Thread Content
0 Introduction Dimethyl ether (DME) boasts excellent properties; it burns cleanly and efficiently. As a fuel for domestic use and in vehicles, it has advantages over fuels such as natural gas, methanol, propane, butane, and diesel, making it an ideal alternative to diesel and liquefied petroleum gas. Dimethyl ether boasts excellent properties and good safety, and can be used as a propellant in aerosols. It serves as a substitute for chlorofluorocarbons (CFCs) and liquefied propane (butane) gases, becoming the main component of fourth-generation propellants. Dimethyl ether is also an important chemical raw material, with wide applications in the pharmaceutical, pesticide, and dye industries. Heteropoly acids (HPAs) possess strong acidity and oxidizing properties, and are widely used as homogeneous and heterogeneous catalysts; they have been listed as one of the new catalyst materials to be vigorously developed. Projects that have been successfully developed using HPA as a catalyst include the hydration of propylene to isopropanol, the hydration of butene and isobutene, and the oxidation of methacraldehyde to methacrylic acid ； Furthermore, fundamental research on HPA-catalyzed organic reactions such as alkylation, etherification, sulfonation, and olefin epoxidation is extremely active. After loading with HPA, the catalyst exhibits many new properties, such as an increased specific surface area, changes in the types and amounts of B-acid and L-acid, as well as an increase in active centers, thereby enhancing the catalyst’s catalytic activity and selectivity. Carriers loaded with HPA that have been reported include molecular sieves, γ-Al2O3, SiO2, and activated carbon, among others. This study developed a novel supported heteropolyacid composite catalyst and used it to catalyze the dehydration of methanol to dimethyl ether. By studying the effect of heteropolyacid content on catalyst activity, the influence of conditions such as reaction pressure, reaction temperature, and mass space velocity on the conversion rate of methanol dehydration to dimethyl ether was investigated, in order to determine the optimal conditions for using this catalyst in the reaction of methanol dehydration to dimethyl ether. 1 Experimental Section 1.1 Preparation of alumina-supported heteropolyacid composite catalysts The impregnation method was used: dodecawolframosilicate (H4SiW12O40·nH2O, abbreviated as HSiW) and the anti-coking component La(NO3)3·xH2O were dissolved in water in a certain mass ratio. This solution was then used to impregnate a certain amount of γ-Al2O3. After drying at 120 °C for 2 hours, it was calcined at 350 °C for 4 hours, thereby yielding the supported heteropolyacid composite catalysts. 1.2 Catalyst characterization The specific surface area, pore structure, and pore distribution of the catalyst were measured using an ASAP2010M surface adsorption analyzer (produced by Micromeritics, USA). Nitrogen was used as the adsorbate, and the adsorption temperature was that of liquid nitrogen. Before the measurement, the sample was evacuated at 300°C for 12 hours. The type of surface acidity of the catalyst was determined using pyridine adsorption infrared spectroscopy, with a Niclet Impact 410 Fourier transform infrared spectrometer being used for this purpose. X-ray powder diffraction analysis was carried out using a D/max-3c automatic X-ray diffractometer (produced by Riko Company, Japan, with a Cu target). X-ray photoelectron spectroscopy testing uses a VG ESCALAB-MKⅡ (manufactured in the UK) as the X-ray source, with MgKa (1253.6 eV), and takes the C1S binding energy of 284.6 eV as the internal standard. 1.3 Evaluation of catalyst activity The reaction experiment for the dehydration of methanol to dimethyl ether was carried out in a fixed-bed reactor system capable of accurately controlling the reaction temperature, reaction pressure, and mass flow rate; the process flow consisted of a feeding system, a reaction section, a separation unit, and a control system. The feeding system includes a methanol metering tube and a methanol metering pump; the reaction system comprises a vaporization chamber and a reactor; the separation system consists of a condenser and a sampling tank; the control system is responsible for the automatic control of the temperatures in the vaporization chamber and the reactor, as well as the control of the pressure in the reaction system. Both gas and liquid samples are analyzed by gas chromatography to calculate selectivity and conversion rate. The gas chromatograph is a Varian-3400 GC equipped with a TCD, and the column is GDX-502. The test procedure is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/0807091729324670.jpg 2 Results and Discussion 2.1 Characterization of the catalyst 2.1.1 General properties of the catalyst The particle size of the catalyst ranges from 8 to 16 mesh, with a bulk density of 0.93–1.00 g/cm3 and a specific surface area of 130.9 m2/g. The mesoporous structure (with pore sizes ranging from 2 to 50 nm) shows a peak distribution at 2.5–7.5 nm, and the average pore size is 6.5 nm. There are two types of acids on the catalyst surface: L-acid and B-acid, with L-acid being the predominant type. 2.1.2 XRD Analysis XRD analysis was conducted on the supported heteropolyacid composite catalyst, the carrier, and HSiW. The results showed that no characteristic peaks corresponding to the heteropolyacid crystal phase were present in the catalyst’s XRD pattern; only a small peak at 2θ = 67° was observed, which is a characteristic diffraction peak of the carrier. This indicates that the heteropolyacid is distributed on the surface of the carrier in a single-layer molecular form. Only when the loading amount of HSiW exceeds the theoretical value for single-layer dispersion can its characteristic diffraction peak be detected in the XRD pattern. 2.1.3 XPS analysis The electron binding energy of W in X-ray photoelectron spectroscopy was 36.1 eV, which is 0.3 eV higher than the binding energy of W in Keggin-structured heteropoly compounds (35.8 eV), indicating an interaction between HSiW and Al2O3. Based on the model of the interaction between heteropoly acids and carriers, it is speculated that the reaction of HSiW with the hydroxyl (OH) groups on the Al2O3 surface will result in the formation of Al(HSiW) species. In this species, the 2P empty orbital of the Al atom accepts electron pairs from the oxygen atoms of the W3O13 trimetallic cluster in the heteropolyacid, resulting in a decrease in the electron density of the W atoms and an increase in their electron binding energy. The electron binding energy value of oxygen determined by X-ray photoelectron spectroscopy is 531.5 eV. Further decomposition of the oxygen peak yields 2 peaks: for peak 1 (531.811 eV), it is considered to be the oxygen peak of adsorbed oxygen (O2－, O2－) ； For peak 2 (530.86 eV), it is considered to be the oxygen peak of lattice oxygen. 2.2 Catalyst activity tests 2.2.1 Effect of HSiW loading on catalyst activity Catalysts with mass fractions of HSiW at 5%, 10%, 16%, and 20% were prepared, and the conversion rate of methanol dehydration to dimethyl ether was determined under reaction conditions of 280°C and a mass space velocity of 1.00–1.12 h⁻¹. The test results show (Table 1) that as the mass fraction of HSiW increases within the range of 5% to 16%, the conversion rate rises significantly ； When the HSiW mass fraction is higher than 16%, the conversion rate shows a downward trend. Therefore, the appropriate mass fraction of HSiW is 10% to 16%. This is because when the HSiW loading is low (mass fraction 5%–16%), HSiW exists in a single-layer molecular dispersion state on the surface of the carrier ； When the load mass fraction is greater than 16%, as the load amount increases, the specific surface area of the catalyst decreases, resulting in a lower conversion rate. http://www.nmtech.com.cn/jishuwang/upload1/0807091730146123.jpg 2.2.2 Gas chromatography of reaction products Through quantitative analysis by gas chromatography, the mass fraction of dimethyl ether in the gas sample was determined to be 99.9% using normalization, meaning that the selectivity for dimethyl ether is 99.9%. This indicates that the catalyst exhibits good selectivity for the reaction of methanol dehydration to produce dimethyl ether, resulting in a product of high purity. 2.2.3 Relationship between dimethyl ether selectivity and temperature The selectivity of dimethyl ether was determined by varying the reaction temperature under conditions of a reaction pressure of 0.8 MPa (gauge pressure) and a mass space velocity of 2.00 h⁻¹. The test results show that within the temperature range of 260–320 °C, the selectivity for dimethyl ether remains above 99.9%, indicating that this catalyst is an excellent one with high selectivity for the production of dimethyl ether from methanol dehydration. The experiment also showed that when the reaction temperature exceeds 320°C, the selectivity for dimethyl ether decreases, due to the further dehydration of dimethyl ether to form hydrocarbons. Therefore, when using this catalyst, the reaction temperature should be kept below 320°C to ensure a dimethyl ether selectivity of over 99.9%. 2.2.4 Effect of mass flow rate on methanol conversion Rate: Under conditions of a reaction temperature of 280 °C and a reaction pressure of 0.8 MPa, the mass flow rate was varied to determine the methanol conversion rate; the test results are shown in Table 2. When the reaction temperature is below 320°C, since the selectivity for dimethyl ether is above 99.9% in all cases, the conversion rate of methanol to dimethyl ether can be regarded as the yield of dimethyl ether. As can be seen from Table 2, at constant reaction temperature and pressure, the methanol conversion rate decreases as the mass space velocity increases. http://www.nmtech.com.cn/jishuwang/upload1/0807091730355782.jpg In addition, the conversion rates under different mass flow rates were also measured at temperatures of 260°C, 300°C, and 320°C. The results show that at higher temperatures (such as 300–320°C), the mass space velocity has a minor effect on the methanol conversion rate. 2.2.5 Effect of reaction temperature on methanol conversion At constant pressure and a mass space velocity of 1.00–1.12 h⁻¹, the effect of reaction temperature on methanol conversion is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload1/0807091730576351.jpg As can be seen from Figure 2, as the reaction temperature increases, the methanol conversion rate rises; however, after 300°C the change is minimal and the conversion rate reaches equilibrium. Therefore, the optimal reaction temperature is 300°C. The effect of reaction temperature on the methanol conversion rate was investigated at a reaction pressure of 0.8 MPa and a mass space velocity of 2.11 h⁻¹; the data are shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload1/0807091731198537.jpg As can be seen from Table 3, as the reaction temperature increases, the methanol conversion rate rises ； After 280 °C, the methanol conversion rate remains relatively constant; at 320 °C, the equilibrium conversion rate is 85.68%. 2.2.6 Effect of reaction pressure on methanol conversion At a reaction temperature of 280 °C and a mass space velocity of 2.0 h⁻¹, the relationship between reaction pressure and methanol conversion was investigated. The test results show (Figure 3) that increasing the reaction pressure raises the methanol conversion rate, with a significant change in the range of 0.40–0.80 MPa, but only a slight change in the range of 0.80–1.00 MPa. Therefore, the appropriate reaction pressure is 0.70–0.80 MPa. http://www.nmtech.com.cn/jishuwang/upload1/0807091731415767.jpg 3 Conclusion The research results show that the γ-Al2O3-supported heteropolyacid composite catalyst is a novel catalyst with high activity and selectivity for the gas-phase dehydration of methanol to dimethyl ether. The optimal process conditions for the dehydration of methanol to dimethyl ether using this catalyst are: a reaction pressure of 0.7–0.8 MPa, a reaction temperature of 280–320°C, and a mass space velocity of 1.5–2.5 h⁻¹.