Catalytic materials, Faujasite zeolite

Owing to the possibility of tuning (1) their acidic and basic properties, (2) their surface hydrophilicity, and (3) their adsorption and shape-selectivity properties, catalytic activity of zeolites was investigated in the production of HMF from carbohydrates. Whatever the hexose used as starting material, acidic pillared montmorillonites and faujasite were poorly selective towards HMF, yielding levu-linic and formic acids as the main products [81-83]. 

Source of Activity in other Siliceous Catalysts.—Although various oxides can be combined with silica to give amorphous, acidic catalysts, the replacement of aluminium in zeolites (specially non-faujasitic zeolites) has proved to be very difficult with any element other than gallium. Materials of ZSM-5 structure with iron or boron in place of aluminium have been claimed recently, but it is not yet certain that either iron or boron is part of the zeolite lattice or that the catalytic activity observed is not due to residual lattice aluminium. 

In small pore zeolites with cage structure, e. g., faujasites, dye molecules encapsulated by in situ synthesis or crystallization inclusion are stable against extraction.1 2 However, these methods fail for MCM-41 due to the channel structure and the wider pore diameter (3 nm) of the host material. Covalent bonding of guests is necessary to obtain diffusion stability. Therefore, anchoring of organic molecules with catalytic functions into MCM-41 by covalent bonding was recently reported by Brunei et al.3... 

Experiments to further demonstrate the critical role of extraframework Al, or another polyvalent cation, have recently been carried out in our laboratory (19.20). A series of faujasite-type zeolites was prepared that had Alf concentrations between 21 and 54 per u.c. At the low end of the range, AHF was used to remove the framework Al, and an H-ZSM-20 zeolite with 42 Alf/u.c. was synthesized. ZSM-20 is an intergrowth of the cubic faujasite structure and the hexagonal variant know as Breck s structure six (BSS) (21). Thus, it is a faujasite-like material. The catalytic activities of these zeolites for hexane cracking are compared in Figure 5 (lower data set) with the activities of zeolites prepared by steaming or by treatment with SiClA (upper data set). The solid lines represent N(0) distributions. The samples without extraframework Al exhibited very modest activity, even though some of them had a favorable N(0) concentration. 

Breck s preparation of type Y faujasite in die late 1950 s still stands as the outstanding success in zeolite synthesis (2). Type X might have had some catalytic applications but I doubt the International Zeolite Association would exist without the interest and support generated by the catalytic applications of the Type Y materials. It didn t seem that critical at the time after all Breck had reproduced a material which exists naturally. Synthetic counterparts of natural zeolites have been prepared dozens of times since (3). But die extra silica content, or perhaps die diminished alumina content, was enough to give high temperature stability in the acid form and to get zeolites into catalysts for petroleum processes (4). 

The chemical transformation of Ru-complexes in faujasite-type zeolites in the presence of water and of carbon monoxide-water mixtures is reviewed and further investigated by IR, UV-VIS spectroscopic and volumetric techniques. The catalytic activity of these materials in the watergasshift reaction was followed in a parallel way. The major observations could be rationalized in terms of a catalytic cycle involving Ru(I)bis and triscarbonyl intermediates stabilized in the supercages of the faujasite-type zeolite. The turnover frequency of this cycle is found to be determined by the nature, number and position of the charge compensating cations, as well as by the nature of the ligands present in the Ru-coordination sphere. 

In view of what precedes, it has been the aim of the present work to identify the Ru-species present in faujasite-type zeolites activated under WGS-conditions, making use of the avail-albe literature data. The activation procedure of Ru(III)hex-ammine in NaY has been related to its catalytic performance as low temperature WGS-catalvst. Subsequently, the basicity of the material was related to its catalytic behavior in the same reaction, by changing the nature of the parent complex, of the charge compensating cations and of the aluminum content of the faujasite-type zeolite. 

Industrial production of synthetic zeolites gives fine powders (<10pm) in their sodium exchanged form. These materials can be used as synthesized , such as for use as detergents, and Figure 16 is a schematic diagram of the production of zeohte ETA (Na form) for this purpose. The manufacture of the synthetic faujasite (FAU) zeolite Y for catalytic nses follows a similar route. 

In this paper we have tried to survey the catalytic properties of mordenite, making comparison where possible with faujasite-type zeolites. Mordenite shows a remarkable selectivity for n-paraffinic material in heavy distillates, and we have shown how this property might be used in a number of petroleum-refining applications. 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

To date, crystalline zeolite catalysts have been most effective in catalyzing carbonium ion reactions such as catalytic cracking and hydrocracking. Other carbonium ion reactions such as alkylation and isomerization also are catalyzed by certain forms of zeolites. I expect to see these applications expand— provided suitable catalyst compositions are developed to allow economically viable processes. Although X- and Y-type faujasite can be used in catalytic cracking and hydrocracking, the Y-type is preferred for paraffin-olefin alkylation. Y-type faujasite is suitable for use in hydroisomerization catalysts, but synthetic mordenite is also a promising material. 

What has to be noted first is that the number of applications for a few molecular sieves is high, however there exist many more stmctures that currently have no application. The widely employed molecular sieves include various forms of faujasites, mordenites, zeolite BETA, ZSM5, TS-1, zeolite L and to a lesser degree S(Me)AP05. For the other molecular sieves examples of utilization are quite scattered and are mainly confined to comparative studies. This suggests that a move to more catalytic chemistry and less material oriented approaches is required. It has to be critically noted that the quality of the materials used often varies quite substantially and this makes it difficult to derive genuine structure-activity correlations. 

Computational modeling was successfully used to identify the locations of cations in zeolites and to clarify their interaction with zeolite hosts Cu" and Cu in ZSM-5 (MFI), ferrierite (PER), and faujasite (FAU) zeolites [134-136] alkali and alkaline-earth cations in MFI [133] Zn " in FAU and MFI [137,138] also some divalent cations in MFI [139]. Yet, these investigations represent only first steps in the exploration of the large variety of both metal cations (especially of transition metals) and zeolite structures. Moreover, the interaction of cations in zeolites with probe molecules or reagents [137,140,141] is much less investigated computationally, despite the fact that these interactions are crucial for interpreting results of spectroscopic methods used to characterize the cations as well as for rationalizing specific catalytic, adsorption, sensor, or other properties of the materials. 

Zeolites of type Y are prepared by either primary or secxindary synthesis. Structures include zeolite Y in t)oth the cubic and hexagonal forms, SAPO-37 and faujasitic frameworks containing Ga or Zn. These materials are characterised using solid state NMR, X-ray powder diffracticai, infrared jectrosccpy, surface aneilysis and sorption. Catalysts are then evaluated for the conversion of n-hexane, cyclohexane and gas-oil. Results are interpreted in terms of the effectiveness of catalytic sites in alkane activaticxi and in the effect of both density and distribution of active sites. 

It is interesting to note that La,Na-X showed a 1.6 times higher conversion than the La,Na-Y zeolite, yielding benzene to diethylbenzene ratios of imity. The ranking of the catalysts with respect to their total acidity did not correlate with the activity in ethylbenzene disproportionation, while the concentration of very strong acid sites (Ho < -8.2) correlated very well with the catalytic activity of the faujasite-type materials. Sr- and Ba-Y samples, which do not possess strongly acidic sites, were not active in this test reaction. 

---