Progress toward Polymerization Reaction Monitoring with Different Dienes: How Small Amounts of Dienes Affect ansa-Zirconocenes/Borate/Triisobutylaluminium Catalyst Systems.

The objectives of this work were to address the fundamental characteristics of ansa-zirconocene catalyzed E/diene copolymerization and E/diene/1-hexene and E/diene/propylene terpolymerizations, and the quantitative relationship between diene structure and polymer chain propagation rate constant in term of quantifiable catalytic active sites. One of the most important but unknown factors in olefins ansa-zirconocene complexes is the distribution of the catalyst between sites actively participating in polymer chain formation and dormant sites. A set of ethylene/dienes copolymerizations, and ethylene/dienes/1-hexene and ethylene/dienes/1-hexene terpolymerizations catalyzed with ansa-zirconocenes/borate/triisobutylaluminium (rac-Et(Ind)2ZrCl2/[Ph3C][B(C6F5)4]/triisobutylaluminium (TIBA) were performed in toluene at 50 °C To determine the active center [C*]/[Zr] ratio variation in the copolymerization of E with different dienes and their terpolymerization with 1-hexene and propylene, each polymer propagation chain ends were quenched with 2-thiophenecarbonyl, which selectively quenches the metal-polymer bonds through acyl chloride. The ethylene, propylene, 1-hexene, and diene composition-based propagation rate constants (kpE, kpP, kp1-H, and kpdiene), thermal (melting and crystalline) properties, composition (mol% of ethylene, propylene, 1-hexene, and diene), molecular weight, and polydispersity were also studied in this work. Systematic comparisons of the proportion of catalytically [Zr]/[C*] active sites and polymerization rate constant (kp) for ansa-zirconocenes catalyzed E/diene, E/diene/1-hexene, and E/diene/propylene polymerization have not been reported before. We evaluated the addition of 1-hexene and propylene as termonomers in the copolymerization with E/diene. To make a comparison for each diene under identical conditions, we started the polymerization by introducing an 80/20 mole ratio of E/P and 0.12 mol/L of 1-hexene in the system. The catalyst behavior against different dienes, 1-hexene, and propylene is very interesting, including changes in thermal properties, cyclization of 1-hexene, and decreased incorporation of isoprene and butadiene, changes in the diffusion barriers in the system, and its effect on kp.

Sulfur doped FeNC catalysts derived from Dual-Ligand zeolitic imidazolate framework for the oxygen reduction reaction.

Iron-nitrogen-carbon (FeNC) catalysts derived from zeolitic-imidazolate frameworks (ZIFs) are worldwide accepted to be the most promising candidates for the oxygen reduction reaction (ORR), but the insufficient stability, the low FeNx exposure and poor density restrict their ORR activity. Here, we demonstrate a strategy to synthesize FeNx sites embedded in a micro/mesoporous N, S co-doped graphitic carbon (FeNC/MUS) by tuning the ligand linkers via the addition of 2-undecylimidazole as a co-ligand in ZIF precursors, and optimizing the electronic structure of Fe center by an in-situ addition of thiourea molecules as sulfur (S) source. 2-undecylimidazole offered an open porous structure to incorporate more FeNx, while the S-doping increased the density of FeNx. Besides, 2-undeclyimidazole cooperatively with S-doping caused favorable changes into the catalyst structure, particularly improved the exposure and density of FeNx sites and doubled the Brunauer-Emmetter-Teller surface area to 1132 m2 g-1 contrasted to the pristine FeNC/M (544 m2 g-1). FeNC/MUS displayed an accelerated ORR activity with a higher half-wave potential of 0.86 V (vs. reversible hydrogen electrode (RHE)) than that of Pt/C (0.84 V) in addition of a longer durability with a 11 % of activity decay after 30000 s in alkaline media. This work offers a new insight to design optimal ZIFs precursor and a facile electron withdrawing S-doping strategy for efficient electrocatalysis.

Dual plasmonic Au and TiN cocatalysts to boost photocatalytic hydrogen evolution.

Employing a suitable cocatalyst is very important to improve photocatalytic H2 evolution activity. Herein, two plasmonic cocatalysts, Au nanoparticles and TiN nanoparticles were in-situ coupled over the g-C3N4 nanotube to form a ternary 0D/0D/1D Au/TiN/g-C3N4 composite via a successive thermal polycondensation and chemical reduction method. The g-C3N4 nanotube acted as a support for the growth of Au and TiN nanoparticles, leading to intimate contact between g-C3N4 nanotube with Au nanoparticles and TiN nanoparticles. As a result, multiple interfaces and dual-junctions of Au/g-C3N4 Schottky-junction and TiN/g-C3N4 ohmic-junction were constructed, which helped to promote the charged carriers' separation and enhanced the photocatalytic performance. Furthermore, loading plasmonic cocatalysts of Au nanoparticles and TiN nanoparticles can enhance the light absorption capacity. Consequently, the Au/TiN/g-C3N4 composite exhibited significantly enhanced photocatalytic H2 evolution activity (596 µmol g-1 h-1) compared to g-C3N4 or binary composites of Au/g-C3N4 and TiN/g-C3N4. This work highlights the significant role of cocatalysts in photocatalysis.

Rapid kinetic evaluation of homogeneous single-site metallocene catalysts and cyclic diene: how do the catalytic activity, molecular weight, and diene incorporation rate of olefins affect each other?

The kinetics and mechanism of ethylene and cyclic diene 5-ethylidene-2-norbornene (ENB) copolymerization catalyzed by rac-Et(Ind)2ZrCl2/[Ph3C][B(C6F5)4]/triisobutylaluminium (TIBA) were investigated using a quench-labeling procedure using 2-thiophenecarbonyl chloride (TPCC). The E/ENB copolymers were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and 1H nuclear magnetic resonance (NMR) spectroscopy and sulfur analysis. To reduce the errors of the ethylene-diene copolymerization for the kinetics study, we selected E/ENB with steric and electronic features that permit us to elucidate the metallocene catalyst behavior against dienes. A quantitative approach of catalyst speciation, stereodynamics, and micro-kinetics assisted the resolution of mechanistic problems, such as the elastomeric synthesis of ethylene propylene diene monomer rubber (EPDM), the catalyst resting state nature, and how much ion-pairing occurs during polymerization. We report here the precise observation of metal-polymer species, explanation of the dynamics of their initiation, propagation, and termination, and ethylene ENB copolymer development. An approach based on acyl chloride was used to selectively quenched transition metal-polymer bonds to evaluate the polymeric catalyst in terms of its reaction rate, R p, propagation rate content, k p, and mole fraction of active centers. It is noted that the decline in catalytic activity in the range of 1800 s, and the active center [Zr]/[*C] fraction significantly increased during the initial 1000 s and then tended towards a steady figure of 86%. It is suggested that nearly complete initiation of all olefins catalysts can be obtained after a sufficiently extended reaction. The quick increase in active sites in the first stage can be described by the immediate initiation of active sites positioned on the surfaces of catalyst particles. The initial polymerization rate, R p, is high and the crystalline properties of the E/ENB copolymer are low due to the greater incorporation of ENB in the polymer backbone, and later the polymerization reaction rates remained stable with a lower mol% of ENB. The melting temperature (T m) ranges from 108 to 127 °C, whereas the crystalline temperature ranges from 63 to 108 (J g-1). In the E-ENB copolymers, the value of k pE is much greater than that of k pENB; at 120 s, the k pE and k pENB values are 9115 and 431 L mol-1 s-1, respectively, implying smaller diffusion barriers in the early stages, which are close to the actual propagation rate constant.

Correction: Rapid kinetic evaluation of homogeneous single-site metallocene catalysts and cyclic diene: how do the catalytic activity, molecular weight, and diene incorporation rate of olefins affect each other?

[This corrects the article DOI: 10.1039/D1RA06243C.].

A 3D nitrogen-doped graphene aerogel for enhanced visible-light photocatalytic pollutant degradation and hydrogen evolution.

Three-dimensional (3D) graphene-based aerogels have attracted widespread interest as promising photocatalysts for dye degradation and hydrogen production. Herein, we have developed a 3D nitrogen-doped graphene aerogel (3DNG) from graphitic carbon nitride combined with graphene oxide (GO). The nitrogen dopant in the 3D aerogel was achieved via a thermal treatment at 1000 °C, and the 3D aerogel catalyst could retain its 3D porous structure after the thermal treatment. The 3DNG was characterized via FTIR, Raman, TEM, UV-vis, XPS spectroscopies and BET analysis, and the results indicated that this 3DNG with a large surface area of 536 m2 g-1 and a band gap of 2.42 eV demonstrated a high adsorption capacity and enhanced methylene blue degradation and hydrogen production under visible light irradiation. Characterization also identified that the porous 3D structure with hydrogen bonding and π-π interactions and better charge transfer resulting from the nitrogen doping are the major reasons for the enhanced photocatalytic performance over this 3DNG catalyst.

N,S-Atom-coordinated Co9S8 trinary dopants within a porous graphene framework as efficient catalysts for oxygen reduction/evolution reactions.

The intrinsic instability and difficulty in controlling the uniform size distributions of cobalt sulfides greatly restrict their application for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as a bifunctional electrocatalyst in regenerative fuel cells and rechargeable metal-air batteries. Herein, we synthesize a stable electrocatalyst of N,S-atom-coordinated Co9S8 trinary dopants within a porous graphene framework (Co9S8@NS-3DrGO), in which Co9S8 nanoparticles show uniform sizes and distributions. The stable Co9S8-based composites are fabricated by a facile soft template-assisted strategy, and the attraction of this method is that the intermediate of melamine formaldehyde resin (MFR) plays trifunctional roles, including (i) it acts as the templated bonding material to crosslink GO sheets together, (ii) it facilitates the formation of a core-shell architecture, and (iii) it acts as the N source for doping. Catalyst composition and performance are largely dependent on the pyrolysis temperature. Extensive investigation elucidates that the mechanism of electrocatalytic activity is attributed to: (i) the unique core-shell structure of the composites, as well as uniform particle sizes and distributions of Co9S8, (ii) the active nitrogen (pyridinic N and graphitic N) contents, and (iii) the large surface area and porous architecture. The composite pyrolyzed at 850 °C exhibits the best electrocatalytic performance, which shows a positive ORR half-wave potential (0.826 V), a small OER overpotential (317 mV) at 10 mA cm-2, and high stability, comparable to the commercial noble catalysts Pt/C and RuO2 in alkaline media. Furthermore, when applied in zinc-air batteries, it also displays a comparable performance to a Pt/C + RuO2 mixture catalyst. This work provides an approach to stabilize cobalt sulfides and control their particle sizes and distributions by ingeniously employing the soft template of MFR and pyrolyzing them at various temperatures.