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]

Abstract

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 °C. The resulting surface [(≡Si−O−Si≡) (≡Si−O) 2 Al (isobutyl) fragment undergoes β‐H elimination at 400 °C leading to [(≡Si−O−Si≡)(≡Si−O−)2Al−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−)2Al−O−W(≡CtBu)(CH2tBu)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−)2Al−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 °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.

A strategy for high ethylene polymerization performance using titanium single-site catalysts

“…If we apply this concept to single-site Ziegler Natta Catalysis, it is well accepted that the propagating species is a metal-alkyl and sometimes with the help of a suitable co-catalyst a cationic metal alkyl (to render metal activity). Transition metal catalysts have been employed in polymerization reactions, and among them, titanium and zirconium have shown improved activity and been intensely investigated 9,10 Inspired by the recently reported tetracoordinate aluminium hydroxide surface ligand [(Si–O–Si)(Si–O) 2 Al–OH] 2 , 11 here we report the prediction, the design, the characterization, and the very high catalytic activity and selectivity of a single site Ti-based catalyst in which the Ti is linked to a tetra-coordinated Al attached to a silica surface. We also demonstrate that TiNp grafted on tetracoordinate Al is more active than TiNp 3 grafted on dehydroxylated silica.…”

“…Furthermore, the insertion of a Lewis acid center (Al site) between the silica surface and the active Ti catalytic site markedly alters the titanium catalyst's performance, as illustrated for ethylene polymerization. The aluminium center was characterized as a Lewis acid center, 11 generating a cationic site on the surface and playing a significant role as a co-catalyst for the polymerization reaction. The [Al-OH] Lewis acid center acts both as an anchoring site for the Ti and modifies the steric and electronic properties of the Ti site.…”