WO2019205101A1

WO2019205101A1 - Process for preparing dispersed pd nanoparticles on a support

Info

Publication number: WO2019205101A1
Application number: PCT/CN2018/084904
Authority: WO
Inventors: Vitaly ORDOMSKY; Stephane Streiff
Original assignee: Rhodia Operations; Le Centre National De La Recherche Scientifique
Priority date: 2018-04-27
Filing date: 2018-04-27
Publication date: 2019-10-31

Abstract

A process for preparing dispersed Pd nanoparticles on a support by fast heating. The process can simplify the reaction steps, equipment and raw materials.

Description

Process for preparing dispersed Pd nanoparticles on a support

TECHNICAL FIELD

The present invention relates to a process for preparing dispersed Pd nanoparticles on a support by fast heating.

BACKGROUD

While support-type metal catalysts have been widely employed in various chemical reactions, it is required to reduce the particle diameter of the metal particles as much as possible for uniformly supporting the metal particles on a support in order to effectively utilize the catalytically active metals especially when such an expensive catalyst metal as a precious metal is employed. In order to attain this requirement, many attempts have been made to support a metal of fine particles on a support.

For example, US Patent No. 5275999 discloses a process for preparing a catalyst supporting highly dispersed metal particles having an average particle size of not more than

utilized in the field of catalyst. In this process, a metal ion in a solution is reduced by means of a reductant to its metallic state to form metal particles. The presence of the carbon monoxide can prevent the agglomeration among the metal particles supported and helpful for providing a catalyst having the metal particles supported and monodispersed on the support with narrow particle size distribution. However, this process needs complicated equipment in order to introduce carbon monoxide, which is desirably conducted by means of bubbling.

Chinese Patent Application No. CN106732558A teaches a method for preparing palladium-carbon catalyst. The method comprises: pre-treating the activated carbon with hydrogen peroxide, and preparing the palladium carbon catalyst. According to this patent application, hydrogen peroxide can increase the mean pore size and mesopore volume of the palladium-carbon catalyst to increase the effective loading area, and can introduce oxygen to the surface of carrier to improve the dispersion of palladium in the carrier so as to improve the catalytic activity. The method has disadvantage due to the application of complex mixture of organic precursors (oxalic acid, dodecyltrimethylammonium bromide) and sodium borohydride for reduction of Pd.

Applied Catalysis A., 2006, 312, 1-11 reports a method for synthesizing highly dispersed Pd alumina supported particles. Specifically, a strongly stabilized complex aqueous phase is used to prepare catalysts with different Pd loading keeping the particle size lower than 1 nm by conventional incipient wetness impregnation method. In this case authors have used very low loading of Pd (0.09-0.56 wt. %) to increase dispersion of Pd nanoparticles which was only slightly higher in comparison with conventional methods.

Catal. Sci. Technol., 2015, 5, 4144-4153 discloses a highly dispersed Pd nanoparticles supported on multi-walled carbon nanotubes, which is pretreated by nitric acid. However, the pretreatment of support makes the whole procedure complicated and increase the cost since large amount of concentrated nitric acid and water is consumed in this step.

There is still a need to provide a process to prepare dispersed Pd nanoparticles on a support, which can simplify the reaction steps, equipment and raw materials.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a TEM image of parent Pd/Al ₂O ₃.

Fig. 2 and Fig. 3 are images showing the dispersed Pd nanoparticles on Al ₂O ₃ prepared by the process according to the present invention as observed by TEM at different magnifications.

DEFINITIONS

For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are collected here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.

The articles “a” , “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The term “and/or” includes the meanings “and” , “or” and also all the other possible combinations of the elements connected to this term.

Throughout the description, including the claims, the term "comprising one" should be understood as being synonymous with the term "comprising at least one" , unless otherwise specified, and "between" should be understood as being inclusive of the limits.

It should be noted that in specifying any range of concentration, any particular upper concentration can be associated with any particular lower concentration.

It is specified that, in the continuation of the description, unless otherwise indicated, the values at the limits are included in the ranges of values which are given.

As used herein, the term "hydrocarbon group" refers to a group which contains carbon and hydrogen bonds. A hydrocarbon group may be linear, branched, or cyclic, and may contain a heteroatom such as oxygen, nitrogen, sulfur, halogen, etc.

As used herein, the term "alkyl" means a saturated hydrocarbon radical, which may be straight, branched or cyclic, such as, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, t-butyl, pentyl, n-hexyl, cyclohexyl.

As used herein, the term "alkenyl" as a group or part of a group denotes an aliphatic hydrocarbon group containing at least one carbon-carbon double bond and which may be straight or branched. The group may contain a plurality of double bonds in the normal chain and the orientation about each is independently E or Z. Exemplary alkenyl groups include, but are not limited to, ethenyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl and nonenyl. The group may be a terminal group or a bridging group.

As used herein, the term "aryl" refers to a monovalent aromatic hydrocarbon group, including bridged ring and/or fused ring systems, containing at least one aromatic ring. Examples of aryl groups include phenyl, naphthyl and the like. The term "arylalkyl" or the term "aralkyl" refers to alkyl substituted with an aryl. The term "arylalkoxy" refers to an alkoxy substituted with aryl.

As used herein, the term "cyclic group" means a closed ring hydrocarbon group that is classified as an alicyclic group, aromatic group, or heterocyclic group. The term "alicyclic group" means a cyclic hydrocarbon group having properties resembling those of aliphatic groups.

As used herein, the term "cycloalkyl" as used herein means cycloalkyl groups containing from 3 to 8 carbon atoms, such as for example cyclohexyl.

The heterocyclic group may also mean a heterocyclic group fused with a benzene-ring wherein the fused rings contain carbon atoms together with 1 or 2 heteroatom’s which are selected from N, O and S.

As used herein, the term "heterocycloalkane" means a saturated heterocycle formally derived from a cycloalkane by replacing one or more carbon atoms with a heteroatom.

As used herein, the terminology " (C _n-C _m) " in reference to an organic group, wherein n and m are each integers, indicates that the group may contain from n carbon atoms to m carbon atoms per group.

DETAILED DESCRIPTION

Through continuous studies concerning improved process for preparing dispersed Pd nanoparticles on a support, the applicant has now surprisingly found that it is possible to advantageously obtain such supported Pd nanoparticles by very simple procedure. The present invention can overcome all the drawbacks of prior art processes.

According to the present invention, the process for preparing dispersed Pd nanoparticles on a support, comprising a step of heating Pd hydride supported on the support from a temperature of -30℃ to 50℃ to a temperature of 300℃ to 600℃ in 0.01-1 second.

As used herein, Pd hydride is in a form of metallic palladium that has absorbed a substantial amount of elemental hydrogen (up to 900 times its own volume) into the interstices of its structure.

The support of Pd nanoparticles or Pd hydride is not particularly limited as long as its presence does not prevent the transfer of Pd hydride to Pd nanoparticles.

The support can notably be a metal oxide chosen in the group consisting of aluminum oxide (Al ₂O ₃) , silicon dioxide (SiO ₂) , titanium oxide (TiO ₂) , zirconium dioxide (ZrO ₂) , calcium oxide (CaO) , magnesium oxide (MgO) , lanthanum oxide (La ₂O ₃) , niobium dioxide (NbO ₂) , cerium oxide (CeO ₂) and any combination thereof. Preferably, said support is aluminum oxide.

The support can also be a zeolite. Zeolites are substances having a crystalline structure and a unique ability to change ions. People skilled in the art can easily understand how to obtain those zeolites by preparation method reported, such as zeolite L is described in US 4503023 or commercial purchase, such as ZSM available from ZEOLYST.

The method for preparing the supported Pd hydride is not particularly limited. The supported Pd hydride is typically formed by gas-phase hydrogen absorption, high-pressure hydrogen insertion, and electrochemical reactions with palladium as reported by S. Kishore, J.A. Nelson, J.H. Adair and P.C. Eklund, J. Alloys Compd., 2005, 389, 234; S. Horinouchi, Y. Yamanoi, T. Yonezawa, T. Mouri and H. Nishihara, Langmuir, 2006, 22, 1880; H. Kobayashi, M. Yamauchi, H. Kitagawa, Y. Kubota, K. Kato and M. Takata, J. Am. Chem. Soc., 2008, 130, 1828; A. Rose, S. Maniguet, R.J. Mathew, C. Slater, J. Yao and A.E. Russell, Phys. Chem. Chem. Phys., 2003, 5, 3220; A. Czerwinski, I. Kiersztyn, M. Grden and J. Czapla, J. Electroanal. Chem., 1999, 471, 190; M. Bernardini, N. Comisso, M. Fabrizio, G. Mengoli and A. Randi, J. Electroanal. Chem., 1998, 453, 221; and M.C.F. Oliveira, Electrochem. Commun., 2006, 8, 647.

The supported Pd hydride can also be prepared in a polyol solution using NaBH ₄ as a hydrogen source as disclosed by Chem. Commun., 2009, 3026–3028.

As previously expressed, the supported Pd hydride is heated from a temperature of -30℃ to 50℃ to a temperature of 300℃ to 600℃ in 0.01-1 second. Preferably, the temperature before heating is room temperature. Preferably, the supported Pd hydride may be heated to a temperature of 300℃ to 500℃ and more preferably a temperature of 300℃ to 400℃.

Preferably, the heating time may be from 0.1 to 0.5 second.

The dispersed Pd nanoparticles may have an average diameter in the range of 1 to 5 nm and preferably from 2 to 3 nm.

The particle size of Pd naoparticles can be evaluated by two methods: The pulse chemisorption analysis or direct calculation of sizes of metal nanoparticles by transmission electron microscopy (TEM) .

The pulse chemisorption analysis determines active surface area, percent metal dispersion, and active metal particle size by applying measured doses of reactant gas to the sample by some commercial apparatuses, such as Micromeritics ASAP 2020.

TEM can measure the diameter of the particles in the image based on magnification of the TEM image. One of ordinary skill in the art will understand how to prepare such a TEM image and determine the particle size based on the magnification. For example, Pd nanoparticles can be characterized by TEM on a JEOL JEM 2100 microscope operated at 200 kV and equipped with Energy Dispersive Spectroscopy (EDS) . The particles to be measured refer to the projection (2D-representation) of the particles on the micrograph. Before performing the measurements, it is necessary to calibrate the image. Size distribution histograms are then plotted as percent Pd nanoparticles versus Pd diameter on the basis of the size measurements obtained from an image processing program, such as ImageJ. The number average is obtained by weighted average method. The measurement should be made on a sufficiently high number of particles, for example at least 25 particles, preferably at least 100 particles, more preferably at least 300 particles, still more preferably at least 500 particles.

TEM is the preferable method to measure the diameter of Pd nanoparticles.

The surface area of the dispersed Pd nanoparticles may be from 50 to 500 m ²/g.

The surface area referred to in the present specification is measured by pulse hydrogen adsorption, which is also the most standard method for measuring the dispersion of metal nanoparticles. The method is described in “Catalyst Characterization Using Thermal Conductivity Detector” described in Chromatographia 2005, 61, March (No. 5/6) 285-290.

The way to fast heat the supported Pd hydrid is not particularly limited. In one preferred embodiment, the fast heating can be realized by exothermic reactions, such as hydrogen peroxide decomposition, reaction of H ₂ with O ₂, decomposition of tert-butyl peroxide on the surface of the Pd hydride particles.

For example, it is a highly exothermic reaction when the supported Pd hydride contacts with hydrogen peroxide, which leads to fast heating of Pd particles from room temperature to 300-400℃during milliseconds. IR camera may be used to detect the temperature on the surface of the Pd hydride particles during the reaction.

Preferably, hydrogen peroxide is in an aqueous solution and the concentration of hydrogen peroxide aqueous solution can be from 30 wt. %to 50 wt. %.

In this embodiment, the weight ratio of hydrogen peroxide aqueous solution to the supported Pd hydride ranges from 100 to 1000.

The process for preparing the dispersed Pd nanoparticles on a support may comprise following steps:

(a) contacting a supported Pd hydride with a hydrogen peroxide aqueous solution under stirring to obtain a solid;

(b) filtering and washing the solid obtain at step (a) with water or an organic solvent;

(c) optionally drying the solid obtained at step (b) .

Advantageously, steps (a) to (c) may be repeated for at least one time after the solid is obtained at steps (b) or (c) . Preferably, steps (a) to (c) can be repeated for two to four times.

In some embodiments, microwave or electric heating can also fast heat the supported Pd hydride in order to obtain target product.

The microwave heating the effect is generally achieved through dipolar polarization and ionic conduction. In comparison with conventional heating, the heating is local in metal nanoparticles with fast increase of the temperature after switching on of microwave irradiation.

It should be understood by the skilled person that the microwave heating can produce target heating rates, depending on the type of reactor and the type of precursor molecules used for nanoparticle synthesis.

The heating rate can be from 300 to 1000℃/s.

The power of the microwave can be from 0.1 to 10 kW.

Advantageously, microwave irradiation can be maintained for a certain period of time, such as 5 to 20 mins after fast heating.

The Pd loading of the dispersed Pd nanoparticles on a support obtained by the process according to the present invention can be from 2 wt. %to 15 wt. %and preferably from 2 wt. %to 10 wt. %.

The present invention also concerns a catalyst comprising the dispersed Pd nanoparticles on a support susceptible of being obtained by the invented process.

The present invention also relates to a catalytic hydrogenation reaction in the presence of a catalyst comprising the dispersed Pd nanoparticles on a support susceptible of being obtained by the invented process.

Advantageously, the catalytic hydrogenation reaction can be conducted in the presence of the dispersed Pd nanoparticles on a support susceptible of being obtained by the invented process.

It can be understood by a skilled person that hydrogenation reaction is a chemical reaction between molecular hydrogen (H ₂) and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds.

The substrate for hydrogenation reaction can be an alkene, an aromatic hydrocarbon, an alkyne, an aldehyde, a ketone, an ester, a carboxylic acid or a nitro.

Preferably, the substrate can be a substituted or non-substituted aromatic hydrocarbon, which is chosen in the group consisting of benzene, toluene, ethylbenzene, naphthalene etc.

In one preferred embodiment, the substrate can be levulinic acid or cyclohexanone.

It has now been surprisingly found that the conversion of the substrates of the hydrogenation reaction can be well increased by using the catalyst comprising the dispersed Pd nanoparticles on a support susceptible of being obtained by the invented process.

The present invention extends to a method for producing a compound which comprises following two steps:

(i) heating Pd hydride on a support from a temperature of -30℃ to 50℃ to a temperature of 300℃ to 600℃ in 0.01-1 second to obtain dispersed Pd nanoparticles supported on the support;

(ii) hydrogenating a substrate in the presence of the dispersed Pd nanoparticles on the support obtained at step (i) , thereby producing the compound.

The substrate has the same meaning as above defined.

The following examples are included to illustrate embodiments of the invention. Needless to say, the invention is not limited to the described examples.

EXPERIMENTAL PART

Raw materials

Toluene Sigma-Aldrich

Levulinic acid Sigma-Aldrich

Cyclohexanone Sigma-Aldrich

5 wt%Pd/Al ₂O ₃ (Johnson Matthey)

Example 1

Put in a 50 ml stainless steel reactor 50 mg of the dry industrial catalyst 5 wt%Pd/Al ₂O ₃, pressurized the reactor with 20 bar H ₂ and heated to 100℃ for 2h. Afterwards the reactor was cooled down with release of hydrogen pressure. The supported Pd hydride was then prepared.

The supported Pd hydride was added to 10 ml of H ₂O ₂ solution 30 wt. %under intensive stirring. It was a highly exothermic reaction, which led to fast heating of metal nanoparticles to 300-400℃during milliseconds with fast evolution of hydrogen. The dispersed Pd nanoparticles was then prepared, and subsequently filtered and washed with water and ethanol. The whole procedure starting from pressurizing by hydrogen was repeated twice.

Fig. 1 is a TEM image of parent Pd/Al ₂O ₃. Fig. 2 and Fig. 3 are images showing the dispersed Pd nanoparticles on Al ₂O ₃ prepared by the process according to the present invention as observed by TEM at different magnifications.

For the dispersed Pd on Al ₂O ₃ prepared by the invented process, magnification shows significant increase of the amount of small size Pd clusters well distributed on the surface of alumina. Parent Pd/Al ₂O ₃ (histogram of Pd nanoparticle distribution) does not show presence of metal nanoparticles smaller than 1 nm in comparison with significant amount of these nanoparticles after it was firstly transferred to the supported Pd hydride and then it was treated by H ₂O ₂.

H ₂ chemisorption measurements were carried out with an AutoChem II 2920 Micromeritics instrument to evaluate the metal dispersion. Before each measurement, the samples were reduced at 200℃ and cooled down to 45℃ for adsorption. According to pulse adsorption of hydrogen, dispersion of Pd (external amount to total amount) increases from 24 to 51 %as shown in Table 1. The size of metal nanoparticles calculated from adsorption results correspond to the size of metal nanoparticles calculated by TEM.

Table 1. Characterization by H ₂ adsorption

Example 2

Put in a 50 ml stainless steel reactor 50 mg of the dry industrial catalyst 5 wt%Pd/Al ₂O ₃, pressurized the reactor with 20 bar H ₂ and heated to 100℃ for 2h.Afterwards the reactor was cooled down with release of hydrogen pressure. The catalyst was fast heated by Mars Classic microwave source under 100W and then maintained for 10 min. Characterization of the metal nanoparticles shows similar results to treatment by H ₂O ₂.

Example 3

Put in a 50 ml stainless steel reactor 50 mg of the dispersed Pd on Al ₂O ₃ prepared by Example 1 together with 2 g of toluene. The reactor was sealed and pressurized with 50 bar of H ₂. The reactor was heated to 50 ℃ for 1 h under continuous stirring. After reaction the products were analyzed by GC and GC-MS. The conversion of toluene is 14.7%. The main product of the reaction is methyl cyclohexane. The selectivity is higher than 99 %.

Example 4

Put in a 50 ml stainless steel reactor 50 mg of the dispersed Pd on Al ₂O ₃ prepared by Example 2 together with 2 g of toluene. The reactor was sealed and pressurized with 50 bar of H ₂. The reactor was heated to 50 ℃ for 1 h under continuous stirring. After reaction the products were analyzed by GC and GC-MS. The conversion of toluene is 11.4%. The main product of the reaction is methyl cyclohexane. The selectivity is higher than 99 %.

Comparative example 1

Put in a 50 ml stainless steel reactor 50 mg of parent catalyst Pd/Al ₂O ₃ (JM 5%Pd) together with 2 g of toluene. The reactor was sealed and pressurized with 50 bar of H ₂. The reactor was heated to 50 ℃ for 1 h under continuous stirring. After reaction the products were analyzed by GC and GC-MS. The conversion of toluene is 1.4%. The main product of the reaction is methyl cyclohexane. The selectivity is higher than 99 %.

Example 5

Put in a 50 ml stainless steel reactor 100 mg of dispersed Pd on Al ₂O ₃ prepared by Example 1 together with 5 ml water and 3 g of levulinic acid. The reactor was sealed and pressurized with 50 bar of H ₂. The reactor was heated to 140℃ for 2.5 h under continuous stirring. After reaction the products were analyzed by GC and GC-MS. The conversion of levulinic acid is 53%. The main product of the reaction is γ-Valerolactone. The selectivity is higher than 99 %.

Comparative example 2

Put in a 50 ml stainless steel reactor 100 mg of parent catalyst Pd/Al ₂O ₃ (JM 5%Pd) together with 5 ml water and 3 g of levulinic acid. The reactor was sealed and pressurized with 50 bar of H ₂. The reactor was heated to 140℃ for 2.5 h under continuous stirring. After reaction the products were analyzed by GC and GC-MS. The conversion of levulinic acid is 34%. The main product of the reaction is γ-Valerolactone. The selectivity is higher than 99 %.

Example 6

Put in a 50 ml stainless steel reactor 50 mg of the dispersed Pd on Al ₂O ₃ prepared by Example 1 together with 2 g of cyclohexanone. The reactor was sealed and pressurized with 50 bar of H ₂. The reactor was heated to 50℃ for 2 h under continuous stirring. After reaction the products were analyzed by GC and GC-MS. The conversion of cyclohexanone is 15.1%. The main product of the reaction is cyclohexanol. The selectivity is higher than 99 %.

Comparative example 3

Put in a 50 ml stainless steel reactor 50 mg of parent catalyst Pd/Al ₂O ₃ (JM 5%Pd) together with 2 g of cyclohexanone. The reactor was sealed and pressurized with 50 bar of H ₂. The reactor was heated to 50℃ for 2 h under continuous stirring. After reaction the products were analyzed by GC and GC-MS. The conversion of cyclohexanone is 2.8 %. The main product of the reaction is cyclohexanol. The selectivity is higher than 99 %.

Claims

A process for preparing dispersed Pd nanoparticles on a support, comprising a step of heating Pd hydride supported on the support from a temperature of -30℃ to 50℃ to a temperature of 300℃ to 600℃ in 0.01-1 second.
The process according to claim 1, wherein the supported Pd hydride is heated to a temperature of 300℃ to 400℃.
The process according to claim 1 or 2, wherein the heating time is from 0.1 to 0.5 second.
The process according to any one of claims 1 to 3, wherein the support of Pd nanoparticles or Pd hydride is chosen in the group consisting of aluminum oxide, silicon dioxide, titanium oxide, zirconium dioxide, calcium oxide, magnesium oxide, lanthanum oxide, niobium dioxide, cerium oxide and any combination thereof.
The process according to any one of claims 1 to 4, wherein the support of Pd nanoparticles or Pd hydride is aluminum oxide.
The process according to any one of claims 1 to 5, wherein the supported Pd hydride is heated by an exothermic reaction on the surface of Pd hydride particles.
The process according to any one of claims 1 to 6, wherein the supported Pd hydride is heated by contacting it with hydrogen peroxide.
The process according to claim 7, wherein hydrogen peroxide is in an aqueous solution and the concentration of hydrogen peroxide in the solution is from 30 wt.%to 50 wt.%.
The process according to claim 7 or 8, wherein it comprises following steps:

(a) contacting a supported Pd hydride with a hydrogen peroxide aqueous solution under stirring to obtain a solid;

(b) filtering and washing the solid obtain at step (a) with water or an organic solvent;

(c) optionally drying the solid obtained at step (b) .
The process according to claim 9, wherein steps (a) to (c) are repeated for two to four times.
The process according to any one of claims 1 to 5, wherein the supported Pd hydride is heated by a microwave.
The process according to claim 11, wherein the power of the microwave is from 0.1 to 10 kW.
The process according to any one of claims 1 to 12, wherein the dispersed Pd nanoparticles have an average diameter in the range of 1 to 5 nm.
The process according to any one of claims 1 to 13, wherein the surface area of the dispersed Pd nanoparticles is from 50 to 500 m ²/g.
A catalyst comprising the dispersed Pd nanoparticles on a support susceptible of being obtained by the process according to any one of claims 1 to 14.
A catalytic hydrogenation reaction in the presence of a catalyst comprising the dispersed Pd nanoparticles on a support susceptible of being obtained by the process according to any one of claims 1 to 14.
A method for producing a compound which comprises following two steps:

(i) heating Pd hydride on a support from a temperature of -30℃ to 50℃to a temperature of 300℃ to 600℃ in 0.01-1 second to obtain dispersed Pd nanoparticles supported on the support;

(ii) hydrogenating a substrate in the presence of the dispersed Pd nanoparticles on the support obtained at step (i) , thereby producing the compound.
The method according to claim 17, wherein the substrate is chosen in the group consisting of an alkene, an aromatic hydrocarbon, an alkyne, an aldehyde, a ketone, an ester, a carboxylic acid and a nitro.