External And Internal Surface Diffusion Barriers Of Zeolites And Their Influence On N-Heptane Hydroisomerization

Zeolites-microporous crystalline aluminosilicates-have found numerious applications,such as the petroleum-refining and petrochemical industries,due to a microporous network,tunable acidity and hydrothermal stability on the one hand,on the other hand,its microporous structures adversely influence the diffusion performance and largely limit the applications in industry.For improving the performance,one approach is to synthesize zeolites with the shortened diffusion path length by various synthesis methods.Besides intracrystalline diffusion resistance,the surface barriers and internal grain boundaries could limit the diffusion performance.Although lots of efforts are being paid for investigating the surface barriers,the nature of surface barriers still is not clear;the limitation from internal grain boundaries is also evidenced,but scientific reports involving the impact of internal grain boundaries on the inhomogeneous catalysis are few.In this work,the diffusion processes occurring in the external surface and internal grain boundaries of zeolites were studied by Zero Length Column,and its effects on inhomogenerous catalysis of n-heptane hydroisomerization over Pt/HBeta were also investigated.Firstly,the molecule diffusion process of external surface of zeolites and its effects on catalysis were studied.Four ZSM-5 zeolites with different particle sizes were synthesized by hydrothermal method and their morphology and structure were characterized.Then ZSM-5 zeolites were successfully modified by surface etching and CLD of TEOS,respectively.Combining the ZLC measurement method,the influence of surface modifications on the diffusion performance of n-heptane in zeolites was studied in order to probe the nature of surface barriers.The results show that only the CLD of TEOS can largely increase the apparent diffusivities at different temperatures,while the surface etching seems not to change the behaviors of n-heptane diffusion.Thus,we conclude that the nature of surface barriers in our research system should be related with the energy barriers of diffusing into or hoping out of the surface microporous,rather than the external surface blockage.Then the effects of surface modification(CLD)on catalysis were investigated for further envidencing the impacts of surface barriers on inhomogenerous catalysis,but the experimental reults show the failure of surface modification on decreasing the surface barriers.Secondly,the impacts of internal grain boundaries on n-heptane diffusion and hydroisomerization were investigated.Two Beta zeoolites with different internal structures but other similar properties were synthesized by hydrothermal method.One sample possesses very orderedly single-crystalline structures and regular external shapes,the other is polycrystalline due to disordered internal structure and ill-defined external shapes,ZLC experimental results show that the single-crystalline Beta zeolites have a beter performance on diffusion compared with polycrystalline ones,because the internal grain boundaries limit the n-heptane diffusion.N-heptane hydroisomerization experiments at different temperatures over two Pt/HBeta catalysts,which were made by the impregnation method,show that their catalytic activities in low temperatures are very similar,while their catalytic differences increase with the increase of reaction temperature,especially in the temperature of 568 K,the TOF for single-crystalline catalysts is 129%higher than the one for polycrystalline catalysts.Therefore,the internal grain boundaries within Beta zeolites can adversely influence the catalytic performance of catalysts.