High-activity sulfur modified TiO2 catalyst and application thereof in preparation of olefin by alcohol dehydration

Abstract

The invention discloses a high-activity sulfur-modified TiO2 catalyst and application thereof in preparation of olefin by alcohol dehydration, and belongs to the technical field of nano-catalysis. According to the invention, the TiO2 (S-TiO2) catalyst of which the surface and the bulk phase both contain the sulfur element is prepared from a sulfur-containing precursor (TiOSO4) through a simple hydrothermal method. The structure of the catalyst contains rich pit defects, sulfur atoms in crystal lattices replace oxygen atoms, surface sulfur species exist in the form of SO4 < 2->, and the strength of surface acid sites is suitable for alcohol dehydration reaction. At 160 DEG C, the conversion rate of S-TiO2 to isopropanol reaches 99.2%, the propylene yield reaches 98.6%, and the conversion rate of S-TiO2 to isopropanol is obviously superior to that of pure TiO2 prepared by a sol-gel method. In addition, the S-TiO2 also shows excellent catalytic performance in the conversion reaction of other C3 and C4 alcohols, and can efficiently convert the alcohols into corresponding olefins.

Description

Technical Field

[0001] The present invention relates to a sulfur (S)-containing nano-TiO2 catalyst for highly selective catalytic conversion of various alcohols to olefins, belonging to the field of nano-catalysis technology. Specifically, the present invention prepares a TiO2 (S-TiO2) catalyst with sulfur elements on both the surface and the bulk phase by a hydrothermal method, and applies it to the conversion reaction of C3 or C4 alcohols. This catalyst can efficiently convert alcohols to olefins (propylene or butene). Compared with pure TiO2 prepared by the sol-gel method, S-TiO2 exhibits significantly excellent catalytic performance and has important industrial application value. Background Art

[0002] C3-C4 light olefins are crucial basic organic chemical raw materials in chemical production. Taking propylene as an example, the global consumption of propylene exceeds 100 million tons, and the annual growth rate is expected to be 4%. Currently, the production methods of propylene mainly rely on petrochemical routes, such as steam cracking of naphtha and light diesel, and fluid catalytic cracking of crude oil. However, these methods have high energy consumption and rely on fossil fuels. Although emerging technologies such as propane dehydrogenation to propylene have made significant breakthroughs, they still cannot avoid the dependence on fossil fuels. Isopropanol, as an important platform molecule, can be used as a precursor for synthesizing propylene. Preparing alcohol chemicals such as ethanol, propanol, and hexanol through biological fermentation technology not only has high economic efficiency but also meets the goal of sustainable development. Therefore, developing a technology for efficient catalytic conversion of bio-isopropanol to propylene has become a very promising alternative to traditional petrochemical technologies.

[0003] During the catalytic conversion of isopropanol, the selectivity of products (such as propylene, isopropyl ether, and acetone) is affected by various factors, including the morphology, pore size, redox properties, and surface acidity and basicity of the catalyst. Typical solid acid catalysts, such as molecular sieves, γ-Al2O3, and TiO2, have been widely used to explore the dehydration reaction of isopropanol. For example, it has been reported that small-pore SAPO-34 mainly promotes the formation of olefins in the dehydration reaction of alcohols (ethanol, n-propanol, and isopropanol), while large-pore SAPO-5 is more conducive to the formation of ethers. In the binary Al2O3-Y2O3 mixed oxide, the adsorption of isopropanol on the Lewis acid sites (LAS) Al δ+ is beneficial to the formation of propylene. With the increase in the content of Y2O3, the adsorption of isopropanol on the Y δ+ LAS will reduce the propylene selectivity and increase the acetone selectivity. Although acid sites play a key role in the alcohol dehydration reaction, there are still controversies in the current mechanism research on the nature of active sites. Therefore, designing and preparing solid acid catalysts with efficient alcohol dehydration performance is of great significance for realizing the value-added conversion of biomass alcohols.

[0004] At present, there is a lack of systematic reports on the catalyst structure and mechanism of sulfur-modified TiO2 for the selective conversion of isopropanol. Therefore, in this invention, a nano S-TiO2 catalyst with relatively uniform particle size was prepared using a sulfur-containing precursor. Fine structure characterization shows that this catalyst has abundant pit defects, active hydroxyl groups (-OH), and unsaturated coordinated Ti 4+ species, where internal sulfur atoms are embedded in the lattice to replace oxygen atoms, and surface sulfur species exist in the form of SO4 2- . These structural features optimize the distribution and strength of surface acidic sites. TiO2 prepared by the sol-gel method was used as a comparative catalyst, and its particle size is similar to that of S-TiO2. Compared with TiO2, S-TiO2 shows higher conversion rate, propylene selectivity and yield in the isopropanol conversion reaction, and also has good thermal stability, which is attributed to the appropriate number and strength of its surface acidic sites. In addition, S-TiO2 also shows excellent catalytic performance and universality for the dehydration of other types of alcohols to olefins. Summary of the Invention

[0005] The purpose of this invention is to provide a sulfur-modified TiO2 catalyst, which is prepared by a simple hydrothermal method. This catalyst can be widely used in the value-added conversion of C3-C4 alcohols such as n-propanol, isopropanol, n-butanol, isobutanol and tert-butanol. Compared with pure TiO2, S-TiO2 exhibits excellent alcohol dehydration activity and has significant application potential.

[0006] A highly active sulfur-modified TiO2 catalyst and its application in the dehydration of alcohols to olefins, characterized in that using TiOSO4 as a precursor, a S-TiO2 catalyst with the best alcohol dehydration performance is prepared by the hydrothermal method.

[0007] The preparation method of the S-TiO2 catalyst is characterized in that TiOSO4 is dissolved in deionized water to obtain a TiOSO4 solution, the TiOSO4 solution is transferred to a hydrothermal autoclave, and the reaction is carried out at 170-190 °C, preferably 180 °C, for 3-5 h, preferably 4 h. The reaction product is centrifuged and washed successively with deionized water and ethanol. The dried product is calcined at 400-600 °C (preferably 400-500 °C) for 3 h to obtain the catalyst. The catalysts calcined at different temperatures (400, 500 and 600 °C) are respectively named S-TiO2-400, S-TiO2-500 and S-TiO2-600. The preferred calcination temperature is 500 °C, and the obtained catalyst S-TiO2-500 is denoted as S-TiO2.

[0008] The concentration of the TiOSO4 solution is preferably 50 mL of deionized water corresponding to every 2.5 g - 3.0 g of TiOSO4.

[0009] Further calcine in a muffle furnace, heating to the corresponding temperature at a heating rate of 5 °C / min.

[0010] The catalyst obtained by the present invention is used for the dehydration of C3 or C4 alcohols to olefins (propylene or butene). Further preferred reaction conditions are: in a nitrogen atmosphere, the reaction temperature is 30 - 300 °C, the alcohol vapor concentration is 0.2 - 1.5 vol%, and the space velocity is 20,000 mL / g·h.

[0011] The S-TiO2 catalyst of the present invention under typical reaction conditions (in a nitrogen atmosphere, the isopropanol concentration is 1.0 vol%, and the space velocity is 20,000 mL / g·h) obtains a 99.2% isopropanol conversion rate at 160 °C. The selectivity and yield of propylene are 99.4% and 98.6% respectively. While on pure TiO2, the isopropanol conversion rate at this reaction temperature is 0.46%, and the propylene yield is only 0.19%. After a long reaction of 25 h at 160 °C, the catalytic performance does not decrease significantly, the isopropanol conversion rate ≥ 99%, and the propylene yield ≥ 99%, showing excellent activity and catalytic stability.

[0012] The performance of S-TiO2 prepared by the hydrothermal method is significantly better than that of pure TiO2 prepared by the sol-gel method.

[0013] Further comparing the conversion performance of S-TiO2 and TiO2 in catalyzing alcohols such as n-propanol, n-butanol, isobutanol, and tert-butanol, it is found that the conversion rate and yield of alcohol dehydration catalyzed by S-TiO2 are far better than those of TiO2, demonstrating its broad-spectrum applicability.

[0014] The present invention has the advantages of cheap and easily available raw materials, simple and controllable preparation methods, etc., and can be widely applied to the reaction of alcohol dehydration to olefins in actual industrial processes, having important industrial application value and broad market prospects. Description of the Drawings

[0015] Figure 1 XRD spectra of TiO2, S-TiO2-400, S-TiO2-500, and S-TiO2-600 samples (a) complete and (b) partially enlarged.

[0016] Figure 2 Trends of (a) conversion rate and (b) propylene selectivity with reaction temperature during the selective catalysis of isopropanol by S-TiO2-400, S-TiO2-500, and S-TiO2-600.

[0017] Figure 3 Scanning electron microscope (SEM) photos of (a) TiO2 and (b) S-TiO2.

[0018] Figure 4High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of (a,b)TiO2 and (c,d)S-TiO2.

[0019] Figure 5 Fourier transform infrared (FT-IR) spectra of TiO2 and S-TiO2.

[0020] Figure 6 Trends of (a) conversion rate, (b) propylene selectivity, and (c) propylene yield with temperature during the selective catalysis of isopropanol by TiO2 and S-TiO2.

[0021] Figure 7 Trends of conversion rate, propylene selectivity, and propylene yield with reaction time during the selective catalysis of isopropanol by S-TiO2.

[0022] Figure 8 Trends of conversion rate and olefin selectivity with reaction time during the selective catalysis of n-propanol (1.0 vol%), n-butanol (0.5 vol%), isobutanol (0.8 vol%), and tert-butanol (1.0 vol%) by TiO2.

[0023] Figure 9 Trends of conversion rate and olefin selectivity with reaction time during the selective catalysis of n-propanol (1.0 vol%), n-butanol (0.5 vol%), isobutanol (0.8 vol%), and tert-butanol (1.0 vol%) by S-TiO2. Detailed implementation mode

[0024] To further illustrate the present invention, the following examples are provided for detailed description, along with the accompanying drawings to describe the catalysts used in the present invention and the implementation process of the tests.

[0025] Example 1: Add 2.7 g of TiOSO4 to 50 mL of deionized water and stir for 3 h until completely dissolved. Transfer the resulting solution to a stainless steel autoclave lined with polytetrafluoroethylene and react at 180 °C for 4 h. After the reaction, centrifuge and wash the product with deionized water and ethanol, and dry it overnight at 80 °C. Finally, place the dried product in a muffle furnace and heat it from room temperature to different temperatures (400, 500, and 600 °C) at a heating rate of 5 °C / min, and calcine it at the corresponding temperature for 3 h. The resulting catalysts are named S-TiO2-400, S-TiO2-500, and S-TiO2-600, respectively.

[0026] Example 2: In a quartz fixed-bed reactor, the performance evaluation of S-TiO2 series catalysts calcined at different temperatures for the selective catalytic conversion of isopropanol was carried out. The specific steps are as follows: 0.050 g of the catalyst (40-60 mesh) was uniformly mixed with quartz sand and loaded into the reactor. By controlling the temperature of the isopropanol solution, 1.0 vol% of isopropanol vapor was carried out by nitrogen bubbling. Nitrogen was used as the reaction balance gas, and the space velocity was 20,000 mL / (g·h). Under the set reaction conditions, the reactants and products were detected online by a gas chromatograph. By comparing the performance of different catalysts, S-TiO2-500 was selected and denoted as S-TiO2.

[0027] Comparative Example 1: A pure TiO2 catalyst was prepared according to the sol-gel method. The specific steps are as follows: 20 mL of tetrabutyl titanate was dissolved in 100 mL of ethanol to form solution A; 20 mL of water, 20 mL of ethanol and 12 mL of acetic acid were mixed to form solution B. Under vigorous magnetic stirring, solution B was added dropwise to solution A, and stirring was continued for 12 h to form a gel. After the gel was left for 12 h, it was washed with deionized water and dried overnight at 110 °C. Finally, the dried product was heated to 400 °C at a heating rate of 2 °C / min in a muffle furnace and calcined for 4 h to obtain a pure TiO2 catalyst.

[0028] Comparative Example 2: According to the activity evaluation scheme of S-TiO2, the performance evaluation of the pure TiO2 catalyst for the selective catalytic conversion of isopropanol was carried out. The specific steps are as follows: 0.050 g of TiO2 catalyst (40-60 mesh) was uniformly mixed with quartz sand and loaded into a quartz fixed-bed reactor. By controlling the temperature of the isopropanol solution, 1.0 vol% of isopropanol vapor was carried out by nitrogen bubbling. Nitrogen was used as the reaction balance gas, and the space velocity was 20,000 mL / (g·h). Under the set reaction conditions, the reactants and products were detected online by a gas chromatograph. In addition, the performance differences between TiO2 and S-TiO2 in the catalytic conversion reactions of n-propanol (1.0 vol%), n-butanol (0.5 vol%), isobutanol (0.8 vol%) and tert-butanol (1.0 vol%) were compared, and the space velocity was 20,000 mL / (g·h).

[0029] The above tests were analyzed, summarized and concluded according to the attached drawings.

[0030] The analysis and summary of the test results in the attached drawings are as follows. X-ray diffraction (XRD) analysis shows that all catalysts are anatase phase. The diffraction peak of the (101) crystal plane of the S-TiO2-500 catalyst moves to the left, presumably due to S with a larger ionic radius 2- replacing O in the TiO2 lattice 2- resulting in ( Figure 1) The results of applying the series of catalysts involved in the present invention to the isopropanol dehydration reaction show that S-TiO2-500 has the best isopropanol conversion performance and achieves extremely high propylene selectivity ( Figure 2 ) Subsequently, further analysis was carried out on S-TiO2-500 (abbreviated as S-TiO2), with TiO2 as the comparative catalyst. The SEM results show that both the TiO2 and S-TiO2 catalysts are relatively regular nanoparticles with similar particle sizes ( Figure 3 ) The HAADF-STEM images show clear lattice fringes, indicating that TiO2 and S-TiO2 have good crystallinity ( Figure 4 ) In particular, the surface of S-TiO2 presents more pit defects, exposing more oxygen vacancies and Ti with unsaturated coordination 4+ , which is due to the induction of sulfur species into the TiO2 lattice. The FT-IR results confirm the vibration of the SO4 4+ group coordinated with Ti on the surface of S-TiO2 2- ( Figure 5 ) Further comparing the performance of S-TiO2 and TiO2 in the catalytic conversion of isopropanol, it is found that the isopropanol conversion rate, propylene selectivity and yield on S-TiO2 are significantly better than those of pure TiO2 ( Figure 6 ), and S-TiO2 shows good catalytic stability ( Figure 7 ) In addition, a series of generality exploration experiments were carried out in the dehydration reactions of other alcohols (n-propanol, n-butanol, isobutanol and tert-butanol). The results show that compared with TiO2 ( Figure 8 ), S-TiO2 shows more excellent catalytic activity and product selectivity ( Figure 9 ).

Claims

1. A highly active sulfur-modified TiO2 catalyst, characterized in that, The catalyst has abundant pit defects, active hydroxyl groups (-OH) and unsaturated coordinated Ti 4+ species, in which internal sulfur atoms are embedded in the lattice to replace oxygen atoms, and surface sulfur species exist in the form of SO4 2- . These structural features optimize the distribution and strength of surface acidic sites.

2. The preparation method of a highly active sulfur-modified TiO2 catalyst according to claim 1, characterized in that, Dissolve TiOSO4 in deionized water to obtain a TiOSO4 solution. Transfer the TiOSO4 solution to a hydrothermal autoclave and react at 170 - 190 °C, preferably 180 °C, for 3 - 5 h, preferably 4 h. Centrifuge the reaction product and wash it successively with deionized water and ethanol. The dried product is calcined at 400 - 600 °C (preferably 400 - 500 °C) for 3 h to obtain the catalyst.

3. The preparation method according to claim 2, characterized in that, The concentration of the TiOSO4 solution is preferably 50 mL of deionized water corresponding to every 2.5 g - 3.0 g of TiOSO4.

4. The preparation method according to claim 2, characterized in that, The calcination is carried out in a muffle furnace with a heating rate of 5 °C / min to reach the corresponding temperature.

5. Use of a highly active sulfur-modified TiO2 catalyst according to claim 1 for the dehydration of C3 or C4 alcohols to olefins (propylene or butene).

6. According to the use described in claim 5, the reaction conditions are: in a nitrogen atmosphere, the reaction temperature is 30 - 300 °C, the alcohol vapor concentration is 0.2 - 1.5 vol%, and the space velocity is 20,000 mL / g·h.

7. According to the use described in claim 5, the C3 alcohols catalyzed include n-propanol and isopropanol, and the C4 alcohols include n-butanol, isobutanol, and tert-butanol.

8. According to the use described in claim 5, with an isopropanol concentration of 1.0 vol%, nitrogen as the balance gas, a space velocity of 20,000 mL / g·h, the isopropanol conversion rate reaches 99.2% and the propylene yield reaches 98.6% at 160 °C.

9. According to the use described in claim 5, after a long reaction of 25 h at 160 °C, the performance of catalyzing the conversion of isopropanol does not decrease significantly, with a conversion rate ≥ 99% and a propylene yield ≥ 99%, showing excellent activity and catalytic stability.

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