Indian Journal of Chemi stry Vol. 41 A, November 2002, pp. 2238-2243

Preparation and characterisation of Zr, Ti and Zr-Ti mixed oxide pillared montmorillonite and their catalytic activity towards nitration of chlorobenzene

D Das, H K Mi shra l, K M Parida* & A K Dalail

Regional Research Laboratory (CSIR), Bhu baneswar 751 01 3, Orissa, India

lCatalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering. Un iversity of Saskatchewa n, I J 0 Science place, Saskatoon SK, S7N 5C9, Canada

Received 10 Septelllber 2001 ; revised 3 1 July 2002

Zr, Ti and Zr-Ti mixed oxide pillared montmorillonite (PILCs) have been prepared by standard method and their spec ific surface area, total acidity, and weight loss have been ca lcu lated. The mi xed ox ide pillared clays retain hi ghest surface area even when ca lcined at 500°C. The tota l number of ac id sites ()..lmol/g) arc highest for 300°C calcined samp les and follow the order Zr-Ti PILC > ZrPILC > TiPILC. Mononitrati on reacti on is carri ed out using all the pillaredmontmorill onites cal cined at differe nt temperatures. The yield of mononitrochlorobenzene from chlorobenzene with different catalys ts also fo llow the sa me order and no ll1etaproduct has been detected for thi s reacti on. All samp les show apprec iably high parase Iecti vity i.e. close to 85%.

Montmorill onite is a class of smectite clay mineral , whi ch is composed of connected sheets or layers of corner linked Si02 tetrahedra and edge- linked AI(O,OH) octahed ra. These negatively charged Tetra·­hedral-Octahedral-Tetrahedral (TOT) aluminosilicate layers are held together due to the electrostati c force of attracti on with the exchangeab le hydrated metal cations present in the interlamellar spaces l

. On pil­laring, these common cations are replaced by the pil­laring agents (primarily large polynuclear cationi c species , usuall y of oxo-hydroxy type). Thermal treat­ment converts these ol igomeric cations into metal ox ide pillars and remains attached firm ly to the sheets", leading to high surface area, thermal stability and more access ibility of the interlayer space. Thereby, it plays a significant role in catalysis.

itration of chlorobenzene is an industrially im­portant industri al reaction and find s application in pharmaceutical s, dye, and perfume industries etc. 3

-5

.

Nitration reacti ons were traditionall y carri ed out using sulphuric acid and nitric acid mixtures in which prod­ucts may undergo violent oxidative degradation. Ow­ing to present environmental concerns, the aggressive acid mixtures employed in the nitration process need to be replaced and there is a huge demand of design­ing an ecofriend ly technology. Solid acids are a good substitute of these liquid acids. And being solid these bear the added advantage of easy to handle, separation by simple filtration and disposal is not at all a prob-

lem for the concerned industri es. Sin ce nitrati on of ch lorobenzene is a Bronsted acid catalyzed reac ti on, catalysts containing hi gher surface density of Bron­sted acid sites will eventually be more active for ni­tration and it can be taken as a good test reacti on to study more characteristics of a material catalyzing thi s reacti on. Several works on mononi trati on reactions have been reported earlier using so lid acid catal ys ts

h I· 6 . . I ' 11 d I 7-~ suc as zeo Ites, transiti on meta pi are cays , sulphated zircon ia lo, etc.

The present paper deal s with the preparation, char­acterization and evaluat ion of catalyti c activity of Zr, Ti and Zr-Ti mixed ox ide pillared montmorill onite for mononitration of ch lorobenzene in presence of aceti c anhydride.

Materials and Methods The montmorillonite (Mainburg, Germany) was

used as the starting material , and before pillaring, it was subjected to exchange with Na+ ions. The process followed for Na+ exchange is as follows: The clay was stirred with 1M Nael so lution fo r 5h and then washed with deionised water till the washings became free of CI - ions (negative AgNO, test). Finall y, the solid was separated by centrifugati cn and dried in air at 110°C. This Na exchanged clay (Na-Mont) was used for pillaring and further study. The chemical composi tion of the parent montmorillonite (in per­centage) is as follows: Si02 62.'); AI 20 3 19.8; Fe20 ,

DAS el 01.: CATALYSIS BY Zr, T i & Zr-Ti MIXED OXIDE PILLARED MONTMORILLONITE 2239

4.2, CaO 3.3, MgO 4 .8, Na20 0.8; K20 1.8 and H20 3.2. The ca tio n exchange capacity of Na-M ont was found to be 85 meqll OOg . Pill ared c lays were pre­pared fo llow in g the procedure proposed by Sterte2. For Ti pillared clay (Ti PILC), the pillaring agent was obtained by adding TiCI4 (F lub, Pract.) to 6 M HCI and diluting it to get the final titanium concentrat ion of 0.5 M. Similarly the pillaring agen t fo r z irconi um pi ll ared c lay (Zr PILC) was obta ined by making an aqueous so luti on of ZrOC I2.8 H20 (Merck, AR) and finally diluting to 0.5 M. But for the Zr-Ti pillared c lay (Zr-Ti PILC), both the pillaring agents (Zr and Ti soluti ons each of 0.5 M concentration) were mi xed in required amounts prior to addi tion for pillaring. The tota l metal ion (Zr and/o r Ti) to c lay rat io was main­tai ned at 10 mmol/g. The pi lIaring agent was added drop wise to a vigorously st irred water suspension of Na-Mont (Sg c lay/L) in an amount 10 mmo le of meta l/g c lay. The resulting product was stirred fo r

furthe r 3h at room temperature (3 0°C), filtered and washed with distilled water till free of cr io n (nega­ti ve AgNO.1 test). This prod uct was dried overni ght at

11 0°C, and ca lcined at req uired temperatures to get the respective pillared c lays (PILCs). The X-ray pow­der diffraction of the pillared clay sampl es (PILCs) were recorded on glass s lides in a Philips Semiauto­matic Diffractometer (X RD 7, Rich. Seitert & Co.,

Freiberg) with Ni-filtered CuKa radiati on. Adsorption of ethy lene glyco l was carried o ut on the same sam­ples to observe any change in basal spacing. The spe­cific surface areas of the PILCs were measured by nitrogen adsorpti o n at 77K by standard BET method (Quantaso rb, Quantachrome, USA). Prior to each

measurement, the samples were degassed at 110°C and 10-4 torr pressure to evacuate any physisorbed moisture.To study the weight loss for different PILCs, the sampl es were heated in an e lec tri c mufne furnace

from 200°C to 900°C w ith an interval of 100°e. The

temperature was rai sed at the rate of lOoC/min and the samples were kept at a parti cular te mperature for one hour. The platinum cruc ibl es were trans ferred into a des iccator in hot cond ition and immed iate ly the weight was taken.

The surface acidity was determined on the basis of irreversible adsorption of organic bases such as pyri­dine (PY, pKa = 5.3) and piperidine (PP, pKa = 11.1) in cyclohexane (Merck, AR) so luti on at 298 K by fol­low ing the method reported earli e r I I. According to their differences in pKa values , PP adsorbed g ives the total ac idity and PY adsorbed represents only the

strong acid sites on the cata lys t surface. The res idual concentrations of the organi c bases in the solution were measured by a Varian Cary IE UV-Vis spec tro­photometer fitted with Cary-IOO software using 10 mm matched quartz cells. Sorption experiments were carri ed out at the wavelength of maxi mum adsorption

(Am",) and in the concentratio n range of the adsorbate w ithin which the Beer-Lambert's law was obeyed . The adsorpti o n of substrate from solution was found to fo ll ow the Lang muir adsorption iso therm . Th is method prov ides both the to ta l concentration of ac id sites and the ir relative strength by the use of organic bases with different pK" values. Prio r to the measure­

ment, all sampl es were kept at lOO°C for 12h .

Solvent free nitration of chlorobenzene to monon i­trochlorobenzene was carried out in a double necked round bottomed flask. In each of the experiment accu­rately weighed 0 . 1 g o f the sampl e was taken into wh ich 4.9 mmol of HN03 (70%, Qualigens) was added and st irred for about 5 min keeping the flask in the ice cold water bath. To the above co ld so luti on. 1.5 ml of acetic anhydride (Merck, GR) was added and st irred for another 5 min. To this 4.9 mmol o f chlorobenzene (Merck, Special product) was added drop wise and the mi xture was a llowed to react for I h under stirring conditio n. The products were separated from the solid by filtration followed by washing with CCI4 (Merck, AR). The yield and selecti vity to differ­ent products were analyzed by GC (CIC Indi a) in FID mode using 15% FFAP on 801100 mesh W (HP) column .

Results and Discussion XRD patterns and specific surface area measure­

ment sugges t the effici ency of the pill aring process .

The PILCs possess a duol diffraction line at 28 = 3. 1

resulting from the fo rmati on of pillars and that at 28 = 9.0 corresponding to the parent clays (graph not g iven) . The changes observed in dool basal spac ing for

500°C calcined PILCs (doo l basal spac ing for Zr-Ti PILC = 28.4 A, Zr PILC = 16.6 A. and Ti PILC = 15.5 A) and increase in surface area (Specific surface area of Na Mont = 89 m2/g) confirms that pillaring is ef­fective . The dool basal spac ing for Zr-Ti PILC is high­est (28.4 A), which indicates that the pill arin g is most effec tive in thi s case. It is seen that in all the samples, interl ayer di stance did not change even upon ethy lene g lyco l adsorption , which indicates the stabi li zat ion clue to strong metal clay inte raction . Figure 1 depicts the change in surface area with calcination tempera-

2240 INDIAN J CHEM . SEC A, NOVEMBER 2002

Table I- Nitration of chlorobenzene using PILCs calcined at different temperatures

Sample code Ca lci nat ion Basal temp. Spacing

Surface area

Pore volume (cc/g)

Pore di ameter

Micropore volume (cc/g)

Reacti on temp.

Yi eld P:O P/O (%)

(UC) (A) (m2/g)

Zr PILC 11 0 208

Ti PILC 11 0 204

Zr-Ti PILC 110 199

Zr PILC 300 244 0.20

Ti PILC 300 236 0.24

Zr-Ti PILC 300 253 0.21

Zr PILC 500 16.6 197 0.19

Ti PILC 500 15.5 170 0.24

Zr-Ti PILC 500 28.4 204 0.20

Zr PILC 500

Ti PILC 500

Zr-Ti PILC 500

Zr PILC 500

Ti PILC 500

Zr-Ti PILC 500

P and 0 represent para and ortho isomers respecti vely

270 .

--<>- Zr P[LC

--+- Ti PfLC ...... Zr-Ti PrLC

1 50 '~------'--------'-------'------~

100 200 300 400 500

Calcination Temperature ('C)

Fig. I- Va ri ation of surfacc arca of PILCs with ca lcination temperature

ture for diffe rent pillared sampl es. Among the three PILCs prepared, the Zr-Ti mi xed ox ide PILC pos­sessed the highest surface area in compari son to the o thers for all calcinati on temperatures. Al l these sam­

pl es mainta in a quite hi gh surface area even at 500°C calcination temperature, but the va lue is highest for mi xed oxide pi ll ared c lay. This may be due to the

(A) (OC)

30 36.3 85: IS 5.66

30 33.8 87: 13 6.69

30 40.0 84: 16 5.25

45 .76 0.016 30 45.0 84: 16 5.25

48.36 0.011 30 43.8 85: 15 5.66

47 .13 0.010 30 73.8 88 : 12 7.33

5 1.50 0.003 30 29.0 8 1: 19 4.26

49.20 0.009 30 2 1.0 8 1: I 8.4 4.40

50.60 0.0007 30 34.5 87: 13 6.69

50 23 .0 80:19.7 4.06

50 31.3 80:20 4.00

SO 30.2 77:23 3.35

100 22.2 77:23 3.35

100 23.9 78:22 3.55

100 23.3 7 1:29 2.45

format ion of more stable pillars in case of mi xed oxide pillaring. A maximum is observed for al l sam­

ples at 300°C , which results from the c lay dehydration without the loss of OH groups . Similar observation was made by Casti llo et 01. 12

, but at lower temperature

(at -200°C) . The decrease in surface area at higher calc ination temperatures may be due to the c lay and pillar dehydroxylation alo ng with the sintering o f the pi llars 13

. The pore vo lume and pore size dis tribution ana lysis (obtained from nit rogen adsorp­tion/desorption isotherms) showed that a ll the samples contained uniform pores with average pore diamete r and total pore vo lume close to 47 .A. and 0 .2 cc/g re­

spectively at 300°C calcinations temperature (Ta­ble I). With increase in calcinatio n temperature to

500DC, there is a slight decrease in to tal pore volume and increase in average pore diamete r close to 50 .A. in al l cases. But, thi s effect could be due to the collapse of mic ropores to form broader pores, which is evident

from decrease in micropore vo lumes for 500°C cal­cined samples in comparison to those calcined at

300°C. Further, it can be observed that pil laring the c lay with different pill aring agents do not have any impact on the original pore structure of the PILCs, as a ll the samples show almost same pore volume and pore diameter. The surface area, pore size distribu tion

~

y

)

I

DAS et (Ii.: CATA LYSIS BY Zr, Ti & Zr-Ti MIXED OX IDE PILLARED MONTMOR ILLONITE 224 1

--------Table 2- Acid ic propert ies o f PILCs ca lcini ed at different temperatures in organic bases

Pyridine (~ mol/g) Ca lci nation temp. (OC)

Sample 11 0°C 300°C 400uC

Zr PILC 387 425 347

T i PILC 34 1 430 398

Zr-Ti PILC 370 474 43 1

and basal spacing values prove that the prepared sam­ple is stable even upto calcination temperature 500°e.

The weight loss pattern of the samples was studied in the temperature range 200°C to 800°C with an in­terval of IOO°e. It is observed that major weight loss takes place in two stages, the first stage is below 300°C and another region is in between 500°C to 700°e. The first stage weight loss is primarily due to removal of adsorbed water molecul es from both clay and pillars. But the second phase weight loss close to 500°C may be attributed to the dehydroxy lati on from pillaring cati ons. Similar observation has been made earli er but at a sli ghtly lower temperaturel~. This hi gh temperature dehydroxy lation may be due to the stronger interacti on between the hydroxy l groups and metal pill ars. This also supports the decrease in sur­face area with calcination temperature, as di scussed in earli er section. The weight loss around 700°C corre­sponds to dehydroxy lation of clay octahedral layers 15.

The total weight loss is observed to be highest for Zr­Ti PILe.

The measurement of ac id sites by the adsorption of organic bases (Table 2) shows th at with increase in calc inati on temperature, both the number of strong ac id sites (measured by adsorpti on of PY) and total ac id sites (measured by adsorption of PP) for all sam­ples increased up to 300°C and thereafter decreased graduall y. The lowest va lue is observed for 500°C calcined sample. Similar observat ions were also re­ported earli er I6

.17

. The loss of acid sites fo r 500°C cal­ci nati on temperature may be due to partial collapse of ox ide pillars and decrease in surface area. Table 2 clearl y indicates the number of both type of acid sites is hi ghest for Zr-Ti PILC ca lcined at 300°C and it follows the order Zr-Ti PILC > Zr PILC > Ti PILC with a littl e va ri ati on. The adsorpti on of PY and PP on the PI LC~ shows that the prepared samples possess higher acid sites in co mpari son to Na-Mont. It further suggests that pi llaring the cbys wi th suitable metal ox ides can enhance the number of ac id sites as well as their strength .

Piperidine ()l mol/g) Ca lci nation temp. (OC)

500"C 110°C 300"C 400uC 500"C

3 16 586 609 614 463

320 557 59 1 547 4 18

385 570 644 6 17 4 15

Most of the heterogeneously catalyzed nitrati on re­act ions are found to depend on both Bronsted and Lewis acid sites present in the catal yst. Bronsted sites are responsible for generating nitronium ion from acetyl nitrate while the Lewis acid si tes design the mechani sm towards the formation of more para iso-

d I· 10 S I . . mers as suggeste ear ler . 0 cata ysts contalnll1g higher surface densi ty of Bronsted acid sites will eventually be more ac tive for nitration subject to the condition that the strength of each sites must be suit­abl e to decompose acetyl nitrate and to generate ni­tronium ion. Since transiti on metal oxides , particu­larly Zr and Ti are very resistant to both acids and alkali s having suitab le acidic properties, pillaring with these metals generate more acid sites and prov ide better thermal stability to the system. The selectivity towards a particular product is also supposed to im­prove because of ex pansion of layers, which give a particu lar direc tion to the nitration product.

The reaction between ch lorobenzene and the ni­trating agent is a three-step process under the studi ed reaction conditions. The steps in volved are as follows:

( I )

CH 3COON02+H+(from catalyst)---jCH}COOH+N02 + (2)

C6HsCI+NO/---jC6H4CIN02+W . .. (3) (Para and OrtlIo)

The yields obtained (calculated from the amount of nitro-deri vati ve formed) by the PILCs at different temperatures are presented in Table I, along with the physico-chemical parameters of the catalysts. It is clear that there ex ists more or less a parallel relati on­ship between the number of strong acid sites (calcu­laled by the adsorption of PY) and the yield (%). The yield is more for 300°C calcined samples hav ing highest number of ac id sites. At the same time, there is a clear jump of yield (%) from nearly 45 % for both lhe single oxide pill ared clays to 73% for Zr-Ti PILC (all calcined at 300°C and for the reaction at 30°C) for

2242 INOIAN J CHEM . SEC A. NOV EMB ER 2002

80 .-------------------------------~ 8

70 7

60 6

50

30 -- Ti PIle

20 -6- Zr-Ti PILe 2 --O-Zr PIle

10 -<r-Ti Pile

---<>- Zr-Ti Pil e u

o 100 200 300 400 500 600

Calcination Temperature ("C)

Fig. 2-COITe l ~ll i o ll or y icld ('If,) ,Ind corresponciing pom to orlho

rali o wilh ca lcination temperature ror nitrat ion or chlorabcllzcllC O il PILes at 3()OC

small increase ill acid si tes. As di sc ussed above, since the reac ti on is catalyzed by Bronsted acid sites, we may here pred ict that the Zr-Ti PILC contains more Bronsted sites. But with increase in cal cination tem­perature from 300 to 5000C, the yield comes down (Fig. 2). The decrease in yield beyond 300°C is per­haps clue to loss of Bronstecl acid sites . As most of the Bronsted acid si tes are associa ted with the clay layers only, change of Bronsted acidi ty is less marked due to I . . I ' 11 17 18 H . . I . d I c lange In OXIC e pi aI's . . owe ve l' , It IS mar"e t lat

the total yie ld va ri es wi th change in ox ide pillars fol­lowing the order Zr-Ti PILC > Zr PILC > Ti PILC for the reac ti on at room telllperalUre for all sampl es . Thi s shows the importa nce of the ox ide pillars in nitrati on. But for hi gher reac ti on temperature, the yield docs not obey any order. Thi s 111ay be due to change in the na­ture of ac ti ve sites or the catal ys t at hi gher reaction tem perature in presence of reacting molecul es . On the other hand , with increase in reaction temperature, un­like the resu lts obtained by Mi shra e/ al. for Fe, Mn and Cr PILCs7

, the yie ld co mes dow n. For the who le se ri es of react ion, no meta product was detected.

It is interesting to note th at , all the samples show d para se lect ivity close to 85% for the reactions at 30°e. Thi s hi gh para se lectiv ity and close ness of the va lues sugges t the presence of pores \-vith uniform pore size dist ributi ons, in side whi ch most part of reac ti on takes place. Thi s was actua ll y observed from nitrogen adso rption/desorption iso th erm that the pore size distributi ons are of monoilloda l type and all the

sampl es contained equi valent pore radius (close to 45 and 50 A fo r 300 and 500°C calci natio n temperature" respec ti vel y), i rrespec ti ve of the na ture of pi lIaring agents. The highest yield with hi ghest .,elec ti vity towards para isomers was observed fo r Zr-Ti PILe calcined at 300°C (Table I ). This could be attributed to the presence of hi gher number of ac id sites fo r thi s sampl e (Table 2) . For hi gher reaction temperatures, both the yield and para selectivity followed a descending path without any order. Though the yield (%) was low, the amount of catalys t used in our case was less (0. I g) . When blank expe rimen ts were carri ed out without taking any catalyst, ::; 5% yield was observed and in presence of Na- 'vlont, yield upto 15 % was observed.

Surface area and basal spacing values of all the PILCs sugges t that pillaring is most effective in case of Zr-Ti mi xed ox ide PILe. Thi s also possesses high­est surface area and hi ghest num ber of acid si tes in co mparison to the other two PILCs for all calcination tempera tures. The pore vo lume and average pore di ­ameter are equi valent irrespec ti ve of the nature of pillaring agents with a monomodal pore size di stribu­tion. The cata lys ts ca lcined at 300"C are more ac ti ve fo r nit rat ion of chlorobenzene th ' 11 110°C and 500°C ca lcined sampl es. The yield (%) of th e nitrated prod­ucts for different sa mpl es follows the order Zr-Ti PILC > Zr PILC > Ti PILe. The higher yield in case of Zr-Ti PILC may be attributed to the hi gher con ten t of Bronsted ac id sites. The selecti vity towards the para isomer fo r all samp les is close to 85 % with Zr-Ti PILC (calcined at 300°C) show ing highest se lectiv ity (88%) .

Acknowledgement The authors are thanHul to Dr. V N Misra, Di ­

rector. RRL, Bhu baneswar, for permitting [ 0 publi sh thi s paper. We are al so thankfu l to Dr. R S Thakur, Head , EM & IC Department fo r is va luab le sugges­tions.

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