SiO2 catalysts for

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Evidence for copper dimers in low-loaded CuOx/SiO2 catalysts for cyclohexane oxidative dehydrogenation Scott L Nauert, Andrew S. Rosen, Hacksung Kim, Randall Q. Snurr, Peter C. Stair, and Justin M Notestein ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02532 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Catalysis

Evidence for copper dimers in low-loaded CuOx/SiO2 catalysts for cyclohexane oxidative dehydrogenation Scott L. Nauerta, Andrew S. Rosena, Hacksung Kimb,c, Randall Q. Snurra, Peter C. Stairb,c, and Justin M. Notesteina,* a

Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA

b

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

c

Department of Chemistry, Center for Catalysis and Surface Science, and Institute for Catalysis in Energy Processes, Northwestern University, Evanston, Illinois 60208, USA

[email protected]

Abstract. Copper oxide catalysts supported on KIT-6 silica were evaluated for cyclohexane oxidative dehydrogenation (ODH) to determine the effects of copper oxide domain size on ODH activity and selectivity. The catalysts were prepared by incipient wetness impregnation of KIT-6 at copper surface densities spanning 0.01-0.7 Cu/nm2 with carefully controlled drying and calcination conditions to systematically vary the average local copper oxide domain size. A

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distinct copper oxide active site exhibiting an order of magnitude higher activity than large copper oxide domains was identified by model cyclohexane ODH studies coupled with in-situ Xray absorption and UV-visible spectroscopies during reduction in H2. The structure of this site is experimentally identified by a combination of extended x-ray absorption fine structure analysis, resonant Raman studies, and modeling by density functional theory. All constraints imposed by these techniques indicate the active site is a mono(µ-oxo)dicopper(II) structure with copper sited in 4-member rings formed by copper insertion into highly strained 3-member siloxane ring defects which form on dehydrated silica. Given the ubiquity of copper oxide sites in selective oxidation catalysis, the understanding of such structures may prove relevant for other oxidation reactions.

heterogeneous catalysis, supported catalyst, copper oxide, active site structure, Raman spectroscopy, DFT calculations 1. Introduction A major goal in heterogeneous catalysis is to develop structure-function relationships linking structural features of the catalytically active site to reactivity metrics such as activity and selectivity. Some reactions, such as olefin hydrogenation, only require an accessible metal site and are insensitive to structural properties such as particle size.1 However, many important catalytic reactions are sensitive to the exact structure of the catalytically active site, whether it is a terrace or step site on a metal nanoparticle for alkane hydrogenolysis,2 an open or closed Sn site geometry for glucose isomerization by Snβ,3-4 or the chromium-silica binding geometry in the Phillips CrOx/SiO2 catalyst for ethylene polymerization.5-6 Ethylene polymerization over


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ACS Catalysis

CrOx/SiO2 is an extreme case of structure sensitivity where it is estimated that only 1-10% of the metal sites in catalysts prepared by traditional incipient wetness are catalytically active.5 Understanding the structural requirements of a reaction can assist in the design of synthesis strategies to maximize the fraction of catalytically relevant species for efficient catalyst design. This in turn makes them easier to study, as bulk spectroscopic techniques become useful for probing catalytically relevant species. Transition metal oxides supported on silica or silicate frameworks form an important class of heterogeneous catalysts active for epoxidation (TS-1),7 ethylene polymerization (Phillips catalyst),5-6 propane dehydrogenation,8-10 and propane oxidative dehydrogenation (ODH) over VOx/SiO2 catalysts.11 In particular, oxidic copper species exhibit interesting ability to activate oxygen in direct methane to methanol conversion over Cu-MOR/MFI12-14 and the oxidation halfcycle in selective catalytic reduction of NOx with NH3 in Cu-CHA15 due to the unique redox properties of Cu. Redox reactions involving small copper clusters and oxygen are inherently structure sensitive due to O2 being a 4e- oxidant, but individual copper atoms typically undergo redox cycles involving only one electron.16 Understanding how copper clusters activate oxygen for use in selective oxidation reactions is an active area of debate for many reactions.16-20 Alkane oxidative dehydrogenation (ODH) is one such unresolved case in that it involves a 2e- redox cycle, and there is debate whether the reaction is structure sensitive.11 We have previously disclosed that CuOx/SiO2 exhibits increasing cyclohexane ODH turnover frequency with decreasing Cu cluster size without change in C—H abstraction selectivity.21 The change in activity was correlated with a new, unassigned Cu+ UV-visible absorption band that appeared at low Cu loadings under reducing environments. An outstanding question from this previous work is whether this Cu+ feature implicates a distinct, highly active catalytic site with a large fraction


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of inactive metal as in the case of CrOx/SiO2,5-6,

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or if intrinsic Cu site activity varies

continuously as a function of copper oxide domain size. In this paper, we investigate how cyclohexane ODH activity and selectivity change as a function of copper oxide cluster size using CuOx/KIT-6 catalysts varying in surface density from 0.01 – 0.7 Cu/nm2, and we find evidence for a distinct Cu site which is highly active for ODH. We first employ UV-visible spectroscopy and in-situ x-ray absorption near edge (XANES) spectroscopy to probe average local copper oxide domain size and Cu2+/Cu+ redox activity as a function of Cu surface density and relate these structural and chemical properties to cyclohexane ODH activity and selectivity. We find that cyclohexane ODH turnover frequency increases continuously with decreasing copper oxide domain size then plateaus below a critical Cu surface density. In-situ UV-visible spectra of the catalysts show a critical change in Cu redox behavior below this critical Cu surface density which points to the existence of a distinct catalytic site for cyclohexane ODH at low Cu loadings. We investigate the Cu active site structure under oxidizing and reducing conditions by extended x-ray absorption fine structure (EXAFS), resonant Raman spectroscopy, and density functional theory (DFT) modeling, and find that all evidence is consistent with a mono(µ-oxo)dicopper (Cu-O-Cu)2+ structure with one of the Cu atoms exchanged in a silica surface defect site. 2. Experimental Methods 2.1 Materials synthesis Mesoporous KIT-6 SiO2 was synthesized by a previously published procedure21,

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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


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ACS Catalysis

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. The KIT-6 3 structure was confirmed using powder X-ray diffraction by the characteristic (220) reflection in figure S1. Copper oxide catalysts were prepared by incipient wetness impregnation with careful drying and calcination procedures before and after impregnation. Before impregnation, KIT-6 was dried under vacuum at 473 K for 16 h to remove all water and partially dehydroxylate the surface. Catalysts were then impregnated to incipient wetness using aqueous copper nitrate trihydrate (Strem, 99.5%), with Cu2+ molarity adjusted to achieve the desired copper loading at incipient wetness. After impregnation, catalysts were dried for 4 h in air to remove bulk water from the pores followed by drying for 16 h at 293 K under dynamic vacuum. The dried materials were then calcined at 823 K for 4 h with a 10 K/min ramp rate. Two CuOx/KIT-6 series with different copper loadings were prepared this way. The first batch was stored in a dessicator and used for H2-XANES linear combination fitting but was subsequently exposed to moist air and hydrolyzed over a period of several months. The second CuOx/KIT-6 series was dried at 293 K under vacuum after calcination before storing under N2 to prevent hydrolysis of the active copper structures, and it was used for all other experiments. 2.2 Catalyst characterization


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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 ICPOES (Thermo iCAP 7600) calibrated with Cu standards of known concentration. Samples were digested with 28.9 M HF then diluted with 0.14 M aqueous HNO3 prior to analysis. Small angle x-ray diffraction (XRD) patterns were collected over 1-3° 2θ in reflection mode using a Rigaku Smartlab diffractometer with Cu Kα radiation, and wide angle XRD patterns were collected over 20-80° 2θ in transmission mode using a STOE STADI MP diffractometer with Cu Kα1 radiation. Diffuse reflectance UV-visible spectra were collected on a Shimadzu UV-3600 spectrophotometer equipped with a Harrick Praying Mantis diffuse reflectance accessory and reaction cell for in-situ measurements. Polytetrafluoroethylene was used as the baseline white light standard, and reflectance data were transformed to pseudo-absorbance units using the Kubelka-Munk transform, ( ), using KIT-6 as the reference:

=

eq. 1

( ) =

( )

eq. 2

A Savitzky-Golay filter with polynomial degree 2 and 11 point frame is applied to all pseudoabsorbance spectra to increase the signal to noise ratio at the expense of minor signal distortion. Absorption edge energies are estimated from the x-intercept of a linear fit of the rising absorption edge in a plot of (( )ℎ) vs. ℎ where the exponent is due to the assumption that the copper oxide species exhibit a direct band gap similar to bulk copper oxide. Cu K-edge x-ray absorption spectroscopy (XAS) was performed at sector 5 of the Advanced Photon Source, Argonne National Laboratory, on the DuPont-Northwestern-Dow Collaborative


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ACS Catalysis

Access Team (DND-CAT) bending magnet D beamline using a Si(111) double crystal monochromator with resolution Δ ⁄ ≈ 1.3 ∗ 10&. Copper standards and CuOx/KIT-6 catalysts with ≥0.1 Cu/nm2 were measured in transmission mode with Canberra ionization chambers, while CuOx/KIT-6 catalysts with

SiO2 catalysts for

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