Sol‐Gel and Porous Glass‐Based Silica Monoliths with Hierarchical Pore Structure for Solid‐Liquid Catalysis

Abstract

This work draws attention to both prospects and challenges for silica monoliths in solid-liquid catalysis at ultrahigh efficiency. The fundamental advantages of silica monoliths as support structure over particulate fixed beds and organicpolymer monoliths are illustrated as well as recent applications as flow-through microreactors for the production of fine chemicals. Preparation routes to sol-gel and porous glass-based silica monoliths featuring a hierarchical pore structure are described and their potential use as short-contact-time reactors is highlighted together with a view on the multiscale characterization and their rational design by establishing quantitative synthesis-morphology-transport relationships.

Pseudomorphic Transformation of Porous Glasses into Micelle‐Templated Silica

“…The incorporation of aluminum and transition metal elements into the silica framework enhances the acidity, ion exchange capacity, and specific catalytic activity, which makes the material even more attractive for the already stated potential applications. . For example Brønsted acid sites can be created by substitution of tetrahedrally coordinated silicon with aluminum ions, which is essential for many catalytic hydrocarbon reactions .…”

High-performance monoliths in heterogeneous catalysis with single-phase liquid flow

“…Employed high-performance monolithic supports exhibit small flow-channel sizes L flow , a radially homogenous distribution of macroporosity ε macro , and also a very little short-range variation in L flow . 20,32,60 In combination with a large external surface area A ext and small skeleton thickness L diff , residence time distributions become so narrow that ideal plug-flow behavior is achieved. This can be illustrated using typical Bodenstein numbers (Bo) characterizing reactor-based liquid-phase transport by flow relative to hydrodynamic dispersion occurring simultaneously along the macroscopic flow direction:…”

“…54 Independent adjustment of macropore size and skeleton thickness allows to decouple hydraulic permeability and advection-dominated macropore-scale hydrodynamic dispersion (L flow ) from diffusion-limited intraskeleton mass transport (L diff ) and mesopore space loading capacity. 20 It gives the materials chemist a wider influence over properties of the final product than possible with traditional (particulate) fixed beds. Fig.…”

“…5,13 Compared with this solid-liquid-gas and also the wellestablished solid-gas processing, where flow-through pores approach millimeter-dimension to guarantee a high permeability and low energy consumption, the application of monolithic supports for solid-liquid catalysis with single-phase liquid flow still emerges from a niche for low-temperature processes, particularly in the continuous-flow production of pharmaceuticals and fine chemicals. 2,6,8,[14][15][16][17][18][19][20] However, for liquid flow operation, millimeter-sized channels are unsuitable, as they lead to excessive longitudinal mixing (due to the parabolic pore-level flow profile under laminar conditions) and long lateral diffusion times for reactants in the flow channels to reach the external surface area and enter the internal pore space with the active surface. This inevitably results in unrealistic reactor residence times and very wide (as well as tailing) residence time distributions.…”

“…Along this direction, organic polymer, polymer-on-glass, and silica monoliths offer a range of flexible morphologies and a portfolio of surface modifications. 2,6,8,[14][15][16][17][18][19][20] With respect to soft polymeric materials, sol-gel and porous glass-based silica monoliths have a higher mechanical and thermal stability and are also more resistant to a wider range of solvents for solid-liquid operation. By taking the drastic step towards sub-micrometer L flow and L diff in hierarchical catalytic fixedbed reactors (with solid-liquid processing), high-performance catalyst supports can be realized and the following goals are envisioned: (i) elimination of external (L flow ) and internal (L diff ) diffusive transport limitations (i.e., due to the diffusion towards and within the porous walls or skeleton), even with liquidphase transport, when diffusion coefficients are four orders of magnitude smaller than in the gas phase and also slug flow cannot be used;…”