(475f) Production of High Value Sustainable Aromatics from Fruit and Nutshell Residues By Direct Depolymerization

Lignin is a complex phenylpropanoid polymer and is the most abundant source of renewable aromatic carbon and can be an alluring platform for sustainable chemicals. Effective utilization of lignin has become a critical subject to enhance the cost-effectiveness and economic viability of the lignocellulose biorefinery. Fruit and nutshell wastes have high potential to contribute to the lignin biorefinery. However, the depolymerization performances on the food/nutshell waste-related biomass remains unclear, owing to the broad physicochemical variations from the edible parts of the fruits and plant species. In this study, the monomer production potentials of ten major fruit and nutshell biomass were investigated with comprehensive numerical information derived from instrumental analysis, such as plant cell wall chemical compositions, syringyl/guaiacyl (S/G) ratios, and contents of lignin substructure linkages (β−O−4, β−β, β−5). A standardized one-pot reductive catalytic fractionation (RCF) process was applied to benchmark the monomer yields, and the results were statistically analyzed. Among all the tested biomass, mango endocarp provided the highest monolignol yields of 37.1% per dry substrates. Positive S-lignin (70-84%) resulted in higher monomer yield mainly due to more cleavable β–O–4 linkages and less condensed C–C linkages. Strong positive relationships were identified between b–O–4 and S-lignin and between b–5 and G-lignin. The analytical, numerical, and experimental results of this study will shed lights to process design of lignin-first biorefinery in food-processing industries and waste management works.

Experiments

Ten phylogenetically diverse fruit and nutshell species (i.e. Peach, Mango, Cherry, Avocado, Date, Pistachio, Peanut, Macadamia, Walnut) were selected to analyze their potential values of monomer production by RCF. The fruits and food nuts were oven dried and removed from the shells. The shell biomass was washed, air dried, and grounded into course powder with particle diameter smaller than 3-5 mm by a shaker mill. The biomass samples were kept in airtight containers before the RCF experiments. The RCF processes were developed to obtain lignin monomers directly from lignocellulosic biomass. As RCF requires the least operational steps in the conversion process and the lignin is still in its original form in the biomass, some researchers have implied the production of monomer in RCF with near-theoretical yield. In RCF, lignin is solubilized and its interunit bonds are cleaved through solvolysis and hydrogenolysis, hence the resulting lignin moieties are stabilized by a redox catalyst, promoting the hydrogenation of C-C bonds, allowing for the solvolytic production of less condensed lignin.

The monomer identification was performed by a GC-FID. Sensitivity factors of the products were obtained by using estimates based on the effective carbon number (ECN) due to lack of commercial standards. The GC peaks were determined by the retention time and each peak was integrated with pre-calibrated data. Identification of monomer peaks was performed initially by a GC-MS and a HP5-MS capillary column. The peaks in the GC-MS chromatogram appear in the same order as those in GC-FID chromatogram due to the use of a similar capillary column. Lignin structures and interunit linkages were determined and quantified using state-of-the-art analytical tools and associated with monolignol properties. The data matrix was analyzed using Mantel’s and Pearson correlation statistics to clarify the relationship between lignin characteristics and the monolignol products.

Results and Discussion

The results of compositional analysis and monolignol yields of tested fruit and nutshell biomass (with corresponding images) are shown in Figure 1. High lignin contents were recorded in most of the samples. The nutshells of macadamia and candlenut from the family Proteales and Fabids show >40% lignin contents followed by peach with 38% lignin on the dry mass basis. Walnut, cherry, and peanut from Fabids family have lignin contents in the range 30-40 % while the remaining samples were near or below 20%.

The high lignin contents in these samples prove them to be ideal biomass for lignin valorization. Along with lignin, the cellulose and hemicellulose contents were also determined. Avocado and date are both rich in carbohydrates. They belong to monocots and have more cellulose (40-60%) than hemicellulose (<5%). Interestingly, the date sample was rich in mannose, which was the only found in all the tested biomass samples. The remaining nutshell sample from the other eudicots have cellulose and hemicellulose in the range 15-40%. Those carbohydrate-rich samples could be more useful in biological conversion rather than RCF. The monomer yields of tested biomass samples are also shown in the bar charts of Figure 1. The results showed difference in the abundance of the G- and S- units along with variation in the monomer yields. Mango seed and walnut shell have the highest monomer yield (>70 mg/g-dry biomass) and the monomer yield of peach, candlenut, cherry, and pistachio biomass were all higher than 50 mg/g-dry biomass. The monomer yields of peanut, macadamia, and date were in the range of 20-30 mg/g-dry biomass, with avocado biomass being the lowest at only 11 mg/g-dry biomass. Meanwhile, significant variation in the abundance of S- and G- monomers was observed in all species. Mango seed demonstrated the highest potential of monolignol yield at 37.1% per dry biomass, followed by walnut, pistachio peach, and cherry biomass. Candlenut shell was with the highest lignin content (~48%) and provided reasonable monolignol production including the special C-type lignin The abundance of different monomers enhances the selectivity of the monomers among species.

In addition, based on the analysis of lignin monomer yield and the information of global production of all the fruit and nutshell selected in this study, the economic analysis of potential monomer product was conducted. Peanuts, peaches, and mangoes are the top three fruits/nuts in terms of their highest annual production (96.76, 40.00, and 39.86 million tons/year) among the 10 selected species (Figure 2a). Additionally, they also have the highest total quantity of inedible parts (26.39, 15.30, and 5.04 million tons/year). Although there is a clear demand for some lignin monomers such as guaiacol and syringol, the low annual production and incomplete downstream development of most of the lignin monomers result in limited market demand. Therefore, we estimated the value of lignin monomers in a fully developed market scenario using vanillin, a common aromatic organic compound, as a reference. Meanwhile, the analysis assumed the Ru/C waste employed in industrial-scale production, with a catalyst recovery rate of 98%. For potential monomer, economic benefits of some species turn out to be negative after eliminating costs of solvent and the catalyst (Figure 2b). Under the set conditions, the recovery of lignin monomers from macadamia, avocado, and peanut is not economically viable. While some species, such as mango, walnut, and pistachio have potential profitability opportunities.

The economic evaluation provides only a preliminary analysis of the potential value of lignin monomers under relatively ideal conditions. In practical industrial production, numerous factors need to be taken into consideration, such as energy consumption, raw material collection, catalyst development, etc. Some of these factors may increase operating costs, such as the construction of reaction equipment, while others may reduce production costs, such as the use of highly efficient and cost-effective catalysts. Although the results indicate some potential profitability in recovering lignin monomers from waste biomass, further research and on-site analysis are needed to fully explore this potential industry.

References

More details of this presentation are now available at Khan, Rabia J., Jianyu Guan, Chun Y. Lau, Huichuan Zhuang, Shazia Rehman, and Shao-Yuan Leu. "Monolignol Potential and Insights into Direct Depolymerization of Fruit and Nutshell Remains for High Value Sustainable Aromatics." ChemSusChem: e202301306.