Fly Ash-Based Geopolymers as Sustainable Bifunctional Heterogeneous Catalysts and Their Reactivity in Friedel-Crafts Acylation Reactions

Kenneth J D Mackenzie,Mohammad I M Al-Zeer

doi:10.3390/catal9040372

Abstract

This study presents the synthesis, characteristics and catalytic reactivity of sustainable bifunctional heterogeneous catalysts derived from coal fly ash-based geopolymer, particularly those with a high Ca content (C-class) fly ash. The developed catalysts were synthesized at room temperature and pressure in a simple ecologically-benign procedure and their reactivity was evaluated in the Friedel-Crafts acylation of various arenes. These catalysts can be produced with multilevel porous architecture, and a combination of acidic and redox active sites allowing their use as bifunctional catalysts. The acidic sites (Lewis and Brønsted acidic sites) were generated within the catalyst framework by ion-exchange followed by thermal treatment, and redox sites that originated from the catalytically reactive fly ash components. The developed catalysts demonstrated higher reactivity than other commonly used solid catalysts such as Metal-zeolite and Metal-mesoporous silicate, heteropolyacids and zeolite imidazole frameworks (ZIF).

Highlights

Geopolymers are an amorphous class of aluminosilicate inorganic polymers which sometimes are described as amorphous analogous of zeolites [1]
The geopolymers their corresponding catalysts were analyzed by X-Ray Diffraction (XRD)
C-class coal fly ash was utilized as a precursor for a reactive, inexpensive and environmentally friendly geopolymer-based heterogeneous catalysts

Summary

Introduction

Geopolymers are an amorphous class of aluminosilicate inorganic polymers which sometimes are described as amorphous analogous of zeolites [1]. Their structure is composed of silicate and aluminate tetrahedral units randomly arranged and joined through their common oxygen atoms forming a 3D framework [2,3]. Geopolymers possess a number of useful features such as their porous structure, ion-exchange capability, low preparation costs, ease of synthesis and environmental friendliness. In the field of heterogenous catalysis, in particular, various catalytically active sites, acidic, basic and redox active centres, can be generated within the geopolymer framework by ion-exchange, allowing their use in a wide range of catalytic applications. Clay-based geopolymers as supports for various catalytically active transition metals and nanoparticles have recently been

Methods

Results

Conclusion