Synthesis of Lipophilic Caffeoyl Alkyl Ester Using a Novel Natural Deep Eutectic Solvent.

In this work, a novel method for lipophilic caffeoyl alkyl ester production was developed using a natural deep eutectic solvent (DES) consisting of choline chloride and caffeic acid (CA) as the caffeoyl donor. Cation-exchange resins were used as the catalyst to catalyze the esterification of fatty alcohols with the DES. Effects of the caffeoyl donor and reaction variables were investigated. Reaction thermodynamics were also analyzed. The results showed that the lipophilic caffeoyl alkyl ester production can be enhanced using the DES as the caffeoyl donor, and cation-exchange resin A-35 showed the best catalytic activity for the reaction. Under the optimized conditions (85 °C, stearyl alcohol/CA 8:1 (mol/mol), A-35 load 5% and 24 h), the maximum octodecyl caffeate (OC) yield (90.69 ± 2.71%) and CA conversion (95.17 ± 2.76%) were obtained with the DES as the caffeoyl donor, which were much higher than those obtained with solid CA as the caffeoyl donor (OC yield 40.97 ± 2.37% and CA conversion 44.26 ± 1.69%). The activation energy of CA conversion (67.57 kJ/mol) with the DES was lower than that with solid CA (90.19 kJ/mol). In addition, the mass transfer limitation can be decreased with the DES. Compared with solid CA as the caffeoyl donor, a fast reaction rate and low mass transfer limitation were obtained using the DES as the caffeoyl donor.

Introduction

Caffeic acid (CA, 3,4-dihydroxycinnamic acid) is widely present in some Chinese herbal medicines, such as peppermint, menthol, and perilla as well as in plant foods, such as tomatoes, carrots, apple juice, and coffee.[1,2] CA can inhibit the oxidative deterioration of food as a food additive.[3] In addition, CA also has some physiological activities, such as antioxidant activity,[4−7] anti-inflammatory activity,[8] antimicrobial activity,[9] anticancer and antitumor activities,[10,11] etc. However, the poor solubility of CA has limited its application in food and chemical industries. Therefore, in order to improve the application of CA, the modifications of CA using some groups have attracted more attention.[12,13]

Caffeoyl alkyl ester is one kind of lipophilic ester of CA, which can be extracted from Chinese herbal medicines, such as comfrey, Halocnemum strobilaceum, and medlar.[14,15] However, the separation and purification processes of the caffeoyl alkyl esters from these materials are complicated, and the yields are very low (∼1.6 mg/g).[15] At present, the synthesis of the caffeoyl alkyl esters is mainly focused on the short carbon-chain alkyl esters of CA,[16,17] and few research studies focused on the synthesis of long-chain alkyl esters of CA are found.[18,19] In these previous modification methods of CA, solid CA was often used as the caffeoyl donor. However, due to the high melting point of solid CA (211-213 °C), low reaction efficiency was obtained in these methods.

Recently, due to the low vapor and stable chemical properties, nontoxicity or low toxicity, biodegradability, simple preparation, low cost, greenness, and environmental protection,[20,21] a deep eutectic solvent (DES) has been used as a new type of green solvent[22,23] and catalyst.[24] For example, a DES consisting of choline chloride and urea was used as the solvent to dissolve some reaction substrates, which can improve lipophilic alkyl ferulate esters[25,26] and other phenolic acid preparations.[27,28] In addition, the DES has also been used as the extraction medium for production of some bioactive substances.[29−31] However, no study focused on the lipophilic caffeoyl alkyl ester production using a DES as the reaction substrate was found.

In this work, in order to prepare lipophilic caffeoyl alkyl esters, a natural DES consisting of choline chloride and caffeic acid (CA) was used as a novel caffeoyl donor. Several environment-friendly catalysts (cation-exchange resins) and fatty alcohols were used as catalysts and caffeoyl acceptors, respectively. The influences of various reaction parameters (reaction temperature, time, substrate ratio, and catalyst load) on the lipophilic caffeoyl alkyl ester formation were investigated. The thermodynamics were also analyzed.

Results and Discussion

Product Identification

Three products were found in the system with the DES as the caffeoyl donor (Figures S1 and S2). These three products were identified using HPLC-MS as caffeoyl choline (CC, peak 1), CA (peak 2), and octodecyl caffeate (OC, peak 3) (Table S1). However, only two products, CA (peak 2) and OC (OC, peak 3), were found in the system with solid CA.

Effect of Fatty Alcohols

Figure A shows that, with the increase of the carbon chain of fatty alcohols, CA conversion decreased using the DES as the caffeoyl donor, which was attributed to the high melting point and the great steric hindrance of long carbon-chain fatty alcohols. Similar results were also found using CA as the caffeoyl donor (Figure B).

Figure 1

Effect of fatty alcohols on CA conversion (A, DES as the caffeoyl donor; B, solid CA as the caffeoyl donor). Reaction conditions: A-35 load 5% (w/w) and substrate ratio 8:1 (fatty alcohol/CA, mol/mol) at 85 °C.

Figure A also shows that, when the DES and octanol were used as the caffeoyl donor and acceptor, respectively, the maximum CA conversion (88.28 ± 2.01%) was achieved at 12 h, which was much higher than that obtained with solid CA as the caffeoyl donor (46.77 ± 0.79%, 12 h). This was due to the fact that the DES was liquid in the reaction system, which can act as both the reactant and solvent, and reduced the mass transfer resistance. A similar effect of the liquid DES can also be found in other reactions.[32]

Catalyst Screening

As one kind of green catalyst, solid acids (heteropoly acids, solid superacids, zeolite/mesoporous molecular sieves, anion/cation-exchange resins) have been used in some reactions[33,34] due to their strong acidities, easy separation, and good selectivity.[35] Compared with other solid acids, cation-exchange resins have high activity and selectivity, reusability, fewer byproducts, and less environmental pollution, which make them widely used in many fields such as esterification and transesterification.[36,37] In this work, A-35 and NKC-9, sulfonic acid-type cation-exchange resins with a large pore diameter (325 Å) were used as catalysts to prepare the lipophilic caffeoyl esters. Figure shows that with the DES and A-35, the OC yield reached 76.54 ± 1.95% at 12 h, which was higher than that obtained with NKC-9 (OC yield 70.69 ± 2.53%). These results were ascribed to the high cross-linking degree, low swelling, and large specific area and pore size of A-35, which can favor substrates to move into the active sites of A-35.[38] In addition, the acid strength (5.1 mmol/g) of A-35 is higher than that of NKC-9 (4.7 mmol/g), which also resulted in the high catalytic activity of A-35. These results were different from the results of our previous report;[39] for the synthesis of caffeoyl structured lipids, the catalytic activity of NKC-9 was better than that of A-35, which was due to formation of more byproduct CC (∼35%) with A-35 as the catalyst. Similar good catalytic performance of A-35 can also be found in the triacetyl glyceride production.[40] With the liquid DES as the caffeoyl donor, the OC yield (76.54 ± 1.95%) was 3 times that obtained with solid CA as the caffeoyl donor (24.59 ± 1.37%), which was attributed to the low mass transfer limitation of the homogeneous reaction system consisting of the liquid DES with liquid stearyl alcohol at reaction temperature.

Figure 2

Effect of A-35 and NKC-9 on CA conversion and OC yield (A, DES as the caffeoyl donor; B, solid CA as the caffeoyl donor). Reaction conditions: A-35 load 5% (w/w) and stearyl alcohol/CA 8:1 at 85 °C for 12 h.

Effect of Temperature

Figure shows that with different caffeoyl donors (liquid DES and solid CA), both CA conversion and OC yield increased, and CA conversion reached the maximum at 24 h when the reaction temperature ranged from 70 to 90 °C. With the DES as the caffeoyl donor, the maximum CA conversions reached 95.17 ± 2.76% (24 h) at 85 °C and 97.57 ± 2.49% (24 h) at 90 °C (Figure A). In addition, OC yields also reached the maximum at 85 and 90 °C (90.69 ± 2.71% and 90.73 ± 1.97%) (Figure C). These results were due to the low viscosity of the DES system, the high catalytic activity of A-35, and the rapid mass transfer rate at high temperature. However, with solid CA as the caffeoyl donor, CA conversion was only 44.26 ± 1.69% (24 h) at 85 °C (Figure B), which was much lower than that obtained with the DES as the caffeoyl donor (95.17 ± 2.76%) at 85 °C. These results were attributed to the high viscosity and the high melting point (211 °C ∼ 213 °C) of CA.

Figure 3

Effect of reaction temperature on CA conversion (A, DES as the caffeoyl donor; B, solid CA as the caffeoyl donor), (C) OC yield, and (D) relationship between the initial reaction rate and reaction temperature. Reaction conditions: A-35 load 5% (w/w) and stearyl alcohol/CA 8:1 (mol/mol).

Effect of the Substrate Ratio

Figure shows that with the increase of the ratio of fatty alcohol from 1:1 to 12:1 (stearyl alcohol/CA), CA conversion and the reaction rate gradually increased, which was ascribed to more OC formation in the presence of more stearyl alcohols. With the DES as the caffeoyl donor and 8:1 substrate ratio, CA conversion (95.17 ± 2.76% at 24 h) and the initial reaction rate (4.83 × 10–4 mol/(L·min)) were 2 and 4 times those obtained with solid CA as the caffeoyl donor (44.26 ± 1.69% at 24 h and 1.16 × 10–4 mol/(L·min)), respectively. With the DES as the caffeoyl donor, the time to reach equilibrium decreased from 12 h of 1:1 to 6 h of 12:1 (Figure A). However, for solid CA as the caffeoyl donor, the time to reach equilibrium was all >24 h (Figure B), which was attributed to the high mass transfer between liquid stearyl alcohol and solid CA.

Figure 4

Effect of the substrate ratio on CA conversion (A, DES as the caffeoyl donor; B, solid CA as the caffeoyl donor). Reaction conditions: A-35 load 5% (w/w) and 85 °C.

Effect of Catalyst Load

With these two caffeoyl donors (DES and CA), the reaction rate and CA conversion both gradually increased with the increase of A-35 load (Figure ). With a further increase of A-35 load, more acid sites can be provided to increase the contact probability between acid sites and substrates, which can enhance the reaction rate. With the increase of A-35 load up to 5%, CA conversion (95.17 ± 2.76%) using the DES as the caffeoyl donor was 2.1 times that obtained using solid CA as the caffeoyl donor (44.26 ± 1.69%) (Figure A,B). Under the same reaction conditions, with the DES as the caffeoyl donor, the effect of A-35 load on CA conversion was more significant than that obtained with solid CA as the caffeoyl donor.

Figure 5

Effect of A-35 load on CA conversion (A, DES as the caffeoyl donor; B, solid CA as the caffeoyl donor), (C) OC yield at 24 h, and (D) relationship between the catalyst concentration and the initial reaction rate. Reaction conditions: stearyl/CA alcohol 8:1 (mol/mol) and 85 °C.

When the A-35 load was 5% with solid CA as the caffeoyl donor, the OC yield reached 40.97 ± 2.37% (Figure C). With a further increase of A-35 load from 5 to 9%, the OC yield maintained at the same level (∼45%). However, when the DES was used as the caffeoyl donor, the OC yield reached the maximum (90.69 ± 2.71%) with 5% catalyst load and 91.35 ± 1.77% with 7% catalyst load at 24 h, which were 2.2 times that obtained with solid CA as the caffeoyl donor (40.97 ± 2.37%). The results were due to the homogeneous reaction system composed of the liquid DES and liquid fatty alcohol, which reduced the mass transfer limitation. In addition, with a further increase of catalyst load up to 9%, the OC yield slightly decreased to 86.72 ± 1.83% (Figure C), which was due to the formation of more byproduct CC in the presence of excess A-35.

Figure D shows that, when the DES was used as the caffeoyl donor, the relationship between the initial reaction rate (V0) and catalyst load suggests that the influence of external mass transfer on the reaction can be negligible. The initial reaction rate of the esterification using the DES as the caffeoyl donor (3.06 × 10–4 mol/(L·min)) was almost 3 times that obtained with solid CA as the caffeoyl donor (1.05 × 10–4 mol/(L·min)).

Reaction Thermodynamics

Figure D shows a good linear relationship (R2 > 0.98) between the initial reaction rate and temperature. With the DES as the caffeoyl donor, the Ea of CA conversion was 67.57 kJ/mol, which was lower than that obtained using solid CA as the caffeoyl donor (90.19 kJ/mol) (Table S2). In addition, both the Ea values were higher than those obtained using solid CA with methanol (17.5 kJ/mol).[17] These results were due to the presence of great steric hindrance of stearyl alcohol.

Reaction Mechanism

Figure shows the esterification mechanism of the DES with fatty alcohol using A-35 as the catalyst as follows. H+ is first released from cation-exchange resin A-35, and H+ attacks the carboxyl carbon atom of CA to form a carbon cation (i). Then, the OH of octadecyl alcohol attacks the carbon cation formed by the reaction (i) to form a regular tetrahedral intermediate (ii). Finally, the tetrahedron formed by the reaction (ii) releases one water and one H+ to form OC (reactions iii and iv).

Figure 6

Mechanism of sulfonic acid-type cation-exchange resins catalyzing the esterification of the DES with stearyl alcohol (R is octadecyl).

Conclusions

In this work, a novel method for the lipophilic caffeoyl alkyl ester production was successfully developed with the DES and cation-exchange resin A-35 as the caffeoyl donor and catalyst, respectively. When the DES was used as the caffeoyl donor, the maximum lipophilic caffeoyl alkyl ester OC yield (90.69 ± 2.71%) and CA conversion (95.17 ± 2.76%) were obtained with the 5% A-35 load and 8:1 molar ratio of stearyl alcohol to CA at 85 °C for 24 h and were 2 times those obtained using solid CA as the caffeoyl donor (OC yield 40.97 ± 2.37% , CA conversion 44.26 ± 1.69%). The activation energy of CA conversion with the DES as the caffeoyl donor (67.57 kJ/mol) was lower than that obtained using solid CA as the caffeoyl donor (90.19 kJ/mol). Compared with those traditional methods using solid CA as the caffeoyl donor, this work using the novel DES as the caffeoyl donor showed some advances as follows: a high lipophilic OC yield, environment-friendly cheap catalyst, fast reaction rate, and low mass transfer limitation.

Experimental Section

Materials

Choline chloride and CA were purchased from Shanghai Macleans (Shanghai, China) and Nanjing Zelang Chemical Co., Ltd. (Nanjing, China), respectively. Cation-exchange resins and fatty alcohols were purchased from Jiangsu Nanda Synthetic Chemical Co., Ltd. (Jiangsu, China) and Tianjin Kemiou Reagent Co., Ltd. (Tianjin, China), respectively.

DES Preparation

ChCl was mixed with CA (2:1, mol/mol) at 80 °C and 90 kPa. After 2 h, a transparent liquid was formed, and the DES was obtained.

Esterification of Different Caffeoyl Donors with Fatty Alcohols

Fatty alcohols were mixed with a caffeoyl donor (solid CA or DES) in a 25 mL flask, which was heated to a certain temperature for 10 min using a magnetic stirrer. After this, the catalyst was added into the system, and the reaction was initiated. At regular intervals, the sample (10 μL) was withdrawn and dissolved with 1 mL of methanol and 2 mL of trichloromethane. Finally, the sample was filtered for HPLC analysis.

HPLC Analysis

According to the previous methods,[41,42] HPLC with a C18 column was used to analyze the samples. The elution was carried out at 0.8 mL/min with 0.5% glacial acetic acid aqueous solution (solvent A) and methanol (solvent B). The samples were eluted with 95% (v/v) B and 5% A for 20 min at 325 nm.

Mass Spectroscopic Analysis of the Products

According to the previous method,[43] reaction products were identified by HPLC-ESI-MS. The voltages of the cone and the capillary were 30 V and 3 kV, respectively. The temperatures of the ion source and the desolvation were 80 and 180 °C, respectively. Gas velocities of cone desolvation and desolvation were 380 and 40 L/h, respectively.