Zeolite crystallization, function

Role of alkali and NH cations in the crystallization of ZSM-5 Introduced in an aqueous (alumino) silicate gel (sol), the bare alkali cations will behave in various ways firstly, they will interact with water dipoles and increase the (super) saturation of the sol. Secondly, once hydrated, they will interact with the aluminosilicate anions with, as a result, the precipitation of the so formed gel (salting-out effect). Thirdly, if sufficiently small, they also can order the structural subunits precursors to nucleation species of various zeolites (template function-fulfilled by hydrated Na+ in the case of ZSM-5 (11,48)). ... 

Equivalent v-sites i have the same probability p, to be occupied by a dye molecule. The occupation probability p is equal to the ratio between the occupied and the total number of equivalent sites. The number of unit cells I1C is controlled by the host while ns is determined by the length of the guest, which means that p relies on purely geometrical (space-filling) reasoning and that the dye concentration per unit volume of a zeolite crystal can be expressed as a function of p as follows ... 

The CgQ surface coverage was determined to be 2.0 10 mol cm . The monolayer can be further modified with monomeric amine reagents, which demonstrates the potential of the self-assembly process for growing three-dimensional fullerene structures. Different surfaces such as quartz. Si-oxide [105] or ITO [102] were coated with multilayers of fullerene up to stacks of 9 layers. An imidirectional electron transfer is possible across the fullerene mulhlayers [102]. Not only can multiple layers of fullerenes be connected to a certain surface but amino-functionalized can also serve as a linker between two different surfaces. 3-Aminopropyl-tethered glass plates could be linked via a Cgg layer to 3-aminopropyl covered zeolite crystals [106]. 

Figure 5. Experimental self-diffusion coefficient of water molecules in NaX as a function of reciprocal temperature, (a) Mean diameter of the zeolite crystals 3 pm ...

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

Fig. 17 Energy as a function of a T-T distance and b T-T-T angle used in the simulation procedure (calculated as smoothing spline fits to Boltzmann equilibrium interpretations of the histogrammed data taken from 32 representative zeolite crystal structures). Only the central portions are shown, c The contribution to the energy sum for the merging of two symmetry-related atoms merging is only permitted when the two atoms are at less than a defined minimum distance [84], Reproduced with the kind permission of the Nature Publishing Group (http //www.nature.com/)...

The classical method of investigation of effects of diffusion on reactions is typically to run a reaction with catalyst particles of various sizes. For zeolites, the resistance of intracrystalline diffusion is normally much larger than that characteristic of molecular diffusion or Knudsen diffusion that could occur in the spaces between the zeolite crystals in a catalyst particle. Thus, the crystal size of the zeolite has to be varied instead of the particle size to determine the effects of diffusion on zeolite-catalyzed reactions. Kinetics of the MTO reaction has been measured with SAPO-34 crystals with identical compositions and sizes of 0.25 and 2.5 pm 89). The methanol conversion was measured as a function of the coke content of the two SAPO-34 crystals in the TEOM reactor. 

Twenty-eight kinetics of crystallization of different types of zeolites have been analysed using kinetic equation fz = K t KQt /(1 - Ka t ) (fzis the fraction of zeolite formed at crystallization time tc, and K, K0, KQ, and q are constants). The analysis showed that the exponent q is a function of the ratio between number Na of the particles formed by the growth of nuclei released from the gel during its dissolution (autocatalytic nucleation) and the number Nq of particles formed by the growth of heteronuclei. Functional dependences between q, K, K0, KQ, NQ, Nq, Kg (growth rate constant) and other factors relevant to zeolite crystallization have been established and the influences of these factors on the characteristics of zeolite crystallizing systems have been discussed. 

The amount of iron incorporated into the zeolitic framework is a function of its content in the starting hydrogel in the case of the TPABr-rich system (type-A system), and it is preferentially introduced into the zeolite if compared to aluminium. If the amount of TPABr in the initial reaction mixture decreases, the iron content in the zeolite crystals also decreases and for the samples synthesized in presence of aluminium, this latter metal is incorporated into the MFI framework. 

A new insight in the population balance of zeolite crystallization is given, by introduction of new functions of nucleation and crystal growth as well as the correction of the crystal growth curves by introduction of the heating function of the reaction mixture. The modified population balance is used for the analysis of the critical processes (nucleation and crystal growth of zeolite) during crystallization of zeolites A, X and ZSM-5. 

In the following pages, the potential effects of organic cations in directing crystallization of specific zeolite structures and techniques for identifying such directing functions will be reviewed briefly. Then, by way of example, the influence of selected cationic polymers on zeolite crystallization will be described. 

For the acid catalysed conversion of hydrocarbons, the reaction mechanisms in absence of sterical hinderance are rather well understood, so that molecular shape-selective effects exerted by constrained environments can be isolated [8,9]. Shape-selective catalysis is also possible when other than acid functions are confined to the intracrystalline void volumes of zeolite crystals, e.g. metal [10,11], bifunctional [12] and basic functions [13]. Nowadays, catalysis on zeolites with organic substrates containing heteroatoms receives much attention. Molecular shape-selectivity seems to be superimposed on electronic factors determining the selectivities [14,15]. 

Today s modem FCC catalyst is composed primarily of zeolite, active alumina, clay or filler, binder plus additional components for specific functions (Figures (25-27). One of the most significant differences found between various commercially available catalyst systems is related to the binding systems. These binding systems include silica sol, alumina sol, alumina gel and insitu technologies. Insitu technologies are based upon the growth of zeolite crystal... 

In this section we will deal with the analysis of adsorption kinetics of a multicomponent system. First we will deal with the case of a single zeolite crystal to investigate the effect of the interaction of diffusion of all species inside a zeolite crystal. This interaction of diffusion is characterized by a diffusivitv matrix which is in general a function of the concentrations of all species involved. This concentration dependence will take a special functional form if we assume that the driving force for the diffusion inside the zeolite crystal is the chemical potential gradient and that the mobility coefficients of all species are constant. Only in the limit of low concentration such that the partition between the fluid phase and the adsorbed phase is linear, the diffusivity matrix will become a constant matrix. 

Solving this set of equations will give the concentration as a function of time as well as distance inside the zeolite crystal. The quantity of interest is the fractional uptake, which is defined as the amount uptaken by the zeolite crystal from time t = 0 up to time t divided by the maximum amount taken by the crystal, that is ... 

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