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Metal-Organic Frameworks as Chemical Reactors for the Synthesis of Well-Defined Sub-Nanometer Metal Clusters

Periodic Reporting for period 1 - MOF-reactors (Metal-Organic Frameworks as Chemical Reactors for the Synthesis of Well-Defined Sub-Nanometer Metal Clusters)

Reporting period: 2019-03-01 to 2020-08-31

The use and development of traditional metals has guided the technological evolution and humankind progress during thousands of years. A paradigmatic shift occurred the last century with the appearance of metal nanoparticles (NPs), that are now heavily utilized in engineering, catalysis and biomedical science,and are still a focus of interest because of their huge potential in nanotechnology. The metal technological roadmap has by–passed a third form of matter with completely different properties, i.e. aggregations of less than 10 atoms that fall in the sub–nanometric regime, here called SNMCs. The present project envisions that SNMCs are irremediably called to trigger unforeseen technological solutions in the next years provided that their design, manufacture and use is properly understood and developed.

The crucial step of any flourishing cluster–based technology is the synthesis and stabilization of well–
defined structures at multi–gram scale. This project aims to achieve this ambitious objective with an innovative synthetic methodology based on the use of metal-organic frameworks (MOFs) as chemical nanoreactors to synthesise, in-situ, large amounts of stable and well-characterised highly reactive SNMCs. The main objective is the preparation of new powerful catalysts capable to generously surpass the state–of–the–art catalysts in some organic reactions of industrial interest such as hydrogenation or organic compounds, with high economic impact.
1. Preparation of new families of new families of highly robust and crystalline MOFs using oxamidato- and oxamato-based ligands derived from biomolecules and robust aromatic amines derivatives of chiral amino acids as ligands. These MOFs show tuneable pore size and functionality, as a direct consequence of the size and chemical nature of the chosen amino acid residues with either sulphur- or oxygen-containing functional groups, which must be also capable to anchor to the pores the desired metallic species in subsequent Tasks. These materials have been also explored with excellent and publishable results in other fields such as water environmental remediation and magnetism. In this context, we have reported a new MOF derived from the amino acid L-serine that can be used in the molecular recognition and sorbent extraction of hydrophilic vitamins in juices. Another MOF derived from the amino acid L-histidine has been also used for the efficient capture and separation of greenhouse gases and their crystallographic visualization. Another oxamato-based MOF showed magnetic ordering. We have also developed a family of multivariate MOFs (which are being used for the in-situ preparation of heterometallic SNMCs) that shows excellent properties in the capture of organic and inorganic contaminants. Finally, some of these MOFs are being processed as mixed matrix membranes (MMM-MOFs) and explored their application in different technological applications. Most of these results have been already published (see 1.1 section).

2. We have carried out the MOF-driven preparation and atomic-level SCXRD characterisation of different SNMCs and single atom catalysts (SACs) using the MOFs described in the previous section. For that, we have followed the programmed strategy, based on the controlled insertion of the desired metal salts that are anchored to the network by interacting with specific functional groups or retained by electrostatic interactions in case of anionic frameworks. In so doing, we have achieved the formation of different well-defined ligand-free subnanometer metal clusters (SNMCs) and single atoms catalysts (SACs) such as Ag2 and Au3 (SNMCs) and Pt and Pd (SACs). In addition, such small species have been characterised by using SC-XRD, being the first examples of SNMCs and SACs characterised crystallographically within MOFs. As planned, we have explored their catalytic activity which is outstanding in some cases. For example, Pd SACs have shown excellent activity and reusability in the selective oxidation of primary alcohols to carboxylic acids. Other SNMCs are showing also excellent results in industrially relevant reactions like the water gas-shift reaction or the selective hydrogenation of olefins and paraffins. Some of these results have been already published (see section 1.1).

3. We have also started to carry out the sequential step-by-step synthesis of homometallic (Task 2) and heterometallic (Task 3) SNMCs. In this sense, we have synthesised unprecedented examples of SNMCs of growing nuclearity (Au, Au3 and Au5 and Pt, Pt2 and Pt4) (Task 2) and heterometallic SNMCs (Task 3) like Ag2Au2, Au2Pd2, Cu2Pd4, Au2Pt, Ag2Pt, Ag2Fe2 and Pd2Pt. All these species show and outstanding catalytic behaviours in different reactions of industrial interest such as the activation of formic acid, hydrogenation reactions, Water Gas Shift Reaction (WGSR), etc. Some of these results, even if overall quite preliminary, are about to be submitted to highly reputed journals.

4. The last point related to the objectives of the project consists of the multigram–scale manufacture of SNMCs and industry scalability. For that, we must take advantage of the results obtained in previous Tasks. As mentioned above, a large plethora of interesting materials have been prepared and tested in catalysis. The next step consists of their large scale preparation. Obviously, this is the less developed step but we have obtained preliminary results. For example, the large scale preparation of MOFs has been achieved in the laboratory. So, 50-100 g of MOF can be obtained in a single operation. Right now we are exploring the large-scale preparation of SNMCs and SACs.
Preparation of well-defined ligand-free SNMCs and SACs attracts attention of many chemists in the world due to the large potential technological applications of these species (i.e. catalysis). However, the gram-scale preparation of stable SNMCs and SACs and their proper characterization constitutes a great challenge. The present approach relies on the use of MOFs to allow their gram-scale formation and to stabilise such naked tiny species. So far, we have achieved different results that can be considered to be beyond the state of the art and they are listed below:

- A high degree of functionalization of the formed MOFs that can be achived pre- (rational programmed methods) or postsynthetically.
- Use of single-crystal X-ray diffraction (SC XRD) as a characterization of such metal species within the MOFs.
- gram-scale preparation of these hybrid materials.
- Preparation of heterometallic ligand free SNMCs.
- The catalytic activities of some of these species surpass the state of the art catalysts.

For the next part of the project we aim to:

1. Prepare even more stable MOFs combined the synthesised ligands with Ti and Zr metals.
2. Produce novel examples of homo- and heterometallic SNMCs which are expected to surpass the catalytic activity of some state of the art catalysts.
3. Focus on the large scale preparation of both MOFs and MOF-driven SNMCs and SACs. We plan to start using the chemical reactor disposable in my institution.
4. Unveil mechanisms governing the catalytic reactions taking advantage of the structures elucidated by SC XRD and using theoretical calculations.
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