Search

Abstract: Under ambient reaction conditions, a one-pot acetalization of carbonyl compounds with methanol was carried out over a series of solid acid catalysts. Ion-exchanged montmorillonite samples were prepared by a metal cation (Ce3+, Fe3+ and Al3+) exchange from the activated K-10 montmorillonite. These materials have been characterized by wide-angle X-ray scattering (WAXS), solid-state nuclear magnetic resonance spectroscopy (solid-state NMR), temperature programmed desorption of ammonia (NH3-TPD) and Brunauer–Emmett–Teller (BET) analysis. The catalytic results of montmorillonites were compared with those of H–Y, Ce,Na–Y, Ce,H–Y zeolites, SiO2 and γ-Al2O3. Irrespective of the catalyst, the reaction yields the corresponding dimethoxyacetal in high yield. Comparison of the catalytic activity indicates that the montmorillonite catalysts are the most active catalysts for the reaction. The molecular size of the carbonyl substrates is one of the significant factors in determining the acetalization ability of the catalysts, and the activity of ketones follow the order cyclohexanone>4-nitroacetophenone>acetophenone>4-methoxybenzophenone>benzophenone. In the aromatic carbonyl compounds, the electron donor or acceptor properties of the substituents are also critical to the reactivity of the acetophenones and benzaldehydes. The aldehydes considered in the present investigation exhibit an activity order of 4-nitrobenzaldehyde>benzaldehyde>4-methoxybenzaldehyde, which substantiates that electron-withdrawing groups favor the reaction. The reaction time studies indicate that the montmorillonite catalysts are more resistant to deactivation, predominantly due to the superior pore diffusional properties of these materials compared to microporous zeolites. [Copyright &y& Elsevier]

Metamaterials have optical properties that are unprecedented in nature. They have opened new horizons in light manipulation, with the ability to bend, focus, completely reflect, transmit, or absorb an incident wave front. Optically active metamaterials in particular could be used for applications ranging from 3D information storage to photovoltaic cells. Silicon (Si) particles are some of the most promising building blocks for optically active metamaterials, with high scattering efficiency coupled to low light absorption for visible frequencies. However, to date ideal Si building blocks cannot be produced by bulk synthesis techniques. The key is to find a synthetic route to produce Si building blocks between 75-200 nm in diameter of uniform size and shape, that are crystalline, have few impurities, and little to no porosity. This Review provides a theoretical background on Si optical properties for metamaterials, an overview of current synthetic methods and gives direction towards the most promising routes to ideal Si particles for metamaterials., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)