Facile adjusting the concentration of siliceous seed to obtain different HZSM-5 zeolite catalysts for effective catalytic depolymerization reaction of lignin

To improve lignin depolymerization reaction performance of HZSM-5 zeolite catalyst, we reported the effective catalytic depolymerization of rice husk lignin (RHL) to lignin phenolic monomer (LP) products over the different Si/Al mole ratios of HZSM-5 zeolite catalyst. Firstly, five samples comprising different Si/Al mole ratio of HZSM-5 zeolite catalyst were prepared by the physical method. The X-ray powder diffraction (XRD), scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET), pyridine adsorption Fourier transform infrared spectroscopy (FT-IR), and temperature-programmed desorption of ammonia (NH₃-TPD) were used to characterize various catalysts. Secondly, gas chromatography–mass spectrometry (GC-MS), 2D heteronuclear single quantum coherence (2D-HSQC), and gel permeation chromatography (GPC) were used to characterize various catalytic depolymerization products. The effect of different Si/Al molar ratios on the lignin conversion to LP products and the structural changes of residual lignin were analyzed. Finally, according to the results of LP products, we discovered that after the use of different Si/Al molar ratios of HZSM-5 zeolite catalysts, the content of acid in the different catalysts which increased with decreasing the Si/Al mole ratios. Among them, the A4P PM products (A4P: having a saturated or unsaturated aliphatic side chain in C4 position) increased from 0.58% (SA560, Si/Al: 560) to 1.83% (SA70, Si/Al: 70), the C6P PM products (C6P: having no side chain in C4 position of G, S, and H structure) increased from 1.53% (SA560, Si/Al: 560) to 3.41% (SA70, Si/Al: 70). Finally, the highest 6.05% PM products yield appeared in the SA70 (Si/Al: 70) catalytic system. It is worth noting that PM products produced by this HZSM-5 zeolite catalyst SA70 have great potential to be a promising renewable alternative to fossil resources.