Stationary-phase contributions to surface diffusion at C8-modified silica mesopores

The structure, dynamics, and mobility of binary solvents and solute molecules at adsorbent surfaces play an important role in adsorption, catalysis, and separation. When investigating chemical systems, information gained by experimental data is often limited to the macroscopic view. Molecular dynamics (MD) simulations allow new insights on molecular processes and offer the possibility to study the molecular-level picture at solid-liquid interfaces in detail

Multiscale diffusion in porous media: From interfacial dynamics to hierarchical porosity

The transport of liquid and solutes in porous media over widely different time and length scales, ranging from specific interactions with the surface (and the associated interfacial dynamics) to the effective pore diffusion through hierarchical porosity, is central to many environmental and technological processes. This interplay between surface functionality and hierarchical porosity requires, on the one hand, a detailed molecular-level picture of sorption, reaction, and mobility, and realistic geometrical models of hierarchically porous media on the other, to establish (and apply) quantitative morphology– functionality–transport relationships for the tailored preparation of ever more selective and efficient materials for storage, separation, and catalysis

Implementation of high slurry concentration and sonication to pack high-efficiency, meter-long capillary ultrahigh pressure liquid chromatography columns

Slurry packing capillary columns for ultrahigh pressure liquid chromatography is complicated by many interdependent experimental variables. Previous results have suggested that combination of high slurry concentration and sonication during packing would create homogeneous bed microstructures and yield highly efficient capillary columns. Herein, the effect of sonication while packing very high slurry concentrations is presented. A series of six, 1 m × 75 μm internal diameter columns were packed with 200 mg/mL slurries of 2.02 μm bridged-ethyl hybrid silica particles. Three of the columns underwent sonication during packing and yielded highly efficient separations with reduced plate heights as low as 1.05

Olefin Ring‐closing Metathesis under Spatial Confinement: Morphology−Transport Relationships

Spatial confinement effects on hindered transport in mesoporous silica particles are quantified using reconstructions of their morphology obtained by electron tomography as geometrical models in direct diffusion simulations for passive, finite‐size tracers. We monitor accessible porosity and effective diffusion coefficients resulting from steric and hydrodynamic interactions between tracers and pore space confinement as a function of λ=d$_{tracer}$/d$_{meso}$, the ratio of tracer to mean mesopore size. For λ=0, pointlike tracers reproduce the true diffusive tortuosities. For λ>0, derived hindrance factors quantify the extent to which diffusion through the materials is hindered compared with free diffusion in the bulk liquid. Morphology‐transport relationships are then discussed with respect to the immobilization, formation, and transport of key molecular species in the ring‐closing metathesis of an α,ω‐diene to macro(mono)cyclization product and oligomer, with a 2nd‐generation Hoveyda‐Grubbs type catalyst immobilized inside the mesopores of the particles

Mesopore etching under supercritical conditions – A shortcut to hierarchically porous silica monoliths

Hierarchically porous silica monoliths are obtained in the two-step Nakanishi process, where formation of a macro microporous silica gel is followed by widening micropores to mesopores through surface etching. The latter step is carried out through hydrothermal treatment of the gel in alkaline solution and necessitates a lengthy solvent exchange of the aqueous pore fluid before the ripened gel can be dried and calcined into a mechanically stable macro mesoporous monolith. We show that using an ethanol water (95.6/4.4, v/v) azeotrope as supercritical fluid for mesopore etching eliminates the solvent exchange, ripening, and drying steps of the classic route and delivers silica monoliths that can withstand fast heating rates for calcination. The proposed shortcut decreases the overall preparation time from ca. one week to ca. one day. Porosity data show that the alkaline conditions for mesopore etching are crucial to obtain crack-free samples with a narrow mesopore size distribution. Physical reconstruction of selected samples by confocal laser scanning microscopy and subsequent morphological analysis confirms that monoliths prepared via the proposed shortcut possess the high homogeneity of silica skeleton and macropore space that is desirable in adsorbents for flow-through applications