Metal-organic frameworks (MOFs)-graphene composites: growth and characterization of HKUST-1 on functionalized graphene layers for gas storage applications

Over the last twenty years Metal-Organic Frameworks (MOFs) have catalyzed the interest of many research groups around the world and, currently, they are well known as a new class of functional solid materials. MOFs are characterized by: i) highly crystalline 1-, 2- or 3-dimensional coordination networks; ii) porous structures with extremely high surface area and with tunable pore size dimensions; iii) network topologies that can be easily functionalized with different chemical groups; iv) many different technological fields of applications that range from gas storage/separation and catalysis to drug delivery and to many others [1]. One of the next challenges in this research area is the preparation of MOF composites in order to improve the properties of the component materials in a synergistic way [2]. MOF composites of graphite oxide (GO) [3] and functionalized reduced graphite oxide (rGO) have been recently reported in the literature. An increase in the CO2 uptake as well has the ability to remove toxic gases, such as H2S, NH3 and NO2, have been documented for some of these materials. More challenging is the obtainment of graphene/MOF composites due to the lack of functional groups on graphene layers that help the growth of MOF on the sp2 carbon layers. The work presented here has the aim to develop new graphene/MOF composites to be used in methane storage technologies. MOFs are characterized by very low thermal conductivity and the preparation of composites with a good thermal conductor, such as graphene, should give the opportunity to develop more energy efficient adsorbing materials. For the preparation of graphene/MOF composites, among all the MOFs prepared so far, we selected HKUST-1 [5] or [Cu3(BTC)2(H2O)3]n (H3BTC = benzene-1,3,5-tricarboxylic acid) for at least two reasons: its synthesis is simple, employ commercial reagents and has been widely investigated under different experimental conditions; recently it has been identified as one of the best porous MOF for methane storage [6]. To facilitate the preparation of the composites we used a benzoic acid functionalized reduced graphite oxide (BFG) that was prepared according to literature procedures [4]. HKUST-1 was synthetized with different methods in the presence of variable amounts of BFG and the obtained materials were characterized by spectroscopic methods, X-Ray diffraction, SEM, TEM (Figure 1) and nitrogen adsorption isotherms. Preliminary results indicate the formation of MOF/BFG composites. In particular, HKUST-1 grows on BFG layers without evident structural modifications even in the presence of large amounts of carbon materials; the morphology and size of HKUST-1 crystallites are affected by the presence of BFG; nitrogen adsorption isotherms on the composites materials show hysteresis not evident in the case of pure HKUST-1 samples [7].References [1] H.-C Zhou, J. Long, O. Yaghi, Guest Eds. 2012 Metal−Organic Frameworks Issue. Chem. Rev., 112 (2012) 673. [2] S. Li, F. Huo, Nanoscale, 7 (2015), 7482. [3] C. Petit, T. J. Bandosz, Adv. Mater., 21 (2009) 4753; C. Petit, T. J. Bandosz, Adv. Funct. Mater., 21 (2011) 2108. [4] M. Jahan, Q.L. Bao, J.X. Yang, K.P. Loh, J.Am.Chem.Soc., 132 (2010) 14487; M. Jahan, Q. L. Bao, K. P. Loh, J. Am. Chem. Soc., 134 (2012) 6707. [5] S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams, Science, 283 (1999) 1148. [6] J. A. Mason, M. Veenstra, J.R. Long, Chem. Sci. 5 (2014) 32; Y. Peng, V. Krungleviciute, I. Eryazici, J. T. Hupp, O. K. Farha, T. Yildirim, J. Am. Chem. Soc., 135 (2013) 11887. [7] This research was supported by Regione Lombardia (Project: NASEMS, Concert_EN-046).