A method for characterizing oxygen species adsorbed by ethylene epoxidation catalysts

Abstract

The present invention relates to a method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst, and to the technical field of catalyst characterization and detection. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst comprises the following steps: pretreating the ethylene epoxidation catalyst with a mixed gas of ethylene / oxygen / helium; adsorbing oxygen on the pretreated ethylene epoxidation catalyst with oxygen, etc. The present invention effectively removes incompletely decomposed Ag precursors by adopting an in-situ reaction induction method in the pretreatment stage of the catalyst. Subsequently, an oxygen adsorption experiment is performed on the Ag catalyst pretreated in situ, and oxygen and other gas phase products generated by programmed temperature desorption are detected by mass spectrometry to detect the composition and characteristics of the ethylene epoxidation catalyst, thereby reducing the interference of Ag precursor salts without increasing the reaction temperature, thereby accurately judging the oxygen active species on the surface of the Ag catalyst.

Description

Technical Field

[0001] The invention relates to the technical field of catalyst characterization and detection, and in particular to a method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst. Background Art

[0002] The epoxidation of ethylene to produce ethylene oxide is an important chemical reaction in industry, and its product ethylene oxide is widely used in the production of antifreeze, medicines, detergents and plastics. Ag / Al2O3 catalyst is used in the industrial preparation of ethylene oxide, and the reaction conditions are 230-270℃ and 1-3MPa (ACS Catal. 2019, 9(12), 10727-10750; DOI: 10.1021 / acscatal.9b03443). The process of ethylene epoxidation to produce ethylene oxide is also accompanied by the generation of by-product C oxygen. The adsorption of oxygen on the Ag catalyst will generate different oxygen species, involving the preparation of ethylene oxide by epoxidation of ethylene and the preparation of C oxygen by the deep oxidation of ethylene as a side reaction. Distinguishing the active oxygen species on the surface of the Ag catalyst is a key factor in improving the selectivity of ethylene oxide in the epoxidation reaction of ethylene.

[0003] Common techniques for characterizing oxygen species, such as XPS and Raman, have the advantage of high precision, but often involve expensive characterization instruments and complex operating methods. In contrast, the temperature-programmed surface oxygen desorption experiment (oxygen-TPD) can determine the strength of oxygen binding on the catalyst surface by analyzing the desorption peaks of oxygen at different temperatures, and link it with the catalytic activity, thereby determining the active oxygen species on the Ag catalyst that are conducive to the formation of ethylene oxide.

[0004] The traditional oxygen-TPD characterization method is usually to pre-treat the Ag catalyst in an inert atmosphere, conduct oxygen adsorption experiments in the range of 100-300°C, and then conduct a temperature-programmed desorption experiment in an inert gas environment. Studies have found that the oxygen desorption peak at 570K corresponds to oxygen adsorption on the step position of Ag, while the low-temperature oxygen desorption peak in the range of 513-523K is attributed to oxygen adsorption on the Ag (111) crystal surface. According to literature reports (J. Catal. 2005, 235 (1), 103-113; DOI: 10.1016 / j.jcat.2005.07.019), the selectivity of oxygen adsorbed on the Ag (111) crystal surface is 57%, which is higher than the selectivity of oxygen adsorbed on Ag (111) for ethylene oxide (33%). Therefore, the intensity of low-temperature oxygen adsorption species is closely related to the selectivity for ethylene oxide, and it is necessary to further enhance the signal intensity and recognition of oxygen-TPD for low-temperature oxygen species.

[0005] However, in the preparation process of industrial Ag catalysts, since the Ag precursor is not completely decomposed, it is usually necessary to release it gradually during the reaction. In the oxygen-TPD characterization experiment, the programmed temperature may release Ag precursor products that are not completely decomposed in the pretreatment stage, such as nitrogen oxides and carbon dioxide, which will interfere with the judgment of active oxygen species. At the same time, although increasing the pretreatment temperature can completely decompose the Ag precursor, this will cause the Ag particle size to increase, thereby reducing the activity of the catalyst.

[0006] Therefore, there is an urgent need to develop a method that can effectively reduce the interference of other substances in order to more accurately characterize the oxygen species on the Ag catalyst, and accordingly design a highly active and highly selective Ag catalyst for the reaction of ethylene epoxidation to prepare ethylene oxide. Summary of the invention

[0007] To solve the above problems, the present invention provides a method for characterizing oxygen species adsorbed by ethylene epoxidation catalysts.

[0008] The present invention provides a method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst, the method comprising the following steps:

[0009] Pretreating the ethylene epoxidation catalyst with a gas mixture of ethylene / oxygen / helium;

[0010] Adsorbing oxygen onto the pretreated ethylene epoxidation catalyst using oxygen;

[0011] Helium was used to conduct oxygen desorption experiments on the ethylene epoxidation catalyst treated with oxygen adsorption to detect oxygen and other gas phase products.

[0012] Furthermore, the ethylene / oxygen / helium mixed gas comprises the following components in volume percentage: 10-30 vol% ethylene, 3-7 vol% oxygen, and 63-87 vol% helium.

[0013] Furthermore, the pressure range of the ethylene / oxygen / helium mixed gas is 0.1-3.0 MPa.

[0014] Furthermore, the pretreatment process includes: heating to 200°C to 300°C at a heating rate of 2°C / min and maintaining for 1.5 to 2.5 hours.

[0015] Furthermore, the flow rate of the mixed gas is 45-55 mL / min.

[0016] Furthermore, the working condition parameters of the oxygen adsorption treatment include: a temperature of 200° C. to 300° C. and a time of 1.5 to 2.5 hours.

[0017] Furthermore, the flow rate of the oxygen is 45-55 mL / min, and the composition of the oxygen is 5-100 vol%.

[0018] Further, the step of conducting an oxygen desorption experiment on the ethylene epoxidation catalyst treated with oxygen adsorption using helium includes the following process:

[0019] The system was cooled to 30°C, switched to helium purging for 25 to 35 minutes, and then heated to 600°C at a heating rate of 10°C / min to conduct an oxygen desorption experiment.

[0020] Furthermore, the ethylene epoxidation catalyst is an Ag-based catalyst.

[0021] Furthermore, the Ag-based catalyst includes Ag / Al2O3 and Ag / Al2O3 catalyst modified by Li, Na, K, Rb, Cs, Re, Mo, Mn, and Sc.

[0022] The above technical solution provided by the embodiment of the present invention has at least the following advantages compared with the prior art:

[0023] The embodiment of the present invention provides a method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst. The present invention effectively removes incompletely decomposed Ag precursors by adopting an in-situ reaction induction method during the pretreatment stage of the catalyst. Subsequently, the Ag catalyst that has been pretreated in situ is subjected to an oxygen adsorption experiment, and the oxygen and other gas phase products generated by programmed temperature desorption are detected by mass spectrometry to detect the composition and characteristics of the ethylene epoxidation catalyst. The interference of the Ag precursor salt can be reduced without increasing the temperature, thereby accurately determining the oxygen active species on the surface of the Ag catalyst. Specifically:

[0024] 1. Compared with the method of using inert gas pretreatment in the existing oxygen-TPD technology, this method can effectively inhibit the generation of nitrogen oxides and significantly reduce the interference caused by the decomposition of Ag precursor salt.

[0025] 2. This method is simple, convenient, fast and efficient, and is suitable for routine experimental procedures.

[0026] 3. This method can significantly improve the recognition of oxygen species and more clearly associate the desorption peak of low-temperature oxygen species with the selectivity of ethylene oxide.

[0027] 4. Using reactive gas induction as a pretreatment step can efficiently decompose the incompletely decomposed Ag precursor and significantly reduce the interference of nitrogen oxides produced by the decomposition of Ag precursor salt on the oxygen signal.

[0028] 5. Reaction gas induction can promote the reconstruction of Ag nanoparticles on the carrier surface, making the adsorption of oxygen species on the Ag particles closer to the actual reaction conditions, thereby more accurately judging the correlation between oxygen active species on the Ag catalyst surface and reaction activity. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0031] Figure 1 A schematic flow chart of a method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst provided in an embodiment of the present invention.

[0032] Figure 2 An oxygen-TPD spectrum of a method for characterizing the adsorption of oxygen species by an ethylene epoxidation catalyst provided in an embodiment of the present invention.

[0033] Figure 3 An oxygen-TPD spectrum of a method for characterizing the adsorption of oxygen species by an ethylene epoxidation catalyst provided as a comparative example of the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0036] The present invention provides a method for characterizing oxygen species adsorbed by ethylene epoxidation catalysts, such as Figure 1 As shown, the method for characterizing the adsorption of oxygen species by ethylene epoxidation catalyst comprises the following steps:

[0037] Pretreating the ethylene epoxidation catalyst with a gas mixture of ethylene / oxygen / helium;

[0038] Adsorbing oxygen onto the pretreated ethylene epoxidation catalyst using oxygen;

[0039] Helium was used to conduct oxygen desorption experiments on the ethylene epoxidation catalyst treated with oxygen adsorption to detect oxygen and other gas phase products.

[0040] The embodiment of the present invention provides a method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst. The present invention effectively removes incompletely decomposed Ag precursors by adopting an in-situ reaction induction method in the pretreatment stage of the catalyst. Subsequently, the Ag catalyst that has been pretreated in situ is subjected to an oxygen adsorption experiment, and the oxygen and other gas phase products generated by programmed temperature desorption are detected by mass spectrometry. The interference of the Ag precursor salt can be reduced without increasing the temperature, thereby accurately determining the oxygen active species on the surface of the Ag catalyst.

[0041] In some specific embodiments, the ethylene / oxygen / helium mixed gas comprises the following components, measured in volume percentage: 10-30 vol% ethylene, 3-7 vol% oxygen, 63-87 vol%.

[0042] In some specific embodiments, the pressure range of the ethylene / oxygen / helium mixed gas is 0.1-3.0 MPa.

[0043] In some specific embodiments, the pretreatment process includes: heating to 200° C. to 300° C. at a heating rate of 2° C. / min and maintaining for 1.5 to 2.5 hours.

[0044] In some specific embodiments, the flow rate of the mixed gas is 45-55 mL / min.

[0045] In some specific embodiments, the working condition parameters of the oxygen adsorption treatment include: a temperature of 200° C. to 300° C. and a time of 1.5 to 2.5 hours.

[0046] In some specific embodiments, the flow rate of the oxygen is 45-55 mL / min, and the composition of the oxygen is 5-100 vol%.

[0047] In some specific embodiments, the step of conducting an oxygen desorption experiment on the ethylene epoxidation catalyst treated with oxygen adsorption using helium comprises the following process:

[0048] The system was cooled to 30°C, switched to helium purging for 25 to 35 minutes, and then heated to 600°C at a heating rate of 10°C / min to conduct an oxygen desorption experiment.

[0049] In some specific embodiments, the ethylene epoxidation catalyst is an Ag-based catalyst.

[0050] In some specific embodiments, the Ag-based catalyst includes Ag / Al2O3 catalyst modified with Li, Na, K, Rb, Cs, Re, Mo, Mn, and Sc.

[0051] It should be noted that the component raw materials involved in the method for characterizing the adsorption of oxygen species by ethylene epoxidation catalyst provided in the embodiment of the present invention, unless otherwise specified or specified, can be directly commercially available products or homemade using existing public preparation methods; at the same time, the steps and parameters involved, unless otherwise specified or specified, can be carried out according to existing processes or directly using existing equipment, and the present invention document will not describe them one by one.

[0052] The present invention will be further described below in conjunction with specific examples. It should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples that do not specify specific conditions are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0053] The catalyst used in the following examples and comparative examples is a silver-based oxide catalyst, and its preparation method is consistent with that reported in the literature (ACS Catal. 2024, 14 (1), 406-417; https: / / doi.org / 10.1021 / acscatal.3c04361). First, silver oxalate is prepared, and then silver oxalate is mixed with ethylenediamine and water to form a solution. The solution is evenly distributed on the AlO support by an equal volume impregnation method, and after drying and calcination, an Ag-based catalyst (Ag / Al2O3) is prepared, which can be used for characterization experiments of oxygen species.

[0054] Example 1

[0055] This example provides a method for characterizing the adsorption of oxygen species on ethylene epoxidation catalysts, including the following process:

[0056] Weigh 0.3g of Ag / AlO catalyst and place it in a U-shaped reaction tube for oxygen-TPD experiment. First, introduce 10vol% ethylene / 5vol% oxygen / 85vol% helium mixed gas (50mL / min) into the reaction tube, heat to 250℃ at a heating rate of 2℃ / min and maintain for 2 hours to complete the pretreatment. Subsequently, switch the reaction gas to pure oxygen (oxygen, 50mL / min) and continue to adsorb at 250℃ for 2 hours. After the pretreatment, cool the reaction tube to 30℃, switch to helium (helium) purge for 30 minutes, and then heat to 600℃ at a heating rate of 10℃ / min for oxygen desorption experiment. The oxygen and other gas products produced during the desorption process are detected and analyzed by mass spectrometry.

[0057] Example 2

[0058] This example provides a method for characterizing the adsorption of oxygen species on ethylene epoxidation catalysts, including the following process:

[0059] Weigh 0.3g of Ag / AlO catalyst and place it in a U-shaped reaction tube for oxygen-TPD experiment. First, introduce 10vol% ethylene / 5vol% oxygen / 85vol% helium mixed gas (50mL / min) into the reaction tube, heat it to 200℃ at a heating rate of 2℃ / min and maintain it for 2 hours to complete the pretreatment. Subsequently, when the system is cooled to 200℃, switch the reaction gas to pure oxygen (oxygen, 50mL / min) and adsorb it at this temperature for 2 hours. After the pretreatment, cool the reaction tube to 30℃, switch to helium (helium) purge for 30 minutes, and then heat it to 600℃ at a heating rate of 10℃ / min for oxygen desorption experiment. The oxygen and other gas products produced during the desorption process are detected and analyzed by mass spectrometry.

[0060] Example 3

[0061] This example provides a method for characterizing the adsorption of oxygen species on ethylene epoxidation catalysts, including the following process:

[0062] Weigh 0.3g of Ag / AlO catalyst and place it in a U-shaped reaction tube for oxygen-TPD experiment. First, introduce 10vol% ethylene / 5vol% oxygen / 85vol% helium mixed gas (50mL / min) into the reaction tube, heat to 300℃ at a heating rate of 2℃ / min and maintain for 2 hours to complete the pretreatment. Subsequently, switch the reaction gas to pure oxygen (oxygen, 50mL / min) and continue to adsorb at 250℃ for 2 hours. After the pretreatment, cool the reaction tube to 30℃, switch to helium (helium) purge for 30 minutes, and then heat to 600℃ at a heating rate of 10℃ / min for oxygen desorption experiment. The oxygen and other gas products produced during the desorption process are detected and analyzed by mass spectrometry.

[0063] Comparative Example 1

[0064] This example provides a method for characterizing the adsorption of oxygen species on ethylene epoxidation catalysts, including the following process:

[0065] Weigh 0.3g of Ag / AlO catalyst and place it in a U-shaped reaction tube for oxygen-TPD experiment. First, an inert gas (helium, 50mL / min) was introduced into the reaction tube, heated to 250°C at a heating rate of 2°C / min and maintained for 2 hours to complete the pretreatment. Subsequently, the reaction gas was switched to pure oxygen (oxygen, 50mL / min) and adsorbed at 250°C for 2 hours. After the pretreatment, the reaction tube was cooled to 30°C, switched to helium (helium) for 30 minutes, and then heated to 600°C at a heating rate of 10°C / min for oxygen desorption experiment. The oxygen and other gas products produced during the desorption process were detected and analyzed by mass spectrometry.

[0066] Comparative Example 2

[0067] This example provides a method for characterizing the adsorption of oxygen species on ethylene epoxidation catalysts, including the following process:

[0068] Weigh 0.3g of Ag / AlO catalyst and place it in a U-shaped reaction tube for oxygen-TPD experiment. First, pure oxygen (oxygen, 50mL / min) was introduced into the reaction tube, heated to 250℃ at a heating rate of 2℃ / min and maintained for 2 hours. Then, the reaction tube was cooled to 30℃, switched to helium (helium) purging for 30 minutes, and then heated to 600℃ at a heating rate of 10℃ / min for oxygen desorption experiment. The oxygen and other gas products generated during the desorption process were detected and analyzed by mass spectrometry.

[0069] The test results are as follows Figure 2 and Figure 3 shown. Figure 2 The oxygen-TPD spectra of Ag / Al2O3 catalyst tested at different pretreatment temperatures of ethylene epoxidation reaction gas species in the embodiment of the present invention are shown in FIG. Figure 3 It is the oxygen-TPD spectrum of the Ag / Al2O3 catalyst in the comparative example tested in an inert gas with or without pretreatment.

[0070] The oxygen-TPD spectra shown in the embodiment are as follows Figure 2 As shown in the figure, ethylene and oxygen were used as treatment gases to pretreat the Ag / AlO catalyst at different temperatures. The oxygen-TPD spectrum mainly shows two oxygen desorption peaks, namely the desorption peak of low-temperature oxygen species (244°C) and the desorption peak of high-temperature oxygen species (about 322°C). As the reaction temperature increased from 200°C to 300°C, the intensity of the desorption peak of low-temperature oxygen species did not change significantly, while the intensity of the desorption peak of high-temperature oxygen species increased slightly. This change may be related to the agglomeration of surface Ag particles caused by the higher treatment temperature. The desorption spectrum of nitrogen oxides obtained by mass spectrometry is shown in Figure 2. Figure 2 As shown in (b), the desorption peak intensity gradually decreases with increasing temperature.

[0071] The oxygen-TPD spectrum shown in the comparative example is as follows Figure 3 As shown, the experimental conditions are different from those in Example 1, in which the Ag / AlO catalyst is pretreated with inert gas helium or there is no pretreatment step. Ag / AlO that has not been pretreated with inert gas shows three oxygen desorption peaks, while Ag / AlO treated with inert gas shows only two oxygen desorption peaks, which are similar to the oxygen-TPD spectrum of Example 1. However, the desorption peak intensity of high-temperature oxygen species of the catalyst treated with inert gas is significantly enhanced. This result shows that the catalyst treated with reaction gas can effectively reduce the generation of high-temperature oxygen species, and this change is more conducive to the generation of ethylene oxide. The TPD spectrum of nitrogen oxides is shown in Figure 3 As shown in (b), compared with the Ag / AlO catalyst of Example 1, the intensity of the nitrogen oxide desorption peak generated by the Ag / AlO catalyst without pretreatment or only treated with helium gas is significantly enhanced.

[0072] Various embodiments of the present invention may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present invention; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0073] The foregoing is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst, characterized in that: The method for characterizing the adsorption of oxygen species by an ethylene epoxidation catalyst comprises the following steps: Pretreating the ethylene epoxidation catalyst with a gas mixture of ethylene / oxygen / helium; Adsorbing oxygen onto the pretreated ethylene epoxidation catalyst using oxygen; Helium was used to conduct oxygen desorption experiments on the ethylene epoxidation catalyst treated with oxygen adsorption to detect oxygen and other gas phase products.

2. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to claim 1, characterized in that: Measured in volume percentage, the ethylene / oxygen / helium mixed gas comprises the following components: 10-30 vol% ethylene, 3-7 vol% oxygen, and 63-87 vol% helium.

3. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to claim 1, characterized in that: The pressure range of the ethylene / oxygen / helium mixed gas is 0.1-3.0 MPa.

4. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to claim 1, characterized in that: The pretreatment process includes: heating to 200°C to 300°C at a heating rate of 2°C / min and maintaining for 1.5 to 2.5 hours.

5. The method for characterizing oxygen species adsorbed by ethylene epoxidation catalyst according to claim 1, characterized in that: The flow rate of the mixed gas is 45-55 mL / min.

6. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to claim 1, characterized in that: The working condition parameters of the oxygen adsorption treatment include: a temperature of 200° C. to 300° C. and a time of 1.5 to 2.5 hours.

7. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to claim 1, characterized in that: The flow rate of the oxygen is 45-55 mL / min, and the composition of the oxygen is 5-100 vol%.

8. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to claim 1, characterized in that: The steps of conducting an oxygen desorption experiment on an ethylene epoxidation catalyst treated with oxygen adsorption using helium include the following process: The system was cooled to 30°C, switched to helium purging for 25 to 35 minutes, and then heated to 600°C at a heating rate of 10°C / min to conduct an oxygen desorption experiment.

9. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to any one of claims 1 to 8, characterized in that: The ethylene epoxidation catalyst is an Ag-based catalyst.

10. The method for characterizing oxygen species adsorbed by an ethylene epoxidation catalyst according to claim 9, characterized in that: The Ag-based catalyst includes Ag / Al2O3 and Ag / Al2O3 catalyst modified by Li, Na, K, Rb, Cs, Re, Mo, Mn, and Sc.

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