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This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol., 2013, 3, 699--705 699

Cite this: Catal. Sci. Technol., 2013,

3, 699

Esterification of 2-keto-L-gulonic acid catalyzed by asolid heteropoly acid

Thu Ha Thi Vu,* Hang Thi Au, Tuyet Mai Thi Nguyen, Minh Tu Pham, Tam Thi Bachand Hong Nhan Nong

The efficacy of a potassium 12-phosphotungstate (KPW) catalyst in the synthesis of methyl 2-keto-L-gulonat

from 2-keto-L-gulonic acid (2-KLGA) and methanol is investigated. The KPW catalyst gives high yields in

short reaction times. The present procedure represents a clean, efficient, practical, simple, mild, time-saving

and eco-friendly method for the synthesis of methyl 2-keto-L-gulonat. The KPW catalyst is found to be a

truly heterogeneous catalyst, highly efficient and reusable in the synthesis of methyl 2-keto- L-gulonat.

1. Introduction

The 2-keto-L-gulonic acid (2-KLGA) esterification reaction plays an

important role in the industrial manufacture of vitamin C.13

Homogeneous acid catalysts, such as sulfuric acid and heteropoly

acids (HPAs), have been used for the esterification reaction.46

However, the use of homogeneous acid catalysts for esterification

causes difficulties in recovery after the reaction has taken place

and produces toxic wastewater.7 To overcome this shortcoming of

homogeneous catalysts, heterogeneous catalysts have been used

in esterification reactions in recent years,8 although the catalyticactivity of heterogeneous catalysts is often lower than that of

homogeneous catalysts.9

Heterogeneous acid catalysis by a HPA with a Keggin structure

is one of the most important and growing areas of research in

recent years because of its potential economic rewards and green

benefits.10,11 HPAs with Keggin structures possess special

characteristics that allow for their use as catalysts in esterification

reactions as they have a very high intrinsic acidity.12 The catalytic

acidity of HPAs is stronger than that of conventional solid acid

catalysts such as acidic oxides and zeolites. The acid strength of

Keggin HPAs decreases in the order: H3PW12O404H4SiW12O404

H3PMo12O404

H4SiMo12O40.13

In addition, the acid sites in HPAare more uniform and easier to control than those in other solid

acid catalysts.14 Usually, phosphotungstic acid (HPW) is the

catalyst of choice because of its stronger acidity, higher thermal

stability and lower oxidation potential compared to phos-

phomolybdic acid.15 However, due to the low surface area

and difficulty in the reutilization of the homogeneous tungsten

HPAs, it is advisable to support tungsten HPAs on a carrier with

a high surface area, such as silica, activated carbon, alumina or

clays.16Another alternative approach is to prepare HPA-salts by

partially exchanging protons of the parent HPAs with large

cations, such as K+ and Cs+, which have high porosity.9,15 Thus,

the HPA-salt not only has a high surface area but can also be

reused in the esterification of methyl 2-keto-L-gulonat.

In our present studies, potassium 12-phosphotungstate

(KPW) was prepared by the partially exchanging protons

process. The Keggin structure of KPW was examined by X-ray

Diffraction Spectra analysis (XRD). An automated BET sorpt-

ometer was used to study the increasing surface area of theHPA-salt compared with that of HPA. The surface area of

Amberlyst 15 was also characterized by a BET sorptometer.

The number of acid sites in the catalyst was examined by TPD-NH3.

Fourier Transform Infrared Spectroscopy (FTIR) was used to study

the Keggin structure of the KPW catalyst. The catalytic activity

of KPW was tested in the esterification of methyl 2-keto-L-gulonat.

The aim of the present work is to compare the KPW catalyst

performance with other known catalyst systems, such as the

homogeneous catalyst 12-phosphotungstic acid and heterogeneous

catalyst Amberlyst 15 on the esterification reaction.

2. Experimental2.1. Materials and catalysts

Monohydrous 2-keto-L-gulonic acid (90 wt%), 12-phospho-

tungstic acid (H3PW12O4021H2O) and methanol (99.5 wt%)

were purchased from Sigma Aldrich. Amberlyst 15 resin was

obtained from Rohm & Hass Co.

The acidic heteropoly salt (KPW) was prepared according to

the literature procedure18 by adding dropwise the required

amount of an aqueous solution of H3PW12O40 (0.1 M) to

an aqueous saturated solution of KCl at 40 1C with stirring.National Key Laboratory for Petrochemical and Refinery Technologies, Hanoi,

Vietnam. E-mail: [email protected]; Fax: +84 439335410; Tel:+84 422189067

Received 16th July 2012,

Accepted 25th October 2012

DOI: 10.1039/c2cy20497e

www.rsc.org/catalysis

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700 Catal. Sci. Technol., 2013, 3, 699--705 This journal is c The Royal Society of Chemistry 2013

The precipitate obtained was filtrated and washed with distilled

water and then dried at 70 1C for 10 hours and after that in an

oven at 120 1C for 5 hours.

2.2. Techniques

Powder X-ray diffraction (XRD) spectra of KPW and H3PW12O40(HPW) were recorded on a D8 Advance diffractometer (XRD) with

monochromatic CuKa

radiation using a Brucker Tensor 37.

The nature of the acid sites of these catalysts was deter-mined by NH3-TPD on an Autochem 2020 (micrometric) at the

temperature range of 100 1C to 600 1C at a rate of 10 1C min1,

using nitrogen as a gas carrier.

The thermal stability of the catalyst was examined by

thermogravimetric/differential thermal analysis (TG/DTA) using

a Perkin Elmer instrument. The samples were heated at a rate of

10 1C min1. The TG and DTA curves were recorded.

The BrunauerEmmettTeller (BET) surface area and porosity

of the catalysts were measured from the N2adsorption isotherm

at 77 K using a BET Sorptometer (Automated BET Sorptometer

201-A, USA).

Infrared spectra were recorded on a Brucker FTIR spectrometerwith samples prepared as KBr disks in the 4004000 cm1 range.

The yield of the esterification reaction was determined by

HPLC chromatography. HPLC was performed using a Model

1200 Agilent, a model UV (210 nm) and a C18 column.

The chemical composition of the KPW catalyst was analysed

using X-ray fluorescence, XRF (Model: Bruker S4 Pioneer, USA).

2.3. Catalytic reactions

The catalytic activity of KPW was tested in 2-keto-L-gulonic acid

esterification, which was carried out under atmospheric pres-

sure in a 250 ml bottom glass reactor. The 2-KLGA was mixed

with methanol at various molar ratios of 2-KLGA : methanol:

1 : 24, 1 : 48, 1 : 96, 1 : 192, 1: 384, 1: 720, 1: 1440 (mol : mol1).The mixtures were heated to 65 1C and then the catalyst was

added to the 2-KLGAmethanol solutions with the weight ratio

of catalyst to 2-KLGA of 1 : 10 (g : g1). The esterification reaction

time was started by charging the catalyst. The product of the

reaction was periodically taken out at 5 min, 15 min, 30 min,

60 min, 120 min, 180 min, 240 min, 300 min, 360 min and

420 min and analyzed by HPLC analysis.

Catalyst recycling experiments were performed as follows:

after the reaction, the heterogeneous KPW catalyst was recovered

from solution by simple filtration. The catalyst was washed with

copious amounts of methanol solvent. Then the catalyst was

reused in the reaction. The efficacy on the esterification reactionwhen using the KPW catalyst was determined after each recycling.

2.4. Scale up reaction

APPARATUS. The experiments were performed in a 2000 ml round

bottom glass reactor dipped in a constant temperature water bath.

The reactor was equipped with a temperature indicator (Pt-100)

and speed monitoring facility. It was also equipped with a

condenser to avoid any possible loss of methanol.

ESTERIFICATION REACTION. In the scaled up esterification reaction,

the molar ratio of 2-KLGA:methanol was 1:24 (mol mol1),

which is the highest concentration of 2-KLGA in methanol at

65 1C. A mixture solution was prepared by mixing 200 g of a

2-keto-L-gulonic acid solution with 1000 ml of a CH3OH solution.

The mixture was stirred at a rate of 500 r min1 and heated to

65 1C. 20 g of the catalyst was then added to the mixture. The

esterification reaction time was started by charging the catalyst.

The products of the reaction were periodically taken out at

5 min, 15 min, 30 min, 60 min, 120 min, 180 min, 240 min,

300 min, 360 min and 420 min. The products were diluted30 times by methanol and were analyzed by HPLC analysis.

3. Results and discussion

3.1. Chemical composition

The chemical composition of the KPW catalyst was determined

by XRF analysis (as shown in Table 1). The results confirm that

the potassium element is present in the catalyst structure. The

presence of potassium in the catalyst structure can change the

physicochemical properties of the catalyst from a homoge-

neous catalyst to a heterogeneous catalysts.15 From the results

in Table 1, we can calculate the content of potassium ions inKPW. The structure formula of KPW is K2.2H0.8PW12O40.

3.2. Catalyst characterization

3.2.1 TG/DTA ANALYSIS. The thermal stability of KPW was

investigated by means of thermogravimetric (TG) and differential

thermal analysis (DTA) (Fig. 1). The TG curve of the KPW catalyst

shows that there is one weight loss step which is attributed to

crystallization water desorption between room temperature to

150 1C. In the DTA curve, one endothermic peak was observed

at 70 1C which associated with the mass losses observed in the TG

curve. No endothermic peaks or weight losses were observed in

the DTA and TG analysis between 150600 1C. These resultsindicate that the KPW catalyst has a high thermal stability which

can be uses in the esterification reaction of 2-KLGA and methanol.

3.2.2 SURFACE AREA. Table 2 summarizes the results from the

BET surface area measurements of the KPW, HPW and Amberlyst

15 catalysts. The specific surface area of the KPW and HPW

catalysts are 96.37 m2 g1 and 11.55 m2 g1, respectively. The

KPW catalyst shows a relatively higher specific surface area

compared to the HPW catalyst. Specifically, the specific surface

area of the heteropoly acid was dramatically increased from

11.55 to 96.37 m2 g1 after the exchange process of protons and

Table 1The chemical composition of the KPW catalyst

Element Chemical composition (%)

W 94.6K 3.67P 1.04Se 0.20As 0.14Si 0.122Al 0.106Ge 590 (ppm)V 430 (ppm)

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potassium cations. This is attributed to the potassium cations of

the KPW, which improved the surface area of the catalyst.

3.2.3 FTIR. The FTIR spectra of the KPW catalyst (as shown

in Fig. 2) exhibits the characteristic frequencies of the Keggin

structure (as shown in Fig. 3) in the range of 1100600 cm1.

It is found that the KPW catalyst presents strong bands at1080.7 cm1 and 986.0 cm1, which are characteristic of the

POa and WOd absorption bands, respectively. The peaks at

891.3 cm1 and 808.0 cm1 are characteristic of WObW bridges

(inter bridges between corner-sharing octahedra) and WOcW

bridges (intra bridges between edge-sharing octahedra), respec-tively (as shown in Fig. 2).10,11,15 The results of the FTIR spectra

indicate that the Keggin structure is retained in the KPW catalyst.

Fig. 1TG/DTA of the KPW catalyst.

Table 2BET specific areas of HPW, KPW and Amberlyst 15 dry

KPW HPW Amberlyst 15 dry

SBET(m2 g1) 96.37 11.55 45.00

Spores(m2 g1) 43.07

Vpores(cm3 g1) 0.06

+pores(nm) 5.20 o2 25

Fig. 2FTIR spectra of the KPW catalyst.

Fig. 3Keggin structure.

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3.2.4 X-RAY DIFFRACTION. The Keggin structure of the catalyst

was also confirmed using XRD analysis. Fig. 4 shows the XRD

patterns of HPW (a) and KPW before use (b). The strong

reflections at 2y values of 19.81, 26.11, 29.51 and 36.21 were

assigned to the Keggin structure of the HPW catalyst (as shown

in Fig. 4(a)).17,18 The characteristic peaks of the KPW catalyst

were at 2y21.11, 26.21, 30.11, and 36.41, which was quite similar

to that of HPW, indicating that the Keggin structure was

retained in the KPW catalyst (as shown in Fig. 4(b)).19 A shiftin the reflections between the KPW and HPW catalysts was due

to the ion exchange between a potassium ion and a proton. This

behavior can be explained by the fact that the atom dimension

of the potassium ion is larger than that of a proton and

therefore caused the shift in the reflection.

3.2.5 ACIDITY MEASUREMENTS TPD OF NH3. The ammonia

adsorptiondesorption technique usually enables the determi-

nation of the strength of the acid sites present on the catalyst

surface together with the total acidity.20 Fig. 5 shows the NH3-

TPD profiles of HPW and KPW before use and Table 3 presents

the amount of NH3 desorbed per g. It was observed that the

number of strong Bronsted and Lewis acid sites of thesecatalysts (desorption peaks are distributed beyond 500 1C) is

more than that of the weak and intermediate acid sites of the

catalyst.5,21,22 In the catalyst, the number of strong acid sites is

about 3 and 2.5 times as high as the number of weak and

intermediate sites, respectively. It is evident from Table 3

that the total acidity for KPW is lower than that for HPW due

to the ion exchange between a potassium ion and proton in the

catalysts.

3.3 Catalyst performance

3.3.1 ELIMINATION OF MASS TRANSFER RESISTANCE. To evaluate the

external mass transfer resistance, the esterification reactionswere carried out at stirrer speeds from 50 to 500 r min1 while

keeping the rest of the reaction conditions the same. The

results are shown in Fig. 6. It was found that the speed of the

rotating rate had no effect on the esterification rate in the range

of 300500 r min1. All further experiments were performed at

500 r min1, at which external diffusion was eliminated.

In the concept of internal diffusion, Xu et al. demonstrated

that the internal diffusion can be considered negligible in

the esterification reaction of 2-KLGA when using the Amberlyst

catalyst with particle sizes of 0.7 mm at a stirrer speed of

500 r min1.23 The smaller the particle size, the lower the

effect of internal diffusion on the esterification reaction. In our

case, the size of the KPW powder catalyst is much smaller

than that of the Amberlyst catalyst. Therefore, the internal

diffusion can be considered negligible in the esterificationreaction of 2-KLGA.

Fig. 4Wide-angle XRD pattern of (a) HPW catalyst, (b) KPW catalyst before using.

Fig. 5NH3 temperature-programmed-desorption profiles of (a) KPW and

(b) HPW.


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3.3.2 EFFECT OF THE INITIAL REACTANT MOLAR RATIOS . 2-KLGA can

be dissolved in methanol at 65 1C with molar ratios of 2-KLGAto methanol up to 1 : 24 (mol : mol1). Therefore, the effect of

the molar ratios of 2-KLGA: methanol on the equilibrium yield

of the esterification reaction was investigated at various molar

ratios of 2-KLGA:methanol: 1:24, 1:48, 1:96, 1:192, 1:384,

1:720 and 1:1440 (mol:mol1). The results are presented in

Fig. 7. It was found that identical results were observed witheach fraction obtained. This indicated that the molar ratios of

2-KLGA : methanol have no effect on the equilibrium yield of

the esterification reaction with ratios up to 1 : 24 (mol mol1).

In addition, by using a large excess of alcohol, the products of

the esterification reaction can be directly pumped into the

column of the HPLC to determine the yield of the reaction

without diluting with methanol. Thus, further experiments

were performed at a 2-KLGA : methanol molar ratio of 1 : 1440

(mol:mol1).

3.3.3 CATALYTIC ACTIVITY STUDIES. The catalytic activity of the

heterogeneous KPW catalyst was demonstrated on the esterification

of 2-KLGA and compared with that of other known catalyst systems:

the homogeneous HPW catalyst and commercial heterogeneous

Table 3Data from the NH3 temperature-programmed-desorption results of the

catalysts

Temperature(1C)

Concentration(mL NH3 g

1)Concentration(mmol NH3g

1)

HPWPeak 1 175.9 1.0367 0.0525Peak 2 591.4 3.1022 0.1570

Total 4.1389 0.2095KPW before use

Peak 1 159 0.1808 0.0092Peak 2 297.9 0.0826 0.0042Peak 3 578.7 0.6447 0.0326

Total 0.9081 0.0460

Fig. 6The effect of the rotating rate on the yield of the esterification reaction

(the molar ratio of 2-KLGA: methanol of 1 : 1440 (mol mol1), reaction time:

180 min, 65 1C).

Fig. 7The yield of the esterification reaction at various molar ratios of 2-KLGA to

methanol.

Fig. 8The effect of different catalysts on the yield of esterification of 2-KLGA with

methanol: (a) HPW catalyst, (b) KPW catalyst, (c) Amberlyst catalyst (the weight

ratio of catalyst to 2-KLGA is 1 : 10 (g : g1)).

Fig. 9Evaluation the catalytic heterogeneity of the KPW catalyst: (a) the

esterification reaction when using the KPW catalyst, (b) the esterification reaction

with the filtrate of the KPW catalyst after 120 min.


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Amberlyst 15 catalyst. The yield of the esterification reaction

when using different catalysts is presented in Fig. 8. From a

comparison of the results between line (a) and line (b) in Fig. 8,

the yield of the esterification reaction using the KPW catalyst is

slightly lower than that of the esterification reaction using

the homogeneous HPW catalyst. In return, the heterogeneous

process is an environmentally friendly process and greatly

minimizes the steps needed for the separation and recovery of

the product.5,14,24,25 In a comparison of the results between line(b) and line (c) in Fig. 8, the yield of the esterification reaction

using the commercial heterogeneous KPW catalyst is slight

higher than that of the reaction using the heterogeneous

Amberlyst catalyst. Thus, due to the useful properties of the

KPW catalyst, it can be regarded as a good heterogeneous

catalyst for the esterification of 2-KLGA.

3.3.4 HETEROGENEITY CATALYST. In order to prove that the

esterification reaction of 2-KLGA using KPW catalysts is truly

a heterogeneous process, we investigated the yield of the

esterification reaction with and without filtration of the KPW

catalyst. Fig. 9(a) shows that the yield of the esterification

reaction using the KPW catalyst depends on time. Another

esterification reaction of 2-KLGA was carried out with the

KPW catalyst at 120 minutes. The KPW catalyst was filtered

out of the reaction mixture and then the reaction was resumed

with the filtrate, in the absence of any externally added catalyst.

The results on the yield of the reaction are shown in Fig. 9(b).

From Fig. 9(a) it was observed that the yield of the reaction

using the KPW catalyst increases with increasing time. Fromthe result in Fig. 9(b), the yield of the reaction also tended to

increase with increasing time from 0 to 120 min. There was no

increase in the yield of the esterification after filtration of the

KPW catalyst from 120 to 300 min. These results prove that the

KPW catalyst is a real heterogeneous catalyst.

3.3.5 CATALYST RECYCLING. To investigate the catalyst recycling,

the KPW catalyst, used in the first cycle of the esterification

reaction for 300 min, was separated by filtration, washed with

methanol and dried 100 1C for 3 h and returned to a fresh

reaction mixture under identical conditions as the first cycle.

The reaction was repeated to check the second and the third

cycle of the KPW catalyst. The results of initial rates of ester-ification are shown in Fig. 10. There was no difference in the

initial rate of the esterification reaction between each cycle,

meaning that the KPW catalysts possess high stability. Thus,

the KPW catalyst can be reuses in the esterification reaction of

2-KLGA.

Fig. 11 shows the XRD patterns of the KPW catalyst after

recycling 3 times. It was found that the characteristic peaks of

KPW after recycling 3 times also tended to be the same as those

of the fresh KPW. There was no difference in the reflections

between the KPW catalyst before and after recycling 3 times,

indicated that the KPW catalyst can be recycled in the ester-

ification reaction of 2-KLGA.

3.3.6 SCALE UP ESTERIFICATION REACTION. Fig. 12 shows the yieldof the esterification reaction with and without scale up. From a

comparison of the results between line (a) and line (b) in Fig. 12,Fig. 10The effect of time on the initial rate of esterification in three cycles.

Fig. 11XRD pattern of the KPW catalyst after recycling 3 times.


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the equilibrium yield of the scaled up reaction is 96% at 360

min, which is quite similar to that of the esterification reaction

without scale up, indicating that with scale up, the esterificationreaction did not affect the equilibrium condition of the reaction.

4. Conclusion

The KPW catalyst was successfully synthesised by an ion exchange

process between a potassium ion and proton. It was found that

the KPW catalyst shows a high efficiency for the esterification of

2-KLGA. The KPW catalyst is a real heterogeneous catalysts in the

esterification reaction. The catalytic activity of the heterogeneous

KPW catalyst is slightly lower than that of the homogeneous

HPW catalyst and is approximate to that of the commercial

heterogeneous Amberlyst catalyst. The KPW catalysts are effective,environmentally friendly and recyclable. The present procedure

also represents a clean, practical, simple and mild method for the

esterification of 2-KLGA with excellent equilibrium yields of

approximately 96% at 360 min.

Acknowledgements

This work was supported by the National Key Programs on

Research of the sciences and technology for developing the

pharmaceutical industry to 2020 of the Ministry of Industry and

Trade, Vietnam.

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Fig. 12Yield of the esterification reaction using the KPW catalyst with and

without scale up: (a) with scale up; (b) without scale up.


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