WO2023059086A1 - Metal phosphide catalyst and method for preparing high value-added compound by using same - Google Patents

Abstract

Disclosed are: a metal phosphide-based catalyst having a yolk-shell structure; and a method for converting, through hydrogenation, an organic acid, especially, an organic acid, such as levulinic acid, derived from biomass, or a derivative thereof into a high value-added compound that can be used as a raw material for a fuel additive, bioplastic, a solvent, or the like.

Description

Metal phosphide catalyst and method for producing high value added compounds using the same

The present disclosure relates to a metal phosphide catalyst and a method for preparing a high addition compound using the same. More specifically, the present disclosure provides a metal phosphide-based catalyst having a yolk-shell structure and an organic acid, particularly an organic acid derived from biomass such as levulinic acid or a derivative thereof, through a hydrogenation reaction using the same as a fuel It relates to a method for converting high-value compounds that can be used as raw materials such as additives, bioplastics, and solvents.

Petroleum energy, which has led the development of mankind, has recently been actively researched to replace petroleum resources in whole or in part with biomass due to problems such as resource finiteness, centralization, and environmental pollution.

Biomass is used broadly to include all substances of biological origin, while used narrowly to mean substances derived primarily from vegetable sources such as corn, soybean, linseed, sugarcane and palm oil. do. However, it can generally be extended to all organisms currently living, or metabolic by-products that play a part in the carbon cycle.

Lignocellulosic biomass can be exemplified as the most widely used biomass, which can be widely used in the production of biofuels and biochemicals. Lignocellulosic biomass is a combination of cellulose, hemicellulose and lignin in a complex and rigid structure. Recently, studies on the production of various chemical substances using materials formed through the saccharification step of lignocellulosic biomass have been actively conducted.

In this regard, organic acids derived from biomass, such as levulinic acid (LA), are used as starting materials for the synthesis of commercially useful compounds, such as acid hydrolysis of biomass (eg, hexose). , can be produced by the dehydration reaction of fructose. Diphenolic acid, acetacrylic acid, 1,4-pentanediol, acrylic acid, aminolevulate, gamma-valerolactone (GVL) and 2-methyltetrahydrofuran (2-MTHF) are compounds that can be prepared from these levulinic acids. ) and the like can be exemplified.

Typically, gamma-valerolactone (GVL) and 2-methyltetrahydrofuran (2-MTHF) can be produced by hydrogenation using a catalyst. In this case, levulinic acid is dehydrated by primary hydrogenation. Dehydration and cyclization are performed to form gamma-valerolactone, and 2-methyltetrahydrofuran is formed by further hydrogenation (secondary hydrogenation) and cyclization along with dehydration. will form

As such, an attempt to use a noble metal catalyst as a hydrogenation catalyst of an organic acid such as levulinic acid (or a derivative thereof) has been reported (eg, US Patent Publication No. 2003/0055270, CN105566258 A, etc.), and nickel phosphide It is also known when a phosphide catalyst of a transition metal is used like a phosphide catalyst (eg, CN112824395 A, etc.).

However, in the case of the noble metal-based catalyst among the above-mentioned prior art, since expensive precious metal components are used, it is not preferable in terms of economic efficiency, and it is necessary to carry out the reaction at a relatively high hydrogen pressure. In addition, in the case of a nickel phosphide catalyst, the conversion rate of organic acids and target products (eg, when preparing gamma-valerolactone (GVL) and 2-methyltetrahydrofuran (2-MTHF) from levlinic acid) Since the selectivity is low, there is a need to improve the overall yield.

Moreover, the conversion reaction using a catalyst according to the prior art is generally carried out in a liquid medium, that is, a solvent, and after completion of the reaction, since the product must be separated from the solvent, an additional separation and purification process is required.

Therefore, there is a need for a hydrogenation catalyst capable of converting organic acids into high value-added compounds with high conversion rate and selectivity even under milder conditions (additionally, solvent-free conditions), overcoming the limitations of the prior art.

In one embodiment of the present disclosure, it is intended to provide a transition metal phosphide-based catalyst capable of producing a high value-added compound from an organic acid in high yield by exhibiting improved hydrogenation activity compared to the prior art and a method for preparing the same.

In another embodiment of the present disclosure, it is intended to provide a conversion process capable of effectively preparing a high value-added compound from an organic acid or a derivative thereof using the above-described transition metal phosphide-based hydrogenation catalyst even in the absence of a solvent.

According to the first aspect of the present disclosure,

Represented by the general formula Ni _x Co _y P (x and y are the molar ratios of Ni and Co, ranging from 1:0.2 to 5), in the presence of a metal phosphide catalyst having a yoke-cell structure, to an organic acid or its derivative A method for converting an organic acid comprising performing a hydrogenation reaction is provided.

According to an exemplary embodiment, the hydrogenation reaction may involve a cyclization reaction.

According to an exemplary embodiment, the organic acid is levulinic acid, and the hydrogenation reaction is carried out in one step or at least two steps,

Here, while gamma-valerolactone (GVL) is formed by the first-step hydrogenation reaction, 2-methyltetrahydrofuran (2-MTHF) may be formed by the at least two-step hydrogenation reaction.

According to an exemplary embodiment, the hydrogenation reaction may be performed without using a solvent.

According to an exemplary embodiment, the first hydrogenation reaction is carried out under conditions of a temperature of 120 to 300 ° C. and a hydrogen pressure of 10 to 50 bar, and

The secondary hydrogenation reaction may be performed under conditions of a temperature of 180 to 320 °C and a hydrogen pressure of 30 to 80 bar.

According to a second aspect of the present disclosure,

A metal phosphide catalyst with a yoke-cell structure represented by the general formula Ni _x Co _y P (x and y are the molar ratios of Ni and Co and range from 1:0.2 to 5),

(i) The overall size (diameter) of the catalyst is 10 to 100 nm, the thickness of the cell layer is 1 to 10 nm, and the size (diameter) of the yoke is in the range of 5 to 50 nm.

According to a third aspect of the present disclosure,

a) preparing a nickel and cobalt precursor solution by dissolving a nickel precursor and a cobalt precursor in a solvent;

b) subjecting the precursor solution to a first hydrothermal synthesis reaction to form a precipitate containing nickel-cobalt;

c) subjecting the precipitate to a second hydrothermal synthesis reaction to form nickel-cobalt hydroxide; and

d) Ni _x Co _y P (x and y are the molar ratio of Ni and Co and range from 1: 0.2 to 5) through a substitution reaction of the nickel-cobalt hydroxide with a phosphide agent under an inert gas atmosphere and elevated temperature conditions (i) conversion to a nickel-cobalt phosphide catalyst having a yolk-cell structure;

Including,

(i) a nickel-cobalt phosphide catalyst in which the overall size (diameter) of the nickel-cobalt phosphide catalyst is in the range of 10 to 100 nm, the thickness of the cell layer is 1 to 10 nm, and the size (diameter) of the yoke is in the range of 5 to 50 nm; A method for preparing the catalyst is provided.

According to an exemplary embodiment, the weight ratio of the yoke in the metal phosphide catalyst having a yoke-cell structure may be in the range of 1:1.5 to 5.

According to an exemplary embodiment, the acid amount (NH ₃ -TPD) of the metal phosphide catalyst may be in the range of 200 to 600 mmol/g.

According to an exemplary embodiment, the molar ratio of the reduced form of the metal to the unreduced form of the metal in the metal phosphide catalyst may be in the range of 1:2 to 5.

A metal phosphide catalyst having a yoke-cell structure according to an embodiment of the present disclosure is an inexpensive transition metal (i.e., base metal) compared to conventional organic acid (specifically, biomass-derived organic acid) hydrogenation technology. While using as an active metal, it is possible to improve economic feasibility by achieving good organic acid conversion and selectivity for target compounds (specifically, compounds that can be used as raw materials for biofuels and/or fuel additives, solvents, and bioplastics). there is. In addition, when carried out as a liquid phase reaction, since the target compound can be obtained by hydrogenating the reactant as it is without requiring the use of a separate solvent, post-treatment steps such as separation of the solvent from the reaction product can be omitted. It is also advantageous in terms of commercialization. Therefore, a wide range of applications is expected in the future.

1 is a diagram showing various reaction mechanisms involved in the hydrogenation reaction of levulinic acid according to an exemplary embodiment;

Figure 2 shows the preparation of the transition metal phosphide catalyst, (a, b) Ni-Co glycerate precursor (generated from the first hydrothermal synthesis reaction), (c, d) Ni-Co phosphide precursor (second hydrothermal synthesis reaction generated from), (eg) Ni ₂ Co ₁ P, (h) CoP precursor, (i) CoP, and (j) NiP, respectively;

Figure 3 shows (a-c) TEM images of nickel-cobalt phosphide catalysts with a yolk-cell structure at different magnifications, (d) HRTEM images recorded from the basal plane, (e, f) HAADF-STEM images and EDS elemental (Ni, Co and P) mapping by (energy-dispersive X-ray spectroscopy);

4 is an HRTEM picture of the Ni ₂ Co ₁ P catalyst;

5 shows (a, b) nickel-cobalt phosphide catalysts (Ni ₂ Co ₁ P-150 °C and Ni ₂ Co ₁ P-100 °C) prepared by performing a second hydrothermal synthesis reaction at 150 ° C and 100 ° C, respectively. , and (c) Ni—Cu/Al ₂ O ₃ TEM images showing each structure;

6 is a graph showing H ₂ -TPD and NH ₃ -TPD results of various transition metal phosphide catalysts, respectively;

7 shows XPS spectra of NiP, Ni ₂ Co ₁ P, and CoP, (a) Ni 2p spectra for NiP and Ni ₂ Co ₁ P, respectively, (b) Co 2p spectra for Ni ₂ Co ₁ P and CoP, respectively. , and (c) P 2p spectra for NiP, Ni ₂ Co ₁ P and CoP, respectively;

8 shows various Ni and/or Co phosphides (NiP, CoP, NiCoP, Ni ₅ P ₄ , NiP ₂ , Ni3Co1P, Ni ₂ Co ₁ P, Ni ₁ Co ₁ P, Ni ₁ Co ₂ P, Ni ₁ Co ₃ P and CoP) XRD patterns for each;

9 is a graph showing the conversion and selectivity measured by hydrogenation of levulinic acid to gamma-valerolactone using various transition metal phosphide catalysts under solvent-free conditions, respectively;

10 is a graph showing conversion and selectivity measured by hydrogenation of levulinic acid to gamma-valerolactone using various solvents in the presence of a Ni ₂ Co ₁ P catalyst;

FIG. 11 is a graph showing the conversion and selectivity measured by hydrogenating reactants (or substrates) of various levulinic acid esters with gamma-valerolactone in the presence of a Ni ₂ Co ₁ P catalyst, respectively; and

12 shows nickel-cobalt phosphide catalysts (Ni ₂ Co ₁ P-100° C. and Ni ₂ Co ₁ P-150° C.) prepared by performing second hydrothermal synthesis reactions at 100° C. and 150° C., respectively, and commercial nickel-copper / Alumina catalyst (Ni-Cu/Al ₂ O ₃ ) Levulinic acid conversion rate and selection for gamma-valerolactone in the reaction of preparing gamma-valerolactone (GVL) from levulinic acid (LA) in the presence of each It is a graph that represents

The present invention can all be achieved by the following description. The following description should be understood as describing preferred embodiments of the present invention, but the present invention is not necessarily limited thereto. In addition, the accompanying drawings are for understanding, and the present invention is not limited thereto, and details of individual components can be properly understood by the specific purpose of the related description to be described later.

Terms used in this specification may be defined as follows.

A “heterogeneous catalyst” may refer to a catalyst that is present in a different phase from the reactants during a catalytic reaction, for example, a catalyst that does not dissolve in a reaction medium. In the case of a heterogeneous catalyst, in order for a reaction to occur, at least one reactant needs to be diffused and adsorbed on the surface of the heterogeneous catalyst, and after the reaction, a product needs to be desorbed from the surface of the heterogeneous catalyst.

"Biomass" usually refers to organic matter produced through photosynthesis, but may be understood as a concept including organic waste such as livestock manure and food waste. Various biomass known in the art (e.g. corn, soybean, linseed, vegetable sources such as sugarcane and palm oil, and more specifically, rice straw, wheat straw, starch-containing grains, corn cobs, corn cob, rice husks, paper products, timber, sawdust, agricultural waste, grasses, sugar sorghum, cotton, flax, bamboo, abaca, algae, fruit husks, algae, palm waste, stems, roots and leaves of plants, etc.). More specifically, carbohydrates obtained by saccharification or degradation from the above-mentioned biomass, such as starch, sugars, specifically monosaccharides (glucose, fructose, galactose, xylose, arabinose, mannose, etc.), disaccharides (sucrose, lactose, maltose, cellobiose, etc.), other (oligo)saccharides, and the like.

The term "crystalline" or "crystalline" can refer to any solid-state material that is typically ordered to have a valence lattice structure (eg, three-dimensional order), typically X - It can be specified by ray diffraction analysis (XRD), nuclear magnetic resonance analysis (NMR), differential scanning calorimetry (DSC), or a combination thereof.

"Catalyst" may refer to a component that increases the rate of a reaction and participates in an electrolysis reaction itself, but can participate in a reaction without being consumed by the reaction itself.

The "hydrothermal synthesis reaction" may refer to a reaction in which a material is synthesized using water or an aqueous solution under high temperature and high pressure conditions as a liquid phase synthesis method.

Terms such as “above” or “upper” and “lower” or “below” may be understood to describe the relative positional relationship between components or elements, and may be understood as “located above” or “below”. The term "located" may be understood to express a relative positional relationship in a non-contact state as well as a state in contact with a specific object.

In this specification, when any component or member is described as "connected" to another component or member, unless otherwise stated, not only when directly connected to the other component or member, but also to the other component. Or it can be understood that it is also included when connected under the interposition of a member.

Similarly, the term "contacts" may also be understood to include not only direct contact, but also contact through the intervening of other components or members.

When a component is referred to as "include", it means that it may further include other components unless otherwise stated.

metal phosphide catalyst

According to one embodiment of the present disclosure, through a hydrogenation reaction, an organic acid (eg, levulinic acid) or a derivative thereof (eg, an ester compound of an organic acid) is converted to gamma-valerolactone (GVL) and/or A binary functional metal phosphide catalyst capable of converting high value compounds such as 2-methyltetrahydrofuran (2-MTHF) is provided. In this regard, in order to convert levulinic acid into a cyclic compound (heterocyclic compound) such as GVL and/or 2-MTHF, a metal active site for hydrogenation and cyclization (specifically, dehydration/cyclic compound) It is necessary to have bifunctional catalytic properties with an acidic site for the reaction.

A metal phosphide catalyst according to one embodiment, specifically, a transition metal phosphide catalyst, and more specifically, a combination of two base metals among group VIII metals on the periodic table may be used. According to certain embodiments, the metal phosphide catalyst may be a nickel-cobalt phosphide catalyst represented by the general formula Ni _x Co _y P. At this time, the molar ratio of nickel (Ni): cobalt (Co) in the catalyst is, for example, 1: about 0.2 to 5, specifically 1: about 0.3 to 3, more specifically 1: about 0.4 to 2, particularly specifically 1: It can be adjusted in the range of about 0.5 to 1. According to certain embodiments, the transition metal phosphide catalyst can be a catalyst represented by the formula Ni ₂ Co ₁ P. At this time, if the relative amount of cobalt in the combination of the two transition metals is too low or too high, it may have an undesirable effect on the selectivity for the target product (specifically, GVL), so it is appropriate to adjust it within the above range. can be advantageous

The metal phosphide catalyst according to the present embodiment may have a spherical shape, and particularly has a yolk-shell structure, wherein the "yoke-shell structure" includes at least one particle in a central compartment ( That is, it may mean a hollow structure having a yoke), and specifically, an empty space may exist because at least a portion of the cell layer and the yoke are spaced apart. In some cases, all surfaces of the yoke may be spaced apart from the cell layer. may be At this time, a point to note is that the cell layer and the yoke are made of the same nickel-cobalt phosphide material. In this case, the space between the cell layer and the yoke can function as a nano-sized reaction region, and the contact area between the yoke and the reactant located while being spaced apart from the inner surface of the cell layer increases, and the hydrogenation reaction performed in the cell layer The product of is transferred to the yoke side, where additional hydrogenation can occur. This yoke-cell structure is a morphological characteristic that differentiates it from nickel phosphide (NiP) catalysts or cobalt phosphide (CoP) catalysts.

According to an exemplary embodiment, the overall size (diameter) of the metal phosphide catalyst may range, for example, from about 10 to 100 nm, specifically from about 20 to 80 nm, more specifically from about 40 to 60 nm, particularly It may have a size of about 50 nm. It may be advantageous to adjust within the above-mentioned range considering that the smaller the catalyst size, the larger the surface area, and the higher the possibility of collision with the reactants, the higher the reaction rate.

According to an exemplary embodiment, the thickness of the cell layer in the core-shell structured metal phosphide catalyst ranges, for example, from about 1 to 10 nm, specifically from about 2 to 8 nm, more specifically from about 4 to 6 nm, In particular, it may be on the order of 5 nm. In this regard, if the thickness of the cell layer is too thin, deactivation of the yoke portion in the cell layer may be induced, whereas if the cell layer is too thick, the mass transfer efficiency of the reactant is reduced because the cell layer is rigid. Due to the increase in the ratio of the cell layer in the catalyst, the space in which the reaction can occur is reduced and the reaction efficiency may be lowered, so it is preferable to properly adjust within the above range.

In an exemplary embodiment, the size (diameter) of the yoke in the catalyst may range, for example, from about 5 to 50 nm, specifically from about 10 to 40 nm, more specifically from about 20 to 30 nm, and particularly from about 25 nm to about 25 nm. It may be at the nm level. In this regard, the yoke functions as a primary active site where the reaction can occur, while the empty space functions as a passage that can increase the mass transfer efficiency and has a significant effect on the effective reaction. It can be adjusted appropriately within the range. In addition, the ratio occupied by the yoke existing inside the cell layer may act as a factor affecting deactivation of the catalyst. Considering the above points, the weight ratio of yoke occupied in the catalyst may be, for example, 1: about 1.5 to 5, specifically 1: about 1.8 to 4, and more specifically 1: 2 to 3. there is.

According to an exemplary embodiment, as described above, for the cyclization (specifically, dehydration/cyclization) reaction of an organic acid or a derivative thereof, the metal phosphide catalyst is required to contain acid sites of a certain level or higher. At this time, the acid amount (or the amount of acid sites) of the catalyst can be measured by NH ₃ -TPD, for example, about 200 to 600 mmol / g, specifically about 250 to 550 mmol / g, more specifically about 300 to 450 mmol/g. However, if the acid amount is too large, it may be advantageous to properly adjust within the above-mentioned range as a phenomenon in which the selectivity may decrease due to additional hydrogenation or dehydration of the target hydrogenation product (specifically, GVL). While the present disclosure is not bound by any particular theory, the acidity of metal phosphide catalysts can be explained by the presence of unreduced metal and/or P-OH species in the catalyst. Illustratively, the molar ratio of the reduced form of the metal to the unreduced form of the metal in the catalyst is in the range of 1 : about 2 to 5, specifically 1 : about 2.5 to 4.5, more specifically 1 : about 3 to 4. can be determined.

Furthermore, the number of acid sites (or acid amount) in the catalyst can have a greater effect on the conversion of GVL to 2-MTHF, which is the second step in the hydrogenation reaction pathway of an organic acid (specifically, levlinic acid) or a derivative thereof, This is because a larger amount of acid sites are required for the conversion of 1,4-PDO to 2-MTHF.

On the other hand, according to an exemplary embodiment, the metal phosphide catalyst having a yoke-cell structure may exhibit crystallinity, and as cobalt is introduced into nickel, it may exhibit an effect of increasing or improving crystallinity, which is XRD It can be identified from well defined peaks in the pattern. In this regard, the metal phosphide catalyst has a lattice fringe ranging, for example, from about 0.1 to 0.4 nm, specifically from about 0.15 to 0.35 nm, more specifically from about 0.2 to 0.4 nm, and particularly from about 0.23 to 0.27 nm. fringe). The above crystal characteristics may correspond to the (111) and (201) planes, for example.

According to an exemplary embodiment, the metal phosphide, specifically, the nickel-cobalt phosphide catalyst may be a bulk catalyst and may be applied in the form of powder or pellets. According to an exemplary embodiment, the bulk catalyst may be a powder catalyst, and its specific surface area (BET) is, for example, about 10 to 80 m ² /g, specifically about 15 to 70 m ² /g, more Specifically, it may be in the range of about 30 to 60 m ² /g, but this may be understood as an exemplary meaning.

On the other hand, the transition metal phosphide catalyst according to an exemplary embodiment, specifically the nickel-cobalt phosphide catalyst, can exhibit good hydrogen adsorption activity, when measured using H ₂ -TPD (hydrogen temperature-programmed desorption), The chemisorbed amount (mol) of hydrogen atoms per mole (mol) of total metal is, for example, about 100 to 250 mmol/g, specifically about 120 to 220 mmol/g, over the temperature range of 50 to 250 °C, More specifically, it may range from about 150 to 200 mmol/g, but this can be understood as an example.

Manufacturing method of metal phosphide catalyst with yoke-cell structure

Hereinafter, a method for preparing a metal phosphide catalyst (bulk type) having a yoke-cell structure using nickel and cobalt as metals will be mainly described.

According to one embodiment, the metal phosphide catalyst is subjected to at least two hydrothermal synthesis reactions (hydrothermal treatment) using a nickel (Ni) precursor and a cobalt (Co) precursor, followed by phosphidation. can be manufactured in this way.

According to an exemplary embodiment, the nickel precursor may typically be a water-soluble nickel compound, specifically a nickel compound having an oxidation number of 2, and particularly in a hydrated or hydrated form. For example, the nickel compound may be at least one selected from nickel halides (specifically, nickel chloride, nickel bromide, etc.), nickel acetate, nickel nitrate, nickel sulfate, nickel carbonate, nickel hydroxide, etc. It can be a hydrate. According to certain embodiments, the nickel precursor is nickel(II) chloride hexahydrate (NiCl ₂ 6H ₂ O), nickel nitrate (Ni(NO ₃ ) ₂ 6H ₂ O) and nickel (II) sulfate heptahydrate ( NiSO ₄ ·7H ₂ O) may be at least one selected from the group consisting of. Meanwhile, the cobalt precursor may also typically be a water-soluble cobalt compound, specifically a cobalt compound having an oxidation number of 2. Illustratively, the cobalt compound may be at least one selected from cobalt nitrate, cobalt sulfate, cobalt acetate, cobalt carbonate, cobalt halide (eg, cobalt chloride, cobalt bromide, etc.), and specifically may be in the form of a hydrate thereof. can More specifically, cobalt(II) chloride hexahydrate (CoCl ₂ 6H ₂ O), cobalt nitrate (Co(NO ₃ ) ₂ 6H ₂ O) and cobalt sulfate (II) heptahydrate (CoSO ₄ 7H ₂ O) may be at least one selected from the group consisting of. As such, when a metal having an oxidation number of 2 is used, it is advantageous in that a larger amount of Lewis acid sites can be formed in the catalyst. In addition, when a hydrate is used as the metal precursor, the metal precursor can be better dissolved in the solvent.

A nickel-cobalt precursor solution may be prepared by dissolving a nickel precursor and a cobalt precursor in a solvent in consideration of the ratio between nickel and cobalt in the final catalyst. At this time, as the solvent for dissolving the nickel precursor and the cobalt precursor, as described above, in forming the yoke-cell structure, a kind that can be converted into a material that provides a self-sacrificing template function can be used. For this purpose, at least It may be advantageous to include a solvent having one hydroxyl group, specifically at least a dihydric alcohol. According to a specific embodiment, as the solvent for preparing the precursor solution, for example, a mixed solvent of at least a dihydric alcohol (ie, a polyhydric alcohol) and a monohydric alcohol can be exemplified. Specifically, the at least dihydric alcohol may be, for example, at least one selected from glycerol and ethylene glycol. Also, the monohydric alcohol may be, for example, at least one selected from ethanol, isopropyl alcohol, and the like.

The volume ratio of at least dihydric alcohol: monohydric alcohol in such a mixed solvent is, for example, 1: about 1 to 10, specifically 1: about 1.5 to 7, more specifically 1: about 2 to 5. It can be. In addition, the total concentration of the nickel precursor and the cobalt precursor in the aqueous metal precursor solution may be, for example, about 30 to 150 mM, specifically about 50 to 100 mM, and more specifically about 60 to 90 mM.

As described above, after preparing the metal precursor solution, a first hydrothermal synthesis reaction may be performed. At this time, the hydrothermal synthesis reaction temperature may be adjusted in the range of, for example, about 150 to 220 ° C, specifically about 160 to 200 ° C, more specifically about 170 to 190 ° C, and the reaction time, for example, about 4 to 10 hours, specifically about 5 to 9 hours, more specifically about 6 to 8 hours. Through the first hydrothermal synthesis reaction, a metal (nickel-cobalt) precipitate may be formed. Optionally, a separation and purification step known in the art may be performed, and the metal precipitate may be washed and dried exemplarily through solid-liquid separation (eg, filtering, centrifugation, etc.). At this time, alcohol (eg, methanol, ethanol, etc.) washing and/or water washing may be performed, or washing may be performed with a mixed solvent of alcohol and water. In addition, drying may be performed by a known drying method such as a method known in the art, heat drying, vacuum drying, or the like. Illustratively, in the case of the thermal drying method, the drying temperature may be adjusted within the range of, for example, about 40 to 90 ° C., specifically about 50 to 80 ° C., and the drying time is typically, for example, about 3 to 10 hours, specifically about 4 to 7 hours.

The metal precipitate thus obtained is affected by the medium used in the first hydrothermal synthesis. As described above, when glycerol is used as a polyhydric alcohol, it may be in the form of metal glycerate.

Then, a second hydrothermal synthesis reaction is performed on the metal precipitate. To this end, the metal precipitate is added to an aqueous medium (specifically deionized water). Illustratively, the content of the metal precipitate is, for example, about 2 to 8% (w / w), specifically about 3 to 6% (w / w), more specifically about 3.5 to 5.5% (w / w) , In particular, it may be specifically adjusted in the range of about 3.75 to 5% (w / w). In the case of the second hydrothermal synthesis reaction, the reaction temperature may be set in the range of, for example, about 130 to 170 °C, specifically about 140 to 165 °C, and more specifically about 145 to 160 °C, particularly in the first hydrothermal synthesis. Compared to the reaction, for example, it may be set at a lower temperature of about 20 to 40 °C, specifically about 25 to 35 °C. In this regard, when the second hydrothermal synthesis reaction temperature falls short of or exceeds a certain level, a cell layer is not formed in the final metal phosphide catalyst, making it difficult to provide a yoke-cell structure. It may be advantageous to carry out hydrothermal synthesis in the range. In addition, the second hydrothermal synthesis reaction time may be adjusted in the range of, for example, about 1 to 8 hours, specifically about 2 to 6 hours, and more specifically about 3 to 4 hours, but this will be understood as an example. can

As described above, a metal hydroxide, specifically, nickel-cobalt hydroxide may be formed by the second hydrothermal synthesis reaction, which corresponds to a precursor or intermediate for forming a metal phosphide. In addition, a separation and purification step may be performed as an optional step, which is similar to that described in relation to the first hydrothermal synthesis reaction product (metal precipitate) (however, water may be used as a washing liquid).

Then, a phosphidation reaction may be performed on the metal hydroxide, and a substitution reaction using a phosphide agent may be used. As an example, the phosphide agent may typically be at least one selected from hypophosphates, phosphine gas, red phosphorus, etc., specifically sodium hypophosphate, and more specifically sodium hypophosphate hydrate. In this regard, the substitution reaction may be carried out under inert atmosphere and elevated temperature conditions. Illustratively, the inert atmosphere gas may be at least one selected from argon, neon, nitrogen, and the like, and more specifically, argon. In addition, the substitution reaction temperature may be set in the range of, for example, about 280 to 400 °C, specifically about 300 to 370 °C, and more specifically about 320 to 360 °C, wherein the temperature increase rate is, for example, about 0.5 °C. to 5 °C/min, specifically about 0.8 to 3 °C/min, and more specifically about 1 to 2 °C/min. In addition, the substitution reaction may be performed over, for example, about 2 to 10 hours, specifically about 2.5 to 8 hours, and more specifically about 3 to 5 hours, but this can be understood as an example.

Although the present disclosure is not bound by a particular theory, in the case of sodium hypophosphate hydrate in the above-described embodiment, under elevated temperature conditions, phosphine (PH ₃ ) gas can be generated, and this phosphi gas reacts with metal hydroxide to It is believed to be capable of forming phosphides. Therefore, instead of using sodium hypophosphate hydrate, phosphine (PH ₃ ) gas may be directly used to perform the phosphidation reaction.

According to an exemplary embodiment, the phosphidation reaction of metal hydroxide with sodium hypophosphate hydrate may be performed in a tubular furnace. At this time, two crucibles are placed at both ends in the tubular furnace, and an inert gas can be flowed. Among the two crucibles, a crucible in which a phosphide agent such as sodium hypophosphate is located is disposed upstream, and a crucible in which metal hydroxide is located Is disposed downstream, and the phosphine gas converted under elevated temperature conditions in the upstream crucible is transferred to the downstream crucible by argon and reacts with the metal hydroxide to form a metal phosphide catalyst.

On the other hand, the metal phosphide catalyst according to the present embodiment is easy to recover the activity to the initial activity or a level close thereto through regeneration. In this regard, the regeneration process includes simple separation (solid-liquid separation such as filter, centrifugation, etc.), washing (e.g., washing at least once with alcohol and/or water), and drying (e.g., about 40 to 90 ° C. for about 3 to 10 hours).

Conversion (cyclization) reaction of organic acids or their derivatives

According to one embodiment of the present disclosure, there is provided a process for producing various high value-added compounds from organic acids or derivatives thereof through hydrogenation using a metal phosphide catalyst having a yoke-cell structure, wherein the metal phosphide catalyst has the general formula Ni It may be a nickel-cobalt phosphide catalyst represented by _x Co _y P.

According to an exemplary embodiment, the organic acid may be an organic acid typically derived from biomass, for example, an organic acid having 1 to 8 carbon atoms, specifically 2 to 7 carbon atoms, more specifically 4 to 6 carbon atoms, , especially an organic acid having 5 carbon atoms. Also, the derivative of an organic acid may be an ester compound. These organic acids can form cyclic compounds while undergoing a hydrogenation reaction in the presence of a metal phosphide catalyst. As an example, the organic acid is levulinic acid, succinic acid, fumaric acid, itaconic acid, aspartic acid, 2,5-furandicarboxylic acid, glutaric acid, lactic acid, etc. It may be at least one selected from. In addition, in relation to the type compound produced through a hydrogenation reaction, when levulinic acid and/or a derivative thereof (specifically, an ester) is used as an organic acid, gamma-valerolactone (GVL) and/or 2-methyltetra Hydrofuran (2-MTHF) can be produced. In addition, when succinic acid or fumaric acid is used as an organic acid, butyrolactone and tetrahydrofuran may be produced. In addition, when itaconic acid is used as an organic acid, 3-methylbutyrolactone and 3-methyltetrahydrofuran can be produced. As described above, since the production of cyclic compounds through hydrogenation of organic acids is known in the art, detailed descriptions thereof will be omitted.

- 1st hydrogenation reaction

In a specific embodiment, the organic acid may be levulinic acid, and as described above, dehydration/cyclization may occur through hydrogenation to form gamma-valerolactone (GVL) (first hydrogenation step). In this regard, two types of reaction routes for converting levulinic acid to gamma-valerolactone are known (as an intermediate, via lactone and via 4-hydroxypentanoic acid). In a specific example, it is believed to be via 4-hydroxypentanoic acid. Gamma-valerolactone (GVL) is a raw material used in the synthesis of adipic acid, a precursor of polyamides such as polyamide 6,6 and polyamide 4,6.

According to an exemplary embodiment, the first hydrogenation reaction may be performed by introducing an organic acid and a catalyst into a reactor, supplying or injecting hydrogen into the reactor until a predetermined pressure is reached, and performing the reaction under elevated temperature conditions. At this time, the hydrogenation temperature may be adjusted in the range of, for example, about 120 to 300 ° C, specifically about 150 to 250 ° C, more specifically about 170 to 200 ° C, and the reaction pressure (hydrogen pressure or hydrogen partial pressure), For example, it is appropriately adjustable in the range of about 10 to 50 bar, specifically about 15 to 40 bar, and more specifically about 20 to 35 bar. As the hydrogenation temperature and pressure affect the conversion rate of the reactants and the selectivity of the product, it may be desirable to properly adjust them within the above-mentioned range, but it can be changed depending on the type of organic acid, the activity of the catalyst, whether or not a solvent is used, etc. . These reaction conditions indicate that the reaction can be carried out at lower temperatures (i.e., milder conditions) due to the intrinsic metallicity and acidic nature of the active phosphide in the catalyst. These are (i) the use of inexpensive base metals, (ii) the ability to increase the conversion rate of reactants and increase the selectivity for target products through a bimetal catalyst system, and (iii) the absence of solvents. (solvent-free) It is equivalent to an additional advantage that differentiates the process according to the present embodiment from the prior art with the fact that it does not require a separation process of the solvent as the reaction is performed. Although the present disclosure is not bound by any particular theory, since the hydrogenation reaction is initiated from the adsorption and activity of hydrogen on the surface of the catalyst, the nickel-cobalt phosphide catalyst according to the present embodiment can adsorb hydrogen at a lower temperature. This can be explained by the fact that it exists and can be activated. In addition, the hydrogenation reaction time may be adjusted in the range of, for example, about 1 to 10 hours, specifically about 2 to 8 hours, and more specifically about 3 to 6 hours.

According to an exemplary embodiment, the reactant organic acid (specifically, levulinic acid) or its derivative: the weight ratio of the catalyst is 1: about 0.015 to 0.03, specifically 1: about 0.018 to 0.026, and more specifically 1: about 0.02 to about 0.026. It may be in the range of 0.025, particularly at the level of 1 : 0.023. Considering that the ratio of reactant to catalyst can affect the conversion of the reactants and the selectivity for the target product, and also that a relatively increased amount of catalyst can provide more active sites for the catalyst. , it is advantageous to adjust within the above range.

According to an exemplary embodiment, the hydrogenation reaction may be performed in a continuous mode as well as a batch mode, for example, a fixed bed reactor, a semi-batch reactor, and the like may be used. More typically, a batch mode may be employed.

When the above reaction is completed, the target compound, specifically gamma-valerolactone, can be separated and recovered from the hydrogenation product. This separation and recovery process is known in the art and may be performed using, for example, distillation, extraction, separation membrane, flash drum, and the like.

According to an exemplary embodiment, the conversion of the organic acid (particularly levulinic acid) in the primary hydrogenation reaction is, for example, at least about 45% (specifically at least about 70%, more specifically at least about 80%, particularly specifically at least about 80%). at least about 95%, substantially up to 100%), and the selectivity for its cyclization products (particularly gamma-valerolactone) is, for example, at least about 70% (specifically at least about 90%, more specifically at least about 95%, particularly up to substantially 100%).

- Secondary hydrogenation reaction

According to an exemplary embodiment, the metal phosphide catalyst according to the present embodiment can be used to convert a hydrogenation product (or cyclization reaction) of an organic acid into another type of high value-added compound or cyclic compound through an additional hydrogenation reaction.

As an example, when levulinic acid is converted to gamma-valerolactone with an organic acid during the first hydrogenation reaction, the converted gamma-valerolactone is converted into the above-mentioned metal phosphide catalyst (new catalyst or regenerated catalyst) in the presence of After forming a decyclization intermediate (eg, 1,4-pentanediol) through an additional hydrogenation reaction, 2-methyltetrahydrofuran (2-MTHF) may be produced by performing a dehydration/cyclization reaction again. (secondary hydrogenation reaction). 2-methyltetrahydrofuran (2-MTHF) can be applied as a biofuel, solvent, etc., and can replace THF, which has been widely used in the past. In particular, it exhibits low water miscibility, high stability and low volatility compared to THF, and can be applied to pharmaceutical manufacturing processes and the like.

In this regard, the hydrogenation reaction temperature may be adjusted in the range of, for example, about 180 to 320 ° C, specifically about 200 to 280 ° C, more specifically about 220 to 250 ° C, and also the reaction pressure (hydrogen pressure or hydrogen partial pressure). ) Is, for example, about 30 to 80 bar, specifically about 40 to 70 bar, more specifically about 45 to 60 bar can be appropriately adjusted in the range. The hydrogenation temperature and pressure are appropriately controlled within the above-mentioned range as they affect the conversion of the first hydrogenation product (eg, GVL) and the selectivity for the second hydrogenation product (eg, 2-MTHF). However, this can be understood for illustrative purposes. In addition, the reaction time may be adjusted in the range of, for example, about 2 to 15 hours, specifically about 5 to 12 hours, and more specifically about 6 to 10 hours.

According to an exemplary embodiment, the weight ratio of the reactant primary hydrogenation product (eg, GVL): catalyst is 1: about 0.015 to 0.04, specifically 1: about 0.016 to 0.033, more specifically 1: about 0.02 to about 0.02. It may be in the range of 0.03, and in particular, it may be at the level of 1:0.026. As such, the ratio of the reactants (i.e., the primary hydrogenation product) to the catalyst can affect the conversion of the reactants and the selectivity for the target compound of the secondary hydrogenation reaction as described above, so it is controlled within the aforementioned range. it is advantageous

According to an exemplary embodiment, the secondary hydrogenation reaction may be carried out in a continuous mode as well as a batch mode similar to the first hydrogenation reaction, and in the continuous mode, for example, a fixed bed reactor, a semi-batch reactor, etc. may be used.

When the above reaction is completed, the target compound, specifically 2-methyltetrahydrofuran (2-MTHF), can be separated and recovered from the hydrogenation reaction product. This separation and recovery process is known in the art and may be performed using, for example, distillation, extraction, separation membrane, flash drum, and the like.

According to an exemplary embodiment, the conversion rate of the cyclic compound (particularly, gamma-valerolactone) as a reactant in the secondary hydrogenation reaction is, for example, at least about 45% (specifically at least about 65%, more specifically at least about 70%, and also up to about 80%), and the selectivity to other cyclic compounds (particularly 2-MTHF) obtained by further hydrogenation is, for example, at least about 40% (specifically at least about 44%, more specifically at least about 48%, but also up to about 55%).

A point to be noted in this embodiment is that the conversion reaction can be carried out in the absence of a liquid medium (solvent) not only in the first hydrogenation reaction, but also in the second hydrogenation reaction. As described above, when no solvent is used, the need to separate the reaction product from the solvent in a subsequent step can be eliminated.

On the other hand, according to an alternative embodiment, when the conversion of levulinic acid is low, a reactant organic acid (for example, levulinic acid) or a cyclic compound (gamma-valerolactone) and a solvent are used together to form primary and / or a secondary hydrogenation reaction can be performed. At this time, the usable solvent may be at least one selected from protic polar solvents and/or aprotic polar solvents, and may act as a hydrogen donor without affecting the dehydration/cyclization reaction accompanying the hydrogenation reaction. It can be selected from possible types. As an example, usable solvents include, for example, aliphatic alcohols having 1 to 5 carbon atoms (specifically, methanol, ethanol, isopropanol, sec-butanol, etc. as primary and/or secondary alcohols), dioxane (specifically, 1, 4-dioxane), water, and the like. In addition, the volume ratio of reactant (an organic acid in the first hydrogenation reaction and a cyclic compound such as GVL in the second hydrogenation reaction): solvent is, for example, 1: about 4 to 15, specifically 1: about 6 to 12, More specifically, it may be adjusted in the range of about 8 to 10.

Hereinafter, preferred embodiments are presented to aid understanding of the present invention, but the following examples are provided to more easily understand the present invention, but the present invention is not limited thereto.

Example

Hereinafter, a phosphide-based catalyst is synthesized and used to synthesize gamma-valerolactone (GVL) and 2-methyltetrahydrofuran (2-MTHF) from levulinic acid according to the reaction mechanism shown in FIG. 1, respectively. An experimental example is described.

Materials used in Examples and Comparative Examples are as follows.

Substances and Reagents

- Nickel nitrate hexahydrate (Ni(NO ₃ ) ₂ 6H ₂ O) and cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ 6H ₂ O) were each purchased from Alfa Aesar.

- Glycerol, isopropyl alcohol and ethanol were each purchased from Alfa Aesar, Daejeong Chemical and Gold, and Daejeong Chemical and Gold.

- Deionized water was prepared in-house in the laboratory.

- Sodium hypophosphate hydrate was purchased from Alfa Aesar.

- Levulinic acid was purchased from Acros Organics.

- All chemicals were used as received without further purification.

analysis device

Sample analysis was performed using the following equipment.

- SEM analysis was performed using a Helios 50 scanning electron microscope.

- TEM analysis was performed using a JEM-F200 microscope.

- EDS (energy-dispersive X-ray spectroscopy) analysis was performed using a JEOL-JEM 2100F Transmission Electron Microscope.

- H ₂ -TPD was performed using a Microtrac MRB BELCAT II. Specifically, in the pretreatment step, pretreatment was performed by adsorbing hydrogen at 200° C. for 2 hours under a flow of 50 mL/min of helium, and then under a flow of 50 mL/min of 10% H ₂ /Ar. In the hydrogen desorption step, the physically adsorbed hydrogen was removed over 1 hour at 100 °C under a 50 mL/min flow of helium. The remaining hydrogen was removed by raising the temperature to 300 °C in a furnace at a heating rate of 5 °C/min under a flow of helium of 30 mL/min.

- NH ₃ -TPD was performed using a Microtrac MRB BELCAT II. Specifically, in the pretreatment step, pretreatment was performed by adsorbing ammonia under a flow of 50 mL of helium at 200° C. for 2 hours, followed by a flow of 5% NH ₃ /He of 50 mL/min. In the ammonia desorption step, physically adsorbed ammonia was removed over 1 hour at 100° C. under a flow of 50 mL/min of helium. The remaining ammonia was removed by raising the furnace temperature to 800 °C at a heating rate of 5 °C/min under a flow of helium of 30 mL/min.

Example 1

Synthesis of nickel-cobalt phosphide catalysts

In this example, a nickel-cobalt phosphide catalyst containing nickel and cobalt in various ratios was synthesized through two hydrothermal synthesis steps and a phosphide step.

A predetermined ratio of nickel precursor (Ni(NO ₃ ) ₂ 6H ₂ O) and cobalt precursor (Co(NO ₃ ) ₂ 6H ₂ O) was mixed with glycerol and isopropyl alcohol at a volume ratio of 1:3.5. It was added to the mixed solvent and dissolved until a transparent and pink solution was obtained. At this time, the concentration of all metals (nickel and cobalt) in the precursor solution was 62.5 mM.

The obtained nickel-cobalt precursor solution was put into a Teflon-lined reactor (200 mL), and a first hydrothermal synthesis reaction (hydrothermal treatment) was performed at 180° C. for 6 hours, followed by cooling. Then, the precipitate, i.e., Ni-Co glycerate, was collected through centrifugation, subsequently washed several times with ethanol and deionized water, and then dried in an oven maintained at 60° C. for 6 hours. .

Thereafter, 4 g of the precipitate of the nickel-cobalt precursor was added to 80 mL of deionized water to prepare a suspension, and a secondary hydrothermal synthesis reaction (hydrothermal treatment) was performed at 150 ° C. for 3 hours to form a precipitate of Ni—Co hydroxide. . Subsequently, similarly to the separation and purification of the primary hydrothermal synthesis reaction product, the precipitate was washed with deionized water and dried to obtain a powder.

1 g of the powder of Ni-Co hydroxide obtained in the previous step and 20 g of sodium hypohydride hydrate (NaH ₂ PO ₂ H ₂ O) were pulverized into fine powder and put into crucibles at different positions in the tubular electric furnace. did At this time, the crucible into which sodium hypophosphate hydrate was introduced was positioned upstream, and phosphide reaction was performed at 350 ° C. for 3 hours at a heating rate of 1 ° C./min while flowing argon gas into the electric furnace. As a result, nickel- Cobalt phosphide powder was obtained. The powder thus obtained was expressed as Ni _x Co _y P (x and y represent the mole fraction of Ni:Co).

Comparative Example 1

Synthesis of cobalt phosphide (CoP) catalysts

A cobalt precursor (Co(NO ₃ ) ₂ 6H ₂ O) was added to a mixed solvent in which glycerol and isopropyl alcohol were mixed in a volume ratio of 1: 4, and dissolved until a transparent and pink solution was obtained. . At this time, the concentration of cobalt in the precursor solution was 62.5mM.

The obtained cobalt precursor solution was put into a Teflon-lined reactor (200 mL), and a hydrothermal synthesis reaction (hydrothermal treatment) was performed at 180° C. for 6 hours, followed by cooling. Then, the precipitate, that is, Co glycerate, was collected through centrifugation, subsequently washed several times with ethanol and deionized water, and then dried in an oven maintained at 60° C. for 6 hours.

Thereafter, 4 g of the precipitate of the cobalt precursor was added to 80 mL of deionized water to prepare a suspension, and a secondary hydrothermal synthesis reaction (hydrothermal treatment) was performed at 150 ° C. for 3 hours to form a precipitate of Co hydroxide. Subsequently, similarly to the separation and purification of the primary hydrothermal synthesis reaction product, the precipitate was washed with deionized water and dried to obtain a powder.

1 g of powder of Co hydroxide obtained in the previous step and 20 g of sodium hypothia phosphate hydrate (NaH ₂ PO ₂ ㅇH ₂ O) were pulverized into fine powder and placed in crucibles at different positions in a tubular electric furnace. At this time, the crucible into which sodium hypophosphate hydrate was introduced was positioned upstream, and phosphidation reaction was performed at 350 ° C. for 3 hours at a heating rate of 1 ° C./min while flowing argon gas into the electric furnace. As a result, cobalt phosphide Pied powder (CoP) was obtained.

Comparative Example 2

Synthesis of nickel phosphide (NiP) catalysts

_{Nickel phosphide powder ( _ _} NiP) was obtained.

Comparative Example 3

Mixed catalyst preparation of cobalt phosphide (CoP) and nickel phosphide (NiP)

CoP and NiP synthesized in Comparative Examples 1 and 2, respectively, were mixed together at a weight ratio of 1:1 and then ground to prepare a uniform physical mixture.

Example 2

Synthesis of gamma-valerolactone (GVL)

In order to synthesize gamma-valerolactone (GVL) by hydrogenating levulinic acid (LA) under solvent-free conditions, a 100 mL stainless steel (SUS 316L) high-pressure reactor was used.

30 mL of LA and 0.8 g of catalyst were charged into the reactor, and an agitator was attached to the top of the reactor. Thereafter, the reactor was sealed, purged with argon gas three times to remove traces of gaseous impurities, and then pressurized with hydrogen gas at 30 bar. Upon reaching the target pressure, the reactor was heated to 180° C. over 4 hours, and stirring was started (stirring speed: 800 rpm). At this time, the starting point of the reaction was recorded as the time when the target temperature was reached. When the hydrogenation reaction was completed, stirring was stopped, the reactor was cooled to room temperature, and the reaction product in the form of a mixture and the catalyst were separated through filtering.

Example 3

Synthesis of 2-methyltetrahydrofuran (2-MTHF)

In order to synthesize 2-methyltetrahydrofuran (2-MTHF) by hydrogenating levulinic acid (LA) under solvent-free conditions, a 100mL stainless steel (SUS 316L) high-pressure reactor was used. - One-pot two-step hydrogenation reaction was performed.

30 mL of LA and 0.8 g of catalyst were charged into a reactor, and a stirrer was installed at the top of the reactor to synthesize gamma-valerolactone (GVL) as in Example 2. Specifically, the reactor was sealed, purged with argon gas three times to remove traces of gaseous impurities, and then pressurized with hydrogen gas at 30 bar. Upon reaching the target pressure, the reactor was heated to 180° C. over 4 hours, and stirring was started (stirring speed: 800 rpm). At this time, the starting point of the reaction was recorded as the time when the target temperature was reached. When the hydroxylation reaction was completed, stirring was stopped, the reactor was cooled to room temperature, and the reaction product in the form of a mixture and the catalyst were separated through filtering. The separated catalyst was washed with a mixture of ethanol and water, dried at 60 °C for 6 hours, and then reused for the secondary hydrogenation reaction. In the first hydrogenation step, LA conversion of 100% and GVL selectivity of 100% were achieved.

In the second hydrogenation step, 30 mL of GVL and 0.8 g of reusable catalyst were filled and sealed in a reactor equipped with an agitator at the top, and then purged with argon gas three times to remove traces of gaseous impurities, followed by 50 bar Pressurized with hydrogen gas. Upon reaching the target pressure, the reactor was heated to 230° C. over 12 hours, and stirring was started (stirring speed: 800 rpm). At this time, the starting point of the reaction was recorded as the time when the target temperature was reached. When the hydrogenation reaction was completed, stirring was stopped, the reactor was cooled to room temperature, the gas generated in the reaction process was discharged, and then the reaction product in the form of a mixture and the catalyst were separated through filtering.

Comparative Examples 4 to 6

Using the metal phosphide catalyst prepared in Comparative Examples 1 to 3, respectively, GVL and 2-MTHF were prepared according to the procedures of Examples 2 and 3, respectively.

product analysis

In each of Examples 2 and 3 and Comparative Examples 4 to 6, the purity of the product was evaluated by 1H, 13C nuclear magnetic resonance (NMR) spectroscopy, using a Bruker Advanced II spectrometer at a rate of 400 MHz and 13 kHz operated at a rotational speed of

The amount of product was measured by High-Performance Liquid Chromatography (HPLC), Shimadzu UHPLC Nexera, SCL-40 equipped with Agilent Hi-Plax Ca (7.7 x 300 mm, 8 μm) and 2410 refractive index detector. was used.

Results and discussion

go. Catalyst characterization

- SEM analysis

In preparing the metal phosphide catalyst, (a, b) Ni—Co glycerate precursor (resulting from the first hydrothermal reaction), (c, d) Ni—Co phosphide precursor (resulting from the second hydrothermal reaction), SEM images of (eg) Ni ₂ Co ₁ P, (h) CoP precursor, (i) CoP, and (j) NiP are shown in FIG. 2 .

2a and 2b, the precipitate formed from the first hydrothermal synthesis reaction (or first hydrothermal treatment) has a spherical shape. In addition, referring to FIGS. 2C and 2D, after the second hydrothermal synthesis reaction (or the second hydrothermal treatment), the precipitate generated from the first hydrothermal synthesis reaction has a spherical shape composed of interconnected nanosheets. You can see that it is going through a change. The change in the shape of the precipitate is such that the precipitate in the first hydrothermal synthesis reaction is hydrolyzed to form a nanosheet. Specifically, Ni ²⁺ and Co ²⁺ ions react with hydroxyl groups of water in the second hydrothermal synthesis reaction to form nanosheets on the surface. It can be seen as synthesizing sheets. At this time, the addition of cobalt plays an important role in maintaining the shape of the precursor.

- TEM analysis

TEM analysis was performed on the nickel-cobalt phosphide catalyst, and TEM images at different magnifications are shown in FIGS. 3a to 3c. In addition, the high-resolution TEM (HRTEM) picture is shown in FIG. 3d, and the HAADF-STEM picture is shown in FIG. 3e.

Referring to the figure, it was confirmed that the nickel-cobalt phosphide catalyst was composed of interconnected nanosheets and had a spherical yoke-cell structure. In this regard, in the case of the yoke-cell structure in the above-described catalyst, it can function as a kind of nano-reactor, wherein the yoke is an active site where a reaction can occur, and the empty space between the cell layer and the yoke is It functions as a pathway to increase mass transfer efficiency. In addition, a hollow shell can encapsulate the active site to prevent deactivation of the catalyst.

In addition, the HRTEM picture of the Ni ₂ Co ₁ P catalyst is shown in FIG. 4 .

Referring to the figure, in the case of the Ni ₂ Co ₁ P catalyst, lattice fringes of 0.24 nm and 0.26 nm were observed, corresponding to the (111) and (201) planes of nickel-cobalt phosphide, respectively. In this regard, several discrete bright spots in the SAED pattern support this result, which can be indexed to the (111), (201), and (210) planes of nickel-cobalt phosphide, respectively.

On the other hand, nickel-cobalt phosphide catalysts prepared by setting the second hydrothermal synthesis reaction temperature to 150 ° C and 100 ° C (Ni ₂ Co ₁ P - 150 ° C and Ni ₂ Co ₁ P - 100 ° C), and Ni 2 Co 1 P - 100 ° C, a commercial catalyst -Cu/Al ₂ O ₃ A TEM image showing each structure is shown in FIG. 5 .

Referring to the figure, in the case of Ni ₂ Co ₁ P-150 ° C, as described above, a clear yoke-cell structure was exhibited, but in Ni ₂ Co ₁ P-100 ° C, solid spherical particles composed of interconnected nanosheets has In addition, it was confirmed that the Ni-Cu/Al ₂ O ₃ catalyst had agglomerated particles located on the upper side of the alumina support.

- EDS (energy-dispersive X-ray spectroscopy) analysis

EDS analysis was performed on the nickel-cobalt phosphide catalyst, and the results are shown in FIG. 3f. From the figure, it was confirmed that cobalt, nickel, and phosphorus atoms were contained in the nickel-cobalt phosphide catalyst.

- Analysis of acid point and hydrogen activity

Considering that the acid properties and hydrogen activity of catalysts act as important factors in synthesizing cyclic compounds by hydrogenation of levulinic acid, H ₂ -TPD and NH ₃ -TPD for various transition metal phosphides analysis was performed. The results are shown in Figures 6a and 6b, respectively.

As can be seen from FIG. 6a, the hydrogen activity and hydrogen adsorption abilities of various metal phosphides can be compared, and single metal cobalt phosphide (CoP) showed the weakest hydrogen adsorption and activity, CoP < Ni ₁ Co ₃ P < NiP < Ni ₁ Co ₂ P < Ni ₁ Co ₁ P < Ni ₂ Co ₁ P < Ni ₃ Co ₁ P In general, nickel-cobalt phosphide exhibits improved hydrogen activating ability compared to single metal cobalt phosphide and nickel phosphide, and nickel is added to phosphide to improve the activating ability of catalysts. It was confirmed that it can act as the main active ingredient in

In addition, as can be compared with the acidity of the catalyst from FIG. 6B, the acidity strength and the distribution of each of the weakly acidic, intermediately acidic and strong acidic regions could be adjusted by varying the molar ratio between nickel and cobalt. As a result of the analysis, it was confirmed that the single metal cobalt phosphide had both a weak acid site and a strong acid site, while nickel phosphide was mainly composed of a weak acid site. In addition, it was confirmed that nickel-cobalt phosphide (Ni ₂ Co ₁ P) contains all of a weak acid site, an intermediate acid site, and a strong acid site. As a result of the acid amount comparison, CoP < Ni ₁ Co ₃ P < Ni ₁ Co ₂ P < Ni ₁ Co ₁ P < Ni ₂ Co ₁ P < Ni ₃ Co ₁ P < NiP. In addition, the metal component in the reduced form functions as a metal active site in the hydrogenation reaction, and the non-reduced metal component (Ni ²⁺ and Co ²⁺ ) provides an acidic site of the phosphide catalyst.

- XPS spectrum analysis

XPS spectra of NiP, Ni ₂ Co ₁ P and CoP are shown in FIG. 7 . At this time, FIG. 7a is the Ni 2p spectrum for NiP and Ni ₂ Co ₁ P, respectively, FIG. 7b is the Co 2p spectrum for Ni ₂ Co ₁ P and CoP, respectively, and FIG. 7c is the NiP, Ni ₂ Co ₁ P and CoP, respectively. is the P 2p spectrum for

Referring to FIG. 7A , peaks located at 857.7 eV and 874.4 eV in the 2p spectrum of nickel correspond to Ni 2p ^3/2 and 2p ^1/2 of Ni ²⁺ , respectively. Peaks located at 853.6 eV and 871.0 eV correspond to Ni 2p ^3/2 and Ni 2p ^1/2 of reduced Ni ^δ+ in Ni—P, respectively.

In FIG. 7B, peaks located at 782.5 eV and 798.7 eV correspond to Co 2p ^3/2 and Co 2p ^1/2 of Co ²⁺ or oxidized Co components, respectively. In addition, the peaks located at 779.3 eV and 793.7 eV correspond to Co 2p ^3/2 and Co 2p ^1/2 of the Co component in Co—P that has a partial positive charge from Co metal (Co ^δ2+ ), respectively.

In addition, as confirmed by the 2p spectrum of phosphorus (P) from FIG. 7c, two components were present on the surface of the single metal phosphide and the double metal phosphide. The peak located at approximately 134 eV corresponds to the unreduced phosphate component (PO ₄ ^3- ) resulting from the oxidation of phosphide, while the peak with lower binding energy of approximately 129 eV is partially reduced. It corresponds to the phosphorus component (P ^δ- ) or phosphide phase. At this time, the double metal Ni ₂ Co ₁ P had similar characteristics to the Ni and Co components of single metal NiP and CoP, and the small shift in the peak was attributed to the electronic interaction between nickel and cobalt in the double metal Ni ₂ Co ₁ P. can see.

From the above XPS results, the coexistence of M ^δ+ (M=Ni, Co) and P ^δ- components in the catalyst led to the formation of covalent bonds between nickel, cobalt and phosphorus in the synthesized metal phosphide, and nickel-cobalt It can be seen that the phosphide was successfully synthesized.

- XRD analysis

Phosphides of various Ni and/or Co (NiP, CoP, NiCoP, Ni ₅ P ₄ , NiP ₂ , Ni ₃ Co ₁ P, Ni ₂ Co ₁ P, Ni ₁ Co ₁ P, Ni ₁ Co ₂ P, Ni ₁ XRD patterns for each of Co ₃ P and CoP) are shown in FIG. 8 .

Referring to the figure, it can be confirmed that the crystallinity of each catalyst is greatly affected by the addition of cobalt through the intensity and width of the XRD peaks. Cobalt-based double metal phosphides (CoP, Ni ₁ Co ₃ P and Ni ₁ Co ₂ P) are amorphous, whereas nickel-based double metal phosphides (NiP, Ni ₃ Co ₁ P and Ni ₂ Co ₁ P) are amorphous. ) showed well-defined peaks, indicating that the crystalline structure was enhanced.

In addition, all of the double metal phosphides exhibited peaks corresponding to the (111) plane and the (300) plane of nickel-cobalt phosphide, from which it was confirmed that nickel cobalt phosphide was successfully synthesized. In particular, it was confirmed that the peak intensity of nickel-cobalt phosphide further increased as the molar ratio of nickel:cobalt increased. In addition, additional peaks located at 44.9ㅀ and 48.1ㅀ related to the (201) and (210) planes of nickel-cobalt phosphide were observed in the XRD patterns of Ni ₂ Co ₁ P and Ni ₃ Co ₁ P, which will be described later. It is noteworthy that the phosphide having high crystallinity has a favorable effect on the activity of the catalyst through the hydrogenation test.

- Specific surface area and pore volume analysis

Phosphides of various Ni and/or Co (NiP, CoP, NiCoP, Ni ₅ P ₄ , NiP ₂ , Ni ₃ Co ₁ P, Ni ₂ Co ₁ P, Ni ₁ Co ₁ P, Ni ₁ Co ₂ P, Ni ₁ The BET specific surface area and pore volume for each of Co ₃ P and CoP) were measured and are shown in Table 1 below.

According to the above table, it was confirmed that the specific surface area and pore volume increased as cobalt was added to the phosphide.

me. Evaluation of activity in the conversion of levulinic acid to gamma-valerolactone

- Analysis of conversion rate and selectivity according to catalyst

9 shows graphs showing conversion and selectivity, respectively, measured in an experiment for preparing gamma-valerolactone from levulinic acid in the presence of the previously prepared metal phosphide catalyst.

Referring to the figure, a binary metal phosphide (particularly, Ni ₂ Co ₁ P) provides better catalytic activity than single metal phosphides, CoP and NiP, and also a physical mixture of CoP and NiP can be obtained from levlinic acid by gamma - It was confirmed that both cobalt and nickel act in the complete conversion to valerolactone. In addition, among the binary metal phosphide catalysts, Ni ₂ Co ₁ P showed the best activity.

As can be seen from the catalyst characterization results, the active site of the Ni ₂ Co ₁ P catalyst comes from a bifunctional nature, contains cobalt and nickel components that provide hydrogenation active sites, and the remaining phosphate contains an acidic site, providing the necessary activity for the dehydration/cyclization reaction. In particular, as the relative content of nickel among the binary metals in the catalyst increased, the activity of the catalyst significantly improved. The overall catalytic activity was CoP < NiP < Ni ₁ Co ₃ P < Ni ₁ Co ₂ P < Ni ₁ Co ₁ P < Ni ₃ Co ₁ P < Ni ₂ Co ₁ P.

Referring again to the catalyst screening results according to FIG. 9, the CoP catalyst showed a rather high GVL selectivity of 74.6%, but the conversion rate of levulinic acid was at a low level of 29.1%. This is because, as confirmed in the H ₂ -TPD results, the CoP catalyst has insufficient hydrogen activating ability to initiate the conversion of levulinic acid. On the other hand, the CoP catalyst contains a large amount of strong acid sites, enabling conversion from HPA to GVL. These results suggest that the hydrogenation reaction from levulinic acid to HPA is related to the hydrogenation ability and metallicity of phosphide, and the dehydration reaction from HPA to GVL is related to the acid properties of the catalyst.

- Optimization of reaction conditions

The hydrogenation reaction was performed under different reaction conditions in the presence of the Ni ₂ Co ₁ P catalyst, which was evaluated to exhibit the best activity among binary metal phosphide catalysts, and the results are shown in Table 2 below.

According to the above table, when the hydrogenation reaction was performed for 4 hours at a temperature of 180 ° C. and a pressure of 30 bar, the conversion of levulinic acid and the selection for gamma-valerolactone reached approximately 100%, and other by-products was hardly formed.

- Evaluate the effect of the solvent in the hydrogenation reaction using the solvent

When a solvent is used in the hydrogenation reaction, the influence thereof was tested to evaluate the versatility of the catalyst under various reaction conditions. To this end, primary alcohols (ie, methanol and ethanol), secondary alcohols (isopropanol and 2-butanol (sec-butanol)), 1,4-diox, and water were used as solvents, Ni ₂ Co ₁ Conversion and selectivity measured by conducting a hydrogenation reaction in the presence of a P catalyst are shown in FIG. 10 together with the results obtained under solvent-free conditions.

According to the figure, in the case of using alcohol as a solvent, quantitative analysis of the conversion rate of levlinic acid and the yield of GVL was possible, but the hydrogenation reaction immediately after esterification of levlinic acid with methyl revlinate, ethyl leblinate, etc. As this occurs, the selectivity for GVL is reduced. However, when secondary alcohol was used, improved results were obtained compared to primary alcohol, wherein the selectivity of GVL for isopropanol and 2-butanol was 78% and 83%, respectively. This is considered to be because the reducing ability of the secondary alcohol is better. In addition, when 1,4-dioxane was used as a solvent, the selectivity for GVL was relatively high at 86%, but considering the toxicity and potential hazards of the solvent, it is judged that there will be limitations in application on a commercial scale.

In this regard, it is noteworthy that the highest conversion of levulinic acid (88%) and selectivity to GVL (98%) can be obtained when environmentally friendly water is used as the solvent. It is believed that this is due to the fact that the reaction activation energy is reduced due to the strong interaction between levulinic acid and water because water has a high hydrogen surface concentration.

On the other hand, the highest levulinic acid conversion and GVL yield were obtained when the hydrogenation reaction was performed under solvent-free conditions. GVL functions as a solvent when LA is hydrogenated into GVL, and it is believed that the produced GVL can promote the formation of GVL from LA. As such, the reaction under solvent-free conditions is advantageous in promoting the sustainability of GVL production.

- Analysis of the effect of reactants

Conversion and selectivity measured by hydrogenating reactants or substrates of various levulinic acid esters with gamma-valerolactone in the presence of a Ni ₂ Co ₁ P catalyst are shown in FIG. 11 , respectively.

Referring to the above figure, it was confirmed that GVL can be obtained even when levulinic acid ester is used as a reactant when a Ni ₂ Co ₁ P catalyst is used. In particular, when methyl leblinate was used as a reactant (substrate), the selectivity for GVL was as high as 82%. On the other hand, butyl and ethyl levellinate showed lower results than methyl levellinate, which is considered to be due to steric hindrance by the long alkyl chain in the ester. In the case of esters, the decrease in GVL selectivity is due to the formation of a bulk intermediate, 4-hydroxy alkyl leblinate, which interferes with lactonization to GVL. Nevertheless, it is noteworthy that the above results show that the Ni ₂ Co ₁ P catalyst also exhibits good activity toward levulinic acid esters.

- Evaluation of the effect of hydrothermal synthesis reaction temperature on hydrogenation reaction when preparing metal phosphide catalyst

Two types of nickel-cobalt phosphide catalysts (Ni ₂ Co ₁ P -100 ° C and Ni ₂ Co ₁ P - 150 ° C) prepared by varying the second hydrothermal synthesis reaction temperature and a commercial nickel-copper / alumina catalyst (Ni- Figure 12 shows the levulinic acid conversion rate and selectivity for gamma-valerolactone in the reaction of preparing gamma-valerolactone (GVL) from levulinic acid (LA) in the presence of Cu/Al ₂ O ₃ ). was At this time, the hydrogenation reaction was performed for 4 hours under conditions of a temperature of 80 °C and a hydrogen pressure of 30 bar. In addition, 30 mL of levulinic acid was reacted under solvent-free conditions, and 0.8 g of catalyst was used. As described with respect to FIG. 5, the Ni ₂ Co ₁ P-150° C. catalyst has a yoke-cell structure, whereas the Ni ₂ Co ₁ P-100° C. catalyst has a non-yoke-cell structure and is rigid (ie, solid). represents the form.

Referring to FIG. 12a, the Ni ₂ Co ₁ P-100° C. catalyst exhibited catalytic performance equivalent to that of the Ni ₂ Co ₁ P-150° C. catalyst in the first reaction. However, when the reaction was continuously and repeatedly performed, the catalytic performance rapidly deteriorated. In addition, the Ni—Cu/Al ₂ O ₃ catalyst exhibited relatively low levulinic acid conversion and GVL yield, and exhibited the most rapid catalyst performance degradation, especially when repeatedly used. The rapid deactivation observed in these two catalysts, especially the Ni—Cu/Al ₂ O ₃ catalyst, is believed to be due to sintering of the metal particles.

On the other hand, in the case of the Ni ₂ Co ₁ P-150 °C catalyst, the catalytic performance was maintained constant due to the active point protection effect of the catalyst resulting from the yoke-cell structure.

all. Evaluation of activity in the conversion of levulinic acid to 2-methyltetrahydrofuran

Preparing 2-methyltetrahydrofuran from levulinic acid in the presence of each of the Ni ₂ Co ₁ P catalyst and a nickel phosphide (NiP) catalyst as a comparative example, which were evaluated to exhibit the best activity among the previously prepared metal phosphide catalysts. Conversion and selectivity measured in the experiment are shown in Table 3, respectively.

From the table above, it can be seen that the Ni ₂ Co ₁ P catalyst contains a sufficient amount of acid sites to facilitate the dehydration reaction from 1,4-PDO to 2-MTHF. However, in the case of the Ni ₂ Co ₁ P catalyst, the selectivity for 2-MTHF was relatively low during the secondary hydrogenation reaction in the one-pot two-step reaction. That is, in the conversion of GVL to 2-MTHF, when the NiP catalyst was used, the selectivity for 2-MTHF was high, but the conversion rate to GVL was low during the first hydrogenation reaction. In this regard, the acid site of NiP can facilitate the dehydration reaction from the formed 1,4-PDO to 2-MTHF, thereby improving the selectivity to 2-MTHF. However, since the metal activity of NiP is insufficient, the conversion to GVL during the primary hydrogenation reaction is insufficient. As such, it is believed that the first hydrogenation step from levlinic acid to GVL depends on the composition of the active metal in the catalyst, whereas the second hydrogenation step from GVL to 2-MTHF is mainly influenced by the amount of acid sites.

Simple modifications or changes of the present invention can be easily used by those skilled in the art, and all such modifications or changes can be considered to be included in the scope of the present invention.

Claims (19)

1. Represented by the general formula Ni _x Co _y P (x and y are the molar ratios of Ni and Co, ranging from 1:0.2 to 5), in the presence of a metal phosphide catalyst having a yoke-cell structure, to an organic acid or its derivative A method for converting an organic acid comprising the step of performing a hydrogenation reaction for
2. 2. The method of claim 1, wherein the hydrogenation reaction involves a cyclization reaction.
3. The method of claim 1, wherein the organic acid is at least one selected from the group consisting of levulinic acid, succinic acid, fumaric acid, itaconic acid, aspartic acid, 2,5-furandicarboxylic acid, glutaric acid, and lactic acid. .
4. The method of claim 3, wherein the organic acid is levulinic acid, and the hydrogenation reaction is carried out in one step or at least two steps,

   Here, gamma-valerolactone (GVL) is formed by the first-step hydrogenation reaction, while 2-methyltetrahydrofuran (2-MTHF) is formed by the at least two-step hydrogenation reaction.
5. The method according to claim 1, wherein the hydrogenation reaction is carried out without the use of a solvent.
6. The method of claim 4, wherein the primary hydrogenation reaction is carried out under conditions of a temperature of 120 to 300 ° C. and a hydrogen pressure of 10 to 50 bar, and

   The secondary hydrogenation reaction is characterized in that carried out under conditions of a temperature of 180 to 320 ℃, and a hydrogen pressure of 30 to 80 bar.
7. 7. The method according to any one of claims 1 to 6, characterized in that the weight ratio of organic acid or derivative thereof:catalyst is controlled in the range of 1:0.015 to 0.03.
8. A metal phosphide catalyst with a yoke-cell structure represented by the general formula Ni _x Co _y P (x and y are the molar ratios of Ni and Co and range from 1:0.2 to 5),

   (i) The overall size (diameter) of the catalyst is in the range of 100 to 500 nm, the thickness of the shell layer is in the range of 20 to 100 nm, and the size (diameter) of the yoke is in the range of 5 to 50 nm.
9. [Claim 9] The catalyst according to claim 8, wherein the weight ratio of the yoke in the yoke-cell structured metal phosphide catalyst is set in the range of 1:1.5 to 5.
10. The catalyst according to claim 8, wherein the metal phosphide catalyst has an acid amount (NH ₃ -TPD) in the range of 200 to 600 mmol/g.
11. 9. The catalyst according to claim 8, wherein the molar ratio of the reduced form of metal to the non-reduced form of metal in the metal phosphide catalyst is in the range of 1:2 to 5.
12. The method of claim 11, wherein the metal phosphide catalyst has a chemical adsorption amount (mol) of hydrogen atoms per mole (mol) of the total metal, as measured by H ₂ -TPD, over a temperature range of 50 to 250 ° C. to 250 mmol/g.
13. a) preparing a nickel and cobalt precursor solution by dissolving a nickel precursor and a cobalt precursor in a solvent;

    b) subjecting the precursor solution to a first hydrothermal synthesis reaction to form a precipitate containing nickel-cobalt;

    c) subjecting the precipitate to a second hydrothermal synthesis reaction to form nickel-cobalt hydroxide; and

    d) Ni _x Co _y P (x and y are the molar ratio of Ni and Co and range from 1: 0.2 to 5) through a substitution reaction of the nickel-cobalt hydroxide with a phosphide agent under an inert gas atmosphere and elevated temperature conditions (i) conversion to a nickel-cobalt phosphide catalyst having a yolk-cell structure;

    Including,

    (i) a nickel-cobalt phosphide catalyst in which the overall size (diameter) of the nickel-cobalt phosphide catalyst is in the range of 10 to 100 nm, the thickness of the cell layer is 1 to 10 nm, and the size (diameter) of the yoke is in the range of 5 to 50 nm; A method for preparing a catalyst.
14. The method of claim 13, wherein each of the nickel precursor and the cobalt precursor is a compound having an oxidation number of 2,

    The nickel precursor is nickel (II) chloride hexahydrate (NiCl ₂ 6H ₂ O), nickel nitrate (Ni(NO ₃ ) ₂ 6H ₂ O) and nickel sulfate (II) heptahydrate (NiSO ₄ 7H ₂ O) at least one selected from the group consisting of, and

    The cobalt precursor is cobalt(II) chloride hexahydrate (CoCl ₂ 6H ₂ O), cobalt nitrate (Co(NO ₃ ) ₂ 6H ₂ O), and cobalt(II) sulfate heptahydrate (CoSO ₄ 7H ₂ A method characterized in that it is at least one selected from the group consisting of O).
15. The method of claim 13, wherein the solvent used in step a) is a mixed solvent of at least a dihydric alcohol and a monohydric alcohol,

    The at least dihydric alcohol is at least one selected from the group consisting of glycerol and ethylene glycol, and

    The method, characterized in that the monohydric alcohol is at least one selected from the group consisting of ethanol and isopropyl alcohol.
16. The method of claim 13, wherein the first hydrothermal synthesis reaction is carried out at a temperature of 150 to 220 ° C. for 4 to 10 hours, and

    The second hydrothermal synthesis reaction is characterized in that it is carried out at a temperature of 130 to 170 ℃ for 1 to 8 hours.
17. 14. The method of claim 13, wherein the phosphide agent is at least one selected from the group consisting of thiaphosphate, phosphine gas and red phosphorus.
18. 14. The method of claim 13, wherein the inert gas in step d) is at least one selected from the group consisting of argon, neon, and nitrogen, and the temperature raising condition ranges from 280 to 400 ° C and a temperature rising rate of 0.5 to 5 ° C / min. A method characterized in that set in.
19. 14. The method of claim 13, wherein step d) is carried out in a tubular furnace with two crucibles disposed at both ends,

    At this time, among the two crucibles, a crucible in which the phosphide agent is located is disposed upstream, a crucible in which nickel-cobalt hydroxide is located is disposed downstream, and an inert gas is introduced into the tubular furnace. Method characterized in that.

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