Indian Journal of Chemistry Vol. 38A, February 1999, pp. 119-123

Surface characteristics and cracking activity of zirconium pillared clays

C Vijaya*t, K Rengaraj & B Sivasankar

Department of Chemistry, College of Engineering, Anna University, Chennai 600 025, India

Received 5 January 1998; revised 17 August 1998

Z irconium pill ared montmorillonite and vermiculite catalysts have been prepared using four different methods by inter­calation with zirconium oxychloride solution . The surface characteristics of the catalysts have been characterized using ni­trogen adsorption , XRD and IR studies. The nature of acid site is identified on the basis of IR spectral data of pyridine and ammonia adsorbed zirconium pillared catalysts. The acidity of the samples was determined by butylamine titration as well as by thermal desorption of pyridine and ammonia. Differential scanning calorimetric technique has also been used to probe the nature of acidic sites, their heterogeneity, acid strength and density. Cumene cracking has been carried out as a test re­action of Bronsted acidity. The product selectivity is towards benzene. The cracking activity has been correlated with the density of Bronsted acid sites.

Pillared clays have attracted attention owing to a wide range of applications in catalysis. Pillared clays are two dimensional zeolite-like materials prepared by exchanging charge compensating cations between the clay layers with oligomeric inorganic hydroxy cations, formed by hydrolysis of metal ions ' . The ca­tions undergo dehydration and dehydroxylation on heating fonning stable metal oxide clusters that act as pillars . The intercalation facilitates the separation of silicate layers and creates an interlayer space (gal­lery) of molecular dimensions2

.J

. Several inorganic hydroxy species have been used for intercalation in­cluding those of aluminium, zirconium, chromium, iron, nickel and titanium4

-9

. In the present work, zir­conium pillared clay catalysts have been prepared using four different preparative methods and the ef­fect of the methods on the acidic properties and cracking activity of zirconium pillared clay catalysts has been investigated.

Materials and Methods MontmorIllonite (Fluka) and venniculite (Tamil

Nadu Minerals Ltd.) clays were used for the prepara­tion of zirconium pillared clay catalysts. Sodium chloride, zirconium oxychloride, cumene, magnesium and zinc carbonates used were of AR / CDH grade. The clays (montmorillonite and venniculite) were

t Present address : Department or Chemi stry, Kongu Engineering College , Perundurai ()38 052, India .

converted to sodium fonn by ion-exchange treatment with 1M NaCI solution followed by centrifugation.

Preparation of zirconium pillared clay catalysts Zirconium pi lIared montmorillonite and vennicu­

lite clay catalysts were prepared using four different methods of intercalation by modifying the procedures given in the literature 'o." . For all the preparative methods sodium fonns of montmorillonite and ver­miculite were taken.

In the first method of intercalation, the sodium fonn of the clays was treated with zirconium oxy­chloride solution (0 .1 M) with slow stirring. The pH of the solution was adjusted to 4 .0 and the slurry was stirred for 24 h for ageing. The residue was collected on a sintered funnel , washed, dried overnight at 100°C and calcined at 250°C for 2 h. In the second preparative method, the sodium fonn of the clays was dispersed in I: I acetone-water medium followed by addition of zirconium oxychloride solution (0.05 , 0.1 and 0.15 M). The mixture was then aged at room temperature for 2 h, filtered, washed and dried over­night at lOS-110°C. It was calcined at 400°C for 16 h.

In the third method of preparation, the sodium fonn of the clays was added to O.IM zirconium oxy­chloride solution which had been previously refluxed for 5 h. The resultant mixture was refluxed further for I h. The product was collected on a filter, washed and dried overnight at 110°C. The dried sample was cal­cined at 400°C for 16 h. In the fourth method of in-

120 INDIAN 1. CHEM ., SEC. A, DECEMBER 1998

tercalation, the sodi um fonn of the clay was added to polY\linyl alcohol solution (10 wt%) with stirring. The z irconium oxychloride solution (0 .1M) was then added to the resultant mixture with stirring. The paste thus obta ined was aged at room tempera ture for 24 h. The mass was co llected on a filter, washed and dried at 60°C for 24 h. The dried sample was calcined at 500°C for I h.

Magnesium oxide (MgO) and zinc oxide (ZnO) impregnated z irconium pillared montmorillonites were prepareu by wet impregnation of the pillared clays with a known weight per cent of magnesium and z inc acetate so lut ions. The slurry was stirred for 2 h fol lowed by dryi ng at II oce fo r 3 h. The dried sample was ca lcined at 500 0 e for I h. The catalysts prepared by eli fferent preparati ve methods in the pre­sen t study are given in Table I.

The ca tal yst sampl es were characterized by various phys ico-chem ica l techniques. The chemica l compo­sition was det ermined by wet chemi ca l analysis. Zir­conium con tent was estimated by ARL 3410 induc­tivel y cou pl ed pla sma emi ss ion spectroscopy at 343 .8 nm . Surface area of the catalyst samples was deter­mined by nitrogen adsorp ti on u.; ing a Micromeritics Pul sc Chem isorb 2700 system. X-ray diffractof,Tfams were reco rded on a Rich Seifert X-ray di ffractometer using ni cke l fi lt ered CuKa radi ation (A."a, = 1.5418 A). The overa ll aci dit y of the c lay samples was de­\ermined by t\\'o different methods involving three differe nt probe molecu les. The first method in vo lved the back titration of excess unadsorbed butylamine in aqueo us med ium aga inst standard hydrochloric acid so lu tion us ing bromothymol blue indicator. The sec­ond method in vo lved the vapour phase adsorption of the prohe mol ec ules ammonia and pyridine fo llowed by thermal desorption by heating the samples to 600°e. In frared spectra of pyridine and ammonia ad­sorbed samples were recorded using se lf-supporting wafers on a Perkin-Elmer 983G spectrophotometer in order to iden ti fy the nature of aci d sites. Differential scanning calorimetric thermograms of the pyridine adsorbed catal yst samples were recorded on a Mettler DSe -20 unit with TA 3000 microprocessor adopting a nullifying technique '2 to detennine the nature of acidic s ites, the acid strength as well as the density of the ac idic sites.

Zi rconium pillared montmorillonite and vennicu­lite catal ysts prepared by different methods were in­vestigated for their crackin g act ivity. The reaction

was carried out in vapour phase, in a fixed bed flow reactor. In the present study cumene cracking was chosen as a test reaction of Bronsted acidity. The re­action was quantitati ve ly studied by varying the tem­perature and contact time [WIF where W is the weight of the catalyst (0.3 g) and F is the flow rate of the reactant]. The flow-rate of cumene was controlled with the help of a microprocessor controlled infusion pump. Products were analyzed by gas chromatogra­phy, using a carbowax 20M column .

Results and Discussion Raw montmorillonite and venniculite have surface

area of 20.0 and 3.5 m2/g respectively . Intercalation of the clays by zirconium polyoxo species increased the surface area in the range of 74-1 50 m 2 /g in case of montmorillonite based catalysts and in the range of 9-42 m2/g in case of vermiculite based catalysts. The layer structure of the clays remained unaltered due to intercalation as indicated by the XRD data. The X-ray diffractogram indi ca ted the enhancement of doo , spaci ng from 9.4 A of raw c lay to 17.2 A for a repre­sentative zi rconium pillared montmorillonite clay [MZrs8o-(A/W)] catalyst.

The relative acidi ties of the samples as determined by butyl amine titration method are given in Table I. It was observed that the relative acidity was higher for the samples prepared by intercalation using ace­tone/wate r medium [MZrS80 - (A/W)]. The acidity values of the samples as detennined by the thennal deSOrPtion of pyridine and ammonia was found to be higher for the sample with higher zirconium weight % (Table 2) . In the case of zirconium pillared ver­miculite catalysts the overall acidity was found to be higher for the sample pillared in the presence of polyvinyl alcohol solut ion . The IR spectra of pyridine adsorbed zirconium pillared samples clearly indicated the presence of three types of acid sites IJ

. The pres­ence of Lewis acid s ites was indicated by strong IR absorption bands at 1445 ,1491 and 1642 em-I while the presence of Bronsted acid sites was indicated by bands at 1642, 1550 and 1491cm-l

. In addition rela­tively weaker acid sites were also found to occur on zirconium pillared clays as indicated by the presence of hydrogen-bonded pyridine. These bands were found to overlap with the other bands assigned to Lewis and Bronsted ac id sites.

Differential scanning calorimetry has been used in the present study to probe the nature of the surface

VIlA Y A et al: SURFACE CHARACTERISTICS OF ZIRCONIUM PllLARED CLAYS 121

acidity, its heterogeneity, acid strength and density of acidic sites. The DSC thermograms of pyridine ad­sorbed montmorillonite samples showed three distinct peaks in the temperature ranges (i) 115-134°C (as­signed to pyridine desorption from relatively weak acid sites), (ii ) 289-309°C (indicative of pyridine de­sorption from Lewis acid sites) and (iii) 431-508°C (assigned to pyridine desorption from Bronsted acid sites) . The DSC thermograms of pyridine desorbed from zirconium pillared vermiculite indicated the presence of acid sites. But quantitative information with regard to the type and density of acidic sites could not be obtained as the desorption bands were very weak .

Cracking activity of zirconium pillared clay catalysts

Cumene cracking was quantitatively studied by varying the temperature and contact time. The cracking activity on zirconium pillared montmorillo­nites and vermiculites with different zirconium con­centra tion s and the effect of time on stream were also studied. Product analys is showed that the main prod­uct was benzene. In the case of vermiculite based catalysts the se lectivity towards benzene was uni­formly low .

The effect of temperature on the percentage con­versi.on of cumene and percentage selectivity towards benzene formation at fixed contact time (WiF = 41.78 g.h/g .mol) was investigated for all zirconium pillared clay samples prepared by di fferent methods in the temperature range of 573-823 K. The percentage conversion was fo und to increase with increase in reaction temperature.

The effect of preparative method on cracking ac­tivity was in vesti ga ted for the zi rconium pillared montmorillonite and vermi culite samples prepared by different methods of intercalation (the concentration of zi rconium oxychloride was maintained constant as 0.1 M). Thc data on the cracking activity of MZr6.63 -

(A), MZr4 95-(NW), MZr787-(R), MZr782-(PV A), VZr I68-(A), MZr,oo-(A/W), VZr2ST(R), and VZr868-(PV A) are given in Table 1. It was found that mont­morillonite b ased catalysts, MZr4 95-(NW) and MZrHT(R) showed hi gher cracking activity (16%) and se lecti vity towards ben zene (63%) at 723 K. This may be attributed to the relatively higher acidity val­ues <;>f MZr4 95-(A/W) and MZr78T(R) . In the case of verm iculite based catalysts, VZrloo-(AlW) inspite of

its lower acidity (0.19 m.mol /g) showed higher cracking activity and selectivity towards benzene compared to other catalysts.

The effect of zirconium concentration on cracking activity was studied for zirconium pillared mont­morillonites and vermiculites prepared with different concentrations of zirconium oxychloride solution (intercalation in acetone/water medium was main­tained constant). The catalytic activity of MZrS80-(NW), MZr49dNW), MZrI60-(NW), YZro6o-(NW), VZr1oo-(AlW) and VZroso-(NW) are given in Table 1. Both the cracking activity and selectivity towards benzene were found to be higher for MZrS80-(NW) and YZr,oo-(NW).

The cracking activity of the samples was found to decrease with increase in time due to coke formation and the catalysts were found to be active up to 3 h.

Kinetics of cumene cracking Kinetics of cumene cracking was quantitati vely

studied by varying the contact time (W iF 13 .92 to 27 .85 g.h/g.mol) and temperature (698-738 K) of the catalyst bed for MZrS80-(AlW). Experimental data were analysed on the basis of the integral method 14 using the equation.

- x'- 2In(l- x') = k PT / R r(WiF) ... ( I )

where x' is the fractional conversion, k is the rate con­stant, Pr is the total pressure, R is the gas constant and r is the temperature. The plot of - x'- 2In(l- x') versus W /F at different temperatures was found to be linear indicating the reaction to be of first order with respect to the concentration of cumene. From the slope of the Anhenius plot of log k vs . 1 / r the acti­vation energy E a (49.6±1 Kl/mol) and the frequency factor A (2 .5 x 103

) were calculated.

The con-elation of surface characteristics with cracking activity was investigated and it was found that in case of montmorillonite based catalysts, MZrS80-(NW) inspite of its slightly lower surface area (92 .9 m2/g ) showed higher cracking activity and selectivity towards benzene at 738 K. The same trend was observed in vermiculite based catalysts. VZr1oo-(NW) with a surface area of 13.2 m2/g showing higher cracking activity at 773 K. Among the four different preparative methods used, the zirconium pillared montmorillonite and vermiculite samples prepared in acetone/water medium was found to be more effective for cracking activity.

122 INDIAN J CHEM, SEC. A, FEBRUARY 1999

Table I-Surface area and cracking activity of zirconium pillared clay catalysts

Catal ysts' Surface Acidity Titrimetry % Cracking area (m 2/g ) (mmol Ig butylamine) activity

Montmor ill onite based catalysts MZro 20.0 0.31 3.5 MZr"bO - (A) 124.3 0.49 5.1 M Zr'80-(A /W) 92.9 0.82 30.9 MZr' 05 - (A /W) 97.6 0.59 15.9 MZr , "0 -(A /W) 99.6 0.47 2.9 MZrw -(R) 149.9 0.48 15.6 MZr"2 -(PYA) 74.5 0.35 5.2 MZr, KO-(A /W)M gO 88.3 0.35 4.1 MZr, 80-( A/W) ZnO 99. 1 0.12 6.3

Y crmi cu I ite based catal ys ts YZro 4.0 0.05 15 YZr l oR -(A) 34.6 0.10 I 1. 8 YZrooo -(A /W) 12.8 0. 16 9.7 YZr I ", -(A/W) 13 .2 0.19 13 .5 YZ r'o ,o-(A /W) 9.4 0.10 8.9 YZr257 -(R) 12.9 0.30 9.4 YZr, "g-(PYA) 41.7 0.39 10.6

% Selectivity towards benzene

52.9 85. 7 62.9

3.4 62.7

2.5 7.3

22.2

1.6 5. 1 7.4 4.4 5.3 1. 8

.• MZr and YZ r indicates 7irconium pill ared montmorillonite and vermiculite. Numeri cal subscripts in the sample code ind icate the f irconium co ntent (wt%). (A) , (A /W) , (R) and (PYA) stand for intercalation in aqueous medium, organ ic/aqueo us med ium, at elcvated temperJtu re and in the presence of pol yv inyl alcohol, respectively. Reaction conditions: W = 0.3 g, W /F = 41. 78 g.h/g. mo l, T = 723 K. Data takcn after I h time on stream.

Tab le 2- Acidi ty determinat ion of zirconium pillared montmorillonite clay catal ysts by thermal anal ytical techn iq ues

Thermogravimetry (m.mol Ig ) DSC data Ca talys ts Ammonia Pyridine Peak temp(°C) 6. Hexo(J/g) Assignment '

MZ r"",-(A) 6.40 1.43 134.0 GI.3 WA

47G.3 202.1 SA MZr, 80-(A /W) G.24 2.93 114.9 51.0 WA

291 .5 13 .0 LA 508.1 195.0 SA

M'Zr, ,,,-(A /W) 5.75 1.71 123 .0 44.0 WA 289.1 6.0 LA 508.1 69.0 BA

MZr l. bO-(A/W) 4.92 1.48 IIG.8 40.0 WA 291. 8 G.O LA 508. 2 70.0 BA

MZr1S7-(R) 7.17 3.24 MZr7>2-(PYA) 2. 89 1.30 125 0 24.4 WA

309.4 0.7 LA 431 .0 93 .4 SA

MZr, 8o-(A/W)M gO 4.07 0.89 11 83 52.2 WA 305 .3 4.G LA 472.1 92.2 BA

MZr, 8o-(A /W)ZnO 4.18 0.76 11 8.9 45.6 WA 303 .3 5.1 LA 478. 1 82.5 SA

• WA , LA and BA indicate pyridi ne desorbed from weak acid sites, lewis aeid sites and Bri:i nsted acid sites, respectivel y.

•

VIlA Y A et af. : SURF ACE CHARACTERISTICS OF Zm.CONIDM PllLARED CLA YS 123

W/F = 41 ·78g· h/Cj.mol 250 T = 738K ~O

~ ;'

.~ " c

~ E " " u

"t> '0 .. ... c ~ 150 20 .§ .Ii " > (; c

0

?: u ... c:; .. 0

50 0 0 ~ 8

W~~ht -I. of Zirconium

Fig. I- Plot of densi ty o f Brbnsted ac idity and % conversion of cumenc versus we ig ht % of z irconium for cUl)1ene cracking on z irconium pi ll ared montmorill onites (A / W)

In case of zirconium pillared clay samples pre­pared with different zirconium oxychloride concen­trations (NW ) the cracking activity was found to be hi gher for MZr,xo-(NW) and VZr ll)O-(A/W). Thus the crac king activity increased with increase in zirconium content. The cracking activity of zirconium pillared montmorill onite was found to increase as a function of overa ll acidity of the ca talysts (Table 1). The com­pari son of acidity va lues of MZr'80-(AlW), MZr4.w (A/W) and MZr 1hO-(A/ W) expressed as m.mol/g of butylam ine, ammotlla/pyr idine (Tables 1 and 2 ) shows that , MZr, xn-(AlW) have higher acidity and cracking activity.

The cracking activity can be correlated with the density of Bronsted acid s ites, in case of zi rconium pi ll ared montmorillonite catalysts. Maximum conver­sion and se lec ti vity towards benzene was observed for MZr, xo-( N W) and it was found to have the maximum density of Bronsted acidic sites as revealed by DSC studies (Table 2). The interrelationship be­t,veen the density of Bronsted acid sites and cracking ac ti vity IS indicated In

fig. 1.

Impregnation of magnesium oxidelzinc oxide on zirconium pillared montmorillonites (A/W) decreased the acidity and thereby the cracking activity. The overall cracking activity of MZrS80-(A/W) MgO and MZrS 80-(A/W)ZnO was found to be lower when com­pared to MZrS80-(AlW). This has been attributed due to the lowering of acidity as shown by DSC data given in Table 2. Thus a direct correlation between the density of Bronsted acidic sites and cracking ac­tivity was observed.

It can be concluded that pillaring of clays with polyoxo metal ions of zirconium seem to enhance Bronsted acidity more than the Lewis acidity. The cracking acti vity could be correlated to the overall acidity and specifically to the density of Bronsted acidity.

Acknowledgement The authors thank the Department of Science and

Technology, New Delhi , for providing financial as­sistance during the period of the work.

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