Tuning Zeolite Properties for a Highly Efficient Synthesis of Propylene from Methanol

A series of nanosized ZSM-5 samples was synthesized at 170, 150, 120, and 100 degrees C. Experimental data show that the decrease of crystallization temperature leads to significant changes in zeolite properties. Crystals synthesized at 100 degrees C exhibit many framework defects with lower acid-site density, strength, and a larger external surface area. The selectivity to light olefins and the propylene-to-ethylene ratio increases as the crystallization temperature decreases. A propylene-to-ethylene ratio of above 6 with the highest selectivity to propylene of 53% was obtained over ZSM-5 catalyst prepared at 100 degrees C. The stability of the nanosized zeolite in methanol to olefins (MTO) was also improved compared to the industrial sample with a similar Si/Al ratio. This catalytic performance is a result of the decrease in the acid-site density, strength, and the crystals' size, providing a shorter diffusion path and larger external surface area. The presence of structural defects and a different external surface in the crystals has been shown to play an important role in the MTO catalyst performance.