Elsevier

Bioresource Technology

Volume 102, Issue 15, August 2011, Pages 7349-7353
Bioresource Technology

High-rate conversion of methane to methanol by Methylosinus trichosporium OB3b

https://doi.org/10.1016/j.biortech.2011.04.096Get rights and content

Abstract

Methanol was produced from methane with a high conversion rate using a high cell density process with Methylosinus trichosporium OB3b in the presence of a high concentration of phosphate buffer. More than 1.1 g/L methanol accumulated in the reaction media under optimized reaction conditions (17 g dry cell/L, 400 mmol/L phosphate, and 10 mmol/L MgCl2) in the presence of 20 mmol/L sodium formate. The conversion rate of methane was over 60%. About 0.95 g/L methanol was produced when the biotransformation was carried out in a membrane aerated reactor into which methane and oxygen were introduced via two separate dense silicone tubing. Our results provide an efficient method and a promising process for high-rate conversion of methane to methanol.

Highlights

► High-rate conversion of methane to methanol by Methylosinus trichosporium OB3b. ► Reactions are conducted at higher cell density in the presence of high concentrations of phosphate. ► More than 1.1 g/L methanol is produced in anaerobic bottles. ► A dense silicone tube membrane aerated bioreactor is applied to highly efficient and safe methane transformation to methanol.

Introduction

Methanol is a basic raw chemical material and an intermediate for production of dimethyl ether and biodiesel. Currently, methanol is made from synthesis gas (CO + H2), obtained mainly from natural gas and coal. This process is characterized by high energy consumption, low conversion rates and high capital costs. In contrast, biotransformation of methane to methanol would require less energy input, be more selective and productive. This process is carried out by whole cells of methanotrophs under mild conditions with a 100% atom economy (Anatas and Warner, 2000, Trost, 1995). The methanol accumulates in the medium provided that suitable methanol dehydrogenase (MDH) inhibitors such as phosphate (Mehta et al., 1987), cyclopropanol (Sugimori et al., 1995), or a high concentration of NaCl (Lee et al., 2004) or CO2 (Xin et al., 2004) are added to the reaction system. In addition, sodium formate is generally added to supply NADH for methane oxidation as well as to prevent further oxidation of methanol (Perdeep et al., 1991). Copper at a concentration of 1.0 μmol/L is also beneficial to cell growth and methane transformation (Markowska and Michalkiewicz, 2009). Takeguchi et al. (1997) determined that Methylosinus trichosporium OB3b at a concentration of 34.6 g dry cell/L, a phosphate and cyclopropanol concentration of 12.9 and 67.0 mmol/L, respectively, produced a maximum amount of 152 mmol methanol/g of dry cell (corresponding to 5.3 mmol/L methanol).

The main challenge faced in high yielding biocatalyzed processes for methanol production is further oxidation by MDH. Mehta et al. (1987) reported that phosphate-dependent inhibition of further oxidation is uncompetitive and reversible. Therefore, to achieve high methanol yields, increases in cell density should be accompanied by increasing concentrations of phosphate buffer to prevent further oxidation of methanol. To our knowledge, studies to test this have not been reported. This study aimed to achieve high yield methanol production using M. trichosporium OB3b whole cells. To this end, a high phosphate concentration was used to inhibit MDH at high cell densities. Parameters including the phosphate buffer, MgCl2, and sodium formate concentrations were optimized. Furthermore, a membrane aerated stirred tank reactor was used to evaluate methanol accumulation under conditions of continuous aeration with methane and oxygen using two separate dense silicone tubes.

Section snippets

Chemicals

Methane (99.9%) was purchased from Beijing Haike Yuanchang Gas Ltd. Oxygen (99.999%) and compressed air was purchased from the Beijing Qianxi Jingcheng gas sale center. All other chemicals were of analytical grade and purchased from commercial sources. Dense silicon tubing (Φ3.4 × 0.2 mm) was purchased from Guangzhou Sanqingda Synthetic Materials Co. Ltd.

Strain and cell cultivation

M. trichosporium OB3b was provided by Prof. Ichiro Okura from the Tokyo Institute of Technology. Nitrate minimal salt was modified to the

Effects of cell and phosphate concentrations on methanol accumulation

Methanol accumulation by M. trichosporium Ob3b whole cells at different cell and phosphate concentrations is shown in Fig. 2. It shows that when the cell concentration was 2.6 g dry cell/L, more than 0.10 g/L of methanol was accumulated after 10 h reaction time at three different phosphate concentrations. However, methanol accumulation decreased with increasing reaction time in these cases (Fig. 2b). Similar phenomena were observed when the cell concentration was 3.9 g dry cell/L. In addition,

Conclusions

This study indicates the significant potential of microorganisms to produce high concentrations of methanol at a high methane conversion rate in anaerobic bottles and bubble free membrane aerated stirring tank reactors. To obtain high methanol yields, it was found that reactions should be conducted at higher cell density in the presence of high concentrations of MDH inhibitor. The maximum methanol accumulation achieved was over 1.12 g/L; 4.5-fold higher than the highest value (0.25 g/L) reported

Acknowledgements

The authors greatly appreciate Prof. Ichiro Okura from the Tokyo Institute of Technology for donating M. trichosporium OB3b. This work was financial supported by the National Natural Science Foundation of China (Nos. 20606036, 20976186), and the Promotive Fund for President Award of the Chinese Academy of Sciences.

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