Study On Catalytic Biomass Pyrolysis To Prepare High Value Added Chemicals At Methanol Atmosphere

Pyrolysis is a technology for efficient conversion of biomass to bio-oil which can be used not only as fuel oil,but also as important source of industrial materials.Chinese medicine is a unique medical culture in China.With the deepening development and utilization of Chinese medicine resources,the Chinese medicine industry has developed rapidly,which has brought about the utilization problem of herb residue of Chinese medicine.The catalytic pyrolysis of herb residue of Chinese medicine can obtain a large amount of high value added chemicals and efficiently realize the resource utilization,which could be expected to become an important way for the utiluzation of Chinese medicine in the future.In this article,lemon grass after extraction（hereinafter referred to as“lemon grass”）was used as the representative to study the pyrolysis of herb residue of Chinese medicine.Experiments were first carried out under nitrogen,ammonia and methanol atmospheres respectively to explore the effect of atmosphere on the distribution of pyrolysis products and the composition of bio-oil.It was found that the yield of bio-oil obviously increased and the components significantly changed under methanol atmosphere.Then experiments were carried out at different temperatures.It was found that under low temperature,the decomposition of raw materials was inadequate,and at high temperature,the decomposition of raw materials would be excessive,both of which would lead to the reduction of bio-oil yield.450°C was the optimal temperature to obtain the highest bio-oil yield.Therefore,“methanol atmosphere”and“450°C”were selected as reaction conditions for subsequent study.Composite catalyst of nickel-aluminum double metal oxide loaded on MCM-41（NiAl-LDO/MCM-41）were prepared with hydrothermal coprecipitation and then applied to lemon grass pyrolysis under methanol atmosphere.The addition of NiAl-LDO/MCM-41 in lemon grass pyrolysis increased the bio-oil yield from 11.25%to12.42%.In addition,the content of non-aromatic oxygeneous compounds sharply decreased from 70.58%to 5.70%and the content of aromatic compounds significantly increased to 93.26%.This result was mainly attributed to combination of NiO and Al₂O₃,which effectively promoted the complete cleavage of chemical bonds of macromolecular alcohols and acids.The increase of loading content on catalyst reduced the specific surface area and pore volumn,and led to the increase in the weak acid amount and the decrease in the strong acid amount.A weak acid facilitated the reaction of the methoxy group with the benzene ring to form a methoxyphenol-containing,medium-strong acid and strong acid contributed to the deoxygenation reaction,which eventually led to a decrease in the content of naphthalenes and esters in the bio-oil and an increase in the content of methoxy-containing phenols.ZSM-5 is a common microporous molecular sieve with strong deoxidation and aromatization effect.ZSM-5 with low ratio of SiO₂/Al₂O₃ had strong acidity and strong deoxy-aromatization ability according to NH₃-TPD test,so ZSM-5（SiO₂/Al₂O₃=27）was selected as catalytic carrier to prepare NiAl-LDO/ZSM-5（x M）.Based on the previous study,ZSM-5 was treated with alkaline solution to form some mesoporous pores and nickel-aluminum double metal oxide were then loaded on the treated ZSM-5to prepare NiAl-LDO/ZSM-5（x M）（x is the concentration of alkaline solution）.NiAl-LDO/ZSM-5（0 M）were applied to lemon grass pyrolysis and obtained 13.42%of bio-oil.NiAl-LDO covered the strong acid sites of ZSM-5,which led to the disappearance of medium-strong acid in the catalyst and the increase of weak acidity and amount resulting in the increase of benzenes content from 34.06%to 43.15%.The naphthalenes content in bio-oil decreased from 63.31%to 23.15%due to the significant decrease in the strong acid amount of the catalyst.As the concentration of alkali solution increasesd,the amount of strong acid and acid in the catalyst increased continuously,but the acid strength gradually decreased.The amount of weak acid increased first and then decreased,and the acid strength increased continuously.As a result,the content of aromatic compounds in pyrolysis bio-oil increased,and the content of oxygeneous compounds decreased.