A New Approach to Characterizing Sorption in Materials with Flexible Micropores

Abstract

Microporous dipeptides, also known as organic zeolites or biozeolites, as examples of small-pore peptide\ud nanotubes provide a convenient set of materials for developing a systematic approach based on 129Xe\ud NMR spectroscopy for the derivation of thermodynamic and molecular scale information on temperature\ud dependent pore filling. The sorption of xenon in the isolated 1D chiral nanochannels of eight microporous\ud dipeptides Ala-Val (AV), Val-Ala (VA), Leu-Ser (LS), Ala-Ile (AI), Val-Val (VV), Ile-Ala (IA), Ile-Val\ud (IV), Val-Ile (VI) (all LL isomers) was monitored in situ with continuous-flow 129Xe NMR spectroscopy\ud over a temperature range of 173-343 K. The materials all showed strongly anisotropic signals, with\ud isotropic chemical shift changing from 95 to 281 ppm depending on the dipeptide used and/or temperature.\ud The isosteric heats of sorption (qst) and entropy factors were determined from two independent models.\ud The sorption process was complicated by reversible phase transformations of some dipeptides and\ud irreversible changes due to aging of samples, both of which may be of considerable importance in\ud applications of soft materials. The interpretation of the line shapes and chemical shift anisotropy as a\ud function of temperature provided information on the structure of the xenon-cavity complex and made it\ud possible taking into account the helicity and flexibility of the nanochannels and the dynamics of xenon.\ud The approach illustrates a powerful way of analyzing pore space in soft microporous materials, yielding\ud a quantitative thermodynamic description of sorption and the characteristics of the pore space and sorption\ud events that occur on molecular-scale level during pore filling

A Channel‐Free Soft‐Walled Capsular Calixarene Solid for Gas Adsorption

“…The position of the signal together with the sign and span of the CSA imply Xe occupies the space within the calixarene capsule. [16] Similar to microporous materials with nanochannels, [17] the signal moves downfield with increased Xe pressure (see the Supporting Information). An estimate of the void space inside the calixarene capsule using single-crystal X-ray diffraction data [12] (Figure 1) allows one to conclude that the deep cavity should not be available for Xe adsorption because it already contains a fragment of one of the acyl arms.…”

“…[18] In our system, this effect can be attributed to the Xe atom in a site affected to some extent by the mobility of the cavity walls: the greater mobility of the acyl chains reduces the ability of the Xe to adsorb into the void space. [17b,c] In this region, the intensity of the NMR signal is determined by the thermodynamics of Xe adsorption by the calixarene, [17] and the isosteric heat of adsorption (q st = 10 kJ mol À1 ) calculated using the equation derived in Refs [17a,19] (see also the Supporting Information) corresponds to the physisorption of Xe. [17a, 19] The isotropic shift of Xe in this region is around 100 ppm, that is, somewhat larger than that in para-tert-butylcalixarene.…”

129Xenon NMR: Review of recent insights into porous materials

“…Microporous dipeptides, also known as organic zeolites or biozeolites, are examples of small-pore peptide nanotubes [165]. These peptides constitute an interesting materials family to get thermodynamic and molecular scale information following the pore filling as a function of temperature based on 129 Xe NMR spectroscopy.…”

Hydrophobic dipeptide crystals: a promising Ag-free class of ultramicroporous materials showing argon/oxygen adsorption selectivity

“…Studies performed using 129 Xe NMR 3,10 showed that sorbate-sorbent interactions in dipeptide crystal micropores are maximised when pore-size precisely matches sorbate size, decreasing for bigger and smaller pores. This is exactly the same behaviour as that observed for hard materials 51 .…”

“…Crystalline hydrophobic dipeptides of the VA-class 1, 2 are a type of ultramicroporous solids that has received considerable attention in recent years [3][4][5][6] . The structure of these molecular crystals is stabilised by H-bonds and hydrophobic interactions, being essentially the same for all VA-class dipeptides.…”