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Factors Affecting the Catalytic Performance of Zr,Al-Pillared Clays in the Synthesis of Propylene Glycol Methyl Ether M. N. Timofeeva,*,†,‡,# V. N. Panchenko,†,# M. M. Matrosova,† A. S. Andreev,† S. V. Tsybulya,†,# A. Gil,*,§ and M. A. Vicente∥ †

Boreskov Institute of Catalysis, Prospect Akademika Lavrentieva 5, 630090 Novosibirsk, Russia Novosibirsk State Technical University, Prospect Karl Marksa 20, 630092 Novosibirsk, Russia § Department of Applied Chemistry, Public University of Navarra, 31006 Pamplona, Spain ∥ Department of Inorganic Chemistry, University of Salamanca, Salamanca, Spain ‡

ABSTRACT: Series of Al-, Ga-, and Zr,Al-pillared clays (Al-MM, Ga-MM, and Zr,Al-MM) were prepared through the intercalation of Al-, Ga-, and Zr-polyoxocations into three layered aluminosilicates with various textural properties and chemical compositions. Zr,Al-phases with the structure of bayerite−boehmite were also synthesized using Al and Zr oligomeric solutions, prepared from aluminum chloride and zirconium oxychloride, respectively, as metal sources, with Zr/Al ratios of 0−0.143. All materials were characterized by X-ray diffraction; elemental analysis; DR-UV−vis, FTIR, and 27Al MAS NMR spectroscopies; and N2 adsorption/desorption analysis. FTIR spectroscopy using pyridine, benzonitrile, and deuterated chloroform as probe molecules was used to investigate the acid−base properties of the solids. The catalytic performance of these materials was investigated in the synthesis of 1-methoxy-2-propanol from methanol and propylene oxide. It was found that the catalytic properties of Zr,Al-pillared clays depended on the textural properties (pore diameter), chemical composition (Si/Al molar ratio and Zr content), and type of Al salt (nitrate or chloride) used as the Al source for the preparation of the pillaring solution. The type of metal cation in the polyoxocations affected the reaction rate and isomer selectivity. Al-MM was found to exhibit a higher catalytic efficiency than Ga-MM and Zr-MM.

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INTRODUCTION Propylene glycol methyl ether (PGME) is widely used as an “alkahest” solvent, detergent, and emulsifier in industrial and consumer products. PGME is generally produced by the propylene oxide method, namely, the reaction of propylene oxide (PO) with methanol

toward II decreased from 99% to 21% and from 61% to 38%, respectively, as the Brønsted acidity was increased by increasing the Zr content in Zr,Al-MMR from 0 to 12.7 wt %. The mass contents of Al and Si in Na-MMR were 5.6 and 32.9 wt %, respectively (Si/Al = 6.09 mol/mol). Interestingly, the effect of the Zr content in Zr,Al-MMK materials prepared from a calciumrich montmorillonite (Tagansk, Kazakhstan) was not great. The conversion of PO and selectivity toward II decreased from 79% to 75% and from 73% to 60%, respectively, when the Zr content in Zr,Al-MMK was increased from 0 to 12.4 wt %. The textural properties of Zr,Al-MMK catalysts were related to the lower changes in the conversion of PO and selectivity toward II compared to those observed when using Zr,Al-MMR. However, the acid−base properties of the Zr,Al-MMK materials were not investigated; therefore, a detailed investigation of the main factors affecting their catalytic properties is still needed. In this work, the effects of the acid−base and textural properties on the catalytic properties of Al- and Zr,Al-pillared clays prepared from three layered aluminosilicates containing 90−95 wt % montmorillonite with different textural properties and chemical compositions were investigated. Al- and Zr,Alpillared clays were prepared through ion exchange of Na+ cations by Al−Zr polyoxocations [OH/(Zr + Al) = 2.4 and Zr/Al = 0− 0.143 mol/mol], followed by calcination at 400 °C. Moreover,

It is well-known that the mechanism of this reaction depends on the type of catalyst.1−3 In the presence of base catalysts (NaOH,4 oxides,3,5,6 or amines),4,7 the reaction proceeds through the ring opening of PO from the sterically hindered position, leading to the predominant formation of 1-methoxy-2propanol (II). At the same time, in the presence of acid-type catalysts (BF3, H2SO4),4,8 the main product is 2-methoxy-1propanol (I). According to Timofeeva et al.,9 the primary alkyl ether of propylene glycol has a much higher toxicity than the secondary alkyl ether. Therefore, a high selectivity toward the secondary alkyl ether of propylene glycol is preferable. In our previous works, montmorillonite from Mukhartala, Russia, intercalated with polymeric Al-containing oxocations (OH/Al = 2.4; Al-MMR)9 and mixed polyoxocations [OH/(Zr + Al) = 2.4 and Zr/Al = 0−0.143 mol/mol]10 was found to be an efficient catalyst for the reaction of PO with methanol. The isomer selectivity of the reaction was controlled by the acid−base properties of the materials. The conversion of PO and selectivity © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13565

March 12, 2014 July 17, 2014 July 19, 2014 July 20, 2014 dx.doi.org/10.1021/ie501048a | Ind. Eng. Chem. Res. 2014, 53, 13565−13574

Industrial & Engineering Chemistry Research

Article

Table 1. Chemical Compositions and Structural and Textural Characteristics of Zr,Al-Containing Materials chemical composition (wt %) sample J

textural and structural data

Si

Al

Zr

Si/Al (mol/mol)

SBET (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

d001 (Å)

1 2 3 4 5

Na-MM Al-MMJCl Na-MMK Al-MMKCl Al-MMKNO3

26.1 23.8 24.1 24.6 24.9

10.9 15.9 10.7 13.5 14.8

0 0 0 0 0

2.48 1.55 2.34 1.89 1.74

8 120 56 170 177

0.05 0.11 0.09 0.11 0.12

0 0.04 0.01 0.07 0.07

17.9−18.9

6

0.4% Zr,Al-MMKNO3

22.0

14.4

0.4

1.58

139

0.11

0.05

18.7

7

1.1% Zr,Al-MMKNO3

23.7

15.3

1.1

1.61

170

0.11

0.07

18.8

8

2.3% Zr,Al-MMKNO3

22.8

14.3

2.3

1.65

171

0.12

0.07

19.0

9

12.4% Zr-MMKNO3

25.6

10.1

12.4

2.63

96

0.08

0.03

10 11 12 13 14

Al-phase 6.9% Zr,Al-phase 17.6% Zr,Al-phase Na-MMR Al-MMRNO3

0 0 0 32.6 31.5

37.0 31.0 21.9 5.6 9.8

0 6.9 17.6 0 0

− − − 5.61 3.10

153 259 170 109 225

0.09 0.15 0.09 0.25 0.22

0.07 0.12 0.08 nd 0.09

20.1 − − − 14

15

12.7% Zr-MMRNO3

30.1

5.6

12.7

5.18

125

0.17

0.06

16

11.9% Ga-MMK

20.5

11.1

11.9

−

180

0.23

0.04

11.0 18.6 11.0 18.9

18 20 18.0

(90 mmol of Al per gram of Na-MMJ). After being mixed , the suspension was diluted to 1/100 wt/wt of Na-MMJ/solution by addition of water. The final suspension was stirred at room temperature for 24 h, and the solid was recovered by filtration, washed repeatedly with water until negative reaction of Cl− using AgNO3 test, and then dried in air and calcined at 400 °C for 4 h. The syntheses of Al-MMKCl, Al-MMRNO3, and Al-MMKNO3 were performed using a procedure described by Timofeeva et al.9 The aluminum hydroxy oligomeric solution (Al solution) was prepared by hydrolysis of 0.1 mol/dm3 AlCl3·6H2O or 0.1 mol/ dm3 Al(NO3)3·9H2O using 1 mol/dm3 NaOH solution with a OH−/Al3+ molar ratio of 2.4. After 14 days of aging at room temperature, the Al solution (7.5 mmol of Al per gram of clay) was slowly added to Na-MMK. After being mixed, the suspension was also diluted to 1/100 wt/wt of Na-MMK/solution by addition of water. The washing, drying, and calcination steps were as reported for the previous series. The syntheses of Zr,Al-MMKNO3 and Zr,Al-MMRNO3 were performed from Zr,Al-intercalating solutions that were prepared by mixing Al and Zr solutions with Zr/Al13 Keggin ion molar ratios of 0.56, 0.93, and 1.86.10 The Al solution was prepared by hydrolysis of 0.1 mol/dm3 Al(NO3)3·9H2O with 1 mol/dm3 NaOH solution (OH−/Al3+ molar ratio of 2.4); then, the solution was held at room temperature for 7 days. Zr solution was prepared dissolving 16.64 g of ZrOCl2·8H2O in 0.5 dm3 of water and heating at 90 °C for 24 h.13 The Al-intercalating solution was added to Na-MMK, and then the Zr-intercalating solution [(Zr + Al)/Na-MMK = 3 mmol/g] was quickly added to the mixture under magnetic stirring. The suspension was also diluted to 1/ 100 wt/wt of Na-MMK/solution by addition of water. The washing, drying, and calcination steps were as reported for the previous series. The chemical composition and textural data of the Na-MM, Al-MM, and Zr,Al-MM materials are listed in Table 1. Synthesis of Al and Zr,Al Phases. The synthesis of Zr,Alphases was performed from solutions containing Al and Zr polycations, prepared using the procedures described in the previous section for the preparation of Zr−Al intercalating solutions and mixed at Zr/Al molar ratios between 0 and 0.143. The Zr,Al-containing solutions were stored at room temperature

Zr,Al-phases with the bayerite−boehmite structure were synthesized by calcination at 400 °C of Zr,Al-composite precursors (5−18 wt % Zr) prepared from the Al-Keggin-type polycation [AlO4Al12(OH)24(H2O)12]7+ and Zr polycations derived from ZrOCl2 as Al and Zr sources, respectively. The acid−base properties of Al- and Zr,Al-pillared clays and Zr,Alphases were studied by IR spectroscopy using pyridine, benzonitrile (PhCN), and deuterated chloroform (CDCl3) as probe molecules. The nature of the original clays and the parameters varied during the preparation of the pillared solids can strongly influence the properties of the final solids. In particular, the Al/Zr ratio can strongly affect the oligomeric state of Zr and, consequently, the acid−base properties of Zr,Alpillared clays, conditioning their catalytic performance in the synthesis of 1-methoxy-2-propanol from methanol and PO. A Ga-pillared montmorillonite was also prepared and used as a catalyst for comparative purposes.

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EXPERIMENTAL SECTION Materials. Propylene oxide (PO) (99.5%) was purchased from Acros Organics. Commercial methanol was used without any further purification. Al(NO3)3·9H2O and AlCl3·6H2O were purchased from Aldrich. Three montmorillonites were used: (i) montmorillonite from the deposit of Tagansk, Kazakhstan, denoted as Na-MMK, which is calcium-rich in its natural form and was converted into the Na-homoionic form by treatment with NaCl (1 mol/dm3 NaCl solution, 100 cm3 solution/1 g of clay, 80 °C for 2 h); (ii) montmorillonite from the deposit of Mukhartala (Buryatia, Russia), denoted as Na-MMR, which is also calcium-rich in its natural form and was converted to Namontmorillonite by the same homoionization treatment; and (iii) purified montmorillonite from Tsukinuno, Japan, denoted as Na-MMJ, used as supplied by The Clay Science Society of Japan. Synthesis of Al- and Zr,Al-Pillared Clays. Al-MMJCl was synthesized by modification of Na-MMJ with aluminum hydroxy oligomeric solutions (Al solutions).11,12 A 1.5 mol/dm3 solution of NaOH was slowly added to a 0.5 mol/dm3 Al-solutions prepared from AlCl3·6H2O (OH−/Al3+ = 2 mol/mol). After 48 h of aging at 50 °C, the Al solution was slowly added to Na-MMJ 13566

dx.doi.org/10.1021/ie501048a | Ind. Eng. Chem. Res. 2014, 53, 13565−13574


Article

of Brønsted acid sites was characterized by the PA values calculated similarly to the previous case, using the equation16

for 7 days. After evaporation of water, the solids were calcined at 400 °C for 6 h and then washed with distilled water and dried in air at room temperature. The chemical compositions and textural data of all of the samples are included in Table 1. Synthesis of Ga-Pillared Clay. The synthesis of Ga-MMK was carried out following the procedure described by Vicente et al.14 The Ga solution was prepared by the hydrolysis of 0.1 mol/dm3 Ga(NO3)3 solution using 1 mol/dm3 NaOH solution, with a OH−/Ga3+ molar ratio of 2.4. After 14 days of aging at room temperature, the Ga solution was slowly added to Na-MMK (7.5 mmol of Ga per gram of Na-MMK). After being mixed, the suspension was diluted to 1/100 of Na-MMK/solution by addition of water. The washing, drying, and calcination steps were as reported for the previous series. Instrumental Techniques and Methods. X-ray diffraction patterns were obtained using an XRD ThermoARL diffractometer with Cu Kα (λ = 1.5418 Å) radiation. The porous structure of the materials was determined from the N2 adsorption isotherms obtained at −196 °C using Micromeritics ASAP 2400 equipment. DR-UV−vis spectra were recorded on a Shimadzu UV-2501 PC spectrometer with an IRS-250A accessory in the 190−900-nm range with a resolution of 2 nm; BaSO4 was used as the standard reference. 27 Al magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) experiments were performed using a Bruker Avance-400 spectrometer at the resonance frequency of 104.26 MHz. MAS spectra were measured using a Bruker MAS NMR probe with 4mm ZrO2 rotors at spinning frequencies of 15 kHz. A pulse length of 1 μs (π/12) and a recycle delay of 0.5 s were used for the acquisition of all 27Al spectra. The spectra were measured at room temperature and referenced to external 0.1 mol/dm3 aluminum chloride. For the analysis of surface functional groups, the samples were pressed into self-supporting wafers (7−20 mg/cm2) and pretreated within the IR cell by heating for 1 h at 300 °C in air and for 1 h at 400 °C under a vacuum before the adsorption experiments. FTIR spectra were recorded on a Shimadzu FTIR8300 spectrometer in the range between 400 and 6000 cm−1 with a resolution of 4 cm−1. The samples were exposed to saturated PhCN vapors at room temperature for the analysis of the Lewis surface acidity. The spectra of the samples were recorded every 10 min until saturation by PhCN. The samples were exposed to saturated CDCl3 vapors for 3 min at room temperature for the analysis of the surface basicity. The spectra were obtained both before and after CDCl3 adsorption, and the difference was calculated. The strength of the base sites was estimated from the shift in νC−D using the equation15 log ΔνC − D = 0.0066PA − 4.36

PA =

log(3400 − νNH) − 51 0.0023

(2)

where PA (kJ/mol) is the energy of proton elimination from the acid residue; 3400 cm−1 is the wavenumber of the band of the undisturbed N−H bond of the pyridinium ion; and νNH (cm−1) is the wavenumber of the center of gravity of the band of the stretching vibration of the pyridinium ion, which depends on the basicity of the acid residue and is determined from the contour of the νNH band. In this case, the smaller the value of PA, the higher the acidity. The chemical analyses of the solids were carried out by means of inductively coupled plasma-atomic emission spectrometry. Catalytic Tests. Al-, Ga-, and Zr-pillared clays and Al- and Zr,Al-phases were tested in the synthesis of 1-methoxy-2propanol from methanol and PO at 60 °C. Before the reaction, the catalysts were activated at 150 °C for 2 h to remove adsorbed water. The reaction was carried out in a stainless steel autoclave reactor with an inner volume of 25 cm3. The standard procedure was as follows: PO (7.4 mmol), MeOH (MeOH/PO = 8 mol/ mol), and catalyst (3 wt %) were introduced into the autoclave. After being run at 60 °C for 6 h under magnetic stirring, the reactor was cooled to room temperature. The products were analyzed on a gas chromatograph (Agilent 7820) with a flame ionization detector on a HP-5 capillary column, after separation of the catalysts by centrifugation. This reaction was also carried out at 40 °C, and in this case, the catalysts were activated at 400 °C for 4 h. The standard procedure was as follows: PO (2.6 mmol), MeOH (MeOH/PO = 16 mol/mol), and catalyst (0.66 wt %) were introduced into the reactor. At various time intervals, aliquots were taken from the reaction mixture and analyzed after separation of the catalysts by centrifugation.

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RESULTS AND DISCUSSION Synthesis and Textural Properties. The chemical compositions and textural properties of all of the samples are reported in Table 1. The use of aluminum nitrate or chloride for the preparation of Al-pillaring solution did not significantly affect the textural properties of the solids, although the solids prepared from aluminum chloride were better ordered (Figure 1). Thus, the solids derived from aluminum chloride showed a welldefined, narrow 001 reflection, whereas those derived from aluminum nitrate showed a broad 001 reflection, indicating that the long-distance order was higher in the first case. The basal value (d001) increased from 11.0 to 18.0−18.9 Å during the pillaring process. The specific surface area (SBET) and microporosity volume (Vmicro) also increased. All of the pillared solids showed specific surface area values typical of this type of solid, reaching 225 m2/g for Al-MMR (Table 1). The chemical compositions, main textural characteristics, and basal values (d001) of Zr,Al-MMKNO3 samples are also included in Table 1 (samples 6−9). The textural data were found to depend on the chemical composition of the pillaring solution: The basal value (d001) increased with increasing Zr content. A broad band at 20−40° in the diffractograms of Zr,Al-MMKNO3 samples indicates the formation of amorphous ZrO2 (Figure 2A). At the same time, diffraction reflections from boehmite and bayerite phases were also observed in the X-ray diffraction patterns of the samples. The intensity of these reflections depended on the Zr

(1)

where ΔνC−D is the shift (cm−1) of the C−D vibration and PA is the proton affinity, such that the larger the value of PA, the higher the strength of the basic sites. IR spectra are given in relative units of absorbance/ρ [where ρ is the amount of the sample per 1 cm2 beam area (g/cm2)], which means that the optical density (Aυ) at wavenumber (υ) is normalized to the wafer thickness ρ. For example, if the value of optical density (Aυ) is 0.05 for a wafer with ρ = 0.01 g/cm2, then the value of absorbance/ρ is 5. For studies of Brønsted surface acidity, the samples were exposed to saturated pyridine vapors at room temperature for 10 min and under a vacuum at 150 °C for 15 min. Then, pyridine was desorbed until a pressure of 10−6 mbar was reached, when pyridine was no longer physisorbed on the wafers. The strength 13567



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According to 27Al MAS NMR spectroscopy (Figure 3), aluminum coordination did not change strongly in the Zr,Al-

Figure 1. X-ray diffraction patterns of Na-MMJ, Na-MMK, and Al-MM synthesized from aluminum chloride and aluminum nitrate.

content, which suggests that the amount of Zr affected the crystallinity of ZrO2. The chemical compositions and textural properties of Zr,Alphases are also listed in Table 1 (samples 10−12). The initial Zr/ Al molar ratio affected the chemical composition of the resulting material. An increase in the Zr amount in solution led to an increase in the Zr amount in the final solid; however, these increases did not show the same tendency, and the Zr/Al ratio was higher in both cases in the final solid than in the precursor solution. Considering the synthesis method followed for the preparation of the samples, this implies the dissolution of some of the aluminum in the final washing step. In other words, despite the calcination at 400 °C, some of the Al still formed soluble phases, being removed in the last washing step, leading to a Zr enrichment of the final solids. The Zr,Al-phases consisted of a mixture of bayerite and boehmite, which was evident from the presence of their diffraction reflections in the XRD patterns (Figure 2B).17 These reflections were very similar in the three solids, whereas no reflections attributable to crystalline Zr-phases or Zr,Al-mixed phases, were detected in the solids containing Zr. The main effect produced by this element was a broad band at 20−40° in the diffractograms of Zr,Al-phases, suggesting that amorphous ZrO2 was formed. The diffraction reflections from boehmite were more intense in the solids containing Zr, which suggests that the presence of Zr influenced its crystallinity.

Figure 3. 27Al NMR (MAS) spectra normalized to the Oh Al intensity of Zr,Al-phase materials.

phases. In the spectrum of the Al-phase, two resonances in the ranges of ca. 0−20 ppm (octahedral AlOh) and ca. 55−75 ppm (tetrahedral AlTd) were clearly present.18,19 The appearance of the broad signal centered at 50−60 ppm can be explained by the existence of five-coordinated aluminum (AlO5) due to the various defects and distortions of the Al-phase structure. The intensity of this signal decreased in the spectra of the Zr,Alphases, which might be due to the better crystallization of bayerite observed in the samples containing Zr. Note that the ratio of integral intensities SAlOh/SAlTd increased with the Zr/Al molar ratio (Figure 4A); the amount of AlTd decreased in the structure of the Zr,Al-phases. According to our previous data,10 the zirconium content in Zr,Al solution affects the stability of the Al13 Keggin ion. The SAlo/SAlt ratio, between the integral intensity of the Alt signal at 62.5 ppm and the Alo signal at 0 ppm in the spectra of Zr,Al-MMRNO3 samples dramatically increased at Zr/Al ≥ 0.141/1.0 mol/mol as a result of the collapse of the Al oligomeric species and the formation of monomeric Al3+ and Zr4+ cations.20 It is reasonable to think that the reduction of the structural distortions of Zr,Al-phases is concerned with this phenomenon.

Figure 2. X-ray diffraction patterns of Zr,Al-MMKNO3 and Zr,Al-phases. 13568



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Figure 4. (A) SAlOh/SAlTd ratios and integral intensities of S2230/S2242 and S2265/S2280−2285 bands versus Zr content in Zr,Al-phases. (B) Integral intensities (Sq) of bands at 2252, 2265, and 2280−2285 cm−1 versus Zr content in Zr,Al-phases.

Acid−Base Properties of Zr,Al-Pillared Clays. Catalytic activity depends on the nature and amount of acid−base sites on the surface of a solids. To better understand the main role of these sites in catalysis, the nature of the acid−base sites of Zr,Alcontaining samples was studied by IR spectroscopy using CDCl3 and PhCN as probe molecules. Basicity. The basicity of the solids was investigated using CDCl3 as a probe molecule.15 This molecule has been successfully used for the analysis of the basicity of various oxides, Al- and Zr,Al-pillared clays, zeotype materials VSB-5/M-VSB-5, and AlPO-34/SAPO-34.9,10,15,21,22 IR spectra of CDCl3 adsorbed on Al-phase and 6.9% Zr,Alphase are shown in Figure 5. Only one band at 2252 cm−1 was

Table 2. Spectral Characteristics (νC−D) of OH Groups for Zr,Al-Containing Materials According to the Adsorption of CDCl3 CDCl3 Na-MMK Al-MMKCl

Al-MMKNO3

2.3% Zr,Al-MMKNO3

12.4% Zr-MMKNO3

12.7% Zr-MMRNO3

Al-phase 17.6% Zr,Al-phase

νC−D (cm−1)

ΔνC−D (cm−1)

PA (kJ/mol)

2268 2265 2262 2265 2260 2255 2265 2260 2252 2265 2260 2254 2265 2260 2254 2265 2260 2254 2252 2252

− 3 6 3 8 13 3 8 16 3 8 14 3 8 14 3 8 14 16 16

− 733 767 733 797 829 733 797 843 733 797 834 733 797 834 733 797 834 843 843

basicity of Al-MMKCl was slightly lower (829 kJ/mol) than that of Al-MMKNO3 (843 kJ/mol), which might be due to the different Al salts (nitrate or chloride) used for the preparation of the Alpillaring solution. Brønsted Acidity. Brønsted acidity of the solids was analyzed using pyridine as probe molecule. The data obtained are summarized in Table 3. The initial Al salt (nitrate or chloride) used for the preparation of the Al-pillaring solution slightly affected the acidity of the Al-MMK materials. At the same time, the amount of Zr in Zr,Al-MMK affected the strength of the acid sites, and PA decreased in the order

Figure 5. FTIR difference spectra of CDCl3 adsorbed on Al- and 6.9% Zr,Al-phases.

observed, due to the weak interactions between CDCl3 and the base sites of the solids. The values of proton affinity (PA) estimated from eq 1 are reported in Table 2. In all cases, these values were close to 843 kJ/mol, indicating the slight effect of zirconium content on the basicity of Zr,Al-phases. Investigation of the basicity of Zr,Al-MMs and Zr,Al-MMRNO3 alludes to the fact that the insertion of Zr into the Al2O3 network of Zr,Al-MM also slightly affects the strength of basic sites (Table 2). The strength of basic sites (PA) is 843 and 834 kJ/mol for AlMMKNO3 and 12.4% Zr-MMKNO3, respectively. Interestingly, the

Zr‐MMK NO3 > Zr, Al‐MMK NO3 > Al‐MMK NO3 (PA, kJ/mol) 1123 >

1144

>

1150

This order is in accord with the trend found for Zr-MMRNO3 prepared from Mukhartala montmorillonite. It also agrees with literature data, as Occelli and Finseth23 and Bagshaw and 13569



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deficient zirconium ions, was observed in the spectra of Zr,Alphases. The variation of the Zr content in the Zr,Al-phases affected both the intensity and the integral intensity (Sq) of these bands. An increase in Zr content favored a decrease in S2230, the most acidic sites. This result agrees with the 27Al NMR spectroscopy data (Figure 4B). Increasing the amount of zirconium in the Zr,Al-phases led to an increase in the integral intensity of this band that can point out an increase in the amount of Zr LAS. Catalytic Properties of M,Al-Pillared Clays (M = Al, Ga, and Zr). The results of the catalytic performance studies of Zr,Al-MMs and Zr,Al-phases are presented in Table 4. Na-MMJ,

Table 3. Surface Brønsted Acidity of Zr,Al-Pillared Clays According to the Adsorption of Pyridine qea (μmol/g)

PA (kJ/mol)

Al-MM Cl Al-MMKNO3

5 5

1148 1150

Al-MMRNO3

5

1150

2.3% Zr,Al-MMKNO3

6

1144

12.4% Zr-MMK NO3

1

1123

Zr-MMRNO3

1

1123

17.6% Zr,Al-phase

2

1123

sample K

12.7% a

qe = amount of Brønsted acid sites.

Table 4. Catalytic Behavior of Zr,Al-Containing Materials in the Reaction of Propylene Oxide with Methanola

Cooney24 demonstrated that Zr,Al-hectorite and Zr,Al-montmorillonite had stronger acid sites than the corresponding Alcontaining counterparts. Lewis Acidity. The Lewis acidity of the Al- and Zr,Alcontaining materials was investigated using benzonitrile as a probe molecule, as this molecule has a high propensity for Lewis acid sites (LASs) and low propensity for Brønsted acid sites (BASs).25,26 The spectrum of PhCN adsorbed on the Al-phase is shown in Figure 6. After deconvolution, three bands at 2230, 2242, and

run 1 2 3 4 5

Na-MMJ Al-MMJCl Na-MMK Al-MMKCl Al-MMKNO3

conversion of PO (%)

selectivity of II (%)

II/I (mol/mol)

2 100 21 100 79

57 31 65 28 73

1.30 0.45 1.82 0.41 2.67

33b 74

61b 66

1.85b 2.63

6 7

0.4% Zr,Al-MM

8

1.1% Zr,Al-MMKNO3

74

64

2.61

9

2.3% Zr,Al-MMKNO3

74

61

2.50

10

12.4% Zr-MMKNO3

75

60

K

NO3

b

2.24 b

1.41b 3.26 2.88 2.23 0.88 1.58

11 12 13 14 15 16

Al-phase 6.9% Zr,Al-phase 17.6% Zr,Al-phase Na-MMR Al-MMRNO3

6 8 12 13 7 99

52 75 72 67 47 61

17

12.7% Zr-MMRNO3

21

38

0.61

18

11.9% Ga-MMKNO3

19b

56b

1.65b

a

Experimental conditions: MeOH/PO 8:1 mol/mol, catalyst 3 wt %, 60 °C, 6 h; before reaction, catalyst was activated at 150 °C for 2 h. b Experimental conditions: MeOH/PO 16:1 mol/mol, catalyst 0.66 wt %, 40 °C, 3 h; before reaction, catalyst was activated at 400 °C for 4 h.

Na-MMK, and Na-MMR showed low catalytic conversions (Table 4, runs 1, 3, and 15). The lower conversion of PO and selectivity toward II over Na-MMs can be related to their low acidity. Modification of Na-MMs with Al-pillaring solution favored an increase in the conversion of PO. Note that the type of Al salt (nitrate or chloride) used for the preparation of pillaring solution, and therefore for Na-MM modification, affected the activity of the Al-MMs. Table 4 (runs 2, 4, and 5) shows that the conversion of PO in the presence of Al-MMJCl and Al-MMKCl was higher (100%) than that in the presence of Al-MMKNO3 (79%). At the same time, the selectivity toward II was higher in the presence of Al-MMKNO3 (73%) in comparison to Al-MMJCl (31%) and Al-MMKCl (28%). Probably, this phenomenon is related to the difference in the strength of the basic sites (Table 2). The addition of Zr ions to the pillaring solution slightly affected the conversion of PO (Table 4, runs 5−10), and the selectivity toward II decreased from 73% to 60% when the Zr content was increased from 0% to 12.4%. A similar effect was also observed for the Zr,Al-phases (Table 4, runs 12−14). The small

Figure 6. FTIR difference spectra of PhCN adsorbed on Zr,Alcontaining materials.

2280 cm−1 were revealed. The band at 2280 cm−1 can be assigned to the interaction between the nonbonding electrons of PhCN and electron-deficient Al ions (LAS) in the solid. The bands at 2230 and 2242 cm−1 can be attributed to the interaction of PhCN with the Al−OH groups. According to Knozinger and Ratnasamy,27 the OH groups bonded to octahedrally coordinated Al ions (AlOh) possess lower acidity than those bonded to tetrahedrally coordinated Al ions (AlTd). Based on this assertion, the bands at 2230 and 2242 cm−1 can be assigned to the AlTd− OH (strong acidic sites) and AlOh−OH (weak acidic sites) groups. At the same time, four bands at 2230, 2242, 2265, and 2280 cm−1 were disclosed after the deconvolution of the spectra of these solids (Figure 6). The band at 2242 cm−1 can also be assigned to the interaction of PhCN with the Zr−OH groups. Moreover, a band at 2265 cm−1, which can be attributed to the interaction of nonbonding electrons of PhCN with electron13570



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-polyoxocations in Na-MMK. The results of the catalytic performance of M-MMKNO3 solids (M = Al, Ga and Zr) are presented in Figure 8 and Table 4 (runs 6, 11, and 18). It is noteworthy that the reaction of PO with methanol (MeOH/PO 16 mol/mol) in the presence of these samples activated at 400 °C proceeded at 40 °C. Kinetic curves are shown in Figure 8A. The reaction rate clearly depends on the type of metal cation. The electronegativity of an ion (Xi) is generally used to represent the acidity of a metal;31,32 this value has very often been used to characterize the acidity and catalytic activity of solids.33,34 Based on Xi values calculated by Li and Xue,35 the correlations between the parameter Xi and the activity of M-MMKNO3 and the molar ratio II/I were estimated. The relationships between the consumption of PO in 180 min based on the specific surface area of the sample and the Xi values and between the II/I molar ratio and the X values, are shown in Figure 8B. According to these correlations, II/I molar ratio and activity of M-MM K NO 3 decreased steadily as the parameter Xi increased in the order

difference in the conversion of PO in the presence of Zr,Alphases (12−13%) and Al-phase (8%) can be related to the difference in SBET (Table 1, samples 10−12). At the same time, the increase in Zr content from 0 to 17.7% leaded to a decrease in selectivity toward II from 75 to 67%, phenomenon that was attributed to the changes in the strength and nature of the acid− base sites. Our group recently demonstrated that the amount of Zr in Zr,Al-MMRNO3 materials prepared from Mukhartala montmorillonite affected the reaction rate and selectivity toward II (Table 4, runs 16 and 17).10 The increase in Zr content led to decreases in the conversion of PO and the selectivity toward II. Interestingly, the changes in the conversion of PO and selectivity toward II in the presence of 12.7% Zr,Al-MMRNO3 were lower than those in the presence of 12.4% Zr,Al-MMKNO3 (Table 4, runs 10 and 17), even though acid−base properties of these materials were similar (Tables 2 and 3). Probably, the differences in the catalytic performance were related to the textural properties (Table 1, samples 3, 5, 9, and 13−15). Na-MMKNO3 (141 Å), Al-MMKNO3 (112 Å), and 12.4% Zr-MMKNO3 (108 Å) exhibited wider pores than Na-MMRNO3 (92 Å), Al-MMRNO3 (68 Å), and 12.7% Zr-MMRNO3 (53 Å). The modification of Na-MMK had a negligible effect on pore diameter. At the same time, the pore diameter decreased more than 1.3−1.7 times after Na-MMR modification. The larger conversion of PO (99%) and lower selectivity toward II (61%) in the presence of Al-MMRNO3 in comparison to Al-MMKNO3 (Table 4, runs 5 and 16) indicates that a significant portion of the Al2O3 formed from Al polyhydroxycations during calcination was localized as an individual Al-phase on the external surface of the material. The narrow pore diameter favored the formation of oligomeric species of ZrO2, in accord with the DR-UV−vis spectroscopy results (Figure 7). The formation of ZrO2 clusters was characterized by a broad band in the 260−500 nm region in the DR-UV spectrum of 12.7% Zr-MMRNO3. Based on literature data,3,22,28−30 it can be proposed that both the reaction rate of PO with methanol and the isomer selectivity depend on the existence of Lewis acid−base site pairs. The contribution of Lewis acid and base sites in these pairs can be revealed from a comparison of the catalytic activities of pillared clays prepared through the intercalation of Ga, Al, and Zr

Al‐MMK NO3 > Ga‐MMK NO3 > Zr‐MMK NO3

that is, the higher Lewis acidity resulted in a higher reaction rate and higher selectivity toward II. The changes in the isomer selectivity toward II and the reaction rate can also provide information about the changes in the basicity of M-MMK. The basicity of these materials can be characterized by the extent of negative charge on the oxygen atom, that is, the ionic covalent nature of the metal−oxygen bond. The ionic−covalent parameter (ICP) is one of the parameters used for the quantitative description of the acid−base properties of solids,36−38 because ICP takes into account not only the type (ionic to covalent) of Me−O bond but also the extent (through polarizability) of the negative charge borne by oxygen. ICP is a dimensionless number and is calculated as ICP = log P − 1.38χ + 2.07

(3)

where χ is the electronegativity (in Pauling-type units35) and P is the polarizing power of the cation (P = e/r2, where e is the formal charge and r is the Shannon ionic radius). For a given cation, ICP decreased in the order Al‐MMK NO3 > Ga‐MMK NO3 > Zr‐MMK NO3 (ICP)

0.95

>

0.78

>

0.57

This order indicates that the polarization of the M−O bond decreases in the opposite order, which should lead to a decrease in the length of the MO bond and polarizing power, that is, a decrease in the negative charge on the oxygen atom. These changes affect the isomer selectivity and reaction rate (Figure 8C). Assuming an anti-Markovnikov mechanism, methoxide (A) and propylene-like (B) species can be formed as intermediates

The formation of similar species was also proposed in the cyanoethylation of alcohols to 3-alkoxypropanenitriles39 and the addition of methanol to allyl alcohol.40 Experimental data (Figure 8) suggest that the great length of the M−O bond and the high polarizing power did not favor a high reaction rate and selectivity toward II.

Figure 7. DR-UV−vis spectra of 12.4% Zr-MMKNO3 and 12.7% ZrMMRNO3. 13571



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Figure 8. (A) Reaction of PO with methanol over M-MMKNO3 (experimental conditions: 2.4 mmol of PO, MeOH/PO 16:1 mol/mol, 0.66 wt % catalyst, 40 °C). (B) Correlations between electronegativity of metal cation (χi) and consumption of PO based on the SBET values of M-MMKNO3 samples and the II/I molar ratios for 180 min. (C) Correlations between ICP and consumption of PO based on the SBET values of M-MMKNO3 samples and the II/I molar ratios for 180 min.

Figure 9. (A,B) Effects of the strength of basic sites (PA) on the (A) conversion of PO and (B) selectivity toward II. (C) Effect of the strength of Brønsted acid sites (PA) on the II/I molar ratio. (Samples are identified by the numberd assigned in Table 1.)

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SUMMARY AND CONCLUSIONS In this work, three montmorillonites were used to prepare Aland Zr,Al-pillared clays with different textural properties and chemical compositions. Microporous Zr,Al-phases with a bayerite−boehmite structure were also synthesized by calcination at 400 °C of Zr,Al solids prepared from the Al-Keggin-type polycation [AlO4Al12(OH)24(H2O)12]7+ and Zr polycations derived from ZrOCl2 as Al and Zr sources, respectively. We found that the structural and physicochemical properties of Zr,Al-containing samples can be adjusted by the zirconium content in the Zr,Al solution. According to the XRD data, an increase in the zirconium content in the solids favored the formation of oligomeric ZrO2 clusters. Data from FTIR spectroscopy pointed out the existence of several sites associated with Al−OH bonds. The nature of the acid sites was investigated by FTIR spectroscopy of adsorbed PhCN and pyridine as probe molecules. Two types of Lewis acid sites were found, namely, strong Al Lewis sites generated by coordinatively unsaturated Al ions and weak Zr Lewis sites generated by coordinatively unsaturated zirconium ions. An increase in zirconium content in the solids led to a decrease in the amount of Al Lewis sites and an increase in the number of Zr

Moreover, according to the literature, the strength of basic sites determines the reaction rate and the isomer selectivity toward II.9,21,22 Materials with moderate basicity (Al2O3, MgO) exhibit a high catalytic activity and isomer selectivity toward II,3 whereas materials with strong basicity (Fe-VSB-5/VSB-5) exhibit a high isomer selectivity and low catalytic activity.22 Our experimental data also point out the key effects of basicity on the reaction rate and the selectivity toward II. An increase in the strength of basic sites leads to a decrease in the conversion of PO and the selectivity toward II (Figure 9A,B). The decrease in the strength of BASs favored an increase in the II/I molar ratio (Figure 9C), because the decrease in Brønsted acidity and the increase in basicity occurred simultaneously. An increase in the strength of basic sites favored the activation of PO and methanol. However, a high strength of these sites can lead to a strong stabilization of species A and B on the surface of the solid and, in this way, lead to a decrease in the reaction rate. Therefore, the highest activity and selectivity toward II can be observed in the presence of Zr,Al-containing materials with basic sites of medium strength. 13572



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(9) Timofeeva, M. N.; Panchenko, V. N.; Gil, A.; Chesalov, Yu. A.; Sorokina, T. P.; Likholobov, V. A. Synthesis of propylene glycol methyl ether from methanol and propylene oxide over alumina-pillared clays. Appl. Catal. B: Environ. 2011, 102, 433−440. (10) Timofeeva, M. N.; Panchenko, V. N.; Gil, A.; Doronin, V. P.; Golovin, A. V.; Andreev, A. S.; Likholobov, V. A. Effect of the acid−base properties of Zr,Al-pillared clays on the catalytic performances in the reaction of propylene oxide with methanol. Appl. Catal. B: Environ. 2011, 104, 54−63. (11) Gil, A.; Trujillano, R.; Vicente, M. A.; Korili, S. A. Analysis of the structure of alumina-pillared clays by means of nitrogen and carbon dioxide adsorption. Adsorpt. Sci. Technol. 2007, 25, 217−226. (12) Lahav, N.; Shan, U.; Shabtai, J. Cross-linked smectites. 1. Synthesis and properties of hydroxyl-aluminium-montmorillonite. Clay Clay Miner. 1978, 26, 107−115. (13) Ben Chaabene, S.; Bergaoui, L.; Ghorbel, A. Zirconium and sulfated zirconium pillared clays: A combined intercalation solution study and solid characterization. Colloid Surf. A 2004, 251, 109−115. (14) Vicente, M. A.; Belver, C.; Sychev, M.; Prihod’ko, R.; Gil, A. Relationship between the surface properties and the catalytic performance of Al-, Ga-, and AlGa-pillared saponites. Ind. Eng. Chem. Res. 2009, 48, 406−414. (15) Paukshtis, E. A.; Kotsarenko, N. S.; Karakchiev, L. G. Investigation of proton-acceptor properties of oxide surfaces by IR spectroscopy of hydrogen-bonded complexes. React. Kinet. Catal. Lett. 1979, 12, 315− 319. (16) Davydov, A. A. Molecular Spectroscopy of Oxide Catalyst Surfaces; Wiley: Chichester, U.K., 2003. (17) Sarkar, D.; Mohapatra, D.; Ray, S.; Bhattacharyy, S.; Adak, S.; Mitra, N. Synthesis and characterization of sol−gel derived ZrO2 doped Al2O3 nanopowder. Ceram. Int. 2007, 33, 1275−1282. (18) Al-Yassir, N.; Le Van Mao, R. Thermal stability of alumina aerogel doped with yttrium oxide, used as a catalyst support for the thermocatalytic cracking (TCC) process: An investigation of its textural and structural properties. Appl. Catal. A: Gen. 2007, 317, 275−283. (19) Welsha, L. B.; Gilson, J.-P.; Gattuso, M. J. High resolution 27Al NMR of amorphous silica-aluminas. Appl. Catal. 1985, 15, 327−331. (20) Kloprogge, J. T. Synthesis of smectites and porous pillared clay catalysts: A review. J. Porous Mater. 1998, 5, 5−41. (21) Timofeeva, M. N.; Panchenko, V. N.; Hasan, Z.; Jhung, S. H. Catalytic potential of the wonderful chameleons: Nickel phosphate molecular sieves. Appl. Catal. A: Gen. 2013, 455, 71−85. (22) Timofeeva, M. N.; Panchenko, V. N.; Jun, J. W.; Hasan, Z.; Kikhtyanin, O. V.; Prosvirin, I. P.; Jhung, S. H. Effect of the acid−base properties of metal phosphate molecular sieves on the catalytic performances in synthesis of propylene glycol methyl ether from methanol and propylene oxide. Microporous Mesoporous Mater. 2013, 165, 84−91. (23) Occelli, M. L.; Finseth, D. H. Preparation and characterization of pillared hectorite catalysts. J. Catal. 1986, 99, 316−326. (24) Bagshaw, S. A.; Cooney, R. P. FTIR surface site analysis of pillared clays using pyridine probe species. Chem. Mater. 1993, 5, 1101−1109. (25) Clark, J. H.; Macquarrie, D. J. Heterogeneous catalysis in liquidphase transformations of importance in the industrial preparation of fine chemicals. Org. Process Res. Dev. 1997, 1, 149−162. (26) Dias, S. C. L.; Macedo, J. L.; Dias, J. A. Acidity measurements of zeolite Y by adsorption of several probes. Phys. Chem. Chem. Phys. 2003, 5, 5574−5579. (27) Knozinger, H.; Ratnasamy, P. Catalytic aluminas: Surface models and characterization of surface sites. Catal. Rev. Sci. Eng. 1978, 17, 31− 70. (28) Cheng, W.; Wang, W.; Zhao, Y.; Liu, L.; Yang, J.; He, M. Influence of acid−base properties of ZnMgAl-mixed oxides for the synthesis of 1methoxy-2-propanol. Appl. Clay Sci. 2008, 42, 111−115. (29) Díez, V. K.; Apesteguía, C. R.; Di Cosimo, J. I. Acid−base properties and active site requirements for elimination reactions on alkali-promoted MgO catalysts. Catal. Today 2000, 63, 53−62.

Lewis sites, whereas the Brønsted acidity increased with the Zr content. The basicity of Zr,Al-containing materials was also studied by IR spectroscopy of adsorbed CDCl3 as a probe molecule, and an increase in Zr content was found to lead to a decrease in the strength of basic sites. The catalytic properties of Zr,Al-containing materials were tested in the synthesis of propylene glycol methyl ether from methanol and propylene oxide. The catalytic properties of Zr,Alpillared clays were found to depend on the nature and textural properties of the clays (specific surface area and pore diameter), the chemical composition of the intercalating solution, and the type of Al salt (nitrate or chloride) used as the Al source for the preparation of the pillaring solution. An increase in Zr content favored a decrease in selectivity toward II as a result of a change in the nature of acid−base sites. The highest conversion and selectivity toward II were observed in the presence of materials with medium-strength basic sites, because high-strength basic sites can lead to a strong stabilization of methoxide and propylene-like species on the surface of the solid. It was demonstrated that the type of metal cation of polyoxocations affects the reaction rate and isomer selectivity. The reaction rate and selectivity toward II were found to decrease in the order Al-MMK > Ga-MMK > Zr-MMK, which correlated with the changes in the Xi parameter and ionic− covalent parameter.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address #

(for M.N.T., V.N., and S.V.T.) Novosibirsk State University, Pirogova Str. 2, 630090, Novosibirsk, Russian Federation Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by SB RAS Project V.44.2.12. We thank A. A. Gorina for several catalytic experiments. REFERENCES

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