# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 2 Synthesis of Porous Metal Phosphonates
### Templated Porous Metal Phosphonates
Some examples of other aluminum diphosphonates based on the atrane route protocol in the presence of different templates such as F127, P123, Brij-56 and F68 are shown in Figure 6A [75; 76]. Interestingly, mesoporous metal phosphonates have also been synthesized with the use of monophosphonates. The synthesis of tin phenylphosphonate in the presence of sodium dodecyl sulfate (SDS) as the template led to the formation of a material that displayed microporosity and mesoporousity, exhibiting a combined type-I and type-IV isotherm. This material was built-up from a layered structure consisting of tin (IV) phosphonate nanoparticles with microporosity. During calcination, thermal shrinkage resulted in the reduction in the inter-particle voids that were between 5 and 8 nm [77]. When the choice of metal was altered to zirconium and reacted with phenyl phosphonic acid in the absence of any supramolecular templates, a mesoporous material was obtained. The origin of the mesoporosity was attributed to the formation of a hydrophobic core by the phenyl groups, which led to the formation of pores with sizes of 5-10 nm [78]. Unique phosphonic linkers such as 1-phosphonomethylproline (H\({}_{3}\)PMP) have been utilized with the purpose of synthesizing mesoporous aluminum phosphonates whose pores were lined by chiral amino acids. The atrane route was utilized to prepare this hybrid material, which possessed a surface area of 422 m\({}^{2}\)/g and a pore size of 3.7 nm [79]. Another methodology that was followed to obtain mesoporous metal phosphonates without the use of templates was based on the use of a complex tetraphosphonic linker that prevented the formation of layered motifs and made it possible to achieve open structured hybrid metal phosphonates. Tetrakis-1,3,5,7-(4-phosphonatop)] adamantane (TPPHA) was one such complex phosphonic linker, which upon non-hydrolytic condensation with titanium and vanadium precursors, resulted in the formation of a hybrid mesoporous material with a surface area of 550 and 118 m\({}^{2}\)/g, respectively [80; 81]. Mesoporous metal phosphonates have also been synthesized by using polyphosphonic acids that feature a hetero-atom bridging the two or more phosphonate units. The hybrid materials synthesized with this strategy allowed to prevent the formation of a lamellar morphology and obtain interesting porous architectures. The 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP), a diphosphonate containing a hydroxy group in the bridging unit of the diphosphonic acid, was reacted with titanium butoxide in the presence/absence of \(\beta\)-cyclodextrinrin and polystyrene (PS) spheres by a hydrothermal method. This material showed no porosity apart from being macroporous due to PS spheres as shown in Figure 6C. This resulted in a low surface area around 23-25 m\({}^{2}\)/g [82]. A higher surface area was achieved by reacting the same inorganic and phosphonic acid reagents under milder synthesis conditions (room temperature) and in the absence of the PS spheres and \(\beta\)-cyclodextrinrin. The result was a hybrid porous titanium phosphonate that showed a combination of a type-II and type-IV isotherm, with a surface area of 158 m\({}^{2}\)/g. The texture of the material showed inter-particle voids with a layered structure as observed by electron microscopy [83]. However, the incorporation of soft templates like pluronic templates (F127/P123) in the condensation reaction of HEDP and titanium butoxide at room temperature, led to a material with a larger surface area of \(\sim\)377-511 m\({}^{2}\)/g, respectively. The results from the nitrogen sorption isotherm and SEM/TEM implied that the texture of the material was arisingfrom inter-particle voids in the mesopore and macropore region [84]. To enhance the surface area further, claw type polyphosphonic acids containing hetero-atoms with titanium precursors can be used, resulting in materials that form hybrid networks with pores containing unreacted P-OH groups, as shown by Figure 6B [85].