Somboon Chaemchuen

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.

Zeolitic imidazole framework-67 as an efficient heterogeneous catalyst for the conversion of CO2 to cyclic carbonates

Zeolitic imidazole framework (ZIF)-67 is an efficient heterogeneous catalyst for the cycloaddition of carbon dioxide to epoxides to afford cyclic carbonates. In this work, ZIF-67 was synthesized by a simple and straightforward method at room temperature. The coupling reaction in the presence of ZIF-67 without any co-catalyst or solvent was studied under different conditions. The reaction conversion was more than 99% and the selectivity toward chloropropene carbonate was considerably higher (>99%) than the previously reported ZIF-8. Moreover, the ZIF-67 catalyst could be recycled at least four times without noticeable loss of catalytic activity.

Carboxylation of Terminal Alkynes with Carbon Dioxide Catalyzed by an In Situ Ag2O/N-Heterocyclic Carbene Precursor System

A carboxylation of terminal alkynes with carbon dioxide (CO2) at ambient conditions was developed in situ using a series of N‐heterocyclic carbene (NHC) precursors and Ag2O. The unique structure of NHCs largely increases the solubility of active Ag species and meanwhile activates CO2 by forming the NHC–CO2 adduct. This novel catalytic system demonstrated quite low Ag loading, very high activities, wide substrate generality and excellent tolerance for a variety of functionalities. In addition, avoiding cumbersome synthesis procedures, processing, and reserving of the photosensitive Ag complex, this system could be stored and operated as straightforward as the inorganic Ag salt catalysts.

A recyclable AgI/OAc− catalytic system for the efficient synthesis of α-alkylidene cyclic carbonates: carbon dioxide conversion at atmospheric pressure

The cyclization of carbon dioxide and propargylic alcohols, especially challenging substrates, were efficiently catalyzed by a green and recyclable AgI/OAc− system under atmospheric pressure, which is shown to be the most recyclable system with 20 recycle rounds and has the lowest loading among all the reported recyclable systems that work under atmospheric pressure.

A simple and robust AgI/KOAc catalytic system for the carboxylative assembly of propargyl alcohols and carbon dioxide at atmospheric pressure

A simple and robust AgI/KOAc system was developed for the cyclization of propargyl alcohols and carbon dioxide under mild conditions, and was identified to have excellent activities for numerous substrates, especially sterically hindered terminal alkynes and internal alkynes. Notably, the Ag loading involved was an unprecedentedly low level of 0.05 mol%.

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.

Zn@ZIF-67 as Catalysts for the Knoevenagel Condensation of Aldehyde Derivatives with Malononitrile

Three different bimetallic zinc-cobalt zeolitic imidazolate frameworks (ZIFs) were successfully synthesized at room temperature in water through a green and simple procedure and tested as catalysts for the Knoevenagel condensation reaction between p-Br-benzaldehyde and malononitrile as model reaction. The materials were characterized with various techniques (XRD, ICP-AES, SEM, N2 adsorption–desorption, TGA) and displayed excellent activities and selectivities. Moreover, the crystalline structure was preserved and the catalysts could be reused at least four times with negligible loss of activity.Graphical Abstract

Chemical and Photochemical Water Oxidation by [RuCl(NC NHC O)(DMSO)(py)]-Type Complexes

In this work, the effects of substitutions in the backbone of imidazolylidene and axial ligands on the reactivity and stability of water‐oxidation catalysts were investigated. Three pincer‐type asymmetric imidazolium salts NCNHCO, NCNHC‐4BrO, and NCNHC‐BO (NHC: N‐heterocyclic carbene), of which the donating ability of the corresponding imidazolylidene decreases in the same sequence, were prepared. Their application in metalation afforded the corresponding Ru complexes 1 a, 1 b, 2, and 3. It was found that the complexes incorporating the stronger donor displayed lower potential for each redox couple and longer lifetimes, but relatively low reaction rates. Under acidic conditions, water oxidation driven by cerium ammonium nitrate resulted in turnover numbers (TONs) of 2322, 1728, 1928, and 2208 for 1 a, 1 b, 2, and 3, respectively. Complex 2 exhibited a good match of reactivity and stability with a TON of 136 in a typical three‐component photocatalysis. Importantly, NHCs could be another powerful tool for tuning the reactivity and stability of water‐oxidation catalysts, in addition to substituted pyridines. Rational orchestration of modified NHCs and pyridines could eventually result in water‐oxidation catalysts exhibiting appreciable effectiveness and considerable robustness.

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.