Methodological review of zeolite synthesis from industrial waste and natural clays and the fabrication of hierarchical pore structures

Zeolites are one of the crucial materials with versatile industrial applications ranging from water purification to gas separation and heterogeneous catalysis. Industrial waste and natural clays provide sustainable and low-cost sources of Si and Al for producing synthetic zeolites. However, the microporous structure of the prepared zeolites limits mass transport during the catalysis of macromolecules in fine chemical synthesis, bio-oil and plastic pyrolysis oil upgrading, which necessitates strategic approaches to tailor pore size towards mesopores. This study presents methodological approaches to synthesising zeolites from industrial waste and natural clays, and strategies for fabricating mesopores during synthesis and post-synthesis. In the synthesis of zeolite from industrial waste or natural clays, structural directing agents can be applied and crystallization conditions controlled to create an assembly of zeolite nanocrystals into mesoporous aggregates with hierarchical frameworks and enhanced chemical properties which increases catalytic activity and selectivity. Consequently, the pore structure, acidity and crystallinity of zeolites can be tuned by altering the Si/Al ratio through dealumination and desilication post-synthesis approach. Fabricated hierarchical zeolites with bi/trimodal levels of pore size distribution offer catalytic benefits in fluidised-bed catalytic cracking (FCC) of vacuum gas oil into gasoline and light olefins. Also, hierarchical zeolites exhibit increased conversion in the range of 15–30 %, and enhanced yields of gasoline, propylene, and olefins by 21 %, 16 %, and 25 %, respectively. This review will help guide an understanding of structure–property–function relationships of zeolites, as well as the optimisation of hierarchical zeolites fabrication strategies and link them to catalytic performance to guarantee application-oriented design.