Surface acid groups, strengths

Boehm and Johnson Matthey titrations. Both titrations give information on the acidic or basic character. The procedures are described in [13] and [14]. Boehm method classifies the surface acidic sites in three types (called phenolic, lactonic, carboxylic), by growing strength of acidity. However, this arbitrary classification fails to attribute one surface acidic group to one type of acidity. Moreover, this method does not give very accurate results with solids containing few acidic sites. Nevertheless, it appears for our active carbon that "phenolic" sites are a majority. The amount of each type of acidic sites does not change very much with the nitric treatment. 

Light olefins especially ethylene ( 2 ) and propylene ( 3 ) can be formed from methanol in the MTO process (Chang et al., 1979) using catalyst SAPO-34. Several other catalysts like ZSM-5 (Marchi and Froment, 1991), and Chabazite (Liu et al.. 1984) have been tested. Physical and chemical properties of the catalyst influence its selectivity to hydrocarbons. The physical factors that affect the selectivity of the catalyst are temperature, pressure of the fixed bed reactor, and space velocity of the feed. Other physical characteristics that influence selectivity are crystal size, crystal size distribution, pore size and pore size arrangement. The chemical characteristics that influence the selectivity are acid site density, strength of acid sites, and type of surface acid groups. 

The distinction between the intrinsic acidity of a solid and the effective acidity displayed when the surface acidic groups are screened by interaction with solvent molecules becomes a topic of prominent importance when the solid has to work in contact with liquids for its practical uses. This is the case of liquid-solid heterogeneous catalysts in which the activity of the catalyst can be modified by the presence of solvent which may establish physical or chemical interactions with the acid sites of the surface. For reactions carried out in liquid phase, the knowledge of the effective acidity (in terms of number and strength of the sites) of the catalyst in given liquids allows determining sound relationships between the catalytic activity and surface acidity. 

Using the multisite complexation model (MUSIC), ° the surface of palladium oxide has been modeled providing useful data on the nature, number, and the acid-base strength of the hydroxyl groups present on the different faces (111) and (10 0) of... 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

After the acid strength of a catalyst surface has been bracketed by means of colors of adsorbed indicators, the next logical step in the determination of surface acidity is the measurement of the number of acidic groups. This is generally done by titrating a suspension of the catalyst with a solution of a suitable amine in an inert solvent the previously described indicators are used to determine endpoints. 

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