1 Activation Mechanism and Surface Intermediates During Olefin

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Activation Mechanism and Surface Intermediates During Olefin Metathesis by Supported MoO/AlO Catalysts x

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Anisha Chakrabarti, and Israel E. Wachs J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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The Journal of Physical Chemistry

Activation Mechanism and Surface Intermediates During Olefin Metathesis by Supported MoOx/Al2O3 Catalysts Anisha Chakrabartiǂ and Israel E. Wachs* Operando Molecular Spectroscopy Laboratory, Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA 18015, USA ǂHoneywell

UOP, 25 E. Algonquin Road, Des Plaines, IL 60017

*Corresponding author: [email protected]

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Abstract The activation of supported MoOx/Al2O3 catalysts and the resulting surface intermediates were examined with in situ DRIFTS during olefin metathesis reaction conditions. The studies were aided with C3D6-C3H6 isotopically labeled switching, C2H4-C4H8 titration, and temperature programmed reaction experiments. Activation of the surface MoOx sites on Al2O3 by propylene initiates by forming surface isopropoxide species that subsequently dehydrogenate to acetone and surface Mo-OH. Desorption of acetone from the catalyst surface reduces the Mo+6 sites to Mo+4 sites and creates a vacancy for coordination of the next CH2=CHCH3 molecule. Oxidative addition from subsequent adsorption of propylene on the surface Mo+4 sites results in formation of surface Mo=CH2 and Mo=CHCH3 reactive intermediates and oxidizes the reduced surface molybdena sites back to Mo+6. This study establishes the activation mechanism and surface intermediates during olefin metathesis by supported MoOx/Al2O3 catalysts under reaction conditions.

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1

Introduction Olefin metathesis for the reaction of propylene to ethylene and 2-butene was first

commercialized in 1966 and dubbed the Phillips Triolefin Process.1 The reversibility of the olefin metathesis reaction also allows for propylene to be produced from ethylene and 2-butene (now known as Olefin Conversion Technology (OCT)).2,3 The Shell Higher Olefin Process (SHOP), established in 1968 and used to produce linear higher olefins starting from ethylene, consists of three main stages: (i) ethylene oligomerization with a homogeneous nickel-phosphine catalyst in a polar solvent to produce a mixture of linear even-numbered α-olefins ranging from C4-C40, (ii) double-bond isomerization of the lighter (< C6) and heavier (> C18) olefins with a solid potassium metal catalyst to yield an equilibrium mixture of internal olefins, and (iii) olefin metathesis by reaction of longer olefins with ethylene to yield linear higher olefins with an alumina-supported molybdenum oxide catalyst.3 The continued importance of olefin metathesis catalysis is further driven by the increasing global demand for on-purpose propylene and higher olefins.4-6 The activation mechanism and surface reaction intermediates during olefin metathesis by Al2O3-supported surface MoOx sites are still poorly understood because of the limited reported studies. Early in situ IR studies7 of propylene adsorption on supported MoOx/Al2O3 catalysts concluded that propylene coordinates by π-bonding to the surface for both C3H6-activated and COreduced catalysts. The detected IR band at 1600 cm-1 was assigned to the C=C bond of π-bonded propylene that was red-shifted from the band at 1640 cm-1 for propylene adsorbed on pure Al2O3. It was concluded that the adsorption of propylene was reversible, and the π-bonded complex is a surface reaction intermediate for propylene metathesis. In contrast, DFT studies8 have suggested that Mo-cyclobutane intermediates anchored to surface AlO6 sites of the Al2O3 support are responsible for high olefin metathesis reactivity. The theoretical studies, however, did not 3



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investigate the activation mechanism of alumina-supported MoOx sites during olefin metathesis and such cyclic intermediates are not usually highly stable surface species9. The adsorption and reaction of propylene on supported MoOx/Al2O3 catalysts was also investigated with in situ IR spectroscopy by Davydov et al.10,

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Propylene adsorption on an

oxidized supported MoOx/Al2O3 catalyst at room temperature exhibited IR bands at ~1090 cm-1 from ν(C-O), 1380 cm-1 from δs(CH3), 1465 cm-1 from δas(CH3), 2885 and 2940 cm-1 from νs(CH), and 2980 cm-1 from νas(C-H) that were assigned to a surface isopropoxide complex since the same bands were observed after adsorption of isopropyl alcohol. The surface isopropoxide complex was assumed to have formed by transfer of a mobile proton from the catalyst Brönsted acid site to the adsorbing propylene molecule. After heating the adsorbed surface isopropoxide intermediate to 100 °C, IR bands appeared at 1250 and 1680 cm-1 from ν(C-C) and ν(C=O) vibrations, respectively, indicating acetone formation. This led to the conclusion that acetone formation on oxidized supported MoOx/Al2O3 catalysts requires strong Brönsted acid sites to protonate the adsorbed propylene to surface isopropoxide that is further oxidized to acetone.10, 11 Although the surface intermediates and acetone formation mechanism were investigated in detail, this study did not address the state or activation mechanism of the alumina-supported molybdena sites during propylene metathesis. The absence of reported in situ spectroscopic characterization studies that directly probe the molecular events during the initial stages of olefin metathesis by supported MoOx/Al2O3 catalysts motivates the present investigation. The objectives of the current study are to establish the activation mechanism and surface reaction intermediates during olefin metathesis for heterogeneous supported MoOx/Al2O3 catalysts by the application of time-resolved in situ IR spectroscopy coupled with the aid of deuterated olefins and olefin titration studies. 4


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2 2.1

Experimental Methods Catalyst Synthesis The supported MoOx/Al2O3 catalysts (20-25wt% MoO3) were previously prepared12 via

incipient-wetness impregnation of an aqueous solution of ammonium molybdate (para) tetrahydrate ((NH4)6Mo7O24•4H2O, Alfa Aesar, 99%, Lot No. 10120802) onto a Sasol Al2O3 support (Sasol, Catalox Lot No. C2939, BET S.A. = 218 m2/g). The supported 20% MoOx/Al2O3 catalyst was synthesized with the molybdena in two separate impregnation steps of 10% MoOx each to improve dispersion of the molybdena on the alumina support. The alumina support was calcined at 500 °C for 16 h under flowing air prior to impregnation. The impregnation step was performed under ambient conditions, and the impregnated mixture was stirred for ~30 min to maximize MoOx dispersion. Using a programmable furnace (Thermolyne, Model 48000), the samples were further dried at 120 °C for 2 h, then calcined by ramping the temperature at 1 °C/min under flowing air (Airgas, Dry grade, 150 mL/min) to 500 °C and holding for 4 h. The final synthesized catalysts are denoted as x% MoOx/Al2O3, where x is the weight percent of MoO3 impregnated on the alumina supports. 2.2

In situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) In situ DRIFTS spectra were obtained using a Thermo Scientific Nicolet 8700 spectrometer

equipped with a Harrick Praying Mantis attachment (model DRA-2). For each experiment, the catalyst was loaded as a loose powder (~20 mg) into an in situ environmental cell (Harrick, HVCDR2). The temperature of the Harrick cell was controlled by a Harrick ATC Temperature Controller. The flow rates were monitored with mass flow controllers (Brooks, Model 5850E). The IR vibrations from the Sasol Catalox Al2O3 support (~1041 cm-1) were used as an internal standard to normalize the signal intensity of the IR spectra. Spectra were collected using a high 5



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sensitive mercury-cadmium-telluride (MCT-A) detector (cooled with liquid N2) with a resolution of 4 cm-1 and an accumulation of 96 scans (total of ~1 min per spectrum) in the range of 650-4000 cm-1. 2.2.1

Dehydration Pretreatment

The experimental protocol was as follows: (1) under flowing 10% O2/Ar (Praxair, certified 10.2% O2/balance Ar, 30 mL/min), initially heat the sample at 10 °C/min from room temperature to 500 °C and hold for 1 h, switch to flowing Ar (Airgas, UHP, 30 mL/min), and cool at 10 °C/min to 30 °C or 120 °C. 2.2.2

Adsorption of Acetic Acid (CH3COOH) and TP with C3H6

After the initial dehydration procedure as outlined above, the experimental procedure was as follows: (2) adsorb acetic acid by flowing CH3COOH/Ar (Glacial, Fisher, certified ACS grade, ≥ 99.7% purity, [H2O] not specified, [O2] < 6.8 x 10-4 (mole fraction)13) (30 mL/min) for 30 min at 30 °C and (3) switch to flowing 1% C3H6/He (Praxair, certified, 1.00% C3H6/balance He, 30 mL/min) and ramp the temperature at 10 °C/min to 500 °C under flowing 1% C3H6/He. 2.2.3

Adsorption of Isopropanol (CH3CHOHCH3) and TP with C3H6

After the initial dehydration procedure as outlined above, the experimental procedure was as follows: (2) adsorb isopropanol by flowing CH3CHOHCH3/Ar (Alfa Aesar, HPLC grade, 99.7% purity, [H2O] ≤ 0.05%, [O2] = 7.78 x 10-4 (mole fraction)14) (30 mL/min) for 30 min at 30 °C and (3) switch to flowing 1% C3H6/He (Praxair, certified, 1.00% C3H6/balance He, 30 mL/min) and ramp the temperature at 10 °C/min to 510 °C under flowing 1% C3H6/He. 2.2.4

Adsorption of Acetone (CH3COCH3) and TP with C3H6

After the initial dehydration procedure as outlined above, the experimental procedure was as follows: (2) adsorb acetone by flowing CH3COCH3/Ar (Pharmco, ACS grade, 99.5% purity, [H2O] 6


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= 0.3% typically, [O2] = 8.71 x 10-4 (mole fraction) [14]) (30 mL/min) for 30 min at 30 °C and (3) switch to flowing 1% C3H6/He (Praxair, certified, 1.00% C3H6/balance He, 30 mL/min) and ramp the temperature at 10 °C/min to 510 °C under flowing 1% C3H6/He. 2.2.5

Adsorption of C4H8 and Titration with C2H4

After the initial dehydration treatment as outlined above, the experimental protocol was as follows: (2) adsorb 2-butene by flowing 1% C4H8/Ar (Praxair, certified 1.00% C4H8/balance Ar, 30 mL/min) for 60 min at 120 °C, (3) flush with flowing Ar for 45 min (Airgas, UHP, 30 mL/min), (4) switch to flowing 1% C2H4/Ar (Praxair, certified, 1.00% C2H4/balance Ar, 30 mL/min) for 45 min at 120 °C, and (5) ramp the temperature at 10 °C/min to 510 °C under the flowing 1% C2H4/Ar (30 mL/min). 2.2.6

Adsorption of C3H6 and TP with C3H6

After the initial dehydration treatment as outlined above, the experimental protocol was as follows: (2) adsorb propylene by flowing 1% C3H6/He (Praxair, certified, 1.00% C3H6/balance He, 30 mL/min) for 45 min at 120 °C, (3) flush with flowing Ar for 45 min (Airgas, UHP, 30 mL/min), (4) switch to flowing 1% C3H6/Ar (Praxair, certified, 1.00% C3H6/balance Ar, 30 mL/min) for 45 min at 120 °C, and (5) ramp the temperature at 10 °C/min to 510 °C under flowing 1% C3H6/Ar (30 mL/min). 2.2.7

Adsorption of C3D6 and TP with C3H6

After the initial dehydration as outlined above, the experimental protocol was as follows: (2) adsorb D-propylene by flowing 10% C3D6/Ar (C/D/N Isotopes, certified, >98% C3D6) (30 mL/min) for 30 min at 120 °C, (3) switch to flowing 1% C3H6/He (Praxair, certified, 1.00% C3H6/balance He, 30 mL/min) for 45 min at 120 °C, and (4) ramp the temperature at 10 °C/min to 510 °C under flowing 1% C3H6/He (30 mL/min). 7



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Results

3.1

In Situ IR Spectra of Reference Molecules (Acetic Acid, Isopropanol, and Acetone) The in situ IR spectra from reference molecules (acetic acid, isopropanol, and acetone)

adsorbed on the dehydrated supported MoOx/Al2O3 catalysts were examined (see Figure S1). These data are presented first to provide clearer assignments of the various surface intermediates detected during olefin exposure of the supported MoOx/Al2O3 catalysts. 3.1.1

In situ IR Spectra of Reference Molecules Adsorbed on Supported MoOx/Al2O3

The in situ DRIFT difference spectrum of acetic acid adsorbed on the supported MoOx/Al2O3 catalyst is presented in Figure S1. During CH3COOH adsorption at 30 °C, bands for gas-phase acetic acid are observed in the bending region at ~1200 for ν(C-O) and ~1425 for δ(C-O-H) cm-1, and at ~2560, 2630, 2700, 2940, 3018, 3065, and 3133 cm-1, which may be for either O-H or C-H stretching vibrations since they overlap for carboxylic acids.15,

17

IR bands also appear at

~1275/1295 from the stretching mode ν(C-O), ~1338/1350 cm-1 from δ(α-CH3) bending mode, ~1725 cm-1 for H-bonded ν(C=O), and ~1765 cm-1 for ν(C=O) from surface acetate. Additionally, bands are observed at ~1470 and 1578 cm-1 from surface acetate stretching vibrations of νs(COO) and νas(COO-), respectively. Bands at ~1725 and 1765 cm-1 for the ν(C=O) vibration of molecularly adsorbed acetic acid are also expected but are overshadowed by the gas-phase acetic acid vibrations. The in situ DRIFT difference spectrum from adsorption of isopropanol at 30 °C on the supported MoOx/Al2O3 catalyst is also presented in Figure S1. Gas-phase CH3CHOHCH3 exhibits bands at 1130, 1160, and 2980 cm-1. The characteristic IR bands of surface isopropoxide species appear in the bending region at ~1275 cm-1 for ν(C-O), ~1338 cm-1 for δ(α-CH3), ~1376 and 1385 8


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for δs(CH3), and ~1465 cm-1 for δas(CH3)16. The corresponding vibrations in the hydrocarbon stretching region for surface isopropoxide species are from the stretching modes of νs(CH3) at ~2885 and 2935 cm-1.16, 17 The in situ DRIFT difference spectrum after acetone adsorption at 30 °C is also presented in Figure S1 and exhibits the characteristic IR bands for CH3COCH3. Gas-phase acetone exhibits bands at ~1210-1230 cm-1, 1720 and 1736 cm-1 from ν(C=O), and 2923, 2970, 3008, and 3030 cm-1 in the C-H stretching region.13 Acetone coordinated to the catalyst surface gives rise to bands at ~1365 and 1425 cm-1 from the δs(CH3) and δas(CH3) deformation modes, respectively.16, 17 The C=O bands for adsorbed acetone may also be present but would be overshadowed by the similar vibrations from gas-phase acetone. 3.1.2

In situ IR Spectra during Subsequent Temperature Programming under Flowing Propylene after Adsorption of Reference Molecules

The in situ DRIFT difference spectra during subsequent temperature programming under propylene after acetic acid adsorption are presented in Figure S2. When the reactant gas is switched to flowing propylene, the vibrations from gas-phase acetic acid (~1425 cm-1 and ~2560~3133 cm-1) and molecularly adsorbed acetic acid (~1725 cm-1) decrease, while the surface acetate vibrations at ~1470 and ~1578 from νs(COO-) and νas(COO-), respectively, increase with temperature under flowing propylene (see Figure S3). The increase in the surface COO- bands demonstrates that more surface acetate is being formed under propylene metathesis conditions even after surface acetate from conversion of molecularly adsorbed acetic acid is decomposed. The in situ DRIFT difference spectra for the subsequent temperature programming under flowing propylene after isopropanol adsorption are presented in Figure S4. When the reactant gas 9



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flow is switched to propylene after isopropanol adsorption and the temperature is increased to 120 °C, the bands for the surface isopropoxide species decrease, while the 1385 cm-1 for δs(CH3) present during acetone adsorption shifts to ~1390 cm-1, and a band appears at ~1700 cm-1 for the ν(C=O) of a carbonyl-containing species. As the temperature is increased to 180 °C under the propylene flow, the carbonyl band increases and red-shifts to ~1683 cm-1 suggesting that this surface species is binding to the surface of the supported MoOx/Al2O3 catalyst. As the temperature is further increased to 210 °C in flowing propylene, the ν(C=O) band at ~1683 cm-1 decreases and broad bands appear at ~1465 and 1555 cm-1 assigned to νs(COO-) and νas(COO-), respectively, arising from oxidative decomposition of acetone to surface CH3COO*.10, 11, 18 The in situ DRIFT difference spectra during subsequent temperature programming after adsorption of acetone are shown in Figure S5. With the introduction of propylene at 30 °C, the IR bands in the ~1210-1750 cm-1 region from gas-phase acetone decrease since acetone flow has been replaced with the flow of propylene. There is little change, however, in the hydrocarbon stretching region (2800-3200 cm-1) since the gas-phase propylene C-H vibrations overlap with those of gasphase acetone in the C-H vibrational region.15 As the temperature is increased to 60 °C (above the boiling point of acetone) under flowing propylene, the bands for the acetone decrease because physisorbed acetone evaporates from the catalyst.19 The presence of a surface carbonyl-containing species is evidenced by the band at 1700 cm-1 that red-shifts to ~1680 cm-1 as the temperature continues to increase, reflecting anchoring of the carbonyl group to the catalyst surface. When the temperature is further increased to 210 °C under the flowing propylene, the IR band at ~1683 cm-1 decreases and broad bands are detected at ~1465 and 1570 cm-1 that are assigned to νs(COO-) and νas(COO-) vibrations, respectively, from the oxidative decomposition of surface acetone to surface acetate.10, 11, 18 10


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3.2

In Situ DRIFTS during C2H4-C4H8 Titration The in situ DRIFT spectra during the C2H4-C4H8 titration of the supported 20% MoOx/Al2O3

catalyst, which corresponds to approximately monolayer coverage of surface MoOx sites as previously shown in Reference

12,

ensuring none of the detected IR bands arise from surface

hydrocarbon species adsorbed on exposed alumina sites, are presented in Figure 1 (Figure 1A exhibits the 1200-1900 cm-1 vibrational range and Figure 1B exhibits the vibrational region of 2800-3200 cm-1). Adsorption of 2-butene at 120 °C gives rise to IR bands at 2864, 2923, and 3035 cm-1 characteristic of gas phase 2-butene15 that are not present during the subsequent Ar flush. Several other bands that appear during 2-butene adsorption persist during the Ar flush indicating that they are related to strongly-bound surface intermediates. The CH2 and CH3 vibrations overlap and cannot be distinguished in this case, thus, the assignment of the IR vibrations are as follows: 1390 and 1460 cm-1 to δs(CH2/CH3) and δas(CH2/CH3) deformation modes, respectively; 2880 and 2939 cm-1 to the corresponding symmetric stretching modes νs(CH2/CH3); and the overlapping IR bands at 2965 and 2980 cm-1 to the corresponding asymmetric stretching modes νas(CH3).17, 20, 21 These IR bands are characteristic of the surface intermediate Mo=CHCH3.16, 22 A strong IR band is also observed at ~1680 cm-1 that is slightly red-shifted to lower wavenumbers from the characteristic gas-phase carbonyl stretching region (~1700-1800 cm-1), suggesting that there is formation of a surface intermediate containing a carbonyl group (i.e. Mo-O=C-R1R2), and assigned to the surface carbonyl stretching ν(C=O) mode. This band overlaps a weaker shoulder band at ~1650 cm-1 assigned to the νs(C=C) of an adsorbed 2-butene π-complex on surface MoOx sites.20 There is an additional weak band at ~1555 cm-1 assigned to νas(COO-) as in the previous section. During ethylene titration of the surface complexes formed by 2-butene adsorption at 120 °C after 11



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the argon flush, little change is observed in the location of the IR bands since the bands from surface intermediates formed by ethylene adsorption, surface Mo=CH2 (νas(CH2) at ~2929 cm-1)7, 21,

would overlap those of the surface Mo=CHCH3 intermediates. The IR bands appearing during

ethylene titration of the surface complexes formed by 2-butene adsorption are, thus, assigned as follows: 1390 and 1460 cm-1 to δs(CH2/CH3) and δas(CH2/CH3) deformation modes, respectively, and 2880 and 2935 cm-1 to the corresponding stretching modes νs(CH2/CH3) and νas(CH2/CH3), respectively. As the temperature is increased during temperature programming under flowing ethylene, the intensity of the bands at ~1650 and ~1680 cm-1 from surface Mo=CHCH3 intermediates decrease. Beginning at ~210 °C, several more changes take place in the IR spectra: the initially weak band at ~1555 cm-1, first observed during 2-butene adsorption and through C2H4 titration, becomes more pronounced, the intensity of a band at ~1465 cm-1 increases, and the intensities of the δs(CH2/CH3) and δas(CH2/CH3) bending modes at 1390 and 1460 cm-1, respectively, decrease. The appearance of the IR bands at 1465 and 1555 cm-1 coincide with the disappearances of the bands at 1680 cm-1 for ν(C=O) and 1390/1460 cm-1 for δs(CH2/CH3)/δas(CH2/CH3). These changes suggest the decomposition of a surface carbonyl species to surface carboxylates (e.g., HCOO* and CH3COO*), allowing assignment of the bands at 1465 and 1555 cm-1 to surface νs(COO-) and νas(COO-) vibrations, respectively.10, 11

12


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Figure 1. In situ DRIFT difference spectra during C2H4-C4H8 titration at 120 °C and subsequent temperature programming at 10 °C/min under flowing ethylene of the supported 20% MoOx/Al2O3 catalyst. (A) 1200-1900 cm-1 and (B) 2800-3200 cm-1. All gas-phase vibrations are labeled with a subscript “(g)” and shown in black and surface CH2/CH3 species arising from 2-butene adsorption and ethylene adsorption are shown in red. The IR spectra were normalized against the Al2O3 support vibration at ~1041 cm-1, and the difference spectra were obtained by subtracting the IR spectrum of the initial dehydrated catalyst from each IR spectrum taken during the titration experiment. 3.3

In Situ DRIFTS during C3H6-Adsorption-TP 13



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In situ DRIFTS of propylene adsorption and temperature programming in flowing propylene (see Figure 2) revealed IR bands similar to those observed during C2H4-C4H8 titration. Gas-phase propylene gives rise to bands at 1339, 1384, and 1466 cm-1 in the bending region and 2920, 2943, 2953, 2985, 3082, and 3103 cm-1 in the C-H stretching region15 that are absent or diminished during the subsequent Ar flush. After the Ar flush, IR bands are still present at 1375 cm-1 for the δs(CH3) deformation mode, 1653 cm-1 for νs(C=C), and 2877, 2929, and 2985 cm-1 for the stretching modes νs(CH3), νas(CH2), and νas(CH3), respectively, from surface complexes.17, 19, 21 These bands from the strongly-bound intermediates are associated with the surface Mo=CH2 and Mo=CHCH3 species16, 22 and are also observed during the C4H8-C2H4 titration. During subsequent propylene TP, the intensities of the IR bands at 1375 cm-1 for δs(CH3) and 1653 and 1687 cm-1 from νs(C=C) and ν(C=O) vibrations, respectively, decrease. The band intensities were plotted with respect to the initial TP temperatures, and the plots were fit with linear best-fit lines. The magnitudes of the slopes of the lines represent the rates of decrease in C=C and C=O and reveal that initially, the C=O band decreases more quickly relative to the C=C band (see Figure S6). Simultaneously, broader bands appear at ~1470 and 1570 cm-1 indicating decomposition of surface carbonyls to surface carboxylates via oxidative breaking of the C2-C3 bond10,

11, 18.

The similarity in the

observed IR bands in the forward (propylene adsorption and TP) and reverse (butene-ethylene titration and TP) directions of the olefin metathesis reaction demonstrate that the activation mechanism of the surface MoOx sites on Al2O3 does not depend on changing the olefin reactants from propylene to ethylene/2-butene. Comparison of the IR spectra taken during adsorption-TP with the reference molecules with those taken of the olefin-exposed catalyst provides additional insight into the activation mechanism. The characteristic IR vibrations for the surface Mo=CH2 and Mo=CHCH3 14


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intermediates are not observed during either acetic acid adsorption or the subsequent temperature programming in flowing propylene. This suggests that the Mo sites may be occupied by surface acetate species that hinder olefin adsorption and metathesis. The shifting of the ν(C=O) band and formation of the νs(COO-) and νas(COO-) bands, from surface carboxylates, are observed during olefin

metathesis

conditions

(C2H4-C4H8

titration

and

C3H6

adsorption-TP)

and

acetone/isopropanol adsorption with subsequent C3H6-TP.

Figure 2. In situ DRIFT difference spectra during propylene adsorption at 120 °C, Ar flush at 120 °C, and subsequent temperature programming at 10 °C under flowing propylene of the 15



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supported 25% MoOx/Al2O3 catalyst: (A) 1250-1900 cm-1 and (B) 2800-3200 cm-1. All gas-phase vibrations are labeled with a subscript “(g)” and shown in black, surface CH3 species arising from propylene adsorption are shown in red, and surface CH2 species arising from propylene adsorption are shown in blue. The IR spectra were normalized against the Al2O3 support vibration at ~1041 cm-1, and the difference spectra were obtained by subtracting the IR spectrum of the initial dehydrated catalyst from each IR spectrum taken during the propylene adsorption-TP experiment.

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3.4

In Situ DRIFTS during C3H6-C3D6 Isotope Exchange The in situ DRIFT spectra collected during C3H6-C3D6 isotope exchange of the supported

20% MoOx/Al2O3 catalyst, which corresponds to approximately monolayer coverage of MoOx, are shown in Figure 3. During initial adsorption of C3D6 at 120 °C, gas phase C3D6 exhibits vibrations at ~1285 cm-1 in the bending region, ~1573-1600 cm-1 for ν(C=C), and ~2348 cm-1 in the hydrocarbon C-D stretching region. After adsorption of C3D6 at 120 °C, the hydrocarbon region exhibits three bands attributed to the two νs(CD3) and νas(CD3) stretching modes at ~2070 and ~2229 cm-1, respectively, for surface Mo=CDCD3 intermediates, and the third at ~2137 cm-1 for νas(CD2) from the surface Mo=CD2 intermediates.16, 22 In the carbonyl region, bands appear at 1650 and 1680 cm-1 assigned to νs(C=C) and ν(C=O), respectively, during C3D6 adsorption, similar to the bands observed in the previous experiments, indicating formation of adsorbed C3D6 and acetone. The hydrocarbon region also exhibits weak bands at ~2935 cm-1 for ν(CH2) vibrations for C3H6 which increase with time-on-stream of C3D6 (see Figure 3) from the minor C3H6 in the C3D6 cylinder. The broad band appearing at ~2600 cm-1 is assigned to ν(OD) vibrations of D-labeled surface hydroxyls that formed by exchange with some D2O by-product. The IR bands for the Dlabeled functionalities are red-shifted to lower wavenumbers compared to the bands for the Hlabeled surface intermediate due to the heavier mass of deuterium relative to hydrogen, which allows discrimination between the surface intermediates formed from these two propylene isotopes.20,

23, 24

When the flow of C3D6 is switched to flow of C3H6, the intensity of the D-

containing bands decrease, while weak bands appear at ~2877 and 2985 cm-1 from νs(CH3) and νas(CH3) vibrations, respectively, from surface Mo=CHCH3 intermediates, and 2929 cm-1 from νas(CH2) vibrations of the surface Mo=CH2 intermediates.16, 17, 21-23 The IR band at ~2877 cm-1 for νs(CH3) is quite weak likely due to IR selection rules that give stronger bands for asymmetric 17



Page 18 of 38

vibrations.20 The IR band for ν(OD) also decreases and shifts to ~3500 cm-1 for ν(OH) arising from exchange of the D-labeled surface hydroxyls by exchange with some H2O by-product.20 During the C3H6-C3D6 TP experiment, the bands associated with D-labeled surface intermediates continue to decrease, while the bands of the H-labeled species at ~2935 and ~2985 cm-1 for νas(CH2/CH3) continue to increase. The formation of the H-labeled surface species and increase in the CH2 and CH3 bands evidence the replacement of reactive surface Mo=CD2/Mo=CDCD3 intermediates by surface Mo=CH2/Mo=CHCH3 intermediates in the presence of gas phase C3H6. Thus, demonstrating that surface Mo=CD2/Mo=CDCD3 and Mo=CH2/Mo=CHCH3 species are reactive intermediates for the propylene metathesis reaction. At ~200 °C, a broad band appears at 1550 cm1, similar to that seen during all of the above results (see Figure 1Figure 2, S1, S2, and S3), assigned

to νas(COO-) of surface acetate. The isotopic shifts and ratios of the detected vibrational bands are presented in Table 1. The calculated ν(CH)/ν(CD) ratios agree well with those calculated for ν(NH3)/ν(ND3) from adsorption of NH3 and ND3 on TiO2 (anatase).23, 24 All the ratios are about the same at ~1.3 and consistent with the IR band assignments given above.

18


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Figure 3. In situ DRIFT difference spectra of supported 20% MoOx/Al2O3 catalyst during C3H6C3D6 isotope exchange at 120 °C after initial adsorption of C3D6 and subsequent temperature programming at 10 °C/min under flowing C3H6: (A) 2000-3800 cm-1, (B) 2700-3200 cm-1, and (C) 19



Page 20 of 38

1200-1900 cm-1. The IR spectra were normalized against the Al2O3 support vibration at ~1041 cm1,

and the difference spectra were obtained by subtracting the IR spectrum of the initial dehydrated

catalyst from each IR spectrum taken during the isotope exchange experiment.

20


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Table 1. Observed Isotopic Shifts

4 4.1

Band Assignment

ν(H) (cm-1)

ν(D) (cm-1)

ν(H)/ν(D)

OH/OD

3500

2600

1.35

CH3/CD3 (symmetric)

2877

2070

1.34

CH2/CD2 (asymmetric) 2935

2137

1.37

CH3/CD3 (asymmetric) 2985

2229

1.39

Discussion Surface Reaction Intermediates In situ DRIFTS of C2H4-C4H8 titration and C3H6 adsorption-TP indicate the appearance of

surface CH2- and CH3-containing species, which is consistent with the metathesis reaction proceeding via reactive surface Mo=CH2 and Mo=CHCH3 carbene intermediates (see Figure 1, Figure 2, and S1-S3). The participation of these surface intermediates in the olefin metathesis reaction is further corroborated by the C3H6-C3D6 isotopic switch study since IR bands are initially observed for the surface Mo=CD2 and Mo=CDCD3 intermediates (see Figure 3) and become replaced by surface Mo=CH2 and Mo=CHCH3 intermediates under flowing C3H6. Such surface carbene intermediates, have been previously proposed for olefin metathesis in the literature, primarily for experimental studies of silica-based supported MoOx16, 22 and supported MoOx/Al2O3 catalysts.10, 11 An adsorbed propylene complex has also been proposed based on in situ IR studies.7 This study concluded that propylene becomes π-bonded to oxidized and CO-reduced supported MoOx/Al2O3 catalysts as evidenced after adsorption and evacuation of propylene over a supported 9% 21



Page 22 of 38

MoOx/Al2O3 catalyst (corresponding to approximately 50% monolayer coverage) by the appearance of an IR band at 1600 cm-1 (red-shifted from the propylene adsorbed on pure Al2O3 at 1645 cm-1).7 From this IR observation, it was concluded that propylene was irreversibly adsorbed and that the active surface intermediate for propylene metathesis was a π-bonded propylene complex. Direct adsorption of propylene on the Al2O3 support was not considered in this study and attempts were not made to distinguish between propylene bonded to surface Mo sites, which may depend on Mo oxidation state, and exposed Al sites from the support. Furthermore, studies were not undertaken to determine if the π-bonded surface propylene complex transforms to other surface complexes during olefin metathesis reaction conditions. In the current studies, catalysts were used that are either approximately at or above monolayer coverage (monolayer ≈ 20% MoOx/Al2O39, 12, 25, 26), which means that any IR bands detected only in the present studies are exclusively related to adsorption of the olefins on the surface MoOx sites.7 The shoulder band at ~1653 cm-1 detected in the present study is assigned to the C=C stretching mode of adsorbed propylene on surface MoOx sites since this band persists during the Ar flush. The difference in position of the IR band in this study compared to that in Reference 7 may be related to the use of vacuum in the earlier studies or the difference in adsorption temperature used (120 °C in the current studies vs. 65 °C in the earlier studies). Handzlik et al.8 concluded from DFT studies that the most likely active reaction intermediates consist of surface Mo-cyclobutane species anchored at surface AlO6 sites of the alumina support. The IR bands observed in the current studies agree with those in other studies concluding surface Mo-carbene species are the reaction intermediates.16,

22, 27

It is possible that surface Mo-

cyclobutane intermediates are formed after further olefin coordination in the presence of gas phase olefins, but surface Mo-cyclobutane species are known to be rather unstable9 and have not been 22


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experimentally confirmed for heterogeneous supported metal oxide catalysts.9, 22, 27 These DFT calculations only examined isolated surface MoOx sites, but previous in situ Raman studies found that oligomeric surface MoOx sites are much more reactive for olefin metathesis than isolated surface MoOx sites for supported MoOx/Al2O3 catalysts.12 DFT studies of the activation mechanism of surface MoOx sites on Al2O3 during olefin metathesis are still absent in the literature and need to be compared with the experimental findings. Well-defined organometallic catalysts, however, with in situ 13C CP MAS NMR of heterogeneous silica-supported Mo-based complexes, such as the d0 Schrock alkylidene catalysts of general structure [(X)(≡SiO)Mo(=NR)(=CHR)], where X = an alkyl ligand, showed that the olefin metathesis reaction does proceed via the surface alkylidene and metallacyclobutane intermediates found for homogenous catalysts.28-31 Such similar studies with conventional heterogeneous supported MoOx/Al2O3 catalysts still need to be performed to determine if olefin metathesis by such commercial-type heterogeneous oxide catalysts proceed via surface alkylidene and metallacyclobutane intermediates. 4.2

Activation Mechanism The current in situ DRIFTS findings demonstrate that surface isopropoxide initially forms via

protonation of propylene by acidic surface hydroxyls (see Figure 2 and S2). The surface isopropoxide subsequently oxidizes to form coordinated acetone (see Figure S2). Formation of acetone is evidenced by the appearance of the IR ν(C=O) stretching vibration at 1680 cm-1 (see Figure S1). Adsorption of acetone demonstrated that the IR ν(C=O) band for gas-phase acetone occurs at ~1700 cm-1 that red-shifts to ~1680 cm-1 when it coordinates to the surface MoOx sites. Some of the acetone subsequently desorbs allowing coordination of another propylene molecule via oxidative addition. This causes the surface Mo metal center to go through a re-oxidation step Mo+4 → Mo+6 with the electrons provided by coordination of propylene to form surface Mo23



Page 24 of 38

carbenes (changes in oxidation state were established in a previous study Ref 12). At temperatures greater than 210 °C, the remaining adsorbed acetone decomposes to surface acetate CH3COO* (see Figure 1Figure 2, and S1-S4). The surface CH3COO* is stable even up to temperatures as high as 450 °C (see Figures S3-S4) and may deactivate the surface MoOx sites by blocking access for the olefin reactants to the MoOx centers. Thus, a temperature above ~200 °C is undesirable for olefin metathesis by Al2O3-supported MoOx catalysts in order to minimize surface acetate CH3COO* formation (commercial operating temperatures are ~25-200 °C 9). Oxidation of propylene to acetone via surface isopropoxy intermediates has previously been shown to take place with Mo-based oxide catalysts of Co3O4-MoO332, SnO2-MoO332, supported MoOx/Al2O310, 11 and supported MoOx/SBA-15.16 The studies with Co3O4-MoO3, SnO2-MoO3 and MoOx/Al2O3 catalyst studies focused on propylene oxidation rather than propylene metathesis. Amakawa et al.16, however, investigated propylene metathesis and detected formation of methyl groups during propylene adsorption at 100 °C on a supported MoOx/SBA-15 catalyst and subsequent evacuation with in situ IR spectroscopy characterization. The IR bands for ν(C=C) vibrations were not detected, leading to the conclusion that there were no strongly bound species with C=C bonds. After propene dosing for 30 min and subsequent evacuation, the IR bands for the stretching modes of C-H bonds in CH3 groups were detected at 2983, 2939, and 2880 cm-1 and the corresponding deformation modes were attributed to surface isopropoxy species adsorbed at surface MoOx sites, which was confirmed by adsorption of isopropanol and acetone. The surface Mo+6-alkylidene species were proposed to be generated by protonation of propylene driven by surface Brönsted acid sites, oxidation of isopropoxide to acetone, and oxidative addition of propylene to create the Mo+6-alkylidene species. Although the proposed mechanism assumes

24


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desorption of acetone, there was no direct evidence reported for acetone desorbing from the catalyst.16 In agreement with the above conclusions, surface methyl groups and surface isopropoxide species were detected during propylene adsorption and during the C2H4-C4H8 titration in the present study with supported MoOx/Al2O3 catalysts. Additionally, the activation mechanism proposed here is similar since acetone desorption was detected during in situ DRIFTS (see Figure 1Figure 2, and S1-S4) and previous C3H6-TPSR-MS with supported MoO3/Al2O3 catalysts during activation of surface MoOx sites on alumina.12 Furthermore, in situ DRIFTS during propylene adsorption and C2H4-C4H8 titration on the supported MoOx/Al2O3 catalyst detected νs(C=C) vibrations at ~1650 cm-1 that persisted during the Ar flush (see Figure 1 and Figure 2), suggesting they are related to strongly bound surface intermediates.9 The difference between the current findings for supported MoOx/Al2O3 catalyst and earlier supported MoOx/SBA-15 catalyst study may be related to the more difficult activation of surface MoOx sites supported on silica that require higher activation temperatures.9 Davydov et al.10, 11 employed in situ IR spectroscopy to elucidate the mechanism of acetone formation and role of Brönsted acid sites for the selective oxidation of propylene by supported MoOx/Al2O3 catalysts. In the present investigation, propylene adsorption on a supported 25% MoOx/Al2O3 catalyst, containing about monolayer surface MoOx coverage, indicated the formation of a surface isopropoxide intermediate with the appearance of IR bands at 1090 cm-1 ν(C-O), 1250 cm-1 ν(C-C), 1380 cm-1 δ(CH), 1465 cm-1 δ(CH), and 2980 cm-1 ν(CH) after 15 min of propylene exposure at room temperature. After 20 h, additional bands appeared at 1680 cm-1 ν(C=O), 2885 cm-1 ν(CH), and 2940 cm-1 ν(CH). These bands were concluded to indicate the presence of a surface isopropoxide complex, which was assumed to have formed by transfer of a 25



Page 26 of 38

mobile proton from the catalyst (Brönsted acid site) to the adsorbing propylene molecule. Adsorption of isopropyl alcohol on the catalyst corroborated the assignment giving similar IR bands. Upon increasing the temperature under flowing propylene, the intensity of the bands for the surface isopropoxide intermediate decreased while those of acetone appeared (ν(C=O) at 1680 cm1

and ν(C-C) at 1250 cm-1). Propylene adsorption on the catalyst at room temperature and

subsequent heating to 100 °C indicated production of acetone since IR bands appeared at 1250 and 1680 cm-1 from ν(C-C) and ν(C=O) vibrations, respectively. As the temperature was further increased to 400 °C, destructive oxidation of acetone to form surface carboxylates was attributed to the appearance of bands at 1480 and 1570 cm-1 assigned to νs(COO-) and νas(COO-), respectively. Thus, it was concluded that acetone formation on supported MoOx/Al2O3 catalysts requires Brönsted acid sites to protonate the propylene to form surface isopropoxide intermediates that are further oxidized to acetone and eventually surface acetate at higher temperatures.10, 11 The current findings agree well with the results and conclusions of Davydov et al. Propylene adsorption on supported the 25% MoOx/Al2O3 catalyst (see Figure 2) and adsorption of isopropanol on 20% MoOx/Al2O3 (see Figure S2) resulted in similar spectral bands to those detected by Davydov et al. and confirm that propylene is protonated by a Brönsted proton to form surface isopropoxides.10, 11 The details of the proposed activation mechanism during propylene metathesis by oligomeric surface MoOx sites on alumina are depicted in Scheme 1.

26


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Scheme 1. Proposed activation mechanism for oligomeric MoOx surface species anchored to acidic OH-µ1/3-AlV/VI surface sites on Al2O3.

5

Conclusions The activation of the surface MoOx sites on Al2O3 during propylene metathesis at 120 °C

proceeds by initial protonation of adsorbed propylene to form surface isopropoxide species (detected with in situ DRIFTS). The surface isopropoxide species oxidize to acetone that coordinates to the surface MoOx sites (detected with in situ DRIFTS). The acetone subsequently readily desorbs from the catalyst due to its low boiling point temperature of 56 °C. Acetone desorption reduces the surface MoOx sites (Mo+6 → Mo+4) and allows subsequent coordination of another propylene molecule by oxidative addition (formation of surface Mo=CH2 and Mo=CHCH3 species) that re-oxidizes the reduced surface molybdena sites back to surface Mo+6Ox. Above 210 °C, the acetone can also decompose to stable surface acetate species that block adsorption of propylene on the surface MoOx sites and prevent the surface MoOx sites from being able to 27



Page 28 of 38

continue the propylene metathesis reaction. This study establishes the activation mechanism of supported MoOx/Al2O3 catalysts and the surface reaction intermediates during olefin metathesis reaction conditions.

Supporting Information. This file contains supporting information. 

In situ DRIFT difference spectra of reference molecules



In situ DRIFT difference spectra of acetic acid adsorption with subsequent propylene-TP



In situ DRIFT difference spectra of isopropanol adsorption with subsequent propylene-TP



In situ DRIFT difference spectra of acetone adsorption with subsequent propylene-TP

This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors acknowledge funding from the Department of Energy Basic Energy Sciences (FG0293ER14350). The authors also acknowledge the free sample of Catalox alumina from Sasol. Additionally, the authors thank Dr. Michael E. Ford and Bin Zhang for performing the in situ DRIFTS experiment of acetic acid adsorption with subsequent propylene-TP.

28


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References

[1] Banks, R. L. In Applied Industrial Catalysis; Leach, B. E., Ed.; Academic Press: Orlando, FL, 1984; pp 215-239. [2] Rouhi, A. M. Olefin Metathesis: Big-Deal Reaction (2002) C&EN 80: 29-33. [3] Mol, J. C. Industrial Applications of Olefin Metathesis. J. Mol. Catal. A: Chem. 2004, 213, 3945. [4] Pieper, L.; Stryk, A. Market Watch. http://www.cbi.com/getattachment/47eebcf7-2176-44ab8f8d-4a1e64505018/Market-Watch.aspx (accessed 2017). [5] Shell. www.shell.com (accessed 2017). [6] Plotkin, J. S. The Propylene Quandary. https://www.acs.org/content/acs/en/pressroom/cutting-edge-chemistry/the-propylenequandary.html (accessed August 2017). [7] Olsthoorn, A. A.; Moulijn, J. A. An In Situ Infrared Spectroscopic Study of the Activity of yAlumina Supported Mo(CO)6 for Metathesis and Ethene Polymerization. J. Mol. Catal. 1980, 8, 147-160. [8] Handzlik, J.; Sautet, P. Active Sites of Olefin Metathesis on Molybdena-Alumina System: A Periodic DFT Study. J. Catal. 2008, 256, 1-14. [9] Lwin, S.; Wachs, I. E. Olefin Metathesis by Supported Metal Oxide Catalysts. ACS Catal. 2014, 4, 2505-2520. [10] Efremov, A. A.; Davydov, A. A. IR Spectroscopic Studies of Adsorption and Conversion of Isopropyl Alcohol on a Sn/Mo Oxide Catalyst. React. Kinet. Catal. Lett. 1981, 18, 353-356.

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[11] Goncharova, O. I.; Davydov, A. A. IR Spectroscopic Studies of the Interaction of Propylene with the Surface of Alumina-Supported MoO3 Catalysts. React. Kinet. Catal. Lett. 1983, 23, 285-289. [12] Chakrabarti, A.; Wachs, I. E. Molecular Structure-Reactivity Relationships for Olefin Metathesis by Al2O3-Supported Surface MoOx Sites. ACS Catal. 2018, 8, 949-959. [13] Sato, T.; Hamada, Y.; Sumikawa, M.; Araki, S.; Yamamoto, H. Solubility of Oxygen in Organic Solvents and Calculation of the Hansen Solubility Parameters of Oxygen. Ind. Eng. Chem. Res. 2014, 53, 19331-19337. [14] Wu, X.; Deng, Z.; Yan, J.; Zhang, Z.; Zhang, F.; Zhang, Z. Experimental Investigation on the Solubility of Oxygen in Toluene and Acetic Acid. Ind. Eng. Chem. Res. 2014, 53, 9932-9937. [15] Afeefy, H. Y.; Liebman, J. F.; Stein, S. E. Neutral Thermochemical Data. In NIST Chemistry WebBook; Linstrom, P. J., Mallard, W. G., Eds; NIST Standard Reference Database Number 69;

National

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of

Standards

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Technology:

Gaithersburg,

MD,

http://webbook.nist.gov (retrieved 2017). [16] Amakawa, K.; Wrabetz, S.; Krohnert, J.; Tzolova-Muller, G.; Schlogl, R.; Trunschke, A. In Situ Generation of Active Sites in Olefin Metathesis. J. Am. Chem. Soc. 2012, 134, 1146211473. [17] Reusch, W. Infrared Spectroscopy. https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/Spectrpy/InfraRed/infrared.htm (accessed 2017). [18] Escribano, V. S.; Busca, G.; Lorenzelli, V. FT-IR Studies of the Reactivity of Vanadia-Titania Catalysts toward Olefins. 3. n-Butenes and Isobutene. J. Phys. Chem. 1991, 95, 5541-5545.

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[19] National Center for Biotechnology Information. PubChem Compound Database. https://pubchem.ncbi.nlm.nih.gov/compound/180 (accessed 2017). [20] Davydov, A. Molecular Spectroscopy of Oxide Catalyst Surfaces; John Wiley & Sons Ltd.: West Sussex, England, 2003. [21] Lwin, S.; Li, Y.; Frenkel, A. I.; Wachs, I. E. Activation of Surface ReOx Sites on Al2O3 Catalysts for Olefin Metathesis. ACS Catal. 2015, 5, 6807-6814. [22] Vikulov, K. A.; Shelimov, B. N.; Kazansky, V. B. IR and UV-Vis Spectroscopic Studies of the

Surface

Mo=CH2

and

Mo=CH-CH3

Carbene

Complexes

Produced

by

Methylcyclopropane Chemisorption Over Photoreduced Silica-Molybdena Catalysts. J. Mol. Catal. 1991, 65, 393-402. [23] Went, G. T.; Bell, A. T. Laser Raman Spectroscopy of NH3 and ND3 Adsorbed on TiO2 (anatase). Catal. Lett. 1991, 11, 111-118. [24] Went, G. T.; Leu, L. J.; Lombardo, S. J.; Bell, A. T. Raman Spectroscopy and Thermal Desorption of NH3 Adsorbed on TiO2 (Anatase)-Supported V2O5. J. Phys. Chem. 1992, 96, 2235-2241. [25] Hu, H.; Wachs, I. E.; Bare, S. R. Surface Structures of Supported Molybdenum Oxide Catalysts: Characterization by Raman and Mo L3-Edge XANES. J. Phys. Chem. 1995, 99, 10897-10910. [26] Tian, H.; Roberts, C. A.; Wachs, I. E. Molecular Structural Determination of Molybdena in Different Environments: Aqueous Solutions, Bulk Mixed Oxides, and Supported MoO3 Catalysts. J. Phys. Chem. C 2010, 114, 14110-14120. [27] Vikulov, K. A.; Elev, I. V.; Shelimov, B. N.; Kazansky, V. B. IR and UV-vis Spectroscopic Studies of the Stable Mo=CH2 Carbene Complexes Over Photoreduced Silica-Molybdena 31



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Catalysts with Chemisorbed Cyclopropane, and Their Role in Olefin Metathesis Reactions. J. Mol. Catal. 1989, 55, 126-145. [28] Blanc, F.; Berthoud, R.; Coperet, C.; Lesage, A.; Emsley, L.; Singh, R.; Kreickmann, T.; Schrock, R. R. Direct Observation of Reaction Intermediates for a Well-Defined Heterogeneous Alkene Metathesis Catalyst. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 1212312127. [29] Blanc, F.; Rendon, N.; Berthoud, R.; Basset, J.M.; Coperet, C.; Tonzetich, Z. J.; Schrock, R. R. Dramatic Enhancement of the Alkene Metathesis Activity of Mo Imido Alkylidene Complexes Upon Replacement of One tBuO by a Surface Siloxy Ligand. Dalton Trans. 2008, 3156-3158. [30] Gordon, C. P.; Yamamoto, K.; Liao, W.C.; Allouche, F.; Andersen, R. A.; Copéret, C.; Raynaud, C.; Eisenstein, O. Metathesis Activity Encoded in the Metallacyclobutane Carbon13 NMR Chemical Shift Tensors. ACS Cent. Sci. 2017, 3, 759-768. [31] Schrock, R. R.; Copéret, C. Formation of High-Oxidation-State Metal-Carbon Double Bonds. Organometallics 2017, 36, 1884-1892. [32] Moro-oka, Y. The Role of Acidic Properties of Metal Oxide Catalysts in the Catalytic Oxidation. Appl. Catal. A: Gen. 1991, 181, 323-329.

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TOC Graphic

33



Figure 1. In situ DRIFT difference spectra during C2H4-C4H8 titration at 120 °C and subsequent temperature programming at 10 °C/min under flowing ethylene of the supported 20% MoOx/Al2O3 catalyst.

(A) 1200-1900 cm-1 and (B) 2800-3200 cm-1. All gas-phase vibrations are labeled with a subscript “(g)” and shown in black and surface CH2/CH3 species arising from 2-butene adsorption and ethylene adsorption are shown in red. The IR spectra were normalized against the Al2O3 support vibration at ~1041 cm-1, and the difference spectra were obtained by subtracting the IR spectrum of the initial dehydrated catalyst from each IR spectrum taken during the titration experiment. 82x127mm (300 x 300 DPI)


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Figure 2. In situ DRIFT difference spectra during propylene adsorption at 120 °C, Ar flush at 120 °C, and subsequent temperature programming at 10 °C under flowing propylene of the supported 25% MoOx/Al2O3

catalyst: (A) 1250-1900 cm-1 and (B) 2800-3200 cm-1. All gas-phase vibrations are labeled with a subscript “(g)” and shown in black, surface CH3 species arising from propylene adsorption are shown in red, and surface CH2 species arising from propylene adsorption are shown in blue. The IR spectra were normalized

against the Al2O3 support vibration at ~1041 cm-1, and the difference spectra were obtained by subtracting the IR spectrum of the initial dehydrated catalyst from each IR spectrum taken during the propylene adsorption-TP experiment. 82x127mm (300 x 300 DPI)



Figure 3. In situ DRIFT difference spectra of supported 20% MoOx/Al2O3 catalyst during C3H6-C3D6 isotope exchange at 120 °C after initial adsorption of C3D6 and subsequent temperature programming at 10 °C/min under flowing C3H6: (A) 2000-3800 cm-1, (B) 2700-3200 cm-1, and (C) 1200-1900 cm-1. The IR spectra were normalized against the Al2O3 support vibration at ~1041 cm-1, and the difference spectra were obtained by subtracting the IR spectrum of the initial dehydrated catalyst from each IR spectrum taken during the isotope exchange experiment. 82x191mm (300 x 300 DPI)


Page 36 of 38

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Scheme 1. Proposed activation mechanism for oligomeric MoOx surface species anchored to acidic OH-µ1/3AlV/VI surface sites on Al2O3. 152x75mm (300 x 300 DPI)



82x12mm (300 x 300 DPI)


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1 Activation Mechanism and Surface Intermediates During Olefin

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