Generation of acid site

Effects of Structural Disorder on the Generation of Acidic Sites in Zeolite L... 

Acidic zeolite materials are the main catalysts in the cracking process, which is the most important process among industrial chemical processes. Broad studies of heterogeneous cracking catalysts, started in the 1950s, discovered that the basic nature of cracking catalysts is acidic, and generation of acidic sites on solids has been extensively studied. 

Acid sites of AgjPW C are generated upon the reduction with hydrogen at 573 K [40,42], After the reduction, the catalytic activity (for example, for o-xylene isomerization) develops. The generation of acid sites has been confirmed by pyridine adsorption and solid state H NMR [42]. 

The acidic properties of zeolites are dependent on their pretreatment and particularly on their activation temperature, which plays an important role in the mechanism of generation of acid sites. 

Nanba T, Uemura A., Ueno A., Haneda M., Hamada H., Kakuta N., Miura H., Ofune H., Udagawa Y. Studies on active species for selective catalytic reduction of NO on alumina-supported cobalt oxide catalysts. Bull. Chem. Soc. Jpn. 1998 71 2331-2337 Nishiwaki K., Kakuta N., Ueno A., Nakabayashi H. Generation of acid sites on finly divided Ti02. J. Catal. 1989 118 498-501... 

Fig. 3.51 Generation of acid sites on MoOs/AlsOj by reduction with hydrogen, and adsorption of ammonia.

Below the chemocline, slow processes of neutralization and metal attenuation dominate over generation of acid and release of metals. Pit Lake, can therefore be considered as a potential long-term repository for different sulfide wastes occurring around the site and the town of Lynn Lake. 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Generation of acidity in silica gel by ionizing radiation was reported by Barter and Wagner (274). p-Dimethylaminoazobenzene was adsorbed out of anhydrous solution in CCI4 in the red, acid form. About 3 /aeq/gm of acid sites were generated by a dose of 3 10 rad, as determined by titration with butylamine. 

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. 

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. 

K Tanabe Solid Acid and Base Catalysts This chapter deals with the types of solid acids and bases, the acidic and basic properties, and the structure ofacidic and basic sites. The chemical principles of the determination of acid-base properties and the mechanism for the generation of acidity and basicity are also described. Howacidid and basic properties are controlled chemically is discussed in connection with the preparation method of solid acids and bases. 

The suitability of this adsorption model to characterize quantitative aspects of surface acidic groups gives no indication, however, about the chemical structure of the reactive sites. Only in combination with the chemical probe reactions is it possible to assign the two types of acid sites to carboxylic acid and hydroxy groups, respectively. It is noted that such an approach can also be used to determine ion exchange capacities for metal ion loading required for the generation of dispersed metal-carbon catalyst systems. 

A solid acid calcined at temperatures above 1000 °C is termed a Ceramic Acid . Ceramics are made by calcination of inorganic materials at elevated temperatures and they possess favorable stability. Brick, usually produced by calcination at temperatures above 1000 °C, is a typical example of one of these materials. Solid acids are generally prepared by calcination at temperatures around 500 °C to generate the highest acidity on the surface, and it is known that the treatment with temperatures above 500 °C causes the surface acidity to reduce. A solid acid prepared by calcination at similar temperatures to a ceramic, would be highly desirable in terms of green chemistry, as a result of the corresponding stability of acid sites. 

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