# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### Templated Porous Metal Phosphonates  
The final part of the synthesis section is committed to porous metal phosphonates wherein the porous texture is created by the incorporation of templates (supramolecular) as structure-directing agents (SDAs). These materials take inspiration from the PMO hybrid system but follow a different approach as the metal oxide and organophosphonate precursors are incorporated into a hybrid network by a heterocondensation step. The protocol to make these mesoporous metal phosphonates is mostly based on the methodologies of porous metal oxide systems [66]. In general, porous metal oxides are obtained by the formation of metal-oxo oligomers around SDAs in solution or alternatively an evaporation-induced oligomer assembly with SDA triggered by the evaporation of the solvent, denoted as the evaporation-induced self-assembly method (EISA). Finally, a thermal step is applied to crystallize the metal oxide and to remove SDA molecules [67]. Alternatively, the SDA can be removed by extraction with/without a thermal step to enhance the condensation/crystallization [66; 67]. Other synthesis methods for this class of materials have been adapted from the sol-gel approaches used to synthesize metal oxides. The sol-gel based materials form in most cases an amorphous metal oxide network that requires a final calcination step for crystallization [66; 67]. However, in the case of hybrid materials possessing organic moieties embedded in the inorganic-organic network, calcination often cannot be applied for crystallization when the temperature required exceeds the thermal stability of the organic functional group. In these cases, extraction with or without a mild thermal treatment step are required. Templated metal phosphonate networks are often characterized with a texture possessing no long-range order (amorphous). One of the earliest mesoporous metal oxides in the presence of an inorganic phosphorus precursor was created by reaction of titanium isopropoxide with phosphoric acid in the presence of octadecyltrimethyl ammonium chloride/bromide as the SDA. This material consisted of an amorphous network of Ti-O-P with a surface area of 700 m\({}^{2}\)/g and pore sizes of 2.7-3.3 nm [68]. There have been many mesoporous metal phosphates made by the use of phosphoric acid without the presence of any organic functional groups [69; 70; 71]. Although valuable materials, these types of materials are not the subject of this review, and focus will be devoted to those materials made from a combination of metal and organophosphonic precursors. Figure 5: The crystal structure of UPG-1 viewed along the c axis. Color code: zirconium octahedra: purple, coordinating PO\({}_{3}\)C tetrahedra: green. Reprinted with permission from [63]. Copyright (2014), The Royal Society of Chemistry. Templated mesoporous aluminum phosphonate can be synthesized via the reaction of aluminum isopropoxide and methylene diphosphonic acid with octadecyltrimethylammonium chloride (ODTMACI) as the SDA in the presence of tetramethylammonium hydroxide. The hybrid structure had an amorphous network with surface area of 738 m\({}^{2}\)/g and a pore size of 1.8 nm. The pores were periodically arranged in a 2D hexagonal mesophase as analyzed by low-angle XRD and TEM. However, these materials were soluble in acidic ethanol solutions and had to be calcined in order to remove the surfactant and enhance condensation, which caused the loss of periodicity of the porous network in the final material [72]. Another method to produce mesoporous aluminum diphosphonates was by the \"atrane route\" where aluminum butoxide is heated with nitrilotriethanol to obtain aluminum atrane complexes and subsequently reacted with ethylene diphosphonic acid in the presence of CTAB as the SDA. This hybrid mesoporous material had a surface area of 674 m\({}^{2}\)/g with a pore size of 3.3 nm. These materials were formed on the basis of S\({}^{+}\) I\({}^{-}\) ionic self-assembly, leading to the creation of a mesoporous aluminum diphosphonate [73; 74].