Characterization of an oxalate-phosphate-amine metal–organic framework (OPA-MOF) exhibiting properties suited for innovative applications in agriculture

Targeting an innovative application for metal–organic frameworks (MOFs) in agriculture, a hydrothermal method is employed to synthesize two compounds of urea-templated iron-based oxalate-phosphate-amine MOFs, OPA-MOF (I) and (II). The compounds, forming powders of highly crystalline masses of platy morphology, crystallize in the orthorhombic system with space group Pccm and subtly different unit cells: a = 10.150(2), b = 11.770(2) andc = 12.510(3) Å for compound (I) and a = 10.170(2), b = 11.886(2) and c = 12.533(3) Å for compound (II). Both compounds are of elemental composition C8Fe8N16O52P8 and consist entirely of the plant nutrient elements P, N and Fe, organized by corner-shared FeO6 and PO4units, which connect to oxalate units in a and c directions to form the framework. The N-containing guest species from the decomposed urea template were found to prefer sites close to the edges of the large (~10 × 8.6 Å) framework pores along the c axis, leaving central pore areas empty. Identification of the guest was challenging due to potential H2O/NH4 mixing from the hydrothermal conditions, in addition to rotational and occupational disorder. While both compounds have suitable N and P contents for their application as fertilizers, compound (I) displays the better oxalate solubility required to initiate bacterial mineralization of the structural oxalate, resulting in structural collapse. This is the proposed release mechanism for the plant nutrients in soil. Compound (II) has unsuitably high oxalate solubility, potentially caused by a higher connectivity of larger macroscopic pores to the surface, which was visible in SEM, i.e., a macroscopic effect rather than a crystallographic effect. The utilization of such MOFs in agriculture might help to address reduced soil fertility from acidification, and provide a pathway for more efficient control of nutrient supply than conventional fertilizers.