Influence of the solid-state modification of MCM-22 with Cs on toluene disproportionation

The catalytic performance of MCM-22 modified with Cs to different extents by Solid-State Ion Exchange has been investigated in toluene disproportionation. It was suggested that protons on the external surface were preferentially replaced by Cs cations. It was observed that the Cs modifications did not provide improved p-xylene selectivity at conversions close to that of the H-form. The remaining unaffected proton sites in the 10 MR channels are considered as responsible for secondary xylene isomerization proceeding upon toluene disproportionation. This revised version was published online in June 2006 with corrections to the Cover Date.     

A study in the developing region of square jet

The developing region of a turbulent square jet is investigated using high-resolution particle image velocimetry (PIV). The mean velocity and turbulence stresses are presented in various horizontal planes, along the jet centerline covering the initial region of the jet as well as the transition to the self-similar region. To study the flow structure away from the central plane, velocity measurements in two additional horizontal planes, one located halfway from the jet central plane toward the edge and the other at the edge of the square jet, are also examined. Analysis of the instantaneous velocity fields reveal the presence of an arrow-like feature in the square jet due to the higher instability generated in the jet shear layer compared with a round jet. To elucidate the imprints of the vortex structures present in the jets, a swirling strength-based vortex identification methodology is applied on a large ensemble of instantaneous velocity fields. Statistical analysis of the number of vortex cores, and their size and rotational strength in the measurement plane is undertaken. Vortex population at the edge was found to be very different compared with that in the central plane.     

Activity and selectivity of zeolite MCM-22 catalysts in the disproportionation of toluene

Zeolite precursor MCM-22 samples were prepared by hydrothermal synthesis. The H-zeolites, obtained from the precursors, were characterized by scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, and by their activity in the title reaction. Despite of the lower Br?nsted acid site concentration of the zeolite having smaller crystallites, showed higher activity, while the zeolite, having larger crystals were more selective for p-xylene formation.