Methanol Production Via CO2 Hydrogenation Reaction: Effect of Catalyst Support

410 Int. J. Nanotechnol., Vol. 14, Nos. 1/2/3/4/5/6, 2017

Methanol production via CO2 hydrogenation reaction: effect of catalyst support

Sara Tasfy Department of Chemical and Petroleum Engineering, American University of Ras Al Khaimah, Ras Al Khaimah, 10021, UAE Fax: +97172210300 Email: [email protected]

Noor Asmawati Mohd Zabidi* and Maizatul Shima Shaharun Department of Fundamental and Applied Sciences, Universiti Teknologi Petronas, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia Fax: +605-3655905 Email: [email protected] Email: [email protected] *Corresponding author

Duvvuri Subbarao Department of Chemical Engineering, Universiti Teknologi Petronas, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia Fax: +605-3655905 Email: [email protected]

Abstract: In this project, Cu-based catalyst was synthesised via impregnation method on various supports such as Al2O3, Al2O3-ZrO2, SBA-15 and Al-modified SBA-15. Samples were characterised by N2 adsorption-desorption, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), temperature-programmed desorption/reduction (TPD/R) and pulse chemisorption. The activity of the supported Cu-based catalyst was evaluated in a fixed-bed micro-reactor at 483 K, 2.25 MPa, and H2/CO2 ratio of 3 : 1. Our results showed that the nature of the oxide support influenced the physicochemical properties of the catalysts as well as the catalytic activity and product selectivity. The size of catalyst nanoparticles ranged from 9 nm to 37 nm, depending on the type of support used. The Cu-based catalyst supported on SBA-15 resulted in 13.96% CO2 conversion and methanol selectivity of 91.32% and these values were higher than those obtained using Cu-based catalyst supported on other oxide carriers.

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Keywords: CO2; hydrogenation; methanol; Cu/ZnO catalyst; support; SBA-15; Al2O3; CuO nanoparticles.

Reference to this paper should be made as follows: Tasfy, S., Mohd Zabidi, N.A., Shaharun, M.S. and Subbarao, D. (2017) ‘Methanol production via CO2 hydrogenation reaction: effect of catalyst support’, Int. J. Nanotechnol., Vol. 14, Nos. 1/2/3/4/5/6, pp.410–421.

Biographical notes: Sara Tasfy recently completed her PhD at the Universiti Teknologi Petronas, Malaysia. She is currently an Assistant Professor at the Department of Chemical and Petroleum Engineering, American University of Ras Al Khaimah. Her PhD project deals with the performance characterisation of Cu/ZnO based-catalyst in CO2 hydrogenation into methanol. She had obtained MSc degree from the same university in 2011 where she had completed a project on development of supported iron catalyst for Fischer- Tropsch reaction.

Noor Asmawati Mohd Zabidi is an Associate Professor at the Department of Fundamental and Applied Sciences, Universiti Teknologi Petronas (UTP). She obtained her PhD in 1995 from the University of Missouri-Kansas City, USA where she had completed research work on the photocatalytic generation of hydrogen from alcohols. She has been at UTP since 2001. Her current research interests are Fischer-Tropsch catalysis and catalytic conversion of CO2 into value added products such as methanol and DME.

Maizatul Shima Shaharun is an Associate Professor at the Department of Fundamental and Applied Sciences, Universiti Teknologi Petronas (UTP). She obtained her MSc in Chemical Process Technology from the University of Durham in 2000 and a PhD in Chemical Engineering from UTP in 2009. Her research interest includes homogeneous and heterogeneous catalysis as well as molecular modelling.

Duvvuri Subbarao has held a professorship at the Department of Chemical Engineering, Universiti Teknologi Petronas (UTP) since 2007. Previously, he was a Faculty Member of IIT, Delhi. He specialises in mass transfer and reactors.

This paper is a revised and expanded version of a paper entitled ‘Methanol production via CO2 hydrogenation: effect of alumina-based catalyst support’ presented at Advanced Materials and Nanotechnology Conference, Nelson, New Zealand, 8–12 February, 2015.

1 Introduction

Global warming, climate change and fossil fuel depletion motivate the investigation of alternative and environmentally friendly fuels [1–4]. Emissions of CO2 from combustion of fossil fuel, industrial waste, and other human activities are considered as main causes for the global warming. Therefore, fixation of CO2 through conversion to value-added chemicals and fuels has become a very attractive area of research not only owing to environmental concerns, but also because CO2 is a renewable, non-toxic and cheap feedstock for many chemical processes [5–7]. Hydrogenation of CO2 to methanol is an important process that offers challenging opportunities for sustainable development in

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energy and environment since methanol can be used as a solvent, as a building block in the synthesis of various chemicals [8], fuel additive or clean fuel [9] and fuel cell [10]. Since the hydrogenation of CO2 is a catalysed reaction, various catalytic systems have been investigated. Cu/ZnO catalysts derived from co-precipitation method are considered to be the most effective methanol synthesis catalyst that has been studied for many years [11,12]. Copper-based catalysts are frequently used in a variety of industrial hydrogenation processes, including methanol synthesis and low-temperature water–gas shift reactions. Several important problems still remain opened such as the low activity and stability of catalysts, which are partly attributed to Cu sintering. To enhance the catalytic performance, various supports, promoters, synthesis parameters and reaction conditions have been examined [13]. This study is aimed at providing a basic assessment of the CO2-hydrogenation pattern of Cu/ZnO catalyst supported on SBA-15, Al-SBA-15, Al2O3, and Al2O3/ZrO2. The variation of products selectivity and catalytic activity with catalyst formulation are presented.

2 Experiment

2.1 Synthesis of catalyst support

Mesoporous Santa Barbara amorphous silica (SBA-15) was synthesised by the hydrothermal method [14]. Typically, a homogeneous mixture consisting of tri-block copolymer Pluronic P123 (EO20PO70EO20, MW = 5800, Aldrich) and tetraethyl orthosilicate (TEOS, 98%, Aldrich) in aqueous hydrochloric acid solution, was stirred at 40°C for 10 h and further treated at 80oC for 48 h. The solid was washed with ethanol, recovered by filtration, dried at room temperature overnight and finally calcined at 550°C for 4 h. To enhance the acidity of the support, SBA-15 was functionalised with Al2O3. The synthesis of Al-SBA-15 was carried out as follows: 4 g of Pluronic P123 was dissolved in a mixture of deionised water and 2 M HCl was added to adjust the pH of the solution to 1 or 2 and stirred at 40°C until complete dissolution of surfactant. Appropriate amount (8.5 g) of TEOS was then added drop wise to the pluronic solution and the mixture was stirred for 2 h. Then, the amount of Al(NO3)3·9H2O required to obtain Si/Al weight ratios of 9/1 was added, followed by stirring for 10 h. The mixture was then aged at 80°C for 48 h. The resultant gel was washed with ethanol and filtered. The sample was dried at room temperature for 24 h and calcined at 550°C for 4 h. Commercial alumina oxide support was purchased from Merck (BET specific surface 2 area 120–190 m /g) and it was treated at 500°C for 4 h to remove impurities. Al2O3/ZrO2 support was supplied by SASOL (PURALOX SBa-200/Zr20) with Al2O3/ZrO2 ratio of 80 : 20 wt%. Thermal treatment was conducted at 500°C for 4 h for removal of impurities from the Al2O3/ZrO2.

2.2 Catalyst preparation

Catalysts were prepared by impregnation method at 15 wt% total metal loading with Cu/ZnO ratio of 3 : 7. Firstly, Cu and Zn precursors (nitrate) were dissolved in deionised water. The solution was subsequently added to the support in a drop-wise manner under

Methanol production via CO2 hydrogenation reaction 413 continuous stirring for 24 h. The solvent was evaporated at 120°C for 12 h, then the catalyst was calcined in air at 350°C for 4 h. Copper/zinc oxide catalysts supported on SBA-15, Al2O3, Al-SBA-15 and Al2O3/ZrO2 were designated as CZS, CZA, CZAS and CZAZ, respectively.

2.3 Catalyst characterisation

The total surface area, pore volume and average pore size for all the synthesised catalysts were determined by N2-physisorption using micromeritics ASAP 2020 gas adsorption equipment. The surface area was calculated according to the multipoint Brunauer Emmett Teller (BET) equation using the adsorption data collected at relative pressure (P/P0) between 0 to 1, while the value for the pore size distribution was determined from the desorption branch of the adsorption isotherm by the Barrett-Joyner-Halenda (BJH) method. The morphology of the synthesised samples was obtained via Zeiss Supra 55 VP field-emission scanning electron microscopy (FESEM). The average particle size and particle size distribution over the support were obtained using high-resolution transmission electron microscopy (HRTEM Zeiss LIBRA 200 FE) at 200 kV accelerating voltage. Typically 20 micrographs were recorded for each sample and at least 100 nanoparticles were counted for determination of the average particle size and the particle size distribution over the support. The reduction behaviour of the calcined catalyst was determined via H2-temperature programmed reduction using a TPD/R/O equipment (Model 1100 CE) under the flow of dilute 5%H2 in Ar. The same equipment was also used to determine the acidity of the samples by ammonia temperature-programmed desorption (NH3-TPD) [15], the metallic copper surface area (SCu) and copper dispersion (DCu) via the multi-pulse N2O chemisorptions [16].

2.4 Catalyst evaluation

The CO2 hydrogenation reactions to methanol were carried out in a micro-activity fixed bed reactor (PID Eng&Tech). Prior to reaction, the calcined catalyst was reduced in –1 5 vol% H2 in He at 20 mL min and 250°C for 4 h under atmospheric pressure. The reactor was cooled to the reaction temperature of 210°C and reactant gas with H2/CO2 of 3 : 1 molar ratio was introduced then the pressure was raised to 22.5 bar. All post-reactor lines and valves were heated to 180°C to prevent product condensation. The feed and gaseous products were analysed by an online gas chromatograph (Agilent 7890A) equipped with HayesepQ and molsieve columns and thermal conductivity detector (TCD) detector for analysis of H2 and permanent gases. Methanol and other hydrocarbons were analysed using DB-1 column and flame ionisation detector (FID) detector. All the catalytic studies were performed at least three times for each catalyst and values of the standard deviations obtained for the conversion and selectivity were in the range of 2–6%.The average values were used to represent the activity and selectivity of the catalyst.

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3 Results and discussion

3.1 Textural and structural properties of the prepared materials

Surface areas and pore sizes of the samples were obtained from N2adsorption/desorption isotherms (77 K) using BET and BJH methods, respectively. The textural data and reduction temperature of the samples are presented in Table 1. The SBA-15 functionalised with Al had the highest surface area. The presence of impregnated Cu species on the supports was found to reduce the surface area and mesopore volume possibly owing to blockage of pores.

Table 1 Textural properties and TPR result for synthesised catalysts

Reducibility (°C) SBET VP DBJH SCu DCu 2 3 2 Samples (m /g) (cm /g) (nm) (m /g) (%) TR1 TR2 SBA-15 510 0.87 6.82 – – – – CZS 458 0.46 6.99 4.2 29 290 337 Al-SBA-15 778 0.85 6.68 – – – – CZAS 558 0.71 6.99 2.9 22 370 -

Al2O3 121 0.25 8.26 – – – CZA 116 0.24 8.20 1.5 4.8 295 350

Al2O3/ZrO2 182 0.38 8.27 – – – CZAZ 171 0.35 8.24 3.1 12 330 400

SBET is specific surface area; VP is mesopores volume; DBJH is average pore diameter; SCu is surface area of Cu metal; DCu is dispersion of Cu; TR1 is lower reduction temperature and TR2 is higher reduction temperature.

The N2 sorption isotherms for the samples are shown in Figure 1. According to the IUPAC classification [17], SBA-15 exhibits a typical type IV isotherm with H1 hysteresis loop which indicate mesoporous material with pores of cylindrical geometry and uniform pore size distributions. The Al2O3 shows a typical type II isotherm exhibiting hysteresis loops of type H4, referring to micropores adsorbent with narrow slit-like pores. Addition of alumina into SBA-15 increased the total surface area but did not change the shape of the isotherm. The Al2O3/ZrO2 shows a typical type IV isotherm exhibiting hysteresis loops of type H2, suggesting mesoporous material with slit-shaped pores. A very slight increase in pore diameters was observed when the supports were impregnated with Cu species. As shown in Table 1, surface area of Cu and its dispersion over the support were significantly influenced by the type of the oxide carrier. Catalyst supported with SBA-15 exhibited the highest values for Cu surface area (4.2 m2/g) and Cu dispersion (29%) compared to those of the other supports which could be due to better distribution of copper oxide nanoparticles on the external surface as well as the internal channels of the SBA-15. In addition, the decrease in the Cu dispersion over the Al-based supports could be attributed to the bigger copper oxide nanoparticles and stronger metal-support interaction compared to those of SBA-15, as shown in the results of the HRTEM and TPR analyses.

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Figure 1 N2 adsorption/desorption isotherms of the supports and catalysts

Figure 2 H2-TPR of the supported catalyst

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Figure 3 FESEM images (a, c, e, and g) and TEM images (b, d, f, and h) for the Cu/ZnO-catalyst supported on SBA-15, Al-SBA-15, Al2O3, Al2O3/ZrO2, respectively

H2-TPR profiles in Figure 2 show some differences in the reduction patterns of the active phase for the Cu/ZnO catalyst supported on various oxides. For Cu/ZnO supported on SBA-15 (CZS), the first peak (290°C) corresponds to reduction of Cu2+ → Cu0 and the second peak (337°C) is for Cu2+ → Cu+ → Cu0 [18]. Similar patterns were also obtained

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for catalyst supported on Al2O3 (CZA) and Al2O3/ZrO2. However, for Cu/ZnO supported on Al-SBA-15 (CZAS), a single and rather broad reduction peak at 370°C was obtained, indicating a strong metal-support interaction. The reducibility of copper oxide species depended on several factors such as particle size, ratio of Cu to Zn and spatial location of the phases [19]. Indeed the average particle size for catalyst supported on Al-SBA-15 (CZAS) was larger and less dispersed compared to those on other supports, thus may retard the reducibility of the copper species. The morphologies of the Cu/ZnO-catalyst on different types of supports are compared in Figure 3. The SBA-15 and Al-SBA-15 exhibited fibre/cylindrical shape morphology whereas the Al2O3 support had an irregular shape. On the SBA-15 support (CZS), the smaller copper oxide nanoparticles were deposited in the channel pores whereas larger nanoparticles were located on the external surface of SBA-15 (Figure 3(b)). Figure 3(c) and (d) show the morphology of the catalyst nanoparticles supported on Al-SBA-15. The presence of aluminium did not alter the morphology of the SBA-15 significantly but agglomeration of catalyst particles was observed. Figure 4 shows the particle size distribution of Cu/ZnO-based catalysts on different supports. The average size of copper oxide nanoparticles on SBA-15 support (CZS) was 9 ± 0.9 nm. Copper oxide nanoparticles having average diameter of 33 ± 1.9 nm were formed on the Al2O3 support and it was much larger compared to the ones observed on the SBA-15 support. The Al2O3/ZrO2 support exhibited ‘corrugated’ morphology and resulted in elongated spherical-shaped CuO nanoparticles with average size of 21 ± 1.5 nm. The Al2O3 and Al2O3/ZrO2supports resulted in bimodal particle size distribution which are indicative of heterogeneous deposition of small and bigger particles. The lack of porosity in Al2O3 and Al2O3/ZrO2 supports induced the agglomeration of CuO nanoparticles on their surfaces resulting in the formation bigger particles with strong-metal support interaction (Figure 2). The particle size and/or the agglomeration of nanoparticles on different oxide carriers are responsible for the changes in the surface area and reducibility behaviour of the catalysts.

Figure 4 Particle size distribution

NH3-TPD profiles of the synthesised catalysts (Figure 5) show regions of weak acid sites (denoted as α at 150–350°C), medium (denoted as β at 400–500°C), and strong acid sites (denoted as γ at >500°C). The intensity of the desorption peak and its position mainly depended on the type of support used. The CZS catalyst showed highest intensity of weak acidic sites and low intensity of the strong acidic sites. On the other hand, large amounts of strong acid sites were observed on samples with alumina-based supports

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(CZAS, CZA and CZAZ). The presence of weak acid sites could improve the adsorption of CO2 molecules and desorption of products, thus enhancing the hydrogenation reaction (Figure 6).

Figure 5 NH3 TPD profile for Cu/ZnO catalysts on different types of support

3.2 Catalytic performance

Hydrogenation of CO2 to methanol was carried out at 210°C, 22.5 bar, and H2/CO2 molar ratio of 3 : 1 in a micro-activity fixed-bed reactor. Methanol, ethanol, methyl formate and trace of methane were the only carbon-containing products obtained under these reaction conditions. The values of CO2 conversion obtained from CO2 hydrogenation reaction varied from 10.77 to 13.97% using Cu-based catalysts on four different supports (Figure 6). The highest methanol selectivity (91.32%) was obtained using Cu/ZnO catalyst impregnated on SBA-15 support. Amongst the samples tested, Cu/ZnO on SBA-15 support (CZS) had the lowest reduction temperature (easily reducible), smallest size of copper nanoparticles, highest Cu metal surface area and well

Methanol production via CO2 hydrogenation reaction 419 dispersed on the support which resulted in catalyst with good activity and selectivity. Modification of SBA-15 support with Al2O3 resulted in a decrease in the methanol selectivity to 80.45% but increased the ethanol and methyl formate selectivity to 8.03 and 7.22%, respectively. The increase in ethanol and methyl formate selectivities could be attributed to an increase in the acidity of the support. The highest ethanol selectivity (14.36%) was obtained over Cu/ZnO supported on Al2O3, possibly owing to the presence of higher amount of medium and strong acid sites (Figure 5) compared to those of the other supports. Modification of Al2O3 with zirconia improved the catalytic performance of Cu/ZnO catalysts where methanol selectivity of 86.47% was obtained which could be attributed to the ‘corrugated’ morphology of the support, larger Cu surface area and dispersion on the Al2O3/ ZrO2 support. These results concur with the previous findings on the influence of ZrO2 on the catalytic performance of Cu-based catalyst [20].

Figure 6 The performance of Cu/ZnO-catalyst on various supports. Reaction conditions: T = 483 K, P = 22.5 bar, and H2/CO2 ratio =3 : 1

4 Conclusion

Cu/ZnO-based catalyst was synthesised on SBA-15, Al-SBA-15, Al2O3 and Al2O3-ZrO2 supports using the impregnation method. The activity of the synthesised catalysts in CO2 hydrogenation reaction to form methanol was investigated. The Cu/ZnO catalyst on SBA-15 support had the largest Cu surface area and dispersion, smallest particle size and was easily reduced which resulted in the highest catalytic activity compared to those of Al-SBA-15, Al2O3 and Al2O3/ZrO2-supported catalysts. The values of ethanol selectivity were influenced by the presence of Al2O3 in the catalyst support. The highest ethanol selectivity (14.36%) was obtained using Cu/ZnO catalyst supported on Al2O3.

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Acknowledgements

The work was supported by Universiti Teknologi Petronas (UTP) and Ministry of Higher Education Malaysia, ERGS No. ERGS/1/2012/SG/UTP and FRGS/2/2014/SG01/ UTP/02/2. The authors are grateful to Sasol Germany GmbH for providing the catalyst support.

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