Chemical Communications Blog

Metal organic frameworks (MOFs) have enjoyed a short but illustrious career to date. Much attention has focussed on their potential for gas storage, but as the field matures an emerging function of these materials is being developed to great success: MOFs as heterogeneous catalysts.

MOFs are highly porous coordination polymers comprised of metallic ‘nodes’ connected in a 3-dimensional lattice by organic ‘linkers’. This structure offers advantages of both homogeneous and heterogeneous catalysis: their large surface area and porosity offers an accessible network of active sites, they can be recovered and recycled, and they are well-characterised and crystalline with a uniformity which facilitates reproducibility, selectivity, and systematic modification.

The authors of the review entitled ‘Tunable nature of metal organic frameworks as heterogeneous solid catalysts for alcohol oxidation’ are tasked with reviewing the literature exploring catalytic MOFs developed to selectively oxidise alcohols to aldehydes and ketones, a reaction with particular relevance to the fine chemical and pharmaceutical industries.

The review divides MOF oxidation catalysts into four categories. The first are defined by having transition-metal complexes attached to the linker, with the nodes having little to no catalytic activity. They compare to the second category, which are constructed with catalytically active metal nodes. The third category comprises photocatalysts, assembled from linkers that facilitate electron transfer to the nodes upon light irradiation, while the fourth category describes MOFs containing stabilised metallic nanoparticles.

This review highlights the most promising catalysts in each category, and MOFs are evaluated on more than catalytic performance alone. Catalysts are examined which contain precious transition metals such as ruthenium and iridium, used under reaction conditions requiring stoichiometric oxidant, base and/or co-catalyst. These are succeeded by MOFs which closely approach the ideal for industry and sustainability: a catalyst with high catalytic activity constructed from earth abundant metals such as copper and iron, which requires no added base or co-catalyst, uses air as the terminal oxidant and can be used under solvent-free conditions. And although we’re not there yet, the challenge has been set.

To find out more please read:

Tuneable nature of metal organic frameworks as heterogeneous solid catalysts for alcohol oxidation
Amarajothi Dhakshinamoorthy, Abdullah M. Asirib and Hermenegildo Garcia
Chem. Commun., 2017,53, 10851-10869
DOI: 10.1039/C7CC05927B

About the author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

A joint group of scientists from China, Japan and Australia recently made a breakthrough in water desalination. They designed and synthesized a multilayered electrode consisting of a graphene nanosheet sandwiched between two porous carbon particle layers. This “sandwich” electrode can be used for capacitive desalination to produce fresh water from seawater, and exhibited the highest desalination capacity among the reported graphene sheet-based electrodes.

Capacitive desalination is an emerging water desalination technique. It removes water-soluble salts, mostly sodium chloride, by applying an electric field to move the salts to the surface of electrodes. Because the amount of ions being removed is directly proportional to the surface area of the electrodes, using electrodes with abundant surface to electro-adsorb ions is critical for excellent desalination performance.

The researchers utilized graphene oxide (GO) and zeolitic imidazolate framework-8 (ZIF-8, a metal organic framework) as the two components (Figure 1a). When dissolved in water, ZIF-8 nanocrystals became attached to the surface of GO and completely covered both sides of the GO nanosheets. This process was driven by the coordination interaction between the two species. The formed ZIF-8/GO/ZIF-8 “sandwiches” were then annealed at near 1000 ^oC in nitrogen gas. The annealing step converted GO nanosheets and ZIF-8 nanocrystals into graphene nanosheets and porous carbon particle layers, respectively. Owing to the presence of pores on the surface of the yielded carbon particles, the carbon “sandwiches” had a high surface area of 1360 m²/g, much higher than that of the graphene sheets alone (150 m²/g).

Figure 1. (a) A schematic diagram displaying the key steps for the synthesis of the carbon “sandwiched” electrodes. 2-MeIM = 2-methylimidazole, a building block for ZIF-8. (b) The change of NaCl concentration collected for a “sandwiched” electrode (NC/rGO) and a graphene sheet electrode (rGO). When an electric field is applied, the concentration of NaCl starts to drop and reaches a plateau; When the electric field dissipates, the concentration of NaCl returns to its initial level. The salt concentration decreased to a much lower level with NC/rGO (red curve) than rGO (black curve).

The desalination capacity of the carbon “sandwich” reaches 17.52 mg/g, meaning 1 gram of the electrode can remove 17.52 mg of sodium chloride. Consistent with the enhanced surface area, the capacity of the “sandwich” is much higher than that of the graphene alone (Figure 1b). More significantly, the “sandwich” electrode outperforms all other previously reported graphene sheet-based electrodes in terms of the desalination capacity.

This work has greatly advanced the development of capacitive desalination, a promising and affordable technique to mass produce fresh water by desalting seawater.

To find out more please read:

High Performance Capacitive Deionization Electrodes Based on Ultrathin Nitrogen-doped Carbon/graphene Nano-Sandwiches

Miao Wang, Xingtao Xu, Jing Tang, Shujin Hou, Md. Shahriar A. Hossain, Likun Pan and Yusuke Yamauchi

Chem. Commun. 2017, 53, 10784-10787

About the blogger:

Tianyu Liu is a Ph.D. in chemistry graduated from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web blogger for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/.