CN1058284C - Light hydrocarbon aromatization catalyst and its preparing process - Google Patents

Abstract

A light hydrocarbon aromatization catalyst, containing Zn, mixed rare earth and HZSM-5 components, the composition is based on the weight of the catalyst: Zn 0.8-3.5% by weight, mixed rare earth (calculated as oxide) 0.2-1.5% by weight , The carrier is 95.0-99.0% by weight. The carrier is composed of 50-80% by weight of HZSM-5 zeolite and 20-50% by weight of γ-alumina. The mixed rare earth contains elements such as lanthanum, cerium, praseodymium, and neodymium. The catalyst uses (Zn(NH ₃ ) ₄ ) ²⁺ complex ion solution as the impregnation solution to introduce Zn, and conducts high-temperature steam treatment under appropriate conditions, so that the catalyst has good aromatization activity, selectivity, and Good stability and regeneration properties.

Description

A light hydrocarbon aromatization catalyst and its preparation method

The invention relates to a light hydrocarbon aromatization catalyst and a preparation method thereof, specifically, a zinc-containing aromatization catalyst suitable for C ₂ -C ₁₂ light hydrocarbons and a preparation method thereof.

A large amount of low-carbon light hydrocarbons and mixed gas containing low-carbon light hydrocarbons are often associated with the production of oil fields, natural gas fields, and refineries. These low-carbon light hydrocarbons are converted into benzene, toluene and xylene (BTX for short) through catalytic reactions. The main aromatics, while by-producing a large amount of cheap hydrogen, have very good economic benefits. The aromatization catalyst prepared by loading zinc or gallium on HZSM-5 zeolite has the advantages of high activity and good selectivity, but due to carbon deposition, the activity of the catalyst decreases rapidly, and the single-pass life is short, usually only a few to tens of hours , it is difficult to promote the use in fixed bed reactors. In addition, zinc is prone to loss in the reducing atmosphere of the aromatization reaction. Although gallium is not as fast as zinc, the price of gallium is quite expensive. Continuous regeneration of moving bed (such as UOP and BP's CYCLAR process) technology abroad can achieve continuous and stable operation, but it is difficult to promote in China due to high equipment investment and large scale. In order to meet the requirements of fixed bed operation and improve the stability of Zn-HZSM-5 aromatization catalyst, the catalyst has been improved mainly from the following aspects.

The second modification component with hydrogenation activity, mainly noble metal platinum, is introduced into the Zn-HZSM-5 catalyst to reduce the carbon deposition of the catalyst. As CN1062100A discloses a light hydrocarbon aromatization catalyst, the active component of the catalyst is platinum, zinc or gallium, and the carrier is ZSM-5 zeolite and/or ZSM-11 and SiO ₂ and/or Al ₂ O ₃ mixture, Carry out the aromatization reaction of C ₅ ~C ₈ light hydrocarbons under the conditions of normal pressure, 500°C, and space velocity 2.0 hr- ¹ . The single-pass life of the catalyst can reach 200 hours, and the yield of aromatic hydrocarbons is 50%. After the catalyst is regenerated many times Can continue to use.

CN1070847A then discloses a kind of HZSM-5 catalyst modified with gallium, zinc, platinum, the active composition of this catalyst is gallium 0.5～2.0%, zinc 0.5～2.0%, platinum 0.1～0.5%, all the other are silicon aluminum ratio is 40-100 HZSM-5. The weight ratio of the active component to the molding binder is 70:30. The preparation method is to impregnate gallium in HZSM-5 first, then extrude the strip, then treat it with an air stream containing 15-20% water vapor at 600-700°C for 1-3 hours to dealuminate, and finally impregnate zinc and platinum. . The catalyst carries out the aromatization reaction of C ₅ -C ₈ light hydrocarbons under the conditions of normal pressure, 550°C, and a weight space velocity of 0.5 hr ^-1 . The single-pass life of the catalyst reaches 240 hours, and the yield of aromatics is not less than 40% by weight.

Although the noble metal modified aromatization catalyst has good stability, the cost of the catalyst will be greatly increased, and the addition of platinum will reduce the aromatization selectivity of the catalyst, which is prone to sulfur poisoning. Therefore, people introduce non-precious metal components into the Zn-HZSM-5 catalyst for modification, as CN1063121A discloses a kind of aromatization catalyst modified with Al and/or rare earth, containing 0.3～15% by weight of Two or three composite activity modifiers selected from Zn, Al and rare earth, 5.0-94.5% by weight of hydrogen-type molecular sieves with a silicon-aluminum ratio of 10-500, including HZSM-5, HZSM-7, etc., and the rest are optional A binder from any one of alumina, silica, and clay. The preparation of the catalyst adopts the conventional impregnation method to introduce the active modifier, and in an example, only a single rare earth element nitrate, such as La(NO ₃ ) ₃ and/or Ce(NO ₃ ) ₂ solution is used to prepare the catalyst.

In order to solve the zinc loss problem of the Zn-HZSM-5 catalyst, CN1057476A discloses a preparation method of a zinc-containing zeolite catalyst resistant to zinc loss. The zinc compound is selected from ZnS, ZnSO ₄ , ZnSiO ₄ , ZnTiO ₄ , ZnAl ₂ O ₄ . The prepared catalyst is used for the aromatization of liquefied petroleum gas, the reaction time is 168 hours, and the yield of aromatics drops from 42% to 32%.

The aromatization process of light naphtha developed by Japan Energy Company (Journal of Petroleum Society Sekiyu Gakkaishi, 37, (1) 77-83 (1994)) adopts hydrothermal synthesis method to directly prepare zinc-containing fine crystals with ZSM-5 structure Granular zinc-silicon-alumina zeolite catalyst (Appl. Catal., V.93, 35 (1992)) is specially treated to reduce the loss of zinc and prolong the single-pass service life of the catalyst. The disadvantage is that tetrapropylammonium hydroxide, an organic template agent, is needed in the synthesis of zeolite, which is costly and has environmental pollution problems, and the preparation technology of the catalyst is also difficult.

In the prior art, for high silica zeolites such as HZSM-5, it is difficult to quantitatively or nearly quantitatively introduce the required metal ions by ordinary impregnation or ion exchange methods, so it is often necessary to use an excessive amount of zinc before the support is formed. The salt solution is heated to 85-95° C. for repeated ion exchange, so that the preparation process of the catalyst is complicated and the yield is low. If the saturated adsorption capacity impregnation method is adopted and the excess impregnation solution is evaporated to dryness after impregnation, it is not easy to make the zinc evenly distributed on the inner and outer surfaces of the preformed carrier particles.

In addition, although steam treatment is often used to modify zeolite catalysts, it is not a commonly used modification method for light hydrocarbon aromatization catalysts. Because the aromatization of light hydrocarbons is a complex process including a series of reactions such as cracking, polymerization, cyclization, and dehydrogenation, especially for the aromatization of alkanes, the control step of the reaction rate is the cracking reaction on the strong acid site, high temperature steam treatment It is possible to destroy the strong acidic active center on the catalyst and greatly reduce the aromatization activity. Therefore, appropriate steam treatment conditions must be selected to coordinate the acidic cracking and dehydrogenation functions of the catalyst, so that the catalyst has good activity, selectivity, and good stability.

The object of the present invention is to provide an aromatization catalyst which does not contain noble metal components and has good activity, selectivity and good stability on the basis of the above-mentioned prior art and a preparation method thereof.

We found that introducing mixed rare earth components into the Zn-HZSM-5 aromatization catalyst, and introducing Zn with [Zn(NH ₃ ) ₄ ] ²⁺ complex ion solution, and then treating the catalyst with high temperature steam, can obtain good performance aromatization catalyst.

The catalyst provided by the invention is composed of Zn, mixed rare earth oxide, HZSM-5 and gamma-alumina as a carrier. The content of Zn in the catalyst is 0.8-3.5% by weight, the content of mixed rare earth oxide is 0.2-1.5% by weight, the content of carrier is 95.0-99.0% by weight, and the carrier is composed of 50-80% by weight of HZSM-5 zeolite and 20-50% by weight % gamma-alumina composition, the mixed rare earth oxide contains 20-40 wt% of lanthanum oxide, 40-60 wt% of cerium oxide, 10-18 wt% of praseodymium oxide and 2-10 wt% of neodymium oxide.

The preparation method of catalyst provided by the invention comprises the steps:

(1) Preparation of carrier: Mix HZSM-5 zeolite and α-alumina monohydrate powder according to the weight ratio of zeolite: Al ₂ O ₃ =50-80:50-20 on a dry basis, and bake at 400-600°C after drying , to prepare the carrier.

(2) Introduce zinc components: impregnate the carrier prepared in (1) with [Zn(NH ₃ ) ₄ ] ²⁺ complex ion solution at room temperature for 2 to 24 hours, and the liquid/solid ratio during immersion is 1.0 to 1.4 ml /g, the amount of [Zn(NH ₃ ) ₄ ] ²⁺ complex ion solution should be such that the Zn content therein accounts for 0.8-3.5% by weight of the total amount of the catalyst, and the zinc-introduced catalyst is prepared after drying.

(3) Introduce rare earth components: the zinc-introducing catalyst prepared in (2) is impregnated with an aqueous solution of chlorinated mixed rare earths for 2 to 24 hours at room temperature, and the amount of chlorinated mixed rare earths is calculated as 0.2% of the weight of the catalyst according to its oxide. ～1.5%, the liquid/solid ratio is 0.6～0.9 ml/g during impregnation, the solid is dried after impregnation, and then roasted at 500～600°C for 2～6 hours in air flow.

(4) Steam treatment: after the catalyst prepared in (3) is heated up to 500-600° C. in air flow, it is treated with pure water steam for 1-6 hours, and the weight ratio of the total water consumption to the catalyst is 1 during the treatment. ~10.

The HZSM-5 zeolite described in step (1) in the above preparation method can also be replaced by ammonia type ZSM-5 zeolite. Ammonia-type ZSM-5 zeolite can be obtained from sodium-type ZSM-5 zeolite after ammonium exchange. Ammonium exchange can be carried out before the carrier is prepared or after the carrier is shaped and roasted. It can be carried out by conventional methods, such as using 1M NH ₄ Cl or NH The _4NO3 solution was exchanged at 85°C for 2 hours _, washed and dried. The sodium content of NH ₄ ZSM-5 obtained after exchange should be less than 0.1% by weight, preferably less than 0.02% by weight. The molding of the carrier also adopts the conventional technology. After the raw materials are mixed, an appropriate amount of water and 1-3% by weight nitric acid or acetic acid are kneaded and extruded, dried at 60-120°C for 2-4 hours, and calcined at 400-600°C for 4 hours.

The silicon-aluminum ratio of the zeolite used in the step (1) is 30-100.

In the step (3), the chlorinated mixed rare earth contains elements such as lanthanum, cerium, praseodymium, and neodymium, and its content is calculated as oxides: lanthanum 20-40% by weight, cerium 40-60% by weight, praseodymium 10-18% by weight %, neodymium 2 to 10% by weight.

The order of steps (2) and (3) in the preparation method can be interchanged. It is best to introduce zinc first, and then introduce mixed rare earths. In this way, after zinc introduction, the carrier only needs to be dried, and can continue to introduce mixed rare earths without roasting. rare earth. It is also possible to introduce mixed rare earths first, and then introduce zinc, but the carrier introduced with mixed rare earths must be roasted before zinc can be introduced. After zinc introduction, the carrier needs to be roasted at 500-600°C for 2-6 hours before steam treatment.

The steam treatment temperature in the step (4) is preferably 520-580°C, the treatment time is preferably 1-4 hours, and the weight ratio of the total water consumption to the catalyst is preferably 2-4 during treatment.

The step (4) can also be carried out after the carrier is prepared in the step (1), that is, the prepared carrier is first subjected to steam treatment, and then zinc and mixed rare earth components are introduced respectively. But it is better to introduce zinc and mixed rare earth components first, followed by steam treatment.

The condition of steam treatment in the step (4) can also be adjusted appropriately according to the silicon-aluminum ratio of the HZSM-5 zeolite raw material used in the step (1), the level of acid cracking activity, to ensure the acidic function and dehydrogenation of the catalyst Functions are at their most coordinated. The standard for adjusting the steam treatment conditions is to make the α value of the used raw material HZSM-5 after steam treatment be between 50 and 250 (the measurement method of α value refers to "Petrochemical Analysis Method (RIPP Experimental Method)" edited by Yang Cuiding et al. , published by Science Press, P255 "The α value of the acidic catalyst is determined by constant temperature method").

The catalyst of the present invention is suitable for the aromatization of C ₂ -C ₁₂ olefins, alkanes, cycloalkanes or mixtures of olefins and alkanes, for example, gas containing C ₂ -C ₅ in refineries, liquefied petroleum gas, pyrolysis gasoline, Light naphtha, oilfield light hydrocarbons, etc. can be used as raw materials for aromatization. The reaction conditions vary with the raw materials, the reaction temperature is 400-600°C, the pressure is normal pressure-1.0MPa, and the feed weight space velocity is 0.2-10 hours ^-1 .

The catalyst of the present invention introduces mixed rare earth components into the zinc-containing HZSM-5 catalyst, adopts [Zn(NH ₃ ) ₄ ] ²⁺ complex ion solution as the impregnation solution to introduce zinc components, and conducts the catalyst under suitable conditions The steam treatment enables the prepared aromatization catalyst to greatly improve the anti-zinc loss ability and stability, while still maintaining good activity and selectivity, and reduces the cost of the catalyst. Using the catalyst of the present invention under the reaction conditions of 520-550°C and the gravimetric space velocity of the feed liquid of 0.6-1.5 hours ^-1 , the mixed _C4 is subjected to aromatization reaction, and the single-pass operation time with the aromatics yield greater than 40% can reach After 300-450 hours, the average yield of total aromatics was 47.5%. After more than 1200 hours of cumulative reaction and multiple regenerations, the zinc content, aromatization activity and stability of the catalyst did not change significantly.

The present invention is further illustrated by the following examples, but the present invention is not limited thereto.

Instance 1

This example is the preparation of the catalyst of the present invention.

(1) Prepare the carrier: get 65 grams of HZSM-5 zeolite powder (Zhoucun Catalyst Factory, Qilu Petrochemical Company) with a silicon-aluminum ratio of 54, and 35 grams of α-alumina monohydrate powder (SB powder, Germany, Condea company), place Grind and mix evenly in a mortar, add 1-2 ml of 1:1 nitric acid and 30 ml of water to form a solution, knead into a dough, extrude into a strip carrier with a diameter of 2 mm in an extruder, and dry at 110 ° C For 4 hours, bake at 540°C for 4 hours.

(2) Introduce zinc components: under constant stirring, add ammonia water (produced by Beijing Chemical Plant) dropwise into 5.0 ml of Zn(NO ₃ ) ₂ aqueous solution containing 270 mg of ZnO until the white precipitate dissolves, then add water to make 12 ml Zn(NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution, take 10 grams of the carrier prepared in (1), and impregnate it with the above Zn(NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution at room temperature for 2 hours. The solid ratio is 1.2 ml/g. After impregnation, the solid is filtered out, rinsed twice with deionized water, and dried at 120°C for 6 hours.

(3) Introduce mixed rare earths: get the above-mentioned catalyst for zinc introduction, use 8 milliliters of chlorinated mixed rare earths containing 100 mg (industrial products, produced by Inner Mongolia Baotou Rare Earth Industry Company, wherein lanthanum oxide accounts for 31%, cerium oxide 51%, praseodymium oxide 14 %, neodymium oxide 4%, X-ray fluorescence analysis) aqueous solution was immersed at room temperature for 2 hours, dried at 110°C for 16 hours, and calcined at 540°C for 4 hours.

(4) Water vapor treatment: Put the catalyst introduced with zinc and mixed rare earth into a tubular reactor, heat up to 540°C in the air flow, and then change to pure water vapor at this temperature for 2 hours, and then pass it into a dry The air cools down. The total water intake during steam treatment is 30 grams.

Containing mixed rare earth oxide 0.34 weight %, Zn 2.1 weight % in the catalyst A made by the above-mentioned method (all adopt X fluorescence quantitative analysis method to analyze).

Example 2

Catalyst B was prepared according to the method of Example 1, except that the amount of mixed rare earth chloride was 200 mg, and the amount of Zn(NO ₃ ) ₂ was 180 mg as ZnO. Catalyst B contained 0.68% by weight of mixed rare earth oxides and 1.4% by weight of Zn.

Example 3

Catalyst C was prepared according to the method of Example 1, except that after the carrier was prepared, steam treatment was performed, and then zinc and mixed rare earths were introduced. Catalyst C contains 0.33% by weight of mixed rare earth oxides and 2.2% by weight of Zn.

Example 4

Catalyst D was prepared according to the method of Example 1, except that steps (2) and (3) were exchanged, that is, mixed rare earths were introduced first, and then zinc was introduced, but after the introduction of zinc, the carrier needed to be roasted at 540 for 4 hours, and then carried out steam treatment. Catalyst D contains 0.34% by weight of mixed rare earth oxides and 2.1% by weight of Zn.

Example 5

Catalyst E was prepared according to the method of Example 1, except that HZSM-5 with a silicon-to-aluminum ratio of 43 was used as a raw material to prepare a carrier, and the carrier was impregnated with zinc and mixed with rare earths and roasted in air at 560° C. for 4 hours, and then water was used at 560° C. Steam treatment was performed for 4 hours, and the total water intake was 30 grams. Catalyst E contains 0.34% by weight of mixed rare earth oxide and 2.1% by weight of Zn.

Comparative ratio

Zn-HZSM-5 aromatization catalyst was prepared by conventional impregnation method.

Get 10 grams of the carrier prepared in Example 1, use 8 milliliters of Zn(NO ₃ ) _aqueous solution containing 280 mg of ZnO, impregnate at room temperature for 24 hours, dry at 80° C., dry at 120° C. for 6 hours, and calcinate in air at 540° C. Hours, catalyst F was prepared. F contains 2.2% by weight of Zn.

The following examples illustrate the excellent properties of the catalysts of the present invention.

Example 6

This example shows that the catalyst of the present invention has good zinc retention ability.

Catalysts A, C and comparative catalyst F were reduced for up to 20 hours in a hydrogen flow at 590°C and a volumetric space velocity of 2000 h ^-1 , and the changes in the zinc content of the catalysts before and after reduction were compared. The results are shown in Table 1. From Table 1, it can be seen that the catalyst of the present invention has good zinc loss resistance, and the catalyst that is impregnated with zinc and mixed rare earth and then treated with water vapor has better zinc retention capacity.

Example 7

This example shows that the catalyst of the present invention has excellent catalytic performance for mixed _C4 aromatization.

Take 10 milliliters of catalyst D weighing 7 grams, put it into a fixed bed reactor, and carry out the mixed _C4 aromatization reaction. The reaction is carried out under continuous flow isothermal conditions. The composition of the mixed _C4 raw material is as follows: _C3 0.1%, C ₄ alkanes 55.5%, C ₄ olefins 44.2%, C ₅ ⁺ 0.1%. The reaction product is separated into gas-liquid two-phase by water cooling, and the metering and composition analysis are carried out respectively. After 20 hours of reaction, the reaction data measured under the conditions of reaction pressure of 0.15 MPa, liquid feed weight space velocity of 1.5 hours ^-1 , and catalyst inlet temperature of 530°C and 550°C are shown in Table 2. The results show that Catalyst D has excellent activity and selectivity for mixed C ₄ aromatization, and the yield of aromatics mainly C ₆ -C ₈ aromatics is higher than 50%, in which BTX content > 90%, and C ₅ ⁺ in the product The aromatic content is as high as 99%.

Example 8

This example shows that the catalyst of the present invention has excellent catalytic performance for the aromatization of C ₅ -C ₈ alkanes.

According to the method of Example 7, catalysts A, B, and C were respectively subjected to the aromatization reaction experiment of _C5 - _C8 alkane mixture, and the reaction raw materials were composed of: _C5 5.1%, _C6 58.2%, _C7 31.9%, _C8 ⁺ 4.8%. The reaction conditions were bed inlet temperature of 550°C, pressure of 0.15 MPa, and liquid feed weight space velocity of 1.5 hours ⁻¹ . The product yields after 20 hours of reaction are shown in Table 3. The results show that the catalyst of the present invention has good aromatization activity and selectivity for C ₅ -C ₈ alkanes, the yield of aromatics is higher than 50% by weight, and the amount of by-product hydrogen can reach about 3% by weight.

Example 9

This example shows that the catalyst of the present invention has good aromatization activity stability.

Catalyst D prepared in Example 4 and Comparative Catalyst F were subjected to accelerated aging experiments of aromatization reaction to evaluate the activity stability of the catalysts. Table 4 shows the test results of using the C ₅ -C ₈ alkane mixture described in Example 8 as the reaction raw material, the reaction pressure is 0.15 MPa, the liquid feed weight space velocity is 5.0 h ⁻¹ , and the continuous operation is 54 hours. It can be seen from Table 4 that the catalyst of the present invention has better stability in the aromatization reaction.

Example 10

This example illustrates that the catalyst of the present invention has good stability for mixed _C4 aromatization reactions.

Take 15 ml of catalyst E, weighing 10 g, for the stability experiment of mixed _C4 aromatization reaction. The composition of the reaction raw materials is the same as in Example 7. Under the reaction conditions of reaction pressure 0.2MPa, temperature 530°C, and liquid feed weight space velocity 0.65 hours ^-1 , the reaction continued for 450 hours, and the yield of aromatics dropped from the initial 50% by weight to the experimental value. 43% by weight at the end, the average yield of aromatics is greater than 47% by weight, and the purity of aromatics in the liquid phase product is high. The changes of BTX and total aromatics yield in 450 hours are shown in Table 5. The test results show that the catalyst of the present invention has good aromatization activity and stability.

Example 11

This example shows that the catalyst of the present invention has good regeneration performance.

Using the catalyst A prepared in Example 1, carry out the aromatization experiment to the mixed C according to the method of Example ₇ , the reaction temperature is 540 ° C, the pressure is 0.2 MPa, the liquid feed weight space velocity is 0.8 hours ^-1 , and the catalyst is continuously reacted for 300 hours. Burn charcoal for regeneration, and continue to carry out the aromatization reaction according to the above conditions after regeneration. The initial reaction activity and average yield of the catalyst after each regeneration are compared with the data of the fresh catalyst in Table 6. It can be seen from Table 6 that the catalyst still has The high reaction activity and the total cumulative reaction time of 1200 hours indicate that the catalyst of the present invention has good activity stability and regeneration performance.

Table 1 catalyst Preparation Features Zn content, wt% Zn loss, wt% Before restore After restoration Example 3 C Zinc and rare earth impregnation after steam treatment 2.2 1.4 0.8 Example 4 A Zinc and rare earth impregnation followed by steam treatment 2.1 1.8 0.3 comparative example f Zn immersion only without steam treatment 2.3 0.9 1.4

Table 2 Inlet temperature, °C Product yield, weight % H ₂ C ₁ +C ₂ C ₃ +C ₄ BTX Total Aromatics C ₅ ⁺ aroma 530550 2.12.6 17.021.5 29.121.8 46.048.7 51.053.7 99.299.7

table 3 Catalyst number A B C Product yield, weight % H ₂ 3.1 2.9 3.2 methane ethane 18.7 16.9 19.0 gaseous olefins 4.8 5.4 4.6 BTX 48.1 47.5 51.1 Total Aromatics 51.5 51.3 54.2 Aromatic content of C ₅ ⁺ products, weight % 94.9 96.1 95.6

Table 4

table 5 Continuous response time, hours Product yield, weight % Aromatic content of C ₆ ⁺ products, weight % BTX total yield 4090138186234282330378450 43.444.043.943.342.442.142.040.739.2 50.249.649.348.447.646.246.545.343.2 99.9999.9999.9999.9999.9999.9999.9899.9599.85

Table 6 Initial 20-40 hours activity, wt% Average yield per pass, wt% Total cumulative time, hours Aromatics yield _H2 yield Butane conversion Response time, hours Aromatics H ₂ fresh catalyst 51.2 2.4 93.4 300 47.7 2.3 300 first regeneration 50.9 2.6 95.1 300 47.5 2.3 600 second regeneration 51.1 2.5 94.8 300 47.2 2.3 900 third regeneration 51.0 2.5 95.2 300 47.9 2.3 1200

Claims (10)

1, a kind of aromatizing catalyst for light hydrocarbon contains Zn, rare earth and HZSM-5 component, it is characterized in that this catalyzer has following composition:

Zn 0.8～3.5 heavy %

Mixed rare earth oxide 0.2～1.5 heavy %

Carrier 95.0～99.0 heavy %

Wherein carrier is made up of the HZSM-5 zeolite of 50～80 heavy % and the gama-alumina of 20～50 heavy %, contains lanthanum trioxide 20～40 heavy %, cerium oxide 40～60 heavy %, Praseodymium trioxide 10～18 heavy %, Neodymium trioxide 2～10 heavy % in the mixed rare earth oxide.

2, the described Preparation of catalysts method of a kind of claim 1 is characterized in that comprising the steps:

(1) preparation carrier: HZSM-5 zeolite and α-water aluminum oxide powder are pressed zeolite: the butt weight ratio mixing moulding of Al2O3=50～80: 50～20, dry back makes carrier 400～600 ℃ of roastings,

(2) introduce the zinc constituent element: with the carrier [Zn (NH of preparation in (1) ₃) ₄] ²⁺Complex ion solution at room temperature floods 2～24 hours, Gu liquid during dipping/than being 1.0～1.4 milliliters/gram, [Zn (NH ₃) ₄] ²⁺The consumption of complex ion solution should make wherein Zn content account for 0.8～3.5 heavy % of catalyzer total amount, makes the catalyzer that draws zinc after the drying,

(3) introduce the rare earth constituent element: the zinc catalyst of making in (2) that draws was flooded 2～24 hours with the aqueous solution of chlorination mishmetal under room temperature, chlorination mishmetal consumption is counted 0.2～1.5% of catalyst weight by its oxide compound, Gu liquid during dipping/than being 0.6～0.9 milliliter/gram, dipping back solid drying, 500～600 ℃ of roastings 2～6 hours in airflow again

(4) steam-treated: the catalyzer of preparation in (3) is warming up to 500～600 ℃ in airflow after, used the pure water steam treatment instead 1～6 hour, the weight ratio of total water amount and catalyzer is 1～10 during processing.

3, in accordance with the method for claim 2, it is characterized in that the described HZSM-5 zeolite of step (1) replaces with ammonia type ZSM-5 zeolite.

4, in accordance with the method for claim 3, it is characterized in that making after ammonia type ZSM-5 zeolite is exchanged through ammonium by sodium type ZSM-5 zeolite, the ammonium exchange is carried out before preparing carriers or after the shaping and roasting, and sodium content is less than 0.1 heavy % in the zeolite after the exchange of gained ammonium.

5, in accordance with the method for claim 4, it is characterized in that sodium content is less than 0.02 heavy % in the zeolite after the ammonium exchange.

6, according to the described any method of claim 2～4, the silica alumina ratio that it is characterized in that used zeolite in the step (1) is 30～100.

7, in accordance with the method for claim 2, it is characterized in that each constituent content in oxide compound is in the middle chlorination mishmetal of described step (3): lanthanum 20～40 heavy %, cerium 40～60 heavy %, praseodymium 10～18 heavy %, neodymium 2～10 heavy %.

8, in accordance with the method for claim 2, it is characterized in that temperature is 520～580 ℃ in the step (4), the steam-treated time is 1～4 hour, and the weight ratio of total water amount and catalyzer is 2～4 during processing.

9, in accordance with the method for claim 2, it is characterized in that introducing mishmetal step (3) earlier, introduce the step (2) of zinc again, drawing the zinc rear catalyst needs to carry out the steam-treated of step (4) again 500～600 ℃ of roastings.

10, in accordance with the method for claim 2, it is characterized in that after step (1) has prepared carrier, promptly to carry out the steam-treated of step (4), and then carry out step (2) and (3).