Solid-State NMR Studies On Methanol Conversion Over Heterogeneous Catalysts

In this dissertation, methanol-to-aromatics (MTA) reaction over silver exchanged HZSM-5 zeolites and methanol selective oxidation over supported vanadium oxide catalysts were studied by using solid-state NMR techniques including probe molecule techniques as well as quantum chemical calculation.Solid-state 13C NMR spectroscopy was used to study the conversion of methanol to aromatics over silver exchanged HZSM-5 zeolites. The experimental results show that in addition to the Br?nsted acid sites the silver oxide in the air-calcined AgZSM-5 zeolite also plays a crucial role for the aromatic selectivity in the MTA reaction. We found that the yield of aromaticson the air-calcined AgZSM-5 reaches 96.3%, while the yields of aromatics on the HZSM-5 and vacuum-calcined AgZSM-5 are only 61.8% and 53.6% at 673 K, respectively. 129Xe NMR spectroscopy was employed to characterize the state of silver species dispersed on the fresh and decayed AgZSM-5 catalysts and its function during the methanol-to-aromatics reaction. Based on our experimental results, A possible reaction mechanism is proposed as follows: the small alkanes (mainly propane and isobutane) were first produced from dimethyl ether, and then the isobutene and propane were dehydrogenated, rearranged and polymerized to form the aromatic products (mainly toluene and benzene). Deactivation of the air-calcined AgZSM-5 catalyst is mainly due to the gradual reduction of the silver oxide into the silver metal with time on stream.Methanol catalytic oxidation over VOx/Al2O3, VOx/ZrO2, and VOx/MgO catalysts has been studied by solid-state NMR spectroscopy. It was found that stronger acid sites in VOx/Al2O3 result in almost the same selectivities for DMM, paraformaldehyde and formic acid, and weaker acid sites in VOx/ZrO2 favor paraformaldehyde synthesis while the VOx/MgO catalyst with the base support shows high selectivity for formate. Supporting VOx species onγ-Al2O3 and ZrO2 leads to the formation of Br?nsted acid sites as revealed by the adsorption of probe molecules. The acid strength of Br?nsted acid sites on the VOx/Al2O3 catalyst is found to be stronger than that of the VOx/ZrO2 catalyst which has the similar acid strength to zeolite HZSM-5. The proposed bridging hydroxyl models accounting for the Br?nsted acid sites formation were also confirmed by quantum chemical calculation.Methanol selective oxidation over NaOH and HNO3 modified VOx/Al2O3 was studied by solid-state NMR spectroscopy. The NMR experimental results indicate that acid sites play a key role in the formation of dimethoxymethane during the methanol oxidation reaction. Comparing to the VOx/Al2O3 catalyst, stronger Br?nsted acid site on the acid-treated VOx/Al2O3 catalyst results in higher selectivity for dimethoxymethane, while the base-treated VOx/Al2O3 catalyst without Br?nsted acid site gives rise to formate instead of dimethoxymethane.