Silica-alumina Catalyst Research Articles

Cyclohexene synthesis by catalytic cracking of cyclohexylarenes.

The cracking behavior of cyclohexylarenes over various catalysts was investigated to develop an effective method for cyclohexene synthesis. The reaction was carried out in the vapor phase under atmospheric pressure using a conventional fixed bed flow apparatus. Cyclohexylbenzene (CHB) was cracked quantitatively over silica-alumina catalysts, but the main products were methylcyclopentenes and only a small amount of cyclohexene was obtained. By loading molybdena on the silica-alumina catalysts, the selectivity of cyclohexene increased but the conversion of CHB decreased, and significant amounts of methylcyclopentenes and some biphenyl, dehydrogenated product of CHB, were found. On the other hand, 2, 4-dimethyl-1-cyclohexylbenzene (2, 4-DMCHB) and 2, 4, 6-trimethyl-1-cyclohexylbenzene (2, 4, 6-TMCHB) compared with CHB were more easily cracked over the MoO3-SiO2-Al2O3 catalysts, but methylcyclopentenes still were considerably produced. The selective cracking of 2, 4, 6-TMCHB to cyclohexene occurred on NaOH-Al2O3 catalysts. Conversion and selectivity under the most suitable reaction conditions were 91% and 95%, respectively. These results suggest that the selective interaction between aryl group and acid sites on the catalyst is important for selective cracking to cyclohexene, and it will occur when the acidity of the catalyst is weak and the basicity of the aryl group is strong. Further, the kinetics of the cracking reaction of 2, 4, 6-TMCHB over NaOH-Al2O3 was studied, and some mechanistic schemes of the reaction were proposed.

Gas-phase formation of methyl and ethyl iodides

Gas phase formation of methyl and ethyl iodides occurs when methanol and ethanol vapors are carried by an inert gas over silica-alumina catalyst supports impregnated with potassium, sodium, cesium, or cadmium iodides at 130–180°. The efficiency of conversion of inorganic to organic iodides by different catalysts was studied using125I-labelled potassium and cadmium iodides. A mechanism of organic iodide formation in the heterogeneous reaction is postulated.

Heat transfer in a fluidized bed. Part I. The effect of thermal driving force on the heat transfer coefficient

The influence of the thermal driving force on the coefficient for heat exchange between a fluidized bed (diam. 30 cm) and part of a submerged vertical cylinder (diam. 7 cm) is described. In the experiments both negative and positive driving forces have been applied (the heat flow being considered positive if directed towards the bed). Beds of glass beads, quartz sand and silica-alumina catalyst powders have been used to cover a range of particle size and density. For powders exhibiting dense phase expansion, heat transfer coefficients are found which are significantly affected by the driving force. At driving forces of about − 70°C heat transfer coefficients in a 40 μm catalyst bed were only half of those at driving forces approaching zero. The behaviour of a transparent ‘two-dimensional’ fluidized bed showed that this might be the result of a reduction of solids mobility in the vicinity of the relatively cold surface. In an attempt to explain this reduction three mechanisms were considered; of these the decrease of the dense phase expansion due to a temperature gradient proved to be the most important one. Powders not exhibiting dense phase expansion show heat transfer coefficients which are only slightly affected by the driving force, the differences resulting entirely from changes in physical properties with temperature.

Gas phase formation of methyl and ethyl iodides

Gas phase formation of methyl and ethyl iodides occurs when methanol and ethanol vapors are carried by an inert gas over silica-alumina catalyst supports impregnated with potassium, sodium, cesium, or cadmium iodides at 130–180°. The efficiency of conversion of inorganic to organic iodides by different catalysts was studied using125I-labelled potassium and cadmium iodides. A mechanism of organic iodide formation in the heterogeneous reaction is postulated.

Isomerization of 2,4-diphenyl-1-butene over silica-alumina catalysts.

Isomerization of 2,4-diphenyl-1-butene over silica-alumina catalysts.

Experimental study of catalyst deactivation and active acid site concentration.

Deactivation of catalyst was dealt with from the viewpoint of active site coverage by coke deposition. Catalyst activity and acidity were simultaneously measured in the isomerization of isopentene on commercial silica-alumina catalyst. This revealed that activity of deactivated catalyst was simply dominated by the concentration of Bronsted acid sites remaining available and that the activity at any time relative to the initial time was equal to the fraction of active sites not covered with coke. Based on this simple rule, an n-th order rate expression was justified to properly represent the catalyst deactivation.

Measurement of surface acid site concentration by carbon-13 NMR

A method of measuring simultaneously the concentration of surface proton and Lewis acid sites is proposed. The method is based on 13C NMR spectroscopy of a suitable adsorbed amine. An investigation of several alkyl-substituted pyridines shows 4-ethylpyridine to be the most satisfactory choice. The method is tested on a silica-alumina cracking catalyst, and found to give good results for total acid concentration. Decomposition of the total acid into proton and Lewis acids requires a higher precision of measurement than presently achieved, but appears possible with existing instruments. Application of the method to silica-alumina with added water shows that the catalyst behavior in the presence of water is more complicated than assumed by previous workers.

The Hydrothermal Activation of Silica–Alumina for Butene Isomerization. A Device to Prepare a Standard Sample

Abstract Three kinds of silica–alumina catalysts were prepared by hydrothermal treatment (heating a sample in hot water) over the range of 25–250 °C for various periods. It was found that the hydrothermal treatment significantly increased its catalytic activity for butene isomerization. The dependencies of various surface properties (the production ratio of 2-butene isomers, the surface acidity, the surface area, the pore-size distribution, and the shape and size of the catalyst particle) on the hydrothermal conditions and the kind of starting catalyst sample were systematically investigated, too. It was concluded that the increase in the catalytic activity might be due to the progress of the Si/Al isomorphic substitution in the tetrahedral surface silica network. Hot water might serve as a solvent for the cations and facilitate their movements. Since the hydrothermal treatment can erase the effects of the blending method on the catalytic activity and other surface properties almost completely, this treatment can be used to prepare a standard sample.

Fluidization of fine powders with air in the particulate and the bubbling regions

Several fractions of a silica—alumina catalyst and an impregnated catalyst were fluidized with air to check published bubble point correlations and to determine the expansion of the dense phase in the bubbling region. The minimum bubbling velocities were fairly close to those reported by Geldart and others, but U mb varied with only the 0.6 power of the particle size. The dense phase expansion was significant even at velocities much above U mb and corresponded to relative velocities up to 2 – 3 times U mf .

Gasification of polyethylene over solid catalysts. 4. Gasification over sodium X zeolite and silica-alumina in a fixed bed tubular flow reactor.

Gasification-degradation reactions of polyethylene (PE) over NaX zeolite and silica-alumina (SA) catalysts were studied in a fixed bed flow reactor system at 452-526°C and under 19.7-37.5g-cat•min/g-PE. The initial values in terms of the extent of gasification (RgF) at 25.0g-cat•min/g-PE were 0.23 and 0.41 at 477°C over NaX and SA, respectively. These values of RgFwere significantly greater than 0.015 for thermal degradation. However, these catalysts were deactivated by coke deposition and the RgFafter 2.5 hours was 33-55% of the initial value of NaX or 65-69% of that of SA. Product distribution in terms of carbon number for SA was almost independent of process time and reaction temperature, but the ratio of olefin to paraffin for the C3 fraction and that for C4 fraction increased gradually with process time and reaction temperature. For NaX, the fractions in the C1-C3 range increased, but other fractions and ratios decreased with increasing process time, while the increase in the reaction temperature caused a complex change in product distribution. The isobutane produced was 1-5mol% or 10-14mol% of the total gaseous products formed over NaX and SA. The main products formed over both of these catalysts were C3 and C4 olefins. The apparent activation energy was 25.2kcal/mol for NaX and it was 14.7kcal/mol for SA. The activity of these catalysts was reduced by pyridine poisoning. Moreover the poisoning of SA catalyst with sodium ion caused sharp drops in the activity levels and brought about marked changes in product distribution. The product distribution obtained over a Na-poisoned SA was quite similar to that over NaX. From the results obtained in the present study, it was suggested that the gasification-degradation reaction of PE over SA proceeded through the carbonium ion mechanism, while the gasification-degradation reaction involved over NaX was complex, competitive reactions of both the radical and carbonium ion mechanisms.

Controlled pore sizes and active site spacings determining selectivity in amorphous silica-alumina catalysts

Controlled pore sizes in amorphous silica-alumina were achieved by employing tetraalkylammonium cations (TMA (methyl) to TBA (butyl)] as the only counterions to the fourfold coordinated aluminate ions in the gels. The width of the pore size distributions and the median size of the pores in the calcined silica-aluminas (now free of nitrogenous cations) decreased as the size of the cations used in the synthesis increased, except in the butyl case. The median values as determined by N 2 desorption were 15, 5, 4.5, 3.7, and 4.2 nm for the Na NH 4 -, TMA-, TEA-, TPA-, and TBA-silica-aluminas, respectively. The surface area per unit mass of catalyst increased with the size of the cations. The average interexchange site distance increased correspondingly, except in the butyl case. This exception is attributed to constraints set by the specific SiO 2 Al 2O 3 ratio used. In the catalytic cracking of heptane the TPA-silica-alumina produced substantially greater proportions of toluene and cyclic C 6 hydrocarbons than the other silica-aluminas. These materials have potential as sorbents, catalysts, and catalyst supports inter alia in the treatment of heavy residue and coal liquids where zeolites cannot be used because of their small pore sizes (< 1 nm).

ChemInform Abstract: THE SELECTIVE ISOMERIZATION OF N‐HEXANE OVER NICKEL ON SILICA‐ALUMINA CATALYSTS

Chemischer InformationsdienstVolume 10, Issue 35 Preparative Organic Chemistry ChemInform Abstract: THE SELECTIVE ISOMERIZATION OF N-HEXANE OVER NICKEL ON SILICA-ALUMINA CATALYSTS R. BURCH, R. BURCHSearch for more papers by this author R. BURCH, R. BURCHSearch for more papers by this author First published: August 28, 1979 https://doi.org/10.1002/chin.197935103AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue35August 28, 1979 RelatedInformation

The selective isomerization of n-hexane over nickel on silica-alumina catalysts

The isomerization of n-hexane to branched chain isomers has been investigated over a range of nickel on silica-alumina catalysts containing between 5 and 16% nickel by weight. It is found that a 7% Ni catalyst has a high activity and selectivity for isomerization, but that catalysts containing more than about 10% Ni produce methane as the main product at temperatures above 633 K. The 7% Ni catalyst has an activity and selectivity which is comparable to a commercial platinum catalyst operating under the same experimental conditions. Hydrogen reduction experiments show that the nickel is present as zerovalent nickel, but X-ray and magnetic measurements do not correspond to bulk nickel metal. It is suggested that the active site in the 7% Ni catalyst is a zerovalent nickel atom probably attached to an aluminium ion. A mechanism is proposed for the isomerization reaction in which a nickel atom first abstracts a hydride ion from a hydrocarbon molecule forming a secondary carbonium ion, which then isomerizes to a tertiary carbonium ion on an adjacent oxide ion, before, finally, addition of the hydride ion gives the saturated isomer.

Model building in complex catalytic reaction systems

Catalytic rate-modelling forms an integral part of Process Development and Design as well as providing insight into the underlying reaction mechanisms. Industrial reactions are usually complex, involving several simultaneous and consecutive reactions, leading to a variety of products. In petro-chemical applications, in particular, the reactions often occur in the presence of continuous deactivation of the catalyst—a complex process which must also be modelled if an optimum plant design and operating policy are to be realized. This paper describes an approach to model building of such systems which requires a multi-step strategy of experimentation and analysis, using different types of laboratory reactor. The approach is applied to the kinetic modelling of simultaneous isomerisation and disproportionation of a mixture of xylenes over a silica-alumina catalyst which is subject to continuous coking.

ChemInform Abstract: DIRECT MEASUREMENT OF THE INTERACTION ENERGY BETWEEN SOLIDS AND GASES. III. COMPARISON OF THE CALORIMETRIC TITRATION METHOD WITH THE AMINE TITRATION METHOD FOR THE DETERMINATION OF ACID STRENGTH DISTRIBUTION OF SILICA‐ALUMINA SURFACE

The acid strength distribution of silica–alumina catalyst (alumina content 13 wt%) evacuated at 400 °C for 5 h was determined by the measurement of heat of adsorption of ammonia, and compared with that determined by amine titration by use of a series of Hammett indicators. The heats of adsorption of ammonia on silica–alumina poisoned with Hammett indicators were also measured in order to obtain the relation between the dissociation constant of the acid (pKa) and the heat of adsorption. When the dissociation constants of acid were equal to −5.6 and +3.3, the differential heats of adsorption of ammonia were observed to be 76.1 and 55.1 kJ/mol, respectively. The number of acid sites calculated from the heat vs. adsorbed amount curve was found to be about twice as large as that determined by amine titration. The discrepancy is discussed.

A study of pyridine adsorbed on silica-alumina catalysts by combined infrared spectroscopy and temperature-programmed desorption

A combined technique employing transmission infrared spectroscopy (IR) and temperature-programmed desorption (TPD) has been used to study the interaction of a gaseous base with a series of acidic microporous catalyst surfaces. This technique was used to establish the firstorder kinetics for pyridine desorption from silica-alumina gel catalysts covering a wide range of compositions. This combined technique has also allowed more detailed information about number, type, and strength of acid sites on catalysts to be obtained in a single experiment. The special measuring system that allows simultaneous transmission infrared monitoring and temperature-programmed desorption from catalysts is described. The results of applying this combined technique to a model silica-alumina catalyst system are shown. Insight into the mechanism for the desorption of both Lewis and Brønsted chemisorbed pyridine was deduced from the TPD results. A model for pyridine desorption is presented and discussed.

Direct Measurement of the Interaction Energy between Solids and Gases. III. Comparison of the Calorimetric Titration Method with the Amine Titration Method for the Determination of Acid Strength Distribution of Silica-Alumina Surface

Abstract The acid strength distribution of silica–alumina catalyst (alumina content 13 wt%) evacuated at 400 °C for 5 h was determined by the measurement of heat of adsorption of ammonia, and compared with that determined by amine titration by use of a series of Hammett indicators. The heats of adsorption of ammonia on silica–alumina poisoned with Hammett indicators were also measured in order to obtain the relation between the dissociation constant of the acid (pKa) and the heat of adsorption. When the dissociation constants of acid were equal to −5.6 and +3.3, the differential heats of adsorption of ammonia were observed to be 76.1 and 55.1 kJ/mol, respectively. The number of acid sites calculated from the heat vs. adsorbed amount curve was found to be about twice as large as that determined by amine titration. The discrepancy is discussed.

Benzene synthesis for radiocarbon dating and study of the catalyst used for acetylen trimerization

The succession of steps undergone by the sample in the benzene synthesis line for radiocarbon dating from its original state to benzene through the intermediate formation of carbon dioxide in the Vycor tube, lithium carbide with molten lithium metal heated to at least 700°C and in absence of air, acetylene by its hydrolysis and its conversion to benzene on the catalyst, is schematically described. The optimum conditions for the above mentioned operations are defined and secondary reactions, responsible for reduced yields in the various steps, are described. Particularly, a silica-alumina catalyst activated by K 2CrO 4 employed for the conversion of acetylene to benzene is studied. Sixty grams of catalyst, dehydrated at a temperature of about 400°C for 1 hr and in absence of air, proved to be sufficient to convert in benzene, in 90 min time, 98–99% of a volume of acetylene averaging 4.51 at P 0 T 0. The benzene is extracted under vacuum at 150°C for 2 hr and its purity checked by gas chromatograph analysis is about 99.0%. The efficiency of the catalyst is maximum when it is activated by Cr(VI) and it is sufficient to expose the poisoned catalyst to an air flow at a temperature of 550–600°C during 60 min to bring it to same efficiency as the fresh catalyst. No cross contamination effects are present when the regenerated catalyst is used since the values of dates are in the range of 3 σ. The reducing action of acetylene on the catalyst promotes the reduction of Cr(VI) to Cr(IV) and Cr(III) with remarkable loss of its efficiency and is responsible for reduced yield of benzene in conversion reaction with formation of by-products which are a mixture of aliphatic and aromatic polymers, the last containing a carbonyl group and an aliphatic chain. The total amount of by-products is about 5% of acetylene used. The order of reaction for conversion of acetylene to benzene, including by-products in respect of the acetylene is 1.5 in the range 20–35 mmole/1 and in the temperature range of 0–55°C. The value of the energy of activation is −4.9 ± 0.3 kcal/mole.

An Infrared Study of Acid Sites on Silica-Alumina Catalysts Poisoned with Sodium Hydroxide

Abstract We investigated the effect of NaOH impregnation on the acidic properties (the type, strength, and amount of acid sites) of silica-alumina by means of IR spectroscopy of chemisorbed pyridine. Most acid sites on the unpoisoned catalyst were of the Lewis type. Some Lewis-bound pyridine was retained on the surface even at 400 °C, but the pyridinium ion was eliminated at 300 °C, indicating that the strong acid sites were also of the Lewis type. Impregnated NaOH affected both Lewis and Brønsted sites. An increase in the amount of NaOH resulted in the elimination of the Brønsted acid sites of medium strength, and also in a significant reduction in the number of Lewis acid sites over the whole range of acid strength. Weak Brønsted acid sites showed no marked change in number. When the NaOH-impregnated catalysts were exposed to pyridine, a new IR band appeared at about 1445 cm−1; this band is attributable to pyridine associated with the Na ions on the surface.

The mechanism of catalytic isomerization of xylenes: Kinetic and isotopic studies

The vapor-phase isomerization of xylenes over a silica-alumina catalyst containing 4 wt% nickel has been investigated in the presence of excess hydrogen at 400 °C and pressures up to 15 atm. The results clearly indicate that the three isomers are interconverted via the consecutive reaction, ortho → meta → para. Hydrogenolysis to toluene and disproportionation to trimethylbenzenes are the two major competing reactions. Additional experiments carried out with deuterium-labeled p-xylene show that the isomerization proceeds almost exclusively by intramolecular 1,2 shifts of the methyl groups around the aromatic ring.