Agricultural waste biomass-assisted nanostructures: Synthesis and application

1.1 Rice husk and straw

Rice husks (or rice hulls) are the hard protecting pods of rice seeds that are 20-22% of total produce of rice, and large amounts of rice husk are currently prepared by rice mill industry as a one of the most abundant agricultural waste which contains cellulose 28.7-35.6%, hemicellulose 12.0-29.3%, and lignin 15.4-20.0% [8]. Interestingly, the high silica content (8.7-12.1%) is observed in its husk, [9]. Mostly in form amorphous hydrated SiO₂ similar to that found in most of the other objects in the biosphere [10]. This section reviewed recent reports in syntheses of rice husks-derived SiO₂ nanomaterials and their various applications, especially in catalysis.

In 2013, Thuc et al. used acid-washed Vietnamese rice husk for the synthesis of silica nanoparticles [11]. Acid treatment removes the small quantities of cations causing the increase of SiO₂ extracted from rice husk by calcination followed by preparation of sodium silicate solution by addition of sodium hydroxide. Subsequently, this sodium silicate solution was slowly added into the cetyltrimethylammonium bromide (CTAB) solution in water/butanol mixture. The precipitation of silica nanoparticles (SiO₂NPs) was performed at pH=4 by adding H₂SO₄. Controlling of the size of nanoparticles was done by surfactant concentration, aging time and aging temperature.

In a separate study, Wang et al. described the use of ionic liquid to extract lignocellulose from rice husk followed by calcination of separated rice husk residue at 700°C for 2 h to prepare 70 nm-sized SiO₂NPs with 241.1 m²/g surface area [12]. Use of HCl instead of ionic liquid 50 nm-sized SiO₂NPs with 283.3 m²/g surface area. Amorphous organo-functionalized SiO₂NPs can be obtained from calcination of nitric acid treated rice husk followed by NaOH treatment, addition of 3-(chloropropyl) trimethoxysilane and pH reduction of resulted solution [13]. Along with this line, it was shown that silica-supported imidazole could be synthesized by these organo-functionalized SiO₂NPs [14]. Mesoporous silicas have been fabricated from sodium silicate solution using rice husk ash, NaOH solution and CTAB as the structure-directing agent where fusion temperature of rice husk plays a crucial role in mesopore sizes and pore volumes [15]. It is proved that the silica materials possessed mesoporous structure with specific surface area, pore volume and average pore diameter in the range of 211-518 m²/g, 1.36-1.92 cm³/g and 11.8-25.8 nm, respectively. In addition, these silica materials can be used as supports for CO₂ adsorption by dispersion of tetraethylenepentamine on the support.

In 2014, Andas et al. prepared sodium silicate solution by stirring the acid-treated and washed rice husk in NaOH solution to yield dark brown solution [16]. Addition of an acidic solution of Cobalt (II) to sodium silicate solution until pH=3 followed by aging resulted in high surface area mesoporous silica-supported cobalt catalysts (Co@SiO₂). This catalyst was studied in the oxidation of phenol with hydrogen peroxide (Scheme 1). A recyclability study of this catalyst has also been successfully performed for five runs.

Scheme 1

Co@SiO₂-catalyzed oxidation of phenol.

Andas, and Adam reported the preparation and characterization of immobilized AgNPs (25 nm) on rice husk-derived silica with 514 m²g^-1 BET surface area [17]. Synthesis of layered sodium silicates from rice husk developed by Lin et al. [18]. Phase transformation from β through δ to α was observed as the temperature and time of synthesis was enlarged. Produced silica with a higher content of the δ phase exhibited greater Mg²⁺ and Ca²⁺ binding capacities providing an appropriate substitute for polluting phosphorus-based detergents. Mukti and coworkers conducted the preparation of hierarchical ZSM-5 below 100oC by silica from rice husks and sodium aluminate in the presence of tetrapropylammonium bromide as structure-directing agent [19]. The synthesized zeolite exhibits hierarchical microporosity (0.55 nm) and the additional intercrystallite mesoporosity (3.5 nm) originated from the spherical morphology of zeolite, which was composed of nanocrystallites. In 2017, Mor and coworkers described the preparation of 10-15 nm rice husk-derived porous silica particles acidification of sodium silicate solution [20].

Hsu and co-workers prepared carbon–silica nanocomposite with 349 m²g^-1 BET surface area by calcination of acid-washed rice husk agricultural waste under an inert atmosphere followed by hydrothermal treatment to decrease inorganic impurities [21]. Carbon–silica composite was mixed with the epoxy resin, curing agent and catalyst which resulted in epoxy/carbon– silica composite. The thermal conductivity and thermal-mechanical properties of CTE and storage modulus of epoxy/carbon-silica composite have been studied and improvement of this properties compared with pure epoxy polymer has been proved. In a separate study, Rangari and co-workers described the preparation of 14 nm sized silica/carbon hybrid nanoparticles from using pyrolysis of rice husk powder in high-pressure/temperature reactor [22]. This nanomaterial has been used to synthesis of nanocomposite from Ecoflex, a biodegradable compostable polymer, using 3D printing technique. This 3D printed biocomposite was further experienced for their mechanical and thermal properties. TGA and Tensile tests showed sensible improvement in thermal stability, due to the addition of silica based nanomaterial.

Silica-coated magnetic nanoparticles (Fe₃O₄NP@SiO₂) have been prepared from dispersion of Fe₃O₄ nanoparticles and rice husk-derived silica in ammonia solution and used for supporting palladium (Fe₃O₄NP@SiO₂-Pd) [23]. This nanomaterial was then studied as catalyst in the Suzuki coupling of electron-rich and electron-poor aryl iodides and bromides in the presence of waste eggshell as inexpensive and green solid base. with phenyl boronic acid (Scheme 2) with Pd loadings of 0.03 mol% (yields: 85-93%). The recovered catalyst could be reused at least four times without loss of activity.

Scheme 2

Fe₃O₄NP@SiO₂-Pd catalyzed Suzuki reaction.

In 2017, carbon quantum dot grafted silica nanoparticles have been prepared by calcination of under a nitrogen atmosphere followed by dispersion in H₂SO₄ and oxidation treatment in HNO₃ [24]. Since the photoluminescence property of this nanomaterial can easily be detected under UV light at a pH=7–8, carbon quantum dot grafted silica nanoparticles can be used in biomedical fields. 34 nm sized SiO₂NPs prepared from rice husk-extracted sodium silicate solution was used for the preparation of nanocomposite with styrene and wood and presented the best improvement in all physical and mechanical properties [25]. Also this nanocomposite considerably growths the decay resistance against white rot fungi.

It is noteworthy that, in addition to silica based-nanomaterials, nanoporous MnO₂ have been prepared using rice husk by Lin and co-workers [26]. Washed and dried rice husk was calcined under an inert atmosphere and treated with NaOH solution to remove to remove of silica impurity. The resulted carbon powder after treatment with KOH solution and calcination was neutralized by HCl. Nanoporous MnO₂ was synthesized by dispersion of MnSO₄ and rice husk-derived carbon in propyl alcohol/water solution followed by addition of KMnO₄. The product is nanocomposite of carbon and MnO₂ with 1645 m²/g surface area. 52% of total pore volume in this nanocomposite is volume of mesopores. Based on cyclic voltammetric data MnO₂ nanocomposite can be used as electrode material for high-performance supercapacitors. Raman et al. reported the application of rice husk-extracted silica nanoparticles as a green additive for the production of sustainable concrete [27].

Burri and co-workers prepared PdNPs of 3.5-4.5 nm in size supported on high surface area nanoporous silica-carbon (PdNP@Si-C) by calcination of acid wash husk rice under inert atmosphere [28]. This catalyst was studied in the carbonylative Suzuki coupling reaction of various types of aryl iodides in 1,4-dioxane at 120°C under carbon monoxide atmosphere (Scheme 3).

Scheme 3

PdNP@Si-C catalyzed carbonylative Suzuki reaction.

In 2018, in a study reported by Mathur et al., the synthesis of 80-85 nm forsterite (Mg₂SiO₄) nanoparticle by rice-husk extracted silica and magnesium oxide through solid-state method has been described [29]. A polymeric membrane nanocomposite was synthesized using rice husk-derived silica nanoparticle as filler and polyether-polyamide block co-polymer for separation of CO₂ from other gases [30]. Chauhan and co-workers prepared silica nanowires diameter in the range of 15-35 nm and length about 0.5 μm using rice husk ash [31]. In addition to the rice husk, 15-20 nm sized SiO₂NPs has been synthesized using rice straw by Yadav and Kauldhar [32].

1.2 Sugar cane bagasse and leaves

Sugar cane bagasse (SCB) is the agricultural waste biomass of sugar and ethanol industries that is abundantly available. Pereira and co-workers in 2015 reported a simple and low-cost technique for modifying the textural properties and stability of mesoporous gamma-alumina using bayerite as aluminum source and SCB as sacrificial template [33]. The effect of waste biomass was evaluated by changing the template/bayerite ratio. The presence of biomass as a template improved surface area and pore volume up to 209 m²/g and 0.44 cm³/g, respectively. In addition, mean pore diameters are adjusted from 5.2 to 7.9 nm by varying the template/bayerite ratio.

Due to high content of silica in SCB, Norhasyimi and co-workers prepared SBA-15 using acid washed SCB ash as silica source and P123 as neutral template [34]. The BET surface area and total pore volume of synthesized SBA-15 were 466 m²/g and 0.14 cm³/g, respectively. In a separate study, SCB has been used for tuning the size of titanium oxide nanoparticles (TiO₂NPs) where the amount of biomass template played an important role in size controlling [35]. TiO₂ sol from titanium tetraisopropoxide in pH=4 was calcined under 200°C for 5 h resulted in TiO₂ powder gel. Various amounts of SCB was added to this gel and stirred for 2 h at room temperature. SCB-supported TiO₂NPs are in the range from 5–20 nm. The photocatalytic activity of SCB-supported TiO₂NPs was tested in the degradation of azo dye methyl orange under visible light.

In 2018, Jabasingh and co-workers prepared 50-200 nm magnetic iron oxide nanoparticles by coprecipitation of Fe(III)/Fe(II) in the presence of ammonia and sugar cane bagasse under inert atmosphere followed by calcination in furnace [36]. This magnetic material can be used as supports for adsorption of Cr⁶⁺. Also, MgO hybrid spongelike carbonaceous composite has been synthesized using magnesium acetate and sugarcane leafy trash as a sacrificial template by thermal decomposition method under N₂ flow [37]. This nanomaterial was comprised of nano-size magnesium oxide flakes and nanotube-like carbon sponge with surface area and pore volume up to 40.61 m²/g and 0.323 cm³/g, respectively.

1.3 Bamboo leaf

From ancient times bamboo was considered to have many applications such as food source and building material. Bamboo species are native to warm and humid climates especially areas in China, Japan, Korea, India, and Australia. Because of silica content of acid washed bamboo leaf ash, it has also been used successfully as silica source in synthesizing of 13.8 nm sized SiO₂NPs by Venkatachalam and Rangaraj [38]. Based on nontoxicity nature of bamboo leaf-derived SiO₂NPs, it has been proposed that these nanoparticles can be considered as a potential candidate for drug delivery and other medical applications. In 2017, aluminosilicate zeolite A has been synthesized from bamboo leaf ash by Ng and co-workers [39]. The catalytic activity of biomass-derived zeolite A was tested in cyanoethylation of methanol with 82% conversion and 100% product selectivity (Scheme 4). The recovered catalyst could be reused at least nine times without significant loss in activity.

Scheme 4

Zeolite A catalyzed cyanoethylation of methanol.

1.4 Egg shell

The eggshell is 95-97% calcium carbonate crystals, stabilized by a protein matrix and may be considered as calcium source [40, 41]. In this regard, Jayasankar and co-workers prepared Dy³⁺-doped calcium silicate (Ca₂SiO₄) nanoparticles by calcined waste egg shell and rice husk through solid-state reaction technique at 1250oC [42]. Dysprosium (III) ions-doped materials are used in making laser materials, optical sensors and solar energy harvesting [43, 44]. Luminescence and excitation spectra of Dy³⁺-doped calcium silicate were investigated and the results indicate that the Dy³⁺-doped calcium silicate phosphors are appropriate for production of low cost white light emitting devices. Hen egg shell has also been used successfully to synthesize calcium oxide nanoparticles by Fulekar and Pandit [45]. Crushed egg shell was decomposed to CaO above 800°C. Resulted calcium oxide was refluxed in water and again calcined in furnace. The average size of calcium oxide nanoparticles were found 75 nm based on XRD pattern. This nanomaterial was then studied as catalyst in the transesterification of dry microalgae biomass (A. Obliquus) into biodiesel.

1.5 Walnut shell

In 2017, we synthesized 11 nm sized walnut shell-supported Cu/Cu₂O nanoparticles (Cu/Cu₂ONP@WS) [46]. Abundance of walnut shell as worthless lignocellulosic waste [47] in nature is main benefit of this protocol. Copper acetate solution in water was mixed with an aqueous mixture of walnut shell powder followed by addition of NaBH₄ resulted in Cu/Cu₂O NPs. This copper nanocomposite was employed in the synthesis of propargylamines via the three-component reaction between benzaldehydes, secondary cyclic amines and phenyl acetylene (Scheme 5). Also, walnut shell-stabilized copper nanoparticles were found to be an efficient, inexpensive, easy to prepare, green and reusable catalyst in the reduction of aromatic nitro and nitrile compounds to their corresponding amines with NaBH₄ at 35°C in an aqueous medium (Scheme 6) [48]. We continued our studies on the application of this nanocomposite in the classic Ullman reaction to synthesize biaryl (Scheme 7) [48]. Also a size-dependence study showed that the catalytic efficiency of larger Cu nanoparticles was much lower than that of smaller ones under similar reaction conditions.

Scheme 5

Cu/Cu₂ONP@WS catalyzed synthesis of propargylamines.

Scheme 6

Cu/Cu₂ONP@WS catalyzed reduction of aromatic nitro and nitrile compounds.

Scheme 7

Cu/Cu₂ONP@WS catalyzed Ullman reaction.

Quite recently, we synthesized high surface area magnesia (MgO) [49], high surface area alumina (Al₂O₃) [50] and ceria (CeO₂) nanoparticles [51] by walnut shell as sacrificial template. Walnut shell powder was mixed with an aqueous solution of magnesium nitrate hexahydrate in different weight ratios [49]. After stirring followed by evaporation of water, the solid was calcined at 500°C. Produced magnesium oxides have BET surface area up to 79 m²/g. Hydrothermal treatment of this magnesium oxide followed by annealing in air surprisingly increase surface area up to 212 m²/g (MgO-212). To test the catalytic activity of magnesia materials as heterogeneous catalysts we selected Meerwein-Ponndorf-Verley reduction of cyclohexanone with 2-propanol (Scheme 8). It is of interest that different surface areas of magnesia materials showed different catalytic activities. Magnesium oxides with the highest surface area exhibited maximum yields. Also, MgO-212 was tested as the support of palladium nanoparticles (PdNP@MgO-212) for the aerobic oxidation of alcohols (Scheme 9). Satisfying results were observed for the PdNP@MgO-212 catalyzed aerobic oxidation of benzylic and aliphatic alcohols under air without the use of exogenous base.

Scheme 8

MgO-catalyzed Meerwein-Ponndorf-Verley reduction.

Scheme 9

PdNP@MgO-212 catalyzed aerobic oxidation of alcohols.

We continued our studies on the synthesis of walnut shell-templated high surface area alumina and boehmite by the same way [50]. Produced aluminum oxides and boehmite has BET surface area up to 218 and 304 m²/g, respectively. Additionally, boehmite was studied as the support of vanadium catalyst for the oxidation of alcohols by hydrogen peroxide. We have found that resulting V-loaded material act as an effective catalytic system for the oxidation of a wide range of alcohols in 1,4-dioxane by hydrogen peroxide. The catalyst can be recovered and reused four times without loss of activity.

Along this line, it was shown that 9-21 nm sized cerium oxide nanoparticles (CeO₂NPs) have been synthesized using cerium nitrate and walnut shell as a sacrificial template by thermal decomposition method [51]. Particle sizes can be adjusted by changing cerium nitrate/walnut shell ratio. To test the catalytic activity of ceria nanoparticles as a solid catalyst, we selected three-component synthesis of 3,4-dihydroquinoxalin-2-amine by tert-butyl isocyanide, acetone, and o-phenylenediamine in an aqueous medium (Scheme 10). We have studied the effect of ceria particle size in this process. It is of interest that smaller ceria NPs showed the highest activity.

Scheme 10

CeO₂NPs-catalyzed synthesis of 3,4-dihydroquinoxalin-2-amine.

1.6 Wheat straw

Recently Patel and co-workers prepared 100-200 nm silica nanoparticles by calcination of acid-washed wheat straw followed by treatment with NaOH solution and neutralization by HCl [52]. This catalyst was studied in the synthesis of pyrano[2, 3-c]pyrazole derivatives by one-pot, four-component process of benzaldehydes, hydrazine hydrate, ethyl acetoacetate, and malononitrile in an aqueous medium (Scheme 11). In a separate study, Chen et al. described the use wheat straw and Fe(NO₃)₃·9H₂O to preparation of porous carbon-supported Fe₂O₃ ultrathin film by calcination of KOH-containing wheat straw under inert atmosphere followed by addition of Fe(II) solution to aqueous mixture of calcined powder and annealing at 200°C [53].

Scheme 11

SiO₂NPs-catalyzed synthesis of pyrano[2, 3-c]pyrazole derivatives.

1.7 Coconut shell

In 2016, Asefa and co-workers prepared magnetic activated carbon by carbonization of coconut shell in the presence of ferric chloride (FeCl₃⋅6H₂O) under an inert atmosphere with different FeCl₃/coconut shell ratios [54]. The BET surface area, total pore volume and average pore size of the resulted porous materials were 238-372 m₂/g, 0.118-0.210 cm³/g, and 1.98-2.26 nm, respectively. Authors proposed that Fe₃O₄ was formed via formation of Fe₂O₃ and reduction of Fe (III) by carbon. It was shown that these nanomaterials are efficient adsorbents for toxic dyes such as Sunset yellow. 10-100 nm sized Iron oxide nanoparticles synthesized from coconut husk extract and ferric chloride by Sebastian and co-workers have been used to remove Ca and Cd from aqueous media [55].

1.8 Banana peel

In 2014, 20-50 nm Mn₃O₄ nanoparticles have been synthesized using banana peel extract by Yan and co-workers [56]. It is believed that banana peel extract plays a dual role, reducing KMnO₄ to form Mn₃O₄ and preventing the agglomeration of nanoparticles during preparation. Also 20 nm SiO₂NPs have been synthesized by addition of banana peel extract to an alkaline solution of tetraethylorthosilicate in ethanol followed by calcination of precipitate [57]. These nanoparticles can be employed for the adsorption of methylene blue.

1.9 Miscellaneous agricultural waste

Ahmaruzzaman et al. synthesized Fe₃O₄ nanocomposites (Fe₃O₄-NC) using papaya leaves as lignocellulosic agricultural waste by a simple thermal decomposition method [58]. Fe(II) and Fe(III) solution were mixed with washed and dried powder of papaya leaves followed by addition of NaOH solution. Resultant black precipitate was dried at 353 K (Fe₃O₄-NC353) and calcined in a furnace under air atmosphere at 573 K or 773 K (Fe₃O₄-NC573 and Fe₃O₄-NC773) to make nanoparticle with size 18-46 nm. Fe₃O₄-NC773 was found to be the most efficient nanocomposite for the removal of chlorazol black E, with an efficiency of 96%.

In 2015, the use of tea waste for the synthesis of supported hydrous aluminium oxide nanoparticles was reported by Wan et al. [59]. This porous nanomaterial has been synthesized by co-precipitation between aluminium sulfate and NaOH in the presence of tea waste and anionic polyacrylamide. Supported porous alumina was employed for defluoridation of drinking water by anion exchange of fluoride with sulfate ions.

In 2017, the use of pine needles for the synthesis of nanocomposites of nanocellulose and NiFe₂O₄ nanoparticles was reported by Singhal et al. [60]. These nanocomposites were fabricated by synthesis of carbon nanofibers (CNF) from pine needles followed by Silanization of these nanofibers (SCNF) by tetraethyl orthosilicate. Ni(II) and Fe(III) solutions were added to the aqueous mixture of CNF or SCNF. After adjusting pH of this mixture to 7.5 and stirring for 2 h, mixture hydrothermally treated at 160°C to make of nanocellulose/NiFe₂O₄ nanocomposites. Authors found that SNCF-supported NiFe₂O₄ (NiFe₂O₄@SCNF) act as an effective catalytic system for the oxidative degradation of Remazol Black 5 (RB5) and reduction of nitrophenols (Scheme 12). The catalyst can be used three times without loss of activity.

Scheme 12

NiFe₂O₄@SCNF-catalyzed reduction of nitrophenols.

Recently Sınağ and co-workers prepared 10 nm titanium oxide nanoparticles supported on hazelnut shell or olive residue activated carbon and employed as a photocatalyst in photodegradation of methylene blue [61]. Tan et al. synthesized NiCo₂O₄ nanocomposites using honey pomelo peel derived porous carbon [62]. Ni(II), Co(II) and urea solution was mixed with porous carbon followed by treatment in 120°C resulted in carbon-supported NiCo₂O₄ nanosheets.