Recent Advances in Dilute Acid Process — Different Acids
The most widely studied and industrially tested acid in the direct saccharification of biomass is sulfuric acid. This may be due to its
stability and availability. Even though direct acid hydrolysis is the simpler route, there are serious limitations such as poor sugar yields due to incomplete hydrolysis of cellulose and formation of inhibitory compounds such as furfural, 5-hydroxymethylfurfural and lev — ulinic acid due to further degradation of sugars. One approach for improving the diluted acid process is the reactor design approach, and the other possibility is changing the acid used. Interestingly, recent studies have shown that further decomposition of sugars to inhibitory compounds like HMF can be controlled by changing the acid [40]. A sound understanding of cid-catalyzed decomposition of hexoses and pentoses in aqueous acid media is the key to control the formation of the undesired products.
Earliest mechanistic studies on acid-catalyzed decomposition of hexose by Van Dam [41], Antal [42], and Kuster [43] were based on the kinetics of the formation of products. These experiments suggested that acid-catalyzed dehydration of hexoses like glucose and fructose could occur through two possible pathways as shown in Figure 7.3.
The use of other mineral acids like H3PO4 [44, 45] and HCl [46] generally gives saccharification products similar to sulfuric acid. However, studies using small organic acids like formic [47], succinic, acetic [48], maleic [48], and oxalic [49] acids have shown that organic acids have certain advantages like reduction in side reactions. In these studies, Ladisch et al. have shown [48] that maleic acid hydrolyzes microcrystalline cellulose Avicel as effectively as dilute
Figure 7.3 Two possible pathways for the acid-catalyzed dehydration of hexoses to
sulfuric acid, but with minimal glucose degradation. Furthermore, maleic acid was found to be superior to other carboxylic acids like succinic and acetic acid and gives higher yields of glucose that is more easily fermented as a result of lower concentrations of degradation products [48]. Amarasekara et al. have tested a series of alkyl and aryl sulfonic acids for the hydrolysis of cellulose [50]. In this study catalytic activities of eight alkyl/aryl sulfonic acids shown in Figure 7.4 were compared with sulfuric acid of the same acid strength (0.0321mole H+ ion /L) for hydrolysis of Sigmacell cellulose (DP ~ 450) in the 140-190°C temperature range by measuring total reducing sugar (TRS) and glucose produced.
The changes in % yields of TRS and glucose produced during the hydrolysis of Sigmacell cellulose (DP ~ 450) in aq. sulfuric acid and eight alkyl/aryl sulfonic acids at different temperatures are shown in Figures 7.5 and 7.6, respectively. Cellulose samples hydrolyzed at 160°C for 3 hr in aqueous p-toluenesulfonic acid, 2-naphthalene — sulfonic acid, and 4-biphenylsulfonic acid mediums produced TRS yields of 28.0, 25.4, and 30.3%, respectively, when compared to 21.7% TRS produced in aqueous sulfuric acid medium. The first order rate constants at 160°C in different acid mediums correlated with octa — nol/water distribution coefficient log D of these acids, except in the case of highly hydrophobic 4-dodceylbenzenesulfonic acid. In the series of sulfonic acids studied, 4-biphenylsulfonic acid appears to
Figure 7.4 Sulfuric acid (SA, 1), methanesulfonic acid (MSA, 2), trifluromethanesulfonic acid (TFMSA, 3), p-toluenesulfonic acid (PTSA, 4), 2-naphthalenesulfonic acid (2-NSA, 5), 10-champhorsulfonic acid (10-CSA, 6), 4-biphenylsulfonic acid (4-BPSA, 7), 4-dodecylbenzenesulfonic acid (4-DBSA, 8), and 1,5-naphthalenedisulfonic acid (1,5-NDSA, 9) used in the cellulose hydrolysis study [50].
be the best cellulose hydrolysis catalyst [50]. These results showed that small aryl sulfonic acid like p-toluenesulfonic acid is a better catalyst than sulfuric acids, and this was explained in terms of interactions between p-toluenesulfonic acid and carbohydrates in water. Later this hypothesis was further supported by 13C NMR studies on cellulose model compound cellobiose and p-toluenesulfonic acid in D2O [51], and cellobiose hydrolysis studies [52].
The same group has extended the studies on the use of aqueous p-toluenesulfonic acid to real biomass forms such as corn stover [40] and switchgrass [53], where they have been able to demonstrate that p-toluenesulfonic acid is a better catalyst than sulfuric acid of the same acid concentration. In one series of experiments, single-step pretreatment saccharification of corn stover was investigated in aqueous p-toluenesulfonic and sulfuric acid media.
Total reducing sugar (TRS, pmol, glucose equivalent) and glucose (pmol) produced during the pretreatment saccharification of
corn stover in dilute aqueous sulfuric acid (SA), and p-toluenesul — fonic acid (PTSA) media as a function of time at different temperatures are shown in Figures 7.7 and 7.8 respectively.
In these corn stover saccharification experiments, the highest catalytic activity enhancement was seen around 150°C; for example, 100 mg corn stover heated at 150°C for 1 h in 0.100 M H+ aqueous sulfuric acid produced 64 pmol of total reducing sugars (TRS), whereas the sample heated in 0.100 M H+ p-toluenesulfonic acid produced 165 pmol of TRS under identical conditions. Glucose yield also showed a similar trend as aqueous sulfuric acid and p-toluene sulfonic acid media produced 29 and 35 pmol of glucose respectively after 2.5 h [40]. In conclusion, dilute aqueous solution of p-toluenesulfonic acid was shown to be a better catalyst than aqueous sulfuric acid of the same H+ ion concentration for singlestep pretreatment saccharification of corn stover at moderate temperatures and pressures.
SA, 140 C SA, 150 C SA, 160 C SA, 170 C PTSA,140 C PTSA, 150 C PTSA, 160 C PTSA, 170 C
SA, 140 C SA, 150 C SA, 160 C SA, 170 C PTSA, 140 C PTSA, 150 C PTSA, 160 C PTSA, 170 C