Composite catalyst for preparing acrylic acid through selective oxidation of propylene and preparation method of composite catalyst

Abstract

The invention discloses a composite catalyst for preparing acrylic acid through selective oxidation of propylene and a preparation method of the composite catalyst, and relates to the technical field of acrylic acid synthesis, and the preparation method of the catalyst comprises the following steps: (1) fully mixing pseudo-boehmite, aluminum hydroxide, a magnesium compound, a reinforcing agent and a pore forming agent to form a mixture, adding nitric acid into the mixture, forming a wet gel blank with plasticity; drying, grinding and sieving the wet gel blank, and then carrying out segmented calcination and nitric acid treatment to obtain a magnesium aluminate spinel carrier; and (2) preparing a salt solution from metal salts containing Mo, V, Cu, Ni, Ce and La, adding ammonia water to prepare a complexing solution, loading the metal in the slurry onto a carrier by a ball milling method, carrying out surface boronizing treatment, drying, and calcining to obtain the active catalyst. The prepared catalyst has good acrylic acid selectivity in one-step propylene oxidation preparation of acrylic acid, and also has very high mechanical strength and better stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalytic synthesis of acrylic acid, and particularly relates to a composite catalyst for preparing acrylic acid by selective oxidation of propylene and a preparation method thereof. BACKGROUND

[0002] Acrylic acid is an important raw material and intermediate in the modern chemical industry, and is an important raw material for the synthesis of resins, plasticizers, and leather. Its demand at home and abroad is increasing, which means that the demand for corresponding catalysts is also increasing. Currently, the production of acrylic acid in the industry mainly adopts a two-step method, which first oxidizes propylene to propylene aldehyde and then further oxidizes it to acrylic acid by additional oxygen. In the first stage, Mo-Bi catalyst is used to oxidize propylene to propylene aldehyde, and in the second stage, Mo-V catalyst is used to further oxidize propylene aldehyde to acrylic acid. Such a process route can have high propylene conversion and acrylic acid yield, and has a mature process route system and corresponding catalyst commercial products. However, there are also drawbacks that cannot be ignored: the process requires two different catalysts, two reactors, different operating conditions and control systems, more monitoring points, resulting in large equipment investment and high energy consumption. Therefore, many technical personnel choose to shift their research focus to the one-step oxidation process for preparing acrylic acid from propylene. This process not only has a simple production route, but also saves energy and equipment investment, and has significant economic benefits and practical significance.

[0003] The core technology of the one-step oxidation process for preparing acrylic acid from propylene is to develop a single catalyst that can complete the preparation of acrylic acid from propylene in one reactor. Patent CN201310033400.8 develops a preparation method for a catalyst for preparing acrylic acid by one-step catalytic oxidation of propylene, which uses molybdenum source, vanadium source, and niobium source materials as raw materials, and contains tellurium, antimony, phosphorus, potassium, and sodium additives. The relative molar ratio of molybdenum, vanadium, niobium, and additive elements A, X, and Y is 12:(1-12):(0-20):(1-10):(0-1):(0-1). The catalyst precursor is prepared by co-precipitation method, and then calcined in an inert gas atmosphere at 300-750℃ for 0.5-4h to obtain a catalyst for preparing acrylic acid by one-step catalytic oxidation of propylene. The propylene conversion rate is 75%-86%, the selectivity of acrylic acid is 70%-85%, and the highest acrylic acid yield can reach 68.54%. Although the catalyst prepared by this method improves the propylene conversion rate in the one-step reaction, the selectivity of acrylic acid is relatively poor.

[0004] Patent CN201310135699.8 discloses a kind of catalyst for preparing acrylic acid reaction of propylene one-step oxidation and its preparation method, which includes adding organic complexing agent to the solution of precursor compound of component used for preparing catalyst, heating and stirring until gel, then drying, calcining to obtain catalyst, or adding one of silica sol or aluminum sol to the solution of catalyst precursor compound with organic complexing agent, using 6%~10% mass fraction of nitric acid solution or 4%~10% mass fraction of ammonia solution to adjust the pH of mixed solution, pH range is 1~6, heating and stirring until gel, then drying, calcining to obtain catalyst.The selectivity of catalyst prepared by this method is improved, but it has the problem of poor mechanical strength. SUMMARY

[0005] The purpose of the present application is to provide a kind of composite catalyst for propylene selective oxidation of acrylic acid and its preparation method, to solve the problem of poor selectivity and poor mechanical strength of existing catalyst for preparing acrylic acid by propylene one-step oxidation.

[0006] The purpose of the present application is achieved by the following technical solutions: A preparation method of a composite catalyst for propylene selective oxidation of acrylic acid, specifically comprising the following steps: (1) mixing pseudo-boehmite, aluminum hydroxide, magnesium compound, reinforcing agent and pore-forming agent to form a mixture, adding nitric acid to the mixture, and finally forming a wet gel body with plasticity; The wet gel body is dried, ground and sieved, then subjected to staged calcination, and then the calcined material is treated with nitric acid to obtain a magnesium-aluminum spinel carrier; (2) preparing a salt solution containing Mo, V, Cu, Ni, Ce and La metal salts, adding ammonia water to the salt solution to prepare a complexing solution; Mixing the magnesium-aluminum spinel carrier with the complexing solution and adding a control agent to prepare a slurry, loading the metal onto the carrier by ball milling to prepare a catalyst precursor, and then surface boronizing the catalyst precursor, washing, drying and calcining to form an active catalyst.

[0007] Further, in step (1), the mass ratio of pseudo-boehmite and aluminum hydroxide, calculated as aluminum oxide, is 1:0.5-1.5; Add nitric acid to the mixture to control the pH of the mixture to 2.5-4.0.

[0008] The mixture of pseudo-boehmite and aluminum hydroxide is selected as the aluminum source in the carrier preparation process, wherein the theoretical value of pseudo-boehmite is 80% in terms of the mass of aluminum oxide, and the actual value is 70% considering the purity and water absorption; the actual value of aluminum hydroxide is consistent with the theoretical value in terms of the mass of aluminum oxide, and can be recorded as 65%; the mass ratio of aluminum hydroxide to pseudo-boehmite in terms of aluminum oxide is 1:0.5-1.5; pseudo-boehmite has high activity, which helps to form abundant mesopores; and aluminum hydroxide decomposes more vigorously during calcination, which helps to produce larger pores or act as a template to guide the formation of large pores; preferably, the mass ratio of aluminum hydroxide to pseudo-boehmite in terms of aluminum oxide is 1:1.05.

[0009] Further, in step (1), the magnesium compound is one of magnesium hydroxide or magnesium oxide, and the total molar ratio of magnesium to aluminum is 1:2-3.5; preferably, the magnesium compound is magnesium hydroxide, and the total molar ratio of magnesium to aluminum is 1:2.

[0010] Further, in step (1), the reinforcing agent is calcium aluminate or calcium silicate, and further, the calcium silicate is selected; the mass of the reinforcing agent accounts for 10-25% of the total mass of the mixture; preferably, the mass of the reinforcing agent is 18% of the total mass of the mixture.

[0011] Further, in step (1), the pore-forming agent is sodium polyacrylate or sesbania powder, and the mass of the pore-forming agent is 0.2-12% of the total mass of the mixture; preferably, the pore-forming agent is sodium polyacrylate, and the mass of sodium polyacrylate is 6% of the total mass of the mixture.

[0012] Sodium polyacrylate is suitable for preparing magnesium-aluminum spinel materials with high strength, high porosity and uniform pore size distribution, and can achieve a balance between high porosity and low apparent porosity, suitable for scenarios requiring high mechanical strength and air permeability, and can also achieve precise control of pore structure, and has good compatibility with inorganic systems.

[0013] Nitric acid is added to the mixture to ensure that the acid is in full contact with the mixture powder (mixed and rolled), and under the combined action of the acid and sodium polyacrylate, the mixture forms a wet gel body with plasticity, which is the basis for obtaining a uniform microstructure.

[0014] Further, in step (1), the wet gel body is dried in a blast drying oven at 100-120°C for 1-2 hours to remove most of the water, and the dried material is ground and sieved through a 100-mesh sieve.

[0015] Further, in step (1), the sieved powder is subjected to staged calcination in an inert atmosphere (nitrogen atmosphere), the first stage calcination temperature is 300-500°C, and the calcination time is 1 hour to remove organic matter and crystal water; the second stage calcination temperature is 900-1100°C, and the calcination time is 2-4 hours, and the heating rate during the staged calcination process is controlled at 1-2°C / min.

[0016] Segmented calcination can promote spinel crystalline phase formation while preserving mesopores, avoiding heat stress-induced channel collapse.

[0017] Further, in step (1), the calcined precursor is post-treated: the calcined material is soaked in 0.1-1 mol / L nitric acid for 4 hours; after soaking, the material is soaked and stirred in deionized water at 50 times the volume of the material, and the washing is repeated 3-5 times until the washing solution has a pH close to neutral (pH = 6-7), and then vacuum dried at 60-80°C for 4-6 hours to prepare a magnesium-aluminum spinel carrier.

[0018] Preferably, the concentration of the nitric acid solution is 0.5 mol / L.

[0019] Nitric acid can selectively dissolve the surface amorphous phase or impurities, exposing mesopores and avoiding pore collapse caused by residual moisture.

[0020] Further, in step (2), the molar ratio of metals Mo, V, Cu, Ni, Ce, and La in the metal salt containing Mo, V, Cu, Ni, Ce, and La is 12:3:1.5:0.5:0.3:0.2. Preferably, ammonia is added to the salt solution to control the pH of the salt solution to 10. Preferably, the total metal ion concentration in the complexing solution is 1.5 mol / L.

[0021] The ammonia added during the preparation of the complexing solution will undergo a coordination reaction with the metal ions to form stable ammonia complex cations, such as [Ni(NH3)6] 2+ , etc.

[0022] Further, in step (2), the control agent is stearic acid, and the mass ratio of the complexing solution to the magnesium-aluminum spinel carrier is 1:4. The mass of stearic acid is 2% of the total mass of the slurry.

[0023] Stearic acid, as an organic surfactant and process control agent, can effectively prevent powder agglomeration, thereby improving the overall fluidity of the slurry, making the grinding more efficient, and the powder particle size more uniform; in addition, the external mechanical force will also promote the partial decomposition of the ammonia complex or the interaction with stearic acid, the carboxyl group of stearic acid binds with the metal ions to form stearate, achieving surface modification of the powder, and then the entire system is placed in a ball mill for reactive ball milling. Further, in step (2), the ball milling medium is selected as zirconia balls, the rotation speed is set to 400-600 rpm, and the ball milling time is 2-8 hours; preferably, the ball milling rotation speed is set to 500 rpm, and the ball milling time is 4 hours, so that the components are dispersed at the nanoscale.

[0024] The wet slurry after ball milling is dried at 50-80℃ for 12-24 hours, and then sieved to 80 mesh; preferably, the wet slurry is dried at 60℃ for 12 hours.

[0025] The functional composite powder obtained after sieving is mixed with a release agent, and the release agent is selected from graphite, and the addition amount of graphite is 1-4%, preferably 2%.

[0026] After sufficient mixing, the mixture is pressed into a cylindrical catalyst precursor with a size of diameter 2-5mm and height 2-5mm, preferably a cylindrical catalyst precursor with a size of diameter 5mm and height 5mm.

[0027] Further, in step (2), the surface of the formed catalyst precursor is boronized: an equal volume impregnation method is used to uniformly distribute the boron source solution on the surface and pores of the catalyst precursor, impregnation is performed for 4 hours, after boronization, the catalyst precursor is washed with deionized water or an organic solvent to remove residual boron source and by-products, and then dried; The surface-treated catalyst precursor is calcined at a temperature of 400-450℃ for 2-8 hours.

[0028] Preferably, the boron source is sodium borohydride, boric acid or boron oxide, the calcination temperature is 400℃, and the calcination time is 4 hours. Preferably, the boron source is sodium borohydride, and the concentration of sodium borohydride is 0.5-2mol / L; more preferably, the concentration of sodium borohydride is 1mol / L.

[0029] The boronization treatment can weaken the overall acid strength, especially the excessively strong acid sites, optimize the distribution of acid centers, and is more suitable for structured catalysts, improves mass and heat transfer, and improves the overall performance.

[0030] The calcination process can remove the process control agent and the binder, and convert the amorphous precursor into an active catalyst with a specific crystal phase structure.

[0031] The application also provides a composite catalyst for the selective oxidation of propylene to prepare acrylic acid, and the composite catalyst is prepared by the above-mentioned method for preparing a composite catalyst.

[0032] The application has the following advantages: 1. The preparation method of a composite catalyst for the selective oxidation of propylene to prepare acrylic acid, pseudo-boehmite and hydroxide are used as aluminum sources, and a reinforcing agent and a pore-forming agent are added at the same time, and the catalyst is prepared by a staged calcination method, so that the prepared catalyst has rich pores and high specific surface area, the mechanical strength is improved compared to the carrier prepared by the coprecipitation method in the prior art, and the difference in strength between different particles (large variance and standard deviation) of the carrier prepared by the existing precipitation method is avoided, thereby affecting the loading effect of the catalyst.

[0033] 2. The preparation method of a composite catalyst for the selective oxidation of propylene to prepare acrylic acid, in the carrier preparation process, the calcined material is subjected to acid treatment by nitric acid, the surface amorphous phase or impurities of the carrier material can be selectively dissolved, mesopores are exposed, and pore collapse caused by residual moisture is avoided; at the same time, the alkaline sites (such as free MgO) on the surface of the carrier are effectively removed, so that the surface of the carrier tends to be weakly acidic or neutral, better adapts to the requirement of moderate acidic sites for the reaction of propylene oxidation to acrylic acid, eliminates alkaline sites, and thus improves the selectivity of acrylic acid.

[0034] 3. The preparation method of a composite catalyst for the selective oxidation of propylene to prepare acrylic acid, by preparing a stable metal component complex solution and adding a control agent, powder agglomeration in the ball milling process can be effectively prevented, so that the fluidity of the slurry as a whole is improved, the ball milling is more efficient, and the powder particle size is more uniform; in addition, the external mechanical force also promotes the partial decomposition of the ammonia complex in the complex solution or the interaction with stearic acid, the carboxyl group of stearic acid is combined with metal ions to form a stearate, surface modification of the powder is realized, and then an organic layer is formed on the surface of the catalyst, which can protect the active sites and inhibit sintering and deep oxidation and other side reactions.

[0035] 4. The preparation method of a composite catalyst for the selective oxidation of propylene to prepare acrylic acid, boronization treatment is performed on the shaped catalyst precursor, the overall acid strength is weakened, especially the strong acid sites, the acid center distribution is optimized, the mass and heat transfer is improved, and the selectivity and stability of the catalyst are improved. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The finished product schematic diagram of the catalyst prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0037] The detailed description of the various exemplary embodiments of the present application should not be considered as limiting the present application, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present application.

[0038] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. Additionally, for a range of values of, for example, concentration, intensity, or other values, an intermediate value of the range can be specifically recited; the inclusion of an intermediate value

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All publications mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any reference is not an admission that it is prior art with respect to the present application.

[0040] Many modifications and variations of this application can be made in the light of the above teachings without departing from the spirit and scope thereof. Additional implementations of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The specification and examples given are exemplary only.

[0041] The reagents and apparatus used in the present application are known products or are obtained by purchasing commercially available products.

[0042] The method for preparing the catalyst of the present application is illustrated by the following examples.

[0043] Example 1 Preparation of magnesium aluminate spinel support The weighed pseudo-boehmite 75 g, aluminum hydroxide 77 g, magnesium hydroxide 65 g, calcium silicate 51 g and sodium polyacrylate 17 g were mixed well, and then about 280 mL of 1 mol / L nitric acid was added dropwise to control the pH of the system below 4, to obtain a wet gel body with plasticity.

[0044] The prepared wet gel body was transferred to a blast drying oven and dried at 120°C for 2 hours to remove most of the water. The dried material was ground and sieved (100 mesh).

[0045] The sieved powder was subjected to staged calcination under a nitrogen atmosphere. The first stage calcination temperature was 300°C, and the calcination time was 1 hour. The second stage calcination temperature was 950°C, and the calcination time was 3 hours. The heating rate during the first and second stage calcination processes was 2°C per minute. After cooling to room temperature, the precursor was obtained.

[0046] Post-treatment of the calcined precursor: the calcined material is soaked with 0.5 mol / L nitric acid for 4 hours, and then soaked and stirred with 50 times the volume of deionized water for cleaning, repeated 5 times until the pH of the washing liquid is close to neutral (pH = 6-7), and then dried at 80°C for 4 hours in vacuum to obtain the magnesium-aluminum spinel carrier.

[0047] Loading of active components Ammonium molybdate 58.8 g, ammonium vanadate 8.8 g, copper nitrate trihydrate 9.1 g, nickel nitrate hexahydrate 3.6 g, cerium nitrate hexahydrate 3.3 g, and lanthanum nitrate hexahydrate 2.2 g are weighed respectively, mixed, and then added with 300 mL of deionized water, stirred and dissolved at 50°C, slowly added with about 30 mL of concentrated ammonia water with a concentration of 15 mol / L, and the pH is accurately controlled at 10 to form a uniform and stable complex solution.

[0048] Take 50 mL of the complex solution and mix with 200 g of magnesium-aluminum spinel carrier powder, add 2% of the total mass of stearic acid, where the total mass refers to the total mass of the complex solution, magnesium-aluminum spinel carrier powder, and stearic acid, evenly divided into 4 parts by mass, put into 4 ball milling tanks (500 mL ball milling tank), add zirconium balls, the ball-to-material ratio is 10:1 (mass ratio of zirconium balls to material), ball mill, the ball milling speed is set to 500 rpm, and the ball milling time is 4 hours.

[0049] Dry the wet paste slurry at 60°C for 24 hours, sieve to 80 mesh, mix the obtained functional composite powder with release agent graphite, the addition amount is 2%, which refers to the addition amount of graphite being 2% of the total mass of the functional composite powder and graphite. After mixing thoroughly, press into shape, the size is a cylinder with a diameter of 5 mm and a height of 5 mm, to obtain the catalyst precursor.

[0050] Perform liquid phase impregnation on the shaped catalyst precursor and perform boronization treatment on the surface: prepare a 1 mol / L NaBH4 solution, and use the equal volume impregnation method to make the sodium borohydride solution uniformly distributed on the surface and pores of the material, and impregnate for 4 hours.

[0051] Calcine the surface treated catalyst precursor, the temperature is 400°C, and the calcination time is 4 hours to obtain the active catalyst. The finished product of the prepared catalyst is shown in the figure.

[0052] Example 2 Preparation of magnesium-aluminum spinel carrier Weigh pseudo-boehmite 75 g, aluminum hydroxide 77 g, magnesium hydroxide 43.5 g, calcium silicate 46 g, and sodium polyacrylate 15.4 g, mix the weighed components thoroughly, and then add about 260 mL of 1 mol / L nitric acid, control the pH of the system to be lower than 4, and prepare a wet gel blank with plasticity.

[0053] The prepared wet gel blank was transferred to a blast drying oven and dried at 120°C for 2 hours to remove most of the water, and the dried material was ground and sieved (100 mesh).

[0054] The sieved powder was subjected to staged calcination under a nitrogen atmosphere. The first stage calcination temperature was 300°C for 1 hour, and the second stage calcination temperature was 950°C for 3 hours, and the precursor was obtained after being cooled to room temperature.

[0055] The calcined precursor was subjected to post-treatment. The calcined material was soaked in 0.5 mol / L nitric acid for 4 hours. After soaking, the material was soaked and stirred in deionized water at a volume of 50 times the material, and the process was repeated 5 times until the pH of the washing liquid was close to neutral (pH = 6-7). After washing, the material was vacuum dried at 80°C for 4 hours.

[0056] The same complexing solution was prepared according to the preparation method of the active metal complexing solution in Example 1, and the same active catalyst was prepared using the same preparation method.

[0057] Example 3 Preparation of magnesium-aluminum spinel carrier 75 g of pseudo-boehmite, 77 g of aluminum hydroxide, 52 g of magnesium hydroxide, 48 g of calcium silicate, and 16 g of sodium polyacrylate were weighed, and the weighed components were mixed and then about 260 mL of 1 mol / L nitric acid was added dropwise to control the pH of the system to be less than 4, thereby preparing a wet gel blank with plasticity.

[0058] The prepared wet gel blank was transferred to a blast drying oven and dried at 120°C for 2 hours to remove most of the water, and the dried material was ground and sieved (100 mesh).

[0059] The sieved powder was subjected to staged calcination under a nitrogen atmosphere. The first stage calcination temperature was 300°C for 1 hour, and the second stage calcination temperature was 950°C for 3 hours, and the precursor was obtained after being cooled to room temperature.

[0060] The calcined precursor was subjected to post-treatment. The calcined material was soaked in 0.5 mol / L nitric acid for 4 hours. After soaking, the material was soaked and stirred in deionized water at a volume of 50 times the material, and the process was repeated 5 times until the pH of the washing liquid was close to neutral (pH = 6-7). After washing, the material was vacuum dried at 80°C for 4 hours.

[0061] Loading of active components Ammonium molybdate 58.8 g, ammonium vanadate 8.8 g, copper nitrate 9.1 g, nickel nitrate 3.6 g, cerium nitrate 3.3 g and lanthanum nitrate 2.2 g were weighed respectively, mixed and then added to 300 mL of deionized water for dissolution. After dissolution, about 30 mL of concentrated ammonia water with a concentration of 15 mol / L was slowly added dropwise to form a uniform and stable complex solution with a pH of 10.

[0062] 50 mL of the complex solution was mixed with 200 g of magnesium aluminate spinel carrier powder, and 2% of the total mass of stearic acid was added. The total mass here refers to the total mass of the complex solution, magnesium aluminate spinel carrier powder and stearic acid, which was evenly divided into 4 parts. The mixture was placed in 4 ball mills (500 mL ball mill) and zirconium balls were added. The ball-to-material ratio was 10:1 (mass ratio of zirconium balls to material). Ball milling was performed at a speed of 500 rpm for 4 hours.

[0063] The wet paste slurry was dried at 60°C for 24 hours, sieved to 80 mesh, and the obtained functional composite powder was mixed with a release agent graphite. The addition amount of graphite was 2%, which refers to the addition amount of graphite being 2% of the total mass of the functional composite powder and graphite. After sufficient mixing, the mixture was pressed into a cylindrical shape with a diameter of 5 mm and a height of 5 mm to obtain a catalyst precursor.

[0064] The shaped catalyst precursor was subjected to liquid immersion for boronization treatment: a 1 mol / L NaBH4 solution was prepared, and an equal volume of sodium borohydride solution was evenly distributed on the surface and in the pores of the material by the equal volume immersion method, and the immersion time was 4 hours.

[0065] The surface-treated catalyst precursor was calcined at a temperature of 400°C for 4 hours to obtain an active catalyst.

[0066] Comparative Example 1 Magnesium aluminate spinel carrier preparation Aluminum source: 703 g of aluminum sulfate, magnesium source: 173 g of magnesium sulfate, mixed and added to deionized water to prepare a 1 mol / L metal salt solution, ultrasonic assisted dissolution, forming a uniform and stable salt solution; prepare a 2 mol / L sodium hydroxide solution as the alkali solution; the salt solution and the alkali solution were added to the bottom liquid containing 100 mL of deionized water at the same time, and the co-precipitation reaction was controlled by a reactor with pH and flow controller interlocking, the pH value of the reaction system was maintained at 9-11, the reaction temperature was maintained at 50-80°C by water bath, the stirring speed was 500 rpm, the reactants were mixed uniformly to avoid local high concentration.

[0067] After the salt and alkali solutions were added dropwise simultaneously, the mixture was aged for 1 hour. The aged precipitate was then soaked and washed with deionized water by stirring, repeating this process multiple times until the pH of the washing solution was close to neutral (pH=6-7). After washing, the material was placed in a Buchner funnel for vacuum filtration until a filter cake was formed and no liquid dripped. The cake was then dried in a vacuum drying oven at 80°C for 4 hours and passed through a 100-mesh sieve to obtain magnesium aluminum spinel precursor powder. The powder was then heated to 900°C at a rate of 2°C / min and held at this temperature for 3 hours to prepare the magnesium aluminum spinel support.

[0068] The same complexing solution was prepared according to the preparation method of the active metal complexing solution in Example 1, and the same preparation method was used to prepare the active catalyst.

[0069] Comparative Example 2 Preparation of magnesium aluminum spinel carrier Weigh 75g of boehmite, 77g of aluminum hydroxide, 65g of magnesium hydroxide, 51g of calcium silicate, and 17g of sodium polyacrylate. After thoroughly mixing the weighed components, add approximately 280ml of 1mol / L nitric acid dropwise, controlling the pH of the system to be below 4, to prepare a plastic wet gel preform.

[0070] The prepared wet gel preform was transferred to a forced-air drying oven and dried at 120°C for 2 hours to remove most of the moisture. The dried material was then ground and sieved (100 mesh).

[0071] The sieved powder was subjected to staged calcination under a nitrogen atmosphere. The first stage of calcination was carried out at 300℃ for 1 hour; the second stage of calcination was carried out at 950℃ for 3 hours. After cooling to room temperature, the precursor was obtained.

[0072] The calcined precursor was post-treated by soaking it in 0.5 mol / L nitric acid for 4 hours. After soaking, it was soaked and stirred in 50 times the volume of deionized water and washed repeatedly until the pH of the washing solution was close to neutral (pH=6-7). After washing, it was vacuum dried at 80℃ for 4 hours to prepare the magnesium aluminum spinel carrier.

[0073] Loading of active components Weigh out 58.8g of ammonium molybdate, 8.8g of ammonium metavanadate, 9.1g of copper nitrate trihydrate, 3.6g of nickel nitrate hexahydrate, 3.3g of cerium nitrate hexahydrate, and 2.2g of lanthanum nitrate hexahydrate. Mix them and add 300mL of deionized water. Stir and dissolve at 50℃. Slowly add about 30mL of concentrated ammonia solution with a concentration of 15mol / L, and precisely control the pH at 10 to form a homogeneous and stable complex solution.

[0074] Mix 50 mL of the complexing solution with 200 g of carrier powder, add 2% stearic acid by weight, divide into 4 equal portions by weight, place in 4 ball mill jars (500 mL ball mill jars), add zirconium balls, the ball-to-material ratio is 10:1 (mass ratio of zirconium balls to material), set the ball milling speed to 500 rpm, and the ball milling time is 4 hours.

[0075] The wet paste was dried at 60°C for 24 hours and sieved to 80 mesh. The resulting functional composite powder was mixed with graphite release agent at a dosage of 2%. After thorough mixing, the mixture was pressed into cylinders with a diameter of 5 mm and a height of 5 mm to obtain the catalyst precursor.

[0076] The catalyst precursor without surface treatment was directly calcined at 400℃ for 4 hours to prepare the active catalyst.

[0077] Comparative Example 3 Preparation of magnesium aluminum spinel carrier Weigh 75g of boehmite, 77g of aluminum hydroxide, 52g of magnesium hydroxide, 48g of calcium silicate, and 16g of sodium polyacrylate. After thoroughly mixing the weighed components, add approximately 260ml of 1mol / L nitric acid dropwise, controlling the pH of the system to be below 4, to prepare a plastic wet gel preform.

[0078] The prepared wet gel preform was transferred to a forced-air drying oven and dried at 120°C for 2 hours to remove most of the moisture. The dried material was then ground and sieved (100 mesh).

[0079] The sieved powder was calcined in a nitrogen atmosphere at a heating rate of 2℃ / min, a calcination temperature of 950℃, and a holding time of 3 hours. After cooling to room temperature, the powder was removed to obtain magnesium aluminum spinel carrier particles.

[0080] Loading of active components Weigh out 58.8g of ammonium molybdate, 8.8g of ammonium metavanadate, 9.1g of copper nitrate trihydrate, 3.6g of nickel nitrate hexahydrate, 3.3g of cerium nitrate hexahydrate, and 2.2g of lanthanum nitrate hexahydrate. Mix them and add 300mL of deionized water to dissolve them. After dissolving, slowly add about 30mL of concentrated ammonia solution with a concentration of 15mol / L, and precisely control the pH at 10 to form a homogeneous and stable complex solution.

[0081] Mix 50 mL of the complexing solution with 200 g of magnesium aluminum spinel carrier powder, and add 2% stearic acid by weight. The total weight refers to the total weight of the complexing solution, magnesium aluminum spinel carrier powder, and stearic acid. Divide the mixture into four equal portions by weight and place them into four 500 mL ball mill jars. Add zirconium balls at a ball-to-material ratio of 10:1 (mass ratio of zirconium balls to material) and ball mill. Set the ball milling speed to 500 rpm and the ball milling time to 4 hours.

[0082] The wet paste slurry was dried at 60°C for 24 hours, sieved to 80 mesh, and the obtained functional composite powder was mixed with a releasing agent graphite, with an addition amount of 2%, which means that the addition amount of graphite is 2% of the total mass of the functional composite powder and graphite. After sufficient mixing, the mixture was pressed into a cylindrical shape with a diameter of 5 mm and a height of 5 mm to obtain a catalyst precursor.

[0083] The shaped catalyst precursor was subjected to liquid immersion and boronization treatment on the surface: a 1 mol / L NaBH4 solution was prepared, and an equal volume of sodium borohydride solution was uniformly distributed on the surface and in the pores of the material by the equal volume immersion method, and the immersion time was 4 hours.

[0084] The surface-treated catalyst precursor was calcined at a temperature of 400°C for 4 hours to obtain an active catalyst.

[0085] The catalysts prepared in Example 1 and Example 3 and Comparative Examples 1-2 were subjected to BET characterization, and the specific results are shown in Table 1.

[0086] Table 1

[0087] As can be seen from the BET characterization data in Table 1 above, the catalyst prepared in Example 1 has a significantly higher specific surface area compared to the catalyst prepared in Comparative Example 1, indicating that when the carrier prepared by the method of the present application and the carrier prepared by the conventional method in Comparative Example 1 are respectively used for the preparation of an active catalyst, the catalyst prepared by the method of the present application is more advantageous for improving the activity of the catalyst. In Table 1, although the magnesium-aluminum spinel carrier prepared by the conventional method in Comparative Example 1 is used for the preparation of an active catalyst, it has a higher total pore volume compared to Example 1 and Example 3, but from the pore size distribution, the catalyst of Comparative Example 1 is basically a mesopore, i.e., the pore size is small, which is not conducive to the entry of the substrate into the pore, and the active sites are reduced, thereby affecting the activity of the catalyst; the catalyst prepared in Comparative Example 2 is not subjected to boronization treatment, although the pore size is improved compared to Comparative Example 1, but the specific surface area and total pore volume of the final catalyst are poor.

[0088] In order to further detect the performance of the catalyst, the catalyst activity and selectivity test conditions were specifically performed: the performance was evaluated in a fixed bed reactor (inner diameter 25 mm), the bed height was 5 cm, the catalyst was loaded with 30 grams, the reaction temperature was 290-320°C, the pressure was 0.1 MPa, the raw gas was propylene: air: water vapor = 1:8:1.5, and the overall feed gas space velocity was 1300 h -1 . The specific detection results are shown in Table 2.

[0089] Table 2

[0090] The bed temperature in Table 2 above is the actual temperature detected by the thermocouple, and the set temperature is the set temperature of the fixed bed heating. The reaction is an exothermic reaction, and therefore the actual temperature is higher than the set temperature.

[0091] As can be seen from the data in Table 2 above, the propylene conversion rate and the selectivity of acrylic acid of the catalyst prepared by the conventional method are both lower than those of Example 1. Although the activity of the catalyst prepared by the catalyst without boronization treatment in Comparative Example 2 is not significantly affected, the selectivity of acrylic acid is affected.

[0092] In the preparation of the carrier, only one calcination is performed, and the precursor after calcination is not post-treated. As shown by the results of Comparative Example 3, the conversion rate of acrylic acid does not change significantly, but the selectivity of acrylic acid is still reduced. This is mainly because the crystallinity of the spinel prepared by direct calcination, i.e. one calcination, can be non-uniform, the pore structure can be irregular, and the formation of magnesium oxide and aluminum oxide impurities is easy, and the spinel phase is not fully formed. The stepwise calcination allows the atoms to fully diffuse, forming a high-purity spinel structure, avoiding impurities, and better controlling the evolution of the pore structure, providing more anchoring sites for active components (such as molybdenum, vanadium, bismuth, etc.), allowing them to be highly dispersed, facilitating the mass transfer and diffusion of the reactant (propylene) and the product (acrylic acid), reducing side reactions, and thus improving the selectivity of the catalyst for acrylic acid. Nitric acid treatment can effectively remove the basic sites (such as free MgO) on the surface, making the surface tend to be weakly acidic or neutral. The reaction of propylene oxidation to acrylic acid requires moderate acidic sites, and basic sites will promote side reactions, leading to excessive oxidation of acrylic acid to carbon dioxide. By eliminating the basic sites through nitric acid treatment, the selectivity of acrylic acid can be improved.

[0093] In the reaction process of preparing acrylic acid from propylene oxidation, the catalyst has high mechanical strength, which can avoid the problem of easy wear and tear during the reaction process, thereby affecting the service life of the catalyst. Therefore, the mechanical strength of the catalyst is specifically detected. The mechanical strength of the catalyst is measured by a particle strength tester, and the particle strength is not counted if the particle is not broken. Here, the lateral pressure strength of the catalyst is measured, which refers to the strength of the radial crushing of the catalyst particle. The sampling test results of the mechanical strength of the catalyst are shown in Table 3 below.

[0094] Table 3

[0095] As can be seen from the detection results of the mechanical strength of the prepared catalyst in Table 3 above, the catalyst prepared by using the carrier prepared by the conventional method in Comparative Example 1 has poor pressure resistance; the average pressure resistance of the catalyst prepared by using the catalyst without boronization treatment in Comparative Example 2 is good, but the pressure resistance of the catalysts is uneven.

[0096] Therefore, the catalysts prepared in Comparative Example 1 and Comparative Example 2 have a large difference in pressure strength, and when loaded into a reactor, the bed layers are easy to have different tightness, appear voids or "bridges", cause uneven fluid distribution in the reactor, produce bias flow or channeling; or broken in loading or running to produce powder, the bed layer pore is blocked, the pressure drop is increased, the local overheating ("hot spot") or the temperature is increased, eventually leading to the decrease of the selectivity of the catalyst, the decrease of the product yield, and the influence on the service life of the catalyst.

[0097] The above is only a preferred embodiment of the present application, of course, cannot be limited by this to limit the scope of the present application, should be noted that, for those skilled in the art, without departing from the principles of the present application, can also make a number of improvements and changes, these improvements and changes are also considered to be within the scope of the present application.

Claims

1. A method for preparing a composite catalyst for the selective oxidation of propylene to acrylic acid, characterized in that, Specifically comprising the following steps: (1) the pseudo-boehmite, aluminum hydroxide, magnesium compound, reinforcing agent and pore-forming agent are mixed to form a mixture, nitric acid is added to the mixture until a wet gel body with plasticity is formed; The wet gel body is dried, ground and sieved, then subjected to staged calcination, and then the calcined material is treated with nitric acid to obtain a magnesium-aluminum spinel carrier; (2) metal salts containing Mo, V, Cu, Ni, Ce and La are prepared into a salt solution, and ammonia water is added to the salt solution to prepare a complexing solution; The magnesium-aluminum spinel carrier is mixed with the complexing solution and then a control agent is added to prepare a slurry, and the metals in the slurry are loaded onto the carrier by ball milling to prepare a catalyst precursor, which is subjected to surface boronization treatment, washing, drying and calcination to form an active catalyst.

2. The method of claim 1, wherein the composite catalyst is prepared by the steps of: In step (1), the mass ratio of the pseudo-boehmite and aluminum hydroxide based on aluminum oxide is 1:0.5-1.5; In step (1), the magnesium compound is one of magnesium hydroxide or magnesium oxide, and the total molar ratio of magnesium to aluminum is 1:2-3.

5.

3. The method of claim 1, wherein the composite catalyst is prepared by the steps of: In step (1), the reinforcing agent is calcium aluminate or calcium silicate, and the mass of the reinforcing agent accounts for 10-25% of the total mass of the mixture; In step (1), the pore-forming agent is sodium polyacrylate or sesbania powder, and the mass of the pore-forming agent is 0.2-12% of the total mass of the mixture.

4. The method of claim 1, wherein the composite catalyst is prepared by the steps of: The sieved powder in step (1) is subjected to staged calcination in an inert atmosphere, specifically: the first stage calcination temperature is 300-500°C, and the calcination time is 1 hour to remove organic matter and crystal water; the second stage calcination temperature is 900-1100°C, and the calcination time is 2-4 hours, and the heating rate during the staged calcination process is controlled at 1-2°C / min.

5. The method of claim 4, wherein the composite catalyst is prepared by the steps of: The calcined precursor is subjected to post-treatment, specifically: The calcined material is soaked in 0.1-1 mol / L nitric acid for 4 hours; after soaking, the material is soaked and stirred in deionized water at a volume of 50 times the material, repeated 3-5 times, until the pH of the washing solution is 6-7, then vacuum dried at 60-80°C for 4-6 hours to prepare a magnesium-aluminum spinel carrier.

6. The method of claim 1, wherein the composite catalyst is prepared by the steps of: In step (2), the molar ratio of metals Mo, V, Cu, Ni, Ce and La in the metal salts containing Mo, V, Cu, Ni, Ce and La is 12:3:1.5:0.5:0.3:0.

2.

7. The method of claim 1, wherein the composite catalyst is prepared by the steps of: In step (2), the control agent is stearic acid, and the mass ratio of the complexing solution to the magnesium-aluminum spinel carrier is 1:4; The mass of stearic acid is 2% of the total mass of the slurry.

8. The method of claim 1, wherein the composite catalyst is prepared by the steps of: In step (2), the ball milling medium is zirconia balls, and the rotation speed is set to 400-600 rpm, and the ball milling time is 2-8 hours; The wet slurry after ball milling is dried at 50-80°C for 12-24 hours, and then sieved to 80 mesh; The functional composite powder obtained after sieving is mixed with a release agent graphite, and the addition amount of graphite is 1-4% of the total mass of the functional composite powder and graphite, and then the mixture is pressed into a cylindrical catalyst precursor with a size of 2-5 mm in diameter and 2-5 mm in height.

9. The method of claim 8, wherein the composite catalyst is prepared by a method comprising: The surface of the catalyst precursor after being pressed is boronized, and the specific boronization method is as follows: the boron source solution is uniformly distributed on the surface and in the pores of the catalyst precursor by using an equal-volume impregnation method, impregnation is performed for 4 hours, the catalyst precursor is washed with deionized water or an organic solvent after boronization to remove residual boron source and by-products, and then drying is performed; The catalyst precursor after surface treatment is calcined at a temperature of 400-450 DEG C for 2-8 hours; The boron source is sodium borohydride, boric acid or boron oxide, and the concentration of the boron source solution is 0.5-2 mol / L.

10. A composite catalyst for the selective oxidation of propylene to acrylic acid, characterized in that, The composite catalyst is prepared according to the preparation method of the composite catalyst in any one of claims 1-9.

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