Silicalite channel system

From a calculation to locate the energy minima of Xe in silicalite, three sets of four symmetry-equivalent minima were found, one in each of the two channel systems and one at the intersections. Pathways connect these minima via saddle points and in addition to pathways from a straight or sinusoidal channel minimum to an intersection minimum, there are pathways that are direct transitions between channel segments, circumventing the intersection minima. 

There are experimental results that show the anisotropic nature of diffusion of methane in silicalite (24, 77). From a stochastic jump model of the diffusion process, Karger et al. (24) found that the ratio of the rate of diffusion in the direction of the two channel systems should not be less than 4.4 times that in the orthogonal direction ... 

The most-tested hydrophobic zeolite in adsorption of organic compounds from water solutions is silicalite-1 [89], This material is a molecular sieve with an MFI structure composed of pure silica. The MFI framework has a 10-MR channel system with elliptical pores having diameters of 5.2 x 5.7 A [83], Additionally, other zeolites, as the all-silica P-zeolite [216] which possesses a three-dimensional, 12-numbered ring, interconnected channel system with pore diameters of 7.1 x 7.3 A [83] have been used in the elimination of methyl tert-butyl ether (MTBE) from water solutions [88],... 

Figure 3. Channel system of silicalite with arrows showing two of the intersections...

Here, we would like to take ZSM-5 as an example to illustrate the relationship between structure and function. ZSM-5 has an interconnected 2-D 10-membered-ring channel system ([100] 10 5.1 x 5.5 <-> [010] 10 5.3 x 5.6 ). Since the Si/Al ratio of ZSM-5 can be varied from 10 to infinity as found in pure-silica silicalite-I, the type, acidity, and distribution of acidic sites can also be controlled accordingly. Furthermore, because of its special channel system, ZSM-5 may function very differently for different molecules. For example, the diffusion, the adsorption/desorption, the reaction rate, and the formation of intermediate and final product of molecules may vary to a great extent. ZSM-5 has been widely used in petroleum refining as a catalyst with good shape-selectivity. 

Titanium silicalite (TS-1) is a porous crystalline titanium silicalite with the MFI structure, analogous to ZSM-5 [1], Catalytic centers are isolated Ti sites in a silica framework [4]. Unlike Ti02/Si02 with a similar elemental composition but an amorphous structure, TS-1 is an effective catalyst for the selective oxidation of different functional groups with dilute aqueous hydrogen peroxide [2]. The structural properties of lattice Ti sites, the hydrophobicity, and the size of the tridimensional channel system (ca 0.55 nm) are thought to be critical factors in determining the unusual catalytic properties of TS-1. 

The synthesis of titanium silicalites TS-1 [10] and TS-2 [84-85], with MFI and MEL structure respectively, opened new opportunities in the oxidations with H O. TS-1 and TS-2, the former being the most studied, show similar properties in catalysis. Catalytic sites are isolated titanium atoms, incorporated into the zeolitic framework, in a channel system of about 0.55 nm average diameter [86-88], Different Ti-peroxo species, at lattice position, result from complex equilibria between TS-1, H O, and protic molecules [89-91],... 

Carbonylation of formaldehyde with carbon monoxide was also performed on various protonated zeolites such as H-ZSM-5, silicalite, H-MOR, H-Y, H-BEA, and MCM-41 (63). l,3-Dioxolan-4-one (l,3-DOX-4) is produced on Br0nsted acid sites of zeolite. H-ZSM-5, H-Y, and H-BEA zeolites with the three-dimensional channel system show the high activity for the formation of l,3-DOX-4. The reaction is carried out under batch conditions in dry methylene chloride solution, and trioxane is used as the formaldehyde source, at 40-180°C, 150-570 atm CO... 

The reduction of Ti4+ to Ti3+ in TS-1 has also been observed with cyclic voltammetry using zeolite-modified carbon paste electrodes. With silicalite, neither anodic nor cathodic processes can be observed. However, TS-1 is electrochemically active, with a reduction process at +0.56 V versus a saturated calomel electrode (SCE) and an oxidation process at +0.65 versus SCE. These observations must be attributed to the redox system Ti4+/Ti3 +. The electrochemical process involves the Ti cations of the inner part of the zeolite crystals, provided that a suitable electrolyte cation can diffuse inside the channels to compensate for the electrical imbalance caused by the redox process in the solid ... 

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

Figure 8 displays some typical FR data of Ci - Ce n-alkanes diffusing in coffin shaped crystals of silicalite-1 (40 x 40 x 260 p,m ). All the spectra in Fig. 8a-f,l can be fitted by the theoretical in-phase and out-of-phase characteristic function curves of the single diffusion model described by Eqs. 3-6, implying that only a simple, single diffusion process is involved in these systems. The diffusivities calculated from the best fit are presented in Fig. 9 and Tables 1 and 2. Equations 5 and 6 were applied since the channel framework structure of sihcahte-1 is comprised of near circular (0.54 x 0.56 nm)... 

Fig.1 The structure of the medium-pore zeolite silicalite-1 (left) and a schematic representation of the pore system which consists of straight and zigzag channels...

The loading of n-hexane in mixtures is somewhat higher than it is expected to be if it were proportional to its partial pressure (Fig. 12). On the contrary, the 2-methylpentane loading is somewhat lower. This points to preferential adsorption of -hexane over isohexane in their mixtures in H-ZSM-5 than in silicalite-1. In earlier experimental [50] and CBMC simulation studies [44] of n-hexane/isohexane mixtures in silicalite-1, a slight preferential adsorption of the linear alkane over the branched one has been found. The most prominent explanation for this preference is the molecular siting of these two hydrocarbon molecules. Whereas -hexane exhibits no clear preference for a position in the micropore system of MFI zeolite, the branched isomer is preferentially located at the channel intersections due to entropic reasons [44]. Consequently, 2-methylpentane will be pushed out from silicalite-1 by -hexane. These effects are even stronger for H-ZSM-5, most likely due to the stronger... 

Table 2 shows the adsorbed concentrations of the pure components. At a partial pressure of 6.6 kPa the amount of n-hexane is just slightly higher than that of isohexane in silicalite-1, while the linear alkane is obviously adsorbed more strongly than 2-methylpentane in H-ZSM-5 due to the stronger interaction with the acid sites. The maximum loading of each component has been measured by a separate adsorption study. The sorption capacity of n-hexane (7 molecules per unit cell), in agreement with earlier studies [48,59-61] exceeds that of 2-methylpentane (4 molecules per unit cell). The latter value equals the number of channel intersections in the MFI pore system per unit cell. Indeed, the sorption of isohexane molecules at... 

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