Reply to “Comment on 'Instantaneous Dissolution of Cellulose in Organic Electrolyte Solutions'”

Reply to “Comment on 'Instantaneous Dissolution of Cellulose in Organic Electrolyte Solutions'

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

Mechanocatalytic Depolymerization of Lignocellulose Performed on Hectogram and Kilogram Scales

Mechanocatalytic depolymerization of lignocellulose constitutes a new frontier in biorefining. In a one-pot process, the combination of mechanical forces and acid catalysis leads to the full conversion of (dried) lignocellulose into water-soluble products (oligosaccharides and lignin fragments). In solution, these products undergo hydrolysis and other reactions, rendering high yields of monosaccharides along with precipitation of a sulfur-free lignin. Therefore, the water-soluble oligosaccharides may serve as a unique replacement for glucose and xylose for the production of platform chemicals. In this work, we report the results obtained from mechanocatalytic depolymerization of α-cellulose, beechwood, and poplar wood performed in Simoloyer mills operating on hectogram and kilogram scales. Irrespective of the process scale, full conversion of the substrate into “water-soluble (ligno)­celluloses”, within milling durations of 1–3 h, is achieved. A phenomenological analysis of parameters for the process upscaling is provided. Moreover, the energy consumption of the process on different scales is assessed. Remarkably, energy consumption significantly decreases (from ca. 200 MWh·t^–1 to 9.6 MWh·t^–1) with upscaling of the experiment from 1 g (planetary mill) to 1 kg. This increase in energy efficiency constitutes key evidence for the feasibility of the mechanocatalytic depolymerization on a large scale

Mechanocatalytic Depolymerization of Dry (Ligno)cellulose As an Entry Process for High-Yield Production of Furfurals

Driven by mechanical forces, the acid-catalyzed depolymerization of solid biomass completely overcomes the problems posed by the recalcitrance of lignocellulose. The solid-state reaction leads to water-soluble oligosaccharides, which display higher reactivity than cellulose and hemicellulose. Here, we show that water-soluble oligosaccharides are useful feedstock for the high-yield production of 5-hydroxymethylfurfural (HMF) and furfural in biphasic reactors. This is because they readily undergo hydrolysis upon microwave heating, selectively forming monosaccharides as intermediates in the aqueous phase. Short reaction times are possible with the use of microwave heating and limit the extent of degradation reactions. This work provides an ionic-liquid-free approach to process lignocellulosic substrates into HMF and furfural with high yields. In fact, starting this novel approach with α-cellulose, yields of HMF of 79% and furfural of 80% at 443 K for 9 min were obtained. The processing of real lignocellulose (e.g., beechwood and sugar cane bagasse) also achieved high yields of HMF and furfural. Thereby, the current results indicate that the process limitation lies no longer in the recalcitrance of lignocellulose, but in the extraction of highly reactive HMF and furfural from the aqueous phase in the biphasic reactor