Barrie M. Lowe

Use of pH-measurements to monitor zeolite crystallization

Abstract The crystallization of zeolites from alumino-silicate gels is usually accompanied by a change in pH and hence pH measurements can be used as a simple rapid method for following the course of the reaction. The method is particularly suitable for investigations into the synthesis of high silica zeolites and can be used both to give mechanistic information and to establish when crystallization of the desired phase has been completed.

Direct measurements of the crystal growth rate and nucleation behaviour of silicalite, a zeolitic silica polymorph

Abstract The method of Zhdanov and Samulevich has been used to analyse the crystal growth rate and nucleation behaviour of silicalite, a silica molecular sieve. Four reactions run under different conditions at 368 K gave values for the crystal linear growth rate (0.5 d l /d t ) in the range (1.9−2.0)×10 -2 μm h -1 . However, the reactions do not show the same pattern of crystal mass increase with time, the variations reflecting differences in nucleation behaviour. All the nucleation rate curves were either bimodal or trimodal, suggesting that at least two separate nucleation mechanisms were operating. Early in the reaction, nucleation is probably heterogeneous and associated with the amorphous gel or colloidal material present in the mixture. Later on, when appreciable quantities of crystalline product have been formed, a further crop of crystals nucleates either by a secondary mechanism or by release of further hetero-nuclei from within the dissolving amorphous component. Nucleation was facilitated by early addition of the mineralising organic “template” (tetrapropylammonium cation), and (probably) by rapid stirring. Although all four reaction compositions were identical, the differences seen in nucleation behaviour were linked to observable differences in chemical parameters (e.g. the pH profile of the reactions) as expected for systems governed by the competitive kinetics of many overlapping reactions. The results confirm that simple growth curves based on, for example, XRD crystallinity are of limited use in helping to understand the complex processes occurring in zeolite synthesis, and that a more detailed analysis is justified.

Piperazine Silicate (EU 19): the Structure of a Very Small Crystal Determined with Synchrotron Radiation

Single-crystal diffraction data have been recorded for a very small crystal of dimensions 8 x 18 × 175 ~rn 3. From these data the structure of piperazine silicate (EU 19) has been determined and refined to R = 0.094 for 490 observed reflections. Data collection for such a crystal has been made practicable by the high intensity of the Daresbury Synchrotron Radiation Source and the Enraf-Nonius FAST area detector diffractometer. The potential of this synchrotron radiation method for other small crystals including proteins is discussed. Crystal data: 2+ • 2

An equilibrium model for the crystallization of high silica zeolites

Abstract A simple equilibrium model is proposed to explain the change in pH which occurs when high silica zeolites are crystallized from gels. The model shows that for a given concentration of quaternary ammonium ions the highest increases in pH are associated with the most stable zeolite, and that the yields depend primarily on the stoichiometry of the reaction mixture. For reaction mixtures which are buffered by amines the model shows that the increase in pH is reduced and the yields are substantially increased.

Effect of organic species on the synthesis and properties of ZSM-5

The synthesis and properties of ZSM-5 zeolites crystallized from the reaction mixture 10Na 2 O Al 2 O 3 60SiO 2 3000H 2 O 10 organic (organic = tetrapropylammonium bromide, hexane-1, 6-diol, hexane-1, 6-diamine or piperazine) at 150°C are compared with those of ‘organic free’ ZSM-5. It is shown that the organic species has a marked effect on the composition and excess cation content of the product. Only the tetrapropylammonium ion is considered to be a true structure directing template, piperazine and hexane-1, 6-diol act as hydrophobic void fillers, and hexane-1, 6-diamine holds an intermediate position.

Crystallisation of zeolitic molecular sieves: direct measurements of the growth behaviour of single crystals as a function of synthesis conditions

A technique is described for the milligram-scale synthesis of zeolitic molecular sieves in sealed capillary tubes. The method is ideally suited to the measurement of the linear growth rates of individual crystals. Spectroscopic characterisation of individual crystals within the sealed capillaries has been demonstrated. Detailed results for growth rates as a function of synthesis temperature and composition are presented for the isostructural zeolite ZSM-5 and silica molecular sieve silicalite (MFI framework). Length growth rates follow the Arrhenius law over a wide temperature range and have an apparent activation energy of 80 ± 3 kJ mol–1. Width growth rates show more complex behaviour and yield apparent activation energies in the range 62–81 kJ mol–1, depending upon the temperature range chosen and the synthesis conditions. The crystal aspect ratio is found to increase with increasing temperature and also when the crystals are growing under chemical constraint. The maximum value observed for the length growth rate {0.5 dl/dt=(1.9–2.0)× 10–2µm h–1} at 368 K agrees well with that found in earlier studies, and it is suggested that this represents a rate limit dependent upon the integration of growth species into the crystal surface. Addition of aluminium compounds, tetramethylammonium salts or ethanol reduces the growth rate, and a number of possible mechanisms are suggested. Some observations have also been made on zeolites EU-1 (EUO), ZSM-39 (MTN) and ZSM-48.

Synthesis and characterization of zeolite Nu—10

The crystallization of the high silica zeolite Nu—10 from the reaction mixtures, 5M2O xAl2O3 60SiO2 3000H2O 20CnDM (M is an alkali metal; CnDM = NH2(CH2)nNH2, 12 ⩾ n ⩾ 2; 1 ⩾ x ⩾ 0) at 180°C has been studied by pH, t.g.a., XRD and SEM measurements. The most suitable compositions for the synthesis of Nu—10 contain potassium or rubidium ions, diamines with 10 ⩾ n ⩾ 6 and have SiO2Al2O3 ∼ 600.35. The products have needle shaped crystals (0.5–4.0 μm) and a characteristic XRD pattern. They can be calcined at 550°C and their aluminosilicate lattice is stable at temperatures ⩾ 1000°C.

Para-selective fries rearrangement of phenyl acetate in the presence of zeolite molecular sieves

Abstract The Fries rearrangement of phenyl acetate is catalysed by acidic zeolites such as H-Nu-2 and H-ZSM-5, with selectivities of 2-6:1 in favour of para-substituted products.

The synthesis and thermal behaviour of zeolite Ω

Abstract The crystallization of zeolite Ω at 95°C from reaction mixtures with the composition aNa2O Al2O3 bSiO2 300H2O c(CH3)4)NBr has been studied as a function of b,(a − 1) b and c (2a + c) by X-ray powder diffraction, scanning electron microscopy, pH and bulk powder density measurements. It was found that for 0.2 b a spherulitic form of zeolite Ω is formed after an induction period of at least 24 h, whereas for ( a − 1) b = 0.40 a mixture of zeolite Ω and zeolite S is formed within 24 h. In the presence of tetramethylammonium ions the zeolite S is metastable with respect to zeolite Ω which continues to grow at its expense to form fibrous crystals. The thermal stability of the spherulitic and fibrous forms of zeolite Ω have been examined by calcination in air, X-ray powder diffraction, d.t.a, and t.g.a. The fibrous form is more thermally stable than the spherulitic one.

Zeolite synthesis in the NH2(CH2)6NH2Al2O3SiO2H2O system at 180 °C

Crystallisation of the reaction mixture 20 C6DM · x Al2O3 · 60 SiO2 · 3000 H2O (C6DM = hexane-l,6-diamine; 0<x<0.5) in stirred reactors at 180 °C has been investigated. In all cases the product has the ZSM-48 framework structure and forms as spherical agglomerates of needle-shaped crystals. The crystallisation of the “aluminium free” (x = 0) reaction mixture was followed by X-ray powder diffraction, pH, thermogravimetric analysis, and bulk density measurements. The product was thermally stable and on calcination it formed a hydrophobic silica molecular sieve.