In conclusion, more efficient and clean solid (acid and superacid) catalysts will be used in the coming years to reduce not only the emission of environmentally harmful products but also the use of noxious catalysts. The optimal catalytic systems will be determined from the nature of acid strength of its active sites, the nature of the reaction, and the reaction conditions.
A first step to reduce the negative impact of liquid acids is to use them impregnated on a metal oxide support, although this is not the only alternative. Metal oxides, especially zeolites, could partially substitute liquid acids. The shape selectivity of zeolites can also help to develop new catalytic processes. However, acid solids or multifunctional acid-solid catalysts with an acid strength higher than or similar to zeolites, respectively, are required for reactions that need large amounts of acid. In this way, the study of sulfated metal oxides and/or heteropoly acids (in which both acid and red-ox mechanisms operate) make if possible to propose a suitable alternative to noxious acids, especially in high-acid-demanding reactions. However, it appears quite clear that the presence of sulfate will always be a limitation for their practical use. So, tungstate or heteropolyoxometalates rather than sulfate will make interesting alternatives. Moreover, nafion-resin-silica composites, with an effective surface area larger than pure nafion, also can be considered to be an effective solid-acid catalyst, especially in fine chemistry application.
In any case, the use of these catalytic systems will depend on their stability. In fact, the still unresolved problem with the application of these catalysts to industrial processes is whether to prolong the life of the catalyst or to continuously regenerate the catalyst in an efficient way.
In addition, a new physicochemical method to characterize the acid strength of acid and superacid solids also will be required in order to evaluate and compare acid strengths with those obtained with liquid acids.
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