WO2019232714A1

WO2019232714A1 - Method for oxidation of alcohol

Info

Publication number: WO2019232714A1
Application number: PCT/CN2018/090097
Authority: WO
Inventors: Jingpeng ZHAO; Vitaly ORDOMSKY; Willinton Yesid HERNANDEZ ENCISO; Wenjuan ZHOU; Mickael Capron; Stéphane STREIFF
Original assignee: Rhodia Operations; Le Centre National De La Recherche Scientifique; Universite Lille 1 - Sciences Et Technologies; Centrale Lille; Ecole Nationale Superieure De Chimie De Lille
Priority date: 2018-06-06
Filing date: 2018-06-06
Publication date: 2019-12-12

Abstract

Disclosed is a method for the oxidation of an alcohol comprising contacting the alcohol with O ₂ in the presence of a catalyst. Furthermore disclosed is the use of free metallic ruthenium nanoparticles as catalyst for the oxidation of an alcohol with O ₂. Additionally, disclosed is a method of preparing metallic ruthenium nanoparticles of controllable size.

Description

Method for the oxidation of alcohol

Technical Field of the Invention

The present invention relates to a method for the oxidation of an alcohol comprising contacting the alcohol with O ₂ in the presence of a catalyst. The invention furthermore relates to the use of free metallic ruthenium nanoparticles as catalyst for the oxidation of an alcohol with O ₂. Additionally, the invention relates to a method of preparing metallic ruthenium nanoparticles of controllable size.

Background

The selective conversion of alcohols to carbonyl compounds is a crucial process in organic chemistry due to the great interest for industry. In the infancy, alcohols have been selectively oxidized into aldehydes or ketones by the costly toxic stoichiometric reagents known as iodine, hyperpermanganate, chromium oxide, generating a huge amount environmentally hazardous metal and organic solvent waste. In recent years, plenty of catalytic systems including supported metal catalysts have been developed for the selective oxidation of alcohols to carbonyls, using Pt, Pd and Au. However, they are commonly well-behaved on oxidative dehydrogenation of aromatic and activated allylic alcohols but invalid for aliphatic alcohols, especially primary ones.

K. Yamaguchi, et al., suggest in Angew. Chem. Int. Ed. 2002, 41, No. 23, 4538-4542 to use ruthenium supported on alumina (Ru/Al ₂O ₃) for the heterogeneous oxidation of alcohols with molecular oxygen.

Also R. Karvembu, et al., describe in React. Kinet. Catal. Lett., Vol. 86, No. 1, 211-216 (2005) the Ru/Al ₂O ₃-catalyzed transfer dehydrogenation of alcohols.

Ruthenium nanoparticles supported on nitrogen-doped carbon nanofibers for the catalytic wet air oxidation of phenol are disclosedby A. B. Ayusheev, et al., in Applied Catalysis: Environmental 146 (2014) 177-185.

Immobilizing of ruthenium nanoparticles onto nitrogen-doped carbon nanotubes for aerobic oxidation of benzyl alcohol under ambient pressure is describedby J. -X. Mao, et al., in Chinese Journal of Inorganic Chemistry, Vol. 28, No. 12 (2012) 2508-2512.

M.A. Hussain, et al., have published a review article regarding different types of nano-sized catalysts for the selective alcohol oxidation to form aldehydes (or ketones) with supported or immobilized metal nanoparticles in Appl. Chem. Eng., Vol. 27, No. 3, 2016, 227-238.

A. Miyazaki, et al., in Chemistry Letters 2001, 1332-1333 describe the supporting of ruthenium nanoparticles on γ-Al ₂O ₃ by suspending the alumina in an ethylene glycol solution of RuCl ₃, heating the suspension and collecting the thus obtained solid phase. The obtained supported ruthenium nanoparticles have an average size of 5 nm.

The tunable preparation of ruthenium nanoparticles is described by Y. Zhao, et al., in Journal of Hazardous Materials 332 (2017) 124-131. Ruthenium nanoparticles with sizes ranging from 2.6 to 51.5 nm were synthesized by controlling the pH and the temperature of a solution of RuCl ₃·nH ₂O in a solution of PVP in n-propanol.

While there is a number of catalysts known which comprise ruthenium supported on various supports, such as alumina, carbon nanofibers and carbon nanotubes, and which are suitable for the oxidation of alcohols with molecular oxygen, there is still a need for further improving such catalysts in particular with respect to the conversion rate and the selectivity of the reaction to the desired aldehyde or ketone. In particular with the oxidation of primary alcohols to aldehydes it is often difficult, if not impossible, to stop the reaction on the aldehyde stage, in which case the aldehyde is further oxidized to acids.

Furthermore, it would be desirable to provide a method for the preparation of metallic ruthenium nanoparticles having a controllable average particle size without the need of, for example, controlling several different parameters, like pH and temperature.

Summary of the Invention

The present inventors now found that the above problems can surprisingly be solved by utilizing a catalyst comprising free metallic ruthenium nanoparticles instead of supported ruthenium catalysts. The present invention therefore relates to a method for the oxidation of an alcohol comprising contacting the alcohol with O ₂ in the presence of a catalyst, characterized in that the catalyst comprises free metallic ruthenium nanoparticles.

Furthermore, the invention relates to the use of free metallic ruthenium nanoparticles as catalyst for oxidizing an alcohol with O ₂.

Additionally, the present invention relates to a method of preparing metallic ruthenium nanoparticles by mixing a first emulsion comprising a ruthenium salt dissolved in water, a surfactant and an oil phase with a second emulsion comprising a reducing agent dissolved in water, a surfactant and an oil phase, and collecting the thereby obtained metallic ruthenium nanoparticles.

Detailed Description

The first method of the present invention relates to the oxidation of an alcohol in the presence of a catalyst. In the method, the alcohol is contacted with molecular oxygen (O ₂) . If the alcohol is a primary alcohol, the desired product is an aldehyde according to the following reaction:

If the alcohol is a secondary alcohol, the desired product is a ketone.

As in these oxidation reactions there is a risk that the reaction does not stop on the level of the aldehyde or ketone but that further oxidation, for example to an acid, occurs, not only the conversion rate but also the selectivity of the method depending on the catalyst used is important.

The present inventors found that both, conversion rate and selectivity can be increased if the ruthenium catalyst is used in the form or metallic ruthenium nanoparticles which furthermore are not supported on any support, such as the known alumina support, i.e. if the catalyst comprises free metallic ruthenium nanoparticles.

In the context of this invention, the term "comprising" includes "consisting essentially of" and "consisting of" .

In addition, if the term "about" is used before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term "about" refers to a±10%variation from the nominal value unless specifically stated otherwise.

The metallic ruthenium nanoparticles used as catalysts in the method of the invention in one embodiment have an average particle size of about 15 nm or less, preferably of about 10 nm or less, more preferably of about 8 nm or less, even more preferably of about 7 nm or less, even more preferably of about 5 nm or less and most preferably of about 3 nm or less. The lower limit of the average particle size of the metallic ruthenium nanoparticles is not particularly limited. In one embodiment, the metallic ruthenium nanoparticles may have an average particle size of at least about 0.3 nm, preferably of at least about 0.5 nm, more preferably of at least about 0.7 nm, even more preferably of at least about 1 nm and most preferably of at least about 2 nm.

The above upper and lower limits of the average particle size of the metallic ruthenium nanoparticles can be combined to suitable ranges using any of the above upper limits in combination with any of the above lower limits. For example, the metallic ruthenium nanoparticles may have an average particle size in the range of about 0.3 to about 15 nm, preferably of about 1 to about 8 nm, more preferably of about 2 to about 7 nm, more preferably of about 2 to about 5 nm, and most preferably of about 2 to about 3 nm.

The average particle size of the metallic ruthenium nanoparticles according to the present invention is measured using transmission electron microscopy analysis (TEM analysis) .

For TEM analysis, a JEOL 2100 with Filament LaB6 having an acceleration voltage of 200 kV equipped with a camera Gatan 832 CCD was used. As support, square 230 mesh TEM support grids (copper) were used. The magnification factor had a range of'10,000～'600,000. For 50 nm: magnification factor was 40,000～50,000; for 20 nm: 60,000～120,000; for 10 nm: 250,000; for 5 nm: 400,000; for 2 nm: 500,000～600,000. Samples of 0.1 wt. %nanoparticles in ethanol were measured. The obtained results were analyzed using the DigitalMicrograph software. For each sample, two pictures were taken and a total of 100 nanoparticles were analyzed for obtaining the described size distribution. From this size distribution, the average particle size of the nanoparticles was obtained. The software used to measure the size of the nanoparticles was ImageJ thereby approximating the particles to be spherical. After setting the scale, the maximum diameter of the particles was manually measured one by one to a total number of particles measured of 100. Every particle has been measured 3 times to obtain an average size.

In one embodiment, the metallic ruthenium nanoparticles used as catalyst in the method of the invention predominantly, preferably essentially, consist of metallic ruthenium. In this context, "predominantly" is to be understood as at least 50 wt. %, preferably at least 70 wt. %, and "essentially" is to be understood as at least 80 wt. %, preferably at least 90 wt. %, each based on the total weight of the nanoparticles.

In a further embodiment, the metallic ruthenium nanoparticles comprise at least about 90 wt. %, preferably at least about 95 wt. %, more preferably at least about 97 wt. %, and most preferably at least about 98 wt. %of metallic ruthenium, each based on the total weight of the nanoparticles. The remaining part of the nanoparticles making up to 100 wt. %, which is not metallic ruthenium, can, for example, be impurities, such as other metals, or ruthenium oxide.

According to the invention, the metallic ruthenium nanoparticles are "free" nanoparticles. In this context, the term "free" is to be understood as defining nanoparticles which are not supported on any support, in particular which are not supported on any solid support, such as, in particular, carbon nanotubes an carbon nanofibers, or other supports, such as alumina.

The oxidation of the alcohol with molecular oxygen can be conducted under usual reaction conditions known to a person skilled in the art and used in the prior art. For example, the contacting of the alcohol with O ₂ in the presence of the catalyst can take place at elevated temperature, such as a temperature above about 50℃, preferably above about 80℃, such as about 100℃.

The pressure at which the reaction takes place is not particularly limited and can be ambient pressure or an elevated pressure, such as a pressure above about 5 bar, preferably above about 8 bar, such as about 10 bar.

The molecular oxygen can be present during the reaction in pure form, or, preferably, as a mixture with other gases, such as nitrogen. In a preferred embodiment, the alcohol is contacted with air.

The heterogeneous oxidation of the alcohol with molecular oxygen can take place in the absence of or in the presence of a solvent. If a solvent is used, it is preferably an inert solvent, which means that the solvent preferably does not react with the alcohol or the molecular oxygen under the reaction conditions. A suitable inert solvent is, for example, toluene.

The alcohol being oxidized in the method of the present invention is not particularly limited. Any organic alcohols, such as aromatic or aliphatic alcohols may be used. Furthermore, the alcohol may be a primary or a secondary alcohol. Primary alcohols being preferred. Non-limiting examples for suitable alcohols are for example octanol, furfuryl alcohol, cyclohexanol, benzyl alcohol, 4-methylbenzyl alcohol, etc. In the method of the invention, a single alcohol or a mixture of two or more alcohols may be employed.

The invention furthermore relates to the use of free metallic ruthenium nanoparticles as catalyst for oxidizing an alcohol with O ₂. For this use, the same preferred features as explained above with regard to the first method of the invention are applicable.

The invention furthermore relates to a method of preparing metallic ruthenium nanoparticles. According to this method, two emulsions are mixed. The first emulsion comprises a ruthenium salt dissolved in water, a surfactant and an oil phase. The second emulsion comprises a reducing agent dissolved in water, a surfactant and an oil phase. The two emulsions can, for example, be mixed by dropping the first emulsion into the second emulsion.

When mixing the two emulsions, the reducing agent reduces the ruthenium ions of the ruthenium salt, thereby forming metallic ruthenium nanoparticles. The thus obtained nanoparticles are collected. Optionally, the collected nanoparticles can, for example, be washed and dried.

In one embodiment, the first and the second emulsion have about the same molar ratio of water to surfactant.

In a further embodiment, the first and the second emulsion are mixed at a volume ratio of about 1: 1.

An advantage of the method of preparing metallic ruthenium nanoparticles according to the invention is that the average particle size of the nanoparticles can be controlled by controlling the concentration of water in the emulsions. For example, in the first and in the second emulsion, the water can be present at a concentration in the range of about 1 to about 50 wt. %, based on the total weight of surfactant, oil phase and water. In this range, metallic ruthenium nanoparticles having average particle sizes in the range of about 2 to about 10 nm are obtained, whereby a lower concentration of water results in a lower average particle size.

The ruthenium salt can be selected according to the requirements provided that it is soluble in water. A suitable ruthenium salt is, for example, RuCl ₃.

Also the surfactant is not particularly limited and can be selected by a person skilled in the art according to his general knowledge. A suitable surfactant is, for example, cetyl trimethyl ammonium bromide.

Also the oil phase is not particularly limited and can be selected by a person skilled in the art according to his general knowledge. Suitable oil phases are, for example, C _6-12 alcohols, such as 1-hexanol and 1-heptanol.

As reducing agent, any agent which is soluble in water and which can reduce to obtain metallic ruthenium can be employed. Suitable reducing agents are, for example, borhydride salts, such as sodium borhydride.

The surfactant and the oil phase in the first and the second emulsion can be selected independently. However, it is preferred that both emulsions comprise the same surfactant and the same oil phase.

The invention will now be explained in more detail with respect to the examples, which are not intended as being limiting.

Examples

Catalyst preparation

Four reverse micellar microemulsions with different ratios of cetyl trimethyl ammonium bromide (CTAB) /1-hexanol/water (as shown in Table 1) were prepared. First, 10g microemulsion A comprising 0.22 g RuCl ₃, CTAB, water and hexanol and 10 g microemulsion B comprising 0.13 g NaBH ₄, CTAB, water and hexanol were prepared. The two microemulsions were mixed by adding microemulsion B into microemulsion A drop by drop for 1 h under vigorous stirring. After that, Ru nanoparticles with uniform particle size distribution were formed inside the micelles. The microemulsion was broken by centrifugation, and the precipitate was washed by ethanol and water for 3 times, and dried at 80℃ in vacuum for 12 h.

The size of Ru nanoparticles in the final product corresponded to the water content in the microemulsion. The smallest water content resulted in the Ru nanoparticles with 2 nm size (Ru NPs-2 nm) . The largest water content resulted in the sample Ru NPs-9 nm with nanoparticles 9 nm diameter.

The reference Ru/γ-Al ₂O ₃ catalyst with 5 wt. %metal loading was prepared by impregnation. 1.90 g γ-Al ₂O ₃ was mixed with an aqueous RuCl ₃ solution, and then the mixture was kept at room temperature for 12 h. The resulting solid was dried at 80℃ in vacuum overnight. Then, the powders were calcined in N ₂ at 400℃ for 4 h with a heating rate of 1℃ min ^-1. TEM analysis identified 5～6 nm supported Ru nanoparticles.

The reference Ru NPs-2 nm/Al ₂O ₃ catalyst with 5 wt. %metal loading was prepared by depositing Ru NPs-2 nm nanoparticles on an alumina support from a suspension in ethanol.

Table 1

TEM analysis of the first five catalysts are shown in the attached Figures 1-5.

Examples 1.1-1.3 and Comparative Examples 2.1 and 2.2

Oxidation of octanol

50 mg catalyst was added into a 30 ml autoclave, and 3 g 1-octanol was injected into the reactor. The reactor was sealed and pressurized with 10 bar of O ₂ and then was heated to 100℃ for 1～8 h under continuous stirring. To test the metallic Ru effect, comparative experiments without catalyst or without O ₂ were carried out by using the same reaction conditions, respectively. After reaction the products were analyzed by GC and GC-MS. No target molecule was observed in the reaction without catalysts or O ₂, which means both were needed. The results are presented in the Table. The selectivity significantly increased with decrease of the size of nanoparticles.

Table 2

*Comparative Example

The above data demonstrate that compared to the catalyst Ru supported on Al ₂O ₃ conversion and selectivity is similar when using the free metallic ruthenium nanoparticle catalyst having an average particle diameter of 5 nm. However, upon decreasing the average particle diameter of the metallic ruthenium nanoparticles, in particular selectivity significantly increases. Also if the metallic ruthenium nanoparticles are supported on alumina (comparative example 2.2) both, conversion and selectivity decrease compared to free metal ruthenium nanoparticles having the same average particle diameter of 2 nm (example 1.1) .

Example 3

Oxidation of furfuryl alcohol

Into an autoclave were placed 50 mg catalyst, 3 g furfural alcohol in the presence of solvent 1 g toluene, while comparative experiments without catalysts and without O ₂ were carried out to make clear Ru effect. The resulting mixture was stirred at 100℃ with 10 bar O ₂ and reaction time was 1～8 h. The progress of the reaction and obtained products were analyzed on GC and GC-MS. For Ru-2nm, the conversion of furfural alcohol was 97%and the selectivity to furfural was 62%in the presence of O ₂. The conversion with Ru-9nm catalyzed was 98%and selectivity was 21%. No product was observed in the reactions without catalysts or O ₂, which means both were necessary. The results are presented in Table 3.

Table 3

Example 4

Oxidation of cyclohexanol

3 g of cyclohexanol was added into a 30 ml reactor; 50mg catalyst was added, together with 1 g toluene as solvent. To start the reaction, the autoclave was sealed and pressurized with 10 bar of O ₂ and the reaction was kept at 100℃for 1～8 h under stirring. Finally, the samples were analyzed by means of GC and GC-MS. The results showed that the conversion of substrate was 95%and selectivity to cyclohexanone was 95%in the case of Ru-2nm; while for Ru-9nm, the conversion and selectivity were 84%and 89%, respectively. No target molecule was observed in the reaction without catalysts or O ₂. The results are presented in Table 4.

Table 4

Example 5

Oxidation of 1, 2-hexanediol

3 g of 1, 2-hexanediol was added into a 30 ml reactor; 50 mg catalyst was added, together with 1 g ethanol as solvent. To start the reaction, the autoclave was sealed and pressurized with 10 bar of O ₂ and the reaction was kept at 100℃ for 1～8 h under stirring. Finally, the samples were analyzed by means of GC and GC-MS. The results showed that the conversion of substrate was 40%and selectivity to 1-hydroxyhexan-2-one was 38%; while for Ru-9nm, the conversion and selectivity were 33%and 20%, respectively. No target molecule was observed in the reaction without catalysts or O ₂. The results are presented in Table 5.

Table 5

Example 6

Synthesis of Metallic Ruthenium Nanoparticles

Metallic Ru (m-Ru) nanoparticles were prepared by a microemulsion method. The procedures for the microemulsion method were as follows: A certain amount RuCl ₃·H ₂O was first dissolved in distilled water, followed by the addition of cetyl trimethyl ammonium bromide (CTAB) as the surfactant and 1-hexanol as the oil phase to obtain microemulsion-1. There are almost identical recipes in microemulsion-2, except using excess reducing agent NaBH ₄ to substitute RuCl ₃·H ₂O. For a given sample synthesis, different emulsion ratios according to Table 5, were altered to investigate the effect on the m-Ru particle size. Both resultant microemulsions were then under constant stirring for 1 h to acquire uniform phases. After that, microemulsion-1 was slowly dropped into microemulsion-2 and stirred for another 2 h. Then the solid metallic catalysts formed was separated, washed with copiously mixed solution (volume ratio=1: 1) of ethanol and dionized water to remove the organic phase and NaBH ₄. The resulting materials were then dried in vacuum oven at 80℃ overnight.

Table 6

Claims

Method for the oxidation of an alcohol comprising contacting the alcohol with O ₂ in the presence of a catalyst, characterized in that the catalyst comprises free metallic ruthenium nanoparticles.
Method according to claim 1, wherein the metallic ruthenium nanoparticles have an average particle size in the range of about 0.3 to about 15 nm, preferably of about 1 to about 8 nm, more preferably of about 2 to about 7 nm, and even more preferably of about 2 to about 5 nm.
Method according to claim 1 or 2, wherein the metallic ruthenium nanoparticles predominantly, preferably essentially, consist of metallic ruthenium.
Method according to claim 3, wherein the metallic ruthenium nanoparticles comprise at least about 90 wt.%, preferably at least about 95 wt.%, more preferably at least about 97 wt.%, and most preferably at least about 98 wt.%of metallic ruthenium, each based on the total weight of the nanoparticles.
Method according to any of the preceding claims, wherein the metallic ruthenium nanoparticles are not supported on any support, preferably on any solid support.
Method according to any of the preceding claims, wherein the contacting takes place at elevated temperature, preferably at a temperature above about 50℃, more preferably above about 80℃.
Method according to any of the preceding claims, wherein the contacting takes place at elevated pressure, preferably at a pressure above about 5 bar, more preferably above about 8 bar.
Method according to any of the preceding claims, wherein the contacting takes place in the presence of a solvent.
Method according to any of the preceding claims, wherein the alcohol is a primary alcohol.
Use of free metallic ruthenium nanoparticles as catalyst for oxidizing an alcohol with O ₂.
Method of preparing metallic ruthenium nanoparticles by mixing a first emulsion comprising a ruthenium salt dissolved in water, a surfactant and an oil phase with a second emulsion comprising a reducing agent dissolved in water, a surfactant and an oil phase, and collecting the thereby obtained metallic ruthenium nanoparticles.
Method according to claim 11, wherein the first and the second emulsion have about the same molar ratio of water to surfactant.
Method according to claim 11 or 12, wherein the first and the second emulsion are mixed at a volume ratio of about 1: 1.
Method according to any of claims 11 to 13, wherein in the first and in the second emulsion the water is present at a concentration in the range of about 1 to about 50 wt.%, based on the total weight of surfactant, oil phase and water.
Method according to any of claims 11 to 14, wherein the surfactant is cetyl trimethyl ammonium bromide, the oil phase is a C _6-12 alcohol and the reducing agent is a borhydride salt.