Polyvinylpolypyrrolidone Supported Brønsted Acidic Catalyst for Esterification

A polyvinylpolypyrrolidone (PVPP) supported Brønsted acidic catalyst ([PVPP-BS]HSO4) was prepared by coupling SO3Hfunctionalized 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-BS]HSO4 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.


Introduction
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 H 2 SO 4 and H 3 PO 4 .However, these acids were reported to exhibit several drawbacks including severe corrosion to equipment, environmental hazards, and difficulty in the aspect of catalyst reusability [3].To overcome these problems, heterogeneous solid acid catalysts such as supported mineral acids, acidic resins, and zeolites were developed [4].Nevertheless, these heterogeneous catalysts were illustrated to exhibit some disadvantages related to product selectivity, catalyst recyclability, and environmental safety [5].As such, developing high catalytic activity, recyclable, environmentalfriendly catalysts for esterification is still an attractive topic in catalyst field [6].
Owing to the negligible selective dissolvability and tunable acidity, ILs (ionic liquids) were utilized as environmentalfriendly solvents and catalysts for chemical reactions [7][8][9][10].Many organic reactions were catalyzed by ILs which were revealed to be promising catalysts for esterification and transesterification.Gui et al. [11] employed three halogenfree 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][15][16][17].Leng et al. [18] 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 [21] used PVPP supported boron trifluoride as the catalyst to prepare N-tertbutyl 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.

Preparation of [PVPP-BS]HSO 4 , PVPP-H 2 SO 4 , and 1-(4-Sulfonic acid) Butylpyridinium Hydrogen Sulfate ([BSPy]HSO 4 ).
All experimental steps were carried out under nitrogen atmosphere.First, [PVPP-BS]HSO 4 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-BS]HSO 4 as a solid.The preparation process of [PVPP-BS]HSO 4 is shown in Scheme 1.
For comparison, two other catalysts PVPP-H 2 SO 4 and 1-(4-sulfonic acid) butylpyridinium hydrogen sulfate ([BSPy]HSO 4 ) were also prepared.The preparation procedure of PVPP-H 2 SO 4 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-H 2 SO 4 as a solid.
[BSPy]HSO 4 was synthesized as reported in the previous literature with a slight modification [22].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 [BSPy]HSO 4 .The preparation process of [BSPy]HSO 4 is shown in Scheme 2.

Catalyst Characterization.
FT-IR spectra were measured on a PRESTIGE-21 FT-IR instrument (KBr discs) in the 2200-400 cm −1 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-BS]HSO 4 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 liquidnitrogen 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-BS]HSO 4 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.

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-BS]HSO 4 (1.19 g, 8% of the total mass of nbutanol 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 30 m × 0.32 mm × 0.25 m).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].

Recycling of Catalysts.
The catalyst [PVPP-BS]HSO 4 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.

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.vibrations of PVPP, respectively [26].The band in Figure 2(b) at 1035 cm −1 , which did not appear in Figure 2(a), was attributed to the S=O symmetric stretching vibrations as reported in a literature [22].This result indicates that BS has been connected to the surface of PVPP.Moreover, comparing H 2 SO 4 with [PVPP-BS]HSO 4 in Figures 2(d) and 2(c), it can be clearly observed that after PVPP-BS reacted with H 2 SO 4 , the symmetric stretching vibration of S-O band had shifted from 610 cm −1 to 617 cm −1 .This is a convincing evidence of the existence of the strong ionic interaction between the polymeric cations and HSO 4 − .It thus suggests that the reaction of SO 3 H-functionalized PVPP with H 2 SO 4 produced a PVPP-based polymeric hybrid via ionic linkages.

Morphological Surface Analysis.
Figure 4 shows the diameters and surface morphologies of (a) PVPP, (b) PVPP-BS, and (c) [PVPP-BS]HSO 4 .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 H 2 SO 4 , the surface of PVPP-BS and [PVPP-BS]HSO 4 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 m 2 /g) and [PVPP-BS]HSO 4 (6.56 m 2 /g) much higher than that of PVPP (3.84 m 2 /g) (Table 1).The elemental analysis results in

Reusability of [PVPP-BS]HSO 4 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-BS]HSO 4 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-BS]HSO 4 .

Conclusions
In conclusion, a Brønsted acidic catalyst [PVPP-BS]HSO 4 was synthesized by coupling SO 3 H-functionalized polyvinylpolypyrrolidone with H 2 SO 4 .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-BS]HSO 4 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.

Table 1
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 Table2.It can be observed that the conversion of acetic acid only reached 20.3% (entry 1) when catalysts were not used, which means
a Weight percentage of S element determined by an elemental analyzer b Determined as KOH mass consumed for neutralization.

Table 2 :
Esterification of n-butanol and acetic acid under different conditions a .CH 3 COOH + CH 3 (CH 2 ) 3 OH → CH 3 COO(CH 2 ) 3 CH 3 + H 2 O. the esterification was difficult to occur with the absence of catalysts.When H 2 SO 4 was used as the catalyst in the esterification, a homogeneous reaction system was formed and H 2 SO 4 exhibited a low conversion of 85.1% and selectivity of 87.1% (entry 2).This homogeneous reaction system made it difficult to separate H 2 SO 4 from the product.Although PVPP-H 2 SO 4 is a heterogeneous catalyst which is easy to prepare, when used as the catalyst in esterification, it exhibited the similar catalytic ability as H 2 SO 4 , and the low conversion of 89.3% and selectivity of 88.5% were obtained (entry 3).Using H 3 PW 12 O 40 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, H 3 PW 12 O 40 had a lower selectivity of reaction (95.8%) than that of [BSPy]HSO 4 and [PVPP-BS]HSO 4 (entry 4).When [BSPy]HSO 4 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 H 2 SO 4 and PVPP-H 2 SO 4 ; and the selectivity of the reaction was 100%, which was higher than that of H 3 PW 12 O 40 (entry 5).However, [BSPy]HSO 4 was easily attached onto the bottom surface of the flask reactor like a gelatinous liquid, which greatly lowered the mass transfer.When [PVPP-BS]HSO 4 was used, a liquid-solid reaction system formed and a high conversion (99.6%) and good selectivity were obtained (entry 6).Moreover, [PVPP-BS]HSO 4 could be easily separated by vacuum filtration and reused.Table3presents the comparison of the catalytic performances of [PVPP-BS]HSO 4 with two reported polymer a n-Butanol (7.12 g); acetic acid (4.80 g); and [PVPP-BS]HSO 4 (8 wt%), 90 ∘ C for 3 h.H 2 SO 4 , H 3 PW 12 O 40 , PVPP-H 2 SO 4 , and [BSPy]HSO 4 were used in a similar proton content as [PVPP-BS]HSO 4 .b Conversion of acetic acid.c Selectivity for ester.that

Table 3 :
Esterification of n-butanol and acetic acid with different polymer supported catalysts.

Table 4 :
[27]rification of different alcohols and carboxylic acids in [PVPP-BS]HSO 4 a .catalystsfor the esterification of acetic acid with nbutanol.It can be seen that [PVPP-BS]HSO 4 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.[18]and Li et al.[27].Therefore, [PVPP-BS]HSO 4 has more potential in industrial application.
b Conversion of acetic acid.c Selectivity for ester.supported

Table 4 .
It can be observed that [PVPP-BS]HSO 4 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 Table4demonstrate that the length of carbon chains merely affected the conversion and the selectivity (Table4, entries 1-6).In addition, satisfactory conversions above 90% (Table4, entries 7, 8) of esterification of aromatic alcohol and carboxylic acids were also obtained.Thus, all these results indicate that [PVPP-BS]HSO 4 catalyst can be applied to various esterification reactions with different substrates.