The Catalyst Review December 2019 - 7

SPECIAL FEATURE

Status of Metal-Organic Frameworks (MOFs) Upscaling for Applied Catalysis
By David Farrusseng, PhD

Metal organic frameworks (MOFs), which are constructed from inorganic nodes and organic linkers by auto-assembly, are the latest
discovered class of nanoporous crystalline materials (e.g., zeolites, AlPOs). According to International Union of Pure and Applied
Chemistry (IUPAC) experts, MOFs are listed among the top ten discoveries in Chemistry that shall impact our future. The discovery
of the first examples in the early 2000s has excited researchers in catalysis. Because of their similarity with zeolites in terms of pore
size and pore structures, it was acknowledged that MOFs could achieve breakthroughs in catalysis in the energy and chemistry
sectors, as nearly an endless number of potential structures exists. MOFs can be designed, for example, to have very high surface
areas, up to 7000 m2/g and usually in the range of 1000-2000 m2/g. Along with high porosity, the crystalline and ordered nature of
MOFs allow for isolated metal or isolated metallic cluster which, according to their molecular counterparts, are acknowledged to
be selective catalytic sites. As indicated by a continuous increase of numbers of scientific publications in catalysis (annual growth
rate of 30% since 2007), MOFs have become key platform for the design of novel catalytic structures and model catalysts which
shall facilitate the characterization of active sites "at work." Nevertheless, despite early excitement and intense efforts by the
catalyst community over the last decade, MOFs have not found their way to a catalytic commercial application. The momentum
gained in these academic studies has not yet been translated to industrial use until now, despite two commercial applications being
announced in 2016 using MOFs as vapor-controlled release devices. MOF Technologies (UK) announced the registration of TruPick
- a post-harvest freshness management device for fruit and vegetables - with the US Environmental Protection Agency (EPA). It
consists of a MOF adsorbent for the storage and release of 1-methylcyclopropene (1-MCP), which is a competitive inhibitor for the
ethylene receptor found on the surface of some fruit. NuMat Technologies announced the launch of the product line ION-X© which
are MOF containing gas cylinders. The application is specific to very toxic gases such as arsine, phosphine and boron trifluoride which
are used in the electronic industry. They were previously stored under high pressure which was a serious security concern. Adsorbed
in the porous structure of the MOF at sub-atmospheric pressure, the device is now much safer and allows a large storage capacity
due to the greater density of adsorbed vapors.
Twenty years after the discovery of MOFs, where are we in terms of innovations for practical application catalysis? Could MOFs
rival well-established catalysts in the future, including cost position? Beyond prerequisite catalytic performances, many other
technical aspects shall be investigated such as production capacity using safe processes, product quality, raw material availability,
production cost to site and so on. While many reviews in the open literature address the state of the art of catalytic MOFs mostly in
academic perspectives, this article focuses on main progresses on synthesis upscaling and shaping which are milestones towards the
commercialization of MOFs.
Starting from Basics

Heterogeneous catalysis is firstly a Surface Science as the mechanism of bond breaking and forming with substrate occurs at the solid
surface. In addition, MOFs exhibiting permanent and high porosity can be regarded as an extended surface as (almost) all atoms can
be accessible from the gas phase. Therefore, the internal surface termination of MOF is of primary importance. For metals oxides,
including zeolites, their surface can be terminated by hydroxyl groups which confer them Brønsted type acidity. For electron poor
noble metals, the coordinatively unsaturated atoms present at the particle surface are typical Lewis catalytic sites. The example of
gold has become famous. It was believed for a long time that gold was inert in catalysis. However, in nanometric particles for which
the surface atoms have low coordination numbers, gold shows intrinsic catalytic properties. Hence, for metal and oxide solids,
the increase of the surface by division of the matter creates sites which are responsible for the catalytic properties. Conceptually,
surface termination, usually vacancies or hydroxyl groups, could be regarded as defects. MOFs are situated at the frontier of solidstate chemistry because of the long-range order which define the porous network and of surface science as most of their atoms are
accessible at the surface. In the following, we will show that same concept of vacancy (e.g., lower coordination number) and hydroxyl
terminated surface can be applied for the definition of MOF defects which are relevant to explain observed, yet "unexpected"
catalytic results.
Catalytic Sites of MOFs Which May Disrupt Current Catalysts

Lewis acidity in MOFs corresponds to an accessible metal site with low coordination number, also known as coordinatively
unsaturated sites (cus). They are obtained by removal of labile ligands which usually are water molecules or electron donor solvents
such as alcohols. MOFs made from dimeric transition metals clusters are representative examples of Lewis acid MOFs. Catalytic
molecular complexes made of dimeric transition metals are well known but can suffer from self-assembly in solution when they are
not hindered by bulky protective groups. The advantage of the MOF scaffold is that the metal clusters are kept isolated from each
other.
The Catalyst Review 										

December 2019

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The Catalyst Review December 2019

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