Article pubs.acs.org/IC
Ionic-Liquid-Based Heterogeneous Covalent Triazine Framework Cobalt Catalyst for the Direct Synthesis of Methyl 3‑Hydroxybutyrate from Propylene Oxide Senkuttuvan Rajendiran, Kwangho Park, Kwangyeol Lee, and Sungho Yoon* Department of Bio & Nano Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Republic of Korea S Supporting Information *
ABSTRACT: β-Hydroxy esters are considered as potential building blocks for the production of fine chemicals and potential drug molecules in various industries. Developing an efficient and recyclable catalyst for the synthesis of β-hydroxy esters is challenging. Here we report the first ionic-liquid-based heterogenized cobalt catalyst, [imidazolium-CTF][Co(CO)4], for the direct ring-opening carbonylation of propylene oxide to methyl 3-hydroxybutyrate (MHB) with 86% selectivity (>99% conversion).
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INTRODUCTION β-Hydroxy esters serve as important key intermediates in the industrial production of value-added chemicals such as 1,3alkanediols,1,2 α,β-unsaturated esters,3 and poly(β-hydroxyalkanoates), which are commonly known as biodegradable polyesters.4,5 Especially, they are shown to be important drug candidates for central nervous system therapies.6−8 Moreover, they are proposed as potential anti-Alzheimer and antiParkinson drugs. β-Hydroxy esters are also directly used to monitor disease statuses such as glycogen storage disease, salicylate poisoning, the severity of alcoholism, and the detection of ketones in the identification of diabetic ketoacidosis in blood.9 To date, many routes are available for the synthesis of β-hydroxy esters, among which catalytic ringexpansion carbonylation (REC) and ring-opening carbonylation (ROC) of epoxide have received great attention because of their ability to produce clean and selective ester products (Scheme 1). Although the REC route has 100% atom economy, it is an indirect, two-step process that utilizes homogeneous and/or heterogeneous bimetallic catalysts to produce β-lactones10,11 and sequential ring openings to form esters. Moreover, the catalyst decomposition and low selectivity
have spurred the need for alternative synthetic routes. Therefore, the direct catalytic ROC of epoxides for the synthesis of β-hydroxy esters was proposed by Howard et al., since it offers a simple, efficient, and inexpensive way to produce the targeted esters (Scheme 1).12 Over the past few years, a vast number of catalytic systems, including a combination of dicobalt octacarbonyl ([Co2(CO)8]) precursors with neutral donor ligands (e.g., 3hydroxypyridine, pyrazole, and imidazole)13−16 and ionic liquid (IL) based tetracarbonylcobaltate complexes [Bmim][Co(CO)4] (Bmim = 1-butyl-3-methylimidazolium) and [CnPy][Co(CO)4] (Cn = alkyl, Py = pyridinium) have been employed as catalysts for direct production of β-hydroxy esters through ROC of epoxide.2,17 Although these homogeneous catalysts showed high activities, their industrial applications are limited because of catalyst decomposition during the reaction and difficulties in product separation from the reaction mixture and catalyst recycling. To overcome these drawbacks, heterogenization of homogeneous active sites is an attractive approach while maintaining the catalytic efficiency of the homogeneous catalyst.11,18−21 Although polymer-supported heterogeneous catalysts have been developed for epoxide carbonylation, product selectivity and poor catalyst recycling has limited their industrial use.22 To overcome such drawbacks, heterogeneous catalysts are expected to be stable under harsh conditions and their pore size should accommodate small molecules for activation along with clean conversion and easy recycling.
Scheme 1. Alkoxycarbonylation of Epoxides via REC and ROC
Received: April 15, 2017 Published: June 5, 2017 © 2017 American Chemical Society
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DOI: 10.1021/acs.inorgchem.7b00974 Inorg. Chem. 2017, 56, 7270−7277
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Inorganic Chemistry Scheme 2. Schematic Representation of the Synthesis of 4 and 5a
Legend: (i) 190 °C, 18 h, neat; (ii) ZnCl2, 400 °C, 48 h; (iii) KCo(CO)4, MeOH, 0.5 MPa of CO, 50 °C, 24 h. Structure 5 was drawn on the basis of the inductively coupled plasma optical emission spectroscopy data. a
Compounds 5 and 6 were synthesized under a CO atmosphere. KCo(CO)4 and [Bmim][Co(CO)4] were prepared by following a reported procedure with slight modifications.2 Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., and further stirred over CaH2 and distilled under an N2 atmosphere by the freeze−thaw method. Research-grade carbon monoxide was purchased from Sinyang Gas Co. with 99.995% purity and used as received. 1H and 13C NMR spectra were recorded on a Bruker Ascend 400 instrument (400 and 125 MHz), and chemical shifts are referenced to the TMS peak. Fourier transform infrared spectroscopy (FT-IR) spectra were collected on a Nicolet iS 50 (Thermo Fisher Scientific) spectrometer using attenuated total reflectance (ATR) or KBr techniques. Scanning electron microscopy (SEM) and energydispersive spectroscopy (EDS) were measured on a JEOL LTD (JAPAN) JEM-7610F instrument operated at an accelerating voltage of 15.0 kV. N2 adsorption−desorption isotherms were measured at 77 K using an automated gas sorption system (Belsorp II mini, BEL Japan, Inc.,). Compounds 4 and 5 were degassed for 24 h before the measurements. The cobalt content in complex 5 was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) (iCAP-Q, Thermo Fisher Scientific) using a microwave assisted acid system (MARS6, CEM/U.S.A). X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCA 2000 instrument (VG Microtech) at a pressure of ∼3 × 10−9 mbar using Al Kα as the excitation source (hν = 1486.6 eV) with a concentric hemispherical analyzer. Powder X-ray diffraction (PXRD) was measured on an RIGAKU D/Max 2500 V instrument using Cu (40 kV, 30 mA) radiation. Elemental analysis was performed with an elemental analyzer (Vario Micro cube, Germany). 13C cross-polarization magic-angle spinning solid-state nuclear magnetic resonance spectroscopy (13C CPMAS ssNMR) data were acquired at ambient temperature on a 400 MHz solid-state NMR spectrometer (AVANCE III HD, Bruker, Germany) at KBSI Western Seoul center with an external magnetic field of 9.4 T. The operating frequency was 100.66 MHz for 13C, and the spectra were referenced to TMS. The samples were contained in an HX CPMAS probe with a 4 mm o.d. zirconia rotor. Thermogravimetric analysis (TGA) was performed on a Q600 analyzer (TA Instrument, US) with a heating rate of 10 °C/min up to 800 °C under a nitrogen atmosphere. LC-MS measurements were performed on an Agilent 6130 single quadrupole LC/MS spectrometer. Carbonylation reactions were performed in a 100 mL
Interestingly, covalent triazine frameworks (CTFs) have attracted a great deal of attention as the best heterogeneous catalytic supports in recent years because of their stability in both acidic and basic media and extreme stability at high temperature and pressure.10,18−37 Their facile structural tunability can incorporate any active site in the framework by maintaining high surface area and large pore volume, which guarantees high catalytic conversion and facile separation in the catalytic process.10,18−21,37−40 Recently, [imidazolium][Co(CO)4] ionic liquids have been significantly studied for epoxide carbonylation.2 Therefore, we hypothesized to introduce [imidazolium]+ motifs in the pore walls of a framework that has the potential to form complexes with [Co(CO)4]− ion by maintaining a homonuclear ion pair (Scheme 2). We anticipate that the heterogenized [imidazolium-CTF][Co(CO)4] catalyst would provide an environment similar to that of the homogeneous catalyst [Bmim][Co(CO)4], and thus the heterogenized catalyst is expected to be active for epoxide carbonylation. Herein, for the first time, we report the synthesis and catalytic activity of the heterogeneous catalyst [imidazoliumCTF][Co(CO)4] for the production of methyl 3-hydroxybutyrate (MHB) from propylene oxide (PO) and CO, which exhibits a much better selectivity than the homogeneous catalyst [Bmim][Co(CO)4]. We also demonstrate the facile separation and sequential recycling of the catalyst for further industrial applications.
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EXPERIMENTAL SECTION
General Considerations and Physical Measurements. All manipulations of air- and water-sensitive compounds were carried out in a glovebox or using standard Schlenk line techniques under an argon atmosphere. All chemicals were purchased from Sigma-Aldrich Chemical Co. and Tokyo Chemical Industry and were used without further purification unless mentioned otherwise. Tetrahydrofuran (THF) was distilled over sodium/benzoquinone, and methanol was distilled over Mg/I under a nitrogen atmosphere before use. Propylene oxide was distilled over CaH2 under a nitrogen atmosphere. 7271
DOI: 10.1021/acs.inorgchem.7b00974 Inorg. Chem. 2017, 56, 7270−7277
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Inorganic Chemistry
Figure 1. (a, b) Scanning electron microscopy images of (a) 4 and (b) complex 5. (c) Co elemental mapping of 5. (d) High-resolution X-ray photoelectron spectra of the Co 2p peaks of complexes (red line) 5 and (black line) 6. 100 mL), THF (3 × 100 mL), and acetone (3 × 100 mL). The resulting black blocks were dried under vacuum at 200 °C for 16 h. Yield: 0.51 g. Synthesis of [imidazolium-CTF][Co(CO)4](5). In a glovebox, a 100 mL stainless steel tube reactor, equipped with a stir bar, was charged with 4 (0.46 g) and KCo(CO)4 (1.25 g, 5.95 mmol) in 10 mL of MeOH. The reactor was cooled to 0 °C and purged with 0.2 MPa CO (twice), then the reactor was pressurized with 0.5 MPa of CO and stirred at 50 °C for 24 h. After the reaction time, the reactor was cooled to room temperature and CO gas was slowly vented. The black blocks were filtered, washed with MeOH (3 × 15 mL), and dried under vacuum overnight. Yield: 0.48 g. Synthesis of 1-Butyl-3-methylimidazolium Cobalt Tetracarbonyl, [Bmim][Co(CO)4] (6). In a glovebox, the tube reactor was equipped with a stir bar and was charged with KCo(CO)4 (0.381 g, 1.813 mmol) and [Bmim]Cl (0.316 g, 1.813 mmol) in 10 mL of MeOH. The reactor was cooled to 0 °C and purged with 0.2 MPa of CO (twice), and then the reactor was pressurized with 0.5 MPa of CO and the contents stirred at 25 °C for 24 h. After the reaction time, the reactor was cooled to room temperature and CO gas was slowly vented. The yellow solution was filtered and the volatiles in the filtrate were removed under vacuum to afford a bluish oily liquid. The resulting liquid was dissolved in THF and filtered to afford a bluish green oil. The bluish green oil was dried under vacuum for 20 h and stored at −20 °C in the glovebox. Yield: 0.495 g, 1H NMR (400 MHz, CDCl3): δ [ppm] 8.97 (s, 1H, imd-NCHN), 7.42 (s, 2H imdNCHCHN), 4.28 (s, 2H, NCH2), 4.06 (s, 3H, NCH3), 1.93 (m, 2H, NCCH2), 1.42 (m, 2H, CH3CH2), 0.99 (m, 3H, CCH3). IR: 1890 cm−1. General Procedure for Epoxide Ring-Opening Carbonylation. A 100 mL stainless steel tube reactor was dried at 80 °C overnight and vacuumed for 3 h. In a glovebox, it was equipped with a magnetic stir bar and charged with the corresponding amount of [imidazolium-CTF][Co(CO)4] (5) and PO in 2.5 mL of dry MeOH. Upon removal from the glovebox the tube reactor was cooled to 0 °C and purged with 0.5 MPa of CO. Then the reactor was immediately pressurized with the desired pressure of CO at room temperature and heated to the mentioned temperature. After the reaction time, the
stainless steel tube reactor, fitted with a pressure gauge and pressure release valve. Carbonylation reactions were carried out in a wellventilated fume hood equipped with a carbon monoxide detector (see the MSDS for proper handling of CO). PO conversion, methyl 3hydroxybutyrate (MHB), 1-methoxypropanol (1-MP), 2- methoxypropanol (2-MP), and acetone were quantified by 1H NMR using naphthalene as an internal standard. Synthesis of 1,3-Bis(5-cyanopyridin-2-yl)-1H-imidazolium Bromide (3). Compound 3 was prepared by modifying a reported procedure.41 1-Methyl-1H-imidazole (1; 0.044 g, 1.0 mmol) and 2bromo-5-cyanopyridine (2; 0.21 g, 2.0 mmol) were placed in a glass ampule, which was sealed under vacuum and heated to 190 °C in a furnace with a heating rate of 1 °C min−1 for 18 h. After 18 h, the ampule was cooled to room temperature, crushed, and ground well. The collected black precipitate was dissolved in hot methanol, filtered, and slowly cooled to ambient temperature to afforded brownish needle-shaped crystals. Yield: 1.5 g. 1H NMR (400 MHz, DMSO-d6): δ [ppm] 11.06 (t, 4JHH = 1.66 Hz, 1H, imd-NCHN), 9.25 (dd, 5JHH = 0.77 Hz, 4JHH = 2.23 Hz, 2H, py-m-CH), 8.85 (dd, 4JHH = 2.23 Hz, 3 JHH = 8.70 Hz, 2H, py-m-CH), 8.84 (d, 4JHH = 1.66 Hz, 2H, imdNCHCHN), 8.50 (dd, 5JHH = 0.77 Hz, 3JHH = 8.70 Hz, 2H, py-o-CH). 13 C NMR (400 MHz, DMSO-d6): δ [ppm] 152.84 (s, py-m-C), 148.03 (s, py-p-C), 144.65 (s, py-m-C), 135.80 (s, imd-NCN), 120.53 (s, imdNC-CN), 116.02 (s, py-p-C), 115.28 (s, py-o-C), 110.53 (s, N-CN). MS (LC-MS): m/z 273.1[M + H]+ (calcd 273.09). Synthesis of [imidazolium-CTF]Cl (4). In a glovebox, compound 3 (1.02 g, 2.81 mmol) and zinc chloride (1.92 g, 14.22 mmol) were mixed in a glass ampule, which was sealed under vacuum and heated to 400 °C in a furnace with a heating rate of 60 °C h−1 and then maintained at 400 °C for 48 h. After the reaction was finished, the ampule was cooled to room temperature with a cooling rate of 10 °C h−1. The hard black solid that formed was crushed but preparation of finely powdered material was difficult; only microsized particles resulted, because of the robustness of the covalent framework. Then the black blocks were stirred with 500 mL of distilled water for 3 h, filtered, and washed with water (150 mL) and acetone (200 mL). The resulting black blocks were refluxed with 1 M HCl (250 mL) overnight and then filtered and washed with 1 M HCl (3 × 100 mL), H2O (3 × 7272
DOI: 10.1021/acs.inorgchem.7b00974 Inorg. Chem. 2017, 56, 7270−7277
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Inorganic Chemistry reactor was cooled to room temperature and CO gas was slowly vented. The crude sample was filtered through Celite, weighed, and analyzed using 1H NMR spectroscopy in CDCl3.
pyridine, and triazine carbon species in 4 (Figure S10 in the Supporting Information). Moreover, the positions of these carbon signals (CN, C−C, CC) were well matched with those of 3, confirming the existence of imidazole, pyridine, and triazine carbon species in 4 as described in Scheme 2. To understand the surface area and pore size of 4, a nitrogen adsorption/desorption isotherm was measured using the Barrett−Joyner−Halenda method (Figure 3a). The material shows a type IV adsorption isotherm with a hysteresis behavior,42 implying that the material exhibits both microporous and mesoporous structure with a total pore volume of 0.37 cm3/g (micropore volume 0.31 cm3/g and mesopore volume 0.06 cm3/g) and a surface area of 650 m2/g (Table S2 in the Supporting Information). The average pore size of 2.30 nm suggests that 4 is mainly arranged with six imidazolium rings, possibly similar to the reported bipyridine moieties in the bpy-CTF system.33 The endogenous anion (Cl−) undergoes facile exchange with exogenous guest anions by treatment of 4 with freshly prepared KCo(CO)4 in methanol under a CO atmosphere, resulting in a black precipitate. The SEM image of 5 shows unchanged irregular block shapes (Figure 1b), and the homogeneous distribution of Co metal and absence of potassium was observed in EDS mapping (Figure 1c and Figure S11 and Table S3 in the Supporting Information). On the basis of ICPOES, the precise content of Co was measured to be 3.62 wt %, which is significantly lower than the theoretical value of full occupancy of 5 (13 wt % of Co). This suggests that approximately 30% of Cl− anions are substituted by Co(CO)4− units. After [Co(CO)4]− immobilization, the surface area and pore volume were decreased to 490 m2/g and 0.27 cm3/g, respectively (Figure 3b and Table S2 in the Supporting Information), indicating the successful exchange of the [Co(CO)4]− ion in 4. Furthermore, elemental analysis of 5 has shown increased carbon content in comparison to 4, which indicates the presence of [Co(CO)4]− ions in the framework (Table 1). XPS was conducted to determine the presence of Co anions in 5, and it has a coordination environment similar to that in homogeneous catalyst 6. The binding energies (BEs) corresponding to the Co− 2p3/2 and Co− 2p1/2 peaks of 6 were found to be 781.4 and 796.9 eV (Figure 1d), respectively. As shown in Figure 1d, the heterogeneous catalyst 5 also has the same BEs (781.4 and 796.9 eV) for the Co− 2p3/2 and Co− 2p1/2 ions, which is well matched with the homogeneous catalyst 6. This affirms that the Co− ions in 6 and 5 have similar coordination environments. Notably, a similar XPS trend for Co− ions was previously demonstrated in the catalyst [bpyCTF-Al(OTf)2][Co(CO)4].11 As mentioned above, preparing the homogeneous sample for IR measurements was also challenging for 5. Hence, the CO stretching peak of [Co(CO)4]− ion could not be observed in the IR analysis (Figure 2). To overcome such drawbacks, the characterization of 5 was limited to ICP-OES, elemental analysis, and XPS measurements, which collectively confirm the existence of [Co(CO)4]− in the frameworks. Notably, the [Co(CO)4]− ions did not completely block the pores of 5, leaving potential channels for small molecules such as PO and CO to pass through the pores to the active sites. The carbonylation of PO using catalysts 6 and 5 was performed in a stainless steel tube reactor using methanol as both the reactant and solvent for the methoxycarbonylation of PO (we used PO as a model epoxide because of its safety and
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RESULTS AND DISCUSSION The IL [Bmim][Co(CO)4] (6), an intense bluish green liquid, was prepared by following the reported procedure (Figures S2 and S3 in the Supporting Information).2 To incorporate the imidazolium motif into the CTF, we chose 3 as a monomer, which was prepared in excellent yield (98%) by treating 1 equiv of 1-methylimidazole (1) with 2 equiv of 2-bromo-5cyanopyridine (2). Using the monomer 3, the porous material 4 was synthesized via an ionothermal method at 400 °C using ZnCl2 as both a solvent and Lewis acid catalyst by maintaining the reactant 3 and ZnCl2 in a 1:5 ratio, which is the optimized condition reported to prepare highly porous material (Scheme 2).22,24,33 The formed black precipitates were subsequently washed with HCl solution to remove excess ZnCl2; during these synthetic routes, most of the Br ions in 4 have been exchanged with Cl ions (Table S1 in the Supporting Information).24,33 The SEM image of 4 shows irregularly shaped blocks with a mean size of >70(5) μm (Figure 1a). The corresponding EDS mapping demonstrated the uniform distribution of carbon, nitrogen, and chloride ions throughout the material (Figure S7 in the Supporting Information), whereas it showed the absence/negligible amount of Zn and Br ions in the CTF matrix (Table S1). Powder X-ray diffraction measurement of solid blocks has shown a broad peak at 2θ = 25°, indicating that 4 has an amorphous nature (Figure S8 in the Supporting Information). The thermal stability of this porous material was revealed by TGA analysis, which showed gradual decay only after 300 °C (Figure S9 in the Supporting Information). The structure and chemical connectivity of 4 were characterized by IR analysis. Because of the hard solid blocks, preparing a homogeneous sample for the IR measurement was challenging, leading to the broad peaks in the spectrum. The absence of a nitrile band at 2200 cm−1 and the weak signals of CN and CC peaks at around 1450−1600 cm−1 suggest the formation of triazine and the presence of imidazolium rings (Figure 2). In addition, 13C CPMAS ssNMR measurement of 4 was performed to confirm the formation of the local structure and compare with 3. The 13C spectrum shows broad signals at around 120−160 ppm, indicating the presence of imidazole,
Figure 2. FT-IR spectra of 3−5. 7273
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Figure 3. Nitrogen adsorption and desorption isotherms of (a) 4 and (b) 5. The inset shows that the pore size of 4 and 5 was ∼2 nm, implying that both samples have mesoporous and microporous structures.
than the rate of side product formation. The controlled carbonylation with equimolar amounts of PO and methanol in THF under 4.0 MPa of CO at 75 °C resulted in 74% conversion and 78% selectivity with a 22% increase in side products (entry 6). To account for the formation of side products, a blank reaction was performed at 75 °C under 4.0 MPa of CO for 11 h, wherein 34% conversion resulted with predominant formation of 1-MP (93%), along with acetone (3%) and 2-MP (4%) as minor products (entry 7). This indicates that the formation of side products (1-MP and 2-MP) was not due to the active catalyst but to the nucleophilic attack of methanol on PO under the reaction conditions, which may be inevitable at this stage. The catalytic activity of the heterogenized catalyst 5 was evaluated using the same reaction conditions of entry 3, resulting in 55% conversion and 67% selectivity (entry 8). To optimize the catalytic efficiency, we varied the reaction conditions, and the results are summarized in Table 2 (entries 9−11). When the reaction temperature and pressure were increased to 90 °C and 6.0 MPa of CO, respectively, complete
Table 1. Elemental Analysis of 4 and 5
a
sample
Ca
Ha
Na
4 5
53.40 63.21
4.16 3.29
18.77 15.47
The weight percent of C, H, N was calculated by elemental analysis.
easy accessibility). Currently, only the carbonylation of ethylene oxide, not other epoxides, has been reported using catalyst 6 (Table 2, entries 1 and 2).2 The carbonylation of PO in methanol at 75 °C under 4.0 MPa of CO using 6 as catalyst resulted in >99% conversion to MHB, 1-methoxypropanol (1MP), and acetone in an 8.5:1.1:0.4 ratio (entry 3). 1-MP and acetone are thought to form as side products only at higher reaction temperatures; the reaction was therefore performed at lower temperatures (34 and 50 °C). Surprisingly, no conversion was observed at 34 °C (entry 4), whereas at 50 °C, the formation of side products increased to 96% (1-MP, 2-MP, and acetone), and the formation of MHB was reduced to 4% (entry 5). This may indicate that the rate of MHB formation is slower
Table 2. Catalytic Ring-Opening Carbonylation of PO Using Catalysts 6 and 5a
selectivity (%)e entry 1 2 3 4 5 6 7 8 9 10 11 12
catalyst b
6 6c 6 6 6 6d 5 5 5 5 5
temp (°C)
pressure (MPa)
time (h)
conversion (%)e
75 75 75 34 50 75 75 75 75 75 90 90
3.7 3.7 4.0 4.0 4.0 4.0 4.0 4.0 4.0 6.0 6.0 0.2
10 10 11 11 11 11 11 11 24 24 24 24
89.2 91.6 >99 42f 74f 34f 55f 73f 90f >99 49f
MHB
85 4 78 67 82 85 86
1-MP
acetone
2-MP
68.5 (3-HPM) 72.4 (3-HPM) 11 4 88 14 93 22 12 11 10 57
5 7 3 10 5 4 4 10
3 1 4 1 1
33
a
Reaction performed in 2.5 mL of MeOH; CO was applied at room temperature followed by heating to specific temperature. Substrate/catalyst (S/ C) ratio 40. bSubstrate ethylene oxide (EO), S/C = 102, solvent MeOH; 3-HPM = methyl-3-hydroxypropionate.2 cSubstrate EO, S/C = 102, solvent THF/MeOH, promoter imidazole.2 dReaction performed in a 1/1 mixture of MeOH and PO in THF. eDetermined by 1H NMR spectroscopy with naphthalene as an internal standard. fRemaining were unreacted PO. 7274
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Inorganic Chemistry conversion was achieved with 86% selectivity, which is comparable to the activity of homogeneous catalyst 6 (Table 2, entry 3). When the reaction was performed under 0.2 MPa of CO at 90 °C for 24 h, no formation of MHB and predominany formation of 1-MP (57%) was observed (entry 12). To understand the stability of 5 during carbonylation, the catalyst after the initial run was separated by simple filtration and washed with methanol. SEM and EDS analyses of the recovered catalyst demonstrate that the distribution of cobalt atoms was uniform throughout the sample (Figure S12 in the Supporting Information). Furthermore, ICP-OES revealed a cobalt content of 3.58 wt %, indicating a very small amount of leaching of [Co(CO)]− ions from catalyst 5. XPS analysis of recovered catalyst showed that the structure of 5 was retained (Figure S13 in the Supporting Information). To check the activity of leached Co species in homogeneous solution, catalyst 5 was heated in 2.5 mL of MeOH at 90 °C for 24 h under an argon atmosphere and the resulting mixture was filtered under hot conditions. Using the filtrate, PO methoxycarbonylation was performed at 90 °C under 6.0 MPa of CO for 24 h, in which no PO conversion was observed. This result indicates that there were no catalytically active Co species in the filtrate for PO carbonylation. The recovered catalyst was used for at least five methoxycarbonylation runs under the same conditions as the first cycle, and the results are compiled in Table 3. In
Figure 4. Proposed mechanism for PO methoxycarbonylation and side products.
activates the PO followed by the nucleophilic attack of [Co(CO)4]− at the less-hindered carbon atom to form the cobalt−alkyl bond a. At higher temperatures, the available CO in the pores of 5 undergoes a rapid migratory insertion of CO between cobalt−alkyl bonds to form the intermediate b, which further undergoes intermolecular addition of methanol to form MHB and regenerate catalyst 5. The delayed CO insertion in a leads to β-hydride elimination followed by enolate protonation and tautomerization to form acetone (c) as a side product (Table 2). However, at higher temperatures, the available PO directly reacts with methanol to form d as unavoidable side products. In summary, for the first time, the heterogenized complex [imidazolium-CTF][Co(CO)4] was synthesized by exchanging the anions on the surface of the CTF. Both [Bmim][Co(CO)4] and the heterogeneous catalyst successfully converted PO to MHB with ca. 86% selectivity, the highest value reported to date. Furthermore, the catalyst stability was revealed via recycling experiments. These results demonstrate that the heterogeneous catalyst 5 might be a good candidate for industrial-scale epoxide methoxycarbonylation. Increasing the reaction rate and selectivity for MHB using a heterogeneous catalyst under various reaction conditions will be the subjects of future studies.
Table 3. Recycling Ability of Catalyst 5a selectivity (%)b cycle
time (h)
conversion (%)
1 2 3 4 5 6c
24 24 24 24 24 24
>99 97 94 90 85 >99
b
MHB
1-MP
acetone
86 83 80 78 74 85
10 10 11 11 11 10
4 7 9 11 15 5
a
Reaction conditions: MeOH 2.5 mL, S/C = 40, CO 6.0 MPa, temperature 90 °C. bDetermined by 1H NMR spectroscopy using naphthalene for reference as internal standard. cAfter the fifth run, catalyst 5 was regenerated with fresh K[Co(CO)4] (see the Supporting Information).
comparison to the first cycle, the activity was maintained at ca. 93% of conversion. Notably, upon recycling, the relative amount of acetone was increased, whereas the amount of 1-MP remained the same. This may be attributed to the previously described discharge of [Co(CO)]− ion without any change in imidazolium ion, leading to the elimination of β-hydride to form more acetone. After the fifth run, to check the possibility of restoring the catalytic activity of the recovered catalyst which has a comparatively reduced amount of [Co(CO)4]−, the recovered catalyst was subsequently treated with fresh K[Co(CO)4] (see the procedure in the Supporting Information). Interestingly, it was found that the regenerated catalyst converts PO into the β-butyrolactone (85%) with an overall conversion efficiency of >99% (entry 6). These results show that the heterogeneous catalyst 5 has an ability to retain its activity for effective carbonylation after regeneration, which makes 5 an attractive catalyst for industrial applications. Here, on the basis of our understanding and the available literature,2,43 we propose a plausible mechanism for conversion of PO to MHB, including the formation of the side products catalyzed by 5 (Figure 4). The imidazolium acidic proton
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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/acs.inorgchem.7b00974. Experimental details, 1H NMR, 13CPMAS ssNMR, TGA, PXRD, XPS, BET analysis, SEM images, and EDS mapping (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for S.Y.:
[email protected]. ORCID
Sungho Yoon: 0000-0003-4521-2958 Notes
The authors declare no competing financial interest. 7275
DOI: 10.1021/acs.inorgchem.7b00974 Inorg. Chem. 2017, 56, 7270−7277
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ACKNOWLEDGMENTS We acknowledged financial support by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3D3A1A01064879).
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