Ferrierite FER

The T-O-T vibrations of cobalt-exchanged ferrierites were monitored by the FUR/KBr-technique in a work by Sobalik et al. [265] who attempted to evaluate quantitatively the intensity of the main band at 1070 cm and, thus, to determine the munber of Co + ions at cationic sites. 

A study on vibrational spectra of the frameworks of natural chabazite by Raman scattering and infrared reflection spectroscopy was conducted by Pechar and 

Rykl [266]. The experiments provided information about the symmetry and the dynamics of the framework and the nature and strength of bond-types in the structure. Also, in investigations of natural zeolites such as natural chabazite the method of Flanigen et al. [112] was adopted [267]. 

Using the IR/KBr pellet technique, Ernst and Weitkamp [275] obtained framework vibration spectra of ZSM-35 (cf. also [276]) and, for the st time, ZSM-57 (MFS). Both zeolites have two characteristic absorbances around 1230 cm, which were assigned to vibrations of five-membered rings. While ZSM-5 exhibited a single band at about 600 cm, ZSM-57 showed a doublet. The additional band was attributed to the presence of four-membered rings in the structure of ZSM-57. 

Sinha et al. [281] investigated the framework vibrations of silicoalumino-phosphate SAPO-39 and of magnesium aluminophosphate MAPO-39 by DRIFT spectroscopy and interpreted them according to the approach of Flanigen et al. [112]. The OH stretching region was studied by FTIR transmission spectroscopy (vide infra. Sect. 5.4.1). 

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The main objective of the present work was to investigate the possibilities of direct (and selective) n-butane dehydroisomerisation into isobutene over Ga-containing zeolites. Another objective was to evaluate the role played by Ga and acid sites in this reaction. For this work such medium pore zeolites, as ferrierite (FER) and theta-1, were chosen because of their superior performance in n-butene isomerisation reaction.3,7 The modifying metal, Ga, was chosen due to the known high dehydrogenation activity of Ga-ZSM-5 catalysts in propane and n-butane conversions. 10 However, Ga-ZSM-5 catalysts were not used in this study because of their high aromatisation activity,8,9 which would not allow to stop the reaction at the stage of formation and isomerisation of butenes. 

Lo et al. used plane wave calculations to examine methanol coupling near a protonated site in chabazite.286,287 Constrained Car-Parrinello molecular dynamics were used to probe potential reaction coordinates for the reaction. These calculations suggest a reaction path that proceeds via formation of stable intermediates of methane and formaldehyde. A novel feature of this work was the use of transition path sampling, a technique to efficiently search for transition states that requires little a priori information regarding the configuration of the final products of a reaction. Methanol coupling in ferrierite (FER) has been examined with periodic DFT calculations by Govind et al.2SS... 

The lifetime of the carbenium ion formed will be limited by transferring a proton back to the zeolite, thus completing the dehydrogenation ofthe hydrocarbon. Hydride abstraction from xylene is assumed to be the initial step in its disproportionation into toluene and trimethylbenzene [9]. The parent compound (7, Fig. 22.9) ofthe carbenium ion formed (6) has such a high proton affinity (1031 kj mohh Table 22.1) that proton transfer back to the zeolite does not occur at all. However, the lifetime of carbenium ions in zeolites is not only limited by proton transfer, but also formation of a C-O bond between the carbenium ion and a framework oxygen atom, yielding an alk-oxide, needs to be considered. In ferrierite (FER) the alkoxide of 6 is found to be 50 to 60 kJ mofi more stable than the carbenium ion [9]. 

Otero Arean et al. (2006) and Nachtigall et al. (2006) used periodic DFT calculations to calculate adsorption enthalpies of molecular hydrogen on Na-, K- and Li-ferrierite (FER), finding good agreement with experimental values derived from variable temperature infrared measurements. In all cases hydrogen was found to bind in a side-on (rf) configuration and up to two molecules could be adsorbed on cations located at the intersection of two channels, whereas only one was adsorbed on cations in the channel wall sites (see Fig. 9.8). Ricchiardi et al. (2007) also concluded that more than one H2 molecule could be adsorbed at specific cation sites, in the titanosilicate ETSIO, on the basis of both IR measurements and molecular mechanic simulations (see Fig. 9.5). 

Restructure (from periodic DFT calculations) of hydrogen adsorbed in ferrierite (FER) (a) on an Na+ cation in a channel wall site and (b) on a cation in a channel intersection site. Adapted from Figure 2 in Otero Arean et al. (2006). 

Figure 7.6. Four of the tubular building units ferrierite (FER), offretite (OFF), ZSM-5 (MFI or pentasil), and mordenite (MOR). Six other tubular building units are shown in Szostak (1998), taken from Gellens et al. (1982).

Many zeolites have been evaluated as supports for Cu in SCR catalysis. Almost all the earlier work, however, is limited to medium- and large-pore zeolites, such as ZSM-5 (MFI, 10-ring), ferrierite (FER, 10-ring), mordenite (MOR, 12-ring), Y (FAU, 12-ring), and beta (BEA, 12-ring). Among them, Cu/ZSM-5 and Cu/beta are the two most studied systems. Of the two, Cu/beta catalysts show better hydrothermal stability and were favored by industry, while Cu/ZSM-5 catalysts were primarily studied by academia. It was not until recently that both industry and academia shifted their interests to small-pore zeolites as supports of Cu SCR catalysts. 

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