INFRARED AND THERMAL-ANALYSIS STUDIES OF HETEROPOLY ACIDS

The thermal stability and acidity of 12-tungstophosphoric acid (HPW) a nd its salts, both fresh and postreaction, have been studied by in sit u and ex situ infrared and thermal analytical methods. The relationshi p between acidity and the lifetime and deactivation behavior in the hi gh-pressure oligomerization of propene has also been investigated as h as been the location of the active sites in heteropoly anions. These s tudies have confirmed a correlation between structural stability and t he salt cation, with the potassium salt of HPW being the most thermall y stable. A correlation has also been established between salt Type (A or B) and the nature of coke deposits formed during reaction. Type A salts produced only Type I(aliphatic) coke during butene isomerisation , butane cracking and short time-on-stream isothermal propene oligomer isation reactions but both Type I and Type II (aromatic) coke in long time-on-stream and/or nonisothermal oligomerisation operation. Type B (high surface area) salts always formed both Type I and Type II coke. In all cases the formation of Type II coke was responsible for catalys t deactivation. This coke blocking of acidic sites Cas confirmed by NH 3 adsorption/desorption studies of postreaction heteropoly anions. On the basis of shifts in IR bands, it is proposed that the coke deposits responsible for deactivation are formed in the so-called cup M-O-M ed ge sites of the Keggin Unit. A mechanism is proposed to describe the t hermal decomposition of HPW. (C) 1995 Academic Press, Inc.