Anomalous Depletion of Pore-Confined Carbon Dioxide upon Cooling below the Bulk Triple Point: An In Situ Neutron Diffraction Study

Discovering Inherent Characteristics of Polyethylenimine-Functionalized Porous Materials for CO2 Capture

CO₂ capture is vital for addressing greenhouse gas (GHG)-based environmental issues worldwide. Amine-polymer/silica sorbents have been extensively studied for CO₂ capture, but the fundamental understandings of polyethylenimine (PEI) loading effect, thermal effect, and CO₂ sorption behavior are still lacking. Small-angle neutron scattering (SANS) offers promising opportunities for characterizing CO₂ sorption behavior of PEI-functionalized SBA-15. Herein, in situ SANS has been used to investigate not only PEI loading distribution but also PEI thermal swelling and temperature-dependent CO₂ sorption behavior of PEI-functionalized SBA-15. The results indicate that PEI could disperse on the mesopore surface for the sample with low PEI loading, while for the sample with high PEI loading, PEI could not only disperse on the mesopore surface but also partially fill in the mesopore as plugs. The sample with high PEI loading shows a two-stage swelling of PEI with increasing temperature from 25 to 120 °C in vacuum, in which the size of the intramolecular voids between PEI chains has no change from 25 to 75 °C but expands from 75 to 120 °C, whereas only a subtle swelling is observed up to 120 °C for the sample with low PEI loading. Besides the fact that in situ SANS successfully detects physisorbed CO₂ on the mesopore surface and chemisorbed CO₂ by the amine groups simultaneously: (1) the amount of physisorbed CO₂ increases with increasing pressure but decreases with increasing temperature, and (2) the amount of chemisorbed CO₂ has a trend of V_CO2 (75 °C) > V_CO2 (120 °C) > V_CO2 (25 °C). The thermal swelling of PEI causes dilation of intramolecular voids and thus increases the accessibility of chemisorption sites, resulting in higher CO₂ sorption capacity. Therefore, temperature and PEI swelling are essential factors for kinetic and thermodynamic controls of CO₂ capture in amine-functionalized porous adsorbents.

Neutron Scattering Measurements of Carbon Dioxide Adsorption in Pores within the Marcellus Shale: Implications for Sequestration

Shale is an increasingly viable source of natural gas and a potential candidate for geologic CO₂ sequestration. Understanding the gas adsorption behavior on shale is necessary for the design of optimal gas recovery and sequestration projects. In the present study neutron diffraction and small-angle neutron scattering measurements of adsorbed CO₂ in Marcellus Shale samples were conducted on the Near and InterMediate Range Order Diffractometer (NIMROD) at the ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory along an adsorption isotherm of 22 °C and pressures of 25 and 40 bar. Additional measurements were conducted at approximately 22 and 60 °C at the same pressures on the General-Purpose Small-Angle Neutron Scattering (GP-SANS) instrument at Oak Ridge National Laboratory. The structures investigated (pores) for CO₂ adsorption range in size from Å level to ∼50 nm. The results indicate that, using the conditions investigated densification or condensation effects occurred in all accessible pores. The data suggest that at 22 °C the CO₂ has liquid-like properties when confined in pores of around 1 nm radius at pressures as low as 25 bar. Many of the 2.5 nm pores, 70% of 2 nm pores, most of the <1 nm pores, and all pores <0.25 nm, are inaccessible or closed to CO₂, suggesting that despite the vast numbers of micropores in shale, the micropores will be unavailable for storage for geologic CO₂ sequestration.

Formation of Periodically Arranged Nanobubbles in Mesopores: Capillary Bridge Formation and Cavitation during Sorption and Solidification in an Hierarchical Porous SBA-15 Matrix

We report synchrotron-based small-angle X-ray scattering experiments on a template-grown porous silica matrix (Santa Barbara Amorphous-15) upon in situ sorption of fluorinated pentane C5F12 along with volumetric gas sorption isotherm measurements. Within the mean-field model of Saam and Cole for vapor condensation in cylindrical pores, a nitrogen and C5F12 sorption isotherm is well described by a bimodal pore radius distribution dominated by meso- and micropores with 3.4 and 1.6 nm mean radius, respectively. In the scattering experiments, two different periodicities become evident. One of them (d1 = 11.5 nm) reflects the next nearest neighbor distance in a 2D-hexagonal lattice of tubular mesopores. A second periodicity (d2 = 11.4 nm) found during in situ sorption and freezing experiments is traced back to a superstructure along the cylindrical mesopores. It is compatible with periodic pore corrugations found in electron tomograms of empty SBA-15 by Gommes et al. ( Chem. Mater. 2009, 21, 1311 - 1317). A Rayleigh-Plateau instability occurring at the cylindrical blockcopolymer micelles characteristic of the SBA-15 templating process quantitatively accounts for the superstructure and thus the spatial periodicity of the pore wall corrugation. The consequences of this peculiar morphological feature on the spatial arrangement of C5F12, in particular the formation of periodically arranged nanobubbles (or voids) upon adsorption, desorption, and freezing of liquids, are discussed in terms of capillary bridge formation and cavitation in tubular but periodically corrugated pores.