Effect of crystal size on the acidity of nanometric Y zeolite: number of sites, strength, acid nature, and dehydration of 2-propanol

Abstract

Little is known about how the variation in crystallite size of zeolites affects acidity. There is currently a tendency to decrease the size of materials because of the resulting improvements in textural properties, and Y zeolite is no exception. Therefore, in this work, the impact of the crystal size of Y zeolite with different Si/Al ratios on its acid properties was studied. In all cases, X-ray diffraction showed the presence of the faujasite phase. Nevertheless, the crystallinity of the acid zeolites decreased as the crystal size got smaller, especially in the protonated zeolites and those that underwent dealumination. Similarly, the nitrogen isotherms showed that there was a loss of microporosity in the smaller acid zeolites. The acidity determined by TPD-NH₃ evidenced a greater number of acid sites for the larger zeolites, while IR-Py showed mostly Brønsted sites in the lanthanide-exchanged zeolites. The dehydration of 2-propanol revealed a greater number of acid sites for larger crystallite size zeolites, which was associated to an improvement in conversion. This test also demonstrated a lower confinement effect of smaller zeolites on the 2-propanol molecule, as shown by a favored shape selectivity towards propene at low temperatures.