Of zeolites Y and ZSM

The occurrence of intergrowths of zeolite Y and ZSM-20, the cubic and hexagonal forms, is analogous to similar intergrowths in SiC and ZnS crystals. Intergrowths in zeolite Y and ZSM-20 do not block channels, as is the case in the erionite-offretite family, where rotation of cancrinite layers blocks the 12MR channels, but are more like intergrowths in the ZSM-5/ZSM-11 family, which modify the channel system. 

Pore volume and surface area of zeolites Y and ZSM-5 are reduced with both silica and alumina binders. 

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

The mixed metal zeolites were prepared according to the procedure of Scherzer and Fort (18). There were, however, several additional preparations similar to those of Scherzer and Fort that we developed. First of all, ruthenium was introduced into zeolite Y and ZSM-5 as the cationic amine complex as reported by Jacobs and Uytterhoeven (19). Ruthenium was also added to the zeolite in an anionic complex form, either K Ru N) or K2Ru(CN) N0. For these anionic preparations, either copper, zinc or cobalt exchanged zeolites Y or ZSM-5 were used. 

The active components of a present commercial FCC catalyst are zeolites Y (0.74 nm) and ZSM-5 (0.54 nm x 0,56 nm) whose pore sizes give limited access to the active centres for long chain and/or bulky molecules. The new extra-large pore crystalline materials MCM-41 (2.0-20 nm), VPI-5 (1.21 nm) or cloverite (1.32 nm) have significantly increased the restricted pore sizes encountered in zeolites Y and ZSM-5 and opened up interesting perspectives for the conversion of heavier feedstocks [1]. 

Multi-nuclear NMR study of the interaction of SiOHAI groups with cationic and neutral guest-molecules in dehydrated zeolites Y and ZSM-5... 

Ziolek and Decyk [935] decomposed ethanethiol and diethyl sulfide over hydrogen and alkali forms of X, Y, and ZSM-5 zeolites, and attempted to identify the sites of the zeolites being active in these reactions and, also, to elucidate individual reaction steps with the help of IR spectroscopy. After ethanethiol adsorption on Na-Y or M, Na-Y (M=Cs, K, Li), bands at 2980,2930,2915 and 2850 cm were observed due to C-H stretching vibrations, the intensity of which paralleled the catalytic activity. Depending on the various possible reaction pathways, either Br0nsted-acid OH groups or alkali metal cations were viewed as the catalytically active centers. 

MeOH OH groups attached to cations which are located in sodalite cages of zeolite Y and in the channels of ZSM-5, hydrogen-bonded 6,239,240,... 

In 1962 Mobil Oil introduced the use of synthetic zeolite X as a hydrocarbon cracking catalyst In 1969 Grace described the first modification chemistry based on steaming zeolite Y to form an ultrastable Y. In 1967-1969 Mobil Oil reported the synthesis of the high silica zeolites beta and ZSM-5. In 1974 Henkel introduced zeolite A in detergents as a replacement for the environmentally suspect phosphates. By 2008 industry-wide approximately 367 0001 of zeolite Y were in use in catalytic cracking [22]. In 1977 Union Carbide introduced zeolites for ion-exchange separations. 

Microwave Synthesis of Zeolites and Molecular Sieves The use of microwaves holds promise for efficiency improvements in zeolite synthesis due to the rapid heating possible when using microwave radiation [166], The first report of microwave synthesis of zeolites was by Mobil Oil in 1988, which broadly claimed the synthesis of zeolite materials in the presence of a microwave-sympathetic material, such as water or other pro tic component [167]. A number of reports have appeared since, including synthesis of zeolites Y, ZSM-5 [168] and metaUoaluminophosphate-type materials, such as MAPO-5 [169], There have also been extensive investigations in using microwaves for zeoHte membrane synthesis. Recent reviews discuss the progress in microwave zeoHte synthesis [170, 171]. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

Many books, reviews and treatises have been pubUshed on related subjects [1-7]. Thus the objective of this chapter is the deUneation of the key features of the catalytic surface and the process conditions which enable better control of the reaction pathways for more efficient and environmentally friendly processes and minimal utiHzation of precious natural resources. As it stands today, hundreds of known framework types have been synthesized and scaled-up [8], but only a handful have found significant application in the hydrocarbon processing industries. They are zeolite Y and its many variants, ZSM-5, Mordenite and zeohte Beta. Other very important crystalline materials (including aluminophosphates (ALPOs),... 

Zeolitic materials have been prominent amongst those so far studied by high resolution powder diffraction using synchrotron X-rays [36]. High definition synchrotron PXD data has been helpful in a number of framework structure determinations and has facilitated studies of planar faulting (see below). Successful Rietveld refinements of the framework structures of zeolite ZSM-11 [37, 38] and silica-ZSM-12 [39], and of the complete structures of zeolite Y containing cadmium sulfide [40] and cadmium selenide [41] clusters have been described. 

Inevitably, the zeolites with a Spaciousness Index between 2 and 16 have a higher Si/Al ratio than zeolites Y or ZSM-20. To separate the effects of the Si/Al ratio and the pore width, two dealuminated samples of zeolite Y, designated YDl and YD2, were tested which resembled in their Si/Al ratios the Beta and EU-1 sample, respectively. Table 1 gives the isomerization selectivities at 40 % conversion. While dealumination of zeolite Y brings about some improvement in selectivity, H-Beta is clearly superior to HYDl, and H-EU-1 is a much better catalyst than HYD2, which indicates that the pore width in the appropriate range is of prime importance. 

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

The pore opening in ZSM-5 is smaller than for zeolite Y and access of the complex gas oil molecules into the pores will be restricted. As a result, ZSM-5 has little effect on the primary cracking of gas oil and, when allowance is made for the slight loss in conversion arising from dilution of the active zeolite Y concentration, there is no significant change in coke, bottoms, or light gas yields. 

---