Reactions intracrystalline

Some authors (e.g., Stimpfl et al., 1999) suggested denoting the above equilibrium coefficient as ku (lowercase k) because it is an intracrystaUine reaction (as opposed to intercrystalline reactions), and K as reaction rate constants of intracrystalline reactions. The suggestion is not adopted in this book because k is used to denote kinetic coefficients. 

In the literature on intracrystalline reactions, another formulation, which is more general than that shown in Table 2-1, has been advanced to treat the kinetics of order-disorder reactions (Mueller, 1969 Ganguly, 1982). The method is outlined below to help readers follow the literature. Those who are not interested in such details may jump to Section 2.1.5. 

Assume that there are two nonequivalent lattice sites in a mineral, referred to as a and (1, and two ions, referred to as i and , may partition between the two sites. The intracrystalline reaction may be written as... 

Some other similar reactions have also been investigated, such as Fe-Mg order-disorder in cummingtonite and olivine. A brief description on how to treat intracrystalline Fe-Mg exchange reaction in cummingtonite can be found in Box 2-4. Because mass balance for intracrystalline reactions with many sites is confusing, it is discussed in Box 2-5. 

Mass balance for intracrystalline reactions must consider the number of moles of the site per formula unit, as shown in Box 2-4. To clarify this point further, here is an example. Consider exchange reaction between... 

Figure 2. Gas chromatogram of complex mixture of products extracted from REX after intracrystalline reaction of ethylene at 213°C (58)...

Cordes [21] extended the theoretical analysis to consider intracrystalline reactions including bimolecular processes. The treatment assumed that the energy of a molecule was not altered when a nearest neighbour became an activated complex, and that the only influence of the product species is to define an interface. In qualitative terms, low values of A were associated with strongly held ("tight") surface complexes and higher values with more mobile complexes. 

Keywords melt reaction, intracrystalline reaction, cyanamide, cyanoguanidine... 

The analysis of the literature data shows that zeolites modified with nobel metals are among perspective catalysts for this process. The main drawbacks related to these catalysts are rather low efficiency and selectivity. The low efficiency is connected with intracrystalline diffusion limitations in zeolitic porous system. Thus, the effectiveness factor for transformation of n-alkanes over mordenite calculated basing on Thiele model pointed that only 30% of zeolitic pore system are involved in the catalytic reaction [1], On the other hand, lower selectivity in the case of longer alkanes is due to their easier cracking in comparison to shorter alkanes. 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

The chemistry of an important group of naturally occurring materials is characterized by surface reactions many clay minerals possess what can be considered surface at its extreme. All clay minerals capable of intracrystalline swelling with separation of the silicate layers are—to overstate it—surface with a silicate layer on each side. Many principles and techniques of surface chemistry were first found with clay minerals. Nevertheless, the clay minerals will not be considered in this article, except for some comparison and analogies with surface compounds. 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Webb, E.B., III and Grest, G. (1998) Influence of intracrystalline diffusion in shape selective catalytic test reactions. Catal. Lett., 56, 95-104. 

Solid-state reactions are known from thermal intracrystalline conversions (isomerizations or loss of volatile fragments), photoreactions, gas-solid reactions, and solid-solid reactions. As all of these relate strictly to the crystal packing (unifying solid-state mechanism) and are not separated in the various sections. Also, nontopotactic (normal) and topotactic (very rare) reactions are not separated in different sections. 

Some other intracrystalline exchange reactions have also been investigated to some extent, such as Fe, Ni, and Mg exchange between Ml and M2 sites in olivine (Ottonello et al., 1990 Henderson et al., 1996 Redfem et al., 1996 Heinemann et al., 1999 Merli et al., 2001), Fe and Mg exchange between Ml + M2 + M3 and M4 sites in amphibole (Ghiorso et al., 1995), order-disorder reaction for Mg and Al, or for Mg and Fe +, between the tetrahedral and octahedral sites (O Neill, 1994 Harrison and Putnis, 1999 Andreozzi and Princivalle, 2002), and... 

Ganguly J. andTazzoli V. (1994) Fe +-Mg interdiffusion in orthopyroxene retrieval from data on intracrystalline exchange reaction. Am. Mineral. 79, 930-937. 

Stimpfl M. (2005) The Mn, Mg-intracrystalline exchange reaction in donpeacorite (Mno.s4Cao.o3Mgi 43Si20e) and its relation to the fractionation behavior of Mn in Ee, Mg-orthopyroxene. Am. Mineral. 90, 155-161. 

Shape-selective catalysis in zeolite requires that the reactants diffuse inwards to the active sites located at the intracrystalline volume (pores), and that products counterdif-fuse after the reaction. At the active sites, presence of a high local electric field may direct the reaction according to steric requirements to yield specific products. Thus, shape-selectivity may be achieved by virtue of geometric factors, Coulombic field at the active sites and/or difference in diffusion rates. Accordingly, three different kinds of shape-selectivity are distinguished (Dwyer, 1984). If the geometric factors are such that... 

Dealumination of HM resulted in common features for both the isopropylation of biphenyl and naphthalene. The dealumination of HM decreases the blocking of the pores by coke deposition in the both reactions because of the decrease in acid sites at intracrystalline and external surfaces. It also reduced the non-regioselective isopropylation at the external surface, and enhanced the reaction to yield the least bulky product molecules. The pressure of propylene is also an important key factor for high yield of 4,4 -DIPB in the isopropylation of biphenyl, because the isomerization of 4,4 -DIPB occurred significantly under low pressure of propylene at the external acid sites. 

Using the monomolecular rate theory developed by Wei and Prater, we have analyzed the kinetics of the liquid-phase isomerization of xylene over a zeolitic catalyst. The kinetic analysis is presented primarily in terms of the time-independent selectivity kinetics. With the establishment of the basic kinetics the role of intracrystalline diffusion is demonstrated by analyzing the kinetics for 2 to 4 zeolite catalyst and an essentially diffusion-free 0.2 to 0.4 m zeolite catalyst. Values for intracrystalline diffusivities are presented, and evidence is given that the isomerization is the simple series reaction o-xylene <= m-xylene <= p-xylene. 

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