Hydrolysis of ZnO in acidic pH provides a facile route for synthesis of layered double hydroxides

(LDH)bearingZn2+withCr3+andAl3+withouthavingcarbonateionintheinter-layer position.The

thermal decomposition of the prepared LDH show that with the increase of Cr3+ content in the

systemthereisanincreaseinthethermalstability ofthecompounds.IncaseofZn–CrLDHthereis

a non-mass loss transition around 420°C. Segregation of parent LDH structure to a bivalent oxide

and a spineltakesplacein both Zn–Cr and Zn–Al LDH only at temperatures above 550°

BARUAH , R K

BHARALI , P

GOSWAMI , R L

SAIKIA , R

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

Journal of Thermal Analysis and Calorimetry, Vol. 78 (2004) 831–838A COMPARATIVE STUDY OF THERMALDECOMPOSITION BEHAVIOUR OF Zn–Cr, Zn–Cr–AlAND Zn–Al TYPE LAYERED DOUBLE HYDROXIDESP. Bharali, R. Saikia, R. K. Boruah and R. L. Goswamee*Regional Research Laboratory (CSIR) Jorhat, Jorhat 785006, Assam, IndiaAbstractHydrolysis of ZnO in acidic pH provides a facile route for synthesis of layered double hydroxides(LDH) bearing Zn2+ with Cr3+ and Al3+ without having carbonate ion in the inter-layer position. Thethermal decomposition of the prepared LDH show that with the increase of Cr3+ content in thesystem there is an increase in the thermal stability of the compounds. In case of Zn–Cr LDH there isa non-mass loss transition around 420°C. Segregation of parent LDH structure to a bivalent oxideand a spinel takes place in both Zn–Cr and Zn–Al LDH only at temperatures above 550°C.Keywords: anionic clays, hydrotalcites, layered double hydroxides, spinel, zinc oxideIntroductionLayered materials are considered to be an important group of solids in the present dayworld, bearing enormous application prospects in various fields. Layered doublehydroxides (LDH) are also known as anionic clay minerals or hydrotalcites. They area group of important materials where elementary layers possess some positive chargedue to substitution of bivalent metal ions in the parent brucite-like structure bytrivalent metal ions [1]; the positive charge thus generated is neutralised by theinterlayer anions like Cl–, SO42– , CO32– , etc. These are the only known class of ioniclayered compounds where the layers are two dimensional crystalline solids [2]. Dueto stereochemical factors, CO32– ion is highly stable in the interlayer position ofLDH [3]. Therefore, it is extremely difficult to synthesize LDH without having CO32–ion in the interlayer positions, which is very easily available in alkaline pH from theatmospheric CO2.One of the easiest methods to have CO32– free LDH is to hydrolyse the bivalentmetal oxides in acidic pH [4, 5]. The method basically involves the reaction of a biva-lent metal oxide with an aqueous salt solution of a trivalent metal ion at room temper-ature. This method is often used to prepare Zn–Cr, Zn–Al, Cu–Cr type LDH [6].1388–6150/2004/ $ 20.00© 2004 Akadémiai Kiadó, BudapestAkadémiai Kiadó, BudapestKluwer Academic Publishers, Dordrecht* Author for correspondence: E-mail: rajibgoswamee@yahoo.comThe thermal properties of LDH draw special attention due to formation of im-mense number of phases like non-stoichiometric oxides, segregated spinels, bivalentoxides, metal nanoparticles, etc. at different temperatures [7]. The composition ofmetal ions, their ratios, the nature and type of interlayer species play very vital andsignificant roles in these transformations [8, 9].It is a common practice to attempt to study the thermal decomposition behaviourof compounds having multiple decomposition steps using constant heating ratenon-isothermal gravimetry, in that case the time required for a set of experiments isrelatively short [10]. Therefore, the present work is done by constant heating rate ex-periments only.Generally speaking, in the non-isothermal treatment starting from the room tem-perature to higher temperatures, the thermal decomposition of an LDH takes place atfour different stages. These can be divided to evaporation of adsorbed water, loss ofinter-layer water, dehydroxylation of brucite-like sheets and loss of inter-layer vola-tile anions like Cl–, NO3– or CO32– [11], etc. However, depending upon the various fac-tors such as the dryness of the sample, the heating rate, chemical composition of themain layer, the nature of the inter-layer anion and its reactivity for possible guest-host interactions these four steps may vary their positions. Pinnavaia et al. [12] haveobserved that at a heating rate of 5°C min–1 the inter-particle pore water begins to dis-appear at a temperature of about 60°C and inter-layer water disappears at a tempera-ture range of 240 to 280°C. After the disappearance of the inter-layer water structuraldehydroxylation commences [13]. Sometimes even a specific decomposition ismulti-stepped e.g. in structural dehydroxylation the decomposition of OH groups at-tached to trivalent metal ions may take place earlier than the same for OH groups at-tached to bivalent metal ions [14, 15]. Acro et al. [16] also have reported that apartfrom mass loss steps thermal decomposition pattern of an LDH may contain somenon-mass loss DTA attributes owing to formation of certain new oxidic phases.ExperimentalIn the present work mainly two types of LDH viz. Zn–Cr and Zn–Al type have beenprepared by the reaction of ZnO with 1 M aqueous CrCl3 and AlCl3 solutions,respectively. The intermediate ternary system of Zn2+ with Cr3+ and Al3+ wereprepared by reacting ZnO with mixtures of corresponding 1 M solutions of Cr3+ andAl3+ with different Cr3+: Al3+ ratios.The products formed were washed and dried at 40°C. Chemical compositions ofthe products and reactants were analysed by AAS and ICP-AES. The dried productswere then tested in IR for the absence of 1400 cm–1 peak corresponding to 3 absorp-tion band of CO32– ion. Powder XRD were carried out in Philips single circle powderdiffractometer at a step size of 0.01° 2 s–1 from CuKsource under 35 kVand 20 mA current with Si as internal standard.The TG/DTG patterns of the products were then carried out at different heatingrates under air atmosphere in SDT-2960 DTA-TG equipment (M/S TA Corpora-tion, USA), with approximately 20 mg sample in platinum crucibles. To orient theJ. Therm. Anal. Cal., 78, 2004832 BHARALI et al.: LAYERED DOUBLE HYDROXIDESlayered materials on their basal planes tightly, samples in the crucible were slightlypressed over a hard base by a flat surface.In order to make the Cl– bearing samples harmless to DSC cell Cl– ions from theinterlayer positions have been exchanged with CO32– ion by dipping the LDHin 30 mL 1 M Na2CO3 solution for 2 to 3 h. The products then washed to free from ex-cess Na2CO3 and dried. DSC runs were then carried out in DSC 2010 (M/S TA Corpo-ration, USA) at a heating rate of 20°C min–1 with a sample size of about 5 mg.In order to compare DTA-TG results, static thermal treatment viz. at 350, 450,550, 650, 750, 950°C in electric muffle furnace for 30 min were carried out. The dif-ferent crystalline phases formed during calcination were then characterized by pow-der X-ray diffractometry (XRD) in Jeol JDXIIP3A single circle goniometer at 20 kVand 10 mA current at a scan rate of 6° 2 min–1.Results and discussionIt has been observed that an LDH type XRD pattern [15] i.e. one having characteris-tic basal (00l) peaks (Fig. 1) free from other recognizable crystalline phases likeZnO, Zn(OH)2, Zn5(OH)8Cl is obtained during hydrolysis of ZnO with aqueousCrCl3 and AlCl3 mixtures. As observed from the width of the peaks there is a gradualdecrease of crystallinity with increase of Al3+ in the system. In case of Zn–Al–ClJ. Therm. Anal. Cal., 78, 2004BHARALI et al.: LAYERED DOUBLE HYDROXIDES 833Fig. 1 XRD patterns of Zn–Cr–Al–Cl LDH at different Al:Cr ratios; a, b, c, d, e, f indicatedifferent samples prepared from reactants with Al:Cr ratios; a – 1:0, b – 0.9:0.1,c – 0.7:0.3, d – 0.5:0.5, e – 0.3:0.7 and f – 0:1, respectively; (=Si internal standard)LDH in order to obtain sharper XRD peaks comparable to that of Zn–Cr–Cl LDH onehas to carry out the reaction under continuous stirring for several weeks at room tem-perature. Which indicates a slower reaction rate of aqueous solution of salt of Al3+than Cr3+ with ZnO. Both Al3+ and Cr3+ form different soluble hydrolytic polycationsin acidic pH [17]. Roussel et al. [18] have shown that in case of Cr3+ there is the for-mation of some hydrolytic complex which is responsible for the formation ofZn–OH–Cr bridges and that forms the initial nuclei for Zn–Cr–Cl LDH. It has beendescribed that in case of Cr3+ the oligomerisation of this hydrolytic complex is slowand that helps in the formation of Zn–OH–Cr bridges. Although, similar study on thedetail mechanism of the reaction of Al3+ salt with ZnO has not been reported so far,yet the fate of Al3+ ion in aqueous medium at acidic pH is well documented. Variousworkers have reported the formation of a tridecameric polycation [19, 20] which isfurther stabilized if solution has Cl– ion in it [21]. Thus, it is possible that slower rateof formation of Zn–Al–Cl LDH is due to this stable polycation. The slower reactivityof Al3+ in comparison to Cr3+ is further evidenced by decreased Al3+ content in theproducts than in the reactants with respect to their Cr3+ contents, e.g. the reactantswith Al:Cr ratios 0.3:0.7, 0.5:0.5, 0.7:0.3, 0.9:0.1 give products with Al:Cr ra-tios 0.28:0.72, 0.41:0.59, 0.63:0.37, 0.85:0.15, respectively.The TG patterns of Zn–Cr–Cl LDH, Zn–Cr–Al–Cl LDH (at Cr:Al ratio 0.5:0.5)and Zn–Al–Cl LDH carried out at an air atmosphere at a heating rate of 8°C min–1show (Fig. 2) mainly the various mass loss transitions characteristic of different LDHdescribed earlier. The total mass loss in two extreme end members of the series i.e. inZn–Cr–Cl is 31.52% while the same in Zn–Al–Cl is 33.16% w/w.When compared from the point of position of DTG peaks of the two decomposi-tion regions viz. the interlayer dehydration and structural dehydroxylation, an idea ofrelative thermal stability of different compositions is obtained. The different DTG peaktemperatures of Zn–Cr–Cl, Zn–Cr–Al–Cl (Cr:Al ratio 0.5:0.5) and Zn–Al–Cl typeLDHs are given in Table 1. It is observed that in Al bearing LDH the decompositionsare gradually taking place at lower temperatures than their Cr bearing counterparts.There is observed a higher amount of mass loss (12.7%) in interlayer dehydra-tion stage of Zn–Cr–Cl LDH than Zn–Al–Cl LDH (7.75%). Which can be describedas due to improved crystallinity of the former, which causes larger interlayer surfacefor accommodation of a larger amount of water.J. Therm. Anal. Cal., 78, 2004834 BHARALI et al.: LAYERED DOUBLE HYDROXIDESTable 1 The DTG peak positions of two structurally important decompositionsLDH speciesInterlayer dehydration Structural dehydroxylationDTG peak/°CZn–Cr–Cl 323.00 453.00Zn–Cr–Al–Cl 278.81 420.70Zn–Al–Cl 187.00 248.21There is an exothermic non-mass loss DTA peak at 416°C immediately preced-ing the structural dehydroxylation peak at 453°C (Fig. 3). While the exact nature ofthis DTA conversion is not known, but Acro et al. [16] have reported that at this stagesome of the Cr(III) species gets partially oxidized to Cr(VI), giving rise to formationof an amorphous zinc hydroxychromate and at higher calcination temperatures it getconverted to ZnO and ZnCr2O4. Similar explanation of oxidation of trivalent speciesis not applicable in Al3+, because of that DTA peak similar to Zn–Cr–Cl is not ob-served in Zn–Al–Cl system.J. Therm. Anal. Cal., 78, 2004BHARALI et al.: LAYERED DOUBLE HYDROXIDES 835Fig. 2TG-DTG patterns of Zn–Cr–Cl, Zn–Cr–Al–Cl and Zn–Al–Cl LDH; a, b, c indicatedifferent samples prepared from reactants with Cr:Al ratios; a – 1:0, b – 0.5:0.5and c – 0:1, respectivelyIt has been observed that apart from trivalent ion present in the system the appear-ance of this DTA peak is dependent upon the interlayer species present in the systeme.g. in CO32– , NO3– form of Zn–Cr LDH the same is not observed. This is probably dueto the formation of certain ‘green-house like’ gas produced from the decomposition ofCO32– and NO3– ion in the inter-layer region, which absorbs the heat produced in theexothermic transition. This is the reason for the absence of the said exothermic peak inDSC patterns of Zn–Cr type LDH (Fig. 4) at around 416°C, as due to the instrumentallimitations the Cl– bearing Zn–Cr–Cl type LDH is initially converted to its CO32– formto avoid any damage to the sample cell from the Cl– species.Other than that, the CO32– form of Zn–Cr LDH prepared by exchanginginterlayer Cl– ion with CO32– ion shows similar TG pattern to Zn–Cr–Cl LDH. How-ever, there is a slight shift in peak positions e.g. first mass loss DTG peak appearsat 95.5°C corresponding to 3.3% mass loss in the range, second mass loss DTG peakappears at 160.10°C with corresponding mass loss of 9.83%, the third mass loss DTGpeak appears at 291.31°C with corresponding mass loss in the range as 13.99%, thefourth DTG peak appears at 480°C with corresponding mass loss being 1.88%. Thusthe total mass loss in four different transitions is 29.0% which is smaller than totalmass loss of 31.52% in Zn–Cr–Cl LDH. Considering the fact that bivalent carbonateion is charge wise equivalent to two monovalent chloride ions and molecular mass ofCO32– being smaller than two chloride ions such a lower mass loss in Zn–Cr–CO3form of LDH appears reasonable.The crystalline phases identified from the XRD patterns of different calcinedproducts show that Zn–Al or Zn–Cr type LDH maintains ZnO type poorly crystallinestructure at least upto 550°C.J. Therm. Anal. Cal., 78, 2004836 BHARALI et al.: LAYERED DOUBLE HYDROXIDESFig. 3 TG-DTG-DTA patterns of Zn–Cr–Cl LDH magnifying the non-mass loss andstructural dehydroxylation stage; a, b, c indicates; a – DTA, b – TG and c – DTGcurves, respectivelyConclusionsLayered double hydroxide prepared from hydrolysis of ZnO with aqueous chromiumchloride is more crystalline than the same prepared by reaction of aqueous aluminiumchloride. Zn–Cr–Cl LDH is thermally more stable than Zn–Al–Cl LDH. Zn–Cr–Cl LDHat around 420°C undergoes a non-mass loss DTA transition. The appearance of the DTApeak is dependent upon the nature of interlayer species. The oxide formed on decomposi-tion of these two LDH initially maintains a ZnO type structure which later on segregatesto ZnO and their corresponding spinel. The DSC pattern is complementary to theirDTA-TG patterns.* * *Authors are grateful to Director R. R. L. Jorhat for his kind permission to publish the work, as wellas acknowledgement is due to Prof. Herbert Poellmann of University of Halle, Germany for somepowder XRD runs. Also, acknowledgement is due to Ministry of Environment and Forests, Govt. ofIndia for some financial assistance.References1 A. Vaccari, La Chimica and l’ Industria, 74 (1992) 174.2 W. Mueller-Warmuth and R. Schoellhorn, in Physics and Chemistry of Materials withLow-Dimensional Structures, Vol. 17, Kluwer Academic Publishers, 1994, ISBN0-7923-2357-2.J. Therm. Anal. Cal., 78, 2004BHARALI et al.: LAYERED DOUBLE HYDROXIDES 837Fig. 4 Comparative DSC patterns of Zn–Cr–Al–CO3 LDH at different Cr:Al ratios; a, b,c, d, e, f indicate different samples prepared from reactants with Cr:Al ratiosa – 1:0, b – 0.9:0.1, c – 0.7:0.3, d – 0.5:0.5, e – 0.1:0.9 and f – 0:1, respectively3 H. F. W. Taylor, Mineralogical Magazine, 37, 287 (1969) 342.4 W. T. Reichle, Solid State Ionics, 22 (1986) 135.5 H. P. Boehm, J. Sterile and C. Vieweger, Angew. Chem. Int. Ed. (Eng.), 16 (1977) 265.6 G. Lagaly, K. Beneke and M. Meyn, Inorg. Chem., 32 (1993) 1209.7 F. Kovanda, V. Balek, V. Dornicak, P. Martinec, M. Maslan, L. Bilkova, D. Kolousek andM. I. Bountsewa, J. Therm. Anal. Cal., 71 (2003) 727.8 R. L. Frost, W. Maters, Z. Ding and J. T. Kloprogge, J. Therm. Anal. Cal., 71 (2003) 429.9 R. L. Frost and K. L. Erikson, J. Therm. Anal. Cal., 76 (2004) 217.10 T. Ozawa, J. Therm. Anal. Cal., 64 (2001) 109.11 F. Malherbe and J.-P. Besse, J. Solid State Chem., 155 (2000) 332.12 Y. S. Kyeong and T. J. Pinnavaia, Chem. Mater., 7 (1995) 348.13 A. de Roy, J. P. Besse and P. Bondot, Mat. Res. Bull., 20 (1985) 1091.14 T. Mark, ‘Porous Structure and Particle Size of Silica and Hydrotalcite Catalyst Precursors;a Thermoporometric Study’, Thesis submitted to Rijk University-Ultrecht, 1993.15 A. Tsuniama, K. Kosuge and K. Kooli, J. Solid State Chem., 118 (1995) 285.16 M. Del Arco, V. Rives, R. Trujilano and P. Malet, J. Mater. Chem., 6 (1996) 1419.17 R. E. Mesmer and C. F. Baes, Review of Hydrolysis Behaviour of Ions in Aqueous Solutions,Mat. Res. Soc. Symp. Proc., 180 (1990) 85.18 H. Roussel, V. Briois, E. Elkaim, A. de Roy, J.-P. Besse and J.-P. Jolivet, Chem. Mater.,13 (2001) 329.19 P. M. Bertsch, G. W. Thomas and R. I. Barmishel, Soil Sci. Soc. Am. J., 50 (1986) 825.20 S. Hans and M. Werner, Inorg. Chem., 22 (1983) 2145.21 D. L. Teagarden, J. L. White and S. L. Hem, J. Pharm. Sciences, 70 (1981) 808.J. Therm. Anal. Cal., 78, 2004838 BHARALI et al.: LAYERED DOUBLE HYDROXIDES

A comparative study of thermal decomposition behaviour of Zn-Cr, Zn-Cr-Al and Zn-Al type layered double hydroxides

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A comparative study of thermal decomposition behaviour of Zn-Cr, Zn-Cr-Al and Zn-Al type layered double hydroxides

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