Self-organising aqueous suspension of layers of cationic montmorillonite and anionic mixed metal layered hydroxides

�MMLH. after thermal treatment at about 5008C gives active and well-dispersed layers of metal oxides over montmorillonite

matrix exhibiting enhanced reactivity with gaseous reactants. Increased absorption of H2S by ZnO supported on montmorillonite

matrix is given as an exampl

AYRAL , ANDRE

BHATTACHARYYA , K G

DUTTA , D K

GOSWAMI , R L

IR@NEIST - North East Institute of Science and Technology (CSIR)

November 2000 .Materials Letters 46 2000 105–108www.elsevier.comrlocatermatletNovel synthesis of active metal oxide surface from aself-organising system of inorganic solidsRajib L. Goswamee a,), Andre Ayral b, Krishna G. Bhattacharyya c, Dipak K. Duttaa Material Science Di˝ision, Regional Research Laboratory, Jorhat, Assam 785006 Indiab Laboratoire des Materiaux et Procedes Membranaires, ENSCM, Montpellier, Cedex 5, 34296 France´ ´ ´c Department of Chemistry, Gauhati Uni˝ersity, Guwahati, Assam 781014 IndiaReceived 23 November 1999; received in revised form 10 May 2000; accepted 11 May 2000AbstractSelf-organising aqueous suspension of layers of cationic montmorillonite and anionic mixed metal layered hydroxides .MMLH after thermal treatment at about 5008C gives active and well-dispersed layers of metal oxides over montmorillonitematrix exhibiting enhanced reactivity with gaseous reactants. Increased absorption of H S by ZnO supported on montmoril-2lonite matrix is given as an example. q 2000 Elsevier Science B.V. All rights reserved.Keywords: Clay composites; Layered hydroxides; Montmorillonite thixotropic suspension; Active oxide surfaces; DesulphurisationNegatively charged layers of montmorillonite claywhen contacted with positively charged layers of .mixed metal layered hydroxides MMLH of hydro-talcite type in aqueous medium gives a highlythixotropic suspension. This remarkable thixotropyw xarising out of the gel formed by self-organised 1bridges of positive MMLH layers between two nega- .tively charged montmorillonite layers Fig. 1 pro-vides a homogeneous distribution of the former overthe latter. Such an interaction of montmorillonitewith other inorganic solids like g-alumina, silica gel,zeolite, etc. cannot be expected as they do not havepositive surface charge.The positive layer charge of MMLH is higher w x.than 2.4 to 4.1 meqrg anion exchange capacity 2) Corresponding author. Tel.: q91-376-370121; fax: q91-376-370011.the negative layer charge of montmorillonite 0.7 to.1.5 meqrg cation exchange capacity , therefore 1 gof the former can electrostatically neutralize morethan 1 g of the latter. Presently, the thixotropicproperty of MMLH–montmorillonite interaction isadvantageously utilized to formulate novel drillingw xfluid for petroleum exploration 3,4 . However, apartfrom this, no report has been made so far about theapplication of MMLH–montmorillonite interactionto design advanced materials for application in otherfields.MMLH on thermal decomposition at about 5008Cgives a bivalent oxide having the layered morphol-w xogy of the precursor 5 but at higher temperatures, aspinel is formed. Due to their layered nature, theseoxides are expected to show high surface area andhence, could be highly reactive for different gas solidw xreactions than granular oxide particles 6 . One of theindustrially important reactions reported for such asystem is the reaction of H S gas with highly dis-200167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. .PII: S0167-577X 00 00150-6( )R.L. Goswamee et al.rMaterials Letters 46 2000 105–108106Fig. 1. Schematic diagram of formation of active metal oxide .ZnO over montmorillonite matrix from MMLH and their reactiv-ity with H S.2persed ZnO particles derived from thermal decompo-w xsition of Zn–Al type MMLH 7–9 . In the presentwork, the synthesis of highly dispersed and activeZnO over montmorillonite matrix and its interaction .with H S gas at high temperature about 6008C has2been studied. .Ca-montmorillonite Mrs LOBA Chemie, India ,w  . xand Zn Cr OH ClPH O synthesised by following2 6 2w xa reported method 10 , were mixed thoroughly in1:1 weight ratio. The mixture was then dispersed in .water under a very high shearing 14,000 rpm andwas set aside for hydration. After about 7 days, thehydrated slurry was carefully dried at 608C andsubsequently calcined at different temperatures viz.5008C, 7508C, 9508C at a heating rate of approxi-mately 38Crmin in an electric muffle furnace underoxygen atmosphere.Sulphurisation experiments were carried out bypassing H S gas at a flow-rate of 40 cm3rmin2through a 2.5-cm length plug of 5.0 g powder -150.mm size of MMLH–clay composite calcined at5008C. This plug was placed in a 1.5-cm diametersilica tube at 6008C for 60 min in a cylindricalfurnace. The flow of hydrogen sulphide gas wasmaintained during cooling and was continued till thetemperature of the reactor came down to 2008C.In order to compare the efficacy of the supportedsystem with a non-supported one, a neat sample ofZn–Cr MMLH was also calcined at 5008C in asimilar way as that of calcined MMLH–clay com-posite. 5.0 g of the calcined product particle size.-150 mm was subjected to sulphidisation reactionwith H S similar to that of the supported system as2described above. Quantitative estimation of sulphuradsorbed in the calcined products of Zn–Cr MMLH .neat and Zn–Cr MMLH–montmorillonite weredone gravimetrically by converting S2y to BaSO4w x11 . The active ZnO surface can be regeneratedTable 1Main crystalline phases formed at different temperatures the corresponding powder diffraction files or literature reference numbers are.given in bracketsSample: Zn–Cr MMLH: Crystalline phases.montmorillonite 50:50 wt.% . w  . x w xDried at 608C Ca-montmorillonite PDF 13-135 , Zn Cr OH ClPH O 102 6 2 .  .Calcined at 5008C Degraded Ca-montmorillonite PDF 13-135 and poorly crystalline ZnO PDF 36-1451 .  .  .Sulphurised at 6008C Cr ZnS PDF 16-507 , degraded Ca-montmorillonite PDF 13-135 , ZnS PDF 39-13632 4 .  .Regenerated at 5008C Degraded Ca-montmorillonite PDF 13-135 , ZnO PDF 36-1451 .  .  .Calcined at 7508C Degraded Ca-montmorillonite PDF 13-135 , ZnO PDF 36-1451 , ZnCr O PDF 22-11072 4 .  .  .Calcined at 9508C Degraded Ca-montmorillonite PDF 13-135 , Zn SiO PDF 24-1469 , ZnCr O PDF 22-11072 4 2 4( )R.L. Goswamee et al.rMaterials Letters 46 2000 105–108 107 .  .Fig. 2. a SEM pattern of calcined MMLH–clay composite and b total EDXA pattern of the sample.from the sulphurised product by heating the latter at .5008C for 1 h in air Table 1 .Fig. 3. XRD pattern of sulphurised product, A sZnO, BsZnS,CsCr ZnS .2 4In the present study, the bivalent, microfine, lay- .ered metal oxide ZnO formed is situated over theceramic matrix of calcined clay and consequently,their reactivity to gaseous reactant is highly en-hanced by the exposed surfaces. The temperatureinvolved in thermal decomposition of MMLH isextremely important because at higher temperature .around 9508C , active metal oxide begins to attack .silica layer of the clay forming Zn SiO Table 12 4which is an inactive species. MMLH calcined at7508C gives segregated phases of ZnO and ZnCr O2 4 .Table 1 , and further lowering the temperature toabout 5008C, a broad XRD peak at around 368 2u.  .CuK of ZnO is obtained Table 1 . The broadnessaof ZnO peak is attributed to the small size of theZnO crystallites and also to the presence of Cr O in2 3the lattice of ZnO. A similar observation was alsoindicated when Al O –MgO solid solution was ob-2 3tained on thermal decomposition of Mg–Al typew xMMLH 12,13 . .Fig. 4. a SEM of calcined Zn–Cr MMLH–clay composite after sulphurisation showing well-grown crystals of ZnS over montmorillonite .matrix and b EDXA of selected area of sulphurised product.( )R.L. Goswamee et al.rMaterials Letters 46 2000 105–108108Table 2Sulphur retention ability of MMLH–clay composite and neatMMLHSample Sulphurisation Amount ofno. condition sulphur inthe product .  .Temperature 8C Time h .wt.%rwt.a1 600 2 13.9%a2 600 2 13.6%a Initial composition of samples No. 1: Clay:MMLH 0:100.  .wt.% ; No. 2: Clay:MMLH 50:50 wt.% .The layered metal oxides formed at 5008C is seen .from scanning electron microscope SEM in Fig. 2aand are projected like small display boards from thesurfaces of montmorillonite layers. Corresponding .energy-dispersive X-ray analysis EDXA patterns .Fig. 2b confirm their chemical composition. Due totheir layered projections, these oxides are highlyreactive. Reaction of H S at 6008C, with such active2oxides derived from Zn–Cr MMLH produces ZnS asindicated by the characteristic XRD pattern of the .  .same Fig. 3 . SEM Fig. 4a shows well-growncrystals of ZnS over the ceramic matrix of montmo-rillonite. EDXA confirms the composition of these .particles as ZnS Fig. 4b . Amount of sulphur chemi-cally adsorbed in calcined products of Zn–Cr MMLH .neat and Zn–Cr MMLH –montmorillonite .MMLH:clay 50:50 wt.% were found to be 13.9% .  .and 13.6% wt.% , respectively Table 2 whichindicates clearly that the latter showed almost twicethe amount of sulphur adsorption than the former.The self-organised aqueous thixotropic suspensionof cationic montmorillonite and anionic MMLH afterthermal treatment gives exfoliated layers of metaloxides over montmorillonite matrix which are veryreactive to gaseous reactants. The technique has beenutilized to enhance by almost two times the absorp-tion of H S over ZnO layers derived from Zn–Cr2MMLH.AcknowledgementsThe authors are grateful to Dr. P.C. Borthakur,Mr. B.P. Baruah, Scientist R.R.L. Jorhat for fruitfuldiscussions and Director R.R.L. Jorhat India forpermission to publish the work.Referencesw x  .1 J. Maselko, Mater. Sci. 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Novel synthesis of active metal oxide surface from a self-organising system of inorganic solids

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Novel synthesis of active metal oxide surface from a self-organising system of inorganic solids

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