Zeolite crystallite size

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. 

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]. 

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

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. 

In the case of these three major areas of applications, the zeolite crystallite size has to be small generally 1 /jm for adsorption and catalysis, the optimal size for ion exchange in detergents being 3-4 /jm. This illustrates the advantage of synthetic zeolites since the precise engineering of their properties (crystal size, composition, polarity...) is now possible in contrast to their natural counterparts. 

In the sub-monolayer range, the amount adsorbed on the external area of a 1 pm cubic zeolite crystal is very small in comparison with the adsorption within the micropore structure (the intracrystalline sorption). Also, apart from a small multilayer adsorption on the external surface, there should be no additional uptake at higher pip0. However, there are three ways in which the non-zeolitic contribution may be increased (a) the binder may have a relatively large specific surface (b) the zeolite crystallite size may be much smaller than 1 pm and (c) the zeolite may contain some amorphous aluminosilicate or silica. In practice, one or more of these effects can result in a significant distortion from the classical form of the Type I isotherm (see Sayari etal., 1991). 

The discussion of MTG kinetic effects just presented is generally applicable to MTO. Olefin selectivity is improved by decreasing methanol partial pressure, increasing temperature, and increasing zeoUte Si02/Al203. An additional effect, that of varying zeolite crystallite size, was reported by Howden et al [61], who found that when the crystallite size was reduced from 30 to 3 pm, ethylene selectivity increased. This was attributed to enhanced diffusivity of li t products, which reduces their opportunity for further reaction. 

The role of the zeolite crystallite size has been clearly demonstrated when different model molecules have been used ( -heptane and 1,3,5-tri-isopropylbenzene) (Fig. 8). 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

All the catalysts contained 1 wt.% platinum, introduced into the zeolite through ion exchange by [Pt(NH3)4)]2+ in competition with NH4+ (NH4+/Pt=100), followed by calcination under dry air flow at 450°C for 4h. The dispersion of platinum, measured using CO adsorption followed by infrared spectroscopy, was about 60%. Whatever the crystallite size, the catalysts were first pelletized, then crushed and sieved to obtain 0.2 - 0.4 mm particles. 

An increase in the crystallites size of the HBEA zeolite brings a neat increase in the activity of the corresponding Pt-exchanged catalyst for n-C.% transformation (Table 1). However, this increase in activity is accompanied by a significant deactivation of the catalyst 90% with the 10-15 pm crystallite size, 60% with the 1-1.5 pm crystallite size, while no deactivation is observed with the 0.02 pm crystallite size. 

The activity of the Pt-exchanged catalyst for n-C f, transformation increases when the crystallites size increases, which was totally unexpected. External diffusional limitations cannot be invoked since the size of the grains of catalyst is the same. Moreover, this would lead to the opposite result. Other experiments showed that the activity of zeolite-... 

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]. 

The effect of crystallite size and shape of K-L zeolite on the dispersion of Pt was examined by a variety of techniques by Resasco and coworkers [143], They obtained multiple overlapping CO bands on these samples and were able to assign them to Pt clusters located inside the zeoHte pores (<2050cm ), near the pore mouth (2050-2075 cm" ) and outside the pores (>2075 cm" ). They were able to correlate high -octane aromatization activity with the K-L zeolite samples with short channels where most of the Pt is inside the pores. 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

The catalysts for xylene isomerization with EB dealkylahon are dominated by MFI zeolite. The de-ethylation reaction is particularly facile over this zeolite. There have been several generations of catalyst technology developed by Mobil, now ExxonMobil [84]. The features in their patents include selectivation and two-catalyst systems in which the catalysts have been optimized separately for deethylation of EB and xylene isomerization [85-87]. The crystallite size used for de-ethylation is significantly larger than in the second catalyst used for xylene isomerization. Advanced MHAI is one example. The Isolene process is offered by Toray and their catalyst also appears to be MFI zeoUte-based, though some patents claim the use of mordenite [88, 89]. The metal function favored in their patents appears to be rhenium [90]. Bimetallic platinum catalysts have also been claimed on a variety of ZSM-type zeolites [91]. There are also EB dealkylation catalysts for the UOP Isomar process [92]. The zeolite claimed in UOP patents is MFI in combination with aluminophosphate binder [93]. 

The tetraethylammonium-Beta (TEA-3) zeolites used in this work have been synthesized following the procedure described in the literature (5). Samples with Si/Al ratios between 7 and 106 (as measured by chemical analysis) and crystallite sizes in the range of 0.2-0.9 ym (as measured by scanning microscopy) were obtained. The H-form of these zeolites was prepared in the following way the TEA-3 samples were heated at 550 C for 3 hours by slowly increasing the calcination temperature (5°C min l), with one-hour intermediate steps at 350 and 450 C. After this treatment all TEA molecules had been removed from the zeolite (as monitored by IR spectroscopy). In a second step, the zeolite was exchanged with 1 M ammonium acetate solution and then heated at 550°C for 3 hours as described. 

Reduced crystallite size of the zeolite and/or breaking or grinding of clusters of zeolite crystals to individual crystals, also serves to enhance accessibility in the face of asphaltenes, nitrogen-containing molecules, destructive and harmful elements, and other molecular "clutter" associated with the bottom of the barrel. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

Crystallite Size Effects upon AP Catalyst Selectivity. Previous studies have shown that with the pellet sizes investigated, gross particle size does not affect activity or selectivity. If there are diffusional limitations, they must be intracrystalline and therefore a function of the crystallite size of the zeolite component. 

Freshly prepared Na+-TMA+-omega zeolite has a very small ultimate crystallite size, as indicated by line broadening in x-ray powder diffraction patterns and microscope studies. 

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 first spectrum could be recorded 25 s after admission of alcohol to the catalyst. For all the zeolite samples of various crystallite sizes (Table I) at 296 K, the adsorption was complete within 25 s for sec- and isobutyl alcohols. The dehydration process of these alcohols in the zeolitic pores was, however, slower. For a given alcohol (/ -, sec-, or iso-) the kinetics of water elimination were identical for catalysts of different crystallite sizes. This firmly establishes the absence of any diffusion limitation for dehydration for these three alcohols. 

For all four alcohols in the zeolitic catalysts with small enough crystallite sizes—when diffusion limitations also disappear—dehydration kinetics are well approximated by the exponental function, a fact that is explicable in terms of the unimolecular decay of molecules of butyl alcohol adsorbed on identical active sites. With isobutyl alcohol, for example, the rate coefficient k may be written... 

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