Pore Size Effect on Methane Adsorption in Mesoporous Silica Materials Studied by Small-Angle Neutron Scattering

Abstract

Methane adsorption in model mesoporous silica materials with the size range characteristic of shale is studied by small-angle neutron scattering (SANS). Size effect on the temperature-dependent gas adsorption at methane pressure about 100 kPa is investigated by SANS using MCM-41 and SBA-15 as adsorbents. Above the gas-liquid condensation temperature, the thickness of the adsorption layer is found to be roughly constant as a function of the temperature. Moreover, the gas adsorption properties, such as the adsorbed layer thickness and the specific amount of adsorbed gas, have little dependence on the pore size being studied, i.e., pore radius of 16.5 and 34.1 Å, but are mainly affected by the roughness of the pore surfaces. Hence, the surface properties of the pore wall are more dominant than the pore size in determining the methane gas adsorption of pores at the nanometer size range. Not surprisingly, the gas-liquid condensation temperature is observed to be sensitive to pore size and shifts to higher temperature when the pore size is smaller. Below the gas-liquid condensation temperature, even though the majority of gas adsorption experiments/simulations have assumed the density of confined liquid to be the same as the bulk density, the measured methane mass density in our samples is found to be appreciably smaller than the bulk methane density regardless of the pore sizes studied here. The mass density of liquid/solid methane in pores with different sizes shows different temperature dependence below the condensation temperature. With decreasing temperature, the methane density in larger pores (SBA-15) abruptly increases at approximately 65 K and then plateaus. In contrast, the density in smaller pores (MCM-41) monotonically increases with decreasing temperature before reaching a plateau at approximately 30 K.

A non-invasive method to directly quantify surface heterogeneity of porous materials

“…Thus the GPSLM can be applied. The density of methane inside kerogen pores at room temperature can be assumed to be bulk methane density in our cases 32 – 34 , i.e. (details are given in Supplementary Note 4 ).…”

Adsorption of ammonia using mesoporous alumina prepared by a templating method

“…The highly regular pore structure and internal surface area of ordered MS make these particles attractive substrates that can be tailored for NH 3 adsorption [8]. For example, MS particles showed very high uptake capacities and selectivities for CH 4 [9], CO 2 [10], H 2 S [11], NH 3 [12], and volatile organic vapors [13]. Few studies on the removal of hazardous gases by adsorption using MA are reported, even though MA and MS share similar pore properties and high thermal stabilities, as seen in comparison to reports about MS in the literature [14,15].…”

Investigation of porous materials with large surface heterogeneity using the generalized Porod's scattering law method

“…GPSLM requires the use of contrast variation. To do contrast variation, SLD of the guest fluid, = , in accessible pores can be changed by mixing hydrogenated and deuterated solvents, such as H2O and D2O, with different ratios [28] for neutron scattering or loading gas with different pressures for both X-ray and neutron scattering [33][34][35]. Thus, the GPSLM can be used for both SAXS and SANS even though we use SANS as the example to demonstrate this method.…”

“…In some works,[39] the surface of large domains is called "envelope" surface. As a simple example, the Porod's scattering from the grain boundaries of mesoporous materials shown at low Q region cannot pick up the mesopore structure because the size of mesopores is too small compared with the Porod's scattering length scale L[33,34,40]. Case 3 can be applied to analyze the Porod's scattering region at low Q for a mixture of mesoporous materials composed of different chemical components.…”