Route to large-pore catalysts
Anyone who has taken a basic chemistry class – or even dabbled a bit in the kitchen – knows that many reactions can be sped by heating or stirring. However, significant increases in reactions to produce chemicals on an industrially relevant scale typically require the use of catalysts. Environmentally friendly microporous zeolite-type materials have increasingly replaced harsher catalysts in chemical synthesis, particularly petrochemical production. Their synthesis typically requires the addition of structure-directing agents (SDAs) to guide crystallisation. Chemical processing of the larger molecular substrates common to the fine chemicals and pharmaceuticals industries relies strongly on extra-large-pore catalysts, but increasing the pore size of zeolite materials is challenging. Scientists sought a solution to this barrier within the context of the EU-funded project NEWSDASFORALPOS (Use of newly designed organic molecules as large and efficient structure directing agents for the synthesis of microporous aluminophosphates). The crux of the problem has been increasing the size of SDAs to increase pore size, because SDA efficiency in directing framework formation decreases at larger molecular sizes. The team combined two important opportunities: microporous aluminophosphates (AlPOs) as a promising alternative to zeolites due to their hydrophilic nature, and the use of supramolecular chemistry to enable self-assembly of large hydrophilic SDAs. Hydrophilic interactions between SDAs and AlPOs can enhance the efficiency of directing framework formation. Hydrophilic SDAs can also interact with water molecules themselves, forming aggregates that can create even larger pores. Researchers thus targeted development of large, hydrophilic SDAs specific for AlPO frameworks. Aromatic amines can self-assemble into multi-molecular aggregates via intermolecular interactions between the rings, providing a route to increasing SDA size. Several SDAs were synthesised and tested, leading to various framework structures. For the first time, a framework based on a very strong supramolecular self-assembly has been observed. Scientists identified chemical factors governing the supramolecular chemistry of aromatic amines and elucidated the way in which that chemistry governs the structure-directing activity of the SDAs formed by them. The NEWSDASFORALPOS team has thus opened the road to rational design of new and efficient self-assembling SDAs for AlPO catalysts, paving the way to expedited development.
Keywords
Catalysts, microporous, zeolite, structure-directing agents, microporous aluminophosphates
