## 5 Light N-Paraffin Separation in Volcanic Glass  
Cuban volcanic glass and volcanic glass with higher Cu adsorption capacity, i.e., the sample without acid treatment activation, were chosen to evaluate their capacity to separate \(n\)-paraffins. The chromatographic profiles of the \(n\)-paraffin mixtures are compiled in Figure 5 for raw volcanic glass and Cu-volcanic glass. Taking into account the profiles of all experiments, it can be observed how each paraffin is eluted at a different retention time, so both samples are appropriate materials for the separation of n-paraffins. In all cases, the chromatographic peaks were sufficiently sharp and symmetric to consider the Henry zone, since the interactions with neighboring molecules must be nonspecific or show low intensity. As indicated, the adsorption of \(n\)-paraffins, which are only composed of C-C and C-H bonds, must be governed by nonspecific interactions. On the other hand, volcanic glass is an amorphous structure, mainly macroporous, although it also displays a certain microporosity (Figure 1). Thus, it is expected that the microporosity of volcanic glass can act as a molecular sieve for the separation of \(n\)-paraffins, as occurs with activated carbon and zeolites. In addition, it must be considered that the structure of volcanic glass expands with increased temperature, which could facilitate accessing n-paraffins inside the pores of volcanic glass. This can improve diffusion of the paraffins along the volcanic glass column, diminishing retention time. The representation of (In Vn) vs. (1/Tc) for each n-paraffin (Figure 6) can be fitted linearly, with \(R>0.99\) in all cases. The differential adsorption heat was determined from the slope, as shown in Table 3. These data reveal that adsorption heat is directly related to the length of the hydrocarbon, with the highest values obtained for nonane. \[Q_{d}\ C_{9}H_{20}>Q_{d}\ C_{8}H_{18}>Q_{d}\ C_{7}H_{16}>Q_{d}\ C_{6}H_{14}>Q_{d} \ C_{5}H_{12}\]  
Table 3 also reports how Cu-volcanic glass reaches higher adsorption heat values compared with raw volcanic glass. It is well known that transition metals, such as Ag, Cu, Co, and Ni, tend to interact with the double bond of the olefins [58]. The presence of these transition metals favors selective adsorption between olefins and saturated hydrocarbons with similar physicochemical properties, as takes place in propane-propylene separation. In the case of paraffins, the interactions between the Cu\({}^{2+}\) species and C-H and C-H bonds should be negligible. Recently, [PERSON] et al. reported the existence of Cu\({}^{2+}\)-alkane interactions by hyperfine sublevel correlation spectroscopy and electron paramagnetic resonance. Thus, it would be expected that the interaction between liquid \(n\)-paraffins and Cu\({}^{2+}\) would lead to greater retention time and increased adsorption heat [59]. Figure 5: Chromatograms of CH\({}_{4}\) + C\({}_{5}\)–C\({}_{9}\) mixture in (**A**) volcanic glass and (**B**) Cu\({}^{2+}\)-volcanic glass. \(T_{C}\) = 400 K + 5 K/min. The contribution of each -CH\({}_{2}\)- of the \(n\)-paraffins can be determined from the slope of the representation (adsorption heat vs. type of n-paraffin) (Figure 7), obtaining a Q\({}_{\rm diff}\) of 5.3 kJ/mol for raw volcanic glass, while Cu-volcanic glass only reaches a Q\({}_{\rm diff}\) of 4.2 kJ/mol (Table 4). In this way, previous research separated propane-propylene by inverse gas chromatography, indicating that propylene, which displays a stronger adsorbent-adsorbate interaction by the presence of the double bond, exhibits lower Q\({}_{\rm diff}\) in comparison to propane [20]. In this sense, the incorporation of Cu species into volcanic glass also causes a stronger interaction with n-paraffins, which could be related to a decrease of its Q\({}_{\rm diff}\) value. The variations of adsorption heat values between the n-paraffins are much higher than 1, confirming that both samples display an excellent capacity for separating \(n\)-paraffins.