Porous Aluminosilicate Inorganic Polymers: A New Class of Heterogeneous Solid Acid Catalysts
Recent increased environmental awareness and the stimulus of greener chemistry has driven the rapid development of heterogeneous catalysts, particularly solid acids, for a wide range of organic synthesis applications. Typical homogenous acids suffer drastic drawbacks in terms of their corrosivity, toxicity, and reusability, in addition to their separation that generates large amounts of industrial wastes which exceeds in many cases the amount of the formed products. Crystalline aluminosilicate inorganic polymers (zeolites) have successfully replaced the typical homogenous Lewis acids in many industrially important applications, the majority of which are in the petrochemical industries, e.g. production of olefins and aromatics. The fine chemical industries, however, are more challenging and still mainly use homogenous catalysts. Typical zeolite catalysts are hindered by their restricted micropores, and the low hydrothermal stability of other mesoporous M-silicates (such as MCM-41) results in structural deformation in aqueous solutions at elevated temperatures. Other highly promising solid catalysts suffer drawbacks of high cost, sophisticated synthesis procedures, and environmental risks from the use of toxic reagents. Thus, there is still a need for new cost-efficient reactive heterogeneous solid catalysts that are also environmentally benign. This thesis reports the development of amorphous aluminosilicate inorganic polymers (known as geopolymers) as a novel class of heterogeneous solid acid catalysts. These geopolymers can be synthesised with the desired acidity and porosity in a very energy-efficient and simple procedure which does not involve lengthy thermal treatments or the use of costly and sometimes toxic structural directing agents that are required for the synthesis of zeolite or other mesoporous aluminosilicates. Microporous, mesoporous and hierarchical geopolymer-based catalysts were synthesised from different precursors with high surface area and acidic sites (Bronsted and Lewis) generated within their structure by ion-exchange with ammonium ions followed by thermal treatment, allowing the nature of these acidic sites to be tailored to specific applications. Furthermore, some of the resulting geopolymer catalysts were subjected to post synthetic treatments (demetallation) which provided improved acidity and porosity. In the first instance, the geopolymer-based catalysts were synthesised from a naturally occurring clay mineral and their catalytic performance was evaluated in the industrially important Beckmann rearrangement of cyclohexanone oxime to 𝜀-caprolactam. High catalytic reactivity and selectivity was achieved over the geopolymer-based catalysts that possess high surface area and weak surface acidities consisting of H-bonded silanol nests and vicinal silanols. The catalytic reactivity of the clay-based geopolymer catalysts was further evaluated in the Friedel-Crafts alkylation of large substituted arenes with benzyl halide as alkylating agent, where typical microporous zeolites show poor reactivity due to diffusional limitations. In this reaction, the thermal treatment was adjusted to generate the required Bronsted and Lewis acidic sites. High reactivity was achieved over several mesoporous geopolymer-based catalysts, with the best performance being observed over a hierarchical geopolymer-based catalyst that exhibits the highest acidity of all these new catalysts. In another approach, highly reactive geopolymer-based catalysts were synthesised from industrial wastes precursors (fly ash). Several fly ashes were collected from different sources and the influence of their chemical and physical properties on the resulting geopolymers was investigated. These fly ash-based catalysts demonstrated excellent catalytic performance in the alkylation of benzene and substituted benzenes and their active sites were ascribed to a combination of Fe2O3 present in the raw fly ash, together with the Bronsted and Lewis acid sites that were generated within the geopolymers framework by the ion-exchange process followed by thermal treatment. The use of the fly ash-based catalysts was also demonstrated in another highly demanding catalytic process, the Friedel-Crafts acylation of aromatics. High reactivity and selectivity was achieved in the acylation reactions of anisole and mesitylene using benzoylchloride as the acylating agent. In addition to their excellent catalytic reactivities, the fly ash-based geopolymer catalysts provide a valuable approach of the utilisation of industrial wastes such as fly ash, the vast production of which is becoming a world-wide concern. The geopolymer-based catalysts developed in this work are reusable without significant loss of reactivity and their catalytic performance is superior to other commonly used solid acid catalysts. The results presented in this thesis demonstrate a great potential for geopolymers as active candidates in the field of heterogeneous catalysis, representing as they do a new class of solid acids with highly desirable features such as catalytic efficiency as well as ecological friendliness, cost effectiveness and ease of synthesis.