Process for Recycling Tungsten from the Leach Solution of Organic

Article pubs.acs.org/IECR

Process for Recycling Tungsten from the Leach Solution of Organic Oxidation Reaction Systems Changming Ye,†,‡ Peng Jin,†,§ Junxia Liu,† Yiqiang Wen,† Huijuan Wei,† Xiaoguang Zheng,*,§ Xiangyu Wang,*,† and Baojun Li*,† †

Institute of Industrial Catalysis, College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, People’s Republic of China ‡ Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, People’s Republic of China § China Pingmei Shenma Group, Pingdingshan 467000, People’s Republic of China ABSTRACT: Recovering tungsten from the leach solution of adipic acid (ADA) synthesis by oxidizing cyclohexene over tungsten peroxide catalyst is a problem. Herein, a novel process is explored to recover tungsten and recycle catalyst in this production process. The reaction mother liquor of ADA synthesis was treated with triethylamine, and the tungsten peroxide decomposed to tungsten. The acidification of mother liquor with HNO3 leads to precipitation of tungsten in the form of H2WO4. The component of recovered reagent was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. A tungsten recovery of 97% was achieved. The regenerated catalyst was prepared from recovered tungsten. The regenerated catalyst provided a yield of ADA of 94.1%, which is comparable to the 94.5% yield of ADA with fresh catalyst. The effective recovery of tungsten and the excellent performance of regenerated catalyst show that an effective recovery approach has been developed.

1. INTRODUCTION Due to their excellent properties in environmental protection and other applications,1−6 tungsten-based catalysts have been a star in green chemistry, especially in green catalytic oxidation with H2O2.7−11 Aqueous H2O2 is considered to be an ideal clean and low cost oxidant, and it often can provide a simple synthetic route, high conversion, and selectivity. Adipic acid (ADA) is conventionally prepared by the catalytic oxidation of cyclohexanol or cyclohexanone with nitric acid.12−14 The N2O released from this process causes serious pollution.15,16 In the synthesis of ADA by oxidizing cyclohexene with H2O2, tungsten compounds show very high H2O2 utilization and selectivity of the target products.17−22 Our group has focused on the synthesis of ADA over homogeneous tungsten peroxide catalyst under organic solvent and phase transfer catalyst free conditions, which has steadily approached industrialization.23−27 The activity of homogeneous tungsten peroxide catalyst decreases seriously after several cycles due to the structural change of the catalyst and the accumulation of impurities in the reaction system. In order to maintain the oxidation reaction, tungsten catalyst has to be recovered and regenerated. In addition, tungsten resources are expensive. With the rapid growth of tungsten consumption, the world’s industry is doomed to face a severe shortage of original tungsten. Tungsten recovering and recycling have become a problem for the development of economical and environmentally benign production processes. Recovering tungsten from solid wastes and catalysts has been researched substantially.28−32 A few research studies also have been carried out on recovering tungsten from homogeneous catalysts. One example is the recovery of ammonium paratungstate from mother liquor by crystallization, which is © 2013 American Chemical Society

an important intermediate in the tungsten smelting process. Normal recovery methods are listed as follows. 1. Precipitation method: The mother liquor is adjusted to alkaline conditions, and then the tungstate is precipitated in the form of CaWO4, followed by conversion of CaWO4 to H2WO4. Finally, the H2WO4 is further decomposed to obtain tungsten oxide. This process is complex, and the recovery efficiency depends mainly on the pH of the tungsten solution.33 2. Anion exchange resin method: The mother liquor is first acidified and then treated with a weak basic anion exchange resin. Subsequently, the resin is eluted using alkali solutions to obtain tungstate solution. Finally, evaporative crystallization and purification are used to get tungstate. This method requires large consumptions of acid and alkali and the long technological line.34 3. Nano- and ultrafiltration membranes driven under pressure:35 Due to the fragility of membranes in strong oxidative, acid environment with organic impurities, the industrial scale application of filtration membranes in this system still faces lifetime and cost limitations. Other approaches have also been reported for tungsten recovery. Latha and co-workers have investigated the electrochemical method for the recovery of tungsten.36 Misra et al. have researched the activated carbon adsorption−desorption method.37 The two methods are suitable for the cases with low organic impurities.36,37 A Japanese patent has shown a method of pumping a gas (nitrogen, etc.) to precipitate tungsten in the Received: Revised: Accepted: Published: 3600

July 12, 2012 February 3, 2013 February 11, 2013 February 11, 2013 dx.doi.org/10.1021/ie301856s | Ind. Eng. Chem. Res. 2013, 52, 3600−3606

Industrial & Engineering Chemistry Research

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form of tungstenic acid followed by separation.38 The recycling of homogeneous catalysts by reaction control phase transfer catalysis have been developed.39−43 Some of these methods exploit the preference of a catalyst for one of two solvents with thermoregulated miscibility;44,45 others exploit a dramatic decrease in catalyst solubility as one reagent is consumed,46,47 or the temperature changed after the reaction.48−52 The synthesis of ADA in our system is carried out under organic solvent and phase transfer catalyst free conditions, which are of low cost and cleaner. After reaction completion, the ADA crystal was separated from the aqueous reaction solution by filtration when cooled to 5−10 °C. The leach fluid was still an aqueous solution after several days, and the tungsten catalyst was also water soluble. When N2 gas was pumped into the leach fluid under heating conditions for a long time, the tungsten recovery rate was not high and more tungstic acid precipitate was attached to the wall of the device and was too small to remove easily. This showed the high stability of our catalyst system, and it also raised the difficulty of catalyst recovery and recycling. The main difficult point is achieving decomposition of homogeneous tungsten peroxide catalyst to precipitate tungsten. The solubility and activity of regenerated catalyst in H2O2 oxidation reaction must be retained. In this article, the pH-controlled method was utilized to precipitate tungsten. A process was developed for recovering tungsten from the leach solution of ADA synthesis by oxidizing cyclohexene with H2O2 over homogeneous tungsten peroxide catalyst. High tungsten recovery can be obtained easily. The regenerated catalyst was prepared from recovered tungsten and employed for ADA synthesis with catalytic activity comparable to that of fresh catalyst.

condition and washing; tungsten was recovered (Figure 1). The tungsten recovery was measured by the following steps. The

Figure 1. Process of tungsten recovery and analysis.

precipitate and filter paper were put into a crucible to be dried at 180 °C for 2 h, and then ashed in a muffle furnace over low heat. Subsequently, the obtained ash was calcined at 750 °C for 2 h. After cooling, tungsten was recovered in the form of WO3 and weighed. The tungsten concentration of the reaction mother liquor was measured by an inductively coupled plasma emission spectrometer and then converted into WO 3 concentration, and the tungsten recovery was calculated according to eq 1. The reproducibility of the recovery data in this research is excellent with an error less than 1%. tungsten recovery =

real mass of obtained WO3 theoretical mass of obtained WO3 ·100%

(1)

The concentration of tungsten contained in the reaction mother liquor could be directly analyzed by an inductively coupled plasma atomic emission spectrometry (ICP-AES; Thermo Scientific iCAP6000, Thermo Fisher) in Ar support gas. Calcinated products (WO3) were dissolved into NaOH solution. The formed dissoluble WO42− was analyzed with liquid state sampling by ICP-AES. X-ray diffracton (XRD) patterns of the recovered precipitates and calcination products were recorded on a X’PertPRO diffractometer (PANalytical Corp.) using Cu Kα radiation in the 2θ range 10−70°, using a scan rate of 6 deg min−1. Fourier transform infrared (FT-IR) spectra of the calcinated products were recorded on a Nicolet 380 FT-IR spectrophotometer (Thermoelectron Corp.) with a resolution of 2 cm−1. The average diameters of recovered tungsten components (H2WO4) were analyzed on a laser particle size distribution instrument (Rise-2002, Jinan, China) in an ethanol dispersion medium. The recovered tungsten was regenerated to catalyze ADA synthesis. The catalytic reaction tests were similar to those in earlier reports from our group.23,26,27 Under the condition of organic solvent and phase transfer catalysts, all free, Na2WO4·2H2O (or H2WO4 or WO3), sulfuric acid, and 30% H2O2 (the molar ratio was 1:1.3:218; H2O2 was added at one time or in batches) were introduced in a round-bottomed flask (500 mL) equipped with a magnetic stirring bar and a reflux condenser. The mixtures were stirred for 30 min to prepare in situ the peroxytungstate catalyst. Then cyclohexene (the total molar ratio to H2O2 was 1:4.36, added in one time or batches) was added and the system was heated to reflux. After reflux, the reaction was stirred for 6 h at 80−95 °C. The resulting hot solution was used for ADA yield determination by high performance liquid chromatography (HPLC). HPLC analyses were carried out using an Agilent 1100 HPLC system with a quaternary pump, a UV detector, and a refractive index detector. HPLC separations were carried out on an Agilent TC-C18 reversed phase column

2. EXPERIMENTAL SECTION After the reaction of synthesis of ADA, the homogeneous solution was allowed to stand at 0 °C for 12 h, and the resulting white precipitate was separated by filtration and washed with cold water (20 mL). The ADA crystal was separated from the pilot test solution of ADA synthesis by filtration. The leach fluid was called the reaction mother liquor, which contained 2−4 mg mL−1 tungsten. The reaction mother liquor (120 mL) was introduced in a round-bottomed flask (three neck, 250 mL) equipped with a magnetic stirring bar and a reflux condenser. The alkaline reagent (or alkaline solution; the molar ratio of alkaline substance to tungsten was 3.8:1) was added dropwise into the reaction mother liquor with stirring to adjust the system to near neutral. The system was stirred at 30 °C for 2 h to decompose the peroxide compound. The alkaline reagent was mainly an organic base, such as aniline, diethylamine, dimethylamine, trimethylamine, triethylamine, tributylamine, ethylenediamine, propylenediamine, and butanediamine. After treating, the system pH was 5−7. The alkaline solution was mainly an inorganic base, such as NaOH (8 mol L−1), KOH (8 mol L−1), and NH3·H2O (14 mol L−1). After treating, the system pH was 7−10. The treated mother liquor was put into a dropping funnel, and then added dropwise into acid solution (CH3COOH, HCl, (1 + 1) H2SO4, and HNO3; the molar ratio of [H]+ to tungsten was 17.1:1) with stirring at 80 °C after acid solution was introduced in a round-bottomed flask (250 mL), for precipitating tungsten in the form of H2WO4. The system became turbid gradually with the addition of the treated mother liquor. Over addition, the system continued to be heated for 30 min. The system was cooled, and then the mixtures were filtered with slow quantitative filter paper under atmospheric 3601

dx.doi.org/10.1021/ie301856s | Ind. Eng. Chem. Res. 2013, 52, 3600−3606


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Figure 2. (a) Effects of various inorganic bases on tungsten recovery rates. (b) Effects of various organic bases on tungsten recovery rates: (A) aniline, (B) diethylamine, (C) dimethylamine, (D) trimethylamine, (E) triethylamine, (F) tributylamine, (G) ethylenediamine, (H) propylenediamine, and (I) butanediamine. For 200 mL of reaction mother liquor in a 500 mL round-bottomed flask, the molar ratio of base to tungsten was 2:1, 2 h with stirring, the organic base was at 30 °C, and the inorganic base was at 50 °C. In the acidification process, the treated reaction mother liquor was added dropwise into a nitric acid solution at 90 °C for 1 h with stirring; the molar ratio of nitric acid to tungsten was 17.1:1.

(25 cm × 4.6 mm, 5 μm particle size) using a 30% methanol in 5 mM ammonium acetate buffer at pH 3.3 and 30 °C. UV data at 225 nm were collected.

liquor with inorganic base and then acidification. It is easy to form a colloid, and has less tendency to precipitate, especially in the leach solution of an organic oxidation reaction containing a lot of soluble organic impurities. In the acidification processes of reaction mother liquors, the color of the turbid liquid was light yellow and the turbidity was low. Tungstic acid precipitate was sticky with an average particle diameter of 1.8 μm, and was not easy to filter and wash. The effects of various organic bases containing primary amine, secondary amine, tertiary amine, and diamine on tungsten recovery rates are demonstrated in Figure 2b. Monoamines from primary amine to tertiary amine show positive effects on tungsten recovery rates, most of which exceed 93%. Tungsten recovery rates obtained with monoamines from primary amine to tertiary amine gradually increase, and the highest one reaches 99.9%. With the growth in the length of carbon chains of aliphatic amines, the tungsten recovery rates rise from 93.3 to 96.1% for secondary amine and from 95.0 to 97.7% for tertiary amine, respectively. The tungsten recovery rates rise from 23.5 to 97.1% with the growth of carbon chains of diamines. When triethylamine or tributylamine was added into the reaction mother liquor, the tungsten recovery rate was very high; the obtained deposit turned yellow and became big grained with an average particle diameter of 2.4 μm (the average particle diameter was 8.8 μm with H2O as dispersion medium). Treatment with organic base could effectively decrease the formation of colloid containing tungsten which was water soluble. More tungsten was transformed into precipitate, which was convenient to filter and wash. Considering the performance in the tungsten recovery rate, processing cost, and other factors, triethylamine can be considered a good choice as an alkaline reagent for treating the reaction mother liquor. 3.3. Effects of Treating Time and Temperature with Triethylamine. An orthogonal experiment about the effects of the treating time and temperature of reaction mother liquors with triethylamine on tungsten recovery rates was designed. As listed in Table 1, all tungsten recovery rates were ≥97.0%; the influences of the treating temperature (30−50 °C) and treating time (2−5 h) on the tungsten recovery rate are not remarkable. Relatively speaking, the condition of treating the reaction mother liquor with triethylamine at 30 °C for 2 h provides better results. Within a certain range, a shorter treating time provides a better recovery ratio, similarly to the treating temperature. Because triethylamine is volatile, with the increase

3. RESULTS AND DISCUSSION 3.1. Design of Experiments. The catalyst used in the synthesis process of ADA is a tungsten peroxide compound.23,26,27 Based on the nature of tungsten peroxide compounds in the reaction mother liquor, a program containing alkaline treatment followed by precipitation with acid has been designed for recycling tungsten. In the first step, alkaline reagent is added into the reaction mother liquor to decompose the tungsten peroxide compound, and then the tungsten peroxide will be transformed into WO42−. Therefore, the suitable alkaline reagent and treatment condition need to be explored. In the second step, acid reagent is added to react with WO42− to form tungstic acid precipitate and then separation by filtering follows. The suitable acid reagent and treatment conditions also need to be studied. Under the conditions of high H2O2 concentration and strong acidity, the structure of the tungsten catalyst may be [WO2(O2)2]2−, [W2O3(O2)4]2−, and so on.53−55 The reaction mother liquor containing tungsten catalyst was still an aqueous solution stably after several days. As we know, in the presence of alkaline, the decomposition of tungsten peroxide (eq 2) could be accelerated and the balance is moving toward the right. [W2O3(O2 )4 ]2 − + 2OH− → 2WO4 2 − + 2O2 + H 2O (2)

Thus alkaline reagents were added into the reaction mother liquor to accelerate the decomposition of peroxotungstate for reducing the concentration of peroxide tungsten. Then the structure of the tungsten peroxide catalyst would be disintegrated toward generating WO42−. After that, acid reagents were added to convert WO42− into H2WO4 (eq 3), which was precipitated in acidic aqueous solution. WO4 2 − + 2H+ → H 2WO4

(3)

3.2. Effects of Different Types of Alkaline Reagents. Figure 2a shows the effects of various inorganic bases used for the treatment of reaction mother liquors on tungsten recovery rates. All are between 63.2 and 80.7% and less than 90.0%. The tungstic acid was generated by disposal of the reaction mother 3602



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Table 1. Orthogonal Results of Treating with Triethylaminea

Table 3. Effects of Acid Reagents on Acidification of Reaction Mother Liquorsa

entry

treat. time (h)

treat. temp (°C)

W recovery rate (%)

reagent

[H]+:W (mol/mol)

W recovery rate (%)

1 2 3 4 range

2 2 5 5 0.9

30 50 30 50 0.6

97.7 97.2 97.4 97.0

CH3COOH HCl (1 + 1) H2SO4 HNO3

17.1:1 17.1:1 17.1:1 17.1:1

37.4 85.0 94.4 97.0

a

For 200 mL of reaction mother liquor in a 500 mL round-bottomed flask, the reaction mother liquor was treated with triethylamine at 30 °C for 2 h with stirring; the molar ratio of triethylamine to tungsten was 1.7:1. In the acidification process, the treated reaction mother liquor was added dropwise into acid solution at 90 °C in 1 h with stirring.

a

For 200 mL of reaction mother liquor in a 500 mL round-bottomed flask, the molar ratio of triethylamine to tungsten was 1.7:1, with stirring. In the acidification process, the treated reaction mother liquor was added dropwise into a nitric acid solution at 90 °C in 1 h with stirring; the molar ratio of nitric acid to tungsten was 17.1:1.

to embed into the sediment, and it is difficult to remove entirely. Hence, nitric acid is suitable for acidification of reaction mother liquor. 3.6. Effects of Nitric Acid. Table 4 indicates that amount of nitric acid has a clear effect on the tungsten recovery rate.

of treating temperature and the extension of treating time, the utilization rate of triethylamine decreases and the tungsten recovery rate also decreases. However, if the treating temperature was too low or the treating time was too short, the reaction would slow down and reduce the tungsten recovery rate remarkably. Therefore, the optimal treating condition of reaction mother liquors with triethylamine is at 30 °C in 2 h. 3.4. Optimization of Triethylamine Amount. The optimized results of the triethylamine amount are listed in Table 2. Table 2 demonstrates that tungsten recovery rates

Table 4. Effect of Nitric Acid on Tungsten Recovery Ratea

Table 2. Effect of Triethylamine on Tungsten Recovery Ratea triethylamine:W (mol ratio)

W recovery rate (%)

0 0.40:1 0.90:1 1.7:1 3.8:1 7.2:1 8.9:1

65.5 95.1 95.6 97.7 99.0 99.4 99.9

[H]+:W (mol/mol)

W recovery rate (%)

precipitate color

0 4.79 6.84 12.0 17.1 25.7 34.1

38.9 45.5 61.3 94.0 97.0 97.7 97.4

pale yellow pale yellow yellow yellow yellow yellow yellow

a

For 200 mL of reaction mother liquor in a 500 mL round-bottomed flask, the reaction mother liquor was treated with triethylamine at 30 °C for 2 h with stirring; the molar ratio of triethylamine to tungsten was 1.7:1. In the acidification process, the treated reaction mother liquor was added dropwise into acid solution at 90 °C for 1 h with stirring.

a

For 200 mL of reaction mother liquor in a 500 mL round-bottomed flask, the reaction mother liquor was treated with triethylamine at 30 °C for 2 h with stirring. In the acidification process, the treated reaction mother liquor was added dropwise into a nitric acid solution at 90 °C for 1 h with stirring; the molar ratio of nitric acid to tungsten was 17.1:1.

With increasing application amount of nitric acid, tungsten recovery rates increase from 38.9 to 97.7%. The acidification benefits from the form of the tungstic acid colloid. Tungstic acid colloid with negative charge has less tendency to precipitate. The leach solution of the organic oxidation reaction also contained a lot of soluble organic impurities. More proton acid could reduce the formation of tungstic acid colloid, in order to achieve the acid−base solution neutralization reaction to generate precipitate. Under low acidity condition, the deposits are often white, sticky, and hard to filter. The deposits are usually yellow and have big particle sizes at higher acidity condition, so they are readily filtered and washed. The appropriate molar ratio of [H]+ to tungsten is 17.1:1, and with more addition of nitric acid, the tungsten recovery rate increases very slowly. The concentrated acidic mother liquor could be used again for acidification of the next batch of reaction mother liquor in order to avoid the rest of tungsten loss and decrease the consumption of nitric acid. 3.7. X-ray Diffraction (XRD). Figure 3 shows XRD patterns of calcined product samples of precipitation obtained by treating mother liquor with triethylamine and ammonia, respectively. As evidenced by these XRD patterns, the crystal structures of the two samples are consistent with that of WO3 (reference pattern: tungsten oxide, PDF No. 83-0951). The sharp peaks of the samples suggest large crystal size and high crystallinity.

increase from 65.5 to 99.9% with increasing the amount of triethylamine. The particle sizes of the obtained precipitate become larger visually. With further increase of triethylamine amount after that ratio, the tungsten recovery rate increases very slowly. Therefore, the appropriate molar ratio of triethylamine to tungsten is 1.7:1. 3.5. Effects of Acid Reagents. The effects of different types of acid reagents on acidification of reaction mother liquors are shown in Table 3. Nitric acid exhibits the best performance on the tungsten recovery rate (97.0%), and acetate acid provides the worst result (37.4%). Light blue precipitate was obtained when hydrochloric acid was used for acidification of reaction mother liquor. This is probably because hydrochloric acid does not have oxidizability, and the tungsten component is easy to reduce and not conducive to regeneration in the reaction system. Under the same conditions, acidification by sulfuric acid more easily formed a colloid and could obtain a stickier precipitate which was more inconvenient to filter and wash than that obtained with the use of nitric acid. Sulfuric acid is hard to break down completely at high temperature, it is easy 3603



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presence of tungsten-based catalyst with hydrogen peroxide as oxidant, such as the Noyori system.8

4. CONCLUSIONS In conclusion, an approach has been developed to recover tungsten catalyst active ingredient from mother liquor generated by the synthesis of ADA. During the treatment of mother liquor with alkali, the different types of alkalis influence tungsten recovery. The recovery is higher using organic alkalis than using inorganic alkalis. Triethylamine is selected as the most suitable alkali, and the optimum usage molar ratio of triethylamine to tungsten is 1.7:1. When nitric acid is added into alkali treatment solution to precipitate tungstic acid, the optimized molar ratio of H+ to tungsten is 17.1:1. Under optimized condition, a tungsten recovery of over 97% is obtained. Moreover, the catalytic performance of regenerated catalyst with recovered tungsten is almost the same as that of fresh catalyst. This is very important for the economical and clean industrialization of the synthesis of ADA in the near future. This method provides an promising route for the recovery and recycling of tungsten in the organic oxidation system because of its high recovery rate, easy operation, and low cost.

Figure 3. XRD patterns of calcined product samples: (a) calcined product of precipitation obtained by treating mother liquor with triethylamine; (b) calcined product of precipitation obtained by treating mother liquor with ammonia.

3.8. FT-IR. FT-IR spectra of calcined product samples of recovered tungsten component are shown in Figure 4. The FT-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.W.); [email protected] (B.L.); [email protected] (X.Z.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the Innovation Fund for Elitists of Henan Province, China (No. 0221001200), the Talent Training Unite Fund of NSFC-Henan (No. U1204203), and the China Postdoctoral Science Foundation (No. 2012M511121) is acknowledged.

Figure 4. FT-IR spectra of calcined product samples: (a) calcined product of precipitation obtained by treating mother liquor with triethylamine; (b) calcined product of precipitation obtained by treating mother liquor with ammonia.

■

IR spectra of the two samples are consistent with that of WO3.56 The characteristic band between 600 and 900 cm−1 is assigned to the stretching vibration of the W−O bond in the WO6 octahedron. The calcined product of the recovered tungsten component is basically considered to be WO3. 3.9. Catalytic Performance of Regenerated Catalyst. The yields of ADA by the regenerated catalyst prepared from the recovered tungsten component (tungsten acid) and fresh catalyst are shown in Table 5. The catalytic performance of regenerated catalyst is also excellent and is nearly the same as that of fresh catalyst. The recovered tungsten component from the solution of ADA synthesis is effective. It could be predicted that this recovery approach is effective for other similar reaction systems in the Table 5. Catalytic Performances of Fresh and Regenerated Catalystsa catalyst

ADA yield (%)

fresh regenerated

94.5 94.1

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