Mordenite acid-leached

Goovaerts, F., Vansant, E.F., De Hulsters, P. et al. (1989) Structural vibrations of acid-leached mordenites - determination of structural aluminum by wave-number and intensity analysis, J. Chem. Soc., Faraday Trans., 85, 3687. 

In a study on acid-leached mordenites, Vansant and coworkers found a linear correlation between the frequency of a skeletal vibration mode and the framework... 

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. 

Mordenite samples (MOR) were obtained from different sources (ref.5). Zeolites beta (BETA), mazzites (MAZ), offretites (OFF) and ZSM5 were synthesized according to procedures described elsewhere (refs.10-12). Some parent offretites, mordenites and mazzites suffered hydrothermal treatments and acid leachings to obtain dealuminated materials (refs.13,14). A wide variety of samples were thus prepared with Si/Al ratios in the range BETA 6.3 < Si/Al < 31.5, ZSM5 13.2 < Si/Al < 44, OFF 3.4 < Si/Al < 26, MAZ 2.5 ( Si/Al < 5, MOR 4.4 < Si/Al < 39.5. The protonic forms of the zeolites, e.g. HZSM5, were obtained by calcination of the ammonium forms at different temperatures. 

Commercial zeolite based hydroisomerization catalysts comprise alumina bound and platinum impregnated dealuminated mordenite. The activity and selectivity of the hydroisomerization of n-paraffins is strongly influenced by acid leaching. The influence of silica to alumina ratio has been studied for pentane isomerization over platinum mordenite many times since one of the first papers published (6). 

L.A.H. Andersson, J.G.M. Brandin and C.U.I. Odenbrand, Selective catalytic reduction of NOx over acid-leached mordenite catalysts. Cat. Today 4 (1989) 173-185. 

The framework-aluminum extracted during acid leaching is not fully washed under the conditions used in this work. Part of it remains inside the mordenitic side-pockets, partially obstructing the free exchange of gaseous molecules between the former and the main channels. 

Clearly acid leaching of mordenite can have a marked effect on properties, but many investigators claim X-ray diffraction patterns indicate that negligible structural changes occur. Adopting a structural pattern similar to... 

Figure 5. J9Si-NMR spectra of mordenite dealuminated by acid leaching and subsequent steaming. (Reproduced from ref. 19. Copyright 1986 American Chemical Society.)...

Table V. Variation of relative HRMAS Si-NMR line intensities of mordenite at various degrees of dealumination by acid leaching...

Because of the small pores in zeolitic catalysts, reaction rates may be controlled by rates of diffusion of reactants and products. Beecher, Voorhies, and Eberly (4) studied hydrocracking of mixtures of n-decane and Decalin with mordenite catalysts impregnated with palladium. They found that acid leaching of the mordenite produces an aluminum-deficient structure of significantly higher catalytic activity. At least part of this improvement appears to be caused by the decrease in diffusional resistance. They observed that with this type of catalyst, the effective catalyst pore diameter appears to be smaller than calculated owing to the strong interaction or adsorption of hydrocarbon molecules on the pore walls. 

Qeveral authors have reported on changes in characteristics of mordenites as aluminum is removed from their structure by leaching with mineral acid. Sand (6), Belen kaya et al. (3), and Frilette and Rubin (5) described the acid leaching of mordenites producing Si/Al ratios up to about 15. Beecher, Voorhies, and Eberly (2) reported on the catalytic activity of palladium supported on a mordenite which had a Si/Al several-fold greater than 5. Weller and Brauer (8) recently described catalytic experiments on mordenite leached to a ratio of about 9. 

P-08 -Transient uptake measurements using an oscillating microbalance effect of acid leaching on the diffusivity of -hexane in Pt/H-mordenite... 

Measurements performed in a tapered element oscillating microbalance (TEOM) reveal that diffusivities can be derived from uptake data monitored under full reaction conditions. In this way the effect of acid leaching on the diffusional behaviour of w-hexane in Pt/H-Mordenite is investigated. It is shown that acid leaching largely enhances the -hexane uptake rate, but does not result in a net increase in the intracrystalline steady state diffusivity. It is concluded that the accelerated uptake after acid leaching merely arises from the shorter intracrystalline diffusion path resulting from the mesoporous structure. 

The FTIR/KBr method was, inter aha, applied by Maache et al. [306] for the characterization of various samples of H-Beta dealuminated by acid leaching. In the case of framework vibrations of zeolite Beta, a good Hnear correlation was found between the wavenumber of the asymmetric O-T-O vibration, Vas(O T-o)> and the aluminum mole fraction, nAi/(nsj+nAi) [263]. Ponthieu et al. [307] attempted to find a correlation between the wavenumbers of framework vibrations of ojfretite and zeolite Omega and the degree of dealumination (aluminum mole fraction) similar to those observed with faujasites and mordenites. However, Hnear correlations were hardly obtained. 

Lewis-acid sites is available. For example in dehydroxylated or acid-leached mordenite up to three types of Lewis site are reported from calorimetric studies as mentioned earlier [16(a)]. The strongest of fliese sites, with q = 170 kJ mol is said to adsorb ammonia dissociatively. It is also clear that several types of Lewis acid could arise from the various components of dislodged aluminium, from detritus silica or even from extraframework amorphous silica/alumina [39]. 

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