Zeolite acid sites

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. 

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. 

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... 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

Characterization is an integral tool for the development of new zeolites and for the development and commercialization of zeolitic catalysts and adsorbents. Single techniques are not sufficient as they rarely provide full details of the system. A combination of selective characterization techniques is required. As suggested by Deka [1] even a single acidity characterization method may be insufficient to provide the necessary detailed information to understand the zeolite acid sites. Thus according to Deka the combination of different experimental techniques is required to shorten the time of development for a new catalyst. 

Likewise, A-(l-propylidene)-l-propanamine is obtained in liquid phase from 1-propanamine on a Cu-containing MFI zeolite, where the zeolite acidic sites selectively converts 1-propanamine to dipropanamine and the dispersed Cu metal dehydrogenates the amine to imine.[22]... 

Maxwell et al. 177, 178) studied the deactivation of reduced Cu2+Y catalysts for butadiene cyclodimerization in some detail. This work showed that the catalyst stability could be markedly improved by using NH3 as a reducing agent and choosing the activation conditions such that excess NH3 remains selectively chemisorbed on the zeolite acidic sites. Further, the Cu2+Y-derived catalyst was thermally stable to 850°C and was therefore able to withstand a regeneration procedure which involved a polymer burn-off at 550°C. By contrast, the catalysts prepared by direct exchange with monovalent copper, i.e., Cu+Y, formed CuO irreversibly when heated above 330°C. 

Aromatization of short alkanes can be carried out on a purely acidic HZSM-5 zeolite. In this case, activation of the alkane is thought to occur by protonation on the zeolite acid sites, with formation of a penta-coordinated carbonium ion (1). In the case of propane, protonation at a C-H bond will lead to the formation of H2 and a sec-propyl cation (dehydrogenation), while protonation at a C-C bond will produce methane and a primary ethyl cation (cracking) ... 

As stated above, the aromatization of short alkanes is carried out in presence of bifunctional catalysts, in where the dehydrogenating function is given by the metal component (Ga, Zn, Pt) and the H-ZSM-5 zeolite carries the acid sites. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step. Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

Data obtained in the catalytic epoxidation of 1-hexene over Ox-Ti-P and other samples are summarized in Table 2. Catalytic properties of Ti-P zeolites were studied by Corma et al. [4,10] and Davis et al. [11,12]. Despite some discrepancies, it is agreed that these catalysts are active in the epoxidation of olefins. Our results also indicate that all of our Ox-Ti-P and Ti-P samples are active in the epoxidation of 1-hexene. The selectivity toward epoxide was very low. The major products were ethers, obtained from solvolysis of glycol by methanol which is catalyzed by the zeolite acid sites. It was found that over Ox-Ti-P samples, the reaction takes place slowly, while the hydrogen peroxide is utilized efficiently. Over Ti-P, the reaction takes place very rapidly and is usually finished in less than 1 hour. It was also found that the parent aluminosilicate P (sample 1) was completely inactive in this reaction. Davis et al. [12] demonstrated that framework Ti is the active site in epoxidation reactions, particularly in aqueous media. It is inferred that our catalysis data provide a strong evidence that Ti(IV) species in our Ox-Ti-P samples are present as isolated framework cations. 

Consequently, in contrast to Ga/MFl cataly sts no complementary synergy between extraframework A1 species and zeolite acid sites is observed in the early stages of propane activation. Aprotonic sites do not play a direct role in propane activation. The correlation observed between the initial rate of propane scrambling and the amount of framework A1 atoms, the dependence of propane activation rate on acid site strength, and the inhibiting effect of co-adsorbed bases ohserved on niire H-MFI siippe< t monofnnctional nronane activation on stronp acid sites... 

Apparently the H-source is elusive to NMR because it is strongly chemisorbed to the solid surface. This implies that secondary carbenium ions do not persist because they are more acidic [12] than the zeolite acid sites. 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

Techniques for the characterization of acid sites in zeolites have progressed much in the past decade. Advances have also been made in the understanding of factors contributing to acidity in such catalysts. No technique can claim superiority in its ability to characterize zeolitic acid sites and, indeed, the technique of choice is most likely to be dictated by the particular catalyst of problem at hand. 

The characterization of acid sites is perhaps better approached using a multitechnique approach, that is using a variety of techniques, each better suited for characterizing a particular acidic site. Clearly, comparisons between various techniques are necessary in order to arrive at a complete description of a zeolitic acid site. 

Water can act in this environment as a Bronsted base to neutralize some of the weaker zeolite acid sites. This effect is not harmful to any appreciable extent to the beta zeolite catalyst at typical feed stock moisture levels and under normal alkylation and transalkylation conditions. This includes processing of feedstocks up to the normal water saturation condition (typically 500-1000 ppm) resulting in 10-150 ppm water in the feed to the alkylation reactor dependent on feed and/or recycle stream fractionation efficiency. 

Additional information about the catalytic performance of the catalysts can be obtained from the analysis of the product distribution, which is affected by the metallic and acid functionalities. Tables 4 and 5 compare the product distributions obtained in the DBT and 4,6-DMDBT reactions with the NiMo/Al203, NiMo/HNaY and NiMo catalysts with 20% of HNaY in their formulation. In the case of DBT, zeolite incorporation into the catalyst changes the contributions of the direct desulfurization (DDS) pathway, which yields biphenyl-type compounds, and of the desulfurization through hydrogenation (HYD) pathway, which gives cyclohexylbenzene-type compounds. Also, the proportion of CHB in the reaction products and the liquid yield decrease with the number of accessible zeolite acid sites in the catalyst. This effect is due to the cracking of CHB on the zeolite acid sites. On the other hand, the formation of DCH is enhanced on the catalysts where Mo precursor phase is more polymerized (NiMo/HNaY-Al203(P) and NiMo/HNaY formulations). 

---