Published: November 28, 2011 r2011 American Chemical Society 834 dx.doi.org/10.1021/jp208709x | J. Phys. Chem. C 2012, 116, 834–843 ARTICLE pubs.acs.org/JPCC Evolution of Structure and of Grafting Properties of γ-Alumina with Pretreatment Temperature Marco Delgado, † Franc -oise Delbecq,* ,‡ Catherine C. Santini,* ,† Fr  ed  eric Lefebvre, † S  ebastien Norsic, † Piotr Putaj, † Philippe Sautet, ‡ and Jean-Marie Basset † † Universit  e de Lyon, Institut de Chimie de Lyon, C2P2, UMR 5265 CNRS - ESCPE Lyon, 43 bd du 11 Novembre 1918, F-69626 Villeurbanne Cedex, France ‡ Universit  e de Lyon, Institut de Chimie de Lyon, CNRS - ENS Lyon, Laboratoire de Chimie, 46 all  ee d’Italie, F-69364 Lyon Cedex 07, France b S Supporting Information 1. INTRODUCTION Electron-poor complexes of group IV metals, grafted on γ- alumina, have been known for a long time. In the early 70s, 1À3 Ballard 4 and Yermakof 5 showed that such alumina-supported zirconium alkyl complexes exhibited a surprisingly high catalytic activity in polymerization or oligomerization of ethylene or propylene. At that period, the structure of the active complex was mostly unknown due to the lack of sophisticated techniques available. Later on, supported metallocenes were used in the homogeneous or heterogeneous phase. 6 In this case, in particular for homogeneous catalysis, the role of the organo-aluminum cocatalyst could be interpreted at a molecular level, and in particular the introduction of methylaluminoxane (MAO) as a cocatalyst was understood as being an alkylating agent and a Lewis center. In molecular chemistry, the formation of cationic species from molecular precursors has been deeply analyzed using fluoroar- ylboranes/borates as a Lewis center able to abstract an alkyl group from early transition metalsÀalkyls. 7,8 The role of the Lewis center was there to create an “ion pair” which was supposed to render the transition metal more electrophilic and more suscep- tible to attract the incoming olefin. In the heterogeneous phase, it was generally assumed from patent literature that MAO is grafted on the silica support, and then the metallocene is allowed to react with the grafted MAO. 9,10 The similarity between the homo- geneous and heterogeneous strategies for metallocene-based polymerization led us to revisit the interaction of Group IV metal alkyls on alumina to see if one could detect the intrinsic Lewis acid role of alumina as it occurs with MAO and metallo- cenes. In the particular case of alumina, the deep knowledge and understanding of the direct environment around the grafted complex seems to be crucial, and temperature pretreatment can be used as a tool to control the surface chemistry. The existence of possible cationic low coordination transition metal structures should, in principle, be allowed by the presence of free Lewis acid sites at the close vicinity of the first formed AlÀOÀMt bond on the surface, as shown for Hf complexes. 11 Moreover, the formation of partially or fully cationic surface complexes has also been proposed based on NMR spectroscopy and on theoretical calculations (Density Functional Theory, DFT). 4,12À23,22,24À26 In particular, γ-alumina exhibits a large variety of hydroxyl groups with several modes of coordination to the surface Al sites. Received: September 9, 2011 Revised: November 22, 2011 ABSTRACT: In this study, the nature of the hydroxyl groups present on γ-alumina, γ-Al 2 O 3 , pretreated at various temperatures has been reinvestigated by 1 H NMR spectroscopy. The peaks are assigned by comparison between experimental and simulated spectra, in agreement with previous IR studies. The lowest chemical shifts δ correspond to OH groups strongly bound to the most acidic Al atoms (Al IV and Al V ). High chemical shifts δ are assigned to OH groups making hydrogen bonds. A large range of values is found depending on the strength of these bonds. The structure of the surface complexes obtained by grafting Hf- (CH 2 tBu) 4 , 1, on γ-Al 2 O 3 at various pretreatment temperatures T (350, 500, 700 °C), referred to as 1-γ-Al 2 O 3‑(T) , and of their thermolysis products has been determined, by a combined experimental (mass balance, in situ IR,) and theoretical (DFT calculations) study. These results unambiguously prove the presence of two kinds of neopentylÀmetal bonds, HfÀCH 2 tBu and AlÀCH 2 tBu for 1-γ-Al 2 O 3‑(500) and 1-γ-Al 2 O 3‑(700) , hence the existence of surface cationic low coordinated hafnium complexes. In contrast, for 1-γ-Al 2 O 3‑(350) , only neutral species exist. Hence, temperature pretreatment has a key role for controlling the chemistry of the alumina surface (density of OH groups, presence of highly Lewis acidic Al), the grafting mode of the Hf precursor, and the formation of cationic low coordinated active centers.