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Eingeladener Vortrag
- nein (3)
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.
Molecular dynamics of 4n-octylcyanobiphenyl confined to nanoporous molecular sieves of SBA-type
(2002)
Unusual relaxation behavior of water inside the sodalite cages of faujasite-type molecular sieves
(2002)
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.
Unusual relaxation behavior of water inside the sodalite cages of faujasite-type molecular sieves
(2002)
Broad-band dielectric spectroscopy (10-2 to 109 Hz) is applied to investigate the molecular dynamics of NaY, a zeolite of faujasite structure. A dc conductivity contribution and three relaxation processes were revealed in the dielectric spectra. The relaxation process in the high-frequency range ascribed to orientational fluctuations of water molecules inside the sodalite cages is studied in detail. As a main result it is found that its mean relaxation time has an unusual saddle-like temperature dependence, which obeys neither an Arrhenius nor a Vogel-Fulcher-Tammann law. A quantitative description was given on the basis of a model recently developed by Feldman et al. (J. Phys. Chem. B 2001, 105, 1845) to analyze the dynamics of water confined in nanoporous glasses. Reasonable values for the activation energies of orientational fluctuations and of defect formation as well as for the defect concentration were obtained.
Improved large mesoporous ordered molecular sieves - Stabilization and acid/base functionalization
(2010)
The preparation of nanoporous materials with enhanced stability using an improved synthesis route using reactive inorganic silica and alumina species is reported. This way improved mesoporous molecular sieves were obtained. The synthesized aluminum substituted mesoporous molecular materials (Al-MMS) contain very large pores of 50200 Å size combined with an improved pore wall thickness. Increased wall thickness and Al substitution lead to an improved chemical stability against alkaline solution. The textural, structural and acid properties are investigated by physico-chemical methods. The catalytic performance acidic materials was tested in the benzoylation reaction; amino functionalized materials were studied in the base catalyzed Michael addition.