Synthesis and structural characterization of HPW-doped niobium pillared Brazilian clay

Natural clay lamellar structure can be controlled through various treatments, such as pillaring with metallic oxides. In this work, lamellar-structured Brazilian montmorillonite was a raw material for developing a new Nb-pillared clay dopped with phosphotungstic acid (HPW). The pristine clay was exchanged with CTAB to obtain an organophilic intermediate, which was further submitted to an Nb source treatment, followed by HPW doping and calcination. XRD, textural and thermal analysis, electronic scanning spectroscopy (SEM) with EDS microscopy, Al-27 and P-31 solid-state nuclear magnetic resonance spectroscopy, and infrared of pyridine adsorption-desorption acidity properties were employed in the characterization of this new class of pillared clay. The XRD analyses showed the maintenance of the lamellar structure after the calcination step at 400 degrees C, as planes 001, 110, 004, and 130 are visible. The P-31 MAS NMR spectrum of the ARGPil-Nb-HPW sample showed a single signal centered at -14.9 ppm, the same chemical shift of the phosphotungstic acid. The absence of additional peaks confirmed the maintenance of the Keggin hetero-polyacid structure in the Nb pillared clay. The pillarization process led to a significant increase in the specific area from 58 m(2)/g to 111 m(2)/g in ARGPil-Nb-HPW, suggesting that niobium acts as a pillar of Nb2O5, covalently binding to the tetrahedral sheets, as evidenced by the FTIR spectra, and keeping them separated, which may favor catalytic processes that require more significant space for molecules to access the internal surfaces of materials and, therefore, to the Br & oslash;nsted sites and especially to the strong Lewis sites. The catalysts showed strong Lewis sites by retaining chemically adsorbed pyridine after heating at 250 degrees C. The pillared catalyst showed activity in the esterification of mandelic acid with methanol at ambient temperature, which was maintained after three cycles.