Electronic structure of porphyrin-based metal– organic frameworks and their suitability for solar fuel production photocatalysis

Metal–organic frameworks (MOFs) can be exceptionally good catalytic materials thanks to the presence of active metal centres and a porous structure that is advantageous for molecular adsorption and confinement. We present here a first-principles investigation of the electronic structure of a family of MOFs based on porphyrins connected through phenyl-carboxyl ligands and AlOH species, in order to assess their suitability for the photocatalysis of fuel production reactions using sunlight. We consider structures with protonated porphyrins and those with the protons exchanged with late 3d metal cations (Fe2+, Co2+, Ni2+, Cu2+, Zn2+), a process that we find to be thermodynamically favorable from aqueous solution for all these metals. Our band structure calculations, based on an accurate screened hybrid functional, reveal that the bandgaps are in a favorable range (2.0 to 2.6 eV) for efficient adsorption of solar light. Furthermore, by approximating the vacuum level to the pore centre potential, we provide the alignment of the MOFs' band edges with the redox potentials for water splitting and carbon dioxide reduction, and show that the structures studied here have band edges positions suitable for these reactions at neutral pH.Royal Society for an International Exchange Scheme grantVia our membership of the UK's HPC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work made use of the facilities of ARCHER, the UK's national high-performance computing services, which are funded by the Office of Science and Technology through EPSRC's High End Computing ProgrammeEuropean Research Council through an ERC Starting Grant (ERC2011-StG-279520-RASPA)MINECO (CTQ2013-48396-P)Andalucía Region (FQM-1851)University of Granad

The Si-Ge substitutional series in the chiral STW Zeolite Structure Type

The whole compositional range (Ge$_f$ = Ge/(Ge+Si)= 0 to 1) of zeolite STW has been synthesized and studied by a comprehensive combined experimental--theoretical approach. The yield of zeolite goes through a maximum and then drops at the GeO$_2$ side of the series, following the inverse of the calculated free energy curve. The unit cell generally expands, roughly linearly, as the Ge$_f$ increases, but a notable resilience to expansion is observed at the high silica side. This can be attributed to the more rigid character of SiO$_2$ and the ability of Ge units to deform. Density functional theory calculations provide a new assignment of the previously controversial $^{19}$F MAS NMR resonances for occluded fluoride, which is based not only in the number of Ge atoms in the double-4-ring units but also on the way they are associated (namely, no Ge, isolated Ge, Ge pairs or closed Ge clusters). While we found an overall good agreement between the experimental and theoretical trends in preferential occupation by Ge of different crystallographic sites, the theoretical models show more sharp and abrupt tendencies, likely due both to limitations of the approach and to kinetic factors that allow metastable configurations to actually exist.Comment: 26 page