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

Abstract

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 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 be 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 explaining 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 Hdonor.

Elucidating the reactivity of methoxyphenol positional isomers towards hydrogen-transfer reactions by ATR-IR spectroscopy of the liquid–solid interface of RANEY® Ni

“…38 Recently, we have studied the adsorption of phenol on RANEY® Ni in presence of cyclohexane as the solvent at 40°C. 39 This study successfully revealed key features of co-adsorption of phenols, alcohols and polyols on the complex surface of this catalyst. Therefore, to provide in-depth insight into the impact of the position of the methoxyl substituent on the adsorption geometries, and their relationships with the selectivity towards H-transfer DMO, we examined the adsorption of the positional isomers at the catalytic solid-liquid interface employing ATR-IR spectroscopy.…”

Catalytic Transfer Hydrogenation of Biomass‐Derived Substrates to Value‐Added Chemicals on Dual‐Function Catalysts: Opportunities and Challenges

“…Several noble metals are known to be active for CÀOc leavage, and thus, supported on as olid acid/basef or deoxygena- [75] They found that the IR signal for the metalÀOb ond was very strong in the presence of 2-PrOH. They also confirmed the substrate inhibition effect,p articularly the strong adsorption of diols on the Ni surfaceduring tandem catalysis.…”

“…Meemken et al. conducted in situ characterization and identified key species on the catalyst surface during dehydrogenation . They found that the IR signal for the metal−O bond was very strong in the presence of 2‐PrOH.…”

“… Plausible surface reaction mechanism for C−O activation, a) on PdFe/Fe 3 O 4 catalyst, and b) possible Ni−O bond vibrational behaviors characterized by in situ IR for 2‐PrOH dehydrogenation . Reproduced with permission from The Royal Society of Chemistry and The American Chemical Society.…”

Catalytic Transfer Hydrogenolysis as an Effective Tool for the Reductive Upgrading of Cellulose, Hemicellulose, Lignin, and Their Derived Molecules

“…2-propanol was used as reaction solvent and hydrogen source and 32 model substrates at temperatures from 80 • C to 120 • C for 3 h were explored: the RANEY ® Ni catalyst shows a high performance under CTH conditions and a good stability in the recycling tests [122]. In the course of years, his research group deeply investigated the CTH of other lignin model molecules (including phenol) in the presence of different heterogeneous catalysts, also elucidating the role of the catalyst's surface on the H-transfer mechanism [123,124].…”