Mesoporous layered aluminosilicates prepared from protozeolitic nanoclusters: Synthesis, physicochemical and catalytic properties
Mesoporous layered aluminosilicates were prepared by a two-stage route using tetrapropylammonium hydroxide (TPAOH) and hexadecylamine (HAD). First, aluminosilicate sols were prepared from very dilute solutions under mild conditions. The sols contained aluminosilicate nanoclusters of the average size 4 – 5 nm. 27Al NMR studies demonstrated that tumbling alone of the small aluminosilicate nanoclusters due to Brownian motions was removing effectively chemical shift anisotropy, dipolar interactions and narrowing quadrupolar interactions; thus, magic-angle-spinning was not required for obtaining good quality 27Al and 29Si NMR spectra of the sols studied. Quantitative analysis of 29Si NMR spectra allowed following evolution of the signals in time, providing insight into dynamical behavior during formation of sols. Second, the sols with the Si/Al ratios of 12 – 70 were used for preparation of the X-ray amorphous aluminosilicate materials. The samples exhibited good sorption properties due to the presence of the ca. 3 nm slit mesopores, coupled with the presence of Brønsted and Lewis acid sites. The initial reaction rate in the liquid-phase isomerization of ?-pinene and the turnover frequency (TOF) were estimated at relatively low temperature of 348 K. The mesoporous aluminosilicate materials turned out to be very active catalytically; the initial reaction rate was surpassing the rate measured over the reference zeolitic and non-zeolitic materials. TOF on mesoporous materials was also high, 0.15 and 0.27 s-1, in comparison with ZSM-5 (0.02 s-1) and ferrierite type zeolites (0.06 – 0.09 s-1).