Research Article | Open Access
Polyvinylpolypyrrolidone Supported Brønsted Acidic Catalyst for Esterification
A polyvinylpolypyrrolidone (PVPP) supported Brønsted acidic catalyst (PVPP-BSHSO4) was prepared by coupling SO3H-functionalized polyvinylpolypyrrolidone with H2SO4 in this work. After the characterization through FT-IR, FESEM, TG, BET, and elemental analysis, it was found that 1,4-butane sultone (BS) and sulfuric acid reacted with PVPP and were immobilized on PVPP surface. The prepared PVPP-BSHSO4 catalyst shows high catalytic activity for a series of esterification reactions and could be separated from the reacted mixture easily. Moreover, this catalyst could be recycled and reused for six times without significant loss of catalytic performance.
Esters have been widely used as lubricating oils, perfumes, plasticizers, biodegradable materials, and so forth [1, 2]. Conventionally, esterification could be catalyzed by mineral acids such as H2SO4 and H3PO4. However, these acids were reported to exhibit several drawbacks including severe corrosion to equipment, environmental hazards, and difficulty in the aspect of catalyst reusability . To overcome these problems, heterogeneous solid acid catalysts such as supported mineral acids, acidic resins, and zeolites were developed . Nevertheless, these heterogeneous catalysts were illustrated to exhibit some disadvantages related to product selectivity, catalyst recyclability, and environmental safety . As such, developing high catalytic activity, recyclable, environmental-friendly catalysts for esterification is still an attractive topic in catalyst field .
Owing to the negligible selective dissolvability and tunable acidity, ILs (ionic liquids) were utilized as environmental-friendly solvents and catalysts for chemical reactions [7–10]. Many organic reactions were catalyzed by ILs which were revealed to be promising catalysts for esterification and transesterification. Gui et al.  employed three halogen-free Brønsted acidic ionic liquids as catalysts for esterification of ethanol with acetic acid. They illustrated that these ILs were effective catalysts for esterification with the conversion rate of ethanol higher than 92%. Nevertheless, ILs were demonstrated to have some disadvantages such as high viscosity, inconvenient reusability, and large amount needed in reaction [12, 13].
In recent years, polymer supported catalysts were developed, which had advantages in product purification, reducing environmental damage, and easy separation from product. These exhibited characteristics made polymer supported catalysts promising for both academic and industrial applications [14–17]. Leng et al.  prepared a polymer supported acidic catalyst by coupling a polymeric IL with heteropolyanions, and their findings showed that when this catalyst was used in the esterification of acetic acid with n-butanol, a conversion rate of n-butanol higher than 97% was obtained. However, due to the complex preparation and polymerization of the monomer, the whole preparation process of polymer supported acidic catalysts is always complicate and less satisfactory.
Polyvinylpolypyrrolidone PVPP is a commercial product usually used as adsorbent [19, 20]. Recently, its application in catalyst field has drawn much attention because it has many advantages such as being insoluble in all kinds of solvents and nontoxic. Mokhtary and Najafizadeh  used PVPP supported boron trifluoride as the catalyst to prepare N-tert-butyl amides by reaction of nitriles with tert-butyl acetate. They found that PVPP supported boron trifluoride was a high efficient Lewis acid. In this study, a commercial PVPP was applied to synthesize a Brønsted acidic catalyst. The catalytic activity and reusability performance of this catalyst used for esterification reactions were also investigated.
2. Experimental Part
PVPP was provided by Gobekie New Materials Science & Technology Co., Ltd, Shanghai, China. 1,4-Butane sultone (BS) (99%), sulfuric acid (98%), and other chemicals (AR grade) were purchased from Sinopharm Chemical Reagent Co. and used as received.
2.2. Preparation of [PVPP-BS]HSO4, PVPP-H2SO4, and 1-(4-Sulfonic acid) Butylpyridinium Hydrogen Sulfate ([BSPy]HSO4)
All experimental steps were carried out under nitrogen atmosphere. First, PVPP-BSHSO4 was prepared in the following steps. Under vigorous stirring, PVPP powder (10 g, monomer molar quantity was about 0.09 mol) was dispersed in toluene and then an appropriate amount of BS (ranging from 0 to 0.09 mol) was added to the mixture. The mixture was slowly heated to 80°C and continuously stirred in a 250 mL round-bottomed flask for 24 h. After filtration, the zwitterions powder (PVPP-BS) was washed three times with ethyl acetate and then dried in vacuum at 60°C to obtain PVPP-BS. Then, sulfuric acid (ranging from 0 to 0.09 mol) in equimolar amount to BS was added to the mixture of PVPP-BS dispersed in methanol at 0°C and stirred for 24 h. On completion, the mixture was filtered and washed three times with methanol, and then water and methanol were removed in vacuum at 60°C to afford the final product PVPP-BSHSO4 as a solid. The preparation process of PVPP-BSHSO4 is shown in Scheme 1.
For comparison, two other catalysts PVPP-H2SO4 and 1-(4-sulfonic acid) butylpyridinium hydrogen sulfate (BSPyHSO4) were also prepared. The preparation procedure of PVPP-H2SO4 was as follows. PVPP powder (10 g) was dispersed in ethyl acetate and then sulfuric acid (0.09 mol) was added to the mixture at 0°C and stirred for 24 h. On completion, the mixture was filtered and washed three times with methanol. Then, water and methanol were removed in vacuum at 60°C to afford the final product of PVPP-H2SO4 as a solid.
BSPyHSO4 was synthesized as reported in the previous literature with a slight modification . Pyridine (0.11 mol) and 1,4-butane sultone (0.1 mol) were introduced into a 100 mL capacity round-bottom flask reactor equipped with a reflux condenser, a magnetic stirrer, and a thermometer. This reaction system was stirred magnetically under reflux at 40°C for 24 h until a white solid zwitterion (BSPy) formed, which was filtered, washed with ethyl acetate three times to remove nonionic residues, and dried in vacuum. Sulfuric acid (0.1 mol) was added dropwise and the mixture was stirred at 80°C for 8 h. The obtained viscous liquid was washed with ether for three times and dried in vacuum to form ionic liquid BSPyHSO4. The preparation process of BSPyHSO4 is shown in Scheme 2.
2.3. Catalyst Characterization
FT-IR spectra were measured on a PRESTIGE-21 FT-IR instrument (KBr discs) in the 2200–400 region. Thermal analysis was performed with a TA Q50 thermogravimetric analyzer in a nitrogen atmosphere with a heating rate of 10°C/min. The surface morphologies of PVPP, PVPP-BS, and PVPP-BSHSO4 were investigated by using a Hitachi SU8000 field emission scanning electron microscopy (FESEM). Elemental analysis was performed on an Elementar Vario EL Cube elemental analyzer. The BET surface area was measured at the liquid-nitrogen temperature using a Micromeritics SSA4200 analyzer. Before the BET surface area analysis, the samples were degassed at 100°C to the vacuum of 0.13 Pa. The multipoint Brunauer-Emmett-Teller (BET) method was used to measure total surface area. The acid value of PVPP-BSHSO4 was determined by acid-base titration [23, 24]. One-half gram of the catalyst powder was dispersed in 50 mL of 0.1 M KCl. The dispersion was stirred for 20 min and titrated with 0.1 M KOH in the presence of phenolphthalein.
2.4. Esterification and Analysis
Taking the esterification of acetic acid with n-butanol as an example, the typical reaction was carried out as follows: acetic acid (0.08 mol), n-butanol (0.096 mol), cyclohexane (8 mL, as a water-carrying agent), and PVPP-BSHSO4 (1.19 g, 8% of the total mass of n-butanol and acetic acid) were introduced into a 100 mL capacity round-bottom flask reactor equipped with a water segregator, thermometer, reflux condenser, and magnetic stirrer. The mixture was heated under reflux at 90°C with vigorous stirring for 3 h. After the reaction completed, the product was analyzed by a gas chromatograph (GC) instrument with FID detector (Agilent 7890 A, HP 5 capillary column ). Other esterification reactions were carried out in the same procedure. The conversion of the carboxylic acid and the selectivity of esters were determined as described in previous literatures [18, 25].
2.5. Recycling of Catalysts
The catalyst PVPP-BSHSO4 was recovered by vacuum filtration and dried in vacuum at 60°C for 2 h. This recovered catalyst was directly used for the next run.
3. Results and Discussion
3.1. Effect of the Ratio of BS to the Pyrrolidone Repeat Unit of PVPP on the Catalytic Activity
BS can only react with the pyrrolidone repeat unit exposed on the surface of PVPP because PVPP is a cross-linking insoluble material. However, due to the lack of reliable method to measure the amount of the pyrrolidone repeat unit exposed on the surface of PVPP, the ratio of BS to the pyrrolidone repeat unit of PVPP, which can greatly affect the catalytic performance of the catalyst, cannot be calculated directly. Therefore, to examine the effect of the ratio of BS to the pyrrolidone repeat unit of PVPP on the catalytic activity, several catalysts synthesized with different mole ratio of BS to the pyrrolidone repeat unit of PVPP were utilized in the esterification of n-butanol with acetic acid. As shown in Figure 1, with the increase of mole ratio of BS to the pyrrolidone repeat unit of PVPP, a higher conversion rate of acetic acid was obtained. When the mole ratio of BS to the pyrrolidone repeat unit of PVPP was increased up to 0.5, the achieved conversion of acetic acid was about 99.6%. With the mole ratio of BS to the pyrrolidone repeat unit of PVPP increased continually, the conversion did not change significantly. This result indicates that the satisfied ratio of BS to the pyrrolidone repeat unit of PVPP is 0.5.
3.2. Catalyst Characterization
3.2.1. FT-IR Spectroscopy
The FT-IR spectra results of PVPP, PVPP-BS, PVPP-BSHSO4, and H2SO4 are presented in Figure 2. The characteristic peaks around 1290 , 1438 , and 1654 which could be clearly observed in Figure 2 were assigned to C-N, C-C, and C=O stretching vibrations of PVPP, respectively . The band in Figure 2(b) at 1035 , which did not appear in Figure 2(a), was attributed to the S=O symmetric stretching vibrations as reported in a literature . This result indicates that BS has been connected to the surface of PVPP. Moreover, comparing H2SO4 with PVPP-BSHSO4 in Figures 2(d) and 2(c), it can be clearly observed that after PVPP-BS reacted with H2SO4, the symmetric stretching vibration of S-O band had shifted from 610 to 617 . This is a convincing evidence of the existence of the strong ionic interaction between the polymeric cations and . It thus suggests that the reaction of SO3H-functionalized PVPP with H2SO4 produced a PVPP-based polymeric hybrid via ionic linkages.
3.2.2. Thermogravimetric Analysis
Thermal stabilities of PVPP, PVPP-BS, and PVPP-BSHSO4 were determined by TG analysis. In Figures 3(b) and 3(c), besides the drastic weight loss at temperature ranging from 350°C to 500°C corresponding to the thermal decomposition of PVPP backbone in the catalyst, there was another weight loss around 200°C. The TG curve of PVPP-BS, curve (b) in Figure 3, shows that there was an approximately 4.5% weight loss of the sample between 150°C and 190°C, while that of PVPP-BSHSO4, curve (c) in Figure 3, exhibits that there was an approximately 3% weight loss of the sample between 200°C and 240°C. The finding that PVPP-BSHSO4 had much higher decomposition temperature and less weight loss than PVPP-BS strongly indicates that PVPP-BSHSO4 is not a physical mixture of PVPP-BS with H2SO4 but an ionic hybrid.
3.2.3. Morphological Surface Analysis
Figure 4 shows the diameters and surface morphologies of (a) PVPP, (b) PVPP-BS, and (c) PVPP-BSHSO4. It can be clearly seen that the amorphous PVPP particles before reaction were aggregated together with size of 3–5 um (Figure 4(a)). However, after reacting with BS or H2SO4, the surface of PVPP-BS and PVPP-BSHSO4 was not as smooth as that of PVPP, and small particles could be seen on the surface (Figures 4(b) and 4(c)). These small particles made the BET surface area of PVPP-BS (6.28 m2/g) and PVPP-BSHSO4 (6.56 m2/g) much higher than that of PVPP (3.84 m2/g) (Table 1). The elemental analysis results in Table 1 demonstrate the difference in S element content in PVPP, PVPP-BS, and PVPP-BSHSO4. The sulfur content in PVPP-BS]HSO4 (8.789%) was higher than that in PVPP-BS (4.121%). In contrast, no sulfur content was found in PVPP. These findings further demonstrate that BS and H2SO4 have already reacted with PVPP.
|Weight percentage of S element determined by an elemental analyzer|
Determined as KOH mass consumed for neutralization.
3.3. Catalytic Activities of Different Catalysts
The catalytic performance of different catalysts for the esterification of acetic acid with n-butanol is presented in Table 2. It can be observed that the conversion of acetic acid only reached 20.3% (entry 1) when catalysts were not used, which means that the esterification was difficult to occur with the absence of catalysts. When H2SO4 was used as the catalyst in the esterification, a homogeneous reaction system was formed and H2SO4 exhibited a low conversion of 85.1% and selectivity of 87.1% (entry 2). This homogeneous reaction system made it difficult to separate H2SO4 from the product. Although PVPP-H2SO4 is a heterogeneous catalyst which is easy to prepare, when used as the catalyst in esterification, it exhibited the similar catalytic ability as H2SO4, and the low conversion of 89.3% and selectivity of 88.5% were obtained (entry 3). Using H3PW12O40 as the catalyst in the esterification, it presented a relative high conversion of 96.9%. Nevertheless, due to its well-known strong acidity caused by the three protons in the cation position, H3PW12O40 had a lower selectivity of reaction (95.8%) than that of BSPyHSO4 and PVPP-BSHSO4 (entry 4). When BSPyHSO4 was used as the catalyst, a liquid-liquid reaction system was formed. The conversion of acetic acid was 96.1% which was much higher than that of H2SO4 and PVPP-H2SO4; and the selectivity of the reaction was 100%, which was higher than that of H3PW12O40 (entry 5). However, BSPyHSO4 was easily attached onto the bottom surface of the flask reactor like a gelatinous liquid, which greatly lowered the mass transfer. When PVPP-BSHSO4 was used, a liquid-solid reaction system formed and a high conversion (99.6%) and good selectivity were obtained (entry 6). Moreover, PVPP-BSHSO4 could be easily separated by vacuum filtration and reused.
|n-Butanol (7.12 g); acetic acid (4.80 g); and [PVPP-BS]HSO4 (8 wt%), 90°C for 3 h. H2SO4, H3PW12O40, PVPP-H2SO4, and [BSPy]HSO4 were used in a similar proton content as [PVPP-BS]HSO4.|
Conversion of acetic acid.
Selectivity for ester.
Table 3 presents the comparison of the catalytic performances of PVPP-BSHSO4 with two reported polymer supported catalysts for the esterification of acetic acid with n-butanol. It can be seen that PVPP-BSHSO4 exhibited similar catalytic performance as the two reported polymer supported catalysts. The conversion of acetic acid exceeded 90%, when these polymer supported catalysts were applied in esterification of n-butanol and acetic acid. However, the difference is that the polymer supports of the reported catalysts are in the stage of laboratory while the PVPP is a commercial product. Using PVPP as catalyst support can greatly simplify the preparation process and lower production costs of the catalyst in contrast to the other two catalysts reported by Leng et al.  and Li et al. . Therefore, PVPP-BSHSO4 has more potential in industrial application.
|Conversion of acetic acid.|
3.4. Catalytic Activity of [PVPP-BS]HSO4 for Esterification of Other Alcohols and Carboxylic Acids
In order to clarify the scope and limitations of PVPP-BSHSO4 catalyst for esterification, different alcohols and carboxylic acids as the reactants for esterification reactions were tested and the results are summarized in Table 4. It can be observed that PVPP-BSHSO4 possessed very high catalytic activity for esterification. Excellent conversions above 90% with perfect selectivity (100%) for corresponding esters were obtained in all investigated reactions. Moreover, the results in Table 4 demonstrate that the length of carbon chains merely affected the conversion and the selectivity (Table 4, entries 1–6). In addition, satisfactory conversions above 90% (Table 4, entries 7, 8) of esterification of aromatic alcohol and carboxylic acids were also obtained. Thus, all these results indicate that PVPP-BSHSO4 catalyst can be applied to various esterification reactions with different substrates.
|Alcohol (96 mmol); carboxylic acid (80 mmol); and PVPP-BSHSO4 (8 wt%), for 3 h.|
Conversion of acetic acid.
Selectivity for ester.
3.5. Reusability of [PVPP-BS]HSO4 for Esterification
Reusability of the catalyst is one of the essential aspects for practical applications. After the reaction of each run, the solid catalyst could be recovered by filtration, dried in vacuum at 60°C for 2 h. The recycling performance of PVPP-BSHSO4 in the esterification of acetic acid with n-butanol is illustrated in Figure 5. The conversion of the acid changed from 99.6% to 96.5% after 5 times of recycling, which indicates the high catalytic performance of PVPP-BSHSO4.
In conclusion, a Brønsted acidic catalyst PVPP-BSHSO4 was synthesized by coupling SO3H-functionalized polyvinylpolypyrrolidone with H2SO4. This catalyst was revealed to be an efficient catalyst for various esterification reactions, presenting the advantages of practical convenience in product preparation, separation, and recovery. Moreover, the conversions for all the investigated reactions were above 90%. Conversions for the reactions could still be satisfactorily maintained above 90% after the catalyst was recycled for 5 times for the synthesis of n-butyl acetate. Thus, the PVPP-BSHSO4 polymer supported catalyst, which was prepared in this study, was proved to be an efficient, reusable, and potential heterogeneous catalyst for the synthesis of carboxylic esters.
The authors declare that they have no competing interests.
The authors acknowledge the financial support of the Shenyang Science and Technology Plan Projects (no. F15-199-1-12 and no. F14-231-1-28) and the Key Laboratory for Catalyst Synthesis of Polymer of the Liaoning Province (no. 2010-36), China.
- Y. Z. Zhao, J. X. Long, F. G. Deng et al., “Catalytic amounts of Brønsted acidic ionic liquids promoted esterification: study of acidity–activity relationship,” Catalysis Communications, vol. 10, no. 5, pp. 732–736, 2009.
- L. E. Hamdaoui, M. E. Moussaouiti, and S. Gmouh, “Homogeneous esterification of cellulose in the mixture N-butylpyridinium chloride/dimethylsulfoxide,” International Journal of Polymer Science, vol. 2016, Article ID 1756971, 7 pages, 2016.
- Y. Yue, H. Liu, Y. Zhou, Z. Bai, and X. Bao, “Pure-phase zeolite beta synthesized from natural aluminosilicate minerals and its catalytic application for esterification,” Applied Clay Science, vol. 126, no. 6, pp. 1–6, 2016.
- J. P. Pariente, I. Díaz, F. Mohino, and E. Sastre, “Selective synthesis of fatty monoglycerides by using functionalised mesoporous catalysts,” Applied Catalysis A: General, vol. 254, no. 2, pp. 173–188, 2003.
- M. Kuzminska, R. Backov, and E. M. Gaigneaux, “Selective synthesis of fatty monoglycerides by using functionalised mesoporous catalysts,” Catalysis Communications, vol. 59, no. 5, pp. 222–225, 2015.
- F. Shi, Q. Zhang, D. Li, and Y. Deng, “Silica-gel-confined ionic liquids: a new attempt for the development of supported nanoliquid catalysis,” Chemistry—A European Journal, vol. 11, no. 18, pp. 5279–5288, 2005.
- M. Nakajima, K. Qiao, N. Kobayashi, Q. Bao, D. Tomida, and C. Yokoyama, “Efficient dehydration of aldoximes to nitriles catalyzed by a Lewis acid ionic liquid,” Chemistry Letters, vol. 40, no. 4, pp. 396–397, 2011.
- R.-J. Li, L. Chen, and Z.-C. Yan, “Synthesis of trimethylolpropane esters of oleic acid using a multi-SO3H-functionalized ionic liquid as an efficient catalyst,” Journal of the American Oil Chemists' Society, vol. 89, no. 4, pp. 705–711, 2012.
- E. D. Bates, R. D. Mayton, I. Ntai, and J. H. Davis Jr., “CO2 capture by a task-specific ionic liquid,” Journal of the American Chemical Society, vol. 124, no. 6, pp. 926–927, 2002.
- A. E. Visser, J. D. Holbrey, and R. D. Rogers, “Hydrophobic ionic liquids incorporating N-alkylisoquinolinium cations and their utilization in liquid–liquid separations,” Chemical Communications, no. 23, pp. 2484–2485, 2001.
- J. Gui, X. Cong, D. Liu, X. Zhang, Z. Hu, and Z. Sun, “Novel Brønsted acidic ionic liquid as efficient and reusable catalyst system for esterification,” Catalysis Communications, vol. 5, no. 9, pp. 473–477, 2004.
- S. Sahoo, P. Kumar, F. Lefebvre, and S. B. Halligudi, “Oxidative kinetic resolution of alcohols using chiral Mn–salen complex immobilized onto ionic liquid modified silica,” Applied Catalysis A: General, vol. 354, no. 1-2, pp. 17–25, 2009.
- Y. H. Liu, L. I. Zhu, X. Q. Sun, J. Chen, and F. Luo, “Silica materials doped with bifunctional ionic liquid extractant for yttrium extraction,” Industrial and Engineering Chemistry Research, vol. 48, no. 15, pp. 7308–7313, 2009.
- X. Z. Liang, “Novel acidic ionic liquid polymer for biodiesel synthesis from waste oils,” Applied Catalysis A: General, vol. 455, no. 2, pp. 206–210, 2013.
- K. Handore, S. Bhavsar, A. Horne et al., “Novel green route of synthesis of ZnO nanoparticles by using natural biodegradable polymer and its application as a catalyst for oxidation of aldehydes,” Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, vol. 51, no. 12, pp. 941–947, 2014.
- F. Liu, L. Wang, Q. Sun, L. Zhu, X. Meng, and F.-S. Xiao, “Transesterification catalyzed by ionic liquids on superhydrophobic mesoporous polymers: heterogeneous catalysts that are faster than homogeneous catalysts,” Journal of the American Chemical Society, vol. 134, no. 41, pp. 16948–16950, 2012.
- H. Li, Q. Zhang, and S. Yang, “Catalytic cascade dehydration-etherification of fructose into 5-ethoxymethylfurfural with SO3H-functionalized polymers,” International Journal of Chemical Engineering, vol. 2014, Article ID 481627, 7 pages, 2014.
- Y. Leng, P. Jiang, and J. Wang, “A novel Bronsted acidic heteropolyanion-based polymeric hybrid catalyst for esterification,” Catalysis Communications, vol. 25, pp. 41–44, 2012.
- Y. Zhou, W. Kong, Y. Li, A. F. Logrieco, J. Xu, and M. Yang, “A new solid-phase extraction and HPLC method for determination of patulin in apple products and hawthorn juice in China,” Journal of Separation Science, vol. 35, no. 5-6, pp. 641–649, 2012.
- A. Kumari, G. Sarkhel, and A. Choudhury, “Effect of polyvinylpyrrolidone on separation performance of cellulose acetate-polysulfone blend membranes,” Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, vol. 50, no. 7, pp. 692–702, 2013.
- M. Mokhtary and F. Najafizadeh, “Polyvinylpolypyrrolidone-bound boron trifluoride (PVPP-BF3); a mild and efficient catalyst for synthesis of 4-metyl coumarins via the Pechmann reaction,” Comptes Rendus Chimie, vol. 15, no. 6, pp. 530–532, 2012.
- L. Li, S.-T. Yu, C.-X. Xie, F.-S. Liu, and H.-J. Li, “Synthesis of glycerol triacetate using functionalized ionic liquid as catalyst,” Journal of Chemical Technology and Biotechnology, vol. 84, no. 11, pp. 1649–1652, 2009.
- L. H. O. Pires, A. N. de Oliveira, O. V. Monteiro Jr. et al., “Esterification of a waste produced from the palm oil industry over 12-tungstophosforic acid supported on kaolin waste and mesoporous materials,” Applied Catalysis B: Environmental, vol. 160-161, no. 5, pp. 122–128, 2014.
- K. A. Shah, J. K. Parikh, B. Z. Dholakiya, and K. C. Maheria, “Fatty acid methyl ester production from acid oil using silica sulfuric acid: process optimization and reaction kinetics,” Chemical Papers, vol. 68, no. 4, pp. 472–483, 2014.
- G. Corro, F. Bañuelos, E. Vidal, and S. Cebada, “Measurements of surface acidity of solid catalysts for free fatty acids esterification in Jatropha curcas crude oil for biodiesel production,” Fuel, vol. 115, no. 8, pp. 625–628, 2014.
- H. M. Fahmy, M. H. Abo-Shosha, and N. A. Ibrahim, “Finishing of cotton fabrics with poly (N-vinyl-2-pyrrolidone) to improve their performance and antibacterial properties,” Carbohydrate Polymers, vol. 77, no. 4, pp. 845–850, 2009.
- H. Li, Y. X. Qiao, L. Hua et al., “Imidazolium polyoxometalate: an ionic liquid catalyst for esterification and oxidative esterification,” ChemCatChem, vol. 2, no. 9, pp. 1165–1170, 2010.
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