First Photocatalytic Synthesis of Cyclic Carbonates from CO2 and

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First photocatalytic synthesis of cyclic carbonates from CO2 and epoxides using CoPc/TiO2 hybrid under mild conditions Pankaj Kumar Prajapati, Anurag Kumar, and Suman Lata Jain ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00755 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Sustainable Chemistry & Engineering

First photocatalytic synthesis of cyclic carbonates from CO2 and epoxides using CoPc/TiO2 hybrid under mild conditions Pankaj Kumar Prajapati,a,b Anurag Kumar,a,b Suman Lata Jaina* a

Chemical Sciences Division, CSIR- Indian Institute of Petroleum, Haridwar Road, Mohkampur, Dehradun, India-248005. b

Academy of Scientific and Innovative Research, New Delhi, India- 110001.

Corresponding Author *Tel: +911352525788. E-mail address: [email protected].

ABSTRACT First report on the photocatalytic coupling of carbon dioxide with epoxides to give cyclic carbonates under extremely mild such as room temperature and atmospheric pressure conditions using a hybrid photocatalyst consisting of cobalt phthalocyanine grafted on titanium oxide (CoPc/TiO2) under visible irradiation is described. The developed protocol provided almost quantitative conversion of various epoxides to corresponding cyclic carbonates in excellent yields without any evidence for the formation of any by-product. At the end of the reaction, the photocatalyst was separated by centrifugation and reused for several subsequent recycling runs without any significant loss in activity, and no leaching had observed during the photocatalytic reactions. 1

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KEYWORDS: Photocatalysis, CO2 utilization, epoxide, cycloaddition, cyclic carbonate, hybrid. INTRODUCTION Maintaining the environmental sustainability to manage a clean and green environment is one of the sounding challenges for humanity in the current scenario. However, due to the fact that rapid development and industrialization, the concentration of greenhouse gases such as CO2 is continuously rising in the atmosphere.1 There are two possible ways for CO2 mitigation i.e., capturing followed by either storage or utilization as feedstock for production of fuel and chemicals.2 However, utilizing carbon dioxide as a feedstock to produce high value chemicals is preferred rather than just dumping of a valuable resource.3-4 In this context, the synthesis of cyclic carbonates resulting from the coupling of epoxides with carbon dioxide constitutes a most successful example of CO2 utilization.5-6 Owing to their fascinating properties such as high boiling point, low toxicity, high solubility and biodegradability, the cyclic carbonates have been widely used as aprotic polar solvents and precursors for biomedical applications as well as for engineering plastics.7 A plethora of reports using various homogeneous, heterogeneous, and metal free catalytic systems including quaternary ammonium and phosphonium salts are known for the synthesis of cyclic carbonates from epoxides and carbon dioxide.8-12 However, in most of the cases, stringent reaction conditions such as higher temperature and pressure have been used to get the higher product yields.13-16 Owing to the growing environmental and energy concerns, the development of synthetic methodologies for cyclic carbonates from CO2 under ambient conditions is gaining particular interest.17-18 In this context, Barkakaty et al. reported an efficient synthesis of cyclic carbonates from the reaction of epoxides and CO2 under mild reaction condition (1 atm pressure and 25-45°C temperature) in reasonable yields (65–83 %) using N-methyl tetrahydro pyrimidine (MTHP) in 2


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the presence of LiBr (25 %) as catalyst.19 Calo et al. described the conversion of CO2 and epoxides to cyclic carbonates by using molten tetraalkylammonium salts bearing halides as counter ions as solvent and catalyst at a 120°C temperature and an atmospheric pressure of carbon dioxide.20 Toda et al. showed tetraarylphosphonium salts (TAPS) as efficient catalysts for coupling reaction of CO2 with epoxides to give cyclic carbonates at atmospheric pressure and high temperature (120°C).21 Nomura et al. reported organometallic halide of group IV, V and VI as catalyst for the cycloaddition of ethyl oxirane with CO2 at atmospheric pressure and 60°C temperature in the presence of a base (i.e. triethylamine).22 Iwasaki et al. reported alkali metal halides as catalyst to synthesise cyclic carbonates at 100°C temperature and atmospheric CO2 pressure.23 Melendez et al. reported one component catalyst system containing bimetallic aluminium complex and tetrabutylammonium bromide for the synthesis of cyclic carbonates from epoxide and CO2 at room temperature and atmospheric pressure under solvent-free condition.24 Motokura et al. reported silica-supported 4-pyrrolidinopyridinium iodide for the synthesis of cyclic carbonates under atmospheric pressure and the elevated temperature.25 Roy et al. recently reported porous zinc stannate nanocrystals for the synthesis of cyclic carbonates from CO2 and epoxide under atmospheric pressure using PEG 600 as solvent at 80°C.26 Besides of these, few metal free systems have also been reported for the synthesis of cyclic carbonates under atmospheric CO2 pressure. For example, Lan et al. reported the production of cyclic carbonate from propylene oxide with CO2 using organo-functionalized graphene oxide as catalyst and TBAB as co-catalyst.27 All the above reported methods although provided conversion of CO2 at atmospheric pressure condition, but required either higher reaction temperatures in the range of 60-120°C or expensive, toxic catalyst components and homogeneous metal complexes. In contrast to the conventional thermal catalysis, photocatalytic 3



processes involve light activation and offer economical advantages regarding milder operating conditions such as room temperature and atmospheric pressure.28 Also for the efficient utilization of solar-energy, a material should work under the visible light as it covers about 45% of solar light energy. Titanium oxide (TiO2) owing to its unique properties such as lower cost, easy availability, nontoxicity, and suitable electronic band edges has been considered as one of the best materials for photocatalytic applications. However, due to the large band gap of TiO2 (∼3.2 eV), it can absorb in the UV region, which contributes only 5 % of total solar light. Hence, modification of TiO2 by several strategies such as doping with metal/non-metal and surface sensitization with organic dyes or metal complexes can enhance its absorption in the visible region of solar light.29 Accordingly, the present paper describes the synthesis of cobalt phthalocyanine modified TiO2 hybrid photocatalyst and its first successful photocatalytic activity for the synthesis of cyclic carbonates via cycloaddition reaction of CO2 with epoxides under visible light irradiation at room temperature (25°C) and 1 atm pressure (Scheme 1).

Scheme 1. Photocatalytic cyclic carbonate synthesis using CoPc/TiO2 photocatalyst. EXPERIMENTAL SECTION 4


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Materials All the substrates and solvents were commercially available and used as received. Cobalt phthalocyanine (CoPc) was synthesized by following the literature procedure.30 The deionized water was used throughout the synthetic procedures. Techniques used Rough surface morphology of materials was determined with the help of Field Emission Scanning Electron Microscopy by using FE-SEM (Jeol Model JSM-6340F). Ultrafine surface morphologies of samples were determined with High-Resolution Transmission Electron Microscopy (HR-TEM) on FEI-TecnaiG2 Twin TEM operating at an acceleration voltage of 200 kV. FT-IR spectra of samples were collected on Perkin–Elmer spectrum RX-1 IR spectrophotometer having potassium bromide window. XRD pattern for determining the crystallinity of materials and phases was carried out at Bruker D8 Advance diffractometer at 40 kV and 40 mA with Cu Kα radiation (λ= 0.15418 nm). UV-Vis absorption spectra of cobalt(II) phthalocyanine complex in acetonitrile and solid UV of TiO2 and CoPc/TiO2 was recorded on Perkin Elmer Lambda-19 UV-VIS-NIR spectrophotometer using a 10 mm quartz cell, using BaSO4 as a reference. Surface properties like BET surface area (SBET), BJH porosity, mean pore diameter, etc. of samples were examined by N2 adsorption-desorption isotherm at 77 K by using VP; Micromeritics ASAP 2010. The thermal degradation pattern of the materials was determined by a thermal analyzer TA-SDT Q-600 in the temperature range of 40 to 800°C under nitrogen flow with 10°C/min heating rate. 1H NMR and

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C NMR of the cyclic carbonates (reaction

products) was collected on 500 MHz by using Bruker Advance-II 500 MHz instrument. Metal loading in the prepared hybrid samples was determined by using Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, DRE, PS-3000UV, Leeman Labs Inc, USA). Samples 5



for ICP-AES were made by digesting an estimated amount of samples with nitric acid followed by filtration and making volume up to 10 mL by adding deionized water. Synthesis of in-situ CoPc/TiO2 photocatalyst31 In the typical synthesis, titanium chloride (0.091 mol, 10 mL) was added drop-wise to a mixture of 9.5 mL of triethanolamine and 20 mL of ethanol in a beaker in an ice bath. The obtained white solid was separated and dissolved in 100 mL of water followed by vigorous stirring of the mixture for half an hour at room temperature. Then to prepare the hybrid, CoPc (200 mg) was added to the above-obtained solution followed by drop-wise addition of the ammonium hydroxide until the pH became 7. This solution was sonicated for 2 h, washed thoroughly with water and finally with ethanol. The precipitate so obtained was separated by filtration, washed with distilled water, and dried in an oven at 60°C for 24 h under vacuum to give CoPc/TiO2 hybrid photocatalyst in 4.2 g yield. Two more samples of different compositions by adding the lower amount of CoPc (100 mg) and higher amount of CoPc (300 mg) were prepared by following the identical procedure for the comparative study. For easy identification and convenience, the samples were labeled as CPT-1, CPT-2 and CPT-3 synthesized by addition of 100 mg, 200 mg, and 300 mg of CoPc complex, respectively. Photocatalytic coupling of epoxides with CO2 In a 50 mL round bottom flask, epoxide (1 mmol), tetrabutylammonium bromide (0.1 mmol), acetonitrile (10 mL) and methanol (5 mL) were added. The obtained reaction mixture was initially purged with N2 gas for 30 min to evacuate the system followed by purging of CO2 gas for additional 30 min to saturate the solution. After that, photocatalyst CPT-2 (100 mg, 0.01 mmol) was added to the reaction mixture, and the flask was sealed with a septum. Then the flask was irradiated with visible light of a 20 W LED (Model No. HP-FL-20W-F-Hope 6


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LED Opto-Electric Co., Ltd, λ>400 nm) with stirring for 24 h. The progress of the reaction was monitored by thin layer chromatography (TLC). After completion of the reaction, the photocatalyst was separated by filtration and solvent was removed by rotary evaporation. The isolated crude product was purified by column chromatography on silica gel by using ethyl acetate and hexane (9:1) as eluent. Conversion and selectivity of the products were determined using GC-FID (Varian CP-3800, Column specification: Varian capillary column, CP Sil 24CB LOW BLEED/MS 30 mm, 0.25 µm # CP 5817) at the flow rate 0.5 mL min-1, injector temperature 250°C, FID detector temperature 275°C. The products were confirmed by comparing their spectral data (1H and 13C NMR) with the authentic samples. Experimental procedure for the recycling experiment After completion of the reaction in a fresh experiment as mentioned in the above section, the photocatalyst was separated by centrifugation, washed thoroughly with ethanol and dried at 60°C under vacuum for 12 h. The recovered catalyst was used with the addition of fresh substrate, TBAB, acetonitrile, and methanol as the solvent in the presence of CO2 under described optimized reaction conditions. The recovered catalyst was tested up to sixth recycling experiments.

RESULTS AND DISCUSSIONS Synthesis and characterization of the photocatalyst The photocatalyst (CoPc/TiO2) labeled as CPT-2 was synthesized by adding CoPc complex (200 mg) during the in-situ synthesis of titanium oxide from titanium tetrachloride as shown in Scheme 2. For comparison, neat TiO2 was synthesized by similar procedure without adding CoPc

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complex. Cobalt content in the prepared photocatalyst CPT-2 was found to be 0.58 wt % (0.1 mmol) as determined by ICP-AES analysis.

Scheme 2: Synthetic route of in-situ CoPc/TiO2 (CPT-2) photocatalyst The diffractogram of the synthesised TiO2 and CoPc/TiO2 (CPT-2) as shown in figure 1, indicates the crystalline nature of the materials.32 The diffraction peaks at 2θ values of 27°, 36° and 55° corresponded to the diffraction planes (110), (101) and (211) were due to the rutile phase of TiO2 which matched well with the JCPDS card number 88-1175 (Fig. 1a).33 Few peaks characteristic to anatase phase found at 2θ values of 25°, 39°, 54°, 56° and 70° were corresponded to the diffractions of (101), (004), (105), (211) and (116), respectively and matched well with JCPDS card no. 21-1272.34 Diffractograms of the hybrid photocatalyst CoPc/TiO2 (Fig.

8


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1a-c) did not reveal the peaks of CoPc, which could be believed due to the lower loading of CoPc or its complete dispersion onto TiO2.35

Figure 1. X-ray diffraction pattern of a) CoPc; b) TiO2 and c) hybrid CPT-2. The FTIR spectra of the synthesized CoPc, TiO2 and CPT-2 are shown in figure 2. In the FTIR spectrum of CoPc, the assigned peaks at 1323, 1377, 1518, 1609 and 1660 cm-1 are attributed to the phthalocyanine ring vibrations. The peak obtained at 743 cm-1 is assigned to the Pc ring vibration. The peaks appeared at 1151, 1609 and 1660 cm-1 are due to the C-H bending vibrations of the aromatic ring, C=N and pyrrole ring of Pc moiety, respectively.36 The characteristic peak at 1518 cm-1 is due to the symmetric C═C stretching for aromatic ring vibrations in Pc ring (Fig. 2a).37 The vibrational band appeared at 1077 cm-1 in FTIR spectrum of TiO2 (Fig. 2b) is assigned to the Ti-O stretching; whereas the peaks at 1402 cm-1 and 1634 cm-1 attributed to the intercalated water in the semiconductor.38 A broad peak observed at 3379 cm-1 is 9



assigned to O-H stretching which suggested the presence of –OH groups on the surface of the insitu TiO2. After immobilization, the resulting hybrid exhibited the shifted characteristic peaks related to C-H, C-C, C=N vibrations of CoPc and Ti-O peaks of TiO2 confirm the successful incorporation of CoPc units onto the TiO2. Also, the significant shifting of peaks suggested the strong interaction of CoPc complex units with the –OH groups located on the surface of TiO2 (Fig. 2c).39

Figure 2. FT-IR spectra of a) CoPc, b) in-situ TiO2 and c) CPT-2. The morphology and the structure of the samples were determined by high-resolution transmission electron microscopy (HR-TEM) analysis as shown in figure 3. HR-TEM image of TiO2 at 10 nm scale showed rough, imperfect structures (Fig. 3d), which remained almost intact in the hybrid (Fig. 3b). The presence of dark spots in the HR-TEM image of hybrid indicated the embedded CoPc units in the TiO2 (Fig. 3b). Furthermore, SAED pattern showed polycrystalline 10


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nature of the hybrid photocatalyst (Fig. 3c). The HR-TEM images of TiO2 exhibited fringes at 10 and 5 nm scale, which confirmed the crystalline nature of the semiconductor support (Fig. 3d-e). The interplanar distance in TiO2 was found to be very fine ~ 0.22 nm at 5 nm scale (Fig. 3e). The polycrystalline nature of the semiconductor was also confirmed by the SAED pattern, exhibited some rings of diffraction planes (Fig. 3f). EDX and elemental mapping confirmed the presence and uniform distribution of the elements such as Co, O, and Ti throughout the hybrid photocatalyst (Fig. 4a-e).

Figure 3. a,b) HR-TEM images and c) SAED pattern of CPT-2; d,e) HR-TEM images and f) SAED pattern of the bare TiO2.

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Figure 4. HR-TEM Elemental mapping of CPT-2: a) carbon, b) nitrogen, c) oxygen, d) titanium, e) cobalt and f) electron image. The electronic absorption spectra of CoPc, TiO2, and CPT-2 are shown in figure 5. The UVvisible spectrum of CoPc exhibited two characteristic regions, the first one is the B band (Soret band) in the wavelength region of 300–340 nm and second one is the Q-band in the wavelength region of 546–665 nm due to the π–π* and n–π* transitions of the Pc ring as per the reported literature (Fig. 5a).40 The Q-band represents the HOMO–LUMO transition, with two split peaks, 12


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corresponding to the monomer absorption (666 nm) and aggregate absorption (546 nm), respectively. A broad peak in the UV-Vis spectrum of TiO2 in the region 236-372 nm, suggested that the pure TiO2 had higher absorption in the UV region as compared to the visible region (Fig. 5b).41 After immobilization of CoPc to TiO2, a significant red shift in the region 300-380 nm suggested a strong interaction between the support (TiO2) and CoPc via Co-O bonding (Fig. 5c). Also, the disappearance of absorption band due to the aggregated CoPc molecules in hybrid suggested the successful immobilization and homogeneous dispersion of complex molecules on the TiO2 surface. Also, the immobilization of CoPc on TiO2 surface made the hybrid photocatalyst active in the visible region.

Figure 5. UV-Visible spectra of a) CoPc, b) in-situ TiO2 and c) CPT-2. Optical band gap values of the synthesized samples were determined with the help of Tauc plot by linear extrapolation (Fig. 6). The homogeneous CoPc complex showed two band gaps, i.e., at 1.75 eV due to the metal to ligand charge transfer (MLCT) and 2.44 eV due to the π→π∗ and

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n→π∗ transition (Fig. 6a). Synthesized TiO2 showed a band gap at 2.93 eV corresponding to the 313 nm of a wavelength which was slightly lesser than the 3.0 eV of rutile TiO2 (Fig. 6b). However, the band gap value in hybrid (CPT-2) was found to be 1.87 eV, which further confirmed the visible light activity of the photocatalyst (Fig. 6c).

Fig. 6. Bandgap determination by Tauc plot for a) CoPc complex, b) in-situ TiO2 and c) CPT-2. The surface chemical properties of synthesized hybrid CoPc/TiO2 (CPT-2) were determined by X-ray photoelectron spectroscopy (Fig. 7). The survey scan of hybrid showed the presence of all desired elements C, O, N and Co, which verified the successful immobilization of CoPc in TiO2 (Fig. 7e). The desired elements C-1s, O-1s, Ti-2p, and Co-2p, showed peaks at their 14


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corresponding binding energies at 282.4, 528.1, 456.0 and 778.4 eV, respectively. In hybrid photocatalyst, the XPS spectrum of C-1s showed three peaks, i.e. at 282.1 eV of the C-N bond, 284.1 eV of C═C and 286.0 eV of C═N bond, which are characteristic of the CoPc complex (Fig. 7a). In the XPS spectrum of O-1s, two peaks at 528.0 eV and 529.9 eV revealed the presence of Ti-O and O-H bond, respectively (Fig. 7b).42 Furthermore, in figure 7c, the peaks observed at 457.2 and 462.7 eV attributed to the 2p3/2 and 2p1/2 of Ti-2p, respectively.43 For cobalt, two characteristic peaks of Co-2p3/2 and Co-2p1/2 were observed as shown in figure 7d. The first peak showed spiliting, i.e., at 778.2 eV due to the octahedral Co-2p3/2 and at 781.5 eV due to the tetrahedral Co-2p3/2, respectively. The peak at 796.9 eV corresponded to Co-2p1/2, also confirmed the presence of cobalt metal in +2 oxidation state (Fig. 7d).44

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Figure 7. High resolution XPS spectra of CPT-2: a) C-1s, b) O-1s, c) Ti-2p, d) Co-2p and e) survey scan. 16


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Thermal stability of the synthesized materials was determined by thermogravimetric analysis (TGA) as shown in figure 8. In CoPc (Fig 8a) an initial weight loss near to 200°C was due to the evaporation of the adsorbed solvent or water molecules. Next, the major weight loss in the region between 500-600°C was attributed to the degradation of the phthalocyanine moiety (Fig. 8a). Thermogram of synthesized TiO2 showed one weight loss of 10 wt % below 200°C due to the removal of adsorbed water molecules followed by a steady weight loss up to 800°C (Fig. 8b). In case of the hybrid CPT-2 photocatalyst, four major weight losses were observed (Fig. 8c). The first weight loss up to 200°C was attributed to the evaporation of adsorbed solvent or water molecules. The second and third weight loses in the region between 250-450°C attributed to the partial decomposition of the complex and loss of oxygen along with the carbonaceous content. The fourth weight loss in the region between 500-600°C was due to the decomposition of the phthalocyanine moiety.

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Figure 8. TG-DTA of a) CoPc; b) TiO2 and c) CPT-2. The surface properties such as BET surface area (SBET), average pore diameter (rp) and total pore volume (Vp) of TiO2 and hybrid are shown in figure 9. Total pore volume and surface area of synthesized TiO2 was found to be 0.0054 m3/g and 7.58 m2/g, respectively. The total pore diameter of TiO2 was found to be 4.2 nm, which confirmed that the material is mesoporous. Further, the adsorption-desorption isotherm was found to be of Type II with H3 hysteresis loop (Fig. 9a). After immobilization of CoPc on TiO2, the values of SBET, pore diameter, and total pore volume were found to be 16.4 m2/g, 7.4 nm, and 0.018 m3/g, respectively (Fig. 9b). The increased surface area in CPT-2 hybrid suggested the attachment of CoPc complex units onto the external surface of TiO2 rather than inside of the pores. Furthermore, hybrid photocatalyst exhibited type IV isotherm with H3 hysteresis loop of the 18


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plate-like structure of pores.

Figure 9. N2 Physico-sorption isotherm of a) in-situ TiO2 and b) CPT-2. The Raman spectral analysis of the synthesized TiO2 and hybrid CPT-2 are shown in figure 10. Raman spectroscopy is an important tool to analyze the structure, electronic properties, and crystal phase of the titania (Fig. 10a).45 Raman frequencies obtained at 439 and 624 cm-1 corresponded to the Eg and A1g modes, respectively represented the bulk structure of the titania (Fig. 10a).46 These values were found to be shifted to 398 and 624 cm-1, respectively in hybrid CoPc/TiO2 (Fig. 10b). The shifting of these peaks confirmed the strong interaction between CoPc and TiO2. Rest of the peaks obtained in the spectrum were attributed to the characteristic peaks of the CoPc complex.47

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Figure 10. Raman spectra of a) in-situ TiO2 and b) CPT-2 photocatalyst.

PHOTOCATALYTIC ACTIVITY The photocatalytic activity of the synthesized CoPc, TiO2 and hybrid CoPc/TiO2 were tested initially for the cycloaddition of epichlorohydrin with CO2 using tetra-butylammonium bromide (TBAB) as a co-catalyst in acetonitrile/ methanol mixture using 20 W white cold LED light as a visible light source. After 24 h of visible light illumination at room temperature (25°C) and 1 atmospheric pressure condition, the desired carbonate was obtained selectively in excellent yield without any evidence for the formation of any by-product. Conversion and the selectivity of the product (cyclic carbonate) were determined by GC-FID and GC-MS respectively. Visible light illumination was found to be essential for the reaction, and a negligible yield of the product was obtained in the dark at room temperature (Table 1, entry 1-3). Further, to confirm that the reaction was not photo-chemically induced transformation, a controlled experiment in the presence of visible light under described reaction conditions was performed. The reaction was continued for 3 h under visible light, and then the light was turned off followed by keeping the 20


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reaction in the dark for 24 h. The reaction mixture collected after 3 h, 12 h and 24 h were analyzed by GC-FID that provided 41, 44 and 48 % conversion respectively. These findings indicated that the reaction was truly photocatalytic not a photochemically induced transformation and a continuous illumination required for the efficient synthesis of cyclic carbonates under described conditions. Further, in comparison to the individual solvents, a mixture of acetonitrile and methanol (2:1) afforded best results (Table 1, entry 2-4). The enhanced activity in the mixture was attributed to the higher solubility of CO2 in acetonitrile; whereas methanol act as a hole scavenger. Further, to investigate the effect of CoPc loading in hybrid on the photocatalytic performance, three different compositions of hybrid (CoPc/TiO2) by varying the amount of CoPc (100, 200 and 300 mg) with a constant amount of TiO2 under identical conditions were synthesized Table 1, entry 4.i-iii). The prepared samples were labeled as CPT-1, CPT-2, and CPT-3 respectively based on the amount of CoPc used for the synthesis. Among the three photocatalysts studied, hybrid (CPT-2; synthesized by addition of 200 mg CoPc) provided maximum conversion and yield of the desired product (Table 1, entry 4.ii). The enhanced photocatalytic activity of CPT-2 was attributed to the higher visible light absorbance, better charge separation, continuous electron supply from CoPc sensitizer to the conduction band of the TiO2 and due to the introduction of plenty of defects in a semiconductor in the form of CoPc molecules. However, CPT-1 afforded poor conversion which can be assumed due to the poor CO2 adsorption and less efficient charge transfer on the photocatalyst surface resulted from the lower concentration of CoPc on the surface (Table 1, entry 4.i). Similarly, in case of CPT-3, a significant reduction in the conversion compared to CPT-2 was observed that could be attributed to the agglomeration of CoPc units at TiO2 surface (Table 1, entry 4.iii). This agglomeration might block the active sites of the photocatalyst for interaction with the substrates. Also, the 21



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higher CoPc concentration might hamper the efficient charge separation and mobility on the photocatalyst surface for the CO2 activation. Therefore, CPT-2 was considered to be optimum photocatalyst for the present study. Furthermore, to evaluate the role of TBAB, an experiment under identical conditions using TBAB alone without photocatalyst was performed under otherwise similar reaction conditions. After 24 h of visible light illumination, no cyclic carbonate was obtained (Table 1, entry 5). Similarly, in the absence of TBAB using photocatalyst alone, the reaction provided a very poor yield of the cyclic carbonate (Table 1, entry 6). Therefore, both photocatalyst and TBAB were played a vital role in the efficient synthesis of the desired cyclic carbonates under the developed methodology. Table 1. Results of optimization of reaction parameters[a] Entry

Yield[b]

Reaction condition CoPc

TiO2

CoPc/TiO2

1

Dark, CH3CN+ MeOH

0

0

Trace

2

Visible Light, CH3CN

04

0

29

3

Visible Light, MeOH

03

0

23

4.i[c]

Visible Light, CH3CN+ MeOH

-

-

57

4.ii[d]


23

0

94

4.iii[e]


-

-

91

5[f]


0

0

07

6[g]


0

0

09

[a]

Reaction conditions: epichlorohydrin (1 mmol), TBAB (0.1 mmol), solvent 15 mL, photocatalyst (100 mg) at 25°C and 1 atmospheric pressure of CO2 for 24 h, using 20 W white cold LED as light source (λ > 400 nm); [b]determined by GC-MS; [c]CPT-1; [d]CPT-2; [e]CPT-3; [f]In the absence of photocatalyst; [g]in the absence of TBAB. 22


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Further, the reaction was generalized to variously substituted epoxides under the optimized reaction conditions (photocatalyst CPT-2 (100 mg), TBAB (0.1 mmol), CH3CN/MeOH (2:1) and visible light illumination). The experimental results of these experiments are summarized in Table 2. Among the various epoxides, the substrates having electron donating groups were found to be more reactive and afforded higher yield as compared to those substituted with electron withdrawing groups. Table 2. Photocatalytic synthesis of cyclic carbonates using CPT-2 [a] S No.

Epoxide

Product

Conversion (%)[b]

Yield (%)[c]

1.

96.7

94

2.

94.2

92

3.

89.0

88

4.

91.1

88

5.

95.2

93

89.8

86

6.

23



7.

Page 24 of 35

95.1

91

[a]

Reaction conditions: epoxide (1 mmol), TBAB (0.1 mmol), solvent (CH3CN 10 mL + methanol 5 mL), photocatalyst CPT-2 (100 mg, 0.01 mmol with respect to Co) at 25oC and 1atmospheric pressure of CO2 for 24 h using 20 W LED as light source, [b]determined by GC-MS, [c]Isolated yield.

Further, to evaluate the recyclability of the photocatalyst, cycloaddition of epichlorohydrin with CO2 was performed under optimum reaction conditions. At the end of the reaction, the photocatalyst was separated by centrifugation, washed with ethanol, and dried at 60°C under vacuum for 12 h. The recovered photocatalyst was used for the subsequent run by adding fresh substrate, co-catalyst, and solvent. The recovered photocatalyst was used for six subsequent runs, and the results of these experiments are summarized in figure 11. As shown, almost similar conversion and product yield were obtained during all recycling experiments, which ascertained that the developed photocatalyst was highly stable without any significant leaching during the reactions. Moreover, the cobalt metal content in recovered photocatalyst after the sixth run was found to be 0.56 wt % as determined by ICP which was almost similar to the fresh one (0.58 wt %). These findings confirmed that the developed photocatalytic reaction was truly heterogeneous and no significant leaching had occurred during the experiments.

24


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Figure 11. Results of recycling experiments.

Although the exact mechanism of the reaction is not known at this stage; based on the previous reports,48 a plausible mechanism of the reaction is depicted in Scheme S1 (see the supporting information). In the hybrid CoPc/TiO2, CoPc unit acts as a photosensitizer, which absorbs strongly in the visible region. Photoexcited electrons (e−) and positive holes (h+) are produced when incident photons are absorbed in CoPc/TiO2. The generated electrons are used to convert absorbed CO2 into radical anion;49 whereas methanol will be acting as a hole scavenger. Also, the electron deficient HOMO can accept electrons from epoxide to convert it into epoxide radical cation.50 Furthermore, the nucleophilic attack of Br– of TBAB from less hindered carbon would facilitate the ring opening of epoxide. During the controlled blank experiments, we found that the reaction did occur in the absence of TBAB but provided very poor yield. Hence, TBAB not only acted as a phase transfer agent but also activated ring-opening of epoxide cation radical which upon coupling with CO2 radical anion yielded corresponding cyclic carbonate. 25



CONCLUSIONS We have demonstrated the first successful photocatalytic synthesis of cyclic carbonates from the coupling of epoxides with CO2 using CoPc/TiO2 photocatalyst in visible light under extremely mild conditions. The developed hybrid was found to be more efficient as compared to the individual components, i.e., CoPc complex and bare TiO2, which was synthesized exactly under identical conditions without adding CoPc. The present methodology represents the first successful use of solar energy for the coupling of CO2 with the organic compounds to produce high-value chemicals under extremely mild conditions other than photoreduction of CO2. We believe that the current work will open several novel opportunities for the chemical fixation of CO2 to valuable chemicals sustainably using solar light. ASSOCIATED CONTENT Supporting Information Schematic representation of the possible mechanistic pathway of the photocatalytic coupling reaction (Scheme S1) and 1H,

13

C NMR of the products (Fig. S1-S12) are provided in the

supporting information. ACKNOWLEDGMENTS Authors are thankful to Director IIP for granting permission to publish these results. PKP and AK are thankful to CSIR, New Delhi, for providing research fellowships. Analytical division of the Institute is acknowledged for providing support in sample analysis.

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For Table of Content Use Only

First successful, visible light assisted photocatalytic coupling of CO2 with epoxides to cyclic carbonates under ambient temperature, and pressure conditions is described.

35


First Photocatalytic Synthesis of Cyclic Carbonates from CO2 and

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