Because of its unique properties, polyethylene wax has been widely applied in many fields. In this contribution, several iminopyridyl nickel catalysts are designed and synthesized. The introduction ...of sterically bulky substituents can significantly enhance the catalytic performances during ethylene polymerization. The generated polyethylene wax is highly branched due to the chain walking characteristic for this system. To further enhance the catalytic behavior of these nickel catalysts, the corresponding supported system is prepared and investigated. The heterogeneous system presents higher activity and higher thermal stability than its homogenous counterpart. Specifically, the nickel catalyst bearing OH moiety is highly robust even at 120 °C with catalytic activity of up to 9.4 × 106 g mol−1 h−1. The molecular weight and microstructure of the generated polyethylene wax can also be affected by ligand substituents and the heterogenization process. This work demonstrates the great potentials of nickel catalysts for the synthesis of polyethylene wax.
Display omitted
•A series of iminopyridyl nickel complexes were designed and well-characterized.•The homogeneous and heterogenous systems were investigated for the synthesis of polyethylene wax.•The nickel catalyst is highly robust even at 120 °C with catalytic activity of up to 9.4 × 106 g mol−1 h−1.
The gasification of biomass produces a syngas that can be used for electricity generation and fuels/chemicals production. However, tar is generated along with the syngas as a by-product which causes ...problematic issues in the end-use of the syngas, such as blockages, plugging and corrosion. Catalytic steam reforming is a suitable option to convert tar into more syngas in the presence of nickel-based catalysts, as the preferred catalyst, mainly due to their activity and low cost. There has been considerable research reported in the literature on modified nickel-based catalysts for steam tar reforming. These modifications have been carried out in order to improve the performance of the Ni-based catalysts for tar reforming, mainly in terms of catalyst stability and activity. Such improvements are achieved by manipulating the properties of the catalyst. This paper therefore presents a critical assessment of these modifications on Ni-based catalysts available in the literature for improved tar reforming. The modifications considered in this review were categorised as: the addition of secondary metal (Fe, Co, Cu, Cr), the addition of noble metals (Pt, Pd, Rh, Au, Rh, Ir), addition of rare earth metals as promoters (Ce, La), alkali and alkaline earth metals (Sr, Ba, Ca, Mg,Ba) and modification of the support material. The paper aims at understanding the properties responsible for the improved performance of the modified Ni-based catalysts in comparison with unmodified Ni-based catalysts. The review paper will serve as a guide for further improvement of Ni-based catalysts for biomass tar reforming.
•Gasification of biomass produces a problematic tar which is a barrier to deployment.•Catalytic steam reforming with Ni-catalysts converts tar to useful gas products.•Modification of Ni-based catalysts can reduced catalyst coking and sintering.•A wide range of catalysts used as Ni-catalyst modifiers are reviewed.•The underlying reasons for enhanced catalyst performance are reviewed.
Display omitted
•Advances in nickel catalyzed copolymerization of olefin with various polar monomers.•N,N, N,O, P,O, and binuclear nickel catalyst for coordination copolymerization.•Correlation ...between catalyst structure and propertyin the presence of polar monomer.•Challenge and opportunity on nickel mediated copolymerization of olefin/polar monomer.
The preparation of well-defined functionalized polyolefins via coordination-insertion polymerization method is a hot topic in polymer science. Late transition organometallic catalysts have attracted widespread attentions due to their application prospects in the synthesis of functionalized polyolefins by direct masking-reagent-free copolymerization of olefin with polar monomers. Numerous improvements in this field have been dominated by precious palladium catalysts, which have been reviewed thoroughly in the past years, while less-expensive and earth-abundant nickel catalysts that are closer to practical application have encountered stronger challenges in this regard because of the lower tolerance toward functional groups. However, with unremitting efforts nickel mediated copolymerization has witnessed rapid development in the past five years. This review thus summarizes the major progress in copolymerization of olefins with various polar monomers using state-of-the-art nickel catalysts bearing N,N-type ligands (such as α-diimine), N,O-type ligands (such as salicylaldimine and α-iminoketone), P,O-type ligands (such as phosphine-sulfonate, phosphine-phenolate, and bisphosphine-monoxide), binuclear ligands and other unclassified ligands, focusing on ligand modifications and catalyst designing strategies that enhance catalytic performance of copolymerization reactions.
Several nickel complexes bearing sterically bulky phosphino-phenolate (P,O) ligands were synthesized and explored as catalysts for olefin (co)polymerization. In the absence of an activator, the ...complexes showed very high catalytic activities (up to 107 g molNi –1 h–1) for ethylene polymerization even at 90 °C or with the addition of a large amount of a polar additive (such as ethyl alcohol, diethyl ether, acetone, or even water), affording linear polymers with high molecular weights (up to 6.53 × 105). In contrast, most of the previously reported nickel catalysts suffer from severe activity suppression at elevated temperature. It is rare that a catalyst has so many good performances simultaneously, including high catalytic activity, good tolerance for polar groups, strong thermal stability, and yielding high molecular weight linear polyethylene. Most importantly, these bulky nickel complexes used in this study also effectively copolymerized ethylene with challenging polar vinyl monomers, including commercially available acrylates and an acrylamide. As we expected, introducing a bulky substituent group on the phosphorus atom of the complex was vital for enhanced catalytic activity and the formation of high molecular weight linear copolymers. Microstructure analyses revealed that the polar functional units were mainly incorporated into the polymer main chain and also located at the chain end with insertion percentages of up to 7.4 mol %. The bulky P,O neutral nickel complexes reported herein are promising alternatives to the well-established palladium catalysts for direct copolymerization of olefins with commercially available polar vinyl comonomers.
A nanoporous PdNi (np‐PdNi) bimetallic catalyst fabricated by electrochemically dealloying a Pd20Ni80 alloy in an acid solution is reported. Residual Ni in the nanoporous alloy can be controlled by ...tuning dealloying potentials and the electrocatalysis of the np‐PdNi shows evident dependence on Ni concentrations. With ∼9 at.% Ni, the np‐PdNi bimetallic catalyst presents superior electrocatalytic performances in methanol and formic acid electro‐oxidation as well as oxygen reduction in comparison with commercial Pd/C and nanoporous Pd (np‐Pd). The excellent electrocatalytic properties of the dealloyed np‐PdNi bimetallic catalyst appear to arise from the combined effect of unique bicontinuous nanoporosity and bimetallic synergistic action.
The nanoporous PdNi (np‐PdNi) bimetallic catalyst is fabricated by electrochemical dealloying of a Pd20Ni80 alloy. Residual Ni in the nanoporous alloy can be controlled by tuning dealloying potentials and the eletrocatalysis of the np‐PdNi shows evident dependence on the Ni concentrations. The superior electrocatalytic properties of the dealloyed np‐PdNi bimetallic catalyst appear to arise from the combined effect of unique bicontinuous nanoporosity and bimetallic synergistic action.
Display omitted
•Highly active nickel pre-catalysts in ethylene oligo-/polymerization.•N-donor ligand effects on ethylene enchainment: oligomers through to polymers.•Influence of multidentate ligand ...frame on the catalytic activity and thermal stability of nickel complex.•Correlation between ligand structure and the degree of branching in the polyethylene.•Using temperature to influence the branching content of the polymer.
Homogeneous nickel catalysts have a considerable track record for mediating ethylene chain growth in the form of oligomerization and more recently polymerization. Within the polymerization arena, high molecular weight materials incorporating various degrees of branching, anywhere from linear to moderately branched through to hyperbranched, highlight the versatility of this type of catalyst. This review focuses on recent progress related to structural modifications made to the pre-catalyst, and in particular to the multidentate Nimine-ligand manifold, and how these changes impact on thermal stability and activity of the catalyst as well as the microstructural properties of the polyethylene and the distribution of the oligomeric fractions. In addition to ongoing process development directed towards commodity-type polyolefinic materials, the emergence of nickel catalysts that can form elastomeric-type materials from a single ethylene feed, without the addition of a high-cost α-olefin such as 1-hexene or 1-octene, offers considerable opportunities for future commercial applications.
Ni-ceria nanoparticles (Ni/Ce = 1/1) in the cage-like pores of SBA-16 were prepared and evaluated in methane dry reforming reactions. Coexistence of ceria in NiCe/SBA-16 resulted in forming uniformly ...sized Ni particles (av. 5.7 nm) within the mesopores of SBA-16, because of the confinement effect from the framework of SBA-16 and the strong interaction between Ni and ceria. Ceria addition facilitated the reduction of NiCe/SBA-16 compared with Ni/SBA-16, and Ce3+ was the dominant species in both fresh and used NiCe/SBA-16 catalysts, as determined by Ce LIII-edge X-ray absorption near-edge structure (XANES). The methane conversion was much more stable on NiCe/SBA-16 than on Ni/CeO2 and Ni/SBA-16 in the methane dry reforming at 973 K during a 100 h reaction period; the deactivation of the Ni catalyst and the collapse of the SBA-16 framework were preferably suppressed for NiCe/SBA-16 under the reaction conditions. The remarkable effect of ceria on the structural stability of both the active Ni particles and the SBA-16 framework led to the consistent catalytic performance of NiCe/SBA-16 in methane dry reforming.
Display omitted
•Nickel catalysts supported on various metal oxides were prepared.•The activity of nickel catalysts for CO2 methanation was evaluated.•The Ni/Y2O3 catalyst exhibited the highest ...activity.•The reaction mainly proceeded via the formation of formate over Ni/Y2O3.
Carbon dioxide methanation is well known to offer some advantages and be catalyzed by Ru, Rh, Pd, and Ni. In this study, Ni catalysts supported on various metal oxides were fabricated and their catalytic activity for CO2 methanation was evaluated. The CO2 conversion for most of catalysts drastically increased at 225–250°C and reached a maximal value at 300–350°C. The order of CH4 yield at 250°C was as follows: Ni/Y2O3>Ni/Sm2O3>Ni/ZrO2>Ni/CeO2>Ni/Al2O3>Ni/La2O3. The catalytic activity could be partly explained by the basic property of the catalysts. Moreover, the chemical species formed on the catalyst surface during CO2 methanation were examined by in situ infrared spectroscopy. From the obtained results, the difference in the activity depending on the support material of Ni catalysts was discussed.
Decline in catalyst performance due to coke deposition is the main problem in diesel steam (SR) and autothermal reforming (ATR) reactions. Good redox potential and strong interaction of CeO2 with ...nickel increase activity and coke resistivity of Ni/Al2O3 catalysts. In this study, mesoporous Al2O3, CeO2/Al2O3, and CeO2/ZrO2/Al2O3 supported nickel catalysts were successfully synthesized. The highest hydrogen yield, 97.7%, and almost no coke deposition were observed with CeO2/ZrO2/Al2O3 catalyst (Ni@8CeO2-2ZrO2-Al2O3-EISA) in SR reaction. The second highest hydrogen yield, 91.4%, was obtained with CeO2/Al2O3 catalyst (Ni@10CeO2-Al2O3-EISA) with 0.3 wt% coke deposition. Presence of ZrO2 prevented the transformation of cubic CeO2 into CeAlO3, which enhanced water gas shift reaction (WGSR) activity. Ni@10CeO2-Al2O3-EISA did not show any decline in activity in a long-term performance test. Higher CeO2 incorporation (20 wt%) caused lower steam reforming activity. Change of synthesis route from one-pot to impregnation for the CeO2 incorporation decreased the number of acid sites, limiting cracking reactions and causing a significant drop in hydrogen production.
Display omitted
•CeO2 and CeO2/ZrO2 successfully incorporated into mesoporous Al2O3.•10 wt% CeO2 incorporation enhanced hydrogen yield in diesel steam reforming.•Incorporation of CeO2 & ZrO2 eliminated coke deposition in diesel steam reforming.•ZrO2 incorporation affected stability of the CeO2–Al2O3 supported catalyst.•10 wt% CeO2 incorporated Ni–Al2O3 showed stability with long-term activity.
![Robust Bulky [P,O] Neutral ...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/2155-5435/small "Robust Bulky [P,O] Neutral Nickel Catalysts for Copolymerization of Ethylene with Polar Vinyl Monomers")
PDF
