Accurate Model for Predicting Adsorption of Olefins and Paraffins on MOFs with Open Metal Sites

Abstract

Metal–organic frameworks (MOFs) have shown tremendous potential for challenging gas separation applications, an example of which is the separation of olefins from paraffins. Some of the most promising MOFs show enhanced selectivity for the olefins due to the presence of coordinatively unsaturated metal sites, but accurate predictive models for such systems are still lacking. In this paper, we present results of a combined experimental and theoretical study on adsorption of propane, propylene, ethane, and ethylene in CuBTC, a MOF with open metal sites. We first propose a simple procedure to correct for impurities present in real materials, which in most cases makes experimental data from different sources consistent with each other and with molecular simulation results. By applying a novel molecular modeling approach based on a combination of quantum mechanical density functional theory and classical grand canonical Monte Carlo simulations, we are able to achieve excellent predictions of olefin adsorption, in much better agreement with experiment than traditional, mostly empirical, molecular models. Such an improvement in predictive ability relies on a correct representation of the attractive energy of the unsaturated metal for the carbon–carbon double bond present in alkenes. This approach has the potential to be generally applicable to other gas separations that involve specific coordination-type bonds between adsorbates and adsorbents.

Ethane/ethylene separation on a copper benzene-1,3,5-tricarboxylate MOF

“…The adsorbent preferentially adsorbed ethylene having an adsorption capacity between 8 and 12 mol·kg -1 . While, the adsorption capacity of ethane ranges from 6 to 9 mol·kg -1 [35]. The ethylene affinity on this material was also observed, in a previous work performed by Wang et al [36].…”

“…The preference for ethylene may be due to the interactions between the ethylene double -bonds and the open metal sites formed by the unsaturated copper atoms of the adsorbent [38,48]. The larger capacity of Cu-BTC for olefin with respect to paraffin was already observed in other previous publications [5,7,35,38,48,49].…”

“…The destruction of crystals during the shaping process is the most likely cause for this loss of capacity. Even so, from Table 6, it can be also noticed that the adsorption capacities of the Cu-BTC beads and powder [35], for ethane and ethylene, are higher than the adsorption capacities of the zeolites [13,23] presented in the same table. Entropic effects could be the reason for the larger adsorption capacity observed on MOFs, since the small octahedral cages of this adsorbent leads to a more efficient molecular packing.…”

“…In the bead form, the Cu-BTC showed a loss of adsorption capacities of 15% and 27% for ethane and ethylene, respectively, when compared with the Cu-BTC in powder form [35] (see Table 6). The destruction of crystals during the shaping process is the most likely cause for this loss of capacity.…”

Insights into CO2 and CH4 Adsorption by Pristine and Aromatic Amine-Modified Periodic Mesoporous Phenylene-Silicas

“…The experimental adsorption measurement technique adopted was previously described 43 and is shortly summarized here. Before performing each adsorption measurement, the void volume was determined by Helium picnometry, as described in our previous publication (43).…”