Gas separation with mixed matrix membranes obtained from MOF UiO-66-graphite oxide hybrids
Graphical abstract
Introduction
Membrane technology provides greater efficiency and simplicity of operation and lower capital and operating costs than traditional separation processes such as adsorption, cryogenic distillation and low-temperature condensation [1], [2]. However, it had been demonstrated that there is a limit to the performance of membrane polymers (the so called Robeson upper limit [3], [4]) due to the inverse relationship between permeability and selectivity, the key parameters in gas separation with membranes. To overcome this upper bound, new materials and procedures for membrane fabrication are under investigation, particularly mixed matrix membranes (MMMs) which, by the incorporation of fillers such as zeolites [5], metal-organic frameworks (MOFs) [6], [7], MOF-silicate hybrids [8], layered silicates [9] and carbon based materials [10], [11], have performed better than pure polymers, approaching or even exceeding the above-mentioned Robeson limit.
Some of the most interesting filler materials for MMMs are MOFs, in which a crystalline structure is generated by linking organic ligands to metal ions [12]. Compared to traditional inorganic fillers, the interaction with the polymer is enhanced due to the organic character of the MOF linkers. In addition, the size, shape, and chemical functionalities of the MOF cavities can be tuned to some extent by choosing the appropriate linker-metal couples [13]. In this case, the MOF chosen was UiO-66, which coordinates Zr ions to terephthalic acid [14]. UiO-66 has emerged as an efficient material for CO2 capture [15], and it has been predicted that its incorporation in a polymeric matrix would help overcome the Robeson limit in CO2/CH4 separation [16].
Graphite oxide (GO), prepared by chemical oxidation of graphite generating carboxylic groups, has been used to prepare a new type of hybrid material in combination with MOFs. The new material is formed by the coordination of the metal centers with the carboxylates present in the GO structure [17], [18]. MOF-GO nanocomposites combine the properties of the individual materials leading to an enhancement in ammonia adsorption in the case of MOF-5 and HKUST-1 [19], and also to the availability of new CO2 capture and NO2 adsorption media [20], [21].
In the present study, UiO-66-GO hybrid materials were obtained by direct solvothermal synthesis of the MOF UiO-66 on previously obtained GO material. The hybrids obtained were then dispersed in the glassy polymers polysulfone and polyimide to obtain MMMs for gas separation. While several works have addressed the preparation and application of graphene oxide [22], [23], UiO-66 [16], [24] and MOF-GO hybrid [25], [26], [27] MMMs, there are no reports on MMMs containing the UiO-66-GO hybrid as a filler. The MMMs are used in the present study to separate H2/CH4 and CO2/CH4 mixtures, in line with two recent publications which reveal the importance of UiO-66-GO hybrids in CO2 adsorption [28], [29].
Section snippets
Synthesis of UiO-66-GO hybrid materials
UiO-66-GO hybrid materials were prepared by adding different amounts of graphite oxide (GO) to the typical synthesis media of UiO-66. Firstly, GO was synthesized following Hummers’ method [30]. 1.5g of sodium nitrate (+99 wt%, Acros Organics) was dissolved in 70 mL of sulfuric acid concentrate (95.0–98.0 wt%, Sigma Aldrich). Next, 3g of graphite (RANCO 9904, 5 µm size, kindly supplied by Richard Anton KG) was added and the mixture was stirred for 30 min to obtain a homogeneous dispersion. 9g of
UiO-66-GO hybrid materials
Table 1 shows relevant characteristics of the GO, UiO-66 and UiO-66-GO hybrid materials. The UiO-66 wt% values in the final sample were obtained by drying the product at the end of the synthesis and considering that GO (constituted initially by both large, up to 10 µm thick agglomerates and fine particles as thin as 15 nm, as observed by SEM and TEM, not shown) was not dissolved during the solvothermal crystallization in DMF. Fig. 1 depicts the SEM characterization of the hybrids. These images and
Conclusions
The hydrothermal synthesis of MOF UiO-66 on GO sheets produced hybrid materials showing SEM, XRD and porosity features of both GO and UiO-66 materials. Moreover, the hybrids exhibit both high BET and external specific surface areas and the presence of nanosized particles of UiO-66. These two properties are of great important for producing MMMs with enhanced filler-polymer interaction and where the crystalline nature of UiO-66 is preserved, as shown by XRD. In fact, the XRD characterization
Acknowledgements
Financial support from Obra Social la Caixa and the Aragón Government (T05 and GA-LC-019/2011), and the ESF is gratefully acknowledged. We also acknowledge the use of the Servicio General de Apoyo a la Investigación-SAI (Universidad de Zaragoza). All the microscopy work was done in the Laboratorio de Microscopías Avanzadas at the Instituto de Nanociencia de Aragón (LMA-INA). The authors acknowledge the LMA-INA for offering access to their instruments and expertise.
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