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Tuning the Selectivity of Single-Site Supported Metal Catalysts with Ionic Liquids Melike Babucci, Chia-Yu Fang, Adam S Hoffman, Simon R Bare, Bruce C. Gates, and Alper Uzun ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02429 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017
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ACS Catalysis
Tuning the Selectivity of Single-Site Supported Metal Catalysts with Ionic Liquids Melike Babucci,†,⊥ Chia-Yu Fang,‡ Adam S. Hoffman,‡,§ Simon R. Bare,§ Bruce C. Gates,‡ and Alper Uzun*,†,⊥ †
Department of Chemical and Biological Engineering, Koç University, Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey ⊥Koç University TÜPRAŞ Energy Center (KUTEM), Koç University, Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey ‡
Department of Chemical Engineering, University of California, Davis, CA 95616, United States
§
SSRL, SLAC National Accelerator Laboratory, Menlo Park, CA 94025 United States
ABSTRACT: 1,3-Dialkylimidazolium ionic liquid coatings act as electron-donors increasing the selectivity for partial hydrogenation of 1,3-butadiene catalyzed by iridium complexes supported on high-surface-area γ-Al2O3. High-energy resolution fluorescence detection X-ray absorption near edge structure (HERFD XANES) measurements quantify the electron donation and are correlated with the catalytic activity and selectivity. The results demonstrate broad opportunities to tune electronic environments and catalytic properties of atomically dispersed supported metal catalysts.
KEYWORDS. Atomically dispersed supported metal catalyst, Selective hydrogenation of 1,3-butadiene, Ionic liquid, SCILL, HERFD XANES electronic environment of the metal,13,14 but incorporation of promoters is often imprecise. An alternative approach is to modify the electron-donating/withdrawing properties of supports,12,15,16 but this approach is limited because the modified supports might fail to maintain the structural integrity of the catalyst.
Atomically dispersed supported metals are an emerging class of catalyst. They offer opportunities for tuning their properties that make them comparable to ligated molecular metal complexes used in homogeneous catalysis.1-3 The atomic dispersion offers advantages of (a) the highest degree of utilization of metals, which are often expensive;3 (b) new reactivities linked with the effects of supports as ligands;4 (c) prospects of high selectivity associated with the structural uniformity of active sites;5,6 and (d) opportunities for precise characterization to facilitate determination of atomic-level structure-performance relationships.7,8
Here, we illustrate an alternative strategy for tuning the properties of single-site supported metal catalysts—by using surface modifiers, as has been done before with conventional supported metal catalysts, to adjust the electronic environment of the active sites.17-20 For example, Chen et al.13 showed that chelation of the surface of ultrathin Pt nanowires with electron-donating ethylenediamine induced high selectivity for the production of Nhydroxylanilines from nitrobenzene by favouring the adsorption of electron-deficient reactants over electronrich intermediates to increase the selectivity for partial hydrogenation. Ionic liquids (ILs) offer especially good potential in this regard, offering an almost unlimited number of possible structures with physical and chemical properties tunable over wide ranges.21-23 An example illus-
The metal sites in these supported catalysts often require stabilizing ligands that intrinsically limit the catalytic performance,9 which is challenging to control. As the catalytic properties depend strongly on the electronic environment of the active site, researchers have attempted to tune these properties by modifying the ligands, as in homogenous catalysis.10 However, this approach is limited by the available classes of ligands in the organometallic precursors.11,12 Promoters can also be used to modify the
1
ACS Paragon Plus Environment
ACS Catalysis 11217.2
R2 = 0.92
2065
1,3-Dimethylimidazolium dimethyl phosphate 1-Butyl-3-methylimidazolium acetate
2055 11216.0
R2 = 0.97
2050 2045 2040 2035
11215.2 3130 3120 3110 3100 3090 3080 3070 3060 3050
ν (C2H) of IL (cm-1)
Figure 1. Variations of Ir LIII edge energy in γ-Al2O3supported Ir(CO)2 complexes resulting from coating with various 1,3-dialkylimidazolium ILs (left vertical axis) and the corresponding ν(CO)sym (right vertical axis) with ν(C2H) of the coating IL.
The ν(CO)sym band shifted to lower wavenumbers upon coating of Ir(CO)2/γ-Al2O3 with various ILs (Figures S2-8, Table S2, SI). For example, when the ILs were [BMIM][NTf2] and [BMIM][Ac], the corresponding red shifts were 11 and 28 cm-1, respectively, consistent with electron donation from each IL to the Ir.23,24,29 The degree of electron donation was quantified by HERFD XANES data recorded at the Ir LIII edge characterizing the uncoated and IL-coated Ir(CO)2 complexes, Figure S9, SI. The edge energy of the uncoated Ir(CO)2/γ-Al2O3 was found to be 11217.8 eV, as expected.29-31 It shifted to lower values as a result of IL coating (Figure 1 and Table S2, SI). The results confirm that 1,3-dialkylimidazolium ILs donate electron density to Ir in the catalysts. Figure 1 illustrates the variation of edge energy (and ν(CO)sym) with the interionic interaction energy in ILs probed by the stretching frequency of the most acidic proton-donor group on the imidazolium ring, ν(C2H).27 Accordingly, the edge energy decreased (almost linearly, R2 = 0.92) from 11217.0 to 11215.4 eV (from [BMIM][NTf2] to [BMIM][Ac]) with a decrease in ν(C2H) of the IL from 3124 to 3056 cm-1, related to an increase in the interionic interaction energy in the IL.27 The results demonstrate that the 1,3-dialkylimidazolium IL layer strongly controls the electron density on Ir, with the strongest interionic interactions in the ion pairs corresponding to the greatest electron donation to the Ir sites.27,32
Table 1. Names and abbreviations of IL coatings.
1,3-Dimethylimidazolium methyl sulfate 1-Butyl-3-methylimidazolium trifluoroacetate 1-Ethyl-3-methylimidazolium diethyl phosphate
2060 11216.4
11215.6
Atomically dispersed supported iridium gem-dicarbonyl complexes were prepared by the reaction of Ir(CO)2(acac) (acac = acetylacetonato), with partially dehydroxylated γAl2O3, as before, with characterization by X-ray absorption spectroscopy and atomic resolution aberrationcorrected scanning transmission electron microscopy.28 IR spectra confirmed the removal of acac ligands from the precursor, and bands at 2074 and 1994 cm−1 characterizing the initial sample (Figure S1, Supporting Information, SI) indicate the terminal CO ligands of supported Ir(CO)2.28 These isolated complexes were coated with ILs (Table 1 and Table S1, SI) such that each resultant sample had approximately 20 and 0.4 wt% IL and iridium loadings, respectively. Pore size distributions of the catalysts before and after coating show that the ILs penetrated even into small pores, 2.8-5 nm in diameter.23 The IL-coating layer thicknesses ranged from 1 to 1.5 nm, corresponding to a maximum of a few molecular layers of ILs.23
1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-Butyl-3-methylimidazolium tetrafluoroborate
2070
11216.8
We report results characterizing the effects on catalyst performance of a family of ILs coating atomically dispersed supported iridium sites, having quantified the electron-donation effects by high-energy resolution fluorescence detection X-ray absorption near edge structure (HERFD XANES). HERFD XANES provides more detailed information than conventional XANES.25,26 Well-defined site-isolated iridium complexes were prepared on γ-Al2O3 and coated with 1,3-dialkylimidazolium ILs, chosen for their varied electron-donor strengths—which we have correlated with the stretching frequencies of the C–H groups on the imidazolium rings.21-23,27 The influence of the IL on catalyst performance was evaluated by the selectivity for partial hydrogenation of BD.
IL Name
2075
Edge energy ν(CO)sym
ν (CO)sym of IL-coated Ir(CO)2 / γ-Al2O3 (cm-1)
trating the benefits of an electron-donating IL, [BMIM][BF4], was reported for a commercial nickel catalyst, giving >95% selectivity for butenes in the industrially important 1,3-butadiene (BD) hydrogenation, irrespective of conversion.24
Ir LIII edge energy (eV)
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Abbreviation [BMIM][NTf2] [BMIM][BF4] [DMIM][MSO4] [BMIM][TFA]
We thus infer that ILs present in coatings can be used to tune the electronic properties of supported metals, and thereby influence their catalytic performance. To test this inference, we investigated the samples as catalysts for BD hydrogenation (at a BD:H2 molar ratio of 1:2) at 333 K and 1 bar in a once-through flow reactor with differential conversions (
