Thermodynamic and kinetic aspects of surface acidity [report]

J.A. Dumesic
1992 unpublished
Our research funded by the Department of Energy in the general area of acid catalysis involves 1) the characterization of solid aadity, 2) the assessment of catalytic performance of acidic materials, and 3) the elucidation of possible relationships between surface thermodynamic and kinetic properties of acidic sites. During the past year we have focused on zeolitic acid catalysts and on methylamine synthesis from ammonia and methanol. Part of our work has also involved elucidation of the role
more » ... ation of the role of acidity in the selective catalytic reduction of nitric oxide over vanadia/ titania. Acidic zeolites find use in a wide variety of catalytic applicatione, ranging from large scale petrochemical processes to the synthesis of specialty chemicals. Quantitative characterization of catalyst acid properties is important for elucidation of the role of surface acidity in determining the catalytic activity and selectivity of these materials. However, solid acid catalysts have long posed a challenge for quantitative characterization. Accordingly, one aspect of our research addresses the problem of quantitative characterization of zeolite acidity. Dehydrative methylamine synthesis from ammonia and methanol is an important example of an acid catalyzed reaction for specialty chemicals production. Amines are used in a number of applications such as pharmaceuticals, herbicides, pesticides, solvents, surfactants and detergents [ 11. Production of methylamines in the US. is approximately 270 Mlb per year [2]. In addition to this commercial importance, the methylamine synthesis reaction is of scientific interest; it poses the classic problem of series selectivity combined with adsorption and readsorption phenomena for each intermediate species. An understanding of the mechanistic details of such a reaction would be of general interest and utility. The objectives of our acid characterization work are based in determining 1) the type of acid site, 2) the strength of the sites, 3) the number of sites of different strength, and 4) the mobility of molecules adsorbed on the acid sites. An accurate measure of acid strength is given by the heat of adsorption of a basic probe molecule on the acid site, AH&. In general, a catalyst may have sites of different strength and there will thus be a distribution of numbers of sites in strength. The mobility of a molecule adsorbed on an acid site is also an important probe of the site. A thermodynamic representation of this mobility is given by 1 * @ e !"%STRWT)ON OF THfS DOCUMENT IS UNLIMI'T'EB
doi:10.2172/5569965 fatcat:z7uiap73mvhongk2xbqoj3qpey