Effects of Exchange Titanium Cations on the Pore Structure and Adsorption Characteristics of Montmorillonite

Journal of Colloid and Interface Science 256, 360–366 (2002) doi:10.1006/jcis.2002.8641

Effects of Exchange Titanium Cations on the Pore Structure and Adsorption Characteristics of Montmorillonite Fu-Chuang Huang,∗ Jiunn-Fwu Lee,∗,1 Chung-Kung Lee,† Wen-Ni Tseng,∗ and Lain-Chuen Juang† ∗ Department of Environmental Engineering, National Central University, Chung-Li, 320, Taiwan, Republic of China; and †Department of Environmental Engineering, Van-Nung Institute of Technology, Chung-Li, 320, Taiwan, Republic of China Received January 17, 2002; accepted July 30, 2002

rillonite were studied based on classical and fractal analysis of the nitrogen adsorption isotherms. It was concluded that a larger exchange cation may decrease the BET surface area of montmorillonite and this result may be ascribed to both the coverage of some surface roughness (surface screening effect) and the inhibition of nitrogen molecules into some pores (pore blocking effect). In this paper, we examine the effects of titanium exchange on the surface, pore structure, and adsorption properties of montmorillonite. The changes in the surface and pore structure of montmorillonite were characterized using classical and fractal analyses of the nitrogen adsorption isotherms as well as the XRD patterns. Adsorption isotherms of hexane and benzene at 288 and 298 K were measured to investigate the effects of exchange titanium on the adsorption selectivity of montmorillonite. The relationship between the alteration in the surface and pore structure and the change in the adsorption characteristics of montmorillonite is discussed.

Ca montmorillonite was exchanged with titanium cations to study the effects of ion exchange on the surface area, pore structure, and adsorption properties of montmorillonite. The revolution of both the surface area and pore structure of montmorillonite was characterized, based on classical and fractal analyses of the nitrogen adsorption isotherms as well as the XRD patterns. The adsorption isotherms of hexane and benzene were then measured to identify the effects of the exchange process on the adsorption characteristics of montmorillonite. It was found that the exchange process might induce an increase in the surface area, pore size, pore volume, and pore connectivity of montmorillonite. Accompanying this was an increase in the basal spacings between the tetrahedral sheets from ˚ The effects of the alteration of both the surface area and 13 to 16 A. pore structure on the adsorption characteristics of montmorillonite are discussed. C 2002 Elsevier Science (USA) Key Words: montmorillonite; titanium; surface area; pore volume; pore connectivity.

INTRODUCTION MATERIALS AND ADSORPTION ISOTHERMS

It is well known that the negative charge of clays is balanced by exchangeable cations, which are usually Na+ and Ca2+ . It is possible to modify the surface properties of clays greatly by replacing natural inorganic cations with other cations. The modification process may induce an enormous change in both the surface and pore structures of clays and then mediate the practical applications of clays (1–3). Modification of the surface characteristics of clay through ion-exchange processes has been studied intensively in recent years and its practical application to removing contaminants from water has been seriously considered (4–7). Given the importance of the pore texture and surface area of clay in its possible applications as an adsorbent or catalyst, it is necessary to analyze the effect of ion-exchange processes on both these parameters. In our previous investigation (3), the size effects of various exchange cations, including manganese, copper, tetramethylammonium, and hexadecyltrimethylammonium on the pore structure and surface fractality of montmo-

1 To whom correspondence should be addressed. Fax: (+886) 34226742. E-mail: [email protected].

0021-9797/02 $35.00

 C 2002 Elsevier Science (USA)

All rights reserved.

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The pure montmorillonite used in this study was purchased from the University of Missouri–Columbia, Source Clay Minerals Repository. The cation exchange capacity (CEC) of pure montmorillonite is 120 meq/100 g. The Ti-saturated clays were made by reacting Ca montmorillonite with 1 M TiCl4 solutions. In 2 L of distilled water, 15 g of Ca montmorillonite was dispersed with the coarse material removed by settlement. The clay suspension was then treated with TiCl4 solutions in an amount five times as great as the CEC (120 meq/100 g). The materials were allowed to stand overnight and then placed in dialysis tubing and dialyzed against distilled water until no chloride appeared in the dialysate, as indicated by negative tests with AgNO3 . The Ti-saturated clays were then freeze-dried and stored in glass bottles until used. XRD patterns of the samples were measured on a Siemens D-500 instrument with CuK α radiation (30 mA and 40 kV). The nitrogen adsorption isotherm and desorption hysteresis loop were measured at 77 K with a Micromeritics ASAP-2000. Clay samples of 1–2 g were outgassed with He for 16 h at 105◦ C prior to the adsorption measurement.

361

Ca MONTMORILLONITE EXCHANGE WITH Ti CATIONS

RESULTS AND DISCUSSION

In this study, we attempted to identify the effects of titanium exchange on both pore structure and adsorption properties of montmorillonite. In what follows, we first provide the basic structural characteristics of the montmorillonite samples evaluated from nitrogen isotherms. Surface Area, Surface Fractal Dimension, and Pore Structural Characteristics Figure 1 shows the nitrogen adsorption–desorption isotherms measured on the examined montmorillonite samples. Some key features may be found directly from this figure. It can be seen that the monolayer capacity, thus the BET surface area, decreases in the order Ti > Ca montmorillonite. This sequence is in contrast ˚ < Ca2+ (1.06 A) ˚ (8)); i.e., a to the cation size order (Ti4+ (0.69 A) larger cation corresponds to a smaller BET surface area. In general, the larger ion may screen some montmorillonite surface rugosity, which becomes inaccessible for the nitrogen molecule and then decreases the BET surface area. On the other hand, it is well known that montmorillonite has the capability of interlamellar expansion and it is often found that the larger organic cations may act as pillars, increasing the spacing between the tetrahedral sheets (6, 7). Although the Ti cation is not very large, it still has the opportunity to intercalate between the clay layers. This may provide more pore spaces and surface area. Accordingly, another possibility for the increase in the BET surface area of Ti montmorillonite may come from the pore opening effect.

220

Ca-montmorillonite

200

Ti-montmorillonite

180 volume adsorbed, cm3/g STP

Benzene and hexane, both containing six carbons, were selected as adsorbates to discuss the adsorption selectivity of montmorillonite in terms of intersheet spacing of adsorbents and molecular shape of adsorbates. The isotherms of hexane and benzene were measured with a Cahn D-200 microbalance in a constant volume system. About 100 mg of the adsorbents were used. Equilibrium was assumed when the sample weight changed by less than 0.01 mg for at least 2 h. A complete adsorption isotherm was constructed by increasing the pressure in a stepwise manner and the desorption branch was generated by a sequential decrease in the pressure. The temperature of the system was controlled to within 1◦ C by recirculating refrigerant from a thermostat. The pressure of the system was measured using two MKS Baratron Type 122A absolute pressure transducers within ranges of 0–10 and 1–1000 Torr, respectively. These pressure transducers have four digits of resolution. The two adsorbates used were GR grade from Merck Co. Originally, the O-ring in the system gave rise to various problems. Even with Viton O-rings, the aromatic compounds were absorbed slowly by the O-rings. This created a system pressure drift as well as problems in the later evacuation process. These problems were finally solved by replacing most parts of the system with stainless steel vessels and all-metal valves. The only O-rings left in the system were those on the balance mechanism.

160 140 120 100 80 60 40 20 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

relative pressure, P/Po FIG. 1. Nitrogen adsorption isotherms of Ca montmorillonite and Ti montmorillonite.

To examine whether or not the surface screening effect exists, we estimated and compared the surface fractal dimension D of Ti and Ca montmorillonite through the nitrogen isotherms. Usually, the surface fractal dimension is between 2 and 3. A surface with D = 2 indicates that it is regular and smooth. A higher D value suggests a greater wiggle and space filling surface. At a D value close to 3, the surface is extremely irregular. Therefore, the D value can be considered an operative measure of the surface roughness (9–11). There are several ways to evaluate the D value from the adsorption data. The easiest way is to fit a single adsorption isotherm to some fractal isotherm equations using D as a parameter (12). The classical Frenkel-Halsey-Hill (FHH) theory on multilayer adsorption was extended to fractal surfaces N /Nm ∼ [RT ln(P0 /P)]−1/m ,

[1]

where N /Nm represents the surface fractional coverage and P and P0 are the equilibrium and saturation pressures of the adsorbate, respectively. Two types of fractal isotherm equations were thus proposed (13). If the van der Waals attraction between the solid and adsorbed film is the dominating factor, then for selfsimilar surfaces   1 D =3 1− . [2] m On the other hand, if the liquid/gas surface tension (capillary force) is more important, then for self-similar surfaces, D =3−

1 . m

[3]

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HUANG ET AL.

2.0

2

Ca-montmorillonite Ti-montmorillonite

Ca-montmorillonite Ti-montmorillonite dV/dlog(Pore Diameter)

1

Ln(N/Nm)

0

-1

1.5

1.0

0.5

-2 0.0 -3

10

100

1000 o

log (Pore Diameter/A) -4 -5

-4

-3

-2

-1

0

1

2

3

FIG. 3. rillonite.

Pore size distributions of Ca montmorillonite and Ti montmo-

Ln(Ln(P0/P)) FIG. 2. Log–log plot of N /Nm vs ln(P/P0 ) showing the linear range where fractal behavior is observed.

In Fig. 2, the evaluation of D from Eqs. [2] and [3] for nitrogen isotherms on Ti and Ca montmorillonite is illustrated and the results are presented in Table 1. For the case of the Ti montmorillonite sample, the desorption isotherm suggests negligible capillary condensation in the mesopore range and we should use Eq. [2] to evaluate D, while for the Ca montmorillonite, we should use Eq. [3] to estimate D. As demonstrated in Table 1, the D values for the two examined montmorillonite samples are distinct, indicating that the surface roughness for Ti and Ca montmorillonite is very different. Because the D of Ti montmorillonite is smaller than that of Ca montmorillonite, the pore opening effect, but not the surface screening effect, may be the dominant factor in the increase in the BET surface area. As presented in Fig. 1, two montmorillonite samples show type II adsorption isotherms based on Brunauer’s classification (14) and a large uptake is observed close to saturation pressure, where capillary condensation in the large voids between the aluminosilicate sheets starts. For Ca montmorillonite, the desorption isotherm does not retrace the adsorption isotherm but rather forms an obvious hysteresis loop, before eventually

rejoining the adsorption isotherm. However, the hysteresis loop of Ti montmorillonite disappears. The observed hysteresis may be the result of an intrinsic property of the phase transition (single pore mechanism) or pore block effects. The structure of the pore network (its connectivity and accessibility), while irrelevant to adsorption, is very critical in the desorption processes. According to the percolation theory, the more highly connected the network, the easier it is for the vapor-filled pores to form a spanning cluster and the knee of the desorption isotherm is to the right (15). The disappearance of the hysteresis loop may imply that the pore connectivity is very high. Accordingly, one possible explanation for the above isotherms is that the pore connection existing in the two examined samples is very different and Ti montmorillonite may possess larger pore connectivity. Figure 1 also suggests that the mean pore size of Ti montmorillonites is larger than that of Ca montmorillonites because the former hardly exhibit the capillary condensation phenomenon in the mesopore range. The pore size distribution calculated from the capillary condensation model is shown in Fig. 3. As presented in Fig. 3, the pore size distribution of Ca-saturated montmorillonite is quite different in character from the distribution associated with Ti montmorillonite. For the former, the distributions decay to zero at about 4 nm. For the latter, the

TABLE 1 Specific Surface Areas, Specific Pore Volumes, Average Pore Diameters, Basal Spacings [d(001)], and Surface Fractal Dimensions for the Examined Samples Sample

BET surface area (m2 /g)

Micropore area (m2 /g)

Total pore volume (cm3 /g)

Micropore pore volume (cm3 /g)

Pore diameter (nm)

d(001) ˚ (A)

Surface fractal dimension (D)

Ca montmorillonite Ti montmorillonite

80.8 259.7

28.8 42.9

0.14 0.29

0.01 0.02

6.97 7.01

13 16

2.73 2.39

Ca MONTMORILLONITE EXCHANGE WITH Ti CATIONS

Count.

Ti-montmorillonite

Ca-montmorillonite

0

10

20

30

40

50

60

70

80

2θ FIG. 4.

XRD patterns of Ca montmorillonite and Ti montmorillonite.

distributions are finite at nearly 95 nm, which was the upper limit of the nitrogen pore size distribution in this experiment. In the absence of a step change in the pore size distribution, it follows that the distribution is also nonzero above 95 nm and, therefore, there exist a number of pores in the network in which nitrogen does not condense at the highest experimental pressure. Finally, the saturation adsorption capacity shown in Fig. 1 also suggests an increase in the total pore volume when Ca montmorillonites

FIG. 5.

363

was exchanged with Ti cations. Obviously, the ion-exchange process leads to the simultaneous widening of the pores and an increase in both the pore volume and connectivity. The increase in pore size and pore volume may be due to the insertion of Ti cations between the clay sheets during the exchange process, which may create a greater interlayer space and, consequently, larger pore diameter and pore space. The porous structure characteristics, including the BET surface area, pore volume, and pore size, obtained from the conventional analysis of nitrogen isotherms are presented in Table 1. The XRD patterns accompanied by the ion-exchange process are demonstrated in Fig. 4. As expected, a severe modification of the mineral emerges from Fig. 4. If the diffraction patterns are compared, it is clearly observed that the Ti montmorillonite is less crystalline than the Ca montmorillonite, indicated by the decreasing intensity of most of the smectite peaks. On the other hand, the maximum peak for Ti and Ca montmorillonite occurs at 5.5◦ and 6.8◦ , respectively. Taking the two values into Bragg’s equation, the distance of the interlayers of Ti and Ca ˚ respectively, montmorillonite is estimated to be 16 and 13 A, indicating that the Ti cation may increase the distance between the tetrahedral sheets and alter the montmorillonite structure. The results obtained from scanning electron microscopy (SEM) also point out that the morphology of Ca montmorillonite is altered substantially upon exchange with Ti cation, as presented in Fig. 5. It is well known that the expansive characteristics of montmorillonite are affected by the nature of adsorbed (or exchangeable)

SEM images (×20,000) of (from left to right) Ca montmorillonite and Ti montmorillonite.

HUANG ET AL.

Adsorption of Hexane and Benzene Figure 6 shows the measured isotherms for hexane and benzene on the two examined adsorbents. The shape of isotherms is closer to type II and is also similar to the nitrogen isotherms, thus indicating that benzene and hexane are adsorbed not only in the mesopores but also in the micropores (22). Moreover, it is found that for the two adsorbents, the adsorption capacity of benzene is larger than that of hexane. This may be ascribed

Adsorption data Desorption data 160

298K

140 adsorption uptake,mg/g

cations. If exchangeable cations could be treated as crystalline, then the d(001) spacings of clay would increase with increasing ion crystalline sizes. However, cations presented in clays are usually hydrated (or partially hydrated) and the basal spacings of montmorillonite are strongly related to the number of adsorbed water layers between the interlamellar (16–19). In general, the smaller ion has the larger hydrated ionic radius and thus, Ti cation may induce more expansion of the structure than Ca cation, as indicated by the XRD patterns. Recently, Ebitani et al. (20) have reported that when Na montmorillonite was used as starting material, a two-dimensional titanium oxide structure might be formed within the layer of the montmorillonite. Since ˚ of their Ti montmorillonite is the d(001) spacing (12.3 A) ˚ however, the proposed novel inconsistent with our result (16 A), structure of titanium oxide may not exist in our Ti montmorillonite. For the preparation of Ti-pillared clays, different intercalation models have been considered. From the different results reported in literature, the d(001) spacings and the specific sur˚ and 225–330 m2 /g face area of Ti montmorillonite were 13–28 A (16–23), respectively. From these results it can be shown that both the size of the interlayer spacing and the total surface area vary with the conditions of preparation. It is generally agreed, however, that the pillaring process leads to the formation of a bidimensional porous network, namely, the coexistence of micropores and mesopores (16). On the other hand, it was established earlier that the microporous network is formed in the interlayer spaces between the polycations anchored to the clay and the pore volume, specific surface area, and pore size of pillared clays are determined by the distance between silicate layers of montmorillonite and between pillars supporting these layers (18, 22). As shown in Fig. 1, both adsorption isotherms are of type I at low pressures, indicating a micropore filling process, and exhibit multiplayer adsorption at higher pressures, implying the presence of a certain mesoporosity (19, 23). Table 1 presents the micropore area and volume obtained from the t method (24). It should be noted that although both samples possess micropores and mesopores, the mesoporosity is more important and makes the difference between the samples. Accordingly, the increase of the surface area induced by the incorporation of the titanium between the silicate layers is mainly contributed from the increase of the interlayer space, but not from the presence of the micropore produced in the interlayer spaces between the polycations pillars supporting these layers.

120 100 80 60 40 20 250

288K

200 adsorption uptake, mg/g

364

150

100

50

0 0.0

0.2

0.4

0.6

0.8

1.0

relative pressure, P/P0 FIG. 6. Adsorption isotherms of benzene and hexane on montmorillonites. The curves from top to bottom are benzene on Ti montmorillonite, hexane on Ti montmorillonite, benzene on Ca montmorillonite, and hexane on Ca montmorillonite, respectively.

to the molecular configuration difference between benzene and hexane (25). Benzene, which has a plate configuration, may possess more effective packing in the montmorillonites than hexane, which has a linear configuration. For both adsorbates Ti montmorillonite possesses larger adsorption capacity than Ca montmorillonite due to its larger surface area and pore volume. Finally, it is found that both benzene and hexane isotherms hardly exhibit hysteresis loops on Ti montmorillonite, which is similar to the nitrogen isotherm and may also be ascribed to the larger pore connection in Ti montmorillonite. Because the isotherms were measured at two temperatures, we could estimate the isosteric heats of adsorption using the Clausius–Clapeyron equation (26) qst = R[∂ ln P/∂(1/T )] M ,

365

Ca MONTMORILLONITE EXCHANGE WITH Ti CATIONS

CONCLUSIONS

100 Benzene on Ca-montmorillonite Benzene on Ti-montmorillonite Hexane on Ca-montmorillonite Hexane on Ti-montmorillonite

q (kJ/mol) st

80

60

40

20

0 0

20

40

60

80

100

120

140

160

M (mg/g) FIG. 7.

Adsorption heat for benzene and hexane on montmorillonites.

where R is the gas constant, P the equilibrium gas pressure, T the temperature, and M the amount of vapor adsorption. Figure 7 shows the isosteric heats of adsorption for the two montmorillonites, plotted as a function of the amount of vapor adsorbed. Some important features may be obtained directly from this figure. Firstly, the adsorption heats of benzene and hexane on Ca montmorillonite are larger than on Ti montmorillonite. Moreover, for Ca montmorillonite the adsorption heat of benzene is larger than that of hexane while for Ti montmorillonite the adsorption heats of the two adsorbates are nearly equal. These results may be interpreted in terms of the pore volume difference between the two examined adsorbents. Because the pore volume of Ti montmorillonite is twice as great as the Ca montmorillonite, the arrangement of benzene and hexane in Ti montmorillonite may be looser and the adsorption heat may be mainly contributed from the adsorbate–host interactions but not from the adsorbate–adsorbate interactions. Accordingly, the adsorption heat of Ti montmorillonite is smaller than that of Ca montmorillonite and the adsorption heats of benzene and hexane on Ti montmorillonite are nearly equivalent. In contrast, when benzene and hexane adsorb onto Ca montmorillonite, both the adsorbate–host and adsorbate–adsorbate interactions must be taken into consideration and the adsorption heat may be larger than for the adsorption on Ti montmorillonite. Moreover, the larger adsorption heat of benzene on Ca montmorillonite may be ascribed to the more close packing of benzene in the montmorillonite and then the shorter distance between benzenes and adsorbent as well as between benzene molecules, which may increase the adsorbate–host and adsorbate–adsorbate interactions.

This study examined the effects of exchanged titanium on the surface area, pore texture, and adsorption properties of montmorillonite. As indicated by the classical and fractal analyses of the nitrogen isotherms, the exchange Ti creates an increase in BET surface area with the possibility of pore opening effects, which is associated with the increase in the spacing be˚ and is confirmed tween the tetrahedral sheets from 13 to 16 A with the XRD patterns. The pore opening effects also induce an increase in the pore size, pore volume, and pore connectivity of montmorillonite. Adsorption isotherms of hexane and benzene were then measured to identify the effects of the above revolutions on the adsorption of montmorillonite. It was found that the increase in surface area might provide larger adsorption capacity for the adsorption of both adsorbates. Moreover, the adsorption heat of Ti montmorillonite was smaller than that of Ca montmorillonite due to its larger pore size and pore volume, which may make the arrangement (or packing) of adsorbates looser and then reduce the adsorbate–adsorbate interactions as adsorption progresses. Finally, the disappearance of the adsorption hysteresis loops for hexane and benzene on Ti montmorillonite may be ascribed to the increase in pore connectivity.

ACKNOWLEDGMENT This work was supported by Grant NSC89-2211-E-008-064 of the National Science Council (Taiwan, ROC).

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