CN108246330B

CN108246330B - Method for constructing monatomic catalyst based on lignin/metal supramolecular assembly

Info

Publication number: CN108246330B
Application number: CN201810029041.1A
Authority: CN
Inventors: 刘云; 周华
Original assignee: Beijing University of Chemical Technology
Current assignee: Beijing University of Chemical Technology
Priority date: 2018-01-12
Filing date: 2018-01-12
Publication date: 2019-12-24
Anticipated expiration: 2038-01-12
Also published as: CN108246330A

Abstract

A method for constructing a monatomic catalyst based on lignin/metal supramolecular assembly belongs to the technical field of catalytic material preparation and lignin application. The method mainly comprises the following steps: 1) uniformly mixing lignin with a certain concentration with a metal ion solution, and forming lignin/metal ion supermolecular assembly precipitate by adjusting the pH; 2) centrifuging and drying to obtain the catalyst precursor. 3) Mixing the precursor with a nitrogen source, and performing high-temperature sintering under the protection of inert gas to obtain the metal monatomic catalyst. Compared with the prior art, the method has the advantages of low raw material cost (lignin is used as a ligand and a carrier), simple process (pH regulation and control of a precursor and no pickling stripping), uniform metal dispersion (coordination and complexation and defect site increase regulation and control), and the like, and is easy to realize the large-scale production of the metal monatomic catalyst.

Description

Method for constructing monatomic catalyst based on lignin/metal supramolecular assembly

Technical Field

The invention belongs to the technical field of catalytic material preparation and lignin application, and particularly relates to a method for constructing a monatomic catalyst based on lignin/metal supramolecular assembly.

Background

The supported heterogeneous catalyst has important significance for improving the recoverability and stability of the catalyst, reducing the cost of the catalyst and reducing the environmental pollution, and is widely applied to the fields of energy storage and conversion, organic synthesis, medicine preparation, environmental improvement and the like. The research shows that: the active component of the supported metal catalyst is mainly metal, and the active sites are concentrated on atoms on the surface of the particles. Therefore, synthesizing smaller sized particles is an effective method for improving the activity and selectivity of the metal catalyst.

In recent years, metal monatomic catalysts have achieved the maximum utilization efficiency of atoms, exhibit high activity and high selectivity, and the catalytic efficiency of some non-precious metal monatomic catalysts has exceeded that of commercial precious metal catalysts. Researchers believe that monatomic catalysts are a bridge that frames heterogeneous and homogeneous catalysis. However, since a single metal atom has high surface free energy, it is very easy to aggregate into clusters or nanoparticles during the preparation process, and it is difficult to obtain the single atom. It follows that the preparation of metal monatomic catalysts still presents many challenges, mainly in how to effectively anchor the metal atoms and increase the interaction between the metal and the support.

Up to now, many techniques for preparing metal monatomic catalysts, such as precipitation, impregnation, have been reported successivelyThe metal ligand and the carrier material relate to metal oxide, graphene, carbon nano tube and the like, and the methods make important contributions to the improvement of the activity of the metal monatomic catalyst and the research of the catalytic reaction mechanism of the metal monatomic catalyst. Patent (CN107346826A, 2017) discloses a preparation method of a monatomic iron-dispersed redox electrocatalyst, which utilizes acid to dissolve nitrogen-doped carbon-supported unstable iron nanoparticles, and iron atoms coordinated with nitrogen are retained, thereby obtaining a monatomic dispersed metallic iron catalyst. The method is a common method for preparing the metal monatomic catalyst and has a simple process. However, this technique generates a large amount of acid waste liquid, and has problems of environmental pollution, corrosion of equipment, and the like. Literature (Yan H₁Remarkable Performance in Selective Hydrogenation of 1,3-Butadiene.J.Am.chem.Soc.2015,137(33): 10484-. The literature (Jones J.et al. thermal stable single-atom-on-ceria catalysts via atom mapping.

Science 2016.353(6295):150-154) reported that a platinum-supported ceria catalyst was prepared by using atomic trapping, which is advantageous in that Pt becomes volatile PtO under the condition of high temperature and oxygen₂Captured by polyploid ceria. However, this method is poor in universality. In conclusion, the existing methods for preparing the metal monatomic catalyst have the defects of complex process, high raw material cost, small yield, poor universality, serious environmental pollution and the like, and no method for realizing low-cost industrial production of the metal monatomic catalyst exists so far.

Disclosure of Invention

The invention aims to develop a method for constructing a monatomic catalyst based on lignin/metal supramolecular assembly. Mainly comprises the following steps:

(1) uniformly mixing lignin with a certain concentration with a metal ion solution, and forming lignin/metal ion supermolecular assembly precipitate by adjusting the pH;

(2) centrifuging and drying to obtain the catalyst precursor.

(3) And mixing the precursor with a nitrogen source, and performing high-temperature sintering under the protection of inert gas to obtain the monatomic catalyst.

The invention utilizes coordination chemistry, and through pH regulation, metal ions and lignin form a supermolecule assembly complex, and the lignin is used as an organic ligand to enable the metal ions to form a single dispersed atom at a spatial position. In order to increase the distance between target metal atoms, by introducing metal zinc ions, when the sintering temperature is higher than 800 ℃, the metal zinc volatilizes, and more defect sites appear on the surface of the lignin carbon. In the high-temperature sintering process, lignin is gradually carbonized, a nitrogen source is gradually decomposed to generate ammonia gas which is doped on carbon to form pyridine nitrogen and pyrrole nitrogen which serve as anchor sites to coordinate with metal, the interaction between a carrier and the metal is enhanced, the agglomeration of metal atoms at high temperature is avoided, and the nitrogen-doped carbon-loaded metal monatomic catalyst is finally obtained.

Compared with the prior art, the invention has the technical advantages that: (1) the lignin derived from biomass is used as an organic ligand and a carbon source, the raw materials are low in price and renewable, and the sustainable development and green manufacturing concept is met; (2) compared with a small molecular ligand, the lignin is a natural aromatic macromolecular polymer, is coordinated with metal ions, and can increase the space distance between metal atoms; (3) the lignin/metal supermolecule assembly is used as a catalyst precursor, can be operated at room temperature in an aqueous solution, and has mild condition, low energy consumption and no pollution; (4) the metal monatomic catalyst can be obtained after high-temperature sintering, the steps of acid pickling, stripping and water washing are not needed, and three wastes are not discharged; (5) simple process, wide universality, low cost and easy industrial production.

Drawings

(taking metallic cobalt as an example):

fig. 1 is an XRD spectrum of metallic cobalt monoatomic and nano-catalyst.

FIG. 2 is a HAADF-STEM diagram of a metallic cobalt monatomic catalyst.

Fig. 3 is a XAFS diagram of a metallic cobalt monatomic catalyst.

Fig. 4 is a TEM image of metallic cobalt nanocatalysts: a) TEM image of the catalyst bulk; b) a locally magnified high resolution TEM image of a single cobalt nanoparticle; c) electron diffraction pattern.

FIG. 5 is the activity evaluation of the cobalt metal monoatomic and nanocatalyst for catalyzing benzyl alcohol to prepare methyl benzoate.

FIG. 6 is a schematic flow chart of the present invention.

The specific implementation mode is as follows:

the invention is further illustrated by the following examples in conjunction with the drawings, but the inventive content is not limited to the examples.

Example 1: synthesis of metal monatomic catalyst precursor

1)8g of lignin was dissolved in 1L of deionized water to prepare a solution A. Zinc nitrate hexahydrate (11.90g,40mmol Zn)²⁺) And cobalt nitrate hexahydrate (2.33g,8mmol Co²⁺) Dissolved in 0.5L deionized water to prepare solution B.

2) And adding the solution B into the solution A, quickly and uniformly mixing by mechanical stirring, wherein the pH is natural (pH is 6.1), continuously stirring the mixture for 1 hour, and then standing at room temperature overnight.

3) Pouring out the supernatant, centrifuging at 5,000 Xg for 10min, precipitating at 80 deg.C, and drying overnight to obtain metallic cobalt single-atom catalyst precursor named Co₁Zn₅-L。

4) Similarly, the substitution of metallic cobalt for other metal ions, such as iron, nickel, copper, manganese, zirconium, molybdenum, etc., gives rise to lignin/metal supramolecular assemblies, designated M₁Zn₅L (M represents a metal ion, such as Fe)³⁺,Ni²⁺,Cu²⁺,Mn²⁺，Zr²⁺,Mo²⁺(ii) a L represents lignin).

To demonstrate the role of metallic zinc in the synthesis of catalyst precursors, we synthesized metallic nanocatalyst precursors, as a comparative example, the following steps:

1)8g of lignin was dissolved in 1L of deionized water to prepare a solution A. Cobalt nitrate hexahydrate (5.82g,20mmol Co²⁺) Dissolved in 0.5L deionized water to prepare solution B.

2) Solution B was added to solution a and mixed rapidly with mechanical stirring, the solution was adjusted to pH 7.4 with 10 wt.% ammonia, the mixture was stirred for 1h and then allowed to stand overnight at room temperature.

3) Pouring out the supernatant, centrifuging at 5,000 Xg for 10min, precipitating, and drying at 80 deg.C overnight to obtain cobalt metal nano catalyst precursor named Co_2.5-L. Other metal nano catalyst precursors are uniformly named as M_2.5L (M represents a metal ion, such as Fe)³⁺,Ni²⁺,Cu²⁺,Mn²⁺，Zr²⁺,Mo²⁺(ii) a L represents lignin).

Example 2: synthesis of metal monatomic catalyst

1) 0.1g of lignin/metallic cobalt catalyst precursor Co is taken₁Zn₅L and 1g of dicyandiamide, mixed by intensive grinding.

2) Putting the ground powder into a porcelain boat, putting the porcelain boat into a tube furnace, wherein the flow rate of argon gas is 75mL/min, and the temperature rising procedure is as follows: raising the temperature from room temperature to 550 ℃ at a speed of 5 ℃/min, keeping the temperature for 1h, raising the temperature to 900 ℃ at a speed of 5 ℃/min, preserving the temperature for 3h, and then naturally cooling to room temperature to obtain the nitrogen-doped carbon-supported cobalt monatomic catalyst.

3) Similarly, the lignin/metallic cobalt catalyst precursor is replaced by other metal ion precursor, such as lignin/metallic supermolecule assembly M of iron, nickel, copper, manganese and the like₁Zn₅L (M represents a metal ion, such as Fe)³⁺,Ni²⁺,Cu²⁺,Mn²⁺，Zr²⁺,Mo²⁺(ii) a L represents lignin), with 1g of dicyandiamide, mixed by intensive grinding. And (3) the mass ratio of the metal precursor to the dicyandiamide is 1:10, the temperature is programmed to 900 ℃, and the metal monoatomic catalyst loaded by nitrogen-doped carbon is obtained by high-temperature sintering.

4) Other conditions are unchanged, and dimethyl imidazole is used as a nitrogen source; or keeping other conditions unchanged, wherein the mass ratio of the metal precursor to the dicyandiamide is 1: 50; or the nitrogen-doped carbon-supported metal monatomic catalyst can be obtained by raising the temperature to 1100 ℃ by programming and sintering at high temperature under the condition that other conditions are not changed.

As a comparative example, a metal precursor M₁Zn₅-L is changed to M_2.5L (M represents a metal ion, such as Fe)³⁺,Ni²⁺,Cu² ⁺,Mn²⁺，Zr²⁺,Mo²⁺(ii) a L represents lignin), fully grinding and uniformly mixing with a nitrogen source, putting into a porcelain boat, putting into a tube furnace, wherein the flow rate of argon gas is 75mL/min, and the temperature rise program is as follows: raising the temperature from room temperature to 550 ℃ at a speed of 5 ℃/min, keeping the temperature for 1h, raising the temperature to 900 ℃ at a speed of 5 ℃/min, preserving the temperature for 3h, and naturally cooling to room temperature to obtain the metal nano catalyst.

Example 3: evaluation of Activity of Metal monatomic catalyst

1) A100 mL round bottom flask was charged with 10mL methanol, 1mmol benzyl alcohol and 2.5 mol% metallic cobalt monatin catalyst, 0.25mmol anisole as an internal standard.

2) Air in the round-bottom flask was replaced with pure oxygen at 1bar O₂(oxygen balloon), reaction at 60 ℃, sampling at regular time, and analyzing the reaction product by HPLC. The analysis conditions were: column C18, column temperature 30 ℃, mobile phase 30% acetonitrile and 70% water (containing 0.05 v% trifluoroacetic acid), flow rate 1mL/min, UV detector 254 nm.

3) The catalyst is changed into other metal single-atom catalysts (such as iron, nickel, copper, manganese, zirconium, molybdenum and the like) to catalyze benzyl alcohol and methanol to prepare methyl benzoate through oxidative esterification, and the reaction system and the product detection condition are unchanged.

4) As a comparative example, the metal monatomic catalyst was replaced with a metal nanocatalyst, which was added in an amount of 5.5 mol%, and the catalytic effects of the metal nanocatalyst were evaluated, and the catalytic effects of the metal monatomic catalyst and the metal nanocatalyst were compared by calculating the conversion frequency (TOF).

As can be seen from the attached figure 1, the XRD spectrum of the metallic cobalt monatomic catalyst can not observe the diffraction peak of the metallic cobalt or the oxide thereof, which indicates that the cobalt is highly dispersed on the carrier. However, the XRD spectrum of the metal cobalt nano-catalyst shows obvious metal cobalt diffraction peaks. To further determine the dispersibility of cobalt on metallic cobalt monatomic catalysts, FIG. 2 observed a single dispersion of cobalt atoms on the support by HAADF-STEM. FIG. 3 further identifies the fine structure of the metallic cobalt monatomic catalyst by XAFS analysis, the catalytically active area being Co-Nx-C. Fig. 4 shows that cobalt on the metallic cobalt nano-catalyst is nano-particles coated by graphite phase carbon through TEM, and the cobalt is determined to exist in the form of metallic cobalt through high-resolution TEM and electron diffraction.

Taking the oxidized esterified methyl benzoate of benzyl alcohol as an example, fig. 5 compares the catalytic efficiency of a cobalt metal monoatomic catalyst and a cobalt nano catalyst. It can be seen that the catalytic rate of the metallic cobalt monatomic catalyst is significantly better than that of the nanoparticle catalyst, and the conversion frequency (TOF) is 18.3 times that of the cobalt nanocatalyst (calculated when the yield of methyl benzoate is 50%).

Claims

1. A method for constructing a monatomic catalyst based on lignin/metal supramolecular assembly is characterized by comprising the following steps:

1) uniformly mixing lignin with a certain concentration with a metal ion solution, and forming lignin/metal supermolecular assembly by regulating and controlling pH; the mass ratio of the amount of the metal ion substances to the lignin is 0.5-50mmol of metal/g of lignin; the pH regulation range is 2-10; the metal ions include manganese, iron, cobalt, nickel, copper, zinc, zirconium, molybdenum, and are a combination of zinc and another metal ion;

2) centrifuging and drying to obtain a catalyst precursor;

3) mixing a catalyst precursor with a nitrogen source, wherein the mass ratio of the nitrogen source to the catalyst precursor is 0.5-50; under the protection of inert gas, high-temperature sintering is carried out, wherein the high-temperature sintering temperature is 900-: raising the temperature to 550 ℃ at the speed of 5 ℃/min, preserving the heat for 1h, then raising the temperature to 1100 ℃ at the temperature of 900-; grinding and crushing the sintered catalyst to obtain the monatomic catalyst.

2. The method of claim 1, wherein the lignin comprises alkali lignin, organosolv lignin, enzymatic lignin, lignosulfonate.

3. The method of claim 1, wherein the solvent for dissolving the lignin and the metal ion solution comprises one or a mixture of water, dimethylformamide, gamma-valerolactone, tetrahydrofuran, dimethyl sulfoxide, ethanol and methanol.

4. The method of claim 1, wherein the nitrogen source comprises ammonia, urea, dicyandiamide, melamine, carbon nitride, cyanamide, dimethylimidazole; the mass ratio of the nitrogen source to the catalyst precursor is 0.5-50.

5. The method of claim 1, wherein the inert gas comprises one of nitrogen, argon, helium, and radon.