Additional information

AI Fig 1 PXRD pattern of precipitate from Exp. 1.9 - showing identifiable Humboldtine (Fe-oxalate) peaks, and otherwise a paucity of peaks besides the D100 at 7.5 Å

AI Fig 2PXRD patternsof precipitates from Exp. 1.7 and 1.13

AI Fig 3PXRD patternsof precipitates from Exp. 1.8 and 1.15

AI Fig 4 Structure of precipitate from Experiment 1.3 - The compounds, crystallize in the orthorhombic system with space group Pccm and unit cell: a = 10.150(2), b = 11.770(2) and c = 12.510(3) Å . The compound’s elemental composition C8Fe8N16O52P8 consists entirely of the plant nutrient elements P, N and Fe, organized by corner-shared FeO6 and PO4 units, 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 9 8.6 Å) framework pores along the c axis, leaving central pore areas empty. FeO6 units are in brown, PO4 units in purple,O in red and N in blue-grey. The C and O atoms are shown as50 % of their refined ADPs. The arrows in the c axis projectionillustrate the large pore sizes of *10 9 8.6 A˚with empty centresand N-containing guests located at the pores’rims. The inset image shows the coordination around Fe atoms. This figure is taken from [46]

AI Fig 5FDS Graph for Design Model

AI Fig 6 RSM of confirmatory experiment A1 - shows ridged response surface for desirability at experimental conditions of 122°C and 22.6 hours reaction time; at the used water ratio of 50 the used urea ratio of 4.1 is right at the edge of the ridge; urea ratios between around 4 and 5 would have resulted in high desirability, while increased dilution would have resulted in further improved desirability

AI Fig 7 RSM of confirmatory experiment A2- shows ridged response surface for desirability at experimental conditions of 134°C and 10 hours reaction time; at increased dilution of 88 the used urea ratio of 3 brings the desirability to almost optimal point

AI Table 1 Model Design Summary (Design Expert 8.0 output)

AI Table 2 part 1 Constraints defined for optimisation (Design Expert 8.0 output)


AI Table 2 part 2Complete optimisation results - 63 synthesis factor combinations and response variable predictions (Design Expert 8.0 output)

AI Table 3 ANOVA and case diagnosticsfor Fe-conversion (Design Expert 8.0 output)


AI Table 4 ANOVA and case diagnostics for Oxalate Solubility (Design Expert 8.0 output)

AI Table 5 ANOVA and case diagnostics for N content (Design Expert 8.0 output)


AI Table 6 ANOVA and case diagnostics for C content (Design Expert 8.0 output)

AI Table 7 ANOVA and case diagnostics for D100 peak position (Design Expert 8.0 output)