rIndian Journal of ChemistryVol. 28A, January 1989, pp. 6-11

Sorption of n-Butylamine on Ferric-Exchanged Y-Zeolites

S J KULKARNI & S B KULKARNI'National Chemical Laboratory, Pune 411 008

Received 11 February 1987; revised 31 December 1987; accepted IS April 1988......The sorption of n-butylamine (n-BA) on ferric exchanged Y zeolite catalysts has been studied. Freundlich equation is

applicable to the sorption data of ferric-exchanged zeolites. The amount of n-BA retained on zeolites under vacuum atdifferent temperatures has been reported which predicts the trend in the acidic strength. FeNa Y (46) zeolite shows maxi-mum number of strong acidic centres. The adsorption data for FeN aY zeolite, activated at 500°C, show migration of fer-ric ions from supercages to sodalite cages or hexagonal prisms, thereby lowering the acidity of zeolite. The isostericheats of sorption of n-BA on FeNaY zeolites have been determined which support the heterogeneity of surface and mi-gration of ferric ions after heating at 500°C.

Attempts I - 5 have been made to determine the num-ber and strength of acidic centres in supercages ofzeolites through the measurements of base adsorp-tion. Ammonia molecules are sorbed in the super-cages and slowly migrate into sodalite units, while n-butylamine (n-BA) and pyridine do not enter intosodalite cages of Yzeolites. The adsorption n-BA isuseful in determining the number and strength ofacidic centres in supercages of Yzeolites where thecatalytic reactions occur.

From the absorption measurements of CO2 andNH3 on various modified zeolites, Coughlan and co-workers's? calculated the thermodynamic quantitiesand reported the sequence in the strength of adsorb-ate-absorbent interaction. Steinburg et a/.R measuredheats of adsorption of some basic molecules on ionexchanged zeolites by calorimetric method. Mishinet a/.Y.lo reported sorption isotherms and heats ofsorption of n-BA on dealuminated Yzeolites. Theyconcluded that the silanol groups formed duringdealumination are not strong acidic centres and the

, number of sorption centres (cations and protonatedcentres) is reduced due to dealumination. In this pa-per, we report the results of a systematic study of /1-

BA sorption on ferric-exchanged Yzeolites,

Materials and MethodsFerric-exchanged Y-zeolites were prepared by

treating Linde NaY zeolite (SK-40, Lot No.96805000) with aqueous solutions offerric acetate-acetic acid at about pH 4 and 30°C. After exchangeall the samples were washed with hot distilled wateruntil the washings were free from acetate ions. Thesamples were oven-dried at 120°C overnight and

tNCL Communication No. 2707.

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/'stored in a constant humidity atmosphere over a sat-urated solution of ammonium chloride. The ferric-exchanged samples were yellow in colour withbrownish tinge at higher exchanges. The X-ray dif-fraction patterns showed that all the samples werecrystalline. The chemical compositions of the sam-ples were determined by the conventional gravimet-ric analysis; sodium by flame photometry and ironby atomic absorption spectroscopy and titrationmethod. The chemical compositions and lattice par-ameters are given in Table 1.The detailed character-isation data were discussed elsewhere II. It was ob-served that the framework structures of FeNa Y (61)and FeNaY (75) were distored during ion exchange.

ProcedureThe isotherms for the sorption of n-BA were mea-

sured in the temperature range of 100-350°C by thegravimetric method using a calibrated silica springbalance. Sample (100 mg) in the form of a pellet wasused and activated under high vacuum ( -10-6 torr)at 350°C for 4 hr to a constant weight and brought tothe desired temperature. The absorbate liquid bulbwas rendered air-free and was separately thermos-tated. The pressure of the adsorbate was measuredaccurately with a cathetometer. The amount sorbedwas estimated from change in weight of sample afterequilibrium was obtained. The adsorption isothermswere obtained by progressive increase of vapourpressure and noting the extent of adsorption. The re-producibility was checked in some experiments. Theamount of adsorbate (n-BA) retained was estimatedby evacuating the sample under high vacuum ( -10-5 torr) for 2 hr to a constant weight at differenttemperatures. The rates of sorption at 25°C wereobtained and equilibrium n-BA sorption capacitiesare given in Table 1.

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rKULKARNI et al.: SORPTIONS OF n-BUTYLAMINE ON Y-ZEOLITES

Zeolite

Table l-Unit Cell Composition and n-BA Sorption Capacity

Unit cell composition" Latticeparameter

alA

n-BAmolecules

per unit cellat 25°C and70mm Hg

44.746.647.129.629.942.245.2

NaYFeNaY (37)*FeNaY (46) ••FeNaY (61)FeNaY (75)FeNaY (82)FeNaY (46, 500°C, 5 hr)"

NaS? (AI02)s? (SiOl)13S (268 HP)Fe, Naw H3(AI02),? (Si02kls (269 H20)FeKA Na2K" H3(AI02)S? (Si02)l.ls (272 HP)FellI>Naly u, (AI02)s? (Si02)l.ls (251 HP)Feu.s Nau H35(AI02),? (Si02)l.ls (228 HP)Fe"" NalO.e(AI02)s? (Si02)l.ls (252 H20)

(a) Figures in parenthesis indicate per cent ion-exchange.(b) FeNaY (46) was heated at 500°C for 5 hr.

24.60124.58924.61724.95424.57324.61424.60

Results and DiscussionTypical families of sorption isotherms of n-butyla-

mine (n-BA) on a series of ferric-exchanged Y-zeo-lites are presented in Figs 1 and 2. The trend in thenumber of n-BA molecules sorbed per unit cell at350°C is NaY"" FeNaY (46» FeNaY (37» FeNaY(82» FeNaY (75). The isotherm 4:>rNaY showedslower increase in sorption capacity compared tothat in FeNa Y samples. Due to strong Na + - n-BAinteraction NaY adsorbs larger amount of n-BA.The rearrangement of molecules should take placeto achieve the saturation capacity at a particulartemperature. On the other hand, in the case of Fe-NaY samples, the n-BA molecules are sorbedstrongly on Fe3 +, Na + and H ". The sorption of n-BAon Fe3+ and H+ is irreversible (up to 350°C) andthe sorption on Fe3 + may have specific orientation.The FeNaY (75) adsorbs smaller number of n-BAmolecules as compared to NaY and FeNaY (46) be-cause of distortion of framework structure as ob-served from the X-ray diffraction and water sorptionstudies II. The unit cell composition, the lattice par-ameter representing crystallinity and n-BA sorbedon the catalysts at 25°C and 70 mm Hg of n-BA, aregiven in Table 1. In the case of FeN aY (46), as a typi-cal example, the number of n-BA molecules sorbedat 25°C is 47.1 molecules per unit cell. As a first ap-proximation, we may consider that the n-BA sorp-tion is localized. The total number of cations in Fe-NaY (46) is 40.0 (8.4 Fe3+ + 28.6 Na + + 3H+) perunit cell as given in Table 1.The number of cations inthe supercages is 20.6 ( - 6 Fe' + + 11.6 Na + + 3H +)per unit cell' I while - 2.4 ferric and 18 sodium ionsare in sodalite cages (site r and II') or hexagonalprisms (site I). The ferric ion being trivalent shouldbe shared by three aluminium tetrahedra. For geom-etric reasons, this results into AI-tetrahedra having

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local anionic charges which hydrolyse water mole-cules to increase the H + ions, and consequently theBrensted acidic centres. The infrared studies 10

showed that n-BA molecules interact strongly withcations like sodium in Y-zeolite. The trend in thenumber of n-BA molecules sorbed at 25°C, i.e. satu-ration capacity, is as follows: FeNaY (46»FeNaY(37» NaY> FeNaY (82» FeNaY (75)""FeNaY(61). The zeolites FeNaY (61) and FeNaY (75) ad-sorb smaller number of n-BA or water molecules ascompared to NaY or FeNaY (46) because of distor-tion of zeolite framework structure as observed fromthe sorption isotherms, XRD patterns and watersorption studies II.The framework distortion may bedue to the lowering of pH from 4 to - 3.7 during the'ion-exchange process.

The following sorption equations were tested forthe data: Langmuir, Freundlich, Temkin, Sipps andDubinin equations. Langmuir equation though fitswell for the data (Fig. 3b), it is not applicable inprinciple due to heterogeneity of the surface. A typi-cal Freundlich plot for n-BA sorption on FeNaY(37) is depicted in Fig. 3a. Freundlich equation is ap-plicable to all the ferric-exchanged Y-zeolites. Line-ar plots with the other sorption equations I~ couldnot be obtained. The analysis suggests that the n-BAsorption is localized and heterogeneity in the surfacepotential exists.

Figure 4 shows the amount of n-BA retained onzeolites under high vacuum (- 10-:' mm Hg) as afunction of temperature. The data indicate the trendin the strength of n-BA interaction with the differentsamples at various temperatures. At about 200°C,the number of irreversibly sorbed n-BA on Na Y andFeNaY (46, 500, 5) decreased than that for FeNaYcatalysts. Above 200°C, the trend in the amount ofn-BA retained was as follows: FeNaY (46» FeNaY

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INDIAN J CHEM, JANUARY 1989

5or-----------------------.-----------------------~

(3)ClJU

'c 50~----------------------~~----------------------~:::l

°O~---------.5~O.---------~O----------~5~O---------1-l00P /mmHg

Fig. I-Sorption isotherms for n-butylamineon (1) NaY,(2) FeNaY (37),(3) FeNaY (46),(4) FeNaY (75)

40~------------------------r-----------------------1

(2 )

..u

50 0 50

p/ mm Hg

Fig. 2-Sorption isotherms for n-butylamine on (I) FeNaY (82),(2) FeNaY (46, 500, 5)

(82»FeNaY (37»FeNaY (61»FeNaY(75»NaY. This indicates that the interaction of n-BA is stronger with ferric ions and acidic protonsavailable in the supercages than that with sodiumions. The maximum number of strong interactingcentres were present in FeNaY (46) among the zeo-lites under study. The total number of interactingcentres responsible for n-BA chemisorption wasmore in FeNaY (46) and NaY as compared to those

in other samples. We may attribute the number of n-BA molecules sorbed approximately corresponds tothe number of sorption centres. Comparativelysharp decrease in irreversibly sorbed n-BA on NaYat about 200°C leads to the conclusion that most ofn-BA molecules sorbed on sodium ions are de-sorbed up to 200-250°C.

In order to evaluate the effect of thermal treat-ment, FeN aY (46) was heated at 500°C for 5 hr in air

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rKULKARNI et al: SORPTIONS OF n-BUTYLAMINE ON Y-ZEOLITES

o· 2

2 :3 4Inp

(.f )o

40 6020P mm Hg

Fig. 3-Freundlich plot (a) for ';-butylamine sorption onFeN aY (37) and (b) the corresponding Langmuir plot

and designated as FeNa Y (46,500, 5). The rate of n-BA sorption and the nature of sorption isothermsfor FeNaY (46,500,5) were found to be similar tothose for untreated FeNaY (46). (Fig. 2). However,the amount of n-BA retained under high vacuumwas much lowerfor FeNaY(46, 500, 5) than that forFeNa Y (46) as shown in Fig. 4. This is due to the lossof strong acidic centres by dehydroxylation and mi-gration of ferric ions from supercages to sodalitecages or hexagonal prisms occupying sites inaccessi-ble to. n- BA molecules II.A comparison of irreversi-bly sorbed n-BA (Fig. 4) on NaY, FeNaY (46) andFeNa Y (46, 500, 5) indicates that the nature of sorp-tion centres is identical in NaY and FeNa Y (46 5005) except some more acidic centres are pres~nt i~FeNaY (46, 500, 5). On the other hand, for FeNaY(46) the irreversibly sorbed n-BA molecules are

~..Q.

30

~20

::'"u•.

"0::E 10

200 300T~mp/~c

Fig. ~-Retention of sorbed n-butylamine sorption un-der high vacuum (-10-5 torr) on (1) NaY, (2) FeNaY(37), (3) FeNaY (46), (4) FeNaY (61), (5) FeNaY (75), (6)

FeNay(82)and(7)FeNay(46,500,5)

larger in number, throughout the temperature rangeup to - 350°C. .

Chemical affinity of adsorbed phaseThe change in chemical potential of n-BA be-

tween gas phase at standard pressure (Po = 760 Torr)and adsorbed state at equilibrium pressure P, is giv-en by

~~=RT (~)

. The mag.nitude of a decrease in chemical poten-tial, - ~~, IS a measure of chemical affinity of n-BAfor a zeol~te sample. Figure 5 shows the plot of - ~~as a function of coverage of n-BA molecules per unitcell at, 200°C. The affinity sequence, throughouttemperature range is NaY>FeNaY (46»FeNaY(46,500, 5»FeNaY (37»FeNaY (82»FeNaY(61) > FeNa Y (75). The trend in affinity is similarthroughout the coverage range as that for equilibri-um adsorption capacity of the zeolites.

Isosteric heat of adsorption.The isosteric heats of adsorption, qst, were ob-

tamed by Clausius-Clapeyron equation's.

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20

-1oE

I--j

sc lEi<,~<f1

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INDIAN J CHEM,JANUARY 1989

8 '----- __ -'- -----l _

20 30Molecules per u.c

Fig. 5-Comparison of affinity curves

Table 2-Isosteric Heats (qst)of n-BA Sorption on Zeo-lites

Zeolite

NaYFeNaY (37)FeNaY(46)FeNaY (61)FeNaY (75)FeNaY (82)FeNaY(46,500,5)

q" (kJ mol- I) at coverages (n-BA molecules!unit cell)

15 18 20 25 30 3582.4 89.5 72.8 67.0 41.8

136.9 177.8 93.3 84.1 60.7136.8 187.4 124.7 46.960.3 20.9

64.9 37.7 20.9171.6 146.0 82.8 56.977.0 79.9 74.1 79.9 52.3

The successive adsorption isotherms were used todetermine qst.The values of qst are given in Table 2.In the case of NaY, at coverage of 18 molecules perunit cell, the heat of n-BA adsorption is 82.4 kJmol- " which decreases to 41.8 kJ. mol- 1 at cover-age of 35 molecules per unit cell. Due to strong inter-action between sodium ion and NH2 group of n-BA9

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the heat of n-BA adsorption is higher as comparedto those for other hydrocarbons". For FeNaY (37),FeNaY (46) and FeNaY (82) at lower coverages (15-20 molecules per unit cell) qSIis above 170 kJ mol- I,which is much higher than that for NaY. The value ofq'l for FeNa Y (46) at coverage of 20 n-BA molecules

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per unit cell attains a maximum value of 187.4 kJmol-I. In the case of FeNaY (61) and FeNaY (75),the adsorption data and low qst values indicate thatthe framework structure is distorted to a great extentduring ion exchange and this is also confirmed byXvray!' and water adsorption'? data.

The presence of [Fe(OH)(H20)2+] in supercageshas been established for untreated ferric-exchangedYzeolites based on spectral and thermal data II. Af-ter activation at 350°C, the coordinated water mole-cules are removed and (Fe(OH))2+ interacts with theframework at site-Il in tetrahedral coordination. At400°C, dehydroxylation of hydroxyl groups att-ached to ferric ions takes place" and simultaneous-ly, ferric ions migrate to inaccessible sites (sites I, I'and II'). It is also evident that up to 50% ion-ex-change, ferric ions preferably occupy site-Il in su-percages, during ion-exchange.

In the case of thermally treated FeNaY (46, 500,5) sample, the heat of adsorption is 75-80 kJ mol- 1

at coverages of 15-25 n-BA molecules per unit cell,which decreases to 52 kJ mol : 1 at coverage of 30molecules per unit cell. Due to thermal treatment,Bronsted acidic centres are destroyed by dehydrox-ylation as confirmed by the TG analysis'rand ferricions migrate into sodalite cages or hexagonal prisms,the sites inaccessible to n-BA molecules. The qst(Table 2) of NaY and FeNaY (46) are comparablebecause in both the cases n-BA molecules interactonly with sodium ions in supercages. Though ad-sorption isotherms and chemical affinity value forFeNaY (46) and FeNaY (46, 500, 5) are approxi-mately same, the qst values and the amount of n-BAretained under vacuum (Fig. 3) indicate that FeNaY(46) has stronger interacting centres. Migration offerric ions into sodalite cages and hexagonal prismsis probably an irreversible process with respect tohydration and n-BA adsorption, due to the forma-tion of Fe3+ - 0 - Fe3+ linkages in sodalite units!",The DTA did not give any support for the existenceof hexaaquo complex of ferric ion after thermaltreatment II.The variation in qst thus helps in under-standing the heterogeneity in surface potential.

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Change in entropy of n-BA sorptionThe entropy changes for n-BA sorption on ferric-

exchanged Yzeolites are given in Table 3, as a func-tion of coverage. The values of tlH and tlS for NaYare approximately constant over a wide range ofcoverage, indicating the homogeneous nature of thecatalytic surface of NaY. The tlS values for FeNa Y(37), FeNaY (46) and FeNaY (82) are 261.2, 278.0and 209.3 JK - mol- 1 at the coverage of 18-20 n-BAmolecules per unit cell. The higher values of tlS in-

dicate strong and localized chemisorption of n-BA

KULKARNI et al.: 50RPTION5 OF n-BUTYLAMINE ON Y-ZEOLITE5

Table 3-Entropy Changes (~Sin JK - I mol-I) of n-BASorption

Zeolite 5, at n-BA coverage (molecules/unit cell)

18 20 2S 30NaY IOS.1 121.8 107.6 108.8FeNaY (37) 261.2 130.2 142.9 119.3FeNaY (46) 183.8 278.0 217.1 7S.8FeNaY (7S) 110.1 26.0FeNaY (82) 209.3 114.3 96.3FeNaY 100.0 98.0 124.3(46, SOO,S)

on ferric ions and Bronsted acidic centres. The mo-bility of n-BA molecules is restricted by the presenceof [Fe{OI-l))+ 2 groups in the supercages. The varia-tion in !'!..S also supports the electrostatic hetero-geneity of the surface. The low values of ( - )!'!..S forFeNaY (75) may be due to distortion of the frame-work during ferric exchange.

The !'!..S values for thermally treated FeNa Y (46,500, 5) zeolite are comparable to those of NaY, andindicate that on thermal treatment, ferric ions migr-ate to inaccessible sites in the soda lite cages and hex-agonal prisms and some Bronsted acidic centres aredestroyed. As <I result, n-BA molecules interactmainly with sodium ions in supercages.

ConclusionBasic molecules like n-butylamine (n-BA) are

chernisorbed on cations (Na + or Fe{IIl)) and acidicprotons in supercages of Yzeolites, The equilibriumsorption values show direct proportionality to num-

ber of interacting centres, including acidic protons,while retention of n-BA sorption under vacuum andheats of sorption data indicate the trend in thestrength of sorbate-sorbent interactions. Among theferric-exchanged Yzeolites, FeNa Y (46) has themaximum number of strong active centres includingacidic protons and is thermally stable. The retentionof n-BA under vacuum support ferric ion migrationinto sodalite cages and reduced acidity after heatingFeNaY (46) sample at 500°C for 5 hr in air.

AcknowledgementWe are thankful to Dr P Ratnasamy, for helpful

discussions and encouragement.

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11 (a) Kulkarni 5 J & Kulkarni S B, Thermochim Acta, 56(1982)93.

(b) Kulkarni S J, Dongare M K & Kulkarni S B, J chem SacFaraday I, (1981) 3019.

12 Kulkarni S J, Ph.D. Thesis, Pune University (1980).

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