MFI-type

Quaternary ammonium bromides and hydroxides (quats) are applied as templates in the synthesis of zeolites with relatively high Si/Al ratio. Examples will be given of the use of mon-, di-, poly- and associated quats as templates in zeolite growth. Templated zeolites of the MFI-type can be grown in a lateral or in an axial way onto metal supports, providing promising composite systems, for separation and catalysis, respectively. 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

Preparation of Mono-Dispersed MFI-type Zeolite Nanoerystals via Hydrothermal Synthesis in a Water/Surfactant/Oil Solution... 

The variation in the lattice vibration of the solid products was examined by utilizing the FT-IR technique at successive DGC process times and the results are presented in Fig. 5. The absorption bands at 550 cm and 450 cm" are assigned to the vibration of the MFI-type zeolite and the internal vibration of tetrahedral inorganic atoms. The band 960 cm" has been assigned to the 0-Si stretching vibration associated with the incorporation of titanium species into silica lattice [4], This indicates that the amorphous wall of Ti-MCM-41 was transformed into the TS-1 structure. 

Role of the organic feed XRD powder patterns and FT-IR spectra confirmed that pure MFI-type Ti-silicalite (TS-1) was obtained [18-23], with a surface area of 530 m g". FT-IR spectra of adsorbed pyridine (Fig. 39.2) showed the presence only of weak Bronsted and Lewis sites [24,25], as confirmed by the complete evacuation from the surface at 373 K. 

The reaction mechanism of SCR of NOx with decane on acid and iron-exchanged MFI-type zeolite was also investigated by operando FTIR spectroscopy and a special reactor cell enabling the use of water-containing gas mixtures. Brosius et al. found that water has a dramatic influence on the reaction pathway, while the formation of organic nitro and nitrite compounds does not proceed via chemisorbed states of NO, as observed in SCR with dry gases [174],... 

Brosius, R., Bazin, P., Thibault-Starzyk, F. et al., (2005) Operando FTIR study of reaction pathways of selective catalytic reduction of NOx with decane in the presence of water on iron-exchanged MFI-type zeolite, J. Catal., 234, 191. 

Similar neutron diffractogram modifications have been al-ready observed several time during our studies concerning the structural proper-ties of confined molecular species (D2, Ar, N2, Kr, CD4, C2D6) in Silicalite-I zeolite. But for such a MFI type of framework porosity, characterized by a two dimensional micropore network, the intensity of the diffraction peaks (101) and (020), observed at small wave vector Q (A1) values, vanishes completely when increasing the confined phase loading ( as shown on figure 6, for the Ar / Silicalite-I system Ld. = 68 % ). 

Fig. 1. The structure of orthorhombic form of silicalite-1 (MFI type) showing the 12 crystallo-graphically distinct T sites. The oxygen atoms are omitted for clarity [Reprinted from Henry et al. (85) with permission. Copyright (2001) American Chemical Society].

The zeolite ZSM-5 has the MFI-type structure and can be obtained with many different Si/Al ratios typically ranging from about 10 to If no aluminum is present (Si/Al = ), a pure siliceous structure is obtained and the resulting material is then called Silicalite-1. The unit cell composition of an MFI-type zeolite can be written as (Na+, H A Si, xOl92-/ H20 withx < 27 (in most cases <9) and n < 16 (the more siliceous a zeolite gets, the higher is its hydrophobicity). 

Figure 4.31 SEM image of MFI-type zeolite, (distance between points of size bar = 50 pm total length 500 pm.)...

Guo, G. and Long, Y. (2001) Static equilibrium studies on separation of dichlorobenzene isomers on binder-free hydrophobic adsorlaent of MFI type zeolite. Sep. Purif. Technol,... 

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

Another recent modification to the secondary growth method as applied to MFI-type zeolite membranes is to perform the hydrothermal growth on the seeded... 

Figure 10.3 SEM cross-section image and XRD spectrum fore-oriented (a), h,0,h-oriented (b), and random (template-free) (c) MFI-type zeolite membranes along with associated XRD patterns (d, e, f) [12],...

For high-quality MFI-type zeolite membranes either Knudsen or configurational diffusion is representative of the flow. Knudsen diffusion occurs in pores where... 

The other component of molecular transport through zeoHte membranes is the equilibrium sorption strength between the molecule and zeoHte. In general, this information is reported in the form of adsorption isotherms, which are tabulated for many species and zeoHte structures [22]. For example, Langmuir isotherm commonly describes the adsorpHon behavior of molecules transporhng through MFI-type zeoHte membranes and can be represented by the foUowing equahons ... 

Figure 10.4 Calculated activation energy of gas diffusion for counter-diffusion CVD modified MFI-type (circle symbols) zeolite membranes and that of counter-diffusion CVD...

Figure 10.6 Temperature dependency of gas permeances for unmodified MFI-type zeolite membrane (closed symbols on solid line gas permeances for single permeation, open...

O Brien-Abraham, J., Kanezashi, M., and tin, Y.S. (2007) A comparative smdy on permeation and mechanical properties of random and oriented MFI-type zeolite membranes, Mesop. Microp. Mater., 105, 140-148. 

Kanezashi, M. and lin, Y.S. (2009) Gas permeation and diffusion characteristics of MFI-type zeolite membranes at high temperatures. J. Chem. Phys. C, 113, 3767-3774. 

MFI-type zeolite membranes. Micropor. Mesopor. Mater., 43, 319-327. 

Xomeritakis, G. and Tsapatsis, M. (1999) Permeation of aromatic isomer vapors through oriented MFI-type membranes... 

Attempts were made to investigate the relaxivity enhancement produced by an MFI-type zeolite with framework-incorporated gadolinium (84). A relaxivity less than 3 s mM was found at room temperature and at 40 MHz. This indicates that the accessibility of gadolinium to water plays a very important role, and that the outer sphere effect might be very small. 

The amount of MFI- and MCM-41-type material were estimated from XRD peak intensities. For the samples containing only small amounts of MFI, the presence of non-XRD detectable structures were identified for all samples by weak intensities in the infrared spectra near a frequency of 550 cm-1, which is characteristic for a skeleton bending mode for the MFI type samples [7]. The hydrothermal stability obtained also varied considerably for the samples, in the following, only three samples revealing the best hydrothermal stability will be discussed in detail. Synthesis and characterization data for the selected samples A, B and C, are summarized in Tables 1 and 2. The corresponding data for a siliceous pure MCM-41 sample synthesized by the same procedure [4,5], have been included for comparison reasons. 

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