Supporting Information

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1 Supporting Information Exploring low-temperature dehydrogenation at ionic Cu sites in beta zeolite to enable alkane recycle in dimethyl ether homologation Carrie A. Farberow, Singfoong Cheah, Seonah Kim, Jeffrey T. Miller, James R. Gallagher, # Jesse Hensley, Joshua A. Schaidle * and Daniel A. Ruddy * National Bioenergy Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States # Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Dept. of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States Dan.Ruddy@nrel.gov; Joshua.Schaidle@nrel.gov Supporting Information Methods Catalyst Synthesis The proton-form of the BEA zeolite, H-BEA, was prepared by calcination of the ammonium form, NH 4 -BEA (SiO 2 /Al 2 O 3 ratio of 27), under flowing air at 500 C. Ionexchanged Cu/BEA (termed IE-Cu/BEA) was prepared using a procedure similar to a reported method. 1,2 H-BEA zeolite (4.0 g) was treated with an aqueous solution of Cu(NO 3 ) H 2 O (2.88 g, 12.4 mmol in 100 ml) corresponding to twice the amount of Cu(II) needed for complete ion-exchange. After ion-exchange for 16 h at room temperature, the catalyst was isolated by filtration, washed twice with 50 ml of deionized water, and dried at 100 C overnight. This yielded a material having 1.8 wt% Cu. Oxidized (500 C under flowing 1% O 2 /He) and reduced (500 C under flowing 1% O 2 /He, then 300 C under flowing 1% H 2 /He) IE-Cu/BEA were prepared giving materials termed ox-ie-cu/bea and red-ie-cu/bea, respectively. 1

2 Silica-supported Cu catalysts were prepared by incipient wetness impregnation of amorphous silica (4.75 g, Sipernat-50, Evonik; surface area 490 m 2 /g) with an aqueous solution of Cu(NO 3 ) H 2 O (0.915 g, 3.93 mmol in 27 ml water) to achieve 5.3 wt% Cu. This material was used to produce CuO/SiO 2 and Cu/SiO 2 catalysts. For CuO/SiO 2 the material was treated in flowing 1% O 2 /He at 500 C for 1 h. For Cu/SiO 2, the CuO/SiO 2 material was subsequently exposed to flowing 2% H 2 /He at 300 C for 1 h. The presence of CuO and Cu were confirmed using X-ray diffraction (Figure S1) with in situ reduction for Cu/SiO 2 to prevent oxidation of the material when exposed to air. Powder X-ray diffraction data were collected using a Rigaku Ultima IV diffractometer with a Cu Kα source (40 kv, 44 ma), and in situ treatments were performed using a Rigaku Reactor X attachment. After preparation of CuO/SiO 2 as described above, the sample (10 20 mg) was supported on a black-quartz sample holder with a recessed sample area and was pressed into the recession with a glass slide to obtain a uniform z- axis height. The pre-oxidized CuO/SiO 2 pattern was recorded at 50 C, and the sample was then heated under flowing 5% H 2 /N 2 to 300 C for 2 h. After cooling to 50 C, the pattern for the reduced Cu/SiO 2 was recorded. Patterns were compared to powder diffraction files (PDFs) from the International Centre for Diffraction Data (ICDD). Methods Isobutane Dehydrogenation Isobutane dehydrogenation experiments were performed in a micro-reactor system, equipped with a gas chromatograph (490 Micro GC, Agilent Technologies) for analysis of the reactor effluent. Approximately 100 mg of catalyst was loaded into a quartz u-tube reactor and supported on quartz wool. Prior to reaction experiments, the catalyst was pre-treated in 1% O 2 /He at 20 sccm for 1 h at 500 C (10 C/min heating rate). Reduced catalysts were exposed to a subsequent pre-treatment in 2% H 2 /He at 20.4 sccm for 1 h at 300 C (10 C/min heating rate). Following pre-treatment, the catalyst was held at the reaction temperature (T = 300 C) in flowing He and then exposed to the 1% isobutane/ar reactant gas at a flow rate of 7 sccm. All experiments were performed at a pressure of 2 atm. GC analysis was used to verify that no H 2 flow was present in the gas stream prior to isobutane exposure. The rate of production at varying time on stream of all quantifiable products except H 2 (propane, propylene, 1-2

3 butene, n-butane, C 5 ) is reported in Figure S3. The H 2 productivity is reported in the main text in Figure 2. We note that small quantities of t-2-butene were detected in the effluent in experiments on ox-ie-cu/bea, red-ie-cu/bea, and H-BEA, but in all cases the amount produced was too small to be quantified (i.e., <0.002%). There was no t-2- butene product detected in isobutane dehydrogenation experiments on Cu/SiO 2 and CuO/SiO 2. Products with a carbon number larger than five could not be detected by the GC. The increase in the propane, n-butane and C 5 productivities measured for the red- IE-Cu/BEA catalyst relative to the ox-ie-cu/bea catalyst are noted and may be due to (1) the accuracy of the measurements, particularly for propane where the signal to noise was low, (2) small differences in the reaction conditions (e.g., reaction pressure), and/or (3) differences in the coordination environment of the Cu(I) species produced from in situ versus ex situ reduction, as described in the main text. A detailed investigation of the cause of these differences in productivities is beyond the scope of this report. Methods X-ray Absorption Spectroscopy X-ray absorption spectroscopy (XAS) measurements were acquired on the insertion device beam line (10-ID) of the Materials Research Collaborative Access Team (MRCAT) at the Advanced Photon Source (APS), located at Argonne National Laboratory. Photon energies were selected using double-crystal Si(111) monochromators. All measurements and harmonics were rejected with a Rh mirror. The X-ray ionization detectors fill gases were selected to keep the detectors in their linear range (i.e., <10 μa). XAS data was acquired in approximately 125 seconds in quick scan mode from 250 ev below the edge to 800 ev beyond the edge. For energy calibration measurements a Cu foil spectrum (edge energy ev) was acquired simultaneously with the sample spectrum. For the operando XAS, a microreactor system was designed and constructed that could be inserted into the X-ray beamline. 3 The reactor consisted of a carbon tube (4 mm ID and 10 mm OD) loaded with catalyst (~100 mg) and contained within a clamshell furnace with a hole through the furnace body to allow for X-rays in and out of the sample. The XAS spectra were taken every 125 seconds during reaction with 1% isobutane/he at a flow rate of 6 sccm. The spectra were collected until there were no 3

4 further changes in the XANES or EXAFS, which was ca. 2 h for the ox-ie-cu-bea and ca. 1 h for the red-ie-cu-bea sample. The catalyst pre-treatments were: 1) 1% O 2 /He at a flow rate of 20 sccm at 500 C, cooled to 300 C and purged with He for 10 minutes to produce ox-ie-cu/bea or 2) 1% O 2 /He at a flow rate of 20 sccm at 500 C, cooled to 300 C and purged with He for 10 minutes, reduced in 3% H 2 /He at a flow rate of 20 sccm at 300 C for 40 minutes to produce red-ie-cu/bea. Normalized energy calibrated XANES spectra were obtained using standard methods. The pre-edge energy reported in Table 1 was obtained from the maximum of the pre-edge peak. The edge energy was determined from the inflection point of the edge by taking the maximum of the first derivative of the spectrum. Standard data reduction techniques were employed to fit the data using the WINXAS 3.1 software program. 4 The EXAFS parameters were obtained by a least square fit in R-space (detailed in Tables S1 and S2) of the k 2 -weighted Fourier Transform (FT) data. Representative fit quality is shown in Figure S2. Experimental phase shift and backscattering amplitudes for oxygen first-shell neighbors were extracted from a model compound, copper(ii) acetylacetonate (4 Cu-O at 1.92 Å). The fits of the k 2 -weighted EXAFS were k = 2.5 to 10.8 Å -1 and R = 1.0 to 2.0 Å. The 95% confidence limits of the EXAFS fits for N (0.1), R (0.005 Å), σ 2 ( Å 2 ) and Eo (0.5 ev) were all within the normal fitting errors for the method. Linear combination fitting of the XANES was performed to estimate the fraction of metallic Cu, Cu(I), and Cu(II) in different environments or at different reaction times. XANES reference spectra and spectral features The XANES reference spectrum for metallic Cu was obtained from Cu foil. The XANES reference spectrum for Cu(II)-zeolite in H-BEA was obtained from the ionexchanged zeolite after pre-treatment in 1% O 2 /He at 500 C for one hour (ox-ie- Cu/BEA). The small pre-edge at ev in this spectrum (indicated by an arrow in the inset of Figure 1A in the main text) is typical of Cu(II) with the low intensity traditionally assigned as the dipole forbidden, quadrupole allowed 1s to 3d transition. 5-8 This pre-edge peak is absent in Cu(I) because Cu(I) has fully occupied d-orbitals (i.e., d 10 configuration). Thus, the presence of this pre-edge feature indicates the presence of 4

5 Cu(II). This spectrum also has an edge energy at ev typical of a Cu(II). EXAFS analysis of this spectrum yielded 4 oxygen first neighbors at 1.91 Å, characteristic of bonds seen in Cu(II). The ev pre-edge feature and the edge energy combined to make this spectrum characteristic of a Cu(II) while the EXAFS results lend further confidence to the assignment of a Cu(II) oxidation state. In contrast, the spectrum of ox-ie-cu/bea after exposure to 1% isobutane/he for 7500 s (125 min) has an intense feature of the rising edge at ev that is characteristic of the 1s to 4p transition of Cu(I). 5,6,9,10 Its edge energy is ev. In a study of Cu(I) and Cu(II) model compounds, Kau et al. found that the Cu(II) complex does not have an edge below 8985 ev. 6 Additionally, EXAFS analysis of our sample indicated two oxygen/carbon neighbors at 1.96 Å (the EXAFS analysis cannot distinguish oxygen and carbon neighbors due to similar backscattering of these two elements). Complexes with one or two ligands are much more common in Cu(I) than Cu(II). 11 In summary, the edge energy (<8485 ev), the high intensity of the 1s to 4p transition at ev, the overall shape of the XANES spectrum, and the finding of two oxygen/carbon neighbors at 1.96 Å all support the conclusion that this spectrum is characteristic of a Cu(I) species. Therefore, this spectrum was used as a XANES reference spectrum for Cu(I)-zeolite in H-BEA. It is worth noting that using the XANES spectrum of the red-ie-cu/bea catalyst (ex situ reduction at 300 C with H 2, Figure 1A in the main text) as a reference for Cu(I)-zeolite resulted in nearly identical Cu speciation values in the operando XAS experiment. Using this reference the Cu fraction values in Table S2 differed by only ±0.01. Methods QM/MM Calculations Quantum mechanics/molecular mechanics (QM/MM) calculations were performed using the Gaussian 09 package 12 and the two-layer ONIOM scheme. 13 The high level QM layer contains the T7 atom, substituted by an Al atom, the four coordination spheres around the substituted atom, and one isobutane molecule. The low level semiempirical layer contains an additional 295 atoms in the zeolite framework. The results reported in the main text of the manuscript were performed using the hybrid wb97xd density functional 14 for the high level calculations, which includes empirical 5

6 dispersion, with the 6-311G(d,p) basis set. The low level was treated with semiempirical PM6. 15 All atoms in the QM layer were allowed to relax, whereas the semiempirical layer atoms were fixed to maintain the crystallographic positions and thus capture the steric and electronic effects of the extended BEA zeolite structure. For optimization of the Cu ion location (Table S3), all framework atoms located in rings that include the T7 site were included in the high level layer. Vibrational frequency calculations were performed for all optimized structures to verify reactants, intermediates, products, and TSs. Intrinsic reaction coordinate calculations 16 were also performed for all transition structures in both forward and reverse directions to determine two relevant minima. All reported energies include zero point energy corrections. Atomic charges were calculated by natural population analysis and are reported for each state along the minimum energy reaction pathway in Table S5. 17 For comparison, we also report the energetics of the minimum energy (primarytertiary) pathway calculated using the hybrid meta generalized gradient approximation M06-2X functional 18 for the high level (Figure S5A). These results differ from those calculated using the wb97xd functional by as much as kj/mol, indicating that dispersion interactions may be important in this system. Convergence with respect to the size of the QM region was verified by repeating the calculations (wb97xd) for the minimum energy pathway with the QM region expanded to include six additional atoms in the zeolite framework near the transition states (Figure S5C). These results are also reported in Figure S5A and indicate that expanding the QM region had a negligible effect (<9 kj/mol) on the reported energetics. Additionally, the energetics of isobutane dehydrogenation at a Cu(I) site in the more open 12 membered ring (MR) site were calculated (Table S4). This Cu(I) site is 61 kj/mol less stable than the 6-MR site modeled and reported in the main text. The calculated barriers at the 12-MR site are within kj/mol of the respective barriers reported for the 6-MR site. 6

7 Figure S1. Powder X-ray diffraction patterns for CuO/SiO 2 and Cu/SiO 2. The peak at θ is from the in situ sample holder. Reference peaks for CuO (JCPDS: ) and Cu (JCPDS: ) are also provided. Figure S2. Representative fit of magnitude and imaginary parts of the k 2 -weighted Fourier Transform. The sample is ox-ie-cu/bea ( k = Å -1 ; R = Å)). 7

8 Figure S3. Plots of all quantifiable products measured during isobutane dehydrogenation (1% isobutane/ar at 7 sccm, 300 C, 2 atm). Small quantities of t-2- butene were detected on ox-ie-cu/bea, red-ie-cu/bea and H-BEA, but not in experiments on Cu/SiO 2 or CuO/SiO 2. Products with a carbon number greater than five could not be detected by the GC. 8

9 Figure S4. (A) Operando XANES spectra and (B) k 2 -weighted Fourier Transform magnitude of red-ie-cu/bea ( k = Å -1 ) as a function of time of exposure to 1% isobutane/he at 300 C. (C) Operando XANES spectra and (D) k 2 -weighted Fourier Transform magnitude of ox-ie-cu/bea ( k = Å -1 ) as a function of time of exposure to 1% isobutane/he at 300 C. 9

10 Figure S5. (A) Reaction energy diagram for the minimum energy pathway (primarytertiary) for isobutane dehydrogenation at a Cu(I) site in BEA zeolite using different density functionals (wb97xd and M062X) and varying the size of the QM region. (B) The QM region used to calculate all results reported in the main text. (C) The QM region used in the expanded QM region calculations to verify convergence with respect to the size of the QM region. Structures depict only the QM region in QM/MM calculations. Blue, red, green and gold spheres represent silicon, oxygen, aluminum, and copper atoms, respectively. 10

11 Figure S6. Optimized structures with bond distances (Å) and activation energy barriers ( E, kj/mol) for the tertiary-primary path for isobutane dehydrogenation at a Cu(I) site in BEA. Structures depict only the QM region in QM/MM calculations. Blue, red, white, grey, green and gold spheres represent silicon, oxygen, hydrogen, carbon, aluminum, and copper atoms, respectively. 11

12 Table S1: Coordination number (N), distance (R), Debye-Waller factor ( σ 2 ), and E 0 shift obtained from EXAFS spectra fitting of red-ie-cu/bea exposed to 1% isobutane/he at 300 C (0 to 3780 s). Discussion of the oxidation state of this sample during operando is provided in the main text. Time N Cu-O R σ 2 E 0 Comment (s) (Å) (x 10-3 Å 2 ) (ev) Small changes in XANES, N and R No further changes in XANES or EXAFS

13 Table S2: Fraction of Cu(II) and Cu(I) obtained from linear combination fitting of XANES, R-factor* of the XANES fit; coordination number (N), distance (R), Debye- Waller factor ( σ 2 ), E 0 shift obtained from EXAFS spectra fitting of ox-ie-cu/bea exposed to 1% isobutane/he at 300 C (0 to 7500 s). Time (s) Fraction Cu(II) Fraction Cu(I) R-factor N Cu-O R (Å) σ 2 (x 10-3 E 0 (ev) Comment Å 2 ) N/A No metallic Cu N/A *R = [Σ(data i -fit i ) 2 ]/ Σ(data i ) 2 Table S3: Relative energy of a Cu(I) ion located in pores nearest to the T7 site in BEA. Four energetically different sites in different five membered rings were identified. Cu(I) Site Relative Energy (kj/mol) 12 membered ring 61 6 membered ring 0 5 membered ring 31, 43, 45, 49 13

14 Table S4: Calculated activation energy barriers for the minimum energy pathway (primary-tertiary) for isobutane dehydrogenation at a Cu(I) site in BEA zeolite at the 6 membered ring site (most stable site) and at the more open, but less stable 12 membered ring site. Cu(I) Site E 1 (kj/mol) E 2 (kj/mol) 6 membered ring membered ring Table S5: Calculated atomic charges for each state along the minimum energy (primary-tertiary) path for isobutane dehydrogenation at a Cu(I) site as determined by natural population analysis. The figure above the table depicts the transition state for the second elementary step (TS2) to illustrate the hydrogen, carbon and oxygen atoms corresponding to the charges reported. Blue, red, white, grey, green and gold spheres represent silicon, oxygen, hydrogen, carbon, aluminum, and copper atoms, respectively. State Cu O1 C1 C2 H1 H2 ic TS INT TS ic 4= +H 2(g) References 1. Bates, S. A.; Verma, A. A.; Paolucci, C.; Parekh, A. A.; Anggara, T.; Yezerets, A.; Schneider, W. F.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H., J. Catal. 2014, 312, Kwak, J. H.; Tonkyn, R. G.; Kim, D. H.; Szanyi, J.; Peden, C. H. F., J. Catal. 2010, 275,

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