CN109850916B - Preparation method and application of metal oxide modified SAPO-34 molecular sieve - Google Patents
Preparation method and application of metal oxide modified SAPO-34 molecular sieve Download PDFInfo
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Abstract
The invention discloses a preparation method of a metal oxide modified SAPO-34 molecular sieve, which comprises the following steps: (1) preparing a formed SAPO-34 molecular sieve; mixing a silicon source, an aluminum source, a structure directing agent and a phosphorus source to obtain a precursor SAPO-34 gel, putting the SAPO-34 gel into a reaction kettle for sealing reaction, taking out, cooling, separating, washing, drying and calcining to obtain a formed SAPO-34 molecular sieve; (2) preparing a bimetallic modified SAPO-34 molecular sieve; mixing and stirring a chromium source, a cobalt source and the SAPO-34 molecular sieve prepared in the step (1), adding a carbonate solution, continuously stirring fully, standing and aging the mixed solution, filtering, washing, drying and calcining to obtain the catalyst molecular sieve CoO-Cr2O3SAPO-34. The molecular sieve prepared by the method is used for preparing ethylene by catalyzing ethanol dehydration, has good catalytic activity and stability, has a conversion per pass of ethanol of 99.3 percent, has ethylene selectivity of 99.4 percent, and has long service life.
Description
Technical Field
The invention relates to the field of catalyst molecular sieves, in particular to a preparation method and application of a metal oxide modified SAPO-34 molecular sieve.
Background
Ethylene is the most important basic feedstock in the petrochemical industry and is referred to as the "parent" for the petrochemical industry. At present, more than 70% of petrochemical products in the world are produced by using ethylene as a raw material. Therefore, the production scale, production and technical level of the ethylene industry have become an important index of the national petrochemical industry level. Historically, fluid catalytic cracking and distillation cracking of steam cracked crude oil have been the primary processes for producing ethylene. However, the rising price of crude oil and the increasing demand for ethylene have resulted in a shortage of ethylene production in China, and there is a need for a method for effectively increasing the yield of ethylene.
In the catalyst sector, Silicoaluminophosphate (SAPO) molecular sieves are used as shape selective catalytic materials and are widely used in industry because of their controlled microporosity and tunable acidity. Among SAPOs, 8-ring SAPO-34 is of great interest due to its unique pore structure, acid-base characteristics and large specific surface area, and high selectivity to light olefins in methanol to olefins processes. However, SAPO-34 molecular sieves are primarily microporous structures that allow only short-chain linear olefins to diffuse out, while aromatics, particularly benzene homologues, remain in the molecular sieve channels, causing the channels to gradually fill and reduce or even deactivate the catalyst activity.
The information disclosed in this background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.
Disclosure of Invention
The invention aims to provide a preparation method of a metal oxide modified SAPO-34 molecular sieve, thereby overcoming the defects that the molecular sieve pore channel is easy to fill, the activity of the catalyst is reduced, and the service life of the catalyst is short.
Another objective of the present invention is to prepare metal oxide modified SAPO-34 molecular sieve for ethylene production as ethylene catalyst to achieve high ethanol conversion and ethylene selectivity.
In order to achieve the purpose, the invention provides a preparation method of a metal oxide modified SAPO-34 molecular sieve, which comprises the following steps:
(1) preparation of shaped SAPO-34 molecular sieves
Mixing a silicon source, an aluminum source, a structure directing agent and a phosphorus source to obtain a precursor SAPO-34 gel, then placing the SAPO-34 gel into a reaction kettle for sealing, then taking out, cooling, separating, washing, drying and calcining to obtain a formed SAPO-34 molecular sieve;
(2) preparation of bimetal modified SAPO-34 molecular sieve
Mixing a chromium source, a cobalt source and the SAPO-34 molecular sieve prepared in the step (1)Mixing and stirring, adding carbonate solution, continuously stirring fully, standing and aging the mixed solution, filtering, washing, drying and calcining to obtain the catalyst CoO-Cr2O3SAPO-34 molecular sieve.
Preferably, in the above technical scheme, in the step (1), the silicon source is tetraethyl orthosilicate, the aluminum source is aluminum isopropoxide, and the phosphorus source is phosphoric acid; the structure directing agent is tetraethyl ammonium hydroxide aqueous solution with the mass concentration of 20-30 wt%.
Preferably, in the above technical scheme, the reaction conditions in the reaction kettle in the step (1) are that the reaction is carried out for 24-72h at the temperature of 120-250 ℃.
Preferably, in the technical scheme, the drying in the step (1) is drying for 5-20h at the temperature of 80-150 ℃; the calcination is carried out for 10-20h at 550-700 ℃.
Preferably, in the above technical solution, the molar mass ratio of the silicon source, the aluminum source, the phosphorus source, and the structure directing agent in step (1) is: 1Al2O3：1P2O5：0.5SiO2：1.0TEAOH：50H2O。
Preferably, in the above technical solution, the chromium source in step (2) is Cr (NO)3)3An aqueous solution; the source of cobalt is Co (NO)3)2An aqueous solution; adding chromium metal simple substance and SAPO-34 molecular sieve with the mass ratio of 0.5-2.5: 1; the mass ratio of the added cobalt metal simple substance to the SAPO-34 molecular sieve is 0.5-2.5: 1.
Preferably, in the above technical solution, the carbonate solution in step (2) is one of a sodium carbonate solution and a potassium carbonate solution. The mass of carbonate needed is calculated according to the reaction equation.
Preferably, in the technical scheme, the stirring in the step (2) is carried out for 0.5-2h at the temperature of 70-100 ℃; standing and aging for 1-5 h; drying at 80-150 deg.C for 5-20 hr; the calcination is carried out for 1-20h at the temperature of 600-700 ℃.
The application of the metal oxide modified SAPO-34 molecular sieve prepared by the method as the catalyst for producing the ethylene by using the catalyst as the ethylene catalyst.
Combining the physical and chemical properties of methanol and ethanol, the molecular dynamics diameter of ethanol is 0.42-0.52nm from direct molecular dynamics, the SAPO-34 is 0.43-0.50nm, and ethanol molecules can enter the pore channels of the SAPO-34 molecular sieve for adsorption and reaction, so that the production of ethylene by using SAPO-34 as an ethylene catalyst is feasible.
Preferably, in the above technical scheme, the method for preparing ethylene comprises: the reaction temperature is 300-500 ℃, and the heavy air time speed is 2.5-3.5h-1Dehydrating with ethanol to prepare ethylene.
Compared with the prior art, the invention has the following beneficial effects:
(1) the metal oxide modified SAPO-34 molecular sieve prepared by the method successfully prepares CoO-Cr by adopting a dipping precipitation method2O3The supported SAPO-34 molecular sieve has CHA structure without altering the original SAPO-34 molecular sieve, weak acid and strong acid supported by metal oxide, strengthened medium acid, increased specific surface area and reduced average pore size.
(2) The metal oxide modified SAPO-34 molecular sieve prepared by the invention is used for preparing ethylene by catalyzing ethanol dehydration, has good catalytic activity and stability, has the conversion per pass of ethanol of 99.3 percent, has the selectivity of ethylene of 99.4 percent, and has the service life of up to 600 min. The catalyst can still maintain high catalytic performance for 600 min.
Drawings
FIG. 1 is Me for different metal contentsxOyXRD spectrum of SAPO-34.
FIG. 2 is an SEM image of MexOy/SAPO-34 molecular sieves of varying metal content according to the invention;
wherein, FIG. 2(a) is SEM picture of SAPO-34 molecular sieve, and FIG. 2(b) is Cr2O3SEM picture of/SAPO-34 molecular sieve, and FIG. 2(c) is SEM picture of CoO-Cr2O3/SAPO-34 molecular sieve.
FIG. 3 is NH of MexOy/SAPO-34 with different metal contents3-a TPD map;
wherein, FIG. 3(a) is NH of SAPO-34 molecular sieve3TPD plot, FIG. 3(b) is Cr2O3NH of SAPO-34 molecular sieve3TPD plot, FIG. 3(c) NH of CoO-Cr2O3/SAPO-34 molecular sieves3-TPD map.
FIG. 4 Me of different metal contents before and after reactionxOyTG-DTG pattern of SAPO-34.
FIG. 5 is a graph of different metal oxide content versus MexOyInfluence graph of SAPO-34 catalytic performance;
wherein FIG. 5(a) shows Cr is different from Cr2O3Graph of the effect of SAPO-34 molecular sieve content on catalyst performance; FIG. 5(b) shows different CoO-Cr2O3Graph of the effect of SAPO-34 molecular sieve content on catalyst performance.
FIG. 6 is reaction temperature vs. MexOyInfluence graph of SAPO-34 catalytic performance;
wherein FIG. 6(a) is a graph showing the reaction temperature vs. Cr2O3Graph of the effect of SAPO-34 molecular sieve on catalyst performance; FIG. 6(b) is the reaction temperature vs. CoO-Cr2O3Graph of the effect of SAPO-34 molecular sieve on catalyst performance.
FIG. 7 is weight hourly space velocity vs. MexOyInfluence graph of SAPO-34 catalytic performance;
wherein, FIG. 7(a) shows the weight hourly space velocity vs. Cr2O3Graph of the effect of SAPO-34 molecular sieve on catalyst performance; FIG. 7(b) is weight hourly space velocity vs. CoO-Cr2O3Graph of the effect of SAPO-34 molecular sieve on catalyst performance.
FIG. 8 is the reaction duration versus MexOyInfluence graph of SAPO-34 catalytic performance;
wherein FIG. 8(a) is the reaction duration vs. Cr2O3Graph of the effect of SAPO-34 molecular sieve on catalyst performance; FIG. 8(b) is the reaction duration vs. CoO-Cr2O3Graph of the effect of SAPO-34 molecular sieve on catalyst performance.
Detailed Description
The following detailed description of the present invention is provided in conjunction with the accompanying drawings, but it should be understood that the scope of the present invention is not limited to the specific embodiments.
Throughout the specification and claims, unless explicitly stated otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element or component but not the exclusion of any other element or component.
Example 1
A preparation method of a metal oxide modified SAPO-34 molecular sieve comprises the following steps:
(1) preparation of shaped SAPO-34 molecular sieves
The silicon source and the aluminum source of the framework element in the catalyst are respectively tetraethyl orthosilicate (TEOS) and aluminum isopropoxide (C) with the mass concentration of 98 percent9H2AlO3). Using a 25 wt% tetraethylammonium hydroxide (TEAOH) aqueous solution as the structure directing agent, 85 wt% H was selected3PO4The aqueous solution serves as a source of phosphorus in the framework, i.e. a phosphorus source. The molar mass ratio of the silicon source, the aluminum source, the phosphorus source and the structure directing agent is as follows: 1Al2O3：1P2O5：0.5SiO2: 1.0 TEAOH: 50H 2O. Mixing a silicon source, an aluminum source, a structure directing agent and a phosphorus source to prepare the alloy with the molar ratio of 1Al2O3：1P2O5：0.5SiO2：1.0TEAOH：50H2Precursor of O SAPO-34 gel. And then placing the SAPO-34 gel into a polytetrafluoroethylene inner stainless steel reaction kettle for sealing, heating for 48h under the autogenous pressure at 180 ℃, cooling, separating, washing for a plurality of times, drying for 12h at 120 ℃, and calcining for 12h at 550 ℃ to obtain the formed SAPO-34 molecular sieve powder.
(2) Preparation of bimetal modified SAPO-34 molecular sieve
With Cr (NO)3)3·9H2The O water solution is a chromium source; co (NO)3)2·6H2The O water solution is a cobalt source. With Cr (NO)3)3·9H2O and Co (NO)3)2·6H2SAPO-34 powder impregnated with aqueous solution of O to prepare SAPO-34 supported CoO-Cr2O3A catalyst. Adding chromium metal simple substance and SAPO-34 molecular sieve with the mass ratio of 0.5-2.5: 1; the mass ratio of the added cobalt metal simple substance to the SAPO-34 molecular sieve is 0.5-2.5:1, namely the mass ratio x(Me/SAPO-34)Are respectively dissolved in V at a ratio of 0.5,1.0,1.5,2.0,2.5(Water)30mL of distilled water). Adjusting the concentration of the precursor solutionTo produce Cr2O32% by weight and 0.5-2.5% by weight of CoO.
Mixing a chromium source, a cobalt source and the SAPO-34 molecular sieve prepared in the step (1), stirring for 1h at the temperature of 80 ℃, stirring and refluxing the mixed suspension, and adding a certain amount of sodium carbonate solution (Na required by calculation according to a reaction equation)2CO3Mass, dissolved in 30mL of water), stirring at 80 ℃ for 1h, standing the mixture for aging for 2h, filtering, washing, drying at 120 ℃ for 12h, and calcining at 550 ℃ for 12 h. Obtaining the catalyst xCoO-2Cr2O3SAPO-34 molecular sieve, wherein x represents weight percent.
Example 2
A preparation method of a metal oxide modified SAPO-34 molecular sieve comprises the following steps:
(1) preparation of shaped SAPO-34 molecular sieves
The silicon source and the aluminum source of the framework element in the catalyst are respectively tetraethyl orthosilicate (TEOS) and aluminum isopropoxide (C) with the mass concentration of 98 percent9H2AlO3). Using a 25 wt% tetraethylammonium hydroxide (TEAOH) aqueous solution as the structure directing agent, 85 wt% H was selected3PO4The aqueous solution serves as a source of phosphorus in the framework, i.e. a phosphorus source. The molar mass ratio of the silicon source, the aluminum source, the phosphorus source and the structure directing agent is as follows: 1Al2O3：1P2O5：0.5SiO2: 1.0 TEAOH: 50H 2O. Mixing a silicon source, an aluminum source, a structure directing agent and a phosphorus source to prepare the alloy with the molar ratio of 1Al2O3：1P2O5：0.5SiO2：1.0TEAOH：50H2Precursor of O SAPO-34 gel. And then placing the SAPO-34 gel into a polytetrafluoroethylene inner stainless steel reaction kettle for sealing, heating for 72h under the autogenous pressure at 180 ℃, cooling, separating, washing for a plurality of times, drying for 20h at 180 ℃, and calcining for 20h at 550 ℃ to obtain the formed SAPO-34 molecular sieve powder.
(2) Preparation of bimetal modified SAPO-34 molecular sieve
With Cr (NO)3)3·9H2O aqueous solutionIs a source of chromium; co (NO)3)2·6H2The O water solution is a cobalt source. With Cr (NO)3)3·9H2O and Co (NO)3)2·6H2SAPO-34 powder impregnated with aqueous solution of O to prepare SAPO-34 supported CoO-Cr2O3A catalyst. Adding chromium metal simple substance and SAPO-34 molecular sieve with the mass ratio of 0.5-2.5: 1; the mass ratio of the added cobalt metal simple substance to the SAPO-34 molecular sieve is 0.5-2.5:1, namely the mass ratio x(Me/SAPO-34)Are respectively dissolved in V at a ratio of 0.5,1.0,1.5,2.0,2.5(Water)30mL of distilled water). Adjusting the concentration of the precursor solution to produce Cr2O32% by weight and 0.5-2.5% by weight of CoO.
Mixing a chromium source, a cobalt source and the SAPO-34 molecular sieve prepared in the step (1), stirring for 2 hours at the temperature of 70 ℃, stirring and refluxing the mixed suspension, and adding a certain amount of potassium carbonate solution (Na required by calculation according to a reaction equation)2CO3Mass, dissolved in 30mL of water), stirring at 50 ℃ for 2h, standing the mixture for aging for 5h, filtering, washing, drying at 80 ℃ for 20h, and calcining at 600 ℃ for 20 h. Obtaining the catalyst xCoO-2Cr2O3SAPO-34 molecular sieve, wherein x represents weight percent.
Example 3
A preparation method of a metal oxide modified SAPO-34 molecular sieve comprises the following steps:
(1) preparation of shaped SAPO-34 molecular sieves
The silicon source and the aluminum source of the framework element in the catalyst are respectively tetraethyl orthosilicate (TEOS) and aluminum isopropoxide (C) with the mass concentration of 98 percent9H2AlO3). Using a 25 wt% tetraethylammonium hydroxide (TEAOH) aqueous solution as the structure directing agent, 85 wt% H was selected3PO4The aqueous solution serves as a source of phosphorus in the framework, i.e. a phosphorus source. The molar mass ratio of the silicon source, the aluminum source, the phosphorus source and the structure directing agent is as follows: 1Al2O3：1P2O5：0.5SiO2: 1.0 TEAOH: 50H 2O. Mixing a silicon source, an aluminum source, a structure directing agent and a phosphorus source to prepare the silicon-aluminum-phosphorus-doped aluminum-silicon alloy with the molar ratio of 1Al2O3：1P2O5：0.5SiO2：1.0TEAOH：50H2Precursor of O SAPO-34 gel. And then placing the SAPO-34 gel into a polytetrafluoroethylene inner stainless steel reaction kettle for sealing, heating for 24h under the autogenous pressure at 180 ℃, cooling, separating, washing for a plurality of times, drying for 5h at 150 ℃, and calcining for 10h at 700 ℃ to obtain the formed SAPO-34 molecular sieve powder.
(2) Preparation of bimetal modified SAPO-34 molecular sieve
With Cr (NO)3)3·9H2The O water solution is a chromium source; co (NO)3)2·6H2The O water solution is a cobalt source. With Cr (NO)3)3·9H2O and Co (NO)3)2·6H2SAPO-34 powder impregnated with aqueous solution of O to prepare SAPO-34 supported CoO-Cr2O3A catalyst. Adding chromium metal simple substance and SAPO-34 molecular sieve with the mass ratio of 0.5-2.5: 1; the mass ratio of the added cobalt metal simple substance to the SAPO-34 molecular sieve is 0.5-2.5:1, namely the mass ratio x(Me/SAPO-34)Are respectively dissolved in V at a ratio of 0.5,1.0,1.5,2.0,2.5(Water)30mL of distilled water). Adjusting the concentration of the precursor solution to produce Cr2O32% by weight and 0.5-2.5% by weight of CoO.
Mixing a chromium source, a cobalt source and the SAPO-34 molecular sieve prepared in the step (1), stirring for 0.5h at the temperature of 100 ℃, stirring and refluxing the mixed suspension, and adding a certain amount of sodium carbonate solution (Na required by calculation according to a reaction equation)2CO3Mass, dissolved in 30mL of water), stirring at 100 ℃ for 0.5h, standing the mixture for aging for 1h, filtering, washing, drying at 150 ℃ for 5h, and calcining at 700 ℃ for 5 h. Obtaining the catalyst xCoO-2Cr2O3SAPO-34 molecular sieve, wherein x represents weight percent.
For CoO-Cr prepared in example 12O3Detection of/SAPO-34 molecular sieve
Characterization of the catalyst
1. X-ray diffraction (XRD) analysis
The phase structure and crystal structure of the sample were measured by Rigaku SmartLab type X-ray diffraction (XRD), with the characteristic radiation CuKa, tube voltage 40Kv, scanning range 3-40 deg., and scanning speed 8 deg./min. The test results are shown in fig. 1.
As can be seen from fig. 1: me of different Metal SupportsxOyThe characteristic diffraction peak positions and peak shapes of the/SAPO-34 molecular sieve are basically consistent, and the characteristic diffraction peaks appear at 10.5 degrees, 16.7 degrees, 21.3 degrees, 26.7 degrees and 31.7 degrees of 2 theta, which shows that the structure of the molecular sieve is not obviously changed by adding the active component, and the CHA structure of the SAPO-34 is kept.
The diffraction peaks appearing in the figure are shifted to the right compared to all reported in the literature and many fine, hetero-peaks appear, indicating that the loading of the metal oxide reduces the relative crystallinity of the molecular sieve, probably because the metal oxide has a certain destructive effect on the crystallinity of the molecular sieve during calcination.
2. Scanning Electron Microscope (SEM) analysis
The morphology of the catalyst was determined using a JSM-6510 Scanning Electron Microscope (SEM) from Peking Procisteice instruments Inc. The test results are shown in fig. 2.
As can be seen from FIG. 2, the synthesized SAPO-34 molecular sieve maintains a relatively intact cubic structure, and Cr is prepared by an impregnation method2O3SAPO-34 and CoO-Cr2O3The cubic structure of the/SAPO-34 molecular sieve is still maintained, but some amorphous particles are accumulated on the surface of the molecular sieve, which indicates that the loading of the metal oxide does not destroy the structure of the original SAPO-34 and is successfully loaded on the surface of the molecular sieve. Relative to Cr2O3SAPO-34 molecular sieve, CoO-Cr2O3The amorphous particles on the surface of the/SAPO-34 molecular sieve are more and more random, probably because of the higher supported bimetallic oxide content or agglomeration upon calcination.
3、NH3Temperature programmed adsorption desorption (NH3-TPD) analysis
Ammonia temperature programmed desorption (NH) using a quartz tube reactor equipped with a residual gas analyzer (RGA200, Agilent)3TPD) experiment to analyze the acid strength of the catalystAnd (4) degree. Before the adsorption experiment, 250mg of the catalyst was pretreated in a He (helium) flow at 200 ℃ for 1 hour, cooled to 110 ℃, and then NH was introduced under the protection of the He (helium) flow3Until the adsorption reached saturation, and then physically adsorbed NH was removed by purging with He gas for 3 hours3The sample was then heated to 800 ℃ at a ramp rate of 10 ℃/min. The NH desorbed from the catalyst was adsorbed with 100mL of HCl (0.01M)3And titrated with NaOH solution (0.01M) to absorb NH3Of HCl (g). The total acidity is calculated from the amount of HCl and NaOH consumed. From NH3-peak areas of the gaussian fit of the TPD curve calculate the distribution of weak, neutral and strong acids. The test results are shown in fig. 3.
As can be seen from FIG. 3, the appropriate acidic sites and acid strength are important factors affecting the performance of the SAPO-34 catalyst, the acidity of the SAPO-34 molecular sieve is mainly generated by Si incorporation into the neutral A1PO-34 framework, and Si atoms are isomorphously substituted into the framework by the mechanisms SM2 and SM 3. From the figure it can be seen that all three catalysts have three acid sites: weak acid, medium strong acid, strong acid; and both the acidity of weak acid and strong acid are SAPO-34 & gt Cr2O3/SAPO-34＞CoO-Cr2O3SAPO-34, SAPO-34CoO-Cr with medium acid acidity2O3/SAPO-34＞Cr2O3SAPO-34 > SAPO-34. This shows that the doping of the metal oxide can improve the acid sites and acid strength of the molecular sieve, so that the molecular sieve maintains proper acid sites and acid strength, which is beneficial to the reaction of converting ethanol into ethylene.
4. Thermogravimetric (TG) analysis
For CoO-Cr2O3The analysis result of the thermogravimetric analysis of the/SAPO-34 molecular sieve is shown in figure 4.
In fig. 4, the weight loss is mainly divided into two stages, the weight loss between 30 ℃ and 300 ℃ is the weight loss of the physically adsorbed water in the sample, the weight loss between 300 ℃ and 800 ℃ is due to the combustion of coke, wherein the weight loss ratio of the coke is lower than that of the physically adsorbed water, which indicates that the sample is not dried sufficiently and the amount of the coke in the used catalyst is less, and the catalyst after use can still maintain good reactivity and long service life by preliminary judgment.
5. Specific surface area (BET) and pore size distribution analysis
The specific surface area and pore size distribution of the catalyst were determined using a NOVA model 2200e adsorption apparatus from Quantachrome, USA. Before testing, the samples were degassed under vacuum for 5h at 200 ℃ under a stream of nitrogen. The relative pressure (P/Po) at which the pore volume was measured was 0.99. As shown in table 1.
TABLE 1 Me for different metal contentsxOySpecific surface area and pore volume of SAPO-34
The data in table 1 show that the doping of the metal oxide can obviously improve the specific surface area of the molecular sieve, increase the reaction surface of ethanol and the molecular sieve, and simultaneously reduce the average pore diameter of the molecular sieve, so that the molecular sieve is converted from a mesoporous molecular sieve to a microporous molecular sieve, thereby effectively inhibiting the secondary reaction and reducing the generation of byproducts.
Secondly, measuring the catalytic performance
The metal oxide modified SAPO-34 molecular sieve prepared by the method is used as an ethylene catalyst to produce ethylene. Namely for catalyzing ethanol dehydration to prepare ethylene. Combining the physical and chemical properties of methanol and ethanol, the molecular dynamics diameter of ethanol is 0.42-0.52nm from direct molecular dynamics, the SAPO-34 is 0.43-0.50nm, and ethanol molecules can enter the pore channels of the SAPO-34 molecular sieve for adsorption and reaction, so that the production of ethylene by using SAPO-34 as an ethylene catalyst is feasible.
The method for preparing ethylene comprises the following steps: the reaction temperature is 300-500 ℃, and the heavy air time speed is 2.5-3.5h-1Dehydrating with ethanol to prepare ethylene.
1. Different metal oxide contents to MexOyEffect of SAPO-34 catalytic Properties
At the reaction temperature of 350 ℃ and the heavy air time rate of 3.5h-1And the catalyst amount is 0.5g, and the ethylene is prepared by ethanol dehydration. In the preparation process, different catalysts are used for reaction, and the reaction is finishedAs shown in fig. 5. As can be seen from the graph, Cr increases with the metal oxide content2O3SAPO-34 and CoO-Cr2O3The reactivity of the/SAPO-34 catalyst shows a tendency of increasing and then decreasing, and is shown in Cr2O3Cr at a content of 2.0 wt%2O3the/SAPO-34 catalyst shows excellent reaction activity, and the ethanol conversion rate and the ethylene selectivity of the catalyst respectively reach 97.3 percent and 97.9 percent. When CoO and Cr2O3When the content of (A) is 1.5% and 2.0%, respectively, CoO-Cr2O3the/SAPO-34 catalyst shows better reaction activity, and the ethanol conversion rate and the ethylene selectivity of the catalyst are improved to 99 percent. This indicates that Cr2O3And the loading of CoO is advantageous for the improvement of the catalyst reactivity because the loading of the metal oxide increases the specific surface area of the catalyst, increases the reaction area of ethanol with the catalyst, and the loading of the bimetallic oxide is more advantageous for the improvement of the catalyst reactivity than the loading of the monometal oxide because a synergistic effect is generated between the two oxides. When the content of the metal oxide is continuously increased, the reason why the catalytic performance of the catalyst is not increased or decreased is that excessive metal oxide can destroy the structure of the molecular sieve, change the active sites of the catalyst and block the pore channels, so that the effective reaction area is reduced and the generation of the target product ethylene is influenced.
2. Reaction temperature vs. MexOyEffect of SAPO-34 catalytic Properties
Keeping the catalyst dosage of 0.5g and the weight-space-time speed of 3.5h-1Dehydrating with ethanol to prepare ethylene. Different catalysts are used for reaction in the preparation process, and the reaction result is shown in figure 6. As can be seen from the figure, Cr2O3SAPO-34 and CoO-Cr2O3The reactivity of the/SAPO-34 catalyst shows a trend of increasing and then flattening along with the increasing of the temperature, and at the reaction temperature of 400 ℃, Cr is added2O3SAPO-34 and CoO-Cr2O3the/SAPO-34 catalysts show excellent reaction activity, the ethanol conversion rate and the ethylene selectivity of the former respectively reach 97.6 percent and 98.2 percent, and the latter respectively reach 99.3 percent and 99.4 percentIt can be seen that under the same reaction conditions, CoO-Cr2O3SAPO-34 to Cr2O3the/SAPO-34 shows better catalytic performance. In the reaction process of preparing ethylene by ethanol dehydration, the dehydration mode is divided into intramolecular dehydration and intermolecular dehydration, both of which need to be carried out in the process of carbonium ion, and at lower temperature, the carbonium ion has longer service life, so that ethanol molecules can have longer time to collide and combine with the carbonium ion, and the intermolecular dehydration is carried out to form ether; ethanol molecules require a large amount of energy to deprive beta-H from the carbenium ion for intramolecular dehydration to occur, and increasing the temperature shortens the lifetime of the carbenium ion, which is not conducive to the displacement reaction, thereby producing ethylene.
3. Weight hourly space velocity vs. MexOyEffect of SAPO-34 catalytic Properties
The ethylene is prepared by ethanol dehydration under the conditions that the catalyst dosage is 0.5g and the reaction temperature is 400 ℃. Different catalysts are used for reaction in the preparation process, and the reaction result is shown in figure 7. As can be seen from the figure, Cr increases with weight hourly space velocity2O3SAPO-34 and CoO-Cr2O3The reactivity of the/SAPO-34 catalyst shows a gradual and then descending trend, Cr2O3SAPO-34 and CoO-Cr2O3The ethanol conversion rate of the/SAPO-34 catalyst is 97.6 percent and 99.27 percent respectively, and the ethylene selectivity reaches 98.2 percent and 99.4 percent respectively. This is because the space velocity is small, sufficient contact time of the reaction raw material with the catalyst is provided, the reaction proceeds sufficiently, and the selectivity of ethylene is high. And when the space velocity is large, the retention time of the ethanol in the catalyst bed layer is short, so that the contact time with the catalyst is short, part of the ethanol does not react or does not react completely, and the conversion rate of the ethanol and the selectivity of ethylene are reduced.
4. Length of reaction to MexOyEffect of SAPO-34 catalytic Properties
The ethylene is prepared by ethanol dehydration under the conditions that the catalyst dosage is 0.5g, the reaction temperature is 400 ℃, and the weight hourly space velocity is 3.5h < -1 >. Different catalysts are used for reaction in the preparation process, and the reaction result is shown in figure 8. As can be seen from the figure, Cr2O3the/SAPO-34 catalyst keeps relatively stable catalytic activity when the catalyst is used for less than 300min, but when the catalyst is used for more than 300min, the catalytic activity of the catalyst is slightly reduced, because coke is accumulated along with the increase of the reaction time to block the pore channels of the molecular sieve, so that the diffusion of macromolecular products is inhibited, and the generation of ethylene is influenced. In contrast, CoO-Cr2O3the/SAPO-34 catalyst can still keep high-efficiency catalytic activity when the reaction time reaches 600min, which shows that the doping of the bimetallic oxide can improve the reaction activity of the catalyst and prolong the service life of the catalyst.
The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and its practical application to enable one skilled in the art to make and use various exemplary embodiments of the invention and various alternatives and modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents.
Claims (6)
1. A preparation method of a metal oxide modified SAPO-34 molecular sieve is characterized by comprising the following steps:
(1) preparation of shaped SAPO-34 molecular sieves
Mixing a silicon source, an aluminum source, a structure directing agent and a phosphorus source to obtain a precursor SAPO-34 gel, then placing the SAPO-34 gel into a reaction kettle for sealing, then taking out, cooling, separating, washing, drying and calcining to obtain a formed SAPO-34 molecular sieve;
(2) preparation of bimetal modified SAPO-34 molecular sieve
Mixing and stirring a chromium source, a cobalt source and the SAPO-34 molecular sieve prepared in the step (1), adding a carbonate solution, continuously stirring fully, standing and aging the mixed solution, filtering, washing, drying and calcining to obtain the catalyst CoO-Cr2O3SAPO-34 molecular sieve;
wherein, in the step (1), the silicon source is tetraethyl orthosilicate, the aluminum source is aluminum isopropoxide, the phosphorus source is phosphoric acid, and the structure directing agent is tetraethylammonium hydroxide aqueous solution with the mass concentration of 20-30 wt%; the molar mass ratio of the medium silicon source to the aluminum source to the phosphorus source to the structure directing agent is as follows: 1Al2O3：1P2O5：0.5SiO2：1.0TEAOH：50H2O；
Wherein, the drying in the step (1) is drying for 5-20h at the temperature of 80-150 ℃; the calcination is carried out for 10-20h at the temperature of 550-700 ℃;
wherein the chromium source in the step (2) is Cr (NO)3)3An aqueous solution; the source of cobalt is Co (NO)3)2An aqueous solution; adding chromium metal simple substance and SAPO-34 molecular sieve with the mass ratio of 0.5-2.5: 1; the mass ratio of the added cobalt metal simple substance to the SAPO-34 molecular sieve is 0.5-2.5: 1.
2. The method for preparing the metal oxide modified SAPO-34 molecular sieve as claimed in claim 1, wherein the reaction conditions in the reaction vessel in the step (1) are 120-250 ℃ for 24-72 h.
3. The method for preparing a metal oxide modified SAPO-34 molecular sieve according to claim 1, wherein the carbonate solution in step (2) is one of sodium carbonate solution or potassium carbonate solution.
4. The method for preparing the metal oxide modified SAPO-34 molecular sieve according to claim 1, wherein the stirring in the step (2) is performed at a temperature of 70 to 100 ℃ for 0.5 to 2 hours; standing and aging for 1-5 h; drying at 80-150 deg.C for 5-20 hr; the calcination is carried out for 1-20h at the temperature of 600-700 ℃.
5. Use of a metal oxide modified SAPO-34 molecular sieve prepared according to the process of claim 1, as an ethylene catalyst for the production of ethylene.
6. Use of the metal oxide modified SAPO-34 molecular sieve according to claim 5, wherein the process for the preparation of ethylene comprises: the reaction temperature is 300-500 ℃, and the heavy air time speed is 2.5-3.5h-1Dehydrating with ethanol to prepare ethylene.
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