Improved process for the preparation of 2,6-dimethylnaphthalene

Abstract

The application relates to the technical field of chemistry and chemical industry, and discloses an improved process for preparing 2,6-dimethylnaphthalene. The method uses 2-methylnaphthalene as raw material, methanol as an alkylating agent, and a molecular sieve containing an active metal as a catalyst, introduces water vapor into the reaction system with hydrogen as a carrier gas in a continuous fixed bed reactor, and controls specific reaction conditions to perform an alkylation reaction. The hydrogen inhibits the generation of carbon deposition precursors, the water vapor controls the reaction selectivity and participates in the carbon removal reaction, the synergistic effect of which prolongs the service life of the catalyst on the basis of maintaining high conversion rate (more than 50%) of 2-methylnaphthalene and high selectivity (more than 55%) of 2,6-dimethylnaphthalene, the conversion rate is reduced by not more than 2% in 50h of continuous reaction, the process is simple, green and environment-friendly, raw materials are widely sourced, and the method has good industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of chemical engineering technology, specifically relating to an improved process for preparing 2,6-dimethylnaphthalene, and more particularly to an improved process for preparing 2,6-dimethylnaphthalene by alkylation reaction of 2-methylnaphthalene with methanol. Background Technology

[0002] Polyethylene naphthalene glycol (PEN) is a high-performance polyester material. The naphthalene ring in its molecular chain exhibits strong conjugation and high rigidity, giving PEN significantly superior mechanical properties, heat resistance, and airtightness compared to traditional polyester materials, making its application prospects very broad. However, 2,6-dimethylnaphthalene, as the core synthetic raw material for PEN polymerization, suffers from high production costs, becoming a key bottleneck restricting the large-scale production and application of PEN. Therefore, developing efficient and low-cost synthetic routes for 2,6-dimethylnaphthalene has significant industrial value and research significance.

[0003] Existing processes for preparing 2,6-dimethylnaphthalene are mainly divided into two categories: direct extraction and chemical synthesis. Direct extraction involves distilling and enriching the 255-275℃ fraction from catalytically cracked diesel or coal tar wash oil to obtain a mixture of dimethylnaphthalenes, which is then purified through melt crystallization, solution crystallization, and other methods. This method can achieve high-value utilization of low-value aromatic resources, but it suffers from problems such as low 2,6-dimethylnaphthalene content, complex composition, and high separation costs, making it difficult to meet the needs of large-scale production.

[0004] In chemical synthesis, the alkylation reaction of naphthalene or 2-methylnaphthalene with methanol is a preferred option for reducing the production cost of 2,6-dimethylnaphthalene due to its abundant and inexpensive raw material sources and short process route, and has become a research hotspot in the field of new chemical materials. Based on electron density theory calculations, when 2-methylnaphthalene is used as a raw material, its electron density at position 6 is higher than that at position 7, exhibiting higher selectivity for 2,6-dimethylnaphthalene compared to naphthalene as a raw material. Currently, numerous publications report the use of molecular sieves such as ZSM-5, ZSM-12, Hβ, MCM-22, MOR, and HY as catalysts for this alkylation reaction. Among them, the MCM-22 molecular sieve catalysis technology jointly developed by ExxonMobil (USA) and Kobe Steel (Japan) has been industrialized.

[0005] However, key technical challenges remain in the existing technology: in the alkylation reaction of 2-methylnaphthalene and methanol, methanol is more prone to self-conversion to form low-carbon olefins, which in turn generate carbon precursors such as polycyclic aromatic hydrocarbons, clogging catalyst pores and active sites, leading to rapid catalyst deactivation. Although modifications such as adjusting the acidity and pore structure of molecular sieve catalysts can suppress side reactions such as isomerization and disproportionation to some extent, reduce carbon formation, and lower the catalyst deactivation rate, it is still difficult to meet the requirements of long-term industrial operation. Therefore, it is urgent to improve the alkylation reaction process of 2-methylnaphthalene and methanol to enhance the overall performance of the catalyst, especially its stability. Summary of the Invention

[0006] To address the problems of poor catalyst stability and short service life in the existing process of preparing 2,6-dimethylnaphthalene by alkylation of 2-methylnaphthalene with methanol, this invention provides an improved process for preparing 2,6-dimethylnaphthalene. While maintaining the catalyst's reactivity and selectivity for 2,6-dimethylnaphthalene, this method significantly extends the catalyst's service life, meeting the requirements for long-term industrial operation.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] An improved process for preparing 2,6-dimethylnaphthalene is disclosed, which uses 2-methylnaphthalene as raw material, methanol as alkylating agent, and molecular sieve loaded with active metal as catalyst. In a continuous fixed-bed reactor, hydrogen is used as carrier gas, and water vapor is introduced into the reaction system to control the reaction conditions for alkylation reaction. The product is then separated and purified to obtain 2,6-dimethylnaphthalene.

[0009] Furthermore, the active metal is selected from at least one of Fe, Co, Ni, Cu, Pt, and Pd, preferably Ni.

[0010] Furthermore, the loading of the active metal on the molecular sieve is 0.5wt% to 20wt%, preferably 2wt% to 5wt%.

[0011] Further, the reaction conditions include: a reaction temperature of 300℃~450℃, a fixed-bed reactor pressure of 0.1MPa~10MPa, a hydrogen carrier gas flow rate of 5mL / min~100mL / min, and a feed liquid mass hourly space velocity of... .

[0012] Further, the reaction temperature is preferably 380℃~420℃, the fixed-bed reactor pressure is preferably 0.1MPa~4MPa, the hydrogen carrier gas flow rate is preferably 10mL / min~40mL / min, and the feed liquid mass hourly space velocity is preferably... .

[0013] Furthermore, the molar ratio of 2-methylnaphthalene to methanol is 1:1 to 1:20, preferably 1:2 to 1:10.

[0014] Furthermore, the molar ratio of water vapor to methanol is 1:10 to 5:1, preferably 2:1 to 4:1.

[0015] Further, the 2-methylnaphthalene is obtained by one of the following methods: direct extraction from coal tar or catalytic cracking petroleum; separation from the alkylation reaction product of naphthalene; or separation from the transalkylation reaction product of naphthalene and polymethylnaphthalene, wherein the separation process includes at least one of distillation, freeze crystallization, and recrystallization.

[0016] Furthermore, the catalyst needs to be activated in a hydrogen atmosphere before use, with an activation temperature of 450℃~650℃ and an activation time of 0.5h~2h.

[0017] Furthermore, the reaction tube of the continuous fixed-bed reactor is a stainless steel tube, the catalyst is loaded in the constant temperature zone of the reactor, and inert quartz sand is filled at both ends of the catalyst.

[0018] The improved process for preparing 2,6-dimethylnaphthalene has the following beneficial effects:

[0019] The raw materials involved in this invention are widely available, and can be directly extracted from coal tar and catalytic cracking petroleum, or separated from the products of alkylation of naphthalene or the transalkylation reaction of naphthalene and polyalkylnaphthalene, thus fully realizing the efficient utilization of low-value-added products. Compared with the international mainstream o-xylene and butadiene reaction route, the technical route adopted in this invention is simpler, easier to operate, more environmentally friendly, and has lower equipment costs. In the process method adopted in this invention, methanol is used as both a reactant and a solvent, avoiding the problem of carbon deposition on the catalyst caused by the cracking of conventional solvents at high temperatures. Hydrogen is used as a carrier gas to hydrogenate and saturate the carbon deposition precursor, inhibiting the formation of carbon deposits at the source. At the same time, the introduction of water vapor increases the selectivity of 2,6-dimethylnaphthalene, inhibits the dehydration of methanol to form low-carbon olefins, and can also participate in the decarbonization reaction, effectively enhancing the stability of the catalyst. Under the 2-methylnaphthalene alkylation process conditions employed in this invention, the conversion rate of 2-methylnaphthalene can reach over 50% and the selectivity of 2,6-dimethylnaphthalene can reach over 55% after 2 hours of reaction. Furthermore, even after 50 hours of continuous reaction, the conversion rate of 2-methylnaphthalene decreases by no more than 2%, and the selectivity is slightly improved. This invention features a simple process route, effectively extends catalyst lifespan, and has promising prospects for industrial application. Attached Figure Description

[0020] Figure 1 This is a process flow diagram for preparing 2,6-dimethylnaphthalene according to the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in detail below with reference to specific embodiments. The following embodiments are for the preparation of 2,6-dimethylnaphthalene by alkylation reaction of naphthalene and methanol, but the present invention includes, but is not limited to, these embodiments.

[0022] Example 1

[0023] 2-Methylnaphthalene was extracted from the alkylation product of naphthalene. The reaction solution was subjected to vacuum distillation to enrich the 2-methylnaphthalene fraction. The fraction was frozen at -10℃ for 3 hours. The precipitated 2-methylnaphthalene was centrifuged to obtain crude 2-methylnaphthalene. Then, it was recrystallized in anhydrous methanol at 0℃ to obtain pure 2-methylnaphthalene.

[0024] The alkylation of 2-methylnaphthalene to prepare 2,6-dimethylnaphthalene was carried out in a continuous fixed-bed reactor using stainless steel tubing with an inner diameter of 10 mm. A catalyst without active metals was loaded into the isothermal zone of the reactor, with inert quartz sand of the same mesh size placed above and below the catalyst. Hydrogen was used as the carrier gas at a flow rate of 30 mL / min, and the pressure in the fixed-bed reactor was maintained at 0.2 MPa. Before the reaction, the catalyst was activated at 500 °C for 2 h in an H2 atmosphere, then the temperature was lowered to 380 °C. After the temperature stabilized, 2-methylnaphthalene and methanol were mixed thoroughly at a molar ratio of 1:10. The feed solution was injected into the reactor using a micro-metering pump, while deionized water was vaporized in a vaporizer and introduced into the reaction system at a molar ratio of 2:1. The alkylation reaction of 2-methylnaphthalene proceeded through the catalyst bed. The mass hourly space velocity (MSV) of the feed solution (based on 2-methylnaphthalene) was [missing value]. The product was collected by condensation and analyzed by gas chromatography. The conversion rate and product selectivity of the reaction were quantitatively calculated, and the results are listed in Table 1.

[0025] Example 2

[0026] 2-Methylnaphthalene was extracted from the alkylation product of naphthalene. The reaction solution was subjected to vacuum distillation to enrich the 2-methylnaphthalene fraction. The fraction was frozen at -10℃ for 3 hours. The precipitated 2-methylnaphthalene was centrifuged to obtain crude 2-methylnaphthalene. Then, it was recrystallized in anhydrous methanol at 0℃ to obtain pure 2-methylnaphthalene.

[0027] The alkylation of 2-methylnaphthalene to prepare 2,6-dimethylnaphthalene was carried out in a continuous fixed-bed reactor using stainless steel tubing with an inner diameter of 10 mm. A catalyst with an active metal Ni content of 2% was loaded into the isothermal zone of the reactor, with inert quartz sand of the same mesh size placed above and below the catalyst. Hydrogen was used as the carrier gas at a flow rate of 30 mL / min, and the pressure in the fixed-bed reactor was maintained at 1.0 MPa. Before the reaction, the catalyst was activated at 500 °C for 2 h in an H2 atmosphere, then the temperature was lowered to 380 °C. After the temperature stabilized, 2-methylnaphthalene and methanol were mixed thoroughly at a molar ratio of 1:10. The feed solution was injected into the reactor using a micro-metering pump, while deionized water was vaporized in a vaporizer and introduced into the reaction system at a water to methanol molar ratio of 4:1. The alkylation reaction of 2-methylnaphthalene occurred through the catalyst bed. The mass hourly space velocity (MSV) of the feed solution (based on 2-methylnaphthalene) was [missing value]. The product was collected by condensation and analyzed by gas chromatography. The conversion rate and product selectivity of the reaction were quantitatively calculated, and the results are listed in Table 1.

[0028] Example 3

[0029] 2-Methylnaphthalene was extracted from the alkylation product of naphthalene. The reaction solution was subjected to vacuum distillation to enrich the 2-methylnaphthalene fraction. The fraction was frozen at -10℃ for 3 hours. The precipitated 2-methylnaphthalene was centrifuged to obtain crude 2-methylnaphthalene. Then, it was recrystallized in anhydrous methanol at 0℃ to obtain pure 2-methylnaphthalene.

[0030] The alkylation of 2-methylnaphthalene to prepare 2,6-dimethylnaphthalene was carried out in a continuous fixed-bed reactor using stainless steel tubing with an inner diameter of 10 mm. A catalyst with an active metal Ni content of 3% was loaded into the isothermal zone of the reactor, with inert quartz sand of the same mesh size placed above and below the catalyst. Hydrogen was used as the carrier gas at a flow rate of 40 mL / min, and the pressure in the fixed-bed reactor was maintained at 1.0 MPa. Before the reaction, the catalyst was activated at 500 °C for 2 h in an H2 atmosphere, then the temperature was lowered to 400 °C. After the temperature stabilized, 2-methylnaphthalene and methanol were mixed thoroughly at a molar ratio of 1:10. The feed solution was injected into the reactor using a micro-metering pump, while deionized water was vaporized in a vaporizer and introduced into the reaction system at a water to methanol molar ratio of 2:1. The alkylation reaction of 2-methylnaphthalene occurred through the catalyst bed. The mass hourly space velocity (MSV) of the feed solution (based on 2-methylnaphthalene) was [missing value]. The product was collected by condensation and analyzed by gas chromatography. The conversion rate and product selectivity of the reaction were quantitatively calculated, and the results are listed in Table 1.

[0031] Example 4

[0032] 2-Methylnaphthalene was extracted from the alkylation product of naphthalene. The reaction solution was subjected to vacuum distillation to enrich the 2-methylnaphthalene fraction. The fraction was frozen at -10℃ for 3 hours. The precipitated 2-methylnaphthalene was centrifuged to obtain crude 2-methylnaphthalene. Then, it was recrystallized in anhydrous methanol at 0℃ to obtain pure 2-methylnaphthalene.

[0033] The alkylation of 2-methylnaphthalene to prepare 2,6-dimethylnaphthalene was carried out in a continuous fixed-bed reactor using stainless steel tubing with an inner diameter of 10 mm. A catalyst with an active metal Ni content of 5% was loaded into the isothermal zone of the reactor, with inert quartz sand of the same mesh size placed above and below the catalyst. Hydrogen was used as the carrier gas at a flow rate of 30 mL / min, and the pressure in the fixed-bed reactor was maintained at 0.2 MPa. Before the reaction, the catalyst was activated at 450 °C for 2 h in an H2 atmosphere, then the temperature was lowered to 380 °C. After the temperature stabilized, 2-methylnaphthalene and methanol were mixed thoroughly at a molar ratio of 1:5. The feed solution was injected into the reactor using a micro-metering pump, while deionized water was vaporized in a vaporizer and introduced into the reaction system at a water to methanol molar ratio of 3:1. The alkylation reaction of 2-methylnaphthalene occurred through the catalyst bed. The mass hourly space velocity (MSV) of the feed solution (based on 2-methylnaphthalene) was [missing value]. The product was collected by condensation and analyzed by gas chromatography. The conversion rate and product selectivity of the reaction were quantitatively calculated, and the results are listed in Table 1.

[0034] Example 5

[0035] 2-Methylnaphthalene was extracted from the alkylation product of naphthalene. The reaction solution was subjected to vacuum distillation to enrich the 2-methylnaphthalene fraction. The fraction was frozen at -10℃ for 3 hours. The precipitated 2-methylnaphthalene was centrifuged to obtain crude 2-methylnaphthalene. Then, it was recrystallized in anhydrous methanol at 0℃ to obtain pure 2-methylnaphthalene.

[0036] The alkylation of 2-methylnaphthalene to prepare 2,6-dimethylnaphthalene was carried out in a continuous fixed-bed reactor using stainless steel tubing with an inner diameter of 10 mm. A catalyst with an active metal Ni content of 2% was loaded into the isothermal zone of the reactor, with inert quartz sand of the same mesh size placed above and below the catalyst. Hydrogen was used as the carrier gas at a flow rate of 30 mL / min, and the pressure in the fixed-bed reactor was maintained at 4 MPa. Before the reaction, the catalyst was activated at 450 °C for 2 h in an H2 atmosphere, then the temperature was lowered to 420 °C. After the temperature stabilized, 2-methylnaphthalene and methanol were mixed thoroughly at a molar ratio of 1:5. The feed solution was injected into the reactor using a micro-metering pump, while deionized water was vaporized in a vaporizer and introduced into the reaction system at a water to methanol molar ratio of 3:1. The alkylation reaction of 2-methylnaphthalene occurred through the catalyst bed. The mass hourly space velocity (MSV) of the feed solution (based on 2-methylnaphthalene) was [missing value]. The product was collected by condensation and analyzed by gas chromatography. The conversion rate and product selectivity of the reaction were quantitatively calculated, and the results are listed in Table 1.

[0037] Example 6

[0038] 2-Methylnaphthalene was extracted from the alkylation product of naphthalene. The reaction solution was subjected to vacuum distillation to enrich the 2-methylnaphthalene fraction. The fraction was frozen at -10℃ for 3 hours. The precipitated 2-methylnaphthalene was centrifuged to obtain crude 2-methylnaphthalene. Then, it was recrystallized in anhydrous methanol at 0℃ to obtain pure 2-methylnaphthalene.

[0039] The alkylation of 2-methylnaphthalene to prepare 2,6-dimethylnaphthalene was carried out in a continuous fixed-bed reactor using stainless steel tubing with an inner diameter of 10 mm. A catalyst with an active metal Ni content of 4% was loaded into the isothermal zone of the reactor, with inert quartz sand of the same mesh size placed above and below the catalyst. Hydrogen was used as the carrier gas at a flow rate of 20 mL / min, and the pressure in the fixed-bed reactor was maintained at 2.0 MPa. Before the reaction, the catalyst was activated at 550 °C for 2 h in an H2 atmosphere, then the temperature was lowered to 380 °C. After the temperature stabilized, 2-methylnaphthalene and methanol were mixed uniformly at a molar ratio of 1:3. The feed solution was injected into the reactor using a micro-metering pump, while deionized water was vaporized in a vaporizer and introduced into the reaction system at a water to methanol molar ratio of 3:1. The alkylation reaction of 2-methylnaphthalene occurred through the catalyst bed. The mass hourly space velocity (MSV) of the feed solution (based on 2-methylnaphthalene) was [missing value]. The product was collected by condensation and analyzed by gas chromatography. The conversion rate and product selectivity of the reaction were quantitatively calculated, and the results are listed in Table 1.

[0040] Example 7

[0041] 2-Methylnaphthalene was extracted from the alkylation product of naphthalene. The reaction solution was subjected to vacuum distillation to enrich the 2-methylnaphthalene fraction. The fraction was frozen at -10℃ for 3 hours. The precipitated 2-methylnaphthalene was centrifuged to obtain crude 2-methylnaphthalene. Then, it was recrystallized in anhydrous methanol at 0℃ to obtain pure 2-methylnaphthalene.

[0042] The alkylation of 2-methylnaphthalene to prepare 2,6-dimethylnaphthalene was carried out in a continuous fixed-bed reactor using stainless steel tubing with an inner diameter of 10 mm. A catalyst with an active metal Cu content of 2% was loaded into the isothermal zone of the reactor, with inert quartz sand of the same mesh size placed above and below the catalyst. Hydrogen was used as the carrier gas at a flow rate of 30 mL / min, and the pressure in the fixed-bed reactor was maintained at 4 MPa. Before the reaction, the catalyst was activated at 450 °C for 2 h in an H2 atmosphere, then the temperature was lowered to 420 °C. After the temperature stabilized, 2-methylnaphthalene and methanol were mixed uniformly at a molar ratio of 1:10. The feed solution was injected into the reactor using a micro-metering pump, while deionized water was vaporized in a vaporizer and introduced into the reaction system at a molar ratio of 2:1. The alkylation reaction of 2-methylnaphthalene occurred through the catalyst bed. The mass hourly space velocity (MSV) of the feed solution (based on 2-methylnaphthalene) was [missing value]. The product was collected by condensation and analyzed by gas chromatography. The conversion rate and product selectivity of the reaction were quantitatively calculated, and the results are listed in Table 1.

[0043] Table 1

[0044]

[0045] In summary, the technical solution of this invention, through synergistic innovation in raw material selection, catalyst design, reaction system optimization, and process parameter control, exhibits significant advantages in multiple dimensions compared to existing technologies. Overall, it can be summarized as "combining efficiency, stability, environmental friendliness, and industrialization potential," which are detailed below:

[0046] The catalyst stability is significantly improved, meeting the needs of long-cycle production: Through the dual-effect synergistic mechanism of "hydrogen carrier gas + water vapor introduction", the formation of carbon deposits is suppressed and surface carbon is removed from the source. Hydrogen saturates the carbon deposit precursor with hydrogen, avoiding the deposition of polycyclic aromatic hydrocarbons. Water vapor not only inhibits the self-conversion of methanol to generate low-carbon olefins (carbon deposit precursors), but also participates in the carbon removal reaction, effectively delaying catalyst deactivation. The conversion rate decreases by no more than 2% after 50 hours of continuous reaction, solving the core pain point of short catalyst life in the existing technology.

[0047] The reaction efficiency and selectivity are balanced and optimized, and the yield of the target product is stable: Using 2-methylnaphthalene as raw material, taking advantage of its high electron density sites, and combining it with a supported metal molecular sieve catalyst (preferably with a Ni loading of 2%~5%), and optimizing key parameters such as reaction temperature and raw material molar ratio (2-methylnaphthalene to methanol 1:2~1:10), the conversion rate of 2-methylnaphthalene is over 50% and the selectivity of 2,6-dimethylnaphthalene is over 55% after 50 hours of reaction. Moreover, the selectivity continues to improve slightly during the reaction process, and the yield of the target product is stable and controllable.

[0048] The raw materials are widely available and of high value, and the resource utilization rate is high: 2-methylnaphthalene can be directly extracted from low-value-added aromatic resources such as coal tar and catalytic cracking petroleum, or obtained through the separation of naphthalene alkylation and transalkylation reaction products, thus fully realizing the efficient utilization of surplus resources; methanol, as an alkylating reagent and solvent, is inexpensive and readily available, and avoids the additional carbon deposition problem caused by high-temperature cracking of conventional solvents, further reducing production costs.

[0049] The process is green and environmentally friendly, easy to operate, and has strong industrial feasibility: the reaction system has no harmful additives, and the synergistic effect of methanol and water reduces side reaction emissions, which is in line with the development trend of green chemical industry; a continuous fixed-bed reactor is adopted, and the catalyst bed is fixed by inert quartz sand. The raw material feeding, reaction process and product separation can be easily automated and controlled. The equipment cost is low, the operation process is simplified, and it is easier to scale up and promote compared with traditional processes.

[0050] With strong adaptability and flexible control, it has a wide range of applications: parameters such as the loading of active metals (Fe, Co, Ni, etc.), reaction pressure (0.1~4MPa), and hydrogen flow rate (10~40mL / min) can be flexibly adjusted according to production needs. It can adapt to small-scale fine production as well as meet the requirements of large-scale industrial production capacity. At the same time, it is compatible with 2-methylnaphthalene raw materials from different sources, with outstanding adaptability and practicality.

[0051] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An improved process for the preparation of 2,6-dimethylnaphthalene, characterized in that, Using 2-methylnaphthalene as raw material and methanol as alkylating agent, molecular sieves supported with active metals were used as catalysts in a continuous fixed-bed reactor. Hydrogen was used as the carrier gas, and water vapor was introduced into the reaction system to control the reaction conditions. The product was separated and purified to obtain 2,6-dimethylnaphthalene.

2. The improved process for the production of 2,6-dimethylnaphthalene according to claim 1, characterized in that, The active metal is selected from at least one of Fe, Co, Ni, Cu, Pt, and Pd, preferably Ni.

3. The improved process for preparing 2,6-dimethylnaphthalene according to claim 2, characterized in that, The loading of the active metal on the molecular sieve is 0.5wt% to 20wt%, preferably 2wt% to 5wt%.

4. The improved process for preparing 2,6-dimethylnaphthalene according to claim 1, characterized in that, The reaction conditions include: a reaction temperature of 300℃~450℃, a fixed-bed reactor pressure of 0.1MPa~10MPa, a hydrogen carrier gas flow rate of 5mL / min~100mL / min, and a feed liquid mass hourly space velocity of... .

5. The improved process for preparing 2,6-dimethylnaphthalene according to claim 4, characterized in that, The preferred reaction temperature is 380℃~420℃, the preferred pressure of the fixed-bed reactor is 0.1MPa~4MPa, the preferred hydrogen carrier gas flow rate is 10mL / min~40mL / min, and the preferred mass hourly space velocity of the feed liquid is... .

6. The improved process for preparing 2,6-dimethylnaphthalene according to claim 1, characterized in that, The molar ratio of 2-methylnaphthalene to methanol is 1:1 to 1:20, preferably 1:2 to 1:

10.

7. The improved process for preparing 2,6-dimethylnaphthalene according to claim 1, characterized in that, The molar ratio of water vapor to methanol is 1:10 to 5:1, preferably 2:1 to 4:

1.

8. The improved process for preparing 2,6-dimethylnaphthalene according to claim 1, characterized in that, The 2-methylnaphthalene is obtained by one of the following methods: direct extraction from coal tar or catalytic cracked petroleum; separation from the alkylation product of naphthalene; or separation from the transalkylation product of naphthalene and polymethylnaphthalene, wherein the separation process includes at least one of distillation, freeze crystallization, and recrystallization.

9. The improved process for preparing 2,6-dimethylnaphthalene according to claim 1, characterized in that, The catalyst needs to be activated in a hydrogen atmosphere before use. The activation temperature is 450℃~650℃ and the activation time is 0.5h~2h.

10. The improved process for preparing 2,6-dimethylnaphthalene according to claim 1, characterized in that, The reaction tube of the continuous fixed-bed reactor is a stainless steel tube, and the catalyst is loaded in the constant temperature zone of the reactor. Inert quartz sand is filled at both ends of the catalyst.

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