Phosphorus induced hydrothermal stability and enhanced catalytic activity of ZSM-5 in methanol to DME conversion

The change in properties of ZSM-5 samples was achieved by treatment with phosphorus compounds (trimethyl phosphite or phosphoric acid) and the resultant materials were characterized by N₂ adsorption, NH₃-TPD, ²⁷Al, and ³¹P MAS NMR techniques. The phosphorus-treated HZSM-5 (P/ZSM-5) samples exhibited lower acidity, higher hydrothermal stability and improved dimethyl ether (DME) selectivity in methanol conversion when compared to the phosphorus-free HZSM-5. ²⁷Al, and ³¹P MAS NMR results revealed that the added P indeed interacted with the ZSM-5 framework and is responsible for the changes observed in the catalytic properties. The interaction caused the decrease in strong acid sites on one hand and creation of new acid sites (NH₃-TPD) on the other, in P/ZSM-5 samples. The studies indicated the need of optimizing the P loading, where the positive role of P on the catalytic activity was observed to be maximum at P/Al molar ratio of 1.05.