# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### Layered Metal Phosphonates
The degree of deprotonation of the phosphonic linker and the affinity of the metal for the amine group was found to dictate the way the hybrid structure evolved during the synthesis [28]. Another example illustrating the importance of reaction conditions was demonstrated by the reaction of cobalt (II) and octylene diphosphonic acid. The room temperature synthesis of the precursors in HF resulted in a clathrate structure made of 1D chains featuring a cobalt atom and undissociated diphosphonic acid in a polymeric network held together by hydrogen bonds. However, when the same system was subjected to a hydrothermal treatment (in the absence of HF), layered cobalt (II) diphosphonate with octyl chains in the inter-layer were formed. Therefore, depending on the synthesis conditions and presence or absence of HF, different structures were formed. The degree of protonation plays an important role in the hydrogen bonded networks in such systems [29]. Due to the closed packing of the phosphonic linkers as pillars, the metal phosphonates did not possess any porosity. Nevertheless, porous systems are known to have higher surface areas and provide better interfacial properties that are crucial for applications such as catalysis, gas adsorption, separation/extraction and others. Therefore, porosity is often a highly desired property and we therefore describe how porosity can be introduced in these layered metal phosphonate systems. These methods involved the use of additives or mixing solvent systems as will be discussed in the following paragraphs. A strategy to impart porosity in the layered materials was by spacing out the closely packed inter-layer spacers through the use of organic linkers with over-crowded diphosphonic acids. This created inorganic layers (e.g., Cd (II) and Ni (II) oxides) with anchored organic groups pointing inwards between the layers but without close packing. Nevertheless, these materials were still not possessing any (accessible) porosity [30]. In another approach to achieve porosity in layered metal phosphonates, zinc (II) precursor and phenylene and biphenylene diphosphonic acid were reacted in the presence of varying amounts of phosphoric acid in the synthesis mixture. It was shown that indeed, a three-component system with a suitable phosphonate/phosphate (1:4 and 1:7) ratio resulted in a material that had inter-particle slit-shaped porosity in the mesopore region (-6.6 nm). It was noted that an equal phosphonate/phosphate ratio resulted in a material that was purely zinc(II) phosphonate, while higher phosphate incorporation led to the formation of zinc phosphate. However, with the right amount of phosphoric acid, phosphate moieties were incorporated between the phenylene linkers at random places without altering the crystal structure, resulting in a mesoporous texture [31]. The use of metals like Ca, Zn, or Mn (valency \(<\) 3) with phosphonic acids usually leads to materials that exhibit a layered morphology [32]. Therefore, the same strategy as in divalent metals was employed to impart porosity in zirconium (IV)-based metal phosphonates. The Zr (IV) precursor was reacted with 4,4\({}^{\prime}\)-biphenyldiphosphonic acid in the presence of phosphoric acid that would act as a spacer between the aryl linkers [20]. A mechanism of a house-of-cards arrangement was proposed, creating porosity between the layered particles of zirconium bisphosphonate-phosphite materials. The inter-particle mesoporous was achieved by balancing the amount of phosphoric acid/diphosphonic acid ratio [33]. However, as tetravalent metals have the problem of insolubility (e.g., zirconium and titanium metals) caused by the rapid hydrolysis and condensation of metal (IV) precursors resulting in rapid precipitation when they have reacted with phosphonic acid species, the crystallinity of these layered metal phosphonates seems to be improved only when the synthesis was performed in the presence of HF as the solubilizer/mineralizer [20].