Dehydration of methanol to dimethyl ether over Nb 2O 5 and NbOPO 4 catalysts: Microcalorimetric and FT-IR studies

Abstract

The unique properties of Nb 2O 5 as a solid acid have led to catalytic applications in various reactions catalyzed by acids. The surface acidity and stability of Nb 2O 5 can be enhanced by the addition of phosphate ions. In this work, amorphous Nb 2O 5 and NbOPO 4 samples were synthesized. The interactions of ammonia, methanol, water and dimethyl ether with Nb 2O 5 and NbOPO 4 were investigated by means of adsorption microcalorimetry, adsorption infrared spectroscopy (FT-IR) and temperature-programmed desorption (TPD) techniques. The results of microcalorimetry and FT-IR for NH 3 adsorption have shown that NbOPO 4 is much more acidic than Nb 2O 5 due to its higher surface area, and that both Brønsted and Lewis acid sites are present on the surface of Nb 2O 5 and NbOPO 4. Water adsorption microcalorimetry results indicate that a small amount of water was strongly chemisorbed on Nb 2O 5 and NbOPO 4 while most of the adsorbed water corresponded to physical adsorption. The results of methanol adsorption microcalorimetry, methanol adsorption FT-IR and methanol TPD suggest that methanol is mainly strongly dissociatively adsorbed on Nb 2O 5 and NbOPO 4 to form methoxy species and DME could be produced from the dehydration of methoxy species. DME adsorption microcalorimetry and FT-IR showed that DME was mainly molecularly chemically adsorbed on Nb 2O 5 and NbOPO 4, while a small amount of DME was dissociatively adsorbed. The probe molecules (NH 3, methanol, H 2O and DME) used in this work were adsorbed more strongly on NbOPO 4 than on Nb 2O 5 because of the stronger acidity of NbOPO 4. In the reaction of methanol dehydration, although Nb 2O 5 and NbOPO 4 were not as active as a H-ZSM-5 zeolite, they exhibited 100% selectivity to the DME product and a good stability of the activity in the temperature range relevant to the reaction (453–573 K), without coke formation.