Protonic zeolites external surface

The effective area of the anionic aluminosilicate framework in the pores of a zeolite is at least 100 times the external surface area, and it can be as high as 1000 m2 g-1. Consequently zeolites are unusually effective as catalysts for reactions that are favored by aluminosilicate surfaces. Substitution of Sj4+ Al3+ in a silica framework makes it acidic and, potentially, coordinatively unsaturated. Suppose, for example, that we heat the NH4+ form of a zeolite. Ammonia is driven off, and one H+ remains to counterbalance each Al3+ that has substituted for a silicon. The protons are attached to oxygens of the aluminosilicate framework ... 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

Figure 5. Catalytic activity of CsxH3 xPW 2O40 for alkylation of 1,3,5-trimethylbenzene (TMB) with cyclohexene as a function of surface acidity. The surface acidity is the number of protons on the surface (external surface for zeolites).

Their total concentration would therefore be essentially proportional to the external surface area of the crystals. Similar surface A1 atoms may be responsible for the low kinetic energy Auger peaks observed in Rgure 12 for high silica zeolites. In this case the sodium cation instead of the proton would be adjacent to the silanol group. 

The process of cation exchange is that by which protons and other cations on the surface or within the structure of the support are replaced by cations of the active metal and this leads, with the noble elements of Groups 8-10, first to atomically dispersed species and then, after an optimal calcination and careful reduction with hydrogen, to extremely small metal particles [18], The procedure is especially effective with zeolites. But the introduction of catal Tically active species into the cavities of these materials, as opposed to placing them on their external surface, presents certain difficulties, namely the lack of suitable cations or cationic complexes [18], Nevertheless, different kinds of Au/zeoiite systems have been prepared this way [78-89],... 

The dominant deactivation process is coke formation, which explains why the catalyst can be regenerated by controlled coke combustion. The work done by Viswanadham, et al. ° and Echevsky, et al. on H/ZSM-5 catalysts indicated that coke tended to accumulate on the binder and external surface of the zeolite and occurred in such a way as to leave the active sites accessible. Deactivation occurred mainly via pore blockage. A decrease in acid strength was caused by the shifting of electron density from the condensed aromatic structures onto the active proton sites. Little loss of mean free pathlength was observed due to coke formation on H/ZSM-5 and coke did not increase diffusion resistance. Yet, the location of the coke deposits led Viswanadam, et al. to postulate that a molecular traffic control mechanism may be operative due to the position of the coke that forms in the zeolite itself. 

ITQ-2, a novel zeolitic structure prepared by swelling and delaminating a MWW precursor, has been studied by IR spectroscopy. The same precursor yields, when calcined, the zeolite MCM-22. Bronsted acidity has been measured as the propensity either to engage in H-bonds or to transfer the proton to unsaturated hydrocarbons. Comparison with MCM-22 shows that dealumination accompanies the process of delamination, but no appreciable change in residual Bronsted acidity takes place. Reaction of propene with Bronsted sites to branched oligomers occurs mainly on the external surface. Oligomers show no tendency to evolve to allylic cationic species, in contrast with MCM-22. 

Recently, it has been shown by a combination of XPS with other surface-analytical methods (UPS, ISS) that Ae Na+ distribution in partially exchanged NaH zeolites can give rise to another difference between surface and bulk properties [2]. After the typical thermal treatments used to obtain acidic protons (calcination of the NH4 form, reduction of reducible cations cf. also [47]), Na+ is depleted in the external surface layer. The effect was interpreted as resulting from an exchange of Na+ at near-surface sites by protons from deeper layers due to a difference in Madelung stabilization for Na ions and the less-charged protons between surface and bulk sites. 

Beutel et al. (2001) studied the interactions of phenol with zeolite NaX using solid-state H and Si MAS and Si CP-MAS NMR spectroscopy. The H MAS NMR spectrum of NaX degassed at 400°C included peaks at 5h= 1.24,2.05,4.05, and 5.11 ppm. The signal at 2.05 ppm is characteristic of silanol protons at the external surface of the zeolite. On NaY zeolite (Si/Al=2.4), a peak at 5h= 1.6 ppm was found after pretreatment in air and in vacuum at 400°C and assigned to traces of water adsorbed on cations. Therefore, the peak at 8h=1.24 ppm was attributed to free hydroxyl protons of water molecules strongly adsorbed on Na+ ions. The peaks at great 8h values could be attributed to water bound to different active sites. 

The nature and location of protonic and cationic sites of Co-H zeolites (FER, MOR, MFl) were studied by UV-vis and IR spectroscopy of adsorbed hindered nitrides [04M3]. They showed, in exchanged Co-H-FER, that Co ions are distributed between the internal and external surface of zeolite. The UV-vis speetia of eobalt were similar in partially exchanged zeolites and on rrricroporous silica-altrrttina (srrrface Co species). The UV-vis spectra of Co-species in exchanged zeolites resrrlt to be quite irrserrsitive to the nature of the errvironmerrt where the ions are located. 

The external surface of protonic zeolites can play a role in acid catalysis, e.g., in alkylaromatics conversions over H-MFI [96], because H-zeolites pro-... 

The overall situation resembles closely that of H-[A1]-MTS, and similar proposals have been made for both H-[A1]-MTS and the external surface of protonic zeolites. For instance, the suggestion has been made that acidic sites at the external surface of protonic zeolites are spectroscopically indistinguishable from terminal silanols [100]. At least for the case of lTQ-2, this hypothesis is disproved in a similar way as done with H-[A1]-MTS, i.e., reversible interaction of ammonia with silanols and the study of the related isotherms show a marginal difference in acidity between silanols in ITQ-2 and pure silica MCM-41 [103,104]. Onida et al. proposed as acidic centers the AIOH(II) species also for H-[A1]-MTS, absorbing at 3660 cm when free, and advanced the same reason for its invisibility (interaction with the sm-roundings). 

In conclusion, the external surface of protonic zeolites bears many resemblances with that of silica-alumina. This is because at the external surface of zeoHtes the rigid nature of the crystalline structure is relaxed, causing instability of the bridging sites. 

The external surface of protonic zeolites can be relevant in acid catalysis. Several data suggest that nonshape-selective catalysis can occur at these sites, like in the case of alkylaromatics conversions over H-MFI [225,226]. On the other hand, H-zeolites also catalyze reactions of molecules, which do not enter the cavities due to their bigger size. Therefore, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk versus surface Si/Al composition of a zeolite could be different and different preparation procedures can allow to modify this ratio [226]. Corma et al. [84] reported data on the accessibility of protonic sites of different zeolites to 2,6-di-ter-butyl-pyridine (DTBP). This molecule has been considered selective for Brpnsted sites, due to its impossible interaction with Lewis sites for steric hindrance. According to these authors, however, the interpretation of the data is not straightforward, for several reasons such as the presence of different cavities and the big size of the probe itself. Surprisingly, Corma et al. found a complete accessibility of the sites of beta zeolite to DTBP. This contrasts the data of Trombetta et al. [197], who showed that protonic sites exist also on the smaller channels of beta, whose access to DTBP seems very unlikely. In a more recent publication, Farcasiu et al. [227] reported an accessibility of 90% of the protons of H-BEA to DTBP, much higher than the 36% for H-MOR and 31% for H-USY. 

All interactions taking place simultaneously contributed to the calorimetrically measured heat, and consequently it is hard to single out the energetics of the individual contributions to the interaction. At the Brpnsted Si(OH)+Al sites H2O molecules are either strongly H-bonded or protonated, whereas at the Lewis acidic sites, i.e. the cus framework Al (III) cations, H2O molecules are oxygen-down coordinated [25]. H2O molecules interacted also via H-bonding with Si-OH nests located within the zeolite nanocavities and, more weakly, with (isolated) Si-OH species exposed at the external surface. Aspeciflc interactions generated by confinement [82-84] also contributed to the overall measured heat. 

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