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Modification of textural and acidic properties of -SVR zeolite by desilication
Catalysis Today 227 (2014) 26–32
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Modification of textural and acidic properties of -SVR zeolite by desilication Martin Kubu˚ ∗ , Maksym Opanasenko, Mariya Shamzy J. Heyrovsky Institute of Physical Chemistry of the ASCR, v. v. i., Dolejˇskova 3, CZ-182 23 Prague 8, Czech Republic
a r t i c l e
i n f o
Article history: Received 14 June 2013 Received in revised form 16 October 2013 Accepted 25 November 2013 Available online 28 January 2014 Keywords: -SVR zeolite Desilication Hierarchical materials Textural and acidic properties
a b s t r a c t Desilication conditions (concentration of NaOH and TPAOH solution, duration of the treatment) of SVR zeolite were varied and related to the textural and acidic properties of the obtained materials. The treatment of -SVR zeolite with NaOH solution led to the formation of mesopores and also the increasing of the concentration of surface acid sites, accessible for bulky molecules (e.g. 2,6-ditertbutyl-pyridine). The volume of mesopores as well as the concentration of accessible acid centres in desilicated materials depends mainly on the concentration of the alkaline solution. The increase in the pH of the treatment resulted in moderate decrease of both micropore volume and Brønsted acid sites concentration, while the volume of mesopores increased significantly. Independently of the duration and pH of the treatment with NaOH solution, all desilicated -SVR zeolites are characterized by broad pore-size distribution in the range of 5–20 nm with the maximum around 14 nm. At fixed pH of the treatment with TPAOH, the volume of mesopores and their average size decreased, while the Si/Al ratio and the concentration of Brønsted acid sites in desilicated materials increased which indicates the inhibiting effect of TPA+ cations on the extraction of Si from -SVR zeolite. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Zeolites, crystalline aluminosilicates with ordered networks of micropores, are utilized in numerous large-scale chemical technologies as heterogeneous catalysts due to their unique properties such as high surface area, high thermal stability, adjustable acidity and shape-selectivity [1,2]. Catalytic applications of zeolites cover a wide range of reactions, from oil refining, petrochemistry, fine chemical synthesis to biomass upgrade and separation processes [3–6]. Despite that, zeolites generally suffer from intracrystalline diffusion limitations because of the molecular dimensions of micropores. The size of zeolite pores (0.3–1.0 nm) also limits the accessibility of the active sites located in the channels for bulky reactants [7], which may negatively impact their catalytic performance for transformation of large molecules. In relation to this issue, big hopes were connected with the mesoporous molecular sieves, firstly synthesized in 1992. However, low thermal and hydrothermal stability as well as the low acidity of mesoporous molecular sieves strongly limit their application in catalysis [8–10]. At the same time, great efforts have been undertaken to obtain zeolites possessing micropores with the diameter higher than 0.85 nm (extra-large pores), which are promising for catalytic
∗ Corresponding author. ˚ E-mail address: [email protected] (M. Kubu). 0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.11.063
transformation of bulky molecules [11–18]. However the synthesis of such zeolites requires specially prepared templates. Recently, it was shown that a lamellar MFI zeolite is formed when using “gemini-type” poly(quaternary ammonium)surfactants composed of a long-chain alkyl group (C18 –C22 ) and several quaternary ammonium groups spaced by a C6 alkyl linkage [19]. Significant effort has been devoted to develop the approaches for the introduction of mesopores in zeolites to combine the perfect properties of both micro- and mesoporous materials. The creation of new materials, including zeolite nanosheets, by post-synthesis modification of layered materials was proposed. However, the application of this strategy is limited to several zeolite structures including MCM-22 precursor [20], pre-ferrierite [21] and germanosilicate UTL zeolite [22,23]. A substantial mesoporosity of zeolite crystals can be obtained by using carbon templating approach where carbon particles are included in the synthesis of the zeolite, thereby leaving pores in the zeolite matrix after combustion [24,25]. Although in this case the optimization of the synthesis parameters is usually required to obtain the material with an appropriate crystallinity. Recently, microwave-assisted hydrothermal method has been reported not only to provide distinct advantages over the conventional synthesis (e.g. rapid heating, homogeneous nucleation, supersaturation by the rapid dissolution of precipitated gels, shorter crystallization time) but also is an efficient way for mesopore generation by desilication [26,27]. Direct synthesis of carbon-templating mesoporous ZSM-5 using microwave heating was shown by Park et al. [25].
M. Kub˚ u et al. / Catalysis Today 227 (2014) 26–32
At the same time, mesopores in zeolite crystals can also be easily created by post-synthesis treatments resulting in dealumination (e.g. steaming, acid leaching or their combination [28]) or in desilication (e.g. base leaching [8]). While removal of aluminium from the framework results in decreasing concentration of acid centres in zeolites, silicon extraction is known as an effective approach to create transport mesopores in various zeolites by preferential extraction of framework Si, preserving the Al environment and the related acidic properties. Unfortunately, the major drawback of desilication remained in limited applicability to zeolites with Si/Al ratio 25–50 in the framework [29], until Verboekend has shown the controlled formation of mesopores in high-silica zeolites through pore-directing agents (PDA, e.g. Al(OH)4 – , TPA+ ) [30]. Since that time, desilication methods were intensively investigated to generate mesopores in zeolites of various chemical compositions and structure types (e.g. MFI [31], Y [32], IFR [33]). Recently synthesized high silica -SVR zeolite is a member of the medium pore zeolite family. It possesses 3-dimensional 10-ring pore system with ordered silicon vacancies – structure defects in tetrahedron surrounding four hydroxyl groups [34]. The aluminosilicate -SVR was active in alkylation and transalkylation reactions of aromatic hydrocarbons, isomerization of C4 –C7 hydrocarbons and olefins to aromatics [35]. The development of intracrystalline mesopores within -SVR crystals upon framework silicon extraction may open new perspectives in the application of this zeolite in catalysis. In this work, we present a detailed study of the influence of the desilication conditions (e.g. NaOH and TPAOH concentration, duration of the treatment) of zeolite -SVR (Si/Al = 41) on the structural, textural and acidic properties of the formed micro/mesoporous materials to identify the crucial parameters for the formation of intracrystalline mesopores, while preserving the original acidity of the zeolite. 2. Experimental part 2.1. 2.1 Synthesis of hexamethylene-1,6-bis-(N-methyl-N-pyrrolidinium) hydroxide 3.5 g of N-methylpyrrolidine (97%, Aldrich) was dissolved in 50 ml of acetone. Then 4.9 g of 1,6-dibromohexane (98%, Aldrich) was added, and the resulting solution was stirred for 3 days at room temperature. The formed solid product was recovered by filtration, washed with diethyl ether and dried. Hexamethylene-1,6-bis-(Nmethyl-N-pyrrolidinium) bromide was converted into hydroxide form by ion exchange with AG1-X8 (Bio-Rad) resin. The successful synthesis of the structure-directing agent (SDA) was confirmed by 1 H NMR spectroscopy after dissolution in methanol-d4 (Fig. 1). 6 signals, attributed to differently shielded hydrogen atoms in the SDA molecule, were found. 1 H NMR (300 MHz, CD3 OD): ␦ 1.51 (m, 4H), 1.88 (m, 4H), 2.24 (m, 8H), 3.12 (s, 6H), 3.44 (m, 4H), 3.60 (t, 8H). The signal at 3.34 ppm (marked with asterisk) belongs to the solvent. 2.2. Synthesis of -SVR zeolite The synthesis of parent -SVR zeolite was performed according to the Ref. [36]. The starting gel had the following composition: 40 SiO2 :1 AlO1.5 :6 SDA(OH)2 :1200 H2 O. In particular, 4.2 g of deionised water, 5.3 g Ludox LS-30 (30% SiO2 ), 11.9 ml of 0.33 M hexamethylene-1,6-bis-(N-methyl-Npyrrolidinium) hydroxide solution and 1.67 g of 15% Al(NO3 )3 solution were mixed and stirred for 30 min. The resulting fluid gel was charged into 25 ml Teflon-lined autoclave and heated at 160 ◦ C
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Fig. 1. 1 H NMR spectrum of hexamethylene-1,6-bis-(N-methyl-N-pyrrolidinium) bromide.
for 15 days under agitation (∼25 rpm). The solid product was separated by filtration, washed with distilled water and dried overnight at 95 ◦ C. The SDA was removed by calcination. Products were heated from room temperature to 120 ◦ C at a rate of 1 ◦ C/min, and the temperature was maintained for 2 h. The next step involved increasing temperature up to 540 ◦ C for 5 h at a rate of 1 ◦ C/min. Finally, the temperature was increased up to 580 ◦ C at a rate of 1 ◦ C/min and the product was kept at this temperature for 5 h. 2.3. Desilication Desilication procedure was based on Refs. [31,37]. Treatments were performed in 10 cm3 glass flasks. The alkaline solution (5 cm3 ) was stirred and heated to 65 ◦ C, after which the zeolite sample (0.167 g) was added. The resulting mixture was left to react under reflux for different periods of time (Table 1) followed by quenching. The solid product was centrifugated and extensively washed out with distilled water to reach neutral pH and then dried overnight in an oven at 60 ◦ C. In the case of TPAOH presence in the alkaline solution, calcination at 550 ◦ C for 5 h (heating rate 5 ◦ C/min) was applied after the treatment to remove occluded TPA+ species. Na+ forms of -SVR samples were converted to NH4 + form by three-fold treatment with 1.0 M NH4 NO3 solution at room temperature for 3 h. The samples, treated with NaOH solution, are designated according with the expression: -SVR/NaOH conc. (M)/time (min). When treated with the mixture of NaOH and TPAOH, the samples are designated as -SVR/NaOH conc. (M) + TPAOH conc. (M)/time (min). 2.4. Characterization 1 H NMR spectrum of the organic SDA was recorded on a Varian Mercury 300 spectrometer at 300.0 MHz in CD3 OD solutions at 25 ◦ C. Chemical shifts (␦/ppm) are given relative to residual CHD2 OD signals (␦H 3.34 ppm). The crystallinity of all samples under investigation was checked by X-ray powder diffraction (XRD) using a Bruker AXS-D8 Advance diffractometer with a graphite monochromator and a position sensitive detector Våntec-1 using CuK␣ radiation in Bragg–Brentano geometry. The size and shape of zeolite crystals were examined by scanning electron microscopy (SEM, JEOL JSM-5500LV microscope). For the measurement crystals were coated with a thin platinum layer by sputtering in vacuum chamber of a BAL-TECSCD-050. Nitrogen adsorption/desorption isotherms were measured on a Micromeritics GEMINI II 2370 volumetric Surface Area Analyzer at
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Table 1 Experimental conditions and respective textural properties. Sample
-SVR/0/0 (parent) -SVR/0.02/15 -SVR/0.02/30 -SVR/0.02/60 -SVR/0.05/30 -SVR/0.10/30 -SVR/0.20/15 -SVR/0.20/30 -SVR/0.05 + 0.05/30 -SVR/0.05 + 0.15/30 a b c
Treatment conditions pH
min
– 12.3
– 15 30 60 30 30 15 30 30 30
12.7 13.0 13.3 12.9 13.2
BET (m2 /g)
Vmic (cm3 /g)
Vmeso (cm3 /g)
Vtot (cm3 /g)
wc (%)
Si/Ala
468.0 389.0 403.2 393.8 391.1 404.8 448.4 451.2 363.7 445.8
0.17 0.14 0.14 0.14 0.12 0.12 0.10 0.10 0.11 0.12
– 0.14 0.16 0.18 0.21 0.21 0.41 0.49 0.20 0.18
0.17 0.33 0.33 0.34 0.34 0.35 0.52 0.59 0.33 0.39
– 6.0 10.6 7.9 9.1 13.4 48.6 52.0 11.1 19.5
41 39 36 n.d.b n.d. n.d. 22 18 n.d. 35
Chemical analysis. Not determined. Weight reduction after desilication [m(parent) − m(desilicated)]/m(parent) × 100%.
liquid nitrogen temperature (−196 ◦ C) to determine surface area, pore volume and pore size distribution. Prior to the sorption measurements, all samples were degassed on a Micromeritics FlowPrep 060 instrument under helium at 300 ◦ C (heating rate 10 ◦ C/min) for 6 h. The chemical composition of the samples was determined by elemental analysis. For this purpose 0.2–0.3 g of zeolite was heated at 70 ◦ C with 5–7 ml of 10 M NaOH in a platinum cup. After the total dissolution of the sample, 10–15 ml of concentrated HCl was added until pH became 0.6–0.7. Then acid solution was heated at 80 ◦ C for 20 min to coagulate the precipitated SiO2 × xH2 O. The precipitate was recovered by filtration on ashless filter, washed out with 1 M HCl, and subsequently with hot water for complete removal of AlCl3 . Precipitated SiO2 × xH2 O was dried at 90 ◦ C, calcined at 1000 ◦ C until the constant mass, and weighed. Al contents were determined by back-complexation titration using the following procedure. 4.0 ml of 0.10 M EDTA was added to the analyzed solution and warmed to boiling, then cooled down and neutralized with NH3 ·H2 O (25% solution) until pH = 3.0 was reached. Then 20 ml of acetate buffer solution (pH = 6.0) was added to adjust the pH. This solution was boiled for 5 min to ensure complete complexation of the Al3+ cations and cooling down to the room temperature. The excess of EDTA was titrated with 0.001 M ZnSO4 in the presence of xylenol orange until the colour change from yellow to violet. Concentration of Lewis (cL ) and Brønsted (cB ) acid sites was determined after adsorption of pyridine (PYR) by FTIR spectroscopy on Nicolet Protégé 460 Magna with a transmission DTGS and MTC/A detector. Zeolites were pressed into self supporting wafers with a density of 8.0–12 mg/cm2 and activated in situ at 450 ◦ C overnight. Pyridine adsorption was carried out at 150 ◦ C for 20 min at partial pressure 600–800 Pa, followed by desorption for 20 min. Before adsorption pyridine was degassed by freezing and thawing cycles. All spectra were recorded with a resolution of 4 cm−1 by collecting 128 scans for a single spectrum at room temperature. Spectra were recalculated on wafer density of 10 mg/cm2 . Concentration of cL and cB were evaluated from the integral intensities of bands at 1454 cm−1 (cL ) and at 1545 cm−1 (cB ) using extinction coefficients, ε(L) = 2.22 cm/mol, and ε(B) = 1.67 cm/mol [38]. A relatively large probe molecule 2,6-di-tert-butyl-pyridine (DTBP) was used to determine the accessibility of acid sites within prepared zeolites [39]. The adsorption of DTBP took place at 150 ◦ C and at equilibrium probe vapour pressure with the zeolite wafer for 15 min. Desorption proceeded at the same temperature for 1 h followed by collection of spectra at room temperature. Extinction coefficients for pyridine [38] were used for the quantitative analysis evaluation of cB .
3. Results and discussion 3.1. The structure of desilicated zeolites -SVR/0/0 zeolite was prepared with Si/Al ratio of 41, which was shown to be optimal [29] for introduction of mesopores by applying treatment with NaOH solutions. Different experimental techniques have been applied to characterize in detail structural properties of -SVR zeolites under investigation. The X-ray diffraction pattern of the parent -SVR zeolite (Fig. 2C, -SVR/0/0) matches well with that one reported in the literature [34,40]. Fig. 2A–C shows XRD patterns of parent and all desilicated -SVR zeolites. It can be seen, that the samples subjected to alkaline treatment in different conditions still display sharp diffraction lines between 5◦ and 40◦ at the characteristic 2-theta positions, which proves the preservation of the long-range crystal ordering during the post-synthesis treatments under experimental conditions. 3.1.1. The effect of the concentration of NaOH solution The decrease in the intensity of the characteristic diffraction lines with increasing concentration of NaOH solution (from 0.02 to 0.20 M) at the same time (Fig. 2A) indicates the progressive decreasing of the framework density during leaching of the framework Si atoms. The concentration of alkaline solution used for the desilication seems to be even more crucial parameter influencing the framework density of the samples. While desilication with 0.02 and 0.05 M NaOH solutions (samples -SVR/0.02/30 and -SVR/0.05/30) did not lead to any essential changes in the
Fig. 2. XRD patterns of -SVR zeolites. The effect of NaOH concentration at the same time of the treatment (A), the time of the treatment at the same NaOH concentration (B) and the concentration of the pore-directing agent (C) compared to the parent zeolite.
M. Kub˚ u et al. / Catalysis Today 227 (2014) 26–32
intensities of the characteristic diffraction lines (Fig. 2A), the increase in the concentration of NaOH up to 0.1 M (-SVR/0.10/30) and especially 0.2 M (-SVR/0.20/30) led to a significant drop in the intensity of corresponding diffraction lines. 3.1.2. The effect of the duration of the treatment The duration of the treatment (at the same concentration of the alkaline solution) had smaller impact on the structural changes proceeded during the silicon extraction from the framework, when compared with previously discussed effect of the concentration of NaOH solution (at the same time). No changes are seen in the intensities of the typical diffraction lines for desilication performed with 0.02 M NaOH after 15 and 30 min (-SVR/0.02/15 and -SVR/0.02/30), while significant decrease was observed when the treatment was prolong to 60 min (Fig. 2B). The substantial decrease in the intensity of the diffraction lines after 30 min for the treatment with 0.2 M NaOH (-SVR/0.20/15 -SVR/0.20/30) supports the idea that the concentration of the alkaline solution is the more crucial parameter influencing the framework density (Fig. 2B). 3.1.3. The effect of the pore-directing agent (PDA) It should be noted, that both desilicated samples obtained during the treatment of -SVR/0/0 with TPAOH + NaOH solution, i.e. -SVR/0.05 + 0.05/30 and especially -SVR/0.05 + 0.15/30 ([TPA+ ]/[OH– ] = 0.75, [OH− ] = 0.20 M), showed more intensive diffraction lines (Fig. 2C) in comparison to -SVR/0.20/30 sample, treated exclusively with 0.20 M NaOH solution. It may be due to the stabilization effect of relatively high polarizable TPA+ cations, which were shown to bind to the zeolite surface in alkaline medium providing a protective layer on the zeolites external surface and act as “pore-growth moderators” in the desilication process [30,31,41]. 3.2. The textural properties of desilicated zeolites Nitrogen isotherms provided valuable information on the textural properties of desilicated samples when compared with the parent one. Obtained isotherms agree well with the structural changes determining by means of XRD. In contrast to parent -SVR/0/0 zeolite (Fig. 3C) which exhibits type I isotherm characteristic for microporous solids, all desilicated samples showed combined type I and type IV isotherm, being typical for hierarchical micro/mesoporous materials [8]. Total pore volume of parent -SVR/0/0 is 0.17 cm3 /g and no presence of mesopores was observed (Table 1). According to the SEM images, the micrographs of the parent and desilicated zeolites represent quite small crystals (ca. 5 m × 8 m × 0.5 m, Fig. 4). Therefore, adsorption of nitrogen in
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the range of p/p0 = 0.8–1.0 can be explained by filling of intercrystalline pores. The results of the chemical analysis indicate decreasing Si/Al ratio in the desilicated samples with increasing duration of the treatment and/or concentration of NaOH solution in the following sequence (see Table 1): -SVR/0/0 (Si/Al = 41) > -SVR/0.02/15 (Si/Al = 39) > -SVR/0.02/30 (Si/Al = 36) > -SVR/0.20/15 (Si/Al = 22) > -SVR/0.20/30 (Si/Al = 18), which is due to the progressive selective extraction of Si atoms from the zeolite framework. At the same time, the sample -SVR/0.20/30 (Si/Al = 18, formed during the treatment of parent sample -SVR/0/0 with 0.2 M NaOH solution) is characterized by significantly lower Si/Al ratio in comparison to -SVR/0.05 + 0.15/30 (Si/Al = 35), obtained in the presence of TPA+ cations ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.2 M). The last result proved the inhibiting role TPA+ cations in the process of silicon extraction from the framework of -SVR zeolite and corresponds to the data, obtained for zeolite MFI [31]. 3.2.1. The effect of the concentration of NaOH solution The increase of the concentration of NaOH solution from 0.02 to 0.05 and 0.10 M led to the enhancement of mesopore volume from 0.16 to 0.21 cm3 /g, while the volume of micropores decreased from 0.17 (-SVR/0/0) to 0.12 cm3 /g (-SVR/0.10/30). At the same time, the treatment of -SVR/0/0 with 0.20 M NaOH solution resulted in significant increase in the volume of mesopores up to 0.41 cm3 /g (-SVR/0.20/15) and 0.49 cm3 /g (-SVR/0.20/30), respectively, while the volume of micropores decreased by 41% to 0.10 cm3 /g (Table 1). These changes are nicely seen from the change of the shape of the nitrogen isotherm (Fig. 3A). 3.2.2. The effect of the duration of the treatment The alkaline treatment of -SVR/0/0 with 0.02 M NaOH for 15–60 min resulted in a slight decrease of the micropore volume (from 0.17 to 0.14 cm3 /g), while the increase in the volume of mesopores (0.14–0.18 cm3 /g) correlates with the duration of the treatment (Table 1). However, under the experimental conditions used, the duration of the treatment had smaller impact on the volume of mesopores formed, than the concentration of the used alkaline solution (Fig. 3B). 3.2.3. The effect of the pore-directing agent (PDA) The mesopore volume of -SVR/0.20/30 sample exceeded almost 3 times (0.49 vs. 0.18 cm3 /g, Table 1) the value for -SVR/0.05 + 0.15/30 obtained by the treatment of parent SVR/0/0 zeolite with mixed NaOH/TPAOH solution for 30 min ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.20 M). These results correspond to the XRD data obtained for respective samples and prove the stabilization effect of TPA+ during desilication. BJH method [42] used for pore-size distribution (Fig. 5) revealed broad distribution of mesopores in the range of 5–20 nm with the maximum around 14 nm for all samples desilicated by using NaOH solutions of different concentrations independently of the time of the treatment. For the sample -SVR/0.05 + 0.15/30 (Fig. 5, wine curve) obtained by the treatment of parent sample (SVR/0/0) with mixed NaOH/TPAOH solution ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.20 M), the maximum is shifted to smaller mesopore size of ca 11 nm. It is in a good agreement with literature data [30]. 3.3. The nature and concentration of acid sites
Fig. 3. Nitrogen adsorption () and desorption (䊉) isotherms. The effect of NaOH concentration at the same time of the treatment (A), time of the treatment at the same NaOH concentration (B) and the concentration of the pore-directing agent (C) compared to the parent zeolite.
The nature of OH-groups within initial and desilicated -SVR zeolites was studied by FTIR spectroscopy. Three different bands were distinguished in the spectrum of parent -SVR/0/0 zeolite in the region of hydroxyl vibrations (Fig. 6A). The most intense bands observed at 3745 and 3724 cm−1 were attributed to external
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Fig. 4. SEM images of parent and treated -SVR zeolites: (A) -SVR/0/0; (B) -SVR/0.02/30; (C) -SVR/0.10/30; (D) -SVR/0.20/30; (E) -SVR/0.20/15; (F) -SVR/0.05 + 0.05/30.
and internal silanol groups, respectively. The band at 3616 cm−1 presents the bridging hydroxyl groups (Si (OH) Al). After pyridine adsorption, the intensities of the bands assigned to the silanol groups slightly decreased or remained almost unchanged. On the other hand, the band assigned to bridging hydroxyl groups having the acid character disappeared, evidencing the accessibility of all Brønsted acid centres for pyridine in the investigated samples. The adsorption of pyridine was accompanied by the appearance of a typical set of adsorption bands in the region 1400–1600 cm−1
Fig. 5. BJH desorption (dV/dD pore volume) pore-size distribution of -SVR samples.
(Fig. 6B). The absorption band around 1546 cm−1 is due to the interaction of pyridine with Brønsted acid sites, while a new band around 1454 cm−1 is characteristic for the pyridine adsorbed on Lewis acid sites. The concentrations of Brønsted and Lewis acid sites calculated from the integral intensities of the bands at 1546 and 1454 cm−1 using extinction coefficients [38] are given in Table 2.
Fig. 6. IR spectra of -SVR zeolites: (A) region of hydroxyl vibrations; (B) region of pyridine vibrations before (bold spectra) and after adsorption of pyridine.
M. Kub˚ u et al. / Catalysis Today 227 (2014) 26–32
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Table 2 The concentration of Lewis and Brønsted acid sites within -SVR zeolites, determined by means of FTIR spectroscopy of adsorbed pyridine and 2,6-di-tert-butylpyridine. Sample
cB (mmol/g) PYR
-SVR/0/0 -SVR/0.02/15 -SVR/0.02/30 -SVR/0.20/15 -SVR/0.20/30 -SVR/0.05 + 0.15/30
0.127 0.120 0.122 0.109 0.107 0.127
cL (mmol/g) DTBP
PYR
0.039 0.040 0.054 0.077 0.079 0.076
0.046 0.081 0.056 0.063 0.064 0.023
3.3.1. The effect of the concentration of NaOH solution and duration of the treatment The increase of the concentration of NaOH solution or the duration of the alkaline treatment was accompanied with the enhancement of the intensity of the band of isolated silanol groups at 3745 cm−1 (-SVR/0/0 ≈ -SVR/0.02/30 -SVR/0.20/15 < SVR/0.20/30), see Fig. 6A, while the intensity of the band at 3724 cm−1 decreased. This indicates the expected growth of the concentration of external SiOH groups in the progress of the formation of mesopores within the zeolite -SVR. As can be seen, with the increasing concentration of NaOH solution used for the preparation of desilicated -SVR zeolites, the concentration of Brønsted acid sites was progressively decreasing (i.e. 0.127, 0.120 and 0.109 mmol/g for -SVR/0/0, -SVR/0.02/15 and -SVR/0.20/30, respectively), while the duration of the alkaline treatment had negligible effect on the amount of Brønsted acid sites in the obtained materials (i.e. 0.120 and 0.122 mmol/g for -SVR/0.02/15 and -SVR/0.02/30, respectively). At the same time the concentration of Lewis acid sites in the desilicated samples, prepared using NaOH solutions of different concentrations, usually exceeded the respective value for parent -SVR/0/0 zeolite. This result may originate from the equilibrium processes, which take place during desilication: (1) releasing of framework Al atoms (which form either Brønsted or Lewis acid sites), resulting in the decreasing of the amount of acid centres within zeolite; (2) realumination of the surface of the material with Al(OH)4 − , which was shown to accompany with the reinsertion of Al atoms into the framework [29]. 3.4. The surface acidity The surface acidity of -SVR zeolites was studied by FTIR spectroscopy of adsorbed 2,6-di-tert-butylpyridine. The size of the DTBP molecule is about 0.79 nm [39], which is higher than the diameter of 10-ring channels in zeolites that makes accessible only acid centres on the outer surface of parent -SVR/0/0 crystals. Indeed, as it can be seen from Fig. 7A, the adsorption of DTBP on the parent -SVR/0/0 zeolite led to only partial disappearance of the band at 3616 cm−1 , corresponding to bridging hydroxyl groups. In the region of chemically adsorbed DTBP a number of bands were detected. According to the previous work of Corma et al. [39] the bands at 3370, 1616 and 1530 cm−1 can be attributed to the formation of DTBPyH+ ions. The absence of a band at 1545 cm−1 confirms no dealkylation of the probe molecule used. It can be seen, that the concentration of accessible (surface) acid sites increases with the increasing of the intensity of the alkaline treatment in the following sequence (Table 2): -SVR/0/0 < -SVR/0.02/30 < SVR/0.20/15 < -SVR/0.20/30, which correlates with the increasing of mesopore volume for the respective samples. 3.4.1. The effect of the pore-directing agent (PDA) In contrast to desilicated samples obtained by the treatment with NaOH solutions, the sample -SVR/0.05 + 0.15/30 prepared
Fig. 7. IR spectra of -SVR zeolites: (A) region of hydroxyl vibrations; (B) region of DTBP vibrations before (bold spectra) and after adsorption of DTBP.
with the mixed NaOH/TPAOH solution ([TPA+ ]/[OH− ] = 0.75, [OH– ] = 0.20 M) contains significantly lower amount of Lewis acid sites (0.023 mmol/g) in comparison with the parent -SVR/0/0 sample (0.046 mmol/g). At the same time, the concentration of Brønsted acid sites for both -SVR/0.05 + 0.15/30 and SVR/0/0 is 0.127 mmol/g. These results may be connected with the “protective” effect of TPA+ cations on the Brønsted acid sites, which leads to the preferential releasing of Lewis acid centres from zeolite during desilication. The mentioned effect is presumably originate from the stronger binding of TPA+ to the anions on zeolite surface due to the additional contribution of hydrophobic and van der Waals forces, which do not act in the case of Na+ interaction with the surface of zeolite. Since the total concentration of acid centres for -SVR/0.05 + 0.15/30 is lower compared with the parent -SVR/0/0 sample, the formation of Lewis acid centres in the progress of realumination in the presence of TPA+ can be hindered. However, this issue requires further investigation. Moreover, despite comparably low volume of the formed mesopores (0.18 cm3 /g, Table 1), the sample -SVR/0.05 + 0.15/30 contains unexpectedly high amount of acid centres, accessible for DTBPy (0.076 mmol/g) if compared with -SVR/0.02/30 (0.054 mmol/g, Table 2), which is characterized by close mesopore volume (0.16 cm3 /g, Table 1). The last result may be connected with the distinctive features of de-/realumination processes, taking place during the silicon extraction from zeolite frameworks in the presence of TPAOH and representing a challenge to be investigated in details. 4. Conclusions In the present work the effect of the concentration of NaOH solution and pore-growth moderator TPAOH as well as the duration of desilication treatment on textural and acidic properties of -SVR zeolite was established. It was shown, that desilication of -SVR zeolite (Si/Al = 41) with 0.02–0.20 M NaOH solutions at 65 ◦ C for 15–60 min results in formation of transport mesopores with broad pore size distribution (5–20 nm). While the volume of the formed mesopores increased with increasing concentration of NaOH solution reaching 0.49 cm3 /g after 30 min treatment with 0.20 M NaOH, the mesopore size distribution was effected neither by the concentration of alkaline solution used nor by the duration of desilication. The increase in the concentration of TPAOH ([TPA+ ]/[OH− ] = 0–0.75) at fixed pH, resulted in decrease of the volume of formed mesopores
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and their average size, confirming the pore-moderating role, which TPA+ cations play during desilication of zeolites. All desilicated -SVR zeolites prepared using NaOH solutions are characterized by increased concentration of Lewis acid centres and higher surface acidity in comparison to parent zeolite. The concentration of acid sites accessible for 2,6-di-tert-butylpyridine correlates with the mesopore volume of desilicated samples, reaching the maximal value for–SVR zeolite, treated with 0.20 M NaOH solution for 30 min. In contrast, desilication of -SVR zeolite (Si/Al = 41) with mixed NaOH/TPAOH solution ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.20 M) results in decreasing of the concentration of Lewis acid sites, while the increasing of the surface acidity could hardly be related to the volume of formed mesopores. The last result may indicate the specific effect of TPA+ cations on the rearrangement of acid centres in zeolites during desilication process, which represents an intriguing issue for further investigation. Acknowledgements The authors thank the Czech Science Foundation for the support (13-17593P) and RNDr. Libor Brabec, CSc. for SEM images. References ˇ [1] J. Cejka, H. van Bekkum, A. Corma, F. Schüth (Eds.), Introduction to Zeolite Science and Practice, Stud. Surf. Sci. Catal., vol. 168, 3rd ed., Elsevier, Amsterdam, 2007. ˇ [2] J. Cejka, A. Corma, S.I. Zones (Eds.), Zeolites and Catalysis: Synthesis, Reactions and Applications, Wiley-VCH, Weinheim, 2010. [3] W. Vermeiren, J.P. Gilson, Top. Catal. 52 (2009) 1131–1161. ˇ [4] J. Cejka, G. Centi, J. Pérez-Pariente, W.J. Roth, Catal. Today 179 (2012) 2–15. [5] D. Kubiˇcka, Collect. Czech. Chem. Commun. 73 (2008) 1015–1044. ˇ [6] D. Kubiˇcka, I. Kubiˇcková, J. Cejka, Catal. Rev. 55 (2013) 1–78. [7] A. Corma, Chem. Rev. 97 (1997) 2373–2419. [8] J. Pérez-Ramírez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen, Chem. Soc. Rev. 37 (2008) 2530–2542. ˇ [9] R.M. Martín-Aranda, J. Cejka, Top. Catal. 53 (2010) 141–153. [10] Ch.T. Kresge, W.J. Roth, Chem. Soc. Rev. 42 (2013) 3663–3670. [11] J. Jiang, J. Yu, A. Corma, Angew. Chem. Int. Ed. 49 (2010) 3120–3145.
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Modification of textural and acidic properties of -SVR zeolite by desilication Martin Kubu˚ ∗ , Maksym Opanasenko, Mariya Shamzy J. Heyrovsky Institute of Physical Chemistry of the ASCR, v. v. i., Dolejˇskova 3, CZ-182 23 Prague 8, Czech Republic
a r t i c l e
i n f o
Article history: Received 14 June 2013 Received in revised form 16 October 2013 Accepted 25 November 2013 Available online 28 January 2014 Keywords: -SVR zeolite Desilication Hierarchical materials Textural and acidic properties
a b s t r a c t Desilication conditions (concentration of NaOH and TPAOH solution, duration of the treatment) of SVR zeolite were varied and related to the textural and acidic properties of the obtained materials. The treatment of -SVR zeolite with NaOH solution led to the formation of mesopores and also the increasing of the concentration of surface acid sites, accessible for bulky molecules (e.g. 2,6-ditertbutyl-pyridine). The volume of mesopores as well as the concentration of accessible acid centres in desilicated materials depends mainly on the concentration of the alkaline solution. The increase in the pH of the treatment resulted in moderate decrease of both micropore volume and Brønsted acid sites concentration, while the volume of mesopores increased significantly. Independently of the duration and pH of the treatment with NaOH solution, all desilicated -SVR zeolites are characterized by broad pore-size distribution in the range of 5–20 nm with the maximum around 14 nm. At fixed pH of the treatment with TPAOH, the volume of mesopores and their average size decreased, while the Si/Al ratio and the concentration of Brønsted acid sites in desilicated materials increased which indicates the inhibiting effect of TPA+ cations on the extraction of Si from -SVR zeolite. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Zeolites, crystalline aluminosilicates with ordered networks of micropores, are utilized in numerous large-scale chemical technologies as heterogeneous catalysts due to their unique properties such as high surface area, high thermal stability, adjustable acidity and shape-selectivity [1,2]. Catalytic applications of zeolites cover a wide range of reactions, from oil refining, petrochemistry, fine chemical synthesis to biomass upgrade and separation processes [3–6]. Despite that, zeolites generally suffer from intracrystalline diffusion limitations because of the molecular dimensions of micropores. The size of zeolite pores (0.3–1.0 nm) also limits the accessibility of the active sites located in the channels for bulky reactants [7], which may negatively impact their catalytic performance for transformation of large molecules. In relation to this issue, big hopes were connected with the mesoporous molecular sieves, firstly synthesized in 1992. However, low thermal and hydrothermal stability as well as the low acidity of mesoporous molecular sieves strongly limit their application in catalysis [8–10]. At the same time, great efforts have been undertaken to obtain zeolites possessing micropores with the diameter higher than 0.85 nm (extra-large pores), which are promising for catalytic
∗ Corresponding author. ˚ E-mail address: [email protected] (M. Kubu). 0920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.11.063
transformation of bulky molecules [11–18]. However the synthesis of such zeolites requires specially prepared templates. Recently, it was shown that a lamellar MFI zeolite is formed when using “gemini-type” poly(quaternary ammonium)surfactants composed of a long-chain alkyl group (C18 –C22 ) and several quaternary ammonium groups spaced by a C6 alkyl linkage [19]. Significant effort has been devoted to develop the approaches for the introduction of mesopores in zeolites to combine the perfect properties of both micro- and mesoporous materials. The creation of new materials, including zeolite nanosheets, by post-synthesis modification of layered materials was proposed. However, the application of this strategy is limited to several zeolite structures including MCM-22 precursor [20], pre-ferrierite [21] and germanosilicate UTL zeolite [22,23]. A substantial mesoporosity of zeolite crystals can be obtained by using carbon templating approach where carbon particles are included in the synthesis of the zeolite, thereby leaving pores in the zeolite matrix after combustion [24,25]. Although in this case the optimization of the synthesis parameters is usually required to obtain the material with an appropriate crystallinity. Recently, microwave-assisted hydrothermal method has been reported not only to provide distinct advantages over the conventional synthesis (e.g. rapid heating, homogeneous nucleation, supersaturation by the rapid dissolution of precipitated gels, shorter crystallization time) but also is an efficient way for mesopore generation by desilication [26,27]. Direct synthesis of carbon-templating mesoporous ZSM-5 using microwave heating was shown by Park et al. [25].
M. Kub˚ u et al. / Catalysis Today 227 (2014) 26–32
At the same time, mesopores in zeolite crystals can also be easily created by post-synthesis treatments resulting in dealumination (e.g. steaming, acid leaching or their combination [28]) or in desilication (e.g. base leaching [8]). While removal of aluminium from the framework results in decreasing concentration of acid centres in zeolites, silicon extraction is known as an effective approach to create transport mesopores in various zeolites by preferential extraction of framework Si, preserving the Al environment and the related acidic properties. Unfortunately, the major drawback of desilication remained in limited applicability to zeolites with Si/Al ratio 25–50 in the framework [29], until Verboekend has shown the controlled formation of mesopores in high-silica zeolites through pore-directing agents (PDA, e.g. Al(OH)4 – , TPA+ ) [30]. Since that time, desilication methods were intensively investigated to generate mesopores in zeolites of various chemical compositions and structure types (e.g. MFI [31], Y [32], IFR [33]). Recently synthesized high silica -SVR zeolite is a member of the medium pore zeolite family. It possesses 3-dimensional 10-ring pore system with ordered silicon vacancies – structure defects in tetrahedron surrounding four hydroxyl groups [34]. The aluminosilicate -SVR was active in alkylation and transalkylation reactions of aromatic hydrocarbons, isomerization of C4 –C7 hydrocarbons and olefins to aromatics [35]. The development of intracrystalline mesopores within -SVR crystals upon framework silicon extraction may open new perspectives in the application of this zeolite in catalysis. In this work, we present a detailed study of the influence of the desilication conditions (e.g. NaOH and TPAOH concentration, duration of the treatment) of zeolite -SVR (Si/Al = 41) on the structural, textural and acidic properties of the formed micro/mesoporous materials to identify the crucial parameters for the formation of intracrystalline mesopores, while preserving the original acidity of the zeolite. 2. Experimental part 2.1. 2.1 Synthesis of hexamethylene-1,6-bis-(N-methyl-N-pyrrolidinium) hydroxide 3.5 g of N-methylpyrrolidine (97%, Aldrich) was dissolved in 50 ml of acetone. Then 4.9 g of 1,6-dibromohexane (98%, Aldrich) was added, and the resulting solution was stirred for 3 days at room temperature. The formed solid product was recovered by filtration, washed with diethyl ether and dried. Hexamethylene-1,6-bis-(Nmethyl-N-pyrrolidinium) bromide was converted into hydroxide form by ion exchange with AG1-X8 (Bio-Rad) resin. The successful synthesis of the structure-directing agent (SDA) was confirmed by 1 H NMR spectroscopy after dissolution in methanol-d4 (Fig. 1). 6 signals, attributed to differently shielded hydrogen atoms in the SDA molecule, were found. 1 H NMR (300 MHz, CD3 OD): ␦ 1.51 (m, 4H), 1.88 (m, 4H), 2.24 (m, 8H), 3.12 (s, 6H), 3.44 (m, 4H), 3.60 (t, 8H). The signal at 3.34 ppm (marked with asterisk) belongs to the solvent. 2.2. Synthesis of -SVR zeolite The synthesis of parent -SVR zeolite was performed according to the Ref. [36]. The starting gel had the following composition: 40 SiO2 :1 AlO1.5 :6 SDA(OH)2 :1200 H2 O. In particular, 4.2 g of deionised water, 5.3 g Ludox LS-30 (30% SiO2 ), 11.9 ml of 0.33 M hexamethylene-1,6-bis-(N-methyl-Npyrrolidinium) hydroxide solution and 1.67 g of 15% Al(NO3 )3 solution were mixed and stirred for 30 min. The resulting fluid gel was charged into 25 ml Teflon-lined autoclave and heated at 160 ◦ C
27
Fig. 1. 1 H NMR spectrum of hexamethylene-1,6-bis-(N-methyl-N-pyrrolidinium) bromide.
for 15 days under agitation (∼25 rpm). The solid product was separated by filtration, washed with distilled water and dried overnight at 95 ◦ C. The SDA was removed by calcination. Products were heated from room temperature to 120 ◦ C at a rate of 1 ◦ C/min, and the temperature was maintained for 2 h. The next step involved increasing temperature up to 540 ◦ C for 5 h at a rate of 1 ◦ C/min. Finally, the temperature was increased up to 580 ◦ C at a rate of 1 ◦ C/min and the product was kept at this temperature for 5 h. 2.3. Desilication Desilication procedure was based on Refs. [31,37]. Treatments were performed in 10 cm3 glass flasks. The alkaline solution (5 cm3 ) was stirred and heated to 65 ◦ C, after which the zeolite sample (0.167 g) was added. The resulting mixture was left to react under reflux for different periods of time (Table 1) followed by quenching. The solid product was centrifugated and extensively washed out with distilled water to reach neutral pH and then dried overnight in an oven at 60 ◦ C. In the case of TPAOH presence in the alkaline solution, calcination at 550 ◦ C for 5 h (heating rate 5 ◦ C/min) was applied after the treatment to remove occluded TPA+ species. Na+ forms of -SVR samples were converted to NH4 + form by three-fold treatment with 1.0 M NH4 NO3 solution at room temperature for 3 h. The samples, treated with NaOH solution, are designated according with the expression: -SVR/NaOH conc. (M)/time (min). When treated with the mixture of NaOH and TPAOH, the samples are designated as -SVR/NaOH conc. (M) + TPAOH conc. (M)/time (min). 2.4. Characterization 1 H NMR spectrum of the organic SDA was recorded on a Varian Mercury 300 spectrometer at 300.0 MHz in CD3 OD solutions at 25 ◦ C. Chemical shifts (␦/ppm) are given relative to residual CHD2 OD signals (␦H 3.34 ppm). The crystallinity of all samples under investigation was checked by X-ray powder diffraction (XRD) using a Bruker AXS-D8 Advance diffractometer with a graphite monochromator and a position sensitive detector Våntec-1 using CuK␣ radiation in Bragg–Brentano geometry. The size and shape of zeolite crystals were examined by scanning electron microscopy (SEM, JEOL JSM-5500LV microscope). For the measurement crystals were coated with a thin platinum layer by sputtering in vacuum chamber of a BAL-TECSCD-050. Nitrogen adsorption/desorption isotherms were measured on a Micromeritics GEMINI II 2370 volumetric Surface Area Analyzer at
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Table 1 Experimental conditions and respective textural properties. Sample
-SVR/0/0 (parent) -SVR/0.02/15 -SVR/0.02/30 -SVR/0.02/60 -SVR/0.05/30 -SVR/0.10/30 -SVR/0.20/15 -SVR/0.20/30 -SVR/0.05 + 0.05/30 -SVR/0.05 + 0.15/30 a b c
Treatment conditions pH
min
– 12.3
– 15 30 60 30 30 15 30 30 30
12.7 13.0 13.3 12.9 13.2
BET (m2 /g)
Vmic (cm3 /g)
Vmeso (cm3 /g)
Vtot (cm3 /g)
wc (%)
Si/Ala
468.0 389.0 403.2 393.8 391.1 404.8 448.4 451.2 363.7 445.8
0.17 0.14 0.14 0.14 0.12 0.12 0.10 0.10 0.11 0.12
– 0.14 0.16 0.18 0.21 0.21 0.41 0.49 0.20 0.18
0.17 0.33 0.33 0.34 0.34 0.35 0.52 0.59 0.33 0.39
– 6.0 10.6 7.9 9.1 13.4 48.6 52.0 11.1 19.5
41 39 36 n.d.b n.d. n.d. 22 18 n.d. 35
Chemical analysis. Not determined. Weight reduction after desilication [m(parent) − m(desilicated)]/m(parent) × 100%.
liquid nitrogen temperature (−196 ◦ C) to determine surface area, pore volume and pore size distribution. Prior to the sorption measurements, all samples were degassed on a Micromeritics FlowPrep 060 instrument under helium at 300 ◦ C (heating rate 10 ◦ C/min) for 6 h. The chemical composition of the samples was determined by elemental analysis. For this purpose 0.2–0.3 g of zeolite was heated at 70 ◦ C with 5–7 ml of 10 M NaOH in a platinum cup. After the total dissolution of the sample, 10–15 ml of concentrated HCl was added until pH became 0.6–0.7. Then acid solution was heated at 80 ◦ C for 20 min to coagulate the precipitated SiO2 × xH2 O. The precipitate was recovered by filtration on ashless filter, washed out with 1 M HCl, and subsequently with hot water for complete removal of AlCl3 . Precipitated SiO2 × xH2 O was dried at 90 ◦ C, calcined at 1000 ◦ C until the constant mass, and weighed. Al contents were determined by back-complexation titration using the following procedure. 4.0 ml of 0.10 M EDTA was added to the analyzed solution and warmed to boiling, then cooled down and neutralized with NH3 ·H2 O (25% solution) until pH = 3.0 was reached. Then 20 ml of acetate buffer solution (pH = 6.0) was added to adjust the pH. This solution was boiled for 5 min to ensure complete complexation of the Al3+ cations and cooling down to the room temperature. The excess of EDTA was titrated with 0.001 M ZnSO4 in the presence of xylenol orange until the colour change from yellow to violet. Concentration of Lewis (cL ) and Brønsted (cB ) acid sites was determined after adsorption of pyridine (PYR) by FTIR spectroscopy on Nicolet Protégé 460 Magna with a transmission DTGS and MTC/A detector. Zeolites were pressed into self supporting wafers with a density of 8.0–12 mg/cm2 and activated in situ at 450 ◦ C overnight. Pyridine adsorption was carried out at 150 ◦ C for 20 min at partial pressure 600–800 Pa, followed by desorption for 20 min. Before adsorption pyridine was degassed by freezing and thawing cycles. All spectra were recorded with a resolution of 4 cm−1 by collecting 128 scans for a single spectrum at room temperature. Spectra were recalculated on wafer density of 10 mg/cm2 . Concentration of cL and cB were evaluated from the integral intensities of bands at 1454 cm−1 (cL ) and at 1545 cm−1 (cB ) using extinction coefficients, ε(L) = 2.22 cm/mol, and ε(B) = 1.67 cm/mol [38]. A relatively large probe molecule 2,6-di-tert-butyl-pyridine (DTBP) was used to determine the accessibility of acid sites within prepared zeolites [39]. The adsorption of DTBP took place at 150 ◦ C and at equilibrium probe vapour pressure with the zeolite wafer for 15 min. Desorption proceeded at the same temperature for 1 h followed by collection of spectra at room temperature. Extinction coefficients for pyridine [38] were used for the quantitative analysis evaluation of cB .
3. Results and discussion 3.1. The structure of desilicated zeolites -SVR/0/0 zeolite was prepared with Si/Al ratio of 41, which was shown to be optimal [29] for introduction of mesopores by applying treatment with NaOH solutions. Different experimental techniques have been applied to characterize in detail structural properties of -SVR zeolites under investigation. The X-ray diffraction pattern of the parent -SVR zeolite (Fig. 2C, -SVR/0/0) matches well with that one reported in the literature [34,40]. Fig. 2A–C shows XRD patterns of parent and all desilicated -SVR zeolites. It can be seen, that the samples subjected to alkaline treatment in different conditions still display sharp diffraction lines between 5◦ and 40◦ at the characteristic 2-theta positions, which proves the preservation of the long-range crystal ordering during the post-synthesis treatments under experimental conditions. 3.1.1. The effect of the concentration of NaOH solution The decrease in the intensity of the characteristic diffraction lines with increasing concentration of NaOH solution (from 0.02 to 0.20 M) at the same time (Fig. 2A) indicates the progressive decreasing of the framework density during leaching of the framework Si atoms. The concentration of alkaline solution used for the desilication seems to be even more crucial parameter influencing the framework density of the samples. While desilication with 0.02 and 0.05 M NaOH solutions (samples -SVR/0.02/30 and -SVR/0.05/30) did not lead to any essential changes in the
Fig. 2. XRD patterns of -SVR zeolites. The effect of NaOH concentration at the same time of the treatment (A), the time of the treatment at the same NaOH concentration (B) and the concentration of the pore-directing agent (C) compared to the parent zeolite.
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intensities of the characteristic diffraction lines (Fig. 2A), the increase in the concentration of NaOH up to 0.1 M (-SVR/0.10/30) and especially 0.2 M (-SVR/0.20/30) led to a significant drop in the intensity of corresponding diffraction lines. 3.1.2. The effect of the duration of the treatment The duration of the treatment (at the same concentration of the alkaline solution) had smaller impact on the structural changes proceeded during the silicon extraction from the framework, when compared with previously discussed effect of the concentration of NaOH solution (at the same time). No changes are seen in the intensities of the typical diffraction lines for desilication performed with 0.02 M NaOH after 15 and 30 min (-SVR/0.02/15 and -SVR/0.02/30), while significant decrease was observed when the treatment was prolong to 60 min (Fig. 2B). The substantial decrease in the intensity of the diffraction lines after 30 min for the treatment with 0.2 M NaOH (-SVR/0.20/15 -SVR/0.20/30) supports the idea that the concentration of the alkaline solution is the more crucial parameter influencing the framework density (Fig. 2B). 3.1.3. The effect of the pore-directing agent (PDA) It should be noted, that both desilicated samples obtained during the treatment of -SVR/0/0 with TPAOH + NaOH solution, i.e. -SVR/0.05 + 0.05/30 and especially -SVR/0.05 + 0.15/30 ([TPA+ ]/[OH– ] = 0.75, [OH− ] = 0.20 M), showed more intensive diffraction lines (Fig. 2C) in comparison to -SVR/0.20/30 sample, treated exclusively with 0.20 M NaOH solution. It may be due to the stabilization effect of relatively high polarizable TPA+ cations, which were shown to bind to the zeolite surface in alkaline medium providing a protective layer on the zeolites external surface and act as “pore-growth moderators” in the desilication process [30,31,41]. 3.2. The textural properties of desilicated zeolites Nitrogen isotherms provided valuable information on the textural properties of desilicated samples when compared with the parent one. Obtained isotherms agree well with the structural changes determining by means of XRD. In contrast to parent -SVR/0/0 zeolite (Fig. 3C) which exhibits type I isotherm characteristic for microporous solids, all desilicated samples showed combined type I and type IV isotherm, being typical for hierarchical micro/mesoporous materials [8]. Total pore volume of parent -SVR/0/0 is 0.17 cm3 /g and no presence of mesopores was observed (Table 1). According to the SEM images, the micrographs of the parent and desilicated zeolites represent quite small crystals (ca. 5 m × 8 m × 0.5 m, Fig. 4). Therefore, adsorption of nitrogen in
29
the range of p/p0 = 0.8–1.0 can be explained by filling of intercrystalline pores. The results of the chemical analysis indicate decreasing Si/Al ratio in the desilicated samples with increasing duration of the treatment and/or concentration of NaOH solution in the following sequence (see Table 1): -SVR/0/0 (Si/Al = 41) > -SVR/0.02/15 (Si/Al = 39) > -SVR/0.02/30 (Si/Al = 36) > -SVR/0.20/15 (Si/Al = 22) > -SVR/0.20/30 (Si/Al = 18), which is due to the progressive selective extraction of Si atoms from the zeolite framework. At the same time, the sample -SVR/0.20/30 (Si/Al = 18, formed during the treatment of parent sample -SVR/0/0 with 0.2 M NaOH solution) is characterized by significantly lower Si/Al ratio in comparison to -SVR/0.05 + 0.15/30 (Si/Al = 35), obtained in the presence of TPA+ cations ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.2 M). The last result proved the inhibiting role TPA+ cations in the process of silicon extraction from the framework of -SVR zeolite and corresponds to the data, obtained for zeolite MFI [31]. 3.2.1. The effect of the concentration of NaOH solution The increase of the concentration of NaOH solution from 0.02 to 0.05 and 0.10 M led to the enhancement of mesopore volume from 0.16 to 0.21 cm3 /g, while the volume of micropores decreased from 0.17 (-SVR/0/0) to 0.12 cm3 /g (-SVR/0.10/30). At the same time, the treatment of -SVR/0/0 with 0.20 M NaOH solution resulted in significant increase in the volume of mesopores up to 0.41 cm3 /g (-SVR/0.20/15) and 0.49 cm3 /g (-SVR/0.20/30), respectively, while the volume of micropores decreased by 41% to 0.10 cm3 /g (Table 1). These changes are nicely seen from the change of the shape of the nitrogen isotherm (Fig. 3A). 3.2.2. The effect of the duration of the treatment The alkaline treatment of -SVR/0/0 with 0.02 M NaOH for 15–60 min resulted in a slight decrease of the micropore volume (from 0.17 to 0.14 cm3 /g), while the increase in the volume of mesopores (0.14–0.18 cm3 /g) correlates with the duration of the treatment (Table 1). However, under the experimental conditions used, the duration of the treatment had smaller impact on the volume of mesopores formed, than the concentration of the used alkaline solution (Fig. 3B). 3.2.3. The effect of the pore-directing agent (PDA) The mesopore volume of -SVR/0.20/30 sample exceeded almost 3 times (0.49 vs. 0.18 cm3 /g, Table 1) the value for -SVR/0.05 + 0.15/30 obtained by the treatment of parent SVR/0/0 zeolite with mixed NaOH/TPAOH solution for 30 min ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.20 M). These results correspond to the XRD data obtained for respective samples and prove the stabilization effect of TPA+ during desilication. BJH method [42] used for pore-size distribution (Fig. 5) revealed broad distribution of mesopores in the range of 5–20 nm with the maximum around 14 nm for all samples desilicated by using NaOH solutions of different concentrations independently of the time of the treatment. For the sample -SVR/0.05 + 0.15/30 (Fig. 5, wine curve) obtained by the treatment of parent sample (SVR/0/0) with mixed NaOH/TPAOH solution ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.20 M), the maximum is shifted to smaller mesopore size of ca 11 nm. It is in a good agreement with literature data [30]. 3.3. The nature and concentration of acid sites
Fig. 3. Nitrogen adsorption () and desorption (䊉) isotherms. The effect of NaOH concentration at the same time of the treatment (A), time of the treatment at the same NaOH concentration (B) and the concentration of the pore-directing agent (C) compared to the parent zeolite.
The nature of OH-groups within initial and desilicated -SVR zeolites was studied by FTIR spectroscopy. Three different bands were distinguished in the spectrum of parent -SVR/0/0 zeolite in the region of hydroxyl vibrations (Fig. 6A). The most intense bands observed at 3745 and 3724 cm−1 were attributed to external
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Fig. 4. SEM images of parent and treated -SVR zeolites: (A) -SVR/0/0; (B) -SVR/0.02/30; (C) -SVR/0.10/30; (D) -SVR/0.20/30; (E) -SVR/0.20/15; (F) -SVR/0.05 + 0.05/30.
and internal silanol groups, respectively. The band at 3616 cm−1 presents the bridging hydroxyl groups (Si (OH) Al). After pyridine adsorption, the intensities of the bands assigned to the silanol groups slightly decreased or remained almost unchanged. On the other hand, the band assigned to bridging hydroxyl groups having the acid character disappeared, evidencing the accessibility of all Brønsted acid centres for pyridine in the investigated samples. The adsorption of pyridine was accompanied by the appearance of a typical set of adsorption bands in the region 1400–1600 cm−1
Fig. 5. BJH desorption (dV/dD pore volume) pore-size distribution of -SVR samples.
(Fig. 6B). The absorption band around 1546 cm−1 is due to the interaction of pyridine with Brønsted acid sites, while a new band around 1454 cm−1 is characteristic for the pyridine adsorbed on Lewis acid sites. The concentrations of Brønsted and Lewis acid sites calculated from the integral intensities of the bands at 1546 and 1454 cm−1 using extinction coefficients [38] are given in Table 2.
Fig. 6. IR spectra of -SVR zeolites: (A) region of hydroxyl vibrations; (B) region of pyridine vibrations before (bold spectra) and after adsorption of pyridine.
M. Kub˚ u et al. / Catalysis Today 227 (2014) 26–32
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Table 2 The concentration of Lewis and Brønsted acid sites within -SVR zeolites, determined by means of FTIR spectroscopy of adsorbed pyridine and 2,6-di-tert-butylpyridine. Sample
cB (mmol/g) PYR
-SVR/0/0 -SVR/0.02/15 -SVR/0.02/30 -SVR/0.20/15 -SVR/0.20/30 -SVR/0.05 + 0.15/30
0.127 0.120 0.122 0.109 0.107 0.127
cL (mmol/g) DTBP
PYR
0.039 0.040 0.054 0.077 0.079 0.076
0.046 0.081 0.056 0.063 0.064 0.023
3.3.1. The effect of the concentration of NaOH solution and duration of the treatment The increase of the concentration of NaOH solution or the duration of the alkaline treatment was accompanied with the enhancement of the intensity of the band of isolated silanol groups at 3745 cm−1 (-SVR/0/0 ≈ -SVR/0.02/30 -SVR/0.20/15 < SVR/0.20/30), see Fig. 6A, while the intensity of the band at 3724 cm−1 decreased. This indicates the expected growth of the concentration of external SiOH groups in the progress of the formation of mesopores within the zeolite -SVR. As can be seen, with the increasing concentration of NaOH solution used for the preparation of desilicated -SVR zeolites, the concentration of Brønsted acid sites was progressively decreasing (i.e. 0.127, 0.120 and 0.109 mmol/g for -SVR/0/0, -SVR/0.02/15 and -SVR/0.20/30, respectively), while the duration of the alkaline treatment had negligible effect on the amount of Brønsted acid sites in the obtained materials (i.e. 0.120 and 0.122 mmol/g for -SVR/0.02/15 and -SVR/0.02/30, respectively). At the same time the concentration of Lewis acid sites in the desilicated samples, prepared using NaOH solutions of different concentrations, usually exceeded the respective value for parent -SVR/0/0 zeolite. This result may originate from the equilibrium processes, which take place during desilication: (1) releasing of framework Al atoms (which form either Brønsted or Lewis acid sites), resulting in the decreasing of the amount of acid centres within zeolite; (2) realumination of the surface of the material with Al(OH)4 − , which was shown to accompany with the reinsertion of Al atoms into the framework [29]. 3.4. The surface acidity The surface acidity of -SVR zeolites was studied by FTIR spectroscopy of adsorbed 2,6-di-tert-butylpyridine. The size of the DTBP molecule is about 0.79 nm [39], which is higher than the diameter of 10-ring channels in zeolites that makes accessible only acid centres on the outer surface of parent -SVR/0/0 crystals. Indeed, as it can be seen from Fig. 7A, the adsorption of DTBP on the parent -SVR/0/0 zeolite led to only partial disappearance of the band at 3616 cm−1 , corresponding to bridging hydroxyl groups. In the region of chemically adsorbed DTBP a number of bands were detected. According to the previous work of Corma et al. [39] the bands at 3370, 1616 and 1530 cm−1 can be attributed to the formation of DTBPyH+ ions. The absence of a band at 1545 cm−1 confirms no dealkylation of the probe molecule used. It can be seen, that the concentration of accessible (surface) acid sites increases with the increasing of the intensity of the alkaline treatment in the following sequence (Table 2): -SVR/0/0 < -SVR/0.02/30 < SVR/0.20/15 < -SVR/0.20/30, which correlates with the increasing of mesopore volume for the respective samples. 3.4.1. The effect of the pore-directing agent (PDA) In contrast to desilicated samples obtained by the treatment with NaOH solutions, the sample -SVR/0.05 + 0.15/30 prepared
Fig. 7. IR spectra of -SVR zeolites: (A) region of hydroxyl vibrations; (B) region of DTBP vibrations before (bold spectra) and after adsorption of DTBP.
with the mixed NaOH/TPAOH solution ([TPA+ ]/[OH− ] = 0.75, [OH– ] = 0.20 M) contains significantly lower amount of Lewis acid sites (0.023 mmol/g) in comparison with the parent -SVR/0/0 sample (0.046 mmol/g). At the same time, the concentration of Brønsted acid sites for both -SVR/0.05 + 0.15/30 and SVR/0/0 is 0.127 mmol/g. These results may be connected with the “protective” effect of TPA+ cations on the Brønsted acid sites, which leads to the preferential releasing of Lewis acid centres from zeolite during desilication. The mentioned effect is presumably originate from the stronger binding of TPA+ to the anions on zeolite surface due to the additional contribution of hydrophobic and van der Waals forces, which do not act in the case of Na+ interaction with the surface of zeolite. Since the total concentration of acid centres for -SVR/0.05 + 0.15/30 is lower compared with the parent -SVR/0/0 sample, the formation of Lewis acid centres in the progress of realumination in the presence of TPA+ can be hindered. However, this issue requires further investigation. Moreover, despite comparably low volume of the formed mesopores (0.18 cm3 /g, Table 1), the sample -SVR/0.05 + 0.15/30 contains unexpectedly high amount of acid centres, accessible for DTBPy (0.076 mmol/g) if compared with -SVR/0.02/30 (0.054 mmol/g, Table 2), which is characterized by close mesopore volume (0.16 cm3 /g, Table 1). The last result may be connected with the distinctive features of de-/realumination processes, taking place during the silicon extraction from zeolite frameworks in the presence of TPAOH and representing a challenge to be investigated in details. 4. Conclusions In the present work the effect of the concentration of NaOH solution and pore-growth moderator TPAOH as well as the duration of desilication treatment on textural and acidic properties of -SVR zeolite was established. It was shown, that desilication of -SVR zeolite (Si/Al = 41) with 0.02–0.20 M NaOH solutions at 65 ◦ C for 15–60 min results in formation of transport mesopores with broad pore size distribution (5–20 nm). While the volume of the formed mesopores increased with increasing concentration of NaOH solution reaching 0.49 cm3 /g after 30 min treatment with 0.20 M NaOH, the mesopore size distribution was effected neither by the concentration of alkaline solution used nor by the duration of desilication. The increase in the concentration of TPAOH ([TPA+ ]/[OH− ] = 0–0.75) at fixed pH, resulted in decrease of the volume of formed mesopores
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and their average size, confirming the pore-moderating role, which TPA+ cations play during desilication of zeolites. All desilicated -SVR zeolites prepared using NaOH solutions are characterized by increased concentration of Lewis acid centres and higher surface acidity in comparison to parent zeolite. The concentration of acid sites accessible for 2,6-di-tert-butylpyridine correlates with the mesopore volume of desilicated samples, reaching the maximal value for–SVR zeolite, treated with 0.20 M NaOH solution for 30 min. In contrast, desilication of -SVR zeolite (Si/Al = 41) with mixed NaOH/TPAOH solution ([TPA+ ]/[OH− ] = 0.75, [OH− ] = 0.20 M) results in decreasing of the concentration of Lewis acid sites, while the increasing of the surface acidity could hardly be related to the volume of formed mesopores. The last result may indicate the specific effect of TPA+ cations on the rearrangement of acid centres in zeolites during desilication process, which represents an intriguing issue for further investigation. Acknowledgements The authors thank the Czech Science Foundation for the support (13-17593P) and RNDr. Libor Brabec, CSc. for SEM images. References ˇ [1] J. Cejka, H. van Bekkum, A. Corma, F. Schüth (Eds.), Introduction to Zeolite Science and Practice, Stud. Surf. Sci. Catal., vol. 168, 3rd ed., Elsevier, Amsterdam, 2007. ˇ [2] J. Cejka, A. Corma, S.I. Zones (Eds.), Zeolites and Catalysis: Synthesis, Reactions and Applications, Wiley-VCH, Weinheim, 2010. [3] W. Vermeiren, J.P. Gilson, Top. Catal. 52 (2009) 1131–1161. ˇ [4] J. Cejka, G. Centi, J. Pérez-Pariente, W.J. Roth, Catal. Today 179 (2012) 2–15. [5] D. Kubiˇcka, Collect. Czech. Chem. Commun. 73 (2008) 1015–1044. ˇ [6] D. Kubiˇcka, I. Kubiˇcková, J. Cejka, Catal. Rev. 55 (2013) 1–78. [7] A. Corma, Chem. Rev. 97 (1997) 2373–2419. [8] J. Pérez-Ramírez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen, Chem. Soc. Rev. 37 (2008) 2530–2542. ˇ [9] R.M. Martín-Aranda, J. Cejka, Top. Catal. 53 (2010) 141–153. [10] Ch.T. Kresge, W.J. Roth, Chem. Soc. Rev. 42 (2013) 3663–3670. [11] J. Jiang, J. Yu, A. Corma, Angew. Chem. Int. Ed. 49 (2010) 3120–3145.
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