Ammonia cracking catalyst acidity

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

The hydrogen sulfide and ammonia can be removed by amine extraction and acid washes respectively. Hydrotreating also removes metals from the feed that would otherwise poison the reforming and cracking catalysts. 

Thorium oxide has a high refractive index and low dispersion and thus finds use in high-quality camera and scientific instrument lenses. Thonum oxide also is used as a catalyst in the conversion of ammonia to nitnc acid, in petroleum cracking, and in sulfuric acid production. 

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-... 

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 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... 

Increased severity of acid treatment continuously increases the surface area and porosity of the clay, but cracking activity shows a maximum at intermediate treating severity (223,325). Mild acid treatment removes a large part of the alkali and alkaline-earth metals. More-severe acid treatment removes increasing quantities of aluminum and iron, as well as other metals still remaining after mild treatment. It has been reported that treatment of clay with ammonium chloride solution, instead of acid, also results in an active cracking catalyst (70). The ammonium ion displaces some of the metal constituents of the clay by base exchange the treated clay is then calcined to drive out ammonia. 

The acidity of the catalyst arises due to interaction of the components (e.g., silica and alumina) during preparation. Pure silica and pure alumina have little or no cracking activity, but the presence of only a few hundredths of a per cent of alumina in silica is sufficient to produce an active catalyst (138,148,320,321). Activity and acidity both increase, up to a certain point, with increased alumina content (320,324). Infrared spectra of ammonia chemisorbed on silica-alumina catalyst indicate that most of the chemisorbed ammonia is in the NHs form, with only a relatively small amount of NH4+ (204). From this evidence it is concluded that most of the catalyst acid is of the Lewis type since, in reacting with a Bronsted acid or a hydrated Lewis acid, the ammonia would be converted to an ammonium ion. 

In many cases the temperature of desorption of the ammonia can be correlated to the acid strength (intensive factor) of the sites. Corma and coworkers correlated the information given by ammonia TPD with the acidity for various zeolite cracking catalysts. Similarly, Figure 3 shows a relationship obtained in the author s laboratory... 

Thoria is an efficient catalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in the production of sulfuric acid. 

This was soon supplanted by a more powerful spectroscopic method, wherein the IR behavior of chemisorbed amines onto acidic surfaces was found to distinguish the relative amounts of Lewis and Bronsted sites. The first report, for ammonia on a silica-alumina cracking catalyst, occurred in 1954 (61). Bands for NH3 and NH4+ were observed upon addition of water, the NH4 bands increased at the expense of NH3 bands. These results indicated two types of acid sites (1) NH3 chemisorbed by coordinate bond formation between the Lewis base (NH3) and a Lewis acid site, and (2) transfer of a proton from a Bronsted site to the base forming NH4" bound via coulombic forces. 

FIG. 23-3 Temperature and composition profiles, a) Oxidation of SOp with intercooling and two cold shots, (h) Phosgene from GO and Gfi, activated carbon in 2-in tubes, water cooled, (c) Gumene from benzene and propylene, phosphoric acid on < uartz, with four quench zones, 260°G. (d) Mild thermal cracking of a heavy oil in a tubular furnace, hack pressure of 250 psig and sever heat fluxes, Btu/(fr-h), T in °F. (e) Vertical ammonia svi,ithesizer at 300 atm, with five cold shots and an internal exchanger. (/) Vertical methanol svi,ithesizer at 300 atm, Gr O -ZnO catalyst, with six cold shots totaling 10 to 20 percent of the fresh feed. To convert psi to kPa, multiply by 6.895 atm to kPa, multiply by 101.3. 

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. 

As expected from the TPD results, Al-sapo was more active for the cracking of cumene on a per weight of catalyst basis than Al-mont. In order to compare the catalytic activity on a basis of active sites, we evaluated the number of active sites on these catalysts. TPD spectra were measured with varying the temperature of ammonia adsorption. Typical results on Al-mont are shown in Fig. 2. By integrating these spectra, the concentration of acid sites corresponding to different strength of acidity can be determined. 

A complete separation of a carbonium ion from the hydride ion is very probably not necessary. It has been shown [73] by MO calculations that any attack by a charged species on an atom bonded to a carbon atom causes activation of the bonds from a /3-carbon atom to the substituents. In this way, the splitting of the Cp—Cy bond can be induced by adsorption of the alkane on a strongly acidic site. The preferential cracking of a saturated hydrocarbon chain in /3-positions to the position where a carbonium ion might be formed was observed early and named the /3-rule by Thomas [2], The question remains open as to which type of acidic centre is able to activate an alkane molecule. The fact that an aluminosilicate catalyst is poisoned for the cracking of alkanes by irreversibly adsorbed ammonia suggests a Lewis site [240], viz. 

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