Behavior of vanadium-phosphorus-oxides catalyst for butane oxidation to maleic anhydride in a single-tube packed bed reactor

Vanadium-phosphorus-oxides catalysts for butane oxidation to maleic anhydri de were prepared in an organic medium in pilot-scale equipment and were tes ted in a single-tube packed bed reactor. To avoid reactor thermal runaway, a nonuniform catalyst loading was adopted to moderate the reactor temperatu re. The precursors and catalysts were characterized by average oxidation st ates of vanadium, PN atomic ratios, X-ray diffraction and pore size distrib ution. VOHPO4. 0.5H(2)O and (VO)(2)P(2)O7 . 9H(2)O in precursors, (VO)(2)P2 O7 and alpha-VOPO4 in catalysts were observed in the present work. After completion of a long run catalytic test, the used catalysts were care fully sectioned and removed from the reactor for characterization. The axia l profiles of average oxidation states of vanadium and PN atomic ratios wer e obtained. The average oxidation states of vanadium were between 4.0 and 4 .3 for the used catalysts and were related to the axial profiles of tempera ture obtained during the test. The PN ratios of the used catalysts were in the range of 1.07-1.08, almost the same as that of the precursor. No phosph orus loss was observed after a long-run test and this enabled the catalyst to maintain a long useful life. The long-run catalytic test results showed that the catalyst was of high activity and good selectivity for butane oxid ation to maleic anhydride. (C) 1999 Elsevier Science B.V, All rights reserv ed.