Delaminated zeolite precursors as selective acidic catalysts

Abstract

Nature © Macmillan Publishers Ltd 1998 8 letters to nature NATURE | VOL 396 | 26 NOVEMBER 1998 | www.nature.com 35360% of the electrons are actually injected into state 3. If we assume that the remaining 40% is then in level 4, and that a w~l 2 , a dependence characteristic of free-carrier absorption, we obtain a theoretical estimate for J 41 th of 3.8 kA cm -2 . The discrepancy with the experimental value probably originates from electrons in higher states of the upper miniband (see also the data of Fig. 4) and from direct scattering from the injector to the lower miniband, which leads to non-unity injection ef®ciency into 3 and 4. At high bias, the doping charges in the injector are no longer suf®cient to keep the superlattices free of electric ®eld. Therefore, the superlattice translation and re¯ection symmetries are spoiled and transitions which are suppressed in an ideal superlattice can acquire large dipole matrix elements. In particular, 4±2 (which is diagonal in k-space) is the one responsible for the l 7:3 mm laser emission.More information on the actual behaviour of our structure can be obtained by analysing the luminescence spectra (Fig. 4). Three peaks with a width of ,15 meV are clearly identi®ed, in correspondence with the energy of each of the lasing transitions. Although the relative intensity of the peaks depends on the wavelength response of the experimental set-up, it is evident from Fig. 4 inset that the intensity of the 4±2 peak increases with current density more rapidly than that of the other peaks, which we expect in view of its oscillator strength increasing (and the other two decreasing) with bias. Emission at larger energies (>220 meV) is also detected, due to transitions from states higher in the miniband.We note that a complete localization of the states over one or two wells induced by the penetration of the electric ®eld 24 would give rise to completely different spectra, and is not compatible with the smooth and monotonic current±voltage curves we observe. Furthermore, the density of carriers stored in the upper miniband at threshold is estimated from the lifetimes to be ,6 3 10 10 cm 2 2 , far less than the sheet density of doping in the injectors, which ensures negligible space-charge electric ®elds caused by the injection. To estimate the threshold for lasing on the 4±2 transition, we cannot use equation (1), because the population n 4 in level 4 is, to a ®rst approximation, locked to its value at the threshold for the 4±1 transition. The threshold for 4±2 lasing is instead reached by a different mechanism (laser action by oscillator strength tuning 17 ). As the penetration of the electric ®eld into the superlattice increases, the dipole matrix element z 42 is enhanced until the gain equals the optical losses (laser threshold condition). Roughly speaking then, J 42 th should be determined by the condition that the applied voltage is such that z 42 < z 41 , which occurs at 7.3±7.4 V, in fair agreement with the measured 7.7 V.M Figure 3 Comparison of the ®rst-order kinetic rate co...

Towards an industrial synthesis of diamino diphenyl methane (DADPM) using novel delaminated materials: A breakthrough step in the production of isocyanates for polyurethanes

“…As has been shown elsewhere, the accessibility of acid sites in zeolitic materials can be drastically enhanced by delamination of a given structure [27][28][29][30]. The exfoliation process can produce single layered structures with large external surface areas (≥300 m 2 g -1 ) and very little, if any, microporosity.…”

“…ITQ-2, ITQ-6 and ITQ-18 catalysts have been prepared by expansion and subsequent exfoliation of the corresponding laminar precursors, respectively, of the MWW structure, Ferrierite (FER) and Nu-6 [27][28][29]. In the case of ITQ-18, two samples of this material were prepared with different levels of delamination.…”

One-pot synthesis of nano-crystalline MCM-22

“…Accordingly, it has been assumed that alkylation takes mainly place over Brønsted acid sites (BAS) located at the external surface of MCM-22. In agreement with this, increasing the external surface by delamination improves the catalytic performance in alkylation reactions [5,16]. Delaminated MCM-22 (ITQ-2) consists of single MWW layers and contains a high concentration of surface BAS, which are accessible to relatively bulky molecules [5].…”

“…The nano-crystalline MCM-22(12) exhibited better stability than the other MCM-22 materials, although the rate of deactivation is higher as compared with ITQ-2. The higher catalytic stability of ITQ-2 compared with bulk MCM-22 has been explained by the larger amount of BAS accessible to benzene at the external zeolite surface [5,67,68]. The deactivation of MCM-22 is usually attributed to carbonaceous deposits that cover these BAS at the external surface [69][70][71].…”

An Overview of Zeolite, Zeotype and Mesoporous Solids Chemistry: Design, Synthesis and Catalytic Properties

“…Corma et al reported on the delamination of a zeolite precursor with a clay-type layered structure, resulting in zeolite sheets with a layer thickness of around 25 Å . [79] The layers consist of a hexagonal array of 'cups' with a 10-membered channel system running through the sheets. Clearly, all (framework-related) acid sites are accessible to substrate molecules which would be too large to fit in the channels of a corresponding three-dimensional material.…”