Engineering Fumed and Precipitated Silica Extrudates: Balancing Surface Area, Porosity, and Mechanical Strength

Abstract

Engineering a process to form high-surface-area, mesoporous, micrometer-sized particles into three-dimensional shapes for heterogeneous catalyst supports is crucial for controlling pressure drop, heat transport, and mass transfer within a reactor. For reactions where minimal acidity is required, silica is considered an attractive support due to its high surface area and porosity. Since catalyst supports may be deployed in either gas-phase fixed-bed reactors or liquid-phase systems, preserving the mechanical strength and mass-transport properties are key design considerations regardless of the reaction medium. However, extruding silica into supports that are mechanically strong with sufficient porosity can be a challenging task. In this work, extrusion formulations with varying calcination temperature, KOH, and methylcellulose content were systematically investigated to produce silica extrudates with sufficient mechanical strength for commercial use while maximizing pore volume and preserving surface area. The silica extrudates were characterized by nitrogen adsorption–desorption measurements (BET surface area and BJH pore size analysis), Hg porosimetry, mechanical strength tests, and scanning electron microscopy (SEM). It was revealed that KOH is the dominant driver for increasing mechanical strength while narrowing the mesopore size distribution. A single optimized formulation was developed and extended across a set of fumed and precipitated silica grades with diverse properties, including surface area, pore volume, particle size, and aggregate structure. The results showed that using the same formulation across different silica types produced a similar crush strength (33–37 N) while still preserving optimal textural properties (high surface area and pore volume). However, the intrinsic properties of each silica were largely preserved and governed the mesoporous structure of the final extrudate. Therefore, the extrusion formulation and processing conditions set the mechanical strength, while the intrinsic textural properties of the silica can be used to engineer the porosity of the silica extrudate. These findings can serve as a foundational guide when designing extruded catalyst support to meet specific requirements, whether for gas- or liquid-phase applications.