Impact of Zeolite Properties on Conversion of Co2 to Aromatics Over Zinc-Zirconia/H-ZSM-5
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Shah, Dhrumil Rajendra
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Abstract
Anthropogenic CO2 emissions have risen dramatically since industrialization, necessitating both CO2 removal and utilization strategies to mitigate climate change impacts and create economically viable solutions for carbon capture. In this regard, the conversion of CO2 to synthesize aromatics (CTA), especially benzene, toluene, and xylene (BTX), has been investigated in this thesis. This can be achieved by one of the two pathways considered in literature: the CO2-Fischer Tropsch (CO2-FTS) pathway and the Methanol-mediated pathway. The methanol-mediated pathway for CO2-to-aromatics (CTA) conversion appears more industrially favorable than the CO2-FTS route due to higher aromatics selectivity, the potential for recycling unconverted reactants, and the ability to reduce CO selectivity through CO co-feeding, despite lower initial CO2 conversion and aromatics yield. Intensified reactors for conversion of CO2 to methanol (via hydrogenation) using metal oxide catalysts coupled with methanol conversion to aromatics in the presence of zeolites (e.g.: H-ZSM-5) in a single step are a current topic of research. However, the high selectivity to aromatics remains a challenge, with CO from the RWGS reaction on metal-oxide catalysts and alkane production being significant co-products. To this end, we are interested in tuning the catalyst properties to favor aromatics production.
Brønsted acid sites (BAS) in H-ZSM-5 are important sites in methanol aromatization reactions, and correlations of the reactivity with zeolite acid properties can guide reaction optimization. A classical way of tuning the acidity of zeolites is via the effect of the isomorphous substitution of the heteroatom in the framework. Hence, in the first objective, we investigated the effect of isomorphous substitution in ZSM-5 in tandem methanol/zeolite catalysts for CTA. H-[Al/Ga/Fe]-ZSM-5 zeolites are synthesized with Si/T ratios = 80, 300, affecting the acid site strength as well as distribution of Brønsted and Lewis acid sites. On catalytic testing of the H-[Al/Ga/Fe]-ZSM-5/ZnO-ZrO2 samples for tandem CO2 hydrogenation and methanol conversion, the presence of weaker Brønsted acid sites improves the aromatics selectivity (CO2 to aromatics selectivity ranging from 13 to 47%); however, this effect of acid strength was not observed at low T atom content. Catalytic testing of H-[B]-ZSM-5/ZnO-ZrO2 provides no conversion of CO2 to hydrocarbons, showing that there is a minimum acid site strength needed for measurable aromatization reactivity. The H-[Fe]-ZSM-5–80/ZnO-ZrO2 catalyst shows the best catalytic activity with a CO2 conversion of ∼10% with a CO2 to aromatics selectivity of ∼51%. The catalyst is shown to provide stable activity and selectivity over more than 250 h on stream.
The use of H-[Fe]-ZSM-5 in CO2 aromatization and the effect of Fe atom density has not been investigated significantly in the literature. Hence, in the second objective, Fe-MFI of varying Si/Fe ratios is synthesized to study the effect of Fe content in the MFI on the aromatic selectivity. The CO2 to aromatics selectivity over H-[Fe]-ZSM-5/ZnO-ZrO2 is maximized at the Si/Fe ratio = 160. At lower Fe contents, the CO2 to aromatics selectivity is affected by limiting the Lewis acid site density. At high Fe loadings, the reduction of Fe sites leads to more production of CO and oxygenates. The reduction during pretreatment and reaction provides reduced Fe2+ extraframework ions, with the appearance of these species concomitant with enhanced CO and oxygenate formation during the reaction.
Finally, in the third objective, we examined the effects of WHSV, methanol partial pressure, and H2, CO, and CO2 co-feeds on MTA reaction over Fe-MFI catalysts. The aim was to understand their impact on MTA reactions at CTA-relevant conditions using tandem metal oxide/H-[Fe]-ZSM-5 catalysts in a single fixed-bed reactor. Over H-[Fe]-ZSM-5-40 catalyst, it was found that a low methanol feed to the zeolitic phase can lead to improved aromatics selectivity. CO and/or CO2 co-feed showed an increase in rate of methylation over aromatics, while H2 co-feed decreased rates of dehydrogenation and led to a decrease in aromatics selectivity as a result.
In summary, this thesis aims to advance the knowledge of zeolite catalysis in the field of CO2 utilization by identifying various structure-property relationships and reaction conditions that affect solid acid catalysis over zeolites.
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2025-01-16
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Dissertation