Synthesis and acid catalysis of nanoporous silica/alumina-clay composites

A novel nanoporous silicoaluminum montmorillonite heterostructured composite (denoted Si/Al-MMT) has been synthesized based on ion exchange and self-assembly techniques. Powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), N2 adsorption–desorption and Fourier transform infra-red (FTIR) spectroscopy were employed to characterize the synthetic Si/Al-MMT. The synthetic Si/Al-MMT composites exhibit a basal spacing of 3.45 nm, gallery height of 2.39 nm, a BET surface area of 502 m2/g and silt width of 2.0 nm. The thermal stability of Si/Al-MMT can reach 750 °C. Compared to silica-montmorillonite heterostructured composites (Si-MMT) prepared through the analogous procedures, the thermal stability of Si/Al-MMT was lower due to the pillar formation of mixed oxide structure. The synthetic Si/Al-MMT composites are potentially effective solid acid catalysts for the Friedel–Crafts alkylation of catechol with tert-butyl alcohol to synthesize butyl 4-tert-butylcatechol (4-TBC). Under experimental condition, the conversion of catechol is 76.8% and the selectivity of 4-TBC is 85.2% over synthetic nanoporous Si/Al-MMT catalysts. Combined with pyridine and ammonium adsorption, the results show that the Lewis acidity can be improved with the addition of aluminum to pillars. Both the acidity and pore structure have effects on the conversion of catechol and the distribution of products.