Role of Support Lewis Acid Strength in Copper-Oxide-Catalyzed

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Role of support Lewis acid strength in copper oxide catalyzed oxidative dehydrogenation of cyclohexane Scott L Nauert, Louisa Savereide, and Justin M Notestein ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00935 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Role of support Lewis acid strength in copper oxide catalyzed oxidative dehydrogenation of cyclohexane Scott L. Nauert, Louisa Savereide, and Justin M. Notestein* Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Technological Institute E136, Evanston, Illinois 60208, United States [email protected]

Abstract. Alkane oxidative dehydrogenation (ODH) over supported redox active metal oxides is highly sensitive to support identity, but the underlying cause of support effects has not been well established. Here, we provide evidence that charge transfer between the support and active oxide phase impacts the rates of C—H bond abstraction and COX formation pathways in the oxidative dehydrogenation of cyclohexane over supported copper oxide catalysts. The surface Lewis acid strength of nine metal oxide supports is quantified by alizarin dye intramolecular charge transfer shifts and compared with supported copper oxide d-d transition energies to determine the relationship between support Lewis acid strength and copper oxide electronic properties. Model cyclohexane ODH reaction studies show that selectivity to C6 products increases with increasing support Lewis acid strength, with selectivities to benzene and cyclohexene over combustion products at zero conversion increasing from 20% over nucleophilic Cu/MgO to over 90% over the more Lewis acidic Cu/Nb2O5 and Cu/Ta2O5. This is ascribed to a linear relationship between the amount of electron density on the copper oxide valence states as described by

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Cu d-d transition energy and the ratio of rates of C—H bond abstraction and COX formation pathways. This approach to quantifying support Lewis acid strength and applying it as a key catalytic descriptor of support effects is a useful tool to enable rational design of next-generation oxidative dehydrogenation catalysts.

heterogeneous catalysis, supported catalyst, metal oxide, oxidative dehydrogenation, copper oxide, cyclohexane, support effects

1. Introduction Alkane oxidative dehydrogenation (ODH) has emerged as a promising class of reactions over supported metal oxide catalysts to overcome the thermodynamic limitations inherent in direct alkane dehydrogenation1-3. Alkane ODH has been most widely studied over supported vanadia catalysts,2, 4 but many reducible metal oxides such as MoO3,5-7 WO3,5 Co3O4,8 and recently CuO9 have been shown to also be active for ODH chemistry. In the absence of 3D crystallites, others have found ODH rates to be insensitive to metal oxide 2D surface structure in vanadia catalysts,10-11 and we have likewise seen weak sensitivity to 2D surface structure in copper oxide catalysts.9 In both cases, the selectivity-conversion profile is structure-independent. Instead, promoting species and support identity have a much greater impact on both rates and selectivity-conversion profiles.2,

10, 12-13

The supporting oxide may play an

active role in catalysis either directly (via reaction at M-O-support species) or by modifying the electronic and structural properties of the active oxide phase.14 One of the challenges in understanding how supports impact the electronic properties of an active oxide phase is the difficulty in directly quantifying the surface Lewis acid strength of catalytic supports


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which may have a variety of surface defects or minor surface impurities. Alizarin dye adsorption has been proposed as a means to quantify surface Lewis acid strength of crystalline, low surface area metal oxides,15 and it has been applied to characterize metal oxide modified alumina supports for selective hydrogenation reactions.16-17 Here, we will demonstrate that it serves as a quantitative metric for support electron withdrawing properties across a wide variety of common support materials, and that it determines the impact of charge transfer on cyclohexane ODH rates and selectivities. In addition, supported copper oxide’s 3d9 valence structure shares many spectroscopic similarities to adsorbed alizarin dye which make it possible to directly relate support electron withdrawing properties to changes in copper oxide electronic structure using UV-visible spectroscopy, which is not possible with supported vanadium oxide catalysts. Supported copper oxide has previously been shown to have similar activity to supported vanadium oxide for cyclohexane ODH,9 and so copper oxide is a useful active metal oxide phase for determining support effects. However, catalyst stability on some supports may limit copper oxide’s industrial viability compared with vanadium oxide.9 Though high ODH rates are important, the major challenge in alkane oxidative dehydrogenation is controlling selectivity to useful alkene products.2, 4 Over-oxidation to COX can occur by sequential C— H bond abstraction in an ODH like pathway, or it can occur by parallel combustion-like reactions involving C—C cleavage, oxygen insertion, or radical processes.4 The vast majority of research on alkane ODH has focused on ethane and propane ODH as industrially relevant reactions, but determination of the factors impacting selectivity is difficult with these reactions because COX is produced as the only over-oxidation product. This makes it very difficult to separate over-oxidation due to sequential C—H bond abstraction from over-oxidation due to combustion-like pathways by product analysis alone.

In contrast, cyclohexane oxidative dehydrogenation is a useful model for alkane

oxidative dehydrogenation because sequential C—H bond abstraction and combustion pathways lead to


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different products.3 Sequential C—H bond abstraction is likely to self-terminate, producing benzene as a product because the C—H bond dissociation energy of benzene is much higher than the C—H bond dissociation energy of either cyclohexane or cyclohexene.18 In contrast, over-oxidation due to C—C cleavage, oxygen insertion, or radical processes will always lead to COX formation. In this report, copper oxide catalysts supported on nine high surface area metal oxide supports comprising a range of Lewis acid strengths are used to determine the effects of support Lewis acid strength on copper oxide reactivity for cyclohexane oxidative dehydrogenation. Support surface Lewis acid strength, as quantified by adsorbed alizarin dye intramolecular charge transfer energy maximum, is compared with the degree of electron density on the copper oxide valence states in the prepared catalysts to determine the degree of charge transfer between support and active copper oxide phase. Model cyclohexane oxidative dehydrogenation reactions are used to compare the change in copper oxide electronic properties with rates of C—H bond abstraction and COX formation reaction pathways. These techniques allow for investigation of support effects using Lewis acid strength as an overarching catalytic descriptor rather than treating each supporting metal oxide on a case by case basis. 2. Experimental Methods 2.1 Materials Synthesis. Anatase TiO2 (Sigma, >99.7% Ti), P25 TiO2 (Sigma, >99.5% Ti), ZrO2, Al2O3 (Evonik, >99.8% Al), ZnO (Strem, >99% Zn), and MgO (Strem, >95% Mg) nanopowders were used as catalyst supports as purchased. High surface area Nb2O5 was prepared by dehydrating niobic acid (CBMM) at 723 K for 2 h under static conditions. Mesoporous Ta2O5 was prepared by a modified ligand assisted templating method of Kondo et al.19 First, 6.15 mmol octadecylamine was added to 12.3 mmol Ta(OEt)5 under Ar and stirred at 333 K for 0.5 h until a homogeneous clear liquid formed. Next, DI water was added dropwise while stirring to 25 mL total volume to form a white precipitate. The precipitate was filtered


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and washed with 50 mL each of DI water, 2x ethanol (Decon Labs), then DI water. The washed material was suspended in 25 mL DI water and aged at 293 K for 24 h followed by aging at 383 K for 48 h under static conditions. The aged material was filtered and surfactant removed by suspending in 25 mL 0.1 M HNO3 in ethanol for 12 h at 293 K. The material was subsequently filtered, washed with 100 mL ethanol, and stirred in 25 mL ethanol for 24 h. The final material was dried and calcined at 823 K for 1 h with a 5 K/min ramp rate before use. Mesostructured KIT-6 SiO2 was synthesized by a previously published procedure9, 20 using a molar ratio of 195 H2O : 1.5 tetraethyl orthosilicate (TEOS) : 1.7 n-Butanol : 1.83 hydrochloric acid (HCl) : 0.017 Pluronic® P-123. Briefly, 19.2 g Pluronic® P-123 (Sigma, Mn = 5800 g/mol) and 29 mL (37 wt %) aqueous HCl (Fisher, ACS grade) were added to 650 mL DI water and stirred at 310 K for 4 h until complete dissolution of the P-123 structure directing agent. Next, 30 mL n-Butanol (Sigma, 99.0%) was added as a co-solvent and stirred for 1 h. Then, 64.3 g TEOS (Sigma, 99.0%) was added, and the mixture was stirred for an additional 24 h. The mixture was aged at 373 K for 24 h under reflux without stirring. Excess structure directing agent was removed by suspending the precipitate in 500 mL 0.1 M HCl in ethanol for 2 h. The precipitate was then filtered and washed with 300 mL ethanol followed by 750 mL DI water. The resulting powder was dried at 373 K for 16 h before calcining at 823 K for 4 h with a 5 K/min ramp rate under static conditions before use. Copper oxide catalysts were prepared by incipient wetness impregnation of dried supports. All supports were dried at 423 K for 16 h under dynamic vacuum before impregnating to incipient wetness with aqueous solutions of Cu(NO3)2·3H2O (Strem, 99.5%). The Cu2+ solution molarity was adjusted for each support to achieve ≈ 1 wt % Cu loading at incipient wetness. Catalysts were then dried under ambient conditions for 4 h followed by drying for 16 h at 293 K under dynamic vacuum before calcination. Catalysts supported on TiO2 (P25), ZrO2, Al2O3, KIT-6 SiO2, and MgO were calcined at 823 K for 4 h with a 10 K/min ramp rate under static conditions. Catalysts supported on TiO2 (anatase),


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Ta2O5, Nb2O5, and ZnO were calcined at 723 K for 4 h with a 5 K/min ramp rate under static conditions due to the reduced thermal stability of these supports as determined by thermogravimetric analysis (TGA) and N2 physisorption. Catalysts were sieved to < 120 µm before use. 2.2 Materials Characterization. Nitrogen physisorption isotherms were collected using a Micromeritics ASAP 2010 instrument. Prior to physisorption measurements, samples were dried at 423 K under dynamic vacuum for 6-12 h until pressure stabilized below 5*10-6 bar. The BET method was used to calculate surface area from N2 adsorption isotherms. Metal content was determined using ICP-OES (Thermo iCAP 7600) calibrated with Cu standards of known concentration. Samples were digested with 28.9 M HF or 12 M aqua regia as appropriate then diluted with 0.14 M aqueous HNO3 and shaken at 300 rpm at 293 K for 24 h prior to analysis. X-ray diffraction (XRD) patterns were collected using a Rigaku Smartlab diffractometer with Cu Kα radiation over the range 20-80° 2θ. Thermogravimetric analysis was conducted on a TA Instruments TGA Q500 with a 10 K/min ramp up to 873 K under a flow of 74.4 µmol/s dry 90% O2 in N2. Temperature programmed reduction experiments were performed on an Altamira AMI-200 chemisorption instrument equipped with a TCD. Raman measurements were performed with a SPID LabRAM Confocal Raman Spectrophotometer using a 532 nm laser, a spectrometer with 600 groove/mm grating, and a synapse detector. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 250Xi equipped with a monochromatic Al Kα (1486.74 eV) X-ray source. A flood gun was used for charge compensation. Spectra were calibrated using the C 1s signal appearing at 284.5 eV. Alizarin dye adsorption experiments were carried out following the method of Jeong et al.15 First, 10 mL of a solution of alizarin dye (Sigma, 97%) dissolved in ethanol was added to a round bottomed flask containing 0.1 g of a metal oxide powder and stirred at 300 rpm for 0.1 h. The alizarin solution molarity


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was adjusted between 10-3-100 mM to achieve final dye surface densities between 3*10-4–3*100 based on BET surface area of the metal oxide. The solids were filtered, washed with ethanol, and dried under dynamic vacuum for 16 h prior to collecting diffuse reflectance UV-Visible spectra. Diffuse reflectance UV-visible spectra were collected on a Shimadzu UV-3600 spectrophotometer equipped with a Harrick Praying Mantis diffuse reflectance accessory. Polytetrafluoroethylene was used as the baseline white light standard, and reflectance data were transformed to pseudo-absorbance units using the Kubelka-Munk transform, F(R∞), using polytetrafluoroethylene as the reference as in equations 1-2. R =

eq. 1

=

eq. 2

A Savitzky-Golay filter with polynomial degree 2 and 11 point frame is applied to all pseudoabsorbance spectra to increase signal to noise ratio at the expense of minor signal distortion. Alizarin intramolecular charge transfer (IMCT) and copper d-d charge transfer energies were determined by fitting the relevant bands to a Gaussian profile to extract the peak maximum energy. Sanderson’s partial metal charges, δM, are calculated for a metal oxide from the Sanderson electronegativity and stoichiometry of the metal and oxygen according to equation 3, where previously published ion electronegativities are used for calculation.15, 21-22 =

" %

"

! #$ % .()

&

eq. 3

2.3 Catalytic Testing. Catalytic measurements were conducted in a packed-bed quartz microreactor using 0.01-0.1 g catalyst diluted with inert SiO2 (Sigma) to a total bed mass of 0.15 g to ensure uniform flow and temperature through the bed. He (Airgas, 99.999%) and O2 (Airgas, 99.999%) flows were supplied by mass flow


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controllers while cyclohexane (Sigma, 99.9%) was supplied by bubbling He through a saturator with fixed pressure and temperature to achieve a known molar flow rate. All lines were heated to 373 K to prevent condensation of vapors. Catalysts were dried at 573 K for 1 h in a 5 % O2 / He stream before reactions. Contact time experiments were conducted at 573 K and atmospheric pressure using a fixed cyclohexane : O2 : He molar ratio of 1.7 : 3 : 95.3 and total flow rates of 16.4-95.1 µmol/s. See Figure S1. The identity and concentrations of reactants and products were analyzed using a gas-chromatograph (Agilent GC 6890) equipped with an HP-Innowax column (30 m x 0.32 mm x 0.5 µm) attached to a flame ionization detector (FID) in parallel with HP-Plot/Q (30 m x 0.53 mm x 40 µm) and HPMolesieve (15 m x 0.53 mm x 50 µm) columns connected to a thermal conductivity detector (TCD). Steady state oxidative dehydrogenation (ODH) turnover frequencies are determined from the slope of the differential region of product yield vs. contact time after initial deactivation and are normalized by total copper content. Instantaneous turnover frequencies are determined for each GC injection as the product yield divided by contact time. Selectivities are determined on a per C6 basis according to equations 4-5, and initial C6 selectivities are determined by extrapolating product yields to zero contact time. Benzene selectivities at 0.5% total product yield were estimated by interpolation of selectivityyield curves. For runs with sufficient cyclohexane conversion to give an accurate value, the carbon balance exceeds 90%. Stoichiometric cyclohexane conversion is calculated from the individual product yields. S+, = -

-#%./0$1"11 2-31411

#%./0$1"11 2-31411 2-#5" /,

S789:898 =

-31411

-31411 2-#%./0$1"11

eq. 4 eq. 5


8

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3. Results and Discussion 3.1 Materials Synthesis and Characterization. Copper oxide catalysts supported on metal oxide supports spanning a wide range of Lewis acid strengths

Table 1. Physicochemical properties of supported copper oxide catalysts. Name

were synthesized by incipient wetness impregnation and calcined prior to use. Copper loadings were determined by ICP-OES and catalyst surface areas were measured by N2 physisorption after calcination. Copper loadings were generally kept near 1 wt%, and corresponding surface densities in table 1 are all below 23

monolayer coverage,

which ensures intimate contact

between the support and copper oxide phase. Neither XRD (Figure S2) nor Raman spectroscopy (Figure S3) show evidence for crystalline copper oxide phases in

Surface area

Cu loading

(m2/g)[a] (wt%)[b] (#/nm2) Cu/Nb2O5

101

1.32

1.25

Cu/Ta2O5

110

0.38

0.33

Cu/TiO2 (Anatase)

55

1.15

2.00

Cu/TiO2 (P25)

57

0.76

1.28

Cu/Al2O3

165

1.12

0.64

Cu/ZrO2

100

0.9

0.86

Cu/SiO2

972

0.99

0.10

Cu/ZnO

16

0.78

4.6

Cu/MgO

28

0.71

2.4

[a] Determined by N2 physisorption using the BET method. [b] Measured by ICP-OES.

any material. This is supported by UV-visible spectroscopy which, for these oxides, is sensitive enough to detect the presence of crystallites too small to detect by XRD or Raman spectroscopy. UV-visible spectra of all fresh catalysts in figures S4-S5 show a lack of absorption between λ=500-600 nm associated with 3D CuO crystallites. Together, XRD, Raman, and UV-visible spectroscopy data are consistent with small, sub-monolayer 2D copper oxide domains on all catalysts.


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3.2 Materials Electronic Properties. Here, we measure the electron-withdrawing properties of nine high surface area metal oxide supports using alizarin dye adsorption studies and correlate the results with copper oxide d-d transition energies of prepared Cu/MOX catalysts using those same supports. Alizarin dye adsorption has been previously proposed for measuring the Lewis acid strength of crystalline oxides,15 but we show that it also holds for defective, high surface area oxides useful as catalytic supports. The lowest energy transition for alizarin is an intramolecular charge transfer (IMCT) depicted in figure 1a, which is sensitive to the identity of a metal oxide surface on which it is adsorbed.24 Because the ground and excited states are localized on alizarin dye, the shift in IMCT peak energy upon adsorption to different metal oxides is a direct measure of the degree of charge transfer between the metal oxide surface and alizarin dye.

As proposed for some single

crystalline oxides, the IMCT peak energy shifts in proportion to the Lewis acidity parameter NM-2δM, where NM is the cation formal oxidation state and δM is Sanderson’s partial charge on the metal

Figure 1. (A) Representative Alizarin IMCT shift upon adsorption to a metal oxide. (B) Alizarin IMCT shift correlated to theoretical Lewis acidity parameter NM-2δM for several metal oxides. Solid symbols are EIMCT at ≈3 dye/nm2, while bars show the range of EIMCT measured between 0.01 to 3 dye/nm2.

calculated by equation 3.15, 24


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Alizarin was adsorbed onto high surface area metal oxide supports and the IMCT peak energy (EIMCT) was measured by UV-visible spectroscopy (Figures S6-S8). The IMCT peak energy scales proportional to NM-2δM (Figure 1b) at surface densities > 0.1/nm2, and it only deviates from expected bulk behavior below this level. High surface area metal oxides used as supports contain a larger proportion of surface defects than low surface area crystallites which may on average be more acidic or basic than the bulk. This behavior can be seen in figure 1b as bars skewed away from being centered on the monolayer alizarin EIMCT. For example, the defects on MgO are typically more basic (lower EIMCT) than the monolayer average, while those on Nb2O5 are more acidic (higher EIMCT) than the monolayer average. In principle, the extent of charge transfer may depend on surface coverage on semiconducting supports, as seen with dye-sensitized TiO2 solar cells.25 However, this is mitigated by choice of alizarin dye, for two reasons: the ground and excited molecular orbitals are localized on the dye rather than a molecular orbital with primarily metal character,24 and the maximum dye shifts are state, and a decrease in electron density causes a shift in the excited state energy with subsequent red shift in Cu(d-d) transition maximum energy. The shift in Cu(d-d) transition energy maximum is proportional to the support Lewis acid strength quantified by alizarin EIMCT in figure 2b. This shows that the supporting metal oxide withdraws


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electron density from the Cu2+ valence states proportional to the Lewis acid strength of the support. This is supported by XPS of several catalysts in figure S9 which show that Cu 2p binding energies shift in the same order as seen in Cu(d-d) peak energy. Just as there is some variation in alizarin EIMCT at low dye surface coverages due to interactions with surface defects, there may be some variation in degree of charge transfer between copper and the support as a function of copper surface coverage. Hence, the monolayer alizarin EIMCT is useful for estimating the mean surface Lewis acid strength, but the Cu(d-d) Emax is a more direct measure of the actual degree of charge transfer between copper and the support at a given copper surface density because it incorporates any effects of copper oxide structure on extent of charge transfer between the support and active copper oxide phase. Cu/MOX catalysts reported in table 1 span a wide range of Cu surface densities, and so variations in charge transfer between the support and copper oxide with copper oxide surface density could explain slight deviations in the Cu(d-d) transition Emax from the general trend in figure 2b as well as outliers such as Nb2O5. All catalyst and support electronegativity metrics are reported in table S1. Next, model cyclohexane ODH studies are used to show how changes in active site electronic structure impact the rates of C—H and COX formation. 3.3 Cyclohexane ODH Kinetics. Alkane ODH has been shown to proceed by a Mars van Krevelen redox mechanism over a variety of redox active metal oxides with several common features across different alkanes and catalytically active phases.5, 27 The rate determining step is initial C—H bond abstraction with two electron reduction of the active site. This is followed by either β-elimination of the alkene or

Scheme 1. Simplified reaction network

cyclohexane

sequential C—H abstraction, and the active site is


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regenerated by dehydration and fast re-oxidation. For cyclohexane ODH, selectivity to cyclohexene or benzene is determined by fast reaction steps after the rate determining initial C—H bond abstraction, and so these products can be summed for analysis to get an overall rate of initial C—H bond activation. The C—H bond dissociation energy in benzene is much higher than cyclohexane or cyclohexene,18 and so it is reasonable to assume that the amount of benzene over-oxidation to COX is negligible. These two assumptions lead to the simplified cyclohexane ODH reaction network in scheme 1. The C6 products, cyclohexene and benzene, are produced by a C—H bond abstraction pathway, while COX is produced primarily by a combustion-like pathway involving C—C bond cleavage, oxygen insertion, or radical processes. This simplified reaction network is used to analyze the effects of varying support Lewis acid strength on rates of the C—H and COX formation pathways. Total product yield and selectivity to C6 products as a function of catalyst contact time are shown in figure 3a-b, and the combined C6 selectivity-yield curve is shown in figure 3c. Bare support activity is

Role of Support Lewis Acid Strength in Copper-Oxide-Catalyzed

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