Trimethyl Borate Regenerated from Spent Sodium Borohydride after

Sep 24, 2010 - New Energy Technology DiVision, Energy and EnVironment Research Laboratories, Industrial Technology. Research Institute (ITRI), Hsinchu...
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Trimethyl Borate Regenerated from Spent Sodium Borohydride after Hydrogen Production Cheng-Hong Liu,† Bing-Hung Chen,*,† Duu-Jong Lee,‡ Jie-Ren Ku,§ and Fanghei Tsau§ Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan, 70101 Taiwan, Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei 10607, Taiwan, and New Energy Technology DiVision, Energy and EnVironment Research Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu 31040, Taiwan

Sodium metaborate (NaBO2) is the hydrolysate of sodium borohydride (NaBH4) for hydrogen production. Trimethyl borate (B(OCH3)3), the major reactant for producing sodium borohydride via the Brown-Schlesinger process, is successfully regenerated from sodium metaborate (NaBO2) via a sequential process including reacting with sulfuric acid, cooling crystallization, and reactive esterification distillation. The metaborate is first converted to boric acid (H3BO3) by reacting with sulfuric acid, bypassing formation of borax (Na2B4O7 · 10H2O) required in a conventional process. Cooling crystallization is utilized to separate and purify boric acid from coexisting sodium sulfate (Na2SO4). Subsequently, trimethyl borate is formed via esterification of boric acid with methanol, in which reactive esterification distillation is adopted to facilitate the esterification and purify the product. Formation of boric acid and trimethyl borate is confirmed with X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FT-IR), and gas chromatography (GC). Approximately 55% of sodium metaborate could be converted to boric acid, along with a production yield from 74.1% to 96.5% realized for trimethyl borate esterified from the as-produced boric acid. catalyst

1. Introduction

NaBH4 + 2H2O 98 NaBO2 + 4H2 v

With a rapid advancement of technology and increasing populations worldwide in recent decades, the development of clean alternative energy is now of utmost importance and significance. Hydrogen, undoubtedly, is a promising potential candidate among various renewable energies as water generated is the main product associated with an extraction of energy through the reaction with oxygen or air in devices like proton exchange membrane fuel cells (PEMFC).1 A few hydrogen storage methods have been extensively researched, such as high-pressure compressed gas tanks, liquefied hydrogen, metallic hydrides, and chemical hydrides.2,3 Liquefaction and high-pressure compressed gas are not preferred because extra energy has to be consumed in the liquefaction and pressurization process. The hydrogen content stored in chemical hydrides can generally exceed that stocked in metal hydrides.3,4 Moreover, hydrogen generated from chemical hydrides, usually via hydrolysis reaction, is ultrapure and could be directly applied to PEMFCs. As a result, chemical hydrides are regarded as a more promising remedy for hydrogen supply and storage.3,4 Owing to its high hydrogen storage capacity, viz. about 10.8 wt %, as well as its great stability in alkaline solution and nonflammability, sodium borohydride (NaBH4) is one of the most studied chemical hydrides for hydrogen storage.3 Controlled generation of hydrogen proceeds through a hydrolysis reaction in the presence of specific catalysts,5–11 shown in eq 1, leaving sodium metaborate as spent product in need of further recycling treatment. * To whom correspondence should be addressed. Tel: +886-6-2757575, Ext. 62695. Fax: +886-6-234-4496. E-mail: bhchen@ alumni.rice.edu. † National Cheng Kung University. ‡ National Taiwan University of Science and Technology. § Industrial Technology Research Institute.

∆H ) -217 kJ · mol-1 (1) In order to make sodium borohydride be widely used as an energy storage medium and to attain sustainable use in hydrogen economy, a total recycling process of spent-NaBH4, viz. sodium metaborate (NaBO2), back to NaBH4 is desired.12 The modified Bayer process was proposed by employing the less expensive reducing metal, e.g., magnesium (Mg), instead of sodium, to fabricate borohydride from dehydrated metaborate.13,14 Additionally, sodium borohydride can also be produced by heating a mixture of Mg and dehydrated sodium metaborate under a very high H2 pressure (i.e., 3.1 MPa) and an elevated temperature at 550 °C through the following reaction15 550 °C

NaBO2 + 2Mg + 2H2 98 NaBH4 + 2MgO

(2)

3.1 MPa

However, a high reaction temperature at 700 °C, much above the decomposition temperature of NaBH4 near 400-500 °C, resulting a high risk in explosion of the reacting system, as well as disposal of silicate waste in large quantity have to be taken care in the Bayer process.13,14 The high-energy ball-milling process was adopted to recycle NaBO2 back to NaBH4 at room temperature with MgH2 as a hydrogen source.16 However, the amount of spent borohydride that could be regenerated in each batch is still limited owing to the mechanical limits of the reacting devices, such as the small volume of the reacting vessel available as well as reactants having to be mixed under a very large centrifugal force field. Hence, a chemical process that can be easily scaled up has to be called forth in regeneration of spent borohydrides in large quantity. Currently, NaBH4 can be commercially produced through a reaction with sodium hydride (NaH) and trimethyl borate

10.1021/ie101309f  2010 American Chemical Society Published on Web 09/24/2010

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(B(OCH3)3), called the Brown-Schlesinger process (eq 3), that has been implemented since 195017 220 - 250°C

B(OCH3)3 + 4NaH 98 NaBH4 + 3NaOCH3 oil bath

(3) Hence, the recycling process of sodium metaborate could be re-emphasized on regeneration schemes from NaBO2 to B(OCH3)3. The reactant B(OCH3)3 in eq 3 is commonly obtained from sequential reactions: (1) first, conversion of mineral borax (Na2B4O7 · 10H2O) and boric oxide (B2O3) by aeration with CO2 or reacted with a mineral acid, such as hydrochloric acid and sulfuric acid, to give boric acid (H3BO3)18 and (2) followed by an esterification reaction with methanol as shown in eq 4.17 Notably, the esterification reaction is reversible; hence, removal of H2O favors production of B(OCH3)3.19 H2SO4

H3BO3 + 3CH3OH a B(OCH3)3 + 3H2O

(4)

Azeotropic distillation on mixture of as-produced B(OCH3)3 and methanol was proposed to drive the esterification reaction of boric acid to near completion.17 However, in practice, the presence of other substances in the reactors, such as sulfuric acid and possibly, dimethyl sulfate, could destroy the azeotrope. Therefore, esterification of boric acid and methanol by a batch reactive distillation is employed to facilitate production and purification of trimethyl borate.20 Hence, the objective of this work is to provide a cost-effective, two-step process to recycle and reuse NaBO2 to yield B(OCH3)3: (1) conversion of NaBO2 with sulfuric acid directly to H3BO3 and (2) esterificationdistillation of as-produced H3BO3 with excess methanol to produce B(OCH3)3 (eq 4). 2. Experimental Section 2.1. Materials. Trimethyl borate (B(OCH3)3, Fluka, > 99%), sodium metaborate (NaBO2 · 4H2O, Alfa Aesar), sulfuric acid (H2SO4, Riedel-de Hae¨n, 95-97%), and anhydrous methanol (CH3OH, Mallinckrodt, 99.9%) were used as received. 2.2. Synthesis of Boric Acid. A 58.54 g (i.e., 0.425 mol) amount of NaBO2 · 4H2O was dissolved in 100 mL of deionized water at 90 °C. Afterward, the metaborate solution was cooled to 70 °C, followed by gentle addition of 24 g (i.e., 0.244 mol) of sulfuric acid under constant stirring for 3 h. After complete reaction of metaborate with sulfuric acid, the bulk solution was rapidly cooled and crystallized at 5 °C. White needle-like crystals of boric acid were observed to precipitate out from the solution. Finally, the precipitate was filtrated and dried in a vacuum oven at 60 °C to get rid of any residual water. 2.3. Fabrication of Trimethyl Borate. Fabrication of trimethyl borate was conducted with a reactive esterification distillation process (Figure S in Supporting Information). Methanol in excess was utilized in order to efficiently consume the boron compounds. Weighted as-synthesized H3BO3 and anhydrous CH3OH in excess were mixed in a round-bottom flask with vigorous agitation in the presence of a few drops of sulfuric acid as catalyst. The reacting system was maintained at a specific temperature in an oil bath. Although the boiling points of CH3OH and B(OCH3) are 64.7 and 68 °C, respectively, they can form a positive azeotrope at a lower temperature near 54.6 °C. The azeotrope contains 48.7 mol % of CH3OH and 51.3 mol % of B(OCH3)3.17 Hence, the esterification reaction and distillation were conducted at various stages: 55 ( 0.5, 64.5 (

Figure 1. XRD analysis of H3BO3 obtained from a reacting system having 58.54 g of NaBO2 · 4H2O (0.42 mol) and 100 mL of H2SO4, followed by cooling crystallization. For comparison, the XRD analyses of H3BO3, NaBO2 · 4H2O, and Na2SO4 of reagent grade are shown, respectively.

0.5, 68.5 ( 0.5, and 71 ( 0.5 °C. The vapor from the reacting system was then conducted through a condenser, where water at the inlet was maintained at 5 °C. After no more distillate was produced, the reactive esterification distillation process was advanced to the next stage at a higher temperature. It is noted that the experimental uncertainty in the production yield of trimethyl borate is estimated at about 10%. 2.4. Characterization of Boric Acid and Trimethyl Borate. Identification of as-fabricated H3BO3 was performed with X-ray diffraction (XRD) analysis (Rigaku RX III) over a range of diffraction angles (θ) from 2θ ) 10° to 80° with Cu KR radiation (40 kV, 20 mA) filtered by a monochromator. On the other hand, Fourier-transform infrared spectroscopy (FTIR) (Varian FTS-2000) was used to determine the existence of B(OCH3)3. A gas chromatograph equipped with a flame ionization detector (GC-FID, Shimadzu GC-2400) and a DB-5 separation column with nitrogen at 30 mL min-1 as the carrier gas was applied to quantify B(OCH3)3 generated at different stages of the reactive esterification distillation. 3. Results and Discussion To identify white needle-like crystals generated from cooling crystallization in NaBO2 solution reacted with sulfuric acid, these crystals were analyzed with the XRD (Figure 1). As a result, boric acid was successfully fabricated according to the XRD analysis. Two characteristic diffraction peaks at 2θ ) 14.97° and 28.02° coincident with those of standard H3BO3 as well as those found in the database for boric acid were observed in Figure 1. The production yield of boric acid converted from sodium metaborate was estimated at about 55%. In addition to boric acid, one would expect other compounds, such as Na2SO4 and unreacted NaBO2, conjointly present in the bulk solution (eq 5). However, none of them could be detected in the products

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obtained from cooling crystallization of the metaborate solution reacted with sulfuric acid by XRD. Hence, cooling crystallization could be truly used to separate the as-produced boric acid from other compounds in the bulk solution. 70 °C

2NaBO2 · 4H2O + H2SO4 98 2H3BO3 + Na2SO4 + 6H2O 3h

(5) The mechanism in the separation of boric acid from coexisting Na2SO4 and unreacted metaborate was mainly based on the significant difference in aqueous solubilities of these substances.21 For example, the aqueous solubilities of H3BO3, Na2SO4, and NaBO2 at 5 °C are 1.77, 6, and 15.7 g per 100 g of H2O, respectively.22 Notably, in the subsequent esterification reaction of boric acid with methanol to produce trimethyl borate, the as-obtained boric acid had to be first dissolved in excess methanol. In addition, it is known that sodium sulfate is almost insoluble in methanol.23 Hence, if sodium sulfate was really crystallized out conjointly with boric acid, one would expect some insoluble solid should have remained in methanol after total dissolution of as-produced boric acid in methanol. Indeed, not any minute precipitate was observed in the prepared boric acid-methanol solution. Consequently, it is fair to conclude that the cooling crystallization process is able to give boric acid in high purity. In addition, FT-IR and GC analyses were utilized to identify product collected from the batch esterification reaction of assynthesized H3BO3 and CH3OH in excess (Figures 2 and 3). Figure 2a(3) shows the IR spectrum of standard B(OCH3)3, in which the stretching C-H bond appears at 2850-3000 cm-1, stretching C-O bond at 1000-1300 cm-1, bending CH3 at 1375 cm-1, and B-O bond at 780, 1200, as well as 1460 cm. Figure 2a(1) exhibits the IR spectrum of standard H3BO3, namely, stretching O-H bond happening at 3200-3400 cm-1 and B-O bond at 780, 1200, as well as 1460 cm-1. Likewise, the IR spectrum of CH3OH shows that the stretching O-H bond is mainly found at 3200-3400 cm-1, stretching C-H bond at 2850-3000 cm-1, bending CH3 at 1450 cm-1, and stretching C-O bond at 1000-1300 cm-1. Together with the IR spectra of boric acid, methanol, and trimethyl borate, seemingly, the characteristic peak at 1375 cm-1 resulting from the bending CH3 bond is present only in trimethyl borate, not found on the other two reactants, boric acid and methanol. Hence, the IR peak at 1375 cm-1 could be selected as a main tool to identify the existence of trimethyl borate. Figure 2b displays IR analysis of the product generated in the batch esterification reaction of boric acid and methanol at 50 °C for 3 h. Notably, 0.12 mol of as-synthesized H3BO3 and 0.48 mol of CH3OH, i.e., a molar ratio of CH3OH/H3BO3 ) 4, were initially introduced into the batch reactor. Obviously, an IR absorption peak at 1375 cm-1 was observed in the product, implying that a successful preparation of B(OCH3)3 from boric acid and methanol was accomplished in this work. Moreover, because the esterification reaction was carried out in a batch reactor, certain unreacted reactants would be detected simultaneously by IR analysis. Accordingly, a broad peak showed up at around 3400 cm-1, which resulted from the stretching O-H bond owing to the presence of methanol in excess. Figure 3a shows the gas chromatogram of trimethyl borate, indicating its retention time at around 6.6 min. However, a feeble peak consistent with that of CH3OH could be observed as well at 5.0 min. The presence of CH3OH mainly resulted from hydrolysis of trimethyl borate, namely, the reverse reaction shown in eq 4. Indeed, in this work, it was found that the

Figure 2. IR spectra of (a) (1) standard H3BO3, (2) standard CH3OH, (3) standard B(OCH3)3 and (b) the product obtained from esterification of 0.12 mol of as-synthesized H3BO3 with 0.48 mol of CH3OH in excess at 50 °C for 3 h.

presence of water even in trace amount, like moisture in the air, could still facilitate the hydrolysis reaction of B(OCH3)3. As a result, H3BO3 and CH3OH were produced and present conjointly with trimethyl borate. Nonetheless, in our GC-FID analysis, the presence of boric acid would not yield any observable peak on the chromatogram, as boric acid could not be further oxidized and ionized by the flame. While the product obtained from esterification of 0.12 mol of H3BO3 and 0.48 mol of CH3OH at 50 °C for 3 h was quickly injected into the GC-FID, two peaks showed up in the chromatogram (Figure 3b). One characteristic peak at 6.3 min was attributable to B(OCH3)3 and the other at 4.9 min arising from CH3OH. Though the gas chromatogram (Figure 3b) confirmed the presence of trimethyl borate in the product, the production yield of trimethyl borate was still low. Evidently from GC-FID and FT-IR analyses, trimethyl borate was successfully fabricated through the esterification reaction of assynthesized boric acid and methanol present in excess. To overcome the low production yield, continuous removal of water from the product side in the esterification reaction (eq 4) seemed to be critical. Consequently, an attempt in azeotropic and reactive distillation was attempted. As aforementioned, methanol and trimethyl borate could form a positive azeotrope at 54.6 °C with an azeotropic composition of CH3OH/B(OCH3)3 ) 48.7/

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Figure 4. Variation in production of B(OCH3)3 during different stages in reactive esterification distillation. The reacting systems contained 0.23 mol of H3BO3 and methanol with an initial molar feed ratio of CH3OH/H3BO3 ranging from 6 to 12. Table 1. Production Yield of Trimethyl Borate and Measured Composition of Distillates Collected from Reactive Esterification Distillation of Boric Acid and Methanol with an Initial Molar Feed Ratio of CH3OH/H3BO3 from 6 to 12a molar ratio of CH3OH/H3BO3 in feed

Figure 3. GC-FID analyses of (a) standard B(OCH3)3 and (b) the product obtained from esterification of 0.12 mol of as-synthesized H3BO3 with 0.48 mol of CH3OH at 50 °C for 3 h.

51.3 by mole.17 The reactor temperature was, therefore, raised to the azeotropic temperature, instead of 50 °C chosen prior. The distillation temperature was first maintained at 55 ( 0.5 °C for 21 h for a reacting system having a molar ratio of CH3OH/H3BO3 from 6 to 12 in initial feed. Surprisingly, no azeotrope was observed at around 54.6 °C. It was surmised that (1) too little B(OCH3)3 was generated to form discernible azeotrope and (2) the presence of a third substance, boric acid, could break formation of the azeotrope. Subsequently, the reactor temperature was gradually increased. As the temperature was raised up to 64.5 ( 0.5 °C (denoted as Stage I), in the vicinity of methanol’s boiling point at 64.7 °C, the reacting system started to boil vigorously. However, the distillate examined with GC analysis was composed of methanol and trimethyl borate, that is, the reactive esterification distillation process was successfully carried out to produce B(OCH3)3 at 64.5 ( 0.5 °C. When the solution stopped boiling and no more distillate could be obtained, the temperature of the reacting system was further increased to 68.5 ( 0.5 °C (denoted as Stage II), at which time the mixture boiled again to yield even more distillate. Noticeably, B(OCH3)3 has a boiling point at 68 °C. Likewise, after boiling ended in the reacting system, the temperature of the reactor was once again increased. Similarly, dynamic boiling was observed again when the reacting system was further heated up to 71 ( 0.5 °C (denoted as Stage III). Figure 4a shows the variation in production of B(OCH3)3 from esterification of methanol and boric acid from Stage I to Stage

CH3OH (g) in feed H3BO3 (g) in feed CH3OH (g) in distillate B(OCH3)3 (g) in distillate production yield of B(OCH3)3 (%) distillation duration (h)

6

8

10

12

44.46 14.31 7.73 17.81 74.09 14

59.78 14.43 20.43 23.40 96.54 16

74.62 14.41 21.80 22.03 91.00 18

88.23 14.20 38.05 22.93 96.11 31

a As-synthesized H3BO3 of 0.23 mol was initially contained in each reacting system.

III. The effect of different molar ratios of CH3OH/H3BO3 in the initial feed on production of trimethyl borate is presented in Figure 4 and Table 1. With an initial feed ratio of CH3OH/ H3BO3 at 6 by mole, the overall distillate collected consisted of 7.73 g (0.24 mol) of CH3OH and 17.81 g (0.17 mol) of B(OCH3)3, that is, mole fractions of CH3OH and B(OCH3)3 in the distillates were 0.58 and 0.42, respectively, slightly different from the azeotrope.17 As aforementioned, no sign of azeotrope formation was observed in the esterification reacting system of trimethyl borate and methanol at the azeotropic temperature. Not until the temperature raised to around 65 °C, i.e., the first boiling point of the reacting system (Stage I), did the distillate started to appear. At this stage, trimethyl borate was produced in the reacting systems with an initial feed ratio of CH3OH/H3BO3 from 8 to 12 by mole (Figure 4). Furthermore, the amount of produced B(OCH3)3 reached the maximum when the temperature increased to about 69 °C (Stage II), in which the boiling in the reacting system could be ascribed to trimethyl borate that has a boiling point at 68-69 °C. Generally, most trimethyl borate was garnered at Stage II (68.5 ( 0.5 °C) in all reacting systems studied in this work. Less B(OCH3)3 could be produced at a higher reaction/distillation temperature near 71 °C (Stage III). Figure 5 depicts the total production yield of B(OCH3)3 collected from esterification of boric acid and methanol during

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Figure 5. Effect of initial molar feed ratio of CH3OH/H3BO3 at 6, 8, 10, and 12 on production yield of B(OCH3)3 from a reactive esterification distillation. The reacting systems contained H3BO3 initially fixed at 0.23 mol.

the three stages. In general, over 90% of the B(OCH3)3 production yield was achieved from the reactive esterification distillation of boric acid and methanol, except that in the system having an initial feed ratio of CH3OH/H3BO3 at 6 by mole. Instead, a lower production yield in B(OCH3)3 near 74% was attained. According to the reaction stoichiometry indicated in eq 4, a molar feed ratio of CH3OH/H3BO3 ) 3 must be utilized in order to get complete esterification of boric acid. Nonetheless, from this work, it is obviously desirable to use methanol in excess to obtain a higher production yield of trimethyl borate. Meanwhile, sulfuric acid, a common catalyst for the esterification reaction, was also present in the reacting system. If too much methanol was added into the reactor, an undesirable side reaction of esterification might take place to give rise to dimethyl sulfate, instead of trimethyl borate, as shown in eq 6.17 2CH3OH + H2SO4 a (CH3)2SO4 + 2H2O

(6)

According to the GC-MS analysis on the product garnered from our preliminary experiments, fragments with a mass-to-charge ratio (m/z) of 127.1 and 95.1, coincident with those of dimethyl sulfate, could be found. These preliminary experiments were performed at 50 °C for 18 h. In these experiments, the initial feed ratio of CH3OH/H3BO3 was set at 4, 6, and 8 by mole. No distillation on the obtained products was employed to get rid of boron compounds other than trimethyl borate. Therefore, the effect of the initial feed ratio of CH3OH/H3BO3 on the production of trimethyl borate through reactive esterification distillation was also studied. It was found that an initial feed ratio of methanol-to-boric acid of 8 by mole could give the highest production yield near 96.5%, a value slightly better than that reported by Schlesinger et al. (viz. 92.6%).17 Furthermore, with more methanol initially present in the esterification system, the total time required for complete distillation from Stage I to Stage III increased accordingly, from ca. 16 for an initial feed ratio of methanol-to-boric acid of 8 to about 31 h for that of 12 (Table 1). In regard to distillation operation and production yield of trimethyl borate, an initial feed ratio of CH3OH/H3BO3 of 8 by mole could lead to an optimum in the reactive esterification distillation of boric acid and methanol. Taking into account the production yield of 55% in boric acid from sodium metaborate (the first step), a total regeneration

efficiency as high as 53% could be achieved from sodium metaborate to trimethyl borate. Subsequently, with the BrownSchlesinger process, the obtained trimethyl borate could be further regenerated back to sodium borohydride, which could store and release H2 on demand for a device like a protonexchange-membrane fuel cell to emit energy. After all, regeneration from spent sodium borohydride, i.e., sodium metaborate, to trimethyl borate apparently has a bottleneck, namely, in the production of boric acid from metaborate. Hence, an improvement made on the crystallization process to obtain boric acid of high purity is certainly in need. In this work, two key steps in fulfilling a total recycling of spent-NaBH4 after hydrogen evolution back to NaBH4 were conceptually developed. The first step of this work has demonstrated successful preparation of boric acid in high purity directly from spent borohydride, viz. metaborate. In contrast, borax and boric oxides were conventionally used as starting materials to produce boric acid. A reactive esterification distillation process was employed to generate trimethyl borate with good yields from as-prepared boric acid and methanol in excess. To realize the hydrogen economy using borohydrides as H2-supply on demand for devices like PEMFCs and H2 internal combustion engines, not only appropriate hydrogen storage systems based on borohydrides should be developed but also these spent borohydrides should be properly handled and recycled to reduce the materials cost of borohydride. As such, sustainability in energy utilization could be possibly attained. 4. Conclusions Trimethyl borate (B(OCH3)3), the major raw material for making a potential hydrogen storage material, sodium borohydride (NaBH4), via the Brown-Schlesinger process, was successfully synthesized from spent sodium borohydride after hydrogen production, i.e., sodium metaborate (NaBO2), through a sequential process, including reacting with sulfuric acid to produce boric acid (H3BO3), cooling crystallization to obtain boric acid of higher purity, and reactive esterification distillation on as-prepared boric acid and methanol (CH3OH) in excess. Syntheses of H3BO3 and B(OCH3)3 were characterized and identified with XRD, FT-IR, and GC-FID. In this work, boric acid was successfully fabricated from NaBO2 directly, in contrast to the conventional process, in which borax (Na2B4O7 · 10H2O) and boric oxide (B2O3) are used as starting materials. Subsequently, a cooling crystallization process at 5 °C was utilized to obtain boric acid of high purity. On average, a production yield of 55% could be accomplished in the regeneration of boric acid from metaborate. The as-obtained boric acid was harvested for subsequent reactive esterification distillation with excess methanol to fabricate trimethyl borate. Distillation of the esterification reacting system was found to begin at around 65 °C (Stage I), not at 55 °C, which was expected from formation of a positive azeotrope between trimethyl borate and methanol. On further heating the reacting system, the distillation process resumed at 69 (Stage II) and 71 °C (Stage III). Trimethyl borate was found in the distillates in all three stages of the reactive distillation process. Consequently, a high production yield of 96.5% was achieved from the reactive esterification distillation on the system containing boric acid and methanol with an initial feed ratio of CH3OH/H3BO3 ) 8 by mole. Acknowledgment This work was financially supported by the Industrial Technology Research Institute (ITRI) and the National Science

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ReceiVed for reView June 18, 2010 ReVised manuscript receiVed August 20, 2010 Accepted September 11, 2010 IE101309F