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Remarkably Efficient Hydrolysis of Cinnamaldehyde to Natural Benzaldehyde in Amino Acid Ionic Liquids

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Remarkably Efficient Hydrolysis of Cinnamaldehyde to Natural Benzaldehyde in Amino Acid Ionic Liquids Korean J. Chem. Eng., 33(12), 3374-3380 (2016) pISSN: 0256-1115 DOI: 10.1007/s11814-016-0204-5 eISSN: 1975-7220 INVITED REVIEW PAPER Remarkably efficient hydrolysis of cinnamaldehyde to natural benzaldehyde in amino acid ionic liquids Shu Xu*, Duan-Jian Tao*,†, Feng-Feng Chen*, Yan Zhou*, Xin Zhao*, Li-Li Yu**, Xiang-Shu Chen*,†, and Kuan Huang*** *College of Chemistry and Chemical Engineering, Jiangxi Inorganic Membrane Materials Engineering Research Center, Jiangxi Normal University, Nanchang 330022, China **Department of Pharmacy, Xi’an Medical University, Xi’an 710021, China ***Department of Chemistry, University of Tennessee, Knoxville TN 37996, U.S.A. (Received 16 April 2016 • accepted 12 July 2016) Abstract−The hydrolysis of cinnamaldehyde to natural benzaldehyde was investigated systematically using tetrameth- ylammonium-based amino acid ionic liquids as homogeneous catalysts. The results indicated that tetramethylammo- nium prolinate ([N1111][Pro]) can be a powerful catalyst for the highly efficient hydrolysis of cinnamaldehyde, in which natural benzaldehyde was obtained with almost 94% yield and over 99% selectivity in 1 h. Moreover, kinetic study showed that compared with other catalysts, the catalytic system of [N1111][Pro] has a lower activation energy of −1 38.30 kJ·mol in the hydrolysis reaction, indicating superior catalytic performance of [N1111][Pro]. Quantum-mechani- cal calculations further manifested that such high performance originates from the cooperative catalysis of the second- ary amino and carboxyl group in the anion [Pro]. Keywords: Natural Benzaldehyde, Cinnamaldehyde, Hydrolysis, Ionic Liquids INTRODUCTION tively [6,7]. The Al-Mg hydrotalcite catalyst was also demonstrated to produce natural benzaldehyde with a considerable yield [8]. In Benzaldehyde is the second most utilized flavor compound world- spite of the preliminary success, these results are not entirely satis- wide and used extensively in the fields of food, beverages, cosmet- factory because of low reaction selectivity and product yield, harsh ics, and pharmaceutical industries, etc [1]. With an increased in- reaction conditions (high temperature of 553 K, high pressure of 15 terest in food safety and natural flavor essence, natural benzalde- MPa), and too complex catalyst preparation. Therefore, many more hyde is widely accepted and is becoming more popular. Thus, the investigations are still required to develop green catalysts with high demand for natural benzaldehyde is growing rapidly, and much efficiency under mild conditions for obtaining natural benzaldehyde. attention has been paid to the production of natural benzalde- Ionic liquids (ILs) have increasingly attracted attention as prom- hyde in recent years [2]. ising catalysts and green solvents [9]. Most ILs have many specific The conventional preparation of natural benzaldehyde is by alka- physical-chemical properties such as structural variability, low vol- line hydrolysis of laetrile. However, this technology could inevita- atility, high thermal and chemical stability, remarkable solubility, bly form highly toxic hydrogen cyanide as a byproduct, which would and effortless reusability. Thus, these features can afford many op- require costly removal methods for safety. With the principle of portunities for the applications of ILs in many fields such as gas green and sustainable processes, the synthesis of natural benzalde- absorption, catalysis, biotransformation, electrochemistry, etc [10- hyde from cinnamon oil, which contains about 80% of cinnamal- 13]. However, to the best of our knowledge, there is no study inves- dehyde, is considered to be an alternative route. But the key problem tigating the application of ILs as catalysts and solvents for the hy- is that cinnamon oil and cinnamaldehyde are poorly soluble in drolysis of cinnamaldehyde to obtain natural benzaldehyde. Among water, resulting in a relatively low yield of natural benzaldehyde. the varieties of ILs, amino acids ionic liquids (AAILs) derived from Thus, several strategies such as near-critical water, modified natu- bioresources are considered to be of low toxicity, high biocompati- ral polymers, and basic hydrotalcite catalyst have been reported to bility and biodegradable characteristics [14-18]. Then the AAILs promote the reaction [3-5]. For example, near-critical water was would have great potential for preserving the highest possible nat- reported to be an excellent solvent to improve the solubility of cin- ural essence of benzaldehyde. namaldehyde and accelerate the hydrolysis reaction [5]. Further- Hence, in this work, a series of tetramethylammonium-based β β more, -cyclodextrin and 2-hydroxypropyl- -cyclodextrin were AAILs ([N1111][AA]) have been used as catalysts for the hydrolysis developed as catalysts for the oxidation of cinnamaldehyde, respec- †To whom correspondence should be addressed. E-mail: [email protected], [email protected] Copyright by The Korean Institute of Chemical Engineers. Scheme 1. Hydrolysis of cinnamaldehyde catalyzed by [N1111][AA]. 3374 Remarkably efficient hydrolysis of cinnamaldehyde to natural benzaldehyde in amino acid ionic liquids 3375 Table 1. Results of hydrolysis of cinnamaldehyde using different of cinnamaldehyde to natural benzaldehyde (Scheme 1). The effect a of different reaction parameters such as reaction time, temperature, catalysts catalyst loading, and molar ratio of reactants were studied in detail. Cinnamaldehyde Benzaldehyde Entry ILs The kinetics for the hydrolysis reaction were explored to obtain the conversion (%) yield (%) kinetics parameters (reaction rate constant, activation energy), and 01[N][Pro] 94 94 a plausible reaction mechanism involving the cooperative catalysis 1111 02[N1111][Gly] 37 34 of [N1111][Pro] was also proposed. 03[N1111][Ala] 8 7 04[N][Val] Trace Trace EXPERIMENTAL METHODS 1111 05[P4444][Pro] 74 66 1. Materials 06[N1111][Ace] <2 <2 Tetramethylammonium hydroxide (25 wt% aqueous solution) 07 [Emim][Pro] 80 72 was obtained from Shanghai Macklin Biochemical Co. Other re- 08NaOH 50 35 09NaCO Trace Trace agents such as amino acid, triethylamine, Na2CO3, and NaOH were 2 3 of analytical grade and used without further purification. Four AAILs 10 Et3N<3<2 a [N1111][Pro], [N1111][Gly], [N1111][Ala], and [N1111][Val] were syn- Reaction conditions: cinnamaldehyde (2.0 mmol), catalyst (5 wt%), thesized via the simple neutralization reaction similar to the reported deionized water (10 g), reaction time (1 h), reaction temperature procedures [19]. The detailed preparation methods and character- (333 K) ization results of these AAILs are listed in the Supplementary data. 2. General Reaction Procedures Typically, cinnamaldehyde (2 mmol, 0.264 g), deionized water to test their catalytic activities. The results are summarized in Table (10 g), and 5 wt% of [N1111][Pro] (0.5132 g) were charged into a 1. [N1111][Pro] catalyzed the hydrolysis reaction smoothly, giving round-bottomed glass flask equipped with a magnetic stirrer and natural benzaldehyde in a good yield (entry 1). However, the other condenser. Then, the reaction mixture was stirred at 333 K for 1 h. three [N1111][AA] ILs catalysts gave unsatisfactory results, with in- Samples were taken from the reactor at regular intervals and ana- complete conversion and low yields of natural benzaldehyde (entries lyzed by GC-MS and GC. For the kinetic study, duplicate experi- 2-4). To gain insight into the catalysis of [N1111][Pro], we calculated ments at least three times were run to obtain averaged values of the natural bond orbital (NBO) charges in the free cinnamalde- natural benzaldehyde yield and selectivity. The averaged uncertainty hyde, [Pro], and the complexes of cinnamaldehyde with [Pro]. As of the kinetic data in this work was well within ±1%. In addition, shown in Fig. 1, compared with the free [Pro], the negative charge large-scale hydrolysis of cinnamaldehyde to natural benzaldehyde of the nitrogen atom in the complexes of cinnamaldehyde with [Pro] was also carried out under optimal conditions as follows: cinnamal- decreases obviously (from −0.054 to −0.01). Meanwhile, both the dehyde (40 mmol, 5.28 g), deionized water (200 g), and 5 wt% of NBO positive charge of the hydrogen atom H (1) and the NBO [N1111][Pro] (10.264 g). negative charge of the oxygen atom O (1) increase, indicating a 3. Analysis strong interaction between secondary amino and aldehyde group. Qualitative analyses of products were examined by a Thermo On the other hand, the carbon atom C (3) and the hydrogen atom Trace 1300 GC-ISQ, and quantitative analyses were carried out by a GC-FID (Agilent 7890B). A capillary column HP-5 (methyl poly- siloxane, 30 m×0.32 mm×1 µm) was used to determine the com- position of the samples with nitrogen as the carrier gas at a flow rate of 3 mL/min. Naphthalene was used as the internal standard to calculate the conversion of cinnamaldehyde and the selectivity of nature benzaldehyde. The temperature of the column, the inlet and the detector was kept at 453, 473, and 523 K, respectively. 4. Computational Methods The geometry optimizations and natural bond orbital (NBO) charge analysis were performed using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level in the Gaussian 09 pro- gram package [20]. Each final optimized structure of [N1111][Pro], cinnamaldehyde and their complexes was checked to be a true mini- mum through frequency calculation at the corresponding levels. RESULTS AND DISCUSSION 1. Catalyst Screening Fig. 1. Optimized structures and NBO charge analysis
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