(720f) Effects of Metal Sites on Co-Reactions of Light Alkanes and Biomass-Derived Oxygenates
AIChE Annual Meeting
2019
2019 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Catalysis for Biomass Upgrading III: Novel Conversion Strategies
Thursday, November 14, 2019 - 5:00pm to 5:18pm
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.