Effects of amino functionality on uptake of CO2, CH4 and selectivity of CO2/CH4 on titanium based MOFs
Graphical abstract
Introduction
In recent years, natural gas becomes one of the alternative resources for transportation and other daily uses of energy. Natural gas is generally composed of up to 90% of methane and various proportions of impurities such as CO2, CO, and other hydrocarbons depending on the source of the gas [1], [2], [3]. Removing and separation of non-hydrocarbon gas contaminants, mainly carbon dioxide (CO2) from natural gas, is one of the important aims in natural gas processing [4]. Separation of carbon dioxide from methane in natural gas can take place through some technologies such as adsorption and absorption [5], [6].
Absorption approach in natural gas industry has been used broadly using aqueous amine solutions through chemisorption. Nonetheless, limitations in absorption process are high operation cost and huge amount of energy requirement for amine regeneration [7]. Porous materials such as zeolites [8] and activated carbon [9] have been used as adsorbents in two main adsorption processes, pressure swing adsorption (PSA) and temperature swing adsorption (TSP) [4], [10]. However, selectivities for such adsorbents are not good enough for CO2 separation from flue gases. Thus, investigation and researching to develop new porous materials with high selectivity and high CO2 adsorption capacity have been carried out. Metal organic frameworks (MOFs) are one class of porous materials being recognized for enhancing gas storage and selectivity over the traditional adsorbents [11].
MOFs materials are synthesized through coordination bonds between metal clusters and organic ligands, acquisition a high specific surface area, high pore volume and low density [12], [13], [14]. These materials have received significant attention by many researchers regarding capture of CO2 and separation of CO2 from CH4 [15], [16]. Some of them can be achieved during the synthesis process, while others may be done via post–synthesis [17], [18]. Functionalized MIL-53(Al) has been found to show good separation of CO2/CH4. Recently, mixed linkers MOFs and mixed metal MOFs have been studied for enhanced adsorption capacity and selectivity of mixtures gases. High separation of CO2/CH4 has been found on mixed linkers MOFs in recent years [19]. Uptakes of CO2 have been investigated on mixed metals such as ZnTi and ZnW [20], [21] as well as on mixed linkers in Al-MOF [22].
Ti-based MOFs (MIL-125) have been prepared by several researchers for different applications such as gas separations and photocatalysis [23], [24]. Moreira et al. used Ti-based MOFs for separation [23] and Zhang et al. tested them as a humidity sensor [25]. However, few investigations have been reported for Ti-based MOFs for CO2/N2 or CO2/CH4 separation. Wiersum et al. proposed an adsorbent performance indicator (API) and evaluated uptake of carbon dioxide and methane adsorptions on various MOFs including Ti-based MOFs [26]. Generally, it is believed that amine-functionalization can improve CO2 adsorption. However, it may also affect adsorption of other gases, influencing separation efficiency. In addition, Ti-MOF by mix-linker has not been reported. Therefore, in this study, we synthesized a mixed linker Ti-MOF [MIX-MIL-125(Ti)] and investigate adsorption capabilities of the MIX-MIL-125(Ti) as well as NH2-MIL-125(Ti) and MIL-125(Ti) toward CO2 and CH4 at different temperatures and high pressures to understand the effect of amino functionality on adsorption and separation.
Section snippets
Chemicals
All chemicals including titanium isoproproxide (Ti(OiPr)4, 99.9%), N, N-dimethylformamide (DMF, C3H7NO, 98%), methanol (Analytic grade, CH3OH, 99%), 1,4-benzenedicarboxylic acid (BDC), 2-aminoterephthalic acids (H2BDC-NH2, 99%), were supplied by Sigma–Aldrich without further purification. Also, commercial activated carbon and zeolite 13X samples were supplied by ChemFFX and UOP, respectively.
Synthesis and activation of various samples
MIL-125(Ti) and NH2-MIL-125(Ti) were synthesized based on the previous reports with some modifications
Results and discussion
XRD patterns of all samples are shown in Fig. 1. It can be seen that the profiles of the as-synthesized samples are similar to the patterns of MIL-125(Ti) and NH2-MIL-125(Ti) as reported in the previous studies [27], [30]. MIX-MIL-125(Ti) sample also showed a similar pattern of MIL-125(Ti) and NH2-MIL-125(Ti). The presence of NH2 groups in the organic linkers in MIX-MIL-125(Ti) does not affect the structure of MIL-125(Ti).
FTIR spectra of samples are shown in Fig. 2. In Fig. 2a, the bands at
Conclusions
Titanium-based metal organic frameworks, MIL-125(Ti), NH2-MIL-125(Ti) and MIX-MIL-125(Ti), were successfully synthesized, characterized and tested as adsorbents for CO2 and CH4 adsorption under high pressure. It was found that addition of amino functional group will significantly improve CO2 and CH4 adsorption. NH2-MIL-125(Ti) can present the highest CO2 adsorption around 10.76 mmol/g and 8.93 mmol/g at 273 and 298 K, and pressure of 988 kPa. However, CH4 adsorptions are less than CO2 uptake giving
Acknowledgments
We thank Ms Elaine Miller for SEM measurements. We also acknowledge the Ministry of Higher Education and Minister of Natural Resources/Kurdistan regional government-Iraq for PhD scholarship.
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