Nayar, B. C.

Rao, K. Subba

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NOTESTABLE 1- MONOLAYERCAPACITIESFORGLASSYSILICA GEL AND FIBROUSSILICA GELTABLE 2- SPECIFICSURFACEAREAS OFGLASSYSILICA GEL AND FIBROUSSILICA GELCONSIDERINGTHE MOLECULESAS SPHERESof glassysilicagel and fibroussilicagelhavebeencalculatedassumingthesecondaryamineandketonemoleculesas spheres4. The detailedprocedureofcalculationshavebeengivenearlier!. TableZgivesthe valuesof sufaceareasalongwith themolecularcross-section.It is observedthat the valuesof specificsurfaceareaarenot constanteitherfor glassysilicagelorfibroussilica gel. Therefore,the assumptionthatthe secondaryamine and ketone moleculesarespheresis ificorrect. As in the caseof normalaliphaticalcohols!,normalaldehydesand amines2,the linear shape of the secondaryamine andketonemoleculesis to be takeninto corsiderationin calculatingthe surfacearea. H.e secondaryamineandketonemoleculescontaintwohydrocarbonchainsattachedto a polargroup,either>NH or>CO. Like normalaliphaticalcohols,aminesoraldehydesthe adsorbedmoleculeof the secondaryamineor ketonecanbeheld on the surfaceof thesorbentat its polar end, either perpendicularorparallelto surface. In the parallelpositionthereare two possibilities:the doublechain strip-likemoleculemay be orientedwith one chain restingonthesurfaceor bothchainsrestingon thesurface.In thesethree possiblemodesof adsorption,-thecross-sectionalareas occupiedby the molecules,andhencethesurfaceareas,will bedifferent. Now,the cross-sectionalarea of a single hydrocarbonchainis ZO'7A2 (ref.4). As thesecondaryaminesandketoneshavetwohydrocarbonchairs, tr.ecrosssectionalareasof thesemoleculesmaybe assumedto be twicethe valuefor a singlechain, i.e. 41·4A2. Assumingthis value for- the cross-sectionalareaof thesecondaryaminesandketonesar:dknow-ing their monolayercapacitie<;,thesurfaceareasofglassyandfibroussilicagelhavebeencalculatedforperpendicularmodeof adsorption. For calculating•1I!Fibrous silicagel0·01780·02210·01260·01870·0241Glassy Fibroussilica gel silica gel437 44'7382 48,7 '364 31-9366 4007353 47·5Specificsurfacearea(m2.g-1)30'5136'9024'3831-0337'41Monolayer capacity(gig of sorbent)Glassy silicagel0·1730·1740·1440·1690·179Molecularcross-sectionA2SorbateDiethylamineDi-n-propylamineAcetoneDiethyl ketoneDi-n-propyl ketoneDiethylamineDi-n-propylamineAcetoneDiethyl ketoneDi-n-propyl ketoneSorbateEvidencefor OrientedAdsorptionin theMonolayersof GlassySilica Gel & FibrousSilica Gel (SantocelC)Studieson theadsorptionof secondaryaminesandketoneson fibroussilicagel (SantocelC) andglassysilica gel revealthat thesesorbatemoleculesareadsorbedperpendicularto the surfacein thecaseofthe former and bothperpendicularand paralleltothesurfacein thecaseof thelatter.8. FIORANI, M., BOMBI, G. G. & MAZZOCCHIN,G. A., ].electroanal.Chern.,13 (1967),167.9. BENJAMIN, L. & GOLD, V., Trans. Faraday Soc., 50(1954),8, 797.10. BOOTHE,J., j. chem.Phys., 19 (1951),1615.11. GLUECKAUF,E., Trans. Faraday Soc., 60 (1965),1637.K. SUBBARAO & B. C. NAYAR*Departmentof Inorganic & Physical ChemistryIndian Institute of Science,Bangalore560012Received15 September1976;accepted7 January 1977INVESTIGATIONS on sorption and desorptionin glassysilicageland fibroussilicagel (SantocelC) of normalalipllatic alcohols!,normalalipllaticaminesandnormalaliphaticaldehydes2haveshownthat thesestraightchainlinearmoleculesareheldin the adsorbedhyer perpendicularto the surface.It wasfelt that a studyof sorption-desorptionwithsorbateshaving two hydrocJ.rbonchains, as insecondaryaminesC1.ndketones,wouldthrowfurtherlighton thenatureof orientationof adsorbedmole-culesonthesurfaceof silicagels. With thisin viewsorptionanddesorptionofdiethylamine,di-n-propyl-amine, acetone,diethyl ketone and di-n-propyl-ketone havebeen studied and the results arepresentedin this paper.Glassysilica gel was preparedaccordingto themethoddescribedearlier!.Fibroussilicagel (Santo-eelC) was obtainedas a gift samplefrom MIsMonsanto Co., USA. The sorbatesused were:diethylamine(Riedel),redistilled,b.p. 56.0°;di-n-propylamine(Koch-Light), redistilled,b.p. 110.7°;acetone(BDH),redistilled,b.p.56'1°; diethylketone(E. Merck),redistilled,b.p. 101'7°;and di-n-propylketone(Fluka), redistilled,b.p. 144·Zo.Sorptionsand desorptionswere studiedat 35°in an air thermostatemployingMcBain-Bakrspringbalance3• Silica gelswereactivatedat 450°for 4hr beforeuse. Duplicateexperimentswere con-ductedand the resultsof one set of experimentsare presented. In eachsystemthe 1st, Znd and3rdsorption-desorptionhysteresisloopsweredrawn.For calculations,the Znd cycleshavebeentakeninto consideration.Equilibrium values were re-cordedeverythreehourly, thoughcompleteequi-libriumhad beenattainedafter 1·5hr.ApplyingtheBET equationthemonolayercapa-cities have been calculatedand the values arepresentedin Table 1. The specificsurfaceareas*Present address:Mis SILANOt., 9th Mile, Old MadrasRoad, Bangalore560049(to whom correspondencemay beaddressed).553INDIAN J. CHEM., VOL. 15A,JUNE 1977TABLE 3 - SPECIFIC SURFACE AREAS (IN m2g-l) OF GLASSY SILICA GEL AND FIBROUS SILICA GELCONSIDERING PERPENDICULAR AND PARALLEL MODE OF ADSORPTIONorbateDiethfamineDi-n- opylamineAcetoeDieth I ketonei-n- opylketonePerpendicularmodeof Parallelmodeof adsorptionadsorption ABGla.ssyFibrous----------- ~-gelgelGlassylassyFibrousgelgelgel59360·675477-15085,242954 3393'4648 ·0618565,91 001 8844282 63912 793(A) Onechainrestingon the surfa.ce.(B) Two chainsrestingon the surface.the aralIel mode of adsorption, the molecule isconS ered as a rectangular rodI. Knowing thethic ness of the hydrocarbon chain, the volumeand he length, the molecular cross-sectionalareasand ence the surface areas are found out. Thesurfa e areas thus calculated for both glassy andfibro s silica gels for the three modesof orientationare iven in Table 3.It s observedfrom Table 3 that the surfaceareasfor brous silica gel calculated for only perpendi-cula adsorptionarenearly the same. This indicatesthat he adsorption of secondaryaminesand ketoneson t e monolayer of fibrous silica gel is of orientedtype and that these linear molecules are heldperp ndicular to the surface in the monolayer.In the case of glassy silica gel, the surface area(calc lated for perpendicularadsorption)with respectto -n-propylamine is less than the value withresp ct to diethylamine. As far as the ketones areconC rned, the value decreasesin the order acetone>d -n-propyl ketone. This decreasein the surfacearea from the lower member to the higher memberof t e secondaryamine and ketone series is similarto t e behaviour of the different members of thenor al aliphatic alcoholsI,normal aliphatic aminesand aldehydes2, Adsorption of normal alcohols,ami es and aldehydeson glassy silica gel has beencan uded to be of mixed type, i.e. both perpendi-cula and parallel to surface. It follows thatadso ption of the secondary amines and ketoneson gassy silica gelmust also be of the mixed type.F m the results obtained, it can be concludedthat (i) the assumption of cross-sectional area ofthe econdary amine and ketone moleculesas twicethe cross-sectional area of the primary normalalco ols, amines or aldehydes is justifiable, (ii)adso ption of the secondaryamines and ketonesonthe urface of glassy and fibrous silica gels is oforie ted type, i.e. either perpendicular or parallelto s rface or both, (iii) adsorption on the surfaceof fi rous silica gel is entirely of perpendicular typeof a sorption, and (iv) adsorption on glassy silicagel urface is of mixed type, i.e. partly perpendi-cula and partly parallel to surface. These generalcan lusions are supported by the fact that glassysilic gel is microporous in structure and fibroussilic gel macroporous.T e authorsare thankful to the CSIR, New Delhi,for nancial support.554References1. SUBBA RAO, K. & BHAGWAN DAS, J. colloid interf. Sci.,32 (1970),24.2. NAYAR, B. c., Investigations on sorption-desorptionhysteresis,Ph.D. thesis,Indian Instituteof Science,Bangalore,1973.3. SUBBA RAO, K. & NAYAR, B. c., J. colloid interf. Sci.,38 (1972),45.4. MOELWYN-HuGHES, E. A., in Kinetics of reactions2nsolution (ClarendonPress,Oxford),1947,7.Acidity & CatalyticActivityof ModifiedZeolite··XL. D. SHARMA, R. P. BADONI, P. C. BORTHAKUR &M. C. UPRETIRegionalResearchLaboratory,J arhatReceived2August1976;accepted19November1976Surfaceacidityand acid stren~thdistributionofvariousforms of zeolite-Xhavebeendeterminedbynon-aqueoustitration technique.The activity foro-xyleneisomerizationhasbeenfoundtodependupontheconcentrationof stron~acidsitesin thezeolite.Na-X containsonly weakacid sitesand is inactivewhereasCa-XandH-X possessin~stron~eracidsitesareactivefor isomerizationofo-xylene.THE acidity of X-zeolites, which exhibit excellentacidic propertiesand possesscarboniogeniccata-lytic activityl-3 has been reported recently4but theacid strength distribution of decationatedX-zeoliteshavenot beenstudiedso far. A completedescriptionof acidic properties of a solid catalyst requires thedeterminationof the nature of acid sites (Bronstedor Lewis type) as well as the acid strength distri-bution pattern. This communication presents ourresults on the determination of the acid strengthdistribution of decationated X-zeolites and theircatalytic activity for a-xylene isomerization.Linde 13 X (sampleA) and 10 X (sampleC) werecommercialUnion Carbidemolecularsievesin powderform. The RRL-X (sample E) in Na-form wassynthesized in our laboratory from paddy husk5•The ammoniumforms of the zeoliteswere preparedby the conventionalion-exchangetechniqueusing an.aqueoussolution of NH4N03 at 80°. The decation-ated samples Gr> G2, G3 and G4 were preparedby subsequent heat treatment of the ammonium

Evidence for Oriented Adsorption in the Monolayers of Glassy Silica Gel & Fibrous Silica Gel (Santocel C)

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Evidence for Oriented Adsorption in the Monolayers of Glassy Silica Gel & Fibrous Silica Gel (Santocel C)

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