Method for preparing surface methoxyl species based on H-ZSM-5 molecular sieve catalytic dimethyl ether conversion

Abstract

The invention belongs to the field of catalysis, and discloses a method for preparing surface methoxyl species by catalyzing dimethyl ether conversion based on an H-ZSM-5 molecular sieve. The method comprises the following steps: by taking an H-ZSM-5 molecular sieve as a catalyst, carrying out reaction on mixed gas of dimethyl ether and nitromethane at 90-120 DEG C under a mild condition, so that the dimethyl ether can be efficiently decomposed into surface methoxy species and methanol. The method disclosed by the invention has the remarkable advantages of simplicity and convenience in operation, mild reaction conditions and the like, provides an innovative solution for controllable preparation and industrial application of surface methoxy species, and has important scientific research value and wide industrial application prospect.

Description

Technical Field

[0002] This invention belongs to the field of catalysis, specifically relating to a method for preparing surface methoxy species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve. Background Technology

[0003] Molecular sieve-catalyzed methanol / dimethyl ether conversion can efficiently convert methanol into high-value hydrocarbons such as olefins and aromatics, providing a sustainable pathway for producing basic petrochemical feedstocks from non-petroleum resources (coal, natural gas, biomass, etc.). This chemical process has always been a research hotspot in both industry and academia. Surface methoxy species, as key active intermediates, not only participate in methanol activation and the formation of the first C / C bond, but also promote the methylation reactions of olefins and aromatics, thereby driving the entire methanol conversion process. Therefore, a deep understanding of the generation and evolution mechanisms of surface methoxy species is of significant scientific importance for elucidating the methanol conversion reaction mechanism and can provide theoretical guidance for optimizing industrial processes.

[0004] Currently, although studies have reported on Lewis acid sites catalyzing the formation of surface methoxy species from methanol at room temperature, Brønsted acid sites, which are the main active centers of molecular sieves, still face significant challenges in catalyzing this process under mild conditions, and related research is scarce. This is mainly because the reaction requires overcoming an activation energy barrier as high as 150 kJ / mol, typically requiring temperatures above 200 °C. Furthermore, at high temperatures, methanol, dimethyl ether, and surface methoxy species can interconvert, further increasing the complexity of the system. The key challenge in addressing this critical scientific issue lies in designing mild conditions to achieve the controllable formation of surface methoxy species. Summary of the Invention

[0005] To address the technical challenges of high formation temperatures and complex reactions at high temperatures for surface methoxy species, this invention aims to provide a method for preparing surface methoxy species through the catalytic conversion of dimethyl ether using H-ZSM-5 molecular sieve. This method innovatively employs dimethyl ether and nitromethane as mixed reactants, and uses nitromethane to regulate the reaction microenvironment, enabling the controllable formation of surface methoxy species at a mild reaction temperature of 90-120℃. This method can effectively reduce the reaction temperature for the dehydration of methanol to produce dimethyl ether.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve, the method comprising: Dehydrate the H-ZSM-5 molecular sieve; Each 13C-labeled dimethyl ether and nitromethane gases were adsorbed into the dehydrated H-ZSM-5 molecular sieve under vacuum. The molar ratio of the adsorption molecular dimethyl ether / nitromethane / Brønsted acid sites was 2 / 2 / 1. The H-ZSM-5 molecular sieve sample after adsorption was vacuum sealed and transferred to a constant temperature oven for heating and reaction. The sample was subjected to solid-state nuclear magnetic resonance analysis.

[0007] Optionally, the method for dehydrating the H-ZSM-5 molecular sieve includes: The H-ZSM-5 molecular sieve samples were placed in glass tubes, tightly connected to the vacuum system, and placed in the center of the heating furnace for heating and dehydration. The temperature was gradually increased from room temperature to the target temperature of 673 K at a heating rate of 1 K / min. At a temperature of 673 K and a pressure of < 10 -3 Under Pa conditions, the sample was continuously dehydrated for 10 hours, and then allowed to cool naturally to room temperature to obtain the dehydrated sample. The dehydration treatment was used to activate the molecular sieve.

[0008] Optionally, the method for adsorbing reactive molecules includes: Open the valve at the top of the sample tube and dispense a quantitative amount. 13 C-labeled dimethyl ether gas and nitromethane gas were adsorbed onto the dehydrated and activated H-ZSM-5 molecular sieve sample, and the sample tube was frozen with liquid nitrogen to accelerate the adsorption process. After adsorption is complete, seal the sample tube with a flame gun.

[0009] Preferably, the temperature of the heating reaction is 80-120 °C.

[0010] Preferably, the heating reaction time is 0.5-2 hours.

[0011] Optionally, the step of performing solid-state nuclear magnetic resonance analysis on the sample includes: Samples after adsorption and reaction of dimethyl ether and / or nitromethane were collected using a solid-state nuclear magnetic resonance spectrometer. 13 CCP / MAS NMR spectroscopy uses chemical shifts to determine whether surface methoxyl species are formed.

[0012] Optionally, comparative experiments may also be included; The comparative experiment differed only in the adsorption reaction molecules, with the molar ratio of the dimethyl ether / Brønsted acid sites in the adsorption molecules being 2 / 1; all other processes were the same.

[0013] Compared to existing technologies, this invention does not require modification of existing reaction devices or catalysts. It innovatively introduces nitromethane molecules to modify the local environment of dimethyl ether in H-ZSM-5 molecular sieves, achieving the generation of methoxy species on the Brønsted acid site catalytic surface under mild conditions of 110 °C. Attached Figure Description

[0014] Figure 1 It is an adsorption of H-ZSM-5 molecular sieve 13 C-labeled dimethyl ether and 13 After reacting a C-labeled dimethyl ether / nitromethane mixture at 80 °C, 90 °C, 100 °C, 110 °C, and 120 °C for 2 hours, 13 CMAS NMR spectra, where (a) represents adsorbed dimethyl ether and (b) represents a mixture of adsorbed dimethyl ether and nitromethane.

[0015] Figure 2 It is an adsorption of H-ZSM-5 molecular sieve 13 C-labeled dimethyl ether and dimethyl ether / nitromethane mixtures were reacted at 110 °C for 0.5, 1.0, 1.5, and 2 hours. 13 CMAS NMR spectra, where (a) represents adsorbed dimethyl ether and (b) represents a mixture of adsorbed dimethyl ether and nitromethane.

[0016] Figure 3 Two atoms are simultaneously adsorbed at each acid site of the catalyst. 13 C-labeled dimethyl ether with two nitromethanes 1 H− 1 1H SQ MAS NMR spectrum. Detailed Implementation

[0017] All embodiments are performed according to the operational steps of the technical solution described above.

[0018] The solid-state nuclear magnetic resonance analysis method in this embodiment is as follows: The experiment used a 3.2 mm magic angle rotation (MAS) probe with a rotation speed of 15 kHz. 13 In the CCP / MAS NMR assay, the contact time was 1.5 ms, the pulse delay was 3 s, and the number of scans was 1024. The chemical shift of dimethyl ether was assigned to 60.0 ppm, the chemical shift of surface methoxy species to 56.5 ppm, and the chemical shift of methanol to 52.0 ppm. The reaction efficiency of dimethyl ether was evaluated by the peak areas of the three species.

[0019] Example 1 H-ZSM-5 molecular sieve adsorption after dehydration treatment 13A mixture of C-labeled dimethyl ether and nitromethane was sealed and placed in a constant temperature oven at 80 °C for 2 hours before being detected and analyzed by a solid-state nuclear magnetic resonance spectrometer.

[0020] The reaction results are as follows Figure 1 (b) and Table 1 are shown.

[0021] Example 2 The implementation process is the same as in Example 1, except for the following differences: The reaction temperature is 90 ℃.

[0022] The reaction results are as follows Figure 1 (b) and Table 1 are shown.

[0023] Example 3 The implementation process is the same as in Example 1, except for the following differences: The reaction temperature is 100 ℃.

[0024] The reaction results are as follows Figure 1 (b) and Table 1 are shown.

[0025] Example 4 The implementation process is the same as in Example 1, except for the following differences: The reaction temperature is 110 ℃.

[0026] The reaction results are as follows Figure 1 (b) and Table 1 are shown.

[0027] Example 5 The implementation process is the same as in Example 1, except for the following differences: The reaction temperature is 120 ℃.

[0028] The reaction results are as follows Figure 1 (b) and Table 1 are shown.

[0029] Example 6 H-ZSM-5 molecular sieve adsorption after dehydration treatment 13 A mixture of C-labeled dimethyl ether and nitromethane was sealed and placed in a constant temperature oven at 110 °C for 0.5 hours, and then analyzed by solid-state nuclear magnetic resonance spectroscopy.

[0030] The reaction results are as follows Figure 2 (b) and Table 1 are shown.

[0031] Example 7 The implementation process is the same as in Example 6, except for the following differences: The reaction time is 1 hour.

[0032] The reaction results are as follows Figure 2 (b) and Table 1 are shown.

[0033] Example 8 The implementation process is the same as in Example 5, except for the following differences: The reaction time is 1.5 hours.

[0034] The reaction results are as follows Figure 2 (b) and Table 1 are shown.

[0035] Example 9 The implementation process is the same as in Example 5, except for the following differences: The reaction time is 2 hours.

[0036] The reaction results are as follows Figure 2 (b) and Table 1 are shown.

[0037] Comparative Example 1 H-ZSM-5 molecular sieve adsorption after dehydration treatment 13 C-labeled dimethyl ether was prepared by sealing the sample and placing it in a constant temperature oven at 80 °C for 2 hours, followed by detection and analysis using a solid-state nuclear magnetic resonance spectrometer.

[0038] The reaction results are as follows Figure 1 (a) and Table 2 are shown.

[0039] Comparative Example 2 The implementation process is the same as Comparative Example 1, except for the following differences: The reaction temperature is 90 ℃.

[0040] The reaction results are as follows Figure 1 (a) and Table 2 are shown.

[0041] Comparative Example 3 The implementation process is the same as Comparative Example 1, except for the following differences: The reaction temperature is 100 ℃.

[0042] The reaction results are as follows Figure 1 (a) and Table 2 are shown.

[0043] Comparative Example 4 The implementation process is the same as Comparative Example 1, except for the following differences: The reaction temperature is 110 ℃.

[0044] The reaction results are as follows Figure 1 (a) and Table 2 are shown.

[0045] Comparative Example 5 The implementation process is the same as Comparative Example 1, except for the following differences: The reaction temperature is 120 ℃.

[0046] The reaction results are as follows Figure 1(a) and Table 2 are shown.

[0047] Comparative Example 6 H-ZSM-5 molecular sieve adsorption after dehydration treatment 13 C-labeled dimethyl ether was prepared by sealing the sample and placing it in a constant temperature oven at 110 °C for 0.5 hours, followed by detection and analysis using a solid-state nuclear magnetic resonance spectrometer.

[0048] The reaction results are as follows Figure 2 (a) and Table 2 are shown.

[0049] Comparative Example 7 The implementation process is identical to Comparative Example 6, except for the following differences: The reaction time is 1 hour.

[0050] The reaction results are as follows Figure 2 (a) and Table 2 are shown.

[0051] Comparative Example 8 The implementation process is identical to Comparative Example 6, except for the following differences: The reaction time is 1.5 hours.

[0052] The reaction results are as follows Figure 2 (a) and Table 2 are shown.

[0053] Comparative Example 9 The implementation process is identical to Comparative Example 6, except for the following differences: The reaction time is 2 hours.

[0054] The reaction results are as follows Figure 2 (a) and Table 2 are shown.

[0055] Table 1. Results of the reaction of dimethyl ether / nitromethane mixture 13 Peak integral percentage of CMAS NMR spectrum Table 2. Results of dimethyl ether reaction 13 Peak integral percentage of CMAS NMR spectrum The NMR results of Comparative Examples 1–5 and 6–9 show that after H-ZSM-5 molecular sieve adsorbs dimethyl ether alone, a characteristic peak appears at 60.0 ppm. This signal can be attributed to dimethyl ether molecules adsorbed at Brønsted acid sites. After heat treatment at 80–120 °C for 2.0 hours and at 110 °C for 0.5–2.0 hours, the spectra still show only a single signal, confirming that dimethyl ether remains chemically inert in the absence of co-adsorbed molecules.

[0056] The NMR results of Examples 1-5 show that when dimethyl ether and nitromethane are co-adsorbed by H-ZSM-5 molecular sieve, a characteristic peak of dimethyl ether is initially observed at 60.0 ppm. After treatment at 80-120℃ for 2.0 hours, two new characteristic peaks of 56.5 ppm and 51.0 ppm appear in the NMR spectrum, and the signal intensity increases with increasing heating temperature. Quantitative analysis of the peak area ratio of each species (Table 1) shows that the proportion of surface methoxy species and methanol gradually increases with increasing reaction temperature. Dimethyl ether decomposition occurs at 90℃, with surface methoxy and methanol peak areas of 7.2% and 9.9%, respectively. The NMR results of Examples 6-9 show that at 110℃ for 0.5-2 hours, the proportion of surface methoxy species and methanol gradually increases with increasing reaction time. These phenomena clearly demonstrate that the introduction of nitromethane significantly promotes the Brønsted acid site-catalyzed decomposition of dimethyl ether into surface methoxy species and methanol. Furthermore, the formation efficiency of surface methoxy species gradually increases with increasing reaction temperature, and at reaction temperatures above 110°C, the formation efficiency further increases with increasing reaction time. Therefore, the introduction of nitromethane allows for the controllable preparation of surface methoxy species.

[0057] Two-dimensional analysis was performed on samples where dimethyl ether and nitromethane were adsorbed at each acid site. 1 H− 1 H SQ MAS NMR sampling analysis, such as Figure 3 As shown, the spectrum shows correlation peaks at (3.6, 4.5) and (4.5, 3.6) ppm, which can be attributed to the methyl H (δ) of dimethyl ether. 1 H = 3.6 ppm) and nitromethane methyl H (δ 1 The correlation between the two molecules (H = 4.5 ppm) indicates that dimethyl ether and nitromethane have spatial proximity. Density functional theory (DFT) calculations revealed that nitromethane can modulate the local microenvironment within the molecular pores, facilitating the protonation of dimethyl ether and significantly reducing its decomposition energy barrier. This result explains the experimental phenomenon of nitromethane promoting the decomposition of dimethyl ether to generate surface methoxyl species from the reaction mechanism.

[0058] The above description is merely a specific embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Any transformations or substitutions that can be conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve, characterized in that, The method includes: Dehydrate the H-ZSM-5 molecular sieve; Each 13 C-labeled dimethyl ether gas and nitromethane gas were adsorbed into the dehydrated H-ZSM-5 molecular sieve under vacuum; The H-ZSM-5 molecular sieve sample after adsorption was vacuum sealed and transferred to a constant temperature oven for heating and reaction. The sample was subjected to solid-state nuclear magnetic resonance analysis.

2. The method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve according to claim 1, characterized in that, The molar ratio of the adsorption molecules dimethyl ether / nitromethane / Brønsted acid sites is 2 / 2 / 1.

3. The method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve according to claim 1, characterized in that, The method for dehydrating H-ZSM-5 molecular sieves includes: The H-ZSM-5 molecular sieve samples were placed in glass tubes, tightly connected to the vacuum system, and placed in the center of the heating furnace for heating and dehydration. The temperature was gradually increased from room temperature to the target temperature of 673 K at a heating rate of 1 K / min. At a temperature of 673 K and a pressure of < 10 -3 Under Pa conditions, the sample was continuously dehydrated for 10 h, and then allowed to cool naturally to room temperature to obtain the dehydrated sample.

4. The method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve according to claim 1, characterized in that, The method for adsorbing molecules includes: Open the valve at the top of the sample tube and dispense a quantitative amount. 13 C-labeled dimethyl ether gas and nitromethane gas were adsorbed onto the dehydrated and activated H-ZSM-5 molecular sieve sample, and the sample tube was frozen with liquid nitrogen to accelerate the adsorption process. After adsorption is complete, seal the sample tube with a flame gun.

5. The method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve according to claim 1, characterized in that, The temperature of the heating reaction is 80-120 ℃.

6. The method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve according to claim 1, characterized in that, The heating reaction takes 0.5-2 hours.

7. The method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve according to claim 1, characterized in that, The step of performing solid-state nuclear magnetic resonance analysis on the sample includes: Samples after adsorption and reaction of dimethyl ether and / or nitromethane were collected using a solid-state nuclear magnetic resonance spectrometer. 13 C CP / MAS NMR spectroscopy, using chemical shifts to determine whether surface methoxyl species are formed.

8. The method for preparing surface methoxyl species by catalytic conversion of dimethyl ether based on H-ZSM-5 molecular sieve according to claim 1, characterized in that, It also includes comparative experiments; The comparative experiment differed only in the adsorption reaction molecules, with the molar ratio of the dimethyl ether / Brønsted acid sites in the adsorption molecules being 2 / 1; all other processes were the same.

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