422991 Incorporation of Light Alkanes into Alkene Oligomerization Cycles on Solid Brønsted Acid Catalysts

Wednesday, November 11, 2015: 12:50 PM
355A (Salt Palace Convention Center)
Michele L. Sarazen and Enrique Iglesia, Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA

Upgrading light alkenes on solid Brønsted acids to larger, fuel-like hydrocarbons involves concurrent oligomerization, isomerization, β-scission, hydride transfer and cyclization reactions [1]. Incorporation of undervalued, light alkanes is mediated by hydride transfer reactions that provide a route of termination from the oligomerization pathway and increase the paraffinic content of the reactor effluent. These reactions, as well as the other aforementioned acid catalyzed hydrocarbon reactions, are mediated by ion-pair transitions states of various sizes, which lead to effects of confinement on turnover rates (per accessible proton) over a wide range of solid acid catalysts, including zeolites and mesoporous aluminosilicates.

Propene oligomerization turnover rates are proportional to propene pressure (5-450 kPa), which reflect kinetically-relevant additions of a propene molecule to a propoxide bound at each proton [1]. The first-order rate constants increase with decreasing zeolitic void size due to preferential solvation of transition states over the smaller alkoxide (and gaseous alkene) via van der Waals contacts with the confining walls. Incorporation of co-fed isobutane into this oligomerization pathway is limited by a hydride transfer step between isobutane and a bound alkoxide resulting in turnover rates that are proportional to isobutane pressure on all zeolitic and mesoporous acids. The first-order rate constants for C6 alkane formation, via hydride transfer to bound propene dimers, are dependent on void diameter, however these transition states are significantly larger than those involved in dimer formation via oligomerization reactions; as a result, the ratio of the first-order rate constant for C6 alkane formation to that for propene dimer formation increases with void size and only becomes larger than one for large pore (12-MR) zeolites and mesoporous aluminosilicates.

The likelihood of a given hydride transfer step is dependent on the stability of both the bound alkoxide and the hydride donor. Thus, measured kinetic rates decrease as the alkoxide becomes smaller, e.g. propane formation from propoxide has lower turnover rates (>0.09 times) than C6 alkane formation from propene dimers because fewer stabilizing van der Waals contacts can be made with the pore walls. Further, on samples where β-scission reactions occur readily, leading to broadening of the carbon number distribution [1], alkoxides of intermediate chain length are available for hydride transfer; again, smaller alkoxides (e.g. C5) lead to lower formation rates of the corresponding alkane. Substitution of the alkoxide also affects its stability, where first-order rate constants for the structural isomers of C6 alkanes (2-methyl pentane, 3-methyl pentane, 2,3-dimethyl-butane and n-hexane) differ despite the fact that isomerization events to form the different backbones are in equilibrium with their corresponding gas phase alkenes [1]. Moreover, the substitution, as well as size, of the hydride donor affects the rate of the hydride transfer step, where isobutane incorporates more easily than n-butane due to its ability to stabilize charge at a tertiary carbon (instead of secondary). Applications of the results and conceptual understandings gleaned here indicate promising routes for the production of higher molecular weight hydrocarbons for fuels by utilizing undervalued, light alkenes and alkanes.

Financial support from BP through the XC2 Program and National Science Foundation Graduate Research Fellowship Programs are gratefully acknowledged.

[1] Sarazen, M. L., Iglesia, E. unpublished results

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