Lignocellulose: A sustainable material to produce value-added products with a zero waste approach—A review

Accepted Manuscript Title: Lignocellulose: A sustainable material to produce value-added products with a zero waste approach—A review Authors: Alejandra Arevalo-Gallegos, Zanib Ahmad, Muhammad Asgher, Roberto Parra-Saldivar, Hafiz M.N. Iqbal PII: DOI: Reference:

S0141-8130(16)32991-9 http://dx.doi.org/doi:10.1016/j.ijbiomac.2017.02.097 BIOMAC 7158

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

21-12-2016 21-2-2017 27-2-2017

Please cite this article as: Alejandra Arevalo-Gallegos, Zanib Ahmad, Muhammad Asgher, Roberto Parra-Saldivar, Hafiz M.N.Iqbal, Lignocellulose: A sustainable material to produce value-added products with a zero waste approach—A review, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2017.02.097 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Lignocellulose: A sustainable material to produce value-added products with a zero waste approach – A review

Alejandra Arevalo-Gallegos1, Zanib Ahmad2, Muhammad Asgher2, Roberto Parra-Saldivar1, and Hafiz M. N. Iqbal1,*

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School of Engineering and Science, Tecnologico de Monterrey, Campus Monterrey, Ave.

Eugenio Garza Sada 2501, Monterrey, N. L., CP 64849, Mexico; 2Industrial Biotechnology Laboratory, Department of Biochemistry, University of Agriculture Faisalabad, Pakistan;

*Corresponding author: [email protected]; [email protected]

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Graphical abstract

Highlights  An emerging technology for revalorization of lignocellulose materials is reviewed. 

Lignocellulose – A potential source to produce high-value marketable and sustainable products.

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Bio-refinery – A potential alternative to petroleum-based refinery.

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High value-added bio-based products from agro-industrial waste ‘materials’.

Abstract A novel facility from the green technologies to integrate biomass-based carbohydrates, lignin, oils and other materials extraction and transformation into a wider spectrum of marketable and value-added products with a zero waste approach is reviewed. With ever-increasing scientific knowledge, worldwide economic and environmental consciousness, demands of legislative authorities and the manufacture, use, and removal of petrochemical-based by-products, from the last decade, there has been increasing research interests in the value or revalue of lignocellulose-based materials. The potential characteristics like natural abundance, renewability, recyclability, and ease of accessibility all around the year, around the globe, all 2

makes residual biomass as an eco-attractive and petro-alternative candidate. In this context, many significant research efforts have been taken into account to change/replace petroleumbased economy into a bio-based economy, with an aim to develop a comprehensively sustainable, socially acceptable, and eco-friendly society. The present review work mainly focuses on various aspects of bio-refinery as a sustainable technology to process lignocellulose ‘materials’ into value-added products. Innovations in the bio-refinery world are providing, a portfolio of sustainable and eco-efficient products to compete in the market presently dominated by the petroleum-based products, and therefore, it is currently a subject of intensive research.

Keywords: Lignocellulose; Biological macromolecules; Green Biotechnology; Bio-refinery; Applications

1. Introduction From the last few years, biorefinery approach is emerging as a promising sector with a considerable potential to capitalize various lignocellulose materials into a variety of industrially relevant bio-products [1]. Undoubtedly, the driving force behind the bio-refineries establishment is a sustainability concept. The sustainability concept is shown in Figure 1. The term ‘bio-refinery’ is defined as: “a sustainable approach to biotransform raw materials e.g. biomass into energy and a wider spectrum of everyday commodities in an economical and ecofriendly manner”. Figure 2 illustrates a schematic overview of a bio-refinery concept. Whereas, some potential advantages and disadvantages related to the biomass utilization are presented in Table 1 and 2, respectively [2]. Research is underway around the globe on the development

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of ‘greener’ technologies. The invention of green chemistry under a green agenda principle has either directed or redirected current search towards the development of high added-value ecofriendly, eco-efficient, and recyclable materials. Words like renewability, recyclability, and sustainability are emphasized in growing scientific knowledge and environmental awareness. In this context, the divergence from non-renewable (petroleum-based resources) to renewable materials (biomass-based resources) is becoming the center of interest for research in industrial communities, worldwide [3, 4]. The fact is that petroleum resources are finite and becoming increasingly costly. Moreover, the consistent depletion of petrochemical resources has pushed up prices in essential sectors, worldwide, including energy, materials, and medical [5]. Therefore, one of the biggest challenges of the modern world is in decreasing the dependency on such petrochemical resources based products. Herein, the bio-refinery approach should ease disputes on eco-pollution and reliance on fossil resources, thus can be considered as an evolution of concepts like “Green Chemistry” [6]. Though the refinery concept is not a new, however, in recent years, bio-refineries, in particular, integrated bio-refineries, are seen as a promising route to meet our aims for sustained prosperity and safeguarding the natural ecosystem. So the focus has been shifted to bio-refineries for target applications in different sectors of the modern society to address the growing environmental concerns where petroleumbased resources are unsustainable. In comparison to the petroleum-based refineries, bio-based bio-refineries have great potential to utilize a variety of feedstocks at larger level using diverse technologies. The present review focuses on various aspects of bio-refinery as a sustainable technology to process lignocellulose ‘materials’ into value-added products. Explicitly, many significant efforts have been devoted to converting these lignocellulosic to value-added products including composite, fine chemical, animal feed, pulp and paper, biofuels and enzymes (Figure 3) [7]. During the last few years, we have demonstrated considerable improvement in many processes related to Lignocellulose biotechnology and triggered in-depth

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studies of lignocelluloses, different fungal consortia, ligninolytic enzymes including Lignin peroxidase (LiP) and manganese peroxidase (MnP), and laccase, their purification and immobilization to present their potential for a wider spectrum of biotechnological applications [8-24]. Innovations in the bio-refinery world are providing, a portfolio of sustainable and ecoefficient products to compete in the market presently dominated by the petroleum-based products, and therefore, it is currently a subject of intensive research. 2. Revalorization of natural biomass materials Interestingly, a bio-refinery has an enormous potential to use/capitalize all types of biomassbased sources that includes agricultural, agro-industrial, algae, and municipal, etc. Moreover, the materials mentioned above are the most promising feedstock as a natural, abundant, and renewable resource essential to the functioning of industrial societies and critical to the development of a sustainable global economy [25]. On the other hand, this should also result in substantial improvement both in quality and quantity integration and process optimization aspects of all types bio-refineries and bio-refinery subsystems. Large amounts of wastes are generated through agricultural and many agro-based industrial practices [7]. For many years, these potential materials were considered among other environmental threats as a major source of ecological pollution and had also been categorized as a global issue. However, in recent years, these materials have gained considerable importance owing to their novel characteristics that include, renewability, recyclability, and sustainability [4, 7, 19]. Their physicochemical and biological characteristics make them a substrate of enormous industrial and biotechnological value to develop a range of value-added products [7, 26-30]. Some potential applications include but not limited to the generation of various energy types like bio-fuels, easily accessible and readily available energy sources for microbial fermentation purposes, production of novel enzymes, platform chemicals, alternative sources for pulp and paper industry, animal feedstuffs, and novel composites. In this background, in the past decade, a

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substantial development in many ongoing practices related to the bio-refinery concept and bioconversion of bio-renewable bio-based natural polysaccharides has been achieved for the future, to develop a variety of different industrially relevant value-added products. This review article mainly focused on the fundamental aspects of the sustainable biorefinery approach, concentrating on the following important areas: (1) Bio-refinery-based bio-economy – considerations, (2) various biorefinery platforms and (3) processing of low-cost agro-industrial wastes ‘natural polysaccharides’ into value-added products. Towards the end, information is also given on the potential future considerations for the sustainability assessment of the current bio-refinery methods. 3. Bio-refinery-based bio-economy – considerations Many considerations are being taken to replace the current, in practice, petrochemical-based approaches to a more sustainable bio-based bio-refinery to enhance the greater use of natural polysaccharides e.g. lignocellulosic materials. Furthermore, the current price hike, increasing demand for eco-friendly practices, bio-based green products, and population growth vs. limited resources, all have opened a lot of opportunities for the development of bio-refinery-based bioeconomy, to jointly produce food, energy, and chemicals [31]. From the historical point of view, bio-based producers have targeted value-added products, platform chemicals, and specialty markets, often where multi-functional characteristics played a critical role to justify the end product goal. This integrated transition from the abovementioned petrochemical-based approaches to a sustainable biorefinery has many of the following justifications [32, 33]. (1) to avoid an over dependency on petrochemicals (2) to avoid the price hike (3) to avoid an over consumption of the oil, gas, coal and other potential minerals (4) to strengthen and diversify the bio-renewable-based energy sources

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(5) to tackle the worldwide climate issues (6) to reduce the greenhouse gasses emission (7) to protect the natural ecosystem (8) to stimulate the greener development of regional and rural areas 4. Bio-refinery – platforms Biorefinery concept is mainly categorized in two platforms, i.e., (1) syngas platform (SGP), also known as a thermochemical platform and (2) sugar platform (SP), also known as a biochemical platform, which is intended to provide basic building blocks for a wider spectrum of bio-based bio-products. Figure 4 illustrates a schematic representation of the abovementioned biorefinery platforms concept. The former type mainly based on either gasification or pyrolysis. Likewise, in later type, bio-refinery normally based on the fermentation of sugars, e.g., C5/C6 extracted from many of the natural polysaccharides-based feed-stocks. Based on the raw materials and processing techniques, various types of bio-refineries have been proposed or developed by researchers [34, 35]. (1) conventional bio-refinery (CBR) (2) advanced bio-refinery (ABR) (3) green biorefinery (GBR) (4) lignocellulosic biorefinery (LCBR) or lignocellulosic feedstock biorefinery (LCFBR) (5) whole crop bio-refinery (WCBR). 4.1. Syngas platform (SGP)/thermochemical platform A thermochemical biorefinery comprised of twelve processes for the production of ethanol, methyl acetate, hydrogen, electricity, and hydrogen was evaluated using dimethyl ether as an intermediate. With an energy efficiency up to 50.2% was achieved. An economic analysis was also carried out for this process concluding that a thermochemical biorefinery is more profitable than the usual thermochemical processes, with the added benefit of producing more than one

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product [36]. During dark fermentation, the substrate can convert to not only hydrogen but also ethanol and butyric acid, among other compounds. Therefore, the acid-rich effluent obtained from the fermentation can be used for the production of other chemicals such as polyhydroxyalkanoates, lactic acid, succinic acid, and methane and it can potentially be used as a fertilizer as well [37, 38]. A study reported a production of polyhydroxy butyrate in anoxic conditions using a VFA-rich effluent, obtaining a yield of 33% PHB [39]. Hydrogen production can be coupled with ethanol production, with varying results depending on several factors, such as carbon source, microorganisms used and the type of bioreactor used for fermentation. For example, in a continuous flow reactor with microbial cultures from a hot spring, 0.32molH2/mol glucose and 0.69 mol ethanol/mol glucose were produced. In a fluidized-bed reactor and a packed-bed reactor with an immobilized anaerobic sludge 0.64mol H2/mol hexose and 0.24 mol, ethanol/mol hexose was produced. However, an expanded granular bed with wastewater and a mixed culture produced up to 3.47 mol H2/mol sucrose and 128 mg of ethanol/L [38]. Several products can be obtained from dates from palm trees, among them hydrogen and bioethanol. The former can be produced through dark fermentation with microorganisms such as Enterobacter, Bacillus, and Clostridium. Hydrogen was produced from rotten dates by a three-stage fermentation process using three different bacteria: E. coli EGY for the dark fermentation stage to consume oxygen and establish anaerobic conditions, followed by a second dark fermentation with Clostridium acetobutylicum, and lastly the third fermentation with Rhodobacter capsulatus for the hydrogen production. This process yielded 7.8mole H2 per mole of sucrose [40]. Also, date pits have been used to produce phosphoric acid using pyrolysis at temperatures ranging from 300 to 700 °C. It was concluded that 500 and 700°C were the optimal temperatures [41]. Hydrogen production during fermentation using glycerol generates a liquid residue that can also be used for the production of propanediol (PD). Using

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pure glycerol as a substrate in a batch process, 0.28 mol H2/mol glycerol, and 0.69 mol PD/mol glycerol [38, 42]. 5. Major classes of bio-refineries and their potential products 5.1. Conventional bio-refinery (CBR) The efficient bio-based value chains such as anaerobic digestion, small scale bio-refinery, and conventional bio-refinery (CBR) systems have a marked regional character. Paper sludge one of the main residues of the paper making process can be divided into primary sludge (celluloserich material), and secondary sludge (high microbial content), originated from wastewater treatment. It has been considered as a substrate for bio-refinery; the possibility of producing bioethanol or biogas from paper sludge has been valorized in several studies, but only at a small scale [43, 44]. The sludge from the pulp and paper industry has also been considered for production of biogas, bioethanol, and other fuels. In the case of bioethanol, concentrations of up to 60g/L have been achieved in bioreactors with saccharification and fermentation [45]. In the same study, it was found that a higher ash content was determined to impact negatively the amount of ethanol produced in the reactor. Waste bio-refineries in arid or semi-arid regions can produce a variety of valuable products, such as fuels (ethanol, butanol), lactic acid, acetic acid or activated carbon. The main challenge in this type of regions is the need to minimize the use of water. In the Middle East, date palms (Phoenix dactylifera) are one of the most important agricultural products. Each palm tree sheds between 20 and 50Kg of leaves per year. This residue is usually burned, or used in house construction or commercial products like ropes or baskets. However, the possibility of using palm tree leaves in a bio-refinery has been assessed [41]. Other platform chemicals have been produced using date palms residues or products. Citric acid has been produced in media enriched with date syrup, with a production of 98.42 g/L. Lactic acid production with added date juice was evaluated. Results indicated that

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supplementing media with yeast extract, sodium acetate, and sweet sorghum increased production up to 0.70g/L/h [46]. Fermentation-produced glutamic acid with C. glutamicum using date waste as a substrate, obtaining up to 36.64mg/mL glutamic acid [40]. Succinic acid was produced from purified glycerol with Crotalaria juncea, which was obtained from a biorefinery using Escherichia Coli in a batch system. It was determined that if crude glycerol was used, a necessary step was the removal of impurities [47]. 5.2. Advanced bio-refinery (ABR) An advanced bio-refinery consists of an upgraded version of a conventional bio-refinery by upgrading one or several steps in the process. Some examples are mentioned below. The degradation of the plant cell wall is a key step in the pretreatment of the material. In one study, termites were used to degrade softwood to release sugar monomers partially. When compared to artificially milled softwood, the amount of five and six carbon sugars released was higher in the chewed softwood [48]. Biodiesel synthesis is based on the transesterification process, which can be catalyzed by lipases. Lipase from Burkholderia sp. immobilized on commercial magnetic nanoparticles was used for the conversion of microalgal oil to biodiesel. It was concluded that the immobilized lipase could be used up to six times without losing its activity. The same enzyme was used for the transesterification of Chlorella biomass for production of biodiesel [49]. An improvement of a paper bio-refinery was established by using fractionation of spent sulfite liquor (SSL) for the production of antioxidants, succinic acid and lignosulfonates [50]. 5.3. Green bio-refinery (GBR) Among other advanced bio-refineries, a green biorefinery system is also considered as a complex but multifunctional and full-integrated that has a great potential to use green biomass, like feeding grass, to produce several products, such as fibers, fuel, animal feed or as a part of a lignocellulose biorefinery. Moreover, it has also been considered among novel resource-

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protecting machinery for a complete use of residual biomasses, as mentioned above. The main characteristic of this kind of bio-refinery is the milling or grinding of the green biomass. Moreover, as mentioned earlier and illustrated in Figures 1 and 2, in light of the sustainability and general bio-refinery concept, the green biorefinery system is strongly based on green principles. Ethanol production is one of the main objectives of a green biorefinery. An ethanol bio-refinery integrated with the production of animal feed was studied from different sources as follows: from starch occurs via grinding (wet or dry) followed by fermentation [51]. The use of faba beans was also proposed, both as a green biorefinery, for the production of ethanol and also as a typical biorefinery for the production of animal feed [52]. A different kind of bean was used in an integrated bio-refinery with field bean seeds for the production of ethanol, feed components and the bonus of an edible and ethanol-producing fungal biomass (N. intermedia) [31]. 5.4. Lignocellulosic bio-refinery (LCBR) or lignocellulosic feedstock biorefinery (LCFBR) Lignocellulosic biomass refers to crops, crops residues or forestry biomass. Lignocellulosic biomass is composed of the carbohydrates cellulose and hemicellulose; since it contains around 70% of sugar, it makes a good substrate for hydrogen production and other biofuels. A key step in this process is the separation of sugars from the lignocellulose. Several pretreatments are applied for this: physical (grinding and milling), chemical (with acid or ionic liquids), physicochemical (steam, hot water or ammonia fiber expansion) and biological (fungi) [53]. Since the separation of carbohydrates from the lignocellulose is a key step in the process, choosing a pretreatment technique is highly important, therefore it can be executed with several enzymes, available commercially, like cellulase from Penicilium purpurogenum, Aspergillus niger and Aspergillus fumigatus. In China, over 670 million tons of agricultural residues and

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crops (both herbaceous and woody) are estimated to be produced annually. If this amount of biomass is used to produce ethanol, it could produce the equivalent to 4 billion barrels of petroleum [54]. The process of producing ethanol from cellulosic materials begins with a pretreatment, followed by hydrolysis and finally fermentation [54]. Clostridium acetobutylicum was used in a bio-refinery for the production of butanol with a fermentation medium consisting of flocculated hemicellulosic wood hydrolysate, and alfalfa juice. The culture containing both alfalfa juice and the wood hydrolysate produced up to 3.80 g/L de acetone-butanol-ethanol, proving that this fermentation medium can be used for the production of biofuels [55]. Basfia succiniproducens for the production of succinic acid (SA) with corn stover, reaching a yield of 0.43 g/L/h SA [56]. Another example of the production of succinic acid was studied using spent sulphite liquor (SSL), with Actinobacillus succinogenes and Basfia succiniproducens. Up to 39 g/L of succinic acid was produced along with almost 32.4 g/L LS and 1.15 g of a phenolic-rich extract after fractionation [57]. 5.5. Whole crop bio-refinery (WCBR) Lignocelluloses have great advantages over annual crops in terms of energy and economy [58]. In the long term, the primary feedstock is only a transitional material and is not sufficient to change China's entire energy structure. To fill that role, straw plant fiber will serve as the main source. Cellulosic biomass production, such as agricultural and forestry residues, major portions of municipal solid waste, and dedicated herbaceous and woody crops, is estimated to be 670 million tons per year in China [54, 59, and could provide a meaningful alternative feedstock for ethanol production that is roughly equivalent to 4 billion barrels of petroleum (100 million tons). It is estimated that the total annual consumption by the Chinese auto industry is approximately 60 million tons of gasoline. Hence, approximately 5 to 10 million tons of fuel ethanol must be supplied within the next few years under an E10 (10% ethanol and 90% gasoline blend) standard. The annual consumption of fuel ethanol is approximately 1

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million tons, and thus there will be a huge commercial market and very bright prospects for the development of cellulosic ethanol. 6. Environmental impact During the past several years, a substantial advancement via green chemistry principles has appeared for a cleaner production and revalorization of naturally occurring lignocellulose biomass. However, owing to the current economic issues at the global level, price hike, greenhouse dilemma, all creating a core demand to reconnoiter comparable alternatives as a strategy to reduce global warming and other problematic issues [4, 7, 19, 20]. Environmental pollution, global warming, and the future of oil production are among major causes of public and private interests in natural bio-based resources as an alternative or substitute for fossil fuel oil. One potential method for the low-cost production of bio-ethanol is to utilize the lignocellulosic or agro-industrial biomass because they contain carbohydrates that must be first converted into simple sugars (glucose) and then fermented into ethanol [60, 61]. Given this reality, nations around the world are investing in alternative sources of energy, including bioethanol. Bioconversions of lignocellulosic biomass to higher-value products normally require multi-step processes that include (i) pretreatment (mechanical, chemical, or biological, etc.) (ii) enzymatic hydrolysis (iii) fermentation process [7, 62]. 7. Potential applications From the industrial and biotechnological standpoint, a wider spectrum of natural polysaccharides-based substrates from agro-industrial and lignocellulosic are available for the (bio)-transformation into high-value and industrially relevancy products. In a recent decade, an extensive development in many integrated procedures related to the biorefinery approach has been achieved for the future. Moreover, owing to the ever increasing socio-economical concerns, for potential applications like alternative energy types, industrially relevant enzymes, platform chemicals, etc., targeted markets have been identified, in recent years. Additionally,

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many of the ongoing research practices around the globe have also discovered that bio-based bio-refinery concept has a great potential to further strengthen the efficiency, yielding, laboring, cost-effective ratio related processes. Whereas, the most striking has been the (bio)transformation of such natural materials to auxiliary resources to full fill the current demands both from industry and modern society [19, 63, 64]. In this context, a part of this review work has focused on prospective applications in the various departments of a current biotechnological era of the modern world. Moreover, the bio-based bio-products, in particular chemicals and energy carriers, needs to be capable of substituting many of the petroleum-based products. Two of the important bio-refinery-based bio-products i.e. (i) platform chemicals, and (ii) energy and bio-fertilizers among many others (Fig. 5) are further summarized in this review under the following sections. 7.1. Platform chemicals In serious consideration of the global issues associated with the extensive use of petrochemicals, from the last decade, there has been increasing research interest in the value of bio-sourced materials. Natural polysaccharides-based biomass represents an alternative candidate material for the production platform chemicals. A considerable attention has been given to many of the agro-industrial or lignocellulosic materials which are carbon neutral, recyclable, renewable and sustainable, in nature, and thus has a noteworthy utilization potential for the production of high-value bio- and non-bio products [4, 7, 19, 20, 65, 66]. Moreover, the bio-refinery breakthrough to produce platform chemicals is a great opportunity to lessen the unavoidable dependence on the petroleum-based chemicals. Based on the recent literature, the production of platform chemicals using raw materials requires a combination of different treatment technologies that additionally involves either single or multiple pre-treatments with an aim to initially separate cellulose, hemicellulose, and lignin fractions. However, based on the raw biomass nature and technological development, the initial pre-treatment scheme

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changes accordingly. Following the initial separation, both cellulose and hemicellulose fractions are subjected to hydrolyze further to sugar monomers whereas, the later lignin fraction is mainly used for the production of aromatic compounds. The sugar monomers, either C5 or C6, are used for the production of xylose, mannose, galactose, acetic acid, ethylene, propylene or butadiene, etc. via the sugar platform of a biorefinery approach. Figure 6 illustrates a schematic conversion of lignocellulose-based biomass into platform chemicals, including xylitol, phenols, glucaric acid, aspartic acid, glutamic acid, syringols, eugenol, toluene, xylene, styrene, 3-Hydroxypropionic acid (3-HPA) and many others [7, 19, 67-73]. 7.2. Bio-energy Likewise, platform chemicals, biomass can also be converted into bioenergy via biomass upgrading approach from the bio-refinery technology. Whereas, biomass upgrading approach involve initial separation and fractionation, liquefaction, gasification, pyrolysis, hydrolysis and fermentation. The term bio-energy is generally referred to as solid, liquid and/or gaseous fuels that has a tendency to be used as an energy source e.g. bio-ethanol and biodiesel and predominantly produced from bio-renewables. In bio-energy production, bio-refinery has numerous advantages/benefits because of the substrate (biomass) diversification and a range of different end-products of interests [74]. Owing to the major environmental benefits, biodiesel production via integrated biorefinery approach is gaining high research interests, in recent years [75]. The classification of bio-renewable bio-fuels is summarized in Table 3, based on their production and/or processing technologies [74]. From the last decade, there is a dire demand to investigate potential alternatives for economical (low-cost) and environmentally friendly fuel resources, owing to the ever increasing price hike, increasing global warming/dimming issues, and cost-effective ratio concerns, etc. Various lignocellulosic materials including bagasse, straws, stalks, and cobs are considered attractive and potential petroleum-alternative materials [7, 19, 20, 63, 64, 76-78]. Apart from their

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availability at a low price, such materials have great potential for their proper utilization as feedstock for biofuel production, because they contain carbohydrates that must be first converted into simple sugars (glucose) and then fermented into bioethanol [60, 61]. The leading nations in bio-ethanol production are Brazil and the USA; the USA is the world’s largest producer of bioethanol [7]. Asian countries altogether account for about 14% of the world’s bioethanol production [7]. So far, many potent processes and technological methods including sequential saccharification and fermentation (SeSF) and simultaneous saccharification and fermentation (SSF) have been developed and reported for maximal bio-ethanol yields from various pre-treated and partially digested natural polysaccharides. Owing to the lignin barrier, the biotransformation of polysaccharide components from lignocellulosic microfibers requires a comprehensive pre-treatment, of the feedstock, for fermenting organisms to convert them into bio-ethanol [7, 19, 79]. To date, in literature many different pre-treatment methods like biological, physiochemical, thermos-mechanical, and enzymatic methods have been reported [19, 80-82]. Most of the in practice pre-treatment methodologies are physiochemical in nature, and among them, hydrothermal-based techniques include a steam explosion, CO2 explosion, or hot H2O treatment. Similarly, chemical-based techniques including dilute acid treatment, alkali treatment, organosolv processes using organic solvents ammonia fiber explosion (AFEX) ammonia recycle percolation and ozonolysis [19, 20]. However, all these pre-treatment methods suffer serious drawbacks. The lack of selectivity often limits the chemical and physical means. Although effective, these processes tend to damage the basic units of these biopolymers by reactions that form new and often unwanted compounds, including toxic and hazardous pollutants. Currently, there is no single pre-treatment method that is perfectly acceptable for the conversion of biomass into biofuel [20]. However, highest ethanol yields have been reported for processes using alkaline hydrolysis of lignin, followed by enzymatic

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digestion of cellulose and fermentation through different process configurations. The bio-based transformation of those mentioned above agricultural and agro-industrial materials into a variety of industrially relevant products require multi-step processes that include pre-treatment, enzymatic hydrolysis, and fermentation [4, 7, 62, 83] (Fig. 7). 8. Considerable limitations Bio-fuel itself has many advantages. However, the feedstock shortage for bio-ethanol is severe. Currently, in China, the most popular non-grain materials are cassava and sweet sorghum, and both of these plants are typically only harvested from small planting areas in dispersed parts of the country. The uncertainty of resources is a key factor that restricts large-scale bio-fuel development, and this uncertainty hardly meets the requirement of a stable supply for continuous industrial production. For lignocellulose, some plants use the method where the manufacturer signs a contract with farmers to guarantee corn stover supplies. Although there are many measures that can be used to solve land and transport problems, most enterprises have still been in short supply of feedstock in recent years. The feasibility and economic efficiency of the industrial production of cellulosic ethanol are determined by the hydrolyzation rate and enzyme costs, which are also keys to fueling ethanol. Pretreatment technology is one of the three bottlenecks in cellulosic ethanol production. The cost of pretreatment is a significant portion of the total cost of producing fuel ethanol using the cellulosic material. Thus, selecting the most effective and clean pretreatment method will be of great significance to reducing the entire production cost. In the context of cellulose ethanol production, the available chemical cellulose material pretreatments are different regarding mechanism, and therefore they all have their respective advantages and disadvantages. Diluteacid and dilute alkali pretreatments have the disadvantages of high cost, equipment corrosion, and environmental pollution. Alkaline solutions, such as ammonia, can break down lignin and the reaction condition is relatively “mild” so that the damage to cellulose and hemicelluloses

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is small. Alkaline-solution-based pretreatments can also eliminate co-products that have the adverse effect on fermentation. However, hemicelluloses are partially lost at high ammonia concentrations. Although ammonia is easy to recycle, the associated costs are still high. Thus, the dilute acid solution is still one of the most widely used technology. In the hydrolysis process, enzymes can repeatedly be used as a kind of catalyst so as to reduce their total required quantities and costs. Research on this technology has far-reaching implications. One way to reuse enzymes is to separate the enzymes from the solution after the reaction via a membrane separation technique. Based on this technique, several technologies have been developed, such as the multi-stage enzymatic hydrolysis technique and immobilized enzymes technique. The main problems of enzymatic hydrolysis are high cost and low conversion rate, which is due to the particularity of material and uncertainty of biochemical reaction process. 9. Concluding remarks and future perspectives In summary, different strategies are in practice for the development of state of the art bio-based biorefinery platform for diverse commercial applications in different divisions or subdivisions of the modern industrial and biotechnological world. The above-discussed data are also revealing about the availability of an abundant lignocelluloses which can be used for onsite platform chemicals production and bio-ethanol production to address the modern energy crises. Furthermore, the integration of green biotechnology into a bio-refinery concept along with the capitalization of natural polysaccharides, low-cost environmental friendly and high impact processing technologies are mandatory, to establish a sustainable future production of the above-mentioned bio-based bio-products. Additionally, green biotechnology, as a set of green principles, has a noteworthy potential to eliminate the consumption and generation of harmful and health-hazardous chemicals and wasteful protection and de-protection steps. Moreover, it also provides a comprehensive set of novel tools with green principles that research community

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either from academia or industry could, and should, consider while developing the next generation of biorefinery platform for the production of genuinely green, carbon-neutral, ecofriendly, recyclable and sustainable products. Enzyme-based processing of various lignocelluloses could also be of particular interest within the biorefinery concept. A wider range of fine chemicals along with other potential products that future biorefineries could produce is extensive. In conclusion, lignocelluloses hold considerable potential to meet the current energy demand of the modern world that are also essential to overcome our excessive dependence on petroleum for liquid fuels that cause environmental pollution or global climate change. Further advanced technologies are crucial for discovery and characterization of new bio-based bioproducts, which are greener and sustainable in nature. Moreover, a noteworthy improvement in the next generation future bio-refineries systems can ultimately lead to the low-cost conversion of lignocelluloses into biofuels and biochemicals. Acknowledgments The literature facilities provided by Tecnologico de Monterrey, Mexico and the University of Agriculture Faisalabad, Pakistan are thankfully acknowledged. The additional support provided by the Emerging Technologies (Focus Group) and the Environmental Bioprocesses from the School of Engineering and Science of Tecnologico de Monterrey, Mexico is thankfully acknowledged. Declaration of interest The authors report no declarations of interest in any capacity, i.e., competing or financial.

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Figure Captions Figure 1 Concept of “sustainability” (Adapted with permission from, [3]). Figure 2 A bio-refinery concept. Figure 3 Lignocellulose bio-conversions into value-added bio-products (Adapted with permission from, [7]). Figure 4 Two platform concept bio-refinery. Figure 5 A schematic representation of lignocellulose-based biomass conversion into bioenergy and bio-fertilizer. Figure 6 Generalized schematic representation of lignocellulose-based biomass conversion into platform chemicals. Figure 7 Generalized schematic representation of lignocellulosic materials bio-conversion into ethanol (Adapted with permission from, [7]).

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List of Figures Figure 1

31

Figure 2

32

Figure 3

33

Figure 4

Biomass

Sugar platform

Syngas platfom

Highvalue products

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Figure 5

35

Figure 6

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Figure 7 Lignocellulosic Materials

Biological Pretreatment

Physical/Chemical Pre-treatment

Enzyme Production and Enzymatic Hydrolysis

Simultaneous Saccharification and Fermentation

Enzymatic Hydrolysis

Enzyme Production

Simultaneous Saccharification and Fermentation

Distillation

Sugar Separation

Fermentation

Distillation

Ethanol

Ethanol

Distillation

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List of Tables Table 1 Major advantages of biomass and biomass fuels (Adapted from Ref. [2], with permission from Elsevier).  Renewable energy source for natural biomass  CO2 neutral conversion and climate change benefits  Transition to low carbon economy, namely from hydrocarbon to carbohydrate and H resources  Use of nonedible biomass  Conservation of fossil fuels  Low contents of ash, C, FC, N, S, Si and most trace elements  High concentrations of volatile matter, Ca, H, Mg and P, structural organic components, extractives, water-soluble nutrient elements  Biodegradable resource with great reactivity and low initial ignition and combustion temperatures during conversion  Huge and cheap resource for production of biofuels, sorbents, fertilizers, liming and neutralizing alkaline agents, building materials, synthesis of some minerals and recovery of certain elements and compounds  Reduction of biomass residues and wastes  Decrease of hazardous emissions (CH4, CO2, NOX, SOX, toxic trace elements)  Capture and storage of toxic components by ash  Use of oceans, seas, low-quality soils and non-agricultural, degraded and contaminated lands  Restoration of degraded and contaminated lands  Diversification of fuel supply and energy security  Rural revitalization with creation of new jobs and income

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Table 2 Major disadvantages of biomass and biomass fuels (Adapted from Ref. [2], with permission from Elsevier).  Incomplete renewable energy resource for biofuels with respect to complete life cycle assessment  Competition with edible biomass (food, feed), fibre and biomaterial productions  Damage of natural ecosystems (water, soil, land use changes, deforestation, biodiversity, land degradation, fertilizers, pesticides, contaminants)  Insecurity of biomass feedstock supply  Indefinite availability of sustainable biomass resources for production of biofuels and chemicals  Omission of sustainable criteria for production of biomass resources for biofuels and chemicals  Lack of global monitoring and control of biofuels production with certification of origin and source  Miss of accepted terminology, methodologies, standards and classification and certification systems  Insufficient knowledge and variability of composition, properties and quality for assessment and validation  High contents of moisture, water-soluble fraction, Cl, K, Na, O and some trace elements (Ag, Br, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Se, Tl, Zn, others)  Low energy density (bulk density and calorific value)  Low pH and ash-fusion temperatures  Low bulk density and fine size of ash with increased dust inhalation risk  Technological problems during processing (agglomeration, deposit formation, slagging, fouling, corrosion, erosion)  Odour, emission and leaching of hazardous components during disposal and processing  Use of extra water, fertilizers and pesticides  Great growing, harvesting, collection, transportation, storage and pre-treatment costs  Regional and seasonal availability and local energy supply  Limited practical experience in biofuel production and unclear utilisation of waste products  Miss of developed biomass markets  High investment cost

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Table 3 Classification of renewable biofuels based on their production technologies (Adapted from Ref. [74], with permission from Elsevier). Generation Feedstock Example First generation biofuels Sugar, starch, vegetable oils, Bioalcohols, vegetable oil, or animal fats biodiesel, biosyngas, biogas Second generation biofuels Non-food crops, wheat straw, Bioalcohols, bio-oil, biocorn, wood, solid waste, DMF, biohydrogen, bioenergy crop Fischer–Tropsch diesel, wood diesel Third generation biofuels Algae Vegetable oil, biodiesel Fourth generation biofuels Vegetable oil, biodiesel Biogasoline

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