Browsing by Author "Fletcher, J"
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- 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.
- 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.
- 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.