Vanadium phosphorus oxide catalyst: Progress, development and applications

With the increasing awareness of carbon neutrality, the global warming issue has been well addressed for the reduction of carbon emissions into the atmosphere. Enormous amounts of carbonaceous gaseous byproducts have already been released into the atmosphere especially after the start of industrial revolution in the last century. Most industrial processes releases low carbon alkanes directly into the environment. Among them petroleum refinery units generate low carbon alkanes especially n-butane as gaseous byproducts. In recent decades, n-butane has been used as a reagent to generate important valuable chemicals such as maleic anhydride with the support of vanadia catalysts. The complex vanadium phosphorus oxide exists in the nature in almost fifteen different phases. Each one has distinguished structural morphology and chemical composition, and exhibits particular physiochemical characteristics such as surface acidity, lattice oxygen, surface oxygen, valance state, P/V ratio, V⁺⁴/V⁺⁵, surface to lattice oxygen ratio, etc., which are highly dependent on the precursor preparation and catalyst activation conditions. Researchers believed that either the addition of various promoters/cocatalysts/metals-dopants, or introducing new preparation techniques such as microwave irradiation, ultrasound, ball milling, barothermal, calcination, sol–gel method electrospinning, hydrothermal and solvothermal synthesis can be used for the improvement of the catalytic selectivity and activity. Aforementioned different aspects are described in the current review along with the description of reaction kinetics and reaction mechanism. Additionally, we have also highlighted the important industrial issues such as air/n-butane pretreatment, deactivation, water supplement, phosphorus supplement, and the strong exothermic reaction, which are influencing the overall catalytic performance.