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Epoxidation of limonene over hydrotalcite-like compounds with hydrogen peroxide in the presence of nitriles
Applied Catalysis A: General 216 (2001) 257–265
Epoxidation of limonene over hydrotalcite-like compounds with hydrogen peroxide in the presence of nitriles Mar´ıa A. Aramend´ıa, Victoriano Borau, César Jiménez∗ , José M. Luque, José M. Marinas, José R. Ruiz, Francisco J. Urbano Departamento de Qu´ımica Orgánica, Facultad de Ciencias, Universidad de Córdoba, Campus de Rabanales, Edificio C-3, Carretera Nacional IV-A, km. 396 E-14014 Córdoba, Spain Received 3 October 2000; received in revised form 5 March 2001; accepted 5 March 2001
Abstract This paper reports the synthesis of an Mg/Al double layered hydroxide (LDH) by coprecipitation, and of its dodecylsulphate (DS) and dodecylbenzenesulphonate (DBS) intercalates by rehydration. The resulting LDHs, also known as hydrotalcite-like compounds, were characterized by using various instrumental techniques including X-ray diffraction (XRD), IR spectroscopy and solid-state NMR spectroscopy. The solids obtained were tested as catalysts in the epoxidation of limonene with hydrogen peroxide. Because the presence of a nitrile was found to be indispensable for the reaction to develop to an acceptable extent, the influence of various nitriles on catalytic activity was examined. The epoxidation reaction yielded limonene oxide. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Epoxidation of limonene; Double layered hydroxide (LDH); Nitrile
1. Introduction Hydrotalcite is a naturally occurring layered double hydroxide present in anionic clay minerals [1]. The structure of an double layered hydroxide (LDH) is similar to that of brucite, Mg(OH)2 ; some Mg2+ ions are substituted by trivalent ions and induce a charge deficiency in the layers that is neutralized by anions in the interlayer region, which also contains crystallization water. LDHs can also have their Mg2+ ions replaced, so their general formula can be expressed as [M(II)1−x M(III)x (OH)2 ]x + (An − )x/n ·mH2 O. According to Miyata [2], calcination of an Mg/Al-CO3 LDH at 773 K produces a mixture of magnesium and aluminium oxides. In the presence of water and appropriate anions, the oxide mixture can ∗
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be rehydrated back to the LDH. This property, known as the memory effect, provides an effective synthetic pathway for the insertion of organic and inorganic anions into LDHs. The reconstruction of an LDH from a mixed metal oxide precursor is believed to occur via a topotactic reaction. In order to improve the crystallinity of the rehydrated product, Carlino et al. [3] recommend heating the precursor LDH by using a slow ramp (ca. 1◦ C/min). This technique prevents the rapid removal of carbon dioxide and water from the LDH caused by direct, sudden heating, which disrupts the layered structure of the calcined product. In this way, a wide variety of organo-LDHs containing carboxylate [4] and phthalocyaninetetrasulphonate anions [5,6], among others, has been prepared. Lately, organo-LDHs have aroused much interest for use in chemical applications such as heterogeneous catalysis via basic sites [5,7], photochemical reactions
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(e.g. the dimerization of various Mg/Al-LDH intercalates [8]), adsorption (e.g. of environmental organic molecules [9]) and electrochemistry [10,11]. One of the greatest disadvantages of LDHs as regards heterogeneous catalysis is the small size of their interlayer spacing (7–9 Å), which restricts the size of the molecules that can be transformed within. On the other hand, stacking of exceedingly large anions can lead to severe loosening of the layers and the eventual destruction of the LDH structure. One must therefore find an acceptable interlayer spacing not disrupting the brucite-like structure while minimizing internal diffusion in the solid. On the other hand, alkenes were initially epoxidized with percarboxylic acids [11] since the use of metals in the presence of hydrogen peroxide led to slow reactions subject to the competition of other processes. Aqueous H2 O2 is an ideal oxidant, because it is a cheap and safe oxidant, and easy to handle, giving only water as a coproduct. In the last years, much effort is devoted to the alquene epoxidation with H2 O2 and basic catalysts based on LDHs [12–14]. Thus Kaneda et al. [15,16] have reported this epoxidation in the presence of nitriles and amides with good conversion and yield results. Moreover, Keggin-type anions modified LDHs [17] were used in the alkene epoxidation but with similar results to that obtained with the free anion. This paper reports on the intercalation of dodecylsulphate (DS) and dodecylbenzenesulphonate (DBS) anions into an Mg/Al-LDH. The starting LDH and the resulting organo-LDHs were characterized by using various instrumental techniques including X-ray diffraction (XRD), IR spectroscopy and solid-state nuclear magnetic resonance. These LDHs have been used in the reaction of limonene epoxidation with hydrogen peroxide to obtain limonene oxide. Moreover, it has been proved that for the reaction to proceed, a nitrile is needed, so its influence has also been studied.
2. Experimental 2.1. Synthesis of the layered-double hydroxides Two solutions containing 0.03 mol of Mg(NO3 )2 · 6H2 O and 0.01 mol of Al(NO3 )3 ·9H2 O in 25 ml of deionized water, were mixed and the mixture was
added dropwise over 75 ml of an Na2 CO3 solution at pH 10 at 333 K under vigorous stirring. The pH was kept constant by adding appropriate volumes of 1 M NaOH during precipitation. The suspension thus obtained was kept at 353 K for 24 h, after which the solid was filtered and washed with 2 l of de-ionized water. Once synthesized, the LDH was exchanged with carbonate in order to remove nitrate ions from the interlayers. For this purpose, 2.5 g of LDH was dispersed in 25 ml of distilled water and the dispersion supplied with 250 mg of Na2 CO3 and refluxed for 2 h, after which the solid was separated by centrifugation and the water discarded. This operation was repeated and the supernatant analysed for nitrate, which gave a negative test. The resulting solid, named HT-Mg/Al, was dried at 373 K and calcined in a nitrogen atmosphere at 773 K for 8 h to obtain a new solid that was designated HT-Mg/Al-773. For intercalated organo-LDHs, an amount of 1 g of solid calcined at 773 K was suspended in 50 ml of a solution containing 0.80 g of DS or 0.90 g of DBS in bidistilled, decarbonated water in a nitrogen atmosphere at 373 K for 6 h. Then, the solids were separated by centrifugation and stored in a nitrogen atmosphere. The two LDHs thus obtained were designated HT-Mg/Al-DS and HT-Mg/Al-DBS (with DS and DBS, respectively, as interlayer anion). 2.2. Characterization The elemental composition of the samples was determined on a Perkin-Elmer 1000 ICP spectrophotometer under standard conditions. XRD patterns were recorded on a Siemens D-5000 diffractometer using Cu K␣ radiation. Scans were performed over the 2θ range from 5 to 80. Fourier transform infrared (FTIR) spectra for the solids were recorded over the wavenumber range from 400 to 4000 cm−1 on a Bomen MB-100 FTIR spectrophotometer. Samples were prepared by mixing the powdered solids with KBr (the blank) in a 15:85 ratio. 27 Al NMR spectra were recorded at 104.26 MHz on a Bruker ACP-400 spectrometer under an external magnetic field of 9.4 T. All measurements were made at room temperature. The samples, held in zirconia rotors, were spun at the magic angle (54◦ 44 relative to the external magnetic field) at 3.5 kHz. Spectra were recorded at an excitation pulse of π /8 (0.6 ms) and
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259
an accumulation interval of 1 s. Al(H2 O)6 3+ was used as external standard. The number of accumulations was 1000. 13 C CP/MAS NMR spectra were recorded at 100.62 MHz, using a spinning rate of 3.5 kHz; the contact time for the transfer of magnetization between protons and 13 C was 6 ms (1000 scans). Chemical shifts for 13 C were measured with respect to tetramethylsilane. 2.3. Oxidation reactions Oxidation reactions were conducted in a 250 ml flask containing the catalyst (0.450 g), limonene (6.2 mmol), benzonitrile (34.3 mmol) and hydrogen peroxide (121.3 mmol). Methanol (80 ml) was used as solvent. The reaction was conducted at 333 K and monitored by gas chromatography, using a 30 m × 0.25 mm i.d. DB-1 column. Reactions products were identified by mass spectrometry.
3. Results and discussion 3.1. Characterization of catalysts The elemental analysis of solid HT-Mg/Al revealed an Mg/Al ratio of 3:1 and the following chemical formula: Mg0.75 Al0.25 (OH)2 (CO3 )0.125 ·0.7H2 O. The organo-LDHs were found to have the following formulae: Mg0.75 Al0.25 (OH)2 (C12 H25 SO4 )0.25 ·mH2 O and Mg0.75 Al0.25 (OH)2 (C12 H25 C6 H4 SO2 )0.25 ·nH2 O, for HT-Mg/Al-DS and HT-Mg/Al-DBS, respectively. The XRD analysis of solid HT-Mg/Al (Fig. 1A) revealed a high crystallinity. As can be seen from the XRD patterns, the solid has a typical structure with stacked layers similar to those previously found by Reichle et al. [18] in Mg/Al-LDHs, as well as by Weir et al. [19] and by our own group [20]. The indexing of the diffraction peaks was obtained by comparison with the reported diagram for synthetic hydrotalcite [21]. The a parameter of the hexagonal unit cell corresponds to the distance between two metals in adjacent octahedral sites while the c parameter corresponds to three times the distance between adjacent hydroxyl layers. The c-axis parameter was calculated to be 23.41 Å from the position of the (0 0 3) peak. The a-axis parameter was calculated to be 3.06 Å
Fig. 1. XRD patterns for solids HT-Mg/Al (A); HT-Mg/Al-500 (B); HT-Mg/Al-DS (C); and HT-Mg/Al-DBS (D).
from the (1 1 0) peak. Since the c-axis parameter was 23.41 Å, the basal spacing of HT-Mg/Al was 7.80 Å. Fig. 1B shows the XRD patterns for the solid obtained by calcining the LDH at 773 K. As can be seen, heating destroys the layered structure and yields a periclase MgO phase in addition to amorphous aluminium oxide where applicable [2,22]. Fig. 1C and D show the XRD patterns for the LDHs following intercalation of DS and DBS anions, respectively. The (0 0 3) diffraction peaks and the higher-order peaks for the two LDH intercalates are shifted to lower 2θ angles relative to those for HT-Mg/Al. The c-axis parameters for HT-Mg/Al-DS and HT-Mg/Al-DBS were calculated to be 37.17 and 39.96 Å, respectively. Consequently, the basal spacing of these two organo-LDHs increased from 7.80 Å in the starting solid (HT-Mg/Al) to 12.39 and 12.32 Å, respectively. This increased interlayer spacing suggests the effective intercalation of both DS and DBS
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Fig. 2. FTIR spectra for solids HT-Mg/Al (A); HT-Mg/Al-500 (B); HT-Mg/Al-DS (C); and HT-Mg/Al-DBS (D).
anions into the LDH structure. Kanezaki et al. [23] found greater spacings (up to 19 Å) for the intercalation of 9,10-anthraquinone-2,6-disulphonates into Mg/Al-LDHs. Fig. 2A and B show the FTIR spectra for HT-Mg/Al and its product of calcination at 773 K. The spectrum for the former solid (Fig. 3A) exhibits a strong band between 3800 and 2700 cm−1 that encompasses the twisting vibrations of physisorbed water [24], OH− structural vibrations [25], OH · · · OH characteristic valency vibrations, and/or characteristic stretching vibrations for Mg2+ –OH− in hydroxycarbonates [24–27]. The band corresponding to the δ HOH vibration mode appears at 1643 cm−1 . Carbonate ion in a symmetric environment exhibits three absorption bands close to those for the free anion (namely, ν4 = 680 cm−1 , ν2 = 880 cm−1 and ν3 = 1415 cm−1 ). Band ν 3 is barely
Fig. 3. 27 Al MAS NMR spectra for solids HT-Mg/Al (A); HT-Mg/Al-500 (B); HT-Mg/Al-DS (C); and HT-Mg/Al-DBS (D).
distinguishable at 1377 cm−1 , and so is ν 2 (848 cm−1 ). On the other hand, ν 4 is observed at 659 cm−1 . The presence of the shoulder at about 1400 cm−1 can be ascribed to a decreased carbonate symmetry [28] that activates vibrational mode ν 1 ; this mode is inactive in wholly symmetric carbonate and appears as a shoulder
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at 1505–1535 cm−1 in the spectra [29]. On the other hand, the band at 1377 cm−1 might also correspond to ν 3 for nitrate ion; however, this is not the case since nitrate ion was checked to be absent following its exchange with carbonate. Calcination of this LDH at 773 K resulted in a markedly different spectrum (Fig. 2B) where the 450–650 cm−1 region exhibited the bands corresponding to the characteristic vibrations of the oxides (MgO and Al2 O3 ) [30]. The broad band at about 1400 cm−1 does not correspond to interlayer carbonate; in fact, it is typical of O–C–O vibrations in carbonate adsorbed on the surface of the oxides formed during calcination [30,31], together with carbonate and reversible adsorbed water on the oxide surface [32]. Finally, one other, weak band is observed in the 3800–3100 cm−1 region due to OH · · · OH2 valency vibrations [27,28]. The FTIR spectra for the organo-LDHs (Fig. 2C and D) are markedly different from that for HT-Mg/Al (Fig. 2A). Thus, the region corresponding to the interlayer anion, 1200–1500 cm−1 , does not contain the band for carbonate (1377 cm−1 ) in the spectra for the organo-LDHs; such spectra, however, exhibit strong bands at 1233 and 1192 cm−1 due to stretching vibrations in sulphate and sulphonate anion [33], respectively. These bands are accompanied by others corresponding to the organic portion of the intercalated molecules, which appear near 3000 cm−1 . Based on these data and on the previous XRD patterns, the organic anions are effectively intercalated into the LDH structure. For further confirmation, however, the solids were subjected to an NMR study. Fig. 3 shows the results of the 27 Al MAS NMR examination. The spectrum for HT-Mg/Al (Fig. 3A) exhibits a single signal, centred at 9.4 ppm, which was assigned to Al octahedrally coordinated to OH groups in the LDH. This is consistent with previously reported results [34,35] and with the assumption that Mg2+ cations are isomorphically substituted by Al3+ ions in the LDH structure. As noted earlier, calcination of the LDH yields a mixture of Mg and Al oxides. The 27 Al spectrum for HT-Mg/Al-773 (Fig. 3B) exhibits two signals at 9.6 and 74.0 ppm assigned to octahedrally and tetrahedrally coordinated Al, respectively [36]. Fig. 3C and D show the 27 Al MAS NMR spectra for the organo-LDHs; as can be seen, both exhibit a single signal, centred at ca. 9 ppm, which suggests that
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Fig. 4. 13 C CP/MAS NMR spectra for the organo-LDHs HT-Mg/Al-DS (A) and HT-Mg/Al-DBS (B).
the whole Al in the LDHs is octahedrally coordinated and hence that rehydration to the organo-LDHs under the experimental conditions used was quantitative. Fig. 4 shows the 13 C CP/MAS NMR spectra for the solids. That for HT-Mg/Al-DS (Fig. 4A) exhibits a series of signals between 8 and 25 ppm corresponding to the hydrocarbon skeleton of DS anion, and another signal at 72 ppm ascribable to the methylene group bonded to the sulphate anion. The spectrum for HT-Mg/Al-DBS (Fig. 4B) is similar to the previous one; again, the region from 8 to 25 ppm contains the signals for the saturated carbon atoms in the DBS anion, whereas that between 125 and 140 ppm exhibits the typical signals for aromatic carbons.
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Table 1 Catalytic activity, conversion and selectivity in the epoxidation of limonenea Catalyst
Blanck 1c Blanck 2d HT-Mg/Al HT-Mg/Al-500 HT-Mg/Al-DSe HT-Mg/Al-DBSe HT-Mg/Al + DSf HT-Mg/Al + DBSf
ra (mmol/h gcat )
Conversionb (%)
Selectivity (%) 1,2-Epoxides
1,2-Glycols
Others
4.8 6.8 2.1 2.2 4.8 9.5
24.1 5.2 86.3 92.1 72.0 63.5 86.5 97.0
54.9 53.5 57.5 55.8 61.1 66.1 48.6 35.1
31.7 32.8 30.6 33.8 27.8 26.8 23.1 17.5
13.4 13.7 11.9 10.4 11.1 7.1 28.3 47.3
Reaction conditions: 6.2 mmol limomene; 34.4 mmol benzonitrile; 121.3 mmol H2 O2 ; 80 ml methanol; 65◦ C. Reaction time = 8 h. c Blank test, in the absence of catalyst. d Blank test, in the absence of a nitrile. e DS and DBS intercalated into LDH. f DS and DBS added to the reaction medium. a
b
3.2. Catalytic activity The epoxidation of limonene with hydrogen peroxide at a low temperature is influenced by a number of operational variables including the reactant concentration, nature of the catalyst, solvent and temperature. In terms of selectivity, the reaction must be controlled in order to (a) accomplish the exclusive epoxidation of the endocyclic double bond and (b) avoid the hydrolysis of the epoxide to the corresponding glycol. Catalytic activity experiments were preceded by blank tests intended to ascertain that the reaction developed to no appreciable extent in the absence of the catalyst and that the presence of a nitrile was essential. As can be seen from Table 1, the reaction developed to an appreciable degree in the absence of the catalyst but not in that of the nitrile. Any effect of the hydrogen peroxide concentration on the reaction rate was avoided by using a very high H2 O2 /limonene mole ratio.
Preliminary tests also included identifying the reaction products. As noted earlier, the process must be conducted under optimum operating conditions if the desired selectivity is to be achieved. This entails determining the origin of the products, i.e. whether they are main products or byproducts, stable or unstable. The main product of the epoxidation of limonene is (cis + trans) limonene oxide; however, in the aqueous medium used, it can be hydrolysed to glycol, an unwanted byproduct in our case. If the reaction takes place according to Scheme 1, then the epoxides would be the unstable main products and the final glycols the stable byproducts. The glycols, however, can form in a different manner, viz. by direct hydroxylation of the endocyclic double bond, a reaction that would compete with the single hydrolysis of the epoxide. In order to clarify the situation, we conducted an experiment by using 6.2 mmol of (−)-limonene oxide (Aldrich ref. 21,833-2, cis/trans ratio = 43:57) instead of limonene. The results are shown in Fig. 5 as
Scheme 1.
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Scheme 2.
Fig. 5. Variation of the yield to double epoxide with time in the presence of hydrogen peroxide and HT-Mg/Al.
the variation of the yield to the double epoxide with time. (cis + trans) limonene oxide gave no glycol, not even at very long reaction times. In fact, the sole product obtained was the diepoxide. Thus, under the reaction conditions used, limonene oxide is not hydrolysed to glycols in the presence of hydrotalcite; rather, the glycols are formed by direct hydroxylation of the limonene (Scheme 2). Consequently, the best way of increasing the selectivity towards the epoxides while avoiding the formation of glycols is to produce the former at as high a rate as possible in order to prevent the limonene from being in contact with the aqueous medium and the catalyst for a long time.
The first experiment series was conducted using the Mg/Al-LDH and the product of its calcination at 500◦ C. Table 1 shows the conversion and selectivity results for the process. As can be seen, the results at 8 h of reaction were very similar with both catalysts; the only differences were slightly higher conversion and lower selectivity with the calcined LDH. The mechanism for this epoxidation reaction involves two steps (Scheme 3) [37]. In the first, peroxycarboximidic acid is formed by reaction of the nitrile with hydrogen peroxide; in the second, oxygen is transferred from the acid to the olefin. The first step is base-catalysed. In previous work, Kaneda et al. [16] found the addition of sodium DS or DBS to the reaction medium to dramatically increase the rate of epoxidation of cyclohex-2-en-1-one. No specific selectivity data were reported, however. This led us to conduct the epoxidation of limonene in two different ways, namely, (a) with the surfactant (DS or DBS) directly added to the reaction medium, and (b) with the surfactant intercalated into the LDH layers. As can be seen from Table 1, the reaction was slower in the absence of the catalyst and the nitrile was indispensable for the process to
Scheme 3.
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Table 2 Influence of the nature of the nitrile on the conversion of limonene and the selectivity towards the different reactions productsa Nitrile
Acetonitrile Butyronitrile Benzonitrile 1,2-Dicyanobenzene 1,3-Dicyanobenzene a
t (h)
8 8 8 2 2
Conversion (%)
Selectivity (%) 1,2-Epoxides
1,2-Glycols
Others
30.9 22.2 86.3 98.0 97.5
52.7 55.4 57.5 74.3 72.3
30.8 30.4 30.6 6.6 8.2
16.5 14.2 11.9 19.1 19.5
Reaction conditions: 6.2 mmol limonene; 34.3 mmol nitrile; 121.3 mmol H2 O2 ; 80 ml methanol; 65◦ C.
develop to an acceptable extent. Also, the two LDHs with an intercalated surfactant (DS and DBS) increased the selectivity towards the 1,2-epoxide and decreased that towards the glycols — albeit in a not significant manner. These two trends were more marked with DBS. The foregoing suggests that intercalation of the surfactant precludes the epoxidation of the exocyclic double bond in limonene. On the other hand, the conversion achieved with the two intercalates was low relative to that provided by the original solid (HT-Mg/Al). The presence of the surfactants in these intercalates decreases the hydrophilicity of the interlayer spacing, not only through the presence of organic matter but also because, as shown by IR spectroscopy, the number of hydroxyl groups from water or interlayer OH groups decreases markedly upon intercalation. Under these conditions, hydrating the epoxide formed must be more difficult, as reflected in the selectivity results. Despite the expanded interlayer spacing, the surfactant may hinder access by the reactant and preclude an increase in the reaction rate. When the surfactant is in the reaction medium, solvation or micellization of the reactants and products may act in combination, so any correlations with the processes taking place at the catalyst layers may be spurious. In any case, it should be noted that the conversion remains high: similar to that of HT-Mg/Al with DS and somewhat higher with DBS. However, the amount of unwanted byproducts increases, particularly with DBS. Consequently, neither the intercalation of surfactants into the layered double hydroxide nor their addition to the reaction medium results in substantial improvements in the results (namely, the obtainment of (cis + trans) 1,2-epoxy limonene) provided by uncalcined HT-Mg/Al.
Because blank tests revealed that a nitrile was required for the epoxidation of limonene with HT-Mg/Al to develop to an acceptable extent, various nitriles were tested for this purpose. As can be seen from Table 2, those with acid ␣-protons (namely, acetonitrile and butyronitrile) provided very low conversions. This was the likely result of the carbanions formed from these nitriles being strongly adsorbed at the highly basic hydrotalcite sites, which gave rise to conversions similar to those obtained in the blank tests (in the absence of the catalyst). On the other hand, all other nitriles, which lacked acid ␣-protons, provided high conversions, even at very short times (e.g. 1,2- and 1,3-dicyanobenzenes exhibited conversions above 95% at 2 h). As regards selectivity, acetonitrile, butyronitrile and benzonitrile provided similar levels; the proportion of 1,2-glycols formed was in the region of 30%. However, when the transformation of limonene was very fast (e.g. with 1,2- or 1,3-dicyanobenzene), the amount of glycols obtained was negligible, which further supports the hypothesis that the diasteromeric glycol results from direct transformation of the limonene in a process that competes with the epoxidation of the starting olefin.
4. Conclusions The results obtained in this work reveal that the insertion of organic molecules of the sulphate or sulphonate type by rehydration effectively increases the interlayer spacing in hydrotalcite-like compounds by effect of carbonate ions being replaced with the organic anions (DS and DBS in our case). All the hydrotalcite-like solids tested in this work were
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found to catalyse the epoxidation of limonene, the best conversion and selectivity being provided by the Mg/Al-LDH containing interlayer carbonate. The LDH intercalates containing organic molecules increase the selectivity towards limonene oxide but exhibit decreased conversion and catalytic activity. On the other hand, when the organic anions are added to the reaction medium rather than intercalated, the reaction rate and conversion are significantly increased, at the expense of the selectivity towards limonene oxide. Finally, the process requires the presence of a nitrile if the activity of the catalysts is to be efficiently exploited; in this respect, 1,2- and 1,3-dicyanobenzene were found to provide the highest conversion — they gave better conversions than benzonitrile in half the time.
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Epoxidation of limonene over hydrotalcite-like compounds with hydrogen peroxide in the presence of nitriles Mar´ıa A. Aramend´ıa, Victoriano Borau, César Jiménez∗ , José M. Luque, José M. Marinas, José R. Ruiz, Francisco J. Urbano Departamento de Qu´ımica Orgánica, Facultad de Ciencias, Universidad de Córdoba, Campus de Rabanales, Edificio C-3, Carretera Nacional IV-A, km. 396 E-14014 Córdoba, Spain Received 3 October 2000; received in revised form 5 March 2001; accepted 5 March 2001
Abstract This paper reports the synthesis of an Mg/Al double layered hydroxide (LDH) by coprecipitation, and of its dodecylsulphate (DS) and dodecylbenzenesulphonate (DBS) intercalates by rehydration. The resulting LDHs, also known as hydrotalcite-like compounds, were characterized by using various instrumental techniques including X-ray diffraction (XRD), IR spectroscopy and solid-state NMR spectroscopy. The solids obtained were tested as catalysts in the epoxidation of limonene with hydrogen peroxide. Because the presence of a nitrile was found to be indispensable for the reaction to develop to an acceptable extent, the influence of various nitriles on catalytic activity was examined. The epoxidation reaction yielded limonene oxide. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Epoxidation of limonene; Double layered hydroxide (LDH); Nitrile
1. Introduction Hydrotalcite is a naturally occurring layered double hydroxide present in anionic clay minerals [1]. The structure of an double layered hydroxide (LDH) is similar to that of brucite, Mg(OH)2 ; some Mg2+ ions are substituted by trivalent ions and induce a charge deficiency in the layers that is neutralized by anions in the interlayer region, which also contains crystallization water. LDHs can also have their Mg2+ ions replaced, so their general formula can be expressed as [M(II)1−x M(III)x (OH)2 ]x + (An − )x/n ·mH2 O. According to Miyata [2], calcination of an Mg/Al-CO3 LDH at 773 K produces a mixture of magnesium and aluminium oxides. In the presence of water and appropriate anions, the oxide mixture can ∗
Corresponding author.
be rehydrated back to the LDH. This property, known as the memory effect, provides an effective synthetic pathway for the insertion of organic and inorganic anions into LDHs. The reconstruction of an LDH from a mixed metal oxide precursor is believed to occur via a topotactic reaction. In order to improve the crystallinity of the rehydrated product, Carlino et al. [3] recommend heating the precursor LDH by using a slow ramp (ca. 1◦ C/min). This technique prevents the rapid removal of carbon dioxide and water from the LDH caused by direct, sudden heating, which disrupts the layered structure of the calcined product. In this way, a wide variety of organo-LDHs containing carboxylate [4] and phthalocyaninetetrasulphonate anions [5,6], among others, has been prepared. Lately, organo-LDHs have aroused much interest for use in chemical applications such as heterogeneous catalysis via basic sites [5,7], photochemical reactions
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 5 7 0 - 1
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(e.g. the dimerization of various Mg/Al-LDH intercalates [8]), adsorption (e.g. of environmental organic molecules [9]) and electrochemistry [10,11]. One of the greatest disadvantages of LDHs as regards heterogeneous catalysis is the small size of their interlayer spacing (7–9 Å), which restricts the size of the molecules that can be transformed within. On the other hand, stacking of exceedingly large anions can lead to severe loosening of the layers and the eventual destruction of the LDH structure. One must therefore find an acceptable interlayer spacing not disrupting the brucite-like structure while minimizing internal diffusion in the solid. On the other hand, alkenes were initially epoxidized with percarboxylic acids [11] since the use of metals in the presence of hydrogen peroxide led to slow reactions subject to the competition of other processes. Aqueous H2 O2 is an ideal oxidant, because it is a cheap and safe oxidant, and easy to handle, giving only water as a coproduct. In the last years, much effort is devoted to the alquene epoxidation with H2 O2 and basic catalysts based on LDHs [12–14]. Thus Kaneda et al. [15,16] have reported this epoxidation in the presence of nitriles and amides with good conversion and yield results. Moreover, Keggin-type anions modified LDHs [17] were used in the alkene epoxidation but with similar results to that obtained with the free anion. This paper reports on the intercalation of dodecylsulphate (DS) and dodecylbenzenesulphonate (DBS) anions into an Mg/Al-LDH. The starting LDH and the resulting organo-LDHs were characterized by using various instrumental techniques including X-ray diffraction (XRD), IR spectroscopy and solid-state nuclear magnetic resonance. These LDHs have been used in the reaction of limonene epoxidation with hydrogen peroxide to obtain limonene oxide. Moreover, it has been proved that for the reaction to proceed, a nitrile is needed, so its influence has also been studied.
2. Experimental 2.1. Synthesis of the layered-double hydroxides Two solutions containing 0.03 mol of Mg(NO3 )2 · 6H2 O and 0.01 mol of Al(NO3 )3 ·9H2 O in 25 ml of deionized water, were mixed and the mixture was
added dropwise over 75 ml of an Na2 CO3 solution at pH 10 at 333 K under vigorous stirring. The pH was kept constant by adding appropriate volumes of 1 M NaOH during precipitation. The suspension thus obtained was kept at 353 K for 24 h, after which the solid was filtered and washed with 2 l of de-ionized water. Once synthesized, the LDH was exchanged with carbonate in order to remove nitrate ions from the interlayers. For this purpose, 2.5 g of LDH was dispersed in 25 ml of distilled water and the dispersion supplied with 250 mg of Na2 CO3 and refluxed for 2 h, after which the solid was separated by centrifugation and the water discarded. This operation was repeated and the supernatant analysed for nitrate, which gave a negative test. The resulting solid, named HT-Mg/Al, was dried at 373 K and calcined in a nitrogen atmosphere at 773 K for 8 h to obtain a new solid that was designated HT-Mg/Al-773. For intercalated organo-LDHs, an amount of 1 g of solid calcined at 773 K was suspended in 50 ml of a solution containing 0.80 g of DS or 0.90 g of DBS in bidistilled, decarbonated water in a nitrogen atmosphere at 373 K for 6 h. Then, the solids were separated by centrifugation and stored in a nitrogen atmosphere. The two LDHs thus obtained were designated HT-Mg/Al-DS and HT-Mg/Al-DBS (with DS and DBS, respectively, as interlayer anion). 2.2. Characterization The elemental composition of the samples was determined on a Perkin-Elmer 1000 ICP spectrophotometer under standard conditions. XRD patterns were recorded on a Siemens D-5000 diffractometer using Cu K␣ radiation. Scans were performed over the 2θ range from 5 to 80. Fourier transform infrared (FTIR) spectra for the solids were recorded over the wavenumber range from 400 to 4000 cm−1 on a Bomen MB-100 FTIR spectrophotometer. Samples were prepared by mixing the powdered solids with KBr (the blank) in a 15:85 ratio. 27 Al NMR spectra were recorded at 104.26 MHz on a Bruker ACP-400 spectrometer under an external magnetic field of 9.4 T. All measurements were made at room temperature. The samples, held in zirconia rotors, were spun at the magic angle (54◦ 44 relative to the external magnetic field) at 3.5 kHz. Spectra were recorded at an excitation pulse of π /8 (0.6 ms) and
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259
an accumulation interval of 1 s. Al(H2 O)6 3+ was used as external standard. The number of accumulations was 1000. 13 C CP/MAS NMR spectra were recorded at 100.62 MHz, using a spinning rate of 3.5 kHz; the contact time for the transfer of magnetization between protons and 13 C was 6 ms (1000 scans). Chemical shifts for 13 C were measured with respect to tetramethylsilane. 2.3. Oxidation reactions Oxidation reactions were conducted in a 250 ml flask containing the catalyst (0.450 g), limonene (6.2 mmol), benzonitrile (34.3 mmol) and hydrogen peroxide (121.3 mmol). Methanol (80 ml) was used as solvent. The reaction was conducted at 333 K and monitored by gas chromatography, using a 30 m × 0.25 mm i.d. DB-1 column. Reactions products were identified by mass spectrometry.
3. Results and discussion 3.1. Characterization of catalysts The elemental analysis of solid HT-Mg/Al revealed an Mg/Al ratio of 3:1 and the following chemical formula: Mg0.75 Al0.25 (OH)2 (CO3 )0.125 ·0.7H2 O. The organo-LDHs were found to have the following formulae: Mg0.75 Al0.25 (OH)2 (C12 H25 SO4 )0.25 ·mH2 O and Mg0.75 Al0.25 (OH)2 (C12 H25 C6 H4 SO2 )0.25 ·nH2 O, for HT-Mg/Al-DS and HT-Mg/Al-DBS, respectively. The XRD analysis of solid HT-Mg/Al (Fig. 1A) revealed a high crystallinity. As can be seen from the XRD patterns, the solid has a typical structure with stacked layers similar to those previously found by Reichle et al. [18] in Mg/Al-LDHs, as well as by Weir et al. [19] and by our own group [20]. The indexing of the diffraction peaks was obtained by comparison with the reported diagram for synthetic hydrotalcite [21]. The a parameter of the hexagonal unit cell corresponds to the distance between two metals in adjacent octahedral sites while the c parameter corresponds to three times the distance between adjacent hydroxyl layers. The c-axis parameter was calculated to be 23.41 Å from the position of the (0 0 3) peak. The a-axis parameter was calculated to be 3.06 Å
Fig. 1. XRD patterns for solids HT-Mg/Al (A); HT-Mg/Al-500 (B); HT-Mg/Al-DS (C); and HT-Mg/Al-DBS (D).
from the (1 1 0) peak. Since the c-axis parameter was 23.41 Å, the basal spacing of HT-Mg/Al was 7.80 Å. Fig. 1B shows the XRD patterns for the solid obtained by calcining the LDH at 773 K. As can be seen, heating destroys the layered structure and yields a periclase MgO phase in addition to amorphous aluminium oxide where applicable [2,22]. Fig. 1C and D show the XRD patterns for the LDHs following intercalation of DS and DBS anions, respectively. The (0 0 3) diffraction peaks and the higher-order peaks for the two LDH intercalates are shifted to lower 2θ angles relative to those for HT-Mg/Al. The c-axis parameters for HT-Mg/Al-DS and HT-Mg/Al-DBS were calculated to be 37.17 and 39.96 Å, respectively. Consequently, the basal spacing of these two organo-LDHs increased from 7.80 Å in the starting solid (HT-Mg/Al) to 12.39 and 12.32 Å, respectively. This increased interlayer spacing suggests the effective intercalation of both DS and DBS
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Fig. 2. FTIR spectra for solids HT-Mg/Al (A); HT-Mg/Al-500 (B); HT-Mg/Al-DS (C); and HT-Mg/Al-DBS (D).
anions into the LDH structure. Kanezaki et al. [23] found greater spacings (up to 19 Å) for the intercalation of 9,10-anthraquinone-2,6-disulphonates into Mg/Al-LDHs. Fig. 2A and B show the FTIR spectra for HT-Mg/Al and its product of calcination at 773 K. The spectrum for the former solid (Fig. 3A) exhibits a strong band between 3800 and 2700 cm−1 that encompasses the twisting vibrations of physisorbed water [24], OH− structural vibrations [25], OH · · · OH characteristic valency vibrations, and/or characteristic stretching vibrations for Mg2+ –OH− in hydroxycarbonates [24–27]. The band corresponding to the δ HOH vibration mode appears at 1643 cm−1 . Carbonate ion in a symmetric environment exhibits three absorption bands close to those for the free anion (namely, ν4 = 680 cm−1 , ν2 = 880 cm−1 and ν3 = 1415 cm−1 ). Band ν 3 is barely
Fig. 3. 27 Al MAS NMR spectra for solids HT-Mg/Al (A); HT-Mg/Al-500 (B); HT-Mg/Al-DS (C); and HT-Mg/Al-DBS (D).
distinguishable at 1377 cm−1 , and so is ν 2 (848 cm−1 ). On the other hand, ν 4 is observed at 659 cm−1 . The presence of the shoulder at about 1400 cm−1 can be ascribed to a decreased carbonate symmetry [28] that activates vibrational mode ν 1 ; this mode is inactive in wholly symmetric carbonate and appears as a shoulder
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at 1505–1535 cm−1 in the spectra [29]. On the other hand, the band at 1377 cm−1 might also correspond to ν 3 for nitrate ion; however, this is not the case since nitrate ion was checked to be absent following its exchange with carbonate. Calcination of this LDH at 773 K resulted in a markedly different spectrum (Fig. 2B) where the 450–650 cm−1 region exhibited the bands corresponding to the characteristic vibrations of the oxides (MgO and Al2 O3 ) [30]. The broad band at about 1400 cm−1 does not correspond to interlayer carbonate; in fact, it is typical of O–C–O vibrations in carbonate adsorbed on the surface of the oxides formed during calcination [30,31], together with carbonate and reversible adsorbed water on the oxide surface [32]. Finally, one other, weak band is observed in the 3800–3100 cm−1 region due to OH · · · OH2 valency vibrations [27,28]. The FTIR spectra for the organo-LDHs (Fig. 2C and D) are markedly different from that for HT-Mg/Al (Fig. 2A). Thus, the region corresponding to the interlayer anion, 1200–1500 cm−1 , does not contain the band for carbonate (1377 cm−1 ) in the spectra for the organo-LDHs; such spectra, however, exhibit strong bands at 1233 and 1192 cm−1 due to stretching vibrations in sulphate and sulphonate anion [33], respectively. These bands are accompanied by others corresponding to the organic portion of the intercalated molecules, which appear near 3000 cm−1 . Based on these data and on the previous XRD patterns, the organic anions are effectively intercalated into the LDH structure. For further confirmation, however, the solids were subjected to an NMR study. Fig. 3 shows the results of the 27 Al MAS NMR examination. The spectrum for HT-Mg/Al (Fig. 3A) exhibits a single signal, centred at 9.4 ppm, which was assigned to Al octahedrally coordinated to OH groups in the LDH. This is consistent with previously reported results [34,35] and with the assumption that Mg2+ cations are isomorphically substituted by Al3+ ions in the LDH structure. As noted earlier, calcination of the LDH yields a mixture of Mg and Al oxides. The 27 Al spectrum for HT-Mg/Al-773 (Fig. 3B) exhibits two signals at 9.6 and 74.0 ppm assigned to octahedrally and tetrahedrally coordinated Al, respectively [36]. Fig. 3C and D show the 27 Al MAS NMR spectra for the organo-LDHs; as can be seen, both exhibit a single signal, centred at ca. 9 ppm, which suggests that
261
Fig. 4. 13 C CP/MAS NMR spectra for the organo-LDHs HT-Mg/Al-DS (A) and HT-Mg/Al-DBS (B).
the whole Al in the LDHs is octahedrally coordinated and hence that rehydration to the organo-LDHs under the experimental conditions used was quantitative. Fig. 4 shows the 13 C CP/MAS NMR spectra for the solids. That for HT-Mg/Al-DS (Fig. 4A) exhibits a series of signals between 8 and 25 ppm corresponding to the hydrocarbon skeleton of DS anion, and another signal at 72 ppm ascribable to the methylene group bonded to the sulphate anion. The spectrum for HT-Mg/Al-DBS (Fig. 4B) is similar to the previous one; again, the region from 8 to 25 ppm contains the signals for the saturated carbon atoms in the DBS anion, whereas that between 125 and 140 ppm exhibits the typical signals for aromatic carbons.
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Table 1 Catalytic activity, conversion and selectivity in the epoxidation of limonenea Catalyst
Blanck 1c Blanck 2d HT-Mg/Al HT-Mg/Al-500 HT-Mg/Al-DSe HT-Mg/Al-DBSe HT-Mg/Al + DSf HT-Mg/Al + DBSf
ra (mmol/h gcat )
Conversionb (%)
Selectivity (%) 1,2-Epoxides
1,2-Glycols
Others
4.8 6.8 2.1 2.2 4.8 9.5
24.1 5.2 86.3 92.1 72.0 63.5 86.5 97.0
54.9 53.5 57.5 55.8 61.1 66.1 48.6 35.1
31.7 32.8 30.6 33.8 27.8 26.8 23.1 17.5
13.4 13.7 11.9 10.4 11.1 7.1 28.3 47.3
Reaction conditions: 6.2 mmol limomene; 34.4 mmol benzonitrile; 121.3 mmol H2 O2 ; 80 ml methanol; 65◦ C. Reaction time = 8 h. c Blank test, in the absence of catalyst. d Blank test, in the absence of a nitrile. e DS and DBS intercalated into LDH. f DS and DBS added to the reaction medium. a
b
3.2. Catalytic activity The epoxidation of limonene with hydrogen peroxide at a low temperature is influenced by a number of operational variables including the reactant concentration, nature of the catalyst, solvent and temperature. In terms of selectivity, the reaction must be controlled in order to (a) accomplish the exclusive epoxidation of the endocyclic double bond and (b) avoid the hydrolysis of the epoxide to the corresponding glycol. Catalytic activity experiments were preceded by blank tests intended to ascertain that the reaction developed to no appreciable extent in the absence of the catalyst and that the presence of a nitrile was essential. As can be seen from Table 1, the reaction developed to an appreciable degree in the absence of the catalyst but not in that of the nitrile. Any effect of the hydrogen peroxide concentration on the reaction rate was avoided by using a very high H2 O2 /limonene mole ratio.
Preliminary tests also included identifying the reaction products. As noted earlier, the process must be conducted under optimum operating conditions if the desired selectivity is to be achieved. This entails determining the origin of the products, i.e. whether they are main products or byproducts, stable or unstable. The main product of the epoxidation of limonene is (cis + trans) limonene oxide; however, in the aqueous medium used, it can be hydrolysed to glycol, an unwanted byproduct in our case. If the reaction takes place according to Scheme 1, then the epoxides would be the unstable main products and the final glycols the stable byproducts. The glycols, however, can form in a different manner, viz. by direct hydroxylation of the endocyclic double bond, a reaction that would compete with the single hydrolysis of the epoxide. In order to clarify the situation, we conducted an experiment by using 6.2 mmol of (−)-limonene oxide (Aldrich ref. 21,833-2, cis/trans ratio = 43:57) instead of limonene. The results are shown in Fig. 5 as
Scheme 1.
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Scheme 2.
Fig. 5. Variation of the yield to double epoxide with time in the presence of hydrogen peroxide and HT-Mg/Al.
the variation of the yield to the double epoxide with time. (cis + trans) limonene oxide gave no glycol, not even at very long reaction times. In fact, the sole product obtained was the diepoxide. Thus, under the reaction conditions used, limonene oxide is not hydrolysed to glycols in the presence of hydrotalcite; rather, the glycols are formed by direct hydroxylation of the limonene (Scheme 2). Consequently, the best way of increasing the selectivity towards the epoxides while avoiding the formation of glycols is to produce the former at as high a rate as possible in order to prevent the limonene from being in contact with the aqueous medium and the catalyst for a long time.
The first experiment series was conducted using the Mg/Al-LDH and the product of its calcination at 500◦ C. Table 1 shows the conversion and selectivity results for the process. As can be seen, the results at 8 h of reaction were very similar with both catalysts; the only differences were slightly higher conversion and lower selectivity with the calcined LDH. The mechanism for this epoxidation reaction involves two steps (Scheme 3) [37]. In the first, peroxycarboximidic acid is formed by reaction of the nitrile with hydrogen peroxide; in the second, oxygen is transferred from the acid to the olefin. The first step is base-catalysed. In previous work, Kaneda et al. [16] found the addition of sodium DS or DBS to the reaction medium to dramatically increase the rate of epoxidation of cyclohex-2-en-1-one. No specific selectivity data were reported, however. This led us to conduct the epoxidation of limonene in two different ways, namely, (a) with the surfactant (DS or DBS) directly added to the reaction medium, and (b) with the surfactant intercalated into the LDH layers. As can be seen from Table 1, the reaction was slower in the absence of the catalyst and the nitrile was indispensable for the process to
Scheme 3.
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Table 2 Influence of the nature of the nitrile on the conversion of limonene and the selectivity towards the different reactions productsa Nitrile
Acetonitrile Butyronitrile Benzonitrile 1,2-Dicyanobenzene 1,3-Dicyanobenzene a
t (h)
8 8 8 2 2
Conversion (%)
Selectivity (%) 1,2-Epoxides
1,2-Glycols
Others
30.9 22.2 86.3 98.0 97.5
52.7 55.4 57.5 74.3 72.3
30.8 30.4 30.6 6.6 8.2
16.5 14.2 11.9 19.1 19.5
Reaction conditions: 6.2 mmol limonene; 34.3 mmol nitrile; 121.3 mmol H2 O2 ; 80 ml methanol; 65◦ C.
develop to an acceptable extent. Also, the two LDHs with an intercalated surfactant (DS and DBS) increased the selectivity towards the 1,2-epoxide and decreased that towards the glycols — albeit in a not significant manner. These two trends were more marked with DBS. The foregoing suggests that intercalation of the surfactant precludes the epoxidation of the exocyclic double bond in limonene. On the other hand, the conversion achieved with the two intercalates was low relative to that provided by the original solid (HT-Mg/Al). The presence of the surfactants in these intercalates decreases the hydrophilicity of the interlayer spacing, not only through the presence of organic matter but also because, as shown by IR spectroscopy, the number of hydroxyl groups from water or interlayer OH groups decreases markedly upon intercalation. Under these conditions, hydrating the epoxide formed must be more difficult, as reflected in the selectivity results. Despite the expanded interlayer spacing, the surfactant may hinder access by the reactant and preclude an increase in the reaction rate. When the surfactant is in the reaction medium, solvation or micellization of the reactants and products may act in combination, so any correlations with the processes taking place at the catalyst layers may be spurious. In any case, it should be noted that the conversion remains high: similar to that of HT-Mg/Al with DS and somewhat higher with DBS. However, the amount of unwanted byproducts increases, particularly with DBS. Consequently, neither the intercalation of surfactants into the layered double hydroxide nor their addition to the reaction medium results in substantial improvements in the results (namely, the obtainment of (cis + trans) 1,2-epoxy limonene) provided by uncalcined HT-Mg/Al.
Because blank tests revealed that a nitrile was required for the epoxidation of limonene with HT-Mg/Al to develop to an acceptable extent, various nitriles were tested for this purpose. As can be seen from Table 2, those with acid ␣-protons (namely, acetonitrile and butyronitrile) provided very low conversions. This was the likely result of the carbanions formed from these nitriles being strongly adsorbed at the highly basic hydrotalcite sites, which gave rise to conversions similar to those obtained in the blank tests (in the absence of the catalyst). On the other hand, all other nitriles, which lacked acid ␣-protons, provided high conversions, even at very short times (e.g. 1,2- and 1,3-dicyanobenzenes exhibited conversions above 95% at 2 h). As regards selectivity, acetonitrile, butyronitrile and benzonitrile provided similar levels; the proportion of 1,2-glycols formed was in the region of 30%. However, when the transformation of limonene was very fast (e.g. with 1,2- or 1,3-dicyanobenzene), the amount of glycols obtained was negligible, which further supports the hypothesis that the diasteromeric glycol results from direct transformation of the limonene in a process that competes with the epoxidation of the starting olefin.
4. Conclusions The results obtained in this work reveal that the insertion of organic molecules of the sulphate or sulphonate type by rehydration effectively increases the interlayer spacing in hydrotalcite-like compounds by effect of carbonate ions being replaced with the organic anions (DS and DBS in our case). All the hydrotalcite-like solids tested in this work were
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found to catalyse the epoxidation of limonene, the best conversion and selectivity being provided by the Mg/Al-LDH containing interlayer carbonate. The LDH intercalates containing organic molecules increase the selectivity towards limonene oxide but exhibit decreased conversion and catalytic activity. On the other hand, when the organic anions are added to the reaction medium rather than intercalated, the reaction rate and conversion are significantly increased, at the expense of the selectivity towards limonene oxide. Finally, the process requires the presence of a nitrile if the activity of the catalysts is to be efficiently exploited; in this respect, 1,2- and 1,3-dicyanobenzene were found to provide the highest conversion — they gave better conversions than benzonitrile in half the time.
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