Preparation,Characterization And Performance Of Mesoporous ZSM-5 Zeolite

ZSM-5 zeolite have several important traits,such as unique crystal structure,special pore structure and good thermal/hydrothermal stability,tunable acidity,specific selectivity for reactants and products molecular due to its relatively narrow pore diameter,therefore ZSM-5 are used in petroleum refining technology world widely as solid acid catalyst.However,the sole presence of micropores in the conventional ZSM-5 zeolites greatly hinders their utilizations in the reactions which involves the large molecule reactants such as heavy oil cracking etc.due to the diffusion and mass transfer limitations.In order to overcome the above disadvantages,many kinds of hierarchical ZSM-5 zeolites were synthesized successfully by various strategies,such as zeolite micro-crystal self-assembling by using mesoporous templates,mesoporous materials in situ zeolitization,nano-zeolite confinement synthesis etc.The hierarchical ZSM-5 zeolites include at least two pore structures,the inherent microporous structure and 2?50 nm mesoporous structure.The mesoporous structure is likely to be generated inside zeolite crystals,or generated between zeolite crystals.The synthesis of the hierarchical zeolite greatly expands the application range of conventional zeolite.Herein,we prepared mesoporous ZSM-5 zeolites based on the introduction of the organic matters in the amorphous walls of ordered mesoporous materials and then carbonization at high temperature.By using the carbide concreted to protect mesoporous channel and support frame construction,the ordered mesoporous silica materials were further recrystallized to mesoporous ZSM-5 zeolites.However,the original ordered mesopore structures could be wrecked during the in situ zeolitization process.Meanwhile,the rapid generation of zeolite nucleus have to be realized during the synthesis process and the growth rate of zeolite crystal must be carefully controlled.In another words,the protection and support of mesopores and the zeolite crystal nucleation and growth could be achieved simultaneously by using carbon materials.Two kinds of mesoporous ZSM-5 zeolites have been synthesized by different method.First strategy is by using organic carbon materials to interact with Si-OH inside in the pore wall and form a layer of carbon in original SBA-15 mesoporous channels.Silicon and aluminum layer in the pore walls are continuous.By connection of the nanosized zeolite crystal and the adjacent crystal in the same lattice orientation during in situ zeolitization process,the"rod-like" ZSM-5 zeolite was obtained.Another method is by using the organic carbon source interacting with Si-OH to form SBA-15 hole walls,and generating the discontinuous silicon and aluminum layer of hole walls.Due to the spatial confinement and blocking effect of carbon materials,the"microspheres" ZSM-5 zeolite was obtained.The cracking reaction of 1,3,5-isopropyl benzene(TIPB)was utilized to evaluate the accessibility of acid sites and the property of mesoporous structure.In spite of the strength of acidity has partly decreased,the cracking activity of TIPB has been enhanced due to the introduction of mesoporous structure which benefits the diffusion of TIPB etc.and the accessibility of acid sites in the channels.TIPB cracking reaction only occurred on the external surface of conventional ZSM-5 zeolite,resulting in a low TIPB conversion and reaction rate constant.However,the conversion of TIPB could be improve by 10?20%on hierarchical ZSM-5 zeolites HPZ-SIC-8h and HPZ-CSI-8h compared with conventional ZSM-5 zeolites.Aromatic hydrocarbon Friedel-crafts benzyl reaction was utilized to evaluate the accessibility of acid sites and the property of mesoporous structure on MZSM-5 zeolite,which was prepared by optimizing synthesis conditions.The conversion of benzyl chloride could reach to 47.3%in the benzyl reaction of naphthalene with benzyl chloride after 4 h in 343 K on the HMZSM-5 zeolite.Even after 4 times reused,the conversion of benzyl chloride still reserves at 43%.Compared with the conventional HZSM-5 zeolite and SBA-15,HMZSM-5 zeolite exhibited higher catalytic activity and better reusability in the catalytic reaction involved large molecules.Due to the fact that the relatively large pore in M-ZSM-5 does not obtain shape selectivity,thus the reaction rate of dibenzyl to isomer of ?-BN and ?-BN is the same on MZSM-5 zeolite.The selectivity of dibenzyl naphthalene increased with benzyl chloride conversion rate increasing.When the benzyl chloride conversion rate is 47.3%,the selectivity of dibenzyl naphthalene increases to 24.2%.