Research progress on biomass catalytic pyrolysis via microwave effects combined with carbon-based catalysts

It is critical to discover a clean energy source to replace fossil fuels such as coal to meet the targets of “emission peak” and “carbon neutrality” in 2030 and 2060, respectively. Biomass is a kind of renewable energy that is rich in reserves and can be directly converted into fuel. Pyrolysis is a common way to maximize the value of biomass, and the composition and distribution of products can be adjusted by the addition of catalysts. Carbon-based catalysts have the advantages of low preparation costs and easy treatment after catalysis. However, they have the disadvantages of easy carbon deposition inactivation and low product selectivity. The competitiveness of carbon-based catalysts can be improved when combined with the microwave effect. Herein, the research status of microwave-assisted carbon-based catalysts for the pyrolysis of biomass is reviewed. This study primarily introduces the microwave heating theory principle as well as the microwave absorber and catalyst effect mechanisms on the microwave for pyrolysis. The limitations of metal catalysts and molecular sieve catalysts are analyzed, and the unique advantages of modified carbon-based catalysts in the field of microwave pyrolysis are proposed. Microwave heating uses microwave radiation to create heat in the internal particles of biomass; therefore, microwave pyrolysis has the advantages of a high heating rate, uniform heating, low energy loss, a high energy conversion rate, and instantaneous adjustment. The effects of different modification methods (metal loading method, chemical method, sulfonation, etc.) on the pore structure, oxygen-containing functional groups, and acidic groups of carbon-based catalysts and the characteristics of catalytic products are analyzed. Among them, the preparation method’s precipitation method is difficult to manage, and the repeatability is low. The impregnation method has the advantages of being an easy preparation process, being inexpensive, and having a large production capacity. The chemical process will remarkably alter the oxygen-containing groups and acidity of the biochar. Too much acidity causes carbon deposition and catalyst deactivation, whereas too few acidic oxygen-containing groups induce pyrolysis rate decreasing. Therefore, the raw materials used in sulfonation are not environmentally friendly. The application progress of microwave-assisted modified carbon-based catalysts in tar reforming and improving the properties of biomass pyrolysis products is summarized. Carbon-based catalysts combined with microwave heating can increase the yield of phenols and syngas and improve the quality of pyrolysis products. Herein, some suggestions on the problems existing in this research direction are put forward and prospected, which provides some reference for the selection and modification of carbon-based catalysts and the high-value utilization of biomass based on microwave catalytic pyrolysis.