JP4165653B2 - Alcohol oxidation solid catalyst and synthesis method thereof - Google Patents

Description

  The present invention relates to an alcohol oxidation solid catalyst, and more particularly to a solid catalyst composed of mesoporous carbon on which palladium is supported in a high concentration and high dispersion, and a synthesis method thereof.

  The oxidation reaction of alcohol to aldehyde or ketone is one of the most important reactions in organic synthesis. Conventionally, these reaction systems have been carried out by a stoichiometric reaction using an oxidizing agent such as chromic acid or manganese dioxide, and have problems in terms of reagent toxicity and waste disposal. Therefore, it is desired to develop a catalytic oxidation method of alcohol using safe and clean molecular oxygen. In particular, the development of a reaction system using a solid catalyst that is easy to separate, recover and reuse is an important issue.

  From such a viewpoint, it has been reported so far that a material in which a noble metal or hydroxide is supported on a metal oxide support has a high function as a solid catalyst (Non-patent Documents 1 and 2).

  On the other hand, as the solid catalyst, there are a commercially available carbon material (Pd / C) in which palladium particles are supported, and a metal catalyst in which palladium or platinum-ruthenium is supported on activated carbon by an impregnation method. Has not been studied (Non-Patent Documents 3 and 4).

In recent years, various materials and synthesis methods have been studied for nanocarbon materials typified by carbon nanotubes. Since mesoporous carbon synthesized using mesoporous silica as a template has a wide specific surface area, it is expected to be used for hydrogen storage materials, adsorbents, electrode materials, catalyst carriers, and the like (Non-Patent Documents 5 and 6). For example, it has been reported that a material in which platinum particles are supported on such mesoporous carbon by an impregnation method has high activity as an electrode catalyst (Non-patent Document 6). However, the alcohol oxidation activity has not been studied at all.
N. Mizuno et al., Angew. Chem. Int. Ed., 23, 4538-4541, 2002 K. Kaneda et al., New. J. Chem., 27, 324-328, 2003 Tomoichi Sasaki et al., Catalyst, 45, 80-82, 2003 Shintaro Asano and Yasukazu Saito, Catalyst, 41, 465-467, 1999 R. Ryoo et al., J. Phys. Chem. B., 37, 7743-7746, 1999 SH Joo et al., Nature, 412, 169-172, 2001

  An object of this invention is to provide the solid catalyst which has high activity with respect to alcohol oxidation reaction.

  The present invention provides a composite material in which palladium particles are supported on mesoporous carbon.

  In a preferred embodiment, the composite material is obtained by immersing mesoporous carbon in a strong alkaline aqueous solution, then immersing in a palladium compound aqueous solution, and further reducing.

  The present invention also provides a solid catalyst comprising the above composite material.

The present invention further provides a method for producing a solid catalyst, the method comprising:
Immersing mesoporous carbon in a strong alkaline aqueous solution;
Immersing the alkali-treated mesoporous carbon in an aqueous palladium compound solution; and adding a reducing agent to the aqueous palladium compound solution to reduce palladium;
including.

  In a preferred embodiment, the strong alkaline aqueous solution has a pH of at least 14.

  In a more preferred embodiment, the immersion in the palladium compound aqueous solution is performed under reduced pressure.

  In another preferred embodiment, the palladium compound is sodium chloropalladate.

  The present invention further provides a method for oxidizing an alcohol, comprising the step of oxidizing the alcohol in an oxygen atmosphere using the composite material or the solid catalyst.

  ADVANTAGE OF THE INVENTION According to this invention, the alcohol oxidation solid catalyst which has high activity with respect to alcohol oxidation reaction is provided. The catalyst is easy to recover after the reaction and can be reused. Further, according to the method of the present invention, the solid catalyst of the present invention can be produced very easily using mesoporous carbon and a palladium compound.

  In this specification, the mesoporous carbon refers to a porous carbon material having substantially uniform and regular pores at a level of several nm to several tens of nm. This carbon material is usually preferably in the form of powder or granules. In the present invention, the mesoporous carbon may be obtained by any means. For example, mesoporous silica having substantially uniform and regular pores at a level of several nanometers to several tens of nanometers may be synthesized as a template (see Non-Patent Documents 5 and 6). Specifically, first, mesoporous silica is immersed in a substance that becomes a carbon source (for example, sucrose or furfuryl alcohol). It is then heated at a suitable temperature such that the carbon source material is converted to carbon, preferably under vacuum or under an inert gas atmosphere. In this heating, sulfuric acid is used as a catalyst as necessary. Finally, the template silica is dissolved and removed with an acid or alkali to obtain mesoporous carbon to which the pores of the template mesoporous silica are transferred.

  The solid catalyst of the present invention is composed of a composite material in which palladium particles are supported on such mesoporous carbon. This composite material can be produced, for example, by the method of the present invention.

  The method for producing a composite material that can be a solid catalyst of the present invention comprises a step of immersing mesoporous carbon in a strong alkaline aqueous solution; a step of immersing the alkali-treated mesoporous carbon in a palladium compound aqueous solution; and a reducing agent in the palladium compound aqueous solution. In addition, a step of reducing palladium is included. Hereinafter, the method of this invention is demonstrated in detail in order of a process.

  First, in the step of immersing mesoporous carbon, the pH of a strong alkaline aqueous solution is usually at least 14. The concentration of the strong alkaline aqueous solution is preferably higher than 0.1M, and may be 0.5M or more, or 1M or more. If the alkali is weak, it is difficult to obtain a solid catalyst having high catalytic activity. The immersion time of the mesoporous carbon in the strong alkaline aqueous solution is usually at least 12 hours, preferably at least 24 hours, more preferably at least 48 hours. After soaking, the alkali-soaked mesoporous carbon is recovered by filtration or the like, washed with distilled water, and then dried at 80 ° C. for 4 hours, for example. In this way, mesoporous carbon pretreated with alkali is obtained.

Next, the alkali-treated mesoporous carbon is suspended in an aqueous palladium compound solution and immersed while stirring. Although the kind of palladium compound used here is not specifically limited, A bivalent palladium compound is preferable. Examples thereof include Na ₂ PdCl ₄ (sodium chloropalladate), K ₂ PdCl ₄ , Na ₂ PdBr ₄ , Na ₂ Pd (SCN) ₄ , and [Pd (NH ₃ ) ₄ ] Cl ₂ . Na ₂ PdCl ₄ (sodium chloropalladate) is preferable because it is easily available. The concentration of the palladium compound aqueous solution is not particularly limited, but the amount of palladium relative to mesoporous carbon is usually 0.5 to 20 wt%, preferably 1 to 10 wt%, more preferably 2 to 5 wt%. Immersion is preferably performed under reduced pressure or in an inert gas atmosphere. Furthermore, it is particularly preferable to carry out under reduced pressure. The immersion time in the palladium compound aqueous solution is usually at least 12 hours, preferably at least 24 hours, more preferably at least 48 hours. In this way, the palladium compound is supported on the mesoporous carbon.

Next, a reducing agent is added to the suspension of mesoporous carbon to reduce the palladium compound supported on the mesoporous carbon to palladium. Examples of the reducing agent include KBH ₄ , NaBH ₄ , LiAlH ₄ , methanol, ethanol and the like. The amount of the reducing agent used is usually 1 to 5 times, preferably 2 to 3 times the number of moles of palladium. These reducing agents are preferably added as an aqueous solution. The reduction of the palladium compound is performed with stirring for 30 minutes at room temperature, for example. After completion of the reduction reaction, the mesoporous carbon is recovered by filtration or the like, and thoroughly washed with distilled water to remove soluble components. Next, for example, it is dried at 80 ° C. for 4 hours. In this way, mesoporous carbon carrying palladium is obtained. The amount of palladium supported on mesoporous carbon can usually be determined stoichiometrically from the concentration of the palladium compound aqueous solution.

  About the obtained palladium carrying | support mesoporous carbon, the dispersion state of the palladium carry | supported by the mesoporous carbon can be observed with a transmission electron microscope (TEM). Usually, palladium is dispersed and supported as particles mainly near the entrance of the pores of the mesoporous carbon or on the outer surface. The particle diameter of the supported palladium particles can be measured by X-ray diffraction (XRD). The crystallite diameter of the palladium particles is about 5 to 30 nm.

  The composite material of the present invention in which palladium particles are supported on mesoporous carbon has catalytic activity for alcohol oxidation reaction. Therefore, it can be used as a solid catalyst in an alcohol oxidation reaction. In particular, the solid catalyst of the present invention has a relatively high selectivity for the production of aldehydes or ketones in alcohol oxidation (ie, the proportion of aldehydes or ketones in the reaction product).

  Therefore, according to this invention, the oxidation method of alcohol including the process of oxidizing alcohol in oxygen atmosphere using this solid catalyst is also provided.

  The kind of alcohol used for oxidation is not particularly limited as long as it is a primary or secondary alcohol. The solid catalyst of the present invention exhibits particularly high activity for an aromatic alcohol and an unsaturated alcohol having a double bond next to carbon having an alcoholic hydroxyl group. Examples of the aromatic alcohol include benzyl alcohol, cinnamyl alcohol, p-anis alcohol, p-methylbenzyl alcohol, o-methylbenzyl alcohol and the like. About aromatic alcohol, it shows high activity with respect to alcohol which has an electron donating group (for example, methyl group, methoxy group) in the meta position or para position of the substituent which has alcoholic hydroxyl group. On the other hand, examples of the unsaturated alcohol include 2-hexen-1-ol and 2-cyclohexen-1-ol.

  The solid catalyst of the present invention is preferably used in a ratio of 1 to 50 μmol, more preferably 3 to 10 μmol, relative to 1.5 mmol of alcohol serving as a substrate for the oxidation reaction.

  Specifically, this oxidation reaction is performed in a suitable organic solvent. The organic solvent that can be used can be appropriately determined according to the type of alcohol of the substrate. Examples of the organic solvent include 1,4-dioxane, toluene, and heptane. The reaction is usually performed in an oxygen atmosphere with stirring at 80 ° C. for 4 hours.

  In this way, the alcohol is oxidized and converted to the corresponding aldehyde. The resulting aldehyde or ketone can be recovered by appropriate means and purified, for example, or subjected to further reaction (eg, oxidation to carboxylic acid).

  On the other hand, the catalyst of the present invention in the reaction solution can be easily recovered by filtration or the like. The recovered solid catalyst after use can be used as a catalyst for a new oxidation reaction after washing. Even when the solid catalyst of the present invention is reused, the catalytic activity is hardly lowered. Therefore, the solid catalyst of the present invention is very useful in that wasteful consumption of resources can be avoided and also in terms of cost.

(Synthesis Example 1: Synthesis of mesoporous silica (SBA-15))
6.9 g of amphiphilic surfactant P123 (manufactured by BASF) was added to 125 mL of 0.9N hydrochloric acid aqueous solution and dissolved, and 14.8 g of tetraethylorthosilicate (TEOS, manufactured by Aldrich) was further added, followed by stirring at 35 ° C. for 24 hours. The obtained cloudy liquid was put into a Teflon (registered trademark) container and hydrothermally synthesized at 100 ° C. for 48 hours. The obtained crystals were filtered, washed with distilled water, and then fired at 550 ° C. for 6 hours to remove the surfactant and obtain 4.5 g of mesoporous silica (SBA-15).

  X-ray diffraction (XRD) of the obtained SBA-15 was measured. The XRD pattern obtained by Philips XP1600 is shown in FIG. Moreover, the adsorption isotherm of nitrogen is shown in FIG. 2, and the physical property values obtained by analyzing the adsorption isotherm are summarized in Table 1.

In the adsorption isotherm of FIG. 2, a hysteresis loop is exhibited on the adsorption side and the desorption side, and the presence of mesopores is clear. As shown in Table 1, SBA-15 mainly had mesopores of about 66 mm in size, and its surface area was 673 m ² / g.

(Synthesis Example 2: Synthesis of mesoporous carbon (CMK-3))
Using SBA-15 obtained in Synthesis Example 1 as a template and sucrose as a carbon source, mesoporous carbon (CMK-3) was synthesized as follows. First, 5 g of sucrose was dissolved in 20.6 mL of 0.28N sulfuric acid aqueous solution, 4 g of SBA-15 was immersed, and heated at 100 ° C. for 6 hours and then at 160 ° C. for 6 hours to obtain a black substance. This material was pulverized in a mortar until powdered, then dipped again in 20.4 mL of a 0.19N aqueous sulfuric acid solution containing 3.2 g of sucrose, and heated at 100 ° C. for 6 hours and then at 160 ° C. for 6 hours. The obtained black material was powdered with a mortar, and then this powder was baked in a vacuum at 900 ° C. for 3 hours. The fired product was washed twice with a 10 vol% hydrogen fluoride aqueous solution to obtain 2 g of mesoporous carbon (CMK-3).

  X-ray diffraction (XRD) of the obtained CMK-3 was measured. The XRD pattern obtained by Philips XP1600 is shown in FIG. 1 together with the SBA-15 pattern. Further, nitrogen adsorption / desorption was also measured. The adsorption isotherm of nitrogen is shown in FIG. 3, and the physical property values obtained by analyzing the adsorption isotherm are summarized in Table 2.

In the adsorption isotherm of FIG. 3, a broad hysteresis loop is seen compared to SBA-15, and it can be seen that it has a wide pore size distribution. This is considered to be caused by a change in regularity due to the shrinkage of the structure when the silica is replaced with carbon or when the carbon is heat-treated. This also appears in the decrease of the analytical line intensity of the XRD pattern. As summarized in Table 2, CMK-3 had about 33 メ mesopores and had a surface area of 1007 m ² / g. Further, a transmission electron microscope (TEM) photograph of CMK-3 is shown in FIG. From the TEM observation, CMK-3 showed a cylindrical structure that copied the cylindrical structure of SBA-15.

(Example 1: Synthesis of palladium-supported mesoporous carbon (Pd / CMK-3))
CMK-3 obtained in Synthesis Example 2 was immersed in 1N NaOH aqueous solution for 24 hours, filtered, washed with distilled water, dried at 80 ° C., and pretreated. The pretreated CMK-3 was suspended in an aqueous solution in which Na ₂ PdCl ₄ was dissolved in an amount such that the amount of palladium relative to the support was 5 wt%, and stirred in an argon atmosphere for 2 days. Then added KBH ₄ aqueous solution containing KBH ₄ 2.4 times the weight with respect to the number of moles of palladium to the suspension, washed with stirring at room temperature for 1 hour, filtered, and distilled water, Pd I got / CMK-3.

  A transmission electron microscope (TEM) photograph of Pd / CMK-3 is shown in FIG. Palladium was present in very high dispersion on CMK-3.

(Example 2: Examination of catalytic activity in alcohol oxidation reaction)
To 5 mL of 1,4-dioxane, 10 mg (4.7 μmol as the amount of Pd) of Pd / CMK-3 obtained in Example 1 and 0.12 mL (1.5 mmol) of benzyl alcohol were added, and at 80 ° C. in an oxygen atmosphere. The reaction was stirred for 4 hours. After completion of the reaction, the amount of residual benzyl alcohol and produced benzaldehyde was measured by gas chromatography (column: DB-5 (manufactured by J &W); column temperature: 170 ° C .; carrier gas: helium; detector: 22D). . In addition, for comparison, two commercially available 5 wt% palladium carbons (Pd / C: manufactured by Nacalai Tesque and Wako) were used in an amount of 10 mg each, and the reaction was performed in the same manner as in the case of Pd / CMK-3. . The results are shown in Table 3. The conversion rate in the table indicates the ratio of the substrate consumed by the reaction, and the selectivity indicates the ratio of the aldehyde in the reaction product.

  In the case of Pd / CMK-3, almost 100% conversion from benzyl alcohol to benzaldehyde was achieved in a reaction time of 4 hours. In contrast, commercially available Pd / C hardly reacted. From this, it was found that Pd / CMK-3 shows much higher activity than the commercially available Pd / C catalyst in the oxidation reaction of benzyl alcohol.

(Example 3: Influence of catalyst carrier type in alcohol oxidation reaction)
Using activated carbon (manufactured by Wako) instead of CMK-3 as a catalyst support, treatment was performed as described in Synthesis Example 2 to synthesize palladium-supported activated carbon (Pd / AC). Using 10 mg each of Pd / CMK-3 and Pd / AC, the reaction was carried out for 2 hours under the same reaction conditions as in Example 2 above. The results are shown in Table 4.

  The synthesized Pd / AC was more reactive than the commercially available Pd / C, but the conversion was considerably lower than that of Pd / CMK-3. Despite the fact that Pd was supported by the same procedure, such a large difference was observed in the catalytic activity. Therefore, it seems that the structure of the support influences how Pd is supported.

  Therefore, XRD was measured for three types of catalysts having different supports, that is, Pd / CMK-3, Pd / AC, and Pd / C (manufactured by Wako). These XRD patterns are summarized in FIG. In any of the catalysts, it was confirmed that Pd was present on the support in a zero-valent state. The crystallite diameter of Pd obtained from the half width of the main peak near 40 ° increases in the order of Pd / AC <Pd / C <Pd / CMK-3, and the correlation between the crystallite diameter and catalyst activity is I couldn't see it. The black circles on the peaks in FIG. 5 indicate zero-valent Pd peaks.

(Example 4: Effect of palladium amount in Pd / CMK-3 synthesis)
The same operation as in Example 1 was carried out using Na ₂ PdCl ₄ aqueous solution with the amount of palladium of 1 wt%, 2.5 wt%, and 5 wt% relative to the carrier, and 1 wt% Pd / CMK-3, 2.5 wt% Pd, respectively. / CMK-3 and 5 wt% Pd / CMK-3 were synthesized. Incidentally, 5 wt% Pd / CMK-3 is the same as the catalyst synthesized in Example 1. 1 wt% Pd / CMK-3 is 50 mg, 2.5 wt% Pd / CMK-3 is 20 mg, and 5 wt% Pd / CMK so that the amount of Pd used in the oxidation reaction is the same based on the amount of Pd used in the synthesis. Oxidation reaction of benzyl alcohol was carried out under the same reaction conditions as in Example 2 using 10 mg of -3, and the residual amount of benzyl alcohol and the amount of benzaldehyde produced were measured over time. The results are shown in FIGS.

  When any catalyst was used, the conversion rate from benzyl alcohol to benzaldehyde exceeded 90% in 3 hours and was almost completely converted in 4 hours. When the thinner Pd salt was used for the catalyst synthesis, it was expected that the catalyst activity would be increased by being supported as smaller Pd particles on CMK-3, but there was no difference in activity among the three types of catalysts. It was. Therefore, it was found that there is no significant difference in the way Pd is supported.

  Therefore, when the state of palladium supported on 5 wt% Pd / CMK-3 was observed in more detail by TEM, palladium particles having an average of about 20 nm were mainly supported in a highly dispersed manner on the outer surface. Was confirmed (FIG. 9).

(Example 5: Examination of CMK-3 synthesis conditions)
In order to improve the dispersibility of palladium, Pd / CMK-3 was synthesized under reduced pressure instead of argon atmosphere in Example 1. Hereinafter, Pd / CMK-3 obtained under reduced pressure is referred to as Pd / CMK-3 (degassing treatment). About this, XRD and the benzyl alcohol oxidation activity were measured. The reaction time was 1 hour. FIG. 10 shows the XRD pattern, and Table 5 shows the results of the benzyl alcohol oxidation reaction.

  When synthesized under reduced pressure (degassing treatment), the tail of the peak of Pd in FIG. 10 is broad, and it is supported in the state of fine particles having a smaller Pd than when synthesized under an Ar atmosphere. Seem. In addition, as can be seen from Table 5, Pd / CMK-3 (degassing treatment) showed a conversion rate of 70% in the reaction of benzyl alcohol for 1 hour and higher activity than that synthesized under Ar atmosphere. Met.

(Example 6: Examination of substrate specificity)
About 1.5 mmol of various substrates shown in Table 6 below, 10 mg each of 5 wt% Pd / CMK-3 (degassing treatment) synthesized in Example 5 and a commercially available 5 wt% Pd / C (manufactured by Wako) as a comparison. In the same manner as in Example 2 above, the mixture was reacted in 5 mL of 1,4-dioxane by stirring at 80 ° C. for 4 hours in an oxygen atmosphere. After the completion of the reaction, the remaining alcohol and the amount of aldehyde produced were each measured.

  As shown in Table 6, generally, Pd / CMK-3 of the present invention tended to have higher activity in alcohol oxidation reaction than commercially available Pd / C. The benzyl alcohol derivative showed high activity for derivatives having an electron donating group (for example, a methyl group or a methoxy group) at the meta or para position. Moreover, the activity was particularly high not only for aromatic alcohols but also for substrates having a double bond next to carbon having an OH group.

(Example 7: Examination of reusability)
The used catalyst was collected, and the catalyst performance when the same reaction was performed again was examined. In one reaction, 1.5 mmol of benzyl alcohol as a substrate and 50 mg of 5 wt% Pd / CMK-3 obtained in Example 2 above as a catalyst were used in 5 mL of 1,4-dioxane at 80 ° C. in an oxygen atmosphere. For 2 hours with stirring. After completion of the reaction, the catalyst was recovered by filtration, washed with ethanol, dried and used for the next reaction. The reaction was performed 5 times. After the completion of each reaction, the amount of remaining alcohol and produced aldehyde was measured. The results are shown in Table 7.

  As is clear from Table 7, the activity of the catalyst was not decreased even when it was used many times. Therefore, it was found that reuse is possible.

  According to the method of the present invention, the palladium fine particles can be present in a high concentration and dispersed on the support, so that expensive noble metal resources can be used effectively. In addition, the solid catalyst of the present invention has a very high catalytic activity for the alcohol oxidation reaction as compared with the conventional stoichiometrically consumed oxidizing reagent, can be easily recovered after the reaction, and can be reused. There is little wasteful energy consumption. Therefore, an environmentally low load type and economical reaction process can be constructed.

It is a figure which shows the XRD pattern of SBA-15 and CMK-3. It is a graph of the nitrogen adsorption isotherm of SBA-15. It is a graph of the nitrogen adsorption isotherm of CMK-3. It is a transmission electron microscope (TEM) photograph of (a) CMK-3 and (b) Pd / CMK-3. It is a figure which shows the XRD pattern of Pd / CMK-3, Pd / AC, and Pd / C (product made from Wako). It is a graph which shows the time-dependent change of the benz alcohol oxidation reaction at the time of using 1 wt% Pd / CMK-3. It is a graph which shows the time-dependent change of the benz alcohol oxidation reaction at the time of using 2.5 wt% Pd / CMK-3. It is a graph which shows a time-dependent change of the benz alcohol oxidation reaction at the time of using 5 wt% Pd / CMK-3. It is a TEM photograph immediately after the start of TEM observation of 5 wt% Pd / CMK-3 (a) and during observation (b). It is a figure which shows the XRD pattern of various Pd / CMK-3.

Claims (4)

An alcohol oxidation catalyst comprising a solid catalyst in which palladium particles are supported on mesoporous carbon . An alcohol oxidation method comprising a step of oxidizing an alcohol in an oxygen atmosphere using an alcohol oxidation catalyst comprising a solid catalyst in which palladium particles are supported on mesoporous carbon . The method for oxidizing alcohol according to claim 2, wherein the alcohol is an aromatic alcohol or an unsaturated alcohol having a double bond next to carbon having an alcoholic hydroxyl group. The method for oxidizing an alcohol according to claim 3, wherein the aromatic alcohol is an alcohol having an electron donating group at a meta position or a para position of a substituent having an alcoholic hydroxyl group.

Images