Indi an Journal of C hemi stry Vol. 41 A. November 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 Chemistry. Cochin Uni versity of Science and Techno logy. Kochi 682 022 . India

Receil'ed 14 lilly 2000; revised 3 lillie 2002

Surface ac idity of phosphate mod ified La~O" CeO~ and Sn02 has been estimated by titrimetri c method using Hammell indicators. Mi xed ox ides of tin and lanthanum have al so been prepared and subj ec ted to phosphate modifi ca ti on. Surface characte ri zation o f the samples has been carried out using XRD, surface area, the rmal analysis and IR spectroscopy. Phosphate content in the samples has been chem ically est imated. The catal yti c activity for benzy lation and esterificati on reaction s has al so been investi gated.

Metal ox ides find wide application in the field of heterogeneous catalysis. Slight modifications of the metal ox ides may result in enhanced catal ytic activity and selecti vity . Considerable attention has been given to anion modified metal ox ides since the first serious report of superac idity came in early 1980sl.2

Numerous studi es have been devoted to in ves tigate on the sulphated metal oxides'-) but only a few reports have appeared on phosphate modi fied ox ideso-8. The present paper deals with a co mparati ve evaluation of the surface properti es of pure and phosphate modified metal ox ides of La, Ce and Sn. Mixed oxides of Sn and La were al so subj ected to study . The catalytic acti vi ty of these ox ides was studied for test reactions such as benzylation and esterification . The data have been compared with those for the unmodifi ed oxide.

Materials and Methods The rare earth oxides were prepared by the

hydroxide method') from their nitrate salts [lanthanum nitrate. cerium nitrate (99.9% pure) IRE, Udyogamandal] . The precipitate was dried overnight at 120"C and then calcined for five hours at 300°C. Tin oxide was prepared fro m stannic chloride solution (prepared from stannous chloride by oxidation with aqua reg ia) by the hydroxide method. Mixed oxides were prepared by co-precipitation method 10 fro m a mixture of rare earth nitrate and stannic salt soluti on in required proportions. The precipitate was filtered after washing, dried overnight at 110°C and calcined at SOO"C for five hours.

Phosphate modified samples were prepared by suspending the oxide samples in H3P04 solution or NaH2P04 solution as the case may be for 20 hours. It was then filtered without washing followed by drying

at I20°C overnight and calcination at 300"C fo r .') hours . Phosphate modificati o n was achi eved by using H,P04 solutions of varying concentrations. H\PO~

solutions, 0.1 M, 0.3 M and O.S M, were used for modifying La20 , and the systems are des ig nated as PLa(O.I) , PLa(0 .3) and PLa(O. S) respectively . CeOc modified with 0.1 M. H,PO~ is denoted as PCe((). 1 ). NaH2PO~/ H3P04 buffer 0.1 M, at pH 4 was also used as modi fier for Ce02 and La20 3 and the sys tems are desig nated as PLa and PCe respectively. The phosphated analogues of mi xed ox ides were prepared by using 0.1 M H3PO~ as modi fier. The various mi xed ox ide systems prepared and the notatio ns used are as follows:

Co mpos iti on Mi xed ox ide Phosphat ed analogue

SnO~( IOO% ) T PT

Sn02(80% ) , La~0.1 (20%) TL82 PTL82

Sn01(60% ).La20 3( 40% ) TL64 PTL6~

Sn02( 40% ).La20 ,(60%) TL46 PTU6

Sn02(20% ). La20 ,(80%) TL28 PTU8

La20 , ( 100% ) L PL

The amount of phosphate in the modified sampl es was estimated chemically 10. Specific surface area was measured by the BET method using low temperature nitrogen adsorption in a Gemini Micromeriti cs surface area analyser. XRD patterns were recorded using CuKa radi ation using Ri gaku-MinirIu x. Thermal analysis was carried o ut using Shimadzu TGA-SO at a heating rate of 10°C /min in nitrogen atmosphere. Surface acidity/basicity measurements were carried out by titrimetri c method using Hammett indicators.

nS2 INDI AN .J CHEM, SEC. A, NOVEMBER 2002

The liquid phase benzy lati on of toluene was carried out by adding 0. 1 g of the catalys t to 10 ml toluene and I ml benzyl chloride in an R.B fl ask. The mixture was reflux ed with stirring for I hour. The esterification was carried out by stirring 0.5 ml acetic acid and 10 ml butanol in presence of 0.3 g of the catalyst in an RB flask at 98"C ror five hours. The produc ts were analysed uSing a Chemito 86 10 Gas chromatograph.

Results and Discussion The concentration of phosphate ions obtained by

the chemical analysis is given in Table 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 the concentrat ion of the H, P04 so lution used. The surface concentration of phosphate ions obtai ned in certai n sa mpl es was higher than the val ues reported in the case or anatasc (l '~ . The phosphate content of La20 .1 samples dec reased when Na H2PO.; was used as the mod ifi er whi le an apprec iable va ri ation could not be observed fo r Ce02. The phosphate uptake by Ce02 was quite low when co mpared to La20 .\. The phosphate content was quite low for Sn02 also. In the case of mixed oxides, the phosphate content gradu ally increased with increase in the amount of La incorporated.

In sp ite of the high phosphate concentration , XRD ana lys is confirmed the locali sation of phosphate ions main ly on the ox ide surface without detectable amou nts of a three dimensional 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 phosphate modification as a res ult of the retardation of crysta lli sa ti on. Such lower degree of crystallinty of modified ox ides as compared to pure ox ides is already reported l

.5 In mixed ox ide systems the intensity of

diffracti on peak corres ponding to Sn02 decreased as the % of La20J increased and new peaks correspondi ng to crysta ll i ne La20 J sta rted appearing.

o co mpound formatio n was observed in these mixed ox ide systems calc ined at SOO"e. Peak intensity of the mi xed ox ide systems was al so reduced due to phosphate modi fication.

The specific surface area of mod ifi ed sa mples was found to be higher than that of the corresponding unmod ifi ed samples, which can also be attributed to the delayed crystallisation]. The phosphate groups being we ll dispersed on the surface prevents the ox ide

Table I - PhosphaIe content of va ri o lls sa mples

Cata lys t % 01' Phosphate phosphorlls iuns/Illllc

PLa(O.I) 2.8 1 7.20 PLa(0.3) 4.02 I 1. 20t) PLa(0.5) 4.95 14.7R PLa 2.13 0.24 PCe(O.I ) 0. 39 .1.02 PCe 0 .42 3.:-:0 PT 0.25 0.52 PTL82 2.86 4.70 PTL64 3.44 5.66 PTL46 3.50 6.48 PTL28 3.90 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 wi th increase in surface phosphate concentrat ion obtained using higher concentrati ons of H.1 PO-l solution . In the case or mixed ox ides, incorpo ration of Lt20 , was found to result in an increase in the surrace area va lue. But ~I S

more and more of La20 .1 was introduced , the su rrace area was found to decrease probably due to agg lomerisa ti on. Thus TL82 showecl the maximu lll and TL28 the minimum surface area. The same trend was also observed for the phosphated analogues.

The thermal stab ility or modified ox ides was tested by thermograv imetri c ana lysis . The TG cu rve of La(OHh shows an initi al weight l o~s at IOO-200"C due to the removal of adsorbed water. This is fo ll owed by formation of hexagonal 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) corresponds to the decom pos iti on or the surface carbonate laye r that i<; formed by the interac ti on of basic lanthanum hvdroxide with the atmospheri c CO2''. In the 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. water fo rmed by the combination of adjacent OH groups was eiiminated leading to the formation of Ce02'

The phosphate mod i Fied sa mpl es in both the cases show a gradual weight loss due to the slow remova l or adsorbed and structural water. There is no prominent weight loss indicating the therm al stability of the phosphate-mod ified samples. Enhanced thermal stability of phosphated anatase has been previously reported6

. The presence of phosphate ions in rhe mod ifi ed samples was further confirmed by IR spectra , whi ch showed a broad peak around 980-1080

SUGU AN ef 01. : CATALYSIS BY METAL OXIDES 2253

cm' \, which can be assigned to the P-O stretching mode of phosphate anion \4 .

The results of the ac idity/basicity measurements by Hammett indicator method are given in Tables 2-5 . Phosphate modified La20~ samples were found to be acidic except in the case where NaH2P04 buffer was used as the mod ifier. Modified catalyst still remained basic but the basicity was lower than that of pure La20 1. The concentrati on of H, P04 solution seemed to have a negli gible effect on the ac idity. ]n spite of the increase in the phosphate content , ac id ity remained more or less the same.

Unmodified CeO} was slightly acid ic with ac idi c

sites of Ho :S 7.2 . But phosphate modificat ion resul ted in the development of ac id sites of Ho between 4.8 and 7.2. Thus the acidity and ac id strength was foun d to increase upon phosphatation. In the case of ceria not much difference was fou nd in the acidity generated by H}P04 and NaH2P04 modifi ers. Even though ceria is more acidic than La20 .l the phosphated samples exh ibited reverse trend wi th phosphated La20 } being more ac idi c. This may be due to the higher phosphate content or due to the difference in the mode of adsorpti on. The acid ic sites in SnO} are present at Ho 4.8 and 7.2 and basic sires at Ho 3.3. Thus Sn02 is found to possess weakly basic and

Table 2- Ac idit y/basicity of La ~O, and phosphate modified La,O, activated at var ious tempe rat ures

S~l1llpk Temp Basicity( mmollg) Acidi ty( mmollg)

(0C) H,,? 3. 3 H,,? 4.8 Ho?' 7.2 H,,~ 3.3 H,,~4 .8

La,O .. 300 1.23 0.25 0.23 500 1.1 0 0.22 0.2 1 700 1.0 I 0.20 0.19

PO/ I La,O, (0. 1 M H, PO-l) 300 0 32 0.32 500 0.22 0.27 700 0.12 0.25

PO/ I La,Ol (0.3M aH , PO-l) 300 0.3 1 0.32 500 0.23 0.25 700 0. 16 0.2 1 300 0.27 0.38

PO-l" I La,O , (0.5M H,PO-l) 500 0.23 0.25 700 0. 18 0.23

PO/ I La,O, (0. 1 M Na H, PO-l1 0. 1 M H, PO-l) 300 1.04 0.15 0.13 500 0.88 0. 14 0.11 700 0.31 0. 13 0.05

Table 3 - Ac idity/basic ity of CeO, and phosphate mod ified CeO, activated at var ious temperat ures

Sample

CeO,

1'0/ 1 CeO, (DIM H,PO-l)

1'0/ 1 CeO, (0. 1 MN aH , PO.tl

Catal ys t

T T T PT PT PT

Temp

(OC) Hn? 3.3 300 0.08 500 0.05 700 0.03 300 0.05 500 0.03 700 0.01 300 0.06 500 0. 12 700 0.01

Basici ty( mmollg)

H,,? 4.8 Hu? 7.2 0.05 0.03 0.02

H,,~3.3

Acidity (mmollg)

H,,~4.8

0.08 0.05 0.0'+ 0.05 0.03 0.02

Tab le 4 - Acid strength di stribution of Sn01 and PO/IS nO, at var ious tcmperaturcs

Temp Basic ity Acid ity

(OC) H,,?3 .3 H,,?4.8 1-I,,?7.2 Ho5:3.3 H.,~4.8

300 0.05 0. 2 1 500 0. 10 0.08 600 0.030 O. t:l 300 0. 16 0.51 500 0.32 0.37 600 0.2 1 0.34

H,,~ 7:1.

0.37 0.35 0.30 0.55 0.36 0.25 0.55 0.38 0. 27

H,,~ 7.2 O. t:l 0. 20 0.03 0.28 0.25 0.05 0.23 0.08 0.04

1I,,~7 2

0.26 0.09 0.29 0.53 0.55 0.63

2254 INDIAN J CHEM. SEC. A, NOVEMBER 2002

Tab le 5 - Acidity/bas icity of Sn-La mixed ox ides and its phosphated analogues ac tivated at 600 DC

Sample Basici ty( mmol/g) Ac idity (millol/g)

Ho? 3.3 Ho? 4.8 Ho? 7.2 HoS, 3.3 HaS, 4.8 H"S, 7.2

T TL82 TL64 TL46 TL28

La PT

PTL82 PTL64 PTL46 PTL28

PLa(O. I)

0.42 0.32 0.26 0.25 1.0 1

0.11 0.04 0.05 0.03 0.20

ac idi c sites. Incorporatio n o f bas ic La20 , to Sn02 decreases the ac idity and the mi xed ox ides were fo und to be bas ic. The phosphated analogues were, however, acidic and fo llowed the same trend as that of pure ox ides.

The increase in ac idity upon phosphatio n can be assigned to the e lectron withdrawing effect of the phosphate group, whi ch makes the metal center more e lectron defic ient. Genera ll y, H, P04 modifi ed samples were fo und to have a greater ac id strength and ac idity than NaH 2PO~ modified ones.

Flaig Bauman ef 0115 have pro posed a mechani sm in whi ch adsorpti on occurs by an exchange process between surface hydroxy l groups and H2P04' io ns in solution, H3P04 being di ssociated o nly to the first degree at pH< 7. The fo llowing struc ture (I ) is postul ated . When NaH2PO~ is used as the modifi er. one of the OH groups is rep laced by ONa.

The second mechani sm in volves the di ssociati ve adsorpti on of H 3PO~ leading to the blockage of Lew is ac id sites and the pro posed structure is denoted as II . On heating, dehyd roxy lat ion occurs leadi ng to

OH 0

"" / 0

0.03 0. 13 0. 29 0.08 0.03 0.02 0.03 0. 19

H

0.2 1 0.44 0.53 0.39 0.24 0. 12

o~ /OH

P- o I r

O-M- O

(I)

0.34 0.52 0.53 0.37 0.32 0.25

0.63 0.55 0.5g 0. 2 1 0.34 0.30

HO OH

"" / p_ o

I o I

O-M- 0 11

(II )

bridged structures as g iven in Scheme 1.ln thi s case Bronsted ac idity is generated . When NaH 2PO~ is used the H+ of the P-OH groups arc converted into P-O Na whi ch wi ll reduce the Bronsted ac idity . Thi s ex pl ains the lowering of ac idity in thi s case.

The cata lytic acti vity was tes ted fo r benzy lati on and esterifi cati o n reacti ons and the results are g ive n in Table 6. Lew is ac id sites are believed to be more acti ve towards benzy lat ion reacti on than Bronsted sites, whereas este rif ication in liquid phase is co nsidered to be mainly cata lysed by Bronsted ac id sites .

The modifi ed La20 .\ and Sn02 sa mples showed a lesser acti vity towards benzy lati on reac tion compared

0 0

0 "- / p __ 0

"" / -- P

·H,O / "" He"1

/ / "" II .. ~ / p "" .-"" 0 0

"" /

.H,o 0 0 0 0 0 0

"" / "" / o -M

"" / o _ M o - tvl

o - -M

Scheme 1

\ '-

'-,

SUGU NAN el al . : CAT ALYSIS BY METAL OXIDES 2255

Table 6- Data o n the catalyti c ac ti vity towards benzylati on and es terification

Catalyst Surface area Benzy lation reac tion Ester ificat ion reactio n

(m2/g) Conversion %

La,O, 44.92 16.78 PLa(O. I) 75 .64 14.08 PLa 66.27 18.74 Ce02 34.80 57. 15 PCe(O I) 40.92 68.65 PCe 4 1A6 79.24 T 40.01 88.82 PT 93.08 5 1.00 T L82 82 .01 72. 17 PTL82 117 .94 74.38 TL64 63.99 92.58 PTL64 104.95 84.76 TL46 6 1.86 85. 15 PTL46 104.87 56.39 TL28 59.87 84.90 PTL28 95 .95 88.32

to pure ox ides irrespec ti ve of the higher acidity and surface area values whereas the reverse trend was observed fo r esteri ficatio n. Thi s leads to the assumpti on that the phosphate modificati on in these cases occurs by the blockage of Lewis sites thereby leav ing them inaccess ible fo r the reactant mo lecul es during benzy lat ion reac ti o n. The Bronsted sites created during the process, cata lyse the es terificati on reacti on. Thi s trend was comple te ly reversed for Ce0 2. In the case of Ce0 2, Lewis ac id sites can be assumed to be more ac ti ve resulting in a bette r acti vity towards benzy lation reactio n.

In the case of mi xed ox ides, TL64 was found to be the most acti ve towards benzy latio n. Thi s is consistent with the fac t that max imum acidity was observed for the same. The modi fied ox ides were less acti ve towards benzy latio n suggesting that the Lewi s ac id sites are weaker compared to pure oxides. However the es teri ficati o n results also sho wed the same trend with the modified oxides .

To conclude phosphati on genera lly leads to an increase in surface area and a decrease in cry stallinity and the samples are thermall y stable. Phosphation can occur lhrough adsorpti o n atco-ordinati vely unsaturated Lew is ac id sites and exchange o f OH groups by H2P04' ions. The catalyti c acti vity and sllrface acid-base properties mainly depend on the amollnt of phosph ate adsorbed and also the nature of adsorptio n.

Rate constant Co nve rsion Rate constant ( 10.7 s· 'm·2) % ( 10.7 s· ' m·2)

11.36 3 1.66 4.69 5.6 1 70.78 9. 10

8.60 1.59 67.98 17.62 1.6 1 59.59 9.8 2.04 15.19 12.30 1.82 2. 13 70.68 7.32 4 .33 65.84 7.28 3.2 1 59.52 4.2() 11 .29 69 .77 10AO 4.98 36.50 2AO 5.16 87AO 18.60 3.72 46.35 3.30 8.77 83 .99 17.00 6.2 1 73.5 1 7.69

Acknowledgement The authors wish to acknow ledge their sincere

gratitude to the CS IR , New Delhi fo r the award o f Junior Research Fellowship to H S and K S

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