Nano mordenite molecular sieve and preparation method and application thereof
Technical Field
The invention belongs to the technical field of molecular sieve catalyst preparation, and particularly relates to a nano mordenite molecular sieve and a preparation method and application thereof.
Background
Methyl acetate is also known as methyl acetate (C)3H6O2) Is slightly soluble in water, is an important chemical product and a good environment-friendly organic solvent, and can replace acetone, butanone, cyclopentane and the like to be applied to the production of resin, leather and the like. The downstream products of the method, such as acetic acid, acetic anhydride, methyl acrylate, ethanol and the like, are important chemical raw materials.
Methyl acetate is mainly produced in the production process of polyvinyl alcohol, acetic acid and terephthalic acid in the form of byproducts. By using the byproduct methyl acetate, the economic benefit of chemical production enterprises can be effectively improved, the discharge amount of waste liquid and sewage can be reduced, and the guarantee is provided for improving the environmental protection benefit and the social benefit of chemical production.
Ethanol is used as a renewable clean energy source, is used as a liquid fuel or is mixed with petroleum according to a certain proportion for use, can effectively reduce the dependence degree of China on the petroleum, and is a potential fuel substitute and additive. Currently, fuel ethanol is mainly produced by a biomass fermentation method. Due to the rising of the price of grains in China in recent years, and the combination of the energy structure characteristics of rich coal, poor oil and little gas in China, the development of a new process for producing ethanol by using coal-based synthesis gas is urgent.
The new process for preparing methyl acetate by dimethyl ether carbonylation and obtaining ethanol by further hydrogenating methyl acetate has the characteristics of high selectivity of target products and mild reaction conditions. At present, the industrial process for preparing ethanol by hydrogenating methyl acetate is mature, and the process for preparing methyl acetate by carbonylating dimethyl ether has the problems of low dimethyl ether conversion rate, high catalyst price and the like.
In order to solve the above problems, researchers have searched for zeolite molecular sieves. The hydrogen mordenite molecular sieve has the advantages of unique pore structure, large specific surface area, strong acidity and the like, and has important industrial application value in the field of dimethyl ether carbonylation. The mordenite has 8-membered ring and 12-membered ring straight channels along the [001] direction, 8-membered ring straight channels between the 8-membered ring and the 12-membered ring and along the [010] direction, for the dimethyl ether carbonylation reaction, the 8-membered ring provides an active site for the dimethyl ether carbonylation reaction, and the active site of the 12-membered ring is related to the inactivation of the molecular sieve. In order to improve the conversion rate of dimethyl ether and the selectivity of methyl acetate, the active sites of 8-membered rings of the molecular sieve are required to be selectively increased, and the effect of the active sites of 12-membered rings in the reaction is weakened. The molecular sieve is treated by inorganic base NaOH to provide more active sites, and mesopores are formed in the molecular sieve, so that the mass transfer rate of reactants and products is increased, and the catalytic activity is improved. However, the molecular sieve framework is largely collapsed by the high-concentration inorganic alkali solution or the long-term treatment, and the catalytic activity is greatly reduced. Therefore, a catalyst with high selectivity and high conversion rate is a problem to be solved in the field.
Disclosure of Invention
The first aim of the invention is to provide a nano mordenite molecular sieve and a preparation method thereof.
The second purpose of the invention is to provide the application of the nano mordenite molecular sieve.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows: a method for preparing a nano mordenite molecular sieve comprises the following steps:
(1) adding a silicon source into an aqueous solution containing an aluminum source, an alkali source and a zinc source, and stirring and aging to form sol;
the formed sol comprises the following components in percentage by weight:
SiO2/Al2O3=5-100;
Na2O/SiO2=0.01-0.50;
H2O/SiO2=10-60;
ZnO/SiO2=0.001-0.2;
wherein the molar weight of the silicon source is SiO2The molar amount of the aluminum source is represented by Al2O3The molar amount of the alkali source is represented by Na2The molar amount of O, the molar amount of zinc source, expressed as the molar amount of ZnO, H2The molar amount of O is expressed as such;
(2) putting the sample prepared in the step (1) into a high-pressure reaction kettle, sealing and crystallizing at the crystallization temperature of 100 ℃ and 200 ℃ for 1-6 days; after crystallization is finished, taking out a product in the high-pressure reaction kettle, washing with water until the pH value is 6.5-7.5, drying and roasting to obtain the Na-MOR molecular sieve;
(3) and (3) uniformly dispersing the Na-MOR molecular sieve prepared in the step (2) in an ammonium salt solution for ammonia exchange, and drying and roasting after 2-3 times of ammonia exchange to obtain the H-MOR molecular sieve.
Preferably, in the step (1), the silicon source is one or more of silica sol, sodium silicate, silicon dioxide or ethyl orthosilicate, the aluminum source is sodium metaaluminate or aluminum oxide, the alkali source is sodium hydroxide or ammonia water, and the zinc source is one or more of zinc nitrate, zinc chloride, zinc acetate, zinc sulfate, zinc hydroxide, zinc sulfide and zinc carbonate.
Preferably, the crystallization temperature in the step (2) is 130-170 ℃, and the crystallization time is 2-4 days.
Preferably, the drying temperature in the step (2) and the step (3) is 100-120 ℃, and the drying time is 8-12 h.
Preferably, the roasting temperature in the step (2) and the step (3) is 500-.
Preferably, the ammonium salt solution in the step (3) is ammonium nitrate solution or ammonium chloride solution, the concentration of the ammonium salt solution is 0.6-1mol/L, and the ammonia exchange conditions are as follows: the exchange time is 2-4h, the exchange temperature is 70-90 ℃, and the solid-to-liquid ratio of the Na-MOR molecular sieve to the ammonium salt solution is 1: 9-11.
The invention also provides the nano mordenite molecular sieve prepared by the method.
The invention also provides the application of the nano mordenite molecular sieve in the reaction of synthesizing methyl acetate by carbonylation of dimethyl ether.
Preferably, in the reaction for synthesizing methyl acetate by carbonylating dimethyl ether, the molar ratio of dimethyl ether to CO as raw materials is 1: 5-50, the reaction temperature is 180--1。
Compared with the prior art, the novel nano-scale mordenite molecular sieve synthesized by the invention not only introduces metal zinc into the framework of the zeolite molecular sieve to regulate and control the strength and the number of acid sites in 8-membered rings and 12-membered rings in the mordenite molecular sieve, but also greatly reduces the particle size of the molecular sieve, reduces the mass transfer path of substances in the pore channel of the mordenite molecular sieve, reduces the formation of carbon deposition and prolongs the service life of a catalyst.
Drawings
FIG. 1 is an XRD pattern of a nano-sized mordenite molecular sieve and H-MOR prepared in examples 1-7 of the present invention.
FIG. 2 is a scanning electron micrograph of a sample of the nano-sized mordenite molecular sieve prepared in example 1 of the present invention.
FIG. 3 is a scanning electron micrograph of a sample of the nano-sized mordenite molecular sieve prepared in example 2 of the present invention.
FIG. 4 is a scanning electron micrograph of a sample of the nano-sized mordenite molecular sieve prepared in example 3 of the present invention.
FIG. 5 is a scanning electron micrograph of a sample of the nano-sized mordenite molecular sieve prepared in example 4 of the present invention.
FIG. 6 is a scanning electron micrograph of a sample of the nano-sized mordenite molecular sieve prepared in example 5 of the present invention.
FIG. 7 is a scanning electron micrograph of a sample of the nano-sized mordenite molecular sieve prepared in example 6 of the present invention.
FIG. 8 is a scanning electron micrograph of a sample of the nano-sized mordenite molecular sieve prepared in example 7 of the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples.
Example 1
Weighing 328.0g of water, 10.4g of sodium hydroxide, 11.48g of sodium metaaluminate and 0.8973g of zinc nitrate hexahydrate, sequentially adding the materials into a beaker, magnetically stirring until the solid is completely dissolved, slowly adding 199.998g of silica sol one drop by one drop into the beaker, uniformly mixing the materials, stirring the materials at room temperature for 2 hours, fully and uniformly mixing the materials to form gel, stirring and aging the gel for 5 hours, and then putting the gel into a hydrothermal kettle. Crystallizing at 170 deg.C for 96 hr, washing with deionized water until pH is 6.5-7.5, drying at 110 deg.C for 12 hr, and calcining at 550 deg.C for 4 hr to obtain Na-type zinc-doped MOR molecular sieve.
And then placing the Na-MOR molecular sieve in a beaker, adding 1.0mol/L ammonium chloride solution into the beaker, carrying out ion exchange for 3 hours at the water bath temperature of 80 ℃, wherein the solid-to-liquid ratio of the Na-MOR molecular sieve to the ammonium chloride solution is 1:11, and repeating the ion exchange once to ensure that the exchange is complete. Then drying at 110 ℃ for 12H, and roasting in a muffle furnace at 550 ℃ for 4H to obtain the H-type zinc-doped MOR molecular sieve.
Example 2
The catalyst was prepared in substantially the same manner as in example 1, except that 1.4895g of zinc nitrate hexahydrate was added in mass.
Example 3
The catalyst was prepared in substantially the same manner as in example 1, except that 2.0853g of zinc nitrate hexahydrate was added in mass.
Example 4
The catalyst was prepared in substantially the same manner as in example 1, except that 2.6811g of zinc nitrate hexahydrate was added in mass.
Example 5
The catalyst was prepared in substantially the same manner as in example 1, except that the zinc source was zinc acetate and the amount added was 1.0971 g.
Example 6
The catalyst was prepared in substantially the same manner as in example 1, except that the zinc source was zinc chloride and the amount added was 0.6957 g.
Example 7
The catalyst was prepared in substantially the same manner as in example 1, except that zinc sulfate was added as the zinc source in an amount of 0.89735 g.
FIG. 1 is a comparison of the XRD patterns of the nano-sized mordenite molecular sieves obtained in examples 1-7 and H-MOR. As can be seen from FIG. 1, the use of different zinc sources and different concentrations of the same zinc source all affect the crystallinity of the mordenite molecular sieve, too low or too high a concentration of zinc source will reduce the crystallinity of the mordenite molecular sieve, and a suitable concentration of zinc source will facilitate the crystallization of the nano-sized mordenite.
FIGS. 2-8 are scanning electron micrographs of samples of the nanoscale mordenite molecular sieve prepared in examples 1-7, respectively. As can be seen from the comparison of fig. 2 to 8, the difference between the morphology and the size of the mordenite molecular sieve can be caused by the type of the zinc source and the concentration of the zinc source, and when the concentration of the zinc source is increased, the composition unit of the mordenite molecular sieve particles is reduced, so that the mass transfer path of the dimethyl ether carbonylation reaction is shortened, which is beneficial to prolonging the service life of the catalyst, but when the concentration of the zinc source exceeds a certain amount, the acid strength of the catalyst is affected, and the conversion rate of the dimethyl ether carbonylation reaction is reduced. The addition of a suitable zinc source concentration can increase the acid strength while enhancing the catalyst life.
Comparative example 1
Under otherwise identical conditions as in example 1, no zinc source was added to the initial gel and the reaction conditions were identical to those of real-time example 1.
Comparative example 2
Under otherwise identical conditions as in example 1, the zinc modified molecular sieve was prepared by ion exchange using zinc without the addition of a zinc source to the initial gel. The preparation method of the zinc modified molecular sieve by the ion exchange method comprises the following steps: weighing 2.27gZn (NO)3)2·6H2Adding O into 100mL deionized water, adding 10g H-MOR molecular sieve into the solution, exchanging in a water bath kettle at 80 ℃ for 3h, and repeating onceAfter suction filtration, the mixture was dried in an oven at 110 ℃ for 12 hours and calcined in a muffle furnace at 550 ℃ for 4 hours.
Comparative example 3
Under otherwise identical conditions as in example 1, the zinc source was not added to the initial gel and the zinc was used to prepare the zinc modified molecular sieve by impregnation. The preparation method of the zinc modified molecular sieve by the impregnation method comprises the following steps: weighing 2.27gZn (NO)3)2·6H2O was added to 100mL of an ethanol solution, then 10g of H-MOR molecular sieve was added to the solution, and the solution was immersed at 40 ℃ for 3 hours, repeated once, suction-filtered, dried in an oven at 110 ℃ for 12 hours, and calcined in a muffle furnace at 550 ℃ for 4 hours.
Example 8: performance testing of the catalyst
Tabletting the prepared H-type zinc-doped molecular sieve, sieving to obtain 40-60 mesh MOR molecular sieve, filling into a stainless steel tube type fixed bed reactor with an inner diameter of 8mm, and filling quartz wool into two ends of a catalyst bed layer respectively. Introducing pure N with the flow rate of 30mL/min from one end2And treating at 300 deg.c and normal pressure for 4 hr for the purpose of eliminating adsorbed water from the molecular sieve. When the temperature is reduced to 220 ℃, the gas is cut into raw material gas, the mixture ratio of the raw material gas is DME/Ar/CO 1/1.5/47.5, and the gas space velocity is 2800h-1And the performance of the catalyst is evaluated at the reaction temperature of 220 ℃ and the reaction pressure of 1.5 MPa. The results are shown in Table 1.
TABLE 1 MOR catalyzed dimethyl ether carbonylation to methyl acetate reaction results with different zinc contents
From table 1, it can be derived: comparative examples 1 to 4: when different qualities of zinc nitrate hexahydrate are used for synthesizing the MOR molecular sieve, the activity of dimethyl ether carbonylation is changed into volcanic type along with the concentration of zinc, namely the conversion rate is increased and then reduced; comparative examples 5 to 7: when different zinc sources are used for synthesizing the MOR molecular sieve, the conversion rate of dimethyl ether is different, but the carbonylation activity of the dimethyl ether can be increased; comparative examples 1 to 3: when zinc is loaded on the MOR molecular sieve by using an ion exchange method and an impregnation method, the two post-treatment methods can also improve the conversion rate of dimethyl ether, which shows that the addition of Zn can improve the activity of the catalyst and increase the active sites for dimethyl ether carbonylation, but compared with the method of directly adding a zinc source into the synthetic gel, the effect of the method is not particularly obvious for improving the conversion rate of the dimethyl ether. Meanwhile, the Zn-MOR obtained by the method of adding a zinc source into the synthetic gel to synthesize MOR has the optimal activity.