STOCCAGGIO CHIMICO DI IDROGENO IN AMMONIACA-BORANO (AB): SINTESI DI CATALIZZATORI DI DEIDROGENAZIONE DI AB E STUDI COMBINATI SPETTROSCOPICI/COMPUTAZIONALI DEL MECCANISMO DI RILASCIO DI IDROGENO

Responsabili di progetto

Maurizio Peruzzini, Elena Shubina

Accordo

RUSSIA - RFBR-suspended - Russian Foundation for Basic Research

Bando

CNR/RFBR 2015-2017

Dipartimento

Scienze chimiche e tecnologie dei materiali

Area tematica

Scienze chimiche e tecnologie dei materiali

Stato del progetto

Nuovo

Proposta di ricerca

The transition from a global economy based on the unsustainable exploitation of fossil fuels to produce energy to a new lifestyle founded on the widespread use of hydrogen as a green energy vector is generally invoked to satisfy the growing energy needs for the whole mankind in the second part of this century. As it is generally accepted, the development of a mature and diffuse "hydrogen economy" is hampered by two severe drawbacks: (i) a "sustainable" hydrogen production not only from coal or natural gas but mainly from renewable energies (such as solar, biomass and wind); (ii) the development of a reliable and cheap hydrogen transportation system. Unfortunately, hydrogen transportation and handling is difficult, making its use for any kind of renewable energy device rather problematic. To solve this problem, hydrogen must be stored into chemical compounds, which easily release hydrogen on demand. Recently, B-N compounds (amine-boranes) have attracted considerable interest in this regard, as they offer an appropriate balance of volumetric and gravimetric energy densities. Among the most promising materials in this field, a special role is taken by the lightweight and hydrogen-rich species ammonia-borane (NH3BH3, AB, 19.3 max.% wt. H). AB is non-flammable and stable under standard conditions; its dehydrogenation/hydrolysis mediated by organometallic catalysts offers the potential to tune both rate and extent of H2 release. Over the past five years, relatively expensive transition metals (Ir, Rh, Ru, Pd) have been exploited as homogeneous catalysts when combined with suitable ligand sets. However, as the final application of these catalysts is the transportation sector, the development of cost-effective catalytic systems based on more Earth abundant metals (e.g. Mn, Fe, Co, Ni, Cu, Zn) are warmly encouraged to make the process really sustainable. On the other hand, heterogeneous catalysis, based on both metallic (supported metal nanoparticles, Metal-Organic Frameworks) and non-metallic (light-heterodoped nanocarbons) materials have been used to promote AB thermal dehydrogenation to produce H2 in a sustainable manner. The combination of AB nanoconfinement into porous and ordered scaffolds (that prevents AB oligomerization and reduces the amount of undesired side-products like ammonia, diborane or borazine) with the presence of exposed metal sites (that strongly interact with AB polar B-H and N-H bonds) represents a convenient and commonly pursued approach to the design of an optimal AB dehydrogenation catalyst featuring the target of reducing the minimum H2 release temperature with respect to pristine AB (that occurs at around 100 °C) and increasing release rates and H2 yields in terms of wt. H %. One main challenge of modern heterogeneous catalysis is re-thinking fundamental metal-based catalytic processes in light of tailored metal-free catalytic architectures, designed and fabricated from cheap and easily accessible building blocks.
The catalysts to be studied are both homogeneous and heterogeneous in nature and will basically consist in: (i) late transition metal (NCP) pincer hydrides (with particular attention to complexes of 3d metals); (ii) metal-free N-doped carbon nanotubes (N-CNTs). In both cases the assorted N-containing heterocycles will be used as part of the pincer ligand or as modifiers of the nanotubes outer surface. Preliminary data collected by the Italian team show their potential for the project purposes.
The polytopic dissymmetrical (NCP) pincer ligands were designed recently at ICCOM and will be used to prepare novel metal hydride complexes as the homogeneous single-site catalysts. Their structure, reactivity and interaction with AB will be studied at both INEOS and ICCOM laboratories. Variable temperature spectroscopic [multinuclear (1H, 13C, 31P, 11B) NMR, IR, UV-Visible] measurements and quantum-chemical calculations will be performed to study the reaction mechanism, possibly leading to H2 formation and evolution after an initial dihydrogen bonding (DHB) between the hydridic and protic H atoms. The data on structure and properties of various reaction intermediates will be obtained, together with the kinetic and thermodynamic parameters of each step. Attempts to isolate the reaction intermediates identified during these studies will be made (if stable at ambient conditions). The metal atom influence on structure, vibrational frequencies, electron density distribution and reactivity of the starting catalysts will be acquired. Special attention will be paid to the study of the solvent effects. The heterogeneously-catalyzed AB dehydrogenation by selected N-CNTs will be performed after an initial nanotube impregnation with AB solutions (at a given molarity) in THF at room temperature, followed by solvent removal. The as-prepared AB@N-CNT solid will be characterized through the ordinary solid-state techniques (powder X-ray diffraction, TG-MS, IR, TEM microscopy, 11B solid-state NMR, BET surface area measurements). The thermal H2 release will be monitored via TPD-MS to determine the release temperatures and yields. The results will be compared with those coming from pristine AB, to clarify the effect of its nanoconfinement into the heterogeneous catalyst. Quantum-chemical calculations will be also exploited to gain insights into the AB@N-CNT interaction mechanism and overall energetics [evaluation of the reaction thermodynamics (DG) and kinetics (DG#, Transition States)].

Obiettivi della ricerca

The objective of this bilateral CNR/RFBR project is the preparation of novel homogeneous and heterogeneous AB dehydrogenation catalysts and the analysis of their catalytic performance, using a comprehensive approach based on the combination of experimental and theoretical methods. A special emphasis will be put on the assessment of the role of the intermolecular dihydrogen bonding that preceeds the H2 release and influence the final reaction pathway. In addition, the effect of the type of substituents on the pincer ligands or the N-containing dangling groups on CNTs on the H2 release temperatures, kinetics and yields will be rationalized, with the aim of creating second-generation systems with improved efficiency. The project involves the Istituto di Chimica dei Composti Organometallici of Florence (ICCOM) of the National Research Council (CNR) and the Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences (INEOS) of Moscow. The participants are largely experienced in this research field, and have a sound track of highly positive international collaborations including the participation, either as Coordinating or Participating team, in numerous international projects (4th RFP, 5th RFP, 6th RFP, INTAS, NATO, GDRI). This collaboration has already produced 11 joint publications and over 25 joint presentations at International and National meetings.

Ultimo aggiornamento: 28/04/2025