�9 1998 Elsevier Science B.V. All rights reserved. Preparation of Catalysts VII B. Delmon et al., editors. 941

Synthesis and physicochemical properties of new hydrotalcite-like anionic clays containing Zr 4+ in the layers.

S.Velu*, A.Ramani, Veda Ramaswamy, Bhanu M.Chanda and S.Sivasanker

National Chemical Laboratory, Pune-411 008, India.

New series of hydrotalcite (HT)-like anionic clays containing Zr 4+ in the brucite-like layer have been synthesised by a simple coprecipitation method at constant pH. The incorporation of Zr in the layer have been confirmed by various analytical methods such as powder X-ray diffraction (PXRD), chemical analysis, UV-Vis diffusereflectance (UV-Vis DR) spectra and thermogravimetric analysis. These Zr containing HT-like materials show very interesting properties as catalyst in the liquid phase hydroxylation of phenol to selectively catechol.

1. INTRODUCTION

Hydrotalcite (HT)-like anionic clays are a new family of interesting materials with applications as catalysts, catalyst supports, ion exchangers and composite materials [1-3]. The structure of these compounds consists of brucite [Mg(OH)z] type octahedral layers in which a part of M(II) cations are isomorphously substituted by M(III) cations. The excess positive charge of the octahedral layers resulting from this substitution is compensated by interstitial layers built of anions such as CO32- and crystal water. These compounds are represented by the general formula; [M(II)I.xM(III)x(OH)2 ]x+ [(A'),~n y.H20] x', where, M(II) is the divalent cations such as Mg, Cu, Ni, Co, Mn, Zn; M(III) is the trivalent cations such as A1, Fe, Cr, Ga, V, Ru, Rh and Y [4-6]; A n is the interlayer anions such as CO32, NO[, SO42- , or organic anions such as terephtalate and the value of 'x ' is in the range from 0.1 to 0.33. A large number of HT-like compounds with a wide variety of M(II)-M(III) cation pairs as well as M(I)-M(III) cation pairs (eg., Li-A1), with different anions in the interlayer region have been reported [7]. However, HT-like compounds with M(II)-M(IV) cation pairs such as Mg-Zr or Mg-Ti are scarcely obtained. Taylor have reported the synthesis of a Co-Ti HT like compound employing an induced hydrolysis method [8]. In the present study we report a new series of ternary HT-like compounds containg Mg, A1 and Zr in the brucite-like layers, synthesised by employing a simple coprecipitation technique. The importance of these Zr containing HT-like compounds as a catalyst in the liquid phase hydroxylation of phenol has been investigated and the preliminary results are presented in this communication.

* Present address: Institute of Chemical Engineering-III, Swiss Federal Institute of Technology, CH- 1015 Lausanne, Switzerland.

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2. EXPERIMENTAL

2.1. Synthesis of Zr-hydrotalcites

Zr containing HT-like compounds with various Mg:AI:Zr atomic ratio were synthesised by coprecipitation method at room temperature by reacting aqueous solutions containing a mixture of Mg(NO3)z, AI(NO3)3 and ZrO(NO3)2, depending on the Mg:AI:Zr atomic ratio and a mixture of NaOH (~ 2M solution) and Na2CO3 (= 0.3M solution) at a contant pH (~ 10) [9]. The Mg/(AI+Zr) atomic ratio in the starting solution was kept as 3 while varying the AI:Zr atomic ratio from 0:1 to 1:0. The resulting precipitate was aged at 338 K for 30 min under stirring in a magnetic stirrer then filtered, washed with distilled water for several times until the pH of the filtrate was 7 and dried in an air oven at 373 K over night.

2.2. Characterisation The chemical composition of the samples were determined by X-ray fluorescence

(XRF) spectroscopy (Rigaku 3070 X-ray wavelength dispersive spectrometer). The powder X-ray diffraction (PXRD) of the samples were obtained using an instrument (Rigaku, Model DMAX-III VC) equipped with a graphite crystal monochromator and Ni-filtered Cu I~ radiation (~ = 1.5418 A). The data were collected in the 20 range 5-70 ~ with a scan rate of 0.5 ~ for calculation of lattice parameters while a speed of 4~ was used for obtaining the PXRD patterns. The observed interplanar 'd' spacing were corrected using elemental Si as an internal standard. The lattice parameters were refined using two different least square fitting programs (HOCT and PDP11). The UV-Vis diffuse reflectance spectra (UV-Vis DRS) of all the samples were obtained using a Shimadzu UV-Vis spectrophotometer 2101 PC model in the range 200 to 400 nm. The thermogravimetry of these samples were recorded in a Setaram TG/DTA 92 model instrument at a scan rate of 10 K/rain under Ar atmosphere. The BET surface area of both uncalcined and calcined samples were determined employing BET N2 adsorption method at 77 K using an Omnisorp 100 CX (Coulter) instrument after degassing the sample at 423 K for 8h.

3. RESULTS AND DISCUSSIONS

3.1. Physicochemical Characterisation

The results of elemental analysis for the samples prepared are included in Table-1. The Mg/(AI+Zr) atomic ratio as determined from X-ray fluorescence spectroscopy is coincident, within the experimental error, with that of the starting mixed aqueous solutions and it is similar (3.1 + 0.16) for all the samples indicating the completion of the precipitation. The PXRD patterns [9] of these compounds with Mg:AI:Zr atomic ratio ranging from 3 1"0 to 3:0"1 indicated that samples with Zr = 0 (Mg:AI:Zr - 3 1:0) to Zr = 0.8 (Mg:AI:Zr = 3:0.2:0.8) exhibited the diffraction patterns characteristic of HT-like structure. However, the crystallinity of the samples decreased with increasing Zr content. This is because of the introduction of large distortions in the brucite-like layers due to the substitution of AI 3+ (Shannon ionic radius 0.53 A) by Zr 4+ (Shannon ionic radius 0.72 A). A binary Zr containing HT without AI forms a poorly crystalline ZrO2 phase indicating that the presence of A1 favours the formation of pure HT-like phase. The crystallographic parameters 'a' and 'c' were calculated employing least square refinement assuming a hexagonal crystal

943

Table-1. Chemical composi t ions , molecular formula and surface area of MgAIZr-HT

Sample Mg:AI:Zr carbon Chemical formula 3 atomic content 2 ratio 1 (%)

BET surface area (m z ~-1)

723 K 4

ZrHT-0 3:0.96:0.00 2.44 ZrHT- 1 3:0.85:0.07 nd ZrHT-2 3:0.78:0.14 2.71 ZrHT-3 3:0.68:0.21 2.91 ZrHT-4 3:0.67:0.33 3.01 ZrHT-5 3:0.57:0.37 nd ZrHT-6 3:0.52:0.50 3.51

Mgo.75Alo.25Zro.oo(On)2(CO3)o.14 0.31.H20 169 - 202

Mgo.75Alo.2oZro.o4(OH)2(COs)o.ls 0.34.H20 241 Mgo.75Alo. 17Zro.05(on)2(CO3)o. 19 0.33.H20 nd Mgo.75Alo. 17Zro.08(OH)2(CO3)0.21 0.40.H20 199

- nd Mgo.75Alo.asZro.13(OH)2(CO3)o.26 0.40.H20 192

1 Determined by X-ray fluorescence spectroscopy 2 Determined by CH microanalysis 3 Calculated based on the chemical analysis and the first weight loss in TG 4 Pretreatment temoerature

,, I I I

3"08

v

I

24"2

- 23-8

- 23 -4

-23"0

Fig.1

3 " 0 7

"3"06

3"05

I I . . . . . . . . I .... l

0 0-18 0 - 5 4 1-22

Zr/AI ATOMIC RATIO

Variation of lattice parameters with Zr content for MgA1Zr-HT

t J

944

system for samples ZrHT-0 to ZrHT-6, wherein the PXRD peaks were intense and sharp enough for the accurate calculation. It can be seen from Fig.1 that the lattice parameters both 'a' and 'c' increases with increase in Zr content and follows the Vegard's law in the region Zr/A1 = 0 to 1.5. The increase in 'a' parameter can be attributed to the isomorphouse substitution of A13+ in the octahedral coordination by Zr 4+ in the brucite-like layer. The increase in 'c' parameter can be attributed to the weakening of the interaction between brucite-like layer and the interlayer due to the increase in the thickness of the brueite-like layer upon incorporation of Zr 4+. Furthermore, since a part of the trivalent cation A1 is being substituted by tetravalent cation Zr, the interlayer should accommodate more amounts of CO32 anions for the charge compensation. This argument is supported by an increase in carbon content from 2.44 % for ZrHT-0 (HT without Zr) to 3.55 % for ZrHT-6 (Table-l). A sharp increase in both 'a' and 'c' parameter is observed upto about 10 % of A13+ is substituted by Zr 4+. This is because the initial addition of Zr 4+ might substitute A13+ which are probably present in the corners of a hexagonal crystal lattice, thereby a sharp increase in the thickness of the brucite-like sheet.

v

I..iJ

z

1313 n- o or) 1::13 ,,~

/ g,

,

0

2 0 0 5 0 0 4 0 0 W A V E L E N G T H (nm)

Fig.2 UV-Vis DR spectra ofMgAIZr-HT: (a) ZrHT-0, (b) ZrHT-1, �9 ZrHT-3, (d) ZrHT- 3cal., (e)ZrHT-5, (f) ZrHT-8, (g) Pure ZrO2, ( .. . . ) ZrHT-0 + pure ZrO2 (physical mixture)

945

The dispersion of Zr in the HT matrix was studied by UV-Vis diffiasereflectance spectroscopy (UV-Vis DRS). It can be seen from Fig.2 that all these samples exhibited a

('"'In)] single narrow band around 210 nm which is attributed to charge transfer involving isolated Zr 4+ species [10, 11]. This electronic transition is clearly distinguishable from that of pure ZrO2 which showed strong absorption bands around 240 and 320 nm (Fig.2, curve-e). These results clearly demonstrates the absence of any ZrO2 specie within the sample and indicates that the Zr 4§ cations are well dispersed in the HT framework similar to that in Zr-silicalites [10, 11].

The thermogravimetric analysis (TGA) of hydrotalcite-like compounds generally exhibits two stages of weight loss

.~ process. Fig.3 shows the differential curves of TGA (DTGA) of some of the

o~ MgA1Zr-HT. All the samples except O -~ ZrHT-10 (without A1) showed two major

weight loss process characteristic of HT- like compounds having layered structure [1,6]. The first weight loss (T1) occurring in the temperature range 423-523 K is attributed to the removal of interlayer water. The second weight loss (T2) observed around 623-723 K is due to the removal of structural water and CO2 from the decomposition of CO32- ions in the interlayer. It is interesting to note that incorporation of Zr in the HT lattice results in broadening of the T1 process (curves b-d), indicating that the weight loss is associated with some complicated process, probably due to the removal of large amount of physisorbed water and CO32 ions. In addition, the peak

, t , . I I

57:5 5~'5 7-f3 temperatures both T1 and T2 are shifted TEMPERATURE (K) towards lower temperature with

Fig.3 DTG traces ofMgAIZr-HT; (a) ZrHT-0 increasing Zr content (eg., ZrHT-0; T1 = (b) ZrHT-1, �9 ZrHT-3, (d) ZrHT-6, 514 K, T2 = 703 K: ZrHT-6; T1 = 436 K, (e) ZrHT-10 (without AI). T2 = 683 K). These results clearly

indicates that the incorporation of Zr in

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the HT lattice facilitates the decomposition process due to the weakening of the interaction between bmcite-like layer and the interlayer (Cf." PXRD results, Fig. 1). A net weight loss of around 40-50 % was recorded in TGA upto 873 K in these samples. From the weight loss recorded for T1 process (373-523 K) and the results of chemical compositions as determined from XRF and CH analysis, the chemical formula of the compounds are derived and they are presented in Table-1 along with the results of chemical compositions. The DTG diagram of ZrHT-10, without A1 shows (Curve-e) a single broad weight loss process probably due to the decomposition of some amorphous hydroxycarbonates since the PXRD of this sample shows a poorly crystalline ZrO2 phase (Fig.l, curve-k). These changes in the thermogravimetric study indicates the incorporation of Zr in the lattice.

d v

>-

F- O3 Z

l- z =_=.

�9 M90 o MgAI203 V Zr02

HT Unidentified

o

v

o

I /

v v

I I I

20 40 60 2e (DEGREE)

Fig.4 PXRD patterns of ZrHT-3 calcined at (a) 423, (b) 573, �9 723, (d) 1073, (e) 1473 K

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Thermal calcination of HT-like compounds results in decomposition of layered structure and forms various m e t a stable phases depending on the calcination temperature. The determination of crystalline phases formed during calcination at various temperatures is useful to understand the thermal stability of the compounds. Fig.4 shows the PXRD patterns of ZrHT-3 calcined at various temperatures for 4h in air. It can be seen that at 423 K, all the peaks corresponding to HT are retained with reduced intensities. At 573 K, the peak corresponding to (006) is completely disappeared while the (003) diffraction line is shined towards higher 20 value (lower interlayer spacing) due to the removal of interlayer water molecules. These results indicates that the HT-like structure in these compounds survive upto at least 573 K. However, at 723 K, the layered structure completely collapses due to the removal of structural water and CO2 from the interlayer. The evolution of these gases during calcination makes craters on the surface hence the surface area increases from around 40 m 2 g-1 for the uncalcined samples to about 200 m 2 g-1 for samples calcined at 723 K (Table-l). The material at this temperature is referred to as a poorly crystalline MgO, whose crystallinity increases with further increase in calcination temperature to 1073 K. At this temperature, in addition to MgO, weak diffraction lines are recorded which are attributed to the formation of MgAI204 spinel and ZrO2 phases. At 1473 K, the intensity of all these diffraction lines increased to form well defined crystalline phases.

It has been reported [12,13] that in the case of MgA1-HT, calcination above 900 K yields a mixture of MgO and MgA1204 phases while a mixture of MgO, MgA1204 and RuO2 phases are reported to be formed in a ternary MgAIRu-HT. On the other hand, Fernandez et al [6] noticed a mixture of MgO, MgAI204 and A12Y409 phases during calcination of MgAIY-HT at 1273 K. However, in the present study we have not noticed any Zr containing phase other than ZrO2 even at 1473 K.

Among the phases formed during calcination of HT-like materials, the one formed around 723 K is of particular interest since in many cases the product formed at this calcination temperature are used potentially as catalyst for many industrially important reactions [ 1-3 ]. Hence we have studied, in detail, the nature of product formed at 723 K by PXRD and UV-Vis DRS. Fig.5 shows the PXRD patterns of various MgAIZr-HT calcined at 723 K for 4h. It can be seen that a poorly crystalline MgO phase is obtained in all cases and the crystaUinity decreases further with increasing Zr content. In addition the formation of a separate ZrO2 phase (poorly crystalline) is evidenced in ZrHT-7.

It has been reported by several authors [7,12,14] that thermal decomposition of MgA1-HT around 723 K yields a poorly crystalline phase corresponding to MgO whose lattice constant 'a' is less than that of pure MgO and has been attributed to the dissolution of a small amount of A13+ in the MgO lattice to form solid solutions. The chemical composition of the resulting solid solution is given by the formula; Mg(1-x)/(2+x)Al2x/(Z+x)[ ]x/(2+x)O; where [ ] is a cation vacancy created due to the dissolution of part of A1 in the MgO lattice [12]. However, in the present study since the PXRD lines are very broad and less intense, the accurate determination of'a' parameter was not possible. Hence, we have calculated only the d(200) values of resulting MgO phase for all the samples and compared them. As expected, the d (200) value increases with increasing Zr content (Fig.6) and follows Vegard's law in the range Zr/A1 = 0 to 0.4, indicating that a part of Mg 2+ (ionic radius 0.66 A) is being

948

"2.

v

>.. I-- CO Z

l- z

�9 M g O .-. 0

~V Zr02_ ~"0 (M (M II

b ~ _ ~ _ _

V

I

20 40 2e (DEGREES)

Fig.5 PXRD patterns of MgA1Zr-HT calcined at 723 K for 4h., (a) ZrHT-O, (b) ZrHT-1, �9 ZrHT-3, (d) ZrHT-5, (e) ZrHT-8

I

60

2-12 -

2.11 -

2.10 -

/ /

t

2"0S 0

I l I

o/ /

/ /

/

/ /

/ /

/ /

/ /

/

! .... I I

0-2 0 -4 0 .6

Zr lA i ATOMIC RATIO

Fig.6 Variation of d(200) of MgO phase with Zr content for MgA1Zr-HT calcined at 723

K for 4 h.

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isomorphousely substituted by Zr 4+ (ionic radius 0.72/~) in the MgO lattice. The decrease in d(200) values at higher Zr content can be attributed to the segregation of Zr 4+ at the surface to form a separate Zr02 phase as evidenced from PXRD (Fig.5, curve-e).

The UV-Vis DR spectra of Zr0.3-HT calcined at 723 K is included in Fig.2 (curve d). It can be seen that the absorbance maxima is more intense and sharper than that of of the uncalcined samples (compare curves c and d) indicating that the Zr 4+ cations are well dispersed in calcined samples.

Some of the Zr containing hydrotalcites have been tested as catalysts in the hydroxylation of phenol using 30 % aqueous solution of H202 as the oxidant. Preliminary studies with H202 ratios from 0.5 to 8 using solvents such as water, CCla or MeOH did not show any catalytic activity. However, appreciable activity (up to 10 % conversion of phenol) was observed when light petroleum (bp 333-343 K) was used as the solvent. Table 2 sumrnarises the results of hydroxylation of phenol over various Zr containing hydrotalcites. It is interesting to note from the Table that the turnover number (TON; moles of phenol converted per mole of Zr atom) increases four-fold (1.0 to 3.8) when I-/202 is added drop- wise using a syringe pump. Similarly, a two-fold increase in catalytic activity (1.0-1.9) is noticed if the sample is calcined at 723 K for 5 h. In all these cases catechol is obtained as a unique product (100 % selectivity). Upon increasing the HzO2/phenol ratio, the TON also increases considerably, but the selectivity of catechol decreases at the expense of hydroquinone. The absence of catalytic activity in the ease of pure MgA1-HT without Zr (Zr0.0-HT) or pure ZrO2 clearly indicates that the Zr incorporated in the HT framework plays a pivotal role in catalytic activity. Surprisingly, under the similar experimental conditions, the Zr-containing silicalite (ZrS-1) which showed appreciable conversion in aqueous medium is found to be inactive in the hydroxylation of phenol. Further work is in progress in order to investigate the above differences and also to compare the catalytic performance with similar Ti-containing hydrotalcites.

Table 2 Catalyst

Hydroxylation of phenol with 11202 over MgAIZr-HT H202/Phenol TON 1 Product selectivity imass %.,)

Catechol Hydroquinone Zr0.0-HT 5.0 - _ Zr0.1-HT 5.0 1.4 100 Zr0.3-HT 5.0 1.0 100 Zr0.3-HT 2 5.0 3.8 100 Zr0.3-CHT 3 5.0 1.9 100 Zr0.3-HT 7.0 29.1 73.8 Zr0.3-HT 10.0 40.5 65.1 ZrS-1 5.0 - _ ZrO2 5.0 - _

26.2 34.9

1 TON = Turnover number (moles of phenol converted per mole of Zr atom. 2 H202 was added drop wise using a syringe pump at a rate of 0.5 ml h-1 3 Sample calcined at 723 K for 5 h. Reaction conditions; solvent: light petroleum (bp 333-353 K) 30 ml. Phenol, 1 g. catalyst 100 mg, temperature 353 K, time 8 h.

950

4. CONCLUSIONS Based on the above experimental evidences, we report that pure and crystalline HT-like

compounds containing Zr in the layer can be easily synthesised by the simple coprecipitation method. The crystallinity of the material depends on the Zr/AI atomic ratio. The Zr 4+ cations are well dispersed in both unealcined as well as in calcined samples (at 723 K) and the resulting Zr-HT like compounds can be used as a potential catalyst in the liquid phase hydroxylation of phenol to selectively catechol.

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