(111a) Autothermal Partial Oxidation of Butanol Isomers

Fermentation of carbohydrates to butanol is a promising technique for generating an energy dense liquid from biomass.   Production of butanol from biomass is desirable because butanol is a versatile platform molecule that can be upgraded to a variety of fuels and chemicals.   In particular, transformation of butanol to syngas is advantageous, either as a starting point for chemical synthesis or to generate hydrogen for fuel cells.  Additionally, generation of C₄ carbonyls and alkenes may be important steps in the synthesis of higher chemicals from butanol.  Autothermal partial oxidation is a promising technology for all of these reactions, as it is both scalable and tunable.  This work aims to demonstrate the ability of autothermal partial oxidation to generate a product stream of either syngas or unsaturated C₄ compounds depending on reaction conditions.   This research also continues our work on reaction network analysis of biomass-based compounds in autothermal reactors.  We demonstrate high yields of syngas and unsaturated C₄ molecules, and investigate the reaction pathways of all four butanol isomers over Al₂O₃-supported Rh, RhCe, Pt, and PtCe catalysts for a range of carbon-to-oxygen (C/O) ratios.

For each isomer, conversion to equilibrium products (CO, CO₂, CH₄, H₂, and H₂O) is essentially complete at C/O = 0.8. At higher C/O ratios, dehydrogenation is the major pathway for the primary and secondary butanols, producing butyraldehyde, isobutyraldehyde, and butanone from 1-butanol, isobutanol and 2-butanol, respectively. At C/O = 2.0, dehydrogenation selectivity approaches 50%, while dehydration to the corresponding butenes represents less than 20% selectivity. tert-butanol behaves differently, selecting mainly for the dehydration product isobutene. Acetone is the main carbonyl product from tert-butanol, but selectivity to acetone is always ≤ 10%. Global mechanisms in an autothermal reactor, based on pyrolysis, combustion and surface science literature, are proposed for each alcohol. Surface chemistry likely accounts for much of the syngas formation, while the carbonyls and alkenes may be formed primarily through homogeneous routes.