Ind. Eng. Chem. Res. 2009, 48, 3685–3691
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Esterification of Salicylic Acid with Dimethyl Carbonate over Mesoporous Aluminosilicate Xiaowei Su,†,‡ Junping Li,† Fukui Xiao,† Wei Wei,† and Yuhan Sun*,† State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
The synthesis of methyl salicylate (MS) from salicylic acid (SA) and dimethyl carbonate (DMC) was performed over a series of mesoporous aluminosilicates. The results showed that mesoporous aluminosilicate was an effective esterification catalyst for the reaction. It was also found that both SA conversion and MS selectivity were closely linked with the intermediate strong acidity and Lewis acidity of catalysts. As a result, the synergistic mechanism was proposed for the esterification of SA with DMC over a mesoporous aluminosilicate. 1. Introduction Methyl esters of carboxylic acids are widely used in fine chemicals, drugs, plasticizers, food preservatives, pharmaceuticals, solvents, perfumes, cosmetics, and chiral auxilliaries.1,2 Methyl ester may be prepared via treatment of carboxylic acids with methylating reagents such as Me2SO4, LiOH · H2OMe2SO4, P(OMe)5, Me3OBF4-iPr2Net, Me3SOH, Li2CO3-MeI, CsF-2-fluoropyridinium salt-MeOH, o-methylcaprolactim, K2CO3-Ph2S+M3BF4-CuBr(iPr2NET), K2CO3-(18-C-6)-Cl3CO2Me, CsCO3-(18-C-6)-MeI, CsF-MeI, aqueous K2CO3Bu4NBr-MeI and Me4NOH.3-11 These methods have limitations, such as the required use of costly reagents that are hazardous or harsh reaction conditions. Therefore, it is desirable to make the reactions catalytic, rather than using stoichiometric amounts of base. The reported catalytic procedures for the synthesis of methyl esters recommend the use of sulfuric acid, HCl, etc.,12 as catalysts and toxic chemicals such as dimethyl sulfate, methyl iodide, or diazomethane as methylating agents. Thus, it is important to find environmentally benign reagents and catalysts for the synthesis of methyl esters, by studying the feasibility of substituting toxic methylating agents (dimethyl sulfate, methyl iodide, etc.) with a nontoxic chemical (such as dimethyl carbonate (DMC))13-19 and designing a heterogeneous catalytic system that involves the use of safe, active, selective, and reusable catalysts (zeolites, metal oxides, their modified forms, etc.). DMC has emerged as a suitable alternative green methylating agent with lower negative environmental impact, with carbon dioxide (CO2) and alcohol being the reaction byproducts in the methylation.13-15 There are few reports on the use of DMC in the esterification of acids.20-25 Lee and Shimizu reported the reaction of mycophenolic acid with DMC over a cesium carbonate catalyst.20 In this case, both the carboxylic and the hydroxyl groups are methylated. Shieh et al. employed DMC to esterify carboxylic acids using 1,8-diazobicyclo[5,4,0]undec7-ene (DBU) as a catalyst, adopting the microwave irradiation technique.21 Selva and Tundo explored the reaction of DMC with several carboxylic acids, such as o- and p-mercaptobenzoic acids, o- and p-hydroxybenzoic acid, and mandelic and phe* To whom correspondence should be addressed. E-mail address:
[email protected]. † State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
nyllactic acids in the presence of NaY faujasite.22 They also reported a comparative study with K2CO3 as a catalyst. Kirumakki et al. reported the esterification of several aromatic carboxylic acids over zeolites Hβ, HZSM5, and HY, using DMC as the methylating agent.23,24 Souza and Nagaraju studied the esterification of salicylic acid with DMC over solid acids such as zirconia, alumina and silica and their sulfate-, phosphate-, and borate-modified forms.25 In addition, they found that the superacid sulfated zirconia is the most suitable catalyst for this reaction. Given these data from the available literature, it is interesting to study the chemoselectivity of DMC in the methylation of salicyclic acid (SA) over a solid acid. Methyl salicylate (MS), which is one of the important esters of SA, is a useful chemical intermediate that is used commercially as a flavor and fragrance agent and as a dye carrier, as well as an ultraviolet (UV)-light stabilizer in acrylic resins.26 MS is produced synthetically for commercial purposes via the catalytic esterification of SA with methanol. This process involves the use of sulfuric acid (H2SO4), HCl, AlCl3, etc., which is undesirable and may be replaced with solid acids that are environmentally benign and can be recycled. There are few reports on the use of DMC to esterify SA to MS and the catalysts that were used in this process, such as NaY, K2CO3, Hβ, HZSM5, HY, zirconia, alumina, silica, and anionmodified metal oxides.22-25 We have focused our research on the use of solid acids in the esterification reactions using DMC as a methylating agent, and we determined that the acid property of catalysts had an important role in the reaction.27-29 Through the research of Brønsted acidic catalysts in this reaction, it was determined that the intermediate to strong acid sites were responsible for the catalytic activity of catalysts.27,28 However, the difference between the effect of Brønsted acid sites and the effect of Lewis acid sites on the catalytic activity of catalysts still is not clear. Therefore, in the present work, mesoporous silica and aluminosilicates were synthesized for the esterification of SA with DMC. Moreover, the influence of acid sites of the catalyst on the reaction was investigated in detail for the mechanism of DMC-involved esterfication. 2. Experimental Section 2.1. Chemical Reagents. Tetraethoxysilane (TEOS, 99%), cetyl trimethylammonium bromide (CTAB, 99%), aluminum nitrate (Al(NO3)3, 99%), ammonia hydrate (NH4OH, 28 wt %), salicylic acid (SA, 99%), and dimethyl carbonate (DMC, 99%)
10.1021/ie801148v CCC: $40.75 2009 American Chemical Society Published on Web 03/03/2009
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were obtained from commercial sources, at analytical grade, and were used without any further purification. 2.2. Synthesis of Catalysts. Mesoporous silica and aluminosilicates with Si/Al ratios of 6, 8, 10, and 20 were synthesized hydrothermally in the following procedure. TEOS and Al(NO3)3 were used as the sources for silicon and aluminum, respectively, and CTAB was used as a structure-directing template. The mesoporous materials were crystallized from a gel with the following composition: TEOS-xAl(NO3)3-0.25CTAB-4NH4OH-200H2O (where x varied with the molar ratio of Si/Al). In a typical synthesis, after CTAB was dissolved in an appropriate amount of deionized water, the pH of the system was adjusted to pH 9-10 by adding a determined amount of NH4OH dropwise under vigorous stirring. The system then was stirred for 30 min. TEOS and Al(NO3)3 were added slowly under vigorous stirring at room temperature until homogenized. After continuous stirring at room temperature for at least 4 h, the resultant gel was then transferred to an autoclave and statically heated at 140 °C for 72 h. The sample was obtained after the precipitates were filtered, washed thoroughly with deionized water, dried, and calcined at 650 °C in air for 6 h. The sample terminology was Si-Al-x, where x represents the molar Si/Al ratio of the samples. 2.3. Characterization. Elemental analysis for the Si:Al molar ratios was performed with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on an Atom Scan 16 instrument. The specific surface area, total pore volume, and average pore diameter were measured via a N2 adsorption-desorption method, using a Model ASAP-2000 instrument (Micromeritics, Norcross, GA). The samples were activated and degassed at 80 °C and 10-6 mmHg overnight and then the adsorption-desorption was conducted by passing nitrogen into the sample that was kept under liquid nitrogen. The surface area was calculated from the linear part of the Brunauer-Emmett-Teller (BET) plot, according to IUPAC recommendations. The pore-size distribution (PSD) was calculated from the N2 adsorption branch, using the conventional Barrett-Joyner-Halenda (BJH) model. The total sample acidities were determined via the temperature-programmed desorption (TPD) of ammonia. A sample of 0.1 g (40-60 mesh) was introduced into the stainless sample tube and pretreated in an argon flow for 2 h at 500 °C. The sample was then cooled to 50 °C and several ammonia pulses were flushed through the sample tube. The saturation of the sample with ammonia was proven by the appearance of a constant peak area on the chart. After saturation, weakly adsorbed NH3 was eliminated by treatment under dry argon at the same temperature, and then the temperature increased to 500 °C with a linear heating rate of 10 °C/min under dry argon. The amount of NH3 evolved from the sample was determined using a mass spectrometer (Omistar GSD 301O3). The flow rate of argon was maintained at 50 mL/min. The acidity of Lewis acid and Brønsted acid of the materials was analyzed by pyridine adsorption using Fourier treansform infrared spectroscopy (FTIR) on an FTIR spectrometer (Nicolet NEXUS 470 (Avatar 360)). The catalyst sample was finely ground and pressed into the form of a self-supporting wafer (10 mg/cm2, diameter of 15 mm), and then it was placed into the FTIR cell with CaF2 windows. The wafers were calcined under vacuum at 500 °C for 4 h, followed by exposure to pyridine vapor. The wafers were allowed to adsorb pyridine for 1 h. Infrared (IR) spectra were recorded after subsequent evacuation at 100 °C.
Figure 1. N2 adsorption/desorption isotherms of samples with different Si/ Al ratios. Table 1. Physicochemical Properties of the Catalysts Si/Al Ratio sample
gel
solid
BET surface area (m2/g)
pore volume (cm3/g)
pore diameter (nm)
Si-Al-6 Si-Al-8 Si-Al-10 Si-Al-20 SiO2
6 8 10 20 ∞
6.20 8.34 10.61 21.11 ∞
247.40 307.14 341.01 361.04 393.22
0.37 0.39 0.41 0.43 0.45
6.54 4.87 4.72 4.61 4.52
2.4. Spectroscopic Techniques. For FTIR experiments, the sample was finely ground and pressed into the form of a selfsupporting wafer (10 mg/cm2, diameter of 15 mm), and then it was placed into the FTIR cell with CaF2 windows. The wafers were calcined under vacuum at 500 °C for 4 h, followed by exposure to DMC vapor. Spectra were recorded on an FTIR spectrometer (Nicolet NEXUS 470 (Avatar 360)) at an appointed temperature. 2.5. Reaction Apparatus and Operation. The esterification of salicylic acid (SA) with dimethyl carbonate (DMC) to methyl salicylate (MS) was conducted in a stainless steel autoclave (150 mL) that was subjected to stirring. The effects of reaction time and catalysts with different amounts of sulfonic acid group were investigated. The samples were analyzed by a gas chromatography instrument that was equipped with a 30-m HP-5 capillary column and a flame ionization detection (FID) detector. 3. Results and Discussion 3.1. Characterization of Catalysts. Figure 1 gives the N2 adsorption-desorption isotherms of the samples. All sorption isotherms showed type IV characteristics of mesostructured materials, according to the IUPAC classification, and a step increase of N2 volume in the P/P0 range of 0.2-0.4, which was typical of capillary condensation within uniform mesopores.30 The textural characteristics in Table 1 clearly indicated that the introduction of aluminum led to a decrease in both the BET surface area and pore volume but an increase in pore diameter, which suggested a partial collapse of the mesostructure. NH3-TPD of the samples illustrated that the amount of adsorbed NH3 increased after the introduction of aluminum (see Figure 2). Furthermore, upon the incorporation of aluminum, a desorption peak was observed and centered at 350 °C, corresponding to the acid sites with medium strength. In addition, no obvious peak shift was observed for the desorption of NH3, although the samples had different
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3687 Table 2. Acidic Properties of the Catalysts Acidity (mmol/m2) sample
weak acid (A)a
medium acid (B)a
Brønsted acid
Lewis acid
total acid
Si-Al-6 Si-Al-8 Si-Al-10 Si-Al-20 SiO2
5.94E-05 3.01E-05 2.84E-05 2.23E-05 1.78E-05
1.56E-04 9.98E-05 8.50E-05 2.09E-05 0
1.04E-04 4.66E-05 3.50E-05 1.79E-05 1.78E-05
1.11E-04 8.34E-05 7.80E-05 2.53E-05 0
2.15E-04 1.30E-04 1.13E-04 4.32E-05 1.78E-05
a
°C.
Desorption temperature regions: A, 200-225 °C; B, 300-400
Table 3. Performance of the Catalysts for the Reactiona Selectivity (%)
Figure 2. Ammonia-temperature-programmed desorption (NH3-TPD) curves of the samples. Si-Al-6 (curve (a)), Si-Al-8 (curve (b)), Si-Al10 (curve (c)), Si-Al-20 (curve (d)), and SiO2 (curve (e)).
Figure 3. Fourier transform infrared spectroscopy (FTIR) spectrum of catalysts containing adsorbed pyridine: Si-Al-6 (spectrum (a)), Si-Al-8 (spectrum (b)), Si-Al-10 (spectrum (c)), Si-Al-20 (spectrum (d)), and SiO2 (spectrum (e)).
aluminum contents. The ratio of Brønsted acidity to Lewis acidity was estimated by FTIR spectroscopy of pyridine adsorption (see Figure 3). Parry reported that the bands at ∼1445 cm-1 characterized hydrogen-bonded pyridine molecules, and the bands at ∼1455 cm-1 and ∼1545 cm-1 monitored the coordination of pyridine molecules to Lewis and Brønsted acid sites.31 According to the relation between the acidity and the area under the peaks in Figures 2 and 3, the acidities of the different acid sites on the catalysts were estimated (see Table 2). It was found that the introduction of Al species greatly improved both the Brønsted and Lewis acidity of samples, because of the charge balance when Al sources were introduced into the SiO2 system. 3.2. Catalytic Performance and the Plausible Reaction Mechanism. Table 3 shows the catalytic performance of SiO2 and a mesoporous aluminosilicate in the synthesis of methyl salicylate (MS) from dimethyl carbonate (DMC) with salicylic acid (SA). The major product was MS and the other product was phenol. Compared to SiO2, the aluminosilicate had high both SA conversion and MS selectivity, and, interestingly, the introduction of alumina improved the
sample
conversion of SA (%)
MS
phenol
Si-Al-6 Si-Al-8 Si-Al-10 Si-Al-20 SiO2
98.6 96.5 92.8 83.4 42.2
77.0 76.7 72.7 70.8 40.0
23.0 23.3 27.3 29.2 60.0
a Reaction conditions: n(SA):n(DMC) ) (catalyst) ) 0.65 g, t ) 8 h, and T ) 200 °C.
1:6
(mol/mol),
w
conversion of SA and the selectivity toward MS. Obviously, this was closely related to the acid properties of catalysts (i.e., the differences in their activity could be attributed to their acid site strength and distribution). As shown in Figure 4, the increase in the conversion of SA and the selectivity of MS clearly was consistent with the increase in the acidity of the intermediate acid sites and Lewis acid sites. Beutel32 envisaged a mechanism for the alkylation of phenol with DMC, where DMC was activated on a Lewis acid site by its carbonyl oxygen and phenol on an adjacent Lewis base site via hydrogen bonding. Bonino et al.33 also observed, via the FTIR technique, that DMC was activated on the solid catalyst and followed a similar reaction process. Jyothi et al.34 had also proposed a similar mechanism for the alkylation of catechol by DMC over hydrotalcites. These had been proposed for the alkylation reactions of DMC involving both acid and basic sites on the catalyst. Kirumakki et al.23 proposed a mechanism for this reaction over zeolites, where DMC was activated on a Brønsted acid site by its carbonyl oxygen and the nucleophilic attack from SA was concerned. However, the effect of the Lewis acid sites was not mentioned. To identify the form of DMC that was adsorbed on the aluminosilicate, a comparative investigation of DMC that had been adsorbed on SBA-15-SO3H, the Si-Al-6 sample, and Al2O3 was performed on the IR spectrum (see Figure 5). Figure 5 suggests that the form of DMC that was adsorbed on the aluminosilicate was more similar to the form on SBA15-SO3H than that on Al2O3. Because SBA-15-SO3H is a Brønsted acidic catalyst and Al2O3 belongs to Lewis acidic catalysts, it can be said that DMC is apt to be adsorbed on Brønsted sites of the aluminosilicate through a hydrogen bond between the carbonyl of DMC and the acidic OH of the aluminosilicate.35 To further comprehend how DMC was activated on the aluminosilicate, DMC that had been adsorbed on the Si-Al-6 sample through stepwise heating was investigated using FTIR spectroscopy (see Figure 6). In Figure 6A, it was observed that, with increases in temperature, the intensity of band at 3495 cm-1 decreased while the strength of band at 3745 cm-1 increased. From the literature, it can be concluded that the strength of the hydrogen bond between the carbonyl of the DMC and the acidic OH of the
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Figure 4. Change in the SA conversion, and MS and phenol selectivity, relative to (A) the medium acidity and (B) the Lewis acidity of the catalysts.
Figure 5. FT-IR spectra of DMC adsorbed on SBA-15-SO3H (spectrum (a)), the Si-Al-6 sample (spectrum (b)), and Al2O3 (spectrum (c)) at 150 °C.
aluminosilicate was reduced and the number of unrestricted OH groups grew.32,35 Bands at 3037 cm-1 and 2968 cm-1, which were ascribed to the antisymmetric and symmetric stretching vibrations of methyl groups, decreased as the temperature increased.36 Simultaneously, some new bands emerged, such as those at 2998 and 2831 cm-1, which suggested the formation of some new species. Figure 6B clearly indicates that there was no other novelty, except a decrease in the intensity of bands in the carbonyl stretching vibration region and methyl group deformation vibration region. The band at 1306 cm-1, which represented the antisymmetric stretching vibration of the CO2 entity in DMC, diminished continuously as the sample was further heated and vanished completely at 350 °C. These experimental observations were consistent with the results of the thermal decomposition of DMC on solid acids that has been reported in the literature.37 From the investigation of DMC that was adsorbed on the aluminosilicate, it can be concluded that the acidic OH group had an important role in the process of activation of DMC on the catalyst and these intermediate strong acid sites also
Figure 6. FTIR spectra of DMC adsorbed on the Si-Al-6 sample ((A) 2800-3800 cm-1 and (B) 1280-1800 cm-1) at various temperatures: 20 °C (spectrum (a)), 50 °C (spectrum (b)), 150 °C (spectrum (c)), 250 °C (spectrum (d)), and 350 °C (spectrum (e)).
Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3689 Scheme 1. Plausible Mechanism for the Esterification of Salicylic Acid with DMC over a Mesoporous Aluminosilicate
dominated the conversion of SA in the reaction. Lewis acid sites might activate SA via the bond between the Al3+ and the carbonyl of SA and then affect the selectivity for MS. Based on our observation, we proposed a mechanism of the esterification of SA, using DMC over mesoporous aluminosilicate (see Scheme 1). First, DMC was absorbed on the Brønsted acid site, which was medium in strength, and was activated through its carbonyl oxygen. Similarly, SA was absorbed on the Lewis acid site with its carbonyl oxygen. Then, through a nucleophilic attack from SA and a subsequent loss of methanol, the formation of an unstable cationic intermediate occurred. As a result, MS formed after a series of steps, including the loss of CO2 from the intermediate. Such a mechanism indicated a close relationship between the catalytic activity and the Lewis acid sites with the medium strength. 3.3. Effect of Reaction Conditions. Table 3 clearly indicated that a mesoporous aluminosilicate was an effective catalyst for the esterification of SA with DMC. The Si-Al-6 sample was chosen as a typical catalyst to investigate the effect of reaction conditions on this reaction. As shown in Figure 7, the reaction was executed with different amounts of catalyst. As the amount of catalyst increased from 0.35 g to 0.8 g, the selectivity to MS increased from 60% to 77.5% and the selectivity for phenol decreased from 40% to 22.5%, while the conversion of SA was >93% in all of the cases. This can be attributed to the number of Brønsted acid sites and Lewis acids sites increasing as the catalyst amount increased. Thus, it suggested that the suitable amount of catalyst was >0.65 g. At the same amount of catalyst, the conversion of SA increased rapidly from 65% to 99%, with the molar ratio of
Figure 7. Effect of the catalyst weight over the Si-Al-6 sample. (Reaction conditions: SA:DMC molar ratio, 1:6; reaction time, 8 h; and reaction temperature, 200 °C.)
SA to DMC changing from 1:2 to 1:8, but the selectivities to MS and phenol were only slightly influenced by the SA: DMC molar ratio (see Figure 8). DMC not only can react with SA but is a solvent that can dissolve SA and accelerate the reaction. Thus, the SA:DMC molar ratio was optimal at 1:6. Figure 9 showed the performance at different temperatures. There was a sharply increase in the conversion of SA from 51% to 99% when the reaction temperature increased from 160 °C to 220 °C. However, the selectivity to MS decreased from 87.8% to 67.1% and the selectivity for phenol increased from 12.2% to 32.9% with the increase in reaction temper-
Figure 8. Effect of molar ratio of the reactants over the Si-Al-6 sample. (Reaction conditions: catalyst weight, 0.65 g; reaction time, 8 h; and reaction temperature, 200 °C.)
Figure 9. Effect of the reaction temperature over the Si-Al-6 sample. (Reaction conditions: SA/DMC molar ratio, 1:6; catalyst weight, 0.65 g; and reaction time, 8 h.)
3690 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009
Figure 10. Effect of the reaction time over the Si-Al-6 sample. (Reaction conditions: SA/DMC molar ratio, 1:6; catalyst weight, 0.65 g; and reaction temperature, 200 °C.)
ature. This could be due to the easy removal of the carboxylic group of SA and then the formation of phenol with increasing reaction temperature. As a result, the optimal reaction temperature should be controlled at ∼200 °C. Furthermore, when the reaction time increased from 4 h to 8 h, the conversion of SA increased from 64.6% to 99% but the selectivity to MS and phenol changed only slightly (see Figure 10). It can be concluded that the selectivity to MS and phenol is independent of reaction time and reaction might reach equilibrium after 8 h. Thus, the optimal reaction time was 8 h. 4. Conclusion Dimethyl carbonate (DMC) was an effective esterifying agent, and mesoporous aluminosilicate was an efficient catalyst for the reaction. Typically, the conversion of salicylic acid (SA) could reach 99% and the selectivity to methyl salicylate (MS) could reach 77% under the optimal conditions. It was also determined that the Lewis acid sites with the medium strength had an important role in the reaction. Thus, a mechanism that involves DMC being activated on acid sites via its carbonyl oxygen and nuleophilic attack by SA on the carboxyl atom was proposed. Acknowledgment The authors acknowledge the financial support from State Key Program for Development and Research of China (No. 2006BAC02A08) and Program of Knowledge Innovation of Institute of Coal Chemistry, Chinese Academy of Sciences. Literature Cited (1) Larock, R. C. ComprehensiVe Organic Transformations; VCH Publishers: New York, 1989; Chapter 9. (2) Cotton, S. Educ. Chem. 1997, 34, 62. (3) Furniss B. R.; Hannaford, A. J.; Rogers V.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Text Book of Practical Organic Chemistry; Longmann: London, 1996. (4) Ho, G. J.; Mathre, D. J. Lithium-Initiated Imide Formation. A Simple Method for N-Acylation of 2-Oxazolidinones and Bornane-2,10-Sultam. J. Org. Chem. 1995, 60, 2271–2273. (5) Denney, D. B.; Melis, R.; Pendse, A. D. Methylation of acids with pentamethoxyphosphorane. J. Org. Chem. 1978, 43, 4672–4673. (6) Yamauchi, K.; Tanabe, T.; Kinoshita, M. Trimethylsulfonium hydroxide: A new methylating agent. J. Org. Chem. 1979, 44, 638–639.
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ReceiVed for reView July 25, 2008 ReVised manuscript receiVed December 30, 2008 Accepted February 21, 2009 IE801148V
