Article pubs.acs.org/EF
Screening of Mixed-Metal Oxide Species for Catalytic Ex Situ VaporPhase Deoxygenation of Cellulose by py-GC/MS Coupled with Multivariate Analysis Pyoungchung Kim, Timothy G. Rials, Nicole Labbé,* and Stephen C. Chmely* Center for Renewable Carbon, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *
ABSTRACT: We present an investigation related to catalytic upgrading of cellulose pyrolysis vapors using mixed-metal oxide catalysts derived from layered double hydroxide precursors. We performed principal component analysis on the pyrolysis-gas chromatography/mass spectrometry data to elucidate changes in the product slate between noncatalytic fast pyrolysis, catalytic pyrolysis using the oxides of magnesium, aluminum, and zinc, and catalytic pyrolysis using our synthesized mixed-metal oxides containing the same cations. Our investigations demonstrate that the mixed-metal species behave differently than even a physical mixture of their monometal counterparts, and that they are capable of producing more furanic compounds by fast pyrolysis of cellulose. We also demonstrate that the metal ratio and identity in these catalysts impart different selectivities to the resulting product slates. Taken together, these data establish the utility of the mixed-metal oxide catalysts in producing a liquid product with low oxygen content and properties that are potentially tunable by metal ion substitutions.
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INTRODUCTION Political and national security affairs and heightened concern over climate change have contributed to growing interest in renewable energy resources. Bio-oil, the liquid product of fast pyrolysis, is a major focus area of research and development because it is a low-value fuel and potential blendstock for crude oil, which makes it a renewable source of not only fuels, but also products and power. During fast pyrolysis, biomass feedstock is rapidly heated in the absence of air, which causes it to vaporize. Condensation of these vapors affords a dark brown, viscous acidic liquid called bio-oil. Despite its superficial similarity in appearance to petroleum, bio-oil is currently underutilized due to a number of undesirable properties, including instability, corrosiveness, low heating value, and immiscibility with crude oil.1 These properties are associated with the presence of oxygen (up to 50 wt %) in the bio-oil.2,3 Bio-oil contains a complex mixture of water and oxygencontaining organic molecules that arise from the depolymerization and defragmentation reactions of the cellulose, hemicellulose, and lignin found in biomass.4 Depolymerization reactions of lignin typically produce phenolic compounds, and those of cellulose and hemicellulose produce sugars and furans, which can further decompose to additional oxygen-containing complexes such as acids, alcohols, aldehydes, and ketones.3 As such, understanding the exact composition of bio-oil produced from whole biomass is a laborious task that requires the identification and quantification of hundreds of compounds. Because cellulose is the major component in plants, it greatly contributes to the complexity of bio-oil. A substantial effort to elucidate the kinetic and mechanistic parameters of cellulose pyrolysis has been exerted over the past 65 years, although many of the details are presently debated.5−7 Nonetheless, an in-depth understanding of the mechanisms of cellulose pyrolysis and the related product slate derived from it could help to make bio-oil a viable source of renewable fuels, © XXXX American Chemical Society
chemicals, and products. The major product of fast pyrolysis of cellulose is levoglucosan (LGA), and a number of recent studies from the groups of Auerbach,7 Dauenhauer,8 Broadbelt,9−11 and Shanks10−12 have provided mechanistic insights pertaining to its formation and the formation of other compounds during fast pyrolysis of cellulose. Although LGA is a kinetically stable product of cellulose pyrolysis, catalysts can be used to convert it into other relevant products. For instance, successive dehydration, decarbonylation, and decarboxylation reactions of LGA and related intermediates catalyzed by HZSM-5 afford furans, which represent bio-oil components with lowered oxygen content.13 Accordingly, a number of publications have investigated the catalytic fast pyrolysis of cellulose in an effort to ameliorate some of the negative qualities of bio-oil.14−19 Although the number of studies using solid acid catalysts vastly outnumbers those using solid amphoteric metal oxides,20 there have been reports detailing the use of the latter to stabilize bio-oil. For instance, Nokkosmäki et al. reported the ex situ vapor-phase upgrading of pine sawdust using ZnO affords a stabilized bio-oil with a lower viscosity than that which did not undergo a catalytic treatment.21 In addition, Stefanidis et al. demonstrated that in situ catalytic fast pyrolysis of beech wood using high surface area Al2O3 increased selectivity for hydrocarbons, although it simultaneously lowered the liquid product yield.22 Fabbri et al. reported that nano-Al2O3 increased the amount of dehydrated C6 monomers during ex situ catalytic fast pyrolysis of cellulose,16 and, in a separate report, indicated that nano-Al2O3 and nanoMgO substantially alter the distribution of anhydrosugars during the in situ catalytic fast pyrolysis of cellulose.15 Furthermore, Lu et al. revealed that various nanometal powders, including ZnO and MgO, were capable of dramatically reducing the number of Received: February 12, 2016 Revised: March 15, 2016
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DOI: 10.1021/acs.energyfuels.6b00347 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
motivated to investigate the ability of mixed-metal oxide species to catalyze these reactions. In the present work, three catalyst species based on LDH precursors containing mixtures of Al, Zn, and Mg were synthesized and characterized. These were investigated in the catalytic ex situ vapor-phase upgrading of cellulose pyrolysis vapors, with our goal being a determination of the extent to which these species can deoxygenate pyrolysis vapors. The gaseous products of this reaction were identified using py-GC/MS. Principal component analysis was applied to reveal statistically significant differences in reactivities between the catalyst species and between those and the noncatalyzed reaction. Although we are unable to isolate a liquid product using py-GC/MS, the data are useful for the rapid screening of these catalysts. From these data, we draw conclusions about the suitability of this family of catalyst species to deoxygenate and stabilize bio-oil.
organic acids during the ex situ catalytic vapor phase upgrading of poplar wood.23 Finally, Zhang et al. demonstrated the production of 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one (a δ-lactone commonly known as LAC) was affected by changes in temperature and catalyst ratio in the presence of Zn−Al mixedmetal oxide catalysts.24 Solid base catalysts show promise in providing a stable, low-oxygen liquid product from the catalytic fast pyrolysis of biomass.25 Layered double hydroxides (LDHs) are part of a class of anionic clays formed by metal hydroxide layers with compensating anions and water molecules in the interlayer region.26 These materials can be expressed by the general formula [M2+(1−x)M3+x(OH)2]x+[An−]x/n·yH2O, where M2+ and M3+ occupy octahedral sites in a brucite-like layer. The modular nature of these materials means that essentially any combination of M2+ and M3+ is allowable, so long as the ionic radii of these two are similar. In addition, calcination of the derived material leads to a mixed-metal oxide, the catalytic reactivity of which can be altered by varying the M2+:M3+ ratio or textural properties of the solid material.27 These properties make LDHs a potential precursor for catalytic species containing a mixture of metal oxide species, which have been shown to be relevant to the stabilization of bio-oil. Rapid analysis of product slates produced from the catalytic fast pyrolysis of cellulose can be achieved using pyrolysis-gas chromatography/mass spectrometry (py-GC/MS). This technique allows for the detection and identification of the vaporous products of fast pyrolysis; however, the complexity of the resulting pyrograms, which can contain hundreds of peaks, makes data analysis challenging. Principal component analysis (PCA) is a common multivariate statistical method used to extract relevant information from large and complex data set (Xvariables) and allows for visualization of the main variability of a data set without the constraint of an initial hypothesis concerning the relationship within samples, or between samples and responses (X-variables). This mathematical transformation compresses the data by removing the redundancy and expresses the main information in the data set by a lower number of variables, so-called principal components (PC) of X.28 Each sample has a score on each principal component, reflecting the sample location along the PC. Plotting these scores against one another can reveal patterns, clustering, or trends within the data set. The scores describe the data structure in terms of samples patterns and more generally show sample differences or similarities. Each score has an associated loading, which provides information about the chemical differences between the samples. PCA is commonly performed on spectroscopic data such as nearinfrared, mid-infrared, and Raman to group and classify samples on the basis of their spectral signature. More recently, PCA has been used to investigate pyrolysis products distributions of biomass.29 In addition to identifying outliers, sample screening, and sample comparisons, PCA has been used to evaluate the impacts of pyrolysis conditions on the composition and yield of products.30 Pattiya et al. demonstrated the impacts of readily available catalysts on the pyrolysis products of cassava rhizome and used this approach to select the appropriate catalysts to perform selective transformations during biomass pyrolysis.31 The combination of myriad experimental techniques with principal component analysis is proving to be an attractive approach to optimize the thermochemical conversion of biomass. Given the relative dearth of studies involving solid-basecatalyzed deoxygenation of pyrolysis oil vapors, we were
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EXPERIMENTAL SECTION
Materials. The hydrates of aluminum, magnesium, and zinc nitrate, urea, and Avicell cellulose were commercially available and used without further purification. Aluminum, magnesium, and zinc oxide were commercially available and were calcined at 773 K in static air for 3 h before use. Hexanes was commercially available as a mixture of structural isomers and used as received. Quartz wool was washed by soaking in hexanes and dried overnight in a vacuum oven (353 K, house vacuum) to remove organic impurities that were detectable by py-GC/MS. No organic components were detectable after this washing step. Synthesis of Catalysts. The synthesis of catalyst materials was accomplished using modified literature preparations.32,33 Briefly, an aqueous solution of precursor metal ions was prepared containing the appropriate ratio of zinc or magnesium and aluminum ions in deionized water. Urea was dissolved in this solution so that the ratio of urea to total nitrate ions was 3:1. The resulting clear, colorless solution was heated to 363 K with vigorous stirring for 24 h. No attempt was made to exclude air from this reaction. The reaction afforded a white precipitate, which was collected by filtration through a medium porosity sintered glass filter, washed thoroughly with deionized water, and dried in ambient air overnight. This layered double hydroxide material was characterized by pXRD and elemental analysis as outlined below. Finally, the material was calcined at 773 K in static air for 3 h to obtain the mixed-metal oxide catalyst, which are denoted by their metal ratios; for example, Mg2Al is a mixed-metal oxide with a stoichiometric ratio of Mg:Al = 2, and so on. Surface area, qualitative basicity measurements, and X-ray diffractometry were carried out on the mixed-metal oxide material as outlined below. Catalyst Characterization. X-ray diffraction profiles were collected using a Bruker D2 Phaser X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm), operated at a beam voltage of 30 kV and a beam current of 10 mA. Scans were collected for 2θ values ranging from 10° to 70° using a scan rate of 2°/min. Nitrogen adsorption/desorption isotherms were collected at 77 K using a Quantachrome NOVA 1000. The samples were degassed under dynamic vacuum at 473 K for 3 h. The specific surface area of the samples was measured using the Brunauer−Emmett− Teller (BET) method. Elemental analyses for Mg, Zn, and Al were performed by ALS Environmental, Tuscon, AZ, and Galbraith Laboratories, Inc., Knoxville, TN, using inductively coupled plasma optical emission spectroscopy. Hammett indicator experiments were conducted to determine the Hrange of basic sites in each catalyst.34 The indicators used were neutral red, phenolphthalein, and alizarin yellow R, and a 1 wt % solution of each in methanol was prepared. Catalyst material was calcined at 773 K under flowing N2 (7 s·dm3/min) for 1 h to remove surface-bound carbonates. This material then was cooled to approximately 373 K under nitrogen and quickly transferred to a vacuum desiccator to cool to ambient. Approximately 10 mg of this material was suspended in 5 cm3 methanol, and a few drops of each indicator solution were quickly added. The basic strength of each catalyst was taken to be higher than the weakest B
DOI: 10.1021/acs.energyfuels.6b00347 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels indicator that underwent a color change and lower than the strongest indicator that underwent no color change.35 Catalytic Pyrolysis and Vapor Analysis. Pyrolysis GC/MS (pyGC/MS) was carried out using a PerkinElmer Clarus 680 gas chromatograph coupled with a Clarus SQ 8C mass spectrometer. This instrument is outfitted with a Frontier EGA/Py-3030 D pyrolyzer and an autosampler that loads samples for pyrolysis contained in stainless steel cylinders (d = 4 mm, h = 8 mm). These cylinders were layered with cellulose (0.5 mg), a plug of washed quartz wool (0.5 mg), catalyst material (1.0 mg), and a final plug of quartz wool (0.5 mg). The layers of quartz wool were used to prevent the catalyst material from physically contacting the cellulose substrate. A blank cellulose pyrolysis reaction, run in the absence of catalyst material, was conducted in a similar manner containing quartz wool to control for its presence in the reaction. Samples were dropped via the autosampler into the pyrolysis furnace that is directly attached to the injection port of a GC/MS instrument. The furnace was constantly held at 773 K for pyrolysis. Pyrolysis residence time was set to 12 s to allow the vapors some time to mix with the catalyst material. At the conclusion of this time, the pyrolysis products were swept into the injector port (unpacked 2 mm quartz liner, split ratio 80:1, injector temperature 543 K) by the GC carrier gas (ultrahigh-purity helium, 99.9999%) that passes through the furnace. The sample was then separated using an Elite 1701 MS capillary column (60 m × 0.25 mm ID by 0.25 μm film thickness) operating under 1 cm3 per minute flow and 17.3 psi pressure. The temperature program was as follows: 4 min at 323 K, ramp 5 K per minute to 553 K, hold at 553 K for 4 min. The compounds were analyzed using MS (source temperature 543 K, 70 eV electron ionization), and compared to the NIST library of fragmentation patterns. All 243 chromatographic peaks (identifiable and nonidentifiable peaks using the NIST library) with S/N ≥ 2000 were extracted from pyrograms using TurboMass GC/ MS software. These peaks were normalized to peak area percentages calculating by the quotient of individual peak area and total peak area. Instead of retention time, a number was assigned to each peak area % and used as a variable for the statistical analysis to alleviate the issue related to retention time shift for the same compound between pyrograms (the complete table containing peak assignments is available in the Supporting Information). Statistical Analysis of Gaseous Products. Pyrograms produced from cellulose pyrolysis with and without catalysts were analyzed using the principal component analysis (PCA) multivariate method (Unscrambler software ver. 9.0, CAMO). PCA generates scores and loadings plots, which are the maps of samples and variables, respectively. The scores plot visualizes the trends between the samples in the new system of axes of principal components (PCs), and the loadings plot displays the degree of contribution of each variable (chromatographic peak area %) to the separation of the samples. Five pyrograms per sample (cellulose with no catalyst, MgO, Al2O3, ZnO, a physical mixture of 4:1 MgO:Al2O3 (2:1 in Mg:Al), Mg2Al, MgAl, and Zn2Al) were set as samples, while individual peak areas % in the pyrograms were set as variables.
Figure 1. Powder X-ray diffraction patterns of commercially available hydrotalcite (HTC) and the as-synthesized layered double hydroxide (LDH) materials. Dotted red lines indicate the corresponding diffraction peaks in HTC.
Table 1. Elemental Analysis for Layered Double Hydroxide Materials
a
sample
M2+a
M3+a
ratio
2:1 Mg-LDH 1:1 Mg-LDH 2:1 Zn-LDH
20.5 27.7 37.8
10.3 27.7 8.9
2.0 1.1 1.8
Percent by weight.
Figure 2. Representative powder X-ray diffraction patterns of Zn2Al LDH prior to calcination (a), calcined Zn2Al mixed-metal oxide (b), commercially available ZnO (c), and commercially available Al2O3 (d). Identical comparisons for the other synthesized catalyst materials are available in the Supporting Information.
constituent oxides, based on the absence of diffraction peaks for these materials (Figure 2c,d). This is in agreement with data that demonstrate the ordered arrangement of the constituent metals in the layered double hydroxide starting materials.37 Moreover, we are unable to detect any Mg- or Zn-containing spinel phases in any pXRD measurements. This is consistent with data reported by Zhao et al. that demonstrate that Zn-containing spinel structures are formed from LDH precursors only at calcination temperatures of 800 °C or above.38 This report further indicates the formation of a mixture of poorly crystalline divalent-metal oxide and an uncharacterized amorphous phase, which is consistent with the pXRD patterns displayed in Figure 2. As shown in Table 2, the surface area of the calcined materials is substantially higher than that of the related monometal oxides. In addition, each oxide sample was qualitatively evaluated for surface basicity using a series of Hammett indicators (Table 2). All of the mixed-metal oxide samples were found to possess Hvalues in the range of 8−10, because they were able to effect a color change of neutral red from red to yellow (pH ≈ 8.0) but were unable to effect a color change of alizarin yellow R from
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RESULTS AND DISCUSSION Catalyst Synthesis and Characterization. Synthesis of layered double hydroxides (LDHs) to be used as catalyst precursors was conducted using the urea method.32 This method has been shown to produce highly crystalline material, which is apparent on the basis of the diffraction patterns for the assynthesized material in Figure 1. A slight shift in the diffraction angles is evident, which is consistent with a change in the lattice constants due to the different concentrations of cations in each sample.36 Ratios of metal ions in each as-synthesized LDH sample were confirmed using ICP-OES (Table 1). Thermal decomposition of the layered materials yields the catalytic mixed-metal oxides of interest. The broad diffraction peaks in the pXRD pattern (Figure 2b) are indicative of the poorly crystalline nature of the calcined material. In addition, the material does not appear to be a physical mixture of the C
DOI: 10.1021/acs.energyfuels.6b00347 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
hundreds of peaks. Deciphering changes in the product slate among the catalysts tested is nearly impossible by simple comparison of the pyrograms (see the Supporting Information for these pyrograms). Quantitative comparisons of pyrograms are possible by constructing standard curves for each compound (which involves multiple injects of multiple compounds), but this process is time-consuming and therefore does not owe itself to rapid screening and comparisons of new catalyst materials. However, principal component analysis (PCA) can be used to investigate the relative effects these solid catalysts have on the vapor products of the fast pyrolysis of cellulose. Although it does not include absolute quantitative analysis of the product slate, PCA reveals qualitative and semiquantitative differences among product slates derived via catalytic upgrading. This technique is especially powerful in determining relative performance among various catalyst species, as it elucidates statistically significant differences among samples that are not detectable by simple comparison of pyrograms. The first PCA was performed with all catalysts, and the data set combined 243 chromatographic peaks detected by py-GC/MS, including all identifiable and nonidentifiable peaks from the NIST library. As shown in Figure 3A, the PCA scores plot visualizes the distribution of catalysts along PC1 and PC2 with 97% and 1% of the total data variance, respectively. This indicates that PC1 takes into account much of the information. The scores plot shows that the product slate produced from the fast pyrolysis of cellulose is compositionally similar to that produced by the catalytic fast pyrolysis of cellulose using MgO, Al2O3, ZnO, or a physical mixture of MgO and Al2O3. This is demonstrated by the close proximity of the related markers in the positive direction along the PC1 axis. In addition, the locations of the markers related to Mg2Al, MgAl, and Zn2Al along the negative PC1 axis indicate differences in the products formed when these are used as catalysts.
Table 2. Textural and Basicity Analysis of the Synthesized Materials sample
surface areaa
basicityb
Mg2Al MgAl Zn2Al MgO Al2O3 ZnO
191.1 111.1 188.7 35.8
