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Influence of the synthesis conditions on the preparation of titanium-pillared clays using hydrolyzed titanium ethoxide as the pillaring agent
Microporous and Mesoporous Materials 54 (2002) 155–165 www.elsevier.com/locate/micromeso
Influence of the synthesis conditions on the preparation of titanium-pillared clays using hydrolyzed titanium ethoxide as the pillaring agent J.L. Valverde a
a,*
, P. S anchez a, F. Dorado a, C.B. Molina b, A. Romero
a
Departamento de Ingenieria Quimica, Facultad de Quımicas, Universidad de Castilla-la Mancha, Campus Universitario s/n, 13004 Ciudad Real, Spain b Area de Ingenieria Quimica, Facultad de Ciencias, UAM Cantoblanco, 28049 Madrid, Spain Received 10 December 2001; received in revised form 25 March 2002; accepted 26 March 2002
Abstract Titanium-pillared clays (Ti-PILCs) have been prepared using a commercial bentonite and a reaction mixture containing titanium ethoxide. The structure and properties of the pillared materials were studied by X-ray diffraction, N2 adsorption, chemical analysis, thermal analysis, and temperature-programmed desorption of ammonia. Materials with a were obtained. When the synthesis was carried high content of micropore area (90%) and a basal spacing of about 24 A out using a molar ratio HCl=Ti ¼ 2:5, a homogeneous pillared sample with a high micropore area was obtained. The structural and textural Ti-PILC properties were enhanced when the process was performed at room temperature and with the slowest addition speed of the pillaring solution to the clay suspension. There is a limit in terms of the number of titanium polynuclear species that can be incorporated as pillars within the clay, and this was found to be 15 mmol Ti/g clay. The use of an aqueous suspension of 0.15 wt.% of clay yielded the best structural and textural properties. The synthesized titanium pillared clays were found to be thermally stable up to 500 °C. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Synthesis; Pillared clay; Titanium ethoxide; Bentonite; Thermal stability
1. Introduction The synthesis of metal oxide pillared clays (PILCs) was first reported in the seventies [1–4] and has been the subject of numerous studies to date [4–10]. These materials contain metal oxide pillars that sustain the clay sheets and lead to the formation of a bi-dimensional porous network. *
Corresponding author. Tel.: +34-9-26295300; fax: +34-926295318. E-mail address: [email protected] (J.L. Valverde).
Several single and mixed oxide pillars have been prepared using polycationic species of Al, Zr, Ti, Fe, and Cr, amongst others [8,11–15]. Properties such as acidity, surface area, and pore size distribution of PILCs offer new shape-selective catalysts that are similar to the zeolites. Nevertheless, thermal stability, which is lower than that of zeolites, limits their use as catalysts to specific reactions at relatively low temperatures. In this sense, the PILC-based catalysts have proven to be excellent for the selective catalytic reduction (SCR) of NOx by NH3 or hydrocarbons
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 7 8 - 5
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[16–19]. Although different metals are able to catalyze the SCR reaction, copper-exchanged titanium-pillared clay (Ti-PILC) has shown good activity in this reaction [19,20]. Moreover, these materials showed excellent thermal stability, high surface area and acidity, and their activity remained almost unchanged in the presence of the poisons SO2 and H2 O, which are present in NOx containing streams. As far as the preparation of the pillared clay is concerned, the ready availability and low price of the host clay make it a desirable starting material. However, the synthetic process mainly takes place in a very dilute aqueous medium (0.51 wt.%) and, as a consequence, the product yield is generally very low. The physicochemical properties and, consequently, the catalytic activities of the pillared clays depend on several synthesis parameters, including the concentration of the metal ion, the temperature, the method of preparation, and the nature of the host clay. Two different procedures for the preparation of Ti-PILCs have been performed: the first method uses a TiCl4 solution in hydrochloric acid [21–23], whereas the second route employs hydrolyzed titanium alkoxide in HCl as the pillaring agent [24,25]. Both methods have proven to be suitable for the preparation of Ti-PILCs, although the first process requires careful handling of the TiCl4 . The aim of the work described here was the synthesis and characterization of Ti-PILCs prepared from a bentonite and a pillaring solution containing titanium ethoxide in a hydrochloric acid medium. The influence of the main synthesis conditions (i.e. HCl/Ti molar ratio, speed of addition of the pillaring solution, synthesis temperature, amount of titanium, and clay dispersion) on the properties of the resulting pillared clays was investigated.
2. Experimental 2.1. Synthesis of Ti-PILC The starting clay used was a purified-grade bentonite supplied by Fisher Scientific, with a cation exchange capacity of 94 meq per 100 g of clay and a chemical composition (wt.%) of: SiO2 , 52.22; Al2 O3 , 16.81; Fe2 O3 , 3.84; Na2 O, 1.26; MgO,
0.88; CaO, 0.74; K2 O, 0.80. Particles sizes of <2 lm were used in the pillaring process. A typical TiPILC synthesis procedure was as follows: titanium ethoxide was added to 5 N HCl solution in an appropriate amount to obtain the required HCl/Ti molar ratio. This solution was aged under stirring at room temperature for 3 h. The above polycationic titanium solution was then slowly added to a suspension of bentonite in deionized water. The mixture was stirred and allowed to react at room temperature for 12 h. The resulting solid was washed by centrifugation with deionized water until it was chloride-free (conductivity lower than 10 lS cm1 ). The suspension was air-dried at 60 °C, and the resulting product was calcined for 2 h at different temperatures (200, 300, 400 and 500 °C). Table 1 shows the notation used for each of the samples and the corresponding experimental conditions used for the synthesis of Ti-PILCs. 2.2. Characterization X-ray diffraction (XRD) patterns were obtained using a Philips model PW 1710 diffractometer and Ni-filtered CuKa radiation. In order to maximize the (0 0 1) reflection intensities, oriented specimens were prepared by spreading the sample on a glass slide. The thermal stability of each material was investigated by exposing the samples to temperatures in the range 200–500 °C for 2 h. Surface areas and pore size distributions were determined using nitrogen as the sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). The pillared clays were previously outgassed at 180 °C for 16 h under a vacuum of 6:6 109 bar. Specific total surface areas were calculated using the BET equation, whereas specific total pore volumes were evaluated from nitrogen uptake at P =P0 ¼ 0:99. The Horvath–Kawazoe method (H–K) was used to determine the micropore surface area and volume. The mesopore size distribution was obtained by applying the Barrett–Joyner–Halenda (BJH) method to the adsorption branch of the isotherm. Thermogravimetric analyses (TGA) were performed with a Perkin–Elmer TGA7 thermobalance under a flow of helium (50 cm3 min1 ) with a heating rate of 20 °C min1 from 30 to 900 °C.
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Table 1 Experimental conditions used for the preparation of Ti-PILCs Sample designation
HCl/Ti (mol/mol)
Addition speeda (cm3 /min)
Synthesis temperature (°C)
mmol Ti/g clay
Aqueous clay suspension (wt.%)
TiPILC-1 TiPILC-2 TiPILC-3 TiPILC-4
1.5 2.0 2.5 3.0
0.4 0.4 0.4 0.4
25 25 25 25
10 10 10 10
0.10 0.10 0.10 0.10
TiPILC-5 TiPILC-6 TiPILC-7
2.5 2.5 2.5
1.3 0.9 0.6
25 25 25
10 10 10
0.10 0.10 0.10
TiPILC-8 TiPILC-9 TiPILC-10
2.5 2.5 2.5
0.4 0.4 0.4
65 10 2
10 10 10
0.10 0.10 0.10
TiPILC-11 TiPILC-12 TiPILC-13
2.5 2.5 2.5
0.4 0.4 0.4
25 25 25
25 15 5
0.10 0.10 0.10
TiPILC-14 TiPILC-15
2.5 2.5
0.4 0.4
25 25
15 15
0.15 0.20
a
Addition speed of the pillaring solution to the aqueous clay suspension.
Total acid site densities of the catalysts were measured by temperature-programmed desorption of ammonia using a Micromeritics TPD/TPR analyzer. The samples were housed in a tubular quartz reactor and pretreated in a helium flow while heating at 15 °C min1 up to 500 °C. After a period of 30 min at this temperature, the samples were cooled to 180 °C and saturated for 15 min in a stream of ammonia. The sample was then allowed to equilibrate in a helium flow at 180 °C for 1 h. Finally, ammonia was desorbed using a linear heating rate of 15 °C min1 up to 500 °C. Temperature and detector signals were recorded simultaneously. The unique area under the curve was integrated to determine, in each case, the total acidity on the catalyst. The titanium and aluminum contents in the assynthesized PILCs were measured with a Spectra 220FS atomic absorption analyzer.
3. Results and discussion 3.1. Influence of the HCl/Ti molar ratio Fig. 1 shows the XRD patterns of Ti-PILCs prepared using pillaring solutions with an HCl/Ti
molar ratio in the range 1.5–3.0 (TiPILC-1 to TiPILC-4). It can be seen that the basal (0 0 1) peak around 2# ¼ 9° (characteristic of natural bentonite) is shifted towards lower 2# values in the pillared samples. This result clearly indicates an enlargement of the basal spacing of the clay as a consequence of the pillaring process. A basal (Table 2) was obtained in all spacing of about 24 A the samples. Small differences in the d0 0 1 peak should not be taken into consideration since the broadness of the (0 0 1) peak, which is characteristic of Ti-PILCs, causes an appreciable error in the measurement of the basal spacing. It should also be noted from Fig. 1 that the increase in the HCl/Ti molar ratio resulted in an increase in the basal (0 0 1) intensity, reaching a maximum for TiPILC-3. A more intense (0 0 1) peak would indicate a more homogeneous pillar distribution. Chemical analysis data (Table 2) showed that the acidity of the pillaring solution had little influence on the titanium content of samples. A slight decrease of the aluminum content was also observed as the HCl/Ti ratio of the pillaring solution increased, a situation attributed to the leaching caused by the acid treatment. The surface areas and pore volumes of the pillared clays (Table 2) increased significantly in comparison to the
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Fig. 1. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, synthesized with an HCl/Ti molar ratio of (a) 1.5, (b) 2.0, (c) 2.5, and (d) 3.0.
parent bentonite. This result is consistent with a successful pillaring process. The sample TiPILC-3, obtained with an HCl/Ti ratio of 2.5, exhibited the highest contribution of micropore area versus total surface area (very similar to the sample TiPILC-2). The total acidity of each material was higher in the pillared samples with respect to raw bentonite. This fact could be due to two reasons: the acid character of the titanium species that act as pillars, and/or the marked increase in the accessibility to the internal surface area after the pillaring process. The preparation of Ti-PILCs appeared to be affected by the acid/alkoxide molar ratio. Del Castillo and Grange [25] demonstrated that the amount of titanium that can be intercalated as the appropriate polycation is related to the pH of the solution intercalation. A pH value greater than 1 would promote the degree of polymerization and the titanium polycation exchange,
whereas a pH value higher than 1.8 might lead to titanium oxide precipitation. In the present study, an HCl/Ti molar ratio ranging from 2.0–2.5 yielded a homogeneous pillared sample with the best textural properties in terms of its subsequent use as a catalyst. Fig. 2 shows the behavior of this sample on increasing the calcination temperature up to 500 °C. A weak diffraction peak at 2# ¼ 4° was observed, and this is typical of pillar formation. A second peak was seen at 2# 7°, and this is generally attributed to either the presence of a nonpillared fraction of the clay or to the insertion of titanium species with a low degree of polymerization [26]. In the present study, some of the synthesized pillared samples exhibit a further peak at 2# ¼ 9°, which is characteristic of the raw clay. The diffraction peak at 2# 7° suggests the presence of titanium polyoxycations with a low degree of polymerization (smaller in size), a situation that leads to a lower opening of the clay layers. Upon increasing the calcination temperature, the intensity of the diffraction line at 2# ¼ 4° increased, whereas that of the line at 2# 7° decreased. This could be due to the polymerization of the titanium polyoxycations, existence of different hydration states, to the loss of adsorbed water of hydration [26] or even to the different structures of titanium oxide [27], all of which could generate a more homogeneous pillar distribution. The differences in the intensities of the peaks at 2# ¼ 4° and 7° with the calcination temperature could explain the peak at 2# 7° as being due to a titanium species with different polymerization degree. Meaningful differences in the position of the 2# ¼ 4° reflection were not observed when the calcination temperature was increased. However, the peak at 2# 7° shifted to higher values when the calcination temperature increased. The sample calcined at 500 °C showed this peak at almost half 2# value of the peak at 2# ¼ 4°, that could be related with (0 0 2), and (0 0 1) reflections, respectively. The attribution of the 2# 7° peak to the (002) reflection does not seem to be the explanation when the calcination temperature is under 400 °C. Textural analysis of the samples showed that the surface and micropore areas are maintained at similar values up to 400 °C. At this temperature a
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Table 2 Basal spacing, textural properties, total acidity and chemical analysis of bentonite and titanium-pillared samples Sample designation
d0 0 1 500 °C ) (A
Surface area (m2 /g)
Micropore area (m2 /g)a
Pore volume (cm3 /g)
Total acidity (mmol NH3 /g)
Ti (wt.%)
Al (wt.%)
Bentonite TiPILC-1 TiPILC-2 TiPILC-3 TiPILC-4
9.7 23.0 23.0 23.8 23.0
24.3 277.8 312.3 310.8 272.3
15.1 (62.2) 248.2 (89.3) 278.6 (89.2) 280.6 (90.3) 240.6 (88.4)
0.060 0.199 0.230 0.229 0.205
0.132 0.434 0.554 0.561 0.544
0.0 29.0 28.3 28.2 27.1
8.9 6.4 6.3 6.0 5.5
TiPILC-5 TiPILC-6 TiPILC-7
22.7 22.0 22.9
310.6 311.3 309.2
247.4 (79.6) 276.7 (88.9) 280.6 (90.7)
0.201 0.213 0.216
0.515 0.538 0.541
25.9 26.9 27.2
6.2 6.0 6.6
TiPILC-8 TiPILC-9 TiPILC-10
26.3 22.9 22.6
283.1 275.9 289.3
244.0 (86.2) 247.9 (89.8) 262.7 (90.8)
0.233 0.198 0.198
0.702 0.517 0.561
28.7 22.5 20.6
5.6 7.6 6.7
TiPILC-11 TiPILC-12 TiPILC-13
16.2 23.8 9.7
338.9 338.6 52.6
287.7 (84.9) 305.1 (90.1) 27.8 (52.7)
0.323 0.263 0.087
0.522 0.572 0.483
29.6 29.2 6.1
5.1 6.0 6.2
TiPILC-14 TiPILC-15
24.0 24.2
353.1 323.8
325.7 (92.2) 295.6 (91.3)
0.250 0.230
0.569 0.451
27.4 26.0
5.8 5.8
a
Values in parentheses represent the percentage of micropores in terms of the total area.
decrease of about 17% of the total surface area was observed. It was noticeable that the contribution of the micropore area to the total surface area remained practically constant (88% at 500 °C). This means that the slight decrease in surface area at 500 °C resulted from the dehydroxylation of the clay and titanium pillars, rather than from sintering of the titanium pillars [25], i.e. the clay structure did not collapse at this temperature. The TGA/DTG curves for sample TiPILC-3 are shown in Fig. 3. The main weight losses occurred in two steps, viz. at 25–150 and 500–725 °C. The first step (7.5 wt.%) corresponds to the loss of physically adsorbed water, whereas the second one (2.1 wt.%) is attributed to either the dehydroxylation of the clay structure, removal of any remaining hydroxide from the pillars [15] or even the transformation of the pillars into crystalline TiO2 [26]. The dehydroxylation of hydroxide groups associated with interlayer pillars began to occur at 150 °C and caused a continuous weight loss (4 wt.%) up to 500 °C. Dehydroxylation continues between 500 and 750 °C. In this way a small step to approximately 600 °C is observed. This step is related with the stability of the pillars stability, since an important decrease in the basal spacing values occurs at this
temperature, indicating the collapse of the clay structure. Therefore, the thermogravimetric analyses are in agreement with the aforementioned thermal stability of the synthesized pillared clay up to 500 °C. 3.2. Influence of the speed of addition of the pillaring solution to the clay suspension Fig. 4 shows the XRD patterns of Ti-PILCs synthesized by varying the speed of addition of the pillaring solution to the clay suspension. It can be seen that the intensity of the (0 0 1) reflection increases upon decreasing the speed of addition, indicating a more homogeneous pillaring process. Neither the basal spacing nor the surface area was affected by modifications in the speed of addition (Table 2). However, it can be seen from the results in Table 2 that the highest addition speed gave the lowest titanium content and the lowest contribution of micropores to the total area surface. The decrease in these two parameters resulted in a slight decrease of the total acidity of the samples, as explained above. The influence of the speed of addition on the pillaring process can be explained on the basis of
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Fig. 2. XRD patterns for TiPILC-3 as a function of the calcination temperature.
Fig. 4. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, synthesized with an addition speed of the pillaring solution to the aqueous clay suspension of (a) 0.4 cm3 / min, (b) 0.6 cm3 /min, (c) 0.9 cm3 /min, and (d) 1.3 cm3 /min.
3.3. Influence of the synthesis temperature
Fig. 3. TGA/DTG curves for the TiPILC-3 sample.
the ion-exchange process. The insertion of the hydrolyzed polynuclear titanium species as pillars was enhanced when a low addition speed of the pillaring solution to the clay suspension was used.
As shown in Fig. 5, the pillaring process seemed to occur even if a temperature lower than room temperature was used for the synthesis. However, the use of a relatively high temperature (TiPILC-8) leads to XRD patterns characterized by broad and ill-defined peaks. This situation indicates that a heterogeneous distribution of the pillars is present and some fraction of the raw clay remains in the sample. Similar results were reported by Bernier et al. [22] upon using TiCl4 as the titanium source. In addition, an acidity value that is anomalously high was found for the sample prepared at the highest temperature (Table 2). On the other hand, even though the XRD pattern of the sample obtained at 10 °C (TiPILC-9) could indicate an improvement in the pillaring process, the surface area and titanium content are low in comparison to the
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Fig. 5. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, synthesized at (a) 2 °C, (b) 10 °C, (c) 25 °C, and (d) 65 °C.
Fig. 6. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, obtained with (a) 5 mmol, (b) 10 mmol, (c) 15 mmol, and (d) 25 mmol Ti/g clay.
pillared clay synthesized at room temperature. This behavior could again be related to the extent of the ion exchange process but also to the nature of the titanium species in solution formed at a certain temperature. TGA analysis of samples obtained below room temperature (not shown) indicates an earlier collapse of their structure (around 500 °C), as well as a weight loss three times higher (6 wt.%) than that obtained for samples synthesized at room temperature. In any case, these results demonstrate that the pillaring process at room temperature is most suitable for the synthesis of Ti-PILCs from titanium ethoxide.
place in the case where 5 mmol Ti/g of clay (TiPILC-13) were used. In this sample, the main diffraction peak (2# ¼ 9°) is characteristic of the raw clay. However, the use of 10 mmol Ti/g clay (TiPILC-3) was found to be sufficient to produce a relatively homogeneous pillared clay. This fact is related to the decrease in the pH produced by the hydrolysis of titanium ethoxide. Thus, for the sample obtained with 5 mmol Ti/g clay, a pH ¼ 2:1 was measured in the reaction mixture, whereas the corresponding pH was 1.6 when 25 mmol of Ti were used. As mentioned above, a pH value greater than 1.8 favors the formation of titanium oxide, a situation that prevents the pillaring process. The intensity of the (0 0 1) diffraction line increases slightly as the Ti/clay ratio increases, until a point is reached beyond which only a very broad peak is obtained. In all cases, well-pillared samples showed an identical (0 0 1) basal spacing
3.4. Influence of the titanium content Fig. 6 shows the XRD patterns of samples synthesized with different amounts of titanium. It can be seen that the pillaring process did not take
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). The great influence of the polycation/clay (23.8 A ratio on the constitution and distribution of the pillars has been indicated previously in the literature [13,26]. The surface area and pore volume of the samples increased as the titanium content in the pillaring solution was increased (Table 2). However, the micropore area reached a maximum for the sample prepared with 15 mmol Ti/g clay and was found to decrease with higher titanium contents. Fig. 7 shows the nitrogen adsorption isotherms and the pore size distribution for the raw and the pillared clays. It can be seen that the slope of the isotherms (P =P0 < 0:4) increases in the pillared samples as a consequence of the pillaring process, whereas sample TiPILC-13, which is hardly pillared, shows an isotherm similar to the starting
material. On the other hand, the hysteresis loop (Type 3 in the IUPAC classification) is consistent with the expected structure for materials prepared by expanding a layer structure [28]. The differences observed in the hysteresis loop are remarkable when the highest amount of titanium was used for the synthesis (sample TiPILC-11). The BJH mesopore size distribution (Fig. 7b) of this sample , whereas the showed a broad peak from 30 to 60 A rest of the samples present a unimodal and narrow . This result is in agreement peak centered at 40 A with the broad diffraction line observed for this sample, indicating a heterogeneous pillaring process that is probably a consequence of the existence of different polynuclear titanium species acting as pillars or due to the fact that the system was not in equilibrium. A similar bimodal micro-
Fig. 7. N2 adsorption isotherms at 77 K for the samples synthesized with (a) 25 mmol, (b) 15 mmol, (c) 10 mmol, and (d) 5 mmol Ti/g clay; (a) isotherms, (b) micropore and mesopore size distribution.
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was pore size distribution centered at 5 and 9 A observed in all the pillared samples (shown for TiPILC-12 in Fig. 7b). This bimodal size distribution for Ti-PILCs is consistent with previously reported distributions for similar clays [29,30]. Apparently, these results led to the textural properties discussed above for the sample obtained using 25 mmol Ti/g clay, i.e. high surface area with a high content of meso- and macropores. As far as the titanium content in the samples is concerned (Table 2), apart from the sample with the lowest titanium content, a very slight increase was observed as the titanium content of the pillaring solution increased. As mentioned previously, the acidity of pillared clays increases owing to the increase in both the titanium content and the micropore area (Table 2). The sample with the highest titanium content showed a low acidity despite the high titanium level in the sample. This behavior could be attributed to the decrease in the micropore area and/or to the formation of bulk TiO2 with almost no acidity [26]. In summary, all these results indicate the existence of a limit to the titanium content that can be introduced as pillars within the clay. Moreover, when the amount of titanium exceeds a value close to 15 mmol Ti/g clay, the physical properties of the resulting pillared clay are clearly modified. This modification is probably due to the formation of several titanium oxide species (of different sizes) in the clay interlayer, or even to bulk TiO2 on the external surface. 3.5. Influence of the clay suspension concentration The conventional synthesis of pillared clays requires a very dilute aqueous medium [15,24–26]. The synthesis of PILCs from highly concentrated clay suspensions using a microwave oven has been reported [28]. In order to reduce the amount of water needed in the synthesis (i.e. to increase the amount of pillared clay obtained in each synthesis), samples involving more concentrated clay suspensions were prepared. All the samples have a (Table 2), indicating a basal spacing of about 24 A large layer opening. The peak at 2# 7° (Fig. 8) was always present. Fig. 8 shows that the highest intensity for the (0 0 1) XRD peak was seen for the
163
Fig. 8. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, obtained with an aqueous clay suspension concentration of (a) 0.20 wt.%, (b) 0.15 wt.%, and (c) 0.10 wt.%.
sample obtained using 0.15 wt.% clay suspension, indicating a high homogeneity of the pillars. On the other hand, both the surface area and micropore area (Table 2) were enhanced in this sample. Chemical analysis (Table 2) indicates a slight decrease in the titanium content as the clay suspension concentration was increased. The total acidity of the samples can be explained on the basis of the micropore area and titanium content. The decrease in both the titanium content and the micropore area is responsible for the lower total acidity of these samples. TiPILC-14 showed a favorable combination of these two effects: higher micropore area and lower titanium content than TiPILC-12, with a similar total acidity seen in both samples. The influence of the clay suspension concentration on the synthesis of Ti-PILCs from titanium ethoxide is clear. In our case a clay suspension concentration of 0.15 wt.% seems to improve the properties of TiPILCs as possible
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suspension preparation should require the re-circulation of the water. The use of acetone–water mixtures as solvent has been reported as an alternative to increase the concentration of clay up to 30–50% [31,32]. 4. Conclusions
Fig. 9. XRD patterns for TiPILC-14 as a function of the calcination temperature.
catalysts. The explanation for this behavior may be related to the fact that the ion exchange process due to the titanium polyoxocation formed should not depend on the clay suspension concentration. Fig. 9 illustrates the evolution of XRD patterns of TiPILC-14, which was prepared using a 0.15 wt.% aqueous clay suspension, with increasing temperature. The intensities of the two peaks obtained change markedly with temperature. As the temperature was increased, the peak at 2# 7° decreased in intensity whereas that at 2# 4° became more intense. As discussed above, this fact suggests the existence of different polynuclear species incorporated within the clay interlayers as the temperature increases. On the other hand, the high intensity of the XRD peaks after treatment at 500 °C provides evidence for an ordered structure with a high thermal stability. Unfortunately, the method proposed here is quite impractical for an industrial synthesis. Obviously, the high amount of water used in the
Ti-PILCs have been synthesized from hydrolyzed titanium ethoxide and a bentonite using different synthesis conditions. Materials with a high contribution of micropore area (90%) and a basal were obtained. The pillared spacing around 24 A samples presented a diffraction peak at 2# 4°, which is typical of pillar formation, and a second peak at 2# 7°. The assignment of this second peak to a polynuclear titanium species (smaller in size) is supported by the presence of a peak at 2# 9° (typical of the raw clay) in several samples and by the variation of the XRD peak intensities during thermal treatment. The total acidity of pillared samples increased as both the titanium content and the micropore area increased. A synthesis using a molar ratio HCl=Ti ¼ 2:5 gave a homogeneous pillared sample with a high micropore area. Structural and textural Ti-PILC properties were enhanced when the process was carried out at room temperature with the slowest addition speed of the pillaring solution to the clay suspension. A limit exists in terms of the amount of titanium polynuclear species that can be incorporated as pillars into the clay, and this was found to be 15 mmol Ti/g clay. When the amount of titanium exceeded this value, the properties of the pillared clay were modified, probably due to the formation of several polynuclear titanium oxide species in the clay interlayer or even to the formation of bulk TiO2 . It was possible to reduce the amount of water in the clay suspension to 0.15 wt.% of clay. Finally, the Ti-PILCs described here showed good thermal stability (up to 500 °C). Acknowledgements Financial support from The European Commission (Contract ERK5-CT-1999-00001) is gratefully acknowledged.
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References [1] G.W. Brindley, R.E. Sempels, Clay Clay Miner. 12 (1977) 229. [2] N. Lahav, U. Shani, J. Shabtai, Clay Clay Miner. 26 (1978) 107. [3] D.E.W. Vaughan, R.J. Lussier, in: L.V.C. Rees (Ed.), Proceedings of 5th International Conference on Zeolites, Heyden, London, Philadelphia, Rheine, 1980, pp. 94–101. [4] D.E.W. Vaughan, R.J. Lussier, J.S. Magee, US Patent 4 176 090, assigned to W.R. Grace & Co., New York, 1979. [5] T.J. Pinnavaia, Science 220 (1983) 365. [6] R. Burch, Catal. Today 2 (1988) 185. [7] F. Figueras, Catal. Rev. Sci. Eng. 30 (1988) 457. [8] J.T. Kloprogge, J. Porous Mat. 5 (1998) 5. [9] K. Bahranowski, M. Gasior, A. Kielski, J. Podobinski, E.M. Serwicka, L.A. Vartikian, K. Wodnicka, Clay Clay Miner. 46 (1998) 98. [10] H. Yamada, H. Nakazawa, K. Yoshioka, T. Fujita, Clay Miner. 26 (1991) 359. [11] J.P. Sterte, Clay Clay Miner. 39 (1991) 167. [12] R. Burch, C.I. Warburton, J. Catal. 97 (1986) 503. [13] J.P. Sterte, Clay Clay Miner. 34 (1986) 658. [14] S. Yamanaka, M. Hattori, Catal. Today 2 (1988) 261. [15] P. Ca~ nizares, J.L. Valverde, M.R. Sun Kou, C.B. Molina, Micropor. Mesopor. Mater. 22 (1999) 267. [16] R.Q. Long, R.T. Yang, J. Air Waste Manage. Assoc. 50 (2000) 436.
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Influence of the synthesis conditions on the preparation of titanium-pillared clays using hydrolyzed titanium ethoxide as the pillaring agent J.L. Valverde a
a,*
, P. S anchez a, F. Dorado a, C.B. Molina b, A. Romero
a
Departamento de Ingenieria Quimica, Facultad de Quımicas, Universidad de Castilla-la Mancha, Campus Universitario s/n, 13004 Ciudad Real, Spain b Area de Ingenieria Quimica, Facultad de Ciencias, UAM Cantoblanco, 28049 Madrid, Spain Received 10 December 2001; received in revised form 25 March 2002; accepted 26 March 2002
Abstract Titanium-pillared clays (Ti-PILCs) have been prepared using a commercial bentonite and a reaction mixture containing titanium ethoxide. The structure and properties of the pillared materials were studied by X-ray diffraction, N2 adsorption, chemical analysis, thermal analysis, and temperature-programmed desorption of ammonia. Materials with a were obtained. When the synthesis was carried high content of micropore area (90%) and a basal spacing of about 24 A out using a molar ratio HCl=Ti ¼ 2:5, a homogeneous pillared sample with a high micropore area was obtained. The structural and textural Ti-PILC properties were enhanced when the process was performed at room temperature and with the slowest addition speed of the pillaring solution to the clay suspension. There is a limit in terms of the number of titanium polynuclear species that can be incorporated as pillars within the clay, and this was found to be 15 mmol Ti/g clay. The use of an aqueous suspension of 0.15 wt.% of clay yielded the best structural and textural properties. The synthesized titanium pillared clays were found to be thermally stable up to 500 °C. Ó 2002 Elsevier Science Inc. All rights reserved. Keywords: Synthesis; Pillared clay; Titanium ethoxide; Bentonite; Thermal stability
1. Introduction The synthesis of metal oxide pillared clays (PILCs) was first reported in the seventies [1–4] and has been the subject of numerous studies to date [4–10]. These materials contain metal oxide pillars that sustain the clay sheets and lead to the formation of a bi-dimensional porous network. *
Corresponding author. Tel.: +34-9-26295300; fax: +34-926295318. E-mail address: [email protected] (J.L. Valverde).
Several single and mixed oxide pillars have been prepared using polycationic species of Al, Zr, Ti, Fe, and Cr, amongst others [8,11–15]. Properties such as acidity, surface area, and pore size distribution of PILCs offer new shape-selective catalysts that are similar to the zeolites. Nevertheless, thermal stability, which is lower than that of zeolites, limits their use as catalysts to specific reactions at relatively low temperatures. In this sense, the PILC-based catalysts have proven to be excellent for the selective catalytic reduction (SCR) of NOx by NH3 or hydrocarbons
1387-1811/02/$ - see front matter Ó 2002 Elsevier Science Inc. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 2 ) 0 0 3 7 8 - 5
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[16–19]. Although different metals are able to catalyze the SCR reaction, copper-exchanged titanium-pillared clay (Ti-PILC) has shown good activity in this reaction [19,20]. Moreover, these materials showed excellent thermal stability, high surface area and acidity, and their activity remained almost unchanged in the presence of the poisons SO2 and H2 O, which are present in NOx containing streams. As far as the preparation of the pillared clay is concerned, the ready availability and low price of the host clay make it a desirable starting material. However, the synthetic process mainly takes place in a very dilute aqueous medium (0.51 wt.%) and, as a consequence, the product yield is generally very low. The physicochemical properties and, consequently, the catalytic activities of the pillared clays depend on several synthesis parameters, including the concentration of the metal ion, the temperature, the method of preparation, and the nature of the host clay. Two different procedures for the preparation of Ti-PILCs have been performed: the first method uses a TiCl4 solution in hydrochloric acid [21–23], whereas the second route employs hydrolyzed titanium alkoxide in HCl as the pillaring agent [24,25]. Both methods have proven to be suitable for the preparation of Ti-PILCs, although the first process requires careful handling of the TiCl4 . The aim of the work described here was the synthesis and characterization of Ti-PILCs prepared from a bentonite and a pillaring solution containing titanium ethoxide in a hydrochloric acid medium. The influence of the main synthesis conditions (i.e. HCl/Ti molar ratio, speed of addition of the pillaring solution, synthesis temperature, amount of titanium, and clay dispersion) on the properties of the resulting pillared clays was investigated.
2. Experimental 2.1. Synthesis of Ti-PILC The starting clay used was a purified-grade bentonite supplied by Fisher Scientific, with a cation exchange capacity of 94 meq per 100 g of clay and a chemical composition (wt.%) of: SiO2 , 52.22; Al2 O3 , 16.81; Fe2 O3 , 3.84; Na2 O, 1.26; MgO,
0.88; CaO, 0.74; K2 O, 0.80. Particles sizes of <2 lm were used in the pillaring process. A typical TiPILC synthesis procedure was as follows: titanium ethoxide was added to 5 N HCl solution in an appropriate amount to obtain the required HCl/Ti molar ratio. This solution was aged under stirring at room temperature for 3 h. The above polycationic titanium solution was then slowly added to a suspension of bentonite in deionized water. The mixture was stirred and allowed to react at room temperature for 12 h. The resulting solid was washed by centrifugation with deionized water until it was chloride-free (conductivity lower than 10 lS cm1 ). The suspension was air-dried at 60 °C, and the resulting product was calcined for 2 h at different temperatures (200, 300, 400 and 500 °C). Table 1 shows the notation used for each of the samples and the corresponding experimental conditions used for the synthesis of Ti-PILCs. 2.2. Characterization X-ray diffraction (XRD) patterns were obtained using a Philips model PW 1710 diffractometer and Ni-filtered CuKa radiation. In order to maximize the (0 0 1) reflection intensities, oriented specimens were prepared by spreading the sample on a glass slide. The thermal stability of each material was investigated by exposing the samples to temperatures in the range 200–500 °C for 2 h. Surface areas and pore size distributions were determined using nitrogen as the sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). The pillared clays were previously outgassed at 180 °C for 16 h under a vacuum of 6:6 109 bar. Specific total surface areas were calculated using the BET equation, whereas specific total pore volumes were evaluated from nitrogen uptake at P =P0 ¼ 0:99. The Horvath–Kawazoe method (H–K) was used to determine the micropore surface area and volume. The mesopore size distribution was obtained by applying the Barrett–Joyner–Halenda (BJH) method to the adsorption branch of the isotherm. Thermogravimetric analyses (TGA) were performed with a Perkin–Elmer TGA7 thermobalance under a flow of helium (50 cm3 min1 ) with a heating rate of 20 °C min1 from 30 to 900 °C.
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Table 1 Experimental conditions used for the preparation of Ti-PILCs Sample designation
HCl/Ti (mol/mol)
Addition speeda (cm3 /min)
Synthesis temperature (°C)
mmol Ti/g clay
Aqueous clay suspension (wt.%)
TiPILC-1 TiPILC-2 TiPILC-3 TiPILC-4
1.5 2.0 2.5 3.0
0.4 0.4 0.4 0.4
25 25 25 25
10 10 10 10
0.10 0.10 0.10 0.10
TiPILC-5 TiPILC-6 TiPILC-7
2.5 2.5 2.5
1.3 0.9 0.6
25 25 25
10 10 10
0.10 0.10 0.10
TiPILC-8 TiPILC-9 TiPILC-10
2.5 2.5 2.5
0.4 0.4 0.4
65 10 2
10 10 10
0.10 0.10 0.10
TiPILC-11 TiPILC-12 TiPILC-13
2.5 2.5 2.5
0.4 0.4 0.4
25 25 25
25 15 5
0.10 0.10 0.10
TiPILC-14 TiPILC-15
2.5 2.5
0.4 0.4
25 25
15 15
0.15 0.20
a
Addition speed of the pillaring solution to the aqueous clay suspension.
Total acid site densities of the catalysts were measured by temperature-programmed desorption of ammonia using a Micromeritics TPD/TPR analyzer. The samples were housed in a tubular quartz reactor and pretreated in a helium flow while heating at 15 °C min1 up to 500 °C. After a period of 30 min at this temperature, the samples were cooled to 180 °C and saturated for 15 min in a stream of ammonia. The sample was then allowed to equilibrate in a helium flow at 180 °C for 1 h. Finally, ammonia was desorbed using a linear heating rate of 15 °C min1 up to 500 °C. Temperature and detector signals were recorded simultaneously. The unique area under the curve was integrated to determine, in each case, the total acidity on the catalyst. The titanium and aluminum contents in the assynthesized PILCs were measured with a Spectra 220FS atomic absorption analyzer.
3. Results and discussion 3.1. Influence of the HCl/Ti molar ratio Fig. 1 shows the XRD patterns of Ti-PILCs prepared using pillaring solutions with an HCl/Ti
molar ratio in the range 1.5–3.0 (TiPILC-1 to TiPILC-4). It can be seen that the basal (0 0 1) peak around 2# ¼ 9° (characteristic of natural bentonite) is shifted towards lower 2# values in the pillared samples. This result clearly indicates an enlargement of the basal spacing of the clay as a consequence of the pillaring process. A basal (Table 2) was obtained in all spacing of about 24 A the samples. Small differences in the d0 0 1 peak should not be taken into consideration since the broadness of the (0 0 1) peak, which is characteristic of Ti-PILCs, causes an appreciable error in the measurement of the basal spacing. It should also be noted from Fig. 1 that the increase in the HCl/Ti molar ratio resulted in an increase in the basal (0 0 1) intensity, reaching a maximum for TiPILC-3. A more intense (0 0 1) peak would indicate a more homogeneous pillar distribution. Chemical analysis data (Table 2) showed that the acidity of the pillaring solution had little influence on the titanium content of samples. A slight decrease of the aluminum content was also observed as the HCl/Ti ratio of the pillaring solution increased, a situation attributed to the leaching caused by the acid treatment. The surface areas and pore volumes of the pillared clays (Table 2) increased significantly in comparison to the
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Fig. 1. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, synthesized with an HCl/Ti molar ratio of (a) 1.5, (b) 2.0, (c) 2.5, and (d) 3.0.
parent bentonite. This result is consistent with a successful pillaring process. The sample TiPILC-3, obtained with an HCl/Ti ratio of 2.5, exhibited the highest contribution of micropore area versus total surface area (very similar to the sample TiPILC-2). The total acidity of each material was higher in the pillared samples with respect to raw bentonite. This fact could be due to two reasons: the acid character of the titanium species that act as pillars, and/or the marked increase in the accessibility to the internal surface area after the pillaring process. The preparation of Ti-PILCs appeared to be affected by the acid/alkoxide molar ratio. Del Castillo and Grange [25] demonstrated that the amount of titanium that can be intercalated as the appropriate polycation is related to the pH of the solution intercalation. A pH value greater than 1 would promote the degree of polymerization and the titanium polycation exchange,
whereas a pH value higher than 1.8 might lead to titanium oxide precipitation. In the present study, an HCl/Ti molar ratio ranging from 2.0–2.5 yielded a homogeneous pillared sample with the best textural properties in terms of its subsequent use as a catalyst. Fig. 2 shows the behavior of this sample on increasing the calcination temperature up to 500 °C. A weak diffraction peak at 2# ¼ 4° was observed, and this is typical of pillar formation. A second peak was seen at 2# 7°, and this is generally attributed to either the presence of a nonpillared fraction of the clay or to the insertion of titanium species with a low degree of polymerization [26]. In the present study, some of the synthesized pillared samples exhibit a further peak at 2# ¼ 9°, which is characteristic of the raw clay. The diffraction peak at 2# 7° suggests the presence of titanium polyoxycations with a low degree of polymerization (smaller in size), a situation that leads to a lower opening of the clay layers. Upon increasing the calcination temperature, the intensity of the diffraction line at 2# ¼ 4° increased, whereas that of the line at 2# 7° decreased. This could be due to the polymerization of the titanium polyoxycations, existence of different hydration states, to the loss of adsorbed water of hydration [26] or even to the different structures of titanium oxide [27], all of which could generate a more homogeneous pillar distribution. The differences in the intensities of the peaks at 2# ¼ 4° and 7° with the calcination temperature could explain the peak at 2# 7° as being due to a titanium species with different polymerization degree. Meaningful differences in the position of the 2# ¼ 4° reflection were not observed when the calcination temperature was increased. However, the peak at 2# 7° shifted to higher values when the calcination temperature increased. The sample calcined at 500 °C showed this peak at almost half 2# value of the peak at 2# ¼ 4°, that could be related with (0 0 2), and (0 0 1) reflections, respectively. The attribution of the 2# 7° peak to the (002) reflection does not seem to be the explanation when the calcination temperature is under 400 °C. Textural analysis of the samples showed that the surface and micropore areas are maintained at similar values up to 400 °C. At this temperature a
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Table 2 Basal spacing, textural properties, total acidity and chemical analysis of bentonite and titanium-pillared samples Sample designation
d0 0 1 500 °C ) (A
Surface area (m2 /g)
Micropore area (m2 /g)a
Pore volume (cm3 /g)
Total acidity (mmol NH3 /g)
Ti (wt.%)
Al (wt.%)
Bentonite TiPILC-1 TiPILC-2 TiPILC-3 TiPILC-4
9.7 23.0 23.0 23.8 23.0
24.3 277.8 312.3 310.8 272.3
15.1 (62.2) 248.2 (89.3) 278.6 (89.2) 280.6 (90.3) 240.6 (88.4)
0.060 0.199 0.230 0.229 0.205
0.132 0.434 0.554 0.561 0.544
0.0 29.0 28.3 28.2 27.1
8.9 6.4 6.3 6.0 5.5
TiPILC-5 TiPILC-6 TiPILC-7
22.7 22.0 22.9
310.6 311.3 309.2
247.4 (79.6) 276.7 (88.9) 280.6 (90.7)
0.201 0.213 0.216
0.515 0.538 0.541
25.9 26.9 27.2
6.2 6.0 6.6
TiPILC-8 TiPILC-9 TiPILC-10
26.3 22.9 22.6
283.1 275.9 289.3
244.0 (86.2) 247.9 (89.8) 262.7 (90.8)
0.233 0.198 0.198
0.702 0.517 0.561
28.7 22.5 20.6
5.6 7.6 6.7
TiPILC-11 TiPILC-12 TiPILC-13
16.2 23.8 9.7
338.9 338.6 52.6
287.7 (84.9) 305.1 (90.1) 27.8 (52.7)
0.323 0.263 0.087
0.522 0.572 0.483
29.6 29.2 6.1
5.1 6.0 6.2
TiPILC-14 TiPILC-15
24.0 24.2
353.1 323.8
325.7 (92.2) 295.6 (91.3)
0.250 0.230
0.569 0.451
27.4 26.0
5.8 5.8
a
Values in parentheses represent the percentage of micropores in terms of the total area.
decrease of about 17% of the total surface area was observed. It was noticeable that the contribution of the micropore area to the total surface area remained practically constant (88% at 500 °C). This means that the slight decrease in surface area at 500 °C resulted from the dehydroxylation of the clay and titanium pillars, rather than from sintering of the titanium pillars [25], i.e. the clay structure did not collapse at this temperature. The TGA/DTG curves for sample TiPILC-3 are shown in Fig. 3. The main weight losses occurred in two steps, viz. at 25–150 and 500–725 °C. The first step (7.5 wt.%) corresponds to the loss of physically adsorbed water, whereas the second one (2.1 wt.%) is attributed to either the dehydroxylation of the clay structure, removal of any remaining hydroxide from the pillars [15] or even the transformation of the pillars into crystalline TiO2 [26]. The dehydroxylation of hydroxide groups associated with interlayer pillars began to occur at 150 °C and caused a continuous weight loss (4 wt.%) up to 500 °C. Dehydroxylation continues between 500 and 750 °C. In this way a small step to approximately 600 °C is observed. This step is related with the stability of the pillars stability, since an important decrease in the basal spacing values occurs at this
temperature, indicating the collapse of the clay structure. Therefore, the thermogravimetric analyses are in agreement with the aforementioned thermal stability of the synthesized pillared clay up to 500 °C. 3.2. Influence of the speed of addition of the pillaring solution to the clay suspension Fig. 4 shows the XRD patterns of Ti-PILCs synthesized by varying the speed of addition of the pillaring solution to the clay suspension. It can be seen that the intensity of the (0 0 1) reflection increases upon decreasing the speed of addition, indicating a more homogeneous pillaring process. Neither the basal spacing nor the surface area was affected by modifications in the speed of addition (Table 2). However, it can be seen from the results in Table 2 that the highest addition speed gave the lowest titanium content and the lowest contribution of micropores to the total area surface. The decrease in these two parameters resulted in a slight decrease of the total acidity of the samples, as explained above. The influence of the speed of addition on the pillaring process can be explained on the basis of
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Fig. 2. XRD patterns for TiPILC-3 as a function of the calcination temperature.
Fig. 4. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, synthesized with an addition speed of the pillaring solution to the aqueous clay suspension of (a) 0.4 cm3 / min, (b) 0.6 cm3 /min, (c) 0.9 cm3 /min, and (d) 1.3 cm3 /min.
3.3. Influence of the synthesis temperature
Fig. 3. TGA/DTG curves for the TiPILC-3 sample.
the ion-exchange process. The insertion of the hydrolyzed polynuclear titanium species as pillars was enhanced when a low addition speed of the pillaring solution to the clay suspension was used.
As shown in Fig. 5, the pillaring process seemed to occur even if a temperature lower than room temperature was used for the synthesis. However, the use of a relatively high temperature (TiPILC-8) leads to XRD patterns characterized by broad and ill-defined peaks. This situation indicates that a heterogeneous distribution of the pillars is present and some fraction of the raw clay remains in the sample. Similar results were reported by Bernier et al. [22] upon using TiCl4 as the titanium source. In addition, an acidity value that is anomalously high was found for the sample prepared at the highest temperature (Table 2). On the other hand, even though the XRD pattern of the sample obtained at 10 °C (TiPILC-9) could indicate an improvement in the pillaring process, the surface area and titanium content are low in comparison to the
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161
Fig. 5. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, synthesized at (a) 2 °C, (b) 10 °C, (c) 25 °C, and (d) 65 °C.
Fig. 6. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, obtained with (a) 5 mmol, (b) 10 mmol, (c) 15 mmol, and (d) 25 mmol Ti/g clay.
pillared clay synthesized at room temperature. This behavior could again be related to the extent of the ion exchange process but also to the nature of the titanium species in solution formed at a certain temperature. TGA analysis of samples obtained below room temperature (not shown) indicates an earlier collapse of their structure (around 500 °C), as well as a weight loss three times higher (6 wt.%) than that obtained for samples synthesized at room temperature. In any case, these results demonstrate that the pillaring process at room temperature is most suitable for the synthesis of Ti-PILCs from titanium ethoxide.
place in the case where 5 mmol Ti/g of clay (TiPILC-13) were used. In this sample, the main diffraction peak (2# ¼ 9°) is characteristic of the raw clay. However, the use of 10 mmol Ti/g clay (TiPILC-3) was found to be sufficient to produce a relatively homogeneous pillared clay. This fact is related to the decrease in the pH produced by the hydrolysis of titanium ethoxide. Thus, for the sample obtained with 5 mmol Ti/g clay, a pH ¼ 2:1 was measured in the reaction mixture, whereas the corresponding pH was 1.6 when 25 mmol of Ti were used. As mentioned above, a pH value greater than 1.8 favors the formation of titanium oxide, a situation that prevents the pillaring process. The intensity of the (0 0 1) diffraction line increases slightly as the Ti/clay ratio increases, until a point is reached beyond which only a very broad peak is obtained. In all cases, well-pillared samples showed an identical (0 0 1) basal spacing
3.4. Influence of the titanium content Fig. 6 shows the XRD patterns of samples synthesized with different amounts of titanium. It can be seen that the pillaring process did not take
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). The great influence of the polycation/clay (23.8 A ratio on the constitution and distribution of the pillars has been indicated previously in the literature [13,26]. The surface area and pore volume of the samples increased as the titanium content in the pillaring solution was increased (Table 2). However, the micropore area reached a maximum for the sample prepared with 15 mmol Ti/g clay and was found to decrease with higher titanium contents. Fig. 7 shows the nitrogen adsorption isotherms and the pore size distribution for the raw and the pillared clays. It can be seen that the slope of the isotherms (P =P0 < 0:4) increases in the pillared samples as a consequence of the pillaring process, whereas sample TiPILC-13, which is hardly pillared, shows an isotherm similar to the starting
material. On the other hand, the hysteresis loop (Type 3 in the IUPAC classification) is consistent with the expected structure for materials prepared by expanding a layer structure [28]. The differences observed in the hysteresis loop are remarkable when the highest amount of titanium was used for the synthesis (sample TiPILC-11). The BJH mesopore size distribution (Fig. 7b) of this sample , whereas the showed a broad peak from 30 to 60 A rest of the samples present a unimodal and narrow . This result is in agreement peak centered at 40 A with the broad diffraction line observed for this sample, indicating a heterogeneous pillaring process that is probably a consequence of the existence of different polynuclear titanium species acting as pillars or due to the fact that the system was not in equilibrium. A similar bimodal micro-
Fig. 7. N2 adsorption isotherms at 77 K for the samples synthesized with (a) 25 mmol, (b) 15 mmol, (c) 10 mmol, and (d) 5 mmol Ti/g clay; (a) isotherms, (b) micropore and mesopore size distribution.
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was pore size distribution centered at 5 and 9 A observed in all the pillared samples (shown for TiPILC-12 in Fig. 7b). This bimodal size distribution for Ti-PILCs is consistent with previously reported distributions for similar clays [29,30]. Apparently, these results led to the textural properties discussed above for the sample obtained using 25 mmol Ti/g clay, i.e. high surface area with a high content of meso- and macropores. As far as the titanium content in the samples is concerned (Table 2), apart from the sample with the lowest titanium content, a very slight increase was observed as the titanium content of the pillaring solution increased. As mentioned previously, the acidity of pillared clays increases owing to the increase in both the titanium content and the micropore area (Table 2). The sample with the highest titanium content showed a low acidity despite the high titanium level in the sample. This behavior could be attributed to the decrease in the micropore area and/or to the formation of bulk TiO2 with almost no acidity [26]. In summary, all these results indicate the existence of a limit to the titanium content that can be introduced as pillars within the clay. Moreover, when the amount of titanium exceeds a value close to 15 mmol Ti/g clay, the physical properties of the resulting pillared clay are clearly modified. This modification is probably due to the formation of several titanium oxide species (of different sizes) in the clay interlayer, or even to bulk TiO2 on the external surface. 3.5. Influence of the clay suspension concentration The conventional synthesis of pillared clays requires a very dilute aqueous medium [15,24–26]. The synthesis of PILCs from highly concentrated clay suspensions using a microwave oven has been reported [28]. In order to reduce the amount of water needed in the synthesis (i.e. to increase the amount of pillared clay obtained in each synthesis), samples involving more concentrated clay suspensions were prepared. All the samples have a (Table 2), indicating a basal spacing of about 24 A large layer opening. The peak at 2# 7° (Fig. 8) was always present. Fig. 8 shows that the highest intensity for the (0 0 1) XRD peak was seen for the
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Fig. 8. XRD patterns for the raw bentonite and Ti-PILCs, calcined at 500 °C, obtained with an aqueous clay suspension concentration of (a) 0.20 wt.%, (b) 0.15 wt.%, and (c) 0.10 wt.%.
sample obtained using 0.15 wt.% clay suspension, indicating a high homogeneity of the pillars. On the other hand, both the surface area and micropore area (Table 2) were enhanced in this sample. Chemical analysis (Table 2) indicates a slight decrease in the titanium content as the clay suspension concentration was increased. The total acidity of the samples can be explained on the basis of the micropore area and titanium content. The decrease in both the titanium content and the micropore area is responsible for the lower total acidity of these samples. TiPILC-14 showed a favorable combination of these two effects: higher micropore area and lower titanium content than TiPILC-12, with a similar total acidity seen in both samples. The influence of the clay suspension concentration on the synthesis of Ti-PILCs from titanium ethoxide is clear. In our case a clay suspension concentration of 0.15 wt.% seems to improve the properties of TiPILCs as possible
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suspension preparation should require the re-circulation of the water. The use of acetone–water mixtures as solvent has been reported as an alternative to increase the concentration of clay up to 30–50% [31,32]. 4. Conclusions
Fig. 9. XRD patterns for TiPILC-14 as a function of the calcination temperature.
catalysts. The explanation for this behavior may be related to the fact that the ion exchange process due to the titanium polyoxocation formed should not depend on the clay suspension concentration. Fig. 9 illustrates the evolution of XRD patterns of TiPILC-14, which was prepared using a 0.15 wt.% aqueous clay suspension, with increasing temperature. The intensities of the two peaks obtained change markedly with temperature. As the temperature was increased, the peak at 2# 7° decreased in intensity whereas that at 2# 4° became more intense. As discussed above, this fact suggests the existence of different polynuclear species incorporated within the clay interlayers as the temperature increases. On the other hand, the high intensity of the XRD peaks after treatment at 500 °C provides evidence for an ordered structure with a high thermal stability. Unfortunately, the method proposed here is quite impractical for an industrial synthesis. Obviously, the high amount of water used in the
Ti-PILCs have been synthesized from hydrolyzed titanium ethoxide and a bentonite using different synthesis conditions. Materials with a high contribution of micropore area (90%) and a basal were obtained. The pillared spacing around 24 A samples presented a diffraction peak at 2# 4°, which is typical of pillar formation, and a second peak at 2# 7°. The assignment of this second peak to a polynuclear titanium species (smaller in size) is supported by the presence of a peak at 2# 9° (typical of the raw clay) in several samples and by the variation of the XRD peak intensities during thermal treatment. The total acidity of pillared samples increased as both the titanium content and the micropore area increased. A synthesis using a molar ratio HCl=Ti ¼ 2:5 gave a homogeneous pillared sample with a high micropore area. Structural and textural Ti-PILC properties were enhanced when the process was carried out at room temperature with the slowest addition speed of the pillaring solution to the clay suspension. A limit exists in terms of the amount of titanium polynuclear species that can be incorporated as pillars into the clay, and this was found to be 15 mmol Ti/g clay. When the amount of titanium exceeded this value, the properties of the pillared clay were modified, probably due to the formation of several polynuclear titanium oxide species in the clay interlayer or even to the formation of bulk TiO2 . It was possible to reduce the amount of water in the clay suspension to 0.15 wt.% of clay. Finally, the Ti-PILCs described here showed good thermal stability (up to 500 °C). Acknowledgements Financial support from The European Commission (Contract ERK5-CT-1999-00001) is gratefully acknowledged.
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