Hydrogen-Rich Syngas Production Through Synergistic Methane

Research Article

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX pubs.acs.org/journal/ascecg

Hydrogen-Rich Syngas Production through Synergistic Methane- Activated Catalytic Biomass Gasiﬁcation † † † ‡ † Amoolya Lalsare, Yuxin Wang, Qingyuan Li, Ali Sivri, Roman J. Vukmanovich, ‡ † Cosmin E. Dumitrescu, and Jianli Hu*, † ‡ Department of Chemical and Biomedical Engineering and Department of Mechanical and Aerospace Engineering, West Virginia University, 395 Evansdale Drive, Morgantown, West Virginia 26505, United States

*S Supporting Information

ABSTRACT: The abundance of natural gas and biomass in the U.S. was the motivation to investigate the effect of adding methane to catalytic nonoxidative high-temperature biomass gasification. The catalyst used in this study was Fe− Mo/ZSM-5. Methane concentration was varied from 5 to 15 vol %, and the reaction was performed at 850 and 950 °C. While biomass gasification without methane on the same catalysts produced ∼60 mol % methane in the total gas yield, methane addition had a strong effect on the biomass gasification, with more than 80 mol % hydrogen in the product gas. This indicates that the reverse steam methane reformation (SMR) reaction is favored in the absence of additional methane in the gas feed as the formation of H2 and CO shifts the equilibrium to the left. Results showed that 5 mol % additional methane in the feed gas allowed for SMR due to formation of steam adsorbates from oxygen in the functional groups of aromatic lignin being liberated on the oxophilic transition metals like Mo and Fe. This oxygen was then available for the SMR reaction with methane to form H2, CO, and CO2. This study was not a detailed catalytic activity evaluation, but it was exploratory research to ascertain the synergy presented in the co-gasification of biomass and natural gas. KEYWORDS: Biomass gasification, Co-gasification of biomass and methane, FeMo/ZSM-5 catalyst

■ INTRODUCTION the world energy needs and the shift to renewable energy Per capita energy demand increased sharply over the last sources is not an easy task, owing to the high costs and unreliability of energy sources like solar and wind. Biomass, century, which caused irreversible changes in the weather − patterns across the globe.1 Climate change, which has long which accounts for 10 14% of the global energy supply, can been perceived as a futuristic phenomenon, is already replace fossil fuels as a reliable, sustainable, and green source of producing adverse climatic changes around the globe. The energy, contingent on the development of technologies that can harness its energy in clean, efficient, and economical (i.e., Downloaded via WEST VIRGINIA UNIV on September 18, 2019 at 20:33:19 (UTC). average annual temperature of the planet has already risen by ff 6 1.5 °C, and the current efforts in reducing emissions are cost-e ective) means. Even today, biomass accounts for most focused on curbing this temperature rise to 2 °C.2,3 At the of the energy utilized in the remotest, underdeveloped, and See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ff current rate of emissions, the planet may see a record developing regions globally. Biomass is available in di erent temperatureriseof2°C by 2050. According to the forms such as agricultural and forestry waste, animal waste, Intergovernmental Panel on Climate Change (IPCC), the biological materials byproducts, wood, and municipal waste. ’ These sources of biomass have vastly different moisture carbon dioxide (CO2) concentration in Earth s atmosphere has ffi crossed the unprecedented mark of 400 ppm.4 The world content and elemental compositions, thus making it di cult to develop a cost-effective technology which utilizes most types of community is diligently working toward moving to cleaner, 7,8 fi green, and sustainable energy sources like solar, wind, and biomass. Although woody biomass such as dry rewood is used in combustion for heating and power generation,9 the biomass to meet energy needs especially in the power and ffi transportation sectors.5 Biomass has been a major source of energy e ciency is low, and air emission control becomes an energy for humankind since the discovery of fire in the issue. In contrast, both heat and power generation requirements using gasification can be met in an efficient, effective, prehistoric era and was the predominant source of fuel for 10 heating and cooking applications until fossil fuels like and clean way. Energy utilization by converting solid petroleum, coal, and natural gas were successfully harnessed feedstock to high-heating-value syngas is one of the cleanest in the 19th century. Fossil-fuel-based technologies enabled the human population to increase sharply from less than 1 billion Received: May 17, 2019 before the 18th century to more than 7 billion by 2020, a span Revised: August 22, 2019 of just three centuries. Fossil fuels account for nearly 80% of Published: September 4, 2019

© XXXX American Chemical Society A DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article ways to harness renewable energy source from woody biomass. abundant source like natural gas has a very high H/Ceff ratio of Biomass fast pyrolysis and gasification holds good potential for 4.39 In recent years, there have been efforts to utilize methane the production of hydrocarbon fuels and value-added and biomass in a single reactor, but mainly for the purpose of chemicals.11,12 Pyrolysis has been extensively researched and fast pyrolysis. In one of the recent studies, pine saw dust applied for producing biofuels for direct use in transportation biomass was impregnated with Ni and gasified with steam at 13−21 ° and related applications. However, the decomposition of 600 C to produce H2-rich syngas with maximum H2 yield of the highly aromatic lignin structure in lignocellulosic biomass 60% in the outlet gas. Steam was replaced by methane to leads to formation of bio-oil at temperatures below 700 perform catalytic methane decomposition (CMD) for 10 h at ° 22−24 ° C. Bio-oil is high in oxygen content and has limited 850 C on Ni/carbon catalyst to further increase H2. About direct application as fuel owing to high acidity, poor resistance 90% methane conversion was reported with the process.35 The to extreme weather conditions, and instability.11,25 These present study focuses on nonoxidative catalytic gasification in a limitations are the result of the highly oxygenated aromatic single stage using co-feeding of methane (5−15 mol %) with structure of biomass. The thermochemical conversion of biomass. Although renewable energy sources have started biomass to energy and fuels have been extensively studied contributing significantly to power generation in the US, for the past few decades.26,27 Biomass decomposes rapidly sources like solar and wind face severe limitations to be between 400 and 600 °C giving devolatilization components considered as mainstream source for power generation. Thus, like carbon monoxide (CO), carbon dioxide (CO2), methane natural gas biomass synergy is of key importance given the 9,11,12,22 (CH4), bio-oil, aerosols, and biochar. power and energy requirements of the US in coming decades. There have been numerous efforts to upgrade the yields With the abundance of natural gas in the US and especially the from biomass to value-added chemicals such as benzene, Appalachian region, efficient and cost-effective utilization of toluene, ethylbenzene, and xylene (BTEX), and produce natural gas for fuels and conversion to valuable chemicals along hydrogen-rich syngas.28,29 In situ tar cracking, hydrodeoxyge- with biomass offers great potential for meeting US energy nation (HDO), and rearrangement to form ketonic inter- needs in the near future. Methane is a major constituent in mediates leading to alkanes are typical reaction steps necessary natural gas (>80 mol %), which makes it an inexpensive source for conversion of biomass to valuable chemicals.28,30,31 of hydrogen for tar reforming in the biomass pyrolysis and Transition metals on catalytic surfaces like zeolite (HZSM- gasification process. Methane can be activated with the use of γ 33,40 5), SiO2, and -Al2O3 have been studied to initiate biomass- synthetic transition metal catalysts like Ni, Fe, Co. upgrading reactions like dehydration, rearrangement, decar- This study chose Fe−Mo/ZSM-5 for the co-gasification of boxylation, decarbonylation, and hydrodeoxygenation.7,28,32 methane and biomass at 750−950 °Cinafixed-bed reactor However, the complexity of reaction chemistry, the require- system. In general, zeolites such as ZSM-5 with acidic function ment of high-pressure hydrogen, and the rapid catalyst can be utilized to crack large molecules such as biomass. deactivation due to coke deposition and poisoning increases Adding metals to zeolite creates bifunctional properties − the cost of the process, rendering it economically nonviable. facilitating dehydrogenation and hydrogen transfer.41 43 Fe Methane-promoted catalytic biomass gasification can achieve on ZSM-5 was shown to assist methane decomposition with in situ tar reformation and cracking, thus producing high less energy intensive cleavage of the C−H bond.39,40,44 Fe- syngas yields with an enhanced H2/CO ratio, suitable for impregnated ZSM-5 catalyst has been subjected to high- chemical synthesis via a conventional Fischer−Tropsch (FT) temperature methane decomposition to produce hydrogen and synthesis process. Air−steam catalytic and noncatalytic gas- carbon nanotubes (CNTs). Fe−Mo/ZSM-5 catalyst has also fi i cation were reported on Ni/CeO2/Al2O3 with catalyst been utilized for methane and ethane dehydroaromatization loadings of 20−40% in a fluidized bed reactor at 725, 825, studies. Iron active sites supported on ZSM-5 have been shown and 900 °C.33 Nishikawa et al. reported that at 900 °C high to cause 3D coke deposition on the catalyst in the form of catalyst-to-biomass loading of 40% was effective in tar cracking CNTs. Coke characterization on the iron-based catalyst and high-purity hydrogen production.33 Ni nanoparticles on showed that crystalline coke formation is pronounced than MCM-41 supports were utilized for biomass gasification to that of amorphous coke. Amorphous coke deposition on the enhance hydrogen production.34,35 Use of a suitable catalyst catalyst leads to the blocking of active metal sites thus causing fi for biomass gasi cation increased the H2/CO ratio in syngas catalyst deactivation. Crystalline coke deposition in the form of 36,37 − − 40 from 1.15/2.15 to 1.87/4.45. Fe Ni/CeO-Al2O3 and CNTs delays Fe Mo/ZSM-5 catalyst deactivation. Iron is noble metals like Pt, Pd, and Rh were synthesized and used known to be conducive for high-temperature methane for biomass steam gasification which showed that Fe-promoted activation, and atomic molybdenum impregnated on ZSM-5 catalysts produce higher gas yields.33 Coconut-shell gasification is highly oxophilic (i.e., having good oxygen-binding affinity). was performed with steam on Pt-, Fe-, and Co-promoted Oxophilicity of Mo on ZSM-5 is key for hydrodeoxygenation catalysts, and it was reported that the use of Fe-promoted and steam methane reforming (SMR) in the methane-activated catalysts increased hydrogen and carbon monoxide composi- biomass gasification reaction system. This was the primary tion by improving the water−gas shift (WGS) reaction.38 reason we selected the Fe−Mo/ZSM-5 catalyst. It is postulated Catalytic fast pyrolysis (CFP) on inexpensive Mo−Ag/ZSM-5 that intermediate hydrogen species participate in cracking the catalysts with methane in a single reactor system operated at oxygenated aromatic structure of lignocellulosic biomass by atmospheric pressure showed that the catalyst deactivation rate hydrodeoxygenation (HDO), decarbonylation, and decarbox- decreased with an increasing H/Ceff ratio. ylation reactions. Fe and FeOx active sites are directly This ratio can be increased by fast pyrolysis/gasification of responsible for high-temperature WGS (HT-WGS) and SMR 41,45−47 biomass with a hydrogen-rich source like methane. H/Ceff ratio reactions. Although there is little evidence that acid of biomass is about 0.3, which is not suitable for producing sites on ZSM-5 directly facilitate WGS and SMR reactions, the hydrogen-rich syngas for downstream production of value- acidity of the ZSM-5 support affects the conversion of CO (in added chemicals. Methane that comes from an inexpensive and WGS) and CH4 (in SMR). Meanwhile, Bronsted sites and

B DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article some Lewis acidic sites on ZSM-5 are known to be effective in ramped from 100 °C to the reaction temperature, initiated while the reactor valves were shut until the temperature ramp was complete conversion of highly oxygenated lignin to value-added − 31,34,48,49 with a ramp rate of 20 °C min 1. This method was adopted for chemicals like BTEX. This study investigated the fl possible synergy between co-gasification of hardwood biomass replication of continuous feeding of biomass in a uidized bed or moving bed reactor at the reaction temperature. and methane. Iron (Fe) and molybdenum (Mo) were used as It was observed through elemental composition analysis of promoters on ZSM-5 zeolite as they are inexpensive and − hardwood lignocellulosic biomass that oxygen accounts of 49 wt % provide active sites for C H bond activation and converting of the biomass on dry basis. Therefore, for hydrogen-rich syngas lignin oxygen, thus minimizing tar formation. The catalyst production, an external source of oxygen is not required for the typical preparation method, type of biomass and its elemental biomass. With the use of external oxygen as gasifying agent, like CO2 composition, type of reactors used, and product analysis or H2O, it was observed that when >5 vol % carbon dioxide was technique are described in the Experimental Section. Biomass added along with 5 vol % methane in the catalyst−biomass system, fi fi fi high concentrations of CO and CO2 were observed in the product gasi cation in TGA, biomass gasi cation tests in a xed bed − without methane, methane−biomass gasification tests, coke gas. However, using 1% CO2 and 5% CH4 with biomass on Fe Mo/ ZSM-5 and Fe−Mo/CNF(carbon nanofiber) catalysts produced a formation, and the synergistic effect of methane addition on “ H2/CO ratio = 2. CO2 thermal activation occurs in neighborhood of H2/CO ratio have been discussed in detail in the Results and ” atomic hydrogen (from methane and biomass) on the catalyst active Discussion section. sites. This finding is being published as an independent study. Moreover, external steam addition would lead to direct SMR, thus ■ EXPERIMENTAL SECTION hindering the possible synergy between natural gas and biomass. − Preliminary biomass gasification studies were also performed using Catalyst Preparation. The catalyst (Zeolyst, Inc.) was NH4 ZSM-5 zeolite with a silica/alumina (SAR) of 23. Ammonium a thermogravimetric analysis (TGA) instrument (TA Instruments, molybdate (VI) tetrahydrate and iron(III) nitrate nonahydrate were Waters LLC, model SDT 650). The product line from this instrument purchased from Acros Organics. The zeolite catalyst was first calcined was connected to a mass spectrometry (MS) instrument (Quantach- at 500 °C in air for 3 h to convert NH −ZSM-5 to H−ZSM-5. The rome) as described in the following section. Reaction conditions in 4 fi conventional incipient wetness technique was used to prepare Mo− TGA instrument were similar to those in the xed-bed reactor. The Fe/ZSM-5. After drying the catalysts at 105 °C to remove the water reaction was performed at several temperatures ranging from 750 to ° overnight, the dry powder catalyst sample was further calcined in air 950 C. Helium (He) was used as the carrier gas for these tests. at 550 °C for 4 h. The chemical composition of the synthesized Continuous product composition analysis was carried out with the catalysts are shown in Table 1. The elemental composition of MS instrument connected downstream. Sample size for a typical test hardwood biomass subjected to all experimental tests presented in the varied from 20 mg for a biomass without catalyst system to 70 mg for − manuscript is shown in Table 2. a biomass ZSM-5 system. Product Analysis. Product gases collected in sampling bags − (SKC) from the fixed-bed reactor tests were analyzed by a four- Table 1. Mo Fe/ZSM-5 Metal Loading of Mo and Fe on column gas chromatograph (Inficon Fusion micro-GC). The four ZSM-5 Catalyst Synthesized by Wet Incipient Method columns consisted of a molecular sieve with a 3 m long PLOT U − catalyst Mo Fe precolumn, an 8 m long RT PLOT U with a 1 m long PLOT Q precolumn, an 8 m long aluminum column, and a 20 m long RTX−1 FeMo1 4 wt % 0.5 wt % column. The four columns allowed for calibrated (ppm level) FeMo2 4 wt % 1.5 wt % detection of hydrogen, methane, carbon monoxide, carbon dioxide (complete syngas profile), ethylene, ethane, acetylene, water, nitrogen, Table 2. Elemental Composition, Moisture, and Ash and ammonia. All gases used for calibration were ultrahigh purity a Content (wt %) of Hardwood Biomass Used in the Study (UHP)-grade (AirGas). carbon hydrogen oxygen ■ RESULTS AND DISCUSSION (C) (H) (O) moisture ash hardwood 45.25 4.65 49.2 7.16 0.32 Conventional Biomass Gasification and Devolatiliza- biomass tion Studies in TGA. TGA-MS was used for the initial aPerformed by National Research Center for Coal and Energy. biomass gasification screening tests to identify a temperature range for hydrogen and carbon monoxide evolution. Figure 1 Reaction Conditions. Catalytic hardwood-pellet biomass gas- shows the concentration profile versus time of the four major ification was performed in a downdraft fixed-bed reactor (12.7 mm syngas components obtained from the TGA-MS studies. For diameter, 915 mm long) stainless steel (316SS) reactor tube fi ° devolatilization and gasi cation at 650 C, only 7% H2 yield (Charleston Valve and Fitting Co.). In a typical experimental test, was obtained. CO and CO made up most of the gas yield 0.75 g of Mo−Fe/ZSM-5 catalyst was premixed with 1 g of 2 obtained at temperatures less than 300 °C with methane lignocellulose hardwood-pellet biomass. Biomass was ground and ° screened to a mean particle diameter of 432 μm. This particle size was evolving beyond 300 C possibly indicating thermal cracking selected to maintain consistency with the 432 μm particles used for a of the array of oxygenated aromatic rings in the lignin structure bench-scale continuous-bubbling fluidized-bed setup that is currently of biomass. being tested under different parametric conditions of temperature, As methane started appearing in the gas products, carbon gasifying agent, feed/bed material ratio, and premixed catalyst. dioxide yield began to fall, and only 6% carbon dioxide Reactor bed temperature was measured using a K-type thermocouple ° remained at 515 C. This suggested that the dry H2 reforming (Omega). Prior to loading into the reactor, the catalyst was subjected reaction dominated between 335 and 515 °C, where all the to reduction with 10 vol % hydrogen (H2) with nitrogen (N2) at 600 hydrogen that was bonded to saturated/unsaturated carbon in °C for 3 h. Catalyst and biomass were mechanically mixed in a ratio of 0.75:1 respectively. A total nitrogen flow of 300 sccm was maintained biomass reacted with the CO2 from devolatilization and as the temperature was raised from room temperature to 100 °C. thermal cracking forming CO. Higher CO concentrations were After moisture and air removal from the reactor, a constant flow of detected at higher temperatures as char oxidation reaction 300 sccm with the desired methane concentration was maintained kicks in. Hydrogen started to appear as a predominant species until the reactor pressure reached 50 psig. Fixed-bed temperature was in the normalized concentrations obtained from the mass

C DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

Figure 1. Biomass gasification (hardwood pellet; heating rate of 20 Figure 3. Biomass gasification (hardwood pellet; heating rate of 20 °C/min, then 30 min at 650 °C; 100 sccm He flow). °C/min, then 30 min at 950 °C; 100 sccm He flow). spectrometry ionization signals only after a threshold temper- Table 3. H2/CO and CO/CO2 Ratios of TGA Biomass ature of about 650 °C. Figures 2 and 3 show that between 650 Gasification Tests Observed at Temperatures in the Range of 750−950 °C ° temperature ( C) H2/CO CO/CO2 750 1.87 16.2 850 1.1 N/A 950 1.1 23

douard reaction (eq 1), which converted carbon to CO due to its highly endothermic nature:9 −1 C+Δ CO2F 2COH° 298 = 172 kJ mol (1)

Similarly, a sharp increase in H2 and improvement in the H2/CO ratio may also be due to continuous heat being supplied driving the endothermic gasification reactions forward. Between 650 and 950 °C, methane was not observed in the product probably because of reforming with water or CO2, which is evidenced by the increase in CO and H2 concentration shown in Figure 3. Steam reforming: −1 Figure 2. Biomass gasification (hardwood pellet; heating rate of 20 CH4++Δ H 2 OF CO H 2H° 298 = 206 kJ mol (2) °C/min, then 30 min at 850 °C; 100 sccm He flow). Dry reforming: −1 CH4++Δ CO 2F 2CO H 2H° 298 = 247 kJ mol and 950 °C hydrogen and carbon monoxide constituted most (3) of the gas yield. This indicated that at a lower temperature and Typically, methane decomposition occurs at temperatures without an oxidative atmosphere like steam or air the higher than 1000 °C in the absence of a catalyst. Due to the devolatilization and biochar formation occurred between 200 presence of mineral materials in biomass, the methane − and 600 °C.50 52 In the TGA tests, the biomass sample size decomposition reaction cannot be ruled out here. The was too small (∼20 mg) to trace any tar formation.53 In the proportion of tar in dry woody biomass falls in the range of alumina crucible used for the tests, only ash residue was 5−12 mg/g (0.5−1.2 wt % on a dry basis) of dry biomass.10 observed with the end weight of the tared crucible measured as With only 20 mg of biomass in the TGA tests, the possibility of − around ∼0.5 mg. The isothermal reaction time at the test traceable tar formation is very low.34,54 56 Endothermic temperatures was sufficient to completely gasify biochar. Char equilibrium reactions in the regime of char gasification and gasification reactions were observed at higher temperatures, homogeneous volatile reactions may have pronounced effects 57 evident by the high H2/CO and CO/CO2 ratios at on the H2/CO and CO/CO2 ratios. temperatures beyond 650 or 700 °C(Table 3). Surprisingly, Figure 4 illustrates the results of a TGA test where biomass no methane formation was observed at char gasification was mixed with ZSM-5 and gasified under identical reaction temperatures (650−950 °C) possibly indicating the Bou- conditions as used in the previous TGA tests. It was observed

D DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

Figure 5. Biomass gasification in the absence of methane operated in a fixed-bed reactor ZSM-5 support and Mo−Fe/ZSM-5 at 850 and Figure 4. Biomass gasification on ZSM-5 (hardwood pellet; heating 950 °C. rate of 20 °C/min, then 30 min at 950 °C; 100 sccm He flow).

fi − ° that the H2/CO ratio improved signi cantly at 750 800 C. The presence of ZSM-5 may have enabled cracking reactions the surface was deficient in methane, which favored the at higher temperatures. Notably, a high concentration of formation of methane and water vapor as the adsorbed CO methane was observed between 450 and 550 °C, indicating and H2 underwent reverse SMR at high temperature. hydrogenation of higher aromatics and some aliphatic chains The results suggest a synergy between methane and biomass, ∼ ° forming CH4 almost as much as CO at 400 C. More CO which shifted the SMR equilibrium to the right and forms and H2 was observed as temperature in the reactor increased. hydrogen-rich syngas. Detailed discussion on the results of This indicated that the presence of a catalyst like zeolite helped methane−biomass gasification studies are discussed in the next to improve the syngas quality with in situ tar reforming and section. Without an external methane feed, typical gasification cracking. The fixed-bed reactor biomass and methane−biomass reactions only yield about 1:1 H2/CO ratio as is the molar gasification had a significantly different reaction environment, ratio of biomass. Biomass and natural gas are both abundant heat flux, and heat flux direction than those in the TGA sample and largely untapped sources of clean and efficient energy crucible. A fixed-bed reactor was operated as a downdraft which can help meet more than 80% of the total energy needs gasifier, whereas gas flow was horizontal in the case of TGA.58 in the US by 2030. Utilization of natural gas with the The interaction between catalyst active sites, char, and gaseous development of new processes will also help revive the products was different in the case of the fixed-bed reactor as Appalachian economy especially that of West Virginia which compared to the TGA. Having said this, TGA provided a strategically lies in the Marcellus shale gas basin.60 Thus, the better study of the evolution and profile of possible gasification synergy between biomass and methane has been explored in reactions occurring in a real reactor like the fixed-bed studies the following sections. presented in following sections. Coke Formation and H2 from CH4 Decomposition. Biomass Catalytic Gasification in the Absence of When using either 0.5 or 1.5% Fe-promoted Mo/ZSM-5 Methane in a Fixed-Bed Reactor. Figure 5 shows that in surface, coke formation was observed during co-feeding of the absence of methane the biomass gasification yields in a biomass and methane. However, in the absence of methane, fixed-bed reactor were (on average) 55 and 58 mol % methane biomass gasification coking was not observed on either catalyst. ° for the reaction at 850 and 950 C, respectively. A H2/CO This indicates that some of the methane in the gas feed is ratio between 0.5 and 1.0 was obtained, which was roughly in decomposed at high temperatures to form coke and produce the range of conventional noncatalytic gasification. However, hydrogen (H2) due to the presence of Fe active sites on ZSM- with almost 55−60% methane in the gas yield and a very low 5. A stoichiometric amount of hydrogen formed from methane − CO2 yield of 2 13%, the results suggested that the reverse decomposition was accounted for based on coke formed from SMR and dry reforming (eq 2) was dominant at high methane decomposition.61 The calculation for hydrogen temperature as both of these reactions were endothermic. obtained from biomass−methane synergy was calculated in Reverse SMR leading to conversion of hydrogen and carbon mol % as follows: monoxide into methane could be due to the presence of acidic HHH2(prod)=− 2(total) 2(coke) (4) active sites on Fe and Mo metals.59 In the case of biomass fi gasi cation on ZSM-5, the methane yield was 29 mol %, which H2/CO ratios obtained for various concentrations of is more than 3 times that seen in typical noncatalytic biomass methane and reaction temperatures were calculated after gasification. The high methane yield in the product was accounting for hydrogen obtained possibly from methane attributed to the thermal cracking of the aromatic array of decomposition. Very little or no tar was recovered from the biomass. Moreover, in the absence of an acidic oxophilic gas−liquid separator at the bottom of the vertical tubular transition metal like Fe or Mo, the oxygenated carbon in the reactor. However, small concentrations of ethane, ethylene, biomass was converted to CO, CO2, and H2 which reacted on and acetylene were seen in the product analysis on the micro- the surface. Also, without additional methane in the gas feed, GC.

E DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

Gas Yield, Tar Formation, and Char Deposition on 84 mol % for the reaction at 850 °C. Gas yield substantially Catalyst. Overall carbon balance for the methane-activated increased from 37.5 mol % at 750 °C (not shown here) to 84 biomass gasification was calculated using the inert gas and mol % at 850 °C and further to 88 mol % at 950 °C. However, methane moles before the temperature ramp and the molar gas yield decreased slightly from 84 to 80 mol % at 850 °C volume occupied by the gas inside the tubular fixed-bed reactor when the gas feed methane concentration was increased from 5 based on reaction temperature and pressure. All of the biomass to 15 vol %. A similar trend was also observed at 950 °C as gas carbon was recovered using TGA studies of spent catalyst for yield decreased from 88 to 82 mol % when methane coke and char characterization. Amount of condensed tar was concentration was increased from 5 to 15 vol %. A marginal estimated using hexane solvent wash of the reactor tube and increase in char yield from 3 to 8 mol % at 850 °C and from 2 condenser. The mole percent and yield of individual products to 7 mol % at 950 °C was also observed when methane were calculated based on gas moles obtained in the product. concentration was increased from 5 to 15 vol %. This could be However, we chose to present mole percent and product yield explained based on the catalyst deactivation phenomenon based on syngas moles as it would be more accurate to do so in possibly occurring due to the high methane coverage on Fe this case. This is because there was coke deposition on the and Mo active sites leading to a paucity of sites available for catalyst as presented in the manuscript, and hydrogen from methane−biomass reaction. Moreover, without methane in the methane decomposition was not considered as hydrogen gas feed, biomass conversion was much higher on the FeMo1 produced from methane−biomass synergy. To be consistent catalyst (87.5% at 850 °C and 90% at 950 °C) compared to with the objective of this work, which is to show natural gas− that in methane-activated biomass gasification tests. However, biomass synergy, mol % and yield was calculated on the basis >80 mol % product gas yield is an indication that low methane of product gas mole. To be fair, if one was to calculate mole concentration leads to high methane and biomass conversion percent on the basis of starting moles of biomass, then the due to Fe and Mo being conducive to methane activation and mole percent of all the species would be smaller by about 10− biomass hydrodeoxygenation. Gas yield, char, and tar 20% depending on the gas, tar, and char yield. components in biomass gasification without methane and methane-activated biomass gasification on FeMo1 catalyst are Yield (mol %) = given in Table 4 and Figure 6. number of moles of species/total product gas moles (5) Table 4. Co-Gasification of Biomass and Methane a It was also observed that normalized product gas Gasification over Fe−Mo/ZSM-5 composition and mole percent calculated from the yield of individual species based on gas moles were similar and thus parameters 850 °C 950 °C were used interchangeably in discussing the results. It was gas yield in biomass, no methane (mol %) 87.5 90.1 observed that all the experimental tests produced greater than 75.6, 5 68.2, 5 or equal to 80 mol % product gas. Tar mole percent ranged coking FeMo1 (mg, % CH4) 135, 10 136, 10 between 10 and 15%, while char was only about 2−8 mol %. 246, 15 204.5, 15 Product gas yield from methane-activated biomass gasification 51.5, 5 can be termed as biomass conversion obtained through the coking FeMo2 (mg, % CH4) 169.2, 10 N/A reaction. Gas yield, tar, and char composition of the biomass 267.3, 15 carbon balance are presented in Figure 6. It was observed that aMeasured amount of coke on catalyst and gas yield for no-methane product gas yield was 88 mol % for the reaction at 950 °C and biomass gasification.

As described in Table 4, coking on the catalyst was between 10−30 wt % of the original weight of catalyst after reduction. Under identical conditions, biomass gasification in the absence of methane in the gas feed yielded almost no recoverable coke on the FeMo1 catalyst. No weight loss was seen in the catalyst either before or after calcination. It can be concluded that coke deposition on both FeMo1 and FeMo2 catalysts was due to methane decomposition when 5−15% was used in the gas feed. The amount of coke from each test using methane is shown in Table 4. No considerable coke formation on the catalyst was observed for biomass gasification without methane, indicating that the aromatic components of lignin with branched functional groups like carbonyl carbon (CO) and hydroxyl carbon (C−OH) undergo surface reactions by latching onto the ZSM-5 acidic sites. Most of the oxygen from the biomass possibly reacted on the surface with the available hydrogen to undergo reverse SMR without external methane (eq 2). The quantifications of moles of hydrogen, carbon monoxide, carbon dioxide, and methane are provided in Table S1. fi Figure 6. Gas, tar, and char yield obtained in biomass gasification Synergistic Methane-Activated Biomass Gasi cation. without methane and in methane-activated synergistic catalytic Synergistic gasification of hardwood biomass with methane fi ° fi biomass gasi cation at 850 and 950 CandCH4 gas feed was studied in the xed-bed reactor on FeMo1 and FeMo2 concentrations 0, 5, 10, and 15 vol % performed on FeMo1 catalyst. catalysts for methane concentrations ranging from 5−15 mol

F DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

%. Figures 7 and 8 show the concentration of typical syngas 11%, respectively, at 850 °C as seen in Figure 5. At 950 °C, components obtained from methane-activated biomass gas- CO yield increased to 25%, and CO2 decreased to 4%. However, the average methane yield was more than 50% of the total moles obtained in the product gas at both 850 and 950 °C. Hydrogen mole percent in biomass that was used for the study is almost equal to the carbon mole percent (see Figure fi 5). Biomass gasi cation typically yields H2/CO less than or close to 1.54 The synergy between methane and biomass reacting together on an active catalyst like FeMo/ZSM-5 was apparent from the multifold increase in the hydrogen yield. The presence of methane in the gas feed suggested an equilibrium shift in some of the typical gasification reactions with SMR possibly occurring on the active sites of Mo and Fe. Both molybdenum (Mo) and iron (Fe) are known to be moderately oxophilic: They have a higher binding affinity to oxygen as compared to those of metals like Zn, Ni, and Cu, but they have a lower oxygen binding affinity as compared to those of Ti, V, and Sc among the transition state or d block metals. Mo is slightly more oxophilic than Fe (0.6 and 0.4), thus having the capability to activate C−OH and CO type of bonds abundantly present in the complex array of aromatics in 62 Figure 7. Methane-activated biomass gasification at 850 °C for lignin component of the hardwood biomass. Hydrodeoxyge- methane concentrations of 5, 10, and 15 mol % using FeMo1 catalyst. nation (HDO) reactions studied on PdZn surfaces have shown that Zn, being an oxophilic metal, latches the oxygen present in the functional groups thus activating C−OH bonds.62 Figure 9 shows a representative structure of aromatic and furfural chains and −OH functional groups in lignin. The

Figure 9. Representation of the type of C−O bonds in lignin. R represents a type of aromatic structure similar to that in the rest of the proposed description of the lignin molecule.64

loosely bonded oxygen and hydroxyl groups on the surface potentially react with the hydrogen obtained from devolatilization gases of biomass to produce H2O adsorbates on the active sites of Mo and Fe. Methane in the feed reacts with the H2O adsorbates on the surface sites undergoing high-temperature Figure 8. Methane activated biomass gasification at 950 °C for SMR to produce hydrogen and carbon monoxide. H2O methane concentration of 5, 10, and 15 mol % using FeMo1 catalyst. adosrbates would evolve by selective adsorption of lignocellulosic oxygenates like alcohols, phenols, furfurals, ethers, and ification. Hydrogen was the dominant species in the product acids. High-temperature thermal cracking of lignoceullosic gas with hydrogen to carbon monoxide (H2/CO) ratio as high components to single-chain phenols, furfurals, and alcohols ° as 7.5 when reaction was performed at 950 C with 5% CH4 in allows for passage through microporous ZSM-5 channels and the gas feed along with 95% carrier gas N2. As the methane selective adsorption of iron and molybdenum active sites in the concentration is further increased to 10 and 15%, the H2/CO ZSM-5 framework. The presence of Fe and Mo on acidic ratio dropped sharply at 950 °C to 3.8 and 3.7, respectively. ZSM-5 in metallic form helps shift the SMR reaction forward ° When the same reaction is performed at 850 C with 5, 10, and thus producing more H2 and CO. Reverse SMR is one of the fi 15% CH4 in the feed, H2/CO drops steadily from 6 to 4.2. In main reactions grouped in biomass gasi cation reaction the absence of a catalyst, typical hardwood biomass gasification chemistry. Although reverse SMR is a highly exothermic − Δ − −1 produced a H2/CO ratio of 0.3 0.5. This sharp increase in reaction with Hrxn,298 K of 206 kJ mol , in the presence of hydrogen gained by adding methane is an interesting aspect of methane as a gas feed, the reaction equilibrium seems to shift this study, which likely shows the synergy between methane to the right to form hydrogen and carbon monoxide. This and biomass at high temperatures on a catalyst surface. The likely scenario explains the more than 85% yield of hydrogen in fi same gasi cation reaction when performed under identical the gas feed with a very high H2/CO ratio of 7.5 in one of the conditions on the same catalyst without methane in the gas cases. feed shows contrasting results. As high as 70 mol % methane The effect of temperature and methane concentration on was seen in the product gas with hydrogen yield averaging at biomass gasification was compared at 750, 850, and 950 °Cin ° 17% at 850 and 950 C. CO and CO2 yield averaged at 19 and the presence of 5 mol % CH4. As seen in Figure10, methane-

G DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

note that the catalyst subjected to higher methane concentrations is not completely deactivated since the H2/CO ranges − ° from 3.7 to 5 for 10 15 vol % CH4 at 850 and 950 C. Without catalyst and methane in the gas feed, the H2/CO ratio reached a maximum of 0.9 which is typical for the given biomass gasification. Another possibility behind decrease in ff H2/CO is an adverse e ect of high concentrations of methane in the gas feed on other hydrogen producing reactions like WGS. This may be due to gradual increase in Fe/O ratio at the wustite−Fe interface when the temperature increases from 700 and 950 °C, leading to the loss of active sites on the surface and consequently leading to decrease in hydrogen production and a slight increase in carbon monoxide production.63 Standard deviations and 95% confidence interval for biomass gasification without methane and methane-activated biomass gasification are shown in Table 5.

Table 5. Statistical Analysis for Product Composition of ff ff fi Syngas Components for Di erent Reaction Temperatures Figure 10. E ect of temperature on biomass gasi cation on FeMo1 − catalyst in the presence of 5 vol % methane at 750, 850, and 950 °C. without CH4 and with 5 15 vol % CH4 syngas standard confidence fi activated biomass gasi cation reaction produces a H2/CO ratio test conditions species average deviation interval fi ± of 1.18 which is higher than that obtained from xed-bed H2 16.47 0.11 0.15 fi ± biomass gasi cation but not suitable for hydrogen-rich syngas ° CO 18.63 4.64 6.43 850 C, no CH4 ± production. The product gas composition obtained from CO2 10.81 2.51 3.48 fi ± methane-activated biomass gasi cation reaction at 850 and 950 CH4 54.09 7.26 10.06 ° ± C with 10 vol % CH4 in the gas feed on FeMo1 catalyst is H2 16.35 1.26 1.43 ± shown in Figure 11. As shown in Figure 10, the H2/CO ratio ° CO 23.78 8.11 9.17 950 C, no CH4 ± CO2 4.60 3.29 3.73 ± CH4 55.27 11.93 13.50 ± H2 72.64 15.33 21.2 ± ° CO 12.29 2.96 4.1 850 C, 5% CH4 ± CO2 0.90 0.32 0.45 ± CH4 0.81 0.29 0.4 ± H2 81.58 0.78 0.88 850 °C, 10% CO 16.42 0.44 ±0.49 ± CH4 CO2 1.05 0.61 0.69 ± CH4 0.94 0.55 0.62 ± H2 68.26 13.28 18.41 850 °C, 15% CO 16.14 0.79 ±1.10 ± CH4 CO2 2.77 0.29 0.40 ± CH4 2.49 0.26 0.36 ± H2 74.90 13.98 19.37 ± ° CO 10.04 1.94 2.69 950 C, 5% CH4 ± CO2 4.73 3.86 5.35 ± CH4 4.26 3.48 4.82 ± H2 63.30 20.41 28.29 ° CO 16.83 6.64 ±9.21 Figure 11. Effect of temperature on biomass gasification on FeMo1 950 C, 10% CH ± catalyst in the presence of 5 vol % methane at 850 and 950 °C. 4 CO2 0.82 0.61 0.85 ± CH4 0.74 0.55 0.76 ± H2 70.84 1.57 2.18 drops sharply when the methane concentration is increased to 950 °C, 15% CO 18.93 1.45 ±2.00 10 mol % at 950 °C, whereas the decrease in H /CO ratio is ± 2 CH4 CO2 2.13 3.01 4.17 not as steep at 850 °C. A decrease in the H /CO ratio from 7.4 2 CH4 3.83 0.00 0 at 950 °C to 3.7 when methane concentration was increased from 5 to 15 vol % is possibly due to partial catalyst deactivation and reaction kinetics. The Fe−Mo/ZSM-5 Proposed Reaction Mechanism of Synergistic Meth- catalyst has a tendency to form CNTs due to carbon ane−Biomass Gasification. The following elementary deposition. It appears from the H2/CO trend that the reaction pathways are proposed to illustrate the interaction optimum methane concentration in the gas feed gives of methane with water molecules produced from biomass over hydrogen-rich syngas. When excess methane is available in the catalyst surface. Reactant molecules are associatively the reaction atmosphere, high-temperature methane decom- adsorbed on a single metal site (Fe or Mo) for − position leads to coke deposition on the catalyst. However, we simplification.65 69 The Langmuir−Hinshelwood adsorption

H DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

Table 6. Proposed Reaction Pathway for Synergistic Methane−Biomass Gasiﬁcation

k6 * CHCH4(g) +*HIoo 4 eq 6: methane adsorption on the active site k‐6 k7 * [RCOH−− ](g) +*HIoo [−− RCOH] eq 7: selective adsorption of functional group oxygen atom of the lignocellulosic biomass on Fe or Mo k‐7 k7 * H2H2(g) +*HIoo eq 8: gas-phase hydrogen dissociation on catalyst surface k‐7 k9 * * +−* [RCOH−− ]+ HHIoo [− RC − O H2] eq 9: coordination of adsorbed oxygen with H2 present in the gas phase k‐9 k10 +−* * ﬀ [RC−− O H2]′+HIooo R HO2 eq 10:H2O molecule breaks o from the phenolic carbon and forms steam adsorbate k‐10 * * k11 * * ﬁ CHHOCHO2HH42+ ⎯→⎯ ++2(g) eq 11: rst equation of methane reforming with steam adsorbate from step 4

k12 CHO* +*HIooo CO* + H* eq 12: intermediate CHO* converting to CO* and H* adsorbates k‐12 k13 * * +−* [RCOH−− ]+ HHIooo [− RC − O H2] eq 13: further phenolic oxygens coordinate with H* atom adsorbate and follows similar kinetics as steps 3−6 k‐13 k14 − COHOCOH*+2 *⎯→⎯*+*22 eq 14: water gas shift reaction k15 * COCOHIooo (g) +* eq 15:CO* desorption k‐15 k16 * * COCO2 HIooo 2(g) + * eq 16:CO2 desorption k‐16 k17 * * 2HHHIooo 2(g) + * eq 17:H atom adsorbates combine to form gas phase H2 k‐17 k18 * * HH2 HIooo 2(g) + * eq 18:H2 desorption k‐18 mechanism is considered in our analysis (Table 6, where ‘*’ the gas phase. Surface-adsorbed atomic hydrogen (H*)is represents an active site): possibly in the neighborhood with other H* adsorbates, thus − − [R C OH] is the type of bond present in the lignin combining to form more hydrogen (H2) in the gas phase. component of hardwood biomass. C−OH bonds are possibly Based on high hydrogen yield in the gas phase, it can be said activated on either Fe or Mo active sites which interact with that k17 ≫ k-17. The remaining eqs 7, 9, 10, 12, and 13 could the surface and methane as described in the equations above. be competent in being the rate-limiting steps. However, among On the basis of the discussion of the experimental results of these five equations, reactions from eqs 9 and 13 could be the methane−biomass gasification, the common conditions for all rate-limiting steps depending on the stability and binding − +− −* methane concentrations and iron loadings were a high H2/CO energy of [R C O H] on the given surface. The − − ratio, 1 5mol%CO2,CH4 yield 50 90% less than mechanistic pathway proposed above for a unique SMR fi conventional biomass gasi cation, and a high hydrogen mole reaction explains the high H2 yield (>80 mol %) in the product percent in the gas yield. This pathway can explain the major gas. distinction between biomass gasification, methane dissociation, Comparison of Catalytic and Noncatalytic Biomass methane SMR, and synergistic methane−biomass gasification. Gasification. Figures 12 and 13 compare the product On the basis of the experimental data, the following composition and H2/CO ratios for hardwood biomass assumptions can be made for the above mechanistic model. gasification without catalyst and external methane, biomass ≫ fi Rate constant k18 k-18, which implies that desorption of H2 gasi cation on ZSM-5 catalytic support without methane, and in gaseous form from the surface is fast step. On the contrary, methane-activated biomass gasification with 5 and 10 vol % based on the product composition, it could be argued that rate CH4 on FeMo1 and FeMo2 catalysts. Biomass-only gas- constant k-16 ≫ k16 and rate constant k-15 ≫ k15, implying ification was performed noncatalytically and on ZSM-5. On − that both CO2 and CO are highly stable on the surface, FeMo1 and FeMo2 catalysts, biomass methane experiments respectively. This could be due to the oxygen in CO and CO2 were performed with methane concentrations of 5 and 10%. binding strongly with oxophilic Fe/Mo metals on the surface. The H2/CO ratio on ZSM-5 more than doubled compared to This could also be the reason behind the 50−90% less yield that in a typical noncatalytic test. This indicated that the obtained for CO and CO2. Equation 17 shows two surface presence of acid sites on ZSM-5 along with Fe active sites * hydrogen atoms forming H2 . Desorption of the surface H2 provide favorable atmosphere for WGS and SMR reactions. appears to be relatively easy if the assumption for eq 17 was This is evident from the decrease in methane concentration true. Among the fast, nonequilibrium reactions eqs 11 and 14, and increase in carbon dioxide formation. The addition of Fe * * the rates of formation of CHO ,H2, and surface-adsorbed H and Mo to ZSM-5 leads to the synergistic interaction between * * ≫ must be higher than that of CO2 and H2 formation (k11 methane and biomass to produce hydrogen-rich syngas. The fi − k14), as apparent from the very low CO2 yield. Experimental H2/CO ratio increases signi cantly when methane biomass studies indicate that water−gas shift does not contribute gasification was performed on FeMo1 and FeMo2 catalyst. fi ∼ signi cantly to the gas yield apparent from low CO2 yield in With 5% CH4 in the gas feed, a H2/CO ratio of 7 was

I DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

oxophilic nature of both iron (Fe) and molybdenum (Mo), which converted the oxygen in the various functional groups of lignin to steam that in turn reacted with the available methane to form more hydrogen. Catalyst surface and active sites on the metals were also favorable to a HT-WGS reaction, as suggested by the multifold increase in the H2/CO ratio. However, the H2/CO ratio dropped by more than 50% when the methane concentration increased from 5−15 vol %, possibly due to partial catalyst deactivation and adverse reaction kinetics. The synergistic biomass−methane nonoxidative gasiﬁcation provided a good motivation for the catalytic conversion of biomass to fuels with natural gas utilization. The synergistic relationship between methane and hardwood biomass will be investigated in detail using transition metals like Ni and Ru along with DFT calculations for getting information on the reaction inter- mediates, activation barriers, transition state pathway, and high selectivity toward hydrogen.

Figure 12. Hardwood biomass gasification without catalyst and ■ ASSOCIATED CONTENT external methane, biomass gasification on ZSM-5 catalytic support * without methane, and methane-activated biomass gasification with 5 S Supporting Information vol % CH4 on FeMo1 and FeMo2 catalysts. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02663. Moles of gas species, SEM characterization of spent FeMo1 catalyst (PDF)

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Cosmin E. Dumitrescu: 0000-0003-1797-4584 Jianli Hu: 0000-0003-3857-861X Notes The authors declare no competing ﬁnancial interest.

■ ACKNOWLEDGMENTS We are pleased to acknowledge U.S. Department of Energy fi Figure 13. Hardwood biomass gasi cation without catalyst and and National Energy Technology Laboratory, Morgantown, external methane, biomass gasification on ZSM-5 catalytic support fi along with Leidos Research Support Team for the continuous without methane, and methane-activated biomass gasi cation with 10 fi vol % CH on FeMo1 and FeMo2 catalysts. support and cooperation for the speci c work and broadly for 4 the NETL Gasifier Support Stand Project (Project No. 10024037). This work was funded by the Department of obtained on FeMo1 which increased further to 10 on the Energy, National Energy Technology Laboratory, an agency of FeMo2 catalyst. With 10% CH4 in the feed, the H2/CO ratio is the United States Government, through a support contract ∼ lower than that for 5% CH4, 5 for FeMo1 and 8 for FeMo2 with Leidos Research Support Team (LRST). Neither the catalyst, respectively. The lower H2/CO ratios observed for United States Government nor any agency thereof, nor any of higher methane concentrations are possibly due to rapid their employees, nor LRTS, nor any of their employees, makes catalyst deactivation due to coke deposition and high any warranty, expressed or implied, or assumes any legal temperature. The comparison of catalytic and noncatalytic liability or responsibility for the accuracy, completeness, or biomass and methane-activated biomass gasification was in usefulness of any information, apparatus, product, or process tune with the proposed reaction mechanism in Table 6. disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial ■ CONCLUSIONS product, process, or service by trade name, trademark, Co-gasification of methane−biomass was conducted on 0.5% manufacturer, or otherwise, does not necessarily constitute or Fe−4% Mo/ZSM-5 and 1.5%Fe−-4%Mo/ZSM-5 catalysts. imply its endorsement, recommendation, or favoring by the The results showed a synergy between additional methane and United States Government or any agency thereof. The views biomass that produced hydrogen-rich syngas with more than and opinions of authors expressed herein do not necessarily 80% hydrogen in the gas yield. The high hydrogen production state or reflect those of the United States Government or any on the Fe−Mo based catalyst was probably due to the agency thereof.

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K DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX ACS Sustainable Chemistry & Engineering Research Article

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L DOI: 10.1021/acssuschemeng.9b02663 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX