Early transition metal organometallics chemisorbed on extremely Brønsted acidic sulfated metal oxides such as sulfated alumina (AlS) and sulfated zirconia (ZrS) produce highly active single-site catalysts for olefin polymerization, alkene and arene hydrogenation, and alkane hydrogenolysis, typically with a large percentage of catalytically significant sites. The low support conjugate basicity allows formation of an extremely electrophilic metal center upon chemisorption of an organometallic, thus highly active catalysts for the aforementioned transformations. This class of catalysts inherits properties from heterogeneous and homogeneous catalysts, possessing well-defined, homogeneous active sites while being immobilized on a support material. This allows for atomistic structural characterization of the active site with a variety of spectroscopic techniques. Structural characterization utilizing x-ray absorption spectroscopy (XANES, EXAFS) shows that Zr-O and Hf-O bond lengths in respective supported organometallic species are significantly longer (by 0.3-0.1 Å) than molecular analogues, providing evidence for a loose surface···metal interaction. While the ion pairing between the metal atom and the support is weak, the support plays a non-innocent role in catalysis. Cp*ZrMe2+ on AlS was found to produce high-Mn isotactic poly-α-olefins, whereas, the molecular analogue ([Cp*ZrMe2]+ [B(C6F5)4]-) produces low-Mn atactic polymers. To further explore the tunability of these single-site heterogeneous catalysts, a series of hafnium-based catalysts supported on AlS and ZrS with varying ligand frameworks were designed to produce a tunable heterogeneous catalytic system. It was shown that altering both the catalyst ligand environment and the support altered polymerization activity by a 10× difference and α-olefin comonomer incorporation by a 28× difference.Similar catalysts tetra(neopentyl)zirconium(IV) and tetra(neopentyl)hafnium(IV) supported on AlS (AlS/ZrNp2, AlS/HfNp2 , respectively) were found to catalyze rapid hydrogenolysis of alkanes under relatively mild conditions, with AlS/ZrNp2 capable of propane hydrogenolysis at a rate of 4000 turnovers per hour at 150°C under flow conditions. These catalysts were also found to be extremely active in the hydrogenolysis of commercial-grade polyethylene-co-1-octene and isotactic polypropylene, as well as postconsumer samples of high-density polyethylene. Additionally, a detailed kinetic and mechanistic investigation was also carried out for these catalysts by utilizing a plug-flow reactor with propane as a model alkane.

Creator

• Mason, Alexander Heath

DOI

• https://doi.org/10.21985/n2-e5mv-6n39

Subject

• Chemistry

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:16529
• etdadmin_upload_984243

Date created

• 2023-01-01

Resource type

• Dissertation

Rights statement

• In Copyright