Acid zeolite catalysts Bronsted type

Ammonium zeolites are transformed into acid zeolites by the decomposition of ammonium cations the zeolite is modified to its acid form by exchanging sodium or any other charge-balancing cation present in the zeolite for ammonium, as follows [18]  

the zeolite is heated in an air flow to obtain the acid form of the zeolite, as follows  

This process should be followed by the ultrastabilization of the acid zeolite. This procedure is one of the basic operations in the industrial production of acid catalysts, consisting of controlled dealu-mination produced by thermal treatment in a water vapor atmosphere, which increases the thermal stability of the zeolite [19], 

The Physical Chemistry of Materials Energy and Environmental Applications 

FIGU RE 9.1 (a) Bridged OH group for aluminosilicate zeolites and (b) bridged OH group in the general case. 

---

Acidity has been ascribed to various combinations of Bronsted, Lewis or defect-type sites. Some people emphasize ordering as a critical property, but on the other hand, a good case can also be made for the importance of lattice defects in providing acidic hydroxyl groups. It can be argued that silica-alumina was active because of partial ordering of a disordered structure, while zeolite catalysts were active because of partial disordering of an ordered structure 41). 

The resulting LH type model builds on this concept by considering that adsorbed NH3 reacts with surface NOx species. It is noted that NH3 adsorption on Fe-zeolite systems is not inhibited by the co-adsorption of water. A simple interpretation of this key observation is that the adsorption of NH3 and H2O occur on different sites. NH3 adsorption on protonated zeolites is known to occur on the Bronsted acid sites, and that has led Tronconi, Nova, and coworkers among others to propose for vanadia-based catalysts the exchange of NH3 between two types of sites. Applying this concept for Fe-zeolite catalysts gives ... 

The effect of adding copper with regard to the acidity of ZSM-5-type zeolites applied in the reduction of NO with methanol was studied by Carvalho et al. [10] and illustrates the application of the technique to catalyst studies. The 1540 cm band in the ZSM-5 sample demonstrates the presence of acidic Bronsted sites... 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Kerr, Plank, and Rosinski reported the preparation and catalytic properties of aluminum-deficient zeolite Y materials 35). Topchieva and co-workers studied the catalytic properties of cationic forms of aluminum-deficient Y zeolites, the aluminum deficiency being effected by the H4EDTA method 36-40). They found that up to 50% aluminum removal increased both stability and cumene cracking activity maximum activity was observed at the 50% removal level. Increased catalytic cracking activity was observed by Eberly and Kimberlin for mordenites from which about 80% aluminum had been removed (. 1). Weiss et al. removed over 99% of the aluminum from a hydrogen mordenite and found the zeolite retained catalytic activity of the type induced by Bronsted acids 42). Although the initial activity of this material was lower than that of more aluminum-rich mordenites, the aging rate was markedly reduced, and in a relatively short time the aluminum-deficient catalyst was the most active. 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

---