Microwave-hydrothermal synthesis and characterization of Co-VSB-5 microporous framework

The crystallization of cobalt metal incorporated microporous nickel phosphate framework, Co-VSB-5, with initial Ni/P gel ratio of about 1.0, 1.5 and 2.5, respectively, has been investigated under microwave-hydrothermal conditions. Attempts are made to incorporate metal ion in the range of 2.5-10 atomic % in the VSB-5 framework having varied Ni/P ratio by varying gel pH. The prepared samples were characterized by means of X-ray diffraction, scanning electron microscopy, UV-visible diffuse reflectance spectroscopy (UV-visible DRS), and nitrogen adsorption-desorption measurements at 77 K. The prepared samples are found to be crystalline in nature. The initial gel pH is found to be critical for crystallization of nickel phosphate framework with varied Ni/P ratio. Like wise, isomorphic substitution is favored with an increase in Ni/P gel ratio. The results for UVDRS demonstrated the incorporation of transition metal ions in the octahedral position for nickel sites in VSB-5 framework. The textural properties, such as surface area and micropore volume, of the Co-VSB-5 samples crystallized from initial Ni/P gel ratio of 2.5, were found to lower than those crystallized from initial Ni/P gel ratio of 1.5 at equivalent metal content level. Interestingly, crystallization of framework having micro-mesoporosity is favored with an increase in Ni/P gel ratio. Furthermore, scanning electron microscopy revealed the crystallization of samples with rod shaped morphology irrespective of the nature of metal ion and its concentration from gel comprising of Ni/P of 1.5 and 2.5. The results of UV-Vis DRS demonstrated the incorporation of transition metal ions in the octahedral position for nickel sites in VSB-5 framework.