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Catalytic Conversion of Cellulose-Based Biomass to Diols

Wu Qiaomei , Yang Qiyue , Zeng Xianhai , Deng Jiahui , Zhang Liangqing , Qiu Jiarong

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (10) : 2173 -2189.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (10) : 2173-2189 DOI: 10.7536/PC220236
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Catalytic Conversion of Cellulose-Based Biomass to Diols
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Abstract

As one of the most potential renewable resources to replace fossil energy, biomass has received increasing attention. Cellulose-based biomass is important feedstocks for catalytic conversion into various fuels and chemicals. In recent years, glycols (including ethylene glycol, propylene glycol, and butanediol, etc.) have been widely used in various fields as fuels and chemicals, which is huge market demand. The traditional method to produce diols was using fossil fuels as raw materials, and there are disadvantages such as non-renewable and large environmental pollution. Therefore, the production of diols by non-fossil feedstocks has attracted great attention. Among them, catalysis conversion of cellulose-based biomass to diols is one of the important approaches to overcome the shortage of fossil fuels and reduce environmental pollution. This review comprehensively summarizes the recent advances of cellulose-based biomass (cellulose, glucose, fructose, and sorbitol) as feedstocks for catalytic conversion of diols, and the reaction pathways, reaction mechanisms, catalytic stability, and reaction solvent categories are in-depth discussed. Based on the above discussion, a future research direction for catalysis conversion of cellulose-based biomass to diols is prospected, which might be helpful for researchers.

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cellulose / C6 carbohydrates / catalysts / diols

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Wu Qiaomei, Yang Qiyue, Zeng Xianhai, Deng Jiahui, Zhang Liangqing, Qiu Jiarong. Catalytic Conversion of Cellulose-Based Biomass to Diols. Progress in Chemistry, 2022, 34(10): 2173-2189 DOI:10.7536/PC220236

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Contents

1 Introduction
2 Preparation of diols from cellulose
2.1 the reaction pathways for The conversion of cellulose-based biomass to diols
2.2 Catalyst types
3 Preparation of diols from C6 carbohydrates
3.1 Preparation of diols from glucose
3.2 Preparation of diols from fructose
3.3 Preparation of diols from sorbitol
4 Economic analysis of yield and paths
5 Conclusion and outlook

1 Introduction

Energy plays an important role in the development of the national economy, and has always been regarded as an important driving force for economic growth[1]. The extensive use of fossil resources (coal, oil, natural gas, etc.) Has greatly promoted the rapid development of economy, but because fossil resources are non-renewable resources and their reserves are limited, they have caused a series of environmental problems in the use process.Therefore, the use of renewable resources as an alternative to fossil resources can reduce the use of fossil fuels and environmental pollution[2⇓⇓⇓⇓~7]. Among the renewable resources, biomass resources are cheap and abundant, and are the only renewable carbon resources on the earth, which can be used to produce a series of materials, health products, chemicals and fuels, while lignocellulose is a cheap and easily available non-food resource in biomass resources.It is composed of cellulose (40% ~ 50%), lignin (15% ~ 20%) and hemicellulose (25% ~ 35%), and has great potential as a substitute for fossil resources[8][9⇓~11][12⇓⇓~15]. Lignocellulose, especially cellulose-based biomass, can be converted into a variety of high value-added chemicals such as diols (including ethylene glycol, propylene glycol, butanediol and pentanediol, etc.), alkyl levulinate, γ-valerolactone, 2,5-furandicarboxylic acid, hydrogel, cellulose-based carbon aerogel and 5-hydroxymethylfurfural[16⇓⇓⇓~20]. Among a series of platform chemicals with high added value, diols are multifunctional and high-value platform chemicals, which have a wide range of uses and a large market demand. Ethylene glycol (EG) is a high value-added chemical raw material, which can be used as antifreeze and coolant. Propylene glycol (PG) is an important chemical in great demand, which is widely used in cosmetics, coatings, pharmaceuticals and other fields[21]. Butanediol can be used in the fields of pharmacy, food additive, fuel and solvent[22]. Pentanediol (PDO) can be used in the fields of polyester synthesis, cosmetics and low toxic fungicides[23]. In traditional industry, EG, 1,2-PG and BDO are converted from ethylene, propylene, butadiene or butylene oxide derived from petroleum. In industry, PDO is usually obtained from petroleum. For example, 1-pentene is the petroleum raw material for preparing 1,2-PDO, while 1,5-PDO can be prepared from glutaric acid and dimethyl glutarate by hydrogenation[24⇓⇓~27][28]. However, petroleum raw materials are non-renewable, so the use of renewable cellulose-based biomass as raw materials to prepare diols can overcome this shortcoming and achieve sustainable development, which has attracted more and more attention. In this paper, the research progress of catalytic conversion of cellulose-based biomass to diols was introduced, the types of catalysts for catalytic conversion of cellulose-based biomass to diols were systematically summarized, and the future development trend was prospected, in order to provide a reference for relevant researchers.

2 Preparation of diol from cellulose by catalytic conversion

The use of cellulose as a raw material for the preparation of diols dates back to 1933[29]. Cellulose is composed of glucan monomers linked by β-1,4-glycosidic bonds[30]. Cellulose has high chemical stability, which benefits from the existence of intramolecular hydrogen bonds and intermolecular hydrogen bonds. In addition, the complex crystal structure of cellulose makes the yield and selectivity of one-pot conversion of cellulose to target diols remain at a low level for a long time. In the past 20 years, great progress has been made in the direct synthesis of diols from cellulose, and the first breakthrough has been made in the synthesis of diols from cellulose catalyzed by 2%Ni-30%W2C/AC-973 catalyst, with the yield of the target product reaching 61%[31].
At present, microcrystalline cellulose is the main raw material for the preparation of diols, and α-cellulose is rarely used as the raw material for the preparation of diols[37⇓⇓⇓~41][52]. This is because α-cellulose is a component of cellulose, which has a high degree of polymerization and a large molecular weight, and is relatively difficult to hydrolyze. Microcrystalline cellulose is a macromolecular compound obtained by the hydrolysis of α-cellulose, with a particle size of 20 ~ 200 μm, and microcrystalline cellulose is not fibrous, has the advantages of strong fluidity, good hydrophilicity and easy degradation, and is more suitable to be used as the main cellulose raw material for the preparation of diols[32]. In addition, cellulose has two chain structures of crystalline region and amorphous region; Compared with cellulose in the crystalline region, the chain macromolecules of cellulose in the non-crystalline region are arranged in a non-directional parallel manner, and hydrogen bonds are not formed or unsaturated between polar hydroxyl groups, which is beneficial to the subsequent catalytic conversion, so reducing the crystallinity of cellulose can promote the degradation of cellulose[33]; The microcrystalline cellulose molecule is mainly composed of crystalline regions, so its crystallinity is relatively high, generally 55% ~ 80%. In the process of preparing diols from microcrystalline cellulose, the pretreatment of microcrystalline cellulose by ball milling can reduce its crystallinity and make the cellulose have a larger specific surface area, thus promoting the hydrolysis of microcrystalline cellulose and facilitating the preparation of diols[57,58][34]. Therefore, it is of great significance to select the appropriate type of cellulose as raw material and develop a green and efficient method to pretreat cellulose to reduce its crystallinity for the preparation of cellulose diols. There are many reports on the preparation of diols from cellulose, including the type of catalyst, the reaction path, and the selective regulation of the target diol (Table 1).

2.1 Reaction Pathway for Catalytic Conversion of Cellulose-Based Biomass to Diols

As shown in fig. 1, the path of cellulose to diol is mainly the hydrolysis of cellulose to glucose, which is dehydrated by the β-hydroxyl of its carbonyl group through Retro-Adol condensation (RAC).The cleavage of the C — C bond between C2 and C3 produces glycolaldehyde and erythrose as intermediates. Glycolaldehyde undergoes hydrogenation to form EG, while erythrose undergoes dehydrogenation, dehydroxylation, or dehydration to produce BDO.
Fructose, the glucose isomer produced by the hydrolysis of cellulose, is also an important route to 1,2-PG (Figure 1). Hydrogenolysis of fructose, an isomer of glucose, to 1,2-PG has two pathways: (1) the β-hydroxyl group of the carbonyl group of fructose undergoes dehydration by RAC, resulting in the cleavage of the C-C bond to form dihydroxyacetone, which is hydrogenated to 1,2-PG; (2) fructose undergoes RAC to form glyceraldehyde, an isomer of dihydroxyacetone, which undergoes dehydration and hydrogenation to form 1,2-PG. Therefore, there is a certain degree of competitive reaction between the production of EG and 1,2-PG from cellulose-based biomass. In addition, EG can also be obtained from glyceraldehyde by dehydration and RAC reaction.

2.2 Catalyst type

The ideal catalyst for the preparation of diols from cellulose requires different functions, including the promotion of hydrolysis of cellulose, RAC of sugars, isomerization of sugars, and hydrogenation of intermediates. Therefore, the design of catalysts with multiple functions can effectively improve the yield and selectivity of target diols. At present, the main types of high value-added diols prepared from cellulose are EG and 1,2-PG.

2.2.1 Non-noble metal catalyst

It is well known that tungsten-containing catalysts play an important role in the preparation of diols from microcrystalline cellulose, which is the focus of current research[41⇓⇓⇓⇓⇓⇓~48]. Various tungsten-containing catalysts have the function of hydrolyzing cellulose and breaking C — C bonds of carbohydrates in the conversion of microcrystalline cellulose. The hydrolysis of microcrystalline cellulose is considered to be the rate-determining step in the conversion of cellulose to diols, but there are few studies on the catalytic effect of tungsten-based catalysts in cellulose hydrolysis. Li et al. Used H3O40PW12, H4O40SiW12 and H2WO4 as catalysts to catalyze the hydrolysis of microcrystalline cellulose[35]; When H3O40PW12 was used as catalyst, the concentration of glucose produced was up to 0. 132 mol/L, which indicated that the tungsten-based catalyst had good catalytic performance in cellulose hydrolysis.
In recent years, multifunctional catalysts composed of tungsten species and transition metals have been used to obtain high yields of ethylene glycol. Among these catalysts, Ni-W catalyst has higher activity for the synthesis of ethylene glycol from cellulose. It is reported that Ji et al. First used 2%Ni-30%W2C/AC-973 as a catalyst to directly convert cellulose into EG at 245 ℃ and 6 MPa, and the yield of EG reached 61%[31]. The catalyst has good reusability, and the EG yield decreases slightly after three times of reuse, but the leaching of Ni and W in the liquid is negligible, so the decrease of catalytic activity may be due to the oxidation of W2C by the reaction solution. In the conversion of cellulose to EG, W has the function of hydrolyzing cellulose and selectively cleaving carbohydrates, while the transition metal Ni plays a dominant role in the hydrogenation of unsaturated intermediates[36]. In addition to the active component, the support of the catalyst also plays a key role. Supports can improve the dispersion and stability of the active metal, thereby improving the conversion of microcrystalline cellulose and the yield of the target diol, so more and more researchers use supports to improve the performance of Ni-W catalysts[37⇓⇓⇓~41]. Cao et al. Achieved 100% conversion of microcrystalline cellulose and 70.7% EG yield under the reaction conditions of 230 ° C, 6 H and 6MPa H2 using SBA-15 as the support and 3%Ni-15%WO3/SBA-15 catalyst prepared by impregnation method[37]. Li et al. Found that under the action of 15% W-5% Ni/MSM catalyst at 245 ° C for 2 H and 6.0MPa H2 reaction conditions, the total yield of microcrystalline cellulose converted into EG and 1,2-PG reached 82.4 wt%, in which EG was 72.9 wt%, while 1,2-PG was only 9.5 wt%[38]. The catalyst was recovered, calcined and reduced for regeneration, but the activity of the catalyst decreased significantly in three cycle tests. The results of deactivation analysis showed that the specific surface area of the used catalyst decreased, and the support structure was partially damaged, which led to mass transfer resistance and space limitation, resulting in carbon deposition. The 15%Ni-20%W/SiO2 catalyst prepared by Xiao et al. Has remarkable activity for the conversion of microcrystalline cellulose into low carbon diols, and the yield of microcrystalline cellulose reaches 61.30% after 2 H of reaction at 513 K and 5.0MPa H2[39]; The severe leaching of nickel and tungsten species in the catalytic system was the main cause of catalyst deactivation in the three reuse tests. It has been reported that the 15%Ni-20%W/SiO2-EEG catalyst prepared by introducing ammonium hydroxide and ethylenediamine as chelating agents and EG as reducing agent has high catalytic activity for microcrystalline cellulose, and the yield of EG can reach 63.3% at 240 ℃ and 5.0MPa H2 for 2 H.However, the cycle stability needs to be improved, and the EG yield of the catalyst decreases sharply in the fourth cycle, which is mainly caused by the cumulative leaching of nickel and tungsten in the reuse[40]. In addition, Xiao et al. Prepared a 15%Ni-20%W/SiO2-OH catalyst by introducing hydroxyl groups on a SiO2 support to catalyze microcrystalline cellulose under the conditions of 5.0MPa H2,240℃ and 2 H, which could obtain a yield of 63.1% of EG and still maintain a high catalytic activity after three cycles[41]. The experimental results show that the introduction of hydroxyl can promote the formation of more Ni-W alloy, which is beneficial to the RAC reaction. The excellent catalytic performance of Ni-W catalyst is attributed to the synergistic effect of active metals Ni and W, the uniform pore structure of the support and the large specific surface area, but the stability of the catalyst needs to be further improved, and the cumulative leaching of nickel and tungsten species leads to the final deactivation of the catalyst.
At present, many studies have been done on tungsten-based catalysts to achieve high conversion and selectivity in the conversion of cellulose to EG, but it is still challenging to develop tungsten-based catalysts with good recyclability and stability in the conversion of cellulose to EG. Chu et al. Prepared a catalyst with adjacent surface Cu0 and anoxic WOx(2<x<3) species by reducing the CuWO4 precursor and reacted with Ni/AC catalyst in tandem to obtain high yield of EG in cellulose (25 μm) conversion[42]. Cellulose (25 μm) conversion was 100% and EG yield was 70.5% over 30%Cu-30%WOx/AC+Ni/AC catalyst at 4MPa H2 and 518 K for 2 H. The introduction of Cu is mainly used to generate more oxygen vacancies on the surface of WOx, which helps to adsorb oxygenated compounds, while Ni rapidly hydrogenates hydroxyacetaldehyde to EG and avoids catalyst coking, which makes its cycle stability good, and its catalytic effect is better than that of physically mixed Ni/C, Cu/C and WO3. Li et al. Obtained Ni-W/M catalyst by calcining Ni-W/MIL-125 (Ti) precursor, and achieved almost complete conversion of microcrystalline cellulose and high EG yield (68. 7%) at 4MPa H2,518 K and 2 H[43]. The reason why the catalyst remained active for 7 cycles was that the tungsten species were linked to the TiO2 support through Ti — O — W bonds, which reduced the loss of tungsten and nickel species. The preparation method of the catalyst also has an important influence on the stability of the catalyst. Wang et al. Synthesized carbon-coated Ni-W bimetallic Ni-W@C700 catalyst by reductive carbonization method to catalyze microcrystalline cellulose, which almost completely converted microcrystalline cellulose within 1 H at 5MPa H2 and 240 ℃, and the EG yield was 60.1%[44]. The coating of carbon can prevent the leaching of Ni and W species, which makes the catalyst highly stable and reusable, and no significant decrease in cellulose conversion and EG yield was observed after five cycles. Xiao et al. Used C source and N source to modify the Ni-W/SiO2 catalyst to improve the hydrothermal stability of the catalyst, and the yield of microcrystalline cellulose to EG was 48.25% at 5.0MPa H2,240℃ and 2.0 H[45]. The CxN layer and Ni-N structure contribute to the stability of the Ni-W/SiO2@CxNy catalyst, and the leaching of nickel and tungsten in the reaction solution is very little during 7 cycles of use of the catalyst. The final deactivation of the catalyst is mainly due to the leaching of active metals nickel and tungsten and carbon deposition.
The introduction of other metals into the Ni-W catalyst can improve the catalytic performance of the catalyst, which can convert microcrystalline cellulose into EG under mild conditions and reduce the cost of expensive and sophisticated reaction equipment. Hamdy et Al. Introduced Al3+ into the Ni-W catalyst system, and 3Ni-15W-3Al catalyst was used for the selective hydrohydrolysis of microcrystalline cellulose to ethylene glycol at 4MPa H2 and 503 K[46]. After 90 min of reaction, the conversion of microcrystalline cellulose was 100% and the selectivity of EG was 76%. 3 Ni-15W-3Al catalyst can be recycled for 4 times without treatment, and the reason for the decrease of catalytic activity is the leaching of nickel and tungsten. Sreekantan et al. Obtained high yields of EG under mild reaction conditions[47]. NaZSM-5 zeolite support was synthesized by sol-gel method, and 5% Al-8% Ni-25% W/NaZSM-5 catalyst was prepared by impregnation method, which showed high EG yield (89 C mol%) and 100% cellulose conversion after 12 H at 220 ℃ and 70 bar H2. The activity of NaZSM-5-based catalyst remained unchanged after three times of reuse, and the structure changed slightly after 12 H of continuous operation. The reason for the decrease of activity after three times of reuse may be the loss of catalyst during recovery. In the above two catalysts, Al3+ promotes cellulose hydrolysis, W5+ promotes RAC reaction, and Ni plays a role in hydrogenation. Yu et al. Developed a 5Ni-15W-15Cu/MgAl2O4 multifunctional catalyst by combining nickel, tungsten and copper with MgAl2O4 as a carrier, and the conversion of microcrystalline cellulose and the yield of EG reached 100% and 52.8%, respectively, under mild conditions (3MPa H2,245℃ and 120 min)[48]; The catalyst was stable for at least three cycles in six cycles under the reaction conditions. The deactivation of the catalyst may be due to the leaching of W species. The decrease of W5+ content on the catalyst surface leads to the decrease of C — C cracking activity of the catalyst.
In the preparation of diol with high added value by using microcrystalline cellulose as substrate, the yield of EG is higher and the yield of 1,2-PG is lower, and the proportion of 1,2-PG in the diol can be increased by adding a cocatalyst. Gu et al. Used β-zeolite supported Ni-W catalyst to catalyze microcrystalline cellulose to achieve good diol yield, and compared the effects of ZnO, Fe3O4 and Ca(OH)2 co-catalysts on diol yield[49]. The results showed that the addition of ZnO not only increased the total yield of diols, but also increased the yield of 1,2-PG. 7 Ni-20W/β at 245 ℃ and 4MPa H2 for 30 min, microcrystalline cellulose was completely converted, and the total yield of diols reached 70. 1%, in which 1,2-PG accounted for 51. 1% of the product. The increase in the proportion of 1,2-PG in the diol was attributed to the isomerization of glucose to fructose promoted by the basic sites on the added cocatalyst ZnO, which in turn produced 1,2-PG. The conversion of microcrystalline cellulose was maintained above 90% and the minimum yield of total diols was 59.7% after 4 times of continuous operation of the catalyst. Deposition of carbon during reuse, structural changes due to mesopore collapse, and leaching of metals resulted in a slight decrease in catalytic activity.
Tungsten-based catalysts show high selectivity for ethylene glycol and relatively low selectivity for 1,2-PG in the conversion of cellulose to diols. Therefore, the development of catalysts with high selectivity for 1,2-PG has attracted the attention of researchers. It has been reported that tin-containing catalysts have high activity for the catalytic conversion of cellulose, and the product selectivity of EG and 1,2-PG can be adjusted to a certain extent by adjusting the valence of tin, such as Pt/Al2O3 catalysts modified by SnOx and bimetallic Ni-Sn catalysts prepared by Sn species and Ni[50][51,52]. It was found that the combination of Ni/AC and metal Sn powder showed a high yield (57.6%) for EG, while the combination of Ni/AC and SnO showed a high activity (32.2%) for 1,2-PG[51]. In addition, Ni and Sn species and some NiSn alloys are active sites for EG and 1,2-PG. Xiao et al. Prepared NiSn/SBA-15 catalyst by incipient wetness impregnation method to realize the directional synthesis of EG and 1,2-PG from α-cellulose, in which 10% Ni-15% Sn/SBA-15 was used as catalyst, under the conditions of 5MPa H2,518 K and 2 H, the cellulose conversion rate was 100%, and the yield of 1,2-PG could reach 11.8% while the yield of EG was 55.4%[52]. The NiSn alloy formed by the catalyst plays a role in promoting the C — C/C — O bond cleavage (or RAC) of monosaccharides and oligosaccharides.
Sun et al. Found that Ni-La (Ⅲ) binary catalyst could effectively convert microcrystalline cellulose into EG and 1,2-PG[53]. Among them, 63.7% overall yield of EG and 1,2-PG could be obtained by using 10%Ni-0.5%Ir/La2O3 as catalyst under the reaction conditions of 518 K,5MPa H2 and 150 min. The catalyst can be recycled for 9 times. The reason for the high stability is that the La2O3, when used as a catalyst support or additive, can stabilize the transition metal. Compared with Ni-W catalyst, Ni-La (Ⅲ) catalyst has lower cost and is more economical.
It has been reported that CuCr catalysts can also catalyze the conversion of cellulose to diols, and CuCr (4) catalysts prepared by sol-gel method (where 4 is the molar ratio of Cu/Cr) show good catalytic performance for the conversion of cellulose to 1,2-PG; Under the reaction conditions of 518 K,6.0MPa H2 and 5 H, complete conversion of cellulose and 42.6% yield of 1,2-PG were achieved with Ca(OH)2 as the cocatalyst, while 31.6% yield of EG could be obtained; CuCr (4) catalyst has good anti-coking performance and can be reused for three times. The increase of copper particle size and the leaching of calcium are the possible reasons for the decrease of activity[54]. In addition to the acidic catalyst, the basic site can also promote RAC, and a small amount of alkali can promote the degradation of glucose without significantly inhibiting the hydrolysis of cellulose, which can effectively reduce the formation of humus in the catalytic process, so the weak base catalytic system has a certain potential in the preparation of diols from cellulose. Li's group prepared a weakly basic Co/CeOx catalyst, which has the advantages of simple synthesis and easy re-preparation[55]. Cellulose was nearly completely converted over 10%Co/CeOx catalyst at 245 ° C and 3MPa H2 for 6 H, yielding about 55.2% of EG and 33.9% of 1,2-PG, respectively.

2.2.2 Precious metal catalyst

Noble metals such as Pt, Ru and Pd have excellent hydrogenation properties and are often used in hydrogenation reactions. The conversion of cellulose to diols involves a hydrogenation step, so many researchers have used precious metals in combination with other metals to prepare multifunctional catalysts for the conversion of cellulose to diols. Catalysts in which noble metals are combined with tungsten species (W,WO3 or H2WO4) are quite effective for diol production[56]. However, the use of noble metals also increases the cost of catalyst preparation, which is not conducive to the large-scale production of diols.
Ru-W bimetallic catalysts have excellent catalytic performance and are often supported on different supports. Carbon nanotube (CNT) is an environmentally friendly support. Ru-W bimetallic catalyst supported on CNT can convert microcrystalline cellulose to ethylene glycol in one pot. Under the conditions of 205 ℃ and 50 bar H2, 0.8% Ru-30% W/CNT can effectively convert microcrystalline cellulose to ethylene glycol in 3 H, and the yield is 40%. It was able to be used for 4 cycles and it was observed that the surface of the used catalyst did not change significantly compared to the fresh catalyst, and no metal leaching was detected in the solution. Compared with the Ru-W catalyst supported on activated carbon, the use of Ru-W/CNT can improve the EG yield[57]. 100% conversion and 41% yield of ethylene glycol were obtained from microcrystalline cellulose over physically mixed Ru/CNT1 (treated with nitric acid) and W/CNT0 (commercial untreated) catalyst under H2 of 205 ℃, 5 H and 50 bar, which is superior to that obtained over Ru-W bimetallic catalyst supported on commercial CNT0 (35%) under the same conditions. The mixed Ru/CNT1 and W/CNT0 catalysts were run for three times with the same conversion of microcrystalline cellulose (100%) and almost no decrease in the yield of EG. The stability of the catalysts in the cycle test was attributed to the strong interaction between the metal and the support, which prevented the metal from leaching into the reaction solution[58]. Compared with carbon nanotubes as support, two glucose-derived carbon materials prepared by the HTC-method: glucose-derived carbon support (CG) and glucose-derived C/CNT hybrid (CG-CNT) are more economical and cheaper, and microcrystalline cellulose can be completely converted by Ru-W/CG and Ru-W/CG-CNT under the reaction conditions of 205℃,5MPa H2 and 5 H, which can achieve 34.2% and 28.1% EG yield, respectively. Under the same reaction conditions, higher EG yield (48.4%) could be obtained by physically mixing Ru/C G H N O 3 and W/CG, which could be recycled for three times in the one-pot conversion of microcrystalline cellulose to EG with high stability, and no metal leaching was detected. The slight decrease in EG yield may be caused by the mass loss during catalyst recovery. The possible reason for the higher yield of EG obtained by the physical mixed catalyst is that there are more acidic sites on its surface, which is beneficial to the formation of glycolaldehyde by RAC reaction and further hydrogenation to EG[59]. Graphene oxide not only has high specific surface area and high stability, but also has — OH groups and acidic — COOH groups on its surface, which are beneficial to the adsorption and hydrolysis of cellulose[60]. With the 5%Ru-30%W18O49/ graphene nanocomposite catalyst, microcrystalline cellulose was almost completely converted at 245 ° C and 6.0MPa H2 for 1 H, resulting in an EG yield of 62.5%, and the catalyst could be reused for three times, and the leaching of tungsten species on the surface of graphene was the reason for the decrease of catalytic performance. The catalyst was synthesized by one-pot solvothermal reduction reaction, which provides a new idea for the preparation of graphene-based nanocomposite catalysts[61]. Li group used solid acid carrier HZSM-5 supported Ru-WOx to catalyze cellulose conversion under mild conditions (235℃,3MPa H2 and 10 H), and the yield of EG was 46.7%[62]. The catalyst support HZSM-5 has poor hydrothermal stability, so it is necessary to seek an acidic support with high hydrothermal stability and improve the metal-support interaction to improve the catalyst stability. Recently, Wiesfeld et al. Used impregnation method to directly load 2 wt% Ru on tungsten ore, and 2 wt%Ru/b-WO3 catalyzed microcrystalline cellulose at 225 ℃ and 45 bar H2 for 1 H, the conversion of microcrystalline cellulose reached 50. 5%, and the selectivities of EG, 1,2-PG and 1,2-BDO were 65. 7%, 5.2% and 5. 9%, respectively[63]. The magnetic catalyst Fe3O4@SiO2/10%Ru-20%WOx synthesized by Lv et al. Can effectively synthesize 1,2-PG from cellulose (50 μm), thanks to the large number of Lewis acid sites provided by the WOx substance, which can effectively promote the isomerization of glucose to fructose[64]. Moreover, the WOx substance can also promote the selective cleavage of C — C bond. Cellulose (50 μm) reacted at 245 ℃ under 5MPa H2 for 2 H, the selectivity of 1,2-PG reached 32. 4%. Compared with the non-magnetic catalyst, the Fe3O4@SiO2/10%Ru-20%WOx magnetic catalyst can be separated and recovered quickly and effectively by an external magnetic field, thereby reducing the loss of the catalyst and maintaining high catalytic activity in six cycle tests; The carbon deposition on the catalyst surface and the loss of active species Ru and WOx resulted in the decrease of catalytic activity with the increase of reuse times. Xu et al. Used Pd-Fe/CNT (Pd/Fe = 1:1) to hydrolyze hydrogenated microcrystalline cellulose into polyols. Under the reaction conditions of 513 K,4MPa H2 and 2 H, the conversion rate of cellulose was 100%, and the yield of 1,2-PG was about 25%[65]. The results of recovery experiment showed that the conversion of cellulose was still 100% after 5 times of continuous operation of the catalyst, but the yield and selectivity of polyols decreased. The reason for the decrease of catalyst activity is the leaching of Fe and the sintering and poisoning of Pd adsorption products. The Pd@W/Al-MSiO2(3.6)YSNSs catalyst prepared by Xin et al. Catalyzed cellulose at 240 ℃, 2 H and 4MPa H2, the conversion of cellulose was 96. 1%, and the selectivity of EG was 56. 5%[66]. The catalyst has a unique core-shell structure, the shell SiO2 is modified by alumina and tungsten oxide, and the interaction of alumina and tungsten oxide enhances the acidity, which is the reason for the high selectivity of EG; The catalyst has high stability, and the protective effect of SiO2 on Pd nanoparticles prevents Pd from sintering and serious loss, and the EG selectivity remains at 48.5% after five cycles. The reason for the decrease in catalytic activity is the loss of tungsten and aluminum from the catalyst.
The catalytic system composed of catalysts with different active sites can also effectively catalyze the preparation of diols from cellulose, and the RAC reaction and hydrogenation reaction were carried out on two catalysts separately. Yu et al. Studied the direct conversion of microcrystalline cellulose to diols catalyzed by M/SBA-15 (M is Ru, Au, Pd, Pt, Rh and Ni) and tungsten compounds. The results showed that these catalysts could not only achieve the complete conversion of microcrystalline cellulose, but also significantly improve the selectivity of diols[67]. The catalyst system composed of Ru/SBA-15 and phosphotungstic acid (TPA) in aqueous solution was operated at 245 ℃ and 5.0MPa H2 for 4 H, and the selectivity of EG reached 55. 5%. Mankar et al. Fixed phosphotungstic acid (PTA) on a ZrO2 to prepare a PTA/ZrO2 solid catalyst, and the catalytic system composed of Ru/C could completely convert microcrystalline cellulose under the conditions of 220℃,50 bar H2 and 5 H.The yield of EG was 40%, and the catalyst was still active after three times of recycling, showing better catalytic performance than other solid acid catalysts. The decrease of catalytic activity may be related to the loss of catalyst caused by catalyst recovery process (washing, drying)[68]. It is worth mentioning that the pretreatment of microcrystalline cellulose by ball milling reduces the crystallinity of cellulose, which is beneficial to the conversion of microcrystalline cellulose into diols. In addition, some researchers have developed a new catalytic system combining homogeneous metal salts with supported Pt-based catalysts for the selective conversion of cellulose to diols. The selected catalytic system with the combination of CeCl3·7H2O and Pt(0.7wt%)/BaZrO3 gave the best selectivity for diols. Under the reaction conditions of 230 ℃ and 25 bar H2 for 12 H, cellulose (50 μm) was converted to ethylene glycol, 1,2-polyglycol and 1,2-dithiobarbituric acid with a selectivity of 21.7%, 19.2% and 2.3%, respectively. The heterogeneous catalyst is easily deactivated under hydrothermal conditions, which limits the application of the catalytic system. The decrease of catalytic activity in the three runs was mainly due to the leaching of barium and the coking of the catalyst[69].

3 Catalytic conversion of C6 carbohydrate to diol

Some intermediate products in the reaction process of preparing diols from cellulose can also be used as direct raw materials for the synthesis of diols. Catalytic conversion of C6 carbohydrates, such as glucose, fructose and sorbitol, to diols opens up a new route for the production of diols from cellulose-based biomass.

3.1 Catalytic conversion of glucose to diol

Glucose is the basic unit of cellulose and is therefore one of the intermediates in the conversion of cellulose to diols. With glucose as the starting material, it can be catalytically hydrogenolyzed to diols, mainly including EG, PG, and BDO (Table 2). Compared with cellulose, glucose has more advantages in the preparation of diols: (1) hydrolysis is not required as the rate-limiting step in the conversion of glucose to diols, so the reaction conditions are mild; (2) glucose is easy to dissolve in water and is easy to be used as a raw material for a continuous reaction; (3) Glucose has a simple structure, so it is more convenient to study its catalytic reaction mechanism, and it can be highly concentrated, which reduces the energy consumption in the reaction process[70,71].

3.1.1 Reaction Pathway of Catalytic Conversion of Glucose to Diol

As shown in Figure 1, the conversion of glucose to EG includes two basic reaction steps, that is, the dehydration reaction of the β-hydroxyl group of the carbonyl group of glucose through the RAC reaction occurs, and the C-C bond is broken to form the intermediate glycolaldehyde, which is then hydrogenated to EG. The product distribution depends on the reaction conditions chosen and the type of catalyst used[72]. The pathway of glucose to BDO is that glucose is converted to erythrose by RAC, and erythrose is further hydrogenated to the corresponding alcohol erythritol, and then BDO (including 1,2-BDO, 1,3-BDO, 1,4-BDO and 2,3-BDO) is formed by dehydrogenation, dehydroxylation or dehydration. Appropriate and abundant Lewis acid sites are beneficial to the isomerization of glucose to 1,2-PG, such as the tunable Imoglite nanocatalyst and the MCM-41 catalyst containing Al, which show good catalytic activity for the isomerization of glucose to fructose[73][74].

3.1.2 Catalyst type

Concentrated glucose can be efficiently converted to diols using various non-precious metal catalysts. Compared with dilute glucose solution, the reaction process using concentrated glucose as raw material consumes less energy in product separation[75]. It has been reported that different types of tungsten-containing catalysts have different yields of glucose to EG, and the EG yield increases in the order of WO3<WO3/SBA-15<(NH4)6W7O24·6H2O, which is mainly related to the particle size of tungsten-containing species, and small particle size is beneficial to obtain more EG. Under the reaction conditions of 175℃,6MPa H2 and 80 min, 10%Ni-15%WO3/SBA-15 could catalyze the complete conversion of glucose, and the yields of EG and 1,2-PG were 29.8% and 2.4%, respectively[76]. Tan et al. Compared the catalytic performance of Ni-based catalysts supported on different solid base supports (MgO and ZnO) for the selective hydrogenolysis of 1,2-PG and EG from glucose[77]. Under the conditions of 493 K, 6 MPa and 3 H, both Ni-MgO and Ni-ZnO catalysts achieved nearly complete conversion of glucose, and the yields of 1,2-PG and EG over Ni-MgO catalyst were 24. 8% and 5. 0%, respectively, while the yields of 1,2-PG and EG over Ni-ZnO catalyst were 27. 9% and 11. 4%, respectively. The MgO support has a lower energy barrier to cleave the bond between C3 and C4, and thus is more favorable for 1,2-PG generation. In a batch reactor, Kirali et al. Made a bifunctional catalyst with spherical mesoporous carbon (MC) supported Ni and Mo, which was able to convert glucose (15 wt%) to EG under mild conditions[78]. At 200 ° C and 40 bar H2 for 6 H, glucose could be completely converted, and the selectivity of EG was 63.2%. The presence of Ni enhances the dispersion of Mo, and the decrease of Mo from + 6 to lower oxidation States (+ 5 and + 4) enhances the acidity of the catalyst, thus promoting the PAC reaction to form a higher yield of EG. The catalyst showed stable activity and EG selectivity in three cycles. The leaching of Ni and Mo and some structural changes in the cyclic operation are responsible for the decrease in EG selectivity.
Like cellulose, isomerization of glucose to fructose is an indispensable step to obtain 1,2-PG with high selectivity. Facilitating this conversion generally requires the presence of a basic substance or the presence of a Lewis acid. The major diol product from the addition of a base promoter to the catalytic system is usually 1,2-PG rather than EG. Alkaline catalyst has a certain catalytic activity for RAC reaction, and its other important function is to promote the catalytic isomerization of glucose to fructose. The reaction follows a proton transfer mechanism, in which the α-hydroxy compound of glucose becomes an enol intermediate catalyzed by the hydroxide group, realizing the shift of the carbonyl group and isomerization to fructose. Therefore, the isomerization of glucose to fructose is the dominant reaction, and fructose undergoes RAC, dehydration, hydrogenation and other reactions to produce 1,2-PG, which improves the yield of 1,2-PG[29]. Xiao et al. Added Ca(OH)2,CuCr catalyst to the catalytic system to catalyze the reaction of glucose under the condition of 413 K,6MPa H2 for 2 H, and then continued the reaction under the condition of 493 K,6MPa H2 for 5 H, glucose was completely converted, and the yield of 1,2-PG reached 52.8%[79]. They found that the addition of an alkaline promoter to increase the alkalinity of the reaction solution could promote the isomerization of glucose and the RAC reaction of fructose, and increase the yield of 1,2-PG. The catalyst can be recycled for three times, and its activity is basically unchanged. The migration of Cr on Cu sites, the covering of active sites by carbon species and the aggregation of metal particles after reaction lead to the deactivation of the catalyst. Yazdani et al. Used Al2O3 catalyst supported Cu-La2O2CO3/Al2O3(Cu/La=6.0) catalyst to show high selectivity in glucose hydrohydrolysis conversion reaction, mainly because the basicity of the catalyst significantly changed the reaction path, which was beneficial to the formation of glucose isomer fructose, and promoted fructose RAC, hydrogenation and other reactions, and finally formed 1,2-PG[80]. Under 200 ° C and 34 bar H2 for 3 H, glucose was completely converted and 1,2-PG selectivity was 29.0%. B2O3 was used to modify the Cu/Al2O3 catalyst to introduce more Lewis acid sites, and B2O3 promoted the dispersion of copper and adjusted the molar ratio of Cuδ+/Cu0(δ≥1), which significantly improved the catalytic activity and selectivity of 1,2-PG. The conversion of glucose over 1CuB/Al2O3 catalyst could reach 96. 6%, and the best selectivity was 49. 5%. The selectivity of glucose conversion to diols can be modulated by the structure and properties of metals and metal oxides. The catalyst can be operated for more than 120 H without significant loss of catalytic activity, and the possible reason for the deactivation of the catalyst is the phase change and coking on the surface of the catalyst after long-term use[81].
The excellent hydrogenation properties of noble metals such as Ru, Pd and Pt also play an important role in the preparation of diols from glucose. Zhao et al. Were the first to convert concentrated glucose to EG in a semi-continuous reactor[82]. In the presence of bifunctional catalyst AMT-4% Ru/AC, when the glucose feed rate was set at 0. 667 mL/min, the yield of EG was 60. 0% and the yield of 1,2-PG was only 6. 9% at 240℃,5MPa H2 and 40 min. Many reports have shown that the synergistic effect of Ru and WOx has an important effect on the selectivity of glucose catalytic conversion to diols. Ru-W catalysts with different supports have great differences in catalytic performance. Liu et al. Used glucose as substrate and Ru-W as catalyst in a continuous flow fixed-bed reactor to study the synergistic effect of Ru and WOx in the aqueous hydrogenolysis of glucose to diols, and compared different supported catalysts including Ru-W/SiO2, Ru-W/AC, Ru-W/CNFs and Ru/WO3.The results show that the abundant acid sites provided by W4+ on the Ru-W/SiO2 catalyst can effectively catalyze the selective cleavage of C — C bond of glucose, and the Ru0 sites and large specific surface area are conducive to hydrogenation to diols.They concluded that a high conversion of 100% for glucose was obtained at 478 K and 4MPa H2 for 10 H, and the selectivities of EG, PG and BDO reached 33.8%, 29.2% and 24.3%, respectively, and the stability was the best among these four catalysts, the strong metal-support interaction prevented the leaching of active metals, and the glucose conversion was about 100% after 50 H of operation, indicating that the Ru-W/SiO2 catalyst was an excellent catalyst for the conversion of glucose to diols[83]. However, the Ru/WO3 catalyst formed more W5+ species, which was beneficial to the production of more EG. Under the same reaction conditions, the glucose conversion over the Ru/WO3 catalyst was 87.5%, and the selectivity of EG and PG was 55.9% and 12.7%, respectively, and no BDO was detected. This illustrates that the yield of the diol is highly dependent on the synergistic effect of the Ru metal site and the WOx acid site. Compared with the traditional Ru-W bifunctional catalyst, polyethylene glycol (PEG) was used to modify the surface of SiO2 support, and the 1Ru10W-PEG catalyst showed higher catalytic activity than the 1Ru10W catalyst, which was mainly due to the highly dispersed WOx crystals and smaller size of the former catalyst, providing more acidic sites and active sites. Using 1Ru10W-PEG catalyst at 463 K and 4MPa H2, the conversion of glucose was 98.2% and the total yield of diols (including EG, 1,2-PG and BDO) was as high as 91.7% in a continuous flow fixed-bed reactor system. The highly dispersed active sites on the catalyst and the strong metal-support interaction organize the active metal sintering or leaching, which still show excellent stability after 50 H of operation[84]. Phosphotungstic acid supported on ZrO2 was used to prepare PTA/ZrO2 solid catalyst, and the Ru/C catalyst system also showed good catalytic activity for the conversion of glucose to diol. Under the reaction conditions of 200℃,50 bar H2 and 5 H, the conversion of glucose was 98%, and the yield of EG was 21%[68].
WOx can promote the RAC reaction in the preparation of diols, and can also promote the isomerization of glucose to fructose due to the presence of Lewis acid sites on its surface. Many studies have used the Lewis acid sites of WOx and other metal active sites to prepare catalysts showing high selectivity for 1,2-PG. Liu et al. Designed the Pd-WOx/Al2O3 catalyst, and the highly dispersed WOx sites provide Lewis acid to promote the isomerization of glucose to fructose[85]. In a fixed-bed reactor, Pd-WOx(5%)/Al2O3 was used as catalyst to catalyze glucose at 180 ℃ and 4MPa H2. The conversion of glucose was 92.2%, and the selectivity of 1,2-PG was 60.8%. The catalyst was stable and could be used for more than 200 H. The decrease in catalytic activity can be attributed to the leaching of active metals. With Fe3O4@SiO2/10%Ru-20%WOx as catalyst, 94.3% glucose conversion and 30.8% 1,2-PG selectivity could be achieved at 215 ° C and 5MPa H2 for 2 H. The selectivity of 1,2-PG was affected by the WOx loading. When the WOx loading was less than 20%, the isomerization of glucose to fructose was promoted by the abundant Lewis sites with the increase of WOx loading, and the isomerization of glucose catalyzed by WOx was dominant, resulting in the increase of 1,2-PG selectivity. But as the WOx loading was further increased to 25%, 1,2-PG selectivity decreased, which was attributed to the dominant role of WOx in the RAC reaction of glucose at this time[64]. In addition, Lv et Al. Prepared a Pd@Al3-MSiO2YSNSs catalyst, in which Al forms a 4-coordinated Al(AlO4) in the mesoporous silica framework to provide abundant Lewis acid sites and promote the isomerization of glucose[86]. However, the Pd shell has excellent hydrogenation activity and strong ability to break C = O, so the catalyst shows high selectivity for 1,2-PG. At 200 ° C and 5MPa H2 for 3 H, glucose conversion was 95.7% and 1,2-PG selectivity was 47.4%. The leaching of Al species after seven consecutive runs resulted in a slight decrease in the catalytic activity of the catalyst, with glucose conversion of 90.4% and 1,2-PG selectivity of 38.5%.
Pang et al. Investigated the RuSn/AC catalyst for the catalytic conversion of glucose to diols[87]. They found that the highly dispersed SnO2 favored the isomerization of glucose, while the RuSn alloy played a dominant role in the RAC reaction and hydrogenation reaction. When the tin content is less than 2%, hexitol is the main product. When the tin content is more than 5%, the humic substances produced in the reaction process increase, and on the 5% Ru3% Sn/AC catalyst, the yields of 1,2-PG and EG are 25.0% and 26.9% at 513 K and 5MPa H2 for 10 min; The catalyst has good stability in five cycles. The slight decrease in yield is mainly due to the minor leaching of tin.
The addition of basic co-catalyst such as soluble base NaOH, Ca(OH)2 or Mg(OH)2 to the catalytic system favors the glucose isomerization and RAC reaction to improve 1,2-PG selectivity. In addition, the catalyst can also be modified with a base to increase the 1,2-PG selectivity. Gu et al. Assembled Mg(OH)2 onto Pt/SiO2 to prepare a Pt/SiO2@Mg(OH)2 catalyst[88]. The alkali-modified catalyst can not only promote glucose isomerization and RAC reaction, but also convert the by-product hexitol into 1,2-PG. When the loading of Mg(OH)2 was 5 wt%, the yield of 1,2-PG was 53.8% under the conditions of 180℃,6MPa H2 and 4 H. The performance of the catalyst is relatively stable and can be recycled for three times; The reason for the decrease in the activity of the catalyst can be attributed to the decrease in the content of Pt and Mg(OH)2 leaching on the catalyst as the number of cycles increases.
Despite numerous studies on the conversion of glucose to diol, it is still difficult to regulate the product selectivity, and how to effectively form the diol product with BDO as the main component is still a great challenge. EG and PG are generally produced in the process of converting glucose into diols, and only trace BDO exists or even does not exist in the products, so more research is needed to synthesize BDO from glucose. Recently, Ji et al. Have made great progress in the synthesis of nanosheet Ru/WO3 catalysts and their application in the preparation of diols from glucose with high BDO selectivity[89]. In a continuous flow fixed-bed reactor, glucose was completely converted to BDO with a yield of 89. 7% by 1Ru/WO3NS(0.3) under 478 K,4MPa H2 within 8 H. Over the 1Ru/WO3NS(0.3) catalyst, glucose was converted to glycolaldehyde by retroacetalization, and glycolaldehyde was converted to erythrose by acetalization, followed by the rehydrogenation of erythritol to BDO. The reverse acetalization reaction and the acetalization reaction exist simultaneously on the catalyst, so that the selectivity of BDO is greatly improved; However, in general, the retroacetal and acetal reactions do not proceed smoothly. The catalyst showed excellent stability within 50 H of operation.

3.2 Catalytic conversion of fructose to diol

Fructose is the second most abundant sugar on the earth, and the main diol produced from fructose is 1,2-PG. The conversion of fructose to 1,2-PG is an important research direction for converting biomass into high value-added chemicals. Compared with other raw materials, it is easier for fructose to produce 1,2-PG. Fructose can produce some C3 intermediates (mainly including glyceraldehyde and dihydroxyacetone) through RAC reaction, and finally produce 1,2-PG through a series of reactions[54][90].

3.2.1 Reaction Pathway for Catalytic Conversion of Fructose to Diols

As shown in Fig. 1, fructose is an isomer of glucose, and fructose generates glyceraldehyde or dihydroxyacetone through RAC. Glycerol produced by glyceraldehyde hydrogenation can be further hydrogenolyzed to 1,2-PG, and lactic acid produced by glyceraldehyde dehydration or isomerization and hydration can be further hydrogenated to 1,2-PG. In addition, dihydroxyacetone can be isomerized, hydrogenated to acetol, and finally hydrogenated to 1,2-PG. It should be noted that if the hydrogenation rate is too fast, 1,2-PG will be over-hydrogenated to propanol, reducing the formation of 1,2-PG.

3.2.2 Catalyst type

The RAC reaction of fructose requires either an acid site or a basic site for the formation of the C3 intermediate, followed by the hydrogenation of the C3 intermediate to 1,2-PG, which requires a metal site. WOx can increase the acid sites required for the RAC reaction of fructose, while noble metals such as Ru and Pd provide hydrogenation sites. EG of 14.3 (mol% C) and 1,2-PG of 37.9 (mol% C) were obtained from fructose over AMT-4% Ru/AC at 240 ° C and 5MPa H2 for 40 min in a semi-continuous reactor (feed rate 0.667 mL/min)[82]. In 2015, Liu et al. Reported that the fructose conversion rate reached 97.7% and the selectivity of 1,2-PG was 62.2% at 180 ° C and 4MPa H2 with Pd-WOx(5%)/Al2O3 as catalyst[85]. The 5Ru-25WOx/HAP catalyst prepared by Li's group using hydroxyapatite (HAP) as a support catalyzed the complete conversion of fructose with a high yield of 91.3% of 1,2-PG at 180 ° C and 1 MPa initial H2 pressure for 8 H. The highly dispersed metal sites have moderate hydrogenation ability, which hinders the excessive hydrogenation of 1,2-PG to propanol, and the HAP support can effectively inhibit the formation of humus. It is worth mentioning that the catalyst is very active in a low hydrogen atmosphere and can catalyze the reaction at a hydrogen pressure of 0.1 bar[91]. The research group also used a weakly basic catalytic system to catalyze fructose to 1,2-PG, and obtained a 1,2-PG yield of 50.8% by 10%Co/CeOx of the catalyst[55].
After exchanging the reaction substrate from glucose to fructose over Fe3O4@SiO2/10%Ru-20%WOx catalyst at 215 ° C and 5MPa H2 for 2 H, the fructose conversion was 95.8% and the 1,2-PG selectivity was 33.4%[64]. The previously mentioned Pd@Al3-MSiO2YSNSs catalyst for catalyzing the hydrogenolysis of glucose to 1,2-PG showed higher conversion and 1,2-PG selectivity in fructose hydrogenolysis. Fructose conversion was 96.5% and 1,2-PG selectivity was 49.1% at 200℃,5MPa H2 and 3 H. From the results of fructose catalyzed by Pd@Al3-MSiO2YSNSs, it can be inferred that the Al(AlO4) in the silica shell of the catalyst has Lewis acid sites, which mainly promote the isomerization of glucose to fructose and also affect the hydrogenolysis of fructose to 1,2-PG[86].

3.3 Preparation of diol from sorbitol by catalytic conversion

Sorbitol is a derivative of hexasaccharides such as glucose and fructose, which can be obtained by hydrogenation of glucose and fructose[92]. Sorbitol can be hydrogenolyzed to diols (mainly EG and 1,2-PG), which provides an economically feasible path for the preparation of diols from biomass feedstock.

3.3.1 Reaction Pathway of Catalytic Conversion of Sorbitol to Diol

As shown in Fig. 2, there are two main routes for the catalytic conversion of sorbitol to diol. First, sorbitol is dehydrogenated to ketohexose and aldohexose. Path one: ketohexose selectively cleaves the C — C bond between C3 and C4 by RAC with base to produce dihydroxyacetone and glyceraldehyde, which is dehydrated to methylglyoxal under alkaline conditions, then hydrogenated to acetol and further hydrogenated to 1,2-PG. Path 2: The C — C bond between C2 and C3 of aldohexose is cleaved by RAC reaction in the presence of base to form glycolaldehyde molecule, and EG can be synthesized by further hydrogenation. The above two paths are the main paths for sorbitol to produce diol.

3.3.2 Catalyst type

Hydrogenolysis of sorbitol to EG and 1,2-PG was carried out in the presence of a metal site and a basic site. Promoting the hydrogenolysis of sorbitol to EG and 1,2-PG using various base promoters in combination with other metal catalysts, such as those for the hydrogenation of sugar alcohols, has been extensively studied (Table 3). In the process of preparing diol from sorbitol, ketohexose and aldohexose are firstly formed by dehydrogenation reaction on a metal catalyst, and the addition of a basic cocatalyst can improve the activity of sorbitol hydrogenolysis and reduce the energy required by C-C bonds,It promotes the reaction of RAC of ketohexose and aldohexose to selectively cleave C — C bonds to form some small molecules (glyceraldehyde, glycolaldehyde, etc.), and glyceraldehyde is dehydrated in alkaline environment and then hydrogenated to 1,2-PG and lactic acid, or further hydrogenated over metal catalysts to form glycerol[93,94]. Glycolaldehyde can be synthesized by hydrogenation[95]. It has been reported that in the presence of a basic co-catalyst such as Ca(OH)2, the conversion of sorbitol hydrogenolysis can be improved[96]. Ru-based and Ni-based catalysts showed good activity for the hydrogenolysis of sorbitol to EG and 1,2-PG, but they also produced more by-products such as methane and glycerol. The supported Ru-based catalyst showed good catalytic activity for the hydrogenolysis of sorbitol to EG and 1,2-PG, and the nature of the support had a significant effect on the catalytic performance. Ru/CNTs showed higher catalytic activity for sorbitol hydrogenolysis than Ru/AC under mild conditions, which may be due to the higher graphitization and electronic conductivity of CNTs. The addition of WOx can improve the catalytic performance and inhibit the agglomeration of Ru metal particles. Under the reaction conditions of 205oC,5.0MPa and Ca(OH)2, 99.6% sorbitol conversion was obtained in 2 H with Ru0.25WOx/CNTs as the catalyst, and the yields of 1,2-PG and EG were 34.6% and 25.6%, respectively, while the yield of 1,2-BDO was only 0.7%. In this reaction system, the alkali promoter increased the catalytic activity of Ru0.25WOx/CNTs, which may be due to the fact that the alkali promoter can not only change the electronic state of Ru surface to facilitate sorbitol dehydrogenation, but also promote the cleavage of C — C bond and RAC reaction. The performance of the Ru0.25WOx/CNTs catalyst was stable, no leaching of Ru or WOx was found in the reaction solution, and the catalyst could be recycled for five times[97]. Guo et al. Used Na (OH) as an alkali promoter to load Ru on carbon nanotubes (AMCN) doped with —NH2 functional groups, which reduced the sintering of Ru and thus improved the activity of Ru in the catalytic process[98]. The addition of Na(OH)2 significantly increased the conversion of sorbitol. At 210 ℃ and 6MPa H2 for 2 H, the conversion of sorbitol was 97.3%, and the total yield of PG and EG was 61.4%. It still has high activity in five consecutive cycle tests, and no leaching of Ru was detected in the reaction solution. The partial agglomeration of Ru species and the deposition of by-products on the Ru surface, which lead to the decrease of the specific surface area of the catalyst, may be the main reason for the deactivation of the catalyst. The doping of some noble metals can promote the stability and activity of Cu-based catalysts. 100% sorbitol conversion was achieved within 4 H using La(OH)3 as co-catalyst and Pd-Cu/ZrO2(Cu/Pd=5) as catalyst for sorbitol conversion at 493 K and 5.0MPa H2, EG and 1.The selectivity of 2-PG was 15.9% and 37.0%;Pd-Cu/ZrO2, respectively, which showed higher activity and selectivity than the monometallic supported Pd/ZrO2 and Cu/ZrO2 catalysts, and the conversion of sorbitol did not decrease significantly after five cycles, which may be due to the strong interaction between Pd and Cu to prevent the aggregation of Cu particles and improve the activity and hydrothermal stability of the catalyst[99]. Wang et al. Used Ca(OH)2 as an alkali promoter to explore the catalytic conversion of sorbitol over Cu/C, and concluded that the conversion of sorbitol was controlled by the particle size of Cu, the selectivity of C3/(C2+C4) products was affected by the equivalent number of Ca(OH)2, and Ca(OH)2 played a role in promoting isomerization and RAC reaction in the preparation of diol from sorbitol[100]. Sorbitol was completely converted under the conditions of 513 K,5MPa H2 and 0.3 equiv of Ca(OH)2 for 24 H, and the total selectivity of diols (including EG and PG) reached a high selectivity of 84.5%. The experimental results showed that Cu/C exhibited excellent catalytic performance for the hydrogenolysis of sorbitol to diol, but the activity of Cu/C catalyst decreased significantly in five runs, and the problem of catalyst deactivation caused by Cu particle aggregation needs to be solved. The PdZn alloy catalyst formed in situ by physically mixing ZnO and Pd/ZrO2 was highly efficient and stable in the presence of solid base Mg3AlOx. Under the reaction conditions of 493 K and 5.0MPa H2 for 17.5 H, the conversion of sorbitol was 96%, the selectivity of PG and EG reached 43.3% and 13.6%, respectively, and the total yield of the two diols was 54.6%. The strong basic site of the solid base Mg3AlOx in the catalytic system is beneficial to catalyzing the hexose intermediate to synthesize a C3-C4 product through RAC, thereby reducing the formation of a by-product lactic acid. There was no significant decrease in sorbitol conversion and diol selectivity after five catalyst cycles, and no leaching of Pd and Zr was detected[101].
In order to avoid the use of soluble base and reduce the damage to the reaction equipment, many researchers have developed a series of bifunctional catalysts with metal active sites and basic sites[102⇓⇓⇓⇓~107]. The Ni-MgO catalyst prepared by Chen et al. By coprecipitation method can effectively hydrogenate sorbitol to diol without basic additives[102]. Under the reaction conditions of 473 K and 4MPa H2, the conversion of sorbitol reached 67. 8%, and the selectivities of EG and 1,2-PG were 26. 0% and 33. 7%, respectively, when the Ni/Mg molar ratio in the catalyst was 3/7 and the reaction time was 4 H. The strong basicity of MgO on the catalyst is beneficial to the RAC reaction, and the abundant basic sites may adsorb a large number of intermediate products in the reaction process, thus accelerating the reaction rate. However, the hydrothermal stability of the catalyst was poor, and the activity decreased significantly after five runs. Jin et al. Supported Cu on Al2O3 and compared the catalytic performance of solid bases doped with CaO, MgO, ZnO, etc[103]. The results showed that Cu/CaO-Al2O3-1 had the best catalytic performance, and the conversion of sorbitol was 98. 1%, and the selectivity of EG and 1,2-PG reached 15. 4% and 46. 1% under the conditions of 230℃,70 bar H2 and 6 H. However, the conversion of sorbitol decreased to less than 70% after three cycles due to the leaching of Cu in the reaction process of the catalyst. Leo et al. Prepared NiRuCa-HT catalyst by directly loading Ni and Ru on alkaline support, and catalyzed sorbitol conversion at 493 K,4MPa H2,4 h, with sorbitol conversion of 89.2% and total selectivity of EG and PG of 44.6%[104]. With the addition of 0.3 g Ca(OH)2 base promoter, the conversion of sorbitol was increased to 93. 6%, and the total selectivity of EG and PG was increased to 49. 5%. Because the basic carrier Ca(OH)2 of the catalyst is easy to dissolve in the reaction solution, the stability of the catalyst is poor, and the activity decreases obviously in the fourth run. The layered Ni-Mg-AL LDH catalyst prepared by Du et AL. Has high activity for the hydrogenolysis of sorbitol under mild conditions[105]. Under the reaction conditions of 473 K,2MPa H2 and 2 H, almost all sorbitol was converted over the Ni3.6/Mg2.4Al2O5.4-SF catalyst, and the selectivities of PG and EG were 28.2% and 18.8%, respectively. The reason for the high stability and high activity of the catalyst in five cycles may be due to the high dispersion of active sites. In the absence of alkali promoter, Shanthi et al. Prepared Ni-Ca (Sr)/HAP catalyst by doping Sr into Hydroxyapatite (HPA) as an alkaline support to catalyze sorbitol conversion at 473 K and 60 bar H2[106]. After 6 H of reaction, the conversion of sorbitol was 64%, and the yields of EG and 1,2-PG were 16% and 23. 7%, respectively. The catalyst can be used continuously for 4 times without any decrease in activity. Cai et al. Found that 10%Ni/5%La2O3/ZrO2 catalyst could hydrogenolyze sorbitol to the target diol, and the conversion of sorbitol was 96. 8%, and the selectivity of EG and 1,2-PG was 20. 3% and 26. 8%, respectively, at 220 ℃ and 40 bar H2 for 4 H[107]. The Ni metal in the catalyst promotes the dehydrogenation and hydrogenation reactions, and the addition of La2O3 increases the basicity of the catalyst, which is beneficial to the breaking of C — C bond. After five cycles, the leaching of the catalyst metal leads to a sharp decrease in activity.

4 Economic analysis of yield and path

Catalytic conversion of cellulose-based biomass (cellulose, glucose, fructose and sorbitol) to diols is a sustainable high-value utilization method of green economy. According to the atom economy rule, the structure of cellulose contains a large number of hydroxyl groups, which makes cellulose an ideal raw material for the production of glycols. The direct preparation of diols from cellulose includes cellulose hydrolysis, saccharide RAC, saccharide isomerization and hydrogenation of intermediate products. Cellulose hydrolysis is the rate-limiting reaction for the preparation of diols from cellulose, but the structure of cellulose is stable and difficult to hydrolyze. The traditional method of inorganic acid hydrolysis is usually used to degrade cellulose, which is not only environmentally unfriendly, but also corrodes equipment and increases the cost of preparation[64]. In recent years, high conversion of cellulose to diols has been achieved, and the diol products mainly include EG and 1,2-PG. From the reaction path (Fig. 1), the RAC reaction of glucose and the isomerization of glucose are competitive reactions, and glucose is further hydrogenated to EG (Route A) through the formation of glycolaldehyde by RAC reaction; After isomerization of grape to fructose, fructose undergoes RAC reaction to form a C3 intermediate, and the C3 intermediate undergoes dehydration, hydrogenation, and other reactions to form 1,2-PG (Route B). Therefore, in order to control whether the main product in the diol product is EG or 1,2-PG, it is necessary to design a suitable catalytic system to effectively control the hydrolysis, isomerization, RAC and hydrogenation reactions, so as to match the rates of each reaction. For example, some transition metal catalysts can not only provide acidity to promote cellulose hydrolysis, but also catalyze RAC reaction, but they are not active for the isomerization of glucose, so EG can become the main diol product[37⇓⇓⇓⇓⇓⇓⇓~45]. Route B, the Route to 1,2-PG, needs to go through more reaction steps than Route A, the main Route to EG, which may be the main challenge to improve 1,2-PG selectivity in diol products[86]. The isomerization of glucose to fructose is the key step in the formation of 1,2-PG. The presence of Lewis acid sites in the catalytic system or the addition of alkali promoter in the catalytic system can promote the isomerization of glucose, which makes the isomerization of dextrose take precedence over the RAC reaction of dextrose, thus improving the yield of 1,2-PG. However, the addition of alkali additive will affect the hydrolysis of cellulose, thus reducing the conversion rate of cellulose.
Glucose has a simpler structure than cellulose. The catalytic pathway for the preparation of diols from glucose is similar to that from cellulose, except that there is no rate-limiting step of cellulose hydrolysis, so the reaction conditions are milder and the energy consumption in the reaction process is reduced[82]. The main diol products synthesized from glucose include EG and 1,2-PG, and BDO is trace or absent in the products. The catalytic system that makes BDO the major diol product requires further investigation.
Diols made from fructose tend to produce 1,2-PG more than those made from cellulose and glucose. Because the catalytic conversion of cellulose and glucose to 1,2-PG also needs to undergo isomerization reaction, it is easier to obtain 1,2-PG from fructose. Under the same reaction conditions and catalytic system, both glucose and fructose can be used to prepare 1,2-PG with high conversion rate, and the yield of 1,2-PG prepared from fructose is higher than that from glucose. However, the preparation of diols from sorbitol usually needs to be carried out under alkaline conditions. Because the C (5) -H bond in sorbitol is preferentially activated, the dehydrogenation of sorbitol to ketohexose is preferred to the formation of aldohexose, resulting in a higher yield of 1,2-PG than EG in the preparation of diols from sorbitol[95]. Compared with glucose and fructose, the dehydrogenation rate of sorbitol is slow, resulting in low conversion rate[29].
In summary, the economic challenge in the preparation of diols is the hydrolysis of cellulose under mild conditions compared to other cellulose-based feedstocks. Cellulose, as the main component of lignocellulose, has a large reserve and a wide range of sources in nature. It will be more economical to develop a green method to replace the traditional inorganic acid hydrolysis method to prepare diols efficiently compared with other cellulose-based[109]. Cellulose-based preparation of diols usually requires the addition of soluble alkali additives, which is easy to cause damage to the reaction equipment and increase the cost of diol preparation and the burden of ecological environment. Therefore, the development of non-noble metal catalysts with good stability, easy recovery and high reusability by using solid base instead of soluble base additives can reduce the cost of cellulose-based preparation of diols, which is green and environmentally friendly, and is more beneficial to large-scale practical application.

5 Conclusion and prospect

In order to meet the major challenge of the current shortage of fossil resources in the world, it is an economical and sustainable way to reduce the use of fossil resources by using biomass to produce high value-added chemicals. The preparation of diols from cellulose-based biomass has the advantages of environmental friendliness and abundant raw materials. At present, great progress has been made in the catalytic conversion of cellulose-based biomass to diols, and various types of catalysts and catalytic systems have been developed for the catalytic conversion of cellulose-based biomass to diols.However, there are still four problems: (1) The catalytic system developed at present shows high catalytic activity for EG and 1,2-PG, but the high conversion rate of substrate, high yield of product and industrial development need further study. (2) In the process of preparing diol from cellulose, the step of cellulose hydrolysis is usually carried out under liquid acid conditions, which is easy to cause corrosion to equipment, and the economy and environmental protection need to be improved. (3) Soluble alkali additives are often added to the reaction system in the preparation of diols from sorbitol and in the preparation of diols from fructose by promoting the isomerization of glucose. The addition of soluble alkali can promote the reaction, but at the same time, it causes the problems of equipment damage and difficult separation of subsequent products. (4) Studies on the catalytic conversion of cellulose-based biomass to other diols such as BDO and PDO are rare. It is still a big challenge to find efficient, green and inexpensive catalytic systems to promote the conversion of cellulose-based biomass into target diols (especially BDO and PDO).
In order to realize the green, economical and efficient production of diols from cellulose-based biomass, the following four suggestions are proposed: (1) further development of non-precious metal catalytic systems;The catalytic activity is improved, and the leaching of active metal components in the operation process is reduced, so that the stability of the catalyst is improved, and the industrial development of preparing the diol from the cellulose-based biomass is facilitated. (2) Replace liquid acid with solid acid and develop a green and economical process to hydrolyze cellulose under mild conditions. (3) Develop multifunctional catalysts, reduce the use of soluble alkali additives, and catalyze the preparation of diols from cellulose biomass under mild conditions. (4) To explore the heterogeneous catalytic system combining biocatalyst and chemical catalyst to promote the efficient and economical production of diols (especially BDO and PDO) from cellulose-based biomass. In addition, some progress has been made in the preparation of PDO from other biomass-based platform compounds, such as levulinic acid, γ-valerolactone, furfural and furfuryl alcohol.

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Funding

National Natural Science Foundation of China(22108038)

National Natural Science Foundation of China(21978248)

Tianjin University-Fuzhou University Independent Innovation Fund Cooperation Project(TF2022-10)

Open Project Fund of Key Laboratory of Marine Biological Resources of Ministry of Natural Resources(HY202201)

Open Project Fund of Key Laboratory of Marine Biological Resources of Ministry of Natural Resources(HY202202)

Fuzhou University Testing Fund of Precious Apparatus(2022T032)

Fuzhou University Testing Fund of Precious Apparatus(2022T037)

Doctoral Scientific Research Foundation of Fuzhou University(511084)

National Key R&D Program of China(2021YFC2101604)

Guangdong Provincial Key Research and Development Program(2020B0101070001)

Natural Science Foundation of Fujian Province of China(2019J06005)

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