CN115970709B - Pd-based catalyst, preparation method thereof and application thereof in aldehyde oxidation esterification reaction - Google Patents
Pd-based catalyst, preparation method thereof and application thereof in aldehyde oxidation esterification reaction Download PDFInfo
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Abstract
The invention relates to the technical field of catalysts, and provides a Pd-based catalyst, a preparation method thereof and application thereof in aldehyde oxidation esterification reaction. The carrier of the Pd-based catalyst provided by the invention comprises gamma-Al 2O3 and a cobalt-containing compound, and the active ingredient is Pd. The invention is based on the carrier gamma-Al 2O3 with high specific surface area, introduces cobalt-containing compound which can provide weak alkaline sites and can form stronger interaction with Pd while ensuring specific surface area, so that the single metal palladium catalyst has better activity and stability, and can be applied to the reaction of preparing MMA by MAL one-step oxidation esterification, and the conversion rate of MAL and the selectivity of MMA are high. The results of the examples show that the conversion rate of MAL can reach 93.8% and the selectivity of MMA can reach 51.8% by catalyzing MAL to perform one-step oxidation esterification by using the catalyst of the invention.
Description
Technical Field
The invention relates to the technical field of catalysts, in particular to a Pd-based catalyst, a preparation method thereof and application thereof in aldehyde oxidation esterification reaction.
Background
Methyl Methacrylate (MMA) is an important organic chemical raw material, is a monomer for synthesizing polymethyl methacrylate (PMMA), and can be used for producing organic glass, molding compound, acrylic fiber, coating, medical functional polymer material and the like.
At present, mature technologies for producing MMA at home and abroad comprise three technologies of a methacrylamide hydrolysis esterification route (an acetone cyanohydrin method and a methacrylonitrile method), an isobutene oxidation route and an ethylene carbonyl synthesis route, wherein the world main production technology is the acetone cyanohydrin method. The acetone cyanohydrin method adopts acetone, hydrocyanic acid, sulfuric acid and methanol as raw materials, and comprises the following reaction steps: cyanohydrination, amidation and hydrolytic esterification. Although this process is mature, there are serious drawbacks. Firstly, the raw material hydrocyanic acid is a highly toxic raw material, and strict protection is required in the processes of storage, transportation and use; secondly, a large amount of acid residual liquid is used as a byproduct, so that the environment is seriously polluted; finally, sulfuric acid is used for the reaction, so that corrosion prevention equipment is required, and the construction cost of the device is high. Therefore, it is necessary to develop a production process route that meets the national strategy and green ecological requirements and meets the atomic economy.
The petrochemical industry in the eastern and southern areas of China and along the river and coastal areas has obvious advantages, and has rich byproduct C4 resources, so the clean production process for developing the MMA by the isobutene oxidation method has obvious advantages. The method has two technological routes. Firstly, an isobutene three-step method, specifically using isobutene or tertiary butanol as a raw material, obtaining methacrylic acid (MAA) through two-step selective oxidation of air, and then esterifying the methacrylic acid with methanol to produce MMA. Secondly, an isobutene two-step method is that isobutene or tertiary butanol is oxidized by gas phase air under the action of a Mo-Bi composite oxide catalyst to generate Methacrolein (MAL), and then MAL is directly generated by liquid phase air oxidation and esterification under the action of the catalyst, wherein MAL oxidation and esterification are key steps of the route. The two-step isobutylene reaction is only needed, the method is simple, most of byproducts are water, and the method is green and environment-friendly, but has higher requirements on the catalyst. As the key step for preparing MMA by MAL oxidation and esterification, the preparation method is important to the research and development design of the catalyst.
For a catalyst for preparing MMA by MAL one-step oxidative esterification, the Xudi formation company develops Pd-Pb bimetallic in 1999 and realizes industrialization, domestic scientific research institutions focus on the development and improvement of Pd catalysts, and many developments are made on the aspects of carriers, active components and the like of the catalysts. According to investigation, the carrier of the Pd-Pb bimetallic catalyst needs larger specific surface area and proper alkaline sites, the commonly adopted carrier is single carrier such as gamma-Al 2O3、MgO、SiO2 or composite carrier, and gamma-Al 2O3 is taken as the carrier, and the carrier has large specific surface area, but the binding force with Pd is weaker due to the limitation of an inert carrier, so that the catalytic activity is poor and the stability is not high; mgO has a certain basic site, but is less stable in aqueous phase reactions; multicomponent supports such as γ-Al2O3-MgO,γ-Al2O3-SiO2、γ-Al2O3-MgO-SiO2,, although improving the loading effect to some extent, do not solve the stability problem of the catalyst, and the activity of the catalyst is still poor, which is reflected by lower conversion rate of MAL and selectivity of MAA.
Disclosure of Invention
In view of the above, the invention provides a Pd-based catalyst, a preparation method thereof and application thereof in aldehyde oxidation and esterification reactions. The Pd-based catalyst provided by the invention has strong combination effect of Pd and a carrier, high catalytic activity and high stability, and can be applied to the reaction of preparing MMA by MAL one-step oxidation and esterification, and the conversion rate of MAL and the selectivity of MMA are high.
In order to achieve the above object, the present invention provides the following technical solutions:
A Pd-based catalyst comprising a support and an active ingredient supported on the support, the support comprising gamma-Al 2O3 and a cobalt-containing compound; the active ingredient is Pd; the mass of the active ingredient is 1-5% of the mass of the carrier; the mass ratio of Co element to Al element in the carrier is 0.05-1:1.
Preferably, the cobalt-containing compound comprises Co 3O4、Co(OH)2 and CoAl 2O4; the Co 3O4 is in a rod-shaped structure.
Preferably, the crystal plane of Pd is a Pd (111) plane, and the Pd (111) plane is between the (100) plane of Co (OH) 2 and the (220) plane of CoAl 2O4.
Preferably, the catalyst comprises weakly basic sites and moderately basic sites, the number ratio of the weakly basic sites is 54% -65%, and the number ratio of the moderately basic sites is 35% -46%.
The invention also provides a preparation method of the Pd-based catalyst, which comprises the following steps:
mixing gamma-Al 2O3 with a soluble cobalt salt solution for impregnation, and then mixing the obtained impregnation system with an alkaline solution for precipitation reaction to obtain a precipitation mixture; roasting the precipitation mixture to obtain a carrier;
and mixing the carrier and the chloropalladate solution, and then reducing to obtain the Pd-based catalyst.
Preferably, the hydroxide concentration of the alkaline solution is 0.5-2.5 mol/L, and the temperature of the precipitation reaction is 20-90 ℃; the roasting temperature is 300-800 ℃ and the roasting time is 2-5 h.
Preferably, the reducing agent is an alkaline formaldehyde solution, and the components of the alkaline formaldehyde solution comprise formaldehyde, alkali metal hydroxide and water; the mass fraction of formaldehyde in the alkaline formaldehyde solution is 2-5%, and the mass fraction of alkali metal hydroxide is 2-5%.
Preferably, the temperature of the reduction is 40-90 ℃ and the time is 10-120 min.
The invention also provides an application of the Pd-based catalyst prepared by the scheme or the preparation method of the scheme in aldehyde oxidation esterification reaction.
Preferably, the aldehyde oxidation and esterification reaction is a reaction for preparing methyl methacrylate by one-step oxidation and esterification of methacrolein.
The invention provides a Pd-based catalyst, which comprises a carrier and an active ingredient loaded on the carrier, wherein the carrier comprises gamma-Al 2O3 and a cobalt-containing compound; the active ingredient is Pd; the mass of the active ingredient is 1-5% of the mass of the carrier; the mass ratio of Co element to Al element in the carrier is 0.05-1:1. The invention is based on the carrier gamma-Al 2O3 with high specific surface area, and introduces the cobalt-containing compound which can provide weak alkaline sites and can form stronger interaction with Pd while ensuring the specific surface area, so that the single metal palladium catalyst has better activity and stability.
Furthermore, the cobalt-containing compound in the carrier comprises Co 3O4、Co(OH)2, coAl 2O4,Co(OH)2 and CoAl 2O4, a certain proportion of alkalescent position and alkalescent position are provided, the alkaline reaction environment is more favorable for the generation of MMA, the alkaline metal oxide in the carrier can transfer electrons to Pd to enable Pd to be in an electron-rich state, and the carrier loses electrons to enable Pd to obtain electrons, so that the alkalinity of the catalyst is enhanced, and the catalytic activity is further increased.
The invention also provides a preparation method of the Pd-based catalyst, which comprises the steps of firstly mixing gamma-Al 2O3 with a soluble cobalt salt solution for impregnation, then mixing the obtained impregnation system with an alkaline solution for precipitation reaction, roasting after obtaining a precipitation mixture to obtain a carrier formed by gamma-Al 2O3 and a cobalt-containing compound, and then mixing the carrier with a chloropalladate solution for reduction to obtain the Pd-based catalyst; the carrier prepared by the invention contains the cobalt-containing compound, the cobalt-containing compound provides alkaline sites for the carrier, and in an acidic medium such as chloropalladate (the pH value is lower than the isoelectric point), the surface of the carrier has positive potential, so that the adsorption capacity to an anion complex [ PdCl 4]2- ] can be improved, and the loading effect of Pd particles is improved. In addition, the preparation method provided by the invention has simple steps and is easy to operate.
The invention also provides application of the Pd-based catalyst in the aldehyde oxidation and esterification reaction, in particular to application in preparing methyl methacrylate by one-step oxidation and esterification of methacrolein. The Pd-based catalyst provided by the invention has high catalytic activity and stability, and can be applied to the reaction of preparing MMA by MAL one-step oxidative esterification, so that the conversion rate of MAL and the selectivity of MMA are high. The results of the examples show that the conversion rate of MAL can reach 93.8% and the selectivity of MMA can reach 51.8% by catalyzing MAL to perform one-step oxidation esterification by using the catalyst of the invention.
Drawings
FIG. 1 is a TEM image of the Pd5/Co-Al (Co/Al=0.1) catalyst prepared in example 1;
FIG. 2 is a TEM image of the Co-Al support (Co/Al=0.1) of example 1;
FIG. 3 is an XRD characterization of the Co-Al support (Co/Al=0.1) and Pd5/Co-Al (Co/Al=0.1) catalysts of example 1;
FIG. 4 is a HRTEM diagram of Pd5/Co-Al (Co/Al=0.1) catalyst prepared in the example;
FIG. 5 is a graph showing the results of a CO 2 -TPD peak fit for the Pd5/Co-Al (Co/Al=0.1) catalyst prepared in example 1;
FIG. 6 is an XPS O1S orbital plot of the Co-Al support (Co/Al=0.1) in example 1;
FIG. 7 is an XPS O1S orbital plot of the Pd5/Co-Al (Co/Al=0.1) catalyst prepared in example 1;
Fig. 8 is the results of the cycle stability test of the Pd5/Co-Al (Co/al=0.1) catalyst prepared in example 1.
Detailed Description
The invention provides a Pd-based catalyst, which comprises a carrier and an active ingredient loaded on the carrier, wherein the carrier comprises gamma-Al 2O3 and a cobalt-containing compound; the active ingredient is Pd; the mass of the active ingredient is 1-5% of the mass of the carrier; the mass ratio of Co element to Al element in the carrier is 0.05-1:1.
In the present invention, the support comprises gamma-Al 2O3 and a cobalt-containing compound, preferably comprising Co 3O4、Co(OH)2 and CoAl 2O4; the Co 3O4 is in a rod-shaped structure; that is, the support of the present invention is preferably a complex of gamma-Al 2O3、Co3O4、Co(OH)2 and CoAl 2O4; the mass ratio of Co element to Al element in the carrier is 0.05-1:1, preferably 0.08-0.8:1, and more preferably 0.1-0.5:1.
In the present invention, the crystal plane of Pd is a Pd (111) plane, and the Pd (111) plane is between the (100) plane of Co (OH) 2 and the (220) plane of CoAl 2O4.
In the present invention, the catalyst comprises weakly basic sites and moderately basic sites, the number ratio of the weakly basic sites is preferably 54% to 65%, more preferably 55% to 62%, and the number ratio of the moderately basic sites is preferably 35% to 46%, more preferably 38% to 42%.
The invention also provides a preparation method of the Pd-based catalyst, which comprises the following steps:
mixing gamma-Al 2O3 with a soluble cobalt salt solution for impregnation, and then mixing the obtained impregnation system with an alkaline solution for precipitation reaction to obtain a precipitation mixture; roasting the precipitation mixture to obtain a carrier;
and mixing the carrier and the chloropalladate solution, and then reducing to obtain the Pd-based catalyst.
The preparation method comprises the steps of mixing gamma-Al 2O3 with a soluble cobalt salt solution for impregnation, and then mixing the obtained impregnation system with an alkaline solution for precipitation reaction to obtain a precipitation mixture. In the present invention, the soluble cobalt salt solution is preferably an aqueous cobalt nitrate solution, the cobalt nitrate is preferably Co (NO 3)2·6H2 O; the concentration of the aqueous cobalt nitrate solution is not particularly required, the cobalt nitrate can be completely dissolved, the mass ratio of the gamma-Al 2O3 to the soluble cobalt salt is preferably determined according to the mass ratio of Co element to Al element in the carrier according to the scheme, the time of the impregnation is preferably 6h, and the temperature of the impregnation is preferably room temperature.
In the present invention, the hydroxide concentration of the alkaline solution is preferably 0.5 to 2.5mol/L, more preferably 1mol/L, and the alkaline solution is preferably a sodium hydroxide solution; the temperature of the precipitation reaction is preferably 20-90 ℃, more preferably 70 ℃, the precipitation reaction is preferably carried out under a protective atmosphere, and the protective atmosphere is preferably nitrogen; preferably, after the impregnation is finished, alkaline solution is dripped into the obtained impregnation system, nitrogen is introduced while dripping, and dripping can be stopped until pink and black floccules are generated; in a specific embodiment of the present invention, the volume ratio of the alkaline solution to the cobalt salt solution is preferably 0.25-2.5:1.
After the precipitation reaction is completed, the obtained product feed liquid is preferably filtered, and the obtained precipitation mixture is washed and dried in sequence; the washing is preferably carried out by adopting deionized water and ethanol in sequence; the drying is preferably vacuum drying, the temperature of the vacuum drying is preferably 100 ℃, and the time is preferably 12 hours.
After obtaining the precipitation mixture, the present invention calcines the precipitation mixture to obtain a support (denoted as Co-Al). In the present invention, the baking temperature is preferably 300 to 800 ℃, more preferably 400 to 600 ℃, the baking time is preferably 2 to 5 hours, more preferably 3 to 4 hours, and the heating rate to the baking temperature is preferably 2 ℃/min; the atmosphere for the calcination is preferably air.
After the carrier is obtained, the Pd-based catalyst is obtained by mixing the carrier with a chloropalladate solution and then reducing the mixture. In the present invention, the chloropalladate solution is preferably an aqueous solution of sodium chloropalladate; the concentration of the chloropalladate solution is preferably 0.0620-0.0650 mol/L, more preferably 0.0645mol/L; the dosage ratio of the carrier to the chloropalladate solution is preferably 1.0-2.0 g:7-15 mL, more preferably 1.5g:11mL; the mixing of the support according to the invention with the chloropalladate solution is preferably carried out under magnetic stirring, the mixing time being preferably from 2 to 8 hours, more preferably 4 hours. In the present invention, the reducing agent is preferably an alkaline formaldehyde solution, and the components of the alkaline formaldehyde solution preferably include formaldehyde, alkali metal hydroxide and water; the mass fraction of formaldehyde in the alkaline formaldehyde solution is preferably 2-5%, more preferably 3-4%, and the mass fraction of alkali metal hydroxide is preferably 2-5%, more preferably 3-4%; the alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide; the volume ratio of the chloropalladate solution to the alkaline formaldehyde solution is preferably 7-15:10-20, more preferably 11:15; the temperature of the reduction is preferably 40-90 ℃, more preferably 70 ℃, and the time of the reduction is preferably 10-120 min, more preferably 30min; in the specific embodiment of the invention, the alkaline formaldehyde solution is dropwise added into the mixed solution of the carrier and the chloropalladate solution at the temperature of 40-90 ℃ preferably, and the mixture is reduced after being dropwise added, and the mixture is preserved at the temperature; the time of the reduction reaction is counted from the completion of the dropwise addition of the alkaline formaldehyde solution.
After the reduction is finished, the invention preferably filters the obtained product feed liquid, and washes and dries the obtained solid product in turn to obtain Pd-based catalyst; the washing is preferably carried out by adopting deionized water and ethanol in sequence; the drying is preferably vacuum drying, the temperature of the vacuum drying is preferably 60 ℃, and the time is preferably 12 hours.
The invention also provides an application of the Pd-based catalyst according to the scheme or the Pd-based catalyst prepared by the preparation method according to the scheme in aldehyde oxidation and esterification reaction, wherein the aldehyde oxidation and esterification reaction is a reaction for preparing Methyl Methacrylate (MMA) by one-step oxidation and esterification of Methacrolein (MAL); in a specific embodiment of the present invention, the reaction for preparing Methyl Methacrylate (MMA) by one-step oxidative esterification of Methacrolein (MAL) preferably comprises the steps of: mixing methanol, MAL, polymerization inhibitor and Pd-based catalyst, and then introducing oxygen to perform oxidation esterification reaction; the temperature of the oxidation and esterification reaction is preferably 50-70 ℃, more preferably 60 ℃, and the pressure is preferably normal pressure; the oxygen is preferably industrial oxygen; the air speed of the oxygen is preferably 20h -1; the ratio of the MAL to Pd-based catalyst is preferably 1mL:0.4g; the polymerization inhibitor is preferably hydroquinone; the ratio of the MAL to the polymerization inhibitor is preferably 1mL:0.002g.
The following description of the embodiments of the present invention will clearly and fully describe the technical solutions of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Example 1
The mass of Pd is 5% of the mass of the carrier, the mass ratio of Co element to Al element in the carrier is 0.1:1, the Pd-based catalyst is Pd5/Co-Al (Co/Al=0.1), and the specific preparation method is as follows:
0.524g of Co (NO 3)2·6H2 O) was weighed into a three-necked flask, 10mL of deionized water was added and stirred into a cobalt nitrate solution, then 2g of gamma-Al 2O3 was added, after 6 hours of immersion, the three-necked flask was slowly dropped into an alkaline solution (sodium hydroxide solution, 1mol/L concentration) at 70 ℃ while inert gas N 2 was passed through until pink and black floc formed, the resulting mixture was suction-filtered, washed with deionized water and ethanol solution, and dried in a vacuum oven at 100 ℃ for 12 hours, and then the resulting dried solid was baked at 600 ℃ for 3 hours at a heating rate of 2 ℃/min to produce Co-Al (Co/Al=0.1).
1.5G of Co-Al (Co/Al=0.1) was weighed into an Erlenmeyer flask, 11mL of Na 2PdCl4 (concentration: 0.0645 mol/L) was added, magnetically stirred for 4 hours, 15mL of an alkaline formaldehyde solution (5% HCHO+5% NaOH) was slowly dropped into the Erlenmeyer flask at 70℃and after 30 minutes, suction filtration was performed, washing with deionized water and ethanol solution, and drying in a vacuum oven at 60℃for 12 hours, to obtain Pd-based catalyst Pd5/Co-Al (Co/Al=0.1).
Example 2
Preparation of Pd5/Co-Al (Co/al=0.05): co (NO 3)2·6H2 O mass was changed to 0.262g, and the other conditions were the same as in example 1.
Example 3
Preparation of Pd5/Co-Al (Co/al=0.3): co (NO 3)2·6H2 O mass was changed to 1.570g, and the other conditions were the same as in example 1.
Example 4
Preparation of Pd5/Co-Al (Co/al=0.5): co (NO 3)2·6H2 O mass was changed to 2.617g, and the other conditions were the same as in example 1.
Example 5
Preparation of Pd5/Co-Al (Co/al=0.7): co (NO 3)2·6H2 O mass was changed to 3.664g, and the other conditions were the same as in example 1.
Example 6
Preparation of Pd5/Co-Al (Co/al=1.0): co (NO 3)2·6H2 O mass was changed to 5.235g, and the other conditions were the same as in example 1.
Example 7
Other conditions were the same as in example 1 except that the calcination temperature was changed to 300℃only, and the resulting Pd-based catalyst was designated Pd5/Co-Al (Co/Al=0.1) -300.
Example 8
Other conditions were the same as in example 1 except that the calcination temperature was changed to 800℃only, and the resulting Pd-based catalyst was designated Pd5/Co-Al (Co/Al=0.1) -800.
Comparative example 1
Pd 5/gamma-Al 2O3: 1.5g of gamma-Al 2O3 is weighed into an Erlenmeyer flask, 11mL of Na 2PdCl4 (concentration: 0.0645 mol/L) is added, magnetic stirring is performed for 4h, 15mL of alkaline formaldehyde solution (5% HCHO+5% NaOH) is slowly added into the Erlenmeyer flask at 70 ℃, suction filtration is performed after 30min, washing is performed with deionized water and ethanol solution, and drying is performed in a vacuum drying oven at 60 ℃ for 12h to obtain the supported catalyst Pd 5/gamma-Al 2O3.
Comparative example 2
0.524G of Co (NO 3)2·6H2 O) is weighed into a three-necked flask, 150mL of deionized water is added and stirred into a cobalt nitrate solution, then 2g of gamma-Al 2O3 is added, mixed and stirred, stirred and evaporated to dryness at 80 ℃, the mixture is placed into a vacuum drying oven at 100 ℃ for drying for 12 hours, and then the obtained dried solid is baked for 3 hours at 600 ℃ with a heating rate of 2 ℃/min, so that Co-Al-s (Co/Al=0.1) is prepared.
1.5G of Co-Al-s (Co/Al=0.1) was weighed into an Erlenmeyer flask, 11mL of Na 2PdCl4 (concentration: 0.0645 mol/L) was added, magnetically stirred for 4 hours, 15mL of an alkaline formaldehyde solution (5% HCHO+5% NaOH) was slowly dropped into the Erlenmeyer flask at 70℃and after 30 minutes, suction filtration was performed, washing with deionized water and ethanol solution, and drying in a vacuum oven at 60℃for 12 hours to obtain a supported catalyst Pd5/Co-Al-s (Co/Al=0.1).
Performance test:
(1) Characterization of
Fig. 1 is a TEM image of the Pd5/Co-Al (Co/al=0.1) catalyst prepared in example 1, and fig. 2 is a TEM image of the Co-Al support (Co/al=0.1) in example 1. As can be seen from fig. 1, in the finally obtained catalyst, the morphology of the carrier is a rod-shaped and plate-shaped structure, the rod-shaped structure is presumed to be Co 3O4, the active component Pd particles are located beside the rod-shaped structure, and as can be demonstrated by combining the TEM characterization diagrams of fig. 1-2, the original carrier has no rod-shaped structure, and the rod-shaped structure appears after the active component Pb is loaded.
FIG. 3 is an XRD characterization of the Co-Al support (Co/Al=0.1) and Pd5/Co-Al (Co/Al=0.1) catalysts of example 1; fig. 4 is a HRTEM image of the Pd5/Co-Al (Co/al=0.1) catalyst prepared in the example. As can be seen from fig. 3: the diffraction peak of Co-Al of the catalyst carrier obtained after alkaline solution precipitation and roasting is attributed to each crystal face of gamma-Al 2O3, and the HRTEM combined with the catalyst can show that the formation of the multi-component carrier containing γ-Al2O3,Co3O4,CoAl2O4,Co(OH)2,Co(OH)2 is attributed to the reaction of alkaline formaldehyde solution used when active components are loaded and the carrier, and the active components Pd are uniformly loaded on the carrier; in the HRTEM image, 0.275nm belongs to the (100) face of Co (OH) 2, 0.223nm belongs to the (111) face of Pd, 0.282nm belongs to the CoAl 2O4 (220) face, 0.198nm belongs to the (400) face of γ -Al 2O3, and Pd (111) is between the Co (OH) 2 (100) face and the CoAl 2O4 (220) face.
The Pd5/Co-Al (Co/Al=0.1) catalyst CO 2 -TPD peak was fitted and the results obtained are shown in FIG. 5; from the results in fig. 5, it can be seen that in the Pd5/Co-Al (Co/al=0.1) catalyst, the weakly basic sites account for 54% and the moderately basic sites account for 46%, so Co (OH) 2 and CoAl 2O4 provide a proportion of weakly basic sites and moderately basic sites.
Fig. 6 is an XPS O1S orbital plot of the Co-Al support (Co/al=0.1) in example 1, and fig. 7 is an XPS O1S orbital plot of the Pd5/Co-Al (Co/al=0.1) catalyst prepared in example 1; table1 shows oxygen vacancy concentration data obtained in accordance with FIGS. 6 to 7.
TABLE 1 oxygen vacancy concentration of Co-Al support and Pd5/Co-Al (Co/Al=0.1) catalyst
From the data in fig. 5-6 and table 1, it can be seen that after Pd loading, the oxygen vacancy concentration increases from 9.9% to 12.8%, which suggests that the catalyst has more oxygen vacancy defects, which helps to increase the electron conductivity of the catalyst, thereby promoting electron transfer between the support and the active component. Therefore, the reasons above allow the catalytic active site Pd (111) to obtain the optimal catalytic effect, and the main component of the carrier Co-Al and the special crystal face position between the carrier and the active component are the reasons for good reactivity of the obtained catalyst in combination with the subsequent catalyst activity characterization data.
(2) Catalyst Activity test
Evaluation conditions: into a three-necked flask, 20mL of methanol solution, 1mL of MAL and 0.002g of hydroquinone as a polymerization inhibitor were added, and finally 0.4g of catalyst was added, and then O 2 gas was introduced for reaction at 60℃under normal pressure at a gas space velocity of 20h -1 in O 2. After 2 hours of reaction, the reaction product was filtered, and the filtered reaction product was quantitatively analyzed by a Shimadzu GC2010 type gas chromatograph to calculate the conversion (X MAL) of MAL and the selectivity of MAA (S MMA).
The evaluation results are shown in table 2.
TABLE 2 evaluation results of catalyst Activity
| Examples | Sample of | XMAL | SMMA |
| Example 1 | Pd5/Co-Al(Co/Al＝0.1) | 93.8 | 51.8 |
| Example 2 | Pd5/Co-Al(Co/Al＝0.05) | 92.0 | 46.7 |
| Example 3 | Pd5/Co-Al(Co/Al＝0.3) | 91.6 | 48.3 |
| Example 4 | Pd5/Co-Al(Co/Al＝0.5) | 89.0 | 46.9 |
| Example 5 | Pd5/Co-Al(Co/Al＝0.7) | 83.4 | 40.6 |
| Example 6 | Pd5/Co-Al(Co/Al＝1.0) | 72.2 | 37.1 |
| Example 7 | Pd5/Co-Al(Co/Al＝0.1)-300 | 91.9 | 32.7 |
| Example 8 | Pd5/Co-Al(Co/Al＝0.1)-800 | 91.6 | 49.3 |
| Comparative example 1 | Pd5/γ-Al2O3 | 91.7 | 30.7 |
| Comparative example 2 | Pd5/Co-Al-s(Co/Al＝0.1) | 72.4 | 32.1 |
From the data in Table 2, it can be seen that the catalyst had the highest catalytic activity at a Co/Al of 0.1 for the multicomponent support Co-Al. When cobalt-aluminum is relatively low, the catalyst carrier cannot provide enough weak alkaline sites and medium alkaline sites, so that the catalytic activity of the catalyst is poor; when cobalt and aluminum are relatively high, the existence of too much cobalt oxide can reduce the specific surface area of the composite carrier, and greatly enhance the number of neutral and alkaline sites of the catalyst carrier, which also can influence the catalytic activity of the catalyst; as can be seen from the data of examples 7 and 8, the calcination temperature was increased, which was advantageous for improving MMA selectivity of the catalyst; in comparative example 1, gamma-Al 2O3 is used as a single component carrier, and poor loading effect leads to poor catalyst activity, especially low selectivity of MMA; in comparative example 2, the catalyst with the active component supported is prepared directly by using a method of mixing, stirring and evaporating to dryness, and has a catalytic activity inferior to that of the catalyst with the active component supported by the composite carrier Co-Al prepared by reacting with an alkaline solution, because the carrier of the catalyst has smaller particle size and poorer crystallinity, contains more weak alkaline sites, and is beneficial to the reactivity.
(3) Catalyst stability test
The Pd5/Co-Al (Co/Al=0.1) catalyst prepared in example 1 was subjected to a cyclic catalytic experiment, and after completion of each reaction, the catalyst was separated by filtration and washed with methanol, and then used in the next reaction under the same reaction conditions as in (2), and the conversion (X MAL) of MAL and the selectivity (S MMA) of MAA were measured for each reaction, and the results were shown in FIG. 8.
As can be seen from the results in FIG. 8, X MAL is not reduced substantially after four times of recycling, and the reduction of S MMA is small, which indicates that the Pd-based catalyst prepared by the invention has excellent stability.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which are intended to be comprehended within the scope of the present invention.
Claims (9)
1. A method for preparing a Pd-based catalyst, comprising the steps of:
mixing gamma-Al 2O3 with a soluble cobalt salt solution for impregnation, and then mixing the obtained impregnation system with an alkaline solution for precipitation reaction to obtain a precipitation mixture; roasting the precipitation mixture to obtain a carrier;
mixing the carrier and a chloropalladate solution, and then reducing to obtain the Pd-based catalyst;
the hydroxide concentration of the alkaline solution is 0.5-2.5 mol/L, and the temperature of the precipitation reaction is 20-90 ℃; the roasting temperature is 300-800 ℃ and the roasting time is 2-5 hours;
The Pd-based catalyst comprises a carrier and an active ingredient loaded on the carrier, wherein the carrier comprises gamma-Al 2O3 and a cobalt-containing compound; the active ingredient is Pd; the mass of the active ingredient is 1-5% of the mass of the carrier; the mass ratio of Co element to Al element in the carrier is 0.05-1:1.
2. The method according to claim 1, wherein the reducing agent is an alkaline formaldehyde solution, and the alkaline formaldehyde solution comprises formaldehyde, alkali metal hydroxide and water; the alkaline formaldehyde solution comprises 2-5% of formaldehyde by mass and 2-5% of alkali metal hydroxide by mass.
3. The method according to claim 1, wherein the reduction is performed at a temperature of 40 to 90 ℃ for 10 to 120 minutes.
4. The Pd-based catalyst prepared by the preparation method according to any one of claims 1 to 3, which is characterized by comprising a carrier and an active ingredient supported on the carrier, wherein the carrier comprises gamma-Al 2O3 and a cobalt-containing compound; the active ingredient is Pd; the mass of the active ingredient is 1-5% of the mass of the carrier; the mass ratio of Co element to Al element in the carrier is 0.05-1:1.
5. The Pd based catalyst of claim 4 wherein the cobalt-containing compound comprises Co 3O4、Co(OH)2 and CoAl 2O4.
6. The Pd-based catalyst of claim 4 wherein the crystal plane of Pd is the Pd (111) plane, and the Pd (111) plane is between the (100) plane of Co (OH) 2 and the (220) plane of CoAl 2O4.
7. The Pd based catalyst according to claim 4, wherein the catalyst comprises weakly basic sites and moderately basic sites, the number ratio of weakly basic sites is 54% -65%, and the number ratio of moderately basic sites is 35% -46%.
8. The use of the Pd-based catalyst of any one of claims 4-7 in an oxidative esterification reaction of aldehydes.
9. The use according to claim 8, wherein the oxidative esterification of aldehydes is a reaction for the preparation of methyl methacrylate by one-step oxidative esterification of methacrolein.
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