Zeolite surface reactions

Though as yet in its infancy, the application of laser Raman spectroscopy to the study of the nature of adsorbed species appears certain to provide unusually detailed information on the structure of oxide surfaces, the adsorptive properties of natural and synthetic zeolites, the nature of adsorbate-adsorbent interaction, and the mechanism of surface reactions. 

The basis of the demonstration can be based on already published data on the surface reaction between NOz and adsorbed organic compounds. Yokoyama and Misono have shown that the rates of N02 reduction over zeolite or silica are proportional to the concentration of adsorbed propene [29], whereas Il ichev et al. have demonstrated that N02 reacts with pre-adsorbed ethylene and propylene on H-ZSM-5 and Cu-ZSL-5 to form nitro-compounds [30], Chen et al [2-4] have observed the same nitrogen-containing deposits on MFI-supported iron catalysts. The question on the pairing of nitrogen atoms is not considered here. 

Based on the above results, ultrasonic irradiation to ion-exchanged [Pd(NH3)4]2+-zeolite powders was performed in an aqueous solution containing 2-propanol. The reduction of [Pd(NH3)4]2+-zeolite to Pd°-zeolite was confirmed by XPS analyses. However, from XPS depth analyses of the prepared samples, it was suggested that the [Pd(NH3)4]2+ complexes in the zeolite pore were not sufficiently reduced even in the presence of 2-propanol. Presumably, the reductants formed from 2-propanol sonolysis could not easily diffuse into the zeolite nano-pore (size 1.2 nm) and/or reductants undergo recombination reactions and quenching reactions with the walls. In addition, the results of XPS spectral analyses of the sonochemically prepared Pd-zeolite powders indicated that the average size of the Pd clusters on the zeolite surface is roughly estimated to be less than 1 nm and composed of several dozen Pd atoms. 

In this work, an attempt is made to show the role of the different metal species present in Co and Co/Pd based FER zeolites, using UV-Vis and H2-TPR experiments, coupled with Temperature Programmed Surface Reaction (TPSR) tests. 

Most mechanistic work has focused on chemical reactions in solution or extremely simple processes in the gas phase. There is increasing interest in reactions in solids or on solid surfaces, such as the surfaces of solid catalysts in contact with reacting gases. Some such catalysts act inside pores of defined size, such as those in zeolites. In these cases only certain molecules can penetrate the pores to get to the reactive surface, and they are held in defined positions when they react. In fact, the Mobil process for converting methanol to gasoline depends on zeolite-catalyzed reactions. 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

Nevertheless, the discussion whether the intermediates involved in the reactions of hydrocarbons over zeolite surface is the alkyl-aluminumsilyl oxonium ion or the carbocation could not be answered with these previous studies. 

To check this possibility we performed experiments with different concentrations of NaBr in the NaY zeolite. Table 2 presents the results. It can be seen that upon increasing the amount of NaBr impregnated on NaY, there is preference to formation of the cyclobutyl bromide over allylcarbinyl bromide, indicating that the relative position between the bromide ions and bicyclobutonium governs the product distribution. Hence, zeolites may act as solid solvent, favoring ionization of alkyl halides and nucleophilic substitution reactions. In contrast to liquid solvents, where solvation is mostly uniform, the zeolite surface seems to provide unsymmetrical solvation of the cations, leading to product distribution that is different from solution. 

However, some characteristics of the reactant and product (and solvent) molecules used in the reactions of fine chemical synthesis have to be considered their large size, their low thermal stability, and their polarity. The small size of the micropores limits the use of zeolites to the synthesis of relatively small molecules solutions to overstep this limitation have been found with the development of larger (mesoporous) molecular sieves and of nanocrystallite zeolites, the reaction then occurring on the large external surface. The low thermal stability of the molecules leads to operation at a low ternperamre, often in the liquid phase. Last, as will be shown in the first example (Section 12.2.3.1), the differences in polarity between reactant(s), product(s), and solvent molecules have to be considered for optimizing both the zeolite catalyst and the operating conditions. 

Alumina, silica, and the aluminosilicates, whether amorphous or crystalline in the form of zeolites, play an important role in catalysis because they are used as supports in a large number of industrial catalysts. In addition to its role in dispersing the catalyst, the support is known to play a significant part in the chemistry of the surface reactions and this is illustrated by the electron transfer mechanism described earlier (Fig. 16). For these reasons, it is important to study the adsorption of oxygen on the support itself. 

Hunger and Wang provide an account of advances in the characterization of solid catalysts in the functioning state by nuclear magnetic resonance spectroscopy. Examples include investigations of zeolite-catalyzed reactions with isotopic labels that allow characterization of transition states and reaction pathways as well as characterization of organic deposits, surface complexes, and reaction intermediates formed in catalyst pores. 

The initial reaction (1) causes no pressure increase in an isolated system but step 2 and subsequent reactions give further evolution of methane and a pressure increase. Although methane is evolved according to this scheme, the surface reaction is more complex, the gray color developed on TMS treatment indicating incorporation of carbon into the zeolite structure. 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

It is most accepted that the cracking of cumene over amorphous silica-alumina or zeolite catalysts takes place by a surface-reaction controlling mechanism. Corrigan et al (8) and Prater (9)... 

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