(660a) Measuring 3D Gas Adsorption Isotherms By X-Ray Computed Tomography

Imaging
techniques constitute a novel paradigm in characterization methods of porous
media; X-ray computed tomography is of particular interest because it allows
for the non-destructive determination of spatially resolved properties such as macropore space^[1],
and distribution of binder content^[2] and minerals^[3].
While the resolution is being continuously increased moving from medical
scanners towards monochromatic synchrotron micro-CT and nano-CT
setups, bench scale laboratory systems are still limited to voxel sizes above
1µm³, and therefore do not allow visualising micro- and meso-porosity (less than 50 nm) that is ubiquitous in both
natural (e.g., clay, coal, shale) and most porous materials used in the
chemical industry (e.g. adsorbent materials, catalysts, membranes). The most
widespread technique for the characterization of nanoporous
solids indeed is gas adsorption. While the interpretation of sub-critical
adsorption isotherms in terms of pore size, pore structure and connectivity is
being continuously refined^[4],
gas adsorption remains a technique that only provides bulk (macroscopic)
information and can therefore not provide any information regarding the
distribution of these properties.

The
images obtained from X-ray CT scans contain spatially resolved quantitative
information of the linear attenuation coefficient: the gray
scale value of each voxel is the volume weighted average of the attenuation of
each component and/or phase present in that specific voxel. Based on this
observation, Pini^[5] showed in a recent communication
that it is possible to extract the excess adsorption from the subtraction of a
scan of a porous solid taken in the presence of an adsorbate, and a scan of
that same solid taken under vacuum or in the presence of an inert (e.g.
Helium). This work has been extended here by developing the relevant
experimental and analysis protocols to measure full excess adsorption isotherms
by X-ray CT imaging for medical and micro-CT setups. By applying the devised
protocols to fixed beds of commercial zeolite 13X pellets (ZeoChem,
Uetikon Switzerland) and activated carbon rods (Norit, USA), we measured spatially distributed adsorption
isotherms of CO₂ -- one isotherm for each cubic millimeter
-- by means of X-ray CT imaging with a medical CT scanner (Universal Systems),
as illustrated in Fig. 1. Pellets can be imaged in a similar way at a larger
resolution by means of micro-CT imaging (Zeiss Versa XRM500): this enables
obtaining adsorption isotherms that are spatially resolved in a sub-millimeter scale.

Fig. 1  X-ray CT
image of a slice positioned at the interface between a Zeolite 13X and an
activated carbon layer at 30 bar CO₂. From difference images between
CO₂ scans taken at various pressures and a helium scan, excess
adsorption isotherms can be reconstructed for every voxel. Two examples are
shown for the rod-like particles (activated carbon, voxels 1 and 3, blue) and
for the spherical particles (zeolite 13X, voxels 2 and 4, red).

Because
this technique ultimately allows to quantitatively probe and visualize the
spatial distribution of typical metrics, e.g. pore volume and pore size
distribution, within individual pellets and across packed beds, it is
anticipated that it can support the development of formulation processes of
adsorbent powders into shaped particles by providing direct structural
information of the properties of interest, i.e., the adsorption isotherms.
Moreover, this work provides a novel way to probe nanoporosity
of heterogeneous porous media in multiple dimensions and in a non-destructive
way. As such, it is expected to bear significant importance in the
characterization of nanoporosity within natural
porous media, such as shales or reservoir rocks, where three-dimensional
information on the pore size, pore volume and pore inter-connectivity are
essential for the accurate description of gas transport.

References:

1.    
Blunt, M. J; Bijeljic, B; Dong, H;
Gharbi, O; Iglauer, S; Mostaghimi, P; Paluszny, A; Pentlan, C. Pore scale imaging and modelling. Advances in Water Resources 2013, 51, 197-216.

2.    
Mitchell, S; Michels, N; Kunze, K; perez-Ramirez, J. Visualization of hierarchically
structured zeolite bodies from macro to nano length
scales. Nature Chemistry 2012, 4, 825-831.

3.    
Lai, P; Moulton, K; Krevor, S. Pore-scale heterogeneity in the mineral
distribution and reactive surface area of porous rocks. Chemical Geology 2015, 411, 260-273.

4.    
Thommes, M; Kaneko, K; Neimark, A.V; Olivier, J. P; Rodriguez-Reinoso,
R; Rouquerol, J; Sing, K. S. W. Physisorption
of gases, with special reference to the evaluation of surface area and pore
size distribution. Pure and Applied
Chemistry 2015, 87, 1051-1069.

5.    
Pini R. Multidimensional
Quantitative Imaging of Gas Adsorption in Nanoporous
Solids. Langmuir 2014, 30, 10984-10989.