Cracking catalysts ammonia studies

The spectrum of ammonia chemisorbed on a silica-alumina cracking catalyst was studied to determine whether the acidity of these catalysts is due to a Lewis (nonprotonic) or a Bronsted type of acid (28, 29). This work was based on the premise that ammonia chemisorbed on Lewis sites would retain a NH3 configuration while ammonia chemisorbed on a Bronsted site would form NHt. The NH3 configuration was expected to have bands near 3.0 and 6.1 p and the NHt near 3.2 and 7.0 p. 

The infrared study of chemisorbed ammonia by Mapes and Eischens (55) was the first to demonstrate the power and utility of the infrared spectroscopic method for determination of surface acidity. These investigators demonstrated that IR spectra of ammonia chemisorbed on cracking catalyst contained H—N—H-bending bands that arose from NH4+ and coordinated NH3 (Fig. 6), a finding that constituted direct evidence for the existence of Br0nsted and Lewis acids on the surface of silica-alumina catalyst. Parry (23) subsequently suggested the use of pyri-... 

As an application of the pressed-salt method to catalysis, French and co-workers studied ammonia on cracking catalysts (57, 58). Other workers (83) have shown that this was an unfortunate choice because ion exchange occurs between the cracking catalyst and the halide salt. As a result of this ion exchange, spectra are observed which are due to ammonium halide rather than chemisorbed ammonia. 

The application of IR spectroscopy to catalysis and surface chemistry was later developed in the fifties by Eischens and coworkers at Texaco laboratories (Beacon, New York) in the USA [7] and, almost simultaneously, by Sheppard and Yates at Cambridge University in the UK [8]. Mapes and Eischens published the spectra of ammonia chemisorbed on a silica-alumina cracking catalyst in 1954 [6], showing the presence of Lewis acid sites and also the likely presence of Br0nsted acid sites. Eischens, Francis and Pliskin published the IR spectra of carbon monoxide adsorbed on nickel and its oxide in 1956 [9]. Later they presented the results of an IR study of the catalyzed oxidation of CO on nickel at the First International Congress on Catalysis, held in Philadelphia in 1956 [10]. Eischens and Pliskin also published a quite extensive review on the subject of Infrared spectra of adsorbed molecules in Advances in Catalysis in 1958, where data on hydrocarbons, CO, ammonia and water adsorbed on metals, oxides and minerals were reviewed [11]. These papers evidence clearly the two tendencies observed in subsequent spectroscopic research in the field of catalysis. They are the use of probes to test the surface chemistry of solids and the use of spectroscopy to reveal the mechanism of the surface reactions. They used an in situ cell where the catalyst sample was... 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

The acidic function of bifunctional catalysts (Ft - Cu alloys in KEY and USY zeoUtes) has also been studied by microcalorimetry, using ammonia as the probe, in relation to the catalytic conversion of n-alkanes to isoalkanes or cracked products. Although the acidity and the bifunctional catalytic performances of the copper-exchanged Pt-REY are improved, these materials are much less efficient than the corresponding USY samples. The activation of USY and the reduction of the Pt - Cu-USY catalysts generate in all cases the same number of protons [259]. 

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