High-Pressure Oligomerization of Propene over Heteropoly Acids

Abstract

Ammonium, 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, NH 4 > 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.