Microwave-assisted catalytic pyrolysis of switchgrass for improving bio-oil and biochar properties

•Catalysts increased microwave biomass heating rate.•Microwave catalytic pyrolysis improved bio-oil and biochar quality.•Bio-oil yield increased while bio-oil viscosity and acidity reduced.•Produced biochar has a high BET surface area and high nutrient content.•Mixing K3PO4 with clinoptilolite/bentonite triggered synergistic catalytic effects.

Solid additives were used as a microwave absorber to improve the low microwave absorption rate of switchgrass going through pyrolysis, and as a catalyst to improve the bio-oil and biochar characteristics. The synergistic effects were manifested in the presence of a mixture of K3PO4 and clinoptilolite or bentonite compared with single catalyst, resulting in increased microwave absorption rate, and improved bio-oil and biochar quality. The sample of microwave heating switchgrass with 10 wt.% K3PO4 + 10 wt.% bentonite reached 400 °C after 2.8 min, compared with 28.8 min through conventional heating, producing biochar with increase in BET surface area from 0.33 m2/g to 76.3 m2/g compared with conventional heating. Furthermore, water content of the bio-oil reduced from 22.7 to 15.0 wt.% compared with biomass mixed with 20 wt.% SiC, a chemically-inert microwave absorbing material used to increase microwave heating. Introducing catalysts showed a great potential for accelerating microwave heating and improving bio-oil and biochar qualities.