H-Beta zeolites with various Si/Al ratios have been prepared for the liquid-phase benzylation of various arenes with benzyl chloride (BzCl). 29 Si and 27 Al MAS NMR spectroscopy revealed the incorporation of Al into the silica framework to form catalytically active Brønsted acid sites (BAS). 1 H MAS NMR spectroscopy investigations demonstrated the BAS density increased with reducing Si/Al ratio, while the BAS strength slightly decreased as probed by CD3CN molecules. These H-Beta zeolites are highly selective to desired monobenzylation products depending on the shape-selectivity induced by the suitable channel system. The catalytic performance is in line with the nucleophilicity and proton affinity (PA) of arenes (xylene > toluene > benzene) on the same catalyst, typically for Friedel-Crafts reaction. A shape-selective effect has been observed to dominate the reaction, lowering the performance of mesitylene compared to xylene, as well as being selective to only the monobenzylation product.The benzylation performance is significantly enhanced with increasing both the BAS density and strength of H-Beta zeolites due to the formation of more aryl cation intermediates by attacking electronegative chlorine atom in BzCl. An alternative reaction mechanism based on the activation of arenes by protonating aromatic ring of BAS is proposed to explain the superior benzylation activity of BzCl with less active arenes.
The influence of size of the scatterer on the image contrast for polarization-based range-gated imaging in turbid media is investigated here by Monte Carlo method. Circularly polarized light would be more efficient to eliminate the noise photons for both the isotropic medium as well as the anisotropic medium, as compared with linearly polarized light. The improvement in contrast is pronounced for isotropic medium using either linear or circular polarization. The plausible explanations for these observations are also presented.
A compact static birefringent imaging spectrometer (BIS) with spectral zooming capability is presented. It based on two identical Wollaston prisms and has no slit. The most significant advantage of the BIS is that we can conveniently select spectral resolution to adapt to different application requirements and greatly reduce the size of the spectral image data for capturing, saving, transferring, and processing. Also, we show this configuration blend the advantage of a grating spectrometer and a Michelson interferometer: extremely compact, robust, wide free spectral range and very high throughput.
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