Bulky Catalyst Research Articles

CO2-SR Cyclic Technology: CO2 Storage and in situ Regeneration with CH4 over a new dual function NiBa unsupported catalyst

A Ni-Ba unsupported catalyst has been developed to be used as a dual function material (DFM) for a new CO2 storage and in-situ regeneration (CO2-SR technology), consisting in a CO2 storage step followed by a CH4 regeneration phase, working in alternating, cyclic and isothermal regime. The heterometallic mixed-oxide catalyst -NiO.BaO- used in this technology was prepared by an ultrasonic assisted coprecipitation using 1:1 atomic ratio (Ni:Ba) and colloidal silica as an agent promoting surface area and improving mechanical properties. The solid obtained maintains the fcc-structure at higher temperatures, demonstrates high reducibility as a bulky catalyst and presents a connected and balanced strong basic sites and CH4 activation centres. It showed a CO2-storage net capacity of 0.232 mmolCO2 g−1 and a CO2 and CH4 removal rate of 13.97 % and 11.04 % respectively, with a remarkable selectivity to hydrogen as an enriched-synthesis gas (H2+CO) stream at 600 °C.

Metal Complexes Containing Redox-Active Ligands in Oxidation of Hydrocarbons and Alcohols: A Review

Ligands are innocent when they allow oxidation states of the central atoms to be defined. A noninnocent (or redox) ligand is a ligand in a metal complex where the oxidation state is not clear. Dioxygen can be a noninnocent species, since it exists in two oxidation states, i.e., superoxide (O2−) and peroxide (O22−). This review is devoted to oxidations of C–H compounds (saturated and aromatic hydrocarbons) and alcohols with peroxides (hydrogen peroxide, tert-butyl hydroperoxide) catalyzed by complexes of transition and nontransition metals containing innocent and noninnocent ligands. In many cases, the oxidation is induced by hydroxyl radicals. The mechanisms of the formation of hydroxyl radicals from H2O2 under the action of transition (iron, copper, vanadium, rhenium, etc.) and nontransition (aluminum, gallium, bismuth, etc.) metal ions are discussed. It has been demonstrated that the participation of the second hydrogen peroxide molecule leads to the rapture of O–O bond, and, as a result, to the facilitation of hydroxyl radical generation. The oxidation of alkanes induced by hydroxyl radicals leads to the formation of relatively unstable alkyl hydroperoxides. The data on regioselectivity in alkane oxidation allowed us to identify an oxidizing species generated in the decomposition of hydrogen peroxide: (hydroxyl radical or another species). The values of the ratio-of-rate constants of the interaction between an oxidizing species and solvent acetonitrile or alkane gives either the kinetic support for the nature of the oxidizing species or establishes the mechanism of the induction of oxidation catalyzed by a concrete compound. In the case of a bulky catalyst molecule, the ratio of hydroxyl radical attack rates upon the acetonitrile molecule and alkane becomes higher. This can be expanded if we assume that the reactions of hydroxyl radicals occur in a cavity inside a voluminous catalyst molecule, where the ratio of the local concentrations of acetonitrile and alkane is higher than in the whole reaction volume. The works of the authors of this review in this field are described in more detail herein.

An easily applicable and recyclable Fenton-like catalyst produced without wastewater emission and its performance evaluation

A thin disc-shaped Fenton-like catalyst sized 28.59*1.39 mm in diameter and thickness was produced by ball milling a hemi-dried silica xerogel with iron (III) acetylacetonate, followed by mold pressing and calcination. The developed compact catalyst facilitated its application and recovery. The whole fabrication process was free of wastewater emission. Plasticizers or binders indispensable for shaping commercial powders into compact green bodies were not necessary in the present method. The bulky catalyst was mesoporous and had a large surface area of 580 m2/g, providing access to the activation of H2O2 by the active iron sites inside the sample. It removed 100% of methylene blue (MB) in solution with a total TOC reduction by 92.13% mainly via the Fenton degradation. No washing, chemical elution of the adsorbed dye before the next application was applied. After five cycles, ca.93% of the removal efficiency was maintained. The TOC reduction increased from the 1st to 3rd cycle and ca.88% of the initial capacity was kept after the 5th cycle, while the iron leaching was only 0.021% of the applied catalyst. The excellent performance of the bulky catalyst was due to its large surface area, uniformly distributed and chemically bonded iron species. The present work provides a scalable and green way to produce Fenton-like catalysts for environmental application.

Elaborate Tuning in Ligand Makes a Big Difference in Catalytic Performance: Bulky Nickel Catalysts for (Co)polymerization of Ethylene with Promising Vinyl Polar Monomers

AbstractTo reveal effect of electronic or steric modification of phosphino‐phenolate nickel complex for preparing optimized catalysts, we take elaborated studies on structure‐performance relationship by finely modifying substituents on ortho‐phenoxy position or phosphorus moiety of this catalyst. It reveals that these newly synthesized complexes are thermally robust, and exhibits very high activity (up to 107 g molNi−1 h−1) in ethylene polymerization even at 120 °C. Associated with stoichiometric experiments, experimental results prove that nickel complexes bearing electron‐withdrawing substituents on ortho‐phenoxy position or electron‐donating substituents on phosphorus atom show higher activity than contrastive catalysts toward ethylene polymerization and ethylene–methyl acrylate (MA) copolymerization. Among these catalysts, 3 g bearing a strong electron‐withdrawing substituent on ortho‐phenoxy position exhibits the highest activity, and produces copolymers with the highest molecular weight and analogous MA incorporation. Various challenging polar vinyl monomers, like polyethylene glycol monomethyl ether acrylate, can be efficiently copolymerized with ethylene.

Moderately branched ultra‐high molecular weight polyethylene by using N,N′‐nickel catalysts adorned with sterically hindered dibenzocycloheptyl groups

Five examples of unsymmetrical 1,2‐bis (arylimino) acenaphthene (L1 – L5), each containing one N‐2,4‐bis (dibenzocycloheptyl)‐6‐methylphenyl group and one sterically and electronically variable N‐aryl group, have been used to prepare the N,N′‐nickel (II) halide complexes, [1‐[2,4‐{(C15H13}2–6‐MeC6H2N]‐2‐(ArN)C2C10H6]NiX2 (X = Br: Ar = 2,6‐Me2C6H3 Ni1, 2,6‐Et2C6H3 Ni2, 2,6‐i‐Pr2C6H3 Ni3, 2,4,6‐Me3C6H2 Ni4, 2,6‐Et2–4‐MeC6H2 Ni5) and (X = Cl: Ar = 2,6‐Me2C6H3 Ni6, 2,6‐Et2C6H3 Ni7, 2,6‐i‐Pr2C6H3 Ni8, 2,4,6‐Me3C6H2 Ni9, 2,6‐Et2–4‐MeC6H2 Ni10), in high yield. The molecular structures Ni3 and Ni7 highlight the extensive steric protection imparted by the ortho‐dibenzocycloheptyl group and the distorted tetrahedral geometry conferred to the nickel center. On activation with either Et2AlCl or MAO, Ni1 – Ni10 exhibited very high activities for ethylene polymerization with the least bulky Ni1 the most active (up to 1.06 × 107 g PE mol−1(Ni) h−1 with MAO). Notably, these sterically bulky catalysts have a propensity towards generating very high molecular weight polyethylene with moderate levels of branching and narrow dispersities with the most hindered Ni3 and Ni8 affording ultra‐high molecular weight material (up to 1.5 × 106 g mol−1). Indeed, both the activity and molecular weights of the resulting polyethylene are among the highest to be reported for this class of unsymmetrical 1,2‐bis (imino)acenaphthene‐nickel catalyst.

Mechanistic Insights into a Chiral Phosphoric Acid-Catalyzed Asymmetric Pinacol Rearrangement.

The first catalytic enantioselective pinacol rearrangement was reported by Antilla and co-workers in 2010. The reaction was catalyzed by a chiral phosphoric acid and resulted in high levels of enantioselectivity (up to 96% ee). The present study uses density functional theory to investigate the mechanism and origins of stereoselectivity of this important reaction and to explain the difference in selectivity between different catalysts. An OH···O hydrogen bond between the intermediate indolyl alcohol and the phosphate group from the catalyst together with a CH···O hydrogen bond between the indole and the phosphate group were observed in the preferred activation mode for the stereodetermining [1,2]-aryl shift. A stronger CH···O interaction in the major transition state was found to contribute to the high levels of enantioselectivity. A more bulky catalyst (TRIP) was found to impede the formation of the key CH···O interaction, leading to lower levels of enantioselectivity.

Enantioselective Michael Reaction of Cyclic β-Ketoesters with Morita–Baylis–Hillman Derivatives Using a Phase-Transfer Catalyst

This study aims to develop a highly enantioselective Michael reaction of cyclic β-ketoesters with Morita-Baylis-Hillman (MBH) derivatives using a phase-transfer catalyst. Cyclic β-ketoesters reacted with MBH derivatives to stereoselectively generate a quaternary carbon center in the presence of cinchona alkaloid-derived bulky ammonium catalyst, and aqueous KOH and Michael adducts bearing an acrylate moiety were obtained in good chemical yields with good enantioselectivity.

Cavity Size Engineering of a β-Barrel Protein Generates Efficient Biohybrid Catalysts for Olefin Metathesis

Incorporation of a synthetic metal catalyst into a protein scaffold yields a biohybrid catalyst, with a remarkable performance in aqueous media and the broad reaction scope of organometallic catalysts. A major challenge for efficient catalysis is the design of the interface between the protein scaffold and the metal catalyst. Until now, protein scaffolds have primarily been engineered by exchanging individual amino acids to anchor metal catalysts and alter their immediate environment. Here, cavity size engineering of the β-barrel protein nitrobindin was performed by duplicating multiple β-strands to generate an expanded variant. The approach of cavity size engineering enabled covalent incorporation of bulky catalysts at excellent coupling efficiencies and yielded excellent conversions in olefin metathesis, including ring-closing metathesis, ring-opening metathesis polymerization, and cross metathesis (conversions up to 99% and turnover numbers up to 10000).

Homo- and Copolymerizations of Ethylene and Norbornene Using Bis(β-ketoamino) Titanium Catalysts Containing Pyrazolone Rings.

A series of bis(β-ketoamino) titanium complexes containing pyrazolone rings (1–3) have been synthesized, characterized, and used as precursors for homo- and copolymerization of ethylene and norbornene. The titanium complexes activated with methylaluminoxane (MAO) exhibited good activities for homopolymerization of ethylene (E) to produce linear polyethylenes (PEs). Ethylene–norbornene copolymers (E–N) were also prepared by these catalysts with moderate activities, and influences of ligand substituents and norbornene addition on copolymer microstructure were studied in detail. Microstructure analysis of the E–N copolymers by 13C NMR and differential scanning calorimetry (DSC) techniques showed that alternating (ENEN) and isolated (ENEE) norbornene predominately appeared in the copolymer chain, and the NN dyad and NNN triad sequences were also present in the copolymers obtained by the less bulky catalyst 1.

Mechanistic Insights into Palladium‐Catalyzed Silylation of Aryl Iodides with Hydrosilanes through a DFT Study

The catalytic cycles of palladium-catalyzed silylation of aryl iodides, which are initiated by oxidative addition of hydrosilane or aryl iodide through three different mechanisms characterized by intermediates R3 Si-PdII -H (Cycle A), Ar-PdII -I (Cycle B), and PdIV (Cycle C), have been explored in detail by hybrid DFT. Calculations suggest that the chemical selectivity and reactivity of the reaction depend on the ligation state of the catalyst and specific reaction conditions, including feeding order of substrates and the presence of base. For less bulky biligated catalyst, Cycle C is energetically favored over Cycle A, through which the silylation process is slightly favored over the reduction process. Interestingly, for bulky monoligated catalyst, Cycle B is energetically more favored over generally accepted Cycle A, in which the silylation channel is slightly disfavored in comparison to that of the reduction channel. Moreover, the inclusion of base in this channel allows the silylated product become dominant. These findings offer a good explanation for the complex experimental observations. Designing a reaction process that allows the oxidative addition of palladium(0) complex to aryl iodide to occur prior to that with hydrosilane is thus suggested to improve the reactivity and chemoselectivity for the silylated product by encouraging the catalytic cycle to proceed through Cycles B (monoligated Pd0 catalyst) or C (biligated Pd0 catalyst), instead of Cycle A.

Highly Enantioselective Oxidation of Nonactivated Aliphatic C-H Bonds with Hydrogen Peroxide Catalyzed by Manganese Complexes.

Monosubstituted cycloalkanes undergo regio- and enantioselective aliphatic C–H oxidation with H2O2 catalyzed by biologically inspired manganese catalysts. The reaction furnishes the corresponding ketones resulting from oxidation at C3 and C4 methylenic sites (K3 and K4, respectively) leading to a chiral desymmetrization that proceeds with remarkable enantioselectivity (64% ee) but modest regioselectivity at C3 (K3/K4 ≈ 2) for tert-butylcyclohexane, and with up to 96% ee and exquisite regioselectity toward C3 (up to K3/K4 > 99) when N-cyclohexylalkanamides are employed as substrates. Efficient H2O2 activation, high yield, and highly enantioselective C–H oxidation rely on the synergistic cooperation of a sterically bulky manganese catalyst and an oxidatively robust alkanoic acid. This represents the first example of nonenzymatic highly enantioselective oxidation of nonactivated methylenic sites. Furthermore, the principles of catalyst design disclosed in this work constitute a unique platform for further development of stereoselective C–H oxidation reactions.

Chemoselective acyl C–O bond activation in esters for Suzuki–Miyaura coupling

The use of palladium–NHC (bulky) catalyst enables selective acylative coupling in esters via blocking the potential C(aryl)–O oxidative addition and decarbonylation processes. Mechanistic studies elucidate the role of bulky carbene catalysts in promoting C(acyl)–O activation, and as a result, the acyl metal intermediates undergo coupling without the loss of carbonyl group.

Remote-Stereocontrol in Dienamine Catalysis: Z-Dienamine Preferences and Electrophile-Catalyst Interaction Revealed by NMR and Computational Studies.

Catalysis with remote-stereocontrol provides special challenges in design and comprehension. One famous example is the dienamine catalysis, for which high ee values are reported despite insufficient shielding of the second double bond. Especially for dienamines with variable Z/E-ratios of the second double bond, no correlations to the ee values are found. Therefore, the structures, thermodynamics, and kinetics of dienamine intermediates in SN-type reactions are investigated. The NMR studies show that the preferred dienamine conformation provides an effective shielding if large electrophiles are used. Calculations at SCS-MP2/CBS-level of theory and experimental data of the dienamine formation show kinetic preference for the Z-isomer of the second double bond and a slow isomerization toward the thermodynamically preferred E-isomer. Modulations of the rate-determining step, by variation of the concentration of the electrophile, allow the conversion of dienamines to be observed. With electrophiles, a faster reaction of Z- than of E-isomers is observed experimentally. Calculations corroborate these results by correlating ee values of three catalysts with the kinetics of the electrophilic attack and reveal the significance of CH-π and stacking interactions in the transition states. Thus, for the first time a comprehensive understanding of the remote stereocontrol in γ-functionalization reactions of dienamines and an explanation to the "Z/E-dilemma" are presented. The combination of bulky catalyst subsystems and large electrophiles provides a shielding of one face and causes different reactivities of E/Z-dienamines in nucleophilic attacks from the other face. Kinetic preferences for the formation of Z-dienamines and their unfavorable thermodynamics support high ee values.

ChemInform Abstract: Readily Accessible Bulky Iron Catalysts Exhibiting Site Selectivity in the Oxidation of Steroidal Substrates.

AbstractBulky iron complexes are able to catalyze the site‐selective oxidation of alkyl C—H bonds with hydrogen peroxide.

An autonomous chemically fuelled small-molecule motor.

Molecular machines are among the most complex of all functional molecules and lie at the heart of nearly every biological process. A number of synthetic small-molecule machines have been developed, including molecular muscles, synthesizers, pumps, walkers, transporters and light-driven and electrically driven rotary motors. However, although biological molecular motors are powered by chemical gradients or the hydrolysis of adenosine triphosphate (ATP), so far there are no synthetic small-molecule motors that can operate autonomously using chemical energy (that is, the components move with net directionality as long as a chemical fuel is present). Here we describe a system in which a small molecular ring (macrocycle) is continuously transported directionally around a cyclic molecular track when powered by irreversible reactions of a chemical fuel, 9-fluorenylmethoxycarbonyl chloride. Key to the design is that the rate of reaction of this fuel with reactive sites on the cyclic track is faster when the macrocycle is far from the reactive site than when it is near to it. We find that a bulky pyridine-based catalyst promotes carbonate-forming reactions that ratchet the displacement of the macrocycle away from the reactive sites on the track. Under reaction conditions where both attachment and cleavage of the 9-fluorenylmethoxycarbonyl groups occur through different processes, and the cleavage reaction occurs at a rate independent of macrocycle location, net directional rotation of the molecular motor continues for as long as unreacted fuel remains. We anticipate that autonomous chemically fuelled molecular motors will find application as engines in molecular nanotechnology.

ChemInform Abstract: Recent Advances in C(sp3)—H Bond Functionalization via Metal—Carbene Insertions

AbstractReview: [45 refs.

Recent advances in C(sp(3))-H bond functionalization via metal-carbene insertions.

The recent development of intermolecular C–H insertion in the application of C(sp3)–H bond functionalizations, especially for light alkanes, is reviewed. The challenging problem of regioselectivity in C–H bond insertions has been tackled by the use of sterically bulky metal catalysts, such as metal porphyrins and silver(I) complexes. In some cases, high regioselectivity and enantioselectivity have been achieved in the C–H bond insertion of small alkanes. This review highlights the most recent accomplishments in this field.

Readily Accessible Bulky Iron Catalysts exhibiting Site Selectivity in the Oxidation of Steroidal Substrates.

Bulky iron complexes are described that catalyze the site-selective oxidation of alkyl C-H bonds with hydrogen peroxide under mild conditions. Steric bulk at the iron center is introduced by appending trialkylsilyl groups at the meta-position of the pyridines in tetradentate aminopyridine ligands, and this effect translates into high product yields, an enhanced preferential oxidation of secondary over tertiary C-H bonds, and the ability to perform site-selective oxidation of methylenic sites in terpenoid and steroidal substrates. Unprecedented site selective oxidation at C6 and C12 methylenic sites in steroidal substrates is shown to be governed by the chirality of the catalysts.

Readily Accessible Bulky Iron Catalysts exhibiting Site Selectivity in the Oxidation of Steroidal Substrates

AbstractBulky iron complexes are described that catalyze the site‐selective oxidation of alkyl C−H bonds with hydrogen peroxide under mild conditions. Steric bulk at the iron center is introduced by appending trialkylsilyl groups at the meta‐position of the pyridines in tetradentate aminopyridine ligands, and this effect translates into high product yields, an enhanced preferential oxidation of secondary over tertiary C−H bonds, and the ability to perform site‐selective oxidation of methylenic sites in terpenoid and steroidal substrates. Unprecedented site selective oxidation at C6 and C12 methylenic sites in steroidal substrates is shown to be governed by the chirality of the catalysts.

Synthesis and characterization of mechanically activated bulky molybdenum sulphide catalysts

Bulky catalysts on the basis of MoS 2 nano-crystallites have been obtained for the first time from commercial molybdenite in one-step synthesis using mechanical-chemical treatment with small additives of polar liquids. Physical and chemical properties of the synthesized catalysts were studied using sedimentation analysis, XRD, XPS, TEM and thermal analysis. The catalytic activity of the catalysts was examined in the course of dibenzothiophene hydrodesulphurization followed by the GC–MS analysis of the reaction products. Correlations between the catalytic activity and the methanol quantity added in the course of mechanical-chemical activation were established. The pathways of dibenzothiophene hydrodesulfurization reactions over the synthesized catalysts were proposed. Pour la première fois, des catalyseurs massiques à base de nano-crystallites MoS 2 ont été synthétisés à partir de molybdénite commerciale par activation mécanique en phase solide en une étape. Des liquides polaires ont été aussi additionnés en petites quantités comme adjuvants. Les propriétés physicochimiques des catalyseurs synthétisés ont été caractérisées par analyses par sédimentation, radiocristallographique et thermique, ainsi que par microscopie électronique à transmission et spectroscopie photoélectronique par rayons X. Les activités des catalyseurs dans une réaction modèle d’hydrogénolyse du dibenzothiophène ont été mesurées par analyse GC–MS. La corrélation entre l'activité catalytique et la quantité de methanol ajoutée au cours de l’activation mécano-chimique a été déterminée et discutée. Des hypothèses ont été émises sur les chemins réactionnels de l'hydrogénolyse des dibenzothiophènes à l’aide des catalyseurs synthétisés.