Strength of acid sites

We can state reasonably activity in alcohol dehydration and high selectivity to ethers of large pore H-zeolites. The concentration and strength of acid sites is important. The mesoporous aluminosilicate showed very little activity despite very large pores because of low number and low strength of acid centers. 

The pillaring process also affected the concentration and the strength of acid sites, as confirmed by NH3-TPD (Table 1). Also, the ion exchange with Ni2+ cations modified the acid properties of surfaces new Lewis (nickel cations) and Bronsted (H+) acid sites have been created during the ion exchange and thermal activation (eq. 1), respectively. 

It can be seen that the catal5dic activity strongly depends on the number and t) pe of the incorporated countercation, which determines the number and strength of acid sites. In addition to this, the existence of mesoporosity (which also depends on the countercation) is also a key factor in the catalytic behavior of these catalysts.In this way, Si02- or MCM-41-supported heteropolyacids also have been studied in order to increase catalytic activity, apparendy without modifying the acid strength. ... 

It is concluded that the proposed model explains well all the observed results with the three different samples and can be considered to be highly reliable. The strength of acid sites in zeolite L correlates well with the Al distribution in the framework, which can be modified by the deviation of Si/AI ratio from the ideal value of 3. The larger the extent of the deviation becomes, the lesser the number of site pattern I, or site D , is. This gives a clue to the generation of strong acid sites in zeolite L. 

The solid acidity of these atom-planted silicalites was examined by ammonia TPD measurement. The ammonia TPD profiles of all these metallosilicates showed one peak at a higher temperature than 453 K. Figure 4 shows the relationship between peak temperatures of ammonia TPD and wavenumber of IR absorption bands due to the acidic SiOH groups of atom-planted silicalites. A straight line was obtained. From these results, the order of strength of acid sites would be as follows ... 

Ammonia Chemisorption. Ammonia gas has been widely employed as a basic adsorbate to count the number and strength of acid sites on various solid surfaces (28,29). The nature and strength of these sites may relate the activity and selectivity character of the catalysts. Further, all acid sites on catalyst surface are easily accessible to the small molecules of NH (kinetic dla 0.26 nm) and... 

Effect of the Dealumination of H-Mordenite - The acid catalysis depends on the number and strength of acid sites on the catalyst. However, the catalytic... 

H-NbMCM-41 sieves appeared to be useful catalysts in the hydrosulfiirization of methanol towards methanethiol thanks to the presence of LAS and BAS pairs, low strength of acidic sites (side reactions are minimized), and the absence of dissociative adsorption of H2S... 

Number and Strength of Acid Sites from Catalytic Titrations 114... 

In the general case, however, both the number and strength of acid sites will vary from one catalyst to another. The determination of both of these parameters by means of model reactions is described in the following section. 

An apparently straightforward method for the determination of number and strength of acid sites consists of the determination of the amount of base required to poison catalytic activity for a model reaction. By means of plots of activity versus amount of added base, the number of acid sites is obtained from the threshold amount of base required to remove catalytic activity acid strength is gauged from the slope of the titration curve. This method can therefore be called a catalytic titration. 

The strength of acid sites can be approximated by determining the heat of adsorption using the Van t Hoff isochore. In this method the pressures P required to give a fixed surface coverage at a series of temperatures T is determined. The derivative of In P with respect to T is related to the isosteric heat of adsorption. This technique is not used routinely. Nor is that of IR spectroscopy, where it is reported that Lewis and Bronsted sites can be differentiated by using substituted compounds. 

Finally, it may be possible to use the rate of a chemical reaction to determine the actual number of active acidic sites. Furthermore, it may be possible to obtain an estimate of the range of strengths of acid sites by using different reactant molecules. For example, it is known that dehydration of alcohols requires only relatively weak acid sites, whereas cracking of alkanes requires very strong acid sites. 

As stated above, shape selectivity due to molecular sieving depends on the relative rates of diffusion and reaction, hence on the respective sizes and shapes of molecules and pores and on the characteristics of active sites (e.g. concentration, nature and strength of acid sites). Obviously the diffusion rate, hence the selectivity depend also on the length of the diffusion path (i.e., on the size of the zeolite crystallites). The selectivity of a zeolite catalyst can be optimized by an adequate... 

TPD using base adsorbents such as ammonia and pyridine has been one of the common techniques to evaluate the amount and strength of acid sites on solid acids. However, the desorption temperature of ammonia and pyridine adsorbed on solid superacids is so high (500 °C and above) owing to the strong interaction that the adsorbed compounds are decomposed or oxidized before desorption. The surface structure can even be destroyed by reactions with probe molecules [120-122]. 

XPS of chemisorbed nitrogen containing basic molecules such as pyridine and ammonia have been employed to monitor the strength of acid sites in H and cationic forms of zeolites. One interesting problem is to follow the changes in concentration of the Brbnsted and Lewis acidic centers with the temperature of thermal treatment. Two mechanisms have been proposed for the thermal degradation. Dehydroxylation occurs through a process first described by Uytterhoven et al. [35]... 

However, it should be emphasized that the measurement and quantification of the acid/base strength of zeolites is complex and that it is difficult to directly compare the aciti base strength of a solid with that of a liquid. This results from the feet that the stabilization of carbocations and carbanions in a microporous solid differs from that in strongly polar acid and base solutions. For zeolites, it can be stated that the concentiatlbn of aluminum in the lattice is directly proportional to the concentration of acid sites and the polarity of the lattice and to a first approximation indirectly proportional to the strength of acid sites [40]. For a given chemical composition of the zeolite, the polarity of the lattice increases with decreasing framework density [41],... 

Table XVII shows the maximum acid strength of Al-Mont catalyst in various organic solvents. In CHjClj or PhCHj, strongly acidic sites (//q < —8.2) were detected on Al-Mont. On the other hand, the acid strength of Al-Mont was weakened to -5.6 < Hq < -3.0 in DME. 1,2-Dimethoxyethane is a relatively basic molecule and thus interacts with the acid sites on montmoril-lonite to reduce their acid strength. The diastereoselectivity of the aldol reaction catalyzed by Al-Mont probably relates to the acid strength of Al-Mont because the degree of interaction between aldehydes (acetals) and acid sites on Al-Mont is affected by the acid strength of acid sites and influences the stabilities of the transition states which involve both enolsilane and aldehyde.

The influence of the pore system structure of the zeolite becomes apparent through experiments with the hexagonal and the cubic modification offaujasite, EMT and Y. These were chosen because the acid sites in both zeolites are reportedly equivalent [8]. Since the Si/Al ratio in the used catalysts is nearly the same (EMT-a = 3.6, Y = 3.5), the numbers of add sites are equal, too. The results are presented in fig. 5. The activity of EMT is slightly higher than that of Y whereas the selectivity (= (3)/(4)) is the same it increases with the catalyst amoimt from 15 to 18. Since the strength of acid sites in Y and EMT is known to be very similar and the numbers of sites are also nearly the same, the higher activity of EMT can only be explained by easier diffusion of reactants and products in the straight channels of EMT than in the cavity system of the FAU-structure. 

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