS-SO$_{3}$H) in a
THF/H$_{2}${O (4/1~v/v) solvent mixture~\cite{53}.  But, although
isomerization with the enzyme at 363~K achieved a promising 61\%}
\textsc{d}-fructose yield, subsequent dehydration required a higher
temperature (403~K), which only led to a HMF 30\% yield, and more
importantly, caused complete denaturation of the glucose isomerase.
Overall, these results highlight the difficulty of integrating these
processes together.

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig08}
{\vspace*{2pt}}
\caption{\label{fig8}General principle of the hybrid
``two-pot/one-step'' simultaneous process applied to the transformation
of  \textsc{d}-glucose to HMF (\textsc{d}-Glc ${=}$ \textsc{d}-glucose,
\textsc{d}-Fru ${=}$ \textsc{d}-fructose, HMF  ${=}$ 5-hydroxymethylfurfural).}
\end{figure*}

As an alternative to what has been proposed, we thought it would be
interesting to turn to an alternative two-pot/one-step process for this
particular situation, as the separation of the reactions could allow
different reaction conditions (pH, temperature, etc.) when run in
parallel, while still benefitting from the possibility of continuously
consuming \textsc{d}-fructose. Indeed, strategies have already been
adopted to combine enzymatic isomerization of \textsc{d}-glucose
to \textsc{d}-fructose and chemical dehydration. These approaches
involve the separation of bio- and chemocatalysis and
\textsc{d}-fructose transportation between the aqueous phase for
isomerization and an organic phase for subsequent reactivity. Huang 
et~al.\ first introduced this concept by adding sodium tetraborate to the
aqueous isomerization medium, forming a fructoboronate complex through
interaction with \textsc{d}-fructose~\cite{54}.  This complex was
then transported to a separate organic phase with the aid of a cationic
quaternary ammonium salt, enhancing \textsc{d}-glucose conversion
by shifting the isomerization equilibrium toward
\textsc{d}-fructose. The dehydration of \textsc{d}-fructose in
the organic phase led to an increase in HMF yield to 63\%, compared to
28\% without borate addition, and by improving the conversion of
glucose from 53 to 88\%. However, the selectivity of the complexation
between \textsc{d}-glucose and \textsc{d}-fructose was
suboptimal, which was later addressed by Delidovich et~al., who
optimized the chemical nature of the boronate species~\cite{55}.  {A
global process exploiting this concept for} \textsc{d}-glucose to
HMF production has concomitantly been proposed by Alipour~\cite{56}. 
Therein, \textsc{d}-fructose is complexed with phenylboronic acid
and transferred to an organic phase, which is then separated and
brought into contact with an acidic ionic liquid phase to release free
\textsc{d}-fructose. The \textsc{d}-fructose-rich ionic liquid
is subsequently used for dehydration to HMF in a biphasic medium, with
HMF being back-extracted into a final low boiling point organic phase.
This method relies on four distinct media and intermediate phase
separations which consequently limits its industrial potential. 
\looseness=-1

Inspired by these sequential steps, we therefore sought to propose a
more streamlined and integrated approach that reduces the number of
steps and avoids the use of costly and difficult-to-recover ionic
liquids. It would be composed of the following steps: (1) isomerization
of \textsc{d}-glucose in a first aqueous phase at neutral or slightly
basic pH, compatible with the enzyme; (2) complexation of the
\textsc{d}-fructose produced with an arylboronic acid, allowing its
solubilization from the first aqueous phase into a second, organic
``transport'' phase; (3) hydrolysis of the complex at the interface of
a second, acidic pH aqueous ``receiving'' phase; in which (4) the
\textsc{d}-fructose is dehydrated to HMF using an acid catalyst. This
concept is illustrated in Figure~\ref{fig8}. 

}

{

Through a first study~\cite{57},  we optimized the various
parameters related to each of the steps independently, starting with
the isomerization step with glucose isomerase. For this, we studied a
temperature range from 323~K to 363~K and tried to find the optimum pH
between 4.5 and 9. These parameters were applied to a commercially
available immobilized glucose isomerase
(Sweetzyme$^{\text{\textregistered}}$ IT Extra, \textsc{d}-xylose
ketol-isomerase), to make the process as scalable as possible. As a
result, we observed that the enzyme exhibited maximum activity around
pH~7.5, at a temperature of 343~K. As the extraction of
\textsc{d}-fructose by formation of a complex
[\textsc{d}-fructose--boronic acid]$^{-}$ is favored if the pH of the
aqueous solution containing \textsc{d}-fructose is larger than the
$\mathrm{p}K_{\mathrm{a}}$ of boronic acid and the
$\mathrm{p}K_{\mathrm{a}}$ of the main boronic acids are lower than
8.5, we chose pH~8. This condition allows us to overcome the
$\mathrm{p}K_{\mathrm{a}}${-related lock of the selected boronic acid
to transport} \textsc{d}{-fructose to the organic phase while
maintaining a relative activity of more than 80\% of the enzyme. Under
these conditions, a final yield of 55\% in} \textsc{d}-fructose
isomerization could be obtained. We then turned our attention to
optimizing the \textsc{d}-fructose extraction from the aqueous
``giving'' phase to the organic ``transport'' phase. For the latter,
methylisobutylketone (MIBK) was selected over other solvents like
methyl-\textit{tert}-amyl ether and dimethyl carbonate based on
solubility, toxicity, and boiling point. Despite similar
\textsc{d}-fructose extraction yields, MIBK's higher boiling point
(117 vs.\ 86~{\textdegree}C for methyl-\textit{tert}-amyl ether)
made it more suitable for chemical catalysis at 80--90~{\textdegree}C.
Then, inspired by a previous study~\cite{58},  we chose the couple
lipophilic arylboronic acid (carrier) along with a quaternary ammonium
salt (Aliquat336$^{\text{\textregistered}}$) as a phase transfer agent
for \textsc{d}-fructose extraction into the MIBK. At the chosen pH
conditions (pH~8), aryl boronic acid ArB(OH)$_{2}$ is actually present
under its hydroxylated anionic form, as a tetrahedral
aryltrihydroxyborate $\mathrm{ArB(OH)}{_{3}}^{-}$~\cite{59}. {At the
interface between the aqueous and organic phases, \textsc{d}-fructose
further reacts with the arylborate to form a tetrahedral fructoboronate
ester. The fructoboronate complex then forms an intimate ion pair with
Aliquat336$^{\text{\textregistered}}$, which enables its transportation
to the organic phase~\cite{60}. {We then studied the influence of the
boronic acid structure to optimize the kinetics and maximize the
selectivity of} \textsc{d}-fructose complexation/transportation. Seven
arylboronic acids, which differ through the electronic properties of
their substituents and thus by the $\mathrm{p}K_{\mathrm{a}}${, were
screened in the complexation with} \textsc{d}-fructose:
3,4-dichlorophenylboronic acid (3,4-DCPBA), 3,5-dichlorophenylboronic
acid (3,5-DCPBA), 2,4-dichlorophenylboronic acid (2,4-DCPBA),
2,3-dichlorophenylboronic acid (2,3-DCPBA),
4-\textit{tert}-butylphenylboronic acid (4-TBPBA), and
4-(trifluoromethyl)phenylboronic acid (4-TFMPBA). With an excellent
extraction yield and rate (46.5\% and 1.48~${{\upmu}}$mol/min,
respectively) under our conditions, 3,4-DCPBA was chosen as the aryl
boronic acid for the continuation of our studies. The
3,4-DCPBA/Aliquat336$^{\text{\textregistered}}$ molar ratio was then
studied, again with a view to maximizing extraction speed and yield.
The best yield was obtained with a 1/2 ratio, as the increase in
Aliquat336$^{\text{\textregistered}}$ concentration meant that no more
\textsc{d}-fructose could be extracted above it. Keeping a molar ratio
of aryl boronic acid/Aliquat336}$^{\text{\textregistered}}$ {equal to
1/2 and at fixed initial} \textsc{d}{-fructose amount, both 3,4-DCPBA
and Aliquat336}$^{\text{\textregistered}}$ concentrations were then
varied. From this screening, we found that the
\textsc{d}-fructose/3,4-DCPBA/Aliquat336$^{\text{\textregistered}}$
ratio of 1/1/2 (with \textsc{d}-fructose used at 100~mM) was the most
effective, even though the extraction yield could not be further
improved compared to previously obtained results. The release of
\textsc{d}-fructose into the aqueous ``receiving'' phase was also
studied as a function of its pH. By changing the pH from 8 to 1, the
release yield was increased from 20\% to 100\%, and over 91\% at pH~5.
We concluded that, as expected, the pH of the recipient phase should be
as low as possible. Finally, the optimal conditions for
\textsc{d}-fructose dehydration to HMF were studied at different
temperatures, as for the isomerization, along with different
catalyst-to-\textsc{d}-fructose mass ratios. For this study we chose,
as an alternative to a strong homogeneous acid that cannot be recycled
in processes, a commercially available acidic resin containing strong
sulfonic groups. This resin, indeed, facilitates a potential
reusability and recyclability of the catalyst. In addition, this
reduces the risk of using and heating strong acids in liquid form. A
temperature of 80~{\textdegree}C was found to be the most effective
with this catalyst. At this temperature, no humins were detected,
avoiding significant resin browning observed at 90~{\textdegree}C. 
\looseness=-1


In the formerly optimized conditions, a sequential process was tested.
The first step involved simultaneous \textsc{d}-glucose
isomerization and \textsc{d}-fructose transport by using a
fructoboronate complex. This was followed by the hydrolysis of the
complex, releasing \textsc{d}-fructose into the aqueous receiving
phase. Finally, the third step dehydrated the released
\textsc{d}-fructose to HMF using the sulfonic resin. All these steps
were performed one after the other, the organic phase \mbox{being} first
isolated, before being put in contact with the aqueous ``receiving''
phase, without the chemocatalyst, the latter being added in a third
time. As a result, only 57\% of \textsc{d}-fructose (based on 100\%
of potential \textsc{d}-glucose to be converted) was extracted from
the ``giving'' phase, followed by a release of 63\% of the extracted
\textsc{d}-fructose into the ``receiving'' phase, and 20\% of this
extracted \textsc{d}-fructose could be dehydrated into the final
product, leading to a very small overall HMF yield of 5\%. However, as
each phase is decoupled from the previous/subsequent one, this
sequential approach benefits little from the shift in the isomerization
equilibrium. The latter is only pulled by the \textsc{d}-fructose
extraction to the organic phase, which also has its own equilibrium.
Not so surprisingly, low yield was obtained. Logically, we then
attempted to couple the three phases and the two catalytic stages, into
a single, very simple system, as described in Figure~\ref{fig9}.\looseness=-1

}

\begin{figure}
{\vspace*{4pt}}
\includegraphics{fig09}
{\vspace*{4pt}}
\caption{\label{fig9}Illustration of the combined process for the
conversion of  \textsc{d}-glucose to HMF using a triphasic system,
combining an immobilized glucose isomerase (IGI) and an acidic
catalyst. Redrawn from Gimbernat et~al.~\cite{57}.}
{\vspace*{1pt}}
\end{figure}

{

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig10}
{\vspace*{-3pt}}
\caption{\label{fig10}H-type reactor concept for HMF production from 
\textsc{d}-glucose in a two-pot/one-step hybrid process combining a
glucose isomerase and a sulfonic resin in three different phases.
Redrawn from Heuson et~al.~\cite{13}.}
{\vspace*{-3pt}}
\end{figure*}

As a result, the isomerization of \textsc{d}-glucose was achieved with
a 70\% yield. Half of the produced \textsc{d}-fructose was successfully
transported to the \mbox{``receiving''} phase, leading to 35\% of
\textsc{d}-glucose transformed to \textsc{d}-fructose available for
further dehydration in the acidic phase, as for the sequential process.
Therefore, it is with no surprise that we again obtained a very low HMF
final yield of 4\%. Despite showcasing the potential of such a
chemoenzymatic catalysis cascade for direct HMF production from
\textsc{d}-glucose, our process was clearly limited by additional
factors, the most preeminent one being probably insufficient phase
agitation. 

This consequently led us to rethink completely the architecture of the
reactor, toward a more complex, but also more efficient system to
promote the continuous extraction of \textsc{d}-fructose toward the
receiving phase. In order to propose radically different \mbox{reaction}
conditions between the two aqueous phases (donor and recipient), while
maximizing diffusion of the \textsc{d}-fructose formed from one to
the other, we switched to a rather unusual ``H''-type reactor topology
(Figure~\ref{fig10})~\cite{61}. 
This reactor incorporates all the stages previously tested, but this
time with two separate compartments that can be independently
thermostated, linked by a bridge for circulation of the organic
transfer phase. With this new reactor at our disposal, we repeated the
tests with the different combined phases we had previously carried out,
but unfortunately did not observe the increase in
\textsc{d}-fructose transfer we hoped. Conversely, we observed a
strong accumulation of the fructoboronic complex in the organic phase
of the first pot. Given the small diameter of the solvent bridge, we
concluded that the main limitation was the lack of circulation of the
organic phase between the two pots. Therefore, we added a fin with
baffles in the opposite direction in both compartments, allowing the
flow of the organic phase created by the rotation of the two stirring
systems to be redirected, so that the organic phase could circulate
freely from one compartment to the other. This improvement enabled us
to increase the rate of \textsc{d}-fructose release into the
receiving aqueous phase by a factor of three. However, in order to
further improve the process, we optimized a number of additional
parameters. Firstly, we \mbox{observed} that, as previously mentioned, a pH
difference between the two aqueous phases was important to enable
efficient continuous extraction of \textsc{d}-fructose from the
donor to the recipient phase. This is because, at basic pH, the
fructoboronic complex can form and dissolve in the organic phase, which
can then be hydrolyzed in the receiving phase at a more acidic pH, and
the greater the pH difference, the more the equilibrium of the two
dissolutions is exceeded in the receiving phase. However, following the
introduction of the fin with baffles, we observed that the pH of the
two phases tended to equilibrate, with the organic phase partially
transferring protons from one phase to the other, helped by the new
MIBK circulation, albeit rather slowly. We therefore \mbox{investigated} the
\mbox{possibility} of buffering the two aqueous solutions in an attempt to
slow down or even eliminate this phenomenon. To this end, we used a
buffer at three different concentrations in each phase, to see whether
the buffer strength also had a predominant role in this limitation,
while maintaining a pH difference of 5.5 between the phases (pH 8.5 and
3 for the donor and recipient phases, respectively). As a result, with
a sufficient buffer concentration (500~mM), we were able to limit the
fall in pH difference to a value of 3.5 over 6~h of reactions, which
showed promise for the efficiency of the hybrid reaction. Obviously,
this efficiency decreases with decreasing buffer concentration. It
should be noted that this type of strategy is obviously not
transferable to a larger scale, and it would then be preferable to set
up active pH control in each of the bases by adding base and acid
respectively, or even to envisage a continuous system directly for the
aqueous phases, with the organic bridge remaining static, to dilute the
pH variation, while still maximizing the effects of equilibrium\break
displacement.

}           

{

In parallel, we also restudied the concentration ratio of
\textsc{d}-fructose/3,4-DCPBA, and determined that with this new
system a new ratio of 1/0.25 was now more efficient (while maintaining
the 3,4-DCPBA/Aliquat336$^{\text{\textregistered}}$ ratio at 1/2) for
\textsc{d}-fructose extraction, while allowing a considerable
reduction in the amount of 3,4-DCPBA and
Aliquat336$^{\text{\textregistered}}$ used, which represents another
advantage of the new reactor. With these latest optimizations in hand,
we tested the new complete hybrid process for HMF production on a scale
of 100~mM initial \textsc{d}-glucose. For this experiment, a
temperature of 70~{\textdegree}C was set in the first pot (enzymatic
isomerization step) while 80~{\textdegree}C was set in the second pot
(chemical dehydration step), and isomerization, extraction and
dehydration efficiencies were then followed over 32~h. At the end of
the reaction, the isomerization, \textsc{d}-fructose extraction and
HMF production yields reached 79\%, 97\%, and 31\%, respectively. The
extraction yield obtained thus demonstrates the efficiency of our new
process and its associated optimized parameters for
\textsc{d}-fructose transportation between the two aqueous phases,
while the isomerization yield clearly highlighted an equilibrium shift
as expected. The carbon balance in both aqueous phases was found to be
equivalent to 90\% in \textsc{d}-glucose, \textsc{d}-fructose
and HMF, proving the efficiency of the release process. The only step
that was really limiting here was therefore the dehydration, with a
final yield of only 30.9\% in the desired product. Although this is on
a par with the best attempts previously described, such as that of
Huang et~al.~\cite{53},  it seems crucial to now turn our attention
to optimizing the dehydration step, probably by choosing a better
catalyst than the sulfonic resin, or perhaps by considering a better
process control so as to be able to further lower the pH and increase,
if possible, the temperature of the second pot, while keeping humins
formation as low as possible.

Nonetheless, this first proof-of-concept clearly highlights the
effectiveness of the hybrid catalysis concept in obtaining molecules of
interest more efficiently, in this case the transformation of
\textsc{d}-glucose, that can easily be obtained from biomass, into the
highly valuable HMF, under mild conditions. More precisely, it shows
how the combination of different catalytic steps within the same system
can greatly help shift the equilibrium of reactions that are normally
highly reversible. Interestingly enough, it would also be possible to
apply this process with the H-shape reactor and organic membrane to an
unrefined \textsc{d}-glucose source, typically a lignocellulosic
biomass extract, insofar as the MIBK could prevent or, at least, limit
the diffusion of polar species that could poison the chemical catalyst
toward the phase where dehydration takes place. If we take the concept
a step further, why not consider direct cellulose and hemicelluloses
depolymerization using an enzymatic cascade in the donor phase, insofar
as the first stage takes place under conditions that are entirely
compatible with the action of cell wall degrading enzymes. But more
than just a synthesis proof-of-concept, this approach, with all the
process-level optimization that it required, illustrates how different
combinations of catalysts, in one-pot/two-steps or two-pots/one-step
systems, can enable radically different catalytic functionalities to be
accessed, and offer specific advantages in each case. This makes the
combination of these processes highly complementary and interesting.

}

\subsection{One-pot conversion of 5-hydroxymethyl{\hubreak}furfural into
5-aminomethyl-2-furan{\hubreak}carboxylic acid}

{

As described above, HMF is a very interesting platform molecule as it
can be transformed into a variety of value-added building
blocks~\cite{62,63,64}, including monomers useful for the production
of biobased polymers such as 2,5-dihydroxymethylfuran (DHMF),
2,5-dicarboxaldehydefuran (DCAF), 5-hydroxymethyl-2-furancarboxylic
acid (HFCA) and 2,5-furandicarboxylic acid (FDCA), thanks to a wide
range of innovative catalytic pathways~\cite{65,66,67}.  The
conversion of HMF to FDCA, for example, has been the subject of
intensive research in recent decades, and numerous approaches have been
\mbox{reported~\cite{67,68,69}.} Noble-metal-based chemical catalysts
demonstrate high efficiency in HMF oxidation, with supported noble
metal nanoparticles exhibiting exceptional catalytic performance in
liquid-phase oxidation under both alkaline and acidic
conditions~\cite{70,71,72,73,74,75,76}.   Moreover, enzymes were also
applied. One of the earliest studies on biological oxidation of HMF was
conducted by Koopman et~al., who identified a novel HMF oxidoreductase
from \textit{Cupriavidus basilensis} HMF14. By introducing the
\textit{hmfH} gene into \textit{Pseudomonas putida} S12, a
whole-cell biocatalyst was created. This system produced 30.1~g/L of
FDCA from HMF in fed-batch experiments, using glycerol as the carbon
source, with an impressive yield of 97\%~\cite{77}.  For their part,
Dijkman et~al.\ utilized an HMF oxidase, expressed in 
\textit{E.~coli}, for a four-step oxidation of HMF to FDCA. This FAD-dependent
enzyme led over 95\% conversion of HMF within 24~h at
25~{\textdegree}C, in an aqueous phosphate buffer solution
(pH~7)~\cite{78}.  Still with the same objective, Carro et~al.\ 
combined an aryl-alcohol oxidase (AAO) with an unspecific peroxygenase
(UPO), both from \textit{Agrocybe aegerita} in a cascade
reaction~\cite{79}.  AAO reduced O$_{2}$ to generate H$_{2}$O$_{2}$,
while enabling stepwise oxidation of HMF to DFF (2,5-diformylfuran) and
FFCA (5-formylfuran-2-carboxylic acid). However, as AAO could not
oxidize the carbonyl groups in FFCA directly, the authors combined it
with the UPO to perform the conversion of FFCA into FDCA using
H$_{2}$O$_{2}$, generated during the first step, to produce FDCA with a
91\% yield after 116~h~\cite{80}.  Finally, and although examples are
rare, a few hybrid attempts have also been made. Liu and coworkers
applied an immobilized laccase on magnetic nanoparticles with TEMPO as
a chemomediator, obtaining a FDCA yield of 90.2\% in 96~h at
35~{\textdegree}C~\cite{81}.  What all these examples have in common
is that they carry out only one reaction type, i.e., oxidation (alcohol
to carbonyl and then carbonyl to acid). Obviously, it is much more
complex to get catalysts to cohabit when the types of functionalization
reactions sought are different, or even in some cases opposed, as is
the case for oxidations and reductions. Nonetheless, with the same
mindset, furfurylamines have been identified as valuable precursors for
biobased polymers such as polyamides, polyimides, polyaspartimides,
polyureas, polyhydroxyurethanes, polyimines, and polyenamines. Their
monomers can be conveniently synthesized from biosourced furfural
derivatives, further supporting their potential in sustainable material
development~\cite{82}.  These compounds also have several
applications including the preparation of benzoxazine derivatives for
flame-retardant resins~\cite{83,84} and, after conversion to
difurfuryl diisocyanates, the replacement of petroleum-based
diphenylmethane diisocyanate in polyurethane systems~\cite{85,86}.
Furfurylamines are typically synthesized through the reductive
amination of the carbonyl group in the furfural structure under mild
conditions with inexpensive reagents~\cite{87}.  However, such
processes often require the use of protecting groups, numerous chemical
steps, and toxic reductive agents~\cite{87,88}.  It is worth
mentioning that the reduction combined with the potential oxidation of
HMF's hydroxyl groups, to obtain the corresponding ${\upomega}$-amino
aldehyde or acid, is obviously particularly challenging. To circumvent
these issues, several chemocatalytic pathways have been recently
developed~\cite{87,88,89,90,91,92}.  However, these are not ideal for
the synthesis of furfurylamines derived from HMF, \mbox{owing} to the
sensitivity of the furan ring to reductive conditions and the tendency
of these compounds to form secondary and tertiary
amines~\cite{65,66,67,93}. An efficient alternative to perform
amination of HMF derivatives is the use of transaminases, their primary
catalytic function being the nonreductive amination (transfer of an
amine) of carbonyl groups in water at low temperatures~\cite{34}.
Recently, several ${\upomega}$-transaminases were employed for the
synthesis of several furfurylamines from HMF derivatives~\cite{93,94}.
Dunbabin et~al.\ reported yields up to 92\% for the aminated products,
starting from different HMF and furfural derivatives. Among them, they
achieved the synthesis of 5-hydroxymethylfurfurylamine (HMFA),
5-aminomethyl-2-furancarboxaldehyde (AMFA), furan-2,5-diyldimethanamine
(FDMA), and the 5-aminomethyl-2-furancarboxylic acid (AMFC). However,
in the case of AMFC, they performed the \mbox{synthesis} directly on
5-aldehyde-2-furancarboxylic acid (AFCA) instead of HMF, as the only
amine that can be directly produced from HMF, in a single step with one
catalyst, is HMFA, the other furfurylamines requiring a coupled
oxidation step, whether before or after the amination. It is precisely
here that hybrid catalysis can be of great interest by proposing the
combination of an oxidation catalyst with a transaminase, as has
already been done for other applications we described earlier, to gain
access to other more complex derivatives of HMF-derived furfurylamines.
We have paid particular attention to the synthesis of AMFC, as it is a
promising building block for polymer synthesis. Being an
${\upomega}$-amino acid, it can be used to produce unnatural peptides
such as cyclopeptides~\cite{95,96},  but could more generally be
integrated in aromatic polyamides. Thus, we attempted to develop the
combination of an oxidative metal chemocatalyst and a transaminase for
the direct synthesis of AMFC from HMF.


To achieve this synthesis, we took full advantage of the
high-throughput screening capabilities of the REALCAT platform in
Lille, enabling us to select the best catalysts, both for the chemical
catalyst and the enzyme, from a panel of catalysts of different
natures, particularly regarding supported metal nanoparticles. Indeed,
depending on the metal used and the properties of the support, the
selectivity of the catalyst is particularly different, which can lead
to the production of undesirable byproducts. In a parallel study, for
example, we demonstrated that under \mbox{certain} conditions, several of our
supported metal catalysts were capable of performing oxidative
deamination of furfurylamines~\cite{97},  which is obviously
problematic in the context of AMFC synthesis, since the latter is then
converted back into purely oxygenated derivatives. Another challenge
was to find a chemical catalyst capable of operating at a relatively
neutral pH, since this is generally the optimum pH for transaminases.
Indeed, many supported noble metal nanoparticle catalysts are more
active at more basic pH, which is typically the case with gold
nanoparticles, for example~\cite{36}.  After screening 15 catalysts
containing nanoparticles of gold, palladium, platinum, ruthenium and
even some bimetals, immobilized on different types of supports, metal
oxides for the majority of them, we were able to demonstrate that one
of the combinations, namely platinum immobilized on silica, was able to
carry out the oxidation of HMF to AFCA (the ${\upomega}$-aldehyde acid
derivative) directly using the oxygen present in the head-space gas
phase of the reaction. This catalyst proved highly effective at pH~8
and at 60~{\textdegree}C, enabling a 100\% substrate conversion in less
than 24~h. Notably, this temperature could in theory also allow for the
use of different transaminases, several of which have already been
described as thermostable~\cite{98}.  A negative aspect of this
catalyst, however, was its lack of selectivity for the oxidation of the
alcohol or aldehyde function of the substrate. Indeed, when oxidation
step takes place before the amination step, apart from the desired
intermediate (AFCA), a byproduct from the complete oxidation of the two
functions to the acid (FDCA) is also found. Despite the previously
mentioned interest of FDCA and the possibility of directly
incorporating it into polymerization matrices, its formation naturally
limits the obtainable yield of AMFC. Therefore, the use of this
catalyst should ideally take place sequentially to the amination, which
would avoid the adoption of a one-pot/one-step system. In our process,
this represented the main limitation since at the time of the study, we
still did not have any thermostable transaminases capable of carrying
out the amination of AFCA, and therefore had to introduce the enzyme
into the reaction medium in a second step, once the reaction had
returned to room temperature. Nonetheless, we could employ one of the
transaminases classically described in the literature for furfural
amination, e.g., \textit{Chromobacterium violaceum}. Notably, we
had immobilized it beforehand on silica too, in order to enhance its
\mbox{removal} from the medium by simple filtration like for the chemical
catalyst, simplifying the posttreatment of the process. As a final
result, by using two different amine donors, methylbenzylamine and
isopropylamine, which are formed through transamination of the
thermodynamically stable acetophenone and the easily removable acetone
product, respectively shifting the reaction equilibrium. Following this
approach, we reached a process leading to a 77\% yield in AMFC in 52~h
and with FDCA as the only byproduct (Figure~\ref{fig11}).

}

\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig11}
{\vspace*{1pt}}
\caption{\label{fig11}One-pot/two-steps hybrid synthesis of
5-aminomethyl-2-furancarboxylic acid (AMFC) combining an oxidation step
catalyzed by platinum nanoparticles immobilized on silica leading to
the formation of an intermediate 5-aldehyde-2-furancarboxylic acid
(AFCA), which is then aminated by the immobilized  \textit{Cv}TA
transaminase in a second step in the same reaction medium. Redrawn from
Lancien et~al.~\cite{99}.}
\end{figure*}

{

Interestingly, this process has paved the way for other attempts by
various research groups. We should mention the one from Giri and
coworkers~\cite{100},  {who recently reported a robust transaminase
from} {\textit{Shimia~marina}} {(SMTA) that enables the scalable
amination of HMF to biobased \mbox{furfurylamines} with high activity and
broad substrate specificity. In their case, they succeeded in
developing a similar cascade reaction, only enzymatically, coupling an
aldehyde reductase from} \textit{Synechocystis}~sp.\ PCC 6906 (SAHR),
in a one-pot/two-steps system, nearly yielding a quantitative
production of AMFC in 30~h. However, as the authors themselves
acknowledged, obtaining the AFCA intermediate by a purely enzymatic
route generally requires the use of several consecutive oxidases (two
in their case) as well as the regeneration of expensive cofactors
(which involve an additional catalase and horseradish peroxidase in
their process), which drastically increases the {complexity} and the cost
of the system, and {consequently,} limits the industrial application of
this approach. This demonstrates, again, the potential of hybrid
catalysis, particularly when chemical catalysts are used to replace the
costliest steps of cofactor regeneration and inhibitor suppression
(e.g., H$_{2}${O}$_{2}$).

}

\subsection{Taking the concept a step further: one-pot/two-steps hybrid
production of furfurylamides from AMFC}
 
{

{Recently, willing to take the concept a step further, we sought to
directly convert AMFC into a new \mbox{family} of molecules. More precisely,
we wanted to take advantage of the polyfunctional character of AMFC,
and even more precisely of the presence of two ionizable functions, one
of them positively (amine) and the other one negatively (carboxylic
acid), to try to produce amphiphilic molecules with different
properties, by attaching a fatty chain to one of these two functions.
Among the many types of new bonds that can be formed on these two
functions to introduce a fatty chain, one of them seemed particularly
promising, as it could potentially be applied to both functionalities
of the molecule: the creation of an amide bond from a fatty alcohol or
acid. This type of bond also seemed relevant because of its rigidity
and \mbox{resistance} to hydrolysis, guaranteeing a certain stability of the
produced compound. More generally speaking, the synthesis of amides is
one of the most important transformations in agrochemical,
pharmaceutical and polymer industries~\cite{101,102}.}  {In recent
decades, there has been growing interest in {amide-producing} processes
after the ACS Green Chemistry Institute pointed out the need for more
sustainable approaches to amide production as a key research area for a
more environmentally friendly chemistry~\cite{103}.}  {The most common
approach for producing amides is the nucleophilic addition of an amine
to an activated carboxylic acid. However, the activating agents
involved in these reactions often lead to the generation of waste and
hazardous byproducts~\cite{104,105}.}  {It should be noted, however,
that many enzymes are also capable of forming amide bonds, such as
hydrolases (e.g., lipases, esterases, and acylases), nitriles
hydratases, and transglutaminases that have already been used at an
industrial scale under milder conditions compared to the traditional
organic synthesis routes~\cite{106}.}  {It is worth mentioning that
other and much less exploited classes of enzymes are also available to
form these bonds, notably acyl-coenzyme A ligases
(ACLs)~\cite{107,108}.}  ACLs catalyze ATP-dependent formation of
acyl-CoA thioesters from carboxylic acids in a two-step reaction: (1)
formation of adenylate derivative from carboxylic acid and ATP and (2)
nucleophilic attack of coenzyme A (HSCoA)~\cite{109}.  Diversion of the
native reaction by adding an extra amine nucleophile in absence of
HSCoA leads to the formation of amides~\cite{110,111,112}.  One of the
main advantages of this reaction, unlike the use of hydrolases for
example, is that the amine used for nucleophilic substitution,
responsible for the formation of the final amide bond, is not taken
over by the enzyme at all, since the substitution takes place
spontaneously thanks to the intrinsic reactivity of the adenylate
intermediate. Thus, virtually any amine could be used, substituted by
an infinite number of substituents, as long as it is sufficiently
nucleophilic to attack the adenylate, implying that the products formed
could therefore be highly chemomodulated. Using these enzymes, it would
therefore be possible to either; (1) activate the AMFC acid and
transform it into an amide by attacking a free amine in the medium; or
(2) consider using AMFC as a nucleophilic amine, by activating another
carboxylic acid, which might also be derived from biomass. In either
case, if the other reagent contains a fatty chain, it is possible to
obtain a relatively apolar AMFC derivative, but one which retains a
polar head, ionizable in acidic or basic media depending on the
strategy chosen. When the amide is formed from the carboxylic acid of
AMFC, it should be noted that it would also be possible to form a
quaternary ammonium at its free amino group, leading to a charged
molecule. In any case, to our knowledge, none of these molecule
families has ever been reported, which makes them a very promising
subject for study. Having at our disposal a library of ACLs produced
from previous work, we verified that AMFC was indeed a substrate for
one of the ACLs. Unfortunately, none of the eight enzymes we tested,
which had shown the most promise in previous assays (activity,
stability, etc.), showed activity toward AMFC, as these enzymes tend to
accept aliphatic substrates with different chain lengths. This stopped,
for now, the acid functionalization pathway for AMFC, and we therefore
turned our attention to the possibility of using the latter as a
nucleophilic amine. Still seeking to branch out into a variety of fatty
chains, and given the estimated substrate selectivity of our enzymes,
we then screened our library on fatty mono- and diacids, ranging from
four to ten carbon atoms. It should be noted that we did include in the
panel some aromatic substrates whose ring was distant from the
carboxylic acid. Different selectivity was observed for short and long
chains depending on the enzyme (Figure~\ref{fig12}). Considering the
{possibility} of using these enzymes in hybrid processes, we carried out
the assays at 60~{\textdegree}C, a temperature which had proved
interesting in our previous study with the chemical catalysts tested.
One particular TsACL displayed a broad substrate range, being very
active toward all the tested monocarboxylic acids excepted the decanoic
acid and revealed to be almost the only ACL accepting a diacid (e.g.,
succinic acid). Thus, we can consider that the production of AMFC
dimers linked by a fatty chain enables to obtain two polar heads
instead of just one on the same molecule. It should also be noted that
these screenings were carried out directly with AMFC as the
nucleophilic amine, which gives an indication of the yields, even
though the assays were carried out on an analytical scale. Also,
looking more specifically at TsACL, yields between 53\% and 71\% for
aliphatic acids and 39\% for succinic acid, respectively, can be
obtained, which is already an excellent result in itself. However,
determined to go one step further by proposing the use of a wider range
of substrates, we were interested to see if we could couple these
enzymes with oxidative catalysts, similar to those we had previously
developed. This time, however, fatty alcohols would directly be used
instead of fatty acids, given the higher stability of the alcohols for
storage purposes rather than the acids. The idea was then to carry out
a first oxidation step, once again using immobilized metal
nanoparticles to transform a fatty alcohol into the corresponding acid,
then to make the ACL act on the formed acid to enable it to be attacked
by AMFC, in a one-pot process (Figure~\ref{fig12}).

}

\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig12}
{\vspace*{-1pt}}
\caption{\label{fig12}General scheme for the synthesis of AMFC-based amides through the
implementation of a one-pot/two-steps process combining supported gold
nanoparticles as oxidative chemocatalyst and a CoA ligase as
biocatalyst. Redrawn from Bisel et~al.~\cite{113}.}
{\vspace*{-1pt}}
\end{figure*}

{

To bring this concept to fruition, a screening of chemical catalysts,
assisted by the REALCAT platform's robots, was carried out to confirm
their \mbox{ability} to oxidize alcohols into their corresponding acid, under
conditions compatible with those of the enzymes. To this end, the
reaction was performed at 60~{\textdegree}C and a family of gold
nanoparticles immobilized on a variety of supports with acidic, basic
or neutral properties tested. As expected, nanoparticles immobilized on
the basic support, and more particularly on calcium oxide (CaO), proved
the most active, with a total conversion in less than 24~h, and perfect
selectivity for the desired acid. These assays were first carried out
on butanol, and then extended to the evaluation of the activity
measurement for the best catalyst (Au/CaO) to the alcohol panel
corresponding to the acids accepted by TsACL. Here, the \mbox{catalyst}
demonstrated an equivalent activity on all substrates, with total
conversion to the desired aliphatic acids, as well as to 1,4-butanediol
leading to succinic acid. This highlights the highly versatile nature
of this new catalyst, making it an ideal candidate for our hybrid
system. On the other hand, recycling tests of this catalyst led to
leach out observation, with the basic sites gradually being eliminated,
leading to its gradual deactivation. This effect was first observed,
albeit more slowly, in water, and then much more rapidly when using a
buffer to prepare the combination of the two catalysts. We screened
different buffers potentially compatible with ACLs (MOPS, TRIS and
CAPS) at different pH values ranging from 8 to 11, and it was observed
that the catalyst activity was greater at more basic pHs, as well as
the recycling activity. Surprisingly, it was also observed that TRIS
and CAPS were strongly degraded by the catalyst, although the
degradation products have not yet been identified. In the end, it was
the 100~mM MOPS buffer that provided the best catalyst activity, with
around 80\% conversion in 24~h for pH 8 and 9. We also took advantage
of this assay to test the activity of the catalyst in the presence of
MgCl$_{2}$ and MnCl$_{2}$, two cofactors of
the enzyme, knowing that the enzyme can use both quite indifferently.
It was found that the activity of the chemical catalyst was
surprisingly greatly diminished in the presence of
Mn$^{2+}$, whereas the Mg$^{2+}$ ion did
not allow us to see any significant difference. Having demonstrated the
activity of the catalyst in a buffered medium, the coupling of our two
catalysts could be achieved. A first assay was carried out on butanol
as substrate, which was first converted to butanoic acid, then
acidified to give 5-(butyramidomethyl)furan-2-carboxylic acid by
introducing TsACL, with AMFC and ATP in the medium in a second step.
Under the previously optimized conditions, a 65\% yield of the desired
product was obtained in 48~h, with alcohol-to-acid conversion always
quantitative, the limiting step being the enzymatic amide formation,
although the obtained yield is in line with the one from the enzyme
screening. We then extended our one-pot/two-steps process to other
alcohols and obtained the corresponding amides in similar yields to
butanol for pentanol and hexanol, and lower yields (26\%) for octanol,
perhaps due to alcohol solubility issues, the latter having been
slightly less oxidized (88\% in acid formation) than the shorter-chain
alcohols by the \mbox{chemical} catalyst. Interestingly, we also managed to
perform the hybrid cascade on 1,4-butanediol, with a modest 24\% yield.
Nevertheless, the presence of two \mbox{products} was confirmed by NMR as
mono- and disubstituted succinic acid, with a preference for the second
one (16\% yield vs.\ 8\% for the monosubstituted). This creates
opportunities for additional valorization pathways and applications for
AMFC. To sum up, this tandem heterogeneous enzymatic catalysis,
performed under mild aqueous conditions, marks a significant
advancement through the integration of gold nanoparticles and enzymes
within a one-pot reaction. Concretely, it represents the first
association of an ACL with a chemocatalyst, showing how such strategies
can offer access to valuable amides. A limitation of the present
process though, is the use of large excess of ATP, but this could be
easily circumvented by setting up an ATP regeneration system previously
reported in the use of ACL \mbox{enzymes~\cite{114}.}  Another avenue for
improvement is the implementation of a fully integrated process in a
single step, ideally using catalyst compartmentalization techniques to
limit their direct interaction. This aspect might be crucial as, after
trying to set up the one-pot/one-step hybrid system, the chemical
catalyst seemed to be poisoned by enzyme---its oxidation capacity
falling to zero as soon as the enzyme was introduced into the medium.
Indeed, it is not a concept detailed in the present article but the
production of ``multicatalytic hybrid materials'' combining the active
centers of the catalysts in a finely compartmentalized manner,
represents one of the major promises of \mbox{hybrid} process. As already
mentioned in several previous reviews, these new materials could
represent an excellent solution for avoiding cross-poisoning, while
maximizing the synergistic effects of the catalysts, and simplifying
processes~\cite{115}.  {In the meantime, a two-pot/one-step system
could also be considered, especially as the molecules generated here
have probably a different partition coefficient with respect to an
organic transfer phase, depending on their oxidation state and level of
functionalization.} 

}

\section{Conclusion}

In conclusion, through these three examples, as well as with the
important work accomplished by the community in hybrid catalysis over
the last decade, we have tried to show that the combination of
catalysts of different natures, and more particularly chemical and
biological ones, could be highly relevant for the valorization of
biosourced molecules. Indeed, the aforementioned recent developments in
hybrid catalysis have shown the possibility of directly transforming
untreated biomass materials into value-added products. In this field,
the production of furfural is one of the most targeted molecules found
in the literature; as many of its derivatives can have potential
applications in different industries (pharmaceuticals, polymers, food,
for instance). It is worth mentioning that the developed systems are
not only simple in terms of substrate and product treatment, notably
without needing any additional pretreatment or intermediate isolation
and \mbox{purification} steps, but also more efficient in terms of atom and
energy economy, thanks to a one-pot set-up. In parallel, research in
hybrid catalysis has also enhanced the exploration of new synthetic
pathways. As a result, structurally rich molecules, often chiral and
therefore interesting to access pharmaceutical molecules, can now be
accessed through such approaches. The wide substrate scopes and the
subsequent obtainment of a range of products show the versatility of
hybrid approaches, which from a future industrial application
perspective, could be very interesting. All these aspects are reflected
on the presented application, showing the whole workflow to develop a
series of hybrid approaches where biosourced \textsc{d}-glucose is
converted into HMF, then to AMFC, a furfurylamine, and then to
amphiphilic furfurylamides that were never produced to the best of our
knowledge. Still, despite this high \mbox{potential,} catalyst combination
remains very limited due to their cross-poisoning or incompatibility
between reaction conditions. Smart processes, based on innovative
chemical engineering, can nonetheless help overcome these limitations
with new equipment (two-pot/one-step process), or in the ideal
situation, tailor-made compartmentalized hybrid materials. Among all,
hybrid catalysis stands as a highly interdisciplinary and emerging
subject, which will increasingly require close collaboration between
\mbox{different} scientific fields, notably chemistry and biology, while also
keeping an engineering perspective to enhance further efficient
applications.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that  could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgments}

This work was supported by an ANRT grant with Sanofi (CIFRE 2024/0663,
PhD grant for Sara Arteche Echeverr\'{i}a). The authors thank the
REALCAT platform funded by the French government grant managed by the
French National Research Agency (ANR) as part of the ``Investments for
the Future'' program (ANR-11EQPX-0037). A CC-BY public copyright
license has been applied by the authors to the present document and
will be applied to all subsequent versions up to the Author Accepted
Manuscript arising from this submission, in accordance with the grant's
open access conditions.

\CDRGrant[ANRT]{CIFRE 2024/0663}
\CDRGrant[ANR]{ANR-11EQPX-0037}

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