Biomass-derived Renewable Carbonaceous Materials for Sustainable

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Perspective

Biomass-derived Renewable Carbonaceous Materials for Sustainable Chemical and Environmental Applications Rajender S. Varma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06550 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Sustainable Chemistry & Engineering

Biomass-derived Renewable Carbonaceous Materials for Sustainable Chemical and Environmental Applications Rajender S. Varma Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký University in Olomouc, Šlechtitelů 27, 783 71 Olomouc, Czech Republic E-mails: [email protected]; [email protected]

KEYWORDS: Carbonaceous materials, Nano-cellulose, Chitosan, Supported nano-catalysts, nanocatalysis, Platform chemicals, Biochar.

ABSTRACT Our Biosphere comprises abundant biopolymers such as cellulose, chitin, and chitosan possessing highly desirable traits with biodegradability and renewability being prominent ones. The presence of amino groups in chitin/chitosan confer these basic polysaccharides numerous advantages to help generate chemical entities with unique and sought-after functional properties, besides nitrogen-enriched carbonaceous materials. This perspective article emphasizes some of the appealing prospects these biopolymers provide because of the recent technological developments in nanotechnology field ranging from traditional catalysis to emerging high-value 1 ACS Paragon Plus Environment

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products. A few examples are presented for sustainable chemical transformations and environmental remediation exploiting the abundant biomass, agricultural residues, and seafood waste. Traditional biomass-derived platform chemicals are also introduced as the potential source of renewable feedstocks from these carbon-based materials, which offer many options for production of sustainable products thus circumventing the traditional use of fossil fuel-derived chemicals.

INTRODUCTION Numerous abundant biopolymers are present in our Biosphere, namely cellulose from woody plants and grasses and chitin predominantly biosynthesized by an array of living organisms such as arthropods and fungi, for example. Majority of these polysaccharides e.g. cellulose, alginic acid, agar, chitin and dextrin’s are naturally occurring molecules and could have numerous industrial applications. Chitosan, deacylated chitin, and chitin have analogous structures, comprise the supporting materials for many animals, and has many attractive properties similar to cellulose (Fig. 1) and starch in terms of biocompatibility, non-toxicity, biodegradability, and more importantly, their renewable nature; they have been exploited in diverse areas e.g. cosmetics, agriculture, biomedical applications and water treatment, among others.1 Unlike cellulose,2 however, the presence of an amino group in chitosan renders it a basic polysaccharide which has the added advantage that it can be easily derivatized to generate chemical entities with useful and desirable functional properties.3,4 This perspective article highlights some of the alluring possibilities these biopolymers offer in view of the recent advancements in nanotechnology domain;2 some sustainable applications in chemical transformation and environmental remediation are exemplified that may offer sustainable solutions, often utilizing 2 ACS Paragon Plus Environment

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abundant biomass and agricultural residues and seafood waste.5 A section on biomass-derived platform chemicals is presented because these carbon-based materials offer many prospects as renewal feedstocks for production of sustainable products including some pre-treatment possibilities facilitating the greener processing of biomass materials.6 Finally, multifold benefits of biochar deployment in various applications are described.

OH OH O

O HO

OH

O O HO

OH

O O HO

OH

OH

O

O HO

NH

n

OH

O

OH

O O HO

NH O

n

O

O

O HO

O

NH2

NH2

n

O

Cellulose

Chitin

Chitosan

OH O O HO OH

O HO

OH

HO

O HO

O

O

HO

OH O O

OH

OH O HO

HO O OH

OH O HO O

HO

OH O

OH

HO

O OH

O n HO

OH O OH O

n

Dextran n

Cyclodextrin

Figure 1. Structures of abundant biopolymers CELLULOSE AND NANOCELLULOSE Cellulose, the most abundant biopolymer on planet, comprises repeating Danhydroglucopyranose monomers, which are connected via -(1-4) glycosidic linkages (Figure 1). The native cellulosic chains form microfibrils, which in turn are bundled in to fibers. During the last decade, several investigations have been conducted that simulate biocomposites by 3 ACS Paragon Plus Environment


melding polymer matrices with natural materials from biomass and agricultural wastes.5 The hydrolysis of cellulose with strong acids cleaves the easily accessible glycosidic chains and generates nano-sized crystalline material which is referred to as nanocellulose; smart processing could generate these nanocelluloses which have been labeled as nanocrystalline cellulose (NCC), cellulose nanofibers (CNF), nanocrystals (CNC), 2 nanowhiskers (CNW), 5 nanocrystallites of cellulose (NCC), and bacterial nanocellulose (BNC), although the definition for each subcategory is still vague. These rod- or ribbon-like entities typically range from 50-1000 nm in length and 3-50 nm in width, with high aspect ratio (10-100), and their morphology is dictated by the source of the starting cellulose and the acidic hydrolytic conditions deployed. Their salient features include high and controllable surface area, crystalline nature and excellent mechanical strength, coupled with sustainable attributes, namely renewability, biodegradability and accessibility on a large scale. The strengthening ability of these nanocelluloses originates from their hierarchical structure. Moores and co-workers have explored applications of nanocellulosic materials as supports for nanoparticles in conjunction with their catalytic utility in the form of metal-nanoparticle-nanocellulose hybrid composites.

Supported Nanoparticles on Nanocellulose-Applications in Catalysis and Environment Cellulose nanocrystals (CNCs) display an array of applications as supercapacitors, flocculants, paper, polymers, aero- and hydrogels, and importantly as chiral materials.7 Although CNCs have been used in catalysis as a support for Pd, Au, and Ag nanoparticles, their use in enantiocatalysis was disclosed, for the first time, by Moores and co-workers.8 Traditionally, cellulose has been pursued as a chiral inducer in enantioselective catalysis, which is often attained via homogeneous organocatalysis. In industrial context, the development of heterogeneous enantiocatalysis is 4 ACS Paragon Plus Environment

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desirable in view of the ease of catalyst separation. CNC’s have been used as 2D chiral inducers and with deposited Pd (PdPs@CNCs) secured 65% ee for the reduction of prochiral ketones in aqueous medium; the structure of the catalysts was established using cryo-TEM.8 Interestingly, PdPs@CNCs could be recycled and reused up to three times via phase separation with no apparent loss of enantioselectivity and activity which bodes well for the use of highly crystalline CNC in asymmetric catalysis. The same research group also showed the direct and inexpensive formation of Ag nanoparticles from bulk Ag metal at room temperature; CNC’s not only provided the high surface area support but also acted as a reducer of Ag wires to afford Ag nanoparticles on biopolymer which were active in the hydrogenation of 4-nitrophenols, alkenes, alkynes and aldehydes.9 The presence of numerous hydroxyl groups on their surface has been exploited for reducing the metal salts thus introducing a greener approach for the assembly of metal nanoparticles (NPs), without the use of any hazardous reducing agents (Fig. 2), a scenario evocative of similar strategy involving plant polyphenols.10,11 The development of advanced characterization techniques for the nanocelluloses and their modified hybrids, will drive additional catalytic applications of nanocelluloses themselves and their composites. The biphasic nanocellulosic suspensions in water not only offer an ionic liquid-like system for metal salts and their NPs but also facilitate the ligand exchange reactions emanating from organometallic precursors in such aqueous CNC suspensions; catalytic reactions pertaining to C–H activations, and C–C coupling reactions are envisaged, among others, in the future.

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Figure 2. Options for metal NP-nanocellulose hybrid composite: (a) external reducing agent, (b) reduction by modified nanocellulose surface, and (c) reduction via nanocellulose, with permission Royal Society of Chemistry from reference 2.

Besides the use of nanocellulosic metal nanocomposites in electrocatalysis for hydrogen evolution or oxygen electroreduction,12 they have found application in photocatalysis which enables the efficient degradation of dyes in aqueous systems; doping these catalysts with nitrogen improved the photocatalytic activity in environmental treatment as exemplified by TiO2 supported on bacterial CNFs for the degradation of methyl orange dye.13 Biodegradable and easily recyclable heterogeneous cellulose–Pd catalyst enables the synthesis of allylic amines via N-allylation of primary and secondary amines; aliphatic and benzylamines upon reaction with a variety of substituted allyl acetates to provide high yield syntheses of target compounds in a ligand-free protocol with simple workup that ensures the recyclability of the 6 ACS Paragon Plus Environment

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catalyst.14 Pd(0) can be readily immobilized on cellulose microcrystals via the reduction of palladium chloride in an aqueous suspension of cellulose with the use of sodium borohydride to afford the cellulose–Pd catalyst (Figure 3); the ensuing catalyst accomplishes the desired Nallylation reaction with ease (Scheme 1).

OH OH OH OH OH OH OH

PdCl2 NaBH4, H2O rt, 15 h

Pd(0)

Pd(0)

OH OH OH OH OH OH OH

Cellulose Cellulose-Pd Catalyst

Figure 3. Synthesis of the cellulose-supported Pd catalyst.

R

OAc +

NH

Cellulose-Pd(0) K2CO3, DMF 100 oC, 15 h

N

R

Scheme 1. Tsuji-Trost N-allylation with allylic acetates using cellulose-Pd catalyst

Enzyme Immobilization on Nanocellulose-Biosensing and other Catalytic Applications Functionalized cellulosic materials are especially sought after in view of their low toxicity and biocompatibility, an ideal situation for immobilization of enzymes and other entities on CNC 7 ACS Paragon Plus Environment


surfaces. A variety of heme proteins (horseradish peroxidase, myoglobin and hemoglobin) have been immobilized on Au NP-adorned bacterial CNF’s; such horseradish peroxidase biosensor is highly sensitive to H2O2 with detection limits approaching 1mM.15 Thiol sensors, implicated in the diagnosis of several metabolic diseases, have been developed and used for sensing 2mercaptoethanol where they exhibit marked color changes;16 the mesoporosity of the matrix facilitates the accessibility of NPs to analytes. Cyclodextrin glycosyl transferase and alcohol oxidase enzymes have been immobilized on Au decorated CNCs which exhibits biocatalytic activity in conjunction with excellent stability,17 a trait that bodes well for their large-scale application in industrial context. More recently, a greener approach has been used to assemble cellulose-based material via treatment with citric acid and cysteine; this conjugation and chemical modification is accomplished by simply soaking cellulose in concentrated aqueous citric acid and cysteine solutions and then drying above 80 oC.18 The ensuing fluorophores exhibit remarkable UV absorption capacity, a selective quenching for chloride ions and excellent fluorescence with potential applications in UV shielding, chemical sensing and anti-counterfeiting; the main fluorophore is thiazolo pyridine carboxylic acid, formed in a complex sequence of dehydration reactions involving citric acid and cysteine.19 The salient features of this strategy are the use of water as the sole solvent during the reaction and processing and modification of cellulose without dissolution thus providing an inexpensive pathway that prevents pollution by avoiding the use of volatile organic solvents. A multifunctional, durable, and superhydrophobic waterproof paper has been prepared from waterborne fluorinated cellulose nanofiber building blocks;20 the protocol avoids the use of organic solvents and make use of fluoroalkyl functional CNFs to incorporate chemical functionalities that withstand mechanochemical insult. A wide 8 ACS Paragon Plus Environment

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variety of paper-based technologies may be a beneficiary of this development wherein the integrity enhancement between the fibers is achieved by simple chemical functionalization of native hydrophilic CNF in an aqueous medium.20 The surface functionalization of CNCs that is devoid of metals has been investigated in recent years. Copper-catalyzed azide-alkyne cycloaddition reaction has enabled grafting of imidazolium salt on the surface;21 ensuing heterogeneous systems, wherein anion exchange is readily feasible, can afford attractive catalytic and ion-exchange systems. The abundant hydroxyl groups on the surfaces of hydrochloric acid treated cellulose nanofibers (CNFs) have been used for the hydrolysis of amides, esters and monophosphates.22 The hydrolytic power of such systems is influenced by the cellulose source and size; potential applications can be in the decomposition of coat proteins thus promoting their use as artificial enzymes, termed cellizymes. Interestingly, chiral-specificity has been revealed for model amino acid substrates with activated amide bonds where specificity significantly associated with amino acids and crystal structure of CNFs has been observed.23 GIFT OF THE SEAS: CHITIN AND CHITOSAN Chitin, the second most abundant natural biopolymer after cellulose, comprise a polysaccharide structure (Figure 1) and is found in the exoskeleton of arthropods such as crustaceans (e.g., shrimp and crabs), and is a byproduct of fisheries industry. Chitin, in close resemblance to cellulose, is an extended-chain co-polymer of β-(1→4)-linked 2-acetamido-2-deoxy-β-D-glucose units. Once the degree of acetylation of chitin falls below 50%, it is referred to as chitosan; this deacylated derivative of Chitin, is a heteropolymer containing both glucosamine units and acetylglucosamine units. Chitin forms a microfibrillar configuration in a protein matrix in living



systems and their sizes reliant on their biological roots. This biopolymer from nature can be used directly as a heterogeneous catalyst without any modification and it can perform important roles as a biodegradable and natural polymer in diverse appliances. Chitin and chitosan exhibit outstanding bioresorbable properties,24 in addition to their other attractive features namely hydrophilicity, biocompatibility, binding to cellular systems, and speeding up of wound healing. Not surprisingly, this justifies their utility in a variety of applications such as, cosmetics, water treatment, membranes, hydrogels, fuel cells, surface conditioners, adhesives, pharmaceuticals and biomedical manufacturing, among others.25 The capacity to produce nanofibers from chitosan, a discarded seafood waste, may provide a means to grow biocompatible skeletons to restore broken or malfunctioning tissues. The advancements in electrospinning techniques has provided ideal pathways to generate such polymer nanofibers with two essential and desirable attributes, namely large surface area-to-volume ratio and high porosity with incredibly small pore size.26 Chitosan as Biodegradable and Efficient Catalyst Numerous catalytic applications27 have been documented for chitosan which include Huisgen cycloaddition,28 Michael addition,29 Ullmann coupling reaction,30 Suzuki cross-coupling,31 aldol and Knoevenagel condensation reactions,32 and Heck reaction.33 Multi-component one-pot synthesis of nitrogen heterocycles has been reported using commercial chitosan in 2% aqueous acetic acid at 60-65 °C; starting materials include aryaldehydes, dicarbonyls, and 2aminobenzothiazole/3-amino-1,2,4-triazole urea/thiourea.34 The underlying noticeable feature of these methodologies are the use of a benign catalyst, ease of operation and recyclability, and processes that are often devoid of organic solvents. A similar eco-friendly method has been reported for the efficient synthesis of α-amino nitriles (Strecker reaction) and imines that used 10 ACS Paragon Plus Environment

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heterogeneous chitosan biopolymer without any modification; low catalyst loading, recyclability of the catalyst and the operational ease are again some of the salient features.35 Thus, chitosan, a natural poly-glucosamine, bearing both primary and secondary hydroxyl groups and amino groups in high concentrations, serves as a mild bifunctional heterogeneous catalyst in organic synthesis. Metals Supported on Chitosan Huisgen 1,3-dipolar cycloaddition reaction has become one of the most important reactions in history,36,37 a representative of click chemistry that can be easily adapted to generate a library of compounds. This reaction typically involves the ligation of azides and terminal alkynes to produce triazoles and often requires a copper salt in presence of a base; catalyst being Cu(I) salt or Cu(I) generated in situ via the reduction of Cu(II) salts. Baig and Varma immobilized copper sulfate on chitosan, termed Chit-Cu, and utilized that for the azide-alkyne cycloaddition reactions in aqueous media at ambient temperature (Scheme 2);38 the reaction could be monitored visually as the product crystallizes out from the aqueous media and the catalyst recycled and reused several times without loss of activity.

R

N3

Chit-CuSO4 + R

H2O, room temp

R

N N N

R

Scheme 2. Chit–Cu, catalyzed dipolar cycloaddition of benzyl azide and phenyl acetylene. The hydration of nitriles, an important conversion in the pharmaceutical industry, has been achieved by several methods including magnetically supported ruthenium catalysts.39 Chitosansupported Ru catalyst, however, could be obtained on a large scale in aqueous media (Scheme 3) 11 ACS Paragon Plus Environment


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and catalyzed—this transformative hydration of nitriles in high yields with superb selectivities; the reaction proceeds exclusively in an aqueous medium under neutral conditions (Scheme 4).40

OH OH O HO

OH O NH2

O HO

OH O

O HO

NH2

O HO

OH O

O

RuCl3

O

O

NH2

H2N Ru

water, NH3 over night

NH2

O

O HO

OH

HO O

O HO

NH2

O HO

O

O

NH2

Scheme 3. Synthesis of Chitosan-Ru (ChRu) catalyst

O CN N

ChRu/H2O

NH2 o

MW, 60 min, 100 C

N

Scheme 4. The hydration of nitriles catalyzed by ChRu

Microbeads and Hydrogels from Chitin Microbeads are spherical material particles (typical size ranging from 5m -1 mm) which have found widespread applications in food science, biomedical diagnostics, and as drug delivery mediums besides a popular means to enhance exfoliation processes in cosmetics and hygiene products.41 However, the adverse environmental impact of microbeads originating from synthetic polymers such as polyethylene, polypropylene, poly(methyl methacrylate) etc., and the resulting 12 ACS Paragon Plus Environment

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menace of microplastics in the water bodies is playing havoc on ecosystems as they enter in to marine food chains, and pollute lakes and oceans.42 A sustainable alternative has been advanced by Rogers and co-workers where they have prepared chitin microbeads using ionic liquid, 1ethyl-3-methylimidazolium acetate ([C2mim][OAc]; which not only extracts the chitin from waste shrimp shells but also produces porous microbeads by a coagulation process deploying eco-friendly, polyethylene glycol (PEG).41 Interestingly, the process generates microbeads of homogeneous narrow size and shape which are not obtainable from commercial grade chitin, underlying the requirement for high molecular weight chitin for proper bead-material formation. The application of ensuing beads to load up cargo of active compounds of varied structural types and its subsequent release (up to ~ 90%) in aqueous medium in a couple of hours augurs well for the technology to be adapted for commercial use of chitin microbeads, thus avoiding the known pitfalls of plastic microbeads. Hydrogels are crosslinked three-dimensional polymeric hydrophilic networks which can swell enormously via absorption of large amounts of biological or aqueous fluids. They comprise beads, microspheres, nanogels from nanoparticles, etc., emanating from polymers and exist in a variety of physical forms including membranes; the term hydrogel is labeled aerogel or cryogel depending on the mode of drying followed via supercritical or freeze-drying process, respectively. In view of the biocompatibility, renewability, processability, and biodegradability of bioploymers such as chitin, chitosan and cellulose, hydrogels made from these natural biopolymers are garnering lots of attention in industrial context. Recent discoveries on their dissolution in ionic liquids, and deep-eutectic solvents help promote the formation of physical hydrogels involving any of the interfaces, namely hydrogen bonds, electronic or hydrophobic interaction, and van der Waals forces. Rogers and co-workers have provided a critical review on 13 ACS Paragon Plus Environment


such hydrogel systems and numerous applications are projected ranging from water purification, medical and pharmaceutical use and to a lesser degree on their utility in electronic and optical fields.3 Future emphasis and challenges have been outlined for injectable hydrogels assembly inside the body for non-invasive surgery aimed at tissue engineering or controlled drug-delivery or exploiting their mechanical strength or photonic properties. The production of high molecular weight native biopolymers may provide an array of potential applications in the future. LIGNOCELLULOSIC RESIDUES AS VALUABLE AND RENEWABLE RESOURCE The persistently increasing oil requirements due to the growing world population and the reserves depletion at a disturbing rate has rekindled the desire to search for renewable alternatives, including biomaterials, bio-fuels and fine chemicals. Lignocellulosic biomass, in this context, has been recognized as a renewable feedstock of great value to produce value-added platform chemicals.

Platform Chemicals from Renewable Resources A catalytic transfer hydrogenation protocol has been reported recently that exploits renewable carbohydrates and lignin derived from biomass as the hydrogen source wherein chemo- and stereoselective hydrogenation of alkenes, alkynes, and carbonyls compounds was revealed;43 several functional groups were readily accommodated and the reduced products could be secured in high yields. Interestingly, the synthesis of stereoisomers could be achieved by simply varying the reaction conditions via an operationally simple method that capitalizes on abundant and renewable materials such as lignin and carbohydrates. In addition to such direct use of biomass as such or via simple modifications, the exploitation of lignocellulosic biomass and agricultural residues as a source for an array of platform chemicals has been actively pursued during the last 14 ACS Paragon Plus Environment

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decade.44 Although the biomass composition may vary depending on its origin, it is generally composed of cellulose (~40-50%), hemicellulose (~25-35%), and lignins (~10-25%). Characteristically, these fractions are segregated and hydrolyzed into their major constituting monomeric sugars (cellulose and hemicellulose into glucose and xylose, respectively), followed by their acid-catalyzed dehydration, to afford products such as 5-hydroxymethylfurfural (HMF) from glucose and furfural (FUR) from xylose.45 These ensuing carbonyl compounds are versatile platform molecules, as they can be converted to an assortment of value-added products, namely, levulinic acid (LA), alkyl levulinates (ALs) and γ-valerolactone (GVL), among others, as summarized in a comprehensive review that summarizes the most recent advances in terms of catalytic upgrading of carbohydrates (Scheme 5).45 Accordingly, the growth of acid catalysts for the valorization of lignocellulosic biomass has proliferated in the catalysis field in the last few years.46,47 The proficiency of versatile zeolites as catalysts in the valorization of biomass and upgrading of the biomass-derived chemicals has been described by Marcio Paixao et al.; 48 with a range of Si/Al ratio’s and inherent three-dimensional medium size pore systems, zeolites in their acidic form or exchanged with transition metals offer unique prospects to exploit the biorenewable materials. Interestingly, magnetic ZSM-5 zeolite with core-shell type structure has been synthetized via encapsulation of the magnetite particles in the ZSM-5 zeolite (Si/Al = 14) grains using a cationic polymer followed by calcination. The ensuing catalyst was used for the valorization of bio-derived furfuryl alcohol; catalytic system with a tunable selectivity to γvalerolactone, alkyl levulinates and even levulinic acid has been achieved by merely changing the reaction conditions. Additionally, the catalyst could be easily recovered and reused for multiple reaction cycles without meaningful losses in activity.49 The approach was extended



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further to synthesis of magnetically recoverable β zeolites exchanged with late transition metals such as Fe, Pd, and Ir and their catalytic activity appraised for the transformation of numerous Lignocellulosic biomass

Cellulose

Depolymerization

Depolymerization

OH O

HO HO

OH

BA ROH

OH -H2O OH

Glucose

O

HO HO

ROH -H 2O OH

-3H2O

BA

O HO

O 5-HMF BA

Levulinic O acid

ROH -H2O

ROH -H2O

Alkyl levulinate O

H2 -AL

O

LA H2

Furfuryl alcohol BA

O

ROH -H2O

OR

2-MeTHF

O

ROH Furfurylalkyl ether

H2 -H2O

LA

2-Methylfuran

O

BA

RO

O

O OH

O

O

LA

H2

LA H2

5-AMF ROH BA H 2O -HCO2R

-H2O

O

LA

O RO

BA

HO

O

BA

OH

Furfural dialkyl acetal

-3H2O

BA

2H2O -HCO2H

O

BA

OH

Alkyl fructoside

Fructose

OR O

2ROH -H2O

Furfural

OH

OH OR

Alkyl xyloside

BA

O

O

HO HO

ROH OH OH -H2O

O

OR O OH

BA

BA

BA -3H2O

LA

OH O OH OH

Xylose

Alkyl glucoside

HO

O

HO HO

OH OR

LA

HO

Hemicellulose

OH

LA

OH

H2 GVL

1,4-petanediol

LA H2

Liquid alkanes

-2H2O

Scheme 5: Value-added chemicals via transformation of biomass-derived carbohydrates. BA, Brønsted Acid; LA, Lewis Acid; 5-AMF, 5-alkoxymethylfurfural; 5-HMF, 5hydroxymethylfurfural; 2-MeTHF, 2-methyltetrahydrofuran; GVL, -valerolactone; 5- -AL, angelica lactone, Adapted from reference 48 with permission from Elsevier 16 ACS Paragon Plus Environment

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bio-derived compounds to other value-added platform chemicals. The transition metal exchanged zeolite matrix displayed remarkable efficiency in the transformation of furfural and furfuryl alcohol to isopropyl levulinate, a platform chemical of immense potential. Additionally, the reduction of furfural to furfuryl alcohol could be accomplished using the same catalyst with sodium formate as a hydrogen source in aqueous medium; interestingly, the Pd-exchanged catalyst could be regenerated via calcination.50 Similarly, valorization,51 chemical modification52 and transformation of lignin,53 a phenolic oligomer, has been pursued for quite some time to generate valuable compounds and fuels54 but the success for pragmatic applications has not appeared yet.55 The general efforts in this area starting from the pretreatment processes6 to biocatalytic and chemical conversion of lignin and sugars in to chemicals, has been explored extensively but the cost-effectiveness has not been realized so far which necessitates that every bit of lignocellulosic components need to be valorized effectively, including the residual biochar as discussed below. As suggested by Sheldon, appropriate sustainability metrics need to be created so that these developments can be realistically compared to the convention fossil resource-derived chemicals;56 their use as applicable to the chemical industry, have been reviewed recently.57

Pyrolysis of Biomass-Carbon Sequestration and Biochar Formation The pyrolysis of biomass and related agricultural- and seafood-derived waste can generate valuable biochar which is essentially a recalcitrant carbonaceous material endowed with interesting physicochemical attributes.58 It has been widely used for numerous applications in



assorted areas such as energy generation, soil augmentation and remediation, catalysis, carbon sequestration, and production of activated carbon and related materials, among others (Figure 4).

Figure 4. Varied applications of biochar, with permission from Springer, reference 58.

The biomass to char transformation can be accomplished by several means namely chemical, biological, and thermochemical processes. Among these options, because of its shorter processing duration, higher yields, and the exploitation of whole biomass, thermochemical approach is often deployed in view of its energy efficiency.59 The heating rate and operational pyrolysis temperature are the constraining issues that influence the ensuing biochar properties


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such as volatile matter, fixed carbon, porosity, surface area and pore size distribution, etc. The origin and the condition of biomass, dry or wet, has been evaluated60 and so are the alternate means of heating such as microwave irradiation.61 Essentially, lower pyrolysis temperatures (~400 °C) with slower heating rate generate biochar in higher yields and volatiles, possessing good cation-exchange capability and electrical conductivity as it allows more time for the secondary reactions to take place.62 On the other hand, higher temperatures (>550-600 °C) via flash pyrolysis at an expeditious heating profile, produce biochar in lower amounts but rich in aromatic carbon, microporous surface area, and alkalinity.63 Hydrothermal Carbonization (HTC) Hydrothermal carbonization (HTC) is an alluring thermochemical process wherein the transformation of biomass admixed with water occurs affording carbonaceous materials, often termed hydrochar, under mild conditions ~100–350 °C;64 ensuing material finds numerous usages from CO2 sequestration and soil amendments to electrochemical products, for instance batteries and capacitors. The salient advantages of this strategy are that wet biomass, including municipal waste can be directly processed thus saving on energy.65 Furthermore, the use of water helps sequester some of the ensuing gases thus reducing the environmental pollution impact as well.66 Unfortunately, the low porosity and surface area of resulting hydrochars precludes some of biochar’s typical applications.67 Ionothermal Carbonization (ITC) Porous carbon materials can be accessed in a single-step process via ionothermal carbonization (ITC). In contrast to hydrothermal carbonization where water is used, ionic liquids (ILs) are deployed as the medium for ITC68 which readily produces porous carbon materials without any 19 ACS Paragon Plus Environment


additional use of scaffolds; besides its role as an efficient catalyst, IL serves as a template for generation of pores.69 In spite of the notable promise of biomass-derived carbonaceous entities generated via thermochemical processes, data on large-scale operations is practically non-existent; no industrial scale adaptation of HTC process has occurred and no efforts have been expanded for the scale-up of ITC in view of the excessive costs of ILs. Applications of Biochar Carbons-Electrochemical, Catalysis and Environmental Remediation In addition to carbon sequestration, energy storage, gas purification, water treatment, and delivery of drugs, biochar has appeared as a practical resource for producing numerous innovative specialty materials such as carbon nanotubes, graphene’s, supercapacitors, as well as coloring vehicles and fillers for composites. Carbon materials created at elevated temperatures (>1200 °C) are likely to have encouraging electrochemical properties;70 the advanced uses of activated carbon has been demonstrated as electrode materials in electrochemical energy and fuel cell catalytic applications. The exceptional physiochemical properties of C-based materials71 have been exploited for use as an electrode as shown recently where corncob material was used to generate porous carbon via a thermal process;72 higher surface area of 1210m2/g, with a vastly stabilized mesoporous configuration, the ensuing carbon electrode displayed an excellent capacitance of 120 F/g in a symmetrical cell which endured 100,000 cycles. In addition to their inexpensive nature and favorable ecological influence, these C-based electrodes exhibited a superior electrochemical function when compared to traditional synthetic polymer-based carbonaceous materials.


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The gasification of biochar is relatively slower than that of biomass as there is a rapid loss of mass from the later due expelled volatile constituents in addition to CO2 and CO [92].73 Some metal catalysts do increase the more active sites on the biochar surface via the adsorption of gasifying entities thus enhancing the potential of biochar to replace coal in power generating stations. The co-gasification of biochar produced, from empty fruit bunches, increased the yield for hydrogen from 5.5 to 28 % along with the carbon conversion from 76 to 84 %.74 As biochar holds substantially greater quantities of stable carbon in the soil relative to other organic materials, it functions as a promising carbon sequestering vehicle; biochar can reside stably in soil for a much extended periods of time independent of mineralization and temperature fluctuations. Plant growth is increased by biochar as a consequence of expanding bioavailability of required nutrients and water thus creating microenvironments for the growth of essential soil microorganisms.75 Additionally, its amendment serves as a soil conditioner by enhancing water retaining capacity of soil and helps assist in the sequestration of hazardous heavy metals; it optimizes the ensuing pH, upsurges the total phosphorus and nitrogen, stimulates plant root growth, and provides home to beneficial bacteria and soil fungi. Heterogeneous catalysts play a crucial role in chemical synthesis and transformations especially when the supporting materials have a desired porosity and high surface area for accelerating the reactions. Although charcoal has been used as a support for metals since ancient times, hydrochars have some of these attributes that have been exploited in the form of carbonaceous nanofibers.76 As reuse and recovery of precious nanocatalysts77 is becoming significant for ecological reasons and pressing environmental awareness, tremendous efforts have been made wherein imparting



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magnetic character to such supports achieves most of the desired objectives;78 the active part of the catalyst resides on the sites over the porous supports. Additionally, the cumbersome activities associated with centrifugation and tedious filtration have been circumvented via a magnetic carbon supported Pd catalyst as exemplified for the hydrogenation reactions (Schemes 6, 7).79

Pd Pd FeSO4.7H2O +

1) Cellulose, 6 h, rt

NH4OH/H2O

Pd Fe3O4 Pd

2) PdCl2, 8 h, rt

o

1h, 50 C

o

Fe2(SO4)3

Fe3O4

3) Calcination, 3 h, 450 C

Pd

Pd Pd

Pd

Fe3O4@CPd- catalyst

Scheme 6. Synthesis of magnetic supported Pd catalyst, reproduced from reference 79

Ph Ph

Ph

Fe3O4@CPd H2-atom, Solvent, rt

carbon

Ph

Scheme 7. Fe3O4@CPd-catalyzed hydrogenation of alkene, reproduced from reference 79

The earlier described carbon-coated magnetic Pd catalyst, synthesized by in situ generation of nanoferrites and incorporation of renewable cellulosic carbon (calcination) could be deployed for 22 ACS Paragon Plus Environment

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the amination reactions, arylation of halides in cross coupling reactions and for the oxidation of alcohols (Scheme 8).80

O H

R

Oxidation OH R

X N R

B(OH)2 R

R

R R

X

H N

R = Pd

Scheme 8. Coupling and oxidative reactions using carbon-coated magnetic Pd catalyst, with permission from Royal society of Chemistry, reference 80

Increasing atmospheric CO2, due to its confirmed influence on climate change via greenhouse effect, needs to be mitigated. Among diverse strategies explored these days, the ready availability of hydrochars as a capturing agent for CO2 has shown merit in view of their controllable porosity and the large surface area as exemplified in a single-step synthetic process for generating microporous carbons by processing biomass with solid KOH.81 In another study, lignocellulosic material, of empty fruit bunch origin, was processed hydrothermally which afforded microporous carbon on KOH activation; the ensuing carbonaceous material is endowed with favorable microporosity (0.55cm3/g) and large surface area (2511 m2/g), attributes which translated to CO2 adsorption capacity of 3.71 mmol/g under ambient conditions.82 Activated carbon, a product derived from activation of the biochar itself,83 is an age-old material that has seen significant advancements in past decades; it has been sourced from lignocellulosic 23 ACS Paragon Plus Environment


biomass such as seeds, bagasse, shell, wood, straw, husk, among other agricultural waste residues and the ensuing biochar from them.83,84 Their numerous applications start from Alchemy days and prominent ones include electrodes for batteries, capacitors, water purification and sewage treatment and of course, catalytic supports for metals and specific biomedical usages. The newer applications and developments for activated carbon are gaining traction in view of the cheaper and renewable earth-abundant resources, namely biochar and its precursor, biomass; the major and critical ascribed attributes being porosity and the surface area. Potable water and wastewater treatment are critical problems worldwide as numerous organic compounds such as dye and pharmaceuticals or their metabolites need to be removed, preferably using air or oxygen to oxidize the pollutants.85 Metal catalysts, deploying earth-abundant Cuand Fe-supported on porous activated carbon, help assist in such oxidative transformation of organic pollutants;86,87 heterogeneous Cu-catalyst on highly porous activated carbon serves as an efficient catalyst to remediate printing and dyeing wastewater from textile industry86 while Fesupported on activated carbons readily oxidize phenols using H2O2.87 Ideally, the biochar obtained from waste resources that are not of value to humans or animals, would be ideal for use and should be preferred. The removal of Pb2+ ions has been accomplished by H2O2-activated hydrochar obtained from peanut hulls.88 Similarly, hydrochar derived from orange peels, via H3PO4 activation, attained a high specific surface area of 618 m2/g and was shown to adsorb a variety of pharmaceutical residues because of its morphology.89 A better separation technique was demonstrated when a magnetic hydrochar was prepared from rice straw by incorporating iron; Triclosan, a common pollutant used in personal care products and often found in water bodies, could be readily adsorbed and the contents isolated using an external magnetic field.90 24 ACS Paragon Plus Environment

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A major component of lignocellulosic resource, lignin, has been utilized as fuel in pulp and paper industry for centuries. Apparently, after extensive explorations over this extended period and continued search for better applications,53,91 there still are no commercially viable uses because of its complexity and variation of structural features; lignin-based adsorbents are being explored for removal of heavy metals in water in view of their stability and abundant nature.92 The use of alternative activation energy such as ultrasound and coupling it with sustainable photochemical pathways may provide some answers.93-97 Nanocellulose and their Functional Hybrid Materials in Environmental Remediation Hybrid microcrystalline cellulose (MC)/TiO2 nanophotocatalyst,13 from benign precursors such as MC and TiCl4, have been prepared via in situ facile synthesis. Utilizing statistics-based factorial design (FD) could optimize the catalytic adsorption of methylene blue (MB) from water; TiO2/MC nanocomposites could photocatalytically diminish 40–90% of MB in 4 h.98 Nanocellulosic fibers, modified via nano-reinforcement of functional groups, have been used for the clean-up of toxic cationic metals from water; augmentation of stability and sorption efficiency for Cr (III), Pb (II), Cd (II), and Ni (II), ions, has been demonstrated.99 The advantageous features of nanocellulosic materials in water filtration membranes and environmental remediation have been exploited which comprise low environmental influence, high surface area-to-volume ratio, ease of surface functionalization, higher strength, and sustainable value because of their renewability and abundance; they surpass carbon nanotubes (CNTs) in terms of several chemical and physical attributes related to cost efficiency and disposal in water treatment technologies.100



Cellulose diacetate (CDA) aerogels with honeycomb-like structural morphology have been prepared by chemical cross-linking followed by cautious solvent interchange to water, and freeze drying; highly porous and ultralight (>4 mg/mL) aerogels display highest oil and water uptake of 112 and 92 g/g, respectively. Further modifications of these CDA aerogels with organosilanes could make them oleophilic and hydrophobic using chemical vapor deposition technique. Such amended CDA aerogel hold oil while their retaining their mechanical integrity during expeditious oil cleanup operations in marine settings.101

CONCLUDING COMMENTS AND OUTLOOK The sustainability evaluation of the overall chemical generation from biomass is of utmost importance in addition to the ready availability of the biomass resources; raw materials explored for chemical processing are not viable if they are currently being deployed as essential feedstocks for humans and non-human food chains. A case-in-point is wheat straw, a common fodder for raising animals and in short supply due to the increase in meat consumption in developing and populous countries of Asia. The economic feasibility plays an essential and vital role in the eventual translation and implementation of commercial production processes. It is imperative that all generated components from biomass valorization need to be fully exploited for high-value purposes with highest conversion and selectivity; preferably under solvent-free conditions and at ambient temperature, wherever possible, deploying earth-abundant and safer catalysts. Besides capital costs and land use, the collection of lightweight biomass materials and cost of transportation, is a labor- and energy-intensive endeavor.102 Consequently, it would be advisable to have modular processing devices that can be easily hauled to the materials’ source where 26 ACS Paragon Plus Environment

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initial processing and reduction of weight can be accomplished. Ideally, waste agricultural residues and discarded foodstuff need to be the first choice, which can exploit the visible light or solar energy in the field.55 The extensive investigations on lignocellulosic materials have been performed during the last several decades as we see the plants and tree growth in common sight. However, practically, two thirds of our planet is water and there are plenty of sea creatures, besides insects, which are covered in very similar or even better biopolymer, Chitin. Rather than exploiting this valuable, abundant and renewable resource, tipping fees are required to discard this valuable material to landfills, which are already in short supply. It is ironic that we meticulously explore nitrogenenriched carbonaceous materials by doping e. g. graphene with nitrogen or explore means to generate graphitic carbon nitrides and yet ignore plentiful nitrogen-rich resource such as chitin and chitosan.103 Additionally, the growth of algae and ensuing plethora of products from brackish or salty ocean water is an enticing proposition for exploration in the future as the journey continues with newer catalytic processes for biomass-derived hexoses and pentoses.104 The demonstrated prowess of biochar to enhance the plant growth and soil fertility via beneficial soil microorganisms is clear but its ecological interactions with microorganisms and plant roots is still undefined. Life-cycle assessment may assist in this context to weigh in on the manifold biochar advantages against any unforeseen drawbacks in addition to delineating the long-term efficacy of biochar-originated carbon in immobilization of inorganic and organic pollutants.

AUTHOR INFORMATION Corresponding Author: E-mails: [email protected]; [email protected] 27 ACS Paragon Plus Environment


ORCID Number: 0000-0001-9731-6228

Conflict of Interest: Author declares no conflict of interest.

ACKNOWLEDGMENT This work was supported by the Operational Program Research, Development and Education European Regional Development Fund, project no. CZ.02.1.01/0.0/0.0/16_019/0000754 of the Ministry of Education, Youth and Sports of the Czech Republic.

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(72) Qu, W.-H.; Xu, Y.-Y.; Lu, A.-H.; Zhang, X.-Q.; Li, W.-C., Converting Biowaste Corncob Residue into High Value Added Porous Carbon for Supercapacitor Electrodes. Bioresour. Technol. 2015, 189, 285-291. doi.org/10.1016/j.biortech.2015.04.005 (73) Di Blasi, C., Modeling Chemical and Physical Processes of Wood and Biomass Pyrolysis. Prog. Energy Combust. Sci. 2008, 34, 47–90. doi.org/10.1016/j.pecs.2006.12.001 (74) Salleh, M.A.M.; Kisiki, N.H.; Yusuf, H.M.; Ghani, W.A.W.A.K., Gasification of Biochar from Empty Fruit Bunch in a Fluidized Bed Reactor. Energies 2010, 3, 1344–1352. doi:10.3390/en3071344 (75) Lehmann, J., Rillig, M.C., Thies, J., Masiello, C.A., Hockaday, W.C., Crowley, D., Biochar Effects on Soil Biota—A Review. Soil Biol. Biochem. 2011, 43, 1812–1836. doi.org/10.1016/j.soilbio.2011.04.022 (76) Hu, B.; Yu, S.-H.; Wang, K.; Liu, L.; Xu, X.-W., Functional Carbonaceous Materials from Hydrothermal Carbonization of Biomass: An Effective Chemical Process. Dalton Transactions 2008, (40) 5414-5423. doi: 10.1039/b804644c (77) Polshettiwar, V.; Varma, R. S., Green Chemistry by Nano-Catalysis. Green Chem. 2010, 12, 743-754. doi:10.1039/B921171C (78) Gawande, M. B.; Branco, P. S.; Varma, R. S. Nano-magnetite (Fe3O4) as a Support for Recyclable Catalysts in the Development of Sustainable Methodologies. Chem. Soc. Rev. 2013, 42, 3371-3393. doi:10.1039/C3CS35480F



(79) Baig, R. B. N.; Varma. R. S. Magnetic Carbon Supported Palladium Nanoparticles: An Efficient and Sustainable Catalyst for Hydrogenation Reactions. ACS Sustain. Chem. & Eng. 2014, 2, 2155-2158. doi:10.1021/sc500341h (80) Baig, R. B. N.; Nadagouda, M.; Varma. R. S., Carbon-Coated Magnetic Palladium: Applications in Partial Oxidation of Alcohols and Coupling Reactions. Green Chem. 2014, 16, 4333-4338. doi:10.1039/C4GC00748D (81) Singh, G.; Kim, I. Y.; Lakhi, K. S.; Srivastava, P.; Naidu, R.; Vinu, A., Single Step Synthesis of Activated Bio-carbons with a High Surface Area and their Excellent CO2 Adsorption Capacity. Carbon 2017, 116, 448-455. doi.org/10.1016/j.carbon.2017.02.015 (82) Parshetti, G. K.; Chowdhury, S.; Balasubramanian, R., Biomass Derived Low-cost Microporous Adsorbents for Efficient CO2 Capture. Fuel 2015, 148, 246-254. doi.org/10.1016/j.fuel.2015.01.032 (83) Azargohar, R.; Dalai, A.K. Biochar as a Precursor of Activated Carbon. Appl. Biochem. Biotechnol. 2006, 129–132, 762–773. doi.org/10.1385/ABAB:131:1:762 (84) Juntgen, H. Activated Carbon as Catalyst Support: A Review of New Research Results. Fuel 1986, 65, 1436–1446. doi.org/10.1016/0016-2361(86)90120-1 (85) Otowa, T.; Nojima, Y.; Miyazaki, T. Development of KOH Activated High Surface Area Carbon and its Application to Drinking Water Purification. Carbon 1997, 35, 1315–1319. doi.org/10.1016/S0008-6223(97)00076-6 (86) Hu, X.; Lei, L.; Chu, H.P.; Yue, P.L. Copper/Activated Carbon as Catalyst for Organic Wastewater Treatment. Carbon 1999, 37, 631–637. doi.org/10.1016/S0008-6223(98)00235-8 40 ACS Paragon Plus Environment

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PHOTOGRAPH AND BIOGRAPHY

Rajender S. Varma Prof. Varma (H-Index 104, Highly Cited Researchers 2016-18; Publons Awardee 2018) was born in India (Ph.D., Delhi University 1976). After postdoctoral research at Robert Robinson Laboratories, Liverpool, U.K., he was faculty member at Baylor College of Medicine and Sam Houston State University prior to joining the Sustainable Technology Division at the US Environmental Protection Agency in 1999. He has a visiting scientist’s position at Regional Centre of Advanced Technologies and Materials, Palacky University at Olomouc, Czech Republic. He has over 45 years of research experience in management of multidisciplinary technical programs ranging from natural products chemistry to development of more environmentally friendly synthetic methods using microwaves, ultrasound, etc. Lately, he is focused on greener approaches 44 ACS Paragon Plus Environment

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to assembly of nanomaterials and sustainable applications of magnetically retrievable nanocatalysts in benign media. He is a member of the editorial advisory board of several international journals, has published over 515 papers, and has been awarded 16 US Patents, 6 books, 26 book chapters and 3 encyclopedia contributions with 36,000 citations.



TOC

Sustainable chemical transformations and environmental remediation strategies are described that exploit the abundant biomass, agricultural residues, and seafood waste.

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Biomass-derived Renewable Carbonaceous Materials for Sustainable

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