CN114632538B - Pd/Ce-F/MCM-48 catalyst and preparation method and application thereof - Google Patents

Pd/Ce-F/MCM-48 catalyst and preparation method and application thereof Download PDF

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CN114632538B
CN114632538B CN202210338228.6A CN202210338228A CN114632538B CN 114632538 B CN114632538 B CN 114632538B CN 202210338228 A CN202210338228 A CN 202210338228A CN 114632538 B CN114632538 B CN 114632538B
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CN114632538A (en
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崔艳红
王彦宏
汪颖军
孙鹏
所艳华
张微
卢志涛
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Northeast Petroleum University
Heilongjiang Bayi Agricultural University
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Abstract

The invention discloses a Ce-F/MCM-48 mesoporous material and a Pd catalyst taking the same as a carrier. The invention synthesizes Ce-F/MCM-48 by utilizing a NaF and Ce in-situ doped hydrothermal synthesis method, and prepares the Pd/Ce-F/MCM-48 bimetallic catalyst loaded by Pd by taking Ce-F/MCM-48 as a carrier by adopting an impregnation method. The catalytic performance of the catalyst on the n-heptane isomerization reaction was examined. The results show that: the molecular sieve prepared by doping NaF has the advantages of increased specific surface area and pore volume, uniform pore size distribution and moderate acidity. The optimal load of the noble metal Pd is 0.4% by mass, the reaction temperature is 280 ℃, the catalytic performance is relatively stable when the reaction time is 120min, the conversion rate of n-heptane isomerization is 79.7%, and the isoheptane selectivity is 91.5%. Is expected to become an important catalyst for the hydroisomerization of the n-heptane.

Description

Pd/Ce-F/MCM-48 catalyst and preparation method and application thereof
Technical Field
The invention belongs to the technical field of catalysts, and particularly relates to a Pd/Ce-F/MCM-48 catalyst, and a preparation method and application thereof.
Background
The quality upgrading of the catalytic cracking gasoline is a main problem which must be faced and solved in the next years of China. The conversion of linear paraffins to branched paraffins is the most important means of guaranteeing the octane number of gasoline. Heptane isomerisation catalyst preparation is also of great interest. Currently, the preparation of heptane isomerization catalysts is most often carried by a carrier-supported metal catalyst. The nature of the support has a very great influence on the catalyst, and scientists in the early 90 th century of the 20 th century report the synthesis of M41S [ Beck J S, vartuli J C, roth W J, et al A new family of mesoporous molecular sieves prepared with liquid crystal templates [ J ]. J.am.chem.soc,1992,114 (27): 10834-10843 ] mesoporous molecular sieves, MCM-48 being one member of the M41S series, have good transport properties due to their double helix three-dimensional pore structure, and these materials are expected to find application in the catalytic field.
However, as the MCM-48 silicon oxygen tetrahedron is a charge balance system, the lattice defect in the pure silicon mesoporous molecular sieve skeleton is few, the concentration of the surface acid center is low, the catalyst has no catalytic oxidation reaction capability, the catalyst cannot be directly used as a catalyst in general, the mesoporous molecular sieve needs to be functionally modified, the most modification method is to carry out heteroatom modification aiming at different catalytic reactions, the other important carrier modification method is to introduce anions into the molecular sieve skeleton, in general, the modification research on the molecular sieve is still less, the catalyst with high quality factors is excavated by utilizing different materials, the coordination environment and the catalytic performance of the catalyst are solved, and the development of a new catalyst is still a target which is always pursued.
Disclosure of Invention
The invention aims to provide a mesoporous material with high anion doping order degree.
The mesoporous material (Ce-F-MCM-48) with high degree of order of anion doping provided by the invention takes CTAB as a template agent, naF and Ce (NO) 3 )·6H 2 O is an additive reagent, TEOS (tetraethyl orthosilicate) is a silicon source, and a hydrothermal synthesis method is adopted under alkaline conditions.
In the initial reaction system, n (TEOS): n (CTAB): n (Ce): n (NaF) =1.0:0.65:0.02 (0-0.15), but the NaF dosage is not 0. Specifically, the n (TEOS): n (CTAB): n (Ce): n (NaF) =1.0:0.65:0.02:0.05 or 1.0:0.65:0.02:0.10 or 1.0:0.65:0.02:0.15.
Further, the alkaline condition can be provided by NaOH, and in the initial reaction system, n (TEOS): n (CTAB): n (H) 2 O) n (NaOH) n (Ce) n (NaF) =1.0:0.65:62:0.5 (0.01-0.05) and (0-0.15), but the NaF dosage is not 0. Specifically, n (TEOS): n (CTAB): n (H) 2 O):n(NaOH):n(Ce):n(NaF)＝1.0:0.65:62:0.5:0.02:0.10。
The hydrothermal conditions in the hydrothermal synthesis method are as follows: the initial gel prepared by the reaction mixture is transferred into a stainless steel reaction kettle with a polytetrafluoroethylene lining, and is put into a drying box for constant temperature crystallization for 12-108 h at 100-140 ℃, specifically, the constant temperature crystallization for 72h at 110 ℃ in the drying box.
After the hydrothermal synthesis method is finished, the method further comprises the step of drying and roasting the product.
The drying conditions are as follows: drying the mixture on an electric heating plate at 70-100 ℃ for 4-24 h and drying the mixture in a drying oven at 105-120 ℃ for 4-12 h.
The conditions of the calcination may be: the dried product is roasted for 2 to 10 hours at 300 to 800 ℃ in a resistance furnace, and can be roasted for 6 hours at 550 ℃.
The specific preparation method of the mesoporous material with high anion doping order degree comprises the following steps:
firstly, dissolving NaOH in deionized water, uniformly stirring, and sequentially adding NaF and Ce (NO) 3 )·6H 2 O is stirred for 15 to 60 minutes, CTAB is added into the mixture for a small amount for many times under the heating of water bath at the temperature of 35 to 45 ℃ and stirred for 60 to 180 minutes, and the solution is transparent; then TEOS is added into the solution dropwise intermittently and slowly, stirring is continued for 30-180 min, the molar ratio of the raw materials of the reaction mixture is n (TEOS): n (CTAB): n (H) 2 O) n (NaOH) n (Ce) n (NaF) =1.0:0.65:62:0.5 (0.01-0.05) (0-0.15); transferring the prepared initial gel into a stainless steel reaction kettle with a polytetrafluoroethylene lining, and placing the stainless steel reaction kettle into a drying oven for crystallization at a constant temperature of 100-140 ℃ for 12-108 h; washing the product with water and filtering; drying for 4-24 h at 70-100 ℃ on an electric heating plate, drying for 4-12 h in a drying oven at 105-120 ℃, and roasting for 2-10 h at 300-800 ℃ in a resistance furnace to obtain the sample marked Ce-F/MCM-48-x.
Another object of the present invention is to provide a catalyst supporting a metal active component and a method for preparing the same.
The active component of the catalyst loaded with the metal active component is noble metal palladium, and the carrier is the mesoporous material Ce-F/MCM-48; the catalyst is Pd-Ce-F/MCM-48; the Pd loading (mass fraction) in the catalyst is 0-0.5% (but not 0).
The catalyst loaded with the metal active component is prepared by adopting an impregnation method and comprises the following steps:
pd was dissolved in deionized water, and then Ce-F/MCM-48-x prepared above was immersed therein; and (3) carrying out ultrasonic dispersion, standing for more than 6 hours at room temperature, drying, and roasting to obtain the Pd-supported catalyst, which is named as Pd-Ce-F/MCM-48.
In the method, the condition of ultrasonic dispersion can be that ultrasonic dispersion is carried out for 10 to 120 minutes at the temperature of 40 to 80 ℃;
in the above method, the drying specifically comprises: drying for 4-24 h at 70-100 ℃ on an electric heating plate, and drying for 4-12 h at 105-120 ℃ in a drying box.
In the method, the roasting condition is that the roasting is carried out for 2 to 10 hours at 300 to 400 ℃, and the roasting can be carried out for 4 hours at 400 ℃ specifically
It is a further object of the present invention to provide the use of the above-described metal active component-supported catalyst.
The application is that the catalyst is used for catalyzing the isomerization of n-heptane.
The invention also provides a method for catalyzing the isomerization of n-heptane.
The method for catalyzing the isomerization of the n-heptane provided by the invention takes the Pd-Ce-F/MCM-48 as a catalyst to catalyze the isomerization of the n-heptane; the reaction conditions are as follows: the reaction temperature is 220-360 ℃ (preferably 260-320 ℃, more preferably 280-300 ℃); the reaction time is 60 to 240 minutes (preferably 100 to 200 minutes, more preferably 120 to 150 minutes).
Preferably, the optimal load of the noble metal Pd in the Pd-Ce-F/MCM-48 is 0.4 percent by mass, the reaction temperature is 280 ℃, and the catalytic performance is relatively stable when the reaction time is 120 minutes.
The Pd-Ce-F/MCM-48 catalyst needs to be activated before use, and the activation conditions are as follows: activating for 1-5 h under the condition of 300-400 ℃ under the pressure of 1.0MPa in the hydrogen atmosphere.
Further, in the isomerization process, the flow rate of hydrogen is 10-50 mL/min, and the weight hourly space velocity is 3.5-8.0 h -1 。
Compared with the prior art, the invention has the following technical effects:
firstly, the invention synthesizes a new mesoporous material, naF is a good structural auxiliary agent, and the invention is characterized in that n (Ce): under the condition that the doping amount of n (Si) is 0.02 molar ratio, determining the optimal n (F) by regulating and controlling the proportion of the fluorine-silicon raw material: the molar ratio of n (Si) is 0.10, and the highly ordered F/Ce-MCM-48 mesoporous material is successfully synthesized by adopting a hydrothermal method. F compared with MCM-48 and Ce-MCM-48 -1 The Ce/F-MCM-48 specific surface area and pore volume are increased after the introduction (see table 1), the pore size distribution range is narrow, the pore size distribution tends to be uniform (fig. 6) due to the fact that the pore size distribution range is narrow and is composed of a large number of mesopores with uniform pore sizes, the carrier-to-element capacity range is wide (see table 2), and the acid quantity and the acid strength are enhanced (fig. 7).
TABLE 1 structural parameters of catalyst samples
Figure BDA0003577455940000031
TABLE 2 elemental data for Ce-MCM-48 catalysts
Figure BDA0003577455940000032
The elemental composition theory and the measured values of the prepared Ce-MCM-48 mesoporous material and 0.4% pd-Ce (F) -MCM-48-x (x=0, 0.10) catalyst are given in table 2. In the molar ratio of n (TEOS) to n (CTAB) to n (H) 2 O) n (NaOH) n (Ce) n (NaF) =1.0:0.65:62:0.5:0.02:x (x=0, 0.10). As can be seen from Table 2, F is introduced -1 The actual loading rate of F element in the rear Ce (F) -MCM-48-0.10 molecular sieve is 63.3%, and the actual loading rate of the first metal Ce is slightly reduced to 95.5%, which shows that F is doped -1 The first metal is not caused to fall off. Pd impregnated 0.4% Pd-Ce-F/MCMThe actual loading rates of Pd, F and Ce in the catalyst are 97.2%, 63.1% and 94% respectively; with Pd impregnation, the F, ce content is not affected, and the phenomenon of falling off does not occur. The capacity range of the illustrative carrier is wide.
TABLE 3 catalyst surface acidity
Figure BDA0003577455940000041
FIG. 7 is NH of MCM-48, ce-MCM-48 and Ce/F-MCM-48 3 The TPD curve, from FIG. 7 and Table 3, can be analyzed that the acid content of the mesoporous material is increased, the acid strength is increased, the acid content of MCM-48 is almost zero, and the acid content of the MCM-48 mesoporous material is increased after the rare earth Ce is introduced. Introduction of F -1 The acid content is increased.
Secondly, the catalyst of loading noble metal palladium on mesoporous materials is not reported in the literature before, and after Pd is loaded by an impregnation method, a high-quality catalyst new material Pd-Ce-F/MCM-48 with good palladium metal dispersity and high sustainable doping amount is obtained.
Again, the present application also focused on the catalytic performance of the catalyst. The catalytic performance of the catalyst is examined on a miniature reaction device by taking the n-heptane isomerization reaction as a probe. The result shows that the optimal load of the noble metal Pd is 0.4% by mass, the reaction temperature is 280 ℃, the reaction time is 120min, the catalytic performance is relatively stable, the conversion rate of n-heptane isomerization is 79.7%, and the isoheptane selectivity is 91.5%. A molecular sieve material is constructed, the octane number of the gasoline is improved by alkane isomerization, and the molecular sieve material has important application prospect along with the enhancement of energy conservation and emission reduction environmental protection consciousness.
Drawings
FIG. 1 is a small angle XRD diffraction pattern for the Ce-F/MCM-48 mesoporous material of example 1.
FIG. 2 is a small angle XRD diffraction pattern of the Pd-/Ce-F/MCM-48 mesoporous material of example 2.
FIG. 3 is a scanning electron microscope image of Ce-F/MCM-48 of example 1.
FIG. 4 is a wide angle XRD diffraction pattern for Pd-Ce-F/MCM-48 of example 2.
FIG. 5 is a transmission electron microscope image of Ce-F/MCM-48 of example 1.
FIG. 6 is a pore size distribution diagram of Ce-F/MCM-48 of example 1.
FIG. 7 is NH of Ce-F/MCM-48 of example 1 3 -TPD map.
FIG. 8 is a transmission electron microscope image of Pd-F/Ce-MCM-48 of example 2.
FIG. 9 is a transmission electron micrograph of non-fluorine doped 0.4% Pd-Ce-MCM-48 of comparative example 1.
FIG. 10 is a bar graph of x% Pd-Ce-F/MCM-48 catalyst performance versus Pd content in example 3.
FIG. 11 is a bar graph of 0.4% Pd-Ce-F/MCM-48 catalyst performance as a function of reaction temperature in example 3.
FIG. 12 is a bar graph of 0.4% Pd-Ce-F/MCM-48 catalyst performance as a function of reaction time in example 3.
Detailed Description
The invention will be further illustrated with reference to the following specific examples, but the invention is not limited to the following examples. The methods are conventional methods unless otherwise specified. The starting materials are available from published commercial sources unless otherwise specified.
Example 1 preparation of anionic doped mesoporous Material Ce-F/MCM-48 with high degree of order
CTAB is used as a template agent, naF and Ce (NO 3 )·6H 2 O is an additive agent, and is prepared by a hydrothermal synthesis method under an alkaline condition.
Firstly, dissolving NaOH in deionized water, uniformly stirring, and sequentially adding NaF and Ce (NO) 3 )·6H 2 O stirring for 30min, adding CTAB into the mixture for multiple times at 35deg.C under heating in water bath, stirring for 90min to obtain transparent solution, adding TEOS (ethyl silicate) into the solution dropwise with 3mL lengthened Pasteur plastic straw, and stirring for 60min continuously, wherein the molar ratio of the raw materials of the reaction mixture is n (TEOS) to n (CTAB) to n (H) 2 O): n (NaOH): n (Ce): n (NaF) =1.0:0.65:62:0.5:0.02:0.10. The prepared initial gel is transferred into a stainless steel reaction kettle with a polytetrafluoroethylene lining, and is put into a drying oven for crystallization at a constant temperature of 110 ℃ for 72 hours. The product is washed with water and filtered. Drying at 90deg.C on electric hot plate for 6 hr, and drying at 105deg.CDrying in a box for 5h, and roasting in a resistance furnace at 550 ℃ for 6h. The prepared sample is marked as Ce-F/MCM-48-x, and x represents the molar ratio n (TEOS) of the raw materials of the reaction mixture, namely n (NaF).
Example 2 preparation of Metal active component-Supported catalyst
The active component of the catalyst loaded with the metal active component is noble metal palladium, and the carrier is mesoporous material Ce-F/MCM-48 prepared in the embodiment 1; the catalyst is Pd-Ce-F/MCM-48.
The catalyst was prepared by an impregnation method, and the Pd loading (mass fraction) was 0.4%. Pd was dissolved in a ceramic crucible of deionized water and then Ce-F/MCM-48-0.10 was immersed therein. And performing ultrasonic dispersion for 20min at 50 ℃. Standing at room temperature for more than 6h, drying at 90 ℃ for 4h on an electric hot plate, drying at 105 ℃ for 5h in a drying oven, and roasting at 400 ℃ for 4h to obtain the Pd-supported catalyst which is recorded as 0.4% Pd-Ce-F/MCM-48.
Comparative example 1 preparation of non-fluorine doped 0.4% Pd-Ce-MCM-48 catalyst
A non-fluorine doped 0.4% Pd-Ce-MCM-48 catalyst was prepared according to the method of example 2, except that the support preparation method was essentially the same as in example 1, but no NaF was added.
Example 3, evaluation of catalytic reactivity of 0.4% Pd-Ce-F/MCM-48 for catalyzing isomerization of n-heptane
The catalytic reaction performance evaluation is carried out on a fixed bed micro reaction device, and the prepared catalyst is subjected to tabletting and screening to obtain 60-80 mesh particles for standby. The filling amount was 0.3g, and the mixture was mixed with quartz sand and then charged into a reaction tube having an inner diameter of 6mm. The catalyst is activated for 4 hours under the condition of the pressure of 1.0MPa and the temperature of 300 ℃ in the hydrogen atmosphere before the reaction, and then the temperature is reduced to be within the range of 220-360 ℃ for modulation. The reaction evaluation process conditions are the pressure n-C 7 Flow rate 6.0mL/H, n-H 2 The flow rate is 30mL/min, and the MHSV is 7.6h -1 . After 30min of stabilization, sampling is started, and the product is separated by a chromatographic column and is analyzed on line by a gas chromatograph FID detector.
The present example examined the relationship between the performance of the 0.4% Pd-Ce-F/MCM-48 catalyst and the reaction temperature (the remaining conditions were the same, the hydrogen flow rate was 10-50 mL/min, weight hourly space in the isomerization process)The speed is 3.5 to 8.0 hours -1 . ) The histogram of the performance of the 0.4% Pd-Ce-F/MCM-48 catalyst as a function of reaction temperature is shown in FIG. 11. As can be seen from fig. 11, the conversion rate increased continuously and the selectivity decreased continuously as the reaction temperature increased. Although the n-heptane isomerisation is a slightly exothermic reaction, if the reaction temperature is too low, the overall reaction rate is too slow, leading in the main to an undesirable conversion result. It is necessary to raise some of the reaction temperature, which is at 280 c, while the cleavage reaction is endothermic, and the raising of the temperature promotes the cleavage reaction.
The example examines the relation between the performance and the reaction time of the 0.4% Pd-Ce-F/MCM-48 catalyst (the rest conditions are consistent, the hydrogen flow rate is 10-50 mL/min, and the weight hourly space velocity is 3.5-8.0 h) -1 . ) The histogram of the performance of the 0.4% Pd-Ce-F/MCM-48 catalyst over the reaction time is shown in FIG. 12. As can be seen from fig. 12, the n-heptane conversion and the isomerization selectivity increased continuously with the increase of the reaction time and then decreased continuously with the increase of the reaction time within 120min, because some carbon deposition was generated on the active center of the flower forcing agent with the progress of the reaction, resulting in a decrease of the conversion. The deactivation may be caused by collapse of the pore channel, blocking of the pore structure, loss of acid sites, etc., and the catalyst is deactivated. When the reaction temperature was 280℃and the reaction time was 120min, the maximum yield of isoheptane was 72.9% and the optimal binding point for conversion and selectivity was obtained. The conversion rate and the selectivity are both good.
The present example also examined the catalytic performance of catalysts with different Pd loadings (0% -0.5%). The performance of the x% Pd-Ce-F/MCM-48 catalyst is shown in FIG. 10 as a histogram of Pd content. As can be seen from FIG. 10, the conversion and selectivity of n-heptane were the lowest when the catalyst contained no Pd, indicating that Ce (F) -MCM-48 exhibited weak catalytic activity without Pd loading. As the n (Pd)/n (Si) ratio is gradually increased from 0.1% to 0.5%, the activity of xPd-Ce (F) -MCM-48 for catalyzing n-heptane isomerization is firstly increased and then decreased. When x=0.4%, the n-heptane conversion of 0.4% pd-Ce (F) -MCM-48 was 79.7% and the isoheptane selectivity was 91.5%. This is because the active center is a palladium atom existing in a low valence coordination state when the palladium loading amount in the catalyst is smallThe activity is better than the reusability, and when the Pd loading is gradually increased, metal Pd particles are aggregated besides the coordinated atomic palladium. When the Pd content is 0.5%, excessive metals are accumulated, and the activity is poor. Thus, to obtain a good heptane isomerisation catalyst, excellent support properties, state of active components, auxiliary agent F -1 Is of critical importance. The Pd content was therefore chosen to be 0.4%.
The results show that the optimal loading of the noble metal Pd is 0.4% by mass, the reaction temperature is 280 ℃, the catalytic performance is relatively stable when the reaction time is 120min, the conversion rate of n-heptane isomerization is 79.7%, and the isoheptane selectivity is 91.5%. A molecular sieve material is constructed, the octane number of the gasoline is improved by alkane isomerization, and the molecular sieve material has important application prospect along with the enhancement of energy conservation and emission reduction environmental protection consciousness.

Claims (6)

1. A process for preparing the anionic doped mesoporous material includes such steps as using CTAB as template, naF and Ce (NO) 3 ）·6H 2 O is an additive agent, TEOS is a silicon source, and is prepared by adopting a hydrothermal synthesis method under an alkaline condition; the mesoporous material is marked as Ce-F/MCM-48;
the alkaline condition is provided by NaOH, and in the initial reaction system,n(TEOS) :n(CTAB):n(H 2 O): n(NaOH): n(Ce): n(NaF)= 1.0:0.65:62:0.5: 0.02:0.10；
the hydrothermal conditions in the hydrothermal synthesis method are as follows: transferring the initial gel prepared by the reaction mixture into a stainless steel reaction kettle with a polytetrafluoroethylene lining, and placing the stainless steel reaction kettle into a drying oven for constant-temperature crystallization at 100-140 ℃ for 12-108 h;
after the hydrothermal synthesis method is finished, the method further comprises the step of drying and roasting the product;
the drying conditions are as follows: drying at 70-100deg.C on electric plate 4-24 h, 105-120deg.C in drying oven 4-12 h;
the roasting conditions are as follows: roasting the dried product in a resistance furnace at 300-800 ℃ for 2-10 h;
the preparation method specifically comprises the following steps of: firstly, dissolving NaOH in deionized water, uniformly stirring, and sequentially adding NaF and Ce (NO) 3 ）·6H 2 O is stirred for 15-60 min, CTAB is added into the mixture for a plurality of times under the heating of water bath at the temperature of 35-45 ℃ and stirred for 60-180 min, and the solution is transparent; then TEOS is added into the solution dropwise intermittently and slowly, and stirring is continued for 30-180 min; transferring the prepared initial gel into a stainless steel reaction kettle with a polytetrafluoroethylene lining, and placing the stainless steel reaction kettle into a drying oven for constant temperature crystallization at 100-140 ℃ for 12-108 h; washing the product with water and filtering; drying at 70-100deg.C on electric plate 4-24 h, drying at 105-120deg.C in drying oven 4-12 h, and roasting at 300-800deg.C in resistance furnace 2-10 h.
2. The anionically doped mesoporous material of claim 1.
3. A catalyst loaded with a metal active component, wherein the active component is noble metal palladium, and the carrier is the anion-doped mesoporous material of claim 2; the Pd loading in the catalyst was 0.4% by mass fraction.
4. A method for preparing the metal active component-supported catalyst according to claim 3, which is prepared by an impregnation method, comprising the steps of: pd is dissolved in deionized water, and then the anion doped mesoporous material is immersed in the deionized water; ultrasonic dispersion, standing for more than 6h at room temperature, drying and roasting to obtain a catalyst loaded with metal active components;
the condition of ultrasonic dispersion is that ultrasonic dispersion is carried out for 10-120 min at 40-80 ℃;
the drying is specifically as follows: drying 4-24 h at 70-90deg.C on electric hot plate, and drying 4-12 h at 105-120deg.C in drying oven;
the roasting condition is that roasting is 2-10 h at 300-400 ℃.
5. Use of the metal active component-supported catalyst according to claim 3 for catalyzing the isomerization of n-heptane.
6. A method for catalyzing isomerization of n-heptane, which comprises catalyzing isomerization of n-heptane by using the catalyst of the supported metal active component according to claim 3; the reaction conditions are as follows: the reaction temperature was 280℃and the reaction time was 120 min.
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