Study on the quality improvement of bio-oil prepared by catalytic pyrolysis of kraft lignin: the role of micropore/mesopore beta zeolites

Single micropore or mesopore commercial zeolites suffer from poor diffusion transfer and deoxygenation efficiency in catalytic pyrolysis of biomass, resulting in easy coking and inactivation of zeolite, and poor bio-oil quality. To solve the above problems, micropore/mesopore beta zeolites with both pore sizes were synthesized by the soft template method in this study and applied to catalytic pyrolysis of Kraft lignin (KL). The results showed that the oxygen content of bio-oil from micropore/mesopore beta zeolite-assisted pyrolysis was 11.12% under optimal conditions, which was a reduction of 9.22% compared to commercial beta zeolite. In addition, possible routes for KL pyrolysis at different pore sizes were proposed. In microporous channels (0.58~0.68 nm), pyrolysis mainly produced oxygenated compounds. A moderate mesopore pore size (2~10 nm) facilitated lignin cleavage as well as deoxygenation and dehydration reactions and promoted the cleavage of phenylpropane side chains to produce small-molecule gases and char. When the pore size was too large (10~30 nm), it would lead to the deactivation of the catalyst by excessive carbon accumulation. The suitable micropore/mesopore pore size range for catalysts used in KL catalytic pyrolysis for bio-oil preparation was 0.68~0.8 nm/2~10 nm. Further, through the chemical characterization of micropore/mesopore beta zeolites and bio-oils, the mechanism of pore size modulation on lignin pyrolysis and catalytic activity of catalysts was elucidated, which promoted the high-value application of lignin, and laid a practical and theoretical foundation for the biomass refining in pulp and paper black liquor.