Preparation method of super-stable supported Pd-based catalyst and application of super-stable supported Pd-based catalyst in acetylene selective hydrogenation reaction

Abstract

The invention discloses a preparation method of an ultra-stable supported Pd-based catalyst and application of the ultra-stable supported Pd-based catalyst in acetylene selective hydrogenation reaction, the catalyst is composed of an active component, a carrier and auxiliaries, the active component is precious metal palladium, the carrier is silicon dioxide, and the auxiliaries are non-active metal silver and alkali metal. The catalytic performance of the catalyst in acetylene selective hydrogenation reaction is effectively improved through the dual modification and isolation effects of alkali metal and non-active metal silver on active components, and the 0.1% Pd, 1% Ag and 0.5% Na / SiO2-R500 supported catalyst gives consideration to acetylene conversion rate and ethylene selectivity within a wide operation temperature window (100-300 DEG C). The catalyst is not obviously inactivated in 100-hour stability tests at 75 DEG C, 100 DEG C, 150 DEG C and 200 DEG C, the selectivity is always maintained at 84% or above, and the catalyst is simple in preparation method and low in cost.

Description

Technical Field

[0001] The invention belongs to the technical field of catalysts, and in particular relates to a method for preparing an ultrastable supported Pd-based catalyst and application of the catalyst in a selective hydrogenation reaction of acetylene. Background Art

[0002] Ethylene is one of the world's largest chemical products and occupies an important position in the national economy. Ethylene can be used in many industries of the national economy, such as rubber, pharmaceuticals, textiles, coatings, and agriculture, and has a wide impact on my country's economic development and people's lives. In industry, the main source of ethylene is catalytic cracking of naphtha. In addition to the required ethylene, the cracking product usually contains 0.5wt% to 2wt% of trace impurity acetylene. The presence of acetylene will reduce the quality of polyethylene and cause the Ziegler-Natta catalyst in the ethylene polymerization reaction to be poisoned and deactivated, thereby affecting the product quality. Therefore, the effective removal of trace acetylene in ethylene is an important link in the ethylene production process.

[0003] Catalytic selective hydrogenation is currently one of the most effective methods for removing acetylene and purifying ethylene raw materials. Pd-based catalysts are widely used due to their high acetylene hydrogenation activity, but due to the easy over-hydrogenation and oligomerization on single Pd catalysts, the ethylene selectivity is poor. It is well known that the adsorption behavior of molecules is closely related to the structure of the catalytic active center. For example, ethylene is preferentially adsorbed on the active center in the form of ethylene dissociative adsorption, 2σ bond configuration, and π bond configuration at the triatomic active site, diatomic active site, and single atom active site, respectively, and the adsorption intensity decreases successively, which helps ethylene to desorb in time and improves the selectivity of the catalyst. Therefore, constructing "isolated active sites" has become an effective strategy to improve ethylene selectivity. At present, alloying active metals with inactive metals or forming intermetallic compounds is widely used in the field of acetylene selective hydrogenation due to its unique surface properties and electronic structure. Alkali metals usually act as electronic and / or structural promoters to regulate the electronic state or hydrogen overflow level of active metals. For example, Lu et al. (Journal of Colloid and Interface Science 2024, 655,584-593.) first prepared ZrO2 supports with different K contents, namely ZrO2-xK, by co-precipitation, where x represents different K contents, and then prepared Pd / ZrO2-xK catalysts by impregnation. The results showed that Pd / ZrO2-16K exhibited good acetylene hydrogenation performance compared with Pd / ZrO2-4K and Pd / ZrO2-10K catalysts. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in-situ synchrotron radiation photoionization mass spectrometry (SRPIMS) and density functional theory (DFT) calculations show that K doping effectively weakens the adsorption of ethylene by adjusting the electronic state of the catalyst, thereby improving the selectivity of ethylene and significantly reducing the hydrogen activation barrier, which is beneficial to hydrogen overflow, thereby improving the stability of the Pd / ZrO2-16K catalyst. However, the work reported in the above literature usually reduces the hydrogenation activity of the catalyst to a large extent as the ethylene selectivity increases, and the stability in the reaction is still a great challenge.

[0004] Based on the shortcomings of the above technologies, the present invention adopts an impregnation method to design a PdAgNa / SiO2 or PdAgK / SiO2 catalyst prepared by dual modification and isolation of active components by alkali metal and inactive metal silver, which effectively improves the catalytic performance in the selective hydrogenation reaction of acetylene. Summary of the invention

[0005] The present invention aims at the problem that it is difficult to balance acetylene hydrogenation activity and ethylene selectivity in an ethylene refining process, and provides a method for preparing an ultra-stable supported Pd-based catalyst and its application in an acetylene selective hydrogenation reaction. The method improves ethylene selectivity by dual modification and isolation of active components by alkali metals and inactive metal silver.

[0006] The present invention adopts the following technical scheme to achieve the above-mentioned purpose, which is a method for preparing an ultra-stable supported Pd-based catalyst, characterized in that the specific process is: palladium salt, silver salt and alkali metal salt precursors are loaded on the surface of a silica carrier by an impregnation method, and then successively subjected to shade drying at room temperature for 6 to 24 hours, drying at 50 to 150° C. for 6 to 24 hours, calcining at 300 to 500° C. for 2 to 5 hours, and reduction treatment at 200 to 600° C. for 0.5 to 3 hours under a hydrogen atmosphere to obtain a PdAgNa / SiO2 or PdAgK / SiO2 supported catalyst, wherein the catalyst consists of an active component, a carrier and an auxiliary agent, wherein the active component is precious metal palladium, the carrier is silica, and the auxiliary agent is inactive metal silver and alkali metal, and the catalytic performance of the catalyst in the selective hydrogenation reaction of acetylene is effectively improved by the dual modification and isolation effect of the alkali metal and the inactive metal silver on the active component.

[0007] It is further defined that the palladium salt is one or more of palladium nitrate, palladium chloride or palladium acetate, the silver salt is one or more of silver nitrate, silver chloride or silver acetate, and the alkali metal includes sodium and potassium, specifically one or more of sodium carbonate, potassium carbonate, sodium chloride, potassium chloride, sodium nitrate or potassium nitrate.

[0008] It is further defined that the hydrogen atmosphere is 50-100 Vol% H2 / Ar, and the flow rate is 20-50 mL / min.

[0009] It is further defined that the loading amounts of the precious metal palladium, the inactive metal silver and the alkali metal in the PdAgNa / SiO2 or PdAgK / SiO2 supported catalyst are 0.01wt% to 0.5wt%, 0.5wt% to 5wt% and 0.1wt% to 10wt%, respectively.

[0010] The method for preparing the ultra-stable supported Pd-based catalyst of the present invention is characterized in that the specific steps are: respectively taking 0.500 mL of a 0.002 g Pd / mL of palladium nitrate solution and 3.000mL of secondary water are added to a beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner; 0.0157g of AgNO3 is weighed and added to the above beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner; 0.0230g of Na2CO3 or 0.0177g of K2CO3 is weighed and added to the above beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner to obtain a metal precursor suspension; 1.0000g of dried SiO2 carrier is weighed and added to the above metal precursor suspension and stirred to mix evenly, dried in the shade at room temperature, and then placed in an electric blast drying oven to dry at 120°C for 12h, and then the dried sample is ground and placed in a box-type high-temperature resistance furnace to calcine at 400°C for 3h; reduction treatment is performed at 500°C for 1h under a hydrogen atmosphere to obtain a PdAgNa / SiO2 or PdAgK / SiO2 supported catalyst, wherein the hydrogen atmosphere is 100Vol% H2, flow rate is 30mL / min.

[0011] The invention discloses an application of the ultrastable supported Pd-based catalyst in the selective hydrogenation reaction of acetylene, characterized in that the ultrastable supported Pd-based catalyst is used at a temperature of 50-300° C., the raw gas composition is: 1 Vol % acetylene, 5-20 Vol % hydrogen, 10-30 Vol % ethylene, argon is the balance gas, and the space velocity is 36000-300000 mL / (g·h).

[0012] The application of the ultra-stable supported Pd-based catalyst in the selective hydrogenation reaction of acetylene is characterized in that: during the selective hydrogenation reaction of acetylene, the 0.1%Pd1%Ag0.5%Na / SiO2-R500 supported catalyst has both high acetylene conversion rate and high ethylene selectivity within a wide operating temperature window (100-300°C), no obvious deactivation is observed in the stability test of up to 100 hours at 75°C, 100°C, 150°C, and 200°C, and the selectivity is always maintained at more than 84%.

[0013] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. Aiming at the problem that the single Pd catalyst is prone to over-hydrogenation and oligomerization, resulting in poor ethylene selectivity, the present invention improves ethylene selectivity by dual modification and isolation of active components by alkali metal and inactive metal silver; 2. The hydrogenation performance of 0.1%Pd1%Ag0.5%Na / SiO2-R500 prepared by the present invention is significantly better than that of 0.1%Pd1%Ag / SiO2-R500, 0.1%Pd0.5%Na / SiO2-R500 and 0.1%Pd / SiO2-R500 supported catalysts, and exhibits ultra-high stability; 3. The preparation method of the present invention is simple, has good repeatability, is easy to operate, and is conducive to large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The graphs showing the changes in acetylene conversion (a) and ethylene selectivity (b) with temperature for the selective hydrogenation of acetylene over 0.1%Pd1%Ag0.5%Na / SiO2-R500, 0.1%Pd1%Ag / SiO2-R500, 0.1%Pd0.5%Na / SiO2-R500 and 0.1%Pd / SiO2-R500 catalysts prepared in Example 1 and Comparative Examples 1 to 3.

[0015] Figure 2 The graphs show the changes in acetylene conversion (a) and ethylene selectivity (b) with temperature for the selective hydrogenation of acetylene over 0.1%Pd1%Ag / SiO2-R500, 0.1%Pd1%Ag0.5%Na / SiO2-R500 and 0.1%Pd1%Ag1%Na / SiO2-R500 catalysts prepared in Comparative Example 1 and Examples 1-2.

[0016] Figure 3 The graph of the variation of acetylene conversion (a) and ethylene selectivity (b) with temperature for the selective hydrogenation reaction of acetylene over 0.1%Pd1%Ag0.5%Na / SiO2-R400, 0.1%Pd1%Ag0.5%Na / SiO2-R500 and 0.1%Pd1%Ag0.5%Na / SiO2-R600 catalysts prepared in Examples 3, 1 and 4.

[0017] Figure 4 The graph showing the variation of acetylene conversion (a) and ethylene selectivity (b) with temperature for the selective hydrogenation of acetylene over 0.1%Pd1%Ag0.5%Na / SiO2-R500 and 0.1%Pd1%Ag0.5%K / SiO2-R500 catalysts prepared in Examples 1 and 5.

[0018] Figure 5 The graphs showing the changes in acetylene conversion (a) and ethylene selectivity (b) over time for the 0.1%Pd1%Ag0.5%Na / SiO2-R500 catalysts prepared in Examples 6 to 8 at different reaction temperatures. DETAILED DESCRIPTION

[0019] The above contents of the present invention are further described in detail below through examples, but this should not be understood as the scope of the above subject matter of the present invention being limited to the following examples, and all technologies implemented based on the above contents of the present invention belong to the scope of the present invention. Example 1

[0020] (1) Preparation of 0.1%Pd1%Ag0.5%Na / SiO2-R500 supported catalyst The PdAgNa / SiO2-R500 supported catalyst with a theoretical Pd loading of 0.1wt%, a theoretical Ag loading of 1wt% and a theoretical Na loading of 0.5wt% was prepared by the impregnation method. First, 0.500mL of 0.002g Pd / mL of palladium nitrate solution and 3.000mL of secondary water were added to a beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner; 0.0157g of AgNO3 was weighed with an electronic balance and added to a beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner; 0.0230g of Na2CO3 was weighed and added to a beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner to obtain a metal precursor suspension; 1.0000g of dried SiO2 carrier was weighed and added to the above metal precursor suspension and stirred to mix evenly, dried in the shade at room temperature, and then placed in an electric blast drying oven to dry at 120°C for 12 hours; the dried sample was then ground and placed in a box-type high-temperature resistance furnace and calcined at 400°C for 3 hours to obtain a 0.1%Pd1%Ag0.5%Na / SiO2 precursor material; a small amount of stone Quartz wool was evenly placed at the middle and lower end of the quartz tube (a dent was made at the middle and lower part of the quartz tube). 50 mg of 0.1% Pd1% Ag0.5% Na / SiO2 precursor material was weighed with an electronic balance and mixed evenly with 150 mg of quartz sand powder. The mixture was then put into the quartz tube covered with quartz wool. A small amount of quartz wool was spread on top of the sample to fix the sample. The sample was treated at 500°C for 1 h in a hydrogen atmosphere with a flow rate of 30 mL / min and cooled to room temperature to obtain 0.1% Pd1% Ag0.5% Na / SiO2-R500 catalyst.

[0021] (2) Evaluation of hydrogenation performance of 0.1%Pd1%Ag0.5%Na / SiO2-R500 catalyst Acetylene selective hydrogenation reaction test, the feed gas is: 1Vol% acetylene, 10Vol% hydrogen, 20Vol% ethylene, argon is the balance gas, the space velocity is 150000mL / (g·h), the catalyst temperature is 50~300℃, the reaction product is detected by gas chromatography analysis, the acetylene selective hydrogenation performance evaluation results are shown in Figures 1 to 4 . Example 2

[0022] (1) Preparation of 0.1%Pd1%Ag1%Na / SiO2-R500 supported catalyst, adjusting the theoretical loading of Na to 1 wt % and the rest being the same as in Example 1.

[0023] (2) The hydrogenation performance evaluation of the 0.1%Pd1%Ag1%Na / SiO2-R500 supported catalyst is the same as that in Example 1. The results of the acetylene selective hydrogenation performance evaluation are shown in Figure 2 . Example 3

[0024] (1) The preparation of 0.1%Pd1%Ag0.5%Na / SiO2-R400 supported catalyst was the same as that of Example 1, except that the reduction treatment temperature was adjusted to 400°C. The rest was the same as that of Example 1.

[0025] (2) The hydrogenation performance evaluation of the 0.1%Pd1%Ag0.5%Na / SiO2-R400 supported catalyst is the same as that in Example 1. The results of the acetylene selective hydrogenation performance evaluation are shown in Figure 3 . Example 4

[0026] (1) The preparation of 0.1%Pd1%Ag0.5%Na / SiO2-R600 supported catalyst was the same as that of Example 1, except that the reduction treatment temperature was adjusted to 600°C. The rest was the same as that of Example 1.

[0027] (2) The hydrogenation performance evaluation of the 0.1%Pd1%Ag0.5%Na / SiO2-R600 supported catalyst is the same as that in Example 1. The results of the acetylene selective hydrogenation performance evaluation are shown in Figure 3 . Example 5

[0028] (1) Preparation of 0.1%Pd1%Ag0.5%K / SiO2-R500 supported catalyst, Na was adjusted to K and 0.0230gNa2CO3 was adjusted to 0.0177gK2CO3, and the rest was the same as in Example 1.

[0029] (2) The hydrogenation performance evaluation of the 0.1%Pd1%Ag0.5%K / SiO2-R500 supported catalyst is the same as that in Example 1. The results of the acetylene selective hydrogenation performance evaluation are shown in Figure 4 . Example 7

[0030] (1) The preparation of 0.1%Pd1%Ag0.5%Na / SiO2-R500 supported catalyst is the same as that of Example 1.

[0031] (2) Evaluation of hydrogenation performance of 0.1%Pd1%Ag0.5%Na / SiO2-R500 supported catalyst The stability test of acetylene selective hydrogenation reaction was conducted by introducing the following raw materials: 1 Vol% acetylene, 10 Vol% hydrogen, 20 Vol% ethylene, argon as the balance gas, space velocity 150000 mL / (g·h), reaction temperature of the catalyst at 75°C, reaction time 108 h, reaction products were detected by gas chromatography, and the evaluation results of acetylene selective hydrogenation performance are shown in Figure 5 . Example 7

[0032] (1) The preparation of 0.1%Pd1%Ag0.5%Na / SiO2-R500 supported catalyst is the same as that of Example 1.

[0033] (2) Evaluation of the hydrogenation performance of the 0.1%Pd1%Ag0.5%Na / SiO2-R500 catalyst. The reaction temperature of the catalyst was adjusted to 100°C. The rest was the same as that of Example 6. The results of the acetylene selective hydrogenation performance evaluation are shown in Figure 5 . Example 8

[0034] (1) The preparation of 0.1%Pd1%Ag0.5%Na / SiO2-R500 supported catalyst is the same as that of Example 1.

[0035] (2) Evaluation of the hydrogenation performance of the 0.1% Pd1% Ag0.5% Na / SiO2-R500 catalyst. The reaction temperature of the catalyst was first adjusted to 150°C for 62 hours, and then the reaction temperature was increased to 200°C for 46 hours. The rest was the same as the evaluation of the hydrogenation performance of the catalyst in Example 6. The evaluation results of the acetylene selective hydrogenation performance are shown in Figure 5 .

[0036] Comparative Example 1 (1) Preparation of 0.1%Pd1%Ag / SiO2-R500 supported catalyst PdAg / SiO2-R500 supported catalysts with a theoretical Pd loading of 0.1 wt% and a theoretical Ag loading of 1 wt% were prepared by impregnation. First, 0.500 mL of 0.002 g of Pd / mL of palladium nitrate solution and 3.000mL of secondary water were added to a beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner; 0.0157g of AgNO3 was weighed with an electronic balance and added to the beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner to obtain a metal precursor solution; 1.0000g of dried SiO2 carrier was weighed and added to the above metal precursor solution and stirred to mix evenly, dried in the shade at room temperature, and then placed in an electric blast drying oven to dry at 120°C for 12h; then the dried sample was ground and placed in a box-type high-temperature resistance furnace and calcined at 400°C for 3h to obtain a 0.1%Pd1%Ag / SiO2 precursor material; a small amount of Quartz wool was evenly placed at the middle and lower end of the quartz tube (a dent was made at the middle and lower part of the quartz tube). 50 mg of 0.1% Pd1% Ag / SiO2 precursor material was weighed with an electronic balance and mixed evenly with 150 mg of quartz sand powder. The mixture was then put into the quartz tube covered with quartz wool. A small amount of quartz wool was spread on top of the sample to fix the sample. The sample was treated at 500°C for 1 h in a hydrogen atmosphere with a flow rate of 30 mL / min and cooled to room temperature to obtain 0.1% Pd1% Ag / SiO2-R500 catalyst.

[0037] (2) The evaluation of the hydrogenation performance of the 0.1%Pd1%Ag / SiO2-R500 catalyst is consistent with that in Example 1. The evaluation results of the acetylene selective hydrogenation performance are shown in Figure 1~2 .

[0038] Comparative Example 2 (1) Preparation of 0.1%Pd0.5%Na / SiO2-R500 supported catalyst: A PdNa / SiO2-R500 supported catalyst with a theoretical Pd loading of 0.1 wt% and a theoretical Na loading of 0.5 wt% was prepared by the same method as in Example 1.

[0039] (2) The evaluation of the hydrogenation performance of the 0.1%Pd0.5%Na / SiO2-R500 catalyst is the same as that in Example 1. The evaluation results of the acetylene selective hydrogenation performance are shown in Figure 1 .

[0040] Comparative Example 3 (1) Preparation of 0.1% Pd / SiO2-R500 supported catalyst: A Pd / SiO2-R500 supported catalyst with a theoretical Pd loading of 0.1 wt% was prepared by the same method as in Example 1.

[0041] (2) The evaluation of the hydrogenation performance of the 0.1% Pd / SiO2-R500 catalyst is consistent with that in Example 1. The evaluation results of the acetylene selective hydrogenation performance are shown in Figure 1 .

[0042] The above embodiments describe the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for illustrating the principles of the present invention. Without departing from the scope of the principles of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing an ultrastable supported Pd-based catalyst, characterized in that The specific process is: palladium salt, silver salt and alkali metal salt precursors are loaded on the surface of the silica carrier by impregnation method, and then dried in the shade at room temperature for 6~24h, dried at 50~150℃ for 6~24h, calcined at 300~500℃ for 2~5h, and reduced at 200~600℃ for 0.5~3h in a hydrogen atmosphere to obtain PdAgNa / SiO2 or PdAgK / SiO2 loaded catalyst. The catalyst is composed of active components, carriers and additives, wherein the active component is precious metal palladium, the carrier is silica, and the additives are inactive metal silver and alkali metals. The catalytic performance of the catalyst in the selective hydrogenation reaction of acetylene is effectively improved through the dual modification and isolation of the active components by alkali metals and inactive metal silver.

2. The method for preparing an ultrastable supported Pd-based catalyst according to claim 1, wherein: The palladium salt is one or more of palladium nitrate, palladium chloride or palladium acetate, the silver salt is one or more of silver nitrate, silver chloride or silver acetate, and the alkali metal includes sodium and potassium, specifically one or more of sodium carbonate, potassium carbonate, sodium chloride, potassium chloride, sodium nitrate or potassium nitrate.

3. The method for preparing the ultrastable supported Pd-based catalyst according to claim 1, characterized in that: The hydrogen atmosphere is 50-100 Vol% H2 / Ar, and the flow rate is 20-50 mL / min.

4. The method for preparing an ultrastable supported Pd-based catalyst according to claim 1, wherein: The loading amounts of the noble metal palladium, the inactive metal silver and the alkali metal in the PdAgNa / SiO2 or PdAgK / SiO2 supported catalyst are 0.01wt% to 0.5wt%, 0.5wt% to 5wt% and 0.1wt% to 10wt% respectively.

5. The method for preparing an ultrastable supported Pd-based catalyst according to claim 1, characterized in that The specific steps are as follows: pipette 0.500 mL of 0.002 g Pd / mL of palladium nitrate solution and 3.000mL of secondary water are added to a beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner; 0.0157g of AgNO3 is weighed and added to the above beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner; 0.0230g of Na2CO3 or 0.0177g of K2CO3 is weighed and added to the above beaker and stirred to mix evenly, and ultrasonicated for 5 minutes using an ultrasonic cleaner to obtain a metal precursor suspension; 1.0000g of dried SiO2 carrier is weighed and added to the above metal precursor suspension and stirred to mix evenly, dried in the shade at room temperature, and then placed in an electric blast drying oven to dry at 120°C for 12h, and then the dried sample is ground and placed in a box-type high-temperature resistance furnace to calcine at 400°C for 3h; reduction treatment is performed at 500°C for 1h under a hydrogen atmosphere to obtain a PdAgNa / SiO2 or PdAgK / SiO2 supported catalyst, wherein the hydrogen atmosphere is 100Vol% H2, flow rate is 30mL / min.

6. Use of the ultrastable supported Pd-based catalyst prepared by the method according to any one of claims 1 to 5 in the selective hydrogenation reaction of acetylene.

7. The use according to claim 6, characterized in that: The use temperature of the ultra-stable supported Pd-based catalyst is 50-300° C. The raw gas composition is: 1 Vol% acetylene, 5-20 Vol% hydrogen, 10-30 Vol% ethylene, argon as the balance gas, and the space velocity is 36000-300000 mL / (g·h).

8. The use according to claim 6, characterized in that: During the selective hydrogenation of acetylene, the 0.1%Pd1%Ag0.5%Na / SiO2-R500 supported catalyst has both high acetylene conversion and high ethylene selectivity within a wide operating temperature window (100-300°C). No obvious deactivation was observed in the 100-h stability test at 75°C, 100°C, 150°C, and 200°C, and the selectivity was maintained above 84%.

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