高熔点蜡合成技术的研究进展

郎雪玲，雷淑桃，李愽龙，李晓红，马冰,＊，赵晨,＊

1华东师范大学，化学与分子工程学院，上海市绿色化学与化工过程绿色化重点实验室，上海 200062

2崇明生态科学研究院，上海 202162

1 Introduction

High-melting hydrocarbon waxes (melting point ＞ 80 °C),consisting of saturated alkanes with carbon number greater than 40 exhibit unique features including high melting points, high stability, low penetration, low mobility, good wear resistance and hardness. These features make them widely used in food,cosmetics, material processing, electronic machinery, national defense, aviation and medical fields, etc. With the economic growth and the development of national defense technology in China, the demand for high-melting waxes has been increasing.For instance, the supply insufficiency of rubber protective waxes reached 48000 tons in China in 2014. There is also a supply insufficiency with 500000 tons in market demand for specialty waxes such as polypropylene wax, anti-rust wax, fruit preservation wax, and power capacitor wax. It is estimated that the supply insufficiency of high-melting waxes and specialty waxes in China will exceed 700000 tons in 20301. Up to 2021,the total production capacity of paraffin wax in China is about 210 × 104t·a-1. Nevertheless, the variety distribution is not reasonable. The specialty waxes and high-melting waxes with high economic value have to rely on import due to the lack of enough supply in China2. In order to ensure the stable development of the national economy and national defense security, production of high-melting waxes with excellent performance is very urgent.

At present, the high-melting synthetic waxes on the market mainly include polyethylene (PE) waxes and Fischer-Tropsch(FT) synthetic waxes. There are a few routes to obtain the polyethylene waxes. The one is the cracking of white plastics(PE) via thermal cracking, catalytic cracking or alkane metathesis. Thermal cracking needs to be carried out at high temperatures, and catalytic cracking makes the reaction conditions milder3. The other one is the polymerization of ethylene. However, the PE waxes obtained from the above routes have more branched chains and display a wide distribution in carbon numbers and melting points4. The FT synthetic waxes are obtained from the FT synthesis processes with syngas or natural gas (CO and H2) as raw materials and the carbon chain is extended according to the following equation nCO + (2n + 1)H2→CnH2n+2+ nH2O. Then, the mixed linear alkanes with high melting point (≥ 85 °C) are achieved after refining and separation. Nonetheless, harsh conditions including high temperature and pressure are employed during the synthesis of FT synthetic waxes. Additionally, both PE waxes and FT synthetic waxes are derived from fossil resources such as petroleum and coal. However, carbon neutrality and petrochemical resource shortage have obvious constraints on the world economy and science & technology. Therefore, synthesis of high-melting waxes using biomass-based monomers as structural units has also become a new competitive green route.This review covers the synthesis of high-melting waxes (melting point ＞ 80 °C) by PE synthesis, FT synthesis and biomass-based synthesis methods, and provides some perspectives and trends in the synthesis of high-melting synthetic waxes.

2 PE waxes via cracking of PE

Synthesis of high-melting waxes via cracking refers to that the waxes are obtained from the cracking of pure PE resin or waste plastics at a certain temperature and pressure. At present, PE waxes in China are mainly produced by the cracking method. In the natural environment, the self-degradation of plastics is extremely slow, which has brought great pressure on the environment and caused “white pollution”. Therefore,transformation of waste PE to waxes provides a better alternative for waxes production, which not only alleviates the environment pressure by reducing the waste emissions, but also makes waste profitable. Furthermore, compared with the polymerization process, this cracking technology is cost-effective.

PE waxes can be divided into thermal cracking PE waxes and catalytic cracking PE waxes. The catalysts for the cracking of PE and the cracked products distribution are summarized in Table 1.

Table 1 Catalysts of cracked polyethylene wax and distribution of cracked products.

2.1 Thermal cracking to PE waxes

The procedures of PE cracking are shown in Fig. 15. There are two main types of PE macromolecules cracking to form hydrocarbons. One is the random scission mechanism to form long-chain hydrocarbons6, and the other is the chain end scission mechanism to produce light hydrocarbons7. Random scission is the main route of thermal cracking to give primary wax products.However, long-chain hydrocarbons will continue to crack into fragments of short-chain molecules, and short-chain hydrocarbons may undergo side reactions such as cyclization and dehydrogenation. If the primary waxes with suitable carbon chains are the goal products, the over-cracking of intermediates should be inhibited. Of course, chain end scission also occurs in PE cracking, which directly produces C1-C4fraction, and also generates aromatics with side reactions such as condensation8.In addition to the cracking reaction, the cracking mechanism(thermal cracking and catalytic cracking) also contains isomerization, hydrogen transfer, cyclization, aromatization or condensation reactions. The enhancement or inhibition of the reactions involved in cracking is affected by operation conditions, acidity and shape selectivity of each catalyst9,10. The secondary reactions can be adjusted by controlling the operation conditions in order to produce high-melting waxes.

Fig. 1 Main reaction pathways in the thermal and catalytic cracking of PE.

During the thermal cracking of polyolefin, the yield of wax is influenced by the reaction temperature. Grause et al.11studied the cracking temperatures can regulate the product distribution of continuous thermal cracking of plastic mixtures (65% polyolefin, 20% polystyrene and 15% poly (ethylene terephthalate)) using a fluidized bed reactor with hard burnt lime(HBL) as bed material. At 600 °C, low boiling oil and waxes were obtained as the main products with about 45% and 36%(mass fraction), respectively. By raising the temperature to 700 °C, the yield of wax drops sharply, from 36% to 9.8%, while the yield of gaseous product increases from 18% to 51%.Therefore, the increased temperature led to the deep thermal cracking. Due to the poor thermal conductivity of the plastic, the excessively high reaction temperature and the prolongation of the reaction time led to coking easily during the reaction.Berrueco et al.12investigated the temperature effect on the product distribution for the continuous thermal cracking of high density polyethylene (HDPE) in a fluidized bed in the range of 640-850 °C. Notably, the cracked product distribution is mainly chain hydrocarbons at 650 °C, in which the proportion of wax and oil reaches 79.7%, and the gaseous products are 20.3%. It is worth noting that the wax yield of C33-C60fraction is 28.8%.With the reaction temperature increasing, the yield of C33-C60fraction obviously decreases. At 685 °C, the C33-C60fraction is hardly detected while the gaseous products are more than twice of those obtained at 650 °C. Clearly, the reaction temperature and time are key factors to affect the PE wax yield in the thermal cracking of PE.

2.2 Catalytic cracking to PE waxes

As mentioned above, the direct thermal cracking of PE to PE waxes requires high reaction temperature. In order to operate the thermal cracking of PE under mild conditions, a suitable catalyst is usually applied during the cracking to reduce the activation energy and further regulate the product distribution. In the cracking of polymers, solid acidic catalysts13, especially zeolites, play an important role in the cracking of PE plastics.The pore structure and acid strength of zeolites have a great influence on the product distribution. Therefore, how to construct zeolite catalysts with suitable microporous network,acid strength and acid quantity for the catalytic cracking of PE is very important14.

Besides the catalysts, the raw materials of PE also affect the properties of the resultant PE wax. Griekenet al.18investigated the catalytic cracking of HDPE and low density PE (LDPE)under mild conditions, respectively. The relevant catalysts included n-HZSM-5, HY, amorphous SiO2-Al2O3, commercial Pd/C and MCM-41. As a result, the PE wax obtained with MCM-41 exhibited better stability, comparable with those commercialized PE wax. In addition, the wax obtained from HDPE cracking possess higher homogeneity than that derived from LDPE.

2.3 New method for synthesis of PE wax

For the past few years, researchers have devoted themselves to exploiting new method to synthesize PE waxes. Cross alkane metathesis (CAM), including alkane dehydrogenation and olefin metathesis, is a highly-effective process to convert hydrocarbons into its lower and higher congeners using low value hydrocarbons as solvents under mild conditions. This method is beneficial to formation of straight chain hydrocarbons under mild conditions. Jiaet al.19developed a dual catalyst system containing a pincer-ligated iridium complex and Re2O7/γ-Al2O3for the CAM of PE. The highly efficient degradation of PE was realized under mild condition in this dual catalyst system. As a result, 120 mg of HDPE was converted to 53 mg of hydrocarbons and 43% of PE was degraded to wax at 150 °C. The relationship between the degradation product distribution and catalyst structure was also explored. The degradation of commercial PE plastic wastes, such as postconsumer PE bottles, bags, and films,to high value liquid fuels and waxes can be accomplished using this CAM method.

Synthesis of PE waxesviasolvent-assisted PE cracking has received great attention. Yuanet al.20designed a green route for the depolymerization of PE. The authors found that using water as solvent was helpful for the decomposition of PEviaavoiding the coke deposition caused by local overheat. However, too much water would increase the separation cost. Under optimal conditions, gray white PE wax was obtained when 50%-75% water was added as solvent. The corresponding PE wax showed high thermal stability with melting point of 104-110 °C, acid value of 0.0143 mg·g-1, and viscosity average molecular weight of 3426.4. In another case, Wanget al.21adopted mixed xylene as solvent to assist the cracking of PE at 425 °C. As a result, 88% yield of PE wax with the melting point of 97.8 °C and theviscosity average molecular weight of 1264.6 was achieved. In addition, the mixed xylene could be totally recovered after utilization. Nevertheless, because PE contained colorant and other additives, there was pungent odour during the decomposition of PE, and the resultant PE wax was faint yellow,which affected the product property inevitably. Considering these influences, Yao et al.22employed supercritical fluid extraction (SCFE) technology to purify waxes which were derived from PE and polypropylene plastics. The SCFE technology can efficiently eliminate the organic and inorganic contaminants and greatly enhance the wax quality. Accordingly,high-value water white waxes were obtained from waste polymeric feedstock using the SCFE technology. Under the optimal conditions, wax with 97% yield was extracted from polypropylene plastics.

3 Polymeric PE waxes

The development of PE technology facilitates the synthesis of PE wax. Compared with the top-down PE cracking to synthesize waxes, polymeric PE wax, derived from the down-up polymerization of ethylene, possesses better qualities in color and performance. The catalysts for synthesis of polymeric PE wax are mainly divided into two categories: metallocene and non-metallocene. The catalysts and properties of the products,including reaction conditions, molecular weights, are summarized in Table 2.

3.1 Metallocene catalysts

Ziegler-Natta catalyst is one of the common catalysts which can be applied to ethylene polymerization for synthesis of PE wax. However, the PE wax products obtained by this route have a wide molecular weight distribution23. In contrast, metallocene catalysts with lower dosage but higher activity have attracted much attention recently.

Finlayson et al.24fabricated a unique metallocene catalyst to synthesize linear PE wax with low molecular weight and high crystallinity by using methylaluminoxane (MAO) as a promoter.Gao et al.25adopted metallocene catalysts containing Ti, Zr or Hf to ethylene polymerization in a new double loop reactor, and the results showed that the resultant PE wax had a narrow molecular weight distribution and excellent performance. Tang et al.26invented a supported metallocene catalyst containing magnesium compound using silica gel as support for the synthesis of PE was in a slurry reactor. With auxiliary of alkyl aluminum, PE wax with controllable molecular weight was afforded over this catalyst system. In addition, this polymerization technology is simple and easy to operate and no impurities were introduced to make the whole reaction environmentally benign. In order to make the catalyst recyclable,Moreira et al.27prepared NaMOR mordenite (Si/Al = 5)-supported metallocene Cp2ZrCl2and applied to the oligomerization of ethylene. The results indicated that PE wax with large molecular weight was obtained over this supported metallocene catalyst, and high amount of zirconium incorporated in the catalyst resulted in lower polymerization activity.

随着经济建设的高速发展，我国已经进入高铁、扫码支付、共享单车和网购新四大发明时代，信息化已经成为这个时代的明显特征。这个时代的学生具有明显的信息化特征，以智能手机为代表的智能终端几乎人手一台。在这个背景下，通过变革传统课堂教师讲、学生听的授课方式，用信息化教学的理念，对课程体系进行重建势在必行。

Although PE wax prepared using metallocene catalyst has excellent performance with controllable molecular weight, the stability, efficiency and high price of metallocene catalysts force researchers to develop more suitable catalysts for ethylene polymerization.

3.2 Non-metallocene catalysts

Non-metallocene catalysts have been focused on by many researchers due to their excellent reactivity and diverse ligand structures, and more and more attention have been paid to out indepth research on non-metallocene catalysts. The characteristics comparison of metallocene catalysts and non-metallocene catalysts is shown in Fig. 228,29.

Fig. 2 Comparison of the characteristics of metallocene and non-metallocene catalysts.

Umare et al.30conducted extensive studies on nonmetallocene catalysts. They developed a titanium-biphenolateethylaluminum sesquichloride based catalyst system for the controllable synthesis of low molecular weight PE waxes (Mw=1800-3400). The results showed that the catalyst activity was significantly affected by the reaction temperature and Al/Ti molar ratios. The resultant PE wax has high crystallinities and narrow PDs, similar to those commercial available synthetic waxes, which brings hope to explore alternative and cheaper catalysts.

Bollmann et al.31for the first time reported a Cr-based catalyst with a variety of diphoshinoamine (PNP) ligands in combination with Cr(III) compounds activated by aluminoxanes, exhibiting an unprecedented ethylene tetramerization to produce 1-octene. In addition, the authors indicated that the Cr-catalyst supported by PNP ligands is favorable for the formation of PE wax with narrow molecular weight distribution during the polymerization of ethylene.Inspired by this study, Huang et al.32synthesized five structurally similar Cr(III) complexes (Cr1-Cr5). The N atoms on each complex are in the different chemical environments leading to the diversity of spatial effects and electronic properties of nitrogen atoms. The results demonstrated that the five chromium complexes showed the best ethylene polymerization performance at 80 °C, and the products exhibited low molecular weight and narrow molecular weight distribution.This series of catalysts exhibit excellent thermal stability while maintaining high catalytic activity.

In addition to Cr element, Co and Ni-based catalysts also can catalyze the growth of ethylene chains to prepare PE wax. Zhanget al.33investigated the catalytic activity and stability of a series of divalent cobalt chloride complexes (Co1-Co6) in the polymerization of ethylene to PE wax. The catalysts treated by MAO or MMAO showed very high ethylene polymerization activity. The catalytic activity of Co4/MAO with cumene as the substituent was the highest. However, decomposition and deactivation of the catalysts at high temperature is unavoidable.

The related nickel-based complexes for catalyzing ethylene polymerization to PE wax have also attracted the attention of researchers. Johnsonet al.34discovered that the dichloride of nickel diimide displayed high activity for ethylene polymerization. Yuet al.35prepared a series of N-(5,6,7-trihydroquinolin-8-ylidene)arylamine derivatives by condensation with 5,6,7-trihydroquinolin-8-one and different aromatic amines, and then the corresponding catalysts were obtained by coordination of these arylamine derivatives with nickel dichloride. The complexes after treatment by alkylaluminum showed high activity (107gPE·molNi-1·h-1), but the PE wax was highly branched.

In addition to the basic researches on ethylene polymerization to PE wax, the relatively mature technologies of ethylene polymerization to PE wax have been developed by some companies in the world. For instance, DuPont in the United States utilized ethylene as raw material to prepare PE wax with peroxide catalysts under 50-100 MPa and at 49-100 °C15.United Chemical produced PE wax with molecular weight of about 2000 by high-pressure polymerization. Although these processes need high cost, they can prepare high-quality and high-performance PE wax products. Up to now, United Chemical Company of the United States is still the largest producer of PE wax in the world. In addition, Japan’s Mitsui,Sanyo Chemical, German companies and Hearst companies also produce PE products with lower molecular weight.

4 Fischer-Tropsch synthesis to FT synthetic waxes

Fischer-Tropsch (FT) synthetic waxes are obtained by distillation of high carbon numbers fraction of Fischer-Tropsch synthesis (FTS). The melting point of high-melting FT synthetic wax, containing linear saturated alkanes with relative molecular weight of 500-1000, is as high as 85-120 °C. The high-melting FT synthetic waxes bear unique features including high stability,hardness, wear resistance, high melting point, fine crystal structure and narrow melting point range, which have unique application in hot-melt adhesives and coatings. Although several sets of FT synthesis units have been built in China and the highmelting FT synthetic waxes can be provided, the properties in product quality and index stability of home-made FT synthetic waxes are inferior to those imported waxes from Shell and Sasol.Hence, it is still necessary to further develop the catalyst system and reactor technology systematically and deeply.

4.1 Mechanism and catalysts for synthesis of highmelting FT synthetic waxes

FTS is an essential catalytic process that converts syngas into fuels and chemicals and has been thoroughly explored and used commercially for decades. However, many details regarding the mechanism are still debatable points. Generally, the carbide mechanism proposed by Fischer and Tropsch36,37(Fig. 338,39)has been widely recognized. Two separate theories,i.e., the carbide mechanism and CO insertion, have been proposed to reveal the carbon initiation and chain growth steps. The carbide mechanism indicated that CO molecules are dissociatively adsorbed on the surface, and the surface carbon atoms are subsequently hydrogenated to form methylene (CH2) groups,which are considered as chain-propagating monomers. Although extensive experimental results support this mechanism, it fails to elucidate the formation of oxygenates. The mechanism of FTS is still ambiguous, although many significant experiments and theories have been done to investigate the reaction mechanism of the complicated FTS.

Fig. 3 Fischer-Tropsch synthesis mechanism, ASF distribution and application of FTS waxes.

There are abundant FTS products, including methane, alkenes and various alkanes with different carbon numbers. Therefore, it is vital to select a suitable catalyst to regulate the content of heavy wax by inhibiting the formation of CH4and other shortchain hydrocarbons in order to obtain high-melting long chain hydrocarbon wax by FTS technology. At present, noble metal Ru-based and base metals Fe and Co-based catalysts are appliedto synthesis of high-melting FT synthetic waxes40,41. For clarity,the catalysts and product distributions of the high-melting FT synthetic waxes are listed in Table 3.

Table 2 The main components of the catalyst and the properties of the product.

Table 3 Catalysts and product distributions of high-melting waxes by Fischer-Tropsch synthesis.

Many studies have demonstrated that Ru-based catalysts show high initial activity42,43, but relatively poor stability due to metal agglomeration, particle oxidation and carbon coking at high temperatures. In addition, restricted by the high cost and scarcity of noble metal Ru, Co and Fe-based catalysts44,45can also convert syngas into high-melting waxes. Ma et al.46prepared Fe-based catalyst via the precipitation method to catalyze FTS to produce high-carbon alkanes. The industrial tests showed that the yield of wax product is about 31% (w). It should be pointed out that Fe-based catalysts can only produce waxes with melting points below 105 °C. The research results from Shell showed that the appearance and performance of the FTS wax using Cobased catalyst are better than those using Fe-based catalyst. They found that higher probability of chain growth was observed over the Co-ZrO2-SiO2catalyst prepared by an impregnation method than that over the conventional catalysts, therefore, high carbon number alkane waxes are likely to be formed over the Co-ZrO2-SiO2catalyst47.

In order to improve the activity of catalysts and product selectivity, the positive effect of promoters was also investigated48,49. Yang et al.50applied K element-modified Fe-Mn catalysts to FTS. The results showed that the addition of K increased the CO/H2molar ratio on the catalyst surface, inhibited the formation of methane and light hydrocarbons, and led to an increase in higher molecular weight hydrocarbons. Shah et al.51studied the effect of Th, Mn, Mg and other promoters on Cobased catalysts on FTS. They found that addition of Th would increase the selectivity to wax, increase the sensitivity of the catalyst to temperature, and decrease the influence of impurities on the catalyst. Modification of Mn to Co-based catalysts improves the wax yield at lower temperature, but it also caused the disadvantages such as reduced hardness and heat sensitivity of the catalyst. Addition of MgO to the catalyst increased the catalyst strength and reduced powder generation during the reaction. Guo et al.52applied density functional theory (DFT)calculations to reveal that the introduction of Ba to the catalyst system can regulate the surface electron density of Co atom in Co/γ-Al2O3, affecting the adsorption of H2and CO on the catalyst surface. Correspondingly, the selectivity to C19+alkanes is significantly improved, and the yield of C19+alkanes reaches above 38%.

Besides the active sites and promoters, the porous structure of the support also affects the wax/oil ratio of FTS products. Yang et al.53constructed two Ru-in/TNT and Ru-out/TNT catalysts,in another word, Ru nanoparticles deposited in the inner and outer pores of TiO2nanotubes, respectively. The Ru-in/TNT catalyst displayed high CO conversion, while the Ru-out/TNT catalyst displayed the deactivation-resist ability. Compared with Ru-out/TNT catalysts, the Ru-in/TNT catalyst facilitated the formation of long-chain (C19+) products. The authors considered that enhanced activity and selectivity of the Ru-in/TNT catalyst was ascribed to strengthen charge transfer and modulate the electronic properties of Ru nanoparticles. Subramanian et al.54designed a nanoreactor with Co nanoparticles as core and porous SiO2as shell (Co@SiO2). Compared with Co/SiO2prepared by an impregnation method, which afforded the solid wax products,the Co@SiO2exhibited a high yield of short-chain hydrocarbons due to the steric restriction of nanoreactor diameter for the formation of long-chain hydrocarbon (C30H62), which changedthe traditional ASF distribution law of products.

4.2 Technology of high-melting FTS waxes

Synthesis of high-melting FT synthetic waxes provides high requirement to catalysts and reactors. At present, only Sasol(South Africa) and Shell (the Netherlands) have their own industrialized FTS technology and large-scale synthesis units55.Synthesis of high-melting wax in Sasol mainly relies on lowtemperature synthesis technology using Arge fixed-bed reactor and SPD™ slurry-bed reactor. The subsequent hydrocracking unit is also employed to produce heavy wax according to market demand. The updated SPD technology adopts more advanced slurry bed reactors, which has a good advantage of excellent material mixing56. Preheated syngas is pumped to the slurry-bed reactor from the bottom. Bubbled syngas passed through the slurry phase and then diffuses and converts into wax. This kind of reactor has many advantages including easy installation,excellent heat transfer and controllable reaction temperature.According to the statistics, the production capacity of FT synthetic wax in Sasol is about 70,000 t·a-157.

Shell successfully developed the Shell middle distillate synthesis (SMDS) technology in 1990, and they built the first natural gas FT synthetic oil unit in Malaysia in 199358. In 2000,after the equipment upgrade of SMDS technology, the production capacity has been increased from 500,000 to 700,000 t·a-1. SMDS adopted a tubular fixed-bed reactor to produce highmelting FT synthetic wax by using low-temperature synthesis technology. At the end of 2011, Pearl GTL Plant based on the second generation SMDS unit was completed and put into operation, on the basis of the improvement of the first set of SMDS, co-built by Qatar and Shell. According to the statistics,the production capacity of FT synthetic wax in Shell is about 50,000 t·a-1.

In the early 1980s, Shanxi Institute of Coal Chemistry of the Chinese Academy of Sciences developed the MFT technology containing a fixed-bed two-stage synthesis technology, in combination with traditional FTS with shape selectivity of zeolite. The wax obtained using precipitated Fe catalyst in the first stage accounts for 31% (w) of total products. This technology has completed the pilot test in Shanxi in 199359. This technology authorized Shanxi Lu’an Coal-based Synthetic Oil Co. Ltd. has already realized the distillation of refinery of highmelting FT synthetic wax and they can provide the standard FT synthetic wax product although they haven’t accomplished the large-scale production yet. In addition, the low-temperature slurry-bed FTS industrial demonstration unit of the National Energy Group has also been successfully operated60.

Synthesis of high-melting waxes by FTS is one of the most common approaches with high economic value in industry at present. The research and development of catalysts and reactors are also relatively complete. However, there are still some deficiencies such as complicated reaction equipment, many impurities in the FT synthetic wax and cumbersome separation technology. For fixed beds, the demand for reaction temperatures and pressure makes the manufacturing cost of the reaction device high and the reaction device relatively complicated. Many side reactions occur during the preparation of high-melting waxes, and the length of the generated carbon chain is different. The obtained product contains many impurities, resulting in a cumbersome separation process.

5 Biomass-based synthesis to wax

The FT synthetic waxes and PE waxes are obtained at the expense of fossil resources such as petroleum and coal.However, the policy of peak carbon dioxide emissions and carbon neutrality restricts the development of fossil resourceconsuming technologies. Renewable biomass resources can be converted into fuels and chemicals, which could be integrated into the current energy system. Transformation of vegetable oil and biomass platform-derived chemicals to biomass-based waxes can replace traditional FT synthetic waxes and PE waxes61.A summary of PE cracking, ethylene polymerization, FTS, and biomass-based methods for synthesizing high-melting waxes is shown in Fig. 4.

Fig. 4 Summary of the processes for preparing high-melting waxes via different routes.

Natural oils containing C12-C18fatty acids are ideal building blocks for biomass-based waxes. Through the ketonization reaction, two molecules of C12-C18fatty acids are converted to C23-C35ketones by eliminating a carbonyl group in the presence of an alkaline catalyst. After subsequent hydrogenation, C23-C35bio-based waxes can be obtained62.

Boekaertset al.63evaluated the ketonization reaction of C12-C18fatty acids on anatase and rutile TiO2catalysts. Rutile-type TiO2, with higher Lewis acid density, exhibited high ketonization activity. However, the catalytic activity suffers from stronger product inhibition by ketones, CO2and H2O. With continuous flush of formed H2O and CO2with inert gas, the conversion was distinctly increased. The kinetic models and experimental data inferred that C-C coupling of intermediate species is the rate-determining step in the ketonization mechanism. Cormaet al.64designed a single-tube double-bed catalytic system, consisting of the first layer of MgO ketonation catalyst and the second layer of Pd/MgO hydrogenation catalyst.This fixed-bed system successfully converted lauric acid into C23straight-chain alkanes with a yield of 63%.

Although the chain growth of long chain hydrocarbon can be achieved by ketolation of fatty acids, the carbon number of the product is between C20-C35and the melting point is only 40-65 °C, which cannot meet the requirements of high-melting waxes. In addition, the ketonation reaction between diacids hardly occurs, so more means to extend the carbon chain need to be further exploited. In recent years, the strategy of extending the carbon chain using furfural as the basic unit to construct fuels and lubricating oils by aldehyde-ketone condensation has been widely reported65-67. Inspired by these findings, Zhao's group68at East China Normal University ingeniously designed a route to prepare long-chain high-melting waxes with excellent performance, starting from cheap and readily available bioderived small molecules such as C1formaldehyde, C3acetone,C5furfural and C18fatty acids. They carried out carbon chain coupling through reasonable carbon chain extension methods such as acylation, ketonation, aldol condensation, etc., and then obtained two types of high-melting wax precursors through the selective hydrogenation/selective hydrodeoxygenation reactions, followed by further hydrodeoxygenation to C45-C49high-melting waxes. The biomass-based high-melting wax product synthesized by this route has controllable carbon chain length and structure, high melting point, less impurities and pure color, and belongs to the category of high-quality biomass-based special wax.

6 Summary and outlook

Considering the ever-growing demand of high-melting waxes in food, medicine, textile and other industries and rational consumption of resource under peak carbon dioxide emissions and carbon neutrality, high efficiency synthesis methods for high-melting waxes are imminent. In this review, we summarized the four synthetic methods for the construction of high-melting wax according to chemical principles and catalytic systems. Four synthetic strategies include PE cracking into waxes, polymerization of ethylene to waxes, syngas to waxes and biomass-based waxes. Despite some of significant progresses have been industrialized, there are still several challenges to be further considered.

PE cracking to waxes with the advantages of low cost can effectively solve “white pollution”, and directly use the existing catalytic cracking units. However, this process needs high energy consumption to achieve waste polymer depolymerization, and exhibits the drawbacks such as wider carbon number distribution and high impurity content in the obtained PE waxes. With regards to PE waxes obtained from ethylene polymerization, the complex technology and expensive catalytic system such as metallocene and non-metallocene complex in the ethylene polymerization method hinder its industrialization. At present, high-melting waxes are generated as by-products of ethylene polymerization. Similar with the PE cracking, the products of ethylene polymerization cannot be regulated selectively to the range of high-melting waxes. The product exhibits wide range melting point and poor quality leading to mismatch with the high-end requirements. Compared with PE waxes, there are no branches and carbon-carbon double bonds in the wax prepared by FTS, so the high-melting FT synthetic waxes show various benefits including lower molecular weight and viscosity, smaller penetration and narrower melting range. With the transformation and upgrading of the domestic manufacturer, the downstream products are developing towards precision and high-end. Demand for highmelting FTS waxes has grown rapidly. Many domestic enterprises such as Shanxi Lu'an Group and Shaanxi Future Energy Company have built and operated several sets of FT synthetic wax production units, and the vacancy of high-melting synthetic waxes has been filled in China. However, the performance of high-melting waxes is inferior to foreign highend products, and the high-end high-melting waxes still rely heavily on import. Related technologies need to be further developed urgently to meet the demand of the domestic market.

Although the high-melting FTS waxes display excellent performance and the technology is gradually mature, FT wax with different melting points are produced by rectification of products with various carbon chain lengths. The products are mixture containing various alkanes with relatively wide melting range. Additionally, raw materials of ethylene polymerization and FTS are derived from petroleum and coal resources, which are not in line with sustainable development concept and peak carbon dioxide emissions and carbon neutrality. Therefore,construction of high-melting waxes with established carbon number and well-defined structure from small molecules on a renewable biomass platform has become an important scientific frontier.

Fatty acids containing long-chain alkyl groups are ideal candidates for the fabrication of high-melting waxes. The carbon growth, by acylation and aldol condensation between fatty acids and biomass-based C5-C6furfural and 5-hydroxymethyl furfural followed by hydrodeoxygenation, provides opportunities for construction of biomass-based waxes. Biomass-based waxes have a narrower melting range due to the precise carbon growth process. Based on the different application demands, the biomass platform small molecules can be functionalized to fabricate biomass-based wax with a special function. More importantly,biomass-based synthesis route is sustainable and accords with peak carbon dioxide emissions and carbon neutrality. We believe that a combination of multi-step reactions, ingenious catalytic system design, together with reaction mechanistic investigation,will make biomass-based high-melting waxes flourishing.