A novel family of solid acid catalysts: substantially amorphous or partially crystalline zeolitic materials Original Research Article

Hydrogels, typically used for the synthesis of highly crystalline ZSM-5 samples at 150°C and under autogenous pressure, were subjected to lower hydrothermal synthesis temperatures, i.e., from 25°C to 130°C. XRD analysis of the samples obtained at the various temperatures showed that for synthesis temperatures of up to 70°C, X-ray amorphous aluminosilicates were obtained, whereas treatment at 90°C produced a material exhibiting a 2% XRD crystallinity. Higher synthesis temperatures resulted in higher levels of % crystallinity. Evaluation of the samples as catalysts for the skeletal isomerization of linear butenes was carried out in a flow reactor at 500°C, MHSV=3 and a butene:nitrogen dilution of 1:10. The results showed that the % conversion of the linear butenes increases and the % selectivity to isobutene decreases with increasing autoclave synthesis temperature, with the highest yield of isobutene (23.4%) being achieved with the 2% XRD crystallinity sample. Higher crystallinities produced considerably lower yields of the branched alkene. The catalytic properties of the 2% XRD crystallinity sample were further evaluated as a function of reaction temperature, 350–500°C, at MHSV=1 and a dilution of 1:10. Highest yield of isobutene was obtained at 500°C and under these reaction conditions, the sample exhibited stable catalytic performance (31% yield) for up to 40 h on-stream. In contrast, under the same reaction conditions, an 86% crystalline ZSM-5 sample gave a 4.9% yield of isobutene. Work on partially crystalline ferrierite-based materials as catalysts for the same reaction was also carried out.