(353g) Enhancement of C02 and CH4 Separation Via Amino-Functionalisation of the MIL-53 Metal-Organic-Framework

A strategy to enhance adsorption of CO2 consists of grafting of amines on surfaces of porous materials[1]. Already several types of Metal Organic Frameworks containing amino groups have been described in scientific and patent literature[2]. In the present work, adsorption and separation of CO2 and CH4 was studied on amino-functionalized MIL-53(Al).

The MIL-53 Metal Organic Framework has a 3-D metal-organic framework containing diamond-shaped channels with pores of free diameter close to 7.5Å, with an adsorption capacity close to 40 wt% for CO2 [3]. By using 2-amino terephtalic acid as organic linker instead of the terephtalic acid used in the synthesis of the parent MIL-53, a structure containing free standing amino groups was synthesised [4,5]. In order to probe the effective pore size of this material, adsorption of alkanes with different degree of branching was studied. Zero coverage adsorption properties of methane, ethane, propane and CO2 were determined on this material using the pulse chromatographic technique. Adsorption isotherms till 30 bar were obtained by volumetry at 5, 15, 30 and 40 °C. The separation of binary CH4/CO2 mixtures was studied by breakthrough experiments at different pressures and mixture compositions. As a reference material, MIL-53(Al), synthesised at the COK (Prof. D. De Vos, KULeuven, Belgium) was used.

The pulse chromatographic method shows that at 30°C, CH4, with a retention time < 5 seconds, is nearly not adsorbed, certainly in comparison to CO2 having a retention time of 5.65 min. This yields a separation factor larger than 60, which is significantly higher than the value of about 5 for the parent MIL-53(Al). On amino-MIL-53, the zero coverage adsorption enthalpy of CO2 (38.4 kJ/mol) is significantly larger than that of methane, ethane and propane. Contrarily, on the parent MIL-53(Al), zero coverage adsorption enthalpy and Henry constant of CO2 are lower than those of ethane. Functionalization with amine groups results in an increase in zero coverage adsorption enthalpy from 20.1 to 38.4 kJ/mol. This high interaction between CO2 and the amino-MIL-53(Al) structure has been demonstrated by in-situ DRIFTS, evidencing the formation of Electron Donor-Acceptor (EDA) complexes between CO2 and the amino and OH groups of the MOF structure [6].

Figure 1: Left: Adsorption isotherms of CO2 and CH4 at 30°C on amino functionalized MIL-53 (closed symbols: adsorption; open symbols: desorption). Right: Separation of an equimolar CO2/CH4 mixture at atmospheric pressure and 30°C.

The CO2 adsorption isotherm shows a 2-step behaviour (see Figure 1), corresponding to framework breathing [3]. In the second isotherm plateau, a capacity of 6.7 mmol/g or 30 wt% CO2 is reached. In contrast to CO2, CH4 is virtually not adsorbed at pressures below 2 bar. This differentiates amino-MIL-53(Al) from MIL-53(Al), which adsorbs significant amounts of CH4 at low pressure [7]. The presence of amino groups on the aromatic ring of the linker in the framework of amino-MIL-53(Al) reduces the number of apolar adsorption sites, leading to reduced CH4 adsorption. This also followed from pulse chromatographic experiments, showing a strong dependency of retention time on injection volume for CH4, but not for CO2. In comparison to MIL-53(Al), the framework re-opens at significantly higher pressures on amino-MIL-53(Al), which is related to the stronger interaction of CO2 with the amino-groups in the structure. Breakthrough experiments indicate an almost infinite selectivity at 1 bar between CO2 and CH4 on amino-MIL-53 (Figure 1), which is a very large improvement compared to MIL-53(Al), with a selectivity of about 7 at 1 bar [8]. For mixtures containing 1vol% of CO2 in methane, the selectivity still exceeds 20. Framework breathing during the adsorption process in breakthrough mode results in unusual breakthrough profiles, with the occurrence of a second step in the CH4 concentration profile right before CO2 breakthrough occurs.

In conclusion, it has been demonstrated that Metal Organic Frameworks can be effectively functionalized with amino groups [9]. The presence of such functional groups drastically enhances the affinity for CO2, resulting in very large selectivity in CO2/CH4 separation.

Acknowledgements

Senter Novem is gratefully acknowledged for financial support through the project EOSLT-04008 (Lange termijn EOS-onderzoeksprogramma). Joeri Denayer is grateful for financial support from FWO Vlaanderen.

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