Characterization and Catalytic Upgrading of Aqueous Stream Carbon

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Characterization and Catalytic Upgrading of Aqueous Stream Carbon from Catalytic Fast Pyrolysis of Biomass Anne Katherine Starace, Brenna A. Black, David D. Lee, Elizabeth Palmiotti, Kellene A. Orton, William E. Michener, Jeroen ten Dam, Michael J Watson, Gregg T Beckham, Kimberly A. Magrini, and Calvin Mukarakate ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03344 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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ACS Sustainable Chemistry & Engineering

Characterization and Catalytic Upgrading of Aqueous Stream Carbon from Catalytic Fast Pyrolysis of Biomass

Anne K. Starace,1 Brenna A. Black,1 David D. Lee,1 Elizabeth C. Palmiotti,1 Kellene A. Orton,1 William E. Michener,1 Jeroen ten Dam,2 Michael J. Watson,2 Gregg T. Beckham,1 Kimberly A. Magrini1 and Calvin Mukarakate1* 1

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA

2

Johnson Matthey Technology Centre, PO Box 1, Belasis Avenue, Billingham, Cleveland TS23 1LB, U.K. Corresponding author *Email: [email protected]

ABSTRACT Catalytic fast pyrolysis (CFP) of biomass produces a liquid product consisting of organic and aqueous streams. The organic stream is typically slated for hydrotreating to produce hydrocarbon biofuels, while the aqueous stream is considered a waste stream, resulting in the loss of residual biogenic carbon. Here, we report the detailed characterization and catalytic conversion of a CFP wastewater stream with the ultimate aim to improve overall biomass utilization within a thermochemical biorefinery. An aqueous stream derived from CFP of beech wood was comprehensively characterized, quantifying 53 organic compounds to a total of 17% organics. The most abundant classes of compounds are acids, aldehydes, and alcohols. The most abundant components identified in the aqueous stream were C1-C2 organics, comprising 6.40% acetic acid, 2.16% methanol, and 1.84% formaldehyde on wet basis. The 1 ACS Paragon Plus Environment


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CFP aqueous stream was catalytically upgraded to olefins and aromatic hydrocarbons using a Ga/HZSM5 catalyst at 500°C. When the conversion yield of the upgraded products was measured with fresh, active catalyst, 33% of the carbon in the aqueous stream was recovered as aromatic hydrocarbons and 29% as olefins. The majority of the experiments were conducted using a molecular beam mass spectrometer and separate GC-MS/FID experiments were used to confirm the assignments and quantification of products with fresh excess catalyst. The recovered 62% carbon in the form of olefins and aromatics can be used to make coproducts and/or fuels potentially improving biorefinery economics and sustainability. Spent catalysts were collected after exposure to varying amounts of the feed, and were characterized using multipoint-Brunauer–Emmett–Teller (BET) adsorption, ammonia temperature programmed desorption (TPD), and thermogravimetric analysis (TGA) to monitor deactivation of Ga/HZSM-5. These characterization data revealed that deactivation was caused by coke deposits, which blocked access to active sites of the catalyst and spent catalysts regained total activity after regeneration.

KEY WORDS: CFP aqueous stream, wastewater treatment, HZSM-5, BTX, coke formation

2 ACS Paragon Plus Environment

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INTRODUCTION Catalytic fast pyrolysis (CFP) is a promising approach for converting non-food biomass, such as agricultural and forestry residues, energy crops, municipal solid waste, and others, into transportation fuels and chemicals.1–9 During this process, biomass vapors are either produced and partially deoxygenated in the presence of a catalyst, a configuration denoted in situ CFP, or are separated from solid biomass and deoxygenated over a catalyst bed in a separate reactor, which is termed ex situ CFP.7,10,11 Dehydration is a prevalent deoxygenation mechanism during both pyrolysis processes, wherein a hydroxyl group and a hydrogen cation are removed to produce water. Thus, CFP produces a liquid product containing a significant amount of water, and the condensed product typically self-separates into an aqueous stream and an organic stream.12 The organic stream contains the majority of the desired fuel precursors and consequently has been the predominant focus of CFP research, while the aqueous stream is typically sent for wastewater treatment, thus comprising a potentially large biorefinery operating cost.13

Previous CFP studies, in Figure 1, show that a significant amount of carbon (3-14 wt% of the original biomass carbon) is retained in the aqueous stream.3–8 Paasikallio et al. reported that in situ CFP of pine using a spray dried HZSM-5 catalyst produced an aqueous stream containing 32 wt% organics accounting for 14 wt% of the original biomass carbon.4 Iisa et al. reported that ex situ CFP of pine using two HZSM-5 catalysts with clay and silica binders generated aqueous streams with 8 wt% (clay) and 15 wt% (silica) organics.6 The organic content in the aqueous stream from the clay binder catalyst accounted for 3 wt% of the biomass carbon and those from silica binder catalyst accounted for 7 wt% of the biomass carbon. Aqueous streams were further



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generated from in situ and ex situ CFP of pine using commercial HZSM-5 catalysts (Nexceris and Johnson Matthey) with comparable acid strengths.7 The in situ CFP aqueous stream exhibited approximately 14 wt% organics accounting for 6-7 wt% of the biomass carbon, while the ex situ stream had about 11 wt% organics accounting for 5 wt% of the biomass carbon. Dayton et al. generated an aqueous stream containing 10 wt% organics accounting for 5 wt% carbon of the biomass from in situ CFP of pine.8 The organic compounds characterized in these various CFP aqueous fractions contained mostly typical carbohydrate pyrolysis products such as acids, aldehydes, ketones, and anhydrosugars; additionally, simple phenols and small amounts of lignin derived methoxyphenols were also observed.3,6,7 A recent detailed characterization of fast pyrolysis and CFP aqueous streams from a range of process conditions reported greater than 75% mass closure.3 These aqueous streams contained additional compounds and different proportions of organics than those normally observed in pyrolysis vapors and bio-oils. In addition to containing biogenic carbon, these studies shows that the CFP aqueous streams contains 57-62 wt% of the total liquid yield.5–9 This is a large volume to send to wastewater treatment, as is typically proposed in biorefinery concept models.13 An alternative approach is to treat the CFP aqueous stream as a revenue stream by recovering and/or upgrading organics to produce coproducts or fuels. Hence, there has been recent interest in characterizing and valorizing aqueous streams to improve overall carbon conversion efficiencies and offset biomass-derived fuel production cost in a biorefinery context.3 The amount of biomass carbon retained in the aqueous streams depends on the CFP process, with the general trend that as the amount of deoxygenation increases, the amount of carbon in the aqueous stream and the organic yield decrease. Since the organic product yield needs to be balanced with the organic product


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oxygen content, the optimum will be determined by the tradeoff between these two competing factors. There have been relatively few reports presented in the literature on the valorization of CFP aqueous streams. One example is from Wilson et al., wherein phenol, cresol, and alkyl phenols were separated from a CFP aqueous stream and used to produce bioderived resol resin.14 Another example is steam reforming CFP aqueous stream to produce hydrogen. Work by Kechagiopoulos et al. achieved approximately 60% hydrogen yield from catalytic steam reforming of a CFP aqueous stream. This yield was lower than that obtained with model compounds (90%), although the authors noted that the low yield likely resulted from the deposition of coke, and thus higher yields would be expected after process improvements are made.15

Conversely, more current studies have addressed valorizing fast pyrolysis (FP) aqueous streams. FP oil consists of biomass pyrolysis products and is typically an emulsion, but can be readily separated into an organic fraction and an aqueous fraction by adding water and mixing. Pollard et al. have developed a condensation train to separate pyrolysis oil into five different fractions so that the addition of water is not necessary to produce an aqueous stream, and a low water content stream is also produced.16 Since the approaches to valorizing both CFP and FP aqueous streams are similar, we also briefly review previous work to valorize FP aqueous streams, which includes many different approaches. The aqueous stream can be used as both the steam source and a hydrocarbon source for catalytic steam reforming to produce hydrogen.17 An acid-rich aqueous stream, produced from the fractional condensation of FP pine vapors, has also been used as a pretreatment technique to wash biomass, to reduce its inorganic content, prior to pyrolysis.18 Another method used a series of separation techniques to remove valuable organic components



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in the aqueous stream. For example, Teella et al. used nanofiltration and reverse osmosis to study the feasibility of separating carboxylic acids and sugars in simulated aqueous streams.19 Another approach is to catalytically convert the organics in the aqueous stream to produce fuels and chemicals.9,20–22 Vispute et al. partitioned an aqueous stream from FP oil, and catalytically converted the organic component of the aqueous stream into polyols using Ru/C in a batch reactor.21 Similarly, Abnisa et al. upgraded the aqueous fraction of FP oil from palm shell by passing it over HZSM-5 from 405-555°C. Between 20 and 30% of the biomass carbon was retained in the upgraded oil stream 9 Mukarakate et al. recently used HZSM-5 with a silicon-toaluminum ratio (SAR) of 30 at 500°C to upgrade an oak FP aqueous stream to aromatics and phenols, recovering 40% of the carbon from the aqueous stream.23

To date, there has been no work reported on direct catalytic conversion of a CFP aqueous stream without preprocessing a priori. We employed an aqueous stream sample generated from in situ CFP of beech wood using a pilot plant at the Center for Research and Technology Hellas (CERTH) in Greece.5 During this process, beech wood was pyrolyzed in a circulating fluidizedbed reactor using HZSM-5 at a catalyst-to-biomass ratio of around 16 to generate an aqueous stream containing 20% organics, accounting for 10% of the biomass carbon. This sample was comprehensively characterized prior to upgrading over a Ga-modified catalyst at 500°C. The resulting products and subsequent catalyst deactivation were analyzed in real time with a molecular beam mass spectrometer (MBMS). Pre-and post-use catalysts at three feed-to-catalyst ratios were characterized to determine the extent of catalyst coking during this process. Additional catalytic upgrading experiments with a GC-MS/FID system were conducted to support the assignments used during the mass-spectrometry based method (MBMS) used to


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quantify products. The GC-MS/FID studies were conducted using fresh excess catalyst. Our goal was to conduct an in-depth characterization of the aqueous stream and perform experiments to determine the feasibility of recovering the waste carbon via catalytic conversion to value-added co-products, benzene, toluene, and xylene (BTX), with no preprocessing. These data are necessary to develop rigorous techno-economic models of CFP processes to determine the yields and products required for catalytic upgrading of the aqueous stream. Valorization of the aqueous stream, instead of sending it to wastewater treatment, may in turn reduce the overall thermochemical biorefinery operating costs, and potentially, reduce the minimum biofuel selling price.

MATERIALS AND METHODS Materials. The aqueous stream used in this work was produced from in situ CFP of beech wood with HZSM-5 catalyst, silica-to-alumina ratio (SAR) of 50, at CERTH, as described previously.5,24 This aqueous stream will be referred to as “CFP aqueous stream” for the remainder of the article. The unmodified HZSM-5 catalyst with a SAR of 30, silica binder, and 1 mm particle size was received from Johnson Matthey (Chilton, UK). The Ga/HZSM-5 catalyst was also produced by Johnson Matthey by adding 5 wt% Ga to the unmodified HZSM-5 catalyst. Ga was loaded to HZSM-5 using incipient wetness and inductively coupled plasma mass spectrometry analysis showed that the Ga loading was 5.0%

Aqueous stream characterization. Carbon, hydrogen, and nitrogen content were measured with a LECO TruSpec CHN module (LECO Corp., St. Joseph, MI) via high temperature combustion



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followed by infrared (IR) and thermal conductivity detector (TCD) analysis. Detailed description of this process is given in the Supporting Information (SI).

The water content of the aqueous stream was measured via Karl Fisher titration according to the standard ASTM E203-08 method. Analysis of the inorganic content of the aqueous stream was conducted by digesting the aqueous stream in 72 wt% nitric acid at 200°C for 15 minutes, filtering, and analyzing the resulting solution using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Total organic carbon (TOC) analysis was measured by a Shimadzu TOC-LCSH analyzer (Shimadzu, Columbia, MD) via a combustion catalytic oxidation method after sample acidification with concentrated hydrochloric acid.

Quantification of individual compounds contained in the organic portion of the aqueous stream was performed using a suite of methods developed by Black et al.3 Briefly, sugars, organic acids, select aldehydes, aromatics, phenolics, and higher molecular weight ketones were analyzed using liquid chromatography (LC) coupled in a system consisting of a Shodex SZ5532 LC column, 6 mm i.d. × 150 mm (Showa Denko America Inc., New York, NY) with evaporative light scattering detection/mass spectrometry (MS); an Aminex HPX-87H LC column, 7.8 mm i.d. × 300 mm column (Bio-Rad Laboratories, Hercules, CA) with refractive index detection/MS; or a YMC C30 LC column, 4.6 mm i.d. × 150 mm column (YMC America, Allentown, PA) with diode array detection/MS. The remaining compounds were analyzed using gas chromatography (GC)-MS with a Stabilwax, 30 m × 0.25 mm i.d., 0.25 µm film thickness (J & W Scientific Inc., Folsom, CA); HP 1 or HP 5 column. Detailed methods using the latter two GC columns have not been previously published and are as follows: an Agilent 7890A GC and Agilent 5975C mass-


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selective detector (Agilent Technologies Inc., Santa Clara, CA) was used for the characterization of analytes in the aqueous stream. Using a splitless injection, 1 µL sample volume was introduced onto a 30 m × 0.25 mm i.d., 0.25 µm film thickness HP 5-MS or HP 1-MS capillary column (J & W Scientific Inc., Folsom, CA) at 280°C. The helium flow was kept constant at 1 mL/min with an oven program as follows: the initial column temperature of 35°C was held for 5 min and then increased to 225°C at 5°C/min, and lastly, to 325°C at 15°C/min with a hold time of 10 min. Electron impact ionization was used at 70 eV electron energy and a mass scan range of m/z 25 – 575. An Agilent Environmental ChemStation G1701DA version D.00.00.38 was used for data analysis.

MS was incorporated with all quantitative analyses as often as possible and individual compounds were often characterized by multiple analytical methods to corroborate calculated concentrations. All analyses were performed in triplicate independent experiments and all quantitative standard curves were maintained with an R2 value of ≥ 0.995 with five or more points of reference ranging between concentrations of 1 to 100 µg/mL. Individual authentic standards were obtained for quantitation in the highest purity available (Table S1) for the construction of each external calibration curve and select internal standards were added to adjust for chromatographic and detector response shift, as outlined in previous work.3

Vertical Reactor-MBMS. A vertical reactor coupled with an MBMS (Extrel CMS, Pittsburgh, PA) was used to upgrade the aqueous stream and characterize the resulting products in real time. A detailed description of this apparatus was reported previously.23 A schematic diagram of the reactor is shown in Figure S1A. Briefly, the aqueous stream was metered into the reactor using a



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syringe pump at a feed rate of 56 µL/min. To create a uniform flow of the aqueous stream, it was fed through a 120 kHz ultrasonic nozzle with a sheath of carrier gas (helium at 600 sccm). The bottom view of the initial design of this sheet is shown in Figure S1B. The theory and design of the sheath, shown in Figure S1C, can be found elsewhere25 and it provides more carrier gas shielding between the liquid in the center of the reactor and the walls of the reactor. The center of the reactor was kept at 500°C by a surrounding tube furnace. 0.5-1.0 g of un-reduced catalyst was held in place in the reactor by two plugs of quartz wool. After passing over the catalyst, the vapors were mixed with helium diluent gas before sampling and analysis by the MBMS. The upgraded products were quantified using response factors calculated from standards as described in the SI. Figure S2 shows typical calibration curves and response factors from the MBMS.

GC-MS/FID system.

A tandem microreactor system (Rx-5030TR, Frontier Laboratories,

Japan) coupled to a Gas Chromatography Mass Spectometer –Flame Ionization DetectorThermal Conductivity Detector (GC-MS/FID) system was used to identify and quantify products from catalytic conversion of organics in the aqueous stream. A detailed description of this system has been reported in previous studies.26–28 For the current study one microliter aliquots of the aqueous stream samples were injected into the reactor instead of biomass. Briefly, this system consists of two reactors connected in series; the first reactor was used for volatilizing organics in the aqueous stream and the second reactor was used for catalytic upgrading. Both reactors were set to 500 °C and 20mg catalyst was loaded in the second reactor. After upgrading the products were measured using the GC-MS/FID system, except for CO, which was quantified using TCD. The GC oven was held at 40 °C for 3 min and heated to 300 °C at a ramp rate of 10 °C·min-1. Standards comprising 25 representative compounds (8 aromatic hydrocarbons, 10


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oxygenates, 5 olefins, and CO and CO2) were used to calculate response factor for quantifying the upgraded products. Compounds without standards were quantified using response factors from compounds with similar functional groups and similar molecular weights.

Catalyst characterization. In experiments with varying aqueous feed-to-catalyst ratios, the post-use catalyst was cooled under a flow of inert gas prior to removal from the reactor for coke analysis. The surface area of the coked, as well as the fresh, catalyst was performed using nitrogen physisorption at 77 K with a Quadrasob SI surface area and pore size analyzer from Quantachrom Instruments (Boynton Beach, FL). More detail on this method is given in the SI. Catalyst coke content was measured by heating in an atmosphere of 20% oxygen in nitrogen to 700°C at a ramp rate of 20°C/min. Mass loss up to 250°C was from water loss and mass loss between 250-700°C was coke loss via combustion. The amount of coke was reported on a g per g of dry catalyst basis. The number of active sites on the fresh and coked catalyst was measured via ammonia TPD on an AMI-390 micro activity test system (Altamira Instruments, Pittsburg, PA), for which the method details are provided in SI.

RESULTS AND DISCUSSION Characterization of the aqueous stream. A summary of the compositional and chemical characterization of the aqueous stream is shown in Figure 2 and the quantification of individual organic compounds and inorganic elements identified in this stream are shown in Tables 1 and 3, respectively. As can be seen in Figure 2, the water content of the aqueous stream was 83.03 ± 1.18% by weight and the inorganic content was negligible at 0.030 ± 0.001%. The total organic content was calculated in two ways; (1) by applying a factor of 2.5 (out of a range of 1.7-2.5) to



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the TOC measurement, as suggested by Iglesias et al.29 and Bader,30 resulting in 16.90% organic content, and (2) by difference from the Karl Fisher water measurement resulting in 17.09% organic content. This resulted in a total mass balance range from 99.96% to 100.15%. Figure 2 also shows that acids were the largest class of compounds in this stream comprising 52.9 wt% of the organic fraction on a dry basis, followed by aldehydes (18.5 wt%), alcohols (15.3 wt%), ketones (7.54 wt%), and aromatics (4.6 wt%). Interestingly, only six compounds were identified as acids compared to 24 compounds identified as aromatics, almost all of them phenols. Table 1 shows that on a wet basis, the most abundant compound present was acetic acid, which comprised 6.40 wt% of the solution, followed by methanol (2.16 wt%), formaldehyde (1.84 wt%), and formic acid (1.01 wt%).The carbon and hydrogen content of the CFP aqueous stream is shown in Table 2. Values are an average of triplicate measurements plus or minus two standard deviations. The carbon and hydrogen content were re-measured three months after the original measurement, verifying that they had not changed significantly.

The ICP-AES analysis of 13 elements, shown in Table 3, found that sulfur was the most abundant element at 304.2 ppm. The next most abundant was sodium (13.7 ppm) followed by aluminum (9.8 ppm), phosphorous (9.4 ppm), calcium (7.1 ppm), iron (3.5 ppm), and magnesium (1.7 ppm). In previous work, where elemental analysis of oils and aqueous streams from FP followed by hydrotreating of eight feedstocks was performed, sulfur and sodium were detected in the aqueous stream, but the amount of aluminum was below the detection limit.31 This result suggests that perhaps the aluminum measured in the CFP aqueous stream came from dealumination of the zeolite catalyst used during the CFP process. This higher abundance of Al


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in the CFP derived aqueous stream compared to in FP aqueous streams was also reported by Black et al.3

CFP aqueous stream upgrading. The data in Table 1 and previous work3 show that the aqueous stream generally contains a large number of organic species, each in small mass fractions. Thus, our work on valorizing this stream was focused on catalytic upgrading, which can achieve a relatively narrow product distribution from a complex feedstock. We first attempted to upgrade the aqueous fraction from FP bio-oils (pine and oak) to olefins and aromatics, and the results from this study are presented in Figure S3 in the SI. As can be seen in Figure S3, the Ga/HZSM5 produced a higher aromatic yield than the unmodified HZSM-5 for both the aqueous stream from the FP of pine (denoted FP pine) and FP of oak (FP oak). Based on these results, we chose to use the Ga/HZSM-5 SAR 30 catalyst for improved carbon recovery from the CFP aqueous stream. Additionally, Ga/ZSM-5 has shown increased aromatic yield compared with unmodified HZSM-5 as previously reported.32

A baseline mass spectrum of the CFP aqueous stream was obtained with the MBMS by passing it over inert sand as a catalyst surrogate at 500°C (Figure 3A). Good agreement was observed between the compounds identified in the baseline mass spectrum and those identified in the detailed LC/MS and GC/MS chemical characterization in Table 1. Note that Figure 3A appear to show fewer compounds compared to those observed in Table 1,however, this is simply because many of the compounds are present in very small concentrations which result in low intensity peaks that are not easily seen when the spectrum is scaled to view the largest peaks. Considering the compounds identified in the chemical characterization, the species in Figure 3A can be



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assigned as: formaldehyde m/z 30, methanol m/z 32, acetic acid and hydroxyacetaldehyde m/z 60, cyclopent-2-en-1-one m/z 82, phenol m/z 94, methyl phenol m/z 108, catechol m/z 110, and 2-methoxyphenol and methylcatechol m/z 124. Additionally, the following fragment peaks could be due to: acetaldehyde and formic acid m/z 29, methanol, butanol, and hydroxyacetic acid fragments m/z 31, butanol fragment m/z 42, propanone and hydroxypropanone m/z 43, and acetaldehyde fragment m/z 44. This agreement between the MBMS characterization at 500°C and the LC/MS and GC/MS characterization at lower temperatures indicates that the chemical composition of the aqueous stream does not undergo significant thermal decomposition at 500°C.

When the aqueous stream containing these oxygenated species (Figure 3A) was upgraded over fresh Ga/HZSM-5, the mass spectrum changed to Figure 3B, which contains completely deoxygenated species, primarily olefins (ethylene m/z 28, propylene m/z 42, butylene and methyl propylenes m/z 56) and aromatic hydrocarbons (benzene and alkyl benzenes m/z 78, 92, 106, indene and alkyl indene m/z 116, 130, naphthalene and alkyl naphthalenes m/z 128, 142, 156, and anthracene and alkyl anthracenes m/z 178, 192, 206). Oxygen loss was achieved via dehydration m/z 18, decarbonylation m/z 28, and decarboxylation m/z 44. The majority of these assignments were confirmed using the GC-MS experiments discussed below. As the catalyst aged, Figure 3C, the quantity of the completely deoxygenated products decreased and breakthrough of compounds found in the aqueous stream, for example phenol m/z 94 and methyl phenol 108 shown in blue were observed. The comprehensive characterization of the aqueous stream prior to upgrading allowed us to identify which compounds were most likely due to breakthrough and also to identify intermediates and/or side products, which are not the


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completely deoxygenated products seen over the completely active catalyst or compounds identified in the starting material. For instance, methyl benzofuran m/z 132, dimethyl benzofuran m/z 146, naphthol m/z 144, and methyl naphthol m/z 158, also in blue, do not correspond to any of the compounds identified in the CFP aqueous stream, as shown in Figure 3C. Benzofurans (m/z 132 and 146) are partially deoxygenated products likely formed due to lack of accessible active sites as the catalyst aged, and naphthols (m/z 144 and 158) as well as phenols have been shown to be formed from reactions of water and coke precursors on the catalyst.26

Figure 4 shows the change in product distribution for selected compounds found in Figure 3 mass spectra, as more aqueous stream was fed over the fixed-bed of Ga/HZSM-5. The single ring aromatics comprised benzene and alkyl benzenes, multi ring aromatics comprised napthalene, alkyl naphthalenes, anthracene and alkyl anthracenes, side products comprised napthol and alkyl naphthols, and breakthrough species comprised acetic acid, catechol, guaiacol, and methyl guaiacol. At the beginning of the experiment, the active sites of the catalyst are accessible and the organics available in the aqueous stream are almost completely deoxygenated to produce olefins and aromatic hydrocarbons shown in mass spectrum in Figure 3B. The catalyst maintains its activity for producing aromatic hydrocarbons for the first 10-15 minutes of the experiment as shown by the profile for multi ring aromatics. The aromatic hydrocarbons then decreases gradually until the end of the experiment.

Conversely, the single ring aromatic

compounds begin high and stay constant for the first 10 minutes and decreases rapidly until the end of the experiment. Some of the starting materials breakthrough almost immediately and they appear to increases at the same rate as the decrease for single ring aromatics. The breakthrough species observed during the first 40 minutes comprised of phenol and methyl phenols as



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indicated by mass spectrum in Figure 3C. Phenol and cresols could also be produced as side products through reactions of water and coke precursors as discussed earlier because they appear in the mass spectrum at the same time as naphthols and alkyl naphthols (Figure 3).26 The mass spectrum in Figure 3C does not show other breakthrough species such as acetic acid and catechol because they were still being upgraded over the catalyst to produce aromatic hydrocarbons or forming coke. Phenol could breakthrough the catalyst because it has been demonstrated to be recalcitrant towards conversion over HZSM-5.33 It strongly adsorbs on acid sites causing deactivation through both active site and pore blockage. The amount of side products formed is relatively constant throughout the experiment. The change in product distributions observed in Figure 4 may be due to coke deposits on the catalyst, which block active sites and this result was supported by the catalysts characterization studies discussed below.

Three separate experiments were conducted to quantify products from upgrading organics in the aqueous stream with Ga/HZSM-5. Each experiment used 56 µL/min of aqueous stream with a carrier gas feed of 600 sccm He, but differed in the total length of the experiment. After each experiment, the catalyst was cooled under a flow of inert gas and collected for further analysis, resulting in coked catalyst samples at 0.5, 1, and 1.5 feed carbon-to-catalyst ratios (F:C) on a mass to mass basis. Averaged spectra of these three experiments are shown in Figures S4-6. Spectra were normalized to the argon tracer signal and m/z 18, from water, was truncated. The mass spectra at the beginning of the experiment are similar for the three experiments and they contain species shown in Figure 3B.


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The average carbon yields from conversion after 10 minutes time on stream, calculated as a percent of the total carbon fed over the catalyst, are given in Table 4. Predominantly one–ring aromatics were formed during the first 10 minutes (Figure 4), including toluene, which formed in the highest quantity (~16.2% of the carbon fed), followed by xylenes (~6.5%), and then benzene (~5.2%). This gives a total BTX (benzene, toluene, xylene) product selectivity of 86% of the 32.5% recovered from the initial carbon fed. The yield of olefins (the sum of ethylene, propylene, and butylene) was approximately 29% of the initial carbon fed. This result is consistent with upgrading FP aqueous stream over Ga/HZSM-5, which also found roughly equal carbon yields for olefins and aromatics (see Figure S2). The amount of CO2 produced over fresh, active catalyst was around 12% of the carbon fed and CO was 23%. This results in a carbon mass balance of 96.5% without including coke yield. The amounts of olefins formed were surprisingly higher than normally formed from catalytic upgrading of biomass with HZSM-5 based catalysts. The high olefin yield could be due to the excess water in the feed, which has been demonstrated to increase conversions of furan,34 and benzaldehyde.35 Gilbert et al. 34 found that increasing the molar ratio of water to furan from 3.5 to 47.7 increased the furan conversion over ZSM-5 at 600°C from 51.8% to 84.8%. Conversion of benzaldehyde with Ga/ZSM-5 at 450°C was found to increase from 55.8 to 67.52% with the addition of water.35 The increased conversion rated could be due to the water reducing coke deposits through steam stripping and making active sites accessible for reactions. Along with the increased conversion of furan with increasing water content, the amount of olefins increased dramatically, while the amounts of aromatics decreased. The presence of water also increased the selectivity to one-ring aromatics and decreased the selectivity to multi ring aromatics. This result agrees with our data in Table 1 showing a much



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larger amount of olefins and more one ring aromatic hydrocarbons compared to two ring aromatic hydrocarbons.

Since the MBMS is a mass spectrometry based analytical technique, additional experiments were conducted using a GC-MS/FID system to complement the results. This system allows us to identify and quantify products from upgrading organics in the aqueous stream using low feed-tocatalyst ratios. It is a batch system, and during the experiment three one microliter samples were separately volatilized at 500 °C and the volatile compounds were upgraded over 20mg Ga/HZSM-5. The upgraded products identified in the GCMS included olefins (ethylene, propylene, 2-methylpropylene) and aromatic hydrocarbons (benzene, toluene, p-xylene, mxylene, 1-ethyl 3-methyl benzene, indene, naphthalene, and methyl naphthalenes). These species are consistent with the products measured from the MBMS, except for the three ring aromatic hydrocarbons which were not observed in the GC-MS studies. Table 4 shows that the amount of olefins produced under these conditions were 43% and the aromatics yield was 19%. Propylene and 2-methylpropylene were the major olefins produced as shown in Table S1. The yield of aromatics from the GCMS is lower than that from the MBMS because the GCMS data was collected at low feed carbon-to-catalyst ratio (0.03 vs 0.2). This could also be due to the GCMS not able to chromatograph heavy aromatic compounds (two to three ring aromatic hydrocarbons). Detailed GC-MS/FID results from three separate aqueous stream injections are presented in Table S1. The carbon balance for the GC-MS/FID study was 87%, which was lower than measured on the MBMS. As mentioned earlier, the missing carbon could be due to losses in the column. In addition, coke yield is missing from these values because our attempt to determine the coke contents on the catalyst was not successful because of the small amount of


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feed and catalyst used in this system and the detection limit of the thermogravimetric analyzer used.

Catalyst characterization. In order to evaluate the effect of coke on accessibility of active sites, spent catalysts from the three experiments described above from the MBMS were collected for analysis. The amount of coke on each spent catalyst is shown in the blue, checkered bars of Figure 5 as a weight percent on a dry-coke free basis, i.e. the mass loss of coke divided by the dry, coke-free catalyst mass and multiplied by 100. The uncertainty of the TGA measurement, as measured with a standard, was less than ± 3%. The possibility of uneven catalyst coking was minimized by mixing the entire coked sample before removing a portion for analysis. The coke data were measured from multiple samples collected after mixing the catalyst. On average, the 0.5, 1.0, and 1.5 F:C post reaction catalysts contained 3.8, 6.0, and 6.7% coke yields. This demonstrated, unsurprisingly, that the rate of coke deposition was highest on the fresh catalyst and that the rate of coke deposition slowed after longer times on stream due to decrease in accessible active sites.

The reduction in accessible active sites was confirmed by measuring the total acid sites on these three spent catalysts using NH3-TPD. As can be seen by the black hashed bars in Figure 5, the fresh catalyst contained 620 µmol/g of accessible acid sites, while the samples collected at F:C ratios 0.5, 1.0, and 1.5 exhibit 450, 300, and 220 µmol/g of accessible acid sites, respectively. Thus the drop in acid sites from F:C of 0.5 to 1 (150) was approximately as large as the drop from fresh to F:C 0.5 (170). Conversely, the decrease in the number of acid sites from F:C of 1 to 1.5 was only 80, which was only slightly above the uncertainty of the measurement (about ± 50).



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Additional evidence can also be seen by the multi-point BET surface area results shown in the solid red bars in Figure 5. Surface areas (m2/g) of the fresh, F:C 0.5, F:C 1.0, and F:C 1.5 catalyst were 354, 280, 199, and 224, respectively. The decrease in surface area from fresh to F:C 0.5 is about the same as the decrease from F:C 0.5 to F:C 1.0. Surface area measurements have the highest amount of uncertainty of any of the coke characterization techniques, around ±10%. With this large uncertainty, the surface area of the F:C 1.0 and F:C 1.5 coked catalyst samples are statistically equivalent. Based on the overall results for coke characterization, it is clear that most coke is deposited up to a F:C of 1.0, which rapidly decreased the surface area and number of acid sites of the catalyst. After a F:C ratio of 1.0 has been achieved, additional feed passed over the catalyst has little effect on the surface area and number of acid sites. Steam can cause dealumination of zeolites resulting in irreversible deactivation of the catalyst. These samples regained their total activity after regeneration indicating that these short runs did not cause significant dealumination. However, dealumination will occur in large scale systems and can be minimized by adding phosphorous to HZSM-5.36

This process recovered a total of 61.5% carbon in the form of olefins and aromatic hydrocarbons, which if sold commercially can generate revenue to potentially improve biorefinery economics. Future work will focus on concentrating organics by removing water from the aqueous stream in a manner that does not add significant cost or energy to the process to reduce the size and cost of the upgrading reactor. For example, Engtrakul et al. recently demonstrated the application of high-performance architectured surface-selective (HiPAS) membranes for concentrating organics from a mixture of water and biomass CFP vapors.37 A preconcentration step of this type will likely be required to make this process economically feasible. Additionally, we will assess the


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impact of catalyst type on products with the goal of tuning catalysts to achieve selective production of biomass derived chemicals more valuable than BTX.



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ASSOCIATED CONTENT Supporting Information. Additional information for experimental conditions used for aqueous stream and catalyst characterization and quantification of upgraded products.

ACKNOWLEDGEMENTS The compositional analysis and catalysis efforts in this study were performed in collaboration with the Chemical Catalysis for Bioenergy Consortium, an Energy Materials Network Consortium funded by the U.S. Department of Energy’s Bioenergy Technologies Office Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory (NREL). We thank Johnson Matthey for providing the catalyst. We thank Konstantinos G. Kalogiannis and Angelos A. Lappas from CERTH, Chemical Process and Energy Resources Institute (CPERI) for providing us with the CFP aqueous stream. We also thank Steve Deutch for running ICP-OES analysis, as well as Matthew Yung and Jonathan Wells for ammonia TPD analysis.

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Table 1 Chemical characterization of in situ CFP aqueous stream presented in g kg-1 on a wet weight basis Compound Acetic acid 64.04 Formic acid 10.09 Butanoic acid 0.13 Propanoic acid 3.27 Acrylic acid 3.89 Glycolic acid 7.37 Acetaldehyde 5.93 Glycoaldehyde 5.01 Crotonaldehyde 0.29 Tiglic aldehyde 0.26 Formaldehyde 18.35 Furfural 0.91 3-Furaldehyde 0.63 Acetone 4.90 Diacetyl 0.33 Acetylpropionyl 0.20 Hydroxyacetone 2.04 Methylolacetone 0.35 Adipic ketone 0.19 Cyclopent-2-en-1-one 3.41 2-Methylcyclopent-2-en-1-one 0.55 3-Methylcyclopent-2-en-1-one 0.29 2,3-Dimethylcyclopent-2-en-1-one 0.09 3-Methylcyclopentane-1,2-dione 0.01 2-Hydroxy-3-methylcyclopent-2-en-10.36 one 2-Acetylfuran 0.02 Phenol 1.93 Toluene 0.01 o-Cresol 0.54 m-Cresol 0.74 p-Cresol 0.40 2,5-Xylenol 0.25 2,6-Xylenol 0.01 3,4-Xylenol 0.01 2,3,5-Trimethylphenol 0.01 Guaiacol 0.16 Syringol 0.06 4-Ethylguaiacol 0.01 Creosol 0.03 2-Ethylphenol 0.22 3-Ethylphenol 0.03 Hydroquinone 0.60 Catechol 1.97 2-Methylhydroquinone 0.14 Vanillic acid 0.37 Homovanillic acid 0.12 Acetosyringone 0.03 Vanillin 0.10 Protocatechuic aldehyde 0.04 Benzofuran 0.01 2-Methyl-1-benzofuran 0.01 Methanol 21.59 Butanol 4.30


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Table 2 Carbon and hydrogen content of CFP aqueous stream date

Oct. 2016

Jan. 2017

wt% C

6.85 ± 0.46

7.15 ± 0.06

wt% H

10.28 ± 0.46

10.61 ± 0.1

Table 3 ICP-AES elemental analysis of CFP aqueous stream presented in parts per million (ppm) on a dry weight basis Element (ppm) Al Ca Cr Cu Fe K Mg Mn Na Ni P S Zn

9.8 7.1 0 0 3.5

Characterization and Catalytic Upgrading of Aqueous Stream Carbon

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