Beyond fractionation in the utilization of microalgal components

C H A P T E R

9 Beyond fractionation in the utilization of microalgal components Michele Aresta1 and Angela Dibenedetto2,3 1

IC2R srl, Lab H124, Tecnopolis, Valenzano, Bari, Italy 2CIRCC, Bari, Italy 3Department of Chemistry, University of Bari, Bari, Italy

9.1 Introduction Microalgae (or algae in general) can be considered source of a wide range of products, including biofuels and added-value chemicals [1 4], and, thus, have attracted much attention as a faster route than land biomass to fix large volumes of CO2 (1 kg of algal biomass would use c.1.8 kg of CO2). Microalgae are considered the third-generation biomass for energy and chemicals production and show important advantages over land crops: (1) they have a higher solar light photoconversion efficiency (6% 8% with respect to 1.8% 2.2%) and more rapid growth; (2) they do not require arable land, do not need herbicides or pesticides application for their cultivation; and (3) they grow in brackish, saline, or wastewater, that means a reduction of freshwater use [5,6]. For their typical cellular composition, microalgal cells accumulate high content of interesting products, such as carbohydrates, proteins, inorganic compounds, amines, lipids, and even fine chemicals. Lipids are considered the most valuable components of microalgal biomass for the production of biodiesel. As a matter of fact, producing microalgae just for extracting biodiesel is not an economic practice, due to the cost associated to microalgae cultivation, harvesting, and processing [3,7], and the low cost of fossil fuels. It is uncertain whether an increase of the price of fossil fuels may make economically viable diesel produced from microalgae. Even with the oil price at over 140 US$/b (2012), microalgae-sourced biodiesel was not competitive with fossil diesel. In order to increase the competitiveness of microalgal biomass, it is necessary to utilize all their components. Conversion of biomass into sustainable energy fuels and added-value chemical products with zero waste means nothing else than the implementation of the concept of biorefinery [8 10]. For applying the biorefinery concept

Bioenergy with Carbon Capture and Storage DOI: https://doi.org/10.1016/B978-0-12-816229-3.00009-0

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9. Beyond fractionation in the utilization of microalgal components

the algal biomass must be fractionated into the three main classes of compounds, namely lipids, proteins, and carbohydrates, plus minority compounds such as antioxidants, valuable chemicals, and enzymes [11,12]. The residual biomass after any process would be used for the production of biofuels by fermentation: bioethanol, biogas, and biohydrogen. In this chapter the use of carbohydrates and lipids as source of fine chemicals and monomers for polymers will be discussed. Most data are derived from our research project aimed to assess the economic value of several microalgae considering their composition and the use of their various components, such as saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), diunsaturated fatty acids (DUFAs), tri-unsaturated fatty acids, polyunsaturated fatty acids (PUFAs) (including omega-3), proteins for animal feed and human food, and cellulosic material for the production of polyols and alcohols. Biotechnology can be eventually used for the conversion of residual biomass into biohydrogen or biogas. Several microalgal strains were grown from lab scale (1 L) to open pond (several cubic meters). Biomass was fully analyzed for its ash, protein, carbohydrates, and lipids content. The FAs profile of microalgae was determined. The ensemble of data represents one of the rare studies in the literature with a complete characterization of the algae and fits our life cycle assessment (LCA) study that compares land and aquatic biomass [13]. In this chapter, we compare our own data on Tetradesmus obliquus and Phaeodactylum tricornutum grown in indoor/outdoor photobioreactors (PBRs) and in open pond (this is the first study on such strains cultivated in local Southern Italy climatic conditions) [14], with figures relevant to commercial Nannochloropsis sp. and Chlorella (literature data). We show how the algal composition varies with the growing conditions (availability of N and P, source of CO2) and how the economic value of the strains is affected by their composition.

9.2 Microalgae strains Microalgae listed in Table 9.1 were cultivated in different conditions at a laboratory scale. Selected strains were cultivated in PBRs under natural light and in local ambient conditions or even in a pond (3000 L) [15] even using wastewater and synthetic or natural seawater (Table 9.2).

TABLE 9.1 Seawater and freshwater microalgae strains tested at laboratory scale. Microalga

Habitat

Microalga

Habitat

Dunaliella salina

Seawater

Botryococcus braunii

Freshwater

Dunaliella tertiolecta

Chlorella sorokiniana

Phaeodactylum tricornutum

Chlorococcum sp. Haematococcus pluvialis Scenedesmus quadricauda Tetradesmus obliquus

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TABLE 9.2 Selected algae and growing conditions. Microalgae

Medium

Production technology

BG11 BG11

100 L outdoor PBR LT 250 L indoor PBR HT

BG11, N starvation BG11

3000 L open pond

BG11, N starvation Wastewater Bristol medium

Phaeodactylum tricornutum

50 L closed sleeve PBR

Jaworski formulation

500 L outdoor tubular PBR

BG11, N starvation

80 L indoor flat plate PBR

Flory Basic Fertilizer 1

20 transparent polyethylene bags

BG11, N starvation

80 L outdoor flat plate PBR

Urban wastewater treatment plant

530 L high rate alga pond

1/2 SWES

100 L outdoor PBR LT

1/2 SWES

250 L indoor PBR HT

1/2 SWES, N starvation 1/2 SWES

3000 L open pond

1/2 SWES, N starvation Wastewater Artificial seawater

Outdoor 51 L PBR

Artificial seawater

Circular pond

Filter-sterilized seawater

Horizontal tubular solar receiver 220 L Externally located PBR bubble column 57 L

Nannochloropsis NANNO 3600

Commercial product from Reed Mariculture

Nannochloropsis sp.

Natural seawater enriched with f/2 nutrients

60 m2 shallow raceway pond

Nannochloropsis sp.

Standard medium

Outdoor raceways

Nannochloropsis sp. F&M-M24

F, N starvation

Outdoors in a 590-L green wall panel (low-cost PBR)

Chlorella F&M-M49

BG11

Green wall panel (low-cost PBR) outdoors in 0.25 m2

F, N starvation Chlorella IAM C-212

BG11 F, N starvation

Chlorella PROD1

BG11 F, N starvation

Chlorella CH2

F F, N starvation

Chlorella

Raw dairy wastewater

HT, High technology; PBRs, photobioreactors.

Outdoor pilot-scale PBRs 40 L

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The following parameters were investigated in detail: 1. Biomass productivity, 2. Ability to grow in habitats other than freshwater (wastewater) in the presence of CO2 or carbonates, 3. Strain robustness and ability to survive for a long time in outdoor culture, 4. Production cost, 5. Ability to produce high lipid content, and 6. Lipid fatty acids profile. T. obliquus and P. tricornutum did show at the laboratory scale (5 L) a good economic potential because of their biomass productivity, lipid content, and cheaper growth conditions. Such strains were selected to be cultivated in indoor PBRs, high technology, in outdoor pilot ponds (3000 L), and PBR low cost/technology in a greenhouse in Southern Italy [14]. Several growth media, namely, standard medium, N starvation, zootechnical or biogas-plant wastewater, were tested for finding the most economical way.

9.2.1 Methodologies for the characterization of biomass Algal biomass selected for a detailed study (Table 9.3) has been fully characterized for its main components by using the literature techniques. The procedures are omitted as they can be found in the relevant literature. Ash content was determined according to the modified CEN TS/14775 methodology [16]. The total ash is expressed as a percentage of dry weight (DW). TABLE 9.3 Microalgae strains retained or discarded and some of their properties. Microalgal strain

Selected strain properties

Rejected strain properties

Botryococcus braunii

Low growth rate, sensible to contamination

Chlorococcum sp.

High ash content: 31%

Dunaliella salina

Under N starvation was obtained a low lipid content of 7.4% (DW)

Dunaliella tertiolecta

Under N starvation was obtained a lipid content of 8% (DW) Low monounsaturated fatty acid content: 8%

Phaeodactylum tricornutum Tetradesmus obliquus

Acceptable lipid content: 19% (DW) High monounsaturated fatty acid content: 40% Using sanitized anaerobic liquid digestate, the strain showed good productivity without any inhibitory effect on growth

DW, Dry weight.

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9.2 Microalgae strains

Lipids were extracted using the Soxhlet technique on dry, ground biomass pretreated with liquid nitrogen. Total lipids are expressed as a percentage relative to the ash-free dry biomass. Fatty acid methylesters (FAMEs) compositions were determined according to the AOCS Method Ce 2-66 using a FOCUS gas chromatograph with a flame ionization detector (FID) detector [17]. Fatty acids identification was done by the comparison of retention times with known standards and by gas chromatography-mass spectrometry (GC MS). Protein content was analyzed by the Kjeldahl method [18], using a nitrogen conversion factor of 6.25. Carbohydrates in algal biomass were determined according to the NREL (National Renewable Energy Laboratory) developed laboratory analytical procedure [19].

9.2.2 Fractionation of algal biomass Algal biomass has captured the interest of researcher since it has a good potential for application of the concept of “biorefinery” for the production of chemicals and energy improving biomass utilization in its entirety (Fig. 9.1). Lipids are considered the most valuable components of algal biomass in the context of a biofuels process, but other biomass components such as proteins and carbohydrates represent a large fraction of the biomass to convert [20]. The composition of the cellulosic FAME/Biodiesel glycerol products

Nonpolar lipids

Pigments and sterols Lipid Polar lipids

AlgaL biomass Fractionation

Nutraceutical, steroids, animal feed Fattyalcohols, fatty aldehydes, fatty acids, Polymers/dimers, polyacids, PUFAs, epoxides, polyols

Amicoacids

Nutrientre cycling animal feed

Secondary metabolites and inorganics

Antibiotics naturalproducts

Protein and residual

Starch and other glucan

Biofuels, bioplastic, commodity chemicals

Carbohydrates Alginates and complex polysaccharides

Food additives

FIGURE 9.1 Algal biomass fractionation and coproduct generation. Source: Adapted from P. Foley, E. Beach, J. Zimmerman, Algae as a source of renewable chemicals: opportunities and challenges, Green Chem. 13 (2011) 1399 1405 with permission of The Royal Society of Chemistry.

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fraction of algal biomass is similar to that of traditional plant crops, but the lack of the structural component lignin facilitates the separation of valuable carbohydrates [21]. Recently, new integrated processes that convert all biomass components into biofuels and chemicals have been investigated. The NREL process assumes high solubilization/ recovery of both carbohydrates and lipids, and their conversion to ethanol and refined, deodorized, bleached oil (RDB) (paraffins targeted in the C13 C20 range), respectively. The process consists of a dilute-acid pretreatment of algal biomass delivered after upstream dewatering to 20 wt.% solids, followed by whole-slurry fermentation of the monomeric sugars to ethanol, followed by distillation and solvent extraction of the stillage to recover lipids [22]. Laurens et al. [23] describe a new route to valorizing algal biomass components with an integrated technology based on moderate temperatures and low pH. The carbohydrates contained in the wet microalgal biomass are converted to soluble sugars for fermentation, the lipids are extracted, and the protein fraction remains at the end. Such method may offer more coproduct flexibility than, for example, hydrothermal liquefaction, which converts the whole biomass without fractionates to selective components [23]. We have applied the above methodology to Chlorella and Tetradesmus. Czartoski et al. [24] propose a new way to extract target classes and products and to recover by-products and recycle critical nutrients and water. The invention describes a method of fractionating biomass, including several steps in which two main fractions (one polar and one nonpolar) are obtained. With the nonpolar fraction are collected triglycerides, free fatty acids, and hydrocarbons, while with the polar fraction are collected all the polar-soluble components (fiber, nutrients, soluble proteins, carbohydrates, residual solid algae cell structural particles, etc.).

9.3 Biochemical composition variability of selected microalgae Table 9.3 presents the strains selected for our study and the reasons why other strains were discarded after preliminary tests. Table 9.4 summarizes the composition of microalgae P. tricornutum and T. obliquus grown in our laboratory (bold characters) on a large scale (100 L—PBR to 3000 L—pond) and that of strains either commercial or investigated by other authors. Unfortunately, literature data are not exhaustive about the characterization of strains, and the scarcity of data does not allow a complete overlapping with our data. Nevertheless, some interesting matches exist, and the whole body of data allows an economic evaluation. Data in Table 9.4 confirm that both environmental factors (temperature and light intensity) and medium growth composition (carbon source, pH, salinity, and P and N sources) drive the microalgae growth rate and composition. Therefore in accordance with literature data [2], lipid abundance and FAs profile, as well as protein and carbohydrates percentage substantially change for the same strain under different growth conditions. The best lipid production for T. obliquus (12.8%) grown in open pond at our premises [14] matches literature data [24 27], but not the unique data reported by Feng et al. [28]. Carbohydrate and protein reach up to 60.9% and 52.9%, respectively. P. tricornutum reaches 23.1% under nitrogen starvation in indoor cultivation, with lower protein content. N limitation or depletion induces the increase of the energy-rich products,

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TABLE 9.4 Dependence of the composition of microalgae upon the growing conditions for several microalgal strains (our data in bold). Microalgae

Medium

Tetradesmus obliquus BG11 BG11 BG11, N starvation BG11 BG11, N starvation

Production technology

Lipids (%)

Carbohydrates Proteins (%) (%)

SFAs (%)

MUFAs (%)

DUFAs (%)

PUFAs (%)

Ref.

100 L outdoor PBR LT

10.4 6 0.64

52.9 6 0.74

36.7 6 0.92

30.8 6 0.32

20.4 6 0.98

13.8 6 0.25

21.6 6 0.40 (C18:3)

[14]

250 L indoor PBR HT

6.97 6 0.06

46.3 6 1.41

46.8 6 1.06

30.1 6 0.21

44.1 6 1.23

10.2 6 0.15

10.7 6 0.28 (C18:3)

10.2 6 0.32

33.4 6 0.42

56.4 6 0.78

23.4 6 0.13

31.4 6 1.02

20.9 6 0.51

14.6 6 0.35 (C18:3)

12.6 6 0.64

37.1 6 1.63

50.4 6 0.28

22.6 6 0.10

22.1 6 0.80

11.6 6 0.21

31.0 6 0.94 (C18:3)

9.10 6 0.35

30.0 6 0.28

60.9 6 1.55

22.1 6 0.21

41.1 6 1.35

10.4 6 0.37

16.1 6 0.23 (C18:3)

5.10 6 0.14

40.0 6 1.34

54.9 6 1.08

23.2 6 0.29

35.0 6 1.17

14.6 6 0.82

14.8 6 0.10 (C18:3)

23.70

23.9

25.7

3.7 (C18:3)

3000 L open pond

Wastewater

Phaeodactylum tricornutum

[25]

Bristol medium

50 L closed sleeve PBR

12.8

Jaworski formulation

500 L outdoor tubular PBR

13.42 6 0.59 35.25 6 3.86

BG11, N starvation

80 L indoor flat 40.1 plate PBR

31.24 6 1.03 29.84 6 0.88 22.94 6 0.90 13.02 6 0.46 (C18:3)

[28]

Flory Basic Fertilizer 1

20 transparent PTE bags

12.3

20.5

24.25

15

21 (C18:3)

[27]

BG11, N starvation

80 L outdoor flat plate PBR

49.6

40.6 6 0.92

38.7 6 1.01

16.4 6 0.73

4.09 6 0.37 (C18:3)

[28]

Urban wastewater treatment plant

530 L high rate alga pond

20.8

1/2 SWES

100 L outdoor PBR LT

6.52 6 0.83

250 L indoor PBR HT

10.8 6 0.94

1/2 SWES

[26]

[31]

50.5 6 0.90

42.3 6 1.07

34.7 6 1.38

38.9 6 1.91

6.42 6 0.35 (C18:3)

[14]

19.6 6 1.23 (C20:5) 58.0 6 1.63

31.2 6 0.10

26.6 6 1.30

46.1 6 1.77

1.07 6 0.13

2.97 6 0.22 (C18:3) 12.6 6 0.87 (C20:5) (Continued)

TABLE 9.4 (Continued) Microalgae

Medium

Production technology

1/2 SWES, N starvation 1/2 SWES 1/2 SWES, N starvation

Carbohydrates Proteins (%) (%)

SFAs (%)

MUFAs (%)

DUFAs (%)

PUFAs (%)

23.1 6 0.35

38.8 6 0.92

50.2 6 2.18

40.9 6 2.19

0.59 6 0.07

1.09 6 0.06 (C18:3)

37.9 6 0.01

Ref.

1.08 6 0.07 (C20:5) 3000 L open pond

Wastewater

Nannochloropsis NANNO 3600

Lipids (%)

5.46 6 0.98

49.4 6 1.24

45.1 6 1.01

39.3 6 2.30

27.2 6 1.48

6.47 6 0.30

19.7 6 0.89 (C20:5)

7.60 6 0.18

41.5 6 0.71

50.9 6 0.64

33.2 6 1.94

33.1 6 1.69

9.24 6 0.61

17.5 6 0.75 (C20:5)

4.45 6 0.21

55.6 6 1.84

39.9 6 0.14

23.4 6 1.50

30.8 6 1.42

10.7 6 0.47

28.7 6 0.95 (C20:5)

52.8

. 15

38.7 6 2.41

23.3 6 1.26

Artificial seawater

outdoor 51 L PBR

27.5

Artificial seawater

Circular pond

25

Filtersterilized seawater

Horizontal tubular solar receiver 220 L

20

44

33

Externally located PBR bubble column 57 L

25

44

28

5.10 6 0.36 (C18:3)

[29]

33.7 6 2.20 (C20:5) 59

. 15

35.8 6 2.22

23.6 6 1.57

5.8 6 0.39 (C18:3)

[29]

34.8 6 2.10 (C20:5)

Commercial product from Reed Mariculture 2

40

[30]

30.5

37.6

2.9

1.9 (C18:3)

[32]

24.3 (C20:5)

Nannochloropsis sp.

Natural seawater enriched with f/2 nutrients

60 m shallow raceway-type ponds

30.2

Nannochloropsis sp.

Standard medium

outdoor raceways

Nannochloropsis sp. F&M-M24

F, N starvation

Outdoors in a 590-L green wall panel (low-cost PBR)

30.1

9.7

27.2

37.4

1.98

26.5 (C20:5)

[33]

28.7

30.96

40.07

1.21

14.3 (C20:5)

[34]

48.2

.40

.40

,5

[35]

Green wall 23.12 6 0.26 47.5 6 5.14 panel (low-cost PBR) outdoors 18.8 6 0.09 20.4 6 4.52 in 0.25 m2

23.8 6 2.39

Chlorella

BG11

F&M-M49

F, N starvation

Chlorella

BG11

27.0 6 0.02

44.4 6 4.88

27.4 6 0.64

IAM C-212

F, N starvation

20.8 6 0.06

18.8 6 3.72

57.8 6 0.77

Chlorella PROD1

BG11

22.8 6 0.18

46.7 6 3.19

28.7 6 0.69

F, N starvation

44.4 6 0.03

29.1 6 3.15

25.1 6 1.64

F

29.9 6 0.02

44.8 6 4.22

23.9 6 1.50

F, N starvation

45.5 6 0.17

29.7 6 3.78

19.6 6 0.67

Chlorella CH2

Chlorella

Raw dairy wastewater

Outdoor pilotscale PBRs 40 L

[36]

54.3 6 0.77

45.3 6 1.80

17.7 6 1.13

9.90 6 0.01

13.94 6 0.79 (C18:3)

[37]

DUFAs, Diunsaturated fatty acids; HT, high technology; LT, low cost/technology; MUFA, monounsaturated fatty acids; PBRs, photobioreactors; PUFAs, polyunsaturated fatty acids; SFAs, saturated fatty acids.

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such as lipids and carbohydrates, a strategy used for maximizing the lipid content, avoiding genetic manipulation, a practice not accepted in several countries, such as Italy, where genetically modified organisms (GMO) and their derivatives are not permitted. The finding by Buono et al. [14] is in agreement with those reported by Benavides [29] and Rebolloso-Fuentes et al. [30], who have used artificial seawater as a growing medium. P. tricornutum cultivated under nitrogen starvation in Southern Italy climatic conditions in open pond reached an interesting 50.9% carbohydrates content, with proteins and lipids within standard limits [14]. Because of the contamination by ameba, it was possible to keep the culture for only 2 months. Nannochloropsis sp., a rotifer feed, has been investigated by several authors. Using a standard medium, Gouveia and Oliveira [34] reported a lipid content of 28.7%, while Biondi et al. [35] and Dibenedetto et al. [32] reported a lipid content of 48.2% under N starvation. Chlorella was investigated by Guccione et al. [36] as source biofuels, biochemicals, and food. Under N starvation, it can accumulate lipids to a maximum of 45.5% and carbohydrates up to 57.8%. It must be emphasized that we have found high heterogeneity in the fatty acid profile even when microalgae were grown in similar conditions, most likely because of their genetic and phenotypic diversity. Such heterogeneity, includes different chain length (from 16 to 25 C atoms) and multiple unsaturations, from 0 up to 3 or 5. The latter property may affect the use of oil as biodiesel. In fact, multiple unsaturations are not suited for a good diesel, as they can originate gums in engines. So for example, T. obliquus, with respect to the other microalgae, shows the highest content of DUFAs and C18:3 fatty acid. Nannochloropsis sp. as well as P. tricornutum show high content of C20:5 under complete medium cultivation, these microalgae are known as the most prominent microalgal eicosapentaenoic acid (EPA) producers. Nevertheless, a decrease of the concentration of such fatty acid was observed in both strains under nitrogen starvation [13,15,35]. Chlorella is rich in SFAs (45%) making it a good candidate for biodiesel production; the MUFAs content is lower than that found in the other microalgae here examined [37]. T. obliquus and P. tricornutum cultivated as reported by Buono et al. [14] have a balanced distribution of the three main fraction, resulting good candidates for bioethanol production, as food and feed supplements, and for chemicals and biodiesel production.

9.4 Cellulose utilization Cellulose is the nonfood candidate to the production of fuels, such as ethanol, and several other platform chemicals and new materials. In land plants, cellulose is joined to hemicellulose and lignin to build up the skeleton that mostly provides the tensile strength of the cell wall. In marine algae, cellulose accounts for a smaller and more variable proportion than in land plants and in some species can even be lacking. In microalgae, cellulose is arranged in microfibrils smaller than in plants (2.5 vs 35 nm [38]), flat ribbons and nonmicrofibrillar rodlets. Furthermore, depending on the strain, algal cellulose frequently contains sugars other than glucose, commonly xylose (mainly ß-1,3 linked) [39a] and mannose

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9.4 Cellulose utilization

183

(mainly ß-1,4 linked) performing a structural function, less rigid than cellulose microfibrils [39b]. In algal cell wall the skeletal polysaccharides are organized in microfibrils, embedded in a rather amorphous and abundant matrix, with variable orientation, from parallel to disperse [39a]. The use of cellulosic hemicellulosic fraction of biomass is the route to avoid the food chemicals energy conflict existing today as crops are used for the production of fuels (ethanol). The key issue is the depolymerization of cellulose hemicellulose (lignin as mentioned above is absent in algal biomass) and further conversion of derived polyols into useful monomers, fuels, and materials. Chemical and biochemical routes exist that afford monomers with good yield [40], and research in such field is still very active in order to maximize the conversion yield. Algal cellulose is, thus, free of lignin and even has a lower molecular mass than cellulose present in trees or land plants in general in which it has a stronger structural function: this makes easier its conversion into glucose and other chemicals.

9.4.1 Glucose conversion into the platform molecule 5-hydroxymethylfurfural Once glucose is obtained, it can be converted into a series of other platform molecules. Among the many products derivable, 5-hydroxymethylfurfural (HMF) plays a key role as a platform molecule. The production of the latter molecule is a two-step process: isomerization of glucose into fructose, which is base catalyzed, and dehydration of fructose, an acid catalyzed process (Scheme 9.1). Bottlenecks in such process are the formation of polymeric humins, which are formed especially when the reaction is carried out in the water. Such solids, in addition to cause loss of the starting reagent reducing the yield of the process, can affect the catalyst activity as they can deposit on the surface of the catalyst (when heterogeneous catalysts are used) and deactivate it [41]. In order to improve the catalyst life and yield of production of 5-HMF, organic solvents can be used to extract the target product. Their correct choice plays a key role, as they increase the efficiency of the catalysts and their recyclability [8]. Research is very active in this field in order to find new active and robust catalysts, which can efficiently and at low cost produce 5-HMF from fructose, or even directly from glucose.

SCHEME 9.1 Conversion of glucose into 5-HMF: the formation of polymeric humins affects the conversion yield and selectivity of the process. HMF, Hydroxymethylfurfural.

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9. Beyond fractionation in the utilization of microalgal components

O

O O

H

O

(DFF)

O HCOOH + HO (FA) (LA)

[O]

[H+] O

O

HO

OH

(HMFCA)

O

[O] O

[O]

O

HO

HO

H

H

5-HMF

[O]

O

O

O O

OH

HO OH

O

[O] O

(FDCA) H

(SA)

O O

OH

+ HO O (OA)

OH

(FFCA)

SCHEME 9.2 Conversion of 5-HMF into other useful compounds, used as fine chemicals or monomers. HMF, Hydroxymethylfurfural.

9.4.2 5-Hydroxymethylfurfural conversion 5-HMF is a platform molecule from which several monomeric compounds can be derived (Scheme 9.2). 5-HMF bears two moieties, the alcoholic and the aldehydic, which can undergo oxidation processes. To perform in a selective way the oxidation of the former or the latter without touching the other is a challenging task. Each of the oxidation products shown in Scheme 9.2 has a specific industrial application, as fine chemical, intermediate, or monomer for polymers. A key issue in such conversion reaction is the choice of the catalyst, oxidant, and reaction medium. Our target is to use cheap and abundant catalysts and oxygen or air as oxidant, working in water [42]. Under such conditions, we have developed catalysts that convert quite selectively HMF into FFCA [42a], DFF [42b], FDCA [42c] or even cleave the ring to afford oxalic and succinic acid [42d]. Each derivative is produced, with yield and selectivity close to 100%, by using a customized catalyst, using oxygen as an oxidant in water. 5-HMF can be also converted into formic acid and levulinic acid that finds several applications, including the use as a precursor of biofuels. The latter conversion requires hydrogen that must be cheap and derived from a nonfossil source, if the reduction of CO2 emission is targeted.

9.5 Use of lipids Lipids represent a variable (from a few units to 70 1 %) fraction of the algal biomass and are constituted of SFAs, MUFAs, DUFAs, PUFAs, including omega-3, esterified with glycerol. Tri-, di-, and monoglycerides exist in the algal biomass. The FA chain length varies over a wide range (14 to 25 1 ). As described earlier, such distribution depends on the

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9.5 Use of lipids

SCHEME 9.3

185

Special uses of lipids according to their nature.

strain and its growth conditions. FAs can find a specific utilization according to their nature. Scheme 9.3 shows the overall view of possible applications. In the following paragraphs each class of FAs (SFA, MFAs, DUFAs, and PUFAs) will be analyzed for its specific uses.

9.5.1 Use of saturated fatty acids The most likely use of SFAs is their conversion into FAMEs that have properties quite similar to those of fossil diesel and can be used as biofuels. As one can see in Table 9.4, the amount of SFAs is quite variable (from 20% to 46% of the lipid fraction or 4% 35% of the biomass) according to the strain and its growing conditions. It is obvious that the energy necessary for the separation of various classes of FAs or FAMEs plays a key role in the utilization of the various fractions of the algal biomass. Below a given concentration, the separation cost becomes too high, and this does not support the use of all fractions. Other oils do exist, which have a high content of SFAs (e.g., palm oil) and are cheaper sources of biodiesel.

9.5.2 Use of monounsaturated fatty acids MUFAs may have a similar abundance as SFAs in algal lipids. Their methylesters can even be used as biodiesel blended with SFAMEs. It must be considered that pure unsaturated

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O HO

O

O

(CH2)n

O3

CH3 (CH2)n

HO

CH3 (CH2)n

(CH2)n

Co/H2O2 O HO

HO

(CH2)n

OH CH3 (CH2)n

O HO

O

O

(CH2)n

OH

HO

CH3 (CH2)n

SCHEME 9.4 Conversion of MUFAs into chemicals: a two-step process. MUFA, Monounsaturated fatty acids.

FAMEs are not the best candidates as biodiesel, as a high concentration may cause, during combustion, the formation of gums that may damage engines. MUFAs can be hydrogenated to SFAs, but this has a cost that may not have sense considered the low value of diesel. MUFAs can find a much better utilization as a source of chemicals. As depicted in Scheme 9.4 (left upper and lower part), MUFAs can be used to produce diols, epoxides (that can originate carbonates) or undergo oxidative cleavage to afford monocarboxylic acid as pelargonic acid (PA) (that has several applications in sectors such as cosmetics and agrochemistry) and azelaic acid (AA), a dicarboxylic acid usable as monomer for polymers, substituting fossil-C-derived phthalic acid. So far, ozone has been used with cobalt as a catalyst in a two-step process that converts MUFAs into the relevant epoxide or diol that is subsequently converted into the mono- and dicarboxylic acids (Scheme 9.4). The key point here is to use cheap oxidants (such as O2) and low-cost and easily available catalysts (based on abundant metals), possibly in a single-step process. The substitution of ozone and cobalt is a key issue in this field, as such materials have a high environmental impact and affect human health. We have used oxygen or air and cheap catalysts based on abundant metal oxides of Earth’s Crust as the solution to this problem, reducing the cost of operation while implementing environmental friendly processes [43]. MUFAs can also undergo metathesis to afford olefins and carboxylic acids (Scheme 9.3, upper right part).

9.5.3 Use of diunsaturated fatty acids DUFAs are present at a lower rate than SFAs and MUFAs (a few units percent, in general). They can easily and selectively be hydrogenated to MUFAs under quite mild conditions [44]. As discussed above, the latter can be used as raw materials for chemicals, justifying the cost of the hydrogenation process. They can, even, be converted into epoxides or polyols and used as constituents of new polymeric materials, such as polycarbonates or polyurethanes. Even in this case, one has to consider the separation costs in order to make a choice about the final use. Fuels are the less remunerative among possible derivatives of FAs, therefore either lipids are all converted into biodiesel at a low cost or if separation is implemented the use of unsaturated fatty acids (UFAs) as a source of chemicals is the best choice, more than the production of fuels.

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9.5 Use of lipids

9.5.4 Use of polyunsaturated fatty acids and omega-3 PUFAs and their alkylesters can be used in many different ways. Omega-3 finds application as food and feed additives. Their abundance in lipids is not so high, amounting to a few units %, but their value is such that the recovery can be economically sustainable. PUFAs can be used as starting materials for the production of new polymeric materials such as multicarbonates, multiols, and multiurethanes (Scheme 9.3, lower right part). Such new materials may find application as biomass-sourced new polymer components, easily biodegradable. Research in this area is very active, and the best exploitation of PUFAs is still to be discovered, giving new impulse to oleochemistry [9.44b].

9.5.5 Use of bioglycerol Lipids extracted from microalgae or algae or even from oils produced from land plant seeds and drupes produce c.10% glycerol when converted into FAMEs or free FAs or their alkylesters (Scheme 9.5). Actually, water-soluble basic catalysts are used in transesterification reactions (Scheme 9.5, A), which produce watery glycerol. Free fatty acids require acid catalysts for esterification (Scheme 9.5, C). This practice increases the cost of FAMEs production as a two-step process is required. Alternatively, glycides would be hydrolyzed and free acids esterified. Even the recovery of glycerol from the watery process requires intensive electric energy for distillation. Conversely, besides making the transesterification esterification in a single pot, the single-catalyst water-free transesterification esterification process (Scheme 9.5, B D) produces cheaper nonwatery glycerol, almost ready to be used in chemical applications [32,45]. The glycerol chemistry is undergoing an interesting expansion these days as the production of biodiesel is putting on the market large volumes of this chemical: new products need to be developed, and new markets require to be explored and exploited for the use of such raw material. Bioglycerol has been used as a platform molecule for several different applications (Scheme 9.6). Its direct use as fuel is not recommended because of its high viscosity, even CH3ONa, H2O CH2OC(O)R

Glycerol(watery) + RCO2CH3

(A)

CHOC(O)R CH2OC(O)R

Glycerol + RCO2CH3

(B)

Mixed oxide Water-free CH3OH CH3OH, protic catalyst RCO2CH3

(C)

RCO2CH3

(D)

RCO2H Mixed oxide, CH3OH

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SCHEME 9.5 Technologies for transesterification of glycides and esterification of fatty acids and production of FAMEs and bioglycerol. FAME, Fatty acid methylesters.

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9. Beyond fractionation in the utilization of microalgal components

SCHEME 9.6 Utilization of glycerol as source of chemicals.

if new engines have been developed, which may burn glycerol [46]. Its conversion into ethers makes easier its combustion, and such application has met the interest of several research groups around the world. Esters are used as solvents. The conversion into carbonate is of interest as the latter may find use as a monomer for polymers or solvent or even reagent [47]. Biotechnology has been used to convert bioglycerol into other chemicals, such as diols, acrolein, acrylic acid [48] used, in turn, as raw materials for the synthesis of fine chemicals, intermediates, or polymers. Diols can be converted into carbonates, monomers for polymers [48c]. This field is in great expansion as the bioglycerol is increasing, and new applications are needed.

9.5.6 Economic evaluation of microalgae The cost of large-scale production of microalgae spans in the broad range of 0.50 6h/kg [49,50]. Detailed studies report that for open ponds, horizontal tubular PBRs, and flat panel PBRs, the production cost including dewatering, is set at 4.95, 4.15, and 5.96h/kg, respectively. Key factors affecting the growth are irradiation conditions, mixing, photosynthetic efficiency of systems, medium-growth and carbon dioxide purity, and distribution techniques. It has been calculated that, optimizing the production with respect to the abovementioned factors, a price as low as 0.68h/kg can be reached as the best value [51], one order of magnitude lower than real costs. It is worth to note that the market price for

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algal biomass and derived components depends on the area of production, the actual market situation, and, most important, the product purity. The production cost in PBRs is higher than in open ponds, due to the higher capital and operational costs. Nevertheless, because of the influence of external factors, such as temperature, photoperiod, CO2 transfer rate, evaporation loss, contamination, the cultivation in PBRs may assure better quality and constant productivity [14]. The cultivation of P. tricornutum in indoor PBR under standard medium brings to the highest biomass productivity [14] and to the highest economic revenue derived from the full biomass fractionation. The microalgae cultivation performed under nitrogen starvation in indoor PBRs affords the highest revenue as biodiesel production. An issue of paramount relevance is the water and nutrients source. If N and P fertilizers must be added, the cost of production would be high enough to discourage the use of microalgae for applications different from fine chemicals production. A strategy to keep low production costs is to use wastewater (municipal or process waters) from which microalgae may uptake inorganic nutrients such as N and P. The use of wastewater rises the issue of contamination of cultures, therefore water should be sanitized before use. In our calculations, we have considered the use of sanitized process water as a growing medium for algae that grow in soft water and marine water for algae growing in salt media. In order to assess the real economic value of algae [49], it is important to adopt a biorefinery scheme [15,52]. It is clear that algae are not viable for only biofuel production, neither considering the lipid fraction nor adopting the biogas technology. Conversely, if the fractionation is applied and the three main components (proteins, carbohydrates, and lipids) are all used, microalgae may turn to be a useful source of bio-based products. As a matter of facts, while biodiesel has a counterpart in fossil fuels that are marketed at a price as low as 0.50h/kg [52], MUFAs as feedstock for the chemical industry has a value of 2h/kg. The market value of proteins used as food or feed supplement ranges around 5 and 0.75h/kg, respectively. The carbohydrates (sugars) used as chemical building blocks are priced at 1h/kg. Also, the market price of PUFAs is quite high, due to the complex downstream processing for pure products. We have recently carried out an economic evaluation [15] of T. obliquus and P. tricornutum cultivated in the local Southern Italy climatic conditions [14], assuming a cost of production of the algal biomass of 1.1h/kg, or 1100h/t DW, which represents a nonfully optimized cost for microalgae grown in the conditions reported by Buono et al. [14]. In summary, Table 9.4 shows that if algal biomass is used only to obtain saturated lipids (SFA) to be used as biodiesel (that means that either SFAs are separated and used or the mixture of UFAs are hydrogenated to SFAs), the economic convenience will never be met. Considering their composition, the higher algal biomass value is around 58h/kg [15]. Conversely, if chemicals are produced from MUFAs or from DUFAs and PUFAs and if the biorefinery concept is applied, it will be possible to have a higher positive return from microalgae (almost 1500h/kg) [15]. The fractionation of the biomass is necessary, as well as the separation of different classes of lipids (saturated for biodiesel, mono- and diunsaturated for the production of chemicals, polyunsaturated for the production of food additives) in order to produce PUFAs, proteins, compounds with pharmacological or nutraceutical properties, chemicals, feed and food supplement, biofuels and bioplastics.

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It is worth to note that, the use of SFAs as feedstock for the chemical industry is much more remunerative than using such biomass for making fuels. In fact from the SFAs, it is possible to extract stearic and palmitic acids for the production of detergents, soaps, cosmetics, shampoos, and shaving cream: all have a higher value than biodiesel, but a much lower market. Other applications of such acids are reported by Lai et al. [53] as plasticizers for zein sheets. The best solution is to make a dedicated use of each fraction. MUFAs can be converted by oxidation into monocarboxylic (PA) and dicarboxylic (AA) acids. AA finds application as co-monomer in the production of polyamides, polyesters, pharmaceuticals, plasticizers, lubricants, hydraulic fluids, textiles, in food packaging, electronics, and automotive [54]. PA is used for the production of lubricants, peroxides, alkyd resin, and perfumes [55]. It is also used as biodegradable herbicide in agriculture. DUFAs (e.g., linoleic acid) can be hydrogenated to MUFAs or even stearic acid, used in the cosmetic and pharmaceutical industry [56]. Both T. obliquus and P. tricornutum are rich in PUFAs such as EPA (C20:5) and α-linolenic acid (C18:3), which have an important role in human metabolism for cancer prevention and cardiac protection and find pharmaceutical and therapeutic applications. Their commercial value is quite high due to the downstream processing for recovery of highly pure product. As said earlier, in addition to lipids, proteins and carbohydrates can be extracted from microalgae. Nutritional and toxicological evaluations have demonstrated the suitability of microalgae as feed [57]. The use as food would require longer and more complex processing with an increase of production costs. Algal carbohydrates are mainly composed of starch, glucose, cellulose/hemicelluloses, and various kinds of polysaccharides. Noteworthy, they do not contain lignin, making easier the hydrolysis of the cellulosic fraction [58]. The sugar fraction derived from cellulose can be used for bioethanol production, while polysaccharides find many applications in food, textiles, cosmetics, as emulsifiers, lubricants, and clinical drugs [2].

9.6 Concluding remarks In order to assess the potential use of T. obliquus and P. tricornutum grown in indoor/outdoor PBRs and in open pond (this is the first study on such strains cultivated in the local Southern Italy climatic conditions), and Nannochloropsis sp. (commercial sample) and Chlorella (literature data), for the production of biofuels, chemicals, and omega-3, and as animal feed and human food, the microalgae were fully analyzed for their proteins, carbohydrates, lipids content, and fatty acids profile. Obtained data confirm that the growth performance and microalgae composition strongly depend on the environmental factors and medium composition. The economic evaluation was carried out on the composition and the use of various components of T. obliquus and P. tricornutum, defining the best conditions for each to have the highest revenue. It was demonstrated that the cultivation of microalgae for the production of only biofuels will not match the economic standards, but if the biorefinery concept is applied (production of fuels, chemicals, proteins, etc.) and wastewater is used as a source of nutrients, it is possible to have a good positive return from microalgae.

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Acknowledgment The work was funded by the MIUR Industrial Research Project PON01_01966 “ENERBIOCHEM” in the frame of the Operative National Programme-Research and Competitiveness 2007 13.

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