Response surface optimisation of an oxalate–phosphate–amine metal–organic framework (OPA-MOF) of iron and urea

Metal–organic framework (MOF) materials are well known for various application fields, such as engineering, and medical sciences. Here, the synthesis, and synthesis-optimisation of a novel oxalate-phosphate-amine MOF (OPA-MOF) for innovative agricultural applications is described, with urea as a structure-directing agent in a hydrothermal synthesis. Product properties conducive to proposed applications included yield, purity, elemental content (N, P, C), and oxalate-solubility, as important driving forces for functionality, which is based on the biomineralisation processes for the material’s decomposition in soil. A four-factors/two levels plus one (42 +1) factorial design included replicated zero-point and factors of time, temperature, urea input rate and dilution factor. 19 experimental runs results provided data for a Response Surface Method optimisation to determine factors resulting in a desired product at highest efficiency. The saddle-ridge shaped response surface highlighted system robustness for two factors (time/urea-input), and sensitivity for temperature and dilution factor. Optimal factor combinations initially appeared counterintuitive compared to expected results from factorial design outcomes, however confirmatory experiments validate model predictions. Consequently, the optimisation process was strongly justified for accurate determination of the optimal OPA-MOF synthesis conditions.