Structural, textural and thermal characterization of a confined nanoreactor with phosphorylated catalytic sites grafted onto a halloysite nanotube lumen

Mesoporous materials are very attractive solids for the fabrication of confined nanoreactors since they overcome the problems of diffusion/deactivation shown by microporous structures. In this work, we present the fabrication of a confined nanoreactor obtained by functionalization with phosphoric acid of the empty lumen (diameter 10–15 nm) of halloysite nanotubes (Hal). Two different halloysite nanoreactors with phosphorylated catalytic sites were prepared by the wet impregnation method followed by thermal activation: 1) phosphate modified Hal (starting with pristine Hal), and 2) phosphate-modified etched Hal (starting with etched Hal obtained by soft-etching pristine Hal using sulphuric acid). The selective grafting of hydrogen phosphate groups onto the aluminol active sites was characterized by solid-state nuclear magnetic resonance (1H, 27Al, 29Si HPDEC and 31P) and Fourier transform-infrared spectroscopies, and thermogravimetric and nitrogen physisorption analyses. Two different aluminum phosphate (Al–O–P) binding modes, monodentate (–Al–O–P[dbnd]O(OH)2) and bidentate (–Al–O)2–P(OH)2, with different distributions were observed in both phosphorylated-Hal nanoreactors. The selective functionalization of the internal lumen was confirmed because no interaction (due to Si–O–P binding mode) was detected between the outer surface of Hal and phosphoric acid. The confined nanoreactors prepared here preserved the main chemical structure and textural properties of Hal. The analysis of the specific surface area and mesopore size showed that, depending on the starting material, grafting the phosphate groups led to the formation of a monolayer or a polycondensation of phosphate moieties inside the tubular mesopore. However, in both prepared nanoreactors, phosphorylation did not result in saturating the material or pore blocking. These mesoporous materials could be used in catalysis and in situ nanoparticle synthesis.