Abstract for the 12th Natural Gas Conversion Symposium. Invited lecture
Conversion of synthesis gas to light olefins: impact of hydrogenation activity on process selectivity over Cr-Zn and Cu-Zn with SAPO-34
Alexey Kirilin1*, Joseph DeWilde2, Adam Chojecki1, Vera Santos1, Kinga Scieranka1 and Andrzej Malek2
1 Dow Benelux B.V., Herbert H. Dowweg 5, building 443 (BBB), 4252 NM, Hoek, Netherlands; 2 The Dow Chemical Company, Midland, building 1776, MI 48674, USA.
*akirilin@dow.com
1. Introduction
Light olefins (C2-C3) are industrially vital feedstocks for production of plastics, functional materials, and as platform chemicals for production of other derivatives. Short chain hydrocarbons can be produced from synthesis gas by using a hybrid catalyst combining Cu-based methanol synthesis and SAPO-34 [1]. Recently it was shown that direct conversion of synthesis gas to light olefins with high selectivity is also possible via the so-called Ox-Zeo process by combining ZnCrOx catalyst with SAPO-34 [2].
In the present study, we aim to compare two bifunctional catalytic mixtures containing a Cu-Zn (low temperature methanol catalyst) [3] or Cr-Zn (high temperature methanol catalyst) [3] in combination with a zeolite component SAPO-34 for the direct conversion of synthesis gas to olefins. The focus of this study is the distribution of products and kinetics of product formation [4].
2. Methods
Cu-based methanol catalyst “HiFuel R120” (Johnson Matthey, sold by Alfa Aesar) and Cr-Zn catalyst (prepared by co-precipitation [5]) were used. SAPO-34 was synthesized according to the literature procedures [6]. Materials were crushed and sieved to 60-80 mesh. Catalyst were characterized by N2 physisorption, XRF, XRD, TPR and SEM techniques. Hybrid catalysts were prepared by mixing Cu-Zn or Cr-Zn with SAPO-34. Catalytic tests were performed in a tubular stainless steel fixed-bed microreactors (i.d. 3 mm) at 370-410°C, 20 bar, GHSV = 1200 h-1 and different H2/CO ratios. Online GC analysis of components (N2, H2, He, CO, CO2, C1-C5 alkanes and olefins) was performed periodically. Mass balance in all experiments was 95-105% based on carbon.
3. Results and discussion
For Cu-Zn/SAPO-34 only saturated hydrocarbons (C1-C5) and carbon dioxide (due to water-gas-shift reaction) were observed while for the Cr-Zn/SAPO-34 system olefins were also present among the reaction products (Figure 1). The C3/C2 ratio is consistently higher for Cr-Zn/SAPO-34 systems compared to Cu-Zn/SAPO-34 systems at the same conversion level for all measured conditions (Figure 1). The difference in C3/C2 ratios demonstrates the role of the relative hydrogenation behavior in affecting SAPO-34 product distributions. We propose that C3/C2 ratio is predominantly controlled by the relative rates of olefin cycle propagation and the rate of cycle termination by the formation of paraffins on the Cr-Zn catalyst – loading additional SAPO-34 at low conversions increases the relative contribution of olefin cycle propagation on the reactor effluent [4].
A kinetic model was built for the Cr-Zn/SAPO-34 system to better understand and describe the observed patterns in product distribution [4]. Model parameter fits were performed using standard Bayesian techniques employing the Athena Visual Studio software package (M. Caracotsios and W.E Stewart, v14.2).
The parity plots for conversion, combined paraffin carbon yield, and combined olefin carbon yield show considerable scatter around perfect model prediction. Considering these kinetic model shortcomings, improving the quantitative description of this reaction network, potentially necessitating SAPO-34 transport restriction analysis, will be a useful avenue of future investigation to predict C3/C2 product distributions as a function of process conditions.
Figure 1. Product composition (C mol%) in conversion of synthesis gas over bifunctional catalysts. Conversion of syngas 40-50%. CO2 selectivity was 45-50%.
Figure 2. Parity plots for (a) conversion (%), (b) combined paraffin (ethane, propane, and butane) carbon yield (%), and (c) combined olefin (ethylene, propylene, and butene) carbon yield (%) for the kinetic model for Cr-Zn/SAPO-34 system data.
4. Conclusions
The choice of methanol synthesis catalyst alters product selectivity and the C3/C2 product ratios. The relative rates of propagation of olefin methylation and cracking (olefin cycle) in SAPO-34 and olefin hydrogenation on the methanol synthesis catalyst dictates the observed C3/C2 ratio. A kinetic model for the hybrid system is proposed to describe the observed selectivity patterns. We identify the balance of methanol synthesis and olefin hydrogenation rates on the mixed-metal-oxide catalysts as a potentially strong factor to control the product distribution in synthesis gas-to-olefin/paraffins technologies.
References
[1] D. Nieskens, A. Ciftci, P. Groenendijk, M. Wielemaker, A. Malek, Ind. Eng. Chem. Res. 56 (2017), 2722-2732.
[2] F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, Z. Zhou, M. Li, S. Miao, J. Li, Y. Zhu, D. Xiao, T. He, J. Yang, F. Qi, Q. Fu, X. Bao, Science 351 (2016), 1065-1068.
[3] J.B. Hansen, P.E. Højlund Nielsen, In Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA: 2008. pp. 2920-2949.
[4] A.V. Kirilin, J.F. Dewilde, V. Santos, A. Chojecki, K. Scieranka, A. Malek, Ind. Eng. Chem. Res. (2017), 56 (45), pp 13392–13401.
[5] Collins, U.S. Patent 3,850,850A, 1974.
[6] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, U.S. Patent 4,440,871A, 1984.
Keywords
Olefins, syngas, mechanism, kinetic model
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