Model dealumination reaction

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

The series of dealuminated samples prepared by AHFS treatment were evaluated for the catalytic decomposition of DCE, which was considered as model reactions of chlorinated VOC destruction. The results of DCE and TCE conversion as a function of of reaction temperature over Y zeolites are shown in Fig. 3. It was noted that all dealuminated samples except H-Y(d64"/o) zeolite exhibited an enhanced performance in comparison with that of the parent material. The 50% dealuminated sample H-Y(d5o%) was the most active catalyst achieving complete conversion at 350 C for DCE and at 550°C for TCE. The following order of activity for chlorinated VOC conversion was observed H-Y(djo%)>H-Y(d32%)>H-Y(di6%)>H-Y>H-Y(d64%). Hence, H-Y(dso%) zeolite showed a light-off temperature or Tso (temperature at which 50% conversion was attained) of 265°C lower than that of H-Y(d32%), H-Y(di6%) and H-Y, 280, 300 and 325"C, respectively. H-Y(d64%), however, showed a less active behaviour with a T50 value of 350 C. Unlike DCE, TCE combustion required significantly higher temperatures [24,25], T50 values were 475, 475, 500, 510 and 520°C over H-Y(d5o%), H-Y(d32%), H-Y(di6 /.), H-Y and H-Y(d64%), respectively. 

Further details on the dealumination procedures, the characterization techniques and the model reaction studies can be found in ref. (23,24). 

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