Zeolites crystallites

The ortho- and meto-isomers are bulkier than the para-iaomer and diffuse less readily in the zeolite pores. The transport restriction favours their conversion into the /lara-isomer, which is fonned in excess of the equilibrium concentration. Because the selectivity is transport influenced, it is dependent on the path length for transport, which is the length of the zeolite crystallites. 

Li and Armor reported that Co-exchanged zeolites present a very high catalytic performance for the CH4-SCR, even in oxygen excess conditions [1, 3], Bimetallic Pt-and Pd-Co zeolites have revealed an increase of activity, selectivity towards N2 and stability, when compared with monometallic Co catalysts [4-8] even in the presence of water in the feed. Recent works show that these catalytic improvements are due to the presence of specific metal species as isolated metal ions, clusters and oxides and their location inside the cavities or in the external surface of zeolite crystallites [9, 10],... 

Hydroisomerization of n-hexadecane on Pt/HBEA bifunctional catalysts effect of the zeolite crystallites size on the reaction scheme. 

The transformation of n-hexadecane was carried out in a fixed-bed reactor at 220°C under a 30 bar total pressure on bifunctional Pt-exchanged HBEA catalysts differing only by the zeolite crystallites size. The activities of the catalysts and especially the reaction scheme depended strongly on the crystallites size. Monobranched isomers were the only primary reaction products formed with the smallest crystallites, while cracking was the main reaction observed with the biggest crystallites. This was explained in terms of number of zeolite acidic sites encountered by the olefinic intermediates between two platinum particles. 

Keywords hydroisomerization, n-hexadecane, platinum, HBEA zeolite, crystallite size... 

An increase in the zeolite crystallites size would very likely produce substantial changes in the physicochemical properties of the catalyst and consequently on the selectivity for hydroisomerisation. Since the effect of the zeolite crystallites size in the nanoscale range cannot be predicted theoretically, n-hexadecane hydroisomerization was carried out on PtHBEA catalysts with different zeolite crystallites sizes. 

Three zeolite crystallite sizes were used, namely 0.02, 1-1.5 and 10-15 pm range The first zeolite sample was obtained from PQ Corporation, the two other were kindly synthesized by the Laboratoire des Materiaux Mineraux , UMR 7016, Mulhouse, France. The three samples had the same framework Si/Al ratio (15) and the same Bronsted acidity (measured by infrared spectroscopy of adsorbed pyridine). They were used in their protonic form (HBEA). 

On the other hand, there is a neat effect of the zeolite crystallite size on the reaction scheme. With the smallest crystallites, the reaction occurs through the process shown in scheme 1. With the intermediate size, the process becomes the one shown in scheme 2, and finally becomes the one shown in scheme 3 with the biggest crystallites. 

The most successful application of microwave energy in the preparation of heterogeneous solid catalysts has been the microwave synthesis and modification of zeolites [21, 22], For example, cracking catalysts in the form of uniformly sized Y zeolite crystallites were prepared by microwave irradiation in 10 min, whereas 10-50 h were required by conventional heating techniques. Similarly, ZSM-5 was synthesized in 30 min by use of this technique. The rapid internal heating induced by microwaves not only led to a shorter synthesis time, and high crystallinity, but also enhanced substitution and ion exchange [22]. 

Zeolite crystallite diffusivities for sorbed gases range from 10-7 to 10-14 cm2/s. These diffusivities generally show a strong increase with the adsorbate concentration that is accounted for by the Darken thermodynamic correction factor... 

The pores of zeolites can be regarded as extensions of their surfaces zeolites have an external surface, i.e., the surface of the zeolite crystallites, and an internal surface, i.e., the surface of their channels and/or cages. In total, the surface areas of zeolites are remarkably large. One gram of a typical Faujasite zeolite expresses a geometric surface area of about 1100 m2/g (specific surface area). The contribution of the external surface area to this number is almost negligible (about 5 m2 g 1 for 1 pm crystallites), and almost the complete surface area is due to the surface of the micropores. 

Information about the zeolite crystallite size can also be inferred from the IR spectrum. Changes in the shape and slope of the baseline of the spectrum between 3600 and 2000 cm" is associated with light scattering from different crystal sizes and shapes. In general, the scattering increases with increased crystal size [102]. 

Fig. 2 displays a set of FTIR spectra obtained for the uptake of benzene into H-ZSM-5 at 415 K employing the experimental device and procedure as described in the Experimental Section. One recognizes the increase in absorbance of the typical benzene band at 1478 cm as a function of time (spectra 1 to 4). The maximum absorbance, A, of such bands can be used as a measure of the amount sorbed, M, at time t into the porous structure of the zeolite crystallites. Therefore, evaluation of the sequence of these spectral uptake curves can provide data which may be used in the appropriate solution (equ. 1) of Fick s second law, and this generates the desired diffusivities [22] ... 

The zeolite crystallites were applied as a ca, 10 /im layer on the surface of fused silica particles of ca. 0.3 mm diameter, which corresponds to a zeolite-quartz-mass-ratio of 1 14 (Fig. 1). No catalyst binder was used. This ideal... 

This type of coke formation occurs in the reaction zone and leads to coverage of the zeolite crystallites and closure of the pore entrances. 

Recently there has been increasing interest in this area, but it is important to remember that early studies were reported as far back as 1978—albeit for relatively simple systems. In 1978, Heink et al. 124) used 1-D profiling to study the time-resolved concentration profile of butane and water in packings of NaCaA and NaX zeolite crystallites, respectively. Other early investigations include the use of imaging to follow the intercalation of AsFs into highly oriented pyrolytic graphite at room temperature 125). 

Effect of Zeolite Crystallite Size on the Selectivity Kinetics of the Heterogeneous Catalyzed Isomerization of Xylenes... 

Let us now assume that isomerization of xylenes is a simple series reaction. Starting with pure o-xylene feed, and in the absence of any diffusional resistance, most molecules of o-xylene which enter the zeolite crystallites leave after, at most, one reaction step (to m-xylene), yielding primarily a single-step product. When substantial diffusion-resistance exists, most molecules remain in the crystallite long enough for several reaction steps to occur (to m-xylene and p-xylene), and the products are mostly multistep products. Wei (4) has shown that in such a case the apparent kinetics—i.e., those determined from the respective reactant product concentrations in the bulk phase—change from that of a simple series reaction... 

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubular flow reactor. Transient data were obtained. The observed rate of reaction passes through a maximum with time, which results from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related to aromatic olefin ratio temperature, and olefin type. The observed rate fits a model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 heal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 

Three obvious models which could describe the observed reaction rate are (a) concentration equilibrium between all parts of the intracrystalline pore structure and the exterior gas phase (reaction rate limiting), (b) equilibrium between the gas phase and the surface of the zeolite crystallites but diffusional limitations within the intracrystalline pore structure, and (c) concentration uniformity within the intracrystalline pore structure but a large difference from equilibrium at the interface between the zeolite crystal (pore mouth) and the gas phase (product desorption limitation). Combinations of the above may occur, and all models must include catalyst deactivation. 

Although other possibilities cannot as yet be absolutely ruled out, the evidence strongly indicates that in this study the desorption of product molecules from the surface (pore mouths) of the zeolite crystallites is a rate-limiting step. Further, product desorption limitations are probably also responsible for the maxima in rates previously reported (7, 8, 9) and may be a more general phenomenon for zeolite systems. Such limitations... 

The porous volumes measured by N2 adsorption are listed in Table 3. After the boronation, the total porous volumes (Vt) of the samples increase, corresponding to the increase of benzene adsorption capacity mentioned above. This should be resulted from the following aspects (1) The average mass of zeolite crystallite decrease and the number of crystal particles in unit weight of sample increases after the boronation owing to a limited introduction of trivalent atoms and Na+cations as counterions, as well as a severe dissolution of silicon. Thus, the total porous volume (mL/g) and the adsorption capacity increase. (2) The transformation of pore size occurs during the boronation. As shown in Table 3, the mesoporous volumes increase and the microporous volumes decrease after the boronation, meaning that some micropores are developed into mesopores due to the removal of silicon from the framework. This is also one of the important reasons why the total porous volumes as well as the adsorption capacities increase after the boronation. 

The reasons for these effects are simple. First, during their diffusion from the inside to outside of the zeolite crystallites, butene molecules inevitably must... 

The silica/alumina mole ratios of the zeolites studied were as follows ZSM-5, 62 large crystal ZSM-5, 75 high silica ZSM-5, 1670 ZSM-11, 78 and dealuminized mordenite, 61. Aluminum analysis was by atomic absorption spectroscopy. Zeolite crystallite sizes were generally less than O.ly, except for the large crystal ZSM-5 sample which was larger than in. 

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