CONTROLLING PHENOLIC HYDRODEOXYGENATION BY TAILORING M-O BOND STRENGTH VIA SPECIFIC CATALYST METAL TYPE AND PARTICLE SIZE SELECTION
Daniel E. Resasco
University of Oklahoma, Norman OK 73019 - USA
Elucidating the detailed reaction mechanisms of the hydrodeoxygenation (HDO) of phenolics has been the goal of a number of recent studies due to the importance of this reaction in biomass conversion.Yet, there are aspects of the mechanism that remain unsettled and require further analysis [1-5]. Microkinetic and theoretical studies have been conducted on a series of different metal supported on various oxide supports with varying degrees of reducibility and acidity.Depending on the metal and support used, different mechanisms are operational.Density functional theory (DFT) calculations show that the energy barrier for the direct dehydroxylation of m-cresol over Pt and Pd surfaces (with d-band centers far from the Fermi level) is too high, indicating that this path is unfavorable. Instead, a path via a ketone tautomer that undergoes hydrogenation of the carbonyl group followed by dehydration to form toluene and water is favorable on these noble metals [2, 4]. By contrast, over the more oxophilic Ru or Fe surfaces (d-band center closer to the Fermi level) the direct dehydroxylation of m-cresol becomes more favorable than the tautomerization route [1]. In addition, the selective deactivation of the different types of sites present on the metal catalyst as well as the effect of this deactivation on HDO selectivity have been investigated on a micro-pulse reactor over a series of metal catalysts of varying particle size and simulated with DFT calculations over FCC metal surfaces of varying defect densities.
Anisole is another interesting oxygenated aromatic molecule that represents the methoxy functionalities typically observed in biomass-derived compounds. In a comparative study, it was observed that over Pt and Ru catalysts, both phenol and benzene are the major products in a phenol/benzene ratio that decreases with the level of conversion.By contrast, over Fe, no phenol formation is detected, even at low conversions. The DFT results show that over all the three metal surfaces the dehydrogenation at the –CH3 side group occurs before the C-O bond breaking. This removal of H atoms from the –CH3 group facilitates the activation of the aliphatic Calkyl-O bond. Therefore, it can be concluded that a common intermediate for the three metals is a surface phenoxy and the significant differences between the three metals is related to the reactivity of this surface phenoxy.That is, in agreement with the experiments, it is concluded that, over Pt and Ru, the phenoxy intermediate is hydrogenated to phenol, which in turn, can undergo further HDO to form benzene. In contrast, over Fe, the strong metal oxophilicity makes the direct cleavage of the C-O bond in the surface phenoxy easier than hydrogenation to phenol. Thus, it is predicted that phenol is not formed over iron, but only benzene should be observed as HDO product at all conversion levels, which is also in agreement with the experimental observations.
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