Ruthenium-Based Macromolecules as Potential Catalysts in

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Ruthenium-Based Macromolecules as Potential Catalysts in Homogeneous and Heterogeneous Phases for the Utilization of Carbon Dioxide Kaiprathu Anjali,† Jayaraj Christopher,‡ and Ayyamperumal Sakthivel*,† †

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Inorganic Materials & Heterogeneous Catalysis Laboratory, Department of Chemistry, School of Physical Sciences, Central University of Kerala Kasaragod, Sabarmati Building, Tejawini Hills, Kasaragod 671316, India ‡ Indian Oil Corporation Limited, R&D Centre, Faridabad 121007, India S Supporting Information *

ABSTRACT: Ruthenium-containing tetraphenylporphyrin (Ru-TPP) molecule was prepared, and the structural elucidation was confirmed using 1H nuclear magnetic resonance (NMR), CHN, and mass spectral analyses. The incorporation of ruthenium ion into the cavities of the macromolecule was confirmed from the disappearance of the 1 H NMR signal, characteristic of the N−H bond (−2.72 ppm in TPP). The CHN and mass spectral analyses of the ligand and metallomacromolecules are consistent with the theoretically calculated values. The homogeneous Ru-TPP macromolecule is grafted on the surface of aminosilane-, diaminosilane-, and iodosilane-functionalized SBA-15 molecular sieves. The successful grafting of Ru-TPP on functionalized mesoporous molecular sieve materials was evident from lowangle powder X-ray diffraction, 13C magic angle spinning NMR, and scanning electron microscopy−energy-dispersive X-ray analyses. The resultant homogeneous and heterogenized Ru-TPP catalysts were used for the utilization of carbon dioxide (CO2) under moderate reaction conditions. The homogeneous Ru-TPP catalyst showed first-order kinetics with respect to epoxide with the exclusive formation of cyclic carbonate (about 98%) and an activation energy of 16.07 kg/mol, which is much lower than some of the reported catalysts. Ru-TPP grafted on aminosilane- and iodosilane-functionalized materials showed better catalytic activity (above 90% conversion and 83−96% cyclic carbonate selectivity) and reusability for the chosen reaction. polymeric resins.6 They are widely used as aprotic polar solvents, organic synthetic intermediates, and in some biomedical applications.19 Recent reports have demonstrated the application of cyclic carbonates as versatile etherifying agents for lignin and tannin functionalization, which are the most important and widely available renewable sources of aromatic structures.20 Reportedly, for the utilization of CO2, both homogeneous and heterogeneous catalysts require vigorous reaction conditions, such as relatively high pressure, temperature, solvents, and expensive cocatalysts, because of the low reactivity of CO2.9−18,21−25 Therefore, researchers are interested in developing efficient catalysts for the utilization of CO2 by the conversion of epoxide into the corresponding carbonate, under comparable reaction conditions. It is well known that biological systems convert CO2 by photosynthesis in the presence of metallomacromolecules. In this regard, various metalloporphyrin molecules have been demonstrated as potential catalysts for several organic

1. INTRODUCTION CO2 is an important constituent of air with a typical concentration of ∼400 ppm. The conversion and utilization of CO2 into gasoline, cyclic carbonates, useful chemicals, and so forth are currently interesting research areas.1 The International Panel on Climate Change has reported that CO2 concentrations have increased by 19%,2 which contributes to climate change via global warming. Consequently, CO2 is considered the most abundant and cheapest source of carbon that can be utilize for the synthesis of various organic products.3 During the last three decades, various materials have been used as CO2 adsorbents, including zeolites, metal organic frameworks, porous carbon, and carbon nanotubes.4 Another important process employs CO2 for electrochemical reduction to methanol, formic acid, methane, and other compounds.5 Similarly, the utilization of CO2 as a renewable carbon feedstock in the synthesis of cyclic carbonate is an important process as well as growing field of research, both from academic and industrial points of view.6−18 Cyclic carbonates are important intermediates and building blocks in the synthesis of various fine chemicals, such as carbonates, glycols, carbamates, and pyrimidines, as well as solvents for © XXXX American Chemical Society

Received: June 12, 2019 Accepted: July 29, 2019

A

DOI: 10.1021/acsomega.9b01741 ACS Omega XXXX, XXX, XXX−XXX

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Scheme 1. Syntheses of TPP and Ru-TPP

transformations, particularly bifunctional porphyrin-based systems, which show high catalytic efficiencies. Ema et al. reported bifunctional Mg porphyrins as efficient catalysts for CO2 coupling to epoxides.23 Noble-metal-based catalysts exhibit high activities toward the activation of CO2, hydrogenation, and other reactions. However, product recovery and purification are difficult when these catalyst systems are applied; therefore, finding an ideal catalyst for the CO2 cycloaddition system7,23 is challenging. Ruthenium-based organometallic complexes are known as noble metal catalysts, which are often used commercially in many reactions26 such as hydrogenation,27−29 olefin metathesis,30−32 C−C bond formation,32,33 C−H bond activation,34 electrochemical reduction,35,36 and steam reformation of hydrocarbons.36,37 Ruthenium chemistry is interesting because of its existence in variable oxidation states from −2 to +8.37 Furthermore, Rubased organometallic complexes are widely used for the reduction of CO2 to formaldehyde and electrocatalytic and photocatalytic reduction of CO2 to formate38−40 and methanol.41−43 Moreover, Ru-based pincer complexes can be used for highly efficient electrocatalytic reductive conversion of CO2.3 Porphyrin molecules are analogous to pincer ligands which form stable metalloporphyrin complexes, a versatile class of homogeneous catalysts suitable for the electrocatalytic reduction of CO244−46 and CO2 coupling with epoxides.47−49 It is also worth mentioning here that ruthenium-based porphyrin complexes were also shown as promising catalysts for alkene epoxidation, cyclopropanation,50,51 oxidation52 reactions, and so forth. In addition, metalloporphyrin (magnesium and aluminum)-based systems are also used for carbon dioxide utilization by epoxide coupling with CO2.53−55 However, to the best of our knowledge, ruthenium tetraphenylporphyrin-based complexes are not explored for CO2 utilization. Herein, ruthenium-containing tetraphenylporphyrin macromolecule was prepared for the first time, and the resulting metallomacromolecule was heterogenized on the functionalized SBA-15 materials. The prepared materials were thoroughly characterized by several spectroscopic and analytical techniques. The homogeneous Ru-TPP macromolecules and surface-grafted materials were used for the carbonylation of epoxides using CO2 at moderate reaction temperature, pressure, and without any solvent.

Scheme 2. Functionalization of SBA-15 with Various Organosilanes

filtered, and washed thoroughly with methanol. The resulting purple crystals were air-dried and recrystallized in dichloromethane. The ruthenium-incorporated meso-tetraphenylporphyrin (Ru-TPP) complex was prepared by dissolving the metal salt (RuCl3·XH2O; 1 mmol) in 5 mL of dry acetone, followed by introducing a solution containing 1 mmol of TPP in 45 mL of dry acetone (Scheme 1). The final mixture was refluxed under N2 atmosphere for 24 h. After the completion of the reaction, the solvent was removed and washed with pentane to remove impurities, after which the product was isolated.56,57 2.3. Synthesis of Functionalized SBA 15. SBA-15 was prepared as per the reported procedure (see Supporting Information), using the pluronic P123 triblock copolymer(poly (ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) EOnPO70EOn as a surfactant (see Supporting Information). The calcined SBA-15 support (6.5 g) was preactivated at 200 °C in an air oven, followed by drying in a vacuum, in a roundbottom flask to remove the physisorbed molecules. Subsequently, a predetermined amount (3 mmol/g of solid support) of N-[3-(trimethoxy silyl)-propyl]-ethylenediamine/ ( 3 - a m i n o p r o p y l ) t r i m e t h o xy s i l a n e / ( 3 - i o d o p r o p y l ) trimethoxysilane was introduced to the activated SBA-15 support material.58−60 The mixture was refluxed in dry toluene (30 mL/g of support) for 24 h under N2 atmosphere. This procedure was repeated thrice to completely convert the surface silanol groups into organofunctionalities. The resultant mixture was cooled to room temperature, washed with dichloromethane, and then dried in vacuum to afford the final product (SBA-X-functionalized materials). The materials that were functionalized using diamino-, amino-, and iodosilane are represented as SBA-D, SBA-A, and SBA-I, respectively (Scheme 2). 2.4. Synthesis of Ru-TPP−SBA-X. The grafting of the RuTPP complex on the functionalized material (SBA-X) was achieved by introducing 0.05 mmol Ru-TPP in 50 mL of dry methanol suspended with 1 g of the support material (SBA-X), followed by refluxing for 24 h under N2 atmosphere. Subsequently, the metalloporphyrin (Ru-TPP)-grafted materials were collected by separating the solvent and washing with

2. EXPERIMENTAL PROTOCOL 2.1. Preparation of the Catalyst. All experiments were conducted using the Schlenk technique. Before the experiments, all the solvents and starting materials were dried by a standard procedure.56 2.2. Synthesis of Ru-Tetraphenylporphyrin Complex. meso-Tetraphenylporphyrin (TPP) was prepared by refluxing freshly dried pyrrole (0.027 mol) and benzaldehyde (0.028 mol) in 100 mL of propionic acid for 2 h (Scheme 1). After the stipulated period, the reaction mixture was cooled to 25 °C, B


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Scheme 3. Synthesis of Ru-TPP−SBA-X

spectra were recorded on a Brücker AVANCE 400 wide bore spectrometer equipped with a superconducting magnet with a field of 7.1 T using a 4 mm double-resonance MAS probe operating at a resonant frequency of 79.4 MHz that was employed for 29Si MAS NMR. In each case, the samples were packed in 4 mm zirconia rotors and subjected to a spinning speed of 10 kHz; single-pulse experiments with a pulse duration of 4.5 μs and a relaxation delay time of 6 s were used for recording all 13C MAS NMR patterns. All chemical shift values are expressed with respect to glycine for 13C nucleus. The textural properties [Brunauer−Emmett−Teller (BET) surface area and Barrett−Joyner−Halenda (BJH) average pore volume] of the heterogenized samples were followed by N2 sorption measurements at −196 °C using an automatic micropore physisorption analyzer (Micromeritics ASAP 2020, USA). The samples were degassed at 110 °C for 10 h under a pressure of 10−3 Torr prior to each run. Powder X-ray diffraction patterns of all the heterogenized materials were collected on a Bruker-D8 high-resolution X-ray diffractometer with Cu Kα radiation (λ = 1.5418), in the 2θ range of 0.5−10° with a scan speed and step size of 0.5°/min and 0.02°, respectively. The morphology and particle size of materials were obtained from the scanning electron microscopy (SEM) images on an EVO MA15 Zeiss microscope operated at 10−20 kV. Finely crushed powder samples were “dusted” on a doublesided carbon tape and mounted over the sample holder. Gold sputtering of the materials was carried out on a gold sputter coater JEC 300, to avoid charging and make the surface conducting. Finally, the specimens were analyzed in secondary electron imaging mode at different magnifications and desired voltages. Thermogravimetric analyses (TGAs) were performed using a TGA instrument (PerkinElmer STA 6000) under a nitrogen atmosphere, with a heating rate of 10 °C/min, ranging from 40 to 850 °C. Elemental compositions present in the final materials were determined using a wavelengthdispersive X-ray fluorescence spectrometer (ZSX PRIMUS Primus, Rigaku). Calibration of the equipment was carried out using standards containing ruthenium with different concentrations. The net intensity (peak-background) of the standards was used for calibration, and ruthenium contents in the catalyst samples were obtained from the calibration curve.

Scheme 4. Carbonylation of Epichlorohydrin Using CO2

methanol and dichloromethane; subsequently, the product was vacuum-dried.56−60 Ru-TPP grafted on diamino-, amino-, and iodo-functionalized SBA-15 are represented as Ru-TPP−SBAD, Ru-TPP−SBA-A, and Ru-TPP−SBA-I, respectively (Scheme 3). 2.5. Catalytic Studies. In a typical reaction, 2 mL (30 mmol) of epichlorohydrin was introduced into a 50 mL autoclave containing 0.01 g (0.0126 mmol) of the homogeneous catalyst (Ru-TPP); subsequently, the autoclave was flushed with CO2 several times. The reaction was charged with the required CO2 pressure of 4 bar. The reaction was performed at 80 °C for 8 h. Afterward, the reactor was cooled to room temperature, and the excess CO2 was vented out. The results were analyzed quantitatively by gas chromatography using an FID detector equipped with an HP 88 column (Mayura Analytical model 2100). The products were further confirmed by gas chromatography (GC), mass spectrometry (MS) and Fourier transform infrared (FT-IR) data. Similarly, the reaction was carried out by using a heterogeneous catalyst in which 2 mL (30 mmol) of epichlorohydrin was introduced into a 50 mL autoclave containing 0.1 g of the heterogeneous catalyst (Ru-TPP−SBA-A/Ru-TPP−SBA-D/Ru-TPP−SBA-I), and the autoclave was flushed with CO2 several times. The reaction was performed at 100 °C for 8 h, with the required CO2 pressure of 4 bar (Scheme 4).6

3. CHARACTERIZATION METHODS FT-IR spectra of all the materials were recorded on a PerkinElmer spectrum-2 FT-IR spectrometer in the range of 400−4000 cm−1 using the KBr method and were collected with 4 cm−1 resolution and 120 scans. The UV−vis spectrum of the homogeneous and DR-UV−vis spectra of the Ru-TPP heterogenized samples were recorded on a PerkinElmer LAMBDA 35 spectrometer. The 1H nuclear magnetic resonance (NMR) spectra of the porphyrin ligand and the metalloporphyrin molecule were recorded using CDCl3 as the solvent and tetramethylsilane as the standard on a Brücker AVANCE III 400 instrument. Electrospray ionization time-offlight mass spectra were recorded on a Brücker maXis mass spectrometer using methanol as the solvent. Elemental analyses were performed on an Elementar vario MICRO cube by grinding the sample into a fine powder and packing in an aluminum foil. Solid-state magic angle spinning (MAS) NMR

4. RESULTS AND DISCUSSION The homogeneous macromolecular ligand (TPP), rutheniumcontaining porphyrin complex (Ru-TPP), and heterogenized Ru-TPP complex (Ru-TPP−SBA-X) were characterized using various analytical and spectroscopic techniques. Ru-TPP and heterogenized ruthenium porphyrin served as potential catalysts for the carbonylation of epichlorohydrin using CO2, under moderate reaction conditions. C


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Figure 1. FT-IR spectra of various homogeneous and heterogeneous systems. (a) TPP, (b) Ru-TPP, (c) Ru-TPP−SBA-D, (d) Ru-TPP− SBA-A, and (e) Ru-TPP−SBA-I.

Figure 2. UV−vis spectra of (a) TPP and (b) Ru-TPP.

formation of Ru-TPP was further confirmed by CHN analysis (Table 1), where the theoretical and experimental values are in agreement with each other. The UV−vis spectrum (Figure 2) of TPP shows peaks at 424 (Soret band), 517, 550, 591, and 649 nm (Q bands), characteristic of the porphyrin macromolecular structure. Upon Ru ion incorporation, the pyrrole nitrogen of TPP is coordinated to the Ru ion to form a Ru-TPP complex. Consequently, the intensities of the Q bands at 517 and 550 nm were decreased and those at 591 and 649 nm disappeared.62 This trend could be because of the improved structural symmetry (D4h point group) and lowered energy gap of the Ru-TPP complex compared to that of TPP (D2h point group). After thorough characterization, the homogeneous catalysts were grafted on the surface of the functionalized SBA15 and were characterized systematically by various spectral and analytical techniques. The FT-IR and 13C MAS NMR spectra were used to confirm the covalent grafting of the Ru-TPP complex on the surface of the functionalized SBA-15 material. The FT-IR spectra (Figure 1) show a vibration band at 2934 cm−1, which corresponds to the C−H stretching of the aliphatic groups of the silane moiety. The peaks which appeared at 1659, 1477, and 694 nm correspond to the NH2 wagging modes of the amine and diamine groups on the functionalized SBA-15 material. All the spectra displayed the typical band characteristics of Si−O−Si stretching, which appeared around 1080− 1200, and 458 cm−1 which is indicative of the binding modes of vibration. Therefore, the molecular structure was intact after the immobilization of Ru-TPP on the functionalized SBA-15. The 13C NMR spectra (Figure 3) of the diaminosilane-grafted Ru-TPP showed peaks at 8.2 (C1), 19.6 (C2), 36.8 (C3), 42.4 (C5), and 51 ppm (C6), which correspond to the aliphatic carbon functionalities arising from the diaminosilane linker. Similarly, aminosilane- and iodosilane-grafted Ru-TPP show

The FT-IR spectra (Figure 1) of TPP and the corresponding ruthenium complex (Ru-TPP) show characteristic bands in the region of 4000−450 cm−1. For TPP, most of the bands appear in the region of 450−970 cm−1 because of the in-plane bending, out-of-plane bending, ring rotation, and ring torsion modes of the TPP skeleton. Additionally, the sharp vibrational bands observed at 3342 and 1595 cm−1 correspond to N−H stretching and symmetric angular deformation in the N−H plane of the pyrrole ring. The incorporation of the ruthenium ion into TPP is supported by the disappearance of these two sharp bands. The above fact confirms that the metal ion is covalently bonded to the nitrogen present in TPP. After ruthenium incorporation, a new shoulder peak was observed at 462 cm−1 which corresponds to the Ru−N bond.61 In addition, there are characteristic shifts in bands from 1472 cm−1 (TPP) to 1483 cm−1 (Ru-TPP) and from 1175 cm−1 (TPP) to 1230 cm−1 (Ru-TPP). This may be because of the coordination of TPP to the Ru center.55 Furthermore, 1H NMR studies support the formation of Ru-TPP (Table 1). The characteristic peak at −2.74 ppm, which is in the highly shielded region, corresponds to the N−H protons of TPP, and the peak is absent in Ru-TPP (Supporting Information Figure S1).62 Therefore, it is confirmed that Ru is bonded to the nitrogen center of TPP. The other peaks, being characteristic of the phenyl porphyrin framework, remain identical for both the TPP ligand (Supporting Information Figure S2) and the RuTPP complex. 13C NMR spectral studies show peaks at 120 (meso-carbon), 127, 126, 142 (phenyl carbon), and 134 ppm (pyrrolic carbon), which are again identical for both TPP and Ru-TPP. The formation of Ru-TPP was further confirmed by mass spectral analysis. In the mass spectrum of Ru-TPP, the parent ion peak appeared at 788.37 (calculated m/z value = 786.07), corresponding to Ru-TPP [C44H28Cl2N4Ru+4H+] (Figure S3 in Supporting Information). The successful Table 1. 1H NMR and CHN Analyses of TPP and Ru-TPP CHN analysis (%)a sample code TPP Ru-TPP

1

H NMR chemical shift (ppm)

C

H

N

pyrrole

N−H

ortho

meta

para

85.97 (85.45) 67.56 (67.35)

4.04 (4.92) 3.64 (3.60)

9.82 (9.11) 7.17 (7.14)

8.87

−2.74

8.25

7.75

7.78

8.80

7.76

7.78

8.96

a

Theoretically calculated values are given in bracket. D


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Figure 3. 13C MAS NMR spectra of (a) Ru-TPP−SBA-D, (b) Ru-TPP−SBA-A, and (c)Ru-TPP−SBA-I.

peaks at 14.2 (C1), 28.8 (C2), and 49.00 (C3) ppm corresponding to the aliphatic carbon functionalities of the aminosilane and iodosilane linkers;56−60 all the grafted samples show peaks at 150, 137, 140, 133, 125, 128, and 122 ppm which are characteristic of TPP.56 The incorporation of bulk Ru-TPP was evident from the low-angle powder X-ray diffraction (XRD) pattern as well as the decrease in the surface area and pore volume, which is observed in N2 sorption studies. The low-angle XRD patterns of Ru-TPP grafted on various functionalized SBA-15 materials are shown in Figure 4. All the samples show the typical peaks of the SBA-15 structure with decreasing relative intensities as well as broadened X-ray reflections. Pure SBA-15 shows X-ray reflections around 2θ values of 0.85, 1.51, and 1.73, corresponding to the (100), (110), and (200) planes of the hexagonal mesoporous structure. Incorporation of bulk RuTPP into functionalized SBA-15 leads to a shift in 2θ values that corresponds to the (100) plane at 1.05, 1.08, and 0.95 for Ru-TPP−SBA-I, Ru-TPP−SBA-D, and Ru-TPP−SBA-A, respectively. The observed shift in the 2θ value supports the fact that bulk Ru-TPP was incorporated on the surface of the functionalized SBA-15. N2 sorption isotherms (Figure 5) of Ru-TPP grafted on the functionalized SBA-15 exhibit a typical type-IV isotherm as per the IUPAC classification. A well-defined, sharp inflection shows the uptake of N2 in the relative pressure range of 0.6− 0.8 because of the capillary condensation of N2 inside the primary mesoporous sample. Ru-TPP−SBA-D, Ru-TPP−SBAA, and Ru-TPP−SBA-I show a drastic decrease in the surface area and pore volume (Table 2), compared to the parent sample (SBA-15); this confirms that Ru-TPP was encapsulated in the mesoporous channels of the functionalized SBA-15. Further, the pore size distribution is shown in the range of 4−7 nm, which supports the intactness of mesoporosity, and is useful for molecular diffusion. The SEM image (Figure 6) confirms the hexagonal morphology, which is retained even after the grafting of Ru-TPP; the presence of an organometallic complex is confirmed by the surface roughness, and EDAX shows the presence of ruthenium ions in the grafted material. To understand the surface environment of the Ru-TPP complex, 29Si NMR spectra of SBA-15, SBA-15-A, and RuTPP−SBA-A were studied (Figure S4). Pure SBA-15 displays three broad resonances with the chemical shifts between −110 and −97 ppm, which can be assigned to the Q4, Q3, and Q2 species58−60 of the silica framework of Qn = Si(OSi)n(OH)4−n. The 29Si NMR spectra of a representative heterogenized sample (Ru-TPP−SBA-A) show two peaks around −67 and

Figure 4. XRD patterns of samples (a) SBA-15, (b) Ru-TPP−SBA-I, (c) Ru-TPP−SBA-D, and (d) Ru-TPP−SBA-A.

Figure 5. N2 adsorption isotherm of (A) SBA-15 (B) Ru-TPP−SBA-I, (C) Ru-TPP−SBA-D, and (D) Ru-TPP−SBA-A.

Table 2. Textural Properties of the Functionalized SBA-15 (Ru-TPP−SBA-D, Ru-TPP−SBA-A, and Ru-TPP−SBA-I) Materials code sample

BET surface area (m2 g−1)

BJH pore volume (cm3g−1)

BJH pore size (nm)

Ru content (wt %)

SBA-15 Ru-TPP−SBA-D Ru-TPP−SBA-A Ru-TPP−SBA-I

824.2 36.7 23.2 55.68

1.05 0.11 0.42 0.18

7.2 6.2 4.4 6.6

0.03 0.38 0.12

E


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Figure 6. SEM−EDAX of Ru-containing porphyrin loaded on the functionalized SBA-15 (Ru-TPP−SBA-I, Ru-TPP−SBA-D, and Ru-TPP−SBA-A.

−110 ppm, which correspond to the T and Q sites of silicon.58−60 The stronger signal at −110 ppm corresponds to the Q4 sites, indicating the complete condensation of organosilane with the surface silanol groups. The peak that appeared around −67 ppm corresponds to the T3 sites, which are due to the covalent bonding of the organosilane on the SBA-15 surface. The 29Si NMR spectrum of SBA-15 is similar to those of heterogenized samples because the chemical environment of silicon remains unchanged upon the metallocomplex grafting.58−60 The above studies confirmed that the introduction of Ru-TPP on the surface of functional molecular sieves does not affect the framework environment. The stability of the Ru-TPP complex and the heterogenized samples was followed by TGA. The TGA curves for Ru-TPP, Ru-TPP−SBA-D, Ru-TPP−SBA-I, and Ru-TPP−SBA-A are shown in Figure S5 (Supporting Information). TGA of RuTPP shows [see Supporting Information Figure S5a] two distinguishable weight losses in the temperature range of 200− 350 and 350−575 °C, respectively, and no weight loss till 200 °C; this suggests that the complex is stable up to 200 °C. The first-stage weight loss of about 9% is due to the decomposition of axial ligands, and the second-stage weight loss of about 80 wt % in the temperature range of 350−575 °C is due to the decomposition of the macrocyclic porphyrin units. In the case of heterogenized catalysts (Ru-TPP−SBA-A, Ru-TPP−SBA-I, Ru-TPP−SBA-D) [see Supporting Information Figure S5b], there is no appreciable weight loss till 200 °C, which suggests that the Ru-TPP-grafted samples are stable up to 200 °C. Furthermore, there is no distinguishable weight loss for the organic linker and the metallocomplex, which suggests that the Ru-TPP macromolecule is strongly grafted on the surface of the functionalized SBA-15.

Figure 7. Effect of temperature on the carbonylation reaction over Ru-TPP (reaction conditions: Ru-TPP (catalyst) = 0.01 g (0.0126 mmol), reactant = 30 mmol, 2 mL, 4 bar CO2 pressure, and 8 h).

Figure 8. FT-IR spectra of the carbonylated product obtained by the reaction: (A) epichlorohyrin and (B) carbonylated product [(4chloromethyl)-1,3-dioxane-2.

5. CATALYTIC STUDIES Ru-TPP and heterogenized Ru-TPP-immobilized materials were fully characterized, and their catalytic activities were assessed using the carbonylation of epichlorohydrin in liquid phase by CO2 (Scheme 4).

The reaction was performed at different temperatures and times using a homogeneous catalyst (Ru-TPP), and the reaction conditions were optimized. The controlled experiment at different reaction intervals was carried out for both F


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The activation energy of the reaction with the Ru-TPP catalyst was determined by examining the carbonylation reaction at different temperatures and times. Furthermore, a straight line with a negative slope was obtained by plotting the log of the concentration of the reactant versus time, which corresponds to the first-order reaction. The best fit of the experimental data was found by using a first-order kinetics (Figure 9) model with respect to epichlorohydrin (substrate). The studies were repeated at various temperatures and times, and the rate constants were calculated using the first-order equation (see Supporting Information, Table S1, for the rate constant calculation). The rate constants were found to be 0.0068, 0.0091, and 0.0092 s−1 at 80, 100, and 120 °C, respectively. The activation energy was then calculated from the Arrhenius equation as follows

Figure 9. Log concentration of the reactant vs time plot for the firstorder reaction.

Table 3. Number of Runs of the Reaction Carried Out by Ru-TPP and Conversion3

ij K yzij T T yz Ea = 2.303R logjjj 2 zzzjjj 2 1 zzz j K zj T − T z 1{ k 1 {k 2

selectivity number of runs on homogeneous catalyst (Ru-TPP)

conversion

P1

P2

1 2 3 4

98.9 89.8 42.0 20.2

98.2 94.0 86.3 62.9

1.8 6.0 13.7 30.7

overall TON

TON per cycle

To obtain the activation energy, each combination of the rate constants was used to calculate the mean activation energy, which was estimated to be 16.07 kJ mol −1 . Furthermore, it was found that the Ru-TPP catalyst could efficiently catalyze the carbonylation of epichlorohydrin with the highest turnover number (TON) of 2335 when the reaction was conducted for 8 h using 0.0127 mmol of catalyst; the catalytic activity of homogeneous catalysts was further continued by only introducing an additional 30 mmol of epichlorohydrin for four subsequent runs (Table 3). It was evident that the substrate conversion decreased steadily over the period of runs (Table 3, as also evident from the TON of individual runs), and the total TON of about 5924 was evident after four cycles. A further catalytic study was carried out with heterogeneous catalysts, that is, heterogenized Ru-TPP catalysts. Both diamino- and iodo-functionalized heterogeneous catalysts showed excellent conversions but poor selectivity for the carbonylated product at 80 °C (Figure 10a). As the temperature increased to 100 °C, the iodo- and aminofunctionalized heterogeneous catalysts (Ru-TPP−SBA-I and Ru-TPP−SBA-A) facilitated the complete conversion with the exclusive formation of the carbonylated product (Figure 10b), and the catalytic activity remained nearly constant up to four cycles (Table 4). In particular, the homogeneous (Ru-TPP)

2335 2121 992 476 5924

a

Reaction conditions: Ru-TPP (catalyst) = 0.01 g (0.0126 mmol), reactant = 30 mmol (2 mL), 4 bar CO2 pressure, 80 °C, and 8 h. TON = no of moles of a reactant consumed in a reaction/no of moles of a catalyst.

homogeneous and heterogeneous catalysts (see Figure S6), and the complete conversion of epoxide was evident around 7−8 h. The effect of the reaction temperature on the catalytic activity and product selectivity is shown in Figure 7. The complete conversion of epichlorohydrin was evident even at 80 °C, with the exclusive formation of the carbonylated product. The formation of the carbonylated product was confirmed by GC−MS (see Supporting Information Figure S7) and FT-IR spectra (Figure 8). The FT-IR spectra show the peak corresponding to the CO stretching at 1798 cm−1, which is characteristic of cyclic carbonates.63 The increase in the reaction temperature facilitates the formation of ring-opened products.

Figure 10. Effect of different catalysts on the carbonylation reaction at (a) 80 and (b) 100 °C (reaction conditions: Ru-TPP (catalyst) = 0.01 g (0.0126 mmol), Ru-TPP−SBA-X = 0.1 g, reactant = 30 mmol (2 mL), 4 bar CO2 pressure, and 8 h). G


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Table 4. Recyclability of Ru-TPP−SBA-I and Ru-TPP−SBA-A Catalystsa selectivity (%)

selectivity (%)

cycle

conversion (%) (Ru-TPP−SBA-I)

P1

P2

conversion (%) (Ru-TPP−SBA-A)

P1

P2

1 2 3 4

100 97.2 94.9 87.5

83.7 80.6 75.9 72.4

16.3 19.4 24.1 27.6

91.7 88.9 85.0 78.5

96.6 87 80.3 85.2

3.4 13.0 19.7 14.8

Reaction conditions: catalyst = 0.1 g, reactant = 30 mmol (2 mL), 4 bar CO2 pressure, 100 °C, and 8 h.

a

Scheme 5. Plausible Mechanism for the Carbonylation of Epichlorohydrin Using CO2 over the Ru-TPP Catalyst

and heterogeneous catalysts (i.e., Ru-TPP−SBA-A and RuTPP−SBA-I) retained their catalytic activity for several cycles (Tables 3 and 4), whereas in the case of Ru-TPP−SBA-D, the activity drastically decreased in each cycle (see Table S2), which may be because of the steric hindrance results from the bulkier diaminosilane functionality, leading to the weak interaction of Ru-TPP and SBA-D. In the first cycle, the catalyst (Ru-TPP−SBA-D) showed 92% conversion with the formation of an 83.8% carbonylated product, and the activity was drastically decreased to 23% in the subsequent cycles (see Table S2 in the Supporting Information). Thus, further studies only focus on using Ru-TPP, Ru-TPP−SBA-A and Ru-TPP− SBA-I catalysts. The ruthenium content present in the heterogeneous catalysts was calculated using the XRF technique, and it shows about 0.38, 0.12, and 0.03 wt % of Ru in Ru-TPP−SBA-A, Ru-TPP−SBA-I, and Ru-TPP−SBA-D,

respectively. The corresponding TON of these heterogeneous catalysts was found to be 9170 (Ru-TPP−SBA-A), 25 000 (Ru-TPP−SBA-I), and 81 176 (Ru-TPP−SBA-D). The plausible mechanism for carbonylation is given in Scheme 5. In the present reaction system, the ring opening of epichlorohydrin was facilitated by the ruthenium center, followed by the reaction with CO2 and subsequent ring closure to produce the corresponding carbonate. The presence of any acidic functionality led to the formation of hydroxylated chloropropane (1,3-dicholoropropane-2-ol), as shown in the mechanism (step 2). The selectivity of the hydroxylated product was increased by the use of the heterogeneous catalyst, probably because of the residual surface hydroxyl functionalities present in the functionalized SBA-15. The homogeneous Ru-TPP and the highly active heterogeneous catalysts were studied for various scopes of the substrate, and the results are H


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Table 5. Conversion of Various Cyclic Epoxides Using the Ru-TPP, Ru-TPP−SBA-I, and Ru-TPP−SBA-A Catalystsa

Reaction conditions: Ru-TPP (homogeneous catalyst) = 0.01 g, reactant = 30 mmol (2 mL), 4 bar CO2 pressure, 80 °C, and 8 h; Ru-TPP−SBAI/Ru-TPP−SBA-A (heterogeneous catalyst) = 0.1 g, reactant = 30 mmol (2 mL), 4 bar CO2 pressure, 100 °C, and 8 h. a

porous molecular sieve materials was evident from low-angle powder XRD, 13C MAS NMR, N2 sorption, and SEM−EDAX analyses. The high thermal stability of the homogeneous as well as heterogeneous catalysts was evident from TGA. The resultant homogeneous (Ru-TPP) and heterogenized Ru-TPP (Ru-TPP−SBA-X) catalysts were used for CO2 utilization at moderate reaction conditions. The carbonylation reaction by Ru-TPP, the homogeneous catalyst, follows first-order kinetics with respect to epoxide, with the exclusive formation of cyclic carbonate, and the energy of activation is much lower compared to the reported catalysts.64 Ru-TPP grafted on aminosilane- and iodosilane-functionalized materials shows promising catalytic activity and reusability for carbonylation. Further, the 13C MAS NMR spectra of used catalysts show that the structure of the catalysts remains intact even after several runs.

summarized in Table 5. Irrespective of the cyclic ring size, the complete conversion of cyclic epoxide into the corresponding carbonate was evident. After the reaction, the structure of the heterogenized catalyst remains intact, and it is evident from the 13C-MAS NMR spectra of the used catalysts (Figure S7 in the Supporting Information). The spectra show peaks at 150, 137, 140, 133, 125, 128, and 122 ppm that are characteristic of TPP and at 14.2, 28.8, and 49.00 ppm, corresponding to the aliphatic carbon functionalities of the aminosilane and iodosilane linkers. To check the metal leaching from the heterogenized catalysts, the hot filtration test was carried out by filtering the catalyst after treating the catalyst and the reactant at 100 °C under atmospheric conditions for 2 h. Subsequently, the reaction was studied using a filtrate solution under a CO2 atmosphere (see Supporting Information in Table S3). The catalysts Ru-TPP−SBA-A and Ru-TPP−SBA-I showed about 1.2 and 1.7% conversions on the filtrate solution, confirming that there is no leaching of the Ru-TPP complex from the heterogeneous surface. In addition, the catalytic activities of our catalytic systems were compared with some of the recent literature reports on similar systems (Table S4). Our catalysts clearly showed similar activity and good cyclic carbonate selectivity, even at a relatively low pressure, when compared with previous studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01741. 1 H NMR spectra of Ru-TPP (solvent: CDCl3, 400 MHz); 1H NMR spectra of TPP (solvent: CDCl3, 400 MHz; mass spectra of Ru-TPP; 29Si MAS NMR spectra of SBA-15, SBA-AM, and Ru-TPP−SBA-AM;TGA curves for Ru-TPP, Ru-TPP−SBA-D, Ru-TPP−SBA-A, and Ru-TPP−SBA-I; Control experimental data on conversion of epoxide using various Ru-TPP catalysts; GC-MS spectra of 4-(chloromethyl)-1,3-dioxolan2-one obtained by the reaction; 13C MAS NMR spectra of used catalysts; calculation of rate constants using the first-order equation; recyclability of the Ru-TPP−SBA-D

6. CONCLUSIONS Ruthenium-containing tetraphenylporphyrin (Ru-TPP) molecule was prepared for the first time and grafted on various functionalized SBA-15, such as amino-, diamino-, and iodofunctionalized SBA-15 materials. The structural elucidation was confirmed by 1H NMR, FT-IR, CHN, and mass spectral analyses. The grafting of Ru-TPP on functionalized mesoI


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catalyst;quenching studies by filtering the solution; and comparative study of the existing reported catalytic system (PDF)

(12) Zhao, Y.; Tian, J.-S.; Qi, X.-H.; Han, Z.-N.; Zhuang, Y.-Y.; He, L.-N. Quaternary ammonium salt-functionalized chitosan: An easily recyclable catalyst for efficient synthesis of cyclic carbonates from epoxides and carbon dioxide. J. Mol. Catal. A: Chem. 2007, 271, 284− 289. (13) Sheng, X.; Qiao, L.; Qin, Y.; Wang, X.; Wang, F. Highly efficient and quantitative synthesis of a cyclic carbonate by iron complex catalysts. Polyhedron 2014, 74, 129−133. (14) Bai, D.; Jing, H.; Liu, Q.; Zhu, Q.; Zhao, X. Titanocene dichloride-Lewis base: An efficient catalytic system for coupling of epoxides and carbon dioxide. Catal. Commun. 2009, 11, 155−157. (15) Bai, D.; Nian, G.; Wang, G.; Wang, Z. Titanocene dichloride/ KI: an efficient catalytic system for synthesis of cyclic carbonates from epoxides and CO2. Appl. Organomet. Chem. 2013, 27, 184−187. (16) Lan, D.-H.; Au, C.-T.; Yin, S.-F. Zn-EDTA complexes as easyto-handle and stable heterogeneous catalysts for cycloaddition of carbon dioxide to epoxides. Curr. Green Chem. 2015, 2, 35−42. (17) Zhang, Y.; Yin, S.; Luo, S.; Au, C. T. Cycloaddition of CO2 to epoxides catalyzed by carboxyl-functionalized imidazolium-based ionic liquid grafted onto cross-linked polymer. Ind. Eng. Chem. Res. 2012, 51, 3951−3957. (18) Chen, Y.; Xu, P.; Arai, M.; Sun, J. Cycloaddition of Carbon Dioxide to Epoxides for the Synthesis of Cyclic Carbonates with a Mixed Catalyst of Layered Double Hydroxide and Tetrabutylammonium Bromide at Ambient Temperature. Adv. Synth. Catal. 2019, 361, 335−344. (19) Schäffner, B.; Schäffner, F.; Verevkin, S. P.; Börner, A. Organic carbonates as solvents in synthesis and catalysis. Chem. Rev. 2010, 110, 4554−4581. (20) Duval, A.; Avérous, L. Cyclic carbonates as safe and versatile etherifying reagents for the functionalization of lignins and tannins. ACS Sustainable Chem. Eng. 2017, 5, 7334−7343. (21) North, M.; Pasquale, R.; Young, C. Synthesis of cyclic carbonates from epoxides and CO2. Green Chem. 2010, 12, 1514− 1539. (22) Decortes, A.; Castilla, A. M.; Kleij, A. W. Salen-complexmediated formation of cyclic carbonates by cycloaddition of Co2 to epoxides. Angew. Chem., Int. Ed. 2010, 49, 9822−9837. (23) Ema, T.; Miyazaki, Y.; Shimonishi, J.; Maeda, C.; Hasegawa, J.y. Bifunctional porphyrin catalysts for the synthesis of cyclic carbonates from epoxides and CO2: structural optimization and mechanistic study. J. Am. Chem. Soc. 2014, 136, 15270−15279. (24) Yin, S.-F.; Shimada, S. Synthesis and structure of bismuth compounds bearing a sulfur-bridged bis (phenolato) ligand and their catalytic application to the solvent-free synthesis of propylene carbonate from CO2 and propylene oxide. Chem. Commun. 2009, 9, 1136−1138. (25) North, M.; Pasquale, R. Mechanism of cyclic carbonate synthesis from epoxides and CO2. Angew. Chem. 2009, 121, 2990− 2992. (26) Kitamura, M.; Noyori, R.; Murahashi, S. Ruthenium in Organic Synthesis; Murahashi, S. I., Ed.; Wiley: Weinheim, 2004; pp 3−53. (27) Noyori, R.; Hashiguchi, S. Asymmetric transfer hydrogenation catalyzed by chiral ruthenium complexes. Acc. Chem. Res. 1997, 30, 97−102. (28) Jessop, P. G.; Ikariya, T.; Noyori, R. Homogeneous catalytic hydrogenation of supercritical carbon dioxide. Nature 1994, 368, 231. (29) Wang, W.; Wang, S.; Ma, X.; Gong, J. Recent advances in catalytic hydrogenation of carbon dioxide. Chem. Soc. Rev. 2011, 40, 3703−3727. (30) Sanford, M. S.; Ulman, M.; Grubbs, R. H. New insights into the mechanism of ruthenium-catalyzed olefin metathesis reactions. J. Am. Chem. Soc. 2001, 123, 749−750. (31) Mol, J. Industrial applications of olefin metathesis. J. Mol. Catal. A: Chem. 2004, 213, 39−45. (32) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. Syntheses and activities of new single-component, ruthenium-based olefin metathesis catalysts. J. Am. Chem. Soc. 1993, 115, 9858−9859.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ayyamperumal Sakthivel: 0000-0003-2330-5192 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank CSIRHRD Project no: 01(2921/18EMRII) for the financial support. Anjali is grateful to DST-Inspire (IF160353) and CUK for the fellowship and lab facilities and Zeswa Catalyst Pvt. Ltd. for their kind support on providing noble metal salts.

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REFERENCES

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