Processes dealumination

Preparation method. Mild acid-dealumination will generally result in a more active material than the parent zeolite due to (a) removal of amorphous materials from the zeolite channels, thus lowering the diffusion resistance for the reacting molecules and (b) generation of stronger acid sites during the dealumination process, which enhances the catalytic activity of the zeolite for acid-catalyzed reactions. However, thermal dealumination will generally result in less... 

Dealumination processes are usually used in conjunction with production of the acid form of zeolite Y for many catalytic apphcations, and zeolites A and X are in most cases no longer used in acid catalytic applications because the high amount of aluminum in their frameworks makes them difficult to stabilize using various dealumination techniques. Successful dealumination and at least partial annealing of defects, resulting from movement of silicon cations to the aluminum vacancies, can be assessed by measurement of the reduction of the unit cell size of the zeolite. This unit cell reduction is a consequence of the relative ionic radii of AF (0.54 A) and Si + (0.40A). 

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. 

This procedure can be succeeded by the ultrastabilization process, which is one of the basic operations in the industrial production of acid catalysts, consisting of a controlled dealumination process produced by thermal treatment in a water vapor atmosphere, which increases the thermal stability of the zeolite. 

In addition to the Bronsted acidity in zeolites, in these materials the Lewis acidity is present as well. According to Lewis, an acid is an electron pair acceptor, a definition which is broader than that given by Bronsted, since a proton is a particular case of an electron pair acceptor. Then, the definition of Lewis covers practically all acid-base processes, whereas the definition of Bronsted represents only a particular type of process [128], The Lewis acidity is related to the existence of an extra-framework A1 (EFAL) species formed during the zeolite dealumination process [128],... 

The results indicate that the mordenite maintains its structure however, the dealumination process which occurs in the zeolite provokes changes in the cell parameters. 

In zeolites, particularly, 29Si, 31P, and 27A1, MAS-NMR has been applied for studying the dealumination processes, determining the Si/Al atomic ratio, characterizing the isomorphic substitutions, and also for identifying reaction intermediates by in situ catalytic reactions [2,81,83-85],... 

In zeolite-based catalysts, the Lewis acidity is related to the existence of extra-framework A1 (EFAL) species formed during the zeolite dealumination process [18], It occurs frequently in zeolite activation, for example, during the calcination process... 

THE DEALUMINATION PROCESS OF ACID ATTACK AND COKING BEHAVIOUR IN ULTRASTABLE V ZEOLITES... 

The enhanced negative charge density (q4) on 04oxygen atom after breaking of an Al —O bond in Table 5 indicates that the most probable site in the second acid attack is 04 for the completion of the dealumination process. Since the 04—Al bond weakening is also significant due to the second acid attack,the 04—Al bond is broken, then the a-luminium atom (T4) is disconnected from zeolite skeleton. 

Fig. 6.7 Schematic representation of the zeolite Y dealumination process steps...

The main applications of Al NMR spectroscopy in connection with zeolitic matmals has been (anoong others) the monitoring of dealumination processes, structure elucidation (for AIPO4 and SAPO molecular sieves) and the detection of extra-framework aluminium. Extra-lattice aluminium is octahedrally coordinated and gives rise to resonance lines at about 0 ppm for zeolites and between -7 and -23 ppm for AIPO4 and SAPO molecular sieves, whereas framework aluminium is tetrahedrally coordinated and resonates between ca. SO and 65 ppm for zeolites and 29 to 46 ppm for AIPO4 and SAPO molecular sieves. 

It is thus concluded that the dealumination process of Ktihl [36] leading to equation (28) did not happen in these samples in agreement with the conclusions of Kazansky [37]. 

Compared to the NaCaA sample the slightly higher stability of the NaY zeolite is caused by the lower content of framework aluminium. With further dealumination of this framework a further stabilization would be expected. However, this effect is not observed. Also the assumption of a strong distortion of the framework after the dealumination process as the sole reason for the reduced stability is incorrect. 

Chemical stability is an extremely important characteristic of HDV s, but one that has received relatively limited mention in the literature. Most reported studies have dealt with just two sorts of environments The first are those encountered in chemical dealumination processes—strong acids, chelating agents (e.g., EDTA, or soluble silicon sources (e.g., amnionium f luorosilicate). [9,10,11] The second deals with catalytic process environaients—ammonia vapor in hydrocrackers [12,13] or vanadic acid in fluid crackers [14]. Essentially no studies directed towards the specific needs of the catalyst manufacturer are available. 

H-mordenites with various Si/Al ratios (5.9 - 16.9) have proved to be active for the SCR of NO with CH4 in the 400-600 C temperature range. However, they suffered an irreversible deactivation after an incursion at 650°C for 1 h under reaction stream, due to a dealumination process. While acid dealumination only affects the free exchange of gaseous molecules between the main channels and the side-pockets (as seen by t29Xe NMR), the aluminum extracted from the lattice of the mordenite during the SCR of NOx at 650°C (without water vapor in the feed) also hinders the diffusive transport along the main channels. 

Table 1 shows that both the dealumination process by reaction at 650 C (compare MH5.9F with MH5.9D) and the acidic dealumination decrease the volume available for the physisorption of N2 and Xe, the effect being even more marked in the case of Xe. The decrease of desorbed NH3 in TPD experiments is related to the decrease of the APV sites observed by 27A1 MAS NMR. i29Xe NMR of physisorbed Xenon, and ethylene diffusion measurements help us to better understand the deactivation phenomena. 

Klinowski et al. (7) argue that a possible reason could be the fact that octahedrally coordinated alumina created during the dealumination process are deposited with equal probabilities in the chaimels formed by eight- and twelve-member rings, respectively. During... 

The Si MAS NMR spectrum (Fig, 3) of the untreated sample shows a main signal at about - 107.7 ppm [Si(OSi)4 in the framework] and a minor one at about -101 ppm [Si(OSi)30Al in the framework]. The additional spectral intensities can be assigned to amorphous silica and amorphous aluminosilicate produced in the hydrothermal dealumination process. 

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. 

Lanthanum is known to improve the hydrothermic stability of Y-type zeolites [20]. A second attempt to stabilize the catalyst was made by preparing La-exchanged EMT-type samples. The dealumination process was stopped but this catalyst did not present any catalytic activity in NO reduction. A La-Cu-EMT-3.8-70-10 sample was also prepared. A maximum extent of reduction of 15% was measured at 500°C. This low activity in NO reduction is probably due to the very low copper content. 

The superior activity and stability of these catalysts are caused by the creation of the meso- and macropores during the 3-DDM dealumination process this greatly enhances access to the twelve-ring pores. 

The location of molybdenum sulfide clusters and M0S2 particles was estimated on the basis of the pore volume of MoSx/zeolite, which was measured by benzene adsorption. Table 3 summarizes the pore volumes of the zeolite before and after the incorporation of molybdenum sulfide clusters. The pore volume of the NaY zeolite is about 0.30 cm g and consistent with the crystallographically calculated volume (0.30 cm g ) of the supercage of NaY zeolite. The pore volumes of USY-Na(3.5) and NaY(630) were slightly lower compared with that of NaY, suggesting a partial destruction of the zeolite framework during dealumination processes. 

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