Role of Catalyst Domain Size in the Hydrogenation of CO₂ to Aromatics over ZnZrO_x/ZSM-5 Catalysts

Abstract

The direct conversion of carbon dioxide into aromatic compounds may provide an environmentally friendly resource for a family of chemical building blocks with high global demandaromatic chemicals. An array of bifunctional catalysts composed of ZnZrO x for the conversion of CO2 to methanol and ZSM-5 for methanol aromatization is evaluated considering multiple operational and catalytic parameters, with a focus on the particle size of each catalyst functionality. Aromatic yields generally increase as the ZSM-5 domain size decreases for different configurations of catalyst domainsmixed powders, mixed pellets, and separate beds. However, for a physical mixture of the catalyst components prior to pelletization (mixed powder case), varying the ZSM-5 crystal size inversely impacts the particle size of ZnZrO x , leading to enlarged metal oxide domains and enhanced intermediate diffusion barriers for very small crystals of ZSM-5. Larger crystals of ZSM-5 can be employed to obtain a higher concentration of benzene, toluene, and xylene (BTX) among aromatics. Upon employing a physical mixture of the ZnZrO x /ZSM-5 catalyst with a Si/Al ratio of 300 and a ZSM-5 crystal size of 300 nm, a total aromatics selectivity of 44% at CO2 conversion of 6% was obtained at 320 °C, WHSV of 7200 mL gcatalyst –1 h–1, and H2/CO2 ratio of 3.

Isomorphous Substitution in ZSM-5 in Tandem Methanol/Zeolite Catalysts for the Hydrogenation of CO2 to Aromatics

“…High acid site densities lead to lower aromatics selectivities, whereas intermediate values of Si/Al (Si/Al = 200−400) improved the performance. 48 As before, the Brønsted acid-rich Si/Al = 300 catalyst gave excellent aromatics selectivity.…”

“…For catalysts with stronger Brønsted acid sites, such as those based on Al substitution, our results are consistent with literature trends, as well as our prior work. High acid site densities lead to lower aromatics selectivities, whereas intermediate values of Si/Al (Si/Al = 200–400) improved the performance . As before, the Brønsted acid-rich Si/Al = 300 catalyst gave excellent aromatics selectivity.…”

“…This represents the balance between H-transfer reactions, producing alkanes, and dehydrogenation reactions, producing H 2 , in the pathways to aromatic products. Prior work by others has shown an impact of zeolite acid site density in these tandem reactions, and our prior work identified an optimal Si/Al range for tandem reactions with ZnO-ZrO 2 /H-[Al]-ZSM-5 . Here, at Si/ T = 80, the Fe substituted zeolite gave the highest selectivity to aromatics while minimizing the paraffin production.…”

“…Prior work by others has shown an impact of zeolite acid site density in these tandem reactions, 33 and our prior work identified an optimal Si/Al range for tandem reactions with ZnO-ZrO 2 /H-[Al]-ZSM-5. 48 Here, at Si/T = 80, the Fe substituted zeolite gave the highest selectivity to aromatics while minimizing the paraffin production. From there, paraffin selectivity seems to correlate with zeolite acid strength, with stronger acids producing more paraffins and less aromatics.…”

“…The overall characteristics of the ZSM-5 samples are summarized in Table . Prior work on methanol conversion over zeolite catalysts has shown that the zeolite topology, crystal size, acid site density, and nature of the acid sites all can influence the reactivity. , In the experimental design here, we sought to hold constant the zeolite topology, crystal size, and total acid site density, while changing the strength and potentially distribution of the acid sites. , With these zeolites in hand, composite catalysts composed of each zeolite individually mixed with a ZnO-ZrO 2 methanol synthesis catalyst were deployed in the tandem hydrogenation of CO 2 to aromatics.…”