Research Article pubs.acs.org/journal/ascecg
Acceptorless Dehydrogenative Coupling of Neat Alcohols Using Group VI Sulfide Catalysts Lauren R. McCullough,† David J. Childers,† Rachel A. Watson,† Beata A. Kilos,‡ David G. Barton,‡ Eric Weitz,§ Harold H. Kung,† and Justin M. Notestein*,† †
Department of Chemical & Biological Engineering and §Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Core R&D, The Dow Chemical Company, 1776 Building, Midland, Michigan 48674, United States S Supporting Information *
ABSTRACT: Group VI sulfides were synthesized via coprecipitation of elemental sulfur and metal hexacarbonyl and characterized with XRD, XPS, and TEM. These materials were then demonstrated as active catalysts for the acceptorless dehydrogenative coupling of neat ethanol to ethyl acetate, rapidly reaching equilibrium conversion and up to 90% selectivity. Other primary alcohols form the corresponding esters, while diols formed the corresponding cyclic ethers and oligomers. KEYWORDS: Oxidative dehydrogenation, Ethanol, Aqueous, Esters, Organic acids, Molybdenum sulfide
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INTRODUCTION Esters are important solvents, fine chemicals, and starting materials in the fragrance and pharmaceutical industries. Ethyl acetate is produced in the highest volume and is currently produced from petrochemical feedstocks via Fischer esterification, the Tishchenko reaction, or addition of ethylene to acetic acid.1 A novel synthesis variant is the one-step acceptorless conversion of neat alcohols to esters in the absence of either oxygen donors or hydrogen acceptors. Acceptorless dehydrogenative coupling (ADC) has been employed for ester formation from alcohols, amide formation from alcohols and amines, and lactone formation from diols but has typically employed relatively exotic soluble precious metal catalysts2,3 or oxide-supported Pt,4 Pd,5 or Cu,6−9 often with a complex set of promoters. Developing simple, robust catalysts for this reaction could accelerate the adoption of this reaction at industrially relevant scales and for other synthetic reactions. For example, this reaction would be an advantaged route to the utilization of fermentation-derived ethanol and other alcohols. ADC yields ethyl acetate from two molecules of ethanol in a single reaction with up to 96% mass efficiency; H2 is the only side product. In contrast, the conventional ethyl acetate production methods described above can require two or more steps, isolation of reactive intermediates such as ethylene or acetaldehyde, the addition of external oxidants, or the stoichiometric removal of water. Scheme 1 shows the products of ADC of alcohols, including the ether side products than can arise from reversible, acidcatalyzed dehydration. Research from a single group showed that a sulfated, supported molybdenum catalyst was capable of ADC.10 While the exact structure of the material used in those studies is far from clear, we hypothesized that bulk metal sulfides would be © 2017 American Chemical Society
Scheme 1. Products of Acceptorless Dehydrogenation of Alcohols and the Formation of Ethers As Side Products
good targets for catalysts, as they are inexpensive, readily synthesized in a variety of morphologies, and are known to be robust catalysts used in demanding petrochemical applications.11 These reactions include hydrotreating, which is the cleavage of C-X bonds at high H2 pressures, and can be thought of as the reverse reaction of ADC. Here, it is shown that unsupported, high surface area sulfides are indeed effective catalysts for ADC of a variety of alcohols.
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EXPERIMENTAL SECTION
Materials. All reagents were used as purchased. Nanoparticulate group VI sulfide catalysts were synthesized via coprecipitation,12 as follows. Received: January 30, 2017 Revised: March 26, 2017 Published: April 19, 2017 4890
DOI: 10.1021/acssuschemeng.7b00303 ACS Sustainable Chem. Eng. 2017, 5, 4890−4896
Research Article
ACS Sustainable Chemistry & Engineering
sample with large amounts of stacking.15 The diffractogram of Cr2S3 was largely featureless, indicating an amorphous material. Transmission electron micrographs support the powder XRD results. The MoS2 has mainly single short layers (Figure 2a), while WS2 shows wide stacked layers (Figure 2b), while the Cr2S3 lacks any visible crystallinity.
MoS2 was synthesized by adding 0.5 g of S8 (15.6 mmol S) to 100 mL of decalin under a N2 atmosphere. Decalin and S8 were heated to reflux for 1 h, and the solution turned bright yellow. 2 g (7.6 mmol) of Mo(CO)6 was added, and a dark precipitate immediately formed. The reaction was allowed to progress under N2 for 24 h, then the solution was cooled and centrifuged at 2500 rpm for 1 h, the solvent was removed, and the remaining material was rinsed with cyclohexane. The centrifugation/rinse cycle was repeated 3 times. The resulting brownblack powder was dried under N2 flow for 48 h. WS2 was synthesized analogously from 0.37 g of S8 (11.5 mmol S) and 2 g (5.7 mmol) of W(CO)6. Cr2S3 was synthesized analogously from 0.44 g of S8 (13.6 mmol S) and 2 g (9.1 mmol) of Cr(CO)6. Thiomolybdate clusters [Mo3S13]2− were synthesized via an established procedure.13 4 g of (NH4)6Mo7O24-4H2O was dissolved in 20 mL of deionized water in a filtering flask with light N2 sweep. 120 mL of a 25 wt % ammonium polysulfide solution was added to the flask, and the flask was covered with a watch glass. The solution was left undisturbed with N2 sweep at 96 °C for 5 days. Dark red crystals precipitated, were filtered, and then rinsed with water, ethanol, and hot toluene. The crystals were dried under N2 for 24 h. Characterization. Nitrogen physisorption was carried out on a Micromeritics ASAP 2010 following sample pretreatment at 250 °C and 3 μmHg for 6 h. Surface areas were calculated using the BET method. Powder X-ray diffraction (XRD) was collected on a Rigaku Ultima for all three samples. Transmission electron microscopy (TEM) was performed on a Hitachi H-8100 TEM. Samples were deposited onto holey carbon grids from a dispersion of ethanol. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 250Xi. Catalysis. Ethanol coupling to ethyl acetate over MoS2 was selected as the model reaction for optimization of reaction conditions for application to other substrates. All reactions were run in a sealed Parr 150 mL stainless steel batch reactor with appropriate pressure relief systems and overhead stirring. Liquid products were identified by GC-MS and quantified against authentic samples.
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RESULTS AND DISCUSSION The sulfide nanoparticles synthesized by this coprecipitation method had excellent surface areas of 140 m2/g, 250 m2/g, and 32 m2/g for WS2, MoS2, and Cr2S3, respectively. XRD (Figure 1) shows the MoS2 to be crystalline, raglike MoS214 with features at 2θ = 14°, 33°, 39°, and 60° corresponding to the [002], [100], [103], and [110] planes. The WS2 had a strong feature at 2θ = 14° corresponding to the [002] plane with a broad feature at 2θ = 33°−45° corresponding to the [100] and [110] planes. These features indicate a poorly crystalline
Figure 2. Transmission electron micrographs of coprecipitated MoS2 (a) and WS2 (b) along with Mo 3d and S 2p XPS spectra of MoS2 (c, e) and W 4f and S 2p spectra of WS2 (d, f).
By XPS, the MoS2 sample has the expected Mo 3d5/2 and 3d3/2 peaks at 229.2 and 232.3 eV, respectively (Figure 2c), which are consistent with a Mo4+ oxidation state. Also visible in Figure 2c is the expected S 2s peak at 226.4 eV. The tungsten sample has W 4f7/2 and 4f5/2 peaks 32.8 and 34.9 eV, respectively, indicating a preponderance of W4+. There are two additional peaks at 35.2 and 38.4 eV indicating a minor contribution from W6+ (Figure 2d). S 2p features were observed at 161.5 and 162.7 eV for MoS2 (Figure 2e) and at 162.4 and 163.5 eV for WS2 (Figure 2f), both consistent with handbook values. For the chromium sample, the Cr 2p spectrum contains 2p3/2 peaks at 574.8 and 577.1 eV corresponding to Cr2S3 and Cr2O3, respectively. The S 2p spectrum contains a peak at 168.4 eV, corresponding to oxidized sulfur (Figure S1).16 Due to the presence of oxidized chromium and sulfur, the sample is likely a mixed oxy-sulfide.
Figure 1. Powder X-ray diffraction patterns of coprecipitated MoS2, WS2, and Cr2S3. 4891
DOI: 10.1021/acssuschemeng.7b00303 ACS Sustainable Chem. Eng. 2017, 5, 4890−4896
Research Article
ACS Sustainable Chemistry & Engineering
As indicated by thermodynamic calculations (SI), high temperatures are necessary to obtain high conversion, and correspondingly, the yield of ethyl acetate at 24 h increased sharply with increasing temperature. At 230 °C and in the absence of added H2, the ethanol conversion was 52% and the acetate selectivity was 84% (Figure 4). Here we note that we
In summary, XRD, TEM, and XPS results confirm the identity of MoS2 and WS2, while the chromium sample is an amorphous mixed oxy-sulfide. Ethanol coupling was studied at a variety of conditions, and optimal conditions were then applied to other reactants. Under all conditions tested, ethanol forms liquid-phase products acetaldehyde, 1,1-diethoxyethane, ethyl acetate, and acetic acid (Scheme 2). No significant amounts of diethyl ether were Scheme 2. Observed Products during Reaction of Ethanol over Group VI Sulfide Catalysts
observed. Ethylene, ethane, and H2 were detected in small quantities dissolved in the product solution, but these products were not quantified. The product distribution from ethanol ADC was studied from 170 to 230 °C under He and H2 partial pressures from 0 to 15 bar. At representative conditions of 230 °C, 10 bar of H2 partial pressure, and 5 bar of He partial pressure, conversion and acetate selectivity were initially studied as a function of reaction time (Figure 3). At 2 h, the conversion is less than
Figure 4. Effect of reaction temperature on product distribution and conversion of ethanol to ethyl acetate over MoS2. Conditions: 10 mL of ethanol, 50 mg of MoS2, 24 h, 5 bar He.
refrain from reporting turnover frequencies or otherwise normalizing rates due to the complex nature of the MoS2 surface − consisting of basal planes, edges, and numerous defects − and the lack of knowledge about what constitutes an active site for this reaction. The effect of H2 partial pressure was also examined for ethanol coupling over MoS2 (Figure 5). Increasing H2 partial
Figure 3. Effect of time on approach to equilibrium conversion of ethanol to ethyl acetate. Conditions: 10 mL of ethanol, 50 mg of MoS2, 230 °C, 10 bar H2, 5 bar He.
20%, and the selectivity is evenly divided between acetates and the acetal side product 1,1-diethoxyethane. Between 2 and 20 h, the conversion increases to approximately 40%, reaching the calculated equilibrium conversion [see the SI for calculations]. Over extended reaction times, the yields of all side products become negligible, and the yield of ethyl acetate reaches equilibrium values. The remainder of the discussion uses product distributions after 24 h reaction, as this appears to be adequate time to reach equilibrium for the experimental catalysts. Product distributions were also studied as a function of stirring speed, but little influence was seen (Figure S2). The overhead stirrer was driven at 430 rpm for the remaining discussion. The lack of a dependence on stirring speed indicates that the following observations are free of significant external mass transfer resistances and are instead limited by catalyst kinetics or by thermodynamics.
Figure 5. Effect of H2 partial pressure on yield and conversion of ethanol. Conditions: 10 mL of ethanol, 50 mg of MoS2, 230 °C, 10 bar H2, total pressure constant at 15 bar, 24 h.
pressure had negligible impact on the selectivity but decreased conversion, consistent with thermodynamics. As previously described on supported copper17 and supported palladium18 catalysts, addition of water to the reactant mixture should shift the equilibrium further toward the desired acetate products by enhancing hydrolysis of ethyl acetate to acetic acid and shifting the equilibrium away from dehydration and acetalization side reactions. After 24 h with 50 vol % water (Figure 6), MoS2 was able to produce ethyl acetate and acetic acid at a total yield of 82%, reaching a final composition of approximately 2:1 ethyl acetate to acetic acid. The 1:1 reaction stoichiometry between 4892
DOI: 10.1021/acssuschemeng.7b00303 ACS Sustainable Chem. Eng. 2017, 5, 4890−4896
Research Article
ACS Sustainable Chemistry & Engineering
Figure 6. Changes in conversion and yield with addition of water. Conditions: 10 mL of total reactant volume, 50 mg of MoS2, 230 °C, 5 bar He, 24 h. Dashed lines are added to guide the eye.
acetic acid and ethanol allows molar yields at equilibrium to increase markedly. The MoS2 was characterized before and after reaction to test for structural changes. In spite of the high temperature reducing conditions and the presence of water in some systems, XRD (Figure 7a), XPS (Figure 7b-e), and BET surface areas indicated no significant changes after use. After reaction, the solids were removed by filtration from the product slurry, and ICP-OES showed that the Mo lost to solution was no more than 0.005% of the original sample. Although this is a very small amount, the presence of active soluble species cannot be ruled out based on the ICP-OES results. Ideally, a hot centrifugation would be performed to determine whether active soluble species are present at operating conditions. However, the high pressure and high temperature conditions prevent removal of solids or maintaining removed liquid at operating conditions. As an alternative method of determining whether soluble species could play a role in reactivity, a model for the MoS2 active site was tested for reactivity. The species is a soluble thiomolybdate [Mo3S13]2− cluster previously reported for high activity in the hydrogen evolution reaction.13 The material was synthesized and characterized according to previously published reports (Figure S3). Table 1 compares the performance of group VI sulfides and oxides in ethanol coupling. Among the Mo-based species, coprecipitated MoS2 had the highest conversion and selectivity to acetate products. The soluble [Mo3S13]2− clusters had similarly high selectivity to ethyl acetate but with much lower conversion. Although the reactivity of these clusters is interesting in its own right, their lower intrinsic reactivity and the very small amount of Mo potentially leached into solution from MoS2 make it unlikely that catalysis by soluble species accounts for a significant fraction of the product when MoS2 is the catalyst. However, a conclusive determination will require further investigation, such as operation under continuous flow. Table 1 also shows that commercially available MoS2 and MoO3 carried out some of these reactions but to a significantly lower extent. MoO2 showed poor conversion and selectivity only to the dehydrogenation products, acetaldehyde and 1,1diethoxyethane, as expected. 19 The coprecipitated WS 2 performed almost as well as the MoS2 sample with high conversion and selectivity to ethyl acetate, while the coprecipitated Cr2S3 sample was much less active, which is unsurprising given its lower surface area and poor crystallinity.
Figure 7. Powder XRD and XPS of MoS2 before and after reaction indicating the absence of any significant changes.
Table 1. Yields of Ethanol Reaction Products over Different Catalystsa catalyst
conversion
ethyl acetate
acetic acid
acetaldehyde
acetal
Cr2S3b WS2b MoS2b MoS2c MoO2c MoO3c [Mo3S13]2−
22% 45% 52%d 22% 1% 10% 26%
73% 83% 82% 52%
