ABSTRACT
The commercial catalyst Amberlyst-15 (A-15) resin and its sister catalyst, Amberlyst-35, presently used in the synthesis of methyl tertiary-butyl ether (MTBE), although giving high performance [1] in the liquid-phase synthesis, still suffer from several drawbacks, such as thermal instability, acid leaching from the resin surface [2], and the production of isobutene oligomers as by-products. The latter aspect constitutes a non-negligible economic disadvantage in terms of loss of isobutene reactant, which cannot be recycled as is methanol in the industrial process [3]. Because of these restrictive factors, considerable efforts have been focused on the search for alternative catalysts. From this perspective, inorganic materials such as acidified clays [4] or zeolites [2] were thus proposed to carry out the etherification. In particular, zeolite catalysts, which have high chemical and thermal stability, do not present the problem of acid leaching at the surface as encountered with the commercial A-15 resin. In addition, the pore systems of these microporous materials, the most representative being the (large-pore-sized) Y and the (medium-pore-sized) ZSM-5 zeolites, result in advanced product shape selectivity, thus reducing sharply the production of bulky isobutene oligomers [2]. However, the relatively lower surface acidity and the microporous nature of the zeolite catalysts require a much higher reaction temperature to compensate for the lower reactivity of the zeolite acidic surface and a more demanding diffusion of reactant and product molecules. Higher reaction temperature also means that the reaction is now controlled by thermodynamics, since decomposition of the product MTBE, occurring at high temperature, increasingly limits the MTBE yield. Attempts were thus made to enhance the surface acidity of the zeolitic materials in order to permit the reaction to be carried out at relatively low temperature. The two main procedures for zeolite modiflcation which have been investigated with some success are the following: loading of the wellknown superacidic triflic acid onto solid silica-based materials; and fluorination of the zeolite surface.
