Effect of alkaline synthesis conditions on mineralogy, chemistry and surface properties of phillipsite, P and X zeolitic materials prepared from fine powdered perlite by-product

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Effect of alkaline synthesis conditions on mineralogy, chemistry and surface properties of phillipsite, P and X zeolitic materials prepared from fine powdered perlite by-product � b, Pavol Hudec c, Adriana Czímerova � b, Dagmar Galuskova � d, Marek Osacký a, *, Helena P� alkova �e Martina Vítkova a

Department of Economic Geology, Comenius University, Mlynsk� a Dolina, Ilkovi�cova 6, 842 15, Bratislava, Slovak Republic Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravsk� a Cesta 9, 845 36, Bratislava, Slovak Republic c Department of Organic Technology, Catalysis and Petroleum, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinsk�eho 9, 812 37, Bratislava, Slovak Republic d Vitrum Laugaricio, Joint Glass Center of the Institute of Inorganic Chemistry, Slovak Academy of Sciences, Alexander Dub�cek University of Tren�cín and Faculty of � Chemical and Food Technology, Slovak University of Technology, Studentsk� a 2, 911 50, Tren�cín, Slovak Republic e Department of Environmental Geosciences, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýck� a 129, 165 00, Prague – Suchdol, Czech Republic b

A R T I C L E I N F O

A B S T R A C T

Keywords: Synthesis NaOH solution Zeolite Volcanic glass Crystallization

The fine powdered perlite, a by-product of processing of raw perlite was used for zeolite synthesis. Perlite byproduct material (PBM) is not suitable for perlite expansion, due to fine particle size, therefore it has very limited application (recently only as additive to concrete). The conversion of PBM into zeolites is proposed to recover this material (i.e. minimize its accumulation) and to obtain value-added material (i.e. zeolites). Volcanic glass was the main PBM component transformed to zeolites after reaction with NaOH solutions. As volcanic glass alteration proceeded, crystallization of zeolites having lower Si/Al ratio was favored. This was likely due to more rapid increase in the solubility of alumina than the solubility of silica with increasing concentration of reacting NaOH solution. Phillipsite, zeolite P and zeolite X were the main reaction products synthesized from PBM. The concentration of NaOH solution had significant impact on the type of synthesized zeolites whereas reaction temperature and time influenced mainly the quantity of synthesized zeolite species. The highest-grade zeolitic material synthesized from PBM contained 77 wt% of zeolites, 16 wt% of unaltered volcanic glass and 7 wt% of accessory minerals. The small amount of zeolites (11–29 wt%) was synthesized even at the lowest tested tem­ perature of 50 � C. After shortest reaction time (24 h), from 6 to 54 wt% of zeolites was formed. Synthesized materials reached maximum total specific surface area (SBET) of 362 m2/g and cation exchange capacity (CEC) of 371 meq/100g.

1. Introduction Zeolites are crystalline, hydrated aluminosilicates of alkali and alkaline earth cations. Specific porous structure of zeolites is responsible for unique physico-chemical properties such as high cation exchange capacity, cation selectivity and molecular sieving, ability to adsorb gases and vapors, low density and large void volume [1]. Due to these prop­ erties, zeolites are considered as an effective adsorption materials with high pollutant removal efficiency. Zeolites have been studied for many potential environmental applications, e.g. for treatment of nuclear,

municipal and industrial wastewaters, acid mine drainage waters, remediation of soils contaminated with heavy metals, etc. [2–8]. Zeolites can be formed from a variety of precursor materials such as volcanic glass (rhyolitic and basaltic) [9–11], aluminosilicate gels [12–14] and aluminosilicate minerals (smectite, kaolinite, illite, chlo­ rite, palygorskite, feldspars, feldspathoids, other zeolites) [9,15–18]. However, nearly all the mineable zeolite deposits in the world are principally formed by alteration of glass-rich volcanoclastics in alkaline environments with high ratio of (Naþ þ Kþ þ Ca2þ)/Hþ [15,19]. In the past decades, waste materials such as fly ashes [3,20–23], rice husk ash

* Corresponding author. E-mail address: [email protected] (M. Osacký). https://doi.org/10.1016/j.micromeso.2019.109852 Received 29 October 2018; Received in revised form 18 October 2019; Accepted 28 October 2019 Available online 31 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.

Please cite this article as: Marek Osacký, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109852

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[17,24,25], aluminum solid waste [26,27], natural clinker [28], fine perlite waste [29] and by-product from expanded perlite [18] have been used as starting materials for synthesis of different types of zeolites (zeolite A, X, P, Linde F, analcime, sodalite, cancrinite). According to some authors [18,29], one of the most significant shortcomings of using natural raw materials and waste materials for zeolite synthesis is their variable composition. As a consequence, the syntheses of zeolites from these kinds of materials are difficult to control and describe and synthesized zeolitic materials are often of variable purity [18,29]. On the other hand, zeolites produced from waste mate­ rials are cheaper than commercial zeolites synthesized from pure chemicals and using industrial wastes/by-products as Si and/or Al sources leads to waste/by-product valorization [30]. Hydrothermal synthesis performed in closed reacting systems (e.g. autoclaves) at temperatures from 100 to 200 � C is the most widely used method to prepare different types of zeolites under laboratory condi­ tions [17,24,34–37]. Reactivity of certain types of precursor materials (e.g. kaolin, rice husk, bauxite tailing, fly ash) can be enhanced prior to the hydrothermal synthesis by calcination/fusion (400–900 � C), micro­ wave and ultrasonic irradiation, or mechano-chemical treatment [13,17, 24,38–43]. Hydrothermal laboratory formation of zeolites usually takes place under supersaturated conditions in alkaline solutions (e.g. NaOH, KOH) [9,10,18,21,28,29]; zeolites never form under acid hydrothermal con­ ditions [31]. The initial aluminosilicate precursor under the combine action of mineralizing (OH , F ) and structure directing agents (Naþ, Kþ, Liþ, tetraalkylamonium cations, etc.) is transformed into crystalline zeolite. The major role of using (high pH) alkaline solutions (e.g. NaOH and KOH) is to bring the Si and Al oxides or hydroxides from precursor into solution at an adequate rate whereas the structure directing agents (inorganic and organic cations) are commonly employed to form and stabilize the framework of newly-forming zeolites [31–33]. The gener­ ally accepted scheme includes the arrangement of SiO4 and AlO4 tetra­ hedra around charged templating species, i.e. hydrated alkali metal cations or positively charged organic cations [32]. The type and quantity of the synthesized zeolites is controlled by experimental conditions such as temperature, time, nature of initial precursor material, reaction solution (chemical composition and con­ centration), solid/liquid ratio, etc. [9,10,14,18,21,22,29]. Controlling the synthesis process also allows tailoring of structure and properties of synthesized zeolites for different applications [44,45]. Where both natural and synthetic forms of the same zeolite are available in com­ mercial quantity, the variations in chemistry and mineralogy of the natural zeolite can make the synthetic zeolite more attractive for specific applications. Conversely, where chemical uniformity and purity are less important, the cheapness of natural zeolite may favor its use. The fine powdered perlite, a by-product of processing of raw perlite ^tka pod Brehmi deposit (Slovak Republic), was used in the from the Leho present study for zeolite synthesis. Due to fine particle size (<100 μm), perlite by-product material (PBM) is not suitable for perlite expansion. As a consequence, PBM has very limited application, recently, only as a partial replacement for cement in concrete. The conversion of PBM into zeolites is proposed to recover this by-product and to obtain value-added material with attractive sorption properties. Zeolite synthesis was per­ formed in batch experiments in a wide range of experimental conditions (temperature, NaOH concentration and time) and the reaction products were characterized for their mineralogy, chemistry and surface properties. Krol et al. [18] investigated the zeolitization of by-product material derived from expanded perlite after reaction with 0.5–5 M NaOH solu­ tion at 30–90 � C for 24–72 h. The authors observed that the zeolitization process is efficient only at temperatures exceeding 50 � C. At 70 � C, zeolite X was the main reaction product formed from expanded perlite but with increasing synthesis temperature and longer reation time the amount of zeolite X decreased in favor of zeolite Na–P1 and hydrox­ ysodalite [18]. Christids et al. [29] studied the formation of zeolites

from perlite fine waste material (Provatas mine, Milos, Greece) at higher temperatures (100–140 � C) in 1–5 M NaOH solution after 2–4 h. Zeolite P was the main reaction product synthesized from fine perlite waste while at higher NaOH concentrations hydroxysodalite formed instead of zeolite P [29]. Barth-Wirsching et al. [46] investigated the synthesis of zeolites from expanded perlite (waste product obtained during the production of expanded perlite) reacted with different alkaline solutions (NaOH and KOH) at 100–140 � C for 2–312 h. The authors reported the formation of phillipsite, zeolite W (merlinoite), G (chabazite) and F (edingtonite) in KOH solutions, while zeolite P, A, hydroxysodalite and analcime were formed in NaOH solutions. Although the purity is one of the key parameters determining the quality and thus possible applications of synthesized zeolitic materials, most of the past studies have not reported the quantitative phase anal­ ysis results on the amount of different types of synthesized mineral phases and the amount of precursor material after synthesis. In contrast to previous studies, in the present paper we determined the quantitative amounts of all mineral phases (including precursor material) present in synthesized samples using full-pattern fitting XRD-based method (RockJock software [47]). The main goal was to find a relationship among the synthesis con­ ditions, mineral and chemical composition of synthesized zeolitic ma­ terials and their surface properties in order to minimize the accumulation of perlite by-product by its transformation to added-value material. A wide range of synthesis conditions was tested in the present study to optimize reaction conditions and to produce zeolites effectively (i.e. low temperature, low NaOH concentration and short reaction time). 2. Materials and methods 2.1. Starting materials and zeolite synthesis The fine powdered perlite, a by-product produced from the milling ^tka pod Brehmi deposit (Slovak process of raw perlite from the Leho Republic) exploited by LBK PERLIT Ltd. was used in the present study for zeolite synthesis. The as-received perlite by-product material (PBM) was dried at 60 � C for 48 h. The >63 μm size fraction was separated from the bulk PBM by dry sieving in a vibratory sieve shaker. The sub-sieve fraction (<63 μm) was used for zeolite synthesis. After separation, 11 g of PBM was placed in 100 mL PP bottles and 80 mL of a 1, 3 and 5 M solution of NaOH was added. For comparison, mixtures of PBM (11 g) with distilled water (80 mL) but without NaOH, designated as “blanks”, were prepared in the same way. The bottles were closed and sealed and agitated for 10 min at room temperature. Then the bottles were put in an oven and kept at 50, 60, 70 and 80 � C for 24, 72 and 144 h. The experimental synthesis conditions are summarized in Table S1. After the synthesis the bottles were cooled down to room tempera­ ture. Solid products were separated from the liquids by centrifugation (4000 rpm for 20 min). After centrifugation the supernatants were slowly siphoned from the bottles and analyzed. The solids were washed by distilled water until the pH was below 10. The samples were dried at 60 � C overnight and passed through a 250 μm sieve. Then the solids were converted to a Na-form through exchange with 1 M NaCl. Excess soluble salts were removed by centrifugation (4000 rpm for 20 min) followed by dialysis. The Na-saturated solids were dried overnight at 60 � C, passed through a 250 μm sieve and analyzed. 2.2. Instrumental methods X-ray diffraction (XRD) patterns of random preparations were recorded using a Phillips PW 1710 diffractometer, with CuKα (λ ¼ 0.15418 nm) radiation and a graphite monochromator, operating at 20 mA and 35 kV. The step size for all analyses was 0.02� 2θ and the counting time 2 s per step. Qualitative analysis, used to identify the phases in the samples, was accomplished using reference patterns from the ICDD powder diffraction files and collection of simulated XRD 2

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patterns for zeolites [48]. Quantitative X-ray diffraction (QXRD) anal­ ysis was performed using RockJock (Version 11) software [47]. Rock­ Jock determines the quantitative content of the minerals in powdered samples by comparing the integrated reflection intensities of the indi­ vidual minerals with the intensities for pure standard minerals and an internal standard (corundum). Samples were prepared according to a method reported by Eberl [47]. A mass of 1.000 g of sieved sample (<250 μm) was mixed with 0.250 g of corundum and ground with 4 mL of ethanol in a McCrone micronizing mill for 5 min using yttria-stabilized zirconia grinding cylinders. The sample/corundum mixture was dried overnight at about 80 � C. Dried mixture was passed through the 250 μm sieve and side loaded into an XRD holder against frosted glass by gently tapping the holder on a hard surface. Samples were scanned in the X-ray diffractometer from 4 to 65� 2θ. The X-ray diffraction intensities extracted from the XRD files using Jade software were entered into the RockJock program and the mineral composition (in wt%) was calculated. New mineral standards for zeolites X (com­ mercial 13X) and P (commercial NaP-1), sodalite and phillipsite were entered into the RockJock following the instructions reported by Eberl [47]. Fourier transform infrared (FTIR) spectra in the wave number region of 4000–400 cm 1 were obtained using a Nicolet 6700 spectrometer (Thermo Scientific). The KBr pressed-disc technique was used for mea­ surement of transmission spectra. Discs were heated in an oven over­ night at 120 � C to minimize water absorption on KBr pellets. For each sample 64 scans were recorded with a resolution of 4 cm 1. All spectral operations were performed using the OMNIC software package. The samples were examined by scanning electron microscopy (SEM) using a Carl Zeiss EVO 40HV operated at 20 kV with a Bruker energy dispersive X-ray (EDX) detector for elemental microanalysis. Prior to the observation, the surface of the samples was coated with carbon film. Solutions after the synthesis were analyzed for major and trace ele­ ments using inductively coupled plasma optical emission spectrometry (ICP-OES; Agilent 730, Agilent Technologies).

3. Results and discussion 3.1. X-ray diffraction (XRD) The perlite by-product material (PBM) consisted predominantly (96 wt%) of volcanic glass with minor amounts (4 wt%) of mica, feld­ spars, quartz, and sometimes opal-CT (Fig. 1 and Table 1). The presence of accessory minerals of the parent PBM (mica, feldspars, quartz and opal-CT) was confirmed by XRD in all samples after synthesis along with variable amount of unaltered volcanic glass and reaction products i.e. zeolites (Fig. 1 and Table 1). This finding was in agreement with pre­ viously published data [29] showing that the crystalline phases (quartz, biotite and plagioclase) of fine perlite waste from Greece resisted the interaction with 1–5 M NaOH solution, even at higher temperatures (100–140 � C) than those used in the present study (50–80 � C). Unam­ biguously, volcanic glass was the main PBM component transformed to zeolites. The main reaction products identified by XRD were zeolites X (FAU type), P (GIS type) and phillipsite (PHI type) while sodalite (SOD type) was present in traces (up to 2 wt%, Table 1) (the three-letter zeolite framework type codes after Baerlocher et al. [50]). No reaction products were observed by XRD for all blank samples (mixtures of PBM with distilled water without NaOH solution) (not shown). This indicated that the addition of NaOH was essential for zeolite formation within the studied experimental conditions. The primary role of highly alkaline reacting solution (e.g. NaOH) used in laboratory zeolite synthesis is twofold: (i) to release Si and Al from precursor material into solution and (ii) to form and stabilize the framework of crystallizing zeolites [31]. The mechanism through which amorphous aluminosilicate reagents (e.g. volcanic glass, aluminosilicate gels) are converted to crystalline zeolite products is still not fully un­ derstood. According to some authors [51], the process of hydrothermal zeolite synthesis in laboratory conditions can be most adequately explained by a mechanism based upon the solution-mediation model. In this model, amorphous precursor material is dissolved and the zeolite product crystallizes from the resulting solution [52,53]. In our experiments, partial dissolution of volcanic glass was indi­ cated by QXRD analysis which showed gradual decrease in volcanic glass content in favor of newly-formed zeolites (Table 1). A direct evi­ dence for volcanic glass dissolution (etch pits on the surface of glass particles) was provided by SEM. With increasing NaOH concentration, temperature and time the dissolution of volcanic glass increased and larger amount of zeolites was formed (Fig. 1 and Table 1). The PBM reacted with 5 M NaOH solution for 144 h at 80 � C contained only 16 wt % of unaltered volcanic glass and 77 wt% of synthesized zeolites (7LB80, Table 1). The experimental synthesis conditions had a significant impact on the type and quantity of synthesized zeolites (Fig. 2). Zeolite X, accompanied with variable amount of zeolite P, was preferably formed in mixtures of PBM with 5 M NaOH solution (Figs. 1 and 2). Generally, with increasing temperature and longer reaction time higher amounts of zeolite X were detected in reacted samples (Fig. 2 and Table 1). The exception was the mixture of PBM reacted with 5 M NaOH for 144 h at 80 � C which contained lower amount of zeolite X than the same PBM mixture reacted at lower temperature (70 � C) (55 wt% of X for 7LB80 vs. 59 wt% of X for 7LB70, Table 1). The largest amount of zeolite X (61 wt %) was observed for mixture of PBM with 5 M NaOH after 72 h at 80 � C (3LB80, Table 1). For comparison, only 10 wt% of zeolite X was detected under the same experimental conditions but lower temperature (50 � C) (3LB50, Table 1). Zeolite P was synthesized predominantly from PBM treated with 3 M NaOH at 80 � C (Figs. 1 and 2). The largest amount of zeolite P (62 wt%) was determined for mixture of PBM with 3 M NaOH at 80 � C after 144 h (6LB80, Table 1). The same PBM-NaOH mixture after 72 h (2LB80) yielded substantially lower amount of zeolite P (24 wt%, Table 1). Even lower zeolite P contents (up to 21 wt%, Table 1) were observed for

2.3. Surface analyses Cation exchange capacity (CEC) determination was based on ion exchange reactions using ammonium acetate (CH3COONH4) solution [49]. An oven-dried sample (105 � C/overnight), approximately 200 mg in weight, was mixed with 10 mL of the 0.1 M CH3COONH4 solution. The suspension was dispersed ultrasonically for 5 min and shaken for additional 30 min. After 24 h and centrifugation, the supernatant was collected, and fresh CH3COONH4 solution was added. The procedure was repeated five times. After each time of the CH3COONH4 saturation, 10 mL of Millipore deionized water was added in order to remove excess salts from the suspension. The collected supernatants were combined, adjusted to a defined volume and analyzed using atomic absorption spectroscopy (AAS). The measured amounts of cations were divided by the mass of dried (105 � C/overnight) sample to calculate the CEC. The reported CEC values are average from two replicates. Adsorption and desorption isotherms were measured with a Micro­ meritics ASAP 2400 device using nitrogen gas adsorption at 77 K. Sample was degassed at 200 � C overnight under vacuum of 5 Pa prior to the measurement. The total specific surface area (SBET) was determined by inversion of the adsorption branch of the isotherm using BrunauerEmmett-Teller (BET) theory in the relative pressure (P/P0) range of 0.05–0.3. The micropore volume (Vmicro) and the surface area from mesopores, macropores and the external specific surface area (St) were calculated using the t-plot method with the Harkins-Jura standard isotherm. The total pore volume (Va) was estimated from the maximum adsorption at a relative pressure of 0.99. The pore-size distribution (PSD) was obtained by the application of the Barret-Joyner-Halenda (BJH) method.

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Fig. 1. XRD patterns of random preparations for starting PBM and synthesized zeolitic materials. M – mica, Op – opal-CT, Q – quartz, F – feldspars, ♥ – phillipsite, ◆ – zeolite X, ● – zeolite P, ♠ – sodalite, * – corundum (internal standard). Table 1 Mineral and elemental composition for starting PBM and synthesized zeolitic materials. The structural formulas of zeolites were obtained by averaging 3 point analyses per sample. Mineral composition (wt%) Sample

Volcanic glass

PBM 96 1LB80 84 2LB80 66 3LB50 85 3LB60 63 3LB70 39 3LB80 19 5LB80 24 6LB60 82 6LB70 48 6LB80 24 7LB50 66 7LB60 42 7LB70 22 7LB80 16 11LB60 91 11LB70 76 11LB80 38 SEM-EDX elemental composition (at%) Sample Mineral phase analyzed PBM Starting volcanic glass 5LB80 Altered volcanic glass 5LB80 Phillipsite 6LB80 Zeolite P 3LB80 Zeolite X Structural formula 5LB80 Phillipsite 6LB80 Zeolite P 3LB80 Zeolite X

Mica

Feldspars

Quartz

2 2 5 3 5 5 5 3 4 5 4 4 6 6 5 2 5 6

1 2 2 1 2 1 3 2 2 1 3 1 1 2 2

<1

O 71.79 73.23 65.91 64.08 68.25

Si 19.60 16.42 18.37 17.27 12.14

1 2

Opal-CT

2

<1

<1

Al 4.24 4.68 7.76 10.60 9.28

K 2.17 2.21 1.71 0.78 0.33

(Na3.12K1.03Ca0.12Mg0.22Fe3þ0.04)[Fe3þ0.27Al4.68Si11.05O32] (Na3.56K0.45Ca0.21Mg0.17Fe3þ0.28Al0.13)[Al6.00Si10.00O32] (Na4.11K0.17Ca0.22Mg0.14Fe3þ0.08)[Fe3þ0.21Al5.03Si6.76O24]

PBM – perlite by-product material. 4

Zeolite X

Zeolite P

3 10 28 45 61 1 9 24 6 25 45 59 55 6 17 47

24 <1 2 10 11

Na 1.26 2.58 5.18 6.15 8.78

Fe 0.40 0.48 0.52 0.49 0.55

3 22 62 4 6 11 21

Phillipsite

Sodalite

2

0.0558 0.0540 0.0627 0.0515 0.0573 0.0785 0.0763 0.0716 0.0544 0.0733 0.0965 0.0618 0.0738 0.0751 0.0882 0.0542 0.0599 0.0664

Mg 0.17 0.08 0.37 0.29 0.25

Si/Al ratio 4.63 3.51 2.37 1.63 1.31

12

68

1

1

1 5 Ca 0.39 0.35 0.20 0.36 0.44

Degree of fit

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Fig. 2. Impact of NaOH concentration, temperature and time on mineralogy of synthesized zeolitic materials. Residual minerals – sum of mica, feldspars, quartz and opal-CT.

mixtures of PBM with 5 M NaOH, where zeolite P accompanied zeolite X (Fig. 2). Phillipsite was identified only for two samples (1LB80 and 5LB80) treated with 1 M NaOH at 80 � C (Figs. 1 and 2). The largest amount of phillipsite (68 wt%) was determined for mixture of PBM with 1 M NaOH at 80 � C after 144 h (5LB80, Table 1) while the same mixture after 72 h (1LB80) contained only 12 wt% of phillipsite (Table 1). Low amounts (up to 2 wt%, Table 1) of sodalite were determined only for mixtures of PBM with 5 M NaOH at 80 � C (11LB80, 3LB80 and 7LB80, Table 1 and Figs. 1 and 2). Overall, the results indicated that the concentration of NaOH solu­ tion had significant impact on the type of synthesized zeolites (phil­ lipsite vs. zeolite P vs. zeolite X) whereas the other experimental conditions such as temperature and time influenced mainly the quantity of synthesized zeolite species. 3.2. Fourier transform infrared spectroscopy (FTIR)

Fig. 3. FTIR spectra for starting PBM (a), 5LB80 (b), 6LB80 (c), 7LB50 (d), 7LB60 (e), 7LB70 (f), 7LB80 (g) and 3LB80 (h).

The starting PBM displayed FTIR spectra characteristic of volcanic glasses, with broad absorption band at 1049 cm 1 assigned to the asymmetric stretching mode of the Si–O vibration (Fig. 3a). The sym­ metric stretching Si–O vibration occurred near 800 cm 1. The spectral shape and shoulder on the lower wavenumbers side of the symmetric stretching Si–O vibration can be related to the presence of tetrahedral Al within volcanic glass structure and/or presence of other mineral admixture in PBM (e.g. feldspars). The band near 470 cm 1 belonged to the bending mode of the Si–O–Si vibration. FTIR spectra of the synthesized materials rich in zeolites showed the asymmetric stretching vibration of TO4 (T ¼ Si and Al) in zeolites in the 1030–1000 cm 1 region (Fig. 3b–h). The position of this band was gradually shifted towards lower wavenumbers (from 1026 to 1010 and 1000 cm 1, Fig. 3b–h) with increasing concentration of NaOH solution

(i.e. as volcanic glass alteration proceeded). These spectral changes indicated that the proportion of Al atoms in the tetrahedral positions of synthesized zeolites increased in the following order: phil­ lipsite < zeolite P < zeolite X, which was in agreement with the EDX results (Si/Al ratios, Table 1). The bending vibration of TO4 in zeolites was observed at the 470–430 cm 1 region. In the FTIR spectra of phillipsite and zeolite P rich samples (Fig. 3b and c), the symmetric stretching vibration of Si–O and Al–O was noticed near 610 cm 1 and in the range of 750–690 cm 1. Simultaneously, the bands in the region of 790–720 cm 1 can be assigned to the vibrations of the single 4-membered rings (S4R) which are the basic structural 5

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elements for phillipsite and zeolite P [54]. The bands in the region of 780–720 cm 1 of zeolite X rich samples (Fig. 3d–h) were assigned to the symmetric stretching vibration of Si–O and Al–O [55]. In the same spectra, an intensive band at 571 cm 1 corresponded to the structural units consisting of double 6-membered rings (D6R) [54]. The intensity of the band near 571 cm 1 in the FTIR spectra of zeolite X rich samples (Fig. 3d–h) varied, likely due to the different quantity of zeolite X among the synthesized materials. This conjecture was supported by the QXRD results and the correlation established between the zeolite X content and the intensity of the band near 571 cm 1 (Table 1 and Fig. S1). A sig­ nificant positive correlation (p ¼ 0.0005 and R2 ¼ 0.995, Fig. S1) indi­ cated that the intensity of the 571 cm 1 band increased with increasing amount of zeolite X in the samples. In the OH stretching region (3700–3000 cm 1), the FTIR spectra for the PBM showed a broad two components band near 3449 and 3621 cm 1 attributed to the hydrogen bonded water molecules and OH vibration from silanol groups present in volcanic glass, respectively (Fig. 3a). Both components were also present in FTIR spectra for phil­ lipsite and zeolite P rich samples (Fig. 3b and c). FTIR spectra for zeolite X rich samples displayed one strong band with maxima near 3465 cm 1 characteristic of molecular water (Fig. 3d–h). The only one strong band at OH stretching region for these samples may be related to the higher amount of molecular water for zeolite X. This assumption was in agreement with higher weight losses determined for the zeolite X sam­ ples compared with those for phillipsite and zeolite P rich samples (19.5 wt% for 3LB80 vs. ~10 wt% for 5LB80 and ~15 wt% for 6LB80, Table 2), after heating to 200 � C for overnight.

glass dissolution which was in line with the chemistry of solutions (see section 3.4. and Fig. 5) and with other studies [10,56]. The SEM-EDX investigation revealed distinct morphology, crystal size and crystal chemistry among different species of synthesized zeo­ lites (Fig. 4c–h). The SEM images of phillipsite synthesized from the PBM in 1 M NaOH at 80 � C after 144 h (5LB80) showed microspheres ranging in sizes from 5 to 10 μm composed of large number of prismatic phillipsite crystals (Fig. 4d). The prismatic crystals were well developed having sharp edges. After shorter reaction time (72 h, 1LB80), the mi­ crospheres of the similar size range were observed, however, the pris­ matic crystals of phillipsite were less developed with round edges compared with those of the 5LB80 (Fig. 4c). The SEM of 1LB80 and 5LB80 revealed some kind of aggregation in between the microspheres. In addition, significantly larger amount of volcanic glass particles was observed for the sample 1LB80 than for the 5LB80, which was in line with QXRD results. The SEM images of the zeolite P synthesized from the PBM in 3 M NaOH at 80 � C after 72 h (2LB80) and 144 h (6LB80) showed knobby surfaced microspheres composed of lath-like zeolite P crystals (Fig. 4e and f). The most of the microspheres had sizes between 5 and 10 μm. The aggregation behavior in between the zeolite P microspheres was very common (Fig. 4f). The sample 6LB80 contained the larger number of zeolite P microspheres and lower number of volcanic glass particles than the 2LB80, which was in agreement with QXRD results. The SEM images of the zeolite X synthesized from the PBM in 5 M NaOH at 80 � C after 72 h (3LB80) revealed smaller particle size for zeolite X (Fig. 4h) compared with that for zeolite P (Fig. 4f) and phil­ lipsite (Fig. 4d). The sample 3LB80 showed rounded aggregates, typi­ cally ~1 μm in size, composed of large number of zeolite X nanocrystals (Fig. 4h). The crystals of zeolite X were well developed with sizes up to several 100’s nm. Zeolite X crystals often displayed octahedral morphology characteristic of FAU-type zeolite [57]. However, reaction of the PBM with 3 M NaOH at 80 � C for 72 h (2LB80) resulted in crys­ tallization of a small amount of zeolite X microspheres with the sizes of about 5 μm (Fig. 4g). The structural formulas of the synthesized phillipsite, zeolite P and X calculated from the EDX analyses are reported in Table 1. For any phillipsite and zeolite X, the sum of tetrahedral (Si þ Al) in the unit cell should equal 16 (half of the oxygens based on a cell of 32 oxygen atoms) and 12 (half of the oxygens based on a cell of 24 oxygen atoms), respectively [22,58]. For phillipsite (5LB80) and zeolite X (3LB80), however, the sum of tetrahedral (Si þ Al) was less than expected value (15.73 for phillipsite and 11.79 for zeolite X, Table 1), but it improved to 16 and 12, respectively, if the Fe3þ was added. The tetrahedral Si/(Al þ Fe3þ) ratio for phillipsite (5LB80) and zeolite X (3LB80) was 2.23 and 1.29, respectively. The framework of tetrahedra for phillipsite (5LB80), zeolite P (6LB80) and zeolite X (3LB80) contained 69, 63 and 56% of Si, respectively. Overall, the EDX results suggested that Fe3þ is part of the tetrahedral framework the 5LB80 phillipsite and 3LB80 zeolite X. Sheppard and Fitzpatrick [58] reported Fe3þ in the tetrahedral frame­ work of phillipsites from saline, alkaline-lake deposits in the south­ western United States. The Na exchangeable cation greatly exceeded the other exchangeable cations, resulting in Na/Ca þ Mg þ K þ Fe3þ ratios of 2.21 for phillipsite (5LB80), 3.21 for zeolite P (6LB80) and 6.74 for zeolite X (3LB80). The predominant exchangeable cation of studied zeolites was Na (Table 1) because the zeolites were synthesized using NaOH solutions and the reaction products were consequently treated by NaCl solution in order to prepare Na-forms of zeolites. The structural formula of zeolite P (6LB80) suggested a small amount of Al as exchangeable cation (Table 1). Derkowski et al. [22] reported Al as exchangeable cation for zeolite X synthesized from fly ash in NaOH solution at room temperature. Overall, the Si/Al ratio of reaction products gradually decreased with increasing concentration of NaOH solution, as volcanic glass alteration proceeded (Table 1).

3.3. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) The SEM examination of samples reacted with 1–5 M NaOH solution showed hemispherical etch pits on the surface of most of volcanic glass particles (Fig. 4b). This etching, consistent with glass dissolution, was not observed for the starting PBM hence it occurred during the synthesis (Fig. 4a). The chemical composition (EDX analysis) for unreacted vol­ canic glass particles differed from that for altered volcanic glass reacted with NaOH. The average atomic ratio Si/Al ¼ 4.63 was determined for unreacted volcanic glass whereas lower value Si/Al ¼ 3.51 was deter­ mined for altered volcanic glass particles (Table 1). These results indi­ cated leaching of Si from volcanic glass after reaction with NaOH due to Table 2 Pore structure characteristics of the starting PBM and synthesized zeolitic materials. Sample

LOD (wt%)

SBET (m2/g)

St (m2/g)

Va (cm3/g)

Vmicro (cm3/g)

PBM 1LB80 2LB80 3LB50 3LB60 3LB70 3LB80 5LB80 6LB60 6LB70 6LB80 7LB50 7LB60 7LB70 7LB80 11LB60 11LB70 11LB80

1.7 4.8 12.8 5.7 11.6 17.3 19.5 10.4 9.1 14.4 15.4 10.4 17.0 19.5 19.0 4.8 7.7 16.1

2.7 6.1 22.8 49.3 131 297 362 11.5 54.7 135 48.8 151 222 324 287 38.6 129 247

2.3 3.9 10.4 11.6 25.4 35.4 38.9 7.5 9.5 21.7 18.6 29.4 38.8 38.0 34.2 10.2 25.8 34.7

0.008 0.020 0.042 0.071 0.147 0.225 0.262 0.023 0.051 0.109 0.081 0.158 0.251 0.285 0.259 0.057 0.119 0.208

0.0002 0.001 0.007 0.020 0.055 0.138 0.170 0.002 0.024 0.060 0.016 0.063 0.096 0.150 0.133 0.015 0.054 0.111

PBM – perlite by-product material, LOD – loss-on-degassing after heating to 200 � C for overnight under vacuum of 5 Pa; SBET – total specific surface area; St – specific surface area from mesopores, macropores and external specific surface area obtained from the t-plot; Va – total pore volume; Vmicro – micropore volume. 6

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Fig. 4. SEM images for starting PBM (a), 5LB80 (b), 1LB80 (c), 5LB80 (d), 2LB80 (e), 6LB80 (f), 2LB80 (g) and 3LB80 (h). Arrows on b indicate etch pits.

3.4. Chemistry of solutions

detected by XRD. The crystallization of zeolites from reacted solutions resulted in the consumption of elements released from volcanic glass. Fig. 5 showed decrease in Al concentrations for 3 M NaOH solution after 72 and 144 h due to zeolite formation. The PBM reacted with 3 M NaOH for 72 h at 80 � C contained 27 wt% of zeolites. The reacted solution contained 954 mg/L of Al (Table S2). After 144 h, the same PBM-NaOH mixture contained 46 and 68 wt% of zeolites at 70 and 80 � C, respec­ tively. The corresponding reacted solutions contained 685 and 465 mg/ L of Al, respectively (Table S2). Similar trend, i.e. decrease in Al con­ centration with crystallization of zeolites, was observed for 5 M NaOH solution after 72 and 144 h (Fig. 5). The QXRD analysis confirmed that increasing amount of synthesized zeolites was accompanied by higher Al consumption from the solution (Fig. 5, Tables 1 and S2).

The chemical composition of reacted solutions (Table S2) was mainly influenced by two processes, dissolution of volcanic glass and crystalli­ zation of reaction products (i.e. zeolites). The gradual increase in Si concentrations with temperature, time and NaOH concentration showed that dissolution rate of volcanic glass increased with increasing NaOH concentration, temperature and time (Fig. 5). The dissolution of volca­ nic glass was indicated also in distilled water (blank samples, Table S2) but the Si concentrations were approximately 100-times lower than those determined in 1 M NaOH (Fig. 5). Similar trend, i.e. increase in concentrations as glass dissolution proceeded, was observed for K and Al, however, only for solutions where no reaction products were 7

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Fig. 5. Concentrations of elements in reacted solutions as a function of temperature, time and solution used (NaOH, water).

The previously published data showed that the pH of reacting solu­ tion (i.e. NaOH concentration in our case) is important parameter affecting the crystallization rate of zeolites [15,31]. Generally, an in­ crease in OH concentration of alkaline solution accelerates the crystal

growth and shortens the induction period before viable nuclei are formed. The high NaOH concentration (i.e. high pH) of reacting solution causes supersaturation of silicate and aluminate and formation of a large number of nuclei. The growth of the nuclei proceeds until the Al is

Fig. 6. Variations of Si/Al and Na/K molar ratios in reacted solutions as a function of NaOH concentration, temperature, time and synthesized solids mineralogy. 8

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mainly on the quantity and type of synthesized zeolites. A series of 7LB samples (i.e. 7LB50, 7LB60, 7LB70 and 7LB80) demonstrated a gradual increase in CEC values (from 174 to 360 meq/100g, Table 3) with higher total zeolite content (from 29 to 77 wt% of zeolites, Table 1). Phillipsite rich sample (5LB80, 68 wt% of phillipsite, CEC ¼ 242 meq/100g) had substantially lower CEC compared to the samples rich in zeolite X (7LB70, 59 wt% of X, CEC ¼ 339 meq/100g) and zeolite P (6LB80, 62 wt % of P, CEC ¼ 371 meq/100g). Based on the published data, the CECs for pure zeolite P, X and phillipsite are 500, 470 and 330 meq/100g, respectively [1,63,64]. The reference CECs based on [1,63,64], corrected for mineral composition of the synthesized samples (QXRD analysis, Table 1) were compared with the CEC values of the synthesized samples (Table 3). The comparison showed very good agreement (the difference being < 9%) for the sam­ ples containing � 68 wt% of synthesized zeolites (not shown). In addition, we calculated the CECs from the structural formulas (CECSF) of synthesized zeolites (based on the exchangeable cations content) reported in Table 1. The CECSF for pure, anhydrous zeolites X, P and phillipsite were 625, 553 and 455 meq/100g, respectively. The similar CECSF values (611–683 meq/100g) were calculated for zeolites X synthesized from fly ash [22]. The cation exchange took place primarily on the negatively charged sites (accessible to ammonium acetate molecules) located at the internal (channels) and external surfaces of zeolites. The distinct framework topology (i.e. dimensions, numbers and spatial configuration of chan­ nels) among different zeolite species synthesized from PBM contributed to different CEC values. The external cation exchange capacity (ECEC, i. e. CEC excluding the cation exchange capacity of internal surfaces) is mainly influenced by zeolite type and content, pH and textural charac­ teristics (e.g. crystal shape, size, crystallinity, aggregation behavior and particles arrangement) [65–67]. For the most widespread natural zeo­ lites, the ECEC does not exceed 25% of CEC value [67].

exhausted. The alkaline solution enables mixing of reactants and facil­ itates the nucleation and crystal growth of zeolites [30–32]. Fig. 6 shows the variation of Si/Al and Na/K molar ratios determined for reacted 1, 3 and 5 M NaOH solutions versus temperature, time and mineral composition of synthesized solids. A similar trend for Si/Al molar ratios was found for all reacted NaOH solutions; the Si/Al molar ratios increased with higher temperature and longer reaction time. For Na/K molar ratios the opposite trend was observed; decrease in Na/K molar ratios with higher temperature and longer time for all reacted NaOH solutions. These observations were in coincidence with stimula­ tion of the leaching rate of Si, Al and K as volcanic glass dissolution proceeded [10,56]. Furthermore, with increasing NaOH concentration decrease in the Si/Al molar ratios of reacted solutions was observed. For Na/K molar ratios, no significant changes were found with NaOH concentration. Phillipsite rich product (5LB80, 68 wt% of phillipsite, Table 1) was found in solution of Si/Al ¼ 200 and Na/K ¼ 24 whereas zeolite P (6LB80, 62 wt% of P, Table 1) and zeolite X (3LB80, 61 wt% of X, Table 1) rich products were found in solutions of Si/Al ¼ 52, Na/K ¼ 21 and Si/Al ¼ 35, Na/K ¼ 25, respectively (Fig. 6). These observations indicated that the Si/Al molar ratios had much stronger influence on the type and quantity of synthesized zeolites than the Na/K molar ratios (Fig. 6). A relationship was observed among the NaOH concentration, the Si/ Al ratio of synthesized zeolites (EDX analysis) and the Si/Al ratio of solutions (ICP analysis). Particularly, the Si/Al ratio of both, synthesized zeolites and solutions in which they were synthesized, decreased with increasing NaOH concentration. The reaction of PBM with more concentrated NaOH solutions tended to produce less siliceous zeolites (i.e. having lower Si/Al ratio) as re­ action products likely because the solubility of alumina increased more rapidly than the solubility of silica with increasing NaOH concetration (i.e. with increasing pH). These findings indicated that the Si/Al ratio of zeolites synthesized from PBM was primarily controlled by the con­ centration of NaOH solution. Similar relationships between the alka­ linity of solution and the Si/Al ratio of zeolites have been noted for natural environments [59,60] and in experimental studies [61,62].

3.6. Nitrogen adsorption isotherms, specific surface areas and pore-size distributions The adsorption-desorption isotherms of nitrogen on starting PBM and synthesized zeolitic materials are shown in Fig. 7. The PBM showed type II isotherm (according to the IUPAC classification [68,69]) with an extremely low amount of gas adsorption and very narrow hysteresis compared with the other samples, indicating the small external surface with negligible micropores (diameter < 2 nm) and mesopores (diameter 2–50 nm) of this volcanic glass rich sample. The isotherms for samples rich in phillipsite (5LB80) and zeolite P (6LB80) showed larger gas adsorption and more pronounced hysteresis but did not show a plateau at high relative pressure (P/P0) like type IV isotherm (Fig. 7). Rouquerol et al. [69] preferred the designation type IIB for such isotherms. The

3.5. Cation exchange capacity (CEC) The CECs determined by ammonium acetate method are reported in Table 3. The starting PBM had the lowest CEC value (6 meq/100g). The reaction products containing >50 wt% of zeolites had CECs in the range from 242 to 371 meq/100g. The CEC of the studied samples depended Table 3 Cation exchange capacity (CEC) of starting PBM and synthesized zeolitic ma­ terials based on the exchangeable cations released. Sample

Na (meq/ 100g)

K (meq/ 100g)

Ca (meq/ 100g)

Mg (meq/ 100g)

Fe (meq/ 100g)

CEC (meq/ 100g)

PBM

3 3 58 57 167 159 312 317 154 153 292 304 316 306 319 309

3 3 10 10 59 56 30 30 2 2 5 4 6 6 10 10

0 0 11 10 17 15 20 20 12 15 17 15 16 16 23 25

0 0 5 4 5 5 6 6 4 5 5 5 6 6 11 12

0 0 0 0 0 1 1 1 0 0 1 0 0 0 0 0

6�0

1LB80 5LB80 6LB80 7LB50 7LB60 7LB70 7LB80

82 � 1 242 � 9 371 � 4 174 � 3 324 � 6 339 � 8 360 � 5

Fig. 7. Adsorption-desorption isotherms for starting PBM and synthesized zeolitic materials.

PBM – perlite by-product material. 9

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Microporous and Mesoporous Materials xxx (xxxx) xxx

shape for the 5LB80 and 6LB80 isotherms indicated that the materials contained mesopores, which were responsible for the hysteresis and macropores which resulted in the steep slopes in the high relative pressure (P/P0 > 0.98). Furthermore, they (mainly 6LB80) had more micropores than the PBM as documented by a larger adsorbed volume of N2 at very low relative pressure (P/P0 < 0.01). The samples rich in zeolite X (7LB series, Fig. 7) showed type I isotherms with a very high amount of gas adsorbed at low relative pressure (P/P0 < 0.01) which indicated the mainly microporous nature of these samples. The narrow hysteresis indicated also the presence of meso-macropores. The com­ parison of the isotherms for the 7LB series showed that the adsorbed gas content for the entire P/P0 range increased with increasing amount of zeolite X in the synthesized samples (Fig. 7). The total BET specific surface area (SBET) values are given in Table 2. The PBM had SBET of 2.7 m2/g whereas the synthesized zeolitic samples had higher SBET (6.1–362 m2/g, Table 3). The SBET values for synthesized materials were controlled by mineralogy, mainly by the type and quantity of zeolites. The highest SBET was determined for zeolite X richest sample (3LB80, 61 wt% of X, SBET ¼ 362 m2/g, Table 2). In contrast, the samples with similar amount of phillipsite (5LB80, 68 wt% of phillipsite) and zeolite P (6LB80, 62 wt% of P) had substantially lower SBET than the zeolite X rich sample (11.5 m2/g for phillipsite rich and 48.8 m2/g for zeolite P rich samples, Table 2). Generally, the SBET values increased with increasing amount of zeolites. The comparison of the total BET specific surface area (SBET) and the tplot specific surface area from mesopores, macropores and external specific surface area (i.e. SBET excluding the volume of nitrogen adsor­ bed in micropores) (St) showed similar SBET and St values only for PBM sample, due to negligible contribution of micropores to SBET (Table 2). Generally, the higher contribution of micropores to SBET values, the larger discrepancy between SBET and St values. Similarly, the comparison of the total pore volume (Va) obtained from the N2 adsorption isotherms and the micropore volume (Vmicro) obtained by t-plot method showed negligible Vmicro value for the PBM (0.0002 cm3/g, Table 2), due to negligible amount of micropores. The synthesized samples rich in zeolite X (7LB70 and 3LB80) displayed the highest Vmicro values (0.15–0.17 cm3/g, Table 2) whereas phillipsite (5LB80) and zeolite P (6LB80) rich samples had substantially lower Vmicro values (0.002 cm3/g for 5LB80 and 0.016 cm3/g for 6LB80, Table 2). A significant positive correlation (p ¼ <0.0001 and R2 ¼ 0.983, Fig. S2) was found between zeolite X content and Vmicro, indicating that with increasing amount of zeolite X in the synthesized materials Vmicro values increased. Fig. 8 shows the pore-size distributions (PSDs) for studied samples obtained by treatment of the desorption branch of the isotherms. The PSDs for the PBM and phillipsite rich sample (5LB80) showed very low pore volume and no significant peak over the measured pore size range. The PSD for the zeolite P rich sample (6LB80) showed a broad peak

around 60 nm and the pore volume higher than that for PBM and phil­ lipsite rich sample (5LB80) but lower than that for zeolite X rich samples (7LB series), across the entire pore size range. The PSDs for the zeolite X rich samples (7LB series) showed higher pore volumes over the measured pore size range compared with the rest of the studied samples. The PSDs for the zeolite X rich samples revealed that the prominent peak shifted from 9 nm (7LB60) to 15 nm (7LB70) and 30 nm (7LB80). The shift towards a higher pore sizes may be related to the gradual increase of zeolite P admixture in 7LB60, 7LB70 and 7LB80 samples. The narrow and intense peak at 4 nm observed in the PSDs of all studied samples is an artifact which does not correspond to real pores [70]. The pore structure characteristics (nitrogen adsorption isotherms, specific surface areas and pore size distributions) along with SEM ob­ servations showed hierarchical porous structures for synthesized zeolitic materials which were associated with specific pore sizes. Micropores (diameter < 2 nm) in the studied samples were related with the pore spaces within channels, located in the zeolite structure. The channel dimensions and configurations differ between different zeolite species. The large dimensions of zeolite X channels (three-dimensional pore system with 12-membered oxygen ring channels of 0.74 � 0.74 nm [50]) enabled the accommodation of large number of N2 molecules in the channels, yielding the highest Vmicro, Va and SBET values for synthesized materials rich in zeolite X. On the other hand, channels dimensions of phillipsite (pore system consisted of three intersecting 8-membered oxygen ring channels of 0.38 � 0.38 nm, 0.30 � 0.43 nm and 0.32 � 0.33 nm in the [100], [010] and [001] directions, respectively [50]) and zeolite P (three-dimensional pore system with two intersect­ ing 8-membered oxygen ring channels of 0.31 � 0.45 nm and 0.28 � 0.48 nm in the [100] and [010] directions, respectively [50]) were too small for N2 to enter the channels. As a consequence, phillipsite and zeolite P rich samples displayed negligible number of micropores accessible by N2 compared with materials rich in zeolite X. Phillipsite and zeolite P rich materials adsorbed N2 mainly in mesopores (diameter 2–50 nm) and macropores (diameter > 50 nm) which were located outside the zeolite structure (e.g. crystal edges, defects and rough sur­ faces). The SEM observations and pore structure characteristics of studied samples indicated that mesopores and macropores accounted for porosity within zeolite aggregates (e.g. microspheres) generated by the arrangement of zeolite crystallites. The different shape, size and arrangement of zeolite P and phillipsite crystallites unambiguously contributed to distinct pore structure characteristics between phillipsite and zeolite P rich materials. Pores in the micron size range were mainly associated with the pore spaces between the aggregates (e.g. micro­ spheres) of zeolites. Similar multiscale porous structures for zeolites NaP1 and ZSM-5 synthesized from gel were reported by Sharma et al. [14] and Jia et al. [71]. 4. Conclusions A different zeolite species, namely phillipsite (PHI), zeolite X (FAU) and zeolite P (GIS) were synthesized from the single starting material, fine powdered perlite-by product material (PBM). The volcanic glass was the main PBM component transformed to zeolites after interaction with NaOH solutions. As volcanic glass alteration proceeded, crystalli­ zation of zeolites having a lower Si/Al ratio was favored. The concen­ tration of NaOH solution had significant impact on the type of synthesized zeolite (phillipsite vs. zeolite P vs. zeolite X) whereas the other experimental conditions such as temperature and time influenced mainly the quantity of synthesized zeolite species. The higher synthesis temperature and longer reaction time yielded higher amount of zeolites. The highest-grade zeolitic material synthesized from the PBM con­ tained 77 wt% of zeolites, 16 wt% of unaltered volcanic glass and 7 wt% of accessory minerals (mica and feldspars). The small amount of zeolites (11–29 wt%) was synthesized even at the lowest tested temperature (50 � C) using 5 M NaOH and longer reaction times (72 and 144 h). The materials containing from 6 to 54 wt% of zeolites were synthesized after

Fig. 8. Pore-size distributions (PSDs) for starting PBM and synthesized zeolitic materials. 10

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the shorter reaction time (24 h) but only when highly concentrated (5 M) NaOH was used. The synthesized zeolitic materials displayed hierarchical porous structures or building blocks, associated with different pore sizes of zeolite channels, zeolite crystallites and zeolite aggregates (micro­ spheres). The highest total specific surface area (SBET) and cation ex­ change capacity (CEC) determined for synthesized zeolitic materials were 362 m2/g and 371 meq/100g, respectively. CEC and pore structure characteristics (nitrogen adsorption isotherms, SBET and pore size dis­ tributions) of synthesized zeolitic materials were influenced mainly by the type and quantity of zeolites, distinct zeolite framework topology (i. e. dimensions, numbers and spatial configuration of channels) and textural characteristics (e.g. crystal shape, size, crystallinity, aggrega­ tion behavior and particles arrangement) among different zeolite species.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was supported by the Slovak Research and Development Agency (APVV-0339-12, APVV-17-0317 and APVV-15-0449) and Slovak Grant Agency VEGA (2/0156/17 and 1/0196/19). The authors � del Sannio, Italy), prof. X. are grateful to prof. A. Langella (Universita Querol (Institute of Environment Assessment and Water research, Spain) � n � a (Constantine the Philosopher University in Nitra, and Dr. J. Stub Slovakia) for providing pure phillipsite, NaP1 and sodalite samples, respectively. LBK PERLIT Ltd. is acknowledged for providing the perlite by-product material samples. Three anonymous reviewers are acknowledged for their constructive comments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109852. References [1] D.W. Breck, Zeolite Molecular Sieves, Wiley-Interscience, New York, 1974. [2] M. Visa, Powder Technol. 294 (2016) 338–347. [3] Y.W. Chiang, K. Ghyselbrecht, R.M. Santos, B. Meesschaert, J.A. Martens, Catal. Today 190 (2012) 23–30. [4] M. Noroozifar, M. Khorasani-Motlagh, H. Naderpour, Microporous Mesoporous Mater. 197 (2014) 101–108. [5] M. Delkash, B.E. Bakhshayesh, H. Kazemian, Microporous Mesoporous Mater. 214 (2015) 224–241. [6] A.K. Vipin, S. Ling, B. Fugetsu, Microporous Mesoporous Mater. 224 (2016) 84–88. [7] O.A.A. Moamen, H.A. Ibrahim, N. Abdelmonem, I.M. Ismail, Microporous Mesoporous Mater. 223 (2016) 187–195. [8] P.M. Nekhunguni, N.T. Tavengwa, H. Tutu, J. Environ. Manag. 197 (2017) 550–558. [9] U. Wirsching, Clay Clay Miner. 29 (1981) 171–183. [10] M. Kawano, K. Tomita, Clay Clay Miner. 45 (1997) 365–377. [11] S. Tangkawanit, K. Rangsriwatananon, A. Dyer, Microporous Mesoporous Mater. 79 (2005) 171–175. [12] V.P. Valtchev, K.N. Bozhilov, J. Phys. Chem. B 108 (2004) 15587–15598. [13] L. Ren, Q. Wu, C. Yang, L. Zhu, C. Li, P. Zhang, H. Zhang, X. Meng, F.-S. Xiao, J. Am. Chem. Soc. 134 (2012) 15173–15176. [14] P. Sharma, J.-S. Song, M.H. Han, C.-H. Cho, Sci. Rep. 6 (2016) 22734. [15] R.L. Hay, R.A. Sheppard, Natural zeolites: occurrence, properties, applications, in: D.L. Bish, D.W. Ming (Eds.), Reviews in Mineralogy and Geochemistry, vol. 45, Mineralogical Society of America, 2001, pp. 217–234. [16] C. Belviso, F. Cavalcante, G. Niceforo, A. Lettino, Powder Technol. 320 (2017) 483–497. [17] A.Y. Atta, B.Y. Jibril, B.O. Aderemi, S.S. Adefila, Appl. Clay Sci. 61 (2012) 8–13. [18] M. Kr� ol, W. Mozgawa, J. Morawska, W. Pich� or, Microporous Mesoporous Mater. 196 (2014) 216–222. [19] D.A. Holmes, in: D.D. Carr (Ed.), Industrial Minerals and Rocks, Society for Mining, Metallurgy and Exploration Inc., Littleton, 1994, pp. 1129–1158.

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