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Chemically Activated Covalent Triazine Frameworks with Enhanced Textural Properties for High Capacity Gas Storage Yoon Jeong Lee, Siddulu Naidu Talapaneni, and Ali Coskun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08930 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017
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ACS Applied Materials & Interfaces
Chemically Activated Covalent Triazine Frameworks with Enhanced Textural Properties for High Capacity Gas Storage Yoon Jeong Lee,† Siddulu Naidu Talapaneni,† and Ali Coskun†,‡* †
Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. ‡
Department of Chemistry, University of Fribourg, Fribourg 1700, Switzerland.
KEYWORDS: pore narrowing, ionothermal synthesis, H2 storage, porous organic polymers, CO2 capture ABSTRACT: The chemical activation of porous/nonporous materials to achieve high surface area sorbents with enhanced textural properties is a very promising strategy. The chemical activation using KOH, however, could lead to broad distribution of pores originating from the simultaneous pore deepening and widening pathways. Accordingly, establishing correlation between the chemical/textural properties of starting porous/nonporous materials and various pore formation mechanisms is quite critical to realize superior porosity and gas uptake properties. Here, we show that the chemical and textural properties of starting porous organic polymers, that is covalent triazine frameworks (CTF), have profound effect on the resulting porosity of the frameworks. The chemical activation of microporous CTF-1 using KOH at 700°C enabled the preparation of chemically activated CTF-1, caCTF-1-700, which predominantly showed pore deepening leading to an increased surface area of 2367 m2 g-1 and significantly enhanced gas adsorption properties with CO2 uptake capacities up to 6.0 mmol g−1 at 1 bar and 1.45 mmol g−1 at 0.15 bar, 273 K along with a isosteric heats of adsorption (Qst) of 30.6 kJ mol−1. In addition, a remarkable H2 uptake capacity of 2.46 and 1.66 wt% at 77 and 87 K, 1 bar along with the Qst value of 10.95 kJ mol−1 at zero coverage was also observed for the caCTF-1-700. Notably, the activation of mesoporous CTF-2 under the same conditions accompanied by a decrease in its surface area and also in the conversion of mesopores into the micropores, thus leading to a pore deepening/narrowing rather than widening. We attributed this result to the presence of reactive weak spots, triazine moieties, for the chemical activation reaction within the CTF backbone. These results collectively suggest the critical role of chemical and pore characteristics of porous organic polymers in chemical activation to realize solid-sorbents for high capacity gas storage applications.
■ INTRODUCTION The development of graphitic materials at various dimensions through bottom-up approach is a promising strategy for the preparation of functional materials, which could, in principle, inherit some of the unique features of graphene while enabling the rational design of these carbonaceous materials with tunable surface properties and functionality.1-3 The introduction of porosity into these materials could be a good approach to increase the accessible surface area for various applications including gas and energy storage as well as heterogeneous catalysis.4-7 Modularity of bottom-up synthetic strategies also facilitates precise incorporation of heteroatoms into the carbonaceous materials, which is impossible achieve via traditional top-down approach. The introduction of heteroatoms, i.e., nitrogen, along with porosity is particularly important as it could offer high affinity binding sites for various guest species.8-9 In this direction, several promising examples of porous organic polymers (POPs), chemically stable covalent organic frameworks10-13 have been prepared and primarily studied in the context of gas capture and separation14-17, heterogeneous catalysis18-21 and energy storage22. In this context, covalent triazine frameworks (CTFs), which are a subclass of POPs, first reported by Thomas and Antonietti23, are highly promising materials. CTFs can be synthesized through the trimerization of aromatic nitriles using ZnCl224-25, triflic acid26 and elemental sulfur27-28 and possess extremely
high thermal and chemical stabilities due to the formation of robust covalent bonds along with high nitrogen contents. However, the preparation of CTF-1 with trifluoromethanesulfonic acid as the catalyst was infeasible, mainly because of the structural rigidity of the 1,4-dicyanobenzene. In addition, CTF-2 prepared using trifluoromethanesulfonic acid showed relatively low specific surface area of 464 m2 g-1 when compared to the
Scheme 1. Synthesis of covalent triazine frameworks (CTFs) starting from 1,4-dicyanobenzene (for CTF-1) and [1,1'-biphenyl]-4,4'-dicarbonitrile (for CTF-2) and their subsequent chemical activation using KOH under different temperatures to form chemically activated CTFs (caCTFs).
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CTF-2 synthesized by using ZnCl2. The high specific surface area, large pore volume and tunable porosities and the moderate conductivity of CTFs make them useful for a number of applications ranging from heterogeneous catalysis, solid supports for metal nanoparticles, metal-free activation of CO2, photocatalysis, gas sensors, gas storage, adsorption of toxic organics, hydrocarbon separation, electrochemical double layer supercapacitor, electrocatalyst, lithium storage, and Li-ion battery.29-38 More recently, the introduction of additional nitrogen sites and/or ultramicroporosity has been shown to be highly effective for CO2 capture and separation.39-41 However, the synthesis temperature of CTFs was found to be highly critical in the resulting porosity and higher reaction temperatures led to the formation of networks resembling activated carbon containing broad distribution of micro-, meso- and macro-pores,25, 38 thus decreasing the control over the textural properties. While hierarchical porosity is important for heterogeneous catalysis to improve mass transport kinetics,42-44 for the capture and storage of small gases such as CO2 and H2, porous materials with high micropore content are beneficial for selective and high affinity gas adsorption. The mechanism of gas adsorption on nanoporous carbonaceous materials is generally physisorptive,45-47 which relies on the specific surface area (SSA), pore characteristics (geometry and volume) and also surface functionalities. Hence, there have been enormous interest in the synthesis of microporous carbon materials with high nitrogen contents for efficient gas adsorption.48-49 Recent studies have shown the usage of physical and chemical activation methods to control the porosity in carbonaceous materials. Among them, chemical activation by potassium hydroxide (KOH) via carbonization of the mixture of carbonaceous species and KOH at high temperatures has been extensively used to achieve high surface areas towards their application in gas storage and supercapacitor applications.5, 50-53 Moreover, KOH activation enhances chemical etching process and has been shown to increase surface areas and create additional pores mainly in the micropore region (
