Mechanocatalytic depolymerization of lignocellulose presents a promising method for the solid-state transformation of acidified raw biomass into water-soluble products (WSPs). However, the mechanisms underlining the utilization of mechanical forces in the depolymerization are poorly understood. A kinematic model of the milling process is applied to assess the energy dose transferred to cellulose during its mechanocatalytic depolymerization under varied conditions (rotational speed, milling time, ball size, and substrate loading). The data set is compared to the apparent energy dose calculated from the kinematic model and reveals key features of the mechanocatalytic process. At low energy doses, a rapid rise in the WSP yield associated with the apparent energy dose is observed. However, at a higher energy dose obtained by extended milling duration or high milling speeds, the formation of a substrate cake layer on the mill vials appear to buffer the mechanical forces, preventing full cellulose conversion into WSPs. By contrast, for beechwood, there exists a good linear dependence between the WSP yield and the energy dose provided to the substrate over the entire range of WSP yields. As the formation of a substrate cake in depolymerization of beechwood is less severe than that for the cellulose experiments, the current results verify the hypothesis regarding the negative effect of a substrate layer formed on the mill vials upon the depolymerization process. Overall, the current findings provide valuable insight into relationships between the energy dose and the extent of cellulose depolymerization effected by the mechanocatalytic process.

The Front Cover illustrates a schematic representation of the mechanical parameters involved in the mechanocatalytic depolymerization of lignocellulosic materials in a ball mill. Owing to the circular jar movement, a ball follows a defined pathway through the milling vial, which allows the estimation of its velocity and, hence, the determination of the kinetic energy transferred to the substrate. This kinematic modeling approach results in an ultimate equation that is primarily dependent on the mill geometry and characteristics, number of the balls, milling time, and rotational speed. In this work, we demonstrate the existence of correlations between the apparent kinetic energy transfer and the yield of water‐soluble products (oligosaccharides and lignin oligomers) depending on the type of (ligno)cellulose substrate employed. More information can be found in the Full Paper by Kessler et al. on page 552 in Issue 3, 2018 (DOI: 10.1002/cssc.201702060).

Invited for this month′s cover is the group of Dr. Roberto Rinaldi from the Department of Chemical Engineering at Imperial College London. The Cover image is an artistic representation of the collision model for the ball mill operation effecting the mechanocatalytic depolymerization of (ligno‐)cellulose resulting in water‐soluble oligosaccharides (and lignin oligomers). The Full Paper itself is available at 10.1002/cssc.201702060.