Chelating dealumination

Reaction with chelating agents. Such reactions have been used primarily for partial dealumination of Y zeolites. In 1968, Kerr (8,21) reported the preparation of aluminum-deficient Y zeolites by extraction of aluminum from the framework with EDTA. Using this method, up to about 50 percent of the aluminum atoms was removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity. Later work (22) has shown that about 80 percent of framework aluminum can be removed with EDTA, while the zeolite maintains about 60 to 70 percent of its initial crystallinity. Beaumont and Barthomeuf (23-25) used acetylacetone and several amino-acid-derived chelating agents for the extraction of aluminum from Y zeolites. Dealumination of Y zeolites with tartaric acid has also been reported (26). A mechanism for the removal of framework aluminum by EDTA has been proposed by Kerr (8). It involves the hydrolysis of Si-O-Al bonds, similar to the scheme in Figure 1A, followed by formation of a soluble chelate between cationic, non-framework aluminum and EDTA. 

Aluminum-deficient Y zeolites prepared by partial removal of aluminum with a chelating agent (e.g. EDTA) also show improved thermal and hydrothermal stability compared to the parent zeolite. The optimum stability was found in the range of 25 to 50 percent of framework A1 extraction (8). However, the maximum degree of dealumination is also affected by the SiO /Al O ratio in the parent zeolite a higher ratio appears to allow more advanced dealumination without loss of crystallinity (8,25,45). Above 50 or 60 percent dealumination, significant loss of crystallinity was observed. Calcination of the aluminum-deficient zeolite resulted in a material with a smaller unit cell size and lower ion-exchange capacity compared to the parent zeolite. 

Barthomeuf and Beaumont (25) have found that the activity for iso-octane cracking remains unchanged for NaHY zeolites in which up to 33 percent of Al has been extracted with chelating agents. This corresponded to a dealumination from 56 to 37 Al/u.c. Further dealumination resulted in a decrease in... 

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. 

In solution, zeolite framework dealumination can be realized through interactions of zeolites with acids, salts, and chelates. 

It seems that dealiunination with chelating agents is essentially an acid-leach-ing process where the effectiveness is enhanced by complexing of the aluminum species formed as reaction products. The process proceeds stoichiometrically (see Fig. 2) so that it can be controlled by the amount of H4EDTA calculated for dealumination to the desired level [24,29]. 

Other acidic chelating agents, e.g., acetylacetone [30,31], tartaric add [32] and oxalic acid [33 - 35], have been successfully applied for the dealumination of zeolites. 

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