H. Lechert

Investigations on the crystallization of X-type zeolites

The Si/Al ratio of crystallizing X zeolites depends distinctly on the alkalinity of the batch but only weakly on its Si/Al ratio. Analyses of the solution phase show that the Si/Al ratio in the crystals is almost constant at OH − /SiO 2 in the solution around the growing crystals above 5 and increases strongly at lower alkalinities. This can be explained by a model of the surface structure, suggesting that the aluminate is attached to the surface only by an ionic mechanism at Si-O − groups formed via an equilibrium with OH − ions in the solution. The silicate is considered to be incorporated via an ionic mechanism at the Al-OH groups at the surface and by condensation at the Si-OH groups. From these assumptions, an equation can be derived, explaining the relation of the Si/Al ratio of the products and the OH − /SiO 2 ratio in the solution

The kinetics of nucleation of X zeolites

Abstract The particle-size distributions and the rate constants for the linear growth of X zeolites were studied for a number of batches with the compositions NaAl02n (NamH4-mSiO4) p H2O, with values for n = 1.4,1.5, 2.0, 3.0, 4.0, and 5.0, for m = 2.8, 3.0, 3.5, 4.0 and 4.4, and for p = 156, 163, 195, 260, 325, and 390. For all batches, the composition of the liquid phase has been analyzed. Using the rate constant of the linear growth, the rate of nucleation can be obtained from the particle-size distribution by an estimation of the time at which the radius had the value zero. It is shown that experimental crystallization curves can be fitted with good accuracy. The nucleation rate of X zeolite has a rather low value below 2 mol/liter NaOH in the solution phase and increases then with a quadratic function of the NaOH concentration. This behavior can be explained by simple models of nucleation using polymerization kinetics. Different possibilities for the formation of precursors of the nuclei are discussed in the solution phase as well as in the synthesis gel.

The crystallization of silicoaluminophosphates with the structure-type SAPO-5

Systematic studies of the crystallization kinetics of the silicoaluminophosphate molecular sieve SAPO-5 are reported for wide ranges of batch compositions in the system SiO 2 , Al 2 O 3 , P 2 O 5 , n -Pr 3 N, and H 2 O and reaction temperatures. The crystallization of SAPO-5 is only weakly sensitive to the nature of the reagents. A high sensitivity is observed, however, to the formation of the byproducts tridymite and cristobalite, depending especially on the kind of aluminum source. Aluminum triisopropylate and hydrargillite favor especially the nucleation of these byproducts, and pseudoboemite favors the nucleation of SAPO-5. The crystallization kinetics have been studied by X-ray powder diffraction, X-ray fluorescence analysis, scanning electron microscopy, and pH measurements. The crystal growth is discussed with common theoretical models.

NMR Investigations on the Structure and Sorption Problems of Faujasite-Type Zeolites

Abstract Nuclear magnetic resonance spectroscopy has grown to a powerful tool for investigations of the molecular structure, the structure of solids, and also of pronominal of molecular motion in liquids and solids. Stimulated by this wide range of possible applications from the very beginning of NMR research, interest has been cast upon questions of sorption and heterogeneous catalysis. The literature available up to 1972 has been reviewed in an excellent article by Derouane, Fraissard, Fripiat, and Stone [1] and in earlier reviews by Resing [2] and Packer [3]. A detailed description of the theoretical background of NMR on adsorption systems has been given by Pieifer [4].

Characterization of mesoporous materials by vibrational spectroscopic techniques

Abstract Vibrational spectroscopic techniques (i.e. infrared, Raman-spectroscopy and inelastic neutron scattering) are employed for the characterization of the wall structure and the nature of active sites in ordered mesoporous MCM-41 materials. Supported by nitrogen physisorption measurements our results provide unambiguous evidence for the destruction of mesoporous pore structures under a pressure usually used for the preparation of wafers for transmission IR spectroscopy. Avoiding pressure treatment of the samples, vibrational spectroscopic studies are suited to yield information about the effects of isomorphous substitution and calcination on the framework structure of mesoporous materials. The utilization of 18 O isotope labelling and N -methylpyrrolidine adsorption revealed the heterogeneity of hydroxyl groups in MCM-41 materials.

Dependence of para-xylene selectivity of pentasils on the Si/Al-ratio

Abstract Using a series of ZSM-5 zeolites with different Si:Al ratios (11, 50, 140, 250 and 500), the disproportionation and alkylation of toluene with methanol was investigated in the temperature range 593–743 K. It was found that with similar values of toluene conversion (between 23 and 30%) the yield of para-xylene increased with increasing Si:Al-ratio in HZSM-5 zeolites.

Possibilities and limitations of the prediction of the Si/Al ratios of zeolites from the batch composition

Abstract In earlier studies, it was shown that the Si/Al ratio of faujasite and some related zeolites crystallizing from batches with the composition NaAlO 2 : n (Na m H 4– m SiO 4 ): p H 2 O can be described by (A) ( Si / Al ) zeolite =1+b([ SiO 2 ]/[ OH − ]) Solution , where [OH − ] is the total alkali content and [SiO 2 ], the silica content of the solution phase in the batch. From Eq. (A) with some assumptions (B) ( Si / Al ) Product ≈ (b+m)n b+nm can be obtained, containing only the parameters n and m of the batch composition and the fitting parameter b from Eq. (A) . For a large number of faujasite batches and for other large pore zeolites with four- and six-membered rings in the structure, a good agreement was obtained between the experimental Si/Al ratios and the values obtained from Eq. (B) using b =2. For zeolites with five-membered ring structures values of b =7–10 proved to be suitable. For high template contents, no common description can be obtained by Eq. (B) . It could, however, be shown that this equation would be a good help for planning syntheses of mordenite, template-free ZSM-5 and BETA. The experiments with BETA were best described with b =14. From kinetic studies, Eq. (A) and the parameter b can be at least qualitatively explained by a model assuming that an amorphous layer is formed in an equilibrium reaction at the crystallizing surface. The formation of the final structure occurs by a reorganization of this layer under the influence of cations and templates. This reaction step is rate determining. The Si/Al ratio is determined in the attachment reaction. From an analysis of the alkalinity in the solution phase, it can be seen that, for the zeolites with four- and six-membered rings, the attachment of monomer silicate and the silico-aluminate species with about one silicate ligand can be expected. For the five-membered ring zeolites oligomer silicates and silico-aluminate complexes with two or more ligands are more probable. A borderline between the two situations can be drawn at about ( nm ) 55.5/p≈[ OH]≈1 mol/l .

The influence of fluoride on the crystallization kinetics of zeolite NaY

Abstract The crystallization time of zeolite NaY can be reduced by up to 80% in the presence of NaF. The optimal fluoride content and the minimal crystallization time can be calculated from the batch composition. Unlike hydroxide, the fluoride ion does not significantly decrease theSi/Al ratio of the framework. The rate constant of linear crystal growth can be expressed as a simple function of the equilibrium concentrations of the complexes Al(OH) 4 − and AlF(OH) 3 − . The incorporation of monomeric aluminate species into the growing crystals seems to be the rate-determining step of zeolite NaY crystallization.

Effect of the rehydration on the acidity and catalytic activity of SAPO molecular sieves

The structural changes occurring in the template-free SAPO-5 and SAPO-34 upon the rehydration are reversible and do not affect their acidity and catalytic activity. The rehydrated SAPO-37 loses irreversibly a considerable part of its initial crystallinity, surface area, acidity and catalytic activity. Nevertheless the remaining acid centers are stable against dehydrationrehydration cycles.

Quantitative relations of the batch composition and the Si/Al ratio in the poduct of zeolites

An analysis of the corelation of the Si/Al ratio of faujasites with the composition of the solution phase from which they were crystallized gives with good accuracy an equation (Si/AI) product = 1 + b · (iO 2 H solution (A1) where b is a constant that can be taken from the experiments. For a series of faujasites b ≈ 2 can be observed. The batch composition can be described formally by NaAIO 2 : n (Na itm H 4-itm SiO 4 ): p H 2 O. (A2) With some simplifying assumptions it can be concluded that (Si/AI) product = ( b + m ) · n /( b + n · m ) (A3) where n is the Si/Al ratio and m the excess alkalinity (NaOHNaAlO 2 )/SiO 2 of the batch. From a comparison with a large number of experimental data it can be seen that Equation (A3) describes the Si/Al ratios for an extended range of batch compositions and different zeolites with good accuracy, allowing a reliable prediction of the composition of the final product from the parameters of the batch composition. Deviations observed for low Si/Al ratios can be corrected by an additional empirical term 0.3/ n 0.3. The validity of the equation and its limits are tested also for data from the literature, including unusual batch compositions.