Insights into the effect of the catalytic functions on selective production of ethylene glycol from lignocellulosic biomass over carbon supported ruthenium and tungsten catalysts
Graphical abstract
A mixture of W and Ru catalysts supported on carbon nanotubes with oxygen-surface groups enhanced the production of ethylene glycol directly from cellulose, tissue paper and eucalyptus.
Introduction
Ethylene glycol (EG) is a valuable industrial chemical due to its important role in the synthesis of high-value chemicals that have a large market demand, such as polymers (e.g. polyester fibers), antifreeze products and cosmetics (Cao et al., 2016, Zada et al., 2017, Zheng et al., 2017). However, EG is produced from petroleum-derived ethylene via multi steps of cracking, epoxidation and hydration. Recent endeavours have also been made for the production of EG from coal, which is also a fossil carbon resource (Zheng et al., 2014). Due to its importance, the production of EG by a sustainable process is vital to be achieved, which is the case of the one-pot hydrolytic hydrogenation of lignocellulosic biomass. Comparing to the petroleum-dependent multistep process, the lignocellulosic biomass path presents noticeable advantages of a renewable feedstock and one-pot process (Byun and Han, 2016, Deng et al., 2015, Kobayashi et al., 2014). However, the complexity of lignocellulosic biomass and its high resistance to chemical transformation make the production of chemicals directly from biomass still a challenge, which makes its investigation even more important. Lignocellulosic biomass is constituted of cellulose (35–50%), hemicelluloses (25–30%) and lignin (15–30%) (Deng et al., 2015, Kobayashi et al., 2012). As the greatest constituent of biomass, cellulose is the most promising natural resource for valuable chemicals production (Han and Lee, 2012, Yabushita et al., 2014). The process of cellulose transformation into EG can follow different reaction routes (Lazaridis et al., 2017, Tan et al., 2016, Zheng et al., 2017), where many types of reactions may occur (e.g. hydrolysis, retro-aldol condensation, hydrogenation, isomerization, dehydration, decarbonylation, dehydrogenation, hydration) and about 20 compounds can be produced as byproducts or intermediates (Zheng et al., 2017). Firstly, cellulose is hydrolysed to glucose, which can be directly hydrogenated to hexitols or break its CC bond to glycolaldehyde (GA) that is subsequently hydrogenated to EG. In the presence of bases (e.g. Ca(OH)2), glucose isomerizes into fructose, which then undergoes degradation to originate glyceraldehyde and/or dihydroxyacetone. These intermediates will then undergo dehydration and hydrogenation to form acetol, which is the precursor to 1,2-propylene glycol (PG) (Zheng et al., 2017). Therefore, the production of hexitols, EG and PG greatly depends on the competitive reactions including hydrogenation, CC bond cleavage and isomerization, catalysed by hydrogenation sites (e.g. Ru), tungsten species and bases, respectively. Accordingly, the major reaction pathway for EG production comprises the following steps: hydrolysis of cellulose to oligosaccharides and glucose, usually catalysed by protons in situ generated reversibly from hot water or by additional acid; retro-aldol condensation (RAC) of glucose to glycolaldehyde (GA), catalysed by tungstic compounds; and hydrogenation of GA to EG, usually catalysed by Ru or Ni supported catalysts (Cao et al., 2016, Wang and Zhang, 2013).
The production of EG from biomass started in 1933 or earlier (Nemours, 1933), and for many decades the yield of the target diol was lower than 40%. Ji et al. reported for the first time an effective cellulose conversion to EG over a W2C promoted nickel catalyst, achieving up to 61% EG yield after 30 min at 245 °C and 60 bar of H2 (Ji et al., 2008, Ji et al., 2009). Since then, the yields of EG have been enhanced to 76% with bimetallic catalysts (Li et al., 2012, Zhang et al., 2010, Zheng et al., 2010) and the catalytic stabilities have been greatly improved using binary catalysts (Tai et al., 2013). The highest EG yield was achieved with Ni-W/SBA-15, but the catalyst could not be reused due to the complete collapse of the SBA-15 mesoporous structure (Zheng et al., 2010). Accordingly, the use of carbon as support is preferred due to its high resistance to acid and base attack and great stability under hydrothermal conditions (Wang and Zhang, 2013). It was also shown that not only W2C, but also W, WO3 or H2WO4 combined with Ni or noble metals (Ru, Pt, Pd) were effective for EG production (Li et al., 2017, Liu et al., 2012, Zheng et al., 2010). WO3 was the most prone to becoming the main component of tungsten species, but there are still some unclear points on how it promotes the transformation (Li et al., 2017). Catalysts with tungstic compound are excellent choices to obtain EG, since tungsten species are highly active in promoting the selective C–C bond cleavage of glucose (Cao et al., 2014, Tai et al., 2013). Meanwhile, based on understanding of the reaction mechanism, the products distribution could also be tuned (Liu et al., 2012, Liu and Liu, 2016, Zheng et al., 2010).
It was shown in our previous work (Ribeiro et al., 2018) that the catalyst containing Ru and W supported on commercial multi-walled carbon nanotubes was more catalytically effective for the one-pot direct cellulose conversion to EG than the physical mixture of the corresponding Ru and W monometallic catalysts. In continuation of that work, the surface chemistry of the carbon nanotubes was modified with nitric acid and its effect was investigated, showing that the EG yield was influenced by the presence of acid groups on the surface of the support. The catalytic reaction pathway for cellulose conversion to EG was proposed. Finally, the catalytic performance was evaluated for the conversion of tissue paper and eucalyptus to EG in aqueous solution, which, to the best of our knowledge, has not been reported anywhere yet. Many advances were made for catalytic conversion of pure cellulose, but the conversion of raw lignocellulosic biomass is still challenging and, herein, an efficient catalytic system is proposed.
Section snippets
Preparation of materials
Microcrystalline cellulose (from Alfa Aesar), tissue paper (Renova) and eucalyptus (Eucalyptus Globulus from Portucel) were ball-milled in a Retsch laboratory equipment (Mixer Mill MM200) during for 4 h at 20 vibrations/s.
Commercial multi-walled carbon nanotubes (Nanocyl-3100, carbon purity > 95) were submitted to oxidative treatment with nitric acid (≥65%, from Sigma-Aldrich) in order to obtain a material with a different surface chemistry. Accordingly, the commercial carbon nanotubes (CNT0)
Characterization of materials
Temperature programmed reduction (TPR) results showed a reduction peak around 250 °C for Ru/CNT0 and a wide reduction range around 600 °C for W/CNT0 catalyst. Accordingly, reaction temperatures of 250 and 700 °C were selected for Ru and W, respectively.
Nitrogen adsorption/desorption experiments at −196 °C allowed determining specific surface areas (SBET) and total pore volumes at P/P0 = 0.98 (Vp), and the results are presented in Table 1. Carbon nanotubes are non-microporous materials, which
Conclusions
The results confirmed that cellulose catalytic conversion to ethylene glycol consists of at least three consecutive reactions. While WO3 promotes the CC bond cleavage for the RAC reaction of glucose to GA, Ru acts on its further hydrogenation to EG. Furthermore, the presence of oxygen-surface groups on CNT favours cellulose hydrolysis to glucose and suppresses glucose isomerization to fructose, which results in an enhanced EG yield (over 40% after 5 h). This catalytic system was then tested for
Acknowledgements
This work is a result of project “AIProcMat@N2020 - Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, with the reference NORTE-01-0145-FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) and of Project POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM funded by ERDF through COMPETE2020 - Programa
Conflicts of interest
The authors declare that they have no conflicts of interest.
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