Hydrogen forms of zeolites

Barrer first described the preparation of hydrogen forms of zeolites by oxidative degradation of ammonium zeolites (12)... 

The chemistry and structure of the hydrogen form of zeolite Y have been thoroughly investigated 82) and are not considered further. The structure of the dehydroxylated zeolite proposed by Uytterhoeven, Christ-ner, and Hall 15) remains unchanged. Recently Ward, on the basis of infrared studies, suggested that this form may be amorphous 27). The extreme instability of dehydroxylated zeolite Y to moisture complicates detailed study 19). The elucidation of the detailed nature of this material lies in the future. At present, completely dehydroxylated Y is little understood and presents a challenging void in our knowledge of the nature of ammonium zeolite Y thermal decomposition products. 

The hydrogen forms of zeolite X and Y are generally more active, but less selective, than their cation-exchanged forms. Side reactions include hydrogen transfer (resulting in the formation of coke and paraffinic products), double-bond migration and disproportionation. 

Hence, the hydrogen forms of zeolites should contain only several definite types of structural hydroxyls. In addition, each of them should predominate for a distinct crystal structure and chemical composition (Si/Al ratio). The exceptions include high-silica-containing zeolites, where the presence of only OH-I groups should be anticipated. 

Solid-state ion exchange does not occur only with the hydrogen form of zeolites but also with the sodium form. Thus, a solid-state reaction between LaCF and Na-Y has also been observed (Karge 1994, Karge et al. 1994). 

Table 6. Hydroxy stretching frequencies in hydrogen forms of zeolites and their proposed assignments ...

Solid-state reactions between halides and hydrogen forms of zeolites were also observed by Clearfield et al. [13] using ESR spectroscopy. For a long time, there was essentially no further activity in the field of solid-state ion exchange in zeolites. However, such studies were resumed more recently by several research groups. 

Similar to solid-state ion exchange with halides, exchange between oxides and hydrogen forms of zeolites can be monitored by TPD-MS of the water evolved upon reaction of the oxides with protons [42]. 

Another approach is to employ probe molecules such as pyridine. Pyridine attached to different cations gives rise to IR bands in the region from 1430 to 1460 cm that are indicative of the nature of the cations [32]. This enables IR spectroscopy to be employed not only when the exchange with hydrogen forms of zeolites is investigated, but also when the starting zeoHte material contains one kind of cations which should be replaced by another. This is dealt with in Sects. 5.2.6 and 5.3.2 where the exchange of Na+ by La + and Cu+, respectively, is discussed. Historically, the IR method was used in the early experiments by Rabo et al. [16,17] mentioned in Sect. 1. 

The relationship between the lattice energy and the reactivity for SSIE of metal compounds in mixtures with zeolites was generally confirmed by the work of Weitkamp and co-workers (cf. [44,45] and Sect. 5.3) concerning the systems of noble metal chlorides/hydrogen forms of zeolites. It should be noted, however, that such a relationship was not foimd in SSIE experiments with Mn oxides as compounds of the in-going cation [46]. 

Solid-state ion exchange is usually conducted at higher temperatures and, if metal halides and hydrogen forms of zeolites are reacted, occurs in the presence of hydrogen halides. Therefore, the question may arise as to whether the integrity of the zeolite structure has been preserved when solid-state ion exchange was carried out There are several ways to check whether the crystallinity of the zeolite material has deteriorated during the solid-state reaction. 

SoHd-state ion exchange between alkali haUdes and hydroxyl groups of the hydrogen form of zeolites was also monitored by infrared spectroscopy. An early example is taken from the study by Rabo et al. [16,17] mentioned earlier and is reproduced in Fig. 8. 

IR studies were also carried out with mixtures of alkaline earth salts and hydrogen forms of zeolites. As an example, IR spectra of H-MOR prior to and after solid-state reaction with CaCl2 are shown in Fig. 12 (cf. [41]). 

Figure 16 displays the TPE profiles [78] obtained during temperature-programmed heating of a finely dispersed sample of the parent zeolite, NH4-Y, (for comparison m/e = 16, deammoniation m/e = 18, dehydroxylation) and a mixture of LaCl3 7H2O with the ammonium (hydrogen) form of zeolite Y (for solid-state reaction m/e = 16, deammoniation m/e = 18, dehydration m/e = 36, evolution of HCl). 

As early as in the work by Clearfield et al. [18] in 1973, solid-state reactions of Fe ", Co ", Ni + and Mn " chlorides with ammonium (hydrogen) forms of zeolites A, X and Y were studied to demonstrate the phenomenon of SSIE. Those authors monitored the reactions through titration of HCl evolved. More recently, interest in zeolites containing these transition metals, especially cobalt and... 

In order to introduce vanadium into hydrogen forms of zeolites (H,Na-MOR,H-MOR with nsi/nAi = 5 and H,Na-ZSM-5, H-ZSM-5 with nsi/nAi = 35), mixtures of V2O5 and the zeolites were subjected to heat-treatment at 1073 K in air [92,190, 191 ]. Electron spin resonance spectroscopy (ESR) yielded a spectrum (as shown for the example of H-ZSM-5 in Fig. 52) exhibiting a well-resolved hyperfine (HF) signal ofvanadyl cations with g = 193,gj = 2.02,A = 19.8 mT,and Aj = 8.3 mT. These parameters are typical of isolated cations in an almost square-planar coordination. 

Similarly, the catalytic behavior of hydrogen forms of zeolites in methanol conversion was completely changed upon loading with 263 via RSSIE. Instead of acid-catalyzed selective dehydration to olefins, dehydrogenation became predominant resulting in products such as H2, CO, CO2 and CH4 [260]. 

Those involving only the removal of framework aluminum by chemical agents or - in case of hydrogen forms of zeolites - by thermal dehydroxylati-on, thereby resulting in lattice defects. 

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