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Composites containing 4-n-octyl-4?-cyanobiphenyl (8CB) either confined to nanopores of molecular sieves with very large pores or coating silica nanoparticles of aerosil type at high silica-to-8CB ratios are investigated by IR spectroscopy. Band shape analysis was performed in wavenumber regions in which the peaks due to CN stretching, CH stretching and CH out-of-plane vibrations appear. Some of molecules confined to molecular sieves show spectroscopic features characteristic to a bulk-like 8CB matter located in the centre of the pores or in the inter-grain space. Other features of the IR spectra are due to 8CB molecules located in the surface layer, mostly forming hydrogen bonds between their CN groups and surface OH groups. Another part of the 8CB molecules in the surface layer may also interact by ? electrons of the aromatic rings. Hydrogen bonding is less hindered for the molecules of the surface layers onto aerosil particles than inside pores of the molecular sieves. Comparison is also made with the case of composites based on molecular sieves with small pores.
Thermal analysis measurements were applied to study the behavior of composite systems containing a nematic liquid crystal (LC), 4-n-octyl-4?-cyanobiphenyl (8CB) and aerosil particles. The silica density ?s (g of silica per 1 cm3 of LC) was varied from 0.20 to 12 g/cm3 in order to approach the monomolecular structure of the surface layer. Thus, thermogravimetric (TG) measurements were performed to get information on the LC content of the samples. Moreover, they allowed an unusual application, to characterize the interaction of 8CB molecules with aerosil surface because the corresponding curves show multistep processes the temperatures of which depend on the silica density. Differential thermal analysis (DTA) also indicates several processes. Some of them, mostly due to desorption, are endothermal. Other processes are exothermal, and these are assigned to chemical transformations. The sample with silica density of 12 g/cm3 has no endothermal desorption peak at low temperatures, and has the highest shift for the DTA peaks. It was supposed that its surface layer has a monomolecular structure. For representative samples, TGDTA studies were supported by FTIR investigations. Thus conventional TGDTA techniques have provided new tools to characterize the nanocomposites. Differential scanning calorimetry gives the behavior of the phase transitions. Samples with low silica density exhibit a depression of the phase transition temperatures characteristic for the bulk 8CB while those with high silica densities do not show any phase transitions.
The influence of the aluminum contents of MCM-41 (Si/Al ratio varied between 2.7 and 69) on the coordination of Al, on the acidity, and on the catalytic properties is studied by 27Al MAS NMR, temperature programmed desorption of ammonia, and the conversion of acetone. Based on TPDA results, an assignment of the peaks of desorption of ammonia is proposed. With low Al contents, the concentration of strong Brønsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al contents, however, the number of strong acid sites decreases again. Besides usual strong Brønsted sites, TPDA reveals the existence of weak Brønsted and Lewis sites and of Lewis sites of a high acidic strength. Separation between the two types of sites of weak acidity is incomplete.
In the temperature programmed decomposition of NH4-exchanged MCM-41 samples, strong Lewis acid sites temporarily re-adsorb ammonia generated by the thermal decomposition of the NH4+ ions located at the Brønsted sites. Ammonia re-adsorption at Lewis sites results in a shift of the maximum of the TPDA peak to a higher temperature. Calcination of samples leads to the formation of strong Lewis sites at the expense of Brønsted sites. To evaluate the acidity of Al-MCM-41, recording of decomposition profiles has been extended to Ga- and Fe-MCM-41.
Catalytic activity in the conversion of acetone reaches its maximum with the sample based on Al-MCM-41 with the molar Si/Al ratio of 6.85. Highly favored formation of isobutene points to a comparably high acidic strength of the active sites.
This work presents some results obtained by confining octylcyanobiphenyl (8CB) to the pores of two SBA-15 type host materials: a silica SBA-15 (pores of 10.2 nm) and an aluminum containing AlSBA-15 (pores of 7.5 nm). The LC molecules were loaded inside the pores from a solution in acetone, in a percentage higher than 70% from the maximum possible loading. It was observed that the 8CB molecules confined to molecular sieves may preserve the phase transitions characteristic for the bulk LC provided that the confining pores are sufficiently large, as offered by SBA materials. However, present guest-host interactions play an important role in these phase transitions. Such a behavior is at variance with the 8CB confined to Controlled Porous Glass with the same pore dimensions, for which the bulk-like phase transitions can be always observed.
Molecular dynamics of 4n-octylcyanobiphenyl confined to nanoporous molecular sieves of SBA-type
(2002)