Surface acidity of zeolites

Humphries et al. [104] have given an excellent description of the surface acidity of zeolites and its influence on FCC. This topic has been comprehensively reviewed also by Shen and Auroux [105] and Auroux [103]. 

The surface acidity of zeolites appears to be an important ingredient for polymerization of acetylene and derivatives. Thus, when diazomethane was used to remove the protonic acidic sites on HZSM-5, no evidence for acetylene polymerization remained, compared to the original acidic form. ... 

The synthesized zeolite ZSM-35 disc was calcinated at 873k for 12 hours to remove ethylenediamine and water. Because inorganic ions Na", was not used in synthesis process, the synthesized zeolite ZSM-35 disc was a zeolite H-ZSM-35. The surface acidity of zeolite H-ZSM-35 was tested by pyridine adsorption IR. From the data in fig. 4, the synthesized zeolite H-ZSM-35 had Bronsted acid site at 1549 cm and Lewis acid site at 1443 cm [24]. Certainly, zeolite ZSM-35 disc from Al203-Si02-EDA-H20 had good acidity and would be a good acid catalyst. 

Lercher, J.A., Gruendling, C. and Eder-Mirth, G. (1996) Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 353. 

Zeolites consist of linked tetrahedra of Si04 and AIO4. The substitution of the aluminum by other trivalent atoms such as B, Ga, and In in order to modify the surface acidity of such solids has aroused considerable interest [34,213-215],... 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

In the following section, we will critically review representative methods for measuring surface acidity of solid catalysts. Recommendations will then be made of the most appropriate methods from the standpoint of the needs of the investigator. The final section is devoted to updating research activities dealing with individual solid catalysts. Particular attention will be devoted to studies of acidities of unusually active catalysts such as crystalline zeolites, synthetic clays, and chlorinated aluminas. 

A simple cluster model of a bridged hydroxyl group in a zeolite is cluster 3. Such a cluster with A = H was used by Chuvylkin et al. (70) as early as 1975 to discuss the properties of possible intermediate structures in the catalytic isomerization of butenes on aluminosilicate surfaces in terms of CNDO/2 approximation. Mikheikin et al. (34) have used a similar cluster with terminal pseudo-atoms A to study the Bronsted acidity of zeolites and its dependence on the Si/AI ratio. 

The ratio between the two main products 24 and 25 depends on the type and acidity of zeolite used. For example, H-BEA with a higher acid outer surface demonstrated a high activity but a low selectivity of 24 in comparison to H-FER or H-US-Y catalysts. 

Characterization of external surface properties of zeolite ZSM-5 modified by 12-tungstosilicic acid... 

Formation of byproducts may partly be caused by the outer surface acidity of the zeolites. If so, deactivation of the outer surface should lead to better selectivities. The outer surfaces of H-mordenite and HB were deactivated by poisoning with triphenylphosphine. For checking outer surface deactivation we have developed a chemical probe molecule [6]. In this way, it was found that the poisoning with triphenylphosphine resulted in a totally inactive outer surface within the time scale of the reactions. 

Understanding the effect of binder on acidity of zeolites would be helpful to select a binder and make a better zeolite-based catalyst. NH3-TPD and FTIR were used to Investigate the acidic properties of silica- and alumina-bound Y and ZSM-5 zeolites with SiOi/A Os ratios of 5 to 280. The BET surface areas and pore volumes were also determined for the bound and unbound zeolite samples. 

In most studies on the use of clays as catalysts or catafyst supports, the surface acidity of the clays has been considered a determinant of their catalytic activity [9,10]. All the supports (of lamellar and zeolitic structures) have acid centers, whose strength and number varied depending on the peculiar structure of the synthesized constituent materials [11]. 

Acetylation of thioanisole with AAN over BEA(25) zeolite can be performed, affording 4-(methylthio)acetophenone in 60% yield (Scheme 4.19). The process can also be efficiently performed under continuous flow conditions. The effect of the surface acidity of the BEA zeolite catalyst is shown to play a crucial role in the regioselectivity of the reaction. In the case of BEA zeolite with 0.94 mmol x g-i surface acidity, 4-(methylthio) acetophenone is obtained in 99.9% selectivity with a thioanisole conversion of 32%. ... 

Sulfided Mo-Y and Ni-Mo-Y catalysts were tested in thiophene hydrodesulfurization and hydrogenation of pentene-1 and cyclopentene. Catalysts were prepared by thermal decomposition of supported Mo(CO)g encaged in Y and stabilized Y zeolites. Cracking ability in both reactions is related to the surface acidity of catalysts but is not parallel to their HDS activity. H S generates protonic acidity over NaY and KY zeolites. Synergetic effect between Ni and Mo sulfided species in HDS reaction was observed. The presence of extra-lattice aluminum in stabilized forms of Y-zeolites favours selectivity towards formation of isopentane and cyclopentane during hydrogenation. 

Several groups have studied the readsorption of water on hydrogen zeolites. Most of these studies have concerned the influence of water on the surface acidity of the zeolite. 

The success of zeolites as catalysts is based on their thermal and hydrothermal stability, together with the strong acidities achieved on their surface. The presence of rare-earth ions plays an important role in both properties, and therefore the impact of R on the stability and acidity of zeolites is presented below. 

P-09 - External surface acidity of modified zeolites ESR via adsorption of stable nitroxyi radicals and IR spectroscopy... 

Thus, ideally, the total number of acid sites is equal to the total number of A1 atoms on framework tetrahedral (T) sites. Because the acidity of zeolites is adversely affected by a small amount of residual Na", Na must be exhaustively removed in order to obtain a highly active solid acid catalyst. Removal of Na from zeolites usually requires repeated ion-exchange steps combined with calcination, in the temperature range of 823-1050 K. Only about 70% of the Na ions are replaced by NH4 ions during the first exchange. It is assumed that as a result of calcination Na atoms, which were not accessible for ion exchange, are redistributed over the zeolite surface and made accessible. Simultaneously, solid-state reactions occur in the zeolite, and framework aluminum is removed. This phenomenon results in stabilization of the zeolite structure the acid forms of low-silica zeolites are inherently unstable. 

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