Direct Oxidation Of Benzene To Phenol By N₂o Over Fe-zsm-5 Catalysts

Phenol is an important organic chemical intermediate and chemical raw material, mainly used in the manufacture of phenolic resin, bisphenol A and caprolactam. At present, the most important method for industrial production of phenol is the so-called cumene process. However, this process has some fundamental and inevitable disadvantages: acetone as byproduct produced in a 1:1 stoichiometry, which in terms of market demand is much smaller than phenol. Therefore, the productivity of phenol via this technology is influenced by the market price of acetone coproduced.Compared with traditional methods, advanced technologies for direct oxidation of benzene to phenol (BTOP) with economical and environmentally friendly advantages have attracted more attention in both academic and industrial affiliations. The Alphox process developed by Solutia company in USA and Boreskov Institute of Catalysis in Russia together is the most attractive process for phenol production. In the Alphox process N2O is used as oxidant, for hydroxylation of benzene to phenol over Fe-ZSM-5 catalysts. The process can not only make a good use of the waste N2O generated in the industrial production of adipic acid, but also be able to integrate effectively phenol production plant with adipic acid production plant, combined with a high selectivity for phenol. Therefore, this is considered as an eco-friendly chemical process. However, the pilot tests of the Alphox process show that the used Fe-ZSM-5 catalysts are unstable and there is still some distance away from industrialization. This is due to the zeolite Fe-ZSM-5 is of microporous material, in which there are some strong mass transfer limitations available for the reactant benzene and the product phenol, easily leading to the catalyst deactivation. To improve mass-transfer properties of zeolites via post-treatments in the refining industry has attracted researchers'attention for years, and in general, hydrothermal-or acid-treatments (dealimination) are used to create mesopores in zeolite crystals. However, recently some experimental investigation has proven that dealuminated species could block the interconnections between created mesopores and zeolite micropores. This cannot, in principle, improve the mass-transport properties of zeolites. On the other hand, recently, a method, the so-called alkaline treatment, has been proved that mesopores formed by extracting framework silicon selectively are connected with micropores, resulting in a reduction of the diffusion path length of molecules through zeolite micropores The micropores structure of crystalline materials and acidity maintain the same after alkaline treatment while larger mesopores offer a molecule transfer path. Micropores can serve as "micro-reactors", which can offer active species or adsorptive sites, and have a molecular sieving effect. Therefore, alkaline treatment is an efficient manner to improve the mass-transfer properties of zeolites. Up to now, to our knowledge, however, research on alkaline treatment for the improvement of Fe-ZSM-5 mass-transfer properties and its application in the BTOP reaction has been very limited.Therefore, our strategy in this work is to compare the catalytic properties of mesoporous Fe-ZSM-5 zeolites obtained via alkaline treatment with those of parent zeolites for the BTOP reaction. The main results in this thesis are summarized as follows:1) The hydroxylation of benzene to phenol over Fe-ZSM-5 catalysts has been investigated. Fe-ZSM-5 catalysts were prepared with ion-exchange and isomorphously substituted methods. Alkali-treated Fe-ZSM-5 catalysts prepared by the isomorphously substituted method show the higher activity and stability. Furthermore, the effects of alkaline treatment conditions such as temperature, duration, and concentration on the activity of the resulted catalyst in the BTOP reaction have been studied. The optimized alkaline treatment conditions are 0.3 M NaOH at 80℃for 2 h, under which the prepared catalyst used in the BTOP reaction shows that the initial conversion of benzene is 22.1% and the conversion still has 20% after the 3-hour continuous reaction at 320℃. This for the first time demonstrates that the mesoporous Fe-ZSM-5 zeolite obtained via alkaline treatment has better catalytic properties in the BTOP reaction, compared to the parent catalyst Fe-ZSM-5. The state of Fe(III) is not altered upon alkaline treatment while the mass-transfer properties of the alkali-treated zeolites are significantly improved, resulting in a better catalytic performance for the BTOP reaction.2) The Fe-ZSM-5 prepared via first alkaline-treatment and then steam-treatment has a lower catalytic activity in the BTOP reaction, compared to the catalyst obtained via only alkaline-treatment. This is due to the transformation of Fe species into oligonuclear Fe3+xOy species during steam-treatment, confirmed by the TEM and UV-vis characterizations. Additionally, the N2 adsorption characterization shows that it is also found that the Fe-ZSM-5 obtained via first alkaline-treatment and then steam-treatment has a lower BET surface area and a smaller micropore volume, perhaps also leading to a lower catalytic activity for the BTOP reaction.