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COFLeaf Report Summary

Project ID: 639233
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - COFLeaf (Fuel from sunlight: Covalent organic frameworks as integrated platforms for photocatalytic water splitting and CO2 reduction)

Reporting period: 2015-09-01 to 2017-02-28

Summary of the context and overall objectives of the project

The efficient conversion of solar energy into renewable chemical fuels has been identified as one of the grand challenges facing society today and one of the major driving forces of materials innovation.
Nature’s photosynthesis producing chemical fuels through the revaluation of sunlight has inspired generations of chemists to develop platforms mimicking the natural photosynthetic process, albeit at lower levels of complexity. While artificial photosynthesis remains a considerable challenge due to the intricate interplay between materials design, photochemistry and catalysis, the spotlights – light-driven water splitting into hydrogen and oxygen and carbon dioxide reduction into methane or methanol – have emerged as viable pathways into both a clean and sustainable energy future. With this proposal, we aim at introducing a new class of polymeric photocatalysts based on covalent organic frameworks, COFs, to bridge the gap between semiconductor and molecular systems and explore rational ways to design single-site heterogeneous photocatalysts offering both chemical tunability and stability.
The development of a photocatalytic model system is proposed, which will be tailored by molecular synthetic protocols and optimized by solid-state chemical procedures and crystal engineering so as to provide insights into the architectures, reactive intermediates and mechanistic steps involved in the photocatalytic process, with complementary insights from theory. We envision the integration of various molecular subsystems including photosensitizers, redox shuttles and molecular co-catalysts in a single semiconducting COF backbone. Taking advantage of the hallmarks of COFs – molecular definition and tunability, crystallinity, porosity and rigidity – we describe the design of COF systems capable of light-induced hydrogen evolution, oxygen evolution and overall water splitting, and delineate strategies to use COFs as integrated platforms for CO2 capture, activation and conversion.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Project Goal: Kinetic Control of COF Growth by Development of Covalent Network Terminating Agents (Step 1 and 2)

COF syntheses which yield highly crystalline materials have predominantly been realized through reversible condensation reactions. The term “self-healing” is often applied to the primary mechanism operating during COF growth when reaction conditions are such that covalent bonds formed by condensation reactions are being formed, broken and re-formed simultaneously. This subproject aims to develop COF growth modulating agents which can reversibly terminate covalent network growth fronts. More controlled growth kinetics will allow “self-healing” to become more prominent, ultimately improving crystallinity in imine based COF systems which are more stable as scaffolds for water splitting catalysis.
Compounds 3C and 3D were selected as synthetic targets for growth modulating agents. Compound 3C has been prepared in its entirety in high purity. 3C possesses relatively short, solubilizing hexyloxy chains and the compound proved straightforward to synthesize and purify according to. Compound 3D possess longer polyether chains designed to solubilize the compound and forming COFs in more polar solvents. Compound 3D has not been fully prepared, and remains at the 3B intermediate stage. The 3B derivative has been prepared on a small scale but yields less high purity material and requires more tedious chromatography steps.
Going forward, compound 3B with longer polyether chains will be reacted with 4-formylphenylboronic acid to yield the final target compound 3D. Both 3C and 3D will be added in variable molar ratios with COF forming reagents 3E and hydrazine under standard COF forming reaction conditions. Resultant COFs will be characterized with regards to crystallinity and surface area as a function of modulator concentration.

Scheme 1:


Project goal: Controlling the crystallinity, layer registry and stacking mode in COFs (Step 2-iii)

The exact stacking sequence of the 2D layers in COFs is of paramount importance for the optoelectronic, catalytic and sorption properties of these polymeric materials. The weak interlayer interactions lead to a variety of stacking geometries in COFs, which are both hard to characterize and poorly understood due to the low levels of crystallinity. Therefore, detailed insights into the stacking geometry in COFs is still largely elusive. In this regard, we could show that the geometric and electronic features of the COF building blocks can be used to guide the stacking behavior of two related 2D imine COFs (TBI-COF and TTI-COF), which either adopt an averaged “eclipsed” structure with apparent zero-offset stacking or a uniformly slip-stacked structure, respectively.[1] These structural features are confirmed by XRPD and TEM measurements. Based on theoretical calculations, we were able to pinpoint the cause of the uniform slip-stacking geometry and high crystallinity of TTI-COF to the inherent self-complementarity of the building blocks and the resulting donor-acceptor-type stacking of the imine bonds in adjacent layers, which can serve as a more general design principle for the synthesis of highly crystalline COFs. Controlling the crystallinity, layer registry and stacking mode in COFs could be used to elucidate structure – activity relationships for photocatalysis.

Figure 1: A: determination of the slipping direction by XRPD. B: Different types of stacking in the TTI-COF.


Project goal: Topotactic Locking of a Covalent Organic Framework (Step 1-iii. Post-synthetic stabilization) – A novel thiazole COF

As a strategy to synthesize highly stable covalent organic framework materials – a prerequisite for robust and long-term photocatalytic action – we investigated the transformation of an imine linkage to a thiazole in a COF by the action of elemental sulfur on the COF.

Figure 2: A: formation of the imine bond and the subsequent reaction to form the thiazole. B: Structural transformation of the TTI-COF to the S@TTI-COF.

Since the reversible formation reaction of the COF is inherently necessary to form a crystalline material, it is not easily possible to imbue high crystallinity and stability in one step. We therefore investigated post-synthetic reaction schemes that would enable the “locking” of the crystalline state once the COF is formed under reaction conditions, and prevents the re-opening of the COF forming linkage (Figure 2). We explored the oxidation reaction between elemental sulfur with the highly crystalline imine-linked TTI-COF.[1, 2] This high-temperature reaction, in which a heterocyclic five-membered ring is formed, fully retains the crystallinity of the framework and is expected to significantly improve the stability of the material against chemical and physical attack with respect to the pristine imine bond.

Figure 3: TEM images of the S@TTI-COF showing the individual crystallites in different orientations (A, B). SAED (C) with logarithmic contrast showing diffraction rings, which are in agreement with the XRPD.


Project goal: New electron-deficient linkers with improved light-harvesting properties (Step 2) and development of new post-synthetic coupling schemes (Step 3) – Tetrazine (Tz)-based COFs

This subproject addresses synthetic challenges of introducing functional, redox-active organic moieties with large optical absorption cross section into COF structures. The function of tetrazine (Tz) based linkers is fourfold: 1) Tz provides reversible (dihydrotetrazine/tetrazine) redox chemistry coupled with the gain or loss of a proton. 2) Tz is a relatively electron poor system which could serve to enhance charge separation as part of donor-acceptor dyad. 3) Tz compounds have large visible light absorption cross section and can therefore sensitize light harvesting catalytic systems. 4) Lastly, Tz cycloaddition reactions allow attachment of additional reaction centers.
Two synthetic pathways have been explored for creating Tz containing organic linkers suitable for formation of COFs (refer to scheme 2). Both strategies began with the synthesis of 1C in moderate yields. 1C proved too reactive for lithiation and too insoluble for Pd catalyzed aryl-aryl coupling reactions, specifically with 4-formylphenylboronic acid. 1C was successfully reacted with bis(pinacolato)diboron to form the Tz- bis(pinacolato)diboron 1D in moderate yield.
With the synthesis of 1D complete, the following stage requires the controlled hydrolysis of the bis(pinacolato)diboron moiety to form the boronic acid in situ. The resultant condensation of this compound will result in the boroxine linked COF-Tz. Following the successful formation of the proposed COF, post-synthetic modification of the pore wall will be attempted. Catalyst free, Diels Alder type “click chemistry” of tetrazines are unique with regard to the mild conditions under which high yielding cycloadditions can occur.

Scheme 2:


Project goal: Tuning the structure – property – activity relationships in COF photocatalysts (Steps 2 and 3-1. Hydrogen evolution (HE))

We developed a new azine-COF platform based on a series of four related COFs that only differ in the number of nitrogen atoms in the central aryl ring (Nx-COF series). These gradual changes in atomic-scale structure, bringing about changes in crystallinity, morphology and electronic structure, enabled us to demonstrate that atomic-scale design of the molecular building blocks directly translates into the photocatalytic hydrogen evolution performance.[3] This design concept was then extended to a pyridine-based photocatalytically active framework, where nitrogen substitution in the peripheral aryl rings reverses the polarity compared to the previously studied materials.[4] We confirmed that simple changes at the molecular level translate into significant differences in atomic-scale structure, nanoscale morphology and optoelectronic properties, which greatly affect the photocatalytic hydrogen evolution efficiency. In an effort to understand the complex interplay of such factors, we have carved out the conformational flexibility of the PTP-COF precursor and the vertical radical anion stabilization energy as important descriptors to understand the performance of the COF photocatalysts. As for the Nx-COFs, we find a complex interplay between extrinsic, i.e. steric and morphology-related factors, and intrinsic, optoelectronic features – specifically the vertical radical anion stabilization energy – determining the photocatalytic activities of PTP-COF. The findings show the importance of precisely controlling the structure, long-range order and morphology of a COF through dynamic covalent chemistry, such that the various factors determining the photocatalytic activity of a COF can ultimately be disentangled.

Figure 4: Schematic representation of the base structure of the phenyl-triphenyl system. For the Nx-COF series ring A was substituted (combination of 0 and a, 1 and a, 2 and a, 3 and a, yield, N0, N1 N2 and N3 COFs, respectively.)

Figure 5: Left: hydrogen evolution under AM1.5 conditions. After an initial activation period of about an hour, a linear increase in hydrogen evolution is seen. Right: Comparison of vertical radical anion and radical cation stabilization energies of Nx-CHO molecules and PTP-CHO, where a lower energy corresponds to better stabilization.


Project goal: Photocatalytic hydrogen evolution with COFs using molecular co-catalysts (Step 3-1. Hydrogen evolution (HE))

A key goal within COFLeaf is the replacement of heterogeneous noble metal co-catalysts with earth-abundant molecular co-catalysts and to tune the COF – co-catalyst interface with molecular precision. Already in the first project phase, photocatalytic hydrogen evolution with COFs using physisorbed cobaloximes as noble metal free molecular co-catalysts has been achieved. Efficient hydrogen evolution (1250 μmol g-1 after 6 hours) is seen with an azine linked N2-COF and chloro(pyridine)cobaloxime co-catalyst in the presence of triethanolamine (TEOA) as a sacrificial electron donor in a water / acetonitrile mixture at pH 8 under AM 1.5 illumination. The activity with chloro(pyridine)cobaloxime is found to be even higher as compared to that using a heterogeneous platinum electrocatalyst (700 vs. 287 μmol g-1 in 6.5 hr under AM 1.5 illumination) in acetonitrile/water (Figure 6). Hydrogen evolution is seen for other azine-linked and hydrazone-linked COFs (that produce hydrogen with platinum co-catalyst) in the presence of a cobaloxime molecular co-catalyst as well.

Figure 6: Photocatalytic hydrogen evolution with N2-COF in the presence of chloro(pyridine)cobaloxime and platinum co-catalyst.


Project goal: Coordinative or covalent attachment of molecular co-catalysts to the COF backbone (Step 3-1. Hydrogen evolution (HE)) – Bipyridine and bispyrimidine COFs for single-site catalyst attachment

Integration of dedicated functional groups for covalent co-catalyst attachment or strong, well defined metal coordination sites is critical for probing the effects of bonding mode and strength of the co-catalyst to the COF backbone on the photocatalytic efficiency. While a few COFs based on the bipyridine linker exist, the doubly chelating linker 2,2’-bipyrimidine has been largely unexplored to date. This subproject aims to synthesize linkers and COFs containing the bipyridine and bipyrimidine units.
In the first reporting period, we have synthesized TT-BPY COF from triazine triphenylamine and 2,2′-bipyridine-5,5′-dicarbaldehyde (Figure 5). In addition, 5,5’-dibromo-2,2’-bipyrimidine was synthesized according to Scheme 3. Preliminary results show photocatalytic hydrogen evolution with TT-BPY COF in the presence of a platinum co-catalyst and triethanolamine as the sacrificial electron donor (Figure 7). Incorporation of cobalt polypyridine based co-catalysts is currently underway. Furthermore, compound 2E (Scheme 3), following partial lithiation, will be reacted with cyanuric chloride and converted to the trialdehyde 2H. 2H will subsequently be reacted with hydrazine to produce the final COF, Scheme 3. The resulting solid will be loaded with metal catalysts and light sensitizing metal complexes by coordination to the bipyrimidine unit. Preliminary photoactivity studies will be completed by end of phase 2.

Scheme 3:


Figure 7: Structure and photocatalytic hydrogen evolution with TT-BPY COF.


Project goal: Covalent organic framework morphology tuning – investigation of thin film synthesis (Step 5-ii. Morphology)

We investigated the solvothermal growth of different COFs (COF-42, HTFG-COF, AB-COF) on different substrates (glass, Si, FTO, spincoated TiO2 nanoparticles, spincoated ZrO2 nanoparticles) and found optimum conditions for the synthesis of homogeneous COF films in all systems. REM and ellipsometry measurements show film thicknesses of about 80 to 120 nm. Precise tuning of the film thickness is an ongoing project. Further, we are investigating the deposition of homogeneous thin films by means of spin-coating techniques. The morphology of colloidally stable suspensions of COF-42 is being examined and suspensions of different concentrations are tested for spin-coating on different substrates.


Project goal: COFs for improved CO2 sorption and pre-concentration (Step 4)

Integration of functional groups that have strong interactions with carbon dioxide is an easy way to improve the CO2 capturing performance of COFs. A platform that is easily modifiable are hydrazone-linked COFs based on 2,5-diethoxyterephthalohydrazide (DETH) such as HTFG-COF (linked with 1,3,5-triformylphloroglucinol) and COF-42 (linked with 1,3,5-triformylbenzene). By a copolymerization approach, 2,5-bis(2-(dimethylamino)ethoxy)-terephthalohydrazide (DtATH) was integrated into both networks in different amounts ranging from 25 % to 100 % and the structural and sorption (esp. CO2 sorption) properties of the systems were studied. The chemical structure of the adsorbed carbon dioxide species was investigated by solid state nuclear magnetic resonance spectroscopy. An increase of the relative CO2 sorption was achieved by the addition of the tertiary amine linker while surface areas and pore sizes decrease. The highly controlled modification of the COF systems opens up the possibility to add functionality that cannot be achieved by conventional COF synthesis.

Figure 8: Relative CO2 adsorption at 273 K and BET surface areas of Amine-COF-42 (blue and purple) and Amine-HTFG (red and orange). BET surface area is indicated by triangles.

References:

[1] Haase, F., Gottschling, K., Stegbauer, L., Germann, L.S., Gutzler, R., Duppel, V., Vyas, V.S., Kern, K., Dinnebier, R.E., and Lotsch, B.V., Materials Chemistry Frontiers, 2017.
[2] Vyas, V.S., Vishwakarma, M., Moudrakovski, I., Haase, F., Savasci, G., Ochsenfeld, C., Spatz, J.P., and Lotsch, B.V., Adv. Mater., 2016, 28, 39, 8749-8754.
[3] Vyas, V.S., Haase, F., Stegbauer, L., Savasci, G., Podjaski, F., Ochsenfeld, C., and Lotsch, B.V., Nat Commun, 2015, 68508.
[4] Haase, F., Banerjee, T., Savasci, G., Ochsenfeld, C., and Lotsch , B. V., Faraday Discuss., 2017, DOI: 10.1039/C7FD00051K.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our work on covalent organic frameworks has not only introduced a new and highly versatile photocatalytic materials platform, but is expected to provide unique insights into the fundamental mechanisms underlying photocatalytic processes at a molecular level. So far, we have generated new understanding of the crystallization of COFs on a morphological and a molecular level that is inherently linked to their photocatalytic activity. Structural modifications developed by us, aiming at generating highly robust and long-term stable systems, as well as the introduction of functionality in the form of molecular co-catalysts drive the tunability of these molecular solids beyond the limits of conventional solid state materials. Continuous improvement of COF photocatalysts with regard to stability, light harvesting and activity could ultimately open the door to tailored commercial photocatalysts for harvesting solar energy on a large scale. The primary benefits of this research, however, will be the advancement of our understanding of what is at the heart of a “good” photocatalyst, and our ability to rationally design and tailor new photocatalytic systems with molecular precision.

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