Conversion of renewable biomass to commodity chemicals has received extensive attention as the depletion of fossil fuel reservoirs has continued over the past two decades. Sugars and sugar alcohols, which can be derived from renewable lignocellulosic biomass instead of fossil resources, are considered as potential feedstocks for the future biorefinery to produce useful chemical products through hydrogenolysis [1, 2]. Along this line, sorbitol hydrogenolysis is a promising “green process” that can produce important chemicals such as ethylene glycol (EG), propylene glycol (PG), and glycerol (GL), which are widely used in the manufacture of polyesters, surfactants, pharmaceuticals, and functional fluids [3].
Sorbitol hydrogenolysis was first conducted in a study where sugars were submitted to the action of hydrogen [4]. Clark [5] then reported this process for the purpose of biomass conversion to lower carbon glycols. Since then, sorbitol hydrogenolysis to glycols has received much attention in industry circles and a raft of patents on the topic have been issued [6, 7, 8, 9, 10, 11]. Most of these patents describe sorbitol hydrogenolysis occurring in the presence of a metal catalyst and base, at a temperature between 180 and 275 °C, and with an elevated H2 partial pressure of 3.4-48.3 MPa. Some academic studies on this process have been reported, mainly focusing on the mechanism of bond cleavage in polyol hydrogenolysis and the development of new catalyst systems. For example, Chen et al. [3] studied sorbitol hydrogenolysis over Ni-MgO catalyst and found that the activity depended strongly on the basicity of catalyst. Banu et al. [12, 13] conducted the sorbitol hydrogenolysis over NaY zeolite-based catalysts and discussed the effect of Ca(OH)2 on the conversion and selectivity. Sohounloue et al. [14] investigated the hydrogenolysis of sorbitol over Ru/SiO2 in a basic medium and discussed the dependence of bond cleavage on temperature with respect to the retro-aldol condensation mechanism. Montassier et al. [15] conducted a study on the modification of Ru catalyst with sulfur and proposed that the bulk reaction causing C-C bond cleavage in sorbitol hydrogenolysis was a retro-Michael reaction under the action of absorbed nucleophilic species OH−. Using 1,3-diols as model compounds, Wang et al. [16] proposed a bond cleavage mechanism (Scheme 1) for polyol hydrogenolysis; retro-aldol condensation was proposed as the dominant mechanism for C-C bond cleavage by thermodynamic and kinetic considerations. The initial dehydrogenation step is thought to occur on the transition metal catalyst.
Previous research in our laboratory [17, 18, 19, 20] has shown that Ru catalyst over carbon nanofibers (CNFs) exhibited high catalytic activity in sorbitol hydrogenolysis to EG and PG owing to the mesoporosity and surface chemistry of the CNFs. The Ru catalysts supported on powdered and structured CNFs showed high performance in a batch reactor (slurry) and in a flow-through continuous reactor, respectively. Moreover, it is generally acknowledged that the use of base can significantly affect the activity and selectivity of this reaction; our preliminary results demonstrated that addition of CaO enhanced the conversion of sorbitol [17]. The use of base in sorbitol hydrogenolysis was first thought to prevent leaching of the metal from the catalyst [7, 8], but further work has suggested a more significant role. Banu et al. [13] found that addition of Ca(OH)2 as a promoter to both Ni and Pt catalysts increased the sorbitol conversion significantly without any significant effect on selectivity. However, studies on the effect of base on glycerol hydrogenolysis found different results that suggested the base aided initial dehydrogenation of glycerol to glyceraldehyde and promoted the dehydration of glyceraldehyde to 2- hydroxyacrolein; hence, the selectivities to different products varied significantly [21, 22, 23]. Sun et al. [24] investigated the selective hydrogenolysis of xylitol to EG and PG over different catalysts in the presence of Ca(OH)2. They revealed that xylitol hydrogenolysis involved the dehydrogenation of xylitol to xylose on the metal surface and subsequent base-catalyzed retro-aldol condensation of xylose to glycolaldehyde and glyceraldehyde. This was followed by direct glycolaldehyde hydrogenation to EG and sequential glyceraldehyde dehydration and hydrogenation to PG. Rass et al. [25] studied the influence of base in the selective oxidation of 5-hydroxymethylfurfural to 2,5- furandicarboxylic acid over Pd/C catalyst, and demonstrated that the cation of the base had no effect while the carbonate base facilitated hydration of the aldehyde.
Little effort has been made to explain the mechnism of this enhancement by base in sorbitol hydrogenolysis, although almost all previous research has used base additives for this reaction. Given that few details are known about how these base additives affect the reaction pathways, the objective of this study was to explore the role of base in sorbitol hydrogenolysis and to optimize the reaction conditions for higher glycol selectivities. Based on our previous work, the effects of the base type and amount on sorbitol hydrogenolysis were systematically investigated over Ru/CNFs in a batch reactor. The role of base is discussed in combination with the reaction mechanism.
CNF catalyst support was synthesized and purified as described by Zhao et al. [17]. 3.0 wt% Ru/CNFs catalyst was prepared by incipient wetness impregnation using RuCl3·3H2O (Heraeus) as precursor [17]. NaOH, KOH, Mg(OH)2, Ba(OH)2, CaO, and sorbitol were purchased from Alfa Aesar China and used as received.
Sorbitol hydrogenolysis was carried out in a 500-mL stainless steel autoclave (Parr 4575A, USA) with magnetic stirring. In a typical run, 330 mL (20 wt% unless specified otherwise) sorbitol aqueous solution, a measured amount of base, and activated Ru/CNFs catalyst were added to the autoclave. After sealing, the autoclave was purged with N2 for 30 min and then H2 for 30 min at a stirring rate of 200 r/min at room temperature. After purging, the autoclave was filled with H2 to 2.0 MPa and then heated to 220 °C by an electrical heater with stirring at 800 r/min. H2 was continuously fed into the reactor to maintain the pressure at 8 MPa. After reaction for 4 h, the autoclave was cooled with tap water flowing through the cooling coil. The product mixture was then collected, filtered, and analyzed.
The products in liquid phase were analyzed qualitatively and quantitatively by high-performance liquid chromatography (HPLC; HP1100, Agilent, USA) equipped with a refractive index detector. A Platisil ODS C18 AQ column was used at 25 °C to separate the five main products of sorbitol, EG, PG, GL, and an unknown product. Redistilled water (0.6 mL/min) was used as mobile phase. Trace products like ethanol, methanol, and methane were detected in the gas phase in the reactor, but selectivity to gaseous products was less than 1%. The liquid products except the unknown substance were quantified using an external standard method. The selectivity to a specific product was expressed as the ratio of sorbitol converted into product to the total sorbitol converted.
To better understand the role of base in sorbitol hydrogenolysis, control experiments were conducted under the same conditions, but contained only the sorbitol substrate and one of: CNF support, CaO, Ru/CNFs, or Ru/CNFs + CaO. The product distributions of controlled sorbitol hydrogenolysis are listed in Table 1. Both the CNF support and CaO showed no catalytic effect for sorbitol hydrogenolysis when they were used alone in the reaction. Sorbitol conversion of 17.5% was achieved with more than 90% total selectivity to glycols and glycerol when 0.5 g Ru/CNFs was used. The addition of CaO led to a significant increase in sorbitol conversion to 43.4% with a slight increase in the glycol selectivity and a decrease in glycerol selectivity. This result was similar to that observed by Banu et al. [13] where the addition of Ca(OH)2 increased the conversion significantly without any effect on selectivity. The difference in the effect of base promoter on the selectivity to glycerol may be caused by reaction conditions and catalyst applied.
It is obvious from Table 1 that without Ru/CNFs, the CaO could not catalyze sorbitol hydrogenolysis. Therefore, CaO is more a promoter than a co-catalyst for sorbitol hydrogenolysis, although it did greatly enhance the sorbitol conversion. According to the mechanism proposed by Wang et al. [16] in Scheme 1, the promotion effect is caused by OH− (released when CaO reacts with water), which could catalyze the retro-aldol condensation of the intermediate aldehyde to generate glycols. However, retro-aldol condensation is the second step in sorbitol hydrogenolysis and only occurs after the sorbitol dehydrogenates to some intermediate aldehyde over Ru catalyst. Thus, only by combining with a metal catalyst, such as Ru/CNFs, can CaO promote sorbitol hydrogenolysis.
The effect of base type on sorbitol hydrogenolysis was investigated by using five bases of equivalent theoretical OH− but with different metal cations. The results (Table 2) showed that all the bases substantially enhanced sorbitol conversion and deteriorated the selectivity to glycols except CaO. CaO was the best base promoter for sorbitol hydrogenolysis by significantly increasing sorbitol conversion with only a slight effect on the glycol selectivity; hence, glycol yields were greatly enhanced.
KOH, NaOH, and Ba(OH)2 added into reactions have equivalent OH− and completely dissolved in the solution under the reaction conditions. As a result, the hydrogenolysis was supposed to take place at a same pH when these three bases were used as promoters. However, they affected the product distribution in the order of NaOH> KOH > Ba(OH)2, indicating that besides OH− in the base, the metal cations might play a role during hydrogenolysis. Moreover, it seemed that the more the base enhanced sorbitol conversion, the more it lowered the selectivity to glycols. Actually, more side products other than EG, PG, and GL were observed when these three bases were used as promoters. This could be explained according to the mechanism of polyol hydrogenolysis [16]. The cleavage of C-C bonds basically relies on a retro-aldol reaction, which is promoted by OH−. However, an overly high OH− concentration also enhances reversible and scrambling aldol reactions, resulting in a decline in selectivities to desired EG and PG and a rise in yield of other undesirable hydrocracked products [26].
Mg(OH)2 and CaO, which have low solubilities in aqueous solution, showed slightly lower enhancement in sorbitol conversion, but only slightly affected the selectivity to glycols. The low solubilities of Mg(OH)2 and CaO limited the concentration of OH− in aqueous solution, although it was still sufficient to accelerate the retro-aldol reaction of intermediates to glycols and glycerol. Excess OHˉ when combined with Ru catalyst would catalyze further degradation of glycol and glycerol and decrease the glycol yields [22]. Therefore, the addition of Mg(OH)2 and CaO not only enhanced sorbitol conversion, but also maintained high selectivity to glycol.
The metal cation of the base promoters, particularly in the case of Ca2+, was probably involved in the reaction. Both sorbitol and the intermediate aldehyde formed after sorbitol dehydrogenation have chelation ability and can form complexes with divalent metal ions such as Ca2+. Kenner et al. [27] studied the degradation of carbohydrates by alkali and proposed that the cation of the alkali played a role in the complexation of aldehydes, leading to the formation of saccharinic acid. This cationic effect of the reacting bases has been also verified in the reaction of d-glycerose-3-C14 with alkali by Sowden et al. [28]. In our case for hydrogenolysis of sorbitol, complexation between the intermediates and Ca2+ was also observed. Figure 1 shows a comparison of HPLC chromatograms for the hydrogenolysis products when different amounts of CaO were employed in the reaction. It is clear that the intensity of peak 1¢ for the unknown product increased with the increasing CaO amount. When oxalic acid was added into the liquid product mixture, the unknown substance (peak 1¢) declined while the concentrations of the other four products did not alter. Simultaneously, a white precipitate was formed and was shown to be calcium oxalate by X-ray diffraction and Fourier transform infrared spectroscopy [29]. Moreover, the unknown substance was the only product that could be detected by ultraviolet detection, indicating the existence of aldehyde or a carbonyl group. Therefore, peak 1¢ was assigned as a complex between Ca2+ and intermediates, although the specific structural formula of this complex has not yet been determined after considerable investigation, including analysis by HPLC-MASS. It is likely that the complexation of Ca2+ with the intermediate would affect the cleavage of C-C bonds and, subsequently, the selectivities to glycols.
Table 3 shows the effect of CaO amount in the reaction system on sorbitol hydrogenolysis. With increased CaO, the conversion of sorbitol gradually increased, while the selectivities to glycols and glycerol gradually decreased. CaO reacts with H2O to give Ca(OH)2, but the solubility of Ca(OH)2 in aqueous solution is very low and decreases with increasing temperature (0.16 g/100 g H2O at 20 °C and 0.07 g/100 g H2O at 100 °C [30]). In the hydrogenolysis reaction, if Ca2+ was present merely in the form of the free ion rather than participating in the reaction by complexing with the intermediate, most of the CaO would exist as undissolved solid because of its low solubility. In such a case, increased CaO would have no effect on the reaction. However, the results in Table 3 show that the CaO amount greatly affected the hydrogenolysis product distribution. This is further evidence that Ca2+ participated in the hydrogenolysis reaction and had an important effect on the selectivities to EG and PG.
Our results suggest that CaO served a bifunctional role in sorbitol hydrogenolysis. As a base additive, it provided a moderate basic medium for accelerating a retro-aldol reaction for C-C cleavage and so enhanced the sorbitol conversion. It also provided the metal center for complexation with intermediate aldehyde and released more OH−, further enhancing the retro-aldol condensation. Because the retro-aldol condensation and the complexation are both reversible reactions under the hydrogenolysis conditions, CaO should reach an equilibrium point between its two roles. As shown in Table 3, when the CaO amount was increased to 5.0 g, the highest yields of glycols were achieved within the investigated range. Further increase in CaO led to severe decline in glycol yields. This is because excess OH− caused the reversible aldol scrambling problem and resulted in a decline in selectivity to glycols, just as in the cases when NaOH and KOH were used as promoters. Therefore, there exists a preferred CaO concentration for the system under which the best glycol yield can be achieved.
Based on the above discussion and previous mechanistic detail reported in the literature, reaction pathways for hydrogenolysis of sorbitol to glycols in the presence of CaO are tentatively proposed in Scheme 2.
As shown in Scheme 2, Ru/CNFs catalyst is necessary for the initial dehydrogenation of sorbitol to intermediate with aldehyde or carbonyl groups, which leads to the cleavage of C-H bonds by dehydrogenation and the subsequent C-C bond cleavage by retro-aldol condensation. The retro-aldol reaction is enhanced under basic conditions because it is catalyzed by adsorbed OH− [16]. A probable mechanistic pathway for sorbitol hydrogenolysis is as follows. The first step is dehydrogenation of sorbitol on the Ru particle surface to the corresponding intermediates with a sugar carbonyl group. Subsequently, the intermediates undergo either the base-involved retro-aldol condensation or complex with Ca2+. The retro-aldol condensation of intermediate aldehyde generates glycolaldehyde and the latter is subsequently hydrogenated to EG. Meanwhile, the retro-aldol condensation of intermediates with a carbonyl group generates glyceraldehyde or dihydroxyacetone, which are then subsequently hydrogenated to GL. GL is further dehydrogenated, dehydrated, and then hydrogenated over Ru particle surface to give PG. The complexation between Ca2+ and intermediates helps to release more OH− and accelerate the retro-aldol condensation to give glyceraldehyde or glycolaldehyde, and these are further hydrogenated on the Ru surface to the desired EG and PG. During this process, CaO acts as base and provides the complexation cation. The amount of added CaO affects the competition between the retro-aldol condensation and complexation with the dehydrogenated intermediates, and dictates the reaction pathways as a consequence.
As discussed above, both the dehydrogenation over Ru and the base-involved retro-aldol condensation dictate the terms of the hydrogenolysis and the eventual generation of the desired products (EG, PG, and GL). Therefore, there should exist a balance between the Ru/CNFs and the CaO promoter to achieve the most favored product distribution. A series of experiments were conducted under different reaction conditions to identify the optimum conditions in terms of the most favored glycol yield. The results showing sorbitol conversion and glycol selectivities are listed in Table 4.
It is generally accepted that increasing the amount of catalyst in batch processing increases the conversion, but its effect on selectivity depends on the nature of the reaction. As shown in Table 4 (entries 2-4), the sorbitol conversion was remarkably increased when more catalyst was used while the other conditions remained the same. The selectivities to EG and GL were slightly decreased, but the selectivity to PG was slightly increased. In addition, it is interesting to note that the change in catalyst concentration imposed inverse effects on the selectivities to PG and GL and the sum of the selectivities to PG and GL was kept about the same with increased catalyst amount. Similar results were also obtained by Casale et al. [6] when Pt/C catalyst was used in sorbitol hydrogenolysis, and suggests that PG is derived from GL as illustrated in the reaction pathways in Scheme 2. The transformation of GL to PG can be rationalized by considering the adsorption ability of polyols on Ru active sites. It has been proposed that PG has a much lower affinity for Ru active sites than GL or EG, while the latter two have relatively equal chemisorption ability [31]. When the amount of catalyst was increased, the higher conversion led to a relatively lower sorbitol concentration in the reaction solution. Meanwhile, more Ru active sites were available for GL to be adsorbed and the adsorbed molecules might be further dehydrogenated and subsequently dehydrated to give PG under the hydrogenolysis conditions.
The effect of initial sorbitol concentration on the hydrogenolysis was also investigated at a fixed ratio of Ru/sorbitol and a specified CaO amount (7 g). The results shown in Table 4 (entries 1, 3, 5, and 7) indicate that the sorbitol conversion gradually decreased, while the selectivities to EG and PG reached a maximum when the initial sorbitol concentration was 20%. In addition, the selectivity to GL monotonically decreased with increasing initial sorbitol concentration. This result can be explained from two aspects. First, the reaction was conducted at a fixed Ru/sorbitol ratio, which means that the number of Ru active sites for each sorbitol molecule was the same. The promoter CaO, however, also played an important role in determining the sorbitol conversion and the selectivities to glycols. When the initial sorbitol concentration increased, the amount of CaO was insufficient for complexation with the dehydrogenated intermediates and resulted in lower concentration of OH−. As a result, enhancement for the retro-aldol condensation was not as significant as it was at lower initial sorbitol concentration. Another possible reason for the decrease in sorbitol conversion is that the reaction order of the sorbitol dehydrogenation is less than 1, which means the higher the initial concentration, the conversion would be lower after the same reaction time.
According to the proposed reaction pathways in Scheme 2, it is likely that Ru/CNFs loading, sorbitol concentration, and the amount of CaO need to be finely balanced to achieve a favorable product distribution. Ru/CNFs catalyzed the dehydrogenation and hydrogenation during hydrogenolysis, while CaO provided OH− for the retro-aldol condensation and dehydration and Ca2+ for complexation with the dehydrogenated intermediates. The compromise between these two processes ultimately determines the product distribution. Concentrated sorbitol solution (40%) was employed to demonstrate the synergy between the actions of Ru/CNFs and CaO. As can be seen in Table 4 (entries 6-9), when 40% sorbitol solution was used as the initial reactant, although sorbitol conversion increased with increasing CaO amount, the enhancement was not as significant as it was when 20% sorbitol solution was used (Table 3). Notably, the selectivity to EG increased with increasing CaO amount. This is because more sorbitol reactant and more Ru/CNFs catalyst in these reactions led to more complexation, which favors the production of EG.
In summary, fine tuning of the reaction parameters allowed sorbitol to be hydrogenolyzed to EG and PG at a total glycol selectivity of 60.1% with a 75% sorbitol conversion. The optimum conditions were: 220 °C, hydrogen pressure 8 MPa, sorbitol solution concentration 20 wt%, 3 wt% Ru/CNFs catalyst 1.0 g, CaO base 7.0 g.
The five bases of NaOH, KOH, Mg(OH)2, Ba(OH)2, and CaO were used to investigate the effect of base on sorbitol hydrogenolysis. The results indicated that all the base promoters significantly enhanced the sorbitol conversion, while the glycol selectivities varied with the base type. CaO was the best base promoter in terms of glycol yields. It served a bifunctional role in sorbitol hydrogenolysis as a source of OH− and metal ions for complexation, enhancing the retro-aldol condensation and dictating the product selectivities. The mechanistic study for sorbitol hydrogenolysis indicated that Ru/CNFs catalyst was necessary for the initial dehydrogenation of sorbitol to intermediates. The intermediates then undergo C-C bond cleavage by retro-aldol condensation, which is enhanced under basic conditions. Pathways for sorbitol hydrogenolysis to EG, PG, and GL in the presence of CaO have been tentatively proposed. We found that there was an optimum ratio among sorbitol concentration, Ru/CNFs catalyst, and CaO to achieve favorable glycol selectivities in sorbitol hydrogenolysis. These findings provide a fundamental basis for the rational development of nonbiological approaches for biomass conversion to commodity chemicals.
从可再生的生物质来源的糖醇氢解制备大宗化学品, 以替代化石燃料工艺路线, 正日益成为近年来的研究热点[1, 2]. 山梨醇是美国能源部认定的可用于生物炼油的基础原料之一, 其氢解制备乙二醇(EG)、丙二醇(PG)和甘油(GL)等已被证明是具有良好应用前景的可持续发展的绿色工艺[3]. EG和PG是重要化工原料, 广泛用于聚酯、表面活性剂、医药和功能液体等领域.
山梨醇氢解反应的研究最早始于考察糖类在氢气作用下的行为[4], 其后Clark[5]首次报道了该工艺可用于从生物质制备低炭二元醇, 自此该过程受到工业界的关注, 在过去的数十年中就此工艺申请了不少专利[6, 7, 8, 9, 10, 11]. 这些专利中山梨醇氢解通常都以过渡金属为催化剂, 并加入碱促进剂, 在180-275 °C和3.4-48.3 MPa的H2中进行. 与此同时, 学术界近年来也对该工艺有所关注, 但研究重点主要集中在糖醇氢解机理和新催化体系的开发. Chen等[3]研究了应用于山梨醇氢解的Ni-MgO催化剂, 发现催化剂的活性与其碱性密切相关. Banu等[12, 13]研究了以NaY为载体的催化剂上山梨醇氢解过程, 并考察了Ca(OH)2添加剂对于反应转化率与选择性的影响. Sohounloue等[14]则在碱性条件下研究了Ru/SiO2催化剂上山梨醇氢解过程, 讨论了温度对于导致C-C键断裂的羟醛缩合的影响. Montassier等[15]研究了S对于Ru催化剂的促进作用, 并提出C-C键的断裂是在吸附的OH-作用下发生逆向迈克尔反应实现的. 而Wang等[16]利用1,3-diol作为模型反应物进行研究, 提出了多元醇氢解反应的断键机理, 如图式1所示, 认为导致C-C键断裂的主导反应是逆向羟醛缩合反应, 而氢解反应的第一步是在过渡金属上发生的脱氢.
本课题组[17, 18, 19, 20]曾采用纳米碳纤维负载的Ru催化剂(Ru/CNFs), 由于纳米碳纤维具有独特微结构和中孔特性, 在山梨醇氢解中表现了优异的催化性能; 同时也发现, NaOH和CaO等碱的加入可显著促进山梨醇的氢解[17]. 最初的研究认为[7, 8], 山梨醇氢解过程中碱的作用是减少金属催化剂从载体上溶析, 但其后发现碱的作用不仅如此. Banu等[13]发现Ca(OH)2作为助剂加入Ni或Pt催化的山梨醇氢解过程, 均可显著提高山梨醇的转化率, 但对选择性的影响很小. 然而, 在甘油氢解反应中, 碱促进了第一步甘油脱氢成为甘油醛以及后续的脱水生成2-羟基丙酮, 因此显著地影响了产物的选择性[21, 22, 23]. Sun等[24]研究了Ca(OH)2促进的不同催化剂上木糖醇氢解生成EG和PG的过程, 发现木糖醇首先在金属催化剂表面发生脱氢反应生成木糖, 然后在碱催化下发生羟醛缩合反应生成乙醇醛和甘油醛, 前者加氢得到EG, 而后者经过进一步的脱水和加氢得到PG. Rass等[25]则研究了Pd/C催化剂上5-羟甲基呋喃选择性氧化生成呋喃-2,5-二羧酸的过程, 发现碱助剂的阳离子对反应几乎没有影响, 但碳酸根可以促进醛脱水过程.
对于山梨醇氢解过程, 尽管此前的研究中均采用了碱助剂, 但有关它在该反应中的作用, 特别是碱助剂对于反应历程的具体影响未见文献报道. 因此, 本文旨在探索碱助剂在山梨醇氢解过程中的作用机制和影响机理, 以进一步优化山梨醇氢解工艺条件, 以期得到更高的二元醇选择性. 基于以前的工作基础, 以Ru/CNFs为催化剂, 系统考察了碱的种类和用量在山梨醇氢解中的影响.
催化剂载体CNFs为实验室自制并纯化后应用[17]. 以RuCl3·3H2O为Ru前体, 采用等量浸渍法制备3 wt% Ru/CNFs催化剂, 催化剂反应前在Ar/H2 (300/100 mL/min)气氛中于300 °C还原5 h进行活化, 具体制备方法及过程可参考文献[17]. NaOH, KOH, Mg(OH)2, Ba(OH)2, CaO和山梨醇购自Alfa Aesar公司.
山梨醇氢解反应在配备磁力搅拌器的500 mL高压反应釜中进行(Parr 4575A, 美国). 典型的操作过程如下: 330 mL山梨醇水溶液(20 wt%或另行指定)、一定量的碱助剂和活化的Ru/CNFs催化剂一起加入反应釜, 密封后搅拌(200 r/min), 采用N2和H2分别吹扫30 min; 然后加入2.0 MPa的H2并在800 r/min搅拌转速下加热至220 °C, 并继续通入H2至8 MPa. 反应4 h后, 通冷却水冷却至室温, 反应液相产物收集后经过滤后分析.
反应的液相产物采用HPLC分析(HP1100, Agilent公司), 示差检测器, Platisil ODS C18 AQ柱, 以超纯水为流动相(0.6 mL/min), 分析温度为25 °C. 液相产物中可分离检测到山梨醇, EG, PG, GL和一种未知产物. 反应器的气相中可检测到少量的甲醇、乙醇和甲烷, 但是所有气相产物的选择性之和小于1%. 除未知产物外, 液相产物均采用外标法定量. 产物的选择性根据碳原子守恒定义为转化成该目标产物的山梨醇与转化的总山梨醇的百分比.
为了更好地理解碱在山梨醇氢解中的作用机理, 设计了一系列的空白和比较实验. 在相同的氢解反应条件下, 将CNFs载体, CaO, Ru/CNFs催化剂以及Ru/CNFs+ CaO分别加入反应釜进行反应, 得到的山梨醇转化率和主要目标产物的选择性见表1. 可以看到, CNFs载体和CaO单独应用时, 都不具有催化山梨醇氢解的能力. 当加入0.5 g Ru/CNFs催化剂时, 山梨醇转化率为17.5%, 二元醇和甘油的总收率达90%; 当加入CaO作为助剂时, 山梨醇的转化率显著升至43.4%, 二元醇的选择性略有升高, 而甘油的选择性则明显降低. 这与Banu等[13]的结果比较类似.
还可以看到, 不加Ru/CNF时, CaO本身并不能催化山梨醇氢解. 因此, 尽管CaO的加入可以大大增加山梨醇的转化率, 但其作用是促进剂而非共同催化作用. 如图式1所示[16], CaO促进作用主要源于其溶于水后释放的OH-催化山梨醇脱氢中间产物发生逆向羟醛缩合反应, 加快了山梨醇向目标产物二元醇的转化. 然而, 羟醛缩合反应是山梨醇氢解过程的第二步反应, 只有当山梨醇在金属催化剂表面发生脱氢反应生成含羰基的中间产物后才可能发生. 因此, 只有当作为助剂与Ru/CNFs共存时, CaO才能促进山梨醇氢解.
采用5种不同种类的碱为促进剂研究碱的种类对山梨醇氢解过程的影响, 在OH-理论引入量相当但具有不同金属离子的情况下, 相同条件下进行了山梨醇氢解反应, 结果列于表2. 可以看出, 所有碱均可显著提高山梨醇的转化率, 但除CaO外, 其他碱促进剂都导致了选择性降低. 对于二元醇作为目标产物而言, CaO是最佳的促进剂, 不仅大大提高了山梨醇的转化率, 对二元醇选择性的影响很小, 因而显著提高了二元醇的收率.
加入氢解体系的KOH, NaOH和Ba(OH)2具有等当量的OH-且3种碱的溶解度足够大, 在反应条件下能完全溶解于反应体系. 因此, 理论上说3种碱促进剂条件下, 氢解反应是在同一pH下发生的. 然而, 它们影响氢解过程的程度为NaOH > KOH > Ba(OH)2. 可见, 不仅仅是OH-在氢解过程发生作用, 其中的金属离子也可能有一定的作用. 由表2还可以看出, 山梨醇的转化率越高, 二元醇的选择性越低. 事实上, 当采用这3种碱作为促进剂时, 反应产物除了EG, PG和GL以外的副产物更多, 这可以根据多元醇氢解机理[16]来解释. C-C键的断裂依赖于OH-促进的逆向羟醛缩合反应, 但是过多的OH-也会加剧氢解过程中异构化反应和不规则C-C键的断裂, 使得副产物增多, 从而降低了目标产物EG和PG的选择性[26].
由于Mg(OH)2和CaO在水溶液中难以溶解, 因此二者对山梨醇转化的促进作用稍低, 但对选择性的影响也较小. Mg(OH)2和CaO在水溶液中的低溶解度使得水溶液中的OH-浓度较低, 但仍足以促进山梨醇氢解中间产物的羟醛缩合, 从而促进山梨醇转化. 过多的OH-与Ru催化剂共存时, 会催化甘油等产物进一步降解, 从而导致EG和PG的选择性降低[22]. 因此, Mg(OH)2和CaO的加入不仅提高了山梨醇的转化率, 而且二元醇的选择性也较高.
另一方面, 碱促进剂的金属离子, 特别是Ca2+很可能也参与了山梨醇氢解过程. 山梨醇及其脱氢后生成的含羰基的中间产物均是螯合分子, 容易与金属离子如Ca2+生成络合物. Kenner等[27]研究了糖类在碱作用下的降解过程, 认为碱分子中的阳离子可与醛类形成络合物, 并导致生成糖酸. Sowden等[28]也在D-glycerose-3-C14与碱的反应中观察到了阳离子效应. 山梨醇氢解过程中也可观察到这一现象. 图1给出了当加入不同量的CaO作为促进剂时山梨醇氢解产物的液相色谱图. 从中可以看出, 当CaO量增加时, 生成的一个未知物(peak1¢)含量也随之增加. 当反应产物溶液中加入一定量的草酸时, 该未知物的含量减少, 而其他4种物质的含量不变, 同时溶液中生成了一种白色沉淀. 该白色沉淀经X射线衍射分析和红外光谱检测为草酸钙[29]. 这表明草酸的加入把一部分的未知产物中的Ca2+置换出来, 生成了草酸钙. 同时, 该未知产物(peak1¢)是产物溶液中唯一一种可以被紫外检测器检测到的物质, 这表明该物质含有可以产生紫外吸收的基团, 如醛基或羰基. 因此, 我们认为该未知产物是一种Ca2+与山梨醇脱氢中间产物形成的络合物. 尽管尝试了采用诸如 HPLC-MASS等多种手段, 仍未能成功确定该物质的分子结构及分子式, 无法准确定量. 但Ca2+参与形成的这一络合过程显然影响到山梨醇的C-C断键过程, 并最终影响目标产物二元醇的选择性.
表3给出了CaO的用量对于山梨醇氢解过程的影响. 可以看出, 随着CaO用量的增加, 山梨醇的转化率逐步增加, 而二元醇和甘油的选择性逐步降低. CaO在水中发生反应生成Ca(OH)2, 但Ca(OH)2在水中的溶解度非常低, 且随着温度的升高而降低(20 °C为0.16 g/100 g H2O, 100 °C时为0.07 g/100 g H2O[30]). 在山梨醇氢解过程中, 如果Ca2+不与中间产物发生络合, 而仅仅以自由离子的形式存在, 则由于其溶解度太低, 绝大部分的CaO都会以固体存在而不参与反应, 那么CaO量的增加将不会影响氢解过程. 事实上, 如表3所示, CaO的用量显著地改变了氢解产物的分布. 这是Ca2+参与氢解过程的又一证据.
可以认为, CaO在山梨醇氢解过程中产生了双重作用: (1) 作为一种碱助剂, 它提供了可促进完成C-C键断裂的逆向羟醛缩合所需要的碱性环境; (2) 作为络合分子, 它提供了与中间产物进行络合的金属阳离子, 进一步促进了羟醛缩合. 由于氢解反应条件下, 羟醛缩合和络合反应都是可逆过程, 因此CaO需要在其两种功能间形成一定的平衡. 正如表3所示, 当CaO的加入量增加到5 g时, 二元醇收率最高; 进一步增加其用量则导致二元醇的收率急剧降低. 这是由于过量的OH-促进了氢解过程中异构化反应和不规则C-C键的断裂, 从而导致副产物增多而降低了目标产物选择性, 与以NaOH和KOH作为促进剂时的情况一样. 因此, CaO浓度存在一个最优值.
基于上述结果并结合文献调研, 提出了CaO作为碱促进剂时山梨醇氢解生成EG和PG的反应历程, 如图式2所示. 山梨醇氢解的第一步是山梨醇脱氢生成含醛基或羰基的中间产物, 该过程必须在Ru/CNFs作用下完成; 然后中间产物在OH-催化下发生逆向羟醛缩合反应. 因此, 反应产物的分布由Ru催化剂和碱促进剂共同决定.
山梨醇氢解可能的反应历程如下: 首先, 山梨醇分子吸附在活性金属Ru晶粒表面, 在高温下发生脱氢反应生成含醛基或羰基的中间产物; 然后, 中间产物一方面可以在溶液中的OH-作用下发生逆向羟醛缩合, 另一方面与Ca2+络合生成络合物, 并帮助释放更多的OH-. 含醛基的中间产物的逆向羟醛反应使得其C-C键断裂, 生成乙醇醛和赤藓糖, 前者在Ru金属表面发生加氢生成EG, 而后者在OH-作用下进一步羟醛缩合生成乙醇醛并加氢得到EG. 而含羰基的中间产物发生羟醛缩合后生成甘油醛或甘油酮, 进一步加氢后得到GL. 甘油在Ru催化剂和OH-的共同作用下, 进一步发生脱氢、脱水和加氢得到PG. Ca2+与中间产物的络合释放了更多的OH-, 促进了羟醛缩合生成乙醇醛和甘油醛的生成, 从而使其进一步加氢生成EG和PG. 在该过程中, CaO同时提供了碱性环境和络合阳离子, 因此CaO的加入影响了羟醛缩合和络合反应之间的竞争和平衡, 并最终影响反应历程和走向, 决定反应产物分布.
如上所述, Ru金属催化的脱氢加氢反应和碱催化的羟醛缩合反应决定了氢解过程的历程, 并最终决定反应产物中EG, PG和GL的分布. 因此, 可以通过优化Ru/CNFs催化剂用量和CaO促进剂用量以调节产物分布, 同时优化反应条件, 以得到更高的二元醇产率, 结果如表4所示.
通常认为, 在间歇反应过程中增加催化剂用量可以提高反应物的转化率, 但是对反应产物选择性的影响则依赖于反应特性的差异. 如表4 (实验2-4)所示, 随着催化剂用量的增加, 山梨醇的转化率显著增加, EG和GL的选择性略有降低, 但是PG的选择性则略有升高, 其中PG选择性的增加量和GL选择性的降低量相当, 使得PG和GL的选择性之和几乎不变. 这与Casale等[6]采用Pt/C催化山梨醇氢解反应观测到的一致. 正如图式2所示, PG是从GL进一步脱氢、脱水并加氢得到的. GL进一步转化为PG的过程与多元醇在Ru活性位上的吸附能力相关. 与EG和GL相比, PG在Ru活性位的吸附能力较弱, 而EG和GL则具有相似的吸附能力[31]. 在多元醇氢解体系中, C-H键的断裂是通过醇类分子的-OH基团在Ru活性位的吸附实现的. 含有较多羟基或浓度较高的多元醇更容易接触到Ru活性位. 对于山梨醇氢解体系, 低转化率时, 山梨醇浓度较高, 其接触到Ru活性位的几率最高. 催化剂用量增加, 体系中Ru活性位相应增加, 山梨醇转化率逐渐增加, 体系中相对浓度降低, 低碳的多元醇更容易接触到Ru活性位. GL在扩散出催化剂的过程中, 由于其在Ru活性位的吸附能力较高, 可进一步发生反应生成PG, 而生成的PG由于对Ru活性位的吸附能力较弱, 不容易发生反应.
由表4 实验1,3,5和7还可以发现, 随着初始山梨醇浓度提高, 山梨醇转化率逐步降低, 而EG和PG的选择性则先增加后降低, 当山梨醇浓度为20%时达到最高; 而GL的选择性则单调降低. 氢解反应在固定的Ru/山梨醇摩尔比下进行的, 这意味着单位山梨醇分子对应的Ru活性位的量是相同的, 但CaO用量不变, 则可能使得当山梨醇浓度增加时, CaO的量不足以提供足够的络合离子, 从而使得系统中的OH-浓度较低, 导致对羟醛反应的促进作用不如其在山梨醇初始浓度较低时. 另一方面, 可能源自反应的动力学因素, 当山梨醇脱氢生成中间产物的反应级数小于1时, 则反应初始浓度越高, 反应相同时间后反应转化率越低.
结合图式2可以认为, 只有当Ru/CNFs和CaO促进剂的用量以及山梨醇的浓度匹配适当的条件下, 才能得到最优的产物分布. Ru/CNFs催化了氢解过程中的加氢和脱氢反应, 而CaO提供了碱性环境和络合离子, 催化了C-C键断裂的羟醛缩合. 这些反应过程之间的相互耦合和平衡最终决定了氢解产物分布. 为了验证该结论, 采用高浓度山梨醇溶液(40%)进行了氢解过程, 如表4实验6-9所示, 虽然山梨醇的转化率仍然随着碱促进剂用量的增加而提高, 但远不及采用20%山梨醇溶液时的效果. 值得注意的是, EG选择性随着CaO用量的增加而增加. 这是由于体系中有了更高浓度的山梨醇分子和更多的Ru活性位, 使得形成了更多的络合物, 从而有利于EG的生成.
研究表明, 通过精细调控反应条件可以优化EG和PG的收率. 综上可见, 在优化的反应条件下(220 oC, 8MPa H2, 20 wt% 山梨醇溶液, 3 wt% Ru/CNFs 1.0 g), 山梨醇的转化率可达75%, EG和PG总选择性高达60.1%.
研究了NaOH, KOH, Mg(OH)2, Ba(OH)2和CaO等5种碱在山梨醇氢解过程中的作用, 发现所有的碱促进剂均可显著增加山梨醇的转化率, 而二元醇的选择性变化则随碱种类的不同有所差异, 其中CaO为促进剂时二元醇的收率最高. CaO在山梨醇氢解过程中提供了OH-以及与中间产物产生络合的阳离子, 从而促进了羟醛缩合, 并影响产物的分布. 山梨醇氢解过程发生的第一步是它在Ru催化剂表面脱氢生成含羰基的中间产物, 然后在碱促进剂催化下发生逆向羟醛缩合造成C-C键断裂, Ru/CNFs催化剂和碱促进剂的共同作用最终决定了反应产物的分布. 在此基础上, 提出了以CaO为促进剂时, 山梨醇氢解生成EG, PG和GL的反应历程, 并可以通过优化山梨醇浓度, Ru催化剂用量和CaO促进剂用量, 得到更高的二元醇选择性. 该结果可以为生物质化学催化转化制备大宗化学品的生产工艺提供基础知识.
2014, Vol. 35 Issue (5): 692-702
PDF (579KB)
2014, Vol. 35 Issue (5): 692-702
PDF (579KB)