Application Of Niobium-containing Solid Acid Catalysts In Friedel-Crafts Reactions

Friedel-Crafts reactions are a kind of important acid catalyzed reactions, which usually take place in the presence of homogeneous catalysts. Although these catalysts are effective in catalysis, they do not accord with the principles of green chemistry. Therefore, it is significant to endeavor to investigate new solid acid catalysts which are environment-friendly in Friedel-Crafts reactions.Recently, niobium-containing materials have attracted more and more attention. They are becoming a new research field of solid acid materials due to their strong hydrophobic characteristic which is in favor of those aqueous systems and high acidity, high selectivity, acceptable stability. During the past decades, an increasing interest and large amounts of researches have been brought in niobium-containing materials, but so far, the understanding of the nature of niobium-containing materials is limited.In the present paper, the reaction of p-methoxyphenol with methyl t-butyl ether, hydroquinone with tert-butanol over supported niobium zeolites and niobium-containing metal oxides were investigated, and the catalysts were characterized by various spectral and physicochemical techniques. The effects of reaction conditions and reaction mechanisms catalyzed by solid acid were further discussed.The main contents and results were as follows:1. Nb2O5 supported on H-DAY zeolites and H1-xNb1-xMo1+xO6 (M=Mo, W) catalysts were prepared separately by impregnation and calcination method with subsequent acid treatments respectively. Their physicochemical properties such as dispersion, specific surface area, and so on were characterized by X-ray diffraction (XRD), N2 adsorption BET, Scanning electron microscopy (SEM) and Raman spectrum. The results showed that:â‘ Nb2O5 was successfully supported on H-DAY zeolites by impregnation method, which exhibited orthorhombic crystals. The surface area, pore volume and acid amount of the synthesized Nb2O5/DAY decreased with the niobium content.â‘¡HNbWO6 showed obvious layered structures, but there were irregular layered structures observed in H1-xNb1-xMo1+xO6 samples. Layered structure was beneficial to the intercalation effect in the substrates.Calcination and acid treatment influenced the structures and characters of the catalysts. The acid amount increased with increasing Nb concentration in H1-xNb1-xMo1+xO6 samples.2. H-DAY zeolite was effective catalysts for the reaction of p-methoxyphenol with methyl t-butyl ether, this paper focused on discussing of the effect of reaction conditions on the Conversion, Yield or Selectivity. In addition, the alkylation of p-methoxyphenol under supported niobium zeolites was studied. The main results were as follows:â‘ Both the pore size and acid amount were critical to the reaction activity. The more the acid amount, the higher the reactant activity as the pore size of zeolite is bigger than the diameter of products. When the pore size is smaller than the diameter of products, the surface acid site is crucial.â‘¡The reaction conditions of p-methoxyphenol over H-DAY zeolite are optimized. It showed that too large amounts of catalysts were not favorable to the forming of 3-TBHA; the selectivity of 3-TBHA decreased with the increasing of the molar ratio of MTBE/MOP; higher selectivity to 3-TBHA was usually obtained at lower temperature and shorter time, whereas a lower conversation was following. Therefore it was better to increase reaction temperature and shorten reaction time in order to obtain a higher yield of 3-TBHA.â‘¢The catalytic activity was significantly influenced by the loading of niobium oxide.59% was considered as the appropriate loading when the main product (3-TBHA) had the best selectivity and yield.3. Reaction of p-methoxyphenol and hydroquinone was studied systematically over H1-xNb1-xMo1+xO6 (M=Mo, W). The main results were as follows:â‘ Metal oxide H1-xNb1-xMo1+xO6 samples were recognized as the suitable catalysts for butylation of hydroquinone, and TBA served as a more excellent alkylating agent than MTBE in this condition.â‘¡The regulation in butylation of p-methoxyphenol catalyzed by H1-xNb1-xMo1+xO6 (x=-0.1-0.3) was corresponded with that in butylation of hydroquinone. H1.05Nb1.05Mo0.95O6.was found to function as the most active solid acid catalyst among H1-xNb1-xMo1+xO6 compounds.2-tert-butyl-1,4-hydroquinone and 2,5-di-tert-butyl-1,4-hydroquinone with little 4-tert-butoxy-phenol were synthesized via hydroquinone and tert-butyl alcohol.â‘¢O-alkylation product 4-tert-butyloxy-phenol was synthesized as the main product via hydroquinone with tert-butyl alcohol over layered HNbWO6. A little 2-tert-butyl-1,4-hydroquinone and 2,5-di-tert-butyl-l,4-hydroquinone were found in some conditions.â‘£lkylation of hydroquinone with TBA was carried out by varying different parameters such as temperature, molar ratio of TBA to HQ, and acid treatment under layered and protonated HNbWO6. It showed that a relatively low reaction temperature was helpful to the formation of O-alkylation product, and a high temperature benifited the C-alkylation products. The maximum conversion of HQ was obtained when the molar ratio of TBA to HQ was 5. The kinds of acids in treatment played an inconsequential role in catalytic activity of catalysts.