(720f) Effects of Metal Sites on Co-Reactions of Light Alkanes and Biomass-Derived Oxygenates

Increasing concern over the environment coupled with energy security issues drive the need for society to diversify its energy portfolio. While it is expected that renewable, non-carbon-based energy sources will play an integral role in future energy infrastructure, the transition to these energy sources will occur over a long time-scale and thus strategies are needed to improve efficiency and sustainability of carbon-based energy technologies in the meantime. Towards this goal, the conversion of biomass is a potentially viable route to supplement petroleum as future energy feedstocks for transportation fuels and chemical intermediates. However, unlike petrochemicals, catalytically upgrading biomass relies on the selective conversion of C-O bonds contained in typical biomass-derived feedstocks, requiring an external source of hydrogen molecules. This hydrogen source is most commonly derived from sacrificing a portion of the feedstock in steam reforming reactions or arene formation during hydrogen transfer reactions. This work aims to improve selectivity and solve the hydrogen deficiency issue by developing new catalysts that can facilitate hydro-deoxygenation reactions using natural gas-derived light alkanes as the hydrogen source. These hydrogen transfer steps have the potential to selectively terminate growing hydrocarbon chains via desorption as less reactive alkanes, thereby producing targeted classes of compounds, as opposed to a wide range of products over a broad carbon number distribution. Furthermore, this strategy has the potential to upgrade a portion of the light alkane feed because the hydrogen transfer steps generate alkenes that can participate in subsequent C-C bond formation reactions.

Reactions of isobutane and n-butanal (either as single reactants or co-feeds) on H-BEA zeolite and H-BEA samples partially exchanged with Zn cations were studied to probe the effects of alkanes as co-reactants during acid-catalyzed reaction of oxygenates. Zn-exchanged BEA zeolite was synthesized via ion-exchange using aqueous solutions of zinc acetate with H-BEA (Si/Al = 12.5) followed by treatment in flowing, dry air at 773 K for 5 h to achieve Zn/Al ratios of up to 0.15. This method has been previously shown to create well-defined Zn²⁺ Lewis acid sites in BEA zeolites while maintaining constant Brønsted acid site density. n-Butanal reactions (1 kPa in the feed) on H-BEA predominantly form 2-ethyl-2-hexenal via aldol-condensation (85% selectivity to the aldol-condensation dimer). Butyl butyrate formation via tischenko-esterification was observed (4% selectivity) while also forming heptenes via subsequent ketonization (5% selectivity). n-Butene formation was also observed (2.5% selectivity) via hydro-deoxygenation of n-butanal. Addition of isobutane as a co-feed on H-BEA led to a similar selectivity to the aldol-condensation dimer (90% selectivity). The deoxygenation of n-butanal and subsequent oligomerization-cracking to form propane was also observed (7% selectivity).

n-Butanal conversion on Zn/H-BEA showed a lower selectivity for 2-ethyl-2-hexenal (67% selectivity). n-Butenes formation was observed at a higher selectivity (11% selectivity) while also forming n-butanes via further hydrogenation (5% selectivity). Co-reaction of isobutane (50 kPa) and n-butanal (1 kPa) on Zn/H-BEA exhibited a different product distribution with a lower selectivity towards the aldol-condensation dimer (selectivity of 77% to 90%) compared to H-BEA. Butyl butyrate and heptenes formation was also observed (6% selectivity and 1% selectivity respectively). Hydro-deoxygenation of n-butanal to form n-butane (1% selectivity) was lower than that of H-BEA. Formation of 1-butanol was observed via hydrogenation of n-butanal (2% selectivity). Reaction of isobutane (50 kPa) was also carried out on H-BEA and Zn/H-BEA to identify the extent of isobutane dehydro-oligomerization reaction paths. Isobutane conversion was only 0.1% on H-BEA and only 0.2% on Zn/H-BEA, indicating that dehydro-oligomerization of isobutane only occurs to a negligible extent in parallel with n-butanal conversion paths. This negligible conversion of isobutane is consistent with the absence of trimethyl-pentane isomers from the product distributions (indicating undetectable isobutane dehydro-oligomerization).

The effect of Zn-content was also probed using a Zn-H-BEA prepared via incipient wetness impregnation to achieve Zn/Al ratios greater than 0.15. Product selectivities for n-butanal conversion on this catalyst were similar to those on H-BEA, suggesting that Zn species introduced at higher Zn/Al ratios and/or using different synthesis techniques are different than those from ion-exchange and are non-catalytic for oxygenate reactions. Physical mixtures of bulk ZnO power with H-BEA also exhibit similar product selectivities as H-BEA for reaction of n-butanal, indicating that bulk ZnO species are not the catalytically active species that lead to n-butanal conversion on Zn-H-BEA with Zn/Al = 0.15.

Ethanol parallel dehydration reactions on H-BEA, Zn/H-BEA, and Zn-H-BEA were studied to observe the role differing Zn sites play in acid-catalyze reactions of oxygenates. Turnover rates for diethyl ether (DEE) normalized to Brønsted acid sites (BAS) for H-BEA and Zn-H-BEA were observed to be 73.1 (moles DEE*moles BAS^-1*s^-1) and 4.4 (moles DEE*moles BAS^-1*s^-1) respectively. Similarly, the turnover rates for ethylene on H-BEA and Zn-H-BEA were observed to be 5.0 (moles ethylene*moles BAS^-1*s^-1) and 0.4 (moles ethylene*moles BAS^-1*s^-1). The difference between these values are indicative that Zn²⁺ Lewis acid sites on H-BEA play a different role than existing BEA Lewis acid sites on acid-catalyzed reactions.