Synthesis and structural characterization of Cu-Cr catalysts for cumene alkylation.

I.M. Sinev 1, O. Shutkina 2, W.Grünert 1, I. Ivanova 2

1 Lehrstuhl Technische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany 2Department of Chemistry, Moscow State University

Copper chromite (CuCr 2O4) has been recently described as an effective catalyst for a broad range of industrially important processes like alcohol dehydrogenation [1] and hydrogenolysis [2]. Partial (reversible) reduction considerably changes its catalytic properties: the reduced form is much more active in acetone [3]. It was observed by HRTEM that after reduction at 250-320 oC reduced forms a Cu(0) phase epitaxially related to the spinel structure, which is not destroyed by the reduction. Inelastic neutron scattering indicated that protons oc- cupy positions in the spinel structure vacated by the Cu ions, thus forming acidic sites. In-situ FTIR spectra show bands between 1400 and 1610 cm -1 from both Brønsted and Lewis acidic sites [3]. Thus, reduced copper-chromite potentially offers two types of active sites – metallic copper and proton substituted spinel. On this basis, the present study aims at creating an effective and stable catalyst for the one-pot synthesis of cumene from benzene and acetone (eq. (1)), in which the two steps required – acetone hydrogenation to isopropanol, which subse- quently alkylates the benzene, proceed over one – bifunctional - catalyst. This catalyst should offer sites for selective hydrogenation (of acetone, in presence of benzene) and acidic sites for alkylation. As described above copper chromite potentially fits these requirements. To enhance the alkylation activityit may be deposited on an acidic support (e.g. zeolite).

O OH -H O 2 (1) + H 2 +

Unsupported CuCr 2O4 was prepared by co-pyrolysis of the corresponding nitrates. The mixture was precipitated with ammonia solution a pH = 7.0, filtered and washed, dried at 110 oC and cal- cined at 400 oC. Its XRD pattern shows, however, the characteristic diffraction peaks of both CuO and CuCr 2O4 phases. Alumina-supported samples were prepared by parallel and sequential preparation routes - by precipitating γ-Al2O3 with a mixture of corresponding nitrate solutions, followed by drying and calcination as above (parallel) or by depositing copper and nitrates on alumina separately with drying and calcination on each step. A Cu/Al 2O3 sample served as a reference. XAFS measurements were made with all samples in two states: as-re- o ceived and after reduction in flowing H 2 at 150 C. In addition, Cu reduction was studied by in- situ XANES (transmission) using a home-made spectroscopic cell. For this experiment, unsup- ported copper chromite (250-350 µm fraction) was fixed by quartz wool in the spectroscopic cell perpendicularly to the direction of the X-ray beam. XANES measurements were done during o o catalyst reduction in flowing diluted H2 (5% in He) after every 25 C step starting from 75 C. XAFS spectra (Cu K edge, 8.979 keV and Cr K edge, 5.989 keV) were measured in transmission mode at the HASYLAB C station using the Si(111) double crystal monochromator. Data treatment was carried out using VIPER for Windows [4]. For the determination of structural pa- rameters theoretical references calculated by the FEFF8.10 [5] code were used. Fig. 1 shows CuK XANES (a) and EXAFS (b) spectra of samples in initial state and after reduc- tion at 150 oC. Absorption edge position, white line height and low intensity of pre-edge feature indicate that the initial state of copper in both supported samples is 2+. The same applies to the copper chromite sample although there the main XANES feature (white line) is less intensive and broader. This difference is most likely caused by different configurations of molecular orbi- tals in copper chromite and copper oxide, although the sample contains a significant amount of

a 9 b Cu-Cr/Al O par. 2 3 CuCr O 2 4 Cu-Cr/Al O seq. 2 3

6 1,5

FT FT amplitude Cu foil 3

Normalizedabsorption CuO Cu O 2 Cu-Cr/Al O par. 2 3 CuCr O 2 4 Cu-Cr/Al O seq. 2 3 0,0 0 8980 9000 9020 9040 0 2 4 6 8 E, eV r, Å Fig. 1 CuK edge XANES (a) and EXAFS spectra of samples in initial state (solid lines) and after reduction at 150 oC (dashed lines) CuO. The EXAFS of samples in the initial state exhibit well-defined Cu-O scattering paths at 1.5 Å (not corrected for phase shifts). Low signal intensity at r>1.9 Å on Cu-Cr/Al 2O3 (parallel) indicate a small size of the supported nanoparticles. Spectra of unsupported copper chromite and Cu-Cr/Al 2O3 (sequential deposition) have more pronounced structure at higher r values indicat- ing thus bimodal distribution of supported particles and spinel microcrystals. After reduction in H2 copper comes very close to the metal state in the unsupported sample. In Cu-Cr/Al 2O3 (paral- lel), its state remains almost unchanged whereas hints for the presence of small amounts of metallic copper can be found in the XANES of Cu-Cr/Al 2O3 (sequential). The EXAFS spectra support the conclusions on the Cu state after reduction by increased Cu-Me scattering paths (Me - Cu, vide infra), for unsupported and sequentially deposited samples and unchanged intensity for parallel deposited sample.

4 a b

Cu-Cr/Al 2O3 par. CuCr O 2 4 2 Cu-Cr/Al O seq. 2 3 3

2

1 FT amplitude FT Cr O

Normalized absorption Normalized 2 3 CrO 1 3

Cu-Cr/Al 2O3 par. CuCr O 2 4

Cu-Cr/Al 2O3 seq. 0 0 6000 6030 6060 0 2 4 6 8 E, eV r, Å Fig. 2 CrK edge XANES (a) and EXAFS spectra of samples in initial state (solid lines) and after reduction at 150 oC (dashed lines) CrK XANES (a) and EXAFS (b) spectra of initial and reduced samples are presented in Fig. 2. XANES plots of initial samples clearly indicate that no CuCr 2O4 phase was formed in both deposition procedures – The Cr state in both samples is equal or very close to 6+, whereas the white line intensity of XANES plot of the unsupported sample indicates Cr3+. Upon reduction Cr changes its state to 3+ in both supported samples. There are no visible changes in Cr state and coordination after reduction in the unsupported sample, which is in agreement to conclusion of retention of the spinel structure after reduction under mild conditions presented in literature.

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