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One-Pot Catalytic Oxidation for Transforming Eugenol to Vanillin Using ZnAl
2O
4Catalyst
Damiana Nofita Birhi,1 Adzkia Qisthi Ismail,1 Elvina Dhiaul Iftitah,1,2* and Warsito1,2
1Department of chemistry, Faculty of Mathematics and Natural Sciences, Brawijaya University, Indonesia
2Essential Oil’s Institute, University of Brawijaya, Indonesia
*Corresponding email: vin_iftitah@ub.ac.id Received 28 September 2021, Accepted 19 December 2021
ABSTRACT
In this study, ZnAl2O4 catalyst was synthesized with the capability of transforming eugenol to vanillin through One-Pot Catalytic Oxidation. ZnAl2O4 was synthesized from Zn(CH3COO)2.2H2O and Al2O3 using the wet-impregnation method, and characterized by FTIR, XRD, and SEM. One-Pot Catalytic Oxidation was conducted by heating under reflux at 150oC using nitrobenzene and a certain amount of ZnAl2O4 catalyst (4% and 7%) for 2 and 3 hours of reaction. Catalytic Oxidation is also carried out without catalyst as a comparison. The vanillin product was confirmed by GC and spectral data achieved from UV-Vis, FTIR, and mass spectrometry. The results revealed that transforming eugenol to vanillin using ZnAl2O4 catalyst provides a better selectivity value than without using the catalyst, is 100% for the use of 4% catalyst in 2 hours, while without catalyst gives 88% in 3 hours. In addition, the use of 4% catalyst in 3 hours gives 94% for selectivity of vanillin, and the use of 7% catalyst gives selectivity values at 82% and 85%, respectively for 2 hours and 3 hours. The conversion rate of the use of catalyst and without catalyst gives the perfect rate at 100%, but the use of catalyst produces better vanillin with percent yield at 2.485 for 2 hours, and 3.22% for 3 hours, while without catalyst have percent yield of vanillin at 1.94%
for 3 hours.
Keywords: Eugenol, Vanillin, ZnAl2O4, Oxidation, Catalytic
INTRODUCTION
Vanilla (Vanilla Planifolia A) is one of the herbs and spices which is widely used in industrial sectors such as food, pharmacy, and cosmetic [1]. This is due to the existence of 4- hydroxy-3-methoxy benzaldehyde or vanillin (C8H8O3) which plays as the main component in the vanilla [2]. Recorded more than 12,000 tons of synthetic vanillin are produced in a year due to its very high market demand while the availability of vanilla pods is limited [2, 3].
Synthesis of vanillin can be conducted in two methods, biotransformation methods using enzymes or bacteria [4–7] and by chemical [8–11] methods using ferulic acid [12, 13], lignin [14, 15], glucose [16], isoeugenol [6, 17, 18], or eugenol [15, 19], as a starting material, with eugenol has the highest percentage of usage due to its low price and commercially available [20]. In addition, modification of the structure of eugenol through isomerization and oxidation was reported to be able to produce vanillin [21]. However, the direct use of eugenol in a one- pot reaction produces a low level of purity, low conversion and requires a long reaction time.
As reported by Mao et al, synthesized vanillin using the one-pot reaction (eugenol) used free additive ligand catalyst Co(OAc)2.4H2O produced 68.5% vanillin within 20 hours [11].
Furthermore, Marquez-Medina synthesized vanillin through two stages of reaction with Al- SBA-15 catalyst and get a vanillin conversion of 57% within 24 hours [10]. It shows a shortage in the duration of the one-pot method which takes a long time to complete the reaction, so that it is necessary to select the right oxidizer to overcome this issue. The usage of nitrobenzene as an oxidizer agent and dimethyl sulfoxide (DMSO) as a solvent in the synthesis of vanillin is one of the methods which is considered easy to conduct with a plenty high level of efficiency [22] due to the DMSO can accelerate the nitrobenzene to react with the sample [23].
Moreover, the usage of a catalyst in the synthesis of vanillin is one of the factors which can increase the selectivity and conversion of vanillin due to its ability to accelerate chemical reactions and can be conducted at low pressure and temperature. Some of the catalysts used in synthesis of vanillin are Nb/Al-SBA-15 [24], Fe/Al-SBA-15 [10], Fe-SBA15-HSO3BM [25], Fe/RGO and Co/RGO [26], CuO/MgFe2O4 and CuO/MgAl2O4 [27]. Furthermore, the usage of spinel oxide catalysts such as ZnAl2O4 can also increase the conversion and the purity of vanillin due to it is known to be inert, hydrophobic, has a high metal dispersion capacity, thermal stability, and low surface acidity [28]. Some examples of catalysts with ZnAl2O4 as support catalyst that have been used are Pt/ZnAl2O4 and Cu/ZnAl2O4. Pt/ZnAl2O4 has been reported to have good activity and selectivity in n-butane dehydrogenation reactions [28], iso- butane combustion, and methanol synthesis [29]. Yet, ZnAl2O4 has not been used as a catalyst in the synthesis of vanillin, therefore researchers are interested in studying further the effectiveness of ZnAl2O4 in the eugenol-based synthesis of vanillin.
EXPERIMENT
Chemicals and instrumentation
The eugenol oil used belongs to Pt. Javagri Inti Lestari, East Java, Indonesia with 100%
purity based on GCMS analysis (Figure 1). Zinc acetate dehydrate, methanol, sodium hydroxide (NaOH), Aluminum oxide (Al2O3), Potassium hydroxide (KOH), Ethylene glycol, Dimethyl sulfoxide (DMSO), Nitrobenzene, Hydrochloric acid (HCl), Sodium bisulfite (NaHSO3), Sulfuric acid (H2SO4), diethyl ether, and anhydrous sodium sulfate (Na2SO4) were obtained from sigma Aldrich Instrumentation applied for analysis should be written all tools are used during research. They can contain instrumentation specification or operational conditions include brand manufacturer. For example: FTIR spectrophotometer (Shimadzu FTIR QP89500, sample was analyzed using NaCl plate or thin film).
Quantitative Result Table
ID R Time m//z Area Conc. Name
1 24.02 164 11481466 100 % Eugenol
Figure 1. GCMS analysis of eugenol
The UV-Vis spectrophotometer uses the SHIMADZU UV-1600 series. IR uses KBR pellet of SHIMADZU IR Spirit-T with serial No. A22415801432 AE. Gas Chromatograph–
Mass Spectrometry (GCMS) SHIMADZU QP2010 SE. X-ray Diffraction (XRD) uses PANANALYTICAL type Expert Pro. Scanning Electron Mycroscopy (SEM) FEI type Inspect- S50 equipped with EDAX. Calcination using Furnace–6000 from Barnstead Thermolgne
Procedure reaction
Catalyst synthesis. ZnAl2O4 was synthesized by ceramic method using ZnO and Al2O3
as precursors. In the first place, Zn(CH3COO)2.2H2O was dissolved in methanol to obtain ZnO by the coprecipitation method. NaOH was added to the solution up to pH 8 after 10 minutes for releasing the precursor salt. This treatment will produce a white precipitate. This precipitate was separated and heated in an oven then calcined at 500oC for 30 minutes.
To the tune of 0.5 grams of ZnO was put into an evaporation dish along with 0.4 grams of Al2O3. The two compounds were mashed and added with water to form a paste, then heated in an oven at 110oC for 6 hours, and calcined for 7 hours at 800oC. The results were analyzed by FTIR, SEM-EDX, and XRD
Vanillin synthesis. The synthesis of vanillin in this study used nitrobenzene as an oxidizing agent, and DMSO as a solvent. Dimethyl sulfoxide is made two times more than nitrobenzene to get optimal results. The use of catalysts was carried out with a loading variation of 4% and 7%. The reaction was carried out at 150oC using reflux with an oil bath for 2 and 3 hours. The resulting k-vanillate was diluted with distilled water and acidified with HCl to pH 2-3 to obtain vanillin. The solution was extracted with diethyl ether which will bind the vanilla to form 2 layers. The top layer was separated from the organic layer, then added sodium bisulfite to form vanillin bisulfite. SO3 was evaporated to form vanillin by adding H2SO4. Vanillin was extracted again with diethyl ether to separate it from impurity compounds. The addition of anhydrous Na2SO4 was carried out to remove water in the solution before evaporated with a rotary evaporator. The result in the form of yellow crystals was purified with hot n-hexane, then tested for solubility in water and ethanol, and analyzed using FTIR spectrophotometer and GCMS.
RESULT AND DISCUSSION Catalyst synthesis
ZnAl2O4 was synthesized using ceramic method and analyzed using FTIR spectrophotometer, XRD, and SEM-EDX. The results analysis of FTIR (Figure 2) show the presence of several functional groups such as –OH group at a wavenumber of 3441 cm-1 and H2O stretching at 1644 cm-1.
Figure 2. FTIR absorption of ZnAl2O4.
The functional group of Zn–O–Zn appears at wave number 1415 cm-1, Al-O type AlO4 at wave number 663 cm-1, and ZnO stretching at wave number 430 cm-1 [29, 30]. The analysis of XRD shows the diffractogram of ZnAl2O4 (Figure 3a) which has a similar pattern to the standard ZnAl2O4 XRD diffractogram (Figure 3b) as reported by Balarini et.al [28]. Some of the appropriate XRD analysis angle positions based on JCPDS cards no. 71–0968 (Table 1) are 31.7927o, 36.2874o, 56.6118o, 59.3444o, 65.5771o, and 77.0325o. However, a peak with strong intensity at 35o indicates the presence of ZnO due to the amount of ZnO used was slightly higher than the initial composition which conduced some of the ZnO was not bound to Al2O3.
a b
Figure 3. X-ray diffractogram of ZnAl2O4 (a) synthesized (b) standard
Table 1. Angle position of 2 theta XRD for ZnAl2O4.
No Angle position (o)
ZnAl2O4 synthesized JPDS card no. 71 - 0968
1 31.7927 31.2
2 36.2874 36.9
3 56.6118 56.8
4 9.3444 59.1
5 65.5771 65.1
6 77.0325 77.1
The analysis using EDX also shows a very low percentage of Al atoms due to the lack of mass of Al2O3 used (Figure 4). Furthermore, the surface morphological characterization of ZnAl2O4
using SEM (Figure 5) with 2 x 104 times magnification shows that ZnAl2O4 is composed of nanoparticles with irregular shapes. Furthermore, particle size analysis was conducted based on the Rietveld method using Rietica and ImageJ shows the particle size of the catalyst is in the range of 8-13 nm (Figure 6).
No Atom Scale 15 Scale 11
% Wt % At % Wt % At 1 O 12.35 35.76 16.19 43.35 2 Al 02.11 03.62 01.83 02.91 3 Zn 85.54 60.62 81.98 53.74
Figure 4. EDX analysis of ZnAl2O4
a b
Figure 5. SEM of ZnAl2O4 at 10000x (a) and 20000x (b)
Figure 6. Distribution of ZnAl2O4 granules
Vanillin synthesis
The resulting oxidation products were examined for the solubility and melting point and analyzed using UV-Vis spectrophotometer, FTIR, and GC-MS. Solubility test in water and alcohol proved that the vanillin produced was in accordance with standard vanillin which the vanillin product has good solubility in alcohol, but is slight/insoluble in water [31]. The melting point test of oxidized vanillin gives various values ranging from 79oC–82oC. This melting point value is close to the melting point of standard vanillin which ranges from 80oC–83oC [32]. The analysis using UV-Vis (Figure 7) shows spectra with 4 different peaks for each variant of the product, with a typical peak indicating the presence of vanillin found at the wavelength of 231 nm.
Figure 7. UV-Vis spectrum of vanillin
200 220 240 260 280 300 320 340 360
0.0 0.5 1.0 1.5
A
wavelength
7% 3h 7% 2h 4% 3h 4% 2h 0%
Table 2. UV-Vis analysis of vanillin.
Peak Wavelength (nm)
Standard 4%, 2h 4%, 3 h 7%, 2h 7%, 3 h Without catalyst
1 204 207 204 204 203 207
2 231 230.9 230 230.9 232 232
3 279 278 278.7 278 278 278
4 310 309.5 308 309 310 308
Table 2 shows that the peaks are close to the standard vanillin peaks with wavelengths of 204, 231, 279, and 310 nm [33]. The results of the FTIR analysis of oxidation products with several variations showed in Figure 8 which indicates that the product is vanillin based on the presence of several functional groups such as –OH group stretching at wave number 3184 cm-1, CH stretching at wave number 2923 cm-1, C=O aldehyde stretching and C=O aldehyde bending at wave number 1667 cm-1 and 631 cm-1 respectively. Absorption at wave number 1028 cm-1 indicates the presence of stretching OCH3 groups, at 1430-1590 cm-1 interprets the presence of C=C–C groups. Furthermore, analysis was continued using GCMS not only for ZnAl2O4 but also ZnO as a comparison catalyst.
Figure 8. FTIR spectra of vanillin Table 3. GCMS analysis of vanillin.
No Catalyst loading
2 hours 3 hours
% yield % conversion % selectivity % yield % conversion % selectivity
1 ZnAl2O4 4% 2.22 100 100 3.11 100 94.035
2 ZnAl2O4 7% 2.75 100 82.555 3.33 100 85.757
3 ZnO 4% 0.5 100 48.31 0.95 100 85.62
3 Without catalyst
- - - 1.94 100 88.843
4000 3000 2000 1000 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
%T
Wavelength (cm-1)
7% 3h 7% 2h 4% 3h 4% 2h 0% 3h
The results (Table 3) show that the vanillin synthesis reaction gave a conversion value of 100%
for all variants with a base peak of m/z 151 (vanillin fragmentation pattern can be seen in Figure 9). Vanillin products treated with 4% catalyst gave the best selectivity values of 100% for a reaction time of 2 hours, and 94.035% for 3 hours. The selectivity values of vanillin decreased to 82.555% when 7% catalyst was given for 2 hours and 85.757% for 3 hours. The use of ZnAl2O4 as a catalyst proved to have higher catalytic activity compared to ZnO seen from the selectivity value and yield percentage of vanillin. Using ZnO, vanillin has a selectivity value of 48.31% for 2 hours reaction and 85.62% for 3 hours, with yields of 0.5% and 0.95%
respectively. The selectivity of the uncatalyzed is 88.843% with a yield value of 1.94%.
Judging from the amount of catalyst, the use of 4% catalyst is considered more effective because too much can cause clumping. The agglomeration causes the closed pores of the catalyst surface so that the performance of the catalyst focusing on the pore surface is reduced.
Apart from the decrement of the selectivity, the percent yield of the product increased along with the addition of the number of catalysts even though the percentage was still quite low. The low percentage of yield produced is due to the inaccurate composition of the sample and the reactants used. Previous researchers have found that the yield percentage of vanillin produced was in very small amounts. Producing a high yield percentage requires a very longtime duration. Mao et al. using a Co(OAc)2.4H2O catalyst and molecular oxygen as an oxidant can produce vanillin with a yield of 68.5% for 20 hours [11]. In the future, further research is needed on the use of sample composition and reactants to produce high yields.
H3CO
O
O H
e
H3CO
O
O H
H
m/z = 152
H3CO
O H3CO O
O
O
H H
m/z = 151
O H
C H
O
m/z = 123
O CH2
-
O
H H
H2C O
-
m/z = 93 m/z = 65
Figure 9. Vanillin fragmentation pattern
Mechanism of vanillin formation from eugenol in Figure 10 shows that the synthesis of vanillin from eugenol does not result in vanillin directly formed from eugenol, but is produced from isoeugenol in the form of an intermediate which is converted to vanillin without stopping the reaction. Isoeugenol is produced in the reaction which is assisted by KOH and facilitated by a catalyst which has been used since the beginning of the isomerization and oxidation
reactions. In the future, the use of ZnAl2O4 can be used continuously, but the synthesis process of ZnAl2O4 needs to be improved by using the right composition, and the purification step needs to be added to the synthesis process.
MeO
HO
Zn O Al
O O
Al O
MeO
O
K OH
MeO
K O
Zn O Al
O O
Al O MeO
K O
H H
H HO
MeO
K O
MeO
O
CH2
K
H OH
MeO
O
CH3
Zn O Al
O O
Al O H
MeO
O ONO
MeO
O
O N OH
Zn O Al
O O
Al O
MeO
O
O N OH
MeO
O
H O
NOH +
H Cl
MeO
HO
H O
Figure 10. Formation of vanillin scheme.
CONCLUSION
The use of ZnAl2O4 as a catalyst proved to have good catalytic activity in terms of selectivity, conversion value, and yield based on GCMS analysis with a reaction time of 2 hours using the one-pot method. In the future, it is necessary to develop the synthesis of ZnAl2O4
catalyst using different compositions.
ACKNOWLEDGMENT
This research was funded by Lembaga Pengelola Dana Penelitian (LPDP) of the ministry of finance of the Republic of Indonesia.
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