J.B. McMonagle

Influence of boria loading on the acidity of B2O3/Al2O3 catalysts for the conversion of cyclohexanone oxime to caprolactam

Abstract Boron oxide catalysts supported on alumina have been prepared with boria loadings in the range 0–20 wt.-%. These materials were characterized by B.E.T. surface area measurements, X-ray diffraction and temperature-programmed desorption of ammonia and were used as catalysts for the Beckmann rearrangement of cyclohexanone oxime to caprolactam by passing the oxime vapour through a reactor containing the catalysts at 300°C. The surface areas of the samples decreased as the boria loading was increased, and all catalysts tested for the rearrangement reaction diminished in activity with time-onstream. There was a direct relationship between the amount of coke which formed on the surface and the loss in catalytic activity. This loss was least for the highest boron oxide loading tested, and this catalyst also featured the least amount of coke formation. Selectivity to caprolactam was associated with the presence of surface acidic sites of intermediate strength. A correlation was observed between the concentration of these surface sites and the selectivity to caprolactam.

Factors affecting selectivity in the rearrangement of cyclohexanone oxime to caprolactam over modified aluminas

Abstract The Beckmann rearrangement of cyclohexanone oxime to caprolactam has been studied over a range of aluminas modified by the addition of chloride, phosphate, sulphate, sodium or boron oxide. The catalysts were characterized by temperature-programmed desorption of ammonia and carbon dioxide and by pyridine adsorption monitored by infrared spectroscopy. Conversion of the oxime declined with time-on-stream for all catalysts studied, but selectivity to caprolactam remained constant during the first five hours of operation. In this period a relationship was observed between the selectivity to cap- rolactam and the surface concentration of intermediate strength acid sites, wherein the selectivity increased as the surface concentration of these sites increased. The boron oxide modified alumina presented the largest concentration of intermediate strength acid sites, and the low level of coke formation on this material was associated with the total absence of surface basicity. A mechanism for the rear- rangement reaction on the modified aluminas is presented.

Selective oxidation of methane to formaldehyde: comparison of the role of promoters in hydrocarbon rich and lean conditions

Abstract The selective oxidation of methane to formaldehyde at atmospheric pressure was investigated over a range of silica supported molybdena and vanadia catalysts promoted with iron, silver, chromium, sodium or cobalt. None of these additives added alone to the silica support assisted in the selective oxidation of methane to formaldehyde, but beneficial effects were observed when molybdenum was subsequently added by impregnation. The vanadium and molybdenum silica systems were compared for a wide range of experimental conditions, including the influence of oxidant (N2O or O2) and the influence of methane content of the feed (methane rich or methane lean conditions). A detailed examination of the influence of vanadia loading on silica was undertaken and this established that vanadia loadings in the range 1–4wt% vanadium gave the best performance.

The relationship between acidity of aluminas and their selectivity in the conversion of cyclohexanone oxime into caprolactam

A range of aluminas modified by addition of sodium, phosphate, sulphate, chloride and boria have been tested for the Beckmann rearrangement of cyclohexanone oxime to caprolactam in the temperature range 300–350 °C. A relationship was observed between the selectivity to caprolactam and the surface concentration of acidic sites of intermediate strength.

Oxidation of methane to formaldehyde over supported molybdena catalysts at ambient pressure: isolation of the selective oxidation product.

Two series of molybdena catalysts supported on a copper treated fumed silica were prepared and their properties for the selective oxidation of methane to formaldehyde were compared with the copper free analogs. The catalysts were tested at ambient pressure in a fixed bed microreactor using a 5:1 CH4:N2O mixture in the temperature range 500–600C. In a second series of tests the propensity of silica and alumina supports to catalyse the decomposition of formaldehyde was assessed. The copper free catalysts lost their selectivity to formaldehyde when the MoO3 loading exceeded 5wt% but loadings up to 15wt% were possible with two copper treated supports without loss of selectivity. Yield of formaldehyde was superior over the copper treated catalysts also. This improved performance was associated with the ability of metallic copper to act as a co-catalyst and increasing the lability of the lattice oxygen in the supported molybdena phase. On all catalysts combustion of formaldehyde in the catalytic reactor occurred and the second series of tests showed that this combustion could occur also on the support material.

Kinetic study of the selective oxidation of butan-2-one to diacetyl over vanadium phosphorus oxide

Abstract The selective oxidation of butan-2-one to diacetyl has been studied in the temperature range 200–380°C over a vanadium phosphorus oxide catalyst. In addition to diacetyl the principal reaction products detected were acetic acid, acetaldehyde, methyl vinyl ketone, propionaldehyde and carbon dioxide. A kinetic analysis indicated that diacetyl formation proceeds through a Mars and van Krevelen type mechanism and that there were two sources of acetaldehyde formation; the first predominating at high oxygen partial pressures involved an enol intermediate and adsorbed molecular oxygen; the second, which predominated at low oxygen partial pressures involved lattice oxygen and a diol intermediate. It is also proposed that acetoin, CH 3 COCHOHCH 3 , is a common intermediate in the formation of diacetyl and methyl vinyl ketone. High selectivities to diacetyl can be achieved by operation in anaerobic conditions.

Rearrangement of Cyclohexanone Oxime to Caprolactam Over Solid Acid Catalysts.

Abstract Beckmann rearrangement of cyclohexanone oxime to caprolactam in the gas phase over a boria catalyst has been investigated. Catalyst testing using a continuous flow fixed bed reactor indicated typical time on stream reaction profiles, to consist of an initiation period, a brief stable operating phase and finally slow deactivation. The fast deactivation of nonselective, strongly acidic catalyst sites is thought to account for the initiation period. In addition to the mechanisms already known (coking and basic by-product re-adsorption) for the loss of caprolactam selectivity during the final deactivation phase, evidence is presented to suggest an additional process whereby some of the selective sites of intermediate acidity are converted to non-selective sites during reaction. The latter are thought to be due to the formation of some amorphous boron species.

Beckmann Rearrangement Over Solid Acid Catalysts

Abstract Beckmann rearrangement of cyclohexanone oxime to caprolactam has been investigated by passing the oxime vapour in helium over a series of modified alumina or zeolites held between 250 and 380°C in a continuous flow fixed bed micro-reactor. The range of solid acids studied included alumina modified with boron, sodium, phosphorus, sulphate or chloride, and the zeolites H-mordenite, H-A, H-Y and HZSM-5. Each catalyst studied exhibited an initiation period, a period of steady state activity and a deactivation period. The duration of each phase of activity depended on experimental conditions but for the boron modified alumina operating at 300°C the initiation period was ca. 1 hour. This phase was characterised by a high conversion of the oxime but very poor selectivity to caprolactam and was accompanied by a build up of coke on the surface of the catalyst. The steady state period was ca. 8 hours and featured high conversion and selectivity. The final phase featured a slow irreversible lowering of conversion and selectivity and was accompanied by the formation of a boron-alumina glassy phase. The boron modified alumina features a high concentration of acidic sites of intermediate acid strength, which are associated with high selectivity to caprolactam. Deactivation and loss of selectivity are associated with the loss of boron through its incorporation into the alumina solid support.

Selective Oxidation of Methyl Ethyl Ketone to Diacetyl Over Vanadium Phosphorus Oxide Catalysts.

Abstract Selective oxidation of methyl ethyl ketone to diacetyl has been studied by passing a mixture of the ketone in artificial air over vanadium phosphorus oxide catalysts in the temperature range 200–350°C. Products observed included diacetyl, methyl vinyl ketone, acetaldehyde, acetic acid and carbon dioxide. C4 products were favoured at low temperatures and at low or zero oxygen partial pressures. These results are rationalised in terms of two pathways for C2 products, namely oxidation of the double bond in the enol form of methyl ethyl ketone to yield acetic acid and acetaldehyde, and acid catalysed hydration of the keto form to yield acetaldehyde only. The C4 products are envisaged to go through a common intermediate, namely, CH 3 COCHOHCH 3 , formed by interaction between methyl ethyl ketone and lattice oxygen.