Abstract
The new zeolite, MCM-22, was invented in Mobil Oil Company in 1990. This zeolite has unique structure which was consisted with two two-dimensional and independent ten-member-ring (10 MR) channels. One channel is sinusoidal through the structure, and the other channel contains twelve-member-ring (12 MR) supercage (7.1Ǻ*7.1Ǻ*18.2Ǻ). Especially, this zeolite contains 12 MR pocket structure on its external surface. This special structure contributes large external surface and the active sites exhibit excellent activity due to no diffusion limitation of reactants and products. In this work, we synthesized this new zeolite successfully under controlling conditions: SiO2/Al2O3 = 25; OH-/ SiO2 = 0.1; Na+/SiO2 = 0.2; HMI/SiO2 = 0.35; H2O/SiO2 = 25. The as-synthesized MCM-22 shows disc-like platelets with a diameter of less than 1 □m and a thickness of less than 0.1 □m. The surface area of this zeolite is 510 m2/g. Comparison the acid strength of ZEM-5 and □ zeolte, it exhibits weaker acid strength. However, we changed the SiO2/Al2O3 mole ratio from 25 to 15 and 35, another zeolites, ZSM-35 and ZSM-5, were synthesized at the same operating conditions. Owing to the molecular size of cyclohexanone oxime (0.73 nm) and trimethylbenzene (0.84 nm), the further research of these reactants over MCM-22 is explored in this work. A comprehensive study has been made on the Beckmann rearrangement of cyclohexanone oxime (CHO oxime), catalyzed by MCM-22 under various operating conditions (temperature, carrier gas, concentration, solvent and WHSV). A reaction pathway was proposed which invokes the main rearrangement reaction of CHO oxime accompanied by two side reactions, namely ring opening reaction of ε-caprolactam to form 5-hexenenitrile (coke precursor) and hydrolysis reaction to form cyclohexanone. The relative rates of these reactions, which would change under various operating conditions, affect catalyst stability and selectivity of products. The incorporation of platinum catalyzes the hydrogenation of 5-hexenenitrile and favor ring opening, which resulted in low ε-caprolactam selectivity. It is concluded that the selectivity of ε-caprolactam and the catalytic stability of H-MCM-22 may be improved by operating at a specified reaction temperature of ca. 653 K and particularly by using cyclohexanone or ethanol as the solvent of CHO oxime feed. By comparing with existing reports for 12-MR zeolites, we conclude that the effect of ethanol solvent over H-MCM-22 behaves more like that of 10-MR zeolites. A comprehensive study has been made on the hydrocarbon transformation of trimethylbenzene catalyzed by MCM-22 under various conditions concluding types of bifunctional catalyst, nature of bifunctional catalyst such as the amounts of metal and acidity of zeolite, and temperature etc.). The reaction pathway was proposed which invokes hydrogenation/dehydrogenation of aromatics/cyclohexane, isomerization and ring opening of C9-naphthene and secondary cracking reaction of C9-paraffins. The relative rates of these reactions decreased in the order of following: hydrogenation > isomerization > ring opening >> secondary cracking reaction under temperature range from 503 to 543 K. The C-C bond secession was the major conducted by acid sites. It is found that the alkyl position of the alkyl-benzene plays an important role on the reaction rate and product distribution during ring-opening reaction. The more reactive 1, 2, 4-TMCH, which converted from 1, 3, 5-TMCH, further proceed ring opening reaction.