Using small angle neutron scattering to explore porosity, connectivity and accessibility, towards optimised hierarchical solid acid catalysts

Abstract

The significant interest in developing hierarchical materials to overcome the traditional limitations of microporous catalysts, has led to a wide range of synthesis protocols being developed. In this work we modify traditional synthetic procedures known to yield highly crystalline microporous materials, by adding a hydrocarbon surfactant, leading to the formation of hierarchical solid-acid zeotypes; silicoaluminophosphates. We show for the first time, that small angle neutron scattering can build a qualitative description of the porosity in hierarchical materials, probing the exact nature of the micropores and mesopores within our system, that can be adapted to any hierarchical system. When combined with positron annihilation lifetime spectroscopy and porosimetry measurements we gain greater insight by exploring the accessibility and interconnectivity of the micropores and mesopores. We show that by varying the quantity of mesoporogen the size and nature of the mesopores can be finely tuned. Further, small angle neutron scattering reveals that mesopores are lined with a silica layer, that strongly influences the accessibility of the micropores. As such we show that our hierarchical materials contain distinct micropores housing stronger Brønsted acid sites, whilst the mesopores are lined with weaker silanol groups. This is complemented with a catalytic study focussing on n-butane isomerisation and ethanol dehydration that highlights the advantages and disadvantages of this design and further probes the influence of these bimodal porous systems on catalytic performance.