Liquid-Phase H-Transfer from 2-Propanol to Phenol on Raney Ni: Surface Processes and Inhibition

Raney Ni is perhaps the most widely used catalyst for the transformation of biogenic molecules in industrial practice (e.g., as in the production of sugar alcohols and hardening of vegetable oils). Currently, Raney Ni has found another key application; the catalytic upstream biorefining (CUB) of lignocellulose in which the soluble products released from the lignocellulosic matrix undergo reductive processes, rendering depolymerized lignin oils in addition to high-quality holocellulosic pulps. Despite the industrial importance of Raney Ni, its surface chemistry is poorly understood. In this study, using the H-transfer reaction between 2-propanol (2-PrOH) and phenol as a model reaction, we studied the influence of various alcohols on the catalytic performance of Raney Ni. For the H-transfer hydrogenation of phenol to cyclohexanol, the inhibition of the catalyst increases in the order of secondary alcohols < primary alcohols < polyols at 130 °C. To better understand the observed inhibition, we also studied the molecular interactions of the various alcohols at the catalytic solid–liquid interface using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The in situ spectroscopic data revealed that 2-PrOH adsorbs on at least two chemically different sites on the surface of Raney Ni. One of these two adsorption sites was attributed to the Ni site responsible for the saturation of the phenolic ring. The ATR-IR spectroscopic data also shows that the adsorption of phenol involves its hydroxyl group. Notably, the phenolic ring was found to be tilted with respect to the surface. Competitive adsorption of various other alcohols was also investigated at the catalytic solid–liquid interface. The presence of methanol inhibited the adsorption of 2-PrOH to a significantly greater degree than phenol. Therefore, it is proposed that hydrogen transfer hydrogenation of the phenolic ring is inhibited in the presence of additional alcohols mainly due to the competitive adsorption with 2-PrOH. Several polyols were found to interact through a bidentate interaction with the catalyst surface, which explains their stronger inhibition compared to primary alcohols. In a broader context, this study proposes the effect of hemicellulose sugars and sugar alcohols, formed in the CUB process, upon the product selectivity of CUB catalyzed by Raney Ni and using 2-PrOH as an H-donor