Zeolites kinetic aspects

Kinetic aspects are significant in ionic exchange in zeolites. The kinetics of ion exchange is represented with a plot of U(t) versus, time t (Figure 7.3), in which... 

Barrer, R.M. 1980. Zeolite exchangers. Some equilibrium and kinetic aspects, p. 273-290. fn... 

The fundamental questions in view of structural and kinetic aspects of organic zeolites are reduced to the following How can structures with stable voids be generated, and by which mechanism can guest exchange occur ... 

Boyd GE, Adamson AW, Myers LS (1947) The exchange adsorption of ions from aqueous solutions by organic zeolites II. Kinetics. J Am Chem Soc 69 2836-2848 Brindley GW, MacEwan DMC (1953) Structural aspects of the mineralogy of clays and related silicates. In Green AT, Stewart GH (eds) Ceramics—A Symposium. The British Ceramic Society, Stoke-on-Trent UK, pp 15-59... 

An intriguing aspect of these measurements is that the values of D determined from NMR and from sorption kinetics differ by several orders of magnitude. For example, for methane on (Ca,Na)-A the value of the diffusion coefficient determined by NMR is 2 x 10 5 cm2 sec-, and the value determined for sorption rates only 5 x 10"10 cm2 sec-1. The values from NMR are always larger and are similar to those measured in bulk liquids. The discrepancy, which is, of course, far greater than the uncertainty of either method, remained unexplained for several years, until careful studies (267,295,296) showed that the actual sorption rates are not determined by intracrystalline diffusion, but by diffusion outside the zeolite particles, by surface barriers, and/or by the rate of dissipation of the heat of sorption. NMR-derived results are therefore vindicated. Large diffusion coefficients (of the order of 10-6 cm2 sec-1) can be reliably measured by sorption kinetics... 

Carbonylation of organic substrates was investigated using these well defined complexes. These carbonyl compounds exhibited catalytic properties in the carbonylation of organic substrates. In particular methanol carbonylation to methyl acetate in the gas phase was successfully attempted. Mechanistic and kinetic studies of this reaction over rhodium and iridium zeolites showed the similarities between the homogeneous and the zeolite mediated reactions. Aromatic ni-tro compounds were also converted to aromatic isocyanates using similar catalytic systems. The mechanistic aspect of this reaction will be also examined. 

Shape selectivity can be induced by differences in the diffusivities of the reactants and/or the products or by steric constraints of the transition state. A schematic representation of the three types of shape selectivity, i.e., the limitations of the access of some of the reactants to the pore system (reactant selectivity), the limitation of the diffusion of some of the products out of the pores (product selectivity) and constraints in forming certain transition states (transition state selectivity) are given in Fig. 8. Differentiation between the latter two is difficult as the kinetic results may be disguised when the overall rate is influenced by the rates of diffusion. In situ IR and NMR spectroscopy have contributed much to our understanding of these complex phenomena. The aspects of shape selectivity have been extensively discussed and excellent reviews exist [242,243,244]. The examples given here should only illustrate what can be achieved by employing a zeolite and why the pathway of a particular reaction is influenced. 

It seems that the zeolites have been well screened in a qualitative sense, for their catalytic properties. This paper is concerned with the quantitative aspects of catalytic reaction rates in zeolites. The question whether the model of coupled surface adsorption and reaction is still meaningful in the case of zeolite catalysis was already raised by Weisz and Frilette (4) when they wrote In conventional surface catalysis the termination of a three-dimensional solid structure is considered to be the locus of activity. For these zeolites the concept of surface loses its conventional meaning.. . It is the purpose of the present article to examine critically some possibile models representing equilibrium and rate phenomena in gas-zeolite systems, in order to obtain an understanding of the kinetics of chemical reactions in zeolites. Sorption equilibria, on the one hand, and rates of sorption/desorption, exchange, and catalytic reaction on the other hand are closely related and therefore have to be represented in terms of the same model. 

Currently the mesoscopic level for many catalytic chemists is their main interest. The catalyst systems are active metal, metal oxide or sulfide particles dispersed on a high-surface-area support. The catalytically active component is usually contacted with the support via impregnation or precipitation reactions. Activation via reduction or oxidation is an important subject of study. The preparation of high-surface-area supports and zeolites, using hydrothermal synthesis, also belongs to this category. Molecular aspects of these processes are only recently becoming clear and will be discussed in chapter 5. The catalytic activity is usually measured in microflow reactors, with the emphasis on kinetics. 

For this process, there are gaps in the literature in aspects such as the establishment of a kinetic model for the main reaction and for deactivation, in order to quantify the product distribution and the effect of operating conditions on this distribution. A kinetic model for deactivation by coke deposition is proposed in this paper, which pays special attention to the role of water on the attenuation of coke deposition, and takes as a reference the recent results in the kinetic modelling of deactivation by coke in the MTG process (methanol to gasoline) on a HZSM-5 zeolite [8] and in the MTO process (methanol to olefins) on a SAPO-34 [9]. 

This book will address the various fundamental aspects of adsorption equilibria and dynamics in microporous solids such as activated carbon and zeolite. The treatment of equilibria and kinetics, when properly applied, can be used for solids other than microporous solid, such as alumina, silica gel, etc. Recognizing that practical solids are far from homogeneous, this book will also cover many recent results in dealing with heterogeneous media. 

Prior to discussing the kinetics of zeolite crystal nucleation and growth it is beneficial to consider several thermodynamic aspects which have bearing on the phase transformation process. Reports on this topic are not abundant in the zeoHte crystallization hterature, but several papers will serve to illustrate various issues which should be important in these systems. 

Zou, W, Han, R., Chen, Z., Jinghua, Z., Shi, J. (2006) Kinetic study of adsorption of Cu(ll) and Pb(II) from aqueous solutions using manganese oxide coated zeolite in batch mode. Colloids and Surfaces A Physicochemical and Engineering Aspects, 279, 238-246. 

---