Study On The Preparation Of Aromatic Compounds By Catalytic Fast Pyrolysis Of Hemicellulose

Biomass energy is a rich renewable energy.However,it is still a great challenge to utilize biomass energy in an economic and effective way.Rapid pyrolysis of biomass is an effective and feasible method to selectively convert lignocellulose into liquid fuel bio oil.Among them,catalytic fast pyrolysis technology can efficiently convert biomass resources into platform chemicals with high added value,which is a technology with great development potential.In this paper,xylan,a model compound of hemicellulose,was selected as raw material,and HZSM-5 molecular sieve was used as catalyst carrier.The effects of catalysts prepared by different methods of modifying HZSM-5 on the product distribution of catalytic pyrolysis of xylan to aromatic compounds were studied The product distribution of catalytic pyrolysis was analyzed by py GC/MS,and the modified zeolites were characterized by X-ray diffraction（XRD）,N₂ adsorption desorption,temperature programmed ammonia desorption（NH₃-TPD）and pyridine infrared（Py-IR）.The main work and conclusions are as follows（1）The effects of HZSM-5 Catalysts Modified by different metals on the rapid pyrolysis of xylan to aromatic compounds were studied.HZSM-5 was modified by metal nitrates（Ni,Ga,Co,Cu and Ca）.The characterization results showed that the metal was evenly dispersed in the internal pores and structural surface of zeolite,and the original MFI structure was not damaged.The total acidity decreased,the number of strong B acid centers decreased,and L acid increased slightly.The results showed that the relative yield of monocyclic aromatics was Ga>Ca>Co>Ni>HZSM-5>Cu,and the selectivity of BTX was Ni>Ga>Cu>Co>Ca>H.The total yield of aromatics was 91.52%,the relative yield of monocyclic aromatics was 54.43%,and the selectivity of BTX was 77.92%.The quality and yield of bio oil were effectively improved.After Ni modification,the selectivity to BTX is the highest,reaching 85.92%.The quality of bio oil is greatly improved,but the relative yield of aromatic compounds is almost not improved.（2）The acid active sites of the catalysts were adjusted and the catalytic effect was improved after modified by different metals.On this basis,the effects of different Ga loading（1,3,10 wt%）on the catalytic pyrolysis of xylan to aromatic compounds were further studied.With the increase of loading amount,the relative yield of total aromatics increased at first and then decreased,and the change trend of monocyclic and polycyclic aromatics was the same.When the loading was 1 wt%,the selectivity of BTX was the highest,reaching 82.11%;when the loading was 3 wt%,the relative yield of aromatics and the selectivity of BTX in bio oil were effectively improved,and the overall catalytic effect was the best;when the loading was 10 wt%,the yield of aromatics and the selectivity of BTX were decreased,indicating that the loading had a certain impact on the quality of bio oil.（3）The effects of different concentrations of Na OH（0.01,0.05,0.1 mol/L）on the preparation of aromatic compounds from xylan were studied.After alkali treatment,the structure and acid distribution of the zeolite were adjusted,the crystallinity decreased,and the metal Ga was evenly dispersed in the internal channels and structural surface of the zeolite.The results show that the introduction of mesoporous can improve the mass transfer performance of reactants and increase the yield of total aromatics.When the base concentration was 0.05 mol/L,the total yield of aromatics was 96.5%,the relative yield of monocyclic aromatics was only 27.74%,and the selectivity of BTX was 46.82%.After metal loading,the total yield of aromatics further increased to 98.05%,but the relative yield of monocyclic aromatics decreased to 26.66%,the selectivity for BTX also decreased,and the selectivity for indene compounds was higher,indicating that the overall acidity and pore size of the catalyst had a great influence on the selectivity of products.To sum up,HZSM-5 catalyst prepared by different methods changed its pore structure and acid distribution,adjusted its catalytic activity,improved the composition and distribution of products,and improved the quality of bio oil.