Synthesis optimisation, characterisation and evaluation of an iron-based oxalate-phosphate-amine MOF (OPA-MOF) for innovative application in agriculture

Porous metal-organic framework materials (MOFs) have found exceptional interest in the engineering and medical communities, due to a large spectrum of valuable properties. These materials may bring solutions to pressing environmental issues, such as gas storage media, new type batteries, or carriers for medical drugs. However, not all MOFs fulfil the specific requirements for these applications, yet may be suitable for alternative applications. In agriculture, for example, increasing soil acidification requires urgent attention. Use of MOFs as innovative slow-release fertilisers may contribute to soil alkalinisation, soil fertility and improved N and P fertiliser use efficiency. This thesis seeks to provide a proof of concept for optimal production, and mode of action for just such a fertiliser: a structure built by plant nutrients N, P and Fe, plus oxalate, which is decomposed by soil bacteria, thereby releasing the nutrients to the soil environment for plant uptake. The structural oxalate provides a source of carbon and energy for oxalotrophic bacteria through adequate solubility in soil solution, which enables biomineralisation of carbonate via the oxalate-carbonate pathway, resulting in increased soil pH. This thesis presents the successful hydrothermal synthesis, with urea as structure-directing agent (SDA), and synthesis optimisation of a mixed-valence iron-based oxalate-phosphate-amine framework (OPA-MOF; C8Fe8N16O52P8), which crystallizes in a Pccm centrosymmetric system. Corner-sharing FeO6 - and PO4 - units are connected by the oxalate ligand in two directions, forming a macro-porous framework, which carries ammonium from decomposed urea as guests. Optimal production factors were defined from a full factorial 42+1 design, and response surface method (RSM), which produced a saddle-ridged optimum function for desirability. Two additional experiments confirmed the robust model. The OPA-MOF displays all properties required for application as fertiliser: high product yields of suitable purity, nutrient contents, and oxalate solubility. The material’s fertilising capacities were tested in a pot trial growing wheat, which showed increased grain yields (compared to unfertilised controls and urea fertilisation), and an unprecedented long-lasting nitrification inhibition effect. Due to a short trial period no soil pH increase could be detected, and P remained limiting for maximum yields. However, microbiological investigations confirmed presence of oxalotrophic bacteria, and their ability to consume OPA-MOF. Scanning Electron Microscopy (SEM) reveals that even after 22 weeks exposure to the soil environment the OPA-MOF particles are almost intact, which suggests that any decomposition of the material is very slow, affording a long-term presence of the incorporated plant nutrients in the soil. The successful proof of concept warrants long-term trials to investigate potential soil alkalinising and P-fertilising effects.