Acid site strengths

Table 1 Acid site strength distribution [pmol g 1] of the fresh and regenerated catalysts.

When the temperature is increased at 550 °C, the water removed from the solid reduces Ni2+ to Ni+. The formation of the monovalent nickel cations by thermal reduction of Ni2+-exchanged Y zeolite has been detected using the IR spectroscopy of CO adsorption [9]. The NH3-TPD measurements have evidenced that the acid sites strength strongly... 

Figure 4.28 Effect of steaming and calcination on Bronsted and Lewis acid site strength distributions of a FAU-type zeolite as determined by pyridine adsorption/desorption IR.

By measuring the shifts of the various hydroxyl bands of the zeolite, a direct measure of the relative acid site strengths can be made without the need for thermal desorption. Table 4.6 lists the measured hydroxyl band shifts for a variety of hydroxyl groups on different zeolites using low temperature CO adsorption. This data indicates that there is indeed a difference in the intrinsic acid strength of the bridging hydroxyl groups in different zeolites as well as in the same zeolite structure with different framework aluminum content. 

In addihon to shape selechvity and acid-site strength, other catalyst characteristics that influence the catalyhc performance of SAPO-34 have also been idenhfied. Variahon in the SAPO-34 gel composition and synthesis condihons have been were used to prepare samples with different median particle sizes and Si contents (Tables 15.3 and 15.4) [104]. In these samples the median parhcle size was varied from 1.4 to 0.6 xm, and the Si mole frachon in the product was varied from 0.14 down to 0.016. A comparison of samples B and E (which have similar parhcle size distributions) shows that reducing Si content decreases propane formation and increases catalyst life. A comparison of samples B and C (which have similar Si contents) illushates an increase in catalyst life with a reduchon in parhcle size. 

The acid sites strength can be determined by measuring the heats of adsorption of basic probe molecules. The basic probes most commonly used are NH3 (pTTa = 9.24, proton affinity in gas-phase = 857.7 kJ/mol) and pyridine (pTTa = 5.19, proton affinity in gas-phase = 922.2 kJ/mol). The center of basicity of these probes is the electron lone pair on the nitrogen. When chemisorbed on a surface possessing acid properties, these probes can interact with acidic protons, electron acceptor sites, and hydrogen from neutral or weakly acidic hydroxyls. 

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

The influence of the nature of the aluminum source on the acidic properties of mesostructured materials (MCM41) has also been studied in the literature [244]. Microcalorimetry experiments using ammonia as a probe molecule have shown that Al insertion into the mesoporous silicate framework affected acid site strength and distribution in a manner controlled by the synthesis conditions (materials prepared... 

Microcalorimetric experiments of NH3 adsorption have shown that the isomor-phous substitution of A1 with Ga in various zeolite frameworks (offretite, faujasite, beta) leads to reduced acid site strength, density, and distribution [250,252,253], To a lesser extent, a similar behavior has also been observed in the case of a MFI framework [51,254]. A drastic reduction in the acid site density of H,Ga-offretites has been reported, while the initial acid site strength remained high [248,250]. 

Microcalorimetry experiments with NH3 and pyridine as probe molecules indicated that insertion of Ga into the offretite aluminosilicate structure increased the overall acid sites strength of the crystals while decreasing its acid sites density [255], The observed heterogeneity of acid site strength distribution of H,Ga,Al-offretites was attributed to some extra-framework Al(Vl) and Ga(Vl) species generated during the ion exchange and calcination procedures used to prepare H-offretite crystals. 

Further acid site strength and concentration measurements were reported by Morita et al. (164), who related the acidity measurements to various catalytic reactions. Using Y zeolite (Linde SK-40, 90% H form) activated at 450°C, they observed no acid sites stronger than an H0 of -8.2, although the total acid site concentration was almost twice that of the former investigations (Fig. 21, curve 4). They also measured acid site concentration as a function of decomposition temperature for NH4Y, and found that n-butylamine titration values paralleled results obtained from pyridine adsorption studies (41,151). The maximum total acidity occurred... 

From a series of experiments in this reactor, the deactivation effect of coke on a complex reaction mechanism may be obtained. This is illustrated for the catalytic cracking of n-hcxane on a US-Y zeolite catalyst. On a faujasite, the coke formation deactivates the main reactions, but not the coking reaction. Moreover, the coke formation induces selectivity changes, which can be explained by the distribution of acid site strength in Y-zeolites and the acid strength requirements of the various reactions. 

Finally, it should be noted that calorimetric measurements can also be used to monitor adsorption phenomena at the solid-liquid interface (in a solvent). This method has been used to measure the adsorption heats evolved upon injection of dilute solutions of pyridine in alkanes ( -hexane, cyclohexane) onto an acidic solid itself in a slurry with -hexane. The amount of free base in solution is measured separately with a UV-Vis spectrometer, leading to an adsorption isotherm that is measured over the range of base addition used in the calorimetric titrations. The combined data from the calorimetric titration and adsorption measurements are analyzed simultaneously to determine equihbrium constants, quantities of sites per gram and acid site strengths for different acid sites on the solid. 

The titration of acid sites in liquids of different polarities and proticities (decane, cyclohexane, toluene, methanol and isopropanol) makes it possible to discriminate the acid site strength distribution more accurately than from the more conventional gas-solid phase titration with ammonia. 

Ru-HPA metal-acid bifunctional catalysts on various supports (siUca, graphite, KL zeolite) have also been characterized by NH3 adsorption calorimetry, which revealed heterogeneous acid site strength distributions varying in the order HPA-SiOj > HPA-graphite > HPA-KL [106]. 

Figure 9.17 Acid site strength distribution of Na,H-mordenites as a function of the exchange level. Pyridine adsorption at 573 K, activation at 773 K.

Measurements of acid site strengths with Hr indicators showed that the Bronsted acid sites were stronger than 70% sulfuric acid. No variation with the degree of cation exchange was observed. 

K on HY zeolites with a Si/Al ratio of approximately 2.4 is found to vary from 130 to 205 kJ mol 91,147,149,150,153-155). The same samples also display different acid site strength distributions, some with strong peaks at heats that vary from 125 to 176 kJ mo whereas others show completely monotonic distributions. These strong differences are caused by other variables such as the sodium content (as discussed above), the activation or dehy-droxylation temperature, and the framework Si/Al ratio. 

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