Olefin oligomerization by main group Ga3+ and Zn2+ single site catalysts on SiO2

Nicole J Libretto,Rhea Nargund,Jeffrey Greeley,Juan Carlos Vega-Vila,Aubrey Quigley,Yinan Xu,Arunima Saxena,Rajamani Gounder,Richard Caulkins,Jeffrey T Miller,Ethan Edwards,Guanghui Zhang

doi:10.1038/s41467-021-22512-6

Abstract

In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni2+ and Cr3+. Here we report that silica-supported, single site catalysts containing immobilized, main group Zn2+ and Ga3+ ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni2+. The molecular weight distribution of products formed on Zn2+ is similar to Ni2+, while Ga3+ forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and β-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes.

Highlights

In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni2+ and Cr3+
These results are consistent with previous single site structures reported for Ga3+, Zn2+, and Ni2+ hydrogenation and dehydrogenation catalysts[22,23,26,27]. These structures differ from organometallic catalysts where metal alkyls, like trimethylgallium, were grafted to a SiO2 support resulting in M–M bonds[28,29,30]
This work demonstrates that isolated, Ga3+ and Zn2+ ion sites on inert SiO2 support catalyze olefin oligomerization following the Cossee–Arlman mechanism which is generally accepted for transition metal catalysts

Summary

Introduction

Olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni2+ and Cr3+. Chevron Philips (Gulf) and others utilize similar catalysts under similar reaction conditions[6] Such homogeneous catalysts often require aluminum–alkyl cocatalysts to form the initial metal–alkyl reaction intermediate. Separation and regeneration of homogenous catalysts is generally not possible; there is interest in development of heterogeneous catalysts, which are readily separated from the products and can be regenerated The latter include immobilized Ni2+ sites on zeolite (BEA, MFI), or mesoporous aluminosilicates (MCM-41) and other high surface area oxide supports which sometimes generate Brønsted acid sites during the reaction, leading to bifunctional catalysis[10,12,13,14,15,16,17]. Ni-alkyl and hydrides are proposed key reaction intermediates, and olefin insertion and β-hydride elimination are the key elementary reaction steps, though limited spectroscopic evidence exists[18,19]

Methods

Results

Conclusion