Tungsten Catalyst Incorporating a Well-Defined Tetracoordinated Aluminum Surface Ligand for Selective Metathesis of Propane, [(≡Si−O−Si≡)(≡Si−O−)2Al−O−W(≡CtBu) (H)2]

A well-defined aluminium-bound hydroxyl group on the surface of mesoporous SBA-15, [( Si-O-Si ) ( Si-O)(2) Al-OH], 3 was obtained by reacting di-isopropyl aluminium hydride with SBA-15 treated at 700 degrees C. The resulting surface [( Si-O-Si ) ( Si-O) (2) Al (isobutyl) fragment undergoes beta-H elimination at 400 degrees C leading to [( Si-O-Si )( Si-O-)(2)Al-O) Al-H]. Further oxidation of this Al-hydride with N2O leads to 3. This acidic support was used to create a well-defined surface organo-tungsten fragment [( Si-O-Si )( Si-O-)(2)Al-O-W( CtBu)(CH(2)tBu)(2)] by reacting 3 with W( C-tBu)(CH2-tBu)(3). A further reaction with hydrogen under mild conditions afforded the tungsten carbyne bis-hydride [( Si-O-Si )( Si-O-)(2)Al-O-W(H)(2)(C-tBu)]. The performance of each of the W-supported catalysts was assessed for propane metathesis in a flow reactor at 150 degrees C. [( Si-O-Si )( Si-O-)(2) Al-O-W( CtBu)(H)(2)] was found to be a single-site catalyst, giving the highest turnover number (TON=800) and the highest reported selectivity for butane (45%) vs. ethane (32%) known for oxide-supported tungsten complex catalysts (with the supports being silica, silica-alumina, and alumina). The results demonstrate that modification of the oxide ligands on silica via the creation of Al Lewis acid center as an anchoring site for organometallic complexes opens up new catalytic properties, markedly enhancing the catalytic performance of supported organo-tungsten species.