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.
Highlights
Flow chemistry represents an enabling technology for various chemical transformations that are difficult or impossible to conduct in batch (Baumann et al, 2020) due to multiple advantages, such as realization of high surface-to-volume ratio, efficient mixing, improved heat and mass transfer, and increased selectivity and safety (Plutschack et al, 2017; Akwi and Watts, 2018)
A practical advantage of hierarchical silica monolith (HSM) is that we present a seamless monolith with the physical dimensions of the container within which the gel was dried
Silica monoliths with a variation of macropore width of 1–27 μm and comparable textural properties such as mean mesopore width (17 nm), total porosity (90%), specific surface area (200 m2 g−1), and specific pore volume (3 cm3 g−1) were successfully synthesized
Summary
Flow chemistry represents an enabling technology for various chemical transformations that are difficult or impossible to conduct in batch (Baumann et al, 2020) due to multiple advantages, such as realization of high surface-to-volume ratio, efficient mixing, improved heat and mass transfer, and increased selectivity and safety (Plutschack et al, 2017; Akwi and Watts, 2018). PBRs often suffer from problems such as inhomogeneous particle size distribution, high pressure drop, catalyst attrition, maldistribution of fluid, hot spot generation, and creation of stagnant fluid zones In a PBR, the particle size can be reduced to decrease mass-transfer limitations and to increase the available surface area; Abbreviations: PBR, packed bed reactor; HSM, hierarchical silica monolith; PNP, p-nitrophenol; PAP, p-aminophenol; MIP, mercury intrusion porosimetry; NP, nanoparticle. It is difficult to tune the space available for convective mass transfer, the surface area, and the diffusion lengths simultaneously in a packed bed
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