Oligomerisations

Dr. Bernd H. Müller

innovative Ligand design

The interaction of metal core and ligand determines the outcome of a homogeneous catalyzed reaction. New ligands result in new catalytic reactions. On these grounds research on selective oligomerization is focused on innovative ligand design. Therefore synthesis of tailor-made ligands is a fundamental task within this topic.

Linear alpha olefins (LAOs) are very useful intermediates for the making of detergents, synthetic lubricants, co-polymers, plasticizer alcohols and many other important products. There are several companies that synthesize such LAOs via catalytic oligomerisation of ethylene. An inherent problem connected to all of these metal-catalysed ethylene oligomerization processes is the production of LAO mixtures following a mathematical distribution (Schulz-Flory or Poisson); these mixtures are difficult to separate and often do not match market demands. To overcome these problems, current research efforts are focused on the development of selective ethylene oligomerisation processes.

Recently these activities have been primarily concentrated on the selective trimerisation of ethylene to yield 1-hexene and also the selective tetramerisation of ethylene to produce 1-octene. A highly topical development in these investigations is the challenge of dealing with the participation of metallacycles in oligomerisation reactions. As an example, in principle the selective trimerisation or tetramerisation of ethylene can proceed via formation of chains or via metallacyclic intermediates; however,  the latter only recently became increasingly interesting from the viewpoint of selectivity (scheme 1):

Scheme 1. Oligomerisation of ethylene via chain or ring formation

 

Several selective processes can be envisaged for the production of the higher value LAOs, such as 1-hexene and 1-octene, which are highly desirable from the industrial point of view. In all cases the formation and stability of metallacycles, e.g. metallacyclopentanes, metallacycloheptanes (Scheme 2) and metallacyclononanes (Scheme 3) was assumed and discussed in mechanistic investigations as an essential postulate for the selectivity with regard to the products that were formed.

Scheme 2. Trimerisation of ethylene to 1-hexene by mononuclear complexes

 

It is supposed that in metallacyclopentanes the geometrical constraints of the ring system prevent an interaction with the beta-hydrogens and the metal center; such an interaction would otherwise lead to the elimination of 1-butene. The coordination and insertion of another ethylene into the metallacyclopentane proceeds as the metallacycloheptane is formed, which is more flexible for the beta-hydrogen elimination; therefore elimination of 1-hexene can occur.

As an alternative, another assumption was made: this involved the insertion of a further ethylene molecule, in the process of a metallacyclononane being formed, as a competing reaction; this gave rise to the selective formation of 1-octene by elimination from the intermediate nine-membered metallacycle After these mechanistic suggestions it is considered to be made relatively easier to eliminate the 1-octene, compared to further ethylene insertion and the formation of higher metallacycles.

Scheme 3. Tetramerisation of ethylene to 1-octene by mononuclear complexes

 

We are investigating the influence of different metals, several innovative ligand systems and the reaction conditions applied.

Papers on catalyst synthesis[1], kinetic[2] and mechanistic[3] models as well as studies on the effects of substituents in ligands[4] and immobilized catalyst systems[5] were published during the last years.

[1] S. Peitz, N. Peulecke, B. H. Müller, A. Spannenberg, H.-J. Drexler, U. Rosenthal, M. H. Al-Hazmi, K. E. Al-Eidan, A. Wöhl, W. Müller, Heterobimetallic Al-Cl-Cr Intermediates with Relevance to the Selective Catalytic Ethene Trimerization Systems Constisting of CrCl3 (THF)3, the Aminophosphorus Ligands Ph2PN(R)P(Ph)N(R)H, and Triethylaluminium. Organometallics 2011, 30, 2364-2370.

[2] W. Müller, A. Wöhl, S. Peitz, N. Peulecke, B. R. Aluri, B. H. Müller, D. Heller, U. Rosenthal, M. H. Al-Hazmi, F. M. Mosa, A Kinetic Model for Selective Ethene Trimerization to 1-Hexene by a Novel Chromium Catalyst System. ChemCatChem 2010, 2, 1130-1142.

[3] S. Peitz, B. R. Aluri, N. Peulecke, B. H. Müller, A. Wöhl, W. Müller, M. H. Al-Hazmi, F. M. Mosa, U. Rosenthal, An Alternative Mechanistic Concept for Homogeneous Selective Ethylene Oligomerization of Chromium-Based Catalysts: Binuclear Metallacycles as a Reason for 1-Octene Selectivity? Chem. Eur. J. 2010, 16, 7670-7676.

[4] S. Heinig, A. Wöhl, W. Müller, M. H. Al-Hazmi, B. H. Müller, N. Peulecke, U. Rosenthal, The Effect of Substituents in PNPN-H Ligands on Activity and Selectivity of a Chromium-Based Catalyst System for Highly Selective Ethylene Trimerization. ChemCatChem 2013, 5, 3107-3113.

[5] N. Peulecke, B. H. Müller, S. Peitz, B. R. Aluri, U. Rosenthal, A. Wöhl, W. Müller, M. H. Al-Hazmi, F. M. Mosa, Immobilized Chromium Catalyst System for Selective Ethene Trimerization to 1- Hexene with a PNPNH Ligand. ChemCatChem 2010, 2, 1079-1081.