Subcritical Aqueous Phase Reforming of Wastepaper for Biocrude

Wei Nan , Anuradha R. Shende , Jordan Shannon , and Rajesh V. Shende ... Fares AlMomani , Moustafa Hussein Ali , Ujjal Ghosh , Shaheen AlMuhtaseb ...
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Subcritical Aqueous Phase Reforming of Wastepaper for Biocrude and H2 Generation Richa Tungal and Rajesh Shende* Department of Chemical and Biological Engineering, South Dakota School of Mines & Technology, Rapid City, South Dakota 57701-3995, United States S Supporting Information *

ABSTRACT: This paper reports subcritical aqueous phase re-forming of wastepaper in presence of homogeneous Ni(NO3)2 catalyst for biocrude and H2 production. Reforming of aqueous wastepaper slurry (15 g/150 mL) was performed using 5 wt % catalyst in high-temperature high-pressure 300 mL SS316 PARR reactor at 200−275 °C. During the progress of the reforming reaction, gas and liquid samples were withdrawn at regular time intervals and analyzed using gas chromatography (GC), gas chromatography mass spectrometry (GC-MS), total organic carbon (TOC) analyzer, and high-performance liquid chromatography (HPLC). At 200 °C, negligible liquefaction of wastepaper was noticed; however, liquefaction was found to increase with temperatures from 225−275 °C. At 250 °C, about 44 wt % biocrude and 3.8 mol % H2 were observed after 120 min of reaction time. The other gases observed in product gas were CO2, CO, and CH4 whereas the liquid phase (biocrude) was found to contain sugars (7.5 wt %), HMF/furfural (∼1 wt %), oxygenated hydrocarbons (42.4 wt %) and monocarboxylic acids (49.1 wt %) such as acetic, formic, propionic, and lactic (2-hydroxypropionic). The effect of Ni(NO3)2 on the production of carboxylic acids as well as H2 from the mixture of sugars (prepared as concentration ratio of the sugars observed during wastepaper reforming) was separately investigated with the viewpoint of developing understanding of possible reaction pathways.

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INTRODUCTION Biocrude and H2 can be produced from thermochemical processing of biomass using gasification,1−3 pyrolysis,4 fast pyrolysis,5 and aqueous phase reforming under subcritical6 and supercritical6,7 conditions. Among these processes, aqueous phase reforming or hydrothermal treatment has several advantages, which include lower energy consumption, direct utilization of biomass without any pretreatment or drying and lower tar and char formation, as compared with other thermochemical processes where significant biochar and tar are generally produced by polymerization of dehydrated products.8−10 The biocrude generated from different biomass feedstocks under hydrothermal conditions has shown lower oxygen content (10−20%) and higher heating value (30−36 MJ/kg) as compared with a biomass, which has higher oxygen content and lower heating value.11 In addition to biocrude, aqueous phase biomass reforming is also capable of generating high energy density fuel, H2 (143 MJ/kg). It is to be noted that under subcritical conditions, the dielectric constant of water is significantly lower, which allows solubilization of organic compounds whereas ionization constant is approximately 3 orders of magnitude higher than at ambient conditions providing acidic medium for the hydrolysis of biomass compounds.6 Consequently, subcritical water not only serves as a reaction medium but also acts as reactant leading to liquefaction of a biomass that proceeds through a series of chemical modifications involving depolymerization, solvolysis, and chemical and thermal decomposition of monomers into smaller molecules.12 This aspect of subcritical water processing of biomass is attractive as it encompasses several competing reaction pathways converting biomass to liquefied biocrude and gaseous fuels such as H2 and/hydrocarbons. The use of a © XXXX American Chemical Society

catalyst further influences the production of biocrude and gaseous fuels. Among different feedstocks investigated so far, the information on homogeneously catalyzed subcritical aqueous phase reforming of wastepaper for biocrude and H2 generation is missing in the literature. Worldwide, in 2005 about 3.6 × 108 tonnes of different variety of paper products were manufactured and in 2015 this number is expected to increase to 4.6 × 108 tonnes.13 In this digital age, the usage of paper products is still very significant leading to paper wastes and their disposable issues.14 Globally wastepaper management is a major issue involving higher disposal costs. Several investigators reported wastepaper management studies addressing life cycle assessment and cost benefits.15−17 Results of these studies indicate recycling of wastepaper is a better option causing less environmental impact as compared to landfilling and incineration. However, a preferred way of managing wastepaper disposal is via landfilling and/or incineration.18 One of the reasons for limited wastepaper recycling activity is the difficulty in manufacturing high quality paper products because of higher pulp fiber content.19 Contrasting to this, the wastepaper life cycle analysis indicates that it is ecologically preferable to utilize wastepaper for energy recovery via recycling.18,20 Recently, Wang et al.21 compared three wastepaper management options, which included bioethanol production, recycling, and incineration with energy recovery, and suggested environmental benefits from making bioethanol from wastepaper over other options. Few investigators suggest that the use of wastepaper (as a Received: December 26, 2012 Revised: April 17, 2013

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use of the H2 produced during aqueous phase reforming, which will improve the overall H2 efficiency of the process. In the present study, subcritical aqueous phase reforming of wastepaper was performed at 200−275 °C in presence of Ni(NO3)2 catalyst for the production of biocrude and H2. Our previous study on using NiSO4 showed higher H2 volume in product gas indicating improved selectivity toward H2.29 It was believed that the nitrate salts would further improve liquefaction because they have been reported to strongly interact with a biomass and influence its decomposition or pyrolysis behavior.37 Therefore, it was thought desirable to make use of Ni(NO3)2 under aqueous phase reforming conditions and investigate biocrude and H2 generation from wastepaper. In addition, this study includes identification of biocrude compounds and possible reaction pathways.

potential partial replacement for coal) for fuel production can reduce of CO2, NOx and SOx emissions.22 Based on these rationales, it was thought desirable to investigate the use of wastepaper for energy and fuel generation. Wastepaper can be a combination of newspaper, which is a lignocellulosic biomass containing cellulose (62%), hemicellulose (16%), and lignin (16%)23 and used office printing papers that consists of mainly cellulose (85−99%) and negligible (0.4%) lignin.24 The production of biocrude and H2 from wastepaper may be achieved using subcritical aqueous phase reforming. As wastepaper contains a significant fraction of cellulose, which is known to produce sugars under hydrothermal conditions,25 production of sugars can thus be expected during aqueous phase reforming of wastepaper. It is well documented in the literature that sugars dehydrogenate to H226,27 and produce a biocrude containing oxygenated hydrocarbons.28 Production of biocrude as well as H2 can be improved in presence of a suitable catalyst. Recently, we reported H2 production during subcritical aqueous phase reforming of cellulose, xylan, kraft lignin, pinewood and waste biomass in presence of 3−7 wt % homogeneous NiSO4 catalyst at 200−250 °C. The investigation revealed formation of H2 in product gas and sugars, furfural, HMF, and oxygenated hydrocarbons29 in liquefied biomass. The catalyst ions from the homogeneous slurry can be removed using the well-known ion-exchange technique; for instance, Amberlite IRC748 from Rohm and Haas is known to remove and recover metal ions from aqueous solutions. Although many biomass feedstocks have been processed by hydrothermal treatment, the use of metal salt catalysts for this purpose is rarely observed in the literature. Cheng et al.8 investigated switchgrass processing in subcritical water and reported H2 yields of 0.17 mol % at 250 °C and 0.9 mol % at 350 °C. The production of H2 was accompanied with CO, CO2, and CH4. Kumar and Gupta11 used K2CO3 for switchgrass processing at 235−260 °C and showed a maximum biocrude yield of 51.1 wt %. Muangrat et al.30 made use of alkali salts for biomass liquefaction and demonstrated H2 rich product gas generation. There is significant information available in literature on the use of heterogeneous catalysts for biomass processing: Raney nickel by Azadi et al.31 for the hydrothermal gasification of glucose, Pt/Al2O3 by Valenzuela et al.32 for aqueous phase reforming of woody biomass (pine sawdust, office wastepaper, D-glucose and ethylene glycol) at 225 °C, and Pt/Al2O3 by Shabaker et al.33,34 for aqueous phase reforming of biomass model compounds. These investigations are mainly focused on H2 production, which was believed to be routed via C−C and C−O cleavage, dehydrogenation, and water−gas shift reaction.32−35 In addition to H2 in product gas, the production of biocrude containing sugars, 5-hydroxomethylfurfural (HMF)/furfural, and organic intermediates have been reported during aqueous phase reforming of a biomass. Sasaki et al.25 studied hydrothermal cellulose processing at 320 °C and 25 MPa and reported 47% conversion into cellobiose, glucose, erythrose 1, 6-anhydroglucose, and HMF. Furthermore, it has also been shown that cellobiose decomposes to glucose in subcritical water and finally to organic acids such as formic, acetic, propionic, and lactic. The biocrude obtained can be further upgraded to higher hydrocarbons; for instance, acids can be reacted with alcohols for the production of biodiesel range hydrocarbons. Very recently, Xing et al.36 reported production of jet and diesel range hydrocarbons from waste hemicellulose derived aqueous solutions. Higher energy density hydrocarbons can be produced from the biocrude by making

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EXPERIMENTAL SECTION

Materials. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 99.9%) was purchased from Alfa Aesar. Sugars (cellobiose, xylobiose, glucose, xylose, and mannose) were bought from Sigma Aldrich. Sulfuric acid (ACS, Fisher Scientific) and ethyl acetate (99.5%, Acros Organics) used were of high-performance liquid chromatography (HPLC) grade. N2 and He from gas cylinders with the minimum stated purity of 99% and 99.99%, respectively, were purchased from Linweld Inc. Distilled water was used to prepare wastepaper slurry. The elemental analysis of the wastepaper is shown in Table 1.

Table 1. Elemental Analysis and Moisture Content of Wastepaper elements

dry weight percent of biomass (%)

C H N O moisture

35.81 5.25 85% confidence level) are summarized in Table 3. The major observed compounds are cyclic ketones and substituted cyclic ketones, quinone derivatives, phenols and substituted phenols, and aromatic alcohols. Among these compounds, 2-cyclopenten-1-one, 2-methyl-2-cyclopenten-1one, 1-methyl-1-cyclopenten-3-one, 3-hydroxy-2-methylpropene, 2-hydroxy-3-methyl-2-cyclopenten-1-one, 2,3-dimethyl2-cyclopenten-1-one, 2,4-dimethyl-1,3-cyclopentanedione, 3ethyl-2-hydroxy-2-cyclopenten-1-one, 2,3,4-trimethyl-2-cyclopenten-1-one, 1,2-benzenediol, and 2-methyl-p-hydroquinone showed larger peaks (>3% to 11%) and contributed 66.8% of the total peak area. The GC-MS analysis of the biocrude obtained after 30 min of reforming reaction at 250 °C was compared with that of the 225 °C, 90 min biocrude sample with the viewpoint of understanding the compounds generated at similar wastepaper to biocrude conversion of about 36%. The compounds observed in the biocrude samples are presented in Table 4. There are 15 oxygenated hydrocarbon compounds observed in the biocrude samples obtained at 225 °C, 90 min and 250 °C, 30 min contributing to 43.5% and 65.2%, respectively, to the total area. These values indicate that the amount of the common compounds are different in two biocrude samples observed at two different reaction temperatures and times although the wastepaper conversion into biocrude was similar to about 36%. The biocrude sample obtained at 225 °C, 90 min showed additional eleven compounds such as 1,2 butylene oxide, 4-methoxy-3-methyl1-butene, 1-(2-ethyl-2-oxiranyl)ethanone, 3,4-pentadienal, 3,5cyclohexadiene-1,2-diol, 1-(1-methylethyl)-cyclopentene, 2,2,4trimethyl-1-pentene, 1-hydroxy-2-methylbenzene, 3,4-dimethyl3-hexen-2-one, 3-ethyl-2-hydroxy-2-cyclopenten-1-one, and 3,5-cyclohexadiene-1,2-diol and contributed 56.5% to the total area. These compounds were not observed in the biocrude sample obtained at 250 °C after 30 min of reaction time. For the biocrude sample obtained at 250 °C, 30 min reaction time about 14 compounds (Table 4) were observed that contributed to 34.8% to the total area. Thus, from the observed product profiles of oxygenated hydrocarbons, it can be very well stated that the reaction temperature influenced the chemistries and reaction pathways. It is believed that the phenolic compounds might have been derived from lignin depolymerization40 whereas other oxygenated compounds might have been originated from cellulose and hemicelluloses fractions of wastepaper. Most of the hemicelluloses are dissolved in subcritical water at or near 180 °C. Suryawati et al.41 believe that the higher solubility of hemicelluose at about 200 °C is due to the presence of side groups such as pentoses, acetic acid, hexuronic acids, and deoxyhexoses. The existence of these side groups increases its solubility under hydrothermal conditions as compared with cellulose and further making it more susceptible for degradation. On the other hand, cellulose is joined together by glycosidic linkages holding together anhydroglucose molecules and constituting highly stable linear chains due to the presence of intra- and intermolecular hydrogen bonding

Table 3. Compounds Observed in Ethyl Acetate Biocrude Extract (>85% Confidence Level)a no.

retention time (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

6.7 7.2 7.5 8.2 8.9 11.2 11.7 11.8 13.1 14.8 15.2 15.5 17.5 17.7 19.0 20.1 20.4 21.1 21.7 22.3 22.6 22.9 23.1 24.5 24.8 26.6 27.3 28.1 29.0 29.1 29.4 32.0 32.4 32.9 33.6 35.5 36.5

38 39 40 41 42 43 44 45 46

39.1 38.1 38.2 38.9 41.7 43.4 46.5 47.8 48.5

compound

% area

2-cyclopenten-1-one 2-methylcyclopentan-1-one 3-methylcyclopentan-1-one 3-(2-oxiranyl)-1-propanol 3-methyl-2-cyclopenten-1-one 2-methyl-2-cyclopenten-1-one 1-(2-furanyl)ethanone dihydrofuran-2(3H)-one 2,5-hexanedione dihydro-5-methyl-2(3H)-furanone 4-methyldihydro-2(3H)-furanone 1-methyl-1-cyclopenten-3-one hydroxybenzene 3,4-dimethyl-2-cyclopenten-1-one 1-(2-furyl)-1-propanone 3,4-dimethyl-2-cyclopenten-1-one 3-hydroxy-2-methylpropene 2-hydroxy-3-methyl-2-cyclopenten-1-one 2,3-dimethyl-2-cyclopenten-1-one 3-hydroxy-2-methylpropene 3-methyl-2-cyclohexene-1-one 2,4-dimethyl-1,3-cyclopentanedione 3-methylphenol 3,4,5-trimethyl-2-cyclopenten-1-one 4-methylphenol 3,4-dimethyl-2cyclohexen-1-one 3,4-dimethyl-2(5H)-furanone 3-ethyl-2-hydroxy-2-cyclopenten-1-one 2,3,4-trimethyl-2-cyclopenten-1-one 3-ethyl-4-methyl-3-penten-2-one 2-acetyl-1-cyclohexanone 2-ethyl-2-methyl-1,3-cyclopentanedione 2-acetonylcyclopentanone 5-hydroxymethyldihydrofuran-2-one 3,4-dimethylphenol 1,2-benzenediol 2-hydroxy-3-methyl-6-propan-2-cyclohex2-en-1-one 4-isopropyl-2-cyclohexen-1-one 2-isopropyl-5-methyl-2-cyclohexen-1-one 1-indanone benzene-1,4-diol 3-isopropyl-6-methyl-2-cyclohexen-1-one 2-methylbenzene-1,4-diol 2-hydroxyacetophenone 2,6-dimethyl-1,4-benzenediol 4-ethyl-1,3-benzenediol

11.40 0.93 0.22 0.27 0.43 10.21 2.40 0.19 1.38 1.49 0.36 4.19 1.78 2.36 0.22 0.92 3.14 4.76 6.73 3.00 0.35 3.05 0.93 0.69 0.82 0.18 0.22 3.43 3.55 0.37 1.43 2.63 1.56 0.42 0.13 9.45 1.95 2.57 0.87 0.23 0.73 0.58 3.93 1.04 0.31 2.20

Biocrude was obtained from wastepaper processing at 250 °C after 120 min of reforming reaction.

a

between the molecules, which makes it less susceptible for degradation.42,43 The reaction chemistry of cellulose under subcritical water has been widely studied, and it is known that the hydrolysis of cellulose leads to the production of sugars such as glucose, fructose, and other products, which include glyceraldehyde, 5-HMF, furfurals, alcohols, and organic acids.44,45 Additionally, it was also demonstrated that the monosaccharides produced under subcritical hydropyrolytic conditions undergo dehydration reactions forming oxygenated E

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Table 4. Compounds Observed in Ethyl Acetate Extracts of Biocrudes Obtained at Similar Wastepaper Conversion of about 36% at 225 °C and 90 min and 250 °C and 30 min no.

retention time (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

10.2 10.5 10.6 12.2 13.8 14.8 15.6 16.1 16.5 20.4 21.5 22.3 24.1 31.1 31.6

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

7.8 11.6 11.7 15.3 26.8 15.7 18.3 20.7 21.6 22.6 26.8 7.7 8.2 11.6 11.7 12.2 12.6 12.9 14.7 15.3 15.8 18.9 19.1 20.4 26.6

cmpds observed at 225 °C, 90 min, and at 250 °C, 30 min Common Compounds 2-methyl-2-cyclopenten-1-one 1-(2-furanyl(ethanone) 4-hydroxybutanoic acid lactone 4,4-dimethyl-2-cyclopenten-1-one 1-methyl-1-cyclopenten-3-one 3-methyl-2(5H)-furanone hydroxybenzene 2,3-dimethyl-2-cyclopenten-1-one 3-methyl-4-hexen-2-one 3-ethyl-2-hydroxy-2-cyclopenten-1-one 3-ethyl-2-cyclopenten-1-one 1-hydroxy-3-methylbenzene 3,4-dimethyl-2(5H)-furanone 2-hydroxy-3-propyl-2-cyclopenten-1-one 1,2-benzenediol Additional Compounds 1,2-butylene oxide 4-methoxy-3-methyl-1-butene 1-(2-ethyl-2-oxiranyl)ethanone 3,4-pentadienal 3,5-cyclohexadiene-1,2-diol 1-(1-methylethyl)-cyclopentene 2,2,4-trimethyl-1-pentene 1-hydroxy-2-methylbenzene 3,4-dimethyl-3-hexen-2-one 3-ethyl-2-hydroxy-2-cyclopenten-1-one 3,5-cyclohexadiene-1,2-diol 2-(hydroxymethyl)furan 2,4-hexadienal 2-cyclohexen-1-one 2,5-hexanedione 1-methylcycloheptane 3-propyl-1,4-pentadiene cycloheptanone 1-(5-methyl-2-furyl)ethanone n-butylfuran 2-methyl-2-cyclohexen-1-one 3-methyl-1,2-cyclopentanedione 2-methyl-1,4-benzenediol 5-hydroxy-2,3-dimethyl- cyclopenten-1-one 4-(1-methylethyl)cyclohexanone

% area 225 °C, 90 min

% area 250 °C, 30 min

7.9 2.7 0.2 1.3 4.1 0.4 0.8 5.2 0.4 4.7 1.5 4.7 0.9 3.4 5.4

8.5 0.2 0.7 0.6 4.0 0.4 2.7 5.5 0.6 15.9 7.0 3.9 0.8 1.7 12.9

0.6 0.2 0.7 0.4 2.4 0.2 26.9 1.8 1.4 21.9 2.4 0.4 0.1 0.2 0.6 0.5 0.3 0.6 0.2 0.6 0.1 21.5 1.9 6.7 1.2

wastepaper liquefaction experiment (refer Table 2) and treating the mixture separately at 250 °C using 5 wt % Ni(NO3)2. Gas samples were withdrawn during the course of reaction and analyzed using GC as per the procedure mentioned in the Experimental Section. The gaseous products observed are shown in Figure 6, which essentially indicates presence of H2, CO, CH4, and CO2. At ‘zero’ reaction time, almost negligible amount of H2 was produced but as the reaction continued; H2 volume increased up to 60 min of reaction and thereafter remained constant up to 90 min. The H2 volume that was observed at zero reaction time was 0.003 mol % (0.78 mol %, N2 free basis), which is much lower than 0.011 mol % (4.78 mol %, N2 free basis) that was observed during wastepaper liquefaction. This suggests that H2 during wastepaper reforming might have been also generated via different reaction routes in addition to dehydrogenation of sugars.

hydrocarbons such as 5-HMF, furfural, organic acids such as acetic, formic, lactic, and propionic, and alcohol.46 In our present investigation, similar products were observed during wastepaper liquefaction at 250 °C in presence of Ni(NO3)2 catalyst. It is believed that as the wastepaper contains mainly cellulose and hemicellulose and small amount of lignin, the presence of sugars and oxygenated hydrocarbons in biocrude are obviously expected. The sugars produced from wastepaper would lead to the formation of H2 via dehydrogenation. Additionally, H2 could also be generated from oxygenated hydrocarbons (Tables 2−4) via complex reforming reactions. The compounds identified using GC, GC-MS, and HPLC and possible reaction pathways are shown in Figure 5. Validation of proposed reaction pathways to limited extent was carried out by preparing a mixture of sugars based on the concentration ratio of different sugars observed at ‘zero’ reaction time of F

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Figure 5. Biocrude compounds and product gas identified during aqueous phase reforming of wastepaper at 250 °C in presence of 5 wt % Ni(NO3)2 catalyst and possible reaction pathways based on the observations made with aqueous phase reforming of wastepaper as well as sugars mixture, which was separately carried out under similar reaction conditions.

observed rapid conversion (55%) at 300 °C in 2 s whereas increased conversion of 90% at 350 °C in 1s of reaction time. The degradation products of glucose and fructose reported in various studies in the temperature range 200−270 °C are glycoaldehyde, pyruvaldehyde, glyceraldehydes, 5-HMF, furfural, levuglucosan, 5-methylfurfural, furfural, erythrose, dihydroxyacetone, hydroxyacetone, and orgnic acids such as formic, acetic, lactic, and levulinic.47−51 Mechanistically, it has been also shown that the degradation of glucose led to glyceraldehyde, which decomposed to pyruvaldehyde and produced lactic acid in presence of metal salts.51 Lactic acid further decomposes into H2 and propionic acid, as shown in Figure 5. Glyceraldehyde was reported to be generated by the retro-aldol condensation after rearrangement of glucose, mannose, and ketose.52 Formic and acetic acids are the decomposition products of glyceraldehyde, which are formed via α-dicarbonyl cleavage reaction.52 Aqueous phase reforming of wastepaper at 250 °C produced approximately 1.4 wt % gaseous products, 43.6 wt % biocrude and 55 wt % residues at the end of 120 min reaction time. The product gas wastepaper reforming was found to contain H2, CO, CH4, and CO2 in the amount of 3.75 mol %, 5.31 mol %, 0.033 mol %, and 12.07 mol % (17.7 mol % H2, 25.1 mol % CO, 0.2 mol % CH4, and 52 mol % CO2, N2 free basis), respectively, after 120 min of reaction time. The product gas obtained from sugars reforming was found to contain H2, CO, CH4, and CO2 in the amount of 0.69 mol %, 0.52 mol %, 0.014 mol %, and 3.05 mol % (16.19 mol % H2, 12.2 mol % CO, 0.32 mol % CH4, and 72 mol % CO2, N2 free basis), respectively, at 250 °C after 90 min of reaction time. Biocrude produced after aqueous phase reforming of wastepaper was found to contain

The TOC results obtained with reforming of sugars mixture indicated about 70% conversion of sugars (or 30% residual sugars) after 90 min reaction time (Figure 6a). In the wastepaper liquefaction experiment, about 22% residual sugars was noticed after 90 min reaction time, which suggests synergistic effect of the biocrude compounds on the degradation of sugars. However, additional experiments will be needed for further validation. The aqueous samples collected at different reaction times during the progress of aqueous phase reforming of mixture of sugars were analyzed using HPLC and the results obtained are shown in Figure 7. All samples showed presence of acetic, formic, propionic, and lactic acids. Among all these acids, lactic acid was observed in higher amount, approximately 864 mg/L at ‘zero’ reaction time. The total amount of acids produced from the mixture of sugars at ‘zero’ reaction was 20.85% where maximum contribution of 17.4 wt % was from lactic whereas formic, propionic, and acetic acids contributed in the amounts of 0.6 wt %, 1.85 wt %, and 1 wt %, respectively. The weight percent of acids was estimated by taking the concentration (mg/L) ratio of acids produced to the sugars originally taken (acids/sugars × 100), and the weight percent of each acid component was determined by taking the concentration ratio of individual acid produced to total acids (individual acid/total acids × 100) generated. The production of acids increased with the progress of the reaction up to 30 min, which reached to 21.6 wt %. The acid profiles (Figure 7) indicate degradation of lactic, propionic, and formic acids after 30 min of reaction time whereas no degradation of acetic acid was observed. The degradation of sugars especially glucose has been widely studied under hydrothermal conditions. Kabymela et al.47 G

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Figure 6. Conversion of sugars mixture (prepared accordingly with sugars concentrations observed at t = 0 during aqueous phase reforming of wastepaper at 250 °C in presence of 5 wt % Ni(NO3)2 catalyst) and H2 generation (a) and CO, CH4, and CO2 formation (b) during aqueous phase processing of sugars mixture performed at 250 °C in presence of 5 wt % Ni(NO3)2.

Figure 8. Bar chart summarizing biocrude products by category, gaseous products, and residue generated after 120 min of aqueous phase reforming of wastepaper at (a) 225 °C, (b) 250 °C, and (c) 275 °C in presence of 5 wt % Ni(NO3)2. Figure 7. Concentration of carboxylic acids observed during aqueous phase processing of sugars mixture at 250 °C in presence of 5 wt % Ni(NO3)2.

than formation of acids from sugars alone. The results on the gas products, biocrude and residue obtained from aqueous phase reforming of wastepaper performed at 225 and 275 °C are shown in Figure 8a and c, respectively. At 225 °C, about 0.3 wt % product gas, 32.3 wt % biocrude, and 67.4 wt % residue were produced after 120 min of reaction time. Significantly higher biocrude of 51.9 wt % and product gas of 17.2 wt % were observed at 275 °C with the residue of only 30.9 wt %. The residue obtained is a potential carbon resource, which can be further recycled to improve the carbon efficiency of the hydrothermal reforming process.53

HMF/furfural, sugars, organic acids, and other oxygenated compounds in the amount of ∼1 wt %, 7.5 wt %, 49.1 wt %, and 42.4 wt %, respectively, as shown in Figure 8b. As total acids of 49.1 wt % observed during wastepaper reforming was higher than 21.6 wt % acids realized in separate experiment on the mixture of sugars, it was believed that additional acids might have been generated following complex reaction routes other H

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Currently, we are upgrading the biocrude obtained from aqueous phase reforming of wastepaper for the production of biodiesel range hydrocarbons.

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CONCLUSIONS Subcritical aqueous phase reforming of wastepaper was successfully accomplished in presence of homogeneous Ni(NO3)2 catalyst, which produced about 44 wt % biocrude at 250 °C after 120 min of reaction time. The biocrude was found to contain ∼1 wt % HMF/furfural, 7.5 wt % sugars, 49.1 wt % acids, and 42.4 wt % oxygenated hydrocarbons. The HPLC analysis of biocrude revealed presence of sugars such as cellobiose, xylobiose, glucose, and mannose and organic acids, which include formic, acetic, propionic, and lactic. GC-MS analysis of biocrude identified forty six oxygenated hydrocarbons with >85% confidence level, which mainly include cyclic ketones and substituted cyclic ketones, quinone derivatives, phenols and substituted phenols, and aromatic alcohols. The product gas showed a maximum of about 0.2 mol % and 3.75 mol % H2 (17.7 mol % N2 free basis) after 120 min of reaction time at 225 and 250 °C, respectively, along with CO, CH4, and CO2. At higher reforming temperature of 275 °C, 10.2 mol % H2 was observed with 51.9 wt % biocrude production. Wastepaper aqueous phase reforming reaction pathways indicated that H2 and carboxylic acids might have been also generated from biocrude compounds other than sugars following complex reforming reaction routes.

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ASSOCIATED CONTENT

S Supporting Information *

More details of the GC-MS analysis, typical representative chromatograms, and tables containing list of compounds observed in biocrude samples obtained at different reaction conditions. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 605-394-1231. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by the Tyndall Air Force Base (Award No. FA4819-11-C-0004) and graduate student fellowship, in part, provided by Chemical and Biological Engineering department, SDSM&T, Rapid City, SD.

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