Studies Of Acid Site And Para-selectivity On Nanoscale ZSM-5

ZSM-5 zeolite has unique property of selectitve catalytsis beacuse of the special pore and channel. Aconsiderable study was carried out on the properties of selectitve catalytsis over ZSM-5, especially in thepara-substituted alkyl-aromatics, in which many kinds of theories of selectitve catalytsis and selectivemodel were proposed. The research of selectitve catalytsis was mainely concentrationed on the microsacleZSM-5. Nanoscale ZSM-5 has some different character with microscale ZSM-5. Compared withconventional microscale HZSM-5, nanoscale HZSM-5 has larger external surface area, higher surfaceenergy, shorter diffusion path length, and anti-coke formation ability. The proportion of external acid sitesover nanoscale HZSM-5 was 32% in the total amount of acid sites, while that of microscale HZSM-5 was10%. There were more pore openings on nanoscale ZSM-5, which resulted in the different acid sites andthe distribution of acid strength compared with the microscale ZSM-5. In this study, the acid site andselective catalysis on nanoscale ZSM-5 were studied. Comparison of catalytic performance over nanoscaleHZSM-5 and microscale HZSM-5 showed that nanoscale HZSM-5 was different from microscale HZSM-5in catalytic performance for the larger reactant molecule such as 1, 3, 5-TMB, 1, 2, 4-TMB and o-Xylene.The different properties in reactivity and products distribution were appeared on the nanoscale HZSM-5because of the more acid sites in the external surface, which was different with microscale ZSM-5. Thecatalytic performance of toluene disproportionation, aklylation of toluene with methanol and isomerizationof p-Xylene were similar because of the small reactant molecule. This difference in reaction betweennanoscale HZSM-5 and microscale HZSM-5 was attributed to the different acid sites and reactantmolecule.Through comparing the catalytic performance with the amount of acid sites with different acidstrengths of HZSM-5, modified HZSM-5, TS-1 and SiO₂, we conclude that only the acid sites with strengthH₀≤+2.27 contribute to toluene disproportionation, although it's only a small part in the total acid amount.The acid sites with acid strength +4.8≤H_0≤+6.8 may be ascribed to silanol group in the HZSM-5.Combining the results measured by the modified titration with the NH₃-TPD curves, we ascribed the LT(Lower Temperature) peak to the acid sites with strength H₀ from +2.27 to -3.0 and HT peak to acid siteswith strength H₀≤-3.0. The silanol group of zeolite contributes to the areas of neither LT peak nor HT(Higher Temperature) Peak. The acid sites related to the reactivity in toluene disproportionation wasstudied and the result showed that the concentration of acid site did not linearly relate to the reactivity intoluene disproportionation.Silanization was widely used to modify the acid sites of ZSM-5 zeolites. The different silyaltion wasstudied by the probe reaction of toluene disproportionation. Comparisons of the CVD method underatmosphere pressure, flowed CVD, ultrasonic treatment, impregnation and chemical reaction deposition,Chemical reaction deposition was an efficient method to passivate the external acid sites of ZSM-5. 91%para-selectivity was obtained over the nanoscale HZSM-5 modified with two cycles of deposition while 92% para-selectivity was achieved on the microscale HZSM-5 modified with single cycle of deposition. Therelative crystailinties and ²⁹Si MAS NMR of modified catalyst were studied and the mechanism ofdeposition was also proposed. The silanization reduced the acid sites severely and relative crystallinityslightly. Most of deposition contributed to elongation of the O-Si bonds along the external surface, which did not redound to eliminate the external acid sites and contributed to the decrease in Q⁴ species of Si onthe surface of nanoscale ZSM-5. There was only a small percent of deposition contributing to annihilate theacid sites. The valid silyation of HZSM-5 with tetraethoxysilane was the partly hydrolyzed molecule oftetraethoxysilane reacting with the bridging hydroxyl groups. After calcined, the partly hydrolyzedtetraethoxysilane formed the new bond of Al-O-Si with the aluminium species of ZSM-5, which reducedthe framework aluminium and then caused the decrease in the reactivity of zeolites.Using modified Hammer indicators and cracking reaction of mesitylene and pseudocumene, the acidsites of catalysts Were studied. The adsorption property and catalytic performance in toluenedisproportionation, toluene methylation with methanol, isomerization of o-Xylene, isornerization ofp-Xylene over silicon modified HZSM-5 and parent HZSM-5 has been studied. It was showed thatnarrowing pore opening size was not a key factor to obtain the high para-selectivity in toluenedisproportionation. The enhancement of para-selectivity over nanoscale ZSM-5 was ascribed to thedecrease in the concentration of external acid sites to less than 0.02 mmol/g and the external acid sites withacid strength H₀≤-3.0 were annihilated. Combined the concentration of external acid sites with reactionperformance, the selective model was proposed that diphenylmethane was formed in the ZSM-5 channelswhen toluene was reacted; and then intermediates decomposed under a concomitant evolution of xylene.The origin of para-selectivity was that the p-Xylene was formed in the internal channel and diffused out ofthe channel and then reacted on the external acid sites. If the external acid sites was passivated, thep-Xylene escgaped from the surface of zeolites and formed the final products. The external acid sites,adsorption property and catalytic performance in toluene disproportionation over silicon and magnesiummodified HZSM-5 was carried out, which validated the correctness of selective model.