Article pubs.acs.org/JPCC
Oxidation, Reduction, and Condensation of Alcohols over (MO3)3 (M = Mo, W) Nanoclusters Zongtang Fang,† Zhenjun Li,‡ Matthew S. Kelley,† Bruce D. Kay,‡ Shenggang Li,† Jamie M. Hennigan,† Roger Rousseau,*,‡ Zdenek Dohnálek,*,‡ and David A. Dixon*,† †
Department of Chemistry, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487, United States Fundamental and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States
‡
S Supporting Information *
ABSTRACT: The reactions of deuterated methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, 2-butanol, and tert-butanol over cyclic (MO3)3 (M = Mo, W) clusters were studied experimentally with temperature-programmed desorption and theoretically with coupled cluster CCSD(T) theory and density functional theory. The reactions of two alcohols per M3O9 cluster are required to provide agreement with experiment for D2O release, dehydrogenation, and dehydration. The reaction begins with the elimination of water by proton transfers and forms an intermediate dialkoxy species that can undergo further reaction. Dehydration proceeds by a β-hydrogen transfer to a terminal MO. Dehydrogenation takes place via an α-hydrogen transfer to an adjacent MoVIO atom or a WVI metal center with redox involved for M = Mo and no redox for M = W. The two channels have comparable activation energies. H/D exchange to produce alcohols can take place after olefin is released or via the dialkoxy species, depending on the alcohol and the cluster. The Lewis acidity of the metal center with WVI being larger than MoVI results in the increased reactivity of W3O9 over Mo3O9 for dehydrogenation and dehydration. However, the product selection of aldehyde or ketone and olefin is determined by the reducibility of the metal center. Our calculations are consistent with the experiment in terms of the dehydrogenation, dehydration, and H/D exchange reactions. The condensation reaction requires a third alcohol with the sacrifice of an alcohol to form a metal hydroalkoxide, a strong gas-phase Brønsted acid. This Brønsted acid-driven reaction is different from the dehydrogenation and dehydration reactions that are governed by the Lewis acidity of the metal center.
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INTRODUCTION Alcohol oxidation and reduction processes using transitionmetal oxides (TMOs) are widely studied, as their available surface acidic and basic sites make them good catalysts. For example, on V2O5, dehydration was reported to be the dominant path over dehydrogenation for 2-propanol.1 Zaki studied ethanol dehydration on Fe2O3, Mn2O3 powders, and their mixtures with Al2O3 and SiO2 in a flow system with a gas chromatograph.2 Dehydration was observed between 200 to 500 °C, and the production of ethylene was found to be dependent on the surface Brønsted acidities. A similar dehydration mechanism was reported on Al2O3 supported on noble catalysts such as Pt.3 Temperature programed desorption (TPD) was used to study alcohol reactions on γ-Al2O3 and revealed that the Lewis acid sites are also active for dehydration; computational studies were in agreement with the experiments.4−6 Alcohol conversion on Al2O3, SiO2, ZnO, and CdO were studied theoretically with the PM3 semiempirical molecular orbital method.7 These workers predicted the generation of alkoxy groups after the adsorption of alcohols. Dehydration is through an alcohol chemisorbed on a Brønsted acid site (a surface OH group). Dehydrogenation takes place © 2014 American Chemical Society
on a basic site through an intermediate with a surface metal ion bound to an alkoxy group and a surface oxygen atom bonded to a proton transferred from the OH group of an alcohol. Dehydration is dominant on Al2O3 and SiO2 surfaces, and dehydrogenation is dominant on ZnO and CdO surfaces. Alcohol conversion reactions with group VIB transitionmetal oxide surfaces were reported on supported molybdenum oxide8 and tungsten oxide catalysts9−12 as well as a WO3 film.13 The dehydrogenation reaction was reported to be 70%−89% of the conversion of ethanol on a MoOx/TiO2 catalyst.8 The acetaldehyde production rate increases with an increase in the MoOx surface densities, which was associated with easier electron delocalization. An activated carbon-supported tungsten oxide catalyst showed catalytic activity for methanol and ethanol conversion. With the TPD method, dimethyl ether was the only product for the reaction of methanol on the catalyst. In contrast, all three reactions, dehydrogenation, dehydration, and condensation, took place with ethanol. The production of Received: July 18, 2014 Revised: September 5, 2014 Published: September 8, 2014 22620
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tionally. The condensation reactions to form an ether with three alcohols are also discussed.
ethylene correlated with the total surface acidities. Iglesia and co-workers 10−12 studied alcohol conversions over WO x domains supported on different metal oxides, such as ZrO2, Al2O3, SiO2, and SnO2. They found that dehydration of the alcohol was not only dependent on the Brønsted acid site densities but also on the interaction between the WOx domains and the support. Brønsted acidity has been also reported to be an important component in catalytic dehydration of alcohols by tungsten polyoxometalate clusters,14,15 where the key intermediate is found to be a W-bound alkoxy species similar to that proposed on WOx films.11 Recent studies on high surface area catalysts have also considered the role played by terminal metal (M = Mo, W) oxo groups, MO, which are common on both SiO2-16 and TiO2-supported MOx nanoparticles.17 We reported alcohol dehydration on WO3 films with both WVIO and O WVIO sites13 and found that the dehydration is active on the OWO site while not on the WO site. In terms of alcohol oxidation and reduction reactions on MO3 (M = Mo, W) clusters, Goncharov and co-workers studied the interactions between small alcohol molecules and MoO3 cationic nanoclusters generated by electron impact in the gas phase.18,19 Dehydrogenation occurred by the insertion of a metal into a C−H bond with the formation of two new bonds, Mo+−H and Mo+−O. The cleavage of a C−O bond in the alcohol led to the formation of the olefin by metal insertion. Our previous studies on the reactions of C1−C4 alcohols over TiO2(110)-supported WO3 clusters showed that both the dehydration and the dehydrogenation reaction channels are active for methanol and ethanol.20,21 Small amounts of dehydrogenation products are detected for the other alcohols, and the condensation ether formation reaction is only observed for methanol. The reactivity is associated with two WO groups on WO 3 /TiO 2 (110). FeO(111)-supported WO3 clusters do not show good catalytic activity for the reduction and oxidation reactions of 2-propanol due to the existence of only one WVIO group in the system.22 Current research is focused on developing structure− reactivity relationships, and the reaction mechanism between alcohols and catalysts is not fully understood at the molecular level. In this context, monodispersed (MO3)3 (M = Mo, W) clusters represent a nearly ideal platform for model catalytic studies that allow for a direct comparison of the activity of dioxo OWO and OMoO species. We recently studied ethanol conversion on cyclic (MO3)3 (M = Mo, W) clusters in an initially frozen ethanol matrix on an inert substrate, a graphene monolayer on Pt(111), with both experimental and computational methods.23 The observed reactions require the addition of two alcohol molecules per cluster. Water is eliminated first with formation of a dialkoxy intermediate for further dehydrogenation and dehydration reactions. Dehydrogenation is found to be dominant on (MoO3)3 in contrast with dehydration, which dominates on (WO3)3. Both channels have comparable reaction barriers. The overall conversion of the alcohol is governed by the Lewis acidity of the metal center, and product selectivities, as determined by the relative weights of dehydrogenation and dehydration, are governed by the reducibility of the metal center. In the current work, we extend the alcohol species to three primary alcohols, methanol, n-propanol, n-butanol; two secondary alcohols, 2-propanol and 2-butanol; and a tertiary alcohol, tert-butanol. The mechanisms for dehydrogenation and dehydration with one and two alcohols were studied computa-
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EXPERIMENTAL AND COMPUTATIONAL METHODS The experiments were performed in an ultrahigh vacuum (UHV) molecular beam scattering chamber (∼1 × 10−10 Torr), as described previously.20 A polished Pt(111) single crystal (disk 10 mm in diameter, 1 mm thick, Princeton Scientific) was mounted on a manipulator cooled by a closed cycle helium cryostat and resistively heated via spot-welded Ta leads. The temperature was measured using a K-type thermocouple spotwelded to the rear side of the Pt(111) sample. The sample was cleaned using standard procedures including a sequence of neon ion bombardment at 300 K, O2 annealing at 1000 K (5 min, 2 × 10−7 Torr), and annealing in UHV at 1200 K. A graphene monolayer was prepared by exposing Pt(111) to decane at high temperatures and its quality confirmed by lowenergy electron diffraction (LEED) and Kr temperatureprogrammed desorption (TPD).23,24 All alcohols employed in this study (methanol-OD, ethanol-OD, 1- and 2-propanol-OD, 1- and tert-butanol-OD and 2-butanol-OH) were transferred into flasks with baked molecular sieves to remove water and purified using several pump−freeze−thaw cycles. Cluster Preparation. A high-temperature effusion cell (CreaTec) was used to sublimate WO3 (99.95% Aldrich) and MoO3 (99%, Alfa) and to prepare (WO3)3 and (MoO3)3 gasphase clusters, respectively. The flux of generated gas-phase clusters was stabilized at ∼1 × 1013 MO3/cm2/s (evaporation temperature of ∼1390 and ∼745 K for WO3 and MoO3, respectively) and monitored using a quartz crystal microbalance (Inficon). The cluster size distribution was determined using infrared reflection absorption spectroscopy (IRAS) in krypton matrix isolation experiments.23,25 The W−O−W and Mo−O− Mo stretching modes were used to determine the cluster size distributions. The analysis yielded 61%, 17%, 15%, and 7% for cyclic (MoO3)3, (MoO3)4, (MoO3)5, and (MoO3)6, respectively, and 89%, 6%, and 5% for (WO3)3, (WO3)4, and (WO3)5, respectively.23 The analysis assumed that all of the clusters have the same infrared oscillator strengths. The deposited MO3 was removed from Pt(111) by repeated Ne+ sputtering. TPD Spectra Deconvolution and Product Yields Determination. TPD experiments were performed using a quadrupole mass spectrometer (UTI) at a constant ramp rate of 2 K/s. A range of mass fragments was collected for each reagent to allow for a detailed analysis of the desorption products. A generalized procedure involved the subtraction of the reagent contribution to the corresponding mass fragments of the products using the cracking pattern determined from the reagent multilayer desorption region at low temperatures (130−170 K). Subsequently, mass spectra and absolute desorption rates of the main products (D2O, H2O, alcoholsOH, alkenes, aldehydes/ketones) were determined by dosing known amounts of these molecules with a flux-calibrated molecular beam and measuring their TPD spectra.26 The resulting measured reference mass spectra (cracking patterns) of the dosed molecules were further utilized to subtract the contribution of D2O to the H2O signal at 18 amu (∼30%) and the contribution of alcohols-OH to aldehydes/ketones and to alkenes when required. Specifically, the exact procedure used for each employed reagent is given in the caption of Figure 1 (for 1-propanol-OD) and in the Supporting Information (SI) 22621
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and Xeon-based Dense Memory Cluster (DMC) and Itanium 2-based SGI Altix systems at the Alabama Supercomputer Center, and the Opteron-based HP Linux cluster at the Molecular Science Computing Facility at Pacific Northwest National Laboratory. Molecular visualization was done using the AGUI graphics program from the AMPAC program package.49
(Figures S1−S6) for the remaining alcohols. Additional details are provided for ethanol-OD in our previous study.23
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RESULTS AND DISCUSSION TPD Spectra. As described in the experimental section and in our prior publications,23,25 the majority of the clusters that are sublimated from our cluster sources are cyclic (MO3)3. To determine their reactivity toward alcohols, we codeposited the clusters with an excess of the alcohol reagent (alcohol to MO3 ratio is ∼10 to 1) on the graphene/Pt(111) substrate at 25 K. As shown by our prior studies, bare graphene/Pt(111) substrate is unreactive toward alcohols.23 After the formation of this reactive matrix, we linearly ramp the substrate temperature and use mass spectrometry to follow the desorption of unreacted alcohol and reaction products in a TPD experiment. OD-labeled alcohols were used when possible (all except 2-butanol) to distinguish whether the hydrogen in the products originates from O−D and/or C−H bond cleavage. As an example, a set of all species (reactant and products) desorbing from (WO3)3 (black traces) and (MoO3)3 (red) clusters codeposited with 1-propanol-OD are shown in the TPD spectra displayed in Figure 1. Similar complete sets of spectra for the other alcohols employed in this study are shown in Figures S1−S6 in the Supporting Information. The desorption rates of all individual products were deconvolved from the mass fragments and quantified (except dipropyl ether) as described in the experimental section. The TPD spectra of unreacted 1-propanol-OD multilayers are shown in Figure 1A. The vast majority of the molecules are observed to desorb between 150 and 200 K with a broad tail extending all the way to 275 K. Since binding of the alcohol molecules in the multilayers is primarily determined by the interactions among their hydroxyl groups, the desorption temperature of unreacted alcohols increases only slightly with their increasing size,26 and as such, their spectra look very similar (see Figures S1−S6 in the SI). The first product, D2O, from 1-propanol-OD is observed on both (WO3)3 and (MoO3)3 between 170 and 300 K (Figure 1B). As discussed previously,1,3,21,23 water formation is a result of the deprotonation of alcohol molecules on the dioxo, O MVIO, moiety of the cluster that can be summarized by the following reaction: 2ROD + OMVIO → (RO)2MVI + D2O. As such, the amount of observed D2O directly corresponds to the amount of 1-propanol-OD that subsequently undergoes further conversion to products and therefore can be used as a measure of the total amount of converted alcohol molecules. The broad TPD H2O feature that appears in the same temperature region (Figure 1B) is an artifact of the subtraction of a large 1-propanol-OD fragment at m/z = 18 amu. As the temperature is increased above 300 K, additional products are seen in the TPD spectra on both (WO3)3 and (MoO3)3. Both H2O (Figure 1B) and 1-propanol-OH (Figure 1C) are observed between 350 and 475 K, indicating C−H bond cleavage and subsequent recombination of liberated H with another OH and/or OR, respectively. As expected, since the H2O formation is a second-order process (cleavage of two C−H bonds) and ROH is a first-order process (cleavage of one
Figure 1. TPD spectra of (A) CH3(CH2)2OD (32 amu), (B) D2O (20 amu) and H2O (amu), and (C) CH3(CH2)2OH (31 amu), CH 3 CH 2 CH 2 (41 amu), CH 3 CH 2 CHO (43 amu), and (CH3CH2CH2)2O (73 amu) as a function of linearly increasing temperature (2 K/s) following coadsorption of 2.0 × 1015 WO3/cm−2 (black) and MoO3/cm−2 (red) with 20 × 1015 of CH3(CH2)2OD on graphene/Pt(111) at 25 K. The contributions of 1-propanol fragments to 18, 20, 31, 41, and 43 amu fragments were subtracted using a fragmentation pattern determined from 1-propanol multilayer desorption. The contribution of mass 18 amu to mass 20 amu was also subtracted using D2O fragmentation pattern.
Computational Methods. We chose to use the B3LYP27,28 exchange−correlation functional for the geometry optimizations at the density functional theory (DFT) level on the basis of extensive studies of the structures and properties of group VIB transition-metal oxide clusters29 and the hydrolysis of transition-metal oxide clusters.30−33 Vibrational frequencies were calculated to characterize the structures as minima or transition states and to obtain the zero-point energy corrections (ZPEs) as well as the thermal corrections at 298 K, which were derived from normal statistical mechanical expressions.34 The transition states, which are characterized by a single imaginary frequency, were optimized by the synchronous transit-guided quasi-Newton (STQN) method.35,36 The aug-cc-pVDZ basis sets were used for H, C, and O,37 and the aug-cc-pVDZ-pp basis sets based on relativistic effective core potentials (RECPs)38−40 were used for Mo and W. This combination of basis sets will be denoted as aD. All of the DFT calculations were performed with the Gaussian 09 program package.41 The B3LYP/aD geometries were further used in single-point calculations at the DFT level with the M06 exchange− correlation functional42 and at the correlated molecular orbital theory coupled cluster CCSD(T)43−46 (coupled cluster with single and double excitations with an approximate triples correction) level with the same basis sets. The CCSD(T) calculations were carried out with MOLPRO 201047 and NWChem.48 The calculations were performed on the local Xeon- and Opteron-based Penguin Computing clusters, the Xeon-based Dell Linux cluster at the University of Alabama, the Opeteron22622
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Figure 2. TPD spectra of carbon-containing products formed as a result of (A) recombination (alcohols), (B) dehydration (alkenes), (C) dehydrogenation (aldehydes/ketones), and (D) condensation (ethers) of alkoxy species on (WO3)3 (black traces) and (MoO3)3 (red traces). All spectra were obtained following the coadsorption of 20 × 1015 alcohols/cm−2 with 2.0 × 1015 WO3/cm−2 (black) and MoO3/cm−2 (red) on graphene/Pt(111) at 25 K. The spectra deconvolution from the measured mass fragments are described for all the molecules in details in Figures 1 and S1−S6 (SI). The dashed lines indicate the onset temperatures of ∼450 and ∼650 K determined previously for (WO3)3 (black) and (MoO3)3 reduction.
Figure 3. Quantified yields of D2O (measure of the total alcohol-OD conversion) and major carbon-containing products (alcohols, alkenes, aldehydes/ketones) following coadsorption of 20 × 1015 alcohol/cm−2 with (A) 2.0 × 1015 WO3/cm−2 and (B) MoO3/cm−2 on graphene/Pt(111) at 25 K. The “D2O” peak for 2-BuOH was obtained by integrating the low-temperature H2O signal.
Figure 4. Desorption temperature maxima, Tmax, for alcohols, alkenes, and aldehydes/ketones observed in TPD spectra (Figures 1 and S1−S6, SI) following coadsorption of 20 × 1015 alcohol/cm−2 with (A) 2.0 × 1015 WO3/cm−2 and (B) MoO3/cm−2 on graphene/Pt(111) at 25 K. The dashed line in B indicates the onset temperature of 450 K for MoO3 reduction.
C−H) with respect to C−H cleavage generated H, the ROH desorption temperatures are somewhat lower as compared to that of H2O. In addition to the formation of 1-propanol-OH, Figure 1C reveals other reaction channels: the formation of propene via
dehydration [for both (WO3)3 and (MoO3)3], the formation of propanal via dehydrogenation [(WO3)3 and (MoO3)3], and the formation of di-n-propyl ether via condensation [only for (WO3)3]. The temperature-dependent desorption profiles for all of the carbon-containing products are very similar, 22623
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+ red + blue bars). Figure 3 shows that generally a good agreement is obtained between the D2O yields and carboncontaining product yields. The only significant discrepancy is observed for 2-PrOD on WO3, most likely as a result of some kind of unaccounted change in the mass spectrometer sensitivity. The yield of ethers is neglected in this comparison, as they represent only a small minority channel. For small primary alcohols on WO3, the overall conversion is ∼1.2 alcohols/WO3 and decreases somewhat for secondary and tertiary alcohols, reaching ∼0.8 for tert-butanol. On MoO3, the conversion drops to ∼0.6 for small primary alcohols and to ∼0.3−0.4 for secondary alcohols and tert-butanol. Thus, on average, the conversion on MoO3 is approximately one-half of that on WO3. The observed alcohol-dependent trends of product yields further reveal that the dehydration channel (blue bars) yielding aldehydes/ketones diminishes with increasing chain length significantly faster on WO3 than on MoO3. This can be further illustrated if the product yields are normalized relative to their sum, as shown in Figure 5. Additionally, it can be seen that
suggesting that the energetics of the rate-limiting steps of all channels are very similar and/or coupled. The products formed on (MoO3)3 desorb at somewhat lower temperatures (peak temperatures between 400 and 410 K) than those from (WO3)3 (peak temperatures of ∼440 K). To facilitate the comparison of the trends in the product formation from different alcohol molecules on both (WO3)3 and (MoO3)3, we plot the TPD spectra of all the carboncontaining products in Figure 2 (black and red traces, respectively). The onset temperatures of ∼450 and ∼650 K determined previously for (WO3)3 and (MoO3)3 reduction are shown.23,50 Detailed analysis and quantification of the spectra give (see experimental section for details) the product yields (Figure 3) and the trends in the maximum desorption temperatures, Tmax (Figure 4). It should be noted that at the temperatures where the carbon-containing products are observed, the excess alcohol forming the matrix has desorbed, leaving behind only RO-functionalized metal centers (e.g., RO−M−OR and RO−MO) on the clusters that are in contact. This certainly complicates direct comparison between the theory and experiment as the exact structure of the clusters can be changing and is less defined than the original (WO3)3 and (MoO3)3 clusters. Nonetheless, our prior studies have shown that the retention of the local structure of the active site (e.g., dioxo vs monooxo) is sufficient to model its activity.13,25 Initial inspection of Figures 2 and 4 shows that the desorption temperatures of all observed carbon-containing products decrease with increasing chain length (e.g., from methanol to 1-butanol) and the number of methyl groups at the primary carbon (e.g., from 1-butanol to 2- and tertbutanol). This effect has been attributed in our previous studies to the increasing inductive effect of the alkyl chains.20,51,52 Interestingly, the desorption temperature maxima of different products from the same alcohol (e.g., 1-propanol-OH, propene, and propanal from 1-propanol-OD) as well as their line shapes are generally similar. This can be easily seen in Figure 4. Figure 4 further demonstrates that on (MoO3)3-based catalysts, the products are systematically observed at lower temperatures than on (WO3)3. While the difference is more significant for small primary alcohols (i.e., ΔT ∼ 75−100 K for methanol and ΔT ∼ 30−40 K for ethanol), it becomes negligible for secondary and tertiary alcohols (i.e., ΔT ∼ 12 K for 2-butanol and ΔT ∼ 7 K for tert-butanol). As the present work focuses on understanding the reactivity of OMVIO species, the thermally induced reduction of both WO3 and MoO3 has been considered. As shown previously by X-ray photoelectron spectroscopy (XPS),50 WO3 is not being reduced until ∼650 K (black dashed lines in Figure 2), well above the formation temperatures of all observed products. The situation is somewhat different for MoO3, where the onset of reduction (450 K, red dashed lines in Figures 2 and 4)23 overlaps with the temperature range where products from methanol and ethanol are formed. Nonetheless, since the onset temperatures of product formation from methanol and ethanol are ∼100 K lower than the onset of MoO3 reduction, we believe that they are also representative of the reactivity of OMoVIO species. The comparison of the overall alcohol conversion and relative yields of different reaction products for all alcohols is shown in Figure 3. As already discussed, the D2O yields (gray bars) directly represent the total conversion yield on a per MO3 basis. The obtained D2O yield should be therefore identical with the sum of yields of all carbon-containing products (green
Figure 5. Relative product fractions of major carbon-containing products (alcohols, alkenes, aldehydes/ketones) following coadsorption of 20 × 1015 alcohol/cm−2 with (A) 2.0 × 1015 WO3/cm−2 and (B) MoO3/cm−2 on graphene/Pt(111) at 25 K.
dehydration is the major channel for all alcohols (except methanol) on WO3. In contrast, on MoO3, all channels (dehydration, dehydrogenation, and recombination) remain competitive for all primary alcohols. For secondary and tertiary alcohols, dehydration is dominant on both WO3 and MoO3. Potential Energy Surfaces. The calculated potential energy surfaces (PESs) for the reactions of M3O9 (M = Mo, W) clusters with two n-propanol molecules and three npropanol alcohols molecules (condensation) are shown in Figures 6−12 at the CCSD(T)/B3LYP level with the aD basis set. The PESs for methanol, ethanol, and 2-propanol, with energies calculated at the DFT/B3LYP, DFT/M06, and CCSD(T) levels are shown in Figures S7−S42 in the Supporting Information. The PESs for the butanols were calculated at the DFT level with the B3LYP and M06 functionals with selected values obtained at the CCSD(T) level. As the DFT/B3LYP calculations give too low initial binding energies for the initial Lewis acid/base or hydrogenbonding interactions,23 the final energies were predicted at the coupled cluster CCSD(T) level. The effects of entropy on the reaction energetics and barriers are small when they are handled in terms of the experimental conditions where the formation of the alkoxy metal oxide complexes occurs in the frozen alcohol matrix at low temperature, as discussed in the Supporting Information. For single alcohol reactions, the barrier to transfer a hydrogen from the OH group of an alcohol to a terminal MO oxygen is above the energy needed 22624
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for ethanol,23 a second alcohol can be either added to the metal via another Lewis acid/base interaction or hydrogen bonded to the other propanol. The physisorption process for two Lewis acid/base additions is −52 kcal/mol exothermic for W and −35 kcal/mol exothermic for Mo relative to the reactive asymptote. The release of D2O has the same reaction mechanism as with ethanol. The first proton transfers from the RO−H(D) to a terminal MO, leading to the formation of a stable metal hydroalkoxide with the second alcohol still attached to the metal. The barrier to transfer a proton to MO is 22 kcal/mol for W and 20 kcal/mol for Mo. This first energy barrier for proton transfer is 29 kcal/mol below the reactant asymptote for W and 15 kcal/mol below for Mo. The energy to form the alkoxyhydroxy intermediate is equivalent to chemisorption of an alcohol and is only slightly more favorable than physisorption. The second proton transfer is from the alcohol to the OH/D group, not to the MO. The barrier for the second proton transfer is smaller than the first proton transfer, 17 kcal/mol for W and 14 kcal/mol for Mo, with both of these barriers lying lower than the initial proton transfer barrier relative to the reactant asymptote. The energy needed to release D2O is 4 and 8 kcal/mol for M = W and Mo, respectively. Proton transfer from the higher energy hydrogenbonded initial complex (Figure S8, SI) has comparable barriers of 19 kcal/mol for W and 23 kcal/mol for Mo and generates a more stable D2O complex intermediate that needs 25 kcal/mol for W and 20 kcal/mol for Mo to release D2O. The final product in either path after D2O release is a dialkoxy intermediate species (B) with two alkoxy groups bound to the active MVI metal center and is energetically significantly below the reactant asymptotic limit. No metal redox is needed to generate intermediate B. Dehydration. After D2O loss, the intermediate B can generate an olefin by breaking a β-C−H bond and forming a CC bond simultaneously, as shown in Figure 7. Consistent with the ethanol dehydration reaction pathway,23 n-propanol follows the same reaction path. This step is a β-hydrogen transfer to the remaining MO oxygen on the metal instead of the OH group, as predicted for the single alcohol dehydration reactions (see the SI). C3H6 is released by breaking a hydrogen bond to an unsaturated hydrocarbon. The final product is identical with the species of the dissociation of a single npropanol on clusters that can further desorb the alcohol (see below) and regenerate the TMO cluster. Again, no redox is taking place for this step. There is no difference in the reaction mechanism for this process on the W3O9 and Mo3O9 clusters. The β-hydrogen transfer barrier for n-propanol is 35 kcal/ mol for M = W, essentially the same value as found for ethanol. Due to the deep initial wells for W, the transition states for npropanol and 2-propanol are still 1.5 and 3.5 kcal/mol, respectively, under the reaction asymptotic limit (Table 2). The dative bond through a hydrogen atom in propene to the nproproxy species is 11 kcal/mol, slightly stronger than the hydrogen bond in the ethylene complex of 8 kcal/mol. The final product is still more than 10 kcal/mol under the reaction asymptotic limit. We note that the reaction to regenerate the catalyst is endothermic [CH3(CH2)2OH → CH3CHCH2 + H2O, ΔH298K = 8.3 kcal/mol].53 For M = Mo, the barrier energy of proton transfer for n-propanol is 35 kcal/mol, close to that for ethanol and comparable to the barriers on the W3O9 cluster. The energy to release C3H6 is 9 kcal/mol, slightly larger than the C2H4 release energy of 6 kcal/mol for the ethanol reaction. The hydrogen bonds in the different olefin complexes
Figure 6. CCSD(T) potential energy surface in kcal mol−1 for D2O elimination generating precursor B from the reaction of 2CH3(CH2)2OD + (MO3)3. Energies in black for W and in red for Mo. Atoms: metal = blue, O = red, C = gray, and H = white.
Figure 7. CCSD(T) potential energy surface in kcal mol−1 for the formation of propene from the precursor state B from the reaction of 2CH3(CH2)2OD + (MO3)3. See Figure 6 legend.
to remove the alcohol for M = Mo (reactant asymptote) and very close to the asymptote for M = W, so reactions with a single alcohol will not be observed for Mo and may not be with W (Table 1). In the experimental conditions with excess alcohol present, the cluster will be able to adsorb a second alcohol and lower the energetics of the initial complex so that the proton transfer barrier is below the reactant asymptote. Before the desorption of both alcohol molecules from the cluster, two labeled alcohol molecules (R−OD) can lose D2O via D transfer with an energy below the reactant asymptote. The details of the reactions with a single alcohol are given in the SI. D2O Elimination. The potential energy surface for D2O loss with two alcohols is shown in Figure 6 for n-propanol (chosen for direct comparison with the experimental data given in the text). We note that the TMO trimers have the metal nominally in the formal +VI oxidation state. The reactions between M3O9 and the alcohols start with an exothermic addition (physisorption) of an alcohol to a metal center with a Lewis acid−base donor−acceptor bond. Consistent with the results 22625
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Figure 8. CCSD(T) potential energy surface in kcal mol−1 for the formation of ethylaldehyde from the precursor state B from the reaction of 2CH3(CH2)2OD + (MO3)3. See Figure 6 legend.
Figure 9. CCSD(T) potential energy surface in kcal mol−1 for H/D exchange to produce alcohols via intermediates A and B from the reaction of 2CH3(CH2)2OD + (MO3)3.
Figure 10. Lowest maximum barriers in kcal/mol for the desorption of alcohols (H/D exchange), alkenes, and aldehydes/ketones calculated at the CCSD(T)/aD//B3LYP/aD level.
comparable energetics for WVI and MoVI. After the release of C3H6, the species of metal hydroalkoxide (A) can either desorb CH3CH2CH2OH (H/D exchange) or release the second olefin by β-hydrogen transfer to the OH group, which is the single
are slightly stronger for M = W than for M = Mo, respectively. Again this reaction step is completely driven by Lewis and Brønsted acid/base chemistry involving hydrogen (proton) transfers from the β-carbon to an terminal MO oxygen with 22626
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asymptotic limit and is above the final product generated by the higher barrier process. Dehydrogenation of n-propanol on a Mo3O9 cluster follows that of ethanol, which is a different reaction mechanism from that on a W3O9 cluster. CH3CH2CHO is produced by an αhydrogen transfer to an adjacent terminal MoO with an energy barrier of 38 kcal/mol. CH3CH2CHO forms a Lewis acid−base donor−acceptor bond with the cluster, and the release of CH3CH2CHO requires 21 kcal/mol. The final metal product has two MoV on the cluster, so a redox process has occurred. The lowest barrier pathway to generate CH3CH2CHO, an α-H transfer to a bridge Mo−O, connects a much higher energy complex. A redox process is also involved as the active metal center becomes MoIV in the product. The reaction barrier and CH3CH2CHO release energy are 31 and 28 kcal/mol, respectively. H/D Exchange To Produce CH3(CH2)2OH. The isotope exchange product CH3(CH2)2OH can be generated from the metal hydroalkoxy species A after the release of propene, as shown in Figure 9. This step is a proton transfer from a M−OH group to an alkoxy group, the reverse of the initial proton transfer step from the Lewis acid/base single alcohol adduct, with the formation of a Lewis acid−base complex with the only difference being the presence of an OH instead of an OD. After CH3(CH2)2OH is desorbed, the TMO cluster is regenerated. The barrier energies for proton transfer are predicted to be 24 and 16 kcal/mol for M = W and Mo, respectively. As these barrier energies (Table 3) are much smaller than that for a βhydrogen transfer to an MO through the dialkoxy species, the rate-limiting step is the earlier dehydration step, so the energy for H/D exchange on an alcohol should be the same as for dehydration. The CH3(CH2)2OH desorption energies from (WO3)3 and (MoO3)3 clusters are 26 and 20 kcal/mol, respectively. Isotopically exchanged CH3(CH2)2OH cannot be readily released from the final product formed in the dehydrogenation step shown in Figure 8 as this is an endothermic process requiring the loss of a MVI−O bond. An alternative pathway of H/D exchange reaction can take place from the dialkoxy species B shown in Figure 9. This exchange occurs via β-hydrogen transfer from one alkoxy group to the other −O(CH2)2CH3 group, leading to the formation of CH3(CH2)2OH and CH3CHCH2 simultaneously with a hydrogen bond between CH3CHCH2 and CH3(CH2)2OH and a Lewis acid−base donor−acceptor bond between CH3(CH2)2OH and metal oxide cluster. CH3CHCH2 and CH 3 (CH 2 ) 2 OH can be desorbed stepwise, leading to regeneration of the TMO cluster. The barrier energies are predicted to be 37 and 35 kcal/mol for M = W and Mo, respectively, which are comparable to the dehydration barriers. The hydrogen-bond energies are 10 and 9 kcal/mol for M = W and Mo. An α-hydrogen transfer from one alkoxy group to the other is possible, leading to the production of an isotopically exchanged product CH3(CH2)2OH (Figure S24, SI). However, this connects to a much higher energy product and generates M3O8, so it is not likely to occur. Thermodynamics of Alcohol Physisorption and Chemisorption. The physisorption and chemisorption energies of the alcohols on the M3O9 clusters as well as the barriers for the dissociation of the alcohol at 298 K at the CCSD(T)/aD//B3LYP/aD level are summarized in Table 1. The DFT/B3LYP and DFT/M06 results are shown in the Supporting Information, and the comparison of the DFT with the CCSD(T) results is discussed there as well. For M = W, the
Figure 11. CCSD(T) potential energy surface in kcal mol−1 for ether formation from the precursor state B (pathway I) from the reaction of 3CH3(CH2)2OH + (MO3)3. See Figure 6 legend.
Figure 12. CCSD(T) potential energy surface in kcal mol−1 for the formation of ether from the precursor state C (pathway II) from the reaction of 3CH3(CH2)OH + (MO3)3. The following step is identical with that in Figure 11. See Figure 6 legend.
alcohol reaction described in the Supporting Information (Figure S46). This endothermic step leads to regeneration of the metal oxide cluster. Dehydrogenation. The dehydrogenation reaction of npropanol on a W3O9 cluster exhibits the same pathway as ethanol. The reaction CH3(CH2)2OH → CH3CH2CHO + H2 is endothermic by 16.1 kcal/mol. CH3CH2CHO release is via an α-hydrogen transfer to the metal bound to the OR group. This reaction has an energy barrier of 35 kcal/mol, comparable to the dehydration step. The CH3CH2CHO release energy is predicted to be 29 kcal/mol and is 10 kcal/mol above the reactant asymptotic limit. There is a second path with a 2 kcal/ mol lower energy barrier, corresponding to an α-hydrogen transfer to an adjacent metal with the formation of a metal− hydride bond. The CH3CH2CHO that is generated also forms a Lewis acid−base adduct with the metal bound to the OR group. In this step, a bridge W−O bond on the adjacent metal is broken and a WO bond on the active metal is formed, so no redox takes place. The final product, W3O8−H− (OC2H5), however, is 13 kcal/mol above the reactant 22627
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Table 1. Calculated Physisorption and Dissociative Chemisorption Enthalpies (ΔHad,298K, kcal/mol) (two alcohols) and Reaction Barriers of the Dissociative Chemisorption of the Alcohols (one and two alcohols) from the Reactant Complexes (ΔH‡298K, kcal/mol) at the CCSD(T)/aD//B3LYP/aD Level ΔHad,298K physisorption cluster
alcohol
1st/ROH addition
2nd/ROH addition
1st H transfer 2 ROH
1st H transfer 1 ROH
1st H+ transfer 2 ROH
W3O9
methanol ethanol 1-propanol 1-butanol 2-propanol 2-butanol tert-butanol methanol ethanol 1-propanol 1-butanol 2-propanol 2-butanol tert-butanol
−23.5 −25.3 −25.6 −24.8 −26.9 −25.9 −26.9 −18.1 −19.6 −19.5 −18.4 −20.6 −19.4 −20.5
−21.9 −21.1 −26.0 −27.9 −25.2 −27.1 −23.9 −18.0 −19.6 −15.5 −23.7 −20.8 −22.6 −20.3
−51.6 −55.7 −56.0 −56.3 −56.8 −57.5 −57.7 −32.8 −36.0 −35.9 −36.4 −36.5 −37.4 −37.8
24.3 25.4 24.8 23.6 25.3 25.1 25.8 27.8 27.8 27.0 26.6 27.7 28.0 29.9
20.6 18.2 22.1 22.5 21.6 21.2 21.3 24.8 24.9 19.5 26.0 24.9 25.0 26.9
Mo3O9
+
dehydration cluster
alcohol
W3O9
methanola,b ethanola,b 1-propanola,b 1-butanola,b 2-propanola,b 2-butanola,b tert-butanola,b methanola,b ethanola,b 1-propanola,b 1-butanola,b 2-propanola,b 2-butanola,b tert-butanola,b
Mo3O9
2ROH after D2O elim c
34.3/37.2 34.9/37.6 34.1/37.0 33.4/34.4 32.0/32.1 32.7/29.5 c
35.6/35.7 35.0/38.2 34.5/38.1 32.8/35.1 30.7/31.3 30.5/28.4
dehydrogenation 2ROH after D2O elim
total after D2O elim
36.7/38.9 33.5/35.0 34.9/36.9 32.5/34.2 33.0/34.2 30.5/32.9
47.2/47.5 43.0/41.0 46.5/43.9 43.8/41.5 42.4/38.5 41.2/36.8
d
d
37.5/40.4 36.6/36.8 37.5/40.6 35.8/38.7 34.5/39.1 32.8/36.9 d
CCSD(T). M06 after slash (/). No β-hydrogen present. dNo αhydrogen present. b
+
ethanol < tert-butanol < 2-propanol, but we note that these differences are small, so the order could easily change for higher accuracy calculations. The physisorption energies are less exothermic than for M = W. Again the second alcohol adsorption enthalpies are also less exothermic than that for M = W, with energies from −16 to −23 kcal/mol. The second alcohol physisorption energies on the metal hydroalkoxides are slightly more exothermic than the other two adsorptions on the metal oxide cluster. The dissociative chemisorption energies are predicted to be −21 to −26 kcal/mol for the single alcohol reactions for M = W. These energies are comparable to the exothermicities of the physisorption enthalpies. In the presence of a second alcohol, the alcohol dissociative chemisorption is very exothermic, −52 to −58 kcal/mol, in the order methanol < ethanol < 1-propanol < 1-butanol < 2-propanol < 2-butanol < tert-butanol. This energy is the sum of the chemisorption of the first alcohol and the physisorption of the second alcohol on the metal hydroalkoxide. For Mo, chemisorption on the clusters is exothermic by only −7 to −12 kcal/mol, much less exothermic than for W. Also the dissociative chemisorption reaction energies are endothermic relative to the alcohol cluster complexes. The dissociative chemisorption energies in the reactions with two alcohols involved are predicted to be −33 to −37 kcal/mol, also less exothermic than for W. The alcohol physisorption energies are more exothermic than the H2O physisorption energies on M3O9 clusters (−12.4 and −16.6 kcal/mol for M = Mo and W, respectively)54 due to the proton affinity of H2O (165.2 kcal/mol)55 being smaller than those of the alcohols (see the SI). Our predictions of the physisorption energies for ethanol, 1-propanol, 2-propanol, and tert-butanol are smaller than those for the γ-Al2O3(110) and -(100) surfaces with adsorption on the acidic sties, ∼50 kcal/ mol predicted by the DFT/B3LYP method.5 The single ethanol physisorption energy on the W3O9 cluster is between the adsorption energies of −30 kcal/mol on an OWO site and −21 kcal/mol on a WO site on a WO3 film.13 The physisorption energies and dissociative chemisorption energies on WO3 film are comparable, which is consistent with what we find on W3O9 cluster. Comparison to TPD Experiments. The first step in the overall reaction is the Lewis acid−base addition to the metal
Table 2. Calculated Lowest Maximum Reaction Barriers of Dehydrogenation and Dehydration Reactions at the CCSD(T)/aD//B3LYP/aD and M06/aD//B3LYP/aD Levels at 298 K
a
ΔH⧧298K proton transfer
ΔHad,298K chemisorption
c
physisorption energy for single alcohol addition is predicted to be −23 to −27 kcal/mol with the order methanol < 1-butanol < ethanol < 1-propanol < 2-butanol < 2-propanol = tertbutanol. For the reactions with two alcohols, the physisorption energies for the addition of the second alcohol are predicted to be −21 to −28 kcal/mol. The second alcohol adsorption energies do not show the same order as the first one, which means that the second alcohol adsorption is not completely dominated by the Lewis acid−base interaction but is a combination of a Lewis acid−base interaction and a hydrogen bond. The physisorption enthalpies of the second alcohol on the corresponding metal hydroalkoxides derived from the chemisorption of one alcohol are −30 to −34 kcal/mol, slightly more exothermic than the adsorption on the original cluster. On the Mo3O9 cluster, the physisorption enthalpies for single alcohol reactions are calculated to be −18 to −20 kcal/mol in the order methanol < 1-butanol < 2-butanol < 1-propanol < 22628
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Table 3. Calculated Reaction Barriers for H/D Exchange Reaction To Generate Alcohol at the CCSD(T)/aD//B3LYP/aD and M06/aD//B3LYP/aD Levels at 298 Ka H/D exchange via hydroalkoxide (2 steps) cluster W3O9
Mo3O9
alcohol methanol ethanol 1-propanol 1-butanol 2-propanol 2-butanol tert-butanol methanol ethanol 1-propanol 1-butanol 2-propanol 2-butanol tert-butanol
2ROH dehydration after D2O elim (1st step)
H/D exchange via dialkoxy (1 step)
after olefin release (2nd step)
c
c
34.3/37.2* 34.9/37.6* 34.1/37.0* 33.4/34.4 32.0/32.1 32.7/29.5
23.3/25.6 23.7/25.5 25.7/27.1 24.6/26.4 23.6/24.5 25.7/26.3
d
c
c
35.6/35.7 35.0/38.2 34.5/38.1 32.8/35.1 30.7/31.3 30.5/28.4
17.0/20.1 15.0/19.0 20.0/23.2 17.4/20.4 17.5/20.5 20.3/22.7
totalb
direct after D2O elim 75.5/75.7* 36.1/40.4 36.4/41.0 34.4/38.4 33.1/34.9* 30.3/32.6* 27.6/28.2* 45.8/46.2*d 34.5/36.3* 34.3/39.2* 31.5/36.6* 30.7/33.2* 22.2/25.7* 22.1/23.6*
c
44.1/46.9 44.3/45.2 43.6/43.8 49.1/49.2 46.7/44.9 52.2/49.2 c
34.5/36.3 34.3/39.2 31.5/36.6 33.9/35.2 30.6/29.9 36.4/34.6
a
Values with an asterisk (*) are the lowest barriers. The order of the energies is CCSD(T)/M06. bCluster regeneration after release of olefin and alcohol. cNo β-hydrogen present. dα-H transfer to the −CH3 group. The other reaction of H/D exchange through the dialkoxy is β-transfer to an OR group.
center. The Lewis acidity of W3O9 is larger than that of Mo3O9 [calculated as the fluoride affinity (FA), FA(W3O9) = 116.2 and FA(Mo3O9) = 102.4 kcal mol−1],29 consistent with the deeper initial well depth for the W complex. This is further consistent with the larger product yield generated for M = W than for M = Mo. The initial well depth might also be expected to track with the Lewis basicity [as measured by the proton affinity (PA)] of the alcohol. As shown by the values in the Supporting Information, where the PAs of the ROH vary over 15 kcal/mol, there is not a clear correlation of the PA’s with the initial welldepth. The first activation process is transfer of a proton from the Lewis acid−base adduct to the MO group. This barrier could correlate with the acidity of the alcohol, as the more acidic the species, the easier it would be to give up a proton. On the basis of the gas-phase acidities reported in the Supporting Information for the alcohols, which vary by about 8 kcal/mol, there is not a strong correlation of the acidity with the first proton transfer step. This could occur because the Lewis acid adduct of the ROH with the MVI center has some character similar to that of a protonated alcohol (the basicity). Hence, the stronger gas-phase bases would not want to give up a proton as easily. As the acidity and basicity both vary with the size of the R group in the same direction (larger R gives a more acidic OH and a more basic OH), the effects will approximately cancel, so there is no real correlation. The barrier for proton transfer to dissociate the alcohol in the presence of one alcohol is very similar for all alcohols, ∼23−26 kcal/mol for M = W. This barrier is close to the exothermicity of the physisorption addition. In the presence of a second alcohol, proton transfer has a slightly lower barrier of 18−22 kcal/mol. For M = Mo, the proton transfer barrier in the single alcohol reaction is predicted to be ∼27−30 kcal/mol, larger than the exothermicitiy of the physisorption addition. Again, different alcohols do not show much difference in the energy barriers. For the reactions with two alcohols, the barriers are slightly smaller than those with only one alcohol, consistent with the results for M = W. The proton transfer barrier in our study also falls between the barrier energies on the two different sites on WO3 film.13
The plots in Figure 10 show the lowest maximum reaction barriers for the desorption of alcohols with H/D exchange, alkenes, and aldehydes/ketones on both (WO3)3 and (MoO3)3 clusters at the CCSD(T)/aD//B3LYP/aD level. The results calculated at the M06/aD//B3LYP/aD and B3LYP/aD levels are shown in Table S2 and Figure S63 (SI). After D2O is released, the aldehyde or ketone is produced by an α-hydrogen transfer via the dialkoxy species B. Dehydrogenation of the other alcohols follows the same mechanism as for ethanol and 1-propanol. For M = W, the release of aldehyde or ketone requires additional energy above the barrier for α-hydrogen transfer. However, for M = Mo, no additional energy is needed to release the aldehyde or ketone as soon as the reaction passes over the transition state. Thus, in the plots, the total barrier to release aldehyde or ketone for M = W is selected instead of the α-hydrogen transfer barrier selected for M = Mo (Table 2). The total barriers are predicted to be 41−47 kcal/mol for M = W. On the (MoO3)3 cluster, the dehydrogenation barriers are 33−38 kcal/mol, smaller than that for M = W. The dehydrogenation barriers follow the order of methanol > 1propanol > 1-butanol > ethanol > 2-propanol > 2-butanol for M = W and methanol > 1-propanol > ethanol > 1-butanol > 2propanol > 2-butanol for M = Mo. Our calculations are generally consistent with the maximum desorption temperatures for different alcohols from experiment, which follow the order of methanol > ethanol > 1-propanol > 1-butanol > 2propanol. The barriers for the dehydration reactions to generate the olefin via dialkoxy species B on the (WO3)3 cluster follow the order of ethanol ≈ 1-propanol > 1-butanol > 2-propanol > 2butanol ≈ tert-butanol. The differences in the barriers are small, less than 3 kcal/mol. The barriers on the (MoO3)3 cluster follow the order of ethanol > 1-propanol > 1-butanol > 2propanol > 2-butanol > tert-butanol with differences less than 5 kcal/mol. The dehydration barriers are smaller than dehydrogenation for M = W and are comparable for M = Mo. The H/D exchange reaction is complicated as there are two paths to produce the alcohol. One path is via metal hydroalkoxide A after the olefin is released and the other path is through dialkoxy species B after D2O is released. The 22629
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Table 4. Calculated Physisorption Energies for the Third Alcohol Addition, Reaction Barriers for the Condensation Reaction and Hydrogen Bond Energies in the TMO Ether Complexes at the CCSD(T)/aD//B3LYP/aD and M06/aD//B3LYP/aD Levels at 298 K pathway I cluster
alcohol
reaction energyc
physisorption (3rd alcohol)
condensation barrier
hydrogen bond energy
physisorption (3rd alcohol)
condensation barrier
H2O release
W3O9
methanola,b ethanola,b 1-propanola,b 1-butanolb 2-propanola,b 2-butanolb tert-butanolb methanola,b ethanola,b 1-propanola,b 1-butanolb 2-propanola,b 2-butanolb tert-butanolb
−3.8 −6.0 −6.8 −5.6 −3.8 −4.0 5.3 −3.8 −6.0 −6.8 −5.6 −3.8 −4.0 5.3
−9.0/−6.8 −7.6/−3.9 −6.5/−2.6 −4.8 −10.2/−7.3 −6.2 −11.2 −7.8/−5.2 −8.5/−8.3 −6.3/−2.8 −4.9 −9.2/−6.1 −6.9 −10.9
22.8/27.4 18.3/20.1 20.0/21.6 22.5 23.0/21.7 22.9
18.6/10.0 15.6/8.5 10.7/5.5 7.3 18.2/13.9 12.8 9.7 13.4/6.5 9.3/6.1 9.6/4.4 6.3 16.2/12.0 12.0 8.6
−4.8/0.1 −1.6/1.7 −1.1/2.3 2.9 −8.5/−4.1 0.3 −3.1 −9.6/−3.9 −9.8/−3.4 −9.6/−2.5 −1.9 −13.9/−7.7 −4.4 −6.7
30.3/30.4 28.0/27.8 26.2/25.6 26.4 45.2/38.7 34.9
8.9/6.6 9.4/7.1 14.0/9.2 8.8 7.2/7.1 18.4 20.2 9.2/8.5 8.5/5.0 5.7/4.5 9.1 6.0/6.0 12.0 13.3
Mo3O9
a
pathway II
b
c
d
21.1/26.4 23.4/26.0 19.6/21.9 22.6 19.3/19.0 22.3 d
d
31.2/30.8 31.7/28.0 28.7/25.8 26.2 39.9/33.2 30.2 d
d
CCSD(T). M06 after slash (/). From experiment. Not studied.
mechanism of the two pathways has been discussed above. In the first pathway, the barrier for H/D exchange is much smaller than that of dehydration. Thus, the reaction barrier of this pathway is dominated by the dehydration step. The second pathway via B generates a complex composed of the TMO cluster, olefin, and alcohol. Then both the alkene and the alcohol are released from the cluster with alkene release first, as the binding energy between the alkene and the cluster is weaker than that between the alcohol and the cluster. Methanol is an exception, as there is no β-hydrogen in the OR group and the only way for H/D exchange via a dialkoxy species is through an α-H transfer to OCH3, which connects to a high-energy product with a reduced MIV metal. This α-H transfer pathway for the other alcohols also involves redox, which is not likely to occur. For the ethanol, 1-propanol, and 1-butanol reactions on the W3O9 cluster, the barriers for the second pathway are larger than the barrier of dehydration in the first pathway, so H/D exchange will take place on the first pathway. This is consistent with experimental observation, in which the desorption temperature maximum, Tmax, for H/D exchanges (alcohols) and alkenes are the same for ethanol, 1-propanol, and 1butanol. H/D exchange for 2-propanol, 2-butanol, and tertbutanol prefers to proceed on the second pathway as the barrier in the second pathway is slightly smaller than the first pathway, which agrees with the experiment. For H/D exchange reactions on Mo3O9, the barriers in the second pathway are smaller than that in the first pathway for all tested alcohols, which is also consistent with the experiment for M = Mo. So H/D exchange reactions on Mo3O9 cluster prefer to take place through the second pathway. In terms of the relative product fractions of major carboncontaining products, formaldehyde is the major product for methanol, as the barrier of H/D exchange is much higher than that of hydrogenation on both (WO3)3 and (MoO3)3 clusters. For M = W, although the barrier of H/D exchange is smaller than that of hydration for 2-propanol, 2-butanol, and tertbutanol, the reaction of H/D exchange also generates alkene, and alkene is more likely to be released than an alcohol. Thus, more alkenes than alcohols are predicted to be observed. The fact that fewer aldehydes or ketones are observed on W3O9
cluster is due to the barriers of dehydrogenation being much larger than that of dehydration. Dehydration is dominant for M = W. For M = Mo, the barriers for dehydration and dehydrogenation are comparable, and the reaction channel selectivity is not only dependent on the reaction barrier but also the reducibility of the metal center. As the electron affinity of the Mo3O9 is 0.26 eV greater than that of the W3O9 cluster, the reactions on the Mo3O9 cluster are more likely to produce aldehydes or ketones than on the W3O9 cluster.56 The ethanol dehydration pathways are consistent with our previous studies of the dehydration reaction on WO3 films with the site of two WO bonds. The barriers for β-hydrogen transfer to generate ethylene are comparable with the first and second olefin formation barrier of 27 and 30 kcal/mol on the film predicted by DFT calculations.13 Our predictions on the dehydration reactions can be compared to the 2-propanol dehydration reactions on the other metal oxide surfaces such as Al2O3, SiO2, ZnO and CdO predicted by the PM3 method.7 They predict that propylene is formed from 2-propanol by breaking a C−O bond via a surface H transfer to an oxygen atom in an OR group. The barriers are predicted to be ca. 44 kcal/mol for Al2O3 and 21 kcal/mol for SiO2, respectively, which are respectively 10 kcal/mol lower or 10 kcal/mol higher than our values for the M3O9 clusters. The dehydrogenation reactions on ZnO and CdO surfaces are predicted to have an αhydrogen transfer to the metal on the surface with the barriers of ca. 23 and 14 kcal/mol, which are lower than for the M3O9 clusters. The dehydrogenation barrier on SiO2 is predicted to be very high, and no dehydrogenation is predicted on the Al2O3 surface. With the DFT/B3LYP method, a slightly lower dehydration barrier for 2-propanol than ours, ca. 30 kcal/mol, has been reported on γ-Al2O3(110) surface modeled as a Al8O12 cluster.5 Alcohol Condensation Reactions. The condensation reaction to generate an ether requires at least two alcohol molecules and releases H2O; the reaction energies for ROH + ROH → ROR + H2O at 298 K are given in Table 4.53,57 We first tried to find a low-energy path for ether generation from the dialkoxy species B (shown in Figures S41 and S42, SI) for methanol and ethanol. The ether is generated by an α-C 22630
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Article
The triple alcohol condensation reaction can follow a different reaction path II starting from complex C, shown in Figure 12. Complex C can physisorb the third alcohol via two hydrogen bonds with one to the second alcohol and the other to an adjacent MO before the release of H2O by the second proton transfer described in the D2O loss section. The physisorption of the third alcohol transforms the Lewis acid− base interaction between the second alcohol and cluster in C into a hydrogen bond. Then the CαHX group (X depending on the alcohol) in the second alcohol “faces” the OH group in the third alcohol by a rotation of the CαHX group in the complex intermediate with two alcohol molecules hydrogen bonded not only together but also to the cluster. In this further activated complex, the rotated alkyl group is ready to form a C−O bond to the third alcohol to produce the ether and the proton bonds to a terminal MO oxygen in cluster. Ether formation on path II also breaks the cluster ring and simultaneously produces H2O, stabilizing it by a hydrogen bond to an MO. After the stepwise release of H2O and the ether, the intermediate cluster can close the broken ring by simple proton transfer, the same reaction as the final step in reaction path I. Again, the metal hydroalkoxide provides a strong Brønsted acid environment and helps ether formation take place via a lower barrier pathway. We discuss the gas-phase Brønsted acidities of those metal hydroalkoxides below. As shown in Table 4, the M06/aD//B3LYP/aD underestimates the physisorption energies of the third alcohol addition, so we predict that the third addition for 1-butanol and 2-butanol is slightly exothermic on the W3O9 cluster and comparable to the other primary alcohols. The addition of the third 2-propanol and tert-butanol has larger physisorption energies due to the electron-withdrawing CH3 groups bound to the α-C making their hydrogen bonds slightly stronger than others. For M = Mo, the physisorption energy of the third alcohol is more exothermic by 5−8 kcal/mol than that for W, because a more stable WVI complex is formed after the addition of the second alcohol due to the Lewis acidity of the metal center with M = W being larger than for M = Mo. Consistent with the results for M = W, slightly more exothermic phsisorption is predicted for 2-propanol and tert-butanol than the others for M = Mo. The energy barriers are generally larger than that on reaction pathway I. The energy barriers on two TMO clusters are comparable and have a broad range from 26 to 45 kcal/mol for M = W and from 26 to 40 kcal/mol for M = Mo. The hydrogen bond of H2O to MO is predicted to be 7−20 kcal/mol for different alcohols. The final steps of ether desorption and ring regeneration are identical with the reaction path I. The condensation reactions can be compared with dimethyl ether formation from the reaction of methanol on a polyoxotungstate cluster.58 The condensation reactions on M3O9 clusters have a similar reaction mechanism with the reaction on the polyoxometallate (POM) clusters. The barriers predicted by periodic DFT with the PW91 functional are comparable with our results on a M3O9 cluster. In the catalytic reactions with the POM, ether formation occurs at an acid site (bridging OH on the cluster), whereas in our study one alcohol has to be sacrificed to form the strong Brønsted acid. Thus, an additional alcohol is necessary to lower the activation energy for ether formation. This acid-driven ether formation reaction is also reported on a zeolite surface with methanol.59 The surface zeolitic protons play the key role in that catalytic process. They predicted that the pathway of direct ether formation without
transfer from one OR group to O in the other OR group bound to the active mental center. The barrier is predicted to be ∼50 kcal/mol at the CCSD(T)/aD//B3LYP/aD level for methanol and ethanol on M3O9 clusters. Another possible pathway with two alcohols is starting from the dihydroalkoxy species with two OH groups and two alkoxy groups bound to the metal center. This intermediate is produced by the proton transfer from OH group in the second alcohol molecule to the remaining terminal MO oxygen atom instead of proton transfer to OH to release H2O. The transition state is still under the reactant asymptotic limit. The condensation reaction from dihydroalkoxy species is also through α-C transfer from one OR group to the other, and the barrier energy is close to the reaction from dialkoxy species. Again this process has a high barrier of ∼50 kcal/mol for ethanol at the CCSD(T)/aD// B3LYP/aD level and ∼60 kcal/mol for methanol at the B3LYP/aD level, so the condensation reaction with only two alcohol molecules always has a large barrier independent of whether water or ether is released first. We thus studied two condensation processes with three ethers. The dialkoxy species B can physisorb a third alcohol through a hydrogen bond to the MO oxygen bound to an adjacent metal, shown in Figure 11, from any excess alcohol or from the formation of alcohol from the H/D exchange path. Ether formation on reaction path I is via an α-C transfer from one OR group to the OH in alcohol, coupled with simultaneous proton transfer from OH in the third alcohol to a MO oxygen. The ether bonds to the cluster through an O to a hydrogen of the OH group in the cluster. A bridge M−O−M bond, which connects two active metal centers, is broken in this process. The two active metals remain MVI in this step and no redox occurs. After ether release, the ring is regenerated by a simple proton transfer from the OH group to the MO, which is the same metal bound to the OR group. The final hydroalkoxide is identical with the single alcohol chemisorption species. We predict the same reaction mechanism for primary and secondary alcohols. The third alcohol physisorption energies, reaction barriers, and hydrogen-bond energies in the TMO ether complexes are summarized in Table 4. On the W3O9 cluster, the physisorption energies for the third alcohol addition are ca. −5 to −10 kcal/ mol. The barriers are similar with values of 18−23 kcal/mol, again showing that the length of the alkyl chain does not substantially influence the barrier. The transition state energy is still below the reactant asymptotic limit. There is a larger difference of energy to remove the different ethers. The hydrogen bond of an OH group to dimethyl ether varies significantly from 10 to 20 kcal/mol. The reaction energy for ring regeneration is slightly exothermic by ∼2 kcal/mol. The third alcohol physisorption on the Mo3O9 cluster is close to that for W3O9 with the differences being less than 1 kcal/mol. The barrier energy for ether formation is comparable to that for W3O9 with differences generally being less than 3 kcal/mol. Again the difference in barrier heights is small, less than 4 kcal/mol. The transition state energy is still below that of the energy of the initial reactants. The energy to release ether is up to 5 kcal/mol less than that for M = W. Ring regeneration energies are 4−8 kcal/mol. Compared to ether formation with two alcohols, this reaction has a much lower reaction barrier for both M = W and M = Mo. One alcohol is sacrificed to produce the active hydroalkoxide cluster, which is a strong Brønsted acid and plays the catalyst role instead of the metal oxide cluster. 22631
dx.doi.org/10.1021/jp5072132 | J. Phys. Chem. C 2014, 118, 22620−22634
The Journal of Physical Chemistry C
Article
Table 5. Gas-Phase Brønsted Acidities of the Metal Hydroalkoxides (in kcal/mol) at 298 K Calculated at the CCSD(T)/aD// B3LYP/aD, M06/aD//B3LYP/aD, and B3LYP/aD Levels M=W M3O8OH(OCH3) M3O8OH (OCH2CH3) M3O8OH (OCH2CH2CH3) M3O8OH (O(CH2)3CH3) M3O8OH (OCH(CH3)2) M3O8OH(OCH(CH3)CH2CH3) M3O8OH (O(CH3)3)
M = Mo
CCSD(T)
M06
B3LYP
CCSD(T)
M06
B3LYP
273.7 275.0 277.3 277.5 276.7 275.9 276.8
272.0 277.1 279.4 280.2 279.2 278.2 279.1
276.9 278.5 281.0 280.1 279.7 279.1 279.8
271.4 272.0 271.4 274.6 276.8 274.4 274.3
272.5 275.0 274.9 278.0 275.9 277.3 277.4
275.4 275.7 276.4 277.7 276.5 279.6 277.8
transfer (hydridic) to a WVI metal center with redox involved for M = Mo and no redox for M = W. The dehydrogenation and dehydration channels have comparable activation energies. H/D exchange to produce ROH alcohols can take place after the olefin generated by dehydration is released or via the dialkoxy species, depending on the alcohol and the cluster. The Lewis acidity of the metal center for clusters with WVI is larger than for MoVI, and the increased acidity results in the increased reactivity of W3O9 over Mo3O9. However, the product selection between aldehyde or ketone formation and olefin formation is determined by the reducibility of the metal center. The electron affinity of Mo3O9 is larger than that of W3O9, so the former is more reducible. The calculations at the CCSD(T)/DFT level are consistent with the experiment in terms of D2O formation, dehydrogenation, dehydration, and H/D exchange reactions. The condensation reaction to generate an ether requires a third alcohol with the sacrifice of this alcohol to form a metal hydroalkoxide, which is a strong gas-phase Brønsted acid. The condensation reaction is driven by the Brønsted acidity of the OH group on the hydroalkoxide-driven reaction, in contrast to the dehydrogenation and dehydration reactions, which are governed by the Lewis acidity and reducibility of the metal center. We show that the product yield for condensation will be impacted by the amount of alcohol present, as three molecules are needed. DFT using both the B3LYP and the M06 functionals gives predictions close to those of the CCSD(T) method for the barrier energies (shown in SI) but the Lewis acid/base interaction and hydrogen bond energies are underestimated by DFT. Thus, CCSD(T) is required to study these types of reactions in terms of predicting the initial well-depths, which determine if D2O elimination leading to the formation of the active intermediate can occur before the alcohols are desorbed back to the reactants.
the alkoxy intermediate involved has lower energetics on the potential energy surface than the one with the formation of an alkoxy intermediate in the first step. This is consistent with our study of the two different pathways I and II as well as the study on the POM cluster. Gas-Phase Brønsted Acidities of Metal Hydroalkoxides. In the condensation reactions, one alcohol is sacrificed to form a metal hydroalkoxide, a Brønsted acid site to lower the reaction barriers. Both CCSD(T) and DFT methods were employed to calculate the gas-phase Brønsted acidities shown in Table 5. At the CCSD(T)/aD//B3LYP/aD level, the gasphase Brønsted acidities for the tungsten hydroalkoxides are between 274 and 278 kcal/mol and follow the order of CH3 < C2H5 < 2-C4H9 < i-C3H7 < t-C4H9 < n-C3H7 < n-C4H9. As the difference is small, the alkyl chain length does not substantially affect the acidities. They are very strong gas-phase acids, stronger than gas-phase H2SO4 (ΔG298K = 304.0 kcal/mol).60 The acidities predicted with the DFT methods generally follow the same order. The differences for those metal hydroalkoxides are small, less than 5 kcal/mol at the B3LYP/aD level and 8 kcal/mol at the M06/aD//B3LYP/aD level. The DFT methods predict slightly larger Brønsted acidities than the CCSD(T)/ aD//B3LYP/aD method, thus weaker acids are predicted. For
