Chronicle of Nanocelluloses (NCs) for Catalytic Applications: Key Advances

Abstract

Nanocellulose (NC) is a biomaterial with growing interest in the field of nanocomposites and sustainable materials. NC has various applications including biodegradable materials, reinforcing agents, packaging films, transpiring membranes and medical devices. Among the many applications, the use of NC functionalized with organic and inorganic groups has found wide use as a catalyst in chemical transformations. The goal of this review is to collect the current knowledge on its catalytic applications for chemical groups conversion. We have chosen to organize the manuscript according to subdivision of NC into Bacterial Nanocellulose (BNC), Cellulose Nanocrystals (CNCs), and Cellulose Nanofibers (CNFs) and their role as inorganic- and organic-functionalized NC-catalysts in organic synthesis. However, in consideration of the fact that the literature on this field is very extensive, we have decided to focus our attention on the scientific productions of the last five years.

1. Introduction

Cellulose is the main component of lignocellulosic biomasses, with an average percentage of 40–50 of the total weight. It is the most abundant polysaccharide on Earth, with an estimated annual production of 7.5 × 10¹⁰ tons [1,2]. From a chemical point of view, it is composed of β-d-glucopyranoses (ca. 5000 and 10,000 units), linked by β-1,4-glycosidic bonds. β-Glycosidic bonds force the entire structure of the polysaccharide to a linear arrangement, due to the pyranose chair conformation and the equatorial disposition of the substituents within the subunits, that corresponds to a minimum of energy of the entire structure [3].

Due to the extensive potential applications of this inexhaustible material [4,5], nanocelluloses (NCs) became in brief time one of the main topics of research worldwide. In particular, NCs have shown excellent properties like strength, high Young’s modulus, biocompatibility, tunable self-assembly, and thixotropic behavior, becoming groundbreaking in plenty of fields like optoelectronics, antibacterial coatings, packaging, engineered polymer composites, medicine (tissue scaffolds, drug delivery, biosensors, etc.), energy storage, catalysis, and environmental remediation [6,7]. Very often the nanocellulose (NC) is subjected to functional modifications through chemical reactions. These modifications are frequently performed to improve the characteristics and performance of nanocellulose-based materials and more specifically to obtain nanocellulose derivatives with enhanced lipophilic characteristics through acetylation, benzoylation or alkylation and with increased hydrophilic features by sulfonation or oxidation reactions. At the same time, it is necessary to preserve the structural integrity of modified nanocellulose to ensure its stability during the use as heterogeneous catalyst in organic transformations. Moreover, the almost always superficial modifications ensure the excellent catalytic performance and the maintenance of its intimate structure for an easy recovery and a possible reuse of the catalyst.

Despite the always growing number of studies about the preparation and their applicative evaluation, NC materials can be classified into just three main nanoforms: bacterial nanocellulose (BNC), cellulose nanocrystals (CNCs), and cellulose nanofibers (CNFs). The parameters which define the classification of these materials are mainly related to the methods of preparation (biological, chemical, and mechanical), the morphological attributes, and the crystallinity degree of the material; for a deeper point of view on the physical properties of NCs, the reader can refer to [8].

In spite of a lot of works have been published about the preparation, the chemical transformation and the general employ of this nanomaterials, to our knowledge literature reports only two papers which examine the role of nanoparticles (NPs)-decorated NC-derivatives as inorganic catalysts [9,10]. On the other hand, the catalytic application of organic-functionalized NCs still have not been properly reviewed. For these reasons, the goal of this work is to provide a complete review on the role of inorganic- and organic-functionalized NC-derivatives in organic synthesis with a particular emphasis on the last five years.

2. Bacterial Nanocellulose

Bacterial nanocellulose (BNC), also defined bacterial cellulose (BC), is an exceptional natural polymer with a great variety of technical applications. BNC was firstly described in a scientific paper by A.J. Brown in 1886, as produced by Bacterium aceti through fermentative processes to produce nanoscale cellulose [11]. Cellulose nanofibers can be also produced, as extracellular secretion, by several species of bacteria, such as Aerobacter, Acetobacter, Agrobacterium, Azotobacter, Rhizobium, and Pseudomonas [12,13]. Acetobacter xylinus, also known as Gluconacetobacter or Komagataeibacter xylinus, a non-pathogenic Gram-negative aerobic bacteria, is considered the most efficient cellulose producer, and the most extensively studied [12]. BNC has the same molecular formula as plant cellulose but is characterized by a 3D porous network structure with unique features. First of all, it has high purity since it is hemicellulose- and lignin-free, with a high water content of 99% and hydrophilicity [13]. It possesses high crystallinity (up to 80%) with a resulting high thermal stability and a high degree of polymerization (up to 20,000). Finally, BNC exhibits high flexibility, with a value of 118 GPa for Young’s modulus of single nanofiber, which is comparable to steel [14]. BNC nanofibers have a high aspect ratio around 20–100 nm in width and 1–9 µm length, with a surface area superior to that of plant cellulose [12]. BNC is very versatile since it can be obtained in different shapes and thicknesses, such as pellicle, disk, or aggregate. Finally, BNC is more environmentally friendly than its vegetal counterpart, due to its high purity, which does not require additional purification procedures. Moreover, the identification of different waste biomass as a useful carbon source for a prospective industrial BNC production, together with its considerable environmental biodegradability and biocompatibility make it an ecofriendly material. The BNC features are influenced by the fermentation conditions such as the carbon and nitrogen sources, the temperature, the incubation time, and agitation. Furthermore, it is possible to alter the BNC structure and the morphology by adding chemical reagents in the culture medium or the growing fibers, to give nanocomposite properties [15,16]. In this way it is possible to obtain a versatile template material for various applications, from medical devices to innovative materials with electrical properties [16,17].

Thanks to its numerous hydroxyl groups it could be chemically functionalized to give materials with enhanced chemical and physical properties. As an example, BNC oxidation enhances its biodegradability and solubility. Instead, esterification of the BNC could be a strategy for improving the capability of integration with other organic polymers, or for the delivery of active pharmaceutical ingredients [12].

Very attractive aerogels can be obtained from BNC, through freeze-drying processes. The resulting porous solid material can be used as a template for numerous catalysts in diverse applications [12,18,19]. It is worth noting that these BNC aerogels also are the starting material in high-temperature pyrolysis processes under an inert atmosphere, to obtain carbon nanofibers with a 3D nanostructure [20]. Although numerous papers reported the use of these advantageous catalysts, this topic is out of scope for the present review, and only pristine BNC, suitably modified without pyrolysis processes will be examined.

Readers can find a complete survey about the advancements in bacterial cellulose applications in previous, very exhaustive reviews [9,21,22,23,24,25,26]. The most interesting bacterial cellulose applications as a catalyst from 2016 until the present, are reported herein.

2.1. Inorganic Functionalization of Bacterial Nanocellulose and Catalytic Applications

Many inorganic compounds, like metal oxides, metal sulfides, silica, or metal nanoparticles, have been integrated into BNC, exploiting its isotropic 3D nanostructure to obtain versatile heterogeneous catalysts. With the aim to prepare nanocomposites with enhanced activity, it is possible to add these inorganic materials through different synthetic strategies. Among them, three strategies have been most used and reported in the literature: (i) the mixing under simple agitation or ultrasound-assisted conditions; (ii) the absorption through solvothermal processes; and (iii) in situ incorporation in the growth medium. The catalytic applications of these new materials are classified in the following sections.

2.1.1. Catalytic Transformation of Organic Compounds for Environmental Purposes

Many efforts have been made in the last years for the treatment of industrial wastewater, to eliminate or limit the pollutants. For example, the textile industries contribute a lot to aquatic pollution with azoic dyes. The reduction of these contaminants in non- or less-toxic compounds was investigated, using transition metal-based nanoparticles, which are more economical than noble metal nanoparticles. Thiruvengadam and Vitta in 2017, proposed the use of BNC as flexible and multifunctional Ni-based nanocomposite to reduce methyl orange (MO) used as a model dye pollutant [27]. The authors obtained Nickel-Bacterial Cellulose (NiBNC) nanocomposite at different nickel concentration using the “inverse chemical reduction” technique. It consisted of the immersion of a BNC hydrogel firstly in a sodium borohydride (NaBH₄) solution and subsequently in a NiCl₂ solution. Finally, the resulting black colored NiBNC was hot-pressed at 70 °C and 4 MPa, to obtain a dried NiBNC sheet with a final formation of highly interconnected metal nanoparticles in a BNC network, with magnetic and electrical properties. The reduction catalytic activity was also tested, and the NiBNC was able to reduce the UV-absorption of a methyl orange solution if compared with pristine BNC. The reusability of this catalyst was checked, and the authors highlighted that a high efficiency was maintained if the catalyst was not stored in an aqueous solution between the cycles. Xu et al., in 2018, developed a novel flexible membrane for the water pollution treatment, based on BNC loaded with graphene oxide (GO) and palladium nanoparticles (Pd-NPs) [28]. At first, GO flakes were introduced as a uniform dispersion in the growing BNC nanofibers, allowing intercalation in a “layer by layer” manner. The resulting material showed more robustness of the pristine BNC, a crucial property for water treatment materials. After the formation and cleaning in alkaline solution, the GO/BNC hydrogel was cut in the desired measures and freeze-dried to give GO/BNC aerogel with a large specific surface area. To add Pd-NPs to the GO/BNC structure, it was immersed in a PdCl₂ solution, dried, and then washed with a NaBH₄ solution leading to in situ formations of Pd-NPs (Figure 1). The newly formed Pd/GO/BNC membrane showed a large loading of Pd-NPs in the final membrane and was used dried for the catalytic test, in which methyl orange dye-contaminated water, in presence of NaBH₄, was filtered through it. The rate of methyl orange degradation was measured with a simple Pd/BNC membrane that was lower compared to that of Pd/GO/BNC membrane. Its powerful activity was highlighted testing this sheet as a filter for a cocktail solution of contaminants with a concentration of 10 mg L⁻¹: methylene blue 1 (MB), 4-nitrophenol 2 (4-NP), and rhodamine 6G 3 (R6G). In all cases, the colored solution became completely colorless after filtration through the Pd/GO/BNC membrane, probably due to the lamellar structure of the membrane that assures an excellent interaction with the Pd-NPs for the reduction activity. It is worth noting that the entire membrane preparation procedure is easily scalable. This feature together with the high spread catalytic activity and the stability showed in the stress test, make this new material interesting for industrial application.

Very recently, the research group of Kamal et al. proposed different examples in which BNC was used as a high surface area support for transition metal nanoparticles stabilized with carboxymethyl cellulose (CMC) like cobalt (CMC-Co-BNC) [29], copper (CMC-Cu-BNC) [30], and nickel (CMC-Ni-BNC) [31]. In these works, metal chloride and CMC (1 wt%) solutions were mixed, then hydrazine hydrate and ascorbic acid were slowly added. Microwave heating of the resulting solution led to the carboxymethyl cellulose-metal (CMC-M) nanoparticles. Finally, the CMC-M-BNC dip-catalyst was obtained by simple dropping and spread of the CMC-M suspension on the BNC sheets previously obtained by a Gluconacetobacter xylinum culture (Scheme 1).

All these catalysts were tested on the reduction of methylene blue 1 (MB) and different nitrophenols: 4-nitrophenol 2 (2-NP), 2,6-dinitrophenol 4 (2,6-DNP), and 2-nitrophenol 5 (2-NP) with CMC-Co-BNC, CMC-Cu-BNC, and CMC-Ni-BNC, respectively. In all cases, the CMC-M-BNC catalyst was able to reduce the dye pollutants, both alone and in combination, even if the colloidal CMC-M was found to catalyze the reaction faster than the heterogeneous CMC-M-BNC. The reusability was also tested with good conversion results after four cycles. The easier separation procedure of the heterogeneous catalyst, compared to the colloidal one that requires long centrifugation times for its recovery, suggests that the supported CMC-M-BNC could be more useful for real applications.

The hypothesized reaction mechanism could explain the different reaction rates measured for the CMC-M and the CMC-M-BNC. Both the reducible molecules and the sodium borohydride move on the nanoparticle surface. The NaBH₄ transfers a hydride on the reducible molecule, while an electron was transferred from it to the catalyst. Then, the reduced molecules leave the catalyst surface and spread in the solution. By then, another reducible molecule could be adsorbed. The authors hypothesized that the anionic surface of the CMC favored a strong interaction with the reducible molecules. Consequently, a fast catalytic reaction occurs (Scheme 2).

After simple oxidation of the pristine BNC, it is possible to highly improve the metal absorption capacity maintaining, at the same time the crystallinity and crystal size of the nanofibers [15]. In addition, BNC TEMPO-mediated oxidation has been combined, also with defibrillation to allow a higher metal nanoparticles inclusion capability. In 2017, Chen et al. exploited this option using the 2,2,6,6-tetramethylpiperidine-1-oxyradical (TEMPO)-mediated oxidation reaction to obtain a BNC with carboxyl groups on the glucose moieties (TOBNC) as support for Au nanoparticles (Au-TOBNC) [32]. Before the oxidation reaction, it was necessary to fibrillate BNC into a slurry, then the reaction proceeded by adding TEMPO, NaBr, and NaClO under mild aqueous conditions. The TOBNC as obtained was then mixed with a HAuCl₄ solution in presence of sodium borohydride to give the Au-TOBNC catalyst (Scheme 3).

The average particle size for AuNP-TOBNC was lower than the unsupported AuNPs, and it was probably due to the carboxyl groups, which in the dissociate form could immobilize tighter the AuNP, allowing a more uniform distribution of the particle size. The influence of the pH on the amount of AuNPs loaded was studied. When pH was 3.2–11.3 it was observed a changing color of the suspension reflecting the increase of the amount of AuNPs. It was probably due to the associate or dissociated form of the carboxyl groups. Then, the Au-TOBNC catalyst was tested for the reduction of 4-nitrophenol (4-NP) 2 by NaBH₄ as a reaction model. The reduction of 4-nitrophenol (4-NP) 2 to 4-aminophenol 6 (4-AP) proceed almost 20-fold faster than the AuNP without TOBNC support. Probably, because the TOBNC allowed a better Au exposure to the reagent when adsorbed on the single nanofiber, it was well dispersed. Finally, the influence of the temperature on the reduction reaction was evaluated, and the rate of substrate activation and the rate of product desorption achieved a balance almost at 65 °C. This work was very important because of the higher catalytic efficiency than the values reported before for analog catalyst [33].

2.1.2. Photocatalytic Applications of BNC-Inorganic Composites for Environmental Remediation

The advanced oxidation processes (AOPs) are another way for the water removal of dyes and pollutants, which were under investigation through heterogeneous catalysis in the last year. Thanks to its unique features, the BNC can be considered an excellent material substrate for the design of a potential Fenton-type catalyst. In 2018, Wibowo et al. synthesized a Fenton-BNC catalyst, by immersing a BNC hydrogel in FeCl₂ and FeCl₃ solutions at different concentrations, with a 1:2 molar ratio between Fe(II) and Fe(III) [34]. After one night, the hydrogel color changed from white to yellow and was immersed in a NaOH (4 M) solution to oxidize the metal, with a final black color for the hydrogel. Finally, the Fenton-BNC hydrogel was freeze-dried to give a Fenton-BNC aerogel that was used for the catalytic degradation studies of methylene blue (MB) 1, used as dye pollutant model. The authors demonstrated that the heterogeneous Fenton catalyst supported by BNC possessed better catalytic activity than the bare Fenton catalyst prepared without BNC. As a further advantage, Fenton-BNC could be easily recovered by using an external magnetic field thanks to its magnetic features.

Very recently, Hu and co-workers proposed a new efficient and recyclable photocatalyst based on TiO₂, non-woven polypropylene (NWP), and BNC [35]. In particular, TiO₂ nanoparticles were loaded onto NWP, and this sample was placed in the culture medium of Gluconacetobacter x., in order to fix the nanoparticles in the growing cellulose (Figure 2). The TiO₂-NWPBNC was used for the MB degradation catalytic tests, confirming that TiO₂ was efficiently embedded into the composite film, maintaining a good degradation performance with an MB removal rate over 92% after 2 h of reaction. Furthermore, after 5 cycles less than 10% of removal rate reduction was measured, showing also good reusability.

2.1.3. Electro-Catalytic Applications of Inorganic-Functionalized Bacterial Nanocellulose

As previously reported [9], BNC can be converted into a material with electrical properties by inorganic functionalization with suitable transition metals. In this work, the authors measured an increase of the room temperature electrical conductance for a flexible BNC sheet functionalized with a 20 vol% of Ni nanoparticles. It is worth noting that the NiBNC membrane could be useful both for biodegradable and bendable electronic devices thanks to its unique properties to conduct both in the hydrogel or dried state, respectively.

Zhou and co-workers in 2019, developed a CuO/Cu nanocomposite electrode supported on BNC as a catalyst for efficient CO₂ electrochemical reduction to CO [36]. This electrode was obtained by applying an in situ chemical reduction process, in which BNC sheets were immersed in a Cu(II) salt solution of ethanol and water. NaBH₄ was added as a reducing agent, together with an NaOH solution (0.1 M), and the mixture was heated at 30 °C for 90 min. Different CuO/Cu-BNC membranes were obtained as a function of the NaBH₄ concentration after washing with deionized water. Electrochemical activity and performance tests were carried out and in particular, the CuO/Cu_4:3-BNC prepared with a 0.3 M concentration in NaBH₄ (with a 4:3 molar ratio between CuSO₄·5H₂O and NaBH₄) was evaluated as an efficient catalyst for electrochemical CO₂ reduction. The CuO/Cu_4:3-BNC catalyst showed high faradaic efficiency of 53% for CO formation at a low overpotential of 490 mV, for over 40 h. Furthermore, it was compared with a conventional CuO/Cu composite catalyst supported on carbon paper and it was highlighted that an enhanced electro-catalytic performance was obtained due to its unique structural advantages, such as high surface area, optimal CuO/Cu composition that allowed a facilitating charge, and mass transport and ability to promote the conversion of CO₂.

2.1.4. Synthetic Applications of BNCs Grafted with Inorganic Catalysts

The use of a heterogeneous catalyst is in great demand, also in organic synthesis, and BNC responds with its peculiar features. In 2019, Jeremic et al. carried out an extensive study on the influence of the BNC production parameters and its use in transition metal nanocomposites for the catalysis of cross-coupling reactions [37]. Firstly, the authors studied the influence of the single parameters, such as pH, glucose concentration, and day of incubation, for the optimization of BNC production. In addition, for the first time, they successfully used a sugar mixture, a product of grass biomass treatment [38], as a carbon source for BNC production, obtaining the valorization of non-food renewable biomass as starting material. Then, the authors evaluated BNC application as solid supports for transition metals for the synthesis of heterogeneous catalysts. They loaded onto BNC, Pd(II) or Cu(II) salts in a sodium borohydride solution in water at 140 and 70 °C, respectively. Differences were observed in the metal catalyst’s distribution, showing that Cu formed a spherical structure on the surface, while Pd nanoparticles were incorporated inside the BNC network. Furthermore, quadrupole inductively coupled plasma mass spectrometry (ICP-QMS) analysis showed that the amount of Cu on Cu/BNC was 10-fold lower than Pd on Pd/BNC, indicating more efficient incorporation for the Pd catalyst than the Cu catalyst. For this reason, the Pd/BNC catalyst was chosen for the catalytic tests. The results of the Suzuki–Miyaura reaction performed with the Pd/BNC catalyst are reported in

Table 1. Pd/BNC catalyzed Suzuki–Miyaura reaction of aryl halogenides with arylboronic acid in water.

Entry	Aryl Halide	Aryl Boronic Acid	Product	Yield (%)	TON a	TOF b
1	9a	10a	11a	14	56	4.5
2	9b			78	312	26
3	9c			87	352	29
4	9c	10b	11b	77	308	26
5	9d	10a	11c	86	344	28.6
6				23 c	92	7.6
7		10c	11d	91	372	31

^a TON: Turnover number; ^b TOF: Turnover frequency; ^c: In the presence of Hg excess (Pd: Hg = 1:300).

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As expected, the coupling reaction gave higher yields using bromobenzene 9a with different arylboronic acids 10a–10c, using water as an ecofriendly solvent, and highlighting the greenness of the whole process. Subsequently, the same recovered catalyst was used to catalyze the reduction of the nitro- to amino group of 11c and 11d with H₂, with 99% yield for both compounds (Scheme 4).

Finally, Cu/BNC catalyst was also tested for the Chan–Lam coupling reaction between benzylamine 12 and phenylboronic acid 10a (Scheme 5). The authors supposed that the moderate observed yield was due to the low Cu incorporation on BNC. Anyway, the easy recovery and reusability, together with the negligible leaching of the metals confirmed these materials as good heterogeneous catalysts.

2.2. Surface Chemical Functionalization of Bacterial Nanocellulose and Catalytic Applications

The introduction of sulfuric acid as functional group on the BNC surface can modify the surface polarity leading to a heterogeneous acid catalyst, with a high crystallinity and surface area. In 2018, Nikoofar and co-workers exploited this chemical modification to catalyze a multicomponent reaction in order to obtain pyrimidine fused heterocycles [39]. Multicomponent reactions (MCR) are very interesting and green tools to obtain useful scaffolds for medicinal chemistry [40]. Many green protocols have been developed in the last years [41,42], and the use of an heterogeneous catalyst can accelerate the reaction times and increases the reaction yields [43]. In this work, biodegradable and stable heterogeneous catalysts were tested in a four MCR (4-MCR). BNC sulfuric acid (s-BNC) together with nanofibrillated cellulose sulfuric acid (s-NFC) were obtained by the authors in mild condition by adding chlorosulfonic acid in n-hexane at 0 °C for 20 min. The mixture was stirred at room temperature to remove the HCl. Then, the catalysts were washed and dried to give s-BNC and s-NFC ready for the MCR reaction to give dihydropyrimido [4,5-b]quinolinetriones (Scheme 6).

The reaction conditions were tested using s-NFC, which showed good results both in water and ethanol. The optimized reaction conditions were applied for both the catalysts, and the results are reported in

Table 2. Multicomponent reactions (MCR) for the synthesis of dihydropyrimido[4,5-b]quinolinetriones 18a-18j catalyzed by s-NFC or s-BNC in refluxing ethanol.

Entry	Aldehyde	Amine	Product	s-NFC		s-BNC
				Time (Min)	Yield (%)	Time (Min)	Yield (%)
1	14a	15a	18a	45	92,90,90 a	40	94
2	14b	15b	18b	90	90	70	91
3	14c	15b	18c	60	85	50	83
4	14d	15c	18d	105	95	90	94
5	14e	15d	18e	90	93	80	90
6	14f	15b	18f	165	93	150	91
7	14f	15e	18g	105	95	95	90
8	14a	15f	18h	75	90	60	88
9	14g	15g	18i	75	90	65	88
10 b	14h	15b	18j	90	55	80	54

^a: Results for recovery and reusability of catalyst; ^b: 14h:15:16:17b at molar ratio of 1:2:2:2 with the double amount of each catalyst.

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Both the catalysts allowed the formation of the products with a range yield of 83–95% in short reaction times. In particular, s-BNC showed a short reaction time in all cases, despite its porosity reduction, if compared with the s-NFC porosity. Finally, the heterogeneous catalysts were successfully recovered and reused for the synthesis of the compound 18a for three times.

Very recently, Said et al. developed an heterogeneous fluoride complex between BNC and tetrabutylammonium fluoride (TBAF) that allowed to be react as highly stable and selective fluoride source for fluorination reactions [44]. Although the use of a complex could be considered as out of scope for this review, this work is very interesting since the BNC was used both in batch and in flow condition with good to excellent results, demonstrating a high stability and versatility that could justify this concise regression. In brief, they tested different polysaccharides, such as BNC, vegetable cellulose, pectine, and starch, as support for (TBAF), in order to find a stable and non-hygroscopic heterogeneous complex for the S_N2 type fluorination reactions performed both in batch and flow conditions (Scheme 7).

3. Cellulose Nanocrystals

Cellulose Nanocrystals (CNCs) are a particular type of nanocellulose that can be easily obtained from strong acid hydrolysis of cellulose derived from biomass or bacters [45,46]. This non-toxic material is renewable, biodegradable and generally is provided at the ton scale through an industrial process carried out on wood pulp [47]. CNCs has numerous mechanical and physical-chemical properties such as: high crystalline order, well-defined size and morphology (generally ca. 5 nm in width and 150–250 nm in length) [48], high specific surface area, controllable surface chemistry and superior mechanical strength. These materials can be applied in different fields, including iridescent film production [49], birefringent film production [50], reinforcing fillers in plastics and polymers [51], chiral templating in carbon and silica materials [52], hydrogels [53], aerogels [54], flocculants [55], and supercapacitors [56]. They are attractive supports for various catalytically active nanoparticles (NPs), such as Pd, Au, Ag, Fe, Cu, and Ru [57], thanks to stable colloidal suspensions that form CNCs in water and for their high specific surface area which make the material highly functionalizable. Several reactions catalyzed by cellulose nanocrystals have been developed over the past five years. Below, in the following subsections, the catalytic activity of cellulose nanocrystals suitably functionalized with inorganic and organic agents will be illustrated in detail.

3.1. Inorganic Functionalization of Cellulose Nanocrystals and Catalytic Applications

Moodley and co-workers have synthesized the triazolidine-3-one derivatives with the nanocellulose (CNC)/hydroxyapatite (HAp) used as catalyst in reaction between aromatic aldehydes 20a–20l and semicarbazide 19 with excellent yields [58]. The obtained results are reported in

Table 3. Triazolidine-3-one synthesis catalyzed by CNC/HAp.

Entry a	R	Product	Yield (%) b
1	2-OMe	21a	96
2	4-OMe	21b	96
3	4-Et	21c	95
4	2-F	21d	90
5	2-Cl	21e	94
6	4-Cl	21f	91
7	4-Br	21g	90
8	2-NO2	21h	92
9	3-OH	21i	93
10	2,3-(OMe)2	21j	94
11	2,5-(OMe)2	21k	94
12	3,4-(OMe)2	21l	91

^a: Reaction conditions: semicarbazide 19 (1 mmol), aldehyde 20a–20l (1 mmol), catalyst (40 mg), ethanol (10 mL), room temperature. ^b: Isolated Yield.

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Notably, the aldehydes with both electron-donating and electron-withdrawing (orto, meta, and para) substituents performed efficiently under the reaction conditions in forming the corresponding target products 21a–21l.

It is possible to propose a mechanism for the cyclocondensation reaction of aldehydes and semicarbazide catalyzed by CNC/HAp (Figure 3), in which the first step starts with nucleophilic attack of NH₂ group of the semicarbazide 19 on the aldehyde carbonyl group 20a–20l that, presumably forming a short-lived zwitterionic intermediate 22a–22l, becomes protonated by the acidic CNC/HAp-protic solvent combination. The cyclization is completed upon elimination of water, presumably assisted by the basic form of catalyst, passing through intermediate 24a–24l, and finally, the catalyst is released back into the catalytic cycle. Lastly, when the system is reused for further applications no significant catalyst degradation was observed for at least five runs.

In 2019, a composite formed by acetylated nanocellulose and nanomagnetite was prepared and tested for the esterification reaction of oleic acid 25 to obtain the corresponding methyl ester 26 for biodiesel application, as shown in Scheme 8 [59].

The optimal reaction conditions produced the final product in very good yields (89%) using a quantity of catalyst of 1.5 wt% and a temperature at 60 °C.

In recent work, Mirosanloo and co-workers developed an particular heterogenous nanocatalyst, composed of cellulose nanocrystals, palladium nanoparticles, and 2-aminopyridine (CNC-AMPD-Pd) and they proved its efficacy in the von Pechmann condensation of differently substituted phenols for the synthesis of coumarin derivatives [60]. The catalyst was synthesized starting from cellulose nanocrystals by a multi-step reaction reported in the Scheme 9.

CNC-AMPD-Pd(4 mol%) was tested in reactions between substituted phenols and ethyl acetoacetate under solvent free conditions to obtain products in high to excellent yield as reported in

Table 4. Pechman condensation of substituted phenols 29a–29m with ethylacetoacetate 30 using CNC-AMPD-Pd(4 mol%).

Entry	Phenols 29a–29m	Reaction Time (h)	Products 31a–31m	Yield (%) a
1		2		94
2		3		96
3		2		97
4		1.5		95
5		0.5		92
6		0.5		95
7		2		90
8		3		89
9		2		87
10		7		45
11		3		86
12		6		84
13		6		85

^a: Yields are referred to isolated product after chromatography.

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The reaction yields are from good to excellent for all the substrates used, but the best yields were obtained with phenols carrying electro-donating substituents.

In another work nanocrystalline cellulose sulfuric acid (s-CNC) was synthesized by treating nanocellulose with chlorosulfonic acid in organic solvent [61]. The obtained catalyst was tested on the synthesis of polyhydroxy pyrimidine-fused heterocyclic compounds (PPFHs), starting from different carbohydrates 32–34, different amines 35–37, and barbituric acid 38 by a multicomponent reaction (Scheme 10).

The reaction was tested using different amount of the catalyst, different solvents, and different temperature. The products were obtained in short times (60–240 min) and with good yields (44–90%). The hypothesized mechanism consists in the activation of the carbonyl group of the carbohydrate by the nanocatalyst that is successively attacked by the enolic form of the barbituric acid through a Knoevenagel condensation, to produce the first intermediate. The latter is attacked by another molecule of barbituric acid by a conjugate addition followed by a dehydration to produce the second intermediate. The elimination of water leads this intermediate to a cyclization to give the final substrate that after the reaction with the amine produces the dehydrated final product.

3.2. Catalytic Trasformation of Inorganic Compounds for Environmental Purposes

An et al., in 2017 [62], developed a ternary composite consisting of cellulose nanocrystal (CNC)/hexadecyltrimethylammonium bromide (CTAB)/silver nanoparticle (AgNP) used as a catalyst for reduction of 4-nitrophenol 2. The latter is a typical polluting compound generally present in industrial and agricultural wastewater and even just a short-term inhalation can cause serious health problems such as chest pains, migraines, vomiting, stomach pain, cyanosis, and drowsiness. AgNPs immobilized on CTAB-adsorbed CNC have higher catalytic efficiency towards the reduction of 4-nitrophenol 2 to 4-aminophenol 6, compared to the control AgNPs and CNC/Ag samples. The improved catalytic performance of AgNPs immobilized on CTAB-adsorbed CNC was attributed to the better dispersity and narrow distribution of silver nanoparticles in the catalytic system thanks to both surfactant ability of the alkyl chain of CTAB that favors hydrophobic interactions with nanoparticles and quaternary ammonium group that increases hydrophilic interaction with CNC. The resulting ternary nanocomposite was applied for highly efficient catalytic reduction of 4-nitrophenol (4-NP) 2 to 4-aminophenol (4-AP) 6. The composite was prepared using cellulose nanocrystals obtained from cellulose pulp subjected to acid hydrolysis using 64 wt% sulfuric acid. The resulting CNC was dispersed in water by the application of ultrasound and, subsequently, the surfactant CTAB at pH = 4.5 was introduced. The loading of the nanoparticles on the composite was favored by the introduction of silver nitrate which was subsequently reduced to silver NPs using NaBH₄. It should be stressed that novel small-sized molecules behaving as amphiphiles are currently being studied thanks to their ability to form ionic liquids [63]. Such molecules, belonging to alkylphosphate, alkylamine [64], or carboxylic acids [65], are stimulating great interest due, for instance, to the peculiar self-assembly that they can trigger [66]. For this reason we believe that interesting uses/ applications can be foreseen if such amphiphiles are used with nanoparticles, as for the example given for CTAB/AgNPs. It should be also said that silver nanoparticles can be easily prepared in conjunction with titania in an interesting nanocomposite with metal-semiconductor junction, which can have, obviously, particular electronic properties [67].

In any case, in the work reported by An et al. [62] the reduction of 4-NP 2 with NaBH₄ was applied as model reaction to determine the catalytic activity of CNC/CTAB/Ag nanohybrid composite as reported in Scheme 11.

Upon the addition of NaBH₄ in the 4-NP 2 solution, the 4-nitrophenolate ion 48 is generated. Then, after the addition of CNC/CTAB/Ag nanohybrid composite, the anion specie is reduced to amino compound 6. The results revealed that CNC/CTAB/Ag nanomaterial possessed the high catalytic activity for the 4-nitrophenol reduction, compared with those of unsupported Ag NPs and CNC/Ag nanocomposite, thanks to the uniformly dispersed Ag NPs with narrow size distribution. The catalytic activity (545 h⁻¹) and pseudo-first order rate constant (k = 1.6 × 10⁻³ s⁻¹), were determined for the CNC/CTAB/Ag nanohybrid composite and these results indicated that the system has great potential in catalytic treatment of industrial and agricultural wastewater.

Another catalyst used for reduction of 4-nitrophenol 2 was synthesized by Zhang et al. in 2018 [68]. A simple and performant one-pot synthesis of well-dispersed hollow CuFe₂O₄ nanoparticles (H-CuFe₂O₄ NPs) in the presence of cellulose nanocrystals (CNC) as the support was described in this study. Based on the one-pot solvothermal condition control, magnetic H-CuFe₂O₄ nanoparticles were in situ grown on the CNC surface uniformly. The catalytic activity of H-CuFe₂O₄/CNC was verified in the reduction of 4-NP 2 (Scheme 12).

The catalytic system exhibited an excellent performance compared to classical CuFe₂O₄ NPs, which was attributed to the introduction of CNC and the special hollow mesostructure of H-CuFe₂O₄ NPs. In addition, the HCuFe₂O₄/CNC was recovered and reused for at least 10 times in the corresponding reactions without significant deactivation in the catalytic activity, confirming relatively high stability.

Moreover, Dhar and co-workers [64] used this catalyst for the reduction of 4-nitrophenol 2 into 4-aminophenol 6 as indicated in Scheme 13, observing quantitative conversion as also proved by UV-visible absorbance analysis.

Another magnetic composite based on MnFe₂O₄/cellulose nanocrystal was synthesized in 2018 for the catalytic decomposition of methylene blue 1 (MB) [69]. The nanocomposite was prepared starting from the cellulose hydrolysis for nanocellulose preparation. The obtained CNC was mixed with MnCO₃ and FeCl₃·6H₂O under alkaline conditions. The resulting material, namely MnFe₂O₄/CNC, displayed high specific surface area and small particle dimension. It showed a higher catalytic activity than MnFe₂O₄ for the decomposition of MB 1 in H₂O₂ solution. Furthermore, MnFe₂O₄/CNC demonstrated a high recovery on application of a magnet in the reaction system, which suggests great potential for wastewater purification.

Cellulose nanocrystals obtained from renewable bamboo pulp were used by Dhar and co-workers as a support for zerovalent iron nanoparticles (CNC-supported ZVIs), for multiple applications [70]. They proved that CNC-supported ZVI catalyst can be used for wastewater remediation for the decomposition of MB 1. The decomposition was proven to be almost quantitative by UV-visible absorbance analysis. The hypothesized decomposition products, suggested by GC-MS studies, was reported in the following scheme (Scheme 14).

3.3. Synthetic Applications of CNCs Grafted with Inorganic Catalysts

Goswami and Das have synthesized cellulose impregnated copper nanoparticles (CNC/CuNPs) used as heterogeneous catalyst for aza-Michael reactions, involving open chained as well as cyclic secondary amines and vinylic compounds directly bonded at electron-withdrawing group, to synthesize N,N-disubstituted 3-aminopropanoate esthers or N,N-disubstituted 3-aminopropanenitriles (

Table 5. aza-Michael reaction between different amines and electron deficient vinylic compounds.

Entry a	Amine 51a–51f	Vinylic Compound 52–53	Product 54a–54f, 55a–55f	Timet (h)	Yield (%)
1				0.75	93
2				1	90
3				1.5	92
4				1.5	88
5				3.5	82
6				2	93
7				1.5	88
8				0.75	95
9				0.75	95
10			No product	24	-
11			No product	24	-

^a 1.1 mmoles of amine 51a–51f react with 1.0 mmole of vinylic compound 52–53. Mass of Methyl acrylate taken is 86.09 mg and mass of Acrylonitrile taken is 53.06 mg. iPr is isopropyl group and Bn is benzyl group.

) [71].

The heterogeneous catalyst demonstrated excellent yield (82–95%) probably due to high reduction potential of copper (indicating strong oxidizing character) which ensures greater compatibility with electron rich groups like amines. However, it was found to be totally inert when used in the case of aromatic amines (Table 5, entries 10 and 11). Finally, the catalyst can be easily recovered from the reaction mixture and reused several times without any significant loss of activity.

In 2017, Chetia et al. used CNC/CuNPs to synthesize a series of 1,4-disubstituted 1,2,3-triazoles 67–80 by azide-alkine cycloaddition carried out in glycerol, an environmentally benign solvent, with reaction yields ranging from 68% to 99% [72]. In particular, 1,2,3-triazoles are five-member N-heterocyclic compounds bearing three nitrogen atoms in the ring that can be easily synthesized in regioselective and eco-friendly way [73,74,75,76], giving products with notable biological activities [77,78,79]. Generally, 1,3-dipolar cycloaddition reaction, are considered a useful approach for the synthesis of bioactive heterocyclic compounds [80,81], sometimes enhancing the biological value already possessed by the precursors involved [82]. However, the results obtained from Chetia and co-workers are summarized in

Table 6. CuNP-CNC-catalyzed 1,3-dipolar cycloaddition reaction between azides and alkyne.

Entry a	R1	R2	R3	Product	Time, t (h)	Yield (%) b
1	Bn	Ph	H	67	1.5	99
2	Ph	Ph	H	68	1.5	98
3	4-Br-Ph	Ph	H	69	4	70
4	4-Cl-Ph	Ph	H	70	4	69
5	4-MeO-Ph	Ph	H	71	2	98
6	4-CN-Ph	Ph	H	72	4	68
7	C8H17	Ph	H	73	2	97
8	4-Br-Ph	C4H9	H	74	4.5	82
9	Bn	C4H9	H	75	3.5	83
10	Ph	C4H9	H	76	1.5	99
11	Bn	CH2OH	H	77	2.5	71
12	4-Br-Ph	CH2OH	H	78	3.5	72
13	Ph	CH2OH	H	79	3	93
14	Ph	COOMe	COOMe	80	3.5	89

^a: Reaction is performed with 1 mmole of azide 56–62, 1.1 mmoles of alkyne 63–66 and 1.05 mol% of heterogeneous catalyst in 2 mL of glycerol at room temperature. ^b: Isolated Yield.

.

The application of this kind of reaction, using CuNPs/CNC as heterogeneous catalyst, was investigated with different azides and alkynes and the best conditions were found at room temperature. Aromatic and aliphatic terminal alkynes readily reacted with the diverse azides to give the corresponding 1,4-disubstituted 1,2,3-triazoles 67–80 in good to excellent yields (68–99%). High reaction yields are observed when phenylacetylene reacts with aromatic or alkylic azide (entries 2, 10, 13, 14). The recyclability of CuNPs/NC was investigated in the cycloaddition of phenylacetylene 63–66 and benzylazide 56 and after each cycle the catalyst was recovered by simple filtration and reused after washing with organic solvent. About 90% of the original activity was obtained even after the fifth run. In a work carried out by Dutta and co-workers, Cu nanoparticles were immobilized on nanocellulose, producing CuNPS/CNC material that was tested as heterogenous catalyst for the oxidation of sulfides in mild reaction conditions, good yields, and discrete reaction time. Sulfides are versatile compounds also present in nature, with important pharmacological activity [83], which can be synthesized through easy approaches [84].

The results are collected in

Table 7. Oxidation of sulfides by CuNPS/CNC in ethanol.

Entry	Substrates 81a–81f	Products 82a–82f	Time (h)	Yield (%) a
1			2	99
2			5	95
3			12	73
4			12	65
5			2	96
6			2.5	95

^a: Isolated yield.

[85].

The developed catalyst was also tested on the oxidation of different alcohols using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as nitroxyl radical and the results are reported in

Table 8. Oxidation on different alcohols by CuNPs/NC and TEMPO.

Entry	Substrates 83a–83h	Products 84a–84h	Yield (%) a
1			99
2			82
3			95
4			95
5			98
6			80
7			45
8			45

^a: Isolated yield.

.

The reaction outcome was excellent predominantly with para-substituted aromatic alcohols. Lower yield was observed for saturated and unsaturated aliphatic alcohols probably due to the steric hindrance of the long alkyl chains. These latter, generally, are introduced on organic molecules to enhance the biological activity [86]. Nanocomposites with diameters less than 100 nm, used as heterogeneous catalysts, are difficult to separate from the reaction mixture. Therefore, the adsorption of magnetic nanoparticles on their surface furnish a facile-apply magnetic material that can be recovered by simple attraction from an external magnet [87]. In this context, Mirjalili and Imani synthesized a magnetic bio-based nanocatalyst, signed as Fe₃O₄@CNC/BF_0.2 (where BF is referred to BF₃ OEt₂) for the synthesis of aryl-2,3-dihydro-1H-perimidines 87a–87l by cyclocondensation of 1,8-diaminonaphthalene 86 with aromatic aldehydes 85a–85l (

Table 9. Synthesis of aryl-2,3-dihydro-1H-perimidine derivatives with Fe₃O₄@CNCs/BF_0.2 used as catalyst.

Entry a	R	Time (min)	Product 87a–87l	Yield (%) b
1	H	5	a	98
2	4-OMe	15	b	91
3	4-NMe2	15	c	86
4	2,4-(OMe)2	15	d	75
5	3,4-(OMe)2	15	e	92
6	4-Cl	10	f	96
7	4-COOH	10	g	99
8	2-NO2	12	h	94
9	3-NO2	10	i	96
10	4-NO2	10	j	99
11	2,3-(OMe)2	20	k	85
12	2,3-(Cl)2	15	l	88

^a: aldehyde 85a–85l (1.0 mmol), 1,8-diaminonaphthalene 86 (1.0 mmol) and Fe₃O₄@NCs/BF_0.2 (0.03 g) are stirred at room temperature for the opportune time. ^b: Isolated Yield.

) [88].

In Table 9, it is possible to observe good and high yields of products under very mild and green conditions and short reaction time. Moreover, the presence of electron-withdrawing groups on the aromatic ring of the aldehyde favors the formation of the product with higher yields than the presence of electron-donating groups. Other advantages of Fe₃O₄@CNCs/BF_0.2 as a catalyst are biodegradation, reusability, environmental friendliness, and easy separation with an external magnet. The recyclability of the catalyst was also investigated on the model reaction (Table 9, entry 1), reusing it at least four times with little loss of catalytic activity. The proposed mechanism is shown in Figure 4.

The proposed mechanism consists of a series of stages. At first, Fe₃O₄@CNCs/BF_0.2 as Lewis acid attacks the carbonyl group of the aldehyde 85a–85l, generating the imine intermediate 92a–92l. The latter cyclizes by intramolecular nucleophilic attack of amino group present on the naphthalene backbone, affording the products 87a–87l. In summary, this method offers different advantages including easy work-up, high yields in short time and the heterogeneous catalyst is eco-friendly because it can be reused for numerous cycles.

Liu et al. proposed new catalysts based on cellulose nanocrystals supported dirhodium (II) to be investigated on the cyclopropanation of styrene with EDA [89]. The catalytic systems were abbreviated in CNC-Rh₂-0.5d, CNC-Rh₂-2.5d, and CNC-Rh₂-4.5d. Among these species CNC-Rh₂-4.5d was selected for testing it on the model reaction because it showed the highest concentration of catalytic sites. The reaction Scheme and the yields of the products are reported in

Table 10. Reaction of styrene with EDA catalyzed by CNC-supported rhodium catalyst.

Entry a	Reaction Time	Cis-97 Yield (%)	Trans-97 Yield (%)	Total Yield (%)
1	15	20	22	42
2	30	21	29	50
3	45	28	34	62
4	60	29	35	64
5	120	33	39	72
6	180	34	40	74

^a: The reaction was tested using the CNC-Rh₂-4.5d catalyst.

.

Finally, the recyclability of the catalyst CNC-Rh₂-4.5d was also evaluated, demonstrating that the yields remained almost unchanged after three cycles.

4. Cellulose Nanofibers

With the term cellulose nanofibers (CNFs), also named cellulose nanofibrils, nanofibrillated cellulose, and nanofibrous cellulose, is intended a wide family of nanocelluloses characterized by a nanometric length comprised between 50 and 3000 nm and a width from 5 to 50 nm. Introduced in early 1980s [90,91], CNFs are obtained from the mechanochemical treatment of cellulosic biomasses, and presents a lower degree of crystallinity with respect to cellulose nanocrystals or bacterial nanocellulose. In addition, they also show interesting mechanical and physicochemical properties: large specific surface area, high strength, and an easy chemically tunable surface.

At the beginning, CNFs were obtained from the simple delamination of delignified and bleached pulps; however, this treatment has been shown to be very expensive in terms of energy consumption, and various other mechanical procedures have been proposed to improve this aspect and making the industrial production of CNFs much more affordable [92,93,94]. Numerous other procedures have been also developed in order to improve further the production of CNFs and can be categorized into two main families: swelling induced by charged groups, enzymes [95,96], or mild acid treatments. In the first case, the main procedures employed to introduce charged groups in the polymeric backbone include chemical oxidation performed via TEMPO (2,2,6,6-tetramethyl piperidinyloxy) [97,98] and carboxymethylation [99].

In this section, we will focus on the catalytic applications of inorganic and organic-functionalized CNFs. For a broader review of the production and applications of CNFs, the reader can refer to [97,100,101,102].

4.1. Inorganic-Functionalized CNFs Employed in Catalysis

The inorganic functionalization of CNFs for catalytic applications is mainly devoted to the use of CNFs as supporting materials for the growth of nanoparticles (NPs). The NPs mainly used as heterogeneous catalysts are made up including noble and transition metals such as Bi, Al, Mo, Ag, Fe, Ti, Ru, Cu, Co, Pt, Pd, Ni, and Au, often in their oxidized state or in presence of another element (i.e., Ni/Fe, CoS, CuS, etc.). The main fields of application reported in literature comprise catalytic degradation of environmental pollutants, electrocatalytic conversion of small molecules, photocatalysis and organic synthesis.

4.1.1. Catalytic Transformation of Organic Compounds for Environmental Purposes

Chemical transformation of organic and inorganic pollutants can be a useful strategy for pollutants removal in aqueous systems. In this sense, Ag-based CNF composite proved to be an efficient medium for the conversion of diverse chemicals having high environmental impact. The principal applications of Ag NP-decorated CNFs consist of degradation of dyes in aqueous systems or reduction of nitrophenolic derivatives. One of the first evidences about aqueous dyes degradation was reported in literature by Jang and co-workers with the evaluation of the role of a deacetylated-CNF template included in AgNP growth and the efficacy of the composite in methylene blue (MB) reduction [103]. The composite was tested evaluating the degradation of MB measuring the intensity of the main absorption peak of the dye at 668 nm, revealing that the full dye degradation took place in 1–10 min.

Recently, Zhang and collaborators developed a new CNF-hydrogel poly(ethylene imine)-based material functionalized with Ag NPs [104]. The material was tested on catalytic continuous discoloration of aqueous cationic and anionic dyes in batch and in flow test obtaining an efficiency greater than 98% maintained for up to 10 cycles of employ. The possibility of using this membrane material under high water flux (5 × 10⁴ L·m⁻² h⁻¹) allowed the treatment of considerable volumes of water.

As for nitrophenols degradation, one of the first evidences was reported by Heidari [105]. In this reported case, the composite was prepared adsorbing Ag NPs on hydrogels of CNFs, which was a non-inert template since it acted also as a stabilizer and reducing agent toward the pollutants employed. Its catalytic potency was evaluated, in particular, toward the degradation of 2 to 6 (

Table 11. Applications of Ag NPs@CNFs catalysts to 4-NP 2 and 2-NP 5 reduction in water.

Nitrophenol (NP)	Catalyst	Rate Constant	Reference
2	Ag/CNFs hydrogel	46.6 × 10−3 s−1	[105]
2	Ag/CNFs	9.05 × 10−2 s−1	[106]
5		11.34 × 10−2 s−1
2	Ag/Au/CNFs	22.83 × 10−2 s−1	[107]
5		15.59 × 10−3 s−1

), obtaining values of rate constants up to 46.6 × 10⁻³ s⁻¹ and catalytic activities up to 2.33 s⁻¹ g⁻¹. In another work, Gopiraman and collaborators realized Ag NP-decorated CNFs that were tested toward the reduction of both 2 and 5 in water. The reported rate constants for the two pollutants were, respectively, 9.05 × 10⁻² s⁻¹ and 11.34 × 10⁻² s⁻¹, showing very important results [106]. Successively, in 2019, they proposed a new Ag@Au bimetallic CNF composite from leaves of Moringa oleifera [107]. The catalytic performance of Ag@Au@CNFs was evaluated on a solution of nitrophenol measuring the UV-vis absorbance of the nitrophenol in the sample at different intervals. Table 11 summarizes the evidence reported in literature about nitrophenols (NP) reduction employing Ag NP-functionalized CNFs.

Iron too was explored as inorganic-functionalizing agent for CNF hybrid material production. Cunha Arantes and collaborators proposed an hybrid catalyst made up of CNFs and iron oxide NPs (magnetite) [108]. This catalyst was employed in MB degradation in a Fenton-like process which degraded 100% of dye in 180 min. Iron-based CNF nanocatalyst was explored also using Fe elements in combination with other metals as reported by Hou, very recently [109]. In this regard, Fe was employed in combination with Co in order to obtain a metal–organic framework (MOF) adsorbed onto the surface of CNFs. This approach enabled a membrane to be obtained useful for the catalytic degradation of tetrabromobisphenol A (TBBPA), a widely dispersed brominated flame retardant (BFR) also known for its hepatotoxicity, cytotoxicity, and immunotoxicity. In this case, Co/Fe bi-MOF/CNF composite was capable of full degradation of TBBPA in 30 min with rate constant of 0.764 min⁻¹ and a degradation of the catalytic membrane of 60% after four cycles of treatment.

In 2018, Pd NPs were employed in the preparation of reusable CNFs water-activated shape memory aerogels [110]. The material was obtained starting from TEMPO CNF-derivatives and applied for the catalytic degradation of both cationic and anionic dyes (MB 1 and Congo red) at concentration of 20 ppm, giving a discoloration efficiency comprised between 91.1 and 99.0% for five cycles of work.

Ujihara and co-workers were capable of preparing in 2018 a new composite where Pt NPs were embedded by a nanostructure of dendrimers and cellulose nanofibers [111]. In this case, the dendrimer was made up of poly (amido amine) and was covalently bonded on 2,2,6,6-tetramethyl-1-piperinyloxy free radical oxidized cellulose nanofibers (TOCNFs). The prepared Pt-based composite were successfully applied to catalytic degradation of formaldehyde and 5. In 2020, a composite material made up by borate-functionalized CNFs and Pt (or Ag and Au) was prepared for application in degrading of 4-nitrophenol 2 [112]. In this study, functionalized CNFs acted as reducing agents in order to obtain the NPs on their surface, giving nanospheres of diameters of 9 nm. Their activity in 4-nitrophenol 2 degradation was quantified calculating their TOFs numbers, giving values comprised between 1038 ± 238 h⁻¹ (Pt NPs/CNFs suspension) and 352 ± 34 h⁻¹ (Ag@Pt NPs/CNF-B(1%) film).

In 2020, other evidence about inorganic elements employed in this context were reported by Idrissi et al. [113]. In this context, the role of Co NPs on the surface of CNFs was investigated for the reduction of 4-nitrophenol 2 that was successfully reduced in 8 min at 25 °C. Gan and co-workers, again in 2020, focused on the role of polyoxometalate in combination with carbonized cellulose nanofibers [94]. They applied this type of composite on oxidative desulfurization reactions, a promising procedure for the removal of sulfur-based heterocyclic derivatives. In this work, the polyoxometalate chosen was [β-SiMo₃W₉O₄₀]ⁿ, which was loaded on the CNF’s surface. The removal of dibenzothiophene, benzothiophene and 4,6-dimethyldibenzothiophene in oil was quantified, giving reduction percentages up to 99%. In a similar way, Zhang et al. reported the hydroxydesulfurization of dibenzothiophenes achieved through mesoporous γ-Al₂O₃/CNFs acting as template for MoNi NPs as catalyst; in this case, k and TOF parameters obtained were up to 16.9 × 10⁻⁴ mol·g⁻¹·h⁻¹ and 5.3 h⁻¹, respectively [114].

4.1.2. Electrocatalytic Applications of Inorganic-Functionalized CNFs

The main application of inorganic-CNF composites in electrocatalysis involves oxygen transformations and, in particular, the so-called oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs) (Figure 5).

OERs are an important class of electrochemical reactions which enable the production of hydrogen from water-splitting, particularly employed in fuel cells, solar cells, and metal–air batteries. In this regard, Tian and collaborators reported in 2019 the first evidence of a CNF-based template for the preparation of electrocatalysts to employ in OERs [115]. In particular, it was realized that a Ni/Fe/CNFs composite where the abundance of carboxyls and hydroxyls groups present on the surface of CNFs helped the fine dispersion of the inorganic moiety. This material exhibited a very high electrocatalytic performance and an excellent long-term stability.

Subsequently, Tao and collaborators successfully employed a Ni derivative to functionalize N-doped CNFs [116], where the term “N-doped” stands for CNFs with a nitrogen content via a pretreatment with dicyandiamide. In this case, Ni@Ni₂P NPs grafted onto a carbonized composite of CNFs and carbon nanotubes, taking advantage from the notable specific surface area present. The obtained catalyst enabled an overpotential of 269 mV to be achieved, with a 1.7% decay after 20 h long-term test. In the same year, Tian et al. [117] developed a CoFeS@CNFs nanocomposite which achieved an overpotential of 257 mV while delivering a current of 10 mA·cm⁻² and decreasing by only 2.82% after 50 h of work.

As for ORRs, Lee et al. [118] and Meng and co-workers [119] proposed important evidence in this field of fuel cells science. In the first case, a nanocomposite based on TEMPO-oxidized N-doped porous carbon CNFs decorated with Co@Co₃O₄ NPs was reported. Even in this case, a three-dimensional network of the CNF template helped the fine dispersion of the catalyst’s NPs, giving performances comparable to the already-used Pt/C catalysts. As for the other reference, a nanocomposite made up by Co NPs loaded on N-doped CNFs was prepared. The catalyst reported notable properties, like a long-term durability, becoming an important alternative to the available Pt/C systems.

4.1.3. Photocalytic Applications of CNFs-Inorganic Composites for Environmental Remediation

The main photocatalytic applications of inorganic-functionalized CNFs concern the chemical transformation of organic and inorganic species considered dangerous pollutants even at low concentrations. In particular, they have been applied in the catalytic degradation of dyes, dangerous cationic metals, and antibiotics.

In 2018, Salama and co-workers prepared a composite material made up from CA-CNFs, multiwall carbon nanotubes (CNT), and NH₂-functionalized TiO₂ NPs [120]. The so-obtained material was successfully employed in photodegradation experiments with MB and indigo carmine (IC), revealing degradation percentages up to 100% for both dyes. In this case, factors such as temperature, pH, and power intensity of the UV lamp were investigated to find the best conditions. Zabihi Sahebi et al. also developed a CNF-based inorganic composite useful in photocatalytic degradation of dyes and harmful inorganic species [121]. In this case, CA/chitosan/single walled CNT/Fe₃O₄/TiO₂ nanofibrils were obtained via electrospinning technique and applied to the removal of MB, Congo red, Cr(VI), and As(V). In particular, this material enabled the complete degradation of both organic and inorganic pollutants up to concentration of 100 mg·L⁻¹ to be achieved. For Cr(VI) and As(V), the composite showed a notable degradation percentage (95%), while the percentage lowered to 85% for both the aza-dyes.

Another approach for Cr(VI) removal was recently proposed by Gan and collaborators [122]. They prepared a series of cellulose-derived carbon nanofibers which were then combined with bismuth oxybromide (BiOBr). In this case, the catalyst generated active oxidative and reductive oxygen species in situ capable of simultaneously degrading Cr(VI) and rhodamine B (RhB) dye at an initial concentration of 10 mg·L⁻¹ for both species. In particular, it was established that the presence of RhB in the aqueous system promoted the removal efficiency of Cr(VI).

RhB was also subject of study in the study proposed by Tian and co-workers [123]. In this case, a composite where CNFs were employed to anchor Ag ions onto carbon nitride (CN), an interesting metal-free photocatalyst. This approach guaranteed an efficient degradation of RhB and tetracycline (TC) under visible light was realized.

Other important evidence useful in dye remediation of waters was recently proposed by several authors employing bimetallic systems. In particular, Gupta and collaborators proposed a novel CNFs-based composite bearing NiFe₂O₄ NPs applied to the photo-Fenton degradation of Remazol Black 5 (RB5) and nitrophenols 2 and 5, obtaining interesting results [124]. Zheng et al., on the other hand, developed a bimetallic system Co-Cu-based (CoS₂-CuS) particularly active in antibiotics degradation [125]. The drugs chosen for this study were ciprofloxacin and ofloxacin: under UV-light, at pH 5 and in 60 min, their CoS₂-CuS@CNFs novel catalyst was capable of degrading up to 98.11% and 88.0%, respectively, of each compound. Zhang and co-workers [126] also recently introduced a novel bimetallic derivative for photocatalytic purposes. The realized material was a membrane consisting of CNFs-CoFe₂O₄/PVDF (polyvinylidene fluoride) used in the cathode of a photocatalytic fuel cell (PFC). In this case, the material was capable of activating peroxymonosulfate (PMS) transformation in order to obtain sulfate radicals (SO₄⁻), hydroxyl radicals (OH), singlet oxygen (¹O₂), and superoxide radical (O₂⁻). The species obtained took part in the subsequent degradation of berberine, an antibiotic, shading new light on PFC/PMS systems applied to water remediation.

Table 12. Resume of the photocatalytic applications of inorganic-containing CNFs.

Substrate	Catalyst	Reference	Substrate	Catalyst	Reference
MB 1	Ag@CNFs TiO2-NH2@CNFs/CNT Fe3O4/TiO2@CNF/Chitosan/CNT	[120] [121]	2 25	NiFe2O4@CNFs	[124]
Indigo carmine	TiO2-NH2@CNFs/CNT	[120]	Cr(VI)	Fe3O4/TiO2@CNF/Chitosan/CNT BiOBr@CNFs	[121] [122]
TC	Ag/CN@CNFs	[123]	As(V)	Fe3O4/TiO2@CNF/Chitosan/CNT	[121]
Congo red	Fe3O4/TiO2@CNF/Chitosan/CNT	[121]	Ciprofloxacin Ofloxacin	CoS2-CuS@CNFs	[125]
RB5	NiFe2O4@CNFs	[124]	PMS	CNFs-CoFe2O4/PVDF	[126]
RhB	BiOBr@CNFs Ag/CN@CNFs	[122] [123]

summarizes the photocatalytic applications of inorganic-containing CNFs described in this section.

4.1.4. Synthetic Applications of CNFs Grafted with Inorganic Catalysts

The role of inorganic-functionalized CNFs in organic synthesis has been greatly extended in the last years, allowing notable development in the synthesis of compounds of high industrial interest. The main applications reported in literature are summarized in Figure 6.

Hydroboration of alkynes 98a–98i using CNFs as template for inorganic catalysts was recently proposed by Zhang et al. [127]. In their work, functionalized CNFs were firstly functionalized via hydrolytic silane condensation in order to obtain their thiolated-form and, subsequently, employed as template for the immobilization of Cu(I) cations. As result, the CNFs-based nanosponges obtained showed high porosity and low density. The material was successfully tested as heterogenous catalyst, giving alkenes 99a–99i in excellent yields and up to 99% of regioselectivity. Furthermore, the reusability of the material was tested guaranteeing optimal catalytic performances until six reaction cycles. The reaction partners involved are reported in

Table 13. Hydroboration of alkynes 98a–98i using a Cu@CNFs heterogeneous catalyst.

Entry	Reactant	Product	Conv. (%)	E/Z
1			92	99:1
2			97	99:1
3			92	97:3
4			97	99:1
5			97	98:2
6			99	>99:1
7			99	99:1
8			92	99:1
9			86	99:1

.

As for alcohol oxidation, an interesting example was presented by Gopiraman and co-workers in 2015 [128]. In this case, they were able to employ acetate-derivatives of CNFs in order to obtain anionic cellulose nanofibers (CNFs) acting as template for noble metal particles adsorption. The metals at the center of this work were Ru and Ag. The catalytic evaluation of the Ru-containing composite was tested on the aerobic conversion of benzyl alcohol to benzaldehyde, furnishing yields up to 89% and with 100% of selectivity. For the Ag-based composite, the model reaction was the aza-Michael coupling between 100 and 101 giving yields of 99% and 100% selectivity. The catalytic cycle proposed for this reaction is illustrated in Scheme 15.

CNF derivatives as heterogeneous catalyst were also tested in Suzuki couplings, as reported by Jebali and collaborators [129]. In this regard, CNFs were employed in their cationic form in order to maximize the adsorption of Pd NPs via in situ reduction of PdCl₂. This simple approach gave a Pd-containing nanocomposite which was tested as catalyst on the coupling between aryl halides 102a–102g and arylboronic acid 10a. The results, listed in

Table 14. Suzuki coupling using Pd@CNF catalyst.

Entry	ArX	Conv. (%)	Entry	ArX	Conv. (%)
1		98	2		98
3		93	4		99
5		87	6		68
7		96

, report an almost quantitative reaction yield; in addition, several reusability tests were successfully performed obtaining the above reported performances up to three catalytic cycles.

A notable application of aerogel CNFs as catalytic templates was reported by Hees et al. in 2017 [130]. In this case, nanocellulose aerogels were obtained freeze-drying CNFs and subsequently impregnated with methylaluminoxane. This organic layer worked as support for the immobilization of bisiminopyridine iron(II) complexes, which served as catalyst for the low-pressure polymerization of ethylene.

Last, but not least, two important catalytic applications of inorganic-functionalized CNFs were proposed by Koga [131] and Ghamari Kargar and respective collaborators [132]. In the first case, the catalyst was prepared from CNFs decorated with Cu(I). The latter was employed with respect to the Huisgen [3+2] cycloaddition reaction involving benzylazide 104 and phenylacetylene 105 obtaining 1,2,3-triazoles 1,4-disubstituted in high yields (Figure 7).

In the second case, the research group of Ghamari Kargar proposed a new approach for the synthesis of 1,2,3-triazole derivatives employing the Fe₃O₄@CNF-ImSalophCu nanocatalyst illustrated in Figure 8. Particularly, it was realized the one-pot three component reaction between phenacyl bromides, sodium azide and alkynes, obtaining 1,4-disubstituted 1,2,3-triazoles in yields up to 98%.

4.2. Organic-Functionalized CNF Employed in Catalysis

The use of CNFs derivatives enriched with organic moieties for catalytic applications have recently seen interesting developments in diverse fields of organic synthesis. In 2018, two important applications were proposed. In the first example, TEMPO-oxidized CNFs (TOCNs) were successfully applied as organocatalysts in hydrolysis of acetals 106 to aldehydes 107 [133] (Scheme 16) with very good performances and quantitative yields.

In the second application, an organocatalytic system based on TOCN and (S)-proline (TOCN-(S)-proline) was realized and successfully applied to the coupling of 4-nitrobenzaldehyde 108 and acetone 109 to 110 (Scheme 17) [134].

In this case, the so-realized reaction gave yields up to 99% with an enantiomeric excess (ee) in the syn form of 89% of the compound 110. The employ of the sole TOCNs without (S)-proline gave a very low yield (18%) and a poorer selectivity (64% ee), confirming the importance of the (S)-proline in the catalytic system. The potential of this kind of catalytic system have been also extended to the Michael addition of ketones to ω-nitrostyrene resulting in good yields and syn:anti selectivity [135].

Furthermore, amine-functionalized TOCNs have been proved to be efficient as cooperative acid–base organocatalysts [136]. In this work, TOCNs were cross-linked with polyethylenimine (PEI) forming a hydrogel (TOCNF-PEI) employed to catalyze the in batch and flow condensation of 4-NBA and acetone.

In 2020, organic-decorated CNFs derivatives were also employed to the ring opening polymerization of lactide [137] and the synthesis of tetraketones in aqueous medium [138]. In the first case, the surface-initiated ring opening polymerization (SI-ROP) with rac-lactide 112 was realized using N,N-dimethyl aminopyridine (DMAP) in dichloromethane (DCM) (Scheme 18).

In addition, the functionalization of cellulose nanofibers with 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) furnished a catalyst able to catalyze the coupling of aldehydes and diketones in water at room temperature. This approach enabled yields up to 96% and to be achieved interesting reusability properties.

5. Conclusions

NCs represent an important contributor to chemical synthesis in terms of eco-sustainability and efficiency. In this regard, this review fills the lack of totally focused overview on inorganic and organic-decorated NCs as heterogeneous catalysts. In particular, BNC and CNFs are shown to possess promising properties in terms of both inorganic and organic-mediated catalysis, while CNCs reveal themselves to be a powerful tool as template for the grow of metallic NPs-catalysts, and in particular for the transformation of chemicals for environmental purposes. Nonetheless, it is conceivable that there is a synergistic contribution between the functional portion of the catalyst (NPs, organic framework, etc.) and its polysaccharidic backbone, making the entire nanomaterial a “non-innocent” template. These aspects, united with the numerous examples in terms of easy disposability, reusability, cheapness, and high chemical tunability of the catalysts reported above, increasingly enhance the central role assumed by NCs in chemistry.