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Ni ratio: synthesis, characterization, and catalysis
Microporous and Mesoporous Materials 33 (1999) 49–59 www.elsevier.nl/locate/micmat
Saponite catalysts with systematically varied Mg/Ni ratio: synthesis, characterization, and catalysis S. Kawi *, Y.Z. Yao Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 8 December 1998; accepted for publication 10 May 1999
Abstract A series of saponite catalysts having systematically varied Mg and Ni ratios in the clay framework (i.e. [Mg Ni ]-saponites) were synthesized using the hydrothermal synthesis method. The synthesized saponite catalysts 6–0 0–6 have similar XRD patterns, indicating that all of the Mg and/or Ni elements have been incorporated as Mg and Ni ions in the octahedral positions of the clay framework, respectively. The results of the isopropanol dehydration reaction show that the saponite containing only Ni (i.e. [Mg Ni ]-saponite) was very active, while the other 0 6 Mg-containing saponites (i.e. [Mg Ni ]-saponite) were inactive. The results of the pyridine-adsorption FTIR 1–6 5–0 further show that the [Mg Ni ]-saponite has Brønsted and Lewis acidic sites, while the [Mg Ni ]-saponites have 0 6 1–6 5–0 only Lewis acidic sites. The possible interaction between Mg and Ni ions in the octahedral positions of saponite catalysts and its influence on the catalytic activity of the catalysts is discussed in this paper. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Bimetallic cations; Catalysts; Clay; Saponite
1. Introduction Clays have been used as catalysts for some time [1–4]. Commonly used clays are the layered clays called smectite (such as montmorillonite and saponite). Although natural clays found in natural deposits have been used as catalysts, synthetic clays still have drawn little attention in clay science. However, it is believed that new types of synthetic clays could be discovered and developed to have catalytic applications [5]. Synthetic clays are of interest because they have unusual advantages over natural clays, as follows. * Corresponding author. Tel.: +65-874-6312; fax: +65-779-1936. E-mail address: [email protected] (S. Kawi)
(1) The catalytic performance of synthetic clays is more versatile than natural clays and could be adjusted systematically to accelerate certain catalytic reactions. This is because, during the synthesis of synthetic clays, catalytically active elements (with no impurities) can be introduced and uniformly distributed in the crystalline framework of clays. (2) By systematically controlling certain synthesis parameters, a series of synthetic clays can be synthesized and developed as catalyst models for understanding the relationship between the catalytic performance and structures or compositions of the clays. (3) Synthetic clays can be produced on a large scale with uniform chemical composition and catalytic properties, but the catalytic properties of natural clays vary with the locations and batches of clay deposits.
1387-1811/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 12 2 - 5
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There have been some recent reports on synthetic clays, especially on synthetic saponites [6 ]. For examples, a Ni-substituted saponite containing only Ni2+ ions (substituting all Mg2+) in all of the octahedral positions of the clay framework had been synthesized [7] and found to be an efficient catalyst for the selective dimerization of ethene owing to its uniform distribution of active Ni ions throughout the clay. An Fe–Ni saponite was synthesized using a similar hydrothermal method [8]; based on the XRD, DTA, IR, TPR, Mo¨ssbauer spectra, and chemical analysis, Fe ions had been shown to be introduced in the tetrahedral position of the clay framework. Some hectoritelike smectites containing Ni have also been synthesized [9], but the substitution and the position of Mg, Ni or Li ions in the clay framework have not been well characterized. Mg and Zn ions have been incorporated in the octahedral layer of saponites and the resulting synthetic saponite catalysts have been shown to be active for Friedel–Crafts alkylation of benzene with propylene to produce cumene [10]. Although bimetallic ions coexisting in the clay framework have been synthesized, the effects of bimetallic ions closely interacting in the clay framework on the catalytic properties of the clays have not yet been reported in the literature. In this paper, a new series of saponites having systematically varied Mg to Ni ratios, with both Mg and Ni in the octahedral positions of the clay framework, were synthesized. The present studies were undertaken to obtain a fundamental understanding about the introduction of nickel in the clay framework, the possibility of introducing bimetallic ions in close proximity to each other in the clay framework, and the effect of bimetallic ions in the octahedral positions on the catalytic properties of the clays. As clay minerals can have both tetrahedral and octahedral positions, the results of this study show that the incorporation of two or even more elements simultaneously in the octahedral or tetrahedral positions can provide ample opportunities to synthesize clay minerals which can then be used as precursors using hydrothermal surfactant treatment to form new high-surface-area clays [11– 13] or clay catalysts having interesting catalytic performance for certain reactions [14].
2. Experimental 2.1. Catalyst preparation A series of Na+-saponites having systematically varied Mg to Ni ratios were prepared using a hydrothermal synthesis method. The amounts of chemicals used in the synthesis of these saponite catalysts are shown in Table 1. By maintaining the total amount of Mg and Ni at the same level, saponites having the formula [Mg · Ni ] x 6−x (Si · Al )O (OH ) ] were synthesised as follows. 7 1 20 4 A buffer solution was prepared by dissolving 3.6 g of NaOH and 6.56 g of NaHCO in 50 ml of 3 deionized water. Solution A was prepared by adding a sodium silicate solution (Merck, Na O: 2 7.5–8.5 wt.% and SiO : 25.5–28.5 wt.%) to the 2 buffer solution and the mixture was completely stirred; solution A had a high pH of about 13. Solution B was prepared by dissolving stoichiometric amounts of AlCl · 6H O (Merck, 99%), 3 2 NiCl · 6H O (Merck, 98%), and MgCl · 6H O 2 2 2 2 (Merck, 99%) in 5.0 ml of deionized water; solution B had a low pH of about 3. Since it was important that the gel for the synthesis of saponites was to be formed at higher pH range, solution B was slowly added to solution A under vigorous stirring until a uniform gel was eventually obtained. The gel was sealed into a 100 ml stainless steel autoclave with Teflon liner and was hydrothermally treated in a furnace at 285°C for 48 h. In order to get rid of the excess electrolytes in the resultant product, the product was repeatedly washed with deionized water and centrifuged (the centrifuge was carried out at a speed of 8000 rpm for 8 min); the washing process was repeated for six times until the product was chloride-free (this was tested using AgNO solution). The resultant 3 product was then dried in an oven at 100°C for 48 h. H+-saponites were prepared by exchanging Na+-saponite samples as follows: 100 ml of 0.5 M of NH NO solution was added to 5 g of 4 3 Na+-saponite sample. The mixture was stirred continuously at room temperature for 1 h before it was centrifuged. The exchange process was repeated for four times. The product was dried in an oven at 100°C for 48 h. The cation-exchanged
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S. Kawi, Y.Z. Yao / Microporous and Mesoporous Materials 33 (1999) 49–59 Table 1 Amounts of chemicals used in the synthesis of [Mg Ni ]-saponites x y Chemical
Na silicatea NaOH NaHCO 3 AlCl · 6H O 3 2 MgCl · 6H O 2 2 NiCl · 6H O 2 2
Amount of chemicals (g) used in an autoclave to synthesize [Mg Ni ]-saponites x y Mg Ni 6 0
Mg Ni 5 1
Mg Ni 4 2
Mg Ni 3 3
Mg Ni 2 4
Mg Ni 1 5
Mg Ni 0 6
7.79 3.60 6.56 1.22 6.16 0
7.79 3.60 6.56 1.22 5.13 1.21
7.79 3.60 6.56 1.22 4.10 2.42
7.79 3.60 6.56 1.22 3.08 3.64
7.79 3.60 6.56 1.22 2.05 4.85
7.79 3.60 6.56 1.22 1.03 6.06
7.79 3.60 6.56 1.22 0 7.27
a Na silicate=sodium silicate solution (27 wt.% SiO ). 2
product was then calcined at 550°C for 24 h. The sample was then ground to a fine powder form. 2.2. Characterization N adsorption–desorption isotherms were 2 obtained at 77 K using a Quantachrome NOVA1000 analyzer. BET surface areas were calculated based on the six adsorption data points in the relative pressure range ( p/p0) of 0.10–0.30. The pore size distribution was derived using a calculation based on the BJH method [15]. X-ray diffraction patterns were obtained on a Philips PW 1710 diffractometer equipped with a Philips PW 1729 X-ray generator, using Cu Ka radiation as the X-ray source and a scanning (2h) range 4–64°. A pyridine-adsorption FTIR study was carried out as follows. A sample (ca. 15 mg) was ground to fine powders and pelletized into a self-supported wafer. The wafer was placed in a quartz cell equipped with two calcium fluoride windows. The wafer was treated under vacuum at 400°C for 2 h. Pyridine adsorption was carried out by exposing the wafer to a He flow saturated with pyridine vapour at 298 K for 20 min. The pyridine desorption was carried out by heating the sample under vacuum at a series of treatment temperatures. The spectra were collected after the sample was treated under vacuum at a certain treatment temperature and cooled to room temperature. The FTIR spectra were recorded on a Shimadzu 8101M FTIR spectrometer. To get some information of the framework structures of the synthesized clays, IR
spectra were measured on the synthesized clay samples which had been mixed with KBr. Isopropanol alcohol (IPA) dehydration and dehydrogenation test reaction was carried out in a microreactor. Typically, 0.20 g of catalyst was loaded into a glass reactor, and pre-treated in a flow of helium at 400°C for 90 min. The reactor was then cooled to the reaction temperatures (160– 250°C ). A He flow (20 ml min−1) saturated with IPA at 12 Torr (which was generated by passing helium through an IPA saturator at 5°C ) was introduced into the reactor. The reaction products were analyzed using an HP 6890 GC on-line system equipped with a packed column (Supelco 10% Carbowax 1540 on 30/60 mesh Chromosorb T ).
3. Results 3.1. Crystalline structure Table 2 lists all the synthetic Mg–Ni saponite catalysts synthesized in this study. A [Mg Ni ] x y saponite (where x or y=0–6 and x+y=6) represents the saponite catalyst in which x and y represent the moles of Mg and Ni in one clay unit cell, respectively (Ni and Mg moles were calculated according to the composition in the starting gel ). Table 3 shows a comparison of the XRD indexes of the reference saponite, whose values are reported in literature, with those of the [Mg Ni ]-saponite, which is one of the saponites 1 5 synthesized in this study. The significant XRD reflections for the [Mg Ni ]-saponite include those 1 5
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Table 2 Unit cell compositions of starting gel and surface areas of [Mg Ni ]-saponites x y Samples
Unit cell composition of starting gel
Surface area (m2 g−1)
[Mg Ni ]-saponite 6 0 [Mg Ni ]-saponite 5 1 [Mg Ni ]-saponite 4 2 [Mg Ni ]-saponite 3 3 [Mg Ni ]-saponite 2 4 [Mg Ni ]-saponite 1 5 [Mg Ni ]-saponite 0 6
Na [Mg · Ni ](Sr · Al )O (OH ) 1 6 0 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 5 1 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 4 2 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 3 3 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 2 4 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 1 5 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 0 6 7 1 20 4
112 144 139 152 124 88 82
Table 3 Comparison of XRD indexes of [Mg Ni ]-saponite with those of reference saponites reported in the literature 1 5 Synthetic [Mg Ni ]-saponite 1 5
Reference synthetic saponite [7]
Reference synthetic trioctahedral smectite [16 ]
˚) d (A
Int
hkl
˚) d (A
Int
hkl
˚) d (A
Int
hkl
13.60 4.51 3.06 2.54 1.71 1.514
100 9.4 15.6 11.5 4.2 15.6
001 020, 004 130, 150, 060,
11.935 4.539 3.186 2.552 1.714 1.526
100 25.7 24.0 34.9 3.4 35.1
001 020, 004 130, 150, 060,
12.6 4.56 3.16 2.58 1.72 1.527
100 13.7 19.0 13.7 2.9 21.2
001 002, 004 130, 150, 060,
110 200 240, 310 330
110 200 240, 310 330
110 200 240, 310 330
at d spacings of 13.60, 4.51, 3.06, 2.54, 1.41 and ˚ ; these reflections are in good agreement 1.514 A with those of the synthetic saponite [7] and the synthetic trioctahedral smectite [16 ]. The comparison shows that the synthesized [Mg Ni ]-saponite 1 5 has the same crystalline structure as the reference saponite. Fig. 1 shows the XRD patterns of all [Mg Ni ]-saponites, which are quite similar to that x y of the [Mg Ni ]-saponite. These results show that 1 5 all [Mg Ni ]-saponites have the same crystalline x y structures as the [Mg Ni ]-saponite (i.e. the sapo6 0 nite containing only Mg ions), demonstrating that Ni ions might have been incorporated into the same sites as the Mg ions and might have possibly been interacting with the Mg ions. 3.2. Catalytic IPA dehydration and dehydrogenation It is well known that IPA can be dehydrated on acidic sites to produce propene and be dehydrogenated on basic sites to produce acetone. In order to examine the possible coexistence and interaction
Fig. 1. XRD patterns of [Mg Ni ]-saponites. x y
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Fig. 2. IPA conversion versus reaction temperatures of [Mg Ni ]-saponite catalysts. x y
between the Mg and Ni ions incorporated in the framework of [Mg Ni ]-saponite catalysts, IPA x y conversion reactions were used to characterize the nature of the acidic and basic sites of these [Mg Ni ]-saponite catalysts [17]. x y Fig. 2 shows the results of the catalytic IPA conversion on [Mg Ni ]-saponite catalysts. The x y [Mg Ni ] saponite catalyst (i.e. the saponite cata0 6 lyst containing only Ni ions) exhibits extremely high catalytic activity for IPA conversion compared with all other [Mg Ni ] saponite cata1–6 5–0 lysts (i.e. saponite catalysts containing both Mg and Ni ions). The effect of reaction temperatures (from 160 to 250°C ) on the catalytic conversion of IPA on [Mg Ni ]-saponite catalysts was also x y examined. At each reaction temperature, the conversion did not increase linearly with an increase in Ni contents of [Mg Ni ]-saponite catalysts. The x y results show that each [Mg Ni ]-saponite catalyst x y is not a physical mixture of [Mg Ni ]- and 6 0 [Mg Ni ]-saponite catalysts, but each [Mg Ni ]0 6 x y saponite catalyst has both Mg and Ni ions interacting with each other in the octahedral positions of the clay framework. Figs. 3 and 4 show significant differences in the product selectivities resulting from the conversion of IPA on the [Mg Ni ]-saponite catalyst and on 0 6 the other [Mg Ni ]-saponite catalysts. The 1–6 5–0 [Mg Ni ]-saponite had almost 100% selectivity 0 6 towards propene, which was formed from the dehydration of IPA on the acidic sites. All of the other [Mg Ni ]-saponite catalysts exhibited 1–6 5–0
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Fig. 3. Effect of reaction temperature on the IPA selectivity to propene using [Mg Ni ]-saponite catalysts. x y
Fig. 4. Effect of reaction temperature on the IPA selectivity to acetone using [Mg Ni ]-saponite catalysts. x y
similar selectivity patterns to each other (i.e. 40– 60% propene, 60–40% acetone). The selectivity results indicate that the [Mg Ni ]-saponite catalyst 0 6 contained mainly acidic sites while the other [Mg Ni ]-saponite catalysts contained both 1–6 5–0 acidic and basic sites. The selectivities to propene and to acetone observed on these catalysts are also shown not to increase with an increase in Ni contents of the [Mg Ni ]-saponite catalysts. 1–6 5–0 The relatively large difference between the selectivities observed at low conversions ( less than 2–3%) is attributed to the larger errors in the determination of the selectivities at low conversions. It was observed that, at a certain reaction temperature (such as at 200°C ), the selectivity of the [Mg Ni ]-saponite catalysts to convert IPA to x y
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propene and to acetone could be adjusted from 30% to almost 100% and from 70% to almost 0%, respectively, by simply changing the composition of the Mg or Ni ions in the framework of the saponite catalysts. This result indicates that the interactions between Mg and Ni ions in the framework of the saponite catalysts play a very important role in regulating the acidity of the saponite catalysts. 3.3. Pyridine-adsorption FTIR To explore further the interesting catalytic behaviors of [Mg Ni ]-saponites, in situ FTIR studies x y for the adsorption of pyridine on [Mg Ni ]-sapox y nites were carried out to characterize the properties of the acidic sites of saponite catalysts. Fig. 5 shows FTIR spectra characterizing the pyridine adsorbed on [Mg Ni ]-saponite catalysts at 200°C. x y All [Mg Ni ]-saponites show similar IR bands x y characterizing the adsorbed pyridine at 1445, 1490 and 1590 cm−1, which are attributed to pyridine adsorbed on Lewis acid sites. As the Lewis acidities of all [Mg Ni ]-saponite samples are quite similar, x y they do not provide any clues to the different catalytic behaviors between the [Mg Ni ]-saponite 0 6 and the other [Mg Ni ]-saponites. However, if 1–6 5–0 Brønsted acid sites are compared, there are substantial differences between the FTIR spectra of the [Mg Ni ]-saponite and the other 0 6
Fig. 5. FTIR spectra of pyridine adsorption on [Mg Ni ]-saponite catalysts (after evacuation at 200°C for 1 h). x y
[Mg Ni ]-saponites. Based on the IR bands at 1–6 5–0 1540 and 1490 cm−1, which are characteristic of Brønsted acid sites, the [Mg Ni ]-saponite had 0 6 more amount of Brønsted acid sites than other [Mg Ni ]-saponites. The results imply that not 1–6 5–0 only were there interactions between Mg ions and Ni ions in the tetrahedral position, but the Mg ions might have also deactivated the Ni ions. 3.4. Surface area, N isotherm, and pore size 2 distribution All [Mg Ni ]-saponites were characterized by x y N adsorption–desorption measurements. The 2 resulting surface areas of [Mg Ni ]-saponites are x y listed in Table 2. Contrary to the IPA reaction results, the differences of the surface areas among these saponite samples are much less than the differences of IPA activities and selectivities using these saponite catalysts. Fig. 6 shows N adsorption–desorption iso2 therms of the [Mg Ni ]-saponite, which has similar 0 6 isotherms to those of other [Mg Ni ]-saponites. x y The adsorption isotherm is of type IV based on the IUPAC classification, indicating the presence of mesoporosity in these saponites. The hysteresis
Fig. 6. Nitrogen adsorption–desorption isotherms of [Mg Ni ]-saponites (upper curve represents desorption and x y lower curve represents adsorption).
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4. Discussion 4.1. Locations of Mg and/or Ni in the framework of saponite
Fig. 7. Comparison of pore size distributions of [Mg Ni ]- and 0 6 [Mg Ni ]-saponites. 6 0
loop is of type h4, indicating the presence of narrow slit-like micropores [18]. Fig. 7 shows pore size distributions of the [Mg Ni ]-and [Mg Ni ]0 6 6 0 saponites; all other saponites also had similar pore size distributions. It is well known that the pore structure of a catalytic material plays an important role in the catalytic performance of the catalyst. Since all [Mg Ni ]-saponites have similar pore structures, x y as they all have similar N adsorption–desorption 2 isotherms and pore size distributions, the pore structures of the saponite catalysts could not cause the marked difference in the catalytic activities for IPA conversion between the [Mg Ni ]-saponite 0 6 and the other Mg-containing [Mg Ni ]1–6 5–0 saponites. Similarly, the differences in the surface areas of the [Mg Ni ]-saponite and the other 0 6 Mg-containing [Mg Ni ]-saponites (Table 2) 1–6 5–0 are much smaller than the differences in their catalytic activities for IPA conversion. As all [Mg Ni ]-saponites have similar pore structures x y and surface areas, the increase of the catalytic activity of the [Mg Ni ]-saponite compared with 0 6 those of the other Mg-containing [Mg Ni ]1–6 5–0 saponites is therefore suggested not to be due to these physical properties but inherently due to the chemical compositions of the catalysts.
A smectite clay generally consists of a 2:1 type structure or T–O–T (tetrahedral–octahedral–tetrahedral ) sheets [19]. One octahedral sheet is sandwiched between two tetrahedral sheets. A natural saponite usually has Si and Al in the tetrahedral positions, Mg in the octahedral position, and no Ni in the framework [1]. Based on this observation, the [Mg Ni ]-saponite does not contain any Ni 6 0 and has a chemical composition similar to that of a natural saponite. In fact, the XRD pattern of the [Mg Ni ]-saponite has been shown to be quite 6 0 similar to that of the natural saponite, implying that all the Mg ions of this synthetic [Mg Ni ]6 0 saponite are in the octahedral positions. For other [Mg Ni ]-saponites, there are several reasons why x y both Mg and Ni ions are suggested to be in the octahedral position [20], as discussed in the following subsections. 4.1.1. Coordination number A coordination number can be used to predict the coordination geometry of a crystalline material. For the case of an oxygen anion as the surrounding anion, the coordination number (CN ) depends on the radii of the cations (R ) and oxygen anions c (R ), where R /R =0.225–0.414 gives CN=4 and a c a R /R =0.414–0.732 gives CN=6 [21]. Taking R c a ˚ , Si (having R /R =0.28) hasa for oxygen as 1.40 A c a CN=4; Al (having R /R =0.41) has CN=4 or 6; c a Mg (having R /R =0.56) has CN=6; and Ni c a (having R /R =0.66) has CN=6. Based on c a R /R of Mg or Ni ions, Mg and Ni ions should c a go into the octahedral positions of all [Mg Ni ]x y saponites. This is the first reason why both Mg and Ni ions are suggested to be in the octahedral positions of all [Mg Ni ]-saponites. x y 4.1.2. Total amount of Mg and Ni in the starting gel The meaning of a dioctahedral or trioctahedral smectite can be used to determine the positions of Mg and Ni ions in [Mg Ni ]-saponites. For smecx y tite clay minerals, one octahedral sheet is sand-
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wiched between two tetrahedral sheets. For a unit cell, there are six available octahedral positions in an octahedral sheet. If the majority of the metal ions in an octahedral sheet is trivalent (such as Al3+ or Fe3+), then there are four octahedrons in a unit cell and 2/3 of the available octahedral positions are occupied; this type of a smectite is called a dioctahedral smectite (an example is a montmorillonite). If the majority of the metal ions in an octahedral sheet is divalent (such as Mg2+ or Zn2+), then there are six octahedrons in a unit cell and 3/3 of the available octahedral positions are occupied; this type of a smectite is called a trioctahedral smectite (an example is a saponite). The relationship between the groups of smectite (dioctahedral or trioctahedral ) and the amount of Mg ions introduced in the starting solution has been studied [22]. A series of smectite-type clay minerals were synthesised by systematically changing the amount of magnesium in the starting synthesis gel (with a molar ratio SiO :Al O :MgO of 8:2:1–6). Based on the results 2 2 3 of XRD, DTA and TGA, it was found that the groups and structures of the smectite samples changed gradually from dioctahedral-group to trioctahedral-group clay minerals when the amount of an octahedral element in the starting gel was increased. It was noticed that, with a high amount of Mg (which is an octahedral element) in the starting gel, it was difficult for Al to be in the octahedral position. In this study, as XRD results have shown that all the [Mg Ni ]-saponites belong to a trioctahex y dral-group smectite, it can be deduced that almost all the octahedral positions are occupied. Since only Mg and Ni ions are available for octahedral positions and the total amount of Mg and Ni ions in the starting gel is exactly 6 for a unit cell, almost all of the Mg and Ni ions should be in octahedral positions. Otherwise, if some Ni ions are not in the octahedral positions, some amount of dioctahedral saponites would be produced, as it has been mentioned previously that the trioctahedral saponites could change to dioctahedral saponites when the amount of octahedral elements is decreased. However, this is not the case for this study, as explained in Section 4.1.3. It is also noticed that when the Mg content in
the starting gel of a smectite was high (>25%), a trioctahedral smectite was produced [22]. For [Mg Ni ]-saponites, the Mg content is 34% for an x y [Mg Ni ]-saponite and the total Mg and Ni 6 0 contents are over 34% for all other [Mg Ni ]-saponites. Considering the fact that 1–5 5–1 Mg and Ni ions are octahedral elements available for [Mg Ni ]-saponites, Ni ions should be in the x y octahedral positions. This is the second reason why Mg and Ni ions are suggested to be in the octahedral positions. 4.1.3. XRD patterns For smectite clay minerals, it was discovered that one of the most important and characteristic reflections in the XRD patterns of smectite clay minerals is the (060) reflection at a d spacing ˚ . The (060) reflection is a (d ) of about 1.500 A 060 criterion to discriminate whether the sample is a dioctahedral-group or trioctahedral-group smectite [16,22,23]. It has been reported that, for synthetic smec˚ and at <1.500 A ˚ tites, d values at >1.500 A 060 were indicative of trioctahedral-group and dioctahedral-group smectites, respectively [23]. Since ˚ the d of [Mg Ni ]-saponite is at 1.514 A 060 1 5 ( Table 3), the result suggests that the [Mg Ni ]-saponite belongs to a trioctahedral1 5 group smectite. Furthermore, based on Fig. 1, it can be seen clearly that each [Mg Ni ]-saponite x y ˚ , indicating has d almost at 1.514 A that each 060 [Mg Ni ]-saponite is a trioctahedral-group smecx y tite. Therefore, the XRD result suggests that all Ni ions in [Mg Ni ]-saponites are in the same x y position as Mg ions. If only part of the Ni ions are incorporated in the octahedral positions, then [Mg Ni ]-saponites cannot have the same XRD x y patterns and d values. This is the third reason 060 why Mg and Ni ions are suggested to be in the octahedral positions. 4.2. Interaction of Mg ions with Ni ions in octahedral positions Based on the consideration of coordination number, the total amount of Mg and Ni ions in the starting gel, and the XRD patterns, both Mg and Ni ions have been shown to be in the octahe-
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dral positions of [Mg Ni ]-saponites. However, an x y issue that needs to be clarified in this study is whether Ni indeed substitutes Mg in the octahedral position of each [Mg Ni ]-saponite or each 1–5 5–1 [Mg Ni ]-saponite is a mixture of the 1–5 5–1 [Mg Ni ]- and [Mg Ni ]-saponites. To clarify this 6 0 0 6 issue, the results of the IPA probe reaction and pyridine-adsorption FTIR are used and discussed in the following subsections. 4.2.1. The effect of Mg ions on the catalytic activity and selectivity of saponites Assuming that each [Mg Ni ]-saponite 1–5 5–1 might contain a mixture of [Mg Ni ]- and 6 0 [Mg Ni ]-saponites, then the IPA conversion 0 6 should increase proportionally with an increase in the amount of Ni ions in [Mg Ni ]-saponite 1–5 5–1 catalysts. If the IPA catalytic activities or selectivities do not show this behavior, it is suggested that there should be some interactions between Mg and Ni ions in the octahedral positions; each [Mg Ni ]-saponite then should not be a mixture 1–5 5–1 of [Mg Ni ]- and [Mg Ni ]-saponites, but it should 6 0 0 6 be a pure saponite. The effect of Ni ion contents in saponite catalysts on the conversion of IPA shows that, among synthesized saponites, the [Mg Ni ]-saponite was 0 6 the most active in the dehydration of IPA, showing that it has more acidic sites than other saponites. It is shown in this study that once one sixth of Ni ions are replaced by Mg ions in the starting gel to form the [Mg Ni ]-saponite, the catalytic perfor1 5 mance of the resulting [Mg Ni ]-saponite drops 1 5 substantially. A further increase of the amount of Mg ions in the starting gel does not substantially decrease the catalytic activities of the resulting Mg-containing [Mg Ni ]-saponites. The cata1–5 5–1 lytic activities of these Mg-containing saponite catalysts do not change linearly with the contents of Ni ions from 0 to 6, but they show quite similar IPA conversion. The reaction results show that there are some interactions between Mg and Ni ions in the octahedral positions of the [Mg Ni ]-saponites, and the Mg ions inter1–5 5–1 acting with Ni ions (possibly by oxygen bridging) have adverse effects on the catalytic performance of the resulting [Mg Ni ]-saponite catalysts for 1–5 5–1 IPA reaction.
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This observation of the interaction between Mg and Ni ions in the octahedral positions is further substantiated by the selectivity study. All Mg-containing [Mg Ni ]-saponite catalysts 1–6 5–0 produce a much smaller amount of propene than the non-Mg-containing [Mg Ni ]-saponite, and 0 6 the selectivities do not decrease linearly with the amount of Mg ions in the Mg-containing [Mg Ni ]-saponites. Similarly, only the 1–6 5–0 [Mg Ni ]-saponite has very low selectivity of IPA 0 6 to acetone and this selectivity does not change linearly with the amount of Ni ions in the catalysts. Therefore, the results of product selectivities imply that each [Mg Ni ]-saponite is not a mixture of x y [Mg Ni ] and [Mg Ni ]-saponites, but a pure sapo6 0 0 6 nite having both Mg and Ni ions in the octahedral positions of the clay framework. In addition, the Mg and Ni ions in the octahedral positions are shown to be interacting with each other. This is because if there is no interaction between these Mg and Ni ions in the octahedral positions, then the catalytic selectivities of [Mg Ni ]-saponite 1–6 5–0 catalysts should be dependent on the amount of the active Ni ions in the framework. Because of these interactions of neighboring Ni and Mg ions, there are two different types of active sites on these saponite catalysts for IPA reaction: one associated with pure Ni ions (i.e. Ni sites) and the other one associated with pure Mg ions or with Mg ions interacting with Ni ions (i.e. Mg sites or Mg–Ni sites, respectively). The IPA reaction results show that the Mg sites have similar activities to the Mg–Ni sites, and the Ni sites are more active than the Mg sites or the Mg–Ni sites. It is worth noting here that the reaction results show the absence of non-crystallized phases such as MgO, NiO, and SiO –Al O in all the synthe2 2 3 sized saponites. This is because, if MgO or NiO did exist in these saponites and their contribution to the catalytic activities for IPA conversion were significant, then the catalytic activity of the saponites would increase or decrease proportionally to the increasing or decreasing amount of Ni of Mg in the saponites. However, this trend could not be observed from the reaction results, which show that the IPA conversions on the Mg-containing saponites were quite similar to each other, and the active sites on [Mg Ni ]-saponite were markedly 0 6
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S. Kawi, Y.Z. Yao / Microporous and Mesoporous Materials 33 (1999) 49–59
different from those on other saponites. If SiO –Al O sites really did exist in these saponites, 2 2 3 they would be expected to have the same catalytic effect to all saponites. These results show that the possible existence of MgO, NiO or SiO –Al O in 2 2 3 the synthesized saponites was negligible. The difference in catalytic activities of the [Mg Ni ]0 6 saponite from other Mg-containing saponites is thus shown to originate mainly from the effect of Mg ions in the octahedral positions of saponites. 4.2.2. The effect of Mg ions on the acidity of saponites The IPA reaction results have shown that the [Mg Ni ]-saponite catalyst is the most active 0 6 catalyst among [Mg Ni ]-saponites and all x y Mg-containing [Mg Ni ]-saponite catalysts 1–5 5–1 have quite similar activities. These results show that the presence of Mg ions in the catalyst is adverse to the conversion of IPA, and Mg and Ni ions are shown to interact with each other. The pyridine-adsorption FTIR results further clarify this Mg–Ni interaction by showing that the [Mg Ni ]-saponite has a strong vibration band at 0 6 1540 cm−1 (characteristic of the Brønsted acid sites) but all other Mg-containing [Mg Ni ]-sapox y nites show a substantial decrease of this band. The pyridine-adsorption FTIR result shows that the Brønsted acid sites have been substantially diminished on these Mg-containing saponites, showing the interactions between Mg and Ni ions in the octahedral positions of the [Mg Ni ]-saponites; 1–5 5–1 otherwise, the Brønsted acidity should increase gradually with the increase of the Ni content in the catalysts. The pyridine-adsorption FTIR result also shows that the interacting Mg ions diminish the Brønsted acidity of the Ni ions. In other words, the interacting Mg ions may have deactivated the active Ni ions. It has been generally accepted that the Brønsted acid sites in saponites mainly come from the SiMOMAl linkage. It should be noted here that the synthesized saponites in this study should also contain these SiMOMAl linkages. The reason is as follows. The XRD patterns of these saponites have similar XRD patterns as those saponites reported in the literature. One of these saponites, i.e. [Mg Ni ]-saponite, was synthesized based on the 6 0 saponite composition reported in the literature.
Since normal saponite has been characterized thoroughly and shown in the literature to have SiMOMAl linkages, it is believed that the synthesized saponites in this study should have these SiMOMAl linkages. One of the interesting phenomena observed from the results of this study is that, although all these synthesized saponites should basically contain similar SiMOMAl linkages, the [Mg Ni ]0 6 saponite shows many more Brønsted acid sites than other Mg-containing saponites. The result indicates that the interaction between the octahedral layer and tetrahedral layer may also contribute to the generation of Brønsted acid sites, which are postulated to be generated possibly through a linkage such as SiMOMAlMOMNi. As a very small amount of Mg ‘would kill’ most of these Brønsted acid sites in Mg-containing saponites, it is postulated that ‘this killer’ may possibly be due to the formation of linkages formed by Mg between the octahedral layer and tetrahedral layer, such as SiMOMAlMOMMg or SiMOMAlMOMNiMOMMg linkages. Further characterization needs to be carried out to show the formation of these linkages.
5. Conclusions A series of synthetic [Mg Ni ]-saponite catax y lysts, having two Mg and Ni ions incorporated in the octahedral positions of the clay framework, were synthesized. Based on the results of XRD, IPA reaction, and pore structure, both Mg and Ni ions incorporated in the octahedral positions are shown to have some chemical interactions in the octahedral sheet, which affects the activity, selectivity, and acidity of the catalysts. The results of this study may offer opportunities to synthesize other interesting clay catalysts having multimetals in either the octahedral or the tetrahedral positions of the clay framework.
Acknowledgement This research work was generously supported from a research grant by the National University of Singapore.
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References [1] R.E. Grim, 2nd ed., Clay Mineralogy, McGraw-Hill, New York, 1968. [2] M. Balogh, P. Laezlo, Organic Chemistry Using Clays, Springer, Berlin, 1993. [3] J.M. Thomas, W.J. Thomas, in: Principles and Practice of Heterogeneous Catalysis, VCH, Weinheim, 1997, p. 624. [4] J.J. Fripiat, in: G. Ertl, H. Knozinger, J. Weitkamp ( Eds.), Handbook of Heterogeneous Catalysis, VCH, Weinheim, 1997, p. 387. [5] Y. Izumi, K. Urabe, M. Onaka, in: Zeolite, Clay and Heteropoly Acid in Organic Reactions, Kodansha, Tokyo, 1992, p. 49. [6 ] J.T. Kloprogge, Ph.D. Thesis, Utrecht University, The Netherlands, 1992. [7] K. Urabe, M. Koga, Y. Izumi, J. Chem. Soc., Chem. Commun. (1989) 807. [8] T. Sun, D.Z. Jiang, Z.Y. Liu, E.Z. Min, H.G. Fu, Chem. J. Chin. Univ. 14 (1994) 1039. [9] Y. Nishiyama, M. Arai, S.-L. Guo, N. Sonehara, N.T. Naito, Appl. Catal. A: Gen. 95 (1993) 171. [10] R.J.M.J. Vogels, M.J.H.V. Kerkhoffs, J.W. Geus, Stud. Surf. Sci. Catal. 91 (1995) 1153.
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[11] S. Kawi, Mater. Lett. 38 (1999) 351. [12] Y.-Z. Yao, S. Kawi, J. Porous Mater. 6 (1999) 77. [13] S. Kawi, Y.-Z. Yao, Microporous Mesoporous Mater. 28 (1999) 25. [14] B. Leliveld, W.C.A. Huyben, A.J. van Dillen, Stud. Surf. Sci. Catal. 106 (1997) 137. [15] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [16 ] V. Luca, D.J. Maclachlan, R.F. Howe, R. Bramley, J. Mater. Chem. 5 (1995) 557. [17] T.J. Pinnavaia, J.R. Butrulle, in: I.E. Wachs, L.E. Fitzpatrick ( Eds.), Characterization of Catalytic Materials, Butterworth-Heinemann, Boston, MA, 1992, p. 149. [18] K.S.W. Sing, D.H. Everett, J. Rouquerol, T. Siemienewska, R.A. Pierotti, L. Moscou, R.A.W. Haul, Pure Appl. Chem. 57 (1985) 603. [19] T.J. Pinnavaia, Science 220 (1983) 365. [20] M. Arai, S.-L. Guo, M. Shirai, Y. Nishiyama, K. Torii, J. Catal. 161 (1996) 704. [21] W.E. Worral, in: Clays: Their Nature, Origin and General Properties, Maclaren and Sons, London, 1968, p. 9. [22] J. Masar, V.S. Fajnor, Silikaty 27 (1983) 45. [23] M.J. Wilson, in: M.J. Wilson ( Ed.), A Handbook of Determinative Methods in Clay Mineralogy, Chapman and Hall, New York, 1996, p. 26.
Saponite catalysts with systematically varied Mg/Ni ratio: synthesis, characterization, and catalysis S. Kawi *, Y.Z. Yao Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 8 December 1998; accepted for publication 10 May 1999
Abstract A series of saponite catalysts having systematically varied Mg and Ni ratios in the clay framework (i.e. [Mg Ni ]-saponites) were synthesized using the hydrothermal synthesis method. The synthesized saponite catalysts 6–0 0–6 have similar XRD patterns, indicating that all of the Mg and/or Ni elements have been incorporated as Mg and Ni ions in the octahedral positions of the clay framework, respectively. The results of the isopropanol dehydration reaction show that the saponite containing only Ni (i.e. [Mg Ni ]-saponite) was very active, while the other 0 6 Mg-containing saponites (i.e. [Mg Ni ]-saponite) were inactive. The results of the pyridine-adsorption FTIR 1–6 5–0 further show that the [Mg Ni ]-saponite has Brønsted and Lewis acidic sites, while the [Mg Ni ]-saponites have 0 6 1–6 5–0 only Lewis acidic sites. The possible interaction between Mg and Ni ions in the octahedral positions of saponite catalysts and its influence on the catalytic activity of the catalysts is discussed in this paper. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Bimetallic cations; Catalysts; Clay; Saponite
1. Introduction Clays have been used as catalysts for some time [1–4]. Commonly used clays are the layered clays called smectite (such as montmorillonite and saponite). Although natural clays found in natural deposits have been used as catalysts, synthetic clays still have drawn little attention in clay science. However, it is believed that new types of synthetic clays could be discovered and developed to have catalytic applications [5]. Synthetic clays are of interest because they have unusual advantages over natural clays, as follows. * Corresponding author. Tel.: +65-874-6312; fax: +65-779-1936. E-mail address: [email protected] (S. Kawi)
(1) The catalytic performance of synthetic clays is more versatile than natural clays and could be adjusted systematically to accelerate certain catalytic reactions. This is because, during the synthesis of synthetic clays, catalytically active elements (with no impurities) can be introduced and uniformly distributed in the crystalline framework of clays. (2) By systematically controlling certain synthesis parameters, a series of synthetic clays can be synthesized and developed as catalyst models for understanding the relationship between the catalytic performance and structures or compositions of the clays. (3) Synthetic clays can be produced on a large scale with uniform chemical composition and catalytic properties, but the catalytic properties of natural clays vary with the locations and batches of clay deposits.
1387-1811/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 12 2 - 5
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There have been some recent reports on synthetic clays, especially on synthetic saponites [6 ]. For examples, a Ni-substituted saponite containing only Ni2+ ions (substituting all Mg2+) in all of the octahedral positions of the clay framework had been synthesized [7] and found to be an efficient catalyst for the selective dimerization of ethene owing to its uniform distribution of active Ni ions throughout the clay. An Fe–Ni saponite was synthesized using a similar hydrothermal method [8]; based on the XRD, DTA, IR, TPR, Mo¨ssbauer spectra, and chemical analysis, Fe ions had been shown to be introduced in the tetrahedral position of the clay framework. Some hectoritelike smectites containing Ni have also been synthesized [9], but the substitution and the position of Mg, Ni or Li ions in the clay framework have not been well characterized. Mg and Zn ions have been incorporated in the octahedral layer of saponites and the resulting synthetic saponite catalysts have been shown to be active for Friedel–Crafts alkylation of benzene with propylene to produce cumene [10]. Although bimetallic ions coexisting in the clay framework have been synthesized, the effects of bimetallic ions closely interacting in the clay framework on the catalytic properties of the clays have not yet been reported in the literature. In this paper, a new series of saponites having systematically varied Mg to Ni ratios, with both Mg and Ni in the octahedral positions of the clay framework, were synthesized. The present studies were undertaken to obtain a fundamental understanding about the introduction of nickel in the clay framework, the possibility of introducing bimetallic ions in close proximity to each other in the clay framework, and the effect of bimetallic ions in the octahedral positions on the catalytic properties of the clays. As clay minerals can have both tetrahedral and octahedral positions, the results of this study show that the incorporation of two or even more elements simultaneously in the octahedral or tetrahedral positions can provide ample opportunities to synthesize clay minerals which can then be used as precursors using hydrothermal surfactant treatment to form new high-surface-area clays [11– 13] or clay catalysts having interesting catalytic performance for certain reactions [14].
2. Experimental 2.1. Catalyst preparation A series of Na+-saponites having systematically varied Mg to Ni ratios were prepared using a hydrothermal synthesis method. The amounts of chemicals used in the synthesis of these saponite catalysts are shown in Table 1. By maintaining the total amount of Mg and Ni at the same level, saponites having the formula [Mg · Ni ] x 6−x (Si · Al )O (OH ) ] were synthesised as follows. 7 1 20 4 A buffer solution was prepared by dissolving 3.6 g of NaOH and 6.56 g of NaHCO in 50 ml of 3 deionized water. Solution A was prepared by adding a sodium silicate solution (Merck, Na O: 2 7.5–8.5 wt.% and SiO : 25.5–28.5 wt.%) to the 2 buffer solution and the mixture was completely stirred; solution A had a high pH of about 13. Solution B was prepared by dissolving stoichiometric amounts of AlCl · 6H O (Merck, 99%), 3 2 NiCl · 6H O (Merck, 98%), and MgCl · 6H O 2 2 2 2 (Merck, 99%) in 5.0 ml of deionized water; solution B had a low pH of about 3. Since it was important that the gel for the synthesis of saponites was to be formed at higher pH range, solution B was slowly added to solution A under vigorous stirring until a uniform gel was eventually obtained. The gel was sealed into a 100 ml stainless steel autoclave with Teflon liner and was hydrothermally treated in a furnace at 285°C for 48 h. In order to get rid of the excess electrolytes in the resultant product, the product was repeatedly washed with deionized water and centrifuged (the centrifuge was carried out at a speed of 8000 rpm for 8 min); the washing process was repeated for six times until the product was chloride-free (this was tested using AgNO solution). The resultant 3 product was then dried in an oven at 100°C for 48 h. H+-saponites were prepared by exchanging Na+-saponite samples as follows: 100 ml of 0.5 M of NH NO solution was added to 5 g of 4 3 Na+-saponite sample. The mixture was stirred continuously at room temperature for 1 h before it was centrifuged. The exchange process was repeated for four times. The product was dried in an oven at 100°C for 48 h. The cation-exchanged
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S. Kawi, Y.Z. Yao / Microporous and Mesoporous Materials 33 (1999) 49–59 Table 1 Amounts of chemicals used in the synthesis of [Mg Ni ]-saponites x y Chemical
Na silicatea NaOH NaHCO 3 AlCl · 6H O 3 2 MgCl · 6H O 2 2 NiCl · 6H O 2 2
Amount of chemicals (g) used in an autoclave to synthesize [Mg Ni ]-saponites x y Mg Ni 6 0
Mg Ni 5 1
Mg Ni 4 2
Mg Ni 3 3
Mg Ni 2 4
Mg Ni 1 5
Mg Ni 0 6
7.79 3.60 6.56 1.22 6.16 0
7.79 3.60 6.56 1.22 5.13 1.21
7.79 3.60 6.56 1.22 4.10 2.42
7.79 3.60 6.56 1.22 3.08 3.64
7.79 3.60 6.56 1.22 2.05 4.85
7.79 3.60 6.56 1.22 1.03 6.06
7.79 3.60 6.56 1.22 0 7.27
a Na silicate=sodium silicate solution (27 wt.% SiO ). 2
product was then calcined at 550°C for 24 h. The sample was then ground to a fine powder form. 2.2. Characterization N adsorption–desorption isotherms were 2 obtained at 77 K using a Quantachrome NOVA1000 analyzer. BET surface areas were calculated based on the six adsorption data points in the relative pressure range ( p/p0) of 0.10–0.30. The pore size distribution was derived using a calculation based on the BJH method [15]. X-ray diffraction patterns were obtained on a Philips PW 1710 diffractometer equipped with a Philips PW 1729 X-ray generator, using Cu Ka radiation as the X-ray source and a scanning (2h) range 4–64°. A pyridine-adsorption FTIR study was carried out as follows. A sample (ca. 15 mg) was ground to fine powders and pelletized into a self-supported wafer. The wafer was placed in a quartz cell equipped with two calcium fluoride windows. The wafer was treated under vacuum at 400°C for 2 h. Pyridine adsorption was carried out by exposing the wafer to a He flow saturated with pyridine vapour at 298 K for 20 min. The pyridine desorption was carried out by heating the sample under vacuum at a series of treatment temperatures. The spectra were collected after the sample was treated under vacuum at a certain treatment temperature and cooled to room temperature. The FTIR spectra were recorded on a Shimadzu 8101M FTIR spectrometer. To get some information of the framework structures of the synthesized clays, IR
spectra were measured on the synthesized clay samples which had been mixed with KBr. Isopropanol alcohol (IPA) dehydration and dehydrogenation test reaction was carried out in a microreactor. Typically, 0.20 g of catalyst was loaded into a glass reactor, and pre-treated in a flow of helium at 400°C for 90 min. The reactor was then cooled to the reaction temperatures (160– 250°C ). A He flow (20 ml min−1) saturated with IPA at 12 Torr (which was generated by passing helium through an IPA saturator at 5°C ) was introduced into the reactor. The reaction products were analyzed using an HP 6890 GC on-line system equipped with a packed column (Supelco 10% Carbowax 1540 on 30/60 mesh Chromosorb T ).
3. Results 3.1. Crystalline structure Table 2 lists all the synthetic Mg–Ni saponite catalysts synthesized in this study. A [Mg Ni ] x y saponite (where x or y=0–6 and x+y=6) represents the saponite catalyst in which x and y represent the moles of Mg and Ni in one clay unit cell, respectively (Ni and Mg moles were calculated according to the composition in the starting gel ). Table 3 shows a comparison of the XRD indexes of the reference saponite, whose values are reported in literature, with those of the [Mg Ni ]-saponite, which is one of the saponites 1 5 synthesized in this study. The significant XRD reflections for the [Mg Ni ]-saponite include those 1 5
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Table 2 Unit cell compositions of starting gel and surface areas of [Mg Ni ]-saponites x y Samples
Unit cell composition of starting gel
Surface area (m2 g−1)
[Mg Ni ]-saponite 6 0 [Mg Ni ]-saponite 5 1 [Mg Ni ]-saponite 4 2 [Mg Ni ]-saponite 3 3 [Mg Ni ]-saponite 2 4 [Mg Ni ]-saponite 1 5 [Mg Ni ]-saponite 0 6
Na [Mg · Ni ](Sr · Al )O (OH ) 1 6 0 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 5 1 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 4 2 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 3 3 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 2 4 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 1 5 7 1 20 4 Na [Mg · Ni ](Sr · Al )O (OH ) 1 0 6 7 1 20 4
112 144 139 152 124 88 82
Table 3 Comparison of XRD indexes of [Mg Ni ]-saponite with those of reference saponites reported in the literature 1 5 Synthetic [Mg Ni ]-saponite 1 5
Reference synthetic saponite [7]
Reference synthetic trioctahedral smectite [16 ]
˚) d (A
Int
hkl
˚) d (A
Int
hkl
˚) d (A
Int
hkl
13.60 4.51 3.06 2.54 1.71 1.514
100 9.4 15.6 11.5 4.2 15.6
001 020, 004 130, 150, 060,
11.935 4.539 3.186 2.552 1.714 1.526
100 25.7 24.0 34.9 3.4 35.1
001 020, 004 130, 150, 060,
12.6 4.56 3.16 2.58 1.72 1.527
100 13.7 19.0 13.7 2.9 21.2
001 002, 004 130, 150, 060,
110 200 240, 310 330
110 200 240, 310 330
110 200 240, 310 330
at d spacings of 13.60, 4.51, 3.06, 2.54, 1.41 and ˚ ; these reflections are in good agreement 1.514 A with those of the synthetic saponite [7] and the synthetic trioctahedral smectite [16 ]. The comparison shows that the synthesized [Mg Ni ]-saponite 1 5 has the same crystalline structure as the reference saponite. Fig. 1 shows the XRD patterns of all [Mg Ni ]-saponites, which are quite similar to that x y of the [Mg Ni ]-saponite. These results show that 1 5 all [Mg Ni ]-saponites have the same crystalline x y structures as the [Mg Ni ]-saponite (i.e. the sapo6 0 nite containing only Mg ions), demonstrating that Ni ions might have been incorporated into the same sites as the Mg ions and might have possibly been interacting with the Mg ions. 3.2. Catalytic IPA dehydration and dehydrogenation It is well known that IPA can be dehydrated on acidic sites to produce propene and be dehydrogenated on basic sites to produce acetone. In order to examine the possible coexistence and interaction
Fig. 1. XRD patterns of [Mg Ni ]-saponites. x y
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Fig. 2. IPA conversion versus reaction temperatures of [Mg Ni ]-saponite catalysts. x y
between the Mg and Ni ions incorporated in the framework of [Mg Ni ]-saponite catalysts, IPA x y conversion reactions were used to characterize the nature of the acidic and basic sites of these [Mg Ni ]-saponite catalysts [17]. x y Fig. 2 shows the results of the catalytic IPA conversion on [Mg Ni ]-saponite catalysts. The x y [Mg Ni ] saponite catalyst (i.e. the saponite cata0 6 lyst containing only Ni ions) exhibits extremely high catalytic activity for IPA conversion compared with all other [Mg Ni ] saponite cata1–6 5–0 lysts (i.e. saponite catalysts containing both Mg and Ni ions). The effect of reaction temperatures (from 160 to 250°C ) on the catalytic conversion of IPA on [Mg Ni ]-saponite catalysts was also x y examined. At each reaction temperature, the conversion did not increase linearly with an increase in Ni contents of [Mg Ni ]-saponite catalysts. The x y results show that each [Mg Ni ]-saponite catalyst x y is not a physical mixture of [Mg Ni ]- and 6 0 [Mg Ni ]-saponite catalysts, but each [Mg Ni ]0 6 x y saponite catalyst has both Mg and Ni ions interacting with each other in the octahedral positions of the clay framework. Figs. 3 and 4 show significant differences in the product selectivities resulting from the conversion of IPA on the [Mg Ni ]-saponite catalyst and on 0 6 the other [Mg Ni ]-saponite catalysts. The 1–6 5–0 [Mg Ni ]-saponite had almost 100% selectivity 0 6 towards propene, which was formed from the dehydration of IPA on the acidic sites. All of the other [Mg Ni ]-saponite catalysts exhibited 1–6 5–0
53
Fig. 3. Effect of reaction temperature on the IPA selectivity to propene using [Mg Ni ]-saponite catalysts. x y
Fig. 4. Effect of reaction temperature on the IPA selectivity to acetone using [Mg Ni ]-saponite catalysts. x y
similar selectivity patterns to each other (i.e. 40– 60% propene, 60–40% acetone). The selectivity results indicate that the [Mg Ni ]-saponite catalyst 0 6 contained mainly acidic sites while the other [Mg Ni ]-saponite catalysts contained both 1–6 5–0 acidic and basic sites. The selectivities to propene and to acetone observed on these catalysts are also shown not to increase with an increase in Ni contents of the [Mg Ni ]-saponite catalysts. 1–6 5–0 The relatively large difference between the selectivities observed at low conversions ( less than 2–3%) is attributed to the larger errors in the determination of the selectivities at low conversions. It was observed that, at a certain reaction temperature (such as at 200°C ), the selectivity of the [Mg Ni ]-saponite catalysts to convert IPA to x y
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propene and to acetone could be adjusted from 30% to almost 100% and from 70% to almost 0%, respectively, by simply changing the composition of the Mg or Ni ions in the framework of the saponite catalysts. This result indicates that the interactions between Mg and Ni ions in the framework of the saponite catalysts play a very important role in regulating the acidity of the saponite catalysts. 3.3. Pyridine-adsorption FTIR To explore further the interesting catalytic behaviors of [Mg Ni ]-saponites, in situ FTIR studies x y for the adsorption of pyridine on [Mg Ni ]-sapox y nites were carried out to characterize the properties of the acidic sites of saponite catalysts. Fig. 5 shows FTIR spectra characterizing the pyridine adsorbed on [Mg Ni ]-saponite catalysts at 200°C. x y All [Mg Ni ]-saponites show similar IR bands x y characterizing the adsorbed pyridine at 1445, 1490 and 1590 cm−1, which are attributed to pyridine adsorbed on Lewis acid sites. As the Lewis acidities of all [Mg Ni ]-saponite samples are quite similar, x y they do not provide any clues to the different catalytic behaviors between the [Mg Ni ]-saponite 0 6 and the other [Mg Ni ]-saponites. However, if 1–6 5–0 Brønsted acid sites are compared, there are substantial differences between the FTIR spectra of the [Mg Ni ]-saponite and the other 0 6
Fig. 5. FTIR spectra of pyridine adsorption on [Mg Ni ]-saponite catalysts (after evacuation at 200°C for 1 h). x y
[Mg Ni ]-saponites. Based on the IR bands at 1–6 5–0 1540 and 1490 cm−1, which are characteristic of Brønsted acid sites, the [Mg Ni ]-saponite had 0 6 more amount of Brønsted acid sites than other [Mg Ni ]-saponites. The results imply that not 1–6 5–0 only were there interactions between Mg ions and Ni ions in the tetrahedral position, but the Mg ions might have also deactivated the Ni ions. 3.4. Surface area, N isotherm, and pore size 2 distribution All [Mg Ni ]-saponites were characterized by x y N adsorption–desorption measurements. The 2 resulting surface areas of [Mg Ni ]-saponites are x y listed in Table 2. Contrary to the IPA reaction results, the differences of the surface areas among these saponite samples are much less than the differences of IPA activities and selectivities using these saponite catalysts. Fig. 6 shows N adsorption–desorption iso2 therms of the [Mg Ni ]-saponite, which has similar 0 6 isotherms to those of other [Mg Ni ]-saponites. x y The adsorption isotherm is of type IV based on the IUPAC classification, indicating the presence of mesoporosity in these saponites. The hysteresis
Fig. 6. Nitrogen adsorption–desorption isotherms of [Mg Ni ]-saponites (upper curve represents desorption and x y lower curve represents adsorption).
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4. Discussion 4.1. Locations of Mg and/or Ni in the framework of saponite
Fig. 7. Comparison of pore size distributions of [Mg Ni ]- and 0 6 [Mg Ni ]-saponites. 6 0
loop is of type h4, indicating the presence of narrow slit-like micropores [18]. Fig. 7 shows pore size distributions of the [Mg Ni ]-and [Mg Ni ]0 6 6 0 saponites; all other saponites also had similar pore size distributions. It is well known that the pore structure of a catalytic material plays an important role in the catalytic performance of the catalyst. Since all [Mg Ni ]-saponites have similar pore structures, x y as they all have similar N adsorption–desorption 2 isotherms and pore size distributions, the pore structures of the saponite catalysts could not cause the marked difference in the catalytic activities for IPA conversion between the [Mg Ni ]-saponite 0 6 and the other Mg-containing [Mg Ni ]1–6 5–0 saponites. Similarly, the differences in the surface areas of the [Mg Ni ]-saponite and the other 0 6 Mg-containing [Mg Ni ]-saponites (Table 2) 1–6 5–0 are much smaller than the differences in their catalytic activities for IPA conversion. As all [Mg Ni ]-saponites have similar pore structures x y and surface areas, the increase of the catalytic activity of the [Mg Ni ]-saponite compared with 0 6 those of the other Mg-containing [Mg Ni ]1–6 5–0 saponites is therefore suggested not to be due to these physical properties but inherently due to the chemical compositions of the catalysts.
A smectite clay generally consists of a 2:1 type structure or T–O–T (tetrahedral–octahedral–tetrahedral ) sheets [19]. One octahedral sheet is sandwiched between two tetrahedral sheets. A natural saponite usually has Si and Al in the tetrahedral positions, Mg in the octahedral position, and no Ni in the framework [1]. Based on this observation, the [Mg Ni ]-saponite does not contain any Ni 6 0 and has a chemical composition similar to that of a natural saponite. In fact, the XRD pattern of the [Mg Ni ]-saponite has been shown to be quite 6 0 similar to that of the natural saponite, implying that all the Mg ions of this synthetic [Mg Ni ]6 0 saponite are in the octahedral positions. For other [Mg Ni ]-saponites, there are several reasons why x y both Mg and Ni ions are suggested to be in the octahedral position [20], as discussed in the following subsections. 4.1.1. Coordination number A coordination number can be used to predict the coordination geometry of a crystalline material. For the case of an oxygen anion as the surrounding anion, the coordination number (CN ) depends on the radii of the cations (R ) and oxygen anions c (R ), where R /R =0.225–0.414 gives CN=4 and a c a R /R =0.414–0.732 gives CN=6 [21]. Taking R c a ˚ , Si (having R /R =0.28) hasa for oxygen as 1.40 A c a CN=4; Al (having R /R =0.41) has CN=4 or 6; c a Mg (having R /R =0.56) has CN=6; and Ni c a (having R /R =0.66) has CN=6. Based on c a R /R of Mg or Ni ions, Mg and Ni ions should c a go into the octahedral positions of all [Mg Ni ]x y saponites. This is the first reason why both Mg and Ni ions are suggested to be in the octahedral positions of all [Mg Ni ]-saponites. x y 4.1.2. Total amount of Mg and Ni in the starting gel The meaning of a dioctahedral or trioctahedral smectite can be used to determine the positions of Mg and Ni ions in [Mg Ni ]-saponites. For smecx y tite clay minerals, one octahedral sheet is sand-
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wiched between two tetrahedral sheets. For a unit cell, there are six available octahedral positions in an octahedral sheet. If the majority of the metal ions in an octahedral sheet is trivalent (such as Al3+ or Fe3+), then there are four octahedrons in a unit cell and 2/3 of the available octahedral positions are occupied; this type of a smectite is called a dioctahedral smectite (an example is a montmorillonite). If the majority of the metal ions in an octahedral sheet is divalent (such as Mg2+ or Zn2+), then there are six octahedrons in a unit cell and 3/3 of the available octahedral positions are occupied; this type of a smectite is called a trioctahedral smectite (an example is a saponite). The relationship between the groups of smectite (dioctahedral or trioctahedral ) and the amount of Mg ions introduced in the starting solution has been studied [22]. A series of smectite-type clay minerals were synthesised by systematically changing the amount of magnesium in the starting synthesis gel (with a molar ratio SiO :Al O :MgO of 8:2:1–6). Based on the results 2 2 3 of XRD, DTA and TGA, it was found that the groups and structures of the smectite samples changed gradually from dioctahedral-group to trioctahedral-group clay minerals when the amount of an octahedral element in the starting gel was increased. It was noticed that, with a high amount of Mg (which is an octahedral element) in the starting gel, it was difficult for Al to be in the octahedral position. In this study, as XRD results have shown that all the [Mg Ni ]-saponites belong to a trioctahex y dral-group smectite, it can be deduced that almost all the octahedral positions are occupied. Since only Mg and Ni ions are available for octahedral positions and the total amount of Mg and Ni ions in the starting gel is exactly 6 for a unit cell, almost all of the Mg and Ni ions should be in octahedral positions. Otherwise, if some Ni ions are not in the octahedral positions, some amount of dioctahedral saponites would be produced, as it has been mentioned previously that the trioctahedral saponites could change to dioctahedral saponites when the amount of octahedral elements is decreased. However, this is not the case for this study, as explained in Section 4.1.3. It is also noticed that when the Mg content in
the starting gel of a smectite was high (>25%), a trioctahedral smectite was produced [22]. For [Mg Ni ]-saponites, the Mg content is 34% for an x y [Mg Ni ]-saponite and the total Mg and Ni 6 0 contents are over 34% for all other [Mg Ni ]-saponites. Considering the fact that 1–5 5–1 Mg and Ni ions are octahedral elements available for [Mg Ni ]-saponites, Ni ions should be in the x y octahedral positions. This is the second reason why Mg and Ni ions are suggested to be in the octahedral positions. 4.1.3. XRD patterns For smectite clay minerals, it was discovered that one of the most important and characteristic reflections in the XRD patterns of smectite clay minerals is the (060) reflection at a d spacing ˚ . The (060) reflection is a (d ) of about 1.500 A 060 criterion to discriminate whether the sample is a dioctahedral-group or trioctahedral-group smectite [16,22,23]. It has been reported that, for synthetic smec˚ and at <1.500 A ˚ tites, d values at >1.500 A 060 were indicative of trioctahedral-group and dioctahedral-group smectites, respectively [23]. Since ˚ the d of [Mg Ni ]-saponite is at 1.514 A 060 1 5 ( Table 3), the result suggests that the [Mg Ni ]-saponite belongs to a trioctahedral1 5 group smectite. Furthermore, based on Fig. 1, it can be seen clearly that each [Mg Ni ]-saponite x y ˚ , indicating has d almost at 1.514 A that each 060 [Mg Ni ]-saponite is a trioctahedral-group smecx y tite. Therefore, the XRD result suggests that all Ni ions in [Mg Ni ]-saponites are in the same x y position as Mg ions. If only part of the Ni ions are incorporated in the octahedral positions, then [Mg Ni ]-saponites cannot have the same XRD x y patterns and d values. This is the third reason 060 why Mg and Ni ions are suggested to be in the octahedral positions. 4.2. Interaction of Mg ions with Ni ions in octahedral positions Based on the consideration of coordination number, the total amount of Mg and Ni ions in the starting gel, and the XRD patterns, both Mg and Ni ions have been shown to be in the octahe-
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dral positions of [Mg Ni ]-saponites. However, an x y issue that needs to be clarified in this study is whether Ni indeed substitutes Mg in the octahedral position of each [Mg Ni ]-saponite or each 1–5 5–1 [Mg Ni ]-saponite is a mixture of the 1–5 5–1 [Mg Ni ]- and [Mg Ni ]-saponites. To clarify this 6 0 0 6 issue, the results of the IPA probe reaction and pyridine-adsorption FTIR are used and discussed in the following subsections. 4.2.1. The effect of Mg ions on the catalytic activity and selectivity of saponites Assuming that each [Mg Ni ]-saponite 1–5 5–1 might contain a mixture of [Mg Ni ]- and 6 0 [Mg Ni ]-saponites, then the IPA conversion 0 6 should increase proportionally with an increase in the amount of Ni ions in [Mg Ni ]-saponite 1–5 5–1 catalysts. If the IPA catalytic activities or selectivities do not show this behavior, it is suggested that there should be some interactions between Mg and Ni ions in the octahedral positions; each [Mg Ni ]-saponite then should not be a mixture 1–5 5–1 of [Mg Ni ]- and [Mg Ni ]-saponites, but it should 6 0 0 6 be a pure saponite. The effect of Ni ion contents in saponite catalysts on the conversion of IPA shows that, among synthesized saponites, the [Mg Ni ]-saponite was 0 6 the most active in the dehydration of IPA, showing that it has more acidic sites than other saponites. It is shown in this study that once one sixth of Ni ions are replaced by Mg ions in the starting gel to form the [Mg Ni ]-saponite, the catalytic perfor1 5 mance of the resulting [Mg Ni ]-saponite drops 1 5 substantially. A further increase of the amount of Mg ions in the starting gel does not substantially decrease the catalytic activities of the resulting Mg-containing [Mg Ni ]-saponites. The cata1–5 5–1 lytic activities of these Mg-containing saponite catalysts do not change linearly with the contents of Ni ions from 0 to 6, but they show quite similar IPA conversion. The reaction results show that there are some interactions between Mg and Ni ions in the octahedral positions of the [Mg Ni ]-saponites, and the Mg ions inter1–5 5–1 acting with Ni ions (possibly by oxygen bridging) have adverse effects on the catalytic performance of the resulting [Mg Ni ]-saponite catalysts for 1–5 5–1 IPA reaction.
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This observation of the interaction between Mg and Ni ions in the octahedral positions is further substantiated by the selectivity study. All Mg-containing [Mg Ni ]-saponite catalysts 1–6 5–0 produce a much smaller amount of propene than the non-Mg-containing [Mg Ni ]-saponite, and 0 6 the selectivities do not decrease linearly with the amount of Mg ions in the Mg-containing [Mg Ni ]-saponites. Similarly, only the 1–6 5–0 [Mg Ni ]-saponite has very low selectivity of IPA 0 6 to acetone and this selectivity does not change linearly with the amount of Ni ions in the catalysts. Therefore, the results of product selectivities imply that each [Mg Ni ]-saponite is not a mixture of x y [Mg Ni ] and [Mg Ni ]-saponites, but a pure sapo6 0 0 6 nite having both Mg and Ni ions in the octahedral positions of the clay framework. In addition, the Mg and Ni ions in the octahedral positions are shown to be interacting with each other. This is because if there is no interaction between these Mg and Ni ions in the octahedral positions, then the catalytic selectivities of [Mg Ni ]-saponite 1–6 5–0 catalysts should be dependent on the amount of the active Ni ions in the framework. Because of these interactions of neighboring Ni and Mg ions, there are two different types of active sites on these saponite catalysts for IPA reaction: one associated with pure Ni ions (i.e. Ni sites) and the other one associated with pure Mg ions or with Mg ions interacting with Ni ions (i.e. Mg sites or Mg–Ni sites, respectively). The IPA reaction results show that the Mg sites have similar activities to the Mg–Ni sites, and the Ni sites are more active than the Mg sites or the Mg–Ni sites. It is worth noting here that the reaction results show the absence of non-crystallized phases such as MgO, NiO, and SiO –Al O in all the synthe2 2 3 sized saponites. This is because, if MgO or NiO did exist in these saponites and their contribution to the catalytic activities for IPA conversion were significant, then the catalytic activity of the saponites would increase or decrease proportionally to the increasing or decreasing amount of Ni of Mg in the saponites. However, this trend could not be observed from the reaction results, which show that the IPA conversions on the Mg-containing saponites were quite similar to each other, and the active sites on [Mg Ni ]-saponite were markedly 0 6
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different from those on other saponites. If SiO –Al O sites really did exist in these saponites, 2 2 3 they would be expected to have the same catalytic effect to all saponites. These results show that the possible existence of MgO, NiO or SiO –Al O in 2 2 3 the synthesized saponites was negligible. The difference in catalytic activities of the [Mg Ni ]0 6 saponite from other Mg-containing saponites is thus shown to originate mainly from the effect of Mg ions in the octahedral positions of saponites. 4.2.2. The effect of Mg ions on the acidity of saponites The IPA reaction results have shown that the [Mg Ni ]-saponite catalyst is the most active 0 6 catalyst among [Mg Ni ]-saponites and all x y Mg-containing [Mg Ni ]-saponite catalysts 1–5 5–1 have quite similar activities. These results show that the presence of Mg ions in the catalyst is adverse to the conversion of IPA, and Mg and Ni ions are shown to interact with each other. The pyridine-adsorption FTIR results further clarify this Mg–Ni interaction by showing that the [Mg Ni ]-saponite has a strong vibration band at 0 6 1540 cm−1 (characteristic of the Brønsted acid sites) but all other Mg-containing [Mg Ni ]-sapox y nites show a substantial decrease of this band. The pyridine-adsorption FTIR result shows that the Brønsted acid sites have been substantially diminished on these Mg-containing saponites, showing the interactions between Mg and Ni ions in the octahedral positions of the [Mg Ni ]-saponites; 1–5 5–1 otherwise, the Brønsted acidity should increase gradually with the increase of the Ni content in the catalysts. The pyridine-adsorption FTIR result also shows that the interacting Mg ions diminish the Brønsted acidity of the Ni ions. In other words, the interacting Mg ions may have deactivated the active Ni ions. It has been generally accepted that the Brønsted acid sites in saponites mainly come from the SiMOMAl linkage. It should be noted here that the synthesized saponites in this study should also contain these SiMOMAl linkages. The reason is as follows. The XRD patterns of these saponites have similar XRD patterns as those saponites reported in the literature. One of these saponites, i.e. [Mg Ni ]-saponite, was synthesized based on the 6 0 saponite composition reported in the literature.
Since normal saponite has been characterized thoroughly and shown in the literature to have SiMOMAl linkages, it is believed that the synthesized saponites in this study should have these SiMOMAl linkages. One of the interesting phenomena observed from the results of this study is that, although all these synthesized saponites should basically contain similar SiMOMAl linkages, the [Mg Ni ]0 6 saponite shows many more Brønsted acid sites than other Mg-containing saponites. The result indicates that the interaction between the octahedral layer and tetrahedral layer may also contribute to the generation of Brønsted acid sites, which are postulated to be generated possibly through a linkage such as SiMOMAlMOMNi. As a very small amount of Mg ‘would kill’ most of these Brønsted acid sites in Mg-containing saponites, it is postulated that ‘this killer’ may possibly be due to the formation of linkages formed by Mg between the octahedral layer and tetrahedral layer, such as SiMOMAlMOMMg or SiMOMAlMOMNiMOMMg linkages. Further characterization needs to be carried out to show the formation of these linkages.
5. Conclusions A series of synthetic [Mg Ni ]-saponite catax y lysts, having two Mg and Ni ions incorporated in the octahedral positions of the clay framework, were synthesized. Based on the results of XRD, IPA reaction, and pore structure, both Mg and Ni ions incorporated in the octahedral positions are shown to have some chemical interactions in the octahedral sheet, which affects the activity, selectivity, and acidity of the catalysts. The results of this study may offer opportunities to synthesize other interesting clay catalysts having multimetals in either the octahedral or the tetrahedral positions of the clay framework.
Acknowledgement This research work was generously supported from a research grant by the National University of Singapore.
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