Characterization of Surface Hydride Hafnium Complexes on Alumina

ARTICLE pubs.acs.org/JPCC

Characterization of Surface Hydride Hafnium Complexes on Alumina by a Combination of Experiments and DFT Calculations Marco Delgado,† Catherine C. Santini,*,† Franc-oise Delbecq,*,‡ Anne Baudouin,† Aimery De Mallmann,† Carmello Prestipino,§ S. Norsic,† Philippe Sautet,‡ and Jean-Marie Basset† †

Universite de Lyon, Institut de Chimie de Lyon, C2P2, UMR 5265 CNRS-ESCPE Lyon, 43 bd du 11 Novembre 1918, F-69626 Villeurbanne Cedex, France ‡ Universite de Lyon, Institut de Chimie de Lyon, CNRS-ENS Lyon, Laboratoire de Chimie, 46 allee d’Italie, F-69364 Lyon Cedex 07, France § ESRF, 6 rue jules Horowitz, 38000 Grenoble ABSTRACT: The hydrogenolysis at 150 °C of the surface complexes obtained by grafting Hf(CH2tBu)4 with γ-alumina yields several hydride complexes. They have been characterized by combining various experimental techniques (IR, DQ 1 H NMR, and EXAFS) and DFT calculations. Two main conclusions are drawn from experiments: first, all Hf
CH2tBu bonds are hydrogenolized in Hf
H ones, while most of the Al
CH2tBu bonds remain intact. Second, there is mainly formation of monohydride surface complex either as a trialuminoxy monohydride (AlO)3HfH or a cationic bisaluminoxy [(AlIVO)(AlSO))HfH]þ[Al
CH2tBu]
. The DFT calculations of the possible species show that all the considered transformations are thermodynamically favorable. Moreover, the calculated bond lengths are in good agreement with the EXAFS values confirming the nature of the hydride complexes obtained at this temperature.

’ INTRODUCTION Ziegler
Natta polymerization is a major process in the chemical industry, and the design of active and selective catalysts remains a key challenge.1 One important area is concerned with supported metal hydrides of group 4, which are known to be active in polymerization at low temperatures (e.g., 100
150 °C)2 and in the hydrogenolysis of polyethylene and polypropylene.3
6 In particular, alumina-supported metal hydrides obtained from the hydrogenolysis of surface alkyl complexes have already been described as very active catalysts, and this has been associated with the presence of cationic species.7
14 However, no process based on these catalysts has been developed, probably due to the fact that the hydrogenolysis reaction of alkyl surface complexes yields several surface hydrides in various proportions depending on the reaction temperature. Nevertheless, in accordance with the current environmental requirements, it is worth noting that these catalysts are active in the absence of any cocatalyst such as methylaluminoxane (MAO) or borane [(C6F5)3B] since alumina plays the dual role of solid support and Lewis acid.14
17 Understanding and controlling their formation is consequently a scientific and economic challenge. For a few years, a combination of the modus operandi developed for the surface organometallic chemistry,18 associated with the contributions of theoretical chemistry in this field of catalysis r 2011 American Chemical Society

with surface organometallic species,19,20 has allowed the characterization of the structure and the quantification of the different surface complexes on oxide supports, in particular alumina. Recently, using this combined experimental and theoretical approach we have reported that the reaction of Hf(CH2tBu)4 (1) with γ-Al2O3-(500) affords several surface species, referred to as 1-γAl2O3(500). Three coexisting surface complexes were fully characterized and quantified, a neutral monoaluminoxy- [(AlIV O)Hf(CH 2tBu)3] (2), a neutral bisaluminoxy [(Al IV O)(AlsO)Hf(CH2tBu)2] (3), and a zwitterionic bisaluminoxy complex [(AlIVO)(AlsO)Hf(CH2tBu)2]þ[(CH2tBu)Als]
(4), in 40%, 26%, and 34%, respectively, Scheme 1. Moreover, their thermolysis and hydrogenolysis reactions have unambiguously proved that the neopentyl
metal bond in Hf
CH2tBu is totally cleaved at 150 °C.17 In this work, the hydride surface complexes formed at this temperature, referred to as 2150-γ-Al2O3(500), have been fully characterized by the combined approach (in situ infrared spectroscopy, solid-state NMR spectroscopy (13C, 1H DQ), and DFT calculations) with additional EXAFS spectroscopy. Received: January 5, 2011 Revised: February 25, 2011 Published: March 22, 2011 6757

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Scheme 1. Grafting Reaction of Hf(CH2tBu)4 (1) on γ-Al2O3(500) at Room Temperature; Grafting Occurs First on a Tetrahedral Al Atom and Can Then Involve Another Surface Aluminum Atom (Al)17

Figure 1. (A) IR spectra along the formation of 2-γ-Al2O3(500) monitored by in situ IR spectroscopy: region 1500
3900 cm
1. (B) ν(Al
OH) region 3400
3900 cm
1. In both cases (a) γ-Al2O3(500); (b) 1-γ-Al2O3(500); (c) b þ H2 150 °C, 17 h. (C) 13C CP-MAS solid-state NMR spectra of (a) 1*-γAl2O3(500); (b) 2*150-γ-Al2O3(500), where * means labeled R-C of neopentyl group. (D) 1H solid-state NMR spectra: (a) 2150-γ-Al2O3(500); (b) γ-Al2O3(500).

’ RESULTS AND DISCUSSION 1. Hydrogenolysis of 1-γ-Al2O3(500) in Batch Reactor at 150 °C. The reaction of 1-γ-Al2O3(500) (500 mg) with H2 has been

performed in a batch system with analysis and quantification of evolved gas and elemental analysis of the residual solid. In this experiment, only methane (C1) and ethane (C2) are detected in the gas phase corresponding to 1.8 equiv of neopentane per Hf, (neoC5/ Hfg), and a residual carbon amount (C/Hfg = 6) has been found by elemental analysis on the support after reaction. 2. In Situ IR Study. In Figure 1A are depicted the spectra of γ-Al2O3(500) (1Aa), 1-γ-Al2O3(500) (1Ab), and 2-γ-Al2O3(500) treated under hydrogen at 150 °C for 17 h (1Ac).When comparing spectra 1Ab and 1Ac, three main changes are observed. The intensity of the vibrations ν(CH) (3000
2600 cm
1) and δ(CH) (1700
1300 cm
1) assigned to the neopentyl group linked to Hf or Al atoms strongly decreases (25% of original intensity), but these vibrations are still present due to the nonhydrogenolyzed

Al
CH2tBu bonds, Figure 1Ac. Indeed, at this temperature, only the Hf
CH2tBu bonds are totally hydrogenolyzed, while the Al
CH2tBu bonds would be fully hydrogenolized only above 250 °C.4,17 A band centered at 1670 cm
1 (1Ac) can be assigned to ν(HfH) vibrations by comparison with literature.21
23 This is related to the hydrogenolysis of Hf-CH2tBu bonds into Hf
H bonds. The concomitant apparition of a band centered at 1915 cm
1, assigned to Als
H vibrations, and of bands at 3687 and 3735 cm
1, assigned to μ2(OH)
Als and μ1(OH)
Als vibrations, Figure 1B, results from the direct reaction of H2 with defects of γ-Al2O3(500)24 Hence, in situ infrared study suggests that after hydrogenolysis of 1-γ-Al2O3(500) at 150 °C hafnium hydrides (Hf
H), aluminum hydrides (Al
H), and aluminum alkyls are present on the surface. The ν(HfH) vibration value at 1670 cm
1 is closer to the one found for the monohydride complex (η5-C5H5)3HfH (1669 cm
1) than the one obtained for the dihydride complex (η5-C5H5)2HfH2 (1572 cm
1), indicating the preferential formation of monohydride.25,26 6758

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Figure 2. 1H DQ MAS spectra of 2150-γ-Al2O3(500) in the region 0
25 ppm.

3. Solid-State 13C CP-MAS NMR Experiments. The 13C CPMAS solid-state NMR of 1-γ-Al2O3(500) shows peaks at 100, 91, 82, and 73 ppm corresponding to Hf
CH2
tBu bonds and a peak at 30 ppm corresponding to Al
CH2
tBu bonds (Figure 1Ca).17,27 In the 13C CP-MAS solid-state NMR spectrum of 2150-γAl2O3(500), all peaks between 70 and 100 ppm disappear, but the peaks corresponding to the resonance of Al
CH2
tBu bonds remain (Figure 1Cb). This confirms that at 150 °C the Hf
CH2
tBu bonds have been totally hydrogenolyzed in Hf
H bonds, while the Al
CH2
tBu bonds do not react completely with hydrogen at this temperature. 4. Solid-State 1H MAS NMR and DQ 1H MAS Experiments. The solid-state 1H NMR spectra of 2150-γ-Al2O3(500) (Figure 1Da) shows a broad massif between
1 and þ5 ppm. The comparison with the 1H NMR spectra of γ-Al2O3(500) indicates that, besides the peaks at 1 and 5 ppm, the other resonances between 0 and 4 ppm correspond to the hydroxyl groups present on the alumina surface (Figure 1Db). The resonance at 1 ppm can be undoubtedly attributed to residual methyl groups of the Al
CH2
C(CH3)3 fragments. The peak at 5 ppm could be assigned to a Al
H resonance by analogy with molecular complexes28 or to a H2O molecule coordinated to the surface, the chemical shift of which having already been reported.29 However this point is still under debate. To further assign these various signals, double-quanta (DQ) proton spectroscopy under magic angle spinning (MAS) was performed on 2150-γ-Al2O3(500). Multiquantum proton spectroscopies under fast MAS have been successfully applied to the characterization of hydrogen-bonding structures, π
π packing arrangements, and defect sites in zeolites30 and for the characterization of surface complexes.18 In such an experiment, correlations are observed between pairs of dipolar-coupled protons. The DQ frequency in the ω1 dimension corresponds to the sum of the two single quantum frequencies of the two coupled protons and correlates in the ω2 dimension with the two corresponding proton resonances. The observation of a DQ peak implies a close proximity between the two protons involved in this correlation.

The 1H DQ MAS spectrum of 2150-γ-Al2O3(500) shows several proton resonances at 20 ( 1 ppm (Figure 2). These resonances could be assigned to several Hf
H species by analogy with the silica-supported hafnium hydrides.22 For all these resonances, no autocorrelation peak at ca. 40 ( 2 ppm in the ω1 dimension is observed. Consequently, the hydrogenolysis of 1-γ-Al2O3(500) at 150 °C affords mainly hafnium monohydride surface complexes in agreement with IR results. Moreover, no correlation peak is observed between the resonance of this hafnium monohydride and other resonances in the 5
0 ppm range. In contrast, the peak around 5 ppm shows an autocorrelation at ω1 = 10 ppm, which is the indication of two close hydrogen atoms, hence the assumption of a water molecule on the surface (see above). 5. Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). The structure of 2150-γ-Al2O3(500) was further studied by Hf LIII-edge extended X-ray absorption fine structure (EXAFS) (Figure 3). The closest contributions (first and more intense peak in the Fourier transform) are reasonably in agreement with the above experimental results, assigned to 2.8(5) σ-bonded oxygen atoms at 1.96 Å. From this assumption, the results are consistent with an average coordination sphere around Hf of two to three oxygen atoms at 1.96(1) Å, which could be attributed to σ-bonded aluminoxy. The distance obtained is in the range of Hf
O σ-bond lengths obtained from EXAFS in silica-supported hafnium complexes (SiO)3HfH (1.943(4) Å) and (SiO)4Hf (1.940(3) Å),31 (SiO)Hf(CH2tBu)3 (1.947(7) Å),22 and from X-ray crystallography studies, i.e., 1.96
1.99 Å for Cp2Hf(c-C5H9)7Si7O11(OSiMe2R),32 1.913(4) Å for Hf(Oi-Pr)10Al2.33 The contribution designated as Hf 3 3 3 O at 2.19(3) Å could be a dative bond between oxygen and Hf center. This distance is in agreement with those observed for dative-linked THF on Hf centers in molecular complexes (2.182
2.190 Å).34,35 So far, the contribution of light elements at 2.70(2) Å could be due to either one carbon atom or one oxygen atom. Finally, the EXAFS data could be improved when the model included some oxygen at 3.11(4) Å and several aluminum atoms at 3.47(3) Å 6759

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Figure 3. Hafnium LIII-edge k3-weighted EXAFS (left) and corresponding Fourier transform (right) with comparison to simulated curves for alumina supported hydrides. Solid lines, experimental; dashed lines, spherical wave theory.

Table 1. EXAFS Parameters for the Alumina Supported Hafnium Hydridesa type of neighbor

number of neighbors

d (Å)

σ2 (Å2)

Hf
OAl

2.8(5)

1.96(1) 1.96(1)

0.011(2)

Hf...O

0.5(4)

2.19(3)

0.003(3)

Hf 3 3 3 C (or O) Hf 3 3 3 O Hf 3 3 3 Al

1.3(8) 0.6(5)

2.70(2) 3.11(4)

0.006(2) 0.005(5)

2(2)

3.47(3)

0.014(6)

The errors generated by the EXAFS fitting program “RoundMidnight” are indicated between parentheses. σ2 designates the Debye
Waller factor. Δk: (2.0
14.1 Å
1)
ΔR: (0.6
3.6 Å); S02 = 1.0. ΔE0 = 3.7 ( 1.0 eV (the same for all shells). Fit residue: F = 3.1%. Quality factor: (Δχ)2/ν = 1.93 (ν = 9/25). a

(see Table 1). However, the results from this analysis have to be considered with caution because there are several surface complexes and EXAFS cannot formally distinguish between O and C light backscatterers surrounding the Hf metal center. Hence, as already described in our previous works,17,19,36 DFT calculations have been performed to improve the characterization of the different surface complexes obtained during the hydrogenolysis reaction of 1-γ-Al2O3(500) at 150 °C. 6. DFT Approach of the Reaction of 1-γ-Al2O3(500) with Hydrogen. It has been shown previously that the reaction of 1 with γ-Al2O3-(500) affords neutral complexes 2 and 3 on the most active AlIV
OH site, while the cationic ones (4) result from the reaction of 1 on defect sites.17 Moreover, IR, 1H NMR, and DQ experiments here suggest that monohydride surface complexes are the most abundant, and EXAFS results seem consistent with a number of aluminoxy ligands in the coordination sphere of the metal superior to 2. Consequently, the calculations were carried out only for the major surface monohydrides 6 and 7, resulting from the most abundant alkyl surface complexes 2, 3, and 4 (Scheme 2). The reaction of H2 with the neutral complex 2 could lead to the intermediate species 5 by two two-step mechanisms: successively, hydrogenolysis of one Hf
CH2tBu bond giving [(AlO)HfH(CH2tBu)2] with a reaction energy of
55 kJ/mol and further grafting onto the surface with elimination of neopentane and formation of a second AlO
Hf bond with a reaction energy of
203 kJ/mol, or the inverse order via 3 with reaction energies of
180 and
78 kJ/mol, respectively. All these processes

are thus exothermic. 5 is not considered as a stable species because, according to the experimental results, all Hf
CH2tBu bonds are hydrogenolyzed at 150 °C. The reaction of 5 with a surface hydroxyl group leading to 7 is favorable by 98 kJ/mol. Besides, 6 is formed from hydrogenolysis of 4 with a reaction energy of
42 kJ/mol. The optimized structures of 6 and 7 are shown in Figures 4 and 5, respectively. In 6 the tetracoordinate hafnium atom is linked to the surface by two short covalent bonds (AlIV
O
Hf, AlV
O
Hf) of 1.94 and 1.90 Å, respectively, and by one dative bond ((Al)3O
Hf) of 2.04 Å. A longer Hf
O contact (2.87 Å) is also found. The distance between the formally cationic Hf and the carbon (CH2) of the counteranion [(CH2tBu)AlS
] is very similar to that obtained for complex 4 (Hf
C 7.2 Å, Al
C 2.0 Å).17 In 7, the hafnium atom is pentacoordinated and linked to the surface by three covalent Hf
O bonds (AlIV
O
Hf, AlV
O
Hf, (AlV)2O
Hf]) of 1.96, 1.97, and 2.07 Å, respectively, and a dative bond of 2.34 Å. The Hf
H bond length is found equal to 1.88 Å in 6 and 1.89 Å in 7. It is worth noting the good agreement between the length values obtained by EXAFS experiment and DFT calculations (Table 2). Indeed, both 6 and 7 are linked to the surface through three short Hf
O bonds. In 7 these bonds are three aluminoxy bonds. In 6, besides two aluminoxy bonds, there is a short dative bond at 2.04 Å which is explained by the highly electron deficient cationic hafnium center. In 7, a longer dative bond is present. The contribution of a light element at 2.70 Å could be assigned definitively to an oxygen atom, ligand of the Hf center in 6. In the second coordination sphere, aluminum atoms are found, indicating a close proximity with the alumina surface. In conclusion, this comparison suggests that the surface hydride complex 2150-γ-Al2O3(500) exists under two main forms, the trisaluminoxy surface complex [(AlO)3Hf
H] (7) and the cationic bisaluminoxy [(AlO)2(AlO)Hf
H]þ (6).

’ CONCLUSION The hydrogenolysis reaction of 1-γ-Al2O3(500) performed at 150 °C affords several surface entities referred to as 2150-γAl2O3(500), which have been analyzed and characterized by several chemical and spectroscopic analyses (IR, DQ 1H NMR, and EXAFS) and DFT calculations. This combination establishes that (i) at 150 °C, all the Hf
CH2
tBu bonds are hydrogenolyzed 6760

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Scheme 2. Possible Reaction Pathways and Stable Intermediates for the Reaction of 1-γ-Al2O3(500) (2, 3, and 4) with Dihydrogena

a

The reaction energies ΔE are obtained by DFT calculations.

Scheme 3. Schematic Representation of the Formation of 6 and 7 Complexes from the Hydrogenolysis of 1-γ-Al2O3(500) at 150 °C

Figure 4. Model structure of the cationic bisaluminoxy hafnium monohydride complex (6) [(AlIVO)(AlsO)Hf (H)]þ with [(CH2tBu)Als]
in the vicinity. The blue atom corresponds to hafnium, white atoms correspond to hydrogen, red atoms to oxygen, and pink atoms to aluminum.

Figure 5. Model structure of the trisaluminoxy hafnium monohydride complex [(AlsO)3HfH] (7). The blue atom corresponds to hafnium, white atoms correspond to hydrogen, red atoms to oxygen, and pink atoms to aluminum.

Table 2. Comparison of Geometric Parameters Obtained by EXAFS and DFT Calculations for the Surface Hydrides Complexes [(AlIVO)(AlsO)Hf (H)]þ[(CH2tBu)Als]
] (6) and [(AlsO)3HfH] (7) Hf
O

Hf
O

1.90

1.94

2.04

7DFT

1.96 1.96

1.97 1.96

2.07 1.96

Short dative bond.

Hf 3 3 3 O

a

6DFT EXAFS

a

Hf
O

2.34 2.19

Hf 3 3 3 O

Hf 3 3 3 Al

2.87

3.20
3.45

2.70

3.23
3.43 3.47

in Hf
H bonds, while the Al
CH2
tBu bonds do not react with hydrogen at this temperature; (ii) only monohydride species are observed; (iii) two surface complexes are present, trialuminoxy monohydride [(AlsO)3HfH] and cationic bisaluminoxy hafnium with monohydride complex [(AlIVO)(AlsO)Hf(H)]þ
[(CH2tBu)Als] in the vicinity. All the transformations are thermodynamically favorable. The similarity of the geometric parameters deduced from EXAFS experiments and the bond lengths obtained with DFT calculations for complexes 6 and 7 enhances our confidence in the proposed structures (Scheme 3).

’ EXPERIMENTAL PART 1. General Procedure. All experiments were carried out using standard air free methodology in an argon-filled Vacuum Atmospheres glovebox, on a Schlenk line, or in a Schlenk-type apparatus interfaced to a high vacuum line (10
5 torr). Hydrogen was dried over a deoxo catalyst (BASF R3-11 þ 4 Å molecular sieves) prior to use. 6761

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The Journal of Physical Chemistry C Gas phase analyses were performed on a Hewlett
Packard 5890 series II equipped with a flame ionization detector and a KCl/Al2O3 on fused silica column (50  0.32 mm). Elemental analyses were performed at the CNRS Central Analysis Department of Solaize or at the LSEO of Dijon. Transmission infrared spectra were recorded on a Nicolet 550FT spectrometer by using a custom infrared cell equipped with CaF2 windows, allowing in situ studies. Typically, 16 scans were accumulated for each spectrum (resolution 2 cm
1). NMR spectra were recorded on an Avance 500 or DSX 300 Bruker NMR spectrometer, using a commercial 4 mm and CPMAS probe from Bruker. The samples were prepared under argon in a 4 or/and 3.2 mm zirconia rotor. The sample spinning speed was ωR = 10 or 18 kHz unless otherwise specified. For the proton DQ experiments, among numerous decoupling sequences, we have chosen the homonuclear symmetry-based post-C7 experiment introduced by Levitt et al.,37 which is robust with respect to chemical shift offset and radio frequency field, rf, in homogeneity,38 and has been extensively used with good efficiency on numerous nuclei including proton.39 In these proton DQ experiments (i) rf field strength was 126 kHz (7*ωR ) during the excitation and reconversion periods which were chosen equal to 222 μs (corresponding to 14 post-C7 basic elements); (ii) 400 increments of 16 scans each were collected with 8 s recycle delay which gave a total experiment time of 14 h; (iii) processing was done with a 50 Hz line broadening in both dimensions and one zero filling in the ω1 dimension. The X-ray absorption spectra (EXAFS) data were acquired on beamline BM29 at ESRF (Grenoble, France) and on station 9.3 at the SRS of the CCLRC (Daresbury, U.K.), at the Hf LIIIedge, between 9.200 and 10.650 keV, in the transmission mode, using a double crystal monochromator, Si(111), detuned to minimize higher harmonics, and ionization chambers as detectors. The calibration in energy was performed with a hafnium foil (E0 = 9.561 keV). The samples were introduced, within a dry and air-preserved glovebox, into a double airtight cell equipped with Kapton windows. The EXAFS spectra were analyzed by standard procedures using the program Athena40and the suite of programs MAX developed by Alain Michalowicz, in particular the EXAFS fitting program RoundMidnight using calculations with spherical waves.41,42 The postedge background subtraction was carefully conducted using polynomial or cubic-spline fittings, and the removal of the lowfrequency contributions was checked by further Fourier transformation. The spectrum fitting was done with the k3-weighted data using the following EXAFS equation, where S02 is a scale factor, Ni is the coordination number of shell i, rc is the total central atom loss factor, Fi is the EXAFS scattering function for atom i, Ri is the distance to atom i from the absorbing atom, λ is the photoelectron mean free path, σi is the Debye
Waller factor, Φi is the EXAFS phase function for atom i, and Φc is the EXAFS phase function for the absorbing atom:   n N F ðk, R Þ
2R i i i i 2 exp expð
2σ 2i k2 Þ χðkÞ = S0 r c ðkÞ kRi2 λðkÞ i¼1

∑

sin½2kRi þ Φi ðk, R i Þ þ Φc ðkÞ The program FEFF843 was used to calculate theoretical values for rc, Fi, λ, and Φi þ Φc based on model clusters of atoms. The refinements were performed by fitting the structural parameters Ni, Ri, σi, and the energy shift ΔE0 (the same for all shells). The fit

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residue, F (%), was calculated by the following formula: F¼

∑½k3 χexp ðkÞ
k3 χcal ðkÞ2 k

∑½k3 χexp ðkÞ2

100

k

As recommended by the Standards and Criteria Committee of the International XAFS Society, an improvement of the fit took into account the number of fitted parameters. The number of statistically independent data points or maximum number of degrees of freedom in the signal is defined as Nidp = (2ΔkΔR/π) þ 2. The inclusion of extra parameters was statistically validated by a decrease of the quality factor, (Δχ)2/ν, and the values of the statistical errors generated in RoundMidnight were multiplied by [(Δχ)2/ν]1/2 in order to take the systematic errors into account, since the quality factors exceeded one. The error bars thus calculated are given in parentheses after each refined parameter. The scale factor, S02, was determined from the analysis of the spectra of reference compounds, HfNp4 diluted in boronitride (4C at 2.20(1) Å). This factor was kept constant in all the fits. 2. Hydrogenolysis of 1-γ-Al2O3(500) Monitored by Transmission Infrared Spectroscopy. γ-Al2O3(500) (75
80 mg) was pressed into a 18 mm self-supporting disk, adjusted in a sample holder, and introduced into a glass reactor equipped with CaF2 windows. γ-Al2O3(500) was calcined under air at 500 °C and partially dehydroxylated at 500 °C. The molecular complex [Hf(CH2tBu)4], 1, was sublimed under dynamic vacuum at 70 °C onto the oxide disk, and the solid was heated at 50 °C for 1 h. The excess of [Hf(CH2tBu)4] was removed by reverse sublimation at 70 °C and condensed into a tube cooled by liquid nitrogen, which was then sealed off using a torch. Hydrogen (500 torr) was introduced in the cell, and the system was heated at 150 °C with a temperature slope of 2 °C 3 min
1 and maintained constant for 17 h. 3. Hydrogenolysis of 1-γ-Al2O3(500) in a Batch Reactor. 1-γAl2O3(500) (ca. 500 mg) was introduced in a reactor of 500 mL under the strict exclusion of air. After evacuation of argon, 500 torr of hydrogen was introduced through a purification trap containing deoxo catalysts and molecular sieves. The system was heated for 17 h at 150 °C, and then the gas phase was analyzed by gas chromatography. 4. Computational Details. The calculations were performed in the framework of density functional theory (DFT) using a periodic description of the system as implemented in the VASP code.44,45 The generalized gradient approximation was used in the formulation of Perdew and Wang PW91.46 Atomic cores were described with the projected augmented wave method (PAW) which is equivalent to an all electron frozen core approach47,48 The one electron wave functions are developed in a basis set of plane waves. With the selected PAW potentials, a cutoff energy of 275 eV is adequate. For the bulk model of alumina, the calculations were done at the Γ point of the Brillouin zone.49 With these parameters a volume of 380 Å3 has been obtained in agreement with the literature.50 The trihydrated (110) surface of γ-Al2O3, corresponding to γ-Al2O3(500), was represented by a four-layer slab. Since we worked with the bulky neopentyl ligands, we had to use a large unit cell with lattice parameters of 16.19  16.83  23 Å3. This corresponds to a vacuum zone of 17.8 Å. Due to the large size of the surface unit cell, a Γ point approach is sufficient. During the optimizations, the two lowest layers are kept fixed in the positions of 6762

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The Journal of Physical Chemistry C the bulk and the two uppermost layers are allowed to relax together with the grafted complex.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (C.C.S.); [email protected] (F.D.).

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