Zeolite acidity

Sodium decreases the hydrothermal stability of the zeolite. It also reacts with the zeolite acid sites to reduce catalyst activity. In the regenerator, sodium is mobile. Sodium ions tend to neutralize the strongest acid sites. In a dealuminated zeolite, where the UCS is low (24.22°A to 24.25°A), the sodium can have an adverse affect on the gasoline octane (Figure 3-7). The loss of octane is attributed to the drop in the number of strong acid sites. 

Improved crystallinity by producing more uniform zeolite crystals, FCC catalyst manufacturers have greater control over the zeolite acid site distribution. In addition, there is an upward trend in the quantity of zeolite being included in the catalyst. 

Another major cause of waste is the use of mineral acids (H2SO4, H3PO4, etc.) and Lewis acids (AICI3, ZnCL), often in stoichiometric amounts, which cannot be recovered and recycled. A typical example is the HNO3/H2SO4 mixture used in aromatic nitrations. Consequently, there is a discernible trend towards the use of solid, recyclable Brpnsted and Lewis acids, e.g. zeolites, acidic clays, etc. (see later) as alternatives to conventional mineral and Lewis acids. 

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

The main focus of this work is on the influence of metal introduction into zeolites on the ring-opening activity and selectivity. The effects of zeolite acidity and structure on the product distribution will be discussed as well. Decalin is used as a representative molecule for ring opening of dinaphthenes formed during hydrogenation of diaromatics in middle distillates. 

Di or trivalent cations are able to induce the dissociation of coordinated water molecules to produce acidic species such as MOH+ (or MOH2+ for trivalent metal cations) and H+. Several infrared studies concerning rare-earth or alkali-earth metal cation exchanged Y zeolites have demonstrated the existence of such species (MOH+ or MOH2+) [3, 4, 5, 6]. However, the literature is relatively poor concerning the IR characterization of these acidic sites for LTA zeolites. The aim of the present work is to characterize 5A zeolite acidity by different techniques and adsorption tests carried on 5A zeolite samples with different ion exchange. 

Keywords SSZ-33, SSZ-35, zeolites, acidity, toluene, alkylation, disproportionation... 

Modulation of zeolite acidity by post-synthesis treatments in Mo/HZSM-5 catalysts for methane dehydroaromatization... 

In this work we modified an HZSM-5 zeolite through different dealumination treatments and partial ionic exchange with Cs+ in an attempt to modulating the zeolite acidity and thus to improving its behavior for the MDA reaction. 

The transformation of n-hexadecane was carried out in a fixed-bed reactor at 220°C under a 30 bar total pressure on bifunctional Pt-exchanged HBEA catalysts differing only by the zeolite crystallites size. The activities of the catalysts and especially the reaction scheme depended strongly on the crystallites size. Monobranched isomers were the only primary reaction products formed with the smallest crystallites, while cracking was the main reaction observed with the biggest crystallites. This was explained in terms of number of zeolite acidic sites encountered by the olefinic intermediates between two platinum particles. 

Hydroisomerization of n-octane over Pt-containing micro/mesoporous catalysts obtained by recrystallization of zeolites BEA and MOR was investigated in the temperature range of 200-250 °C under 1-20 bar. Composite materials showed remarkably high activity and selectivity with respect to both pure microporous and pure mesoporous materials. The effect is due to high zeolitic acidity combined with improved accessibility of active sites and transport of bulky molecules provided by mesopores. 

Thus, zeolites may replace such environmentally unfriendly acid catalysts as A1C13 or H2S04 in organic transformations, contributing to cleaner and safer methodologies. The strength and concentration of the acid sites can be modified by controlling the Si/Al ratio, and therefore the zeolite acidity can be adjusted for a particular application. 

The successful application of 170 NMR of zeolite acid sites has been discussed in Sect. 2.1.4 in the context of the 170 chemical shift scale. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

The analysis of the structural properties of zeolitic acid sites based on dipolar interactions has further improved the understanding of acidity. Grey and Vega were the first to apply the 1H 27A1 TRAPDOR technique [36]. The REAPDOR method was first applied by Kalwei and coauthors [236-238] on bare acid sites and also on zeolites loaded with probe molecules. These methods allow one to distinguish... 

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