Solvent-free synthesis of a zirconium-carbon coordination catalyst for efficient aqueous-phase production of lactic acid from xylose
Graphical Abstract
Introduction
Lactic acid, with two active functional groups (hydroxyl and carboxyl) in its structure, is regarded as an important platform molecule during biomass valorization [1], [2], [3], [4], [5], [6]. On one hand, it can be used as the feedstock to produce various valuable fine chemicals including acrylic acid, pyruvic acid, alkyl lactates, propanediol, 2,3-pentanedione, propylene oxide, et al. On the other hand, it has a wide application in the production of biodegradable polymers (such as polylactide) which can be used as the promising alternatives to replace the current non-biodegradable plastics. It has been estimated that the global market of LaA was 750 kilotons in 2017 and this value is projected to reach 1845 kilotons by the end of 2022 [7]. However, the industrial fermentation process of the LaA production from carbohydrates inevitably suffers from strict reaction conditions, slow reaction kinetics, low productivity, and complicated separation process [8], [9]. Therefore, the chemoselective production of LaA is of significance in the efficient utilization of lignocellulosic biomass.
Homogeneous catalytic systems adopting the transition metal ions (Ni2+, Al3+, Sn2+, Y3+, et al.) have been developed for the conversion of carbohydrates or lignocellulosic biomass to LaA [10], [11], [12]. For instance, Prof. Hu’s group found that Y3+ was highly active for the simultaneous conversion of hemicellulose and cellulose contents in raw biomass and the yield of LaA was up to 66.3 % at 240 °C for 1.5 h in water [12]. Although these homogeneous catalysts exhibit excellent performances for the LaA production, some inherent shortcomings such as difficult recycling of metal ions, inevitable corrosion of reactors, and harsh reaction conditions greatly hinder wide application in a large scale. With this in mind, heterogenous catalytic systems have been attracted significant attention recently. Lots of solid catalysts including metal oxides [13], [14], [15], heteropolyacids supported on metal oxide [16], metal-organic frameworks (MOFs) [17], [18], [19], and metal-modified zeolite [20], [21], [22], [23] have been designed and applied in the catalytic production of LaA from carbohydrates. For example, Xia et al. reported that the incorporation of tin (Sn) and indium (In) could enhance the LaA production from glucose and 53 % yield of LaA was obtained over the resultant In-Sn-Beta zeolite. Deep studies revealed that Sn sites promoted the isomerization and retro aldol condensation steps, and In sites inhibited the side reactions [20]. Nevertheless, low productivity of metal oxide catalysts, high cost of MOFs, or potential toxicity of Sn precursor used to prepare Sn-based catalysts greatly retards their wide applications in industry. Alternatively, Zr-based materials as the efficient Lewis acidic catalysts have attracted great attention in the heterogeneous reactions recently [24], [25], [26]. For instance, Prof. Lin’s group found that the commercial ZrO2 can be used as the catalyst for the LaA production from xylose, however, 42 % yield of LaA was just obtained at 200 °C for 40 min in water. Therefore, it is highly desirable but of challenge to develop a more facile and efficient process for the LaA production in water.
Recently, we have developed a facile solvent-free method to prepare carbon-based materials and the resultant catalysts exhibit excellent activities in the conversion of carbohydrates and lignin-derivatives to the corresponding furan-based platform chemicals and biofuels [27], [28], [29], [30], [31]. Here, we extend this method to prepare a series of zirconium-carbon coordination materials. The systematically characteristic results show that Zr is successfully incorporated into the framework of the resultant sample. Then these catalysts are tested in the conversion of xylose to LaA for the first time. Under the optimal reaction conditions, a LaA production rate of 1.8 h−1 is achieved, outperforming the recent works reported in the literature (below 1.1 h−1) [15], [17], [19], [26]. Further investigation results reveal that the presences of both Lewis acidic sites and base sites not only lower the energy barrier for xylose conversion but also enhance the efficient conversion of the generated intermediates, resulting in a higher yield of LaA from xylose.
Section snippets
Chemicals
Zirconyl chloride octahydrate (ZrOCl2⋅8H2O, 99%), glucose (99 %), sodium chloride (NaCl, 99 %), ethanol (99.5 %), methanol (99.5 %), zirconyl nitrate (98 %), terephthalic acid (99 %), acetic acid (99 %), N,N-dimethylformamide (DMF, 99 %), urea (99 %), xylose (99 %), furfural (98 %), lactic acid (LaA, 90 %), 1,3-dihydroxyacetone (DHA, 99 %), pyruvaldehyde (PAL, 40 %), arabinose (99 %), fructose (99 %), sucrose (99.5 %), benzoic acid (99.5 %), commercial ZrO2 (99 %, monoclinic polytype), tin (IV)
Characterization of prepared samples
As depicted in Fig. 1a, BC was synthesized through a direct solvent-free carbonization of glucose without addition of Zr precursor, and Zr-BC was prepared via a solvent-free coordination route using ZrOCl2⋅8H2O and glucose. SEM images of the resultant BC and Zr-BC in Fig. 1b and e presented that these two samples did not have uniform morphology, and both were composed of irregular particles with rough surfaces. TEM images (Fig. 1c and f) confirmed the nonuniform morphologies of these two
Conclusions
In summary, we have constructed an active Zr-BC catalyst via a facile solvent-free coordination route. This catalyst exhibited superior activity in the production of LaA from xylose in water. 58.2 % of LaA yield with 99.9 % of xylose conversion can be achieved over Zr-BC at 190 °C for 3 h. Moreover, LaA yield can be further promoted in a GVL/water system. Detailed studies revealed that the excellent catalytic performance of Zr-BC was attributed to the presences of Lewis acidic sites and basic
CRediT authorship contribution statement
Qin Wang: Investigation, Writing – original draft. Dan Luo: Investigation, Writing – original draft. Jiansu Ran: Methodology, Visualization, Investigation. Jie Zheng: Conceptualization, Writing – review & editing. YuntongCui: Methodology, Visualization, Investigation. Ruixue Yangcheng: Methodology, Visualization, Investigation. Shuang Luo: Methodology, Visualization, Investigation. Jianjian Wang: Supervision, Writing – review & editing, Investigation, Conceptualization, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
J.W. thanks to the final support from National Natural Science Foundation of China (21902016) and Fundamental Research Funds for the Central Universities (2019CDQYHG026). This research used resources of Analytical and Testing Center of Chongqing University.
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These authors are equally contributed to this work.