Catalytic Valorization of Cellulose and Cellobiose ...

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Catalytic Valorization of Cellulose and Cellobiose with Nanoparticles Hu Lia,b, Qiuyun Zhanga, Anders Riisagerb and Song Yanga,*,# a

Center for Research and Development of Fine Chemicals, State-Local Joint Engineering Laboratory for Comprehensive Utilization of Biomass, State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering (Ministry of Education), Guizhou University, Guiyang 550025, P.R. China; b Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark Abstract: Cellulose considered as one of the most abundant renewable resources have great potential for the production of bio-fuels and chemical building blocks bearing a diverse range of applications. Among various approaches for the efficient transformation of cellulose, nanoparticles on ordered porous materials with high surface area and unique particle morphology employed as heterogeneous catalysts exhibit dramatic improvement of catalytic activity and selectivity. In this review, selective conversion of cellulose as well as cellobiose through different types of reactions including hydrolysis, isomerization, dehydration, hydrogenation/hydrogenolysis, oxidation, hydrogenation-dehydration, and gasification/pyrolysis promoted by mono- or bi-functional nanocatalysts has been described. Emphasis is also paid to discuss plausible reaction pathways catalyzed by functionalized nanoparticles in these catalytic processes.

Keywords: Cellulose, cellobiose, nanoparticles, heterogeneous catalysis, biofuels, renewable chemicals. #

Author’s Profile: Dr. Song Yang was born in 1974 in Yangzhou, P.R. China. He received a B.Sc. degree at East China University of Science and Technology in 1995, in Shanghai, China. He then obtained his M.Sc. and Ph.D. degrees at Guizhou University, Guiyang, China, where he had been working on the synthesis of novel green pesticides (with Prof. B.A. Song). In 2005, he accepted a post-doctoral position in the University of Texas Southwestern Medical Center at Dallas, Dallas, USA (with Prof. W.H. Li), where he developed molecular probes for biological research. Since Nov. 2005, he has been a professor in the Center for Research and Development of Fine Chemicals, Guizhou University. He has authored 12 patents and more than 100 scientific papers in international journals. His research interests include the rational design of heterogeneous catalysts for conversions of biomass, and discovery of new antiviral agents. 1. INTRODUCTION Fossil fuels (especially petroleum) with finite reserves and negative impact on the planet through pollution and global warming have brought about an increased awareness towards CO2 as a greenhouse gas [1], greatly stimulating the research into development of efficient routes for the utilization of renewable biomass resources [2, 3]. Lignocellulosic biomass, the most abundant and non-edible renewable source, is expected to be a promising alternative for the production of biofuels and fine chemicals [4-6]. The constituents of lignocellulose are typically composed of 40-50% cellulose, 16-33% hemicelluloses, 15-30% lignin, and small amounts of other components including triglycerides, alkaloids, pigments, resins, sterols, terpenes, terpenoids, and waxes [7]. In particular, cellulose composed of glucose or cellobiose unit homo-polymerized with β-1,4-glycosidic bonds has an average molecular weight ranging from 300000 to 500000 (g/mol) and relatively higher contents compared *Address correspondence to this author at the Center for Research and Development of Fine Chemicals, State-Local Joint Engineering Laboratory for Comprehensive Utilization of Biomass, State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering (Ministry of Education), Guizhou University, Guiyang 550025, P.R. ChinaTel: (+86) 851 8292171; Fax: (+86) 851 8292170; E-mail: [email protected]

1573-4137/15 $58.00+.00

to other components in lignocelluloses [8], which is essential for establishing the sustainable production of chemicals. Nevertheless, the tight microcrystallinity structure attributed to extensive intermolecular hydrogen bonding and van der Waals forces caused difficulties in efficient activation of cellulose under mild conditions. Generally, the direct utilization of cellulose can be divided into two major steps involving selective C−O and/or C−C bond cleavage. In this regard, cellulose is firstly hydrolyzed into glucose, which is then capable of being converted into fructose, 5-hydroxymethylfurfural (HMF), polyols, gluconic acid, isosorbide and syngas/hydrogen/alkanes via isomerization, dehydration, hydrogenation/hydrogenolysis, oxidation, hydrogenation-dehydration, and gasification/pyrolysis, respectively (Scheme 1) [9]. Over the past few decades, an impressive amount of catalytic studies on the transformation of cellulose into the valuable chemicals have already been carried out. Numerous review articles regarding on such an important research theme have been published [10-16]. In addition to homogeneous catalysts and enzymes mediated processes, solid acids with much more attention on separation, reusability, lower catalyst loading and corrosion effects are described in detail within these reviews. © 2015 Bentham Science Publishers

2 Current Nanoscience, 2015, Vol. 11, No. 1

Li et al. Light alkanes , H 2 , CO , C O 2, etc.

OH O

HO HO

HO O

OH

OH OH O

OH

H ydr ogen olys is [H ]

OH C ello bio se H 2O

O ther po ly ols

+ OH E th ylen e gly col

H 2O H y droly sis

OH O HO

HO O

OH

OH

OH O

H 2O

n/2

OH C ellu los e

O

HO HO

O

OH

OH

[O ]

OH

OH

O

HO

OH

O x id atio n OH

Glu cose

OH

Glu con ic acid I so merizatio n

OH

H OH2C

O O

O ther molecules

D ehy dration

O HO

OH

CH2O H

OH Fru cto se

HMF

OH

[H ] H ydr ogen atio n

OH OH

HO OH OH S orb itol

H y drog enation D ehyd ration OH

OH OH

HO

HO

H O

OH OH Ma nn itol

O

H OH I s oso rbid e

Scheme 1. Possible reaction pathways for the direct transformation of cellulose.

Among various heterogeneous catalysts, ordered porous materials supported nanoparticles exhibit dramatic improvement of catalytic activity and selectivity in oil refinery, petrochemistry, and energy production [17]. It seems that the nature and distribution of final products can be adjusted by modification of chemical processes with suitable nanocatalysts [18]. Thereby, the use of nanoparticles as heterogeneous catalysts may be one of the prospective routes to efficient degradation of cellulose into specific molecules with high selectivity. As a simplified model of cellulose, cellobiose is often selected to evaluate the efficacy of the used catalyst. In this review, we try to depict the state-of-the-art of selective conversion of cellulose as well as cellobiose to important/profitable chemicals through different types of reactions including hydrolysis, isomerization, dehydration, hydrogenation/hydrogenolysis, oxidation, hydrogenationdehydration, and gasification/pyrolysis promoted by monoor bi-functional nanocatalysts. Emphasis is also paid to discuss plausible reaction pathways catalyzed by functionalized nanoparticles in these catalytic processes. 2. NANOPARTICLES CATALYZED HYDROLYSIS AND HYDROLYTIC DEHYDRATION Catalytic hydrolysis of cellobiose and cellulose to glucose is considered as an important technology for efficient

utilization of lignocellulosic biomass, since glucose has the potential to be further selectively transformed into biofuels and value-added chemicals [19-22]. Over the past decades, considerable efforts have been made to study efficient and selective hydrolysis of cellulose in the presence of enzymes, acids, and sub- or super-critical water [23-26]. Although the hydrolytic process can be achieved with above methods, significant drawbacks such as high cost of enzymes, difficulty in separation of homogeneous catalysts, corrosion of reactors, waste effluents and severe reaction conditions are always involved. In contrast to physical and chemical processes generally requiring extreme treatments (e.g., high pressure and strong acids) that may lead to undesirable byproducts, enzymeassisted processes for cellulosic conversion are usually performed in relatively mild reaction conditions with high specificity [27-29]. For example, a high glucose yield of 70% could be obtained from catalytic hydrolysis of cellulose in the presence of free cellulase at 45 ºC [30]. However, cellulase was easily deactivated by environmental factors such as high reaction temperature, which largely hindered its practical application in industry [31, 32]. Immobilization of cellulase onto solid materials was thus adopted as a feasible way to overcome this difficulty by enhancing the stability of this enzyme. In particular, mesoporous silica materials with

Catalytic Valorization of Cellulose and Cellobiose with Nanoparticles

Current Nanoscience, 2015, Vol. 11, No. 1

large specific surface areas, high mechanical strength, and tunable surface functionality have attracted much attention [33, 34]. In 2008, Sakaguchi et al. demonstrated that mesoporous silica SBA-15 with a pore diameter of around 8.9 nm showed the best performance for the enzymatic activity of cellulase in cellulose hydrolysis. Nevertheless, the long 2D-hexagonal structure of SBA-15 greatly decreased the adsorption amount of cellulase due to the inner surface of the support inaccessible for the enzyme [35]. As an improved approach, mesoporous silica FDU-12 possessing a pore size of about 25.4 nm was employed to immobilize cellulase [36]. It was found that vinyl-functionalized FDU-12 could provide a relatively stable environment for immobilization of cellulase owning to the presence of hydrophobic groups, as well as high glucose yields (up to 80%).

to-glucose conversion, affording above 80% glucose yield at 50 ºC for 24 h. Recently, Bae et al. [41] reported three cystein-tagged cellulases including endo-glucanase (EGIVCBDII), exo-glucanase (CBHII), and β-glucosidase (BglB) coimmobilized on AuNP ((gold nanoparticles) or Au-MSNP (gold-doped magnetic silica nanoparticles) for the hydrolytic degradation of cellulose, and the procedures for the synthesis of AuNP and Au-MSNP are illustrated in Scheme 3. With respect to thermal stabilities of free and co-immobilized cellulases, the lifetime of thermal inactivation for cellulases coimmobilized on Au-MSNP (36 h) was a little longer relative to both the free enzymes and those co-immobilized on AuNP (24 h) at 80 ºC. Importantly, the magnetic nanoparticles immobilized biocatalyst (Au-MSNP) could be easily recovered by applying an external magnetic field to concentrate the nanoparticles, and cellulases co-immobilized on Au-MSNP were able to withstand seven times while retaining significant residual activity. Similarly, iron oxide magnetic nanoparticles were reported to immobilize β-glucosidase (BGL) from Aspergillus niger by covalent binding, which showed 93% immobilization binding and retained more than 50% enzyme activity up to the 16 cycles of cellobiose hydrolysis [42]. The catalytic activity of immobilized BGL on the hydrolysis of cellobiose was comparable to that of the free enzyme, and more than 90% cellobiose conversion could be obtained at 60 ºC after 16 h in the presence of immobilized BGL.

Instead of conventional bulk supports, nanomaterials with high surface areas were recently illustrated to be capable of increasing enzyme loading [37, 38]. On the other hand, covalent immobilization was reported to provide the most stable bonding between amino groups of enzyme and carbonyl groups of the glutaraldehyde activated nanomaterials supports [39]. For instance, Wu et al. [40] revealed that cellulase chemically linked with (3-triethoxysilylpropyl) succinic acid anhydride (TESP-SA)-functionalized LPMSNs (large pore mesoporous silica nanoparticles) exhibited excellent stability (Scheme 2) and catalytic activity for celluloseO

O O H 2O

3

O OH OH

O

NH2 ED C /N H S

O

H C H2C

N H H N

O T ES P-S A lin ked cellu las e

T ES P-S A f u nct io na lized LPMS N

T ES P-S A : ( 3-trietho xys ily lpro pyl)s uccinic acid an hyd ride L PMS N : larg e p ore meso por ous silica n an opar ticles E DC : N -( 3-dimeth ylaminop rop yl)-N ' -ethy lcar bod iimid e hy dro chlorid e N HS : N -hy dro xys uccinimd e

Scheme 2. Schematic for the connection of TESP-SA functionalized LPMSN with cellulase [40].

Scheme 3. Overall schemes for the synthesis of the cellulases immobilized on (a) AuNP and (b) Au-MSNP [41].

4 Current Nanoscience, 2015, Vol. 11, No. 1

Li et al.

Selective and efficient hydrolysis of cellulose to glucose and even fructose could be well achieved by cellulase enzymes [43]; however, one of the drawbacks of the enzymatic method is a low reaction rate. In contrast, solid acids may be favorable choices for cellulose and cellobiose hydrolysis, because heterogeneous catalysts can be applied under a wide range of conditions, and recycled in repeated reactions [44]. In this regard, SO3H functionalized solid catalysts such as sulfonated activated carbon (AC-SO3H) [45], sulfonated carbon [46], and sulfonated silica/carbon [47] generally exhibited high selectivity to glucose in the hydrolysis of cellulose or cellobiose under hydrothermal conditions, which were superior to zeolites [48], mixed-oxides TiO2-ZrO2 [49], layered niobium molybdates [50], mesoporous carbonsupported Ru catalysts [51], activated hydrotalcite nanoparticles [52], and ionic liquids [53]. However, these carbonsupported SO3H catalysts were still proved to be difficultly recovered after completion of the reaction. Moreover, hydrolysis carried out at the interface between the solid surface and macromolecular cellulose always resulted in poor accessibility of the substrate with active sites. As such, the use of nanoparticles as solid catalysts had great potential to be a powerful approach to overcome the difficulty of the solidsolid reaction. Considering the practical process for production of glucose from hydrolysis of cellulose, a magnetic solid acid catalyst was prepared through a modified direct synthetic method involving the co-condensation of tetraethoxysilane (TEOS) and mercaptopropyl trimethoxysilane (MTPMS) and oxidation of mercapto groups in the existence of magnetic Fe3O4 nanoparticles (MNPs), triblock copolymers and hydrogen peroxide, which was demonstrated to be not only separable but also durable under hydrothermal conditions [54]. The obtained sulfonic group functionalized magnetic SBA-15 catalyst (Fe3O4-SBA-SO3H) with ordered Table 1.

Entry[a]

mesopores and MNPs were found to be sufficiently robust to hydrolyze 1,4-glycosidic bonds, giving glucose in 96% yield from cellobiose and a medium yield from amorphous cellulose (Table 1). Notably, the used Fe3O4-SBA-SO3H could be easily separated from the resulting mixture in a magnetic field, and the regenerated catalyst could be repeatedly used for the hydrolysis of amorphous cellulose with almost constant glucose yields decreasing from 50% to 48% after three recycling runs. In a similar manner, CoFe2 O4-embedded silica nanoparticles containing sulfonic acid groups were found to be efficient for the hydrolysis of cellobiose and cellulose, giving 88% and 7% glucose yield with 30% yield of TRS (total reducing sugars), respectively [55]. Silica-protected cobalt spinel ferrite nanoparticles functionalized with perfluoroalkylsulfonic acid (PFS), alkylsulfonic acid (AS), and butylcarboxylic acid (BCOOH) groups were also synthesized for pretreatment and hydrolysis of lignocellulosic biomass [56]. It was demonstrated that cellobiose conversion of 78%, 75%, and 60% could be achieved at 175 ºC for 1 h when AS, PFS, and BCOOH functionalized nanoparticles were used as the catalyst, respectively, which was higher than that of the control experiment (52%). Among these catalysts, AS functionalized nanoparticles retained more than 60% of sulfonic acids groups after successive three runs; however, glucose yields (up to 49.5% during the first run) were still quite low at the temperature of 175 ºC. Similarly, core-shell Fe3 O4 @C-SO3 H nanoparticles with a magnetic Fe3O4 core encapsulated in a sulfonated carbon shell could be employed as a recyclable catalyst for the hydrolysis of cellulose, and 48.6% cellulose conversion with 52.1% glucose selectivity was obtained under moderate conditions of 140 ºC after 12 h reaction [57].

Results of catalytic hydrolysis of cellulose and cellobiose to glucose with various acids [54].

Substrate

Catalysts

Acid sites amount

Yield

[b]

Cellobiose (1.0 g)

Fe3O4-SBA-SO3H

1.09 mmol/g

96%(98%)[c]

2

Cellobiose (1.0 g)

0.05 M H2 SO4

--

32%(54%)[c]

3

Cellobiose (1.0 g)

0.11 M H2 SO4

--

36%(60%)[c]

4

Amorphous cellulose (1.0 g)

Fe3O4-SBA-SO3H

1.09 mmol/g

50%

5

Amorphous cellulose (1.0 g)

SBA-SO3H

0.44 mmol/g

24%

6

Amorphous cellulose (1.0 g)

SBA-SO3H

0.56 mmol/g

31%

7

Amorphous cellulose (1.5 g)

SBA-SO3H

1.50 mmol/g

52%

8

Microcrystalline cellulose (1.5 g)

Fe3O4-SBA-SO3H

1.09 mmol/g

26%

9

Microcrystalline cellulose (1.5 g)

AC-SO3H

0.72 mmol/g

21%

10

Microcrystalline cellulose (1.5 g)

Amberlyst-15

4.40 mmol/g

15%

11

Microcrystalline cellulose (1.5 g)

γ-Al2O3

0.05 mmol/g

3%

12

Microcrystalline cellulose (1.5 g)

SBA-15

0 mmol/g

< 1%

1

[a]

Reaction conditions: Solid acid catalyst (1.5 g), H2O (15 mL), T = 150 ºC, t = 3 h

[b]

Reaction conditions: Solid acid catalyst (1.5 g), H2O (15 mL), T = 120 ºC, t = 1 h

[c]

The value in parentheses is the conversion of cellulose

Catalytic Valorization of Cellulose and Cellobiose with Nanoparticles

Current Nanoscience, 2015, Vol. 11, No. 1

In the presence of suitable acid catalysts, glucose produced from cellulose through hydrolysis can be further dehydrated to useful molecules such as alkyl glucosides, 5hydroxymethyl furfural (HMF), and levulinic acid in one-pot [58-60]. Amongst these molecules, HMF with identical carbon skeleton to cellulose has been under particular scrutiny as a future platform chemical for the production of liquid transportation fuels [61-63] and polyester building block chemicals [64, 65]. In this conversion process, heterogeneous acid catalyst always played an important role [66]. For instance, ZrO2, TiO2, SO42-/ZrO2, and TiO2-ZrO2 were found to be effective for HMF production from fructose or glucose at a reaction temperature of around 200 ºC [67-70]. Selfassembled mesoporous TiO2 nanoparticles (NPs) catalyst synthesized by using sodium salicylate as template was found to have high surface area and unique particle morphology and could be utilized as an efficient solid acid catalyst for cellobiose dehydration reaction in the microwave reactor with microwave power of 300 W, producing 14.5% and 18.7% yield of HMF after 5 min in water and DMSO, respectively [71]. It was speculated that surface Lewis acidity together with high surface area played a crucial role in this dehydration reaction, and that the higher microwave energy absorbing ability (tan δ) of DMSO resulted in a little higher HMF yield than water. Moreover, mesoporous TiO 2 NPs catalyst could be recycled for four times without significant loss of its catalytic activity. Meanwhile, Wu et al. [72] demonstrated that hydrothermally treated mesoporous zirconia nanoparticles (HT-MZrN) exhibited superior HMF yield (29.2%) to hydrothermally treated mesoporous titania nanoparticles (HT-MTN, 18.2%) in one-pot cellulose-toHMF conversion at 120 ºC within 3 h, and the better performance of HT-MZrN was proposed to arise from its relatively strong acidity at higher temperature of 450 ºC. In this regard, Saha et al. [73] reported an acidic hierarchical macro/mesoporous titanium phosphate MTiP-1 synthesized through a slow evaporation method by using titanium isopropoxide and orthophosphoric acid as inorganic sources, and pluronic P123 as the structure directing agent. The prepared MTiP-1 catalyst showed very good catalytic activity in the microwave (300 W) assisted conversion of sugarcane bagasse to HMF, and a higher yield of HMF (26%) was achieved in DMA-LiCl at 140 ºC after 5 min. Similarly, an acidic TiO2 nanoparticles catalyst (SO4/TiO2) was synthesized via modified precipitation of TiOSO4·xH2SO4 (Alfa

Aesar) with ammonia solution under acidic condition at 85 ºC [74]. An enhanced catalytic performance for conversion of cellobiose and cellulose to methyl levulinate (ML) rather than HMF was observed in the presence of the nanocatalyst SO42-/TiO2, successively giving 58% and 42% yield of ML at 175 ºC after 9 h and 20 h. In comparison to mono-functionalized heterogeneous catalysts, solid bi-functional catalysts were illustrated to be efficient and easily handled catalytic systems for sequential cellulose hydrolysis and glucose dehydration in a single pot under mild conditions [75-77]. Recently, Wang et al. [78] reported a Brønsted-Lewis-surfactant-combined heteropolyacid (HPA) catalyst Cr[(DS)H2PW12O40]3 (DS = OSO3C12H25, dodecyl sulfate) capable of enabling cellulose depolymerization and glucose conversion to HMF in a single-pot. This micellar HPA catalyst with 200-300 nm particles exhibited 77.1% glucose conversion and 52.7% yield of HMF within 2 h at 150 ºC, and it showed high stability and could be recycled by simple separation process. Mesoporous silica nanoparticles (MSNs) having large pore size around 30 nm functionalized with both acid-base (SO3H and NH2) functional groups were also synthesized for the production of HMF from cellobiose and cellulose [79], in which acidic groups could facilitate the hydrolysis of cellulose and dehydration of mono-carbohydrates whereas basic groups were helpful for isomerization of glucose-to-fructose [80, 81]. 3. NANOPARTICLES CATALYZED HYDROLYSISHYDROGENATION The hexitol is an important platform chemical for the production of H2, liquid alkane fuels, and various valueadded chemicals such as sorbitan, isosorbide, glycerol and lactic acid [82-84]. Likewise, bi-functional catalytic systems containing both acidic sites and metal nanoparticles were necessary and efficient for the transformations of cellulose and cellobiose into sorbitol via the successive hydrolysishydrogenation process (Scheme 4). As one of pioneering researches, Kou et al. [85] reported that four different metal nanoclusters including Ru, Rh, Pd, and Pt stabilized by poly(N-vinyl-2-pyrrolidone (PVP) had similar diameters of about 3 nm but showed completely different catalytic performance in the conversion of cellobiose to sorbitol (Table 2). It was clearly pointed out that the combination of acidity of the catalytic system (pH 2) and Ru nanoclusters could

OH HO HO

OH

HO O

O

OH O

OH OH C ellob iose H yd rolys is

C at alys t A OH O HO

HO O

OH OH C ellulo se

O

H 2O

OH

OH O

H 2O

n /2

H yd rolys is

5

HO HO

OH O OH

Glu cos e

OH

[H ] OH

H yd rog enation

OH

HO

C at alys t B

Scheme 4. Direct conversion of cellulose or cellobiose into sorbitol via a hydrolysis-hydrogenation route.

OH

OH

So rbito l

6 Current Nanoscience, 2015, Vol. 11, No. 1

Table 2.

Entry

[c]

Catalyst

pH

Selectivity (%)

Conv.(%) Sorbitol

Glucose

A[c]

Other polyols

Pd

2

100

0

100

0

--

2[a]

Rh

2

100

6.9

66.9

0

--

3

[a]

Pt

2

100

18.5

42.6

0

--

4

[a]

Ru

2

100

100

0

0

0

5[a]

Ru

7

87.8

26.4

1.6

64.8

7.2

[a]

Ru

10

75.6

24.0

3.2

55.7

17.1

[b]

Ru/C

7

100

99

0

7 [b]

Effect of different reaction parameters for catalytic hydrogenation of cellobiose under different conditions [85].

1[a]

6

[a]

Li et al.

-3

Reaction conditions: metal (1.67 × 10 mol/L); PVP/metal = 10 (mole ratio), cellobiose (7.31 mmol), H2O (30 g), H2 pressure (40 bar), T = 120 ºC, t = 12 h Reaction condition: the same as (a) except 0.1 g of 1% Ru/C was used as catalyst A = 3-β-D-Glucopyranosyl-D-glucitol.

promote quantitative production of sorbitol (yield of 100%) from cellobiose in a Parr autoclave at 120 ºC and 4 MPa H2. In contrast, other metal nanoclusters as well as neutral or basic conditions (pH 7 or 10) resulted in low yields of sorbitol. In neutral water medium, Deng et al. [86] observed that acids modified carbon nanotubes (CNTs)-supported ruthenium catalysts could efficiently promote the direct conversion of cellobiose into sorbitol in the presence of hydrogen. It was clearly demonstrated that the catalysts with larger mean sizes of Ru particles (above 8.7 nm) and abundant acidic sites (e.g., CNTs pretreated by concentrated nitric acid) exhibited better sorbitol yields (up to 87%) at 185 ºC. On the contrary, the catalyst with a smaller mean Ru size and lower acidity afforded as high as 93% yield of 3-β-Dglucopyranosyl-D-glucitol at the initial reaction stage. It should be noted that only a small amount of glucose, mannitol and degradation products were generated, implying the partial hydrolysis of cellobiose catalyzed by the acid sites but high selectivity to 3-β-D-glucopyranosyl-D-glucitol or sorbitol in the set reaction conditions. A two-step reaction pathway was finally elucidated, in which cellobiose was proposed to be first transformed into 3-β-D-glucopyranosylD-glucitol via the hydrogenolysis of C−O bond in one of glucose rings over Ru nanoparticles, and sorbitol would be formed from the subsequent hydrolysis of 3-β-Dglucopyranosyl-D-glucitol in the presence of concentrated nitric acid modified CNTs (Scheme 5). Recently, Ran et al. [87] modified multi-walled CNTs with or without waterassisted chemical vapor deposition (CVD) during the CNT growth process, and they found that the Ru/CNTs containing H2O not only showed highly dispersed Ru nanoparticles owing to the existence of oxygen-containing groups and uniform defects on the tube-walls, but also exhibited much higher yield of sugar alcohols (39%) and cellobiose conversion (73.6%) than those without water (21.3% and 28.9%, respectively). Besides, the authors further illustrated that Ru naoparticles supported on interior CNTs surface displayed better stability as well as higher catalytic activity than Ru particles on the exterior surface of CNTs [88].

Cellulose, the most abundant source of biomass, is generally considered as a promising alternative to cellobiose for production of polyols. However, the crystalline cellulose with robust structure is not easily accessible to the surface acid sites of solid acids, which consequently suppress the sequential hydrolysis and hydrogenation of cellulose to form sorbitol and other polyols [89]. Given that the acid could be in situ formed from liquid water at elevated temperature (> 200 ºC) [90, 91], Liu et al. [92] reported a green and efficient catalytic system for the production of polyols from cellulose by combination of hydrolysis using H+ ions generated in situ in hot water with instantaneous hydrogenation on carbonsupported Ru clusters. In the presence of Ru/C, cellulose was rapidly converted into hexitols (sorbitol and mannitol at a molar ratio of about 3.6:1) in 22.2% yield with 38.5% conversion in 5 min at 245 ºC and 6 MPa H2. Upon prolonging the reaction time from 5 to 30 min, the conversion of cellulose and the yield of hexitols sharply increased to 85.5% and 39.3%, respectively. At a little lower reaction temperature of 185 ºC, Ru/CNT catalyst was demonstrated to be a robust and efficient catalyst for the direct conversion of cellulose into sorbitol in aqueous media, and a sorbitol yield of 36% and 69% could be achieved under 5 MPa H2 after 24 h in the conversion of cellulose with a crystallinity of 85% and 33%, respectively [93]. On the other hand, cellulose could be transformed into C2-C6 polyols with a yield up to 38% at the conversion of 81.3% catalyzed by Ru/CNTs with 4 wt% Ru loading at 225 ºC and 6 MPa H2 within 30 min [94]. It was proposed that both the higher concentration of adsorbed hydrogen species and the acidic functional groups on CNT surfaces which were largely influenced by the reaction temperature and time as well as H2 pressure played key roles in sorbitol formation. Recently, dual-functionalized catalysts containing both acidic groups and metal active sites were thus prepared for hydrolysis and hydrogenation, respectively, showing high efficiency for the direct and selective conversion of cellulose into sorbitol. For example, Ru nanoparticles on carbon supports treated with sulfuric acid (Ru/AC-SO3H) could afford a maximum sorbitol yield of 71.1% from the direct and selective conversion of cellulose in neutral aqueous solution and

Catalytic Valorization of Cellulose and Cellobiose with Nanoparticles OH O

HO HO

Current Nanoscience, 2015, Vol. 11, No. 1

OH

HO O

OH O

OH OH

H2O OH

Hydrolysis

Cellobiose

O

HO HO

H2

O

OH

OH

Glucose

OH HO HO

7

OH

HO O

OH

H2O

H2

Hydrogenolysis

OH

OH OH

OH

3- ! -D-glucopyranosyl- D-glucitol

OH

OH OH

HO OH

OH OH

HO OH

OH

OH

Mannitol

Sorbitol

Degradation products

Scheme 5. Plausible reaction pathways for the conversion of cellobiose over the Ru/CNT catalysts [86].

50 bar H2 at an intermediate temperature of 165 ºC after 24 h [95]. Other bifunctional heterogeneous catalysts such as Ru nanoparticles loaded on a Keggin-type polyoxometalate (Cs3PW12O40) [96], phosphotungstic acid (PTA)/metal– organic-framework-hybrid supported ruthenium catalyst (Ru-PTA/MIL-100(Cr)) [97], and mesoporous niobium phosphate supported Ruthenium (Ru/NbOPO4) [98] were also efficient for the conversion of cellulose into sorbitol (up to 69% yield) in neutral water in the presence of H2 under relatively mild conditions. Apart from supported Ru nanoparticles, other metal nanoparticles such as Pt and Ni on selected supports were also capable of efficiently converting cellobiose or cellulose into sugar alcohols by environmentally friendly processes. For instance, a comparable yield of sugar alcohols (~25%) could be obtained over HUSY(20) supported Pt and Ru nanoparticles under initial H2 pressure of 5 MPa at 190 ºC after 24 h, which was much higher than that of Pd, Ir, and Ni catalysts [99]. Unexpectedly, Pt/γ-Al2O3 catalyst gave a much higher yield of sugar alcohols (sorbitol: 25%, mannitol: 6%) than Pt nanoparticles immobilized on other different types of supports including HZSM-5, Hβ, HUSY(20), HUSY(30), SiO2, HY(2.6), HUSY(40), HUSY(15), SiO2Al2O3, FSM-16, ZrO2 and HMOR under the identical reaction conditions, which obviously indicated the importance of supports materials. Nevertheless, the supports with different acidity didn’t show too much difference in the yield of glucose (0~4%) produced from cellulose, suggesting that the acid sites for the hydrolysis of cellulose were generated in situ from H2 [100]. In an attempt to enhance the affinity of cellulose for the catalyst, a fibrous 3D carbon material mimicking the shape of the sea urchin Echinometra mathae was fabricated, which was further used as a support for the preparation of Pt-based catalyst [101]. Without extra acid treatment of the catalyst with H+ species, the Pt-based catalyst on 3D carbon was very active for the hydrolysis of cellulose, giving hexitols in a total yield of ~80% under 30 bar of H2 pressure at 180 ºC within 24 h. It was again confirmed that the metallic Pt per-

formed not only hydrogenation but also the in situ generation of protons by H2 dissociation or the splitting of water on the surface, resulting in the hydrolysis of cellulose [102-104]. Most recently, Ma et al. [105] employed carbon monoxide (CO) and liquid water as an expedient alternative source of hydrogen gas for the catalytic conversion of cellulose, in consideration of a well-established industrial process for the large-scale production of hydrogen via the water-gas shift (CO + H2O → CO2 + H2, WGS) reaction. Over Pt-Mo2C/C catalyst (0.15 g), 1,2-alkanediols in a yield of 28.7% with a selectivity of 39.4%, 32.0%, 15.0% and 13.6% towards ethylene glycol (EG), 1,2-propanediol (1,2-PD), 1,2-butanediol (1,2-BD) and 1,2-hexanediol (1,2-HD) respectively could be obtained directly from cellulose (0.5 g) in water (60 mL) at 250 ºC under 4.5 MPa CO after 15 min. The Pt-Mo2C domains in the tandem Pt-Mo2C/C catalyst were proposed to be responsible for the formation of active hydrogen species from the WGS reaction, while the Pt-C domains might catalyze the subsequent hydrogenation/hydrogenolysis reactions. Unfortunately, only trace amount of hexitol was observed with the passivated Pt-Mo2 C/C catalyst, possibly due to the existence of molybdenum oxide that facilitated the C–C bond cleavage reaction [106]. Interestingly, both a higher total yield of polyols (42.1%) and a higher selectivity to hexitols (~60%) were achieved in the presence of unpassivated Pt-Mo2C/C catalyst, indicating that the surface properties of Pt-Mo2C/C indeed had a major impact on the behavior of the catalyst. In order to reduce the pyrophoricity of Raney Ni that was reported to be efficient for the aqueous phase hydrogenation of sugars to sugar alcohols [107-109], Ni nanoparticles supported on aluminium hydroxide (NiNPs/AlOH) was developed to be a stable catalyst for the production of sugar alcohols from cellobiose [110]. The NiNPs/AlOH catalyst not only exhibited high activity with 89% yield of sugar alcohols at cellobiose conversion of 90% under 2.0 MPa of H2 pressure in H2O at 130 ºC after 24 h, but also could be reused for at least five consecutive runs without any significant loss of its catalytic performance. Considering the excellent stability

8 Current Nanoscience, 2015, Vol. 11, No. 1

Li et al.

of carbon material under hydrothermal conditions as well as its large surface area for dispersing active components, Zhang et al. [111] reported a series of Ni-based bimetallic catalysts supported on mesoporous carbon (MC) or activated carbon (AC) for the conversion of cellulose to hexitols. It was discovered that MC supported Ni (20 wt%) catalyst exhibited very good performance in terms of cellulose conversion (84.5%) and hexitol yield (42.1%) at 245 °C and 6 MPa H2 for 30 min, which was more active than 20%Ni/AC catalyst with 61.9% cellulose conversion and 19.7% yield of hexitol under the same reaction conditions. On the other hand, the authors illustrated that the nickel-based bimetallic catalysts consistently showed enhanced catalytic activity and stability as compared with corresponding sole metals including Ni, Pt, Ru, Pd, Rh and Ir, which might be aroused by the tunable electronic and chemical properties superior to those of the parent metals [112, 113]. The highest hexitol yield of 59.8% was obtained over the 1%Rh-5%Ni/MC catalyst, and the hexitol yields followed the order of 1%Pt-5%Ni/MC < 1%Pd-5%Ni/MC < 1%Ru-5%Ni/MC < 1%Ir-5%Ni/MC < 1% Rh-5%Ni/MC, in the range of 47% to 60%. Furthermore, the bimetallic catalysts gave a much improved stability than the counterpart monometals in hydrothermal conditions for cellulose conversion.

including Ni/KB, Ni/XC, Ni/SiO2, Ni/Al2O3, Ni/TiO2, Ni/ZrO2 and Ni/CeO2 could be significantly improved as the content of Ni was increased from 10 wt% to 70 wt%; while Ni/CNF was durable for 3 times regardless of the low loading amount of 3 wt%, which implied that larger crystalline Ni particles were more resistant to sintering and the surface coverage caused by Ni oxide species [124]. In the presence of a tungsten-based catalyst that is effective to promote the C−C bond cleavage reaction, ethylene glycol (EG) instead of hexitols was always obtained as the major product in the hydrolytic hydrogenolysis of cellulose [125]. The cascade hydrolysis, cleavage of C−C bonds, and hydrogenation of cellulose could take place in one-pot to afford EG and other related products (Scheme 6) [126]. Amongst the applied catalysts, tungsten carbide (WCx) and tungsten phosphide (WP) were demonstrated to be capable of directly catalyzing the hydrolysis of cellulose and the subsequent hydrogenolysis of sugars without promotion of a transition metal, in which acid sites arising from both hot water and the surface tungsten oxides or phosphates were responsible for the initial hydrolysis of cellulose [127], and the platinum-like electronic properties of WCx or WP played a role in hydrogenation [128, 129]. For instance, a high EG yield of 72.9% could be achieved in water at 245 ºC and 6 MPa H2 within 30 min, when a three-dimensional mesoporous carbon (MC) supported WCx nanoparticles was used as a multifunctional catalyst [130]. In this case, the promotional effect of Ni on the WCx/MC was negligible, and the EG yield only increased from 72.9% to 74.4% after 2 wt% Ni was introduced. The 3D interconnected mesoporous structure of MC support was proposed to partially facilitate the dispersion and accessibility of active component WCx as well as the transportation of molecules, thereby enhancing its hydrogenation activity for unsaturated intermediate [131]. On the other hand, tungsten and tungsten oxide species such as W, WO3, H2WO4 and heteropoly acids were only active for C−C cleavage of cellulose but inactive for the subsequent hydrogenation, which was mandatorily used in combination with a transition metal by physical mixing or co-reduction to form M−W bimetal catalysts for EG production [132-135].

Unlike previous studies processing of cellulose with porous materials [114-118], Sels et al. [119] recently put forward a basic concept of catalyst design relying on the entanglement of threadlike carbon nanofibers around the waterinsoluble cellulose matrix. The authors illustrated that the Ni-containing carbon nanofibers (Ni/CNF) prepared by catalytic vapor deposition (CVD) of methane over Ni nanoclusters supported on γ-alumina were efficient for the degradation of microcrystalline cellulose to hexitols, producing sorbitol and mannitol in a yield of 30% and 5%, respectively, at 87% cellulose conversion under 6 MPa of O2 pressure at 210 ºC after 24 h. Most notably, negligible Ni leaching of 3.8 ppm was observed by ICP-AES analysis under such reaction conditions. Afterwards, Sels and his researchers [120] further found that the anchoring and dispersion of Ni precursors onto inert CNFs could be well achieved by oxidative activation of the CNFs with oxidizing acid HNO3 [121-123]. On the other hand, the acid strength/concentration of Ni/CNF catalyst could be simultaneously tailored by this preparation method, and an improved yield in hexitols (76%) as well as cellulose conversion (93%) was observed during the hydrolytic hydrogenation of cellulose in the presence of 7.5 wt% Ni/CNF at 190 ºC within 24 h. Interestingly, the yields of hexitols and durability of supported Ni catalysts OH O HO

HO O

OH OH C ellulo se

In addition to above discussed platform molecules such as glucose, HMF and polyols, gluconic acid (GA) is also an important intermediate widely applied in the food and pharmacecutical industrials [136]. Generally, GA was produced OH

OH O O

4. NANOPARTICLES CATALYZED OTHER REACTIONS

n

H 2O H ydro lys is

HO HO

O OH Gluco se

O H H ydro geno lysis [H ]

OH

+ O ther po lyols OH E th ylene glyco l

Ligh t alkan es , H 2 , CO , CO 2, etc.

Scheme 6. General reaction route for the production of ethylene glycol (EG) from cellulose [126].

Catalytic Valorization of Cellulose and Cellobiose with Nanoparticles

Current Nanoscience, 2015, Vol. 11, No. 1

by the fermentation of glucose [137] or catalytic oxidation of glucose with transition-metal particles [138-142]. In recent years, increasing attention has been paid to the direct conversion of cellobiose or cellulose to GA via the successive hydrolysis and oxidation step catalyzed by supported nanoparticles (Scheme 7). Gold nanoparticles loaded on nitric acidpretreated carbon nanotubes (CNTs) were reported to be efficient for the selective oxidation of cellobiose by molecular oxygen to GA in aqueous medium without pH control, and a high GA yield of 80% could be obtained at 145 ºC and 0.5 MPa O2 after 3 h [143]. In contrast, Au/SiO2, Au/HZSM-5, Au/AC, Au/graphite and Au/XC-72 only exhibited lower catalytic performance in terms of cellobiose conversions (< 60%) and GA selectivity (< 40%). On the other hand, Al2O3, MCM-41 and MgO employed as supports could provide higher cellobiose conversions (93-100%) but lower GA selectivity (10-20%). These results clarified that the CNTs was relatively inert toward the conversion of GA, as compared with other solid supports. Gratifyingly, a nearly complete cellobiose conversion of 97.5% with a high selectivity of 98.9% toward gluconic acid could be achieved under the identical conditions (i.e., 145 ºC, 0.5 MPa O2, and 3 h), when Cs2HPW12O40 was used to support Au nanoparticles with particle sizes of 1–3 nm [144]. The phosphotungstate support was proposed to play multiple crucial roles in this reaction, which firstly worked as a heterogeneous supOH HO

O

HO HO

n

OH

OH H 2O

Or

OH

As discussed in Section 3, sorbitol could be produced from cellulose through hydrolytic hydrogenation catalyzed by various metal nanoparticles [146]. Interestingly, the sorbitol could be transformed into isosorbide via 1,4-sorbitan through two stepwise acid-catalyzed dehydration reactions (Scheme 8). The final product isosorbide was illustrated to have promising potential to be used as a pharmaceutical in-

O O

OH C ellu los e

porting material for Au nanoparticles, allowing the catalyst to be easily recycled. Importantly, it could provide solid acid sites for hydrolysis as well as modulate Au redox sites for selective oxidation. Afterwards, the authors further demonstrated that Au/CsxH3-xPW12O40 system could also catalyze the transformation of cellulose into GA with good efficiency (47-60% yields), while the cellulose conversion and GA yield largely decreased from about 70% and 60% to 41% and 32%, respectively, after three cycles [145]. The serious deactivation of the catalyst could be overcome by using Au/Cs3.0PW12O40 in combination with H3PW12O40, and a high GA yield of 85% for the conversion of the ball-milled cellulose (97% conversion) was achieved at 145 ºC after 11 h under 1.5 MPa O2. Notably, after sixth cycles, the cellulose conversion and the selectivity to GA only decreased slightly from 87% and 89% to 81% and 81%, respectively, over the H3PW12O40–Au/Cs3.0PW12O40 catalyst at 145 ºC and 1.0 MPa O2 after 11 h.

OH

HO O

O

HO O

OH

H ydro lys is

OH

HO HO

OH

OH O

[ O]

OH

O xidation

OH Glu cos e

OH

O

HO

OH OH OH Gluco nic acid

O

OH C ellob iose

Scheme 7. Reaction pathways for the conversion of cellulose or cellobiose to gluconic acid (GA). OH O HO

HO O

OH

OH O O n /2

OH C ellulo se Or

OH HO HO

OH H 2O

O

HO O

OH

H yd rolys is OH OH

HO HO

9

OH O

OH

[ H]

OH

H ydro genation

OH

OH

HO OH

Glu cos e

OH

So rbito l

O

OH C ellob ios e

D eh ydr atio n

HO

H

OH O D ehy dration

O

H

OH

Is os orb ide

Scheme 8. Reaction pathway for the catalytic conversion of cellulose to isosorbide.

H+

H+

O

HO

HO

OH

1 ,4 -So rbita n

10 Current Nanoscience, 2015, Vol. 11, No. 1

termediate, an additive to enhance the strength and rigidity of polymers, and a monomer for biodegradable polymers [147]. Recently, Li et al. [148] reported a strategy for the efficient conversion of cellulose into isosorbide over a bifunctional Ru catalyst supported on mesoporous niobium phosphate (mNbPO) without further addition of any soluble acid. Under H2 pressure of 6.0 MPa, more than 50% yield of isosorbide with almost 100% cellulose conversion could be achieved at 220 ºC in 1 h. It was proposed that the strong acidity, good hydrothermal stability, and mesoporous character of the mNbPO support facilitated the hydrolysis of polymeric cellulose to glucose and the dehydration of sorbitol to isosorbide [149]. Syngas, a mixture of CO and H2, can be used as an intermediate in the production of chemicals (e.g., methanol) and liquid fuels (e.g., dimethyether and diesel fuel) or as a fuel to produce heat and power [150-152]. Biomass gasification is considered to be one of the most promising options for the production of syngas; however, undesirable products, especially tar that is a complex mixture of condensable organic compounds [153], always cause severe operational problems associated with their condensation and polymerization. Employment of a catalyst in a secondary gas conditioning reactor at the gasifier downstream was ever used for tar reduction in biomass gasification systems, whereas the catalyst was mixed with the feed and a downstream gas cleaning process was subsequently required [154]. Instead, directly using catalysts in the gasifier could simplify the subsequent downstream cleanup process [155]. Among various supported catalysts efficient for both syngas production and downstream tar removal [156-158], nickel with a good activity/price compromise has been widely utilized in industrial manufacture involving hydrocarbon reforming reactions [159]. Nevertheless, the major challenge in Ni-based catalyst mediated processes is the loss of activity [160, 161]. An alternative strategy to the classical supported catalyst was developed, in which catalyst precursors were directly used as fuel additives inserted into the biomass solid fuel before thermochemical conversion [162, 163]. For instance, direct usage of nickel as well as iron nitrates salts in the pyrolysis of wood at 700 ºC was found to be efficient for reducing tars and increasing H2 yield, and an increase of H2 production of up to 260% could be obtained with the nickel catalyst, compared to the reference (catalyst-free) sample [164]. Such high catalytic performance might be attributed to the formation of nickel crystalline metal NPs during the pyrolysis of nickel(II) impregnated wood in the temperature range of 400–500 ºC [165]. Nano SnO2 particles with 3-4 nm synthesized by hydrothermal method were reported to make a more positive influence on selective pyrolysis of hazelnut shell compared with other catalysts including red mud (byproduct of Aluminum factory), HZSM5 and K2CO3, and a maximum gas yield of 43.7% could be obtained by nano SnO2-hazelnut shell interaction at 700 ºC [166]. With respect to catalytic hydrothermal conversion of cellulose to gaseous products through water-gas shift reaction (WGS), Sınağ et al. [167] illustrated that the effect of nano ZnO on cellulose conversion was more remarkable at 300 ºC, whereas nano SnO2 showed higher catalytic efficiency for cellulose conversion at 400 ºC and 500 ºC. As to Al2O3 supported Pt catalyst (Pt/Al2O3), no

Li et al.

CO was observed in the process of biomass reforming; Whereas, Chang et al. [168] found that the carbon fiber supported catalyst (e.g., Pt/C and PtRu/C nanoparticles) could provide a good reforming environment for the bio-hydrogen as well as CO generation from biomass. In fact, supported ruthenium nanoparticles also exhibited high catalytic activities for the C−C bond cleavage and hydrogenation, affording mainly methane and lower alkanes rather than water soluble organic compounds, such as diols and alcohols, which were formed with the use of the other catalysts [169]. 5. SUMMARY The catalytic conversion of cellulose and cellobiose into fuels and chemicals via different types of reactions has attracted considerable concern in both scientific and industrial communities. Higher yield and selectivity of specific molecules could be achieved over nanoparticles, which are showing great potential for efficient utilization of cellulose or even lignocellulose towards more favorable economics and greener processing. However, the biomass refinery is still one of the challenging subjects in green and sustainable chemistry. For instance, the separation of heterogeneous catalysts from a solid mixture containing biomass residues as well as insoluble humins is a serious problem. More active, selective and durable nano-sized catalysts are eagerly required for the production of tailored bio-oils direct from biomass resources. With the rapid development of the scientific research in material and catalysis applicable to biomass transformations, we believe that further progress of solid catalysis especially nanoparticles mediated catalytic systems will overcome such tough issues for practical applications. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS The authors thank anonymous reviewers and editors for their invaluable comments and suggestions which help improve the article. We wish to thank the financial support from the International Science & Technology Cooperation Program of China (2010DFB60840), Key Technologies R&D Program (2011BAE06B02), the Research Project of Chinese Ministry of Education (213033A), Guizhou Provincial S&T Program ([2012]6012, [2011]3016, and [2009] 3011), and State Scholarship Fund (No. 201306670004). REFERENCES [1]

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Received: March 25, 2014

Revised: September 03, 2014

Accepted: October 05, 2014

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