Unearthing a Well-Defined Highly Active Bimetallic W/Ti Precatalyst Anchored on a Single Silica Surface for Metathesis of Propane

Abstract

Two compatible organometallic complexes, W(Me) (1) and TiNp (2), were successively anchored on a highly dehydroxylated single silica support (SiO) to synthesize the well-defined bimetallic precatalyst [(≡Si-O-)W(Me)(≡Si-O-)Ti(Np)] (4). Precatalyst 4 was characterized at the molecular level using advanced surface organometallic chemistry (SOMC) characterization techniques. The strong autocorrelation observed between methyl of W and Ti in H-H multiple-quantum NMR spectra demonstrates that W and Ti species are in close proximity to each other. The bimetallic precatalyst 4, with a turnover number (TON) of 9784, proved to be significantly more efficient than the silica-supported monometallic catalyst [(≡Si-O-)W(Me)] (3), with a TON of 98, for propane metathesis at 150 °C in a flow reactor. The dramatic improvement in the activity signifies the cooperativity between Ti and W and indicates that the key step of alkane metathesis (C-H bond activation followed by β-H elimination) occurs on Ti, followed by olefin metathesis, which occurs on W. We have demonstrated the influence and importance of proximity of Ti to W for achieving such a significantly high activity. This is the first report demonstrating the considerably high activity (TON = 9784) in propane metathesis at moderate temperature (150 °C) using a well-defined bimetallic system prepared via the SOMC approach.

A strategy to convert propane to aromatics (BTX) using TiNp4 grafted at the periphery of ZSM-5 by surface organometallic chemistry

“…1B). 26,30 The formation of Ti-H confirms our strategy to immobilize the Ti-H strongly on the surface of [H-ZSM-5 300 ].…”

“…24 This hydrogenolysis is assumed to come from a Sigma bond metathesis step between the C-H bond of the paraffin and the d 0 metal hydride. [25][26][27][28] In order to fully control these steps, we decided to immobilize Ti outside the pore system of the [ZSM-5] which could lead to olefin by Sigma bond metathesis. Then, it was expected that the formed olefin could migrate into the pore system and undergo oligomerization, cyclisation and aromatization inside the cavities.…”

Exploiting Confinement Effects to Tune Selectivity in Cyclooctane Metathesis

“…Works for improving the TON are currently ongoing in our laboratories using bimetallic catalysts. [46][47]…”

Metathesis of Functionalized Alkane: Understanding the Unsolved Story

“…Since the discovery of alkane metathesis reaction by well-defined silica supported [Ta]-H catalyst [1], many groups, including us, have been continuously working on the development of well-defined catalysts for "functionalized" alkanes metathesis reaction or alkyl chains bearing any functionality, along with improved reactivity in alkane metathesis reaction [2][3][4][5][6][7][8]. While significant progress was achieved in improving the catalyst activity (e.g., by changing catalyst support and by employing a tandem system, where two catalysts act back to back for cascade reaction), metathesis of a functionalized alkane remains elusive in the scientific community [7,[9][10][11][12]. To metathesize a functionalized alkane, the reactant should undergo dehydrogenation to a functionalized olefin, functionalized olefin metathesis, and finally reduction of the newly-formed, functionalized olefins to new functionalized alkanes.…”