US-Y zeolite

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

From a series of experiments in this reactor, the deactivation effect of coke on a complex reaction mechanism may be obtained. This is illustrated for the catalytic cracking of n-hcxane on a US-Y zeolite catalyst. On a faujasite, the coke formation deactivates the main reactions, but not the coking reaction. Moreover, the coke formation induces selectivity changes, which can be explained by the distribution of acid site strength in Y-zeolites and the acid strength requirements of the various reactions. 

This concept was used for the study of the deactivation of n-hexane catalytic cracking on a US Y zeolite catalyst. The interpretation of the flow patterns in the recycle reactor, necessary for the quantification of the degree of mixing, was based upon tracer experiments. 

The novel reactor was used to study the deactivation of n-hexane cracking on an US-Y zeolite catalyst. These experiments showed that on a faujasite the coke formation deactivates the main reactions and not the coking reaction itself, in contrast with previous observations on pentasil zeolites. The coke deposition also modifies the product distribution of n-hexane cracking. This effect can be explained by the non-uniform strength of the acid sites in the Y-zeolite and the acid strength requirements of the various reactions. 

Surprisingly, it was found (7) that some H-US-Y zeolites having a high amount of mesopores are very suitable for this kind of reaction. At the given conditions at a temperature of 0°C, best selectivities of about 75% at a conversion of 30% were obtained with a commercially available, highly dealuminated H-US-Y zeolite (A), kindly provided by The Zeolyst Corporation (Fig. 15.3). 

The performance of different H-US-Y zeolites strongly depends on the bulk Si02/Al203 ratio, as shown in Figure 15.3 for catalysts B to D. The activity as well as the selectivity to campholenic aldehyde 10 increases with decreasing aluminum content. The Bronsted acid sites do not seem to be responsible for the desired reaction, since the number of these sites is equal to the number of aluminum atoms... 

The 27A1 and 29Si NMR measurements (7) showed that after treatment with 0.01 molar HC1 most of the amorphous silica-containing material is removed from the parent catalyst A. This can be understood easily since the maximum solubility of silica (16) is reached at pH = 2. Although the improved performance of the treated catalyst cannot be entirely explained by the removal of less active material, i.e. the increase of the number of Lewis acid sites per mass unit, it is believed that these silica species block most of the catalytically active centers, i.e. the highly dispersed Lewis acidic alumina sites in the micro- and mesopores of the parent US-Y zeolite. 

Acid zeolite catalysts offer a very good alternative for the clean synthesis of these sulfur containing substances. A suitable feed stock is the 4-isopropenyl-l-methyl-1-cyclohexene. In the presence of a commercial beta-zeolite (25) hydrogen sulfide is added to the autoclave at a reaction temperature of 50°C at a pressure of 17 bar. The conversion is 65.1% and the selectivity to 1-p-menthene-8-thiol is 43.9%. These are very promising results and they can be improved by using a commercial H-US-Y zeolite which rendered a conversion of 76.8% and a selectivity of 64.3% (62). 

Figure 1. Hydroxyl and Deuteroxyl bands for a "US-Y" zeolite. A. US Sieve steamed 65 hr at 500 C and dried in vacuum at 500 C. B.- A. after exchange with D20 and redrying at 500 C.

Y. H. Lin, P. N. Sharratt, A. A. Garforth, and J. Dwyer, Deactivation of US-Y Zeolite by Coke Formation dnring the Catalytic Pyrolysis of High Density Polyethylene, Thermochim. Acta, 294, 45-50 (1997). 

Figure 7.1 TGA graphs of HDPE at various polymer-to-US-Y zeolite ratios. Heating rate, 5 K/min nitrogen flow, 50 mlN/min. (From [8]. Reproduced by permission of the American Chemical Society)...

Figure 7.2 Conversion and liquid selectivity during degradation of LLDPE over US-Y zeolite at different ratios and a step temperature programme (0-5 min 573 K, 5-10 min 633 K, 10-15 min 673 K)...

Figure 7.3 Liquid formation during degradation of LLDPE over US-Y zeolite at different ratios...

However all the samples heated in the presence of US-Y catalyst (polymer-to-catalyst mass ratio 2 1) showed a deviation from the original polymer molar mass distribution in the region of lower molar masses. In the first experiment, the polymer/US-Y-zeolite sample was exposed at a temperature of 378 K, which is below its melting point, for 120 min and then for 30 min at 418 K. No volatile products were initially observed, but traces of isobutane and isopentane were detected when the temperature was raised to 418 K. Although these conditions were much milder than in the equivalent experiment with pure polymer (curve number 2), the molar mass distribution, curve number 5 in Figure 7.5, was different from that of the original polymer. 

Figure 7.7 Boiling point distribution of liquid fuel formed over US-Y zeolite, a commercial cracking catalyst, a pillared clay (polymer-to-catalyst ratio 2 1) and comparison with a commercial gasoline sample...

The mesoporosity of these materials has been established by BET measurements and gas adsorption experiments. As the chain length of the surfactant was increased from Cs to G 5, the amount of adsorbed benzene was increased, indicating that there was a relationship between the size of the surfactant and the amount of gas adsorbent taken up by the MCM-41 material. In terms of a comparison to zeolite materials, experiments were done at 60 torr pressure and at 25< C. llie US-Y zeolite sample had an uptake that was about 4 times less than that of MCM-41. The above mentioned MCM-41 materials all show pore size distributions with broad bands centered around 40 A. The pore size distribution measurements are a true indication of the size of the pores and can be used to verify the existence of mesopores. Further evidence of mesoporosity comes from X-ray powder difraction experiments which were done to determine the crystallinity of these materials. The position of the (100) reflection was found to correlate with the amount of uptake by the different materials, or in therwords, with the mesoporosity of these systems. Pores of the MCM-41 materials were shown to form in a hexagonal shape by using high resolution transmission electron microscopy data. 10... 

So far these processes have been modeled in terms of lumps. In catalytic cracking the 3-lump -and the 10-lump model [Nace et al, 1971 Jacob et al, 1976] are still widely used although the lumps are based on boiling ranges rather than on chemical nature. These models contain in general only one deactivation function of an empirical nature for the reactions of the various lumps, b their study of the catalytic cracking of n-hexane on a US-Y-zeolite in an electrobalance with recycle Beimaert et al, [1994] derived an empirical deactivation function of the type (2) for the various reactions, but with different a-values, as illustrated in Table 2 for the isomerizations. 

Martens, G. G. Hydrocracking on Pt/US-Y zeolites Fundamental kinetic modeling and industrial reactor simulation , PhD Thesis, Ghent University (2000). 

A new process for the heterogeneous catalyzed production of campholenic aldehyde out of a-pinene oxide has been found. By using low reaction temperatures of 0°C and below in combination with HCl pretreated highly dealuminated H-US-Y zeolites we have been able to achieve up to 80 % yield of the desired aldehyde. Thefore this process is competitive with the homogeneous ZnBr2 system. 

With heterogeneous catalysts the selectivity depends on adsorption effects. Hdl-derich and coworkers found, that some H-US-Y zeolites with many mesopores are suitable catalysts for this reaction [29]. 

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