Effect of reaction conditions on the activity of novel carbon nanofiber-based Cu/ZrO<sub>2</sub> catalysts for CO<sub>2</sub> hydrogenation to methanol

Israf Ud Din; Maizatul S. Shaharun; A. Naeem; Mshari A. Alotaibi; Abdulrahman I. Alharthi; Qazi Nasir

doi:10.5802/crchim.6

Communication

Effect of reaction conditions on the activity of novel carbon nanofiber-based Cu/ZrO₂ catalysts for CO₂ hydrogenation to methanol

Israf Ud Din ^1,² ; Maizatul S. Shaharun ³ ; A. Naeem ² ; Mshari A. Alotaibi ¹ ; Abdulrahman I. Alharthi ¹ ; Qazi Nasir ⁴

¹ Department of Chemistry, College of Science and Humanities, Prince Sattam Bin Abdulaziz University, P.O. Box 173, Al-Kharj 11942, Saudi Arabia
² National Centre of Excellence in Physical Chemistry, University of Peshawar, Pakistan
³ Department of Fundamental and Applied Sciences, Universiti Teknologi Petronas, Malaysia
⁴ Department of Chemical Engineering, University of Nizwa, Oman

Comptes Rendus. Chimie, Volume 23 (2020) no. 1, pp. 57-61.

Résumé

The increasing concentrations of CO₂ in the environment result in catastrophic phenomena like global warming. Hydrogenation of emitted CO₂ to methanol is one of the economically viable strategies to tackle the problem. In the current work, methanol synthesis by CO₂ hydrogenation was studied over carbon nanofiber-based Cu/ZrO₂ catalysts. The effect on rate of reaction by varying reaction temperature, reaction pressure, and feed gas ratio were investigated. Rate of reaction increased with increasing temperature and 220 °C was found as an optimum reaction temperature. The study revealed a linear relationship between the rate of reaction and applied pressure of the feed gases. The rate of reaction was accelerated by increasing feed gas ratio and the highest activity was recorded for H₂/CO₂ $= 3$ .

Métadonnées

Reçu le : 2019-09-30
Révisé le : 2019-10-30
Accepté le : 2019-10-30
Publié le : 2020-05-06

DOI : 10.5802/crchim.6

Mots clés : CO₂ conversion, Effect of reaction conditions, Temperature effect, Feed gas ratio, Pressure effect

Affiliations des auteurs :

Israf Ud Din ^{1, 2} ; Maizatul S. Shaharun ³ ; A. Naeem ² ; Mshari A. Alotaibi ¹ ; Abdulrahman I. Alharthi ¹ ; Qazi Nasir ⁴

Licence :

CC-BY 4.0

Droits d'auteur : Les auteurs conservent leurs droits

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     author = {Israf Ud Din and Maizatul S. Shaharun and A. Naeem and Mshari A. Alotaibi and Abdulrahman I. Alharthi and Qazi Nasir},
     title = {Effect of reaction conditions on the activity of novel carbon nanofiber-based {Cu/ZrO\protect\textsubscript{2}} catalysts for {CO\protect\textsubscript{2}} hydrogenation to methanol},
     journal = {Comptes Rendus. Chimie},
     pages = {57--61},
     publisher = {Acad\'emie des sciences, Paris},
     volume = {23},
     number = {1},
     year = {2020},
     doi = {10.5802/crchim.6},
     language = {en},
}

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%A A. Naeem
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%A Abdulrahman I. Alharthi
%A Qazi Nasir
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Israf Ud Din; Maizatul S. Shaharun; A. Naeem; Mshari A. Alotaibi; Abdulrahman I. Alharthi; Qazi Nasir. Effect of reaction conditions on the activity of novel carbon nanofiber-based Cu/ZrO₂ catalysts for CO₂ hydrogenation to methanol. Comptes Rendus. Chimie, Volume 23 (2020) no. 1, pp. 57-61. doi : 10.5802/crchim.6. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.5802/crchim.6/

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1. Introduction

Global warming, due to emissions of greenhouse gases such as CO₂, is one of the catastrophic phenomena faced by the world today. Many strategies have been devised for the reduction of CO₂ emissions; however, CO₂ conversion to methanol provides a vital route as it diminishes CO₂ concentration on the one hand and produces valuable fuel like methanol on the other [1, 2, 3, 4].

Methanol synthesis is generally carried out by using a mixture of CO₂ and syngas at the industrial level [5]. However, pure CO₂ hydrogenation to methanol is more preferable than traditional syngas gas route from the environmental point of view. Methanol synthesis by syngas route has been extensively investigated; however, literature on the former route is very limited [6]. The performance of catalysts for pure CO₂ conversion to methanol has been affected by various reaction parameters like reaction pressure, temperature, and composition of feed gases. In general, rate of reaction is directly proportional to the reaction temperature. Nevertheless, reaction temperature for CO₂ reduction to methanol has not been univocal in the literature. Rate of methanol synthesis by CO₂ hydrogenation was increased by increasing reaction temperature and 220 ^°C was recorded as an optimum reaction temperature [7]. Likewise, according to An et al. [8], 250 ^°C was reported as the maximum reaction temperature for best methanol selectivity and CO₂ conversion. Similarly, both methanol production and CO₂ conversion were enhanced by increasing temperature up to 250 ^°C [9].

Thermodynamically, according to Le Chatelier’s principle, high reaction pressure is favorable for CO₂ hydrogenation to methanol. Liu et al. [10] claimed that CO₂ hydrogenation to methanol could be synthesized at a low reaction pressure of 20 bar. Another study carried out by Liaw et al. [11] summarized that at 210 ^°C methanol yield was higher when increasing the total reaction pressure.

CO₂ reduction to methanol is also affected by variation of feed gas ratio. From the thermodynamic point of view, H₂∕CO₂ = 3 is considered favorable for the process. Nevertheless, this factor has also been quite debatable in the literature. Methanol synthesis by CO₂ hydrogenation at various feed gas ratios and constant reaction temperature 260 ^°C and pressure 360 bar over Cu∕ZnO∕Al₂O₃ catalyst was investigated by Bansode and Urakwa [12]. They observed remarkable increase in the rate of methanol synthesis as well as CO₂ conversion by increasing feed gas H₂∕CO₂ ratio from 3 to 10. Such results were also documented by Kim et al. [13] for CO₂ hydrogenation to methanol carried out in a slurry phase reactor. In summary, CO₂ reduction to methanol is affected by the magnitude of reaction conditions and hence performance of the catalysts is determined by the reaction medium.

The current work describes CO₂ reduction to methanol over Cu/ZrO₂ catalysts supported by CNFs. Furthermore, the effect of reaction parameters such as pressure, temperature, and feed gas composition on the performance of the catalysts were also studied.

2. Experimental section

2.1. Catalysts synthesis

Deposition precipitation method was adopted to synthesize CNF-based Cu/ZrO₂ catalysts. The detailed procedure for catalysts synthesis has been documented elsewhere [14, 15, 16]. Catalysts Cu⋅ZrO₂/CNFs with 15 wt% each of Cu and ZrO₂ with CNFs as a support were synthesized. The prepared catalysts were cooled, filtered and dried overnight at 100 ^°C. The dried catalysts were calcined under N₂ flow at 450 ^°C for 3 h. A 0.5 g of pre-reduced catalysts (reduced in H₂ for 6 h with 2000 cm³ h⁻¹ flow rate at 380 ^°C) was used for reaction studies.

2.2. Variation of reaction parameters

Parr autoclave slurry reactor model, Parr 4593, was employed for the activity studies of the catalysts. The detailed procedure is reported elsewhere [17, 18]. Methanol synthesis via CO₂ hydrogenation was first optimized in terms of reaction temperature. Hence, a range of reaction temperatures: 180, 200, 220, and 240 ^°C, was selected at constant reaction pressure of 30 bar. After an optimized reaction temperature was identified, methanol synthesis was tested at different reaction pressures of 20, 30, 40, and 50 bar at a constant reaction temperature of 220 ^°C. Similarly, the effect of feed gas ratio was also studied by carrying out methanol synthesis at different H₂∕CO₂ ratios of 0.8, 1.5, and 3 at 220 ^°C and 30 bar reaction temperature and pressure respectively. Methanol content was evaluated by using flame ionization detector (FID). Gas chromatography GC Agilent 6890 was employed for the current study.

Methanol synthesis rate was calculated by the following equation:

\begin{matrix} Rate of methanol yield \\ = g of methanol produced ∕ kg of catalyst \times h \end{matrix}

3. Results and discussions

3.1. Effect of reaction temperature

The rate of methanol synthesis was studied in a slurry reactor at fixed pressure of 30 bar in a range of 180–240 ^°C. The correlation of reaction temperature and methanol synthesis rate is displayed in Figure 1.

\begin{matrix} {CO}_{2} + 3 H_{2} ⇌ {CH}_{3} OH + H_{2} O \\ Δ {H^{0}}_{298 K} = - 49.5 kJ ∕ mol \end{matrix}

(reaction 1)

As shown by (reaction1), reduction of CO₂ to methanol is exothermic in nature so that low temperature is thermodynamically favorable. However, keeping in mind the higher stability of CO₂, some adequate energy is required for breaking such a stable molecule. Moreover, methanol synthesis rate increases kinetically with the increasing reaction temperature. Therefore, the methanol productivity is controlled by kinetics as well as thermodynamics. A maximum methanol synthesis rate is obtained at the critical point where the reaction transitions from kinetics to thermodynamics. This critical point of reaction depends on a number of reactivity factors like the nature of the catalyst, the number of active sites and the exposure of such catalyst sites to the approaching reactant molecules. Therefore, literature regarding the critical point is not univocal and different degrees of temperature are reported by different catalysts and operating systems. Similarly, reverse water gas shift reaction (RWGS) is also associated with the main CO₂ hydrogenation to methanol (reaction2).

\begin{matrix} {CO}_{2} + H_{2} ⇌ CO + H_{2} O \\ Δ {H^{0}}_{298 K} = + 41.2 kJ ∕ mol \end{matrix}

(reaction 2)

As RWGS is endothermic in nature, an increase in reaction temperature facilitates this reaction. Likewise, an increase in reaction temperature can also bring about the decomposition of methanol to CO, according to (reaction3).

\begin{matrix} {CH}_{3} OH ⇌ CO + 2 H_{2} \\ Δ {H^{0}}_{298 K} = + 90.64 kJ ∕ mol \end{matrix}

(reaction 3)

In the current case, methanol synthesis rate was increased by increasing reaction temperature. The optimum rate was observed at 220 ^°C before it declined at the highest reaction temperature of 240 ^°C. A similar trend of methanol synthesis rate as a function of reaction temperature was documented by Xin et al. [8] over Cu/Zn/Al/Zr fibrous catalyst. Similarly, methanol yield as a function of reaction temperature also revealed a similar dependency of methanol synthesis by CO₂ reduction route [19]. The decline of methanol synthesis rate at the highest reaction temperature could be due to the selectivity of RWGS reactions. This can also be further justified by the fact that RWGS reaction has higher apparent activation energy as compared to the methanol formation [20, 21]. Therefore, the CO formation rate was faster as compared to methanol formation at higher reaction temperature. Likewise, the decomposition of methanol to CO at higher reaction temperature could also be one of the reasons of decreasing methanol synthesis rate.

Figure 1.
Correlation of reaction temperature and methanol synthesis rate.

Figure 2.
Correlation of reaction pressure and methanol synthesis rate.

3.2. Effect of pressure

Methanol synthesis by CO₂ hydrogenation was also studied at different pressures like 20, 30, and 40 bar. Figure 2 shows the correlation of reaction pressure and methanol synthesis rate. As observed, CO₂ reduction to methanol is a molecule decreasing reaction so that it is thermodynamically facilitated at higher pressure. In the current study, a linear relationship was observed between pressure and methanol synthesis rate. Rate of methanol synthesis was increased from 45 to 57 g∕kg⋅h when pressure was increased from 20 to 30 bar. A similar trend of increasing rate was continued with further increase in pressure and maximum rate was obtained with the highest reaction pressure.

The obtained observations were in good agreement with the work of An et al. [8] and Arena et al. [22]. Although the process is favorable at high reaction pressure, too high pressure demands higher requirement of materials strength, high cost of operation and it is associated with safety problems.

3.3. Effect of feed gas composition

Rate of methanol synthesis by CO₂ hydrogenation was also studied by varying feed gas composition. Figure 3 displays the rate of methanol synthesis as a function of increasing H₂∕CO₂ ratio. The methanol synthesis rate was increased from 32 to 41 g∕kg⋅h when the H₂∕CO₂ ratio was increased from 0.8 to 1.5. A similar trend was observed with further increase in H₂∕CO₂ ratio. This trend is quite understandable keeping in mind the stoichiometric chemistry of CO₂ hydrogenation reaction.

Figure 3.
Correlation of H₂∕CO₂ ratio and methanol synthesis rate.

Even though H₂∕CO₂ ratio of unity has been favorable for formation of CO as the main product, the rate of methanol synthesis was enhanced by increasing H₂∕CO₂ ratio. Equation (reaction1) rightly justifies this trend, where higher H₂∕CO₂ ratio was desirable for better methanol yield. This could be one of the reasons of lower activity of catalyst to methanol synthesis with H₂∕CO₂ ratio <3.0. In addition, Shen et al. [23], reported similar observations for methanol synthesis via CO₂ hydrogenation over Cu∕Zr∕Al₂O₃ catalyst.

4. Conclusion

The current work documented the effect of reaction variables on the CO₂ reduction to methanol. Rate of methanol synthesis was significantly altered with variation in reaction temperature. It was initially raised with increasing temperature and the highest rate was observed at 220 ^°C before it declined with further increase in temperature. Likewise, the influence of reaction pressure was investigated by conducting the methanol synthesis at different magnitudes of total pressure. It was concluded from the pressure variation study that the rate of methanol synthesis progressively enhanced as a function of increasing total pressure. Finally, rate of methanol synthesis was also optimized in terms of feed gas ratio. The study concluded that a linear relationship existed between the rate of methanol synthesis and the increasing H₂∕CO₂ ratio.

Acknowledgment

The financial support provided by Ministry of Higher Education Malaysia via FRGS No: FRGS/1/2011/SG/UTP/02/13 is appreciated. Support provided by Chemistry Department, Prince Sattam bin Abdulaziz University, Saudi Arabia is also highly appreciated.

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