HIGH-PRESSURE OLIGOMERIZATION OF PROPENE OVER HETEROPOLY ACIDS

Ammonium, potassium, nickel, copper, cobalt, iron, cerium, and alumini um salts of 12-tungstophosphoric acid (HPW) as well as the ammonium an d 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 t he HPAs could be divided into two types: Type A, low surface area salt s with multiple endothermic mass losses, and Type B, high surface area salts with a single endothermic mass loss. The surface acidity of som e of these catalysts was evaluated using butane cracking and butene is omerization 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 th e order: Al much greater than Co > Ni, NiH, NH4 > H, Cu > Fe, Ce > K. Premature deactivation as a result of substantial film temperature gra dients occurs due to the inability to dissipate the large heat of reac tion in the undiluted catalyst bed. Diluting the catalyst with acid-wa shed sand (1 part catalyst to 10 parts sand) dramatically increased th e liquid product yield and catalyst lifetime but the activity order re mained the same as the pure powder form. The pure aluminium salt of HP W, viz. AlPW, was found to be the most active, achieving 90% conversio n at a WHSV of 12 h-1, 230-240-degrees-C, and 5 MPa. The main product of propene oligomerization was the trimer. The sand-diluted AlPW catal yst achieved 100% conversion under identical conditions with no sign o f deactivation after 150 h on stream. Pure AlPW yielded a catalyst uti lization value (CUV) of 540 g . (liquid product)/g . catalyst with a d istillate fraction cetane number of 40. The CUV of the diluted AlPW ca talyst was in excess of 1800 g . (liquid product)/g . catalyst. The re lationship between the catalytic activity, surface area, and structure of the catalysts is discussed. (C) 1994 Academic Press, Inc.