A method for directional regulation of crystal reconstruction of coox species and its application in propane dehydrogenation

Abstract

This invention belongs to the field of catalytic materials and catalyst preparation technology, specifically relating to a method for preparing a Co / Al2O3 propane dehydrogenation catalyst. This method employs a crystal reconstruction method to realize the presence of CoO in Co / Al2O3. x Targeted regulation of species. Using the preparation method described in this invention, the cobalt species in the catalyst can be precisely controlled, resulting in a catalyst with high propylene selectivity (93%) and high propane conversion (23%) in the propane dehydrogenation reaction. Simultaneously, this method effectively solves the problem of coking in traditional dehydrogenation catalysts, opening up new avenues for improving catalytic performance.

Description

Technical Field

[0001] This invention belongs to the field of catalytic materials and catalyst preparation technology, specifically relating to a method for preparing a four-coordinate Co-containing catalyst by encapsulating cobalt species into an alumina precursor lattice using a crystal reconstruction method. 2+ Methods for using Co / Al2O3 catalysts. Background Technology

[0002] Propylene, a key raw material in the chemical industry, plays a crucial role in the synthesis of various chemical products such as polyurethane, polypropylene, acetone, acrylonitrile, and propylene oxide. With the large-scale development of shale gas resources in recent years, low-cost propane resources have been fully utilized, driving the large-scale development of propane dehydrogenation (PDH) to propylene technology. Although this process currently mainly relies on Pt-based and CrO-based materials... x Pt-based catalysts are available, but these catalysts face many challenges: Pt-based catalysts are expensive and prone to sintering and deactivation, while CrO2-based catalysts are more susceptible to degradation. x Traditional catalysts are limited by their high toxicity. Therefore, there is an urgent need to develop novel catalysts that are both cost-effective and environmentally friendly to meet the needs of industrial production and overcome the shortcomings of existing technologies.

[0003] The transition metal cobalt has attracted attention due to its excellent CH bond activation ability and environmental friendliness. Patent CN115007200 discloses a method for preparing a sub-nano cluster Co catalyst, which exhibits good activity in dehydrogenation reactions. Patent CN 111589449 A discloses a hydrothermal synthesis method for a Co-based catalyst for propane dehydrogenation. This catalyst uses Al₂O₃ as a support and cobalt as the active center, but its propylene selectivity is low. This is attributed to the formation of various CoO₂ groups on the catalyst surface. x Species (including Co3O4, CoO) x Microcrystals and CoAl2O4). Specifically, Co3O4 and CoO x Microcrystals are easily reduced to metallic Co, which leads to cracking and coking during the dehydrogenation reaction; in contrast, due to the four-coordinate Co... 2+ It is not easily reduced and can exist stably on the catalyst surface, thereby promoting the effective dehydrogenation process.

[0004] Therefore, the CoO on the Co / Al2O3 surface can be directionally controlled. x The design and preparation of highly active, highly selective, low-cost, and environmentally friendly propane dehydrogenation catalysts is an urgent problem to be solved. Summary of the Invention

[0005] Therefore, the technical problem to be solved by this invention is to achieve moderate destruction of the precursor crystal at a suitable temperature, impregnate an appropriate amount of cobalt species onto the support, and encapsulate the cobalt species at crystal defects using crystal reconstruction technology. Finally, through a suitable calcination temperature, a propane dehydrogenation catalyst with high activity, high selectivity, low cost, and environmental friendliness is prepared.

[0006] To solve the above technical problems and achieve the preparation of the catalyst, the following parameters need to be controlled:

[0007] For alumina supports, high-temperature decomposition of the alumina precursor can effectively generate a large number of crystal defects, providing ideal spaces for the intercalation of active metals. However, it must be noted that as the decomposition temperature increases, the precursor may irreversibly transform into the high-temperature phase of alumina, a transformation that is detrimental to the crystal reconstruction process. Therefore, precisely controlling the pyrolysis temperature of the precursor is crucial for the preparation of highly efficient catalysts.

[0008] For catalyst precursors: (1) While increasing the cobalt loading helps increase the number of active sites and fully utilize the generated crystal defects, the limited number of crystal defects means that excess cobalt may aggregate at other non-target sites and significantly inhibit crystal reconstruction. Therefore, it is necessary to select an appropriate loading based on the number of crystal defects. (2) Increasing the reconstruction temperature is beneficial for the forward reaction, but when the concentration of the second modifier is low, high temperature will promote the aggregation of cobalt species and inhibit the reconstruction reaction. Conversely, when the concentration of the second modifier is too high, cobalt species on the support may be excessively lost, thereby affecting the number of active sites and reducing the activity of the catalyst. Therefore, adjusting the balance between the reconstruction temperature and the concentration of the second modifier is crucial.

[0009] In terms of calcination conditions, this process is beneficial for the decomposition of the catalyst precursor, allowing cobalt species to infiltrate into the alumina lattice and thus expose more active sites. However, high-temperature calcination may lead to excessive accumulation of cobalt species in the bulk phase, which is not conducive to improving catalyst activity. Therefore, selecting a suitable calcination temperature is a key factor in ensuring excellent catalyst performance.

[0010] Therefore, the present invention describes a method for targeted regulation of CoO x A method for reconstructing species using crystals and its application in propane dehydrogenation, characterized by the following preparation steps:

[0011] (1) Using ammonium salts or amide compounds as the first modifier, the aluminum source and the first modifier are subjected to a hydrothermal reaction at a certain temperature to obtain an alumina precursor;

[0012] (2) The alumina precursor is decomposed at a certain temperature to obtain an alumina support, and then impregnated with a certain amount of cobalt-containing compound solution. After drying, a crystal reconstruction precursor is obtained.

[0013] (3) Using ammonium salt compounds as the second modifier, the crystal reconstruction precursor is reconstructed at a certain temperature to obtain the catalyst precursor;

[0014] (4) The catalyst precursor is calcined at a certain temperature to obtain a propane dehydrogenation catalyst.

[0015] Specifically, in step (2), the pyrolysis temperature of the alumina precursor is between 200-1000℃;

[0016] Specifically, in step (2), the amount of the cobalt-containing compound is 1.5-15 wt% based on the mass fraction of cobalt in the catalyst;

[0017] Specifically, in step (3), the reconstruction reaction temperature is 30-90℃;

[0018] Specifically, in step (3), the second modifier is one or more of ammonium citrate, ammonium tartrate, ammonium carbonate, ammonium bicarbonate and ammonium oxalate;

[0019] Specifically, in step (3), the concentration of the second modifier is 0.05-4 mol / L;

[0020] Specifically, in step (4), the calcination temperature of the catalyst precursor is between 400-800℃;

[0021] This invention also discloses a pure Co / Al₂O₃ catalyst prepared by the method, wherein the cobalt content in the catalyst is between 1.5 wt% and 15 wt%, thus obtaining a Co / Al₂O₃ catalyst for propane dehydrogenation. The cobalt in the catalyst is only tetracoordinated with Co. 2+ It exists in its original form and remains stable after reduction and reaction. Using this catalyst for propane dehydrogenation, propane conversion reaches 24.6%, and propylene selectivity is as high as 93.6%.

[0022] The Co / Al2O3 catalyst for propane dehydrogenation described in this invention utilizes inexpensive, readily available, and environmentally friendly alumina and a modifier. Under the action of a second modifier, a crystal reconstruction process is employed to transform CoO... x Encapsulated within AACH crystals. During calcination, these highly dispersed CoO... x It is more conducive to entering the alumina lattice, thus synthesizing only tetracoordinated Co. 2+ The Co / Al2O3 catalyst effectively inhibited the formation and sintering of metallic cobalt during reduction and the reaction, thereby improving propylene selectivity. Attached Figure Description

[0023] Figure 1Thermogravimetric curves of the carrier precursors in Examples 1, 2, 3, 4 and Comparative Example 1;

[0024] Figure 2 X-ray diffraction patterns of the supports in Examples 1, 2, 3, 4 and Comparative Example 1;

[0025] Figure 3 X-ray diffraction patterns of the catalyst precursors in Examples 1, 2, 3, 4 and Comparative Example 1;

[0026] Figure 4 H2-TPR chromatograms of the catalysts in Examples 1, 2, 3, 4 and Comparative Example 1;

[0027] Figure 5 UV-Vis spectra of catalysts in Examples 1, 2, 3, 4 and Comparative Example 1;

[0028] Figure 6 Example 2: Low-temperature CO infrared spectra of the catalyst in Comparative Example 1 under different CO pressures;

[0029] Figure 7 Propylene selectivity and propane conversion of catalysts in Examples 1, 2, 3, 4 and Comparative Example 1; Detailed Implementation

[0030] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0031] Example 1

[0032] The preparation method of the propane dehydrogenation catalyst described in this embodiment includes the following steps:

[0033] (1) At room temperature, 30 mL of 13 mol / L ammonium tartrate solution was mixed with 2 g of boehmite and crystallized at 120 °C for 24 hours. The resulting solid was filtered, washed, and dried at 80 °C for 12 hours to obtain the alumina precursor.

[0034] (2) The alumina precursor was heated to 400°C in a muffle furnace at a heating rate of 5°C / min and calcined for 2 hours to obtain alumina. 0.114 g of cobalt nitrate hexahydrate was impregnated onto 2 g of support and dried to obtain a crystal reconstruction precursor.

[0035] (3) The crystal reconstruction precursor was mixed with 400 mL of 0.5 mol / L ammonium bicarbonate solution and reacted at 30 °C for 24 hours. After filtration, washing, and drying, the catalyst was obtained by calcination at 400 °C for 6 hours.

[0036] Example 2

[0037] The preparation method of the propane dehydrogenation catalyst described in this embodiment includes the following steps:

[0038] (1) At room temperature, 30 mL of 10 mol / L ammonium tartrate solution was mixed with 2 g of boehmite and crystallized at 100 °C for 24 hours. The resulting solid was filtered, washed, and dried at 80 °C for 12 hours to obtain the alumina precursor.

[0039] (2) The alumina precursor was heated to 200°C in a muffle furnace at a heating rate of 1°C / min and calcined for 2 hours to obtain alumina. 0.57 g of cobalt nitrate hexahydrate was impregnated onto 2 g of support and dried to obtain a crystal reconstruction precursor.

[0040] (3) The crystal reconstruction precursor was mixed with 400 mL of 1 mol / L ammonium citrate solution and reacted at 50 °C for 24 hours. After filtration, washing, and drying, the catalyst was obtained by calcination at 800 °C for 6 hours.

[0041] Example 3

[0042] The preparation method of the propane dehydrogenation catalyst described in this embodiment includes the following steps:

[0043] (1) At room temperature, 30 mL of 5 mol / L ammonium tartrate solution was mixed with 2 g of boehmite and crystallized at 150 °C for 24 hours. The resulting solid was filtered, washed, and dried at 80 °C for 12 hours to obtain the alumina precursor.

[0044] (2) The alumina precursor was heated to 800°C in a muffle furnace at a heating rate of 4°C / min and calcined for 2 hours to obtain alumina. 2.28 g of cobalt nitrate hexahydrate was impregnated onto 2 g of support and dried to obtain a crystal reconstruction precursor.

[0045] (3) The crystal reconstruction precursor was mixed with 400 mL of 4 mol / L ammonium oxalate solution and reacted at 90 °C for 24 hours. After filtration, washing, and drying, the catalyst was obtained by calcination at 600 °C for 6 hours.

[0046] Example 4

[0047] The preparation method of the propane dehydrogenation catalyst described in this embodiment includes the following steps:

[0048] (1) At room temperature, 30 mL of 0.5 mol / L ammonium tartrate solution was mixed with 2 g of boehmite and crystallized at 80 °C for 24 hours. The resulting solid was filtered, washed, and dried at 80 °C for 12 hours to obtain the alumina precursor.

[0049] (2) The alumina precursor was heated to 600°C in a muffle furnace at a heating rate of 3°C / min and calcined for 2 hours to obtain alumina. 3.42 g of cobalt nitrate hexahydrate was impregnated onto 2 g of support and dried to obtain a crystal reconstruction precursor.

[0050] (3) The crystal reconstruction precursor was added to 400 mL of 0.05 mol / L ammonium carbonate solution and reacted at 50 °C for 24 hours. After filtration, washing, and drying, the catalyst was obtained by calcination at 400 °C for 6 hours.

[0051] Comparative Example 1

[0052] Loading cobalt using the conventional equal-volume impregnation method includes the following steps:

[0053] The modified alumina precursor was heated to 400°C in a muffle furnace at a heating rate of 2°C / min and calcined for 2 hours to obtain alumina. 0.57 g of cobalt nitrate hexahydrate was impregnated onto 2 g of the modified alumina support, and then dried to obtain the catalyst precursor (2.5% cobalt content in the catalyst). The catalyst precursor was calcined at 600°C for 6 hours to obtain the catalyst.

[0054] The prepared catalyst was evaluated for activity using the following method:

[0055] Catalyst activation: 0.2 g of catalyst was reduced at 400 °C for 1 hour in an H2 stream of 40 mL / min.

[0056] Catalyst activity evaluation: Propane dehydrogenation of the catalyst was evaluated at 550 °C in a reaction gas flow of 30 mL / min (propane / nitrogen = 5 / 95).

[0057] Characterization:

[0058] Figure 1 Thermogravimetric curves of the catalyst precursors obtained after treatment with the second modifier in Examples 1, 2, 3, 4 and Comparative Example 1 are shown. Figure 2 X-ray diffraction patterns of boehmite after treatment at different activation temperatures; Figure 3 The image shows the X-ray diffraction pattern of the catalyst precursor obtained after treatment with the second modifier. Figure 4 The above are H2-TPR diagrams of the catalysts in Examples 1, 2, 3, 4 and Comparative Example 1. Figure 5 The UV-Vis spectra of the catalysts in Examples 1, 2, 3, 4 and Comparative Example 1 are shown. Figure 6 The low-temperature CO infrared spectra of the catalyst after reduction in Comparative Example 1 under different CO pressures are shown in Example 2. Figure 7 Propane conversion and propylene selectivity of the catalysts in Examples 1, 2, 3, 4 and Comparative Example 1.

[0059] according to Figure 1It can be observed that, compared with the comparative example, the catalyst precursor prepared by the crystal reconstruction method exhibits a higher weight loss. This weight loss process mainly consists of two stages: water desorption (temperature below 130°C) and AACH decomposition (temperature above 130°C). Thermogravimetric analysis revealed that the weight loss rate of the obtained catalyst precursor ranged from 45% to 54% within the temperature range of 130 to 500°C. Compared with previous studies, the theoretical weight loss of AACH is approximately 60%, indicating that the structure of AACH can still be effectively recovered by the crystal reconstruction method after Co impregnation. However, the effect of the second modifier on the AACH reconstruction process differs.

[0060] according to Figure 2 It can be seen that AACH transforms into amorphous alumina at low temperatures and into γ-Al2O3 at high temperatures.

[0061] Figure 3 The data demonstrate the effect of different cobalt loadings in the catalyst precursor on the reconstruction process of basic ammonium aluminum carbonate (AACH). Analysis of the X-ray diffraction patterns of Examples 1 and 3 revealed significant changes in diffraction peak intensities due to differences in loading. With increasing cobalt loading, the intensity of diffraction peaks attributable to AACH gradually decreased, indicating that cobalt inhibited the AACH reconstruction process to some extent.

[0062] according to Figure 4 As can be seen, in the comparative sample, cobalt species mainly exist in the form of Co3O4, indicating that their interaction with the support is relatively weak, and therefore they can be reduced to metallic Co at a lower temperature. In contrast, the cobalt species in the examples have a stronger interaction with the support, resulting in a reduction temperature of the main cobalt species exceeding 900°C. Furthermore, although some cobalt species with weaker interactions with the support are generated on the catalyst surface with increasing cobalt loading, the number of these species is significantly less than that in the control sample.

[0063] according to Figure 5 It can be seen that the main cobalt species in the comparative example is Co3O4, which has a weak interaction with the support; while in the examples, the observed cobalt species are mainly tetracoordinated Co, which has a strong interaction with the support. 2+ It exists in form.

[0064] Figure 6 The results clearly show that, in the comparative examples, cobalt species were reduced to metallic cobalt after catalytic reduction. In contrast, in Examples 1, 2, 3, and 4, the four-coordinated Co on the catalyst prepared by the crystal reconstruction method... 2+It remained stable after reduction, and no metallic cobalt formation was observed. This finding highlights the effectiveness of crystal reconstruction methods in directionally controlling the occurrence of cobalt species on catalyst surfaces.

[0065] according to Figure 7 It can be seen that the catalyst performance of the examples is significantly better than that of the comparative examples. The propylene selectivity of the examples is as high as 94%, and the propane conversion rate is 23%, while the propylene selectivity and conversion rate of the comparative examples are only 60% and 19%, respectively. This indicates that the reaction performance of the catalyst can be significantly improved by directionally controlling the cobalt species occurrence form on the catalyst.

[0066] This invention discloses a method for preparing a catalyst. The method uses an ammonium salt or amide compound as the first modifier and one or more of ammonium citrate, ammonium tartrate, ammonium carbonate, ammonium bicarbonate, and ammonium oxalate as the second modifier. A catalyst precursor is prepared through an AACH crystal reconstruction process, and a calcination process yields a catalyst containing only tetracoordinated Co. 2+ This method effectively achieves targeted regulation of cobalt species on the catalyst surface, revealing the relationship between cobalt species distribution on the catalyst surface and the performance of propane dehydrogenation reaction, providing an important strategy for developing a new generation of environmentally friendly and efficient industrial catalysts for propane dehydrogenation.

[0067] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and enhancements can be made without departing from the principle of the present invention, and these improvements and enhancements should also be considered within the scope of protection of the present invention.

Claims

1. A method for directional control of CoO2 via crystal reconstruction x The application of species in propane dehydrogenation is characterized by The preparation steps include the following: (1) Using ammonium salts or amide compounds as the first modifier, the aluminum source and the first modifier are subjected to a hydrothermal reaction at a certain temperature to obtain an alumina precursor; (2) The alumina precursor is decomposed at a certain temperature to obtain an alumina support, and then impregnated with a certain amount of cobalt-containing compound solution. After drying, a crystal reconstruction precursor is obtained. (3) Using ammonium salt compounds as the second modifier, the crystal reconstruction precursor is reconstructed at a certain temperature to obtain the catalyst precursor; the reconstruction reaction temperature is 30-90℃; the second modifier is one or more of ammonium citrate, ammonium tartrate, ammonium carbonate, ammonium bicarbonate and ammonium oxalate. (4) The catalyst precursor is calcined at a certain temperature to obtain a propane dehydrogenation catalyst, wherein the calcination temperature of the catalyst precursor is between 400-800℃; The crystallographic reconstruction removes the Co3O4and CoO from the surface of the catalyst x microcrystalline, the cobalt species on the surface of the catalyst only exist in the form of tetrahedral Co 2+ .

2. The application according to claim 1, characterized in that... In step (2), the pyrolysis temperature of the alumina precursor is between 200-1000℃.

3. The application according to claim 1, characterized in that... In step (2), the content of the cobalt-containing compound is 2.5-15 wt% based on the mass fraction of cobalt in the catalyst.

4. The application according to claim 1, characterized in that... In step (3), the concentration of the second modifier is 0.05-4 mol / L.

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