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Acetic acid conversion reactions on basic and acidic catalysts under biomass fast pyrolysis conditions
Molecular Catalysis 465 (2019) 33–42
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Acetic acid conversion reactions on basic and acidic catalysts under biomass fast pyrolysis conditions
T
A.C. Psarrasa, C.M. Michailofa, E.F. Iliopouloua, K.G. Kalogiannisa, A.A. Lappasa, ⁎ ⁎ E. Heracleousa,b, , K.S. Triantafyllidisa,c, a
Laboratory of Environmental Fuels and Hydrocarbons (LEFH), Chemical Process and Energy Resources Institute (CPERI) – Centre for Research and Technology Hellas (CERTH), 6th km Charilaou-Thermi Road, P.O. Box 60361, 57001 Thessaloniki, Greece b School of Science & Technology, International Hellenic University (IHU), 14th km Thessaloniki – Moudania, 57001 Thessaloniki, Greece c Department of Chemistry, Aristotle University of Thessaloniki, University Campus, P.O. Box 116, 54124 Thessaloniki, Greece
ARTICLE INFO
ABSTRACT
Keywords: Biomass fast Biomass Bio-oil Acetic acid ZSM-5 MgO Reaction pathways
The objective of this study was to investigate the reaction pathways of acetic acid, the most abundant organic acid in bio-oils, over acidic and basic catalysts under typical biomass fast pyrolysis conditions. The pyrolysis/ cracking experiments were performed in a fluidized bed reactor at 500 °C over a ZSM-5 zeolite formulation, as the acidic catalyst, and MgO as the basic catalyst. Both exhibit high activity (≥ 70 wt.% conversion) but distinctly different product selectivity. The primary reaction scheme on both acid and basic sites involves the ketonization of acetic acid to acetone, water and CO2, via different elementary routes. However, the consecutive reactions of acetone differ significantly depending on the nature of the active sites, leading to different product selectivity over the two types of catalysts. Aromatics, phenols, light olefins and CO were the main by-products over ZSM-5, while MgO favored the formation of higher ketones. TPD studies on acetic acid and acetone-saturated catalysts indicated that acetone undergoes self-condensation and oligomerization reactions at temperatures > 300 °C on MgO, yielding mainly CO2 and coke. On ZSM-5, both the direct dehydration of acetic acid and the secondary cracking of acetone condensation products play an equally important role, leading to the production of light olefins and small aromatic hydrocarbons. Phenols were also produced over ZSM-5 via acetone condensation and cyclization routes.
1. Introduction
cases completely replaced by the in situ upgrading of biomass pyrolysis vapours, i.e. as soon as they are formed via thermal pyrolysis, by catalytic fast pyrolysis (in situ CFP). In in situ CFP a heterogeneous catalyst is used instead of an inert heat carrier (i.e. silica sand) in mixtures with the biomass particles within the pyrolysis reactor. Alternatively, a separate catalytic reactor can be coupled with the fast pyrolysis reactor (ex situ CFP) for the catalytic upgrading of the produced biomass pyrolysis vapours [2,3]. CFP is a relatively simple and efficient process that avoids the use of hydrogen and aims to promote depolymerization of the initially formed oligomers and to tune product selectivity depending on the nature of the catalyst used. Over the last two decades, numerous types of catalysts have been investigated as candidates for the CFP process, such as zeolites, mesoporous aluminosilicate materials with uniform pore size distribution (MCM-41, MSU, SBA-15, etc.), hierarchical zeolites with combined
Despite its great potential for the large-scale production of biofuels and chemicals in an efficient and cost-effective manner, fast pyrolysis of lignocellulosic biomass suffers from the low quality of the produced oil, the so-called bio-oil or pyrolysis oil. Bio-oil typically contains significant quantities of water and hundreds of oxygen-containing compounds which render it unstable, corrosive and immiscible with petroleum-based fuels [1]. Hence, further upgrading is required, targeting to elimination of undesirable compounds, such as organic acids, or to even deeper deoxygenation towards aromatics (BTX) or hydrocarbon transportation fuels. Several downs-stream catalytic reactions/processes have been investigated as means of bio-oil upgrading, including pyrolysis/cracking, esterification, ketonization, hydrotreating/hydrodeoxygenation, etc [2]. These processes can be combined or in some
Corresponding authors at: Chemical Process and Energy Resources Institute (CPERI) – Centre for Research and Technology Hellas (CERTH), 6th km CharilaouThermi Road, P.O. Box 60361, 57001 Thessaloniki, Greece (E. Heracleous) and Department of Chemistry, Aristotle University of Thessaloniki, University Campus, P.O. Box 116, 54124 Thessaloniki, Greece (K.S. Triantafyllidis). E-mail addresses: [email protected] (E. Heracleous), [email protected] (K.S. Triantafyllidis). ⁎
https://doi.org/10.1016/j.mcat.2018.12.012 Received 3 November 2018; Received in revised form 13 December 2018; Accepted 17 December 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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micro- and mesoporosity, metal doped zeolites as well as various oxides with acid-base properties [1,4,5]. Focus has been largely placed on catalysts with acidic properties that favor dehydration, decarbonylation, cracking and aromatization reactions towards BTX, with zeolite ZSM-5 being the most widely studied material [1,4,6]. Acid catalysts, however, typically suffer from fast deactivation due to acid site poisoning and extensive coking that leads to pore blockage and loss of reactivity and selectivity to the desired products [7]. The application of basic materials in CFP has been much scarcer. Studies from our group have highlighted the promising potential of materials with basic properties. In a screening study of various acidic and basic catalysts for the in-situ catalytic upgrading of biomass pyrolysis vapors in a fixed bed reactor, it was shown that basic catalytic materials lead to high CO2 yields and produce a bio-oil with decreased acid concentration and high content of ketones via ketonization and aldol condensation reactions [8]. In a subsequent study it was shown that MgO catalysts, derived from natural magnesite mineral, exhibit, despite the lack of acidity, comparable activity in the catalytic pyrolysis of biomass to that of a diluted, industrial ZSM-5 zeolite catalyst, with enhanced deoxygenation due to acids ketonization and CeC coupling reactions [9]. Pütün [10] employed MgO as catalyst for the fast pyrolysis of cotton seeds and reported the production of bio-oil with much lower oxygen content than the thermal bio-oil. Similar results were also published by Lin et al. using CaO as catalyst for the pyrolysis of white pine biomass [11]. MgO has also been considered as a suitable dopant for zeolite-based biomass pyrolysis catalysts. The incorporation of 10 wt.% MgO in lamellar and pillared ZSM-5 zeolites allowed tailoring the zeolite activity to avoid the excessive cracking of bio-oil in the in-situ catalytic upgrading of eucalyptus woodchips fast pyrolysis vapors. This in turn resulted in higher yield of organics and decreased the formation of undesired polyaromatic hydrocarbons and coke [12]. Analogous findings were reported also for MgO-doped hierarchical ZSM-5 and Beta zeolites. The enhanced performance of the MgO-loaded catalysts was attributed to the adequate balance of Lewis acid and basic sites [13]. Despite the intense research efforts, biomass pyrolysis and bio-oil upgrading still remain challenging, owing mainly to the complex nature of bio-oil. Depending on the feedstock, pyrolysis conditions and catalyst, biomass pyrolysis oil consists of oxygenates corresponding to different families: alcohols, phenols, aldehydes, ketones, acids, and esters [14]. The determination of optimum conditions for the catalytic transformation of bio-oil requires detailed knowledge of the reactivity and the mechanistic pathways followed during the upgrading reactions. According to a recent review on the mechanism of thermal and catalytic lignocellulosic biomass pyrolysis [15], further research is needed to explore the fundamental reaction mechanisms of the reactants on the catalyst’s active sites. To this end, model compounds, being present in biomass fast pyrolysis oils, can be very useful in understanding and identifying the reaction steps involved in the upgrading of bio-oils. In this context, the present study aims to elucidate reaction pathways of acetone conversion, a representative model compound of the carboxylic acids contained in bio-oil, over acidic and basic catalysts under typical biomass fast pyrolysis conditions. Tests were conducted over typical catalysts with well-defined acidic and basic properties, namely ZSM-5 and MgO, in a small-scale fluidized bed reactor unit at a wide range of residence times. Temperature-programmed desorption (TPD) studies on acetic acid and acetone-saturated samples were also undertaken in an effort to elucidate the stepwise formation and transformation of primary and secondary products.
impurities) produced via beneficiation and rotary kiln calcination (800–1200 °C) of natural magnesite minerals, kindly provided by Grecian Magnesite S.A. [9]. Prior to characterization and testing the catalysts were calcined at 500 °C for 2 h in air. Powder X-ray diffraction (XRD) was applied to verify the crystal structure of ZSM-5 zeolite and MgO using a Siemens Diffractometer D5000 equipped with Cu Kα X-ray radiation and a curved crystal graphite monochromator operating at 45 kV and 100 mA; counts were accumulated in the range of 5-75° 2θ every 0.02° (2θ) with counting time 2 s per step. The crystal size (L) was also determined by measuring the width at half maximum, β1/2, of the MgO’s main peak at 2θ = 42.9° as an input to the Scherrer equation (L [nm] = K λ/β1/2 cosθ). N2 adsorption-desorption experiments at −196 °C were performed on an Automatic Volumetric Sorption Analyzer (Autosorb-1, Quantachrome) for the determination of surface area (BET method), total pore volume at P/Po = 0.99, micropore volume (t-plot method), and pore size distribution (BJH method) of the samples that were previously outgassed at 150 °C for 16 h under 5 × 10−9 Torr vacuum. Fourier-Transform Infrared (FT-IR) spectroscopy experiments, combined with in situ adsorption of pyridine were performed on a Nicolet 5700 FTIR spectrometer (resolution 4 cm−1) using the OMNIC software for the determination of the Brønsted (band at 1545 cm−1 attributed to pyridinium ions) and Lewis (band at 1450 cm−1 attributed to pyridine coordinated to Lewis acid sites) type acid sites of the catalysts. Data processing was carried out via the GRAMS software and the quantitative determination of acid sites was performed by adopting the molar extinction coefficients proposed by Emeis [18]. The samples were initially outgassed at 450 °C under high vacuum (10−6 mbar) for 1 h, followed by adsorption of pyridine (added in pulses) for 1 h at 1 mbar equilibrium pressure. All spectra were collected at 150 °C in order to eliminate the possibility of pyridine condensation. Basicity was determined by CO2-temperature programmed desorption (TPD-CO2). In a typical experiment, 0.2 g of the sample were loaded in a fixed bed quartz reactor and pretreated at 600 °C in He for 1 h, followed by cooling to 80 °C under He flow and subsequent treatment with a flow of 40% CO2/He for 1 h at 80 °C. Flushing with pure He at 80 °C for 3 h was then applied to remove the physisorbed CO2. TPD analysis was carried out from 80 to 600 °C at a heating rate of 10 °C/ min and a He flow rate of 50 cm3/min. The composition of the exit gas was monitored online by a quadrupole mass analyzer (Omnistar, Balzer). Quantitative analysis of the desorbed CO2 was based on the fragment m/z = 44. 2.2. Acetic acid conversion experiments The pyrolysis experiments were carried out in a fully automated bench-scale microactivity (MAT) test unit, utilizing a fluidized bed reactor configuration. Details on the design of the unit can be found in previous communications [19]. All experiments were conducted at constant temperature of 500 °C. Variation of the conversion level was attained by modifying the catalyst-to-reactant (acetic acid) mass ratios, corresponding to a WHSV range of 16 to 48 h−1. In all tests, the liquid and gaseous products were separated, collected and analyzed. Gas products were analyzed on a gas chromatograph (HP-6890) equipped with two thermal conductivity detectors (TCD). The liquid products were analyzed on a HP5890II GC with a polar DB-WAX column and equipped with a flame ionization detector (FID). The qualitative and quantitative determination of the compounds was performed by means of calibration lines using external standard solutions of acetone, cyclopentanone, hydroxyacetone, acetic acid and phenol (in order of elution) at five concentration levels. The liquid samples were also analyzed by GC–MS (5975C Mass spectrometer with 7890 Gas Chromatograph using a HP-5MS 5% Phenyl Methyl Silox column) in an attempt to identify the compounds related to unassigned peaks observed in the GC-FID chromatograms. To this end, the aqueous samples were dispersed in CH2Cl2 at proper dilution ratio. In order to avoid the
2. Experimental 2.1. Catalysts and catalyst characterization The catalysts used were a commercial equilibrium ZSM-5 zeolite formulation diluted with silica-alumina (containing 30 wt. % crystalline zeolite) [16,17] and a MgO material (with < 2% SiO2 and CaO 34
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formation of emulsions and the introduction of water in the apolar GC–MS column, Na2SO4 was added to the mixture. The samples were filtered through a 0.2 μm nylon filter prior to injection in the GC–MS. The water content of the liquid products was quantified via volumetric Karl Fischer titration (ASTM E203). Finally, the amount of coke deposited on the catalyst surface was measured on a LECO CS400 elemental analyzer.
that may act as Lewis acid sites [16,17,22]. With regard to the MgO catalyst, it shows an appreciable surface area, being attributed solely to textural/intercrystal meso/macroporosity [9]. As expected, it exhibits only basic surface properties and no acidity. 3.2. Acetic acid conversion under biomass fast pyrolysis conditions The performance of the two investigated catalysts in the fast pyrolysis of a common woody lignocellulosic biomass, i.e. beech wood sawdust, was typical of an acidic ZSM-5 zeolite and a basic MgO [9,16,17]. Representative data on product yields and selectivity to selected types of compounds is shown in Table S1 (Supporting information). It can be clearly seen that the acidic ZSM-5 catalyst induces the deoxygenation of the organics in bio-oil via dehydration, decarbonylation and decarboxylation reactions, while it favors the production of aromatics. On the other hand, the basic MgO causes also substantial deoxygenation of the bio-oil, but with negligible formation of aromatics and increased formation of ketones. The CO2/CO ratio is also significantly higher with MgO compared to the performance of the acidic zeolite. Furthermore, the acids in the organic bio-oil, which comprise mainly of acetic acid derived from the acetyl groups in hemicellulose as well as from the decomposition of sugars [23], decrease on both catalysts (Table S1). It has been previously suggested, without direct experimental proof, that carboxylic acids may contribute to the formation of aromatics on ZSM-5 and the formation of ketones on MgO and other basic catalysts [24]. These differences in product yields and selectivities imply the different reaction pathways that are being catalyzed by the acidic ZSM-5 and basic MgO during the in situ upgrading of biomass pyrolysis vapors. Thus, in this work, we investigated the conversion of acetic acid on ZSM-5 and MgO under representative biomass fast pyrolysis conditions in a fluidized bed reaction unit at 500 °C in a WHSV range of 16–48 h−1. Fig. 1 shows the conversion of acetic acid as a function of space velocity over the two materials. High conversion levels ranging between 70–90 wt.% were recorded over both catalysts, with ZSM-5 appearing slightly more active than MgO. However, the turnover frequencies (TOFs) normalized per active site (considering all acid and basic sites for ZSM-5 and MgO respectively) and determined at isoconversion show an order of magnitude higher reactivity for ZSM-5 (see inset of Fig. 1). Although the accessibility to the active sites might differ over the two materials, the superior reactivity of the acidic sites is indisputable and it is clear that the various reaction steps proceed much faster than on basic sites. This has been further verified by comparing the performance of the commercial ZSM-5 formulation with that of a pure ZSM-5 zeolite with higher total acidity as well as higher ratio of Brønsted to Lewis acid sites, with the pure zeolite being significantly more reactive compared to the commercial equilibrium ZSM-5 formulation of the present study (Fig. S-1, Supporting information).
2.3. Temperature-programmed desorption (TPD) studies Temperature-programmed desorption (TPD) studies of acetic acid and acetone-saturated catalysts were performed on a home-made TPD unit comprising of a quartz fixed bed reactor, heating and gas flow controllers and on-line quadrupole mass analyzer (Omnistar, Balzers). The fresh ZSM-5 and MgO samples (100 mg) were first saturated with either acetic acid or acetone (2 cm3) and then dried overnight in an oven at 40 °C. The saturated catalyst samples were placed in the fixed bed reactor and were heated to 800 °C with a heating rate of 5 °C/min under a flow of He (40 cm3/min). Mass spectra were recorded using a SEM amplifier operating at 1400 V and an ionization potential of −50 eV. The signals of the following mass-to-charge (m/z) were recorded: 60 (CH3COOH), 58 (CH3OCH3), 44 (CO2), 18 (H2O), 56 (iC4H8), 41 (C3H6) and 26 (C2H4). The intensity of the signal for each reactant/product was normalized with respect to helium and corrected for contributions from other components, based on the cracking patterns of each molecule. 3. Results and discussion 3.1. Catalyst properties The two catalysts, i.e. commercial acidic ZSM-5 zeolite formulation and basic MgO, have been previously investigated by our group in biomass fast pyrolysis [9,16,17]. Detailed physicochemical characterization has been reported in these studies, while a summary of the most important properties is also shown in Table 1. The commercial ZSM-5 catalyst exhibits relatively low surface area and micropore volume, compared to a pure crystalline ZSM-5 zeolite, as it is diluted with amorphous silica-alumina matrix with enhanced meso/macroporosity (average pore size of 4 nm). The number of Brønsted acid sites and the Brønsted to Lewis acid sites ratio are also significantly lower compared to those of a pure H-ZSM-5 zeolite due to the dilution and the nature of silica-alumina which possesses mainly Lewis acid sites [20,21]. In addition, being in equilibrium state, i.e. having been subjected to multiple cycles of biomass pyrolysis - regeneration in a recirculating fluid bed reactor, the high temperature steaming conditions during regeneration induce dealumination of the zeolitic framework, thus lowering the Brønsted acid sites and generating extra-framework aluminum species Table 1 Physicochemical characteristics of the ZSM-5 and MgO catalysts. Catalysts
ZSM-5a MgO a b c d e f
Porosity characteristics (N2 porosimetry)
Acidity (FTIR - pyridine)
Basicity (TPD-CO2)
BET areab (m2/g)
Micropore volumec (cm3/g)
Meso/macropore volumed (cm3/g)
Meso-pore size (nm)e
Brønsted sites (μmol/g)
Lewis sites (μmol/g)
(μmol/g)
138 64
0.037 –
0.071 0.360
4.0f 28.9
36.5 –
18.1 –
∼0 244
Commercial equilibrium ZSM-5 zeolite formulation in silica-alumina matrix (contains ∼30 wt. % crystalline zeolite). Multi-point BET method. t-plot method. Difference of total pore volume (at P/Po = 0.99) minus the micropore volume. BJH analysis using adsorption data. Due to the mesoporosity of the silica–alumina matrix, in addition to the typical microporous structure of ZSM-5 zeolite with ∼0.55 nm channels. 35
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Fig. 1. Acetic acid conversion as a function of WHSV at 500 °C (inset: TOF at 74 wt.% iso-conversion).
Fig. 2. Product selectivities at 74 wt.% acetic acid iso-conversion.
Selectivity to the different liquid, gaseous and solid reaction products at constant conversion is presented in Fig. 2. The main liquid products over both catalysts (MgO and ZSM-5) are acetone and water, with the main gaseous product being CO2. Thus, the reaction pathway that prevails on both acid and basic materials is the ketonization of acetic acid with production of acetone, water and CO2 as primary products. When the equilibrium ZSM-5 formulation of the present work is compared to the pure ZSM-5 zeolite (Fig. S-1, Supporting information), it can be seen that the selectivity (or yield) to the various products has similar trend for the two catalysts but is different (case of CO2) or changes more steeply (case of acetone and water) with the increase of conversion, thus indicating differences either in the intrinsic reactivity of their active (acid) sites and/or diffusional and confinement effects of microporosity (pure ZSM-5) versus the combined micro- and meso/macroporosity (due to the silica – alumina matrix of the ZSM-5 formulation). Stoichiometrically, the ketonization reaction leads to an iso-molecular mixture of the three components, corresponding to a weight based selectivity of 48.3 wt.% to acetone, 15 wt.% to H2O and 36.7.% to CO2. However, the results shown in Fig. 2 deviate from this stoichiometry; the acetone selectivity is much lower while the concentration of H2O is notably higher, evidencing the occurrence of extensive secondary reactions of acetone. The distribution of the rest of the products, i.e.
except acetone, water and CO2, is considerably different over the two catalysts. In the presence of ZSM-5, C2-C4 light olefins and CO are formed as by-products in the gas phase. In addition, GC–MS analysis of the organic liquid phase revealed, besides acetone, a considerable amount of phenolic and aromatic compounds, such as methyl- and dimethyl- phenols, xylenes, alkyl-benzenes etc. (Fig. 3 and Table S2 in Supporting information). On the contrary, over basic MgO the nongaseous major by-products consist of higher ketones (pentenones, hexenones etc.), coke and a very small amount of phenols and aromatics (almost exclusively tri-methyl benzenes). It should be mentioned that coke production is slightly higher over MgO than ZSM-5 (Fig. 2). These indications clearly point out that the reaction pathways over acidic catalysts are different in comparison to those on basic catalysts, being numerous and more complex on the former. The yield of the major products as a function of acetic acid conversion for the two catalysts is shown in Fig. 4. Such graphs/correlations are typical in the field of catalytic pyrolysis/cracking of biomass or petroleum fractions, depicting the effect of all reaction parameters on product yields and conversion. In the present work, conversion was varied by progressive increase of WHSV (via increase of catalyst to feed ratio). Acetone production monotonously decreases with increasing conversion, confirming that it undergoes successive reactions leading ultimately to the production of coke, H2O and CO2 over both basic and 36
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Fig. 3. Concentration (GC–MS peak area, %) of major by-products present in the organic phase of the liquid products, at 74 wt.% acetic acid iso-conversion.
Fig. 4. Yields of acetone and H2O (top left), coke (top right), CO2 (bottom left) and other gases (bottom right) as a function of acetic acid conversion at 500 °C.
acid sites, in addition to CO and light olefins that are only formed on the acid sites of ZSM-5. Comparatively, MgO appears more selective to acetone than ZSM-5; at the same time, the decreasing rate of acetone yield is also slightly lower than that with ZSM-5. This confirms the lower reactivity of MgO towards further acetone conversion. Despite the more extensive decomposition reactions on ZSM-5 (as evidenced by the formation of light olefins whose concentration increases with conversion), the coke yield is greater on MgO.
3.3. Temperature-programmed desorption (TPD) studies The ZSM-5 and MgO catalyst samples were saturated with acetic acid or acetone and were then heated in a temperature programmed manner under helium flow in order to obtain information on the primary and secondary transformations that occur on the catalyst surface. Fig. 5 shows the TPD profile (MS spectra) obtained for the acetic-acid saturated MgO sample. At temperature < 150 °C, water and small 37
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Fig. 5. Temperature-programmed desorption profile of acetic acid-saturated MgO.
amounts of acetic acid were detected that were likely due to the desorption of catalyst’s moisture and evaporation of physisorbed acetic acid, respectively. The main transformation reactions occur in the temperature range of 275–400 °C and lead almost exclusively to the production of acetone, CO2 and water. No acetic acid desorption was observed at the temperature range of ca. 150 to 400 °C, pointing out to strong interaction of the carboxylic acid with the MgO surface that inhibits its evaporation and allows its transformation on the catalyst surface sites. The evolved gaseous product exhibited a doublet with maxima at ˜305 °C and 330 °C. Although all products were detected at both temperatures, the evolution of acetone was more pronounced at 305 °C, whereas formation of CO2 prevailed at 330 °C indicating the simultaneous occurrence of secondary acetone conversion reactions that potentially have higher activation energy than the ketonization reaction. This observation is supported by the TPD profile obtained on the acetone-saturated MgO, shown in Fig. 6, which is dominated by large production of H2O and CO2 at ˜ 335 °C. MgO has been reported to exhibit promising performance in the gas phase aldol condensation of
acetone for the synthesis of mesityl oxide and isophorone [25–27]. Selfaldol condensation of acetone initially forms diacetone alcohol which subsequently dehydrates to form mesityl oxide and water. At > 200 °C, aldol condensation of acetone was reported to show more complex reaction pathways to yield acyclic, cyclic and aromatic trimers [28]. Therefore, the large production of water observed at 335 °C in the TPD of the acetone-saturated sample could be associated with self-condensation and oligomerization reactions of acetone. This temperature coincides with that of the high-temperature peak in the TPD of the acetic acid-saturated MgO and can thus be attributed to secondary conversion of acetone that primarily forms on MgO from the carboxylic acid. The corresponding TPD profiles for the acetic acid and acetone-saturated ZSM-5 catalyst are presented in Figs. 7 and 8 respectively and exhibit considerable differences in comparison with MgO. At low temperature (< 200 °C), the profile is dominated by large desorption of water, together with some CO2 and unreacted physisorbed acetic acid, at ˜130 °C. This initial dehydration step could be related with the
Fig. 6. Temperature-programmed desorption profile of acetone-saturated MgO.
38
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formation of acyl intermediates via the interaction of acetic acid with the strong Brønsted acid sites of the zeolite, as previously suggested by Gumidyala et al. [29] on the basis of similar studies on HZSM-5. Following this initial dehydration step, the reaction zone is broader on the acidic catalyst compared to MgO and extends from ˜250 to 500 °C. There is a single desorption peak of acetone at 310 °C, accompanied by evolution of CO2 and H2O. Ketonization probably occurs via the interaction of the formed acyl species with either acetates or carboxylates, as discussed in detail in the following section. The products profile is however dominated by the formation of light olefins, namely isobutene, propene and ethene, in accordance with the results of the reactivity tests presented above. No formation of aromatics and phenols is observed, as the mass-to-charge (m/z) ratios monitored during the TPD studies was < 100. It is also unlikely to observe oligomerization/cyclization products in such studies due to the low residence time at each temperature and the low partial pressures of the reactants (acetic acid/ acetone). The production of the light olefins exhibits a doublet, with temperature maxima at 310 °C (together with the formed acetone) and 350 °C. Temperature-programmed desorption studies of acetone preadsorbed on HZSM-5 showed that acetone is converted at low temperatures by zeolitic acid sites to mesityl oxide that undergoes further condensation and cracking at higher temperature, releasing mainly isobutene in the gas phase [30]. Significant sequential condensation of acetone was also observed at temperature > 300 °C during acetic acid ketonization tests on ZSM-5 [29]. Considering that similar evolution of olefins was also observed at similar temperature range in the TPD profile of the acetone-saturated ZSM-5 (Fig. 8), it can thus be deduced that acetone is formed via ketonization of acetic acid on the zeolite acid sites and then undergoes acid-catalyzed condensation to higher molecular weight products that finally crack to yield olefins and light gases.
pathway. Several reaction routes have been proposed in literature. One possible pathway is via the adsorption of the acid as CH3COO− and H+ on an electron–rich surface and the subsequent formation of the ketone via interaction of two such surface acetate ions (Reaction 3) [34,35]. It is however anticipated that the strongly acidic protons of bridging hydroxyls in the zeolitic framework promote the formation of surface acyl species during adsorption of acetic acid to form CH3CO+ and OH− [31]. Acetone would then be formed via interaction with an acetate ion as shown in Reaction 4. Alternatively, the recombination of a surface acyl with a carboxylate could form β-ketoacid that decomposes to yield the ketone through a similar route to the mechanism of ketonization on metal oxide surfaces [31,36]. Reaction 3: Ketonization via surface acetate ions 2Zeol-OH + 2 CH3COOH → 2 Zeol+ — CH3COO− +2 H2O 2Zeol+ – CH3COO− → CH3COCH3 + CO2 + ZeolO− + Zeol+ ZeolO− + Zeol+ + H2O → 2Zeol-OH Reaction 4: Ketonization via acylium ion formation Zeol-OH+ + CH3COOH → Zeol-O− — CH3CO+ + H2O Brønsted sites Acylium ion Zeol-OH + CH3COOH → Zeol+ — CH3COO− + H2O Acetate ion Zeol-O− — CH3CO+ + Zeol+ — CH3COO− → CH3COCH3 + CO2 Zeol-O− + Zeol+ + H2O → Zeol-OH+ + Zeol-OH
Zeol -OH+ + CH3 COOH
Zeol -O --- CH3 CO+ + H2 O Brønsted sites Acylium ion Zeol -OH + CH3 COOH Zeol+ --- CH3 COO + H2 O Acetate ion Zeol -O --- CH3 CO+ + Zeol+ --- CH3 COO CH3COCH3 + CO2 Zeol -O + Zeol+ + H2 O Zeol -OH+ + Zeol -OH
4. Mechanistic pathways of acetic acid conversion on acidic and basic sites Despite the obvious differences between the active sites of the two investigated catalysts, being acidic in nature on ZSM-5 and basic on MgO, the primary reaction in the conversion of acetic acid is the ketonization to the corresponding ketone, acetone, accompanied by the formation of CO2 and H2O, as evidenced by the results presented in the previous sections. The reaction follows however a different underlying mechanistic pathway over the two materials. It has been well established that oxides with low lattice energy (or very high basicity) such as alkali and alkali earth metal oxides, including MgO, CaO, BaO, SrO and CdO, interact very strongly with acetic acid. This interaction results in the formation of bulk carboxylate salts that decompose upon thermal treatment to acetone, water and CO2 according to Reaction (1) [31]. As demonstrated by the fluidized bed experiments and the TPD study of this work, the initially formed acetone undergoes secondary reactions over MgO, with the main final products being higher ketones, water and coke. Based on the GC–MS analysis, these ketones are mainly pentenones and hexenones (Table S2) that probably form from acetone oligomerization via aldol self-condensation reactions. The ketones may evolve to coke through cyclization and condensation reactions (Reaction 2) [32,33]. Reaction 1: Bulk Ketonization
The performance of ZSM-5 and MgO was largely differentiated concerning the distribution of the secondary products, as shown in the present study. The formation of CO, C2-C4 olefins, phenols and aromatics that was mainly observed with ZSM-5 indicates the occurrence of more complex reaction scheme of acetone over the acidic active sites. Deoxygenation could be progressing over the ZSM-5 through the production of i-butene from acetone, by dimerization with water abstraction (aldol condensation) and successive cracking of the mesityl oxide which is an intermediate product (Reaction 5) [30,36,37]. The i-butene molecule may further oligomerize to higher olefins, which ultimately form aromatic molecules, such as alkyl benzenes and xylene (Reaction 6), possibly via a Diels Alder mechanism or other cyclization and dehydrogenation reactions [14,38,39]. The cracking of higher olefins towards ethylene and propylene which may also be converted to aromatics on ZSM-5 is also a possible pathway. The detection of propylene and other C4 hydrocarbons in the reactions of acetic acid and acetone over ZSM-5 in the present study may also be attributed to the formation of alkyl aromatics and their consecutive dealkylation [14]. It should be noted that, although the reaction of acetone to i-butene and successively to aromatics is negligible over MgO (based on the observed products), the initial aldol condensation to mesityl oxide and its limited thermal cracking cannot be completely excluded. Reaction 5: i-Butene production
2CH3COOH + MgO → Mg(CH3COO)2 + H2O Mg(CH3COO)2 → MgO + CH3COCH3 + CO2 Reaction 2: Aldol Condensation and Polymerization to higher ketones and coke
CH3COCH3 + CH3COCH3 → (CH3)2C = CH(CO)CH3 + H2O
CH3COCH3 → pentenone, hexenone → coke
Aldol condensation
Ketonization has been much less studied on strong acidic materials, such as the ZSM-5 zeolite. As opposed to MgO and other low lattice energy oxides, ketonization follows in this case a surface catalyzed
2(CH3)2C = CH(CO)CH3 2ZeolO− — CH3CO+ 39
+
2Zeol-OH+
→
2(CH3)2C = CH2
+
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Fig. 7. Temperature-programmed desorption profile of acetic acid-saturatedZSM-5.
Fig. 8. Temperature-programmed desorption profile of acetone-saturated ZSM-5.
2ZeolO− — CH3CO+ + H2O → 2ZeolOH+ + CH3COCH3 + CO2
The phenolic molecules, detected in the product distribution of the ZSM-5 catalyzed reaction, are expected products of acetone polymerization [40,41]. As reported above (see reaction 5), acetone is converted at low temperatures by zeolitic acid sites to mesityl oxide [30], which can undergo cyclization reactions to isophorone [42] as shown in Reaction Scheme 7. Alternatively, the reaction of mesityl oxide with acetone can yield phorone that has been reported to convert
2(CH3)2C = CH(CO)CH3 → 2(CH3)2C = CH2 + CH3COCH3 + CO2 Mesityl oxide cracking Reaction Scheme 6: i-Butene reaction network [adapted from 14]
40
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to isophorone over zeolite catalysts [43]. Isophorone is a key intermediate for the production of 3,5-xylenol via catalytic rearrangement [44]. The dimethyl-phenol can further react to methyl-phenols with loss of methane, in agreement with the formation of methane that was detected in the reaction products of the acetone reaction over the ZSM-5 catalyst in this study.
5. Conclusions Acetic (or other) acids in bio-oil can be effectively removed during in situ or ex situ upgrading via pyrolysis with either acidic (ZSM-5) or basic (MgO) catalysts via different reaction pathways. The primary route involves in all cases the deoxygenation of acetic acid via ketonization towards acetone, CO2 and water. The successive reactions are
Reaction Scheme 7: Phenols production [adapted from 42] The CO detected in the ZSM-5 catalyzed reaction is unlikely to originate from the decarbonylation of acetic acid, as this route has been reported to be insignificant compared to decarboxylation and dehydration under similar conditions and catalysts as this work [14]. However, decarbonylation was found to have an important contribution in the transformation of acetone over HZSM-5 and therefore CO could be a product of the secondary decarbonylation of the acetone produced in the process [14]. There is also a possibility for direct dehydration of acetic acid to ketene and successive decomposition of ketene to methylene radicals and CO [45]. The ethylene observed in the gaseous products over ZSM-5 supports the direct ketene decomposition route, as ethylene is produced by the combination of two methylene radicals (Reaction 8) [37]. Ethylene can then be readily converted to benzene or alkylated benzene via oligomerization, cyclization and dehydrogenation on the Brønsted acid sites of ZSM-5. The polymerization of ketene to coke species could also contribute to the CO yield [33]. Reaction 8: Dehydration to Ketene, decomposition to ethylene and CO, and oligomerization-cyclization-dehydrogenation of ethylene to aromatics
however different, altering significantly the final product distribution on the two catalysts. Higher ketones are mainly produced on MgO, while aromatics, phenolic compounds and light olefins are observed in the products of ZSM-5 catalyzed reaction. Temperature-programmed desorption studies on acetic acid and acetone-saturated catalyst amples show that at high temperature, acetone undergoes secondary self-condensation and oligomerization reactions yielding mainly CO2 and coke on MgO. On ZSM-5, both the direct dehydration of acetic acid and the cracking of the acetone condensation products play an important role, leading to the production of light olefins which are then converted to phenols and aromatics. The obtained catalytic results and the reaction mechanisms proposed in the present work can be utilized for the design of improved catalysts in bio-oil upgrading via catalytic pyrolysis/ cracking, selecting between the production of a bio-oil rich in aromatics and phenolics or a bio-oil that contains mainly higher ketones and related compounds that could be hydrodeoxygenated towards gasoline or diesel range alkanes. Acknowledgments This research was co-financed by European Union (European Social Fund) and Greek national funds through Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF)-Research Funding Program: THALES-Investing in knowledge society through the European Social Fund (MIS 380405).
CH3COOH → CH2CO + H2O CH2CO → :CH2 + CO 2[:CH2] → C2H4 3 C2H4 → C6H6 + 3 H2
Appendix A. Supplementary data
Finally, the presence of methane in the product slate of ZSM-5 could be the result of the decomposition of surface acetates to methane [38], according to reaction 9, supporting the surface acetate formation route. Reaction 9: Acetate Decomposition
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2018.12.012. References
CH3COO− + zeol-OH→ CH4 + CO2 + zeol-O− The above described reaction pathways, proposed on the basis of the obtained experimental data, the specific characteristics of the acidic ZSM-5 zeolite and the basic MgO, and the available literature regarding related mechanistic sutdies of various individual reactions, can be utilized in order to elucidate the product yields and distribution in the upgrading (in situ or downstream) of bio-oil under typical catalytic pyrolysis conditions. More specifically, the aim and future work is focused on the development of catalysts that can effectively eliminate acetic acid (and other acids) present in the bio-oil and at the same time tune the selectivity towards the desired product in each case.
[1] A.A. Lappas, K.G. Kalogiannis, E.F. Iliopoulou, K.S. Triantafyllidis, S.D. Stefanidis, WIREs: Energy Environ. 1 (2012) 285–297. [2] D.A. Bulushev, J.R. Ross, Catal. Today 171 (2011) 1–13. [3] E.F. Iliopoulou, K.S. Triantafyllidis, A.A. Lappas, WIREs Energy Environ. (2018), https://doi.org/10.1002/wene.322; S. Wan, Y. Wang, Front. Chem. Sci. Eng. 8 (2014) 280–294. [4] E.F. Iliopoulou, P.A. Lazaridis, K.S. Triantafyllidis, Nanocatalysis in the fast pyrolysis of lignocellulosic biomass, chapter 27, pp. 655–714, in: B. Sels, M.l Van de Voorde (Eds.), Nanotechnology in Catalysis: Applications in the Chemical Industry, Energy Development, and Environment Protection, Wiley, 2017. [5] C. Liu, H. Wang, A.M. Karim, J. Suna, Y. Wang, Chem. Soc. Rev. 43 (2014) 7594–7623. [6] A. Zheng, L. Jiang, Z. Zhao, Z. Huang, K. Zhao, G. Wei, H. Li, WIREs: Energy and
41
Molecular Catalysis 465 (2019) 33–42
A.C. Psarras et al. Environ. (2016), https://doi.org/10.1002/wene.234. [7] M. Guisnet, P. Magnoux, D. Martin, Stud. Surf. Sci. Catal. 111 (1997) 1–19. [8] S.D. Stefanidis, K.G. Kalogiannis, E.F. Iliopoulou, A.A. Lappas, P.A. Pilavachi, Bioresour. Technol. Rep. 102 (2011) 8261–8267. [9] S.D. Stefanidis, S.A. Karakoulia, K.G. Kalogiannis, E.F. Iliopoulou, A. Delimitis, H. Yiannoulakis, T. Zampetakis, A.A. Lappas, K.S. Triantafyllidis, Appl. Catal. B 196 (2016) 155–173. [10] E. Putun, Energy 35 (2010) 2761–2766. [11] Y. Lin, C. Zhang, M. Zhang, J. Zhang, Energy Fuels 24 (2010) 5686–5695. [12] J. Fermoso, H. Hernando, P. Jana, I. Moreno, J. Prech, C. Ochoa-Hernandez, P. Pizarro, J.M. Coronado, J. Cejka, D.P. Serrano, Catal. Today 277 (1) (2016) 171–181. [13] H. Hernando, I. Moreno, J. Fermoso, C. Ochoa-Hernández, P. Pizarro, J.M. Coronado, J. Čejka, D.P. Serrano, Biomass Conver. Bioref. 7 (3) (2017) 289–304. [14] A.G. Gayubo, A.T. Aguayo, A. Atutxa, R. Aguado, M. Olazar, J. Bilbao, Ind. Eng. Chem. Res. 43 (2004) 2619–2626. [15] W. Shurong, D. Gongxin, Y. Haiping, L. Zhongyang, Prog. Energy Combust. Sci. 62 (2017) 33–86. [16] E.F. Iliopoulou, S.D. Stefanidis, K.G. Kalogiannis, A. Delimitis, A.A. Lappas, K.S. Triantafyllidis, Appl. Catal. B 127 (2012) 281–290. [17] E.F. Iliopoulou, S.D. Stefanidis, K.G. Kalogiannis, A.C. Psarras, A. Delimitis, K.S. Triantafyllidis, A.A. Lappas, Green Chem. 16 (2014) 662–674. [18] C.A. Emeis, J. Catal. 141 (1993) 347–354. [19] A.C. Psarras, E.F. Iliopoulou, L. Nalbandian, A.A. Lappas, C. Pouwels, Catal. Today 127 (2007) 44–53. [20] V.G. Komvokis, S. Karakoulia, E.F. Iliopoulou, M.C. Papapetrou, I.A. Vasalos, A.A. Lappas, K.S. Triantafyllidis, Catal. Today 196 (2012) 42–55. [21] P. Lanzafame, S. Perathoner, G. Centi, E. Heracleous, E.F. Iliopoulou, K.S. Triantafyllidis, A.A. Lappas, ChemCatChem 9 (2017) 1632–1640. [22] A.A. Gusev, A.C. Psarras, K.S. Triantafyllidis, A.A. Lappas, P.A. Diddams, Molecules 22 (2017) 1784–1804. [23] A. Demirbas, Energ. Convers. Manage. 41 (2000) 633–646.
[24] J.D. Adjaye, R.K. Sharma, N.N. Bakhshi, J.S. Kevin, C.S. Emerson (Eds.), Stud. Surf. Sci. Catal. vol. 73, Elsevier, 1992, pp. 301–308. [25] C.X. Ma, G. Liu, Z.L. Wang, Y.F. Li, J. Zheng, W.X. Zhang, M.J. Jia, React. Kinet. Catal. Lett. 98 (2009) 149–156. [26] Z. Liu, F. Peng, X. Liu, Adv. Mater. Res. 550–553 (2012) 424–428. [27] M. Leon, L. Faba, E. Diaz, S. Bennici, A. Vega, S. Ordonez, A. Auroux, Appl. Catal. B 147 (2014) 796–804. [28] L. Wu, T. Moteki, A.A. Gokhale, D.W. Flaherty, D. Toste, Chem 1 (2016) 32–58. [29] A. Gumidyala, T. Sooknoi, S. Crossley, J. Catal. 340 (2016) 76–84. [30] L. Kubelkova, J. Novakova, Zeolites 11 (1991) 822–826. [31] T.N. Pham, T. Sooknoi, S.P. Crossley, D.E. Resasco, ACS Catal. 3 (11) (2013) 2456–2473. [32] R. Martinez, M.C. Huff, M.A. Barteau, J. Catal. 222 (2004) 404–409. [33] M. Watanabe, H. Inomata, R. Lee Smith Jr., K. Arai, Appl. Catal. A Gen. 219 (2001) 149–156. [34] E. Kukulska-Zając, K. Góra-Marek, J. Datka, Microporous Mesoporous Mater. 96 (2006) 216–221. [35] A.G. Gayubo, A.T. Aguayo, A. Atutxa, B. Valle, J. Bilbao, J. Chem. Technol. Biotechnol. 80 (2005) 1244–1251. [36] A. Pulido, B. Oliver-Thomas, M. Renz, M. Boronat, A. Corma, ChemSusChem 6 (1) (2013) 141–151. [37] C.D. Chang, A.J. Silvestri, J. Catal. 47 (1977) 249–259. [38] M.A. Hasan, M.I. Zaki, L. Pasupulety, Appl. Catal. A Gen. 243 (2003) 81–92. [39] A. Corma, G.W. Huber, Angew. Chem. Int. Ed. 46 (2007) 7184–7201. [40] S. Lippert, W. Baumann, K. Thomke, J. Mol. Catal. 69 (1991) 199–214. [41] K.V. Ramanamurty, G.S. Salvapati, J. Sci. Ind. Res. 59 (2000) 339–349. [42] K.V. Ramanamurty, G.S. Salvapati, Indian J. Chem. 38B (1999) 24–28. [43] O.I. Kusnetsov, G.M. Panchenkov, A.M.D. Guseim, T.A. Chasova, Nefte- pererab Neftekhzm 3 (1973) 28. [44] K.V. Ramanamurty, G.S. Salvapati, M. Janardanarao, J. Mol. Catal. 54 (1989) 9–30. [45] X. Li, S. Wang, Y. Zhu, G. Yang, P. Zheng, Int. J. Hydrogen Energy 40 (2015) 330–339.
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Acetic acid conversion reactions on basic and acidic catalysts under biomass fast pyrolysis conditions
T
A.C. Psarrasa, C.M. Michailofa, E.F. Iliopouloua, K.G. Kalogiannisa, A.A. Lappasa, ⁎ ⁎ E. Heracleousa,b, , K.S. Triantafyllidisa,c, a
Laboratory of Environmental Fuels and Hydrocarbons (LEFH), Chemical Process and Energy Resources Institute (CPERI) – Centre for Research and Technology Hellas (CERTH), 6th km Charilaou-Thermi Road, P.O. Box 60361, 57001 Thessaloniki, Greece b School of Science & Technology, International Hellenic University (IHU), 14th km Thessaloniki – Moudania, 57001 Thessaloniki, Greece c Department of Chemistry, Aristotle University of Thessaloniki, University Campus, P.O. Box 116, 54124 Thessaloniki, Greece
ARTICLE INFO
ABSTRACT
Keywords: Biomass fast Biomass Bio-oil Acetic acid ZSM-5 MgO Reaction pathways
The objective of this study was to investigate the reaction pathways of acetic acid, the most abundant organic acid in bio-oils, over acidic and basic catalysts under typical biomass fast pyrolysis conditions. The pyrolysis/ cracking experiments were performed in a fluidized bed reactor at 500 °C over a ZSM-5 zeolite formulation, as the acidic catalyst, and MgO as the basic catalyst. Both exhibit high activity (≥ 70 wt.% conversion) but distinctly different product selectivity. The primary reaction scheme on both acid and basic sites involves the ketonization of acetic acid to acetone, water and CO2, via different elementary routes. However, the consecutive reactions of acetone differ significantly depending on the nature of the active sites, leading to different product selectivity over the two types of catalysts. Aromatics, phenols, light olefins and CO were the main by-products over ZSM-5, while MgO favored the formation of higher ketones. TPD studies on acetic acid and acetone-saturated catalysts indicated that acetone undergoes self-condensation and oligomerization reactions at temperatures > 300 °C on MgO, yielding mainly CO2 and coke. On ZSM-5, both the direct dehydration of acetic acid and the secondary cracking of acetone condensation products play an equally important role, leading to the production of light olefins and small aromatic hydrocarbons. Phenols were also produced over ZSM-5 via acetone condensation and cyclization routes.
1. Introduction
cases completely replaced by the in situ upgrading of biomass pyrolysis vapours, i.e. as soon as they are formed via thermal pyrolysis, by catalytic fast pyrolysis (in situ CFP). In in situ CFP a heterogeneous catalyst is used instead of an inert heat carrier (i.e. silica sand) in mixtures with the biomass particles within the pyrolysis reactor. Alternatively, a separate catalytic reactor can be coupled with the fast pyrolysis reactor (ex situ CFP) for the catalytic upgrading of the produced biomass pyrolysis vapours [2,3]. CFP is a relatively simple and efficient process that avoids the use of hydrogen and aims to promote depolymerization of the initially formed oligomers and to tune product selectivity depending on the nature of the catalyst used. Over the last two decades, numerous types of catalysts have been investigated as candidates for the CFP process, such as zeolites, mesoporous aluminosilicate materials with uniform pore size distribution (MCM-41, MSU, SBA-15, etc.), hierarchical zeolites with combined
Despite its great potential for the large-scale production of biofuels and chemicals in an efficient and cost-effective manner, fast pyrolysis of lignocellulosic biomass suffers from the low quality of the produced oil, the so-called bio-oil or pyrolysis oil. Bio-oil typically contains significant quantities of water and hundreds of oxygen-containing compounds which render it unstable, corrosive and immiscible with petroleum-based fuels [1]. Hence, further upgrading is required, targeting to elimination of undesirable compounds, such as organic acids, or to even deeper deoxygenation towards aromatics (BTX) or hydrocarbon transportation fuels. Several downs-stream catalytic reactions/processes have been investigated as means of bio-oil upgrading, including pyrolysis/cracking, esterification, ketonization, hydrotreating/hydrodeoxygenation, etc [2]. These processes can be combined or in some
Corresponding authors at: Chemical Process and Energy Resources Institute (CPERI) – Centre for Research and Technology Hellas (CERTH), 6th km CharilaouThermi Road, P.O. Box 60361, 57001 Thessaloniki, Greece (E. Heracleous) and Department of Chemistry, Aristotle University of Thessaloniki, University Campus, P.O. Box 116, 54124 Thessaloniki, Greece (K.S. Triantafyllidis). E-mail addresses: [email protected] (E. Heracleous), [email protected] (K.S. Triantafyllidis). ⁎
https://doi.org/10.1016/j.mcat.2018.12.012 Received 3 November 2018; Received in revised form 13 December 2018; Accepted 17 December 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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micro- and mesoporosity, metal doped zeolites as well as various oxides with acid-base properties [1,4,5]. Focus has been largely placed on catalysts with acidic properties that favor dehydration, decarbonylation, cracking and aromatization reactions towards BTX, with zeolite ZSM-5 being the most widely studied material [1,4,6]. Acid catalysts, however, typically suffer from fast deactivation due to acid site poisoning and extensive coking that leads to pore blockage and loss of reactivity and selectivity to the desired products [7]. The application of basic materials in CFP has been much scarcer. Studies from our group have highlighted the promising potential of materials with basic properties. In a screening study of various acidic and basic catalysts for the in-situ catalytic upgrading of biomass pyrolysis vapors in a fixed bed reactor, it was shown that basic catalytic materials lead to high CO2 yields and produce a bio-oil with decreased acid concentration and high content of ketones via ketonization and aldol condensation reactions [8]. In a subsequent study it was shown that MgO catalysts, derived from natural magnesite mineral, exhibit, despite the lack of acidity, comparable activity in the catalytic pyrolysis of biomass to that of a diluted, industrial ZSM-5 zeolite catalyst, with enhanced deoxygenation due to acids ketonization and CeC coupling reactions [9]. Pütün [10] employed MgO as catalyst for the fast pyrolysis of cotton seeds and reported the production of bio-oil with much lower oxygen content than the thermal bio-oil. Similar results were also published by Lin et al. using CaO as catalyst for the pyrolysis of white pine biomass [11]. MgO has also been considered as a suitable dopant for zeolite-based biomass pyrolysis catalysts. The incorporation of 10 wt.% MgO in lamellar and pillared ZSM-5 zeolites allowed tailoring the zeolite activity to avoid the excessive cracking of bio-oil in the in-situ catalytic upgrading of eucalyptus woodchips fast pyrolysis vapors. This in turn resulted in higher yield of organics and decreased the formation of undesired polyaromatic hydrocarbons and coke [12]. Analogous findings were reported also for MgO-doped hierarchical ZSM-5 and Beta zeolites. The enhanced performance of the MgO-loaded catalysts was attributed to the adequate balance of Lewis acid and basic sites [13]. Despite the intense research efforts, biomass pyrolysis and bio-oil upgrading still remain challenging, owing mainly to the complex nature of bio-oil. Depending on the feedstock, pyrolysis conditions and catalyst, biomass pyrolysis oil consists of oxygenates corresponding to different families: alcohols, phenols, aldehydes, ketones, acids, and esters [14]. The determination of optimum conditions for the catalytic transformation of bio-oil requires detailed knowledge of the reactivity and the mechanistic pathways followed during the upgrading reactions. According to a recent review on the mechanism of thermal and catalytic lignocellulosic biomass pyrolysis [15], further research is needed to explore the fundamental reaction mechanisms of the reactants on the catalyst’s active sites. To this end, model compounds, being present in biomass fast pyrolysis oils, can be very useful in understanding and identifying the reaction steps involved in the upgrading of bio-oils. In this context, the present study aims to elucidate reaction pathways of acetone conversion, a representative model compound of the carboxylic acids contained in bio-oil, over acidic and basic catalysts under typical biomass fast pyrolysis conditions. Tests were conducted over typical catalysts with well-defined acidic and basic properties, namely ZSM-5 and MgO, in a small-scale fluidized bed reactor unit at a wide range of residence times. Temperature-programmed desorption (TPD) studies on acetic acid and acetone-saturated samples were also undertaken in an effort to elucidate the stepwise formation and transformation of primary and secondary products.
impurities) produced via beneficiation and rotary kiln calcination (800–1200 °C) of natural magnesite minerals, kindly provided by Grecian Magnesite S.A. [9]. Prior to characterization and testing the catalysts were calcined at 500 °C for 2 h in air. Powder X-ray diffraction (XRD) was applied to verify the crystal structure of ZSM-5 zeolite and MgO using a Siemens Diffractometer D5000 equipped with Cu Kα X-ray radiation and a curved crystal graphite monochromator operating at 45 kV and 100 mA; counts were accumulated in the range of 5-75° 2θ every 0.02° (2θ) with counting time 2 s per step. The crystal size (L) was also determined by measuring the width at half maximum, β1/2, of the MgO’s main peak at 2θ = 42.9° as an input to the Scherrer equation (L [nm] = K λ/β1/2 cosθ). N2 adsorption-desorption experiments at −196 °C were performed on an Automatic Volumetric Sorption Analyzer (Autosorb-1, Quantachrome) for the determination of surface area (BET method), total pore volume at P/Po = 0.99, micropore volume (t-plot method), and pore size distribution (BJH method) of the samples that were previously outgassed at 150 °C for 16 h under 5 × 10−9 Torr vacuum. Fourier-Transform Infrared (FT-IR) spectroscopy experiments, combined with in situ adsorption of pyridine were performed on a Nicolet 5700 FTIR spectrometer (resolution 4 cm−1) using the OMNIC software for the determination of the Brønsted (band at 1545 cm−1 attributed to pyridinium ions) and Lewis (band at 1450 cm−1 attributed to pyridine coordinated to Lewis acid sites) type acid sites of the catalysts. Data processing was carried out via the GRAMS software and the quantitative determination of acid sites was performed by adopting the molar extinction coefficients proposed by Emeis [18]. The samples were initially outgassed at 450 °C under high vacuum (10−6 mbar) for 1 h, followed by adsorption of pyridine (added in pulses) for 1 h at 1 mbar equilibrium pressure. All spectra were collected at 150 °C in order to eliminate the possibility of pyridine condensation. Basicity was determined by CO2-temperature programmed desorption (TPD-CO2). In a typical experiment, 0.2 g of the sample were loaded in a fixed bed quartz reactor and pretreated at 600 °C in He for 1 h, followed by cooling to 80 °C under He flow and subsequent treatment with a flow of 40% CO2/He for 1 h at 80 °C. Flushing with pure He at 80 °C for 3 h was then applied to remove the physisorbed CO2. TPD analysis was carried out from 80 to 600 °C at a heating rate of 10 °C/ min and a He flow rate of 50 cm3/min. The composition of the exit gas was monitored online by a quadrupole mass analyzer (Omnistar, Balzer). Quantitative analysis of the desorbed CO2 was based on the fragment m/z = 44. 2.2. Acetic acid conversion experiments The pyrolysis experiments were carried out in a fully automated bench-scale microactivity (MAT) test unit, utilizing a fluidized bed reactor configuration. Details on the design of the unit can be found in previous communications [19]. All experiments were conducted at constant temperature of 500 °C. Variation of the conversion level was attained by modifying the catalyst-to-reactant (acetic acid) mass ratios, corresponding to a WHSV range of 16 to 48 h−1. In all tests, the liquid and gaseous products were separated, collected and analyzed. Gas products were analyzed on a gas chromatograph (HP-6890) equipped with two thermal conductivity detectors (TCD). The liquid products were analyzed on a HP5890II GC with a polar DB-WAX column and equipped with a flame ionization detector (FID). The qualitative and quantitative determination of the compounds was performed by means of calibration lines using external standard solutions of acetone, cyclopentanone, hydroxyacetone, acetic acid and phenol (in order of elution) at five concentration levels. The liquid samples were also analyzed by GC–MS (5975C Mass spectrometer with 7890 Gas Chromatograph using a HP-5MS 5% Phenyl Methyl Silox column) in an attempt to identify the compounds related to unassigned peaks observed in the GC-FID chromatograms. To this end, the aqueous samples were dispersed in CH2Cl2 at proper dilution ratio. In order to avoid the
2. Experimental 2.1. Catalysts and catalyst characterization The catalysts used were a commercial equilibrium ZSM-5 zeolite formulation diluted with silica-alumina (containing 30 wt. % crystalline zeolite) [16,17] and a MgO material (with < 2% SiO2 and CaO 34
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formation of emulsions and the introduction of water in the apolar GC–MS column, Na2SO4 was added to the mixture. The samples were filtered through a 0.2 μm nylon filter prior to injection in the GC–MS. The water content of the liquid products was quantified via volumetric Karl Fischer titration (ASTM E203). Finally, the amount of coke deposited on the catalyst surface was measured on a LECO CS400 elemental analyzer.
that may act as Lewis acid sites [16,17,22]. With regard to the MgO catalyst, it shows an appreciable surface area, being attributed solely to textural/intercrystal meso/macroporosity [9]. As expected, it exhibits only basic surface properties and no acidity. 3.2. Acetic acid conversion under biomass fast pyrolysis conditions The performance of the two investigated catalysts in the fast pyrolysis of a common woody lignocellulosic biomass, i.e. beech wood sawdust, was typical of an acidic ZSM-5 zeolite and a basic MgO [9,16,17]. Representative data on product yields and selectivity to selected types of compounds is shown in Table S1 (Supporting information). It can be clearly seen that the acidic ZSM-5 catalyst induces the deoxygenation of the organics in bio-oil via dehydration, decarbonylation and decarboxylation reactions, while it favors the production of aromatics. On the other hand, the basic MgO causes also substantial deoxygenation of the bio-oil, but with negligible formation of aromatics and increased formation of ketones. The CO2/CO ratio is also significantly higher with MgO compared to the performance of the acidic zeolite. Furthermore, the acids in the organic bio-oil, which comprise mainly of acetic acid derived from the acetyl groups in hemicellulose as well as from the decomposition of sugars [23], decrease on both catalysts (Table S1). It has been previously suggested, without direct experimental proof, that carboxylic acids may contribute to the formation of aromatics on ZSM-5 and the formation of ketones on MgO and other basic catalysts [24]. These differences in product yields and selectivities imply the different reaction pathways that are being catalyzed by the acidic ZSM-5 and basic MgO during the in situ upgrading of biomass pyrolysis vapors. Thus, in this work, we investigated the conversion of acetic acid on ZSM-5 and MgO under representative biomass fast pyrolysis conditions in a fluidized bed reaction unit at 500 °C in a WHSV range of 16–48 h−1. Fig. 1 shows the conversion of acetic acid as a function of space velocity over the two materials. High conversion levels ranging between 70–90 wt.% were recorded over both catalysts, with ZSM-5 appearing slightly more active than MgO. However, the turnover frequencies (TOFs) normalized per active site (considering all acid and basic sites for ZSM-5 and MgO respectively) and determined at isoconversion show an order of magnitude higher reactivity for ZSM-5 (see inset of Fig. 1). Although the accessibility to the active sites might differ over the two materials, the superior reactivity of the acidic sites is indisputable and it is clear that the various reaction steps proceed much faster than on basic sites. This has been further verified by comparing the performance of the commercial ZSM-5 formulation with that of a pure ZSM-5 zeolite with higher total acidity as well as higher ratio of Brønsted to Lewis acid sites, with the pure zeolite being significantly more reactive compared to the commercial equilibrium ZSM-5 formulation of the present study (Fig. S-1, Supporting information).
2.3. Temperature-programmed desorption (TPD) studies Temperature-programmed desorption (TPD) studies of acetic acid and acetone-saturated catalysts were performed on a home-made TPD unit comprising of a quartz fixed bed reactor, heating and gas flow controllers and on-line quadrupole mass analyzer (Omnistar, Balzers). The fresh ZSM-5 and MgO samples (100 mg) were first saturated with either acetic acid or acetone (2 cm3) and then dried overnight in an oven at 40 °C. The saturated catalyst samples were placed in the fixed bed reactor and were heated to 800 °C with a heating rate of 5 °C/min under a flow of He (40 cm3/min). Mass spectra were recorded using a SEM amplifier operating at 1400 V and an ionization potential of −50 eV. The signals of the following mass-to-charge (m/z) were recorded: 60 (CH3COOH), 58 (CH3OCH3), 44 (CO2), 18 (H2O), 56 (iC4H8), 41 (C3H6) and 26 (C2H4). The intensity of the signal for each reactant/product was normalized with respect to helium and corrected for contributions from other components, based on the cracking patterns of each molecule. 3. Results and discussion 3.1. Catalyst properties The two catalysts, i.e. commercial acidic ZSM-5 zeolite formulation and basic MgO, have been previously investigated by our group in biomass fast pyrolysis [9,16,17]. Detailed physicochemical characterization has been reported in these studies, while a summary of the most important properties is also shown in Table 1. The commercial ZSM-5 catalyst exhibits relatively low surface area and micropore volume, compared to a pure crystalline ZSM-5 zeolite, as it is diluted with amorphous silica-alumina matrix with enhanced meso/macroporosity (average pore size of 4 nm). The number of Brønsted acid sites and the Brønsted to Lewis acid sites ratio are also significantly lower compared to those of a pure H-ZSM-5 zeolite due to the dilution and the nature of silica-alumina which possesses mainly Lewis acid sites [20,21]. In addition, being in equilibrium state, i.e. having been subjected to multiple cycles of biomass pyrolysis - regeneration in a recirculating fluid bed reactor, the high temperature steaming conditions during regeneration induce dealumination of the zeolitic framework, thus lowering the Brønsted acid sites and generating extra-framework aluminum species Table 1 Physicochemical characteristics of the ZSM-5 and MgO catalysts. Catalysts
ZSM-5a MgO a b c d e f
Porosity characteristics (N2 porosimetry)
Acidity (FTIR - pyridine)
Basicity (TPD-CO2)
BET areab (m2/g)
Micropore volumec (cm3/g)
Meso/macropore volumed (cm3/g)
Meso-pore size (nm)e
Brønsted sites (μmol/g)
Lewis sites (μmol/g)
(μmol/g)
138 64
0.037 –
0.071 0.360
4.0f 28.9
36.5 –
18.1 –
∼0 244
Commercial equilibrium ZSM-5 zeolite formulation in silica-alumina matrix (contains ∼30 wt. % crystalline zeolite). Multi-point BET method. t-plot method. Difference of total pore volume (at P/Po = 0.99) minus the micropore volume. BJH analysis using adsorption data. Due to the mesoporosity of the silica–alumina matrix, in addition to the typical microporous structure of ZSM-5 zeolite with ∼0.55 nm channels. 35
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Fig. 1. Acetic acid conversion as a function of WHSV at 500 °C (inset: TOF at 74 wt.% iso-conversion).
Fig. 2. Product selectivities at 74 wt.% acetic acid iso-conversion.
Selectivity to the different liquid, gaseous and solid reaction products at constant conversion is presented in Fig. 2. The main liquid products over both catalysts (MgO and ZSM-5) are acetone and water, with the main gaseous product being CO2. Thus, the reaction pathway that prevails on both acid and basic materials is the ketonization of acetic acid with production of acetone, water and CO2 as primary products. When the equilibrium ZSM-5 formulation of the present work is compared to the pure ZSM-5 zeolite (Fig. S-1, Supporting information), it can be seen that the selectivity (or yield) to the various products has similar trend for the two catalysts but is different (case of CO2) or changes more steeply (case of acetone and water) with the increase of conversion, thus indicating differences either in the intrinsic reactivity of their active (acid) sites and/or diffusional and confinement effects of microporosity (pure ZSM-5) versus the combined micro- and meso/macroporosity (due to the silica – alumina matrix of the ZSM-5 formulation). Stoichiometrically, the ketonization reaction leads to an iso-molecular mixture of the three components, corresponding to a weight based selectivity of 48.3 wt.% to acetone, 15 wt.% to H2O and 36.7.% to CO2. However, the results shown in Fig. 2 deviate from this stoichiometry; the acetone selectivity is much lower while the concentration of H2O is notably higher, evidencing the occurrence of extensive secondary reactions of acetone. The distribution of the rest of the products, i.e.
except acetone, water and CO2, is considerably different over the two catalysts. In the presence of ZSM-5, C2-C4 light olefins and CO are formed as by-products in the gas phase. In addition, GC–MS analysis of the organic liquid phase revealed, besides acetone, a considerable amount of phenolic and aromatic compounds, such as methyl- and dimethyl- phenols, xylenes, alkyl-benzenes etc. (Fig. 3 and Table S2 in Supporting information). On the contrary, over basic MgO the nongaseous major by-products consist of higher ketones (pentenones, hexenones etc.), coke and a very small amount of phenols and aromatics (almost exclusively tri-methyl benzenes). It should be mentioned that coke production is slightly higher over MgO than ZSM-5 (Fig. 2). These indications clearly point out that the reaction pathways over acidic catalysts are different in comparison to those on basic catalysts, being numerous and more complex on the former. The yield of the major products as a function of acetic acid conversion for the two catalysts is shown in Fig. 4. Such graphs/correlations are typical in the field of catalytic pyrolysis/cracking of biomass or petroleum fractions, depicting the effect of all reaction parameters on product yields and conversion. In the present work, conversion was varied by progressive increase of WHSV (via increase of catalyst to feed ratio). Acetone production monotonously decreases with increasing conversion, confirming that it undergoes successive reactions leading ultimately to the production of coke, H2O and CO2 over both basic and 36
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Fig. 3. Concentration (GC–MS peak area, %) of major by-products present in the organic phase of the liquid products, at 74 wt.% acetic acid iso-conversion.
Fig. 4. Yields of acetone and H2O (top left), coke (top right), CO2 (bottom left) and other gases (bottom right) as a function of acetic acid conversion at 500 °C.
acid sites, in addition to CO and light olefins that are only formed on the acid sites of ZSM-5. Comparatively, MgO appears more selective to acetone than ZSM-5; at the same time, the decreasing rate of acetone yield is also slightly lower than that with ZSM-5. This confirms the lower reactivity of MgO towards further acetone conversion. Despite the more extensive decomposition reactions on ZSM-5 (as evidenced by the formation of light olefins whose concentration increases with conversion), the coke yield is greater on MgO.
3.3. Temperature-programmed desorption (TPD) studies The ZSM-5 and MgO catalyst samples were saturated with acetic acid or acetone and were then heated in a temperature programmed manner under helium flow in order to obtain information on the primary and secondary transformations that occur on the catalyst surface. Fig. 5 shows the TPD profile (MS spectra) obtained for the acetic-acid saturated MgO sample. At temperature < 150 °C, water and small 37
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Fig. 5. Temperature-programmed desorption profile of acetic acid-saturated MgO.
amounts of acetic acid were detected that were likely due to the desorption of catalyst’s moisture and evaporation of physisorbed acetic acid, respectively. The main transformation reactions occur in the temperature range of 275–400 °C and lead almost exclusively to the production of acetone, CO2 and water. No acetic acid desorption was observed at the temperature range of ca. 150 to 400 °C, pointing out to strong interaction of the carboxylic acid with the MgO surface that inhibits its evaporation and allows its transformation on the catalyst surface sites. The evolved gaseous product exhibited a doublet with maxima at ˜305 °C and 330 °C. Although all products were detected at both temperatures, the evolution of acetone was more pronounced at 305 °C, whereas formation of CO2 prevailed at 330 °C indicating the simultaneous occurrence of secondary acetone conversion reactions that potentially have higher activation energy than the ketonization reaction. This observation is supported by the TPD profile obtained on the acetone-saturated MgO, shown in Fig. 6, which is dominated by large production of H2O and CO2 at ˜ 335 °C. MgO has been reported to exhibit promising performance in the gas phase aldol condensation of
acetone for the synthesis of mesityl oxide and isophorone [25–27]. Selfaldol condensation of acetone initially forms diacetone alcohol which subsequently dehydrates to form mesityl oxide and water. At > 200 °C, aldol condensation of acetone was reported to show more complex reaction pathways to yield acyclic, cyclic and aromatic trimers [28]. Therefore, the large production of water observed at 335 °C in the TPD of the acetone-saturated sample could be associated with self-condensation and oligomerization reactions of acetone. This temperature coincides with that of the high-temperature peak in the TPD of the acetic acid-saturated MgO and can thus be attributed to secondary conversion of acetone that primarily forms on MgO from the carboxylic acid. The corresponding TPD profiles for the acetic acid and acetone-saturated ZSM-5 catalyst are presented in Figs. 7 and 8 respectively and exhibit considerable differences in comparison with MgO. At low temperature (< 200 °C), the profile is dominated by large desorption of water, together with some CO2 and unreacted physisorbed acetic acid, at ˜130 °C. This initial dehydration step could be related with the
Fig. 6. Temperature-programmed desorption profile of acetone-saturated MgO.
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formation of acyl intermediates via the interaction of acetic acid with the strong Brønsted acid sites of the zeolite, as previously suggested by Gumidyala et al. [29] on the basis of similar studies on HZSM-5. Following this initial dehydration step, the reaction zone is broader on the acidic catalyst compared to MgO and extends from ˜250 to 500 °C. There is a single desorption peak of acetone at 310 °C, accompanied by evolution of CO2 and H2O. Ketonization probably occurs via the interaction of the formed acyl species with either acetates or carboxylates, as discussed in detail in the following section. The products profile is however dominated by the formation of light olefins, namely isobutene, propene and ethene, in accordance with the results of the reactivity tests presented above. No formation of aromatics and phenols is observed, as the mass-to-charge (m/z) ratios monitored during the TPD studies was < 100. It is also unlikely to observe oligomerization/cyclization products in such studies due to the low residence time at each temperature and the low partial pressures of the reactants (acetic acid/ acetone). The production of the light olefins exhibits a doublet, with temperature maxima at 310 °C (together with the formed acetone) and 350 °C. Temperature-programmed desorption studies of acetone preadsorbed on HZSM-5 showed that acetone is converted at low temperatures by zeolitic acid sites to mesityl oxide that undergoes further condensation and cracking at higher temperature, releasing mainly isobutene in the gas phase [30]. Significant sequential condensation of acetone was also observed at temperature > 300 °C during acetic acid ketonization tests on ZSM-5 [29]. Considering that similar evolution of olefins was also observed at similar temperature range in the TPD profile of the acetone-saturated ZSM-5 (Fig. 8), it can thus be deduced that acetone is formed via ketonization of acetic acid on the zeolite acid sites and then undergoes acid-catalyzed condensation to higher molecular weight products that finally crack to yield olefins and light gases.
pathway. Several reaction routes have been proposed in literature. One possible pathway is via the adsorption of the acid as CH3COO− and H+ on an electron–rich surface and the subsequent formation of the ketone via interaction of two such surface acetate ions (Reaction 3) [34,35]. It is however anticipated that the strongly acidic protons of bridging hydroxyls in the zeolitic framework promote the formation of surface acyl species during adsorption of acetic acid to form CH3CO+ and OH− [31]. Acetone would then be formed via interaction with an acetate ion as shown in Reaction 4. Alternatively, the recombination of a surface acyl with a carboxylate could form β-ketoacid that decomposes to yield the ketone through a similar route to the mechanism of ketonization on metal oxide surfaces [31,36]. Reaction 3: Ketonization via surface acetate ions 2Zeol-OH + 2 CH3COOH → 2 Zeol+ — CH3COO− +2 H2O 2Zeol+ – CH3COO− → CH3COCH3 + CO2 + ZeolO− + Zeol+ ZeolO− + Zeol+ + H2O → 2Zeol-OH Reaction 4: Ketonization via acylium ion formation Zeol-OH+ + CH3COOH → Zeol-O− — CH3CO+ + H2O Brønsted sites Acylium ion Zeol-OH + CH3COOH → Zeol+ — CH3COO− + H2O Acetate ion Zeol-O− — CH3CO+ + Zeol+ — CH3COO− → CH3COCH3 + CO2 Zeol-O− + Zeol+ + H2O → Zeol-OH+ + Zeol-OH
Zeol -OH+ + CH3 COOH
Zeol -O --- CH3 CO+ + H2 O Brønsted sites Acylium ion Zeol -OH + CH3 COOH Zeol+ --- CH3 COO + H2 O Acetate ion Zeol -O --- CH3 CO+ + Zeol+ --- CH3 COO CH3COCH3 + CO2 Zeol -O + Zeol+ + H2 O Zeol -OH+ + Zeol -OH
4. Mechanistic pathways of acetic acid conversion on acidic and basic sites Despite the obvious differences between the active sites of the two investigated catalysts, being acidic in nature on ZSM-5 and basic on MgO, the primary reaction in the conversion of acetic acid is the ketonization to the corresponding ketone, acetone, accompanied by the formation of CO2 and H2O, as evidenced by the results presented in the previous sections. The reaction follows however a different underlying mechanistic pathway over the two materials. It has been well established that oxides with low lattice energy (or very high basicity) such as alkali and alkali earth metal oxides, including MgO, CaO, BaO, SrO and CdO, interact very strongly with acetic acid. This interaction results in the formation of bulk carboxylate salts that decompose upon thermal treatment to acetone, water and CO2 according to Reaction (1) [31]. As demonstrated by the fluidized bed experiments and the TPD study of this work, the initially formed acetone undergoes secondary reactions over MgO, with the main final products being higher ketones, water and coke. Based on the GC–MS analysis, these ketones are mainly pentenones and hexenones (Table S2) that probably form from acetone oligomerization via aldol self-condensation reactions. The ketones may evolve to coke through cyclization and condensation reactions (Reaction 2) [32,33]. Reaction 1: Bulk Ketonization
The performance of ZSM-5 and MgO was largely differentiated concerning the distribution of the secondary products, as shown in the present study. The formation of CO, C2-C4 olefins, phenols and aromatics that was mainly observed with ZSM-5 indicates the occurrence of more complex reaction scheme of acetone over the acidic active sites. Deoxygenation could be progressing over the ZSM-5 through the production of i-butene from acetone, by dimerization with water abstraction (aldol condensation) and successive cracking of the mesityl oxide which is an intermediate product (Reaction 5) [30,36,37]. The i-butene molecule may further oligomerize to higher olefins, which ultimately form aromatic molecules, such as alkyl benzenes and xylene (Reaction 6), possibly via a Diels Alder mechanism or other cyclization and dehydrogenation reactions [14,38,39]. The cracking of higher olefins towards ethylene and propylene which may also be converted to aromatics on ZSM-5 is also a possible pathway. The detection of propylene and other C4 hydrocarbons in the reactions of acetic acid and acetone over ZSM-5 in the present study may also be attributed to the formation of alkyl aromatics and their consecutive dealkylation [14]. It should be noted that, although the reaction of acetone to i-butene and successively to aromatics is negligible over MgO (based on the observed products), the initial aldol condensation to mesityl oxide and its limited thermal cracking cannot be completely excluded. Reaction 5: i-Butene production
2CH3COOH + MgO → Mg(CH3COO)2 + H2O Mg(CH3COO)2 → MgO + CH3COCH3 + CO2 Reaction 2: Aldol Condensation and Polymerization to higher ketones and coke
CH3COCH3 + CH3COCH3 → (CH3)2C = CH(CO)CH3 + H2O
CH3COCH3 → pentenone, hexenone → coke
Aldol condensation
Ketonization has been much less studied on strong acidic materials, such as the ZSM-5 zeolite. As opposed to MgO and other low lattice energy oxides, ketonization follows in this case a surface catalyzed
2(CH3)2C = CH(CO)CH3 2ZeolO− — CH3CO+ 39
+
2Zeol-OH+
→
2(CH3)2C = CH2
+
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Fig. 7. Temperature-programmed desorption profile of acetic acid-saturatedZSM-5.
Fig. 8. Temperature-programmed desorption profile of acetone-saturated ZSM-5.
2ZeolO− — CH3CO+ + H2O → 2ZeolOH+ + CH3COCH3 + CO2
The phenolic molecules, detected in the product distribution of the ZSM-5 catalyzed reaction, are expected products of acetone polymerization [40,41]. As reported above (see reaction 5), acetone is converted at low temperatures by zeolitic acid sites to mesityl oxide [30], which can undergo cyclization reactions to isophorone [42] as shown in Reaction Scheme 7. Alternatively, the reaction of mesityl oxide with acetone can yield phorone that has been reported to convert
2(CH3)2C = CH(CO)CH3 → 2(CH3)2C = CH2 + CH3COCH3 + CO2 Mesityl oxide cracking Reaction Scheme 6: i-Butene reaction network [adapted from 14]
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to isophorone over zeolite catalysts [43]. Isophorone is a key intermediate for the production of 3,5-xylenol via catalytic rearrangement [44]. The dimethyl-phenol can further react to methyl-phenols with loss of methane, in agreement with the formation of methane that was detected in the reaction products of the acetone reaction over the ZSM-5 catalyst in this study.
5. Conclusions Acetic (or other) acids in bio-oil can be effectively removed during in situ or ex situ upgrading via pyrolysis with either acidic (ZSM-5) or basic (MgO) catalysts via different reaction pathways. The primary route involves in all cases the deoxygenation of acetic acid via ketonization towards acetone, CO2 and water. The successive reactions are
Reaction Scheme 7: Phenols production [adapted from 42] The CO detected in the ZSM-5 catalyzed reaction is unlikely to originate from the decarbonylation of acetic acid, as this route has been reported to be insignificant compared to decarboxylation and dehydration under similar conditions and catalysts as this work [14]. However, decarbonylation was found to have an important contribution in the transformation of acetone over HZSM-5 and therefore CO could be a product of the secondary decarbonylation of the acetone produced in the process [14]. There is also a possibility for direct dehydration of acetic acid to ketene and successive decomposition of ketene to methylene radicals and CO [45]. The ethylene observed in the gaseous products over ZSM-5 supports the direct ketene decomposition route, as ethylene is produced by the combination of two methylene radicals (Reaction 8) [37]. Ethylene can then be readily converted to benzene or alkylated benzene via oligomerization, cyclization and dehydrogenation on the Brønsted acid sites of ZSM-5. The polymerization of ketene to coke species could also contribute to the CO yield [33]. Reaction 8: Dehydration to Ketene, decomposition to ethylene and CO, and oligomerization-cyclization-dehydrogenation of ethylene to aromatics
however different, altering significantly the final product distribution on the two catalysts. Higher ketones are mainly produced on MgO, while aromatics, phenolic compounds and light olefins are observed in the products of ZSM-5 catalyzed reaction. Temperature-programmed desorption studies on acetic acid and acetone-saturated catalyst amples show that at high temperature, acetone undergoes secondary self-condensation and oligomerization reactions yielding mainly CO2 and coke on MgO. On ZSM-5, both the direct dehydration of acetic acid and the cracking of the acetone condensation products play an important role, leading to the production of light olefins which are then converted to phenols and aromatics. The obtained catalytic results and the reaction mechanisms proposed in the present work can be utilized for the design of improved catalysts in bio-oil upgrading via catalytic pyrolysis/ cracking, selecting between the production of a bio-oil rich in aromatics and phenolics or a bio-oil that contains mainly higher ketones and related compounds that could be hydrodeoxygenated towards gasoline or diesel range alkanes. Acknowledgments This research was co-financed by European Union (European Social Fund) and Greek national funds through Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF)-Research Funding Program: THALES-Investing in knowledge society through the European Social Fund (MIS 380405).
CH3COOH → CH2CO + H2O CH2CO → :CH2 + CO 2[:CH2] → C2H4 3 C2H4 → C6H6 + 3 H2
Appendix A. Supplementary data
Finally, the presence of methane in the product slate of ZSM-5 could be the result of the decomposition of surface acetates to methane [38], according to reaction 9, supporting the surface acetate formation route. Reaction 9: Acetate Decomposition
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2018.12.012. References
CH3COO− + zeol-OH→ CH4 + CO2 + zeol-O− The above described reaction pathways, proposed on the basis of the obtained experimental data, the specific characteristics of the acidic ZSM-5 zeolite and the basic MgO, and the available literature regarding related mechanistic sutdies of various individual reactions, can be utilized in order to elucidate the product yields and distribution in the upgrading (in situ or downstream) of bio-oil under typical catalytic pyrolysis conditions. More specifically, the aim and future work is focused on the development of catalysts that can effectively eliminate acetic acid (and other acids) present in the bio-oil and at the same time tune the selectivity towards the desired product in each case.
[1] A.A. Lappas, K.G. Kalogiannis, E.F. Iliopoulou, K.S. Triantafyllidis, S.D. Stefanidis, WIREs: Energy Environ. 1 (2012) 285–297. [2] D.A. Bulushev, J.R. Ross, Catal. Today 171 (2011) 1–13. [3] E.F. Iliopoulou, K.S. Triantafyllidis, A.A. Lappas, WIREs Energy Environ. (2018), https://doi.org/10.1002/wene.322; S. Wan, Y. Wang, Front. Chem. Sci. Eng. 8 (2014) 280–294. [4] E.F. Iliopoulou, P.A. Lazaridis, K.S. Triantafyllidis, Nanocatalysis in the fast pyrolysis of lignocellulosic biomass, chapter 27, pp. 655–714, in: B. Sels, M.l Van de Voorde (Eds.), Nanotechnology in Catalysis: Applications in the Chemical Industry, Energy Development, and Environment Protection, Wiley, 2017. [5] C. Liu, H. Wang, A.M. Karim, J. Suna, Y. Wang, Chem. Soc. Rev. 43 (2014) 7594–7623. [6] A. Zheng, L. Jiang, Z. Zhao, Z. Huang, K. Zhao, G. Wei, H. Li, WIREs: Energy and
41
Molecular Catalysis 465 (2019) 33–42
A.C. Psarras et al. Environ. (2016), https://doi.org/10.1002/wene.234. [7] M. Guisnet, P. Magnoux, D. Martin, Stud. Surf. Sci. Catal. 111 (1997) 1–19. [8] S.D. Stefanidis, K.G. Kalogiannis, E.F. Iliopoulou, A.A. Lappas, P.A. Pilavachi, Bioresour. Technol. Rep. 102 (2011) 8261–8267. [9] S.D. Stefanidis, S.A. Karakoulia, K.G. Kalogiannis, E.F. Iliopoulou, A. Delimitis, H. Yiannoulakis, T. Zampetakis, A.A. Lappas, K.S. Triantafyllidis, Appl. Catal. B 196 (2016) 155–173. [10] E. Putun, Energy 35 (2010) 2761–2766. [11] Y. Lin, C. Zhang, M. Zhang, J. Zhang, Energy Fuels 24 (2010) 5686–5695. [12] J. Fermoso, H. Hernando, P. Jana, I. Moreno, J. Prech, C. Ochoa-Hernandez, P. Pizarro, J.M. Coronado, J. Cejka, D.P. Serrano, Catal. Today 277 (1) (2016) 171–181. [13] H. Hernando, I. Moreno, J. Fermoso, C. Ochoa-Hernández, P. Pizarro, J.M. Coronado, J. Čejka, D.P. Serrano, Biomass Conver. Bioref. 7 (3) (2017) 289–304. [14] A.G. Gayubo, A.T. Aguayo, A. Atutxa, R. Aguado, M. Olazar, J. Bilbao, Ind. Eng. Chem. Res. 43 (2004) 2619–2626. [15] W. Shurong, D. Gongxin, Y. Haiping, L. Zhongyang, Prog. Energy Combust. Sci. 62 (2017) 33–86. [16] E.F. Iliopoulou, S.D. Stefanidis, K.G. Kalogiannis, A. Delimitis, A.A. Lappas, K.S. Triantafyllidis, Appl. Catal. B 127 (2012) 281–290. [17] E.F. Iliopoulou, S.D. Stefanidis, K.G. Kalogiannis, A.C. Psarras, A. Delimitis, K.S. Triantafyllidis, A.A. Lappas, Green Chem. 16 (2014) 662–674. [18] C.A. Emeis, J. Catal. 141 (1993) 347–354. [19] A.C. Psarras, E.F. Iliopoulou, L. Nalbandian, A.A. Lappas, C. Pouwels, Catal. Today 127 (2007) 44–53. [20] V.G. Komvokis, S. Karakoulia, E.F. Iliopoulou, M.C. Papapetrou, I.A. Vasalos, A.A. Lappas, K.S. Triantafyllidis, Catal. Today 196 (2012) 42–55. [21] P. Lanzafame, S. Perathoner, G. Centi, E. Heracleous, E.F. Iliopoulou, K.S. Triantafyllidis, A.A. Lappas, ChemCatChem 9 (2017) 1632–1640. [22] A.A. Gusev, A.C. Psarras, K.S. Triantafyllidis, A.A. Lappas, P.A. Diddams, Molecules 22 (2017) 1784–1804. [23] A. Demirbas, Energ. Convers. Manage. 41 (2000) 633–646.
[24] J.D. Adjaye, R.K. Sharma, N.N. Bakhshi, J.S. Kevin, C.S. Emerson (Eds.), Stud. Surf. Sci. Catal. vol. 73, Elsevier, 1992, pp. 301–308. [25] C.X. Ma, G. Liu, Z.L. Wang, Y.F. Li, J. Zheng, W.X. Zhang, M.J. Jia, React. Kinet. Catal. Lett. 98 (2009) 149–156. [26] Z. Liu, F. Peng, X. Liu, Adv. Mater. Res. 550–553 (2012) 424–428. [27] M. Leon, L. Faba, E. Diaz, S. Bennici, A. Vega, S. Ordonez, A. Auroux, Appl. Catal. B 147 (2014) 796–804. [28] L. Wu, T. Moteki, A.A. Gokhale, D.W. Flaherty, D. Toste, Chem 1 (2016) 32–58. [29] A. Gumidyala, T. Sooknoi, S. Crossley, J. Catal. 340 (2016) 76–84. [30] L. Kubelkova, J. Novakova, Zeolites 11 (1991) 822–826. [31] T.N. Pham, T. Sooknoi, S.P. Crossley, D.E. Resasco, ACS Catal. 3 (11) (2013) 2456–2473. [32] R. Martinez, M.C. Huff, M.A. Barteau, J. Catal. 222 (2004) 404–409. [33] M. Watanabe, H. Inomata, R. Lee Smith Jr., K. Arai, Appl. Catal. A Gen. 219 (2001) 149–156. [34] E. Kukulska-Zając, K. Góra-Marek, J. Datka, Microporous Mesoporous Mater. 96 (2006) 216–221. [35] A.G. Gayubo, A.T. Aguayo, A. Atutxa, B. Valle, J. Bilbao, J. Chem. Technol. Biotechnol. 80 (2005) 1244–1251. [36] A. Pulido, B. Oliver-Thomas, M. Renz, M. Boronat, A. Corma, ChemSusChem 6 (1) (2013) 141–151. [37] C.D. Chang, A.J. Silvestri, J. Catal. 47 (1977) 249–259. [38] M.A. Hasan, M.I. Zaki, L. Pasupulety, Appl. Catal. A Gen. 243 (2003) 81–92. [39] A. Corma, G.W. Huber, Angew. Chem. Int. Ed. 46 (2007) 7184–7201. [40] S. Lippert, W. Baumann, K. Thomke, J. Mol. Catal. 69 (1991) 199–214. [41] K.V. Ramanamurty, G.S. Salvapati, J. Sci. Ind. Res. 59 (2000) 339–349. [42] K.V. Ramanamurty, G.S. Salvapati, Indian J. Chem. 38B (1999) 24–28. [43] O.I. Kusnetsov, G.M. Panchenkov, A.M.D. Guseim, T.A. Chasova, Nefte- pererab Neftekhzm 3 (1973) 28. [44] K.V. Ramanamurty, G.S. Salvapati, M. Janardanarao, J. Mol. Catal. 54 (1989) 9–30. [45] X. Li, S. Wang, Y. Zhu, G. Yang, P. Zheng, Int. J. Hydrogen Energy 40 (2015) 330–339.
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