Bauxite residue as a catalyst for microwave-assisted pyrolysis of switchgrass to high quality bio-oil and biochar

Bauxite residue (BR) is a highly alkaline type of solid waste generated by the aluminum industry that poses a significant environmental risk upon disposal. However, BR is abundant in metals, especially iron, that offer the desired catalytic activity for microwave pyrolysis. Thus, this study aimed to use BR as a low-cost microwave absorber and catalyst for biomass microwave pyrolysis to obtain higher quality bio-oil and biochar. The addition of BR to switchgrass, the representative biomass, did not facilitate microwave absorption because most of the iron in the BR was in the form of goethite and hematite. However, the addition of an efficient microwave-absorbing catalyst (e.g., K3PO4 or clinoptilolite) to the BR triggered synergistic effects, increasing the micro-wave heating rate by similar to 346% compared to K3PO4 or clinoptilolite alone, which was attributed to the reduction of hematite and goethite to maghemite and/or magnetite. The addition of 10% BR to a mixture of 10% K3PO4 and 10% bentonite further triggered synergistic effects that resulted in the highest microwave heating rate of 439 degrees C/ min, which was a 211% increase compared to using 10% K3PO4 and 10% bentonite without BR, and doubled the Brunauer-Emmett-Teller (BET) surface area of the biochar, reduced the bio-oil acidity by up to 71% compared to that obtained using a single catalyst, and increased the alkylated phenols contents in the bio-oil by 339% compared to that produced without a catalyst. These results demonstrated that the synergistic effects of BR can only be triggered when mixed with another efficient microwave-absorbing catalyst.

Automated summary

The study demonstrates that the synergistic effects of BR can only be triggered when mixed with another efficient microwave-absorbing catalyst, such as K3PO4 or clinoptilolite. This mixture significantly increases the microwave heating rate, enhances the quality of biochar and bio-oil, and improves the overall efficiency of biomass microwave pyrolysis.