Synthesis and characterisation of organo-silica hydrophobic clay heterostructures for volatile organic compounds removal

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Microporous and Mesoporous Materials 111 (2008) 612–619 www.elsevier.com/locate/micromeso

Synthesis and characterisation of organo-silica hydrophobic clay heterostructures for volatile organic compounds removal Carla D. Nunes a, Joa˜o Pires a, Ana P. Carvalho a, Maria Jose´ Calhorda a, Paula Ferreira a b

b,*

Department of Chemistry and Biochemistry, CQB Faculty of Science, University of Lisbon Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, Campus Universita´rio de Santiago, 3810-193 Aveiro, Portugal Received 15 June 2007; received in revised form 4 September 2007; accepted 5 September 2007 Available online 14 September 2007

Abstract Mesostructured materials belonging to a new class of solid acids known as porous clay heterostructures (PCHs) have been prepared by chemical modification of a natural clay, by using a cationic surfactant, a neutral amine, and an equimolar mixture of bis(triethoxysilyl)benzene (BTEB) and tetraethyl orthosilicate (TEOS). The effect of different polymerisation times of the silica sources and of the hydrocarbon chain length of the neutral amine was studied. The materials retained their layered structure after the formation of stable pillars by the polymerisation of hydrolysed TEOS and BTEB. All materials were characterised by low temperature nitrogen adsorption isotherms, 13C CP MAS, 29Si MAS and CP MAS NMR spectroscopy, thermal analyses and infrared spectroscopy. The specific surface BET areas of the materials were in the range 550–800 m2 g1 and the corresponding microporous volume were near 0.2–0.3 cm3 g1. The reduction of the reaction time from 12 to 4 h avoids the extra-gallery polymerisation, contributing for a larger specific surface area. The increase of two carbon atoms in the neutral amine chain does not show much effect on the available surface area. These materials were very effective as adsorbents of volatile organic compounds (VOCs), according to tests on methanol, methyl ethyl ketone, toluene and trichloroethylene. The water adsorption isotherms proved the hydrophobicity of the materials, suggesting their capabilities for VOC adsorption in the presence of water. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Porous clay heterostructure (PCH); Hybrid organic–inorganic materials; Mesoporous materials; Adsorption; VOCs

1. Introduction Volatile organic compounds (VOCs) are widely applied in industrialised countries in many different fields, such as chemical industry (biocides, plastics and solvents), automotive and aerospace industry, dry cleaning solvents in the garment industry, and solvent cleaning in the electronic industry. However, VOCs are frequently found in municipal and industrial wastes and can easily contaminate soil and groundwater. In the evolution toward a sustainable chemistry, and taking into account the increasingly stringent environmental regulations, it is important to minimise their use, finding new materials that can act as environmen*

Corresponding author. Tel.: +351 234 370 264; fax: +351 234 425 300. E-mail address: [email protected] (P. Ferreira).

1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.09.008

tal friendly catalysts and/or adsorbents for undesired compounds. Surface modification of inorganic matrix materials offers an efficient way to achieve this aim, allowing easy tailoring of the properties of the resulting materials towards specific applications. From the plethora of inorganic matrix materials available, clays are attractive materials for catalysis and adsorption. They have the big advantage of being easily available, coupled with appropriate physicochemical properties, their most obvious flaw being the variable nature of their pore structure. Attempts to solve this problem included the development of Pillared Inter-Layered Clays (PILCs), after a procedure that led to constant distances between the layers of the clays. These materials can be prepared from natural clays by exchanging the inter-layered cations by larger and bulkier polyhydroxymetallic cations, followed by their calcination. In this way,

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rigid metal oxides are formed and sustain the layers at constant distances (pillars), defining specific spaces between clay layers, which will be responsible by the adsorption and catalytic properties. The pillars themselves will have a strong influence on such properties. Reports on the activity of PILCs as acid catalysts [1], potential adsorbents of volatile organic compounds [2,3], and several catalytic reactions have been published [4]. It is expected thus that the incorporation of organic groups into inorganic pillars will modify their adsorptive properties, improving their affinity for organic compounds. A successful answer to this synthetic challenge was given by Galarneau et al. [5] who reported a new class of solids, known a porous clay heterostructures (PCHs). The interlayered space of the clays is distended by mean of an ionic structure-directing agent around which a silicon source is polymerised. The modification of the polymerisation source allows the control of acidity of the intra-gallery [6]. The acidic properties can also be tuned by post-synthesis reaction with the reactive silanols of surface [7]. The main advantage of this type of materials is the possibility of designing the pore size, distribution and chemical nature. Indeed, they combine the structural elements of a mesostructure with the intrinsic features of lamellar smectite clays, with pore sizes in the rarely observed supermicropore to small mesopore domain, and specific surface areas in the range of 700–1000 m2 g1. Ishii and co-workers reported in 2006 the development of new microporous organic–inorganic hybrid nanocomposites by alkoxysilylation of 4,4 0 biphenyl-bridged alkoxysilane compounds in ilerite, a crystalline layered silicate [8]. Organo-modified clays or activated carbon have been used for VOCs adsorption, as reported in several examples [9]. It has been shown that activated carbon present some disadvantages, such as fire risk, potential pore blockage and problems of regeneration. In many of the different processes where VOCs are liberated, water vapour is usually present. Therefore, besides high adsorption capacities, the materials for VOCs adsorption normally also require some degree of hydrophobicity. Potential advantages of using organic–inorganic hybrid PCHs as adsorbents of VOCs are that such materials can retain high volumes of VOCs prior to oxidations, they sustain usually quite high temperatures and they can be tailored to possess hydrophobic properties. Recently, we have shown that it is possible to prepare a porous benzene–silica hybrid clay heterostructure by cocondensation of bis(triethoxysilyl)benzene (BTEB) and tetraethyl orthosilicate (TEOS) in the presence of octylamine as co-surfactant [10]. The resulting material had a high specific surface area and the capability to adsorb volatile organic molecules. Here, we report the synthesis of organo-silica clay heterostructures using different amines (octylamine and decylamine) as co-surfactants, in order to study the influence of the hydrocarbon chain size on final porous clay. The effect of the reaction time on the final product was also investi-

613

gated. The clay materials were characterised by 13C and Si MAS and CPMAS solid-state NMR spectroscopy, thermal analyses, low temperature nitrogen sorption isotherms and infrared spectroscopy. The hydrophobicity of the materials was characterised by water adsorption. The VOCs adsorbent capabilities of the clays were tested using toluene, trichloroethylene, methanol and methyl ethyl ketone as probe molecules. 29

2. Experimental 2.1. General Cetyltrimethylammonium bromide (CTAB), TEOS (98%), octylamine (99%), decylamine (95%), 1,4 dibromobenzene (98%) and magnesium were obtained from Aldrich and were used as received. Solvents were dried by standard ˚ procedures, distilled under nitrogen, and kept over 4 A molecular sieves. The organosilicon precursor, 1,4-bis(triethoxysilyl)benzene (BTEB), was prepared via a Grignard reaction and distillation in vacuum as described in the literature [10,11]. Microanalyses were performed at the University of Aveiro. Powder X-ray diffraction data was collected on a Phillips PW 1730 diffractometer using Cu-Ka radiation. FTIR spectra were measured using 2 cm1 resolution with a Nicolet Nexus 6700 FTIR spectrometer using KBr pellets in transmission mode. 1H solution NMR spectra were obtained at 400.13 MHz with a Bruker Avance 400 spectrometer using CDCl3 as solvent. Chemical shifts are quoted in ppm from TMS. The thermal studies were performed using a TG–DSC 111 from Setaram at a heating rate of 10 °C min1 under nitrogen. 29Si solid-state NMR spectra were recorded at 79.49 MHz, on a (9.4 T) Bruker Avance 400 P spectrometer. 29Si MAS NMR spectra were recorded with 40° pulses, a spinning rate of 5.0 kHz, and 35 s recycle delays. 29Si CP MAS NMR spectra were recorded with 4 ls 1H 90° pulses, 8 ms contact time, a spinning rate of 5 kHz, and 4 s recycle delays. 13C CP MAS NMR spectra were recorded with a 4.5 ls 1H 90° pulse, 2 ms contact time, a spinning rate of 7 kHz, and 4 s recycle delays. Chemical shifts are quoted in ppm from TMS. 13C CP MAS NMR spectra were also recorded in the solid state at 125.76 MHz on a Bruker Avance 500 spectrometer. Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4100 microscope. Nitrogen adsorption isotherms at 196 °C were measured in a conventional volumetric apparatus, with a pressure transducer (Datametrics 600) previously calibrated against a mercury manometer, equipped with a rotary/diffusion system, which allowed a vacuum better than 102 Pa. The adsorption isotherms of the organic vapours were obtained by the gravimetric method in a microbalance (C.I. Electronics) with a precision of 10 lg. Pressure read-

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ings were made with transducers from Pfeiffer (CMR 261 and 262) and the vacuum system included a turbomolecular pump (Pfeifer mod. TMH 071 P). The adsorption temperature (25 °C ± 0.1 °C) was maintained with a water bath (VWR Scientific). Prior to the adsorption measurements the vapours were purified by freeze–vacuum–thaw cycles. Water adsorption isotherms were measured in an automatic apparatus from Coulter (Omnisorp 100Cx). The adsorption temperature (30 ± 0.1 °C) was maintained with a recycle bath equipped with a temperature controller (Eurotherm 2216 L). Water was bi-distilled, de-ionised and purified by freeze–vacuum–thaw cycles. Before the adsorption measurements, the samples were heated under vacuum for 2.5 h. All the amounts adsorbed are expressed per gram of outgassed sample. 2.2. Preparation of porous heterostrutured materials A natural Portuguese clay from Porto Santo (Madeira archipelago) with structural formula (Si3.70Al0.30)IV (Al1.16Fe0.51Mg0.45)VI(Ca/2,K,Na)0.39 was used (PTS) [12,4]. The fraction <63 lm was decarbonated and washed, in a dialysis tube, until the conductivity was lower than 1 mS m1 [12]. For the synthesis of the porous clay heterostructure containing organo-silica pillars, the clay gallery was opened up by stirring overnight, at 50 °C, a suspension of clay (1 g in 100 ml of water) with a 0.5 M solution of the ionic surfactant, CTAB. The solid was then separated from the solution and washed with water until pH 7. The air-dried ion-exchanged clay was then stirred overnight with a mixture of 1:1 of TEOS and BTEB in the presence of a neutral amine (octylamine or decylamine) as co-surfactant. After 4 or 12 h of mixing, the solid was filtered and dried. The surfactant template was extracted using a solution of 4.5 g of 37% HCl in 125 ml ethanol during 24 h and at 70 °C, following procedures reported in the literature [13]. For comparison of the adsorption isotherms, a pillared inter-layer clay (PILC), obtained by conventional methodologies of ion exchange with aluminium polyhydroxy-cations, as described elsewhere [12,4], was also used. PTS-BTEB-Octyl-12 h (1) Elemental analysis found: C 16.64, H 2.81. IR (KBr/cm1): 3428 (vs), 2979 (m), 2929 (m), 1637 (s), 1386 (m), 1155 (vs), 1043 (vs). 13 C CP MAS NMR (d ppm): 132.6 (Ph), 57.4, 16.7 (OEt groups not polymerised). 29Si CPMAS NMR (d ppm): 78.1 (T3), 70.4 (T2), 61.7 (T1), 107.2 (Q4), 101.6 (Q3), 91.4 (Q2 and Qm). PTS-BTEB-Decyl-12 h (2) Elemental analysis found: C 17.20, H 3.31. IR (KBr/cm1): 3468 (vs), 3006 (m), 2954 (m), 1648 (s), 1391 (m), 1163 (vs), 1067 (vs). 13 C CP MAS NMR (d ppm): 133.1 (Ph), 57.7, 16.4 (OEt groups not polymerised). 29Si MAS NMR (d ppm): 78.4 (T3), 71.6 (T2), 62.9 (T1), 109.7 (Q4), 103.4 (Q3), 93.2 (Q2 and Qm).

PTS-BTEB-Octyl-4h (3) Elemental analysis found: C 11.71, H 3.33. IR (KBr/cm1): 3408 (vs), 2927 (m), 2872 (m), 1631 (s), 1383 (m), 1154 (vs), 1044 (vs). 13 C CP MAS NMR (d ppm): 132.8 (Ph), 59.0, 16.3 (OEt groups not polymerised). 29Si MAS NMR (d ppm): 79.1 (T3), 72.6 (T2), 64.2 (T1), 108.8 (Q4), 104.2 (Q3), 92.7 (Q2 and Qm).

3. Results and discussion As previously reported [10], organic–inorganic mesostrutured moieties were assembled in the galleries of the natural occurring clay, as depicted in Scheme 1. Initially, the interlamellar Na+ cations were exchanged by cetyltrimethylammonium ions. Subsequently, the hydrated clay galleries were swelled with octylamine or decylamine (Table 1) that act as co-surfactants, in order to allow the co-condensation of an equimolar mixture of TEOS/BTEB in the inter-gallery space. After a certain reaction time (Table 1), at room temperature, the resulting clays were filtered and dried in air for several days giving rise to materials 1–3. Removal of the surfactant templates from the material was accomplished by mild extraction with an ethanolic/ HCl solution at 70 °C during 24 h, in order to prevent destruction of the aromatic moieties. It was not possible to detect an X-ray diffraction pattern at low angles when using either oriented or non-oriented mounts. This observation may be related with poor long range order, an explanation given before for other authors for materials prepared by similar methods [7,10,14]. FTIR and 13C CPMAS NMR spectra confirmed the organo-silica nature of the materials. The presence of the benzene moieties of BTEB is identified in the FTIR the spectra of all three materials (Fig. 1) through the observation of the bands at 2929–3006 cm1 assigned to mC–H, and in the 1383– 1391 cm1 range assigned to mC=C and bC–H modes, respectively. The very weak bands at around 2930–2850 cm1 correspond to mC–H modes of the ethoxy groups not polymerised. Peaks at around 1630–1650 cm1 and 3408– 3468 cm1 are assigned to H–O–H and O–H vibrations confirming the presence of water molecules. Special attention should be given to mSiOSi, mSiOAl and mAlOAl modes in the 800–1200 cm1 range, which give reliable information on the local structure of the materials. Bands assigned to the mAlOAl modes are observed at 817 (1), 812 (2), and 815 (3) cm1, and to mSiOSi at 1155, 1043 (1), 1163, 1067 (2), and 1154, 1044 (3) cm1. In addition, clear differences in the full width at half height (FWHH) distinguish materials 1 and 2 from material 3, which shows the narrowest band and was obtained after only 4 h reaction time. The narrow band indicates that the distribution of local environments is more homogeneous in 3. In a recent work addressing the identification of the mSi–C vibration mode in several hybrid organic–inorganic matrix materials, it was found that this particular mode

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615

Scheme 1.

Table 1 Surfactant, co-surfactant and time of reaction used for the synthesis of materials 1–3 Materials

Surfactant

Co-surfactant

Time of reaction (h)

1 2 3

CTAB CTAB CTAB

C8NH2 C10NH2 C8NH2

12 12 4

* * 3

*

* * PTS

*

2

*

*

*

* 1 *

1

*

2

250

200

150

100

50

0

-50

δ (ppm from TMS)

3

Fig. 2. 13C CPMAS NMR spectra of the extracted materials 1–3 (*denote spinning sidebands). 4000

3200

2400

1600

800

wavenumber (cm-1) Fig. 1. FTIR spectra for samples 1–3 and the parent clay (PTS).

is not easily assignable, particularly in the case of BTEB [15], so no assignment was attempted. The solid-state 13C CPMAS NMR do not show any resonances that could be assigned to long carbon chains of CTAB and co-surfactants (d = 10–33 ppm) in the solvent extracted samples (Fig. 2), reflecting a successful extraction

procedure. On the other hand, the presence of the phenyl group is confirmed by a strong resonance at d = 132 ppm with a shoulder at d = 127 ppm assigned to the aromatic carbons, thus confirming that the integrity of the organic fragment was preserved after surfactant removal. Low intensity peaks corresponding to non polymerised ethoxy groups of the organic fragment at d = 57.4 (OCH2) and 16 (CH3) ppm can be observed. These signals at these values are in good agreement with other reported in the literature [16]. 29 Si MAS and CPMAS NMR spectra of the materials are shown in Fig. 3, and is possible to see three distinct

616

C.D. Nunes et al. / Microporous and Mesoporous Materials 111 (2008) 612–619 2

T

4

Q

3

T 1

T

Q 2

1

-5

3

2

m

Q +Q

3

MAS

TG (%)

CP MAS

3

PTS -15

CP MAS

2

-25 MAS

CP MAS

1

-35 0

200

400 T / ºC

600

MAS

Fig. 4. Thermogravimetric curves for the parent PTS clay and materials 1–3. -25

-50

-75

-100

-125

-150

δ (ppm from TMS) Fig. 3. 3.

29

Si MAS and CPMAS NMR spectra of the extracted materials 1–

resonances of silicon environments attributed to siloxane [Qn = Si(OSi)n(OH)4-n, n = 2–4], organosilicon [Tm = RSi(OSi)m(OH)3-m, m = 1–3] and silicon of tetrahedral layer of the clay [Qm = Si(OSi)3Al]. Two resonances assigned to Q4 and Q3 species appear at d ca. 110 and 102.5 ppm, respectively. The T3, T2 and T1 organosilicon environments are observed at d ca. 80, 70 and 62.5 ppm, respectively. The resonance at 92.5 ppm is assigned to both Q2 species and Qm of the clay. In material 3, the band of the T1 species is not so strong as in the other materials, probably because of the shorter reaction time. This is consistent with the results from elemental analyses, showing only 11.71% of carbon content in material 3, while it reaches 16.64% in material 1, obtained after 12 h of reaction. Fig. 4 displays the TG curves of the clay and prepared materials. A weight loss can be observed in the materials 1–3 between 100 and 200 °C related with the release of the water molecules physically adsorbed into the pores. The small weight loss taking place between 200 and 400 °C can be assigned to the release of a small portion of not extracted surfactant. Above 400 °C, a significant weight loss occurs, much higher than what is noticed for the parent PTS clay, which can be attributed to the decomposition of the organic groups and to dehydroxylation. Indeed, for the PTS clay, only the first weight loss between 100 and 200 °C, related with the release of the water molecules is observed, and there is not decomposition of the clay. The PTS maintains the structure and composition. The SEM images show that the as-synthesised extracted solvent materials have essentially the same form as the Portuguese clay mineral (Porto Santo) and reveal random particle shapes. The treatment required to introduce new pillars does not seem to destroy the clay.

Materials 1 and 2 have the same form as PTS, while material 3 is more irregular. According to the SEM image, material 3 presents a porous morphology, probably because the material reacted only 4 h, unlike the other materials (12 h). For materials 1 and 2, random shapes appeared at the surface of the PTS clay, indicating that the mixture TEOS/BTEB grew around the pillars of the clay particles [17]. As the reaction time increases, shapes become more uniform, reflecting its effect on the shapes of the particles (see Fig. 5). The low temperature nitrogen adsorption isotherms for these materials are shown in Fig. 6, (desorption points are not shown for clarity). The curves of the initial clay (PTS) and a pillared clay (PILC) are also shown. Table 2 summarises the adsorption data. The isotherms of materials 1 and 2 have similar profiles. A gradual increase in N2 adsorption at low to medium partial pressures (0.05 < p/p0 < 0.4) suggests that these materials possess supermicropores and small mesopores. At higher relative pressures, the isotherms present a positive slope which will most probably correspond to the adsorption in the external area of the particles. Sample 3 shows a similar profile but the adsorbed amounts in the low pressure region are the highest. Consequently, this material also has the highest value of microporous volume (0.3 cm3 g1) and the equivalent specific surface area (723 m2 g1). The specific surface areas of materials 1 and 2 are very similar and significantly lower than that of material 3. The pore sizes lie in the 1.7–4 nm range, confirming the presence of supermicropores and small mesopores. Similar results have been reported in the literature for a phenyltrietoxysiloxane derivatised clay [19]. On the other hand, as can be seen from Table 2, in the three cases the surface area was significantly increased relative to the parent clay. This stresses the success of this pillaring procedure towards formation of hybrid organic–inorganic structure. The values are also higher than those obtained for the PILC sample, which was prepared by the more usual

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617

Fig. 5. Scanning electron micrographs of parent clay PTS and materials 1–3.

2

Table 2 Equivalent specific surface areas (ABET), microporous volumes (Vmicro, from DR-plots), mesoporous volumes (Vmeso) [18], and carbon content for samples 1–3, obtained from the nitrogen adsorption isotherms at 196 °C and elemental analysis

1

Samples

ABET (m2 g1)

Vmicro (cm3 g1)

Vmeso (cm3 g1)a

%C

PTS PILC 1 2 3

97 306 561 560 723

– 0.13 0.24 0.26 0.30

0.09 0.06 0.16 0.14 0.10

– – 16.64 17.20 11.71

0.4

3

w (cm3 g -1 )

0.3

0.2

PILC PTS

0.1

0 0

0.2

0.4

0.6

0.8

1

p/p 0

Fig. 6. Nitrogen adsorption isotherms at 196 °C of samples 1–3, the initial clay (PTS) and a pillared inter-layered clay (PILC) prepared from PTS.

process of ion-exchange with polyhydroxyaluminium cations and subsequent calcination. The sequence of the values obtained for specific surface areas and porous volumes of materials 1–3 may be related with the reaction times used. In fact, the time for preparing material 3 was one third (4 h) of the time used for preparing materials 1 and 2 (12 h). It is possible that longer reaction times induce further modifications such as surface coating and thus originate the blockage of the pores, reduc-

PTS and PILC (prepared with PTS) data [4] is shown for comparative reasons. a Obtained subtracting the microporous volume to the total adsorbed volume at p/p0 = 0.97.

ing the surface area and the micropore volume. This speculation is supported by the carbon elemental analyses. Material 3 prepared in 4 h has the lowest carbon content, which may indicate that the condensation occurs only in between the clay layers. On the other hand, the size of the hydrocarbon chain of the neutral amine does not seem to influence the properties, as seen in similar values of surface area, pore volume and even carbon content of materials 1 and 2. The adsorption properties of the three prepared materials toward selected vapours, such as water, methanol,

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methylethylketone (MEK), trichloroethylene (TCE), methanol, and toluene were investigated. The adsorption isotherms are shown in Figs. 7 and 8 with the adsorbed amounts expressed in volume per gram of material. As can be seen from Fig. 7, the total amount of each VOC adsorbed in material 1 tends to a value which is near the total sum of the micro and mesoporous volumes determined with nitrogen (Table 2). Considering the total amount of adsorbed vapours, it is possible to say that material 1 adsorbs high amounts of all samples, specially MEK, where the amounts are comparable to those found in the literature for materials with superior specific surface areas [20]. This result indicates that these materials are indeed potential adsorbents for the abatement/recovery of VOCs. This observation is better supported when the isotherms of all the VOCs are considered in conjugation with the water adsorption data. In fact, it is noteworthy also the difference in the shape of the isotherms for the VOCs molecules, on one hand, and for the water, on the other hand. This difference shows that the prepared materials have hydrophobic properties, which has important practical implications. In fact, as stated above, in different

0.4

4. Conclusions

3 -1

w (cm g )

0.3 MEK 0.2

toluene TCE methanol

0.1

water

0 0

0.2

0.4

0.6

0.8

1

p/p 0 Fig. 7. Adsorption isotherms of the indicated vapours for material 1 (MEK and TCE stand for methylethylketone and trichloroethylene, respectively).

0.4

0.3 w (cm3g-1)

processes where VOCs are liberated, water vapour is also usually present, so that, besides high adsorption capacities, some degree of hydrophobicity is convenient. It can be observed in Fig. 7 that even for intermediate water partial pressures, the adsorption of VOCs is clearly favoured over the water adsorption, a fact that is less pronounced in the case of methanol, which is more similar to water. For the materials 2 and 3 (Fig. 8) the same patterns, namely the differences in the shapes of the water and VOCs adsorption isotherms, also apply. Nevertheless, the limiting values for the VOCs adsorption are now somewhat lower and closer to the microporous volumes measured with nitrogen adsorption (Table 2). As already noticed when the results from nitrogen adsorption were presented, the isotherms for material 3 have the highest rectangular character. This means that for corresponding adsorbates, the adsorbed amounts at low relative pressures are higher for material 3 than for material 2. This behaviour should not result from using of a shorter chain amine (octylamine), as it was not noticed in material 1 synthesised with the same amine. A more plausible explanation may arise from the fact that, according to the higher surface area and micropore volume, the pillaring procedure was more efficient in material 3 than in materials 1 and 2. The low temperature N2 isotherms of material 3 suggest that no surface coating with consequent pore blockage occurs.

toluene

0.2

methanol water 0.1

toluene methanol

0 0

0.2

0.4

0.6

0.8

1

p/p 0 Fig. 8. Adsorption isotherms of the indicated vapours for materials 2 (open) and 3 (filled).

Phenylene-bridged porous clay heterostructures were prepared from a natural Portuguese clay using a salt of cationic quaternary ammonium surfactant as structure directing agent, a neutral amine (octylamine or decylamine) and an equimolar misture of bis(triethoxysilyl)benzene and tetraethyl orthosilicate. The organosilica nature of the prepared materials was well supported by FTIR, 13C CPMAS NMR 29Si CPMAS and MAS NMR spectra. It was shown that the time used in the polymerisation can affect the porosity characteristics of the samples. The reaction time of 4 h led to a material (material 3) with lower carbon content, higher specific surface area (723 m2 g1) and higher adsorption capabilities at low relative pressures. The materials 1 and 2 prepared in 12 h have specific surface areas around 560 m2 g1, a value ca. 25% lower than that of material 3. The materials presented hydrophobic characteristics and the adsorbed amounts of different types of organic vapours reached values of 0.3 cm3 g1. The effect of using different neutral amines has not proven to be relevant, while the reaction time is primordial. The obtained samples were stable up to 400 °C. The structural modifications introduced in the final materials by the presence of organic aromatic moieties in the pillars were successful in terms of leading to a highly adsorbent species favouring VOCs over water, and thus tailored for the adsorption of hydrophobic VOCs. Water and methanol were found to be the least adsorbed at low relative pressures, but even water can be highly adsorbed at

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high relative pressures. The results for the adsorption of VOCs show that these materials are very promising for remediation of environmental pollutants, namely material 3 as discussed above. Therefore, short reaction times should be encouraged. Acknowledgments The authors are grateful to FCT, FEDER and POCTI for financial support (POCTI/QUI/44654/2002). C.D. Nunes acknowledges FCT for a post-doctoral fellowship (SFRH/BPD/14512/2003). References

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