Distinctive Spectral and Microscopic Features for Characterizing the

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Distinctive Spectral and Microscopic Features for Characterizing the Three-Dimensional Local Aluminosilicate Structure of Perlites Maria Roulia, Thomas Michael Mavromoustakos, Alexandros A. Vassiliadis, and Gregor Mali J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 14 Oct 2014 Downloaded from http://pubs.acs.org on October 15, 2014

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The Journal of Physical Chemistry

Distinctive Spectral and Microscopic Features for Characterizing the Three-Dimensional Local Aluminosilicate Structure of Perlites

Maria Roulia,*,1 Thomas Mavromoustakos,2 Alexandros A. Vassiliadis,3 and Gregor Mali4,5

1

Inorganic Chemistry Laboratory, Department of Chemistry, University of Athens, Panepistimiopolis, 157 71 Athens, Greece

2

Organic Chemistry Laboratory, Department of Chemistry, University of Athens, Panepistimiopolis, 157 71 Athens, Greece

3

Dyeing and Finishing Laboratory, Department of Textile Engineering, Technological Education Institute of Piraeus, 250 Thivon st., 122 41 Athens, Greece 4

Laboratory for Inorganic Chemistry and Technology, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia

5

EN-FIST Centre of Excellence, Dunajska 156, SI-1000 Ljubljana, Slovenia

_____________________________________________________________________

*Corresponding author. Tel: +30 210 7274780; fax: +30 210 7274782 E-mail address: [email protected] (M. Roulia).

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ACS Paragon Plus Environment


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Distinctive Spectral and Microscopic Features for Characterizing the Three-Dimensional Local Aluminosilicate Structure of Perlites

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ABSTRACT: Expanded perlite and raw perlites of various origins were examined using solid-state NMR and FTIR, as well as scanning electron microscopy, in an attempt to correlate their expansion properties with the characteristics of aluminosilicate framework. The

27

Al MAS NMR spectra indicated that aluminum

atoms in perlite are mostly tetrahedrally coordinated in Q4 structures not participating in the expansion process; two different octahedral Al species were also identified in perlite. Tetrahedra–octahedra transformations may include perlite into metastable materials. The 29Si MAS NMR and 1H-29Si CPMAS NMR spectra showed that Q3 and Q4 units are mostly abundant, a fact that explains the low cation exchange capacity of perlites. Deconvoluted FTIR and 29Si MAS NMR spectra are consistent regarding the relative Qn intensities which, together with the network geometry, determine the expansibility of raw perlites. The calculated variations of Si–O–Si angle are more significant than those of the Si–O bond length, remarkably affecting the structure of the aluminosilicate network. Thus, perlite samples of increasingly fragmentary framework exhibit the highest expansion ratios. SEM micrographs monitored the expansion process differentiating between the low- and high-expansibility perlites.

Keywords: Expanded perlite; Solid-state NMR;

27

Al MAS NMR;

29

Si MAS NMR;

FTIR; Expansion; SEM

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INTRODUCTION Among other glassy igneous aluminosilicates, perlite (shaped as lava cools and immediately hardens leaving no time for crystals to form and for water to escape) meets a wide range of applications,1–4 thanks to its amorphous, chemically inert, low sound-transmitting, fire-retarding nature and the trapped, encapsulated water.5 By rapidly heating6 at 970–1470 K the perlite ore softens and the entrapped water vaporizes. The resulting white, granular, frothy, hydrophilic, environmentally safe expanded perlite (of four- to twentyfold volume compared with the raw material) does not deteriorate, is characterized by low density (30–150 kg m–3), a neutral pH, sterility and increased porosity. Surface broken openings and tiny cavities, as well as clusters of minute, trapped glass bubbles and closed air cells, are responsible for water and air holding capacity offering an extensive surface area.7 Emulsifier-modified4 expanded perlite has been used for sorption of oil spills to assist environmental issues. United States and Greece are the world’s largest perlite producers. Perlite has been used as a plant growing medium,8–10 to evaluate organic nutrient sources11and to determine cadmium microlocalization,12 for the acclimatization of embryos-derived plants,13 in soil to trap moisture, to maintain the air–water balance and to reduce temperature fluctuations;7 also, to adsorb dyes14 and metals,15,16 as a nanosized iron oxide-coated adsorbent,17 a composite adsorbent for arsenic removal18 and a chitosancoated biosorbent.19 Perlite-based fungus-inoculated biofilters20 and ceramic supports for microfiltration membranes21 have been tested. Catalytically-obtained adsorbed species have been developed onto aluminosilicates;22–24 similarly, a catalytically active zeolite has been prepared from perlite,25 perlite-immobilized titanium dioxide nanoparticles have been photocatalytically employed26 and phosphate-accumulating bacteria on perlite carrier have been successful in phosphorus removal.27 Shape-

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stabilized phase change energy-storage composites of expanded perlite have been prepared.28,29 Perlite has been employed to regulate the delivery of cutaneous actives through the skin30 and as a useful in tissue engineering compound usable in bone substitution.31 As perlite possesses pozzolanic properties, it affects the microstructure and durability of mortars.32 Lithium admixtures into perlite-contained concrete inhibit alkali-aggregated expansion,33 perlite pellets are useful in tobermorite crystallization34 and surface-vitrified perlite microspheres have been prepared to obtain controlled particle size distributions.35 Also, perlite has been reinforced by a glassy–ceramic surface coating,36 polyaniline–perlite composites are electrically conductive37 and polylactide–perlite composites exhibit improved mechanical and thermal properties.38 Chemistry and microstructure of perlite are both essential for the correlation of structural features with the expansion process. 29Si- and 27Al NMR have been used to identify structural configurations and aluminum atoms of different coordinations in clays.39–41 On the basis of their expansion characteristics,5 the molecular fingerprint derived from FTIR could predict the utilization and applicability of perlites while SEM can bring new dimensions to the understanding of expansion. Many papers have been published on the properties and applications of perlite,42–44 just a few are concerned with interrelations between its microscopic and spectral features, but none is focused on the use of MAS NMR and FTIR to study the structure of perlites or comparable volcanic glasses. Hence, questions related to differences in local structure of perlite, specifically associated to its expansion, remain open. In the present work, the nanostructural, morphological and textural properties of perlites are correlated to systematic framework characterization. MAS NMR results combined with data from both FTIR spectroscopy and microscopic observation yield information on the aluminosilicate backbone leading to a better understanding of

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perlite under expansion conditions. SEM is applied to record real-time information of the heating dehydration process, to illustrate the morphology and dynamic behavior of perlites during expansion and to clarify their interactions with the escaping water.

MATERIALS AND METHODS Sample Preparation. Perlite samples of different origins, i.e., Tsigrado (GRs), Trachilas (GRr) and Provatas (GRp) regions (Milos island, Greece), Italy (IT), Hungary (HU), Turkey (TU) and China (CH), were supplied by Silver and Baryte Ores S.A., Greece. The chemical compositions of both raw and expanded (EX) perlites, determined from EDXRF measurements, have been presented elsewhere.5 Prior to NMR and FTIR experiments, all samples were crushed to pass a 25-µm sieve. Sample Characterization. Solid-state 1H-,

27

Al- and

29

Si NMR spectra were

recorded on a 600 MHz Varian NMR spectrometer with a 3.2-mm Varian MAS probehead. The rotation frequency used was 16 kHz for 1H and 27Al and 10 kHz for 29

Si. The numbers of transients used in the experiments were 16, 3000, and 2000, the

repetition delays were 5 s, 1 s, and 30 s, and the excitation pulses were 2.2 us (90°), 0.5 us (10°), and 3.2 us (45°) for 1H-, 27Al-, and 29Si nuclei, respectively. The spectra were recorded using standard single pulse experiments. Chemical shifts are reported relative to tetramethylsilane for 1H and 29Si, and relative to 1M solution of AlCl3 for 27

Al. 1

H-29Si cross-polarization (CP) magic-angle spinning (MAS) experiments

employed RAMP during CP block and high-power continuous-wave heteronuclear decoupling during acquisition. Duration of the CP block, acquisition time, repetition delay, and number of repetitions were 0.8 ms, 15 ms, 5 s and 45000, respectively. CP

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block was short as 1H relaxation under spin-lock conditions was very fast (T1ρ was on the order of several hundred microseconds). FTIR spectra (average of 50 scans) were recorded at room temperature with the use of a Shimadzu IR Affinity-1 spectrophotometer; pellets of the specimens were prepared after mixing with dry KBr. To monitor the morphological and textural differences of perlites thermally treated under expansion-simulating conditions (1470 K), perlite was placed in a Pt crucible and processed in a preheated programmable laboratory furnace with a Shimaden FP 21 Programmable controller. On industrial scale, the expansion of perlite in vertical furnaces requires less than 3 s after entering the high-temperature area. The temperature of the grains should lie within the softening range (1070–1270 K) increasing the plasticity of the material to achieve maximum expansion. If perlite remains longer or at higher temperatures the grains melt and the foamy structure of expanded perlite collapses. Calculations of the most influential parameters, as the particle temperature and viscosity, have been reported.45 Our procedure, being quite different from the industrial one, requires higher temperatures but, on the other hand, offers the advantage of reproducible results and best control of the expansionaffecting parameters, i.e., time and temperature. In these experiments, treatment times varied from 5 to 20 s. Micrographs of gold-coated heat-modified perlite samples were acquired using a Jeol JSM-5600 scanning electron microscope.

RESULTS AND DISCUSSION

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Al MAS NMR Spectroscopy. The

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Al MAS NMR spectra acquired (see

Figure 1) highlight the presence of both tetrahedral and octahedral aluminum in the perlite samples. The main resonance peak centered at around 52.8 ppm corresponds to 4-coordinated aluminum atoms46 and the peak at about 13.8 ppm in Figure 1 is attributed to the octahedral coordination of perlitic aluminum.

EX GRs GRr GRp HU IT TU CH

100

80

60

40

20

0

-20

Chemical shift (ppm)

Figure 1. 27Al MAS NMR spectra of perlites.

For samples IT, TU and CH (see Table 1) the 13.8-ppm peak is accompanied by another signal at 4.2, 2.8 and 2.7 ppm, respectively, suggesting a heterogeneity of Al(VI) sites in perlite related to distorted geometries. These are due to different OH orientations near the aluminum sites,47 to extraframework species48 or a substitution of atoms in the octrahedra.49 In general, the next-nearest-neighbor of the Al atom is affecting the chemical shift; varying next-nearest-neighbor atoms and, also, the number of hydroxyl groups in the Al(VI) coordination environments could result in 8


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two different octahedral geometries and this can explain why the 13-ppm Al peak is a bit shifted in the positive direction compared with most octahedral Al signals. This shift may suggest a distorted octahedron slightly resembling a five-coordinated geometry, i.e., one of the ligands might be at a larger distance from Al than the others, yielding an aluminum environment somewhere between 5- and 6-coordinated. Both intensities (of ~13.8 and ~3.5 ppm) in Table 1 correspond to the total Al(VI) content of perlite.49 The Al resonances are quite broad, as expected, as

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Al nuclei have a

nuclear spin of 5/2 and are subjected to a quadrupole interaction, resulting from the coupling between their quadrupole moment and electric field gradients.

Table 1. Al(IV) and Al(VI) peaks and their relative peak areas from spectroscopy δ (ppm) Perlite Al(IV) EX 52.2 GRs 53.1 GRr 52.5 GRp 53.0 HU 52.8 IT 53.0 TU 52.8 CH 53.4 a Total Al(VI) peak area.

Al(VI)1 13.7 13.4 13.4 13.7 13.4 14.2 13.6 14.0

Al(VI)2 4.2 2.8 2.7

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Al MAS NMR

Al(IV)/Al(VI)a peak area ratio 29.9 45.7 67.0 63.2 57.7 22.3 31.2 11.9

From Figure 1 and the relative areas of Al(IV) and Al(VI) peaks (see Table 1) it is concluded that most of aluminum atoms afford a tetrahedral geometry, a feature also observed in open-framework aluminosilicates, e.g., zeolites,47,50 explaining the feasibility of the zeolitization process undergone by perlite.51 This coordination of Al atoms is a property distinguishing perlites from phyllosilicate clays, where aluminum atoms are mostly octahedrally coordinated. On the other hand, as Al atoms are mainly forming tetrahedral networks in perlite, Al is structurally conforming to Si (contained in larger quantities and solely as

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tetrahedra), a pattern also observed in the tetrahedral sheets of clay minerals. The calculated (see Table 1) semiquantitative Al(IV)/Al(VI) peak-area ratios from the NMR spectra indicate that samples IT and CH contain higher amounts of Al in octahedral configuration compared with the other perlites. It has been reported52 that octahedral Al is due to the transformation of the tetrahedral one by effect of water coordination; on the other hand, a calcination-induced transition from octahedral to tetrahedral Al has been observed53 in certain aluminosilicates. This may actually support the idea that perlite, which is formed during sudden freezing of lava and can slowly devitrify, is a metastable material. Polymerization, i.e., degree of interconnection, of the aluminum tetrahedra is also affecting the chemical shift. In principle, aluminosilicate networks consist of tetrahedral units with varying degrees of interconnection. Such a unit is commonly described in terms of a “Qn” notation, where Q denotes the central atom, i.e., Si or Al, bound to four oxygens. The superscript n (varying from 0 to 4) is used to indicate the number of neighboring Q structures attached through oxygen atom to the unit in question. In aluminosilicates, Q4 tetrahedral aluminum resonates in the range +56 to +69 ppm, whereas Q3 resonates at higher (+70 to +77 ppm) chemical shifts.47 The spectra in Figure 1 indicate that most aluminum contained in the perlite framework is located at the centers of fully interconnected distorted tetrahedra. It has been shown previously,5 that Q1, Q2 and Q3 units condensate during the expansion process leading to an increasingly cross-linked aluminosilicate network based on interlinked Q4 species. Thus, we suggest that the regions of Al-centered Q4 structures in the framework of perlite inhibit expansion.

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1

H MAS NMR Spectroscopy. The 1H MAS NMR spectra (see Supporting

Information, Figure S1) demonstrate a broad envelope in the range of 2.5 to 4.5 ppm. Signals at 2.8 and 4.9 ppm have been assigned to hydroxyl groups and water, respectively.54–57 29

Si MAS NMR Spectroscopy.

29

Si MAS NMR results (see Figure 2) yield

information on the local silicate structure of perlites. As a general rule, shifts of around –65 ppm typically correspond to Q0 units in monosilicates, changing in steps of about 10 ppm for each additional bonded Si tetrahedron, up to about –110 ppm for the Q4 units of fully interconnected silica polymorphs (as in quartz or cristobalite). Thus, the 29Si chemical shift becomes increasingly negative with each additional Si– O–Si linkage, due to increased electronic shielding of the central Si. Substitution by Al of each of the four silicon atoms surrounding the central Si of a Q4 unit results in a change in the 29Si chemical shift of about 5 ppm towards less negative values.46


0

-50

-100

-150

-200

Chemical shift (ppm)

Figure 2. 29Si MAS NMR spectra of perlites. 11



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All perlite samples exhibit a characteristic broad NMR signal peaking at around –105 ppm (see Table 2), an indication that the nanostructure is interlinked. Also, the aluminosilicate structure of perlites should comprise a number of Si environments; from the broad asymmetric peak (see Figure 2) it is assumed that, apart from the Q4 (0Al) and Q4 (1Al) species, important contributions may originate from Q3 (0Al), Q3 (1Al) and, possibly, Q2 (0Al) structures. The peak at around –112 ppm (see Supporting Information, Figure S2) is assigned to fully polymerized silicon-atom species58,59 and, therefore, the peak at –107 ppm should be ascribed to Q4 (1Al) units. The contribution of both Q4 (0Al) and Q4 (1Al) structures is, as expected, substantially suppressed in the 1H-29Si CPMAS NMR spectra compared with the respective

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Si MAS NMR ones (see Supporting Information, Figure S3). The major

component at around –103 ppm, observed in both

29

Si- and 1H-29Si CPMAS NMR

spectra, is attributed to Q3 (0Al) units being, as also expected, 10 ppm downshifted with respect to Q4 (0Al) resonance. Further downfield lie two peaks at around –99 and –94 ppm (see Supporting Information, Figure S2), accounting for structures Q3 (1Al) and Q2 (0Al), respectively. The 94-ppm peak is the most enhanced in 1H-29Si CPMAS NMR spectra supporting the existence of Q2 units.

Table 2. Chemical shifts in original and simulated intensities of deconvoluted peaks Perlite Original peak Deconvoluted peaks δ (ppm) Q2+Q3 (1Al) δ (ppm) Area (%) EX –105.0 –98.2 23.1 GRs –105.1 –98.0 33.7 GRr –104.9 –98.6 32.2 GRp –105.3 –98.0 29.3 HU –105.1 –98.9 29.2 IT –104.3 –97.0 25.2 TU –106.4 –99.9 37.6 CH –104.2 –96.8 30.8

Q3 (0Al) δ (ppm) –103.7 –104.3 –104.3 –104.2 –103.6 –104.2 –105.9 –103.9

Area (%) 43.8 40.2 34.2 41.4 38.4 46.4 29.2 28.9

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Si MAS NMR spectra and

Q4 δ (ppm) –108.7 –108.2 –109.2 –108.3 –108.9 –108.2 –111.2 –107.6

Q2+Q3 (1Al) : Q3 (0Al) : Q4 Area (%) 33.1 26.1 33.6 29.3 32.4 28.4 33.2 40.3

5 7 6 6 6 5 8 6

:9 :8 :7 :8 :8 :9 :6 :6

12


:7 :5 :7 :6 :6 :6 :7 :8

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Three components (Q2–4) were selected for the deconvolution of the 29Si MAS NMR broad peaks in Figure 2 (see Supporting Information, Figures S4, S5 and S6) as individual resonances overlap one another, due to the proximity of the Qn peak centers. The analysis was carried out assuming Gaussian lineshape for each peak to obtain quantitative data on the relative content of the Qn perlite structures. Simulation of

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Si MAS NMR spectra of aluminosilicates and quantification of the results have

been discussed in the literature.59–61 The simulated 29Si MAS NMR spectra verify that the signal resonating at around –108 ppm represents the sum of Q4 (0Al) and Q4 (1Al) contributions, the –104-ppm peak corresponds to Q3 (0Al) units while the third peak at about –98 ppm accounts for the sum of Q3 (1Al) and Q2 (0Al) lower-order units (see Table 2). It must be noted here, that the inclusion of a fourth downshifted component did not importantly alter the results, a feature consistent with the less significant contribution of the Q2 units to the perlite nanostructure. The same stands for the addition of a fourth upshifted line (see Supporting Information, Figure S7). The relative areas of these deconvoluted peaks, indicating the content of each Qn unit in the network (see Supporting Information, Figures S4, S5 and S6), are also listed in Table 2, shedding light on the construction of different perlites. It is obvious from Figure 2 that the perlitic framework consists mainly of Q3 and Q4 units. The limited number of surface non-bridged oxygen atoms successfully explains the low cation exchange capacity of perlites (~25 meq/100 g raw material). Framework topology is greatly affected by the location of aluminum atoms; according to the Löwenstein’s rule,62 aluminum atoms participating in the perlitic structure cannot occupy second-neighbor sites and, thus, they are relatively distant to each other. On the other hand, aluminum is readily incorporated in fully interconnected Al- or Si-

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centered structures (see Figures 1 and 2) but, due to its limited amount in the network composition (~12.5% w/w Al2O3),5 Al is preferentially distributed in Qn (1Al) units. The

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Si chemical shift values are directly related to the shielding of the

29

Si

nucleus, which, in turn, is affected by the electronic structure of its immediate environment, i.e., the degree of s-hybridization of the oxygen bond orbitals. Linear relationships between the chemical shift and the Si–O bond length have been reported, based on both empirical and theoretical approaches; the following Eq. (1), derived by Smith et al.,63 was selected to calculate the mean Si–O bond length of perlites

δ = 875(d Si −O ) − 1509 ,

(1)

where δ is the chemical shift in ppm and dSi–O is the mean bond length in Å. Additionally, the electronegativity of the Si–O bond has been related to the tetrahedral Si–O–Si bond angle and various simple relationships between δ and the mean tetrahedral bond angle α, in degrees, (or some trigonometric function of the bond angle) have been derived. Smith and Blackwell,64 based on the assumption that the electronegativities of the s-hybridized oxygen bond orbitals are related to the Si– O–Si bond angles α by a cosine function, extracted the following Eq. (2),

δ = −55.7

1 − 176 . cos a

(2)

From Eqs. (1), (2) and the δ values in Table 2, the mean Si–O bond length and the mean Si–O–Si bond angles can be calculated for the Qn structures in perlites. It is realized that the variations of Si–O–Si angle are far more significant compared with those of the Si–O bond length, i.e., a shift of 1 ppm results in 1° decrease in the Si–O– Si angle but only 0.001 Å increase in the Si–O bond length. As the Si–O bond length is practically constant, it becomes obvious that, for any two adjacent Si tetrahedra sharing one or more oxygen atoms, the smaller the Si–O–Si angle the closer the Si atoms come. Thus, the reduced Si–O–Si angles of samples CH and IT (both 14


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presenting a downshifted

29

Si MAS NMR spectrum in Figure 2) indicate a tight

aluminosilicate network with a compact nanostructure and a limited capacity for expansion-effective water. In addition, instability of the network may result from electronic repulsions due to the proximity of the Si atoms, inducing microstructural gaps and cracks. These can explain why water can readily escape at the beginning of thermal treatment which, in turn, may lead to a significant decrease in the water content before the critical expansion stage and to a low expansibility of samples CH and IT. On the other hand, slightly decreased mean Si–O bonds and increased mean Si–O–Si angles were calculated for sample TU (as it exhibits an upshifted 29Si MAS NMR spectrum compared with the other samples in Figure 2), supporting the concept of a spacious-cell silicate network with a higher content of expansion-effective water. The above observations are of significant importance as they actually reveal the key role of the aluminosilicate structure in the expansion process; the compact samples IT and CH barely expand while the loose TU perlite demonstrates high expansion ratios. FTIR Spectroscopy. Figure 3 shows the low-frequency FTIR spectra of the perlites studied. Among other vibrations, the bands at 470 and 785 cm–1 represent motions of the oxygen atom of the Si–O–Si bridge, rocking perpendicularly and bending along the bisector, respectively, and the 1630 cm–1 peak is due to the deformation mode of molecular water. Both the most significant spectral region (main peak in Figure 3) and the accompanying shoulder at about 1160 cm–1 are the asymmetric stretching vibrations of the Si–O–Si and Si–O–Al planes. The entire 900– 1300 cm–1 envelope also includes vibrations of the tetrahedral Qn silicate units having either Si–OH bonds or non-bridging oxygen atoms.5

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400

600

800

1000

1200

1400

1600

1800

-1

Wavenumbers (cm )

Figure 3. Low-energy FTIR spectra of perlites.

To elucidate the molecular-scale structure of perlite, the broad peaks in Figure 3 were decomposed to disclose the distinguished bands corresponding to the Qn units that form the perlitic network (see Supporting Information, Figures S8, S9 and S10). For this purpose, four components, i.e., 980, 1060, 1155 and 1210 cm–1, were selected on the basis of the second derivative of the original spectra,65 assuming a Gaussian lineshape of peaks; an excellent fit was obtained in all cases.

Table 3. Wavenumbers in original and simulated FTIR spectra and intensities of deconvoluted peaks Perlite Original peak Deconvoluted peaks ν (cm–1) Q2+Q3 (1Al) ν (cm–1) Area (%) EX 1059 985 18.2 GRs 1068 984 31.7 GRr 1056 982 33.1 GRp 1066 981 31.2 HU 1063 981 32.2 IT 1045 980 25.9 TU 1049 984 34.6 CH 1052 985 31.3

Q3 (0Al) ν (cm–1) Area (%) 1057 45.2 1068 41.5 1060 32.9 1065 37.9 1063 39.0 1045 45.7 1050 29.1 1051 30.7

Q4 (1Al) ν (cm–1) 1153 1164 1157 1156 1162 1152 1158 1152

Area (%) 24.5 13.5 21.4 18.3 14.1 19.8 25.5 34.7

Q4 (0Al) ν (cm–1) 1205 1211 1208 1214 1207 1210 1210 1206

Q2+Q3 (1Al) : Q3 (0Al) : Q4 (1Al)+Q4 (0Al) Area (%) 12.1 13.3 12.6 12.6 14.7 8.6 10.8 3.3

4 6 7 6 6 5 7 6

:9 :8 :7 :8 :8 :9 :6 :6

:7 :5 :7 :6 :6 :6 :7 :8

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The data calculated from the simulated spectra are presented in Table 3. The relative intensities of these four peaks are denoted by their area in the deconvoluted spectra (see Supporting Information, Figures S8, S9 and S10) and reflect the contribution of the respective Qn structures to the network. Tables 2 and 3 clearly indicate the excellent consistence between the

29

Si

MAS NMR and FTIR deconvolution analyses. This agreement should be emphasized here as the peak ratios found from the FTIR spectra decomposition almost coincide with those calculated from the 29Si MAS NMR simulated spectra (see Figure 4).

CH IT

GRs

HU

TU

GRr GRp

EX TU CH

CH

HU IT

EX

6

GRp GRr

GRr GRp

GRs

8

FTIR

HU

TU

GRs

NMR

IT

EX

10

Peak ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

2

0 2

3

Q +Q (1Al)

3

Q (0Al)

4

4

Q (1Al)+Q (0Al)

Figure 4. Peak ratios of Qn species from simulated 29Si MAS NMR and FTIR spectra.

Bands at 770–800, 800–850, 900–950 and 1000–1100 cm–1 have been assigned to Si–O– stretching vibrations in Q0, Q1, Q2, and Q3 species, respectively.66 Therefore, the high-frequency FTIR band at 1210 can be attributed only to the 17



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interconnected Q4 (0Al) units and, as the substitution of Al for Si in aluminosilicate glasses results in a systematic decrease of the main absorption frequency,67 the peak at 1155 cm–1 represents Q4 (1Al) structures. The main band at 1060 cm–1 is characteristic of Q3 (0Al) units while the lowest-frequency peak may correspond to the presence of tetrahedral Q3 (1Al) and Q2 (0Al) structures. An important feature here, in perfect agreement with the

29

Si MAS NMR results, is the dominant

contribution of Q3 and Q4 units to the perlite nanostructure, indicating a cross-linked interconnected amorphous silicate network. Furthermore, as already mentioned, perlite framework alterations under expansion-simulating thermal treatment are mainly due to condensation reactions (attributed to the dehydroxylation of Si–OH groups), resulting in an increasingly polymerized network, i.e., the total contribution of Q3 (1Al) and Q2 (0Al) units in expanded perlite (sample EX) is below 23% as shown in Tables 2 and 3. With the exception of the IT perlite, consisting of only 25% Q3 (1Al) and Q2 (0Al) units, the samples of raw perlite generally exhibit a higher relative abundance of the Q3 (1Al) and Q2 (0Al) units, ranging from 29 to 38%. As a result, the lowest expansion ratio was observed for sample IT while sample TU, with the highest contribution of Q3 (1Al) and Q2 (0Al) units (38% in Table 2), displayed maximum expansion. GRs, GRr, GRp and HU perlites demonstrated a 31–32% abundance of Q3 (1Al) and Q2 (0Al) units and intermediate expansion ratios; contrarily, CH perlite showed a very low expansion ratio despite its 31% content of Q3 (1Al) and Q2 (0Al) units. In fact, examination of the Qn proportions in Tables 2 and 3 reveals that the contribution of the Q4 structures in the CH perlite is the highest (38–40%), remarkably higher than that in the interconnected expanded perlite (sample EX), a nanostructural characteristic explaining the weak expansibility of the CH perlite. On the other hand,

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the contribution of Q4 structures in the silicate network of the EX perlite represents 33–36% of the total Qn units, consistent with the 26–33% relative abundance of Q4 units observed for the other raw perlites. Deconvolution of the FTIR spectra was also carried out with the introduction of a fifth component to isolate the Q2 contribution. The simulated spectra revealed a newly observed low-intensity peak at 915 cm–1 pointing out an insignificant contribution of the Q2 units, a confirmation that the perlite’s aluminosilicate network consists mostly of Q3 units. As expected, the 915-cm–1 peak in the expanded sample was eliminated; thus, the Q2 units are lacked. Another observation, further supporting the above drawn conclusions, is that the single intensity from four-component deconvolutions (Q3 (1Al) and Q2 (0Al) together) equals the twin-intensity sum obtained in five-component spectra (Q3 (1Al) and Q2 (0Al) separately). Scanning Electron Microscopy. Features involving the surface and threedimensional aspects of perlites, such as the morphology, fabric and texture of the particles, are readily observed on the scanning electron microscope. The expansion ratio of raw perlite depends on silicate microstructure and grain morphology, which are greatly affected, as mentioned above, by the relative Qn proportions calculated from both NMR and FTIR spectra and by the network geometry derived from NMR results, respectively. Also, as the content and behavior of the expansion-effective water is critical, a deep understanding of dynamic interactions between the hydrophilic perlite and moisture is of great importance. In fact, the amount of water trapped depends on the perlite nanostructure which, in turn, is determined from NMR and FTIR studies. Nevertheless, the influence of water content on the expansion has to do with how the detected local structures allow the generation of microcavities that retain relatively large quantities of water. Perlitic structures, fragmented or not,

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behave in a different manner during heating and consequent escape of water, a microscale phenomenon that SEM micrographs record and illustrate. The onionskin construction of the TU perlite is responsible for the difficulty of dehydration during heating, as water cannot escape through the successive layers. Therefore, the increasing pressure of trapped water vapor results in a sudden explosion of the grain and leads to a larger expansion. Conversely, the vesicular, pumiceous GRr perlite and, mainly, the granular, compact IT perlite appear less expandable as shown in Figure 5a–i.

a

b

c

d

e

f

g

h

i

Figure 5. Scanning electron micrographs of the TU, GRr and IT perlites during the expansion process. Sample TU after heating for 5 s (a), 10 s (d) and 20 s (g); sample GRr after heating for 5 s (b), 10 s (e) and 20 s (h); sample IT after heating for 5 s (c), 10 s (f) and 20 s (i).

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The TU perlite, after 5 s of heating, still presents an onionskin pattern and, compared with the GRr perlite, exhibits fewer scales and sparse, tiny holes. The sealed surface of TU particles permits the internal circulation of water molecules, but not the escape of steam (see Figure 5a). Almond-shaped holes resembling the TU perlite are relatively bigger, apparently allowing the escape of water vapor from the surface of the GRr perlite (see Figure 5b). It seems that at 5 s heating period the internal steam pressure does not increase excessively, thus, the softening process is not accompanied by a remarkable expansion. Sample IT heated for 5 s in Figure 5c is characterized by a uniform structure without holes, but with numerous cracks creating an integrated net of discontinuity. A potential breaking of particle surface along the cracks is expected during the consequent heating and plasticizing of the material. The micrographs of samples TU, GRr and IT as the heating is taking place for 5 s show that the IT perlite dissipates the expansion of the vapor pressure through the lattice of channels and cracks, the GRr perlite allows the escape of water only through the surface holes, while the TU perlite retains almost all of its water between the adjacent successive layers. At 10 s of heating, the TU perlite shown in Figure 5d has very few but slightly bigger amygdaloid vents on the surface and its expansion is clearly evident. Such an almond-shaped mouth of cavity represents a potential steam-escaping pathway. However, the waterproof surface is maintained and the water molecules are still encapsulated. Sample TU exhibits a nodulous surface, an indication of rapid internal transformation. The scales of sample GRr are detached and separated, the number of elongated cavities is larger (see Figure 5e) and steam starts to escape from the surface, retarding the expansion. It is obvious that, even after 10 s of heating, only a limited thinning of viscoelastic particle walls is achieved. At the right of sample GRr a

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growing vesicle, with a diameter of about 100 µm, is starting to expand. Another region of expansion (center), 80 µm in diameter, is clearly visible. As shown in Figure 5f, almost no expansion is observed in the IT perlite within 10 s of heating. The formation of holes is commenced, the cracks are more distinct and deep, the escape of steam seems to be increased and uniformly distributed through the entire particle surface (see Supporting Information, Figure S11). At this stage, the softening of the IT perlite is not yet accompanied by a sufficiently high internal pressure of steam and the expansion is still negligible. Some deformation is observed after heating for 10 s in the sample GRr, an indication of incipient surface break, while the TU perlite is expanded steadily, thanks to the trapped water inside, without vapor relaxation. Plasticized and resolidified, sample TU heated for 20 s shows a huge broken vesicle, about 550 µm in diameter (see Figure 5g) with bulky internal space, in which the typical fragmented pattern of expanded perlite is detected; thus, an open structure is generated. Heating causes steam pressure to rise and the water is finally escaped after it has induced a significant expansion (see Supporting Information, Figure S12). The surface of sample GRr, after 20 s of heating, appears not particularly fragmented, as the gradual escape of steam does not allow the formation of sizeable internal steam cavities. Its expansion is moderate and an imperfect open structure is observed in Figure 5h. A bubble of about 150 µm in diameter with a 15 µm orifice on its top (lower center of the micrograph) can be seen. In spite of the softening during heating for 20 s (see Figure 5i), the escaping water vapor prevents the structure of sample IT from expanding sufficiently. Local wall expansions and vesicles of 50–100 µm, ideal starting points for burst under the internal pressure of evaporated water, are noticed.

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The net of surface bowl-shaped cavities (25 µm craters made by explosion of shells) is more extensive, but the material remains relatively compact. In conclusion, the locally expanded external surface of the IT perlite reveals no indication of grain explosion and maintains a solid structure. The GRr perlite presents a moderate expansion, as opposed to sample TU that expands remarkably and obtains a characteristic open structure.

CONCLUSIONS Solid-state NMR, FTIR and SEM were engaged to visualize for the first time both the three-dimensional nanoscale characteristics and the microscopic features of raw and expanded perlites. As perlite expansion represents a systematic series of changes, taking place under heating, exceptionally significant from both scientific and economic point of view, to study and optimize this process, to correlate the expansibility of raw material with its physicochemical properties and to distinguish between perlites of low and high expansibility are all of great importance. In the aluminosilicate skeleton of perlites, most aluminum nuclei are tetrahedrally coordinated in Q4 configurations (structurally conforming to Si) as in open-framework zeolites, explaining the feasibility of perlite zeolitization. These Q4 tetrahedrals do not participate in the expansion process. A small percentage of aluminum atoms may adopt two different distorted octahedral coordinations. Tetrahedra–octahedra transformation routes suggest that perlite is a metastable material. Signals of 1H MAS NMR spectra associated with the hydoxyl and water contents could be correlated to the expansion process.

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Silicon atoms (exhibiting a characteristic broad NMR signal peaking at around –105 ppm) form mainly Q3 (0Al) and fully interconnected Q4 (0Al or 1Al) units (~70%), an observation explaining the low cation exchange capacity of perlites. Deconvoluted FTIR and 29Si MAS NMR spectra coincide allowing the calculation of relative Qn ratios which, in turn, are related to the expansibility, i.e., raw perlites with increased amounts of Q2 and Q3 (1Al) units (TU perlite) exhibit higher expansion ratios than those consisting of polymerized aluminosilicate networks with large quantities of Q4 units (CH perlite). The mean Si–O–Si bond angles vary more significantly than the mean Si–O bond length revealing the key role of the perlite network in the expansion process; reduced Si–O–Si angles (CH and IT perlites) indicate a network of a compact aluminosilicate nanostructure with a limited expansion-effective water capacity and a low expansibility due to microstructural gaps and cracks through which water can escape at the beginning of thermal treatment. On the other hand, increased mean Si– O–Si angles (TU perlite) suggest higher contents of expansion-effective water in a spacious-cell network. As the expansion ratio of raw perlite is affected by the relative Qn proportions and by the network geometry, the differences in grain morphology, fabric and texture of the particles monitored by SEM perfectly distinguish the process characteristics. A granular, compact, non-expandable perlite (sample IT) maintains a solid structure while a vesicular, pumiceous one (sample GRr) presents a moderate expansion. An onionskin construction, however, inhibits dehydration on early heating; then, increased pressure of vapor trapped in the spacious-cell network (TU perlite) results in both a sudden explosion of the grain and a larger expansion, generating an open structure.

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ASSOCIATED CONTENT Supporting Information S1: 1H MAS NMR spectra of perlites; S2: Representative 1H-29Si CPMAS NMR spectra; S3: 1H-29Si CPMAS NMR and 29Si MAS NMR spectra drawn together; S4, S5, and S6: Deconvolution into three peaks and simulated 29Si MAS NMR spectra of samples EX, IT and TU, respectively; S7: Deconvolution into four peaks and simulated 29Si MAS NMR spectrum of sample IT; S8, S9, and S10: Simulated FTIR spectra and the four deconvoluted peaks of the EX, IT and TU perlites, respectively; S11: Scanning electron micrograph with surface details of the IT perlite within 10 s of heating; S12: Scanning electron micrograph showing several characteristic broken vesicles of sample TU after 20 s of heating. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS M. R. wishes to thank Mr. E. Michailidis (Department of Geology, University of Athens) for the acquisition of SEM micrographs. G. M. was financed for a part of this work by the Slovenian Research Agency through project P1-0021.

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Distinctive Spectral and Microscopic Features for Characterizing the

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