Calorimetric study of mesoporous solids at room temperature

Microporous and Mesoporous Materials 156 (2012) 45–50

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Calorimetric study of mesoporous solids at room temperature J.C. Moreno-Piraján a,⇑, L. Giraldo b a b

Research Group on Porous Solid and Calorimetry, Department of Chemistry, Faculty of Sciences, Universidad de Los Andes, Carrera 1 No. 18 A 10, Bogotá, Colombia Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Ciudad Universitaria, Carrera 30 No. 45 03, Edificio 451, Bogotá, Colombia

a r t i c l e

i n f o

Article history: Available online 10 February 2012 Keywords: Immersion calorimetry Zeolites Enthaphy Thermodynamics Mesoporous

a b s t r a c t Mesoporous aluminosilicate molecular SBA-15, SBA-16, MCM-41 and MCM-48 were synthesized using standard procedures. The products were characterized by powder X-ray diffraction (XRD), diffuse reflectance Fourier-transform infrared (DRIFT), nitrogen sorption and immersion microcalorimery. The walls of the mesopores are essentially crystallinity, and the local atomic arrangement is similar to that in amorphous aluminosilicates. On the other hand, the immersion enthalpies in solvents polar and non-polar are a function of the accessible area of each of mesoporous solids. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Mesoporous materials are inorganic or mixed organic–inorganic solids which are synthesised by incorporating specific molecular species from solution into their structure. Their layered or threedimensional structural frameworks are porous in one or more directions, with correspondingly low densities. Their structural frameworks may be crystalline or amorphous, and the template or structure-directing agent (SDA) is co-crystallised in pores, channels, or interlayers [1–5]. Additional solvent molecules (usually, but not necessarily water) are also often present in the structure. Recently, a variety of calorimetric approaches has been applied to study the stability of these frameworks, their interactions with organic templates, and their evolution during synthesis and processing [1,2–5]. The purpose of this research is to investigate the relationship between immersion enthalpies and structural porosity in synthesised mesoporous solids, using immersion microcalorimetry and non-polar and polar probe molecules. In addition, we explore the possible relationship between immersion enthalpies and the physicochemical properties of the molecules used in this study.

2. Experimental section 2.1. Sample preparation For synthesis of mesoporous materials, a specialised reactor was designed, as illustrated in Fig. 1. The reactor was constructed ⇑ Corresponding author. Tel.: +57 1 3394949 2786. E-mail address: [email protected] (J.C. Moreno-Piraján). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2012.02.007

out of Teflon, and was provided with a controllable agitation system and pressure gauge. A safety valve was also installed, which enables the system to release excess pressure automatically, balancing the internal and external pressure [3].

2.1.1. Synthesis of SBA-15 For the synthesis of SBA-15 [4], 8 g of pluronic P123 triblock copolymer (EO20PO70EO20) was dissolved in 300 mL of 2 M HCl. Subsequently, 17 g of tetraethyl orthosilicate (TEOS) was added under vigorous stirring at 45 °C. The molar ratio of the synthesis solution, TEOS:HCl:H2O:P123, was 1:5.87:194:0.017. After 7.5 h, stirring was stopped and the solution was aged for 15 h at 80 °C. Then, the resulting white powder was filtered, washed, and dried. Finally, the powder was calcined in air by heating from ambient temperature to 550 °C for 6 h, using a heating rate of 1 °C/min.

2.1.2. Synthesis of SBA-16 SBA-16 was prepared as described in [5]: triblock copolymer Pluronic F127 was added to an aqueous solution of HCl, and stirred at a certain temperature overnight. Then, TEOS was added to this solution under vigorous stirring. After 10 min of stirring, the mixture was kept under static conditions at the aforementioned temperature for 3–72 h. The resulting solid products were collected by filtration, washed with ethanol, dried, and calcined in air at 550 °C in air for 5 h, using a heating rate of 1 °C/min. Preparation of SBA16 crystals relies on precise control of the reaction temperature and the ratio of reactants. Typical synthetic conditions were 1.00TEOS:0.0041F127:5.00HCl:180 H2O at 28 °C, to obtain 1.00TEOS:0.0053F127:4.10HCl:150 H2O at 32 °C, resulting in a rhomb-dodecahedron shape.

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bonding from the identification of functional groups in the infrared absorption spectra. Adsorption–desorption nitrogen isotherms were measured at the temperature of liquid nitrogen, using an Autosorb 3B (Quantachrome, Boyton Beach, MI). The surface area was determined by applying the Brunauer–Emmett–Teller (BET) equation, and desorption points were used for pore size analysis by applying the NLDFT [8–12]. Before adsorption, samples were outgassed by heating at 100 °C in a <3  10 2 mm Hg vacuum for 12 h. To calculate of pore volume, Vp, a novel method based on the nonlocal density functional theory (NLDFT) of adsorption and capillary condensation in cylindrical pores was used. The NLDFT method allows one to calculate the mesopore size distribution, and to evaluate the pore wall thickness and the level of intrawall porosity. The NLDFT method was applied for isotherms with an H1 hysteresis loop, and the two kernels were used for calculations from the experimental (a) desorption branches and (b) adsorption branches [12]. XRD, FTIR (are not show here) and adsorption isotherms measurements were performed to complete the respective analysis. 2.3. Calorimetry Fig. 1. Reactor designed for the synthesis of mesoporous solids.

2.1.3. Synthesis of MCM-41 MCM-41 has been successfully prepared using different synthesis procedures and conditions [6,7]. For this study, MCM-41 powder was crystallised from an alkaline solution containing cetyltrimethylammonium bromide (C16TABr, 99%, Merck), sodium silicate solution (Na2O, 7.5–8.5%, SiO2, 25.8–28.5%, Merck), sulfuric acid (98%, Merck), and deionised water; using a molar ratio of 1 CTABr:1.76 Na2O:6.14 SiO2:335.23 H2O:1.07 H2SO4. After 24 h of crystallisation at room temperature, the MCM-41 powder was filtered, washed, and dried, before it was calcined in air, in a furnace at 450 °C for 4 h, to remove the organic template. 2.1.4. Synthesis of MCM-48 MCM-48 was synthesised in polypropylene bottles under ambient pressure. 1 M NaOH solution (Baker) was added to the bottle, followed by cetyltrimethylammoniumbromide (CTABr, aldrich) and water, under continuous stirring over a waterbath at 40– 80 °C. After, tetraethoxysilane (TEOS, Merck) was added and stirring was continued until the gel was homogeneous. The final molar gel composition of the synthesis mixture was TEOS:0.15 CTABr:0.5 NaOH:80 H2O. The final gel was divided into two parts and poured in Teflon autoclaves for microwave heating, using an MDS-2000 microwave-oven system equipped with a temperature controller and adjustable power output (maximum 650 W). The gel was heated to 100 °C for 1 and 2 h. The resulting solid white products were separated by filtration, washed with a mixture of H2O, ethanol and HCl (molar ratio 18:1:2), and dried at room temperature; following a similar procedure described in the literature [8–10]. Surfactant inside the synthesised material was removed by at 100 °C for 12 h. After that the samples were calcined at 540 °C under flowing nitrogen for 1 h, followed by 6 h under flowing air to remove template with a heating rate of 1 °C/min. 2.2. Sample characterisation Crystalline phases of the synthesised mesoporous materials were identified by X-ray diffractometry (XRD). Spectra were scanned from 2h = 1.0 to 20 °C, at a rate of 1°/min. Fourier transform infrared spectra (FT-IR) were collected on the samples from 400 to 4000 cm 1; enabling determination of inter-species

In the present work, experimental immersion enthalpies of the synthesised mesoporous solids in different solvents (with respect to mass) were determined, to estimate the energetic interactions that occur when the solids are in contact with the different solvents. A heat conduction microcalorimeter, equipped with a calorimetric cell made of stainless steel, was used for determination of the experimental immersion enthalpies [10,13,14]. Prior to the experiment, the sample was outgassed under vacuum (10 3 Pa) at 250 °C for 4 h in a glass bulb. The bulb was then sealed in vacuum and inserted into a calorimetric chamber containing the immersion liquid. Once thermal equilibration was reached, the brittle end of the glass bulb was broken and the heat released was followed with time. A detailed description of the experimental setup can be found elsewhere [15,16]. Approximately 8 mL of each solvent (previously kept at 25 °C in a thermostat) was placed inside the cell. Between 50 and 800 mg of mesoporous solid samples were put in a bulb was then sealed in vacuum and inserted into a calorimetric chamber containing the immersion liquid inside the calorimetric cell, and the microcalorimeter was assembled. When the device reaches a temperature of 25 °C, the output potential is recorded for approximately 15 min, with data points recorded every 20 s. After 15 min, the glass bulb breaks, and the resulting thermal effect is recorded, while potential readings are taken for an additional 15 min. Finally, the device is electrically calibrated. 3. Results and discussion 3.1. DRX, FTIR and textural analysis of different mesoporous solids The FTIR spectrum of SBA-15 (not shown here) has bands at 1069 cm 1 (T–O asymmetric stretching) and 800 cm 1 (T–O symmetric stretching); these bands are due to TO4 vibrations (T = Si, Al) and are assigned to the bending Al–O–Si, indicating incorporation of Al into SBA-15 [17,18]. Only one signal was observed at 3740 cm 1 for SBA-15, due to the terminal Si–OH. The nitrogen ( 196 °C) sorption isotherms was used to analyze of mesoporous solids synthesized in this work shown in Fig. 2a and b to obtain surface and pore size characteristics for the SBA-15, SBA-16, MCM-41 and MCM-48 silica materials. The results are summarized in Table 1. BET surface areas were determined from nitrogen isotherms at 196 °C in a range of relative pressures (P/ P0) prior to the occurrence of pore condensation for each sample by assuming a cross-sectional area of 0.162 nm2 for N2 at a range

J.C. Moreno-Piraján, L. Giraldo / Microporous and Mesoporous Materials 156 (2012) 45–50

Fig. 2. (a) Nitrogen sorption isotherms at 196 °C in SBA-15 and SBA-16. (b) Nitrogen sorption isotherms at 196 °C in MCM-41 and MCM-48.

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[23–27]. The pattern of diffraction peaks confirms a high level of crystallinity (or long-range structural order) in all samples. Signals were observed at 2.3°, 2.6°, and 3° (2h), corresponding at 2 0 0, 2 1 0, and 2 1 1 (hkl) index reflections, based on the SBA-15 mesoporous silica Pm3n cubic mesostructure, as reported for pure cubic SBA15 (using another synthesis route) [8,26,28]. N2 adsorption and desorption isotherms for SBA-16 are consistent with type IV behaviour with a hysteresis loop (not shown). Analyses of the data in the relative pressure (P = P0) range between 0.04 and 0.20 yields a BET (Brunauer–Emmett–Teller) surface area of 790 m2 g 1 and a pore volume of 0.60 cm3 g 1 for the SBA-16 obtained. The calculated adsorption-pore size distribution is in agreement with the characteristic cage-like pore structures reported for SBA-16, with a spherical cage cavity diameter of 8.3 nm. XRD analysis reveals that under the experimental conditions used in this study, SBA-16 displays well-resolved X-ray diffraction patterns, suggesting an ordered mesostructure. The purity and structure of the synthesised MCM-41 materials were analysed by low-angle XRD. XRD patterns for MCM-41 exhibit a strong peak in the 1 0 0 reflection, which is shifted to a higher value, indicating contraction of the lattice caused by template removal and subsequent condensation of silanol groups, as previously reported for MCM-41 materials [28]. N2 adsorption isotherms for SBA-15, SBA-16, MCM-41 and MCM-48 materials exhibit a combination of type I and type IV isotherms, according to the IUPAC classification system (data not shown). In all samples, a high N2 uptake at low relative pressures (p/p0 < 0.1) was observed, together with characteristic capillary condensation in the mesopores at p/p0  0.32, p/p0  0.34, p/ p0  0.65 and p/p0  0.68, for MCM-41, MCM-48, SBA-15 and SBA-16, respectively. The higher relative pressure observed for capillary condensation on SBA-15 and SBA-16 is consistent with its larger pore diameter (4.56, 4.32, 8.0 and 8.3 nm, as obtained from NLDFT). Interestingly, while capillary condensation and evaporation were completely reversible for MCM-41, SBA-15 exhibited a type H1 hysteresis loop. 3.2. Calorimetry

Table 1 Properties of the synthesised mesoporous solids. Adsorbents

SBA-15

SBA-16

MCM-41

MCM-48

SBET (m2 g 1) dp, N2-NLDFT (nm) Vp,N2(cm3 g 1)

660 8.0 0.67

790 8.3 0.60

990 4.32 0.75

1100 4.86 0.96

of relative pressures (P/P0) before the pore condensation point for each sample (SBA-15: 0.05–0.21; SBA-16: 0.04–0.20; MCM-41: 0.05–30; and MCM-48: 0.05–0.32), by assuming a cross-sectional area of 0.162 nm2 for N2 [19–22]. The nitrogen sorption isotherms for MCM-41 and MCM-48 materials (Fig. 1a.), did not show sorption hysteresis. Fig. 1b show the N2 adsorption–desorption isotherms for the SBA-15 and SBA-16 materials. All samples exhibit type IV isotherms with type H1hysteresis loops according to the IUPAC nomenclature, typical of mesoporous materials with 1D cylindrical channel [19–22]. Pore volumes were calculated for the primary mesopores of SBA-15, SBA-16, MCM-41 and MCM-48 silica samples, from the plateau of the sorption isotherms after pore condensation occurred. Nitrogen sorption isotherms for MCM-41and MCM-48 did not display sorption hysteresis. The NLDFT models used for this pore size analysis were developed [16] for nitrogen sorption at 77 K in siliceous pores, based on cylindrical pore geometry. The results of XRD analysis of SBA 15 are in agreement with literature data for materials synthesised by conventional methods

As can be seen in Table 2, the heat of immersion (J/g) into different non-polar liquids is larger for MCM-41 and MCM-48 versus SBA-15 and SBA-16. This behaviour is attributed to the larger accessible surface area for the different molecules in MCM-41 and MCM-48, and consequently, the accessible surface area of each mesoporous solid. In addition, the accessible surface area can be related to the size and shape of the probe molecules. Fig. 3 shows the relationship between surface area and immersion enthalpy: values between 59 J/g and 121 J/g were obtained for SBA-15 and MCM-48, respectively. The larger enthalpy of immersion (J/g) obtained for MCM-48 and MCM-41 must be due to the higher degree of interaction between hydrocarbon molecules and the silica surface inside the micropores (larger adsorption potential). Based on a search of the literature, our immersion calorimetry results for microporous solids are novel under the experimental conditions used. Interestingly, n-pentane, a larger hydrocarbon molecule with a kinetic diameter of 0.546 nm, still provided an increased areal enthalpy of immersion for all mesoporous solids tested, suggesting the absence of steric restrictions for this molecule, allowing it to freely access the sample microporosity (the increased enthalpy versus benzene can be attributed to a higher packing density). However, the situation is different for molecular probes with a diameter of 0.711 nm, such as n-decane. In fact, a marked decrease in the enthalpy of immersion was observed (see Fig. 4) after immersion in molecules with a diameter of 0.65 nm. This decline was most pronounced in SBA-15 and MCM-48 samples.

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Table 2 Immersion enthalpy of non polar molecules probe used. Probe molecule

Diameter (nm)

Intrinc volume, cm3 g 1

Immersion enthalphy (J g 1) SBA-15

Immersion enthalphy (J g 1) SBA-16

Immersion enthalphy (J g MCM-41

Benzene n-pentane Toluene n-hexane n-heptane n-octane n-decane

0.526 0.546 0.568 0.587 0.623 0.655 0.711

71.6 81.3 85.7 95.4 109.5 123.6 151.8

64 69 59 66 68 74 60

70 77 63 84 91 97 85

81 89 75 91 97 104 86

This result confirms that, in addition to the normal decrease in hydrocarbon uptake expected for larger molecules, SBA-15 exhibits molecular sieve effects for molecules with diameters greater than 0.65 nm. Thus, the micropores on our SBA-15 material should be below 0.65 nm. Using immersion calorimetry and various nonpolar molecules as probes, the present study provides a novel contribution to our knowledge of the existing economic structure of mesoporous solids. Fig. 5 shows the behaviour of intrinsic volume as a function of immersion enthalpy. The intrinsic volume is considered as ad135

Benzene n-pentane toluene n-hexane n-heptane n-octane n-decane

-Immersion enthalpy. J.g

-1

125 115 105 95 85 75 65 55 650.0

750.0

850.0

950.0 2

Surface area. m .g

1050.0

1150.0

1

)

Immersion enthalpy (J g MCM-48

1

)

89 96 78 103 112 121 109

sorbed molecule occupies that not only the site on the surface but also some of the neighboring volume, similarly to how is define the partial molar volume, which referred to according to the solution, in this case is defined respect the adsorbent. This thermodynamic property enables verification that the immersion enthalpies in non-polar solvents are a function of not only the solid, but also the area accessible to the probe molecules. Fig. 5 shows that the enthalpy of immersion was increased for samples immersed in molecules with intrinsic volumes >105 cm3/g, followed by a decrease at even higher values (approximately 150 cm3/g). Table 3 shows immersion calorimetry results obtained using polar probe molecules; revealing an immersion heat increase. This behaviour can be attributed to the larger accessible surface area, as discussed for immersion in non-polar solvents. Resulting values were significantly greater than values obtained from immersion of mesoporous solids in non-polar solvents; due to interactions of the polar probe molecules, and in particular their dipole moments. Fig. 6 shows the relationship between immersion enthalpies and the dipole moment of the probe molecules used; demonstrating that immersion heats are a function of the characteristics of the molecules in which the solid is immersed. Note that, even as the dipole moment increases, the resulting immersion enthalpy is higher. Interestingly, immersion enthalpy values for SBA-15 and MCM-48 have the lowest dispersion, which we attribute to the presence of more organised structure.

-1

140

Fig. 3. Immersion enthalpy as a function of surface area using non-polar solvents.

120

-Im m ersion enthaphy.J.g-1

140

- Immersion enthalpy.J.g-1

120 100 80 60

100 80 60 40

SBA-15 SBA-16

SBA-15

40

20

SBA-16 20

MCM-41

MCM-41 MCM-48

0 0.5

0.55

0.6

0.65

0.7

0.75

Pore size of probe molecules. nm Fig. 4. Immersion enthalpy as a function of the pore size of the probe molecules using non-polar solvents.

MCM-48 0 65

85

105

125 3

145

165

-1

Intric.volume . cm .g

Fig. 5. Immersion enthalpy as a function of intrinsic volume area using non-polar solvents.

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J.C. Moreno-Piraján, L. Giraldo / Microporous and Mesoporous Materials 156 (2012) 45–50 Table 3 Immersion enthalpies of polar solvents and thermodynamic characteristics. Probe molecule

Diameter (nm)

Intrinc volume (cm3 g 1)

n-

dipolar moment (ua)

Immersion enthalphy (J g 1) SBA-15

Immersion enthalphy (J g 1) SBA-16

Immersion enthalphy (J g 1) MCM-41

Immersion enthalpy (J g 1) MCM-48

propanol

0.515

59.0

1.68

123

135

propanol

0.516

59.0

1.66

132

144

73.1 73.1 73.1 73.1 pentanol

1.66 1.64 1.70 1.66 0.582

143 152 162 164 87.2

177 187 198 209 173

197 204 218 220 198

pentanol

0.594

87.2

182

216

pentanol

0.597

87.2

197

225

266 276 287

261 282 298

176 156 i189 167 2-Butanol i-butanol t-butanol n-butanol t-

0.552 0.557 0.548 0.558

154 166 177 183

231 221 n-

1.70

242 234 i256 243 n-hexanol n-octanol n-decanol

0.627 0.685 0.727

101.3 129.5 157.6

1.8 1.76 2.11

214 252 278

243 253 264

350

-Immersion enthalphy. J.g-1

300 250

Immersion enthalphy (J.g) . SBA-15

200 150

Immersion enthalphy (J.g) . SBA-16

100

Immersion enthalphy (J.g) . MCM-41

50

Immersion enthalphy (J.g) . MCM-48

0

1 .66 4

1 .7 6 4

1 .86 4

1 .9 6 4

2 .06 4

2 .16 4

Dipolar moment.ua

dimensions of channels and cavities of the mesoporous or microporous solids framework. The above results show some of the importance of immersion calorimetry to assess the surface properties of porous solids as the synthesized here. From them, it can be concluded that, due to the complexity of the as zeolitic systems, care has to be taken in the choice of the wetting liquid to try to separate the different possible contributions to the total energy of interaction. 4. Conclusions Cubic mesoporous silica SBA-16 and MCM-48 samples with single crystal morphologies have been synthesized and the other hand hexagonal mesoporous silica SBA-15 and MCMC-41 have been synthesized. The immersion calorimetry allows characterize the different mesoporous solids synthesized in this research in terms of its porosity. The immersion enthalpies in solvents polar and non-polar are a function of the accessible area of each of mesoporous solids. Acknowledgments

Fig. 6. Immersion enthalpy as a function of dipolar moment using polar solvents.

This shows that immersion calorimetry is a powerful technique for the characterization of mesoporous solids and as mentioned by other authors microporous solids. Several properties of the solid surface can be well assessed by properly choosing the wetting liquids. In the case of our porous materials (SBA-15, SBA-16, MCM-41 and MCM-48), immersion into non-polar or low polarity liquids with different molecular sizes can be estimation of micropore size distributions, and immersion into liquids with different polarity (n-propanol, i-propanol, 2-butanol, ibutanol, etc.) can permit the characterization of the degree of surface hydrophilicity. As demonstrated by other authors, in the case of this type mesoporous solid, the polarity of the solid surface can be assessed by measuring the interaction with selected molecules, with different dipolar moments and polarizabilities. Furthermore, immersion into non-polar liquids such as alkanes can help, together with other known techniques (XRD, etc.) to estimate the

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