Pore-mouth catalysis

Ethylbenzene (EB) transformation was carried out on bifonctional catalysts based on 10MR zeolites (ZSM-5, Ferrierite, ZSM-22, EU-1) and compared to Mordenite based catalysts. This work shows that monodimensional (ID) 10MR channels or large cavities are highly selective towards isomerization. For 10MR(1D) zeolites, this selectivity is attributed to microporosity blockage suggesting a pore mouth catalysis. 

These observations could be interpreted as a pore mouth catalysis. It was suggested that EU-1 fresh catalyst comprises two types of active sites, inner and external acid sites, the first ones which are non selective to isomerization and sensitive to deactivation, the second ones selective to isomerization but non sensitive to deactivation. Selectivity of inner acid sites could be estimated by difference between results obtained after 45 minutes and those obtained after 8 hours. These results are shown on table 3. 

Because molecular graphics suggested that isomerization of decene intermediates could not occur entirely inside the TON channels, it was theorized that this reaction occurred at the mouth of the micropores. This pore-mouth catalysis was confirmed 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... 

Even emerging concepts such as those related to the shape selectivity of the external surface of the crystallites (nest effect, pore mouth catalysis,. ..) have already been applied to develop new catalytic processes isodewaxing, selective... 

When the hydrogenation function is embedded in the crystal voids of an MFI topology, the formation of trans-isomers is strongly reduced. After partial reduction of soy bean oil with such catalyst from an iodine value of 140 to 80, virtually no trans-isomers are obtained (56). This is the result of pore mouth catalysis combined with zeolite shape selectivity. Due to the bent character of the cts-isomer chains in triglycerides, trans-configured chains preferentially enter the pore mouths for hydrogenation. In this environment, metal-catalyzed cis-trans isomerization is restricted for steric reasons as multiple readsorption is minimal. 

The catalytic properties associated with the molecular shape-selectivity exhibited by ZSM-5 are now well known. Recent work by Martens et al. (1995) has revealed that the external surfaces of zeolite crystals have also to be considered as potential shape-selective environments. Thus, strong evidence has been obtained for a lock-and-key model, which involves a form of pore mouth catalysis with bulky long-chain molecules that cannot penetrate into the intracrystalline micropores. The proposed lock-and-key model for n-alkane isomerization over ZSM-22 zeolite (with tubular pore openings of 0.55 x 0.45 nm) seems likely to be valid for other catalytic reactions. 

ABSTRACT. The amount of published work on molecular shape-selective catalysis with zeolites is vast. In this paper, a brief overview of the general principles involved in molecular shape-selectivity is provided. The recently proposed distinction between primary and secondary shape-selectivity is discussed. Whereas primary shape-selectivity is the result of the interaction of a reactant with a micropore system, secondary shape-selectivity is caused by mutual interactions of reactant molecules in micropores. The potential of diffusion/reaction kinetic analysis and molecular graphics for rationalizing molecular shape-selectivity is illustrated, and an alternative explanation for the cage and window effect in cracking and hydrocracking is proposed. Pore mouth catalysis is a speculative mechanism advanced for some systems (a combination of a specific zeolite and a reactant), which exhibit peculiar selectivities and for which the intracrystalline diffusion rates of reactants are very low. 

For the classic types of molecular shape-selectivity in zeolites, the reader is referred to the excellent review papers in literature [18-25]. In this paper we elaborate on the recently proposed distinction between primary and secondary shape-selectivity [26], and on the more or less abused concept of cage and window effects in cracl g and hydrocracking. In addition, some evidence available in literature for the speculative mechanism of pore mouth catalysis is presented. 

Katzer [42] concluded that the alkylation of benzene with propylene in the liquid phase on H-mordenite catalysts is an example of pore mouth catalysis. This conclusion was based on the observation that the counterdiffusion of benzene and cumene under the reaction conditions is extremely slow. 

One way to rationalize the behavior of Pt/ZSM-22 is via a branching mechanism, where the reacting molecules enter the zeolite pores partially, are branched at the end which does not penetrate the pores, and consequently are obliged to desorb. However, the high selectivity for 2,7-dimethyloctane in the dibranched isomers from decane [51,52] points to the existence of a site which is particularly suited for branching the end of hydrocarbon chains. As 2-methylalkanes in reaction conditions cannot enter the pores of ZSM-22, the hypothesis of "pore mouth" catalysis is preferred in this case over product shape-selectivity. 

Figure 12.4 Schematic representation of methyl branching at the chain end of n-alkanes via pore-mouth catalysis (PMC) and dibranching at specific positions via molecular recognition in neighboring pore mouths (key-lock catalysis (KLC) [55].

A review on cage and window effects in mainly hydroconversion of alkanes with zeolites [73] shows that bi- and even trinodal distributions of product carbon numbers can be formed. In the erionite (ERI) cage Cs hydrocarbon fragments are selectively trapped, thus undergoing intense secondary cracking. This effect was confirmed in the ketonization of carboxylic acids [74]. Alternatively, in cases of slow diffusion (and counterdiffusion), viz. in the liquid-phase propylation of benzene in mordenite, the possibihty of having pore-mouth catalysis was advanced [75]. Multinodal product distributions from... 

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