Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid

CLAY-02990; No of Pages 12 Applied Clay Science xxx (2014) xxx–xxx

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Research paper

Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid Aleksandra Pacuła a,⁎, Katarzyna Pamin a, Adam Zięba a, Joanna Kryściak-Czerwenka a, Zbigniew Olejniczak b, Ewa M. Serwicka a, Alicja Drelinkiewicz a a b

Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland

a r t i c l e

i n f o

Article history: Received 27 May 2013 Received in revised form 11 April 2014 Accepted 12 April 2014 Available online xxxx Keywords: Montmorillonite H3PW12O40 Keggin-like structure Dehydration of ethanol Transesterification of castor oil

a b s t r a c t Background: The catalytic performance of supported heteropolyacids in various liquid–solid and gas–solid heterogeneous reactions may be affected by the choice of support and/or the method of heteropolyacid deposition. Main methods: Radioligand binding assays were used to quantify affinity, density and ratio of ET receptors. Design/methodology: 12-tungstophosphoric acid (HPW) was dispersed throughout three matrixes: Na, H-exchanged montmorillonites (Na-Mt, H-Mt) derived from Polish bentonite and commercially available K10 montmorillonite (Mt-K10). Two series of catalysts were prepared via conventional and modified impregnations. Conventional procedure based on the incipient wetness impregnation method involved stirring of the aqueous dispersion of HPW and montmorillonite, followed by drying at elevated temperature. Modified preparation method employed ultrasonication as a means of the dispersion homogenization, followed by freeze-drying. The catalysts were characterized by XRD, N2 sorption, TG analysis, FT-IR and 31P solid-state MAS-NMR spectroscopy. The acidic properties of HPW/montmorillonites were examined in the conversion of ethanol, whereas their catalytic performance was evaluated in the transesterification of castor oil with methanol. Findings: Intact Keggin anions were preserved on H-Mt and Mt-K10, while lacunary Keggin anions dominated on Na-Mt (especially after deposition aided by ultrasonication). Appropriate choice of the montmorillonite support and/or pretreatment allowed tailoring of the acidic function of the heteropolyacid supported catalysts. The use of H-Mt prepared under mild conditions resulted in enhanced acidity and high catalytic performance of acid catalysts, whereas the use of Na-Mt, aided by ultrasonication diminished their acidic function and catalytic activity. The most important achievement was to obtain HPW/montmorillonite system more active than unsupported HPW, via spreading heteropolyacid on H-Mt derived from Polish bentonite. © 2014 Published by Elsevier B.V.

1. Introduction Heteropolyacids with the Keggin-type structure have been extensively studied for the use as acid and redox catalysts. They have been examined in bulk or supported forms in both homogeneous and heterogeneous reactions. Supporting the heteropolyacids on porous solids with high specific surface area may improve their catalytic performance in various liquid–solid and gas–solid heterogeneous reactions. A lot of studies have been published on the immobilization of heteropolyacids on various supports including silica, alumina, titania and zeolites (Haber et al., 2003; Kozhevnikov et al., 1996; Nowińska et al., 2003; Pamin et al., 2000; Pizzio et al., 1998; Sulikowski and Rachwalik, 2003). Among them, clay minerals have been also explored as the supports for heteropolyacids. The early use of heteropolyacid/montmorillonite systems as acid catalysts for production of phenol and acetone, synthesis ⁎ Corresponding author at: Niezapominajek 8, 30-239 Kraków, Poland. Tel.: +48 12 6395131; fax: +48 12 4251923. E-mail address: [email protected] (A. Pacuła).

of tert-butylamine and hydration of olefins was described in patents (Atkins, 1997; Knifton, 1990; Knifton and Grice, 1992). H3PW12O40 (HPW) deposited on natural and acid-treated montmorillonites was studied for industrially important reactions such as conversion of alcohols and/or isomerization (Bokade and Yadav, 2011; Marme et al., 1998; Rožić et al., 2011; Trolliet et al., 2001). The synergism of clay minerals and heteropolyacids was discussed for the development of many green processes with potential industrial applications (Yadav, 2005). Montmorillonites activated with acid have high specific surface areas, suitable pore diameter and appropriate surface acidity. They can be used as the efficient supports for heteropolyacids (Ahmed and Dutta, 2005; Hart and Brown, 2004). The catalysts containing 20 wt.% of HPW on acid-treated montmorillonite were reported to be active, selective and stable in many reactions such as etherification including among other the synthesis of methyl tertbutyl ether (MTBE), alkylation of aniline or hydroquinone, acylation of mesitylene, transesterification of vegetables oils, esterification of acetic acid and disproportionation of ethylbenzene (Bhorodwaj and Dutta, 2010; Bhorodwaj et al., 2009; Bokade and Yadav, 2009; Yadav and

http://dx.doi.org/10.1016/j.clay.2014.04.016 0169-1317/© 2014 Published by Elsevier B.V.

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

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A. Pacuła et al. / Applied Clay Science xxx (2014) xxx–xxx

Asthana, 2002; Yadav and Doshi, 2000, 2003; Yadav and Kirthivasan, 1995; Yadav and Krishnan, 1998). In the present work, two series of the catalysts containing H3PW12O40 were prepared according to a well-established and commonly applied impregnation procedure and a new modified impregnation method. Montmorillonite derived from Polish bentonite and commercially available K10 montmorillonite were used as supports for HPW. The conversion of ethanol and the transesterification of triglycerides to form bio-esters were selected to evaluate their acidic function and to demonstrate that not only the form (Na or Hexchanged) of montmorillonite but also the method of HPW deposition affects their catalytic performance. 2. Experimental 2.1. Materials 12-Tungstophosphoric acid, H3PW12O40·xH2O (denoted as HPW) was supplied by Merck. The number of structural water molecules (x) in HPW was determined by thermal gravimetric analysis. Polish bentonite (Milowice in Poland) was used as the raw material to prepare montmorillonite support. Commercially available K10 montmorillonite was purchased from Fluka. 2.2. Preparation of the supports Na-exchanged montmorillonite (denoted as Na-Mt) was prepared using Polish bentonite, in particular, the fraction of montmorillonite containing particles with the size less than 2 μm. Na-Mt was obtained via a few cycles of the ion-exchange reaction carried out with raw montmorillonite and 1 M solution of sodium chloride (NaCl). The mixture was filtered by centrifugation and the solid was washed with distilled water until the washings no longer showed detectable levels of chlorides ions (test with AgNO3). The final product was dried in the air at 50 °C. H-exchanged montmorillonite (denoted as H-Mt) was prepared via the ion-exchange reaction using Na-exchanged montmorillonite and dilute hydrochloric acid (HCl). The reaction was carried out as follows: 1 g of Na-Mt powder was mixed with 100 cm3 of 5 wt.% HCl. The resulting dispersion was stirred for 30 min. Then, the slurry was centrifuged, and the same amount of HCl solution was added to the residue. The ionexchange reaction was continued for 30 min. The mixture was then filtered by centrifugation and the solid was washed with distilled water until the washings no longer showed detectable levels of chlorides (test with AgNO3). The final product was dried in the air at 50 °C. According to the literature (Cseri et al., 1995), commercially available K10 montmorillonite (denoted as Mt-K10) was obtained from raw material by treatment with mineral acid at high temperature which resulted in ion exchange, dealumination, and extraction of iron and magnesium. 2.3. Preparation of the catalysts HPW was deposited on Na and H-exchanged montmorillonites (Na-Mt, H-Mt) derived from Polish bentonite and on commercially available K10 montmorillonite (Mt-K10). The amount of HPW used for the deposition was calculated assuming the formation of one monolayer HPW coverage on Na-Mt (HPW content = 12 wt.%) and Mt-K10 (HPW content = 41 wt.%), whereas 0.7 monolayer on H-Mt (HPW content = 12 wt.%). Two series of HPW-containing catalysts were prepared using conventional and modified impregnation methods. 2.3.1. Conventional impregnation Impregnation procedure was based on the incipient wetness method. A montmorillonite powder was first heated at 120 °C for 18 h. Then, an appropriate volume of aqueous HPW solution (1–2 cm3 per 1

g of support) containing calculated amount of HPW was added to the hot montmorillonite powder. The resulting dense paste was mixed for 10 min using a spatula. The final product was dried in the air at 90 °C for 18 h. Dried sample was ground using an agate mortar. 2.3.2. Modified impregnation Modified impregnation involved the preparation of dispersion by mixing 1 g of montmorillonite powder (Na-Mt, H-Mt, Mt-K10) with 150 cm3 of distilled water and stirring the obtained dispersion for 30 min using a magnetic stirrer. Then, the mixture was ultrasonically agitated for 100 min. Ultrasonic treatment was performed with a POLSONIC ultrasonic bath Sonic-2 (100 W, 40 kHz). Next, an appropriate amount of HPW powder was added to this dispersion. The resulting slurry was mixed for 30 min. and then ultrasonically agitated for 20 min. The final product was dried after immersion in liquid nitrogen in a CHRIST freeze dryer Alpha 1–4/LD (at − 70 °C, in a vacuum 4 × 10 − 3 Tr). Dried sample was ground using an agate mortar. 2.4. Characterization of the physicochemical properties Powder X-ray diffraction (XRD) patterns were obtained with a Philips X'Pert powder diffractometer using Cu-Kα radiation operating at 35 kV and 30 mA. The patterns were recorded from 2 to 72° 2θ with a scan step time of 2 s and a step size of 0.05°. Nitrogen sorption analysis was carried out at − 196 °C using an Autosorb-1 QUANTACHROME sorptometer. Prior to analysis, the samples were degassed at 200 °C over night. Adsorption/desorption of nitrogen was performed in the relative pressure range of 0.01 b p/p0 b 1.00, using ca. 0.2 g of sample. Specific surface area (SBET) was calculated using the Brunauer–Emmett–Teller (BET) method based on adsorption data in the partial pressure of 0.1 b p/p0 b 0.35. Total pore volume (Vtot) and average pore diameter (Φ) were determined by the amount of nitrogen adsorbed at p/p0 = 0.99. Micropore contribution (Stmicro, Vtmicro) was assessed with t-method, and mesopore analysis (SBJHmeso, VBJHmeso) was done using the Barrett–Joyner–Halenda (BJH) method. Scanning electron micrographs were obtained using a PHILIPS XL 30 scanning electron microscope. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 800 spectrometer using KBr disk technique. Solid state Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) spectra were measured on a Tecmag APOLLO pulse NMR spectrometer in the magnetic field of 7.05 T generated by a Magnex superconducting magnet. A Bruker HP-WB high-speed MAS probe equipped with a 4 mm zirconia rotor and a KEL-F cap was used to record the MAS spectra at the spinning speed of 8 kHz. The solid state 31 P MAS-NMR spectra were measured at 121.28 MHz, using a single 2 μs radio-frequency (rf) pulse, corresponding to π/6 flipping angle. The acquisition delay used in accumulation was 20 s. The 31P chemical shifts were referenced using 85% H3PO4 solution (0 ppm). The conversion of ethanol was conducted in a quartz flow reactor using 0.3 cm3 of catalyst (0.2–0.5 g) granulated into particles with size ranging from 0.2 to 0.4 mm. The reaction was performed according to previously reported procedure (Haber et al., 2002). The reactor was heated to the required temperature (150–300 °C) with a heating rate of 2 °C min−1 under a flow of helium (30 ml min−1) saturated with ethanol vapor (5.7 mol%). The reaction temperature was maintained for 30 min. Prior to catalytic test, the samples were activated in the reactor under helium flow at 300 °C for 2 h. The products were analyzed by means of a Perkin–Elmer 900 gas chromatograph (GC) equipped with a flame ionization detector and a Porapak S column. Thermal stability of HPW deposited on montmorillonite supports was estimated by means of a STA 409PC (Netzsch) thermal analyzer. For comparison, thermal stability of montmorillonites and pure HPW was also determined. Thermogravimetric (TG) and differential scanning

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

A. Pacuła et al. / Applied Clay Science xxx (2014) xxx–xxx

The transesterification of castor oil with methanol was carried out in a glass reactor (100 cm3) equipped with a reflux condenser, magnetic stirrer, and a tube for sampling the solution. The reaction was performed according to previously published procedure (Zięba et al., 2009, 2010a, 2010b). After heating the mixture of castor oil (6 g), methanol (7.6 cm3) and internal standard (eicosane) up to 60 °C, the catalyst (0.6 g) was added. The concentration of the catalyst was 43 g dm−3, whereas the molar ratio of methanol to triglyceride was 29:1. The methanolysis was carried out at 60 °C for 3 h at atmospheric pressure. The catalytic tests were performed twice (in selected cases 3 times) and the average values were calculated. The activity of the catalysts was expressed as the yield to methyl esters (Y) determined after 3 h of reaction. The yield to methyl esters formed in the methanolysis of castor oil was taken into consideration as it is commonly used in the methanolysis of vegetable oils. According to GC analysis, castor oil was mainly composed of triglyceride of ricinoleic acid (87.44 wt.%) but it also contained traces of glycerol (below 0.1 wt.%), mono- and diglycerides of ricinoleic acid, and free ricinoleic acid (below 0.1 wt.%). The following composition of castor oil: triglycerides of ricinoleic acid (87.44 wt.%), triglycerides of other fatty acids: linoleic (5.05 wt.%), oleic (3.88 wt.%), stearic (1.40 wt.%), palmitic (1.28 wt.%), linolenic (0.56 wt.%), and other acids (0.39 wt.%) was taken into account while estimating the average molecular weight of castor oil (928 g mol−1) (Zięba et al., 2009). 3. Results and discussion 3.1. Powder X-ray diffraction Fig. 1(a–c) shows the XRD patterns of the clay minerals (Na-Mt, H-Mt and Mt-K10) and the catalysts containing HPW spread on the clay minerals. For comparison, the XRD pattern of unsupported H3PW 12 O40·12H2O is also demonstrated. The XRD patterns of the clay minerals exhibit a set of reflections ascribed to the (001), (020), (004), (005), and (060) diffractions originating from the montmorillonite structure. All the clay minerals contain quartz impurities (denoted as Q) reflected in the XRD patterns by five reflections at 2θ = 21, 27, 37, 50 and 68°. The structure of Na-Mt and H-Mt supports derived from Polish bentonite significantly differs from that of commercially available K10 montmorillonite. In particular, the reflections ascribed to the (001) and (004) diffractions in the XRD patterns of NaMt and H-Mt are narrower and more intense than their analogs in the XRD pattern of Mt-K10, implying that both samples are more crystalline

Intensity (a.u.)

H3PW12O40·12H2O (001)

HPW/Na-Mt/m

(020) Q

Q(004) (005) Q

Q

(020) Q

Q (004) (005) Q

Q

(060)

Q

(001)

(001)

(020) Q

2

12

HPW/Na-Mt

32

Q

Na-Mt

Q (005) (004) Q

22

(060)

Q

42

52

(060)

Q

62

72

2 theta (degree)

(b) Q

(001) (020) Q

Intensity (a.u.)

2.5. Characterization of the catalytic properties

(a)

HPW/H-Mt/m

(004) (005) Q

Q

(060)

Q

Q

(001) (020) Q (001)

HPW/H-Mt

(004) (005) Q

Q

(060)

Q

Q (020) Q

2

12

(004)

22

H-Mt

(005) Q

32

Q

42

52

(060)

Q

62

72

2 theta (degree)

(c) M (001) (020) M Q

Intensity (a.u.)

calorimetric (DSC) curves were recorded during heating the samples (10 mg) from the ambient temperature up to 1000 °C with a heating rate of 10 °C min−1 under static air condition. Montmorillonites exhibit four degradation steps. The first endothermic DSC peak at ca. 100 °C is attributed to the elimination of water adsorbed on the surface and interlayer water associated with cations (Na+, H+). The second endothermic DSC peak at ca. 700 °C is attributed to the dehydroxylation of montmorillonite layers. The third endothermic DSC peak at ca. 900 °C is attributed to the decomposition of dehydrated and defective montmorillonite structure. The exothermic DSC at ca. 950 °C is related to the crystallization of new phases from the residue after decomposition of montmorillonite. Generally TG and DSC profiles for HPW-containing catalysts are similar to those corresponding to appropriate montmorillonite supports. Thermal effects associated with the decomposition of HPW are obscured by the degradation steps of montmorillonite. Only for HPW/Mt-K10 and HPW/Mt-K10/m the exothermic DCS peak ca. 560 °C attributed to the decomposition of Keggin-like structure can be observed. Similar DSC peak for unsupported HPW is observed at ca. 610 °C. It shows that HPW (anchored and loosely deposited) dispersed throughout Mt-K10 is less stable than pure HPW.

3

Q (004) (005) Q

HPW/Mt-K10/m Q

(060)

Q

M (001)

(020) M Q

Q (004)

HPW/Mt-K10

(005) Q

Q

(005) Q

Q

(060)

Q

Q M (001)

2

(020) Q M

12

22

(004)

32

Mt-K10

42

52

(060)

62

Q

72

2 theta (degree) Fig. 1. The XRD patterns of the catalysts containing H3PW12O40·xH2O (HPW) deposited on various montmorillonite supports (a) Na-Mt, (b) H-Mt, (c) Mt-K10. The XRD pattern of H3PW12O40·12H2O is also displayed.

than Mt-K10. It confirms that the montmorillonite structure is partially destroyed due to treating naturally occurring montmorillonite with strong acid at high temperature (Cseri et al., 1995). Therefore acidtreated montmorillonite is less crystalline than raw montmorillonite (Yadav and Asthana, 2002; Zhao et al., 2013). In addition, Mt-K10 contains mica impurities (denoted as M) reflected in the XRD pattern by two reflections at 2θ = 9 and 18° (Villegas et al., 2005). The XRD pattern of Mt-K10 (Fig. 1c) is similar to that reported by other authors (Bokade and Yadav, 2011; Yadav and Asthana, 2002). Higher crystallinity of Na-Mt and H-Mt than commercial Mt-K10 is accompanied by their smaller specific surface areas (42 and 63 m2 g−1, respectively) compared to Mt-K10 (211 m2 g−1) (Table 2).

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

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3.2. Nitrogen sorption

Table 1 Ratio of integrated (001) and (020) reflections intensities I001/I020 from Fig. 1. Catalysts prepared via conventional impregnation

I001/I020

Catalysts prepared via modified impregnation

I001/I020

HPW/Na-Mt HPW/H-Mt HPW/Mt- K10

0.9 1.2 2.2

HPW/Na-Mt/m HPW/H-Mt/m HPW/Mt-K10/m

4.0 2.2 3.0

Deposition of HPW on the clay minerals influences their structures as observed by others (Bhorodwaj and Dutta, 2011; Yadav and Asthana, 2002). This phenomenon is especially pronounced in the case of Na-Mt and H-Mt. The relative intensity of the reflections assigned to the (001) and (020) diffractions from the montmorillonite structure varies differently for the HPW-containing catalysts prepared either by conventional or modified impregnations (Table 1). An increase in the ratio of the integrated (001) and (020) reflection intensities I001/I020 for the samples obtained according to modified impregnation procedure may be explained by the influence of the ultrasound treatment on the montmorillonite grain organization along the direction parallel to the (001) basal plane, which causes a partial ordering in polycrystalline montmorillonite sample (Table 1) (Pacuła et al., 2006). Furthermore, after impregnation the clay minerals with HPW, their chemical compositions may also change. As a consequence of acid treatment, not only the interlayer cations may be replaced with protons but also the layer ions such as Mg, Al, Fe, located in the octahedral sheets may be partially removed (Bhorodwaj and Dutta, 2011; Cseri et al., 1995). The reflections characteristic of crystalline HPW are only detected in the XRD patterns of the catalysts based on Mt-K10. The amount of deposited HPW, which is proportional to specific surface area of support, is significantly higher for Mt-K10 (41 wt.%) than for Na-Mt (12 wt.%) and H-Mt (12 wt.%). It implies that deposited HPW is not well spread over Mt-K10 and the applied quantity of heteropolyacid is sufficient to form HPW crystallites. The traces of high-intensity reflections of crystalline HPW can be also observed in the XRD patterns of HPW/Na-Mt and HPW/H-Mt — the catalysts prepared by conventional impregnation. Different positions of these peaks suggest that supported HPW has various degrees of hydration. These reflections may be assigned to HPW crystallites containing a various number of water molecules, i.e. H3PW12O40·3H2O and H3PW12O40·6H2O (File Nos. 90637 and 92217 of Inorganic Crystal Structure Database (ICSD), respectively). Whereas in the XRD patterns of the catalysts prepared by modified impregnation, the reflections originating from crystalline HPW are either not visible or barely visible. This observation clearly demonstrates that modified preparation method leads to more dispersed HPW in the catalysts than conventional impregnation.

Nitrogen sorption results are summarized in Table 2. All the supports are mesoporous with a little contribution of micropores. H-Mt has larger specific surface area (63 m2 g− 1) and lower average pore diameter (7 nm) than Na-Mt (42 m2 g−1 and 10 nm, respectively). Furthermore, Na-Mt and H-Mt derived from Polish bentonite have smaller specific surface areas than commercially available K10 montmorillonite (211 m2 g− 1). Acid-treated montmorillonites have more micropores formed due to some changes in structure and composition of natural montmorillonite after acid treatment, as revealed by XRD analysis (Fig. 1). Impregnation of all the clay minerals with HPW reduces their specific surface area and porosity. Similar observations were reported by other authors, who applied various montmorillonites as supports for heteropolyacids (Bhorodwaj and Dutta, 2011; Marme et al., 1998; Trolliet et al., 2001; Yadav, 2005; Yadav and Asthana, 2002; Yadav and Bokade, 1996). As shown in Table 2, the extent of changes after HPW deposition depends on the type of the support and the method of HPW deposition. The reduction in specific surface area and porosity (the contribution of mesopores) is especially pronounced for Mt-K10. HPW/Mt-K10 and HPW/Mt-K10/m have significantly smaller specific surface areas (89 and 84 m2 g− 1, respectively) than Mt-K10. They contain less mesopores as illustrated in Fig. 2 but they have more micropores (Table 2). Micropores may be formed during the impregnation of Mt-K10 with acidic HPW solution causing a partial destruction of its structure, revealed also by XRD analysis (Fig. 1), and also as a result of a partial blockage of mesopores. Deposition of HPW on H-Mt has less significant impact on specific surface area and porosity than in the case of Mt-K10. Irrespective of the preparation methods, the catalysts have only slightly smaller specific surface areas (57 and 53 m2 g−1) than H-Mt (63 m2 g− 1). The mesopores are partially filled with HPW. The effect of clogging the pores with HPW is more pronounced for the catalyst prepared by modified method (HPW/H-Mt/m) (Fig. 2). Conventional impregnation diminishes specific surface area of Na-Mt but less significantly than modified impregnation. The differences in porosity are mainly related to a decrease in the contribution of mesopores. During the impregnation of Na-Mt the acidity of heteropolyacid is to some extent neutralized due to sodium ions coming from Na-Mt. In addition, HPW molecules are partially hydrolyzed and transformed to other, not well identified species, such as lacunary ions observed in the FT-IR and NMR spectra. Therefore the discussion of that observation seems to be more difficult than in the case of H-exchanged montmorillonite. SEM images illustrate that the montmorillonite grains consist of irregular aggregation of plate-like particles (Fig. 3a–c). Heteropolyacid seems to be preferentially deposited on the external surface of the clay minerals, filling the voids between the particles within a grain,

Table 2 Textural properties of the catalysts prepared by H3PW12O40∙12H2O deposition on motmorillonite supports Na-Mt, H-Mt, Mt-K10 using conventional and modified impregnations. Specific surface area (SBET), micropore surface area and volume (Stmicro, Vtmicro), mesopore surface area and volume (SBJHmeso, VBJHmeso), total pore volume at p/p0 = 0.99 (Vtot) and average pore diameter (Φ), were calculated from nitrogen adsorption isotherm. Sample

SBET (m2 g−1)

Stmicro (m2 g−1)

SBJHmeso (m2 g−1)

Vtmicro (cm3/g)

VBJHmeso (cm3/g)

Vtot (cm3/g)

Φ (nm)

Na-Mt HPW/Na-Mt HPW/Na-Mt/m H-Mt HPW/H-Mt HPW/H-Mt/m Mt-K10 HPW/Mt-K10 HPW/Mt-K10/m

42 27 5 63 57 53 211 89 84

6 10 0 19 21 25 16 51 45

42 15 5 41 33 26 205 47 43

0.003 0.005 0 0.010 0.011 0.014 0.006 0.026 0.023

0.106 0.039 0.030 0.093 0.078 0.126 0.311 0.114 0.084

0.108 0.046 0.030 0.106 0.091 0.142 0.321 0.139 0.107

10 7 24 7 6 11 6 6 5

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

A. Pacuła et al. / Applied Clay Science xxx (2014) xxx–xxx

Adsorption Ds(d) (m2Å-1g-1)

(a)

5

3.0

Na-Mt HPW/Na-Mt HPW/Na-Mt/m

2.0

1.0

0.0 10

100

1000

10000

Pore diameter (Å)

Adsorption Ds(d) (m2Å-1g-1)

(b)

3.0

H-Mt HPW/H-Mt HPW/H-Mt/m

2.0

1.0

0.0 10

100

1000

10000

Pore diameter (Å)

Adsorption Ds(d) (m2Å-1g-1)

(c)

8.0

Mt-K10 HPW/Mt-K10 HPW/Mt-K10/m

6.0 4.0 2.0 0.0 10

100

1000

10000

Pore diameter (Å) Fig. 2. The pore size distribution (data obtained from nitrogen sorption, adsorption branch from nitrogen isotherm).

which reflects in the reduction in specific surface area and porosity (Fig. 2). The external surface (mesopores) in montmorillonites by covering with HPW is partially converted into as internal surface (micropores). 3.3. Fourier transform infrared spectroscopy The subtracted FT-IR spectra of HPW-containing catalysts are displayed in Fig. 4a–c. For comparison, the spectrum of pure HPW is also shown. The FT-IR spectrum of bulk heteropolyacid exhibits four bands at 1080, 980, 890 and 800 cm− 1 corresponding to νas(P\Oa), ν as(W_O d ), νas (W\O b\W) and ν as(W\O c \W) stretching vibrations, respectively (Fig. 4a), characteristic of (PW12O40)3− anion with the Keggin-like structure (Bhorodwaj and Dutta, 2011; Okuhara et al., 1996; Rožić et al., 2011). The subtracted spectra of HPW deposited on Mt-K10 are very similar to that of bulk HPW. However, in the spectrum of the sample prepared by modified route a splitting of the band at 1080 cm− 1 resulting in the shoulder at 1050 cm−1 is observed. Its presence indicates that the symmetry of some heteropolyanions is changed and differs from the Td symmetry characteristic of the hydrated Keggin anions. According to the literature (Bielański and Lubańska, 2004; Bielański et al., 2003; Okuhara et al., 1996), several possible reasons of the distortion of the heteropolyanion symmetry may occur.

Fig. 3. SEM images of various montmorillonite supports (a) Na-Mt, (b) H-Mt, (c) Mt-K10.

They are described to be a result of a) a change in interaction between proton and anion being a consequence of lowering the hydration degree, b) a strong affection between anions and surface groups of support, c) a presence of lacunary anions. When a decrease in the symmetry of heteropolyanions is not accompanied by the degradation of the Keggin-like structure the vibrations which can be shifted are mainly those connected with X\O vibrations and vibrations associated with the most external oxygen atoms such as M_Od and M\Oc\M in the molecules of Hn(XM12O40). For lacunary anions the symmetry distortion of heteropolyanions is higher so a more complex FT-IR spectrum with a greater number of bands should be expected. For instance, in the spectrum of sodium salt containing (PW11O39)7 − anion six bands are observed in the wavenumber range of 1200–600 cm− 1 (Pizzio et al., 1998). In the spectrum of HPW/Mt-K10/m besides the absorption bands characteristic of intact Keggin anions and a shoulder at 1050 cm−1, there are no other bands. Thus, the presence of this weak band is likely not a consequence of depolymerization, but rather a change in the symmetry of heteropolyanions due to their interaction

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

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Fig. 4. The subtracted FT-IR spectra of the catalysts containing HPW deposited on various montmorillonite supports (a) Na-Mt, (b) H-Mt, (c) Mt-K10. The FT-IR spectrum of H3PW12O40·12H2O is also displayed.

characteristic broad band at 840 cm− 1 (Trpkovska and Šoptrajanov, 1997). The band at 620 cm−1 may indicate that also small amount of WO3 is formed during HPW deposition on Na-Mt (Teoh et al., 2005).

3.4. Nuclear magnetic resonance Fig. 5 shows the 31P MAS-NMR spectra of HPW-containing catalysts prepared via conventional and modified impregnations. In the spectra of HPW deposited on Mt-K10 and H-Mt only one broad resonance line at ca. −15.7 ppm is recorded. This signal is characteristic of phosphorous

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with support surface groups, which was also observed by other authors (Bielański et al., 2003). This may be additionally confirmed by the fact that this band is noticed for the sample obtained by modified impregnation, in which HPW is more dispersed. The similarity between the spectra of HPW/Mt-K10, HPW/Mt-K10/m and the spectrum of pure HPW implies the presence of HPW crystallites in both catalysts, confirmed by the XRD patterns (Fig. 1). In the case of HPW deposited on H-Mt (Fig. 4b), besides a splitting of the band associated with vibration of P\Oa (shoulder at ca. 1050 cm−1) another splitting of the band associated with vibration of W_Od (shoulder at 930 cm−1) is also observed. It implies that HPW species are more dispersed on H-Mt than on Mt-K10. In addition, the interaction between HPW particles (species) and montmorillonite surface groups seems to be enhanced in the sample prepared by modified impregnation and therefore the shoulders in the spectrum of HPW/H-Mt/m are more pronounced. According to the literature (Brückman et al., 1994), at low heteropolyacid coverage (less than one monolayer loading), Keggin anions may be perturbed by the support probably by the electrostatic forces resulting from the protonation of the surface hydroxyl groups by strong acidic protons of heteropolyacid. As a consequence the vibration band associated with W\Oc\W should disappear. However, in the spectra of HPW/H-Mt and HPW/H-Mt/m the band at 810 cm− 1 attributed to W\Oc\W vibration is still observed. So the Keggin anions seem not to be perturbed in such a way. The FT-IR subtracted spectra of HPW/Na-Mt significantly differ from that of bulk HPW (Fig. 4c). It is likely that after HPW spreading on Na-exchanged montmorillonite, due to alkaline nature of the support, Keggin anions are partially decomposed, similarly to HPW deposited onto natural montmorillonite (Marme et al., 1998). The most possible reason for the presence of absorption bands at 1040, 960, 860, 810 and 760 cm−1 (Fig. 4c) is the occurrence of lacunary Keggin anions (PW11O39)7− (Pizzio et al., 1998). Not only lacunary anions but also intact heteropolyanions interacting with the support or another type of defected anions with formula (P2W21O72)6− may be responsible for the presence of the bands with shifted positions in comparison to the bands in the spectrum of bulk HPW. (P2W21O72)6− anions are formed according to the following degradation reaction (PW12O40)3 − ↔ (P2W21O72)6 − ↔ (PW11O39)7 − (Pope, 1983). The absorption bands characteristic of (P2W21O72)6− should be observed at 1100, 1050, 950, 800 and 780 cm−1 (McGarvey and Moffat, 1991). The partial decomposition of heteropolyanions could be a source of tungstate ions, with a

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Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

A. Pacuła et al. / Applied Clay Science xxx (2014) xxx–xxx

of one layer thickness and interlayer distance) estimated from the position of the reflection due to the (001) plane is ca. 12.4 Å for Na-Mt. If all sodium cations in Na-Mt are replaced by protons d-spacing should be close to 14.8 Å, characteristic of H-exchanged montmorillonite (like observed for H-Mt or Mt-K10), being a consequence of a bimolecular water layer associated with hydrogen (Barshad, 1950). After the deposition of HPW onto Na-Mt, d-spacing moderately decreases to ca. 11.3 Å, which is rather a result of lower water content and therefore rearrangement of a monomolecular layer of water associated with sodium (Barshad, 1949). However, we cannot exclude that some sodium ions may be replaced by protons. Sodium cations, released from Na-Mt, may compensate the negative charge of heteropolyanions (Na)+(H2PW12O40)− by analogy with (SiOH2)+(H2PW12O40)−. It seems that heteropolyanions bonded to Na+ may be less stable than those connected to (SiOH2)+. In addition, the release of sodium ions at Na-Mt/HPW interface, may lead to an increase in the pH, which facilitates the transformation of anions with the Keggin-like structure into defective anions as revealed by FT-IR and NMR analysis.

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with the symmetry observed for heteropolyacid with the following formula H3PW12O40·6H2O (Edwards et al., 1998). This confirms that anions with the Keggin-like structure are preserved on the acid-treated montmorillonite supports. A similar NMR signal at ca. −15 ppm was reported for HPW deposited on K10 montmorillonite (Yadav and Asthana, 2002). In the 31 P MAS-NMR spectrum of HPW dispersed throughout Na-exchanged montmorillonite by conventional impregnation, a broad resonance line at ca. −14.3 ppm is observed. There are several possible reasons for the shift of the 31P NMR signal. However, the most possible explanation for this effect is the formation of various species due to a partial decomposition of heteropolyanions. Such observations are in agreement with the FT-IR data (Fig. 4). As discussed before, it is very likely that during the impregnation process a partial degradation of heteropolyanions via hydrolysis takes place. According to the literature (Pope, 1983), the anions, i.e. (PW12O40)3 −, (P2W21O72)6 − and (PW11O39)7 − give NMR signals at − 14.9, − 13.3 and −10.4 ppm, respectively. A gradual degradation of heteropolyacid causes the subsequent shift of the 31P NMR signal towards the region of lower values of the chemical shift. The NMR signal at ca. −13 ppm was observed for (P2W21O72)6 − (Kozhevnikov et al., 1996; Yadav and Asthana, 2002). In the spectrum of HPW/Na-Mt the resonance line may be also assigned to the species being a consequence of the strong interaction between heteropolyanions and the support surface groups. Such phenomenon was described for HPW deposited on silica, the NMR signal observed at − 14.2 ppm was assigned to the interacting species (SiOH2)+(H2PW12O40)− (Lefebvre, 1992; Mastikhin et al., 1990). The position of the NMR resonance line of H3PW12O40·nH2O depends also on its degree of hydration. For example, when n = 6 the NMR signal occurs at −15.6 ppm, whereas the loss of the crystallization water (n = 0) results in the shift of the NMR signal, which is observed at − 11.0 ppm (Misono, 2001). Because all the investigated catalysts in priory to analysis were standardized under controlled humidity conditions, the shifts of the NMR signal cannot be explained by the change of their degree of hydration. In the spectrum of HPW/Na-Mt/m a broad asymmetric signal, being a superposition of two NMR signals, with maximum at ca. −12.6 ppm is observed. The use of the ultrasound treatment results in more dispersed HPW in HPW/Na-Mt/m and facilitates interaction not only between heteropolyanions and surface groups of Na-Mt but also between heteropolyanions and sodium anions, which ends up with the higher degradation of anions with the Keggin-like structure and the formation of lacunary anions (Marme et al., 1998). Also in this case, the NMR data confirm the above-mentioned results based on the analysis of the FT-IR spectra (Fig. 4). According to FT-IR and NMR spectra, the mechanism of interaction between HPW and the clay mineral supports depends on the nature of the support, HPW loading and the impregnation mode. HPW in HPW/Mt-K10 and HPW/Mt-K10/m is not well dispersed and only a certain amount of HPW is attached to the support surface by electrostatic attraction, i.e. (SiOH2)+(H2PW12O40)−. The remaining part of HPW is present in the form of crystallites with characteristics of unsupported (bulk) HPW. In the catalysts based on H-Mt, HPW is more dispersed especially in HPW/H-Mt/m. So, the extent of HPW anchoring to the support surface via (SiOH2)+(H2PW12O40)− is higher for H-Mt than for Mt-K10. The interaction of heteropolyanions with surface hydroxyl groups may affect their symmetry but they still have Keggin-like structure. In the catalysts based on Na-Mt, HPW interacts with OH groups and with interlayer exchangeable Na+. Some protons are engaged in the protonation of the surface hydroxyl groups and some protons may replace interlayer sodium ions. It seems that in HPW/Na-Mt and HPW/Na-Mt/m less protons derived from HPW is consumed for the protonation in comparison to HPW/H-Mt and HPW/H-Mt/m and less intact HPW remains on the surface. Moreover, there is no evidence in XRD patterns that sodium ions in Na-Mt are completely replaced with protons. According to XRD patterns, d-spacing (which corresponds to the sum

7

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Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

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A. Pacuła et al. / Applied Clay Science xxx (2014) xxx–xxx

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Fig. 7. The selectivity to diethyl ether and ethylene in the presence of (a, b) the catalysts prepared via conventional impregnation (c, d) the catalysts prepared via modified impregnation (e, f) montmorillonites and pure HPW.

3.5. Ethanol conversion The conversion of ethanol is selected as a test reaction to identify the nature of catalytic sites in the samples. According to the literature (Okuhara et al., 1996), heteropolyacids can absorb polar molecules like alcohols (this is called a pseudoliquid phase behavior). Thus, ethanol can readily penetrate the bulk of heteropolyacid replacing water molecules and/or expanding the distance between polyanions. Diethyl ether, ethylene and acetaldehyde are the products of ethanol conversion over HPW-containing catalysts, montmorillonites (Na-Mt, H-Mt, Mt-K10) and pure HPW (Figs. 6 and 7). Diethyl ether and ethylene are formed on acid sites via dehydration of ethanol, whereas acetaldehyde is produced on redox sites through the dehydrogenation of ethanol. According to the literature (Okuhara et al., 1996; Varisli et al., 2007), ethylene is formed by the dehydration of (C2H5OH)H+ intermediates, whereas diethyl ether is formed by the dehydration of (C2H5OH)2H+ intermediates. Although ethylene and diethyl ether may be produced through parallel routes, ethylene may be also formed via dehydration

of diethyl ether. Based on the literature (Cseri et al., 1995; Okuhara et al., 1996), transition metal ions, i.e. W6+ in HPW and Fe3+ in montmorillonites (located in octahedral sheet) may act as redox sites and they may be responsible for the formation of acetaldehyde. The selectivity to acetaldehyde is very low, less than 5% for HPW and HPW-containing catalysts or less than 10% for montmorillonites. The dehydration products (diethyl ether and ethylene) dominate the products of ethanol conversion in the presence of studied samples. It indicates that HPW-containing catalysts predominantly show acidic properties, which are relevant to transesterification of castor oil with methanol. Therefore, only the dehydration of ethanol is discussed in details as it represents acidic properties of the investigated samples. Fig. 6(a–c) illustrates that the activity (conversion) of HPWcontaining catalysts in the ethanol dehydration reaction increases with increasing reaction temperature. HPW/Mt-K10 and HPW/MtK10/m show the highest activity among studied samples. The catalytic performance of HPW/Mt-K10 and HPW/Mt-K10/m is comparable. The

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

A. Pacuła et al. / Applied Clay Science xxx (2014) xxx–xxx

catalysts containing HPW deposited on H-Mt exhibit lower activity in comparison to the Mt-K10-based samples. At low temperatures, the catalyst prepared via modified impregnation (HPW/H-Mt/m) is more active than the sample obtained by conventional impregnation. The catalysts containing HPW deposited on Na-Mt are active but they show low activities. HPW/Na-Mt/m is less active than HPW/Na-Mt because it contains highly defective heteropolyanions or heteropolyanions interacting with sodium cations, which are less active than heteropolyacid (Okuhara et al., 1996). Fig. 6(a–c) demonstrates that the hydrogen forms of montmorillonite (H-Mt, Mt-K10) are less active than the corresponding HPW-containing catalysts and the sodium form of montmorillonite (Na-Mt) is scarcely active. Fig. 7(a–d) displays the selectivity to diethyl ether and ethylene for HPW-containing catalysts and illustrates that at lower reaction temperatures diethyl ether is a dominant product, whereas ethylene is produced in small amounts. As the reaction temperature grows, the contribution of diethyl ether decreases, whereas the quantity of ethylene increases. Such trend in the catalytic performance of all the studied samples is similar to HPW-containing catalysts investigated by others (Haber et al., 2002; Varisli et al., 2007). According to the literature (Hayashi and Moffat, 1982), methanol conversion into hydrocarbons over HPW passes through dimethyl ether as an intermediate. Therefore we suppose that ethanol as well as methanol may convert into ethylene via ether (diethyl) as an intermediate. The conversion of ethanol to ethylene is a preferred reaction pathway. Therefore, the catalysts facilitate the production of ethylene. The temperature of 50% selectivity to ethylene (T50) for the catalysts prepared via conventional and modified impregnations are compared in Fig. 8. Among studied samples, the lowest T50 (~190 °C) is observed for the catalysts containing HPW deposited on Mt-K10. HPW/Mt-K10 and HPW/Mt-K10/m have the highest HPW loadings (41 wt.%) providing large number of the acid sites to engage ethanol molecules in the conversion to ethylene. Moreover, the catalytic performance of both catalysts is comparable, which confirms that the environments of HPW deposited on Mt-K10 in both cases are alike. The catalysts containing HPW deposited on H-Mt are able to convert ethanol into ethylene at higher T50 (~ 210–230 °C) compared with the above-described Mt-K10-based samples. Since HPW loading is lower (12 wt.%) the nominal number of available active sites is smaller. T50 (210 vs. 230 °C) is lower for the catalyst prepared via modified impregnation (HPW/H-Mt/m). HPW in HPW/H-Mt/m is more dispersed on H-Mt and heteropolyanions are better anchored to protonated surface hydroxyl groups than HPW in HPW/H-Mt. Such arrangement of HPW on the surface of H-Mt seems to be beneficial for the production of ethylene. The catalytic behavior of HPW/H-Mt/m demonstrates that the acidic properties of deposited HPW may be enhanced by the 300 conventional

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9

interaction of HPW with the surface of H-Mt aided with modified impregnation. H-Mt and Mt-K10 have similar T50 (~260 °C) and they show comparable selectivity to ethylene but much lower than for pure HPW (Fig. 7f). It seems that the presence of HPW in the catalysts based on H-Mt and Mt-K10 provides them with more acidic properties in comparison to the supports (H-Mt and Mt-K10). It also confirms that if the supports have comparable acidic properties like in the case of H-Mt and Mt-K10, the catalysts containing more HPW are more active. The catalysts containing HPW deposited on Na-Mt have the highest T50 ~280 °C, which implies that their activity in the ethanol dehydration reaction is the lowest among studied samples (Fig. 6c). Their (selectivity) reactivity towards the formation of ethylene is also the lowest (Fig. 7(b,d)) probably due to a partial decomposition of anions with the Keggin-like structure, due to suppressing of acidity by alkaline support. It also indicates that the dispersion of HPW plays an important role in the catalysts based on H-Mt and Na-Mt as the stability and properties of HPW are determined by the interaction between heteropolyacid and montmorillonite. The results displayed in Fig. 6, do not provide straightforward information on the reactivity of HPW because HPW/montmorillonites have different HPW loadings. As described before, the amount of HPW deposited on montmorillonites was calculated assuming the formation of one monolayer HPW coverage on Na-Mt and Mt-K10, whereas 0.7 monolayer on H-Mt. As a result, the catalysts have different HPW loadings, i.e. 12 wt.% on Na-Mt and H-Mt and 41 wt.% on Mt-K10. This “monolayer coverage” model for the deposition of HPW was adopted from the literature (Marme et al., 1998; Trolliet et al., 2001; Yadav and Bokade, 1996) and theoretically it should provide a uniform distribution of HPW species on the surface of montmorillonite support. In order to illustrate the reactivity of HPW species in HPW-containing systems in a more informative way, the activity of the studied catalysts is expressed as the rate of ethanol conversion referred to 1 g of HPW (Fig. 9(a–c)). It should be stressed that this method of calculation assumes that the total amount of deposited HPW participates in the ethanol conversion. In the case of the supports (without HPW) the rate of ethanol conversion −1 is calculated per 1 g of support (mmol g−1 ). cat h The catalysts containing HPW deposited on commercially available K10 montmorillonite, irrespective of preparation methods, show slightly higher activity referred to 1 g of HPW (0.33 × 106 mmol g−1 h−1 and 0.27 × 106 mmol g−1 h−1 for HPW/Mt-K10 and HPW/Mt-K10/m, respectively) (Fig. 9(a–c)) than bulk HPW (0.25 × 106 mmol g−1 h−1) (Fig. 9a). Fig. 9(a–c) illustrates that the nature of the support plays an important role in the reactivity of HPW deposited on Na-Mt and H-Mt. HPW dispersed on H-Mt via conventional impregnation (Fig. 9b) exhibits a relatively high activity towards ethanol dehydration. Above 175 °C, the activity per 1 g of HPW is significantly higher than that of bulk HPW (Fig. 9a) due to spreading of HPW on H-Mt. Deposition of HPW on Na-Mt via both impregnation routes (Fig. 9(b–c)) leads to less active catalysts because of the formation of lacunary anions. Deposition of HPW on H-Mt via modified impregnation (Fig. 9c) results in enhanced catalytic properties. Furthermore, HPW/H-Mt/m shows the highest activity referred to 1 g of HPW among all the investigated catalysts. Whereas HPW/Na-Mt/m is less active than HPW/Na-Mt. Discussion of the catalytic performance of HPW/Na-Mt/m seems to be difficult, because of a “complex” state of HPW species on Na-Mt. The catalytic data clearly show that the application of modified impregnation results in obtaining the catalysts with HPW species more distributed throughout the clay mineral layers. It appears that modified impregnation is especially beneficial for spreading HPW on H-Mt. Irrespective of impregnation methods Mt-K10-based catalysts contain HPW crystallites with catalytic characteristics of bulk HPW. The procedure of HPW spreading influences not only the acidic function but also such properties as specific surface area, porosity and crystallinity of the catalysts. Therefore a literature model assuming that

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

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Temperature (oC) Fig. 9. The activity of (a) montmorillonite supports and HPW (b) the catalysts prepared via conventional impregnation (c) the catalysts prepared via modified impregnation.

“monolayer coverage” of montmorillonite support provides molecularly dispersed heteropolyacid species seems to be questionable. Nevertheless, H-Mt derived from Polish bentonite is very promising as a potential support for HPW especially that H-Mt is more crystalline than Mt-K10 and it may facilitate filtration which is useful in practice. 3.6. Transesterification of castor oil with methanol In the transesterification of castor oil with methanol, triglycerides of fatty acids are converted into respective fatty acid methyl esters and glycerol. The transesterification reaction is catalyzed by enzymes, bases (e.g. clay minerals modified with KOH was used as alkaline

catalysts for the transesterification of palm oil (Soetaredjo et al., 2011)), acids including heteropolyacids and their salts (Zięba et al., 2010a, 2010b). For instance, H3PW12O40 deposited on K10 montmorillonite was investigated for the transesterification of vegetable oils with methanol (Bokade and Yadav, 2009). Among the catalysts with various HPW loadings, the one containing 20 wt.% of HPW was the most active as it showed the highest acidity. That catalyst was also the most stable in reaction conditions. Its high stability was related to the fact that HPW was covalently bonded to the surface groups of K10 montmorillonite (Yadav and Asthana, 2002). As a result, HPW was not leached from the catalyst by methanol. HPW/H-Mt, HPW/H-Mt/m and HPW/Mt-K10 catalysts were examined in the transesterification of castor oil with methanol. For comparison, the catalytic response of the supports was also tested. As shown in Table 3, the clay minerals (H-Mt and Mt-K10) are scarcely active in the methanolysis of castor oil. A very low yield to methyl ester (less than 1%) implies that the acid-activated montmorillonites are not acidic enough to catalyze transesterification of triglycerides. The catalysts containing HPW (12 wt.%) deposited on H-Mt (HPW/H-Mt and HPW/H-Mt/m) show roughly comparable activities (Y = 6.7 and 4.7%, respectively), whereas the catalyst containing HPW (41 wt.%) deposited on Mt-K10 is ca. twice more active (Y = 11.0%). However, the comparison of the activities referred to 1 g of HPW demonstrates the same relation as that obtained in the conversion of ethanol. The catalysts containing HPW deposited on H-Mt are more active than HPW/Mt-K10. The reactivity of HPW/montmorillonites may be determined by the accessibility of the bulky triglyceride reactants to the existing acid sites. Since HPW is more dispersed on H-Mt than on Mt-K10, the acid sites in HPW/H-Mt and HPW/H-Mt/m could be easily accessible and also could be better utilized than those in HPW/Mt-K10. In addition, a similar catalytic performance of HPW/H-Mt and HPW/H-Mt/m may imply that the transesterification of castor oil occurs rather on the outermost surface of the catalysts as bulky triglycerides molecules have restricted ability to penetrate into the pores of the catalysts. Therefore the extent of HPW dispersion plays a more significant role in the transesterification of bulky triglycerides than in the conversion of ethanol. HPW/H-Mt is slightly more active than HPW/H-Mt/m probably because of higher contribution of mesopores (Table 2), which could improve the access of the reactants to the active sites. The recycling experiments were performed for HPW/H-Mt and HPW/Mt-K10 as recycling of heterogeneous catalysts in the transesterification reaction is one of the key factors for their practical applications. The recycling reactions were carried out subsequent to the catalytic test applying reused catalysts and fresh reagents (castor oil, methanol and eicosane). The used catalysts were washed with acetone/THF and dried in the air. The yield to methyl esters for reused HPW/H-Mt is only slightly lower than that for fresh HPW/H-Mt (Y = 6.0 and 6.7%, respectively) (Table 3). On the other hand, the yield to methyl esters for reused HPW/Mt-K10 is significantly lower than that observed for fresh HPW/Mt-K10 (Y = 3.3 and 11.0%, respectively) (Table 3). A drastic decrease in the catalytic performance may be caused by a HPW leaching from fresh HPW/Mt-K10 by methanol. The presence of HPW crystallites on Mt-K10 may facilitate their dissolution. It implies that bulky HPW species are less stabilized via interaction with Mt-K10 than more dispersed HPW species on H-Mt. The possible impact of HPW soluble in methanol on the transesterification reaction was also studied. To check this, HPW/H-Mt was vigorously stirred under methanol reflux (at 60 °C for 3 h). Then, methanol collected during filtration was used in the transesterification of castor oil. However, no formation of methyl esters was observed. The catalytic behavior of dissolved (unsupported) HPW has been also evaluated in transesterification of castor oil with methanol in our previous work (Zięba et al., 2009). Under homogeneous conditions,

Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016

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Table 3 The catalytic activity for the transesterification of castor oil with methanol. Samples

Yield (%)

−1 Activity (mol g−1 ) × 10−4 cat min

−1 Activity (mol g−1 ) × 10−4 HPW min

H-Mt Fresh HPW/H-Mt (12 wt.%) Reused HPW/H-Mt (12 wt.%) Fresh HPW/H-Mt/m (12 wt.%) Mt-K10 Fresh HPW/Mt-K10 (12 wt.%) Fresh HPW/Mt-K10 (41 wt.%) Reused HPW/Mt-K10 (41 wt.%) Amberlyst-15 Nafion-SAC-13

0.4 6.7 6.0 4.7 0.3 1.3 11 3.3 3.4 0.97

0.014 0.252 0.217 0.171 0.011 0.047 0.400 0.120 0.125 0.035

– 2.10 1.80 1.43 – 0.39 0.97 0.29 – –

the activity of HPW (referred to H+) was comparable to that of commonly studied H2SO4. The activity (referred to 1 g of HPW) for −1 HPW/H-Mt (2.10 × 10−4 mol g−1 ) is only slightly lower than HPW min −4 −1 for dissolved HPW (2.82 × 10 mol gHPW min−1). Although the activity of solid acid catalysts is lower than the activity of dissolved HPW, in practice heterogeneous catalysts would be more recommended. The activity of the montmorillonite-supported catalysts was also compared with commercial catalytic materials, i.e. Amberlyst-15 and Nafion-SAC. Both catalysts are commonly considered as so called “standards” in the transesterification of triglycerides. As shown in Table 3, the yield to methyl esters obtained for fresh and reused HPW/H-Mt catalysts (Y = 6.7 and 6.0%, respectively) is approximately twice as high as that in the presence of Amberlyst-15 (Y = 3.4%). Hence, the catalyst containing HPW on H-Mt exhibits promising catalytic behavior in the transesterification of triglycerides similarly to HPW deposited on K10 montmorillonite already described in the literature (Yadav and Asthana, 2002). 4. Conclusions Stability of the Keggin-like structure heteropolyanions depends on the nature of the support. Intact Keggin anions were preserved on the surface of hydrogen exchanged forms of montmorillonite (H-Mt, Mt-K10), while lacunary Keggin anions dominated on the surface of sodium exchanged montmorillonite (especially after deposition aided by ultrasonication). Appropriate choice of the montmorillonite support and/or pretreatment allowed tailoring of the acidic function of the heteropolyacid supported catalysts. The use of H-exchanged montmorillonite prepared under mild conditions resulted in enhanced catalytic performance of acid catalysts, whereas the use of Na-exchanged montmorillonite, aided by ultrasonication diminished their acidic function and therefore catalytic activity. The most important achievement was to obtain H3PW12O40/montmorillonite system more active than unsupported HPW, via spreading heteropolyacid on H-exchanged montmorillonite derived from Polish bentonite. Acknowledgments This work was financially supported by the Polish Ministry of Science and Higher Education within the research project 4 T08D 051 24. Adam Gaweł (AGH-University of Science and Technology), Elżbieta Bielańska and Zofia Czuła (Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences) are gratefully acknowledged for XRD, SEM and nitrogen sorption measurements, respectively. References Ahmed, O.S., Dutta, D.K., 2005. Friedel–Crafts benzylation of benzene using Zn and Cd ions exchanged clay composites. J. Mol. Catal. A 229, 227–231. Atkins, M.P., 1997. US Patent 5 629 459 to BP CHEM INT LTD.

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Please cite this article as: Pacuła, A., et al., Physicochemical and catalytic properties of montmorillonites modified with 12-tungstophosphoric acid, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.04.016