Dehydration of methanol to dimethyl ether over Nb2O5 and NbOPO4 catalysts: Microcalorimetric and FT-IR studies

The unique properties of Nb2O5 as a solid acid have led to catalytic applications in various reactions catalyzed by acids. The surface acidity and stability of Nb2O5 can be enhanced by the addition of phosphate ions. In this work, amorphous Nb2O5 and NbOPO4 samples were synthesized. The interactions of ammonia, methanol, water and dimethyl ether with Nb2O5 and NbOPO4 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 NH3 adsorption have shown that NbOPO4 is much more acidic than Nb2O5 due to its higher surface area, and that both Brønsted and Lewis acid sites are present on the surface of Nb2O5 and NbOPO4. Water adsorption microcalorimetry results indicate that a small amount of water was strongly chemisorbed on Nb2O5 and NbOPO4 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 Nb2O5 and NbOPO4 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 Nb2O5 and NbOPO4, while a small amount of DME was dissociatively adsorbed. The probe molecules (NH3, methanol, H2O and DME) used in this work were adsorbed more strongly on NbOPO4 than on Nb2O5 because of the stronger acidity of NbOPO4. In the reaction of methanol dehydration, although Nb2O5 and NbOPO4 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.

Graphical abstractThe probe molecules (NH3, methanol, H2O and dimethyl ether) used in this work were adsorbed more strongly on niobium phosphate (NbOPO4) than on Nb2O5 because of the stronger acidity of NbOPO4. In the reaction of methanol dehydration, although Nb2O5 and NbOPO4 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.Figure optionsDownload full-size imageDownload as PowerPoint slide