Sulfamic acid functionalized PVC: a remarkably efficient heterogeneous reusable catalyst for the acid-catalyzed reactions

Sulfamic acid functionalized polyvinyl chloride catalysts (PVC-N-SO3H, N = EDA, DTA, TTA, TPA, PHA) were prepared as efficient heterogeneous solid acid catalysts via two-step treatment processes. The prepared catalysts were characterized by X-ray diffraction analysis (XRD), TG thermograms/derivative thermogravimetric (TG/DTG), transmission electron microscope (TEM), elemental mappings and energy-dispersive spectroscopy analyses (STEM-EDS), and FTIR measurements (FTIR) as well as acid–base back-titration. The acetalization of aldehydes (ketones) and alcohols to acetal (ketal) was selected to evaluate the acid catalytic performance of PVC-N-SO3H. The results showed that PVC-EDA-SO3H exhibit excellent activity and reusability due to its highly exposed sulfonic acid sites, high surface acid density (2.28 mmol g−1) for the conversion of aldehydes (ketones) to acetal (ketal). This new solid acid has obvious advantages in reusability and catalytic activity over traditional homogeneous concentrated sulfuric acid and heterogeneous sulfonated resin catalysts. Furthermore, PVC-EDA-SO3H exhibited an excellent catalytic performance in the synthesis of 12 acetals (ketals) as well as a good compatibility in the hydroxyalkylation of phenol with formaldehyde to bisphenol F (BPF) and the esterification reaction of oleic acid and methanol to biodiesel. More importantly, it could also be recovered easily and used repeatedly at least nine times without an obvious decrease in the activity.


Introduction
Acid-catalyzed reaction is a powerful and direct method for the synthesis of organic compounds [1][2][3].Conventionally, boron trifluoride, aluminum chloride and liquid protonic acids such as phosphoric acid, hydrochloric acid, sulfuric acid or other inorganic acids are widely used as efficient acid catalysts for many homogeneous liquid phase reactions due to their high activity and low cost, such as hydration, esterification, Friedel-Crafts reaction and hydrolysis [4][5][6][7].These homogeneous acid catalysis systems, however, suffer from several drawbacks such as strong corrosion, high toxicity, spent acid disposal, serious environmental pollution, difficult separation and recovery, which seriously limit their application in industry [8][9][10][11].In recent years, the reports of solid acid catalysts for acid-catalyzed reactions have shown a downward trend year by year, which poses a severe challenge to the position of acidcatalyzed reactions in organic synthesis.It is, therefore, of great practical significance to develop efficient, easy-to-separate and reusable solid catalysts having high activity for acid-catalyzed reactions to avoid the above several drawbacks.Recently, many researchers have devoted themselves to the development of solid acid catalysts and developed a variety of solid acid catalysts, such as biomass carbon sulfonic acid [12][13][14][15], sulfonic acid functionalized solid catalyst [16][17][18][19][20], metal supported molecular sieve [21], metal sulfate or metal oxide [22], heteropoly acid [23], solid material with metal organic framework structure [24,25], acid site modified silica [26,27] and zeolite [28].Among these catalysts, carbon-based solid acid catalysts using biomass as raw materials have attracted much attention from scholars due to their wide range of raw material sources and rich surface functional groups [12,14].The biomass was first prepared by pyrolysis to obtain biochar, and then, the biochar sulfonic acid catalyst was prepared by sulfonation reaction with concentrated sulfuric acid [15].Those carbon-based solid acid catalysts exhibited excellent catalytic activity in acid catalytic reaction due to the large number of sulfonic acid groups covered on the surface.However, the stability of biochar sulfonic acid is poor due to the shedding of sulfonic acid groups during the acid-catalyzed reaction, resulting in the catalytic efficiency of carbon-based solid acid catalysts decreased significantly with the increase in continuous catalytic reaction cycles.Environmentally friendly and recyclable solid acid catalysts have been widely and successfully applied to various organic transformations such as esterification, etherification, acetal (ketal), rearrangement, dehydration, oxidation, alkylation and acylation.These catalysts have the advantages of high acid strength and acid density, large specific surface area, large pore space and good thermal stability, making them efficient catalysts for various acid-catalyzed reactions.Unfortunately, not only are the synthesis routes of these catalysts complex, but their stability is also poor, which will severely restrict its application in industrial acid-catalyzed reactions.Therefore, the development of stable, efficient, reusable and highly active solid acids has attracted great interest from scholars [29][30][31].During the last few years, sulfamic acid (NH 2 SO 3 H, SA), as a dry nonhygroscopic, nonvolatile, and odorless solid, has been considered as an 1 3 Sulfamic acid functionalized PVC: a remarkably efficient… efficient heterogeneous catalyst to replace traditional acidic catalysts [32][33][34][35].It is widely used in organic synthesis reactions such as ketal or acetalization [36], esterification [32], nitrile formation [37], tetrahydropyranylation of alcohols [38] and transesterification of β-ketoesters [39].
Herein, we report an simple and efficient approach for the preparation of PVC bearing sulfamic acid (PVC-EDA-SO 3 H) under the guidance of green chemistry concepts such as low cost, ease-inexpensiveness, environmental friendliness, high efficiency, and use it as a stable and efficient heterogeneous catalyst for the acetalization reaction of aldehydes (ketones) with alcohols to acetal (ketal) and the hydroxyalkylation of phenol with formaldehyde to BPF as well as the esterification reaction of oleic acid and methanol to biodiesel.As shown in Scheme 1, the sulfonated sites were introduced into the carbon chain of PVC by the N-alkylation reaction of EDA and PVC to obtain EDA grafted PVC material (PVC-EDA), and then the target catalyst was successfully prepared by the chlorosulfonation reaction of PVC-EDA and chlorosulfonic acid (LHS).A series of characterizations confirmed that the newly constructed solid acid catalyst has high acid density and high exposure and stable amino sulfonic acid sites on the outer surface of the catalyst, which can significantly accelerate the conversion of the substrate on the surface of the catalyst, showing excellent catalytic activity and reusability in acid-catalyzed reactions.

Preparation of the catalyst
Preparation of PVC-EDA 2 g PVC and excessive ethylenediamine (20 mL) were added to a single-necked round-bottom flask equipped with a condenser and the resulting mixture was continuously magnetically stirred at 80 °C for 8 h.The reaction mixture was then cooled to room temperature and filtered through a vacuum glass filter.The filter cake was washed with excessive deionized water and vacuum dried at 60 °C for 12 h to obtain the desired precursor PVC-EDA, which was an umber powder.

Preparation of PVC-EDA-SO 3 H
Referring to the preparation of covalently bonded zwitterionic sulfamic acid onto the SBA-15 sulfamic acid catalyst (SBA-15/PrEn-NHSO 3 H) [40], chlorosulfonic acid (10 mL) was slowly added to the mixture of 1 g PVC-EDA and 10 mL CH 2 Cl 2 at room temperature.The reaction was stopped and filtered through a vacuum glass filter after continuous magnetic stirring for 8 h at room temperature.The filter cake was washed with excessive deionized water and vacuum dried at 60 °C for 12 h to obtain the target catalyst PVC-EDA-SO 3 H, which was a dark brown powder.

Preparation of PVC-N-SO 3 H catalysts
PVC-N-SO 3 H catalysts (N = DTA, TTA, TPA, PHA) were obtained by the same procedure used for the preparation of PVC-EDA-SO 3 H except that EDA was used instead of DTA, TTA, TPA, PHA as a starting material.

The acetalization reaction of aldehydes (ketones) with alcohols to acetal (ketal)
In a typical procedure, 0.1 mol aldehydes or ketones, 0.15 mol diols, 20 ml cyclohexane and fresh catalyst (0.043 g, 0.1 mol%, based on the acid density of the catalyst) were mixed together in a three necked round bottomed flask (50 mL) equipped with a magnetic stirrer, reflux condenser and a Dean-Stark apparatus was used to remove the water continuously from the reaction mixture (Scheme 2).The resulting solution was stirred at refluxing temperature for the specified reaction time.After the desired reaction time had elapsed, the reaction was stopped and cooled to room temperature; then, the solid catalysts were separated by filtration, washed with aqueous ethanol, and then was dried overnight at 60 °C in a vacuum drying oven for reuse.The qualitative analysis of the liquid reaction mixture was carried out on a GC-MS with an HP-5 column with helium as carrier gas.The column temperature was raised from 1 3 Sulfamic acid functionalized PVC: a remarkably efficient… 40 to 260 μm at a heating rate of 10 μm min −1 .The quantitative analysis of the reaction solution was analyzed using Agilent 6890 N gas chromatograph (GC) with a DB-17 polysiloxane capillary column (30 m × 0.32 mm × 0.50 μm) and flame ionization detector (FID) using ethylbenzene as an internal standard.
The activity of catalyst was evaluated in terms of the yield of acetal (ketal), which were calculated by dividing the actual produced molar mass of acetal (ketal) by the theoretical molar mass of acetal (ketal).

The hydroxyalkylation of phenol with formaldehyde to BPF
The hydroxyalkylation of phenol with formaldehyde to BPF was performed in a magnetically stirred glass reactor fitted with a reflux condenser and an arrangement for temperature control.In a typical procedure, 0.2 mol of phenol, 0.01 mol of formaldehyde and catalyst (0.0044 g, 0.1 mol%, based on the acid density of the catalyst) were added into the reactor (Scheme 3).No other solvent was needed.The reaction mixture was heated to 60 °C.After 1 h, the reaction was stopped and cooled down to room temperature.
The unreacted formaldehyde was determined using an UV-2450 spectrophotometer (Shimadzu, Kyoto, Japan) using an acetylacetone spectrophotometric method.The compositions of the hydroxyalkylated products were determined on an Agilent 1100 HPLC (Agilent, USA) with an external standard method and the specific operation conditions were as follows: 4.6 × 250 mm of Agilent SB-C 18 chromatographic column, methanol/water 65:35 (v/v) of mobile phase, 1.0 mL/ min of flow rate, the UV detector 270 nm, column temperature 30 °C, injection volume 20 μL.The conversion of formaldehyde was calculated by dividing the actual added molar mass of formaldehyde by the reacted molar mass of formaldehyde obtained from the oxygenated products.The selectivity of BPF was calculated by dividing the molar mass of oxygenated products by the molar mass of BPF.
The esterification reaction of oleic acid and methanol to biodiesel 10 mmol oleic acid, 120 mmol methanol and fresh catalyst (0.043 g, 0.1 mol%, based on the acid density of the catalyst) were mixed together in a three necked round bottomed flask equipped with a magnetic stirrer, reflux condenser (Scheme 4).After the desired reaction time had elapsed, the reaction was stopped and the samples were analyzed by Agilent 6890 N gas chromatograph (GC) equipped with a DB-17 polysiloxane capillary column (30 m × 0.32 mm × 0.50 μm) and flame ionization detector (FID) using dodecane as an internal standard.The oven temperature is programmed at 70 °C for 2 min and then it is raised from an initial value of 70-190 °C at a ramp rate of 10 °C/min.The injector and detector temperature are maintained at 250 and 300 °C, respectively.The analytical uncertainty of GC is less than 4%, which is in accordance with the acceptable limits.The biodiesel yield is calculated by dividing the actual produced molar mass of methyl oleate by the theoretical molar mass of methyl oleate.

Characterization of catalyst
X-ray diffraction spectroscopy (XRD) was conducted on a Bruker D8-Advance X-ray diffractometer using Cu-Kα radiation (λ = 0.15405 nm) with the scanning angle (2θ) of 5-80˚, under the following conditions: scanning voltage 40 kV, scanning current 40 mA; scanning speed 0.2 s, scanning step 0.02°.Fourier transform Infrared spectroscopy (FTIR) was performed on a Nicolet Avatar370 using KBr pellets over the frequency range 400-4000 cm −1 .Thermogravimetric analyses (TG/ DTG) were carried out on a NETZSCH-STA 409 PC thermoanalyzer from room temperature to 800 °C with a heating rate of 10˚C/min under nitrogen atmosphere.The morphology of the PVC-EDA-SO 3 H catalyst was determined by transmission electron microscopy (HR-TEM; JEM 2100, JEOL, Japan).The EDS was measured using a JEOL JSM-2000EM scanning electron microscope.The acidity of PVC-N-SO 3 H was measured by traditional chemical titration referring to the literature [41].

FTIR
The FTIR spectra of PVC-EDA-SO 3 H and its precursors are shown in Fig. 1.The FTIR spectrum of PVC shows some characteristic absorption bands, including two bands at 690 cm −1 and 613 cm −1 , respectively, originating from C-Cl bond stretching, two absorption bands at 2909 and 1426 cm −1 , respectively, assigned to the stretching and wagging vibrations of C-H from CH 2 groups, as well as three absorption bands at 2968, 1331 and 1254 cm −1 , respectively, attributed to the stretching and deformation of C-H of CHCl groups [42,43].PVC-EDA shows two new broad absorption bands at 3300 cm −1 and 1652 cm −1 attributed to the bending vibration of N-H bond and a disappearance of the C-Cl peak at 615, 2968 cm −1 is found in its FTIR spectrum comparing with PVC.And the peak at 2909 cm −1 is weaker than that of PVC, perhaps because the grafting of C-N bond leads to the breakage of C-Cl bond after N-alkylation, resulting in the weakening of C-Cl bond peak.The peak at 1709 cm −1 can be attributed to the C-N stretching vibration peak of -CH 2 -NH 2 , indicating that EDA is successfully grafted on the carbon chain of PVC via N-alkylation.The FTIR spectrum of PVC-EDA-SO 3 H shows the absorption band at 3430 cm −1 is attributed to the OH group stretching vibrations of SO 3 H group, and the peak at about 1110 cm −1 is attributed to the typical vibration bands of SO 3 − stretching and the stretching absorption modes of O=S=O of supported SO 3 H group. Notably, the absorption peak at 1652 cm −1 corresponded to the bending Fig. 1 FTIR spectra of PVC, PVC-EDA, PVC-EDA-SO 3 H and recovered catalyst vibration of the N-H bond suffered a slight shift to the direction of low wavenumber after chlorosulfonation of PVC-EDA, these finding confirm that the sulfonic groups are successfully grafted on PVC-EDA by chlorosulfonation reaction [44][45][46][47].These characteristic vibrations clearly indicate the formation of desired catalytic system.

TG/DTG
The thermal decomposition behavior of the above PVC, PVC-EDA, PVC-EDA-SO 3 H samples under nitrogen atmosphere was investigated by a thermal gravimetric analysis (TGA) technique and the obtained TG and differential TG (DTG) curves are shown in Fig. 2. It can be seen from the TG (Fig. 2a) and DTG curves (Fig. 2b) of pure PVC that there were two obvious weight loss peaks at 298 °C and 511 °C, which are, respectively, attributed to the dehydrochlorination of PVC and the further thermal degradation of its dechlorination residue [48][49][50].From the TG and DTG curves of PVC-EDA, it can be seen that the weight loss attributable to the dehydrochlorination of PVC is significantly weakened after PVC grafting EDA.In addition, a new weight loss occurs about 100 °C, which is likely due to the evaporation of the adsorbed water.These findings indicate that most C-Cl bonds of PVC have been successfully substituted by C-N bonds via N-alkylation and such substitution can increase the hydrophilicity of PVC-EDA.After PVC-EDA further grafting sulfonic acid group by chlorosulfonation reaction with LHS, its weight loss peak for the adsorbed water is slightly strengthened owing to the grafting of stronger polarity sulfonic acid group.In addition, a weight loss between 150 and 380 °C could be assigned to removal of organic motif (ethylenediamine and sulfamic acid) from the structure of PVC-EDA-SO 3 H [40], which proves the successful formation of aminosulfonic acid sites on the PVC carbon chain.Sulfamic acid functionalized PVC: a remarkably efficient… XRD Powder X-ray diffraction (XRD) spectra of these samples are shown in Fig. 3.The amorphous structure of PVC is indicated by the broad peak appearing in the region of 15-30 o in its XRD spectrum, consistent with previous reports [51,52].In addition, the XRD spectra of PVC-EDA and PVC-EDA-SO 3 H samples are similar to those of PVC, which suggests that these samples are also amorphous materials.The diffraction peaks of PVC-EDA and PVC-EDA-SO 3 H samples in the 15-30° region were significantly stronger than those of PVC, indicating that the structure of PVC has undergone slight changes after amination modification with ethylenediamine and PVC-EDA grafting sulfonic acid groups.

TEM
The morphology of PVC-EDA-SO 3 H catalyst was characterized by high resolution transmission electron microscopy (HRTEM) and its four HRTEM micrograph are given in Fig. 4. Figure 4a demonstrates that the PVC-EDA-SO 3 H catalyst is an organic whole tightly packed together; however, the regions marked with red circles in Fig. 4b-d show that the catalyst seems to possess a denser layered structure.This explains the reason why there are highly exposed sulfonic acid groups on the surface of the PVC-EDA-SO 3 H catalyst is that the reaction sites of PVC and PVC-EDA in the amination modification and chlorosulfonation are carried out on the outer surface of those two precursors, while the probability of amination and chlorosulfonation of C-Cl bonds and amino groups in the inner layer of the highly covered precursor is greatly reduced, respectively.These findings are highly consistent with the characterization results of STEM-EDS.

STEM-EDS
The distribution and content of the catalyst surface elements were further investigated using high resolution scanning transmission electron microscopy (HRSTEM) equipped with a high-angle annular dark field (HAADF) and energy-dispersive spectroscopy (EDS) detectors.Figure 5a, c shows the TEM images corresponding to the element distribution mappings of PVC-EDA-SO 3 H catalyst.It can be seen from Fig. 5d, h that the single elemental mappings of C and Cl elements on the surface of PVC-EDA-SO 3 H show relatively uniform distributions within the tested regions.Notably, the distributions of N, O and especially S elements in the three mappings (Fig. 5e-g) almost overlaps with the distribution of C elements, suggesting that N-alkyl and sulfonic groups are homogeneously grafted on the PVC in successive.The actual content of Cl element on the surface of PVC-EDA-SO 3 H catalyst (2.3%) is much lower than the theoretical content of Cl element in PVC (56.8%), further supporting that most C-Cl bonds of the surface of PVC have been substituted by C-N bonds via N-alkylation.Notably, the amount of SO 3 H groups on PVC-EDA-SO 3 H calculated from EDS analysis of Fig. 5b is 5.57 mmol g −1 , which is 2.5 times of the acid density measured via chemical analysis (2.26 mmol g −1 ), indicating that most of the -SO 3 H groups are highly exposed on the surface of the catalyst, only a small amount of SO 3 H groups are enclosed in the dense layer of the PVC-EDA-SO 3 H.These findings suggest that the STEM-EDS characterization results of highly exposed sulfonic acid groups on the catalyst surface are in good agreement with its high catalytic activity in acid catalytic reaction.Sulfamic acid functionalized PVC: a remarkably efficient… reaction conditions.Under the standard conditions, the reaction was carried out with cyclohexane as water-carrying reagent, 0.1% mol PVC-EDA-SO 3 H as catalyst at reflux temperature for 1 h, achieving 99.9% ketal yield and 999 h −1 TOF (Entry 1), representing a mild and efficient character.Even if shortening the reflux time to 15 min, the catalyst still exhibited a good efficiency, affording 67.5% ketal yield.Notably, the TOF was significantly increased to 2700 h −1 (Entry 2).In addition, nearly 50% ketal yield was achieved and TOF was significantly increased to 3300 h −1 when the catalyst concentration was reduced to 0.01 mol% (Entry 3).These experimental results show that PVC-EDA-SO 3 H has fast reaction rates for the conversion of substrates and high catalytic activity.The current ketalization reaction was further studied in the absence of water-carrying agent in order to make clear the effect of water-carrying agent on the ketalization reaction of cyclohexanone and ethylene glycol.As shown in Entry 4, the yield of ketal in the absence of water-carrying agent is significantly lower than that in the presence of water-carrying agent.These findings indicate that the water-carrying agent is crucial to the complete progress of the current ketalization reaction.The water-carrying agent can carry the water generated during the reaction away from the reaction system, making the chemical reaction equilibrium move to the positive reaction and improving the conversion of the cyclohexanone.The catalytic ketalization performance of PVC-EDA-SO 3 H was compared to those of four newly prepared PVC-bonded N-alkylaminosulfonic acid catalysts (PVC-DTA-SO 3 H, PVC-TTA-SO 3 H, PVC-TPA-SO 3 H, PVC-PHA-SO 3 H) and two reference catalysts (the existing concentrated sulfuric acid (H 2 SO 4 , 98.3%) and sulfonic acid resin) as well as PVC, PVC-EDA, and the results are shown in Entries 5-12, PVC-EDA-SO 3 H catalyst exhibits an outstanding ketalization activity, which can achieve 99.9% ketal yield and 999 h −1 TOF.While PVC, PVC-EDA catalysts show a poor catalytic activity, achieving a extremely low ketal yield (Entries 5-6).It is worth mentioning that the acetalation (ketalation) reaction can be carried out smoothly under both acid and base catalyst conditions.The precursor PVC-EDA contains basic groups (amino groups), so it is not surprising that it can catalyze the ketalization reaction of cyclohexanone and ethylene glycol.Entries 7-10 show that these PVC-bonded N-alkylaminosulfonic acid catalysts possess a high acid density 1 3

The ketalization reaction under different conditions
Sulfamic acid functionalized PVC: a remarkably efficient… and catalytic activity for this ketalization in cyclohexane, providing 89.5-99.9%ketal yield and 895-999 h −1 TOF and the cyclohexanone ethylene ketal as the only product.Among them, PVC-DTA-SO 3 H exhibits a very high ketal yield (ca. 99.9%) owing to its high acid density and highly exposed sulfamic acid sites.Notably, the ketal yield and acid density gradually decreases from 99.9 to 89.5% and 2.12 to 1.93 mmol g −1 with PVC-DTA-SO 3 H to PVC-PHA-SO 3 H, respectively, which should be due to a fact that the increase in the steric hindrance of the amination reagent reduced the nucleophilic substitution reaction activity of the amino group with PVC [53][54][55][56] and the growth of the amino-linked carbon chain reduces its nucleophilic ability [57,58], resulting in a decrease in the number of amino groups grafted on the PVC carbon chain, which eventually led to a decrease in the number of sulfonic acid groups connected by the chlorosulfonation reaction.The catalytic ketalization performance of PVC-EDA-SO 3 H was compared to the existing concentrated sulfuric (H 2 SO 4 , 98.3%) and sulfonic acid resin and the results are shown in Entries 11-12, PVC-EDA-SO 3 H catalyst exhibits an outstanding ketalization activity, while homogeneous H 2 SO 4 and heterogeneous sulfonated resin catalysts provide 95.4 and 86.8% ketal yield under the same reaction conditions, respectively.Furthermore, the selectivity of ketal (96.3%) using sulfuric acid as catalyst is lower than that using PVC-EDA-SO 3 H (100%) as catalyst, which might be due to a fact that strong oxidation of concentrated sulfuric acid leads to carbonization of reactants or increased side reactions (Figure S1).As a heterogeneous solid acid catalyst, PVC-EDA-SO 3 H has an absolute advantage over concentrated sulfuric acid in the reusability.With a higher dispersibility, PVC-EDA-SO 3 H shows a better catalytic activity than sulfonic acid resin.These findings support that PVC-EDA-SO 3 H, as a newly constructed solid acid catalyst, has an overwhelming advantage in ketalization performance compared to the two existing reference catalysts, expecting that it has extensive application prospects in acid catalysis reactions.The blank experiment conducted in the absence of catalyst confirmed that PVC-EDA-SO 3 H is necessary for the ketalization (Entry 13).

The effect of variables on ketalization of cyclohexanone catalyzed by PVC-EDA-SO 3 H
The molar ratio of ketone and alcohol: The influence of molar ratio of ketone and alcohol on ketalization reaction was examined under catalysis of PVC-EDA-SO 3 H.As shown in Fig. 6, this catalyst shows a good low molar ratio of ketone and alcohol (1:1) activity and can provide 84.6% yield of cyclohexanone glycol ketal and 564 h −1 turnover frequency (TOF).As molar ratio of ketone and alcohol is elevated to 1:1.3, the yield and TOF continuously and remarkably increase to 99.6 and 664 h −1 , respectively.However, the yield and TOF are gradually reduced to 96.2% and 641 h −1 , with molar ratio of ketone and alcohol consequently increasing from 1:1.3 to 1:1.5.The reason is mainly due to the increase in the amount of ethylene glycol so that the reaction system is diluted, reducing the probability of collision between the reactants molecules, and ultimately reduce the yield.Here, 1:1.3 was selected as the preferred molar ratio of ketone and alcohol for the ketalization reaction of cyclohexanone and ethylene glycol.

Effects of catalyst concentration:
The effect of catalyst concentration on ketalization was also checked using PVC-EDA-SO 3 H as a catalyst.As shown in Fig. 7, the activity of catalyst continuously and markedly increases with a gradual increase in its concentration, which can achieve 49.5% yield and 3300 h −1 TOF at 0.01 mol% catalyst concentration.The yield of cyclohexanone glycol ketal increases from 49.5 to 99.3% and the TOF reduces from 3300 to 662 h −1 , with catalyst concentration Fig. 6 Effect of molar ratio of ketone and alcohol on the ketalization of cyclohexanone with ethylene glycol a .a Reaction conditions: 0.1 mol cyclohexanone; Mole ratio of cyclohexanone/ethylene glycol, 1:1-1:1.5;Water-carrying reagent, 20 mL cyclohexane; Catalyst concentration, 0.1% mol; Reaction temperature, Reflux; Reaction time, 1.5 h Fig. 7 Effect of catalyst concentration on the ketalization of cyclohexanone with ethylene glycol a ..aReaction conditions: 0.1 mol cyclohexanone; Mole ratio of cyclohexanone/ethylene glycol, 1:1.3;Water-carrying reagent, 20 mL cyclohexane; Catalyst concentration, 0.01-0.50%mol; Reaction time, 1.5 h; Reaction temperature, reflux 1 3 Sulfamic acid functionalized PVC: a remarkably efficient… consequently increasing from 0.01 to 0.1% mol.Upon further increase in catalyst concentration, the yield of cyclohexanone glycol ketal only shows a minor change.Thus, 0.1% mol is selected as the suitable catalyst concentration.

Effects of reaction time:
The influence of reaction time on ketalization was examined under catalysis of PVC-EDA-SO 3 H and the results shown in Fig. 8.Such catalyst exhibits an outstanding acceleration effect on ketalization, which can afford 67.5% cyclohexanone glycol ketal yield and 2700 h −1 TOF at 15 min.When the time is prolonged from 15 to 60 min, the yield further increases to 99.9% and TOF reduces to 999 h −1 .However, the yield of cyclohexanone glycol ketal shows no significant increase upon further extend the reaction time to 1.5 h.On the contrary, TOF decreased significantly to 666 h −1 .Therefore, the optimal reaction time is 1 h.

Effects of water-carrying agent type:
In order to study the effect of different water-carrying agents on the catalytic performance of PVC-EDA-SO 3 H, benzene and toluene were also selected as water-carrying agents for ketalization reaction and the experimental results are shown in Fig. 9.The yield of cyclohexanone glycol ketal is only 88.4% for 1 h and the TOF is also much lower than that of cyclohexane and toluene when benzene is selected as the water-carrying agent.At the same time, benzene and toluene are toxic, the waste liquid produced after the reaction is harmful to the environment and the reaction temperature is higher using toluene as a water-carrying agent.Considering safety and energy consumption, cyclohexane is more suitable as a water-carrying agent.
Effects of cyclohexane dosage: The effect of cyclohexane dosage on the yield of cyclohexanone ethylene ketal was checked using PVC-EDA-SO 3 H as a catalyst and the results are shown in Fig. 10.The yield of cyclohexanone ethylene ketal increased from 87.9 to 99.9% as well as the TOF increased from 879 to 999 h −1 with Fig. 8 Effect of reaction time on the ketalization of cyclohexanone with ethylene glycol a .a Reaction conditions: 0.1 mol cyclohexanone; Mole ratio of cyclohexanone/ethylene glycol, 1:1.3;Water-carrying reagent, 20 mL cyclohexane; Catalyst concentration, 0.1% mol; Reaction temperature, Reflux; Reaction time, 15-90 min Fig. 9 Effect of water-carrying reagent on the ketalization of cyclohexanone with ethylene glycol a .a Reaction conditions: 0.1 mol cyclohexanone; Mole ratio of cyclohexanone/ethylene glycol, 1:1.3;Watercarrying reagent, 20 mL; Catalyst concentration, 0.1% mol; Reaction temperature, Reflux; Reaction time, 1 h Fig. 10 Effect of water-carrying reagent on the ketalization of cyclohexanone with ethylene glycol a .a Reaction conditions: 0.1 mol cyclohexanone; Mole ratio of cyclohexanone/ethylene glycol, 1:1.3;Watercarrying reagent, 10-30 mL cyclohexane; Catalyst concentration, 0.1% mol; Reaction temperature, reflux; Reaction time, 1 h 1 3 Sulfamic acid functionalized PVC: a remarkably efficient… the increase in the amount of cyclohexane from 10 to 20 mL, respectively.However, the yield and TOF gradually and slowly decreased to 92.2% and 922 with the further increase in cyclohexane dosage.Therefore, too much or too little cyclohexane is not conducive to the reaction.If the amount is too small, the effect of carrying water is poor, resulting in increasing the reaction temperature, the reactants are easily carbonized and blackened as well as the side reactions increase.If the amount is too much, the reaction system is diluted and the yield of cyclohexanone ethylene ketal is reduced.Therefore, the optimum amount of cyclohexane is 20 mL.

The acetalization reaction of various substrates catalyzed by PVC-EDA-SO 3 H
In order to verity the adaptability in acetalization (ketalation) reaction of the above catalyst, the catalytic activity of PVC-EDA-SO 3 H for the synthesis of 12 kinds of acetals (ketones) was also checked by use of the above optimal reaction conditions.The adaptability of PVC-EDA-SO 3 H and the mass spectrometry characterization results of 12 kinds of acetals (ketones) are shown in Table 2.It can be seen from Table 2 that PVC-EDA-SO 3 H exhibited high catalytic activity in the synthesis of 12 acetals (ketones), achieving 92.2-99.9%yield of acetals (ketals) and 922-999 h −1 TOF.In the acetalation reaction of n-butyraldehyde and diol, more than 99% acetal yield can be obtained, which is caused by the small steric hindrance of n-butyraldehyde.The reason why the yield of the condensation product of benzaldehyde and diol is lower may be that the benzene ring is directly connected to the carbonyl group to form a π-π conjugated system.In addition, the phenyl group is an electrondonating group, which reduces the positive electricity of the carbonyl carbon and makes the carbonyl group not conducive to the attack of the alcohol hydroxyl group.The reason for the lowest yield of furfural and diol condensation products may be that the furan ring forms a π-π conjugated system with the carbonyl carbon, which reduces the positive charge of the carbonyl carbon, and another possible reason is that the furfural itself is very unstable and prone to self-polymerization.The yield of cyclohexanone and diol condensation product is as high as 99%.This may be due to the fact that the cyclohexanone carbonyl group is not on the same plane with the adjacent two methylene groups, which has less hindrance to the attack of the alcohol hydroxyl group on the carbonyl carbon.The above results showed that PVC-EDA-SO 3 H had a good applicability for the synthesis of acetals (ketones).

Application of PVC-EDA-SO 3 H in hydroxyalkylation and esterification
In order to verify the applicability of PVC-EDA-SO 3 H catalyst in other acid-catalyzed reactions, the esterification of oleic acid with methanol to biodiesel and the hydroxyalkylation of phenol with formaldehyde to BPF were examined using the optimal reaction conditions (Fig. 11) and the results of condition optimization experiments are shown in Figure S2-7.Under the optimal conditions, the esterification of oleic acid with methanol to biodiesel was carried out with 0.5% mol PVC-EDA-SO 3 H as catalyst, oleic acid dosage of 10 mmol, molar ratio of oleic acid to methanol of 1:12 under reflux temperature for 2 h (Figure S2-4), Sulfamic acid functionalized PVC: a remarkably efficient… the esterification catalyzed by PVC-EDA-SO 3 H could achieve 98.7% oleic acid conversion and 98.1% biodiesel yield.This newly constructed solid acid catalyst also offers several promising advantages in the hydroxyalkylation of phenol with formaldehyde to BPF, which could achieve a remarkable formaldehyde conversion (99.6%) and selectivity of bisphenol F (92.8%) under optimal reaction conditions (10 mmol formaldehyde, mole ratio of phenol/formaldehyde, 20; Catalyst concentration, 0.1%mol; Reaction time, 1 h; Reaction temperature, 70 °C.Figure  -7).Based on the above experimental results, it is suggested that the highly exposed sulfonic acid sites and high acid density make PVC-EDA-SO 3 H exhibit excellent conversion efficiency and catalytic activity in the above two types of acid-catalyzed reactions, and it is expected to further explore its potential applications to other important acid-catalyzed reactions in the industry.

Reusability of the catalyst
Finally, the stability of PVC-EDA-SO 3 H was tested by performing a recycle experiment and the testing results are presented in Fig. 12.The performance of the catalyst shows no significant reduction even after nine successive runs, 96.3% yield of cyclohexanone ethylene ketal can still be achieved, and the acid density of PVC-EDA-SO 3 H catalyst was still as high as 1.87 mmol g −1 after repeated use for 9 times.Moreover, PVC-EDA-SO 3 H also showed good reusability in the hydroxyalkylation of phenol and formaldehyde to BPF and the esterification of oleic acid with methanol under the optimal reaction conditions (Figure S8), but the catalytic activity decreased slightly (ca.6%) after repeated use for 7 times.In order to reveal the reason for the catalytic activity of the PVC-EDA-SO 3 H catalyst in the above three acid-catalyzed reactions decreases slightly with the increase in the number of repeated use.The PVC-EDA-SO 3 H recovered after the ninth run was also characterized by FT-IR, TG/DTG, XRD, TEM and STEM-EDS methods and the results indicated that the recovered catalyst has almost the same characteristic IR absorption bands, XRD patterns and TEM morphology as the fresh sample (Figs. 1 and 3-4).The characterization results of TG/DTG (Fig. 2) and STEM-EDS (Figure S10) showed that the sulfonic acid groups on the surface of PVC-EDA-SO 3 H catalyst were partially shed with the increase in reuse times.In general, PVC-EDA-SO 3 H is considered to be an excellent and stable recyclable solid acid catalyst for the current acid catalytic reaction.

The plausible mechanism for formation of acetals(ketones)
A plausible mechanism of the ketalization reaction of cyclohexanone and ethylene glycol is presented in Scheme 5.The carbon atom in the carbonyl group exhibits strong positive electricity and the oxygen shows strong negative electricity since the carbonyl group is a strong polar group.A carbocation intermediate RCHOH + (A) is formed by protonation of carbonyl with H + released by -SO 3 H in PVC-EDA-SO 3 H, and then the oxygen atom of another molecule of alcohol combines with the carbocation to form intermediate B by lone pair electrons, which is deprotonated to obtain hemiacetal C. The hydroxyl group of hemiacetal is unstable, and it is easy to combine with hydrogen ion to form intermediate D, which takes off a molecule of water and forms carbocation intermediate E. Similarly, the oxygen atom on the other alcohol hydroxyl group attacks the carbonyl carbon with lone pair electrons to form intermediate F, this unstable intermediate then eliminates a proton, resulting in the final formation of stable acetals (ketals).In particular, the acetalization of aldehydes (ketones) with alcohols is a reversible reaction, which is easily hydrolyzed to aldehydes (ketones) in acidic aqueous solution.Therefore, this reaction is often used to protect the carbonyl group in organic synthesis.

Conclusion
In conclusion, PVC-EDA-SO 3 H catalyst with high exposed sulfonic acid sites and high acid density has been successfully synthesized via a two-step reaction of N-alkylation reaction and chlorosulfonation reaction.The outstanding advantages of PVC-EDA-SO 3 H catalyst are summarized as follows: (1) Use cheap and readily available organic polymer PVC as a raw material for the catalyst; (2) the reaction conditions are mild and the catalyst is easy to separate; (3) possessing highly exposed sulfonic acid sites and high acid density; (4) achieving 99.9% cyclohexanone glycol ketal yield and 3300 h −1 TOF, 98.7% oleic acid conversion and 98.1% biodiesel yield, 99.6% formaldehyde conversion and 92.8% bisphenol F selectivity under optimal reaction conditions, respectively; (5) having a good adaptability for the synthesis of 12 acetals (ketals) as well as a good compatibility in the hydroxyalkylation of phenol with formaldehyde to bisphenol F (BPF) and the esterification reaction of oleic acid and methanol to biodiesel.With this renewable solid acid in hand, we are interested in further exploring its potential applications in other important acid-catalyzed reactions in industry, such as alkylation, rearrangement, hydrolysis, acylation and nucleophilic addition.

Scheme 4 3
Scheme 4The synthesis of biodiesel catalyzed by PVC-EDA-SO 3 H

Fig. 4
Fig. 4 TEM images of fresh PVC-EDA-SO 3 H and recovered catalyst (a, b, c and d, fresh catalyst; e, f, g and h, recovered catalyst)

Fig. 5
Fig. 5 Elemental mappings and energy-dispersive spectroscopy analyses of PVC-EDA-SO 3 H catalyst

Table 1
lists the data for the ketalization of cyclohexanone with ethylene glycol to cyclohexanone ethylene ketal catalyzed by PVC-EDA-SO 3 H under various

Table 1
The ketalization reaction of cyclohexanone and ethylene glycol under different conditions

Table 2
Adaptability of PVC-EDA-SO 3 H in the synthesis of acetals (ketals)