Browsing by Author "O'Connor, C"
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- ItemRestrictedAromatization of 1-hexene and 1-octene by gallium/H-ZSM-5 catalysts.(Elsevier, 1996) Nash, R; Dry, M; O'Connor, CGallium/H-ZSM-5 zeolite catalysts were tested for the aromatization of long-chain alkenes, namely 1-hexene and 1-octene. Aromatic yields of ca. 70% were obtained, which are similar to those reported in the literature for propene aromatization. The catalysts tested were prepared by impregnation by incipient wetness of gallium nitrate, ion-exchange with gallium nitrate and by physical mixing with β-gallium oxide crystallites. Hydrogen pretreatment improved the selectivity to aromatic products of gallium/H-ZSM-5 catalysts prepared by physical mixing with β-gallium oxide, but decreased aromatic selectivity for catalysts prepared by ion-exchange and impregnation.
- ItemRestrictedThe characterization and elimination of the external acidity of ZSM-5.(Elsevier, 1996) Weber, R; Fletcher, J; Möller, K; O'Connor, CThe external acidity of ZSM-5 was modified either by coating the crystals with a silicalite shell or by chemical vapour deposition (CVD) of tetraethoxysilane (TEOS). Temperature programmed desorption (TPD) techniques were used to characterize the acidity changes arising from these modifications. Ammonia and pyridine were used as probes for the total acidity, and 4-methylquinoline was used to probe the external acidity of the modified and unmodified ZSM-5 samples. Si/Al ratios of the modified samples were also used to characterize the samples. Both methods of modification were able to make the external acid sites inert. Chemical vapour deposition was found to be a better method, because the external surface acidity was eliminated without significantly changing the catalyst in terms of total acidity. Optimum elimination of the external surface acidity by CVD was obtained for the sample with the higher deposition time (16 h) and temperature (400°C). Optimum elimination of the external surface acidity using the silicalite shell method was obtained using the detemplated NH4-form of the parent crystal as seed material. The channel structures of crystals modified by both processes remained accessible to ammonia and pyridine.
- ItemRestrictedThe chemical vapour and liquid deposition of tetraethoxysilane on the external surface of ZSM-5(Elsevier, 1998) Weber, R W; Möller, K P; Unger, M; O'Connor, CThe external acidity of ZSM-5 was modified by chemical vapour deposition (CVD) and chemical liquid deposition (CLD) of tetraethoxysilane [Si(OC2 H5 ) 4 ] using a static vacuum system, a vapour phase flow system and liquid phase deposition. Temperature programmed desorption (TPD) techniques were used to characterise the acidity changes arising from these modifications. Pyridine was used as a probe for the total acidity and 4-methyl quinoline (MQ) was used to probe the external acidity. The adsorption capacities of the samples were measured using n-hexane, p-xylene, o-xylene and 1,2,4-trimethyl benzene. The extent of Si(OC2 H5 ) 4 deposition was strongly dependent on temperature in both vapour phase flow and static vacuum systems. Continuous Si(OC2 H5 ) 4 deposition was observed in the presence of H2 O at relatively high temperatures when decomposition products were removed from the sample. It is proposed that physisorbed species need to be removed by evacuation or calcination to re-expose active sites, thereby enabling complete inertisation of the external surface acidity to occur, and that a more uniform covering can be obtained when a gradual deposition process is used. Such a process may be achieved by the use of diluents or by preventing overexposure of Si(OC2 H5 ) 4 to the sample under conditions where continuous deposition may occur. It was shown that it is possible to reduce the relative external surface acidity by 97% without significant changes in acidity as measured by Py-TPD or adsorption capacity.
- ItemRestrictedEffect of catalyst modification on the conversion of methanol to light olefins over SAPO-34.(Elsevier, 1996) van Niekerk, M; Fletcher, J; O'Connor, CThe 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 C5+ 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.
- ItemRestrictedEffects of in-situ chemical vapour deposition using tetraethoxysilane on the catalytic and sorption properties of ZSM-5.(Elsevier, 1998) Röger, H; Krämer, M; Möller, K; O'Connor, CThe external surface of HZSM-5 crystals was gradually modified by in-situ CVD of tetraethoxysilane. Using the particular procedures described in the paper, it was shown from probe reactions in combination with sorption studies that the external surface was deactivated and the pore-opening size was reduced while the intracrystalline pore space remained unmodified. The positive effect of the CVD treatment procedure on the shape selective properties of HZSM-5 was demonstrated by an enhancement in p-selectivity during toluene disproportionation. The paper discusses the relation between the effects of the CVD treatment procedure on catalytic properties, sorption properties and structural properties and conclusions are made on the use of the conversion of 1,2,4-trimethylbenzene, 1,3,5-triisopropylbenzene, toluene and n-hexane, respectively, to probe the effects of external surface modifications on properties of ZSM-5.
- ItemRestrictedHigh-pressure oligomerization of propene over heteropoly acids(Elsevier, 1994) Vaughan, J; O'Connor, C; Fletcher, JAmmonium, potassium, nickel, copper, cobalt, iron, cerium, and aluminium salts of 12-tungstophosphoric acid (HPW) as well as the ammonium and aluminium salts of 12-tungstosilicic acid have been synthesized and characterized by TG-DTA, nitrogen adsorption, XRD, electron microscopy, ammonia temperature programmed desorption, and FTIR. Consistent with the findings of other heteropolyacid (HPA) characterization studies the HPAs could be divided into two types: Type A, low surface area salts with multiple endothermic mass losses, and Type B, high surface area salts with a single endothermic mass loss. The surface acidity of some of these catalysts was evaluated using butane cracking and butene isomerization as probe reactions. These indicated that the Type B salts had strong acid sites on the surface as they were capable of cracking butane and butene. The Type A salts were inactive for these reactions. The propene oligomerization activity of the HPW salts decreased in the order: Al ⪢ Co > Ni, NiH, NH4 > H, Cu > Fe, Ce > K. Premature deactivation as a result of substantial film temperature gradients occurs due to the inability to dissipate the large heat of reaction in the undiluted catalyst bed. Diluting the catalyst with acid-washed sand (1 part catalyst to 10 parts sand) dramatically increased the liquid product yield and catalyst lifetime but the activity order remained the same as the pure powder form. The pure aluminium salt of HPW, viz. AlPW, was found to be the most active, achieving 90% conversion at a WHSV of 12 h−1, 230-240°C, and 5 MPa. The main product of propene oligomerization was the trimer. The sand-diluted AlPW catalyst achieved 100% conversion under identical conditions with no sign of deactivation after 150 h on stream. Pure AlPW yielded a catalyst utilization value (CUV) of 540 g · (liquid product)/g · catalyst with a distillate fraction cetane number of 40. The CUV of the diluted AlPW catalyst was in excess of 1800 g · (liquid product)/g · catalyst. The relationship between the catalytic activity, surface area, and structure of the catalysts is discussed.
- ItemRestrictedInfluence of preparation variables on the dispersion of platinum on zeolite KL.(Elsevier, 1997) M'Kombe, C; Dry, M; O'Connor, CVarious preparation conditions were investigated to evaluate their effect on the dispersion of Pt on zeolite KL. The highest dispersion was obtained by calcining in O2 at about 350 °C. Calcining in N2 or H2 resulted in lower dispersions. Irrespective of the gas atmosphere, heating at 600 °C resulted in sintering of the Pt. The duration of calcination, whether at 350 °C or 600 °C, had little effect on Pt dispersion. The rate at which the temperature was increased up to the calcination temperature and the rate of the O2 flow also had no effect. Subsequent to identical calcination procedures, reduction with H2 resulted in much higher Pt dispersions than reduction with CO. Below 350 °C the temperature of the H2 reduction did not have a marked effect on dispersion. At the 1.5% Pt level, different loading techniques, viz., liquid ion exchange, solid-state ion exchange, and incipient wetness impregnation, resulted in similar Pt dispersions. There was a good correlation between the percentage of Pt dispersion and n-hexane conversion in the aromatization reaction. Pt dispersions were determined by CO chemisorption, and the observed trends were confirmed by TEM. Hydrogen chemisorption was a less satisfactory method of measuring dispersion. © Elsevier Science Inc. 1997
- ItemRestrictedUse of TPR/TPO for characterization of supported cobalt catalysts.(Springer, 1996) Sewell, G; van Steen, E; O'Connor, CThe extent of reduction of supported cobalt catalysts is difficult to determine using TPR due to the unknown stoichiometry of reduction and due to the dynamic nature of the measurement. A method is described where, by using a combined TPR/TPO technique, it is possible to determine the extent of cobalt reduction and obtain an estimate of the extent of cobalt-support species formation. The results showed that the extent of metal reduction following hydrogen reduction at 500°C is affected considerably by the type of metal carrier. In particular, the extent of metal reduction decreased with increasing aluminium content of the support material. Decreasing extents of metal reduction could be correlated with an increase in the temperature required for reduction of the nitrate ion during TPR. Increasing the time and temperature of hydrogen reduction results in increased extents of metal reduction.