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Article

Efficient Copolymerization of CO2 and Propylene Oxide via ZnGA/Zn-Co DMC Composite Catalysts: Synergistic Catalysis for High-Performance Polypropylene Carbonate

by
Shuqin Miao
1,
Xiaojiong Wu
2,
Delong Ding
2,
Chunliang Ge
2,
Weihua Shen
1,*,
Yi Yang
2 and
Yunjin Fang
1,*
1
State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
2
Zhejiang Zheneng Technology & Environment Group Co., Ltd., Hangzhou 311100, China
*
Authors to whom correspondence should be addressed.
Reactions 2025, 6(2), 30; https://doi.org/10.3390/reactions6020030
Submission received: 23 March 2025 / Revised: 20 April 2025 / Accepted: 25 April 2025 / Published: 29 April 2025
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Abstract

:
Polypropylene carbonate (PPC) is a biodegradable material derived from propylene oxide (PO) with the renewable resource CO2. In this study, PPC was prepared by the catalytic polymerization of CO2 with PO using a zinc glutarate/zinc cobalt double metal cyanide (ZnGA/DMC) composite catalyst prepared from two heterogeneous catalysts, zinc glutarate (ZnGA) and zinc cobalt double metal cyanide (Zn-Co DMC). High selectivity of PPC was achieved among the polymer and propylene carbonate. The prepared PPC had high molecular weight. The thermal stability of the PPC product was obviously improved by the optimization of the reaction conditions. The catalytic effect of the composite catalyst was superior to that of individual ZnGA and Zn-Co DMC, overcoming the shortcomings of those two catalysts. And the composite catalyst also stimulated some synergistic effects between the two composites, which significantly improved the catalytic effect.

1. Introduction

As non-renewable resources such as fossil fuels and natural gas are rapidly depleting, research on alternate and renewable resources is becoming more and more prevalent worldwide [1,2]. Carbon dioxide is one of the main gases released during the combustion of fuels. And it is also one of the important greenhouse gases that play a major role in causing global warming [3]. Controlling the emission of carbon dioxide from source and using chemical reactions to fix and store carbon dioxide are two means to reduce its content [4]. On the other hand, carbon dioxide can be a renewable raw material to synthesize several molecules [5,6]. Polypropylene carbonate, which is prepared from propylene oxide (PO) and carbon dioxide, has an excellent performance in strength, hardness, high transparency, and high barrier properties [7,8]. Therefore, this material has great potential for applications in optical media, electronics, sheet metal, glass, etc. [9]. The reaction between carbon dioxide and PO can both fix carbon dioxide and generate biodegradable polymers, which not only reduce carbon dioxide content but also alleviate the problem of white pollution, playing a dual role [10].
Inoue [11] used the catalyst ZnEt2/H2O (1:1) in PO and CO2 copolymerization in 1969. However, the ZnEt2/H2O catalyst has low catalytic efficiency, product selectivity, and yield [12,13]. Many researchers have paid attention to this subject after that, and some catalysts have been developed. These catalysts are divided into two types, homogeneous catalysts and heterogeneous catalysts. In particular, N,N′-bis(salicylidene)ethylenediamine (Salen) Co [14], Salen Cr [15], porphyrin Al [16,17], β-diimide Zn [18,19] complexes were used as homogeneous catalysts. Compared with homogeneous catalysts with high manufacturing costs, heterogeneous catalysts offer the processing advantages of lower costs and easy product separation [20]. Heterogeneous catalysts include zinc dicarboxylate [21], rare earth ternary [18], and bimetallic cyanide [22,23]. The development of these catalysts has improved the polymerization reaction to some extent. However, there are still some problems, such as the inevitable appearance of by-products cyclic propylene carbonate, and polyethers [24,25,26]. These impurities cause the product to have a lower initial decomposition temperature, thereby affecting the thermodynamic performance of the polymer and limiting its application range [10,11,12,13,14,15,16]. Therefore, improving the thermodynamic properties of polymers is an important area of research.
As a heterogeneous catalyst, the zinc glutarate (ZnGA) catalyst suffers from a long reaction cycle, a low molecular weight of the product, and a low glass transition temperature [27,28]. The catalyst of zinc cobalt double metal cyanide (Zn-Co DMC) was remarkably selective for the ring-opening of epoxy compounds [29]. As a result, more by-products of polyether was contained in the product while the reaction rate was faster [30], which was not conducive to the formation of fully alternative polypropylene carbonate and was more conducive to the ring-opening homo-polymerization of propylene oxide [31]. The mixed catalyst of ZnGA and Zn-Co DMC was successfully used for this polymerization with the result of high molecular weight of product [32]. However, the systematic investigation of the catalyst preparation was rarely reported. At the same time, a large amount of reaction heat will be released during the polymerization process, which is extremely demanding for the reactor [33].
In this work, ZnGA/DMC catalysts were prepared by combining ZnGA and Zn-Co DMC. Their effects on polymerization were investigated by varying molar ratios of composites. Additionally, their effects on the temperature and pressure of the polymerization were examined to increase the molecular weight of the products. And high performance PPC was prepared in the optimized conditions. The prepared catalyst were characterized by various methods.

2. Experimental

2.1. Materials

Propylene oxide of 99.5% purity was obtained from Aladdin Company, Shanghai, China. Guaranteed reagent CO2 (>99.99% purity) was obtained from the Air Liquide Company, Shanghai, China. Zinc oxide powder and glutaric acid (GA) with a purity of 99% were purchased from Adamas Beta. Zinc chloride with 98% purity was obtained from Aldrich, Shanghai, China. Potassium hexacyanocobaltate(III) (>99%) was obtained from Aladdin Company. Tert-butyl alcohol, toluene, acetone, dichloromethane, hydrochloric, and ethane were purchased from Sinopharm Chemical Reagent Company, Shanghai, China.

2.2. Catalyst Preparation

2.2.1. ZnGA Synthesis

A three-necked flask of 250 mL and a Dean-stark separator was filled with 50 mmol of glutaric acid. The acid was then dissolved with 80 mL of toluene and the flask was shaken at 40 °C for 1 h. Then, 50 mmol of zinc oxide powder was added and the reaction was agitated for six hours at 60 °C. After that, the temperature was raised to 110 °C to remove the produced water. Eventually, the temperature was cooled to ambient temperature, and there was filtering and three acetone washes before the material was dried to a consistent mass at 80 °C in a vacuum oven.

2.2.2. Preparation of Zn-Co DMC

The following procedure was used to prepare the Zn-Co DMC catalyst. A total of 46 mmol of anhydrous ZnCl2 was dissolved in 90 mL solvent mixture (V(t-BuOH)/V(H2O) = 1:2) to give solution A. Solution B was created by dissolving 6 mmol of K3Co(CN)6 in 20 mL deionized water. A 250 mL three-necked flask was filled with solution A, and it was shaken for 30 min at 40 °C. Then, solution B was slowly added to the solution A dropwise with vigorous stirring, which took about 1 h until solutions A and B were completely mixed. After that, the white viscous material was reacted under vigorous stirring at 40 °C for 3 h. After that, it was cooled to ambient temperature and filtered. The white product was washed with a mixture of deionized water and t-BuOH with the volume ratios of 1:1, 1:0.5, and 1:0.3, and finally washed with pure t-BuOH two times. Then, the resulting white solid was vacuum-dried overnight at 80 °C and stored under vacuum conditions before use. The yield of Zn-Co DMC was 96% based on the cobalt addition while accepting Zn3[Co(CN)6]2 as a molecular formula of the product.

2.2.3. Preparation of the ZnGA/DMC Catalyst

A given molar ratio of ZnGA and Zn-Co DMC was dissolved in 100 mL of toluene and reacted for 12 h at 60 °C. Following the reaction, the precipitate was filtered and then washed with t-BuOH three times to remove excess water and residual unreacted raw materials. The solid precipitate was then dried under vacuum at 60 °C overnight.

2.3. Copolymerization

A 250 mL stainless steel autoclave with a mechanical stirrer was vacuum-heated to 100 °C for 3 h, during which the reactor was alternately flushed with nitrogen to remove oxygen and water. After that, it was cooled to ambient temperature. After a certain amount of CO2 was added to ensure that in a pressurized state, the appropriate amount of catalyst and PO was added. Finally, the mixture was stirred at 400 rpm and heated to reaction temperature, filled with CO2 to the set pressure, and then kept for 24 h. After polymerization, the autoclave was rapidly cooled to room temperature in an ice bath and then released the unreacted CO2. Except for a small amount of crude product for 1H-NMR and GPC detection and analysis, the other major product was dissolved in propylene oxide, a certain amount of 5 wt% HCl was used to wash the residual catalyst. The solvent and small molecules contained in the product were removed by using a rotary evaporator [34]. Subsequently, the product was washed three times with ethanol to remove the residual by-products, such as polyether and propylene carbonate. After two final washes in distilled water, the product was vacuum-dried for 24 h at 40 °C.

2.4. Characterizations

An X-ray diffraction (XRD) pattern was obtained on an 18 KW/D max 2550VB PC (Rigaku Corporation, Tokyo, Japan) to examine the structure of catalysts under atmospheric conditions, with a scan range of 10–80°, scanning rate of 5°/min, and a scanning step of 0.02°. A scanning electron microscope (SEM, Nova SEM 450, Hillsboro, OR, USA) was used to observe the surface morphology of the catalysts. The N2 sorption was carried out on the Quadrasorb SI adsorber (USA) to calculate the specific BET surface area of catalysts. The X-ray photoelectron spectroscopy (XPS) was performed on an Axis Ultra DLD Kratos Axis Supra (Shimazu, Kyoto, Japan) to analyze the elemental composition and chemical states on the surface of the catalyst. Fourier infrared spectroscopy (FTIR) was measured by a Nicolet 6700 Fourier infrared spectrometer (Thermo Scientific, Waltham, MA, USA) to detect changes in functional groups in the molecular structure of catalysts and polymer PPC in the wavenumbers range of 4000–400 cm−1. Gel permeation chromatography (GPC, Waters 1515, Milford, MA, USA) was used to determine the molecular weight and molecular weight distributions (PDI) of copolymers, using polystyrene as a calibration standards sample and chloroform as an eluent (1 mL/min rate at 35 °C). The Thermal Gravimetric Analysis (TGA) was determined on TGA 8000 (PerkinElmer, Waltham, MA, USA) to obtain the 5% thermal decomposition temperature (Td–5%), with the range of 20 to 600 °C at a heating rate of 10 °C/min and a nitrogen flow of 100 mL/min. The differential scanning calorimetry (DSC) was measured by a Perkin Elmer DSC6000 (Waltham, MA, USA).

3. Result and Discussion

3.1. ZnGA Catalysts

FTIR was used to characterize the prepared ZnGA. As depicted in Figure 1, the absorption bands of ZnGA catalyst are summarized as follows: the adsorption band at 2957 cm−1 is assigned to CH stretching; the band at 1537 cm−1 is attributed to COO symmetric stretching; COO antisymmetric stretching is assigned to 1406 cm−1; and the band at 1455 cm−1 is assigned to CH2 scissoring. The absence of a pronounced characteristic absorption peak of ZnO at 436.9 cm−1, suggested that ZnO reacted completely with glutaric acid. The result showed that the prepared ZnGA had the groups generated from the anion of GA.
The grain size and crystallinity of the catalyst are the main factors influencing catalytic activity. As shown in Figure 2, ZnGA prepared at different temperatures possessed three sharp peaks (peak a: 12.7°, peak b: 22.6°, peak c: 23.0°). According to Scherrer’s equation [35], dxrd = Kλ/(βcosθ) peaks with higher intensity represent substances with better crystallinity and better quality (grain size and crystal structure), where K is the shape factor (0.89), λ is the X-ray wavelength (λ = 0.15406 nm), β is the full width at half maximum (FWHM), and θ is the Bragg’s angle. It was calculated that when the preparation temperature was raised from 60 °C to 90 °C, the mean crystallite dimension (dxrd) of ZnGA increased from 191.8 Å to 293.2 Å (the detailed calculation is shown in Supporting Information, Table S1). However, it was found that the polymerization with this catalyst required a long reaction period. The catalyst was extremely selective for the production of polycarbonates and high FCO2 (FCO2 means the ratio of CO2 to PO in PPC product).

3.2. Zn-Co DMC Catalyst

XRD was utilized to characterize the structure of Zn-Co DMC as depicted in Figure 3a. The analysis revealed that this complex exhibited three broad characteristic peaks at 14°, 17.1°, and 23.5°, indicating poor crystallinity. The catalyst effectively promoted the polymerization of carbonate dioxide with propylene oxide, resulting in a significantly faster reaction rate compared to that of the ZnGA catalyst and a shorter reaction time. However, it should be noted that the reaction process was susceptible to strong exothermic reactions, resulting in temperature fluctuations that adversely affected the reaction process and led to lower yields of the target product PPC. The reaction phenomenon indicates that the Zn-Co DMC catalyst has a higher selectivity for the propylene oxide ring-opening reaction, making it more advantageous for the ring-opening of propylene oxide during the polymerization process [36].

3.3. Analysis of ZnGA/DMC Catalysts and Its Effect on PPC Synthesis

The ZnGA catalyst has exceptional selectivity for PPC synthesis, whereas the Zn-Co DMC catalyst is highly active in opening the rings of propylene oxide. To produce catalysts for the polymerization reaction that exhibited high activity and good selectivity, a series of composite catalysts consisting of ZnGA and Zn-Co DMC were prepared. The catalysts were then subjected to reactions at various temperatures and pressures, as well as different catalyst usage, to determine their effects on the polymerization reaction.
Figure 3a shows the XRD patterns for Zn-Co DMC, ZnGA, and ZnGA/DMC composite catalysts. It was demonstrated that the ZnGA/DMC catalyst contained all of the typical Zn-Co DMC and ZnGA peaks. Figure 3b shows the entire XPS spectra of the catalysts. Zn, Co, C, N, and O are present in the hybrid structure of the ZnGA/DMC catalyst, as shown by the full XPS spectrum. At 1045.1 eV and 1021.9 eV, respectively, the XPS 2p1/2 and 2p3/2 orbital peaks of Zn species in ZnGA were seen. Zn species were observed at 1045.8 eV and 1022.7 eV in Zn-Co DMC and 1045.5 eV and 1022.4 eV in ZnGA/DMC composite catalyst, respectively (Figure 3c). Although the principal Zn species in the ZnGA/DMC composite catalyst were from ZnGA and Zn-Co DMC, the XPS 2p orbital of Zn species in ZnGA/DMC differed greatly from ZnGA and Zn-CoDMC, suggesting that the composite catalyst were different from the physical mixture of two raw materials. ZnGA may coordinate with C≡N in Zn-Co DMC, producing Zn2+-CN-Co3+ [36]. 2p1/2 and 2p3/2 orbital peaks of Co species in Zn-Co DMC emerged at 796.1 eV and 781.2 eV, respectively, whereas those of Co species in ZnGA/DMC appeared at 796.2 eV and 781.2 eV. Because the predominant Co species in the ZnGA/DMC composite catalyst is Zn-Co DMC, the XPS 2p orbital peaks in ZnGA/DMC and Zn-Co DMC were identical (Figure 3d).
Figure 4 shows the SEM image of ZnGA (a, b), Zn-Co DMC (c, d), and ZnGA/DMC (e, f). The ZnGA catalyst displays flake particles with 470 nm × 250 nm size; the Zn-Co DMC catalyst also shows a large rhombic structure with strong agglomeration; whereas the ZnGA/DMC composite catalyst shows a uniform distribution of rods with smaller particle size. These results indicated that the ZnGA/DMC composite catalyst is not just a physical mixture of the two powders, but a new type of catalyst with the mixing in atomic scale.
The N2 absorption–desorption isotherms (Figure 5) were used to access the surface areas and porosity of those catalysts. The BET surface area of ZnGA was 20.44 m2/g with a small pore volume of 0.134 cm3/g, and Zn-Co DMC had a BET surface area of 172.63 m2/g and a pore volume of 0.331 cm3/g. While ZnGA/DMC had a BET surface area of 107.55 m2/g and a pore volume 0.141 cm3/g, respectively. Furthermore, The ZnGA/DMC composite catalyst helped to increase the reactant contact area and improved catalytic activity since the Zn-Co DMC catalyst was amorphous [37].
As shown in Figure 6, small amount of PPC can be obtained with the catalysis of ZnGA. The activity of Zn-Co DMC for self-polymerization of PO was too high, so only some viscous liquid can be obtained for the Zn-Co DMC catalyst when the fly temperature occurred (Figure 6b). An abundance of solid product can be formed when mixed ZnGA/DMC catalyst was used (Figure 6c). For the ZnGA catalyst, the yield of the product was only 50 gpolymer/gcatalyst, and the weight-average molecular weight was just 22.9 kg/mol and the Td–5% was only 210 °C shown in Table 1. For the Zn-Co DMA catalyst, the catalytic efficiency was only 8.48 gpolymer/gcatalyst, the molecular weight of product was 16.07 kg/mol, and the Td–5% was 170 °C, as shown in Table 1.
As the molar ratios of ZnGA and Zn-Co DMC gradually increase, the catalytic effectiveness of the composite catalyst tends first to grow and subsequently decline, as displayed in Table 1. Mn, and Mw increase significantly, indicating that Zn-Co DMC doping in the ZnGA system can effectively improve the product yield and improve its quality. Secondly, the catalytic system can also significantly shorten the induce period of the polymerization reaction. The reaction typically exhibits an obvious temperature rise and pressure drop phenomenon after 2.5 h, indicating that the reaction has commenced at that time. The reaction rate was faster between 3 and 12 h, and then gradually slowed down before 24 h, which addresses the issue of the long reaction period associated with the ZnGA catalyst. By extending the reaction time, the yield can be increased. At a reaction time of 24 h, the catalytic yield can reach up to 951 gpolymer/gcatalyst. The molecular weight of the products increased significantly with varying ratios of ZnGA and Zn-Co DMC, reaching a maximum value of Mw = 512.79 kg/mol, and PDI = 2.44 as shown in Figure 7b when the molar ratio of ZnGA and Zn-Co DMC was 10:1. The Td–5% was significantly increased with the catalysis of ZnGA/DMC, with a comparison of them shown in Table 1 and Figure 7a. The selectivity of polymer was as high as 99% (Table 1) in all cases, indicating that the formation of small molecular of propylene carbonate was inactive on these catalysts under the present reaction conditions.
The molecular weight of the PPC products was significantly impacted by varying molar ratios of the ZnGA/DMC catalyst, and the thermal characteristics of PPC with various molecular weights are displayed in Figure 7a. As the molecular weight of the polymer increases, the thermal decomposition temperature of PPC increases noticeably, and the maximum Td–5% value reaches 284 °C when the molecular weight is 512.79 kg/mol, which is much larger than that of the Zn-Co DMC-catalyzed product. Simultaneously, the polymerization of CO2 with PO was accelerated and product quality was raised by the combination of ZnGA and Zn-Co DMC catalyst outperforms the standalone catalytic system and demonstrates a strong synergistic effect [38].
A large number of studies have shown that the rate control step of the polymerization of CO2 and propylene oxide is the insertion of propylene oxide, and the key step of the catalytic cycle is the combination of metal carbonate and open-ring cyclic oxide [39]. Bimetallic cyanide based on Zn-Co has a higher catalytic activity for the ring-opening of propylene oxide in the polymerization of CO2 and PO, but it has lower carbon dioxide fixation. The zinc glutarate catalyst can effectively fix carbon dioxide, and the nucleophilic binding ability of propylene oxide is poor. Combined with the advantages of the two catalysts, the ZnGA/DMC composite catalyst was prepared to improve the catalytic performance by stimulating the synergistic ability of the two catalysts as much as possible.
The structure of PPC was detected by Fourier infrared spectroscopy (FTIR) as shown in Figure 8a. The C=O absorption peak in polycarbonate is at 1747 cm−1; 1230 cm−1 is the C-O stretching vibration peak; while the weak peaks at 1064 cm−1 and 787 cm−1indicate that the carbonate contains polyether. It was proved that the PPC product from the catalytic copolymerization reaction by the ZnGA/DMC composite catalyst contains a small amount of by-product of polyether, reducing the complete alternation of PO and CO2. From the 1H-NMR spectra, it is known that in solvent CDCl3, δ (ppm) 5.00 (m, 1H, -CH-), 4.20 (m, 2H, -CH2-), and 1.33 (d, 3H, -CH3-) are the main characteristic peaks in carbonate, confirming the synthesis of PPC product. While δ (ppm) 3.55–3.60 ppm and 1.16 ppm are the main characteristic peaks of the polyether by-products. It is clear from Figure 8b that the PPC product as a result of the ZnGA/DMC catalyst contains a small amount of polyether by-products, which represents an improvement in product conversion over that of the individual Zn-Co DMC and ZnGA catalysts. The DSC result of PPC is shown in Figure 8c. The glass transition temperature was 29.32 °C.
The impacts of the catalyst dosage were investigated to better understand the catalytic activity of the ZnGA/DMC composite catalyst. The results are shown in Table 2. The amount of PO was maintained at 65 g with the usage of the ZnGA/DMC (10:1) composite catalyst increasing. The optimum catalytic efficiency was 1510 gpolymer/gcatalyst when using 0.05 g ZnGA/DMC. Although the product had a relatively high molecular weight, the molecular weight distribution was wide. The molecular weight of the obtained PPC significantly decreased from 512.79 kg/mol to 229.86 kg/mol with increasing amounts of ZnGA/DMC above 0.1 g. When the catalyst dosage exceeds 0.2 g, the molecular weight of the product decreases from 200 kg/mol to 15 kg/mol, and the molecular weight distribution becomes much wider. The result indicates that increasing the amount of catalyst increases the rate of chain transfer rather than the chain growth rate [39]. Additionally, severe exotherm was observed in the early stage of the reaction with high catalyst dosage. The severe exotherm caused the reaction temperature to be out of control and resulted in the reaction to be terminated within 4 h. Therefore, using excessive catalysts could lead to a sharp increase in temperature, which could cause PPC degradation. That would also significantly reduce the yield of the product. It is necessary to control the catalyst dosage to improve the reaction yield. When the amount of catalyst is excessive, the homogeneous polymerization rate of PO increases, which also impacts the selectivity and molecular weight of the product.
There was a greater effect of reaction temperature on the polymerization of PO and CO2. As shown in Table 3, the molecular weight and yield of the products tended to rise and subsequently fall as the reaction temperature increased. At 70 °C, the polymerization procedure produces the optimum results. A higher temperature appears to be more favorable for the creation of cyclic products, as evidenced by the large rise in the polymerization rate at reaction temperatures over 90 °C and the resulting increased amount of by-product PC in the final product. However, the polymer chain is prone to back-biting, which may affect the selectivity of the polymerization reaction [40]. On the other hand, decreasing the reaction temperature causes the molecular weight and polymerization reaction rate to drop significantly. The polymerization reaction will not occur at a temperature lower than 60 °C.
The impact of pressure on the molecular weight of the products is displayed in Table 4. The polymerization reaction failed to occur when the pressure was below 2.0 MPa. Increasing the pressure from 2.0 MPa to 4.0 MPa resulted in an increase in both the molecular weight (from 101.83 kg/mol to 512.79 kg/mol) and the yield of the product. However, when the pressure exceeded 4.0 MPa, the polymerization rate slowed down, and the yield and molecular weight of the product decreased. This implied that a moderate increase in pressure causes the CO2 level to rise, which is advantageous for CO2 insertion to form the carbonate linkage [40]. However, if the pressure was too high, it reduced the concentration of PO and catalyst and slowed down the reaction rate, thereby reducing the product yield and molecular weight.
The selectivity in this reaction always refers to the ratio of the polymer product in polymer and the other small molecular product. And the fraction of polycarbonate (FCO2) means the ratio of alternative structure of CO2 and PO in the polymer product of PPC. For the ZnGA/DMC catalyst, the selectivity of the polymer was more than 98%, as shown in Table 1 and Table 2. This means that the side product of propylene carbonate was very little. However, as mentioned in the results of FTIR and H-NMR, polyether was formed during the copolymerization. The fraction of polycarbonate (FCO2) with alternative structure of CO2 and PO was in the range of 60.46–72.53% (see Table 3 and Table 4). The high activity of the ring open catalyst of Zn-Co DMC was considered to attribute to these results.

4. Conclusions

Inexpensive and easy-to-prepare catalysts, ZnGA and Zn-Co DMC, were chosen for this work to investigate how the ratio of the ZnGA/DMC composite catalyst and the polymerization condition affected the characteristics of the final product. Comparing ZnGA and Zn-Co DMC as individual catalysts in the copolymerization of CO2 and PO, the results demonstrated that ZnGA/DMC composite catalysts could considerably shorten the polymerization time and exhibit outstanding catalytic activity and selectivity. It showed that the ZnGA/DMC catalyst enhanced the synergistic impact between the two catalysts, which generally increased the catalytic activity, rather than just combining the catalytic performance of ZnGA and Zn-Co DMC. The maximum catalytic efficiency of the composite catalyst reached 1510 gpolymer/gcatalyst when n(ZnGA)/n(DMC) = 10:1. The molecular weight of PPC product reached to 512 kg/mol, and the Td–5% climbed to 284 °C under optimum reaction conditions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/reactions6020030/s1, Table S1: WXRD results of ZnGA catalysts prepared at different temperatures.

Author Contributions

Conceptualization, S.M., W.S. and Y.F.; methodology, S.M., D.D. and W.S.; software, S.M.; validation, X.W., D.D. and C.G.; formal analysis, X.W.; investigation, S.M., Y.Y. and D.D.; resources, C.G.; data curation, S.M., X.W. and C.G.; writing—original draft preparation, S.M. and W.S.; writing—review and editing, W.S., Y.Y. and Y.F.; supervision, Y.F.; project administration, W.S. and Y.F.; funding acquisition, W.S. and Y.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by Zhejiang Zheneng Technology & Environment Group Co., Ltd. (Project No. KH-KJ-24-004).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflict of interest. Xiaojiong Wu, Delong Ding, Chunliang Ge are employees of Zhejiang Zheneng Technology & Environment Group Co., Ltd. The paper reflects the views of the scientists, and not the company.

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Figure 1. FTIR spectra of the prepared ZnGA.
Figure 1. FTIR spectra of the prepared ZnGA.
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Figure 2. XRD patterns of ZnGA prepared at different temperatures.
Figure 2. XRD patterns of ZnGA prepared at different temperatures.
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Figure 3. (a) XRD patterns for pure Zn-Co DMC, ZnGA, and ZnGA/DMC (10:1) catalysts. (b) XPS spectra of Zn-Co DMC, ZnGA, and ZnGA/DMC (10:1). (c) XPS of Zn 2P, (d) XPS of Co 2P of Zn-CoDMC and ZnGA/DMC (10:1).
Figure 3. (a) XRD patterns for pure Zn-Co DMC, ZnGA, and ZnGA/DMC (10:1) catalysts. (b) XPS spectra of Zn-Co DMC, ZnGA, and ZnGA/DMC (10:1). (c) XPS of Zn 2P, (d) XPS of Co 2P of Zn-CoDMC and ZnGA/DMC (10:1).
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Figure 4. The SEM mages of ZnGA (a,b), Zn-Co DMC (c,d), and ZnGA/DMC (10:1) (e,f). The letter “d” means the dimension.
Figure 4. The SEM mages of ZnGA (a,b), Zn-Co DMC (c,d), and ZnGA/DMC (10:1) (e,f). The letter “d” means the dimension.
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Figure 5. (a) Nitrogen absorption-deportation isotherms, (b) pore size distributions derived suing the DFT method for ZnGA, Zn-Co DMC and ZnGA/DMC (10:1).
Figure 5. (a) Nitrogen absorption-deportation isotherms, (b) pore size distributions derived suing the DFT method for ZnGA, Zn-Co DMC and ZnGA/DMC (10:1).
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Figure 6. (a) PPC made from ZnGA catalyst. (b) Zn-Co DMC catalyzed products at fly temperature; (c) ZnGA/DMC catalytic product (10:1).
Figure 6. (a) PPC made from ZnGA catalyst. (b) Zn-Co DMC catalyzed products at fly temperature; (c) ZnGA/DMC catalytic product (10:1).
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Figure 7. (a) Thermogravimetric curves, and (b) GPC curves of PPC prepared with different molars of ZnGA and DMC in the composition.
Figure 7. (a) Thermogravimetric curves, and (b) GPC curves of PPC prepared with different molars of ZnGA and DMC in the composition.
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Figure 8. (a) FTIR spectrum of PPC catalyzed by ZnGA/DMC (10:1). (b) 1H-NMR spectra of PPC. (c) DSC result of PPC catalyzed by ZnGA/DMC (10:1).
Figure 8. (a) FTIR spectrum of PPC catalyzed by ZnGA/DMC (10:1). (b) 1H-NMR spectra of PPC. (c) DSC result of PPC catalyzed by ZnGA/DMC (10:1).
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Table 1. Results on ZnGA, Zn-Co DMC, and ZnGA/DMC catalyst.
Table 1. Results on ZnGA, Zn-Co DMC, and ZnGA/DMC catalyst.
CatalystReaction a Time/hSelectity b
/%		Yield c
/(gpolymer/gcatalyst)		Mn d
/(kg/mol)		Mw d
/(kg/mol)		PDI dTd–5% e/(°C)
ZnGA f40995016.9422.901.35210
DMC f30998.4813.7916.071.16170
ZnGA/DMC (1:10) g249955.87136.12518.663.81256
ZnGA/DMC (1:1) g249916040.2365.831.63279
ZnGA/DMC (5:1) g249968056.44105.861.87281
ZnGA/DMC (10:1) g2499951210.07512.792.44284
ZnGA/DMC (15:1) g2499750139.49296.582.12280
a Reaction temperature is 70 °C, reaction pressure is 4.0 MPa, and agitation rate is 400 rpm. b Selectivity was calculated by the formulas in 1H-NMR, S = A4.9–5.1/(A4.9–5.1 + A4.85) ×100%. It means the content of polymer in the sum of polymer and propylene carbonate. c The moral ratio of polymer and byproduct calculated the yield. d Mn, Mw, and PDI were determined by GPC, which used tetrahydrofuran as the solvent and polystyrene as the internal standard. e Td–5% is detected at a temperature increase rate of 20 °C/min in the 20–600 °C range in a nitrogen atmosphere. f Catalyst addition: ZnGA: 0.15 g, DMC: 0.1 g. g all the catalysts were the ZnGA/DMC composite, and the addition was 0.05 g (ZnGA and DMC molar ratios were 1:10, 1:1, 5:1, 10:1, 15:1).
Table 2. Copolymerization of CO2 and PO over ZnGA/DMC with various m(PO)/m(catalyst).
Table 2. Copolymerization of CO2 and PO over ZnGA/DMC with various m(PO)/m(catalyst).
ZnGA/DMC
/(g)		PO
/(g)		m(PO)/m(ZnGA/DMC)Selectity b
/%		Yield c
/(gpolymer/gcatalyst)		Mn d
/(kg/mol)		Mw d
/(kg/mol)		PDI dTd–5% e
/(°C)
0.206532598398.598.46229.862.33278
0.186536199433.3200.98263.001.31281
0.156543398570121.10288.692.38280
0.106565099885.0210.07512.792.44284
0.05651300981510137.25472.973.44280
The reaction was carried out at 70 °C, pressure of 4.0 Mpa, time of 24 h, catalyst n(ZnGA)/n(DMC) = 10:1, agitation rate of 400 rpm. b Selective was calculated by the formulas in 1H-NMR, S = A4.9–5.1/(A4.9–5.1 + A4.85) × 100%. It means the content of polymer in the sum of polymer and propylene carbonate. c The moral ratio of polymer and by-product calculated the yield. d Mn, Mw, and PDI were determined by GPC, which used tetrahydrofuran as the solvent and polystyrene as the internal standard. e Td–5% is detected at a temperature increase rate of 20 °C/min in the 20–600 °C range in a nitrogen atmosphere.
Table 3. Copolymerization of CO2 with PO over ZnGA/DMC with various temperature.
Table 3. Copolymerization of CO2 with PO over ZnGA/DMC with various temperature.
Temperture/°CFCO2
/% b		Yield b
/(gpolymer/gcatalyst)		Mn c
/(Kg/mol)		Mw c
/(Kg/mol)		PDI cTd–5% d
/(°C)
6070.5545046.7293.702.01278
7072.53885210.07512.792.44284
8067.6468075.41185.552.46280
9064.4834045.93106.842.32276
Catalyst dosage of 0.05 g PO dosage of 40 mL, reaction pressure of 4.0 MPa, stirring rate of 400 rpm, and reaction time of 24 h. b The FCO2 and yield were computed by data from 1H-NMR (CDCl3-400 HZ). c Mn, Mw, and PDI were determined by GPC, which uses tetrahydrofuran as the solvent and polystyrene as the internal standard. d Td–5% is detected at a temperature increase rate of 20 °C/min in the 20–600 °C range in a nitrogen atmosphere.
Table 4. CO2 and PO copolymerization using a ZnGA/DMC composite catalyst at varying pressures.
Table 4. CO2 and PO copolymerization using a ZnGA/DMC composite catalyst at varying pressures.
Pressure
/MPa		FCO2 b
/%		Yield b
/(gpolymer/gcat)		Mn c
/(kg/mol)		Mw c
/(Kg/mol)		PDI cTd–5% d
/(°C)
4.566.81885118.75243.352.05279
4.072.53951210.07512.792.44284
3.565.8873070.60218.023.08278
3.063.6348055.05133.582.42278
2.560.4643455.06137.872.50279
2.061.9434046.70101.832.18278
Catalyst dosage of 0.05 g, PO dosage of 40 mL, reaction temperature of 70 °C, stirring rate of 400 rpm, and reaction time of 24 h. b The FCO2 and yield were calculated by data from 1H-NMR (CDCl3-400 HZ). c Mn, Mw, and PDI were determined by GPC, which uses tetrahydrofuran as the solvent and polystyrene as the internal standard. d Td–5% is detected at a temperature increase rate of 20 °C/min in the 20–600 °C range in a nitrogen atmosphere.
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Miao, S.; Wu, X.; Ding, D.; Ge, C.; Shen, W.; Yang, Y.; Fang, Y. Efficient Copolymerization of CO2 and Propylene Oxide via ZnGA/Zn-Co DMC Composite Catalysts: Synergistic Catalysis for High-Performance Polypropylene Carbonate. Reactions 2025, 6, 30. https://doi.org/10.3390/reactions6020030

AMA Style

Miao S, Wu X, Ding D, Ge C, Shen W, Yang Y, Fang Y. Efficient Copolymerization of CO2 and Propylene Oxide via ZnGA/Zn-Co DMC Composite Catalysts: Synergistic Catalysis for High-Performance Polypropylene Carbonate. Reactions. 2025; 6(2):30. https://doi.org/10.3390/reactions6020030

Chicago/Turabian Style

Miao, Shuqin, Xiaojiong Wu, Delong Ding, Chunliang Ge, Weihua Shen, Yi Yang, and Yunjin Fang. 2025. "Efficient Copolymerization of CO2 and Propylene Oxide via ZnGA/Zn-Co DMC Composite Catalysts: Synergistic Catalysis for High-Performance Polypropylene Carbonate" Reactions 6, no. 2: 30. https://doi.org/10.3390/reactions6020030

APA Style

Miao, S., Wu, X., Ding, D., Ge, C., Shen, W., Yang, Y., & Fang, Y. (2025). Efficient Copolymerization of CO2 and Propylene Oxide via ZnGA/Zn-Co DMC Composite Catalysts: Synergistic Catalysis for High-Performance Polypropylene Carbonate. Reactions, 6(2), 30. https://doi.org/10.3390/reactions6020030

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