Microporous and Mesoporous Materials 162 (2012) 159–167

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Microporous and Mesoporous Materials

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The effect of synthesis gel dilution on the physico-chemical properties of acidsaponite clays

Daniele Costenaro a, Giorgio Gatti a, Fabio Carniato a, Geo Paul a, Chiara Bisio a,b,⇑, Leonardo Marchese a

a Dipartimento di Scienze e Innovazione Tecnologica and Nano-SISTEMI, Interdisciplinary Centre, Università del Piemonte Orientale ‘‘A. Avogadro’’, V. Teresa Michel 11,15121 Alessandria, Italyb ISTM-CRN Istituto di Scienze e Tecnologie Molecolari, via G. Venezian 21, Milano, Italy

a r t i c l e i n f o

Article history:Received 5 April 2012Received in revised form 2 May 2012Accepted 18 June 2012Available online 27 June 2012

Keywords:ClaySaponiteNanoparticlesAcidityPhysico-chemical characterization

1387-1811/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.micromeso.2012.06.023

⇑ Corresponding author at: Dipartimento di Scienzand Nano-SISTEMI, Interdisciplinary Centre, UniversitAvogadro’’, V. Teresa Michel 11, 15121 Alessandria, Ifax: +39 0131 360250.

E-mail address: chiara.bisio@mfn.unipmn.it (C. Bis

a b s t r a c t

Acid saponite samples prepared by varying H2O/Si molar ratio of the synthesis gel are reported in thiswork aiming to tune their particles size and surface properties. In particular, H2O/Si molar ratio of 20,50, 100 and 150 were used and the obtained samples were submitted to an ion exchange procedure in0.01 M HCl solution in order to replace the interlayer Na+ ions by protons. A combined experimentalapproach (XRD, HRTEM, N2 physisorption and solid-state MAS NMR) was used to finely characterizethe synthesized materials.

FTIR spectroscopy of adsorbed probe molecules with different basicity (e.g., CO and NH3) was used tomonitor the surface acid properties (i.e. number and strength of acid sites) and to assess the acid site dis-tribution from weak to strong acidity. Structural data evidenced that samples prepared by using H2O/Siratio of 20, 50 and 110 are characterized by a layered structure typical of saponite. Moreover, the increaseof synthesis gel dilution leads to a progressive reduction of the particles sizes along with cation exchangecapacity (CEC), which depends on the gradual decrease of aluminum concentration in the tetrahedralsheets as found by 27Al MAS NMR. In addition, the presence of an amorphous phase was observed forthe saponite sample prepared with H2O/Si = 150.

IR spectroscopy of adsorbed NH3 and CO probes pointed out that the H-exchanged samples preparedwith higher synthesis gel dilution presented a reduced concentration of Brønsted acid sites, accordingto CEC data.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

In the last years, natural clays have been considered for a largenumber of applications in relation to their peculiar physico-chem-ical properties, large availability and low costs. Classical uses ofclays range from agriculture and surface coatings to environmentalpurposes (as adsorbents, ion exchangers, water decontamination).Nevertheless, in the last decades clays have been also employed asnew heterogeneous catalysts for fine chemicals production [1] andas additives for the preparation of polymer nanocomposites [2,3].More recently, clays have also been used as support for the immo-bilization of luminescent entities rendering the final compositematerials promising candidates for optical imaging [4] and opto-electronic applications [5]. Clays have been also successfullyexploited as additives for the stabilization of liquid electrolytesneeded for the preparation of organic-based photovoltaic devices

ll rights reserved.

e e Innovazione Tecnologicaà del Piemonte Orientale ‘‘A.taly. Tel.: +39 0131 360216;

io).

(dye-sensitized solar cells), achieving interesting results in termsof solar conversion efficiency and device stability [6,7].

Despite the large availability and low price of natural clays,their technological use is very often limited by the fact that,depending on their origin and genesis processes, they may havevariable chemical composition and physico-chemical properties[2]. In addition, the use of natural clays requires several extensivepurification steps due to the presence of organic and/or inorganicimpurities.

In this respect, large attention has been paid to the develop-ment of synthetic methods allowing a fine control of chemicalcomposition, particles size, aggregation and textural features ofclay materials [8].

Among the family of smectite clays, saponite was largely stud-ied due to its high specific surface area, surface acidity (wellexploitable for catalytic purposes) and thermal stability (of greatinterest for polymer science applications).

Different hydrothermal and microwave-assisted methods havebeen proposed in the literature to prepare synthetic saponite withcontrolled chemical composition and tunable physico-chemicalproperties [8–12]. Essentially, two types of synthetic approaches

160 D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167

have been proposed: in the first, a gel with stoichiometric mixtureof silica, Al and Mg salts and a source of interlayer cations (typi-cally sodium or ammonium) is treated under hydrothermal condi-tions at temperatures ranging from 493 to 723 K [13,14]. Amodification of hydrothermal methods concerned the use ofmicrowave assisted processes allowing to decrease both the syn-thesis temperature to 393 K and the preparation time [15].

A second type of synthetic approach is represented by sol–gelmethods, that are normally carried out at low temperature (around373 K) and atmospheric conditions [16–19]. However, as suggestedby Kloprogge, the major success of hydrothermal techniques to ob-tain pure products is due to the high hydrolysis rates of the octahe-dral cations precursors at high temperature, which favornucleation and growth of saponite materials [10].

Crystallization models were developed to describe the variousfactors (chemical composition, crystallization rate, pH, temperature,etc.) influencing the growth of saponite materials, [8,14] and theirmorphology, textural and surface properties. In 1993, Kloproggeand co-workers reported that different ammonium-exchanged sap-onites with variable cristallinity, platelets dimensions, specific sur-face areas and cation exchange capacities could be produced byvarying synthesis gel composition and crystallization temperature[14]. Following these works, our research group studied the phys-ico-chemical properties of synthetic saponites prepared by varyingthe relative amount of water with respect to Si content in the synthe-sis gel, which has proved to be a crucial parameter in the hydrother-mal synthesis of aluminosilicates [20]. Morphology, textural,structural and surface features of saponites prepared by usingH2O/Si molar ratio ranging from 20 to 50 were specifically investi-gated [8]. It was shown that the higher the dilution of the startinggel, the smaller the lamellae of the products obtained after crystalli-zation and the higher is the specific surface area.

These data prompted us to investigate the effect of more dilutedsynthesis gel conditions (till reaching a H2O/Si of 150) aiming atfurther reducing saponite particle size and modifying texturalparameters.

For this reason, in this work, two novel saponite solids havebeen prepared by using highly diluted synthesis gel conditions(H2O/Si ratio of 110 and 150) and their particle size and aggrega-tion, and textural properties have been compared with those of sol-ids previously prepared using H2O/Si molar ratio of 20 and 50.Although the investigation of the effect of synthesis gel dilutionon the structural, textural and morphological properties of the sol-ids is the main objective of this work, all materials presented herehave been submitted to ion exchange in mild acid conditions (HCl0.01 M solution, following the approach proposed by our group[21]) and the acid surface features of prepared materials have beensystematically investigated by using IR spectroscopy of adsorbedmolecules.

2. Experimental section

2.1. Materials preparation

2.1.1. - Synthesis of acid saponite samplesSynthetic saponite samples with a nominal cationic exchange

capacity (CEC) of 104.9 meq/100 g were prepared by modifyingthe method indicated by Kloprogge and co-workers [14] for thepreparation of ammonium-saponite. Thus, a gel with the composi-tion of 1SiO2:0.835MgO:0.056Al2O3:0.056Na2O:xH2O (x = 20, 50,110 or 150) was prepared. For that, 11.9 g of amorphous silica(SiO2 99.8%) were gradually dispersed in a solution prepared by dis-solving 0.92 g of NaOH in 20, 50, 109 and 148 mL of water for x = 20,50, 110 and 150, respectively. The obtained gel was then mixedaccurately with a Teflon rod and a mechanical stirrer. After 1 h,35.8 g of magnesium acetate tetrahydrate (Mg(CH3COO)2

⁄4H2O

99%) and aluminum isopropoxide (4.7 g Al[OCH(CH3)2]3 98 + %)were added to the remaining water (4, 10, 22 and 30 mL of H2Ofor x = 20, 50, 110 and 150, respectively). After 2 h, the gel wasintroduced in a Teflon cup (125 mL capacity) of an autoclave (AntonPAAR 4748) and heated in an oven for 72 h at 513 K.

The water amount used to prepare saponite gels was varied inrelation to the quantity of silicon within the gel: H2O/Si molar ra-tios of 20, 50, 110 and 150 were adopted.

After hydrothermal treatment, the products were filtered,washed with ca. 2 L of deionized water up to neutrality and driedin an oven for 36 h at 393 K. The so-produced material (1 g) wassubmitted to ion exchange procedure in 100 mL of saturated NaClsolution for 36 h at r.t. to replace all possible cations (i.e. hydratedaluminum and magnesium) with sodium ions in the interlayerspace. Then, the solids (hereafter named NaSAP20, NaSAP50, Na-SAP110 and NaSAP150 in relation to the H2O/Si ratio used for syn-thesis gel) were filtered and washed with deionized water until thecomplete elimination of chlorides (tested by AgNO3 solution).

Finally, these samples (1 g) were submitted to ion exchange in100 mL of acid solution (using 0.01 mol L�1 HCl) under stirringfor 36 h at r.t. to replace Na+ ions with protons. The H-exchangedsaponite samples were obtained after filtration and washing ofthe solid materials until the disappearance of chlorides. The sam-ples will be named HSAPx, where x represents the H2O/Si ratioused for the preparation of the synthesis gel.

2.2. Characterization techniques

– The chemical composition of saponite samples was determinedby inductively coupled plasma-mass spectrometry (ICP-MS)and atomic absorption spectroscopy (AAS) by ITECON labora-tory (Nizza Monferrato, Italy). The cationic exchange capacity(CEC) of the materials was determined by the method reportedin literature [22–23].

– X-ray diffractrograms (XRD) were collected on unorientedground powders with a Thermo ARL ‘XTRA-048’ diffractometerwith a Cu Ka (k = 1.54 Å) radiation. Diffractograms wererecorded at r.t. between 2� and 65� 2h deg with a step size of0.02� and a rate of 1� 2h/min.

– TEM images were collected on a JEOL 3010 High ResolutionTransmission Electron Microscope operating at 300 kV. Speci-mens were prepared by dispersing the samples by sonicationin isopropanol and by depositing few drops of the suspensionson carbon-coated grids. The microscope was equipped with anOxford Instruments INCA TEM 200 system for energy dispersiveX-ray (EDS) analysis.

– N2 physisorption measurements were carried out at 77 K in therelative pressure range from 10�6 to 1 P/P0 by using a Quanta-chrome Autosorb 1MP/TCD instrument. Before the analysisthe samples were outgassed (residual pressure p < 10�6 Torr)at 573 K for 3 h. Specific surface areas were determined byusing Brunauer–Emmett–Teller equation, in the residual pres-sure range from 0.01 to 0.1 P/P0. Pore size distributions wereobtained by applying the NLDFT method (N2 silica kernel basedon a cylindrical pore model).

– Infrared spectra were collected on a Thermo Electron CorporationFT Nicolet 5700 Spectrometer (resolution 4 cm�1). Self-support-ing pellets were placed into an IR cell with KBr windows perma-nently connected to vacuum line (residual pressure:1.33� 10�4 Pa, 1 Pa = 0.01 mbar), allowing all treatments andadsorption–desorption experiments to be carried out in situ.Before the gas adsorption tests, the saponite samples were out-gassed at 573 K with an heating ramp of 5 K/min for 3 h, using aspecial oil-free apparatus and grease-free vacuum line. The sam-ples were cooled at room temperature for the collection of IRspectra upon NH3 adsorption, while the CO adsorption

D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167 161

experiments were made at 100 K. IR spectra of the various sam-ples were then normalized taking as reference the intensity ofthe overtones and combination modes of the silica framework(bands in the 2200–1600 cm�1 range, which is not shown in theIR figures). In this way, differences of the bands intensity amongdifferent samples related to intrinsic oscillators of the materials(e.g. hydroxyl groups) can be associated to actual differences inthe amount of such species in the samples. As a consequence ofthe normalization, the absorbance values are reported as arbi-trary unit (a.u.).

– Solid state NMR spectra were acquired on a Bruker Avance III500 spectrometer and a wide bore 11.7 Tesla magnet with oper-ational frequencies for 29Si and 27Al of 99.35 and 130.33 MHz,respectively. A 4 mm triple resonance probe with MAS wasemployed in all the experiments and the samples were packedon a Zirconia rotor and spun at different MAS rate. The 27Al 1DMAS spectra have been acquired on large sweep width withsmall pulse angle (p/12) to ensure quantitative interpretation.The 27Al 2D triple quantum magic angle spinning (3QMAS)experiments have been acquired using z-filter pulse sequence[24,25]. The symmetric triple quantum excitation and conver-sion pulses of 4.8 and 1.2 ls were used, respectively. The centraltransition selective soft pulse was 25 ls with a z-filter delay of20 ls. A 2D Fourier transformation followed by a shearingtransformation gave pure adsorption mode 2D map where theF1 axis gives isotropic spectra while the quantum filteredMAS spectra on the F2 axis. All the 29Si MAS NMR spectra wererecorded under high-power proton decoupling conditions. Therelaxation delays, d1, between accumulations were 120 and1 s for 29Si and 27Al NMR, respectively. All chemical shifts arereported using d scale and are externally referenced to TMS at0 ppm. All NMR spectra were fitted with DMFIT functions forquantitative deconvolution of overlapping peaks [26].

3. Results and discussion

3.1. Structural, morphological and textural properties of acid saponitenanoparticles

The chemical composition (expressed as wt.%) of saponite sam-ples after ion exchange in NaCl solution is reported in Table 1.

NaSAP20 shows a Na content of 0.50 mmol/g (3.1 wt.% of Na2O),lower than the amount used in the synthesis gel (ca. 1.0 mmol/g ofNa), which can be ascribed to the lower cation exchange capacity(CEC) of this synthetic solid (73.0 meq/100 g, as determined exper-imentally by using the method reported in the literature [8]) withrespect to the theoretical value (104 meq/100 g) [8]. The aluminumconcentration of NaSAP20 is 1.1 mmol/g (11.3 wt.% of Al2O3), inagreement with data reported in the literature for parent saponites[8]. The amount of Na ions in NaSAP50, NaSAP110 and NaSAP150(0.11, 0.39 and 0.31, respectively) is significantly lower than thatof NaSAP20 sample. Nevertheless, the variability in Na+ contentmight be also associated to the presence of interlayer Mg2+ cations.

Table 1CEC (meq/100 g) and chemical composition of synthetic saponite samples exchangedin NaCl solutions (NaSAPx). Values are reported as wt.% of oxides in dry sample. SiO2

wt.% has been calculated to reach 100 wt.%.

Na2O Al2O3 MgO SiO2 CEC (meq/100 g)

NaSAP20 3.1 11.3 27.0 58.6 73NaSAP50 6.8 14.3 26.9 52.0 34NaSAP110 2.4 13.9 28.0 55.7 29NaSAP150 1.9 14.5 30.1 53.5 33

A significant increase of Al ions is observed passing from Na-SAP20 (1.1 mmol/g, 11.3 wt.% of Al2O3) to samples prepared withhigher H2O/Si molar ratio (Al content close to ca. 1.4 mmol/g,14 wt.% of Al2O3), suggesting that the chemical composition ofthe final solids is influenced by the adopted synthetic conditions.

The ion exchange procedure in 0.01 mol.L�1 HCl solution, car-ried out to prepare HSAPx samples, results in a significant decreaseof Na content (passing from 0.50, 0.11, 0.39 and 0.31 mmol/g forNaSAP20, NaSAP50, NaSAP110 and NaSAP1150, respectively to0.04–0.06 mmol/g for all HSAP samples), thus indicating that alarge part of Na ions are replaced by protons. This treatment doesnot alter the amount of aluminum and magnesium in saponitesamples, which remains similar to that of Na-parent samples, thusconfirming the efficacy of the adopted procedure (i.e. the structureis retained after exchanging the samples in the acid solution used).

As a result of the increase of H2O/Si molar ratio in the synthesisgel, a significant decrease of the cation exchange capacity (CEC) isobserved passing form 73 meq/100 g for NaSAP20 to 34 for Na-SAP50. Samples prepared with higher H2O/Si molar ratio (Na-SAP110 and NaSAP150) showed CEC similar to that observed forNaSAP50, with values of 29 and 33 meq/100 g for NaSAP110 andNaSAP150 samples, respectively.

The structural properties of saponite samples exchanged in acidconditions have been studied by XRD and the obtained results arereported in Fig. 1. According to the data observed in previousworks, [8] the H2O/Si ratio of the synthesis gel influences theXRD profile of the samples. (001), (110) and (060) reflections at8.1�, 19.4� and 60.6� 2h, respectively are typical of saponite struc-ture, [27,28] and their width and intensity are sensitive to the com-position of synthesis gel. In particular, the basal (001) reflection isintense and almost resolved for HSAP20 thus suggesting a regularorganization of the saponite layers in this sample (Fig. 1, curve a);the (001) reflection decreases in intensity and become broader inthe case of HSAP50, HSAP110 and HSAP150 and this suggests thatthe structural order along the C-axis is reduced for these samples.As a matter of fact, the dilution of synthesis gels favors theformation of saponite materials with a more disordered lamellaestacking.

The morphology of all saponite samples prepared with differentH2O/Si ratio has been investigated by transmission electronmicroscopy (TEM, Fig. 2). As a general feature, all samples are char-acterized by a sheet-like morphology. Nevertheless, the dilution ofsynthesis gel has a strong effect on the saponite particles size.While HSAP20 shows crystals with dimensions over 200 nm(Fig. 2A), the samples prepared with higher H2O/Si molar ratioare characterized by particles whose dimensions are around

Fig. 1. XRD patterns of HSAP20 (curve a), HSAP50 (b), HSAP110 (c) and HSAP150 (d)samples.

Fig. 2. TEM micrographs of HSAP20 (A), HSAP50 (B), HSAP110 (C) and HSAP150 (D) samples.

162 D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167

100 nm for HSAP50 (Fig. 2B) and below 50 nm for HSAP110(Fig. 2C) and HSAP150 (Fig. 2D) samples. In addition, TEM micro-graphs of HSAP150 sample evidence that the material is not totallycomposed by layered phases. Regions with not well defined mor-phology and of amorphous origin, as determined by selected areaelectron diffraction (SAED), are indeed observed.

Interestingly, for samples with smaller particle dimensions,lamellae appear randomly organized, forming tactoids composedby a reduced number of platelets with respect to HSAP20.

N2 physisorption analyses have been carried out to verify howthe particles size influences the textural features of saponite mate-rials. Fig. 3 shows the physisorption isotherms (Frame A) and poresize distribution (Frame B) for all samples. The specific surface areavalues are reported in Table 2.

Fig. 3. N2 adsorption–desorption isotherms at 77 K (A) and pore size distribution deterHSAP150 (-}-) samples.

The adsorption isotherms for all samples are of type IV based onthe IUPAC classification, indicating the presence of both micro- andmesoporosity in the materials (Fig. 3A). This has been already ob-served in the literature for parent layered solids [29]. Moreover,while HSAP20 shows a hysteresis loop of H4 type, [30] that is anindication of the presence of slit-pores, the isotherms of samplesprepared with higher H2O/Si molar ratios are characterized byloops of H3 type, with larger N2 adsorption at high P/P0 values, thusindicating the formation of aggregates of plate-like particles givingorigin to mesopores formation. This reflects the presence of disor-dered and smaller aggregates for samples prepared with highersynthesis gel dilution (from H2O/Si ratio 50 to 150).

BET algorithm has been used to estimate the specific surfacearea (SSA) of the solids: HSAP20 shows a SSA of 216 m2/g, whereas

mined by NLDFT methods (B) of HSAP20 (-s-), HSAP50 (-d-), HSAP110 (-N-) and

Table 2Specific surface area of acid saponite samples.

Samples Specific Surface Area (m2/g)

HSAP20 216HSAP50 339HSAP110 369HSAP150 314

Fig. 4. 27Al MAS NMR of HSAP20 (A), HSAP50 (B), HSAP110 (C) and HSAP150 (D)samples. Each spectra includes experimental (solid line curve) and its deconvolutedspectrum (dashed line) with individual contribution from each 27Al sites.

Fig. 5. 27Al 2D 3QMAS NMR of HSAP20, HSAP50, HSAP110 and HSAP150 samples.

D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167 163

a significant increase of the surface area is observed for HSAP50and HSAP110 samples, characterized by a SSA of 339 and369 m2/g, respectively. This effect should be ascribed to the pres-ence of lamellae with sizes smaller than the HSAP20 sample (ingood agreement with TEM observations), that give origin to smalltactoids spatially distributed in a random way.

Nevertheless, in the case of HSAP150, which shows particlessize close to 50 nm (see TEM analysis above), the specific surfacearea is lower in comparison to HSAP50 and HSAP110 materials(SSA = 314 m2/g), probably due to the presence of amorphousphases (vide infra) with lower surface area.

The pore size distribution of all samples has been also deter-mined by NLDFT model using the desorption branch of the iso-therms (Fig. 3B). While HSAP20 shows a narrow pore sizedistribution, with maximum centred at 54 Å, HSAP50 and HSAP110samples are characterized by a broader pore size distribution withdiameter values in the range 50–70 Å (Fig. 3B). This means that thesmall platelets of these saponites are randomly packed in a largervariety of porous aggregates than HSAP20.

HSAP150 sample deserves some additional comment: a com-pletely different pores size distribution, characterized by the pres-ence of two main maxima centred at 52 and ca. 90 Å is observed forthis sample (Fig. 3B). This feature should be related to the hetero-geneity of HSAP150 sample which contains both a crystalline part,typical of the saponite, and an amorphous phase probably formedbecause the high gel dilution used to prepare this solid does not al-low a complete crystallization.

Additional information on the structural properties of saponitesamples have been obtained by using solid-state NMR. 27Al MASNMR investigation (Fig. 4) shows different contributions in therange 45–75 and �10–15 ppm, ascribed to coordinated aluminumin the tetrahedral (AlIV) and octahedral (AlVI) sheets of the saponiteframework, respectively [21].

For all samples, the band due to octahedrally coordinated Al isbroad and characterized by a shoulder associated to a second orderquadrupolar line shape, due to the disorder in the aluminum sites,leading to a distribution in the isotropic chemical shift and quadru-polar interaction. Concerning tetrahedral Al, all samples exhibittwo components: the first in the 75 ppm range (Fig. 4, component(a)) typical of saponite structure (AlIVa) and the second at ca.60 ppm (Fig. 4, component (b)), which can be ascribed to the pres-ence of tetrahedral sites with different chemical environment(AlIVb) [9].

Nevertheless, some differences in the abundance of tetrahedraland octahedral Al sites and in their chemical nature occur passingfrom HSAP20 to HSAP150 samples. In this sense, 27Al 2D 3QMASNMR helps us to estimate the quadrupolar interaction parametersand the isotropic chemical shifts for the different Al sites as a func-tion of the synthesis gel dilution.

27Al 2D 3QMAS NMR spectra of the four samples are shown inFig. 5. HSAP20 shows two resolved distinct regions correspondingup to two partially overlapping tetrahedral and octahedral coordi-nated aluminum species. The isotropic chemical shifts, diso, and thequadrupolar coupling constant, CQ, of these signals, obtained fromdeconvolution of the 1D MAS spectra, are given in Table 3. Gener-ally, the asymmetry parameters cover a range of 0 6 g 6 1. Thetrue isotropic chemical shifts of the tetrahedral coordinated

aluminum species (AlIVa and AlIVb), derived from 3QMAS NMRexperiments, are at 68 and 58 ppm, respectively [31]. From the2D NMR data, it can be seen that there are two different octahedralcoordinated aluminum species (named AlVIa and AlVIb) that are at13 and 8 ppm, respectively. The peak at 13 ppm is assigned towell-ordered octahedral Al species and the broad peak at 8 ppmis probably due to distorted Al sites [8]. The quadrupolar couplingconstant also vary between 3–5 MHz for all the Al sites. In all cases,we observe Gaussian broadening for those undergoing larger quad-rupolar couplings (due to distribution of isotropic chemical shift)

Table 3Isotropic chemical shifts, diso, quadrupolar coupling constant, CQ and concentration oftetrahedral and octahedral 27Al sites in saponite samples. ⁄Values reported in the lastcolumn (AlIV/AlVI) are calculated with the following equation (AlIVa + AlIVb)/(AlVIa + AlVIb).

AlIVa AlIVb AlVIa AlVIb AlIV/ AlVI⁄

HSAP20 diso/ppm 71 69 13 8CQ/MHz 3.2 5.1 4.5 3.0% 36 33 23 8 2.2

HSAP50 diso/ppm 74 71 12 12CQ/MHz 4.1 5.6 4.5 1.4% 35 27 34 4 1.6

HSAP110 diso/ppm 75 71 12 12CQ/MHz 4.2 5.6 4.1 1.2% 27 32 38 3 1.4

HSAP150 diso/ppm 76 72 11 /CQ/MHz 4.3 5.7 4.0 /% 21 35 44 / 1.3

Fig. 6. 29Si MAS NMR of HSAP20 (A), HSAP50 (B), HSAP110 (C) and HSAP150 (D)samples. Each spectra includes experimental (solid line curve) and its deconvolutedspectrum (dashed line) with individual contribution from each 29Si sites.

164 D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167

and a Lorentzian broadening for the sites undergoing the smallerquadrupolar couplings.

The most striking feature of the 27Al 2D 3QMAS NMR experi-ments is the gradual decrease (passing from HSAP20 to HSAP150)in the AlIV/AlVI integral ratio (data expressed as relative percentagein Table 3). The AlIV/AlVI ratio decreases from 2.2 to 1.3, passingfrom HSAP20 to HSAP150 supporting the fact that the synthesisgel dilution affects the distribution of aluminum sites in the tetra-hedral and octahedral sheets of the saponite samples. Quantitativeanalysis shows that an increase of the aluminum ions that prefer-ably occupy octahedral sites by replacing six coordinated Mg is ob-served by increasing the H2O/Si ratio, in agreement with the lowcation exchange capacity data obtained for HSAP50, HSAP110and HSAP150 samples.

The local environment of Si atoms in saponite samples aredetermined by 29Si MAS NMR spectra and are shown in Fig. 6, to-gether with the corresponding fitted spectra and their individualcomponents from each silicon site. All samples show resonancesat around �96, �92 and �86 ppm due to [Q3Si(0Al)], [Q3Si(1Al)]and [Q3Si(2Al)] species, respectively. In addition, a resonance ataround �98 ppm is also visible for samples except HSAP20. Therelative intensity of this peak increased up to 25% in HSAP150and from the chemical shift point of view can be assigned to[Q3Si(0Al)] with different environment with respect to the signalat �96 ppm. Bands in the range 100–110 ppm are also observedfor all samples and are assigned to the presence of amorphous sil-ica (Q4Si). This band was mainly evident in HSAP150 sample(around 20% of the total amount of Si sites) thus supporting thefact that the high dilution of the synthesis gel does not allow toachieve a complete crystallization of saponite sample under thehydrothermal conditions used. Thus, 29Si NMR data helped to clar-ify the nature of non-layered aggregates that are observed by TEMfor HSAP150 data and to support the results coming from N2 phys-isorption analysis, suggesting a different textural properties forthis specific sample.

Fig. 7. FTIR spectra in the mOH region of saponite samples outgassed at 573 K for3 h: (a) HSAP20, (b) HSAP50, (c) HSAP110 and (d) HSAP150 samples.

3.2. Surface acid properties of saponite materials

Fig. 7 shows the IR spectra of saponite samples in the 3800–3000 cm�1 region after thermal treatment at 573 K. As a generalfeature, the IR spectra of all samples are characterized by threemain components located at ca. 3740, 3675 and 3595 cm�1. Theband at 3740 cm�1 is due to the stretching mode of OH speciesbelonging to isolated Si-OH groups located on the surface of sapo-nite lamellae. The signal at 3675 cm�1 is associated to the OHstretching mode of structural Mg(OH)2 groups present in the octa-hedral layers of saponite, [32] and finally the signal at 3595 cm�1 is

due to the OH stretching vibration mode of SiO(H)Al groups gener-ated by the substitution of Si(IV) with Al(III) ions in the tetrahedralsheets of saponite structure [33].

In addition, the spectra show a broad absorption in the 3550–3000 cm�1 region associated to surface Si–OH species in interac-tion each other through the H-bonding (i.e. H-bond donor silanolsas described in Ref. [34]). Interestingly, the intensity of such broadband is higher in HSAP150 with respect to HSAP20. In HSAP150sample, H-bond donor SiOH species are especially abundant prob-ably because of the presence of amorphous silica formed duringthe saponite growth (vide supra, XRD, TEM and SS-NMR analyses).

The characterization of surface acid sites was made by FTIRspectroscopy of adsorbed probe molecules (i.e. NH3 and CO). TheNH3 was used as a probe molecule to quantify the Lewis and

Table 4Concentration of Brønsted acid sites in saponitesamples.

Sample [H+] mmol/g sample

HSAP20 0.32HSAP50 0.20HSAP110 0.22HSAP150 0.21

D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167 165

Brønsted acidity in the different saponite samples, while the COwas used to study distribution and strength of acid sites.

Fig. 8 shows the IR spectra collected after admission of100 mbar of NH3 in the cell and subsequent evacuation a r.t. for180 min. Only IR spectra related to NH3 irreversibly adsorbed onsaponite samples are reported in this work, because the completeprocess of NH3 adsorption/desorption was already described in theliterature [21].

As a general feature, the IR spectra of all saponite materialsshow a series of bands with maxima at 1445, 1505, 1565 (morepronounced in HSAP150 (curve d)), 1618 and 1680 cm�1 (the lattermore pronounced in HSAP20 (curve a) and HSAP110 (curve (c)).

The intense band at 1445 cm�1, with an evident shoulder at1505 cm�1, is due to the asymmetric bending mode of ammoniumions [21,35] formed through interaction of basic NH3 withBrønsted acid sites of saponite solids. The formation of this bandis a clear-cut indication of the presence of SiO(H)Al acid sites (i.e.proton on oxygen sharing silicon and aluminum), along with Al-OH on the saponite surface. The complexity of this band suggeststhat ammonium ions experience several coordination sites on thesaponite surface where they have different local symmetry [36].

The band at 1618 cm�1 is related to the bending mode ofammonia adsorbed on Lewis acid sites present at the surface ofsaponite samples. Finally, the broad band at 1565 cm�1 is due toSi–NH2 species formed by the interaction of ammonia with dis-torted siloxane bridges produced by condensation of vicinal sila-nols during the thermal activation of the sample [37,38].

The total concentration of Brønsted acid sites (Table 4) can beestimated from the IR data of adsorbed NH3 using the extinctioncoefficient of the 1445 cm�1 band reported in the literature [35].

The sample HSAP20 contains the highest amount of Brønstedacid sites (0.32 mmol H+/g) whereas the samples prepared byincreasing the synthesis gel dilution are characterized by a similaramount of Brønsted acid sites, that is around half of the valuedetermined for HSAP20 sample. These data are in rough agreementwith cationic exchange capacity of the synthetic clays. Indeed,samples prepared by using H2O/Si of 50, 110 and 150 have similarCEC (ca. 30 meq/100 g). The CEC of HSAP20 is around 73 meq/100 g, and this explains the high content of Brønsted acid sites ofthis sample with respect to the other saponite solids. These resultscan be correlated to the 27Al NMR data that showed for the sampleprepared with high H2O/Si ratio a marked reduction of the tetrahe-dral aluminum sites, which are responsible of the Brønsted acidityin SiO(H)Al species.

Fig. 8. FTIR spectra, in the 1800–1300 cm�1 region, of NH3 (100 mbar) adsorbedand subsequently evacuated a r.t. on (a) HSAP20, (b) HSAP50, (c) HSAP110 and (d)HSAP150 materials outgassed at 573 K. Spectra are reported after subtraction ofbare samples, before ammonia adsorption, that were used as a reference.

CO adsorption at 100 K was employed to have a scale of theacidity of Brønsted acid sites of saponite samples [39,40].

Fig. 9 shows the IR spectra collected upon CO adsorption at100 K on HSAP20 (Frames A and A’), HSAP50 (Frames B and B’),HSAP110 (Frames C and C’) and HSAP150 (Frames D and D’)samples.

CO admission on HSAP20 sample (Fig. 9 A) results in the conver-sion of the original bands due to surface OH groups at 3742 and3595 cm�1 (appearing as negative bands in the spectrum) intobroader adsorption located at 3630 (DmOH � 110 cm�1) and3250 cm�1 (DmOH � 340 cm�1), respectively. Moreover, a largeabsorption in the 3500–3200 cm�1 range is also observed. Theband at 3630 cm�1 is due to the surface SiOH groups interacting,through H-bonding, with CO probe molecule. As already observed,the shift of 110 cm�1 of the OH vibration after CO interaction indi-cates that the saponite SiOH groups have higher acidity withrespect to silanols in amorphous silica [41].

The shift of ca. 340 cm�1 for the Si(OH)Al species indicates thatthe acidity of these Brønsted sites in saponite is comparable withthose of strong acid zeolites [40].

The broad band at around 3400 cm�1 formed upon CO adsorp-tion should be related to the presence of extra-framework Al–OHspecies with medium acidity (generally absorbing at ca.3650 cm�1) [21,35,42].

In the mCO stretching region (Fig. 9 A’), the spectrum of HSAP20after CO admission (100 mbar) at 100 K is dominated by threemain bands at 2178, 2160 and 2135 cm�1. The band at2178 cm�1 is due to the stretching mode of CO in interaction withBrønsted Si(OH)Al acid species, [20] while the band at 2160 cm�1 isdue to the CO adsorption on silanol groups and the band at2135 cm�1 is assigned to CO in liquid-like state condensed in theporosities of HSAP20 sample. By decreasing the CO pressure, allabsorptions progressively decrease in intensity, along with thebands in the OH stretching region. At relatively low-pressure(p < 1 mbar) bands at 2178, 2160 and 2135 cm�1 are strongly de-pleted while a wide band in the range 2200–2190 cm�1 becomesmore evident. This band can be ascribed to the to the presence ofMg2+ ions and Al3+ ions located in the interlamellar space of thesaponite sample.

In general, the adsorption of CO at 100 K on HSAP50, HSAP110,and HSAP150 samples are characterized by the presence of thesame bands already discussed for HSAP20 sample. Nevertheless,some differences in the IR spectra of the samples prepared withhigh H2O/Si ratio occur and deserve some comments. Interestingly,a progressive decrease of the bands at ca 3400 and 3250 cm�1 as-signed to CO interacting with Al–OH and Si(OH)Al acid species isobserved passing from HSAP20 to HSAP150 solids (Fig. 9). More-over, in the mCO stretching region, a new band at 2188 cm�1, whichbecome more evident at low CO pressures, is visible for HSAP50,HSAP110 and HSAP150 samples. The position and its higher stabil-ity to the outgassing procedure with respect to the band of CO ininteraction with Si(OH)Al sites at ca. 2180 cm�1 strongly supportthe assignment of this band to the presence of cations (i.e. Al orMg) in defect positions of saponite materials. This is in generalagreement with the fact that the increase of H2O/Si ratio leads toa reduction of platelets size of saponite materials, which displaymore defective sites, as clearly demonstrated in this work.

Fig. 9. IR spectra of CO adsorbed at 100 K on HSAP20 (A and A’), HSAP50 (B and B’), HSAP110 (C and C’) and HSAP150 (D and D’). The arrows indicate a decreasing of COpressure. Spectra are reported after subtraction of the spectrum of the bare sample (before CO interaction) used as a background.

166 D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167

4. Conclusions

Saponite samples were prepared by hydrothermal procedureusing different water/silicon ratio in the synthesis gel. Textural,adsorption properties and surface acidity of these materials werestudied by a combination of different experimental techniques(XRD, TEM, N2 physisorption measurements, SS-MAS NMR, FTIRspectroscopy of adsorbed NH3 and CO).

XRD profiles indicated that all samples showed a lamellar struc-ture, nevertheless the crystallinity of the samples was influencedby the synthesis gel dilution, in particular passing from HSAP20to HSAP150 sample. The differences observed in the XRD patternscollected for all samples were correlated with a decrease of the

lamellae size, according to TEM images. Indeed, it was shown thatby increasing the synthesis gel dilution the dimensions and spatialorganization of saponite platelets are modified. HSAP20 material iscomposed by lamellae with dimension of ca. 200 nm, denselypacked, whereas HSAP50 sample displayed smaller particles in tac-toids randomly distributed. A further decrease of lamellae dimen-sions was observed for HSAP110 sample, showing particles of ca.50 nm.

The textural properties of prepared samples are related to thedilution of synthesis gel, the specific surface area passing from216 m2/g for HSAP20 to 369 m2/g for HSAP110.

The HSAP150 sample contained a significant fraction of amor-phous silica, as indicated by TEM, SS-NMR and FTIR spectroscopy

D. Costenaro et al. / Microporous and Mesoporous Materials 162 (2012) 159–167 167

probably due to the fact that the high synthesis gel dilution used inthis case does not allow to reach a complete crystallization of thesaponite structure.

Moreover, SS-MNR data clearly demonstrated that the increaseof synthesis gel dilution leads to a reduction of the AlIV/AlVI ratiobetween aluminum in octahedral sheet and aluminum in the tetra-hedral sheets of the solids, in agreement with the reduced cationicexchange capacity of these synthetic clays. FTIR data of NH3

adsorption confirmed the presence of higher concentration ofBrønsted acid sites in HSAP20 sample, whereas the use of CO asprobe molecule allowed to make a scale of acidity of surface acidsites present in the saponite samples prepared with differentH2O/Si ratio.

Acknowledgments

The authors acknowledge the European research Project ‘‘INNO-VASOL’’, No. FP7-ENERGY-NMP-2008-1 ‘‘Innovative Materials forFuture Generation Excitonic Solar Cells’’ for the financial support.G. P., F.C. and G.G. acknowledge Regione Piemonte for a post-docfellowship. The financial support from ‘‘Fondazione Compagnia diSan Paolo’’ for the acquisition of Bruker Avance III 500 SS-NMRspectrometer is gratefully acknowledged. The authors thank Dr.Luca Bertinetti and Federica Franconieri (University of Turin) andJames Bendall (University of Cambridge) for HRTEM micrographsanalyses.

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