Jithin John Varghese

The aim of this work is to develop insights into adsorption of hydrogen and carbon dioxide in a zeolitic imidazolate framework, ZIF-8, using high-pressure adsorption studies, adsorption isotherm model fitting, and DFT investigation of preferential adsorption sites and binding energies. The robustness of ZIF series metal–organic frameworks has drawn interest towards its utility in large scale applications in gas storage and separation. We use room temperature synthesis of ZIF-8 using DMF as a solvent, and benchmarked it against typical solvothermal synthesis. The resulting material is characterized using XRD, SEM, TG–DSC and N2 adsorption isotherm. High-pressure volumetric adsorption of the activated materials is conducted to analyze the hydrogen and carbon dioxide storage capacities up to 50 and 40 bar, respectively. ZIF-8 shows maximum H2 storage capacity of 3.13 wt% at 50 bar and 77 K, and CO2 storage capacity of 46 wt% at 40 bar and 300 K. The parameters of Unilan adsorption isotherm are estimated from the equilibrium adsorption data and isosteric heats of adsorption for H2 and CO2 on ZIF-8 are computed. DFT calculations are used to obtain preferential adsorption sites of H2 and CO2. Adsorption enthalpy values were calculated from DFT as − 7.08 and − 25.98 kJ/mol, respectively for H2 and CO2 at the most preferred sites. We found a close agreement between isosteric heat of adsorption of hydrogen (− 4.68 kJ/mol) and the enthalpy of hydrogen adsorption from DFT (− 6.04 kJ/mol) at 77 K.

Propane oxidative dehydrogenation (ODH) was studied over VOx/CeO2 nanoparticle catalysts for various vanadia nuclearities using Density Functional Theory simulations. On monomers and dimers, propene formation involved V=O, bridged (V–O–Ce, V–OV) and ceria surface oxygens but with low selectivity, while on trimers and possibly higher oligomers it was kinetically favourable over ceria surface oxygens, with higher selectivity than on monomers and dimers. On trimer, overoxidation via acetone formation was inhibited, but at the expense of overall catalyst reactivity. Doping of ceria support with Ni induced strong Ni–VOx interactions, stabilizing the vanadia monomer. The V–O–Ni bridged oxygen was less accessible for overoxidation, resulting in an increase in activation energy barrier for acetone formation by 0.69 eV on VO2/Ce0.89Ni0.11O2(1 1 1), while there was a minimal impact on V3O6/Ce0.89Ni0.11O2 (1 1 1). Hence, transition metal doping of ceria support is a potential strategy to improve propene selectivity over VOx/CeO2 catalysts for a wide range of V loadings.