Effect on zeolite crystallization

Our first change in synthesis procedures was to repeat some of the earlier experiments while applying a hydrostatic pressure of about 2,000 psig. This additional pressure had no apparent effect on zeolite crystallization. 

Silica-Alumina Ratio Effects on Zeolite Crystallization in the Presence of Trioctylamine... 

Since some effects, observed during the crystallization of zeolites from gels, as for instance, the autocatalytic nature of zeolite nucleation (2,3,12, Katovic, A. Subotic, B. mit, I. Despotovic, Lj. A. Zeolites, in press) cannot be readily explained only by the classical approaches to the nucleation processes in the liquid phase (2,6,10,12), the objective of this work is to analyse the crystallization kinetics of different types of zeolites by the kinetic equation derived on the basis of Zhdanov s idea on autocatalytic nucleation (2), in order to explain the influence of autocatalytic nucleation on zeolite crystallization processes. 

Figure 5. Effect of silica source on zeolite crystallization. (Reproduced with permission from Ref. 8. Copyright 1988 Chem. Lett.)...

Instead of the seed zeolite crystals, a mixture of one part y-alumina ( 0.2 pm ) and three parts a-alumina ( ca. 1 im ) was used as seed material. Fortunately, a similar effect on the crystallization was observed, although about 2 times the quantity of the seed materials was necessary (5)). The metal pre-loaded y and a-alumina mixture was then tried as the seed material and confirmed the same effect on crystallization. In the case of 0.4 wt% Ru ( ) and 0.7 wt% Rh (12) in the catalyst product a longer catalyst life was obtained, i.e., 1.32 times and 1.65 times, respectively, without any significant change in the activity and selectivity compared with the non metal-loaded zeolite catalyst. 

Figure 4. Effect of zeolite crystal size on relationship between methanol conversion and selectivity toward para-xy enQ.

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 some solids as zeolites, excessive milling can have negative effects on the crystal structure affecting conversion and selectivity [66]. BaU milling is one of the most used techniques to reduce particle size. 

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

Large zeolite crystals with dimensions of tens and hundreds of micrometers have proven to be irreplaceable as model materials for reactivity and diffusion studies in the field of zeolite science and heterogeneous catalysis [1-3], These large crystallites often possesses complex structures consisting of several intergrown subunits and since the pore orientations of the different elements are not always aligned, this phenomenon can have a considerable effect on the accessibility of the pores in different crystallite regions [4]. 

In recent years, there has been a growing interest in the synthesis and application of nano-scale zeolites. Zeolites with a crystal size smaller than 100 nm are the potential replacement for existing zeolite catalysts and can be used in novel environmentally benign catalytic processes. It is well known that the crystal size of zeolites has a great effect on their catalytic properties. The improved catalytic activity and selectivity as well as lower coke formation and better durability can be obtained over nano-sized zeolite crystals [2]. 

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 effect of temperature on diffusivities in zeolite crystals can be expressed in terms of the Eyring equation (see Ruthven, gen. refs.). 

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