Acid strength distribution

The density of Bronstcd and Lewis acid sites was determined by IR spectroscopy (Nicolet 710) of adsorbed pyridine, after desorption at 250°C, using the molar extinction coefficients previously obtained by Emeis [11]. The acid strength distribution of selected zeolites was studied by NH3-TPD in an Autochem 2910 Equipment (Micromeritics) coupled to a quadrupole mass spectrometer. First, NH3 was adsorbed at 175°C until saturation and then desorbed by increasing the temperature up to 800°C at a heating rate of 10°C/min. 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

It was also shown than DAY zeolites prepared by different methods have different acid strength distributions (18). This was related to structural differences between these zeolites. 

White28 indicated that the adsorption of pyridine molecule can be used to determine the concentration of Bronsted and Lewis acid sites. When IR is used in conjunction with thermal desorption, an estimation of the acid strength distribution can be obtained. 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

It is known that the activation temperature can influence the acid strength distribution. For example, measurements of the differential heats of ammonia adsorbed at 150°C for a HY zeolite have led to the conclusion that stronger acid sites, in the 150-180 kJ/mol range, are formed upon increasing the activation temperature from 300 to 650°C. Dehydroxylation at high temperature resulted in the formation of strong Lewis acid sites and the disappearance of intermediate and weak Brpnsted sites [62]. 

Fig. 4. Acid strength distribution for the clays pillared with oxide sols heated at 500°C O, TiOo F3, Si02 TiO A. Si02-Feo03 pillared clays.

The work of Misono et al. (55) illustrates how acid strength distributions for silica-alumina catalyst can be deduced from catalytic titration measurements by use of an appropriate series of reactants. Surface concentration of amine, pyridine in this case, was adjusted by proper choice of amine partial pressure and desorption temperature while carrier gas flowed over the catalyst sample. At each level of chemisorbed pyridine, pulses of the reactants were passed over silica-alumina at 200°C and the products analyzed. The reactants were t-butylbenzene, diisobutylene, butenes, and f-butanol. It was concluded that skeletal transformations require the presence of very strong acid sites, that double-bond isomerization occurs over moderately strong acid sites, and that alcohol dehydration can occur on weak acid sites. 

Yoshizumi et al. (70) determined acid strength distributions on silica-alumina catalyst calorimetrically by measuring the heat adsorption of n-butylamine from benzene solution. They found that the differential heat of adsorption of n-butylamine ranged from 3.7 kcal/mole (weak acid sites) to 11.2 kcal/mole (strongest acid sites). 

The size of the indicator molecules most probably affects also the observed acid strength distributions. 

In their surface model, Zecchina et al. (145,146) propose the presence of five distinct types of Cr3 + ions which differ in the coordination number (4 or 5) and in the nature of their ligands. Nevertheless, only one set of infrared bands for the PyL species could be observed. This indicates that the differences in the acid strength of the different Cr3+ ions are small and/or that the vibrational modes of the coordinated pyridine do not respond sensitively enough to the intrinsic acid strength distribution. 

Acidity and acid strength distribution were evaluated by temperature-programmed desorption of ammonia. The acid sites were classified as weak < 300°C, medium (M) 300 < T < 450° 0 and strong (S) 450 < T < 550° 0. Ultra strong acid sites (S in a. u ) are defined as a ratio of peak height for catalyst to alumina (Ni cat./AfjOj) at 550° C,... 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

Nonaqueous methods include the use of amine titration and adsorption of indicators for visual measurement of acid strength. This procedure allows both the determination of the total amount of acid sites and also the acid strength distribution. A disadvantage is that bulky molecules (amines and indicators) arc used and these may be excluded from entering small pores. With zeolites, the slow rate of diffusion and equilibration has to be taken into account. Spectroscopic measurement of acid strength may also be performed using amine titration and indicator adsorption. Ultraviolet or fluorescent indicators may be used. 

Acid Strength Distribution After Stabilization Treatment. The acid strength distributions of the zeolites were determined by temperature-programmed desorption of ammonia equipped with a thermal conductivity detector. The detailed procedure has been given in the early paper (6). 

The calorimetric results discussed above demonstrate that mordenite behaves qualitatively as HY zeolite with equivalent modifications to the acid strength distribution caused by changing the same relevant variables. Under similar conditions it appears that mordenite zeolites are slightly stronger than HY zeolites. 

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