Upgrading of biomass transformation residue: influence of gas flow composition on acetic acid ketonic condensation

Abstract

Acetic acid, a common biomass transformation residue, can be converted into acetone by direct ketonization in the gas phase over γ-Fe₂O₃ (maghemite) pre-activated at 450 °C under air. This reaction was performed in a continuous flow reactor with varying temperature from 250 to 300 °C and different carrier gases. Using N₂ as carrier gas, the activity of the iron based catalyst normally increased with increasing temperature and contact time and remained constant under stationary operating conditions. A slight decrease of activity was observed using a N₂/CO₂ mixture as carrier gas. A strong enhancement of the activity was observed when H₂ was introduced in the carrier gas, as a N₂/H₂ or CO₂/H₂ mixture. This improvement could be attributed to the modification of the oxidation state of the iron catalyst used for which in situ characterisation is still to be performed.

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1. Introduction

The use of biomass as a raw material for the synthesis of fuels and chemicals is currently a major challenge because of the scarcity of fossil fuels and the global climate change concerns. However, only a very minor fraction of this biomass (triglycerides, sugars and starches) can be effectively transformed into fuels (biodiesel, ethanol). Furthermore, these transformations compete with the human food chain.^1–3

Lignocellulosic biomass (the non-edible portion of biomass) is the most abundant form of biomass. However, the lignocellulosic biomass components (i.e. cellulose, hemicellulose and lignin) are not yet easy to transform into valuable organic molecules. Thus, biomass conversion techniques such as thermochemical and biochemical processes are used.^1,4 Most of these treatments lead to the formation of low-rank organic molecules of low molecular weight and high oxygen/carbon ratio.

For instance, fast pyrolysis is being actively developed for producing liquid fuels.^5,6 The yields and properties of the generated liquid product, bio-oil, depend on the feedstock, the process type and operating conditions, and the product collection efficiency. The liquid product is a mixture of oxygenated compounds and water, with the overall oxygen content being in the range of 35–40%.^7–9 The multicomponent mixtures are derived primarily from depolymerisation and fragmentation reactions of the three key building blocks of lignocellulose: cellulose, hemicellulose and lignin.⁵ The high acidity of bio-oils is attributed to the large quantities of carboxylic acids, mainly acetic acid, with a concentration varying between 2 and 12 wt%.

Dark-fermentation of organic compounds is a promising method of producing hydrogen through biological processes.^10,11 Heterotrophic anaerobic bacteria can convert pure substrates, including glucose, starch and cellulose, as well as different organic waste materials directly to hydrogen via fermentative butyrate or acetate metabolism and the maximum theoretical hydrogen yield is, respectively, 2 or 4 mol/mol hexose. However, the hydrogen yield from sucrose ranged from 1.25 to 2.4 mol/mol hexose for different bacterial species owing to the formation of volatile fatty acids during the fermentation process which contribute to the high COD and a low pH value in fermentation effluents and carry a potential threat to the environment.^10,12

Acetic acid is an important bulk commodity chemical with world annual production capacity of 9 million tonnes. Today, production is dominated by carbonylation of methanol¹² and its principal use (∼40%) is in the manufacture of vinyl acetate, a monomer of great importance in the polymer sector. Acetic acid produced by biomass conversion techniques could not be considered as an alternative feedstock due to the relatively low yield of acetic acid from biomass and the expensive separation procedures to give acetic acid of sufficient purity. Thus, the development of new catalytic processes for the upgrading of biomass transformation residue is a particular challenge and processes for the carboxylic acid valorization are in high demand.

The condensation of acetic acid to form acetone (ketonisation, eqn (1)) has been known for many years.¹³

2 CH₃CO₂H → CH₃COCH₃ + CO₂ + H₂O (1)

The process has historically been realised by the pyrolytic decomposition of metal carboxylates, mostly calcium salts.¹³ More recently, a great improvement in such syntheses of ketones has been made by the direct ketonization of carboxylic acids in the gas phase over solid catalysts under continuous flow conditions.^14–19 Acetone could be transformed into raw materials for the chemical industry such as methyl isobutyl ketone (MIBK),²⁰ isopropanol by hydrogenation or propylene by dehydration of isopropanol over an acid catalyst or can be used as a building block for the production of biofuels through aldol reaction.²¹

Most previous works on carboxylic acid condensation reactions have involved experiments using several heterogeneous catalysts: oxides such as Cr₂O₃, Al₂O₃, TiO₂, ZrO₂, CeO₂, iron oxide and manganese oxide as well as Mg/Al hydrotalcites.^15,18 Although ketonization of carboxylic acids has been studied extensively, no agreement has yet been reached concerning the mechanism. Ponec et al.¹⁴ have presented a detailed mechanistic investigation in which lattice energy has a influence on the mechanism.

Iron oxide based catalysts have already been used for the catalytic transformation of acetic acid to acetaldehyde^22,23 or to acetone.^14,24,25 With the above mechanistic considerations in mind, the present investigation was designed to examine the ketonisation of gaseous acetic acid over iron oxide catalysts using a continuous flow reactor and to study the effect of the oxidation state of the iron on reactivity. This paper reports the results obtained so far.

2. Experimental

2.1 Reagents and catalysts

Powder iron oxide (FeO) was purchased from Aldrich (−10 MESH). The catalytic reactor consisted of a packed-bed operated in a upflow mode, constructed from a 200 mm height and 5 mm diameter stainless steel tube filled with 20 g of FeO between two plugs of 5 mm thick quartz layers. The reactor was connected to an acetic acid evaporator, i.e. a saturator unit (Fig. 1). The evaporator consisted of a tube (same dimension as for the reactor tube) filled with SiC power. Such a saturator unit allows us to obtain stable and constant mass flow rates of acetic acid. Liquid acetic acid (>99.7%, provided by Aldrich) was injected via a dosing pump (HAVARD APPARATUS PHD-4400) at the evaporator bottom and the evaporator top was swept with gas mixtures. Mass-flow controllers (provided by ANALYT) were used to control gas flow rates. Hydrogen, carbon dioxide and nitrogen were provided by Air Liquide. The acetic acid vapour pressure, and thus the gas phase concentration, was controlled by the evaporator temperature which was set in order to have the evaporator exit gas saturated but no liquid was entrained. The reactor and evaporator tubes were coil heated. Gases or premixed gas mixtures were pre-heated before the reactor. The reactor and evaporator temperatures were measured and controlled using K-type thermocouples introduced in a thermowell at the centre of catalytic bed and in the gas phase at the evaporator bed top, respectively. The pressures were measured at the evaporator and reactor outlets.

Fig. 1 Experimental set-up—process flow diagram.

The feed and gas-phase reaction mixtures were analysed on-line using an Agilent 5890 N gas chromatograph equipped with three columns in parallel and two detectors (Fig. 1). The outlet line was heat traced to the heated injection valve of the gas chromatograph to avoid product condensation. For gas analysis two columns, Molecular sieve Varian CP7534 (10 m × 0, 32 mm × 10 mm) and Porabond Q Varian CP7351 (25 m × 0, 32 mm × 5 mm), were both connected to a TCD detector while for organic compounds a single column, Agilent HP-5 19091J-413 (30 m × 0, 32 mm × 0, 25 mm), was connected to an FID detector. These three columns were all connected to the injection valve.

2.2 Catalytic reaction

The ketonization of carboxylic acid was carried out in a fixed bed reactor under atmospheric pressure, the contact time (W/F, i.e. catalyst weight/total molar flow) varied between 170 and 280 kgcat s mol⁻¹. Prior to the reaction, the catalyst was oxidised in air at 450 °C for 30 minutes at a flow rate of 50 mL min⁻¹ and was stabilized in N₂ at working temperature for 30 minutes at a flow rate of 50 mL min⁻¹.

3. Results

Fig. 2 shows the acetic acid conversion to acetone as a function of temperature using N₂ as carrier gas. At any temperature no other products were detected. Conversion at 250 °C was low but regularly increases with temperature to reach 85% at 300 °C with selectivity higher than 99%. The same conversions were found when decreasing the temperature from 300 °C to 250 °C, suggesting that, under those conditions, the oxidation state of iron was stable.

Fig. 2 Temperature effect on acetic acid conversion to acetone (P = 1 bar, N₂ = 95 vol%, acetic acid = 5 vol%, W/F = 250 kgcat s mol⁻¹).

Regarding the effect of contact time on acetic acid conversion (Fig. 3), the acetic acid conversion to acetone increased when the contact time increased, as expected. Conversion is a linear function of contact time, indicating a zero order reaction due to the strong adsorption of acetic acid. This simplified zero order reaction kinetics could be taken as a hypothesis to calculate an activation energy of 101 kJ mol⁻¹ (±0.5 kJ mol⁻¹).

Fig. 3 Effect of contact time on acetic acid conversion to acetone (P = 1 bar, N₂ = 95 vol%, acetic acid = 5 vol%, T = 300 °C).

As the particular catalyst showed high conversion and selectivity to acetone, the stability of the iron oxides was evaluated. Thus, experimental conditions were chosen in order to have a conversion close to 50%, i.e. 300 °C, W/F of 200 kgcat s mol⁻¹ (Fig. 2 and 3). Acetic acid conversion as a function of time under these conditions is shown in Fig. 4.

Fig. 4 Acetic acid conversion as a function of time (P = 1 bar, N₂ = 95 vol%, acetic acid = 5 vol%, T = 300 °C, W/F = 200 kgcat s mol⁻¹).

It can be seen that during the first 50 minutes on stream, acetic acid conversion to acetone was increasing from 30 to 50%. A stable conversion was obtained after 1 hour following which an almost stable conversion could be observed. Such 50% conversion was expected from the plot shown in Fig. 2 and 3. We believed that the oxidation state of the iron during the ketonisation of acetic acid vapour could vary during the continuous flow reaction. The exact oxidation state of the iron during the reaction was unknown and could have been determined using in situ Mössbauer spectroscopy.²² However, such in situ spectroscopy was not available in the course of the study. However, at the end of the reaction, Mössbauer spectroscopy suggested that the catalyst is γ-Fe₂O₃ (maghemite).

The reproducibility of the experiments was also checked. The catalyst which had been used in the 6 hours run was oxidised in air at 450 °C for 30 minutes at a flow rate of 50 mL min⁻¹ and then stabilized in N₂ at 350 °C for 30 minutes at a flow rate of 50 mL min⁻¹. Then, the same experimental conditions were used and the same curve shape (±5%) was obtained. The conversion evolution during the first hour of the experiment (up to 50%) could therefore be put down to the catalyst and set-up stabilisation after catalyst regeneration and evaporator operating conditions changes.

According to the results obtained and the reaction pathways suggested in the literature, ketonization of acetic acid leads to acetone and CO₂ as by-product of the ketonization reaction. The effect of CO₂ on acetone production was thus studied (Fig. 5).

Fig. 5 Acetic acid conversion as a function of time (P = 1 bar, N₂ = 42.5 vol%, CO₂ = 42.5 vol%, acetic acid = 5 vol%, T = 300 °C, W/F = 200 kgcat s mol⁻¹).

For a high concentration of CO₂ in the carrier gas and as already observed in the case of N₂, there was a stabilisation period during which the conversion of acetic acid to acetone over regenerated iron catalysts was increasing as the function of time on-stream and a conversion close to 50% was obtained after 50 min (Fig. 5). However, a slow decrease of the acetic acid conversion (less than 10%) could be observed until 250 min on-stream where the conversion almost stabilizes. After more than 7 hours on-stream, the catalyst was then regenerated in air and the same experimental conditions were used. Under those conditions, the same curve (±5%) was obtained.

The effect of gas flow composition on the oxidation state of iron was further studied using a mixture of N₂/H₂ as carrier gas (Fig. 6).

Fig. 6 Acetic acid conversion as a function of time (P = 1 bar, acetic acid = 5 vol%, T = 300 °C, W/F = 200 kgcat s mol⁻¹).

As shown in Fig. 6, the presence of H₂ in the gas flow has a strong influence on the activity of the regenerated iron catalyst. The conversion was increasing as a function of the time on-stream. During this experiment, no reduced compounds issue from the reactant (i.e. acetaldehyde, ethanol) or product (i.e. isopropanol or propylene obtained by dehydration of isopropanol) could be detected in the effluent. To date, no information on the composition, i.e. oxidation state, of the iron catalyst is available. However, one could expect an equilibrium of the iron oxidation state as constituents of the feed can reduce (H₂) or oxidise (acetic acid) the catalyst.^22,26 The possibility of using biogas, a mixture of H₂ and CO₂ produced by dark fermentation of organic waste, was then evaluated for the conversion of acetic acid to acetone (Fig. 7).

Fig. 7 Acetic acid conversion as a function of time (P = 1 bar, CO₂ = 42.5 vol%, H₂ = 42.5 vol%, acetic acid = 5 vol%, T = 300 °C, W/F = 200 kgcat s mol⁻¹).

Within experimental error, the same acetic acid conversion was observed using simulated biogas CO₂/H₂ or N₂/H₂ as the carrier gas (Fig. 7). Under those conditions, no deactivation due to the presence of CO₂ was seen. The poisoning of the catalyst by CO formed by reverse water-gas shift reaction (RWGS) has been reported for platinum group metals.²⁷ However, under our catalyst pretreatment and reaction conditions CO formation could be neglected.²² Furthermore, CO was never detected by on-line GC analysis.

The previous experimental results with different gas flow compositions (Fig. 4 and 7) were obtained with the regenerated catalyst, i.e. γ-Fe₂O₃ (maghemite). It has not yet been possible to characterise the oxidation state of the iron during the experiments. However, one could expect an evolution of the oxidation state with reaction time despite the observed conversion rates of acetic acid to acetone being similar. The conversion of acetic acid to acetone was therefore studied with changes of gas flow composition during a single experiment (Fig. 8).

Fig. 8 Acetic acid conversion as a function of time and carrier gas (P = 1 bar, acetic acid = 5 vol%, T = 300 °C, W/F = 200 kgcat s mol⁻¹, carrier gas composition: □ N₂ = 95 vol%; ▲ H₂ = 42.5 vol%, N₂ = 42.5 vol%; ○ H₂ = 42.5 vol%, CO₂ = 42.5 vol%; ▼ N₂ = 42.5 vol%, CO₂ = 42.5 vol%).

After a 50 minutes catalyst and set-up stabilization period with N₂ as carrier gas where acetic acid conversion to acetone was increasing from 30 to 50%, a stable conversion, close to 50%, was obtained until 100 min as expected from Fig. 4. Switching to N₂/H₂ led to an increasing conversion for the time period 100 min to 250 min in agreement with Fig. 6. Then, switching to biogas (CO₂/H₂) for the period 250 min to 375 min again led to increasing conversion in line with Fig. 7. The replacement of H₂ by N₂ induced a decrease of the acid conversion with the mixture CO₂/N₂ from 375 min to 500 min, as expected from Fig. 5. Finally, a stable acetic acid conversion close to 70% was obtained under pure N₂.

4. Conclusion

The transformation of low-rank carboxylic acids obtained by conversion of lignocellulosic biomass into chemicals was studied. The experiments performed in a continuous flow reactor showed that acetic acid condensation to acetone can be catalysed by iron oxide catalysts with high conversion and selectivity. The conversion was found to be strongly influenced by the composition of the carrier gas. No inhibition by the CO₂ produced during the acetic acid condensation was seen, but when a mixture of CO₂/N₂ was used as carrier gas a decrease in the acetic acid conversion was observed. The presence of H₂ in the gas flow strongly increased the conversion of acetic acid even in the presence of a high amount of CO₂ (biogas). These results suggest that acetic acid condensation occurs by the way of iron atoms present at the surface of the iron oxide catalyst. Since constituents of the feed can reduce (H₂) or oxidise (acetic acid) the iron catalyst, the conversion seems to be dependent on the iron oxidation state. To date, the exact oxidation state of the iron during the reaction is still unknown and will be determined using in situ Mössbauer spectroscopy in due course.

The results presented here give new understanding of the reaction of different volatile fatty acids when converted to ketones over iron based catalysts, and might help pave the way for the development of new processes for conversion of biomass transformation residue to “platform” molecules.

Acknowledgements

We are thankful to Government of Pakistan (Higher Education Commission) for supporting this research work through doctoral grant to A. A. T. and to Jean-Marc M. Millet, Institut de recherches sur la catalyse et l'environnement de Lyon (IRCELYON) for Mössbauer spectroscopy.

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