A tandem Catalyst for hydrogenation of CO2 to light olefins — The role of the zeolite component

Abstract: The catalytic conversion of waste CO2 into light olefins offers a sustainable pathway for green chemicals production in the future. Over a tandem catalyst with the bifunctional active sites for methanol synthesis (CTM) and methanol to olefins (MTO), CO2 can be efficiently converted via intermediate methanol into a mixture of light olefins (ethylene, propene, butene). In the study of CO2 hydrogenation, the moderately acidic SAPO-34 molecular sieve is often used as the MTO catalyst component. SAPO-34 performs well for the formation of C-C bonds without fast coking, and minimal further hydrogenation of olefins. These qualities give the catalyst a long lifetime and high selectivity for light olefins. Unfortunately, under high-pressure hydrothermal conditions, it easily suffers structural damage. In this context, the SSZ-13 zeolite, an alternative MTO catalyst with higher hydrothermal stability, but also stronger acidity, was systematically investigated and modulated with the aim to achieve high selectivity for light olefins and high stability during CO2 hydrogenation. Firstly, in order to identify the effect of zeolite acidity on product distribution and coke deposition, two types of SSZ-13 zeolites with similar bulk composition, but different protonic acid site distributions, were synthesized. They were combined with a stable CTM catalyst (bulk indium oxide, In2O3) as a tandem catalyst and were evaluated in CO2 hydrogenation. The SSZ-13 with isolated acid sites had a lower Brønsted acid site (BAS) density and exhibited a higher selectivity for light olefins compared with the one with paired protonic acid sites. By exchanging Na+ cations to tailor the BAS density of SSZ-13 zeolite, the comparative experiments further indicated that the BAS density, rather than BAS distribution, had a high correlation with the selectivity for light olefins, which proved that the BAS density had the primary impact on the product distribution. The high BAS density promoted hydrogenation which reduced the selectivity for light olefins, while low BAS density tended to accumulate excessive coke leading to catalyst deactivation, but with improved selectivity for olefins. Thereafter, over the tandem catalysts with the optimized BAS density, a transient experiment with varying reaction conditions was carried out to investigate the coke evolution during CO2 hydrogenation. The results indicated that the coking behavior of SSZ-13 zeolite was significantly affected by reaction conditions. By manipulating the reaction temperature and pressure, the active coke species, or so-called hydrocarbon pool species (HCPS), can be deposited inside the zeolite in a targeted manner, thereby modifying the catalyst to achieve a higher MTO activity and lower olefin hydrogenation activity. Continuous transient experiments further revealed a dynamic equilibrium between the formation and degradation of coke inside SSZ-13 zeolite. This balance is established under the appropriate BAS density and optimized reaction temperature and pressure. Using the conditions of 20 bar and 375 ℃, with a H2 to CO2 mole ratio of 3, the results obtained for the pre-coked tandem catalysts of In2O3 and SSZ-13 (BAS density = 0.23 mmol'g-1) exhibited very stable activity, with selectivity for light olefins around 70% ± 2% (among hydrocarbon products), and low average coke deposition rate of 0.016 wt.%'h-1 over 100 h time-on-stream. This result also experimentally confirmed the success of pre-coking modification and verified the balance mechanism of coke accumulation.