Study On Catalyst Of Benzene Oxidation By N₂O And Inorganic Membrane Reactor For Controllable Oxidation

The partial oxidation of benzene to phenol (BTOP) by nitrous oxide in a single step has very important industrial and social significances .Although the reaction process has been studied for over ten years and many catalysts with high activity and selectivity has been developed , the fast deactivation of catalyst is still a big problem in industrial application. The aim of this paper was to study the cause and its regular of the catalysts deactivation and to expect to develop more active and stable ZSM5 catalysts.At the same time the subject of the reaction engineering in BTOP has been studied and the possibilities of increasing activity and stability of catalyst has been explored in the view of reaction engineering. The main research works and its results in this thesis are summarized as below:1. Zeolite ZSM5 with different Si/Al ratios was synthesized and was treated by steaming under different conditions. The composition, physical structure and surface character of those samples were analyzed and tested .Meanwhile its reaction performance and regular in the BTOP were evaluated studied in order to select a catalyst with high activity of BTOP reaction . The results indicate that in the range of Si/Al ratios from 40 to 120, activity of the sample decreases with increasing of their Si/Al ratios. After treated by high temperature steam, the initial selectivity of phenol increased obviously. Selected those samples and its high temperature steam treating conditions, a sample with Si/Al ratios=40 and treated in the 650-700℃ has high reaction activity: its initial yield of phenol achieves 4.0³.16 mmol.g^-1.h^-1, selectivity of phenol is 96%⁹8%.2. Coking and deactivating experiments of the catalysts selected were carried out in a fixed-bed reactor for benzene to phenol oxidation. The catalysts undergoing different reaction time were analyzed and characterized. The amount of coke was measured by TG. The coke components were detected by infrared and C¹3 NMR. At the same time, the main physics properties of the catalysts with carbon deposition were characterized. The pores of the catalysts with carbon deposition were measured by N₂ adsorption at low temperature. The amount of hexane and cyclohexane adsorption was measured by using a continuous flow adsorption unit. The acid sites of Bronsted and Lewis were characterized by pyridine adsorption-infrared spectroscopy. The above results show that the main cause to the deactivation of ZSM5 catalyst is that the carbon deposition on the catalyst surface causes choking in the mouth of pores of the catalyst. The Bronsted acid sites are the active sites which caused coke on ZSM5 in the experiments. The main components of coke on the catalysts are consist of aromatic with alkyl and multiring hydrocarbon. In addition, the coke also contains small amount of aromatic with hydroxy group which maybe caused by further oxidation, dehydrogenation and polymerization of phenol formed on Lewis acid sites.3. The catalyst selected above was further modified by chemical deposition of silicon, the effect of modification condition on the catalyst activation was studied. The chemical composition and surface Si/Al ratio of the samples, before and after modification, were detected by XRF and EPS techniques. The crystal phase was detected by XRD, and the crystal structure was detected by TEM. The pore structure of the catalysts ware measured by Nt adsorption at low temperature. The acid sites of Bronsted and Lewis were characterized by pyridine adsorption-infrared spectroscopy. The amount of hexane and cyclohexane adsorption was measured by using a continuous flow adsorption unit. In addition, using the method of Hammett, in which n-butylamine and cyclohexylamine are used as the probe molecules, to measure the total amount of acidity and the surface amount of acidity of the samples. All the above results show that the even SiOi membrane can be formed on ZSM5 crystal surface under moderate modification conditions. The SiO2 membrane covers the ZSM5 surface acid sites consequently inhibits surface coking, and avoids or decreases the possibility of ZSM5 pores choking. Meanwhile this method can keep the acid centers in pores unchanged so that the catalyst activity and stability can be improved efficiently. The opium siliconiting conditions were determined by these experiments. Compared with the samples without siliconiting treatment, the sample? treated under the above opium conditions can increase the productivity of phenol by 14% at 3h reaction time and by 41 % at 6hr reaction time.4. The kinetic of BTOP reaction over the catalyst modified by silicon was studied in a fixed-bed reactor and the experimental kinetic equation with power law was obtained. Based on the feature of the kinetic equation a novel inorganic membrane reactor for controllable oxidation was designed and operation modes of the reactor and its reaction performance were studied. The research results show that by permeating N2O across the porous membrane tube into the reaction region gradually the high concentration of benzene in the reaction region was maintained and the reaction rate was risen and the contact time of reactant with the catalyst was increase. By using a non-uniform membrane tube, it is possible to plan the axle permeation of this membrane tube according the need of the reaction so the performance of the membrane reactor was increased further, for example as the rate of benzene was 12 g/h, the maximum yield of phenol were 3.57, 4.48. 5.09 mmol-g'^h'1 separately for fixed-bed reactor, ordinary membrane reactor and membrane reactor with non-uniform membrane tube, that was that comparing with fixed-bed reactor the maximum yield of phenol of the membrane reactor with non-uniform membrane tube was increased 42.6%, comparing with ordinary membrane reactor, increased 13.6%.