External surface modification of as-made ZSM-5 and their catalytic performance in the methanol to propylene reaction

Post-synthetic treatment of high-silica as-made ZSM-5 with organic template in the micropores was explored to reduce/remove the external surface acid density of ZSM-5. It is found that Na2H2EDTA treatment can selectively remove the surface Al atoms, but generates new acid sites (likely silanol nests) on the external surface. H3PO4 treatment is unable to remove surface Al atoms, while small amount of P is left on the external surface, which effectively decreases the acid density. The catalytic performance of the resultant materials is evaluated in the methanol conversion reaction. H3PO4 treatment can effectively improve both the catalytic lifetime and the stability of propene selectivity. This occurs due to a combination of the increased tolerance to the external coke deposition and the depressed coking rate (reduced side reactions). Na2H2EDTA treatment only prolongs the catalytic lifetime, resulting from the improved tolerance to the external coke deposition. Under the optimized H3PO4 treatment condition, the resultant ZSM-5 gives a catalytic lifetime of about 1.5 times longer than the precursor. Moreover, the propene selectivity is improved, showing a slight increasing trend until the deactivation.