CALIXARENE MOLYBDENUM BASED (ELECTRO)CATALYSTS FOR NH3 PRODUCTION UNDER MILD CONDITIONS – CalixMo
The project involves the following tasks: 1) the synthesis of the calixarene ligands and of the corresponding Mo complexes, 2) studies of the reactivity of the complexes with N2 (and other ‘probe’ substrates like CO or hydrazine) and 3) the investigation of the reactivity of the N2-adducts with protons and electrons, towards homogeneous catalysis and electrocatalysis for N2 reduction.
In the first 18 months of the CalixMo project we mainly focused on tasks 1a and 1b, and we approached task 2, i.e. the synthesis of ligands and of the corresponding metal complexes (ideally, the catalysts or pre-catalysts for N2 activation and reduction). Concerning task 1a, Figure 1 displays (i) the calixarene ligands that we were able to synthesize during this period (in green, C6-N3, C6-P3, C4-P4), (ii) the ligands whose synthesis is in progress (in blue, C6-P4) and (iii) the only one that we failed to prepare (in red, C6-N3P). Even if the multi-step synthesis of calixarene ligands has revealed to be time-consuming and not trivial, the fulfillment of task 1a can be considered as successful.
The synthesis of metal complexes with the obtained calixarene ligands has revealed to be more challenging. We cannot evidence the coordination of the C6-N3 cryptand to MoIII, and neither with VIII or FeII ion, before or after deprotonation of the amino groups (by ESI-mass spectrometry and spectroscopic techniques). We thus decided to prepare P-rich derivatives like C6-P3 or C4-P4 in order to stabilize low oxidation states of the metals (Mo0, Fe0). The major drawback of these ligands is the lack of preorganization in the lower rim of the calixarene. Actually, after adding metal ions (MoIII, FeII) and reducing agents (KC8, Na/Hg, Mg), or directly by using Mo0 precursor complexes, uncharacterized mixtures of products were formed.
Trials to coordinate N2 (task 3) to Mo and Fe in the presence of C6-P3 or C4-P4 and of different reducing agents led to insoluble materials, probably corresponding to coordination polymers not containing M-N2 bonds (as probed by infrared spectroscopy).
The CalixMo project should have a strong impact not only in the scientific domain, but also in the human’s everyday life. In fact, the development of a more sustainable process for ammonia production, alternative to the industrial Haber-Bosch process, will have enormous industrial, economic, environmental and societal consequences. Currently, more than 130 million tonnes of ammonia per year are produced by the Haber-Bosch process, which contributes to N2 fixation comparably to the biological process catalysed by nitrogenase. The availability of fixed nitrogen represents frequently the limiting factor for agricultural production throughout the world. As a consequence, nitrogen fertilizers produced by the Haber-Bosch process indirectly feed about half of the present world population. It is a common belief that stopping or limiting the use of synthetic fertilizers would lead to mass starvation. Furthermore, a huge increase of ammonia production will be required in the next decades to support the continuous increment of world’s human population. However, the consequent need to intensify ammonia production by the Haber-Bosch process in the next future could be problematic, as: 1) the Haber process is high energy consuming (1-2% of the world’s annual primary energy supply is consumed by this process); 2) H2 is produced in-situ by steam reforming of natural gas, with economic and environmental implications: H2 production generates more than 300 million metric tons of fossil-derived carbon dioxide per year. Haber-Bosch plants are very expensive, as high pressures cost a lot to produce and maintain. For these reasons, the development of an alternative method to the Haber–Bosch process - more environmentally and economically sustainable, will become soon an urgence. In this context, the achievement of a “mild” electrochemical process to reduce N2 into ammonia, bypassing the need of high temperature and pressure and high fossil fuel energy inputs, would be highly desirable.
coming soon (2017)
Ammonia is essential for the synthesis of fertilizers, which provide food for about half of the present world population, and as precursor of several N-containing chemicals. Moreover, its use as an energy vector is emerging. Ammonia is mainly produced from dinitrogen, the most abundant molecule in the atmosphere. Unfortunately, the energetic cost for the 6-electron reductive cleavage of N2 into NH3 is very high. The main obstacle for this process is mainly kinetic in nature. A further complications comes from the competitive reduction of protons into H2. NH3 is commercially produced by the energy-consuming Haber–Bosch process, based on the reduction of N2 by H2 under drastic conditions of temperature and pressure. In nature, N2 reduction into NH3 – in presence of protons and electrons - is efficiently catalysed under mild conditions by Mo-containing nitrogenase, with a [Fe7S9MoC; homocitrate] cluster in the active site. Only two synthetic catalysts that selectively reduce N2 into NH3 at ambient temperature and pressure, both containing Mo, have been reported by Schrock and Nishibayashi. In addition, Peters has recently reported on the first iron catalyst for NH3 production, but at -78 ºC. Although the performance of these systems is low, these results are very promising. On the other hand, none of the attempts at developing a mild electrochemical process for N2 reduction has been very successful in terms of current efficiencies (0.1-8%) and yields of ammonia. Most of these systems involve purely inorganic (noble metals) electrode materials and no molecular systems. In this context, the goal of the CalixMo project is the development of a series of new mononuclear molybdenum complexes with mixed N/P/S ligands using a calixarene-based molecular edifice, as potential catalysts or electrocatalysts for N2 reduction into NH3 at ambient pressure and temperature. It has been already shown that a single metal center is enough to catalytically reduce N2 into NH3, and Mo represents an adapted choice for catalytic N2 reduction, as it can switch between four different oxidation states. Most importantly, calixarene molecular edifices, and - more specifically - capped calix[6]arenes, will assure different benefits for N2 reduction: 1) preorganization and nuclearity control, 2) a relative flexibility, 3) protection of unstable intermediates, 4) potential molecular recognition of N2, and 5) the presence of ROAr, as possible proton relay. The electrochemical approach will be preferential in this project, due to its potential industrial exploitation. Our project involves the following tasks: 1) the synthesis of the calixarene ligands and of the corresponding Mo complexes, 2) studies of the reactivity of the complexes with N2 (and other ‘probe’ substrates) and 3) investigation of the reactivity of the N2-adducts with protons and electrons, towards homogeneous catalysis and electrocatalysis for N2 reduction. Two further tasks, complementary to tasks 2 and 3, are the spectroscopic (task 4) and electrochemical (task 5) characterization of the Mo complexes, N2-adducts and N2-reduction intermediates. The potential impact of the CalixMo proposal in the field of catalytic N2 reduction is enormous: 1) it will contribute to the obtention of the first competitive molecular catalyst for mild NH3 production; 2) the electrochemical approach should permit to open the doors to the development of electrochemical devices for NH3 production ; 3) the reactivity studies on the targeted complexes will allow to get further insights into the mechanism(s) of N2 reduction. Thanks to the complementarity of the different members of the consortium, all the competences required to fulfill the tasks of the project are available: organic and inorganic synthesis, NMR and EPR spectroscopy, metal-based activation of small molecules; electrochemistry and electrocatalysis.
Project coordination
Marcello Gennari (Département de Chimie Moléculaire - Université Joseph Fourier)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partner
DCM - UJF Département de Chimie Moléculaire - Université Joseph Fourier
Help of the ANR 198,394 euros
Beginning and duration of the scientific project:
September 2014
- 42 Months
