B-ZSM-5 catalysts were prepared by various modification methods with boric acid, including ion-exchange, impregnation and direct synthesis. The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), NH3-Temperature Programmed Desorption (NH3-TPD), N2 adsorption-desorption, Fourier Transform Infrared spectrometry (FT-IR), 27Al and 11B MAS NMR spectra. The results revealed that the weak acidity of catalysts was significantly increased by modification. The catalytic activity was measured in a fixed bed at 460°C for methanol to propylene (MTP) reaction. The results of MTP reaction showed a great increment of the propylene selectivity for the boron modified samples, especially for the directly synthesized B-ZSM-5 which also displayed high activity and selectivity towards C2 =-C4 = olefins. It was found that the remarkable selectivity strongly depended on the intensity of weak acidity.
Density functional theory (DFT) calculations were performed to study the configuration and electronic states of N2O and H2O adsorbed on the ion-ex changed ZSM-5 using the Gaussian98 program. We also calculated the adsorption energy of NO2 species and H2O ad sorbed on various ion exchanged ZSM-5. The calculated results indicated that Cu+, Co2+ and Ti3+ sites on ZSM-5 have strong affinity to N2O and these metal cations are stable against water poisoning.
Activity and selectivity of Ni/ZSM-5+Al2O3 catalyst (8 wt.% NiO) containing 30 wt.% Ni,H-ZSM-5 were compared to those of catalysts to which USY zeolite, Ni,H-ZSM-5 zeolite, and heteropoly acid were added (20%). The ZSM-5 containing catalyst modified with heteropoly acid was less active than those modified by zeolite (USY, ZSM-5). N-Hexane conversion was highest for the catalyst contg. 50 wt.% zeolite. The yield of hydrocracking products was higher for the catalyst contg. 50 wt.% ZSM-5 than that of catalyst containing 30 wt.% ZSM-5 and 20 wt.% USY zeolites. Modification of the ZSM-5 containing catalyst did not result in higher yield of isomerization products.
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