The group research investigates all aspects of nanoporous solids, including synthesis, structure determination and applications of zeolites, metal organic frameworks and mesoporous solids. In particular, our research areas include the following.
Synthesis of Novel Zeolite structures by Template Design
The catalytic performance of microporous aluminosilicate zeolites and their aluminophosphate analogues depends on their framework structure and the geometry and dimensions of their pore shape, so that the synthesis of novel structure types gives rise to new catalysts. The use of organic molecules as structure directing agents (SDAs) is an important route to new materials, which we have exploited to generate the ‘STA’ family of zeolites, including structures with small to large pores (4 to 8Å). This work has involved the use of macrocycles and metal amine complexes as templates and the first application of the co-templating approach (2 SDAs) in designed synthesis of the SAPOs STA-7 and -14.
Synthesis of Novel Metal Organic Frameworks
Metal Organic Frameworks (MOFs) are a fast-growing and structurally- and chemically-diverse family of materials which possess exciting potential in a whole range of applications. They are built from metal cations linked in two or three dimensions by organic molecules (most typically carboxylates and amines) with two or more binding groups. We have synthesised novel MOFs with unique properties following three new approaches: using scandium as the framework forming metal; using bisphosphonates as the linker molecules; via the use of combinations of linkers. The most significant of these include scandium carboxylates with unprecedented thermal stability, framework flexibility and Lewis acidity, and mixed linker zeolitic imidazolates decorated with extra framework cations. In particular, our frequently cited work on porous metal phosphonates (with main group, transition and rare earth metals) has given the first permanently porous phosphonate MOFs (pore sizes from 3-20 Å), the first example of isoreticular synthesis in phosphonate MOFs (mesoporous STA-16); structures that demonstrate reversible topotactic transformation and conformational ligand isomerisation upon dehydration, and coordinatively unsaturated metal cation sites.
Carbon Capture on Zeolites and MOFs
The selective adsorption of carbon dioxide is of great importance in natural gas upgrading and is the first step in carbon capture from pre- and post-combustion gas streams in power plants and refineries. In collaboration with the Chemical Engineering Department at Edinburgh University, we are investigating a range of porous solid adsorbents for these applications, and highlights include good selectivity and uptake over the phosphonate MOF STA-12 and most recently a deep understanding of the action of ‘sentinel’ cations in zeolite Rho to give adsorbents with high capacity and CO2/N2 and CO2/CH4 selectivity. This selective adsorption over zeolites via cation gating, first described in our JACS 2012 paper, is expected to be widespread and useful. We also recently reported a similar effect over a functionalised scandium MOF.
Catalysis over Microporous solids
Many of the novel materials we prepare have catalytically active cations of groups distributed evenly over their internal surfaces, and so have potential as shape selective single site heterogeneous catalysts. In collaboration with industrial support and other academic research groups, at St Andrews and elsewhere, we investigate the catalytic performance of our novel solids. Recently published examples of zeolitic catalysts include the selective conversion of methanol to light olefins over SAPOs STA-7 and STA-14, the use of Cu SAPOs STA-7 and SAPO-34 prepared by direct synthesis using copper amine complexes as SDAs in the selective catalytic reduction of NOx with NH3. We are also investigating possible catalytic applications of MOFs, and have reported the use of cobalt phosphonates in selective oxidation and of scandium MIL-100 as the most active MOF in Lewis acid catalysed C-C and C-N bond-forming reactions. This work was highlighted in the January 2012 issue of The Catalyst Review.
Multitechnique approach to Structure Determination and Characterisation
Much of our research makes use of state-of-the-art structural methods to characterise fully the new materials. Where single crystals are unavailable, powder diffraction analysis combined with electron microscopy, computer modelling and solid state NMR can determine structure (as shown in the journal Nature for TNU-9) and dynamics (we were the first to show the rotational dynamics of the prototype MOF-5 material). We typically use synchrotron-based diffraction studies to study the structural response to catalyst activation and gas adsorption (for example for carbon capture on zeolites and MOFs).
Synthesis and Applications of Mesoporous Solids
Our work on mesoporous silicas comprises two areas, the structural characterisation and computer simulation of complex structures and the utilisation of large pore mesoporous silicas, suitably modified, as structured supports for enzymatic catalysts. The latter has been very well cited, with four papers achieving over 100 citations each.