This A cracking was shown to be 20 times faster than the rearrangement of type B involved in n-alkane hydroisomerization.
As a consequence, over ideal bifunctional Pt HFAU catalysts, that is, over catalysts on which acid isomerization and cracking of alkene intermediates are the limiting steps, the maximum yield of isodecanes that could be obtained is not very high (~50%).16 The maximum yield is even lower (~20%) with HMFI as the acidic component of the bifunctional catalyst; this can be ascribed to limitations in the diffusion of isodecene intermediates, which undergo secondary cracking during their long residence time in the narrow pores of HMFI. However, with another medium-pore zeolite (TON) as acid component of the bifunctional catalyst, an exceptionally high yield in isodecane can be obtained,17 and this practically without deactivation. This seems all the more surprising as (1) TON is a monodimensional zeolite and the pore blockage by carbonaceous deposits of these type of zeolites is known to be very fast, and (2) TON has pore openings smaller than the three-dimensional MFI zeolites (Table 12.1), which should be even more favorable for secondary cracking. Whereas the classic reaction scheme found with bifunctional catalysts, can also be observed with Pt TON, the maximum in monobranched (n-C9), bibranched (dm-C8) are higher and much better separated . Therefore, long-chain n alkanes can be very selectively transformed into isomers, but also to monobranched or to multibranched isomers. Since it is only the first methyl group that causes a significant decrease in the pour point of alkanes (from 30°C to 60°C), this possibility to selectively form monobranched isomers seems in favor to the use of Pt TON as isodewaxing catalyst. However to minimize the pour point, the methyl branch has to be preferentially located toward the center of the chain,15 although isomerization of n-alkanes was found to result in selective 2-methylbranching up to high conversion levels.17 Subsequent isomerization of 2-methylalkanes by alkyl shift (which occurs very fast over bifunctional large-pore-based catalyst) seems desirable.
Because molecular graphics suggested that isomerization of decene intermediates could not occur entirely inside the TON channels,18 it was theorized that this reaction occurred at the mouth of the micropores. This pore-mouth catalysis was confirmed19 by characterization of Pt TON samples recovered after various time-on-stream (TOS). The quasi-immediate retention of 1.5-2.0 wt % of carbonaceous compounds on the catalysts could be observed, these compounds causing a quasi-total blockage of the access of nitrogen (hence, of reactants) to the TON
n-C10 O m-C9 O dm-C8 O Cracking products
channels. These carbonaceous compounds were demonstrated to be essentially constituted by C12-C20 linear and monobranched alkanes trapped within the zeolite channels.
In agreement with this pore blockage, it was suggested that isomerization would be catalyzed by protonic sites at the mouth of the channels; however, another possibility might be that the molecules of products trapped within the micropores near the external surface were the active species. Indeed, a simple mechanism, in which active sites are tertiary carbenium ions formed at the pore mouth by adsorption of methylalkenes trapped in the zeolite pores during the first minutes of reaction, can be theorized.19 This mechanism involves only very simple steps: A isomerization (through alkyl shift), hydride transfer, A cracking, which is simple than the limiting step of isomerization (B-type) catalyzed by the protonic-acid sites located at the pore mouth. Moreover, contrary to this latter mechanism, it requires no diffusion of alkene intermediate molecules in the narrow zeolite channels (Figure 12.3).
High selectivity in hydroisomerization of long-chain alkanes can also be obtained with the other medium-pore zeolites or silicoaluminophosphates similar to TON, that is, having a monodimensional pore system with small apertures: MTT, FER, SAPO 11. Commercial processes were developed using these molecular sieves as catalysts, in particular Isodewaxing by Chevron, Mobil's Selective Dewaxing (MSDW), and Wax Isomerization (MWI) by Exxon Mobil. These
processes are more efficient than those based on solvent extraction or on selective hydrocracking. Thus, results obtained on pilot plants showed that for similar values of the pour point of the product (— 12°C), the yield was much higher (90.5 wt %) with isodewaxing than with dewaxing by hydrocracking; moreover, the viscosity index VI (the temperature dependence of the viscosity, a high VI indicating a low dependence) was of 105 instead of 92.15
18.104.22.168 Generalities. Alkylation of aromatic compounds is practiced commercially on a large scale. The major products are ethylbenzene converted into styrene for polymer use and cumene (isopropylbenzene), a precursor to acetone and phenol.20, 21 Alkylation of benzene by ethylene or propylene is an electrophi-lic substitution catalyzed by protonic acid sites. Acidic halides such as AlCl3, which are typically Lewis acids, have little or no activity when used in a pure state; they become active by addition of co-catalysts such as hydrochloric acid, which interact with AlCl3 to generate a strong protonic acidity (AlCl—H+).
The mechanism of catalytic alkylation is later in the example of benzene isopropylation:
Ethylation, which involves an unstable ethylcarbenium ion as intermediate, is much slower (1500 times over AlCl3) than isopropylation. It is also the case in benzene alkylation with propene for the undesired formation of n-propylbenzene, which involves a primary n-propyl carbocation. Furthermore, as alkyl substituents activate the aromatic ring, consecutive alkylation of the primary product occurs with a greater rate than the first alkylation step (k2 > k1).
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