The catalyst was highly efficient and could be reused for four reaction cycles without losing its activity. Also for the acetalization reaction of crude glycerol into fuel additives, Kiakalaieh and Tarighi [115] used faujasite zeolite material with phosphotungstic acid as a catalyst. This catalyst showed high thermal stability, large specific surface area, large capillary diameter and strong acidity. The reaction yield reached 97.8% and the glycerol conversion reached 100% when the reaction was carried out at 40 o C with 10% catalyst mass for two hours of reaction. The results confirmed that the HPA catalyst immobilized on faujasite zeolite could be a potential material for further development studies on a larger scale.
Another important application of heteropoly acids immobilized on supports is catalyzing the photodegradation of organic pollutants in aqueous solutions [95]. Studies have shown that HPA/support catalysts are effective for photodegradation of organic pollutants in polluted water. However, further studies on the reuse of materials and increasing the durability of catalysts in different environments are still needed.
1.3.2. Fructone synthesis reaction
1.3.2.1. General introduction to fructone synthesis reaction
Fructone (ethyl (2-methyl-1,3-dioxolan-2-yl) acetate) is a synthetic apple flavoring agent. Fructone is widely used in the perfume, beverage, cosmetic, food, pharmaceutical, detergent, and lacquer industries [116, 117]. Fructone flavoring agents are usually synthesized by the acetalization of ethyl acetoacetate and ethylene glycol (polar medium) using an acid catalyst. The reaction scheme and mechanism [118] are shown in Figures 1.14 and 1.15.
Figure 1.14. Schematic diagram of the reaction to form fructone.
The reaction mechanism takes place in the following stages:
Step 1. Protonation of the carbonyl group.
Stage 2. Attack of the nucleophile.
Stage 3. Water separation process.
Stage 4. Second nucleophile attacks.
Figure 1.15. Schematic diagram of the reaction mechanism for fructone formation.
Acetalization reaction is widely used for the synthesis of fructone because it is carried out from very simple starting materials and gives high yield of fructone. Many fructone synthesis reactions use homogeneous catalysts such as H 2 SO 4 , HCl, p-toluenesulfonic acid, pyridine salts and Lewis acids such as ZnCl 2 [119- 122]. These homogeneous acids are toxic, highly corrosive, difficult to recover and the excess acid needs to be neutralized after the reaction. A quantity of salt produced from the neutralization process will be discharged into the environment, polluting water sources. The problem is to use a heterogeneous catalyst with high acidity but can be recovered after the reaction and can be reused to avoid disposing of the catalyst into the environment, reduce the cost of the catalyst for the reaction and reduce environmental pollution.
In general organic synthesis reactions, there are many published results of scientists around the world on the synthesis of heterogeneous catalysts with high activity efficiency. However, the research on the synthesis of heterogeneous catalysts for the synthesis reaction of the flavoring agent Fructone has not been published much up to now. Therefore, there are still many issues that can be exploited for in-depth research in the direction of synthesizing different heterogeneous catalysts to improve the efficiency of the reaction.
Various acidic heterogeneous catalysts have been reported for the synthesis of Fructone including:
- HSO 3 - functionalized catalysts based on ionic liquids with Bronsted acid centers and on carbon materials [116, 117, 123].
- Polymeric acid catalysts were synthesized through the copolymerization of p-toluenesulfonic acid and para-formaldehyde using sulfuric acid [124].
- Solid acid catalysts such as acid-modified MQTB materials Al- MCM-41, Al-SBA-15, zeolite Beta, ZSM-5 [125, 126].
- Heteropoly acids immobilized on USY zeolite, activated carbon, silica gel carriers [127- 130].
The synthesis of fructone in liquid phase using heterogeneous catalysts of ionic liquids with Bronsted acid centers functionalized by HSO 3 was reported by Y. Liu et al.
[117] reported that the conversion rate was below 60%, lower than that of the homogeneous sulfuric acid catalyst (71.2%). With the heterogeneous carbon-HSO 3 catalyst, the conversion and product selectivity were both 95-98% with the ability to reuse 6 reaction cycles without significantly reducing the efficiency and composition of the product [123]. This result shows that the active phase carrier HSO 3 based on carbon materials is highly effective in the fructone synthesis reaction.
In addition to carbon carriers, according to the study of the Climent group [125], zeolite-beta catalyst has shown very good catalytic ability for the fructone synthesis reaction with a raw material conversion rate of 97% and a product selectivity of 99%, this catalytic activity is higher than that of the ionic liquid catalyst of Y.Liu [117], equivalent to the heterogeneous catalysts carbon-HSO 3 , and polymer p-toluenesulfonic acid according to the results published by G. Shan and colleagues [124]. However, the disadvantage of zeolite-beta catalyst in the fructone synthesis reaction is that it loses its activity after 3-4 reaction cycles, the efficiency is only about 60% after the catalyst has been regenerated.
Acid-modified SiO 2 MQTB materials such as Al-SBA-15 and Al- MCM-41 were used by A. Vinu and colleagues [126] as catalysts for the synthesis of fructone. The results showed that although zeolite ZSM-5 had a higher acidity, it gave a significantly lower conversion rate than the medium-acidity Al-SBA-15 catalyst. The explanation was based on the observation that in addition to the acidity, the contribution of the MQTB system of Al-SBA-15 helped to increase the contact ability of the reactant with the catalyst, significantly improving the catalytic activity of this material compared to highly acidic materials such as ZSM-5. This study found
40
Determination of both the acidity and the MQTB structure of the material is very important in the synthesis of fructone.
1.3.2.2. Heteropolyacid catalyst HPA for fructone synthesis reaction
In the fructone synthesis reaction, in addition to simply using zeolite as a catalyst, the research group of F. Zhang and colleagues [127] studied the synthesis of catalysts using zeolite USY (superstable zeolite Y) as a catalyst carrier and dispersed on it a heterogeneous superacid catalyst HPA used in organic synthesis reactions in non-polar environments. Accordingly, zeolite USY is a substance that can both catalyze the reaction and immobilize the HPA catalyst. The efficiency of the HPA-zeolite catalyst for the fructone synthesis reaction is significantly higher than studies using only zeolite alone. Specifically, the optimal amount of catalyst used is only about 0.6% by mass compared to the reactant, much lower than the figure of 3% when using zeolite catalyst, but still gives a conversion rate of 98.7% with a fructone product selectivity of over 97%. However, the reusability of this catalyst is not high due to the significant loss of active phase in the environment after 5 reaction cycles.
The authors also compared the catalytic activity of HPA acid attached to a capillary support with that of Cs salt of HPA attached to the same support [128]. These catalysts had high conversion and selectivity in the fructone synthesis reaction. HPA acid catalysts on the support tended to lose their activity due to the leaching of heteropoly anions in the polar reaction medium. In contrast, Cs salt catalysts of HPA on the support were hydrophilic and gave high activity. In particular, Cs salt catalysts of HPA on DUSY support gave higher activity stability than on other supports with the lowest amount of active substance washed away.
From the studies on heterogeneous catalysts for fructone synthesis reactions analyzed above, it can be seen that the role of acidity and MQTB structure of zeolite materials and MQTB materials is very important in the fructone synthesis reaction. Therefore, the research objective of the thesis is to synthesize HPA active phase catalyst dispersed on carriers based on MQTB materials. The efficiency of the synthesized catalyst will be increased due to the ability to easily distribute and fix HPA on these carriers, helping to increase the contact surface of HPA with the reactants. In addition
It is the catalytic ability of the acidic carrier itself, together with the effective support of the general porous structure and the MQTB structure of the carrier in particular, that is expected to significantly increase the catalytic ability for the synthesis reaction of the fructone flavoring agent.
CHAPTER 2. EXPERIMENTS AND RESEARCH METHODS
2.1. Experiment
2.1.1. Chemicals
The chemicals used in the thesis are listed in Table 2.1.
Table 2.1. List of main chemicals.
STT
Chemical | Source | |
1 | Pluronic P123, (PEO)20(PPO)70(PEO)20, 99% | Sigma-Aldrich |
2 | TetraEthylOrthoSilicate (TEOS), Si(OC 2 H 5 ) 4 | Fluka |
3 | TetraPropylAmmoniumBromide (TPABr), C 12 H 28 BrN | Merck |
4 | Ludox (LUDOX® HS-40 colloida silica), 99% | Aldrich |
5 | Aluminum sulfate, Al 2 (SO 4 ) 3 .18H 2 O | China |
6 | Sulfuric acid, H 2 SO 4 98% | China |
7 | Sodium hydroxide, NaOH | China |
8 | Ammonia, NH 3 25% | China |
9 | Phosphotungstic acid, H 3 PW 12 O 40 , 99% | Sigma-Aldrich |
10 | 3-aminopropyl-triethoxysilane (APTES), 99% | Sigma-Aldrich |
11 | Ammonium nitrate, NH 4 NO 3 99% | China |
12 | Hydrogen peroxide, H 2 O 2 30-32% | China |
13 | Phosphoric acid, H 3 PO 4 85% | China |
14 | Hydrochloric acid, HCl 36.5-38 % | China |
15 | Ethanol, C 2 H 5 OH 99.7% | China |
16 | Ethyl acetoacetate, C 6 H 10 O 3 99.8 % | Aladdin |
17 | 1,2-Ethandiol (Ethylene glycol), C 2 H 6 O 2 99.8 % | Merck |
18 | 1,2-Propandiol, C 3 H 8 O 2 99% | Merck |
19 | 1,4-Butanediol, C 4 H 10 O 2 99% | Merck |
20 | Caesium chloride, CsCl | Merck |
21 | Tetradecane, C 14 H 30 | Merck |
22 | Toluene , C7H8 99 % | China |
23 | Cyclohexane , C6H12 99.5 % | China |
24 | Iso-octane, C 8 H 18 99% | China |
25 | p-Toluenesulfonic, C 7 H 8 O 3 S | Merck |
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2.1.2. Experiment
The synthesized materials in the thesis include: Al-SBA-15 and ZSM-5/SBA-15 carriers with different Si/Al ratios; HPA immobilized onto the carriers by different methods. The synthesized materials were characterized and tested for activity in the fructone synthesis reaction.
2.1.2.1. Synthesis of Al-SBA-15 carrier with different Si/Al ratios
Al-SBA-15 carrier was synthesized according to document [131] and the process was carried out as follows:
Dissolve 1.75 grams of P123 in distilled water, stir magnetically for 3 hours at room temperature. Then add 0.288 grams of Al 2 (SO 4 ) 3 .18H 2 O and continue stirring for 1 hour until a homogeneous solution is obtained. Then, 4.5 grams of TEOS is added to the homogeneous solution and the mixture is continued to stir for 15 hours at room temperature. Next, the mixture is continuously stirred at 40 o C for 24 hours.
Then, the pH of the mixture was adjusted to 5.5 with 25% NH3 solution diluted with distilled water in a volume ratio of 1:1. The resulting mixture was transferred to an autoclave and aged at 90 o C for 2 days. After 2 days, the solid product was collected and the product was filtered and washed to pH=7. The resulting solid was filtered, washed several times with distilled water and dried at 90 o C overnight.
The amount of Al 2 (SO 4 ) 3 .18H 2 O added was adjusted to obtain samples with different Si/Al ratios. The synthesized samples are denoted as Al-SBA-15- n (n is the Si/Al ratio, n = 10, 15, 20, 25, 30).
2.1.2.2. Synthesis of HPA/Al-SBA-15 catalyst material
HPA/Al-SBA-15 material was synthesized by many processes with different steps (given in Table 2.2) to investigate and find the optimal method to fix HPA on Al-SBA-15 carrier.
Each process is carried out in three stages: stage 1 is the type of structure-directing agent (done by one of two methods: calcination method - step 1 or oxidation method with H 2 O 2 - step 2); stage 2 is the creation of functional groups on the carrier (NH 4 + group - step 3, NH 2 group - step 4, or both groups); and stage 3 is the introduction of HPA onto the carrier (by HPA impregnation method