Precombustion CO2 capture in natural-gas plants predominantly involves the sep­aration of CO2 from H2 at high pressures, resulting in a pure H2 stream which is used in energy generation [12]. This process has a component with a higher con­centration of CO2, existing at elevated pressures, resulting in the relatively-low energy penalty for carbon capture of 10-16 % [13]. Additionally, due to the large differences in the polarisability and quadrupole moment between CO2 and H2, the two gases are more easily separated via chemical methods than other gases such as CO2 and N2 [61]. Porous materials with a high density of localized charge, such as achieved through open-metal sites, are particularly promising for this type of separation. Additionally, variances in gas properties such as diffusion rate may also be exploited by adsorbents to increase selectivity. Aside from selectivity for CO2, the working capacity of the adsorbent is another major factor in determining the effectiveness of candidate materials for precombustion capture processes. However, these factors are generally inversely related as a material with high selectivity will generally suffer low regenerability as the guest molecules are strongly bound to the adsorbent and are difficult to remove via a mild pressure-swing approach.

Whilst little work exploring H2 separation specifically from H2/CO2 mixtures has been performed using neutron scattering, more work using neutron scattering to study H2 confined in porous materials has been published than for any other guest molecule. This is a direct consequence of the ease of structural characterization of H (as D) using neutron diffraction as well as the unique information that can be gained for H2 using neutron spectroscopy [62]. This work is extensive and covered in publications concerning H2 storage [63, 64], where the interaction of H2 (D2) with Zn4O(bdc)3 (also known as MOF-5) [65, 66], Cu3(btc)2 [67-71], Mg2(dobdc) and Fe2(dobdc) as well as its oxidized analogue [72], Zn2(dobdc) [73], Al2(OH)2(bptc) [74], Zn(mIm)2 where mIm = 2-methylimidazolate and also known as ZIF-8 [75], Cu3[Co(CN)6]2 [76], as well as many carbonaceous materials and zeolites, have all been elucidated using neutrons. Such work is the subject of Chap. 8.