Porous aluminosilicate inorganic polymers (geopolymers): a new class of environmentally benign heterogeneous solid acid catalysts

• New environmentally-benign aluminosilicate geopolymer catalysts are described.
• The incorporated acid sites are an integral part of the geopolymer structure.
• The weakly acidic silanol and Bronsted sites are ideal for Beckmann rearrangements.
• The catalysts can be re-used for at least five cycles without performance degradation.
• The catalysts are produced easily in a low-energy environmentally friendly process

Aluminosilicate inorganic polymers (geopolymers) were developed as a new class of cost-efficient, environmentally friendly, solid acid catalysts and their performance evaluated in a model liquid-phase Beckmann rearrangement reaction (cyclohexanone oxime to ε-caprolactam). The active sites were generated within the structure of the geopolymers by ion-exchange with NH4+ followed by thermal treatment. The effect of varying the starting composition on the textural and acidic properties of the geopolymer catalysts was studied and its influence on the catalytic activity was investigated. Catalytic performance was significantly improved by the use of post-synthetic treatments. No significant decrease in the yield of ε-caprolactam after recycling for five times suggesting that geopolymer-based catalysts are advantageous over supported catalysts which often lose their catalytic activity due to leaching of the active sites from the support. The catalytic activities obtained in this study are comparable, and sometimes superior, to other solid catalysts suggesting that geopolymers have a great potential as environmentally benign heterogeneous catalysts.

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