Acetyl-CoA generated via the Wood-Ljungdahl pathway serves as key intermediate for syn­thesis of cell mass as well as products. All acetogens are described to produce acetate, in or­der to gain energy via SLP to compensate for the energy invested in activating formate in the Western branch of the reductive acetyl-CoA pathway. Acetate and ATP are formed via acetyl-phosphate through the successive actions of Pta and Ack. pta and ack are arranged in the same operon and they were reported to be constitutively expressed [100]. With CO2 and H2 as substrate, only acetate has been observed as major product [44], with minor amounts of ethanol produced in rare cases with C. ljungdahlii [101], C. autoethanogenum [53], or "Moor — ella sp." [102, 103]. Using the more reduced substrate CO, production of a range of other products have been reported, such as ethanol, butanol, butyrate, 2,3-butanediol [104], and lactate (Figure 4.) [105]. From a biofuel perspective, ethanol and butanol are of particular in­terest. Ethanol and butanol have even been described as the main fermentation products over acetate in some acetogens under specific conditions. Ethanol producers include C. ljungdahlii [62, 63], C. autoethanogenum [53], "C. ragsdalei" ("Clostridium strain P11") [106, 107], "Moorella sp." [102, 103], Alkalibaculum bacchii [44], C. carboxidivorans ("Clostridium strain P7") [54, 55], and B. methylotrophicum [49, 108]. The latter two have also been descri­bed to produce butanol.

Due to historical roles in ABE fermentation, organisms like C. acetobutylicum, C. beijerinckii, C. saccharobutylicum, and C. saccharoperbutylacetonicum have been much more extensively characterized than acetogenic Clostridia [95]. Since C. acetobutylicum was the first Clostridium to be fully sequenced [109] and it remains the most commonly used species for industrial production of solvents to date [110], it provides a model for study of solventogenesis. Al­though sugar — and starch-utilizing ABE Clostridia and acetogens exhibit clear distinctions in substrate utilization and thus metabolism, they share some similarities in the biochemical pathway and genetic organization of product synthesis and can be used as model for com­parison. Structure of key genes and operons (except for the absence of acetone biosynthetic genes) have been found to be very similar in sequenced acetogen C. carboxidivorans [54], and in respect of acetate and ethanol genes to some extent also in C. ljungdahlii [62]. For instance, the operon structure of pta-ack, ptb-buk and the bcs cluster of acetogen C. carboxidivorans are highly similar to starch-utilizing C. acetobutylicum and C. beijerinckii [54, 109] (Figure 5). Due to these reasons, solventogenic genes from starch-utilizing Clostridia are ideal targets for heterologous expression in acetogens for improvement of product yield and expansion of product range.

(A) c. carboxidivorans, C. Ijungdahlii, C. autoethanogenum, C. acetobutylicum, C. beijerinckii

(В) C. carboxidivorans, C. acetobutylicum, C. beijerinckii

Similar to sugar — and starch-utilizing ABE Clostridia, acetogens such as C. carboxidivorans [111, 112], C. Ijungdahlii [113], and C. autoethanogenum [27] also typically undergo biphasic fermentation under autotrophic conditions. The first phase involves the production of car­boxylic acids (acidogenic), H2 and CO2 during exponential growth. This is followed by the solventogenic phase in which part of the produced acids are reassimilated or reduced in­to solvents, which usually occurs during stationary growth phase [114]. This shift from acidogenesis to solventogenesis is of industrial importance and several transcriptional analysis on C. acetobutylicum [100, 115], and C. beijerinckii [116] have been performed to shed light on this process. In both organisms, the onset of solventogenesis coincides with an increase in expression of master sporulation/solventogenesis regulator gene spo0A, sol — ventogenic genes such as ald, ctfA-ctfB, and adc, as well as down-regulation of chemotaxis/ motility genes [100, 115, 116]. Physiologically, the signals that induce solventogenesis were hypothesized to involve temperature, low pH, high concentrations of undissociated acetic and butyric acids, limiting concentrations of sulphate or phosphate, ATP/ADP ratio and/or NAD(P)H levels [117].

For Clostridia such as acetogen C. carboxidivorans [54], which harbour the genes thiolase (thlA), 3-hydroxybutyryl-CoA dehydrogenase (hbd), crotonase (crt) and butyryl-CoA dehy­drogenase (bcd), the two carbon acetyl-CoA can be converted to four carbon butyryl-CoA [95]. ThlA compete with the activities of Pta, Ald (aldehyde dehydrogenase), and PFOR to condense two acetyl-CoA into one acetoacetyl-CoA, and plays a key role in regulating the C2:C4 acid ratio [110, 118]. Since the formation of acetate yields twice as much ATP per mole of acetyl-CoA relative to butyrate formation, thiolase activity indirectly affects ATP yield [118]. Under physiological conditions, Crt catalyzes dehydration of p-hydroxybutyryl-CoA to crotonyl-CoA [119]. Bcd was shown to require a pair of electron transfer flavoproteins (Et­fA and EtfB) to convert crotonyl-CoA to butyryl-CoA [120]. Furthermore, the Bcd was dem­onstrated to form a stable complex with EtfA and EtfB, and they were shown to couple the reduction of crotonyl-CoA to butyryl-CoA with concomitant generation of reduced ferre — doxins, which can be used for energy conservation via Rnf complex [94, 119]. Subsequent actions of phosphotransbutyrylase (ptb) and butyrate kinase (buk) then generate ATP and butyrate from butyryl-CoA [118].

Under low extracellular pH of 4-4.5, the secreted undissociated acetic acid (pKa 4.79) and/or butyric acid (pXa 4.82) diffuse back into cell cytoplasm and then dissociate into the respec­tive salts and protons because of the more alkaline intracellular conditions. Without further interventions, the result of this is abolishment of the proton gradient and inevitable cell death [95]. The conversion of acetate and butyrate into solvents increase the pH, thus pro­vide some time for the organism to sporulate and secure long term survival. However, the solvents produced are toxic because they increase membrane fluidity and disrupt critical membrane-associated functions such as ATP synthesis, glucose uptake and other transport processes [114, 121]. In C. acetobutylicum, it has been demonstrated that the addition of 7-13 g/l of butanol, or up to 40 g/l of acetone and ethanol resulted in 50% growth inhibition [122]. The bacterium is likely to experience a different cytotoxic effect from endogenously pro­duced solvents because the organism has time to adapt to increasing amount of solvents.

The reassimilation of acetate and butyrate into the respective acyl-CoA and acetoacetate is catalyzed by acetoacetyl-CoA:acetate/butyrate CoA transferase (CtfA and CtfB) [110, 117, 118]. Acetoacetate is deconstructed by acetoacetate decarboxylase (Adc) into acetone and CO2. This enzyme is missing in acetogenic C. carboxidivorans compared to the ABE strains [54, 123]. Some ABE strains such as C. beijerinckii NRRL B593 also possess a primary/secon- dary alcohol dehydrogenase that converts acetone to isopropanol [124]. In acetogenic "C. ragsdalei", reduction of acetone to isopropanol was also observed although the mechanism of this reduction is as yet unknown [124, 125]. Again, C. carboxidivorans lacks this activity [125]. The recycled acetyl-CoA and butyryl-CoA can be converted to ethanol and butanol through the actions of coenzyme A-acylating aldehyde dehydrogenase (Ald) and alcohol de­hydrogenase (Adh) [110, 118]. Ald converts acyl-CoA into aldehydes, and the enzyme has been purified from C. beijerinckii NRRL B593 and was shown to be NADH-specific, exhibit higher affinity with butyraldehyde than acetaldehyde, but possess no Adh activity [126]. In C. ljungdahlii, two variants of aldehyde:ferredoxin oxidoreductases (AOR) are present in the genome, and they are hypothesized to couple reduced ferredoxin from CO oxidation via the CODH (see above) to perform the reversible reduction of acetate into acetaldehyde, which can be further reduced into ethanol [62].

The final step of solventogenesis utilizes Adh to reduce acetaldehyde and butyraldehyde in­to ethanol and butanol, respectively. For ethanol synthesis, transposon mutagenesis and en­zymatic assay in C. acetobutylicum showed the involvement of a specific Ald that does not interact with butyryl-CoA, and a NAD(P)H-dependent Adh [127, 128]. The production of butanol by C. acetobutylicum is mainly due to the action of butanol dehydrogenase A and B (BdhA and BdhB), and bifunctional butyraldehyde/butanol dehydrogenase 1 and 2 (AdhE1 and AdhE2) [95]. In C. carboxidivorans [54] and C. ljungdahlii [62] both adhE1 and adhE2 are arranged in tandem and separated by a 200bp gap which contains a putative terminator [62,

111]. This is likely the result of gene duplication [62]. qRT-PCR analysis from C. carboxidivor — ans fed with syngas showed that the two adhE showed differential expression, and the more abundant adhE2 was significantly upregulated over 1000 fold in a time span that coincided with the greatest rate of butanol production [111].

Pyruvate is a central molecule for anabolism and it is predominantly generated from glycol­ysis during heterotrophic growth. But under autotrophic growth, this four carbon molecule can be synthesized by PFOR and potentially also the pyruvate-formate lyase (PFL). Two variants of PFOR were reported in C. autoethanogenum, and transcriptional analysis showed that they were differentially expressed when grown using industrial waste gases (containing CO, CO2 and H2) [104]. Unlike PFL from most other microorganisms that only catalyze the lysis of pyruvate into formate and acetyl-CoA, clostridial PFL (C. kluyveri, C. butylicum, and C. butyricum) were reported to readily catalyze the reverse reaction (i. e. pyruvate formation) [129]. Apart from roles in anabolism, pyruvate is also a precursor to other products such as lactic acid and 2,3-butanediol. Small amounts of lactic acid are converted from pyruvate in acetogens, a reaction which is catalyzed by lactate dehydrogenase (Ldh) [104, 118]. Recently, Kopke et al. (2011) reported the production of 2mM 2,3-butanediol from acetogenic bacteria (C. autoethanogenum, C. ljungdahlii, and C. ragsdalei) using industrial waste gases (containing CO, CO2 and H2) as feedstock [104]. Pyruvate is first converted into a-acetolactate by the en­zyme acetolactate synthase, followed by acetolactate decarboxylase which split acetolactate into acetoin and CO2, before a final reduction of acetoin into 2,3-butanediol by 2,3-butane — diol dehydrogenase [104] (Figure 4).

From technological viewpoint, fermentation can be divided into two well separable phases: acid formation phase and, after reaching an autoinhibition limit value of the acids, solvent formation phase. These steps can be performed in separated technological environments as well [70].

Hydrogen formation takes place in the acidogenic phase, so the composition of the gases (CO2 H2) changes during the fermentation process. The larger part of the carbon dioxide is formed in the pathway of acetone formation. Presence of hydrogen and carbon dioxide has large influence on each metabolic step. The effect of H2 and CO2 as product gases on solvent production was studied in a continuous culture of alginate-immobilized C. acetobutylicum. Fermentations were carried out at various dilution rates. With 10% H2 and 10% CO2 in the sparging gas, a dilu­tion rate of 0.07 h-1 was found to maximize volumetric productivity (0.58 g*L-1xh-1), while maxi­mal specific productivity of 0.27 g-1*h-1 occurred at 0.12 h-1. Continuous cultures with vigorous sparging of N2 produced only acids. It was concluded that in the case of continuous fermenta­tion H2 is essential for good solvent production, although good solvent production is possible in an H2-absent environment in case of batch fermentations. When the fermentation was carried out at atmospheric pressure under H2-enriched conditions, presence of CO2 in the sparging gas did not slow down glucose metabolism; rather it changed the direction of the phosphoroclastic reaction and, as a result, increased the butanol/acetone ratio [71].

Klei et al. [72] studied the effect of pure CO2 on the second phase of ABE fermentation. CO2 pressures up to 100 psig were used in a batch fermentor using glucose as substrate. Maximal solvent production occurred near 25 psig CO2 at the expense of cell growth. In addition, the BuOH:Me2CO ratio changed sharply at 40 psig from 5:1 to 20:1 and EtOH production was eliminated at >50 psig. As the pressure increased, both conversion rates of organic acids to solvents and the utilization rate of substrate glucose decreased.

Pressurization of the fermentation vessel with H2 appeared to decrease, rather than increase, the formation of neutral solvents in batch fermentations [73]. However, increasing H2 partial pressure increased BuOH and EtOH yields from glucose by an average of 18% and 13%, respectively, and the yields of acetone and of endogenous H2 decreased by an average of 40% and 30%, respectively, and almost no effect was observed on the growth of the culture. The BuOH-to-acetone ratio and the fraction of BuOH in the total solvents also increased with H2 partial pressure. There were no major differences in the observed pattern of change with pressurization at either t = 0 or t = 18 h [74].

Redox active additives such as carbon monoxide have important influence on the ABE fermentation processes. Addition of CO inhibited the hydrogenase activity of cell extracts and viable metabolizing cells. Increasing the partial pressure of CO (2 to 10%) in unshaken anaerobic culture tube headspaces significantly inhibited (90% inhibition at 10% CO) both growth and H2 production. The growth was not sensitive to low partial pressures of CO (~15%) in pH-controlled fermentors (pH 4.5). CO addition dramatically altered the glucose fermen­tation balance of C. acetobutylicum by diverting carbon and electrons away from H2, CO2, acetate and butyrate production and towards production of EtOH and BuOH. The BuOH concentration increased from 65 to 106 mM and the BuOH productivity (the ratio of BuOH produced/total acids and solvents produced) increased by 31% when glucose fermentation was maintained at pH 4.5 in presence of 85% N2-15% CO vs. N2 alone [75]. Carbon monoxide sparged into batch fermentations of C. acetobutylicum inhibited production of H2 and enhanced production of solvents by making available larger amounts of NAD(P)H2 to the cells. CO also inhibited biomass growth and acid formation as well. Its effect was mostly pronounced under fermentation conditions of excess carbon — and nitrogen-source supply [76]. When continuous, steady-state, glucose-limited cultures of Clostridium acetobutylicum were sparged with CO, complete or almost complete acidogenic fermentations became solvento — genic. Alcohol (butanol and ethanol) and lactate production at very high specific production rates were initiated and sustained without acetone, and little or no acetate and butyrate formation. In one fermentation strong butyrate uptake without acetone formation was observed. Growth could be sustained even with 100% inhibition of H2 formation. Although CO gasing inhibited growth up to 50%, and H2 formation up to 100%, it enhanced the rate of glucose uptake up to 300%. These results support the hypothesis that solvent formation is triggered by an altered electron flow [77]. The metabolic modulation by CO was particularly effective when organic acids such as acetic and butyric acid were added to the fermentation as electron sinks. The uptake of organic acids was enhanced, and increase in butyric acid uptake by 50-200% over control was observed. H2 production could be reduced by 50% and the ratio of solvent could be controlled by CO modulation and organic acid addition. Acetone production could be eliminated if desired. BuOH yield could be increased by 10-15%. Total solvent yield could be increased by 1-3% and the electron efficiency to acetone-BuOH-EtOH solvents could be increased from 73% for controls to 80-85% for CO — and organic acid — modulated fermentations. The dynamic nature of electron flow in this fermentation was elucidated and mechanisms for metabolic control were hypothesized [78].

Even though liquid biomass is currently being exploited as a renewable feedstock for fuels pro­duction, its characteristics are far beyond suitable for its use as fuel. More specifically liquid bi­omass, just as other types of biomass, has a small H/C ratio and high oxygen content, lowering its heating value and increasing CO and CO2 emissions during its combustion. Moreover liq­uid biomass contains water, which can cause corrosion in the downstream processing units if it’s not completely removed, or even in the engine parts where its final products are utilized. In addition to the above, liquid biomass has an increased concentration in oxygenated com­pounds, mainly acids, aldehydes, ketones etc, which not only reduce the heating value, but al­so decrease the oxidation stability and increase the acidity of the produced biofuels. For all the aforementioned reasons it is imperative that liquid biomass should be upgraded and specifi­cally that its H/C should be increased while the water and oxygen removed.

The effectiveness of catalytic hydroprocessing towards improving these problematic char­acteristics of liquid biomass is presented in Table 1, where the H/C ratio, the oxygen con­tent and density before and after catalytic hydrotreatment of basic liquid biomass types are given. The H/C ratio exhibits a significant increase that exceeds 50% in all cases. This is due to the substitution of the heteroatoms by hydrogen atoms as well as in the saturation of

double bonds that enriches the H/C analogy. The oxygen content (including the oxygen contained in the water) from over 15%wt can be decreased down to 5wppm. Actually the deep deoxygenation achieved via catalytic hydrotreatment is the most significant contribu­tion of this biomass conversion technology, as it improves significantly the oxidation stabil­ity of the final biofuels. Furthermore significant improvement is also observed in the biomass density, which is never below 0.9 kg/l while after hydrotreatment it reduces to val­ues less than 0.8 kg/l

Catalytic hydroprocessing has been proven as the most efficient technology for the upgrad­ing of liquid biomass as it achieves to increase the H/C ratio and to remove oxygen and wa­ter. However the effectiveness of this technology is also shown in other parameters. For example the distillation curve of raw liquid biomass shows that over 90% of its molecules have boiling points exceeding 600°C and only 5% are within diesel range (220-360°C), while after catalytic hydrotreatment upgrading most of 90% of the product molecules are within diesel range [13].

Diesel

range

In the following sections the basic types of liquid biomass and their corresponding products via catalytic hydrotreatment are presented.

The process includes alcohol purification, hydrocarbon purification and a main reaction that uses acid resins. The reactants are converted at temperatures lower than 90 oC and pressures lower than 2 MPa. Then, the main effluents are purified for further applications or recycle.

The reactor column uses CATACOL technology that combines catalysis (in a well-controlled liquid phase) and distillation in separated sections.

1.1. Phillips Etherification process (by Philips Petroleum Co.)

This process uses olefins (i. e. isoamylene and IB) to react with EtOH or MeOH over acidic ion-exchange resin. Mixed olefins from a fluid catalytic cracking unit (FCCU) or steam cracker, along with fresh alcohol are fed to the reactor section. The reactor operation is liq­uid phase at mild temperature and pressure.

In case of MTBE, high purity MTBE is removed as a bottom product from the fractionator and all the unreacted MeOH is taken overhead. The overhead product is then stripped of MeOH in an extractor using H2O. The extract is sent to the fractionator, while the denuded H2O are returned to the MeOH extractor.

Liquefaction is a process of converting biomass into a bio-oil in the presence of a solvent— usually water, an alcohol, or acetone—and a catalyst [22]. Liquefaction operates at milder temperatures than gasification, but requires higher pressures. Liquefaction can be indirect, wherein biomass is converted into gas and thence into liquid, or direct, in which biomass is converted directly into liquid fuel [23]. Bio-oils produced in direct liquefaction processes usually produce heavy oils with high heating values and value-added chemicals as by-prod­ucts. Direct liquefaction also produces relatively little char compared to other thermochemi­cal processes that do not utilize solvents. In addition, liquefaction has the advantage that the method is not hindered by the water content of the biomass, giving credence to utilizing this method for water based biomass. The use of water as a solvent can significantly reduce op­erating costs, and recent studies with sub-and super-critical water have demonstrated in­creased process productivity by overcoming heat-transfer limitations [24, 25]. Operating parameters and feed quality significantly influence the overall quality of the oil produced by these processes. A recent review presented an exhaustive comparison of the operational var­iables that affect the liquefaction of biomass and concluded that a well-defined temperature range is the most influential parameter for optimizing bio-oil yield and biomass conversion [22]. Similarly, catalyst choice can alter the heating value of the final liquefaction product and reduce the quantity of solid residue [25].

In a product recovery technique termed pervaporation, a membrane that directly comes in contact with fermentation broth is used to selectively remove volatile compounds such as ethanol and butanol [219, 222]. The volatile compounds diffuse through the membrane as vapour and are then collected by condensation. To facilitate volatilization of permeates into vapour, a partial pressure difference across the membrane is usually maintained by apply­ing a vacuum or inert gas (e. g. N2) across the permeate side of the membrane [219]. Polydi- methylsiloxane (PDMS) is the current material of choice for the membrane, but other materials such as poly(l-trimethylsilyl-l-propyne) (PTMSP), hydrophobic zeolite mem­branes, and composite membranes have also been investigated [225].

1.7. Gas stripping

Gas stripping is an attractive product recovery method for gas fermentation because the exit gas stream from the bioreactor can be used for in situ/online product recovery [219]. Following product recovery via condensation, the effluent and gas can be recycled back into the bioreactor. In sugar-based fermentation using C. beijerinckii mutant strain BA101, in situ gas stripping was shown to improve ABE productivity by 200%, complete sub­strate utilization and also complete acid conversion into solvents, when compared to non-integrated process [226].

Biobutanol has excellent fuel properties compared to ethanol, thus it can be used directly as fuel or blending component for both diesel and gasoline powered internal combustion engines [217—221]. Butanol has no corrosive properties and its miscibility with gasoline and water tolerance is higher than the appropriate properties of ethanol or methanol [222]. Butanol can also be used as hydrogen source for fuel cells [223] and proved to be useful as esterification alcohol in fatty acid ester type biodiesel production [229—233] or as raw material in the production of dibutyl ether [236] butoxylated butyl diesels [237] or can be converted into aromatic hydrocarbons on zeolite catalysts [238—241].

6.1. Butanol as fuel and blending component in fuel mixtures

Although ethanol as a gasoline extender has received a great deal of attention, this fluid has numerous problems, such as aggressive behaviour toward engine components and a relatively low energy content, the properties of butanol or butanol containing gasoline, diesel and biodiesel fuel compositions are more advantageous than the analogous properties of ethanol or ethanol containing fuels [222]. The performances of gasoline and diesel engines powered with gasoline contained 0-20% BuOH and diesel fuel contained 0-50% BuOH were evaluated. Tests showed that BuOH can be used as a gasoline or diesel fuel supplement in amounts of <20% and <40%, respectively, without significantly affecting unmodified engine performance. BuOH slightly decreased the octane rating of a blend of 20% BuOH in gasoline but in diesel fuel <40% BuOH had no detectable effect on the ignition of the fuel blend [217]. Diesel engines can be powered with 25-75% of a Bu-alcohol and 25-75% of vegetable oil mixtures which were normally liquids under operating conditions. A fuel mixture composed of 50% corn oil and 50% n-BuOH was used as the fuel for 2 tractors when the engine performance in both tractors and the behaviour of the fuel was entirely satisfactory, the engine running smoothly and evenly without significant smoke or odor, with quick acceleration and smooth idling. The above blend could be mixed in any proportion to no. 2 diesel oil without significant change in engine performance [218]. A diesel precombustion chamber engine powered with 70% BuOH-30% diesel fuel had, at an av. 5.9-bar pressure, an ignition delay of operation which was only 10% more than that when operated with diesel alone. The maximum pressure increase during the operation remained higher in both combustion chambers in operation with 70 vol.% BuOH than in operation with diesel alone. There is high potential of improvement of the exhaust gas quality with BuOH-diesel fuel mixtures, especially with regard to smoke value, particulate emissions, and nitrogen oxides. The engine performance under such conditions is similar to that with diesel fuel alone. The starting problem of the engine powered with diesel-BuOH mixture is avoided by using an electrically heated spark plug which maintains ~1000 °C in the precombustion chamber. More than 200 h of satisfactory operation was attained in a BuOH — diesel mixture powered engine [219]. Substitute diesel fuel compositions consist of gas oil (b. 167-359 °C) 20-55, a 75:25 (wt.) mixture BuOH-Me2CO 30-40, fatty acid esters 15-40 wt.%. Thus, substitute diesel fuel composition containing gas oil 20, BuOH-Me2CO mixture 40, and gas oil and BuOH-Me2CO mixture 40 wt.% had cetane no. 40.6 and resulted in normal tractor operation for 50 h [220].

Coupled biodiesel and ABE production technology proceeds by extraction of the ABE contain­ing broth with biodiesel oils forms a mixture which can directly be applied as fuel for diesel en­gines [224]. Using soybean-derived biodiesel as the extractant with an aqueous phase volume ratio of 1:1, butanol recovery ranged from 45 to 51% at initial butanol concentration of 150 and 225 mM, respectively. Using biodiesel-derived glycerol as feedstock for butanol production, the production of a biodiesel/butanol fuel blend could be a fully integrated process within a biodie­sel facility [225]. The presence of surfactants had important influence on the amount of extract­ed butanol with biodiesel oil prepared from waste cooking oil [226]. This extraction was integrated into the fermentation process, when large quantity of gas (H2 and CO2), was released and the produced butanol and acetone were brought into extractant phase. Surfactants de­creased the tension of gas-liquid interface and made the large bubble break down, therefore, the releasing gas passed through the extractant phase in form of small bubbles. The mass transfer rate of products from the aqueous phase to the extractant phase was enhanced and the balance time was shortened accordingly by addition of surfactants, consequently, the fermentation pro­ductivity was improved. Using waste cooking oil derived biodiesel as extractant the butanol concentration in the extractant phase was increased by 21.2% as compared to the control, while the concentration of surfactant (Tween-80) in culture medium was 0.140% (w/v). Under these conditions, gross solvent productivity was increased by 16.5% [226]. When the biodiesel de­rived from crude palm oil was used as extractant, the fuel properties of the biodiesel-ABE mix­ture were comparable to that of No.2 diesel, but its cetane no. and the boiling point of the 90% fraction were higher [227]. Biodiesels prepared from some waste oils proved to be somewhat toxic toward C. Acetobutylicum. Under this condition, the butanol concentration in the biodie­sel phase also reached a level of 6.44 g L-1 [228].

Biomass components have a great potential as building block intermediates. Indeed, sugars, vegetable oils and terpenes can be employed for synthesizing products with a high added value, such as chemicals and fine chemicals. There are hundreds of different processes to ob­tain chemicals from biomass origin building blocks. This chapter deals with those processes involving hydrotreating for the removal of oxygen. In the first part of this section, some ex­amples of significant hydrogenolysis reactions in the valorization of platform chemicals will be given, while the last part will be focused on one of the most studied hydrogenolysis proc — cesses; the conversion of glycerol into propanediols (PDO).

As it has been previously stated, platform chemicals coming from biomass usually contain higher O/C ratio than most commodity chemicals; thus main valorization processes require the removal of oxygen. One widely used process to remove oxygen is hydrogenolysis. Hy — drogenolysis is a type of reduction that involves chemical bond dissociation in an organic substrate and simultaneous addition of hydrogen to the resulting molecular fragments [33]. Therefore, reaction for oxygen removal involves the cleavage of the C-O bond and the addi­tion of hydrogen (oxygen is removed in the form of H2O). This is a significant aspect, be­cause, in those processes where the starting and target molecule have the same number of carbons it is important to use catalytic systems that present high activity in C-O bond hydro — genolysis while low activity in C-C bond hydrogenolysis.

At the whole world, the solids wastes from different sources are generating negative environ­mental impact to the nature, the biodiversity and life in the planet. This is caused by the inappropriate disposition of wastes, the increase of population, the processes of industrial transformation, agroindustrial and life habits of people [3]. At the present time, one charac­teristic of the society is the increase unbridled of the production and accumulation of solids wastes, which are generated without a solution to its final disposition. In the most of cases, this produced an inappropriate final disposition, an increase in the environment deterioration (air, surface water and groundwater, soil, landscape), problems in the public health and personal security [2].

The characteristics of solids wastes changed in function of the main activity (industry, trade, tourism and others), the habits of the population, type of fed, consumption models, environ­ment conditions and others. The solids wastes can be classified according to: the source (domestic activities, institutional, commercial, industry, farming, municipal services and construction); the constitution (recyclable material and non-recyclable) and grade of danger (commons and dangerous).

The present chapter shows the energetic potential of the solid organic wastes generated in Colombia and its capacity to produce biohydrogen by anaerobic fermentation; additionally is presented a research carried out at the Laboratory of Agricultural Mechanization of the National University of Colombia in Medellm between the years 2009 and 2012, which main aim was determinate the initial feasibility to generate biohydrogen from urban organics wastes and to establish some conditions to operate a bioreactor type batch.

Because butanol has a higher boiling point than water, therefore, distillation is not suitable for butanol recovery. Other processes such as adsorption, pervaporation, membrane pertrac — tion, reverse osmosis and gas stripping have been developed to improve recovery perform­ance and reduce costs (Oudshoorn et al., 2009; Ezeji et al., 2004b).

4.1. Adsorption process

Adsorption is the technology operating easily for the butanol separation. Butanol can be ad — sorpted by the adsorbents in the fermenter and then the butanol was obtained by desorp­tion. A variety of materials can be used as adsorbents for butanol recovery and silicalite is the common one used (Qureshi et al., 2005b; Ezeji et al., 2007). Silicalite is a form of silica with a zeolite-like structure and hydrophobic properties, it can selectively adsorb small or­ganic molecule like C1-C5 alcohols from dilute aqueous solutions (Zheng et al., 2009). How­ever, adsorption separation process is not suitable on an industrial or semi-technical scale because the capacity of adsorbent is very low.