Indi an Jo urn al of C he mi stry
Vo l. 41 A. Novembe r 2002 , pp. 225 1-2255
Catalysis by some metal oxides modified with phosphate ions
S Sugunan *, H Suja, C S Oeepa, K Sreejarani & M V Ouseph
Department of Applied Chemi stry. Cochin Uni ve rsity of Science a nd T ec hno logy. Kochi 682 022 . India
Receil'ed 14 lilly 2000; revised 3 lilli e 2002
Surface ac idity of phosphate mod ifi ed La ~ O " CeO~ and Sn02 has bee n estimated by titrim etri c me thod using Ha mm ell
indicato rs. Mi xed ox ides of tin and lanth anum have al so been prepared and subj ec ted to phosphate modifi ca ti o n. Surface
cha racte ri zati o n o f the sa mpl es has been carried o ut using XRD, surface a rea, the rmal a nal ysis and IR spectrosco py.
Phosphate co nt e nt in the sa mpl es has bee n chem ica ll y estimated. Th e catal yti c activit y fo r benzy latio n a nd este rificati o n
reaction s has al so been investi gated.
Metal ox ides find wide application in the field of
heterogeneous cataly sis. Slight modification s of the
metal ox ides may result in e nhanced catal ytic activity
and selecti vity . Considerable attention has been given
to anion modifi ed metal ox ides since th e first seriou s
report of supe rac idity came in early 1980sl. 2
Numero us studi es have been devoted to in ves tigate on
th e sulphat ed metal oxides'-) but o nl y a few reports
have appeared o n phosphate modi fied ox ideso-8 . Th e
present paper deals w ith a co mparati ve evaluation of
the surface prope rti es of pure and phospha te modified
metal ox id es of La, Ce and Sn. Mixed oxides of Sn
and La were al so subj ected to study . Th e cata lytic
acti vi ty of these ox ides was studi ed for test reactions
such as benzylation and esterification . The data have
been compared with those for the unmodifi ed oxide.
at I20°C overni g ht and calcination at 300"C fo r .')
ho urs . Phos phate modificati o n was achi eved by usin g
H, P0 4 so lutions of varyin g concentrations. H\ PO~
solutions, 0.1 M, 0.3 M and O.S M, were used for
mod ifyin g La20 , a nd th e systems are des ig nated as
PLa(O.I) , PLa(0 .3) and PLa(O. S) respectively . CeO c
modified with 0.1 M. H, PO~ is den oted as PCe((). 1).
NaH2PO~/ H 3P0 4 buffer 0.1 M, at pH 4 was also used
as modi fier for Ce02 and La20 3 and the sys te ms are
desig nated as PLa and PCe respectivel y. The
phosphated analogues of mi xed ox ides were prepared
by using 0.1 M H 3 PO~ as modi fier. Th e various mi xed
ox ide systems prepared and th e no tati o ns used are as
follows:
Co mpos iti o n
Mi xed ox ide
Phosphat ed
analog ue
Materials and Methods
SnO~( IOO % )
T
PT
The rare earth oxides were prepared by the
hydroxide method') from the ir nitrate salts [lantha num
nitrate.
cerium
nitrate
(99.9 %
pure)
IRE,
Udyoga mandal] . The precipitate was dried overnight
at 120"C and th e n calcined for five ho urs at 300°C.
Tin oxide was prepared fro m stannic chloride so lution
(prepared from stannou s chloride by oxidation with
aqua reg ia) by th e hydroxide method. Mix ed oxides
were pre pared by co-precipitation method 10 fro m a
mixture of rare earth nitrate and stannic salt soluti o n
in required propo rtion s. The precipitate was filtered
after washing, dried overnight at 110°C and calcined
at SOO"C for five ho urs.
Phosphate modified samples were prepared by
suspending the oxide samples in H 3 P0 4 solution or
NaH 2 P0 4 solution as the case may be for 20 hours. It
was then filtered without washing followed by dryin g
S n02(80 % ) , La ~ 0 .1 (20 % )
TL82
PTL82
Sn01(60 % ).La 20 3(40% )
TL64
PTL6~
Sn02( 40% ).La20 ,(60% )
TL46
PTU6
Sn0 2(20 % ). La 20 ,(80%)
TL28
PTU 8
L
PL
La 20 , ( 100% )
The amount of ph osphate in the modified sa mpl es
was estimated che mically 10. Specific surface area was
measured by the BET method usin g low temperature
nitrogen adsorption in a Gemini Microme riti cs
surface area analy ser. XRD pattern s we re reco rded
using CuKa radi ation using Ri ga ku-MinirIu x.
Thermal analysis was carried o ut using Shimadzu
TGA-SO at a heatin g rate of 10°C /min in nitrogen
atmosphere. Surface acidity/basicity measure me nts
were carried out by titrimetri c method usin g Hammett
indicators.
nS2
INDI AN .J CHEM, SEC. A, NOVEMBER 2002
The liquid phase be nzy lati on of toluene was carri ed
out by adding 0. 1 g of th e catalys t to 10 ml toluene
and I ml be nzyl chl orid e in an R.B fl ask. The mixture
was reflux ed with stirrin g for I hour. The
esterification was carried out by stirring 0.5 ml acetic
acid and 10 ml butanol in prese nce of 0.3 g of the
ca talyst in an RB flask at 98"C ror five hours. The
produc ts we re analysed uSing a Chemito 86 10 Gas
chromatograph.
Results and Discussion
The co ncentratio n of phos phate ions obtain ed by
the chemical analysis is given in Tabl e I . Surface
concen trat ion of phosphate ions per unit surface area
was also ca lculated. The amou nt of phosphate ions
was fo und to increase with an increase in th e
concent rat ion of the H, P0 4 so lution used. The surface
conce ntration of phosphate ions obtained in certai n
sa mpl es was high er th an th e va lues reported in the
case or anatasc (l'~ . The phosphate content of La 20 .1
samples dec reased when Na H2PO.; was used as th e
mod ifi er whi le an app rec iable va ri ation co uld not be
observed fo r Ce02. The phosphate uptake by Ce02
was quite low when co mpared to La20 .\. The
phospha te content was quite low for Sn02 also. In th e
case of mixed oxides, th e phosphate co ntent gradu all y
increased with increase in th e amo unt of La
incorporated.
In sp ite of the high phosphate co ncentration , XRD
ana lys is co nfirmed th e locali sati on of phosphate ions
main ly on the ox ide surface without detectab le
amou nts of a three dim ensional phosphate phase.
Phosphate ions can be considered to be finely
dispersed on the oxide surface. The intensity of XRD
peaks was found to decrease upon phos phate
modification as a res ult of the retardation of
crysta lli sa ti on. Such lowe r degree of crystallinty of
modified ox ides as co mpared to pure ox ides is already
repo rted l . 5 In mix ed ox ide sys tems the intensity of
diffracti on peak corres ponding to Sn02 decreased as
the % of La 20J increased and new peaks
correspo ndi ng to crystall ine La 20 J sta rt ed appearing.
o co mpound formatio n was observed in these mixed
ox ide sys tems calc ined at SOO"e. Peak intensity of th e
mi xed ox ide syste ms was al so red uced du e to
phosphate modi fication.
The specific surface area of mod ifi ed sa mpl es was
found to be hi gher than that of th e corresponding
unmod ifi ed sa mpl es, whi ch can also be attributed to
th e delayed crystallisation]. The phosphate groups
being we ll dispersed on th e surface prevents the ox ide
Table I -
PhosphaIe conte nt of va ri olls sa mpl es
Ca ta lys t
% 01'
phosphorlls
Phosphate
iun s/Illll c
PLa(O.I)
PLa(0.3)
PLa(0.5)
PLa
PCe(O.I )
PCe
PT
PTL82
PTL64
PTL46
PTL28
2.8 1
4.02
4.95
2.13
0. 39
0 .42
0.25
2.86
3.44
3.50
3.90
7.20
I 1. 20t)
14.7R
0.24
.1.02
3.:-:0
0.52
4.70
5.66
6.48
7. 89
particles from comi ng closer duri rig calc inat ion. It
was al so observed th at the surface area of the
phosphated samples decreased with increase in
surface phosphate conce ntrat ion obtained using higher
co nce ntrati ons of H.1 PO-l solution . In th e case or
mixed ox ides, incorpo ration of Lt20 , was found to
res ult in an increase in th e surrace area va lue. But ~I S
more and more of La 20 .1 was introduced , th e su rrace
area was found to decrease probabl y due to
agg lomerisa ti on. Thus TL82 showecl th e maximu lll
and TL28 th e minimum sur face area. The same trend
was also observed for th e phospha ted analogues.
The th erm al stab ility or modified ox ides was tested
by thermograv imetri c ana lysis . The TG cu rve of
La(OH h shows an initi al we ight l o~s at IOO-200"C
due to the removal of adsorbed wa ter. This is
fo ll owed by formation of hexago nal LaOOH
intermed iate (2S0-3S0"C) wh ich is finall y con verted
into La20 .1 (3S0-420"C) 11.12. The fi nal weight loss
(4S0-800"C) co rresponds to the decom pos iti on or the
surface carbo nate laye r that i<; form ed by the
interac ti on of basic lanthanum hvdro xide with the
atm ospheri c CO 2''. In th e case of 'Ce(O H)2 an initial
weight loss due to loss or adsorbed water "vas
observed. During th e temperature range of 2S0-4S0"e.
wa ter fo rmed by the co mbination of adjacent OH
gro ups was eiiminated leading to the formation of
Ce02'
The phosphate mod iFied sa mpl es in both the cases
show a gradu al we ight loss du e to the slow remova l or
adsorbed and structural wa ter. Th ere is no prominent
we ight loss indicating th e therm al stability of the
phos phate-mod ified sa mples. Enhanced th ermal
stability of phosp hated anatase has been previou sly
reported 6 . The presence of phosphate ion s in rh e
mod ifi ed sampl es was further co nfirmed by IR
spectra, whi ch showed a broad peak around 980-1080
SUGU AN
ef
01. : CATALYS IS BY METAL OXIDES
cm' \, which can be assign ed to the P-O stretching
mode of phosphate anion \4 .
The res ults of the ac idit y/bas icity meas urements by
Hamm ett indi cator method are given in Tables 2-5 .
Phosp hate modified La2 0~ samples were found to be
acidi c except in th e case where NaH 2P0 4 buffer was
used as the mod ifier. Modified catalyst still remained
basic but th e basicity was lower than that of pure
La 20 1. The concentrati on of H, P0 4 solution see med
to have a negli gible effec t on the ac idity. ]n spite of
the increase in the phosphate co ntent, ac id ity
remained more or less th e same.
Unm od ified CeO} was slightl y acid ic with ac idi c
Table 2- Ac idit y/basic ity of
La ~ O,
S~l1llpk
PO/ I La,O, (0. 1M H, PO-l)
PO/ I La,O l (0. 3M
aH , PO-l)
PO-l" I La,O , (0.5M H, PO-l)
PO/ I La,O, (0. 1M Na H, PO-l1 0. 1M H, PO-l)
sites of Ho :S 7.2 . But phosphate modificat ion res ul ted
in the devel opment of ac id sites of H o betwee n 4.8
and 7.2. Thus the acidity and ac id strength was foun d
to increase upon phos phatation . In the case of ceria
not much difference was fou nd in th e acidity
generated by H}P0 4 and Na H2 P0 4 mod ifi ers. Even
though ceria is more aci di c than La20 .l th e phosphated
sam ples exh ibited reverse trend wi th phosphated
La 2 0 } being more ac idi c. Thi s may be due to the
hi gher phosphate co ntent or due to the difference in
the mode of adsorpti on. The acid ic sites in SnO} are
present at H o 4.8 and 7.2 and basic sires at H o 3.3.
Thus Sn02 is found to possess weakly basic and
and phosphate modified La,O , activated at var ious tempe rat ures
Basicity( mmollg)
Temp
La,O..
2253
Acidi ty( mm ollg)
(0C)
H ,,? 3. 3
H ,,? 4.8
H o?' 7.2
300
500
700
300
500
700
300
500
700
300
500
700
300
500
700
1.23
1.1 0
1.0 I
0.25
0.22
0.20
0.23
0.2 1
0.19
H,,~
H,,~4 .8
3.3
0 32
0.22
0.12
0. 3 1
0.23
0. 16
0.27
0.23
0. 18
1.04
0.88
0.31
0.15
0. 14
0. 13
H,,~
0.32
0.27
0.25
0.32
0.25
0.2 1
0.38
0.25
0.23
7:1.
0.37
0.35
0.30
0.55
0. 36
0.25
0.55
0.38
0. 27
0.13
0.11
0.05
Table 3 - Ac idit y/basic it y of CeO, and phos phate mod ified CeO, activated at var ious temperat ures
Sa mple
CeO,
1'0/ 1 CeO,
(DIM H,PO-l)
1'0/ 1 CeO,
(0. 1MN aH , PO.tl
(OC)
300
500
700
300
500
700
300
500
700
Tab le 4 -
Catal ys t
T
T
T
PT
PT
PT
Te mp
(OC)
300
500
600
300
500
600
Ac id ity (mm ollg)
Basici ty( mmo llg)
Te mp
Hn? 3.3
H,,? 4.8
0.08
0.05
0.03
0.05
0.03
0.01
0.06
0. 12
0.01
0.05
0.03
0.02
Hu? 7.2
H,,~3.3
H,,~4 .8
0.08
0.05
0.0'+
0.05
0.03
0.02
H,,~
7.2
O. t:l
0. 20
0.03
0.28
0.25
0.05
0.23
0.08
0.04
Ac id stre ngth di stributi o n of Sn01 and PO/I SnO, at var io us tc mperaturcs
Basic it y
H,,?3 .3
H ,,?4. 8
Acid ity
1-I,,?7.2
Ho5:3.3
H.,~4.8
1I,,~ 7 2
0.030
0. 16
0.32
0.2 1
0. 2 1
0.08
O. t:l
0.51
0.37
0.34
0.26
0.09
0.29
0.53
0.55
0.63
0.05
0. 10
INDIAN J C HEM. SEC. A, NOV EMBER 2002
2254
Tab le 5 - Ac idity/bas icity of Sn-La mi xed ox ides and its phosphated analog ues ac tivated at 600 DC
Bas ici ty( mmo l/g)
Samp le
T
T L82
TL64
TL46
T L28
La
PT
PT L82
PT L64
PTL46
PTL28
PLa(O. I)
Ac id ity (millol/g)
Ho? 3.3
Ho? 4.8
Ho? 7.2
0. 42
0.32
0.26
0.25
1.0 1
0. 11
0.04
0.05
0.03
0.20
0.08
0.03
0.02
0.03
0. 19
ac idi c sites. Inco rpo rati o n o f bas ic La 2 0 , to Sn02
dec reases th e ac idity a nd the mi xed ox ides were
fo und to be bas ic. The phosphated a na log ues were,
however, acidi c and fo ll o wed the same trend as th at
of pure ox ides.
The inc rease in ac idity upo n phosphati o n can be
assig ned to the e lectron w ithdrawing effect of the
phos phate gro up, whi ch mak es the meta l center mo re
e lectron defic ient. Ge nera ll y, H, P0 4 modifi ed
sa mpl es were fo und to have a g reater ac id streng th
and ac idity than Na H 2 PO~ modifi ed o nes.
15
Flaig Bauman e f 01 have pro posed a mechani sm
in whi ch adsorpti o n occurs by an excha nge process
between surface hydroxy l g roups and H 2 P0 4 ' io ns in
so luti o n, H3P0 4 bein g di ssociated o nl y to the first
degree at pH < 7. The fo ll ow ing struc ture (I ) is
postul ated . Wh en Na H 2 PO~ is used as the modifi er.
one of the OH groups is rep laced by ON a.
Th e second mecha ni sm in vo lves the di ssoc iati ve
adso rpti o n of H 3 PO~ leading to the bl ockage of Lew is
ac id sites and the pro posed stru cture is de no ted as II.
On heatin g, dehyd roxy lat io n occurs leadi ng to
/
·H,O
II
/
..
0
o
"" ""
/
""
H"S, 7.2
0.03
0. 13
0. 29
0.2 1
0.44
0.53
0.39
0.24
0. 12
0.34
0.52
0.53
0.37
0.32
0.25
0.63
0.55
0.5 g
0. 2 1
0.34
0.30
/OH
P-
I
HO
o
r
o
0 11
(II )
bridged structures as g iv e n in Scheme 1.ln thi s case
Bro nsted ac idity is ge ne rated . When Na H 2 PO~ is used
the H+ of the P-OH g roups arc co nverted into P-O Na
whi ch wi ll redu ce the Bro nsted ac idity. Thi s ex pl ain s
th e lo werin g of ac idity in thi s case.
Th e cata lytic acti vity was tes ted fo r benzy lati o n
a nd esterifi cati o n reacti o ns and th e results are g ive n in
T able 6. Lew is ac id sites are be lieved to be more
acti ve toward s be nzy lat io n reacti o n th an Bro nsted
sites, whereas este rificatio n in liq ui d phase is
co nsidered to be mainly cata lysed by Bro nsted ac id
sites .
T he modifi ed La 2 0 .\ a nd Sn02 sa mp les showed a
lesser acti vity towards be nzy lati o n reac tio n co mpared
0
0
""
/
He"1
.-
.H,o
o - -M
/
Scheme 1
"-
0
p
/
o
__ 0
""
/
0
0
0
0
""
p_
/
I
o
O-M -
(I)
/ p ""
~
""
OH
I
O-M- O
0
0
-M
HaS, 4.8
H
0
OH
o~
HoS, 3.3
""
_
M
-- P
/
/
""
/
0
0
""
o -
tvl
\'-
2255
SUGU NAN el al . : CAT ALYSI S BY METAL O X ID ES
Ta ble 6 - Data o n the catalyti c ac ti vity towa rd s benzy lati o n and es terifi catio n
Ca talyst
'-,
La,O,
PLa(O. I)
PLa
Ce02
PCe(O I)
PCe
T
PT
T L82
PT L82
TL64
PTL64
TL46
PTL46
TL28
PTL28
S urface area
(m2/g)
44.92
75 .64
66.27
34.80
40.92
4 1A6
40.01
93.08
82 .01
117 .94
63.99
104.95
6 1. 86
104.87
59.87
95 .95
Esterificat io n reacti o n
Benzy latio n reac ti o n
Co nve rsio n
%
Rate co nstant
( 10. 7 s·'m·2)
16.78
14.08
18.74
57. 15
68.65
79.24
88.82
5 1.00
72. 17
74.38
92.58
84.76
85. 15
56.39
84.90
88.32
to pure ox ides irres pec ti ve of th e hi g he r ac idity and
surface area va lu es whe reas the reverse trend was
o bserved fo r esteri ficati o n. Thi s leads to the
assumpti o n th at the phos ph ate modifi cati o n in th ese
cases occurs by the bl oc kage of Lewis sites th ereby
leav ing them in access ibl e fo r the reactant mo lecul es
d urin g benzy lat io n reac ti o n. Th e Bro nsted sites
created durin g the process, cata lyse the es terificati o n
reacti o n. Thi s tre nd was comple te ly reversed for
Ce0 2. In the case of Ce0 2, L ew is ac id sites can be
ass umed to be mo re ac ti ve resultin g in a bette r
acti vity towards be nzy latio n reacti o n.
In the case of mi xed ox ides, TL64 was fo und to be
th e most acti ve towards benzy lati o n. Thi s is
co nsistent with the fac t that max imum ac idity was
o bserved for th e same. Th e modi fied ox ides we re less
acti ve toward s benzy lati o n suggesting th at the Lewi s
ac id sites are weake r co mpared to pure oxides.
However th e es teri ficati o n results also sho wed the
sa me tre nd with th e modifi ed ox ides .
To co nclude ph osphati o n genera ll y leads to an
increase in surface area and a decrease in cry stallinity
and the sampl es are therm all y stabl e. Phosphation can
atco-ordinati vely
occur
lhro ugh
adsorpti o n
unsaturated Lew is ac id sites a nd exch ange o f OH
groups by H 2P0 4 ' io ns. The catalyti c acti vity and
sllrface ac id-base pro perties ma inl y depend o n the
amollnt of phosph ate adso rbed a nd also the nature of
adsorpti o n.
Co nve rsio n
%
11.36
5.6 1
8.60
1.59
1. 6 1
2.04
15.19
2. 13
4 .33
3.2 1
11 .29
4.98
5. 16
3.72
8.77
6.2 1
Rate co nsta nt
( 10.7 s·' m· 2)
3 1.66
70.78
4.69
9. 10
67.98
59 .59
17.62
9.8
12.30
70.68
65.84
59.52
69 .77
36.50
87AO
46.35
83 .99
73.5 1
1. 82
7.32
7.28
4.2()
10AO
2AO
18.60
3.30
17.00
7.69
Acknowledgement
Th e autho rs w ish to ac kn ow ledge the ir sin cere
g ratitude to the CS IR , Ne w De lhi fo r the award o f
Juni o r Research Fe ll ows hip to H S a nd K S
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