Mohammed H. Al-Hazmi

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.

Comparative study of new chromium-based catalysts for the selective tri- and tetramerization of ethylene

Two new ligands, Ph2PN(iPr)P(Ph)N(c-Hex)(CH3) (2) (Ph = phenyl, iPr = isopropyl, c-Hex = cyclohexyl) and Ph2PN(c-Hex)P(Ph)NEt(CH3) (Et = ethyl) (3) were synthesized. 2 was characterized by X-ray analysis. To compare their applicability with the previously known Ph2PN(iPr)P(Ph)N(iPr)H (1) these ligands were investigated together with chromium(III)acetylacetonate, different co-catalysts and solvents for the catalytic oligomerization of ethylene. The in situ prepared catalysts proved to be highly active, yielding highly pure 1-octene as the main product and 1-hexene as the main byproduct. It was possible to reach productivities of several hundred kg product per gram chromium and hour, up to almost 65 wt% 1-octene with about 99% purity and overall 1-hexene plus 1-octene yields of more than 85 wt%.

The Effect of Substituents in PNPNH Ligands on Activity and Selectivity of a Chromium‐Based Catalyst System for Highly Selective Ethylene Trimerization

The influence of ligand substituents on a highly selective homogeneous catalyst system for the trimerization of ethylene to 1‐hexene consisting of chromium(III) acetylacetonate, a ligand with a PNPNH backbone, tetraphenylphosphonium chloride as chlorine source, and triethyl aluminum as activator was investigated. These N‐modified and P‐modified ligands, which differ strongly in their electronic and steric properties of the substituents, were tested in the ethylene trimerization at three temperatures (50, 70, and 90 °C). Interestingly, the substituents at the N atoms of the PNPNH‐backbone had only little effect on the catalytic activity, on 1‐hexene selectivity, and on thermal stability of the catalyst system, whereas the substituents at the P atoms significantly influenced the efficiency of the system. This is even more surprising as the PNP ligand from SASOL for ethylene tetramerization is much more sensitive against substituent variation at its N atom.

Oxidative dehydrogenation of ethane over movmnw oxide catalysts

Catalysts based on mixed oxide of MoVMn are active at relatively low temperature for oxidative dehydrogenation of ethane. Incorporation of tungsten into MoVMn oxides enhances the catalytic activity. Enhancement of the activity is explained in the light of acid-base interaction accompanied with a redox mechanism of surface reoxidation.

Influence of Ph2PN(R)P(Ph)N(R)H (R=c‐Hex, iPr) Ligand Substituents, Modifier, and Process Parameters on Reaction Kinetics of a Chromium‐based Catalyst for the Selective Trimerization of Ethylene

The influence of ligand substituents on a homogeneous, highly selective catalyst system for the ethylene trimerization to 1‐hexene, consisting of chromium(III)acetylacetonate as a chromium source, tetraphenylphosphonium chloride ([Ph4P]Cl) or dodecyltrimethylammonium chloride ([CH3(CH2)11N(CH3)3]Cl) as a modifier and chlorine source, Ph2PN(iPr)P(Ph)N(iPr)H (L0) as a ligand (Ph=phenyl, iPr=isopropyl), and triethylaluminum as an activator, was investigated. It was shown that the phosphonium or ammonium salts act as both chlorine source and as modifier for the activator triethylaluminum. Additionally, a systematic parameter study using Ph2PN(c‐Hex)P(Ph)N(c‐Hex)H (L1) as the ligand (c‐Hex=cyclohexyl) was performed and important performance figures for catalyst systems using L0 and L1 ligands, respectively, were compared under different reaction conditions. As a result, both catalyst systems demonstrate very similar kinetics at identical ethylene pressures, temperatures, and ligand concentrations. Additionally, the influence of the modifiers on the performance of the catalyst systems at different ethylene pressures and temperatures was studied in detail. The results suggest that the modifier has a greater influence on the reaction mechanism than expected. Moreover, the influence of the modifier on the catalytic performance was found to be more significant than the impact of the ligand’s chemical nature.

Preparation of Zirconium Oxide Powder Using Zirconium Carboxylate Precursors

Zirconia was prepared at low temperatures (<450°C) using single several source precursors based on zirconium carboxylates where the R groups were systematically varied. The combination of density functional theory (DFT) calculations and extensive characterization of the precursors (i.e., X-ray diffraction, thermal gravimetric analysis, infrared spectroscopy, and scanning electron microscopy) indicated that the carboxylic acid complexes may link the zirconium metal with a cis bidentate configuration. Periodic DFT calculations were performed to examine the interaction between monoclinic ZrO2 and propanoic acid. Dissociative adsorption takes place through the cis bidentate structure with an adsorption energy of −1.43 eV. Calculated vibrational frequencies using the optimized structure are in good agreement with experimental findings.

Synthesis of Differently Substituted N- and P-Aminodiphosphinoamine PNPN-H Ligands

Abstract On the basis of the known aminodiphosphinoamine ligand Ph2PN(i-Pr)P(Ph) N(i-Pr)-H (3a), differently substituted aminodiphosphinoamine PNPN-H ligands (3) were prepared. By using different synthetic methods, the N-substituted ligands Ph2PN (i-Pr)P(Ph)N(c-Hex)-H (3b), Ph2PN(c-Hex)P(Ph)N(i-Pr)-H (3g), and Ph2PN(i-Pr)P(Ph) N[(CH2)3Si(OEt)3]-H (3c), in addition to the formerly described Ph2PN(n-Hex)P (Ph)N (i-Pr)-H (3h), Ph2PN(i-Pr)P(Ph)N(Et)-H (3d), Ph2PN(i-Pr)P(Ph)N(Me)-H (3e), and Ph2PN(c-Hex)P(Ph)N(c-Hex)-H (3f), were obtained. In addition, Ph2PN(i-Pr)P(Me)N(i-Pr)-H (3i), (cyclopentyl)2PN(i-Pr)P(Ph)N(i-Pr)-H (3j), (-O-CH2-CH2-O-)PN(i-Pr)P(Ph)N(i-Pr)-H (3k), and (1-Ad)2PN(i-Pr)P(Ph)N(i-Pr)-H (3l) were prepared with different P-substitutions. All compounds were characterized and the molecular structures of the intermediates Ph2PN(i-Pr)P(Ph)Cl (1a) and (cyclopentyl)2PN(i-Pr)P(Ph)Cl (1e) and the ligand (1-Ad)2PN(i-Pr)P(Ph)N(i-Pr)-H (3l) were investigated by single-crystal X-ray diffraction. Supplemental materials are available for this article. Go to the publishers online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. GRAPHICAL ABSTRACT