In situ synchrotron IR microspectroscopy of CO2 adsorption on single crystals of the functionalized MOF Sc2(BDC-NH2)3.

Synchrotron radiation (SR) IR microspectroscopy has enabled determination of the thermodynamics, kinetics, and molecular orientation of CO2 adsorbed in single microcrystals of a functionalized metal-organic framework (MOF) under conditions relevant to carbon capture from flue gases. Single crystals of the small-pore MOF, Sc2 (BDC-NH2 )3 , (BDC-NH2 =2-amino-1,4-benzenedicarboxylate), with well-defined crystal form have been investigated during CO2 uptake at partial pressures of 0.025-0.2 bar at 298-373 K. The enthalpy and diffusivity of adsorption determined from individual single crystals are consistent with values obtained from measurements on bulk samples. The brilliant SR IR source permits rapid collection of polarized spectra. Strong variations in absorbance of the symmetric stretch of the NH2 groups of the MOF and the asymmetric stretch of the adsorbed CO2 at different orientations of the crystals relative to the polarized IR light show that CO2 molecules align along channels in the MOF.

Probing hysteresis during sorption of cyclohexane within mesoporous silica using NMR cryoporometry and relaxometry.

The conventional data analysis methods for obtaining a pore size distribution (PSD) from gas sorption data make several critical assumptions that impact significantly on the accuracy of the PSD thereby obtained. In particular, assumptions must be made concerning the nature of the pore-filling or emptying process in adsorption, or desorption, respectively. The possibility of pore-pore interactions is also generally neglected. In this work, NMR cryoporometry and relaxometry have been used to study the adsorption and desorption of cyclohexane within a mesoporous, sol-gel silica catalyst support pellet with the aim of assessing the impact of the aforementioned problems for gas sorption PSDs and developing solutions. The advanced melting effect makes cryoporometry a particularly sensitive probe of adsorbate ganglia spatial distribution. It has been demonstrated that utilising gas sorption scanning curves provided insufficient additional information to alleviate the aforementioned problems with interpreting gas sorption data. The NMR data has shown how the nature of the sorption hysteresis changed with amount adsorbed, due to detectable variations in the mechanisms of pore-filling and emptying along the isotherm. Hence, relating a particular condensation or evaporation pressure to a specific characteristic pore size is not as straightforward as assumed in typical pore size analysis software. However, the NMR techniques reveal the additional information required to improve pore size estimates from gas sorption for disordered solids.

Probing the impact of advanced melting and advanced adsorption phenomena on the accuracy of pore size distributions from cryoporometry and adsorption using NMR relaxometry and diffusometry.

The accuracy of pore size distributions (PSD) obtained from gas adsorption and cryoporometry is compromised by the presence of advanced adsorption and advanced melting effects, respectively. In order to improve PSD accuracy, it is necessary to know the extent of such effects. In this work cryoporometry and adsorption have been combined to study the onset of advanced melting effects in a sample partially-saturated with different volumes of condensate, in turn, by pre-equilibration with different vapour pressures of adsorbate. NMR relaxometry and diffusometry have been used to independently study the size and connectivity of adsorbed liquid ganglia at different molten fractions. It has been found that the onset of significant advanced melting coincided with abrupt changes in levels of individual pore-filling and ganglia inter-connections determined by NMR. These findings also highlighted where significant advanced adsorption processes were occurring, where larger pores were being filled with condensate before smaller pores. These studies have enabled the critical pores governing the advanced processes to be identified, and the likely errors in PSDs arising from advanced effects to be quantified.