Chlorodiethylaluminum supported on silica: A dinuclear aluminum surface species with bridging μ2-Cl-ligand as a highly efficient co-catalyst for the Ni-catalyzed dimerization of ethene
Graphical abstract
Introduction
Many catalytic systems require co-catalysts or activators to form the highly reactive intermediates necessary for efficient catalytic cycles [1], [2]. In olefin oligomerization/polymerization, alkylaluminums have long been recognized as superior co-catalysts [1], [3], [4], [5], [6], [7], [8], [9]. For example, methylaluminoxane (MAO) is among the best co-catalysts for metal-catalyzed olefin polymerization, despite its unknown structure [10], [11], [12], [13], [14], [15], [16]. In most cases, MAO is used in very large excess (100–2000 equiv. compared to the metal catalysts) [17], [18], [19], [20], [21], [22], which remains a severe problem for industrial processes.
There have been tremendous efforts aimed toward developing alternatives to MAO, in particular heterogeneous variants that could simplify chemical processes [23]. One approach is to support molecular aluminum compounds on large surface area supports [24], [25], [26]. Despite the promise of this method, most materials reported to date result in poor catalytic activity. For example, grafting trialkylaluminum reagents on dehydroxylated silica results in well-defined alkylaluminum environments [27], [28], [29], but generally do not activate transition metal pre-catalysts [30]. One of the reasons for the poor co-catalytic activity is that the silica support is heavily modified by reaction with the trialkylaluminum by alkyl transfer of AlR groups to nearby siloxane bridges to form surface alkylsilane moieties, which are unreactive toward metal pre-catalysts. The surface Al species are “incorporated” into the silica matrix as Al(OSi)n species [28], [29], [31]. In contrast, very promising alternative co-catalysts have been found for the ethene oligomerization or polymerization based on chloroalkylaluminum derivatives (2–15 equiv per metal pre-catalysts ratio) [32], [33]. Combining the alkylating properties with the high Lewis acidity from the ClAl in the chloroalkylaluminum is essential to react with the metal pre-catalyst [34], [35], [36] and to generate the active sites [37], [38].
Here, we describe the preparation of supported diethylaluminum chloride (DEAC) on partially dehydroxylated silica using Surface Organometallic Chemistry [39], [40], [41], their use as co-catalyst in the NiCl2(PBu3)2-catalyzed dimerization of ethene [42], and the detailed characterization of surface species by aluminum-27 solid-state NMR spectroscopy at high-fields and ultrafast spinning rates combined with first principle calculations [29], [31]. We found that the dominating species on the silica surface are bis-grafted dinuclear aluminum surface species with bridging Cl-ligands, which likely account for the very high activities in these materials because the core DEAC structure is conserved on the surface.
Section snippets
Materials
The grafting reaction was carried out under inert atmosphere using dry and freshly distilled solvents. Tetraethoxysilane (TEOS), pluronic P123, pentane, 1 M hexane solution of Et3Al (TEA) and Et2AlCl, (DEAC) were purchased from Sigma–Aldrich. Mesoporous SBA-15 and MCM-41 were synthesized following the procedure previously reported in the literature.[43], [44] Aerosil SiO2 was Aerosil Degussa 200 selling Silicon Dioxide. Elemental analyses were performed at Mikroanalytisches Labor Pascher. Gas
Results and discussion
The solid co-catalysts were prepared by grafting diethylaluminum chloride (DEAC) in pentane/hexanes mixtures on high specific surface area silica, SBA-15 [57], partially dehydroxylated at 500 °C (SBA500). SBA500 has a silanol coverage of 1.3–1.4 OH nm−2 (1.5–1.6 mmol g−1; surface area 690 m2 g−1). Contacting SBA500 with a solution of DEAC results in the evolution of ca. 1 equiv of ethane per surface silanol. Aluminum and carbon elemental analysis were 5.59 and 6.53 wt%, respectively; values which
Acknowledgments
This publication is based on work supported by Award No.UK-C0017, made by King Abdullah University of Science and Technology (KAUST), and by the TGE RMN THC Fr3050. The authors thank the PSMN at ENS of Lyon for the attribution of CPU time.
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