Khalid Al-Bahily

Towards selective ethylene tetramerization

And the magic number is … 4! A large amount of oligomer-free 1-octene (99.9?%) was produced from ethylene by a catalytic system based on chromium during the formation of polyethylene wax [see GC–MS chromatogram; the other three significant peaks are methanol (quenching agent), ethyl acetate (needle-rinsing agent), and toluene (solvent)].

A Highly Selective Ethylene Tetramerization Catalyst

Small change, big difference: The introduction of an additional CH2 group into the bridge of ligands with two P/N units leads to a different selectivity of the corresponding chromium-based catalysts. Whereas 1 produces an ethylene trimerization system, 2 provides an unprecedented ethylene tetramerization system that produces 1-octene with high purity and little or no polymer side products (see scheme; DMAO=Me3Al-depleted methylaluminoxane).

A chromium ethylidene complex as a potent catalyst for selective ethylene trimerization

Going one, twice… Reaction of the mononuclear complex [{?5-(tBu)2C4H2N}CrCl2(thf)] with AlEt3 afforded the dinuclear species [{[?5-(tBu)2C4H2N]CrEt}2(µ-Cl)2] (see picture; Cr purple, Cl green, C sticks/gray, H white). The complex acts a single-component selective trimerization catalyst; a higher loading of AlEt3 activator afforded isomerization of 1-hexene to cis, trans 2-hexene

Preparation and Characterization of a Reduced Chromium Complex via Vinyl Oxidative Coupling: Formation of a Self-Activating Catalyst for Selective Ethylene Trimerization

Reaction of the divalent [(t-Bu)NP(Ph)(2)N(t-Bu)]CrCl(2)Li(THF)(2) (1) with 1 equiv of vinyl Grignard (CH(2)=CH)MgCl reproducibly afforded the triangulo {π-[(t-Bu)N-P(Ph)(2)-N(t-Bu)]Cr}(2)(μ,μ,η(4),η(4)-C(4)H(4)){σ-[(t-Bu)N-P(Ph)(2)-N(t-Bu)]Cr} (2) containing a σ-/π-bonded butadiene-diyl unit. The diene-diyl moiety was generated by an oxidative coupling and deprotonation of two vinyl anions. The crystal structure revealed that of the three chromium atoms, each bearing one NPN ligand, two are perpendicularly bonded to the two sides of the π-system of the butadiene-diyl residue in a sort of inverted sandwich type of structure. The third is instead coplanar with the doubly deprotonated C(4) unit and σ-bonded to the two terminal carbon atoms. Despite the appearance as a Cr(II)/Cr(I) mixed valence species, DFT calculations have revealed that the structure of 2 consists of three divalent chromium atoms, while the additional electron resides on the π-system of the bridging organic residue. Complex 2 behaves as a single component selective catalyst for ethylene trimerization.

Vinyl Oxidative Coupling as a Synthetic Route to Catalytically Active Monovalent Chromium

Reaction of the deprotonated form of cis-{(t-Bu)N(H)P[μ-N(t-Bu)](2)PN(H)(t-Bu)} with CrCl(3)(THF)(3) afforded the trivalent cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li (THF)])CrCl(2) (1). Subsequent reaction with 2 equiv of vinyl Grignard (CH(2)=CH)Mg Cl gave the butadiene derivative (cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li(THF)])Cr(cis-η(4)-butadiene) (3) formally containing the metal in its monovalent state. The presence of the monovalent state was thereafter confirmed by DFT calculations. The coordination of the butadiene unit appears to be rather robust since reaction with Me(3)P afforded cleavage of the dimeric ligand core but not its displacement. The reaction formed the new butadiene complex [(t-Bu)N-P-N(t-Bu)]Cr(cis-η(4)-butadiene)PMe(3) (4) containing a regular NPN monoanion. In agreement with the presence of monovalent chromium, complexes 3 and 4 act as single-component self-activating catalysts for selective ethylene trimerization and dimerization, respectively.

Highly Ordered Nitrogen-Rich Mesoporous Carbon Nitrides and Their Superior Performance for Sensing and Photocatalytic Hydrogen Generation

Mesoporous carbon nitrides (MCN) are fascinating materials with unique semiconducting and basic properties that are useful in many applications including photocatalysis and sensing. Most syntheses of MCN focus on creating theoretically predicted C3 N4 stoichiometry with a band gap of 2.7u2005eV using a nano-hard templating approach with triazine-based precursors. However, the performance of the MCN in semiconducting applications is limited to the MCN framework with a small band gap, which would be linked with the addition of more N in the CN framework, but this remains a huge challenge. Here, we report a precursor with high nitrogen content, 3-amino-1,2,4-triazole, that enables the formation of new and well-ordered 3D MCN with C3 N5 stoichiometry (MCN-8), which has not been predicted so far, and a low-band-gap energy (2.2u2005eV). This novel class of material without addition of any dopants shows not only a superior photocatalytic water-splitting performance with a total of 801u2005μmol of H2 under visible-light irradiation for 3u2005h but also excellent sensing properties for toxic acids.

Energy efficient synthesis of highly ordered mesoporous carbon nitrides with uniform rods and their superior CO2 adsorption capacity

An energy efficient route for the synthesis of mesoporous carbon nitride (MCN) materials with highly ordered mesopores and a rod shaped morphology from uncalcined mesoporous SBA-15 (SEW-SBA-15) templates with a controlled morphology through a nanocasting technique using ethylenediamine and carbon tetrachloride as carbon and nitrogen sources is introduced. Porosity in the SBA-15 templates is created by washing with ethanol whereas the controlled rod shaped morphology in the nanotemplates is obtained by modifying the synthesis conditions from stirring to static conditions. The prepared MCN from the SEW-SBA-15 templates retains the morphological and structural order of the template. By tuning the pore diameter of SEW-SBA-15, it is possible to prepare MCN with tuneable pore diameters, which exhibits a specific BET surface area of 596–655 m2 g−1, pore diameter of 2.8–5.7 nm, and specific pore volume of 0.49–0.89 cm3 g−1. These values are similar to those of MCN-1 prepared from the calcined SBA-15 template with an irregular morphology. The SEW-MCN-1-T samples are used as CO2 adsorbents at 0, 10 and 25 °C and pressures from 1 up to 30 bar. Among the samples, the SEW-MCN-1-130 sample with the highest specific surface area, uniform particle size and morphology, and the largest pore volume exhibits the highest CO2 uptake capacity of 15.4 mmol g−1 at 0 °C and 30 bar, which is similar to the sample prepared by the calcination route but higher than that of activated carbon and multiwalled carbon nanotubes. This is the first report of the MCN prepared from uncalcined SBA-15 which helps to avoid the required energy intensive calcination step of the template and offers a promising system for CO2 capture.

Energy efficient synthesis of ordered mesoporous carbon nitrides with a high nitrogen content and enhanced CO2 capture capacity

Highly ordered mesoporous carbon nitrides (MCN) with 3D structure and a high nitrogen content are successfully prepared for the first time using uncalcined mesoporous silica template, KIT-6 and 3-amino-1,2,4-triazole as a single molecular carbon and nitrogen precursor. The prepared MCN with C and N stoichiometry of C3 N5 shows unique CN framework and exhibits the CO2 capture capacity of 5.63u2005mmolu2009g-1 at 273u2005K and 30u2005bar, which is higher than that of MCN with 2D structure and C3 N4 stoichiometry.

Diaminotetrazine based mesoporous C3N6 with a well-ordered 3D cubic structure and its excellent photocatalytic performance for hydrogen evolution

Novel nitrogen enriched diamino-s-tetrazine based highly ordered 3D mesoporous carbon nitride (MCN-9) hybrid materials with a body centered cubic Ia3d structure having high specific surface areas, large pore volumes, and tunable pore diameters were prepared by employing 3D body centered cubic KIT-6 mesoporous silica having a gyroidal porous structure and various pore diameters as the sacrificial hard template through a simple self-condensation followed by polymerization reaction of aminoguanidine hydrochloride inside the nanochannels of the KIT-6 template. Characterization results reveal that the prepared materials exhibit a 3D porous structure with well-defined mesopores and possess excellent physical parameters including high surface areas (157–346 m2 g−1), large pore volumes (0.36–0.63 cm3 g−1), different pore diameters (5.5–6.0 nm) and a high N/C ratio of 1.87, which is much higher than that of ideal C3N4 (1.33). The deep yellow colored MCN-9 with a 3D porous structure also shows good absorption properties with a tunable narrow bandgap of 2.25–2.5 eV, which is again much lower than that of C3N4 (2.7 eV) and helps to achieve much higher photocatalytic water splitting activity than non-porous C3N4 and other carbon nitrides under visible light irradiation.

A family of cis-macrocyclic diphosphines: modular, stereoselective synthesis and application in catalytic CO2/ethylene coupling

A family of cis-macrocyclic diphosphines was prepared in just three steps from white phosphorus and commercial materials using a modular synthetic approach. Alkylation of bicyclic diphosphane 3,4,8,9-tetramethyl-1,6-diphosphabicyclo(4.4.0)deca-3,8-diene, or P2(dmb)2, produced phosphino-phosphonium salts [R-P2(dmb)2]X, where R is methyl, benzyl and isobutyl, in yields of 90–96%. Treatment of these salts with organolithium or Grignard reagents yielded symmetric and unsymmetric macrocyclic diphosphines of the form cis-1-R-6-R′-3,4,8,9-tetramethyl-2,5,7,10-tetrahydro-1,6-DiPhospheCine, or R,R′-DPC, in which R′ is methyl, cyclohexyl, phenyl or mesityl, in yields of 46–94%. Alternatively, symmetric diphosphine Cy2-DPC was synthesized in 74% yield from the dichlorodiphosphine Cl2P2(dmb)2. As a first application, these cis-macrocyclic diphosphines were used as ligands in the nickel-catalyzed synthesis of acrylate from CO2 and ethylene, for which they showed promising catalytic activity.