Synthesis, modification and catalytic evaluation of high-silica zeolites
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Saudi Digital Library
High-silica zeolites of MFI type were synthesized by rapid crystallization method. The major factors affecting the hydrothermal synthesis were screened using Plackett-Burman experimental design. The pH of the crystallizing mixture was found to be most important factor. A pH of 10.0+0.5 was found to be the optimum for the crystallization method were compared with that of conventional method. A narrow particle size distribution was found in the case of rapid crystallization method. This finding was rationalized on the basis of nucleation rate ralative to the crystallization rate. The synthesized zeolites were modified by varying Si/A1 ratio, metal incorporation and steaming. A quantitative relationship was found between the Si/A1 ratio of the gel and that of the zeolite product. The least square fit to the data gave: (Si/A1)zeolite = 3.53 + 0.777(Si/A1)gel with a correlation coefficient of 0.999. This was consistent with the silica having higher solubility than aluminosilicate species and that the pH of the hydrogel, within the range of Si/A1 ratios investigated, was constant. The synthesized and modified zeolites were characterized by FTIR, XRD, SEM, ESR, thermal analysis and particle size distibution analysis. Acidity measurements of some of the synthesized and modified zeolites were performed by TPD of ammonia and FTIR of absorbed pyridine. A bench scale high-pressure flow reactor system was designed and fabricated for the catalytic evaluation of the high-silica zeolites. Some of the selected zeolite catalysts were evaluated for MTBE synthesis and methanol to olefins (MTO) conversion. In the case of MTBE, the effect of temperature, Si/A1 ratio, space velocity and methanol to isobutene molar ratio was investigated for the conversion of isobutene. Higher yields of MTBE were obtained at 80°C for lower Si/A1 ratio. The space velocity and molar ratio had minor effect. The acidity of the zeolite was correlating with the conversion of isobutene. Steam-modified zeolite performed better in term of selectivity for MTBE than commercial Amberlyst-15 resin catalyst. The yield of MTBE was comparable for both catalysts. The higher activity of the steamed catalyst was explained on the basis of enhanced acidity. Steaming at higher severity caused reduction in activity resulting from excessive loss of acid sites. The results of catalytic evaluation for MTO showed that Fe-containing synthesized zeolite gave highest selectivity for the C₂ - C₄ olefins. This was attibuted to the dilute acidity of the catalyst.