# Advances and Challenges in the Creation of Porous Metal Phosphonates
## 3 Applications
### Catalysis
Similarly, the enantioselective addition reaction to obtain optically active phenylpropanol has also been catalyzed by hybrid titanium phosphonates [111]. Zirconium phosphonates were employed as a solid-acid catalyst in the synthesis of methyl-2,3-o-isopropylidene-\(\beta\)-d-ribofuranoside. The hybrid material was compared alongside commercial acidic resin, NKC-9 and concentrated hydrogen chloride. The reaction was carried out at 70\({}^{\circ}\)C and yielded 28% after 2 h when using the hybrid zirconium phosphonate, which levelled off at 35.6% after 3 h. With the commercial resin NKC-9, the yield of 21.1% and 33% was reached after 2 h and 3 h of reaction, respectively. Thus, the hybrid zirconium phosphonate performed similar to the commercial resin even though the resin had substantially higher proton concentrations [112]. In another example, porous zirconium phosphonates were evaluated as a solid-acid catalyst for the esterification of acetic acid with cyclohexanol as well as for the hydrolysis of ethyl acetate in aqueous media. The results were compared with commercial resin NKC-9 and ZrPO\({}_{4}\). After 10 h of reaction between acetic acid and cyclohexanol at 100 \({}^{\circ}\)C, the NKC-9 showed the highest yield of 87.9% followed by the zirconium hybrid phosphonates synthesized with HEDP, ATMP, and EDTMP phosphonic acids providing yields of 76.8%, 65.4% and 75.3%, respectively. The inorganic ZrPO\({}_{4}\) gave a yield of 72.9% under similar reaction conditions. However, the TOF (measured at 100 \({}^{\circ}\)C) for the hybrid zirconium phosphonates based on EDTMP, HEDP and ATMP were 310.2, 234.0 and 306.1, respectively. These TOF were higher than those of the commercial resin, NKC-9 (118.7) and the inorganic ZrPO\({}_{4}\) (210.1). As a side note: these reported TOF values were calculated based on the products formed, hence their activity calculation also includes the occurring diffusion/adsorption effects that might be rate determining. Moreover, the authors seem to have calculated turnover number (TON) according to the definition of [PERSON] et al., 2012 rather than TOF values [113]. The acid content of the hybrid zirconium phosphonates was 1.92, 1.25, and 1.42 mmol/g for the HEDP, ATMP and EDTMP-based hybrids (NKC-9 = 4.33 and ZrPO\({}_{4}\) = 2.03 mmol/g), respectively. The higher TOF values for the hydrolysis reaction in the presence of hybrid zirconium phosphonates was attributed to higher specific surface areas and the surface hydrophobicity of the catalyst. When the same hydrolysis reaction of ethyl acetate was performed in aqueous media, the catalytic activity per acidic proton was 51.1, 21.5, 30.6, 35.2, and 15.1 mmol/mol\({}_{\text{acid}}\)/min for Zr-EDTMP, Zr-HEDP, Zr-ATMP, NKC-9 and inorganic ZrPO\({}_{4}\), respectively [114]. In another example, hybrid iron (III) phosphonate, created by the reaction of iron (III) chloride with BTP precursors, was employed as a solid-acid catalyst for transesterification reactions. Amongst many reactants, ethyl cyanoacetate showed the highest conversion of 88.3% with a selectivity of 100% (at 60 \({}^{\circ}\)C, 6 h). It was observed that as the electron withdrawing effect (CN \(>\) C(=O)CH\({}_{3}\)\(>\) Cl) of the ester reactant decreased, the conversion to the product decreased. The electrophilicity of the carbonyl carbon of the ester reactant was attributed as the main driving force for the reaction. Negatively charged free phosphonic acids in the framework of the hybrid material could prevent molecules enriched with p-electron clouds entering the porous channels. Thus, providing the catalyst with selectivity for the transesterification of esters that contain fewer mobile electrons [115]. Next to the phosphonate moiety discussed in the previous paragraph, the organic group and metal center can also play an important functional role in catalysis.