Elucidating the Structure of Bimetallic NiW/SiO₂ Catalysts and Its Consequences on Selective Deoxygenation of m-Cresol to Toluene

Abstract

As a non-noble metal, Ni could offer significant economic advantages if used as a catalyst for hydrodeoxygenation (HDO) of lignin-derived phenolics to produce aromatics. However, on unmodified Ni catalysts, the desirable direct deoxygenation reaction must compete with high rates of phenyl-ring hydrogenation and C–C hydrogenolysis reactions, which lead to low aromatics yields. Here, we report on a bimetallic NiW/SiO2 (W/Ni = 1) prepared by coimpregnation that shows an HDO reaction rate of m-cresol almost an order of magnitude higher than that on Ni/SiO2 at 350 °C and 1 atm H2. More importantly, under these conditions, this catalyst exhibits a complete inhibition of CH4 formation, while at a temperature as low as 250 °C, the dominant product is still toluene, with minimal formation of ring-saturation products. To elucidate the structure of this catalyst, a detailed characterization was performed by combination of several techniques. It was found that the calcined NiW/SiO2 exhibits a large extent of Ni–W oxide interaction. After reduction at 500 °C, a thin NiW alloy shell with a small Ni core and WO x in close proximity are formed, with a strong interaction between Ni and adjacent W species. The electronic modifications of Ni and W species were monitored by X-ray photoelectron spectroscopy and it was found that these interactions alter the surface properties of the alloy, resulting in significantly weakened CO chemisorption. This unique structure provides a balanced hydrogenation, oxophilicity, and C–O cleavage activity, which result in a significantly improved rate and selectivity toward toluene with inhibition of CH4 and hydrogenation product formation.

The role of Ni sites located in mesopores in the selectivity of anisole hydrodeoxygenation

“…In addition, a high yield of aromatic hydrocarbons (BTX: benzene, toluene, and xylene isomers) was detected, which was derived from the deoxygenation of the above oxygenates. The different product distribution over similar catalysts was also observed in studies by Zhu 60,61 and Hong. 62 Zhu investigated the HDO of anisole over a Pt/HBeta catalyst with aromatics detected as the main products, while cycloalkanes were observed as the main products in the HDO of phenol over a Pt/HY catalyst in Hong's studies.…”

“…The direct deoxygenation of phenol, anisole and guaiacol to aromatics is commonly promoted by low H 2 pressure, high reaction temperature and metal species with high oxophilicity. 61,[73][74][75][76]…”

Role of MoOx/Ni(111) interfacial sites in direct deoxygenation of phenol toward benzene

“…In recent years, the HDO of phenolics has been explored on various types of catalysts. [16][17][18][19][20][21][22][23] Among them, Ni-based catalysts have been intensively investigated, [24][25][26][27][28][29][30][31][32][33] since Ni shows excellent ability to dissociate H 2 and is much cheaper than other typical noble metal based hydrogenation catalysts. However, monometallic Ni catalysts exhibit at low temperatures a low deoxygenation activity but high hydrogenation activity, resulting in saturation of the phenyl ring, and at high temperatures a hydrogenolysis activity toward the C-C bond, resulting in a high methane yield and lowered aromatic selectivity.…”

“…Density functional theory (DFT) calculations have been performed to understand the mechanism of HDO of phenolics on different metal surfaces. 24,26,[39][40][41] On less oxophilic closed packed metal surfaces, such as Pt(111), Ni(111) and even Ru(0001), the DDO of phenol has a high activation barrier which makes it unlikely to take place under mild conditions. 24,39,41 Doping an oxophilic metal into those metals results in a surface alloy, which was found to lower the activation barrier of DDO.…”

“…34,42,43 For example, the activation barrier of phenol dehydroxylation was lowered from 1.82 eV on pure Ni(111) to 0.98 eV on a Re-doped Ni(111) surface, which makes the DDO channel accessible under mild conditions. 26 A stronger oxophilic metal is likely to react with O, forming metal oxides and segregating from the hydrogenating metal as oxide particles under the reaction conditions. However, few theoretical studies have been performed to understand the HDO of phenolics on oxophilic (reducible) metal oxide modified or supported metals.…”