Concerted Functions of Surface Acid–Base Pairs and Supported

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Concerted Functions of Surface Acid−Base Pairs and Supported Copper Catalysts for Dehydrogenative Synthesis of Esters from Primary Alcohols Hiroki Miura,*,†,‡,§ Karin Nakahara,† Takahiro Kitajima,† and Tetsuya Shishido*,†,‡,§ †

Department of Applied Chemistry, Graduate School of Urban Environmental Sciences and ‡Research Center for Hydrogen Energy-based Society, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan § Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan S Supporting Information *

ABSTRACT: Dehydrogenative synthesis of esters from primary alcohols proceeded efficiently over a ZrO2-supported copper catalyst. A variety of esters were obtained from primary alcohols as well as diols in good to high yields. The key to the dehydrogenative synthesis of esters is the concerted effect of the acid−base pairs on ZrO2 and metallic copper.

1. INTRODUCTION Ester is one of the most important functionalities, and is found in a wide range of valuable chemicals such as fibers, inks, solvents, surfactants, fragrances, lubricants, food additives, and pharmaceuticals.1 Although the condensation of alcohols with activated carbonyl compounds, such as acid chlorides and acid anhydrides, is the most convenient method for the preparation of esters, a stoichiometric amount of salt is inevitably formed as waste (eq 1 in Scheme 1). Also, in dehydrative condensation of carboxylic acids with alcohols promoted by acid catalysts, the addition of dehydrating agents is indispensable for the highyield production of esters (eq 2). On the other hand, the catalytic transformation of alcohols to esters in a dehydrogenative manner is a promising method thanks to the coproduction of only molecular hydrogen (eq 3).2,3 In 2005, Milstein and coworkers first demonstrated an efficient and acceptorless dehydrogenative synthesis of esters from alcohols through the use of ruthenium complexes bearing a pincer-type ligand as catalysts.4 Following this breakthrough, a series of homogeneous catalysts based on transition-metal complexes, such as Ru,5−10 Rh,11 Ir,12 Re,13 Os,14 and Mn,15 have been developed. On the other hand, efficient organic synthesis with heterogeneous catalysts is of great significance from the perspective of green and sustainable chemistry.16−20 Shimizu and co-workers reported that supported Pt catalysts were effective for the dehydrogenative synthesis of esters.21,22 Recently, much attention has also been paid to the development of an efficient catalytic system with abundant first-row transition metals. Particularly, many examples have demonstrated that supported © 2017 American Chemical Society

Cu catalysts are highly effective for the dehydrogenative transformation of alcohols to the corresponding carbonyl compounds.23−25 Dehydrogenative synthesis of esters from alcohols has been achieved by the use of supported Cu catalysts,26−30 and the availability of ZrO2 as a support for Cucatalyzed dehydrogenative synthesis of esters has also been explored. However, the correlations of the surface property of ZrO2 with the activity of Cu catalysts have not been fully understood. Herein, we describe the dehydrogenative synthesis of esters from primary alcohols in the presence of supported Cu catalysts. The supports for the catalysts remarkably dominated the reaction efficiency and selectivity, and ZrO2-supported catalysts gave the corresponding esters with a high ester yield and selectivity. NH3- and CO2-temperature-programmed desorption (NH3-TPD and CO2-TPD) profiles revealed that metallic Cu and acid−base pair sites on the surface of ZrO2 promoted the catalytic transformation of primary alcohols to the corresponding esters with high selectivity.

2. RESULTS AND DISCUSSION Supported Cu catalysts were prepared by a simple impregnation method. An oxide support was impregnated in an aqueous solution of Cu(NO3)2·3H2O at 80 °C for 2 h. After drying, the resulting powder was calcined at 500 °C for 3 h under an air Received: August 7, 2017 Accepted: September 14, 2017 Published: September 26, 2017 6167

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Scheme 1. Synthetic Routes to Esters from Alcohols

flow. Before being used in catalytic reactions, the supported Cu catalysts was pretreated under a H2 flow (10 mL min−1) at 400 °C for 1 h to reduce the Cu cation to metallic Cu.31 Table 1 summarizes the results of the reaction of 1-octanol (1a, 1.5 mmol) in the presence of supported Cu catalysts (1

When only ZrO2 was subjected to the catalytic reaction, no conversion of 1a was confirmed. Although several Cu salts, such as Cu(OAc)2, Cu(NO3)2·3H2O, CuCl, CuCl2·2H2O, and Cu(OH)2, were employed as soluble or insoluble catalyst, no ester was obtained. These results indicate that the combination of metallic Cu and a ZrO2 support is essential for the selective formation of esters from primary alcohols in a dehydrogenative manner. Note that no leaching of Cu species into the solvent after the reaction with Cu/ZrO2 catalysts was confirmed by the atomic emission spectroscopic analysis. Under the optimized reaction conditions, a series of alcohols were subjected to the dehydrogenative synthesis of esters over Cu(5 wt %)/ZrO2 catalysts (Table 2). The reactions of

Table 1. Dehydrogenative Synthesis of Ester with Supported Catalystsa

yield (%) entry

catalyst

conv. of 1a (%)

2a

3a

1 2 3 4 5 6 7 8 9 10 11 12 13

Cu/ZrO2 Cu/CeO2 Cu/Al2O3 Cu/SiO2 Cu/TiO2 Co/ZrO2 Ni/ZrO2 Ru/ZrO2 Pd/ZrO2 Ag/ZrO2 Ir/ZrO2 Pt/ZrO2 ZrO2

99 63 96 28 100 15 61 27 12 35 14 33 11

68 24 8 1 4 1 11 9 0 19 2 3 1

3 7 2 8 0 0 1 1 0 2 1 1 0

Table 2. Scope of Substratesa

a Reaction conditions: 1a (1.5 mmol), supported catalyst (0.01 mmol as Cu), mesitylene (1.0 mL), at 170 °C, 24 h, under Ar. bYields (2a and 3a) and conversion (1a) were determined by gas−liquid chromatography based on 1a.

mol % as metal) at 170 °C for 24 h in mesitylene. Among the supported Cu catalysts tested, ZrO2-supported catalysts showed the highest conversion of the alcohol and the highest yield of the corresponding ester, octyl octanoate (2a). In all of the cases, the formation of a small amount of octanal (3a) was confirmed. Despite the high conversion of the alcohol, Al2O3supported Cu catalysts resulted in a very low yield of the ester because Lewis acidic surface property of Al2O3 dominantly promoted dehydrative coupling to give octyl ether as a main byproduct. On the other hand, TiO2- and CeO2-supported catalysts gave α,β-unsaturated ketone as a main byproduct via the self-condensation of octanal. The effects of the supported metallic species on the present reaction were also investigated, whereas the reaction with Co, Ni, Ru, Pd, Ag, Ir, or Pt catalysts supported on ZrO2 resulted in a lower conversion of the alcohol and a lower yield of the ester than that with Cu/ZrO2.

a

Gas chromatography (GC) yields. bReaction for 48 h. cIsolated yield. Cu/CeO2 was used as a catalyst. eSolvent 2 mL. fSolvent 3 mL.

d

aliphatic primary alcohols (1a−f) with a straight alkyl chain gave the corresponding esters (2a−f) in moderate to high yields. Cyclohexylmethanol (1g) and 3-phenyl-1-propanol (1h) could also participate in the present catalytic system to afford 2g and 2h in high yields. The reaction of benzyl alcohol (1i) resulted in 35% yield of benzyl benzoate (2i), whereas an increased yield of 52% was obtained in the reaction with Cu/ CeO2 catalyst. This is due to the higher reactivity of benzyl alcohol than that of aliphatic alcohols. The rapid conversion of alcohol to aldehyde reduces the amount of alcohols as the 6168

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coupling reagent to form hemiacetal, leading to a low yield of the corresponding ester. In this respect, Cu/CeO2 is a suitable catalyst for the dehydrogenative synthesis of esters from benzyl alcohol because the catalyst shows moderate catalytic activity for the dehydrogenation of alcohols. The reactions of diols were also examined, and the intramolecular cyclization of 1,4butanediol (1j) and 1,5-pentanediol (1k) took place to give the corresponding lactones (2j and 2k) in high yields under diluted conditions. Figure 1 shows the time course of the reaction of 1a in the presence of Cu/ZrO2 at 170 °C. At the initial stage of the

Scheme 3. Reaction of 3a with Cu/ZrO2

general considerations for very low equilibrium concentration of hemiacetals, we can assume that the dehydrogenation of hemiacetal formed by the condensation of aldehyde with alcohol is the rate-determining step. It has become widely accepted that the activity of supported Cu catalysts for alcohol dehydrogenation is enhanced by the combination of metallic nanoparticles and base sites on the support because the basic nature of surface hydroxyl groups facilitates the dissociation of the O−H bond of alcohol to form copper alkoxide species, which are key intermediates for dehydrogenation.33−37 In sharp contrast, the formation of hemiacetals via the condensation of carbonyl compounds with alcohols can be promoted by Lewis or Brønsted acid catalysis. On the basis of these facts, the coexistence of acid and base sites on the surface of the catalyst must be crucial for the selective formation of ester from primary alcohols. Hence, the surface acid−base amounts for each supported Cu catalyst were estimated by NH3- or CO2-temperature-programmed desorption (NH3-TPD or CO2-TPD).38 Figure 2 shows the relationship between the amounts of both acid and base sites for the supported Cu catalysts and the yields

Figure 1. Time course of the reaction by Cu/ZrO2 catalyst.

reaction, the rapid formation of octanal 3a was observed, and the yield of the aldehyde gradually decreased after 5 h. In contrast, the yield of ester 2a gradually increased with the passage of time. This strongly suggests that aldehyde 3a was an intermediate for the ester in the present reaction. Two reaction pathways from aldehyde to ester can be assumed (Scheme 2); Scheme 2. Reaction Pathway from Alcohol to Ester

Figure 2. Relationship between the yield of 2a (•) in Table 1 and the amount of acid (white squares with black background) and base (chessboard pattern) on the catalysts.

of ester 2a. Al2O3-, SiO2-, and TiO2-supported catalysts had acid sites and much fewer base sites than ZrO2- and CeO2supported catalysts. In contrast, the large amounts of both acid and base sites were present on Cu/ZrO2 catalyst, implying that the acid−base pair sites and adjacent metallic Cu species worked in concert as efficient catalysts for the formation of esters. Figure 3 shows the correlation between the yield of ester 2a and the amount of acid−base pair sites per unit surface area in each supported Cu catalyst. In this case, the amount of acid− base pair sites was defined as the lesser acid and base sites. For example, the amounts in the ZrO2-, Al2O3-, TiO2-, and SiO2supported catalysts were equivalent to those of base sites,

dimerization of aldehyde, the so-called Tishchenko reaction (path A),32 and the formation of hemiacetal via the condensation of aldehyde with alcohol, followed by dehydrogenation (path B). The reaction of octanal (3a) in the presence of Cu/ZrO2 under the same reaction conditions as in Table 1 resulted in a very low yield of 2a (Scheme 3). This strongly suggests that the ester was furnished through path B. Furthermore, the formation of aldehyde was observed (Figure 1), which indicates that the dehydrogenation of 1-octanol was faster than the formation of ester. From this result and the 6169

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it is generally accepted that a Zr center and surface hydroxyl groups serve as the acid and base sites, respectively.39−41 On the basis of these considerations, a possible reaction mechanism for the transformation of primary alcohols to the corresponding esters is proposed, as follows (Scheme 4). The base sites on the ZrO2 surface promote the dissociation of the O−H bond to form copper alkoxide. Subsequently, β-hydride elimination gives aldehyde together with copper hydride species, followed by the coupling of hydride with a proton on the surface hydroxyl group to generate molecular hydrogen. The Lewis acidic Zr center activates aldehyde, which promotes the condensation with alcohols to furnish hemiacetal. Finally, the thus-formed hemiacetal is dehydrogenated by the adjacent surface base and metallic Cu to give the final product ester. A detailed mechanistic investigation by spectroscopic techniques is currently underway in our laboratory. Finally, reusability of Cu/ZrO2 catalyst was investigated (Table 3). Unfortunately, severe deactivation of the Cu/ZrO2 Table 3. Reuse of Cu/ZrO2 Catalysts

Figure 3. Relationship between the yield of 2a and the amount of acid−base pair sites on the catalysts.

whereas the amount of acid−base pair sites in Cu/CeO2 was equivalent to that of the acid sites. As shown in Figure 3, the yield of ester is closely correlated with the amount of acid−base pair sites in each supported Cu catalyst, suggesting that the selective formation of esters from primary alcohols must be dominated by the synergetic function of the surface acid−base pair sites with adjacent metallic Cu species. Such amphoteric functions of ZrO2 have been widely applied in the transformation of organic molecules, such as Meerwein−Ponndorf− Verley reduction39−41 and CO2 fixation into alcohols,42−44 and

yield (%)a procedure for regeneration none calcination and reduction a

fresh 2nd use 2nd use

conv. (%)

2a

3a

99 14 51

68 12 30

3 2 2

GC yields.

Scheme 4. Proposed Reaction Mechanism

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performed using Cu Kα radiation and a one-dimensional X-ray detector (SmartLab, Rigaku). The samples were scanned from 2θ = 10−70° at a scanning rate of 10° s−1 and a resolution of 0.01°. The Brunauer−Emmett−Teller specific surface area was estimated from the N2 isotherms obtained using a BELSORPmini II (BEL Japan, Osaka, Japan) at 77 K. The analyzed samples were evacuated at 573 K for 2 h prior to the measurement. The contents of copper species leached into reaction solvent were determined by the atomic emission spectroscopic analysis with a SHIMADZU AA-6200. 4.3. Experimental Procedure. 4.3.1. Typical Preparation of a Support Cu Catalyst. Supported catalysts were prepared by the impregnation method. A 1.0 g of support was added to the aqueous solution of Cu(NO3)2·3H2O in air at 353 K. After impregnation, the resulting powder was calcined in air at 773 K for 3 h to afford a supported Cu catalyst. Supported Ni, Co, Cu, Pd, Ag, Ir, and Pt catalysts were prepared in a similar manner. 4.3.2. Representative Procedure for Dehydrogenative Synthesis of Ester from Primary Alcohol. Dehydrogenative coupling of 1-octanol was carried out in a batch-type reactor (20 mL Pyrex tube). Cu/ZrO2 catalyst (100 mg, 1 mol % as Cu) was reductive pretreated in a H2 flow (10 mL min−1) at each temperature for 1 h in the reactor (Cu, Pd, and Pt catalysts at 423 K, Co, Cu, and Ir catalysts at 673 K, Ni catalyst at 773 K). Then, 1-octanol (1.5 mmol) and mesitylene (1.0 mL) were added to the reactor. The reaction was carried out at 443 K under Ar atmosphere. The products were quantified by gas chromatography using an internal standard technique. 4.3.3. Recycling of the Cu/ZrO2 Catalyst. After the reaction, the solid was separated from the reaction mixture by centrifugation and washed with 10 mL of diethyl ether, methanol/H2O (1:1), and again by diethyl ether. The resulting solid was dried overnight at 80 °C and calcined in air at 400 °C for 30 min to recover the Cu/ZrO2 catalyst for reuse. Before catalytic reactions, the supported Cu catalyst was subjected to reductive pretreatment in a H2 flow at 400 °C for 1 h.

catalyst was observed after the catalytic reaction. The reaction of 1a by the recovered Cu/ZrO2 without any regenerative treatment resulted in very low yield of 2a. In contrast, before being subjected to the repeated use, the recovered catalyst was treated with air calcination and H2 reduction. As a result, the regenerated Cu/ZrO2 catalyst gave 2a in an improved yield, whereas the activity of the catalyst was not fully recovered. As described above, a number of acid−base pair sites are crucial for high activity of the catalyst and high selectivity of the reactions. Such pair sites might be diminished during the catalytic reactions in an organic solvent, and they cannot be recovered even though the used catalysts were calcined at high temperatures under the air-flow conditions. Further investigation on the catalyst regeneration is now in progress.

3. CONCLUSIONS In summary, the dehydrogenative synthesis of esters from primary alcohols under mild reaction conditions was achieved by the use of a ZrO2-supported Cu catalyst. A variety of esters were obtained from primary alcohols as well as diols in good to high yields. Both NH3-TPD and CO2-TPD profiles revealed that the concerted functions of the acid−base pair sites and metallic Cu on the surface of ZrO2 should be responsible for the selective formation of esters. 4. EXPERIMENTAL SECTION 4.1. Materials. Cu(NO3)2·3H2O, Cu(OAc)2, CuCl, CuCl2· 2H2O, and Cu(OH)2 were purchased from Wako Chemicals. Al2O3 (Sumitomo Chemical Co., Ltd, AKP-G015; JRC-ALO-8 equivalent), TiO2 (JRC-TIO-4), ZrO2 (JRC-ZRO-3), CeO2 (JRC-CEO-2), and SiO2 (JRC-SIO-9A) were obtained from the Catalysis Society of Japan. Other organic substrates, alcohols and aldehydes, were of analytical grade and used as received from TCI without further purification. 4.2. Physical and Analytical Measurements. The products of the catalytic Cuns were analyzed by gas chromatography−mass spectrometry (GC−MS) (Shimadzu GCMS-QP2010, CBP-10 capillary column, i.d. 0.25 mm, length 30 m, 50−250 °C) and gas chromatography (Shimadzu GC-2014, CBP-10 capillary column, i.d. 0.25 mm, length 30 m, 50−250 °C). NMR spectra were recorded on a JMN-ECS400 (FT, 400 MHz (1H), 100 MHz (13C)) instCument. Chemical shifts (δ) of 1H and 13C{1H} NMR spectra are referenced to SiMe4. The supported catalysts were analyzed by temperatureprogrammed desorption (TPD), N2 adsorption, and X-ray diffraction (XRD). NH3-TPD and CO2-TPD were carried out to estimate the amount of acid and base on the catalysts. The TPD was performed on a MicrotracBEL BELCAT-II in the following manners: 100 mg of powder was reduced with H2 (50 mL min−1) gas at 400 °C for 1 h. Then, the temperature was kept for 1 h in He (50 mL min−1) gas (CO2-TPD: the sample was heated at the rate of 10 °C min−1 up to 500 °C, and the temperature was kept for 1 h in He gas). The sample was cooled down to 100 °C in He gas. Gaseous NH3/He (5/45 mL min−1) was adsorbed for 1 h and then removed in He gas for 1 h. Consecutively, NH3-TPD was started at 100 °C, and the temperature was raised to 800 °C at a ramping rate of 10 °C min−1 under the He gas flow. The products desorbed were determined by using a BELMass and recorded on an online personal computer. CO2-TPD was also carried out in a similar manner using CO2 gas. X-ray powder diffraction analyses were

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01142. Characterization of supported Cu catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.M.). *E-mail: [email protected]. Tel: +81-42-677-2850. Fax: +81-42-677-2821 (T.S.). ORCID

Hiroki Miura: 0000-0002-2488-4432 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by the Program for Element Strategy Initiative for Catalysts & Batteries (ESICB), Platform for Technology and Industry, a Grant-in-Aid for Young Scientists (B) (No. 26820353), and Grants-in-Aid for Scientific Research (B) (Nos. 26289305 and 17H03459), commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. 6171

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DOI: 10.1021/acsomega.7b01142 ACS Omega 2017, 2, 6167−6173