Article pubs.acs.org/EF
Online Analysis of Biomass Pyrolysis Tar by Photoionization Mass Spectrometry Liangyuan Jia,† Yann Le Brech,† Guillain Mauviel,† Fei Qi,‡ Matthias Bente-von Frowein,§ Sven Ehlert,§,∥,⊥ Ralf Zimmermann,∥,⊥ and Anthony Dufour*,† †
Reactions and Process Engineering Laboratory (LRGP), National Centre for Scientific Research (CNRS), University of Lorraine, National School of Chemical Industries (ENSIC), 1 Rue Grandville, 54000 Nancy, France ‡ Key Laboratory for Power Machinery and Engineering of Ministry of Education (MOE), Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China § Photonion GmbH, Hagenower Straße 73, 19061 Schwerin, Germany ∥ Joint Mass Spectrometry Centre, Chair of Analytical Chemistry, Institute of Chemistry, University of Rostock, 18059 Rostock Germany ⊥ Comprehensive Molecular Analytics (CMA), Helmholtz Zentrum München, 85764 Neuherberg, Germany ABSTRACT: The online analysis of volatiles from biomass pyrolysis (or gasification or combustion) is interesting because it has the ability to sample the volatiles directly from their reactive environment. The photon ionization (PI) is an efficient and soft ionization method for online analysis of biomass pyrolysis volatiles. Here, we review recent developments conducted in our groups on PI−mass spectrometry (MS) analysis of biomass pyrolysis volatiles by (1) synchrotron light PI−MS and (2) various commercial PI−MS techniques combined with various pyrolysis reactors. The fundamentals of PI−MS applied to biomass tar are briefly presented. The effect of photon energy on mass spectra from biomass volatiles is studied by synchrotron PI−MS. Different sources of PI−MS are then compared on vapors produced from fast pyrolysis in a microfluidized bed, namely, argon electron-beam-pumped excimer light (EBEL) vacuum ultraviolet (VUV) lamp single photon ionization (SPI)−MS (126 nm and 9.8 eV), laser Xe cell−SPI−MS (118 nm and 10.5 eV), laser resonance-enhanced multiphoton ionization (REMPI)−MS (266 nm). The suitability of these different ionization techniques for tar online analysis is discussed. The high potential of PI−MS to unravel the mechanisms of biomass pyrolysis is highlighted by some examples of applications. A VUV lamp SPI−MS has been combined to a fixed bed reactor to study the evolution of chemical markers from lignin, cellulose, and hemicelluloses as functions of biomass types and temperature of pyrolysis. It has also been combined to a microfluidized bed to study the fast pyrolysis of different sizes, shapes, and composition of biomass particles. Principal component analysis of the various MS “fingerprints” reveals interesting markers of some pyrolysis regimes.
1. INTRODUCTION Condensable vapors from biomass pyrolysis are complex mixtures containing a wide range of oxygenated aromatic compounds (e.g., furans, ketones, phenolics, organic acids, etc.), which make the bio-oil unstable and acidic for transportation, storage, and industrial use.1,2 Detailed information about the composition of pyrolysis vapors and their reaction mechanism is required to understand and optimize the processes of biomass pyrolysis.1 It is also important to monitor online the vapors to assess the transient phenomena occurring during the pyrolysis of individual particles or on industrial processes (e.g., during the commissioning of the unit or during the changes of operating conditions). For this reason, there is a strong need on the direct online analysis of the biomass vapors. Mass spectrometry (MS) is a fast and sensitive technique, and it can be used for online, real-time analysis. It has provided important insights into the structure and composition of pyrolysis products from biomass in current analytical tools.1,3,4 Meanwhile, the rapid development of MS benefits from the innovation of ionization techniques. The traditional electron impact (EI) ionization source is a “hard” ionization method and is often used for the pyrolysis coupled with gas chromatography © XXXX American Chemical Society
mass spectrometry (Py−GC−MS) or pyrolysis−molecular beam mass spectrometry (Py−MBMS) for online analysis of pyrolysis vapors of biomass.5−10 The overlapping ionization fragments usually make the EI mass spectra difficult to be interpreted even if the low-energy EI (12.0−16.0 eV) is used.11−13 On the other hand, Evans et al. studied the mass spectrometric behavior of levoglucosan under different ionization conditions and stated that all Py−MS studies of cellulose or levoglucosan pyrolysis must consider the contribution of ionization fragmentation because pyrolysis and ionization processes create the same species at several important m/z.14 Thus, the drawback of EI has led to the development of “soft” ionization methods to produce the fragment-free ions. Several soft ionization techniques, including chemical ionization (CI),10,15 electrospray ionization (ESI),16,17 matrix-assisted laser desorption/ionization (MALDI),18 and Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: September 30, 2015 Revised: December 23, 2015
A
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Energy & Fuels photon ionization (PI),13,19,20 have been used to identify the biomass pyrolysis products. However, more and more studies support the view that PI may become a promising universal and standard soft ionization technology in the future.21−23 Until now, PI−MS is more widely used for the analysis of combustion products than for biomass pyrolysis.24−26 The PI technique is obtained by means of an ultraviolet (UV) or vacuum UV (VUV) light source.27 Laser-based resonanceenhanced multiphoton ionization (REMPI) using UV wavelengths is generated by standard pulsed lasers.28,29 The REMPI processes typically involve a resonant single or multiple photon absorption to an excited intermediate state, followed by another photon, which ionizes the atom or molecule. Dependent upon the required number of photons for the excitation to a vibronic state prior to ionization, REMPI processes can be divided into different classes: (1 + 1) REMPI, (2 + 1) REMPI, etc. (see Figure 1).30 In REMPI processes, only atom or molecules with
produce the best performing VUV light, which has some unique abilities in terms of high flux, broad energy range, continuous tunability, and high energy resolution.25 Synchrotron-based SPI−MS was used for the first time to study the biomass pyrolysis by Qi and Dufour groups. The results show that the analysis of the biomass vapors depends strongly upon the photon energy.20,37,38 A novel intermediate compound (m/ z 128) produced from cellulose pyrolysis has been evidenced by means of the synchrotron-based SPI−MS.37 VUV light generated from different sources corresponds to various photon energies, which are relevant for the different classes of biomass products. In Figure 2, the available ionization
Figure 1. Simplified scheme of SPI and REMPI principles (summarized from refs 28−30).
a vibronic structure can be ionized and identified; therefore, it has been proven to be particularly selective for detecting ligninderived phenolic compounds.19,31−33 Unlike REMPI processes, single photon ionization (SPI) involves the process that a photon with a short wavelength or high energy can ionize the atoms or molecules in one step. The ionization threshold of the molecule has to be below the photon energy of the applied VUV light (see Figure 1). SPI requires the suitable sources and optics for VUV light generation and emission in the required photon energy range (7.5−16.0 eV). Continuous VUV light can be excited from discharge lamps filled with various noble gases (e.g., Kr, Ar, He/H2, etc.). Their photon energy distributes on broad spectral lines usually centered between 8.4 and 11.8 eV.21 Lyman-α light (10.2 eV) exhibits a high photon flux, which is irradiated from a microwave discharge lamp filled with He/H2. This light can be used for SPI.34 Lampbased SPI−MS has been successfully used for the characterization of biomass vapors.19,33,35 Pulsed-laser-based sources are also capable of emitting a stable VUV light with a very narrow energy spread for SPI. The most commonly used 118 nm VUV light can be produced from frequency tripling in a xenon cell of the third harmonic (355 nm) of a Nd:YAG laser. Brown et al. applied laser SPI−MS to analyze the biomass pyrolysis vapors and demonstrated that the mass spectra obtained by SPI showed a markedly lower degree of fragmentation than the mass spectra obtained by EI.36 Finally, synchrotron can
Figure 2. Versatility of VUV SPI as a soft ionization technique: photon energy distribution of some VUV light sources (in part adapted from ref 21) and IEs of some compounds from biomass pyrolysis. These IEs are from the National Institute of Standards and Technology (NIST) and theoretical calculations conducted at the CBS-QB3 level of theory (see ref 37). U10 and U14c represent two beamlines in the NSRL (Hefei, China) before its upgrade. The relative intensities of different VUV light sources displayed here do not reflect their real photon flux.
energies (IEs) of biomass vapors are shown as a function of their molecular mass along with the VUV emission profiles of a commonly used light source, and most of them are distributed in the energy range of 7.8−10.0 eV. Some convenient VUV sources, such as a molecular fluorine laser (F2 laser) and hydrogen laser (H2 laser), can emit short pulses at 7.87 and 7.75 eV, respectively.21,39 However, these low photon energies are below the IEs of most biomass vapors (see Figure 2). Photon energies of VUV light from an Ar- or Kr-excimer lamp and from a Nd:YAG laser (9.0−10.5 eV) well cover the IEs of most biomass vapors.21 The tunable synchrotron VUV (SVUV) light allows for the near-threshold PI of almost all biomass vapors, including permanent gases (e.g., CO, CO2, CH4, and H2) and water. SVUV light has a potential ability to provide the information on the unknown IEs of biomass vapors by scanning the photon energies.40−43 The main issue on SVUV is that synchrotron sources are scarce and not movable. Fendt et al. B
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Energy & Fuels stated that SPI cross-sections for different compounds are on the same order of magnitude (especially for similar molecules and higher masses), but this is not the case for REMPI analysis. Indeed, the different REMPI cross-sections of molecules have to be assessed to compare the absolute signal intensity of individual peaks.19 For example, the REMPI cross-sections are relatively low for syringol (S) derivatives but high for the guaiacol (G) derivatives. The detailed information about the detectability of biomass vapors by means of REMPI at 266 nm has been reported.33 The aim of this paper is to point out the fundamentals of “soft” PI on the basis of various light sources and the advantages of PI−MS in the application of the biomass pyrolysis study. PI−MS allows for the real-time and online monitoring of the pyrolysis vapors. It is an attractive technique to reveal the chemical mechanism of biomass thermal conversion. Besides, some examples are given for the applications of PI−MS in the biomass pyrolysis study.
2. EFFECT OF PHOTON ENERGY ON MASS SPECTRA FROM BIOMASS PYROLYSIS AS STUDIED BY SYNCHROTRON SPI−MS The optimal photon energy for the ionization of biomass pyrolysis vapors has been studied by means of SVUV PI−MS thanks to the continuous tunability of SVUV light at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. A new setup based on SVUV PI−MS was developed by Weng et al. for online analysis of solid material pyrolysis in the NSRL.20,37 More details about the pyrolysis setup and synchrotron source have been given in details elsewhere.20 Briefly, synchrotron using undulator radiation from an 800 MeV electron storage ring produces VUV light covering the photon energy of 7.8−24.0 eV. The pyrolysis setup consists of a homemade chamber with a Shimadzu furnace, a PI chamber, and a modified Qstar pulsar hybrid quadrupole time-of-flight (QTOF) mass spectrometer. Biomass samples were directly introduced into the preheated furnace, and the pyrolysis vapors were then ionized by SVUV light. Finally, the SPI mass spectra of biomass vapors at various temperatures and photon energies were recorded by SPI−MS. Meanwhile, MS/MS spectra of given ions were also obtained by collision-induced dissociation (CID) experiments. During the experiment, the pyrolysis chamber was maintained at a low pressure of 2.8 Torr. The effect of photon energy on cellulose pyrolysis primary volatiles was discussed in our previous work.37 Major cellulose vapors (e.g., m/z 97, 98, 126, and 144) exhibit an increase from 8.5 to 10.0 eV as a result of a better ionization accompanied by a fragmentation of higher m/z (especially m/z 162) into lighter species. The low signal intensity of levoglucosan (m/z 162) has been explained by a dissociative PI of levoglucosan ions. The detailed mechanism of this dissociative ionization has been discussed on the basis of ab initio calculation at the CBS-QB3 level of theory.37 Figure 3 presents the mass spectra of Miscanthus pyrolysis vapors at a reactor temperature of 350 °C as a function of the photon energy from 9.0 to 12.0 eV. The main products from Miscanthus pyrolysis whose chemical structures were characterized by our tandem mass spectrometry (MS/MS) and two-dimensional gas chromatography (GC × GC)/MS analysis are listed in Table 1.44 Besides, MS/MS analysis after SVUV ionization provides more accurate structural information on biomass vapors.20,37 For instance, in cellulose pyrolysis, the proposed structure of m/z 128 (or position isomers) was deduced from its MS/MS spectrum and
Figure 3. Effect of photon energy (9.0−12 eV) on the mass spectra from Miscanthus pyrolysis (pyrolysis reactor at 350 °C) obtained by tunable SVUV PI−MS (in part from ref 37).
the peak at m/z 86 should be from hemicelluloses based on MS/MS spectra of m/z 114.37 As seen in Figure 3, as the photon energy increased, pyrolysis vapors and fragments with different IEs were observed successively in SPI mass spectra (see panels a−d of Figure 3). At the photon energy of 9.0 eV, an almost fragment-free mass spectrum was obtained and most pyrolysis vapors (e.g., m/z 110, 114, 120, 126, 150, 154, etc.) could be observed because their IEs are lower than 9.0 eV (see Figure 2). When photon energy increased from 9.0 to 9.5 eV, there is no remarkable difference in the two mass spectra, except for the newly appeared m/z 70, probably corresponding to 2,5dihydrofuran (IE = 9.16 eV). At a higher photon energy of 10.5 eV, fragments from polysaccharide products (e.g., m/z 43, 57, and 85) were detected by SPI−MS. When the photon energy reached 12.0 eV, no new compounds could be detected, expect for the increase in fragments (e.g., m/z 69, 85, 97, etc.). Furthermore, the previous work20 also indicates that the IEs of some lignin markers (e.g., m/z 150, 154, 164, 180, and 194) are lower than 8.0 eV. The IEs of major carbohydrate markers (e.g., m/z 110, 114, 126, and 162) and some lignin markers (e.g., m/ z 94, 124, 152, 166, and 178) distribute in the energy range of 8.0−9.0 eV. Other markers, such as m/z 86, 96, 98, and 112, could be detected between 9.0 and 10.5 eV. These results agree with the IE values given in Figure 2. SVUV PI mass spectra for xylan (from birch, Sigma-Aldrich) and organosolv lignin (extracted from the same Miscanthus as analyzed above) pyrolyzed at a reactor of 350 °C were obtained at 9.5 and 10.5 eV (see Figure 4). 3-Hydroxy-2-penteno-1,5lactone (m/z 114) is one of the major markers for xylan C
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Table 1. Mass Assignments and Molecular Structure of Main m/z Markers from Carbohydrates and Lignin in Miscanthus (from Ref 44)
Figure 4. SPI mass spectra for extracted xylan (from birch) and organosolv lignin (from Miscanthus) at the same temperature of 350 °C but different photon energies of 9.5 and 10.5 eV (in part from ref 37).
pyrolysis. Please note that SPI mass spectra of extracted xylan also show some lignin markers (e.g., 124, 138, 152, 164, etc.), which are lignin chemical moieties associated with the extracted xylan.45 Xylan is usually prepared by removal of lignin with
sodium chlorite or by several treatments with chlorine in ice water, followed by extraction of chlorinated lignin. An ideal delignification should remove lignin exhaustively without breaking the polysaccharides, but few delignification procedures D
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Energy & Fuels can satisfy this requirement.46 As seen in panels a and b of Figure 4, the signal of m/z 114 increased considerably with rising the photon energy from 9.5 to 10.5 eV, indicating that 10.5 eV is more suitable for hemicellulose markers than 9.5 eV. The lignin extracted from Miscanthus by the organosolv method exhibits similar markers as the lignin present inside the complex matrix of Miscanthus (panels c and d of Figure 4 versus Figure 3). However, the photon energy (9.5−10.5 eV) has almost no effect on the SPI mass spectra of lignin markers. Therefore, the best compromise between an efficient ionization of the majority of biomass pyrolysis volatiles and a reduced fragmentation is photon energy between 9.5 and 10.5 eV. On the basis of this finding obtained with SVUV PI−MS, the most appropriate commercial ionization sources for biomass pyrolysis would be Ar electron-beam-pumped excimer light (EBEL) VUV lamp (126 nm) or laser SPI (118 nm).
with a selection of different PI sources: (1) Ar excimer VUV lamp system (EBEL), which is modified from an E-Lux 126 lamp (Optimare, Germany), pumped by an electron beam to produce VUV light with a photon energy of 9.8 eV for SPI;49 (2) VUV laser system, which can generate VUV light with photon energy of 10.5 eV (118 nm) through the ninth harmonic of the Nd:YAG fundamental laser frequency,30,36 with the repetition rate of the laser being 20 Hz; and (3) UV laser system, which has two doubling units at 266 nm, employed for REMPI. Actually, VUV and UV lights are emitted from the same Nd:YAG. There is a laser unit for simultaneous generation of UV and VUV laser pulses and switching between these two ionization sources. Optical devices are used to uncouple the different laser beams into the ionization region. All of these ionization systems are developed by the Photonion Company (Germany). Figure 6 displays the comparison between mass spectra obtained from Miscanthus fast pyrolysis in MFBR set at 400 °C
3. EFFECT OF THE IONIZATION TECHNIQUE ON PI MASS SPECTRA FROM BIOMASS PYROLYSIS The combination of the selectivity of UV-based REMPI and VUV-based SPI enables comprehensive analysis of biomass pyrolysis volatiles. However, because of the different nature of the ionizing photons and their generation, i.e., short laser pulses and high-frequency VUV beam, experiments were needed for investigating the effect of the ionization technique on the mass spectra of biomass vapors under similar well-controlled pyrolysis experiments. For this purpose, we have combined several commercial PI sources to a microfluidized bed reactor (MFBR) for the production of fast-pyrolysis vapors under wellcontrolled conditions. A simplified scheme of the experimental setup is shown in Figure 5, and more details can be found
Figure 6. Comparison between mass spectra obtained for Miscanthus pyrolysis in a microfluidized bed at 400 °C by (a) Ar EBEL VUV lamp (9.8 eV), (b) laser SPI (10.5 eV), and (c) laser REMPI (266 nm).
Figure 5. Microfluidized bed coupled to MS with various ionization techniques (adapted from ref 47): lamp SPI (9.8 eV), laser SPI (10.5 eV), and laser REMPI (266 nm).
by different ionization methods. In general, mass spectra recorded by lamp- and laser-based SPI−MS display very similar m/z peaks, except for some more intense fragment ions (i.e., 43, 57, 85, etc.) in the latter case for laser SPI (10.5 versus 9.8 eV) (panel a versus panel b of Figure 6). There is no major difference in the ionization behavior between lamp- and laserbased VUV light. Nonetheless, for the laser-based VUV light, the laser energy and the delay between the ion extraction pulse and ionization pulse should be well-optimized because an intense laser beam can produce heat and further lead to more thermal fragments. Besides, the comparison between laser- and synchrotron-based SPI mass spectra at 10.5 eV indicates that
elsewhere.47 In short, the MFBR with an inner diameter of 20 mm was designed in CNRS, Nancy, for biomass fast pyrolysis, following the recommendations of Xu et al.48 The MFBR is heated by an electric furnace with a quartz window, which allows for the visualization of the MFBR in real time and temperature. Samples in different shapes and sizes could be fixed or stored in specially designed injection rods and then introduced into the MFBR. The pyrolysis vapors from biomass were analyzed by a reflectron time-of-flight (RTOF) mass spectrometer equipped E
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Energy & Fuels detected main products are similar, except for the weak signals of m/z 86, 96, and 98 but strong signal of m/z 137 in the latter case (synchrotron SPI) (Figure 6b versus Figure 3c). However, this slight difference may also be explained by the use of different reactors and different conditions (e.g., temperature, pressure, etc.). With regard to REMPI analysis, the absence of carbohydrate products from Miscanthus fast pyrolysis in Figure 6c confirms that the REMPI process is highly selective and sensitive for lignin-derived products with H, G, and S subunits when compared to other ionization methods.32,33 The majority of monomeric aromatics (e.g., 94, 120, 150, 162, 164, 180, etc.) detected by REMPI−MS were characteristically derived from lignin decomposition. Few odd mass peaks at m/z 77, 91, 135, etc., which were observed in REMPI mass spectra of Miscanthus pyrolysis, probably correspond to the photofragments of some monomeric aromatics according to previous literature.50 Despite the soft REMPI process, photofragmentation may occur as a result of the excess laser power employed in the REMPI−MS or because of the direct irradiation of the quartz capillary (used for vapor sampling) by the laser beam, which may further lead to the electron ionization effect. In conclusion, each ionization source has its own advantage and disadvantage. VUV lamp is the simplest VUV source without the need of tedious laser correction, and it is possible to vary the ionization energy by replacing the argon filling with other rare gases or rare gas mixtures; however, its photon flux is relatively low, and its lifetime is short (usually lower than 1000 h). The laser VUV source is relatively expensive and difficult to operate compared to the lamp VUV source, but it produces VUV light with a higher photon flux and has a longer lifetime compared to the lamp VUV source. Laser-based SPI−MS has thus a better detection limit and is more likely to be used for the detection of trace gas than lamp-based SPI−MS. The tunable SVUV light is the best SPI−MS source in terms of performance (tunability, accuracy, photon flux, etc.), while its drawbacks are scarce, expensive, and unmovable. REMPI is highly sensitive and selective for aromatic detection and allows for obtaining the much “cleaner” spectra. It is a method of choice for combustion and gasification reactors because, in these reactors, the primary tars are converted into aromatic secondary and tertiary products with a lower concentration. Nevertheless, sensitivity is not the priority for biomass fast pyrolysis because the concentration of products is always high enough. Indeed, dilution is even often required for a proper PI−MS analysis of pyrolysis vapors, while for some application, such as thermogravimetry−SPI−MS,33 the sensitivity (or detection limit) of SPI−MS may be an issue as a result of the small sample mass used and the slow heating rate, leading to a low concentration of volatiles in the carrier gas. Different PI sources can be selected to meet experimental needs, but the combination of SPI− and REMPI−MS could be the best versatile option for online analysis of biomass or other solid materials (e.g., coal, plastics, etc.) and vapors and for various thermochemical reactors and conditions.
Figure 7. Evolution profiles of main selected markers for Miscanthus slow pyrolysis (fixed bed reactor, 5 K/min) obtained by VUV lamp SPI−TOFMS (9.8 eV). Abbreviations: C, cellulose; H, hemicellulose; L-1, lignin-1; L-2, lignin-2; and L-3, lignin-3 markers (from ref 44).
main selected markers for Miscanthus slow pyrolysis. Markers from different components (cellulose, hemicelluloses, and lignin) are released at different temperature ranges. Among them, lignin markers can be further classified into three classes (lignin-1, lignin-2, and lignin-3) according to their different ways of formation as well as temperatures of formation.44 The characteristic peak at m/z 120 (4-vinylphenol, typical compound of the lignin-1 marker) was detected from 210 to 380 °C, with the maximum release rate at around 270 °C (see Figure 7). This marker was followed by the release of the marker for hemicelluloses (m/z 114, 4-hydroxy-5,6-dihydro(2H)-pyran-2-one) starting at about 220 °C and with a maximum at around 305 °C. Then, the release of m/z 124 (2methoxy phenol, lignin-2 marker) starts at 250 °C and reaches its maximum at around 330 °C. Its formation occurs on a large range of the temperature until 450 °C. The main decomposition of cellulose inside the native network of Miscanthus occurred in a narrower temperature range of 260−370 °C than hemicelluloses and lignin. Finally, m/z 94 (phenol, lignin-3 marker) is released on a broad range of the temperature from 250 °C to temperatures higher than 500 °C. The detailed chemical mechanism for lignin and cellulose pyrolysis has been proposed on the basis of this advanced online analysis during the slow pyrolysis of various biomasses (Miscanthus, oak, and Douglas fir).44 A more simplified chemical mechanism is shown in Figure 8. Globally, cellulose degradation undergoes two different pathways with or without the catalytic effect of minerals. For the uncatalyzed pathway, levoglucosan (m/z 162) can be produced via transglycosylation and the rupture of the glycosidic bond. Other oxygenated compounds, such as 5-hydroxymethyl-2-furaldehyde (m/z 126), 2-furanmethanol (m/z 98), etc., can be obtained through open-ring reactions and dehydration. The formation of cyclopentanone-based compounds from cellulose is promoted by the catalytic effects of minerals (such as potassium present in Miscanthus) through the aldol reaction (see Figure 8). Concerning lignin degradation, it is first converted to a primary char (char 1) via the rupture of ether bonds (mainly α-O-4 and β-O-4) and leading to the release of the lignin-1 class species. Char 1 could be further converted to another more cross-linked char (char 2), by the cleavage of C−C linkages within and between the alkyl chains, leading to the formation of “lignin-2”
4. SPI−MS AS A PIVOTAL TOOL TO UNRAVEL THE MECHANISMS OF BIOMASS PYROLYSIS VUV lamp SPI−MS has been also combined with a U-shape fixed bed reactor for slow pyrolysis with a good control of mass and heat transfers.44 The reactor made of quartz is heated by an electrical furnace from room temperature to targeted final pyrolysis temperatures. Figure 7 gives the evolution profiles of F
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PCA results indicate that different temperatures (T1 and T2) and shapes (cylinder and powder) could be obviously separated (see Figure 9) and three pyrolysis regimes (A, B, and C) could
Figure 9. PCA of SPI−MS spectra of all investigated products from Douglas fir pyrolysis (from ref 47). Abbreviations: (1) D, Douglas fir; (2) C1, cylinder, 6 × 12 mm; C2, cylinder, 6 × 20 mm; L, lamella, 0.5 × 12 mm, and P, powder; (3) T1, 400 °C; and T2, 500 °C; and (4) B, 670 mL/min; M, 300 mL/min; and S, 200 mL/min.
Figure 8. Simplified chemical mechanism for lignin and cellulose pyrolysis in biomass with the catalytic effect of inorganic material for cellulose pyrolysis (adapted from ref 44).
be defined as follows: (A) m/z 128 is located on the upper part of biplots (positive loading values for PC2), indicating that this carbohydrate volatile is much easier to be formed in cylinder samples at a lower reactor temperature (400 °C). m/z 128 exhibits a fragile chemical structure and is very sensitive to the temperature. The importance of m/z 128 for pyrolysis mechanisms highlights the interest of our online soft ionization MS analysis. Heat transfer into the cylinders also decreases the overall temperature of pyrolysis and promotes the formation of species with a lower activation energy in their kinetic rate of formation. (B) m/z 144, 126, and 114 are located on the left side in biplots (negative loading values of PC1). These markers are also more favored by a low temperature (400 °C) but with a high flow rate (low gas-phase residence time, without secondary conversion) and powder samples. Therefore, the formation of these species is promoted by a faster heating rate that favors a higher pyrolysis temperature range. These labile species may also be converted in the hot nascent char in bigger particles (lamella and cylinders). (C) The third “pyrolysis regime” corresponds to the high-temperature pyrolysis (500 °C) of the powder particle. This is the fast heating rate and hightemperature reactor regime. This regime is even enhanced if the flow rate of fluidization gas is decreased (more secondary gasphase reactions). This regime is correlated mainly with m/z 110 (carbohydrate markers) and m/z 124 (lignin marker). These three regimes are affected by the catalytic effect of minerals (mainly K and Ca). Indeed, only two pyrolysis regimes could be defined for oak and Miscanthus pyrolysis as a result of their higher content of minerals.47
class species. Char 2 can also be further converted to “lignin-3” class markers and a more cross-linked solid structure. Secondary reactions could also occur depending upon the temperature and residence time of the gas phase in the reactor. Concerning specifically Miscanthus pyrolysis (see Figure 7), carbohydrate markers are 3-hydroxy-2-penteno-1,5-lactone (m/ z 114) (mainly from thermal degradation of hemicelluloses and also at a minor yield from cellulose37). Furfuryl alcohol and 2cyclopentene-1-one, 2-hydroxy (m/z 98), are the main markers from cellulose pyrolysis (see Figure 7). m/z 126 may be rather assigned to levoglucosenone (detected by GC−MS) because 5hydroxymethyl-2-furaldehyde was not detected by GC−MS analysis for Miscanthus pyrolysis. Concerning lignin markers, 4vinylphenol (m/z 120) with an unsaturated propyl chain is a characteristic marker from grass-derived lignin and could be formed by the rupture of acetate functions. Guaiacol (m/z 124, a compound without an unsaturated propyl chain) comes from the direct cleavage of C−C linkage within and between the alkyl chains. Phenol is formed by demethoxylation reactions until a temperature higher than 500 °C.51 We have to point out that m/z 162 in Miscanthus pyrolysis may correspond to 2′methoxycinnamaldehyde19 rather than levoglucosan, because levoglucosan formation is inhibited by the high ash content (notably potassium) in Miscanthus.44 The pyrolysis vapors formed by a fast pyrolysis in the microfluidized bed for various biomasses (Miscanthus, oak, and Douglas fir) were also characterized by means of lamp-based VUV SPI−MS. Meanwhile, the effect of the particle geometry and operating conditions on the product distribution was studied by principle component analysis (PCA), and some related pyrolysis regimes could be defined.47 In this paper, we only present briefly the results obtained for Douglas fir pyrolysis. Concerning Douglas fir pyrolysis, particles with three shapes (cylinder, powder, and lamella) were pyrolyzed at two temperatures (400 and 500 °C) and three fluidizing gas flow rates (670, 300, and 200 mL/min).
5. SUMMARY AND PERSPECTIVES In this paper, the fundamentals and the main features of different “soft” PI techniques are briefly introduced. The effects of photon energy and ionization techniques on product distribution for biomass pyrolysis are discussed on the basis of the mass spectra obtained by SVUV PI−MS (in Hefei, China), lamp/laser SPI−MS, and laser REMPI−MS (in Nancy, G
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France). Finally, some applications of PI−MS in the biomass pyrolysis study are also displayed. Our results can be summarized as follows: (1) The best photon energy for the ionization of biomass pyrolysis vapors is in the range of 9.5− 10.5 eV. This range of energy is a good trade-off between comprehensive detection and reduced ionization fragmentation of biomass vapors. (2) Mass spectra of biomass vapors obtained by Ar EBEL VUV SPI−MS and laser SPI−MS do no exhibit a major difference, even though a slightly higher fragmentation for laser SPI−MS can be noticed. However, comparing to lampbased SPI−MS, laser SPI−MS exhibits a better sensitivity since laser produces VUV light with higher photon flux. Therefore, the selection of VUV commercial sources should rather take into consideration their cost, lifetime, and detection limit. The combination of SPI− and REMPI−MS should be the best option for analysis of biomass vapors in different reactors and operating conditions, ranging from fast pyrolysis to combustion. (3) PI−MS is a powerful method to unravel the mechanisms of biomass pyrolysis because a soft and online analysis method is required to analyze some fragile but important markers (such as m/z 128). Nevertheless, thus far, online PI−MS has to be combined with GC−MS analysis for a better interpretation of the chemical structure of m/z peaks. PI−MS offers an important potential to monitor thermochemical reactors, for instance, during their commissioning or during the change in the composition of the input (change in biomass or waste fuels).19 It may also be very useful to study the fundamentals of biomass pyrolysis, for instance, if it is combined with thermogravimetry or laser pyrolysis/desorption.32 Despite all of these features and advantages, there is still the need of further works to improve PI−MS analysis of biomass pyrolysis vapors: (1) The quantification of the compounds by online PI−MS is still challenging. A practical online method for absolute quantification in PI−MS is needed by adding an internal standard in the pyrolysis gas outlet flow and by an adequate calibration. (2) The lack of IE data for biomass products impedes the identification of some isomers by SPI− MS (even by synchrotron MS). Isomers are often identified by scanning photon energies with synchrotron PI−MS, for instance, in combustion science.25,52 Comprehensive IE data can be obtained by scanning photon energies on biomass vapors. (3) PI sources, which would be simpler to operate and cheaper, still need to be developed to promote a wider application of PI−MS in the energy sector.
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REFERENCES
(1) Bahng, M.-K.; Mukarakate, C.; Robichaud, D. J.; Nimlos, M. R. Current technologies for analysis of biomass thermochemical processing: A review. Anal. Chim. Acta 2009, 651 (2), 117−138. (2) Olcese, R. N.; Lardier, G.; Bettahar, M.; Ghanbaja, J.; Fontana, S.; Carre, V.; Aubriet, F.; Petitjean, D.; Dufour, A. Aromatic chemicals by iron-catalyzed hydrotreatment of lignin pyrolysis vapor. ChemSusChem 2013, 6 (8), 1490−1499. (3) Kanaujia, P. K.; Sharma, Y. K.; Agrawal, U. C.; Garg, M. O. Analytical approaches to characterizing pyrolysis oil from biomass. TrAC, Trends Anal. Chem. 2013, 42, 125−136. (4) Reale, S.; Di Tullio, A.; Spreti, N.; De Angelis, F. Mass spectrometry in the biosynthetic and structural investigation of lignins. Mass Spectrom. Rev. 2004, 23 (2), 87−126. (5) Shin, E. J.; Nimlos, M. R.; Evans, R. J. Kinetic analysis of the gasphase pyrolysis of carbohydrates. Fuel 2001, 80 (12), 1697−1709. (6) Shin, E. J.; Nimlos, M. R.; Evans, R. J. A study of the mechanisms of vanillin pyrolysis by mass spectrometry and multivariate analysis. Fuel 2001, 80 (12), 1689−1696. (7) Herring, A. M.; McKinnon, J. T.; Gneshin, K. W.; Pavelka, R.; Petrick, D. E.; McCloskey, B. D.; Filley, J. Detection of reactive intermediates from and characterization of biomass char by laser pyrolysis molecular beam mass spectroscopy. Fuel 2004, 83 (11−12), 1483−1494. (8) Agblevor, F. A.; Evans, R. J.; Johnson, K. D. Molecular-beam mass-spectrometric analysis of lignocellulosic materials: I. Herbaceous biomass. J. Anal. Appl. Pyrolysis 1994, 30 (2), 125−144. (9) Akalin, M. K.; Karagoz, S. Analytical pyrolysis of biomass using gas chromatography coupled to mass spectrometry. TrAC, Trends Anal. Chem. 2014, 61, 11−16. (10) Pouwels, A. D.; Tom, A.; Eijkel, G. B.; Boon, J. J. Characterisation of beech wood and its holocellulose and xylan fractions by pyrolysis-gas chromatography-mass spectrometry. J. Anal. Appl. Pyrolysis 1987, 11, 417−436. (11) Evans, R. J.; Milne, T. A.; Soltys, M. N. Direct massspectrometric studies of the pyrolysis of carbonaceous fuels. 3. Primary pyrolysis of lignin. J. Anal. Appl. Pyrolysis 1986, 9 (3), 207−236. (12) Atkinson, D. J.; Lehrle, R. S. Trimer formation studies in polystyrene pyrolysis reveal that time-resolved mass-spectrometry can be misleading. J. Anal. Appl. Pyrolysis 1991, 19, 319−331. (13) Genuit, W.; Boon, J. J.; Faix, O. Characterization of beech milled wood lignin by pyrolysis-gas chromatography photoionization massspectrometry. Anal. Chem. 1987, 59 (3), 508−513. (14) Evans, R. J.; Milne, T. A.; Soltys, M. N.; Schulten, H. R. Massspectrometric behavior of levoglucosan under different ionization conditions and implications for studies of cellulose pyrolysis. J. Anal. Appl. Pyrolysis 1984, 6 (3), 273−283. (15) Vanderhage, E. R. E.; Mulder, M. M.; Boon, J. J. Structural characterization of lignin polymers by temperature-resolved in-source pyrolysis mass-spectrometry and curie-point pyrolysis-gas chromatography mass-spectrometry. J. Anal. Appl. Pyrolysis 1993, 25, 149−183. (16) Jarvis, J. M.; McKenna, A. M.; Hilten, R. N.; Das, K. C.; Rodgers, R. P.; Marshall, A. G. Characterization of pine pellet and peanut hull pyrolysis bio-oils by negative-ion electrospray ionization fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2012, 26 (6), 3810−3815. (17) Liu, Y.; Shi, Q.; Zhang, Y. H.; He, Y. L.; Chung, K. H.; Zhao, S. Q.; Xu, C. M. Characterization of red pine pyrolysis bio-oil by gas chromatography-mass spectrometry and negative-ion electrospray ionization fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2012, 26 (7), 4532−4539. (18) Lunsford, K. A.; Peter, G. F.; Yost, R. A. Direct matrix-assisted laser desorption/ionization mass spectrometric imaging of cellulose and hemicellulose in populus tissue. Anal. Chem. 2011, 83 (17), 6722− 6730. (19) Fendt, A.; Streibel, T.; Sklorz, M.; Richter, D.; Dahmen, N.; Zimmermann, R. On-line process analysis of biomass flash pyrolysis gases enabled by soft photoionization mass spectrometry. Energy Fuels 2012, 26 (1), 701−711.
AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support from the National Agency for Research (ANR, French agency) through the Project “PYRAIM” is gratefully acknowledged. Funding from the French State and Lorraine Region through the Project CPER MEPP is also kindly acknowledged for a part of the purchase of the PI− TOFMS system at CNRS, Nancy. The authors also thank Michel Mercy (CNRS, Nancy) for his technical support and kind help with some experimental measurements. H
DOI: 10.1021/acs.energyfuels.5b02274 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
synchrotron photoionisation-mass spectrometry. RSC Adv. 2013, 3 (14), 4786−4792. (38) Weng, J.; Jia, L.; Sun, S.; Wang, Y.; Tang, X.; Zhou, Z.; Qi, F. On-line product analysis of pine wood pyrolysis using synchrotron vacuum ultraviolet photoionization mass spectrometry. Anal. Bioanal. Chem. 2013, 405 (22), 7097−7105. (39) Finch, J. W.; Toerne, K. A.; Schram, K. H.; Denton, M. B. Evaluation of a hydrogen laser vacuum ultraviolet source for photoionization mass spectrometry of pharmaceuticals. Rapid Commun. Mass Spectrom. 2005, 19 (1), 15−22. (40) Ghosh, D.; Golan, A.; Takahashi, L. K.; Krylov, A. I.; Ahmed, M. A VUV photoionization and ab initio determination of the ionization energy of a gas-phase sugar (deoxyribose). J. Phys. Chem. Lett. 2012, 3 (1), 97−101. (41) Pan, Y.; Zhang, L.; Guo, H.; Deng, L.; Qi, F. Photoionisation and photodissociation studies of nonvolatile organic molecules by synchrotron VUV photoionisation mass spectrometry and theoretical calculations. Int. Rev. Phys. Chem. 2010, 29 (2), 369−401. (42) Pan, Y.; Hu, Y.; Wang, J.; Ye, L.; Liu, C.; Zhu, Z. Online Characterization of Isomeric/Isobaric Components in the Gas Phase of Mainstream Cigarette Smoke by Tunable Synchrotron Radiation Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometry and Photoionization Efficiency Curve Simulation. Anal. Chem. 2013, 85 (24), 11993−12001. (43) Kleeblatt, J.; Ehlert, S.; Holzer, J.; Sklorz, M.; Rittgen, J.; Baumgartel, P.; Schubert, J. K.; Zimmermann, R. Investigation of the photoionization properties of pharmaceutically relevant substances by resonance-enhanced multiphoton ionization spectroscopy and singlephoton ionization spectroscopy using synchrotron radiation. Appl. Spectrosc. 2013, 67 (8), 860−872. (44) Le Brech, Y.; Jia, L.; Cisse, S.; Mauviel, G.; Brosse, N.; Dufour, A. Mechanisms of biomass pyrolysis studied by combining a fixed bed reactor with advanced gas analysis. J. Anal. Appl. Pyrolysis 2015, DOI: 10.1016/j.jaap.2015.10.013. (45) Pouwels, A. D.; Boon, J. J. Analysis of beech wood samples, its milled wood lignin and polysaccharide fractions by curie-point and platinum filament pyrolysis-mass spectrometry. J. Anal. Appl. Pyrolysis 1990, 17 (2), 97−126. (46) Pouwels, A. D.; Boon, J. J. Analysis of lignin and chlorolignin residues in a beech xylan fraction by pyrolysis-gas chromatographymass spectrometry. J. Wood Chem. Technol. 1987, 7 (2), 197−213. (47) Jia, L. Y.; Le-Brech, Y.; Shrestha, B.; Frowein, M. B.; Ehlert, S.; Mauviel, G.; Zimmermann, R.; Dufour, A. Fast pyrolysis in a microfluidized bed reactor: effect of biomass properties and operating conditions on volatiles composition as analyzed by online single photoionization mass spectrometry. Energy Fuels 2015, 29 (11), 7364−7374. (48) Liu, X. H.; Xu, G. W.; Gao, S. Q. Micro fluidized beds: Wall effect and operability. Chem. Eng. J. 2008, 137 (2), 302−307. (49) Muhlberger, F.; Wieser, J.; Morozov, A.; Ulrich, A.; Zimmermann, R. Single-photon ionization quadrupole mass spectrometry with an electron beam plumped excimer light source. Anal. Chem. 2005, 77 (7), 2218−2226. (50) Fabbri, D.; Adamiano, A.; Torri, C. GC−MS determination of polycyclic aromatic hydrocarbons evolved from pyrolysis of biomass. Anal. Bioanal. Chem. 2010, 397 (1), 309−317. (51) Collard, F.-X.; Blin, J. A review on pyrolysis of biomass constituents: Mechanisms and composition of the products obtained from the conversion of cellulose, hemicelluloses and lignin. Renewable Sustainable Energy Rev. 2014, 38, 594−608. (52) Battin-Leclerc, F.; Herbinet, O.; Glaude, P. A.; Fournet, R.; Zhou, Z. Y.; Deng, L. L.; Guo, H. J.; Xie, M. F.; Qi, F. Experimental Confirmation of the Low-Temperature Oxidation Scheme of Alkanes. Angew. Chem., Int. Ed. 2010, 49 (18), 3169−3172.
(20) Weng, J.; Jia, L.; Wang, Y.; Sun, S.; Tang, X.; Zhou, Z.; KohseHoeinghaus, K.; Qi, F. Pyrolysis study of poplar biomass by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry. Proc. Combust. Inst. 2013, 34, 2347−2354. (21) Hanley, L.; Zimmermann, R. Light and molecular ions: The emergence of vacuum UV single-photon ionization in ms. Anal. Chem. 2009, 81 (11), 4174−4182. (22) Zhou, Z.; Guo, H.; Qi, F. Recent developments in synchrotron vacuum ultraviolet photoionization coupled to mass spectrometry. TrAC, Trends Anal. Chem. 2011, 30 (9), 1400−1409. (23) Zimmermann, R. Photo ionisation in mass spectrometry: light, selectivity and molecular ions. Anal. Bioanal. Chem. 2013, 405 (22), 6901−6905. (24) Cool, T. A.; McIlroy, A.; Qi, F.; Westmoreland, P. R.; Poisson, L.; Peterka, D. S.; Ahmed, M. Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source. Rev. Sci. Instrum. 2005, 76 (9), 094102. (25) Qi, F. Combustion chemistry probed by synchrotron VUV photoionization mass spectrometry. Proc. Combust. Inst. 2013, 34, 33− 63. (26) Mitschke, S.; Adam, T.; Streibel, T.; Baker, R. R.; Zimmermann, R. Application of time-of-flight mass spectrometry with laser-based photoionization methods for time-resolved on-line analysis of mainstream cigarette smoke. Anal. Chem. 2005, 77 (8), 2288−2296. (27) Butcher, D. J. Vacuum ultraviolet radiation for singlephotoionization mass spectrometry: A review. Microchem. J. 1999, 62 (3), 354−362. (28) Ledingham, K. W. D.; Singhal, R. P. High intensity laser mass spectrometry a review. Int. J. Mass Spectrom. Ion Processes 1997, 163 (3), 149−168. (29) Grotemeyer, J.; Boesl, U.; Walter, K.; Schlag, E. W. A general soft ionization method for mass-spectrometry - resonance-enhanced multiphoton ionization of biomolecules. Org. Mass Spectrom. 1986, 21 (10), 645−653. (30) Muhlberger, F.; Hafner, K.; Kaesdorf, S.; Ferge, T.; Zimmermann, R. Comprehensive on-line characterization of complex gas mixtures by quasi-simultaneous resonance-enhanced multiphoton ionization, vacuum-UV single-photon ionization, and electron impact ionization in a time-of-flight mass spectrometer: Setup and instrument characterization. Anal. Chem. 2004, 76 (22), 6753−6764. (31) Vanderhage, E. R. E.; Boon, J. J.; Steenvoorden, R.; Weeding, T. L. Resonance-enhanced multiphoton ionization mass-spectrometric analysis of lignin using laser pyrolysis with entrainment into a supersonic jet. Anal. Chem. 1994, 66 (4), 543−550. (32) Mukarakate, C.; Scheer, A. M.; Robichaud, D. J.; Jarvis, M. W.; David, D. E.; Ellison, G. B.; Nimlos, M. R.; Davis, M. F. Laser ablation with resonance-enhanced multiphoton ionization time-of-flight mass spectrometry for determining aromatic lignin volatilization products from biomass. Rev. Sci. Instrum. 2011, 82 (3), 033104. (33) Fendt, A.; Geissler, R.; Streibel, T.; Sklorz, M.; Zimmermann, R. Hyphenation of two simultaneously employed soft photo ionization mass spectrometers with thermal analysis of biomass and biochar. Thermochim. Acta 2013, 551, 155−163. (34) Kuribayashi, S.; Yamakoshi, H.; Danno, M.; Sakai, S.; Tsuruga, S.; Futami, H.; Morii, S. VUV single-photon ionization ion trap timeof-flight mass spectrometer for on-line, real-time monitoring of chlorinated organic compounds in waste incineration flue gas. Anal. Chem. 2005, 77 (4), 1007−1012. (35) Streibel, T.; Fendt, A.; Geissler, R.; Kaisersberger, E.; Denner, T.; Zimmermann, R. Thermal analysis/mass spectrometry using soft photo-ionisation for the investigation of biomass and mineral oils. J. Therm. Anal. Calorim. 2009, 97 (2), 615−619. (36) Brown, A. L.; Dayton, D. C.; Nimlos, M. R.; Daily, J. W. Characterization of biomass pyrolysis vapors with molecular beam, single photon ionization time-of-flight mass spectrometry. Chemosphere 2001, 42 (5−7), 663−669. (37) Dufour, A.; Weng, J. J.; Jia, L. Y.; Tang, X. F.; Sirjean, B.; Fournet, R.; Le Gall, H.; Brosse, N.; Billaud, F.; Mauviel, G.; Qi, F. Revealing the chemistry of biomass pyrolysis by means of tunable I
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