Surface properties and porous texture of montmorillonite-(Ce or Zr) phosphate cross-linked compounds

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surface science ELSEVIER

Applied Surface Science 120 (1997) 340-354

Surface properties and porous texture of montmorillonite-(Ce or Zr) phosphate cross-linked compounds F. del Rey-Bueno *, A. Garcla-Rodriguez, A. Mata-Arjona, F.J. del Rey-P6rez-Caballero, E. Villafranca-Sfinchez Department of lnorganic Chemistry, Faculty of Sciences, Universityof Granada, Avd. Fuentenuevas.n., 18071 Granada, Spain Received 11 September 1996; accepted 22 July 1997

Abstract In this work, pore texture characteristics of a series of Ce(IV) or Zr(IV) montmorillonite phosphate cross-linked compounds obtained by precipitation of cerium or zirconium phosphate with dilute HaPO4 on the micelles of an aqueous montmorillonite suspension, previously submitted to ion-exchange processes to replace its exchange ions with Ce(IV) or Zr(IV), are studied. Surface areas and pore volumes of the different materials prepared are determined by N 2 adsorption at 77 K and mercury porosimetry techniques. Analysis of the N 2 adsorption isotherms by the t-De Boer and DubininRadushkevich methods, revealed the presence of a certain degree of microporosity in all the materials studied. Moreover, analysis of the Hg intrusion data permitted to determine the contribution of the macro- and mesopores to the total surface area and pore volume of the prepared compounds. The results reveal a greater specific surface area for these compounds than for montmorillonite and the evolution of this parameter with thermal treatment is related to the nature and content of phosphate in the different samples. However, the changes recorded in the Vp and S/Vp parameters during the thermal process suggest that surface diffusion is the dominant transport mechanism in the sintering process. © 1997 Elsevier Science B.V. Keywords: Surface propertier; Porous texture; Montmorillonite; Layered phosphates

I. Introduction There is a growing interest in the application and study of clays as precursors of cross-linked compounds and 'pillars', especially since, with these, one can now obtain solids of controllable porosity of possible use as adsorbents, catalysts or ideal matrices to house ionic or molecular species [1,2]. The low thermal stability of these pillared clays and related

* Corresponding author. Tel.: + 34-958-243173; fax: + 34-958248526.

compounds, however, leads to an even greater interest in the factors that influence their thermal degradation. Two recent papers [3,4] described the obtention and physico-chemical surface properties of a series of Ce(IV) or Zr(IV) montmorillonite phosphate cross-linked compounds, structurally related to the tetravalent metal laminar phosphates and pillared clay. Analysis of the properties of the different fractions obtained, according to their different preparation conditions and relative amounts of clay-phosphate revealed a change in properties related to the

0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All fights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 3 8 1-4

341

F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354 Table 1 Chemical composition of montmorillonite and some MP-montmorillonite compounds (wt%) Sample

SiO 2

AI203

Fe203

MgO

CaO

Na20

K20

P205

MO2

C.L.

Montmorillonite 5C 5C' 5Z 10Z'

57.45 48.07 45.36 48.32 38.62

21.84 15.43 14.16 16.84 9.98

2.81 1.91 1.68 2.40 1.48

5.67 2.69 2.46 3.02 1.81

1.29 0.14 0.31 0.27 0.06

0.37 0.06 0.10 0.07 0.13

0.28 0.10 0.09 0.12 0.07

6.34 10.54 9.16 21.46

10.89 10.94 7.53 12.62

9.66 9.59 9.22 9.90 9.24

The content in Ti and Mn of the montmorillonite, expressed as wt.% of TiO 2 and MnO, is 0.19 and 0.14 respectively, and remains roughly constant along the series of cross-linked compounds. MO 2 = CeO 2 or ZrO 2. C.L = Calcination lost.

pure components with increasing relative amounts of phosphate. Thus, at proportions of phosphate greater than a certain quantity per unit mass of clay, the mixture compound is stable in water, and, unlike montmorillonite, does not peptize or expand. Also, the degree of phosphate hydrolysis considerably decreases. Given the importance of pore texture and surface area of a solid in its possible applications as an

adsorbent or catalyst, this study focuses on the analysis and the subsequent effect of thermal treatment on both these parameters.

2. Experimental 2.1. Materials

Various series of montmorillonite-cerium phosphate (labelled C, C' and C R) and montmorillonite-

5C' 10 crn39-1

C' 5CR 5C

(71

Mont.

5C eP rP M CeP ZrP

0

O. 2

O.Z,

0.6

O.8

283K-0'

0

•.2

0./,

0.6

0.8

1.0

1.0

,~

Fig. 1. N 2 adsorption-desorption isotherms.

Fig. 2. Heat treatment influence on N 2 adsorption-desorption isotherms.

F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354

342

A

20cm3 g-1

,.) r,s

t~

>2C'-0

2C

C'-O10CR-0/-CR-0LC-02C-0

0.2

0.-'-

0.6

0.8

1.0

1.2

Ii/-

f.6

t (nm.) Fig. 3. t-de Boer plots for N 2 adsorption on some montmorillonite-Ce phosphate samples.

zirconium phosphate systems (labelled Z, Z' and Z R ) were prepared by using the following procedure [3]: an amount of 5 g of montmorillonite ('pure clay fraction', O < 2 /xm) was dispersed in 50 ml of a solution of the corresponding quadrivalent cation (Ce(IV) or Zr(IV)) at an appropriate concentration to saturate the exchange capacity of the clay (90 meq/100 g) in samples C and Z, and a concentration exceeding such a capacity by a factor of 2, 4, 5 and 10 in samples 2C and 2Z, 4C and 4C, 5C and 5C, and 10C and 10Z, respectively. The suspensions thus obtained were continuously stirred at ambient temperature for 9 h and then allowed to stand for 12 h. Next, they were refluxed under constant stirring as a volume of 50 ml of phosphoric acid at a concentration twice higher than that of the quadrivalent cation (Ce or Zr) used to saturate the clay was gradually added at a rate of 2.5 ml/min. The mixture was refluxed for a further 7 h and allowed to stand at ambient temperature for 12 h. The samples thus obtained were washed with distilled water to complete absence of sulphate (Ce samples) or chloride (Zr samples) in the washing water, followed by drying at room temperature and then at 383 K to a constant weight. The sample series labelled with a prime symbol

was also obtained by using the above-described procedure; however, the concentration of phosphoric acid used was twice higher. The series labelled with subscript R was successively treated with three fresh saturating solutions containing the quadrivalent ion prior to refluxing. The materials thus obtained were analyzed chemically by X-ray fluorescence spectroscopy and characterized by thermogravimetric analysis (TGA and DTG), X-ray diffraction and IR spectroscopies, and scanning electron microscopy (SEM-EDX). By way of example, Table 1 gives the values for the main components determined in the starting montmorillonite and four of the most representative samples [3]. From the results it follows that both Ce(IV) and Zr(IV) are present essentially as exchange cations; however, a substantial portion occurs as hydrogen phosphate that acts as a binder among clay particles. While no fibrous or crystalline phase typical of the Ce(IV) or Zr(IV) phosphate structures was detected, increasing their proportions led to the gradual disappearance of the typical XRD reflections of clay and the appearance of a large, bell-shaped peak that encompassed the stronger reflections, of variable spacing, and those for the different clayphosphate phases, which were formed simultane-

I 2 0 cm3g -I

-

: .....

~o.o.o.~..o..o_-~o10ZR 5ZR

~

:# 10ZR-0 5 ZR-04ZR-0" 10Z -0" 5Z-0Z,Z-O

0:2 0[i' 026 018 '5

i[ 2 '[i'

~j6

t (nm) Fig. 4. t-de Boer plots for N 2 adsorption on some montmorillonite-Zr phosphate samples.

F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354

ously with the disappearance of crystalline order in montmorillonite [3]. We also determine the surface acidity of the samples from ammonia adsorption measurements. The results were in the region of 1021 c.a.g -1 . Based on the IR spectra for the corresponding ammonia phases, acid sites were largely of the Br~Snsted type [4]. 2.2. Thermal treatment

Four samples, of different phosphate concentration, of stable surface area and pore volume, were used to study the influence of temperature on changes of surface properties. These samples are 5C and 5Z, of equal phosphate content, 5Z', with double the phosphate concentration of 5Z, and 10C', with the highest phosphate content, tetravalent ion content and one of the greatest surface areas. These samples, together with a montmorillonite

343

sample, were heated to 523, 773, 1023 and 1273 K for 12 h, in air, in a Heraeus furnace, left to cool and stored in a desiccator with PzOs.

2.3. N 2 adsorption-desorption isotherms

Specific surface area and volume of pores were determined from the N 2 adsorption isotherms at 77 K, obtained in a volumetric adsorption system, equipped with a Balzer TPG-300 pressure gauge, with one Pirani TPR 010 and one Penning IRK 020 head, with measuring ranges of 10000 Pa > P > 0.1 Pa and 0.5 Pa > P > 5 × 10 -5 Pa, respectively. The adsorption system was also equipped with a Baratron 390 head for pressure measurements in the 1-10000 Pa range. The nitrogen used was 99.998% pure. The samples were degassed previously at 95°C until the system had reached a pressure of 10 - 4 Pa.

Table 2 Available surface area and pore volume of montmorillonite-(Ce or Zr) phosphate cross-linked compounds Sample

SBET (m2 g -1 )

St (m2 g - l )

CBET

V0.98 (ml g - l )

Vo.1 (ml g - l )

V0 (ml g -1 )

Vt (ml g -1 )

Montmorillonite C 2C 4C 5C 10C C' 2C' 4C' 5C' 10C' 4C R 5C R 10C R 4C~ 5C~ 10C~t Z 2Z 4Z 5Z 10Z 4Z R 5Z R 10Z R

109.8 165.1 153.0 178.0 194.2 175.0 151.4 167.9 189.1 195.4 239.6 173.4 176.9 213.0 183.0 184.0 207.0 129.8 144.2 199.7 194.4 195.6 191.6 183.0 205.5

116.0 161.8 157.0 160.0 160.2 173.0 160.0 176.6 195.1 198.1 251.1 176.9 187.8 216.0 143.6 161.9 176.7 134.8 151.6 212.0 200.6 202.6 197.0 189.6 196.6

521 364 319 395 303 326 429 440 391 238 362 378 259 335 297 221 378 506 841 266 352 203 391 402 189

73.6 98.4 99.0 103.1 116.6 88.1 97.0 97.0 101.7 103.4 116.4 98.0 95.1 109.6 103.0 97.7 99.4 129.8 144.2 199.7 194.4 195.6 191.6 183.0 205.5

0.043 0.064 0.059 0.070 0.076 0.068 0.060 0.067 0.075 0.075 0.094 0.068 0.068 0.083 0.071 0.071 0.081 0.052 0.058 0.078 0.076 0.075 0.075 0.072 0.078

0.045 0.070 0.063 0.074 0.079 0.067 0.062 0.072 0.078 0.077 0.094 0.069 0.071 0.084 0.072 0.072 0.083 0.055 0.060 0.083 0.080 0.087 0.075 0.073 0.081

0.042 0.075 0.069 0.078 0.094 0.094 0.073 0.078 0.097 0.105 0.095 0.089 0.095 0.124 0.068 0.109 0.091 0.061 0.066 0.097 0.090 0.077 0.095 0.098 0.126

SBET = BET surface. V0.9S = N 2 adsorbed volume at P/Po = 0.98. V0.t = N 2 adsorbed volume at P/Po = 0.1. St = t-de Boer surface. Vt = t-de Boer micropore volume. CBET = BET constant. V0 = Dubinin-Radushkevich micropore volume.

344

F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354

5CR, C' and 5C' and Fig. 2 shows those corresponding to samples 5C (383 K), 5C (523 K), 5C (773 K), 5C (1023 K) and 5C (1273 K).The isotherms corresponding to the compounds of montmorillonitezirconium phosphate are qualitatively analogous to the cerium compounds. Figs. 3 and 4 show the results of applying the t-De Boer method [5] to the two sample series 2C, 4C, 4C R, 10C R, C', 2C' and 4Z, 5Z, 10Z, 4Z R, 5Z R and 10Z R respectively using the t-multilayer values obtained by Lippens, Linsen and De Boer [6,7]. Table 2 compiles the textural parameter values of the samples obtained from analysis of the adsorption isotherms using the t-De Boer [5] and BET methods [8]. The latter was applied to N 2 adsorption data at 77 K in the relative pressure range of 0.05-0.25 with 0.162 nm 2 for the N 2 molecule section. Fig. 5 shows the SBET values of the samples dried at 383 K and Fig. 6 gives the values for montmorillonite and samples 5Z, 5Z', 5C and 10C' heated to 383, 523, 773, 1023 and 1273 K, respectively. Pore volume distribution curves in the interval

2.4. Mercury porosimetry

Further information on the pore texture of the prepared materials, including the contribution of the meso- and macropores to the total pore surface area and volume, was obtained using a Quantachrome Autoscan 60 with maximum pressures of 60 000 Psi that covers spectra between 1.8 and 10000 nm. The system is doted with a data processor which facilitates analysis of the experimental extrusion and intrusion curves of mercury, pore size distribution, accumulative volume and equivalent surface area of pores, in addition to other parameters.

3. Results 3.1. N 2 adsorption-desorption isotherms

Fig. 1 shows the adsorption-desorption isotherms of N 2 at 77 K of the montmorillonite, cerium phosphate, zirconium phosphate, and the samples C, 5C,

~"

3oo

(~

E 200 I-- 1 0 0 ILl m U) 0

MP

•

r..ri.

[]

c ....

[]

z.°,..

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c ..,i.

Fig. 5. BET surfacearea of montmorillonite-(Ceor Zr) phosphatecross-linkedcompounds.

F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354

345

30O

E 20o I-m

IO0

o

I

[]

10c"

[]

[1~)

5c

1273

i

sz"

[]

5z

[]

.o.,

Fig. 6. Heat treatment influence on BET surface area of montmorillonite (Ce or Zr) phosphate cross-linked compounds.

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rp (nm) Fig. 7. Pore size distribution for the C series samples evaluated with Hg intrusion porosimetry.

F. del Rey-Bueno et al. /Applied Surface Science 120 (1997) 340-354

346 0,6

C' .....

/\ I

/ I I a I ,

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rp ( n m ) Fig. 8. Pore size distribution for the C' series samples evaluated with Hg intrusion porosimeLry.

1 0 0 0 0 > R > 2 nm (Figs. 7 and 8) were obtained from the mercury intrusion data. Tables 3 and 4 show the contributions of the different ranges of pore

size to the volume and surface area of the materials studied. This can also be evaluated by applying a graphical model, that does not assume a geometrical

Table 3

Volume and surface of pores Sample

Vm~cro a (m] g - l )

Vmes° b (ml g - l )

S..... a (m 2 g - l )

S.... b (m 2 g - l )

Smicroc (m2 g-I)

Montmorillonite C 2C 4C 5C 10C 4C R 5C R 10C R C' 2C' 4C' 5C' 10C' 4C~ 5C~ 10C~

0.19 0,40 0.40 0.44 0.55 0,33 0.36 0.38 0.38 0.38 0.38 0.38 0.35 0.50 0.36 0.35 0.35

0.05 0.06 0.06 0.07 0.08 0.02 0.05 0.06 0.04 0.07 0.04 0.06 0.04 0.05 0.05 0.06 0.03

1.10 2.32 3.53 5.53 8.43 2.23 2.14 2.47 2.27 2.90 2.08 3.34 1.97 4.41 1.85 1.72 1,90

20.10 26.25 26.13 22.00 28.96 8.50 26.17 31,06 17.24 31.97 22.87 25.09 19.84 21.15 28.76 31.14 15.03

109 137 123 150 157 164 145 143 193 117 143 161 174 214 152 151 190

a50 n m < ~ < 8500 nm.

b4 n m < ~ < 50 nm. cSBET -- (Smacro q- S.... )'

F. del Rey-Bueno et al. /Applied Surface Science 120 (1997) 340-354 Table 4 Volume and surface of pores Sample Vr~cro a (ml g- l)

Vmes ° b

Z 2Z 4Z 5Z 10Z 4Z R 5ZR 10ZR Z' 2Z' 4Z' 5Z' 10Z' 4Z'a 5Z'R 10Z'R

0.05 0.06 0.07 0.06 0.07 0.06 0.04 0.05 0.05 0.07 0.08 0.07 0.02 0.08 0.03 0.05

0.42 0.40 0.41 0.39 0.40 0.39 0.38 0.40 0.40 0.46 0.49 0.53 0.36 0.44 0.39 0.42

(ml g- 1)

S. . . . . 2.05 2.16 2.72 2.38 7.74 2.66 1.95 2.18 2.66 5.23 5.71 6.98 1.86 3.55 1.92 2.31

a (m 2

g- 1)

S. . . .

b (m 2

22.68 28.50 30.02 29.45 22.11 29.09 22.89 26.92 24.11 26.13 28.55 25.10 7.68 26.40 15.69 27.77

g- 1)

347

Smicro c

(m2 g- l)

105 114 167 163 166 160 158 176 102 121 150 166 164 134 154 236

a50 nm< • < 8500 nm. b4 nm< O < 50 nm. CSBET -- (Smacro q- S.... )'

distribution model for the pores, to the intrusion curves [9]. Taking into account the limitations of mercury porosimetry in pore volume measurements with R < 1.8 nm, in all cases the micropore volume is taken as the volume Vp o f N 2 adsorbed at P / P o = 0.1, which, for the samples studied, is in good agreement with the m i c r o p o r e v o l u m e values o f D u b i n i n Radushkevich [10,11]. Likewise, for the contribution of microporosity to the internal surface area of the samples the value SBET -- (Smacro + Smeso), preferentially that of the equivalent monolayer area, defined by Barrer [11] has been taken. Some of these results are given in Figs. 7 and 8 and Tables 3 and 4.

4. Discussion

4.1. Porous texture The curves in Fig. 1 correspond to type II isotherms of the B D D T classification [12] which are characteristics o f non-porous or macroporous solids. Nevertheless, a sharp decline in the region of low relative pressure suggests the presence of micropores in the prepared materials. The hysteresis cycle, how-

ever, corresponds to the H-3 type that is usual in adsorbents comprised of aggregated plane particles in the form of a slit [13]. There is a smooth junction between the adsorption and desorption branches o f the isotherm due to the gradual approach of the parallel layers that comprise the pore as desorption progresses. This meeting point of the two isotherm branches takes place at P / P o values near to 0.40, or, as the proportion of phosphate in the different materials increases, this union occurs at slightly higher values and the area inside the hysteresis curve decreases. Analysis of the N 2 adsorption isotherms of the series C, C R, C' and C R by the t-de Boer method, gives a family of straight lines in the region of low relative pressure (Figs. 3 and 4) which pass through the origin. Values of the total surface area of the corresponding materials can be calculated from the slope of these lines. A t values of P / P o > 0.35, the points begin to deviate from the straight line, suggesting the presence of pores with a R < 1.5 nm that are filled at lower relative pressures thus reducing the pore surface area through which adsorption can take place. In the region where 0.75 < P / P o < 0.98, (7 < t < 1.4 nm), the adsorption data fit a second straight line

348

F. del Rey-Bueno et al. /Applied Surface Science 120 (1997) 340-354

of lower slope which, extrapolated to the ordinate axis, gives a value for micropore volume. This value is in good agreement with that obtained using the Dubinin-Radushkevich method [10] and with the volume of N 2 adsorbed at P / P o = O . 1 at 77 K (Table 2). From the slope of this second section of the t-de Boer representation the contribution of the pores with radii in the interval 4 < R < 1.5 nm to the total area of the prepared materials can be calculated; these values range from 11 to 26% depending on the material studied. As shown in Table 2, the SBEa- and St.De Boer values are in good agreement, with only slight variation between both sets of values. From Table 2 and Fig. 5 we can see that values of the surface area accessible to N 2 at 77 K are greater than for montmorillonite and its corresponding initial phosphates [14]. Similarly, as the proportion of each phosphate increases in the samples there is a corresponding increase in surface area which stabilizes at a certain quantity of phosphate present. Surface area values however, are higher in the samples from the C series, although the maximum values, corresponding to those with the greatest phosphate contents, are very similar in both series although these are reached first in the Z series. Samples with the largest surface areas are those in which there is most interaction between the montmorillonite and its corresponding phosphates since this influences the properties of both, even permitting them to be used in aqueous suspension. Changes in the surface areas of the M' samples (Series C' and Z') are very similar to those recorded in the M samples (Series C and Z) and in the MR and MR' samples (Series C R, C~, Z R and Z~), with the exception of samples 10MR and 10MR', for which the maximum surface area values are recorded. The same pattern is observed with the surface areas of the samples determined by ammonia adsorption at 239.8 K [4]. Analysis of the pore volume distribution curves in the range studied (2-10000 nm) (Figs. 7 and 8) reveals a bimodal distribution with two acute peaks at 1000 and 2 nm. In the C and Z' series, although the bimodal character is preserved there is a progressive displacement of the main peak towards values of smaller pore radius with increasing phosphate content in the samples.

Since the efficiency of an adsorbent depends, to a great extent, on the accessibility of its surface, it is thus pertinent to determine the contribution of the different size ranges of pores on the total surface area of the materials. From Figs. 9 and 10, we can see that the micropores significantly contribute to the specific surface area of these materials, increasing in series C' and Z' with the phosphate content of the samples. The contribution of the meso and macropores to the surface area is not so significant and there is no clear correlation between this parameter and the phosphate content of the studied materials. 4.2. Thermal influence on porous texture The N z adsorption isotherms of heated samples at 523, 773 and 1023 K (Fig. 2) are qualitatively similar to those of their dry precursors at 383 K. In those heated to 1273 K, however, there is a sharp decline in N 2 adsorption and the isotherms appear to follow Henry's Law [15]. On the other hand, the C BET values are markedly lower than those of untreated samples which seems to suggest a significant weakening of the solid-adsorbate interactions. The SBET data of the heat activated samples are given in Fig. 6, together with values of the dry precursor at 383 K as a reference. The montmorillonite initially suffers an increase in surface area of 9% at 523 K followed by a decrease in this parameter to almost undetectable levels at 1273 K. This is comparable to the behaviour observed at the start of the dehydration or dehydroxylation process of clay, with an initial increase in the homogeneity of the mineral crystals followed by gradual destruction of the crystalline structure that can ultimately lead to vitrification of the material [16]. Similar behaviour to that of montmorillonite is shown by sample 10C', that undergoes an increase in surface area of 17% at 523 K followed by a progressive decrease in surface area at higher temperatures. In the remaining samples (5C, 5Z and 5Z'), instead of an initial increase in surface area, this parameter decreases from the start and continues to decline with increasing severity of experimental conditions until it is almost undetectable at 1273 K. It is worth noting the greater surface area of the cerium compounds compared to their zirconium counterparts and the less pronounced degradation of the former with temperature.

F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354

349

250

200

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150

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¢

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5C

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A

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5C"

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Mont

Samples 41 rn I o r o p o r e e

•

S m eeoporee

•

8 m aoroporee

Fig. 9. Pore surface distribution of montmorillonite-Ce phosphate cross-linked compounds.

An estimate of the degree of sample sintering can be evaluated from the relationship between the surface area of the material treated at 773 K and the corresponding dry material at 383 K. For samples 5C, 10C', 5Z and 5Z' these values are shown in Table 5, showing that the greatest loss in surface area occurs in samples 5Z and 5Z' and the lowest in sample 10C'. This loss in surface area could be related to the amount and nature of the tetravalent cation. Therefore, sample 10C', which has the greatest Ce(IV)

content, loses less surface area because most of its Ce is present in a fibrous phosphate form, wrapped around the clay particles [3], the smaller loss in the degree of cross-linkage in this compound thus leads to a smaller loss of surface area. In sample 5C however, only a small amount of the Ce is present as phosphate and thus contributes less to the thermal stability of the sample. In samples 5Z and 5Z', in spite of the fact that the second has twice the amount of Zr(IV) than the former, they both experience a similar loss in surface

350

F. del Rey-Bueno et al. /Applied Surface Science 120 (1997) 340-354

J 250

J

200

J

/ / J

t5o C~

/

t00

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J

i

Z

2Z

4Z

5Z

10Z

4ZR

5ZR 10ZR

Mont

5ZR" IOZR"

Mont

Sam pies

250 -

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200- ~

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~

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2Z"

4Z °

tOZ"

5Z"

JIZ,R "

Samples []

S

mlcroporo

~

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rne¢opore

•

$ mlcropore

Fig. 10. Pore surface distribution of montmorillonite-Zr phosphate cross-linked compounds.

area. This could possibly be explained by the crystalline nature of zirconium phosphate, which has less cross-linkage than cerium phosphate therefore does not significantly contribute to the thermal stability of the samples in which it is present. Table 5

5773/S383 % loss

5C

10C'

5Z

5Z'

0.357 64.3

0.793 20.7

0.159 84.1

0.142 85.8

This explanation is supported by evidence from micrographic analysis of the materials [3] that shows that both cerium and zirconium phosphate form a type of lattice between the clay particles that increases the strength of the connection between them. The IR spectra of the 5C series samples reveal a weakening in the absorption bands corresponding to the OH and H20 vibration modes, whereas a new /-Psim band appears at 735 cm -1 which increases in intensity with increasing temperature, this is ascribed to pyrophosphate. The Uas band at 950 cm -~ is

F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354

probably concealed by the asymmetrical deformation of the S i - O - S i of montmorillonite situated at approximately 1100 cm -1 [17,18]. The X-ray diffractograms of the initial clay sampies and the thermally activated samples 5C, 10C', 5Z and 5Z' confirm the above results, since these show progressive loss of crystallinity accompanied by a loss in surface area until 1273 K when the samples are composed of a mixture of oxides. Analysis of the pore distribution curves gives a more detailed picture of the evolution of sample texture with temperature. Table 6 shows the total pore volumes and the volumes and surface areas assigned to the meso- and macropores from mercury porosimetry experiments of the thermally treated samples. Also, micropore volumes corresponding to P/Po = O.1 and the sur-

351

face areas, calculated from the difference between the SSEv and (Smacro + Smeso) value, the latter being obtained from mercury porosity data corresponding to pores with radii greater than 2.0 nm. From Table 6 we can see that in samples 5C, 10C', 5Z and 5Z' the macropore volume varies relatively little with heat treatment with values ranging from 0.50 to 0.40 cm 3 g-1 in the temperature interval 523-1273 K. On the contrary, the micropore volume sharply decreases at 1023 K with simultaneous decreasing of specific surface area, indicating the thermal instability of the micropore texture in this temperature range. It is worth mentioning the difference in behaviour between the samples from the cerium series and the zirconium series with temperature: whereas the macropore texture of the former is only slightly

Table 6 Heat influence on porous texture Sample Montmorillonite

5C

10C'

5Z

5Z'

T (K)

Vp (mlg -1)

Vm. . . . a

Vines ° b

V0.1

S .....

( m l g -1)

(mlg - l )

(mlg -l)

(m2g -1)

( m 2 g -1)

(m2g -l)

383 523 773 1023 1273 383 523 773 1023 1273 383 523 773 1023 1273 383 523 773 1023 1273 383 523 773 1023 1273

0.25 0.42 0.42 0.18 0.08 0.64 0.59 0.43 0.45 0.68 0.55 0.48 0.44 0.41 0.44 0.46 0.49 0.51 0.45 0.20 0.61 0.54 0.51 0.49 0.42

0.19 0.37 0.37 0.16 0.07 0.55 0.55 0.40 0.43 0.68 0.50 0.43 0.42 0.40 0.44 0.39 0.43 0.48 0.43 0.20 0.53 0.43 0.48 0.42 0.37

0.05 0.05 0.05 0.02 0.00 0.08 0.03 0.03 0.02 0.00 0.05 0.04 0.02 0.00 0.00 0.06 0.05 0.03 0.02 0.00 0.07 0.04 0.03 0.02 0.00

0.04 0.05 0.05 0.02 0.00 0.08 0.07 0.06 0.02 0.00 0.09 0.10 0.10 0.04 0.00 0.08 0.06 0.05 0.01 0.00 0.08 0.06 0.05 0.01 0.00

1.10 2.00 1.91 0.73 0.07 8.43 3.94 5.38 5.34 1.54 4.41 2.62 2.70 2.84 0.98 2.38 5.37 4.26 3.77 0.64 6.98 3.92 3.92 3.59 1.02

20.10 21.07 22.06 7.30 0.00 28.96 15.17 8.50 3.19 0.00 21.15 21.12 7.01 0.38 0.00 29.45 18.20 7.06 3.25 0.00 25.10 14.64 5.19 2.59 0.00

89 96 92 51 1 157 156 134 61 1 256 259 117 25 1 163 133 126 24 0 166 130 113 22 1

Vp = total pore volume Hg porosimetry evaluated. VO. 1 = N 2 adsorbed volume at P/Po = 0.1. a50 nm < ~ < 8500 nm. b4 n m < O < 50 nm. CSBET -- ( S m . . . . q- S . . . . ).

a

S....

b

Smicro c

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F. del Rey-Bueno et al. / Applied Surface Science 120 (1997) 340-354

modified in the temperature range studied, in the latter series there is a significant decrease in macropore volume at 1273 K. In contrast with the above, in the initial montmorillonite samples the contribution of the micro- and mesopores to the specific surface area remains almost constant up to 773 K, after this temperature, however, it decreases sharply to reach almost undetectable values at 1273 K.

4.3. Pore texture sintering mechanism The loss of surface area of the study materials during thermal treatment can be explained by the formation of fillets between the particles produced by the transport and deposition of matter in temperature conditions that permit ions and atoms to be displaced from the surface of the micelles by uncompensated attractive forces. In a material comprised of small agglomerate particles, the region of minimum potential energy is in their zones of contact, where the radius of curvature is smallest and the concavity most pronounced. Following thermodynamic reasoning, the vapour pressure of a substance is lower on a concave compared to a convex or plane surface and, if other conditions remain constant, decreases with decreasing radius of curvature. The transfer of matter from a plane or convex surface to a concave one is thus energetically favourable since it reduces the extension of the solid's surface. Bearing these facts in mind, we would thus expect the condensation of matter between the micelles of a porous solid to lead to the formation of aggregates of particles and the filling of interstitial cavities with a loss of both pore volume and surface area of the material. The most important mechanisms in the sintering process of a porous solid are evaporation-condensation transport (ECT), surface diffusion (SD) and volume diffusion (VD), the latter by migration of lattice defects or vacancies. The temperature threshold that must be surpassed in order to activate these mechanisms is approximately 0.3Tm, for the two former, and 0.5-0.6T m for the latter, with Tm being the fusion temperature of the solid in K. Given the nature of the materials studied, it seems that ECT

and SD are the main mechanisms involved in the degradation of their pore texture. Kuczynsky [19] and Herring [20] in studies on solid systems comprised of metallic or ceramic spherical particles and Lee and Parravano [21], among others, from their research into sintering of metallic oxides, established methods of kinetic analysis which enable to get some insight on the transport mechanism operating in a solid system made up of regular particles during sintering using optical techniques to determine the growth of the contact zone between two particles or between one spherical particle and a plane surface of the same material. However, it is difficult to apply these methods to our system since the growth rate of the particles or of their contact zones (necks) would have to be determined; two very difficult tasks in an aggregate comprised of very small O < 2/xm layered micelles. In spite of the lack of information on these two parameters, however, we can still gain some insight into the transport mechanisms that probably determine degradation of the pore texture of these materials from analysis of the changes induced in surface BET values and the SBET/Vp ratio in these materials by processes of ECT and SD as a result of thermal treatment [22]. If we assume that the ECT mechanism predominates over SD, then pore filling by condensation of solid matter would produce a quicker loss of volume than of internal surface area, thus the ratio SBET/Vp would increase with increasing degree of sintering. In the opposite case, i.e. if SD is the dominant mechanism, then, transport of matter from the surface to the inside of the solid via intercommunicative pores would cause the smaller particles to aggregate and weld together forming larger structures, and there would be a correlative decrease in surface area and S B E T / V p ratio, as can also be deduced from simple geometric calculations [23]. Table 7 shows SBET, Vp and SBET//Vp values for 5C, 10C', 5Z and 5Z' at different thermal treatments. For all the series, both SBET and S B E T / V p values fall gradually until temperatures of 773 K are reached and then more sharply at 1023 K, suggesting that surface diffusion is the dominant process in texture degradation. Nevertheless, in the lower temperature range, in which the loss of interlaminar water, the start of

F. del Rey-Bueno et al./Applied Surface Science 120 (1997) 340-354 Table 7 BET surface and pore volume ratio Sample T (K) SBET (m 2 g - l ) V0.9s ( m l g - 1 ) S~ V (mZm1-1) 5C

10C'

5Z

5Z'

383 523 773 1023 1273 383 523 773 1023 1273 383 523 773 1023 1273 383 523 773 1023 1273

194.2 175.4 147.9 69.3 2.6 239.6 279.7 268.9 120.2 1.8 194.4 156.5 137.1 31.0 2.0 197.9 148.9 122.3 28.2 1.9

0.18 0.15 0.14 0.09 0.00 0.18 0.21 0.22 0.12 0.00 0.17 0.15 0.15 0.07 0.00 0.14 0.14 0.12 0.06 0.00

1067 1154 1064 762 0 1346 1307 1228 1045 0 1157 1057 933 477 0 1384 1071 994 455 0

SBET = BET surface. V0.98= N 2 adsorbed volume at P / P o = 0.98.

dehydroxylation and the formation of pyrophosphate (as shown by IR) with separation of water in the vapour phase take place, the ECT mechanism may also play a role in the sintering process.

5. Conclusions The study of pore texture characteristics of a series of Ce(IV) or Zr(IV) montmorillonite phosphate cross-linked compounds by N 2 adsorption at 77 K and mercury porosimetry techniques emphasizes the considerably greater values of surface area and pore volume of these compounds compared to montmorillonite and higher values for the cerium compared to the zirconium series. Similarly, the texture of the former is less sensitive to thermal treatment than that of the latter compounds. Analysis of the differential intrusion curves of mercury show a bimodal distribution with peaks at 10000 and 2 nm, the first of these displaces towards smaller radii as the relative cerium or zirconium phosphate content of the samples increases. At 773 K only moderate changes occur in the

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pore texture of the montmorillonite-(Ce or Zr) phosphate compounds but these increase markedly after 1023 K. The changes in Vp and S/Vp during thermal treatment suggest surface diffusion to be the predominant transport mechanism in the sintering process.

Acknowledgements This research was financially supported by the Spanish Ministry of Education and Science (Grant DGIYT, PB 89-047-0).

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