Ind. Eng. Chem. Res. 1993,32, 588-593
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Deactivation and Acidity Deterioration of a Si02/A1203 Catalyst in the Isomerization of &-Butene Ana G. Gayubo,*Jose M. Arandes, Andres T. Aguayo, Martin Olazar, and Javier Bilbao Departamento de Ingenieria Qulmica, Universidad del Pais Vasco, Apartado 644, 48080 Bilbao, Spain
The evolution of acidity of a SiOz/A1203catalyst in the isomerization of cis-butene has been studied. The catalyst deactivation takes place by blockage of active sites by slightly evolved coke, which is partially soluble in dichloromethane and pyridine and is slightly aromatic. The blockage of active sites has no incidence on the porous structure and on the catalyst surface area. By combining the results for the catalyst in different deactivation states, corresponding to calorimetric and thermogravimetric measurements of tert-butylamine desorption a t a programmed temperature, the evolution of the average value of acidity strength has been determined. In addition, from tertbutylamine adsorption measurements at 200 "C, the evolution of surface acidity strength distribution has been attained. The fast deactivation of very strong acidic sites is concluded. 1. Introduction The deactivation of acidic catalysts in processes of great economic importance (cracking, hydrocracking, isomerization of olefins, alkylation of aromatics, transformation of methanol into gasolines, etc.) takes place mainly by blockage of active sites due to coke deposition, which is aprocess that occurs in short reaction times. Pore blocking due to coke deposition generally has a secondary importance as it takes place at long reaction times and with a smaller incidence on activity, except for great depositions. Due to the fact that deactivation by active-site blocking is fast and conditions the global process economy, it is the object of study from different perspectives (Butt and Petersen, 1988). In studies on a microscopic scale, either of catalyst or of coke,the determination of the relationship between catalyst properties and deposition magnitude and deposited coke nature is pursued (Guisnet and Magnoux, 1989; Kulkarni et al., 1989). By means of kinetic studies, the mechanisms of coke formation and evolution and the intervention of the main reaction components in these mechanisms are the objects of this study (Naccache, 1985). Several techniques have been developed to characterize the physical properties of deactivated catalysts, which include the physical adsorption of an inert gas (Kirszensztejn et al., 1991), optic and electronic microscopy (Behrsing et al., 1989),and small-angle scattering by X-ray and neutrons (Acharya et al., 1990). In deactivation of acidic catalysts, the preferential mechanism is coverage of active sites, sothe techniques mentionedgive secondary information and the study of the deactivated catalyst consists in monitoring the surface acid function. From infrared (IR) analysis of dehydroxylated HY zeolites deactivated by coke, it has been proven that coke formation on these catalysts is linearly proportional to the disappearance of hydroxylated Bronsted acidic sites (Blackmond et al., 1982). Nevertheless, coke formation is partially attributable also to Lewis acidic sites, which have an inductive effect on the remaining Bronsted sites, as they generate sites of higher acidity than even the Bronsted ones (Eisenbachand Gallei, 1979). Langner and Meyer (1980) have proposed a coke formation mechanism from butadiene on zeolites. In this mechanism the DielsAlder reactions are catalyzed by Lewis sites and Bronsted sites take part in hydrogen-transfer reactions. Fajula and Gault (1981a-c) also maintain that both types of sites take part in the coke formation mechanism. In the same way, it has also been determined, from IR analysis of Y zeolites, that the internal OH groups corresponding to strong acidic
sites disappear in the first reaction minutes, in accordance with the initial fast coke increase, while the external OH groups (weak acidity) are preferably subsequently consumed, with a smaller rate (Langner, 1981). In this paper, the aim is to contribute to the study of acidic catalyst deactivation (basically carried out in the literature by IR analysis) by applying acidity measurement techniques that allow for the study of the evolution of the average acidity strength and of the distribution of the catalyst acidity strength during deactivation. Deactivation of a silica/alumina catalyst used in the isomerization of cis-butene has been studied. Measurement of average acidity strength has been carried out by combining the calorimetric and thermogravimetric techniques used in the measurement of chemical desorption of tert-butylamine. The application of these techniques in the measurement of tert-butylamine adsorption isotherm allows for determining the surface acid distribution (Aguayo et al., 1991). 2. Experimental Section
2.1. Reaction Conditions and the Catalyst. The isomerization of cis-butene has been carried out in an isothermal integral reactor, in reaction equipment previously described (Arandes et al., 1990; Gayubo, 1992). The operation conditions have been the following: pressure, 1.0 atm; temperature, 220 "C; time: 1, 2, and 4 h; space time, 0.5 g of catalyst h/mol; gas linear velocity, 6 cm/s. The conversion has been measured by gas chromatography. The catalyst used, SiOz/A1203, has been prepared by impregnation of Si02 gel (Aguayo et al., 1987) and with -0.5+0.32-mm particle size. The alumina content, 12.3 wt 5% , has been determined following conventional methods. The calcination temperature, 550 "C,corresponds to the maximum value of total acidity (pK = 6.8 titrated by Benesi's method (1956,1957)), and for this temperature and with a calcination time of 4 h, the strong acidity (more sensitive to thermal treatment) is noticeably attenuated (Aguayo et al., 1987). The physical properties of the catalyst (surface area, 208 m2/g;pore volume, 0.62 cm3/g) have been calculated from the NZadsorption-desorption isotherms obtained in a Micromeritics AccuSorb 2100 E, while the real density (2.16 g/cm3)and the apparent density (0.92 g/cm3) have been measured by He-Hg pycnometry. The pore volume distribution shows the micro-mesoporous nature of the catalyst (rp< 10nm, 5 5% of the pore volume; 10 < rp < 100 nm, 65%).
0SSS-5SS5/93/2632-05~S~Q4.O0/0 0 1993 American Chemical Society
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c -butene
difference between the signal generated by the sample and that of an inert reference system. The system is provided with mass flowmeters to control the streams of -
