Shape-selective hydrocracking

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

A single-event microkinetic description of complex feedstock conversion allows a fundamental understanding of the occurring phenomena. The limited munber of reaction families results in a tractable number of feedstock independent kinetic parameters. The catalyst dependence of these parameters can be filtered out from these parameters using catalyst descriptors such as the total number of acid sites and the alkene standard protonation enthalpy or by accounting for the shape-selective effects. Relumped single-event microkinetics account for the full reaction network on molecular level and allow to adequately describe typical industrial hydrocracking data. 

Dewaxing is an important specific hydrocracking process used to improve diesel and heating oils by pour-point reduction.88 This is achieved by shape selectivity of certain zeolites allowing selective hydrocracking of long-chain paraffinic waxes to C3—C5 alkanes in the presence of other paraffins.89 Platinum H-mordenite is used in an industrial process.90... 

In principle, all the kinetic concepts of intercalation introduced for layer-structured silicates hold for zeolites as well. Swelling, of course, is not found because of the rigidity of the three dimensional frame. The practical importance of zeolites as molecular sieves, cation exchangers, and catalysts (cracking and hydrocracking in petroleum industry) is enormous. Molecular shape-selective transport (large differences in diffusivities) and micro-environmental catalysis (in cages and channels)... 

The cage or window effect was proposed by Gorring (48) to explain the nonlinear effect of chain length observed in hydrocracking of various n alkanes over T zeolite, chabazite (CHA) and erionite (ERI). Thus, when a nC22 alkane is cracked over erionite, there are two maxima in the size distribution of the product molecules at carbon numbers of 4 and 11 and a minimum at carbon number of 8. The diffiisivities of n-alkanes also change in a similar periodic manner by over two orders of magnitude between the minimum at C8 and the maxima. This shows that for diffusion, and hence for shape selective effects, not only the size but also the structure of the reactant and product molecules need to be considered. 

N. Y. Chen (Mobil Research Development Corp., Princeton, N. J. 08540) It might be of interest to the audience, particularly to those who are not familiar with the application of zeolites in industrial catalytic processes, to mention that since the discovery of catalysis over shape-selective zeolite first published by Weisz and Frilette in I960, a commercial process based on selective hydrocracking reactions similar to that reported in this paper has been in operation on a large scale in more than four of our refineries since 1967. A technical paper describing this process, known as the Selectoforming process, was published in 1968. 

An interesting related study introduced the concept of inverse shape selectivity in molecular sieves [99]. Relative computed adsorption heats for n-hexane and 2,2-dimethylbutane in a series of zeolites with 1-dimensional channels were compared with corresponding experimental adsorption data and data for the relative selectivity to production of these two C6 isomers in hydrocracking of n-Ci6H34. A peak in the relationship between 2,2-dimethylbutane n-hexane selectivity and channel diameter at intermediate pore sizes indicated a channel size domain in which the branched isomer was... 

M. Niwa, K. Kawashima, and Y. Murakami, A Shape-Selective Platinum-Loaded Mordenite Catalyst for the Hydrocracking of Paraffins by the Chemical Vapour Deposition of Silicon Alkoxide. J. Chem. Soc., Faraday Trans. 1, 1985, 81, 2757-2761. 

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

In catalytic de-waxing, special shape-selective catalysts are used to selectively hydrocrack only the straight-chain alkanes to low boiling point by-products. Since it is the linear alkanes that comprise the bulk of the waxy components in the lube boiling range, the oil is effectively de-waxed. There are differences in the composition and properties of base oil de-waxed by catalytic solvent processes because of different selectivities. 

Naphtha reforming catalysts are mostly based on metals (Pt, Pt-Re, Pt-lr, Pt-Sn, Pt-Re-Sn) supported on chlorinated-ALOs or on a KL zeolite. Non-acidic KL zeolite in combination with Pt has been applied in a new reforming process. The non-acidic zeolite support inhibits undcsircd isomerization and hydrocracking reactions leading to enhanced aromalization selectivities [69]. Besides the absence of acidity, the presence of highly dispersed Pt clusters inside the zeolite channels and the shape-selective effects imposed by the monodirectional channel structure (0.71 nm diameter) of the zeolite may also contribute to the excellent aromatization performance of Pt/KL catalysts. 

The efficiency of the catalytic reforming process is determined by the relationship between the octane number (ON) and the liquid yield. For improvement of the ON of reformates the n-alkanes can be hydrocracked shape-selectively on narrow-pores zeolites, i.e. in case of the Selectoforming process (ref. 1) on metal containing H-erionite. During the past 15 years efforts were directed towards the integration of the shape-selective catalyst into the reforming unit (ref. 2). 

A large amount of information has been published on catalytic cracking with molecular shape-selective catalysts, such as natural or synthetic crystalline aluminosilicates. Several excellent reviews have appeared on the chemistry of catalysis with zeolites (52, 53). Literature on hydrocracking of pure hydrocarbons and simple mixtures of hydrocarbons with zeolite-containing catalysts is limited. However, numerous patents on the use of zeolites in hydrocracking catalysts and published... 

For example, Weisz et ah (8, 9,58) studied acid- and metal-catalyzed reactions of n-paraffins in the presence of branched paraffins and aromatics. They report that, by a proper balance of both shape-selective acid and metal functions, the n-paraffins can be hydrocracked from the mixture without simultaneously hydrocracking other paraffin isomers or by incurring the loss of aromatics through hydrogenation or hydro-genolysis. 

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