Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Direct Synthesis of Alternating Polycarbonates from CO2 and Diols by Using a Catalyst System of CeO2 and 2‑Furonitrile Yu Gu,† Keitaro Matsuda,† Akira Nakayama,‡,§ Masazumi Tamura,*,†,‡ Yoshinao Nakagawa,† and Keiichi Tomishige*,†
ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/05/19. For personal use only.
†
Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan ‡ JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo, 001-0021, Japan S Supporting Information *
ABSTRACT: The control technique of polymer molecular weight is required for the synthesis of versatile polymers with various properties. In our previous work, we found that CeO2 + 2-cyanopyridine catalyst system was effective for the direct synthesis of alternating polycarbonates from CO2 and diols, however, the maximum average molecular weight was ∼1000 g mol−1 (degree of polymerization = 7−8). In this study, we succeeded in the synthesis of alternating polycarbonates with higher molecular weight from CO2 and diols by using a catalyst system of CeO2 + 2-furonitrile. The average molecular weight reached up to 5000 g mol−1 and could be controlled by adjusting the amount of diols and 2-furonitrile. Moreover, polycarbonate diols, polycarbonates without capping of OH groups at the ends, were obtained with the average molecular weight of ∼2000 g mol−1. The catalyst system was applicable to the direct polymerization of CO2 and various α,ω-diols, providing the corresponding alternating polymers. Comparison of CeO2 + 2-cyanopyridine and CeO2 + 2-furonitrile catalyst systems based on the kinetics and DFT calculations showed two main causes for the formation of polycarbonates with higher molecular weight in the CeO2 + 2-furonitrile catalyst system: First, the reactivity of 2-furamide, which was formed from 2-furonitrile, with produced polycarbonate diols was lower than that of 2picolinamide, which was formed from 2-cyanopyridine, leading to decrease of formation of ester-capped polycarbonates. Second, the adsorption of 2-furonitrile on CeO2 was weaker than that of 2-cyanopyridine, leading to low steric hindrance at the active sites of CeO2 and enabling the reaction of longer diols, such as polycarbonate diols with CO2. KEYWORDS: Carbon dioxide, Polycarbonate, Cerium oxide, Heterogeneous catalyst, Diol
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one. Syntheses of ureas,13−16 carbamates,17−27 organic carbonates,28−40 and polycarbonates41−51 using CO2 as a raw material have been investigated. Among these chemicals, polycarbonates are a promising target because it is one of the most widely used engineering plastics because of its excellent physical, chemical, and mechanical properties. Conventionally, polycarbonates have been produced by using hazardous phosgene as a raw material in industrial process,52,53 which brings a large amount of waste salts. As alternative methods, processes using organic carbonates, such as ring-opening polymerization of cyclic carbonates54−58 and transesterification between diols and dialkyl carbonates,59−61 have been developed, however, organic carbonates are typically produced by using toxic phosgene or CO. Direct polymerization from
INTRODUCTION Utilization of CO2 as a C1 source for production of valuable chemicals is one of the most attractive topics because CO2 is inexpensive, nontoxic, and abundant in nature, and the chemical transformation of CO2 is a promising way to alleviate global warming caused by increase of atmospheric CO2 concentration. Chemical transformation of CO2 can be generally divided into two types: reductive transformation and nonreductive one. Great efforts have been made on the reductive CO2 transformation to methanol, formic acid, methane, or light alkanes due to the potential utilization of these molecules as fuels or hydrogen carriers.1−12 Nonetheless, the market price of these compounds is low in industry and production of these compounds requires high energy input, such as use of highly reactive reductants such as hydrosilanes and boranes or use of H2 at high temperature. On the other hand, nonreductive transformation of CO2 will be promising because of the lower energy input compared with the reductive © XXXX American Chemical Society
Received: December 31, 2018 Revised: February 8, 2019 Published: February 15, 2019 A
DOI: 10.1021/acssuschemeng.8b06870 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
bonitrile (purity > 98.0%), 5-chloro-2-pyridinecarbonitrile (purity > 98.0%), 3-methyl-2-pyridinecarbonitrile (purity > 98.0%), 2-pyridineacetonitrile (purity > 98.0%), 3-cyanopyridine (purity > 98.0%), 1,10-decanediol (purity > 97.0%), trans-1,4-cyclohexanedimethanol (purity > 98.0%), and 2,5-hexanediol (purity > 98.0%) were purchased from Tokyo Chemical Industry, Co., Ltd., and benzonitrile (purity > 98.0%), acetonitrile (purity > 99.5%), 1,4-butanediol (purity > 98.0%) were purchased from FUJIFILM Wako Pure Chemical Corporation. General Procedure. All the reactions were carried out in a 190 mL stainless steel autoclave reactor with an inner glass cylinder. Catalyst and reagents were put into the reactor with a magnetic spinner, and then the reactor was sealed and weighed. One MPa CO2 (Taiyo Nippon Sanso Corporation) was charged and purged for three times to replace air. After that, the reactor was charged with CO2 to the desired pressure (typically 5.0 MPa, ∼22 g) at room temperature and weighed, and the weight change was recorded as the amount of charged CO2. The reactor charged with CO2 was set into a heating block equipped with a magnetic stirrer and heated to the desired temperature (typically 403 K). The reaction mixture was stirred constantly during the reaction. When the reaction was finished, the reactor was cooled to room temperature in water bath. After the gas in the reactor was purged, 20 mL of THF was used to wash the inner of the reactor and glass cylinder of the reactor, and the washing solution was added to the reaction mixture with 0.4 mL of 1,4-dioxane (FUJIFILM Wako Pure Chemical Corporation, purity > 99.5%) as an internal standard for quantitative analysis. CeO2 was then removed from the mixture by filtration, and the obtained clear solution was used as a crude sample in analyses. In the catalyst reusability test, the reaction mixture diluted with THF (collecting solvent) was centrifuged, and the liquid phase was removed carefully. The remaining solid was then washed with 20 mL of fresh THF, and the mixture was centrifuged again. The collected spent catalyst was dried at 333 K in air for 1 h, and then was used for the next run under the same reaction conditions. Analysis. The amount of nitriles and products derived from nitriles was analyzed by a gas chromatograph (Shimadzu, GC-2014) equipped with an FID detector and a CP-Sil5 CB column (Agilent, length = 50 m, i.d. = 0.25 mm, film thickness = 0.25 μm). Since a part of produced polymers thermally decompose into diols in the injection chamber of GC, the amount of 1,6-hexanediol was determined by HPLC (Shimadzu, Prominence HPLC System) equipped with an RI detector (RID-10A) and a Phenyl-Hexyl Luna column (Phenomenex, particle size = 5 μm, length 250 mm, inner diameter 4.6 mm, conditions: eluent, methanol/water = 30/70, 0.5 mL min−1, 313 K). The crude sample was diluted 30-fold with 30 vol % methanol(aq.). The qualitative analysis of the products was conducted by a gas chromatograph equipped with a quadrupole mass spectrometer (Shimadzu, GCMS-QP5050) using the same capillary column and NMR (Bruker, AV400). For the preparation of NMR sample, the mixture after reaction was dissolved by 20 g CHCl3 (FUJIFILM Wako Pure Chemical Corporation, with ethanol 0.5−0.9% as stabilizer, purity except ethanol >99.7%) and then collected (the produced 2furamide in the mixture was partly dissolved). CHCl3 in the received solution was removed by rotary evaporation, and CDCl3 (MagniSolv, 0.03 vol % TMS, deuteration degree >99.8%, stabilized with silver) was used to dissolve the remain and make an NMR sample which contained ∼10 wt % polymer. The polymerized products were analyzed by MALDI-TOF mass (AB SCIEX, TOF/TOF 5800). For the preparation of MALDI-TOF mass samples, 1,8,9-trihydroxyanthracene (Tokyo Chemical Industry, purity >95.0%) was used as the matrix, and sodium trifluoroacetate (Tokyo Chemical Industry, purity > 98.0%) was used as the ionization agent. The matrix and cationizing agent were dissolved separately in THF at a concentration of 10 mg mL−1. The collected reaction solution was diluted by THF to make the concentration of polymer in the solution 10 mg mL−1. The diluted reaction solution, dissolved matrix and cationizing agent were combined in a 1:1:2 ratio, and the mixture was spotted on the MALDI plate and left to dry. All the samples were analyzed in reflector mode. Size exclusion chromatography, SEC (Shimadzu,
CO2 as a carbonyl source will be ideal, and copolymerization of epoxides or oxetanes with CO 2 has been vigorously investigated;48−51 however, these processes have many drawbacks, such as high price, unstable chemical properties, and limited scope of epoxides and oxetanes. Direct synthesis of polycarbonates from CO2, diols, and halides was also reported,41,42,44−47 but the use of hazardous halides is problematic. Therefore, direct polymerization of CO2 and diols to polycarbonates will be the most promising because water is the only byproduct and diols are easily available, however, the reaction has big problems of severe equilibrium limitation and low reactivity of CO2.32,62,70 Recently, we reported that the combination of CeO2 and 2cyanopyridine was an effective catalyst system for the direct polymerization of CO2 and diols,43 which is based on our achievement on high yield and selective synthesis of organic carbonates directly from CO2 and alcohols by the same catalyst system.29,31−33,62−70 In the catalyst system of CeO2 and 2cyanopyridine, the produced water was removed from the reaction media by CeO2-catalyzed hydration of 2-cyanopyridine to 2-picolinamide,71−73 shifting the reaction equilibrium to the product side. In addition, we reported that the addition of 2-cyanopyridine to CeO2 can contribute to increase of the carbonate formation rate (32-fold compared with only CeO2), indicating that 2-cyanopyridine worked as a cocatalyst for the carbonate formation reaction.31 In regard to the dehydration of 2-picolinamide to 2-cyanopyridine, we found that Na2O/SiO2 was an effective catalyst, however, the formation rate was very low (0.12 mmol h−1 gcat−1).29 Recently, the reaction was improved by using diphenyl ether solvent and a reactive distillation apparatus, and the formation rate was increased to 6.1 mmol h−1 gcat−1.74 As a serious problem for the direct polymerization of CO2 and diols by CeO2 and 2-cyanopyridine catalyst system, the polymerization was stopped below degree of polymerization (DP) of 10 (average molecular weight (Mn) = 900−1700 g mol−1). One possible cause for that is esterification reaction of the produced polycarbonate diols with 2-picolinamide, which was produced by hydration of 2cyanopyridine during the reaction, giving ester-capped polycarbonates with no ability for further chain elongation. The development of the control method of polycarbonate properties is essential, and for that, the synthesis method of polycarbonates with various chain lengths is strongly required. After addressing further investigation on effective catalyst systems which can obtain polycarbonates with high molecular weight, we found that the catalyst system of CeO2 and 2furonitrile substantiated the synthesis of polycarbonates with high molecular weight by the direct polymerization of CO2 and diols. The obtained polycarbonates are alternating ones of CO2 and diols with average molecular weight up to 5000 g mol−1 (DP ≈ 35) and high yields (>99%), and the molecular weight is higher than that in the case of CeO2 + 2-cyanopyridine catalyst system.
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EXPERIMENTAL SECTION
Materials. CeO2 was prepared by calcining CeO2−HS (Daiichi Kigenso Kagaku Kogyo CO., Ltd., purity of CeO2 = 99.97%) for 3 h under air atmosphere at 873 K because we have already reported that CeO2 calcined at 873 K was the best catalyst for the synthesis of organic carbonates from CO2 and alcohols.29,64,65,68−70 The specific surface area of the CeO2 catalyst is 84 m2 g−1. All the reagents used in the experiments were used without further purification. 2-Furonitrile (purity > 98.0%), 1,6-hexanediol (purity > 97.0%), 2-cyanopyridine (purity > 99.0%), cyanopyrazine (purity > 99.0%), 2-pyrimidinecarB
DOI: 10.1021/acssuschemeng.8b06870 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Polymerization of CO2 and 1,6-Hexanediol with CeO2 and Various Nitrilesa
a
Reaction conditions: CeO2, 0.09 g; 1,6-hexanediol, 5 mmol; nitrile, 10 mmol; CO2, 5 MPa (at r.t.); 403 K; 24 h. Nitrile-based data are shown in Table S1. bPolystyrene equivalent molar mass based on crude samples. cDetermined by SEC with THF as eluent. dProduced from amide and 1,6hexanediol. Prominence HPLC System), equipped with an RI detector (RID10A) and a SEC column KF-805L (Shodex, particle size = 10 μm, 8.0 mm × 300 mm, flow rate = 1.0 mL min−1) using THF (FUJIFILM Wako Pure Chemical Corporation, purity > 99.5%) as eluent was also used to determine the Mn and Mw/Mn of produced polymers (the calibration curve with 7 points was obtained by using polystyrene as standards). For preparation of an ICP sample, the collected reaction mixture (5 times diluted by weight with THF) was further diluted with water 10 times by weight because high volatility of THF is not ideal for ICP-AES analysis, and the catalyst was separated by filtration. The filtrate was used as a sample in the ICP-AES (ThermoFisher, iCAP6500) analysis, and the collected catalyst was analyzed by XRD (Rigaku, MiniFlex600).
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stoichiometry. Moreover, reaction of 1,6-hexanediol with the produced amide also takes place, providing the corresponding ester (eq 2). In the case of only CeO2 (without any nitriles, entry 1), no polymeric product was obtained. 2-Cyanopyridine (entry 3) was effective for the polymerization as reported in our previous work,43 giving 93% conversion with Mn of 1400 g mol−1 (DP = 10). Cyanopyrazine and 5-chloro-2-pyridinecarbonitrile (entries 4 and 6) also gave over 90% conversion and Mn of 1100 g mol−1 (DP = 8). In the cases of 2-pyridinylacetonitrile, 3cyanopyridine, benzonitrile, and acetonitrile (entries 8−11), almost no conversion was observed. 2-Pyrimidinecarbonitrile and 3-methyl-2-pyridinecarbonitrile (entries 5 and 7) showed moderate conversion with Mn around 500−600 g mol−1 (DP = 3−4); however, decomposition of these nitriles was observed during the reaction (Table S1), and the molar balance of 2pyrimidinecarbonitrile and 3-methyl-2-pyridinecarbonitrile was very low (12% and 38%, respectively). In addition, the selectivity to the ester (eq 2) was higher (18−19%) than other cases (up to 1%). The activity tendency of nitriles is similar to
RESULTS AND DISCUSSION Performance of CeO2 + 2-Furonitrile Catalyst System in the Direct Polymerization of CO2 and Diols. Considering our previous preliminary result that one plausible cause for the suppression of chain elongation is the esterification reaction of produced polycarbonate diols with 2-picolinamide, which was produced by hydration of 2cyanopyridine, selection of nitriles in the combination catalyst system of CeO2 and nitriles can be a key factor for the synthesis of longer polymer chain. At first, effect of nitriles was investigated in the direct polymerization of CO2 and 1,6hexanediol (Table 1). Conversion and selectivity were calculated on 1,6-hexanediol basis. The details of nitrilebased data are shown in Table S1. In this reaction, hydration of nitriles to amides over CeO2 catalyst also occurs at the same time (eq 1),71−73 and hence equivalent amount of nitriles to 1,6-hexanediol is required to transform 1,6-hexanediol and CO2 into polycarbonates on the basis of the reaction C
DOI: 10.1021/acssuschemeng.8b06870 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Table 2. Effect of CeO2 Amount on the Polymerization of CO2 and 1,6-Hexanediol Using CeO2 and 2-Furonitrilea selectivity (%) entry
amount of CeO2 (mmol)
conv. (%)
polymer
monoesterd
others
Mnb,c (g mol−1)
Mw/Mnc
1 2 3 4 5 6
0 0.13 0.25 0.50 1.0 2.0
99 98 >99
>99 >99 >99 99 99
