Non-hydrolytic sol-gel synthesis of zirconium phosphonates with controlled mesoporosity

Abstract

We disclose the preparation of high-surface-area mesoporous zirconium phosphonates by the non-hydrolytic sol-gel reactions (NHSG) of Zr(NEt2)4 with trimethylsilylated phosphonates RP(O)(OSiMe3)2 (R = Me, tBu, Ph, OSiMe3), and bis-phosphonates (Me3SiO)2(O)P–X–P(O)(OSiMe3)2 (X = CH2, C6H4, CH2(C6H4)CH2, and CH2(C6H4)2CH2) in dry toluene under ambient pressure. Zirconium phosphonate xerogels are prepared by condensation reaction with the elimination of silylamine Me3SiNEt2. This irreversible reaction provides the amorphous xerogels and drives the formation of the porous structure. The influence of organic moieties bonded to phosphorus atoms on porosity was examined in this study. The final surface area of dried xerogels reached 720 m2 g−1 without any extra templating agent. Xerogels were characterized by 13C, 29Si, and 31P solid-state NMR to define atomic homogeneity and evidence the presence of organic phosphonate substituents and residual amido and trimethylsilyl groups. The amount of residual reactive groups was determined by gravimetric measurements and the thermal analysis (TG-DSC) method. These groups may be applied in post-synthetic surface modification, such as controlling the hydrophobic/hydrophilic properties.The catalytic properties of NHSG-prepared xerogels were tested on a model reaction of aminolysis of styrene oxide in a batch mode employing relatively bulky molecules. Products were identified and quantified by the 1H NMR spectroscopy. Catalyst performance parameters, such as selectivity, conversion, turnover frequency, and others, were exceeded compared to benchmark microporous layered zirconium phosphonate catalysts.