Mass Transfer in Hierarchical Silica Monoliths Loaded With Pt in the Continuous-Flow Liquid-Phase Hydrogenation of p-Nitrophenol

Abstract

Sol-gel-based silica monoliths with hierarchical mesopores/macropores are promising catalyst support and flow reactors. Here, we report the successful preparation of cylindrically shaped Pt-loaded silica monoliths (length: 2 cm, diameter: 0.5 cm) with a variable mean macropore width of 1, 6, 10, or 27 μm at a fixed mean mesopore width of 17 nm. The Pt-loaded monolithic catalysts were housed in a robust cladding made of borosilicate glass for use as a flow reactor. The monolithic reactors exhibit a permeability as high as 2 μm2 with a pressure drop below 9 bars over a flow rate range of 2–20 cm3 min−1 (solvent: water). The aqueous-phase hydrogenation of p-nitrophenol to p-aminophenol with NaBH4 as a reducing agent was used as a test reaction to study the influence of mass transfer on catalytic activity in continuous flow. No influence of flow rate on conversion at a fixed contact time of 2.6 s was observed for monolithic catalysts with mean macropore widths of 1, 10, or 27 µm. As opposed to earlier studies conducted at much lower flow velocities, this strongly indicates the absence of external mass-transfer limitations or stagnant layer formation in the macropores of the monolithic catalysts.

Experimental and modeling of tetracycline degradation in water in a flow-through enzymatic monolithic reactor

“…They observed that productivity was increased by increasing the flow rate from 0.01 to 0.03 mL min -1 . In another study, Jatoi et al, (2021) studied the effect of flow rate on external mass transfer limitations with silica monoliths loaded with Pt in the continuous-flow liquid-phase hydrogenation of p-Nitrophenol. They observed that for the range of flow rate from 1 to 8 mL min -1 there was no effect on mass transfer limitations in macropores and attributed this behavior to the relative "high" flow rate applied.…”

Treatment of Wastewater Containing Pharmaceutical Micropollutants by Adsorption under Flow in Highly Porous Carbon Monoliths

“…Highly porous carbon monoliths (CM) with well-controlled hierarchical micro/meso/macropores were obtained by replicating silica monoliths by hydrothermal nanocasting. , CM proved to be very suitable for high flow, a low drop pressure, and a homogeneous contact time. ,, This family of monoliths, prepared by spinodal decomposition, can have different natures (silica, zeolites, titania, etc.) and has proven to be extremely efficient in flow chemistry in many applications such as high-performance liquid chromatography and electrochromatography, , heterogeneous catalysis, − photocatalysis, biocatalysis, ,, oil purification, decontamination of water containing radioactive elements, metal capture, , etc. In this study, CM were used to deplete some antibiotics from waters by adsorption under flow.…”

Powdered Hierarchically Porous Silica Monoliths for the Selective Extraction of Scandium

“…However, this solution has its own drawbacks, namely monolith containment, which is itself a nontrivial obstacle to monolith scale-up. Each individual monolith, regardless of size, must be strictly contained within some type of external sleeve (e.g., a glass or metal cylinder, resin coating, or heat shrink tubing) , to retain the liquid feedstock within the system without leaking. If the containment system is not precisely fitted to the monolith, the recovery process will suffer from wall effects and/or channeling and exhibit reduced extraction efficiency.…”