Adsorption and Fixation into Cyclic Carbonates - American Chemical

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Hydroxylamine-Anchored Covalent Aromatic Polymer for CO Adsorption and Fixation into Cyclic Carbonates 2

Seenu Ravi, Pillaiyar Puthiaraj, and Wha-Seung Ahn ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01588 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Sustainable Chemistry & Engineering

Hydroxylamine-Anchored Covalent Aromatic Polymer for CO2 Adsorption and Fixation into Cyclic Carbonates Seenu Ravi, Pillaiyar Puthiaraj and Wha-Seung Ahn* Department of Chemical Engineering, Inha University, Incheon, Korea *Corresponding author: [email protected]

Abstract Hydroxylamine-anchored covalent aromatic polymer (CAP-DAP) was synthesized from p-terphenyl and 1,3,5-benzene tricarbonyl chloride, followed by subsequent functionalization with 1,3-diamino-2-propanol for CO2 capture and metal-free catalysis in CO2–epoxide cycloaddition reactions. The novel CAP-DAP material was characterized using various analytical techniques. It showed a very good CO2 adsorption capacity of 153 mg/g along with a high (CO2/N2) selectivity of 86 at 273 K/1 bar, in contrast to bare CAP, which exhibited moderate CO2 adsorption of 136 mg/g with a CO2/N2 selectivity of 47. CAP-DAP also displayed high catalytic activity for CO2–epoxide cycloaddition reactions under mild and solvent-free conditions. The synergistic effect between metal-free CAP-DAP and tetrabutylammonium bromide (n-Bu4NBr) enabled a high epoxide conversion of 98% coupled with an excellent product selectivity of 99% at 60 ℃, 1 bar CO2, and a reaction time of 12 h. Faster reaction kinetics with reaction times < 6 h was possible at 80 ℃. The catalyst also showed excellent reusability and no leaching of active species was observed from the spent catalyst. Based on experimental results, a plausible reaction mechanism for CO2– epoxide cycloaddition reaction over CAP-DAP catalyst has been proposed. Key words: Covalent organic polymer, hydroxylamine, CO2 adsorption, carbon dioxide fixation, cyclic carbonate synthesis

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Introduction Climate change by global warming is largely due to excessive CO2 emission into the atmosphere, mainly associated with anthropogenic processes such as fossil-fuel-fired power plants, vehicular emissions and deforestation.1 Among these, fossil fuel-fired power plants are responsible for about 77% of the total emissions, which is ~ 12 billion tons of CO2 every year; moreover, this value continues to increase.2,

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In this scenario, carbon capture and

storage (CCS) methods are considered the most effective means to mitigate the direct emission of CO2 from power plants. On the other hand, CO2 is also nontoxic, ubiquitous, and can be used as a sustainable C1 resource to produce high-value commodity chemicals such as urethane,4 formic acid,5 methanol,6,

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and cyclic carbonates.8 Ideally, accomplishing CO2

capture and CO2 fixation, with the same active material serving both as adsorbent and catalyst, would be highly desirable. Several materials have been investigated for their potential for CO2 capture or for catalytic CO2 conversion including supported silica,9, 10 metal organic frameworks (MOFs),1113

covalent organic polymers (COPs),14-18 and porous carbons.19, 20 Among these materials,

COPs, in view of their numerous advantageous properties including high surface areas, tunable skeletons, excellent physicochemical stabilities, and flexibility for rational designing, have been shown to be highly promising for efficient CO2 adsorption and organic transformation.21-24 COPs have been prepared via several different synthetic routes such as coupling reactions,25, 26 Friedel-Crafts reaction,22, 23 Co(0)-catalyzed trimerization of alkynes,27 ZnCl2-catalyzed trimerization of nitrile,28 Schiff-base reaction,29 nucleophilic substitution reaction,30 and co-condensation/polycondensation reactions.31 Among these, Friedel-Crafts reaction is a simple and facile synthetic route to prepare the high surface area COPs from commercially available chemicals.

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To date, various COPs with different active functional sites for CO2 adsorption have been investigated; for example, a BODIPY-containing COP was reported, which adsorbed 99.0 mg g−1 of CO2 at 273 K/1 bar with a maximum CO2/N2 selectivity of 38.6.32 Porous aromatic frameworks (PAF-1) have shown 89.2 mg g−1 of CO2 adsorption capacity at 273 K/1 bar.33 Recently, Yang et al. developed a nitrogen-rich hyper-crosslinked porous polymer (FCDTPA) and its carbonized form and reported CO2 uptakes in the range 92.8–128.5 mg g−1 (273 K/1 bar).34 Kaskel et al. reported a porous polyimine, which showed a CO2 uptake of 86.7 mg g−1 at 273 K/1 bar with CO2/N2 selectivity of up to 25.35 A series of aminefunctionalized microporous aromatic polymers (CBAP-EDA) have also been reported by Puthiaraj et al. where CO2 adsorption in the range 124–131 mg/g with CO2/N2 selectivity in the range of 87–97 was observed.36 In general, with the exception of CBAP-EDA, most other reported materials have shown rather low CO2/N2 selectivities despite having high CO2 adsorption capacities. Thus, it seems that a systematic control of pore dimensions and appropriate surface functionalization using “CO2-philic” organic groups within the COP network are necessary conditions to achieve high CO2 capture with enhanced CO2 selectivity. Interestingly, certain functionalized COPs have also been applied as catalysts for CO2 fixation via cycloaddition reactions to various oxirane groups, where functional moieties that can act either as a Lewis/Brønsted acid or as a nucleophile are necessary to promote the reaction.37-39 Zn/HAzo-POP,40 Zn(Por)OP,41 Zn/POP-TPP,42 CBAP-1(EDA-Zn),43 Cu/POPBpy,44 and Bp-Zn@MA45 have been reported to be effective catalysts for CO2 and oxirane cycloaddition reactions under mild reaction conditions; however, all of them used transition metals to provide catalytic Lewis sites during the reaction. On the other hand, metal-free porous polymer catalysts have only been rarely studied under ambient reaction conditions.46, 47

Zhi et al. reported a metal-free covalent organic framework structured material for the

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chemical fixation of CO2 into cyclic carbonates, where 92% conversion and 99% selectivity were obtained at 40 ℃ and 0.1 MPa after a reaction time of 48 h.48 Wu et al. reported a triphenylphosphine-based porous polymer catalyst, which yielded 97% conversion with 99% selectivity at 120 ℃ after 4 h reaction time.49 Nevertheless, these protocols have shortcomings such as high cost of material synthesis, slow reaction kinetics, or the need for a high reaction temperature. In this regard, the synthesis of a high porosity polymer catalyst using economically viable precursors followed by a post-synthesis grafting of high hydroxyl and amine comprised functional groups is necessary, so as to develop a more efficient metalfree organocatalyst, which can efficiently capture and transform CO2. With this objective, herein we report a new porous covalent aromatic polymer (CAP) with narrow pore size distribution, synthesized from p-terphenyl and 1,3,5-benzene tricarbonyl chloride, which was further functionalized with diaminopropanol via postsynthesis grafting (scheme 1). In our study, CAP-DAP was applied for CO2 adsorption in the temperature range of 273–298 K and also as a catalyst for CO2 cycloaddition to various epoxides under different reaction conditions.

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Scheme 1. Synthesis schemes of a new porous covalent aromatic polymer (CAP-DAP) functionalized with 1,3-diamino-2-propanol

Experimental Chemicals and reagents 1,3,5-Benzenetricarbonyl trichloride, p-terphenyl, 1,3-diamino-2-propanol (DAP), anhydrous aluminum chloride (AlCl3), methanol, ethanol, dichloromethane (DCM), toluene, tetrabutylammonium bromide (n-Bu4NBr), tetrabutylammonium chloride (n-Bu4NCl), tetrabutylammonium iodide (n-Bu4NI), potassium iodide (KI), sodium borohydride (NaBH4), propylene oxide (PO), epichlorohydrin (ECH), 1,2-epoxy-5-hexene (EH), styrene oxide (SO),

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allyl glycidyl ether (AGE), and cyclohexene oxide (CHO) were purchased from Sigma Aldrich, South Korea. All the chemicals were of analytical grade and were used as received. CAP synthesis The CAP was synthesized by Friedel-Crafts benzoylation reaction between 1,3,5benzenetricarbonyl trichloride and p-terphenyl using anhydrous AlCl3 as the catalyst. A 250mL round bottom flask was charged with 5 mmol of 1,3,5-benzenetricarbonyl trichloride and 7.5 mmol of p-terphenyl in 100 mL of DCM; the mixture was degassed and kept under N2 atmosphere. Next, 5.0 mmol of AlCl3 was added and the reaction mixture was refluxed at 80 °C overnight with the help of a cooling condenser. After cooling the mixture, the precipitate was collected by filtration and washed successively with DCM, methanol and water. The filter cake was dried under vacuum at 120 °C, to obtain CAP in the form of a brown powder. Post-synthesis functionalization of CAP to CAP-DAP One gram of CAP and 0.8 g of DAP were dispersed in 50 mL of methanol in a 100mL round bottom flask fitted with a condenser and refluxed at 75 °C for 12 h under magnetic stirring. After cooling to room temperature, the obtained Schiff-base intermediate was reduced by reaction with NaBH4 in methanol for 1 h. The solution was then filtered and washed with methanol and water. The obtained solid was dried at 120 ℃ for 6 h, when the pale brown powder of CAP-DAP was obtained. Characterization The powder X-ray diffraction (XRD) patterns were measured on a Rigaku diffractometer using CuKα (λ=1.54 Å) with 0.5o min-1. N2 adsorption-desorption isotherms were measured at 77 K using BELSORP-Max (BEL, Japan). Prior to analysis, the samples were degassed at 120 °C for 8 h under vacuum of 10-2 kPa. The surface area and pore size 6


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distribution were calculated using Brunauer–Emmett–Teller (BET) and Non-linear density functional theory (NLDFT) methods, respectively. Fourier-transformed infrared (FT-IR) spectra were collected in transmission mode by dispersing the sample in KBr disks using a OTSUKA IG-2000 FT-IR spectrometer. Thermogravimetric analysis (TGA) was performed using a TG 209 F3/ NETZSCH instrument. The samples were heated at the rate of 10 °C/min under nitrogen atmosphere up to 800 °C.

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C cross polarization magic angle

spinning nuclear magnetic resonance (13C CP/MAS NMR) spectra were recorded on a WB 400 MHz Bruker AVANCE III spectrometer with a contact time of 2 ms (ramp 100) and a pulse delay of 3 s for porous materials. The cyclic carbonate products were analyzed by a liquid NMR (Bruker AVANCE III) spectrometer using CDCl3 as a solvent and tetramethylsilane as an internal standard. The surface morphology and size of the materials were analyzed by field emission-scanning electron microscopy (SEM: Hitachi S-4300) and transmission electron microscopy (TEM; Philips CM200 UT instrument. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, USA) was performed using a monochromatic Al Kα X-ray source and a hemispherical analyzer. Elemental analysis (EA) was carried out on a Thermo EA1112 Elemental Analyzer (USA). Results and discussion Characterization of the synthesized materials The powder XRD patterns of CAP and CAP-DAP (Figure S1) showed two different broad peaks at 2θ = 10.7 and 22.5° attributed to the direct phenyl-phenyl ring interactions favored by the intrinsic flexibility of the amorphous polymer network.50 The textural properties of CAP and CAP-DAP were examined by N2 adsorption-desorption measurements at 77 K. As shown in Figure 1 a, both CAP and CAP-DAP show a steep rise in amount of N2 adsorbed in the low relative pressure region (P/P0

Adsorption and Fixation into Cyclic Carbonates - American Chemical

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