Studies On Supported Silver Catalysts Of Methanol Dehydrogenation To Anhydrous Formaldehyde And The Reaction Mechanism

Studies on supported silver catalysts of methanol dehydrogenation to anhydrous formaldehyde and the reaction mechanismThe formation of anhydrous formaldehyde by methanol dehydrogenation under the no-oxygen & catalytic conditions is studying by many reasearchers presently and the selection of catalyst plays a key role in the research process. The reasearchers have studied many elements and compounds to look for the catalyst of methanol dehydrogenation, such as metal, oxide, carbonate and zeolite and so on. But none of the catalysts has been applied into industry for some reasons. In addition, the reaction mechanism of methanol dehydrogenation is not clear for the various catalysts, which hampering us to optimize the catalyst ulteriorly. Silver catalyst has been widely applied into the formaldehyde formation from oxidative dehydrogenation of methanol. So it may be naturally used into the formation of anhydrous formaldehyde from methanol dehydrogenation. Therefore, the main purpose of this dissertation is to develop some new kinds of supported-silver catalysts and resolve the problems by other catalysts and then realize the industrial production of anhydrous formaldehyde. All the catalysts can be divided into two types according to the characterization results, one of which is named as non-Al-type with no Al element exists in the catalyst and the other is Al-type with Al element being in the catalyst. In addition, in order to elucidate the molecular reaction mechanism of direct dehydrogenation of methanol, electrolytic silver catalyst has been used as a probe for its simplest structure. The translation process of adsorbed oxygen species has been studies by using in situ Raman method and the active center is found. Then the methanol direct dehydrogenation mechanism will be completed and the molecular reaction mechanism of the oxidative dehydrogenation of methanol to formaldehyde will also be integrated. These results will be helpful to the industrial process.1. Non-Al type catalyst used in the methanol dehydrogenation to formaldehydeA series of supported-silver catalysts---Ag/SiO2-ZnO and Ag/SiO2-MgO were prepared by sol-gel method and the optimal preparation conditions were confirmed as follows: mixed alcohol and water as the solvent, water content fixed at 30%, calcined at 1000(C with flow air. Ag/SiO2-ZnO catalyst with Ag/Zn(mol)=10:1 shows the highest activity. But for the Ag/SiO2-MgO catalyst,the best condition is Si/Mg=10:1 and the proper silver content was 20%(wt.). Methanol conversion and formaldehyde yield are 82.3%and 73.2%, respectively with Ag/SiO2-ZnO catalyst applied into the reaction of methanol direct dehydrogenation and reacted at 700℃, which obvious higher than the patent reported. As for the Ag/SiO2-MgO catalyst, the highest methanol conversion and formaldehyde yield are 96% and 75%, respectively with the reaction temperature as 650(C, which is lower than that of Ag/SiO2-ZnO catalyst. All the catalysts have been characterized by a series of general method, such as BET, XRD, SEM and so on. It is found that these catalysts all have low BET area, which very appropriate to be applied into the methanol dehydrogenation to anhydrous formaldehyde. Silver exists as Ag0 state either before or after reaction in all the non-Al type catalysts. The difference is that silver particles on the Ag/SiO2-ZnO catalyst will be sintered after reaction for the high reaction temperature. As for the Ag/SiO2-MgO catalyst, silver particles just become a little bigger than that before reaction but not sintering for the lower reaction temperature. The special state of silver indicates that the active center of the non-Al type catalyst is Ag-O species, which is similar to that of electrolytic silver catalyst. So an effective way to regenerate the inactive catalysts is to calcine it in oxygen atmosphere under high temperature. Ag/SiO2-ZnO catalyst gains only 55% activity of the fresh one after regeneration because of the sintering after reaction. But the Ag/SiO2-MgO catalyst can get above 90% activity of the fr...