One-pot Synthesis of 6-Hydroxyhexanoic Acid from Cyclohexanone Catalyzed by Dealuminated HBEA Zeolite with Aqueous 30% H2O2 Solution

Xia Changjiu; Yang Yongjia; Zhao Yi; Lin Min; Zhu Bin; Peng Xinxin; Dai Zhenyu;Luo Yibin; Shu Xingtian

(State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083)

Abstract: The one-pot synthesis of 6-hydroxyhexanoic acid from cyclohexanone via the integrated Baeyer-Villiger oxidation and ring opening reaction catalyzed by dealuminated HBEA zeolite has been developed. Under optimized conditions,the cyclohexanone conversion and 6-hydroxyhexanoic acid selectivity are over 95%, respectively. The excellent catalytic performance is attributed to the activation of carbonyl group of cyclohexanone and the fast hydrolysis and ring opening of ε-caprolactone by both Lewis acid and Brönsted acid sites under aqueous conditions.

Key words: Baeyer-Villiger oxidation; cyclohexanone; 6-hydroxyhexanoic acid; zeolite; hydrolysis

1 Introduction

Cyclohexanone oxidation is of ultra-importance for the commercial production of chemical intermediates, i.e.ε-caprolactone, adipic acid and ε-caprolactam, which are extensively employed to synthesize polymers and plastics in industry[1-5]. However, highly toxic and polluted oxidants and catalysts are introduced to the conventional cyclohexanone oxidation routes. Such as peracids are the oxidants for the synthesis of ε-caprolactone and the high-concentration nitride acid (HNO3) solution is used as catalyst and oxidant for the industrial production of adipic acid, causing many serious environment and health problems. To overcome these drawbacks, unswerving efforts have been devoted to the heterogeneously catalytic transformation of cyclohexanone under moderate conditions. Therefore great progresses have been achieved by using solid catalysts in the academic and industrial researches. For example, the Baeyer-Villiger (B-V)oxidation of cyclohexanone for producing ε-caprolactone catalyzed by the Sn-β zeolite was reported by A. Corma,using 30% of H2O2as oxidant in the presence of organic solvents[6-8]. Under optimized reaction conditions, the selectivity of lactone is over 98%, thus this reaction process is apparently much cleaner than the oxidation route using peracids. The said process is contributed to the activation of carbonyl groups in cyclohexanone molecules under the effect of the tetrahedral Sn Lewis acid sites in the BEA structural framework. With respect to conversion of cyclohexanone to adipic acid[9-11], solid acidic materials (such as H2WO4and Na2WO4) have been demonstrated as the effectively heterogeneous catalyst operating under mild reaction conditions. This can be achieved, because the H2WO4can be oxidized by H2O2solution to form the stable H2[WO(O2)2(OH)2] species,which are more reactive than the original H2O2molecules during the catalytic oxidation process. However, these heterogeneous catalysts are still far beyond to be employed in commercial scale. For example, the Sn-β zeolite is very difficult to be synthesized in the fluoride media, which need high organic-template content and long hydrothermal reaction time (usually over 6 weeks). Since the crystal size of Sn-β zeolite is very large, it is prone to be deactivated during oxidation reactions[12]. Furthermore,huge amount of organic solvents (about 20―30 times the mole of cyclohexanone) is used to prevent the ring-opening reaction of ε-caprolactone. Until now, the TS-1 zeolite has been commercially synthesized and applied in many industrial processes, but the selectivity of target product catalyzed by the TS-1 zeolite is very low in this reaction[13-14].

Herein, we provide a novel one-pot synthesis route to produce 6-hydroxyhexanoic acid with 30% H2O2catalyzed by dealuminated HBEA zeolite (DHBEA) in the absence of organic solvents. Most importantly, this catalyst is water-tolerant and cheaply available in industry, and this reaction is carried out in the base-, acid- and additive-free condition, realized at low reaction temperature (＜100 °C)without hazardous wastes discharged. 6-Hydroxyhexanoic acid can serve as a potential chemical intermediate for producing other chemicals. ε-Caprolactone and adipic acid can be produced from oxyacid via dehydrated ring-closing reaction and deep oxidation, respectively.Moreover, 1,6-hexanediol can be obtained through the hydrogenation of 6-hydroxyhexanoic acid. Thus, this study is of significance to explore alternative routes for catalytic transformation of cyclohexanone under mild conditions.

2 Experimental

The commercial nano-sized β zeolite, with the BEA topological structure and a Si/Al ratio of about 22.24,was provided by the Research Institute of Petroleum Processing (RIPP), SINOPEC. The dealuminated nanosized β zeolite was prepared according to the following method. Firstly, 3 g of water was homogeneously dissolved in 7 g of aqueous 98% H2SO4solution.Secondly, 5 g of commercial nano-sized β zeolite was mixed with this 10 g of aqueous 68.6% H2SO4solution under continuous magnetic stirring at 80 °C for 6 hours.The product was washed and filtered for 3 times prior to being calcined at 550 °C for 6 hours in air. The final product was labeled as DHBEA zeolite (with the molar ratio of Si/Al equating to 69.2).

The cyclohexanone oxidation reaction catalyzed by DHBEA zeolite was carried out in a three-necked flask under continuous stirring and heating. In a typical reaction system, 0.63 g of HBEA zeolite was suspended in a mixture of cyclohexanone (0.1 mol) and solvents in a glass reactor at 80 °C. Then, 0.1 mol of aqueous 30% H2O2solution was added into this mixture. Small amount of mixture was collected at set intervals,and the products were separated from the catalyst by centrifugation. The final products were analyzed by using gas chromatography (GC), equipped with a hydrogenflame ionization detector and a 3-meter-long weak polar HP-5 column.

The charge and optimized structure of active sites were calculated by using the DMol3and PW91/DND techniques. The adsorption and activation of cyclohexanone and hydrogen peroxide on the Lewis acidic Al sites were calculated by the Adsorption Locator module in MS software.

3 Results and Discussion

The catalytic activity and target product selectivity of cyclohexanone oxidation catalyzed by DHBEA zeolite in different solvents are listed in Figure 1. It is observed that organic solvents are not preferential for the high cyclohexanone conversion and the selectivity of target product under the same reaction conditions.Therefore, we infer that water is the best solvent in the cyclohexanone oxidation, and under optimized conditions both the cyclohexanone conversion and the selectivity of 6-hydroxyhexanoic acid are greater than 95%,respectively. It is of interest to note that the selectivity of total target product in methanol media is decreasing with the increase of reaction time, which is attributed to the esterification between 6-hydroxyhexanoic and methanol molecules. Although the corresponding ester is also a very useful chemical for organic synthesis, it would significantly reduce the cyclohexanone conversion and selectivity of single target product, leading to the difficulty in separation and purification.

Furthermore, to deeply understand the reaction pathway, the distribution of target product selectivity of cyclohexanone oxidation in three solvents is illustrated in Figure 2. It can be seen from Figure 2 that the selectivity of ε-caprolactone is very low (close to 0) in the absence of organic solvent, which means that water is in favor of the ring-opening of ε-caprolactone molecules.Meanwhile, there is almost no adipic acid formed in each medium, which infers that the hydroxyl group in the 6-hydroxyhexanoic acid is too inert to be activated and transformed by acidic DHBEA zeolite. Thus, it is confirmed that the absence of organic solvent addition is the best choice for our reaction, resulting in a high 6-hydroxyhexanoic acid selectivity of 95%.

Figure 1 Catalytic activity (a) and target product selectivity(b) of cyclohexanone oxidation reaction in different solvents

To reflect the catalytic mechanism and optimize the reaction parameters, the catalytic performance of different catalysts in cyclohexanone oxidation under various conditions is shown in Table 1. It is interesting to see that, in the absence of catalyst, the selectivity of 6-hydroxyhexanoic acid and adipic acid is 66.5% and 15.2%, respectively, as listed in entry 3 of Table 1. When the TS-1 zeolite is added as catalyst in this reaction,the total target product selectivity is about 88.6%along with a higher selectivity of adipic acid (20.9%).It is inferred that TS-1 zeolite can promote the deep oxidation of 6-hydroxyhexanoic acid via enhancing the nucleophilic capability of H2O2solution[14]. As reported by A. Corma[6-8], when the Sn-β zeolite is used as catalyst in the cyclohexanone B-V oxidation, the major product ε-caprolactone selectivity is over 98% in the presence of a large amount of MTBE or dioxane solvent. This can occur because the organic solvents can prevent the hydrolysis of ε-caprolactone. However, from the viewpoint of application, the separation of ε-carprolactone from a huge amount of organic solvent would consume a large quantity of energy and capital cost. Moreover, the high solvent content causes the decreasing of reaction rate, which is accompanied with the reduction of cyclohexanone and H2O2concentration at the same time.As a result, the cyclohexanone conversion catalyzed by the Sn-β zeolite is 52%, which is lower than that achieved by the DHBEA zeolite (usually over 90%) in the absence of organic solvents under the similar conditions.

Figure 2 Target product selectivity distribution of cyclohexanone oxidation reaction in different solvents

It has been demonstrated that lots of Brönsted acid sites and framework defects exist in the DHBEA zeolite,which can catalyze the hydrolysis and ring-opening reaction of ε-caprolactone in aqueous solution. Thus, the major product of cyclohexanone oxidation catalyzed by the DHBEA zeolite is 6-hydroxyhexanoic acid (with its selectivity exceeding 95%), rather than ε-caprolactone.Judging from the entries 4―9 of Table 1, we can see that both cyclohexanone conversion and 6-hydroxyhexanoic acid selectivity become larger as a function of the increasing reaction temperature and catalyst content.Under optimized conditions, both the cyclohexanone conversion and the selectivity of main product are greater than 95%, as illustrated in entry 9 of Table 1. Then, we can infer that the carbonyl groups in cyclohexanone molecules can be highly reactivated, while the H2O2and terminal hydroxyl groups cannot be activated by the acid sites of DHBEA zeolite. Furthermore, the reaction rate of B-V oxidation is much faster than that of deep oxidation of 6-hydroxyhexanoic acid, making the selectivity of adipic acid close to 0. In a word, this study provides an effective route for cyclohexanone oxidation to controllably produce high value-added chemicals, which shows great industrial application potential. Therefore, it can be in good agreement with the requirements of green and sustainable chemical processes.

Compared to the Sn-β zeolite, both Brönsted and Lewis acid sites are observed in the DHBEA zeolite, as confirmed by the pyridine adsorption IR spectroscopy.In order to distinguish the roles of Brönsted and Lewis acid sites in the cyclohexanone oxidation reaction,DFT methods were introduced to calculate the electron properties, geometric structures and energy profiles of these possible mechanisms. It is well accepted that the Brönsted acid site are assigned to the ≡Si(OH)+Al-≡species, formed due to the isomorphous substitution of framework Si atoms by tetrahedral Al atoms[15-17].Although the actual nature of Lewis acid sites is still in doubt, the trigonal Al atom in the zeolite framework,which is similar to the anhydrous AlCl3salt, is widely considered as the modelled Lewis acid site of HBEA zeolite in some literature reports[18-21]. Thus, the Brönsted and Lewis acid site models are selected as the active sites for cyclohexanone oxidation in molecular simulation, as illustrated in Scheme 1.

The charge of carbon atoms in the carbonyl groups coordinated to Brönsted acid and Lewis acid sites is+0.519 e and 0.496 e, respectively, which are much larger than that in the original cyclohexanone molecule (+0.405 e). It is indicated that both Brönsted and Lewis acid sites can enhance the reactivity of the carbonyl group, and then two responding possible B-V oxidation mechanisms are proposed, as illustrated in Scheme 1. After the activation of cyclohexanone by Brönsted acid (H+) sites or Lewis acid sites, the carbonyl groups are easier to be nucleophilically attacked by H2O2molecules to produce the tetrahedral Criegee intermediate (CI) species. In the second step, the intramolecular rearrangement reaction of CI species takes place, with the formation of H2O and ε-caprolactone molecules. After that, 6-hydroxyhexanoic acid is formed in the presence of H+ions via hydrolysisand ring-opening reaction. On the other hand, both the Lewis and Brönsted acid sites work poor in the activation of H2O2and hydroxyl group of 6-hydroxyhexanoic acid.Consequently, there is very low adipic acid selectivity observed in the DHBEA catalyzed reaction pathway.

Table 1 Catalytic performance of the catalyst-free case, the Sn-β, TS-1, and DHBEA zeolites in the cyclohexanone oxidation under moderate conditions

Scheme 1 Possible reaction mechanisms of Baeyer-Villiger oxidation of cyclohexanone catalyzed by Brönsted acid sites (a)and Lewis acid sites (b) of DHBEA zeolite

4 Conclusions

The DHBEA zeolite shows great catalytic performance in the one-pot cyclohexanone oxidation to produce 6-hydroxyhexanoic acid under moderate conditions. It is demonstrated that the water is the best solvent, and high reaction temperature and catalyst content are preferential for the conversion of cyclohexanone. Under the optimized reaction conditions, cyclohexanone conversion and 6-hydroxyhexanoic acid selectivity are both over 95%, respectively. It is attributed to the carbonyl groups in cyclohexanone molecules that can be activated by Brönsted (≡Si(OH)+Al-≡) and Lewis acid sites. Then two responding BV oxidation mechanisms are proposed, and 6-hydroxyhexanoic acid is obtained via the hydrolysis and ring-opening reaction of ε-caprolactone. However,the DHBEA zeolite has no effect on the activation of the alcoholic hydroxyl groups and H2O2molecules, leading to the highest selectivity of 6-hydroxyhexanoic acid.

Acknowledgement:This work was financially supported by the National Basic Research Program of China (973 Program,2006CB202508), the China Petrochemical Corporation Program(SINOPEC Group ST417004), and the National Key Research and Development Program of China (2017YFB0306800).