Characterization and Catalytic Upgrading of Aqueous Stream Carbon

Oct 23, 2017 - Michael B. Griffin , Kristiina Iisa , Huamin Wang , Abhijit Dutta , Kellene A. Orton , Richard J. French , Daniel M. Santosa , Nolan Wi...
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Research Article pubs.acs.org/journal/ascecg

Characterization and Catalytic Upgrading of Aqueous Stream Carbon from Catalytic Fast Pyrolysis of Biomass Anne K. Starace,† Brenna A. Black,† David D. Lee,† Elizabeth C. Palmiotti,† Kellene A. Orton,† William E. Michener,† Jeroen ten Dam,‡ Michael J. Watson,‡ Gregg T. Beckham,† Kimberly A. Magrini,† and Calvin Mukarakate*,† †

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Johnson Matthey Technology Centre, P.O. Box 1, Belasis Avenue, Billingham, Cleveland TS23 1LB, United Kingdom

‡

S Supporting Information *

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 wt % organics. The most abundant classes of compounds were 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 CFP aqueous stream was catalytically upgraded to olefins and aromatic hydrocarbons using a Ga/HZSM-5 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. KEYWORDS: CFP aqueous stream, Wastewater treatment, HZSM-5, BTX, Coke formation

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INTRODUCTION Catalytic fast pyrolysis (CFP) is a promising approach for converting nonfood 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 either are 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 © 2017 American Chemical Society

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 Received: September 19, 2017 Revised: October 18, 2017 Published: October 23, 2017 11761

DOI: 10.1021/acssuschemeng.7b03344 ACS Sustainable Chem. Eng. 2017, 5, 11761−11769

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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 the 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-watercontent 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 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 Abnisa et al. upgraded the aqueous fraction of FP oil from palm shell by passing it over HZSM-5 from 405 to 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-to-aluminum 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 fluidized-bed 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/HZSM-5 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

Figure 1. Distribution of carbon from catalytic fast pyrolysis of biomass. The data were taken from refs 4 and 6−8.

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 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 show that the CFP aqueous streams contain 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 oxygen content, the optimum process conditions will be determined by the tradeoff between these two competing factors. 11762

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increased to 225 °C at 5 °C/min, and last, 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 and 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 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 elsewhere,25 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. A 0.5−1.0 g portion of unreduced 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) coupled to a gas chromatography mass spectrometer−flame ionization detector−thermal 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 1 μL 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 20 mg of 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. Standards comprising 25 representative compounds (8 aromatic hydrocarbons, 10 oxygenates, 5 olefins, CO, and CO2) were used to calculate the 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 analyzed using nitrogen physisorption at 77 K with a Quadrasob SI surface area and pore size analyzer from Quantachrome 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 and 700 °C was coke loss via combustion. The amount of coke was reported on a gram per gram of dry catalyst basis. The number of active sites on the fresh and coked catalyst was measured via ammonia TPD on an AMI-

GC-MS/FID system were conducted to support the assignments used during the mass-spectrometry-based method (MBMS) used to 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 coproducts, 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.

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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-toalumina 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, U.K.). 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 contents were measured with a LECO TruSpec CHN module (LECO Corp., St. Joseph, MI) via high-temperature combustion 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 min, 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 TOCLCSH 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 MS (GC-MS) with a Stabilwax, 30 m × 0.25 mm i.d., 0.25 μm film thickness (J & W Scientific Inc., Folsom, CA); and 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-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 of 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 11763

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Table 1. Chemical Characterization of in Situ CFP Aqueous Stream Presented in g/kg on a Wet Weight Basis

390 microactivity test system (Altamira Instruments, Pittsburgh, PA), for which the method details are provided in SI.

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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

Figure 2. Overall composition and chemical group characterization of the CFP aqueous stream, on a wet and dry weight basis, respectively. Inorganic content was negligible.

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 1.7−2.5) to 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 6 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 contents were remeasured 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), phosphorus (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

Table 2. Carbon and Hydrogen Content of CFP Aqueous Stream date wt % C wt % H

Oct, 2016 6.85 ± 0.46 10.28 ± 0.46

Jan, 2017 7.15 ± 0.06 10.61 ± 0.1

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 11764

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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 appears 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 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 changes 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

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