Adsorption of fermentation inhibitors from lignocellulosic biomass hydrolyzates for improved ethanol yield and value-added product recovery
Introduction
The growing energy need, limited reserve of crude oil and rising green house emissions have brought attention to renewable biofuels. For example, United States and several other countries have set goals to partially replace transportation oil with biomass-derived ethanol [1], [2]. Existing bio-ethanol plants mostly use biomaterials like corn (in United States) and sugar cane (in Brazil) [3]. Excessive consumption of these starch or sugar based plants driven by increasing demand of ethanol has led to increasing food cost [2] and rapid deforestation. As a result, ethanol produced from abundantly available lignocellulosic biomass has emerged as sustainable and cleaner alternative [4], [5], [6] as it has higher potential for green house gas emission reduction (according to some estimates, 88% compared to 18% in the earlier case) [2].
The lignocellulosic biomass, obtained from grass and plant residue, primarily consists of polysaccharides (hemicellulose and cellulose: polymers of 5 and 6 carbon chain sugar) and lignin. These polysaccharides are usually depolymerized into sugars by a thermo-chemical pretreatment step, such as dilute-acid or auto-hydrolysis, which precedes the enzymatic fermentation of the hydrolyzate. But this process faces several challenges prohibiting the use of lignocellulosic biomass in any commercial ethanol plant until now. A major challenge faced in commercial production of lignocellulosic bio-ethanol is the inhibitory compounds, generated during acidic hydrolysis of biomass, which are toxic to fermenting species [7], [8], [9]. Each of these compounds (see Fig. 1) may affect cell growth, sugar uptake, individual metabolic pathways, or all three [10]. Though 5-hydroxymethylfurfural (HMF), furfural and vanillin are inhibitors to the fermentation process, they have also been identified as platform chemicals for producing alternative fuels, drugs and polymeric materials. For example, the catalytic dehydration of biomass-derived sugars into HMF and furfural for producing fuels, chemicals and plastics has been demonstrated [11], [12].
A number of technologies have been employed to remove inhibitory compounds from lignocellulosic hydrolyzates. These include over-liming [13], treatment with activated charcoal [14], ion exchange resins [15], and solvents [16]. Treatment with activated charcoal or anion exchange resins effectively removes inhibitors and can lead to hydrolyzates that show a fermentation performance similar to that of an inhibitor-free model substrate [9], [14], [17]. Anion exchange resins work most effectively when the hydrolyzate is adjusted to a pH of 10 [9]. This adjustment not only requires significant quantities of acid and base chemicals, but also results in up to 26% loss of fermentable sugars [9]. Treatment with activated charcoal does not require adjustment of the pH [14] and sugars have a relatively low affinity towards charcoal [18]. However, activated charcoal application is expensive since powdered activated charcoal cannot be regenerated and granular activated charcoal usually incurs a 10% loss during each thermal reactivation cycle.
Here, we present a pretreatment method to improve ethanol yield by removing these inhibitors from hydrolyzates. Adsorption using hydrophobic zeolites has been studied in this work for inhibitor removal and ensuring negligible loss of sugar in the process. We also demonstrate the potential of recovering these inhibitors to be used as industrially valuable compounds by preferential adsorption on appropriately chosen zeolites. By looking at the volume of ethanol production worldwide and typical amounts of inhibitors present in hydrolyzates, recovery of HMF, furfural and vanillin may become economically significant.
Section snippets
Adsorbates and adsorbents
Several types of zeolites (MFI, β, faujasite and FER) were investigated as adsorbents of HMF, furfural, vanillin and xylose. Except silicalite-1, which was synthesized using a recipe from literature [19] (refer to Fig. 16(d) in the corresponding reference), all zeolites were purchased from Zeolyst and were calcined prior to each adsorption experiment. The reported Si/Al ratios of all zeolites was as provided by the supplier (Zeolyst) except for the Si/Al ratios of zeolites β which was
Fermentation improvement by zeolite treatment
Fig. 2a and b show the ethanol production and sugar consumption during fermentation of model media solutions containing sugar and representative compounds that inhibit fermentation. Due to presence of these inhibitory compounds we see a reduced sugar consumption and very low ethanol production when compared to the reference solution having no inhibitors. When pretreated with zeolite β, the media solutions showed enhanced fermentation results, matching closely the reference solutions with no
Conclusion
Zeolite pretreatment improves the ethanol yield during fermentation by removing fermentation inhibitors from the media with almost no loss of fermentable sugar. Single component and mixture adsorption experiments indicate the exclusion of sugars and strong adsorption of inhibitors like HMF, furfural and vanillin by zeolites β and faujasite. Stronger adsorption is observed for siliceous zeolites and the adsorbed amounts decrease with decreasing Si/Al ratio indicating competition between
Acknowledgments
The authors acknowledge the financial support from the Biotechnology Institute, University of Minnesota and a Discovery Grant from the Institute on the Environment, University of Minnesota. Partial support for this work is also provided by an NSF award: CBET 085563.
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2021, Renewable EnergyCitation Excerpt :Despite the relatively straightforward application, studies have shown that several process conditions such as pH, temperature, contact time, and loading ratio can be optimized to deal with specific inhibitor profiles and concentrations. This procedure is necessary since the hydrolysate characteristics are strongly dependent on the type of lignocellulosic materials [16,192]. Zhang et al. [193] have reported that the oxygen functional groups on the surface of activated charcoal also significantly influence its sorption capacity, which represents another factor for the fine-tuning of a treatment.