Recycling Strategy for Bioaqueous Phase via Catalytic Wet Air Oxidation to Biobased Acetic Acid Solution

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University of Groningen

Recycling Strategy for Bioaqueous Phase via Catalytic Wet Air Oxidation to Biobased Acetic

Acid Solution

He, Songbo; Bijl, Anton; Barana, Patryk Kamil; Lefferts, Leon; Kersten, Sascha R.A.; Brem,

Gerrit

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10.1021/acssuschemeng.0c05946

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He, S., Bijl, A., Barana, P. K., Lefferts, L., Kersten, S. R. A., & Brem, G. (2020). Recycling Strategy for Bioaqueous Phase via Catalytic Wet Air Oxidation to Biobased Acetic Acid Solution. ACS Sustainable Chemistry and Engineering, 8(39), 14694–14699. https://doi.org/10.1021/acssuschemeng.0c05946

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Recycling Strategy for Bioaqueous Phase via Catalytic Wet Air

Oxidation to Biobased Acetic Acid Solution

Songbo He,

*

Anton Bijl, Patryk Kamil Barana, Leon Leﬀerts, Sascha R. A. Kersten, and Gerrit Brem

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ABSTRACT: The bioaqueous phase generated during biomass conversion to biofuel and biochemicals, e.g., fast pyrolysis and ex situ catalytic pyrolysis, contains a large number of organics, leading to a high chemical oxygen demand (COD) for its treatment. In this study, we demonstrate its catalytic conversion to bioacetic acid solution and propose a recycling strategy thereof. We found that the diluted bioaqueous phase (e.g., C content <0.5 wt %) can be selectively (>90%) converted to acetic acid with nondetectable impurities in solution. The solution contains 1.3−1.5 wt % acetic acid and can be directly used for demineralization of biomass in the bioreﬁneries. This recycling strategy enhances the sustainability of the biobased economy and sheds light on production of biobased acetic acid, which has been recognized as a smart drop-in chemical.

KEYWORDS: Catalytic fast pyrolysis, Biomass, Biochemicals, Advanced oxidation, Circular economy, Leaching, Bioreﬁnery, Wastewater

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INTRODUCTION

Biobased bulk chemical production from renewable sources (e.g., lignocellulosic biomass) could signiﬁcantly save fossil energy and reduce environmental issues (e.g., CO₂emission).1 A variety of biobased fuels and chemicals have been emerging, of which bioacetic acid could be a valuable commodity chemical for the biobased industry.2A market research report has revealed that the global acetic acid market was worth USD 8.1 billion in 2018 and will generate a revenue of USD 11.4 billion by 2024.3Acetic acid is mainly used to produce a vinyl acetate monomer (VAM, a monomer to polymerize as polyvinyl acetate (PVA)) and acetic anhydride, followed by as a solvent for the manufacture of puriﬁed terephthalic acid (PTA, a raw material for the production of polyethylene terephthalate (PET)).4,5Vinegar, a diluted acetic acid solution, is also commonly used in food and beverage.6 The biobased acetic acid has the same properties as the conventional one but with lower environmental impact and has been recognized as a smart drop-in chemical.7Bioacetic acid is therefore anticipated to play a vital role in propelling the acetic acid market increase in terms of environmental sustainability.

Acetic acid is commercially produced by the conventional petrochemical processes, e.g., methanol carbonylation (Mon-santo or Cativa process),8 acetaldehyde oxidation,9 butane oxidation,10ethylene oxidation (Showa Denko K.K. process),11 and partial oxidation of ethane.12 The foreseen shortage of fossil energy and stringent regulations to reduce fossil CO₂ emissions have driven the proﬁcient players (e.g., Wacker, ZeaChem, AFYREN, Lenzig, and Godavari) to shift toward

biobased acetic acid production (e.g., by microbial fermenta-tion, currently accounting for ca. 10% global acetic acid production13). The fermentation approach has the advantage of using renewable sources. However, there are challenges such as eﬃcient separation of acetic acid from the diluted mixture and minimization of the other organic acids impurities.4,13 Recently, some cutting-edge technologies to convert CO₂ to renewable acetic acid (e.g., in an ionic liquid-based catalytic system14or by electrocatalytic reduction of CO215) have also emerged.

Alternatively, the liquiﬁed “bio-oil” in the biobased industry, e.g., the aqueous phase of pyrolysis liquid from biomass fast pyrolysis,16contains a certain amount of acetic acid and could be a renewable source for the production of bioacetic acid. Recovery of bioacetic acid from this alternative has been explored by reactive extraction5 and by nanoﬁltration and reverse osmosis membranes.17Furthermore, we have noticed that the biobased industry also generates abundant aqueous byproducts, which sometimes are regarded as wastewater, during e.g., the pretreatment of biomass (e.g., acid leaching)18 and post-treatment of“bio-oil” (e.g., fractionation of pyrolysis liquid by water extraction).19 This wastewater needs to be Received: August 14, 2020

Revised: September 10, 2020 Published: September 16, 2020

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treated before draining, and a few treatment technologies (e.g., adsorption,20 photocatalytic ozonation,21 electrochemical oxidation,22 and biological treatment23) have been inves-tigated, though the degradation efficiency is rather low. Instead of treatment, valorizing the above aqueous streams (termed as bioaqueous phase in this contribution) to fuels and chemicals may offset the wastewater treatment cost and enhance the sustainability and profitability of the biobased industry. Several developments by the authors and others have been reported, e.g., bio-H2 by steam reforming,

24,25

aqueous-phase reform-ing,26,27 CH4 by supercritical gasiﬁcation,

28

and fuels by catalytic upgrading.29

Herein, we explore a catalytic conversion of the bioaqueous phase to bioacetic acid solution for recycling in the biobased industry (Figure 1). The research interest originated from the boosted acetic acid usage in the biobased industry, e.g., for acid leaching pretreatment (or demineralization) of lignocellulosic biomass to remove alkali and alkaline earth minerals (AAEM, the main precursor for char formation)18,30 before the downstream processing (e.g., torrefaction,31 hydrothermal carbonization,32and fast pyrolysis33). Minimizing the conven-tional fossil acetic acid usage and producing addiconven-tional biobased acetic acid will lead to a more sustainable biobased industry. We have applied a well-known catalytic wet air oxidation (CWAO) approach for this demonstration, which is one of the advanced oxidation processes (AOPs) widely used for degradation of highly concentrated, toxic, and refractory organic pollutants in wastewater in the existing petrochemical industry.34Acetic acid is one of the organics with the highest resistance to oxidation and is generally a dead-end for the CWAO process,35which aims to mineralize organic C to CO2 for an ultimate treatment.34 However, the strategy in this demonstration is to minimize mineralization and maximize oxidation to acetic acid with minimal impurities in the solution.

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EXPERIMENTAL SECTION

Bioaqueous Phase. Two types of representative bioaqueous phases investigated in this work were obtained from fast pyrolysis (FP,

Figure 1b) of pine wood on a bench-scaleﬂuidized bed unit36and catalytic conversion (CC, Figure 1d) of paper sludge via ex situ catalytic pyrolysis on a pilot Pyros demonstrator37integrated with a side ﬁxed-bed catalytic reactor.38 The one-phase liquid product of biomass fast pyrolysis was washed with water (Figure 1c) to separate the bio-oil (bottom layer) and aqueous phase (top layer, termed as bioaqueous phase FP,Figure 2a, ca. 500 mL). The liquid product of ex situ catalytic pyrolysis of biomass contains two phases, viz.,

high-grade bio-oil (top layer,Figure 2b) and aqueous phase (bottom layer, termed as bioaqueous phase CC,Figure 2c, ca. 50 mL).

CWAO Catalysts. ZrO2is one of the most common and stable

carriers for CWAO catalysts,39among which Ru- and Pt-based ones are the most active and stable.40Therefore, a commercial tetragonal ZrO2(Figure S1Ca, SBETof 13 m2g−1and Vporeof 0.03 cm3g−1) with

macrospores (Figure S1Ba) was applied to prepare Ru/ZrO2 (Ru

loading of 2.51 wt %) and Pt/ZrO2(Pt loading of 0.54 wt %) catalysts

by an incipient wetness impregnation method (SI).

CWAO of Bioaqueous Phase. CWAO of the bioaqueous phase (Figure 1e) was performed in a microstirred reactor (25 mL, Model 4590/4848, Parr, loaded with 10 mL of liquid and 50 mg of catalyst) at 270°C for 3 h. A low pressure and low O2dose were applied (viz.,

an initial air pressure of 20 bar) as acetic acid tends to be mineralized to CO2 at a high O2 partial pressure (e.g., >10 bar).35,41 The

bioaqueous phase and the liquid products were analyzed by GC-MS, COD, and CHN elemental analyses (SI).

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RESULTS AND DISCUSSION

Properties of Bioaqueous Phase. The raw bioaqueous phase FP wasﬁrst ﬁltered to obtain a homogeneous solution (Figure 2a). It has a C content of 17.6 wt %, accounting for an extremely high chemical oxygen demand (COD) of 477 g O2 L−1when considering a complete treatment by oxidation. The product from catalytic conversion of paper sludge is a biphasic liquid with a high-grade bio-oil (H₂O of 0.7 wt %, O content of 3.2 wt %, and higher heating value (HHV) of 40.9 MJ kg−1,

Figure 2b)38 on the top layer. The bioaqueous phase CC (Figure 2c) has a C content of 2.1 wt %, equivalent to an COD Figure 1.Scheme of the bioaqueous phase source and its conversion to bioacetic acid solution via a catalytic wet air oxidation approach: (a) acid leaching, (b) fast pyrolysis, (c) washing, (d) catalytic conversion (pyrolysis/upgrading), and (e) catalytic wet air oxidation.

Figure 2.(a) Bioaqueous phase FP from fast pyrolysis of pine, (b) high-grade bio-oil, and (c) bioaqueous phase CC from ex situ catalytic pyrolysis of paper sludge, and the compositions of the organics in (d) bioaqueous phase FP and (e) bioaqueous phase CC.

ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Letter

https://dx.doi.org/10.1021/acssuschemeng.0c05946 ACS Sustainable Chem. Eng. 2020, 8, 14694−14699

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of 72 g O2 L−1. The C contents in these two types of bioaqueous phases are comparable with those reported for the aqueous streams from fast pyrolysis (19.4 wt %) and catalytic fast pyrolysis (2.5 wt %) of oak.16

GC-MS analyses of the components in these two types of bioaqueous phases are given inTables S1 and S2. The major compounds are shown inFigure 3. It needs to be noted that the oligomeric molecules in the bioaqueous phase were not analyzed because the MS detection range is only from m/z 25 to 550 (SI, Section 1.4). For the sake of simplicity, the components are grouped to carboxylic acids, carbonyls, furans,

phenols, aliphatics, esters, alcohols, and N-containing com-pounds according to the chemical functionalities. The semiquantiﬁed analyses based on peak area percentage for the detectable organic components are shown inFigure 2. The bioaqueous phase FP (Figure 2d) mainly consists of acetic acid, carbonyls (e.g., hydroxyacetone), furans (e.g., 2-Furanone), and N-containing compounds (e.g., 2-isopropox-yethylamine), originated from the soluble components of the pyrolysis liquid from biomass fast pyrolysis. Diﬀerently, the bioaqueous phase CC (Figure 2e) comprises carbonyls (e.g., butanone), phenol, alcohols (e.g., 1-butanol), and also N-Figure 3.Chemistry for catalytic wet air oxidation of bioaqueous phase to bioacetic acid, showing the representative components.

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containing compounds (e.g., acetamide). It has to be noted that the composition of the bioaqueous phase diﬀers with the raw biomass and the conditions for processing (e.g., pretreat-ment, liquiﬁcation, and catalytic conversion). Nevertheless, the complexity of these two types of bioaqueous phases in this demonstration (e.g.,Figure 3) could be representative of those major organic components in most of the aqueous streams in the biobased industry.

CWAO of Bioaqueous Phase. Considering the most suitable COD range for CWAO technology (e.g., 10−100 g O₂ L−1),42 the bioaqueous phase FP was diluted seven times (Table 1a, H-0, which has a similar COD as that for the bioaqueous phase CC). A blank experiment using Ar and without using a catalyst (Table 1b, H-1) shows that ca. 45% of C was mineralized according to COD removal, which is ca. 42% based on elemental analysis. This indicates that a significant thermal decomposition (also called thermolysis) occurred at high temperature and pressure. Indeed, thermolysis has often been reported as a pretreatment (e.g., reducing COD) before applying CWAO.43When using air (viz, wet air oxidation, WAO,Table 1c, H-2), only a slightly higher COD reduction (ca. 49%) was obtained. However, acetic acid solution purity was dramatically increased to 86.1% (Table 1c, H-2). CWAO over Ru/ZrO₂ (Table 1d, H-3) showed the same COD removal. Comparatively, the Pt/ZrO2 catalyst (Table 1e, H-4) is more active to reduce COD (e.g., to 59%). However, some of the impurities (e.g., 2,5-Hexanedione, ethylene glycol, and phenol,Table S7) were still retained in the acetic acid solution, probably related to the very low oxygen dose. This speculation was further confirmed by performing CWAO of the same solution an additional two times by only refilling the same amount of air, which resulted in a colorless and transparent acetic acid solution with negligible impurities which is not detectable by GC-MS (Table 1f, H-5). Similarly, three times of CWAO of bioaqueous phase CC over Pt/ZrO2 catalysts produced a 100% purity acetic acid solution with a COD reduction of ca. 88%.

For the more diluted bioaqueous phase FP (e.g., 42 times,

Table 2a, L-0), WAO removes 9% COD, keeping most of the C in the solution (Table 2b, L-1). However, the purity of the obtained acetic acid solution was rather low (23.8%). This

issue may be solved by applying CWAO, e.g., using Ru/ZrO2 (Table 2b, L-2) and Pt/ZrO2 (Table 2c, L-3) catalysts, over which all the organic components were oxidized to acetic acid as well as CO₂ (Figure 3). Of great interest is that no additional C was mineralized by CWAO over the Ru/ZrO₂ catalyst as compared with WAO, and all the carbonyls were converted to acetic acid. These results have shown a great potential of catalytic conversion of the diluted bioaqueous phase to a bioacetic acid solution with negligible impurities by a CWAO approach.

The bioacetic acid solution obtained in this demonstration (e.g., H-5 inTable 1f and L-2 inTable 2c) has ca. 0.5−0.6 wt % C (equivalent to 1.3−1.5 wt % acetic acid) and might be directly recycled in the biobased industry, e.g., for acid leaching (Figure 1a). Several reports have used a diluted acetic acid solution (e.g., 0.1−1 wt %) to demineralize the biomass prior to torrefaction or fast pyrolysis for a reduction of char yield and an enhanced reactivity of biomass.44−46 In a few works, a higher concentration of acetic acid solution was applied (e.g., 5−10 wt %).33,47For this application, the diluted bioacetic acid solution could be concentrated by, for example, distillation and nanoﬁltration membrane.48

The conversion of two types of bioaqueous phases (from fast pyrolysis of pine wood and ex situ catalytic fast pyrolysis of paper sludge) to bioacetic acid solution via the catalytic wet air oxidation (CWAO) approach using ZrO2supported Pt and Ru catalysts was investigated. For the high-concentration bioaqu-eous phase (e.g., C content of 2.6 wt %), most of the organic carbon could be mineralized to CO₂ by CWAO, while the diluted bioaqueous phase containing a low C content (e.g., 0.5 wt %) could be selectively oxidized to acetic acid solution with a low mineralization (<10%). The bioacetic acid solution obtained in this demonstration contains ca. 1.3−1.5 wt % acetic acid with nondetectable impurities and could be directly recycled in the biobased industry for, for example, deminer-alization of biomass. This work brings a recycling strategy that valorizing the complex aqueous streams generated in the biobased industry, which contains a low amount of organics and can be regarded as wastewater, to a valuable biobased acetic acid solution.

Table 2. CWAO of Bioaqueous Phase FP Diluted 42 Times

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ASSOCIATED CONTENT

*

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acssuschemeng.0c05946. Detailed information about experimental methods and additional data for catalyst characterizations and GC-MS analyses (PDF)

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AUTHOR INFORMATION

Corresponding Author

Songbo He − Catalytic Processes and Materials, Faculty of Science& Technology and Thermal Engineering, Faculty of Engineering Technology, University of Twente, 7500 AE Enschede, The Netherlands; Green Chemical Reaction Engineering, Engineering and Technology Institute Groningen, University of Groningen, 9747 AG Groningen, The

Netherlands; orcid.org/0000-0002-5001-6620; Email:songbo.he@rug.nl

Authors

Anton Bijl − Alucha Management B.V., 6827AV Arnhem, The Netherlands

Patryk Kamil Barana − Catalytic Processes and Materials, Faculty of Science& Technology, University of Twente, 7500 AE Enschede, The Netherlands

Leon Leﬀerts − Catalytic Processes and Materials, Faculty of Science& Technology, University of Twente, 7500 AE Enschede, The Netherlands; orcid.org/0000-0003-2377-5282

Sascha R. A. Kersten − Sustainable Process Technology, Faculty of Science& Technology, University of Twente, 7500 AE Enschede, The Netherlands; orcid.org/0000-0001-8333-2649

Gerrit Brem − Thermal Engineering, Faculty of Engineering Technology, University of Twente, 7500 AE Enschede, The Netherlands

Complete contact information is available at:

https://pubs.acs.org/10.1021/acssuschemeng.0c05946

Notes

The authors declare no competingﬁnancial interest.

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ACKNOWLEDGMENTS

S. He and G. Brem are grateful for the ﬁnancial support (Project Demonstrator Biofuels, No. 209.36.203) from Technology Foundation STW, The Netherlands. S. He also thanks C. Sun and W. Zhao (Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P. R. China) for suggestions and discussions on CWAO catalysts and reaction parameters.

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