Hexane, cracking

Another example of performance enhancement using a zeolite/TUD-1 catalyst is shown in n-hexane cracking using a series of zeolite-Beta-embedded TUD-1 catalysts (29) 20, 40 and 60 wt% zeolite Beta in Al-Si-TUD-1 (Si/Al = 150). These are compared to pure zeolite Beta, and to a physical mixture of 40% zeolite Beta and 60% Al-Si-TUD-1. These catalysts were tested in a fixed bed reactor, at atmospheric pressure, with constant residence time at 538°C. The pseudo-first-order rate constants are shown in Figure 41.8. Note that the zeohte-loaded catalysts were clearly superior to both the pure zeolite Beta catalyst and the zeohte-TUD-1 physical mixture. Again, this is evidence that catalyst performance benefits from a hierarchical pore stracture such as zeolite embedded in TUD-1. 

Figure 41.8 Pseudo-first-order rate constants based on the mass of zeolite for n-hexane cracking at 538°C over zeolite Beta-TUD-1 catalysts.

Formation of products in paraffin cracking reactions over acidic zeolites can proceed via both unimolecular and bimolecular pathways [4], Based on the analysis of the kinetic rate equations it was suggested that the intrinsic acidity shows better correlation with the intrinsic rate constant (kinl) of the unimolecular hexane cracking than with the apparent rate constant (kapp= k K, where K is the constant of adsorption equilibrium). In... 

Likewise, the value for hexane cracking at 538°c, the a-value, is proportional to the intrinsic value for toluene disproportionation and xylene isomerization, and is much easier to obtain since it is not affected by crystal size or diffusive alteration (5 ). 

J.R. Katzer and P.G. Rodewald, R.M. Lago for the hexane cracking measurements, F.G. Dwyer for supplying some of the zeolites used in this study, and N.H. Goeke for his experimental assistance. 

Over HZSM-5 catalyst at 538°C, n-hexane cracking follows a first order kinetic rate... 

Fig. 2. Hexane cracking with HZSM-5 (Si/AI = 35) at 538°C. a. First order rate constant determined at different hexane pressures, b. First order plot of conversion (e) at different contact times, hexane pressure = 10 torr.

Fig. 3. Yields of paraffin products from hexane cracking (H2SM-5, Si/AI = 35, 538°C, 10 torr hexane).

The conditions where the bimolecular reaction path predominates are low temperature and high olefin concentration. Although both mono- and bimolecular limiting conditions can be experimentally realized to a good approximation, experiments are often carried out under conditions were both mechanisms contribute to product formation and the kinetics is complex. For example, kinetic evaluation of hexane cracking at 370°C and 150 torr hexane pressure shows that initially the reaction is slow and then accelerates (Fig. 4). 

Fig. 4. Autocatalysis in hexane cracking (HZSM-5 Si/AI = 35, 370°C, 150 torr hexane), a. Conversion vs. contact time. b. First-order plot.

Several different test reactions have been suggested to evaluate the catalytic activity of an acid catalyst as a measure of the number and strength of the active sites. The ideal test reaction is experimentally easy, fast, reproducible, requires only a small amount of catalyst, has simple kinetics, and should show little deactivation. It should also not be diffusion limited and affected by the particle or crystal size. While no one reaction fits all these criteria perfectly, we and apparently others - find that hexane cracking comes closer to the ideal than most other reactions. 

The rate constant for hexane cracking is calculated from k = -FA/V In(l-E) where F is the total gas flow rate in ml s, at the reaction temperature, W is the weight of catalyst in... 

While medium pore zeolites such as ZSM-5 do not deactivate significantly during hexane cracking at 538°C, large pore zeolites usually do. For maximum accuracy of results in these cases we found it advisable to use a low hexane partial pressure of about 10 torr. This not only completely eliminates catalyst deactivation during the test (Fig. 7),... 

As a means to characterize the activity of acid catalysts, hexane cracking offers unique advantages and is best carried out at high temperature, such as 538°C, and low pressure. 

Note that k in this example is defined as the real-time constant for hexane cracking to C5 -. In the selectivity matrix for this system this constant is unity. Thus, Eq. (10), written for C5- formation, becomes... 

Step 1. Fit selectivity rate coefficients in the C6 reversible submatrix relative to the hexane cracking to Cs- rate coefficient using C6 charge data... 

An earlier report from this laboratory (7) noted that in a series of mildly extracted mordenites, the hexane cracking activity in a continuous-flow test went through a marked maximum with increasing severity of extraction, while the f-butane to n-butane ratio continuously increased. The activity and product distribution were measured after 10 min on stream. Since catalyst deactivation was rapid, it was not possible to... 

As indicated above, overall activity for hexane cracking was expressed, for each pulse, in terms of an apparent first-order rate constant, k. The activity declined substantially with increasing pulse number (i.e., with increasing total amount of n-hexane fed) for all samples. 

Catalytic activities for n-hexane cracking were performed using an isothermally operated flow reactor. The feed stream of nitrogen was saturated at 3°C with hexane. With the help of a bypass it was possible to determine both the reactor inlet and outlet concentration of hexane using a gas chromatograph (Varian Star 3400) with FID-detector. 

Figure 5 n-hexane cracking activity of MFI composite materials as a function of the crystallinty parameter Qai... 

Catalytic test reactions (n-hexane cracking) prove that partially crystallized biphasic ZSM-5 containing silicate materials are catalytically active composites. The activity of the prepared biphasic silicates is related to the crystalline fraction in the pellets. 

Fig. 20. Effect of activation temperature on catalytic activity. O, ethylene-benzene alkylation (160) , toluene disproportionation (157) A, n-hexane cracking (161) O, 1-methyl-2-ethylbenzene isomerization (158).

The effect of steaming and of extensive poisoning by alkali metal ions is not limited to Y-type zeolites, as Lago et al. (12) have observed similar phenomena with mildly steamed H-ZSM-5. The activity for hexane cracking increased by about a factor of four upon mild steaming of the catalyst. Selective Cs poisoning indicated that the concentration of a more active site in the steamed sample was only about 6% of the tetrahedral framework aluminum. These sites exhibited a specific activity 45-75 times greater than that of a normal site in H-ZSM-5. 

Figure 3. The dependence of hexane cracking activity on the framework A1 content for four series of Na+-poisoned catalysts. Each Na+ is assumed to poison one framework Al atom. ( ) A series, ( ) B series, ( A ) C series, ( ) D series. The solid line represents the theoretical activity based on isolated Al atoms (6). Reproduced with permission from Ref. 10.

Experiments to further demonstrate the critical role of extraframework Al, or another polyvalent cation, have recently been carried out in our laboratory (19.20). A series of faujasite-type zeolites was prepared that had Alf concentrations between 21 and 54 per u.c. At the low end of the range, AHF was used to remove the framework Al, and an H-ZSM-20 zeolite with 42 Alf/u.c. was synthesized. ZSM-20 is an intergrowth of the cubic faujasite structure and the hexagonal variant know as Breck s structure six (BSS) (21). Thus, it is a faujasite-like material. The catalytic activities of these zeolites for hexane cracking are compared in Figure 5 (lower data set) with the activities of zeolites prepared by steaming or by treatment with SiClA (upper data set). The solid lines represent N(0) distributions. The samples without extraframework Al exhibited very modest activity, even though some of them had a favorable N(0) concentration. 

The amount of extraframework Al that is needed to develop a strongly acidic H-ZSM-20 zeolite is actually quite small, as indicated by the results of Figure 6. Dealumination of an H-ZSM-20 zeolite from an original concentration of 41 Al /u.c. to 31 Alf/u.c. resulted in the maximum increase in activity for hexane cracking. This increase is almost the mirror image of the decreases that were noted previously for Na+ poisoning. The 10 Al atoms that have been removed from the framework correspond to about 1.3 Al per small cavity. As more Alf atoms are removed, the activity decreased in a manner consistent with the N(0) distribution that is indicated by the solid line in Figure 6. 

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