Title : Ni2+ and Ni+ counterions in MFI zeolite as single and multiple coordination sites for small molecules
A relatively new field in heterogeneous catalysis is aiming at developing catalysts formulation where platinum group metals are replaced by more abundant and cheap 3d base metals for industrial and environmental adsorption and catalytic applications. An important requirement for such systems is that the 3d metal is in the form of coordinatively unsaturated (CUS) cation state having the proper energetics for adsorption and/or catalytic transformation of molecules. The stabilization of such state can be achieved when cations are ion-exchanged in zeolite structures. A very suitable analytical method to study the CUS-state of cations is IR spectroscopy of probe molecules.
In this presentation we summarize our extensive work on IR studies of adsorption of probe/guest molecules such as CO, NO, N2, O2, H2O along with isotopically labeled molecules to determine the state of Ni cations in ZSM-5 zeolite The IR studies are combined with density functional theory (DFT) modeling to justify the assignment of vibrational modes of adsorbed molecules and thus to prove the state of metal.
Ni2+ ions in the ZSM-5 matrix are highly electrophilic due to their low coordination. The theoretical calculations find that the Ni2+ ions are coordinated to four zeolite oxygen centers. As a result Ni2+ forms relatively stable monoligand complex with NO, CO and N2. The binding energies (BE) of these ligands calculated by DFT are -173, -112 and -57 kJ/mol, respectively.
Another consequence of the low coordination of Ni2+ cations in ZSM-5 is their ability to coordinate two small molecules. Our theoretical prediction find preferential formation of Ni2+(NO)3 and Ni2+(CO)(NO)2 to occur. The IR experiments, however, reveal the formation of Ni2+(NO)2 and Ni2+(CO)(NO) complexes.
Ni+ ion has larger ionic radius than Ni2+ and our DFT calculations showed that Ni+ ions are bonded to two O atoms of the zeolite. As a result Ni+ can bind up to tree CO molecules. BEs of successively added CO molecules are calculated to be -168, -94, and -26 kJ/mol. The Ni+-CO species are much more stable as compared to Ni2+−CO because of the formation of a π bond and the synergism between the σ and π bonds. All monocarbonyls can be converted into dicarbonyls but even at low temperature and high CO pressure, di-carbonyls are not completely converted into tricrabonyls, likely due to the existence of steric restrictions for specific positions Ni−ZSM-5 channels. Ni+ ions are able to form Ni+(N2)2 complexes. The latter can lose their ligands stepwise, producing two types of linear Ni+−N2 species. Calculated BE of the first and the second N2 ligand is -86 and -27 kJ/mol. As predicted by DFT calculations, formation of mixed ligand Ni+(CO)(N2) complexes is observed by IR. Complexes with NO ligands are not experimentally registered even at low temperatures as Ni+ is spontaneously oxidized by NO to Ni2+. No mixed ligand complexes of Ni2+ and Ni+ with aqua ligands are observed.