The determination of acidity in fluid cracking catalysts (FCCs) from adsorption microcalorimetry of probe molecules

The introduction of fluid cracking catalysts (FCCs) for the transformation of crude oil into gasoline and other fuel products has revolutionized the petroleum refining industry. It is generally admitted that the initial step in a cracking reaction is the formation of a carbocation. Consequently, in the field of catalytic cracking of petroleum by zeolite-containing catalysts, the terms "active site" and "acid site" are synonymous [1]. All FCC catalysts, regardless of their specific applications, are designed to have the following catalytic properties: activity, selectivity, and stability [2]. Catalytic activity is due to the presence of acidic sites, and is determined by the zeolite content and by the types of zeolite and matrix in the FCC catalyst. Catalytic selectivity is determined by the zeolite type, the nature (Bronsted or Lewis), strength, concentration and distribution of the acid sites, the pore size distribution, the matrix surface area and activity, and the presence of additives or contaminants. Stability is affected by both the composition and the structural characteristics of the catalyst components [2]. Therefore, the acidity of FCC catalysts is designed to meet specific requirements, and a full characterization of the acidity is necessary; this gives a great importance to the information gathered by direct methods such as the monitoring by microcalorimetry or by temperature-programmed desorption (TPD) of the adsorption or desorption of gaseous bases, particularly ammonia or pyridine. In TPD experiments, the peak maxima temperatures are influenced by the acid site strength, the number of acid sites, the zeolite structure and the heating rate [I]. Additional information can be obtained concerning the nature of the acid sites and their strength, using respectively infrared spectroscopy and calorimetry. In particular, adsorption microcalorimetry gives access to the number, strength, and strength distribution of the acid sites in a single experiment. The purpose of this article is to describe the different types of active acid sites present in fluid cracking catalysts and their components. The factors influencing the acidic properties of these materials are discussed, and details are given concerning the characterization of the number and strength of the acid sites of FCCs by direct thermal methods such as TPD and calorimetry. Finally, the correlations that exist between the acidity of a fluid cracking catalyst and its catalytic properties are also mentioned.