Indi an Journal of Chemistry
Vol. 40A, August 200 1, pp. 837-840
Partial oxidation of benzene catalysed by heteropolycompounds
B Louis 1, B Viswanathan 2•t, I louranov 1 & A Renken1
'Institute of Chemical Engineering, Swiss Federal Institute of Technology, CH-10 15 Lausanne, Switzerland
2Departme nt of Chemistry, Indi an Institute of Technology Madras, Chennai 600 036, Indi a
tE-mail: [email protected]
Received 20 March 200 1; revised 30 Apri/2001
Vanadium substituted phosphomolybdic ac ids could hydroxy late benzene with nearly 100% selecti vi ty at apprec iable
co nversion levels in the presence of oxidi sin g age nts like H20 2 and CH 3COOOH. There appears to be a correlatio n, for the
li quid phase, between phenol yield and its rate of formation with va nadium content in the phosphomolybdic acid.
Unfortun ately, these materials do not show the sa me act ivity in gas-phase with N 20 as oxidant. Therefore, it has been
demonstrated th at the hi gh acidi ty of these solids is no t in volved in the reaction pathway.
Oxidation is the widely used process m the
conversion of aromatic substrates to industri al
chemicals like phenols and qui nones 1. Direct
hydroxy lation of aromatic hydrocarbons to phenol is
one of the reactions th at have received considerable
atten tion in recent times with a view to replacing the
multi-step industrial cumene process2 • Heteropoly
compounds, especially those containing molybdenum,
have been examined for the oxidation of alkyl
aromatics 3.4 . Homogeneous oxidation of alkyl
aromatics with hydrogen peroxide using vanadium
substituted heteropoly compounds has been recently
reported 5. In view of these reports, it is considered
worthwhile to examine the activity and selectivity of
these materi als for the direct hydroxylation of
benzene. In add ition, the objectives of thi s study
include:
(i) In vestigati on of the rel ative hydroxylatio n ability
of oxidants like H20 2 and CH 3COOOH in liquid
phase.
(ii) Study of the oxidation by N20 in vapour phase.
(iii) Evaluation of the role of solvent.
(iv) Elucidation of the mechani sti c pathway of the
hydroxylation reacti on.
Materials and Methods
12-Tungstophosphoric and 12-Molybdophosphoric
ac ids (Fluka) were used without further purificati on.
Cs2.sHo.5 PW 12 0 40 was prepared from aqueous solution
of parent acid H3PW 120 40 and Cs203 (Merck, 99.5
wt%) accordi ng to the procedure reported 111
literature6 .
Heteropolyvanadates
(HPV)
were
sy nthesised, in the same manner as reported by
Tsigdinos et al. 7 , by addition of aqueous solutions of
sodium mol ybdate dihydrate (Na2Mo04.2H 20, Fluka,
99.5 wt%), sodium hydrogenphosphate dihydrate
(Na 2HP04.2H20, Fluka, 99 wt%) and sodium
metavanadate (NaV0 3, Fluka, 98 wt%). The
concentrations of V, Mo and P salts are given in Table
I with the yield in each synthesis.
Under vigorous stirrin g, concentrated sulphu ric
ac id (36 wt%) was added dropwise. The solution was
cooled to 273K, kept 30 min at this temperature
before being extracted with ether. The resulting
HPY-etherate complex was dri ed at 333K overnight
in order to remove the solvent.
Surface area and pore volume were determined
from N2 adsorption isotherms measured by the BET
method (Carl o Erba Sorptomatic 1900).
Table 1- Feed compositio n for the synthesis of vanadi um contai ning phosphomolybdates
[VO, -]
(mo l.1" 1)
[HPO/"] (mol.l" 1)
[MoO/ ] (mo lT' )
H4PVMo , ,04o
0.4
0.4
H5 PV 2Mo1004o
2.0
0.4
H6 PV 3 Moq04o
1. 5
0.4
Catalyst
(a) Yie ld of the synthesi s based on mo lar content of V
V H2S04
(ml)
Yield (a)
%
2.5
17
63
2.5
85
35
1.5
30
7
838
INDIAN J CI-IEM, SEC A, AUGUST 200 1
Oxidation procedures
• Gas-phase: I Og of catalyst was activated under
nitrogen at 623K during one hour. The reaction
mixture contained 25 mol % benzene and 75
mol % N 20 . T he weight hourly pace velocity
(WHSV) based on the aromatic substrate was 17
h-1• The reaction was carried out at 573K and at
atmospheric pressure.
• Liquid-phase: The catalyst (0.6 mmol) was
di ssolved in 10 ml of acetonitrile (or a 50/50
water-acetic acid mixture) in a round-bottomed
fla sk. To thi s, the substrate benzene (II mmol)
was added fo llowed by a dropwi se addition,
during 10 min , of a 30% solution of H 20 2 or
CH 3COOOH (65 mmol) under continuous stirring
at ambient temperature. The kinetics of the
reaction was followed by monitoring the
concentration of phenol formed as a function of
time.
Products analysis
After 2 hours, the products were extracted into
dichloromethane, washed with water to re move all the
catalyst and analysed by GC off-line (Shimadzu
GC-I4A, 30m HP-5, FID). In case of gas-phase
reaction , quantities of aromatics were determined
on-line with the same apparatus. An IR detector
(S iemens Ultramat-22) followed C0 2 formation . The
products were confirmed both by comparison of
retention times of the authentic samples and also by
GC-MS (HP-G 1800A, 30m HP-5).
Results and Discussion
Characterisation
The values of BET surface area of these materials
were between 4 and 6 m 2/g, which are in agreement
However,
with
previous
investi gations8 .
2
Cs2.sHo.s PW 120 4o exhibited I28 m /g and a total pore
Consequently, the porous
volume of O.II2 cm 3/g.
Cs salt containing highly di spersed ac id sites is
obtained 9 .
Figure I shows the XRD patterns for the different
HPV sample. Despite an increase in diffraction s due
to V species, the relative intensity is hardly decreased
from the VI to the V3-compound. In fact, the stability
o f the structure is limited to three V atoms per Keggin
umt 8 .
ac id, its cesium salt and phosphovanadomolybdic
acid. The conversion levels are in all cases less th an
2% . Secondly, the only major product obtained is
carbon dioxide, due to comp lete combustion. Even the
porous Cs salt, which has a hi gher specific area and a
more access ible number of acid sites on the surface, is
not able to produce phenol. It is, therefore, deduced
that heteropoly compounds are not capable of
generating the active species, which leads to phenol
of
Bro nsted
ac idity,
formation.
In
terms
heteropolyacids are more acidic th an other common
so lid acids (zeolites, amorphou s sili ca-alumin a, yAI203). Unfortunately, these catalysts were not useful
for hydroxylation of aromati cs in gas--phase and the
surface-type chemi stry does not occur 8 .
Liquid-phase reaction
The reaction was also carried out in liquid ph ase at
298 K for 2 hr, on various heteropolycompounds and
H20 2 and the results obtained are given in Table 2.
The following points emerge from the data g iven in
Table 2:
(i) Only ph osphomolybdic systems are active for the
hydrox ylation o f benzene.
(ii) Substitution o f Mo by V resu lts in an increase in
catalytic acti vity.
(iii ) Between H20 2 and CH 3COOOH, th e latter
appears to be a better hydroxy lating agent. In fact,
peracetic ac id is able to stabili ze V spec ies to a
greater ex tent than H 20 2 (ref. I 0). Moreover,
H20 2 deco mposes faster than CH 3COOOH.
Nevertheless, the se lectivity towards phenol is
decreased (99 to 92 %). This cou ld be explained
by an acidic attack from the peraceti c ac id to the
aromatic ring leading to C0 2 formation.
(iv) Acetonitrile is a better so lvent compared to other
polar ones like acetone or acetic ac id . V
complexes are known to deco mpose in aproti c
so lvents rather than in protic ones 6·' 0 . So they
0
Gas-phase reaction
The vapour phase hydroxyl ati on of benzene by
nitrous oxide was studi ed on acid ic phosphotungstic
5
10
15
20
25
30
35
40
45
50
2 theta
Fig. !- Diffractio n pattern s for the different hcleropoly vanadatcs
LOUIS eta/.: OXIDATION OF BENZENE CATA LYSED BY HETEROPOLYCOMPOUNDS
839
Table 2-0x idati on of benzene on heteropoly compunds
)Experimental conditions: TR=298 K, IR=2 h. 10 ml solvent, I ml benzene, 2 ml ox idant (30%)]
Catalyst
Ox idant
Solven t
Phenol Yield
Selecti vi ty
(%)
(%)
0.1
*
H.1 PW1104o
H10 1
CH1CN
Cs1.oHo5 PW u04o
H10 1
CH.1 CN
0.9
99
H.1 PMo1 104o
H10 2
CH-'CN
5.9
99
99
H4PV 1Mo1104o
H5 PV 1Mo 1004o
H10 2
CH.1 CN
13.4
H20 2
99
H20 2
CH1CN
CH3CN
20.5
H6 PY.1 Mo904o
35.5
99
H, PV 2Mo 100 4o
H20 1
CH1COOOH
CH1COOH- H20
5.8
99
CH 3CN
26.7
92
H5 PV 1Mo 100 4o
*not measurable
could more eas ily lead to the desired intermedi ate
in aprotic media. The inhibitory effect of the
aceti c acid-water mixture on th e yield of reaction
could be clearly seen.
(v) On hi ghl y acidic Cs salt of phosphotungstic acid,
the observed conversions were low, showing that
Bronsted acidity alone is not enough for the
promotion of thi s reaction.
(v i) Since the selecti vities achi eved are hi gh, it ts
possible to develop thi s into a viable process.
The variati on of the yield of phenol as a function of
vanadium co ntent in th e phosphomolydic ac id is
shown in Fig.2. It is seen that there is a correl ation
between the phenol yield and the vanad ium content in
the catalyst. Furthermore, th e kin etics of the reaction
was also studi ed; the convers ion of benzene is given
as a functi on of time (Fig. 3).
It is seen th at th e act ivity is enhanced with the
number of V atoms in these materi als.
The be neficial effect of th e concentration of V on
the rate of reacti on can be clearly seen.
In fac t, th e rate of benzene co nsumption
(determi ned by th e slope of the curve) is enh anced
with an increase in the total amount of Y: from 3
mmol/1 of benzene co nverted per minute for th e VI
compou nd to 6.60 for th e Y3 com pound.
Unfortunately, deco mpos iti on of hydroge n peroxide is
a competiti ve reacti on that limits the obtainable
yield 11 . In fact, after 40 min , the quantity of phenol
produced is nearly co nstant (Fig. 3).
Nevertheless, th e correlation between th e total
amoun t of V in the catalyst and the rate of reacti on
has been clearly demonstrated. Therefore, it can be
concluded th at V atoms are the ac ti ve sites of th e
catalyst, although H3 Mo 12 P0 40 also exhibi ts a sli ght
catalytic act ivity. Nomiya et a /. 11. 12 demonstrated a
40
_ 35 1
~
g
30 l
25 .
i
I
20
J
.!: 15 j
'0
o; 10 I
;,;: 5 1
0
•I
0
1
I
Number of V atoms
Fig. 2-Dependence of the phenol yield on the number of V
atoms
--; 3 5
"c0
·~
~ 20
0
u
.,
• t. t.
t.
15
"'
t.
•• • •
~ 10
~
• • •
•
30
25
5
t.
• •
0
0
20
40
60
80
Tim< ( mi n)
100
12 0 14 0
Fig. 3-Evolution of benzene con version with time
cooperative effect between V and Mo in th e
framework.
These systems may promote this reacti on by
generation of peroxo species. A possible reaction
mechani sm, similar to the one proposed by Mimoun
10
et a /. for V-peroxo complexes, is proposed in th e
scheme.
Ox ygen transfer from the peroxo species to
hydrocarbon may occur in a bimolecul ar fashion and
the hydroxy lation of benzene takes pl ace by the
homolytic addition of th e electrophi li c rad ical speci es
to the aromatic ring.
840
INDI AN 1 CH EM, SEC A, AUGUST 200 1
I
M---0 '--...
•H202
.-/o- o -
H
-
M--0 '--...
-------0- 0
v
M-0 /
M---0 '--...
/
-------0-0
V
>CJ
~~
H
0
M-0
In o rde r to confirm the validity of thi s mechani sm,
the isoto pe effect kH/k 0 was dete rmined fro m
co mpetiti ve hyd roxy lati o n o f be nzene -d 6 (99.5%) by
H c, P V~ M o904o in aceto nitril e. Th e rati o between the
yields of C 6 H50H and C 6 D 50D was fo und to be equ al
to 1±0. 1, indi cating that the C -H bo nd c leav age is no t
the rate-dete rmini ng step o f the reacti o n. Furth ermo re,
the o nly product detected by GC-MS was C 6 D 50D.
T hi s impli es th at an ac id-cata lysed mechani sm via the
[+OH20H] intermedi ate, as re po rted by Ol ah et a /. 13
fo r liquid superacids, can be excluded o ver these
cata lysts. Indeed, C 6 D 50H could no t be seen
indi catin g th at H 20 2 was no t acting as a n e lectrophile
(whi ch g ives OW to the substrate) but reall y as an
14
ox idant (0 do no r). Altho ug h Mi sono et a/. c laimed
th at so me heteropo lyac id s were so li d supe rac id s, thi s
work demo nstrates th at in th is case those ac idi c
fun cti o ns are no t used .
T hi s ex pe riment also s uppo rts the mechani sm
in volvin g a l VOO]* peroxo species gene rated by H 20 2
deco mpos iti o n. Vanad ium peroxo compl exes, whi ch
cl eave ho mo lytica ll y to gene rate reacti ve e lectro phili c
s pec ies, behave di ffe rentl y fro m known simil ar
complexes o f mo ly bdenum th at a re kno wn to be
effec ti ve reagents fo r e poxidati o n o f o le fin s. This
could be the reaso n fo r th e lower effic ie ncy of the
15
H3PM o 120 4ofH20 2 syste m fo r thi s reacti o n . This
d iffe re nce accounts for th e hi g her acti vity o bserved
wi th vanad ium substituted syste ms. Huy brechts et
16
a/. have also pro posed a simil ar mechani sm o ver
T S - 1.
Conclusion
One of the striking features of V substi tuted
heteropo ly co mpo unds, fo r the ox ida ti o n o f aromatics,
is the ability to tra nsfer (0 ) to the substrate at room
te mpe rature.
The acti vity and se lecti vity that can be achi eved by
hydrogen perox ide in liquid-ph ase appears to be
favo urabl e. It can also be conc luded that HPVs a re
acting
in
a
bulk-ty pe
manner,
whi ch
is
co mprehe nsible due to the ir low surface area.
Ho wever, since the reacti o n has to be carri ed o ut in
ho mogeneous ph ase, suitabl e process steps have to be
evolved to offset the loss of catalyst, separati o n
demands a nd to reduce H 20 2 deco mpositi o n.
Acknowledgement
Fin anci al s uppo rt fro m the S wiss Nati o nal Found of
Research is g rate full y ack nowledged . The authors
th ank M . P. M oeckl i and E . Casa li fo r techni ca l
ana lysis.
References
I Sheldo n R A, Chem Tech, ( 199 1) 576.
2 Motz J L, Heini chen H & Holderich W F. J molec Cared. A
136 (1998) 175.
3 Attanasio D, Suber L & Thorslund K, lnorg Chem. 30 (1 99 1)
590.
4 Neumann R & de Ia Vega M, J molec Catal, A 84 (1993) 93.
5 Athil akshmi & Viswanathan B, Indian .I Chem. 378 ( I 998)
439.
6 Tatematsu S, Hibi T, Okuh ara T & Misono M. Chem Le11.
( 1984) 865.
7 Tsigdinos G A & Hallada C 1, In org Chem. I ( 1968) 437.
8 Lee K Y & Mi sono M, Handbook of hererogeneous
catalysis, Vol I ( 1997) 11 8.
9 Moffat J B, McMonagle J B & Tay lor D, Solid Srare lonics.
26 (1988) 10 1.
10 Mimoun , H Saussine L. Dai re E, Postel M. Fischer J &
Weiss R, .I Am chem Soc, I05 ( 1983) 3 10.
II Nomi ya K, Nemoto Y. Hasegawa T & Matsuoka S. .I molec
Catal, A 152 (2000) 55.
12 Nomi ya K, Yagishita K, Nemoto Y & Ka mataki T, J molec
Catal, A 126 ( 1997) 43.
13 Olah G A & Ohni shi R, .I org Chem, 43 ( 1978) 865.
14 Mi sono M & Okahura T. Chem Tech. 23 ( 1993) 23.
15 Kozhevnikov I V, Catal Rev Sci Eng, ) 7 (1 995) 3 11.
16 Huybrechts D R C, Buske ns P L & Jacobs P A . J molec
Catal, A 7 1 ( 1992) 129.