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VOI. 40 NO. 11
Perovskite-like mixed oxides La2- ,(Sr, Th) ,Cu04 * 1 -A LIU Chibiao
novel catalyst for phenol hydroxylation
(B S ), YANG Xiangguang ([email protected]
%), (Dt%3A) and WU Yue (% B ) "
(Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China) Received May 30, 1997
Two groups of mixed oxides La2- ,Th,CuO,
0 . 5 , a wide band at 670 cm- appears with a distinct shoulder around 675 cm-' which may be due to the appearance of T * phase in 50 La2-,SrxCu04+A. Band at 410 cm-' can be as40 signed to the bending vibration of La-0-Cu in La2-,.Sr,Cu04+A.
When some A atoms ( ~ a ) ~are' replaced ' , IR vibration peak around 590 with ~ h ~ new cm-' appears, and the band near 690 cm-' is split into two peaks (720 and 685 c m - I ) . However, T h o 2 has no signals in these areas, which shows that the appearance of these vibration bands might be caused by the incorporation of ~ h in~ framework.
' -201 200
2 . 1 . 3 Average valence of Cu ions and nonstoiAccording to the chemical chiometric oxygen. analyses, the structure, average valence of copper and the nonstoichiometric oxygen ( A ) of La2- ,-
Wave nurnber/cm-' Fig. 4.
IR spectra of La2- ,Th,Cu04
PEROVSKITE-LIKE MIXED OXIDES La2- ,( Sr. T h ) ,Cu04 *
are shown in table 1. Table 1 Composition, structure, average valence of Cu ions and nonstoichiometric oxygen ( A )
Composition of catalyst LaSlCuO, La1 1sr0.9cuo4 La1 3 Sro.7 Cu04 L~I.sS~O.~CUO~ La1 1S ~ O 3 cuo4 . La1 9 Sro. I Cu04 La2Cu04 La, 9 T b . 1CuO, La, 8 T b . 2 C ~ 0 4 La1 7 T b .2Cu04 ,Tb.4CuO4
Average valence of Cu ions
K2NiF,(T+ T " )"' K2NiF4(T+T ' ) K 2 N i F 4 ( T + T ") K2NiF4(T+T * ) K2NiF, ( T ) K2NiF4( T ) K,N~F~(o)~' K2NiF4( 0 ) K2NiF4( 0 ) K2NiF4(0) K2NiF4( 0 )
2.24 2.24 2.25 2.24 2.17 2.14 2.02 1.98 1.95 1.91 1.89
Nonstoichiometric oxygen( A )
-0.38 -0.33 -0.22 -0.13 -0.07 0.01 0.02 0.05 0.07 0.11 0.14
a) T , tetragonal K2NiF4 phase; T ' , another tetragonal K2NiF4phase whose coordinating number of oxygen for B ion is 5; b) 0 , orthorhombic K2NiF4 phase. c) tr, true.
2 . 2 Phenol hydroxylation 2 . 2 . 1 Catalysis of La2-, (Sr, Th),Cu04 A in phenol hydroxylation. Compared with simple metal oxides and their mechanical mixtures, results of phenol hydroxylation catalyzed by La2- , (Sr, T h ) ,CuO4 + A are shown in table 2. Table 2 Catalyst LaSKuO, La1 S ~ 9OCu0.1 Lat . 3 S ~ OCuO, . L ~ Is S. ~ O s CuQ . 1sro 3 CUO, La,. 9 S ~ OCuO4 La2CuO, 9 T b 1CuO4 La1 , , T b . ~ C u 0 4 La,. 1T b cue, La.6Tb.4CuO4 LazQ ThQ
sro CuO La2Q-CuOSrO La2Q-CuOThQ
Activity of different catalysts in the phenol hydroxylation
Observed phenol conversion( % )
2.2 10.5 15.6 20.2 30.4 40.8 50.9 30.4 25.6 23.4 20.7 0.0 0.0 0.0 16.5 12.5 11.2
Observed product distribution( % ) HQ BQ
63.6 61.0 60.1 59.5 58.0 57.5 57.5 57.6 58.0 57.4 57.3
36.4 39.0 39.8 40.0 40.6 40.7 40.0 40.7 40.4 41.0 41.0
0 0.1 0.5 1.4 1.8 2.5 1.7 1.6 1.6 1.7
60.4 61.0 60.5
29.8 30.2 30.1
9.8 8.8 9.4
Volume of 9 evolved/mL
100 85 65 50 25 19 12 9 7 5 3 0 0 0 75 78 76
CAT, catechol; HQ, hydroquinone; BQ, bemquinone; reaction time, 2 h; temperature, 70C ; medium water, p H = 7 . 0 ; phenol/H2Q/~20(molar ratio), 1 : 1 :60; phenol used, 1 g; catalyst used, 0 . l g .
Table 2 shows that simple metal oxides La203SrO, T h o 2 have no catalytic activity for phenol hydroxylation and H202decomposition, CuO and the prepared mixed oxides have some activity for these reactions, which means that c u 2 + ions are the active centers for the discussed reactions. Mixed oxides formed by simple oxides have specific structure and physicochemical properties, which make them have better activity in phenol hydroxylation. For La2NiF4(A2B04)perovskite-
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like mixed oxides, when A is partially replaced with s?+and ~ h ~ the + structure, , average valence of copper and the nonstoichiometric oxygen ( A ) of La2- ,( Sr, T h ) ,Cu04 + A change with x , leading to change in catalysis of the obtained mixed oxides in phenol hydroxylation (table 2 ) . In the 2 . 2 . 2 Mechanism of phenol hydroxylation catalyzed by perovskite-like mixed oxides. above-discussed phenol hydroxylation catalyzed by the mixed oxides, H202 can get one electron from cU2,+ [ 13,141 ions to produce .OH and OH- . OH can further react with phenol to produce diphenols. According to the above discussion and our previous studies[15g161, a proposed reaction mechanism could be suggested as follows:
+ cu2+ + H + (3a)
This mechanism shows that steps 1 , 2 and 3 are very important for phenol hydroxylation. There are two reasons for lower phenol conversion in the pheno1 hydroxylation catalyzed by La2 - =Th,Cu04 +_ . On the one hand, positively nonstoichiometric oxygen ( table 1 ) in these mixed oxides does not favor adsorption of H202 on catalyst surface; in this way reaction 1 proceeds slowly to produce OH; on the other hand, the lower average valence ( < 2 . 0 ) leads to weaker oxidisability of the obtained mixed oxides. The weak oxidisability is
favorable for reaction 1, but not for steps 3 and 4 ; so their catalysis in both the diphenol production and [email protected]
decomposition is lower.
When too much oxygen defiencies H ~ Q +.OH , HO; + H,O (4a) appear in mixed oxides ( Laz- ,Sr,Cu04 + , s > 0 . 5 ) , H202 molecule cu3' + H Q c u 2 + + ~ ~ + " (4b) is absorbed on the oxygen deficiencies ( existing quantitatively ) drastically. 0 +2H20 ( 5 ) The .OH produced on CU" centers HO OH OH= 0 meet and react with adjacent H202 molecules to produce HO; easily ( step (6) 4 ) , which will further react with cu3+ (existing quantitatively) to produce 02 a great deal. In this way, there is not enough *OHleft for the diphenol production.
For La2- ,Sr,Cu04 +_ A , when x = 0 . 0 , 0 . 1 and 0 . 3 , there are a few oxygen defiencies in these mixed oxides, which do not adsorb H202too much. The adsorbed H202around CU" is re-
PEROVSKITE-LIKE MIXED OXIDES La2 - ,(Sr, Th) , C U O *~1
duced to produce .OH (reaction 1). .OH further reacts with phenol to produce diphenols following steps 2 and 3 .
The above investigation shows that some mixed oxides with specific structures have better catalytic activity in phenol hydroxylation than that of TS-1. Furthermore, easy preparation and high stability of these mixed oxides will accelerate their industrial application in diphenol production. Meanwhile, the close relation between structure, physicochemical properties and catalytic activity will give some enlightenment and provide a scientific basis for catalyst design.
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