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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 8: 1191 - 1198

DOI: https://doi.org/10.7536/PC221209

Review

Selective Oxidative Lactonization of 1,6-Hexanediol into ε-Caprolactone

  • Xiaoyu Shen 1 ,
  • Zhongtian Du , 1, * ,
  • Bairui Guo 1 ,
  • Zhongxu Guo 1 ,
  • Changhai Liang , 1, 2, *
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  • 1 School of Chemical Engineering, Dalian University of Technology,Panjin 124221, China
  • 2 State Key Laboratory of Fine Chemicals, Dalian University of Technology,Dalian 116023, China
*Corresponding author e-mail: (Zhongtian Du);
(Changhai Liang)

Received date: 2022-12-15

  Revised date: 2023-05-08

  Online published: 2023-07-18

Supported by

National Natural Science Foundation of China(22172010)

Fundamental Research Funds for the Central Universities(DUT2021TD103)

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Abstract

ε-Caprolactone is a key monomer for the synthesis of poly(ε-caprolactone) (PCL) with good biocompatibility and biodegradability, and relevant polymer materials could be applied in pharmaceutical, medicinal, and packaging applications. Green and economic synthesis of ε-caprolactone is vital to popularize such eco-friendly polymers, and selective oxidative lactonization of 1,6-hexanediol into ε-caprolactone remains to be developed. In this review, different routes for the synthesis of ε-caprolactone such as Baeyer-Villiger oxidation of cyclohexanone and oxidative lactonization of 1,6-hexanediol are comparatively analyzed. According to whether electron acceptors (oxidants) are added to the reaction systems, the related advances of oxidative lactonization of 1,6-hexanediol are summarized, and the advantages and disadvantages of the corresponding reaction systems and catalysts are reviewed. The development trend of oxidative lactonization of 1,6-hexanediol into ε-caprolactone is also proposed.

Contents

1 Introduction

2 Catalytic oxidation processes

2.1 Carbonyl compounds act as electron acceptors

2.2 Molecular oxygen acts as the electron acceptor

2.3 H2O2acts as the oxidant

3 Catalytic dehydrogenation

3.1 Homogeneous catalysts

3.2 Heterogeneous catalysts

4 Conclusion and outlook

Key words： poly(ε-caprolactone); ε-caprolactone; 1,6-hexanediol; catalytic oxidation; catalytic dehydrogenation; molecular oxygen

Cite this article

Xiaoyu Shen , Zhongtian Du , Bairui Guo , Zhongxu Guo , Changhai Liang . Selective Oxidative Lactonization of 1,6-Hexanediol into ε-Caprolactone[J]. Progress in Chemistry, 2023 , 35(8) : 1191 -1198 . DOI: 10.7536/PC221209

1 Introduction

ε-caprolactone is an aliphatic lactone with important application value, and the product of its ring-opening polymerization, polycaprolactone (PCL), has good biodegradability, biocompatibility and polymer compatibility[1]. ε-caprolactone can also be copolymerized with other monomers such as lactide, and polycaprolactone can also be blended with a variety of organic polymers to obtain a variety of polymer materials with excellent properties[1⇓~3]. This kind of material can be used in biomedicine, environmental protection packaging and other fields, including drug release carriers, absorbable surgical sutures, food packaging materials and so on[4,5]. In recent years, with the deepening of the concept of sustainable development and the introduction of environmental protection policies at home and abroad, the market demand for ε-caprolactone and its polymers has gradually increased[6]. Green and cheap access to ε-caprolactone is the key factor to promote the wide use of this kind of degradable polymer materials.
At present, the main production method of ε-caprolactone in industry is Baeyer-Villiger oxidation method, which uses organic peroxy acid as oxidant to oxidize cyclohexanone to obtain ε-caprolactone (Figure 1).The production, storage, transportation and use of a large number of organic peroxides require high safety in this method, and the reaction process produces a large number of organic waste acid by-products, so the development of a safer and more environmentally friendly production process of ε-caprolactone has been paid more and more attention[7⇓~9]. In addition, at present, the manufacturers of ε-caprolactone include multinational enterprises such as Pasto in Sweden, BASF in Germany and Daicel in Japan. Hunan Juren Chemical in China also has a large production capacity. The demand for ε-caprolactone in China used to rely mainly on imports[1,9]. With the expansion of the application field of ε-caprolactone and the expansion of China's market scale, the research and development of green and efficient new preparation process of ε-caprolactone has important economic value and social significance for improving China's chemical material industry chain and realizing the sustainable development of society.
图1 有机过氧酸氧化环己酮制备ε-己内酯

Fig. 1 Oxidation of cyclohexanone into ε-caprolactone via Baeyer-Villiger oxidation

1,6-Hexanediol is an important aliphatic linear diol, which is widely used in polyester, polyamide, coatings and other fields. At present, 1,6-hexanediol is mainly produced by dimethyl adipate hydrogenation process in industry, and the market supply is stable. In addition, more research progress has been made in the preparation of 1,6-hexanediol from biomass in recent years[10][11⇓~13]. Both 1,6-hexanediol and ε-caprolactone contain six carbon atoms and two oxygen atoms in their molecular structures, and the O/C ratio is the same. The conversion of 1,6-hexanediol to ε-caprolactone requires the oxidation of an alcoholic hydroxyl group at one end to form a carbonyl group.The formation of cyclic lactone structure mainly involves the oxidative cleavage of C — H bond and the formation of C — O bond, but not the oxidative cleavage of C — C bond, so the conversion of 1,6-hexanediol to ε-caprolactone can be carried out in a mild reaction environment. In contrast, cyclohexanone and ε-caprolactone have the same H/C ratio, and the conversion of cyclohexanone to ε-caprolactone via Baeyer-Villiger oxidation requires the introduction of an oxygen atom to build the lactone structure, thus involving oxidative cleavage of the C — C bond (Fig. 2). There is no hydrogen atom on the carbonyl carbon of cyclohexanone, and it has a certain degree of electropositivity. It usually requires stoichiometric strong oxidants (such as peracetic acid, potassium permanganate, nitric acid, etc.) to oxidize the C-C bond between the carbonyl group of cyclohexanone and the α carbon, and the use of such oxidants usually produces a large amount of waste. With the enhancement of people's awareness of environmental protection and the improvement of national laws and regulations, green and safe reaction process is bound to be the future direction of chemical industry development. Replacing stoichiometric inorganic oxidants with oxygen and hydrogen peroxide, and exploring new raw materials and their corresponding process routes are important ideas for the development of new ways to prepare lactones, among which the oxidation of 1,6-hexanediol to ε-caprolactone has broad prospects for development[9,14⇓⇓~17].
图2 从结构和组成角度对比分别从1,6-己二醇和环己酮制备ε-己内酯的路线

Fig. 2 The preparation routes of ε-caprolactone from 1,6-hexadiol and cyclohexanone compared from the perspective of structure and composition

The oxidation of cyclohexanone to ε-caprolactone has been summarized and reviewed in many literatures, but the preparation of ε-caprolactone from 1,6-hexanediol has not been reported[7⇓~9,15⇓~17]. In the process of oxidative lactonization of 1,6-hexanediol to ε-caprolactone, 1,6-hexanediol is usually subjected to selective oxidation to form a carbonyl function, and then the intermediate is cyclized to form a lactone, in which the selective oxidation is the key step (fig. 3). ε-caprolactone is a seven-membered heterocyclic compound, which has greater intramolecular tension than five-/six-membered ring compounds, and is more difficult to form under the same conditions. There are many reports on the oxidative lactonization of diols (such as 1,4-butanediol, 1,5-pentanediol, etc.) In the literature, but these systems can not be simply extended to the oxidative lactonization of 1,6-hexanediol[18⇓⇓⇓~22]. Selective oxidative lactonization of 1,6-hexanediol to ε-caprolactone is very challenging.
图3 1,6-己二醇氧化内酯化制备ε-己内酯常见的反应路径

Fig. 3 Reaction pathway from 1,6-hexanediol to ε-caprolactone

Although 1,6-hexanediol can be easily oxidized to ε-caprolactone using stoichiometric inorganic oxidants (such as BaMnO4, NaBrO2, etc.), such methods produce large amounts of inorganic waste, which is not in line with the requirements of sustainable development, and such research is not included in this paper[23⇓~25]. In the process of oxidation reaction, there must be electron transfer. The oxidation of 1,6-hexanediol to ε-caprolactone will lose electrons, so there must be electron acceptors, and the oxidant is the electron acceptor. In the dehydrogenation type reaction, no additional electron acceptor is added, and hydrogen is generated after electron transfer. In this paper, according to the presence or absence of electron acceptor (oxidant) in the reaction system, the recent studies on the catalytic conversion of 1,6-hexanediol to ε-caprolactone were classified and summarized, the reaction system and catalysts for the preparation of ε-caprolactone from 1,6-hexanediol were reviewed, and the future development trend was prospected.

2 Catalytic oxidation process with electron acceptor in the reaction system

In this method, carbonyl compound, oxygen, hydrogen peroxide, etc. Are used as the final electron acceptor (oxidant) to directly oxidize and lactonize 1,6-hexanediol to obtain ε-caprolactone under the action of catalyst.

2.1 Carbonyl compound as electron acceptor

The conversion of 1,6-hexanediol to ε-caprolactone via Oppenauer oxidation usually requires carbonyl compounds as electron acceptors (hydrogen acceptors), such as methyl isobutyl ketone (MIBK), acetone, etc. In 2011, Heeres et al. Used a catalyst prepared in situ from [Ru(cymene)Cl2]2 and 1,1 '-bis (diphenylphosphino) ferrocene (DPPF) to achieve efficient conversion of 1,6-hexanediol to ε-caprolactone in methyl isobutyl ketone solvent with a yield of 99%, and they also demonstrated a route to bio-based 1,6-hexanediol and bio-based ε-caprolactone using biomass as the starting material (Fig. 4)[26,27]. One of the disadvantages of this method is that methyl isobutyl ketone is in large excess and the corresponding reduction product, methyl isobutyl carbinol, is partially formed, and about 20 mol% of potassium carbonate is needed[26]. In 2020, Funk et al. Were able to convert 1,6-hexanediol to ε-caprolactone using iron carbonyl compound ([2,5-(SiMe3)2-3,4-(CH2)4(η4-C4C=O)]Fe(CO)3) with cyclopentadienone as ligand as catalyst, excess acetone as solvent and hydrogen acceptor, and TMAO as activated catalyst, but the yield needed to be further improved (21%)[28]. 2022 Sooknoi et al. Catalyzed the oxidation of 1,6-hexanediol to ε-caprolactone in the presence of potassium carbonate using a (p-cymene)RuCl2(L) complex of phosphine and pyridine as a catalyst, toluene as a solvent, and MIBK as an electron acceptor, with a yield of 41.7%[29]. In this reaction system, the methyl isobutyl carbinol produced after the reduction of MIBK is easy to cause the deactivation of the catalyst. 1,6-Hexanediol can be efficiently converted to ε-caprolactone by oxenol oxidation, but the structure of the catalyst used in the reported studies is usually complex, the amount of electron acceptor carbonyl compound added is large, and the same amount of reduction by-products will be produced, which may hinder the popularization of this method.
图4 [{Ru(cymene)Cl2}2]/DPPF催化氧化1,6-己二醇制备ε-己内酯[26,27]

Fig. 4 [{Ru(cymene)Cl2}2]/DPPF catalyzed oxidation of 1,6-hexanediol to ε-caprolactone[26,27]

2.2 Molecular oxygen as electron acceptor

When oxygen is used as an oxidant, the corresponding by-product is usually water, and green and cheap oxygen is an ideal oxidant in the selective oxidation reaction. Direct preparation of ε-caprolactone by oxidation of 1,6-hexanediol with molecular oxygen has attracted much attention in recent years because of its high atom economy and few by-products. The ground state oxygen molecule is in a triplet state, and this structure makes it show a certain chemical stability in kinetics, which usually requires a catalyst to activate oxygen[30]. In this part, enzymatic catalysis, homogeneous catalysis and heterogeneous catalysis are reviewed respectively.

2.2.1 Enzyme catalyzed oxidation system

In 2013, Kara et al. Studied the oxidative lactonization of 1,4-, 1,5-, and 1,6-diols catalyzed by Horse liver alcohol dehydrogenase (HLADH) through a laccase (LMS) and nicotinamide cofactor (Horse liver alcohol dehydrogenase) mediated system, using molecular oxygen as the final electron acceptor to achieve the oxidative lactonization of diols, and the only by-product produced in this process was water[31]. In the HLADH-LMS system, 1,4- and 1,5-diols can be converted into the corresponding lactones smoothly, but the conversion of 1,6-hexanediol is very slow, even if the reaction time is prolonged, the conversion is only 26%, which indicates that the reaction is significantly affected by different carbon chain lengths. They also reported a convergent tandem bienzyme catalytic system composed of Baeyer-Villiger monooxygenase (BVMO) and alcohol dehydrogenase (ADH), and continuously improved the system and the types of monooxygenases. Cyclohexanone and 1,6-hexanediol (molar ratio of 2:1) were used as common substrates, and cyclohexanone and 1,6-hexanediol were oxidized to ε-caprolactone simultaneously through the coupling of redox reactions[32⇓⇓~35]. In 2019, Opperman et al. Found that AaSDR-1 alcohol dehydrogenase, which belongs to the short-chain dehydrogenase/reductase family, can catalyze 1,6-hexanediol to obtain ε-caprolactone[36]. In 2020, Pyo et al. Used Gluconobacter oxydans to selectively oxidize 1,6-hexanediol to obtain 6-hydroxycaproic acid at pH 6 ~ 7, and then used molecular sieve (MS) and cation exchange resin DR-2030 to catalyze the lactonization of 6-hydroxycaproic acid to obtain ε-caprolactone in DMF solution at 140 ℃[37]. More importantly, combining chemical and biological transformation processes, they also demonstrated the development potential of preparing C6 compounds such as 6-hydroxycaproic acid and ε-caprolactone from bio-based 5-hydroxyfurfural (Fig. 6). In 2021, Zhu Chenjie's research team established a new system of horse liver alcohol dehydrogenase (HLADH) combined with synthetic flavin cofactor for the synthesis of lactone. 1,6-hexanediol can produce ε-caprolactone in 50% yield in this catalytic system[38]. From the existing cases, the reaction conditions of enzyme-catalyzed oxidation system are usually mild, but there are few reports on the oxidative lactonization of 1,6-hexanediol, and the catalytic activity needs to be further improved.
图5 环己酮和1,6-己二醇耦合氧化生成ε-己内酯[32]

Fig. 5 The coupled oxidation of 1,6-hexanediol and cyclohexanone to produce ε-caprolactone[32]

图6 从5-羟甲基糠醛出发制备ε-己内酯[37]

Fig. 6 Preparation of ε-caprolactone from 5-hydroxymethylfurfural[37]

2.2.2 Homogeneous catalytic oxidation system

In 2011, B Bäckvall et al. Reported a biomimetic oxidation system, using a composite catalyst system composed of dimeric ruthenium complex, 2,6-dimethoxybenzoquinone and cobalt complex to achieve biomimetic electron transfer, using air as the final oxygen source, which can efficiently catalyze 1.Oxidative lactonization of 6-hexanediol gave ε-caprolactone in 86% yield (based on NMR analysis). Fig. 7 shows the proposed redox cycle. They also suggested that one of the hydroxyl groups in the diol molecule was first oxidized to an aldehyde group, which was then further oxidized to the lactone after cyclization[39]. Stahl et al. Used a Cu/ABNO catalytic system composed of 2,2 '-bipyridine, N-methylimidazole, [Cu(CH3CN)4]OTf and ABNO (ABNO = 9-azabicyclo [3.3.1] -nonan-N-oxyl) to catalyze the oxidation of 1,6-hexanediol to ε-caprolactone at room temperature, but the yield was only 22%, and the main product was adipaldehyde (yield 74%)[18]. In addition, Gao Shuang et al disclosed in the Chinese invention patent that the conversion of 1,6-hexanediol was 85% and the yield of ε-caprolactone was 82% by using Fe(NO3)3·9H2O/ABNO two-component catalyst system and acetonitrile as solvent at room temperature for 15 H[40]. The key step in the oxidation of 1,6-hexanediol to ε-caprolactone is the selective oxidation of a terminal hydroxyl group to an aldehyde group, which is essentially a selective oxidation reaction of primary aliphatic alcohols, and has always been an important difficulty in the selective oxidation of alcohols. The development of efficient catalytic systems for the selective oxidation of primary aliphatic alcohols is an important approach to the oxidation of 1,6-hexanediol to ε-caprolactone.
图7 仿生催化氧化1,6-己二醇制备ε-己内酯[39]

Fig. 7 Biomimetic aerobic oxidation of 1,6-hexanediol into ε-caprolactone[39]

2.2.3 Heterogeneous catalytic oxidation system

Although homogeneous catalytic systems have high catalytic activity, heterogeneous catalysts that are easy to separate and recycle have always been attractive. In 2013, Davilin et al. Prepared a gold nanocatalyst deposited on manganese sesquioxide (Mn2O3), which oxidized 1,6-hexanediol to ε-caprolactone at 120 ℃ in liquid phase with air as oxidant and tributyl phosphate as solvent, but the yield was only 9%.Under the same conditions, the catalyst can efficiently catalyze the preparation of γ-butyrolactone from 1,4-butanediol (the yield is 98%), which shows that the carbon chain length of diol has a significant effect on the oxidative lactonization reaction, and the alcohol oxidation catalyst can not be simply extended to the oxidative lactonization reaction of 1,6-hexanediol[41]. In 2023, Zhao Guixia et al. Catalyzed 1,6-hexanediol to ε-caprolactone in aqueous solution at 15 ℃ with titanium-based oxide materials loaded with Pt nanoparticles as catalyst and oxygen as oxidant. The conversion rate of 1,6-hexanediol was 73.5%, and the yield of ε-caprolactone was 43.4%[42]. In terms of non-precious metal catalysts, in 2015, Li Yingwei et al. Created a cobalt-based catalyst Co @ C-N (800) embedded in nitrogen-doped graphite, which can catalyze the oxidation of 1,6-hexanediol to ε-caprolactone at room temperature and atmospheric air[43]. The heterogeneous catalyst was prepared with ZIF-67 as the precursor without additional inorganic base. Under the optimized conditions, the yield of ε-caprolactone was as high as 92% (Fig. 8), and the other main by-product was inner ether. However, the catalytic system often required a long reaction time (such as 96 H). In addition, the Co @ NOSC catalyst anchored with N, S, O doped carbon can also catalyze the oxidation of 1,6-hexanediol to ε-caprolactone, and the conversion of 1,6-hexanediol is 65% and the selectivity of ε-caprolactone is 91% at 100 ℃ for 22 H[44]. In that catalyst, carrageenan is use as a carbon source, an oxygen source and a sulfur source, urea is used as a nitrogen source, and cobalt nitrate is use as a cobalt source. The above studies show that cobalt-based catalysts exhibit special properties and can catalyze the oxidative lactonization of 1,6-hexanediol to ε-caprolactone using molecular oxygen as the oxygen source under mild liquid phase reaction conditions. The main deficiency in the existing reports is that the amount of cobalt-based catalysts used is large, and more efficient non-precious metal heterogeneous catalysts need to be further developed.
图8 Co@C-N(800)催化1,6-己二醇转化为ε-己内酯反应[43]

Fig. 8 Co@C-N(800) catalyzed oxidative lactonization of 1,6-hexanediol into ε-caprolactone[43]

2.3 H2O2 as oxidant

In addition to molecular oxygen, the by-product produced when hydrogen peroxide is used as oxidant is also water, and hydrogen peroxide has a high content of "active oxygen". In 1988, Ishii et al., a Japanese scholar, used cetylpyridinium chloride and phosphotungstic acid to prepare a [π-C5H5N+(CH2)15CH3]3(PW12O40)3- catalyst, which was refluxed for 24 H with H2O2 as the oxidant and t-BuOH as the solvent. The conversion of 1,6-hexanediol was 53%, and the selectivity of ε-caprolactone was 70% (Fig. 9)[45]. In 2006, Bamoharram et al. Realized the efficient conversion of 1,6-hexanediol to ε-caprolactone using a Preyssler-type polyacid H14[NaP5W29MoO110] catalyst[46]. Using chloroform as solvent and 30% hydrogen peroxide as oxidant, the yield of ε-caprolactone reached 98. 5% at reflux temperature for 4 H. It should be pointed out that although this report does not mention side reactions, Preyssler-type polyacids are strongly acidic, and water in hydrogen peroxide and water produced by the reaction may cause hydrolysis of ε-caprolactone. Hydrogen peroxide oxidation system usually has strong oxidizability, but there are few reports on the oxidation of 1,6-hexanediol to ε-caprolactone, which may be related to the complexity of the reaction.
图9 氯化十六烷基吡啶/磷钨酸催化1,6-己二醇制备ε-己内酯反应[45]

Fig. 9 Cetylpyridinium chloride/tungstophosphate-catalysed 1,6-hexanediol to ε-caprolactone[45]

To sum up, the oxidative lactonization of 1,6-hexanediol to ε-caprolactone with carbonyl compounds, molecular oxygen and hydrogen peroxide as electron acceptors requires the addition of catalysts. In the existing literature, organometallic complex catalysts are mostly used in the oxidation of fenol in the presence of carbonyl compounds, which is less atom-economical. The catalytic oxidation process with molecular oxygen or hydrogen peroxide as the oxidant has high atom economy and the theoretical by-product is water, which has significant advantages. Most of the reported hydrogen peroxide catalytic oxidation systems are heteropolyacid salt systems, while enzyme catalysis, homogeneous catalysis and heterogeneous catalysis systems in the process of molecular oxygen catalytic oxidation have their own advantages and disadvantages, and show broad prospects for development.

3 Catalytic dehydrogenation reaction system

Compared with the external oxidant as the electron acceptor, the dehydrolactonization strategy without acceptor is more atom-economical, and molecular hydrogen is the only by-product in theory (Fig. 10). The reported catalytic systems include homogeneous catalytic systems and heterogeneous catalytic systems. The former is usually carried out in liquid phase reaction system, while the latter is usually carried out in solvent-free or high temperature gas phase dehydrogenation environment.
图10 1,6-己二醇催化脱氢内酯化示意图

Fig. 10 Catalytic dehydrogenative lactonization of 1,6-hexanediol

3.1 Homogeneous catalytic dehydrogenation system

In 2015, Beller et al. Reported an iron (II) complex catalytic dehydrogenation system, which used Fe-MACHO-BH complex [carbonylhydrido (tetrahydroborato) [bis (2-diisopropylphosphinoethyl) amino] iron (II)] as catalyst and toluene as solvent to catalyze the dehydrogenation of 1,6-hexanediol to ε-caprolactone at 150 ℃ with the addition of 10% potassium carbonate, with a yield of 63%[47]. Fig. 11 shows the proposed reaction scheme for the dehydrogenation of 1,6-hexanediol to ε-caprolactone, which results in a moderate yield of ε-caprolactone due to the reaction equilibrium of the dehydrogenation reaction.
图11 Fe-MACHO-BH催化1,6-己二醇制备ε-己内酯[47]

Fig. 11 Fe-MACHO-BH catalyzed dehydrogenative lactonization of 1,6-hexanediol to ε-caprolactone[47]

3.2 Heterogeneous catalytic dehydrogenation system

The heterogeneous catalytic dehydrolactonization of 1,6-hexanediol to ε-caprolactone is usually carried out at higher reaction temperatures. In the 1960s, the dehydrogenation process of 1,6-hexanediol was reported[48,49]. For example, Shinzaburo et al. Used a composite oxide catalyst composed of CuO-Cr2O3-ZnO to achieve 78% conversion of 1,6-hexanediol and 68% yield of ε-caprolactone at 210 ~ 220 ℃, but there are few subsequent reports[48]. When Sato et al. Used rare earth metal oxide catalysts to study the high-temperature gas-phase reaction of diols such as 1,6-hexanediol, the production of ε-caprolactone was detected, but the yield was very low[50]. Shimizu et al. Obtained a heterogeneous Pt-SnO2 catalyst by loading 1 mol% platinum on tin dioxide. Under the conditions of 180 ℃, nitrogen atmosphere, no solvent and no additional electron acceptor, the conversion of 1,6-hexanediol was 93%, the isolated yield of ε-caprolactone was 81%, and hydrogen was produced as a by-product[51]. Wang Yuzhong et al., Sichuan University, prepared CuO/Cr2O3/Al2O3 as catalyst to convert 1,6-hexanediol into ε-caprolactone by high temperature gas phase catalytic dehydrogenation (reaction temperature is 300 ℃). The report shows that under the optimal reaction conditions, the conversion of 1,6-hexanediol can reach 100%, and ε-caprolactone is 66.2%[52]. In 2015, Wu et al. Prepared CuO/ZnO/Al2O3/Cr2O3 catalyst by coprecipitation method and carried out gas-solid dehydrogenation reaction in a fixed-bed reactor at 265 ℃, and reported that the conversion rate of 1,6-hexanediol was ≥ 99% and the selectivity of ε-caprolactone was ≥ 98% under the optimal conditions[53]. It should be pointed out again that although there have been many studies on the heterogeneous high-temperature dehydrogenation of diols, and many catalysts can efficiently catalyze the dehydrolactonization of 1,4-butanediol and 1,5-pentanediol, they cannot be directly applied to the dehydrogenation of 1,6-hexanediol to ε-caprolactone, which may be related to the long carbon chain of 1,6-hexanediol and the seven-membered aliphatic ring of ε-caprolactone[54,55]. In view of the high atom economy of the reaction process, the catalyst for 1,6-hexanediol dehydrolactonization needs to be further developed.

4 Conclusion and outlook

ε-caprolactone is an aliphatic compound with ester structure in the molecule, which has important applications in the preparation of degradable polymers. The widely used Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone with peroxide as oxidant has many problems, such as potential safety hazards and large amount of waste. The catalytic conversion of 1,6-hexanediol to ε-caprolactone based on new raw materials does not involve C — C bond oxidative cleavage reaction, and the reaction characteristics are different from those of Baeyer-Villiger oxidation, which should be paid attention to.
In this paper, the research progress in the catalytic conversion of 1,6-hexanediol to ε-caprolactone is reviewed according to the presence or absence of electron acceptors, including the oxidative lactonization of 1,6-hexanediol with carbonyl compounds as electron acceptors, the catalytic oxidative lactonization with molecular oxygen and hydrogen peroxide as electron acceptor, and the catalytic dehydrogenation without electron acceptor. Each of these reaction systems has its own advantages and disadvantages. Considering various factors such as environmental protection, safety and atom economy (Table 1), under alkali-free and mild reaction conditions, the heterogeneous catalytic oxidative lactonization reaction with molecular oxygen as oxidant and the heterogeneous catalytic dehydrolactonization reaction without electron acceptor are ideal conversion methods.It is in line with the principles of green chemistry and sustainable development, and has broad prospects for development. The key is to develop cheap and efficient heterogeneous oxidation/dehydrogenation catalysts, especially non-precious metal Co-based, Cu-based and other catalytic materials.
表1 1,6-己二醇氧化内酯化为ε-己内酯反应的原子经济性、理论副产物、可能安全隐患的对比1)

Table 1 Comparison of atom economy, theoretical by-products, and possible safety hazards in oxidative lactonization of 1,6-hexanediol into ε-caprolactone1)

Electron acceptors Atom utilization [%] Theoretical by-product Possible safety hazards
Methyl isobutyl ketone2) 35.8% 4-Methyl-2-pentanol Volatile solvent
O2 87.7% H2O O2 and organic mixture
H2O2 44.2% H2O Storage and transport of H2O2
Electron acceptor-free (Dehydrogenation reaction) 96.6% H2 Explosion limit of H2

Table note:1) Catalysts and additives are not discussed in this table;2) Methyl isobutyl ketone (MIBK) is used as the example of carbonyl compounds.

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