Prabir K. Dutta

Olefin oxidation by zeolite-encapsulated Mn(salen)+ complexes under ambient conditions

Zeolite-encapsulated manganese complex of the salen ligand [N,N′-ethylenebis(salicylidene-aminato)] was synthesized, and its catalytic activity toward various olefins was examined. Iodosylbenzene was chosen to be the terminal oxidant. Oxidation products, which included the epoxide, alcohol, and aldehyde, are formed from cyclohexene, styrene, and trans- and cis-stilbene. In comparison with catalysis by Mn(salen)+ complexes in solution, encapsulation lowers the rate of reaction considerably, but provides for selective oxidation based on olefin size and ability to penetrate the zeolite cages.

Storage of light energy by photoelectron transfer across a sensitized zeolite-solution interface

A COMMON strategy for storage of solar energy involves the photoexcitation of a donor molecule D followed by electron transfer to an acceptor A. To exploit this strategy in a practical context, a way must be found to impede the back-reaction in which the electron is transferred from A to D (ref. 1). Previous attempts to achieve long-lived charge separation have involved the use of D–A combinations held in well defined geometries by spacer groups2,3 or immobilized on supports such as porous media4–12. Immobilization of the redox species poses problems, however, for their subsequent separation in order to reclaim the stored energy. Here we report a system that achieves efficient D–A electron transfer, a slow back-reaction and easy separation of the products. We trap the photosensitizer donor, trisbipyridine ruthenium(II), in the supercages of zeolite Y, and use as the acceptor a neutral, zwitter-ionic viologen in the surrounding solution. Electron transfer from the ruthenium centre to the viologen is mediated by N,N′-tetramethylene-2,2′-bipyridinium ions loaded into the zeolite by ion exchange. Isolation of the donor within the zeolite from the acceptor in the solution outside makes the photochemically generated products easily accessible. Practical utilization of this trimolecular redox assembly will, however, require improvement of the quantum yield.

Strategies for total NOx measurement with minimal CO interference utilizing a microporous zeolitic catalytic filter

A strategy is presented for determining total NOx (NO þ NO2) in gas streams at temperatures greater than 400 8C. By using a Pt-loaded zeolite Y as a catalyst filter bed placed before a sensor, NOx species in the gas stream are brought to an equilibrium concentration of NO and NO2 that is determined by the background oxygen concentration and the filter temperature. The equilibrated NOx is then measured with a yttria stabilized zirconia (YSZ) sensor using a chromium oxide sensing electrode. In the absence of the filter NO and NO2 produced responses in opposite directions, while in the presence of the filter, NO and NO2 gave signals of the same magnitude and direction. By varying the temperature of the filter relative to the sensor, the signal magnitude could be increased. Another advantage of the filter is that CO is oxidized to CO2 and thus little interference of CO upon NOx was observed. By introducing a layer of the Pt-zeolite catalyst filter material directly onto a Pt electrode, a pseudo-reference to NOx was formed. This design provides the opportunity for miniaturization as well as removes the need for an air reference. # 2002 Elsevier Science B.V. All rights reserved.

Correlation of framework Raman bands of zeolites with structure

A correlation is developed between the ring sizes and average T-O-T angles present in zeolites, with the prominent oxygen-bending frequency observed in the Raman spectra of these materials. Based on this correlation, predictions are made about the structure of a synthetic, magadiite-type aluminosilicate mineral.

Structural stability of titania thin films

This paper studies the stability of thin films of nanostructured titania at elevated temperatures. Thin films of titania are intended for use in many applications, including high-temperature gas sensing devices. The initial structure of the films consists of nanocrystalline anatase (∼8nm). Exposure of the films to temperatures > 400°C results in the nucleation and subsequent rapid growth of rutile grains (final grain size > 300nm). In-situ, hot-stage experiments in the TEM were carried out, which revealed details about the nucleation and abnormal grain growth processes in this system. It is argued that coarsening of the nano-structure following the transformation is a characteristic of polymorphic reactions from metastable to stable phases. The nucleation process is a critical rate-controlling process for maintaining nanosize grains in the transformed material.

Raman spectroscopic studies of the synthesis of faujasitic zeolites: Comparison of two silica sources

In this study, we have examined the influence of two common commercial silica sources, Ludox and N-Brand, on the formation of faujasitic zeolites. The preparation of zeolite X is readily accomplished from both these sources. The Raman spectra indicate that the gel preceding the formation of crystals is composed of a solid phase with disordered four-membered aluminosilicate rings and soluble monomeric and dimeric silicate species. The Ludox preparation has a more rapid crystallization rate than does N-Brand, and zeolite X crystals are observed in the Raman spectrum at earlier times than by X-ray diffraction, indicating more extensive nucleation. Synthesis of zeolite Y was only possible with Ludox, whereas preparation with N-Brand as the silica source and identical in composition as that of Ludox led to the formation of zeolite P. Raman spectroscopy in this composition indicated that the structure of the solid phase in the initial stages of aging is different for the Ludox and N-Brand preparations of zeolite Y. At later stages, the spectra are dominated in both cases by soluble silicate species. It is possible to direct the N-Brand preparation to zeolite Y either by addition of tetramethylammonium ions or by creating conditions under which zeolite X nuclei can form in the initial stage

Decavanadate ion-pillared hydrotalcite: spectroscopic studies of the thermal decomposition process

Abstract In this study, we have focused on the thermal decomposition of the material generated by ion-exchange of decavanadate ions into the layers of a magnesium aluminum hydroxide hydrotalcite-like compound, over the temperature range from ambient to 650°C. Using powder diffraction data and Raman and XANES spectroscopy, it was found that the intercalated decavanadate ions transform into cyclic and chain-like metavanadate species at temperatures between 160–350°C. The metal hydroxide layer structure remains intact until 350°C. Reaction between the metal oxide and the metavanadate species to form magnesium vanadates is only observed for samples heated in excess of 450°C.

Mechanism of zeolite formation: Seed-gel interaction

Abstract The mutual interactions between zeolite Y seed crystals and amorphous aluminosilicate gels in influencing zeolite synthesis has been examined. Reactant compositions that result in the synthesis of faujastic- and gismondine-type frameworks is the focus of this study. A reactor system in which seed crystals and gel are separated by a 0.2 μm membrane has been used for the synthesis experiments. The membrane allows for ready passage of soluble species, while keeping the solids separate. The results indicate that the seed crystals modify the crystallization process if they are in physical contact with the gel, but do not influence it if they are kept separated via the membrane. Thus, it appears that the viable nuclei generated either from the seed or the gel do not travel readily through the 0.2 μm membrane, suggesting a minimal size of 2000 A. But, at the same time, crystal growth occurs readily on both sides of the membrane, suggesting incorporation of solution species that travel freely across the membrane.

Ceramic-based chemical sensors, probes and field-tests in automobile engines

The monitoring and control of combustion-related emissions is a top priority in many industries. The major methods used to detect combustion gases fall short of practical applications for in-situ measurements in industrial environments involving high temperature and chemical contaminants. The real challenge is not only to develop highly sensitive and selective sensors, but to maintain long-term stability in such aggressive environments. This article presents an overview of a multidisciplinary research effort in ceramic-based chemical sensors, highlighting opportunities as well as challenges. The group of sensors (CO, NOx, O2, and CO2) selected for this article can, in general, be used to determine the state of combustion in a wide variety of applications. Fabrication of sensor probes and their field-test results in automobile engines are also presented.

Sintering and Dielectric Properties of Hydrothermally Synthesized Cubic and Tetragonal BaTiO3 Powders

The sintering behavior of hydrothermally synthesized cubic and tetragonal BaTiO3 powders was studied as a function of temperature. In addition, the dielectric properties of the sintered samples were examined from room temperature to 200°C at a frequency of 1 kHz. The influence of powder characteristics and sintering temperature on the microstructure development and dielectric properties were investigated. It was found that tetragonal powders showed limited grain growth and higher final density upon sintering, whereas the cubic powder exhibited abnormal grain growth and lower density. This was attributed to more disperse and homogeneous particles in the tetragonal powders with fewer defects, as compared to the cubic powder. Correlations between the microstructure and the dielectric properties were developed. As expected, samples with finer grain size and higher density exhibited higher dielectric constant and lower loss.