Resumen de Integrating pretreatment techniques in a "benign-by-design strategy" in the context of biomass valorization
Circular bioeconomy concept was used as an anchor in the valorization of walnut shell. Even though, upstream processes such as microwave, mechanocatalysis and deep eutectic solvents have shown potential in biorefinery processes, they exhibit some limitations when they are used in isolation. Here, we demonstrate the application of these upstream processes in a combination in an integrated approach. In this thesis, a cheap and easily available resource (walnut shell) was selected as feedstock for input of the biorefinery set up. Microwave process was used to deconstruct walnut shell to obtain xylose as the major intermediate compound. The optimum condition to give high yield of xylose achieved without the assistance of homogeneous and heterogenous catalyst. The thermal and chemical effect of microwave process on walnut shell were monitored to reveal the viability of the residue left after the reaction. Autohydrolysis assisted by microwave processes was used to depolymerize hemicellulose and amorphous cellulose in walnut shell to yield 63.5% w/w of xylose in a liquid fraction (hydrolysate) with a minimum yield of glucose. The optimum reaction conditions to obtain a high xylose yield with a low byproduct titer were 190 °C for 25 min. The xylose and glucose obtained from walnut shell was transformed into L-lactic acid for the first time with a productivity of 0.2g/L/h through homolactic fermentation by Bacillus coagulans DSM 2314. Prior to the fermentation of the sugars, various media were tested for the best productivity and growth for Bacillus coagulans DSM 2314. It was found a simple Yeast, Peptone and sugar substrate were rich enough to promote the growth of the bacteria. Microwave pretreatment and the adoption of thermophilic bacteria prevented the vigorous sterilization required for most fermentation processes.
Subsequently, the residue obtained after microwave processes was characterized and used in other experiment as a substrate to immobilized Bacillus coagulans DSM 2314. This experiment was carried out to investigate the potential maximation of the productivity of lactic acid and re-usability of the immobilized bacteria.
Secondly, poplar wood which is readily available in Europe was used as a feedstock for production of bioethanol. At first stage of biorefinery processes, mechanocatalysis technique was employed to depolymerized poplar to produce high quality glucose for the production of bioethanol. Common yeast saccharomyces cerevisiae was used in the fermentation process. For the first time, sugars streams from mechanocatalytic processes of poplar wood were investigated for their fermentability. Mechanocatalytic process gave a high glucose yield of ca. 80 % with accompanying inhibitory compounds. The most significant inhibitory compounds found in the crude hydrolysate after mechanocatalysis and hydrolysis of water-soluble products were furfural, HMF and dissolved phenolics. A novel and easy approach of polybenzene as an adsorbent was adopted to purify the crude hydrolysate. It removed 99.9% of furfural, HMF, and a significant amount dissolved phenolics. The purified hydrolysate was used in the fermentation process.
Cellulosic content could easily be accessible, after deconstruction of walnut shell through DESs by cleavage of ether bonds linking carbohydrate and lignin, hence freeing lignin into solution. The lignin dissolved in solution is subsequently precipitated and recovered. The combination of ball milling (BM), microwave irradiation (MI) and Deep Eutectic Solvents (DES) results synergistic for an efficient, selective, and very rapid (10 minutes) delignification of materials with high lignin content (ca. 50 wt%) such as walnut shells (WS). Lignin is dissolved in the DES media, whereas the polysaccharide fractions remain suspended with reduced degradation, due to the rapid pretreatment. After ball milling procedure (3 h), biomass loadings in the range of 100-200 g L-1 are selectively delignified in 10 minutes at 150 ºC by using choline chloride – formic acid DES (1:2 molar ratio), rendering lignin yields of 60-80 % (ca. ~60 g lignin L-1). Ball milling, microwave irradiation and DES result much more efficient in comparison to BM, conventional heating, and DES. This recovered lignin can also serve as feedstock to produce bio-renewable platform chemicals and biofuels.
The inhibitory compounds (furans) which were recovered after purification of hydrolysate could be converted into bio-jet fuel precursors such as 2,5-bis(2-furylmethylidene) cyclopentanone (F2Cp) and 2-(2-furylmethylidene) cyclopentanone (FCp). Here, we evaluated the feasibility this proposal, commercial furan derivatives such as furfural and cyclopentanone in a cross- aldol condensation reaction in the presence of mixed oxide catalyst. Two major problems associated with the production of biojet fuel precursors during aldol condensation are (i) solidification of products and (ii) longer reaction time. We solve this problem by using microwave as source of heat supply and performing the reaction in binary mixture (ethanol:water) with monophasic configuration.
Justification of Research 1. To use a renewable feedstock (walnut shell) as replacement for the dwindling petroleum resources.
2.To reverse effects of climate change on the environment by producing net-zero carbon biofuels.
3. To mitigate plastic pollution by reversing linear economy practices into circular bioeconomy.
4. To produce biodegradable polymers/chemicals/materials from sustainable resources.
5. To enhance and increase the market share of bio-based products of high value to compete with fossil-based products.
6. To test the viability and reliability of pretreatment techniques in biorefinery processes against the conventional petroleum refineries.
Methodology This thesis work involved broad application of scientific methodology to upgrade walnut shell as model biomass.
Three major pretreatment methodology was investigated, In thesis, extensive study has been carried out on three deconstruction processes, namely, Microwave processes: Deep eutectic Solvent and Mechanocatalysis. These are green and sustainable methods that were used on renewable vegetal resources to meet some relevant pillars established for sustainable green chemistry evaluation.
Microwaves (MW) are a form of electromagnetic energy that causes heating of substances when microwave radiation encounters polar molecules or ions causing dipole rotation and ionic conduction to generate the heat . Microwave processes possess many advantages over conventional heating system, such as 1) there is a wide range of feedstock for valorization and higher quality products; (2) heating is non-contact and volumetric (3) energy is transferred not heat; (4) energy is saved; (5) heating is rapid and efficient (6) material is selectively heated; (7) start-up and shut down are quick; (8) there is a higher level of safety and automation. Microwave processes was used as an effective pretreatment to deconstruct lignocellulosic biomass (walnut shell) to xylose and glucose with minimal side products. Subsequently, the sugars were converted into L-lactic acid through fermentation process.
Mechanocatalysis or mechanochemistry is chemical reaction that is induced by mechanical energy [6]. This process offers deep depolymerization of lignocellulose biomass compared to physical or chemical pretreatment. The process is solvent free and requires no external heat to drive the reaction .
Deep Eutectic Solvents (DESs) are mixtures with a rare constituents of two or more phase immiscible solid compounds which transform to liquid phase at a precise temperature [8] or in a broader scope, DESs are formed by mixing a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), where the resultant mixture has melting temperature lower than its constituents formed from. DESs offers several advantages such as (1) are easy to prepare, (2) biocompatible, (3) biodegradable and (4) non-toxic.
Two downstream processes such as fermentation and aldol condensation was applied for upgrading downstream process.
Two major conclusions 1.To begin, a novel strategy was taken in which lignin was isolated first. The walnut shell was deconstructed using deep eutectic solvents, exposing the abundant holocellulose for subsequent valorization. After 3 hours of ball milling and 10 minutes of microwave irradiation, a good yield of unaltered lignin was achieved. This procedure proved critical in overcoming the recalcitrance of lignin. This paves the door for easy access to cellulose substrates for enzymes. The lignin recovered is of excellent quality and can be used as a feedstock for the synthesis of biofuels and biochemicals. Thus, both the lignin and holocellulose recovered are beneficial in the closed-loop process.
2. We demonstrate in Chapter 6 that mechanocatalytic processes can provide a high yield of glucose, which can then be transformed to bioethanol. The acid hydrolysate presented no significant difficulties due to its neutralization with Mg (OH)2. MgSO4 in-situ was created at no additional expense. MgSO4 is a necessary mineral for yeast fermentation to occur during the synthesis of bioethanol. To purify and recover furans and phenolics, an effective purification procedure was used. These furans serve as useful building blocks for the synthesis of biojet precursors. All chemicals and residues retrieved have the potential to be valuable in sustaining a closed-loop system.
