Micropore occupancy

The large difference in reactivity of the zeolites arises primarily from the large difference in micropore occupancy in the different zeolites. The large variation in micropore occupancy is deduced from the large differences in adsorption equilibrium constants shown in Table 4.5. The similarity of the elementary rate constants for isomerization implies that there is little variation in the corresponding activation energies. The overall rate of the reaction is found to be a maximum for the zeolite in which the adsorption concentration of the reactant is a maximum. 

Eq. (4.2), is the behavior of this system at low micropore occupation 0 -C 1) where the overall rate is linear in pressure. The apparent activation energy imder these conditions... 

At 1 kPa, for strongly adsorbing Cio alkanes in MEL-type zeolite, there is a large preference for adsorption of the linear alkane. This preference is much less than for the MFI zeolite. Differences appear at high micropore occupation. Competitive adsorption suppresses the formation of i-Cio in MEL owing to the difference in the channel cross-section geometry, where branched alkanes prefer to adsorb. As a consequence, the rate of n-Cio conversion is low towards i-Cio. The MFI zeolite, therefore has the superior rate since the rate of iCio formation is higher. The reaction products are the result of consecutive reactions of i-Cio. In contrast, as one notes from Fig. 4.39, in MEL at 10 kPa for C7 there is no such preference in adsorption for the n-C7 versus i-C7 molecule since under these conditions the adsorption concentration is still too low. 

The dominant interactions between the zeohte framework and the hydrocarbon are van der Waals disp>ersion interactions that take place between the pwlarizable zeohte oxygen atoms and the adsorbed hydrocarbon. These interactins are in addition to those between the hydrocarbon and the protons or cations which lead to the activation of the hydrocarbon. The overall van der Waals interaction between hydrocarbon and zeohte cavity depends strongly on the match of hydrocarbon size and shape and that of the zeohte cavity. As a consequence, at the same partial pressiue and temperature, the microp)ore occupation of different zeohtes may vary signihcantly for the same adsorbents. This has an important consequence on zeohte catalysis that depends on the concentration of reactant molecules adsorbed at reaction centers. Second, it wih strongly affect the rates of diffusion which are strongly micropore occupation dependent. 

Dedecek, J., Kaucky, D. and Wichterlova, B. (2000) Co2+ ion siting in pentasil-containing zeolites, part 3. Co2+ ion sites and their occupation in ZSM-5 A VIS diffuse reflectance spectroscopy study, Microporous Mesoporous Mater., 35-6, 483. 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

Fig. 5.2 Occupation density p(q) of hexane vapour in APET at partial pressure P/P =0.4 ( ), and immersed in liquid hexane (A). With water vapour at relative humidity RH = 0.5 (-I-), the occupation density does not exceed 0.6 in the micropores...

Zeolite polarity and reaction rate The competition between sulfolane, PA and product molecules for the adsorption on the active protonic sites is sufficient enough to explain the differences in reaction orders and catalyst stability and selectivity between PA transformation in sulfolane and in dodecane. However, the competition for the occupancy of the zeolite micropores plays a significant role as well. This was demonstrated by studying a related reaction the transformation of an equimolar mixture of PA with phenol in sulfolane solvent on a series of H-BEA samples with different framework Si/Al ratios (from 15 to 90).[49] According to the largely accepted next nearest neighbour model,[50,51] the protonic sites of these zeolites should not differ by their acid strength, as furthermore confirmed by the... 

Autoinhibition of arene acetylation, i.e. the inhibition by the acetylated products and also by the very polar acetic acid product seems to be a general phenomenon. The effect is much more pronounced with hydrophobic substrate molecules such as methyl- and fluoro- substituted aromatics because of the larger difference in polarity between substrate and product molecules.[56,57] The occupancy of the zeolite micropores by these substrate molecules as well as their chemisorption on... 

Figure 2.5 Relative occupancy (%) of the intracrystalline volume of a H-BEA zeolite during the transformation of a 2 1 molar anisole - acetic anhydride mixture in a batch reactor, assuming no adsorption of acetic acid and full occupancy of the micropores. Anisole ( ), acetic anhydride (o) and 4-methoxyacetophenone (x). Reprinted from Journal of Catalysis, Vol. 187, Derouane et al., Zeolite catalysts as solid solvents in Fine Chemicals synthesis 1. Catalyst deactivation in the Friedel-Crafts acetylation of anisole, pp. 209-218, copyright (1999), with permission from Elsevier...

Adsorption experiments The method developed for the analysis of carbonaceous compounds formed and trapped within the zeolite micropores during catalytic reactions1581 can be adapted for determining the occupancy of micropores by reactant, solvent and product molecules. However, this method cannot be used with compounds sensitive to hydrolysis, such as AA, because of the step of dissolution of the zeolite in a hydrofluoric acid solution necessary for the complete recovery of the organic molecules located within the zeolite micropores.[58] This method was used to determine the composition of the organic compounds retained within the micropores of three different zeolites [H-BEA (zeolite Beta), H-FAU (zeolite Y), and H-MFI (zeolite ZSM-5)] after contact in a stirred batch reactor at 393 K for 4 min of a solution containing 20 mmol of 2-methoxynaphthalene (2-MN), 4 mmol of l-acetyl-2-methoxynaphthalene (1-AMN) and 1 ml of solvent (sulfolane or nitrobenzene) with 500 mg of activated zeolite.[59 61] From the comparison of... 

F. Kapteijn, W.J.W. Bakker, G. Zheng, and J.A. Moulijn, The temperature and occupancy dependent diffusion of n-butane through a silicalite membrane, Microporous Mater. 3(3) 227 (1994). 

For a given adsorbent-adsorbate system the relationship between s and the fractional occupation of the micropore volume is defined by the characteristic curve , which is assumed to be independent of temperature (see Fig. 5). Such an assumption should be valid for systems dominated by dispersion-repulsion forces (which are temperature independent), but cannot be expected to hold when electrostatic forces (which are temperature dependent) are important. The characteristic curve generally has a Gaussian form leading to an isotherm of the form ... 

With the exception of iron, this procedure is simple and cheap, but the complete prevention of TM hydrolysis and Bronsted acid site generation is not allways possible. Residual acidity will catalyse peroxide decomposition and is therefore detrimental to the use of the material as oxygenation catalyst. The preferential occupation of hidden sites of (bivalent) TM ions in presence of an excess of monovalent alkali cations, will often cause incomplete reaction with DCB. Residual TM in oxygenation reaction conditions will cause radical decomposition of the peroxide reagent and be at the origin of radical chain reactions of the substrate with dioxygen thus formed. Rnally, only microporous materials with cation exchange capacity can be used as support. 

NMR spectroscopy." Xe NMR spectroscopy has been used to detect the microporous structure of nanosized HZSM-5 zeolite." The preferential occupation of pores by xenon in zeolite MCM-22 has been revealed by Xe NMR spectroscopy." The mechanical properties of MCM-41, SBA-15 and me-soporous silicas have been studied by Xe NMR spectroscopy." A (3/MCM-41 composite molecular sieve has been investigated by Xe NMR spectroscopy." Xe NMR spectroscopy has been used to characterise [Pt(NH3)2] on MCM-41." Al-MCM-48 and Si-MCM-48 have been studied using Xe... 

In which b° is the mechanical mobility for a molecule on an otherwise empty lattice, the concentration of available sites, R the universal gas eonstant, T absolute temperature, and 0 <6 < 1 is the average site occupation. Equilibrium between the site occupation and feed eoncentration on both sides of the membrane can be used as the boundary condition to solve the above eqnation. Non-eqnilibrinm correction factor 0 < j < 1 is often very close to nnity bnt can be very small for the molecule of low occupancy (low 0) when the other molecnle occnpies the sites strongly and hinders the movement of the former molecnle. The above generic expression leads to the following two types of microporous and surface diffusion separation ... 

---