Zeolite physical mixture

Another example of performance enhancement using a zeolite/TUD-1 catalyst is shown in n-hexane cracking using a series of zeolite-Beta-embedded TUD-1 catalysts (29) 20, 40 and 60 wt% zeolite Beta in Al-Si-TUD-1 (Si/Al = 150). These are compared to pure zeolite Beta, and to a physical mixture of 40% zeolite Beta and 60% Al-Si-TUD-1. These catalysts were tested in a fixed bed reactor, at atmospheric pressure, with constant residence time at 538°C. The pseudo-first-order rate constants are shown in Figure 41.8. Note that the zeohte-loaded catalysts were clearly superior to both the pure zeolite Beta catalyst and the zeohte-TUD-1 physical mixture. Again, this is evidence that catalyst performance benefits from a hierarchical pore stracture such as zeolite embedded in TUD-1. 

Another example is the synthesis of supported oxide catalysts by spreading of one oxide across the surface of another (support) oxide in physical mixtures. Also, the phenomenon of solid-state ion exchange in zeolites may be discussed within the framework of the wetting and spreading concept. 

Jacobs has described (249a) these two different approaches in terms of secondary (physical mixtures) and primary (FT function in zeolite matrix) effects. In the former case the results obtained can be quite well understood in terms of the separate behavior of each component. However, in the latter case the results may be different since the primary FT products are formed inside the spatially restricting pores of the zeolite. 

On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffin [2], hydrocracking of hydrocarbons [3] or methanol conversion to hydrocarbon [4] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [5]. Recently Masai et al. [6] and Steinberg et al. [7] showed that the physical mixtures of protonated zeolite and R/AI2O3 showed high hydrocracking activities of paraffins and skeletal isomerization to some extent. 

The dramatic increase in 2,4-didehydroadamantane (tetracyclo[3.3.1.13,7.02,4]de-cane, 22) production when diazirine 16 is photolyzed in CyDs and FAU zeolites (Table 1) is demonstrative of the powerful influence these hosts can have on reaction outcomes. Since there are no solvent molecules with which supramolecular carbene 21 can react and because a 1,2-H shift by carbene 21 is prohibited, decay via 1,3-CH insertion is greatly enhanced. In contrast, cyclopropane 22 is only formed in trace amounts from diazirine 16 in organic solvents.101,103 When a physical mixture... 

The catalyst was prepared by kneading of a physical mixture of precipitated cobalt hydroxycarbonate, MgO and NaZSM-5 (Si02/Al20 = 38) with water used as a binder. The paste was dried at 120°C. It contained 32 wt.% Co, 3 wt.% MgO and the rest was the ZSM-5 zeolite. 

Support for the existence and stability of the bridged La-0 structure was provided by X-ray diffraction (Smith et al. 1967, Mauge et al. 1986, Lee and Rees 1987a) and XPS (Griinert et al. 1993) studies. In the last case, it has been excluded that lanthanum is present in the zeolite in a physical mixture or as a supported bulk phase. 

The Na-form of zeolites were synthesized by usual methods. Modification of ZSM-5 zeolites was carried out by two different methods (i) conventional ion exchange in Cu -acetate solution, and (ii) heat treatment of the physical mixtures of CuCU and H-forms of zeolites at 873 K (solid-state ion exchange). 

Fig. 10. AuL, XANES spectra of a metallic Au foil b AuCl c AU2CI6 d a physical mixture of AujClg and Na-zeolite Y as well as of e-g physical mixtures of AU2CI6 and Na-zeohte Y treated at different temperatures e 323 K /338 K and 353 K. After [69]...

Fig. 14. Extra- and intra-zeolite Cu detected by CuLjVV Auger spectroscopy. Chemical transport of CuCl into Na-ZSM-5. Insets Cu 2p XPS BE a a, eV. a bulk CUCI2, b bulk CuCl, c CuCl2/Na-ZSM-5 physical mixture, d (c) after microwave treatment, e (c) after Nj, 673 K, 24 h, / (e) after vacmun, 823 K, 1 h, g (/) after air, 673 K, 1 h h CuCl/Na-ZSM-5 physical mixture, k (h) after N2,823 K, 4 h. Intra-zeolite Cu is indicated by the line at KE=911-913 eV (acu= 1844-1846 eV). Taken from Griinert W, Liese T, Schobel C (1997) Stud Surf Sci Catal 105 1517, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands...

However, XRD is also a suitable technique to provide evidence for solid-solid cation exchange itself. Thus, Kokotailo et al. [40] and Fyfe et al. [14] were able to show by analysing the powder diffraction patterns of a mixture of Li-A and Na-A that the initial doublet splitting of many reflections vanished when the physical mixture was allowed to equilibrate. Instead, a pattern was obtained with one set of peaks. Since replacement of Li" bjf Na causes a shrinkage of the unit cell dimension of zeolite A, the peak positions in the XRD pattern of the equilibrated sample are shifted with respect to those of the original physical mixture. Peaks of the initial mixture which were split into "doublets ... 

Modified zeolites for use in an industrial reactor should have the right physical form and shape implied by the choice of the reactor (Chapter 9). FCC in a fluid-bed reactor requires spherical particles (70-100 [im), obtained by spray drying of the [im-sized USY zeolite crystals in the presence of a binder, viz. silica, alumina, and clays [21]. Extrusion requires a binding agent that attributes to the zeolite/binder mixture tixotropic properties in the extruder [22]. As the catalyst binder may be active, it exceeds the role of diluent agent for the active zeolite [21]. Indeed, the steam stability of some zeolites is known to be enhanced in a binder[Pg.244]

SSIE requires two primary elements. The first element is an intimate physical mixture, the second is a driving force to push the ingoing metal into the zeolite. We will discuss each of these factors. 

Almost all studies utilizing SSIE to prepare catalysts use a mortar and pestle to make the physical mixture of the ingoing cation compound with the zeolite. This is not an efficient method for making these mixtures and certainly could not be implemented in industrial practice. A more efficient and reproducible method is highly recommended. 

The coupling between a NOx-trap catalyst and a NH3-SCR material located downstream the first one or in a double layer on the monolith, was firstly patented by Ford in 2004. In other recent studies, two main SCR catalysts are usually associated in such system, namely Cu-ZSM-5 and Fe-ZSM-5 catalysts. For instance, it is reported that using Cu-ZSM-5 material, about 15-50 % of supplementary NOx conversion can be achieved, depending on the working conditions. Whatever the zeolite used, higher performances are reported with H2 as reductants, since it enhances NH3 formation in the first NSR catalytic bed. Physical mixture of NSR and SCR catalysts gives better performances than dual bed system. It is concluded that a close proximity of both materials was required for a better use of ammonia produced during the rich pulses. 

B. A mechanical (physical) mixture of the FT synthesis catalyst and zeolite [87], The size and shape of the mixed particles affects the performance of the resulting mixture [88]. a-Olefins formed as primary FT synthesis products are isomerized or cracked on acid sites of the zeohte, which prevents the participation of these a-olefins in the further chain growth via readsorption. When the powdered components are mixed, the activity enhancement is most pronounced [89]. 

Na-Y zeolites containing gold spedes were prepared by SSIE by subjecting a physical mixture of Na-Y and AuClj to a thermal treatment under vacuiun. The product was investigated by XAFS spectroscopy. The XANES spectra provided evidence for the presence of Au , AuCl and AU2CI6 (cf. [131] and Vol. 4, Chap. 5, this Series, see [69] therein). 

---