Applications of lignin-derived catalysts for green synthesis

This review intends to introduce the application of lignin-derived catalyst for green organic synthesis over latest two decades and aims to present a renewable alternative for conventional catalyst for future industry application. The structure of lignin is initially introduced in this review. Then, various pretreatment and activation technologies of lignin are systematically presented, which includes physical activation for the formation of well-developed porosity and chemical activation to introduce catalytic active sites. Finally, the catalytic performances of various lignin-derived catalysts are rationally assessed and compared with conventional catalysts, which involves lignin-derived solid acids for hydrolysis, hydration, dehydration(trans)esterification, multi-component reaction and condensation, lignin-derived solid base for Knoevenagel reaction, lignin-derived electro-catalysts for electro-oxidation, oxygen reduction reaction, and lignin-derived supported transition metal catalysts for hydrogenation, oxidation, coupling reaction, tandem reaction, condensation reaction, ring-opening reaction, Friedel-Crafts-type reaction,Fischer–Tropsch synthesis, click reaction, Glaser reaction, cycloaddition and(trans)esterification. The above lignin-derived catalysts thus successfully promote the transformations of organic compounds, carbon dioxide, biomass-based cellulose, saccharide and vegetable oil into valuable chemicals and fuels. At the end of this review, some perspectives are given on the current issues and tendency on the lignin-derived catalysts for green chemistry.

Numerical Investigation on Downstream Increase in Peak Discharge of Hyperconcentrated Floods in the Lower Yellow River

Hyperconcentrated floods in the Yellow River usually accompanied with some peculiar phenomena that cannot be explained by general conceptions of ordinary sediment-laden flow (e.g., downstream increase in peak discharge, instability flow, ripping up the bottom). Up to date, the mechanisms for the abnormal phenomena are not well understood. The aim of this paper is to facilitate a new insight into the abnormal downstream increase in peak discharge of hyperconcentrated floods in the lower Yellow River. Numerical model experiments have been conducted on a typical flood occurred in August 1992 in the Lower Yellow River during which the peak discharge at Huayuankou station was 1690 m3/s larger than the value at Xiaolangdi station at upstream. It is found that a fully coupled model that incorporates the contribution of bed evolution to the mass conservation of the water-sediment mixture, can reasonably well capture the characteristics of peak discharge rise and severe bed scour, while separate numerical experiment using a decoupled model, which ignores the feedback effects of bed evolution, shows no rise in the peak discharge. This leads us to comment, if only briefly, that the entrainment of sediment due to bed erosion is the main reason for causing peak discharge increase along downstream course.