蒸汽機的出現加速工業發展，全球經濟快速成長的情況下，能源已變成人類不可或缺的一部分，伴隨而來所必須面臨的重大危機是能源短缺、環境汙染及氣候變遷。生質柴油比起傳統石化柴油具有許多優點，例如安全、可再生、無毒、生物可分解性、潤滑性較好，是一種很有發展潛力的替代能源，轉酯化反應是普遍使用於生產生質柴油的方法。
本研究使用Beta型沸石作為催化反應的觸媒，利用三酸甘油酯和過量甲醇在觸媒環境下生產生質柴油。藉由改變不同的溫度和時間合成出Beta型沸石，並且使用離子交換法進行觸媒的表面改質提升其催化能力，透過改變離子交換參數 (如鹼金屬離子、鈉來源、濃度、時間及擔體種類等等)及轉酯化參數找出有利於轉酯化反應的條件，而大部分油料來源都含有影響轉酯化反應的水份和游離脂肪酸，所以本實驗將探討不同含量的游離脂肪酸和水份，觸媒在轉酯化反應中對的催化效果，並利用FT-IR, XRD, SEM, BET, NH3-TPD, CO2-TPD等儀器分析觸媒。
實驗結果顯示Beta型沸石於濃度1 mmol/g cat.的氫氧化鈉溶液下進行離子交換半小時，並在觸媒油醇重量比1 : 1 : 25，溫度60 C情況下反應一小時後，產率可達95%以上，而使用後的觸媒可以回收再利用或進行再生程序，對於降低成本有不錯效果。而觸媒分析結果顯示Beta型沸石為酸性觸媒，觸媒經改質後會降低酸性，增強鹼性特性，進而提升催化效果。

Study on Catalyzed Transesterification of Triglycerides with Ion-Exchanged Zeolite Beta

Ti-Lung Chuang
Bing-Hung Chen
Department of Chemical Engineering, National Cheng Kung University, Taiwan

SUMMARY

Transesterification of triglycerides to methyl esters was studied by using as-prepared zeolite Beta as a heterogeneous catalyst. Zeolite Beta was synthesized hydrothermally at 160C from a system containing Na2O•Al2O3, SiO2, TEAOH (tetraethylammonium hydroxide) and H2O. The as-obtained Zeolite Beta was further treated with an ion-exchange process with alkali ions to improve its catalysis in transesterification. The as-prepared catalyst was characterized with FT-IR, XRD, SEM, BET, NH3-TPD, and CO2-TPD. Parameters affecting the performance of transesterification, such as reaction time and the loadings of catalyst would be discussed. The yields of triglyceride to biodiesel by using zeolite Beta with proper Na+ ion-exchange for 0.5 h reaches more than 95% in 1 h. Moreover, the yield of transesterification of triglycerides with conjoint presence of water and free fatty acid could still reach ca. 80% under proper reaction conditions. More importantly, the catalysts after reaction could be reused and regenerated.

Keywords: Biodiesel, Transesterification, Ion exchange, Zeolite Beta, Vegetable oil.

INTRODUCTION
With rapid industrialization, demands of energies have increased dramatically, leading to extensive combustion of fossil fuels which emits significant amount of anthropogenic CO2 that caused global warming and severe climate changes. To remediate these crises, the needs to search for renewable and environmentally friendly energy have climbed up. Biodiesel, a mixture of long-chain fatty acid methyl esters (FAMEs) or ethyl esters (FAEEs), is regard as one kind of promising alternative fuels to petroleum. Biodiesel not only has many advantages, such as proper viscosity, boiling point, and high cetane number, but also degradable, nontoxic, and without additional carbon dioxide production.
Transesterification is a common method of reacting triglycerides with alcohol to produce biodiesel and glycerol. Transesterification is a reaction of triglyceride with short-chain alcohol, commonly methanol, in presence of catalyst. The equation of transesterification is expressed as followed:
(1)
The catalysts usually employed to catalyze transesterification reaction are homogeneous alkaline catalysts and heterogeneous alkaline catalysts, because the alkali-catalyzed transesterification is faster than that catalyzed by the same amount of an acid catalyst. However, the homogeneous catalyst has some drawbacks and limitations, such as the sensitivity to the purity of reactants, especially in water and free fatty acids, the high cost of product separation, and environmental pollution by the liquid wastes. Since the reaction using homogeneous alkaline catalysts (such as NaOH) needs high-quality feedstock, various heterogeneous catalysts to catalyze transesterification are developed recently.
In this study, Zeolite Beta prepared in house was chosen as heterogeneous catalysts in catalyzed transesterification of triglycerides in excess methanol. A modification with alkali ion was employed to increase the catalysis of zeolite Beta in transesterification. The influence of metal loadings, duration of ion exchange, and sources of metal ions were evaluated. And several reaction parameters have been studied to establish better transesterification reaction conditions. Finally, the practical use of the catalyst for the industrial biodiesel production, such as catalytic durability, regeneration, content of free fatty acids (FFAs) and water content of oil sources were discussed.

MATERIALS AND METHODS

Synthesis
Zeolite Beta was prepared hydrothermally at 160°C in the following molar ratio: 30 SiO2 : 1 Al2O3•Na2O : 15 TEAOH (tetraethylammonium hydroxide): 400 H2O, where TEAOH is the structure-directing agent. The products were filtered and rinsed with deionized water and were subsequently dried. Finally, the as-synthesized zeolite materials were calcined for 8 h in air at 500C.

Modification of the catalysts
The Zeolite Beta powder obtained from the previous synthesis procedure was used as the catalyst support. Zeolite powder was firstly soaked in alkali ion solutions for a certain period, and these modified zeolite samples were rinsed thoroughly with deionized water at ambient temperature to remove any undoped ions and other impurity. Subsequently, these modified zeolite catalysts were dried in an oven and calcined at 500C in air for 8 h.

Catalyst characterization
Characterization and identification of the internal structure, bulk phase, and composition were determined by the X-ray diffraction (XRD), inductively coupled plasma - optical emission spectrometry (ICP-OES), and scanning electron microscope - energy dispersive X-ray (SEM-EDX). The morphology of crystalline materials was observed using a Hitachi SU8000 scanning electron microscope, and the specific surface area was determined by the Brunauer-Emmet-Teller (BET). The acidity and basicity of the zeolite were determined using temperature-programmed-desorption (TPD) of NH3 and CO2, respectively. NH3 and CO2 were used as the probe molecule. Fourier transform infrared spectrophotometer (FTIR) was used to determine the framework vibrations of zeolite Beta, and pyridine has been used as a probe of acid material.

Transesterification reaction (biodiesel production)
Transesterification of triolein in excess methanol with catalyst to produce methyl oleate as biodiesel was performed in a batch reactor, the reaction temperature was always maintained constant at 60C. After reaching a preset reaction time, the reaction was quickly quenched. The reacting mixture was centrifuged at 6000 rpm for 20 min to separate the solid catalyst, and then the liquid phase with biodiesel was extracted by using hexane. The production yield of biodiesel was mainly measured by using a GC-FID.

RESULTS AND DISCUSSION

Characterization of as-synthesized zeolite Beta
The structure of these catalysts and support were examined with power XRD. The zeolite Beta in Figure 1 is synthesized without aging at 160°C with hydrothermal reaction time for 2 days. The morphology of zeolite and catalyst support is shown in Figure 2, and shows that as-prepared zeolite Beta was in oval-like shape, and the particle sizes of zeolite Beta ranged from 150 to 300 nm.

Figure 1. XRD pattern of zeolite Beta with hydrothermal reaction at 160°C for 2 days.

Figure 2. SEM image of zeolite Beta with hydrothermal reaction at 160°C for 2 days
Transesterification of triglyceride using as-prepared modified zeolite
Before transesterification, as-obtained zeolite BEA was modified by alkali ion exchange method to render better catalysis. For example, ion exchange from NaOH achieved higher conversion efficiency than other ion sources, and conversion efficiency of triolein to biodiesel increased with increasing Na-loading of zeolite. The result (Figure 3) showed that the yield of triglyceride to biodiesel catalyzed over zeolite Beta loaded with 1 mmol Na per gram of catalyst could reach more than 95% in 1 h of reaction.
The better transesterification reaction conditions used were: 25:1 ratio of methanol-to-oil (by mass), 1:1 ratio of oil-to-catalyst (by mass), 60°C reaction temperature, and reaction time of 1 h. And under the condition, the Zeolite Beta could be used for at least three cycles. Finally, the effect of free fatty acids and water to transesterification were studied by varying amounts of oleic acid and water content in the transesterification reaction system. The result shows that presence of oleic acid, < 3 wt%, in triolein will not affect the transesterification over the Zeolite Na-Beta catalyst, and when the ratio of water to triglycerides is increased to 0.2:1 (by mass), the yield of transesterification in the presence of water still could reach 80%.

Figure 3. Transesterification reaction with Na-Zeolite Beta with different ion loadings

Figure 4. Transesterification reaction at different temperature (1 mmol Na/g cat.)

Figure 5. Transesterification reaction at 60°C with water in reactants (Na-Beta with 1 mmol Na/g cat.).

Figure 6. Transesterification reaction at 60°C with oleic acid in reactants (Na-Beta with 1 mmol Na/g cat.).

Figure7. Cyclic durability of catalyst.

CONCLUSION

The as-synthesized zeolite Beta had higher crystallinity with a hydrothermal reaction time from 1 to 3 days, and was in oval-like shape. From SEM images, the particle sizes of zeolite Beta ranged from 150 to 300 nm. Zeolite Beta modified with NaOH solution give the better catalytic activity, and the conversion of triglyceride to biodiesel increases with the loadings of sodium ion. The yield of biodiesel was higher than 95% in 1 h by using modified zeolite Beta which is ion exchanged in 1mmol Na/g cat. NaOH solution, and Zeolite Beta could be used for three cycles and regenerated under this condition. Moreover, presence of oleic acid £ 5 wt% and water £ 20 wt% in triolein will not affect the transesterification significantly.

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