Subscriber access provided by UNIV OF NEWCASTLE
Article
Lignin Pyrolysis and In Situ Hydrodeoxygenation over MoO3: Interaction between MoO3 and Lignin Yong Huang, Yalun Hu, Fenghui Ye, and Yunming Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01490 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 28
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
Energy & Fuels
Lignin Pyrolysis and In Situ Hydrodeoxygenation over MoO3: Interaction between MoO3 and Lignin Yong Huang, Yalun Hu, Fenghui Ye, and Yunming Fang*
National Energy Research Center for Biorefinery, College of Chemical Engineering, Beijing University of Chemical Technology, 100029, Beijing, China.
Abstract: Lignin pyrolyses and in situ hydrodeoxygenation over MoO3 with varying lignin/MoO3 mass ratios (L/M) at different temperatures were studied in a free-falling reactor. The presence of MoO3 not only facilitated the depolymerization of lignin to liquid and gas products but also catalyzed the in situ hydrodeoxygenation of the resulting liquid product to phenol, both of which were further intensified with the increases of temperature and the MoO3 amount. Negative apparent solid yield and over 100% apparent gas yield were obtained in some cases because of the reaction of MoO3 during lignin pyrolysis. To clarify this phenomenon, detailed TG/MS, XRD, and XPS investigations on spent MoO3 were carried out. The results indicate that Mo6+ was partly reduced to Mo4+ and Mo5+ during the lignin pyrolysis, mainly caused by the reaction with carbon from lignin rather than hydrogen from carrier gas.
1. Introduction
ACS Paragon Plus Environment
Energy & Fuels
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
Page 2 of 28
Lignin, a three-dimensional amorphous and heterogeneous polymer consisting of three primary monolignol building blocks (p-coumaryl, coniferyl, and sinapyl alcohols), is a potential resource for aromatics that are mainly derived from fossil resource currently.1 Lignin fraction in biomass usually accounts for 10–35% by weight and up to 40% by energy, varying with different feedstocks.2 Numerous outstanding investigations have been executed in cellulose and hemicellulose valorization and a handful of prominent progression has been achieved,3–5 whereas lignin conversion to value-added chemicals or fuels is underexploited.6 Indeed, lignin is characterized by higher carbon content
and less oxygen
content compared
with
cellulose/hemicellulose fractions.7 However, lignin is also endowed with recalcitrant properties by its intricate structures.8 What’s more, the decrease of ether bonds and subsequent formation of new C-C linkages generally lead to difficulties in depolymerization of the lignin extracted from the original biomass. 9 Catalytic fast pyrolysis (CFP) is regarded as a promising technology since CFP can selectively convert biomass into target chemicals or fuels in a single-step process, which integrates pyrolysis of biomass to intermediates and subsequent catalytic upgrading of intermediates to platform chemicals or fuels.10–12 Furthermore, liquid products derived from CFP have potentials to be compatible with current petroleum refinery infrastructures.11 Considering those distinct advantages mentioned above, CFP draws increasing attention of researchers from both academic and industrial sectors. Zeolite is the most widely used catalyst in lignin CFP, however, it suffers from severe deactivation by coke deposition.13 More applicable catalyst for lignin pyrolysis is thus desired. Recently, Prasomsri et al. reported that earth-abundant MoO3 can selectively cleave
ACS Paragon Plus Environment
Page 3 of 28
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
Energy & Fuels
the C-O bond in lignin under low H2 pressure (≤1 bar).14 Shetty et al. also performed hydrodeoxygenation (HDO) of m-cresol at 593 K and H2 pressure ≤1 bar with 10 wt% MoO3 dispersed on several supports, like SiO2, γ-Al2O3, TiO2, ZrO2 and CeO2, and a high conversion of 78 C-mol % with 99% selectivity to hydrocarbons was obtained in the case of MoO3/ZrO2.15 Additionally, employing co-condensation method, molybdenum was loaded over KIT-5 mesoporous silica support and used for production of furans and phenols from pine, cellulose, and lignin pyrolyses.16 Tandem reactors including separate sections for pyrolysis and bio-oil upgrading were commonly employed in the above described MoO3-catalyzed lignin pyrolysis, which can be regarded as ex situ lignin catalytic pyrolysis.17 Under that circumstance, lignin pyrolysis vapors (intermediates) were upgraded when went through catalyst bed by complete/partial deoxygenation. On the other hand, in situ catalytic pyrolysis, where lignin directly mixes with MoO3, not only upgrades the pyrolytic intermediates prior to processing18, but also has a possibility to facilitate the depolymerization of lignin because of the potential interaction between MoO3 and lignin.19,20 However, the reports on in situ catalytic pyrolysis are very rare, and more investigations are needed to gain comprehensive insights into its catalytic behaviors and the interaction between the catalyst and sample. In this contribution, lignin conversion with MoO3 was investigated using an in situ pyrolysis mode, in which MoO3 and lignin were mechanically mixed together. The effects of pyrolysis temperature and lignin to MoO3 mass ratios (L/M) were examined, and the derived products (i.e., solid, liquid, and gas) were subjected to detailed analysis. 2. Experimental
ACS Paragon Plus Environment
Energy & Fuels
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
2.1. Materials Corncob derived lignin used in this study was obtained from Shandong Longlive Bio-Technology Co., LTD, China. The result of its elemental analysis is listed in Table 1. Prior to each run, the lignin was dried at least 3 days at 60 °C in air. MoO3 (AR, 99.5%) was purchased from Aladdin, calcined in muffle furnace at 550 °C for 4 h, and finally subjected to partial reduction conducted at 400 °C lasting for 70 min under a H2 flow rate of 250 mL/min. The acetone used as solvent for GC/MS analysis was purchased from J. T. Baker. 2.2. Lignin pyrolysis with MoO3 The lignin pyrolysis experiments were carried out in a free-falling reactor, which consisted of a screw feeder coupled with a hopper, a high temperature resistance quartz tube (inner diameter, 28 mm; length, 800 mm), furnace, temperature control system, and carrier gas system. The schematic diagram of the free falling reactor is shown in Figure S1 (Supporting Information). Lignin and MoO3 were pre-mixed with a L/M of 1/2, 1/4, or 1/6 and pelleted. The pellets were then pulverized and sieved to obtain a fraction with particle size ranging from 20 to 60 mesh. In each run, the weight of lignin was kept at 8-10 g. Prior to reaction, the prepared feedstock (i.e., lignin and MoO3 mixtures) was loaded into the hopper, and the reaction temperature was ramped at a rate of 10 °C/min under a H2 flow (500 mL/min) to the 400, 500, or 600 °C. When the temperature stabilized at the set value, the feedstock was conveyed into the reaction zone by screw feeder for pyrolysis at a rate of 1.5–6.6 g/min depending on the L/M ratio. The overall reaction time was kept at 30 min for every run. The condensable vapor (lignin oil) was trapped by two consecutive condensers at about 0 and -70 °C, respectively, and the remaining
ACS Paragon Plus Environment
Page 4 of 28
Page 5 of 28
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
Energy & Fuels
non-condensable gases were collected by gasbag. The weight of the solid product was detected by difference of lignin and MoO3 mixture before and after the experiment and corrected for the initial mass of MoO3. Liquid product was subjected to GC/MS analysis, and its mass was determined by mass differences of the condensers before and after experiment. Gas product excepting H2 was quantified by GC analysis using external standard method. Considering MoO3 participates in the reaction of lignin depolymerization, an apparent yield was employed in this paper to describe the lignin-derived product distribution. The apparent yield was calculated by the following equation: Apparent product yield = Weight of product/Weight of lignin loaded
(1)
2.3. MoO3 characterization and products analysis Both fresh and spent MoO3 were characterized by the following techniques. PXRD patterns were collected using a Bruker diffractometer with Cu radiation (40 kV, 40 mA), and corresponding data were recorded in the 2 Theta range of 5–90° with an angular step size of 0.02°. XPS was performed employing a Thermo Fisher ESCALAB 250 equipped with a monochromatic Al kα radiation and high-resolution and survey scans were executed at pass energies of 30 and 200 eV, respectively. Binding energy was corrected to 284.8 eV (C 1s), and a 9-point Shirley background correction was introduced to Mo 3d XPS spectra. Deconvolution process of Mo 3d was conducted under several constraints: (1) Splitting energy of Mo 3d5/2-3d3/2 was set to 3.15 eV; (2) Area ratio for Mo 3d5/2:3d3/2=3:2; (3) Full width at half maximum (FWHM) of Mo 3d5/2-3d3/2 stayed the same.
ACS Paragon Plus Environment
Energy & Fuels
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
Thermogravimetric analysis (TG, NETZSCH STA 449 C) and TG coupled with mass spectroscopy (MS, Balzers MID) were utilized to analyze the carbon residues upon spent MoO3. TG-MS experiments were conducted by using O2/N2 (20/80, v/v) with a total flow rate of 50 mL/min. Programmed temperature began from 50 °C to 650 °C with a heating rate of 10 °C/min. CO2 (m/z=44) evolved during the experiment was detected by MS. TG experiments were carried out by introducing air at a flow rate of 50 mL/min, and temperature programming were the same with TG-MS described above. The water content of liquid products (lignin-oil) derived from lignin pyrolysis were determined by Karl Fisher Moisture Titrator (MKV-710, KEM), where working medium was HYDRANAL®-Solver (Crude oil, Sigma-Aldrich) and reagent HYDRANAL®-composite 5 (Sigma-Aldrich). Lignin oil, after dilution by acetone, was analyzed by gas chromatography/mass spectrometry (GC/MS) with a gas chromatograph (Agilent 7890A) coupled to a mass spectrometer (Agilent 5975C) using a scan range of m/z 40-500. A column (Agilent HP-5 MS; 30 m × 0.32 mm × 0.25 µm) was employed for separation of compounds with the following temperature profile: temperature started from 40 °C and sustained for 4 min, and then ramped to 300 °C at a rate of 5 °C/min and stayed for 4 min. The interpretation of the mass spectra was mainly based on an automatic library search (NIST11, version 2.0). 3. Results and discussion Lignin pyrolyses with (a L/M ratio of 1/4) and without MoO3 were firstly investigated under H2 atmosphere at 500 °C. As shown in Table 2, the apparent solid yield from the pyrolysis with
ACS Paragon Plus Environment
Page 6 of 28
Page 7 of 28
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
Energy & Fuels
MoO3 (2.8 wt%) is significantly lower than that without MoO3 (47.9 wt%), while the apparent liquid and gas yields (30.2 and 69.0 wt%, respectively) are obviously higher than those without MoO3 (25.4 and 26.7 wt%, respectively). This result suggests that the presence of MoO3 favors the conversion of solid to liquid and gas. The total ion chromatograms (TICs) of the liquid products from both cases are exhibited in Figures 1a and 1c. As can be seen in Figure 1, the composition of pure lignin derived liquid is more complicated, supporting that a lot of liquid products were converted when MoO3 was employed. The GC/MS identified compounds were mainly composed of phenolic compounds and aromatic hydrocarbons (see Supporting Information). Noteworthy, phenol is the most abundant compound in both cases, and its relative content increases from 9.6 to 27.5% when MoO3 is present, indicating that MoO3 influences the liquid product distribution. After confirming the effect of MoO3 on product distribution, further investigation of lignin catalytic pyrolyses at different temperatures (400, 500, and 600 °C) and with varying L/M ratios (1/2, 1/4, and 1/6) were performed, and their products were analyzed in details. 3.1 Gas product Apparent gas yields during lignin pyrolysis in the presence of MoO3 are listed in Table 2, and the results indicate that both higher temperature and lower L/M ratio favor gas production, which is easily understood by the deep conversion of primary lignin pyrolysis products under those conditions. It should be noted that the oxygen in MoO3 also contributes to the formation of COx, resulting in the apparent gas yield higher than 100 wt% in some case. Figure 2 shows the gas composition from all runs, which is composed of CO2, CO, CH4, and C2-C4 compounds. H2 is
ACS Paragon Plus Environment
Energy & Fuels
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
not shown in the result because it plays roles as both carrier gas and reactant, which leads to difficulties in quantification. Overall, CO2 is the most abundant compound in gas products from all runs followed by CO. The contents of CH4, CO, and C2-C4 increase when reaction temperature rises and, on the contrary, that of CO2 drops. The change in CH4 content is in line with the results from Yang et al.,21 which reported that the temperature for the maximum CH4 formation rate was between 500 and 600 °C in lignin pyrolysis. In addition, the improving HDO of phenols by MoO3 with an increasing temperature also leads to the change in CH4 content, and it will be discussed later.22 The increase in CO content at the expense of CO2 is probably due to the reverse water-gas shift reaction and reduction of CO2 by coke, which are favored at elevated temperature.23,24 3.2 Liquid product As listed in Table 2, the apparent liquid yield increases with a rise of MoO3 amount at the same temperature, while that reduces with the same L/M ratio when the temperature goes up. It seems that a moderate temperature and a small L/M ratio should be employed if liquid products are desired. As an example, the apparent liquid yield can be as high as 37.1 wt% at 400 °C with a L/M of 1/6. However, as exhibited in Figure 3, the water content in the liquid product also increases with a decreasing L/M ratio. The highest one, 92.7 wt%, is from the pyrolysis performed at 600 °C with a L/M of 1/6. Similar to the formation of COx mentioned in Section 3.1, a portion of water can be derived from the reaction between MoO3 and H2. The oil (water-free phase) yield is also shown in Table 2. Except the oil from the pyrolysis of pure
ACS Paragon Plus Environment
Page 8 of 28
Page 9 of 28
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
Energy & Fuels
lignin, which is characterized by dark color and high viscosity, a relatively high yield of clear oil with yellow color is obtained from the pyrolysis at a L/M of 1/4. Recently, MoO3 has been successfully tested in HDO of lignin derived model compounds without saturating the aromatic ring.15,22 Lignin oils mentioned above were analyzed by GC/MS. The TICs are shown in Figure 1 and Figures S2–S4 (Supporting Information), and the corresponding peak attributions are listed in Tables S1–S3 (Supporting Information). As displayed in Figure 1, a similar product composition is found for the pyrolyses of pure lignin and that with a L/M of 1/2 at 500 °C. In addition, the apparent liquid yields from these two runs are also close (see Table 2), suggesting a limited effect of MoO3 on the liquid product when the L/M is large as 1/2. The identified organic compounds primarily consist of benzene, alkyl benzene, phenol, alkyl phenols, and alkoxy phenols, and most of them are originated from the lignin building blocks, e.g., coniferyl and sinapyl alcohols. As the L/M reduces from 1/2, to 1/4, and to 1/6, the composition is increasingly simpler and concentrated to phenol, of which the relative contents increases from 15.7, to 27.5, and to 51.5%, respectively (see Table S1), while most alkoxy phenols are hydrodeoxygenated, because the bond dissociation energies of Ph-OMe and Ph-O-Me are lower than that of Ph-OH.22 In addition, the enhanced contact for lignin derived fragments (e.g., oxygenated phenols) and MoO3 with a less L/M ratio is also contributed to the simple composition in Figures 1c and 1d. For a better insight into the impacts of MoO3 and temperature on lignin pyrolysis, oil composition is summarized into 7 categories, which are composed of alkylbenzene, O1-phenols, O2-phenols, O3-phenols, O4-phenols, benzofurans, and others, as shown in Table 3. Both L/M
ACS Paragon Plus Environment
Energy & Fuels
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
Page 10 of 28
ratio and temperature significantly influence the oil composition. At 400 °C, the relative contents of O3-phenols, O4-phenols, and benzofurans slightly increases when MoO3 is employed, suggesting that MoO3 favors the depolymerization of the recalcitrant lignin to those compounds. When the temperature goes up to 500 °C, significant increases in the relative contents of alkylbenzene and O1-phenols are found, while those of O2-phenols, O3-phenols, O4-phenols, and benzofurans decrease or even some of them disappear, and this phenomenon becomes more obvious at 600 °C. It indicates that MoO3 is mainly contributed to HDO of O2-phenols, O3-phenols, and O4-phenols at 500 and 600 °C.25 The relatively high temperature is also beneficial to the deoxygenation of O2-phenols, O3-phenols, and O4-phenols. When 600 °C and a L/M of 1/6 is employed, the HDO of O1-phenols is enhanced to form more alkylbenzene (46.7%, see Table 3). The intensified HDO of the resulting liquid intermediates with the rises of MoO3 amount and temperature is further demonstrated by the reduced oxygen content in the oils, which can be found in Table S4 (Supporting Information). 3.3 Solid product It is easily understood that the apparent solid yield decreases with the rises of temperature and MoO3 amount due to a deeper depolymerization. As shown in Table 2, however, it is interesting to find that the apparent solid yield drops to negative when a L/M ratio of 1/6 or even 1/4 at 600 °C was employed. Noting that the solid product weight in this study was calculated by difference of lignin and MoO3 mixture before and after the experiment and corrected for the initial mass of MoO3, the negative apparent solid yield suggests the weight of MoO3 also reduced, which makes the calculation of char yield difficult. Therefore, a series of analysis techniques such as XRD,
ACS Paragon Plus Environment
Page 11 of 28
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
Energy & Fuels
XPS, TG/MS, and TG were performed for solid product to understand its behavior during pyrolysis. XRD patterns of both fresh and spent MoO3 are shown in Figure 4. Referring to the investigation on the conversion of bulk MoO3 into MoO2 from Prasomsri et al.,22 the peaks at 2θ=26.04°, 37.00°, and 53.52° are assigned to crystalline MoO2 particles. XRD patterns of pre-reduced and raw MoO3 were presented in Figures 4d and 4e, respectively, indicating that phase transformation of MoO3 does not occurred during partial reduction procedure under H2 atmosphere. However, it was obvious that the spent MoO3 has been changed into MoO2 (see Figures 4a–4c and 4f–4k). These results suggest that the lattice oxygen in MoO3 can be more easily withdrawn by carbon from lignin than hydrogen from carrier gas under the operational conditions. Theoretically, MoOxCy can be formed after the oxygen in MoO3 is replaced by carbon originated from lignin.26 However, no signal assigning to MoOxCy substance is found in Figure 4, probably because these species are below the detection limit of the diffractometer.22 XPS profiles are depicted in Figure 5, which present oxidation state of the surface Mo species of bulk MoO3. It shows that the Mo (3d) doublet comprised of Mo4+, Mo5+, and Mo6+ oxidation states, and no signals related to Moδ+ (2
