Dehydration and Oxidation of Alcohols by Supported Polyoxometalates

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Kinetics, Catalysis, and Reaction Engineering

Dehydration and oxidation of alcohols by supported polyoxometalates: effects of mono- and multi-valent cation exchange on catalyst acidity and activity Trenton Wilke, and Mark A Barteau Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03084 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 28, 2019

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Industrial & Engineering Chemistry Research

Dehydration and oxidation of alcohols by supported polyoxometalates: effects of mono- and multi-valent cation exchange on catalyst acidity and activity

Trenton Wilkea,1, and Mark A. Barteaua,2,* aDepartment

of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109

1 Present

address: Sironix Renewables, Seattle, WA 98103

2 Present

address: Department of Chemical Engineering, Texas A&M University, College Station,

TX 77843 * Corresponding Author Email: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract The effects of cation exchange on methanol oxidation and dehydration by supported H3PMo12O40 catalysts were investigated by exchanging polyoxometalate (POM) protons with cations such as Na+, Mg2+, Cu2+, and Al3+. The activity decreased dramatically for both oxidation and dehydration pathways with cation addition. The measured oxidation and dehydration turnover frequencies over Na, Mg, and Cu exchanged POMs were primarily a function of the H+/POM ratio of the catalysts, and differences in the charge of the exchanged cations had little impact. Similar behavior with cation exchange was also observed for ethanol dehydration and oxidation. This study suggests that there are limited benefits of cation-exchanging supported POMs for reactions at mild conditions, as changes to POM reducibility induced by cationexchange were not catalytically relevant, and decreasing the number and strength of acid sites crucial to selective oxidation and dehydration catalysis decreases activity at these conditions.


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1. Introduction Polyoxometalates (POMs) of the Keggin-type supported on a high surface area support (Scheme 1) are attractive model transition metal oxide catalysts for which one can tune the composition and therefore the catalytic properties,1,2 all while maintaining a constant structure with different composition. POMs are active both for acid and selective oxidation catalysis, and relationships have been developed between POM composition and the POM acid and redox properties.3–5 The effect of varying the POM heteroatom and framework metal atoms on POM properties and catalytic behavior is well documented.1,3,6 As for the counter-cations, we have previously demonstrated a relationship between counter-cation identity and POM reducibility, based largely on spectroscopic probes.1,7 Ideally, one could use cation exchange to carefully tune the POM reducibility.1 Cations that improve POM reducibility might be expected to enhance oxidation activity, while their exchange for protons would decrease POM acidity, thereby increasing oxidation selectivity further.



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Scheme 1: Idealized schematic for POMs on a high surface area support. Examples of Keggintype POM constituent atoms have been included for the heteroatom, polyatom, cation and the heterogeneous support

Catalytic studies, however, have suggested that such effects may be difficult to demonstrate. Studies of the competitive oxidative dehydrogenation and dehydration of methanol by supported phosphomolybdate Keggin ions on silica have shown that the rates of these two reaction channels (illustrated in Scheme 2) are highly correlated.2,8,9 Iglesia and co-workers2 varied the coverage of H3PMo12O40 on silica over a range of surface coverages, and found that the methanol oxidation/dehydration selectivity was invariant, and that the turnover frequencies (TOFs) of the two reaction channels were essentially independent of coverage. In those studies the H+/POM ratio was held constant. Our studies8 of the same chemistry in which the H+/POM ratio was varied by exchange of Na+ for H+ showed a similarly constant selectivity, but surprising variations of activity for both oxidation and dehydration with the level of sodium exchange. While one would expect the rate of acid catalyzed reactions to be diminished by Na+ 4 ACS Paragon Plus Environment

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exchange for protons, we observed that the turnover frequencies for both methanol oxidation and dehydration were dramatically decreased by cation exchange.

Scheme 2: Methanol reaction channels Dehydration:

2 CH3OH → (CH3)2O + H2O

Oxidative dehydrogenation: CH3OH + ½ O2 → HCHO + H2O

Taken together, these studies demonstrate the importance of acid sites in methanol oxidation over these supported polyoxometalate catalysts. One possibility is that, in addition to catalyzing methanol dehydration, the protons interact with the various intermediates and transition states along the oxidation channel, resulting in a lower barrier to oxidation than for methanol molecules unperturbed by acid sites. Iglesia and co-workers have carried out extensive Density Functional Theory (DFT) calculations that support this explanation.10 In the simplest variant of this scheme, the dehydrogenation and dehydration pathways may share a common intermediate, e.g., protonated methanol. If this species is sufficiently mobile to access the POM sites that carry out oxidation, then the selectivity invariance can be explained, provided that POM coverage and the extent of counter-cation exchange do not influence the redox properties of the POM anions.

In the present work, cation-exchanged POMs deposited on a high surface area support were used to investigate the relationship between POM acid and redox properties and methanol reactivity. The effect of cation exchange on ethanol dehydration and oxidation was probed as 5 ACS Paragon Plus Environment


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well, to determine whether the dependence of methanol oxidation on POM acid properties is unique to methanol or may be extended to other alcohols. POMs in their acid form (i.e., heteropolyacids, HPAs) were exchanged with Na+, Mg2+, Cu2+, and Al3+ cations chosen to yield a range of effects on POM acid and redox properties. In the case of exchange of higher-valent cations such as Mg2+, Cu2+ and Al3+, one might expect a more rapid decrease in catalytic activity owing to the replacement of multiple protons per cation exchanged, as well as possible perturbation of POM redox properties. POM reducibility was probed using UV-Vis of cationexchanged POMs, where the UV-Vis adsorption edge energy may be correlated with reducibility, as demonstrated previously.7 POM acid sites were quantified using titration with DTBP under reaction conditions. Surprisingly, Mg2+ and Cu2+ exchanged catalysts behaved quite similarly to Na+ exchanged catalysts; Al3+ exchanged catalysts exhibited somewhat different chemical and physical properties that appear to make direct comparisons less relevant.

2. Experimental

2.1. Catalyst synthesis Silica-supported cation-exchanged catalysts were prepared with a PMo12O403- loading of 0.2 POM/nm2 (10.3 wt % POM), and cations (Na, Mg, Al, or Cu) in quantities between 0 and 6 cations per POM. Catalysts will be denoted as XxPMo, where X is the identity of the cation (Na, Mg, Al, or Cu) and x is the nominal number of cations per POM (ex. Al2PMo was prepared using two equivalents of Al (2 units of Al(NO3)3 · 9H2O) per POM (1 unit of H3PMo12O40), to yield a catalyst with 0.4 Al/nm2 and 0.2 POM/nm2). The un-exchanged supported POM (H3PMo12O40 on 6 ACS Paragon Plus Environment

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SiO2 at 0.2 POM/nm2) is denoted at HPMo. Synthesis procedures were adapted from the literature2 and have been described previously.8 Briefly, fumed silica (Cab-O-Sil M5, 189 m2/g, 1.5 mL/g wet point) was washed with ultrapure water, dried at 110°C, and calcined at 300°C overnight. Aqueous solutions for use in incipient wetness impregnation that contained the appropriate quantity of POM (Phosphomolybdic acid hydrate, Nippon Inorganic Color and Chemical Co.) and cation (sodium carbonate monohydrate, 99.5%, Fisher; magnesium carbonate hydroxide hydrate, 99%, Sigma-Aldrich; aluminum nitrate nonahydrate, 99.997% trace metals basis, Sigma-Aldrich; copper(II) carbonate basic, reagent grade, Sigma-Aldrich) were prepared, using nitric acid addition when required to keep solutions sufficiently acidic to prevent POM decomposition11 or to decompose insoluble carbonates. The POM solutions were added dropwise to the previously prepared silica, with intermittent mixing using a vortex mixer, stored in closed vials for 24 hours, and then dried under vacuum at 50°C for 24 hours.

2.2. Catalyst analysis ATR-FTIR spectra of as-prepared catalyst samples at ambient conditions were obtained using a Nicolet iS50 spectrometer from Thermo Scientific, with a built-in diamond window ATR with a DTGS detector. UV-Vis spectra were obtained using a ThermoFisher Scientific Evolution 300 spectrophotometer with a Harrick Praying Mantis diffuse reflectance accessory, equipped with a Harrick High Temperature Reaction Chamber. Catalyst samples in the reaction chamber were kept at 200°C in flowing air for 30 min. prior to and during UV-Vis measurements. The UV-Vis adsorption edge energy was calculated by applying the Kubelka-Munk function with MgO as the



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reference.2 The XRD diffraction patterns of the as-prepared catalysts were recorded using a Rigaku Miniflex 600 with a Cu-Kα radiation source with a D/teX Ultra silicon strip detector.

2.3. Methanol and Ethanol Dehydration/Oxidation Catalyst activity was tested using methanol and ethanol as probe reactants. Catalyst samples were loaded in a ¼” quartz tube with a quartz wool plug and placed in a resistively heated vertical split tube furnace (Mellen Microtherm). He (99.99%, Purity Plus) and O2 (99.99%, Purity Plus) were supplied using mass flow controllers (GF80/GF40, Brooks) and methanol (99.99%, Fisher), ethanol (99.5%, Acros Organics), and water (Ultrapure, Millipore Milli-Q) were supplied using syringe pumps (NE-300, New Era Pump Systems) and vaporized in a home-built manifold in flowing He. Catalysts were pretreated in 2.7 kPa H2O, 20 kPa O2, balance He, at reaction temperature (200°C for methanol, 160°C for ethanol) for 30 min. The feed was then switched to a reactant mixture containing 2.7 kPa H2O, 4 kPa of either CH3OH or C2H5OH, 20 kPa O2, balance He. The catalyst loading (20 – 200 mg) and total flow rate (10 – 100 sccm) were adjusted to maintain alcohol conversions below 10%. Products were analyzed using an online gas chromatograph (Agilent 7890B) equipped with a DB-1 capillary column connected to a FID and packed Hayesep Q and ShinCarbon ST columns connected to a TCD.

The dehydration and oxidation TOFs are defined for methanol in Scheme 3a and for ethanol in Scheme 3b. Methanol may undergo dehydration to dimethyl ether (DME) over acid sites or oxidative dehydrogenation to formaldehyde (HCHO) over redox sites. Formaldehyde may react further with methanol to form dimethoxymethane (DMM) and methyl formate (MF). The 8 ACS Paragon Plus Environment

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methanol dehydration TOF is defined as the molar production rate of DME per acid site, and oxidation TOF is defined as the sum of the molar production rates of HCHO, DMM, and MF normalized per POM, as each requires the formation of one HCHO, the relevant oxidation product.2

Ethanol may undergo dehydration to either ethylene (C2H4) or to diethyl ether (DEE) over Brønsted acid sites, or oxidative dehydrogenation to acetaldehyde (AcH). Only trace quantities of secondary products were observed. The ethanol dehydration TOF is defined as the sum of the molar production rate of the dehydration products, ethylene and diethyl ether, normalized per H+. The ethanol oxidation TOF is defined as the molar production rate of the sole oxidation product, AcH, normalized per POM.

Scheme 3a: Methanol TOFs Oxidation TOF (s ―1) =

mol HCHO + mol DMM + mol MF POM ∙ s

Dehydration TOF (s -1) =

mol DME H+ ∙ s

Scheme 3b: Ethanol TOFs mol AcH

Oxidation TOF (s ―1) = POM ∙ s

Dehydration TOF (s -1) =

mol C2H4 + mol DEE H+ ∙ s



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2.4. DTBP Titration The quantity of Brønsted acid sites present under reaction conditions was determined using titration with 2,6-di-tert-butylpyridine (DTBP, 97%, Sigma-Aldrich). Similar to steady state activity testing, catalysts (40mg) were pretreated in 2.7 kPa H2O, 20 kPa O2, balance Argon, 30 sccm total flow, at reaction temperature (200°C for methanol, 160°C for ethanol) for 30 min. The feed was then switched to a reactant mixture containing 2.7 kPa H2O, 4kPa of either CH3OH or C2H5OH, 20 kPa O2, balance Argon, 30 sccm total flow, for four hours. At this point the titration was started by switching the alcohol feed from a syringe containing alcohol only to one containing DTBP dissolved in methanol or ethanol, resulting in an identical reaction mixture except with the addition of 2 Pa DTBP in the feed. The concentrations of the reagents fed as liquids (CH3OH or C2H5OH, H2O, DTBP) and the primary acid-catalyzed products, (DME or DEE and C2H4), were monitored with an on-line mass spectrometer (Hiden HPR-20 R&D). The quantity of acid sites was determined with a linear fit of the DME or DEE rates as a function of the quantity of DTBP added per POM, as described previously.8

3. Results

3.1. Catalyst Characterization 3.1.1. FTIR The as-prepared catalysts were analyzed by ATR-FTIR to confirm the presence of intact POMs on the silica surface. Figure 1 contains the FTIR spectra for a series of catalysts with the addition


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of one cation per POM, X1PMo 0.2, as well as bare silica and the unexchanged POM supported on silica, HPMo, as references. The four characteristic POM bands, located at 1050, 960, 880, and 790 cm-1, corresponding to Op, Ot, Ob, and Oc,12,13 (defined in Scheme 1) respectively, may be distinguished from the silica bands at 1060 and 800 cm-1, indicating the presence of intact POMs on the surface of the as-prepared catalysts.

Figure 1: ATR-FTIR for X1H2PMo12O40/SiO2 0.2 POM/nm2 where X is H, Na, Mg, Al, or Cu. The bare silica support is included as a reference. Op, Ot, Ob, and Oc are defined as shown in Scheme 1.

No significant shifts of the characteristic POM bands were observed with the addition of one cation per POM, or with the addition of up to three cations per POM (Supplemental Information, Figure S1). Analysis of POM FTIR bands post-reaction (Supplemental Information, Figure S2) generally indicated that all POM bands were retained after reaction, although some weaken and broaden, especially for HPMo, which was observed to be significantly reduced post-reaction, as 11 ACS Paragon Plus Environment


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indicated by the deep blue color of the used catalyst. Cation-exchanged catalysts tended to be green in color after reaction, or yellow for fully-exchanged catalysts (ex. Na3PMo), consistent with the fact that they were much less active for methanol oxidation.8

3.1.2. XRD XRD patterns for X1PMo (X = H, Na, Mg, Al, Cu) as well as the bare silica support are provided in Figure 2. The primary observable features are the broad peak at ~ 22° corresponding to the amorphous fumed silica support and the peak at 7-9° corresponding to small 2D POM clusters8,14 on the order of 2-3 nm in diameter.2,8 These results are in excellent agreement with those obtained in our previous study at different extents of Na exchange,8 as well as with the XRD and TEM results of Iglesia and co-workers2 for silica-supported H3PMo12O40 catalysts at comparable loadings. The absence of large diffraction peaks at higher angles (in particular from 15-35°) indicates that, with the addition of one cation per POM, the distribution of POMs on the support is largely unchanged, although there is evidence for a small quantity of bulk POMs for Na1PMo, indicated by a small peak at 26°. We have previously shown that the formation of POM crystallites is easily detectable by XRD on supports as varied as silica, carbon and carbon nitride, even to the extent of identifying crystallite formation in cases where it had not previously been recognized.9 The shifts in the location of the low angle peak may reflect changes in inter-POM spacing with cation addition, as hydrated protons, (H2O)2H+, are replaced 12 ACS Paragon Plus Environment

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with cations. At higher levels of cation addition than that characteristic of the catalysts in Figure 2 (up to 3 cations per POM) very similar XRD patterns were obtained (Supporting Information, Figure S3), with the exception of Al, for which growth of bulk POM peaks and shifts in the low angle peak with the addition of 2 or more Al/POM were observed. Analysis of catalysts post reaction (Supporting Information, Figure S4) indicated that XRD patterns did not significantly change after reactions with methanol.

Figure 2: XRD for X1H2PMo12O40 0.2 POM/nm2/SiO2 where X is H, Na, Mg, Al, or Cu. The bare silica support was included as a reference.

3.1.3. Titration results The quantity of Brønsted acid sites present under reaction conditions was determined using DTBP titrations after catalysts had attained steady state conversion, typically four hours. The



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details of the titration for Na-exchanged POMs have been discussed in detail previously.8 The titration results and corresponding quantity of acid sites per POM are presented in Figure 3. For supported POMs exchanged with Na+, Mg2+, or Cu2+, the quantity of Brønsted acid sites decreased linearly with cation addition, at an approximate rate of ~ 0.5 ± 0.1 H+ removed per addition of one cation, whether monovalent or divalent. Clearly, stoichiometric exchange was not observed (e.g., Mg2+ is not observed to exchange with 2 H+), suggesting that the exchanged cations have significant interactions with silanol groups on the silica surface. POM protons are known to interact strongly with silanols,2,6,15–18 the interaction of which results in the unexchanged POM having two titratable protons under conditions used in this study.8

Figure 3: DTBP uptake per POM for [Na,Mg,Al,Cu]xH3-xPMo12O40/SiO2 0.2 POM/nm2 where x is 0 for H; 0.25, 0.5, 1, 2, or 3 for Na; 0.5, 1, 1.5, or 3 for Mg; 0.5, 1, 2, or 3 for Al; 0.1, 0.5, 1, 1.5, or 3 for Cu.


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With the inclusion of Al3+ in the supported POM catalysts, the quantity of acid sites remained nearly constant at ~ 2 H+/POM for POMs with one or less Al, and decreased to ~ 1.4 H+/POM for Al2PMo and Al3PMo. At all levels of exchange examined here, Al exchanged catalysts retained ~ 0.5 more H+/POM than the catalysts exchanged with other cations. This higher level of acidity may be associated with either the generation of Brønsted acid sites via interaction of water with Lewis acidic Al cations19 (Scheme 4) and/or with the formation of bulk POM crystallites observed by XRD at excess levels of Al-exchange (Figure S3). In any case, the effect of increasing Al exchange on catalyst acidity is qualitatively different from that of the mono- and di-valent cations examined. Scheme 4 Al3+ + H2O → Al3+(OH)- + H+ The titration of sodium-exchanged POMs during reaction with ethanol at 160°C gave nearly identical results as those obtained during reaction with methanol at 200°C on identical catalysts. During reaction with ethanol, the unexchanged POM supported on silica, HPMo, retained 1.95 H+/POM, and cation addition resulted in a decrease of 0.5 H+ per Na+. The nominally stoichiometrically exchanged POM, Na3PMo, retained 0.53 H+/POM during reactions with ethanol. Under the conditions used in this study, the quantity of POM acid sites was not particularly sensitive to the reaction temperature (160°C vs. 200°C), the probe molecule (methanol or ethanol), or the identity of the cation (with the exception of Al3+). The most important factor in determining the quantity of acid sites appears to be the number of cations per POM. 15 ACS Paragon Plus Environment


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3.1.4. UV-Vis results UV-Vis was used to determine the effect of cation addition on redox properties, as shown previously.7 The UV-Vis adsorption edge energy reflects the LMCT transition between the POM HOMO (located on the terminal oxygens, generally does not change with POM composition changes) and the POM LUMO (delocalized primarily on the POM Mo, is sensitive to POM composition changes).7 Thus one can probe the changes in the LUMO energy, which will reflect the reducibility of the POM. Altering POM reducibility may impact the rate of selective oxidation of methanol over POMs,1,20 as the rate-limiting step for this reaction has been suggested to involve simultaneous C-H bond breaking and transfer of the H to the POM, resulting a in reduced Mo center (MoVI to MoV) and the formation of a new O-H bond.2 Edge Energy shifts relative to HPMo (eV) Cation/POM

Na

Mg

Al

Cu

0

0.00

0.00

0.00

0.00

1

0.16

0.14

-0.08

-0.03

2

0.19

0.14

-0.08

-0.32

3

0.22

0.10

0.00

-0.26

Table 1: UV-Vis adsorption edge energy shifts for supported cation exchanged POMs, [Na,Mg,Al,Cu]xH3-xPMo12O40/SiO2 0.2 POM/nm2, where x is 0, 1, 2, or 3. Shifts are relative to H3PMo12O40/SiO2 0.2 POM/nm2. Spectra were taken after 0.5 hours in flowing air at 200°C in a Harrick high temperature cell.

The UV-Vis adsorption edge energy shifts for cation-exchanged POMs are contained in Table 1 above. The measurements were obtained with the catalysts in flowing air at 200°C in a Harrick high temperature cell, in order to take measurements at a consistent degree of hydration and oxidation. It can be seen that the edge energy for Na- and Mg-exchanged catalysts increased by 16 ACS Paragon Plus Environment

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0.1 – 0.2 eV, indicating these cations render the POMs less reducible. Al-exchanged POMs exhibited subtler effects, decreasing the edge energy by 0 – 0.1 eV, perhaps making the POMs slightly more reducible. Cu appeared to have the strongest effect, decreasing the edge energy by up to 0.3 eV, which would be expected to render the POMs much more reducible. These observations are consistent with the expected influence of cation exchange on POM reducibility.7,21,22 For reference, changing the POM from H3PMo12O40 to H3PW12O40 increases the edge energy by 0.7 eV ( the W-containing POM is less reducible) and exchanging a few Mo atoms in the framework for V (H5PV2Mo10O40) decreases the edge energy by 0.3 eV (the Vcontaining POM is more reducible).7,20 However, as shown below, the rates of alcohol oxidation depend strongly on catalyst acidity under the reaction conditions of this study, and the effects of cation exchange on POM reducibility are not evident from the catalytic reaction data. 3.2. Methanol Dehydration Both methanol dehydration rates and turnover frequencies decrease with cation substitution for protons, as previously demonstrated for silica-supported NaxH3-xPMo12O40 catalysts.8 Figures 4 and 5 illustrate the effects of higher-valent cations in the present work, compared to that of sodium. Dehydration TOFs were obtained by normalizing dehydration rates per proton; proton populations were determined by the DTBP titration method described above. The mass balances around the reactor varied with a standard deviation of 10% from run to run, due to inconsistencies in the methanol feed rate of the syringe pumps. Overall the average closure of all runs was within 2.5% of unity and no trends in mass balance closure were observed with cation identity, quantity, etc., demonstrating the decrease in TOF with increasing cation 17 ACS Paragon Plus Environment


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content is not due to accumulation of product. These values are comparable to those reported previously with Na-exchanged POM catalysts in reference 8, Supplemental Information. When dehydration TOFs are plotted vs. proton population per POM, the results essentially fall on a common curve. We have previously suggested that the reduction in TOFs with increasing extents of cation exchange is due to a reduction of acid site strength with increasing exchange.8 Figure 4 shows that this effect is similar, whether monovalent (Na+) or divalent cations (Mg2+, Cu2+) are substituted for protons. This suggests that the identity of the cations is relatively unimportant in their influence on acid site strength.

Figure 4: Methanol dehydration turnover frequencies for cation-exchanged POMs as a function of the quantity of acid sites per POM as determined by DTBP titration. The catalysts used were [Na,Mg,Al,Cu]xH3-xPMo12O40 0.2 POM/nm2 where x is: 0 for H; 0.25, 0.5, 1, 2, 3, or 6 for Na; 0.5, 1, 1.5, or 3 for Mg; 0.5, 1, 2, or 3 for Al; 0.1, 0.5, 1, 1.5, or 3 for Cu. Reactions were carried out at 200°C in 2.7 kPa H2O, 4 kPa MeOH, 20 kPa O2, balance He. Dashed line is a smooth curve though the NaxPMo points, included to guide the eye.


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The results for exchange of the trivalent cation (Al3+) are less definitive. The difference between Al3+ and the lower-valent cations is even clearer when dehydration TOFs are plotted against nominal extents of cation addition, as shown in Figure 5. Unlike the other cases, TOFs for methanol dehydration are much less strongly affected by Al3+ substitution. It should be noted that if stoichiometric exchange occurred, all protons in H3PMo12O40 should be removed by the exchange of a single Al3+ cation per POM. Thus, most of the results for Al3+ exchange in Figure 5 represent cases where excess aluminum has been added to the catalyst.

Figure 5: Methanol dehydration turnover frequencies for several series of cation-exchanged POMs: [Na,Mg,Al,Cu]xH30.2 POM/nm2 where x is: 0 for H; 0.25, 0.5, 1, 2, or 3 for Na; 0.5, 1, 1.5, or 3 for Mg; 0.5, 1, 2, or 3 for Al; 0.1, 0.5, 1, 1.5, or 3 for Cu. The x-axis represents the nominal quantity of cations added per POM. Reactions were carried out at 200°C in 2.7 kPa H2O, 4 kPa MeOH, 20 kPa O2, balance He. xPMo12O40



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In contrast to the other cation-exchanged catalysts, Al exchanged POMs, which retain higher levels of acid sites with increasing cation addition, also retain higher TOFs with cation exchange as well. In fact, the dehydration TOF for Al-exchanged POMs barely decreases until 3 Al/POM, where we observe a significant quantity of bulk POMs (Supporting Information, Figure S3c). The near constant TOF for Al quantities of 0.5 to 2 per POM suggests that these catalysts have similar acid site strengths, even though the quantity of acid sites decreases by over 0.5 H+/POM over this range. The maintenance of acid site strength for these catalysts may be a result of Al cations generating Brønsted acidity (Scheme 4), or the Al cations may exchange poorly with the POM protons and therefore do not disturb the acid sites already present. 3.3. Methanol Oxidation Similar to the results for methanol dehydration presented in the previous section, the oxidation rates and TOFs decreased rapidly with the addition of Na+, Mg2+, Al3+, or Cu2+ cations to the POMs, with little dependence on cation identity, as shown in Figures 6 and 7 below. The dependence of oxidation rates on POM acid properties was established previously for supported NaxH3-xPMo12O40 catalysts.8 Oxidation rates are normalized per POM; proton populations were determined by DTBP titration under reaction conditions. Figure 6 shows the oxidation TOFs as a function of the quantity of acid sites per POM, as determined by DTBP titrations. These fall on the same curve when plotted as a function of H+/POM, indicating that the quantity of acid sites is the primary reactivity descriptor for methanol oxidation over cationexchanged polyoxometalates. Additionally, as proposed previously,2,8,10,15 the oxidation of methanol over polyoxometalates may involve protons and may be sensitive to the acid strength 20 ACS Paragon Plus Environment

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of POM protons. Moving left across Figure 6, the oxidation TOF decreases, regardless of the cation used. The acid site strength also decreases as one moves left across this Figure 6, as the decreasing acid site quantity is indicative of increasing cation interaction with the POMs, which in turn will decrease acid strength.6,8,23,24



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Figure 6: Methanol oxidation turnover frequencies for cation-exchanged POMs as a function of the quantity of acid sites per POM as determined by DTBP titration. The catalysts used were [Na,Mg,Al,Cu]xH3-xPMo12O40 0.2 POM/nm2 where x is: 0 for H; 0.25, 0.5, 1, 2, 3, or 6 for Na; 0.5, 1, 1.5, or 3 for Mg; 0.5, 1, 2, or 3 for Al; 0.1, 0.5, 1, 1.5, or 3 for Cu. Reactions were carried out at 200°C in 2.7 kPa H2O, 4 kPa MeOH, 20 kPa O2, balance He. Dashed line is a smooth curve though the NaxPMo points, included to guide the eye.

Significantly, these results are in contrast to the trends demonstrated for the UV-Vis adsorption edge energy for POMs cation-exchanged with Na, Mg, Al, and Cu. Based on those trends, one might expect POMs exchanged with Cu to be more active for selective oxidation catalysis, as Cu rendered the POM more reducible, and the reduction of a POM Mo has been suggested to play a crucial role in the rate-limiting step for methanol oxidation.2 Similarly, Na and Mg exchange rendered POMs less reducible and therefore, in principle, less capable of carrying out the redox cycles necessary for an active oxidation catalysts. In contrast to these trends, very little difference in the TOFs of POMs exchanged with Cu, Na, and Mg was observed.


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The differences that were observed are mostly easily visualized by plotting the oxidation TOF as a function of the nominal quantity of cations per POM, as shown in Figure 7. The Al-exchanged POMs likely retain a higher oxidation TOF due the increased quantity of acid sites observed with Al exchange (see Figure 3). However, the extent to which Al retains higher activities with cation exchange is much smaller than that observed for dehydration TOF.

Figure 7: Methanol oxidation turnover frequencies for several series of cation-exchanged POMs: [Na,Mg,Al,Cu]xH3-xPMo12O40 0.2 POM/nm2 where x is: 0 for H; 0.25, 0.5, 1, 2, or 3 for Na; 0.125, 0.25, 0.5, 0.75, 1, 1.5, or 3 for Mg; 0.057, 0.11, 0.22, 0.33, 0.5, 0.66, 1, 2, or 3 for Al; 0.1, 0.5, 1, 1.5, or 3 for Cu. The x-axis represents the nominal quantity of cations added per POM during synthesis. Reactions were carried out at 200°C in 2.7 kPa H2O, 4 kPa MeOH, 20 kPa O2, balance He.

It is worth emphasizing that the shifts in UV-Vis edge energy, and therefore the changes in POM redox properties induced by cation-exchange, do not appear to play a role in the observed changes in oxidation TOF with cation-exchange. Rather, any effect of the alterations to the POM redox properties are far outweighed by the effect of cation addition on the POM acid properties.



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The importance of protons in both methanol dehydration and oxidation and subsequent lack of catalytic impact of altering redox properties with cations may be easily visualized by plotting the oxidation TOF as a function of the dehydration TOF, as shown in Figure 8.

Figure 8: Methanol oxidation TOF plotted as a function of the methanol dehydration TOF for [Na,Mg,Al,Cu]xH3-xPMo12O40 0.2 POM/nm2 where x is: 0 for H; 0.25, 0.5, 1, 2, 3, or 6for Na; 0.5, 1, 1.5, or 3 for Mg; 0.5, 1, 2, or 3 for Al; 0.1, 0.5, 1, 1.5, or 3 for Cu. Reactions were carried out at 200°C in 2.7 kPa H2O, 4 kPa MeOH, 20 kPa O2, balance He. Dashed line is a line of constant selectivity corresponding to the selectivity of the unexchanged catalyst, HPMo (black circle).

Here one can see that the majority of the cation-exchanged POMs fall near the line of constant selectivity (dashed line) with respect to the unexchanged catalyst, HPMo, indicating that both the oxidation and dehydration pathways are perturbed to a similar extent, perhaps as both pathways involve intermediates including interactions with protons as a key component. The notable exception is AlxPMo, which as indicated earlier, exhibited unique acid site characteristics as compared to the other cation-exchanged POM catalysts.


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3.4. Ethanol Dehydration and Oxidation The dehydration and oxidation reactions of ethanol over sodium-exchanged POMs were investigated in order to determine if the dependence of methanol reactivity on acid site population was unique to methanol or may be extended to other alcohols. The reactions were carried out at 160°C, rather than 200°C as was used for methanol, in order to maintain conversion under 10% with otherwise similar reaction conditions, as the supported POM catalysts were significantly more active for ethanol conversion than for methanol. The only oxidation product observed was acetaldehyde, and two dehydration products, diethyl ether and ethylene, were observed, typically occurring in an approximate ratio of 4:1 on a molar basis, for all catalysts investigated. Dehydration rates are normalized per proton and oxidation

Figure 9: Ethanol a) dehydration TOF and b) oxidation TOF for sodium exchanged POMs, NaxH3-xPMo12O40 0.2 POM/nm2, where x is 0, 0.5, 1, 2, or 3. The TOF are plotted as a function of the quantity of acid sites per POM, as determined by DTBP titrations under reaction conditions. Reactions were carried out at 160°C in 2.7 kPa H2O, 4 kPa C2H5OH, 20 kPa O2, balance He. Error bars reflect standard deviations from multiple runs.



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rates are normalized per POM; proton populations were determined by DTBP titration during reaction with ethanol at 160°C. The ethanol dehydration and oxidation TOFs decreased rapidly with increasing quantities of sodium exchanged for POM protons, in good agreement with the results for methanol presented in Figures 4-7. When the ethanol dehydration and oxidation TOFs are plotted as a function of the quantity of acid sites per POM, as shown in Figure 9, they decrease rapidly as the quantity of acid sites approaches 1 H+/POM, mirroring the results for methanol presented earlier. The dependence of alcohol dehydration and oxidation TOFs on POM acid properties clearly is not limited to methanol, although alcohols larger than ethanol have not been investigated.

Figure 10: Ethanol oxidation TOF as a function of the ethanol dehydration TOF. Reactions were carried out at 160°C in 2.7 kPa H2O, 4 kPa C2H5OH, 20 kPa O2, balance He.

Figure 10 contains the ethanol oxidation TOF plotted as a function of the ethanol dehydration TOF. The linear correlation between ethanol oxidation and dehydration TOF for sodium-


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exchanged POMs is in good agreement with the behavior found for methanol conversion over these materials, as was shown in Figure 8.

4. Discussion Our previous investigation of supported POMs exchanged with sodium established that acid sites are of paramount importance for a high TOF for both methanol dehydration and oxidation.8 Theoretical and kinetics studies by Iglesia and coworkers2,10 for methanol reaction over HPMo confirmed that DME formation proceeds through protonated methanol monomers and dimers. The simplest explanation for the strong correlation of methanol oxidation and dehydration rate is that both involve a common protonated intermediate. This correlation has been observed whether proton concentration has been varied by varying POM coverage,2,10 by exchanging protons for Na+ with supported POM catalysts8 or by using basic supports to neutralize acidity.9

The current work expands upon these findings by examining cation exchange with cations other than sodium. Very few differences were observed in the physical or catalytic properties of silica-supported POMs when exchanged with Na+, Mg2+, or Cu2+. As observed previously for Naexchanged catalysts, acid site quantities decreased linearly with cation addition at a rate of 0.5±1 H+/POM per cation added. Both we8 and Iglesia and co-workers2 have shown that deposition of H3PMo12O40 results in the loss of one proton to the silica support. The similar extents of proton removal with exchange of mono- and di-valent cations in the present work suggests the possibility of cation interactions with neighboring silanols, thereby altering the 27 ACS Paragon Plus Environment


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apparent exchange stoichiometry. In addition to behaving similarly with respect to acid site quantity, the catalytic activity for acid-catalyzed reactions was also similar for Na, Mg, and Cu. With the addition of these cations, the DME TOF rapidly decreased, presumably due to decreasing acid site strength with cation addition. Furthermore, the oxidation activity of these catalysts was very similar, mirroring the trends observed for dehydration, indicating that the alteration of POM acid sites with the addition of Na, Mg, or Cu results in a general decrease in reactivity for methanol over these materials. This is demonstrated by plotting the TOF as a function of H+/POM (Figure 4 and Figure 6) and by the direct correlation between the oxidation and dehydration TOF (Figure 8). The behavior of Al-exchanged, supported POMs was different than that of the other cations used in this work. These catalysts retained a higher quantity of acid sites with cation addition (~0.2 H+ were removed on average per 1 Al3+). Bulk POM diffraction lines were observed in XRD for high levels of cation exchange, and this catalyst series exhibited a higher, yet still generally decreasing, dehydration and oxidation TOF with Al addition compared to other cation-exchanged catalysts. As depicted in Scheme 4, reactions of aluminum cations with water may produce additional acid sites on the catalyst not associated with the supported POMs. This is analogous to the presence of extra-framework aluminum in aluminosilicate zeolites.

In order to eliminate the possibility that the dependence of oxidation activity on POM acid sites is unique to methanol, ethanol oxidation and dehydration was investigated at comparable conditions and with identical catalysts. Parallel behavior was observed for ethanol dehydration and oxidation, with decreasing TOFs with cation addition, and a direct correlation between the 28 ACS Paragon Plus Environment

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TOF of the two pathways (Figure 10). This suggests that over POMs both methanol and ethanol oxidation proceed via protonated intermediates, as described by Iglesia for methanol.2,10

Variations in the reducibility of these cation-exchanged catalysts inferred from spectroscopic measurements of the solid catalysts do not appear to play a distinguishable role in determining the oxidation activity, which is surprising considering the proposed transition state involves reduction of POM framework metal atoms (MoVI to MoV).2 As suggested by the strong correlation of acid and oxidation activities above, the interaction of methanol with protons, rather than oxidation of the protonated complex, appears to be the primary factor controlling oxidation activity under the conditions of our experiments.

The catalytic and spectroscopic results can be reconciled by a framework in which the various ionic species are solvated under our reaction conditions. This behavior for supported POM catalysts is analogous to the well-known pseudoliquid catalysis by bulk POMs. As we have suggested previously,8 solvation of protons and exchanged cations can influence the electronic interactions between them and enhance the mobility of protonated intermediates that undergo oxidation at POM oxidation sites. This description also helps to explain the absence of cation effects on POM oxidation behavior that might be expected from differences in the UV-vis spectra of cation-exchanged solid catalysts. We have previously shown that in aqueous environments, cation exchange does not affect POM absorption edge energies7, in contrast to the results in Table 1 for supported catalysts dried at elevated temperature. This again suggests that, under our reaction conditions in which water is co-fed as well as being produced 29 ACS Paragon Plus Environment


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by both reaction channels, the surface environment of the supported POM catalyst is best described as solvated.

Based upon these results, the feasibility of using POM cations to improve POM oxidation activity is quite limited. Kozhevnikov characterized the acid strength of supported and cation exchanged POMs via ammonia adsorption microcalorimetry.23 Those workers found that both supporting POMs on a high surface area supports (SiO2, TiO2, ZrO2, Nb2O5, Cs3PW12O40) and cation-exchanging POMs with Cs led to a significant decrease in acid strength, and ultimately that the initial heat of ammonia adsorption, a measure of acid site strength, correlated very well with the rate of ethanol dehydration. Janik also investigated cation-exchanged and supported POMs via DFT calculations, reaching similar conclusions.6 Similarly, Moffat investigated the acid strength of cation-exchanged POMs using Hammett indicators,24 and determined that cation addition decreases acid strength and that the relative acid strength of cation-exchanged POMs is determined by the polarizing power of the cation (i.e., H+>>Al3+>Mg2+>Na+).

In contrast, other studies have found beneficial effects associated with POM cation exchange at industrially relevant conditions,4,19,21,25 including in the patent literature.1,4 However, one must consider the possibility that POMs may not be intact at these harsh conditions.20,27–30 At mild conditions where POMs are confirmed to be intact (in our studies for T ≤ 200°C with co-fed H2O) there is little catalytic benefit of cation-exchange (although cation exchange with cations like Cs may improve stability, surface area, and solubility for some applications4,31), as at these 30 ACS Paragon Plus Environment

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mild conditions activation and coordination of reactants and reaction intermediates via interactions with acid sites control the kinetics of both dehydration and oxidation pathways.



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5. Conclusions

The dehydration and oxidation reactions of methanol over cation-exchanged, supported POMs were investigated, and few differences in activity were found for different cations at comparable levels of cation exchange. Rather, the acidic properties of the cation-exchanged POMs were the primary descriptor for both dehydration, a well-known Brønsted acid catalyzed reaction, and oxidative dehydrogenation. This critical role of protons in the oxidative dehydrogenation route, as well as the similarity in TOF trends with cation-exchange for methanol and ethanol, suggests that small alkanols may undergo oxidation via a protonated intermediate. Additionally, although it has been proposed that the rate-limiting step in methanol oxidation over POMs involves POM reduction, evidently this electron transfer is facile enough to not impact oxidation rates in light of other changes to POM properties, in particular their acidity. Together these results suggest that, at mild conditions where POMs are intact, the surface environment may best be described as solvated, and the opportunity to use countercation exchange to tailor POM properties to improve selectivity and activity is quite limited.

Associated Content Supporting Information FTIR of all catalysts in this work; Post reaction FTIR of selected catalysts; XRD of all catalysts in this work; post reaction XRD of selected catalysts (PDF)


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Dehydration and Oxidation of Alcohols by Supported Polyoxometalates

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