Hydroxy-functionalized hyper-cross-linked ultra-microporous organic polymers for selective CO2 capture at room temperature

Hydroxy-functionalized hyper-cross-linked ultra-microporous organic polymers for selective CO2 capture at room temperature Partha Samanta1, Priyanshu Chandra1 and Sujit K. Ghosh*1,2 Letter Address: 1Indian Institute of Science Education and Research (IISER), Pune. Dr. Homi Bhabha Road, Pashan, Pune-411008, India. Fax: +91 20 2589 8022; Tel: +91 20 2590 8076 and 2Centre for Research in Energy & Sustainable Materials, IISER Pune, Pashan, Pune, India Open Access Beilstein J. Org. Chem. 2016, 12, 1981–1986. doi:10.3762/bjoc.12.185 Received: 09 July 2016 Accepted: 19 August 2016 Published: 02 September 2016 Email: Sujit K. Ghosh* - This article is part of the Thematic Series "Organic porous materials". * Corresponding author Guest Editor: S. Bräse Keywords: carbon dioxide capture; hyper-cross-linked polymer; metal-organic framework; microporous organic polymer © 2016 Samanta et al.; licensee Beilstein-Institut. License and terms: see end of document. Abstract Two hydroxy-functionalized hyper-cross-linked ultra-microporous compounds have been synthesized by Friedel–Crafts alkylation reaction and characterised with different spectroscopic techniques. Both compounds exhibit an efficient carbon dioxide uptake over other gases like N2, H2 and O2 at room temperature. A high isosteric heat of adsorption (Qst) has been obtained for both materials because of strong interactions between polar –OH groups and CO2 molecules. Introduction The increase in the earth’s average temperature, also termed as global warming, is mainly due to the effects of greenhouse gases. The impacts of global warming includes rising sea level, more likelihood of extreme events (like floods, hurricanes etc.), widespread vanishing of animal population, loss of plankton due to warming seas. There are many heat-trapping greenhouse gases present in the atmosphere (from methane to water vapour), but CO2 puts us at the greatest risk if it continues to accumulate in the atmosphere. This is due to the fact that CO2 remains in the atmosphere in a time scale of hundred years in contrast to other greenhouse gases which leave the atmosphere with relatively smaller time scale [1]. The CO2 long life in the atmosphere provides the clearest possible rationale for carbon dioxide capture and storage. Previously, different types of amine solvents were employed to study the CO2 capture, but the need of high energy to regenerate the amine solutions after CO2 capture, hinders their applications further [2]. In the domain of porous materials, zeolites, metal-organic frameworks (MOFs), cage molecules, etc. have been introduced for selective uptake of CO2 [3-5]. In terms of surface area, tuneable porosity and feasible host–guest interaction, MOFs have scored over other above mentioned porous materials [6]. But the less hydrolytic stability of metal-organic frameworks limits their real time application [7,8]. So the search for new materials having high sur- 1981 Beilstein J. Org. Chem. 2016, 12, 1981–1986. face area and feasible interaction with carbon dioxide like MOFs and with high chemical stability have become one of top priority for researchers. Microporous organic polymers (MOP) are a relatively new class of porous materials, constructed from light elements like H, C, B, N, O etc. having a large surface area, small pore size and low skeletal density [9-12]. This type of materials has already been used for various purposes of applications such as gas storage, gas separation, catalysis, sensing, clean energy, etc. [13-18]. Relatively weaker coordination bonds in MOFs have been replaced with stronger covalent bonds in this type of porous compounds. This results in a high chemical stability of the microporous organic polymers, which is an essential condition for the real-time application of any compound. The last decade has witnessed advancements in synthesizing various types of microporous organic materials including covalent organic frameworks (COFs), conjugate microporous polymers (CMPs), porous polymeric networks (PPNs), porous aromatic frameworks (PAFs), covalent triazine framework (CTFs), etc. [1924]. Hyper-cross-linked microporous organic polymers (HCPs) are a subclass of this type of porous materials. Recently, hypercross-linked MOPs are emerged as a new subclass, synthesized by hyper-cross linking of basic small organic building blocks by Friedel–Crafts reaction in the presence of the Lewis acid FeCl3 (as catalyst) and formaldehyde dimethyl acetal (FDA) as the cross linker [25-27]. Here, aromatic small organic compounds are used to polymerise via C–C cross coupling to produce the targeted porous and physicochemical stable organic hyper- cross-linked polymeric materials. One huge advantage of this material is the low-cost synthesis, the cost-effective formaldehyde dimethyl acetal (FDA), FeCl3 and that organic small molecules can produce very low cost materials with high yield [28]. Hyper-cross-linking prevents the close packing of polymeric chains in this type of material to impart the intrinsic porosity. Hyper-cross-linked polymers have been applied in the field of gas storage, catalysis, separation and recently also in CO 2 capture [29-32]. The increasing environmental pollution due to carbon dioxide, urges us to develop new materials with high stability, which are cost-effective and demonstrate a high efficiency in CO2 capture. Based on the interaction of Lewis basic sites with carbon dioxide it has been observed that porous materials functionalised with –NH2 groups or –OH groups exhibit a selective uptake of CO 2 in contrast to other gases [33,34] (Scheme 1). Inspired by this we have designed and synthesized two hydroxy-functionalised hyper-cross-linked microporous organic polymers for selective CO2 capture at room temperature. Both compounds (HCP-91 and HCP-94) were synthesized via hyper-cross-linked C–C coupling of hydroxyl-functionalised aromatic rings by using a Friedel–Craftys reaction. At different temperatures (273 K and 298 K) gas (CO2, N2, H2 and O2) adsorption experiments were carried out for both compounds. HCP-91 and HCP-94 showed selective CO2 capture at both temperatures over other flue gases. Results and Discussion For the synthesis of HCP-91 and HCP-94, we used 4-phenylphenol and 9-(hydroxymethyl)anthracene, respectively Scheme 1: Schematic representation of selective CO2 capture in a porous material. 1982 Beilstein J. Org. Chem. 2016, 12, 1981–1986. (Figure 1). HCP-91 and HCP-94 have been synthesized by using a Friedel–Crafts alkylation reaction. The thus obtained as-synthesized compounds were washed repeatedly with dimethylformamide (DMF), methanol, water, chloroform, dichloromethane and tetrahydrofuran (THF) to obtain phase-pure hyper-cross-linked polymers. Both compounds were immersed in a CHCl3–THF (1:1) mixture and kept for 4–5 days to exchange the high boiling solvents occluded inside the framework with low boiling CHCl3 and THF. The solventexchanged phases of HCP-91 and HCP- (...truncated)