Synthesis, Characterization And Catalytic Performance Of MCM-41 And Si-Al Micro-Mesopore Composited Zeolite

A series of MCM-41 and Si-Al composited zeolites were synthesized byhydrothermal method and characterized by XRD, BET, IR, SEM, TEM,TG-DTA and NH₃-TPD techniques. The catalytic cracking of 1,3,5-triiso-propylbenzene were carried out over the zeolites.(1) The best synthesis conditions of MCM-41 zeolites synthesized byhydrothermal method in laboratory were nSiO₂/nAl₂O₃=70, nCTAB/nSiO₂=0．12, pH=9, crystal temperature 120℃and crystal time 56h; The bestsynthesis conditions of Si-Al composited zeolites by hydrothermal method inlaboratory were nSiO₂/nAlO₃=90, nCTAB/nAl₂O₃=22, nTEAOH/nAl₂O₃=30,nH₂O/nAl₂O₃=1400, crystal temperature 120℃and crystal time 12～84h。(2) The result of N₂ adsorption-desorption showed that MCM-41 zeolitehad symmetrical aperture of 2．8nm and Si-Al composited zeolite had bothmicropore and mesopore structure. The surface area of Si-Al compositedzeolite reached 1120m²/g, larger than 782 m²/g of MCM-41 zeolite.(3) The SEM photos showed that crystalloid of MCM-41 zeolites grew all round, the grain of crystalloid was homogeneous and the dimension of themwas nearly 10μm. The crystalloid grain of Si-Al composited zeolite assembled,small grain grew over big grain, and the aggregation of some grain made thecircumscription blurry.(4) The TEM photos showed that the aperture section of MCM-41 zeolitelike dactylogram and aperture was bigger than 2nm, which was similar to2．8nm of N₂ adsorption-desorption result. The Si-Al composited zeolite haddense aperture and inhomogeneous dimension, which validated that the Si-Alcomposited zeolite was really micro-mesopore structure.(5) Pyridine adsorbed IR showed that, the Si-Al composited zeolite hadless B and L acidic sites and weaker acidity than ZSM-5 zeolite. The result ofNH₃-TPD indicated the acidic amount of Si-Al composited zeolite decreasedgradually with the increase of nSiO₂/nAl₂O₃．(6) The TG-DTA result demonstrated that the framework of MCM-41zeolite collapsed at 780℃, while that of Si-Al composited zeolite collapsed at900℃, so the Si-Al composited zeolite had better heat stability.(7) Initial catalytic convertion of the HMCM-41 zeolite for the crackingof 1,3,5-triisopropylbenzene was 56．1%, and the convertion declined with theincrease of nSiO₂/nAl₂O₃．The selectivity of benzene, m-diisopropylbenzeneand p-diisopropylbenzene decreased with the increase of nSiO₂/nAl₂O₃．The1,3,5-triisopropylbenzene had been converted almost completely over the Si-Al composited zeolite with nSiO₂/nAl₂O₃ of 30～150．The selectivity ofbenzene and isopropylbenzene decreased and the selectivity ofm-diisopropylbenzene and p-diisopropylbenzene increased gradually with theincrease of nSiO₂/nAl₂O₃．(8) Initial catalytic activities for the cracking of 1,3,5-triisopropylbenzeneover HZSM-5, HMCM-41, Si-Al composited zeolites and mechanical mixedzeolites with same nSiO₂/nAl₂O₃ indicated that the Si-Al composited zeolitepossessed the highest activity, and almost all 1,3,5-triisopropylbenzene hadbeen converted absolutely, and the selectivity of all production were reallyhigh, which prove the Si-Al composited zeolite has pure mico-mesoporestructure, and it was different from HMCM-41 and mechanical mixed zeolites,its catalytic cracking performance was superior to micopore zeolites,mesopore zeolites and mechanical mixed zeolites. The initial convertion ofHMCM-41 was 34．6%, the selectivity of production benzene, m-diisopropyl-benzene and p-diisopropylbenzene were not high. HZSM-5 had the lowestconvertion 15．9%, and the production only had m-diisopropylbenzene. Themechanical mixed zeolites ZM1 showed higher convertion than ZM2 becausethe content of ZSM-5 in ZM1 was lower than that in ZM2．...