Transition metal pillared clay 4. A comparative study of textural, acidic and catalytic properties of chromia pillared montmorillonite and acid activated montmorillonite

APPLIED CATALYSIS A: GENERAL

ELSEVIER

Applied Catalysis

A: General

166 (1998) 123-I 33

Transition metal pillared clay 4. A comparative study of textural, acidic and catalytic properties of chromia pillared montmorillonite and acid activated montmorillonite Trilochan Mishra, Kulamani Parida* Regional Research Laboratory, Bhubaneswar-751013, Received

Orissa, India

19 March 1997; received in revised form 25 June 1997; accepted

8 July 1997

Abstract The preparation, characterisation and catalytic activities of chromia pillared montmorillonites were investigated. The Naexchanged montmorillonite shows higher uptake of chromium(II1) acetato complex than that of acid activated clay. However, in both the cases, stepwise ion-exchange process results in better intercalation compared to the single step process. Both the pillared materials are found thermally stable up to 773 K. Materials prepared from Na-exchanged clay show higher surface area, micropore volume and basal spacing, whereas the materials prepared from acid-activated clay possess higher mesopore volume, average pore diameter and surface acidity. A substantial increase in acidity due to acid activation before pillaring is observed in the 2-propanol and methanol dehydration reactions. Increase in cracking activity indicates the formation of an extra number of Bronsted acid sites due to the acid activation. However, with the increase in reaction temperature above 673 K, cracking activity decreases and dehydrogenation activity increases owing to the loss of Bronsted acid sites. Surface poisoning studies proved that alcohol dehydration and cumene cracking depend on the Bronsted acid sites, while dehydrogenation of cumene occurs on Lewis acid sites. 0 1998 Elsevier Science B.V. Keywords:

Chromia pillared montmorillonite;

Alcohol; Cumene

1. Introduction Pillared clays are materials prepared by intercalation of metal oxide precursors between the silicate layers smectite

of smectite clays

used successfully

type

pillared

of clays by metal

as acid catalysts

*Corresponding author. [email protected]

Fax:

(+91)

[ 11. In particular, oxides

have

been

in several organic 674-581637;

E-mail:

0926-860x/98/$19.00 c 1998 Elsevier Science B.V. All rights reserved. PZI SO926-860X(97)00247-0

reactions [2]. In this context, chromia pillared materials are of particular interest as the interlayer pillaring agent is itself catalytically active [3]. Various polycationic species of metal ions have been intercalated in clays [4,5] leading to a variety of materials with high basal spacings and surface areas. Chromiapillared clays are mostly prepared using cationic oligomeric species as the pillaring agents [6]. Recently the successful preparations of chromiumpillared montmorillonite using trinuclear chromium-

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Ctrmly.sis A: Gerwrcd 166 (1998)

(III) acetato complex as the pillaring agent with high thermal stability and basal spacings were reported [7,8] by different groups of researchers. As the type of polynuclear metal unit intercalated in the interlayers determines the characteristics of the pillared material, like surface area, porosity, acidity and hence the catalytic activities, it is worthwhile to study in detail the characterisation and catalytic activities of chromia-pillared clay prepared using trinuclear chromium(II1) acetato complex as a pillaring agent. In pillared clays, in addition to the clay layers, the metal oxide pillars also possess a certain amount of acidity [9, lo]. The combination of host layer and oxide pillars yield materials which possess both Lewis and Brcdnsted acid sites 191. Of course at high temperature (673-773 K) the acidity is predominantly of the Lewis type [lo]. The generally accepted view regarding the acidity of the pillared clays is that Lewis acid sites are mainly resident on the metal oxide pillars [ 1 I], whereas Bransted acid sites are associated with the structural OH groups present on the layers of the host clay [ 11,121. However, Bodoardo et. al. [ 131 have reported that the pillars can also contribute a significant number of Bronsted acid sites. Most of the attempts made so far at improving and controlling the acidity of the pillared clays are concentrated on changing the type of host matrix [ 14,151 and varying the identity of the pillars [ IO,1 11. Because most of the Bransted acid sites on pillared clays are believed to be associated with the clay layers [ 11,121, we have attempted to enhance the number of Brclnsted acid sites of the host matrix by acid treatment prior to pillaring. In this context, our earlier work on iron(II1) and manganese(II1) pillared montmorillonite showed an increase in acidity due to the acid activation before pillaring [16,17]. Alcohol dehydration and cumene cracking reactions are generally used as the probe reactions to characterise the acid-base properties of the catalysts [ 1g-201. Alcohol dehydration usually gives two types of products: olefins and ether. The selectivity to a particular product is determined by the strength of the acid sites, alcohol structures (primary, secondary or tertiary) and reaction temperatures. On the other hand, cumene cracking mainly depends on Bronsted acid sites, whereas dehydrogenation occurs on Lewis acid sites [21,22]. Thus it is possible to compare both

123-133

Brensted and Lewis acid sites in a catalyst through cumene conversion reaction. The present paper deals with the preparations, characterisations and catalytic activity of chromia pillared materials prepared from both the Naexchanged and acid-activated montmorillonite. The alcohol (2-propanol and methanol) dehydration and cumene cracking/dehydrogenation reactions are studied as test reactions to compare acid-base and catalytic activities of all the materials.

2. Experimental 2.1. Materials The Na-exchanged (Na-Mont) parent montmorillonite (Mainburg, Germany) and acid-activated montmorillonite (H-Mont) were used as the starting materials. Acid activation was carried out with sulphuric acid solution, maintaining the conditions adopted by Bovey et. al. [23]. According to this method the solution to clay ratio of 20 ml g-’ and acid to clay ratio (w/w) of 0.3 were maintained. Chemical analysis showed that nearly 25% of the metal ions (Al, Fe, and Mg) from the octahedral sites of the parent clay are removed due to acid treatment (Table 1). Ion-exchange capacity of the parent clay and acid activated clay were found to be 86 and 65 meq gP’, respectively. Trinuclear chromium(II1) acetato complex was prepared by a reported method [24] and was used as a pillaring agent. Ion exchange was carried out at 298 K for 3 h from a fixed volume of

Table 1 Chemical composition nite dried at 393 K

of parent and acid-activated

montmorillo-

Composition

Parent montmorillonite (wt.%)

Acid-activated montmorillonite (wt.%)

SiOz Al203 FezOJ cao

62.0 19.7 4.1 3.3 4.8 0.8 1.8 3.5

72.0 14.9 3.2 2.3 3.2 0.5 I .4 2.5

MgO Na?O K20 Hz0

7: Mishra,

K. Parida/Applied

Catalysis

complex and clay suspension by varying the complex to clay ratio from 5 to 40 mmol g-‘. In addition to the single step ion exchange process, multi-step ion exchange was also carried out, taking 3 mmol of Cr(III)/g of clay in each time. The resulting samples were washed and dried at room temperature. The airdried samples were calcined at 383, 573, 673 and 773 K, collected in air-tight bottles and stored in a desiccator for further use. Total chromium in the complex as well as in the pillared material was estimated by Atomic Absorption Spectrometry. 2.2. Textural properties The XRD patterns of the oriented pillared clay samples on glass slides were recorded on a Philips semiautomatic diffractometer with Ni-filtered Cuk,, radiation from 20 = 2 to 20 . Basal spacing was calculated from d’,,,’ values. Adsorption of ethylene glycol vapour was also carried out on the same samples to observe any change in basal spacing. TG/DSC analysis of the room temperature dried samples were carried out in dry air using a Stanton Redcraft (STA 625) thermal analyser in the temperature range of 303 to 873 K at a heating rate of IOKmin’. IR spectra of the pillared samples calcined at 383 and 673 K were recorded with a Jasco FTIR spectrometer in the range of 4000400 cm-’ in KBr phase. All the samples were degassed at 383 K in vacuum ( 1 x 1O--” Torr) before analysis. Solid state UV/VIS spectra of trinuclear chromium(III) acetato complex, chromium(III) pillared samples calcined at 383, 573 and 773 K, were recorded on a Varian spectrophotometer using BaS04 as the reference. Surface areas (BET), pore volumes, average pore diameters and pore size distributions were determined by nitrogen adsorption-desorption method at liquid nitrogen temperature using Quantasorb (Quantachrome, USA). Prior to the adsorption desorption measurements, all the samples were degassed at 393 K and lops Torr for 5 h. Since the validity of the r-plot based on the C values of the BET equation has been criticized, the use of a, plot, defined as o, = (n/n,),’ (n, is the amount adsorbed by the reference solid at P/P0 = s) has been proposed [25-271. According to Sing [28] it is convenient to

A: Grtmal

I66 (199X) 123-133

125

place cv, = 1 at P/P” = 0.4, since monolayer coverage and micropore filling occurs at P/P’) < 0.4, while capillary condensation takes place at P/P0 > 0.4. Here we have used Na-montmorillonite as the reference material with the assumption that it has no microporosity. 2.3. Surjke

acidity

Surface acidity was determined by a spectrophotometric method [29] on the basis of irreversible adsorption of organic bases such as pyridine (PY), piperidine (PP) and 2.6-dimethyl pyridine (DMPY). In all the cases, adsorption was carried out in cyclohexane solution at 298 K according to a detailed procedure described earlier [29.30]. 2.4. Catalytic activity Catalytic activities of all the samples for alcohol conversions (2-propanol and methanol) were studied in a fixed bed catalytic reactor (10 mm) with on line CC. Prior to the reactions, all the catalysts were preheated in nitrogen atmosphere at 573 K for 1 h. The conversion of 2-propanol was carried out in the temperature range of 413 to 573 K whereas methanol conversion was tested within 523 to 723 K. Alcohol was quantitatively supplied to the reactor from a continuous microfeeder (Orion. U.S.A) through a vapouriser using nitrogen as the carrier gas (flow rate 70-90 ml/min). To avoid condensation of alcohol and liquid products in the apparatus, all the connections from reactor to Gas Chromatograph were heated at 423 K by a heating tape. All the reaction products are analysed by means of on line CC (CIC, India) in FID mode using Paropak Q and T columns. Rates of reactions were calculated under steady state conditions by applying the equation: x = K,(W/F)

(1)

Here K:, is the rate of conversion in mol g ’ SC’. W is the weight of the catalyst in g, X is the percentage of conversion and F is the flow rate of the reactant in mol 5s’. The rate of reaction was calculated from the slopes of the straight lines obtained by plotting X vs. F-‘. Cumene cracking/dehydrogenation reactions were carried out in a micropulse reactor using nitrogen as

126

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Catalysis A: General 166 (1998) 123-133

the carrier gas in the temperature range of 573 to 773 K. All the catalysts were activated at 573 K before the catalytic reactions. The volume of each cumene pulse was maintained at 1 ml (7.2 mmol). Products were analysed by the Gas Chromatograph on line with the microreactor using the 10 ft SS column with 10% TCEF? Rates of reaction was calculated using the Bassett-Habgood equation for the first order kinetic processes [31].

3. Result and discussion 3.1. Intercalation The uptake of trinuclear chromium(II1) acetato complex by both Na-exchanged and acid activated montmorillonite are presented in Fig. 1. It is seen that the uptake of Cr(II1) increased with the increase in its concentration up to 20 mmol g-’ of clay and thereafter decreased on further increase in concentration. The values are low in the cases of acid-activated clay in comparison to those of the Na-exchanged clay. A similar trend was also observed in cases of Fe(II1) and Mn(II1) complex uptake [16,17] which can be correlated to the low cation exchange capacity of the acid activated clay. But in both the cases higher intercalation is observed when stepwise ion-exchange is car-

3

0 0

10

20

30

40

50

Cr(II1) added in mmol/g of clay Fig. 1. Uptake of Cr(III) in single step ion-exchange Na-Mont and (b) H-Mont, and in multistep process: and (d) H-Mont.

process: (a) (c) Na-Mont

ried out taking 3 mmol of complex/g of clay in each time and repeating for five times. (Tables 2 and 3). It is also observed that the basal spacings and surface areas increase with the increase in Cr(II1) concentration up to 20 mm01 g- ’ and thereafter decrease, although there is only a little decrease in Cr(II1) uptake. So it seems that at higher Cr(II1) concentration in solution, a trinuclear Cr(II1) complex polymerises to larger size, which hinders intercalation. So instead of going inside the clay layer, a part of it is adsorbed on the surface which rather blocked the pores, resulting in low surface area. However, the samples prepared by a stepwise ion-exchange process (Cr-NaMont-15s and CrHMont-15s) show the highest basal spacings and surface areas. For further comparison studies, only samples prepared by stepwise ion-exchange process are used. 3.2. Textural properties The daai basal spacings of Cr-NaMont-15s and CrHMont- 15s samples decrease with the increase in calcination temperature. However, both the samples are stabde up to 773 K having the gallery height of 7.7 and 8 A, respectively. Moreover, both the samples after ethyleneglycol adsorption show the same interlayer distances, which indicate the stabilization of oxide pillars due to strong complex-to-clay interactions. TG/DSC thermograms (Fig. 2) of both the samples show an initial endothermic peak within 423 K, indicating the loss of interlayer water. A second intense exothermic peak within 538 to 628 K indicates the decomposition of the acetyl group of the complex. FT-IR spectra of both the air-dried samples (Fig. 3) show the characteristic bands at 1550 and 1448 cm-‘, corresponding to the COO- asymmetrical (n,,) and symmetrical (n,) stretching vibrations, respectively. The difference (nas - n,) of 102 cm-’ is the characteristic of the bidentate acetate group [32]. This indicates that the intercalated complex still retains its bidentate acetate group inside the clay layers. But the sample calcined at 673 K does not show the above characteristic peak, indicating the complete decomposition of the complex forming the oxide pillars. UV-VIS spectra of 383, 623 and 773 K calcined samples presented in Fig. 4 indicate that the samples

7: Mishra, K. Parida/Applied

Catalysis A: General 166 (1998) 123-133

127

Table 2 Variation of Cr(lI1) uptake, surface area and basal spacing with change in complex to clay ratio for acid-activated Sample no.

I

2 3 4 5 6 7 x Sample prepared

Sample code

Cr(II1) added (mmolg

‘)

2 5 I0 I5 20 30 40 IS

Cr-HMont-2 Cr-HMont-5 Cr-HMont- IO Cr-HMont- I5 Cr-HMont-20 Cr.HMont-30 Cr.HMont-40 Cr.HMont- 1%‘ by stepwise ion-exchange

-

montmorillonite

Cr(III) in the solution (mol I ‘)

Cr(II1) in the pillared sample (wt.7c)

area (m’ g-

0.004 0.0 I 0.02 0.03 0.04 0.06 0.08 0.006

6.5 8.0 10.4 I I.1 12.0 I I.91 II.88 12.15

179 202 240 251 26.5 242 220 270

Surface

Sample code (mm01 g ‘)

I 2 3 4 5 6 I X ’ Sample prepared

2

Cr-NaMont-2 Cr-NaMont-5 Cr-NaMont-IO Cr-NaMont- I5 Cr-NaMont-20 Cr-NaMont-30 Cr-NaMont-40 Cr.NaMont-1%’

5

IO IS 20 30 40 I5

by stepwise ion-exchange

montmorillonite

Cr(II1) in the solution (mol I ‘)

Cr(III) in the pillared sample (wt%)

Surface area (m’y-‘)

Basal spacing at 773 K(w)

0.004 0.0 I 0.02 0.03 0.04 0.06 0.08 0.006

6.7 x.9 I I I.5 12.21 13.00 12.96 12.91 13.1

I81 209 2s I 269 280 263 232 286

14.4 15.8 l6.S 17.1 17.5 17.0 16.6 17.8

process (taking 3 mmol of Cr(III)/g of clay) in each step.

calcined at 383 and 773 K correspond to the characteristic spectra of Cr(III), whereas the same samples calcined at 573 K show the formation of two new bands at 277 and 374 nm. These may be due to the presence of Cr(V1) with Cr(II1). Therefore it seems that this complex decomposes to Cr203 via Cr(VI), as presented below. +3/20, 2Cr(III) + 40R-

14.2 IS.5 16.3 16.X 17.2 16.6 16.0 17.5

process (taking 3 mmol of Cr(III)/g of clay)

Table 3 Variation of Cr(II1) uptake, surface area and basal spacin g with change in complex to clay ratio for Na-exchanged Sample no.

’1

Basal spacing at 773 K (A,

2Cr0, + H,O + 2H’

I WA

-3120,

The disappearance of the exothermic peak due to the oxidation of Cr(II1) to Cr(V1) in the DTA curve [33] may be owing to the presence of the intense exothermic peak associated with the decomposition of acetyl group within the same temperature range.

The changes in surface areas of both the samples (Cr-NaMont- 1.5s and Cr-HMont- 15s) with respect to the calcination temperatures are presented in Table 4. They indicate an increase in surface areas with the calcination temperatures. Similarly, the pore volume also increases with the calcination temperature. There is a marked increase in the surface area in both the pillared materials in comparison to the parent montmorillonite. The formation of new pores inside the clay layers is probably responsible for the increase in the surface areas. The adsorption-desorption isotherms of the material prepared from Na-exchanged clay is nearly of type I, whereas the material prepared from the acid-activated clay are nearly of type II or IV in BDDT classifications [34]. Mokaya et al. [35] also observed this type of change in the hystersis loop due to acid activation prior to pillaring. According to the IUPAC classification, these hysteresis loops can be

T. Mishra, K. PariaWApplied Catalysis A: General 166 (1998) 123-133

128

12%

1650 Wave

92 -

number

2050

(cm-‘1

Fig. 3. m-IR spectra of Cr-HMont calcined at (a) 383, (c) 673 K and Cr-NaMont calcined at (b) 383 and (d) 673 K

30 -

Fig. 2. TCVDSC analysis of Cr(II1) pillared samples dried at room temperature (K).

included in type Hs. Materials that show this type of hysteresis generally possess slit-shaped pores and plate-like particles with spaces between the parallel plates. The high Cnnr values of all the pillared samples show the presence of a microporous structure [36]. Assuming the pores to be cylindrical, the average pore diameter is calculated using the formula d = 4V,/S,, where d is the average pore diameter, VP is the pore volume and 5, is the specific internal surface area of the pores. The decrease in average pore diameter with the increase in calcination temperature (Table 4) is also supported by the mesopore size distribution curve calculated by BJH equation [37] and presented in Fig. 5. Interestingly, the mesopore size distributions of acid-activated pillared samples are found within a broad range, whereas for the material prepared from Na-exchanged clay, the distributions are found only in a small range. This type of discrimination may be due to the formation of extra mesopores due to the acid-activation, which clearly

u

1

300 A (nml

Fig. 4. Diffuse reflectance spectra of (A) Cr(III) acetato complex and Cr(II1) pillared samples calcined at (B) 383, (C) 623 and (D) 773 K.

indicates that the pore size can be better controlled using Na-exchanged clay as the starting material. The acid-activated materials show somewhat low basal spacings and surface areas but possess significantly higher total pore volumes. However, the material prepared from the Na-exchanged clay shows a higher micropore volume (V,). Specific surface areas (S,)

T. Mishra, K. Parida/Applied

Table 4 Textural parameters Sample

of chromia-pillared Calcination temperature

Catalysis A: General

clay at different calcination

383 513 673 773 383 513 613 773

123-133

129

temperature

&ET

SC,

(m2 gg’)

Cm2 g-9

Pore volume (ml cme3)

95 98 241 280 286 110 108 242 265 270

-

-

235 -

0.232 -

643 832

(K) NaMont Cr-NaMont Cr-NaMont Cr-NaMont Cr-NaMont HMont Cr-HMont Cr-HMont Cr-HMont Cr-HMont

166 (1998)

CBET

287

0.245

-

-

230 -

0.254 -

468

264

0.273

702

Basal spacing

Average pore diameter

(A)

(A)

22.0 18.3 17.9 17.8 -

0.168

21.5 17.9 17.5 17.5

0.122 0.141

Table 5 Surface acidities of chromia-pillared temperatures Sample code

Calcination temperature

Micropore volume (V,) (ml cm-3)

samples calcined

at different

PY (pm01 g- ‘)

PP (urn01 go ‘)

DMPY (pm01 gg’)

280 310 620 600 410 380 390 655 650 430

450 520 900 910 680 770 810 950 950 710

115 120 300 300 60 170 180 315 310 95

(K) Na-Mont Cr-NaMont Cr-NaMont Cr-NaMont Cr-NaMont HMont Cr-HMont Cr-HMont Cr-HMont Cr-HMont

Pore

diameter

(iI

Fig. 5. Distribution of mesopore sizes as a function of pore diameter of Cr-NaMont calcined at (A) 573, (B) 773 K and CrHMont calcined at (C) 573, (D) 773 K.

calculated from the a, plots (Table 4) are very close to the SBET values, indicating the accuracy of the reference method. 3.3. Surface acidity Surface acidity of all the samples show a decreasing trend with the increase in calcination temperature from 573 to 773 K (Table 5). On the basis of pK,

28 283 573 673 773 283 283 573 673 773

of the bases, piperidine (PP, pK, = 11.1) measures the total acidity, whereas pyridine (PY, pK, = 5.3) measures only the strong acid sites. However, 2,6-dimethyl pyridine (DMPY) can be preferentially adsorbed on the Bronsted acid sites [38]. So, to estimate the Bronsted acidity, DMPY is used as the adsorbate. Adsorption of DMPY is found to be the highest in case of samples calcined at 573 K, whereas it has the lowest amount at 773 K calcination. This supports the earlier report that Bronsted acidity of pillared clays decreases to a low value at 773 K. At all calcination temperatures, Cr-HMont-15, showed higher acidity (total as well as Bronsted) than Cr-NaMont-15, sample. This shows the importance of the use of acid activated clay for the preparation of acidic chromia pillared clays.

3.4. Catalytic activities 3.4.1. Alcohol dehydrrrtion

The 2-propanol conversion produces propene as the major product, while acetone (3 mot%) and isopropyl ether (4 mol%) are formed as the minor products. It is seen from the Arrhenius plot (Fig. 6) that the 2propanol dehydration activity decreases with the increase in calcination temperature of the catalysts, which can be correlated to the decrease in the number of surface active sites. Samples prepared from the acid-activated clay show higher rates of propene formation at all calcination temperatures (Table 6). This increase in the rate of propene formation may be due to the increase in the number of acid sites due to the acid activation. Rates of propene formation and the activation parameters calculated from the Arrhenius plot are summarised in Table 6. In all the cases, a least square regression analysis shows a correlation coefficient over 0.99 and all the data are reproducible within ~t6%. It is observed that irrespective of the variations in rate of propene formation, the activation energy is within a range of 9.8-12.6 kcal mol-’ for all the catalysts. This justifies the conclusion that the increase in the calcination temperature does not much affect the nature of the reactive sites. The drastic decrease in the

Rate of reaction montmorillonite

(K.,) and activation

Calcination

Catalyst5 2-Propanol + Cr-NaMont Cr-NaMont Cr-NaMont Cr-HMont Cr-HMont Cr-HMont Cumene Cr-NaMont Cr-HMont Methanol Cr-NaMont Cr-HMong

parameters

I

?B ?

xD

AC

xE

OF

-11 I 2.2

2.1

2.4

2.3 l/TX

2.5

2.6

1000

Fig. 6. Arrhenius plot for 2-propanol comersion over Cr-NaMont calcined at (A) 573, (B) 673. (C) 773 K and Cr.HMont calcined at (D) 573. (E) 673, (F) 773 K.

rate of propene formation for 773 K calcined samples is mainly due to the loss of Bronsted acid sites. At the same time dehydrogenation activity of all the chromia pillared catalysts is found to be very low in comparison to pure CrzO-+ which may be due to the homo-

(E, and In A) for 2.propanol.

temperature

oA

methanol

and cumene

conversion

on chromia-pillared

Ed,

In A”

10.3 * 0.4 Il.2 * 0.2 12.6 zt 0.3 9.8 IL 0.5 10.9 It 0.4

PI.1 -2.1 ~ I .7 ~I.5 -2.0

Ii .h + 0.2

-1.3

115.5’

23.0 + 0.4 24. I T 0.2

-0.x

276’ 381’

19.3 * 0.3 18.9 i 0.4

-1.x - 1,s

K,, IO”

(K)

(molg

573 673 773 573 673 773

34sh 27hh 69” 448h 379h I7Y

573 573

s73 573

‘s

‘)

propene

benzene 75.X’

-2.x

hydrocarbones

” A is expressed in mol/g s. ’ At a reaction temperature of 4 I3 K. ’ At a reaction temperature of 573 K.

geneous grafting of chromia pillars to the silica layers [391. Generally, alcohol dehydrates on acid sites to give ether and oletins. As the basicity of 2-propanol is higher than the primary alcohols, it can give olefin as a major product even on weak acid sites, whereas methanol gives olefins only on strong acid sites. So to differentiate between the weak and strong acid sites of the catalysts, methanol conversion was carried out. Over all the catalysts, both dimethyl ether (DME) and CT hydrocarbons are found as major products. In addition to both of these products, very small amounts of decomposition products (CO + CHJ are also found, particularly at higher reaction temperatures. Table 6 shows that the rate of hydrocarbon formation decreases with the increase in the calcination temperature of the catalyst. Particularly, the decrease is more pronounced above 673 K. This is mainly due to the loss of Bronsted acid sites associated with the clay layers. However, at all the calcination temperatures, the rate of hydrocarbon formation is high for the sample prepared from the acid-activated clay. Selectivity of the products in alcohol conversion reaction is presented by their corresponding optimum performance envelope (OPE) curves. Such values are obtained by plotting the fractional conversion (X) of a particular product against the total conversion (Xi). This is obtained by varying catalyst to alcohol ratio (W/W) as described by Ko and Wojciechowski [40]. The OPE curves represent the conversion selectivity behavior of active sites present on a catalyst; the slopes of these curves at origin represent the initial selectivity for those products. For obtaining the product distribution as a function of conversion, we have included experiments performed by varying the weight of the catalyst, so that conversion as high as 40-50s can be achieved. The product profiles for 2propanol dehydration on all the catalysts show only one product (propene) but methanol conversion reactions (Fig. 7) show two types of products (DME and Cl hydrocarbons). Within 40-50 mol% conversion the decomposition product is negligible. One can observe in the Fig. 7 that both DME and Ct hydrocarbons are present from the onset of the reaction. This indicates the formation of both the products by direct dehydration. The marked downward deviation in the case of DME plots indicates the unstability of the product. At the same time, an upward deviation is

4

IO

0

30

40

on Cr.NaMont

calcined

20 X, (mol%)

Fig. 7. OPE plot for methanol conversion at 573 K.

marked in the C,’ hydrocarbon plots. This means that DME further dehydrates to Ct hydrocarbons in a secondary reaction at higher reaction temperature. Thus the pathway for the formation of C, hydrocarbons on chromia pillared clays is a combination of parallel and consecutive (primary and secondary) reactions. 3.4.2. Cumrne crtrckinig/dehvdrogenurion On all the catalysts, cumene conversion resulted in cracking as well as in dehydrogenation products (Table 7). Benzene and propene are the cracking products, whereas o-methylstyrene is the dehydrogenation product. However, benzene and propene are found to be the major products on all the catalysts. The rates of benzene and o-methylstyrene formation follow first-order kinetics. It is also well marked that, in

Table 1 crackingidehydrofenation different reaction temperatures

Cumene

over chromia-pillared

clays at

Reaction temperature

Total conversion

(K)

NaMont

Cr-NaMont

HMont

Cr-HMont

573 623 613 723 773

3 5 9 IS 22

6 12 22 37 48

7 II I9 28 39

IO IO 3s 51 62

(2.0) (1.5) (1.2.5) (0.66) (0.57)

Figures in the parentheses methylstyrene in the product.

(molp)

(3.00) (4.3) (5.0) (3.1) (1.66)

are the ratio

(2.5) (2.66) (1.37) (0.86) (O.Sh)

of benzene

(4.0) (8.0) (9.5) (S.4) (2.6) to o-

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Catalysis A: General 166 (1998) 123-133

cases of acid-activated pillared clay, cracking activity is high at all reaction temperatures. As the cracking reaction mostly takes place on the Bronsted acid sites, the increase in cracking activity indicates the presence of more Bronsted acid sites owing to the acid activation before pillaring. Of course, this observation is also supported by the number of acid sites determined by the spectrophotometric method. Moreover, the catalysts calcined at 573 and 673 K show better cracking activities in comparison to those of the samples calcined at 773 K (Table 6) which may be due to the decrease in Bronsted acid sites above 673 K calcination temperature. At the same time dehydrogenation activities increase with the increase in calcination temperature of the catalysts. So also the ratio of benzene to a-methylstyrene increases with the reaction temperature up to 673 K and thereafter decreases to a low value (Table 7). This supports the conclusion that the dehydrogenation reaction proceeds on the Lewis acid sites [22] which are unaffected at higher calcination temperature (> 673 K). In both the cases total conversion is quite higher for pillared materials in comparison to those of parent clays. Probably the low number of acid sites and the unavailability of interlayer pores are responsible for low cumene conversion. It is observed that selectivities of both the catalysts towards a-methylstyrene at all reaction temperatures are nearly the same. This indicates that acid treatment before pillaring has no effect on the Lewis acid sites, whereas it increases the Bronsted acid sites. The OPE plot (Fig. 8) shows that both the lines are straight without any deviation, which indicates the formation of benzene and a-methylstyrene from the on-set of the reaction. Therefore, both are primary stable products. 3.4.3. Poisoning studies The poisoning of active sites on chromia pillared materials for 2-propanol and cumene conversion reactions were performed by presaturation of the acid sites with 2,6_dimethylpyridine(DMPY). The catalysts were saturated with the probe molecules in the nitrogen stream. Then the catalyst bed was flushed with nitrogen at 573 K for 45 min to remove the unreacted base from the catalyst surface. In this way, DMPY is bonded to the Bronsted acid sites [41-43]. It is found that the dehydration activity for 2-propanol decreases up to 60% owing to the DMPY adsorption. As DMPY

0

10

20

30

40

x, (mol%) Fig. 8. OPE plot for cumene conversion 573 K.

on Cr-NaMont

calcined at

is preferentially adsorbed on the Bronsted acid sites, the decrease in activity shows that 2-propanol dehydration mainly occurs on the Bronsted acid sites, thus strengthening the carbenium ion mechanism. On the other hand, cumene cracking activities drastically decrease to a negligible value for all the catalysts, owing to the DMPY adsorption, whereas the dehydrogenation activities remain the same. This indicates that the cracking reaction over chromiapillared clay mainly occurs on the Bronsted acid sites through a carbenium ion mechanism, whereas the dehydrogenation occurs on Lewis acid sites via a free radical mechanism. Above all, the poisoning effect is more pronounced for cumene cracking reaction than the 2-propanol dehydration, although both the reactions proceed on Bronsted acid sites. This indicates that cumene cracking require stronger Bronsted acid sites which are mainly poisoned due to DMPY adsorption. Probably DMPY is not capable of poisoning effectively the weaker Bransted acid sites which are mainly responsible for dehydration of 2-propanol.

4. Conclusions From the forgoing results we can conclude that acid-activated montmorillonite is more suitable than the Na-exchanged clay as the starting material for the preparation of acidic chromia-pillared clay. However, the pillared material prepared from Na-exchanged montmorillonite shows higher Cr(II1) uptake, micro-

Z Mishra, K. Parida/Applied Catalysis A: General 166 (1998) 123-133

pore volume and controlled pore size than those of acid-activated materials. Alcohol conversion reaction proved the increase in acid sites due to acid-activation before pillaring. In all the cases, 573 K calcined samples show higher alcohol conversion. From the selectivity study, it is found that alcohol dehydration on chromia-pillared material follows a combination of parallel and consecutive pathways. Cumene conversion reaction confirmed the substantial increase in Bronsted acid sites due to the acid treatment before pillaring, thus increasing the cracking activity. However, Bronsted acid sites on the material are not stable at higher temperature(> 673 K). Further poisoning study confirmed that most alcohol dehydration and cumene cracking take place on the Bronsted acid sites, whereas the dehydrogenation of cumene proceeds on the Lewis acid sites.

Acknowledgements The authors are thankful to Professor H.S. Ray, Director, Regional Research Laboratory, Bhubaneswar, for his permission to publish this paper and to Dr. S.B. Rao, HOD, IC group for his encouragement throughout the work. One of the authors TM is thankful to CSIR, New Delhi, for providing a fellowship.

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[Ill [121 v31 1141 1151 [I61 [I71 [I81 [I91 WI

Pll L=l ~231 1241 1251 [261 v71

WI 1291 [301 1311 ~321 1331

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