Rare earth metal doped H-M-ZSM-48 molecular sieve as well as preparation method and application thereof

Abstract

The invention discloses a rare earth metal doped ZSM-48 molecular sieve (H-M-ZSM-48), a preparation method and application thereof, and application of the rare earth metal doped ZSM-48 molecular sieve to improvement of the performance of the ZSM-48 molecular sieve. According to the present invention, the specific rare earth metal is introduced into the synthesis liquid during the ZSM-48 molecular sieve synthesis stage, the rare earth metal is doped in the ZSM-48 molecular sieve skeleton through the hydrothermal synthesis, the acidic sites of the molecular sieve are moderately reduced, the acidic site distribution of the molecular sieve is changed, and the rare earth metal doped H-M-ZSM-48 molecular sieve is prepared. Furthermore, the H-M-ZSM-48 molecular sieve doped with the rare earth metal is used as a carrier, and is used for a hydroisomerization reaction of long-chain n-alkanes after loading noble metal (such as platinum). Due to the low acid strength, the concentration of B acid sites and appropriate acid site distribution, the target product isoparaffin can be obtained with high activity and high selectivity, the generation of cracking products is inhibited, and the proportion of single-branched chain products is remarkably increased.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical and chemical heterogeneous catalysis, and specifically relates to a rare earth metal-doped HM-ZSM-48 molecular sieve, a preparation method and application thereof. Background Art

[0002] Normal alkane hydroisomerization catalysts are generally bifunctional catalysts, with molecular sieves as carriers and providing acidic sites, and loaded with precious metals to play the role of hydrogenation and dehydrogenation. The topological structure type of ZSM-48 molecular sieve is *MRE, with a one-dimensional ten-membered ring straight channel and a pore size of 0.53nm×0.56nm. These channel characteristics are suitable for the hydroisomerization reaction of long-chain normal alkanes, and its special channel structure has a high selectivity for isomerized products.

[0003] However, the acid sites of ZSM-48 molecular sieve crystals synthesized according to the prior art method are too many and too strong. For the hydrogenation isomerization reaction of long-chain normal alkanes, the molecular sieve has more acid sites and too strong acid strength, which increases the possibility of the isomerized product contacting the acid sites again, causing it to further isomerize or even crack, making the reaction have a lower selectivity for isomerized products.

[0004] Patent CN 116332201 A prepares a ZSM-48 molecular sieve with high dispersibility and low acidity by adding phenolic molecules after homogenizing and mixing the raw materials, and then undergoing processes such as crystallization, calcination, and ion exchange. The catalyst using the molecular sieve as a carrier can increase the yield of isomerized products in the hydrogenation isomerization reaction of long-chain normal alkanes, but the in-situ addition of phenolic molecules will greatly prolong the crystallization time of the molecular sieve.

[0005] Therefore, people hope to prepare ZSM-48 zeolite with lower acidity through a simple method, and further load it with precious metal platinum for use in the hydrogenation isomerization reaction of long-chain normal alkanes, thereby reducing the possibility of the product further contacting the acidic sites, inhibiting the further isomerization and cracking of the isomerized products at the acidic sites of the zeolite, and improving the isomerization selectivity of the reaction.

[0006] In order to solve the above problems, the present invention is proposed. Summary of the invention

[0007] The present invention aims to provide a method for preparing rare earth metal-doped ZSM-48 molecular sieve framework aluminum using simple hydrothermal synthesis, and loading precious metal platinum on rare earth metal-doped HM-ZSM-48 molecular sieve for hydrogenation isomerization reaction of long-chain normal alkanes.

[0008] The present invention discloses a rare earth metal doped ZSM-48 molecular sieve (HM-ZSM-48), a preparation method and application thereof, and a purpose of improving the performance of ZSM-48 molecular sieve by rare earth metal doping. The present invention introduces a specific rare earth metal into a synthetic liquid during the synthesis stage of ZSM-48 molecular sieve, realizes the doping of rare earth metal in the framework of ZSM-48 molecular sieve by hydrothermal synthesis, moderately reduces the acid sites of the molecular sieve, changes the distribution of the acid sites of the molecular sieve, and prepares the HM-ZSM-48 molecular sieve doped with rare earth metals. Further, the HM-ZSM-48 molecular sieve doped with rare earth metals is used as a carrier, and loaded with precious metals (such as platinum) for the hydrogenation isomerization reaction of long-chain normal alkanes. Due to its lower acid strength and the concentration of B acid sites and the appropriate acid site distribution, the target product isoparaffin can be obtained with high activity and high selectivity, the generation of cracking products is suppressed, and the proportion of single branched products is significantly increased.

[0009] The method adopted by the present invention is: the synthesis of HM-ZSM-48 molecular sieve requires components such as silicon source, aluminum source, alkali, template, water, rare earth metal source, etc. After the above components are fully mixed, the mixture is then transferred to a hydrothermal synthesis kettle for hydrothermal reaction. After the reaction is completed, washing, drying, demoulding, ion exchange and other procedures are performed to obtain a hydrogen molecular sieve. After the hydrogen molecular sieve is loaded with precious metal platinum, it can be used for the hydrogenation isomerization conversion reaction of long-chain normal alkanes after roasting, reduction and other processes.

[0010] On the one hand, the present invention provides a rare earth metal doped HM-ZSM-48 molecular sieve, such as Figure 2 As shown, the HM-ZSM-48 molecular sieve has an aggregate of 2 to 3 μm, and the aggregate is composed of crystal clusters growing in multiple directions. The length of the larger crystal cluster is more than 1 μm, and the length of the smaller crystal cluster is less than 300 nm. The single crystal in the crystal cluster is columnar, with a diameter of 10 to 50 nm and a length of 300 to 900 nm. More preferably, the width of a single crystal is about 30 nm.

[0011] The second aspect of the present invention provides a method for preparing the rare earth metal-doped HM-ZSM-48 molecular sieve according to the first aspect of the present invention, which comprises the following steps:

[0012] (1) Water, alkali solution, template solution, silicon source, aluminum source and rare earth metal source are added to the crystallization kettle in sequence and stirred to mix evenly, and then the mixed solution is subjected to crystallization reaction. After the hydrothermal reaction is carried out for 48 hours, the mixture is naturally cooled to room temperature after the crystallization is completed.

[0013] (2) washing the product after the crystallization reaction with water until it is neutral, centrifuging or filtering, and drying to obtain a rare earth metal-doped M-ZSM-48 molecular sieve;

[0014] (3) calcining the molecular sieve formed after rare earth metal doping in a muffle furnace to remove the template agent to obtain Na / KM-ZSM-48 molecular sieve;

[0015] (4) Converting the Na / KM-ZSM-48 molecular sieve into the HM-ZSM-48 molecular sieve to obtain the rare earth metal-doped HM-ZSM-48 molecular sieve.

[0016] Preferably, in step (1), the aluminum source is selected from one or more of silica-alumina sol, sodium aluminate or alumina sol; the silicon source is selected from one or more of silica sol, silica aerosol, tetraethyl orthosilicate (TEOS) or sodium silicate with a SiO2 content of 30% to 40%; the alkali solution is an aqueous solution of NaOH or KOH with a molar concentration of 0.6 to 1.2 mol / L; the rare earth metal source is selected from one or more rare earth metal salts of yttrium nitrate hexahydrate, scandium nitrate hexahydrate, europium nitrate hexahydrate, terbium nitrate hexahydrate, erbium nitrate hexahydrate, cerium nitrate hexahydrate, and lanthanum nitrate hexahydrate; the template is selected from one or more of hexamethylammonium bromide, pentamethylammonium bromide, and 1,6-hexanediamine. More preferably, the template is selected from hexamethylammonium bromide.

[0017] Among them, silica-alumina sol, such as 056 silica-alumina sol or 007 silica-alumina sol (Nalco); silica sol, such as AS-30, AS-40, etc.

[0018] Preferably, in step (1), the molar ratio of the aluminum source, water, alkali solution, template solution, silicon source and rare earth metal source is:

[0019] scope <![CDATA[SiO2 / Al2O3]]> 10-400 <![CDATA[H2O / SiO2]]> 30-80 <![CDATA[OH - / SiO2]]> 0.2-0.5 <![CDATA[Q / SiO2]]> 0.05-0.15 <![CDATA[Al2O3 / M2O3]]> 9-0.5

[0020] The amount of alkali solution is OH - The aluminum source is calculated as Al2O3, the silicon source is calculated as SiO2, the rare earth metal is calculated as M2O3, and wherein Q represents a template.

[0021] Preferably, in step (1), the crystallization reaction temperature is 120-200° C., and the crystallization reaction time is 2-3 days.

[0022] Preferably, in step (2), the drying temperature is 80 to 140° C. and the drying time is 2 to 6 hours.

[0023] Preferably, the calcination conditions in step (3) are: first calcining at 450-600°C for 5-8h, and then calcining at 550-650°C for 4-6h.

[0024] Preferably, in step (4), the method for converting the Na / KM-ZSM-48 molecular sieve into the HM-ZSM-48 molecular sieve is as follows: according to the mass ratio of ammonium chloride to molecular sieve being 10, the Na / KM-ZSM-48 molecular sieve is dispersed in a 0.6-1.5 mol / L aqueous solution of ammonium chloride, refluxed at 65-85°C for 2-6 hours, and dried at 80-140°C for ion exchange, and then calcined in a muffle furnace at 400-600°C for 2-6 hours to obtain the HM-ZSM-48 molecular sieve.

[0025] Preferably, the ion exchange step in step (4) can be performed multiple times to ensure complete ion exchange.

[0026] The third aspect of the present invention provides a HM-ZSM-48 molecular sieve catalyst loaded with precious metal platinum, using the rare earth metal doped HM-ZSM-48 molecular sieve described in the first aspect of the present invention as a carrier, which is loaded with precious metal platinum and used for the hydrogenation isomerization reaction of long-chain normal alkanes.

[0027] The fourth aspect of the present invention provides a method for improving the isomerization performance of ZSM-48 molecular sieve by rare earth metal doping, characterized in that the present invention reduces the acid strength and B acid site concentration of ZSM-48 molecular sieve by adding rare earth metal in situ during the hydrothermal synthesis stage of ZSM-48 molecular sieve, and replaces the skeleton aluminum atom site with rare earth metal, thereby enhancing the isomerization performance of the catalyst using ZSM-48 molecular sieve as a carrier. This method can also be called a method for reducing the acid strength and B acid concentration of ZSM-48 molecular sieve by adding rare earth metal to the hydrothermal synthesis solution.

[0028] The fifth aspect of the present invention provides an application of the rare earth metal-doped HM-ZSM-48 molecular sieve described in the first aspect of the present invention for the hydrogenation isomerization reaction of long-chain normal alkanes. The rare earth metal-doped HM-ZSM-48 molecular sieve described in the first aspect of the present invention is used as a carrier to load it with precious metal platinum and then used for the hydrogenation isomerization reaction of long-chain normal alkanes, which can improve the selectivity of isoalkanes and increase the proportion of single-branched products.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. The present invention reduces the acid strength and B acid concentration of the molecular sieve by adding a rare earth metal source in situ during the hydrothermal synthesis stage of the ZSM-48 molecular sieve, and replaces the framework aluminum atoms with rare earth metals, thereby preparing a HM-ZSM-48 molecular sieve with low acidity. Molecular sieves are usually synthesized by a hydrothermal method. During the synthesis process, the mixed silicon precursor, aluminum precursor and rare earth metal precursor form a primary amorphous mixture; during the hydrothermal process, the components of the solid phase and the liquid phase are redistributed and reach a state of equilibrium, and then a partial regular structure (crystal nucleus) is formed on some "ordered islands" containing templates, and the rare earth metals and aluminum atoms added thereto form a framework structure with silicon. The crystalline units decomposed from the amorphous part grow in connection on the crystal nucleus until the amorphous matrix is ​​consumed and the crystals reach equilibrium with the mother liquid components. In the present invention, by adding rare earth metals during the hydrothermal process, the skeleton structure composition of the molecular sieve is adjusted, and the acid strength and B acid site concentration of the molecular sieve crystals are reduced.

[0031] 2. The present invention regulates the acid strength and B acid concentration of HM-ZSM-48 molecular sieve crystals by controlling the type and amount of rare earth metal added. Wherein, when the rare earth type is the rare earth metal source and the amount of rare earth metal added is 0.2-1.0wt% of the mass of the silicon source, HM-ZSM-48 molecular sieve crystals with suitable acid strength and B acid concentration are prepared.

[0032] 3. The preparation method of the present invention requires the addition of rare earth metals to change the molecular sieve framework structure.

[0033] 4. Experiments have shown that at the respective optimal isomerization reaction temperatures, that is, at the reaction temperatures at which each catalyst reaches the highest isomerization rate, the isomerization rate and isomerization selectivity of the rare earth metal-doped Pt / HM-ZSM-48 catalyst are greatly improved compared to the normal synthesized ordinary catalyst. The molar ratio of multi-branched products to single-branched products in the comparative example is 0.5157, while that in Example 1 drops to 0.3624. The above results show that the rare earth metal-doped HM-ZSM-48 molecular sieve catalyst prepared by the present invention is applied to the hydrogenation isomerization reaction of normal alkanes. Due to its lower acid strength and B acid site concentration, it can highly selectively produce an isomerized alkane mixture, inhibit the formation of cracking products, and significantly increase the proportion of single-branched products. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 XRD (X-ray diffractometer) analysis of Example samples 1-6 and Comparative Example sample 1;

[0035] Figure 2 The SEM images of Example Samples 1-6 and Comparative Example Sample 1;

[0036] Figure 3The relationship between temperature and conversion rate of the catalyst obtained by using molecular sieves of comparative example 1 and example samples 1-6 as carriers and loading precious metal platinum in the hydrogenation isomerization reaction of n-hexadecane;

[0037] Figure 4 The relationship between the conversion rate and isomerization selectivity of the catalyst obtained by using the molecular sieves of comparative example 1 and example samples 1-6 as carriers and loading precious metal platinum in the hydrogenation isomerization reaction of n-hexadecane;

[0038] Figure 5 The relationship between the conversion rate and the isomerization rate of the catalyst obtained by using the molecular sieves of comparative example 1 and example samples 1-6 as carriers and loading precious metal platinum in the hydrogenation isomerization reaction of n-hexadecane;

[0039] Figure 6 The catalyst obtained by using the molecular sieves of comparative example 1 and example samples 1-6 as carriers and loading precious metal platinum is used in the hydrogenation isomerization reaction of n-hexadecane, and the ratio relationship diagram of the conversion rate isomerization product between the single-branched isomerization product and the multi-branched isomerization product;

[0040] Figure 7 This is a graph showing the relationship between temperature, isomerization rate and isomerization selectivity of the catalyst obtained by using the molecular sieve of Example Sample 1 as a carrier and loading precious metal platinum in the hydrogenation isomerization reaction of n-hexadecane. DETAILED DESCRIPTION

[0041] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the following examples are illustrative, not restrictive, and the scope of protection of the present invention cannot be limited by the following examples. The raw materials required in the following examples and comparative examples are all commercially available.

[0042] Example 1: (Y-doped ZSM-48 is denoted as HY-ZSM-48)

[0043] Completely dissolve 2.36g NaOH in 41.63g deionized water, then slowly add 5.0g white carbon black to the solution and stir for 30min. Then add 3.01g hexamethylammonium bromide and stir for 30min. Finally, add 21.42g of a mixed aqueous solution of aluminum sulfate and yttrium nitrate (prepared with 0.30g Al2(SO4)3·18H2O, 0.06g Y(NO3)3·6H2O, 18.82g deionized water and 1.78g concentrated sulfuric acid) dropwise and stir for 30min. Seal the polytetrafluoroethylene liner in a stainless steel high-pressure hydrothermal autoclave, fix the hydrothermal autoclave in a homogeneous reactor, and crystallize for 2 days at an autogenous pressure of 160°C.

[0044] The crystallized product was taken out, cooled, filtered and washed until the filtrate was neutral, and dried in an oven at 120°C; then placed in a muffle furnace, heated to 540°C at a heating rate of 1°C / min, maintained for 3h; heated to 595°C at a heating rate of 1°C / min, maintained for 3h; cooled to room temperature at a cooling rate of 1°C / min. The obtained powder is sodium-type M-ZSM-48. The powder was then dispersed in a 1mol / L aqueous solution of ammonium chloride with a liquid-solid ratio of 10, refluxed at 80°C for 4h, filtered and dried in an oven at 120°C, and then the ion exchange was repeated twice. Finally, the sample was calcined in a muffle furnace: heated to 550°C at a heating rate of 5°C / min, maintained for 4h, and then cooled to room temperature at a cooling rate of 1°C / min. The obtained powder is a hydrogen-type molecular sieve. The Y-substituted HY-ZSM-48 molecular sieve sample 1 was obtained.

[0045] The obtained product was analyzed by XRD (X-ray diffractometer) and its structure was determined to be HY-ZSM-48 molecular sieve, see Figure 1 .

[0046] Example 2: (Tb-doped ZSM-48 is denoted as H-Tb-ZSM-48)

[0047] The specific implementation conditions are similar to those of Example 1, but at the end of the hydrothermal synthesis, 21.42 g of a mixed aqueous solution of aluminum sulfate and yttrium nitrate (prepared from 0.30 g of Al2(SO4)3·18H2O, 0.07 g of Tb(NO3)3·6H2O, 18.82 g of deionized water and 1.78 g of concentrated sulfuric acid) was added dropwise and stirred for 30 min. The molecular sieve obtained was determined to be H-Tb-ZSM-48 molecular sieve by XRD analysis. Figure 1 .

[0048] Example 3: (Sc-doped ZSM-48 is denoted as H-Sc-ZSM-48)

[0049] The specific implementation conditions are similar to those of Example 1, but at the end of the hydrothermal synthesis, 21.42 g of a mixed aqueous solution of aluminum sulfate and scandium nitrate (prepared from 0.30 g of Al2(SO4)3·18H2O, 0.04 g of Tb(NO3)3·6H2O, 18.82 g of deionized water and 1.78 g of concentrated sulfuric acid) was added dropwise and stirred for 30 min. The molecular sieve obtained was determined to be H-Sc-ZSM-48 molecular sieve by XRD analysis. Figure 1 .

[0050] Example 4: (Eu-doped ZSM-48 is denoted as H-Eu-ZSM-48)

[0051] The specific implementation conditions are similar to those of Example 1, but at the end of the hydrothermal synthesis, 21.42 g of a mixed aqueous solution of aluminum sulfate and yttrium nitrate (prepared from 0.30 g of Al2(SO4)3·18H2O, 0.07 g of Eu(NO3)3·6H2O, 18.82 g of deionized water and 1.78 g of concentrated sulfuric acid) was added dropwise and stirred for 30 min. The molecular sieve obtained was determined to be H-Eu-ZSM-48 molecular sieve by XRD analysis. Figure 1 .

[0052] Example 5: (Er-doped ZSM-48 is denoted as H-Er-ZSM-48)

[0053] The specific implementation conditions are similar to those of Example 1, but at the end of the hydrothermal synthesis, 21.42 g of a mixed aqueous solution of aluminum sulfate and yttrium nitrate (prepared from 0.30 g of Al2(SO4)3·18H2O, 0.07 g of Er(NO3)3·6H2O, 18.82 g of deionized water and 1.78 g of concentrated sulfuric acid) was added dropwise and stirred for 30 min. The molecular sieve obtained was determined to be H-Er-ZSM-48 molecular sieve by XRD analysis. Figure 1 .

[0054] Example 6: (Ce-doped ZSM-48 is denoted as H-Ce-ZSM-48)

[0055] The specific implementation conditions are similar to those of Example 1, but at the end of the hydrothermal synthesis, 21.42 g of a mixed aqueous solution of aluminum sulfate and yttrium nitrate (prepared from 0.30 g of Al2(SO4)3·18H2O, 0.07 g of Ce(NO3)3·6H2O, 18.82 g of deionized water and 1.78 g of concentrated sulfuric acid) was added dropwise and stirred for 30 min. The molecular sieve obtained was determined to be H-Ce-ZSM-48 molecular sieve by XRD analysis. Figure 1 .

[0056] Comparative Example: (ZSM-48 without rare earth element doping is recorded as ZSM-48)

[0057] The specific implementation conditions are similar to those in Example 1, but at the end of the hydrothermal synthesis, 21.42 g of aluminum sulfate aqueous solution (prepared from 0.30 g of Al2(SO4)3·18H2O, 18.82 g of deionized water and 1.78 g of concentrated sulfuric acid) was added dropwise and stirred for 30 min. The obtained molecular sieve was determined to be ZSM-48 molecular sieve by XRD analysis. Figure 1 .

[0058] Example 7: Hydroisomerization reaction of n-hexadecane

[0059] The rare earth metal-substituted molecular sieve samples 1-6 obtained in the above examples 1-6 and the ordinary ZSM-48 molecular sieve comparison samples obtained in the comparative example were used as carriers, loaded with precious metal platinum, and applied to the hydrogenation isomerization reaction of n-hexadecane. The loading amount of platinum is 0.3wt%. The loading method is to first take 0.5g of the molecular sieve sample for saturated water test, then add 700μl of chloroplatinic acid aqueous solution (0.1mol / L) to 4.5g of HM-ZSM-48 molecular sieve, and after the equal volume impregnation method, stand for 12h, then roast in an oven, and dry at 120°C for 4h. After taking it out and cooling to room temperature, use a tablet press to shape it into particles of 20 to 40 mesh sizes. The catalyst is loaded in a micro fixed bed reactor and filled with silicon carbide at both ends. Before the evaluation begins, it is reduced at 400°C for 4h under a hydrogen atmosphere. The reaction conditions are: pressure 3MPa, air velocity 2h -1 , hydrogen to oil ratio 1000, temperature 260~320℃.

[0060] The SEM images of the seven catalysts of Comparative Example 1 and Example Samples 1-6 are as follows: Figure 2 As shown, from Figure 2 It can be seen that the HM-ZSM-48 molecular sieve prepared by the present invention has aggregates of 2 to 3 μm, and the aggregates are composed of crystal clusters growing in multiple directions. The length of the larger crystal cluster is more than 1 μm, and the length of the smaller crystal cluster is less than 300 nm. The single crystal in the crystal cluster is columnar, with a diameter of 10 to 50 nm and a length of 300 to 900 nm. More preferably, the width of a single crystal is about 30 nm.

[0061] The conversion rate, isomerization selectivity, isomerization rate, and ratio of single-branched isomerization product to multi-branched isomerization product in isomerization product of the seven catalysts of comparative example 1 and example samples 1-6 are shown as follows: Figure 3 , Figure 4 , Figure 5 , Figure 6 shown.

[0062] Figure 7 The catalyst obtained by using the molecular sieve of Example 1 as a carrier and loading precious metal platinum is used in the hydrogenation isomerization reaction of n-hexadecane. The relationship diagram between the reaction temperature and the isomerization rate (shown by ■) and the isomerization selectivity (shown by ●) is shown.

[0063] The product analysis results at the respective optimal isomerization reaction temperatures are shown in Table 1.

[0064] It can be seen from Table 1 that at the respective optimal isomerization reaction temperatures, that is, at the reaction temperatures at which each catalyst reaches the highest isomerization rate, the isomerization rate and isomerization selectivity of the rare earth atom-doped Pt / HM-ZSM-48 catalyst are greatly improved compared to the normal crystallized ordinary catalyst. The molar ratio of multi-branched products to single-branched products in the comparative example is 0.5157, while that in Example 1 drops to 0.3624. The above results show that the HM-ZSM-48 molecular sieve catalyst with low acidity and B acid concentration prepared by the present invention is applied to the isomerization and decondensation reaction of normal alkanes. Due to its fewer acidic sites, it is possible to obtain an isomerized alkane mixture with high selectivity, inhibit the formation of cracking products, and significantly increase the proportion of single-branched products.

[0065] Table 1 Comparison of catalytic performance results

[0066]

[0067]

[0068]

Note

[0069] The present invention is described above by way of example. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by those skilled in the art without inventive effort falls within the protection scope of the present invention.

Claims

1. A rare earth metal doped HM-ZSM-48 molecular sieve, characterized in that: Rare earth metals are substituted in the aluminum atoms of the ZSM-48 molecular sieve framework. The rare earth metal-doped HM-ZSM-48 molecular sieve has aggregates of 2 to 3 μm, which are composed of crystal clusters growing in multiple directions. The crystals in the crystal clusters are columnar, with a diameter of 10 to 50 nm and a length of 300 to 900 nm.

2. A method for preparing the rare earth metal-doped HM-ZSM-48 molecular sieve according to claim 1, characterized in that: It includes the following steps: (1) adding deionized water, alkali solution, template solution, silicon source, aluminum source, and rare earth metal source into a crystallization kettle, stirring and mixing, and then subjecting the mixture to a crystallization reaction; (2) washing the product after the crystallization reaction with water until it is neutral, centrifuging or filtering, and drying to obtain a rare earth metal-doped M-ZSM-48 molecular sieve; (3) calcining the molecular sieve formed after rare earth metal doping in a muffle furnace to remove the template agent to obtain Na / KM-ZSM-48 molecular sieve; (4) Converting the Na / KM-ZSM-48 molecular sieve into a hydrogen-type M-ZSM-48 molecular sieve to obtain a rare earth metal-doped HM-ZSM-48 molecular sieve.

3. The preparation method according to claim 2, characterized in that: In step (1), the aluminum source is selected from one or more of silica-alumina sol, sodium aluminate or aluminum sol; the silicon source is selected from one or more of silica sol with a SiO2 content of 30% to 40%, silica aerosol, tetraethyl orthosilicate or sodium silicate; the alkali solution is an aqueous solution of NaOH or KOH with a molar concentration of 0.6 to 1.2 mol / L; the rare earth metal source is selected from one or more rare earth metal salts of yttrium nitrate hexahydrate, scandium nitrate hexahydrate, europium nitrate hexahydrate, terbium nitrate hexahydrate, erbium nitrate hexahydrate, cerium nitrate hexahydrate, and lanthanum nitrate hexahydrate; the template agent is selected from one or more of hexamethylammonium bromide, pentamethylammonium bromide, and 1,6-hexanediamine.

4. The preparation method according to claim 3, characterized in that: The molar ratio of the aluminum source, water, alkali solution, template solution, silicon source and rare earth metal source in step (1) is: The amount of alkali solution is OH - The aluminum source is calculated as Al2O3, the silicon source is calculated as SiO2, and the rare earth metal source is calculated as M2O3, wherein Q represents a template.

5. The preparation method according to claim 2, characterized in that: In step (1), the crystallization reaction temperature is 120-200° C., and the crystallization reaction time is 2-3 days; in step (2), the drying temperature is 80-140° C., and the drying time is 2-6 hours.

6. The preparation method according to claim 2, characterized in that: The calcination conditions in step (3) are: first calcining at 450-600°C for 5-8h, and then calcining at 550-650°C for 4-6h.

7. The preparation method according to claim 2, characterized in that: The method for converting the Na / KM-ZSM-48 molecular sieve into the HM-ZSM-48 molecular sieve in step (4) is to disperse the Na / KM-ZSM-48 molecular sieve in a 0.6-1.5 mol / L aqueous solution of ammonium chloride / ammonium nitrate, reflux at 65-85° C. for 2-6 hours, and dry at 80-140° C. to perform ion exchange, and then calcine in a muffle furnace at 400-600° C. for 2-6 hours to obtain the HM-ZSM-48 molecular sieve; the ion exchange step can be performed multiple times to ensure complete ion exchange.

8. A HM-ZSM-48 molecular sieve catalyst loaded with precious metal platinum, characterized in that: The rare earth metal-doped HM-ZSM-48 molecular sieve according to claim 1 is used as a carrier to load precious metal platinum for the hydrogenation isomerization reaction of long-chain normal alkanes.

9. A method for improving the isomerization performance of ZSM-48 molecular sieve by rare earth metal doping, characterized in that: Rare earth metals are added in situ during the hydrothermal synthesis stage of ZSM-48 zeolite. The acid strength and B acid site concentration of ZSM-48 zeolite are reduced by replacing the skeleton aluminum atom sites with rare earth metals, thereby enhancing the isomerization performance of the catalyst with ZSM-48 zeolite as the carrier.

10. Use of the rare earth metal-doped HM-ZSM-48 molecular sieve according to claim 1 for the hydroisomerization reaction of long-chain normal alkanes, characterized in that: The rare earth metal-doped HM-ZSM-48 molecular sieve described in claim 1 is used as a carrier, loaded with precious metal platinum and used for the hydrogenation isomerization reaction of long-chain normal alkanes, which can improve the selectivity of isoalkanes and increase the proportion of single-branched products.

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