Abstract
Solid acid Zr(SO4)2/TiO2 catalyst has highly catalytic activity, and has non-corrosiveness to equipment. It is separated from production expediently. As the above advantages, the influence of Zr(SO4)2 loading amount, calcination temperature, and calcination time on the solid acid Zr(SO4)2/TiO2 catalyst preparation process is discussed. The experimental condition is optimized by orthogonal test, the result indicate that Zr(SO4)2 load is 65%, calcination temperature is 430°C, and calcination time is 2.5 h. Solid acid catalyst Zr(SO4)2/TiO2 is analyzed and characterized by FT-IR, XRD and SEM. The results will provide the experimental condition for enlarging experimental study.
Introduction
Fatty acid methyl esters are the raw materials of anionic surfactant fatty acid methyl ester sulfonate (MES) and nonionic surfactant fatty acid methyl ester ethoxylate (FMEE) (1, 2). There are many ways to prepare fatty acid methyl ester, such as physical method, biological method and chemical method, and so on. As a result of the layered structure (3-4-5-6), the solid acid Zr(SO4)2·4H2O shows higher catalytic activity and selectivity without a strong catalyst center (7, 8). The TiO2 is used as the support of the catalyst to improve the specific surface area and hydrophobic in order to improve the catalytic activity. As the catalyst for esterification reaction, the solid acid Zr(SO4)2/TiO2 has high catalytic activity, convenient product separation, no corrosion to equipment, no acidic wastewater, and can be reused – it is a kind of environmental friendly green catalyst.
Preparation and characterization of catalysts
Titanium dioxide (AR) was bought from Tianjin Fuchen Chemical Reagent Factory; 98% four zirconium sulfate hydrate was bought form Sinopharm Chemical Reagent Co. Ltd. The sample was taken by analytical balance from German Sartorius Company and was roasted by muffle furnace from Sida Instruments Company. The Zr(SO4)2 was put in hot water and stirred to dissolve, then along with TiO2, both were stirred into evaporating dish. After being completely dry, the powder was put in the pot and roasted in the muffle furnace. In the end, the solid acid catalytic Zr(SO4)2/TiO2 was prepared.
The surface functional groups of the solid acid catalyst Zr(SO4)2/TiO2 were analyzed by the Fourier transform infrared spectrometer (FT-IR) from American Nicolet company, in the range of 2000 cm−1∼4000 cm−1. The grain of the solid acid catalyst Zr(SO4)2/TiO2 was analyzed by the D8 type x-ray powder diffraction (XRD) from Bruker company in Germany with the test condition: copper target, working voltage: 40 kv, working current: 30 mA, scanning speed is: 3°/min, the scanning range: 2θ = 20°∼ 80°. The surface microstructure of the solid acid catalyst Zr(SO4)2/TiO2 was analyzed by JMS-5610 type scanning electron microscopy (SEM) from Japanese electronics company. An energy-dispersive spectroscopy analyzer (EDS) attached to the SEM was used to determine the chemical composition of the samples.
Results and discussion
Catalytic activity in esterification
Effect of calcination temperature on the activity of the catalysts
The catalytic behavior of the 65% Zr(SO4)2/TiO2 with different calcination temperature is presented in Figure 1A. There are stronger interactions between Zr(SO4)2 and water molecules. Dehydration can increase the interactions between SO42- and Zr. As shown in Figure 1B, when the calcination temperature is below 430°C, the transformation rate of palmitic acid increases with the temperature rises. When the calcination temperature is 430°C, it corresponds to the highest catalytic activity. As a result, the catalytic activity is improved with the appropriate increase of the temperature, along with the increase of number of L-acid and the catalytic activity of Zr(SO4)2/TiO2. However, the conversion rate of palmitic acid reduces with further increasing of the calcination temperature. With the temperature rises, Zr(SO4)2 is decomposed completely and Zr(SO4)2/TiO2 loses catalytic activity. Above all, the best calcination temperature is 430°C in the preparation process of solid acid catalyst Zr(SO4)2/TiO2.
The effect of different operating conditions of Zr(SO4)2/TiO2 on the esterification: calcination temperature (
Effect of Zr(SO4)2 load on the activity of the catalysts
Figure 1B displays the esterification activities of the samples as a function of the Zr(SO4)2 loading for catalysts pretreated at 430°C. It is interesting to note that the conversion rate at different Zr(SO4)2 load in Zr(SO4)2/TiO2 is quite different. When the Zr(SO4)2 was 65 wt.%, it corresponds to the highest catalytic activity. The catalyst with lower concentration of Zr(SO4)2 shows poor water resistance and serious catalyst attrition. Furthering increasing of Zr(SO4)2 concentration did not increase the esterification conversion rate. Zr(SO4)2/TiO2 with furthering increasing of Zr(SO4)2 concentration shows lower pore volume and surface area with serious aggregation, which leads to lower catalytic activity. Above all, the best Zr(SO4)2 load is 65% in solid acid catalyst Zr(SO4)2/TiO2 (9, 10).
Effect of calcination time on the activity of the catalysts
The catalytic behavior of the 65% Zr(SO4)2/TiO2 with different calcination time at 430°C is presented in Figure 1C. As shown in Figure 1C, when the calcination time is less 2.5 h, the transformation rate of palmitic acid increases with the time rises. When the calcination time is 2.5 h, it corresponds to the highest catalytic activity. The catalytic activity of Zr(SO4)2/TiO2 is improved with the appropriate increase of the time, along with the strong force between Zr(SO4)2 and TiO2 after dewatering. While the calcination time keeps longer, Zr(SO4)2 falls off from TiO2, which reduces the catalytic activity of Zr(SO4)2/TiO2. Above all, the best calcination time is 2.5 h in the preparation process of solid acid catalyst Zr(SO4)2/TiO2.
The orthogonal experiment optimization of the preparation conditions of solid acid catalytic Zr(SO4)2/TiO2
Optimization of catalytic reaction conditions by orthogonal experiment is based on the capacity of Zr(SO4)2 in solid acid catalyst Zr(SO4)2/TiO2, calcination temperature and calcination time on the esterification reaction. Three factors three levels orthogonal experiment table is designed and the experiments are arranged according to the L9(33) orthogonal table. The factors and levels are shown in Table I and the orthogonal experimental results are shown in Table II.
Factors and levels of orthogonal experiment
Results of orthogonal experiment
First of all, calculate the sum K1 and the average k1 under each factor and level of test index, The K1 is the sum of palmitic acid conversion rate for various factors in the first level, the K2 is the sum of palmitic acid conversion rate for various factors in the second level, the K3 is the sum of palmitic acid conversion rate for various factors in the third level, k1 = K1/3, k2 = K2/3, k1 = K3/3. Second, calculate the gap R on every column, the value is R, which is the maximum minus the smallest of k1, k2 and k3 in the same column. The values of the R can reflect the index influence of the selected factors level changes, the larger of R, the deeper of influence of factors level changes on index. The largest R in that column factor is the main factors to consider.
Table II shows that gap R is A>B>C, indicate the load of Zr(SO4)2 of solid acid catalyst Zr(SO4)2/TiO2 has the most important influence on the test results. The optimal level A2B2C2, that the optimum condition of preparation solid acid catalyst Zr(SO4)2/TiO2 is 65% Zr(SO4)2 load, 430°C calcination temperature, 2.5-h roasting time, can easily be got through the comparison of K and K1.
Characterization of solid acid catalyst Zr(SO4)2/TiO2
Infrared spectra analysis for solid acid catalyst Zr(SO4)2/TiO2
Solid acid catalyst Zr(SO4)2/TiO2 is roasted for 2.5 h in different temperature, and the infrared spectrum is shown in Figure 2A. Each curve of the S = O vibrational absorption is caused by SO42- and Zr4+ chelate nearby 1339 cm−1, 1245 cm−1, 1060 cm−1 and 956 cm−1. Also the absorption peak is due to SO42- and TiO2 chelate. Because of the water-absorbency of the sample surface, the S = O vibrational absorption doesn't appear at the range of 1370 cm−1∼1380 cm−1 in infrared spectrum.
The spectra of different samples: Fourier transform infrared spectroscopy (
With the temperature rises, interactions between Zr and SO42- are stronger, and the symmetry of SO42- is lower. Spectral peak splitting appears in infrared spectrum. At the same time, the catalytic activity of Zr(SO4)2/TiO2 is improved with more electron lost from Zr and the increase of the number of L-acid (11, 12).
Analysis of morphology and structure of solid acid catalyst Zr(SO4)2/TiO2
Figure 2B shows x-ray powder diffraction (XRD) spectra of different calcination temperature of solid acid catalyst Zr(SO4)2/TiO2 after 2.5 h. The calcination temperature of samples is 450°C, 430°C, 400°C, which ranges from top to bottom. Diffraction peak can be indicated in Figure 2B. Zr(SO4)2 is parceled by TiO2, after solid acid catalyst Zr(SO4)2/TiO2 being calcinated at 450°C, 430°C, 400°C, TiO2 becomes anatase TiO2. As the calcination temperature increased, the peak intensity of anatase TiO2 increased. This is due to the small crystalline grain, intercrystalline structure disorder and crystal defects make continuous change of lattice spacing.
Scanning electron microscopy (SEM) of solid acid catalyst Zr(SO4)2/TiO2
Figure 3A, B and C show the clear structure of solid acid catalyst Zr(SO4)2/TiO2. At calcination temperature 430°C, solid acid catalyst Zr(SO4)2/TiO2 has uniform particle size distribution, the average size is about 20 μm, and catalyst particle dispersion is better. So it has advantages of small particles, large specific surface area. Figure 3D shows a typical EDS spectrum of Zr(SO4)2/TiO2 catalyst, which obtained zirconium, sulfur, and titanium. Figure 3D confirms that the Zr(SO4)2 exist in the sample and atomic ratio of zirconium and sulfur is 1.84:1, which approximately meets the formula of Zr(SO4)2.
Scanning electron microscopy (SEM) spectra of Zr(SO4)2/TiO2 calcinated at 430°C with different magnification: 500 times (
Conclusion
Solid acid catalyst Zr(SO4)2/TiO2 is prepared by Zr(SO4)2 and TiO2. Esterification reaction is researched and Zr(SO4)2/TiO2 is taken as the catalyst. The optimal preparation condition for Zr(SO4)2/TiO2 is discussed, result indicate that Zr(SO4)2 load is 65%, calcination temperature is 430°C, and calcination time is 2.5 h. Solid acid catalyst is analyzed and characterized by FT-IR, XRD and SEM. Research result in this article provides the effective catalyst in the process of esterification reaction.
Footnotes
Financial support: This work was supported by the Key Discipline Construction of 211 Project IV in China (grant number 1843202543), Research Innovation Program for College Graduates of Jiangsu Province (grant number KYLX15_0728) and Science and Technology Program of Jiangsu Province (grant number BY2012003).
Conflict of interest: None.