# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### Layered Metal Phosphonates
The porosity in these materialswas mainly dictated by the packing density of the organic linker between the inorganic metal oxide layers, acting as spacers, and on the presence of ingredients such as solvent/additives in the synthesis recipe [9; 10]. Attempts to make narrow pore size lamellar metal phosphonates were achieved by [PERSON] et al. by replacing around 30% of the butane and benzene diphosphonic acid moieties by a phosphite moiety in between the zirconium oxide layer. This way, mesoporosity was observed with pore sizes around 2-4 nm, depending on the amount of phosphite incorporated [18]. Similarly, complex pillaring moieties were incorporated, such as the 3,3',5,5'--tetramethylbiphenyldiphosphonic acid that has bulky ends with narrow biphenyl units [19]. This material possessed a high surface area of 375 m\({}^{2}\)/g with inter-layer microporosity (with a pore size of \(\sim\)0.5 nm) [34]. Tin (IV) phosphonates were prepared that consist of small nanoparticles with microporosity due to inter-particle voids that created tunnels in dimensions of 1.1-2.4 nm, depending on the size of the organic group of the phosphonate [35]. A similar attempt to synthesize porous layered material with tin (II) was unsuccessful even with the inclusion of mixed phosphonic ligands [36]. In contrast, a mixed phosphonic acid precursor of phenyl phosphonic acid and methyl phosphonic acid with tin (IV), resulted in mixed micro/mesoporosity. The stacking of tin nanoparticles of tin (IV) phosphonates that was arranged in a house-of-cards fashion contributed to the mixed porosity. The porosity was found to be dependent on the ratio of phenyl phosphonic/methyl phosphonic acid amount and on the presence or absence of HF. In the absence of HF, the materials were mesoporous and in the presence of HF, the materials showed microporosity [37]. In another study, layered tin (IV) phosphonates that were synthesized from diphosphonic acids had completely different structures in comparison with tin (IV) phosphonates from monophosphate acids. Tin (IV) phosphonates with diphosphonic acids presented larger particles with sizes \(\sim\)20 \(\upmu\)m, while tin (IV) phosphonates with monophosphate acids were \(\sim\)2 \(\upmu\)m in size. Thus, the nature of the phosphonic acids is crucial to particle growth in forming tin (IV) phosphonates. Along with the mono- or di-functionality of the phosphonic acid, the metal source plays an important role. With zirconium (IV), diphosphonic acids were inserted between the inorganic layers as pillars, whereas in the case of tin (IV) they were found to be incorporated in between inorganic layer as pillars as well as inserted as a part of the inorganic-organic chains. Because of this dual role, when tin (IV) was the metal, not all diphosphonic moieties were present as pillars and therefore voids were present in the layers which resulted in porosity in the final material [38]. In another study, the growth of porous layered tin (IV) phosphonates was explained by random cross-linking of layered species providing inter-layer porosity. A possible hypothesis for the underlying mechanism that caused the random stacking instead of layer growth was the presence of stacking defects by the combination of diphosphonate and monophosphate ligands or the presence of protonated phosphonic acids [39]. Based on these results, a generalized growth model for metal (IV)diphosphonate coordination networks was proposed, resulting from rapid precipitation of unevenly grown capped plates, leading to formation of structure as illustrated by Figure 2[39; 40]. Apart from other factors such as the choice of the phosphonate ligand and the presence or absence of HF, also the type of solvent used plays a role in obtaining porous metal phosphonates. The obtained porosity was found to be dependent on the solvent system employed. Zr (IV) precursors are soluble in DMSO but poorly soluble in an alcohol/water medium, and the vice versa for Sn (IV) precursors.