J.C.Q. Fletcher

Effect of catalyst modification on the conversion of methanol to light olefins over SAPO-34

Abstract The catalytic activity and selectivity of as-prepared and modified samples of SAPO-34 and Me-APSO-34 ( Me = Co, Ni ) for the conversion of methanol to olefins has been investigated. The catalytic performance for the conversion of methanol to light olefins of all the catalyst samples prepared was found to be closely related to the number of strong acid sites present. Mild steaming, encountered during deep-bed calcination, increased the lifetime of SAPO-34 due to the formation of stronger acid sites probably on the external surface of the crystallites. Selectivities to light olefins were typical of those previously reported and was essentially constant for all the catalysts investigated. The absence of C 5+ olefins is ascribed to the ‘cage effect’. Dilution of the methanol with water as opposed to nitrogen increased the catalyst utilization value threefold and reduced the rate of coke formation during reaction. Treatments such as steaming, silanization and poisoning of strong sites by ammonia all reduced the number of strong acid sites and, thus, reduced catalytic performance.

Experimental approaches to the preparation of supported metal nanoparticles

Supported metal particles play an important role in heterogeneous catalysis. It has been shown lately that the size of the metal crystallites in the supported metal catalysts has a profound effect on the catalytic activity, thus necessitating the need for synthesis methods aimed at a strict control of the metal crystallite size in these catalysts. The classical methods used to synthesize supported metal catalysts typically yield a wide metal crystallite size distribution, and average crystallite sizes which are difficult to control. Suitable techniques have been developed to obtain supported metal catalysts with defined crystallite size distributions, inter alia impregnation of reverse micelle microemulsions, colloid impregnation following reverse micelle precipitation or crystallization, and deposition-precipitation. Using these techniques, a series of supported Ru/γ-Al2O3, Co/SiO2, Fe/γ-Al2O3, Fe/C, and Au/ZnO catalysts have been prepared and characterized.

Characterization of dealuminated large-port mordenites

Abstract Four different types of large-port mordenite were studied. Three of these catalyst samples were dealuminated by treatment with nitric acid and were characterized by temperature-programmed desorption (TPD) of ammonia, infrared spectroscopy, cyclohexane adsorption, solid-state NMR, and X-ray diffraction (XRD). It was found that the extent of dealumination and the amount of octahedral aluminum remaining in the zeolite channels was strongly influenced by crystallite size. Extensive dealumination resulted in materials with surface areas greater than the parent materials, yet still retaining their characteristic mordenite structure as determined by XRD. Ammonia TPD was shown to uniquely determine the framework aluminum content, irrespective of the presence of extraframework aluminum. The previously disputed origin of the infrared absorption band at 955 cm −1 is assigned to new SiOSi bonds formed upon framework dealumination. A sequence of dealumination events is proposed.

Acidity and catalytic activity of synthetic mica-montmorillonite: Part II: Propene oligomerization

Abstract The effect of varying calcination and reaction conditions on the oligomerization of propene over synthetic mica-montmorillonite, SMM, was studied in a fixed bed reactor at 61 atm. Lewis acid sites gave high activity and long catalyst lifetime, while the presence of moisture led to a temperature runaway and rapid deactivation. Reaction temperatures above 470 K produced longer chainlength hydrocarbons and aromatic compounds with reduced lifetime. The findings are related to a previous study characterizing the acidity of SMM using infrared spectroscopy and pyridine temperature programmed desorption.

Selective Fischer-Tropsch wax hydrocracking : opportunity for improvement of overall gas-to-liquids processing

Abstract Fischer-Tropsch (F-T) based Gas-to-Liquids (GTL) processing is recognized as an industrially proven and economically competitive route to high quality diesel. Furthermore, it is generally accepted that for this purpose, GTL processing is most effective when comprising an F-T synthesis driven to wax production, followed by hydrocracking to produce middle-distillate products. Applying a CoMo/SiO 2 -Al 2 O 3 catalyst, optimised for hydrocracking crude oil refinery feedstocks in a sulphur-containing environment, to the processing of a linear paraffin F-T wax model compound, n-tetradecane, shows that a significant opportunity exists for utilisation of base metal catalyst, having the advantage of producing less branched hydrocracking products, i.e. high cetane number diesel via a hydrogenolytic cracking mechanism. A drawback of such a catalyst, if applied in non-sulphided form and in a non-sulphur containing environment, is the comparably high yield of light gases, in particular methane. It is shown, and proved by a simple kinetic model, that methane is formed via ‘methanolysis’, i.e. successive hydrogenolytic demethanisation reaction of the feed compounds, presumably of islands of metallic cobalt on the catalyst.

Internal and external transport effects during the oxidative reforming of methane on a commercial steam reforming catalyst

Abstract In order to establish an isothermal catalyst bed it was necessary to dilute both the catalyst particles (1:10 in α-Al 2 O 3 ) and the feed mixture (2% reactants in He). Under these conditions, a linear velocity of about 6 cm/s was sufficient to remove the effect of film diffusion at 600°C. Experiments with particle sizes in the range 125–710 μm revealed that internal diffusion did not play a role in methane conversion at a fixed space velocity. No change in the apparent activation energy was found over the range 575–625°C which confirmed the absence of diffusion limitations; at 650°C, however, a slight decline in E a was observed which may be indicative of the presence of concentration gradients. Correlations taken from the literature and which are normally used to test the absence of internal and external temperature and concentration gradients, were applied to our operating conditions and confirmed our conclusions.

Oxidative reforming of methane to syngas over a NiO-CaO catalyst

Abstract Methane was successfully converted to synthesis gas by oxidative reforming over a reduced NiO-CaO catalyst first prepared by Choudhary et al. [1]. In order to compare this work with previously reported results, the effect of space velocity and temperature was investigated. Additionally, the lifetime of the catalyst was tested with both a dry and a wet feed, as well as the influence of CH4/O2 ratio on catalyst performance. In accordance with the results of Choudhary on this and other catalysts, the syngas yield was found to deviate from the expected equilibrium values. This may be attributed to the catalyst temperature being higher than the measured bulk gas temperature. Lifetime tests ( 500°C for 50 hours and 700°C for 20 hours) showed that the catalyst slowly lost activity and selectivity. In contrast, the inclusion of low steam feed rates resulted in stable operation (no deactivation and constant bed temperature). Decreasing the CH4/O2 ratio resulted in improved CH4 conversion while, contrary to the expected equilibrium trend, CO selectivity remained constant. When space velocity was increased from 200 000 to 500 000 h−1, CH4 conversion and CO selectivity remained unchanged, but both values decreased when space velocity was again lowered to 200 000 h−1.

Acidity and catalytic activity of synthetic mica-montmorillonite: Part I: An infrared and temperature programmed desorption study

Abstract The nature and distribution by strength of acid sites present in activated synthetic mica-montmorillonite, SMM, has been studied. Bro˝nsted acidity disappears at about 700 K while Lewis acidity is present strongly even at 950 K. Apparent Bro˝nsted acidity as detected by pyridine using infrared spectroscopy arises in part from pyridine associated with ammonium ions and strongly polarized water. Rehydration of Lewis sites restores Bro˝nsted acidity and the presence of water vapour in general is found to enhance significantly the amount of pyridine adsorbed on SMM. Pyridine desorbing from rehydroxylated SMM between 350 and 450 K in temperature programmed desorption is not physisorbed pyridine but is proposed to be bound to polarized structural water. Subsequent to calcination sites located in the interlayer spacings are inaccessible to pyridine. The number of sites as determined by both infrared and temperature programmed desorption studies are in good agreement in the absence of polarized physisorbed water.

The effect of different Si/Al ratios and reaction conditions on the phenol methylation activity of MCM-22

Abstract MCM-22 samples of different Si/Al ratios were prepared using various silica sources and agitation conditions and were characterised using BET, SEM, XRD and elemental analysis. Two samples with Si/Al ratios of ∼10 and ∼100 were used in a study of the alkylation of phenol with methanol in a flow reactor. Phenol conversions did not vary much with Si/Al ratio in the gas phase but were greater at high Si/Al ratios in the liquid phase. For the Si/Al = 10 sample, selectivities to anisole and cresols were hardly influenced by the reaction phase but varied considerably at Si/Al = 100. p-Cresol selectivity increased significantly in liquid phase reactions but was not greatly influenced by changes in the Si/Al ratio. A series –parallel mechanism is proposed to explain these results. The primary products are anisole, formed via a rapid alkylation of the side chain, and o-cresol, formed via a slower alkylation of the aromatic ring. Secondary reactions, which may occur, involve a slow monomolecular transalkylation to convert anisole to o-cresol or a reaction between anisole and phenol to form cresol and phenol.

Propene Oligomerization and Xylene and Methyl-Pentene Isomerization over SAPO-11 and MeAPSO-11

The molecular sieves SAPO-11 and MeAPSO-11, where Me = Co, Fe, Mn and Ni, were synthesized and the acidity of these catalysts and modified forms thereof tested using the isomerization of m-xylene and 2-methyl-2-pentene (2M2P) as probe reactions. The activity of the catalysts for high pressure propene oligomerization was also evaluated. The modifications included steaming, acid washing, metal impregnation, silanization, pelletization and extrusion. In the conversion of m-xylene the catalysts all showed the high para/ortho ratio expected for 10MR structures. In the case of 2M2P isomerization the ratio of methyl shift products (c/t-3M2P) to double bond shift (c/t-4M2P) indicated the presence of strong acid sites. In both these reactions diffusional resistances due either to the presence of non-framework intraparticular matter or to amorphous acidic material on the external surface of the crystallites masked the activity of the catalysts. This was indicated by the unusually high extent of disproportionate of the m-xylene and the poor correlations between the activity and the ammonia-TPD results. SAPO-11 modified by mild steaming yielded a remarkably high catalyst utilization value of over 1000 g liquid/g-catalyst for propene oligomerization. More than 50% of the reaction products were dimers of propene.