Multiscale Evaluation of Catalytic Upgrading of Biomass Pyrolysis

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Multi-scale evaluation of catalytic upgrading of biomass pyrolysis vapors on Ni- and Ga-modified ZSM-5 Matthew M. Yung, Alexander R. Stanton, Kristiina Iisa, Richard J. French, Kellene A. Orton, and Kimberly A. Magrini-Bair Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01866 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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Multi-scale evaluation of catalytic upgrading of biomass pyrolysis vapors on Ni- and Ga-modified ZSM-5 Matthew M. Yung,*Alexander R. Stanton, Kristiina Iisa, Richard J. French, Kellene A. Orton, and Kimberly A. Magrini National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado, 80401 USA *Corresponding Author: [email protected]; Fax: 1-303-384-6363; Phone: 1-303-384-7771

ABSTRACT Metal-impregnated (Ni or Ga) ZSM-5 catalysts were studied for biomass pyrolysis vapor upgrading to produce hydrocarbons using three reactors constituting a 100,000x change in the amount of catalyst used in experiments. Catalysts were screened for pyrolysis vapor phase upgrading activity in two small-scale reactors: i) a Pyroprobe with a 10 mg catalyst in a fixed bed and ii) a fixed-bed reactor with 500 mg of catalyst. The best performing catalysts were then validated with a larger scale fluidized-bed reactor (using ~1 kg of catalyst) that produced measureable quantities of bio-oil for analysis and evaluation of mass balances. Despite some inherent differences across the reactor systems (such as residence time, reactor type, analytical techniques, mode of catalyst and biomass feed) there was good agreement of reaction results for production of aromatic hydrocarbons, light gases, and coke deposition. Relative to ZSM-5, Ni or Ga addition to ZSM-5 increased production of fully deoxygenated aromatic hydrocarbons and light gases. In the fluidized bed reactor, Ga/ZSM-5 slightly enhanced carbon efficiency to condensed oil, which includes oxygenates in addition to aromatic hydrocarbons, and reduced oil oxygen content compared to ZSM-5. Ni/ZSM-5, while giving the highest yield of fully deoxygenated aromatic hydrocarbons, gave lower overall carbon efficiency to oil but with the lowest oxygen content. Reaction product analysis coupled with fresh and spent catalyst characterization indicated that the improved performance of Ni/ZSM-5 is related to decreasing deactivation by coking, which keeps the

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active acid sites accessible for the deoxygenation and aromatization reactions that produce fully deoxygenated aromatic hydrocarbons. The addition of Ga enhances the dehydrogenation activity of the catalyst, which leads to enhanced olefin formation and higher fully deoxygenated aromatic hydrocarbon yields compared to unmodified ZSM-5. Catalyst characterization by ammonia TPD, surface area measurements, and post-reaction TPO also showed that the metal-modified zeolites retained a greater percentage of their initial acidity and surface area, which was consistent between the reactor scales. These results demonstrate that the trends observed with smaller (mg-g) catalyst reactors are applicable to larger, more industrially-relevant (kg) scales to help guide catalyst research towards application. Keywords: catalytic pyrolysis, biomass, vapor phase upgrading, acid sites, ZSM-5, gallium, nickel, multi-scale reactors


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1. INTRODUCTION The world’s dependence on petroleum for its liquid transportation fuels and the uncertainty associated with its long-term supply have led to significant interest in developing alternative, sustainable sources for these fuels. Biomass is an attractive source for renewable hydrocarbon fuels production and perhaps the only source which is currently feasible. Fast pyrolysisconverts solid biomass into a liquid bio-oil through rapid heating, in the absence of oxygen, to 450-600°C followed by condensation.1,2,3 Fast pyrolysis of biomass is capable of achieving up to a 70% mass yield to liquid product.4,5 The resulting liquid product, pyrolysis oil or “bio-oil”, contains a complex mixture of oxygenated compounds of various functionalities (e.g., water, phenols, acids, ketones, aldehydes, and methoxyphenols) and it retains a similar elemental composition as the solid biomass that was used to produce the pyrolysis oilThe oxygenates species in pyrolysis oil are largely responsible for many of its undesirable properties: high acidity, corrosivity, immiscibility with petroleum products, low heating value as compared to fossil fuels, significant residues left after distillation, and reactivity during storage.1,2,3 The instability during storage may be attributed to polymerization and condensation reactions that are likely initiated by residual char and alkali particles and can also cause phase separation. A promising method to reduce the oxygen content of pyrolysis oil is to pass the pyrolysis vapors over a catalyst that promotes deoxygenation prior to the condensation of the oil, and this process is known as catalytic fast pyrolysis (CFP).6,7,8, 9,10,11,12 Heterogeneous catalysts that contain surface acidity and well-define pore structures, zeolites and HZSM-5 in particular, have been used for pyrolysis vapor upgrading as they allow for deoxygenation and molecular transformations to produce a stabilized liquid oil through dehydration, aromatization, cracking, and isomerization reactions. 7,9,11,13,14,15,16

Catalytic pyrolysis may be classified as either in situ (pyrolysis and upgrading catalyst placed

in the same reactor) or ex situ (biomass pyrolysis in a first reactor followed by catalyst upgrading in a second reactor), and both of these methods have been evaluated. A commercial catalytic pyrolysis process is likely to utilize circulating fluidized bed reactors with continuous catalyst regeneration, similar to the reactors used for fluid catalytic cracking (FCC) units, as the catalysts and process conditions are similar to petroleum processing and the catalysts may also suffer from rapid deactiavtion.17,18,19 3 ACS Paragon Plus Environment

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Solid acid zeolite catalysts like ZSM-5 typically deactivate during pyrolysis vapor upgrading due to coke formation, and the liquid product formed over the deactivated catalyst more closely resembles an uncatalyzed, raw pyrolysis oil of high oxygen content and poor stability. 20 A compositional strategy to improve both the stability and reactivity of zeolites incorporates metals into the zeolite where they can assist in hydrogen transfer reactions and concurrently reduce coking. Studies on modified zeolites, particularly ZSM-5, have shown partially conflicting results suggesting that metal addition can lead to either enhancements in hydrocarbon production and catalyst stability or decreases in product oil yields. Additions of various metals, including Ni, Ga, Co, Fe, Zn, Ca, Ce, and Na, have been investigated, and studies have been performed in several scales from model compound investigations to pilot-scale studies of biomass pyrolysis vapor phase upgrading. French and Czernik11 compared forty catalysts, including twenty-two metal-modified ZSM-5 zeolites, for upgrading of biomass pyrolysis vapors by inserting boats filled with biomass and catalyst into a hot furnace and analyzing the product vapors by a molecular-beam mass spectrometer. Ni/ZSM-5, Ga/ZSM-5, Co/ZSM-5, Fe/ZSM-5 and unmodified ZSM-5 gave the highest aromatic hydrocarbon yields; however, there is insufficient information on the catalysts (base ZSM-5, metal loadings, preparation method) to draw conclusions on the impacts of the metal additions. Ga incorporation has been found in many cases to increase the formation of aromatics and decrease coke formation.21,22,23 Hydrocarbon yields during the upgrading of wood pyrolysis vapors were reported to have increased by more than a factor of twoin a fixed bed of Ga/ZSM-5 as compared to a bed of unmodified ZSM-5.21 In another study, several Ga-modified ZSM-5 catalysts were compared for upgrading of furan by analytical pyrolysis-gas chromatography (Py-GCMS), with mixed results.22 Catalysts to which Ga was added by ion exchange or incipient wetness increased the yields of aromatic compounds and reduced coke yields compared to unmodified ZSM-5 whereas Ga in the framework decreased the aromatics yields. A catalyst with approximately 1% Ga/ZSM-5 (SiO2:Al2O3 = 30, SiO2:Ga2O3 = 108) prepared by incipient wetness was further evaluated for in situ catalytic upgrading of pine pyrolysis vapors in a fluidized bed reactor and the aromatic yield was observed to increase relative to ZSM-5. A third study evaluated a mesoporous ZSM-5 4 ACS Paragon Plus Environment

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catalysts to which Ga ranging from 0.25-3% was added for upgrading of poplar wood by Py-GCMS.23 The addition of ≤1% of Ga improved the yields of aromatics but higher Ga loadings had no or a negative impact on aromatics yields. In addition to aromatic yields, Ga affected the composition of the aromatics by enhancing the formation of less alkylated one-ring aromatics (benzene and toluene) and 2-ring aromatics and suppressing the formation of more alkylated one-ring aromatics (xylene of and other C9+).22,23 The improvements by Ga have been attributed to Ga enhancing dehydrogenation reactions and thus production of olefins, which can then participate in aromatization reactions,23 and to Ga increasing the rates of decarbonylation and olefin aromatization.22 Other metals intensively studied include Ni and Co. Recent work surveyed various weight loadings (1-10%) of these metals on ZSM-5 for upgrading beech wood pyrolysis vapors in a fixed-bed reactor. 24 In most cases, the metal additions produced oil with lower oxygen content but decreased the overall organic liquid yield. These changes were associated with reduced coke, enhanced light gas, and despite the decreased overall organic liquid yields - higher aromatic hydrocarbon and phenol formation. The impact of Ni on both oil yield and oil oxygen content was more pronounced than that of Co. In another study from the same group, bench-scale results with a 5% Co/ZSM-5 catalyst were validated at the pilot-scale for in situ biomass vapor upgrading.18 It was proposed that the metallic Ni or Co was able to utilize hydrogen either i) produced during pyrolysis or ii) externally added to enhance hydrogenation reactions to produce light alkanes and that Ni can additionally promote dehydrogenation reactions, which favors tproduction of aromatic hydrocarbons over acidic zeolites. Other recent work showed that, compared to unmodified ZSM-5, both Ni- and Ga-modified ZSM-5 with 0.5% metal loadings slightly decreased aromatic hydrocarbon production but increased aliphatic hydrocarbon production from Jatropha pyrolysis vapors in Py-GCMS experiments.25 The Ni/ZSM-5 catalyst exhibited both enhanced coke reduction and higher hydrocarbon content in vapors compared with the Ga containing catalyst. In contrast to the previous studies, the incorporation of 1.2% Ni to ZSM-5 for upgrading of microalgae or duckweed vapors in a fixed-bed reactor gave both lower oil yield and higher oxygen content than the base ZSM-5


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did.26 In that same study, 2% Cu, 3.5% Fe on ZSM-5 also gave poorer performance than the base HZSM5. The impact of hydrogen on metal-promoted ZSM-5 has been evaluated in some studies. The addition of 2.5-10% Ni to ZSM-5 reduced one-ring aromatic hydrocarbon yields in the presence of H2 at atmospheric pressure for upgrading of Miscanthus x giganteus vapors in Py-GCMS experiments. 27 Ni promoted depolymerization of lignin and enhanced the formation of light phenols. In the presence of H2 at 27 bar (400 psi), the addition of 5% Co, Mo, or Ni or 0.5% Pt enhanced aromatic hydrocarbon yields.28 Lower H2 pressures with Mo/ZSM-5 resulted in reduced hydrocarbon yields. The current work aims to compare metal-modified ZSM-5 catalysts for upgrading of biomass pyrolysis vapors in multiple reactors in order to assess the agreement across reactor scales and contribute to the often-conflicting body of research on metal-modified upgrading catalysts. Ga- and Ni-modified catalysts were selected for the comparison based on literature results and preliminary screening experiments. Our recent work investigated various zeolites and their properties such as the effect of binder material29 and the SiO2-to-Al2O3 ratio (SAR) 30,31 for upgrading biomass pyrolysis vapors. From these studies, a ZSM-5 catalyst with a SiO2 binder and zeolite SAR = 30 was selected as the baseline material with requisite acidity. This ZSM-5/SiO2 (SAR=30) was impregnated with various metallic promoters (Ni, Ga, Co, Cu, and Pt) at constant molar loadings (1 mol metal/2 mol Al) and assessed for pine vapor upgrading in both inert atmospheres and atmospheres containing added H2. These screening reactions showed that Ga/ZSM-5 (in inert atmosphere) and Ni/ZSM-5 (in H2-containing atmosphere) led to marked improvements in aromatic hydrocarbon production relative to unmodified ZSM-5. Because of this enhanced selectivity for fuel components, these catalysts and process conditions were selected for ex situ testing in a larger fluidized bed reactor system. The fluidized-bed reactor allowed for production and collection of bio-oil and is more representative of an industrial process than the fixed-bed reactors. This contribution discusses the hydrocarbon yields using ex situ catalytic pyrolysis, including the characterization of fresh and spent catalysts, across reactors of three scales utilizing 0.010 g, 0.50 g, or 1000 g of catalyst. 6 ACS Paragon Plus Environment

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2. EXPERIMENTAL 2.1. Materials and catalyst synthesis. The ZSM-5 zeolites used in this study had a SiO2-to-Al2O3 (SAR) of 30. The ZSM-5 used in the microscale experiments was supplied by Nexceris and had a SiO2 binder (20wt%) and particle size range of 500-1000 µm. The Ga and Ni promoted catalysts were prepared by impregnation to the incipient wetness point of the zeolite with an aqueous solution containing the Ni or Ga nitrate precursors (Aldrich). An equal molar quantity of metal was added to each catalyst, corresponding to (1 mol metal)/(2 mol aluminum), which resulted in different weight loadings on the catalysts: 3.1wt%Ni/ZSM-5 and 3.7wt%Ga/ZSM-5. After impregnation with the metal nitrate aqueous solutions, the catalysts were dried for at least 3 hours at 110°C and then heated at 2K/min to 550°C in air and held for 3 hours. For the experiments in the 2” fluidized-bed reactor, separate 1 kg batches of catalysts were prepared in a similar fashion, but the ZSM-5 support (SAR = 30) was purchased from Zeolyst International (CBV 3024E). The pellets were ground and sieved, and particles sieved from 3001000 µm were used in the experiments. The biomass used in the experiments was southern pine ground to < 0.5 mm, supplied by Idaho National Laboratory. The biomass had an elemental composition of 49.6% carbon, 43.1% oxygen, 6.3% hydrogen, , 0.1% nitrogen on a wet basis; the moisture content was 2.3%. 2.2 Catalyst characterization. Surface area and pore volume measurements were obtained using N2 physisorption on Quadrasorb SI (Quantachrome Instruments). Samples were degassed at 350°C, under vacuum, overnight, and then immersed in a liquid nitrogen bath at 77K.The multi-point BET analysis was used for surface area determination and pore volume analysis was performed with the BJH (BarrettJoyner-Halenda) method. The micropore contribution to the total surface area was calculated using the tplot method, with the DeBoer model. Post-reaction catalysts were analyzed for their coke content using thermogravimetric analysis (TGA) on a Setaram instrument. Samples were heated at 20oC/min in air from 25oC to 850oC. The mass loss below 250°C was attributed to water and weakly adsorbed organics, whereas the loss from 250-850oC was attributed to coke. Separate experiments coupled the effluent gas from the TGA with FTIR gas spectroscopy to validate these assumptions on similar materials. 7 ACS Paragon Plus Environment

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Ammonia temperature-programmed desorption (TPD) was used to measure the total acidity of the catalysts, using a flow system (AMI-390) with a thermal conductivity detector. Fresh samples were pretreated by heating to 500°C in 10% O2/Ar, and held for 60 min. Samples were then cooled in helium flow to 120°C, where they were exposed to ammonia (10% NH3/He) for 30 min, and then flushed with He. The TPD step involved heating at at 30K/min from 120-600 °C, with a 30 min hold at 600°C. The gas flow rate in all steps was 25 sccm. A sample loop of known volume (5 mL) was used to calibrate the thermal conductivity detector (TCD) response for NH3. For the spent samples, pretreatment was performed in inert (as opposed to oxygen) to avoid combustion of coke/organics on the catalyst surface and the TPD maximum temperature was 500°C to prevent desorption of heavy species (e.g., polyaromatics) that maybe remain on the catalyst surface as a result of the pyrolysis vapor upgrading reaction at 500°C.Triplicate experiments on the fresh ZSM-5 catalyst were performed for surface area and acidity measurements, and the standard deviations of these measurements were scaled to provide error bars to estimate uncertainty in the measurements on the other samples. 2.3 Catalytic pyrolysis experiments. Three different reactor systems were used in this study to evaluate catalysts. Schematics of the main components of each system are shown in Figure 1 and Table 1 compares the reactor systems’ properties. Each of these reactor systems is described in greater detail below. 2.3.1 Pyroprobe reactor. A commercially-available analytical pyrolyzer (Pyroprobe model 5200, CDS Analytical Inc.) was coupled with a gas chromatogram-mass spectrometer (GC/MS) for quantitative catalytic pyrolysis experiments. The Pyroprobe system utilizes a computer-controlled, resistively-heated coil for sample pyrolysis, and a constantly-heated fixed-bed for catalytic upgrading. A sorbent tube trap, filled with Tenax-TA™ (poly (2,6 diphenyl-p-phenylene oxide)), was in-line following the upgrading zone. The trap adsorbed the condensable components of the upgraded pyrolysis vapors, and allowed the non-condensable gases to be vented. For all experiments, 10 mg of catalyst was loaded into the upgrading zone and reduced for 1 hour at 300oC in 100% H2, prior to the start of the experiment. Ten (10) successive 1 mg pine samples were 8 ACS Paragon Plus Environment

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then pyrolyzed at 500oC and the vapors carried over the catalyst, also at 500oC, by 15 sccm of carrier gas (100% He or H2). A total of 10 mg of pine were pyrolyzed for each experiment, resulting in a cumulative biomass-to-catalyst ratio of 1:1. There is a well-known discrepancy between the Pyroprobe’s heating coil set point and the temperature experienced inside the quartz sample tube. A set point of 595oC was used to achieve a pyrolysis temperature of 500oC. At the conclusion of each pyrolysis/upgrading experiment, the trap was heated to 400oC to desorb the condensable components of the pyrolysis, which were carried to the GC/MS by GC carrier gas (54 mL/min of He). For product identification and quantification, an Agilent G1530A gas chromatograph coupled with an HP 5973 mass spectrometer was used to analyze both the Pyroprobe products and the oil from the 2” fluidized-bed reactor. For the Pyroprobe, the products were desorbed from the trap and carried directly to the GC/MS. For the 2” fluidized-bed reactor analysis, a 1 µL sample of oil diluted with acetone in the ratio 10:1 was manually injected into the GC/MS inlet. The GC contained a 30 meter capillary column (Agilent 190915-433) with a 5% phenyl and 95% dimethyl polysiloxane stationary phase, operated at a constant volumetric flow of 2 mL/min. The GC interface was held at 300°C, with a 50:1 split ratio. The oven temperature was held at 40oC for 3 minutes, before ramping to 240°C at 6°C/min, followed by a 10oC/min ramp to 300oC. The NIST98 MS library was used for product identification. For quantification, the GC/MS was calibrated for 20 compounds, and functional group matching and molecule size was used to best approximate non-calibrated products. 2.3.2 Fixed-bed reactor. The fixed-bed reactor system, which has been previously described, utilized a horizontal quartz reactor with concentric flow path (inner tube and outer tube) coupled to a molecular beam mass spectrometer (MBMS).11,20 Catalysts were pretreated in 33% H2 at 500°C for 30 minutes prior to reaction experiments. Pyrolysis and vapor upgrading occurred in the inner tube,whichhad a flow of 300 sccm He (ZSM-5 and Ga/ZSM-5) or 100 sccm H2 + 200 sccm He (Ni/ZSM-5) at 500°C and atmospheric pressure. Biomass was introduced using quartz boats containing pine (35.5 mg) at a rate of approximately one boat every 150 seconds into the pyrolysis zone of the inner tube which was maintained at 500°C. In each experiment, 14 boats containing 35.5 mg of pine were consecutively pyrolyzed and the 9 ACS Paragon Plus Environment

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vapors were passed over a fixed-bed of 500 mg catalyst. The pulse of pyrolysis vapors from 35.5 mg of pine took approximated 45 seconds to eluted, from start to finish. 2.3.3 Two-inch (2”) fluidized-bed reactor. A simplified diagram of the reactor system is presented in Figure 1. The system includes two bubbling fluidized bed reactors: a pyrolyzer (5.2 cm inner diameter x 43 cm tall) and a vapor phase upgrading reactor (5.2 cm inner diameter x 15 cm tall, followed by a 7.8-cm inner diameter x 35.6 cm tall disengagement section). Biomass and fluidizing gas were fed into the pyrolyzer, which contained quartz sand (~330 g) as the bed material. The pyrolysis vapors were passed into the upgrading reactor via a cyclone, in which char was separated. Fresh catalyst was continuously fed into the upgrading reactor and spent catalyst removed via an overflow tube. Any entrained catalyst fines were removed in a stainless steel 2 µm hot gas filter. The vapors were condensed in a system consisting of an air-cooled condenser, an electrostatic precipitator, and 1-2 dry-ice traps followed by a coalescing filter cooled with dry ice to 0°C. The exit gases were metered and analyzed for H2, CO, and CO2 by non-dispersive infra-red-analyzers and for H2, CO, CO2, and C1-C4 hydrocarbons by a micro-GC (Varian GC with Molecular Sieve 5A, Porabond Q, and CP-Sil columns). In addition, gas bag samples were collected of the exit gas and analyzed by GC/MS/FID (Agilent 7890A with 5975C MSD and FID) for the presence of condensable organics. – Further details of the experimental system are available elsewehere.29 In the current experiments, the pyrolysis and upgrading reactors were both held at 500°C. Biomass was fed into the pyrolyzer at a rate of 420 g/h and fluidizing gas at a rate of 14 l/min (STP). 7501000 g of biomass was fed in each experiment. The catalyst feed rate was 300 g/h, and the mass ratio of biomass feed to catalyst feed was 1.4. The catalyst mass in the upgrading reactor was 60 g; this corresponded to a weight hourly space velocity of 7 h-1. The weight hourly space velocity (WHSV) of the fluidized bed reactor and the other reactors tested here (Table 1) are lower than those in typical FCC units, which have WHSVs of 100-250 h-1.32 However, the WHSVs in the current reactors resemble closely those for other reactors used for testing of FCC catalysts such as Advanced Cracking Evaluation


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(ACE) reactors.32,33 ACE units have been found to correctly rank feeds and catalysts even though the yields differ from those obtained in pilot scale circulating riser reactors.33 The fluidizing gas was N2 in the experiments with ZSM-5 and Ga/ZSM-5 and a mixture of 30% H2 and 70% N2 in the experiment with Ni/ZSM-5. For the experiment with Ni/ZSM-5, the catalyst was pre-reduced in 10% H2/90% N2 at 500°C for 30 minutes. The ZSM-5 and Ga/ZSM-5 catalysts were used without pretreatment. The experiment with Ga/ZSM-5 was performed in duplicate. The oil from the condensation train was separated via a separatory funnel into a top organic liquid, middle aqueous liquid, and a bottom organic liquid. Each liquid product was analyzed for C/H/N/O/S by ultimate analysis, water by Karl Fisher titration, carboxylic acids (CAN) by potentiometric titration, and for composition by GC/MS as described above. The total oil yield was determined as the sum of the top and organic layers and any organics detected by GC/MS/FID in the gas bag samples. 3.0 RESULTS AND DISCUSSION In an effort to identify materials and process conditions that could improve the yield of liquid-range hydrocarbon fuel molecules, several catalysts were evaluated for the vapor phase upgrading of pine pyrolysis vapors in inert or H2-containing atmospheres. The fixed-bed reactor was used as an initial screening tool to qualitatively compare catalysts. From the materials that were evaluated against the baseline ZSM-5 material, two promising catalysts were identified: i) Ga/ZSM-5 in inert and ii) Ni/ZSM-5 in H2. These metal impregnated zeolites led either to improvements in the aromatic hydrocarbon yield or reduction in the oxygenate breakthrough so that the oxygen content of a condensed oil would be decreased relative to one that was obtained using ZSM-5 as the catalyst. The results for these two catalysts, as well as ZSM-5, are shown in Figure 2. All of the catalysts showed a significant decrease in aromatic production with increasing exposure to biomass pyrolysis vapors, which is typical of zeolitebased catalysts and can be attributed to coke formation, which causes blockage of active sites. The Ga/ZSM-5, however, had higher initial activity than ZSM-5. The high activity of Ga-impregnated catalysts has been attributed to dehydrogenation activity of Ga which increases the quantity of olefins that can further participate in aromatization reactions.,23 The Ni/ZSM-5 in H2 showed lower initial activity 11 ACS Paragon Plus Environment

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than ZSM-5 but was able to retain its initial activity longer than ZSM-5 as it did not show significant deactivation until a biomass-to-catalyst ratio of ~0.7. The increased stability may result from hydrogenation activity of Ni; coke-forming precursors from the catalyst surface become hydrogenated into molecules that can diffuse out instead of remaining in the hydrocarbon pool, thereby slowing coke formation and the rate of deactivation as compared to unmodified ZSM-5. Because of these promising results, the materials were also studied in a micro-scale Pyroprobe reactor, and a 2” fluidized-bed reactor. These additional reactor systems provided detailed speciation and quantification of product yield and selectivity. 3.1 Fluidized-bed reactor experiments. The experiments in the fluidized-bed reactor were performed with a biomass-to-catalyst mass ratio of 1.4. The yields of each phase – oil, aqueous, gas, char, and coke – and the overall mass balance closure are summarized in Table 2. The mass balance closures were good (92-96%); the major sources of losses are likely light organics such as acetaldehyde and furan or water vapor that did not condense in the liquid collection system. The duplicate runs with Ga/ZSM-5 demonstrated excellent reproducibility with the yields of all phases within one percentage point of each other. In all experiments, the liquid phases accounted for approximately 40% of the biomass feed but over half of those consisted of the aqueous liquids. The organic oil yields ranged from 16 to 19%, with the highest oil yields measured for Ga/ZSM-5. This result confirmed the high hydrocarbon production observed for Ga/ZSM-5 in the fixed-bed experiments. Ni/ZSM-5 produced the lowest liquid yields (both oil and aqueous) and highest gas yields in accordance with the high activity of Ni for gasification reactions.34 The yields of biomass carbon in the oil are also depicted in Table 2, and they similarly indicate decreasing oil C yields in the order Ga/ZSM-5 > ZSM-5 > Ni/ZSM-5. The compositions of the oils are summarized in Table 3. The oxygen contents on a dry oil basis varied in the range of 14-19%. They were lowest for the oil produced over Ni/ZSM-5, which highlights the high deoxygenation of the oil and the slow deactivation of this catalyst as already demonstrated in the fixed-bed tests. The acid numbers including only carboxylic acids (CAN) were below 10 for all oils,


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which indicates high acid destruction compared to non-catalytic pyrolysis, for which CAN was measured to be 40-100 for oils produced in the same fluidized-bed reactor.35,36 3.2 Aromatic hydrocarbon yield and selectivity across reactor scales. The yields of liquidrange hydrocarbons from the vapor-phase upgrading of pine pyrolysis products, normalized to the yields obtained from ZSM-5, are shown in Figure 3. The liquid-range hydrocarbons produced were found to be almost entirely aromatic in nature for all three catalysts in all three reactors. The selectivity to 1-, 2-, and 3-ring compounds is shown in Figure 4. For all three reactors, adding Ga to the ZSM-5 catalyst increased liquid-range hydrocarbon yields. In both the fixed-bed reactor and the fluidized-bed reactor, the Ga-modified ZSM-5 resulted in approximately 30% increase in liquid-range hydrocarbons compared to the oil from ZSM-5. The large improvements in the hydrocarbon yields for Ga/ZSM-5 are in line with earlier reports: Park et al.21 found a two-fold increase in aromatics in catalytic pyrolysis of sawdust in a fluidized-bed reactor and Cheng et al.22 reported increases of 40% in aromatics yields. All three reactors showed an increased selectivity to 1-ring aromatics with metal-modified ZSM-5 catalysts. Cheng et al.22 and Park et al.21 also reported higher enhancements for 1-ring aromatics than for other hydrocarbons in accordance with the higher selectivities of 1-ring compounds found here. Zeolite catalysts undergo deactivation during upgrading of pyrolysis vapors, and this deactivation is caused by the formation of graphitic coke and polynuclear aromatics. 37 In this context, the higher selectivity towards 1-ring aromatics on the Ni/ZSM-5 and Ga/ZSM-5 relative to ZSM-5 is an indication that they deactivate less than ZSM-5. The reduced deactivation on Ni/ZSM-5 and Ga/ZSM-5, as compared to ZSM-5, is also apparent in the catalyst characterization results, which will be discussed in subsequent sections. The fixed-bed reactor showed a greater selectivity towards 3-ring aromatics with all catalysts, compared to the Pyroprobe and the 2” fluidized-bed reactor. This is thought to be a result of the detection methods used with the reactors. The MBMS, which is used with the bench-scale reactor, does not require a cooling of the products before reaching the detector and therefore even heavy compounds are detected. 13 ACS Paragon Plus Environment

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However, both the Pyroprobe and the analysis of oil produced in the 2” fluidized-bed reactor used a GC/MS for product identification and quantification. The GC/MS required the vapors to be cooled to 40°C at the beginning of the GC cycle, followed by heating to 300oC. Consequently, the majority of 3&4 ring aromatics, with boiling points near or greater than 300oC, do not reach the detector. 3.3 Light gas formation in fixed-bed and fluidized-bed reactors. The light gas production during the ex situ catalytic pyrolysis experiments is shown in Figure 5. The experiments in the Pyroprobe did not allow for the analysis of light gases due to the sorbent tube trap used. There was good agreement in the relative gas yields for the fixed-bed and fluidized-bed reactors. Namely, ZSM-5 produced the lowest quantity of light gases, whereas Ga/ZSM-5 and Ni/ZSM-5 gave higher relative yields on both reactor scales (10% and 40% higher yields, respectively). The highest yield in the Ni/ZSM-5 (H2) experiment can be attributed to the hydrogenation activity of Ni, which is evidenced by the substantial increase in CH4 production on both scales. Methane is likely produced via hydrogenation of surface alkyl groups by activated hydrogen, and this same hydrogenation reaction pathway can account for the increased production of C2+ hydrocarbons over Ni/ZSM-5 as compared to the other catalysts. Additionally, the enhancement in CO and CO2 production may be caused by the higher gasification activity of the nickel catalysts, leading to CO production; CO can also undergo water-gas shift to produce CO2 with the steam that is present. 3.5 Catalyst characterization 3.5.1 Coke on spent catalysts. The spent catalysts from the fixed-bed and 2” fluidized-bed reactor were collected and analyzed for coke content (Figure 6). The relative amount of coke on the catalysts agreed between the two reactor systems, with the coke yield increasing in the order ZSM-5 > Ga/ZSM-5 > Ni/ZSM-5. The formation of coke can block catalyst pores and active sites and is a major cause of catalyst deactivation for pyrolysis vapor upgrading over zeolites. This explains why higher aromatic hydrocarbons yields were observed on Ni- and Ga-impregnated catalysts as compared to ZSM5; less coke on the catalyst resulted in sustained catalyst activity and contributed to the higher yields. Initial small-scale screening experiments showed little difference in coke formation between He and H2 14 ACS Paragon Plus Environment

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atmospheres for the ZSM-5 and Ga/ZSM-5 catalysts, indicating that they do not activate gaseous H2 to mitigate coking. A decrease in coke formation, however, was observed for Ni/ZSM-5 in both H2 and inert atmospheres as compared to the other catalysts, indicating the ability of Ni to activate H2 produced in situ during pyrolysis of co-fed, and hydrogenate coke precursors on Ni/ZSM-5 to from aromatic hydrocarbons, which can diffuse out of the catalyst pores. Therefore the difference in coke formation between Ga/ZSM-5 in inert and Ni/ZSM-5 in H2-containing atmosphere is attributed both to the impact of Ni and the gas atmosphere. 3.5.2 Surface area and acidity measurements. To further understand how the catalysts changed during the course of the reaction and how coking might influence the catalyst properties, the surface area and acidity were measured for fresh and spent catalysts. The results from the fixed-bed and fluidized-bed reactors are shown in Figure 7. There was not a sufficient quantity of catalyst in the Pyroprobe experiments to allow for post-reaction characterization. The surface area on the fresh materials did not vary significantly, indicating that the addition of Ga or Ni did not lead to any significant blockage of pores as probed by N2 physisorption. The surface area on the spent catalysts, however, was significantly less than the fresh catalysts. In the fixed-bed experiments, ZSM-5 and Ga/ZSM-5 retained only 32% and 26% of their surface areas, respectively, as compared to the fresh materials, whereas the Ni/ZSM-5 catalyst retained 51% of its initial surface area (Figure 7a). The catalysts used in the fluidized-bed reactor retained a greater percentage of their surfaces areas as compared to the catalysts used in fixed-bed experiments but showed the same trends in retained surfaces area as compared to the fresh catalysts. In examining the total acidity of the fresh catalysts, which can be considered as the number of active sites for aromatic hydrocarbon production, there was little variation amongst the fresh materials (Figure 7b). This indicated that the addition of Ga or reduced Ni to ZSM-5 did not lead to any substantial blockage of existing acid sites or net creation of new acid sites, which was consistent with conclusions drawn from surface area measurements. Although the total acidity as measured by NH3 TPD did not change amongst the materials, it is possible that the addition of Ga or Ni to ZSM-5 may have led to increased Lewis acidity, which changed the nature of the acidity. In the spent materials from the fixed-bed reactor, the 15 ACS Paragon Plus Environment

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ZSM-5, Ga/ZSM-5, and Ni/ZSM-5 retained 35%, 47%, and 76% of their initial acidity, respectively. In the fluidized-bed reactor, the retained acidity was 37% for ZSM-5, 57% for Ga/ZSM-5, and 68% for Ni/ZSM-5. Again, the trends for the relative acidity retained showed good agreement from the fixed-bed and fluidized-bed reactor. The addition of Ni to ZSM-5 led to significantly more retention of surface area and acidity as compared to unpromoted ZSM-5. The influence of Ga addition to ZSM-5 on retained surface area and acidity was rather minor. These findings are consistent with the role of Ni helping with hydrogenation of coke precursors which results in decreased coke deposition and rate of deactivation, whereas the main role of Ga is to enhance aromatization reactions but it does not reduce the rate of deactivation by coke deposition. In viewing the coke, surface area, and acidity characterization results in the context of the reaction data for aromatic hydrocarbon production, the higher production of aromatics on the Ni and Ga catalysts relative to ZSM-5 may be due to a protective effect of the active sites from deactivation via coking. In the case of Ni, this may be attributed to hydrogenation of surface alkyl groups in coke forming precursors. For the Ga catalyst, the anti-coke protective function is not as strong as for the Ni catalyst, but the Ga may serve to increase dehydrogenation to produce more olefins which can participate in aromatization reactions, resulting in the enhanced aromatic yield relative to unmodified ZSM-5.23 It has been proposed that Ga selectively catalyzes olefin aromatization and reduces Diels-Alder and alkylation reactions leading to coke formation.38 4.0 CONCLUSIONS The addition of either Ga or Ni to ZSM-5 enhanced the performance of ZSM-5 towards aromatic hydrocarbon formation during vapor phase upgrading of pine vapors (ex situ catalytic pyrolysis). With Ni addition, the improved hydrocarbon yields were associated with reduced coke and increased light gas formation. The lower carbon deposition as coke reduced catalyst deactivation rates by keeping the active acid sites accessible for the deoxygenation and aromatization reactions that produce fully deoxygenated, aromatic hydrocarbons. Compared to ZSM-5, oil with increased aromatic hydrocarbon yield and lower oxygen content was produced. 16 ACS Paragon Plus Environment

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Slightly reduced coke deposits were observed with Ga/ZSM-5 as well, and this can contribute to the higher aromatic yields compared to ZSM-5. However, with Ga/ZSM-5, improved aromatics yields were also observed initially when the catalyst was fresh. The Lewis acid sites provided by Ga enhance dehydrogenation activity, which results in increased olefin formation. The presence of olefins increases aromatics production and Ga has also been reported to enhance the rate of olefin aromatization. As a result, the overall aromatics production is increased and Ga/ZSM-5 produced the highest overall oil yields as well as the highest hydrocarbon yields. The reactions were studied in three reactor systems from a micro-scale Pyroprobe reactor (10 mg catalyst) through a fixed-bed reactor (0.5 g catalyst) to a fluidized-bed reactor with continuous catalyst feed (~1 kg catalyst fed). Good qualitative agreement with respect to oil, hydrocarbon, light gas, and coke yields were obtained across all the scales despite inherent differences in the configurations and methods of product analysis. The improvements in hydrocarbon production and catalyst deactivation that were observed in the smaller scale fixed-bed reactors were validated in the fluidized-bed reactor. Further scaleup to industrially-relevant circulating reactors with shorter catalyst retention times and catalyst regeneration is required; however, the current results suggest that trends observed in laboratory reactors with mg-g quantities of catalysts are well translated to larger scales. ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy under Contract No. DE-AC3608GO28308 with the National Renewable Energy Laboratory, and funding was provided by U.S. DOE Bioenergy Technologies Program. The authors would like to thank Scott Palmer for performing the fluidized-bed reactor experiments. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. REFERENCES


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Pyrolysis Vapors, Technical Report NREL/TP-5100-62455, PNNL-23823, National Renewable Energy Laboratory: Golden, CO, 2015; 275 pp. (20) Mukarakate, C.; Zhang, X.; Stanton, A.; Robichaud, D. J.; Ciesielski, P. N.; Malhotra, K.; Donohoe, B. S.; Gjersing, E.; Evans, J. R.; Heroux, D. S.; Richards, R.; Iisa, K.; Nimlos, M. R., Green Chem. 2014, 16, 1444-1461. (21) Park, H. J.; Park, Y.-K.; Kim, J.-S.; Jeon, J.-K.; Yoo, K.-S.; Yim, J.-H.; Jung, J.; Sohn, J. M. Stud. Surf. Sci.Catal. 2006, 159, 553-556. (22) Cheng, Y.-T.; Jae, J.; Shi, J.; Huber, G. W Angew. Chem. Int. Ed. 2012, 51, 1387 –1390. (23) Kelkar, S.; Saffron, C. M.; Li, Z.; Kim, S. S.; Pinnavaia, T. J.; Miller, D. J.; Kriegele, R. Green Chem. 2014, 16, 803-812. (24) Iliopoulou, E.F.; Stefanidis, S.D.; Kalogiannis, K.G.; Delimitis, A.; Lappas, A. A.; Triantafyllidis, K.S. Appl. Cat., B 2012, 127, 281-290. (25) Vichaphund, S.; Aht-ong, D.; Sricharoenchaikul, V.; Atong, D. Renewable Energy, 2014, 65, 70-77. (26) Campanella, A.; Harold, M. P. Biomass Bioenergy 2012, 46, 218-232. (27) Melligan, F.; Hayes, M. H. B.; Kwapinski, W.; Leahy, J. J. Energy Fuels 2012, 26, 6080−6090. (28) Thangalazhy-Gopakumar, S.; Sushil Adhikari, S.; Gupta, R. B. Energy Fuels 2012, 26, 5300−5306. (29) Iisa, K.; French, R. J.; Orton, K. A.; Budhi, S.; Mukarakate, C.; Stanton, A. R.; Yung, M. M.; Nimlos, M.R., Top. Catal. 2016, 59, 94-108. (30) Engtrakul, C.; Mukarakate, C;, Starace, A. K.; Magrini, K. A.; Rogers, A. K., Yung, M. M. Catalysis Today 2016, 269, 175-181. (31) Mukarakate, K.; Watson, M. J.; ten Dam, J.; Baucherel, X.; Budhi, S., Yung, M. M.; Ben, H.; Iisa, K.; Baldwin, R. M.; Nimlos, M. R. Green Chem., 2014, 16, 4891 – 4905. (32) Hu, R.; Weatherbee, G.; Ma, H.; Roberie, T.; Cheng, W-C. Grace Catalagram 2008, 103, 22-31. (33) Lappas, A.A.; Iatridis, D.K.; Papapetrou, M.C.; Kopalidou, E.P.; Vasalos, I.A. Chem.Eng. J. 2015, 278 (2015) 140–149.


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Table 1. Comparison of reactor system attributes. B:C = cumulative biomass to catalyst ratio at end of the experiment. System Pyroprobe

Catalyst 0.010 g

B:C 1:1

WHSV 36 h-1

Analytical GC/MS (quantitative), discrete pulses and analysis

Reactor, feed Packed bed, discrete feed

Fixed-bed reactor

0.500 g

1:1

6 h-1

MBMS (qualitative), discrete pulses and analysis

Packed bed, discrete feed

2” Fluidizedbed reactor

1000 g

1.4:1

7 h-1

GC/MS of condensed oil, end-of-run analysis (cumulative)

Fluidized bed, continuous catalyst and biomass feed

Table 2. Yields of major phases in the fluidized bed experiments and the carbon yield in oil Catalyst/ Mass yield, g/g ZSM-5 Ga/ZSM-5 Ga/ZSM-5 Ni/ZSM-5 biomass Run 1 Run 2 (H2) Oil 17.1% 18.5% 19.3% 15.9% Aqueous 28.1% 26.4% 27.2% 22.7% Gas 29.5% 31.6% 31.8% 39.8% Char 9.5% 9.8% 9.7% 9.5% Coke 8.5% 8.5% 7.8% 6.2% Total 92.7% 94.7% 95.8% 94.1% Oil C yield 25.1% 26.4% 28.5% 24.0% g C/g C in biomass

Table 3. Normalized composition of the oils from the fluidized-bed reactor experiments on dry oil basis. ZSM-5 Catalyst C, wt% (dry oil) H, wt% (dry oil) O, wt% (dry oil) S, wt% (dry oil) H2O, wt%

74.5 7.0 18.4 0.0 4.0

Ga/ZSM-5 Ga/ZSM-5 Run 1 Run 2 75.2 75.6 7.1 7.1 17.7 17.1 0.0 0.0 4.8 3.8

Ni/ZSM-5 (H2) 78.2 7.2 14.4 0.0 3.2


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Figure Captions Figure 1. Schematic diagrams of the main components of the three reactor systems: (a) Pyroprobe, (b) fixed-bed reactor, (c) 2” fluidized-bed reactor. Figure 2. Comparison of aromatic hydrocarbon production for individual pulses of biomass catalytic pyrolysis in the fixed-bed reactor. Figure 3. Hydrocarbon yields, normalized to ZSM-5, during pine pyrolysis vapor upgrading over various catalysts on the Pyroprobe, fixed-bed reactor, and 2” fluidized-bed reactor. Figure 4. Selectivity among hydrocarbons products during pine pyrolysis vapor upgrading over various catalysts in the Pyroprobe, fixed-bed reactor, and 2” fluidized-bed reactor. Figure 5. Cumulative light gas production during catalytic pyrolysis over various catalysts on the (a) fixed-bed reactor and (b) 2” fluidized-bed reactor. The C2+ includes C2, C3 and C4 alkanes and olefins. Figure 6. Coke measurements on post-reaction catalysts collected in the fixed-bed reactor and 2” fluidized-bed reactor, reported on a coke-free, dry-basis. Figure 7. Characterization of (a) surface area and (b) acidity by NH3 TPD on fresh (blue) and spent catalysts used in the fixed-bed (red) and fluidized-bed reactors (green).


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Figure 1. Schematic diagrams of the main components of the three reactor systems: (a) Pyroprobe reactor, (b) fixed-bed reactor, (c) 2” fluidized-bed reactor.

Figure 2. Comparison of aromatic hydrocarbon production for individual pulses of biomass catalytic pyrolysis in the fixed-bed reactor.


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Figure 3. Hydrocarbon yields, normalized to ZSM-5, during pine pyrolysis vapor upgrading over various catalysts in the Pyroprobe, fixed-bed reactor, and 2” fluidized-bed reactor.

Figure 4. Selectivity among hydrocarbons products during pine pyrolysis vapor upgrading over various catalysts in the Pyroprobe, fixed-bed reactor, and 2” fluidized-bed reactor.


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Figure 5. Cumulative light gas production during catalytic pyrolysis over various catalysts in the (a) fixed-bed reactor and (b) 2” fluidized-bed reactor. The C2+ includes C2, C3 and C4 alkanes and olefins.

Figure 6. Coke measurements on post-reaction catalysts collected in the fixed-bed reactor and 2” fluidized-bed reactor, reported on a coke-free dry-basis.


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Figure 7. Characterization of (a) surface area and (b) acidity by NH3 TPD on fresh (blue) and spent catalysts used in the fixed-bed (red) and fluidized-bed (FBR) reactors (green). Error bars reported are scaled from the standard deviation of triplicate analyses of the fresh ZSM-5 catalyst.


Multiscale Evaluation of Catalytic Upgrading of Biomass Pyrolysis

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