Production of alcohols from cellulose by supercritical methanol

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Production of alcohols from cellulose by supercritical methanol depolymerization and hydrodeoxygenation Peter Galebach, Daniel J McClelland, Nathaniel M Eagan, Ashley Wittrig, J. Scott Buchanan, James A. Dumesic, and George W. Huber ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04820 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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Production of alcohols from cellulose by supercritical methanol depolymerization and hydrodeoxygenation Authors: Peter H. Galebach†, Daniel J. McClelland†, Nathaniel M. Eagan†, Ashley M. Wittrig‡, J. Scott Buchanan‡, James A. Dumesic†, George W. Huber† † Department of Chemical and Biological Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, Wisconsin 53706, United States. ‡ ExxonMobil Research and Engineering, 3545 Route 22 East Clinton Township, Annandale, NJ 08801, United States. Corresponding author: George W. Huber Email: [email protected] Keywords: Supercritical methanol, hydrodeoxygenation, methanol incorporation, retro-aldol condensation, cellulose, biomass conversion, fuels Abstract The reaction pathway and products of cellulose supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHDO) were investigated. Mono-alcohols, diols, alcohol ethers, and methyl esters were produced from cellulose at 300°C with a CuMgAl mixed metal-oxide catalyst. Time-course experiments show that cellulose is rapidly solubilized and depolymerized within 1 hour with C2-C4 diols being intermediates. Experiments with glucose-13C6 show that methanol is incorporated in all liquid products accounting for approximately 30% to 40% of the carbon in these products. Experiments with model compounds (dihydroxyacetone, isosorbide, and 5-hydroxymethylfurfural), indicate that the reaction pathway for cellulose occurs primarily through retro-aldol condensation of solubilized cellulose followed by recondensation with methanol. Methanol produces H2, CO, and CO2 through reformation with 30% of the generated H2 being incorporated into the liquid products. Analysis of the liquid products with Fourier transform ion cyclotron resonance MS (FT-ICR MS) measured C7-C12 partially oxygenated species with 2-6 double bond equivalence which could not be detected via gas chromatography (GC). We conclude that the reaction pathway occurs through rapid solubilization and depolymerization of cellulose followed by retro-aldol condensation to C2-C4 oxygenates. Retroaldol condensation products undergo hydrodeoxygenation and extensive carbon-carbon coupling to produce C2-C7 alcohols or other oxygenates. Introduction There is a growing interest in transportation fuels derived from sustainable sources both to provide an additional source of fuel to meet increasing demand and to reduce carbon emissions.1, 2 Currently biofuels are produced in large volumes from first generation sources such as corn starch, sucrose from sugarcane, and oil from soybeans. Non-food biomass feedstocks for second generation biofuels are more abundant and less expensive, but require difficult conversion steps to depolymerize the constituent components of lignocellulose.3 A preferred process would convert lignocellulosic biomass into a liquid transportation fuel, or fuel precursors, in a single catalytic step. Ford and coworkers demonstrated a single pot process to produce fuel-range mono-alcohols from biomass using supercritical methanol and a copper/magnesium/aluminum mixed metal

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oxide catalyst.4 In this process biomass was solubilized with supercritical methanol at 300 to 320°C and converted with 1:1 catalyst to biomass loading (mass basis) in a single reactor. The products from this reaction were reported to be a mixture of C2-C7 single alcohols with reported yields up to 121 wt% (additional mass from incorporated methanol) that could be used as a gasoline replacement or coupled to produce long chain alkanes to produce distillate-range fuels. The reaction was understood to occur through activation and depolymerization of cellulose and lignin to monomeric units that were hydrodeoxygenated over the catalyst to linear or cyclic single alcohols. Hydrogen for reduction of the biomass-derived oxygenates was provided from reforming of methanol over the CuMgAl mixed metal oxide. This presents a number of advantages over other biomass conversion technologies including: • Single process to convert biomass to final product • Inexpensive and reusable catalyst derived from earth abundant metals • High carbon yield to valuable fuel grade compounds • Methanol used instead of hydrogen, which is easier and cheaper to transport. The Ford group also demonstrated how this technology could be used on lignin.5 The effects of varying catalyst compositions, solvent systems, and feedstocks on the liquid product yield have also been investigated. Heeres and Barta used a CuMgAl mixed oxide catalyst with identical metal ratios to the Ford group in a study examining the conversion of sugars from pyrolysis oil and methanol.6 They reported a 95% yield to a mixture of alcohols, diols, esters, ethers, and furans. They also observed deactivation of the catalyst after reaction due to sintering of the Cu particles and leaching of Mg possibly caused by acids and water in their feed. Palkovits and co-workers studied the performance of a CuO/ZnO/Al2O3 catalyst in 245℃ water for cellulose conversion.7 They produced liquid products from cellulose at 95% yield with 65% selectivity to C1-C3 alcohols and diols. CuO/ZnO/Al2O3 was used by Wu et al. in supercritical methanol with similar results to the Ford group.8 The products from the study by Wu were C4-C7 alcohols with small amounts of esters and furanics. The similarity between these studies suggests a common reaction pathway to C2-C7 alcohols that has not been fully elucidated. Studies to date using supercritical organic solvents with the copper mixed metal oxide have provided excellent data on the effects of reaction conditions and catalyst composition on the conversion to liquid products. However, the results to date have primarily been empirical without a detailed understanding of the reaction pathway of this system, especially related to the role of methanol. In this study, we used 13C labelled glucose to quantify the amount of methanol incorporated into products and used FT-ICR MS to measure high molecular weight (MW) products that do not appear in a GC. We also tested SCM-DHDO with model compounds to recreate the product distribution with possible intermediates. We propose a new reaction pathway that accounts for the role of methanol. Experimental Catalyst Synthesis The catalyst used in this study is a copper magnesium aluminum mixed metal oxide derived from a hydrotalcite type precursor. Hydrotalcites are layered double hydroxides with the x+

nII general form of MII1-x MIII x OH2 (Ax/n ) ·mH2 O in which M is a divalent metal (Mg, Cu, Ni, etc.), MIII is a trivalent metal (Al, Fe, etc.), An- is the anion (CO3-2) and the value of x is typically 0.16 to 0.33.9 CuMgAl mixed metal oxide was prepared using co-precipitation according to methods in the literature.9-12 In a typical synthesis, three solutions were prepared: 1) 150 mL


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solution of DI water, 0.06 moles of MgCl2·6(H2O), 0.025 moles of AlCl3·6(H2O), and 0.015 moles of Cu(NO3)2·3(H2O); 2) 187.5 mL solution of DI water, and 0.025 moles of Na2CO3; and 3) 250 mL solution of DI water and 0.25 moles of NaOH. The Na2CO3 solution was placed in a 1L beaker and heated to 60°C on a stir plate and stirred with a 2-inch stir bar. The Mg, Al, Cu solution and the NaOH solution were added to the Na2CO3 solution using two syringe pumps for 1 hour. The pH and temperature were monitored with an Ohaus ST20 temperature/pH probe and kept at 10 and 60°C, respectively, during mixing. Catalysts were aged while stirring at 60°C for 1 day in a closed 1L bottle. After 1 day the catalyst was vacuum filtered. The filter cake was dispersed in a 150 mL solution of 1M Na2CO3, stirred for 1 hour and filtered. The filter cake was washed with DI water until it passed a chloride test using AgNO3. The catalyst was dried overnight at 110°C. The dried catalyst was crushed into a powder and calcined at 460°C for 12 hours with a 5°C/min temperature ramp. MgCl2·6(H2O), AlCl3·6(H2O), Cu(NO3)2·3(H2O), Na2CO3, and NaOH were purchased from Sigma Aldrich. Experimental Setup Batch reactors and analysis steps are included in the supplemental section in Figure S1. Reactors were made with tube fittings and bleed valves from Swagelok. In a typical reaction, 100 mg of CuMgAl mixed metal oxide, 100 mg PH-101 cellulose (Sigma Aldrich), and 3 mL of high pressure liquid chromatography (HPLC) grade methanol (Fisher Chemical) were added to an opened Swagelok union reactor. Reactors were alternatively pressurized and depressurized with ultra high purity (UHP) grade helium (Airgas) to remove air. Reactors were placed into a fluidized sand bath pre-heated to 300°C for a specified reaction duration. Reaction duration was measured from when the sand bath temperature returned to 300°C after the reactor was added. Typical heat up time for 6 reactors was 15 minutes while the reactions were held isothermally for 1 to 480 minutes. As shown in Table 1, low yields of products (4.2% carbon yield) are observed at 1 minute reaction time demonstrating that the reactions that occur during the heat up time can be neglected for this study. After the reaction, reactors were quenched by submersion in water. For gas analysis, the reactor bleed valve was attached to the bottom of a graduated inverted water cylinder and opened. Gas volume was measured, and the gaseous products were transferred to a gas bag by refilling the water cylinder. Liquid samples were taken from opened reactors and filtered using 0.22 µm syringe filters. Solids were washed out of reactor with acetone and collected in a 25 mL vial. Solids were isolated by evaporating the acetone under vacuum. Product analysis Gas products were analyzed with a Shimadzu GC-2014 equipped with 4 sample loops for studying refinery gas samples. Loop 1: 80/100 Hayesep N column, RTX Q-bond column with He carrier for CO2 analysis by thermal conductivity detector (TCD). Loop 2: 80/100 Hayesep N column, RT-Msieve 5A column with He carrier for O2, N2, CH4, and CO analysis by TCD. Loop 3: RTX alumina column, 10% OV-1 on Chromosorb WHP column with He carrier for analysis by flame ionization detector (FID). Loop 4: RT-Qplot column, RTX-Msieve 5A column with N2 carrier for He and H2 analysis by TCD. The GC oven was held at 40°C for 1.65 minutes then ramped to 90°C at 20°C/min and held for 1 minutes then ramped to 150°C at 30°C/min and held for 6.85 minutes. Liquid products were analyzed by FID using a Shimadzu GC-2010 equipped with a RTX-VMS column, by GC MS using a Shimadzu GCMS-QP2010 mass spectrometer equipped with a RTX-VMS column, and by HPLC using a Shimadzu HPLC equipped with an Aminex HPX-87H column, a RID-10A Refractive Index Detector, and a SPD-M20A Diode



Array Detector. The GC FID oven was held at 40°C for 5 minutes then ramped to 240°C at 7.5°C/min and held for 15 minutes. Products were quantified via external calibrations or using effective carbon number for GC products for which standards were not available.13 Dodecane was used as an internal standard to measure methanol conversion. Cellulose conversion was measured by removing the solids from the reactor, drying in a vacuum oven, and measuring the mass difference before and after reaction. Mass spectrometric data were acquired with a Bruker solariX XR FT-ICR MS with a 15 T actively shielded superconducting magnet. Instrument control, data acquisition and preliminary processing were performed on Bruker Daltonics ftmsControl 2.1.0 and Bruker Compass DataAnalysis 4.5 software. Atmospheric pressure chemical ionization (APCI) was used to investigate each sample. Samples were taken directly from the liquid after reaction. All samples were diluted 5 times by volume in LCMS grade methanol prior to analysis. Flow rates were controlled by the instrument syringe pump at 300 µL/h. The conditions were set to an APCI temperature of 400°C, corona needle current of 1250 nA, dry gas flow of 3.5 L/min, and dry gas temperature of 200°C. Nitrogen was used as the drying gas. The data was processed by using the sine-squared apodization method in absorption mode. Each mass spectrum is an average of 200 scans. After acquisition, elemental formulas for each peak were assigned by PetroOrg software.14 For the quantitative 13C nuclear magnetic resonance (NMR) analysis, methanol solvent was evaporated from the cellulose SCM-DHDO products and the resulting products were dissolved in DMSO-d6. The experiments were completed on a Bruker Biospin (Billerica, MA) AVANCE III 500 MHz spectrometer fitted with a DCH (13C optimized) cryoprobe with the standard ‘zgig30’ Bruker pulse sequence. The experiment was performed with a sweep width of 240 ppm centered at 110 ppm per 800 scans of 59520 data points using a 1s acquisition time and a 12s inter-scan relaxation delay. The spectra were processed with Mestrelab Research’s MestReNova software and were referenced to the residual DMSO solvent peak at 39.5 ppm. Catalyst Characterization Catalysts were characterized by X-ray diffraction (XRD) before and after calcination and reaction (Figure S3 a-i) with a Bruker D8 Discover diffractometer using a Cu Kα source (λ=1.54184 Å). Scans were collected via an area detector with 300 s acquisition time to obtain data from 5 to 80° 2θ in 3 steps of 25°. No major sintering of the Cu particles was observed from the XRD. Mg, Al, and Cu content of the catalyst were determined after dissolution in 3M HNO3 with inductively-coupled plasma atomic emission spectroscopy (ICP-AES) using a Perkin Elmer Plasma 400 at wavelengths of 279.079, 396.152, 324.754 nm for Mg, Al, and Cu, respectively. Acid and base sites were determined by temperature programmed desorption (TPD) of NH3 and CO2, respectively, using a Micrometrics AutoChem II 2920. In TPD of NH3, 100 mg of catalyst was loaded in a quartz tube and dried at 150°C for 2 hours with 50 mL/min He flow. NH3 was adsorbed using 30 mL/min of 15% NH3/balance He at 150°C for 30 minutes then desorbed using 10 mL/min He flow and a 10°C/min temperature ramp to 600°C. NH3 concentration was measured with TCD. In TPD of CO2, 100 mg of catalyst was loaded in a quartz tube and heated to 400°C for 2 hours with 20 mL/min He flow. CO2 was absorbed by injecting 8x0.5 mL pulses of 100% CO2 into a 50 mL/min He flow at 50°C or until the area of CO2 after each pulse became constant. CO2 was desorbed using 50 mL/min He flow and a 5°C/min temperature ramp to 600°C. CO2 concentration was measured with TCD. Surface area was determined by the Brunauer–Emmett–Teller (BET) method using a Micrometrics ASAP 2020 Plus. Cu sites of reduced catalyst were determined using N2O oxidation assuming a 2:1 Cu:O ratio and 6.8·10-20


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m2·Cu atom-1.15, 16 During N2O adsorption, 100 mg of catalyst was reduced at 350°C for 2 hours in 10 mL/min flow of 10% H2/balance Ar then cooled to 90°C. A pulse of 0.5 mL of N2O was injected into a 25 mL/min stream of He and passed over the catalyst. The catalyst was dosed with N2O 5 times to ensure complete oxidation of the surface copper. Dispersion was calculated using the moles of surface copper from N2O adsorption and the total moles of copper from ICP. The catalyst reducibility and hydrogen uptake were measured with temperature programmed reduction (TPR) using 25 mL/min 10% H2/balance Ar flow from 50°C to 900°C at 10°C/min. The catalyst characterization results are shown in the supplemental section. Results and Discussion Time Course Study A time course study of cellulose conversion was performed both with and without catalyst to identify intermediates from cellulose depolymerization and subsequent deoxygenation steps. Table 1 shows the identified liquid product carbon concentrations measured with GC FID from cellulose time-course reactions with CuMgAl mixed metal oxide catalyst and methanol. Each reaction in Table 1 was performed in triplicate. Reactions were run from 1 min to 60 min without catalyst and 1 min to 480 min with catalyst. The main reaction products were monoalcohols, diols, ketones, esters, and ethers. No organic acid acids or aldehydes were observed in the liquid products likely due to high reactivity. Aldehydes are likely esterified via the Tishchenko reaction with adsorbed methoxy groups on Cu sites or undergo aldol condensation over basic Mg-Al sites.9, 17 At these conditions Cu-catalyzed alcohol-aldehyde interconversion is also rapid with thermodynamic equilibrium favoring alcohols. No products with unsaturated carbons bonds are observed due to hydrogenation over Cu sites. The Ford group only reported observing single alcohols and ethers for this reaction.4 More recent work by Heeres and Barta6 on SCM-DHDO of sugars obtained a similar product distribution to those observed here. Dimethyl ether (DME) was also produced in the reaction, but is not included in Table 1 as it is solely a product from intermolecular methanol coupling. A large amount of the products could not be assigned in the GC MS (from ~28.5 mmol C/L for 1 min reactions up to 578.6 mmol C/L for 480 min reactions). Most unassigned compounds were observed at retention times higher than 1-hexanol (20 min). Close matches in the GC MS indicate these are likely C5+ alcohols, ethers, and esters that have methyl branching with multiple isomers, making positive identification difficult. The concentration of unassigned compounds was estimated by applying the sensitivity factor of identified compounds to the unknown area. The identified products range in carbon number from C2 to C6. C3 and C4 compounds account for most (up to 34.1%) of the identified carbon. The C5+ alcohols, ethers, and esters, which are observed in the GC, but are not calibrated, account for up to 41.6% of the total carbon yield. The overall carbon yield of the reaction increased from 4.2% at 1 min to 92.6% at 480 min and did not reach a maximum at the reaction times tested. Methanol conversion increased from 4.8% at 1 min to 19.0% at 480 min. All products increased in concentration from 1 min to 60 min. After 60 min 1,2-ethanediol and 1,2-propanediol began to decrease and after 240 min 1,2-butanediol and 2,3-butanediol decreased as well. Mono-alcohols, ketones, esters, and ethers increase at all reaction times. The primary liquid products were isobutanol (up to 158.7 mmol C/L) and 2,3-butanediol (up to 158.1 mmol C/L). The most abundant mono-alcohols are isobutanol, ethanol, 2-methyl 1butanol, and 1-propanol. All possible isomers of linear alcohols were observed while branched alcohols only contained methyl branching at the β carbon. Methyl branched alcohols such as isobutanol, 2-methyl-1-butanol, and 2-methyl-1-pentanol increased the most of any alcohol from



60 min to 480 min. The only observed diols were vicinal diols such as 1,2-ethanediol, 1,2propanediol, 1,2-butanediol, and 2,3-butanediol. Diols account for 65% of the carbon yield of identified liquid products at 1 min, 54.6% at 30 min, and 56.9% at 60 min. After 60 min, the overall percentage of diols in the identified liquid products decreases, although 2,3-butanediol still increases until 240 min. The most abundant ketone is acetone (up to 2.9 mmol C/L), while other ketones such as 2-butanone are only present in trace quantities and are not listed. All observed esters are C2 to C5 methyl esters. The most abundant ester is methyl acetate (up to 21.3 mmol C/L). Each ester is roughly proportional to the n-alcohol of the same chain length which suggests that esters are produced via dehydrogenation followed by esterification of the corresponding n-alcohol with methanol derivatives.17 The most abundant ethers are 2methoxyethanol and 2,5-dimethyltetrahydrofuran (DMTHF). 2-methoxyethanol is likely formed from etherification of 1,2-ethanediol with methanol through a similar mechanism to dimethyl ether formation. All observed acyclic ethers contain a hydroxyl group in addition to a methoxy group. The evolutions of mono-alcohols, polyoxygenates, C5+ compounds, and overall carbon yield over time are shown in Figure 1. The overall carbon yield increases at all reaction times up to 92.6% at 480 min. The C5+ alcohols, ether, and ester carbon yield increases up to 41.5% at 240 min before decreasing to 37.3% at 480 min. Polyoxygenates (which include diols, esters, and ethers) increase up to 24.2% at 120 min before decreasing to 19.9% at 480 min. Mono-alcohols increase at all reaction time up to 35.2% at 480 min. The decrease in polyoxygenates and C5+ alcohols, ethers, and esters suggests that the diols and some of the unassigned compounds are intermediates to mono-alcohols. The conversion of cellulose and consumption of methanol are shown in Figure 2. After 60 min cellulose conversion is complete and methanol consumption is 15.6%. Methanol consumption slows at longer reaction times reaching 19.0% at 480 min Cellulose conversion in Figure 2 increases much faster than the carbon yield of products in Figure 1 suggesting that depolymerized fragments of cellulose cannot be observed in the GC. Methanol consumption and cellulose conversion appear to follow a similar trend, suggesting that methanol is largely consumed during cellulose depolymerization. The products from conversion of cellulose without a catalyst are shown in Figure 3. The observed products include methyl glucoside (up to 1.6% carbon yield), methylated dimers (up to 6.9% carbon yield), levoglucosan (LGA, up to 6.6% carbon yield), and HMF (up to 1.0% carbon yield). The overall carbon yield increased up to 15.6% at 60 min. Cellulose conversion increased from 15% at 1 min to 23% at 30 min and further to 42% at 60 min. The cellulose conversion without catalyst was lower than with catalyst, indicating that the catalyst helps depolymerize soluble cellulose. No methanol consumption was observed without catalyst present. No monoalcohols, diols, or other deoxygenated products were observed. Conversely, in reactions with catalyst, no methyl glucoside, HMF, or LGA were detected. The products without catalyst are similar to those observed in cellulose conversion studies performed by Saka at 350°C with methanol.18 Reactions were also run with only catalyst and methanol. The results from these reactions are included in the supplemental section in Figure S6. The yield of alcohols from methanol accounts for less than 1% of these products. The evolution of products over time points to the primary reaction pathway being retroaldol condensation. The products from retro-aldol condensation are C2-C4 oxygenates and the intermediates in this reaction are 1,2-ethanediol, 1,2-propanediol, 1,2-butanediol, and 2,3butanediol. After retro-aldol condensation there appears to be continuous C-C addition and


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deoxygenation as the yield to C2 and C3 species remains essentially unchanged after 60 minutes, but the yield to C5 and C6 species continuously increases. Since no monomeric sugars or triols/tetrols are observed when catalyst is added, it appears that retro-aldol condensation and the conversion of retro-aldol intermediates to diols is rapid. The absence of C5 and C6 diols also supports the retro-aldol condensation pathway rather than successive hydrodeoxygenation.



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Table 1. Identified liquid phase products as a function of reaction time for SCM-DHDO of cellulose from GC FID. Reaction conditions: 300°C, 100 mg CuMgAl mixed metal oxide with 100 mg Avicel PH-101 cellulose, 2.4 g MeOH with dodecane internal standard, each reaction run in triplicate. Standard error is shown in parentheses. Reaction Times (standard error) Products (mmol C/L) 1 min 4 min 10 min 20 min 30 min 45 min 60 min 120 min 240 min 480 min Mono alcohols 3.2 (1.0) 6.9 (2.5) 7.3 (0.7) 13.7 (4.1) 19.9 (3.2) 23.4 (1.3) 36.2 (2.8) 51.4 (8.6) 70.2 (2.5) 79.7 (0.0) Ethanol 3.1 (1.2) 3.4 (0.3) 6.2 (2.0) 8.6 (1.6) 10.4 (0.8) 17.6 (1.0) 27.0 (5.9) 39.1 (2.5) 46.8 (0.0) 0.9 (0.8) 1-propanol 0.2 (0.4) 0.6 (0.0) 1.1 (0.4) 1.7 (0.6) 1.9 (0.1) 3.0 (0.3) 4.4 (1.0) 6.5 (0.4) 7.2 (0.0) 0.0 (0.0) 2-propanol 0.3 (0.5) 2.5 (1.0) 2.6 (0.2) 4.6 (1.0) 7.1 (1.2) 8.3 (0.5) 12.8 (0.8) 17.9 (3.1) 25.4 (1.3) 28.5 (0.0) 1-butanol 0.7 (0.6) 2.2 (1.0) 2.3 (0.3) 4.6 (1.6) 6.5 (1.6) 7.1 (0.6) 11.9 (1.1) 17.0 (5.5) 25.9 (2.6) 35.1 (0.0) 2-butanol 3.0 (1.5) 3.1 (0.3) 6.4 (2.5) 10.6 (2.9) 12.8 (1.4) 25.0 (2.5) 46.1 (14.8) 96.3 (17.4) 158.7 (0.0) 0.9 (0.9) isobutanol 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 18.7 (7.0) 13.0 (0.0) 0.0 (0.0) 1-pentanol 1.0 (0.7) 1.0 (0.5) 2.6 (0.8) 3.3 (0.6) 3.5 (0.3) 5.8 (0.5) 8.3 (2.5) 14.4 (1.8) 20.8 (0.0) 0.1 (0.2) 2-pentanol 0.4 (0.8) 1.7 (1.6) 2.3 (0.2) 3.7 (1.2) 4.9 (1.2) 5.8 (0.6) 11.0 (1.2) 21.8 (6.5) 44.4 (8.1) 76.3 (0.0) 2-methyl-1-butanol 0.0 (0.0) 1.7 (0.6) 1.8 (0.2) 3.6 (1.3) 5.0 (1.4) 6.0 (1.3) 10.3 (1.3) 12.0 (0.6) 14.4 (0.1) 17.1 (0.0) 1-hexanol 0.0 (0.0) 0.0 (0.0) 0.9 (0.8) 1.5 (1.3) 3.2 (0.6) 3.5 (0.5) 0.0 (0.0) 1.7 (3.0) 7.4 (0.6) 9.9 (0.0) 2-hexanol 0.0 (0.0) 1.0 (1.8) 2.7 (0.4) 5.2 (1.2) 6.3 (1.0) 7.3 (0.8) 7.4 (6.5) 8.6 (7.6) 16.2 (0.0) 16.6 (0.0) 3-hexanol 0.0 (0.0) 1.1 (0.5) 1.3 (0.2) 2.7 (0.9) 3.7 (0.7) 4.5 (0.7) 5.6 (4.9) 12.1 (2.1) 22.0 (3.1) 35.9 (0.0) 2-methyl-1-pentanol Diols 6.5 (2.5) 9.7 (2.3) 23.8 (5.2) 34.2 (5.4) 44.2 (4.0) 57.2 (2.8) 47.1 (11.7) 4.8 (6.8) 0.0 (0.0) 2.0 (1.1) 1,2-ethanediol 9.5 (2.2) 12.7 (2.4) 24.9 (4.8) 35.1 (4.7) 44.3 (2.8) 55.3 (2.2) 46.5 (12.0) 24.2 (5.0) 10.1 (0.0) 3.9 (1.0) 1,2-propanediol 0.6 (0.7) 3.6 (1.1) 4.5 (0.6) 8.4 (1.6) 12.2 (1.9) 15.3 (0.9) 21.7 (1.4) 27.3 (0.7) 28.5 (1.1) 26.6 (0.0) 1,2-butanediol 11.6 (3.1) 24.2 (4.9) 28.0 (2.0) 46.7 (8.9) 64.9 (8.7) 80.3 (5.6) 116.9 (5.9) 155.4 (2.3) 158.1 (12.5) 115.6 (0.0) 2,3-butanediol Ketones 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.6 (0.5) 0.0 (0.0) 0.0 (0.0) 0.5 (0.9) 0.0 (0.0) 2.2 (0.2) 2.9 (0.0) Acetone Esters 0.0 (0.0) 6.0 (2.3) 6.1 (0.6) 9.7 (2.3) 10.1 (0.9) 12.3 (0.6) 23.7 (1.3) 29.6 (4.2) 36.0 (0.0) 35.5 (0.0) methyl acetate 2.1 (0.5) 2.4 (2.1) 4.6 (0.2) 6.2 (0.7) 6.3 (1.3) 6.1 (0.2) 6.6 (5.7) 3.8 (6.5) 11.9 (0.4) 12.2 (0.0) methyl propanoate 0.0 (0.0) 0.0 (0.0) 0.5 (0.9) 2.3 (0.7) 1.7 (1.5) 3.5 (0.2) 6.0 (0.4) 5.7 (5.1) 10.7 (0.1) 11.6 (0.0) methyl butyrate 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.0 (0.0) 0.5 (0.7) 1.8 (0.0) methyl pentanoate Ethers 0.0 (0.0) 0.6 (1.1) 1.2 (0.2) 3.7 (1.2) 6.7 (2.4) 7.8 (0.7) 13.3 (1.0) 14.7 (13.0) 31.3 (0.6) 34.7 (0.0) 2-methoxy ethanol 0.0 (0.0) 0.0 (0.0) 0.4 (0.4) 0.9 (0.2) 2.1 (2.1) 1.0 (0.0) 0.0 (0.0) 2.4 (4.2) 9.3 (0.7) 11.6 (0.0) 1-methoxy-2-propanol 0.0 (0.0) 0.3 (0.0) 0.3 (0.3) 0.8 (0.7) 0.0 (0.0) 1.5 (1.3) 1.8 (2.4) 1.9 (0.3) 2.7 (0.0) 0.0 (0.0) 2-methoxy-1-propanol 0.3 (0.5) 0.8 (0.7) 2.6 (0.9) 3.3 (0.9) 3.5 (0.3) 1.9 (3.3) 0.0 (0.0) 9.8 (1.3) 14.8 (0.0) 0.2 (0.3) 2-methyl THF 0.0 (0.0) 0.0 (0.0) 7.3 (0.7) 7.7 (6.7) 15.0 (3.3) 16.6 (1.0) 0.0 (0.0) 11.1 (16.1) 33.5 (0.9) 30.9 (0.0) 2,5-dimethyl THF C5+ alcohols, ethers, and 28.5 (4.7) 81.7 (26.4) 90.7 (10.9) 165.4 (32.5) 242.7 (30.9) 287.8 (25.8) 473.4 (39.8) 588.4 (31.5) 650.7 (39.0) 578.6 (0.0) esters* 0.4 (0.2) 1.0 (0.4) 1.2 (0.2) 2.6 (0.7) 3.7 (0.5) 4.6 (0.3) 6.3 (0.4) 6.7 (0.2) 4.8 (0.6) 5.1 (0.0) Total C2 Yield (%) 0.4 (0.1) 1.4 (0.5) 1.8 (0.2) 3.3 (0.7) 4.3 (0.6) 5.2 (0.3) 7.6 (0.4) 8.7 (1.3) 8.9 (0.1) 8.9 (0.0) Total C3 Yield (%) 1.2 (0.4) 2.8 (0.5) 3.4 (0.2) 5.5 (1.1) 7.6 (1.2) 8.9 (0.5) 13.1 (1.1) 18.6 (2.3) 22.8 (0.4) 25.2 (0.0) Total C4 Yield (%) 0.1 (0.1) 0.2 (0.2) 0.3 (0.1) 0.8 (0.3) 0.9 (0.1) 1.1 (0.1) 1.7 (0.3) 2.5 (0.9) 6.3 (0.2) 8.8 (0.0) Total C5 Yield (%) 0.0 (0.0) 0.3 (0.2) 1.0 (0.1) 1.5 (0.4) 2.3 (0.4) 2.6 (0.3) 1.6 (0.8) 3.2 (1.8) 6.0 (0.1) 7.2 (0.0) Total C6 Yield (%) 2.1 (0.3) 6.0 (1.9) 6.7 (0.8) 11.6 (2.2) 16.7 (1.9) 19.5 (1.5) 31.7 (2.7) 41.0 (1.9) 41.6 (2.6) 37.3 (0.0) Total Others Yield (%) 4.2 (1.0) 11.7 (3.5) 14.4 (1.5) 25.3 (4.8) 35.6 (4.7) 41.8 (3.0) 61.9 (4.7) 80.7 (3.8) 90.3 (1.4) 92.6 (0.0) Carbon Yield (%) 4.8 (1.1) 5.9 (0.6) 7.6 (0.3) 10.4 (1.0) 11.6 (1.4) 13.3 (0.8) 15.6 (0.7) 12.7 (2.1) 19.1 (1.1) 19.0 (0.0) Methanol Conversion (%) *Compounds detected in the GC but not identified. These mostly consist of high boiling compounds above 1-hexanol. These appear to be branched C5+ alcohols, ethers and esters. These compounds were assumed to have the average sensitivity of assigned compounds.


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Figure 1. Carbon yield of liquid phase products as a function of reaction time for SCM-DHDO of cellulose from GC FID. Reaction conditions: 300°C, 100 mg CuMgAl mixed metal oxide with 100 mg Avicel PH-101 cellulose, 2.4 g MeOH with dodecane internal standard, each reaction run in triplicate. Error bars indicate standard error.



Cellulose Conversion

Methanol Consumption

Figure 2. Methanol loss and cellulose conversion as a function of reaction time for SCMDHDO of cellulose from GC FID. Reaction conditions: 300°C, 100 mg CuMgAl mixed metal oxide with 100 mg Avicel PH-101 cellulose, 2.4 g MeOH with dodecane internal standard, each reaction run in triplicate. Error bars indicate standard error.


Page 10 of 29

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Carbon yield of liquid phase products as a function of reaction time for SCM-DHDO of cellulose from GC FID without catalyst. Reaction conditions: 300°C, 100 mg Avicel PH-101 cellulose, 2.4 g MeOH with dodecane internal standard, each reaction run in triplicate. The gas phase products of SCM-DHDO are shown in Table 2. H2, CO, CO2, and CH4 were analyzed using a GC equipped with a TCD and FID. Insufficient quantities of gases were produced at early reaction times (1 and 4 min) for collection. The majority of the dimethyl ether was in the liquid phase. H2, CO and CO2 are likely produced from methanol reforming although H2 could also be produced from dimethyl ether reforming.19 CH4 is likely produced from methanation of CO.20 At longer reaction times the ratio of CO to CO2 decreases likely due to the reaction between CO and water from dehydration reactions via water gas shift. We defined hydrogen selectivity as the amount of reformed hydrogen that is in incorporated into the liquid products (Equation 1). =

, , !" ,

=

#$% &'$%$( ,)*→) , !" #$% &'$%$(

(1)

The hydrogen selectivity increases from -3.9% at 10 min to around 30% after 60 min and remains constant from 60 min to 480 min. Negative selectivity may be due to hydrogen released from methanol adsorbing onto the catalyst surface and forming methoxy species at early reaction times before hydrogenation occurs or during esterification reactions. The volume of reformed gas is nearly constant between 60 min and 480 min. Dimethyl ether increased over the entire time and did not appear to reach a maximum. The steady increase in dimethyl ether shows the acid sites on the catalyst remain active at all reaction times. As noted earlier, the methanol conversion rate slowed after 60 min. The increase in methanol conversion from 60 min (15.6%) to 480 min (19.0%) was close to the increase in dimethyl ether over the same time (0.9% to 3.2%) and after 60 min methanol appeared to be lost primarily to dimethyl ether. There is a



Page 12 of 29

difference at all reaction times between the methanol consumption and the sum of conversion products, which could be explained by methanol incorporation into products as described later in this publication.

Table 2. Gaseous products as a function of reaction time for SCM-DHDO of cellulose from GCTCD. Reaction conditions: 300°C, 100 mg CuMgAl mixed metal oxide with 100 mg Avicel PH-101 cellulose, 2.4 g MeOH with dodecane internal standard, each reaction run in triplicate. Values in parenthesis represent the standard error. Methanol consumption is calculated from dodecane internal standard area after reaction. Reaction Time 1 min 4 min 10 min 20 min Conversion of methanol 0.1 (0.03) 0.3 (0.03) 0.3 (0.02) 0.4 (0.04) to dimethyl ether (%) Conversion of methanol N/A N/A 0.0 (0.02) 0.1 (0.01) to methane (%) Conversion of methanol N/A N/A 1.5 (0.13) 2.3 (0.19) to CO (%) Conversion of methanol N/A N/A 0.6 (0.01) 0.9 (0.06) to CO2 (%) Hydrogen after reaction N/A N/A 3.60 (0.11) 4.34 (0.33) (mmol)

30 min

45 min

60 min

120 min 240 min 480 min

0.4 (0.02)

0.5 (0.06)

0.9 (0.02)

1.4 (0.08)

2.1 (0.17)

3.2

0.6 (0.00)

0.6 (0.00)

0.2 (0.01)

0.2 (0.02)

0.4 (0.04)

0.6

2.4 (0.09)

2.5 (0.05)

2.7 (0.06)

2.5 (0.11)

2.8 (0.07)

2.5

1.2 (0.06)

1.5 (0.04)

1.5 (0.07)

1.8 (0.11)

2.4 (0.03)

3.0

4.91 (0.11) 5.05 (0.04) 4.96 (0.17) 5.48 (0.30)

6.5 (0.21)

7.04

N/A N/A H2 selectivity (%)* -13.7 (4.8) 13.6 (2.9) 17.0 (1.8) 23.1 (0.7) 28.9 (1.9) 24.3 (0.9) 25.9 (0.7) Methanol consumption 4.8 (0.63) 5.9 (0.32) 7.6 (0.17) 10.4 (0.55) 11.6 (0.82) 13.3 (0.55) 15.6 (0.43) 12.7 (1.19) 19.1 (0.81) (%)

27.2

* H2 selectivity accounts for hydrogen required to reduce CuO sites to Cu0.

Isotopic Study of 13C Glucose An isotopic study was performed with 13C6 (99%) glucose (Sigma Aldrich) to study methanol incorporation in the products. SCM-DHDO reactions with glucose showed identical products to those from cellulose, as shown in Figures S4 and S5. Isotopic fitting was performed by finding the least squares of the difference between observed and calculated peaks. This approach is similar to those reported elsewhere.21, 22 The 13C carbon contents of select SCMDHDO products are shown in Figure 4. 13C carbon content ranges from 0-100% which represents variation between 100% carbon from methanol and 100% carbon from glucose. Isotopic results were obtained at 1 hour, 2 hours, and 4 hours. CO2 was analyzed by injecting a gas sample directly into a GC MS. CO and CH4 eluted too quickly for analysis. CO2 is composed almost entirely of 12C from methanol, confirming that methanol reforming accounts for gas products and that decarboxylation of cellulose-derived species is negligible. Dimethyl ether is composed of 12C which confirms that dimethyl ether is produced exclusively from methanol. Surprisingly, all liquid products contain 12C from methanol in varying amounts. Compounds with methyl branching or methoxy groups, such as methoxy ethanol, isobutanol, and 2-methyl-1butanol show higher 12C incorporation than linear alcohols, and almost all of these contain at least 1 12C. Methanol incorporation does not strictly increase with chain length, as ethanol and 1pentanol contain fewer 12C than 1-propanol or 1-butanol. Secondary alcohols contain more 12C than linear alcohols. Both 1,2-propanediol and 2,3-butanediol contain 12C. Short chain diols fragment easily such that the molecular ions of the diols could not be observed, thus the ionized ethanol fragment was studied for both diols. 12C content increases at all reaction times studied, showing that methanol is constantly incorporated into the products.


19.0

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Table 3 shows the 13C in each product calculated from the fitting procedure. At 1 hour, 68.9% of the carbon in liquid products comes from glucose, decreasing to 57.7% at 4 hours. These results suggest that much of the increase in carbon yield after 1 hour comes from incorporation of methanol. The carbon yield from cellulose when accounting for methanol incorporation decreases from 60.5% to 41.7% at 1 hour, 80.1% to 52.6% at 2 hours, and 88.6% to 51.1% at 4 hours. Since few 13C species are detected in the gas phase, the unaccounted carbon from cellulose is likely high MW species seen in the FT-ICR MS that cannot be quantified by GC FID. These results show that as much as 50% of the carbon from cellulose is contained in these heavy C7 to C25 species. These results provide further evidence of retro-aldol condensation being the primary reaction pathway and also suggest that methanol incorporation takes place to a greater extent than expected. If the reaction proceeded through hydrodeoxygenation then long chain alcohols such as 1-butanol or 1-pentanol and diols such as 1,2-propanediol and 2,3-butanediol would not contain carbon from methanol. Methanol incorporation was expected to only occur in methyl esters, methyl ethers, or methyl branched alcohols which is not the case. The high 12C content in 1,2-propanediol and 2,3-butanediol suggests that methanol incorporation takes place early in the reaction pathway. 12C and 13C in all the products suggests that the reaction pathway involves rapid scrambling of the carbon from the feed and methanol. Another unexpected result is the presence of fully 13C methyl branched alcohols. The methyl group being located at the β carbon suggests that branching is due to condensation of methanol to a linear alcohol. This mechanism would result in a similar result to methoxy ethanol in which no fully 13C product was observed. However, the presence of 4 13C isobutanol and 5 13C 2-methyl-1-butanol shows that an unaccounted mechanism may cause branching. These results show that C-C coupling may be a more integral part of the reaction mechanism than what has been discussed in the literature. CO2

dimethyl ether

ethanol

n-propanol



Methoxy Ethanol

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n-butanol

2-butanol

isobutanol

n-pentanol

2-methyl-1-butanol

1,2-propanediol

2,3-butanediol

Figure 4. 13C content in select liquid and gaseous products from an isotopic study with 13C6 Dglucose. Reaction conditions: 300°C, 100 mg D-glucose, 100 mg CuMgAl mixed metal oxide. 13 C content calculated by fitting sample MS spectra to reference spectra. OI 1 hour reaction, OI 2 hour reaction, OI 4 hour reaction. Analyzed fragment is shown in brackets.


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Table 3. 13C content of liquid and gaseous products from isotopic SCM-DHDO experiments. Reaction conditions: 300°C, 1-4 hours, 100 mg 13C (99%) glucose, 100 mg CuMgAl mixed metal oxide, 3 mL methanol. Gas Products (13C %) Carbon dioxide Dimethyl ether Liquid Products (13C %) Ethanol 1-propanol 1-butanol 2-butanol Isobutanol 1-pentanol 2-pentanol 3-pentanol 2-methyl-1-butanol 2-methyl-1-pentanol 1,2-ethanediol 1,2-propanediol 1,2-butanediol 2,3-butanediol Methyl acetate Methyl propanoate Methyl butyrate 2-methoxyethanol Total 13C content (%)

1 hour

2 hours 4 hours 4.4 3.8 1.6 1.8 0.5 0.3

81.6 73.4 71.8 69.8 59.0 96.1 83.0 62.3 67.5 NA 76.4 76.3 73.2 62.6 62.9 70.0 NA 63.4 68.9

80.0 74.3 74.2 70.9 58.4 84.6 75.4 63.6 59.1 61.0 66.3 68.9 77.6 61.4 62.1 65.3 67.8 56.0 65.6

73.4 66.1 70.0 65.4 54.4 79.3 67.9 56.8 52.7 55.5 NA NA NA 50.2 57.1 58.7 68.2 50.2 57.7

High Molecular Weight Compounds Comparison of the carbon yields in Figure 1 with the cellulose conversions in Figure 2 shows that there is a large amount of unaccounted carbon at short reaction times. This carbon could be lost to either gas phase products via C-C cleavage reactions, partially depolymerized cellulose fragments, or polymerized degradation products formed from intermediates during reaction. Since the isotopic results showed that CO2 and DME were almost entirely from methanol, unaccounted carbon is likely in the liquids. To analyze this missing carbon, we studied the liquid product functionality and molecular weight distribution using quantitative 13C NMR and FT-ICR MS, respectively. NMR analyses of the liquid products from SCM-DHDO are shown in Table 4. A monomeric unit of cellulose has 6 C-O aliphatic carbons (100%). The number of C-O and C=O carbons should be indicative of the degree of oxygenation of the products. As cellulose is depolymerized and converted to alcohols, carbon in the C-O region shifts to the C-C region. NMR shows that the liquid products are largely deoxygenated after 1 hour with 58.2% of the carbon identified as C-C aliphatic and only 37.6% of the carbon identified as C-O aliphatic compared to cellulose which would have 100% of the carbon in the C-O aliphatic region. There



Page 16 of 29

is also a small amount of C=O carbonyl carbons present (4.2%) likely as ketones or methyl esters. The comparative functionalities of identified products in GC shown in Table 4 were determined by counting the carbonyl, C-O aliphatic, and C-C aliphatic carbons observed in the liquid products using GC FID. The functionality of all products determined by NMR agrees with the functionality of identified products from GC FID. This suggests that unassigned and any undetected compounds are similar in functionality to the identified products. The NMR data also show the trend of deoxygenation of these compounds. Cellulose conversion is very rapid in the first hour, and the products are mostly deoxygenated. After 1 hour, deoxygenation slows abruptly and the composition of liquid products is largely unchanged as C-O aliphatic carbons only decrease from 37.6% to 33.7%. Table 4. Liquid product functionalities quantified using Quantitative 13C NMR. Reaction Conditions: 300°C, 1, 2, 4 hours, CuMgAl mixed metal oxide. Carbonyl Aromatic C-O Aliphatic C-C Aliphatic Region Region Region Region NMR Results (155-220 ppm) (107-155 ppm) (55-107 ppm) (0-55 ppm) Theoretical Cellulose (%) 0.0 0.0 100.0 0.0 1hr Cellulose SCM DHDO 4.2 0.0 37.6 58.2 2hr Cellulose SCM DHDO 3.4 0.2 38.6 57.8 (%) 4hr Cellulose SCM DHDO 3.5 0.4 33.7 62.4 (%) Functionality of identified products in GC

1hr Cellulose SCM DHDO 2hr Cellulose SCM DHDO (%) 4hr Cellulose SCM DHDO (%)

3.1 3.2 3.8

0.0 0.0 0.0

33.9 33.6 32.8

63.1 63.2 63.4

In addition to NMR we also studied the liquid products using FT-ICR MS to approximate the molecular weights and molecular formulas of undetected and unassigned compounds. The results from FT-ICR MS analyses are shown in Figure 5 below. The products in highest concentration at 1 hour contain 7 to 15 carbons, 2 to 4 oxygens and 2 to 5 double bond equivalence (DBE). Each double bond and ring contribute 1 DBE. The structures of these compounds are unknown, but these results suggest deoxygenated cyclic monomers with methanol incorporation, as evidenced by the large amount of C7 compounds which would likely result from a single methylation of a glucoside. After 4 hours, the most abundant species detected by FT-ICR have a composition of C12H18O2, with only low amounts of C7 species present. C12 species are either dimers from incomplete cellulose depolymerization or condensation products from retro-aldol products. From 1 hour to 4 hours there is a small decrease in oxygen content and DBE due to hydrogenation. C25 species increase after 1 hour which may be due to conversion of C7 species, increasing the relative C25 concentration. No C6 species are observed since a low MW cutoff of 100 Da was used. Although Figure 5 gives a strong indication of the molecular weights of species undetected by GC, it does not provide information about how much of the carbon from cellulose is converted to these species due to differences in ionization efficiencies of the various species. The pathway to these high MW compounds is also not yet understood. These species could be produced by hydrodeoxygenation of soluble cellulose fragments directly or from condensation of monomeric sugars or retro-aldol products. FT-ICR MS analysis of the products from model compounds may be able to further elucidate the pathway to these species.


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Max (a)

(b)

C7H12O3

C12H18O2

Min

(c)

(d)

C12H18O2

C25H34O4

C9H16O2

Figure 5. FT-ICR MS plots of liquid products from SCM-DHDO of cellulose after (a,b) 1 hour and (c,d) 4 hours. The molecular weight is plotted as number of carbons, oxygens, and double bond equivalence. Reaction conditions: 300°C, 100 mg CuMgAl mixed metal oxide with 100 mg Avicel PH-101 cellulose, 2.4 g MeOH with dodecane internal standard. Model Compound Studies Previous research on hydrodeoxygenation of glucose and sorbitol show that hydrodeoxygenation to alcohols takes place through three initial reaction pathways: 1) hydrogenation of glucose to sorbitol followed by dehydration to isosorbide, 2) dehydration of glucose to 5-hydroxymethylfurfural (HMF), and 3) retro-aldol condensation of glucose to erythrose and glycoaldehyde or isomerization of glucose to fructose followed by retro-aldol condensation to glyceraldehyde and dihydroxyacetone (DHA).23, 24 We tested these possible reaction pathways in the SCM-DHDO system using isosorbide, HMF, and DHA as model compounds. The products from each of these model compounds are compared to cellulose in Figure 6.



Dihydroxyacetone DHA was used as a model compound for the retro-aldol condensation of glucose. Conversion of DHA was complete at all reaction times tested. The liquid products from dihydroxy acetone SCM-DHDO are similar to the liquid products from cellulose showing that the wide distribution of C2-C6 alcohols, esters, and ethers can also be produced from a single C3 reactant. The most abundant species for both these reactions are 2,3-butanediol, isobutanol, and 2-methyl-1-butanol. No furans and few cyclic alcohols are seen in the reactions with DHA, while small amounts of both are seen in reactions with cellulose. DHA is more selective to esters, ethers and C2-C4 compounds than cellulose. Although most liquid products are C2 to C4 alcohols, C5 and C6 products are also observed. C5 and C6 products could be formed from coupling between feed derivatives or multiple C-C additions via methylation reactions. Methylation appears more likely as most linear C5 and C6 products from DHA are secondary alcohols which could result from aldol condensation. These results are consistent with the isotopic results that show methanol incorporation in all products. The yield of linear C5 and C6 alcohols is higher for cellulose than DHA which suggests that some longer chain alcohols, such as 1-hexanol and 1pentanol, can come from a dehydration route rather than retro-aldol condensation. All other C5 and C6 alcohols can be produced via C-C addition to retro-aldol intermediates. The carbon yields from DHA reactions are below 100%, indicating that coupling to high MW compounds occurs. Isosorbide The primary products from isosorbide are cyclic alcohols such as cyclopentanol, cyclohexanol, and various methyl branched isomers of these cyclic species. The high number of possible isomers makes identification and calibration of specific compounds difficult. However, close matches of these unassigned peaks were found using GC MS and characterized as either furans or cyclic alcohols. The response factors for 2-methylfurfural alcohol and cyclohexanol were used to calculate the yields of unassigned furans and cyclic alcohols, respectively. Cyclic alcohols and furans account for 8.5% and 10.5% of the carbon yield from isosorbide, respectively, the highest of any type of compound. The presence of C2-C4 alcohols suggests that retro-aldol condensation of isosorbide still occurs. Many of the products from isosorbide match with products from cellulose, however, the distribution of products from isosorbide is much different than from cellulose. Single alcohols only account for 6.9% of the carbon yield from isosorbide vs. 25.6% for cellulose. In addition, the carbon yield to diols is only 1.9% compared to 14.6% for cellulose. Based on these results the primary reaction pathway likely does not pass through isosorbide. These results also show that cyclohexyl alcohols can be produced via reaction pathways from cellulose not only from lignin. Cyclic alcohols could be produced from intramolecular aldol condensation of two carbonyl groups after a ring opening step. Furans could be produced from dehydration/ring-closing of the ring opened form of isosorbide. Methanol addition to any of these intermediates is also possible, which would explain the large number of isomers of cyclic compounds observed in the GC MS. 5-Hydroxymethylfurfural HMF conversion is very selective to only a few liquid products under SCM-DHDO conditions. The primary products are 2,5-dimethyltetrahydrofuran (DMTHF), 2-hexanol, 2,5dimethylfuran, and 5-methylfurfuryl alcohol. These products come from hydrogenolysis of the


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aldehyde and alcohol group on HMF and ring opening of DMTHF to 2-hexanol.25 The distribution of products shows that the conversion of cellulose likely does not occur through HMF. The remaining carbon yield from HMF is to longer chain alcohols such as methylbranched 2-hexanol, 3-heptanol, and 1,2-hexanediol from ring opening of 5-methylfurfuryl alcohol. Based on the results from these model compounds, retro-aldol condensation of early reaction intermediates followed by successive C-C addition reactions to C4+ alcohols appears to be the primary reaction pathway for producing mono-alcohols and diols in SCM-DHDO of cellulose.

Figure 6. Carbon yield of identified products from SCM-DHDO of cellulose (black), dihydroxy acetone (green), isosorbide (red), and HMF (blue). Reaction conditions: 300°C, 4 hours, 100 mg CuMgAl mixed metal oxide with 100 mg model compound, 2.4 g MeOH with dodecane internal standard. Discussion These results highlight a number of previously undiscovered intricacies key to understanding the SCM-DHDO of cellulose. The Ford group suspected methanol incorporation occurred in their study of biomass and cellulose conversion due to the large mass of products observed (121% of the feed mass), which our studies definitively prove.4 Isotopic studies show a high degree of C-C addition occurring. Possible routes of methanol incorporation are shown in Figure 7. The primary route for methanol incorporation into alcohols is likely coupling of a dehydrogenated methanol surface species with an alcohol after Cu-catalyzed dehydrogenation.26 For C3+ primary alcohols, the products will be primary alcohols with a methyl group on the β carbon. The product 2-methyl branched alcohols are unreactive toward further condensation reactions due to the lack of two β hydrogens.27 A second possible route for C-C addition is



through methylation of a C=C bond after dehydration. Ethers are formed from intermolecular coupling over acid sites.9 The catalyst does not fully deoxygenate any observed product. Most observed products contain a hydroxyl group with the exception of dimethyl ether and esters. This may be due to most species binding to copper sites which primarily perform redox reactions rather than dehydration reactions. FT-ICR MS shows evidence of high MW species that could explain the discrepancy between the carbon yield (91%) and the amount of carbon from methanol that is incorporated (up to 42.3%). The results from the isotopic study and analyses with FT-ICR MS suggest that cellulose is fully converted in SCM-DHDO, but a large amount of the carbon remains in high MW products. These high MW products could come from deoxygenation of oligomers from cellulose or from coupling of intermediates produced from full depolymerization. Since the product distribution from DHA is similar to that from cellulose, and C6+ compounds were observed using GC MS in both cases, the coupling of heavy oxygenated intermediates is highly likely. The color of the liquid recovered after a 5 min reaction with DHA was bright orange, indicating the liquid contained unsaturated species which could not be accounted for via GC FID. Further investigation of the reaction pathway to these high MW products could be performed by studying the liquid products from model compounds studies of DHA, glucose and cellobiosan with FT-ICR MS. Isotopic studies of C3 compounds may also provide insight on the C-C addition mechanism.

Figure 7. Possible methanol addition reactions via C-C coupling, etherification, and esterification during SCM-DHDO reactions. We have put together a potential reaction pathway to account for the observed liquid products, shown in Figure 8. The reaction pathway first progresses through solubilization of cellulose in supercritical methanol. The solubilized cellulose is depolymerized over the catalyst to monomeric sugars. Retro-aldol condensation after depolymerization produces C2-C4 oxygenates. These intermediates can then undergo several possible reactions to produce the observed liquid products. Esterification can occur through a Tishchenko reaction between an aldehyde and an adsorbed methoxy species or through an internal Cannizarro reaction to produce an organic acid that reacts with methanol.17, 28 Hydrodeoxygenation reactions produce 1,2propanediol or hydroxyacetone. Methylation produces 1,2-butanediol and 2,3-butanediol. The diols can undergo hydrodeoxygenation to produce primary and secondary alcohols or


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methylation to incorporate methanol. Methylation of primary alcohols such as 1-propanol or 1butanol induces methyl addition at the β carbon. Methylation of secondary alcohols such as isopropanol or 2-butanol either produces branched secondary alcohols (e.g. 3-methyl-2-butanol) or longer chain secondary alcohols (e.g. 3-pentanol). Ethanol and 1,2-ethanediol can be produced via retro-aldol condensation or decarbonylation of C3 compounds. The high MW products are shown as C7+ oxygenates in Figure 8. These can be produced through oligomerization of monomeric sugar species or products of retro-aldol condensation. Reactions without catalyst show that methylated sugars are produced during depolymerization, but methanol consumption without catalyst is also negligible. This suggests that methanol consumption occurs primarily during the reaction steps after depolymerization. Future research will focus on model compound studies with intermediates from the various pathways in Figure 8 to further elucidate the overall reaction pathway.



Figure 8. Proposed reaction pathway for alcohol production from cellulose in supercritical methanol with CuMgAl mixed metal oxide catalyst. Challenges The SCM-DHDO process has many advantages over other approaches for biomass conversion. All reactions occur in one reactor, liquid fuel range products are produced, lowercost methanol is used instead of hydrogen, and inexpensive catalysts are used. Despite the advantages there are several challenges that need to be addressed to make this technology feasible including decreasing the methanol consumption, increasing the yield of liquid products


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from cellulose and ensuring the catalyst is stable after recycling. As shown here up to 20% of the methanol is converted into hydrogen gas or liquid products by C-C coupling reaction. The hydrogen gas could be recycled and re-used to produce liquid products. It could be advantageous to increase the hydrogen pressure further to promote hydrogenation and increase selectivity to alcohols. While the focus here is on conversion of the biomass, the conversion of methanol to liquid fuel range molecules may actually be desirable since the price of methanol is often lower than the price of liquid transportation fuels.29, 30 The liquid products are also potential jet and diesel fuel precursors which could be coupled via dehydrogenation+aldol condensation or dehydration+oligomerization, as outlined by Kunkes et al. for upgrading of sugar hydrodeoxygenation products.23 More work is needed to understand how to control the CC coupling reactions. Multiple studies have been performed on the stability of the CuMgAl mixed metal oxide over the course of multiple runs in the literature.4, 6 However, there has not been a study of time-on-stream stability of the catalyst in a continuous reactor. To date these reactions have not been performed in continuous reactors, though this would be essential for any process targeting fuel production. The low catalyst:substrate ratios typically used in these microreactors could not be industrially feasible. Figure 9 shows a schematic of a possible process to enable larger-scale SCM-DHDO. Since our work to date has only focused on cellulose this schematic is preliminary as the lignin and hemicellulose fractions of real biomass would affect product yields. However, based on previous literature these products should be similar in structure to the products from cellulose.4 In addition, the challenges of solid handling and gas/liquid recycling should apply to both cellulose and whole biomass conversion. In this process, dry biomass is sent to feed processing where it undergoes size reduction and inorganics/ash removal. The pretreated biomass is then mixed with fresh and recycled methanol to produce a slurry. The slurry is fed to a continuous hydrodeoxygenation reactor where it is heated to supercritical conditions and converted to products. The catalyst is removed from the reactor and regenerated by an oxidation treatment. The gaseous products and liquid products are flashed after reaction with the gaseous products recompressed and recycled to the reactor (with a small purge stream burned to produce process energy). Methanol is separated from the liquid products and recycled to the beginning of the process to reduce the solvent cost. Water is then separated from the products while the fuel precursor oxygenates are sent for further processing. West et al. demonstrated processes to upgrade mono-functional compounds to fuel substitutes.31 Since many of the products from SCM-DHDO of cellulose are single alcohols, they could be coupled to diesel or jet range molecules via dehydration + oligomerization or dehydrogenation + aldol condensation.



Figure 9. Block flow diagram for production of renewable mono-oxygenates by supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHDO) of biomass. Conclusions Liquid products from SCM-DHDO of cellulose are a much more complex mixture of liquid products than previously described. In addition to mono-alcohols, the liquid products contain diols, ethers, and esters. Isotopic studies indicate that methanol is incorporated into all of the liquid products, accounting for up to 40% of the carbon in the liquid products. Methanol incorporation occurs both early in the reaction pathway during cellulose depolymerization and later in the reaction pathway. The gas phase products contain CO, CO2, CH4 and H2, all of which are produced from methanol. Approximately 30% of the hydrogen produced from the methanol is either incorporated into the liquid products or used in catalyst reduction. FT-ICR MS and NMR data show that deoxygenation occurs rapidly within the first 1 hour of reaction, but stabilizes after about 60-70% removal of oxygen. A large amount of carbon in the liquid phase cannot be identified with the GC techniques used in this study. FT-ICR MS studies indicate that this missing carbon is primarily within C12-C24 compounds containing mainly 2-3 oxygens. Liquid products from SCM-DHDO of dihydroxyacetone are similar to products observed from SCM-DHDO of cellulose. Liquid products from SCM-DHDO of isosorbide were primarily C4C7 cyclic alcohols and furans. HMF was found to be highly selective to 2,5dimethyltetrahydrofuran and 2-hexanol, which were only found in trace quantities from reactions with cellulose. The general reaction mechanism is hypothesized to involve the slow depolymerization of cellulose followed by rapid retro-aldol condensation of soluble monomers to C2 to C4 oxygenates which then undergo various hydrodeoxygenation and methylation reactions to produce primarily C3 to C5 oxygenates composed of 30-40% carbon from methanol. Nearly half of carbon from cellulose is partially deoxygenated to high MW species which were not detected by GC. The amount of and pathway to form these species has not yet been fully elucidated, but may occur through either polymerization of C3 to C4 intermediates or through incomplete depolymerization of cellulose.


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Acknowledgements This work was supported by ExxonMobil. The authors gratefully acknowledge use of facilities and instrumentation supported by NSF through the University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288). Supporting Information Batch reactor description and sampling procedure, catalyst characterization results, and sample chromatograms from SCM-DHDO reactions with glucose, cellulose, and methanol with catalyst. References 1. The Outlook for Energy: A View to 2040; ExxonMobil, 2014. 2. Perlack, R. D.; Stokes, B. J., U.S. Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry; ORNL/TM-2011/224; Oak Ridge National Laboratory: Oak Ridge, TN., 2011; p227. 3. Humbird, D.; Davis, R.; Tao, L.; Kinchin, C.; Hsu, D.; Aden, A., Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover; NREL/TP-5100-47764; National Renewable Energy Laboratory: 2011. 4. Matson, T. D.; Barta, K.; Iretskii, A. V.; Ford, P. C., One-Pot Catalytic Conversion of Cellulose and of Woody Biomass Solids to Liquid Fuels. J. Am. Chem. Soc. 2011, 133 (35), 14090-14097. DOI: 10.1021/ja205436c 5. Barrett, A. B.; Gao, Y.; Bernt, C. M.; Chui, M.; Tran, A. T.; Foston, M. B.; Ford, P. C., Enhancing Aromatic Production from Reductive Lignin Disassembly: in Situ O-Methylation of Phenolic Intermediates. ACS Sustainable Chem. Eng. 2016, 4 (12), 6877-6886. DOI: 10.1021/acssuschemeng.6b01827 6. Yin, W.; Venderbosch, R. H.; Bottari, G.; Krawzcyk, K. K.; Barta, K.; Heeres, H. J., Catalytic upgrading of sugar fractions from pyrolysis oils in supercritical mono-alcohols over Cu doped porous metal oxide. Appl. Catal., B 2015, 166-167, 56-65. DOI: 10.1016/j.apcatb.2014.10.065 7. Tajvidi, K.; Pupovac, K.; Kukrek, M.; Palkovits, R., Copper℃Based Catalysts for Efficient Valorization of Cellulose. ChemSusChem 2012, 5 (11), 2139-2142. DOI: 10.1002/cssc.201200482 8. Wu, Y.; Gu, F.; Xu, G.; Zhong, Z.; Su, F., Hydrogenolysis of cellulose to C4–C7 alcohols over bi-functional CuO–MO/Al2O3 (M = Ce, Mg, Mn, Ni, Zn) catalysts coupled with methanol reforming reaction. Bioresource Technol. 2013, 137, 311-317. DOI: 10.1016/j.biortech.2013.03.105



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For Table of Contents Use Only

Synopsis Cellulose is depolymerized in supercritical methanol and converted over CuMgAl mixed metal oxide to a mixture of alcohols, esters, and ethers that can be used as a sustainable fuel.


Production of alcohols from cellulose by supercritical methanol

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