I.G. Dalla Lana

Infrared studies of the adsorption and surface reactions of hydrogen sulfide and sulfur dioxide on some aluminas and zeolites

Abstract Adsorption of hydrogen sulfide, sulfur dioxide, and their mixtures on four different catalysts has been studied by infrared spectroscopy of the catalyst surfaces. The four catalysts, which show a wide range of acidity and are all active for the Claus reaction (2H 2 S + SO 2 → 3S + 2H 2 O), were γ-alumina (the main constituent of commercial bauxite catalysts), γ-alumina doped with NaOH, sodium Y zeolite, and hydrogen Y zeolite. All catalysts showed physical adsorption of both reactants with strong hydrogen bonding to surface OH groups. This would suggest that the role of the catalyst is primarily to bring the reactants together in suitable orientation. On the other hand, γ-alumina shows, on heating with SO 2 , a chemisorbed SO 2 species which may be a reaction intermediate. The NaOH-treated γ-alumina shows a second chemisorbed SO 2 species which is irreversibly adsorbed and thus may be a catalyst poison.

The nature and catalytic influence of coke formed on alumina: Oxidative dehydrogenation of ethylbenzene

Abstract Catalytic properties of alumina doped with 0.1 to 1.0 wt% NaOH, as well as that of pure alumina, were studied for the oxidative dehydrogenation of ethylbenzene and for some acid-catalyzed model reactions. The largest contribution to the formation of coke catalytically active for styrene formation was found to come from sites of moderate and weak acid strengths. Stronger acid centers produce cokes lower in hydrogen content, whereas weaker centers form cokes of a more saturated chemical nature. On very very weak centers, cokes with the ratio of C H are formed and such cokes show low catalytic activity for oxidative dehydrogenation. Based on ESR measurements, it has been suggested that paramagnetic centers are the active sites for styrene formation in the process of ethylbenzene oxidative dehydrogenation.

Surface properties of sulphate-containing aluminas

Abstract The chemical properties of the surface of an alumina containing an admixture of basic aluminium sulphates were compared to those of a comparable pure alumina. The sulphated alumina exhibited increased total acidity and oxidizing properties. Strong Bronsted-acid sites were formed, such as are absent on pure alumina. Conversely,the presence of sulphates decreased the basicity and reducing ability. The catalytic properties of this sulphated alumina are noteworthy.

A study of the kinetics and mechanism of COS hydrolysis over alumina

The role of surface sites on γ-alumina in the mechanism of hydrolysis of carbonyl sulfide was investigated using a combination infrared cell-recycle flow reactor. Zero-order and first-order behavior were observed for H2O and COS, respectively. The use of acidic (acetic acid) and basic (NaOH, NH3, and pyridine) site-blocking agents reveal basic sites to be essential for the hydrolysis surface reaction. Competitive adsorption of COS and H2O base centers was evident; however, an Eley-Rideal surface reaction mechanism involving adsorbed H2O seemed most compatible with the experimental observations. Catalytic deactivation with time and with exposure to oxygen were evaluated and explanations put forward.

The oxidizing properties of γ-alumina: Infrared studies of the adsorption of H2S and CS2

Abstract In adsorption or surface reaction studies on γ-aluminas which have been pretreated with oxygen, one may unexpectedly encounter oxidation products. The evidence from H 2 S and CS 2 adsorption studies suggests that molecular oxygen, in some adsorbed state, is responsible. To obtain unambiguous reproducible results, the p etreatment of γ-Al 2 O 3 with oxygen should always be followed by a pretreatment with hydrogen.

An FTIR spectroscopic view of the initiation of ethylene polymerization on Cr/SiO2 catalyst

Abstract The onset of polymerization of ethylene on Cr/SiO 2 catalyst activated by high-temperature treatment was studied at room temperature using fast-scanning FTIR spectral measurements. Initial rapid adsorption of molecular ethylene on the pre-activated catalyst preceded the onset of polymerization. A time-delay between adsorption and polymerization was observed. The experimental evidence suggests that initiation of polymerization follows dissociation of ethylene, presumably on two Cr sites. Propagation and other mechanistic steps then follow as suggested earlier.

On the role of surface hydroxyls during the Cr/SiO2-catalyzed polymerization of ethylene

Abstract The onset of polymerization of ethylene over a 0.5% chromium supported on silica catalyst was studied using a diffraction-type IR spectrophotometer. Detailed investigations emphasizing the hydroxyl region of the IR spectrum suggested the importance of silanol groups in the processes of reaction initiation, termination, and catalyst deactivation. The formation of chromium-bound hydroxyls and their role in catalyst deactivation were suggested.

Base properties of zeolite catalysts

Abstract Distributions of base strengths were determined using indicator techniques and i.r. spectroscopic studies of acetic acid adsorbed on samples of faujasite-type zeolites. The results show that base sites of variou zeolites differ significantly in both their strength and nature. Among sodium forms of the zeolites, NaX appeared to exhibit higher strength and concentrations of base sites than NaY. For hydrogen-sodium forms, the situation was reversed: HNaY prevailed over HNaX. The total basicities of the hydrogen—sodium zeolites were considerably higher than those of the sodium forms, due to contributions from both hydroxyl groups and oxygen ions of (AlO 4 ) − tetrahedra; whereas the latter exhibited Lewis basicity alone. Nonetheless, the higher basicities of the HNaY and HNaX zeolites do not correlate with their catalytic activity for decomposition of diacetone alcohol to acetone, a model base-catalysed reaction. The relatively poor conversions to acetone are examined within the context of base strengths and the high acidity of HNaX and HNaY since another reaction pathway, acid-catalysed formation of mesityl oxide, was also observed.

Role of reduction sites in vapor-phase hydrolysis of carbonyl sulfide over alumina catalysts

Abstract The activity of alumina catalysts at 230 °C decreased upon exposure to O2 at elevated temperatures contrary to when pretreated in a stream of either N2 or N2 + H2 mixture. The concentration of reduction sites at the surface of the catalysts, which was estimated from the amount of SO2− formed, decreased obviously after exposure to O2 whereas surface basicity or acidity changed slightly. Pentene isomers poisoned the alumina catalyst under reaction conditions; the poisoning effect, despite being scarcely affected by the feed concentration of COS, decreased with increasing feed concentration of H2O and rather increased with increasing electron-donating nature of pentene isomers. Furthermore, the catalyst was temporarily poisoned by CO2 as well as by SO2; the equilibrium adsorption constants calculated from the kinetics followed the order: SO2 ⪢ COS ⪢ CO2. A mechanistic interpretation suggests that both reduction and Lewis acidic sites are involved in the surface reaction. It is proposed that COS is adsorbed on the former sites to form a negatively charged species, which is then hydrolyzed by water adsorbed on the latter sites.

Copolymerization of ethylene and 1-butene using A Cr-silica catalyst in a slurry reactor

A novel slurry reactor was used to investigate the copolymerization behavior of ethylene and 1-butene in the presence of 1 wt % Cr on Davison silica (Phillips-type) catalyst over the temperature range of 0–50°C, space velocity of about 0.0051 [m3 (STP)]/(g of catalyst) h, and a fixed ethylene to 1-butene feed mole ratio of 95 : 5. The effect of varying the ethylene to 1-butene feed ratios, 100 : 0, 96.5 : 3.5, 95 : 5, 93 : 7, 90 : 10, 80 : 20, and 0 : 100 mol/mol at 50°C was also studied. The addition of 1-butene to ethylene typically increased both copolymerization rates and yields relative to ethylene homopolymerization with the same catalyst, reaching a maximum yield for an ethylene: 1-butene feed ratio of 95 : 5 at 50°C. The incorporation of 1-butene within the copolymer in all cases was less than 5 mol %. The average activation energy for the apparent reaction rate constant, ka, based on total comonomer mole fraction in the slurry liquid for the ethylene to 1-butene feed mole ratio of 95 : 5 in the temperature range of 50–30°C measured 54.2 kJ/mol. The behavior for temperatures between 30 to 0°C differed with an activation energy of 98.2 kJ/mol; thus, some diffusion limitation likely influences the copolymerization rates at temperatures above 30°C. A kinetics analysis of the experimental data at 50°C for different ethylene to 1-butene feed ratios gave the values of the reactivity ratios, r1 = 27.3 ± 3.6 and r2 ≅ 0, for ethylene and 1-butene, respectively.