Controlling phenolic hydrodeoxygenation by tailoring metal–O bond strength via specific catalyst metal type and particle size selection

Abstract

Experimental and theoretical studies of hydrodeoxygenation (HDO) of m-cresol show that on metals of low oxophilicity, such as Pt or Pd, the direct CeO bond cleavage is not possible. Therefore, HDO requires an indirect pathway via tautomerization. In contrast, on metals of higher oxophilicity, such as Ru or Rh, the direct CeO bond cleavage is possible and toluene can be directly obtained from m-cresol. These studies show that the HDO activity correlates directly with the metal oxophilicity, which in turn depends on the position of the d-band center relative to the Fermi level. This catalytic descriptor depends on (1) the type of metal and (2) the extent of coordination of the metal atoms. The present contribution investigates the effect of the second factor. It is demonstrated that a Rh/SiO 2 catalyst with small particles (low-coordination sites) is more active for HDO than one with larger particles (high-coordination sites). At the same time, the low coordination site is more prone to deactivation and loss of HDO selectivity. These experimental results are combined with the density functional theory calculations, which show that the energy barrier for direct CeO bond cleavage is lower on a step site than on a terrace site. At the same time, it is shown that a step site requires a higher energy barrier to remove adsorbed molecular fragments to regenerate the activity than a terrace site, which explains the faster deactivation rate observed on catalysts with small more defective metal clusters.

Relationship between Atomic Scale Structure and Reactivity of Pt Catalysts: Hydrodeoxygenation of m-Cresol over Isolated Pt Cations and Clusters

“…We also considered the possibility that HDO of m -cresol over Pt iso could be hindered by the difficulty in performing C–O bond scission over these sites. In previous studies, we have shown that the barrier for the kinetically relevant C–O scission step in HDO correlates strongly with the oxygen binding energy of the metal . Therefore, weaker interaction between Pt iso and cresol could render it inactive for catalyzing this reaction step.…”

“…In previous studies, we have shown that the barrier for the kinetically relevant C−O scission step in HDO correlates strongly with the oxygen binding energy of the metal. 23 Therefore, weaker interaction between Pt iso and cresol could render it inactive for catalyzing this reaction step. We have previously shown that the binding energy of small adsorbates over stable Pt iso sites is significantly lower than it is over extended Pt surfaces.…”

“…In previous studies, we explored the mechanistic routes for HDO of cresol and related molecules on extended metal surfaces. − We have shown that while oxophilic metals are able to catalyze HDO directly on the metal surface, the reactivity of noble metals, such as Pt and Pd, are promoted by the existence of exposed oxophilic cations on reducible oxide supports . On catalysts such as Pt and Pd on TiO 2 or ZrO 2 supports, HDO occurs at the metal-support interface with O in cresol binding to exposed Ti or Zr cations and the metal catalyzing H 2 activation and hydrogenation .…”

Size Dependence of Vapor Phase Hydrodeoxygenation of m-Cresol on Ni/SiO2 Catalysts

“…For HDO of phenolics, a few pioneering works devoted to the effect of metal particle size have been reported, with particular attention paid to the noble metal based catalysts. − Newman et al performed HDO of phenol at 300 °C and 600 psi H 2 and 50 psi N 2 over two Ru/TiO 2 samples . They reported that benzene selectivity on Ru/TiO 2 with Ru particle size of 1.5 nm (86% selectivity at 12% conversion) is dramatically higher than that with Ru particle size of 33 nm (18% selectivity at 4% conversion).…”

“…Since the reducible TiO 2 and ZrO 2 supports may participate in the deoxygenation, it is difficult to draw a clear conclusion on the effect of metal particle size. Recently, Resasco et al reported HDO of m -cresol in a pulse reactor over two Rh/SiO 2 catalysts with Rh particle sizes of 4 and 2 nm, achieved by varying Rh loadings . The turnover number (TON) for toluene formation was moderately improved when Rh size was reduced from 4 to 2 nm, while the TON for the formation of cracking products of C2–C6 was found to be little affected by the change of size.…”