Abstract
Catalytic oxidation is a key technology for the conversion of petroleum-based feedstocks into useful chemicals (e.g., adipic acid, caprolactam, glycols, acrylates, and vinyl acetate) since this chemical transformation is always involved in synthesis processes. Millions of tons of these compounds are annually produced worldwide and find applications in all areas of chemical industries, ranging from pharmaceutical to large-scale commodities. The traditional industrial methods to produce large amounts of those compounds involve over-stoichiometric quantities of toxic inorganic reactants and homogeneous catalysts that operate at high temperature, originating large amounts of effluents, often leading to expensive downstream processes, along with nonrecovery of valuable catalysts that are loss within the reactant effluent. Due to the increasingly stringent environmental legislation nowadays, there is considerable pressure to replace these antiquate technologies, focusing on heterogeneous catalysts that can operate under mild reactions conditions, easily recovered, and reused. Parallelly, recent advances in the synthesis and characterization of metal complexes and metal clusters on support surfaces have brought new insights to catalysis and highlight ways to systematic catalysts design. This review aims to provide a comprehensive bibliographic examination over the last 10 years on the development of heterogeneous catalysts, i.e., organometallic complexes or metal clusters immobilized in distinct inorganic supports such as zeolites, hierarchical zeolites, silicas, and clays. The methodologies used to prepare and/or modify the supports are critically reviewed, as well as the methods used for the immobilization of the active species. The applications of the heterogenized catalysts are presented, and some case-studies are discussed in detail.
Highlights
Catalytic oxidation reactions are of high industrial relevance since many important commodities have synthesis paths involving oxidation
The catalysts were tested for geraniol epoxidation, and while the homogenous assays resulted in a total conversion of the substrate after 1h of contact time, when the catalyst obtained by direct immobilization was used, only 17% of substrate conversion was obtained after 48 h of reaction
The catalytic oxidation of hydrocarbons is a key industrial process that allows the production of important chemicals from petroleum-based feedstocks, with application in all areas of chemical industries
Summary
The direct oxidation of alkanes is an attractive alternative to oxidation via olefins; only two industrial processes have been implemented, and other alkanes oxidations are only at the research or pilot plant status One of these reactions is the production of maleic anhydride from n-butane (Figure 1). This process uses supported (VO) P2 O7 as heterogenous catalyst and achieves high weight yields Fixed, fluidized, and transport bed reactors technologies have been implemented in different industrial plants to address different technical difficulties [4] Another example of alkane oxidation but in the liquid phase with homogeneous catalysis is the oxidation of cyclohexane into a mixture of cyclohexanol and cyclohexanone ( known as KA oil), which are intermediates in the manufacture of nylon-6 and nylon-6,6. Since the 1960s, terephthalic acid has been mainly produced by the Amoco process; this homogeneous catalytic process uses soluble cobalt salt (acetate or naphthenate) simultaneously with manganese or bromide ions [3,4]
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