Clinoptilolite, dealumination

Fig. 3.70 (a)Change in wavenumber of the hydroxyl vibrating in the large cavities as a iunc-tion of Si/Al ratio. From left to right A, GeX, X, Y, chabazite, L, Q, dealuminated Y, dealuminated Y, offretite, mordenite, clinoptilolite, dealuminated Y, dealuminated mordenite, ZSM—5. 

Zeolite dealumination was first reported by Barrer and Makki (1), who progressively removed aluminum from clinoptilolite by treating the zeolite with hydrochloric acid of different strengths. Subsequent dealumination studies were carried out primarily on mordenite (2-5) and Y zeolites. 

Reactions with acids. Hydrochloric acid was used in the dealumination of clinoptilolite (1), erionite (14) and mor-denite (2,3,15,92). In the case of Y zeolite, dealumination with mineral acids was successful only after conversion of the zeolite into the ultrastable form (vide infra). Barrer and Makki (1) were the first to propose a mechanism for the removal of aluminum from mordenite by mineral acids. It involves the extraction of aluminum in a soluble form and its replacement by a nest of four hydroxyl groups as follows ... 

TTigh silica zeolites attract great attention since they are characterized by relatively high thermal stability and considerable acid resistance. Physicochemical properties of high silica zeolites, despite a number of investigations, have not been sufficiently studied. The same is true for L- and clinoptilolite zeolite. The data on synthesis, structure, adsorption properties, decationization, dealuminization, adsorption heats, and other properties of the above-mentioned zeolites have been given (1-15). Results of studies of physicochemical properties of L zeolites and of natural and modified clinoptilolite are given here. 

Natural zeolite was enriched with potassium ions and dealuminized. Enrichment of exchangeable ions of L zeolites and of clinoptilolite was done by multiple treatment with 0.52V solutions of the corresponding nitrates. Decationization and dealuminization were done by treating the natural zeolite with solutions of hydrochloric acid 0.25-12.02V. The Si02/Al203 ratio increased from 8.0 to 69.5, and the CaO content decreased from 6.30 to 1.00 wt % (Table II). 

We have studied the effect of chemical modification on adsorption properties of natural clinoptilolite (18). Studies of water vapor adsorption show a decrease in adsorption for Dzegvi clinoptilolite, decationized and dealuminized on the water bath, with increased acid concentration, compared with the adsorption of the natural clinoptilolite. The main contribution to adsorption is from primary porosity. 

Dealumination and Heat of Immersion. Treatments of mordenite, clinoptilolite, and ferrierite with dilute acid in the first instance remove metallic cations and yield the hydrogen forms 4, 12, 13). Such forms also result by heating the ammonium zeolites (24). Treatment with stronger acid solutions removes increasing amounts of A1 4, 13, 26) (Table IV). The marked effect this has upon qn is shown in the table. 

Influence of Dealumination. Clinoptilolite and mordenites free of metallic cations and with Al/Si ratios which were progressively reduced gave heats of immersion which diminished with the A1 content (Table IV). Each A1 atom removed reduces the framework charge by 1, according to the reaction (4)... 

The final product is a crystalline silica rich in hydroxyl defects but having essentially the framework structure of the parent zeolite 4, 13, 30). The defects render it polar, but the electrostatic components of the heats of immersion are reduced. The extent to which smooth correlations are obtained for qn and Qh is shown in Figure 6 for clinoptilolite and mor-d ite, respectively. Equally clear correlations between A1 content and AH have been observed for CO2 m these 2 dealuminized zeolites, whereas for the nonpolar Kr molecule, AH remained virtually independent of the... 

Usually, inorganic and organic acids can be used for framework dealumination of zeolites, and the acids include hydrochloric acid, nitric acid, formic acid, acetic acid, and so on. According to its acid-resistance ability, hydrochloric acid can be used for high-silica zeolites such as mordenite, clinoptilolite, erionite, etc. We will take mordenite as an example to describe this dealumination method (Table 6.4). The first step in the treatment of mordenite using hydrochloric acid is to convert the zeolite into H-type, and further acid treatment can then enlarge the pore diameter through dealumination. After partial dealumination, the Si/Al ratio of the zeolite is increased and the heat-resistance, water-resistance, and acid-resistance abilities are enhanced. 

Because the aforementioned technique shows excellent effects for dealumination and silicon-addition, this technique has been applied to the adjustment of Si/Al ratio for various zeolites such as erionite, zeolite L, clinoptilolite, and chabazite. 

Clinoptilolite has a monoclinic symmetry and its typical formula is (Na2,K2)0.Al203.10SiO2.8H2O. Its porous structure may resemble mordenite.The dimensions of its channels are 0.75 x 0.30, 0.43 X 0.33 and 0.31 x 0.33 nm. However, the apparent pore size of the nondecationized clinoptilolite is close to that of small-pore zeolites. Its pore size can be enlarged by decationization and dealumination. 

For most catalytic applications activated zeolites are required. There are two standard routes how this activation can be achieved. Fairly well understood is ion-exchange to clinoptilolite-NH4 with subsequent release of H2O and NH3 upon heat treatment above 843 K leading to anhydrous clinoptilolite-H with Brpnsted centers [e.g. 30, 31]. Upon dehydroxylation at higher temperature the concentration of Br0nsted sites (acidic hydroxyl groups) decreases and Lewis sites are formed. Heat treatment of clinoptilolite-NH4 above 673 K leads also to partial dealumination of the framework and migration of Al to extraframework sites [32, 33]. 

Activity and selectivity of natural (pure and dealuminated clinoptilolite and mordenite) and A synthetic zeolites modified with cations of the transition metals (Cu, Co ", Cr, Zn and Pd ) have been tested in vapor phase oxidation of wo-propyl and -propyl alcohols. The efficient catalyst, CuPdH-mordenite has been selected for the oxidation of iso-propyl alcohol to acetone. It has been shown that this catalyst is not efficient in the oxidation of n-propyl alcohol. The catalyst prepared from A synthetic zeolites and containing Pd " and Cu shows the highest activity in this reaction. 

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