Fuad Mosa

An Alternative Mechanistic Concept for Homogeneous Selective Ethylene Oligomerization of Chromium‐Based Catalysts: Binuclear Metallacycles as a Reason for 1‐Octene Selectivity?

An alternative concept for the selective catalytic formation of 1-octene from ethylene via dimeric catalytic centers is proposed. The selectivity of the tetramerization systems depends on the capability of ligands to form binuclear complexes that subsequently build up and couple two separate metallacyclopentanes to form 1-octene selectively. Comparison of existing catalytic processes, the ability of the bis(diarylphosphino)amine (PNP) ligand to bridge two metal centers, and the experimental background support the proposed binuclear mechanism for ethylene tetramerization.

Activation and Deactivation by Temperature: Behavior of Ph2PN(iPr)P(Ph)N(iPr)H in the Presence of Alkylaluminum Compounds Relevant to Catalytic Selective Ethene Trimerization

Coordination, deprotonation, rearrangement, and cleavage of Ph(2)PN(iPr)P(Ph)N(iPr)H (1) by trialkylaluminum compounds R(3)Al (R=Me, Et) are reported that are relevant to the selective ethene trimerization system consisting of the ligand 1, CrCl(3)(THF)(3) and Et(3)Al that produces 1-hexene in more than 90% yield and highest purity. With increasing temperature and residence time first the formation of an adduct [Ph(2)PN(iPr)P(Ph)N(iPr)H][AlR(3)] (2), second the aluminum amide [Ph(2)PN(iPr)P(Ph)(AlR(3))N(iPr)][AlR(2)] (3) and third its rearrangement to the cyclic compound [N(iPr)P(Ph)P(Ph(2))N(iPr)][AlR(2)] (4) were observed. The cleavage of 3 by an excess of R(3)Al into an amidophosphane and an iminophosphane could be the reason for its rearrangement to complex 4, as well as to the cyclic dimer [R(2)AlN(iPr)P(Ph)(2)](2) (5). The chemistry of ligand 1 in the presence of alkylaluminum compounds gives hints on possible activation and deactivation mechanisms of 1 in trimerization catalysis.

A Kinetic Model for Selective Ethene Trimerization to 1‐Hexene by a Novel Chromium Catalyst System

A numerical model for the kinetics of the selective trimerization of ethene to 1‐hexene has been developed on the basis of mechanistic investigations and extensive experimental parameter studies. The reaction is catalyzed by a homogeneous catalyst system, comprising the chromium source [CrCl3(thf)3], a Ph2PN(iPr)P(Ph)N(iPr)H ligand, and triethylaluminum as activator. The kinetic model is designed as a tool for laboratory data evaluation, design and planning of meaningful experiments in the multidimensional parameter space, and parameter identification, and, moreover, it includes all features needed to eventually facilitate the transfer of the laboratory results into the technical environment. In particular, the model is designed to deliver the intrinsic chemical kinetics of the homogeneous catalytic system and to rule out any undetected influence of phase‐transfer limitations. Key kinetic parameters are determined by fitting the numerical simulations to the experimental results. In general, the model calculations and experimental data are in excellent agreement. In conjunction with mechanistic investigations, the model helps to elucidate the complex reaction network.

Immobilized Chromium Catalyst System for Selective Ethene Trimerization to 1‐Hexene with a PNPNH Ligand

Immobilization of transition metal complexes on solid supports is well known as an efficient method to handle and recover catalysts. Among these supports, polystyrene-based materials are widely used to create recyclable catalysts. The immobilization of transition metals on these supports offers a number of advantages over traditional solution-phase chemistry. Covalent binding is the most commonly used technique for immobilization, due to its broad applicability, the fact that significant leaching does not usually occur, and that stable, active catalysts are formed. Binding is usually achieved in one of two ways: a) by grafting the catalyst (via a ligand) onto the prederivatized support, or b) by copolymerization of the active species with styrene and divinylbenzene (DVB). Linear a-olefins have found extensive applications in the manufacture of fine chemicals such as detergents, plasticizers and lubricants, in addition to being used as comonomers for the production of linear low-density polyethene. Industrially prevalent in the production of linear a-olefins are mainly the SHOP, Chevron, and Amoco processes. In the field of selective production of linear a-olefins, only the Chevron–Philips selective trimerization process is currently industrially used, although there are several other known selective oligomerization systems. Even fewer examples of immobilized chromium-based selective trimerization catalysts for the production of 1-hexene have been reported to date. We recently described the development of a novel homogeneous catalyst for selective ethene trimerization to 1-hexene, which is based on a new class of aminophosphine ligands with a Ph2PN(iPr)P(Ph)N(iPr)H (PNPNH) backbone, in conjunction with [CrCl3(thf)3] and Et3Al as a cheap and well-defined aluminum–alkyl activator (Scheme 1). It is important to mention that especially the terminal secondary amine function is crucial for selectivity towards 1-hexene. Deprotonation of this group by the aluminum–alkyl activator, to yield an amide ligand backbone, is very likely to occur under catalytic conditions. Detailed kinetic investigations of this new homogeneous trimerization system were very recently reported. Herein we report the immobilization of this homogeneous system to yield an effective heterogeneous catalyst. The latter combines the advantages of the homogeneous catalyst system, such as selectivity and moderate conditions, with the advantages of a heterogeneous process setup, such as easy catalyst separation and recyclability. An excess of the ligand precursor Ph2P(iPr)NP(Ph)Cl [7] reacted with the amino groups of the amino-modified styrene polymer to yield the desired PNPNH ligand structure (Scheme 2). Among various supports investigated, such as aminomethyl polystyrene resin, TentaGel MB NH2, aminomethyl ChemMatrixR resin, and diethylenetriamine polystyrene resin, tris(2-aminoethyl)amine polystyrene resin gave the best results in terms of loading and catalysis. Gel-phase P NMR spectroscopy of the solid support suspended in C6D6 gave clear evidence for the covalently bonded PNPNH ligand (d= 40.6, 72.5 ppm). According to elemental analysis, approximately Scheme 1. Composition according to the homogeneous system.