Indian Journal of Chemistry Vol. 39A April2000, pp. 439-441 Catalytic liquid phase oxidation of p-xylene using transition metal substituted polyoxometalates Nirupa A Alekar, Sarada Gopinathan C Gopinathan • Pun e 411 008 , India Received 30 Mar c h 1999; revised 26 October 1999 Liquid phase oxidation of xylene h as b ee n ca rried out in the presence of pot ass ium sal t s of monosub s tituted het e ropoly compounds with tungsten as the addenda atom and h av ing the general formulae [XW 11 ML 0 39 ] K • . x H 2 0 where X = P and Si , M + = Co 2 , Cu 2 + and Mn 2 + and L = H 2 0. The reactions are carried out in a halide free environment u s ing ai r as the oxidant and water as the so lvent at a temp e r at ur e of 180 ° C. High values of p-xylene conversion to th e order of 70% are ob tained u sing K 6 [SiW 11 Co ( H 2 0 ) 0 39 ].14 H 2 0 as cat a l ys t The s ilicotun gstate deriv a tives hav e b ee n found to b e m o r e efficient ca talyst s th a n the phosphotungstate analogues for th e ox idati o n of p-xylen e. Th e product obtained is mainly p-toluic a nd ter eph th a lic acids with minor quantitie s of p-tolualdehyde and p-methyl benzylalcohol. Influen ce of re ac t io n p aramete r s s uch as t e mp e rature, solvents and catalyst co nc e ntr at i o n o n th e ox id a ti o n r eac ti o n and i ts effect on product distributi on h ave been exami ned . Industrially p-xylene i s ox idized in acetic acid medium in th e pr e se nc e of a Co-Mn-Br catalyst to produce ter e phthali c ac id . Various catalytic pro ce dures for thi s oxida ti o n o f p-xy l e n e t o ter e phthalic aci d h ave bee n described in p a t e nt lit e rature as th is i s a n industrially important r eac ti on. US pat e nt No. 4892 , 970 , ass ign e d to Amoco Corporation, d e scribes a process for th e oxidation of p-xylene se le c tiv e y to ter e phthalic acid in the presence of a mixtur e of coba lt acetate, m a ngan ese acetate and HBr as catalys t s and ace ti c acid as s olvent. Zaidi ha s re p o rt e d t h e use of low concentrations of co b a lt acetate and sod ium bromide as ca t a l ys t s in the liquid ph ase ox idation of p-xylene by air a t a t e mper at ur e of 60-105 ° t o affo rd o nl y p- toluic ac id 1 • A major di sa dvanta ge of th e a b ove described pro cess is the necessity t o combat the co rro s i ve nature o f co balt/ m a n gane s e/ bromide/ acetic acid mi x tur es a nd consequent i nc ease i n cost of capital equipment. Another di sa d va nt age i s th e l oss of about I 0 p a rt s by weight o f acetic acid p er I 00 part s of aromatic ca rbox y li c acid pr o duc e d , b y conversion to carbon dioxide. Other reports · on oxidation of p-xylene include the use o f transition I I f F 3 C 2 N ·2 + C 3 . b . . . meta sa ts o e , u , 1 or r m com matton with cobalt salts as catalysts. Cobalt catalyzed xylene oxidation in aqueous medium at temperature in the range I 00-150° in the pre sen ce of phase transfer catalysts has also been reported 2 . This process yielded mainly p-toluic acid as the oxidation product. In this paper we report the liquid phase oxidation of p-xylene using transttton metal s ubstituted Keggin type heteropoly anion in a halide free system in aqueous medium to give a mixture of p-toluic and terephthalic acids. xperimental p-Xylene , I , 2-dichloroethane and 70 aqueous tert- but yl hydroperoxide were procured from Aldric h and were used as suc h for the reactions. The catalysts K 5 [PW11Co ( H 2 0 ) 0 39 ] . 16 H 2 0 , K s [PW11Mn ( H 2 39 ]. 15 H 2 0 , K 5 [PW11Cu(H 2 39 ]. 23 H 2 0 , W C o H 0 ) 0 9 ] 14 H z O K 6 [SiW11Mn(H 2 3 9 ]. 21 H20 and ~ [ S i W C u H z 0 ) 0 3 9 ] . 14 H z O , were fre s hl y prepared by methods described b e low . Pr e p a rati o n o f K 6[Si W 11 Co H 2 0 0 J 9 ] . 14 H 2 0 Hydr a t e d 1 2-s ilicotung s tic aci d ( 11 . 5g) was disso l ved in water (2 5 rnl) containing gla cia l acetic aci d (3 rnl ). The pH was adjusted to 6 with potassium bicarbonate. Th e so lution was b oi l e d and coba lt ace tat e t e trahydrat e ( 0 . 8 g in one ml water) wa s a dd e d with stirring, followed by the addition of pot assium ace tat e ( 15 g in 8 ml water a nd 0.5 rn1 g laci a l acetic ac id ) . The hot so lution wa s filtered and k ept i n a r ef ri ge r a t o r overnight. Deep red crystals se p arate d were ai r-dri ed . The yie ld of the potassium salt was abo ut I 0 g . Similarly K 6 [SiW Mn ( H 2 0 ) 0 39 ]. 21 H 2 0 an d K 6 [S iW Cu(H 2 3 9 ].14 H 2 0 wer e prepared in good y i e ld s . The phosphotungstate sa lt s were a l so obtained by the a b ove proc e du r e e xc e pt th a t the pH of the so lution was a djusted to 5 u s ing potassium bicarbonate befor e the addition of the co rr es p o nd i n g tr a nsition· metal sa lt s.
