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\DOI{10.5802/crchim.239}
\datereceived{2023-04-05}
\datererevised{2023-06-07}
\dateaccepted{2023-06-28}
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\dateposted{2023-08-22}
\begin{document}

\begin{noXML}

\RubricEF{Research article}{Article de recherche}

\title{Synthesis of acrylonitrile functionalized hydroxymethylfurfural
derivatives with Mg(OH)\tsub{2} under solvent-free conditions}

\alttitle{Synth\`{e}se de d\'{e}riv\'{e}s d'hydroxym\'{e}thylfurfural
fonctionnalis\'{e}s \`{a} l'acrylonitrile \`{a} l'aide de
Mg(OH)\tsub{2} dans des conditions sans solvent}

\author{\firstname{Sonia} \lastname{Mancipe}\CDRorcid{0000-0001-9206-5386}\IsCorresp}
\address{Escuela de Ciencias Qu\'{i}mica-Grupo de Cat\'{a}lisis,
Universidad Pedag\'{o}gica y Tecnol\'{o}gica de Colombia, Avenida
Central del Norte 39-115, Tunja, Colombia}
\email[S. Mancipe]{sonia.mancipe@uptc.edu.co}

\author{\firstname{Valentina} \lastname{Coca}}
\addressSameAs{1}{Escuela de Ciencias Qu\'{i}mica-Grupo de
Cat\'{a}lisis, Universidad Pedag\'{o}gica y Tecnol\'{o}gica de
Colombia, Avenida Central del Norte 39-115, Tunja, Colombia}
\email[V. Coca]{diana.coca01@uptc.edu.co}

\author{\firstname{Juan-Carlos} \lastname{Castillo}\CDRorcid{0000-0002-5841-0518}}
\addressSameAs{1}{Escuela de Ciencias Qu\'{i}mica-Grupo de
Cat\'{a}lisis, Universidad Pedag\'{o}gica y Tecnol\'{o}gica de
Colombia, Avenida Central del Norte 39-115, Tunja, Colombia}
\email[J.-C. Castillo]{juan.castillo06@uptc.edu.co}

\author{\firstname{Hugo} \lastname{Rojas}\CDRorcid{0000-0003-3906-4522}}
\addressSameAs{1}{Escuela de Ciencias Qu\'{i}mica-Grupo de
Cat\'{a}lisis, Universidad Pedag\'{o}gica y Tecnol\'{o}gica de
Colombia, Avenida Central del Norte 39-115, Tunja, Colombia}
\email[H. Rojas]{hugo.rojas@uptc.edu.co}

\author{\firstname{Mar\'{i}a Helena}~\lastname{Brijaldo}\CDRorcid{0000-0002-9437-5086}}
\address{Escuela de Ciencias Administrativas y Econ\'{o}micas- Grupo de
Investigaci\'{o}n de Farmacia y Medio Ambiente (FARQUIMA), Universidad
Pedag\'{o}gica y Tecnol\'{o}gica de Colombia, Avenida Central del Norte
39-115, Tunja, Colombia}
\email[M. H. Brijaldo]{maria.brijaldo@uptc.edu.co}

\author{\firstname{Claudia} \lastname{Casta\~{n}eda}\CDRorcid{0000-0002-5360-2756}}
\addressSameAs{1}{Escuela de Ciencias Qu\'{i}mica-Grupo de
Cat\'{a}lisis, Universidad Pedag\'{o}gica y Tecnol\'{o}gica de
Colombia, Avenida Central del Norte 39-115, Tunja, Colombia}
\email[C. Casta\~{n}eda]{claudia.castaneda@uptc.edu.co}

\author{\firstname{Jos\'{e} Jobanny} \lastname{Mart\'{i}nez}\CDRorcid{0000-0002-4906-7121}}
\addressSameAs{1}{Escuela de Ciencias Qu\'{i}mica-Grupo de
Cat\'{a}lisis, Universidad Pedag\'{o}gica y Tecnol\'{o}gica de
Colombia, Avenida Central del Norte 39-115, Tunja, Colombia}
\email[J. J. Mart\'{i}nez]{jose.martinez@uptc.edu.co}

\author{\firstname{Gustavo Pablo} \lastname{Romanelli}\CDRorcid{0000-0002-3529-4753}}
\address{Centro de Investigaci\'{o}n y Desarrollo en Ciencias Aplicadas
``Dr. Jorge J. Ronco'' (CINDECA-CCT La Plata-CONICET-CIC-PBA),
Universidad Nacional de La Plata, Calle 47 No 257, B1900AJK La Plata,
Argentina}
\address{Centro de Investigaci\'{o}n en Sanidad Vegetal (CISaV) /
C\'{a}tedra de Qu\'{i}mica Org\'{a}nica, Facultad de Ciencias Agrarias
y Forestales, Universidad Nacional de La Plata, Calles 60 y 119 s/n,
B1904AAN La Plata, Argentina}
\email[G. P. Romanelli]{gpr@quimica.unlp.edu.ar}

\keywords{\kwd{HMF}\kwd{Active methylene}\kwd{Brucite}\kwd{Solvent-free}\kwd{Knoevenagel reaction}}

\altkeywords{\kwd{HMF}\kwd{M\'{e}thyl\`{e}ne actif}\kwd{Brucite}\kwd{Sans solvent}\kwd{R\'eaction de Knoevenagel}}

\shortrunauthors

\thanks{SM acknowledge the financial support from the Universidad
Pedag\'{o}gica y Tecnol\'{o}gica de Colombia (Internal Grant
SGI-3344).}

\begin{abstract}
Hydroxymethylfurfural (HMF) derivatives that contain an acrylonitrile
group result in interesting scaffold molecules that can be obtained
from biomass. However, the synthesis of these types of molecules has
not been extensively studied. In this study, we investigated the
catalytic activity of Mg(OH)\tsub{2} and MgO materials. Specifically,
we evaluated the Knoevenagel reaction between HMF and malononitrile as
a test reaction under solvent-free conditions. The fresh and used
catalysts were evaluated using various techniques. Our results indicate
that the combination of synthesis methods influences the crystalline,
basic, and textural properties of the catalysts. We found that water
plays an essential role in obtaining high yields. By using this simple
and inexpensive method, we were able to achieve yields near 90\% in
short reaction times ($<$30~min).
{\vspace*{-4pt}}
\end{abstract}

\begin{altabstract}
Les d\'{e}riv\'{e}s de l'hydroxym\'{e}thylfurfural (HMF) contenant un
groupement acrylonitrile conduisent \`{a} des architectures
remarquables obtenues \`{a} partir de la biomasse. La synth\`{e}se de
ce type de mol\'{e}cules ayant \'{e}t\'{e} peu examin\'{e}e, la
pr\'{e}sente \'{e}tude a explor\'{e} l'activit\'{e} catalytique des
mat\'{e}riaux Mg(OH)\tsub{2} et MgO. La r\'{e}action de Knoevenaegel
entre le HMF et le malononitrile a \'{e}t\'{e} \'{e}valu\'{e}e comme
r\'{e}action-test dans des conditions sans solvant. Les catalyseurs
frais et usag\'{e}s ont \'{e}t\'{e} correctement \'{e}valu\'{e}s \`{a}
l'aide de techniques vari\'{e}es. Les r\'{e}sultats montrent que la
combinaison de la m\'{e}thode de synth\`{e}se influe sur les
propri\'{e}t\'{e}s cristallines, basiques et texturales. Les
r\'{e}sultats confirment que le r\^{o}le de l'eau est essentiel pour
obtenir des rendements \'{e}lev\'{e}s. Cette m\'{e}thode simple et peu
co\^{u}teuse a produit des rendements proches de 90~\% avec des temps
de r\'{e}action courts ($<$30~min).
\end{altabstract}

\maketitle

{\pagebreak}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

For several years, the primary source of carbon for obtaining fuels and
chemical products was fossil fuels. However, at present, biomass has
become one of the main sources of carbon, in particular carbohydrates
such as glucose and fructose that present the largest natural basis of
carbon~\cite{1}. The dehydration of fructose in an acid medium produces
the preferential formation of 5-hydroxymethylfurfural (5-HMF). This
substance is one of the most promoted platform chemicals derived from
biomass, due to the variety of applications that are provided by the
presence of hydroxyl, aldehyde, and furan moieties~\cite{2}. 5-HMF is
widely used as an intermediate in pharmaceutical products, flavor
enhancers in the food industry and for the synthesis of
polymers~\cite{3}. Many efforts have focused on the organic synthesis
of HMF derivatives with active methylene compounds in order to obtain
high-value-added products through environmentally friendly catalysts
and renewable raw materials~\cite{4}. However, these classical
reactions have been scarcely studied using 5-HMF. Among them, the
Knoevenagel condensation is one of the important reactions for
obtaining C--C bonds~\cite{5}. This reaction is a typical
base-catalyzed reaction that involves the abstraction of a proton from
activated methylene by basic sites to obtain the corresponding
carbanion. Studying basic solid materials in these reactions has made
it possible to contribute to green and sustainable chemistry~\cite{6}.
Hydrotalcite (HT) or hydrotalcite-like compounds are commonly employed
as base catalysts~\cite{7}. However, Knoevenagel condensation reactions
over metal oxide catalysts have been less investigated~\cite{8},
although these materials can be easily obtained at lower costs than
hydrotalcite-type compounds.\looseness=-1 

MgO and Mg(OH)\tsub{2} are well known to be basic and inexpensive
solids that have been used as catalysts for a wide variety of organic
transformations including oxidation, reduction, epoxidation,
condensation, and notable heterocyclic reactions~\cite{9,10}. MgO
exhibits good catalytic activity in the Knoevenagel condensation
depending on the method and conditions of {its} preparation 
{which affects} their surface and {which can lead}
to increased CO\tsub{2} chemisorption and higher concentration of
strong basic sites~\cite{11,12}. To increase the catalytic activity of
MgO, which has a low surface area (near 20~m\tsup{2}/g) and low
\mbox{concentration} of basic sites (around 0.4~mmol/g), it is
supported on materials such as carbon~\cite{13}, Al-MCM-41~\cite{14},
ZrO\tsub{2}~\cite{15},  {or also by modifying the synthesis of MgO
using surfactants (sol--gel method), with the thermal treatment of
Mg(OH)$_2$, or using hydration--dehydration procedures which alter
surface characteristics of MgO}~\cite{12}.

In the sol--gel synthesis, it has been observed that the crystallinity
of the magnesium oxide depends on the surfactant concentration
used~\cite{11}, and consequently, the Knoevenagel condensation of
benzaldehyde with ethyl cyanoacetate using MgO prepared with the
highest surfactant concentration (Pluronic P123) led to the highest
catalytic activity due to the highest quantity of basic sites. 

The simple calcination of precursors as rehydrated Mg(OH)\tsub{2}
allows obtaining MgO with a high surface area and activity for
base-catalyzed reactions~\cite{16}. Temperatures between
350 and 450~\textdegree C are sufficient to obtain surface areas of about 
300~m\tsup{2}/g. The formation of a particle-internal nanostructure
comprised of newly crystallized strictly aligned, cube-shaped, and
nanometer-sized crystals of MgO is the main cause of this increase in
the surface area~\cite{17}. \looseness=-1

The basic sites can be controlled by the densified nanostructured MgO
material in the synthesis method~\cite{18} due to the creation of
atomic disorder formed at the interface of crystal grains (grain
boundaries) between MgO nanocrystals. Another strategy used was
the use of MgO-stabilized Pickering emulsion. Sadgar \etal~\cite{19}
studied the emulsion-stabilizing ability of MgO {{depending on}}
the method of preparation, calcination temperature, and solvents. These authors
found that the high emulsion stability and small droplet size have a
considerable effect on the catalytic activity in the Knoevenagel
condensation. For the adduct condensation of furfural and methylene
compounds such as malononitrile, it has been reported that it almost always
requires short times of reaction using MgO or Mg(OH)\tsub{2} (15~min,
yield $>$~83)~\cite{20} and lower temperatures of reaction
(20~\textdegree C in 5~min) to reach yields of 96\%~\cite{21}. However,
no report has been described for obtaining hydroxymethylfurural (HMF)
derivatives containing an acrylonitrile moiety which could result in
interesting molecules obtained from biomass.

Curiously, the Knoevenagel condensation between HMF and malononitrile
has only been \mbox{described} by three research groups. Rani \etal~\cite{22}
described the use of an amine-functionalized MOF (UiO-66-NH\tsub{2}) to
obtain quantitative yields of the adduct HMF with malononitrile.
Mancipe \etal~\cite{23}, used hydrotalcite doped with boric acid
allowing to obtain yields near 86\%, while imidazole-functionalized
3D-cobalt metal--organic framework led to quantitative yields under mild
reaction conditions (40~\textdegree C in 30~min). The absence of a
catalyst requires that the Knoevenagel reactions occur at higher
temperatures for prolonged times of reaction to afford the adduct in
only 30\% yield~\cite{24}.

The Mg\tsup{2+} cation and O\tsup{2-} anion can be considered as an
acid--base pair~\cite{12} leading to the preferential formation of
Knoevenagel products. However, some studies highlighted the importance
of the hydroxyls on the MgO surface to stabilize the generated anionic
intermediate R\tsup{-} after abstraction of the proton~\cite{26}. For
this reason, the presence of MgO and Mg(OH)\tsub{2} materials for the
solventless Knoevenagel condensation of HMF and malononitrile was\break
\mbox{studied.}

\section{Experimental section}\label{sec2}
\subsection{Preparation of Mg(OH)\tsub{2} and MgO}\label{ssec21}

For the synthesis of magnesium hydroxide by the precipitation method,
the magnesium precursor solution (0.0857~mol of
Mg(NO\tsub{3})\tsub{2}$\cdot$6H\tsub{2}O) was placed in a glass reactor
containing an alkaline solution (0.1715~mol of NaOH); the solutions
were simultaneously left under permanent agitation and aged for one day
at room temperature. The suspension was washed several times with
distilled water and dried at 80~\textdegree C. The obtained solid was
named BPC.

Mg(OH)\tsub{2} by sol--gel method was synthesized using 10~mL of CTAB
(cetyltrimethylammonium bromide) as a cationic surfactant. This was
added with constant stirring to a 1~M solution of
Mg(NO\tsub{3})\tsub{2}$\cdot$6H\tsub{2}O for one hour. After this time,
a 1~M ammonia solution was added dropwise until the clear solution
forms the Mg(OH)\tsub{2} sol. The sol was left to age for 2 days so
that the sol particles condense in the gel. The gel particles were
collected by filtering and washing the material several times with
distilled water, and the final product denominated BSG was dried at
80~\textdegree C. 

In both cases, for obtaining the magnesium oxides, the hydroxides
synthesized by precipitation and sol--gel methods were calcined at
500~\textdegree C and named MPC and MSG, respectively. 

\subsection{Characterizations}\label{ssec22}

{

\advance\baselineskip by -.15pt

X-ray powder diffraction patterns (XRD) of the catalysts were recorded
with a PANalytical diffractometer model X'PERT-PRO with Co K${\alpha}$
radiation (${\lambda} = 1{.}78901$~\AA{}) and a 1D pixel detector. The
data were collected with a step increment of 0.013\textdegree\ in the
range of 5\textdegree\ $\leq \Theta \leq$ 80\textdegree. 

FTIR spectra were obtained by the ATR method using a Thermo SCIENTIFIC
model NICOLET iS50 FT-IR equipment with a
wavenumber range of 4000--400~cm\tsup{-1} and a resolution of 
4~cm\tsup{-1}. 

XPS analysis was obtained using a Thermo Scientific ESCALAB 250 XI
photoelectron spectrometer with a step of
0.05~eV, pass energy of 25~eV, and dwell time of 50~ms. The pressure
was kept around 10\tsup{-8}~mbar inside the analysis chamber. The
spectra were calibrated employing the C~1s peak (284.4~eV).

For the textural analysis measurements, a Micromeritics ASAP 2020
equipment was used. The samples were previously degassed at
250~\textdegree C for 4~h with a vacuum of 10\tsup{-3}~mmHg to
eliminate gases or molecules adsorbed on the surface. Nitrogen
adsorption--desorption isotherms were measured at ${-}$193.15~\textdegree
C, obtaining data on the volume of adsorbed gas (cm\tsup{3} STP/g
sample) versus the relative pressure of nitrogen ($P/P_{0}$).

CO\tsub{2} adsorption was measured by the DRIFTS method using a
Thermo SCIENTIFIC model NICOLET iS50 FT-IR spectrometer. The
oxides and hydroxides were prepared with a 30~mL/min flow of He at
100~\textdegree C for 30~min. After the flow of He, the adsorption of
CO\tsub{2} was evaluated at room temperature. DRIFT spectra of the
CO\tsub{2}-adsorbed species on the surface were obtained by subtracting
the background spectra.\looseness=-1

NMR spectra were recorded on a Bruker Avance 400 spectrophotometer
(Bruker BioSpin GmbH, Rheinstetten, Germany), operating at 400.1~MHz
(\tsup{1}H) and 100.6~MHz (\tsup{13}C) at 298~K using tetramethylsilane
(0~ppm) as the internal reference. NMR spectroscopic data were recorded
in CDCl 3 using as internal standards the residual non-deuterated
signal ($\delta=7.26$ ppm) for \tsup{1}H NMR and the deuterated solvent
signal ($\delta = 77.16$ ppm) for \tsup{13}C\{\tsup{1}H\} NMR
spectroscopy. Chemical shifts ($\delta$) and coupling constants (J) are
given in ppm and Hz, respectively.

}

\subsection{Catalytic studies}\label{ssec23}

Mg(OH)\tsub{2} and MgO synthesized by precipitation and sol--gel
methods were studied in the solventless synthesis of the Knoevenagel
reaction. A mixture of 5-(hydroxymethyl)furfural (0.4~mmol),
malononitrile (0.4~mmol), and catalyst (20~mg) was placed in a glass
tube with constant stirring. Under solvent-free conditions, the tube
was heated to 60~\textdegree C for 3~h. Once the reaction was finished,
2.0~mL of ethyl acetate was added to solubilize the Knoevenagel
product and the catalyst was readily recovered by simple filtration.
The monitoring of the reaction was done in a gas chromatograph coupled
to mass spectroscopy (GC-MS), using a column \mbox{HP-5.} A sample was
purified by a flash chromatography column on silica gel using
dichloromethane as an eluent to afford
2-((5-(hydroxymethyl)furan-2-yl)methylene)malononitrile 3a. Brown
solid. Rf $=$\ 0.16 (DCM). M.p. 76--77~\textdegree C (amorphous). 1H
NMR (400 MHz, CDCl3): $\delta = 2.85$ (br s, 1H, OH), 4.73 (s, 2H),
6.62 (d, $J=3.6$ Hz, 1H), 7.27 (d, $J=3.6$ Hz, 1H), 7.46 (s, 1H)
ppm. \tsup{13}C\{\tsup{1}H\} NMR (101 MHz, CDCl\tsub{3}): $\delta =
57.6$  (CH\tsub{2}), 112.0 (CH), 113.0 (C), 114.0 (C), 125.2 (CH),
142.9 (CH), 147.7 (C), 162.6 (C) ppm.

\section{Results and discussion}\label{sec3}
\subsection{Characterization} \label{ssec31}

The structural phase of MgO and Mg(OH)\tsub{2} was determined by XRD.
Figure~\ref{fig1} shows the diffraction patterns {for}
MgO at 36.6\textdegree, 42.90\textdegree, 62.30\textdegree, 74.9\textdegree,
and 78.5\textdegree\ in $2\Theta$ for (111), (200), (220), (311), and
(222) planes, respectively~\cite{27}. The crystalline phase of MgO
corresponds to cubic structure (lattice parameters $a,b,c =
4.2130$~\AA{}) with space group Fm3m (JCPDS card 45-0946)~\cite{28}.
The sharp peaks indicate the presence of well-crystallized MgO
particles under precipitation and sol--gel synthesis conditions. 
{X-ray diffraction patterns for magnesium hydroxide are dependent on
crystalline Mg(OH)$_2$ nanoparticles}~\cite{29}, and the planes can
be indexed as a hexagonal structure with space group P-3m1 and lattice
parameters $a$ of 3.1442~\AA{} and $c$ of 4.7770~\AA{} (JCPDS card
07-0239)~\cite{30,31}. In the MPC material, a signal is shown at
30\textdegree\ in $2\Theta$, which can be attributed to a change from
brucite to a pseudomorphic periclase that originates once the
dehydroxylation ends~\cite{32}. The average crystallite size of the
catalysts evaluated from diffraction planes was measured using the
Scherrer equation. The values of BPC, BSG, MPC, and MSG are 12.56,
15.27, 9.71, and 19.54 nm, respectively, showing slightly larger
crystallite size in solids synthesized by sol--gel than those prepared
by the precipitation method.

\begin{figure}
\includegraphics{fig01}
{\vspace*{-4pt}}
\caption{\label{fig1}X-ray diffraction of (a) BSG, (b) BPC, (c) MSG,
(d) MPC.}
{\vspace*{-4pt}}
\end{figure}

The FTIR spectra of Mg(OH)\tsub{2} and MgO materials synthesized by
sol--gel and precipitation methods are shown in Figure~\ref{fig2}. The
hydroxides present an intense peak at 3700~cm\tsup{-1} due to the OH
stretching vibrations in Mg(OH)\tsub{2}~\cite{33}. It is known that MgO
can chemisorb H\tsub{2}O and CO\tsub{2} on the surface; the signal
around 1400~cm\tsup{-1} may correspond to carbonate ions
(CO\tsub{3}\tsup{2-}) adsorbed on the surface, even after the
calcination process~\cite{34}. In the Mg(OH)\tsub{2} sol--gel catalyst,
the signals at 2918 and 2858~cm\tsup{-1} correspond to asymmetric
$\upnu_{\mathrm{as}}$(CH\tsub{3})~\cite{35}. The band at 
3700~cm\tsup{-1} disappears in the spectrum of the calcined sample, which is
due to the decomposition of Mg(OH)\tsub{2} to MgO particles. The bands
below 1000 cm\tsup{-1} correspond to M--O and M--OH (M $=$\ metal)
characteristics of Mg--O and Mg--OH, respectively. 

\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig02}
{\vspace*{2pt}}
\caption{\label{fig2}FTIR spectra of synthesized materials.}
{\vspace*{3pt}}
\end{figure*}

{Table~\ref{tab1} summarizes the binding energies and ratios of signals
of Mg 1s and O 1s. The XPS spectra are shown in Figure~\ref{fig3}. The
signal characteristics of Mg(OH)\tsub{2}, MgO, and
Mg(NO\tsub{3})\tsub{2} are observed at 1303.08~eV and 1304.48~eV, and
1305~eV, respectively~\cite{36}. The proportion of these signals
depends on the method of preparation and consequently on the
calcination temperature. Thus, Mg(OH)\tsub{2} was only observed in
sol--gel synthesis. This is consistent with X-ray diffraction results.
The precipitation method allows obtaining 
{{a mixture of Mg(OH)$_{2}$ and the precursor used Mg(NO$_{3}$)$_{2}$.}}
Once the calcination
takes place at 500~\textdegree C, the proportion of MgO and Mg\tsup{2+}
is dependent on the method employed. A shift {{to}}
the highest binding energy could indicate the highest proportion of
Mg\tsup{2+} species on the surface. The binding energies of the signals
assigned to oxygen in lattices (O$_{\mathrm{b}}$) and oxygen in OH
groups (O$_{\mathrm{s}}$) are shown in 
{Table~\ref{tab1}.} The quantitative ratio
O$_{\mathrm{b}}$/O$_{\mathrm{s}}$ shows that the precipitation method
favors the formation of bulk oxygen in Mg(OH)\tsub{2}, which could be
related to the lowest hydration of brucite obtained for this method.
This ratio drops when MgO is formed by calcination from Mg(OH)\tsub{2},
but this same behavior is observed in the ratio between the two main
signals. In consequence, it is possible to suppose that the sol--gel
method favors the hydration of brucite and MgO on the surface.}

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig03}
{\vspace*{2pt}}
\caption{\label{fig3}XPS spectra of Mg 1s and O 1s for Mg(OH)\tsub{2} 
and MgO materials.}
{\vspace*{6pt}}
\end{figure*}

%tab1
\begin{table*}
\caption{\label{tab1}Binding energies of Mg 1s and O 1s, quantitative
ratio O$_{\mathrm{b}}$/O$_{\mathrm{s}}$ and O 1s/Mg 1s, and surface area
($S_{\mathrm{BET}}$) and pore size}
\begin{tabular}{ccccccc}
\thead
Catalyst & Mg 1s & O 1s & O$_{\mathrm{b}}$/O$_{\mathrm{s}}^{*}$ & O
1s/Mg 1s & $S_{\mathrm{BET}}$ (m\tsup{2}/g) & Pore size (nm) \\
\endthead
BSG & \parbox[t]{4.5pc}{\raggedright 1301.7(66), 1300.8(34)}\vspace*{4pt} &
\parbox[t]{4pc}{\raggedright 532.2(64), 530.56(36)}    & 1.77 & 0.08 &
25 & 22.4 \\ 

BPC & \parbox[t]{4.5pc}{\raggedright 1303.9(44), 1301.6(56)}\vspace*{4pt} &
\parbox[t]{4pc}{\raggedright 530.05(79), 528.75(21)}   & 3.76 & 0.42 & 
35 & 33.7 \\ 

MSG & \parbox[t]{4.5pc}{\raggedright 1302.9(63), 1304.8(37)}\vspace*{4pt} &
\parbox[t]{4pc}{\raggedright 532.2(12), 530.7(88)}     & 0.13 & 0.16 &
26 & 14.8 \\ 

MPC & \parbox[t]{4.5pc}{\raggedright 1304.47(29), 1302.89(71)}\vspace*{2pt} &
\parbox[t]{4pc}{\raggedright 531.85(25), 529.97(75)} & 0.33 & 0.24 & 35
& 22.3
\botline
\end{tabular}
\tabnote{$^{*}$O$_{\mathrm{b}}$/O$_{\mathrm{s}}$ quantitative ratio
between bulk and surface oxygen.}
\end{table*}

In the textural analysis, the specific surface area
($S_{\mathrm{BET}}$) of the solids Mg(OH)\tsub{2} and MgO is shown in
Table~\ref{tab1}. No evident changes are observed before and after the
calcination process in the $S_{\mathrm{BET}}$ values. The nitrogen
adsorption--desorption isotherms are characteristics of type IV with H4
hysteresis loop~\cite{33}, showing sharp peaks at high relative
pressures, indicating that the materials contain large mesopores and
macropores, which can be verified in the pore size distribution. In
general, the precipitation method to obtain Mg(OH)\tsub{2} led to the
highest density of pores (dV/dD) with a minor pore diameter compared
with sol--gel synthesis (Figure~\ref{fig4}).  Besides, the calcination
of Mg(OH)\tsub{2} sol--gel showed a broad pore diameter. This means
that the porosity is highly dependent on the synthesis method and
calcination process. 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}BJH pore size distribution curves of
Mg(OH)\tsub{2} and MgO materials.}
\end{figure}

CO\tsub{2} adsorption followed by DRIFT spectra of MgO and
Mg(OH)\tsub{2} synthesized by the two methods is shown in
Figure~\ref{fig5}. In the spectra taken at room temperature, signals
can be observed between 3800 and 3300~cm\tsup{-1} corresponding to the
hydroxyl groups on the surface that may be in variable coordination
with the cation, while the signal at 800~cm\tsup{-1} may be due to the
bulk infrared absorption of MgO~\cite{38}. With the adsorption of
CO\tsub{2}, the formation of monodentate species can be seen at 1572,
1501, and 1411~cm\tsup{-1}. The signals at 1647 and 1331~cm\tsup{-1}
can be assigned to bidentate species~\cite{39}. In the adsorption
process, CO\tsub{2} chemisorbs on the active sites as unidentate,
bidentate, and bridged carbonates; \mbox{however,}
the \mbox{formation} of bidentate
species is favored on the most active sites, reducing the number of
free carbonate ions. The higher formation of bidentate species in
Mg(OH)\tsub{2} can be explained by the fact that some of the active
sites have conformational characteristics. Furthermore, this
description is consistent with the fact that the bidentate carbonate is
less basic than the unidentate and bridged carbonates~\cite{40}. Taking
into account the aforementioned and according to Figure~\ref{fig5}, it
could be affirmed that MgO solids present a higher basicity in
comparison with the hydroxides that show signs of adsorption of
bidentate species. On the other hand, the signals at 2924 and 2854
cm\tsup{-1} in the magnesium hydroxide can be attributed to the
H\tsup{+} ions resulting from the dissociation of water molecules that
can be trapped {{by}} the surface O\tsup{2-} ions~\cite{41}. \looseness=-1

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}DRIFT spectra of MgO and Mg(OH)\tsub{2} materials
at room temperature.}
\end{figure}

\subsection{Evaluation of the catalytic activity}\label{ssec32}

The Knoevenagel reaction between 5-HMF ({1a}) and malononitrile ({2a})
was studied as a test reaction under solvent-free conditions at
60~\textdegree C. The Knoevenagel adduct ({3a}, $m/z=174$) formed by a
condensation process and the aldehyde ({3b}, $m/z=172$) obtained by the
dehydrogenation of the hydroxyl group into adduct 3a were the main
products observed (Scheme~\ref{sch1}). Figures~\ref{fig6}a
and~\ref{fig6}b display the yields of 3a and 3b, respectively, as the
reaction \mbox{progresses} with each catalyst used. 
{{It is observed that they show}}
the preferential formation of adduct 3a with good
yields. Although this reaction can typically be conducted in
H\tsub{2}O, in the absence of catalyst, and moderate temperatures,
the Knoevenagel adduct is obtained in lower yields. Thus, the use of
catalysts under solvent-free conditions {appears interesting}
to understand the role of water on the surface\break of MgO.

\begin{scheme*}
{\vspace*{-3pt}}
\includegraphics{sc01}
\caption{\label{sch1}Products formed in the condensation of Knoevenagel
between 5-HMF and malononitrile.}
{\vspace*{-2pt}}
\end{scheme*}

\begin{figure*}
\includegraphics{fig06}
{\vspace*{2pt}}
\caption{\label{fig6}(a) Yield of 3a and (b) yield of 3b as a
function of reaction time with MgO and Mg(OH)\tsub{2} materials at room
temperature at 60~\textdegree C, 100 mg of catalyst, and 600~rpm.}
{\vspace*{3pt}}
\end{figure*}

It can be observed for the uncalcined and calcined {catalysts}
that the catalytic behavior is highly dependent on the preparation method. 
The preferential formation of 3a is observed with Mg(OH)\tsub{2} obtained
by the sol--gel synthesis. The Knoevenagel product 3a requires 
deprotonation of the $\alpha$-methylene hydrogen atoms by basic sites.
The hydroxyl groups on the surface of the solid stabilize the anionic
intermediate formed on the surface~\cite{26}. BSG and BPC display a
similar basicity but a distinct proportion of surface hydroxyls. Thus the 
{catalysts}
with the lowest ratio of O$_{\mathrm{b}}$/O$_{\mathrm{s}}$ present the
highest yield for adduct {3a}. However, the formation of the adduct 3b
in moderate yields is unexpected. The lowest amount of surface hydroxyls
and the presence of two phases of Mg\tsup{2+} in BPC could
favor the formation of adduct {3b}, which can be formed by two
pathways, including the faster formation of diformylfuran (DFF) that
condenses with malononitrile or the dehydrogenation of adduct~{3a}.
\looseness=-1

MgO {catalysts}
show a distinct behavior. As the reaction progresses
and by release of water in the reaction medium, the catalyst
surface is modified and the Knoevenagel reaction is consequently
favored for MPC solid. This phenomenon was not \mbox{observed} with MSG, where
the yield of adduct 3a drops, favoring the formation by dehydrogenation
toward product 3b. These {catalysts} present the highest basicity and
consequently could also favor the formation of this product. Thus, the
role of surface hydroxyls in the reaction mechanism to obtain 3a is
evident. The {MSG catalyst}
with a higher proportion of surface hydroxyls
shows a better performance toward the Knoevenagel reaction 
at the start of the reaction.

The effect of temperature was studied using MPC. An increase in
temperature should avoid the presence of water
on the surface favoring the formation of 3a, as can be observed in
Figure~\ref{fig7}. However, at the lowest temperatures, the reaction
also allows the formation of {3b}. In summary, the selectivity
of the reaction could be controlled by surface hydroxyls due to the
stabilization of anionic intermediates.

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Effect of temperature in the Knoevenagel
reaction with MPC (a) 3a adduct and (b) 3b product.}
\end{figure*}

To confirm that those results were obtained without external mass transfer,
the reaction was performed at different catalyst amounts (5--20~mg) at
60~\textdegree C for 24~h. As shown in Table~\ref{tab2} (entries 1--4),
the behavior \mbox{follows} a linear trend 
when the catalyst mass is increased,
suggesting the absence of external mass-transfer resistance.
Besides, the recyclability of the MPC catalysts (entries 5--8) was
studied by separation from the mixture by simple filtration, washing
with hot acetone, and drying at 80~\textdegree C for 4~h. The catalyst
was reused up to 4 times without appreciable loss of catalytic
activity. In the fourth reuse, the mass loss of the catalyst was
approximately 5\%.

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}\tsup{1}H and \tsup{13}C\{\tsup{1}H\} NMR spectra
of the Knoevenagel adduct 3a.}
\end{figure*}

%tab2
\begin{table*}
\caption{\label{tab2}The recycling capability of MPC catalyst in the
Knoevenagel synthesis of 3a at different catalyst amounts at
60~\textdegree C for 24~h}
\begin{tabular}{ccccccc}
\thead
Entry & Catalysts & Loading (mg) & $T$ (\textdegree C) & Time (h) &
Reuse cycle & Yield (3a) (\%) \\
\endthead
1 & MPC & \05& 60 & 24 & -- & 50 \\ 
2 & MPC & 10 & 60 & 24 & -- & 60 \\ 
3 & MPC & 15 & 60 & 24 & -- & 68 \\ 
4 & MPC & 20 & 60 & 24 & -- & 77 \\ 
5 & MPC & 20 & 60 & 24 & 1   & 77 \\ 
6 & MPC & 20 & 60 & 24 & 2   & 75 \\ 
7 & MPC & 20 & 60 & 24 & 3   & 70 \\ 
8 & MPC & 20 & 60 & 24 & 4   & 68
\botline
\end{tabular}
\end{table*}

The structure of Knoevenagel adduct 3a was confirmed by \tsup{1}H
and \tsup{13}C\{\tsup{1}H\} NMR spectroscopy (Figure~\ref{fig8}).
The \tsup{1}H NMR spectrum shows two singlets at 4.73 and 7.46
ppm assigned to methylenoxy (CH\tsub{2}O) and vinyl (${=}$CH) protons,
respectively. To our delight, the proton signal of the hydroxyl group
was observed at 2.85 ppm, indicating that the dehydrogenation of the
CH\tsub{2}OH moiety was not successful. The \tsup{13}C\{\tsup{1}H\} NMR
spectrum of 3a displays eight carbon signals, consisting of one
methylene, three methines, and four quaternary carbons. In particular,
the methylenoxy (CH\tsub{2}O) and vinylic (${=}$CH) carbons can be
observed at 57.6 and 142.9~ppm,\break respectively.

\section{Conclusions}\label{sec4}

{\vspace*{2pt}}

In the present study, the efficient synthesis of MgO and Mg(OH)\tsub{2}
materials was carried out by the sol--gel and precipitation methods.
The XRD characterization of the solids revealed formation of 
brucite and periclase phases. IR studies showed signals typical of
hydroxyl groups and M--OH and M--O vibrations. The
O$_{\mathrm{b}}$/O$_{\mathrm{s}}$ 
ratios obtained from XPS analysis indicate that synthesis
by precipitation results in a lower hydration of the brucite phase
compared to the sol--gel method. The greater formation
of bidentate species on the surface of Mg(OH)\tsub{2} indicates lower
basicity compared to MgO. Finally, orienting the catalytic activity in
the Knoevenagel condensation reaction between HMF and malononitrile
under environmentally friendly solvent-free conditions toward the
formation of product 3a requires the deprotonation of the
$\upalpha$-methylene hydrogen atoms and {catalysts}
with a lower O$_{\mathrm{b}}$/O$_{\mathrm{s}}$ 
ratio present higher yield of adduct 3a. On the other hand, the
lower presence of hydroxyls on the surface of BPC favors\break product 3b.

{\vspace*{2pt}}

\section*{Conflicts of interest}

{\vspace*{2pt}}

Authors have no conflict of interest to declare.

\CDRGrant[UPTC]{SGI-3344}

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{\vspace*{2pt}}
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