Titanic acid nanotube loaded sub-nanometer palladium catalyst, and preparation method and application thereof

Abstract

The invention provides a titanic acid nanotube loaded sub-nanometer palladium catalyst, a preparation method and application, the titanic acid nanotube is used as a carrier of the catalyst, an active material palladium is dispersed and loaded on the carrier in a sub-nanometer cluster form, the average particle size of the active material palladium is less than 1 nm, and the loading capacity of the active material palladium is 1.9-3.9 wt%. The method comprises the following steps: carrying out heat treatment on a titanic acid nanotube carrier in an air atmosphere to regulate and control the surface hydroxyl density of the titanic acid nanotube carrier; and putting the titanic acid nanotube carrier subjected to heat treatment into an atomic layer deposition reactor, and synthesizing the titanic acid nanotube loaded sub-nano palladium catalyst by taking palladium hexafluoroacetylacetonate as a precursor and formaldehyde as a precursor and adopting an atomic layer deposition method. According to the method, along with the increase of the heat treatment temperature, the density of hydroxyl groups on the surface of the H2Ti3O7 carrier is increased, the hydroxyl groups can effectively inhibit Pd site agglomeration, a Pd-O-carrier covalent bonding structure is formed, migration of Pd particles in the reaction can be inhibited, and the Pd cluster size reaches the sub-nanometer level.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of catalyst technology, and relates to a palladium catalyst, in particular to a titanium acid nanotube loaded sub-nanometer palladium catalyst, a preparation method and application. BACKGROUND

[0002] Hexabenzylhexaazaisowurtzitane (HBIW) is an important precursor for the synthesis of CL-20, which can be prepared by condensation of benzylamine or substituted benzylamine with glyoxal. HBIW undergoes the first hydrogenolysis reaction under the action of Pd / C catalyst, i.e. removal of 4 benzyl groups connected with the cage structure, C-N chemical bond rupture, to obtain tetraacetyl dibenzyl hexaazaisowurtzitane (TADBIW). Due to the instability of the cage structure of HBIW, the hydrogenolysis reaction needs to be carried out at low temperature. Pd / C catalyst has good catalytic activity and selectivity at low temperature, and becomes a commonly used catalyst for the debenzyl reaction of HBIW hydrogenolysis, but the loading amount of noble metal is as high as 3-20wt% (US5693794A, 1997). In addition, Pd / C catalyst has poor stability, and in the hydrogenolysis reaction, Pd particles will migrate on the surface of the carrier, causing agglomeration and loss, resulting in catalyst deactivation, low repeated utilization rate, thereby limiting the large-scale production of CL-20.

[0003] To reduce the amount of Pd used, existing technologies employ strategies such as alloying, strong metal-support interactions, and spatial confinement to construct catalysts. For example, Yang Shuang et al. used transition metals to partially replace noble metals, synthesizing PdFe / TiO2, PdFe / SiO2, and PdFe / Al2O3 alloyed catalysts via deposition-precipitation-reduction methods. In the PdFe / TiO2 catalyst, the Pd particle size was 5 nm. When the mass ratio of Pd to HBIW was 1.97 wt%, the yield of TADBIW, the product of the first hydrogenobenzyl dehydrogenation reaction, was 76% in the PdFe / TiO2 catalyst. Although the proportion of noble metals was reduced by partially replacing Pd with Fe, the large Pd particles (5 nm) resulted in low Pd atomic activity (Catalysis Communication, 2018, 109, 28-32). Furthermore, Yang Shuang et al. prepared titanium oxide-supported Pd catalysts using a deposition-precipitation method, utilizing strong metal-support interactions to anchor active sites, while still achieving a Pd particle size of 5 nm. When the mass ratio of Pd to substrate HBIW is 1.0 wt%, the product yield of the single hydrogenobenzyl dehydrogenation reaction reaches 89%. Although the strong metal-support interaction can improve the dispersion, the particle size is still relatively large (molecular catalysis, 2019, 477, 110556). Chen Kun et al. used the spatial confinement effect of two-dimensional material C3N4 to prevent Pd site aggregation and prepared Pd / g-mpgC3N4 catalyst by chemical reduction method. The average Pd particle size is 4.5 nm. The results show that when Pd is 1.0 wt% of substrate HBIW, the TADBIW yield reaches 81.7%. However, after use, the Pd particle size of the catalyst increases to ~8.6 nm, resulting in a decrease in catalyst activity (Applied catalysis A, general, 2021, 624, 118310). Qiu Wenge et al. used a lignocellulose derivative carbon support and loaded metal Pd onto the support using a deposition precipitation method. The average Pd particle size was 2-3 nm. The results showed that when the mass ratio of Pd to HBIW was 1.4 wt‰, the TADBIW yield reached 90%, indicating that small particle size showed better catalytic activity. However, the 2-3 nm particle size was still much higher than the sub-nanometer scale (<1 nm) (Catalysts, 2023, 13, 637).

[0004] The traditional wet chemical preparation processes used for the Pd-based catalysts mentioned above, such as deposition-precipitation-reduction, liquid-phase impregnation, and deposition-precipitation, generally suffer from insufficient control over the particle size and dispersion of the Pd active component. The above studies indicate that the activity of HBIW hydrogenobenzyl dehydrogenation strongly depends on the size and dispersion state of the Pd active sites. Sub-nanometer scale (<1 nm) Pd clusters or single atoms exhibit the highest atom utilization and intrinsic activity. However, traditional methods struggle to achieve atomic-level precise control over the nucleation and growth of Pd species, resulting in Pd particles that are typically large (reported in the literature to be mostly in the range of 2-5 nm or even larger), unevenly distributed, and prone to aggregation. This leads to a limited number of effective active sites in existing catalysts and low Pd atom utilization. Therefore, the large Pd particle size, poor dispersion, low atom utilization, and the resulting high cost in existing technologies remain core technical obstacles restricting the large-scale, low-cost synthesis of CL-20. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a sub-nanometer palladium catalyst supported on titanate nanotubes and its preparation method, thereby solving the technical problem that the average particle size of palladium, the active material in existing catalysts, needs to be further reduced.

[0006] Another objective of this invention is to provide an application of a sub-nanometer palladium catalyst supported on titanate nanotubes, thereby solving the technical problem that the amount of palladium, the active material of the catalyst, needs to be further reduced in the primary hydrogenodebenzyl hexaazaisowurtzite reaction in the prior art.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A titanate nanotube supported sub-nanometer palladium catalyst, wherein the catalyst uses titanate nanotubes as a support, and the active material palladium is dispersed and loaded on the support in the form of sub-nanometer clusters, the average particle size of the active material palladium is <1nm, and the loading amount of the active material palladium is 1.9wt% to 3.9wt%.

[0008] The present invention also has the following technical features: This invention also protects a method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes as described above. The method uses titanate nanotubes as a support, heat-treating the titanate nanotube support in air at 300–400°C for 1–3 hours to regulate the hydroxyl density on the surface of the titanate nanotube support. The heat-treated titanate nanotube support is then placed in an atomic layer deposition reactor, using palladium hexafluoroacetylacetone as a precursor and formaldehyde as a precursor, to synthesize the sub-nanometer palladium catalyst supported on titanate nanotubes using atomic layer deposition.

[0009] Preferably, in the atomic layer deposition reactor, a vacuum is drawn to control the chamber pressure of the atomic layer deposition reactor at 100-200 Pa, and the reactor is heated to 150-200°C; the preheating temperature of the palladium hexafluoroacetylacetone is 50°C-60°C; in the atomic layer deposition method, the flow rate of the inert carrier gas is 100-200 ml / min, and then the active material palladium is deposited on the titanate nanotubes for 1-5 cycles using the atomic layer deposition method.

[0010] Most preferably, the heat treatment time is 2 hours; in the atomic layer deposition reactor, a vacuum is drawn to control the chamber pressure of the atomic layer deposition reactor at 130 Pa, and the atomic layer deposition reactor is heated to 200°C; the preheating temperature of the palladium hexafluoroacetylacetone is 60°C; in the atomic layer deposition method, the flow rate of the inert carrier gas is 100 ml / min.

[0011] Specifically, one cycle of the atomic layer deposition method includes the following four steps: Step 1: Inject palladium hexafluoroacetylacetonate precursor to induce an interfacial chemical reaction with titanate nanotubes. The injection time of palladium hexafluoroacetylacetonate precursor is 100-500 seconds.

[0012] Step 2: Purge with inert carrier gas for 500-1000 seconds.

[0013] Step 3: Inject palladium hexafluoroacetylacetonate onto the surface of the formaldehyde precursor. The formaldehyde precursor injection time is 200-1000 seconds.

[0014] Step 4: Purge with inert carrier gas for 500-1000 seconds.

[0015] Preferably, one cycle of the atomic layer deposition method includes the following four steps: Step 1: Inject palladium hexafluoroacetylacetonate precursor to induce an interfacial chemical reaction with titanate nanotubes. The injection time of palladium hexafluoroacetylacetonate precursor is 200-400 seconds.

[0016] Step 2: Purge with inert carrier gas for 500-1000 seconds.

[0017] Step 3: Inject palladium hexafluoroacetylacetonate onto the surface of the formaldehyde precursor. The formaldehyde precursor injection time is 400-800 seconds.

[0018] Step 4: Purge with inert carrier gas for 500-1000 seconds.

[0019] Preferably, one cycle of the atomic layer deposition method comprises the following four steps: Step 1: Inject palladium hexafluoroacetylacetonate precursor to induce an interfacial chemical reaction with titanate nanotubes. The injection time of the palladium hexafluoroacetylacetonate precursor is 300 seconds.

[0020] Step 2: Purge with inert carrier gas for 500 seconds.

[0021] Step 3: Inject palladium hexafluoroacetylacetonate onto the surface of the formaldehyde precursor. The injection time of the formaldehyde precursor is 600 seconds.

[0022] Step 4: Purge with inert carrier gas for 500 seconds.

[0023] This invention also protects the application of the titanate nanotube-supported subnanometer palladium catalyst described above in the primary hydrogenodebenzylhexaazaisowulzane reaction. The amount of palladium, the active material, in the titanate nanotube-supported subnanometer palladium catalyst is 0.76 wt‰ to 1.56 wt‰ of the hexabenzylhexaazaisowulzane.

[0024] Preferably, in the titanate nanotube-supported sub-nanometer palladium catalyst, the number of deposition cycles of the active material palladium is 1 or 2.

[0025] Preferably, in the primary hydrogenodebenzyl hexaazaisowurtzite reaction, the yield of the product tetraacetyl dibenzyl hexaazaisopentane is 85.6% to 89.1%.

[0026] Specifically, the reaction temperature for the primary hydrogenodebenzyl hexaazaisowurtzite reaction is 17°C to 30°C.

[0027] Preferably, the reaction temperature for the primary hydrogenodebenzyl hexaazaisowurtzite reaction is 25°C.

[0028] Compared with the prior art, the present invention has the following technical effects: (I) This invention uses atomic layer deposition (ALD) technology to synthesize metallic Pd on the surface of an air-heat-treated H2Ti3O7 support. As the heat treatment temperature increases, the hydroxyl group density on the H2Ti3O7 support surface increases. The hydroxyl groups can effectively inhibit the aggregation of Pd sites, forming a Pd-O-support covalent bond structure, which can inhibit the migration of Pd particles during the reaction and achieve Pd cluster size down to the sub-nanometer scale (<1 nm).

[0029] (II) The H2Ti3O7 support for supporting sub-nanometer Pd cluster catalysts in this invention, after being heat-treated at medium and high temperatures, can still maintain a certain mesoporous structure, inhibit particle surface migration and agglomeration, which is conducive to the diffusion of reactants and products, improves the conversion rate, realizes the atom-economic design of HBIW debenzylidene catalyst, and reduces the synthesis cost of CL-20.

[0030] (III) The present invention constructs sub-nanometer Pd clusters by atomic layer deposition on heat-treated titanate nanotube carriers, which can reduce the amount of palladium, the active material of the catalyst in the primary hydrogenodebenzyl hexaazaisowurtzite reaction, to 0.68wt‰~1.48wt‰. Attached Figure Description

[0031] Figure 1 The images show the morphological changes of HTO supports after different heat treatment temperatures. Among them, (a, b) are TEM images of HTO; (c, d) are TEM images of HTO at 300℃; (e, f) are TEM images of HTO at 400℃; and (g, h) are TEM images of HTO at 500℃.

[0032] Figure 2 The pore structure of the HTO, HTO-300 ℃, HTO-400 ℃ and HTO-500 ℃ supports were characterized. Among them, (a) is the N2 isotherm adsorption-desorption curve; (b) is the pore size distribution curve.

[0033] Figure 3 XRD patterns of HTO, HTO-300℃, HTO-400℃ and HTO-500℃ supports.

[0034] Figure 4 Characterization results of the Pd / HTO catalyst are shown. Among them, (a) is a TEM image and a statistical diagram of the Pd particle size distribution; (b) is an HRTEM image.

[0035] Figure 5 Characterization results of the Pd / HTO-300℃ catalyst. Among them, (a) is a TEM image and a statistical diagram of Pd particle size distribution; (b) is an HRTEM image.

[0036] Figure 6 The aberration-corrected HAADF-STEM characterization of the Pd / HTO-400℃ catalyst is shown. Among them, (a) is a low-magnification image and a statistical diagram of the Pd particle size distribution; (b) is a high-magnification image; and (c) is a HAADF-STEM image and the corresponding EDX elemental distribution diagram.

[0037] Figure 7 The aberration-corrected HAADF-STEM characterization of the Pd / HTO-500℃ catalyst is shown. Among them, (a) is a low-magnification image and a statistical diagram of the Pd particle size distribution; (b) is a high-magnification image; and (c) is a HAADF-STEM image and the corresponding EDX elemental distribution diagram.

[0038] Figure 8ATR-FTIR spectra of supports and corresponding catalysts at different heat treatment temperatures are shown. Among them, (a) represents HTO, HTO-300℃, HTO-400℃, and HTO-500℃ supports; (b) represents Pd / HTO, Pd / HTO-300℃, Pd / HTO-400℃, and Pd / HTO-500℃ catalysts.

[0039] Figure 9 Performance curves of Pd / HTO, Pd / HTO-300℃, Pd / HTO-400℃, and Pd / HTO-500℃ catalysts are shown. Among them, (a) is the N2 isotherm adsorption-desorption curve; (b) is the pore size distribution curve.

[0040] Figure 10 The characterization results of the Pd / P25 catalyst are shown below. Among them, (a) is a TEM image and particle size distribution diagram; (b) is an HRTEM image; and (c) is a HAADF-STEM image and corresponding EDX elemental distribution diagram.

[0041] Figure 11 The images show HAADF-STEM images of the recovered catalyst and the Pd particle size distribution. (a) represents Pd / HTO; (b) represents Pd / HTO at 300℃; (c) represents Pd / HTO at 400℃; and (d) represents Pd / HTO at 500℃.

[0042] The specific content of the present invention will be further explained in detail below with reference to the embodiments. Detailed Implementation

[0043] It should be noted that, unless otherwise specified, all materials and equipment used in this invention are those known in the art. For example, the atomic layer deposition reactor is an atomic layer deposition reactor known in the art. The titanate nanotubes are titanate nanotubes known in the art.

[0044] Atomic layer deposition (ALD) involves the saturation of chemisorption between a gaseous precursor and the active sites on a support. Through precise cycle number control, the size of Pd clusters is reduced to the sub-nanometer scale (<1 nm), resulting in an in-situ Pd-O-support covalent bond structure. This suppresses particle surface migration and aggregation, optimizing the catalyst's activity, selectivity, and stability. Based on this, this invention designs a mesoporous H2Ti3O7-supported sub-nanometer Pd cluster catalyst: the H2Ti3O7 support surface -OH density increases after air heat treatment, effectively suppressing Pd site aggregation. This represents the first atom-economical design of an HBIW debenzylidene catalyst, reducing the synthesis cost of CL-20.

[0045] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0046] Comparative Example 1: This comparative example provides a method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes. The method includes the following steps: placing a titanate nanotube (H2Ti3O7, abbreviated as HTO) support into an atomic layer deposition reactor, sealing the reactor, evacuating the chamber to control the pressure at 130 Pa, heating to 200 °C, preheating the hexafluoroacetylacetone palladium precursor to 60 °C, and using inert carrier gas nitrogen at a flow rate of 100 ml / min, and then starting to deposit the active material palladium on the titanate nanotubes.

[0047] The four steps of one cycle of atomic layer deposition are the same as the four steps of one cycle of atomic layer deposition given in Example 1.

[0048] In this comparative example, the atomic layer deposition period of the active material palladium is 1, and the sub-nanometer palladium catalyst supported on titanate nanotubes is denoted as 1cPd / HTO.

[0049] This comparative example also illustrates the application of a titanate nanotube-supported sub-nanometer palladium catalyst in the primary hydrogenolysis-benzylation reaction of HBIW. The catalyst used in this comparative example is the 1cPd / HTO catalyst with titanate nanotube-supported sub-nanometer palladium catalyst prepared in Comparative Example 1.

[0050] The specific process of the HBIW primary hydrodebenzylation reaction is the same as that given in Example 1. The loading of palladium, the amount of palladium used, the yield of TADBIW product, and the number of conversions (TON) are listed in Table 2.

[0051] Example 1: This embodiment provides a method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes. The method includes the following steps: First, titanate nanotube (H2Ti3O7, abbreviated as HTO) support is placed in a high-temperature furnace and calcined at 300°C in air atmosphere for 2 hours (the support heat-treated under this condition is denoted as HTO-300°C) to obtain the heat-treated titanate nanotube support.

[0052] The heat-treated titanate nanotube carrier was then placed in an atomic layer deposition reactor, the reactor was sealed, and a vacuum was drawn to control the chamber pressure at 130 Pa. The reactor was heated to 200 °C, the hexafluoroacetylacetone palladium precursor was preheated to 60 °C, and the inert carrier gas nitrogen flow rate was 100 ml / min. Then, the active material palladium was deposited on the titanate nanotube.

[0053] One cycle of atomic layer deposition (ALD) consists of the following four steps: Step 1: Inject palladium hexafluoroacetylacetonate precursor to induce an interfacial chemical reaction with titanate nanotubes. The injection time of the palladium hexafluoroacetylacetonate precursor is 300 seconds.

[0054] Step 2: Purge with inert carrier gas nitrogen for 500 seconds.

[0055] Step 3: Inject palladium hexafluoroacetylacetonate onto the surface of the formaldehyde precursor. The injection time of the formaldehyde precursor is 600 seconds.

[0056] Step 4: Purge with inert carrier gas nitrogen for 500 seconds.

[0057] In this embodiment, the atomic layer deposition period of the active material palladium is 1, and the sub-nanometer palladium catalyst supported on titanate nanotubes is denoted as 1cPd / HTO-300℃.

[0058] This embodiment also provides the application of a titanate nanotube-supported sub-nanometer palladium catalyst in the primary hydrogenolysis-benzylation reaction of HBIW. The catalyst used in this embodiment is the titanate nanotube-supported sub-nanometer palladium catalyst 1cPd / HTO-300℃ catalyst prepared in Example 1.

[0059] The specific process of the HBIW hydrogenolysis reaction is as follows: 0.25 g HBIW, 8 mg catalyst, 0.63 mL N,N-dimethylformamide, 5 μL bromobenzene, and 0.375 mL acetic anhydride were sequentially added to a 100 mL high-pressure reactor. After sealing the reactor, it was purged three times with inert nitrogen carrier gas, then H2 was introduced and purged three more times. Finally, H2 was introduced to ensure that the pressure displayed on the reactor was at least 0.4 MPa. The reaction was carried out at 600 rpm at 25 °C for 24 h. After filtration, washing, and drying, TADBIW was obtained. The loading of palladium, the amount of palladium used, the yield of TADBIW, and the conversion number (TON) are listed in Table 2.

[0060] Example 2: This embodiment provides a method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes. The method includes the following steps: First, the titanate nanotube support is placed in a high-temperature furnace and calcined at 400°C in air for 2 hours (the support heat-treated under this condition is denoted as HTO-400°C) to obtain the heat-treated titanate nanotube support.

[0061] The heat-treated titanate nanotube carrier was then placed in an atomic layer deposition reactor, the reactor was sealed, and a vacuum was drawn to control the chamber pressure at 130 Pa. The reactor was heated to 200 °C, the hexafluoroacetylacetone palladium precursor was preheated to 60 °C, and the inert carrier gas nitrogen flow rate was 100 ml / min. Then, the active material palladium was deposited on the titanate nanotube.

[0062] The four steps of one cycle of atomic layer deposition are the same as the four steps of one cycle of atomic layer deposition given in Example 1.

[0063] In this embodiment, the atomic layer deposition period of the active material palladium is 1, and the sub-nanometer palladium catalyst supported on titanate nanotubes is denoted as 1cPd / HTO-400℃.

[0064] This embodiment also provides the application of a titanate nanotube-supported sub-nanometer palladium catalyst in the primary hydrogenobenzyl dehydrogenation reaction of HBIW. The catalyst used in this embodiment is the titanate nanotube-supported sub-nanometer palladium catalyst 1cPd / HTO-400℃ catalyst prepared in Example 2.

[0065] The specific process of the HBIW primary hydrodebenzylation reaction is the same as that given in Example 1. The loading of palladium, the amount of palladium used, the yield of TADBIW product, and the number of conversions (TON) are listed in Table 2.

[0066] Comparative Example 2: This embodiment provides a method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes. The method includes the following steps: First, the titanate nanotube support is placed in a high-temperature furnace and calcined at 500°C in air for 2 hours (the support heat-treated under this condition is denoted as HTO-500°C) to obtain the heat-treated titanate nanotube support.

[0067] The heat-treated titanate nanotube carrier was then placed in an atomic layer deposition reactor, the reactor was sealed, and a vacuum was drawn to control the chamber pressure at 130 Pa. The reactor was heated to 200 °C, the hexafluoroacetylacetone palladium precursor was preheated to 60 °C, and the inert carrier gas nitrogen flow rate was 100 ml / min. Then, the active material palladium was deposited on the titanate nanotube.

[0068] The four steps of one cycle of atomic layer deposition are the same as the four steps of one cycle of atomic layer deposition given in Example 1.

[0069] In this comparative example, the atomic layer deposition period of the active material palladium is 1, and the sub-nanometer palladium catalyst supported on titanate nanotubes is denoted as 1cPd / HTO-500℃.

[0070] This comparative example also illustrates the application of a titanate nanotube-supported sub-nanometer palladium catalyst in the primary hydrogenolysis-benzylation reaction of HBIW. The catalyst used in this comparative example is the titanate nanotube-supported sub-nanometer palladium catalyst 1cPd / HTO-500℃ catalyst prepared in Comparative Example 2.

[0071] The specific process of the HBIW primary hydrodebenzylation reaction is the same as that given in Example 1. The loading of palladium, the amount of palladium used, the yield of TADBIW product, and the number of conversions (TON) are listed in Table 2.

[0072] In Comparative Example 1, Example 1, Example 2 and Comparative Example 2, TEM was used to characterize the effect of heat treatment temperature on the morphology of HTO. Figure 1 The a in the figure shows that the morphology of HTO is a tubular nanostructure with a length ranging from tens to hundreds of nanometers. Figure 1 The inner and outer diameters of HTO were measured to be approximately 5.40 and 9.91 nm, respectively. Figure 1 The values ​​of c to h indicate that the heat treatment temperature has a significant impact on the morphology of titanate nanotubes: as the calcination temperature increases to 300℃, the titanate nanotubes still maintain their tubular morphology; however, the inner and outer diameters of the samples decrease to 4.09 and 8.43 nm, respectively, and the length of the nanotubes shortens, indicating that the titanate nanotubes undergo dehydration at 300℃. Figure 1 (c and d in the text); when the temperature reaches 400℃, some nanotubes begin to break, the tube walls collapse, and anatase nanocrystals appear, as shown in the figure. Figure 1 As shown in e and f in the figure. Continuing to increase the temperature to 500℃, the titanium dioxide nanotubes completely collapse and transform into nanorods, as shown in the figure. Figure 1 As shown in g and h, anatase nanocrystals exhibit good crystallinity.

[0073] Figure 2 The effects of different heat treatment temperatures (300–500 °C) on the BET specific surface area and pore size distribution of HTO were shown. All samples exhibited type IV isotherm curves with an H3 hysteresis loop. Figure 2 (a) in the middle.

[0074] As shown in Table 1, the specific surface areas of HTO, HTO-300℃, HTO-400℃, and HTO-500℃ are 274 m², respectively. 2 / g、186 m 2 / g、125 m 2 / g and 90 m 2 / g indicates that the specific surface area of ​​nanotubes decreases with increasing temperature.

[0075] Table 1. Changes in pore structure of HTO support and loaded Pd metal at different heat treatment temperatures.

[0076] from Figure 2Figure b shows that the pore size distribution of HTO is affected by the calcination temperature: the pore size distribution of untreated HTO is narrow and shows a peak at 11.3 nm; when the temperature increases to 300℃, the pore size peak increases to 15.0 nm; when the temperature continues to increase to 400℃, the pore size distribution widens and expands to 21.9 nm; further increasing the calcination temperature to 500℃ causes the pore size to gradually expand to 26.6 nm. The above results reflect the wide pore size distribution caused by the transformation of titanate nanotubes into nanoparticles.

[0077] like Figure 3 As shown, the untreated HTO sample exhibits H2Ti3O7 diffraction peaks at 24˚ and 48˚. When the annealing temperature increases, the characteristic peaks of HTO disappear and gradually transform into anatase.

[0078] The morphology and dispersion state of Pd on titanate tube supports after air calcination at different temperatures were systematically studied using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX). Figure 4 As shown in a and b, Pd nanoparticles were successfully loaded onto the HTO tube wall, with an average particle size of 4.3 ± 2.3 nm. As the calcination temperature increased to 300℃, the average particle size of Pd in ​​the Pd / HTO-300℃ catalyst decreased to 2.9 ± 1.3 nm. Figure 5 (a and b in the image) indicates that appropriate heating helps to obtain smaller Pd nanoparticles. When the calcination temperature is further increased to 400℃, the HAADF-STEM image ( Figure 6 Sub-nanometer Pd clusters with an average size of approximately 0.9 nm can be clearly observed in (a) and (b) in the diagram, and they account for 44% of the total. The corresponding EDX elemental distribution diagrams ( Figure 6 c) indicates that the Pd element is uniformly distributed on the support. Further increasing the temperature to 500℃, as... Figure 7 The HAADF-STEM results for a and b in the figure show that the average size of Pd clusters is further reduced to 0.8 nm, accounting for more than 70%. The corresponding EDX elemental mapping ( Figure 7 c) in the above results also confirms that Pd is highly uniformly dispersed. These results indicate that as the heat treatment temperature of the HTO support increases, the particle size of the Pd particles it supports gradually decreases, and the size distribution becomes more concentrated, demonstrating that the heat treatment temperature has a significant regulatory effect on the metal dispersion behavior.

[0079] Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was used to characterize the hydroxyl groups on the surface of calcined HTO and ALD-deposited Pd / HTO catalysts. Figure 8 The value of 'a' in the figure is 3100-3600 cm.-1 The broad peak at this location is attributed to the stretching mode of the -OH groups, indicating that the H2Ti3O7 surface has abundant hydroxyl groups. Due to dehydration after heating, the number of -OH groups decreased after calcination at 300℃, but regenerated after calcination at 400–500℃, increasing the hydroxyl density. Figure 8 As shown in b, the number of -OH groups significantly decreased after Pd deposition on the HTO support, indicating that Pd undergoes chemisorption via -OH bonds. The Pd / HTO-500℃ catalyst exhibited the minimum -OH signal, indicating that Pd is fully bound to the surface hydroxyl groups, thus the Pd clusters are highly dispersed on the HTO-500℃ support. These results show that HTO heat treatment generates a large number of hydroxyl groups, which effectively anchor Pd sites, resulting in atomically dispersed Pd clusters on the HTO-400℃ and HTO-500℃ supports.

[0080] Figure 9 Figure a shows the N2 adsorption-desorption isotherms of Pd / HTO, Pd / HTO-300℃, Pd / HTO-400℃, and Pd / HTO-500℃ catalysts. All catalysts exhibit typical Type IV isotherm characteristics and have a Type H3 hysteresis loop, with specific surface areas of 187, 170, 116, and 84 m² / g, respectively. The gradual decrease in specific surface area can be attributed to the deposition of Pd species within the support pores. From the corresponding pore size distribution curves (… Figure 9 As can be further seen in b), the mesoporous structure of the catalyst changes significantly after Pd loading. As the Pd species occupy the pores of the support, the pore size and pore volume decrease.

[0081] Example 3: This embodiment provides a method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes. This method is basically the same as the method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes given in Example 2. The only difference is that in this embodiment, the atomic layer deposition period of the active material palladium is 2, and the sub-nanometer palladium catalyst supported on titanate nanotubes is denoted as 2cPd / HTO-400℃.

[0082] This embodiment also provides the application of a titanate nanotube-supported sub-nanometer palladium catalyst in the primary hydrogenolysis-benzyl reaction of HBIW. The catalyst used in this embodiment is the titanate nanotube-supported sub-nanometer palladium catalyst 2cPd / HTO-400℃ catalyst prepared in Example 3.

[0083] The specific process of the HBIW primary hydrodebenzylation reaction is the same as that given in Example 1. The loading of palladium, the amount of palladium used, the yield of TADBIW product, and the number of conversions (TON) are listed in Table 2.

[0084] Comparative Example 3: This comparative example provides a method for preparing a TiO2-supported palladium catalyst. The method includes the following steps: First, the catalyst support TiO2 (P25) is placed in an atomic layer deposition reactor, the reactor is sealed, a vacuum is drawn to control the chamber pressure at 130 Pa, the temperature is heated to 200°C, the palladium hexafluoroacetylacetone precursor is preheated to 60°C, the flow rate of inert carrier gas nitrogen is 100 ml / min, and then the active material palladium is deposited on P25s.

[0085] In this comparative example, the four steps of one cycle of atomic layer deposition are the same as the four steps of one cycle of atomic layer deposition given in Example 1.

[0086] In this comparative example, the atomic layer deposition period of the active material palladium is 1, and the TiO2-supported palladium catalyst is denoted as 1cPd / P25.

[0087] This comparative example also illustrates the application of a TiO2-supported palladium catalyst in the primary hydrodebenzene reaction of HBIW. The catalyst used in this comparative example is the TiO2-supported palladium catalyst 1cPd / P25 catalyst prepared in Comparative Example 3.

[0088] The specific process of the HBIW primary hydrodebenzylation reaction is the same as that given in Example 1. The loading of palladium, the amount of palladium used, the yield of TADBIW product, and the number of conversions (TON) are listed in Table 2.

[0089] The HBIW primary hydrogenobenzyl dehydrogenation reaction was evaluated using Pd catalysts supported on HTO series supports and commercial P25. Catalytic performance data are listed in Table 2. The Pd / HTO-400℃ catalyst exhibited the highest catalytic activity, achieving a TADBIW product yield of 88.1% and a turnover number (TON) of 160. The Pd / HTO-300℃ catalyst showed lower activity, with a TADBIW product yield of 85.6% and a TON of 70. Unmodified Pd / HTO exhibited the lowest catalytic efficiency, with a yield of only 0.2% and a TON of 0.23. The superior performance of Pd / HTO-400℃ is attributed to its highly dispersed sub-nanometer-scale Pd active sites. Although Pd / HTO-500℃ also possesses atomically dispersed Pd species, no TADBIW product was generated. The lower specific surface area and nanoparticles of the HTO-500℃ support reduced the reactant concentration at the active sites, severely hindering the effective collision frequency between HBIW reactant molecules and Pd catalytic centers. The Pd / P25 catalyst in Comparative Example 1 was also prepared using the ALD method, with small-scale, highly dispersed Pd nanoparticles deposited on the surface of the P25 support TiO2. Figure 10(a to c) in the above-mentioned samples did not exhibit catalytic activity after undergoing a single hydrogenobenzyl dehydrogenation reaction with HBIW, demonstrating that the TiO2 spherical nanoparticles on the support can only adsorb reactant molecules on a limited surface.

[0090] The catalyst recovered from the HBIW hydrodebenzylation reaction was then structurally characterized. HAADF-STEM ( Figure 11 (a to d) in the figure reveals the morphology and particle size distribution of Pd species after the catalytic reaction. Figure 11 Figure 'a' indicates that the Pd nanoparticle size of the Pd / HTO catalyst after the reaction even decreased to 2.9 ± 0.8 nm, and Pd was redispersed on the nanotubes under a hydrogen atmosphere. From... Figure 11 Figures b to c show that the Pd particle size of the Pd / HTO-300℃, Pd / HTO-400℃, and Pd / HTO-500℃ catalysts increased to 3.5±1.4 nm, 3.0±1.8 nm, and 2.5±1.4 nm, respectively, after the reaction. This indicates that the small-particle, sub-nanometer-sized Pd clusters undergo slight aggregation during the reaction, but still maintain a highly dispersed state. The surface hydroxyl groups of the support have a good anchoring effect on Pd species.

[0091] Table 2 Catalyst performance evaluation results

Claims

1. A sub-nanometer palladium catalyst supported on titanate nanotubes, characterized in that, The catalyst uses titanate nanotubes as a support, and the active material palladium is dispersed and loaded on the support in the form of sub-nano clusters. The average particle size of the active material palladium is <1 nm, and the loading of the active material palladium is 1.9 wt% to 3.9 wt%.

2. A method for preparing a sub-nanometer palladium catalyst supported on titanate nanotubes as described in claim 1, characterized in that, This method uses titanate nanotubes as a carrier. The titanate nanotube carrier is heat-treated in an air atmosphere at 300-400℃ for 1-3 hours to regulate the hydroxyl density on the surface of the titanate nanotube carrier. The heat-treated titanate nanotube carrier is then placed in an atomic layer deposition reactor. Using palladium hexafluoroacetylacetone as a precursor and formaldehyde as a precursor, a sub-nanometer palladium catalyst supported on titanate nanotubes is synthesized by atomic layer deposition.

3. The method for preparing the sub-nanometer palladium catalyst supported on titanate nanotubes as described in claim 2, characterized in that, In the atomic layer deposition reactor, a vacuum is drawn to control the chamber pressure of the atomic layer deposition reactor at 100-200 Pa, and the reactor is heated to 150-200 °C. The preheating temperature of the palladium hexafluoroacetylacetone is 50-60 °C. In the atomic layer deposition method, the flow rate of the inert carrier gas is 100-200 ml / min, and then the active material palladium is deposited on the titanate nanotubes for 1-5 cycles using the atomic layer deposition method.

4. The method for preparing the sub-nanometer palladium catalyst supported on titanate nanotubes as described in claim 3, characterized in that, The heat treatment time is 2 hours; in the atomic layer deposition reactor, a vacuum is drawn to control the chamber pressure of the atomic layer deposition reactor at 130 Pa, and the atomic layer deposition reactor is heated to 200°C; the preheating temperature of the palladium hexafluoroacetylacetone is 60°C; in the atomic layer deposition method, the flow rate of the inert carrier gas is 100 ml / min.

5. The method for preparing the sub-nanometer palladium catalyst supported on titanate nanotubes as described in claim 2, characterized in that, One cycle of the atomic layer deposition method described above includes the following four steps: Step 1: Inject palladium hexafluoroacetylacetonate precursor to induce an interfacial chemical reaction with titanate nanotubes. The injection time of palladium hexafluoroacetylacetonate precursor is 100-500 seconds. Step 2: Purge with inert carrier gas for 500-1000 seconds; Step 3: Inject palladium hexafluoroacetylacetonate onto the surface of the formaldehyde precursor. The formaldehyde precursor injection time is 200-1000 seconds. Step 4: Purge with inert carrier gas for 500-1000 seconds.

6. The method for preparing the sub-nanometer palladium catalyst supported on titanate nanotubes as described in claim 5, characterized in that, One cycle of atomic layer deposition (ALD) consists of the following four steps: Step 1: Inject palladium hexafluoroacetylacetonate precursor to induce an interfacial chemical reaction with titanate nanotubes. The injection time of palladium hexafluoroacetylacetonate precursor is 200-400 seconds. Step 2: Purge with inert carrier gas for 500-1000 seconds; Step 3: Inject palladium hexafluoroacetylacetonate onto the surface of the formaldehyde precursor reduction agent. The formaldehyde precursor injection time is 400-800 seconds. Step 4: Purge with inert carrier gas for 500-1000 seconds.

7. The method for preparing the sub-nanometer palladium catalyst supported on titanate nanotubes as described in claim 6, characterized in that, One cycle of atomic layer deposition (ALD) consists of the following four steps: Step 1: Inject palladium hexafluoroacetylacetonate precursor to induce an interfacial chemical reaction with titanate nanotubes. The injection time of the palladium hexafluoroacetylacetonate precursor is 300 seconds. Step 2: Purge with inert carrier gas for 500 seconds; Step 3: Inject palladium hexafluoroacetylacetonate onto the surface of the formaldehyde precursor. The injection time of the formaldehyde precursor is 600 seconds. Step 4: Purge with inert carrier gas for 500 seconds.

8. The application of the titanate nanotube-supported sub-nanometer palladium catalyst as described in claim 1 in the primary hydrogenolysis reaction of hexabenzylhexaazaisowurtzite; wherein the amount of palladium as the active material in the titanate nanotube-supported sub-nanometer palladium catalyst is 0.76 wt‰ to 1.56 wt‰ of the hexabenzylhexaazaisowurtzite.

9. The application as described in claim 8, characterized in that, In the titanate nanotube-supported sub-nanometer palladium catalyst, the number of deposition cycles of the active material palladium is 1 or 2; in the hexabenzylhexaazaisowulzane primary hydrogenodebenzyl reaction, the yield of the product tetraacetyldibenzylhexaazaisopentane is 85.6% to 89.1%; the reaction temperature of the hexabenzylhexaazaisowulzane primary hydrogenodebenzyl reaction is 17℃ to 30℃.

10. The application as described in claim 9, characterized in that, The reaction temperature for the primary hydrogenodebenzyl hexaazaisowurtzite reaction is 25°C.

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