[()TD$FIG]
1
36
Y. Xiao et al. / Chinese Chemical Letters 22 (2011) 135–138
CH3
COOH
CH3
T(p-Cl)PPMnCl / Co(OAc)2
O
+
+
Other products
+
2
4
63 K, 1.2 MPa
CH3
PX
COOH
TPA
COOH
PTA
2
Scheme 1. Solvent-free oxidation of PX to TPA with air over T( p-Cl)PPMnCl/Co(OAc) .
cooled to room temperature. The products were quantified with an Agilent HPLC with UV (254 nm) detector and a
Zorbax SB-C18 (4.5 Â 250 mm) column. Eluent consisting of 25% CH OH, 20% CH CN and 55% H PO buffer
3
3
3
4
À1
solvent (1%) with a flow rate of 1.0 mL min was used. T( p-Cl)PPMnCl was synthesized and purified according to
documented procedures [20].
The products of solvent-free oxidation of PX over T( p-Cl)PPMnCl/Co(OAc) under the optimum conditions of
2
1
90 8C and 1.2 MPa, identified by LC–MS and also by HPLC co-injection of commercially available authentic
samples, consisted mainly of terephthalic acid (TPA), p-toluic acid (PTA), and a small amount of other products
including 4-carboxybenzalaldehyde (CBA), p-toluic aldehyde (TALD) and p-toluic alcohol (TALC), as shown in
Scheme 1.
Results of solvent-free oxidation of PX using different catalyst systems were summarized in Table 1. Indeed, PX
can be oxidized by molecular oxygen under the catalysis of Co(OAc) or T( p-Cl)PPMnCl, but the TPA yield is not
2
satisfying. To our delight, both the yield and selectivity of TPA could be increased significantly when T( p-Cl)PPMnCl
and Co(OAc) were used together. Moreover, TPAyield did not change much as Mn(OAc) was added into the reaction
2
2
mixture over Co(OAc) . Above results suggest that there is synergy between T( p-Cl)PPMnCl and Co(OAc) , and co-
2
2
catalysis may be operative under solvent-free conditions. T( p-Cl)PPMnCl/Co(OAc) can effectively catalyze the
2
oxidation of PX with air in absence of any solvent, and the reaction of PX oxidation catalyzed by T( p-Cl)PPMnCl
(
5.6 ppm) and Co(OAc) (535 ppm) at 190 8C gave 74% of PX conversion and 38.9% of TPA selectivity in 5 h.
2
Compared with other solvent-free methods, our catalytic process has the advantages of giving higher TPA yield,
operating under milder reaction conditions, being more environmental friendly and it do not require the use of any
additive or initiator.
Chemically, the oxidation of PX to TPA involves the oxidation of PX to PTA and the subsequent oxidation of PTA to
TPA. Thestepof PTA oxidation isthe rate-determiningstepof the oxidationprocess ofPXtoTPA [3,21]. Fig. 1showsthe
time evolution of yields for PTA and TPA in different catalytic reactions. T( p-Cl)PPMnCl/Co(OAc) catalyst system
2
gavehigher TPAyieldallthe time, moreover, the TPA slopeofthe tangentline for T( p-Cl)PPMnCl/Co(OAc) was higher
2
than that for Co(OAc) . So it is reasonable to conclude that the addition of T( p-Cl)PPMnCl into the reaction mixture over
2
Co(OAc) accelerated the formation rate of TPA derived from PTA. Moreover, two different catalyst systems have the
2
same PTAyield at 240 min, thereafter, PTAyield for T( p-Cl)PPMnCl/Co(OAc) was lower than that for Co(OAc) . So
2
2
one might conclude that PTA consumption rate was also accelerated by the addition of T( p-Cl)PPMnCl. The PTA
formation rate was equal to the PTA consumption rate when the PTAyield reached the maximum, and thereafter the PTA
consumption rate was faster than the PTA formation rate. The reaction time until PTAyield reached the maximum was
postponed for 90 min when the catalyst was changed from T( p-Cl)PPMnCl/Co(OAc) to Co(OAc) . From all analysis,
2
2
we can see that the addition of T( p-Cl)PPMnCl into the reaction mixture over Co(OAc) accelerated the whole reaction
2
especially for rate-determining step of the oxidation process of PX to TPA.
Table 1
a
Investigation of the catalyst system composition on solvent-free oxidation of PX.
Catalyst (ppm)
Co(OAc)
Conversion (%)
TPA yield (%)
Selectivity (%)
2
T( p-Cl)PPMnCl
Mn(OAc)
2
TPA
CBA
PTA
TALD
TALC
5
35
69.7
18.8
74.0
70.0
19.8
0.3
28.4
1.7
5.2
2.0
5.7
5.3
66.4
73.2
55.4
65.7
0
15.1
0
0
5
.6
8.0
0
5
5
35
35
5.6
28.8
20.3
38.9
29.0
5.6
0
0
a
Reaction conditions: PX, 3 mol; temperature, 190 8C; reaction time, 5 h.