# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 3 Applications
### Catalysis
Porous tin phosphonate made from tin(IV) chloride and pentaethyleneexamine-octakis-(methylphosphonic acid) hexadecenoic sodium was employed as Baeyer-Villiger catalyst for the oxidation of cyclohexanone in the absence of peroxides as an oxidant. The product of this reaction was the formation of adip acid with a yield of \(\sim\)74%. When the hybrid material was replaced with an inorganic tin(IV) phosphate, the yield was only of the order \(\sim\)22%, suggesting the key role played by the organic group providing free -NH\({}_{2}\) groups in the porous framework, aiding in stabilizing the keto-enol tautomerization. In addition, the enol form is also stabilized by the Sn-centers. Furthermore, the Sn-center was found to activate the molecular oxygen and helped to form the cyclic six-membered transition state compound that later rearranged to form a cyclic ester as a second intermediate [116]. In another example, hybrid tin(IV) phosphonates synthesized from tin (IV) chloride and BTP were used as catalysts for the synthesis of 1,4-dihydropyridines from 1,3-dicarbonyls and ammonium acetate under solvent-free conditions (Hantzsch ester reaction). It was observed that the 1,4-addition was selectively facilitated over the 1,2-addition to the unsaturated C=O compound in the presence of the hybrid tin(IV) phosphonate catalyst. The authors attributed this to the adsorption of unsaturated C=O to the hydrophilic surface of the porous framework containing Sn-centers. Since there were no free C=O groups, the enamine exclusively attacked the C=C bond resulting in the formation of only 1,4-dihydropyridine as illustrated in Figure 9A [117]. Alternatively, porous molybdenum(V) phosphonate made with MoCl\({}_{5}\) and BTP was employed as a catalyst for the one-pot synthesis of 2-aryl-benzimidazoles at room temperature by the condensation of o-phenylenediamine and substituted aromatic aldehydes (yield for 2-phenyl benzimidazole = 93%). When the same reaction was catalyzed by pure H\({}_{3}\)PO\({}_{4}\) and MoCl\({}_{5}\), lower yields of 53% and 40%, respectively, were obtained. Therefore, the presence of vacant d-orbitals of the Mo metal centers along with the free P-OH groups were correlated to the activation of the C=O group of the aldehydes [118]. Fenton-like heterogeneous catalysts from hybrid cobalt phosphonates, made from CoCl\({}_{2}\) and DTPMP, were employed for the catalytic oxidation of methylene blue (MB). The optimal pH for the degradation reaction was around \(\sim\)7, and the complete degradation of MB occurred within 25 min, with a degradation rate of 0.136 min\({}^{-1}\). Decreasing the pH to 5 and 2 resulted in a notable deterioration of the MB degradation rate of 0.0331 and 0.0203 min\({}^{-1}\), respectively. Similarly, at higher pH \(\sim\)9, the degradation reduced to a rate of 0.0931 min\({}^{-1}\). Under acidic conditions, electrostatic repulsion between the positively charged hybrid framework and MB molecules prohibited effective adsorption. Under alkaline conditions, the lower degradation was attributed to the fact the sulfate radicals generated in the reaction system were quenched by the OH\({}^{-}\) ions and possibly also by the formation of Co(OH)\({}_{2}\) that might form on the surface of the hybrid cobalt phosphonates [119]. In another example, hybrid iron phosphonate was employed as a Fenton-like catalyst for the catalytic oxidation of cyclohexanone to adip acid in an aqueous medium at 75 \({}^{\circ}\)C for 10 h under oxygen atmosphere and in the absence of peroxide. The conversion to adip acid was determined to be 72% with a selectivity of 96%. The hybrid iron phosphonate catalyst had comparable results to other Fe-based catalysts with the important difference that other classical heterogeneous Fenton catalysts required hydrogen peroxide as the oxidant (Figure 9B). The authors explained that the Fe(II) sites are responsible for the activation of molecular oxygen, in contrast to the strong oxidizing capability of Fe(III) [120].