Advances in the Catalytic Production and Utilization of Sorbitol

Review pubs.acs.org/IECR

Advances in the Catalytic Production and Utilization of Sorbitol Jun Zhang, Ji-biao Li, Shu-Bin Wu,* and Ying Liu* State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong, 510640, China ABSTRACT: Recently, research on the production and transformation of sorbitol has become exciting in chemical industry and in catalysis studies for its broad applications. It opens up a new path for achieving sustainable energy supply and chemicals production. Here we mainly review the catalytic routes for the synthesis of sorbitol and conversion of sorbitol into high valueadded compounds such as lower alcohols, paraffins, isosorbide, and other derivatives. Meanwhile, some promising and valuable research directions are suggested based on the major challenges emerged in current research, such as the development of efficient magnetic catalysts, microwave heating, and other hydrogen sources.

1. INTRODUCTION For energy and economic reasons, extensive research has been carried out worldwide to study the efficient conversion of biomass resources into valuable biofuels and chemical materials in the last decades (Figure 1), since they have great merits such as abundance, renewable, and wide distribution when compared to other raw materials.1−10 Among these explorations, one attractive route is the preparation and utilization of sorbitol, since it is known as one of the 12 important target chemicals in their biomass program.11 Sorbitol being the most commonly used sugar alcohol (it is the least costly) holds the biggest market share among similar polyols, which is widely used in food, drugs, cosmetics, toothpaste, and so on. For example, it is an important precursor for the manufacture of L-ascorbic acid that consumes almost 15% of world sorbitol production.12 Most importantly, sorbitol can be further degraded into polyols that are the downstream products in the petrochemical industry.13 Meanwhile, it can be used for the synthesis of lactic acid under alkaline hydrothermal conditions.14 Recent studies showed that the structure and the catalytic performance of some catalysts were significantly enhanced with the addition of sorbitol during catalyst preparation.15−18 As a selective dehydration product of sorbitol, isosorbide has a special application in cosmetic, biomedicine, and polymers materials due to the rigid molecular structure and chiral centers.19−21 In brief, the general preparation and conversion routes of sorbitol are clearly shown in Scheme 1. Usually the production of sorbitol is accomplished in a hydrogenation process;22−25 however, reactions like hydrolysis and hydrogenation may be involved in the same reaction system due to the rapid development of research. Because biomass materials such as starch26 and cellulose27−29 are receiving increasing interest in recent work, leading to a great need in the improvement in the catalysts and/or reaction systems. Recently, the ruthenium catalysts showed higher hydrogenation activity than that of nickel and alloy catalysts.23,30,31 With in-depth studies, unavoidable phenomenon happens that sorbitol will be easily degraded in the presence of H2 under high temperatures. Then lower alcohols, including glycol, 1,2-propylene glycol, and methanol, are formed after reaction.32,33 Notably, these chemicals can be used to synthesize many high value-added products for the replacement © 2013 American Chemical Society

of oil resources. As concerns the dehydration product, isosorbide is obtained by 2-fold dehydration of sorbitol via sorbitan under acidic conditions. It was reported that sulfuric acid and other inorganic acids were first used in the synthesis of isosorbide.34−36 Due to high corrosion and environmental pollution of inorganic acids, some pollution-free and effective catalysts such as solid acids and acidic ion exchange resins are developed. Through the above analysis, we can see that a plethora of useful molecules will be obtained from multifunctional sorbitol via a series of reactions by using various catalysts. Although some published work deals with the topic of sorbitol chemistry to a certain degree, including conversion of cellulose into sorbitol and hydrogenolysis and dehydration of sorbitol,37−39 this Review concentrates mainly on describing and analyzing all aspects of the work on sorbitol chemistry reported up to date. The improvement in catalytic synthesis and conversion of sorbitol with suitable reaction systems are discussed in greater detail, and some of the existing limitations and unsolved challenges are put forward at the same time. Owing to the rapidly expanding nature of this interesting field, we hope that this Review provides a helpful overview and insight to readers in this exciting research area.

2. SORBITOL PRODUCTION In commercial terms, sorbitol is an ideal, versatile compound that has been widely used in the fields of food and chemistry. The detailed information for physical properties of sorbitol is shown in Table 1. Three techniques are mainly introduced in the industrial production, namely, batch, semicontinuous, and continuous technology. It begins with raw materials like cassava, corn, or wheat that are first converted into dextrose through enzymatic hydrolysis and then was hydrogenated into sorbitol at 403−423 K with H2 pressure ranging from 4.0 to 12.0 MPa. Among the manufacturers, Roquette Freres is the biggest sorbitol producer around the world, together with Cargill and SPI Polyols they hold a market share of over 70%. By the way, the yield of mannitol is accompanied during Received: Revised: Accepted: Published: 11799

April 14, 2013 July 28, 2013 July 29, 2013 July 29, 2013 dx.doi.org/10.1021/ie4011854 | Ind. Eng. Chem. Res. 2013, 52, 11799−11815

Industrial & Engineering Chemistry Research

Review

Figure 1. Concept of a biorefinery.

Scheme 1. Specific Network for the Preparation and Utilization of Sorbitol

catalytic activity and stability of Raney-nickel were not desirable due to the leaching of Ni. Follow-up studies revealed that the activity of Raney-nickel could be significantly enhanced by the addition of some promoters.41−43 For the Mo modified sample, the specific surface area and hydrogenation activity increased from 56 to 77 m2/g and 0.35 to 0.46 (kg·s)−1, respectively.44,45 Mo was present in the form of Mo3Al or Mo3Al8 in Ni2Al3, and the leaching of Al and Ni was effectively avoided. Hoffer et al. reported that the activity of Raney-nickel modified with and without Mo decreased from 0.46 to 0.32 (kg·s)−1 and from 0.35 to 0.18 (kg·s)−1, respectively, in the third run.27 Note that Mo showed excellent stability without leaching into reaction solution. With in-depth studies, the specific surface area increased by 30%−35% in Cr modified Raney-nickel catalyst, and the activity depended on Cr content. The Cr replaced Ni in Ni2Al3 to form homogeneous alloy at certain degree when less than 2 wt % Cr was added, and a new phase of Al9Cr4 would be created with further increasing Cr loading.45,46 With respect to Fe modified catalyst, the specific surface area and activity were obviously promoted, which increased from 56 to 112 m2/g and from 0.35 to 0.90 (kg·s)−1, respectively. But more Ni, Al, and

Table 1. Physical Properties of Sorbitol

a

molecular weight

refractive indexa

density (kg/m3)b

melting point (K)

boiling point (K)

182.17

1.3477

1489

361−375

569

Value given for 10 wt % aqueous solution. bValue given for 268 K.

sorbitol production, which has wide application in many fields as well like medicine, chemical industry, and so on. The formation of mannitol is a result of the epimerization of glucose to mannose in the case of alkaline medium. The following sections summarize the research advances in the catalytic synthesis of sorbitol using various biomass carbohydrates. A series of efficient nickel and ruthenium catalysts are explored in glucose hydrogenation, as listed in Table 2. 2.1. Production of Sorbitol from Glucose. 2.1.1. Nickel Catalysts. Sorbitol was introduced early by glucose hydrogenation with the presence of suspension catalyst in 1942 for the first time, and then fixed bed reactor was developed in combination with Raney-nickel catalyst.40 However, the 11800

dx.doi.org/10.1021/ie4011854 | Ind. Eng. Chem. Res. 2013, 52, 11799−11815


Review

Table 2. Catalytic Hydrogenation of Glucose with Various Catalysts

a

glucose concn (wt%)

catalyst

T (K)

Pa (MPa)

glucose conversion (%)

sorbitol selectivity (%)

ref

10 10 10 10 5 50 50 40 50 50 50 50 50

Ru/MCM-41 Pd/C Al−Ni Ru/C Ni1.85Cu1Al1.15 Raney Ni−P Ru−B Ru/C Ru/Cc Ru−B/SiO2 Ru/HMT Ru−Cr−B NiMoAl

393 393 393 393 398 393 353 373 393 373 373 353 408

3 3 3 4 3 4 4 8 4 4 3 4 4

100 41.1 20.4 ―b 78.4 55.8 95.1 100 99.9 100 72.9 99.7 100

94.4 39.9 20.5 >98 93.4 99.5 ∼100 99.2 98.2 ∼100 ― ∼100 >99

29 29 29 45 51 53 54 55 56 57 62 63 64

Initial hydrogen pressure measured at room temperature. bNot reported. cAdding carbonyl group promoted organic assistant.

efficient accessibility of the Ni particles attached at the tip of the supports allows for immediate hydrogenation of glucose units. Additionally, considerable effort should be directed to the relationship between Ni particle size and related catalytic activity. Hydrotalcite-like compounds (HTlcs) were used as precursors for the preparation of hydrogenation catalysts in previous study.50 In our recent work, a series of catalysts were prepared from Ni/Cu/Al hydrotalcite precursors and were applied in glucose hydrogenation. The obtained catalysts exhibited great sorbitol selectivity (93.4%) and glucose conversion (78.4%) when the precursor was pretreated under H2 atmosphere at high temperatures. It was proposed that the existing Al may be used as support for active Ni and Cu in reduced samples and Ni species were mainly responsible for high activity. For recycling experiments, Ni was obviously leached into the solution, while Cu showed excellent stability according to the ICP-AES analysis of reaction solutions. With the modification of various metallic elements, an interesting phenomenon came to us that the Co and Fe modified catalysts were magnetic. It is known that products separation and catalyst reuse can be easily realized via the use of magnetic catalysts. With the further research, Fe content also played a great important part in catalyst activity. It was inferred that the synergy effect among metallic elements was weakened with the further increase in Fe content; future studies on calculating reaction activation energy and leaching of Fe will effectively address the present problems. When the magnetic catalyst was applied in glucose hydrogenation, the optimal yield and selectivity of sorbitol were 88.2 and 94.2% at reaction temperature of 398 K with a catalyst dosage of 35%. For fructose hydrogenation, the desired yields of sorbitol and mannitol on magnetic catalyst were 42.9 and 56.9% at reaction temperature and initial H2 partial pressure of 383 K and 3.0 MPa, respectively. Concerning the conversion of biomass materials, the one-step synthesis of sorbitol could be realized over magnetic catalysts in combination with extremely low acids. For catalyst regeneration, various methods were investigated in detail to remove the adsorbed carbohydrates and prolong the catalyst life, such as calcination or elution. Notably, the used catalyst after acetone elution showed better catalytic activity when compared to that of other organic solvents. For methanol or ethanol elution, Ni was easily leached into the solution due to the chelation between Ni and hydroxyl.51 Above all, the

Fe were lost during the reaction, which would raise the cost for the purification of sorbitol. As discussed above, it is speculated that the high activity and stability of Mo and Cr modified catalysts may be explained by the synergy among these active metals, probably providing a lower activation energy of CO in glucose molecule. Much more work should focus on the synergy effect among used metals and calculating reaction activation energy through performing kinetics experiments. The phase of metal promoters may play a critical role in the performance of modified catalysts. Overall, Mo and Cr modified catalysts hold great potential for possible application in industrial-scale production after comprehensive comparison. Considering the environment for producing sorbitol on modified Raney-nickel catalysts, particular attention should be directed to the following aspects. First, proper H2 pressure is deserved to achieve great glucose conversion, and higher reaction temperature leads to glucose carbonization and generates some byproducts. Second, the desired pH value of reaction solution was 8.0−9.0 when using Raney-nickel catalyst. However, glucose can be easily isomerized into mannose in alkaline condition and then hydrogenated into mannitol.47 Therefore a suitable pH value should be chosen so as to prolong catalyst life and prevent glucose isomerization, and a pH of around 7.5 is proposed in industrial production. Another method also attracts significant interest in catalyst preparation, i.e., the use of support to increase metal dispersion. Various carriers were examined to enhance catalyst activity, such as SiO2, TiO2, Al2O3, and so on. The research group of Claus had made great effort to the development in supported Ni catalysts. They reported that the activity followed the sequence Al2O3 > TiO2 > SiO2 > C, while all these tested catalysts were leaching.48 In most cases, catalysts prepared by impregnation exhibited higher activity compared to those prepared by incipient wetness. With in-depth study, the Ni precursors were found to play an important role in the performance of obtained catalysts. By comparison, Ni catalysts prepared by impregnation with nickel ethylenediamine complexes revealed small nickel particles (mean diameter: 2− 3 nm) and showed almost no Ni leaching when compared to a commercial Ni/SiO2 catalyst, which confirmed that small nickel particles were more resistant to leaching than larger ones.49 In current research, some other carriers such as MCM-41 and HZSM-5 molecular sieves are of high interest that can provide high BET surface area, desirable porosity, and excellent metal dispersion for supported metal catalysts. In this manner, an 11801



Review

Scheme 2. Reaction Mechanism for the Hydrogenation of D-Glucose to D-Sorbitol

preparation cost of HTlcs is lower compared to that of noble metal catalysts, and the hydrogenation catalysts derived from HTlcs exhibit exciting activity for merits of smaller crystal size and higher surface area. Amorphous hydrogen storage alloy developed in recent years has gained increasing interest as an alternative target for its hydrogenation/dehydrogenation properties. It was reported that high yield and selectivity of sorbitol could be achieved under mild reaction conditions when using amorphous hydrogen storage alloy in glucose hydrogenation. For example, the glucose conversion reached 97% at 313 K and 834 kPa for 24 h when using LaNi5 as catalyst and HAc as additive. However, the conversion was only 35% under the same conditions over Raney nickel.12,52 In 2000, Li et al.53 reported that a skeletal Ni−P amorphous alloy (Raney Ni−P) was formed by alkali leaching of amorphous Ni−Al−P precursor. This catalyst gave higher turnover rates than Raney Ni, apparently as a result of promotion of Ni-active sites by phosphorus. 55.8% glucose (50 wt % feed concentration) was hydrogenated, compared to that of 17.2% in Raney nickel and 1.1% in commonly used Ni−P. Meanwhile, only less than 1.0 ppm Ni was leached, indicating

the superior stability of Raney Ni−P. It was speculated that the disorder degree of catalyst was obviously increased by adding phosphorus, which facilitated hydrogen flowing during the reaction. For amorphous NiMoAl alloy, it exhibited excellent stability that could be recycled five times, and glucose was completely converted with sorbitol selectivity of 99%.54 It was assumed that the diffusion rate of Ni active species decreased due to greater atomic radius of Mo (0.139 μm) than Ni (0.124 μm), which would prevent catalyst crystallization. The added Mo increased the disorder degree and hydrogenation activity of catalyst. Follow-up studies are proposed to introduce supports (molecular sieves, SiO2, active carbon, and γ-Al2O3) to improve metal dispersion. Detail work on properly increasing disorder degree of amorphous structure by adding various additives (Fe, Zn, La, Ce, Co, etc.) is still necessary. 2.1.2. Ruthenium Catalysts. To completely overcome the problem of nickel leaching and further increase sorbitol selectivity, the increasing requirement for new catalysts is essential. Ruthenium catalysts have attracted a great deal of attention in recent years for high stability and superior activity.31,47,55 Hoffer et al. first reported that carbon supported Ru catalyst was a promising alternative for Raney-type Ni in the 11802



Review

Table 3. One-Pot Aqueous Hydrolytic Hydrogenation of Cellulose and Cellobiose Using Various Metal/Carrier Catalysts or Mixture Systems catalyst 2.5 wt % Pt/γ-Al2O3 1.0 wt % Ru/CNT 4.0 wt % Ru/C HCl-0.2 wt % Ru/H-USY CsHPA-5.0 wt % Ru/C H2SO4-5.0 wt % Ru/C 1.0 wt % Ru/CNT 1.0 wt % Ru/CsPW12O40 1.0 wt % Ru/CsHPW12O40 Ru, pH 2.0 Ru/C

substrate b

MC MCc MCb BMCd BMC α-cellulose cellobiose BMC cellobiose cellobiose cellobiose

concn (wt%)

time (h)

T (K)

Pa (MPa)

sorbitol yield (wt%)

ref

0.8 0.8 2 2 10 5 0.85 0.67 2 8.3 1

24 24 0.5 3 8 1 3 24 6 12 1

463 458 518 463 463 433 458 433 413 393 458

5 5 6 5 5 5 5 2 2 4 3

26 69 29.6 66e 59e 33.2 87 40 93 100 87.1

23 24 68 69 70 71 76 77 77 78 79

a

Initial hydrogen pressure measured at room temperature. bMC = microcrystalline cellulose. cMicrocrystalline cellulose pretreated in 85% H3PO4 at 323 K for 40 min. dBMC = ball-milled microcrystalline cellulose. eSugar alcohol yield.

of Cr2O3 improved the dispersion of Ru−B alloy particles by acting as the support, preventing the coalescence of active Ru.64 However, excess Cr-dopant led to a rapid decrease in hydrogen uptake rate due to negative effect on the intrinsic activity, since Cr3+ may accept partial electrons from Ru atom. Concerning the existing B element, it afforded the transformation of electrons into the active Ru, resulting in an increase in the specific area of catalyst. Through the above analysis, we can give inspiration to the following three points: first, the supports of high pore and BET surface area are more favorable for the reaction. Second, particular attention should be paid to the preparation methods of Ru catalysts. Lastly, the selectivity to sorbitol is obviously improved over Ru catalysts and the refining process will be simple and clean. Therefore, subsequent studies on exploring effective carrier for Ru catalysts should be focused, and regeneration of Ru catalysts must be a second important point, given the high price of metallic Ru. Only in this way can we improve catalyst life and cut down the actual production cost. 2.2. Production of Sorbitol from Biomass Feedstocks. Recently increasing effort has been devoted to find ways to utilize biomass as feedstocks for the production of organic chemicals due to advantages of abundance, renewability, and worldwide distribution. Discarding or burning away these renewable resources will not be a feasible approach because of environmental problems. Hence, efficient utilization of the existing natural resources is now an urgent research to be explored in great depth. In commercial terms, two steps involving biomass hydrolysis and glucose hydrogenation are generally adopted as an effective way in yielding sorbitol. With the rapid development of research, one-pot conversion of biomass into sorbitol captures our attention. As the most abundant source, cellulose, a linear polymer of glucose with β-1,4-glycosidic bonds, can be hydrolyzed into glucose and subsequently hydrogenated into sorbitol, while starch that consisted of glucose by α-1,4-glycosidic bonds also can be used as an ideal source with excellent hydrolysis rate.26 However, starch should primarily be considered as a source of food, so a great effort has been directed to the conversion of cellulose. Above all, this conversion route involves both hydrolysis and hydrogenation reactions that play an important role in yielding sorbitol. Table 3 lists the main data on the most recent advances in the catalytic conversion of cellulose into sorbitol.

selective hydrogenation of glucose, providing clear direction toward catalyst development in relevant fields. It was found that Ru/C showed nearly 100% selectivity to sorbitol under 4 MPa H2, above all, Ru exhibited excellent stability that effectively addressed the issue of leaching in nickel catalysts.45 The activity of Ru/C was proportional to Ru surface area (representing metal dispersion) and independent of the preparation methods, suggesting that great yield and selectivity could be achieved over catalysts of high Ru dispersion. A novel anionic deposition method rendered catalyst with Ru dispersion of up to 40%.45 In case of mass transfer and kinetics on glucose hydrogenation, a first-order dependency with respect to hydrogen was observed for low glucose concentrations (up to ca. 0.3 mol/L), while at higher concentrations the reaction rate changed to zero-order behavior when the kinetic experiments were carried out in the absence of mass transport limitations.30 Also, it was proposed that the conversion route involved the formation of an ionized β-pyranose species, adsorbed on the Ru surface by coordination of O-1, O-5, and O-6. Follow-up studies focus on the influence of preparation methods, supports (SiO2 and γ-Al2O3), and ruthenium precursors (ruthenium acetate and ruthenium trichloride) on catalyst activity.56−60 With different ruthenium complexes as the precursors, the essential properties of catalyst may be of considerable difference. For example, by comparison with RuCl3, the glucose conversion increased by approximately 5% when using ruthenium acetate as precursor.61 When Ru3+ was supported on expanded graphite with HMT (hexamethylene tetramine) as ligand by method of ultrasonic impregnation, the obtained catalysts exhibited great dispersion and smaller grain size (below 10 nm). A glucose conversion of 72.9% was attained at 373 K under 3 MPa H2, while only 34.7% glucose was converted by similar catalyst without HMT as ligand.62 In another study, Ru/MCM-41 prepared by an impregnationformaldehyde reduction method showed almost 100% glucose conversion and 94.4% selectivity toward sorbitol at 393 K under 3 MPa H2.25 The mechanism for glucose hydrogenation over Ru/MCM-41 involved a gas−liquid−solid three-phase catalytic reaction, in which the carbonyl group in glucose reacted with activated H (Scheme 2). On the other hand, ultrafine Ru−B amorphous alloy reduced by KBH4 was found to show perfect selectivity (∼100%) to sorbitol compared to crystallized Ru−B, pure Ru powder, and Raney Ni catalysts.63 Cr-promoted Ru−B amorphous alloy gave ∼98% glucose conversion at Cr content of 3.3%. The formation 11803



Review

by forming hydride compounds to increase H2 solubility.73 Zhu et al.74 reported that an IL connected to an additional binding agent with a boronic acid-containing functional group could be used for the stabilization of Ru nanoparticles, and yields of up to 94% for the conversion of cellulose to sorbitol (main product) and mannitol were observed. By using metal nanoparticles as catalysts, ILs feature certain advantages as solvents, such as the stabilization of the nanoparticles to avoid aggregation to maintain high surface area and catalytic activity.75 However, the use of ILs poses other problems, such as the separation of high polar products from equally highly polar ILs. Usually all chemicals in the reactions described above have high boiling points or decompose at elevated temperatures, which hinders separation by distillation. Therefore, biphasic systems have been discussed, which enable the extraction of the products from the IL to facilitate a more efficient product separation. However, most extraction solvents exhibit low separation coefficients, which result in inefficient extraction. An advanced separation of products and catalysts could be an important factor for an efficient catalytic conversion of cellulose to isosorbide with the present system. Besides, the corrosion of ILs to the commonly used stainless steel equipment has to be avoided. On the basis of the above discussion, there are still some very intriguing questions about this novel process yet to be answered. Recently, a sorbitol yield of 87% was attained at 458 K, and little mannitol (around 2%) was detected using Ru/CNT catalyst with cellobiose as raw material.76 During the reaction, the catalysts with larger mean sizes of Ru nanoparticles and higher acidity presented a better sorbitol yield. While those with smaller Ru nanoparticles and less acidic sites afforded about 93% 3-β-D-glucopyranosyl-D-glucitol, and smaller Ru particles accelerated the degradation of sorbitol. Polyoxometalate-supported Ru nanoparticles were also served as bifunctional heterogeneous catalysts for the conversions of cellobiose and cellulose into sorbitol under mild conditions. Liu et al. confirmed that H2-originated Brønsted acid sites played a key role in the conversions, and Keggin-type Ru/Cs2PW12O40 showed outstanding activity and stability.77 Increasing the pH value from 2.0 to 10.0 resulted in an acceptable decrease in the conversion rate of cellobiose, and a new chemical with formula of C6H14O4 (2,6-dideoxy-hexitol) was formed in neutral and basic mediums.78 In our work, the deactivation mechanism of Ru/C catalyst was discussed in conversion of cellobiose to sorbitol in extremely low phosphoric acid. Based on the ICP-AES analysis, Ru could stably exist in acidic reaction solution without any loss. XRD and XPS tests of recycled catalysts indicated that almost all the Ru element was presented in zero valence state. The study on surface properties of used Ru/C indicated that the BET surface area and pore volume decreased significantly due to the adsorption of organic chemicals, resulting in an obvious decrease in catalyst activity.79 This work may provide a remarkable level of understanding for catalyst activation and recycling in future work. On the whole, efficient catalysts and reaction systems are still in high demand in one-pot conversion of biomass carbohydrates. The Ru and Pt based catalysts are to be preferred for the excellent performance in previous work. Otherwise, some magnetic catalysts can be properly considered through the introduction of Fe or Co in noble metal catalysts, and efficient supports are deserved to be explored to enhance the activity of

In an earlier research, Fukuoka and Dhepe investigated a new green catalytic process for the conversion of cellulose into sugar alcohols. Among the metals examined, Pt/γ-Al2O3 gave high yields (sorbitol: 25%, mannitol: 6%) in sugar alcohols at 463 K and 5 MPa H2, compared to Pd, Ir, and Ni catalysts.23 Cellulose hydrolysis was found to be a rate-determining step, and the Pt catalysts promoted both hydrolysis and hydrogenation steps.65 It was suggested that the metal−support match made hydrogen spill over from the metal onto the support, which generated protic sites on the support surface and led to an increased pool of H+ in the system consequently.66 Subsequently, Essayem and co-workers performed a series of controlling reactions to clarify the functional details of Pt/γ-Al2O3 catalyst in cellulose degradation.67 It was demonstrated that the presence of Pt/γAl2O3 increased the initial rate of dissolution-conversion significantly as well as the total yields of monomer sugars. Pt associated with hydrogen was proposed to intervene not only in glucose hydrogenation but also most likely in H+ generation via H2 heterolytic dissociation and/or hydride transfer steps. Much work still focuses on the development of Ru catalysts for the hydrolytic hydrogenation of cellulose due to superior catalytic performance in glucose hydrogenation. For example, a two-step transformation of cellulose into polyols was conducted in hot water by using Ru/C catalyst. The elevated temperatures resulted in the formation of H+ from water that was capable of performing acid-catalyzed reactions; finally a 29.6% yield in sorbitol was achieved at 518 K and 6 MPa H2.68 However, the use of around 0.1 wt % inorganic acids will be of great effectiveness, and the acidity of reaction system can be enhanced in an almost environmentally friendly manner. Desirable hexitols yield of 66% (mainly consisted of sorbitol and mannitol) at 463 K for 24 h was presented in 106 ppm HCl.69 Subsequent studies focus on the exploration of multifunctional Ru catalysts for the replacement of inorganic acids that will address the issues of catalyst reuse and green environment. The hydrolysis activities of functional catalysts are found to be very dependent on the presence of acidic functional groups on the acid-pretreated surface of the supports. With the introduction of acidic supports, one-step conversion of bioresources into sorbitol can be accomplished under the catalyzing of supported noble metal catalysts. For instance, a 40% yield of hexitols (including 36% sorbitol) was achieved over Ru/CNT for the conversion of commercial cellulose (crystalline, 85%) at 458 K for 24 h.28 Decreasing cellulose crystallinity favored the formation of sorbitol, and a 69% yield was obtained at crystallinity of 33% due to the rapid rate in hydrolysis of amorphous cellulose. In related research, a series of heterogeneous heteropoly acids, hydrotreated cesium salts of heteropoly acids and the like combining with metal supported catalysts were examined,29,70,71 but the main problems presented in achieving high sorbitol from cellulosic materials are low cellulose dissolution and complex side products. In recent years, ionic liquids (ILs) have gained increasing interest as alternative solvents as they enable the complete dissolution of cellulose to facilitate hydrolytic depolymerization.72 Hence, the hydrolytic hydrogenation reaction that results in sugar alcohols has been actively investigated by using ILs as the media.37 For instance, the combination of a heterogeneous Pt or Rh catalyst with a homogeneous Ru complex in 1-butyl-3-methylimidazolium chloride under a H2 atmosphere achieved full cellulose conversion and 51%−74% selectivity to sorbitol.73 The Ru complex acted as a H2 carrier 11804



Review

magnetic catalysts. Inspired by the related work, noble metal catalysts also show excellent performance in the production of H2 from 2-propanol and formic acid,80−84 so the attempts for the use of other hydrogen sources are suggested to be given increasing attention. In this manner, the production cost and equipment requirement for high H2 pressure will be lowered. In our current study, various hydrogen donors were tested in glucose hydrogenation by using supported noble metal catalysts. It was found that sodium formate was much better than other formate salts (HCOOK and HCOONH4) and other agents of donating-hydrogen (CH3COOH, HCOOH, and (CH3)2CHOH) for the catalytic transfer hydrogenation of glucose. Among the examined noble metal catalysts, the catalytic transfer hydrogenation activity for Pd/C was far superior to that of Pt/C and Ru/C. At the reaction conditions of using sodium formate as hydrogen donor at 363 K for 2.5 h, the conversion of glucose and selectivity to sorbitol reached 71.7 and 94.1%, respectively. It can be observed that low reaction temperature and low N2 pressure are advanced in sorbitol production.

Subsequently, a series of supported Ni catalysts are extensively developed. Clark used Ni supported diatomite to catalyze sorbitol hydrogenolysis at temperatures of 488 to 513 K under 13.78−38.58 MPa H2 for the production of glycerol. However, many byproducts such as ethylene glycol, propylene glycol, and xylitol were created at the same time.86 According to the patents of DuPont, under the catalyzing of Ni/silica/ alumina catalyst, basic promoter Ca(OH)2 favored the proceeding of hydrogenolysis; however, the selectivities toward ethylene glycol and 1,2-propylene glycol decreased significantly by using Cl−C4 alkoxides as solvents.87,88 Follow-up study further demonstrated that the use of Ca(OH)2 remarkably increased sorbitol conversion, and the selectivity to 1,2propylene glycerol was obviously improved by using Pt-NaY as compared to Ni-NaY.89 The addition of Ce into Ni/Al2O3 slightly lowered the reduction temperature of nickel oxide but considerably enhanced the H2-chemisorption amount. The overall selectivity to glycols was around 55% at sorbitol conversion of 90%.90 It is concluded that high H2 pressure is required to achieve desirable conversion when using Ni-based catalysts. Another point to consider is that selecting one or two chemicals as the main products is a must, with the aim of increasing corresponding yield and selectivity. Low ruthenium loading catalysts are found to be a good replacement to address the present problems in combination with the use of alkaline mediums. Various supports were investigated to increase the activity of Ru catalysts, such as Al2O3, SiO2, TiO2, activated carbon, and so on. The maximum 1,2-propylene glycol yield reached up to 37.8% at desirable Ru dispersion of 2.5% Ru/CTC-20.91 Sorbitol hydrogenolysis was found to show a dependence on pH and reaction temperature. In a basic medium, the main reaction was a reverse aldolization which gave, among others, a mixture containing two or three carbon atoms independently of the temperature.92 As a comparison, the application of neutral medium and low temperature led to an increase in selective hydrogenolysis in the middle carbon chain of sorbitol, giving mainly glycerol and 1,2-propanediol. Montassier et al. proposed that the cleavage of C−C bond belonged to a retro-Michael reaction under the action of adsorbed nucleophilic species.93 In recent years, active carbon nanofibers (CNFs and CNF/ GF) were used in sorbitol hydrogenolysis, which provided high Ru dispersion and proper porosity.94−96 Supported Ru catalysts are shown to have an attracting behavior compared to commercial Ru/C, especially in terms of selectivity to glycols. Ru/CNF/GF presented higher selectivities to ethylene glycol, propylene glycol, and glycerol (79.1% in total). The addition of Ca(OH)2 remarkably increased the selectivities to glycols, as compared with that of soluble NaOH. With the progress of research, biomass materials are explored for direct synthesis of lower alcohols via sorbitol production during the past years, due to the advantages of broad source, splendid reserve, and cheap cost. Ji et al. investigated single-step conversion of cellulose into ethylene glycol using nickelpromoted tungsten carbide catalysts. The Ni−W2C/AC gave a remarkable selectivity toward ethylene glycol than that of Pt/ Al2O3 and Ru/C, and the highest yield of 61% was achieved at a temperature of 463 K.97 Ni5−W25/SBA-15 gave 75.4% ethylene glycol at 100% conversion.98 They claimed that the addition of Ni into tungsten phosphide (WP) promoted catalytic hydrogenation due to a remarkable synergy between Ni and WP, leading to an increase in ethylene glycol yield to 46.0 mol %,

3. SORBITOL CONVERSION Sorbitol has a considerable potential for the production of versatile chemicals from renewable resources as its hydroxyl groups allow for further functionalization or direct processing via a series of hydrogenolysis, dehydration, and aqueous-phase reforming reactions. In the following sections, the most important advances in the catalytic conversion of sorbitol with regard to potential fields of applications are summarized. 3.1. Synthesis of Lower Alcohols. Lower alcohols, including ethylene glycol, propylene glycol, and glycerol, are important platform molecules with extensive applications. For example, 1,2-propylene glycol is primarily used as a monomer in polyesters and as an antifreeze or cooling liquid. Strikingly, glycerol is an interesting starting material for further chemical derivatization as it can be used for the synthesis of ethylene glycol and propylene glycol as well by deep hydrogenolysis. The state of the art of processing of downstream petrochemical products (oxirane and epoxypropane) into these chemicals is of incredible efficiency after so many years of research. One can reasonably argue that today’s society is not only addicted to oil as a fuel but also to its products. Generally, the mass productions of ethylene glycol and 1,2-propylene glycol are accomplished through a hydration process at ∼473 K under high pressure. Due to gradual diminishment of fossil fuel reserves and further deterioration of environment and ecosystem, the switch from oil-derived chemicals to biorenewable ones calls for a considerable effort in recent years. Onestep rapid hydrogenolysis of biomass holds great advantage that offers a new route for lower alcohols production under a relatively lower reaction pressure, principally addressing the issues of cheap material and diverse sources. On the other side, it opens up a feasible pathway in the synthesis of these highgrade alcohols and accords with the demands of sustainable development strategy in converting cellulosic biomass into bulk chemicals. The C−C and C−O bonds in polyols tend to be cleaved at high temperatures under H2 atmosphere. The first evidence on sorbitol hydrogenolysis was made by Zartman and co-authors, when they studied the degradation of sugar alcohol (sucrose, glucose, maltose, sorbitol, and mannitol) at 30 MPa H2 using Cu/Cr2O3 catalyst.85 11805



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Scheme 3. Reaction Mechanism of Sugar and Sugar Alcohol Hydrogenolysis

Scheme 4. Route for Continuous Conversion of Biomass into Useful Chemicals

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studies showed that H2 was produced from ethylene glycol and methanol by aqueous-phase reforming (APR) using supported noble metal catalysts.110,111 With a detailed study, sorbitol can also be directly used for the production of H2 and alkanes that opens up a new field in sorbitol chemistry. The conversion of sorbitol into clean fuel is of great significance for the shortage of energy and partial replacement of oil resource, given the rising demand for fossil resource and natural gas. The catalytic route for the production of H2 and alkanes by APR of oxygenated hydrocarbons involves cleavage of C−C bonds as well as C−H and/or O−H bonds to form adsorbed species on the catalyst surface. Cleavage of these bonds occurs readily over Group VIII metals, such as Pd and Rh.112 The production of alkanes by APR of sorbitol is presented in a bifunctional pathway, involving the formation of H2 and CO2 on metal catalysts and then dehydration of sorbitol on a solid acid catalyst. In 2002, the research group of Dumesic declared that H2 was formed by APR of sugar alcohols over Pt/Al2O3 catalyst.113 Although paraffin mixtures were synthesized by onestep reforming of sorbitol, the specific reaction network was complex due to the presence of parallel and cascade reactions. The selectivities of H2 and alkane reached up to 66 and 15% separately, and the addition of ethanol into sorbitol enhanced the formation of H2.114 For the non-noble metal catalyst, the Raney Ni−Sn catalyst gave selectivities of H2 and alkane at 65 and 19%, respectively, that may be a good choice to replace Pt/Al2O3.115,116 The essential features of bifunctional pathway for production of alkanes from sorbitol were briefly described in Scheme 5.117

compared to a 25.4 mol % yield over 20% WP/AC at 6 MPa H2 and 518 K.99 In 2010, a novel one-pot approach for alkaline hydrolysis and hydrogenation of cellulose on supported Ru catalysts was proposed by Deng et al.100 A wide range of bases including solid bases, e.g., Ca(OH)2 and La2O3, and phosphate buffers were examined. It was found that the cellulose conversions and products distribution depended largely on the basicity and pH values of the solutions. The findings may provide a scientific basis for efficient conversion of cellulose into targeted polyols by using various alkaline conditions. Similar research is found in recent studies on the conversion of cellulose into polyols using supported noble metals catalysts.101−103 As is well-known that the basic compounds are used as the promoters, the properties of base (NaOH, KOH, Ba(OH)2, Mg(OH)2, CaO, and so on) play a critical role in conversion rate and selectivities. Originally, it was assumed that the base could prevent the dissolution of metal ions from the catalyst.104−106 However, subsequent experiments showed that the base played a more important role in the reaction process. Several hydrogenolysis mechanisms were advanced to illustrate the cleavage of C−C and C−O bonds in polyols in the following research.93,107−109 On the basis of previous work, Wang et al.109 had systematically studied the mechanism of breaking bonds in the reaction of polyols hydrogenolysis by using 1,3-diol as the model. The results had fully demonstrated that the addition of bases facilitated the proceeding of reverse aldol condensation and dehydration, thus leading to the cleavage of C−C and C−O bonds (as shown in Scheme 3). According to the above analysis, noble metal catalysts are widely used in the production of lower alcohols, especially Ru based catalysts. Improving metal dispersion by selecting excellent supports can be a pronouncing research direction. Nickel catalysts also are promising ones if the problem of leaching can be well addressed. Suitable alkaline conditions are conducive to the formation of these valuable chemicals. Most work focuses on cellulose conversion, while the utilization of existing resources, such as secondary fiber, agricultural waste (cotton stalks, saw dust, coconut husks, and rice husks), and forestry residues (bark, branches, and trunk) has not been given enough attention. Besides, an efficient and recyclable solid base is deserved to be investigated in more detail. In practical production, various high value-added chemicals can be achieved by a method of continuous processing (Scheme 4). For the initial step, biomass materials are hydrolyzed into glucose in acidic mediums under high temperature. Then the hydrolysates are hydrogenated into sorbitol under H2 atmosphere or using other H2 sources. The obtained mixtures will be purified by crystallization to produce high purity sorbitol. Furthermore, the final solutions in Reactor 2 can be successively used for deep hydrogenolysis to obtain useful lower alcohols without separation and purification. Besides, the metal catalysts developed in the second step are proposed to be further applied in Reactor 3 without any treatment. Usually alkaline mediums offer good selectivities toward these valuable platform chemicals in Reactor 3, so properly increasing pH value of the reaction solutions provides a good choice for sorbitol hydrogenolysis. 3.2. Synthesis of Hydrogen and Alkane. Processing of renewable carbohydrates to produce fuels and/or chemicals invariably requires the removal of oxygen to form compounds with lower molecular weights and higher volatility that can be subsequently upgraded using gas-phase processes. Previous

Scheme 5. Reaction Network for the Production of Alkanes from Sorbitol over Bifunctional Catalysts

Hydrogen is produced on the metal by cleavage of C−C bonds followed by the water-gas shift reaction. Dehydrated species such as ring compounds (e.g., isosorbide) or enolic species are formed on acid sites,118 which migrate to metal sites where they undergo hydrogenation reactions. Repeated cycling of dehydration and hydrogenation reactions under H2 atmosphere leads to the production of heavier alkanes (such as hexane). Formation of lighter alkanes takes place by cleavage of C−C bonds compared to hydrogenation of dehydrated reaction 11807



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Scheme 6. Pathways for Sorbitol Dehydration

Table 4. Catalytic Preparation of Isosorbide with Different Catalysts

a

catalyst

substrate

concn

t (h)

T (K)

isosorbide yield

ref

SnPO 0.8P/NBO-400 CuSO-650 SZ-0.05 S-TiO2 30% PW/SiO2 none Amberlys 35a 5 wt % Ru/C H4SiW12O40

sorbitol sorbitol sorbitol sorbitol sorbitol sorbitol sorbitol sorbitol celluloseb lignocellulosec

10 wt % 10 wt % 10 wt % 20 wt % 20 wt % 10 wt % 1 mol/dm−3 ― 2 wt % 2 wt %

2 5 4 2 2 6 1 5 6 1

573 498 473 483 483 423 590 413 488 483

47.2 mol % 62.5 mol % 67.3 wt % 61 wt % 75 wt % 56 mol % 57 wt % 70 wt % 49.5 mol % 63 wt %

135 136 137 138 139 140 143 144 145 148

Microwave heating. b6 MPa H2 cWheat straw pulp (CIMV, delignified), 5 MPa H2.

liquid water, because the existing liquid water led to a severe decrease in metal dispersion even at room temperature.122 The steaming pretreatment of Pt/SiO2−Al2O3 allowed a first aging of the catalyst that limited the modification induced by the hydrothermal environment on metal dispersion and support texture. Thereby new catalysts with better hydrothermal stability and appropriate metal/acid sites ratios are still necessary for further advances in sorbitol chemistry. Metal catalysts toward improving certain product selectivities may be an interesting research direction. Last but not least, attention to the reuse and regeneration of supported metal catalysts will be a promising direction in one-step aqueous-phase reforming of sorbitol. Recently, biomass gasification/pyrolysis has attracted huge interest by producing a gas rich in H2 and CO, which can be further processed to produce liquid alkanes by Fischer− Tropsch synthesis.123,124 An advantage of the above process is that it is relatively simple, usually requiring only one reactor (thus having a low capital cost); however, this process is nonselective, producing a wide range of products under various temperatures. Based on our previous work, typical products made from thermochemical processing of biomass include H2, CO, CH4, tars, acids, chars, alcohols, aldehydes, esters, ketones, and aromatic compounds.125−128 Unfortunately, unacceptable levels of tars produced in this process can cause operational problems in downstream processes by blocking gas coolers, filter elements, and engine suction channels. Moreover, tars may deposit on the surface of the catalysts used in the downstream process (reforming, shift and methanol or Fischer−Tropsch synthesis), deactivating them. Most applica-

intermediates. Lighter alkanes can also be formed by hydrogenation of CO and/or CO2 on metals such as Ni and Ru.119 The selectivities of alkanes depend on the relative rates of C−C bond cleavage, dehydration, and hydrogenation reactions, which can be varied by changing the catalyst composition, the reaction conditions, and modifying the reactor design.117 In addition, these selectivities can be modified by cofeeding H2 with the aqueous sorbitol feed, leading to a process in which sorbitol can be converted to alkanes and water without the formation of CO2. Meanwhile, the production of alkanes can be accomplished by replacing the solid acid with a mineral acid that is cofed with the aqueous sorbitol. The alkane distribution shifts to heavier alkanes when the pH of the aqueous sorbitol feed is lowered by the addition of HCl. By changing the nature of the metal component in the catalyst, one can vary the relative rates of C−C bond cleavage versus hydrogenation, thereby controlling the selectivities for the production of different alkanes. For example, using the optimized Pd/SiAl catalyst, the selectivity to n-hexane, n-pentane, and n-butane increased to 56%, 28%, and 8%, respectively.117 The selectivities to heavier alkanes increased obviously when more solid acid sites (SiAl) were added to a nonacidic Pt−Al catalyst. Zhang et al. investigated the aqueous-phase processing of sorbitol to isoparaffins under the catalyzing of Ni/HZSM-5 catalyst, and the maximal i-C6H14 selectivity of 45.4% and the total yield of i-C6H14 and i-C5H12 in 32.3% were obtained over calcined catalyst.120,121 Higher calcination temperature resulted in a decrease in catalyst activity that may be caused by the sintering and pore changes. In addition, Vilcocq first observed that Pt sintering occurred in the atmosphere of steam and 11808



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Scheme 7. One-Pot Conversion of Cellulose to Isosorbide in a Molten Salt Medium

obviously over phosphated Nb2O5, due to the increment in surface acid strength after phosphoric acid modification.136 The excellent performance with 100% conversion and 62.5% isosorbide selectivity was achieved over 0.8P/NBO-400 at a temperature of 498 K. It was revealed that the modification of phosphoric acid could well prevent the crystallization of Nb2O5. Furthermore, the catalytic activity of used phosphated Nb2O5 could be retained by calcination compared to an unmodified sample.136 When using 30% PW/SiO2 as catalyst, over 56% isosorbide selectivity was achieved at a 95% sorbitol conversion under the temperature of 423 K.141 The acidity of the supported PW catalyst was determined by the PW species presenting on the supports and interaction between PW and supports. Through dichloromethane elution, the regenerated PW/SiO2 showed no loss after recycling five runs. Employing NiO/AC as catalyst, the preparation methods were found to have a significant influence on NiO distribution and catalyst surface acidity. The catalysts prepared through the simultaneous loading-reduction procedure had stronger acidity and higher NiO dispersion on the outer surface of activated carbon, which contributed jointly to high sorbitol conversion and slightly low selectivity to anhydro sugar alcohols (ca. 66%). The stronger acidity and greater moderate acid amount of catalysts were beneficial to high sorbitol conversion and excellent selectivity to isosorbide, respectively, especially for the one reduced by NaBH4.142 In near future study, a green process was applied in sorbitol dehydration without using any acidic catalysts. Based on the properties of high temperature liquid water, high isosorbide yield could be realized by properly controlling reaction temperature and time, and the maximum yield of isosorbide reached up to 57% at 590 K.143 They ascribed the success to the higher concentration of H3O+ and OH− owing to the temperature-dependent autoprotolysis of water. In spite of the higher energy consumption at high temperatures, no obvious byproducts were identified in the reaction. With in-depth study, a more environmentally friendly and economical process was introduced for the conversion of sorbitol to isosorbide, and energy consumption and reaction time were significantly reduced with the use of microwaves. Meanwhile, the obtained isosorbide yield could be up to 70%.144 As for the kinetic study in sorbitol dehydration under microwave heating, several models considering first-order reactions or Langmuir−Hinshelwood type equations were detailed investigated by Polaert et al. They claimed that kinetics of the reaction was only correctly described by a Langmuir−Hinshelwood type model, including the adsorption−desorption equilibrium of sorbitol.144 Based on the resulting kinetic parameters, a continuous microwave reactor could be rationally designed after further chemical engineering calculations.

tions also require removal of at least part of the dust and tar before the gas can be used. Hence, the tar removal is a key issue for a successful application of biomass generated gases. Catalytic cracking/reforming is currently one of the most effective ways for both reducing tar content and enhancing H2 content in syngas at relatively low temperature. Several kinds of catalysts were developed and applied in this process, such as mineral, Ni-based, and noble metal catalysts.129−132 Therefore one-step conversion of biomass into clean fuels is receiving increasing interest for both environmental and economic reasons, and the APR of high value-added sorbitol also provides a promising avenue for the production of high-quality fuel molecules. 3.3. Synthesis of Isosorbide. Isosorbide is known as an important biobased industrial chemical like polylactic acid in the future. The conversion of sorbitol into isosorbide includes two steps that are cyclodehydration to 1,4-sorbitan and 3,6sorbitan and afterward dehydration to isosorbide (Scheme 6). In earlier studies, various inorganic acids such as HF, H2SO4, and HCl were extensively used in sorbitol dehydration at 293− 408 K.35,133,134 However, the neutralization treatment is required to remove those hazardous inorganic acid, and the separation of dehydration products from the salt solutions will be a problem. For this reason, a considerable research effort has been directed to the environmentally benign techniques. A series of efficient catalysts and reaction systems are developed in isosorbide production, as summarized in Table 4. Metal(IV) phosphates of tin, zirconium, and titanium synthesized by the hydrothermal method were used for selective dehydration of sorbitol to isosorbide.135 Among the three catalysts studied, SnPO showed the highest stability and selectivity to isosorbide (65.4%) with a 72.1% sorbitol conversion at 573 K. Interestingly, TiPO exhibited the excellent conversion (97.1%), but over 50% side products were detected during the reaction. It was found that the deactivation rate was in accordance with coke deposition by thermal analysis, and the acidity provided by functional groups of catalysts was the key factor for prolonging catalyst lifetime. Thereby much work should be directed to prevent coke deposition and retain the acidity, and the reasons for coke deposition must be investigated in detail to avoid such phenomenon. Meanwhile, the acid types of synthesized catalysts are required to be identified clearly that will be beneficial to the development of new catalysts in latter studies. As for the determination of acid amount and strength for solid catalysts, NH3-TPD analysis is suggested to be mastered. As the study develops in depth, several solid acid catalysts are developed with the aim of improving selectivity and conversion, such as phosphated Nb2O5,136 sulfated copper oxide,137 sulfated zirconia,138 sulfated titania,139 silicotungstic acid,140 and PW/ SiO2.141 For instance, the selectivity to isosorbide increased 11809



Review

Scheme 8. Route for Continuous Conversion of Biomass Feedstocks into Isosorbide

Another interesting development in the field of chemocatalytic conversion of carbohydrate is the recent reported onepot conversion of cellulose into isosorbide using hydrogenation catalyst, in combination with ZnCl2 as both Lewis acid catalyst and molten salt reaction medium, as depicted in Scheme 7.8 During this cascade procedure, cellulose was first converted into glucose in the presence of ZnCl2 hydrate and afterward hydrogenated into sorbitol using Ru/C catalyst under 5 MPa

H2. A subsequent 2-fold dehydration in the ZnCl2 hydrate media, alone or with additional CuCl2 and NiCl2 catalysts, followed to form isosorbide. The authors claimed that the hydrolysis and hydrogenation steps could occur with complete conversion, while the dehydration exceeded 95% conversion. In another work, under the circumstances of dilute HCl and superheated water, Ru/C showed excellent selectivity toward isosorbide compared to Pt/C and Pd/C, and a isosorbide yield 11810



Review

of 49.5% was obtained at 488 K.145 It was proposed that much more produced glucose was converted into sorbitol under a hydrogen atmosphere due to the superior hydrogenation activity of Ru/C, so more isosorbide was attained under the same acidic system. For Pt/C and Pd/C catalysts, levulinic acid and other degradation products accounted for the major parts by means of the acid−catalyzed degradation of glucose. As cellulose is one of the most abundant biomass materials on earth, the conversion of cellulose into important fine chemicals presents a great challenge and opportunity in the fields of energy and environment.146,147 Obviously, formation of isosorbide from biomass resources instead of glucose will be preferred from an availability point of view. In recent work, three different lignocellulosic biomass feedstocks were used: hardwood (Populus x canadensis), softwood, and wheat straw. It was shown that hemicellulose, lignin residues, and other impurities impeded cellulose conversion to isosorbide and lignocellulose pretreatment methods were of high importance as they could increase accessibility of cellulose for the catalyst. As a result, the delignified CIMV (Compagnie Industrielle de la Matière Végétale, organosolv) wheat straw pulp was rapidly and efficiently converted to 63% isosorbide after 1 h of reaction over SiW and Ru/C catalysts. Notably, the disadvantage of a high crystallinity index was overturned by the advantage of a small particle diameter and large specific surface area, originated from the activity comparison between ball-milled Avicel PH101 cellulose and delignified CIMV wheat straw pulp.148 According to the above work, a reaction diagram in converting biomass feedstocks into isosorbide was generally described in Scheme 8, which provides a promising direction for the future study in isosorbide production.

functional catalysts should deserve the priority in the following research. Furthermore, the supports of multifunctional catalysts provide acidity and high metal dispersion. Meanwhile cellulosic materials can be dissolved significantly by using acidic solvents or ionic liquids, thus leading to an obvious increase in the conversion and yield. With the introduction of the concept of green chemistry, efforts in biomass conversion should be devoted to the economical, rapid, and eco-friendly production of sorbitol based chemicals. High temperature liquid water and microwave heating are of great interest in catalytic reactions. A feasible pathway for the industrial conversion of biomass into these chemicals is still needed to be progressed in depth. This suggests that steps involving material pretreatment and subsequent sectional processing can be clearly considered. Another point worth noting is that the activation and reuse of the catalyst and effective separation of the target products are always the hottest topics for catalytic processing research.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86 020 22236808. E-mail: [email protected]. cn (S.-B.W.), [email protected] (Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the supported by National Key Basic Research Program of China (No. 2013CB228101), National Natural Science Foundation of China (No. 31270635), and the National High Technology Research and Development Program of China (863 Program, 2012AA101806).

4. CONCLUSIONS The production and utilization of sorbitol hold great potential due to the rapid development of food and chemical industry, especially in the fields of energy and fuel. In this review, we focus on developing effective methods for the synthesis of sorbitol and its derivatives. In summary: (1) glucose and biomass feedstocks can be converted into sorbitol by using nickel-based catalysts, hydrogen storage alloy, and ruthenium catalysts; (2) for lower alcohols, the major routes include sorbitol hydrogenolysis and one-pot hydrolytic hydrogenation of biomass resources with multifunctional catalysts; (3) in particular, hydrogen and alkanes are generally produced through aqueous phase reforming of sorbitol; (4) isosorbide is attained by 2-fold dehydration of sorbitol or one-step conversion of lignocellulose materials with designed reaction systems. Although great progress has been achieved for the catalytic preparation and transformation of sorbitol, further improvement in productivity and selectivity are still necessary in many cases for achieving the goal of industry production of those processes. On the one hand, metal catalysts, especially Ni, Ru, and Pt, exhibit excellent catalytic performance in sorbitol production and downstream chemicals. Much work should focus on the realization of regulation mechanism of target products. However, efficient and cheap catalysts or rational reaction systems are also in great need for further commercialization. As far as one-pot reactions on the production and conversion of sorbitol are concerned, that is, several steps are completed in one reactor, such as biomass hydrolysis, glucose hydrogenation, and sorbitol hydrogenolysis, aqueous-phase reforming and dehydration reactions, so multi-

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REFERENCES

(1) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. The path forward for biofuels and biomaterials. Science 2006, 311, 484. (2) Naik, S. N.; Goud, V. V.; Rout, P. K.; Dalai, A. K. Production of first and second generation biofuels: A comprehensive review. Renewable Sustainable Energy Rev. 2010, 14, 578. (3) Zhao, H. B.; Holladay, J. E.; Brown, H.; Zhang, Z. C. Metal chlorides in ionic liquid solvents convert sugars to 5-Hydroxymethylfurfural. Science 2007, 316, 1597. (4) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. Green chemicals: A kinetic study on the conversion of glucose to levulinic acid. Chem. Eng. Res. Des. 2006, 84, 339. (5) Christensen, C. H.; Rass-Hansen, J.; Marsden, C. C.; Taarning, E.; Egeblad, K. The renewable chemicals industry. ChemSusChem 2008, 1, 283. (6) Gallezot, P. Catalytic conversion of biomass: challenges and issues. ChemSusChem 2008, 1, 734. (7) Takagaki, A.; Tagusagawa, C.; Domen, K. Glucose production from saccharides using layered transition metal oxide and exfoliated nanosheets as a water-tolerant solid acid catalyst. Chem. Commun. 2008, 5363. (8) de Almeida, R. M.; Li, J. R.; Nederlof, C.; O’Connor, P.; Makkee, M.; Moulijn, J. A. Cellulose conversion to isosorbide in molten salt hydrate media. ChemSusChem 2010, 3, 325. (9) Zhang, J.; Wu, S. B.; Li, B.; Zhang, H. D. Advances in the catalytic production of valuable levulinic acid derivatives. ChemCatChem 2012, 4, 1230. (10) Balat, M.; Balat, H. Progress in biodiesel processing. Appl. Energy 2010, 87, 1815.

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Advances in the Catalytic Production and Utilization of Sorbitol

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