Abstract
Hierarchical porous catalysts offer highly connected architectures for enhanced transport of bulky molecules and the sustainable manufacturing of bio-derived platform chemicals and fuels.
Highlights
Porous solids nd widespread practical application in catalysis, sorption, and separation science wherein pore dimensions and network connectivity control the attendant internal surface area and accessibility.[1,2] Micro- and mesopores (0.5–50 nm diameter) confer high areas that can maximise the density of surface chemical functions such as catalytically active sites, while macropores (>50 nm) enhance the rate and extent of uid permeation through a pore network
Macropore size control was achieved through the synthesis of monodispersed polystyrene nanospheres of tunable diameter (200 to 580 nm) as sacri cial hard templates and their subsequent compaction into a colloidal crystal assembly
Catalytic performance of the resulting PrSO3H/MMSBA-15 family towards the liquid phase esteri cation of short and long-chain carboxylic acids with methanol revealed that turnover frequencies for propanoic acid were structure invariant, whereas those for palmitic and erucic acids exhibit striking increases with macropore size
Summary
Considering heterogeneous catalysis, simulations suggest that in the Knudsen diffusion regime, where reactants/products can enter/exit mesopores but experience attendant diffusion limitation, hierarchical bimodal pore networks can signi cantly improve active site accessibility, with larger pores acting as ‘molecular superhighways’[15] to accelerate transport from the bulk media to mesopore domains.[16,17,18,19] In practice, such enhanced accessibility is facilitated by truncation of mesopore channels in ordered hierarchical porous solids (commonly prepared by dual-templating routes) compared with their mesoporous counterparts;[20,21] shorter mesopores may mitigate any attendant diffusional resistance. Porous silicas were prepared by a modi ed true liquid crystal templating technique incorporating a range of polystyrene nanospheres as macropore-directing hard templates.[34] Pluronic P123 (2 g, Aldrich average Mn $ 5800) was sonicated with HCl-acidi ed deionised water (2 g, pH 2) at 40 C to form a homogeneous gel. Tetramethoxysilane (4.08 cm[3], Acros 99%) was added to the gel and rapidly stirred for 5 min (800 rpm) until a homogeneous liquid formed Following this phase change, polystyrene nanospheres of a particular size (6 g, independent of bead diameter, as a ne powder) were added with agitation (100 rpm) for 1 min. Pulsed- eld gradient (PFG) NMR diffusion measurements were performed to assess the diffusive behaviour of liquids in the unrestricted bulk and within PrSO3H/MM-SBA-15 catalysts.
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