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Solid Base Catalyst For Grease Transesterification
Dec 28, 2017

Abstract: Biodiesel is an environment-friendly renewable resource. The preparation methods are direct mixing, microemulsion, pyrolysis and transesterification. Among them, transesterification is one of the most commonly used methods. Homogeneous catalysis has the advantage of high reaction yield, but waste catalyst poses environmental problems; but with high activity and selectivity, easy to separate, recyclable and reusable, non-toxic and cheap, to avoid the advantages of byproducts Heterogeneous catalysis has become a research hotspot in recent years. This article briefly introduces the preparation of biodiesel, summarizes the multiphase base

Research Status of Catalytic Preparation of Biodiesel.

Key words: biodiesel; transesterification; heterogeneous catalyst; solid base

At present, with the increasingly serious global energy crisis and environmental deterioration, the search for renewable energy and the protection of the natural environment have become the major problems to be solved urgently by mankind. From the perspective of environmental protection and energy strategy, all countries in the world are actively seeking renewable petroleum fuels alternatives. Biodiesel has the advantages of renewable, biodegradable, non-toxic, high flash point, high cetane number and environmental friendliness. Therefore, biodiesel is an environmentally friendly green fuel as an alternative to petrochemical diesel, and its development and application have far-reaching significance Economic and social benefits? Biodiesel can be used not only directly as a fuel, but also as a diesel fuel additive.

There are a lot of recent reviews on solid bases. Ond summarizes recent developments in solid base materials and their associated reactions, and lists some methods and applications for the preparation of solid base catalysts. Jiang Shaoliang ¨ and so on the recent review of solid base catalyst

Research status of the agent, and prospects of the development and application of the solid base catalyst. This paper summarizes and summarizes the bio-diesel transesterification reaction of heterogeneous solid base catalyzed fatty acids, and points out some existing problems and prospects its development in the future.

1. Biodiesel research status

Biodiesel is a monoalkyl ester oxygen-containing, clean fuel that contains no mineral oil and can be processed directly from animal or vegetable fats and oils, a form of biomass energy. The preparation methods include direct mixing method, microemulsion method, thermal cracking method and transesterification method. The first two methods belong to the physical method; the latter two methods belong to the chemical method. Although the physical method can reduce the viscosity of animal and vegetable oils and fats, but there are carbon soot and oil pollution problems; the main pyrolysis products

Is a bio-gasoline, and its reaction temperature is high and difficult to control, in contrast, transesterification is a better method.

Transesterification is a method of converting high viscosity animal and plant oils to low viscosity fatty acid esters by transesterification. Its main raw materials are all kinds of natural vegetable oils and animal fats and waste oil and other food industries. Methanol used for transesterification mainly methanol, ethanol, propanol and butanol, etc., of which the most commonly used is methanol, which is due to its low price, short carbon chain and polarity, can quickly react with fatty acid glycerides: The disadvantage is the poor solubility of glycerol with fatty acids. At present, biodiesel is mostly prepared by homogeneous transesterification. Although the catalytic efficiency is very high, it is difficult to separate the catalyst from the product after the reaction, and a large amount of waste acid and lye are discharged to pollute the environment. The use of solid base catalyst instead of homogeneous catalyst, the product and catalyst separation easy, thus effectively preventing environmental pollution. Therefore, the heterogeneous catalysis catalyzed preparation of biodiesel has become a hot topic in recent years.

At present, all countries in the world are actively developing the biodiesel industry. The United States, Canada, Brazil, Japan and the European Union are all developing the industry. Among them, the United States is the earliest country to study biodiesel. Its concern with biodiesel was caused by the 1991 Clean Air Act: but it has only taken shape in recent years, and biodiesel has become the country's fastest-growing alternative fuel. In the meantime, in order to ensure the healthy development of the industry, legislation in this area was also strengthened and a biodiesel standard was formulated (Table 1). China has also successfully developed the use of animal and vegetable fats and oils as raw materials, pre-esterified, re-esterified biodiesel production process. Biodiesel is a true green diesel fuel. Its superior environmental protection, lubricity, safety and recyclability are universally valued by all over the world. Biodiesel has broad application prospects as a diesel alternative fuel.

2. Introduction of solid base

Solid base mainly refers to the solid reactants can be given as a solid catalyst as a catalyst active center has a strong electron donor or accept the ability of electrons from a surface anion hole, the free electron center by the surface O} or O} -OH composition. Solid base also refers to a solid that can chemically adsorb the acid or discolor the acid indicator. According to different classification methods, solid base catalysts can be divided into a variety of catalytic systems, which are mainly divided into solid and non-supported solid base, while non-supported solid base catalysts mainly include metal oxide solid base, molecular sieve, clay mineral and basic ion exchange Resin and so on. Solid base has the following characteristics: (1) make the indicator discolored alkaline color, you can use} L_ to characterize its strength. (2) acidic molecules and H20 poisoning effect on the catalyst activity, so that the catalytic activity is reduced or lost. (3) Similar catalytic activity as homogeneous base catalyst. (4) The study of reaction mechanism, product distribution and spectrum analysis of surface adsorbed species all strongly indicate that anion intermediates participate in the reaction process.

Based on the above characteristics, the solid base characterization methods are as follows: (1) titration; (2) anion exchange; (3) CO2 or NO adsorption; (4) calorimetry; xPS method; (7) reaction rate method. Solid base has the advantages of high reactivity, mild reaction conditions, good selectivity, easy product separation, reusable, less corrosive reaction equipment, etc. However, the solid base preparation is more complicated, expensive, less strength, easily affected by the atmosphere CO2 and water impurities such as pollution, and the surface are relatively small. In addition, the solid base catalytic reaction also has two important features: First, the solid base catalytic reaction shows a characteristic reactivity and selectivity through the formation of a carbanionic intermediate, which catalyzes or forms with the solid acid that forms the carbanionic intermediate Metal-catalyzed reactions of free-radical intermediates are different; secondly, solid base catalysis does not have the high rate of by-product formation as catalyzed by solid acids.

Study on solid base catalyzed transesterification of grease

In addition to being easily poisoned by acidic molecules such as H20 and CO2, solid bases are more heterogeneous because of their catalytic properties and thus react more slowly than conventional acid-base catalyzed transesterification. However, when the solid base catalyzes the transesterification of the oil, the conversion rate is generally higher than 95%, which is much higher than that of the conventional acid-base catalyzed transesterification. In addition, in practical applications, heterogeneous catalysts have many advantages, also prompted people to conduct more in-depth study.

3.1 Supported solid base

At present, among the commonly used solid base catalysts, supported solid bases are the most popular catalysts due to their advantages of simple preparation, relatively large specific surface area, strong basicity and even pore size. Mengxin et al prepared by impregnation KF / CaO catalyst, and applied to catalyze the methanolysis of soybean oil solution. The results showed that the yield of biodiesel reached 90% when the molar ratio of alcohol to oil was 12: 1, the amount of catalyst (mass ratio of catalyst to oil) was 3%, the reaction temperature was 333 K ~ 343 K and the reaction time was 1 h %, Compared with the results of CaO catalytic reaction, KF / CaO catalyst catalytic activity was significantly improved. Xie Wenlei, et al. (Q) Study on methanolysis of soybean oil catalyzed by KNO3 / A1203 and KF / ZnO solid base and the corresponding optimum reaction conditions were obtained.When KNO3 / A1203 solid alkali was used, the molar ratio of alcohol to oil was 15: 1 and the amount of catalyst was 6.5 , The reaction time 7 h, the loading 0.35 (g / g), the calcination temperature 500 ~ C calcined 5 h, the highest conversion 87.43%; KF / ZnO solid base: alcohol oil molar ratio 10 : 1, the amount of catalyst 3 wt.%, The reaction time 9 h, the conversion rate of 85.29%.

In addition, Cui et al. Conducted a study on the transesterification of vegetable oils catalyzed by K20 / ~ -A1203 and Cs20 / A1203 solid base. The results showed that the yield of biodiesel reached over 95.8%. Kim et al. "Found that the supported sodium-based catalyst Na / NaOH / A12O3 has the same transesterification catalytic activity as the homogeneous NaOH." Ebiura et al. "Studied the use of aluminum oxide as a carrier to carry different metal salts as catalysts for transesterification, The results show that potassium fluoride and potassium carbonate have higher transesterification activity in different loadings. JiTPUtti et al studied the transesterification of palm oil and coconut oil catalyzed by KNO3 / KL zeolite and KNO3 / ZrO2 solid base. The results showed that the conversion rates of both palm oil and coconut oil were over 70%.

4.2 non-supported solid base

4.2.1 Metal oxides

In metal oxides, alkaline earth oxides are relatively low in specific surface area and readily absorb H20 and CO2, which tends to form a slurry in the reaction mixture; in addition, pretreatment must be performed under high temperature and high vacuum

Base catalytic activity, therefore, subject to some restrictions. Bancquart and other solid base using MgO, ZnO, CeO2 and La203 catalysis of methyl stearate and glycerol at 493 K transesterification of monoglycerides, the study found that these solid alkaline region alkaline (per unit area of the basic) the stronger catalytic The higher activity. In addition, Dossin et al. "Studied the kinetics of MgO catalyzed transesterification and obtained the corresponding kinetic model.

4.2.2 Clay minerals

Clay minerals mainly include hydrotalcite and sepiolite, etc., of which hydrotalcite is usually prepared by calcination of its corresponding precursor, the precursor structure is generally cited: [M (II) 1 a x M (III) (OH) 2] (An2) x / · mH20, M (II) and M (III) are respectively metal cations and x = M (III) / {M Substitution of the metal cation with a trivalent metal cation can change the layer structure, thereby changing the properties of the catalyst such as alkali strength. Hydrotalcite structure Mg-AI anionic layered compound is calcined hydrotalcite into Mg-AI composite oxide (Mg (AI) O). It is a mesoporous material with strong basicity, large specific surface area, high stability and structural and alkaline adjustability. Its activity depends on the x value in the precursor and the sintering temperature. Wu Yu-Xiu et al "prepared and characterized Mg-AI composite oxide hydrotalcite and used it as a catalyst to catalyze the transesterification between vegetable oil and methanol. It was found that hydrotalcite calcined had higher activity with the optimum reaction temperature 333 K ~ 343 K, When the conversion rate of methyl ester of more than 90%. Li Weimin and other coprecipitation prepared hydrotalcite, calcined Mg-AI composite oxide, and as a catalyst for rapeseed oil transesterification, the optimum process conditions for The reaction temperature was 65 ℃, the molar ratio of alcohol to oil was 6: 1, the reaction time was 3 h, the amount of catalyst added was 2% of rapeseed oil, and the yield of fatty acid methyl ester (biodiesel) was 95.17%.

In addition, the AI-Mg hydrotalcite prepared by Corma et al. Has a strong ability of catalyzing the exchange reaction of triglyceride and glycerol, and the conversion rate reaches 92% after reacting for 5 h at 240 ° C. Xie Wenlei et al. "Also studied the methanolysis reaction of Mg-AI hydrotalcite catalyzed soybean oil. The optimum reaction conditions were as follows: molar ratio of alcohol to oil 15: 1, catalyst dosage 7.5% (g / g), reaction time 9h, 65.74%.

4.3 basic ion exchange resin

The advantage of the alkali metal exchanged zeolite molecular sieve is that it has a larger specific surface area and a channel selective shape. It is selective and easy to use. It is easily stored in the air because adsorbed H20 and CO2 are easily removed. Bayense et al. Found that ETS-4 and ETS-10 also catalyzed the methanolysis of soybean oil, and the conversions reached 85.7% and 52.6% respectively at 220 ~ C for 1.5 h. Galen and other alkali metal exchange zeolite NaX and ETS-10 catalyzed alcoholysis of soybean oil and found that when the reaction temperature is below 125 ℃, the reaction after 24h conversion rate can exceed 90%, and the conversion rate increases with the reaction temperature increases . Since ETS-10 has a larger pore size, the alkali oxide is better dispersed internally so that the ETS-10 series has a higher catalytic activity than the NaX series. ETS-l0 series of catalysts can be reused without any loss of activity, under the same reaction conditions, the catalytic conversion rate of soybean oil alcoholysis reaction is close to 90%. Barrault et al. Studied the transesterification of glycerol and fatty acid methyl esters catalyzed by Mg-MCM-41 and Mg-AIMCM-41 mesoporous molecular sieves. It was found that Mg-MCM-41 and Mg-AIMCM-41 are not only catalytically active, In the same reaction condition and the same conversion, the selectivity to monoglyceride decreases with the increase of carbon chain of fatty acid methyl ester, and the reactivity decreases with the increase of carbon chain of fatty acid methyl ester. In addition, Xie Wenlei study of strong base anion exchange resin (OH type 717 anion exchange resin) heterogeneous catalytic refining of soybean oil and pig fat transesterification.

4.4 Other solid base forms

Filippis studied phosphate as a transesterification catalyst for olefins, and found that Na3PO4 and Na3PO4 · 12H20 have higher catalytic activity, of which Na3PO4 is larger than Na3PO4 · 12H20. The possible mechanism of its catalytic reaction is as follows:

Na3PO4 + CH3OH-Na2HPO4 + CH3ONa (1)

TG + CH3ONa + HE0-ME + CH3OH + NaOH + DG (2)

NaOH + Na2HPO4 - * Na3PO4 + H2O (3)

In addition, Schuchardt et al., Which incorporated an organic base guanidine into the support to make an organic solid base, also found that the solid base was also able to catalyze the lipid transesterification, but the activity was not high when used homogeneously.

5. Conclusion

Biodiesel is a clean and renewable energy source. However, compared with the development of mature petrochemical diesel oil, biodiesel is relatively late in development. It started to rise only in the 1990s and inevitably has some problems, mainly manifested in the poor start-up performance at low temperature , The use of high levels of NOx emissions and the use of trace amounts of methanol and glycerol. In addition, the current production of biodiesel mainly uses homogeneous production methods, heterogeneous methods are mainly in the research stage, there are still some gaps from the industrial application. Heterogeneous catalysts have the advantages of mild reaction conditions, easy separation of products, recycle and many other advantages, which can be a new generation of environment-friendly catalytic materials. At present, the study of heterogeneous catalysts should mainly in the following aspects: (1) to clarify the reasons for the heterogeneity of catalyst activity and selectivity, the relationship between the amount of alkali and alkali strength, and the related factors. (2) Combined with the surface science, the method of solid base characterization is deepened, and the interaction between the surface properties and the surface of the support is described from atomic and electronic structures. Only in this way, we can effectively control the solid alkali active center reaction environment, thereby enhancing its catalytic activity and selectivity.