Kui Zhou

Characterization and properties of Zn/Co zeolitic imidazolate frameworks vs. ZIF-8 and ZIF-67

A novel Zn/Co zeolitic imidazolate framework (ZIF) has been constructed by an easy and straightforward room temperature technique. Several characterization techniques such as SEM, TEM-EDX, single-crystal XRD and ICP have been applied to confirm that the structure formed is a sodalite (SOD) cage type structure. The Zn/Co-ZIF possesses a high nano-crystallinity and porosity with a large surface area. By tuning the amount of Co and Zn in the Zn/Co zeolitic imidazolate framework, the physical and chemical properties have been improved compared with those of the single metal frameworks (ZIF-8 and ZIF-67). Consequently, the Zn/Co-ZIF was investigated for two different applications; gas adsorption (CO2, CH4 and N2) and catalysis (CO2 conversion to cyclic carbonates) and the obtained results were compared with the performance of previously reported single metal frameworks (ZIF-8 and ZIF-67). Additionally, hydrolytic stability tests under ambient conditions and immersed in water at 75 °C were performed and pointed out that Zn/Co-ZIF exhibits a higher stability. Moreover, based on these results, the Zn/Co-ZIF demonstrates better properties compared with ZIF-8 and ZIF-67.

Chemical and photochemical water oxidation catalyzed by novel ruthenium complexes comprising a negatively charged NCNHCO ligand

Incorporation of negatively charged moieties into complexes is a promising strategy to develop water oxidation catalysts with high stability and low over-potentials. We reported here the synthesis of a novel pincer-type ligand precursor, 1-(pyridin-2-ylmethyl)-3-methylcarboxylate-imidazolium (1). Deprotonation of C2-H of 1 resulted in formation of NCNHCO, a negatively charged asymmetric N-heterocyclic carbene (NHC). Complexes [Ru(NCNHCO)(terpy)]+ (2) and RuCl(NCNHCO)(DMSO)(pic) (3) were prepared and capable of decomposing water chemically and photo-chemically. Under acidic conditions, 4-picoline/H2O exchange in 3 was detected by UV-vis and 1H NMR, forming the corresponding Ru–aqua complex. In a typical three-component light-induced water oxidation, a turnover number of 273 was attained employing 3, which demonstrated catalytic ability comparable to those state-of-the-art catalysts.

CO2 Cycloaddition to Epoxides by using M‐DABCO Metal–Organic Frameworks and the Influence of the Synthetic Method on Catalytic Reactivity

Abstract A series of high‐quality M2(BDC)2(DABCO) metal–organic frameworks (abbreviated as M‐DABCO; M=Zn, Co, Ni, Cu; BDC=1,4‐benzene dicarboxylate; DABCO=1,4‐diazabicyclo[2.2.2]octane), were synthesized by using a solvothermal (SV) method, and their catalytic activity for the cycloaddition of CO2 to epoxides in the absence of a co‐catalyst or solvent was demonstrated. Of these metal–organic frameworks (MOFs), Zn‐DABCO exhibited very high activity and nearly complete selectivity under moderate reaction conditions. The other members of this MOF series (Co‐DABCO, Ni‐DABCO, and Cu‐DABCO) displayed lower activity in the given sequence. Samples of Zn‐DABCO, Co‐DABCO, and Ni‐DABCO were recycled at least three times without a noticeable loss in catalytic activity. The reaction mechanism can be attributed to structural defects along with the acid–base bifunctional characteristics of these MOFs. Moreover, we illustrate that the synthetic method of M‐DABCO influences the yield of the reaction. In addition to the SV method, Zn‐DABCO was synthesized by using spray drying due to its industrial attractiveness. It was found that the synthesis procedure clearly influenced the crystal growth and thus the physicochemical properties, such as surface area, pore volume, and gas adsorption, which in turn affected the catalytic performance. The results clarified that although different synthetic methods can produce isostructural MOFs, the application of MOFs, especially as catalysts, strongly depends on the crystal morphology and textural properties and, therefore, on the synthesis method.

Water Oxidation by In Situ Generated [RuII(OH2)(NCNHCO)(pic)2]+

A dinuclear ruthenium complex [RuII (NCNHC O)(pic)2 ]22+ (2) was firstly prepared and characterized spectroscopically and electrochemically. Instead of the conventional ligand exchange, complex 2 dissociates in situ to afford two single-site Ru aqua complexes, [RuII (OH2 )(NCNHC O)(pic)2 ]+ , which mediates water oxidation through proton-coupled electron transfer events. In electrokinetic studies, complex 2 demonstrated a TOF of 150.3 s-1 comparable to those state-of-the-art catalysts at neutral conditions. TONs of 2173 and 217 were attained in chemical and photochemical water oxidation when 2 was used as a catalyst, exhibiting good stability. Notably, a TOF of 1.3 s-1 was achieved at CAN-driven water oxidation, which outperformed most of the reported single-site Ru complexes, indicating that complex 2 is one of most active water oxidation catalysts (WOCs) to date. The unique coordination configuration and outstanding catalytic performance of complex 2 might shed light on the design of novel molecular WOCs.

Synthesis and characterization of [Ru(NCNHCO)(bpy)L]+ complexes and their reactivity towards water oxidation

While the role of negatively charged axial ligands (e.g. Cl−, Br− and I−) in tuning the reactivity and stability of molecular Ru water oxidation catalysts was well-documented, less was explored when axial ligands are neutral molecules. In this work, we reported the synthesis and full characterization of [Ru(NCNHCO)(bpy)L]+ complexes (L = DMSO, 1; L = 4-picoline, 2), where L is a neutral axial ligand. The effects of these axial ligands on the stability and reactivity of the catalysts were thoroughly investigated. Both complexes were competent to oxidize water with turnover numbers up to 463 and 107, respectively. Complex 1 initiated the water oxidation involving a PCET process. It was found that complex 2 afforded its corresponding Ru–aqua complex owing to the carboxylate dissociation, while complex 1 exhibited considerable stability in acidic aqueous solutions. On the other hand, in the presence of an oxidant, for complex 1 DMSO/H2O exchange occurred, facilitating water oxidation. In contrast, the Ru–O bond cleavage of complex 2 was inhibited, leading to an inferior catalytic performance.

Spray drying of zeolitic imidazolate frameworks: investigation of crystal formation and properties

High-quality zeolite imidazole frameworks (ZIF-8, ZIF-67, and bimetallic Zn/Co-ZIF) were synthesized using the spray-drying technique and the mechanism of crystal growth during the whole process is discussed. It is also demonstrated that the crystallization mechanism for N-donor containing ditopic imidazolate (IM) ligands forming ZIF structures is divergent from MOF formation applying carboxylate ligands, and their formation using the spray dry method occurs via a unique mechanism. An intermediate amorphous phase was obtained after spray drying the feed solution, and the crystallographic ZIF structure was obtained during immersion in a solvent. Several characterization techniques, such as FTIR, XRD, SEM, porosity analysis, elemental analysis etc., were applied to examine the characteristic properties of materials, including, intermediate compounds and final products. The results proved that the spray drying procedure is a facile, continuous and effective synthetic route to produce MOFs (ZIFs) in higher quantities with high-quality properties such as high porosity, a large surface area, more active catalyst sites, etc. applying a minimum amount of time and energy which is unusual in common synthesis methods of MOFs.

Ring‐Opening Polymerization of l‐Lactide to Cyclic Poly(Lactide) by Zeolitic Imidazole Framework ZIF‐8 Catalyst

The catalytic activity of ZIF-8 in the ring-opening polymerization of l-lactide without solvents or cocatalysts is presented for the first time. Two different synthetic strategies have been applied for synthesizing ZIF-8, either under solvothermal condition or by spray-drying procedure. Their catalytic activities are found to be correlating with the presence of open active sites in ZIF-8 structure. The structural defects that afford active acid and basic sites are supposed to cooperatively catalyze the reaction. ZIF-8 assembled by spray-drying technique, displays a superior catalytic activity at temperature of 160 °C, leading to the formation of high molecular weight cyclic polylactide. The ZIF-8 catalysts could be recycled and reused without any significant loss of catalytic activity.