A comparative study on conversion of porous and non-porous metal–organic frameworks (MOFs) into carbon-based composites for carbon dioxide capture
Graphical abstract
We report four novel carbon-based materials, converted from a non-porous Mg-MOF and a porous Zn-MOF for carbon dioxide capture. This comparative study highlights the relationship of the porosity generation via carbonization and the thermal/crystallographic stability.
Introduction
Carbon dioxide is the largest anthropogenic greenhouse gas contributing to global warming with combustion of fossil fuels being the major emission source. A range of technologies such as solvent scrubbing, membrane, and adsorption [1], have been developed to capture CO2 from flue gas mixtures. Therein, post-combustion carbon capture predominantly involves CO2/N2 gas separation at low pressures [2]. In particular, adsorption based technology is very promising due to its advantages including low cost, simple operation, versatile processes, etc. Adsorbent materials with high CO2 selectivity and capacity are critical to the improvement of separation efficiency and reduction of energy consumption of the separation processes.
Porous carbon and carbon–metal hybrid materials, featuring high thermal and chemical stability, have been intensively studied in gas adsorption and separation [3], [4], [5]. To achieve better control of the amorphous structure of the carbon materials, a synthetic strategy known as templating has been developed and is attracting increasing attention [6]. A variety of adsorbents (such as zeolites, alumina, silica, etc.) have been employed as templates for the synthesis of porous carbon materials [7], [8], [9], [10], [11], [12]. Recently, growing interest has been drawn to the synthesis of porous carbon materials from metal-organic frameworks (MOFs) or PCP (porous coordination polymers) which are constructed from metal nodes and organic linkers [13], [14]. On one hand, MOFs feature ultra-high porosity, structural diversity and tunability, providing great advantages as a starting material for carbon-based compounds. Templating with MOFs offers an exotic carbon source, low solubility, and non-volatility, along with pre-arranged organic ligand to enhance the controllability of chemical/physical properties of the carbon-based materials [15]. On the other hand, since a great number of MOF materials are not structurally stable and exhibit non-porous properties, it is a logical attempt to convert those MOFs into compounds with permanent porosity for gas adsorption and separation [16], [17], [18]. For example, nanoporous carbons with accessible N dopants were synthesized via the pyrolysis of pyridinedicarboxylate containing Zn-MOFs, showing high CO2 capacities and excellent cyclic performance over repeated CO2 adsorption/desorption [19]. Yamauchi et. al. reported a nano-porous carbon (NPC), converted from Al(OH)(1,4-NDC)·2H2O (1,4-NDC = 1,4-naphthanedicarboxylate), exhibiting extremely high surface area (5500 m2 g−1) [20]. However, the limited understanding of the role played by templates in MOF-derived carbon materials in adsorptive gas separation restricts its further application. One important factor that requires detailed investigations is the impact of thermal stability of MOF templates on the porosity and thus the gas sorption properties of the derived pyrolysis products as adsorbents.
Herein, we report the preparation and characterization of four novel carbon-based materials converted from a non-porous Mg3(BPDC)3·4DMF and a porous Zn3(BPDC)3(BiPY) based on the same bridging linkers (where BPDC = 4,4′-biphenyldicarboxylate, BiPY = 4,4′-dipyridyl, DMF = N,N-dimethylformamide). The as-obtained carbon materials selectively adsorb CO2 over N2 at low pressure and moderate temperature, which could be promising adsorbents for post-combustion carbon dioxide capture.
Section snippets
Chemicals
All reagents were purchased from commercial sources and used without further purification. Specifically, Zinc acetate dehydrate, Mg(CH3COO)2·4H2O, Zn(CH3COO)2·2H2O, 4,4′-biphenyldicarboxylic acid (BPDC), and 4,4′-dipyridyl (BiPY) (98%) were purchased from Sigma–Aldrich. N,N-dimethylformamide (DMF) (99.9%) was purchased from PROLABO CHEMICALS.
Preparation of MOF precursors Zn3(BPDC)3(BiPY) and Mg3(BPDC)3·4DMF
Zn3(BPDC)3(BiPY), denoted as Zn-MOF, was prepared following a similar procedure in the literature except for a few minor changes [21]. Specifically, a
Results and discussion
The high purity and crystallinity of the two as-synthesized MOF structures were confirmed by the synchrotron powder XRD (Fig. 1a and c). Their structures are similar to the ones reported in the literature, in spite of some variance in the reaction conditions in this work compared with the reported synthetic procedure [21], [22] (ESI, Table S1). The crystal structure of the Zn-MOF consists of trinuclear Zn clusters which are connected to the carboxylate groups of six BPDC bridging linkers to
Conclusion
In summary, we prepared four novel carbon-based products including Zn/Mg nano-particle imbedded carbon matrix and nano-porous carbons from MOFs via carbonization process. Carbons derived from the non-porous Mg-MOF template showed substantially greater surface area than that of the carbons converted by the porous Zn-MOF. This study provides insightful understanding of the relationship between the porosity generated by carbonization and the thermal/crystallographic stability of the template MOFs.
Acknowledgments
Y.H. acknowledges the Chinese Scholarship Council (CSC). P.A.W and J.S. acknowledge Australian Research Council (ARC) for providing the funding (DP2013000024). G.L. is the recipient of an Australian Research Council Discovery Early Career Researcher Award (DE140101824). Part of work was undertaken on the PD beamline, Australian Synchrotron.
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