A microwave assisted green process has been developed for production of sugars through liquefying holocellulose catalyzed with sulfonated char derived from the lignin-rich residue produced during pretreatment of lignocellulose. Various reaction parameters including the hydrolysis temperature, hydrolysis time, catalyst content, and the ratio of water to feedstock were evaluated. The maximum sugars yield of 82.6% (based on the dry mass of holocellulose) was obtained under the optimum reaction conditions. The sulfonated char showed superior catalytic performance to that of dilute sulfuric acid in converting holocellulose into sugars under microwave irradiation.
Hydrolysis of holocellulose (cellulose and hemicellulose) is a key technology to obtain the sugars which are pivotal platform compounds for a range of industrially important chemicals, such as ethanol, butanol, and hydrocarbon [
The move toward more environmentally benign processes has stimulated the development of solid acid catalysts, which are nontoxic and easy in separation and have a high strength of acidity. Solid acid catalysts, including inorganic oxides [
In this study, we developed an environmentally benign hydrolysis approach for the saccharification of holocellulose. As shown in Figure
Schematic route of the holocellulose hydrolysis catalyzed by sulfonated char derived from lignin-rich pretreatment residues.
The hybrid poplar was collected from West Virginia University Experimental Forest. The holocellulose was separated from raw poplar particles and the composition of holocellulose was analyzed (mass percent: cellulose 63.2%, xylan 32.7%, and others 4.1%). Sulfuric acid and other chemicals (if applicable) were purchased from Sigma-Aldrich, Inc.
According to the typical procedure [
The specific surface area of the sulfonated char was obtained by Brunauer Emmett Teller surface analyzer (BET, ASAP2020M, Micromeritics). Scanning electron microscopy (SEM) analysis was conducted on a Hitachi 3400-1 electronic microscope working at 30 kv. FTIR spectrum was recorded on a Fourier transform infrared spectroscopy (FTIR, I80, Nicolet) using the standard KBr disc method. The samples were scanned between 400 and 4000 cm−1 with a resolution of 0.4 cm−1. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Focus Advance diffractometer using Cu K
The holocellulose was separated from raw poplar (
The aqueous filtrate was analyzed using the Dionex Capillary Ion Chromatography System (ICS5000 Thermo Fisher) with a pulsed amperometric detector and a CarboPac PA-10 (4 mm) column. NaOH (18 mM) aqueous solution was used as elution solvent at a flow rate of 1.0 mL min−1. The yield of total sugars (based on the dry mass of holocellulose) was calculated using the following equation: Total sugars yield (%) = [amount (g) of oligose + amount (g) of monose]/amount (g) of holocellulose.
The catalytic performance of the sulfonated char was examined by the microwave assisted hydrolysis of holocellulose under various conditions. The experimental parameters were evaluated as follows: hydrolysis temperature from 373 K to 403 K, reaction time from 20 min to 100 min, catalyst/substrate (holocellulose) ratio from 0.25 to 1.5, and liquid/solid (catalyst + holocellulose) ratio from 2.5 to 12.5. The reusability of the sulfonated char was also studied and the respective catalytic performance was compared with that of dilute sulfuric acid.
The BET analysis showed that the specific surface area of sulfonated char was 2 m2 g−1, which is in the range of a typical lignin carbon processed at similar temperatures [
The SEM image (a), FTIR spectrum (b), and XRD pattern (c) for the prepared sulfonated char.
As shown in Figure
Effect of reaction temperature on the sulfonated char catalyzed hydrolysis of holocellulose. Reaction conditions: 0.2 g holocellulose, 0.2 g catalyst, 3 mL H2O, and microwave irradiation (300 W) for 60 min.
The effects of catalyst/substrate (holocellulose) ratio and liquid/solid (catalyst + holocellulose) ratio were evaluated using the optimal reaction temperature and time. As shown in Figure
(a) Effect of catalyst/substrate ratio on the sulfonated char catalyzed hydrolysis of holocellulose. Reaction conditions: 0.2 g holocellulose, 3 mL H2O, 393 K, and microwave irradiation (300 W) for 60 min. (b) Effect of liquid/solid ratio on the sulfonated char catalyzed hydrolysis of holocellulose. Reaction conditions: 0.2 g holocellulose, 0.2 g catalyst, 393 K, and microwave irradiation (300 W) for 60 min.
To evaluate reusability, sulfonated char samples (reused up to three times) were investigated by reacting a mixture of 0.2 g catalyst, 0.2 g holocellulose, and 3 mL water under 300 W microwave irradiation for 60 min. The unreacted sulfonated char and those recovered once, twice, and 3 times were designated SC0, SC1, SC2, and SC3, respectively. Additionally, the catalytic performance of the sulfonated char was compared with that of dilute sulfuric acid. After the first run at 393 K for 60 min, the sulfonated char was recovered from the hydrolytic solution by filtration followed by washing with distilled water. The recovered catalyst was reused by directly mixing with fresh holocellulose and water. As shown in Table
Comparison of the catalytic performance of reused sulfonated char and dilute sulfuric acid.
Yield/% | SC0a | SC1a | SC2a | SC3a |
|
|
---|---|---|---|---|---|---|
Monose | 26.8 | 23.9 | 20.4 | 16.1 | 12.2 | 4.9 |
Oligose | 55.8 | 54.6 | 52.5 | 49.1 | 10.3 | 27.2 |
Sugars | 82.6 | 78.5 | 72.9 | 65.2 | 22.5 | 31.7 |
An environmentally friendly hydrolysis process alternative to dilute sulfuric acid was developed to efficiently convert holocellulose into useful sugars under microwave irradiation. The catalyst, a sulfonated char, was produced from lignin-rich residues from pretreatment of lignocellulosic biomass. The catalyst showed excellent catalytic performance in hydrolysis of holocellulose and low toxicity for microbial activity, which is different from sulfuric acid. This approach may also be exploited for the direct hydrolysis of lignocellulosic biomass into sugars and other useful chemicals.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by National Nonprofit Institute Research Grant of CAFINT2013C05.