Indian Journal of Chemical Technology
Vol. II, July 2004, pp 569-574
Oxidative coupling of methane over La-promoted MgO catalysts: Influence of
precursors and method of catalyst preparation
V R Choudhary"*, V H Rane h & S T Chaudhari"
;'Chemical Engineering Divi sion , "Homogeneou s Catalysis Division,
Nati o nal Chemical Laboratory, Pune 411 008 , Indi a
Received 21 August 2003; revised received 17 February 2004; accepted 30 April 2004
The inlluence of prec urso rs fo r La10) (viz. lanthanum acetate and lanthanum nitrate) and MgO (v iz. magnes ium acetate,
magnesi um carbonate and magnesium hydrox ide) and catalyst preparation methods (viz. physical mixing orthe catal yst precursors
and coprecip itati o n using different precipitating agents such as ammonium hydroxide, amm oni um carbonate and sodium carbonate)
o f La-promoted MgO (La/Mg = 0. 1) catalyst o n its surface area, total a nd strong basicity (measured in te rms of COl c he mi sorbed
at 50 and 500"C, respect ive ly) and catalyt ic perfo rman ce in oxidative cou pling o f methane (OCM) to C ,-hydrocarbons at differe nt
temperat ures (750-850"C) , C H/ 0 1 rati os (4.0 and 8.0) for a hi g h space ve locity (5 1,360 e m).g-'.IY') h as been investigated. Th e
catalys t prepared using the cop recipitation method ha ve hi g her surface area and basicity (total and strong basic sites measured in
te rms of COl c he mi so rbed at 50 and SOO"C, res pectively) but lower activity/selectivity, whereas, the catalyst prepared using th e
physica l mi xing method have lower su rface area and basicity (both total a nd strong basic sites) but show hi gher act ivity and
se lecti vit y in the OC M process, depending upo n the La- and Mg- salts used as th e catal ys t precursors . The catalyst, prepared by
phys ica l mi xi ng usin g lanth anum nitrate and magnes ium carbonate (prepared by precipitation from magnes ium nitrate and
sodi um carbonate) as the catal ys t precurso rs s howed best catalytic performance in the OCM at different process cond itions.
IPC Code: BO Il 21/00
Keywords: Oxidati ve coupling, meth ane, La-promoted MgO catalys ts, catalyti c selectivity
Oxidative couplin g of methane (OCM) to ethan e and
eth ylene is a process of great practical importance. In
the past one and half decade, worldwide efforts I 12 have
been made for co nv ertin g methane by its ox idative
couplin g to C 2 -hydrocarbons, using various catalysts
co ntai nin g alkali and alkaline earth oxides , rare ea rth
ox ides and even transiti on meta l ox ides.
In prev ious studi es", La-promoted MgO (La/Mg
= 0.1) showed hi gh catal yti c activity and C 2-selectivity
and C2-productivity and also long catal yst life in th e
OCM reaction at a low co ntact time (G HSV = 105,600
cmo.g I.h-I). Because of the addition of La to MgO, its
basi c ity (bo th th e total and strong bas ic sites) and
activity/se lecti vity in th e OCM process is in c reased l ~.
Since thi s catal ys t shows hi gh promise for the OCM
process, it is interesting to improve it further by studyin g
the influence 01" precurso r for La 20 , (v iz. . lanthanum
acetat e and la nthanum nitrate ) and MgO (v i z. .
ma gnes iul11 ac e tat e, magnesium ca rb o nat e and
" For correspo nd cncc (E-mai l: vrc @ems.ncl.res.in ;
Fax: +91 -20-58(3041)
magnesium hydroxide) used in the catalyst preparation
and al so of the method of catal yst preparation on th e
catalyst perforl11ance in the OCM process . This stud y
was undertak en for thi s purpose.
Experimental Procedure
The La-promoted MgO catal ysts with La/Mg mole
ratio of 0. 1 were prepared from phys ically mix ed catal yst
precursors and also from coprecipitated lanthanum and
magnesium carbonates or hydrox ides, usin g different
catalyst precurso rs and prec ipitating agents (Tables I
and 2).1n case of th e physica ll y mixed precursor met hod ,
the precursor for L~ Oo and MgO were th oroughl y mi xed
with grinding in presence of water just su ffic ient to form
a thick paste, which was dri ed at 120"C for 12 h. In case
of the co prec ipitation me thod s, lanth anum and
ma g nes ium carbonates o r hydro x id es we re
coprecipitated us in g different precipitating agen ts
(ammonium carbonate, ammonium hydroxide or sod iulll
carbonate) at pH of 8-9 frolll an aqueous so lu ti on
containing lanthanum and magnes lul11 sa lts. Th e
570
INDIAN J. CHEM. TECHNOL. , JULY 2004
coprecipitated lanthanum and magnesium compounds
were filtered, washed thoroughly with deionized water
and dried at 120"C for 12 h. The dried precursor catalyst
mass pressed binder-free, crushed to 22-30 mesh size
particles and then calcined at 950"C in static air for 10
h.
The surface area of the catalysts was determined
by the singly point BET method by measuring the
adsorption of nitrogen (30 mol % balance helium) at
liquid nitrogen temperature using a Monosorb Surface
Area Analyzer (Quantachrome Corp ., USA). The
surface basicitylbase strength distribution was measured
by the step-wi se thermal desorption (STO) of CO 2
(c hemi so rbed at 50"C) from 50 to 980"C in two different
te mperature ste ps (50-500"C and 500-9 80"C) using
helium as a carrier gas (3 0 cm' .min-' ) and measuring
th e des orbed CO 2 quantitative ly. The detailed procedure
for measuring basicity di stribution is given elsewhere L1 •
In the present case, the chemisorption is defined as the
amount of adsorbate retained on the presaturated catalyst
when it was swept with pure nitrogen (20 cm'. mil'-')
for a period of 30 min at the chemisorption temperature.
The OCM reac tion over La-promoted MgO
catal ysts was carried out in a continuous flow tubular
qu artz reac tor packed with the catalyst partic les (22-30
mes h size) between quartz wool plugs. The reacti on
te mp e rature was measured by a Chrome l-Al ume l
thermocouple located in th e catalys t bed. The feed
co nsists of pure methane and oxygen. The OCM reaction
was ca rri ed out at the following reaction conditions:
amount of cataly st, 0 .2 g; temperature, 600-850"C ; CHi
0 , ratio in feed, 4.0 and 8.0; and gas hourly space
velocity (GHSY) at STP, 51 ,360 cm' .g-I .I,- I. The feed
a nd produ c t gases, afte r the re mo val of wate r by
co ndensation at O"C, were analyzed by an on-lin e gas
c hromat og raph with th ermal co nducti v it y de tec to r
(TCO ) and flam e ioni za tion de tec tor (FlO) uSJll g
Poropak-Q and Spherocarb columns.
Results and Discussion
Catalyst characterization
A co mpari so n of th e surface a rea o f th e Lapromoted MgO (La/Mg = 0.1 ) catalysts, prepared using
the different catalyst precurso rs and methods, is given
in Table I. The results show a strong influence of both
th e catal ys t precursor and method of cataly st preparation
on the sinterin g properties of th e catalysts. The surface
area of th e catalysts prepared by the coprecipitation
method is found to be much higher.
The catalyst prepared usin g the different
precursors for the La,Ol and MgO and/or methods of
catalyst preparation (physical mlxll1g and
coprecipitation) are compared for their total and strong
basicity (measured in terms of CO 2 chemisorbed at 50
and 500"C, respectively) in Table I. A co mparison of
the results reveal s that both the total anel strong basicity
of the catalysts are strongly influenced by the precursors
(for MgO and La?Ol) and the meth ods (viz . phys ical
mixing of precursors and coprecipitation) used in the
catalyst preparation. The catalyst prepared usin g the
lanthanum nitrate and magnes ium ni trate as catalyst
precursors and ammonium hydroxide as a precipitating
agent showed the hi ghest basicity (both the total and
strong). In general, the basicity for the eatal ysts obtained
by the coprecipitation method is fOllnd to be hi gher,
probably because of the ir hi gher s urfac,~ area.
Comparison of La-promoted MgO catalysts for their activity/
selectivity in OCM
The results showing the influence of catalyst
precurso rs and catalyst preparati on method on th e
catalytic activity/selectivity (at 800 'C. CH/ 0 2 rati o =
4.0 and GHSY = 51 ,360 c m3.g-' .IY' ) of th e La-promoted
MgO (LalMg = 0.1) catal ysts in OCM are in cluded in
Table I . The res ults show that the catalytic activity/
se lectivity is stron g ly de pe nd e nt on th e ca tal ys t
precursors and also on the catal yst preparation meth od.
The catalysts prepared usin g the meth od of ph ys ica l
mixin g showed better pe rforman ce (i.e. C,-y ie ld or
selectivity). The hi ghest C,-yield ( 18.0 with 64 .2% C, selectivity) is obtained for-the ca tal ys t prepared by th-e
method of physical mi xin g of lantha num nitrate and
magn es ium carbonate (I) (p repa red by th e prec ipitati on
of magnesium ca rbonate from an aqu eous so luti on of
magnes ium nitrate by sodium ca rbona te), whereas. the
lowest catal ytic performance ( 10.9 % C"-yi e ld with 42.6
% C"-selecti vity) in th e OCM is shovvn by the catalyst
pre pared by th e coprecipitation me th od , Ll s in g
lanthanum nitrate and magnes ium ni trate as th e catalyst
precurso rs and sodium carbonate as a prec ipitating agent.
The influence of catalyst prec ursor and meth od of
catalyst preparation on the ethyl ene/et:'1ane product rati o
is also significant (Table I).
The three catalysts , prepared by the ph ys ica l
mixin g of lanthanum nitrate and magnes ium nitrate,
lanthanum ace tate and ma gnes ium hydrox ide an d
lanthanum nitrate and magnes ium ca rb onate (1) are
furth er compared for their performan ce in OCM process
l
Table I -
Influence of the catalyst precursors (for Lap, and MgO) and method used in the preparation of Lap)-M gO (La/Mg
(calcined at 950'C ) on its activity/selectivity in th e OCM process
(temperature = 800"C , CH/0 1 ratio =4.0 and GHSV = 51 ,360 cm' .g·l.h· 1)
= O. I) catalyst
(")
:I:
0
C
"»:I:
Precursor
For La1O ,
For MgO
::0
-<
Method of catalyst
preparation
Surface area
(m 2.g. 1)
C 2-yield (%)
C 2selectivity
C 2H/C 2H6
ratio
~
s:,
:-
COl
chemisorbed
(mmol.g· 1)
at 50'C
at 500°C
CH.
conversion
(%)
(%)
54.2
15.8
1.0
<
tTl
0
><
"~
La(NO),
Mg(NO))1
Coprecipitation (by NHPH)
32.2
0.540
0.200
29 .2
La(NO))
Mg(NO,)l
Coprecipitation (by Na 2CO))
30.0
0.438
0.097
25 .7
42.6
10.9
I.8
0
La(NO,),
Mg(NO)1
Coprecipitation (NH)lCO,
30.8
0.327
0.080
26 .7
52.0
13.9
1.1
C
(CH,COO),La
(CH,COO)lMg
Coprecipitation (by NHPH)
56 .0
0.330
0.096
26.6
43 .2
11.5
1.3
0
0
(CH,COO)jLa
(CH)COO)lMg
Coprecipitation (by Na1CO))
40.0
0.381
0. 124
26.6
59 . 1
15 .7
0.9
~
La(NOj)j
Mg(NO))l
Physical mixing
2.9
0.098
0.042
29.6
58 .3
17 .2
1.2
:I:
La(NO)))
Mg(OH) 2
Physical mixing
20.9
0.187
0.040
28.6
58 . 1
16.6
1.1
(")
C
"t!
z
"I1
£:.l
»z
tTl
0
(C H)COO))La
Mg(OH\
Physical mixing
14.5
0.173
0.040
29.5
58.3
17.2
1.1
<
tTl
La(NO)))
MgCO/I)
Ph ysical mixing
10.6
0.213
0.052
28 .0
64.2
18.0
1.0
r
La(NO)))
MgCO/")
Physical mixing
15.6
0.171
0.058
26.4
64.8
17. 1
1.1
::0
'"
.;,
::0
0
~
(CH)COO))La
MgCO/1)
Ph ysical mixing
23. 1
0.369
0.072
26.0
53.9
14.0
1.4
0
(CH)COO\La
MgCO)(II)
Physical mixing
28.3
0.209
0.058
27 .2
58.6
15 .9
1.2
tTl
-l
"s:
(Jq
0
MgCO/1) Prepared by precipitation of magnesium nitrate by sodium carbonate
MgCO)(II) Prepared by precipitation of magnesium nitrate by ammonium carbonate
(")
»~
~
CI:l
-l
CI:l
Ul
--.)
572
INDIAN J. CHEM . TECHNOL., JULY 2004
Table 2 -
Intluence of the catalyst precursors (for Lap3 and MgO) of LapfMgO (La/Mg =0. 1) catalyst (calcined at 950'0
on its activity/selectivi ty in the OCM process (G HSY =51 ,3 60 cm).g·l.lr l )
Precursor
For MgO
For Lap)
a)
Temp. = 850"C, CHiOl
Method of catalyst
CH 4
preparation
conversion
(%)
Cl-selectivity
(%)
C ~ - yidd
(%)
C l H/C 2 H"
ratio
=4.0
La(N O) )
Mg( NO)l
Physical mixing
29.2
56.5
16.5
1.1
(C HF OO»)La
Mg(OH)l
Physical mixing
28 .5
55.1
15 .7
1.4
La(N O))
MgCO)(I )
Physical mixin g
28 .5
64.9
18.5
1.1
La(N O)l)
Mg( NO) l
Physical mixing
19.9
74. 8
14.9
0.8
(C HFOO\ La
Mg(OH \
Physical mixing
21.0
68.3
14 .3
0.9
La(NO)l)
MgCO)(I)
Physical mixing
20.5
75.1
15.4
0.9
La(NO))
Mg( NO) l
Physical mixing
18.9
73.9
14.0
0.7
(CH)COO\La
Mg(OH)l
Physical mixin g
20.2
65.6
13.2
0.8
La(NO)
MgCO/1)
Physical mixin g
20. 1
74.2
14.9
0.8
La(NO,»)
Mg(NO)\
Physical mi xing
30.3
58.0
17.6
1.0
(C H)COO» La
M g(O H ) ~
Physical mi xing
29.0
57. 1
16.6
1.1
La(NO),
MgCO)(I)
Phys ica l mi xing
29.2
6 1.0
17.8
0.9
b) Temp.
= 850"C, CHiOz= 8.0
c) Temp. = 800"C, CH/O l = 8.0
d) Temp.
=75(}'C, CHiOl =4.0
MgCO)(I) PrepJrcd by precipitati on of magnesi um nitrate by sodi um carbonate
at different process co nditi o ns in Tabl e 2. Among these
cata lysts, the cata lyst prepared using lanthanum nitrate
and mag nes ium carbonate (I) showed the best catalytic
performance in the OCM process at all th e process
conditi o ns (Tabl es I a nd 2).
Th e basicity of the La-promoted MgO is muc h
hi ghe r th an that of the La 2 0 , or M gO obtai ned under
ident ical condition s from the sa me precursors used in
th e co rrespo ndin g La-prom o ted MgO cata lysts ' Ii 17 .
He nce, th e observed hi g h strong basic ity of th e Lapromoted M gO cata lysts (Table I ) cannot be due to the
presence of a separate La10, phase (obse rved by XRD
for the La-promoted M gO cataly sts with La/Mg = 0 . 1)
in th e cata ly sts. Th e obse rved hi g h strong bas ic ity of
the La·· promoted MgO ca tal ys ts is expec ted most ly
because o f the formation of additional low coo rdin ated
0 2. surface s ites by th e in corporati o n of La' + cati on in
the MgO latti ce. Since, th e radiu s of La' + ca ti o n (0. 102
11m) is larger than th at ofMg"+(0.065 nm ), the lanthanum
cation cannot fit into the M gO latti ce without causing
its structural di stortion. Thi s is expec ted to result in
surface oxygen species of low coord ination IX . It has been
shown earli er 'li a poss ibi lity of an involve ment of aciclbase pair (M "+ LC 0 \ " where subsc ri pt LC de notes low
coordi nati o n) o n the surface in th e abs tracti on of th e Hatom from adso rbed CH~ in the OCM process .
OCM over the hest La-promoted MgO ( Ol talyst at different
process conditions
The influe nce o f te mpe rature (750-850"C) at a
space ve loc ity of 5 1,360 c m.1 .g ·'.h · ' o n the cata lyt ic
activity/se lec tivity in OCM ove r La·promoted MgO
cata lys t [prepared by th e me th od o f physical mi xin g
usin g la nth anum nitrate and mag nes ium carbonate (I)
as catalyst precursors] has been stud ied . The results sho w
that the methan e convers ion is inc reased fro m 27.2 to
28 % and the C 2-se lecti vity is a lso increased fro m 6 1 to
64.9 % with increas in g th e react ion te mpe rature fro m
CHOUDHARY
el
al.: OXIDATIVE COUPLING OF METHANE OVER La-PROMOTED MgO CATALYSTS
573
radicals and/or thermal cracking of more ethane
molecules at higher temperature, as follows:
1.2
- - - > C2H 4 + H.
- - - - > C 2H4 + H2
.~
;:;
... (2)
It may also be due to an increase in the rate of
the following reaction,
o.~
p::
... ( I)
r.-G
eJ'
;:~
eJ'
... (3)
0.4
O.c)+-----r--~--._----___.--__1
xo
~ 60
[J
VJ
o 40
X
G
20
0-1--- - ; r - - _ - - - - , - - - _--,---.....j
'"
6
CHi02 Ratio
Fig. I - Erfect o r CH /0 , ratio on the methane conversion , C ,selectivity and C1H/C)-l h ratio in OCM over La- promoted MgO (La/Mg = 0.1) , prepared by the physical mixing of
lanthan um nitrate and magnesiulll ca rbonate (I) (X C IU = CH.
co nversion and SCI = C1-selec ti vity )
750 to 850"C. An increase in the C 2-se lec tivity with
te mpe rature has also been observed earlier in the OCM
Process over La 2 ~ 1').20 , Sm 2 3 21' K-Sb 2 .1 22 ' Li-ZnOMgO and Li-CdO-Mg02:1 catalysts. The increase in the
C 2-se lec tivity is ex pec ted to be mostly due to a decrease
in th e formation of carbon dioxide by gas phase
decomposition of methy l peroxy radicals (C HP? ), the
formation of which by a gas phase reaction be tween
free oxyge n with meth y l radi ca ls (w hich are formed on
the cata lyst s urfa c e ) is n o t favoured at hi g h e r
temperatures I') As expected, the e th y \ene/ethane rati o
is increased from 0.9 to 1.1 with in c rease in the
temperature frolll 750 to 850"C. The increase in the C 2H/
C 2H(, product rati o with temperature suggests that th e
co nversio n of etha ne [which is formed by couplin g of
meth y l radi ca ls 24] to e thylen e is favoured at hi g her
temperatures; it is cons istent with that observed in the
ea rl ie r studies 1 6.1~ . 21.2-'.26. The inc rease in C 2H /C 2 H 6 rati o
is expected due to th e decomposition of mo re et hy l
°
°
°
and the oxidative dehydrogenation of ethane on the
catalyst surface.
The results showing the influence of CH/02
ratio (4.0-8.0) at a space velocity of 51,360 cm 3 .g-' .h-'
on the catalytic activity/selectivity of La-promoted MgO
catalyst [prepared by the method of physical mixing
using lanthanum nitrate and magnesium carbonate (I)
as catalyst precursors] in the OCM , are presented in
Fig. I . The results show that the methane conversion is
decreased but the C 2-selectivity is increased when the
CH/0 2 ratio is increased from 4.0 to 8.0. The increase
in the C 2-selectivity with increasing CH/02 ratio (or
decreasing 0 2 concentration in feed) has also been
,
observed earlier for the rare earth oxide catalysts (,,20,
rare earth promoted Mg027, MgO l7 and several o th er
oxide catalysts 2(1.2X. The observed decrease in the C 2H/
C 2H(1 ratio with increasing the CH/ 0 2 ratio in the feed
is consistent with that observed in the earlier
s tudies'6. ' 7.26-2~_ Thi s is expected mostly because of the
availability of 0 2 at low e r concentration for th e
following gas phase reaction s involved in the formation
of ethyl radical s and ethylene frolll ethane:1 O:1 I,
C 2H 6 + 0 2
> C 2H, + H0l'
C 2H, + 0 2 - - - - > C 2H 4 + H02·
... (4)
.. .(5)
The formation of e than e in th e OCM process is
expected to be mostly by the gas phase couplin g of
methyl radi cals formed on the cataly st s urface 24 .
Conclusions
The surface area, tota l and stron g bas icity and
cata lytic act ivity/se lectivity of La-pro moted MgO (La/
Mg = 0.1) a re s ig nificantl y influ e nc ed by cataly st
preparation method (physical mixing and coprecipitation
method) and also by the catalyst precursors (La- and
Mg-salts used in th e catalyst prepa rati o n). The Lapromoted MgO ca tal ys t prepared lI s i ng th e
coprec ipitation me thod ha ve hi g he r surface area and
574
INDIAN 1. CHEM. TECHNOL., JULY 2004
12
Guczi L, van Santen R A & Sanna K Y, Ca/ol Rev - Sci Ellg,
38 (1996) 249.
Choudhary Y R, Chaudhari S T, Rajput A Ivl & Raile Y H, .I
Chelll Soc Chelll COIllIllItIl, ( 1989) 555 .
Choudhary Y R, Chaudhari S T, Rajput A M & Rane Y H,
Calaf Lell, 6 ( 1990) 95.
basicity (total and strong basic sites measured in term s
of eo, chemisorbed at SO and 500 e, respectively) but
lower ilctivity/selectivity in the OeM process. Whereas,
the La-promoted MgO catalyst prepared using th e
physical mixing method have lower surface area and
basicity (both total and strong basic sites) but show
higher activity and selectivity in the OeM process,
depending upon the La- and Mg-salts used as the catalyst
precursors. Among the catalysts prepared using different
methods and precursors, the one prepared by the physical
mixing method and using the lanthanum nitrate and
magnesium carbonate (prepared by precipitation it from
an aqueous solution of magnesium nitrate by sodium
carbonate) showed best performance in the OeM
process at different process conditions .
20
Choudhary Y R & Rane Y H, .I Chelll Soc Faraday Trails, 90
(1994) 3357.
References
21
Otsuka K, Jinno K & Morikawa A, J COWl. 100 (1986) 353.
I
2
3
22
23
Lo M Y, Agarwal S K & Marcelin G, J Cow l, 112 (1988) 1611.
Choudhary Y R, Rajpllt A M, Akolekar D B & Seleznev Y A,
Appl Cowl, 62 ( 1990) 171.
Ito T, Wang J X, Lin C H & Lunsford J H,./ Alii. Chell/ Soc, 107
(1985) 5062.
Kimble J B & Kolts J H, Chell/lech, ( 1987 ) 50 I.
1l
4
5
6
7
8
9
10
II
Pitchai R & Kli er K, Calal Rev-Sci Eng, 28 ( 1986) 13.
Lee J S & Oyama S T, Calal Rev -Sci Eng , 30 ( 1988) 249.
Hutchings G J, Woodhouse J R & Scurrell M S, J Chelll Soc
Faraday TrailS, 85 (1989) 2507.
Hutchings G J, Scurrell M S & Woodhouse J R, Chelll Soc
Rev, I X ( 1989) 251.
Anderson J R, Appl Catal, 47 (1989) 177.
Garibyan T A & Margolis L Ya, Calal Rev - Sci Eng, 31 (198990) 355.
Sokolovskii Y D, Calal Rev -Sci Ellg, 32 ( 1990) I.
Dubois J L & Cameron C J, AWl Cowl, 67 ( 1990) 49.
Lunsford J H, Cawl Today, 6 (1990) 235.
Mleezko L & Baerns M, File! PlVcess Tecllllol, 42 (1995)
2 17.
Lunsford J H, Angell' Chelll 1111 Ed Ellgl, 34 (1995) 970.
13
14
15
Choudhary Y R & Rane Y H, Calal Lell, 4 ( 1990) 101 .
16
Choudhary Y R & Rane Y H, J Cala l, 130 ( 199 1) 411.
17
Choudhary Y R, Rane Y H & Gadre R Y, .I C({wl, 145 (1994)
300.
18
Nunan J, Cunningham J, Deane A M, Co lbourll E A &
Mackrod t W C, Adsol]Jlion and Catalysis 0 11 (}xides SlIIfaces ,
Surface and Catalysis Series (Elsevier, Am ~ terdam), 21 (1985)
90.
19
Lin C H, Campbell K D, Wang J X & Lun sford J H, J Phys
Chem, 90 ( 1986) 534.
24
25
26
27
28
29
30
3I
Rane Y H, Ph.D. Thesis, Universily of POO/w, 1992.
Choudhary Y R, Rane V H & Chaudhari S T, Appl Calal, 1511
(1997) 121.
Challdhari S T, Ph.D. Thesis, Ulliversilv BUII/ba)' ( 1993).
Choudhary V R, Chaudhari S T, Rajput A M & Rane V H, .I
Chem Soc Chem COII/I/Utll, (1989) 1526.
Morales E & Lunsford J H, J Cowl, Illl ( 19B9) 255.
Geibrech! R A & Daubar! T E, Illd Ellg ChclIl PlVcess Des
Del', 14 (1975) 159.
or