Response to Extreme Temperatures of Mesoporous Silica MCM-41: Porous Structure Transformation Simulation and Modification of Gas Adsorption Properties

Abstract

Molecular dynamics (MD) and Monte Carlo (MC) simulations were applied together for the first time to reveal the porous structure transformation mechanisms of mesoporous silica MCM-41 subjected to temperatures up to 2885 K. Silica was experimentally characterized to inform the models and enable prediction of changes in gas adsorption/separation properties. MD simulations suggest that the pore closure process is activated by a collective diffusion of matrix atoms into the porous region, accompanied by bond reformation at the surface. Degradation is kinetically limited, such that complete pore closure is postponed at high heating rates. We experimentally observe decreased gas adsorption with increasing temperature in mesoporous silica heated at fixed rates, due to pore closure and structural degradation consistent with simulation predictions. Applying the Kissinger equation, we find a strong correlation between the simulated pore collapse temperatures and the experimental values which implies an activation energy of 416 ± 17 kJ/mol for pore closure. MC simulations give the adsorption and selectivity for thermally treated MCM-41, for N 2 , Ar, Kr, and Xe at room temperature within the 1−10 000 kPa pressure range. Relative to pristine MCM-41, we observe that increased surface roughness due to decreasing pore size amplifies the difference of the absolute adsorption amount differently for different adsorbate molecules. In particular, we find that adsorption of strongly interacting molecules can be enhanced in the low-pressure region while adsorption of weakly interacting molecules is inhibited. This then results in higher selectivity in binary mixture adsorption in mesoporous silica.

Temperature Activates Contact Aging in Silica Nanocontacts

“…The density of the amorphous silica thus created is of 2.22 g=cm 3 , in agreement with the experimental value [71] of 2.20 g=cm 3 and previous MD simulations [40,41,57,58,63,[65][66][67]69,71]. The amorphization of the sample has been confirmed by calculating the radial distribution functions of Si─Si, Si─O, and O─O pairs whose first peaks occur, respectively, at 3.13, 1.65 and 2.67 Å, in agreement with previous MD simulations as well as with experimental results [40,41,57,58,63,[65][66][67]69,71]. The root-mean-square roughness of the surface thus obtained is of 1.56 Å, which is similar to a previously reported MD simulation whose protocol was followed here [2] and close to the experimental value [45].…”

“…This parametrization provided results in excellent agreement with experimental data [63]: namely, the structural properties and energies of the various crystalline phases as well as the amorphous phase for silica [57,63], radial distribution functions, and phonon density of states of silica glass [63]. Furthermore, it has also been experimentally validated to describe atomic rearrangements in silica glass surfaces induced using aberration-corrected transmission electron microscopy [37], transformation mechanisms [65] and thermal conductivity [65,66] in porous silica, and also the Hugoniot curves [58,67] describing the dynamic shock-induced response of silica. Additionally, a good agreement with experimental results has also been found in other mechanical properties such as the deformation behavior of amorphous silica upon indentation [40,41].…”

“…[68]. Then, in order to remove the superperiodicity of 5 nm, we improved SiO 2 amorphization by following the standard melting-quenching procedure [2, 12,13,40,41,57,58,63,[65][66][67]69]. This protocol consisted of melting SiO 2 by heating it up to 5000 K in 250 ps at zero pressure followed by a quenching to 300 K with a cooling rate of 11 K=ps.…”

Preparation and thermal properties of shape‐stabilized polyethylene glycol/mesoporous silica composite phase change materials for thermal energy storage

“…Therefore, mesoporous matrix stabilized CPCMs is one type of the most promising materials for thermal energy storage applications. As the most important type of materials, porous silica has attracted extensive interests owing to its excellent properties, such as low cost, small density, large specific surface areas, high adsorption capacity, satisfying thermal conductivity, and environment‐friendly nature, which suggests that mesoporous silica (MS) can be utilized as a good supporting material …”

“…As the most important type of materials, porous silica has attracted extensive interests owing to its excellent properties, such as low cost, small density, large specific surface areas, high adsorption capacity, satisfying thermal conductivity, and environment-friendly nature, which suggests that mesoporous silica (MS) can be utilized as a good supporting material. [28][29][30][31] In this study, MS was prepared through a one-step selfassembly process using tetraethyl orthosilicate (TEOS) as silica precursor and cetyltrimethyl ammonium bromide (CTAB) as the template under alkaline condition. Lately, a series of shape-stabilized PEG/MS composite phase change materials (PEG/MS ss-CPCMs) with different mass percentage of PEG6000 were prepared through a vacuum impregnation technique.…”