Strategies to improve the conversion of sugarcane bagasse into second generation ethanol
Carregando...
Data
2021-03-25
Autores
Igbojionu, Longinus Ifeanyi
Título da Revista
ISSN da Revista
Título de Volume
Editor
Universidade Estadual Paulista (Unesp)
Resumo
O etanol de segunda geração a partir da biomassa lignocelulósica é uma solução renovável e sustentável para os problemas ambientais decorrentes das grandes demandas de energia da sociedade moderna. Bagaço de cana-de-açúcar (SB), uma lignocelulose composta por dois grandes polímeros de açúcar (celulose e hemiceluloses) ligados à lignina. Devido à natureza recalcitrante do SB, o pré-tratamento é considerado uma etapa fundamental do processo antes de ser convertido em biomoléculas. SB bruto consiste em celulose (40,4% em peso), hemicelulose (23,8% em peso), lignina (27,2% em peso), cinzas (3,0% em peso) e extrativos (4,4% em peso). O pré-tratamento de SB com NaOH a 3% e NaOH a 8% em autoclave (121 oC) e à temperatura ambiente, respectivamente, levou a aumentos nas frações celulósicas (aproximadamente 62,0% em peso), enquanto o conteúdo de lignina diminuiu para valores inferiores a 11,0% em peso. O NaOH sequencial em duas etapas e o pré-tratamento com ácido resultaram em aumentos substanciais nas frações celulósicas (aproximadamente 80,0% em peso), que foi cerca de duas vezes o valor obtido a partir de SB não tratado (40,4% em peso). O conteúdo de hemicelulose de SB diminuiu substancialmente após o pré-tratamento da segunda etapa com ácido (ácido maleico e sulfúrico). No entanto, o índice de cristalinidade aumentou substancialmente após diferentes pré-tratamentos foram aplicados ao SB. A sacarificação enzimática do SB pré-tratado levou a rendimentos de glicose acima de 75% (g/g) em comparação com 27% (g/g) obtidos do SB não tratado após 72 h. A otimização da hidrólise enzimática resultou em rendimento máximo de glicose de 85,9% (g/g) pela aplicação de 1,8 FPU/g de celulose de celulose, 5,3% de carregamento de sólidos e 48 h de tempo de hidrólise. Por outro lado, a hidrólise ácida em duas etapas do SB pré-tratado resultou em um rendimento máximo de glicose de 56,8% (g/g). A otimização da hidrólise ácida em duas etapas aplicando ácido sulfúrico a 1,5%, FeSO4 40,0 mmol/L e 120 min na segunda etapa de hidrólise resultou em um rendimento máximo de glicose de 90,0% (g/g) e um valor de rendimento de glicose previsto de 88,9%. Após a validação, os valores experimentais e preditos de rendimento de glicose foram de 88,6% e 86,5%, respectivamente. Além disso, os hidrolisados ácidos obtidos continham baixa quantidade de compostos inibidores (furfural e 5-hidroximetilfurfural). Tanto o ácido quanto o hidrolisado enzimático foram eficientemente utilizados por Saccharomyces cerevisiae IQAr/45-1 para atingir rendimentos de etanol de 0,43 g/g e 0,46 g/g, respectivamente. No entanto, quantidades consideráveis de açúcar residual permaneceram no final da fermentação, indicando a incapacidade de S. cerevisiae IQAr/45-1 em metabolizar o açúcar pentose presente nos hidrolisados. Assim, o uso de levedura geneticamente modificada com a capacidade de utilizar simultaneamente os açúcares hexose e pentose melhorará ainda mais o processo de fermentação. Em conclusão, os processos que envolveram o uso de reagentes químicos baratos para superar a recalcitrância de SB e liberar açúcares fermentáveis poderiam potencialmente apoiar uma biorrefinaria e reduzir significativamente o custo do etanol de segunda geração.
Second generation ethanol from lignocellulosic biomass is a renewable and sustainable solution to the environmental problems arising from the large energy demands of the modern society. Sugarcane bagasse (SB), a lignocellulose is made up of two major sugar polymers (cellulose and hemicelluloses) connected to lignin. Due to the recalcitrant nature of SB, pretreatment is considered a key process step before it can be converted to biomolecules. Raw SB consists of cellulose (40.4 wt%), hemicellulose (23.8 wt%), lignin (27.2 wt%), ash (3.0 wt%) and extractives (4.4 wt%). Pretreatment of SB with 3% NaOH and 8% NaOH in autoclave (121 oC) and at room temperature respectively led to increases in cellulosic fractions (approximately 62.0 wt%), while the lignin contents decreased to values less than 11.0 wt%. Two-step sequential 3% NaOH and acid pretreatment resulted to substantial increases in cellulosic fractions (approximately 80.0 wt%) which was about two-fold the value obtained from untreated SB (40.4 wt%). The hemicellulose content of SB decreased substantially after the second-step pretreatment with acid (maleic and sulfuric acid). However, crystallinity index increased substantially after different pretreatments were applied to SB. Enzymatic saccharification of pretreated SB led to glucose yields above 75% (g/g) compared to 27% (g/g) obtained from untreated SB after 72 h. The optimization of enzymatic hydrolysis resulted to maximum glucose yield of 85.9% (g/g) by applying 1.8 FPU/g cellulose of cellulase, 5.3% of solids loading and 48 h of hydrolysis time. On the other hand, two-step acid hydrolysis of pretreated SB resulted to a maximum glucose yield of 56.8% (g/g). The optimization of two-step acid hydrolysis by applying 1.5% sulfuric acid, 40.0 mmol/L FeSO4 and 120 min in the second hydrolysis step resulted to maximum glucose yield of 90.0% (g/g) and predicted glucose yield value of 88.9%. After validation, the experimental and predicted values of glucose yield were 88.6% and 86.5% respectively. Besides, the acid hydrolysates obtained contained low amount of inhibitory compounds (furfural and 5- hydroxymethylfurfural). Both the acid and enzymatic hydrolysates were efficiently utilized by Saccharomyces cerevisiae IQAr/45-1 to achieve ethanol yield of 0.43 g/g and 0.46 g/g respectively. Nevertheless, considerable amounts of residual sugar remained at the end of fermentation, indicating the inability of S. cerevisiae IQAr/45-1 to metabolize pentose sugar present in the hydrolysates. Thus, the use of genetically engineered yeast with the ability to simultaneously utilize both hexose and pentose sugars will further improve the fermentation process. In conclusion, processes which involved the use of cheap chemical reagents to overcome SB recalcitrance and liberate fermentable sugars could potentially support a biorefinery and greatly reduce the cost of second generation ethanol.
Second generation ethanol from lignocellulosic biomass is a renewable and sustainable solution to the environmental problems arising from the large energy demands of the modern society. Sugarcane bagasse (SB), a lignocellulose is made up of two major sugar polymers (cellulose and hemicelluloses) connected to lignin. Due to the recalcitrant nature of SB, pretreatment is considered a key process step before it can be converted to biomolecules. Raw SB consists of cellulose (40.4 wt%), hemicellulose (23.8 wt%), lignin (27.2 wt%), ash (3.0 wt%) and extractives (4.4 wt%). Pretreatment of SB with 3% NaOH and 8% NaOH in autoclave (121 oC) and at room temperature respectively led to increases in cellulosic fractions (approximately 62.0 wt%), while the lignin contents decreased to values less than 11.0 wt%. Two-step sequential 3% NaOH and acid pretreatment resulted to substantial increases in cellulosic fractions (approximately 80.0 wt%) which was about two-fold the value obtained from untreated SB (40.4 wt%). The hemicellulose content of SB decreased substantially after the second-step pretreatment with acid (maleic and sulfuric acid). However, crystallinity index increased substantially after different pretreatments were applied to SB. Enzymatic saccharification of pretreated SB led to glucose yields above 75% (g/g) compared to 27% (g/g) obtained from untreated SB after 72 h. The optimization of enzymatic hydrolysis resulted to maximum glucose yield of 85.9% (g/g) by applying 1.8 FPU/g cellulose of cellulase, 5.3% of solids loading and 48 h of hydrolysis time. On the other hand, two-step acid hydrolysis of pretreated SB resulted to a maximum glucose yield of 56.8% (g/g). The optimization of two-step acid hydrolysis by applying 1.5% sulfuric acid, 40.0 mmol/L FeSO4 and 120 min in the second hydrolysis step resulted to maximum glucose yield of 90.0% (g/g) and predicted glucose yield value of 88.9%. After validation, the experimental and predicted values of glucose yield were 88.6% and 86.5% respectively. Besides, the acid hydrolysates obtained contained low amount of inhibitory compounds (furfural and 5- hydroxymethylfurfural). Both the acid and enzymatic hydrolysates were efficiently utilized by Saccharomyces cerevisiae IQAr/45-1 to achieve ethanol yield of 0.43 g/g and 0.46 g/g respectively. Nevertheless, considerable amounts of residual sugar remained at the end of fermentation, indicating the inability of S. cerevisiae IQAr/45-1 to metabolize pentose sugar present in the hydrolysates. Thus, the use of genetically engineered yeast with the ability to simultaneously utilize both hexose and pentose sugars will further improve the fermentation process. In conclusion, processes which involved the use of cheap chemical reagents to overcome SB recalcitrance and liberate fermentable sugars could potentially support a biorefinery and greatly reduce the cost of second generation ethanol.
Descrição
Palavras-chave
Bagaço de cana-de-açúcar, Hidrólise ácida, Pré-tratamento, Saccharomyces cerevisiae IQAr/45-1, Fermentação de etanol, Sugarcane bagasse, Acid hydrolysis, Pretreatment, Ethanol fermentation