**1.1 Methanol**

Today, methanol is mainly produced from synthesis gas. It is a very versatile basic chemical. Processes to further process methanol include the following ones:

• methanol to gasoline (MtG)


The most used strain to obtain methanol from methane by biochemical conversion is *Methylosinus trichosporium. M. trichosporium* was found to produce up to 4101 mg methanol/L/day [1]. Methanol is of interest, for example, for fuel cells and combustion, apart from being a basic chemical for synthesis.

In Ref. [2] immobilized *Methylocystis bryophila* were used to produce methanol by repeated batch fermentation from a simulated biogas mixture.

#### **1.2 Biopolymers**

Biopolymers are defined as either being biobased and/or biodegradable according to certain standards. They are used as thermoplastics, elastomers, and thermosets, to replace conventional polymers. Methanotrophs have been described to produce, for instance, PHB [1].

#### *1.2.1 Polyhydroxyalkanoates (PHA) including PHB*

The common energy storage compound in microorganisms is glycogen. When a shortage of essential nutrients occurs, particularly nitrogen or phosphorus, several microorganisms are able to store their energic carbon in a different compound, which is polyhydroxybutyrate (PHB). That PHB, which is accumulated intracellularly in granules up to 50% dry cell weight and above, can be extracted and used as a thermoplastic material. PHB is a biopolymer that is both biobased and biodegradable. The biodegradability can occur aerobically or anaerobically, in different environments. This property, together with the property set that is comparable to the commodity polyolefin PP (polypropylene), makes PHB an interesting bioplastics material. PHB belongs to the class of PHA (polyhydroxyalkanoates), which are polyesters and as such are naturally occurring compounds [3]. In plants, only PHB can be found [4], whereas microorganisms can produce a wide variety of PHA copolymers depending on available comonomers.

PHB formation was seen with type II methanotrophs. For instance, *Methylocystis parvus OBBP* was found to accumulate 60% of PHB on nitrogen limitation (ammonium), as opposed to only 36% accumulation with nitrate limitation. The strain *Methylosinus trichosporium IMV3011* could accumulate 47% PHB with both nitrogen sources, nitrate and ammonium. *Methylocystis hirsute* could accumulate up to 51% PHB on ammonium (again after N-limitation). A total of 51% of PHB accumulation were found with a mixed culture dominated by *Methylocystis GB25* on ammonium [1], see **Table 1**.

The yield that was obtained varied between 0.22 and 0.67 g of PHB per gram of methane.

EPS production can limit the yield. There were no extracellular products (EPS) under methane-limited growth conditions [5].

In **Table 2**, several bioreactor configurations to produce PHB were compared.

While PHB generally resembles PP in its property set, the material has a low elongation at break and due to its high crystallinity is brittle. Adding a few percent of valeric acid as a comonomer to PHB, yielding PHBV, makes the material softer and thereby more versatile. PHBV can be synthesized by methanotrophs, too.


*Value-Added Products from Natural Gas Using Fermentation Processes: Products… DOI: http://dx.doi.org/10.5772/intechopen.104643*

#### **Table 1.**

*Yield of PHB found for selected strains of methanotrophs (type II).*


#### **Table 2.**

*How different fermenters fare in PHB accumulation studies.*

Another important PHA copolymer is PHBH, with a certain content of hexanoic acid being incorporated in the polymer.

Biopolymers can be an environmentally benign material class. For a life cycle assessment (LCA) for biopolymers made from biogas, see Ref. [6].

In Ref. [7], the yields were found to be 1.13 g of PHB per gram of methane for *Methylosinus trichosporium OB3b* and 0.88 g of PHB per gram of methane for *Methylocystis parvus OBBP* [7], which is significantly above the figures reported in **Table 2**, see **Table 3**.

Methanotrophic strains producing PHB are listed in **Table 4**.

Yield figures have a broad spread. PHB will form copolymers when comonomers, such as valeric acid, are supplied in the medium.


#### **Table 3.**

*Production of PHB from methane.*


**Table 4.** *PHB production by methanotrophs.* *Value-Added Products from Natural Gas Using Fermentation Processes: Products… DOI: http://dx.doi.org/10.5772/intechopen.104643*

## *1.2.2 PHBV and other PHA*

As stated above, polyhydroxyalkanoates (PHAs) are (bio)polyesters of hydroxy acids. They can be naturally synthesized by bacteria with the purpose of hoarding carbon under N- and P-limitation [9]. PHA consists of 3-, 4-, 5-, and 6 hydroxycarboxylic acids [10]. Lactic acid, citric acid, glycolic acid, malic acid, mandelic acid, and tartaric acid, by contrast, are alpha hydroxy acids (1-hydroxycarboxylic acids). Thereby, their polymers are no PHA. An example of a 2-hydroxycarboxylic acid (beta hydroxy acids, where the acid and hydroxy functional groups are separated by two carbon atoms) is salicylic acid. For hydroxybutyric acid, there are three isomers—alpha, beta, and gamma. When we talk about PHB, we typically mean poly(3-hydroxybutyric acid). Valeric acid (petanoic acid) has four isomers. It is found in some foods.

Polyhydroxyalkanoates (PHA) are established biopolymers. Some well-known brands and producers are given below in alphabetic order:


The organization Go!PHA [12] is promoting PHA.

To synthesize the copolymer PHBV, valeric acid is supplied as a comonomer to the fermentation broth.

It was found that above valerate concentrations of 0.7% (by volume), the PHBV accumulation in *Methylocystis sp. WRRC1* was inhibited [1]. While the standard content of PHB, without any valerate present, was found to be 30%, it reached 15% only with high valeric acid concentrations. At 0.34% (by volume) of valerate, the PHBV content in the cells reached 60%, with 50% comonomer content [1]. *Methylocystis parvus OBBP* could be used to make different PHA from various supplied comonomers. 3-hydroxy-butyrate (3HB), butyrate, valerate, hexanoate, and octanoate were added and nitrogen limitation was applied [13]. The products included P(3HB-co-4HB), P(3HB-co-5HV-co-3HV), and P(3HB-co-6HHx-co-4HB). **Table 5** shows PHBV production by methanotrophs.

**Figure 1** shows how various PHA can be obtained by methanotrophs through suitable comonomer addition.

In **Table 6**, details on PHA yields from *M. parvus OBBP* with different comonomers are given.

**Table 7** lists the physical properties of the obtained PHA.

As **Table 7** shows, the elongation at break is strongly increased by the comonomers, improving the biopolymer properties compared to heat PHB. Applications of PHA are described in Ref. [14].

#### *1.2.3 Other biopolymers*

Several microorganisms, also methanotrophs, produce EPS (extracellular polymeric substances, exopolysaccharides) [15, 16], which might be used potentially for


#### **Table 5.**

*PHBV accumulation from various substrates.*

#### **Figure 1.**

*Production of different PHA by comonomer choice. (a) Shows co-substrates without the hydroxy group and (b) With the hydroxyl group. Source: [13].*

biopolymer applications. A higher oxygen concentration increases the excretion of EPS by methanotrophs [8]. *Methylobacter luteus, Methylomonas rubra, Methylococcus thermophilus*, and *Methylobacter ucrainicus* were found to produce EPS from methane in the range of 0.5–0.8 g/L. In general, it is extreme conditions that promote EPS production. *M. alcaliphilum* 20Z, under moderately saline conditions, gave 1.8 g EPS/g of biomass when grown in a bubble column bioreactor for the treatment of dilute methane emissions [8].

A well-known biopolymer is PLA (polylactic acid). Its monomer, lactic acid, has been obtained from methanotrophs, too, see **Table 8**.


*Value-Added Products from Natural Gas Using Fermentation Processes: Products… DOI: http://dx.doi.org/10.5772/intechopen.104643*

#### **Table 6.**

*Synthesis of different PHA through a variation of fatty acid co-substrates with the strain* M. parvus *OBBP.*


*Mn = number average molecular weight; Mw = weight average molecular weight; Tm = melting temperature; ΔHm = apparent heat of fusion; Tg = glass transition temperature; E = Young's modulus; σ<sup>t</sup> = tensile strength; ε<sup>b</sup> = elongation at break; Source: [13].*

#### **Table 7.**

*Properties of the PHA materials derived by methanotrophic fermentation.*

PLA is biobased and degradable (however, it requires 70°C for disintegration, so "home compost" standards are not met by PLA-based bioplastics products; The material is only "industrially compostable," see EN 13432 standard). PLA is suitable for food packaging (FDA rating as gras = generally recognized as safe). Today, PLA is made from sugar-derived LA, which requires agricultural starch or sugar production with the associated food/feed competition over land, fertilizer and water consumption, etc. Significant efforts are undertaken to make the enzymatic hydrolysis of lignocellulosic biomass economically viable, however, no breakthrough has been achieved in that field yet (compare 2nd generation biofuel production attempts). Methane-derived PLA can offer a lower environmental footprint and be produced with less price volatility than agriculture-based material. Of PLA, several


#### *Natural Gas - New Perspectives and Future Developments*

**Table8.**

 *Organic acids from methanotrophs.* copolymers and blends exist, e.g. with glycolic acid (PLA + GLA, PLGA), as well as compounds. For LA production by methanotrophs, see [17, 18].
