Effect of Pyrolysis Temperature on Acidic Oxygen-Containing

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Effect of Pyrolysis Temperature on Acidic Oxygen-Containing Functional Groups and Electron Storage Capacities of Pyrolyzed Hydrochars Nepu Saha, Danhui Xin, Pei Chiu, and M. Toufiq Reza ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00024 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Sustainable Chemistry & Engineering

Effect of Pyrolysis Temperature on Acidic Oxygen-Containing Functional Groups and Electron Storage Capacities of Pyrolyzed Hydrochars Nepu Saha1,2, Danhui Xin3, Pei C. Chiu3, M. Toufiq Reza1,4* 1

Institute for Sustainable Energy and the Environment, 350 W State St., Athens, OH 45701

2

Department of Chemical and Biomolecular Engineering, 171 Stocker Center, Ohio University, Athens,

OH 45701 3

Department of Civil and Environmental Engineering, 468 Harker ISE Lab, 221 Academy Street,

University of Delaware, Newark, DE 19716 4

Department of Mechanical Engineering, 183 Stocker Center, Ohio University, Athens, OH 45701 *

Corresponding Author: Email: [email protected], Tel: +1 740 593 1506

1 ACS Paragon Plus Environment

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Abstract Hydrothermal carbonization (HTC) is a thermochemical process, where biomass is treated with subcritical water. Hydrochar, the solid product of HTC, is a carbon-rich material containing acidic functional groups. In order to increase total surface area, one of the common practices is to pyrolyze it. However, dehydration occurs during pyrolysis, which may affect the acidic functional groups on hydrochar. In addition, biochars have recently been shown to possess significant electron storage capacities (ESC), but it was unknown whether pyrolyzed hydrochars also possess ESC and to what extent. In this article, the effect of pyrolysis temperature on acidic oxygen-containing functional groups and ESC of pyrolyzed hydrochars are evaluated. Hydrochars were prepared from cellulose and wood at 220 and 260 °C. These hydrochars were then pyrolyzed at 400, 500, and 600 °C under an N2 atmosphere. Afterward, the changes in functional groups were evaluated by ESC, BET analysis, ultimate analysis, ESC measurement, pH, pH at point of zero charge (pHPZC), Boehm titration, and FTIR analysis. The hydrochars showed relatively low surface areas mostly due to the lack of pores or pores clogged with volatiles. Surface area was increased by an order of magnitude after pyrolysis; however, acidic oxygen-containing functional groups decreased significantly with increasing pyrolysis temperature. ESC was also decreased with increasing pyrolysis temperature, ranging from 1.44 (cellulose at 600 °C) to 3.25 (wood at 400 °C) mmol/g. This result suggests a portion of the ESC of the pyrolyzed wood hydrochars originated from cellulose. A linear correlation between ESC and lactonic group was observed for pyrolyzed hydrochars. Keywords: Hydrothermal carbonization; Hydrochar; Pyrolysis; Boehm titration; Electron storage capacity; Acidic oxygen-containing functional groups


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Highlights Hydrochar produced at 260 °C was more thermally stable than that at 220 °C Polarity index of pyrolyzed hydrochar decreased with increase in pyrolysis temperature Pyrolysis at lower temperature yielded higher quantities of acidic oxygen-containing functional groups Lactonic group was positively correlated to electron storage capacity (ESC) A portion of the ESC of pyrolyzed wood hydrochars was derived from cellulosic components



Introduction In recent years, hydrochar, the solid product from hydrothermal carbonization (HTC), has been proposed for various applications including solid fuel, soil amendment, adsorbent, carbon sequestration, nutrient recovery, and water retention.1-6 Especially, hydrochar as a low-cost adsorbent is attracting more attention for targeted contaminants (e.g., heavy metals, triclosan, tetracycline, etc.).4, 7-8 Despite relatively lower surface area compared to activated carbon or biochar, hydrochar has proven effective to adsorb chemical contaminants from water due to its abundant oxygen-containing surface functional groups.9-11 Now, the oxygen-containing functional groups are measured per unit surface area of the hydrochar. Therefore, a larger surface area of hydrochar might lead to increase sorption sites for targeted contaminants. Pyrolysis, similar to HTC, is another thermochemical treatment, where dry biomass is treated at around 400-800 °C under an inert atmosphere and ambient pressure.12 Biochar produced through pyrolysis is carbon-rich and has higher surface area than hydrochar.13-14 High surface area and corresponding surface chemistry are the main consideration for biochar’s beneficial applications as soil amendment, carbon sequestration, adsorbent, etc.15 Moreover, surface functionality of biochar plays an important role in its redox properties (e.g., electron exchange or storage capacity (EEC or ESC).16-19 ESC is the amount of electrons that can be stored within a unit mass of char through chemical redox reactions with a reductant or an electron transfer mediator.18-21 It is important to quantify ESC for optimal design of biocharbased treatment processes involving redox transformation of contaminants.22 Klüpfel et al. and Emmerich et al. proposed that quinone functional groups are primarily responsible for the ESC of biochar.18, 23


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Pyrolysis process promotes volatiles removal from lignocellulose structure and increases surface area.13 As a result, pyrolysis of hydrochar may allow to expose maximum functional group on the surface. To date, it is a challenge to obtain the surface functionality of pyrolyzed hydrochar. It has been recently reported that HTC maximizes the surface functionality on hydrochar.24 On the other hand, pyrolysis increases the surface area of the chars. So far, few studies have investigated the ESC and oxygen-containing functional groups of pyrolyzed hydrochar. Liu et al. found that pyrolysis of hydrochar make them more effective as an adsorbent.25 Saquing et al. found that biochar ESC could be a source of reversible electron donor and acceptor for microbial transformation.19 Since the carboxylic, lactonic groups are available on the hydrochar 24 and upon their exposure due to pyrolysis, the ESC of the pyrolyzed hydrochar might be different than corresponding hydrochar. However, dehydration is one of the major reactions of pyrolysis,26 which leads to the loss of the oxygen from char. Pyrolysis of hydrochar, especially at higher temperatures might remove oxygen-containing functional groups. For instance, Chen et al. reported that, C=O bonds were diminished from biochar, when the pyrolysis temperature maintained between 500 - 700 °C.12 To the authors’ knowledge, the effect of pyrolysis on oxygen-containing functional groups of hydrochar has not been extensively studied. A lack of understanding of the types and abundance of oxygen-containing functional groups of pyrolyzed hydrochar and their contributions to ESC prevents one from designing an effective adsorbent from biomass. Therefore, the main objectives of this study were to determine the effect of pyrolysis temperature on the pH, pH at which the surface is neutral (pHPZC), surface area, acidic oxygencontaining functional groups and ESC of pyrolyzed hydrochars. In order to understand the phenomena, a model biopolymer (cellulose) and a biomass (wood) were used for HTC and 5 ACS Paragon Plus Environment


subsequent pyrolysis at various temperatures. ESC of various pyrolyzed hydrochars were correlated with acidic functional groups. Studying the ESC and oxygen-containing functional groups of pyrolyzed hydorchars allowed us to better understand how they vary with pyrolysis temperature.

Materials and methods Materials Microcrystalline cellulose (extra pure, avg. particle size 90 µm) was purchased from Acros Organics (Fair Lawn, NJ). Meanwhile, debarked hardwood (Prunus avium) was harvested from Ohio University Ridges Land Lab (Athens, OH). Wood was chopped with a power saw to make particle size less than 5 mm. The initial moisture contents of cellulose and wood were measured as 3.9±0.1 and 4.9±1.4%, respectively. In order to perform Boehm titration, 0.01 N HCl and 0.01 N NaOH were purchased from Fisher Scientific. Meanwhile, 0.1 M NaHCO3 and 0.1 M Na2CO3 were purchased from Sigma Aldrich. To maintain the same concentration of all acid and bases, NaHCO3 and Na2CO3 were diluted 10 times from their original concentration. For ESC measurements, titanium(III) chloride (20% w/v, in 2 N HCl) and sodium citrate (99%) were acquired from Acros Organics (Morris Plains, NJ) and used as received. Each pyrolyzed hydrochar sample (detailed below) was individually ground and sieved, and the

Effect of Pyrolysis Temperature on Acidic Oxygen-Containing

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