abstracts - Институт катализа им. Г.К. Борескова

OP-III-2Result and discussionThe infrared spectra recorded during the reaction for a molar ratio anisole: acetic anhydrideequal to 2 (ratio close to that used in the industrial process) are shown in fig1 (left), togetherwith the evolution of the concentration at the surface and in the gas phase versus time onstream (right). At the initial stage (time on stream 0-10 min.), the conversion of anisole ishigh and is related to the formation of PAA at the surface (ν(C=O) band at 1680 cm-1). In asecond step, a strong deactivation is deduced from gas phase analysis and is related to theformation of a secondary reaction product only observed at the surface of the solid, the ß-diketone, formed by acylation of PAA by acetic anhydride . The ß-diketone is adsorbed on thesurface under chelated form on Lewis acid sites (νC=C band at 1530 cm-1) and in interactionwith Brønsted acid sites via the carbonyl group (νCO band at 1638 cm-1).Under the same experimental conditions, the use of a beta zeolite with bigger crystallitesgives rise to a stronger deactivation and a lower activity in acylation. The IR spectra recordedduring the reaction (not shown here) demonstrate that the mechanism of the deactivation isclearly different. Although PAA is the only product detected in the gas phase, IR spectrareveal that the deactivation is related to the condensation of two molecules of PAA, givingrise to the formation of dypnone.1680 cm -11638 cm -11510 cm -1 1500 cm -1 Conversion (%)1551 cm -11530 cm -1CH 3 OC CH 3O1591CH 3O15101500C CH 2 C CH 3OO0.5 u.A12108Band areaPAA conversion (Gas phase)PAA ( 1680 cm -1 )β-diketone (1638 cm -1 )16801638155115306421700165016001550(cm -1 )150014501400135000 50 100 150 200 250 300 350 400Time on stream (min)Figure 1. Left: Infrared spectra of the catalyst during the reaction at maximum conversion(full line) and during deactivation (dotted line). Right: Anisole conversion, surfaceconcentration of PAA (1680 cm-1) and ß-diketone (1638 cm-1) during anisole acylationreaction at 393 K.The use of a lower anisole:anhydride molar ratio (1 and 0.15) shows that the nature of theadsorbed species formed during the deactivation are strongly influenced by the gas phaseconcentration of acetic anhydride: the anhydride condensates and builds up on the surface,310