Highly efficient metal salt catalyst for the esterification of biomass derived levulinic acid under microwave irradiation

Abstract

The esterification of levulinic acid (LA) to alkyl levulinates has been investigated in the presence of various metal salt catalysts under microwave irradiation. The reaction obtained 99.4% yield of methyl levulinate (ML) in the presence of Al₂(SO₄)₃ catalyst in methanol solution under microwave conditions. The optimized reaction conditions were 110 °C and 10 minutes with a 20 mol% catalyst loading. Alcohols with longer carbon chains showed lower reactivities in the microwave electromagnetic field due to their poorer abilities to absorb and transmit microwave energy. Moreover, microwave irradiation provided a significantly higher reaction rate compared to conventional oil bath heating. LA aqueous solution was also converted to ML with high yields. The Al₂(SO₄)₃ catalyst was successfully applied to the esterification of other biomass derived organic acids to their corresponding esters in high yields. Finally, the catalyst was recycled 5 times without much decrease in activity.

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Introduction

The production of fuels and chemicals from renewable lignocellulosic feedstocks has been considered as a sustainable way to replace the traditional fossil resource based industrial systems. Among the various chemicals produced from biomass, levulinic acid (LA) and its esters are gaining wide attention as versatile platform chemicals, due to their potential applications as biofuel, polymer monomers and pharmaceutical intermediates.¹ Levulinic acid can be produced by the acid hydrolysis of (ligno) cellulose and it can be further converted into a range of important derivatives such as levulinate esters,² γ-valerolactone³ and diphenolic acid.⁴ In particular, levulinate esters, especially ethyl levulinate (EL), can be a diesel miscible fuel up to 5 wt% in regular car engines.⁵ Therefore, the esterification of levulinic acid with different alcohols to produce alkyl levulinates is highly desirable.

In general, the esterification of LA with alcohols can be accomplished in the presence of Brønsted acid catalysts such as H₂SO₄.⁶ However, the use of liquid mineral acids always suffer from corrosion and disposal problems. On the contrary, heterogeneous catalysts are more favourable due to their easier separability and recyclability. Sulfated oxides,⁷ heteropolyacids,⁸ zeolites⁹ and sulfonic resins⁵ have been widely used as acid catalysts for esterification reactions. For example, Kuwahara et al.^7a used sulfated Zr–SBA-15 to catalyze the esterification of levulinic acid in ethanol and 80% yield of ethyl levulinate (EL) was obtained at 70 °C for 24 h. Baronetti et al.^8c employed silica-supported Wells–Dawson heteropolyacid (H₃PW₁₂O₄₀/ZSM-5) as the catalyst to synthesize ethyl levulinate at 78 °C. They obtained 76% conversion of LA within 5 h. These studies indicate that the acid strength and the dispersibility of the acid sites have a big impact on the overall activity of the catalysts. Recently, Fernandes et al.⁵ used several different types of zeolites, sulfated oxides and commercially available resins in the esterification reaction. They found that Amberlyst-15 showed the highest activity. Sulfonated carbon nanotubes have also been employed in the esterification of LA and achieved moderate to good product yields.¹⁰

On the other hand, multivalent metal salts, such as CrCl₃, SnCl₄, ZnCl₂, and AlCl₃, have been widely used in the conversion of carbohydrates (e.g. cellulose, glucose and fructose) into platform molecules (e.g. HMF) through their Lewis acidity and Brønsted acidity.¹¹ The catalytic effect of these metal salts can be compared to that of homogeneous or heterogeneous acid catalysts under specific reaction conditions. Especially, the Brønsted acidic site can be generated through the hydrolysis/alcoholysis of metal ions in solvents at higher reaction temperatures.¹² Apart from that, the metal ions of these salts are also preferable catalytic sites for esterification reactions. Combing these two advantages, it is appealing to establish a new catalytic system for the esterification of LA over metal salts catalysts, especially the earth-abundant metal salts.

Herein, we demonstrate that some inexpensive metal salts show outstanding activity, efficiency and stability in the esterification of LA with different primary alcohols. Microwave irradiation, which is an efficient and green technology for biomass conversion, has also been employed in the model reaction to accelerate the reaction rate. Among all the metal salts we have tested, the Al₂(SO₄)₃ catalyst obtained the highest yield of methyl levulinate (99.4%). In addition, heating from microwave irradiation dramatically increases the reaction rate compared to conventional oil bath heating. Finally, the effects of catalyst loading, reaction time, temperature, and reusability of catalyst on the esterification of LA were also investigated.

Experimental

Materials

Levulinic acid (97%), methyl levulinate (99%), ethyl levulinate (98%) and butyl levulinate (98%) and were obtained from TCI Chemicals Co. Ltd (Shanghai China). Amberlyst-15 was purchased from Alfa Aesar (H⁺, ∼4.8 mmol g⁻¹). Alcohols and Al₂(SO₄)₃·18H₂O and other metal salts of analytical grade were obtained from Adamas-beta Chemical Reagent Co., Ltd (Shanghai China), and used without further purification.

Esterification of levulinic acid

Esterification of levulinic acid was carried out in a MILESTONE Ethos A microwave reactor (MA039). In brief, levulinic acid (116 mg, 1 mmol), MeOH (14 mL), and a given amount of catalyst were loaded into a 100 mL sealed Teflon tube and the tube was then heated in the microwave reactor to the desired temperature. Zero time was defined as the time when the reactor was heated to the set temperature. Then, the reaction was performed for a given reaction time. After the reaction was finished, the reactor was cooled down to room temperature. All experiments were performed in duplicates and the average values reported.

For the conventional oil heating reaction, a Parr reactor with a temperature probe was used and the reactor was placed in an oil bath, and zero time was defined as the time when the reaction mixture was heated to the desired reaction temperature. After the reaction was finished, the rector was quickly cooled down in an ice bath.

Analytic methods

Analyses of the reaction products were conducted using a GC system (Agilent, 7890A) equipped with a flame ionization detector (FID) and a DB-5 capillary column (30 m × 0.25 mm × 0.25 μm, Agilent). The injection port and the detector were operated at 250 °C. The temperature of the column was programmed to increase from 80 °C to 200 °C at a rate of 10 °C min⁻¹. Naphthalene was used as the internal standard to calculate the yield of the reaction products.

Recycle and reuse of catalyst

Recycle of the catalyst was as follows: methanol and methyl levulinate were removed from the reaction mixture by rotary evaporation. The catalyst was then washed with dichloromethane (10 mL × 3). Residual dichloromethane was then evaporated. The dried catalyst was directly dissolved in MeOH and transferred to the reactor.

Measurement of dielectric properties

Measurement of the dielectric properties of different alcohols was carried out on an Agilent E5071C Vector Network Analyzer with an Agilent dielectric probe 85070E. The sample was measured at room temperature and the frequency was 2.45 GHz.

Results and discussion

Esterification of LA with different catalysts

Initially, esterification of LA was carried out with different metal salt catalysts in methanol under microwave irradiation and the results are presented in Table 1. A blank experiment was firstly carried out and only a trace amount of methyl levulinate (ML) was detected, which suggests that the LA esterification reaction cannot proceed without any catalyst (Table 1, entry 1). When CrCl₃ was used as the catalyst, the esterification reaction afforded 91.5% yield of ML with 96.9% LA conversion at 110 °C for 10 minutes. This result suggested that CrCl₃ is a preferable metal salt, which has both good Lewis and Brønsted acidity in MeOH. A much lower ML yield was observed when Cr₂(SO₄)₃ was used, which is due to its poor solubility in methanol. A similar result was obtained for SnSO₄. On the other hand, the soluble SnCl₄ obtained a much higher ML yield (96.8%). This result is consistent with many previous literature, where Sn⁴⁺ was a good Lewis acidic metal ion.^11d In the case of Fe³⁺ and Cu²⁺ salts, moderate ML yields were obtained. The different Lewis acidity strengths of these metal salts may be the main reason for their activities in the esterification reaction. It can be noted that a near 100% ML yield was obtained when Al₂(SO₄)₃ was submitted to the esterification reaction, which could be attributed to the excellent Lewis acidity of Al³⁺ and the in situ generated Brønsted acidity, as reported in the previous study.^12a When the anion was changed to Cl⁻ or NO₃⁻, the Al³⁺ salts provided lower ML yields compared to that of SO₄²⁻. This result may result from the different acid strengths of these metal salts. Finally, Zn²⁺ salts were tested in the reaction and only achieved a much lower ML yield, which is due to their weak Lewis and Brønsted acid strengths in the reaction media. These results further demonstrate that the Lewis and Brønsted acidity of the reaction system are the key factors to the esterification of LA to alkyl levulinates.

Table 1 Esterification of LA over different metal salts^ab

Entry	Catalyst	Conv. (%)	ML yield (%)
a Conditions: LA 1.0 mmol, catalyst (metal ions, 0.4 mmol), MeOH 14 mL (MeOH/metal ions = 864 (mole ratio)), 400 W, 110 °C, 10 min.b Analysed by GC.c Catalyst 80 mg.d Catalyst 80 mg, 30 min.
1	—	5.0	1.8
2	CrCl3	96.9	91.5
3	Cr2(SO4)3	45.2	4.1
4	SnCl4	100	96.8
5	SnSO4	58.6	15.7
6	FeCl3	87.6	89.6
7	Fe2(SO4)3	92.7	81.7
8	CuSO4	70.9	49.5
9	CuCl2	71.8	66.1
10	AlCl3	83.7	83.9
11	Al2(SO4)3	100	99.4
12	Al(NO3)3	79.1	70.9
13	ZnCl2	41.4	8.1
14	ZnSO4	44.0	10.8
15^c	HNS–PrSO3H	100	98.0
16^d	Amberlyst-15	100	99.6

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For comparison, the commercially available solid catalyst, Amberlyst-15, was also tested in the esterification reaction, and the reaction achieved a 99% ML yield with an extended reaction time, which might be caused by the mass transfer limitation of reactants with the acid sites on the surface of the solid catalyst. However, the structure of Amberlyst-15 was destroyed to a powder, which was probably caused by the “hot spots” generated under the MW condition. For the solid catalyst, HNS–PrSO₃H,¹³ the reaction could also achieve a 98% ML yield in 10 min. However, its high cost may prevent it from wider applications in industrial production. It is noteworthy that the auto-esterification of methanol in these reactions can be ignored because the reaction temperature was relatively lower. In our previous study, we revealed that a higher reaction temperature (150 °C) would lead to the auto-etherification of alcohols in the reaction. In spite of this, the metal salt catalyst provided minimum esterification side reactions compared to traditional liquid or solid acid catalysts.

Effect of reaction temperature

The effect of reaction temperature on the esterification of LA was investigated using Al₂(SO₄)₃ as the catalyst under microwave irradiation from 90 °C to 130 °C (Fig. 1). It can be found that the esterification of LA can proceed at lower temperatures, with an 84.8% ML yield at 90 °C. Upon further increase in the reaction temperature to 110 °C, a maximum ML yield of 99.4% was obtained, which suggests that a higher reaction temperature can promote the reaction rate. The ML yield remained almost unchanged as the reaction temperature was further increased to 130 °C. Thus, the optimum reaction temperature for the esterification of LA is 110 °C.

Fig. 1 Influence of reaction temperature. Conditions: LA of 1.0 mmol, Al₂(SO₄)₃ of 0.2 mmol, MeOH of 14 mL (MeOH/metal ions = 864 (mole ratio)), 400 W and 10 min.

Effect of catalyst loading

The catalyst loading clearly determines the amount of Lewis and Brønsted acid sites of the methanol solution, and subsequently affects the yield of ML. To determine the optimized catalyst loading, experiments were carried out by changing the Al₂(SO₄)₃ loading from 10 mol% to 30 mol%, whereas the other conditions remained unchanged. As shown in Table 2, the yield of ML firstly increased from 87.3% to 99.4% when the catalyst loading was increased from 10 mol% to 20 mol%. Further increase of the catalyst loading did not make a significant improvement in the ML yield. In addition, an excess amount of catalyst loading decreased the product yield slightly, which could be due to the formation of side products. In summary, the catalyst loading of 20 mol% was sufficient to achieve a high ML yield.

Table 2 Effect of catalyst loading^a

Entry	Catalyst loading (mol%)	Conv. (%)	ML (%)
a Conditions: LA 1.0 mmol, MeOH 14 mL (MeOH/metal ions = 864 (mole ratio)), 400 W, 110 °C and 10 min.
1	10	89.4	87.3
2	15	95.2	94.3
3	20	100	99.4
4	25	100	99.6
5	30	100	98.5

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Comparison of two different heating methods

The esterification reactions with microwave heating and conventional oil bath heating were also investigated to compare their heating efficiency and the results and presented in Fig. 2. When the esterification reaction was carried out under microwave irradiation, 99.4% yield of ML was obtained in just 10 minutes. For conventional oil bath heating, we used a Parr reactor to carry out the reaction with a temperature probe to monitor the reaction temperature of the reaction mixture. The ML yield was lower (74.1% for 10 min, 91.4% for 60 min and 96.6% for 90 min) than that with microwave heating. Even when the reaction time was further prolonged, the yield of ML remained almost unchanged (97.1% for 120 min and 96.6% for 150 min). Moreover, some side products were detected in the reaction mixture as the reaction time was extended and this led to a decrease in reaction selectivity. Thus, an appropriate reaction time is crucial to maintain high selectivity of the esterification of LA. These results demonstrate that microwave radiation not only promotes the esterification reaction rate but also improves the selectivity of the target product. The microwave irradiation heating method provides internal heating, which can directly couple microwave energy with molecules (solvents, reagents and catalysts) and promote the reaction rate.

Fig. 2 Time course of the esterification of LA using microwave and conventional heating. Conditions: 1.0 mmol of LA, 0.2 mmol of Al₂(SO₄)₃, 14 mL of MeOH (MeOH/metal ions = 864 (mole ratio)), 110 °C and 10 min.

Preparation of different alkyl levulinates

Different alkyl levulinates were synthesized with the same catalyst and various alcohols. Alcohols with different carbons (e.g. ethanol, n-propanol and n-butanol) were tested in this study and the results are shown in Table 3. The complete conversion of LA and high selectivity of alkyl levulinates were achieved for all the tested alcohols. However, the reaction conditions need to be varied with different alcohols to achieve higher product yields. This may be caused by two reasons: the different reactivities of alcohols and their dielectric properties. As known, dielectric property of a material determines its heating efficiency when it is subjected to reaction under microwave irradiation. Dielectric constant (ε′) and dielectric loss (ε′′) are two key parameters to reflect a medium's abilities to absorb microwave energy and convert the energy to heat, respectively.¹⁴ Therefore, to determine the relationship between dielectric properties and reaction efficiency, we carried out measurements of dielectric properties of the different alcohols used in our reactions (Table 3). The dielectric constants of alcohols followed the order of methanol > ethanol > n-propanol > n-butanol, as well as the dielectric loss at the commonly used microwave frequency of 2.45 GHz. This result indicates that alcohols with a larger polarity have larger dielectric constants/loss, which then can easily absorb microwave energy and generate internal heat, perhaps with more hot pots (local high temperature). Moreover, the collision frequency of molecules (nonthermal effects) may also be greatly increased in alcohols with higher dielectric constants.¹⁵ Thus, for alcohols with lower dielectric constants/loss, the “promotion effect” of microwave irradiation was relatively weaker compared with those with higher dielectric constants/loss. Therefore, to achieve higher product yields, the reaction conditions need to be varied. By increasing the reaction temperature or prolonging the reaction time, high alkyl levulinate yields can be obtained when ethanol, n-propanol and n-butanol are employed as the solvent.

Table 3 Esterification of LA with different alcohols of different dielectric properties^ab

Entry	Alcohols	Polarity	ε′	ε′′	Ester yield/%
a Conditions: LA 1.0 mmol, Al2(SO4)3 0.2 mmol, and solvent 14 mL.b Dielectric properties were measured at room temperature.c 110 °C, 5 min.d 130 °C, 15 min.e 150 °C, 20 min.f 150 °C, 35 min.
1^c	MeOH	6.6	26.95	14.55	99.4
2^d	EtOH	4.3	9.21	8.26	97.7
3^e	n-PrOH	4.0	5.29	3.83	96.5
4^f	n-BuOH	3.7	4.77	2.64	96.2

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Esterification of levulinic acid in aqueous solution

In general, the esterification of pure LA is not practical for the industrial production of alkyl levulinates because of the high cost to separate LA from the aqueous solution, which is produced by the hydrolysis of sugars. To investigate whether the Al₂(SO₄)₃ system could be applied to the direct esterification of carbohydrate hydrolysate, the esterification of LA aqueous solution with methanol was also carried out. Our previous study on the hydrolysis of glucose in water showed that about 42 wt% of LA aqueous solution could be obtained after simple treatment.¹⁶ Therefore, an LA aqueous solution of the same concentration was prepared to simulate the carbohydrate hydrolysate that results from the hydrolysis of glucose and this mixture was submitted it to esterification reaction with the Al₂(SO₄)₃ system. The results are presented in Fig. 3. The amount of methanol and the mole ratio of the Al₂(SO₄)₃ catalyst to acid LA were constant. With the increased loading of the LA a.q. in MeOH solution, the ML yield first decreased slightly, but still remained at high levels (>80%) until the weight ratio reached 17.5 : 100, which indicates that a proper percentage of water in the reaction system does not have much effect on the esterification reaction. However, when the ratio was further increased to 22.5 : 100, the ML yield decreased to 65.3%. However, extension of the reaction from 10 min to 40 min provided a much higher ML yield of 96.7%, which implies that the addition of water to the esterification system might decrease the reaction rate but the reaction could reach a high product yield at prolonged reaction time. The above mentioned results show that the Al₂(SO₄)₃ system has great potential for use in the esterification of the acids in carbohydrate hydrolysate.

Fig. 3 Esterification of LA aqueous solution. Conditions: LA : H₂O = 42 : 58 (weight ratio), Al₂(SO₄)₃ : LA = 0.2 (mole ratio), MeOH 11.06 g (14 mL), 400 W, 110 °C, 10 min. (a): Reaction time 40 min.

Esterification of different carboxylic acids

Given the high conversion and yield of the esterification of LA, this method was applied to the esterification of other biomass derived acids. Straight-chain aliphatic acids could be converted to the corresponding esters with high yield under optimized conditions (Table 4, entries 1, 2 and 4). For isovaleric acid, an increased temperature of 130 °C was necessary to achieve a higher product yield. The esterification of aromatic acid benzoic acid was much more difficult due to the steric hindrance effect of the phenyl ring compared to aliphatic acid. Therefore, a higher reaction temperature and longer reaction time were used to obtain better results. A good product yield could also be achieved at 150 °C for 25 minutes for 2,5-furandicarboxylic acid. The above mentioned results further suggest that Al₂(SO₄)₃ is an effective catalyst with good compatibility for the esterification of different acids.

Table 4 Esterification of different carboxylic acids^a

Entry	Product	Conv. (%)	Yield (%)
a Conditions: acid 1 mmol, Al2(SO4)3 0.2 mmol, MeOH 14 mL (MeOH/metal ions = 864 (mole ratio)).b 400 W, 110 °C, 10 min.c 400 W, 130 °C, 15 min.d 600 W, 150 °C, 25 min.
1^b		100	99.7
2^b		100	99.5
3^c		100	98.1
4^b		100	97.7
5^d		100	98.2
6^d		100	98.5

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Alcoholysis/hydrolysis of metal salts in alcohols

The high catalytic activity of Al₂(SO₄)₃ in the esterification reaction may result from the alcoholysis/hydrolysis of metal salts in alcohols, which can generate H⁺ and then participate in the esterification reaction. Our previous study on the alcoholysis of furfuryl alcohol with metal salt catalysts showed that the solvent becomes acidic when the reaction mixture is heated to higher temperatures, which indicate the generation of H⁺.¹⁷ To obtain further insight into this process, the changes of the dielectric property of the reaction solvent, MeOH, and MeOH with the catalyst, were monitored using a Network Analyzer. The dielectric constants/loss of pure MeOH and MeOH + Al₂(SO₄)₃ was measured at the fixed temperature of 60 °C (Fig. 4). For the dielectric constant, pure MeOH remained almost unchanged during the measurement time. When Al₂(SO₄)₃ was added to MeOH at 60 °C, the dielectric constant of the mixture gradually decreased during the first 7 min of the mixing time, and then it remained almost unchanged as the time was prolonged. A similar trend was also found for the dielectric loss of these two systems. The changes of the dielectric properties of the mixture suggest that the metal salt underwent an alcoholysis/hydrolysis process in MeOH. The alcoholysis of Al³⁺ in the mixture led to a gradual change in the mixture's composition (more H⁺ and Al species formed); therefore, causing a change in the mixture's dielectric property. When this process reached equilibrium, the composition of the mixture remained unchanged and therefore the mixture's dielectric properties did not change either. These results provide more information about the dielectric properties of the alcohol systems with metal salt catalysts, which might be useful when designing microwave assisted reactions with metal salt catalysts.

Fig. 4 Dielectric properties of MeOH and MeOH + 0.2 mmol Al₂(SO₄)₃ at 60 °C. Dielectric constant (left) and dielectric loss (right).

Reusability of the catalyst

The reusability of catalysts is crucial for building a green, economical, and sustainable catalytic system. The reusability of Al₂(SO₄)₃ was evaluated using the esterification of LA in methanol as a model reaction. The catalyst was recycled and reused 5 times and the results are presented in Fig. 5. It was found that the yield of ML can maintain at a high level (99.4% yield for the fresh catalyst and 97.8% after five runs). The recycling experiments further demonstrate that metal salts are economic and efficient catalysts for the esterification of LA for the production of alkyl levulinates.

Fig. 5 Recycling of the catalyst. Conditions: 1.0 mmol of LA, 0.2 mmol of Al₂(SO₄)₃, 14 mL of MeOH (MeOH/metal ions = 864 (mole ratio)), 400 W, 110 °C and 10 min.

Conclusions

In summary, the esterification of LA using commercially available metal salts as catalysts was studied. The highest ML yield of 99.4% could be achieved over the Al₂(SO₄)₃ catalyst at 110 °C for 10 min. The comparison of two heating methods, namely, microwave and conventional heating, in the esterification reaction indicates that microwave radiation could significantly enhance the efficiency of the esterification of LA. Other organic carboxylic acids were also successfully converted to their esters with high yields. The esterification of LA aqueous solution in methanol was achieved, which provides a potential solution to the esterification of carbohydrate hydrolysate to generate alkyl levulinates. Finally, the catalyst can be easily recycled and reused without much decrease in activity. The current findings provide a new economically viable and environmentally friendly system for the esterification of LA or its aqueous solution to alkyl levulinates.

Acknowledgements

The authors are grateful for the financial support by the National Natural Science Foundation of China (NSFC 21502095), the Natural Science Foundation of Jiangsu Province (BK20150872) and the Jiangsu Specially-Appointed Professor program of the State Minister of Education of Jiangsu Province.

Notes and references

1. G. M. G. Maldonado, R. S. Assary, J. Dumesic and L. A. Curtiss, Energy Environ. Sci., 2012, 5, 6981.
2. (a) D. R. Fernandes, A. S. Rocha, E. F. Mai, J. A. M. Claudio and V. T. da Silva, Appl. Catal., A, 2012, 425, 199; (b) J. Zhang, S. Wu, B. Li and H. Zhang, ChemCatChem, 2012, 4, 1230.
3. (a) D. J. Braden, C. A. Henao, J. Heltzel, C. C. Maravelias and J. A. Dumesic, Green Chem., 2011, 13, 1755; (b) Z. Yang, Y.-B. Huang, Q.-X. Guo and Y. Fu, Chem. Commun., 2013, 49, 5328.
4. K. Li, J. Hu, W. Li, F. Ma, L. Xu and Y. Guo, J. Mater. Chem., 2009, 19, 8628.
5. D. R. Fernandes, A. S. Rocha, E. F. Mai, C. J. A. Mota and V. T. da Silva, Appl. Catal., A, 2012, 425–426, 199.
6. (a) H. J. Bart, J. Reidetschlager, K. Schatka and A. Lehmann, Ind. Eng. Chem. Res., 1994, 33, 21; (b) Y. Liu, E. Lotero and J. G. Goodwin Jr, J. Catal., 2006, 242, 278; (c) E. I. Gürbüz, D. M. Alonso, J. Q. Bond and J. A. Dumesic, ChemSusChem, 2011, 4, 357.
7. (a) Y. Kuwahara, T. Fujitani and H. Yamashita, Catal. Today, 2013, 476, 186; (b) Z. L. Li, R. Wnetrzak, W. Kwapinski and J. J. Leahy, ACS Appl. Mater. Interfaces, 2012, 4, 4499.
8. (a) S. Dharne and V. V. Bokade, J. Nat. Gas Chem., 2011, 20, 18; (b) K. Y. Nandiwale, P. S. Niphadkar, S. S. Deshpande and V. V. Bokade, Appl. Catal., A, 2013, 460–461, 90; (c) G. Pasquale, P. Vázquez, G. Romanelli and G. Baronetti, Catal. Commun., 2012, 18, 115; (d) K. Yan, G. Wu, J. Wen and A. Chen, Catal. Commun., 2013, 34, 58; (e) F. Su, L. Ma, D. Song, X. Zhang and Y. Guo, Green Chem., 2013, 15, 885; (f) F. Su, Q. Wu, D. Song, X. Zhang, M. Wang and Y. Guo, J. Mater. Chem. A, 2013, 1, 13209; (g) J. Zhao, H. Guan, W. Shi, M. Cheng, X. Wang and S. Li, Catal. Commun., 2012, 20, 103; (h) Y. Leng, P. Jiang and J. Wang, Catal. Commun., 2012, 25, 41.
9. (a) J. A. Melero, G. Morales, J. Iglesias, M. Paniagua, B. Hernandez and S. Penedo, Appl. Catal., A, 2013, 466, 116; (b) C. R. Patil, P. S. Niphadkar, V. V. Bokade and P. N. Joshi, Catal. Commun., 2014, 43, 188; (c) K. Y. Nandiwale, P. S. Niphadkar, S. S. Deshpande and V. V. Bokade, J. Chem. Technol. Biotechnol., 2014, 89, 1507.
10. B. L. Oliveira and V. T. da Silva, Catal. Today, 2014, 234, 257.
11. (a) Y. Yang, C. Hu and M. M. Abu-Omar, Green Chem., 2012, 14, 509; (b) S. de, S. Dutta and B. Saha, Green Chem., 2011, 13, 2859; (c) N. Shi, Q. Liu, Q. Zhang, T. Wang and L. Ma, Green Chem., 2013, 15, 1967; (d) S. Hu, Z. Zhang, J. Song, Y. Zhou and B. Han, Green Chem., 2009, 11, 1746.
12. (a) L. Zhou, H. Zou, J. Nan, L. Wu, X. Yang, Y. Su, T. Lu and J. Xu, Catal. Commun., 2014, 50, 13; (b) M. Schwiderskia and A. Kruse, J. Mol. Catal. A: Chem., 2015, 402, 64.
13. B. Lu, S. An, D. Song, F. Su, X. Yang and Y. Guo, Green Chem., 2015, 17, 1767.
14. (a) J. D. Moseley and C. O. Kappe, Green Chem., 2011, 13, 794; (b) M. Nüchter, B. Ondruschka, W. Bonrath and A. Gum, Green Chem., 2004, 6, 128.
15. S. Tabasso, D. Carnaroglio, E. Calcio Gaudinob and G. Cravotto, Green Chem., 2015, 17, 684.
16. Y. Huang, J. Dai, X. Deng, Y. Qu, Q. Guo and Y. Fu, ChemSusChem, 2011, 4, 1578.
17. Y. Huang, T. Yang, M. Zhou, H. Pan and Y. Fu, Green Chem., 2015 10.1039/C5GC01581B.

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