Mechanical Characterization of Hierarchical Structured Porous Silica by in Situ Dilatometry Measurements during Gas Adsorption

Abstract

Mechanical properties of hierarchically structured nanoporous materials are determined by the solid phase stiffness and the pore network morphology. We analyze the mechanical stiffness of hierarchically structured silica monoliths synthesized via a sol–gel process, which possess a macroporous scaffold built of interconnected struts with hexagonally ordered cylindrical mesopores. We consider samples with and without microporosity within the mesopore walls and analyze them on the macroscopic level as well as on the microscopic level of the mesopores. Untreated as-prepared samples still containing some organic components and the respective calcined and sintered counterparts of varying microporosity are investigated. To determine Young’s moduli on the level of the macroscopic monoliths, we apply ultrasonic run time measurements, while Young’s moduli of the mesopore walls are obtained by analysis of the in situ strain isotherms during N2 adsorption at 77 K. For the latter, we extended our previously reported theoretical approach for this type of materials by incorporating the micropore effects, which are clearly not negligible in the calcined and most of the sintered samples. The comparison of the macro- and microscopic Young’s moduli reveals that both properties follow essentially the same trends, that is, calcination and sintering increase the mechanical stiffness on both levels. Consequently, stiffening of the monolithic samples can be primarily attributed to stiffening of the backbone material which is consistent with the fact that the morphology on the mesopore level is mainly preserved with the post-treatments applied.

In Situ Small-Angle Neutron Scattering Investigation of Adsorption-Induced Deformation in Silica with Hierarchical Porosity

“…This strain is roughly 0.65% for the as-prepared sample, while the calcined sample exhibits about a factor of two, and the sintered sample nearly an order of magnitude smaller strain. This is in qualitative agreement with the results from N 2 adsorption in a similar series of samples in ref (37), although the absolute values of the strains appear to be higher for water adsorption. For further discussion, it is important to keep in mind that the strain evaluated from SANS is because of deformation of the mesopore lattice, corresponding to the radial strain of the single struts, while dilatometry determines the strain of the macroporous strut network.…”

“…This procedure gives a reasonable description of the N 2 adsorption isotherms from the sintered and the calcined samples, respectively, except for the detailed shape and width of the hysteresis (see Figures S1 and S2, Supporting Information). We note that the parameter b (Table 2) is considerably smaller than the values found for samples with similar thermal history in ref (37), and also the hysteresis part of the present sample is less well described by the DBdB theory using the same reference isotherm. We attribute these differences to the aging of the present sample series at 50 °C/74% humidity for several weeks, as this procedure may have significantly changed the micropore size distribution and possibly also the interaction of the micro- and mesopores with N 2 .…”

“…For the quantitative description of the experimental strain isotherms obtained from in situ dilatometry and in situ SANS, we apply the theoretical framework presented in detail in refs (36) and (37). A major challenge for this approach in the present work is the unknown adsorption properties of the silica surface for water at 17 °C, which are very sensitive to the amount and quality of the surface groups still present after the sintering process.…”

“…It needs to be noted that the axial (σ a ) and radial (σ r ) stresses in the mesopores are uniquely determined by the modeling of the adsorption isotherms (eqs S7–S10, Supporting Information), and that the stress σ mic within the micropores given by (eq S14, Supporting Information)contains only one adjustable parameter ∂b/∂ε mic , which describes the variation of the adsorbate–micropore interaction b with the micropore strain according to the Shuttleworth relation. 67 From these stresses, the total linear strain in the radial mesopore direction is given by 37 …”

“…We compare the SANS strain isotherms with macroscopic dilatometry strain isotherms from the same samples. Moreover, we use the N 2 adsorption isotherms in conjunction with the water adsorption isotherms to estimate the water–silica interaction, and apply the theoretical model from refs (36) and (37) to quantitatively describe both, the adsorption and the strain isotherms.…”