Time：2015.07.23 (Thursday), 03:00 pm
Location：Conference Room of Energy Sciences No.1 Building 1st Floor
Lecturer：Prof. Daniel E. Resasco, University of Oklahoma,
School of Chemical, Biological and Materials Engineering
Introduction：
Professor Daniel Resasco holds the Douglas Bourne Chair of the School of Chemical, Biological and Materials Engineering at the University of Oklahoma. He is also George Lynn Cross Professor, the highest research honor bestowed by the University. He received his B.S. in Chemical Engineering at the Universidad Nacional del Sur, Argentina (1975) and his Ph.D. from Yale University (1984).
Abstract：
Elucidating the reaction mechanism of the hydrodeoxygenation of phenolics has been the goal of recent studies. Most of the proposed deoxygenation mechanisms reported in the literature can be grouped into two major pathways: (a) hydrogenation/deoxygenation (HDO) route that starts with hydrogenation of the aromatic ring of phenolic compounds to their corresponding alcohols, subsequently gets deoxygenated via dehydration. This route requires a bifunctional catalysts with a metal function that hydrogenates the aromatic ring and an acid function that catalyzes the removal of the O atom by dehydration. (b) direct deoxygenation via direct cleavage of the C(sp2)-O bond which could only take place at very high temperatures due to the high energy barrier required to brake this strong bond. Here, we discuss the different reaction paths that may occur on different metals. For example, on Pt a fast enol-keto tautomerization step precedes the ring hydrogenation. The unstable ketone intermediate forms over the catalyst surface, which could be hydrogenated via two possible paths: When the CO of this intermediate gets hydrogenated instead of the ring, a very reactive unsaturated alcohol (3-methyl-3,5-cyclohexadienol) is formed and it can be readily dehydrated to toluene. By contrast, when the C=C bonds are hydrogenated, methylcyclohexanone is formed and is further hydrogenated to methylcyclohexanol. Over acidic supports, the alcohol dehydrates to methylcyclohexene and a sub-sequent dehydrogenation would produce toluene. However, on inert supports dehydration of saturated alcohols does not occur to a great extent. This mechanism explains how toluene can be obtained at mild temperatures and in the absence of strong acid sites. It also explains why toluene is formed as a primary product, before methylcyclohexane, and why methylcyclohexene is not observed on this catalyst, even at low conversions. By contrast, on Ru a very different product distribution is obtained. That is, while ring hydrogenation to 3-methylcyclohexanone is dominant over Pt, deoxygenation to toluene and C-C cleavage to C1-C5 hydrocarbons prevail over Ru. To understand the differences in reaction mechanisms responsible for this contrasting behavior, the conversion of m-cresol over the Pt(111) and Ru(0001) surfaces has been analyzed using density functional theory (DFT) methods. The DFT results show that the direct dehydroxylation of m-cresol is unfavorable over the Pt(111) surface, with an energy barrier for the 242 kJ/mol. In turn, the calculations support the tautomerization path. At the same time, a low energy barrier for the ring hydrogenation path towards 3-methylcyclohexanone compared to the energy barrier for the deoxygenation path towards toluene over the Pt(111) surface is in agreement with the experimental observations, which show that 3-methylcyclohexanone is the dominant product over Pt/SiO2 at low conversions. By contrast, the direct dehydroxylation of m-cresol becomes more favorable than the tautomerization route over the more oxophilic Ru(0001) surface. In this case, the deoxygenation path exhibits an energy barrier lower than that for the ring hydrogenation, which is also in agreement with experimental results that show higher selectivity to the deoxygenation product, toluene. Finally, it is proposed that a partially unsaturated hydrocarbon surface species C7H7* is formed during the direct dehydroxylation of m-cresol over Ru(0001), becoming the crucial intermediate for the C-C bond breaking products, C1-C5 hydrocarbons, which are observed experimentally over the Ru/SiO2 catalyst.
Contacts：Group DNL0602 Lu Wan (9371)