Abstract

The new desilicated forms of the zeolites X, Y and ZSM-5 were prepared by the selective removal of silicon atoms from the parent zeolites in an aqueous solution of base. This treatment, which decreases the Si/Al ratio while keeping the zeolite framework almost unmodified, enhanced the ion exchange capacities of all the zeolites, a property which can be exploited industrially for the X and Y zeolites since the desilicated forms had higher total ion removal rates of calcium and magnesium ions from hard water. Similarly, desilication results in a higher density of acid sites as shown in the following. $\sp1$H MAS NMR studies were done on anhydrous samples of desilicated ZSM-5 and have detected a signal which was not observed for the parent sample and is believed to arise from the interaction of Bronsted acid sites hydrogen bonded to framework Aluminum atoms. This corresponds to the creation of a second generation of Bronsted Acid Sites (BAS II) distinguishable from the normal bridging hydroxyl BAS (BAS I) associated with tetrahedrally coordinated aluminum in the zeolite framework. In catalytic testing studies, the desilicated ZSM-5 zeolite was found to display a higher total conversion in the dehydration of absolute ethanol into ethylene, however the selectivity for ethylene was higher for the parent zeolite at lower reaction temperatures. This was a direct consequence of the higher density of acid sites on the desilicated zeolite, which is comprised of BAS II in the presence of "unusual" Lewis acid sites of type II (LAS II). These Lewis acid sites were also detected by $\sp1$H MAS NMR during the rehydration phase and are believed to be produced by dehydroxylation of BAS II which were generated by the "desilication-healing" phenomena. LAS II are distinguishable from LAS I since the latter are easily rehydrated back to the corresponding BAS I. The nature and strength of these different acidic sites on the parent and those created on the desilicated ZSM-5 were also studied in more detail by the techniques of XPS, FTIR, NH$\sb3$ TPD, nitrogen and argon adsorption and desorption (BET), DTA/TGA and poison testing using pyridine and 2,6-dimethylpyridine. Upon steam treatment of the desilicated zeolite, both the total conversion and selectivity for ethylene improved significantly in the dehydration of absolute ethanol. Steaming was capable of rehydrating the LAS II into BAS II, a reconversion which was previously too demanding to be achieved under the conditions of catalytic testing, hence the low selectivity for ethylene at lower reaction temperatures. Further evidence of this reconversion is provided by catalytic testing with aqueous ethanol for which the desilicated zeolite achieved higher total conversions and selectivities for ethylene than the parent at all reaction temperatures, and the selectivity for ethylene was always significantly higher than was achieved with absolute ethanol. This has important implications for the Bioethanol to Ethylene Process. A cut off temperature was also confirmed for the dehydration of ethanol, at which the reaction mechanism changed from the two step conversion into diethyl ether and ethylene, into the one step, direct conversion to ethylene. These temperature ranges were determined to be 200-225$\sp\circ$C and 225-250$\sp\circ$C for absolute and aqueous ethanol respectively.