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Catalytic polymerization of CO2 to polyureas over K3PO4 catalyst
Journal of CO₂ Utilization 28 (2018) 403–407
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Catalytic polymerization of CO2 to polyureas over K3PO4 catalyst a
Peixue Wang , Yuqing Fei
a,b
a,b
, Yan Long
a,⁎
, Youquan Deng
T
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, State Key Laboratory of Solid Lubrication, Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China University of Chinese Academy of Sciences, Beijing, 100049, China
b
ARTICLE INFO
ABSTRACT
Keywords: Catalysis CO2 Polyureas Polymerization
A series of potassium alkali salts (K3PO4, K2HPO4, KH2PO4, K2CO3, KHCO3) were applied in CO2 conversion to polyureas. It was observed that the catalytic activity was related to the alkalinity of salts and solvents. K3PO4 was found to be a highly active catalyst for the polymerization of CO2, and various polyureas were achieved from inferior to superior yields. Characterization of spent catalyst by IR and XRD demonstrated that the K3PO4 was completely transformed into less active K2HPO4, KH2PO4 and KHCO3 via reaction with CO2 and water during the polymerization process. The products were examined by IR, 13C NMR, TGA and DSC, indicating that the product obtained from CO2 possesses the ureas structure and good thermal stability.
1. Introduction Polyureas (PUs) were a new type of polymer including the urea linkage in their structure, thus indicating strong insolubility to organic solvents and good thermal stability. It may therefore be applicable for the preparation of fibers, films, membranes, and coatings [1]. In addition, it can also be degraded in the presence of urea or dialkyl carbonates for producing carbamates, which regarded as the indirect synthesis of carbamates by CO2 [2,3]. Isocyanates are frequently-used for PU production in the industrial processes; however, its toxicity has led researchers to explore other means [4,5]. The synthesis of PUs through the direct polymerization of diamines with CO2 has received growing interest in consideration of CO2 utilization. Although several methods for the preparation of polyureas from CO2 under drastic conditions (high temperature and pressure) have been described [6–12], more effective catalyst systems are still needed in terms of the energy-saving. As a non-toxic, inexpensive, readily available compound, potassium phosphate (K3PO4) has been widely used in organic reactions as a catalyst, additive, and base [13–15]. Among many useful K3PO4-catalyzed reactions, the carboxylation of amines attracted our attention [16]. We have developed two efficient catalysts (P4,4,4,4,6ATriz and KATriz) to convert CO2 and diamines into corresponding polyureas under mildness condition in our previous investigation [17,18]. However, the complex preparation and purification of the above catalysts restricted its application. Besides, the costs of the above catalysts and reaction temperature (170 °C) were higher than K3PO4 (160 °C). Although the thermal stability of PUs was better than the products
⁎
prepared by K3PO4, the alcoholysis of PUs became difficult. After all, the PUs was mainly used for carbamates production via alcoholysis in our laboratory. The PUs obtained at low temperature may have a low degree of polymerization, which was favorable to the alcoholysis process. In the process of constant efforts to develop highly efficient catalyst for the polymerization reaction, here we report that K3PO4 could be employed for the polymerization of diamines under a lower reaction temperature. The influence of reaction parameters of catalyst dosage, CO2 pressure, temperature, and reaction time was examined for the K3PO4 catalyzed polyurea synthesis. The performance of K3PO4 catalyst as also investigated for the synthesis of polyurea with different diamines. Besides, the reusability of K3PO4 and the structural change of K3PO4 after use were also discussed. 2. Experimental 2.1. Chemicals All of the chemicals for the synthesis of PU were used as received without further purification. Potassium phosphate (K3PO4, AR) was purchased from Shanghai Macklin Biochemical Co., Ltd. Diamines and other potassium salts including hexanediamine (HDA, purity > 99%), ethylenediamine (EDA, purity > 99%), isophoronediamine (IPDA, purity > 99%), 1,3-benzenedimethylamine (MXDA, purity > 98%), methylene diphenylamine (MDA, purity > 98.5%), 2,4-tolylene diamine (TDA, purity > 98.5%), potassium bicarbonate (KHCO3, purity > 99.5%), potassium carbonate (K2CO3, purity > 99%),
Corresponding author. E-mail address: [email protected] (Y. Deng).
https://doi.org/10.1016/j.jcou.2018.10.020 Received 7 September 2018; Received in revised form 22 October 2018; Accepted 28 October 2018 2212-9820/ © 2018 Published by Elsevier Ltd.
Journal of CO₂ Utilization 28 (2018) 403–407
P. Wang et al.
potassium phosphate monobasic (KH2PO4, purity > 99.5%), dipotassium hydrogenphosphate (K2HPO4, purity > 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Trimethylhexamethylenediamine (2,2,4- and 2,4,4- mixture, TMDA), butanediamine (BDA), dicyclohexyl methane diamine (HMDA), 1,3cyclohexanedimethanamine (HMXDA) were analytical reagent supplied by Tokyo Chemical Industry Co., Ltd. Solvents including tetrahydrofuran (THF), dimethyl formamide (DMF), 1,4-dioxane, acetonitrile (CH3CN), N-Methyl pyrrolidone (NMP), isopropanol (IPA), octane, obtained from Tianjin Chemical Reagent Factory (AR, > 99%). CO2 (purity > 99.9%) was obtained from Qingdao dehai weiye technology Co., Ltd.
Table 1 Syntheses of PU-HDA from CO2 with various catalysts and solvents.a
2.2. Characterization of the catalyst and product The CP/MAS 13C NMR spectra was carried out on a Bruker AVANCE II WB 400 spectrometer provided with a 4 mm standard bore CP/MAS probe head whose X channel was tuned to 100.62 MHz for 13C, using a 9.39 T magnetic field at 24 °C. Infrared spectroscopy (IR) transmission data was gathered with a Nicolet 5700 from 400 to 4000 cm−1 using KBr pellet pressing method. X-ray diff ;raction (XRD) was measured on a Siemens D/max-RB powder using Cu Kα radiation with a 2θ angle from 10° to 80°. Differential Scanning Calorimetry (DSC) was tested on a Mettler-Toledo DSC822e instrument under N2 with a heating rate of 10 °C/min from 0 °C to 300 °C. Thermograms recorded from the second heating run. Thermal analysis (TGA) measurements were performed on a METTLER TG1 system under N2 at a heating rate of 10 °C/min from 30 °C to 800 °C.
Entry
Catalyst
Solvent
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13
None K3PO4 K2CO3 KHCO3 KH2PO4 K2HPO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4
NMP NMP NMP NMP NMP NMP None Octane 1,4-dioxane IPA CH3CN DMF THF
20 96 77 70 63 76 21 5 14 21 42 42 56
a b
Reaction conditions: 10 mmol HDA, 3 mL NMP, 160 °C, 8 h, 4 MPa. Isolated yield.
molecules, and led to a higher polymerization yield [21]. Based on previous research [17], diamines could be reacted with CO2 spontaneously to form ammonium carbamate intermediate (Scheme 1). This reaction should be reversible as it decomposed easily. K3PO4 catalyst plays a role in the dehydration process of ammonium carbamate. The PO43- acted as a base to active the ammonium carbamate cation and K+ had a stable capability to anion, and one molecule of water is removed to produce a disubstituted urea. Then, the PUs would be formed by constant dehydration. In order to ensure the high PU-HDA yield, an appropriate reaction solvent is needed for this process. Only 21% PU-HDA yield achieved in the absence of any solvent (entry 7). The PU-HDA yields in polar solvents e.g. NMP, CH3CN, dimethyl formamide (DMF), tetrahydrofuran (THF), isopropyl alcohol (IPA) and 1,4-dioxane were much higher than in apolar solvents (for instance, octane). In all solvents, NMP with strong basicity received the highest yield of PU-HDA. The alkali strength sequence of these solvents in water was NMP (pH 8.5–10) > THF (pH 7–8) ≈ DMF (pH 7) > 1,4-dioxane (pH 6–8) was correlated with the PU-HDA yield [18]. In addition, the hydrogen bonding formation between the carbonyl group in NMP with diamines, carbamate salt, may be favorable for the dehydration processes and the nucleophilic attack of the diamines [16,22]. As a comparisons, CH3CN and DMF were found not stable in the reaction process, some co-products were generated and therefore leading to a significant decrease of the PU-HDA yields. While, the nonpolar solvent had adverse effects on this process, the PU-HDA yield was only 5%. Hence, the alkalinity and polarity of solvent both have significant impact on the polymerization of CO2.
2.3. General processes for the syntheses of PU from CO2 The test was conducted in a 100 mL autoclave. 10 mmol diamines, 3 mL NMP and 1 mmol catalyst were added to the autoclave, the reactor was purged with CO2 twice, and then the reactor was pressurized to 4 MPa CO2 at room temperature. The autoclave remains at 160 °C for 8 h under magnetically stirred. After reaction, product was washed with distilled water, received by vacuum filtration, dried in an oven and weighed to calculate the yields. 3. Results and discussion 3.1. Hexamethylenediamine polyurea (PU-HDA) syntheses from 1,6hexanediamine (HDA) and CO2 As listed in Table 1, several alkali metal salts were selected as catalysts for direct synthesis of PU-HDA from CO2 and HDA. A lot of water soluble carbamate salt was produced without catalyst (entry 1). Insoluble products (20%) collected from HDA and CO2 were analyzed with IR (Fig. S1). The characteristic absorption peaks i.e. stretching vibration of C]O (1625 cm−1), NeH (3330 cm−1), bending vibration of NeH in COeNeH (1562 cm−1), respectively, suggesting urea species were existed in products [19]. This could be further proved by the presence of 159.7 ppm peak on 13C NMR (Fig. S2) [8]. As compared with the free NeH (∼3450 cm−1) and free C]O (∼1690 cm−1) the red shift in their stretching vibration were found [20], which indicated the presence of the ordered hydrogen bond. Alkali metal salts chosen were attested effective catalysts for the polymerization of CO2 with HDA, forming corresponding PU-HDA in general to perfect yields (63%–96%). The order of alkalinity strength of these catalysts was K3PO4 (pKb = 1.6) > K2CO3 (pKb≈3.75) > K2HPO4 (pKb = 6.8) > KHCO3 (pKb≈7.63) > KH2PO4 (pKb = 11.9), which consistent with the yields of PU-HDA. The K3PO4 catalyst with strong basicity displayed the highest catalytic activity, which demonstrated that a more basic catalyst would be much easier in the activation of CO2 and diamines. This was probably because more basic catalysts would interact more strongly to amino groups and therefore facilitated the insertion of CO2into the NeH bond of diamine
3.2. Influence of reaction conditions The effects of the temperature, catalyst amount and CO2 pressure on the yield were studied (Table 2). The PU-HDA yield increased rapidly with the temperature increasing. But decreased slightly with the temperature further increasing to 170 °C, which could be because of the reverse reaction between PU-HDA and water at high temperatures (entries 1–3) [23]. As expected, the PU-HDA yield increased when the CO2 pressure was raised from 1 to 5 MPa (entries 4–6). Besides, the effect of the catalyst amount on the PU-HDA yield was also investigated (entries 7–8). As the catalyst amount was increased from 0.5 mmol to 1.5 mmol, the PU-HDA yield increased from 85% to 96% and then decreased to 87%. Too much catalyst would react with CO2 to form K2PO3(OCO2K) [16], which resulted in the decrease of the concentration of CO2. This shows that a considerable amount catalyst is 404
Journal of CO₂ Utilization 28 (2018) 403–407
P. Wang et al.
Scheme 1. Possible mechanism of PUs formation from diamines and CO2 over K3PO4. Table 2 Optimization the polymerization conditions of HDA and CO2.a
Entry
Catalyst (mmol)
Temp. (°C)
Time (h)
PCO2 (MPa)
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12
1 1 1 1 1 1 0.5 1.5 1 1 1 1
150 160 170 160 160 160 160 160 160 160 160 160
8 8 8 8 8 8 8 8 2 4 6 12
4 4 4 1 3 5 4 4 4 4 4 4
49 96 93 15 63 98 85 87 35 56 84 97
a
Reaction conditions: 10 mmol HDA, 3 mL NMP. Isolated yield.
b
Fig. 1. The recyclability of the K3PO4 catalyst for PU-HDA synthesis.
disadvantageous for the formation of PU-HDA. Finally, the change in PU-HDA yield over time was studied (entries 9–12). In the first 8 h, PUHDA yield increased rapidly and then tardily with prolong reaction time, which may be due to the decomposition of PU in the presence of water. 3.3. PUs syntheses from CO2 and diamines K3PO4 was also employed for the polymerization of several diamines in NMP at 160 °C for 8 h (Scheme 2). For the aliphatic diamines, the PU yield increased with the carbon chain growth. While the yield of PU-TMDA was slightly lower than PU-HDA due to the high steric
Scheme 2. Syntheses of polyureas with various diamines.
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Fig. 4. TGA curves of the PUs.
Fig. 5. DSC curves of the PUs.
hindrance of the multi-branched. Similarly, the steric hindrance of cyclic diamines (such as HMXDA, IPDA and HMDA) also resulted in the decrease of PU yield. Since the low basicity of aromatic diamines MXDA, a relatively low yield 51% was obtained. Besides, TDA and MDA were completely nonreactive toward the polymerization of CO2 under this reaction conditions due to their extremely low alkalinity.
Fig. 2. Comparison of XRD patterns of the spent catalyst with those of K2CO3, KHCO3, K3PO4 K2HPO4 and KH2PO4.
3.4. Recyclability of the K3PO4 catalyst After reaction, the catalyst in the water was collected by distillation, dried under a vacuum, and then reused five times for synthesis of PUHDA (Fig. 1). Interestingly, the amount of recycle-catalyst increased to 0.27 g (initial amount was 0.21 g) in the first run, and then the recovered catalyst is reduced reversely with the increasing times. After five times reuse, the amount of catalyst mixed into the final product of polyurea was about 20%. As can be seen in Fig.1, only 86% PU-HDA yield was obtained over recovered catalyst under 160 °C, 8 h, which was slightly lower than the fresh catalyst (96% PU-HDA yield). However, the PU-HDA yield was remained ≥82% during subsequent reuse runs. The catalytic activity of the mixture of KHCO3, K2HPO4 and KH2PO4 (1 mmol KHCO3, 0.5 mmol K2HPO4, 0.5 mmol KH2PO4) was also tested in run number six, which shows the similar activity as the recovery catalyst (85% yield PU-HDA was obtained). To explain the deactivation of the K3PO4 catalyst, the recovered catalyst was characterized by XRD and IR. XRD patterns of the K3PO4, K2HPO4, KH2PO4 and K3PO4-recovered were shown in Fig. 2A. Obviously, the K3PO4 characteristic peaks disappeared in the K3PO4-recovered, suggesting that K3PO4 was converted into other species under
Fig. 3. Comparison of IR spectra of the spent catalyst with those of KHCO3, K2HPO4 and KH2PO4.
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the reaction process, maybe by reaction with CO2 and H2O. It has been reported that K3PO4 reacts with CO2 to produce KHCO3 and K2HPO4 in the presence of water Eq. (1) [24]. Since, water was produced during polymerization of CO2 and diamines, it was conjectured that K3PO4 was changed into K2HPO4 and KHCO3. The XRD pattern of K3PO4-recovered was compared with those of K2CO3, KHCO3, KH2PO4 and K2HPO4, indicating that the recovered catalyst was a mixture of KHCO3, KH2PO4 and K2HPO4. The existence of KH2PO4 was evidence that K2HPO4 could be further reacted with CO2 and water under the polymerization condition Eq. (2) [16]. Besides, no K2CO3 phase was observed in the recovered K3PO4, demonstrating that the decomposition of KHCO3 under CO2 pressure was not occurred [16]. These results explain why the mass of the catalyst increases after first use and then does not increase after another. K3PO4+CO2+H2O→K2HPO4+KHCO3
(1)
K2HPO4+ CO2+H2O→KH2PO4+ KHCO3
(2)
2 KHCO3→K2CO3+H2O+CO2
(3)
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2018.10.020. References [1] Y. He, D. Xie, X. Zhang, The structure, microphase-separated morphology, and property of polyurethanes and polyureas, J. Mater. Sci. 49 (2014) 7339–7352. [2] J. Shang, S. Liu, X. Ma, L. Lu, Y. Deng, A new route of CO2 catalytic activation: syntheses of N-substituted carbamates from dialkyl carbonates and polyureas, Green Chem. 14 (2012) 2899–2906. [3] Q. Li, P. Wang, S. Liu, Y. Fei, Y. Deng, Catalytic degradation of polyurea: synthesis of N-substituted carbamates with CuO–ZnO as the catalyst, Green Chem. 18 (2016) 6091–6098. [4] D.J. Primeaux, 100% Solids aliphatic spray polyurea elastomer systems, J. Elastomers Plast. 24 (1992) 323–336. [5] A. Sanchez-Ferrer, D. Rogez, P. Martinoty, Synthesis and characterization of new polyurea elastomers by sol/gel chemistry, Macromol. Chem. Phys. 211 (2010) 1712–1721. [6] N. Yamazaki, T. Tomioka, F. Higashi, Studies on reactions of the N-phosphonium salts of pyridines. XVIII. Polyureas from carbon dioxide and diamines by use of phosphorus chlorides, J. Polym. Sci. Part C: Polym. Lett. 14 (1976) 55–57. [7] G. Rokicki, Direct method of synthesis of polyureas by N-acylphosp horamidites, Macromol. Chem. Phys. 189 (1988) 2513–2520. [8] C. Wu, J. Wang, P. Chang, H. Cheng, Y. Yu, Z. Wu, D. Dong, F. Zhao, Polyureas from diamines and carbon dioxide: synthesis, structures and properties, Phys. Chem. Chem. Phys. 14 (2012) 464–468. [9] Z. Ying, Y. Dong, J. Wang, Y. Yu, Y. Zhou, Y. Sun, C. Zhang, H. Cheng, F. Zhao, Carbon dioxide as a sustainable resource for macrocyclic oligourea, Green Chem. 18 (2016) 2528–2533. [10] Z. Ying, C. Wu, S. Jiang, R. Shi, B. Zhang, C. Zhang, F. Zhao, Synthesis of polyurethane-urea from double CO2-route oligomers, Green Chem. 18 (2016) 3614–3619. [11] Z. Ying, C. Zhang, S. Jiang, Q. Wu, B. Zhang, Y. Yu, M. Lan, H. Cheng, F. Zhao, Synthesis of a novel hydrophobic Polyurea gel from CO2 and amino-modified polysiloxane, J. CO2 Util. 15 (2016) 131–135. [12] S. Jiang, R. Shi, H. Cheng, C. Zhang, F. Zhao, Synthesis of polyurea from 1,6-hexanediamine with CO2 through a two-step polymerization, Green Energy Environ. 2 (2017) 370–376. [13] A. Muthukumar, N.C. Mamillapalli, G. Sekar, Potassium phosphate-catalyzed chemoselective reduction of α-keto amides: route to synthesize passerini adducts and 3phenyloxindoles, Adv. Synth. Catal. 358 (2016) 643–652. [14] Y. Urawa, K. Ogura, A convenient method for preparing aromatic ketones from acyl chlorides and arylboronic acids via Suzuki–Miyaura type coupling reaction, Tetrahedron Lett. 44 (2003) 271–273. [15] D.M. Pore, M.S. Soudagar, U.V. Desai, T.S. Thopate, P.P. Wadagaonkar, Potassium phosphate or silica sulfuric acid catalyzed conjugate addition of thiols to α,β-unsaturated ketones at room temperature under solvent-free conditions, Tetrahedron Lett. 47 (2006) 9325–9328. [16] Y. Choi, H. Kim, S.H. Shin, M. Cheong, Y.J. Kim, H.G. Jang, H.S. Kim, J.S. Lee, K3PO4-catalyzed carboxylation of amines to 1,3-disubstituted ureas: a mechanistic consideration, Appl. Catal. B: Environ. 144 (2014) 317–324. [17] P. Wang, X. Ma, Q. Li, B. Yang, J. Shang, Y. Deng, Green synthesis of polyureas from CO2 and diamines with a functional ionic liquid as the Catalyst, RSC Adv. 6 (2016) 54013–54019. [18] P. Wang, Y. Fei, Y. Deng, Transformation of CO2 into polyureas with 3-amino-1,2,4triazole potassium as a solid base catalyst, New J. Chem. 42 (2018) 1202–1207. [19] M.M. Coleman, M. Sobkowiak, G.J. Pehlert, P.C. Painter, T. Iqbal, P.C. Painter, Infrared temperature studies of a simple polyurea, Macromol. Chem. Phys. 198 (1997) 117–136. [20] J. Mattia, P. Painter, A comparison of hydrogen bonding and order in a polyurethane and poly(urethane−urea) and their blends with poly(ethylene glycol), Macromolecules 40 (2007) 1546–1554. [21] Y.-S. Choi, Y.N. Shim, J. Lee, J.H. Yoon, C.S. Hong, M. Cheong, H.S. Kim, H.G. Jang, J.S. Lee, Ionic liquids as benign catalysts for the carbonylation of amines to formamides, Appl. Catal. A Gen. 404 (2011) 87–92. [22] D.S. Nguyen, K.C. Jin, S.H. Shin, D.K. Mishra, Y.J. Kim, Reusable polystyreneFunctionalized basic ionic liquids as catalysts for carboxylation of amines to disubstituted ureas, ACS Sustain. Chem. Eng. 4 (2016) 451–460. [23] A. Ion, V. Parvulescu, P. Jacobs, D.D. Vos, Synthesis of symmetrical or asymmetrical urea compounds from CO2 via base catalysis, Green Chem. 9 (2007) 158–161. [24] J. Lu, H. Zhang, M. Cheng, L. Wang, CO2 capture through membrane gas absorption with aqueous solution of inorganic salts-amino acid salts, J. Fuel Chem. Technol. 37 (2009) 77–81.
The spent catalyst was further investigated by IR spectroscopy (Fig. 3). The characteristic absorption peaks at 1657 and 1402 cm−1 were appeared in recycled-K3PO4, which may be assigned to the C]O, CeOeC (asymmetric carbonate) of the carbonate group derived from KHCO3 [16]. While the band at 1084 cm-1 was attributed to P]O in K2HPO4 and KH2PO4. By comparing of IR spectra of the recycled-K3PO4 with those pure KHCO3, K2HPO4 and KH2PO4, it could be seen that the recycled-K3PO4 was a mixture of KHCO3, K2HPO4 and KH2PO4. This was consistent with the XRD results. So, the composition and structural change of K3PO4 lead to the activity decrease of the spent catalyst. 3.5. The thermal properties of PUs The thermal properties of PUs were analyzed by TGA (Fig. 4). It can be found that the synthesized PUs exhibits good thermal stability, their initial degradation temperature were about 200 °C. For the chain PUs, the initial decomposition temperature of PU-HDA was higher than PUBDA, showing that as the length of the carbon chain increases, the thermal stability of the PUs increases. An increase in the degree of substitution also leads to a decrease in thermal stability. For the PUMXDA, the decomposition temperature was lower than aliphatic PUs. This may be due to their low degree of polymerization and relatively weak hydrogen bond interactions. DSC results (Fig. 5) indicated that short chain PUs and cyclic PUs without melting state in the heating process. Only long chain PU-HDA have a melt state at about (274 °C), which may be due to the flexibility of the PU-HDA increased with the long chain carbon structures [8]. 4. Conclusions K3PO4 was a simple and efficient catalyst to realize the direct polymerization of diamines with CO2. The relatively strong basicity of K3PO4 catalyst and NMP solvent may be related to the high catalytic activity. XRD and IR results indicated that the K3PO4 was converted into KHCO3, K2HPO4 and KH2PO4 during the polymerization, which lead to the slightly decrease of activity. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 21802148, 21761132014, 91745106, 21633013) and CAS "Light of West China" Program.
407
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Catalytic polymerization of CO2 to polyureas over K3PO4 catalyst a
Peixue Wang , Yuqing Fei
a,b
a,b
, Yan Long
a,⁎
, Youquan Deng
T
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, State Key Laboratory of Solid Lubrication, Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China University of Chinese Academy of Sciences, Beijing, 100049, China
b
ARTICLE INFO
ABSTRACT
Keywords: Catalysis CO2 Polyureas Polymerization
A series of potassium alkali salts (K3PO4, K2HPO4, KH2PO4, K2CO3, KHCO3) were applied in CO2 conversion to polyureas. It was observed that the catalytic activity was related to the alkalinity of salts and solvents. K3PO4 was found to be a highly active catalyst for the polymerization of CO2, and various polyureas were achieved from inferior to superior yields. Characterization of spent catalyst by IR and XRD demonstrated that the K3PO4 was completely transformed into less active K2HPO4, KH2PO4 and KHCO3 via reaction with CO2 and water during the polymerization process. The products were examined by IR, 13C NMR, TGA and DSC, indicating that the product obtained from CO2 possesses the ureas structure and good thermal stability.
1. Introduction Polyureas (PUs) were a new type of polymer including the urea linkage in their structure, thus indicating strong insolubility to organic solvents and good thermal stability. It may therefore be applicable for the preparation of fibers, films, membranes, and coatings [1]. In addition, it can also be degraded in the presence of urea or dialkyl carbonates for producing carbamates, which regarded as the indirect synthesis of carbamates by CO2 [2,3]. Isocyanates are frequently-used for PU production in the industrial processes; however, its toxicity has led researchers to explore other means [4,5]. The synthesis of PUs through the direct polymerization of diamines with CO2 has received growing interest in consideration of CO2 utilization. Although several methods for the preparation of polyureas from CO2 under drastic conditions (high temperature and pressure) have been described [6–12], more effective catalyst systems are still needed in terms of the energy-saving. As a non-toxic, inexpensive, readily available compound, potassium phosphate (K3PO4) has been widely used in organic reactions as a catalyst, additive, and base [13–15]. Among many useful K3PO4-catalyzed reactions, the carboxylation of amines attracted our attention [16]. We have developed two efficient catalysts (P4,4,4,4,6ATriz and KATriz) to convert CO2 and diamines into corresponding polyureas under mildness condition in our previous investigation [17,18]. However, the complex preparation and purification of the above catalysts restricted its application. Besides, the costs of the above catalysts and reaction temperature (170 °C) were higher than K3PO4 (160 °C). Although the thermal stability of PUs was better than the products
⁎
prepared by K3PO4, the alcoholysis of PUs became difficult. After all, the PUs was mainly used for carbamates production via alcoholysis in our laboratory. The PUs obtained at low temperature may have a low degree of polymerization, which was favorable to the alcoholysis process. In the process of constant efforts to develop highly efficient catalyst for the polymerization reaction, here we report that K3PO4 could be employed for the polymerization of diamines under a lower reaction temperature. The influence of reaction parameters of catalyst dosage, CO2 pressure, temperature, and reaction time was examined for the K3PO4 catalyzed polyurea synthesis. The performance of K3PO4 catalyst as also investigated for the synthesis of polyurea with different diamines. Besides, the reusability of K3PO4 and the structural change of K3PO4 after use were also discussed. 2. Experimental 2.1. Chemicals All of the chemicals for the synthesis of PU were used as received without further purification. Potassium phosphate (K3PO4, AR) was purchased from Shanghai Macklin Biochemical Co., Ltd. Diamines and other potassium salts including hexanediamine (HDA, purity > 99%), ethylenediamine (EDA, purity > 99%), isophoronediamine (IPDA, purity > 99%), 1,3-benzenedimethylamine (MXDA, purity > 98%), methylene diphenylamine (MDA, purity > 98.5%), 2,4-tolylene diamine (TDA, purity > 98.5%), potassium bicarbonate (KHCO3, purity > 99.5%), potassium carbonate (K2CO3, purity > 99%),
Corresponding author. E-mail address: [email protected] (Y. Deng).
https://doi.org/10.1016/j.jcou.2018.10.020 Received 7 September 2018; Received in revised form 22 October 2018; Accepted 28 October 2018 2212-9820/ © 2018 Published by Elsevier Ltd.
Journal of CO₂ Utilization 28 (2018) 403–407
P. Wang et al.
potassium phosphate monobasic (KH2PO4, purity > 99.5%), dipotassium hydrogenphosphate (K2HPO4, purity > 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Trimethylhexamethylenediamine (2,2,4- and 2,4,4- mixture, TMDA), butanediamine (BDA), dicyclohexyl methane diamine (HMDA), 1,3cyclohexanedimethanamine (HMXDA) were analytical reagent supplied by Tokyo Chemical Industry Co., Ltd. Solvents including tetrahydrofuran (THF), dimethyl formamide (DMF), 1,4-dioxane, acetonitrile (CH3CN), N-Methyl pyrrolidone (NMP), isopropanol (IPA), octane, obtained from Tianjin Chemical Reagent Factory (AR, > 99%). CO2 (purity > 99.9%) was obtained from Qingdao dehai weiye technology Co., Ltd.
Table 1 Syntheses of PU-HDA from CO2 with various catalysts and solvents.a
2.2. Characterization of the catalyst and product The CP/MAS 13C NMR spectra was carried out on a Bruker AVANCE II WB 400 spectrometer provided with a 4 mm standard bore CP/MAS probe head whose X channel was tuned to 100.62 MHz for 13C, using a 9.39 T magnetic field at 24 °C. Infrared spectroscopy (IR) transmission data was gathered with a Nicolet 5700 from 400 to 4000 cm−1 using KBr pellet pressing method. X-ray diff ;raction (XRD) was measured on a Siemens D/max-RB powder using Cu Kα radiation with a 2θ angle from 10° to 80°. Differential Scanning Calorimetry (DSC) was tested on a Mettler-Toledo DSC822e instrument under N2 with a heating rate of 10 °C/min from 0 °C to 300 °C. Thermograms recorded from the second heating run. Thermal analysis (TGA) measurements were performed on a METTLER TG1 system under N2 at a heating rate of 10 °C/min from 30 °C to 800 °C.
Entry
Catalyst
Solvent
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13
None K3PO4 K2CO3 KHCO3 KH2PO4 K2HPO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 K3PO4
NMP NMP NMP NMP NMP NMP None Octane 1,4-dioxane IPA CH3CN DMF THF
20 96 77 70 63 76 21 5 14 21 42 42 56
a b
Reaction conditions: 10 mmol HDA, 3 mL NMP, 160 °C, 8 h, 4 MPa. Isolated yield.
molecules, and led to a higher polymerization yield [21]. Based on previous research [17], diamines could be reacted with CO2 spontaneously to form ammonium carbamate intermediate (Scheme 1). This reaction should be reversible as it decomposed easily. K3PO4 catalyst plays a role in the dehydration process of ammonium carbamate. The PO43- acted as a base to active the ammonium carbamate cation and K+ had a stable capability to anion, and one molecule of water is removed to produce a disubstituted urea. Then, the PUs would be formed by constant dehydration. In order to ensure the high PU-HDA yield, an appropriate reaction solvent is needed for this process. Only 21% PU-HDA yield achieved in the absence of any solvent (entry 7). The PU-HDA yields in polar solvents e.g. NMP, CH3CN, dimethyl formamide (DMF), tetrahydrofuran (THF), isopropyl alcohol (IPA) and 1,4-dioxane were much higher than in apolar solvents (for instance, octane). In all solvents, NMP with strong basicity received the highest yield of PU-HDA. The alkali strength sequence of these solvents in water was NMP (pH 8.5–10) > THF (pH 7–8) ≈ DMF (pH 7) > 1,4-dioxane (pH 6–8) was correlated with the PU-HDA yield [18]. In addition, the hydrogen bonding formation between the carbonyl group in NMP with diamines, carbamate salt, may be favorable for the dehydration processes and the nucleophilic attack of the diamines [16,22]. As a comparisons, CH3CN and DMF were found not stable in the reaction process, some co-products were generated and therefore leading to a significant decrease of the PU-HDA yields. While, the nonpolar solvent had adverse effects on this process, the PU-HDA yield was only 5%. Hence, the alkalinity and polarity of solvent both have significant impact on the polymerization of CO2.
2.3. General processes for the syntheses of PU from CO2 The test was conducted in a 100 mL autoclave. 10 mmol diamines, 3 mL NMP and 1 mmol catalyst were added to the autoclave, the reactor was purged with CO2 twice, and then the reactor was pressurized to 4 MPa CO2 at room temperature. The autoclave remains at 160 °C for 8 h under magnetically stirred. After reaction, product was washed with distilled water, received by vacuum filtration, dried in an oven and weighed to calculate the yields. 3. Results and discussion 3.1. Hexamethylenediamine polyurea (PU-HDA) syntheses from 1,6hexanediamine (HDA) and CO2 As listed in Table 1, several alkali metal salts were selected as catalysts for direct synthesis of PU-HDA from CO2 and HDA. A lot of water soluble carbamate salt was produced without catalyst (entry 1). Insoluble products (20%) collected from HDA and CO2 were analyzed with IR (Fig. S1). The characteristic absorption peaks i.e. stretching vibration of C]O (1625 cm−1), NeH (3330 cm−1), bending vibration of NeH in COeNeH (1562 cm−1), respectively, suggesting urea species were existed in products [19]. This could be further proved by the presence of 159.7 ppm peak on 13C NMR (Fig. S2) [8]. As compared with the free NeH (∼3450 cm−1) and free C]O (∼1690 cm−1) the red shift in their stretching vibration were found [20], which indicated the presence of the ordered hydrogen bond. Alkali metal salts chosen were attested effective catalysts for the polymerization of CO2 with HDA, forming corresponding PU-HDA in general to perfect yields (63%–96%). The order of alkalinity strength of these catalysts was K3PO4 (pKb = 1.6) > K2CO3 (pKb≈3.75) > K2HPO4 (pKb = 6.8) > KHCO3 (pKb≈7.63) > KH2PO4 (pKb = 11.9), which consistent with the yields of PU-HDA. The K3PO4 catalyst with strong basicity displayed the highest catalytic activity, which demonstrated that a more basic catalyst would be much easier in the activation of CO2 and diamines. This was probably because more basic catalysts would interact more strongly to amino groups and therefore facilitated the insertion of CO2into the NeH bond of diamine
3.2. Influence of reaction conditions The effects of the temperature, catalyst amount and CO2 pressure on the yield were studied (Table 2). The PU-HDA yield increased rapidly with the temperature increasing. But decreased slightly with the temperature further increasing to 170 °C, which could be because of the reverse reaction between PU-HDA and water at high temperatures (entries 1–3) [23]. As expected, the PU-HDA yield increased when the CO2 pressure was raised from 1 to 5 MPa (entries 4–6). Besides, the effect of the catalyst amount on the PU-HDA yield was also investigated (entries 7–8). As the catalyst amount was increased from 0.5 mmol to 1.5 mmol, the PU-HDA yield increased from 85% to 96% and then decreased to 87%. Too much catalyst would react with CO2 to form K2PO3(OCO2K) [16], which resulted in the decrease of the concentration of CO2. This shows that a considerable amount catalyst is 404
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Scheme 1. Possible mechanism of PUs formation from diamines and CO2 over K3PO4. Table 2 Optimization the polymerization conditions of HDA and CO2.a
Entry
Catalyst (mmol)
Temp. (°C)
Time (h)
PCO2 (MPa)
Yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12
1 1 1 1 1 1 0.5 1.5 1 1 1 1
150 160 170 160 160 160 160 160 160 160 160 160
8 8 8 8 8 8 8 8 2 4 6 12
4 4 4 1 3 5 4 4 4 4 4 4
49 96 93 15 63 98 85 87 35 56 84 97
a
Reaction conditions: 10 mmol HDA, 3 mL NMP. Isolated yield.
b
Fig. 1. The recyclability of the K3PO4 catalyst for PU-HDA synthesis.
disadvantageous for the formation of PU-HDA. Finally, the change in PU-HDA yield over time was studied (entries 9–12). In the first 8 h, PUHDA yield increased rapidly and then tardily with prolong reaction time, which may be due to the decomposition of PU in the presence of water. 3.3. PUs syntheses from CO2 and diamines K3PO4 was also employed for the polymerization of several diamines in NMP at 160 °C for 8 h (Scheme 2). For the aliphatic diamines, the PU yield increased with the carbon chain growth. While the yield of PU-TMDA was slightly lower than PU-HDA due to the high steric
Scheme 2. Syntheses of polyureas with various diamines.
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Fig. 4. TGA curves of the PUs.
Fig. 5. DSC curves of the PUs.
hindrance of the multi-branched. Similarly, the steric hindrance of cyclic diamines (such as HMXDA, IPDA and HMDA) also resulted in the decrease of PU yield. Since the low basicity of aromatic diamines MXDA, a relatively low yield 51% was obtained. Besides, TDA and MDA were completely nonreactive toward the polymerization of CO2 under this reaction conditions due to their extremely low alkalinity.
Fig. 2. Comparison of XRD patterns of the spent catalyst with those of K2CO3, KHCO3, K3PO4 K2HPO4 and KH2PO4.
3.4. Recyclability of the K3PO4 catalyst After reaction, the catalyst in the water was collected by distillation, dried under a vacuum, and then reused five times for synthesis of PUHDA (Fig. 1). Interestingly, the amount of recycle-catalyst increased to 0.27 g (initial amount was 0.21 g) in the first run, and then the recovered catalyst is reduced reversely with the increasing times. After five times reuse, the amount of catalyst mixed into the final product of polyurea was about 20%. As can be seen in Fig.1, only 86% PU-HDA yield was obtained over recovered catalyst under 160 °C, 8 h, which was slightly lower than the fresh catalyst (96% PU-HDA yield). However, the PU-HDA yield was remained ≥82% during subsequent reuse runs. The catalytic activity of the mixture of KHCO3, K2HPO4 and KH2PO4 (1 mmol KHCO3, 0.5 mmol K2HPO4, 0.5 mmol KH2PO4) was also tested in run number six, which shows the similar activity as the recovery catalyst (85% yield PU-HDA was obtained). To explain the deactivation of the K3PO4 catalyst, the recovered catalyst was characterized by XRD and IR. XRD patterns of the K3PO4, K2HPO4, KH2PO4 and K3PO4-recovered were shown in Fig. 2A. Obviously, the K3PO4 characteristic peaks disappeared in the K3PO4-recovered, suggesting that K3PO4 was converted into other species under
Fig. 3. Comparison of IR spectra of the spent catalyst with those of KHCO3, K2HPO4 and KH2PO4.
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the reaction process, maybe by reaction with CO2 and H2O. It has been reported that K3PO4 reacts with CO2 to produce KHCO3 and K2HPO4 in the presence of water Eq. (1) [24]. Since, water was produced during polymerization of CO2 and diamines, it was conjectured that K3PO4 was changed into K2HPO4 and KHCO3. The XRD pattern of K3PO4-recovered was compared with those of K2CO3, KHCO3, KH2PO4 and K2HPO4, indicating that the recovered catalyst was a mixture of KHCO3, KH2PO4 and K2HPO4. The existence of KH2PO4 was evidence that K2HPO4 could be further reacted with CO2 and water under the polymerization condition Eq. (2) [16]. Besides, no K2CO3 phase was observed in the recovered K3PO4, demonstrating that the decomposition of KHCO3 under CO2 pressure was not occurred [16]. These results explain why the mass of the catalyst increases after first use and then does not increase after another. K3PO4+CO2+H2O→K2HPO4+KHCO3
(1)
K2HPO4+ CO2+H2O→KH2PO4+ KHCO3
(2)
2 KHCO3→K2CO3+H2O+CO2
(3)
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2018.10.020. References [1] Y. He, D. Xie, X. Zhang, The structure, microphase-separated morphology, and property of polyurethanes and polyureas, J. Mater. Sci. 49 (2014) 7339–7352. [2] J. Shang, S. Liu, X. Ma, L. Lu, Y. Deng, A new route of CO2 catalytic activation: syntheses of N-substituted carbamates from dialkyl carbonates and polyureas, Green Chem. 14 (2012) 2899–2906. [3] Q. Li, P. Wang, S. Liu, Y. Fei, Y. Deng, Catalytic degradation of polyurea: synthesis of N-substituted carbamates with CuO–ZnO as the catalyst, Green Chem. 18 (2016) 6091–6098. [4] D.J. Primeaux, 100% Solids aliphatic spray polyurea elastomer systems, J. Elastomers Plast. 24 (1992) 323–336. [5] A. Sanchez-Ferrer, D. Rogez, P. Martinoty, Synthesis and characterization of new polyurea elastomers by sol/gel chemistry, Macromol. Chem. Phys. 211 (2010) 1712–1721. [6] N. Yamazaki, T. Tomioka, F. Higashi, Studies on reactions of the N-phosphonium salts of pyridines. XVIII. Polyureas from carbon dioxide and diamines by use of phosphorus chlorides, J. Polym. Sci. Part C: Polym. Lett. 14 (1976) 55–57. [7] G. Rokicki, Direct method of synthesis of polyureas by N-acylphosp horamidites, Macromol. Chem. Phys. 189 (1988) 2513–2520. [8] C. Wu, J. Wang, P. Chang, H. Cheng, Y. Yu, Z. Wu, D. Dong, F. Zhao, Polyureas from diamines and carbon dioxide: synthesis, structures and properties, Phys. Chem. Chem. Phys. 14 (2012) 464–468. [9] Z. Ying, Y. Dong, J. Wang, Y. Yu, Y. Zhou, Y. Sun, C. Zhang, H. Cheng, F. Zhao, Carbon dioxide as a sustainable resource for macrocyclic oligourea, Green Chem. 18 (2016) 2528–2533. [10] Z. Ying, C. Wu, S. Jiang, R. Shi, B. Zhang, C. Zhang, F. Zhao, Synthesis of polyurethane-urea from double CO2-route oligomers, Green Chem. 18 (2016) 3614–3619. [11] Z. Ying, C. Zhang, S. Jiang, Q. Wu, B. Zhang, Y. Yu, M. Lan, H. Cheng, F. Zhao, Synthesis of a novel hydrophobic Polyurea gel from CO2 and amino-modified polysiloxane, J. CO2 Util. 15 (2016) 131–135. [12] S. Jiang, R. Shi, H. Cheng, C. Zhang, F. Zhao, Synthesis of polyurea from 1,6-hexanediamine with CO2 through a two-step polymerization, Green Energy Environ. 2 (2017) 370–376. [13] A. Muthukumar, N.C. Mamillapalli, G. Sekar, Potassium phosphate-catalyzed chemoselective reduction of α-keto amides: route to synthesize passerini adducts and 3phenyloxindoles, Adv. Synth. Catal. 358 (2016) 643–652. [14] Y. Urawa, K. Ogura, A convenient method for preparing aromatic ketones from acyl chlorides and arylboronic acids via Suzuki–Miyaura type coupling reaction, Tetrahedron Lett. 44 (2003) 271–273. [15] D.M. Pore, M.S. Soudagar, U.V. Desai, T.S. Thopate, P.P. Wadagaonkar, Potassium phosphate or silica sulfuric acid catalyzed conjugate addition of thiols to α,β-unsaturated ketones at room temperature under solvent-free conditions, Tetrahedron Lett. 47 (2006) 9325–9328. [16] Y. Choi, H. Kim, S.H. Shin, M. Cheong, Y.J. Kim, H.G. Jang, H.S. Kim, J.S. Lee, K3PO4-catalyzed carboxylation of amines to 1,3-disubstituted ureas: a mechanistic consideration, Appl. Catal. B: Environ. 144 (2014) 317–324. [17] P. Wang, X. Ma, Q. Li, B. Yang, J. Shang, Y. Deng, Green synthesis of polyureas from CO2 and diamines with a functional ionic liquid as the Catalyst, RSC Adv. 6 (2016) 54013–54019. [18] P. Wang, Y. Fei, Y. Deng, Transformation of CO2 into polyureas with 3-amino-1,2,4triazole potassium as a solid base catalyst, New J. Chem. 42 (2018) 1202–1207. [19] M.M. Coleman, M. Sobkowiak, G.J. Pehlert, P.C. Painter, T. Iqbal, P.C. Painter, Infrared temperature studies of a simple polyurea, Macromol. Chem. Phys. 198 (1997) 117–136. [20] J. Mattia, P. Painter, A comparison of hydrogen bonding and order in a polyurethane and poly(urethane−urea) and their blends with poly(ethylene glycol), Macromolecules 40 (2007) 1546–1554. [21] Y.-S. Choi, Y.N. Shim, J. Lee, J.H. Yoon, C.S. Hong, M. Cheong, H.S. Kim, H.G. Jang, J.S. Lee, Ionic liquids as benign catalysts for the carbonylation of amines to formamides, Appl. Catal. A Gen. 404 (2011) 87–92. [22] D.S. Nguyen, K.C. Jin, S.H. Shin, D.K. Mishra, Y.J. Kim, Reusable polystyreneFunctionalized basic ionic liquids as catalysts for carboxylation of amines to disubstituted ureas, ACS Sustain. Chem. Eng. 4 (2016) 451–460. [23] A. Ion, V. Parvulescu, P. Jacobs, D.D. Vos, Synthesis of symmetrical or asymmetrical urea compounds from CO2 via base catalysis, Green Chem. 9 (2007) 158–161. [24] J. Lu, H. Zhang, M. Cheng, L. Wang, CO2 capture through membrane gas absorption with aqueous solution of inorganic salts-amino acid salts, J. Fuel Chem. Technol. 37 (2009) 77–81.
The spent catalyst was further investigated by IR spectroscopy (Fig. 3). The characteristic absorption peaks at 1657 and 1402 cm−1 were appeared in recycled-K3PO4, which may be assigned to the C]O, CeOeC (asymmetric carbonate) of the carbonate group derived from KHCO3 [16]. While the band at 1084 cm-1 was attributed to P]O in K2HPO4 and KH2PO4. By comparing of IR spectra of the recycled-K3PO4 with those pure KHCO3, K2HPO4 and KH2PO4, it could be seen that the recycled-K3PO4 was a mixture of KHCO3, K2HPO4 and KH2PO4. This was consistent with the XRD results. So, the composition and structural change of K3PO4 lead to the activity decrease of the spent catalyst. 3.5. The thermal properties of PUs The thermal properties of PUs were analyzed by TGA (Fig. 4). It can be found that the synthesized PUs exhibits good thermal stability, their initial degradation temperature were about 200 °C. For the chain PUs, the initial decomposition temperature of PU-HDA was higher than PUBDA, showing that as the length of the carbon chain increases, the thermal stability of the PUs increases. An increase in the degree of substitution also leads to a decrease in thermal stability. For the PUMXDA, the decomposition temperature was lower than aliphatic PUs. This may be due to their low degree of polymerization and relatively weak hydrogen bond interactions. DSC results (Fig. 5) indicated that short chain PUs and cyclic PUs without melting state in the heating process. Only long chain PU-HDA have a melt state at about (274 °C), which may be due to the flexibility of the PU-HDA increased with the long chain carbon structures [8]. 4. Conclusions K3PO4 was a simple and efficient catalyst to realize the direct polymerization of diamines with CO2. The relatively strong basicity of K3PO4 catalyst and NMP solvent may be related to the high catalytic activity. XRD and IR results indicated that the K3PO4 was converted into KHCO3, K2HPO4 and KH2PO4 during the polymerization, which lead to the slightly decrease of activity. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 21802148, 21761132014, 91745106, 21633013) and CAS "Light of West China" Program.
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