One-pot catalytic conversion of biomass and alkanes: Kinetically coupling deoxygenation and dehydrogenation pathways
The stoichiometric hydrogen deficiency of carbohydrate feedstock implies that all conversion processes must remove oxygen and concurrently increase hydrogen content to form liquid hydrocarbon products. With support from the DOE Early Career program (DE-SC0008418) we have investigated the use of transition metal carbides for hydrodeoxygenation (HDO) chemistries using CH4 and H2 as co-reactants. We report that concurrently processing CH4 and C1-C3 oxygenates (oxygenate/CH4 = 0.01-0.1) at 950 K on molybdenum carbide clusters encapsulated in zeolite micropores results in a staged bed with upstream CH4 reforming and downstream CH4 dehydroaromatization. Forward rates of CH4 pyrolysis limited by CH4 activation are unperturbed by the introduction of oxygenates with a (C/H)eff <0.25 after rigorously accounting for the thermodynamic reversibility caused by the H2 produced in oxygenate reforming reactions and the fraction of the active catalyst bed deemed unavailable for CH4 dehydroaromatization. A binomial distribution of 12C/13C isotopologues of CO is observed upon introduction of CH313COOH/CH4 reactants suggesting that the introduction of oxygenates with a (C/H)eff <0.25 results in complete fragmentation of the oxygenate at these temperatures which in turn results in the invariance of forward rates of CH4 pyrolysis when using CO2, HCOOH, or CH3COOH co-feeds. The stoichiometric loss of oxygen as CO when co-processing C1-C3 oxygenates and CH4 at these temperatures demonstrates that (C/H)eff =(C-O)/H is a single valued descriptor for oxygen removal on Mo/HZSM-5 catalysts. Separately, we have demonstrated the utility of transition metal carbides for vapor phase catalytic hydrodeoxygenation using molecular hydrogen with high selectivity for C-O cleavage and near absence of sequential hydrogenation reactions at atmospheric hydrogen pressure and illustrated that the faculty of transition metal carbides for selective hydrodeoxygenation arises due to oxygen-modification of the catalyst in situ. The oxophilic characteristics of these materials engender both the observed acid-metal catalytic bifunctionality and selectivity for hydrodeoxygenation. The evolution of catalyst surface composition and catalytic function under oxidative reaction environments proffers unique challenges in structural characterization of these materials and in this context, we developed and demonstrated the use of chemical titration and chemical transient studies to assess catalytic site identity and densities.
- Research Organization:
- Univ. of Minnesota, Minneapolis, MN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0008418
- OSTI ID:
- 1507124
- Report Number(s):
- DOE-UMN-0008418
- Country of Publication:
- United States
- Language:
- English
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