On the underestimated impact of the gelation temperature on macro- and mesoporosity in monolithic silica

Abstract

The preparation of monolithic SiO with bimodal porosity using a special sol-gel procedure ("Nakanishi process") generally shows a pronounced sensitivity towards several physico-chemical parameters of the initial solution (concentrations, precursors, pH, temperature, etc.). Thus, temporal and spatial variations of these parameters during the sol-gel reactions can affect the final meso- and macropore space with respect to the pore size distributions and homogeneity. In this study we thoroughly examine the sol-gel reaction in terms of the impact of temperature accuracy and homogeneity during the gelation and their effect on meso- and macropore space. The in-depth characterization of the macroporosity in monolithic SiO rods, prepared by utilizing a highly homogeneous and accurate temperature profile, shows that a decrease of only 1.5 °C during the reaction doubles the mean size of the macropores in the analyzed temperature ranges (22.0-28.0 °C and 33.5-36.5 °C). Rheological measurements of the gelation points and the viscosity of the starting solutions prove that a higher reaction rate is the main reason for this marked temperature-sensitivity. Furthermore, the mesoporosity is affected to a surprising extent by the applied small temperature differences during the gelation reaction. This phenomenon is shown to be mainly caused by the temperature-dependent differences in macropore and skeleton dimensions and an inhomogeneous distribution of mesopore sizes within the skeleton. In essence, our study reveals that the impact of temperature on the formation of meso- and macroscale dimensions during the sol-gel process has been underestimated so far. The impact of a poor temperature homogeneity during monolith synthesis is exemplarily demonstrated by the application of monolithic silica capillary columns in HPLC.

Immobilization of a copper complex based on the tripodal ligand (2‐aminoethyl)bis(2‐pyridylmethyl)amine (uns‐penp)

“…As described above monoliths based on mesoporous silica were applied as well to test for further applications in a flow reactor. Monoliths were synthesized by sol‐gel procedures (“Nakanishi process”) as described previously [47–49] . In contrast to the procedure applied with silica powder described above the monoliths were not treated with concentrated HCl because these rough conditions could cause damage to the cylindrical shape.…”

“…Monoliths were synthesized by sol-gel procedures ("Nakanishi process") as described previously. [47][48][49] In contrast to the procedure applied with silica powder described above the monoliths were not treated with concentrated HCl because these rough conditions could cause damage to the cylindrical shape. No further purification for the monoliths was performed to avoid deformation.…”

DMAP Immobilized on Porous Silica Particles and Monoliths for the Esterification of Phenylethanol in Continuous Flow

“…The synthesis of the monolith was adapted from Meinusch et al [26] 1.200 g polyethylene glycol (10,000 g mol À 1 ) and 0.900 g urea were dissolved in 10 mL 0.01 M ChemCatChem acetic acid and stirred for 35 min at 0 °C. 5.6 mL tetramethyl orthosilicate were added and the mixture was stirred at 0 °C for 20 min.…”

“…The second silica material are mesomacroporous monoliths synthesized after a modified "Nakanishi process". [24][25][26] This synthesis can be performed with good reproducibility, and thus these materials are commercialized for HPLC separation. [27][28][29][30][31][32] With their bimodal porosity comprising macroporous flow-through space and a large surface area due to their mesoporosity, these SiO 2 monoliths are also promising solid supports for heterogeneous catalysis in continuous flow.…”

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

“…Urea proved to be a very suitable pore controlling agent, especially with respect to the mean mesopore width due to homogeneous pore widening during hydrothermal treatment (Kohns et al, 2018). However, to independently alter mean macropore width and to keep the mean mesopore width constant, the present synthesis procedure was modified via tuning of the gelation temperature (Meinusch et al, 2017).…”