J Polym Res (2015) 22: 220 DOI 10.1007/s10965-015-0849-5

ORIGINAL PAPER

Silica-supported zinc glutarate catalyst synthesized by rheological phase reaction used in the copolymerization of carbon dioxide and propylene oxide Lijun Gao 1 & Yucheng Luo 1 & Yongjia Lin 1 & Ting Su 1 & Riping Su 1 & Jiuying Feng 1

Received: 1 June 2015 / Accepted: 23 September 2015 / Published online: 26 October 2015 # Springer Science+Business Media Dordrecht 2015

Abstract In order to improve the catalytic efficiency of zinc glutarate (ZnGA) used in the CO2/propylene oxide (PO) copolymerization, silica-supported zinc glutarate catalysts (ZnGA/SiO2) are prepared via a rheological phase reaction with a little toluene as a reaction medium. Wide-angle X-ray diffraction (WAXD) tests of the catalysts illustrate that the crystal quality increases and the overall crystallinity declines after supporting. The scanning electron microscope (SEM) images show that the proportion of supported ZnGA increases with increasing the SiO2 proportion. Accordingly, the polymer yields increase from 12.7 Kg polymer/mol Zn of ZnGA to 25.6 of ZnGA/SiO2. And the almost alternating poly(propylene carbonasstes) (PPCs) with the carbonate content of above 97 % and the Mns of above 10, 000 g/mol are obtained. This work illustrates that the catalytic efficiency does not increase sharply like the supported catalysts used in olefin polymerization because of the decline in overall crystallinity of ZnGA. The catalytic efficiency is expected to be enhanced significantly if it can be improved for ZnGA/SiO2.

Keywords Zinc glutarate . Supported catalysts . Rheological phase reaction . Poly(propylene carbonate)

* Lijun Gao [email protected] * Jiuying Feng [email protected] 1

School of Chemistry and Chemical Engineering, Development Center for New Materials Engineering & Technology in Universities of Guangdong, Lingnan Normal University, Zhanjiang, People’s Republic of China 524048

Introduction PPC has a potentially wide range of applications due to its excellent biodegradability, adhesion, transparency, low permeability for oxygen and water and burning property: burning gently in air only producing CO2 and water without producing an ash residue [1]. So the synthesis of PPC from CO2 and PO has been a hot issue in view of economic and environmental benefits [2]. The catalyst is key technique in CO2/PO copolymerization. Over the past several decades, various catalysts such as dicarboxylate zinc, β-diiminate zinc, rare earth complexes, double metal cyanide complex and SalenCo(III) complexes have been developed [3–8]. In particular ZnGA is considered to be one of the commercial suitable catalyst benefiting from its simple preparing process, relatively low cost and the high molecular weight of PPC obtained with it [9, 10]. Thereafter, it was widely studied by several research groups [11–15]. However, the relatively low catalytic efficiency of ZnGA with 10 Kg polymer/mol Zn prevent it being applied extensively; for comparison, the catalytic efficiency can be well over 1000 Kg polymer per mol of metal for polyolefins [16]. So many efforts have been devoted to improve the catalytic efficiency of ZnGA. It was early found that the efficiency is strongly influenced by the synthetic approach. Kobayashi et al. first reported the ZnGA catalyst in 1973, which was prepared using diethylzinc and glutaric acid, and the efficiency was only 16.5 g polymer/mol Zn [17]. Ree et al. explored the influence of different zinc sources including zinc oxide, zinc hydroxide, zinc nitrate and diethylzinc used in preparing ZnGA on the catalytic efficiency [13]. They found that ZnGA from zinc oxide exhibited the highest yield with 70 g polymer/g ZnGA. Later investigations for single crystal X-ray structure of ZnGA showed that overall efficiency is restricted to the outer surface of the ZnGA-particles [15, 18, 19], which is actually an inherent limitation for a

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heterogeneous catalyst. The strategies to increase the catalytic efficiency of ZnGA have been focused on increasing the surface area using ultrasonic technique [14], amphiphilic template [10] and supporters [20–22]. They are effective in improving the catalytic efficiency. For instance, the silica-supported ZnGA catalyst was patented with an activity of 358.8 g polymer/g Zn, which was prepared through grinding ZnGA and silicon dioxide together in a planetary ball grinder under vacuum [21]. The MCM-41 supported ZnGA also has a yield of 89.5 g polymer/g ZnGA in producing PPC [22]. Nevertheless, the morphology of the supported ZnGA is still unknown, which has important influence on the catalytic efficiency. The rheological phase reaction method is often used for preparing inorganic salts for electrode materials [23, 24] and metal-organic complexes [25, 26]. Zinc glutarate is actually a kind of metal-organic complex based on its single crystalline [15, 19]. Here we report a facile and effective synthesis method for the ZnGA/SiO2 catalyst by a rheological phase reaction. Thus prepared ZnGA/SiO2 exhibits an enhanced catalytic efficiency as well as a high molecular weight and carbonate content of PPC obtained with it. The catalysts were characterized by IR, wide-angle X-ray diffraction (WAXD) and scanning electron microscope (SEM). The relationship between the morphology of ZnGA and the catalytic efficiency is mainly investigated.

Experimental Materials Tetraethyl orthosilicate (98 %), ammonium hydroxide solution (25 %), glutaric acid (GA, 99 %), ethanol absolute (99.7 %), toluene (99.9 %) and dichloromethane (99.5) are analytical grade and were used without further purification. Zinc oxide (99.99 %) is nanosize of 30 ± 10 nm. PO (99.5 %) was pre-treated by KOH and refluxed over CaH2 for 24 h and then distilled under N2 and stored over 0.4 nm molecular sieves. CO2 gas is 99.99 % pure. SiO2 was prepared by the hydrolysis and following condensation of tetraethyl orthosilicate (TEOS) in the water/ethanol mixed solvent catalyzed by ammonium hydroxide. The feed mole ratio of TEOS, water, ammonium hydroxide and ethanol is 1:6.2:1.12:30.6. The above reagents were all purchased from Shanghai Aladdin Reagent Co., Ltd. except for CO2 which was from Zhanjiang Oxygen Factory.

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and dried at 80 °C under vacuum. The catalyst yield is calculated in according to the unreacted GA which was removed by acetone. The unsupported ZnGA (as ZnGA) was prepared using the typical method [13] with GA and zinc oxide as raw materials. Typical polymerization A 100 mL autoclave reactor charged by the catalyst ZnGA (0.1 g) was purged with N2 and evacuated alternatively for three times, followed by charging 30 mL PO and CO2. The polymerization was performed at 60 °C and 50 bar of CO2 pressures under stirring for 36 h. The reactor was cooled and the pressure was released. The polymer was dissolved in chloroform and filtered through Celite under vacuum. The obtained transparent polymer solution was added slowly to ethanol precipitating the product, which was washed by ethanol several times to remove byproduct, propylene carbonate and dried at 80 °C under vacuum to a constant weight [27]. Measurements The catalysts were characterized by an attenuated total reflectance (ATR) infrared spectrophotometry using Nicolet 6700 Fourier transform infrared spectrometer. The crystallization and phase identification of catalysts were carried out by a wide-angle X-ray diffraction (WAXD, Rigaku D-MAX 2200 VPC) using Cu Kα (λ=0.15406 nm) radiation operating at 30 kVand 30 mA. The morphology of catalysts were observed using a Philips XL-30 scanning electron microscope. The average molecular weights of polymers were determined by gel permeation chromatography (GPC) in tetrahydrofuran using a Waters1515 apparatus with polystyrene standards. The 1H NMR were recorded on a Bruker AV 400 apparatus.

Catalysts synthesis ZnGA/SiO2 were prepared by stirring the mixture of SiO2, zinc oxide and GA to a rheological phase with a little toluene as dispersion medium followed being heated at 70 °C in a hydrothermal reactor. The feed mole ratio of GA to zinc oxide is 0.98. The obtained white powder was washed with acetone

Fig. 1 The IR spectra of the samples

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shown in Fig. 1, the characteristic peaks are listed as follow: 1584.69 and 1535.06 cm − 1 (ν a s (COO − )), 1403.78 cm −1 (ν s (COO − )), 1445.98 cm −1 (δ(CH 2 )), 1089.58 cm−1 (νas(SiOSi)) and 955.72 cm−1 (δ(SiOH)). The νas(COO−) at 1584.69 cm−1 and at 1535.06 cm−1 is due to a syn-anti and syn-syn bridging bidentate bonding mode, respectively [18]. They are two coordination modes of the carboxylates with zinc metal ion. The syn-syn mode is relatively higher in population than the syn-anti mode based on their absorption band intensity. A presence of glutaric acid could hinder the catalytic efficiency of ZnGA. For the catalysts synthesized, the characteristic peak of νas(C=O) at 1697 cm−1 from glutaric acid was not detected. It was reported that the level of crystal quality (namely, size and perfectness of crystal) and crystallinity of ZnGA affects the catalytic efficiency in the CO2/PO copolymerization [13, 28]. The crystal morphology is related to the coherence length (LC) and the ratio of the intensity to the full width at half-maximum (Intensity/FWHM) of X-ray absorption peaks. WAXD patterns of the catalysts are shown in Fig. 2. The data are listed in Table 1. All the catalysts exhibit the characteristic peaks at around 12.7, 22.6, 23.0, 26.8, 37.5, 38.8 and 39.8°, denoted by (a), (b), (c), (d), (e), (f) and (g),

Fig. 2 The WAXD diffraction patterns of the samples

Results and discussion Catalysts characterization The IR (ATR) measurements of the supported ZnGA with the weight ratio of SiO2/ZnGA of 3 were carried out. As Table 1

The X-ray diffraction results of the catalysts

Peaks paramerers

Peak a 2θ (O) d-spacing (Å) Intensity/FWHM LC (Å) Peak e 2θ (O) d-spacing (Å) Intensity/FWHM LC (Å) Peak f 2θ (O) d-spacing (Å) Intensity/FWHM LC (Å) Peak g 2θ (O) d-spacing (Å) Intensity/FWHM LC (Å) Average value of LC (Å) a

The unsupported

The weight ratio of ZnGA to SiO2a

ZnGA

1:1

1:2

1:3

1:4

12.74 6.94 12,332

12.72 6.95 8158

12.50 7.08 2260

12.66 6.99 2453

12.56 7.04 930

219

276

215

253

265

37.56 2.39 1702 179

37.52 2.40 1010 205

37.28 2.41 411 194

37.24 2.40 259 171

37.36 2.40 147 218

38.86 2.32 1434 257

38.82 2.32 877 355

38.60 2.33 369 355

38.83 2.32 364 418

38.60 2.33 306 589

39.84 2.26 1422 347 251

39.78 2.26 1031 290 282

39.62 2.27 544 537 326

39.72 2.27 328 460 326

39.76 2.27 225 559 408

The weight of ZnGA is the theoretical yield based on the feed weight of GA

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The synthesis of ZnGA/SiO2 by rheological phase reaction and the results in the CO2/PO copolymerization Time/h

Catalyst yield /%b

1

The weight ratio of ZnGA to SiO2a 1:0

6

2

1:1

3

1:2

4 5 6

Entry

a

99.8

Polymer yield /Kg mol−1Zn 12.7

The carbonate percentage/mol %c 97.7

Number average molecular weight×10−3/g mol−1 11.5

Polydispersity index

6

98.5

16.0

97.5

11.4

2.5

6

98.4

17.8

97.7

10.6

2.7

1:3 1:3

6 9

98.8 99.4

23.5 24.0

97.5 97.7

12.7 13.1

3.4 3.1

1:4

6

98.5

25.6

98.0

14.5

3.3

3.1

The weight of ZnGA is the theoretical yield based on the feed weight of GA

b

The Catalyst yield is calculated based on the unreacted GA which is removed by acetone

c

Determined by 1 H NMR spectroscopy

Fig. 3 The SEM mages of a ZnGA (5000 times magnification) and ZnGA/SiO2 (10,000 times magnification) with the weight ratio of b 1:1, c 1:2, d 1:3 (Table 2 entry 4) and e 1:4

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which are in agreement with those reported by other researchers [15, 18]. This demonstrates that ZnGA from ZnGA/SiO2 have the same crystal lattice structure of the unsupported ZnGA. The peaks (a), (e), (f) and (g) are selected to calculate LC from FWHM according to the Scherrer equation. The strong diffraction peaks (b), (c) and (d) are not selected because these peaks overlap with that of the supporter, SiO2. The average Lc of each peak increases, however, the Intensity/FWHM of the peaks all decrease with increasing the content of SiO2. These suggest that the catalysts with relatively larger size and lower overall crystallinity can be obtained after supporting. Catalysts and polymers properties The synthesis of ZnGA/SiO2 by rheological phase reaction and the results in the CO2/PO copolymerization are listed in Table 2. It is observed that the SiO2 feed proportion has little influence on the catalyst yields that are above 98 % when the reaction time exceeds 6 h, but significantly affects the efficiency of catalysts in the CO2/PO copolymerization. It increases 100 % from 12.7 to 25.6 Kg polymer per mol of Zn with increasing the SiO2 proportion. When the reaction time in preparing catalyst is extended from 6 h to 9 h, the catalyst Fig. 4 The 1H NMR and 13C NMR spectra of ethanol soluble polymer with the closeup view of the 13C NMR spectrum of the carbonate carbon in the lower-left position

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yield and the polymer yield only increase 0.6 and 2.1 %, respectively (Table 2 entry 4 and 5). Figure 3 shows the scanning electron microscope photographs of the catalysts. the unsupported ZnGA displays flake particles with above 2 um of size. The ZnGA/SiO2 exhibit small spherical particles with size of 0.5~1 um. The core of these particles is SiO2, and ZnGA is supported on SiO2 and displays a rough surface. So the size of ZnGA is actually smaller than the above value. After supporting on SiO2, the outer surface of the ZnGA-particles obviously increases, which results in the high catalytic efficiency. When the weight ratio of SiO2 to ZnGA is less than two, the flake particles of ZnGA that are not supported on SiO2 still generate, which becomes less with increasing the SiO2 proportion. Accordingly, the catalytic efficiency increases slowly (Table 2 entry 2 and 3). When the weight ratio of SiO2 to ZnGA is more than three, ZnGA are almost completely supported on SiO2 (Fig. 3c and d) and the polymer yield accordingly increases rapidly (Table 2 entry 4). After this point, a slow rise tendency in catalytic efficiency is observed (Table 2 entry 6). Figure 4 shows the 1H NMR and 13C NMR spectra of the ethanol insoluble polymer. 1H NMR (σ, CDCl3), 5.0 (1H, CH2 CH(CH3)OCO), 4.2 (2H, CH2CH(CH3) OCO), 3.5~ 3.8 (2H, CH2CH(CH3)O), 1.3 (3H, CH2 CH(CH3)OCO) and 1.2 (3H, CH2CH(CH3)O). 13C NMR (σ, CDCl3), 154.3

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(CH 2 CH(CH 3 )OCO), 72.5 (CH 2 CH(CH 3 )OCO), 69.2 (CH2CH (CH3)OCO) and 16.4 (CH2CH(CH3)OCO). The 1 H NMR spectrum shows that the polymer chain consists mainly of carbonate linkages and contains a small fraction of ether units. The carbonate content (fC) is determined by the integration (I) of the correlated proton signals in 1H NMR according to the Eq. 1. .  f C ðmol%Þ ¼ ðI 5:0 þ I 4:2 Þ I 5:0 þ I 4:2 þ I 3:5e3:8  100%

ð1Þ

They all exceed 97 % (Table 2), indicating that the PPCs obtained are almost alternating copolymers from CO2 and PO. The 13C NMR spectrum of the carbonate carbon includes three regions at 154.7, 154.2 and 153.7 ppm. They were assigned as TT, HT and HH junctions between the carbon dioxide and propylene oxide monomers, respectively [29]. Based on their intensity, the ratio of TT, HT and HH of the PPCs is estimated to be about 1:3:1. And also, the number average molecular weights are all above 10, 0000 g/mol with the molecular weight distributions (PDI) of around 3, which are poorly affected by the SiO2 proportion.

Conclusions For ZnGA, as a heterogeneous catalyst in the CO2/PO copolymerization, preparing nanoparticles can improve the catalytic efficiency, as one expects, even though the increase is strongly limited due to limitations in further downsizing the particles [15]. And what’s more, a new problem of particle agglomeration will arise and has to be tackled. While it is an appropriate way in which ZnGA is supported on other cheap supporters like silica. The main advantage is a rise in the surface per weight of ZnGA as well as a good crystal quality. However the overall crystallinity declines after supporting. The improvement in crystallinity need to be continued explored.

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16. Acknowledgments We are grateful for financial support from the National Nature Science Foundation of China (51403183), Nature Science Foundation of Guangdong Province (S2013010012917 and 2015A030313778), Scientific and Technological Innovation Project Foundation in Higher Education of Guangdong (2013KJCX0122), China Spark Program (2014GA780060) and the Key Programs of Lingnan Normal University (LZL1401).

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