Dealumination moderate

Degree of dealumination. Moderate dealumination generally increases the catalytic activity or leaves it unchanged, while advanced dealumination leads to a decrease in activity. Such a decrease is due to a loss of active sites with advanced framework dealumination. 

Figure 16. Dealumination of zeolite Na-Y using SiClk vapor studied by 27Al-MAS NMR at 104,2 MHz. Key a, parent Na-Y zeolite b, after treatment with SiClk and moderate washing with dilute acid and c, a sample similar to b, but after extensive washing. (Reproduced from Ref. 30. Copyright 1982, American Chemical...

Figure 4. Al-MASNMR spectra of (a) NaY zeolite (b) partially dealuminated NaY with SiCl, and moderate washing (c) same sample as in (b) but after extensive washing (40).

They found that aluminum-deficient mordenite (SiO /Al O ratio of 18), pre-calcined in a flow of dry air, is twice as active for toluene disproportionation and coke formation as compared to regular mordenite. The aluminum-deficient form also has a higher rate of deactivation. It is assumed that the formation of new, strong acid sites during the process of moderate dealumination is responsible for the observed change in activity. 

Expectedly, increasing amounts of AHFS added led to increased degrees of dealumination of the samples. For moderate dealumination levels (<50%) in which 15, 30 and 50% of the original aluminium was calculated to be removed, respectively, the reaction was stoichiometric within the experimental error. Hence, the actual dealumination degrees obtained were 16, 32 and 50%, respectively. However, for higher degrees (75%) the reaction was not complete any more since only 64% dealumination was achieved. 

Cerium-, copper-cerium coexchanged ZSM-5, copper-MCM-22, copper- and cerium-EMT type zeolite, copper-FAU type zeolite and copper-Beta exhibit an activity of the same order as that of copper-ZSM-5 in NOx reduction under simulated Diesel exhaust conditions. Propene was used as the reducing agent. The catalysts were used in a powder form and their activities tested in a fixed-bed flow reactor at a space velocity of 50 000 H . Copper-SAPO-34 and cerium- and gallium-EMT type zeolite have a moderate activity under the same conditions. The presence of water vapor inhibits the activity of EMT-zeolites. When an ageing procedure is carried out on copper-EMT type zeolite, dealumination occurs. The increase of the Si/Al ratio of the zeolite does not limit the dealumination process. The exchange of the zeolite with lanthanum prevents the zeolite from dealumination but leads to a loss of the catalytic activity. 

Most of the physico-chemical measurements (XRD, SEM, N2 adsorption, framework vibrations) show little difference between fresh and hydrothermally treated (air + 10 % H2O mixture) Cu-ZSM-5 solids. There is no clear destruction of the zeolite framework the decrease in micropore volume remains moderate and it is difficult to observe dealumination in the aged solids, even after treatment at 1073 or 1173 K. Significant changes in catalytic activity are observed, however, even after treatment at 923 K, and the activity becomes negligible after treatment at 1073 or 1173 K. 

The third kind of spectrum, pictured on Figure 7c, is found on dealuminated zeolites Y15 and Y 5 at medium and high nickel loadings. This spectrum reveals that only a small part of nickel is stabilized in the cationic sites. But the bands at 23460 cm-1 and 5500 cm-1 are inconsistent with an octahedral coordination. Such bands may be attributed to coordinatively unsaturated nickel ions. A similar type of nickel species was recently found on an alumina surface after moderate heating (16) with characteristic bands at 5830 and 23500 cm 1. 

It is well known that silanol groups are formed upon dealumination of Y zeolites. The excellent catalytic performance of moderately dealuminated ultrastable... 

Early quadrupole nutation experiments concerned the chemical status of nonframework A1 in dealuminated zeolite Y of static samples or with MAS at moderate rates [137-139,154). However, it has been shown that Al quadrupole nutation with fast (12.5 kHz) MAS and radiofrequency excitation fields up to 220 kHz provides significantly better spectral resolution [155), enabling, for example, the resonances at 0, 30, 56, and 62 ppm observed in ultrastable zeolite Y to be assigned to different aluminous species. The various quadrupolar para-... 

There is a great similarity in the catalytic transformation of ethanol and of methanol on a HZSM-5 zeolite, either concerning the reaction mechanism [4], or in the spectra of products [5,6], Nevertheless, the high content of water in the feed in the BTG process markedly influences the distribution of products and catalyst deactivation [5-7], It has been proven that water attenuates deactivation by coke at moderate temperatures [6], but causes irreversible deactivation by dealumination of the zeolite at high temperatures. 

In toluene disproportionation, two toluene molecules react to yield benzene and xylenes by transferring a methyl group from one toluene to the other. This reaction has to be done at moderate temperatures to avoid cracking and dealkylation of toluene. Therefore, reactions are mostly carried out at around 400 °C. Rhodes and Rudham [154] studied this reaction over steam-and EDTA-dealuminated Y zeolites and found initially an increase of conversion with time, followed by a maximum after 5 to 20 min. Subsequently a complete deactivation of the catalysts occurred after 10 to 25 minutes. This behavior resulted from the formation of catalytically active coke during the reaction. The maximum activity over all catalysts was linearly related... 

Removal of framework silicon from Y zeolite dealuminated with (NH4)3 [ AlFg] and virtually free of extra-framework aluminum spedes was also suggested by Liu et al. [224], who observed a gradual decrease of the framework Si/Al ratio upon treatment with 0.25 M KOH solution at temperatures between 40 and 100°C and the appearence of soluble silicate in the fluid. Moderate desiUcation (about 10%) of H-ZSM-5 upon treatment with 0.08 N NaOH at reflux temperature was observed by Lietz et al. [227]. 

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