Chemical conversion of lignocellulosic biomass is an attractive method to convert renewable carbon to value chemicals and fuels, but it requires hydrodeoxygenation (HDO) as a final step to upgrade biomass-derived oxygenates to suitable liquid transportation fuels . Understanding the role of HDO catalysts from fundamental aspects for selective furan ring hydrogenation and C-O bond hydrogenolysis has yet to be sufficiently achieved in order to develop finely tuned, bifunctional catalytic systems for biorefinery applications. Oxygenates targeted for HDO typically contain furan groups since the C-C bond coupling chemistry exploits the functional handles of hydroxymethylfurfural and furfural, which are biomass-derived platform chemicals . Therefore 2,5-dimethylfuran (DMF) is a model substrate conducive to study over HDO catalysts.
From our previous work, we have established that certain hydrogenation metals can produce 2-hexanone by selective C-O bond hydrogenolysis of DMF in liquid phase under mild conditions. Methyl ketones are considered synthons in condensation reactions to produce aviation fuel and lubricants . It is valuable to understand the kinetics and the conditions to directly ring open DMF to produce methyl ketones and also to consider what other catalytic components are necessary to fully hydrodeoxygenate the furan ring. Although full HDO can be achieved over noble metals supported on carbon alone, harsher conditions are demanded of the process . Milder conditions can be used when HDO catalyst involves a combination of both metal sites and acidic sites for C-O bond hydrogenolysis. The selection and design of HDO catalysts plays an important role to effectively transform biomass derived oxygenates.
In our previous studies, Pt/C exhibits the highest C-O bond hydrogenolysis activity of DMF at mild reaction conditions (80oC , 80psi H2) when compared to other typical hydrogenation metals (Pd, Rh, Re, Ni). In this study, we examine the kinetics of the reaction network of DMF hydrogenation over Pt/C and the influence of reaction conditions and catalytic components. Under mild reaction conditions, the rate observed for DMF conversion was zeroth order with respect to substrate and between ½ and 1st order to partial pressure of hydrogen. This observed rate law leads us to propose that the 2nd addition of H may be the rate-limiting step in the hydrogenation of DMF. Upon further tuning of the reaction conditions, we can maintain high selectivity to 2-hexanone production (>85%) at high temperatures and low pH2 pressures over Pt/C.
Although the rate of C-O bond hydrogenolysis of DMF was greater than the rate of hydrogenation of 2-hexanone over Pt/C, this was not always the case for Pt metal on other supports. We extend our kinetic studies to consider the influence of Pt particle size, metal density, and support effects on the reaction network of DMF hydrogenation. Enhancement in 2-hexanone hydrogenation and removal of oxygen from 2-hexanone and DMTHF occurred very effectively when Pt was supported on niobium phosphate, NbOPO4, which contain strong Lewis acid centers. We therefore continued our study to evaluate the importance of Lewis acid sites in the HDO process of furanyl compounds.
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