CN114247460B

CN114247460B - Method for preparing (Co-MOF-g-CTS) -900 by finite field pyrolysis strategy

Info

Publication number: CN114247460B
Application number: CN202111178967.5A
Authority: CN
Inventors: 胡晓丽; 王东升; 刘百玲; 李晓; 高凤伟; 潘清清
Original assignee: Changchun University of Science and Technology
Current assignee: Changchun University of Science and Technology
Priority date: 2021-10-08
Filing date: 2021-10-08
Publication date: 2023-04-25
Anticipated expiration: 2041-10-08
Also published as: CN114247460A

Abstract

The invention relates to a method for preparing (Co-MOF-g-CTS) -900 by a finite field pyrolysis strategy, which comprises the steps of synthesizing Co-MOF (-COOH); synthesizing Co-MOF-g-CTS; synthesis of a (Co-MOF-g-CTS) -900 catalyst; the process of the invention synthesizes a carboxyl functional [ Co (TPT) using the nitrogen-containing ligand TPT and the carboxyl ligand bptc _2/3 (bptc)]·3H ₂ O (Co-MOF (-COOH)). Further grafting CTS on the surface of Co-MOF by EDC/NHS reaction one-pot method to prepare Co-MOF-g-CTS, and calcining at 900 ℃ by taking Co-MOF-g-CTS as a precursor to finally obtain the catalyst (Co-MOF-g-CTS) -900. Shows excellent performance in the catalytic reduction reaction of 4-NP, and can be completed in 80s only by 0.1mg of catalyst, and the reaction rate constant k= 0.0512s ^‑1 The catalytic efficiency is still kept at about 95% after 10 cycles.

Description

Method for preparing (Co-MOF-g-CTS) -900 by finite field pyrolysis strategy

Technical Field

The invention belongs to the field of chemical material synthesis, relates to MOFs-derived carbon material technology, and particularly relates to a method for preparing (Co-MOF-g-CTS) -900 by a finite field pyrolysis strategy.

Background

The carbon-based material has high specific surface area, high conductivity and chemical stability, and has wide application prospect in the fields of environmental remediation, energy storage systems, catalysts, drug delivery and the like. Particularly when the carbon-based material has a narrow pore size distribution and has an associated doping of heteroatoms (e.g., cu, co, N, S, etc.), the performance is easily improved and enhanced to some extent. Carbon-based materials are typically carbonized from organic precursors, and despite the high specific surface area of the synthesized product, there is also an poorly designed porous structure whose chemical composition is difficult to customize. In recent years, MOFs-derived carbon materials solve the above problems well by virtue of their nano-pore structure, high specific surface area and adjustability, and have attracted a great deal of attention in the fields of environmental remediation, catalysis, and the like.

Although MOFs-derived carbon materials have excellent physicochemical properties, they still have many common problems in carbonization: the carbon material obtained by the intrinsic characteristics of MOFs is mainly micropores, which is unfavorable for proton transfer in the catalytic process; the evolution of the structure of MOFs in the carbonization process is not well controlled, and the microstructure is easy to collapse; this can affect the properties of the material to a large extent; most importantly, irreversible aggregation of the metal particles may occur, thereby reducing the dispersibility of the metal particles. To address these limitations, a number of new approaches are currently emerging to facilitate the development of MOFs-derived carbon-based nanostructures, such as with SiO ₂ New chemical aids such as polymers and surfactants; such as with SiO ₂ As a template, zn/Co bimetallic MOFs (SiO2@ZIFs) are self-assembled on the surface of the template, and the hollow sphere carbon material with the surface full of carbon tubes is obtained after carbonization, and the influence of Zn/Co proportion on the formation of the carbon tubes is systematically studied. Although SiO ₂ The rigidity induction method can reach the carbon material with special structure, but the method generally needs to etch SiO ₂ This affects the properties of the material to some extent. However, the polymer itself carbonizes when it is assisted in carbonization, thereby simplifying the post-treatment process, and the doping of heteroatoms in the polymer also improves the properties of the carbon material. There are publications describing the preparation of ZIF-8 nanoparticles, coating mesoporous RF on the surface thereof, and then carbonizing to obtain a mesoporous carbon material due to the confinement effect of RF, and in addition, the study of metal/carbon composite materials [; in the prior art, the ZIF-8@PDA is prepared by coating the surface of the ZIF-8 with the PDA, and then the structural change of the carbon material at different carbonization temperatures is systematically researched.

The above publications describe that MOFs as precursor derived carbon materials have made some important breakthroughs in terms of morphology and composition, but further development of functional MOFs derived carbon materials is of great significance, and also expands the field of carbon material chemistry application.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a method for selecting a nitrogen-containing

ligand

2,4, 6-tri (4-pyridyl) -1,3, 5-triazine (TPT) and a tridentate carboxylic acid ligand [1,1' -biphenyl ]]3,4', 5-tricarboxylic acid (bptc) A carboxyl functionalized [ Co (TPT) was synthesized _2/3 (bptc)]·3H ₂ O (Co-MOF (-COOH)), the surface of the Co-MOF has a large number of non-coordinated carboxyl groups, the carboxyl groups are further grafted with amino groups on Chitosan (CTS) through an EDC/NHS reaction one-pot method to form a Co-MOF-g-CTS material, and the optimal calcination temperature is 900 ℃, so that a limited-domain pyrolysis strategy is obtained to prepare the (Co-MOF-g-CTS) -900 catalyst.

The invention solves the technical problems by adopting the following technical scheme:

a method for preparing (Co-MOF-g-CTS) -900 by a finite field pyrolysis strategy, which comprises the following steps:

synthesis of Co-MOF (-COOH):

20.8mg,0.07 mmole TPT,28.6mg,0.1mmolbptc and 29.1mg,0.1 mmole Co (NO) ₃ ) ₂ ·6H ₂ The mixture of O is dissolved in the mixed solution of 2mLDMA and 3mL deionized water, then the mixture is filled into a 15mL polytetrafluoroethylene high-pressure reaction kettle, the mixture is reacted for 3 days under self-pressure in a 100 ℃ oven, after the reaction is finished, the reaction kettle is naturally cooled to room temperature, a brown yellow massive crystal is obtained, the obtained crystal product is washed three times by using the mixed solution of DMA/water, and is dried at room temperature for standby after being filtered;

synthesis of Co-MOF-g-CTS:

firstly, 1000mg of CTS is weighed and dissolved in 200mL of deionized water, 0.5% formic acid is added to adjust the pH of the solution to be between 3 and 4, the solution is heated and stirred until the CTS is completely dissolved and then cooled to room temperature for standby, then 10mL of CTS solution is taken and added with 30mL of deionized water and 20mL of DMA solution, 50mgCo-MOF (-COOH), 20mg of EDC and 20mg of NHS are respectively and sequentially added into the mixed solution dissolved with the CTS after uniform stirring, stirring is carried out for 2 hours at room temperature, the product is obtained after the reaction is finished, the product is washed by methanol for 3 times, and the Co-MOF-g-CTS material is prepared by a one-pot method;

third step (Co-MOF-g-CTS) -900 catalyst Synthesis:

placing Co-MOF-g-CTS in a magnetic boat, transferring into a tube furnace, and performing heat treatment under 0-0.2MPa nitrogen environment at 900 deg.C at a heating rate of 5 deg.C for min ^-1 Naturally cooling to room temperature after heat treatment to obtain the (Co-MOF-g-CTS) -900 catalyst.

Furthermore, in the step (A): the volume ratio of water is 1:2.

Moreover, in step C ₂₇ H ₂₂ N ₄ O ₉ The Co element analysis calculated values are calculated as percentages: c,45.92%; h,4.20%; n,14.61%, measured values in percent: c,43.55%; h,3.64%; n,9.26%.

The invention has the advantages and positive effects that:

the invention uses the nitrogen-containing ligand TPT and the carboxyl ligand bptc to synthesize a carboxyl-functionalized [ Co (TPT) _2/3 (bptc)]·3H ₂ O (Co-MOF (-COOH)). Further grafting CTS on the surface of Co-MOF by EDC/NHS reaction in one pot to prepare Co-MOF-g-CTS, calcining Co-MOF-g-CTS as precursor at 900 ℃ to obtain the final catalyst (Co-MOF-g-CTS) -900, and performing excellent performance in catalytic reduction reaction of 4-NP, wherein the reaction can be completed in 80s only by 0.1mg of catalyst, and the reaction rate constant k= 0.0512s ^-1 The catalytic efficiency is still kept at about 95% after 10 cycles. Compared with direct carbonization and physical mixed carbonization, (Co-MOF-g-CTS) -900 prepared by the limited-area pyrolysis method can enable internal particles to have smaller particle size and higher dispersibility at higher heat treatment temperature. The regulation of the structure of the pyrolysis of the outer limit region ensures that the (Co-MOF-g-CTS) -900 has larger specific surface area, larger specific pore volume and smaller pore diameter. Furthermore, the protective effect of CTS makes (Co-MOF-g-CTS) -900 still exist in Co-N-C active center and surface active site at high temperature. These characteristics are more favorable for proton transfer and reactant adsorption, and promote the catalytic efficiency of the catalyst. CTS finite field pyrolysis provides a novel auxiliary method for preparing MOFs derived carbon-based materials.

Drawings

FIG. 1 is a flow chart of a one-pot method for preparing Co-MOF-g-CTS from Co-MOF (-COOH) and a flow chart of a Co-MOF-g-CTS derived carbon-based material according to the invention;

FIG. 2 is a chemical structure diagram of an intermediate in the present invention, wherein FIG. 2 (a) is an asymmetric unit of Co-MOF (-COOH), FIG. 2 (b) is a three-dimensional structure diagram of Co-MOF (-COOH), and FIG. 2 (c) is a topological structure diagram of Co-MOF (-COOH);

FIG. 3 is a graph showing XRD data of Co-MOF-g-CTS and experimental and simulated XRD data of Co-MOF in the present invention;

FIG. 4 is a graph showing the results of TGA curves of Co-MOF (-COOH), co-MOF-g-CTS and CTS in the present invention;

FIG. 5 is a graph showing the results of infrared absorption spectra of Co-MOF (-COOH), co-MOF-g-CTS and CTS in the present invention;

FIG. 6 is a graph showing the results of C1 s spectra of Co-MOF (-COOH) and Co-MOF-g-CTS high resolution XPS in the present invention;

FIG. 7 is a digital image of each intermediate form in the present invention, wherein FIG. 7 (a) is a digital image of Co-MOF (-COOH), FIG. 7 (b) is a digital image of CTS, and FIG. 7 (c) is a digital image of Co-MOF-g-CTS in methanol;

FIG. 8 is a graph of particle dispersibility and particle size transmission electron microscopy of carbon-based nanomaterials prepared from different precursors according to the present invention, FIG. 8 (a) is (Co-MOF) -600, FIG. 8 (b) is (Co-MOF) -900, FIG. 8 (c) is (Co-MOF-g-CTS) -600, FIG. 8 (d) is (Co-MOF-g-CTS) -900, FIG. 8 (e) is (Co-MOF/CTS) -600 and FIG. 8 (f) is (Co-MOF/CTS) -900;

FIG. 9 is a scanning electron microscope image of (Co-MOF-g-CTS) -900 in different scales, with FIG. 9 (a) being 1 μm magnification and FIG. 9 (b) being 500nm magnification;

FIG. 10 is an XRD plot of carbon-based materials obtained by carbonizing different processing precursors at 600℃in FIG. 10 (a) and 900℃in FIG. 10 (b) according to the present invention;

FIG. 11 shows a different processing precursor N according to the present invention ₂ The adsorption-desorption results are shown in FIG. 11 (a) as (Co-MOF) -900 and FIG. 11 (b) as N of (Co-MOF-g-CTS) -900 ₂ Adsorption-desorption isotherms; FIG. 11 (c) is (Co-MOF) -900 and FIG. 11 (d) is the BJH mesoporous size distribution of (Co-MOF-g-CTS) -900; FIG. 11 (e) is (Co-MOF) -900 and FIG. 11 (f) (Co-MHK micropore size distribution OF-g-CTS) -900;

FIG. 12 shows XPS spectrum full spectrum of (P) -T, (Co-MOF) -600 (black), (Co-MOF) -900 (red), (Co-MOF-g-CTS) -600 (blue) and (Co-MOF-g-CTS) -900 (pink) in the present invention;

FIG. 13 is a high resolution Co 2p spectrum of (Co-MOF) -600, (Co-MOF) -900, (Co-MOF-g-CTS) -600 and (Co-MOF-g-CTS) -900 in the present invention;

FIG. 14 is a high resolution N1 s spectrum of (Co-MOF) -600, (Co-MOF) -900, (Co-MOF-g-CTS) -600 and (Co-MOF-g-CTS) -900 in the present invention;

FIG. 15 is a high resolution C1 s spectrum of (Co-MOF) -600, (Co-MOF) -900, (Co-MOF-g-CTS) -600 and (Co-MOF-g-CTS) -900 in the present invention;

FIG. 16 shows the 4-NP absorbance structure of the invention, and FIG. 16 (a) shows the characteristic absorbance peak (black) of 4-NP and the characteristic absorbance peak (red) after adding NaBH4 and adding NaBH420 minutes; FIG. 16 (b) is a characteristic absorption peak of 4-AP after addition of the catalyst;

FIG. 17 is a UV-vis spectrum of the catalytic reduction 4-NP of the present invention as a catalyst, FIG. 17 (a) is (Co-MOF) -600, FIG. 17 (b) is (Co-MOF) -900, FIG. 17 (c) is (Co-MOF-g-CTS) -600, FIG. 17 (d) is (Co-MOF-g-CTS) -900, FIG. 17 (e) is (Co-MOF/CTS) -600 and FIG. 17 (f) is (Co-MOF/CTS) -900;

FIG. 18 (a) shows different catalysts C/C in the present invention ₀ A graph of time; FIG. 18 (b) is a bar graph of reaction rate constants for different catalysts;

FIG. 19 is a graph showing the reduction efficiency of 4-NP of the present invention and a microscopic electron microscope, FIG. 19 (a) shows the reduction efficiency of 4-NP in 10 cycles of the experiment, FIG. 19 (b) shows the XRD curve of (Co-MOF-g-CTS) -900 after 10 cycles, and FIG. 19 (c) shows the SEM image and the TEM image after cycles.

Detailed Description

The invention is further illustrated by the following examples, which are intended to be illustrative only and not limiting in any way.

The method selects a nitrogen-containing ligand of2, 4, 6-tri (4-pyridyl) -1,3, 5-triazine (TPT) and a tridentate carboxylic acid ligand of [1,1' -biphenyl ]]3,4', 5-tricarboxylic acid (bptc) A carboxyl functionalized [ Co (TPT) was synthesized _2/3 (bptc)]·3H ₂ O (Co-MOF (-COOH)), the Co-MOF surface has a large number of non-coordinated carboxyl groups, and the carboxyl groups are further grafted with amino groups on Chitosan (CTS) to form Co-MOF-g-CTS material by a one-pot method of EDC/NHS reaction as active sites.

The Co-MOF surface obtained by the method is coated by the nitrogen-containing CTS, and then carbonized at relatively low and relatively high temperature, so that the influence of CTS finite field effect on the structural evolution of the Co-MOF is studied, and meanwhile, the Co-MOF is compared with a sample (Co-MOF/CTS) obtained by physically mixing the Co-MOF and the CTS. The catalytic performance of the (P) -T is evaluated by taking the material (P) -T obtained by carbonizing different precursors at different temperatures as a catalyst for the catalytic reduction reaction of 4-NP. The (Co-MOF-g-CTS) -900 prepared under this method showed the best catalytic performance with good stability during recycling.

The specific experimental process is as follows:

1. experimental reagent

The chemicals and solvents used in the method are all commercially available and are all analytical grade, and can be used without further purification: co (NO) ₃ ) ₂ ·6H ₂ O, 2,4, 6-tris (4-pyridyl) -1,3, 5-triazine (TPT), [1,1' -biphenyl]-3,4', 5-tricarboxylic acid (bptc), 4-nitrophenol (4-NP), sodium borohydride (NaBH) ₄ ) Rhodamine B (RhB) and Methylene Blue (MB) were purchased from Anaglycone Chemicals, 1-ethyl- (3-dimethylaminopropyl) carbodiimide (EDC), N-hydroxysuccinimide (NHS) were sourced from Michael Chemicals, inc., and N, N-Dimethylacetamide (DMA) was supplied by Sigma-Aldrich.

2. Experimental equipment and characterization method

Single crystal data were collected on a bruker smartapexiiiccdx diffractometer at room temperature, mono-color MoK alpha radiation by graphite monochromator

As an incident light source, the SHELXTL-97 package was used for crystal structure analysis and the SADABS program was used for absorption correction of the data. By usingThe morphology of the material was observed by field emission scanning electron microscopy (FE-SEM) and Transmission Electron Microscopy (TEM). Mirror image of the material was studied with an X-ray diffractometer (Bruker) with a scan range of 5-80℃and a scan speed of 5℃min ^-1 . The changes in the composition and chemical state of the surface elements were studied by X-ray photoelectron spectroscopy (XPS). Thermogravimetric analysis (TGA) was performed on a TASTD-Q600 thermal analyzer. The specific surface area of (P) -T was calculated by Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated by Barrett-Joyner-Halenda (BJH) model. Ultraviolet spectral data were obtained using a Jasco V-770 type spectrophotometer.

Synthesis of 3 Co-MOF (-COOH)

TPT (20.8 mg,0.07 mmol), bptc (28.6 mg,0.1 mmol) and Co (NO ₃ ) ₂ ·6H ₂ A mixture of O (29.1 mg,0.1 mmol) was dissolved in a mixture of 2mL of LDMA and 3mL of deionized water. The mixture was then charged into a 15mL polytetrafluoroethylene autoclave and reacted in an oven at 100℃under self-pressure for 3 days. And after the reaction is finished, naturally cooling the reaction kettle to room temperature to obtain brown yellow blocky crystals. The crystalline product obtained is washed three times with a DMA/water (v: v=1:2) mixed solution, filtered and dried at room temperature for use. C (C) ₂₇ H ₂₂ N ₄ O ₉ Calculated value of Co elemental analysis (%): c,45.92; h,4.20; n,14.61, found (%): c,43.55; h,3.64; n,9.26.IR (KBr, cm) ^-1 )：440.21(w)，715.32(w)，1061.08 (w)，1444.59(w)，1615.46(w)，498.96(m)，719.32(m)，1061.08(m)，1444.59(m)， 1615.46(m)，640.46(s)，785.82(s)、1373.45(s)，1514.59(s)，1615.46(s)。

Synthesis of 4 Co-MOF-g-CTS and Co-MOF/CTS

1000mg of CTS is firstly weighed and dissolved in 200mL of deionized water, 0.5% formic acid is added to adjust the pH of the solution to be between 3 and 4, and the solution is heated and stirred until CTS is completely dissolved and then cooled to room temperature for standby. Then 10mL of the solution is taken and added with 30mL of deionized water and 20mL of DMA solution, 50mgCo-MOF (-COOH), 20mg EDC and 20mg NHS are respectively and sequentially added into the mixed solution dissolved with CTS after being uniformly stirred, the mixture is stirred for 2 hours at room temperature, the product is obtained after the reaction is finished by centrifugation, and the product is washed 3 times by methanol, thus preparing the Co-MOF-g-CTS material by a one-pot method. Co-MOF/CTS is obtained by physically mixing Co-MOF and CTS in a mortar according to the grafting ratio of Co-MOF-g-CTS.

Synthesis of 5 (P) -T catalysts

Placing Co-MOF (-COOH), co-MOF-g-CTS and Co-MOF/CTS (which are physically ground and mixed samples of Co-MOF and CTS) into a magnetic boat, transferring into a tube furnace, and heat treating under 0-0.2MPa nitrogen atmosphere at 600 or 900deg.C at a temperature rising rate of 5deg.C for min ^-1 After heat treatment, naturally cooling to room temperature, the resulting carbon-based material is designated (P) -T, where P represents the precursor of the material and T represents the pyrolysis temperature.

6. Catalytic reduction 4-NP test

The catalytic activity of the carbon-based material (P) -T was monitored with an ultraviolet-visible spectrophotometer (UV-vis). The 4-NP catalytic reduction was performed in quartz cuvettes at room temperature. First, 0.2mL of 2.5mM 4-NP solution and 2.5mL of deionized water were added to a quartz cuvette, followed by dropwise addition of 0.2mL of 0.2M freshly prepared NaBH ₄ The solution at this point changed from pale yellow to bright yellow. Finally, 1mg mL is dripped ^-1 0.1mL of a mixed solution of the catalyst and water uniformly dispersed in ultrapure water. The absorbance of the solution in the cuvette was measured at different times and the rate and progress of the reduction reaction was monitored by recording the UV-vis spectra of the reaction system at different time intervals.

7. Results and discussion

7.1 Structural analysis of Co-MOF (-COOH)

TABLE 1 crystallographic data of Co-MOF (-COOH)

Analysis of X-ray single crystal test data shows that Co-MOF (-COOH) has three-dimensional structure, and is crystallized in R-3 space group of triclinic system with molecular formula of [ Co (TPT) _2/3 (bptc)]·3H ₂ O. The crystallographic parameters of this compound are shown in table 1. As shown in FIG. 2 (a), the asymmetric unit of Co-MOF (-COOH) comprises two Co (II) units, which are Co-shared with4N atoms, 6O atoms, and these coordination atoms are from 4 TPT ligands and 4 bptc ligands. Wherein each Co (II) is in penta-coordinated form, coordinated with three oxygen atoms (O1, O2, O3) from the carboxyl group and two nitrogen atoms (N1, N2) from the pyridine

FIG. 2 (b) is a three-dimensional block diagram of Co-MOF (-COOH) where many Co dinuclear units are shown to form a planar network structure by three-linking of TPT ligands, and the linkage between planes is accomplished by linking the Co dinuclear units with two carboxyl groups meta to the bptc ligands. It is noted that the carboxyl group in the bptc ligand represented in green in FIG. 2 (a) is not involved in coordination, and this non-coordinated carboxyl functionality provides an active attachment point for further expansion of Co-MOF. FIG. 2 (c) is a topological structure diagram of Co-MOF (-COOH), which is a network structure of two nodes 3,8-c, with a topological symbol {4 } ¹² ·6 ¹⁰ ·8 ⁶ } ₃ {4 ³ } ₄ 。

7.2 Characterization of Co-MOF (-COOH) and Co-MOF-g-CTS

As shown in FIG. 3, the XRD pattern of the Co-MOF (-COOH) is better in agreement with the simulated XRD data, which shows that the prepared Co-MOF (-COOH) has high crystallinity and better purity, the XRD peak of the Co-MOF-g-CTS is unchanged after the grafting reaction, which shows that the crystal phase is not changed before and after the reaction, the crystal structure is not destroyed, and the Co-MOF has good physical and chemical stability.

FIG. 4 shows the TGA curves of Co-MOF (-COOH), co-MOF-g-CTS and CTS, from which it can be seen that there is about 8.7% mass loss before 341℃due to the loss of solvent at the Co-MOF surface and in the channels. About 80.6% of the mass loss from 341℃to 479℃is due to collapse of the Co-MOF structural framework. Whereas it can be seen from the TGA curve of CTS that its thermal decomposition temperature starts approximately at 233 ℃. Thus, from the TGA gradient of Co-MOF-g-CTS we can understand that the thermal decomposition of Co-MOF-g-CTS is subject to three phases, as shown by solvent removal, pyrolysis of CTS and pyrolysis of Co-MOF, respectively, we can see that with increasing temperature in Co-MOF-g-CTS is carbonized before Co-MOF, the carbon layer formed by CTS plays a role in limiting the domain protection against the subsequent pyrolysis of Co-MOF, preventing agglomeration of metal particles. Furthermore, it can be calculated from the TGA curve that the grafting ratio of CTS in Co-MOF-g-CTS after removal of the solvent is approximately 19.8%, so that the comparative sample of Co-MOF/CTS physical blend was also blended in terms of grafting ratio (Co-MOF: CTS=80.2:19.8).

Fourier transform IR spectra of Co-MOFs are shown in FIG. 5 at 1711.34 and 1061.08cm ^-1 The absorption peaks at the bptc ligand are due to the vibration of c=o and c—oh, respectively, in the non-coordinated carboxyl structure. At 1615.46 and 1373.45cm ^-1 The characteristic peaks at these points are attributed to the vibrations of c=n and C-N in the TPT structure. Notably, in the infrared spectrum of Co-MOF-g-CTS, at 1711.34 and 1061.08cm ^-1 The characteristic absorption peak of carboxyl groups disappeared, which indicates that CTS was successfully grafted to the Co-MOF surface by EDC/NHS one-pot reaction, and the amino groups in CTS formed peptide bonds with non-coordinated carboxyl groups in Co-MOF. Unlike Co-MOF, 1373.45cm in CTS ^-1 The absorption peak at that point is derived from the C-N structure in the polymer. A comparison of the C1 s spectra of Co-MOF (-COOH) and Co-MOF-g-CTS high resolution XPS shows that the peak area of C-O at 288.5eV of Co-MOF-g-CTS is significantly lower than that at 288.5eV of Co-MOF288.5eV, which is caused by the formation of peptide bonds to carboxyl groups during the grafting reaction, while the peak area of C-N at 285.5eV of Co-MOF-g-CTS material is significantly increased, which is caused by the presence of CTS in Co-MOF-g-CTS, which indirectly suggests that Co-MOF-g-CTS was successfully synthesized. As shown in FIG. 7, co-MOF (-COOH) and CTS were seen as brown and white powders, respectively, in a methanol solution, co-MOF-g-CTS was in a white flocculent state, which also indicated from a macroscopic point of view that CTS was coated on the surface of Co-MOF, and Co-MOF-g-CTS was successfully synthesized.

7.3 Microcosmic morphology of (P) -T

And observing the particle dispersibility and particle size of the carbon-based nano materials prepared by different precursors by using a TEM. The internal details of the (P) -T material can be visually observed as shown in fig. 8, with the metal ions in the precursor being converted into nanoparticles during the pyrolysis process. It can be seen in FIG. 8 (a) that the particles in (Co-MOF) -600 appear to be more uniformly distributed in the carbon layer, with example sizes between 12-39nm, whereas the particles in (Co-MOF) -900 in FIG. 8 (b) aggregate together and even overflow the carbon layer, forming larger size particles (129-742 nm), indicating that low temperature carbonization tends to produce small size particles. As can be seen in FIG. 8 (c), the particles in (Co-MOF-g-CTS) -600 have a carbon shell protection, forming a core-shell structure, and the particles are very uniformly dispersed, with the particle size range of 8-40nm. FIG. 8 (c) is a graph of a material formed by carbonizing Co-MOF-g-CTS at 900. It can be seen that particles are also more uniformly dispersed at 900. It is more concentrated in particle size range (47-81 nm), and the particle dispersion and size of (Co-MOF-g-CTS) -900 at high temperature is improved to a large extent compared to (Co-MOF) -900, which illustrates that CTS domain pyrolysis strategy is advantageous for forming relatively small and highly dispersed particles at high temperature. FIG. 8 (e) shows that the resulting material of the physical blend of Co-MOF/CTS sample carbonized at 600 ℃, (Co-MOF/CTS) -600 and (Co-MOF) -600 particles had a slightly increased particle size (26-54 nm) and slightly decreased dispersibility. FIG. 8 (f) is a graph showing a particle distribution of (Co-MOF/CTS) -900, in which the particle dispersion degree is slightly better than that of (Co-MOF) -900, the particle diameter range is 111 to 271nm, the particle diameter distribution range is smaller without generation of an extremely large particle diameter, and the particles can be coated with a carbon layer. In general, the physical mixture samples are used as precursors for controlling the morphology of the carbon-based materials, and have a great difference from the Co-MOF-g-CTS used as the precursors.

SEM image of (Co-MOF-g-CTS) -900 as shown in FIG. 9 (a), the (Co-MOF-g-CTS) -900 presents rugged porous structure with gaps due to morphology of CTS grafted on Co-MOF surface, details are shown in FIG. 9 (b), and the porous structure is favorable for adsorption of reactants and proton transfer, and has positive effect on many catalytic reactions.

7.4 XRD analysis of (P) -T

XRD analysis was performed on (P) -T to analyze their phase composition. As shown in FIG. 10 (a), XRD curves for Co-MOF, co-MOF-g-CTS and Co-MOF/CTS were obtained by carbonization at 600 ℃. The (Co-MOF) -600, (Co-MOF-g-CTS) -600 and (Co-MOF/CTS) -600 all exhibited diffraction at 44.22℃and 51.52℃and these peaks were attributed to the (111) and (200) planes of Co, respectively, and the (Co-MOF-g-CTS) -600 exhibited a characteristic peak of the CoO (111) plane at 36.49℃probably due to the presence of oxygen atoms in CTS, resulting in the formation of CoO, with flat peaks around 20-25℃being characteristic peaks of graphitic carbon. FIG. 10 (b) is an XRD plot of Co-MOF, co-MOF-g-CTS and Co-MOF/CTS carbonized at 900℃to give carbon-based materials, with characteristic peaks of (Co-MOF) -900, (Co-MOF-g-CTS) -900 and (Co-MOF/CTS) -900 appearing at 44.22℃and 51.52℃being assigned to the (111) and (200) crystal planes of Co, characteristic peaks at 36.49℃and 42.39℃being assigned to the (111) and (200) crystal planes of CoO, as well as diffraction peaks of graphitic carbon around 20-25 ℃.

7.5 Nitrogen adsorption analysis of (Co-MOF) -900 and (Co-MOF-g-CTS) -900

Table 2 specific surface area, pore volume and pore size of (Co-MOF) -900 and (Co-MOF-g-CTS) -900

N ₂ Adsorption-desorption isotherms show the porosity of the material, as shown in FIG. 11 for both (Co-MOF) -900 and (Co-MOF-g-CTS) -900, showing typical type IV adsorption isotherms and type H3 hysteresis loops. Compared with (Co-MOF) -900, (Co-MOF-g-CTS) -900 has larger specific surface area, larger specific pore volume and smaller pore diameter in the pyrolysis process due to the effect of CTS (Table 2), and the optimization of the pores has important significance for catalytic reaction.

7.6 XPS analysis of (P) -T

The composition of the surface elements of the (P) -T carbon-based material is studied by using X-ray photoelectron spectroscopy (XPS) to investigate the influence of different precursors and different temperature carbonization on the state of each element in the carbon-based material. Four elements Co, N, C and O can be observed in the total spectra of (Co-MOF) -600 and (Co-MOF-g-CTS) -900 as shown in FIG. 12; three elements Co, C and O can be observed in (Co-MOF) -900; while in (Co-MOF-g-CTS) -600, three elements, N, C and O, were observed.

The high resolution spectra of Co 2p are shown in FIG. 13, with peaks at 777.5, 779.7 and 780.1eV for (Co-MOF) -600 and (Co-MOF-g-CTS) -900 being due to metallic Co, co-N and Co-O bonds. Of these (Co-MOF) -900 only one Co-O peak at 780.1eV and one weaker metallic Co peak at 777.5eV are present, mainly due to the (Co-MOF) -900 particle overflow spring layer protection, which is oxidized by oxygen in air. It is interesting that no Co 2p signal was observed at (Co-MOF-g-CTS) -600, because CTS coated on the surface of Co-MOF carbonizes first, forming a protective carbon shell, encapsulating the particles in a carbon layer, and thus no Co 2p signal was detected at the surface.

The high resolution spectrum of N1 s is shown in FIG. 14, from which it can be seen that Co-MOF forms Co-N bonds at 399.0eV when carbonized at 600℃, which is an important active site for catalytic reactions. However, when Co-MOF is carbonized at 900 ℃, the high-resolution spectrum of N1 s does not detect the presence of N, which indicates that high temperature carbonization affects the doping of N element. (Co-MOF-g-CTS) -600 the peak of pyridine-N at 398.3eV was formed due to the introduction of CTS in the precursor. In the high-resolution N1 s spectrum of (Co-MOF-g-CTS) -900, co-N bonds are formed, because the finite field pyrolysis strategy plays a role in protecting pyrolysis materials, so that important catalytic active sites are reserved in high-temperature carbonization.

The high resolution spectra of C1 s are shown in fig. 15, with peaks at 284.6 and 285.5eV for (Co-MOF) -600 and (Co-MOF-g-CTS) -600 caused by C-C and C-N bonds, 288.5eV being the result of c=o and O-c=o groups from carboxyl groups. The peaks for (Co-MOF) -900, which were only C-C bonds (284.6 eV) and weak C-N bonds (285.5 eV), were shown to be completely peak-to-carboxyl at 288.5eV, indicating that the high temperature caused some of the carboxyl species on the carbon-based material surface to be completely destroyed. The peaks at 284.6 and 285.5eV for (Co-MOF-g-CTS) -600 are due to C-C and C-N bonds with a higher C-N bond peak intensity, and under high temperature treatment of CTS confinement effect, peaks of C-O and O-c=o at 286.6 and 289.1eV are formed.

7.7 Analysis of catalytic Performance of (P) -T

4-NP is a common environmental contaminant, currently in NaBH ₄ In the presence ofReduction of 4-NP to 4-AP with a catalyst is the most effective treatment, a reaction that is also often used to evaluate the catalytic performance of the catalyst. We therefore used this reaction to evaluate the catalytic properties of (P) -T. As can be seen from FIG. 16 (a), pure 4-NP exhibited a characteristic absorption peak at 317nm, and NaBH was added to the 4-NP solution ₄ After this, the characteristic absorption peak shifted to 400nm and the solution changed from pale yellow to bright yellow due to the formation of 4-nitrophenol ions. The intensity of the absorption peak at 400nm remained unchanged after 30 minutes, which means that only NaBH was added ₄ The reaction cannot be started.

As shown in FIG. 16 (b), when the 4-NP and NaBH are contained ₄ When a small amount of catalyst is added to the solution, the reaction starts rapidly and a new absorption peak of 4-AP appears at 300 nm. When the absorption peak at 400nm completely disappeared, it showed that 4-NP was completely converted to 4-AP, and the solution was also changed from bright yellow to colorless, so we monitored the progress of the reductive conversion by UV-vis absorption spectroscopy.

As shown in FIG. 17, the UV-vis spectra of (Co-MOF) -600, (Co-MOF) -900, (Co-MOF-g-CTS) -600, (Co-MOF-g-CTS) -900, (Co-MOF/CTS) -600 and (Co-MOF/CTS) -900 as catalysts for catalytic reduction of 4-NP, only 0.1mg of catalyst was needed to complete the complete conversion of 4-NP to 4-AP in 150, 280, 400, 80, 150 and 240s, respectively, with nearly 100% conversion.

C/C as shown in FIG. 18 (a) ₀ The relation between the reaction time t and the reaction time t can more intuitively reflect the whole catalytic reduction reaction process, wherein C represents the concentration of 4-NP in a reaction system and C/C ₀ Can be obtained from the absorption peak intensity ratio A/A at 400nm ₀ Obtained. Since the excessive reducing agent NaBH is added into the reaction system ₄ Therefore, the catalytic efficiency of (P) -T can be estimated by using the first order kinetic constant k, and the formula is ln (C _t /C ₀ ) = -kt. The reaction rate constants of (Co-MOF) -600, (Co-MOF) -900, (Co-MOF-g-CTS) -600, (Co-MOF-g-CTS) -900, (Co-MOF/CTS) -600 and (Co-MOF/CTS) -900 were calculated to be 0.0222, 0.0173, 0.0114, 0.0512, 0.0258 and 0.0179s, respectively ^-1 (FIG. 18 b).

The low-temperature carbonized (Co-MOF) -600 has better catalysis rate due to small-sized metal particles and Co-N-C active sites. The physical mixed (Co-MOF/CTS) -600 and (Co-MOF) -600 have similar catalytic efficiency, and the physical mixed CTS does not greatly improve the catalytic performance. At high temperature heat treatment temperatures, the (Co-MOF-g-CTS) -900 of the confined pyrolysis strategy possesses a larger specific surface area, a larger specific pore volume and a smaller pore size than the directly carbonized (Co-MOF) -900, which are more favorable for proton transfer and reactant adsorption. And the (Co-MOF-g-CTS) -900 of the finite field pyrolysis strategy has smaller metal particle size and dispersibility than the directly carbonized (Co-MOF) -900 and the physically mixed (Co-MOF-g-CTS) -900, which are important factors for improving the catalytic performance. Interestingly, (Co-MOF-g-CTS) -900 had the fastest reaction rate, whereas (Co-MOF-g-CTS) -600, which had small particle size at low temperature carbonization, was the slowest to catalyze the reaction, since the heteroatom N in the (Co-MOF-g-CTS) -900 catalyst was in the form of Co-N-C with high catalytic activity, whereas the N atom in the (Co-MOF-g-CTS) -600 was in the form of pyridine-N, and on the other hand, most importantly, the active metal particles of (Co-MOF-g-CTS) -600, due to the limited domain pyrolysis strategy, were surrounded by carbon shells formed by CTS carbonization, and could not be in sufficient contact with the reactants in solution, so XPS did not detect the signal of Co element. While (Co-MOF-g-CTS) -900 maintains good dispersion of metal particles without aggregation into large particle size particles, and at the same time, partial exposure of the particle surface can be seen from a projection electron microscope, and the exposed active sites can be fully contacted with reactants, so that high-efficiency catalysis is realized. CTS limited pyrolysis may provide a new way for preparing catalytic materials by carbonization at high temperature.

7.8 Cyclic analysis of (Co-MOF-g-CTS) -900

In view of practical application, the recycling property is an important characteristic of the catalyst, so that (Co-MOF-g-CTS) -900 with optimal catalytic performance is selected and tested for stability. As shown in FIG. 19 (a), after 10 cycles, (Co-MOF-g-CTS) -900 had a slight decrease in catalytic efficiency, but the conversion of 4-NP could be maintained at about 95%. The diffraction peaks for Co (111), co (200), coO (111), coO (200) and C (002) that were identical were still present in the XRD profile after 10 cycles of the experiment, indicating that the catalyst composition was unchanged (FIG. 19 b). And the SEM image (fig. 19 c) and the TEM image (fig. 19 d) after cycling, it can be seen that the surface morphology and the internal particle state of the catalyst are not changed either. This indicates that (Co-MOF-g-CTS) -900 has a certain cyclic stability and does not undergo physicochemical changes after catalytic reaction.

8. Conclusion(s)

In summary, the invention uses the nitrogen-containing ligand TPT and the carboxyl ligand bptc to synthesize a carboxyl-functionalized [ Co (TPT) _2/3 (bptc)]·3H ₂ O (Co-MOF (-COOH)). Further grafting CTS on the surface of Co-MOF by EDC/NHS reaction in one pot to prepare Co-MOF-g-CTS. And (3) carbonizing Co-MOF, co-MOF-g-CTS and Co-MOF/CTS serving as precursors at relatively low and high temperatures to obtain the carbon-based material (P) -T. Wherein (Co-MOF-g-CTS) -900 shows optimal performance in the catalytic reduction reaction of 4-NP, and the reaction can be completed in 80s only by 0.1mg of catalyst, and the reaction rate is constant k= 0.0512s ^-1 The catalytic efficiency is still kept at about 95% after 10 cycles. Compared with direct carbonization and physical mixed carbonization, (Co-MOF-g-CTS) -900 prepared by the limited-area pyrolysis method can enable internal particles to have smaller particle size and higher dispersibility at higher heat treatment temperature. The regulation of the structure by the external limit domain pyrolysis ensures that the (Co-MOF-g-CTS) -900 has larger specific surface area, larger specific pore volume and smaller pore diameter. Furthermore, the protective effect of CTS makes (Co-MOF-g-CTS) -900 still exist in Co-N-C active center and surface active site at high temperature. These characteristics are more favorable for proton transfer and reactant adsorption, and promote the catalytic efficiency of the catalyst. CTS finite field pyrolysis provides a novel auxiliary method for preparing MOFs derived carbon-based materials.

Although embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and the appended claims, and therefore the scope of the invention is not limited to the disclosure of the embodiments.

Claims

1. A method for preparing (Co-MOF-g-CTS) -900 by a finite field pyrolysis strategy, which is characterized by comprising the following steps: the method comprises the following steps:

synthesis of Co-MOF (-COOH):

20.8mg,0.07mmol TPT,28.6mg,0.1mmol bptc and 29.1mg,0.1mmol Co (NO ₃ ) ₂ ·6H ₂ The mixture of O is dissolved in the mixed solution of 2mL of DMA and 3mL of deionized water, then the mixture is filled into a 15mL polytetrafluoroethylene high-pressure reaction kettle, the mixture is reacted for 3 days under self-pressure in a baking oven at 100 ℃, after the reaction is finished, the reaction kettle is naturally cooled to room temperature, a brown yellow blocky crystal is obtained, the obtained crystal product is washed three times by the mixed solution of DMA/water, and is dried at room temperature for standby after being filtered;

synthesis of Co-MOF-g-CTS:

firstly, weighing 1000mg of CTS, dissolving the CTS in 200mL of deionized water, adding 0.5% formic acid to adjust the pH of the solution to be between 3 and 4, heating and stirring until the CTS is completely dissolved, cooling to room temperature for standby, then taking 10mL of the CTS solution, adding 30mL of deionized water and 20mL of DMA solution, uniformly stirring, sequentially adding 50mg of Co-MOF (-COOH), 20mg of EDC and 20mg of NHS into the mixed solution dissolved with the CTS, stirring for 2 hours at room temperature, centrifuging after the reaction is finished to obtain a product, washing 3 times by methanol, and preparing the Co-MOF-g-CTS material by a one-pot method;

third step (Co-MOF-g-CTS) -900 catalyst Synthesis:

2. A method of preparing (Co-MOF-g-CTS) -900 according to claim 1, wherein: DMA (direct memory access) in the following steps: the volume ratio of water is 1:2.

3. A method of preparing (Co-MOF-g-CTS) -900 according to claim 1, wherein: c in the step ₂₇ H ₂₂ N ₄ O ₉ The Co element analysis calculated values are calculated as percentages: c,45.92%; h,4.20%; n,14.61%, measured values in percent: c,43.55%; h,3.64%; n,9.26%.