Vanadium phosphorus oxide catalyst and preparation method and application thereof
Technical Field
The invention relates to the technical field of preparation of vanadium phosphorus oxide catalysts, in particular to a preparation method of a vanadium phosphorus oxide catalyst capable of improving the specific surface area of the catalyst.
Background
The vanadium-phosphorus-oxygen catalyst is a catalyst composed of three elements of vanadium, phosphorus and oxygen, has good electron transfer function and oxygen transfer performance, is commonly used in the reaction for preparing maleic anhydride by oxidizing carbon tetrahydrocarbon, is the most effective catalyst applied to the reaction system for preparing maleic anhydride by oxidizing n-butane, and has the main active phase component of vanadyl pyrophosphate, and the catalytic performance is closely related to the preparation method. Since the first commercial use of VPO catalysts by Monsanto in 1974, the preparation process was studied intensively, see catal.rev. -sci.eng.27 (1985): 373.
the preparation of vanadium phosphorus oxide catalyst has been studied in many ways, its catalytic performance depends on its preparation method, the general preparation process is divided into adding raw materials phosphoric acid, vanadic anhydride, isobutanol and other reaction solvents, the pentavalent vanadium is reduced to tetravalent vanadium under the action of organic alcohol, the precursor crystal is finally generated, the reaction generated crystal is filtered, dried, roasted, shaped and activated, and the active component of the finally prepared vanadium phosphorus oxide catalyst is (VO) 2 P 2 O 7 。
For the vanadium phosphorus oxide catalyst, the surface property of the catalyst, the type of additive, the drying and roasting method, the method for activating the catalyst and the like have obvious influence on the catalyst performance. At present, the vanadium phosphorus oxide catalyst is also required to further improve the activity and selectivity, further improve the catalyst performance and increase the maleic anhydride yield.
The performance of the vanadium phosphorus oxygen catalyst has obvious relation with the specific surface area of the catalyst after activation molding, and the larger the specific surface area is, the better the catalyst performance is. In the process, different students adopt a method of adding a pore-expanding agent into a reaction kettle in the reaction process, and the addition and removal of the pore-expanding agent are controlled to improve the finished catalyst
US patent 5275996 discloses the removal by high temperature heat treatment under stripping gas conditions using stearic acid, citric acid, and the like as a pore template. The patent CN106582744A improves the pore canal structure of the catalyst and improves the specific surface area and the performance of the catalyst by adding a nonionic surfactant with low melting point as a pore canal template agent. The patent CN108339558A adopts trimethylolethane, trimethylolpropane, phthalic anhydride, maleic anhydride, tartaric acid, citric acid and the like as pore-expanding agents, and improves the specific surface area of the catalyst, thereby improving the activity of the catalyst.
Although the specific surface area of the catalyst can be increased by adding and removing the pore-expanding agent, the reaction operation steps are increased from the other hand, the reaction control difficulty is increased, and if the pore-expanding agent removing operation is problematic, the pore diameter collapse of the catalyst is easily caused, and the catalyst performance is reduced. In addition, the addition of a pore-expanding agent also increases the cost of the reaction.
Disclosure of Invention
Aiming at the problems that the forming effect of a vanadium phosphorus oxide catalyst precursor is poor during forming in the prior art and the improvement of the specific surface area of the vanadium phosphorus oxide catalyst is generally limited in the aspects of selecting pore canal template agents, pore-forming agents and the like, the reaction operation difficulty and the reaction cost are increased, the invention provides the vanadium phosphorus oxide catalyst, which can effectively improve the catalyst tabletting forming effect, improve the specific surface area of the activated catalyst and improve the catalyst activity through the improvement of a preparation method.
The technical purpose of the first aspect of the invention is to provide a preparation method of a vanadium phosphorus oxide catalyst, which is to dry and bake a vanadium phosphorus oxide catalyst precursor, then place the vanadium phosphorus oxide catalyst precursor in an aqueous atmosphere to absorb moisture, and then perform tabletting, forming and activating to obtain the vanadium phosphorus oxide catalyst.
It is generally understood by those skilled in the art that during the production of vanadium phosphorus oxide catalysts, operational attention is paid to avoiding water absorption, as the vanadium phosphorus oxide catalyst precursor component VOHPO is generally considered 4 ·0.5H 2 O and active phase component (VO) 2 P 2 O 7 Decomposition can occur in water, thereby deactivating the catalyst. However, in the invention, the technical prejudice is overcome, and the method innovatively adopts a small amount of moisture absorption of the calcined non-formed catalyst precursor in an atmosphere containing a certain humidity, so that the catalyst forming effect is improved, a small amount of water absorbed in the activation process can be removed from the formed catalyst, the specific surface area of the activated catalyst can be improved, and the catalyst performance is improved.
Further, the moisture absorption rate of the catalyst precursor is 0.01% -80%, preferably 0.5% -20%, and most preferably 1% -10%, and the moisture absorption rate refers to the ratio of the moisture absorption amount of the catalyst precursor after moisture absorption to the weight of the catalyst precursor before moisture absorption.
Further, the relative humidity of the gas in the aqueous atmosphere is 1% -100%, preferably 10% -90%, and most preferably 50% -80%.
Further, the gas of the aqueous atmosphere is selected from at least one of air, nitrogen or inert gas, preferably air or nitrogen.
Further, the moisture absorption is performed at a temperature of 0 ℃ to 100 ℃, preferably 10 ℃ to 50 ℃, and most preferably 15 ℃ to 35 ℃.
Further, the time of moisture absorption is 2-48 hours, preferably 4-24 hours, and most preferably 6-12 hours.
Further, the moisture absorption operation is performed such that the thickness of the catalyst precursor powder placement layer is 1cm to 30cm, preferably 1 to 10cm, more preferably 2cm to 5cm.
Further, the moisture absorption can be performed in an atmospheric environment, preferably in a constant temperature and humidity machine with adjustable humidity and temperature.
Further, the vanadium phosphorus oxide catalyst precursor is prepared by a liquid phase method, the preparation of the vanadium phosphorus oxide catalyst precursor by the liquid phase method is a technology well known to a person skilled in the art, and the method generally refers to a method for preparing a vanadium phosphorus oxide catalyst precursor solution by taking phosphoric acid and vanadium oxide as raw materials and reacting the raw materials in an organic solvent in the prior art. Specifically, the catalyst is generally prepared by taking an organic solvent as a solvent and taking vanadium pentoxide and phosphoric acid as raw materials, and as a more preferable implementation mode, an auxiliary agent is also added in the preparation process of the vanadium phosphorus oxide catalyst precursor, wherein the auxiliary agent comprises, but is not limited to, at least one salt containing Co, ni, zn, bi, zr, cu, li, K, ca, mg, ti, la, mo, nb, B, fe, cr or Ce; the organic solvent is at least one selected from isoamyl alcohol, isobutanol, isopropanol, benzyl alcohol and n-octanol.
As a more specific preferred embodiment, the preparation of the vanadium phosphorus oxide catalyst precursor includes, but is not limited to, the preparation methods disclosed in patent applications CN105749941A, CN104549393A, CN104607220a and CN104549392 a.
Further, the drying temperature is 80-170 ℃ and the drying time is 6-12 hours; the roasting temperature is 100-600 ℃, preferably 150-450 ℃, and most preferably 200-300 ℃; the roasting time is 1-24 hours, preferably 3-10 hours. The roasting atmosphere is air, nitrogen, inert gas or a mixture of inert gas and air.
Further, the activation is a technique well known to those skilled in the art, specifically, is carried out under an atmosphere of one or a combination of a mixture of nitrogen/air, a mixture of water vapor/air, a mixture of n-butane/air or a mixture of cyclohexane/air, the activation temperature is 350 to 450 ℃, preferably 375 to 425 ℃, the activation time is 5 to 40 hours, preferably 12 to 20 hours
The technical purpose of the second aspect of the invention is to provide the vanadium phosphorus oxide catalyst prepared by the method. The catalyst prepared by the method firstly absorbs moisture of the precursor in a certain humidity atmosphere, so that the catalyst forming effect can be improved, and in the catalyst activation process, a small amount of moisture contained in the formed catalyst can be separated from the catalyst again in a water vapor form, and the specific surface area in the catalyst can be increased in the separation process, so that the catalyst performance is improved.
The technical object of the third aspect of the present invention is to provide the use of the above vanadium phosphorus oxide catalyst for the reaction of n-butane oxidation to maleic anhydride. Specifically, the reaction conditions are generally: the reaction temperature is 380-450 ℃, the pressure is normal pressure-0.5 MPa, and the space velocity of the n-butane mixed gas is 1000-3500 h -1 The concentration of the n-butane is 1.0-1.8% (volume percent).
Compared with the prior art, the technical scheme of the invention has the following advantages:
(1) The invention overcomes the technical prejudice, increases the moisture absorption operation after roasting the vanadium phosphorus oxide catalyst precursor, on one hand, can improve the forming effect of the catalyst precursor, on the other hand, in the activation process, the moisture absorbed by moisture can be separated from the catalyst again in the form of water vapor, and the specific surface area of the catalyst can be increased in the separation process, so that the catalyst performance is improved.
(2) The specific surface area of the catalyst obtained by the method is increased, the influence of the precursor crystallization process caused by the process of adding and removing the pore canal template agent and the pore expanding agent can be avoided, and the crystal performance and the catalyst activity performance of the catalyst precursor are ensured.
(3) The water is used as a safe and environment-friendly substance, so that the environmental pollution caused by other pore canal templates and pore expanding agents can be effectively avoided, the moisture-containing gas is used as a medium, and the small amount of moisture absorption method of the baked catalyst precursor powder does not have extra operation influence on the subsequent forming and activating processes.
Additional features and advantages of the invention will be set forth in the detailed description which follows.
Detailed Description
The following non-limiting examples will enable those of ordinary skill in the art to more fully understand the invention and are not intended to limit the invention in any way.
The specific surface area and pore size of the catalysts prepared in the following examples and comparative examples were measured using an AUTOSORB3B type full-automatic specific surface area and pore size distribution instrument of Quantachrome company, U.S.A.
Vanadium phosphorus oxide catalyst precursors prepared in a liquid phase process were used in the following examples and comparative examples:
in a reaction kettle, adding 649mL of isobutanol solution, 29.53g of vanadium pentoxide, 0.3g of auxiliary ferric nitrate hexahydrate, 0.5g of auxiliary zirconium nitrate, 34.98g of concentrated phosphoric acid, stirring, raising the reaction temperature and keeping at 100 ℃, carrying out reflux reaction, keeping the reflux time for 6 hours, cooling the reaction liquid to room temperature, carrying out vacuum filtration, and leaching a filter cake with a small amount of isobutanol three times. And drying the filter cake in a 100 ℃ oven for 12 hours, and finally roasting in a muffle furnace for 5 hours at 250 ℃ to obtain the black brown catalyst precursor.
The above vanadium phosphorus oxide catalyst precursor was subjected to different subsequent treatments in the following examples and comparative examples, respectively:
example 1
The obtained catalyst precursor is placed in a constant temperature and humidity machine, a gas medium in the constant temperature and humidity machine is air, the thickness of a catalyst layer is 2cm, the temperature is 25 ℃, the air humidity is 80%, the moisture absorption time is 12h, and the moisture absorption rate is measured to be 12%. The catalyst precursor after moisture absorption is pressed into tablets for molding, and the forming rate of the catalyst precursor after moisture absorption is higher than that of the catalyst precursor after moisture absorption, namely, the catalyst precursor after moisture absorption is pressed into tablets for molding in the tablet molding process, so that the catalyst precursor after moisture absorption is easy to mold and the molding effect is good; after molding, the mixture is placed in a tubular reactor and is treated with nitrogen in an inert gas nitrogen atmosphereGas space velocity 800h -1 Heating to 3 ℃/min, heating the activation temperature from room temperature to 425 ℃ for roasting treatment, and keeping the temperature at 425 ℃ for 6 hours, and ending the activation process to obtain the vanadium phosphorus oxide catalyst C1 in a green state. The specific surface area and pore size results are shown in Table 1.
Example 2
The obtained catalyst precursor is placed in a constant temperature and humidity machine, a gas medium in the constant temperature and humidity machine is air, the thickness of a catalyst layer is 4cm, the temperature is 25 ℃, the air humidity is 50%, the moisture absorption time is 8 hours, and the moisture absorption rate is measured to be 5%. The subsequent operation is the same as in example 1, the hygroscopic precursor powder has good molding effect and high molding rate; and obtaining the vanadium phosphorus oxide catalyst C2. The specific surface area and pore size results are shown in Table 1.
Example 3
Placing the obtained catalyst precursor in a tray, wherein the thickness of a material layer is 2cm, placing the tray in an indoor atmospheric environment, and standing for 12 hours; the temperature changes from 15 ℃ to 20 ℃ during standing, the indoor humidity changes from 40% to 50%, and the moisture absorption rate is measured to be 6% after the completion of standing. The subsequent operation is the same as in example 1, the hygroscopic precursor powder has good molding effect and high molding rate; and obtaining the vanadium phosphorus oxide catalyst C3. The specific surface area and pore size results are shown in Table 1.
Example 4
The obtained catalyst precursor is placed in a constant temperature and humidity machine, the gas medium in the constant temperature and humidity machine is nitrogen, the thickness of a catalyst layer is 4cm, the temperature is 25 ℃, the relative humidity of gas is 50%, the moisture absorption time is 10 hours, and the measured moisture absorption rate is 7%. The subsequent operation is the same as in example 1, the hygroscopic precursor powder has good molding effect and high molding rate; and obtaining the vanadium phosphorus oxide catalyst C4. The specific surface area and pore size results are shown in Table 1.
Example 5
The obtained catalyst precursor is placed in a constant temperature and humidity machine, a gas medium in the constant temperature and humidity machine is air, the thickness of a catalyst layer is 6cm, the temperature is 25 ℃, the relative humidity of gas is 40%, the moisture absorption time is 4 hours, and the measured moisture absorption rate is 2%. The subsequent operation is the same as in example 1, the hygroscopic precursor powder has good molding effect and high molding rate; and obtaining the vanadium phosphorus oxide catalyst C5. The specific surface area and pore size results are shown in Table 1. Example 6
The obtained catalyst precursor is placed in a constant temperature and humidity machine, a gas medium in the constant temperature and humidity machine is air, the thickness of a catalyst layer is 2cm, the temperature is 25 ℃, the relative humidity of gas is 80%, the moisture absorption time is 24 hours, and the moisture absorption rate is 22% measured. The subsequent operation is the same as in example 1, the hygroscopic precursor powder has good molding effect and high molding rate; and obtaining the vanadium phosphorus oxide catalyst C6. The specific surface area and pore size results are shown in Table 1.
Comparative example 1
The obtained precursor powder is directly subjected to tabletting and activating operations which are the same as those of the embodiment 1 without moisture absorption, the precursor powder is not easy to form in the tabletting process, the operation difficulty is increased, and the forming rate is low; and obtaining the vanadium phosphorus oxide catalyst D. The specific surface area and pore size results are shown in Table 1.
Table 1.
The performance of the catalyst in the reaction of preparing maleic anhydride by catalyzing n-butane oxidation is measured: the conversion rate of proper interval exists in the reaction of generating maleic anhydride by n-butane under the catalysis of vanadium phosphorus oxide catalyst, and partial maleic anhydride is peroxided to generate CO along with the increase of the conversion rate of the reaction 2 And CO, the reaction selectivity gradually decreases. Since the overall yield of maleic anhydride was highest at about 85% conversion of n-butane, the overall yield at 85% conversion of n-butane was used as an evaluation criterion in the examples and comparative examples.
Crushing and screening the catalyst, taking 5mL of catalyst particles (10-20 meshes), diluting quartz sand with the same mesh number according to a ratio of 1:1, filling the catalyst particles into a stainless steel reaction tube with an inner diameter of 10mm, reacting at a temperature of 410 ℃ and a reaction pressure of 0.1MPa, wherein the reaction gas is n-butane/air mixed gas with butane volume concentration of 1.5%, and the gas space velocity is 1600h -1 The catalytic performance evaluation test was conducted under the reaction conditions of (2). The results of evaluating the catalyst performance are shown in Table 2.
Table 2.
From the results, the method provided by the invention has the advantages that after the moisture absorption operation is added to the vanadium phosphorus oxide catalyst precursor, the vanadium phosphorus oxide catalyst precursor is easier to form, the forming effect is good, the forming rate is high, and the forming operation difficulty is reduced; in addition, under the moisture absorption condition of the invention, the catalyst does not show deactivation phenomenon due to moderate moisture absorption, and the prepared catalyst has at least equivalent catalytic effect to the catalyst in the prior art.