H-mordenite aluminum-deficient

TT-mordenite, which contained 11.2 wt % alumina, was dealuminated by A A heating for 4 hours at 700° C and then leaching under 6N HC1 reflux to 0.1 wt % alumina content (1) (aluminum-deficient H-mordenite). The intrinsic cracking activities of the parent H-mordenite and acid-leached H-mordenites are proportional to aluminum content (2) (i.e., the associated Bronsted acid centers). The deactivation rate of the aluminum-deficient H-mordenite is much lower than that of the parent H-mordenite. 

Selectivity as a Function of Alumina Content. At a reaction temperatures of 204° and 343°C, Csicsery (7, 8) found that the major reaction of o-ethyltoluene over H-mordenite was isomerization to the m- and p-isomers. Results of our study, listed in Table I and plotted in Figure 7, show that this is also so at 360°C. Isomers are the major reaction products over both the parent H-mordenite at 360°C and aluminum-deficient H-mordenite at 360°-505°C. At the higher temperatures, H-mordenite cracking selectivity becomes predominant. 

Figure 7 shows that dealkylation using the parent H-mordenite before deactivation occurred to a much greater degree than when using aluminum-deficient H-mordenite, which behaved as an isomerization catalyst. 

A comparison of the (CnH2n+i)+ and (CnH2n i)+ ion series obtained near 360° C from both parent and aluminum-deficient H-mordenite treated with cumene near 360°C is shown in Figures 4 and 5. Aluminum-deficient H-mordenite which did not deactivate apparently desorbs a smaller quantity of material than did parent H-mordenite. 

The activity of parent H-mordenite is essentially Bronsted activity, and aluminum sites are effective proton counters (2). In parent H-mor-denite (11.2 wt % alumina), acid centers are present at a density of 100 times those in aluminum-deficient H-mordenite (0.1 wt % alumina). 

On heating deactivated parent H-mordenite (80 to 33% cumene conversion), quantities of desorbate are so low (30.6 g/gram catalyst) that the desorbable deactivants, and hence the catalyst activity, must be at the pore mouth in the deactivated material. Non-desorbable polynuclear aromatics fill the mordenite tube. On the other hand, aluminum-deficient H-mordenite did not deactivate significantly for the same cumene treatment. Activity of this catalyst could be throughout the tube, but because of the disperse nature of the alumina sites, the high activity of parent H-mordenite, only active at its mouth, is not approached. 

Karge and Ladebeck (90) studied the alkylation of benzene with olefins over aluminum-deficient, beryllium exchanged mordenite and found a considerable extension of the lifetime of the catalyst, as compared to H-mordenite. The authors were able to carry out quite efficiently the alkylation reaction as well as the transalkylation of ethylbenzene at relatively low temperatures. 

The differential heats of adsorption on Bronsted sites and Lewis sites (cationic species) are not easily comparable [53]. For the former it is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of ammonia, while for Lewis sites the differential heat of adsorption represents the energy associated with the transfer of electron density towards an electron deficient, coordinatively imsaturated site, and probably an energy term related to a relaxation of the strained smface. Micro calorimetric studies of several zeohtes (H-mordenite, USY, H-ZSM-5) treated in such a way as to contain a noticeable amount of extra-framework aluminum have shown that the distribution of the sites with respect to the differential heats of NH3 adsorption is exponential for the Lewis sites (Freund-lich isotherm) and hnear for the Brbnsted sites (Temkin isotherm) [53]. 

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