Conversion processes, cellulose hydrolysis

Abstract Polyfunctionality of carbohydrates and their low solubility in conventional organic solvents make rather complex their conversion to higher value added chemicals. Therefore, innovative processes are now strongly needed in order to increase the selectivity of these reactions. Here, we report an overview of the different heterogeneously-catalyzed processes described in the literature. In particular, hydrolysis, dehydration, oxidation, esterification, and etherification of carbohydrates are presented. We shall discuss the main structural parameters that need to be controlled and that permit the conversion of carbohydrates to bioproducts with good selectivity. The conversion of monosaccharides and disaccharides over solid catalysts, as well as recent advances in the heterogeneously-catalyzed conversion of cellulose, will be presented. 

Figure 1. The centrality of cellulose hydrolysis in the conversion processes...

The applications of enzymes can be classified into three major categories industrial enzymes, analytical enzymes, and medical enzymes. In this chapter, we review several industrial processes, utilizing industrial enzymes such as starch conversion and enzymatic hydrolysis of celluloses. Before we discuss the enzymatic hydrolysis of starch and cellulose, we review the organic chemistry of carbohydrates. 

A great amount of time, money and effort is being devoted to the use of cellulose as a feedstock for the production of ethanol. The studies incorporate chemical or enzymatic conversion of the cellulose to glucose and the conversion of this to ethanol with yeast (Saccharomyces) or bacteria (Zymomonas). However, a third process is presently under development at Massachusetts Institute of Technology whereby the direct conversion of cellulose to ethanol is being attempted without a separate hydrolysis step. ... 

At high concentrations, corrosion-resistant reactors and an effective acid recovery process are needed, raising the cost of the intermediate glucose. Dilute acid treatments minimize these problems, but a number of kinetic models indicate that the maximum conversion of cellulose to glucose under these conditions is 65 to 70 percent because subsequent degradation reactions of the glucose to HMF and lev-ulinic acid take place. The modem biorefinery is learning to exploit this reaction manifold, because these decomposition products can be manufactured as the primary product of polysaccharide hydrolysis (see below). 

Any treatment of wood must take account of the differing accessibilities and reactivities of the principal wood constituents. Further, any chemical or microbial method of breaking down wood has to devise conversion pathways for cellulose, the hemicelluloses and lignin, and if necessary consider ways of isolating the individual reaction products so that they can be processed separately. Hydrolysis has proved to be a most effective method of opening up the wood structure for subsequent treatments. The expression hydrolysis of wood is used rather loosely. It is not technically correct since the reactions affect primarily the carbohydrate fraction of wood. Lignin is largely unaffected. 

A continuous counter-current reactor system has shown great promise as a process reactor in the dilute acid hydrolysis of cellulose. However, the findings on this unique reactor system have been limited to the theoretical aspects and the proof-of-concept laboratory experiments. It has to be developed into an upscale process reactor before it is adopted into the biomass conversion process. To this end, a pilot-scale process study is being conducted at NREL. This is only the first step. To be noted here is that reactors of similar design are being used in industry it took years of developmental work, however, before they were put into commercial service. It would probably take about the same degree of investment in this case. In addition to the reactor issue, there are other important issues that need to be addressed in the dilute acid process. 

Many factors influence the reactivity and digestibility of the cellulose fractions of lignocellulose materials. These factors include Hgnin and hemicellulose content, crystalhnity of cellulose, and the porosity of the biomass materials. Pretreatment of Hgnocellulosic materials prior to utiHzation is a necessary element in biomass-to-ethanol conversion processes. The objective of the pretreatment is to render biomass materials more accessible to either chemical or enzymatic hydrolysis for efficient product generation. The goals of the pretreatment are ... 

Enzyme activity loss because of non-productive adsorption on lignin surface was identified as one of the important factors to decrease enzyme effectiveness, and the effect of surfactants and non-catalytic protein on the enzymatic hydrolysis has been extensively studied to increase the enzymatic hydrolysis of cellulose into fermentable sugars [7, 9 19]. The reported study showed that the non-ionic surfactant poly(oxyethylene)2o-sorbitan-monooleate (Tween 80) enhanced the enzymatic hydrolysis rate and extent of newspaper cellulose by 33 and 14%, respectively [20]. It was also found that 30% more FPU cellulase activity remained in solution, and about three times more recoverable FPU activity could be recycled with the presence of Tween 80. Tween 80 enhanced enzymatic hydrolysis yields for steam-exploded poplar wood by 20% in the simultaneous saccharification and fermentation (SSF) process [21]. Helle et al. [22] reported that hydrolysis yield increased by as much as a factor of 7, whereas enzyme adsorption on cellulose decreased because of the addition of Tween 80. With the presence of poly(oxyethylene)2o-sorbitan-monolaurate (Tween 20) and Tween 80, the conversions of cellulose and xylan in lime-pretreated com stover were increased by 42 and 40%, respectively [23]. Wu and Ju [24] showed that the addition of Tween 20 or Tween 80 to waste newsprint could increase cellulose conversion by about 50% with the saving of cellulase loading of 80%. With the addition of non-ionic, anionic, and cationic surfactants to the hydrolysis of cellulose (Avicel, tissue paper, and reclaimed paper), Ooshima et al. [25] subsequently found that Tween 20 was the most effective for the enhancement of cellulose conversion, and anionic surfactants did not have any effect on cellulose hydrolysis. With the addition of Tween 20 in the SSF process for... 

The bioconversion of cellulose into ethanol with conventional methods is usually achieved in two steps first being the enzymatic saccharification of the polysaccharide to monosaccharide and secondly the bioconversion of monosaccharides into ethanol. A combination of enzymatic hydrolysis and ethanol production in the same reactor has been attempted using different cellulases and ethanol producing microbial species to improve process efficiency [46-53]. The production of ethanol from cellulose in a simultaneous saccharification and biological conversion process alleviates the problem of end product inhibition, since glucose does not accumulate in this system and is converted to ethanol immediately following saccharification [46]. 

The separation of ethanol from water can also be effected by freezing. The effect of ethanol concentration on the freezing point is given in Table 5.4. Thus, a Uter of fermented brew with 12.5% ethanol by volume was completely frozen and then allowed to thaw. The first 500 mL of solution was 17% ethanol. When this 500 mL solution was frozen and allowed to thaw, again the first 250 mL was 23% ethanol. Various freeze-thaw cycles can thus concentrate ethanol. Another process which has been studied extensively for more than 70 years is the conversion of cellulose from wood and straw to glucose by enzyme or by acid hydrolysis. 

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