te. When the TEOS was added at high rates, the mean size and standard deviation of the silicacoated nanoparticles were 653 nm and 56 nm, respectively, based on total of 226 nanoparticles (Figure 2(c)). A TEM image of SiMNPs obtained after dropwise addition of the TEOS is shown in Figure 2(d). The mean size of SiMNPs was 3.5 nm with a standard deviation of 1.6 nm based on 629 nanoparticles. The mean size of SiMNPs aggregates was 2.2 µm with a standard deviation of 0.3 µm. Acid functionalization changed the dispersability of the nanoparticles because larger aggregates were found after this step. The material functionalized with PFS acid groups showed a bimodal distribution; the smaller distribution could be attributed to silica-coated

Figure 2. TEM images of (a) cobalt iron oxide (CoFe2O4) nanoparticles; (b) CoFe2O4 aggregates of nanoparticles; (c) SiMNPs synthesized at high addition rates of TEOS; and (d) SiMNPs synthesized at low addition rates of TEOS.

iron oxide that escaped the functionalization. The smaller material had a mean of 2.3 (±0.1) µm, which is the same as that obtained for silica-coated iron oxide aggregates. The functionalization with PFS had promoted the formation of larger aggregates with a mean size of 74.0 (±4.1) µm. AS nanoparticles also showed a bimodal distribution with mean diameter of 13. (±2.5) µm for the larger aggregates and mean diameter of 0.8 (±0.1) µm for the smaller aggregates.

FTIR spectra for acid-functionalized nanoparticles are shown in Figure 3. The broad band in the wavelength 500 - 600 cm–1 appears in all the spectra. This band has been associated to Fe-O bonds [44,45]. The bands at 820 and 960 cm–1 in SiMNPs, AS, and PFS spectra have been attributed to the stretching vibrations of Si–O–Si and Si–O–H groups [46]. Similarly, a broad band at 1120 cm–1 from the Si–O bond appears in all these spectra [47]. The peak at 1370 cm–1 has been assigned to the O=S=O stretching vibrations [48-51]. The peaks at 1425 cm–1 in the spectra of AS and PFS nanoparticles have been attributed to undissociated SO3H groups [48,50]. The peaks at 1080, 1150, and 1190 cm–1 in the spectrum of PFS nanoparticles have been assigned to the C-F bond [52-54]. The results from FTIR analysis showed that all of the nanoparticles had the functional groups expected.

XPS profiles for PFS and AS functionalized nanoparticles were used to analyze their atomic concentration (Figure 4). A peak at 104 - 108 eV was observed for both acid-functionalized nanoparticles. This peak is associated with silicon bonds from the silica layer that covers the CoFe2O4 nanoparticles [51,55]. The peaks associated with carbon, silicon, and sulfur were observed in the AS spectrum. The surface composition of both PFS and AS nanoparticles is shown in Table 1. For AS, the C/Si theoretical value was calculated from the molecular formula of the propylsulfonic acid attached to the silica surface as 3. The C/Si ratio of 3.4 from the XPS experiment agrees with the theoretical value. The C/S ratio of 26.6 was much larger than the theoretical value, which is also 3.0. The large amount of carbon with respect to sulfur suggests that the loss of the sulfonic acid groups might have occurred during the synthesis procedure. The peaks that correspond to F, O, C, S, and Si were found in the XPS profiles for PFS nanoparticles. As in the AS case, the C/S ratio of 10.4 was greater than the theoreticcal value of 3.0. The large amount of carbon also suggests the loss of the sulfonic acid group during the synthesis procedure; however, the ratio of fluorine to carbon (0.8) was lower than expected (3.0), which indicates that the acid sulfonic group leached out along with other fluorine atoms of the HFP sultone moieties.

The thermal decomposition scans of the nanoparticles before and after acid functionalization were used to evaluate the thermal stability of the nanoparticles and

Figure 3. FTIR spectra of CoFe2O4, SiMNPs, AS-SiMNPs, and PFS-SiMNPs after synthesis. All the samples were dried at 45˚C under vacuum before taking the spectra.

Figure 4. XPS profiles of PFS and AS nanoparticles.

Table 1. Atomic concentration and organic load of the acidfunctionalized nanoparticles.

their total organic loading (Figure 5). The DTA curves show that the drying step occurred before the samples reached 150˚C. SiMNPs had absorbed more water than the acid-functionalized nanoparticles. The moisture content of SiMNPs, AS, and PFS nanoparticles was 3.9%, 2.6%, and 2.6%, respectively. The incorporation of alkylsulfonic and perfluoroalkylsulfonic acids groups increased

Figure 5. TG/DTA profiles of SiMNP, PFS, and AS functionalized nanoparticles.

the hydrophobicity of the nanoparticles. Similar results were found during functionalization of silica with perfluoroalkylsulfonic acid [51]. An increase in the hydrophobic properties of the nanoparticles can affect their dispersability in the biomass slurry and could promote aggregation of nanoparticles, thus reducing the available area for the reaction. The TGA profile of SiMNPs showed a total weight loss of 4.2% during heating from 150˚C to 800˚C. The weight loss between 150˚C and 400˚C was 1.7% and it has been attributed to bound water. The weight loss at temperatures higher than 400˚C was measured as 2.5% and it has been explained as the weight lost due to a ferrite crystallization process [44,56]. PFS and AS nanoparticles showed similar thermal stabilities; when heated to 450˚C, PFS nanoparticles lost 22% of their weight, or about 60% of their original organic content; AS lost 10% of their weight, or about 55% of their original organic content. The first derivative profile for PFS nanoparticles shows four peaks at 225˚C, 292˚C, 417˚C, and 492˚C, which indicates that the perfluoroalkylsulfonic acid group splits into smaller moieties. The sulfonic acid groups and the CF2-chains decompose at different temperatures [51]. The first derivative profile for AS nanoparticles shows a main peak at 437˚C that corresponds to the alkyl-sulfonic acid. The left shoulder on this peak could be attributed to mercapto-propyl groups that were not completely oxidized to the sulfonic acid [57]. The total organic content was counted as the total mass lost between 150˚C and 600˚C; these values can be seen in Table 1 along with the number of acid sites on the nanoparticles. The hydronium ion concentration per mass of catalyst was calculated from the values of total organic content on the nanoparticles and the surface sulfur concentration.

3.2. Biomass Pretreatment

Table 2 shows the biomass composition on a dry mass basis, and the total hemicellulose sugars recovered at 80˚C and 160˚C from wheat straw are shown in Figure 6.

At temperatures as low as 80˚C, solubilization of hemicellulose rather than cellulose hydrolysis is expected [8]. The amount of sugars solubilized (24.0% ± 1.1%) at 80˚C in the presence of PFS nanoparticles was greater

Figure 6. Wheat straw hemicelluloses recovered after pretreatment with PFS and AS acid-functionalized nanoparticles. The left columns correspond to results of the experiments carried out at 80˚C for 24 h with a catalyst load of 1.5% and biomass load of 2.5%. Results of the experiments carried out at 160˚C for 2 h with a catalyst load of 0.25% and biomass load of 2.5% are shown in the right columns. The error bars represent the standard errors of two replicate experiments.

Table 2. Whole biomass composition.

than the sugars released from the control (7.7% ± 0.8%); but the amount of hemicelluloses recovered with AS (9.1% ± 1.7%) was not significantly different from the control. PFS experiments used 2 mmol H+/L based on the values obtained for organic content and sulfur atomic concentration; this is equivalent to using 0.02% (w/w) sulfuric acid solutions. By a similar calculation, the load of AS nanoparticles provides 1.02 mmol H+/L, which is equivalent to using 0.01% (w/w) sulfuric acid solutions. Hemicellulose yield of 20% can be considered moderate because low temperature and acidity levels were used. Complete (100%) hemicellulose solubilization requires temperatures higher than 160˚C using 1% sulfuric acid [8]. Previous work reported that complete xylan solubilization was reached at 130˚C using 3% (w/w) sulfuric acid solutions for 4 h [58]. Higher temperatures have been used (180˚C - 220˚C) for hemicellulose hydrolysis with acid concentrations between 0% to 0.1% sulfuric acid (w/w) [59-61].

Very few monosaccharides were obtained after pretreatment of biomass with PFS or AS nanoparticles. PFS and AS solubilized the xylan fraction into its monomeric form at very low levels: 3.5% ± 0.1%, 1.0% ± 0.2% at 80˚C. Xylose monomer units were not detected in the solution for the control experiment. In the experiments carried out at 160˚C, only 0.3% and 1.2% of the original xylan was found in the solution as xylose for PFS and AS nanoparticles, respectively. From the initial arabinan, 49.5% ± 2.1% and 57.9% ± 11.2% was found as arabinose monomer in the pretreatment liquor of AS and PFS nanoparticles. In the control experiment, 36.0% ± 1.6% of arabinan was hydrolyzed to the monomeric form. These results agree with previous findings in which acidfunctionalized amorphous carbon was used to hydrolyze cellulose, the yield of monomers was only 4%, and most of the sugars obtained were in oligomeric form [49]. About 30% monomers yield was obtained from loblolly pine hemicelluloses using 1% sulfuric acid at 150˚C and 60% at 200˚C [62]. At 160˚C, the low percentage of xylose found in the solution could be explained by degradation of the xylose units to other products such as furfural and formic acid [63-67]; in this experiment, 16.0% ± 0.2%, 20.0% ± 1.3%, and 15.7% ± 1.2% of the original xylan was found in solution as furfural for AS, PFS, and control, respectively (Figure 7). Formic acid was also found in the pretreatment liquor, but it may have been a degradation product of either cellulose or xyloglucans. The results of this paper agree with the hemicellulose hydrolysis models proposed in the literature [59,68-71]. These models suggest that either hemicelluloses or xylan are made of two different fractions that are hydrolyzed at different rates, one slow and one fast. In the present work, the fast-hydrolyzing fraction of hemicellulose is degraded to decomposition products, and that is why very few

Figure 7. Wheat straw hemicelluloses solubilized after pretreatment with PFS and AS acid-functionalized nanoparticles. The pretreatment was carried out at 160˚C for 2 h with a catalyst load of 0.25% and biomass load of 2.5%. The error bars represent the standard errors of two replicate experiments.

monomers were found in the pretreatment liquor. The slow-hydrolyzing fraction is found in solution as oligomers or remains in the non-hydrolyzed solid fraction.

In the experiments performed at high temperature, a 0.25% (w/w) catalyst load was used; the PFS and AS nanoparticle dispersions are equivalent to using 0.003% and 0.002% (w/w) sulfuric acid solutions, respectively. The total hemicellulose (oligosaccharides and monosaccharides) recovered from wheat straw at 160˚C reached 46.3% ± 0.4% and 45.0% ± 1.2% using PFS and AS, respectively. The control experiment recovered 35.0% ± 1.8% of the original hemicellulose in the wheat straw sample. Most of the sugars found in solution came from the xylan fraction 38.6% ± 0.9% and 40.4% ± 0.2% using AS and PFS, respectively. Hemicellulose solubilization could have been affected by the buffering effect of the biomass, which has been said could reduce 1% (w/w) acid concentration in half [8]. Low acid levels are frequently used along with high temperatures; hemicellulose conversions of up to 97% (w/w) have been reported when using 0.1% (w/w) sulfuric acid at 180˚C, but still more than 50% was oligomers of hemicellulose [72]. In a another study, 96% (w/w) conversion of hemicellulose was obtained with 0.8% (w/v) sulfuric acid from yellow poplar sawdust at 175˚C [61]. PFS and AS nanoparticles converted 66.3% ± 1.7% and 61.0% ± 1.2% of the hemicelluloses at acid levels 50 and 400 fold lower than those acid sulfuric solutions at 160˚C.

PFS and AS nanoparticles gave similar amounts of recovered hemicellulose; and both catalysts gave more sugars than the control. A better performance of PFS nanoparticles was expected because perfluorosulfonic acids are known as superacids and can be more acidic than sulfuric acid [27,48,73]. The acid strength of these acids has been explained by the electron-withdrawing properties of the Fluorine atoms [74], but the leveling effect of water could have an effect on the catalytic activity of PFS nanoparticles [74]. Nanoparticles functionalized with alkyl-sulfonic acid show a significant improvement on hemicelluloses solubilization when using 160˚C instead of 80˚C, although the acid loading for this catalyst was relatively low. Similar acid capacity was reported previously for propyl-sulfonic acid-functionalized materials [38]. Low catalytic activity also could have been a consequence of the low water affinity observed for this catalyst in TGA experiments. These results agree with the findings of Van Rhijn et al. [75], who synthesized mesoporous silicas functionalized with propylsulfonic acid and obtained moisture contents of less than 1%. The aggregation of both PFS and AS nanoparticles also could have an effect on their capacity to hydrolyze hemicelluloses due to a considerable reduction of their surface area; however, the attachment of PFS and AS acid functions could have stabilized the acid and allowed moderate levels of hemicellulose hydrolysis compared with sulfuric acid solutions of similar acid strength.

4. Conclusion

Acid-functionalized magnetic nanoparticles were synthesized as catalysts for pretreatment and hydrolysis of lignocellulosic feedstock. TEM images confirmed that the synthesis of cobalt spinel ferrite yielded particles with diameters less than 10 nm. Coating the cobalt spinel ferrite particles did not significantly change the size distribution of the nanoparticles, although some particles agglomerated upon coating. FTIR and XPS spectra confirmed the covalent bonding between the magnetic core and the silica layer and the presence of sulfonic acid groups following functionalization. Analysis of sugars in the liquid fraction after pretreatment revealed a significant amount of oligosaccharides compared with the hydro-thermolysis when using PFS or AS nanoparticles at 160˚C. The acid-functionalized nanoparticles broke down the non-soluble polysaccharides into oligomeric forms.

5. Acknowledgements

This project is supported by the NSF award CNET- 1033538 and the NSF EPSCoR Kansas Center for Solar Energy Research. This material is also based upon work supported by NSF Grant # 0903701: “Integrating the Socioeconomic, Technical, and Agricultural Aspects of Renewable and Sustainable Biorefining Program awarded to Kansas State University.” Contribution number 12- 367-J from the Kansas Agricultural Experiment Station.


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