CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. § 371 of International Patent Application No.: PCT/CN2018/088175, filed May 24, 2018, which claims priority to Chinese Patent Application No. 201710379007.2, filed May 25, 2017, the entire contents of all of which are hereby incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to a preparation method for an olefin epoxidation catalyst, specifically a preparation method for a SiO2—Ti-MeO—SiO2composite oxide catalyst and a catalyst prepared according to the method, and further relates to use of the catalyst in catalyzing propylene epoxidation to prepare propylene oxide.

BACKGROUND

Co-oxidation methods, also known as Halcon methods, are important methods for producing propylene oxide, comprising isobutane co-oxidation method (PO/TBA) and ethylbenzene co-oxidation method (PO/SM). First of all, peroxide is obtained by oxidizing isobutane or ethylbenzene respectively, then propylene oxide is obtained by oxidizing propylene, and tert-butyl alcohol or styrene is co-produced.

Co-oxidation methods overcome the disadvantages such as strong corrosiveness, large production of polluted water of the chlorohydrin method, and have advantages such as low product cost (co-products share the cost) and less environmental pollution. Co-oxidation methods have been developed rapidly around the world since the methods were industrialized in 1969. At present, the capacity of producing propylene oxide by a co-oxidation method accounts for about 55% of the total capacity in the world. In the co-oxidation methods, PO/SM process co-produces styrene, which is a chemical product produced in bulk and is an important monomer for synthesizing resin and rubber. Due to the relatively broad market of the co-product, this process is spreading faster and faster.

The PO/SM processes can be divided into homogeneous PO/SM processes and heterogeneous PO/SM processes according to the different catalysts used in the key procedures (epoxidation procedures) of the processes. The catalysts used in the epoxidation procedures of the heterogeneous PO/SM processes are Ti—SiO2composite oxide catalysts. The main steps of the preparation methods disclosed in U.S. Pat. No. 3,829,392 and US2003166951A1 as well as Chinese Patent Applications CN1894030A and CN1720100A are as follows: a silica gel carrier was dried first, and then using N2or other inert gases to introduce titanium halide vapor into a reaction tube to react with silica gel (this step is called a chemical vapor deposition), high temperature calcination, and finally a catalyst was prepared through steps such as water washing. The catalyst prepared by this method has problems such as the Ti active center is easily lost during use, the catalyst activity decreases rapidly and the catalyst has a short life.

In order to solve the above problems, it is necessary to seek a new preparation method for propylene epoxidation catalyst to reduce the loss of Ti active center and improve the service life of the catalyst.

SUMMARY OF THE DISCLOSURE

The object of the present disclosure is to provide a preparation method for an olefin epoxidation catalyst. The olefin epoxidation catalyst prepared by the preparation method of the present disclosure can reduce the loss of Ti active center during use and improve the service life of the catalyst.

In the present disclosure, the catalyst is obtained as follows: an auxiliary metal salt is loaded onto a silica gel carrier first, dried, and then a chemical vapor deposition is carried out, the thus obtained product is calcined to load Ti species, washed with water, then a chemical vapor deposition and calcination are carried out again to load a SiO2layer onto the surface of Ti-MeO—SiO2, and finally a silylanization treatment is carried out. The catalyst prepared by the preparation method of the present disclosure can reduce the loss of the Ti active center and improve the service life of the catalyst during use, especially when applied for catalyzing propylene epoxidation to produce propylene oxide.

Another object of the present disclosure is to provide use of the catalyst (also referred to as a SiO2—Ti-MeO—SiO2composite oxide catalyst) prepared by said method, the catalysts can be used as a catalyst for olefin epoxidation to produce an epoxide, especially a catalyst for propylene epoxidation to produce propylene oxide, and the catalyst not only has good catalyst activity, the catalyst also has high average selectivity to PO (propylene oxide).

In order to achieve the above objects, the present disclosure adopts the following technical solutions:

A preparation method for an olefin epoxidation catalyst, which comprises the following steps:

(1) loading an auxiliary metal salt onto a silica gel carrier to obtain a silica gel carrier A modified by the auxiliary metal salt; in some specific embodiments, the auxiliary metal salt can be loaded onto the silica gel carrier using a impregnation method, such as an incipient-wetness impregnation method;

(2) carrying out a drying treatment for the A obtained in step (1);

(3) carrying out a chemical vapor deposition for the dried A using a titanium salt vapor, preferably TiCl4vapor to obtain a silica gel B on which the auxiliary metal salt and the titanium salt, preferably TiCl4are loaded; in some preferred embodiments, in step (3), the dried A is charged into a reaction tube (preferably a fixed bed reactor), and preferably nitrogen is used to introduce the titanium salt vapor into the reaction tube to carry out the chemical vapor deposition;

(4) calcining the B obtained in step (3) to obtain a silica gel C on which the auxiliary metal salt and Ti species are loaded; preferably, the calcination is carried out in N2atmosphere;

(5) carrying out a water vapor washing for the C obtained in step (4), the product obtained by washing is referred to as Ti-MeO—SiO2composite oxide;

(6) carrying out another vapor deposition for the Ti-MeO—SiO2composite oxide obtained in step (5) using an alkyl silicate vapor, preferably tetraethyl orthosilicate vapor to obtain the Ti-MeO—SiO2composite oxide D having a silicon-containing compound loaded on the surface of the composite oxide; preferably, the vapor deposition is carried out in a reaction tube, and the alkyl silicate vapor enters the reaction tube to carry out the vapor deposition for the Ti-MeO—SiO2composite oxide; further preferably, the alkyl silicate vapor is introduced into the reaction tube using nitrogen to carry out the vapor deposition;

(7) calcining the D obtained in step (6) to obtain a Ti-MeO—SiO2composite oxide having a SiO2layer coated on the surface of the composite oxide, this product is referred to as SiO2—Ti-MeO—SiO2;

(8) carrying out a silylanization treatment for the SiO2—Ti-MeO—SiO2obtained in step (7).

The product codes “A”, “B”, “C”, “D” appeared in the preparation method of the present disclosure do not have special meanings and are used as product codes only for ease of description. The Ti species described herein are terms commonly used in the art, the Ti species comprise tetra coordinate skeletal titanium, free titanium dioxide.

Further, in some specific embodiments, steps (3)-(8) are all carried out in the same reaction tube.

The auxiliary metal salt in step (1) of the present disclosure can be one or more of Ce(NO3)3, Pr(NO3)3, Tb(NO3)3and La(NO3)3. Using the amount of the metal oxide of the auxiliary metal salt for calculation, the auxiliary metal salt is added in an amount preferably ranging from 0.6-2.4 wt % based on the mass of the silica gel carrier. Said metal oxide refers to an oxide obtained from the decomposition after high-temperature calcination of the auxiliary metal salt, for example, the oxides corresponding to Ce(NO3)3, Pr(NO3)3, Tb(NO3)3and La(NO3)3are CeO2, Pr6O11, TbO2and La2O3respectively.

The impregnation method in step (1) can be an incipient-wetness impregnation method or other impregnation methods, and is not particularly limited as long as the method can load the auxiliary metal salt on the silica gel carrier. The specific operations of loading the auxiliary metal salt using an incipient-wetness impregnation method, for example, can be carried out as follows: first of all, measuring the water absorption per unit mass of the silica gel carrier, calculating the saturated water absorption of the silica gel carrier, and recording the saturated water absorption as a; weighing a certain amount of auxiliary metal salt and dissolving the salt in a grams of water, and recording the water solution as b solution; then evenly spraying b solution on the surface of the silica gel carrier, then allowing to settle, drying specifically, such as allowing to settle for 2-5 hours then drying at 80-120° C.; specific impregnation operations are conventional means in the art, and will not be described in detail.

An auxiliary metal salt is added in step (1) of the present disclosure, and the auxiliary metal salt is preferably one or more of Ce(NO3)3, Pr(NO3)3, Tb(NO3)3and La(NO3)3. The purpose of adding an auxiliary metal salt is to utilize the alkalinity of the oxide formed from the auxiliary metal salt during the subsequent calcination process, thereby neutralizing the acidity of the free TiO2which is inevitably produced in step (4) and the subsequent steps, reducing self-decomposition reactions of peroxides and improving catalyst selectivity.

The silica gel carrier used in step (1) of the present disclosure is a C-type silica gel, and the specific shape thereof can be but is not limited to a spherical shape or a block etc., and there's no special requirement for the specific shape, for example, the silica gel can be but is not limited to an irregular blocky C-type silica gel. Preferably, the silica gel carrier used in step (1) has a specific surface area of 100-350 m2/g, an average pore diameter of 8-11 nm, a pore volume of 0.7-1.2 ml/g, a Na2O impurity content of <100 ppm, a Fe2O3impurity content of <500 ppm and a size of a spherical equivalent diameter of 0.5-2 mm.

The main purpose of drying in step (2) of the present disclosure is to remove free water in the silica gel carrier, preferred drying temperature is 150-240° C. and preferred drying time is 120 min-240 min. Preferably, in step (2), the auxiliary metal salt modified silica gel carrier A obtained in step (1) is dried using a drying gas, and the drying gas used for drying can be any gas that does not react with silica gel, such as air or nitrogen; the drying in step (2) can be carried out specifically in a reaction tube, and the flow rate of the drying gas in the reaction tube is preferably 1.0-3.0 cm/s.

The step (3) of the present disclosure is a chemical vapor deposition, specifically, the chemical vapor deposition of this step can be carried out in a reaction tube. The titanium salt vapor, preferably TiCl4vapor in the vaporization tank is introduced into the reaction tube using an inert gas, preferably N2. The TiCl4vapor interacts with the associated hydroxy on the surface of the silica gel to carry out the following reaction:
˜O—Si—OH+TiCl4→˜O—Si—O—Ti—Cl

wherein Ti is loaded on the silica gel carrier in an amount ranges from 0.1-5.0 wt %, preferably 2.5-4.5 wt % based on the weight of the silica gel carrier (i.e., the amount of loaded Ti is calculated based on the weight of the silica gel carrier used in step (1)), the Ti described herein is based on the Ti element in the used titanium salt vapor. In some specific embodiments, the titanium salt is contained in a vaporization tank to provide titanium salt vapor, and the titanium salt vapor, preferably TiCl4vapor has a temperature of 137° C.-150° C., i.e. the titanium salt vapor is preheated to this temperature; the flow rate of the inert gas, preferably N2in the reaction tube is 0.05-2.0 cm/s, preferably 0.50-1.35 cm/s, the temperature of the reaction between the titanium salt vapor, preferably TiCl4vapor and the associated hydroxy on the surface of the silica gel is 150-300° C., and the deposition time is 120-240 min.

In some preferred embodiments, the calcination carried out in N2atmosphere in step (4) of the present disclosure is carried out at a calcination temperature ranges from 450-700° C., preferably 500-600° C., the temperature is increased in a rate ranges from 1.5-3° C./min, and the calcination time is 30-240 min, preferably 120-180 min, the flow rate of N2is 0.05-2.0 cm/s, preferably 1-2.5 cm/s. The main purpose of calcination is to introduce the Ti species adsorbed on the surface of the silica gel into the silica gel skeleton to form a tetra-coordinated skeletal Ti (or a Ti═O tetrahedron) active center, to stabilize the active center, and some of the Cl elements form HCl gas during calcination process and are removed from the surface of the silica gel carrier.

The step (5) of the present disclosure is water washing, and the purpose thereof is to remove the Cl element adsorbed on the surface of the silica gel carrier using water vapor, reducing or eliminating the influence of Cl element on the performance of the catalyst; the water washing process can be carried out specifically in the reaction tube. Preferably, a certain amount of water vapor is introduced into the reaction tube using an inert gas, preferably N2, and the Cl element on the surface of the silica gel carrier is interacted with the water vapor to form HCl gas to be removed from the surface of the silica gel carrier. In some specific embodiments, the water vapor comes from a vaporization tank, which has a temperature of 100-200° C. (i.e., the water vapor is preheated to this temperature), preferably 120-180° C.; the water washing time is 180-240 min, and the flow rate of the inert gas, preferably N2is 1-2.5 cm/s. The molar ratio of water vapor to Ti is 20-150:1, preferably 50-100:1, and the amount of Ti in this molar ratio is based on the amount of the Ti element in the titanium salt vapor used in step (3); water washing with a preferred molar ratio of water vapor can remove Cl effectively and completely.

The step (6) of the present disclosure is also a chemical vapor deposition, and the main purpose thereof is to load an alkyl silicate, preferably tetraethyl orthosilicate onto the surface of the silica gel carrier. Preferably, the chemical vapor deposition of this step is carried out in a reaction tube, and an alkyl silicate vapor, preferably tetraethyl orthosilicate vapor is introduced into the reaction tube using an inert gas, preferably N2; in some specific embodiments, the alkyl silicate vapor, preferably tetraethyl orthosilicate vapor comes from a vaporization tank that is heated to a temperature of 166-200° C., the flow rate of the inert gas, preferably N2in the reaction tube is 0.05-2.0 cm/s, preferably 0.5-1.0 cm/s, the reaction temperature is 166-200° C., the reaction time is 120-180 min, and the weight ratio of alkyl silicate, preferably tetraethyl orthosilicate to silica gel carrier is 0.5-1:1, the weight of the silica gel carrier in this weight ratio is based on the weight of the silica gel carrier used in step (1).

The purpose of the step (7) of the present disclosure is to calcine and remove the ligand alkyl ester in the alkyl silicate in air atmosphere, the ligand alkyl ester forms an oxycarbide and is then removed, a SiO2film (or a SiO2layer) is further formed on the surface of the silica gel carrier, and a SiO2—Ti-MeO—SiO2sandwich structure is formed, which can protect Ti species, reducing (decreasing) the loss (loss rates) of Ti active centers, and improving the stability of the catalyst. In some preferred embodiments, in step (7), the calcination is carried out with a heating rate of 1.5-3° C./min, a calcination temperature of 500-700° C., a calcination time of 30-120 min, and the flow rate of air is 0.5-1 cm/s.

The step (8) of the present disclosure is a silylanization treatment, and the silylanization treatment can be specifically carried out in a reaction tube, a silylanization reagent vapor is introduced into the reaction tube using an inert gas, preferably N2, and the chemical reaction is carried out as follows:
˜O—Si—OH+Si(CH3)3—NH—Si(CH3)3→˜O—Si—O—Si(CH3)3

The purpose of the silylanization treatment is to increase the hydrophobicity of the catalyst surface, reducing the decomposition ability of the catalyst to peroxide, and improving the selectivity of the catalyst; the silylanization reagent is preferably hexamethyl disilylamine, hexamethyl disilylamine which is used in an amount of 5 wt %-15 wt % based on the weight of the silica gel carrier (i.e., based on the weight of the silica gel carrier used in step (1)); preferably, the temperature of hexamethyl disilylamine is 126-150° C., the silylanization temperature is 200-300° C., the flow rate of the inert gas, preferably N2in the reaction tube is 0.5-1 cm/s, and the silylanization time is 60-180 min.

The catalyst prepared by the preparation method of the present disclosure is used for catalyzing propylene epoxidation to prepare propylene oxide. The preferred process conditions are as follows: the reaction temperature is 40-120° C., the pressure is 2-4.5 MPa (gauge pressure), the molar ratio of propylene to ethylbenzene hydroperoxide (EBHP) is 3-10:1, and the mass space velocity is 1-5 h−1.

The present disclosure has the following advantageous effects:

(1) A layer of SiO2is further deposited on the surface of Ti-MeO—SiO2composite oxide by chemical vapor deposition, a layer of protective SiO2shell is formed, and a SiO2—Ti-MeO—SiO2sandwich structure is formed, which can protect Ti species, reducing (decreasing) the loss (loss rates) of Ti active centers, and improving the stability of the catalyst; (2) by modifying the silica gel carrier though adding a metal auxiliary thereto, the metal auxiliary synergizes with Ti active centers to improve the activity and the selectivity to propylene oxide of the catalyst, the average selectivity of the catalyst to PO is up to 96.7%, which reduces the consumption of only propylene of PO products; (3) The process flow of the present disclosure is simple to control, easy to industrialize, and has great prospects for industrialization.

