Solid Phase Extraction of (+)-Catechin from Cocoa Shell Waste Using Dual Ionic Liquid@ZIF8 Covered Silica

Abstract

(+)-catechin is one category of flavonoids in cocoa shell waste and it has been reported to have many health benefits. In order to isolate it from aqueous extracted solution of cocoa shell waste by solid phase extraction (SPE), a series of dual ionic liquids@ZIF8-covered silica were prepared as the sorbents. Regarding the operation conditions of SPE and the characteristic structure of (+)-catechin, ZIF8-covered silica was synthesized to establish a stable and porous substrate, and various dual ionic liquids with multiple properties were immobilized on substrate to obtain a high adsorption capacity. Different adsorption conditions were investigated and the highest adsorption capacity (58.0 mg/g) was obtained on Sil@ZIF8@EIM-EIM at 30 °C during 60.0 min. When the sorbent was applied in the SPE process, 96.0% of the total amount of (+)-catechin from cocoa shell waste can be isolated after several washing and elution steps. The satisfactory recoveries of 97.5–100.2% and RSDs of 1.3–3.2% revealed that the SPE process was accurate and precise. The stability of Sil@ZIF8@EIM-EIM was tested in water and the reusability was tested using repeated adsorption/desorption process. The results revealed that Sil@ZIF8@EIM-EIM as an efficient sorbent can isolate (+)-catechin from cocoa shell waste.

1. Introduction

Cocoa bean is one of the most famous food raw materials in our world. It contains large amounts of beneficial bioactive compounds such as phenolic compounds [1], butter [2], and a high content of polyphenols and flavonoids [3]. Cocoa shell is a byproduct of the cocoa industry but it still contains useful bioactive compounds. In order to reduce the food waste, several researchers extracted these compounds for further applications. Soares et al. extracted flavanols, alkaloids and protein using isopropanol [4], Rebollo-Hernanz et al. obtained 15 phenolic compounds and other flavonoids using water [5], and Okiyama et al. used ethanol with liquid extraction to obtain two main compounds from cocoa shell [6]. (+)-Catechin is one of the flavonoids in cocoa bean and shell (Figure S1). It has a high value for research and a wide application prospect. Some reports indicated that (+)-catechin can be applied to inhibit the growth of harmful microbes and may help to prevent cardiovascular diseases and cancers [7].

There were several extraction techniques involved in the separation and isolation of catechins, such as liquid extraction (LE), solid-phase extraction (SPE), microwave-assisted extraction (MAE), and aqueous two-phase extraction (ATPE). The SPE, with an appropriate sorbent, is a quite common and convenient assistant method to isolate target compounds from solvents [8]. Chu et al. used SPE to extract eight catechins with a good linearity, and the absolute recovery was greater than 85% [9]. Song et al. used a C₁₈ cartridge to pretreat three analytes and the RSD value of catechin was less than 2.23% [10]. In order to increase the isolation efficiency of SPE, several functional materials used as sorbents were developed. For example, Liu et al. used cellulose and pectin to adsorb catechin; the adsorption capacities were 2.4 mg/g and 20.7 mg/g, respectively [11].

When the properties of sorbents in SPE were tuned, more interactions were created between the sorbents and target compounds [12]. Metal-organic frameworks (MOFs), one of several emerging porous materials, were reported. They contained abundant porous structures and were simply synthesized [13,14,15,16]. The potential application prompted bioactive compounds to be isolated. For example, Jiang et al. evaluated the interactions between MOFs and catechin and found that the hydrogen bond and van der Waals force were the two main interaction forces [17,18]. In order to improve the isolation performance of MOFs, sorbent modifiers were involved to modify their textural properties [19]. Ionic liquids (ILs) are one of the most efficient sorbent modifiers. They have unique physicochemical properties because of the adjustable cations or anions [20,21]. When the ILs were modified on sorbents, their hydrophilicity/hydrophobicity properties can provide chemisorption with various chemical forces (hydrogen bond force, ionic force, π–π bond, et al.) to increase the interactions between sorbents and target compounds [22]. Our previous research revealed that IL-immobilized ZIF67 increased the isolation efficiency in the SPE process [23]. However, the low stability of ZIF67 in aqueous solution limited its application. Hence, to isolate (+)-catechin in solution, the type of MOF should be considered, as should the sorbent needed to be composited to increase its stability.

In Hu’s report, ZIF8 as a high-stability material was selected; its composite material was synthesized and constructed an electrochemical sensor for catechins [24]. In our previous research, dual IL-immobilized silica had high stability in various aqueous solutions [25]. Hence, in this research, ZIF8-covered silica was prepared as the substrate, and various dual ILs were immobilized to obtain several composited sorbents. The adsorption efficiencies of sorbents were evaluated and optimized. Finally, the sorbent was applied in the SPE process to isolate (+)-catechin from an aqueous extracted solution of cocoa shell waste.

2. Materials and Methods

2.1. Chemicals

Silica (15–31 μm), (3-chloropropyl)trimethoxysilane (98.0%), 2-methylimidazole (98.0%), zinc nitrate hexahydrate (99.0%), 1,2-dichloroethane (99.0%), 1,4-dichlorobutane (98.0%), 1,6-dichlorohexane (99.0%), imidazole (99.0%),1-methylimidazole (99.0%), 1-ethylimidazole (98.0%), 1-butylimidazole (98.0%), and (+)-catechin (97.0%) were purchased from Aladdin Inc. (Shanghai, China).

HPLC-grade acetonitrile and methanol were obtained from CINC High Purity Solvents Co. Ltd. (Shanghai, China). Methanol, ethanol, triethylamine, hydrochloric acid and acetic acid were obtained from Beilian Company (Tianjing, China), of purities higher than 99.0%. Ultrapure water was produced from a purification machine (UPH-I-5, Youpu, China).

2.2. Apparatus

Scanning electron microscopy (SEM) images were obtained using a MIRA3 scanning microscope (TESCAN, Brno, Czech Republic). Fourier transform infrared (FT-IR, Nicolet 6700, Thermo Fisher, Waltham, MA, USA) spectroscopy was performed using KBr pellets in the range of 400.0–4000.0 cm⁻¹ with a scan rate of 20.0 scans/min. Thermogravimetric analysis (Labsys evo, Setaram, Caluire Et Cuire, France) was used with a heating rate of 10 °C/min under N₂. The BET surface area (in N₂ atmosphere) was measured by ASAP 2020HD88 (Micromeritics, Norcross, GA, USA)

The analysis was performed on a TC-C18 column (4.6 × 150.0 mm, 5.0 μm, Agilent, Santa Clara, CA, USA) with a HPLC (LC3000, CXTH, Beijing, China). The mobile phase, flow rate, UV wavelength, injection volume and column oven temperature were acetonitrile/water (10:90, v/v, containing 1.0% vol. acetic acid), 0.5 mL/min, 280.0 nm, 10.0 μL and 30 °C, respectively.

2.3. Preparation of Dual Ionic Liquids-ZIF8-Covered Silica

First of all, in a 10.0% vol. hydrochloric acid aqueous solution, silica was stirred for 24 h to active the -OH group. After washing with water until pH = 7.0, the activated silica was dried and synthesized as follow: (Figure 1)

In a flask, 30.0 g of silica and 40.0 mL of (3-chloropropyl)trimethoxysilane were mixed with 80.0 mL of toluene. The mixture was heated to 80 °C for 8 h to afford 3-chloropropyl immobilized silica (Sil@Cl). Subsequently, 30.0 g of Sil@Cl, 30.0 g of 2-methylimidazole, 20.0 mL of triethylamine and 80.0 mL of toluene were stirred in a flask for 8 h at 80 °C to produce the 2-methylimidazole-immobilized silica (Sil@2-methylimidazole).

Based on a previously reported method, the covered ZIF8 was prepared with some modifications [19]. To start, 30.0 g of Sil@2-methylimidazole, 25.8 mL of triethylamine and 20.0 g of zinc nitrate hexahydrate were dissolved in 100.0 mL of ultrapure water. At the ambient temperature, the obtained mixture was stirred for 8 h and aged for 16 h. Then the precipitate was collected and washed by methanol three times. Finally, after drying at 60 °C for 12 h, 42.3 g of Sil@ZIF8 were obtained.

Three flasks were prepared and numbered as #1, #2 and #3. Then 10.0 g of Sil@ZIF8, 100.0 mL of toluene, 14.0 mL of 1,2-dichloroethane were mixed in #1 flask; 10.0 g of Sil@ZIF8, 100.0 mL of toluene, 14.0 mL of 1,4-dichlorobutane were mixed in #2 flask, and 10.0 g of Sil@ZIF8, 100.0 mL of toluene, 14.0 mL of 1,6-dichlorohexane were mixed in #3 flask. The three flasks were simultaneously heated to 80 °C for 10 h, and three Sil@ZIF8@ILs (Sil@ZIF8@EIM, Sil@ZIF8@BIM and Sil@ZIF8@HIM) were obtained.

Finally, 3.2 g of Sil@ZIF8@IL was synthesized with the same molar weights of imidazole or 1-methylimidazole or 1-ethylimidazole or 1-butylimidazole in 80.0 mL of toluene at 80 °C for 8 h. After washing with 50.0 mL of methanol, 12 types of Sil@ZIF8@IL-IL, shown in Figure 1, were obtained.

2.4. Adsorption Isothermal and Kinetics Studies

Firstly, the calibration curve was created using (+)-catechin aqueous solution with seven different concentrations (from 0.0005 mg/mL to 1.2 mg/mL).

Then, the maximum adsorption amounts of all 12 sorbents were evaluated. First, 0.015 g of each sorbent was mixed with 2.0 mL of 1.0 mg/mL of (+)-catechin aqueous solution for 10 h at 30 °C. After fully adsorption, the residual (+)-catechin was detected by HPLC.

After that, 0.015 g of each selected sorbent was immersed in 1.0 mL of (+)-catechin aqueous solution with different concentrations (from 0.001 mg/mL to 1.0 mg/mL) at 10, 20, 30 and 40 °C. The concentrations of all solutions before and after the adsorption process were detected by HPLC, and the adsorption isothermal and adsorption efficiencies were calculated by Equations (1) and (2)

$$Q_e=\frac{\left(C_0-C_e\right)\times V}{m}$$

(1)

$$ER\%=\frac{\left(C_0-C_e\right)\times V}{C_0\times V}\times 100\%$$

(2)

where Q_e (mg/g) is the adsorption capacity of (+)-catechin adsorbed onto sorbent at equilibrium, and C₀ and C_e (mg/mL) are the initial concentration before adsorption and residual concentration after adsorption, respectively. V (mL) is the total volume of solution and m (g) is the weight of the sorbent. ER% is the adsorption efficiency of sorbent.

For the adsorption kinetics study, 0.015 g of sorbents was immersed in 1.0 mL of (+)-catechin in water with initial concentrations of 1.0 mg/L at 30 °C for different times (from 1.0 min to 120.0 min). Then, the pseudo-first-order (Equation (3)) and pseudo-second-order (Equation (4)) kinetics models were used to fit the experimental data to evaluate the kinetics of the adsorption process.

$$\ln (Q_e-Q_t)=\ln Q_e-k_{1}t$$

(3)

$$\frac{t}{Q_t}=\frac{1}{k_2{Q}_e^2}+\frac{t}{Q_e}$$

(4)

Q_t (mg/g) is the amount of (+)-catechin adsorbed at different times. k₁ (1/min) and k₂ (g/mg·min) are the rate constants for pseudo-first-order and pseudo-second-order models, respectively.

2.5. Stability and Reusability Test

Base on the stability analysis method in other literatures [26,27,28,29], 5.0 g of Sil@ZIF8@IL-IL sorbents were dispersed in 50.0 mL of distilled water, and the solutions were kept at 30 °C for three days under constant stirring. Then, the sorbents were washed by 10.0 mL of water twice and dried at 80 °C for 24 h before performing further BET analysis.

In order to test the reusability of the sorbent, 0.015 g of sorbent was immersed in 1.0 mL of (+)-catechin in water at a concentration of 1.0 mg/mL at 30 °C in 120.0 min. After the residual (+)-catechin was detected by HPLC, the sorbent was desorbed and washed more than ten times. Then, the same adsorption and desorption processes were repeated on the 0.015 g of sorbent under the exactly same conditions. The adsorption capacity was analyzed after each repeated process.

2.6. Isolation of (+)-Catechin from Cocoa Shell Waste Using SPE

To start, 10.0 g of cocoa shell waste powder was dipped into 20.0 mL of water at 65 °C. After 8 h, the extracted solution was collected and filtered through a 0.2 μm membrane. Next, 0.3 g of sorbent was packed into a standard SPE cartridge (Ø 0.9 cm) and washed with 6.0 mL of methanol. Then the extracted solution was poured into the cartridge and the outflow from the bottom was collected. After that, 3.0 mL of solution with different elution abilities, such as water, acetonitrile, methanol, and methanol/water (contained 1% vol. of acetic acid), were used to separate (+)-catechin from interferences with a flow of 0.6 mL/min.

3. Results and Discussion

3.1. Characterization

The results of SEM analyses of the synthesized composites were presented in Figure 2. In Figure 2A, ZIF8 was covered on the surface of silica. The particle size of silica was less than 30.0 μm and the regular polygon-shape of ZIF8 was about 250.0 nm. When the dual ILs were immobilized, the particle size of silica was not changed; however, the maximum polygon-shape of ZIF8 was increased to 300.0 nm.

The FTIR of materials are shown in Figure S2. Two peaks at 689.0 cm⁻¹ and 758.8 cm⁻¹ are shown on ZIF8, and one peak at 805.5 cm⁻¹ was shown on Sil. On Sil@ZIF8, three peaks appeared at the same positions. This phenomenon also applied to the peaks at 1309.1 cm⁻¹ and 1457.9 cm⁻¹. This result revealed that ZIF8 was successfully covered on silica. Compared to Sil@ZIF8, Sil@ZIF8@IL-IL showed one more peak at 670.0 cm⁻¹, which belonged to Cl- on ILs groups. Additionally, on Sil@ZIF8@IL-IL, the peak areas of the -C-N group at 951.0 cm⁻¹ and the -C-H group in the range of 3000–3300 cm⁻¹ were increased obviously. The results proved that Sil@ZIF8@IL-IL were well prepared.

TGA can determine the thermal degradation of Sil, ZIF8, Sil@ZIF8 and one of Sil@ZIF8@IL-IL (Figure S3). When the temperature was higher than 600 °C, Sil was stable enough, but the ZIF8, Sil@ZIF8 and Sil@ZIF8@IL-IL lost 37.4%, 26.2% and 21.9% of their weight, respectively. The weight loss of ZIF8 was due to the decomposition of the ligand, and the lesser weight loss of Sil@ZIF8@IL-IL was owed to the stability of IL groups.

The surface area was determined by BET, and the blank silica was 50.4 m²/g. Because of the porous structure and nanoscale size of ZIF8, the surface area of Sil@ZIF8 was obviously increased to 164.2 m²/g. Sil@ZIF8@IL-IL exhibited a larger surface area (176.6 m²/g), revealing that ionic liquid groups were successfully immobilized on the surface of sorbents once again.

3.2. Comparison of the Maximum Adsorption Ability

The correlation equation was y = 0.19x + 0.001 (y and x are the peak area and the concentration in solution, respectively) and R² = 0.97 revealed the good linearity of the calibration curves.

In order to prove the optimum adsorption capacity of Sil@ZIF8@IL-IL, the maximum adsorption amounts of Sil, ZIF8, Sil@ZIF8 and 12 ILs-immobilized sorbents were evaluated. Figure 3 showed that the maximum adsorption amounts on ILs-immobilized sorbents were much higher than Sil, ZIF8 and Sil@ZIF8. The results were caused by the increasing BET surface area as well as the chemical interactions between ionic liquid groups and (+)-catechin. However, the maximum adsorption amounts of four Sil@ZIF8@HIM-IL sorbents were obviously lower than the others because of the excess length of the carbon chain in IL groups that increased their hydrophobic properties.

The ER% of Sil@ZIF8@BIM-IL and Sil@ZIF8@EIM-IL sorbents are shown in Figure 4 and of Sil@ZIF8@HIM-IL sorbents in Figure S4. The ER% of all sorbents increased with the C₀ increasing. When the C₀ was higher than 0.2 mg/mL, the (+)-catechin on the surface of sorbents tended to saturate, so the ER% decreased gently. The standard deviations of all data points in Figure 4 were in the range of 1.7–3.8% and in Figure S4 were in the range of 2.4–4.3%. The maximum ER% for four Sil@ZIF8@EIM-IL, for four Sil@ZIF8@BIM-IL and for four Sil@ZIF8@HIM-IL sorbents were up to 92.5%, 91.6%, and 88.0%, respectively. Figure 4 also showed the relationship between the ER% and temperature. With the increasing of temperature, the ER% increased until 30 °C and then decreased. Hence, the adsorption equilibrium could be disturbed at high temperature. In conclusion, four Sil@ZIF8@EIM-IL sorbents had higher adsorption efficiencies than the others, and the optimized temperature was selected as 30 °C.

3.3. Adsorption Isothermal and Kinetics Studies

For further investigation, the isothermal adsorptions were tested on Sil@ZIF8@BIM-IL and Sil@ZIF8@EIM-IL sorbents to explain the relationship between the adsorption capacities and concentrations. The adsorption capacities on Sil@ZIF8@BIM-IL and Sil@ZIF8@EIM-IL sorbents at four temperatures with different C₀ are shown in Figure 5.

The -OH and -O- groups interacted with the imidazole group through a hydrogen bond. (+)-catechin can bind to positive ion [30], so the interaction of the ionic force between IL and (+)-catechin resulted in a larger adsorption capacity. Additionally, the π-π bond force appearing between the benzene ring and imidazole ring can increase the selectivity of sorbents. However, the polarity of the dual IL layers decreased with carbon chain length increasing. The optimum equilibrium of the interaction was based on the hydrophilic/hydrophobic property of (+)-catechin and the carbon chain length of the dual IL layers. Compared to the results of four Sil@ZIF8@BIM-IL sorbents in Figure S5, Sil@ZIF8@EIM-EIM had the highest adsorption amount of all the sorbents, and Figure 5 proved that 30 °C was the optimized temperature for adsorption.

Figure 6 showed the relationships between adsorption time and adsorption amount (Q_t) of (+)-catechin on Sil@ZIF8@EIM-IL and Sil@ZIF8@BIM-IL sorbents at 30 °C. With time increasing, the adsorption amount on all sorbents increased. Among the results, Sil@ZIF8@EIM-EIM showed the highest adsorption amount (58.0 mg/g) and the shortest equilibrium time (60.0 min). After adsorption, the surface area of Sil@ZIF8@EIM-EIM was decreased to 148.3 m²/g, revealing that the molecules of (+)-catechin were captured by ionic liquid groups and filled the pore structure of the sorbent.

3.4. Other Effectives on Adsorption Amounts of the Four Sil@ZIF8@EIM-IL Sorbents

Other effectives on adsorption amount such as ratio of organic solvent and pH were tested. Figure 7 showed the effect of ratio of methanol in (+)-catechin aqueous solution. With the ratio of methanol increasing, the adsorption amounts on Sil@ZIF8@EIM-IM, Sil@ZIF8@EIM-MIM and Sil@ZIF8@EIM-BIM decreased, especially on Sil@ZIF8@EIM-BIM. Because the structure of the IL group on Sil@ZIF8@EIM-EIM perfectly fitted the structure of (+)-catechin, the adsorption amount on it was almost unaffected. Figure S6 showed the pH effect of (+)-catechin aqueous solution on adsorption amount. When the pH was increased from 3.0 to 6.0, the adsorption amounts increased from 52.1 mg/g to 58.0 mg/g. However, when the pH was higher than 7.0, the adsorption amount rapidly decreased. This variation trend can be explained by Ahmadijokani’s and Molavi’s theories: in acidic solution, the electrostatic repulsion forces between (+)-catechin and ZIF8 increase. When the pH is in the range of 5.0–7.0, the repulsive forces decrease and the attractive force becomes dominant [28,29]. With the pH increasing, the solution environment was harmful for the structural stabilities of sorbent or (+)-catechin. Hence, the pH in the range of 5.0–7.0 was suitable for adsorption. Further, from previous results it can be derived that methanol and acidic solution can be used to desorb (+)-catechin from the sorbent in the SPE process.

3.5. Solid Phase Extraction of (+)-Catechin from Cocoa Shell Waste

Firstly, the extracted solution of cocoa shell waste was analyzed, and the concentration of (+)-catechin was 0.46 mg/g (Figure 8 (A)). Then, in an incubator at 30 °C, 0.3 g of Sil@ZIF8@EIM-EIM was packed into the SPE cartridge and the extracted solution was poured into it. After 60.0 min, water was used to wash the sorbent, and Figure 8 (B) showed that the water removed a large amount of interference from the sorbent. Figure 8 (C) showed that acetonitrile as a washing solvent can remove lipophilic and polar organic interferences. Combined with the results in Figure 7 and Figure 8 (D), methanol as the washing solvent caused desorption of (+)-catechin from sorbent, and 18.4% of the total amount of (+)-catechin was obtained in the step. The final elution step in Figure 8 (E) showed that methanol/water (containing 1% vol. of acetic acid) can isolate 77.6% of the total amount of (+)-catechin. Totally, 96.0% of the total amount of (+)-catechin can be isolated from cocoa shell waste by the SPE process.

3.6. Stability and Reusability of Sil@ZIF8@EIM-EIM, and Validation of SPE

After three days of dipping in water, the surface area of Sil@ZIF8@IL-IL was slightly decreased from 176.6 m²/g to 175.2 m²/g. During the repeated adsorption and desorption processes, the adsorption capacity of Sil@ZIF8@EIM-EIM decreased by 2.3% after six repetitions, and it decreased by 9.6% after 10 repetitions. The results revealed that the stereostructure of Sil@ZIF8@EIM-EIM was stable enough and that dual IL groups protected substrate and provided chemical interactions at the same time.

The LOD (signal-to-noise ratios of three) and LOQ (signal-to-noise ratios of 10) shown in

Table 1. Validation results of analysis and SPE process.

(+)-Catechin in Cocoa Shell Waste (mg/g)	Spiked (mg/g)	Found (mg/g)	Relative Recovery (%)	RSD (%)		LOD	LQD
				Intra-Day	Inter-Day
0.46	0.35	0.79	97.5	1.3	2.6	0.006 mg/L	0.01 mg/L
	0.45	0.91	100.2	1.5	3.2
	0.55	1.00	99.3	1.7	2.9

proved that the analysis condition was precise enough. Then, a certain concentration was spiked into extracted solution of cocoa shell, and the recoveries of SPE were analyzed and calculated using the HPLC peak areas and the correlation equation [10]. Moreover, the relative standard deviations (RSDs) in Table 1 were analyzed by repeating the SPE process five times per day (intra-day RSD) under exactly the same conditions over five consecutive days (inter-day RSD). The satisfactory recoveries of 97.5–100.2% and RSDs of 1.3–3.2% revealed that the SPE conditions with Sil@ZIF8@EIM-EIM was accurate and precise for isolating (+)-catechin from cocoa shell waste.

In

Table 2. Comparison of the methods or proposed material for catechin.

Sorbent	Capacity (mg/g)	LOD (mg/L)	Recovery (%)	RSD (%)	Recycle	Ref
-	1.7	-	-	-	-	[5]
-	0.14				-	[6]
-	-	0.4	94.6–100.0	-	-	[10]
Cellulose and pectin	20.7	-	-	-	-	[11]
MOF sensor	-	0.01	-	-	-	[24]
IL resin	101.7	-	-	-	5	[31]
IL micellar	-	0.2	98.0–104.5	0.2–1.8	-	[32]
Sil@ZIF8@IL-IL	58.0	0.006	97.5–100.2	1.3–3.2	6	This work

, the performance of proposed methods or sorbents for catechin were compared with previous studies. Some literature reported optimized conditions for extraction or analysis of catechin from cocoa shell. Rebollo-Hernanz et al. optimized the extraction conditions (100 °C, 90 min) of catechin from cocoa shell, and 1.7 mg/g of catechin were obtained [5]. Okiyama et al. used ethanol with a pressurized liquid extraction method to obtain catechin from cocoa bean shell with an extraction yield of 0.14 mg/g [6]. Song et al. used the SPE method to detect catechin with a recovery in the range of 94.6–100.0% [10]. Moreover, adsorption materials for catechin were also reported. Liu et al. adsorbed catechin onto cellulose and pectin. The adsorption capacities of pectin and cellulose for catechin were 20.7 mg/g and 2.4 mg/g for 24 h at 37 °C, respectively [11]. Hu et al. prepared a ZIF-8 composite material for a catechin sensor with a wide detection range and a low LOD [24]. Pei et al. prepared ionic liquid-modified hypercrosslinked polystyrene resins to adsorb catechin from aqueous solution, and 101.7 mg/g of adsorption capacity was obtained [31]. El-Hady and Albishri prepared ionic liquid-based sweeping-micellar and successfully applied it in the detection of catechin in human plasma [32]. Overall, the sorbent Sil@ZIF8@EIM-EIM developed in the present study exhibited better performance than those reported in other literature.

4. Conclusions

In this study, Sil@ZIF8@EIM-EIM as the optimized sorbent was successfully applied in SPE and isolated (+)-catechin from extracted solution. Sil@ZIF8 as the substrate provided a stable and porous structure, and dual ILs exhibited strong interactions with (+)-catechin. The maximum adsorption capacity of Sil@ZIF8@EIM-EIM was 58.0 mg/g at 30 °C in 60.0 min. The surface area of Sil@ZIF8@EIM-EIM was slightly decreased after three days of dipping in water, and the adsorption capacity declined insignificantly in six repeated adsorption and desorption processes. In the SPE process, 96.0% of the total amount of (+)-catechin can be isolated from cocoa shell waste with satisfactory recoveries (97.5–100.2%) and RSDs (1.3–3.2%). Overall, Sil@ZIF8@EIM-EIM was a potential sorbent to isolate (+)-catechin, and it could be an ideal sorbent for recycling other plants waste containing (+)-catechin.