Tin-Functionalized Wood Biochar as a Sustainable Solid Catalyst for

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Tin-Functionalized Wood Biochar as a Sustainable Solid Catalyst for Glucose Isomerization in Biorefinery Xiao Yang,†,‡ Iris K. M. Yu,† Dong-Wan Cho,† Season S. Chen,† Daniel C. W. Tsang,*,† Jin Shang,§ Alex C. K. Yip,∥ Lei Wang,† and Yong Sik Ok*,‡

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†

Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China ‡ Korea Biochar Research Center, O-Jeong Eco-Resilience Institute (OJERI) & Division of Environmental Science and Ecological Engineering, Korea University, Seoul 02841, Republic of Korea § Joint Laboratory for Energy and Environmental Catalysis, School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, P. R. China ∥ Energy and Environmental Catalysis Group, Department of Chemical and Process Engineering, University of Canterbury, Christchurch 8041, New Zealand S Supporting Information *

ABSTRACT: This study tailored a novel engineered biochar as a solid catalyst for glucose isomerization by pyrolyzing Sn-functionalized wood waste under varying hypothesis-driven selected conditions (i.e., 650, 750, and 850 °C in N2 and CO2 atmosphere). The results showed that properties of biochar support (e.g., porosity and acid/base property) and chemical speciation of Sn were highly related to their catalytic performance. Variations in pyrolysis temperature and feed gas modified the porous structure and surface functionality of biochar as well as the valence state of doped Sn on the biochar. For the N2 biochars, higher pyrolysis temperature enhanced the fructose yield yet had trivial effect on the selectivity, where 12.1 mol % fructose can be obtained at 150 °C and 20 min using biochar produced at 850 °C. This was plausibly attributed to the increased fraction of amorphous Sn structures and metallic Sn that were more reactive than its oxide form. At the pyrolysis temperature of 750 °C, the use of CO2 increased the surface area by 40%, enlarged the pore volume from 0.062 to 0.107 cm3 g−1, and enriched the amorphous Sn structures compared to those for N2 biochar. This probably accounted for the better catalytic performance of CO2 biochar than that of N2 biochar (∼50% and 100% enhancement in fructose yield and selectivity, respectively). The Sn-biochar catalysts may have promoted glucose isomerization via both the Lewis acid and Brønsted base pathways. This study paves a new way to design biochar as a sustainable and low-cost solid catalyst for biorefinery applications. KEYWORDS: Engineered biochar, Waste valorization/recycling, Biobased value-added chemicals, Sugar conversion, Lignocellulosic biomass, Lewis acid

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INTRODUCTION Sugar isomerization is known as one of the most important tactics in the catalytic bioeconomy chain.1 For example, the isomerization of glucose into fructose not only plays a role in high-fructose corn syrups production,2 but serves as a thermodynamically favorable pathway in the synthesis of 5hydroxymethyl furfural (HMF)3,4 and levulinic acid (LA).5 These building blocks produced by biomass valorization can be employed as renewable alternatives of fossil fuel derivatives as well as feedstocks in the industrial production of fragrances, food additives, and resins,6−9 which relieve our heavy reliance on conventional petrochemical resources. The increasing use of abundant and renewable biomass resources (approximately 220 billion tons of dry biomass a year, which is equal to 4500 EJ of energy content)10 is driven by both economic incentives and environmental benefits.7,11−13 © XXXX American Chemical Society

The glucose isomerization is an endothermic (ΔH = 3 kJ/ mol) and reversible reaction (Keq ≈ 1 at 298 K) and the use of biological or chemical catalysts is necessary to accelerate isomerization.14 Although the enzymatic process is recognized as an effective method that can achieve 42 wt % fructose yield,15 it incurs high cost owing to the demand for purified feeding material, narrow operating conditions (pH and temperature), and irreversible deactivation of spent enzymes.16 In comparison, chemical catalysts show the advantages of significantly shorter reaction time and greater endurance to impurities. Glucose isomerization can be catalyzed by Lewis acids (i.e., electron pair acceptor such as Sn4+ and Al3+) via Received: October 15, 2018 Revised: February 7, 2019 Published: February 12, 2019 A

DOI: 10.1021/acssuschemeng.8b05311 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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intramolecular hydride shift from C-2 to C-1 or by Brønsted bases (i.e., OH−) via hydrogen transfer from O-2 to O-1.3 Homogeneous catalysts provide good catalytic performance,17 but often raise concerns about equipment corrosion as well as downstream product separation and solvent treatment, which has promoted the development of heterogeneous solid catalysts for biomass conversion.18,19 Recent studies have suggested that the incorporation of Lewis acid metal species into porous supporting materials (e.g., zeolite, silica−alumina composite, metal−organic framework, and covalent organic framework)20,21 can manufacture high-performance solid catalysts for isomerizing glucose to fructose.22 However, chemical-/energy-demanding synthesis conditions and use of unsustainable chemicals and organic solvents may restrain the large-scale application of these advanced solid catalysts. Biochar is a low-cost pyrolytic solid produced from waste biomass, which possesses tunable pore structure and surface chemistry. These advantages render biochar attractive in a variety of environmental applications such as soil remediation, wastewater treatment, energy production (gasification), and storage, as well as CO2 capture.23−25 Recent research demonstrated that biochar-based materials could serve as catalysts for transforming biomaterials to high-value products, including esterification/transesterification of waste oils for biodiesel production,26 tar cracking for improved syngas production,27 and biomass hydrolysis for sugar and biofuel synthesis.12 Hence, there is a high potential for biochar serving as a sustainable and economical solid support of catalysts for biorefineries as an emerging and value-added application. The major challenge in designing a high-performance biochar-based catalyst is to increase the selectivity toward useful products, by accelerating the desired reaction pathways and suppressing the side reactions. While soluble Sn4+ catalyst provides Lewis acid sites for catalytic glucose isomerization, it is uncertain that if the Lewis acid sites remain active when Sn is immobilized on a biochar support. The forms of impregnated metal species are contended to play an important role in determining the active sites,28 which may vary with the pyrolysis conditions such as temperature and purge gas. In comparison to inorganic materials such as zeolite, biochar as an organic carbon support is more reactive.26 Therefore, more chemical interactions between biochar and metal can be expected, which may also alter the speciation of the impregnated metals. As biochar by itself contains alkali and alkaline earth metals that are naturally present in waste biomass, we hypothesize that base sites may appear in Snfunctionalized biochar composite, promoting glucose isomerization via the base-driven pathway. In addition, the physical properties of biochar-based catalysts such as porosity and surface area are critical to their performance.29 This work aims to synthesize Sn-functionalized biochar (SnBC) catalysts using a facile and simple impregnation method. A series of Sn-BCs with distinctive physicochemical properties were produced under different conditions (pyrolytic temperature of 650−850 °C in N2 or CO2 atmosphere). The Sn-BCs were evaluated as a solid catalyst for the conversion of glucose to fructose in water, to investigate the correlation between the physicochemical properties of Sn-BCs and their catalytic activity. The reusability of Sn-BCs was also examined. This work elucidates the critical properties of biochar-based catalyst in determining their catalytic performance, which can advance the development of engineered biochars for sustainable biorefineries in a wide context.

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EXPERIMENTAL SECTION

Sample Collection and Chemical Agents. Mixed waste wood biomass generated from the Industrial Centre, the Hong Kong Polytechnic University, was employed as initial feedstock for biochar production. The wood waste was first pulverized into 2 mm size and dried at 60 °C before use. The SnCl4·5H2O (98%, Sigma-Aldrich) was used as the impregnating agent for Sn-BC synthesis. Pure glucose (≥99.5%, Sigma-Aldrich, St. Louis, MO, U.S.A.) was used as the substrate in the evaluation of catalytic performance of the biochars. Model compounds including glucose, fructose (≥99%, Wako, Tokyo, Japan), cellobiose (≥98%, Alfa Aesar, Ward Hill, MA, U.S.A.), maltose monohydrate (≥98%, Wako), HMF (≥99%, Sigma-Aldrich), levoglucosan (Fluorochem, Hadfield, UK), levulinic acid (LA; 98%, Alfa Aesar), formic acid (FA; 98%, Alfa Aesar) and furfural (FF; 99%, Sigma-Aldrich) were used for instrument calibration based on the reported protocol.30 All the chemicals were used without any further purification. Synthesis of Sn-Functionalized Biochar. Wood waste (WW) was soaked in a Sn aqueous solution (prepared from SnCl4·5H2O) at a solid-to-liquid mass ratio of 1:5, with the Sn dosage equivalent to 20 wt % of biomass. The mixture was continuously agitated for 1 h, followed by drying at 105 °C in an air-flowing oven until achieving constant mass. The resulting dry solid (i.e., SnCl4-treated WW) was pyrolyzed using a horizontal tubular furnace. Three types of biochars produced at 650, 750, and 850 °C under N2 purging were denoted as Sn-B650N, Sn-B750N, and Sn-B850N, respectively. The temperature of 750 °C was selected to produce Sn-BC in CO2 environment (i.e., Sn-B750C) because Boudouard reaction is feasible at ≥710 °C, enabling the CO2-promoted development of porosity and syngas enrichment.31,32 The temperature of 850 °C in CO2 was excluded to avoid the complete devolatilization of C (i.e., no black carbon but ashlike residue was observed in the solid product in preliminary experiments). The heating rate was set at 10 °C min−1, and the designated temperature was held for 2 h with the gas flow rate maintained at 150 mL min−1. After pyrolysis, the reaction zone was naturally cooled down to ambient temperature with continuous gas purging. All Sn-BCs were stored in a desiccator before subsequent use. Characterization of Sn-Functionalized Biochar. Thermogravimetric analysis (TGA, Rigaku Thermo plus EVO2) was performed to investigate the weight loss of SnCl4-treated WW and Sn-BCs with temperature increasing from 100 to 1000 °C at a ramp rate of 10 °C min−1 in Ar environment. Surface morphology, pore structure, and surface elemental distribution were analyzed using a scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEMEDX, TESCAN VEGA3 XM). The crystalline structure of pyrolyzed products was characterized by an X-ray diffractometer (XRD, Rigaku SmartLab) with a scanning zone ranged from 20° to 70° at 45 kV and 200 mA. The Brunauer−Emmett−Teller (BET) surface area and porosity (pore size, pore volume) of Sn-biochars were measured using a gas adsorption−desorption analyzer (Micromeritics, Gemini VII) using N2 at 77 K. Before the measurement, the biochars were subjected to degassing under N2 purging at 200 °C for 6 h. Acid−base properties were determined using an autotitrator (Mettler, Easy Plus) based on a simplified Boehm titration.33,34 Typically, 0.1 g of the sample was suspended in 25 mL of 0.025 M sodium hydroxide or 0.025 M hydrochloric acid. The mixture was placed in a shaker for 24 h at 200 rpm. The liquid phase was then collected via filtration and titrated against 0.025 M HCl and NaOH, respectively. The number of base sites (proton acceptor) was calculated based on the assumptions that HCl neutralizes the basic fraction of biochar. Similarly, the number of acid sites (proton donor) was calculated from the amount of NaOH consumed. The inductively coupled plasma mass spectrometry (ICP−MS, SPECTROBLUE FMX36) was used to quantify the doped Sn in each sample, which was completely digested using US EPA Method 3051A.35 Catalytic Conversion Tests and Products Identification and Quantification. The catalytic conversion of glucose was carried out in a 100 mL microwave autoclave with the Ethos UP Microwave B


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ACS Sustainable Chemistry & Engineering Reactor (Milestone, maximum power 1900 W). The microwaveassisted system was operated in the same manner as in our previous work.36,37 A temperature probe was inserted into one of the reaction vessels during each run of experiment, which enables the programmed control of the reaction temperature. Typically, 0.25 g biochar catalyst was added into the 10 mL solution of 5 wt/v % glucose (substrate) in deionized water (reaction medium). A 5 min magnetic agitation was conducted to make a homogeneous mixture before reaction. The reaction mixture was then subjected to microwave radiation at a temperature range of 140−160 °C for 1−60 min. After conversion, 40 min of air-flow ventilation was carried out to cool the reactor to ambient temperature. The reusability of Sn-BC was examined at 160 °C for 20 min following the above-mentioned protocol. After reaction, the solid catalyst was separated from the mixture by filtration, followed by washing with distilled water for four times to remove water-soluble substances. The washed solid was oven-dried at 105 °C overnight, giving the recycled solid catalyst that was directly used in the next run of catalytic conversion of glucose without regeneration. For the product analysis, the collected liquid phase was diluted and filtered through a 0.22-μm membrane. The prepared liquid samples were injected into a high-performance liquid chromatography (HPLC, Hitachi) equipped with an Aminex HPX-87H column (Biorad). The mobile phase was 0.01 M H2SO4 at a flow rate of 0.5 mL min−1 at 50 °C. The yield and selectivity of each product were calculated based on the carbon number.38 The unit Cmol % allows for the estimation of carbon efficiency and carbon balance, which are important consideration in devising high-performance catalytic systems.

yield p (Cmol%) =

C p × Vol × Np/M p Gi × Vol × NG/MG

fructose selectivity (Cmol%) =

× 100%

(1)

Cf × Vol × Nf /M f × 100% (Gi − Gf ) × Vol × NG/MG

(2) where Cp and Cf (mg mL−1) denotes the concentrations of the product and fructose after the reaction; Np, Nf, and NG represent the numbers of C in the product, fructose, and glucose (substrate); Gi and Gf (mg mL−1) are the concentrations of the glucose at the initial and final stage; Mp, Mf, and MG refer to the molecular weights of the product, fructose, and glucose; and Vol refers to the volume of the reaction mixture, i.e., 10 mL, respectively.

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RESULTS AND DISCUSSION Thermogravimetric Analysis. Different stages of thermal decomposition of SnCl4-treated WW can be described by its TGA curve (Figure 1a), which could illustrate the thermalinduced changes during Sn-BC synthesis. The mass decay at 650 °C, because most of the thermally susceptible components were lost during the synthesis of Sn-BCs. For Sn-BCs produced under N2, approximately 74−82 wt % residues remained at 1000 °C, which may comprise graphitic carbon and noncombustible metal salts. Increasing pyrolysis temperature would result in more solid residues of biochars in

Figure 1. TG, DTG, and DTA curves of SnCl4-treated WW (a) and TG and DTG patterns of Sn-BCs (b,c).

the TGA analysis. The Sn-BC pyrolyzed in CO2 (i.e., SnB750C) gave the least residue at 1000 °C among the samples (67 wt %), suggesting the presence of more thermally unstable moieties in the biochar. Physicochemical Property of Sn-BCs. The SEM images show that the ordered and vascular structure of raw WW (Figure 2a,b) collapsed after the SnCl4 treatment before pyrolysis, producing particles ≤50 μm (Figure 2c,d). This finding is contradictory to our recent study on Al-impregnated biochars, which reported smaller changes in the vascular structures (i.e., particle >100 μm) even after pyrolysis at 500− 700 °C.28 Such comparison suggests that the selection of metal precursor plays a more important role than the pyrolysis temperature in causing the collapse of biomass structure. As the hydrolysis constant of Sn4+ (pK1 = −1.6) is much higher C


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Figure 2. SEM-EDX results of WW at magnification of 1500 and 5000 (a,b), SnCl4-treated WW (c,d), Sn-B650N (e,f), Sn-B750N (g,h), SnB850N (i,j), and Sn-B750C (k,l).

than that of Al3+ (pK1 = ∼5),40,41 more acidic environment can be created when SnCl4 undergoes partial hydrolysis releasing corrosive hydrogen chloride during the pretreatment. It has been demonstrated that mineral acids (e.g., H2SO4 and H3PO4) would partly disrupt the fibrous ridged structure of wood-based materials, altering the surface porosity and morphological feature.29,38 The EDX results validated the successful impregnation of Sn element onto the surface of SnCl4-treated WW (Figure 2d). After pyrolysis in N2 at 650−850 °C, the presence of Sn was observed in two major forms, i.e., the Sn species attached to biomass structures (72−87% C and 1.7−14% Sn) as well as the Sn-rich spherical particles (7.6−10% C and 83% Sn) (Figure 2e−j). The abundance and size of the latter increased with the increasing pyrolysis temperature (i.e., ≤ 5 μm at 650 °C, 5−8 μm at 750 °C, and up to 20 μm at 850 °C). It is speculated that the progressive biomass devolatilization with increasing temperature results in the partial loss of carbon support and thereby the aggregation of Sn into spherical structures. There was a low oxygen content of 0−7.3 wt % in Sn-B750N and Sn-B850N (Figure 2h,j) implying the presence of metallic Sn, which was confirmed by the XRD patterns (Figure 3). In contrast, a mixture of SnO2 and Sn crystals was formed in Sn-B650N. It has also been reported that solid− solid and solid−gas carbothermal reduction of SnO2 was feasible at 700 °C or above in Ar environment:42

Figure 3. XRD patterns of the synthesized Sn-BCs and SnCl4-treated WW.

was disrupted at higher temperatures to give amorphous structures that cannot be detected by XRD. The differential thermal analysis (DTA) of SnCl4-treated WW shows an endothermic peak starting from ∼500 °C where the mass change was relatively minor (Figure 1a), suggesting the possible occurrence of phase transition that involved bondbreaking reactions without significant mass loss. A drop in the crystallinity of Si−Sn−C ternary alloy has been similarly observed as the heating temperature reached 700−800 °C depending on the elemental proportion in a recent study.43 Further investigations are needed to trace the thermal stability of SnO2 in the biochar matrix under the pyrolysis conditions. The formation of Sn-rich components was also observed in the CO2 biochar (i.e., Sn-B750C) according to the SEM-EDX

SnO2 + C → Sn + CO2 ; SnO2 + 2C → Sn + 2CO (3)

SnO2 + 2CO → Sn + 2CO2

(4)

In the current study, the carbon from WW could have reduced the SnO2 that was formed at the initial stage of pyrolysis to its metallic form, as the pyrolysis temperature increased to 750− 850 °C. Another possibility is that the crystalline order of SnO2 D


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ACS Sustainable Chemistry & Engineering Table 1. Surface Properties of Synthesized Sn-BCs

Sn-B650N Sn-B750N Sn-B850N Sn-B750C

base site density

acid site density

BET surfaces area

t-plot micropore area

t-plot external surface area

pore volume

average pore size

mmol g−1

mmol g−1

m2 g−1

m2 g−1

m2 g−1

cm3 g−1

nm

0.300 0.365 0.550 0.715

0.615 0.465 0.685 0.700

375.1 438.6 360.3 614.8

303.0 346.1 285.7 451.9

72.1 92.5 74.6 163.0

0.047 0.062 0.054 0.107

2.67 2.82 3.10 2.55

Figure 4. Product yields (a) and fructose selectivity (b) in the conversion of glucose at 140−160 °C for 20 min over Sn-BC catalyst produced at 650, 750, and 850 °C under N2.

the microporous area, indicating that biomass devolatilization was excessive at 850 °C causing the collapse of micropores.39 Compared to Sn-B750N, Sn-B750C showed a higher BET SA (615 m2 g−1) and larger pore volume (0.107 cm3 g−1), i.e., increases by 40.2% and 72.5%, respectively. The use of CO2 promoted the formation of both micropores (452 m2 g−1) and meso/macro-pores (163 m2 g−1), which may enhance the site accessibility of biochar-based catalysts. Effect of Pyrolysis Temperature on the Catalytic Performance of Sn-BCs. The synthesized Sn-BCs were examined as the solid catalyst in glucose conversion. The fructose yield generally increased with the increasing pyrolysis temperature as well as the catalytic reaction temperature (Figure 4a). For instance, the fructose yield for Sn-750N at 160 °C was approximately three times higher than that at 140 °C, while Sn-B850N gave fructose of 10.6−12.1 Cmol % at 150−160 °C which was notably higher than 5.4 Cmol % at 140 °C (20 min). However, a small amount of secondary products such as LA, FA, and FF was also produced over Sn-B750N and Sn-B850N at 150 and 160 °C, suggesting the promotion of side reactions (e.g., rehydration) by high reaction temperatures. Changing the catalysts and reaction temperature exerted marginal impact on the fructose selectivity (Figure 4b). The product profiles of glucose conversion over Sn-B650/ 750/850N for 60 min are illustrated in Figure 5. The three kinetic patterns were similar, where fructose was produced at the expense of glucose during the first 10 min of reaction, and the fructose yield reached an equilibrium at ∼20 min. However, the decrease of glucose content during the initial stage of reaction was the steepest for Sn-B850N, followed by Sn-B750N, and then Sn-B650N. This observation reinforces the increasing catalytic activity of Sn-BCs with the pyrolysis temperature. The carbon balance in the catalytic systems (160 °C, 20 min) followed the descending order: Sn-B650N (81.3 Cmol %) > Sn-B750N (59 Cmol %) > Sn-B850N (45.3 Cmol %), suggesting that Sn-BCs with a higher catalytic activity also promoted side reactions generating undetected byproducts.

results (Figure 2k,l). However, their oxygen content (27.6 wt %) was significantly higher than that of Sn-B750N (0−11 wt %). The XRD results confirmed a higher proportion of SnO2 to metallic Sn in Sn-B750C (Figure 3). The presence of CO2 may have suppressed the reduction of SnO2 to metallic Sn by serving as an oxidizing agent in the O-containing environment, although the enriched CO content in the syngas can serve as a reductant (eq 4). These results highlight that by changing the pyrolysis temperature and purge gas we can tune the form of impregnated metal species, besides the porous structure of biochar support. In the case of Sn-BCs, the formation of metallic Sn was favored by high pyrolysis temperature and suppressed by CO2 purging. The acid−base properties of the Sn-BCs are shown in Table 1. Their total acid site density ranged from 0.465 to 0.7 mmol g−1, which may be derived from the acidic surface functional groups such as carboxyl and hydroxyl groups.12,29 The base sites (0.3−0.715 mmol g−1) may be attributed to the alkaline earth metals naturally present in WW. For the N2 biochars, both acidity and basicity increased with the increase of pyrolysis temperature. The CO2-pyrolyzed Sn-B750C had the highest acid and base site density of 0.7 and 0.715 mmol g−1, respectively, among the samples. When pyrolysis was performed in CO2 at 750 °C, the Boudouard reaction (CO2 + C(s) → 2CO; thermodynamically feasible at >710 °C) could occur and intensify the thermal degradation of biomass.31 This may increase the relative content of the naturally occurring alkaline minerals. According to the porosity analysis, the BET surface area (SA) increased from 375 m2 g−1 for Sn-B650N to 439 m2 g−1 for Sn-B750N (Table 1). The external surface area and average pore size also increased from 72 to 93 m2 g−1 and 2.67 to 2.82 nm, respectively, suggesting the more significant formation of mesopores (2−50 nm) at 750 °C. The increase in pyrolysis temperature is known to promote biomass decomposition and improve porosity development.44 However, Sn-B850N showed a decrease in BET SA to 360 m2 g−1 along with a decrease in E


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BCs. Therefore, we uphold the speculation that the content of amorphous Sn species increased at higher pyrolysis temperatures, which were probably conducive to the improved glucose conversion (Figures 4 and 5). This speculation is in good agreement with the latest study on glucose conversion using Sn-based composite catalysts that comprised amorphous components.46 As for the metallic Sn existing in the effective Sn-B750N and Sn-B850N, further investigation would be needed to evaluate its catalytic activity for glucose isomerization, which has not been reported in the literature. It is plausible that Sn as an elemental metal is more reactive than its oxide forms and partially leaches as soluble Sn2+ (with a stronger Lewis acidity than Sn4+) and Sn4+ to the liquid phase via the reactions: Sn + SnCl4 → 2SnCl2, 6Sn2+ + O2 + 4HCl → 2SnCl4 + 4Sn(OH)Cl,47 facilitating glucose conversion in the reaction medium. The investigation of glucose isomerization over Sn-modified biochar catalysts is reported for the first time in this work. The fructose yield of 12.1−15.2 Cmol % obtained over Sn-B850N or Sn-B750C (150 °C, 20 min) is comparable with that of morpholine as a homogeneous base catalyst (∼17% fructose),48 although it is lower than that of triethylamine and a few costly solid catalysts such as zeolite- and hydrotalcite-based catalysts (∼30%).1,16,49 Using biomass waste as a low-cost feedstock via fast and simple preparation process makes the biochar-based catalyst a sustainable alternative to the traditional heterogeneous catalysts. This study articulates the linkages among the pyrolysis conditions, physicochemical properties, and catalytic performance, providing new insights for the future development of highperformance biochar-based catalysts. Effect of Pyrolysis Atmosphere on the Catalytic Performance of Sn-BCs. Figure 6 illustrates the product

Figure 5. Product yields in the conversion of glucose at 160 °C as a function of reaction time over Sn-BC catalyst produced at 650, 750, and 850 °C under N2.

The catalytic performance of the N2-pyrolyzed Sn-BCs may be associated with their average pore size in the descending order: Sn-B850N > Sn-B750N > Sn-B650N (Table 1). A larger pore size could have promoted the diffusion of glucose and products for enhancing the glucose conversion.28 Yet there is no clear trend of fructose yield as a function of SA (Table 1), suggesting SA as an insignificant factor in the current study. The chemical properties may also contribute to the catalytic performance, because the least active Sn-B650N contains a mixture of SnO2 and metallic Sn, in contrast to Sn-B750N and Sn-B850N where metallic Sn and/or amorphous Sn species are abundant (Figure 3). These results highlight the unfavorable presence of SnO2, which corroborates our recent findings that SnO2 in conventional and green solvents was inactive toward catalytic glucose isomerization.45 As the pyrolysis temperature increased, SnO2 may have been reduced to metallic Sn by carbothermal reduction and/or transformed into amorphous structures that were undetectable in the XRD analysis (Section 3.2). It has been suggested that Al-impregnated biochars contained Lewis acidity in amorphous structures, which gave a higher fructose yield of 21.5 Cmol % compared to the Sn-BCs in this study.28 This points to the importance of the crystallinity to the catalytic activity of Sn-

Figure 6. Product yields and fructose selectivity in the conversion of glucose at 140−160 °C for 20 min over Sn-BC catalyst produced at 750 °C under N2 and CO2.

yields and fructose selectivity over the Sn-BC catalysts produced at 750 °C in N2 and CO2 environment. It is interesting to note that Sn-B750C presented better catalytic performance in terms of the fructose yield of 14.7−15.2 Cmol % compared to its N2-pyrolyzed counterpart (i.e., 3.1−10.8 Cmol % for Sn-B750N) at the studied temperatures (140−160 °C, 20 min), suggesting that CO2 atmosphere facilitates the production of active Sn-BCs. However, Sn-B750C is dominated by SnO2 according to the XRD pattern, in contrast to Sn-B750N that possesses metallic Sn as the major species F


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ACS Sustainable Chemistry & Engineering (Figure 3). This comparison suggests that the elemental Sn crystals may be the secondary contributor to a high catalytic activity. It should be noted that Sn-B750C is more amorphous than Sn-B750N in view of the peak broadening in the XRD pattern of the former, probably because more aggressive biomass degradation by the Boudouard reaction in CO2 environment reduces the crystallinity of Sn structures. This further substantiates our postulation that the amorphous Sn species can play a more important role than the crystalline fraction in the catalytic glucose isomerization (Section 3.3.1). The enrichment of both micropores and meso/macro-pores of Sn-B750C in comparison to Sn-B750N (Table 1) could be favorable for the improved accessibility of active Sn sites on the biochar-based catalyst. It is known that the micropore imparts stronger adsorption potential,50 while the meso/macro-pores promote faster molecular diffusion of adsorbate and products. The well-established hierarchical porous surface can enhance the contact between the solid catalyst and dissolved reactants, and hence potentially improve the electron transfer between the entities.51 Moreover, it is noteworthy that fructose selectivity of Sn-B750C was two times higher than that of Sn-B750N (140 °C, 20 min) (Figure 6). The improved mass transfer may have mitigated the retention of sugar products at the active sites of Sn-BC, which helps to suppress the side reactions due to excessive catalytic activity. The finding that Sn-B750C has the base site density two-fold higher than that of Sn-B750N appears as another possible reason for the better performance of the former (Table 1). In particular, the base site density of Sn-B750C (0.715 mmol g−1) is comparable to that of the effective base catalysts reported in the literature,52 suggesting that the Brønsted base-driven pathway of glucose-fructose isomerization may also be feasible over Sn-B750C. Therefore, the synergy between Lewis acid sites and Brønsted base sites as well as the favorable site accessibility renders Sn-B750C the most active among the prepared biochar samples. A small amount of HMF (70% Sn retained in the catalyst. Therefore, activating the G


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Figure 8. Thermogravimetric analysis (a,b) and XRD pattern (c,d) of Sn-B750N and Sn-B750C and their reused samples, i.e., RSn-B750N and RSn-B750C, respectively, after three cycles of reaction.

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spent Sn and removing the humins deposit would be vital for regeneration of Sn-functionalized biochar catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05311.

CONCLUSIONS

We proposed to valorize wood waste through a fast and simple preparation process to produce a new biochar-based catalyst as a low-cost and sustainable alternative to the traditional heterogeneous catalysts in biorefinery. We fabricated different types of Sn-BCs under hypothesis-driven design of pyrolytic temperatures and N2/CO2 atmosphere to elucidate the linkages among the pyrolysis conditions, physicochemical properties, and catalytic performance. In an N2 environment, increasing the pyrolysis temperature resulted in the formation of more reactive metallic Sn phase and increased the fraction of amorphous Sn structures, which were found to be conducive to higher catalytic activity for the isomerization of glucose to fructose. Compared to N2 biochars, the use of CO2 created larger porosity, enriched surface reactivity, and mediated structure crystallinity, which further improved the catalytic activity of Sn-BCs. Sn-750C gave the highest yield (15.2 Cmol %) and selectivity (29 Cmol %) of fructose at 160 °C in 20 min. Both Lewis acid and Brønsted base were present in SnBCs to simultaneously catalyze glucose conversion. The reusability test revealed that active site leaching and potential pore clogging by humins and Sn precipitates compromised the catalytic function of Sn-BCs but the performance remained stable in the second and third cycle. This study devises Sn-BCs for catalytic glucose isomerization for the first time, and more importantly, provides new fundamental insights for fostering the tunable design of engineered biochar as sustainable catalysts in emerging applications for circular bioeconomy.

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The Sn content in pristine and reused catalysts; SEM images of reused catalysts; and N2 isotherms of pristine and reused catalysts (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.C.W.T.). * E-mail: [email protected] (Y.S.O.). ORCID

Daniel C. W. Tsang: 0000-0002-6850-733X Jin Shang: 0000-0001-5165-0466 Alex C. K. Yip: 0000-0003-4042-7589 Yong Sik Ok: 0000-0003-3401-0912 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Hong Kong Research Grants Council (PolyU 15217818) and Hong Kong Environment and Conservation Fund (K-ZB78, 2016).

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