Original Article
chemtechnol Ind J, Volume: 11( 6)

One Step Acetic Acid Formation Through Simultaneous Activation of Methane and Co2 over Cu Exchanged Zsm-5 Catalysts.

*Correspondence:
Jingying Pan , Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027 Zhejiang, China,
Tel:
+86-571-87951237; E-mail: [email protected]

Received: October 06, 2016; Accepted: November 09, 2016; Published: November 14, 2016

Citation: Pan J. One Step Acetic Acid Formation Through Simultaneous Activation of Methane and Co2 Over Cu Exchanged Zsm-5 Catalysts. Chem Technol Ind J. 2016;11(6):108.

Abstract

The ultimate goal of our study is the development of strategies to exploit methane and carbon dioxide resources more efficiently and cleanly. Due to the high stabilities of both methane and CO2, simultaneous C-H bond activation of methane and CO2 is the one of the toughest challenge in catalysis. Formation of acetic acid was achieved by the concurrent feed of methane and carbon dioxide over Cu loaded basic cation ZSM-5 catalysts under the continuous flow microreactor system. The formation of acetic acid was observed in the broad temperature range of 425°C to 525°C with a co-feed unimolar ratio of CO2/CH4, under low space velocity of 360 ml hr-1 based on methane. The formation rate of acetic acid was observed to be low in Cu°-H-ZSM-5, however, the cation exchanged M+-ZSM-5 catalyst showed remarkable increment in the formation rate of acetic acid. The basic cations showed the catalytic activity in the following order K>Na>Ca>Li. This indicates that basic cations helps in providing surface active CO2 in the form of carbonates, which led to efficient CO2 insertion into activated C-H bond from methane over reduced Cu metallic nanoparticles. The bi-functional Cu and basic cationic species, together ascribes to the outstanding formation rates of acetic acid. The Cu0-K-ZSM-5 catalyst exhibited the highest formation rate of acetic acid of 12424 μ mole gcat-1h-1 at the initial activity and 395 μ mole gcat-1h-1 at steady state, for 10 h time-on-stream at 500°C.

Keywords

Co-activation of CH4 and CO2; Acetic acid; Cu0-K-ZSM-5; Continuous system

Introduction

Valuable chemicals from abundant resources, which are global warming gasses like methane and CO2, evoke great attention not only from the viewpoints of both energy and environmental issues, but also due to its potential utilization as an alternative and economical feedstock [1-4]. For the conversions of methane and CO2, surface activation using heterogeneous catalyst has been proposed as one of the plausible ways to overcome very high thermodynamic and kinetical barriers [5] due to their high stabilities and chemical inertness [6]. Moreover, the simultaneous activation of both molecules looks attractive but even tougher [7]. C-H bond activation of methane has been applied to give aromatics, olefins, and oxygenates through the non-oxidative or oxidative conditions with the help of oxidants [8] by loosening or dissociating of C-H bond of methane [9]. And CO2 activation has been believed to need a reductant to lose oxygen or at least should be activated molecularly with the help of metallic Lewis acid sites or basic sites [10]. Even though simultaneous activation of methane and CO2 seems to be very difficult [11], there have been reported such as CO2 (dry) reforming of methane to synthesis gas [12] and oxidative coupling of methane with CO2 at high temperature, but quite less on oxidative conversion of methane into methanol with CO2 and/or CO, and carboxylation of methane to acetic acid as an insertion of CO2 into C-H bond of methane [13].

When it comes to the acetic acid, the most prevailed acetic acid manufacturing process is methanol carbonylation process, which has three steps starting from methane to methanol via synthesis gas followed carbonylation with CO homogeneous conditions over catalyst system such as rhodium and iridium complexes with HI [14] among (1) carbonylation of methanol; (2) isomerization of methyl formate; (3) syngas to acetic acid; and (4) vapor phase oxidation of ethylene etc. So, the direct one-step formation of acetic acid via the carboxylation of methane with CO2 evokes great interest but very challengeable for simultaneous activation of both reactant molecules and overcoming thermodynamic barrier. Also, there was no loss of any atoms after the reaction. So, the simultaneous activation of these two molecules on the same catalyst system for forming acetic acid is subjected to have great merits comparing to current major acetic acid processes.

Despite of this advantage, the simultaneous conversions of methane and CO2 was hardly achieved, mainly due to thermodynamic unfavourability in terms of free energy [5]. There were several trials to get over thermodynamic unfavourability by adding O2 [15], and dielectric barrier discharge [13]. Acetic acid from CH4 and CO2 was firstly reported by Pd/Cu complexes [15] in CF3COOH solvent and K2S2O8 (15 mol%) acts as an oxidizing agent homogeneously, which furnished 7% yield on acetic acid. However, it was explained later that no CO2 participated but CF3COOH acted as a carboxylate source due to more thermodynamically favorable reaction between CH4 and CF3COOH. Heterogeneous approaches mostly have adopted the two-step, stepwise, periodic or cyclic reaction modes by using Co-Cu, V2O5-PdCl2/Al2O3, [16] Pd and Rh/TiO2, Pd and Rh/SiO2, [17] Pd/C and Pt/alumina [18] as catalysts with less than 45 μ mole gcat-1 h-1 of formation rate and low selectivity as well. Huang et al. proposed two-step process for the synthesis of acetic acid over bimetallic Co-Cu catalysts by alternative feeding of CO2 and CH4 with H2 sweeping between the cycles to give various oxygenates including poor acetic acid selectivity, wherein the first step formation of surface carbonaceous species took place via the dissociation of methane over Co to give Co-CHx species, then CHx inturn produced CHx-Cu species which rearranged to metal-O-C-O-CHx followed by addition of hydrogen to give acetic acid [19].

However, a couple of theoretical and spectroscopic studies have shown the possibility of the direct conversion of CH4 and CO2 to acetic acid using heterogeneous catalysts. Liu et al. demonstrated the route via CO2- thermodynamically favoured than the route of CO, which could be formed by dielectric-barrier discharge [20]. Zhang et al. proposed a two-step technology but only by the DFT studies, by showing the conversion of both CH4 and CO2 directly to acetic acid at low temperature (100°C to 400°C) over Cu (111) surface and proposed the Eley-Rideal meachnism between the gaseous CO2 and metal-CHx, which was formed dehydrogenation of CH4 on metals by the first-principle DFT-GGA calculations [21]. Recently, Limtrakul et al. proposed the concerted bifunctional mechanism by the DFT (M06-L) over Au(I)-exchanged ZSM-5 zeolite, which draw out the metal-Broensted acid synergistic effect on the activation of CH4 and CO2 by the homolytic dissociation of CH4 followed by the insertion of CO2 activated on B-sites [6].

However, Spivey et al. reported the formation of acetate as an intermediate from 50% CO2/50% CH4 mixture over 5% Pd supported carbon and Pt supported alumina catalysts by using (DRIFTS) [18]. Zhu et al. confirmed by FTIR the formation of CHx and CHxO species over Cu/Co supported oxide catalysts [22]. Recently, a new strategy was investigated using C13-NMR for co-conversion of CH4 and CO2 over Zn/H-ZSM-5 catalyst. Where, the zinc sites were observed to effectively activate CH4 to form (-Zn-CH3), followed by the insertion of CO2 into the Zn-CH3 to form zinc acetate species [23]. Ding et al. investigated and confirmed the effectiveness of Pd/SiO2 and Rd/SiO2 in this carboxylation reaction by step-wise isothermal routs [17]. Narsimhan et al. reported the mechanistic studies for direct conversion of CH4 and CO2 onto acetic acid using Cu-exchanged zeolites (ZSM-5 and MOR.) [24]. Wannakao et al. illustrated the possibility of methane and CO2 activation and their corresponding adsorption capacity over some metal ion exchanged zeolites [25].

However, to the best of our knowledge, there is no report which deals with the experimental study for the direct synthesis of acetic acid from CH4 and CO2 under continuous fixed-bed reactor using concurrent feeding. In this work, we reported the use of copper and basic cation-loaded ZSM-5 catalyst for the direct synthesis of acetic acid from CO2 and CH4 under continuous fixed-bed reactor after reduced with H2 in a reactor before reaction. So, we presumed the role of metallic Cu species and the basic cations as a bifunctional catalyst for the simultaneous activation of CH4 and CO2 which enabled the continuous formation of acetic acid.

Experimental

Ion-exchanged Cu-ZSM-5 preparation

Commercial H-ZSM-5 procured from (Zeolyst) was cation-exchanged three times using 0.1 M of LiNO3, NaNO3, KNO3, and Ca(NO3)2 and then calcined at 500°C for 5 h. The prepared M+-ZSM-5 was then ion exchanged with 0.25 M of Cu(NO3)2. 3H2O. In a typical synthesis procedure 1 g of M+-ZSM-5 was charged into 50 ml round bottom flask with a certain amount of Cu(NO3)2.3H2O and refluxed at 60°C for 6 h. The solid was then filtered and washed several times with de-ionized water dried and calcined at 550°C for 5 h.

Catalytic activity

The direct reaction between CH4 and CO2 was carried out in stainless fixed-bed reactor loaded with appropriate amount of catalyst (300 mg) and was subjected to He flow (30 ml/min) for 20 min at room temperature to remove the air. Subsequently, He flow was switched to hydrogen (5 ml/min) and the catalyst was reduced at 450°C for 2 h. The reduction was carried out using a temperature program from 30°C to 400°C in 30 min and then the temperature was raised to 450°C in 30 min and maintained for 2 h. Then the desired temperature for the reaction was adjusted under hydrogen flow 2 ml/min. After the reduction, at the desired temperature hydrogen flow was switched to a mixture of 1% CH4 in He (1.5 ml/min) and CO2 (1.5 ml/min) simultaneously. The reaction was conducted at different temperature ranges from 425°C to 525°C. The products were analyzed by online gas chromatography (Younglin Instrument, Acme 6000 series, Korea) equipped with FID and TCD.

Characterization of the catalysts

Powder X-ray diffraction patterns (XRD) of catalysts were obtained using Rigaku Miniflex X-ray diffractometer with CuKα radiation source (λ=0.154 nm) at 30 kV, 15 mA from 5° to 90°. UV-vis-NIR diffuse reflectance spectra (DRS) were performed with a Shimadzu UV-2501PC spectrophotometer equipped with a reflectance attachment and BaSO4 was used as the reference material. The NIR spectra were recorded in the reflectance mode at room temperature. The FTIR spectra were recorded using a Nicolet Impact 410 spectrometer over a range of 400 cm-1 to 4000 cm-1. The X-ray photoelectron spectra were obtained using an ESCALAB MK II spectrometer provided with a hemispherical electron analyzer and Al anode X-ray exciting source (Al Kα=1487.6 eV). TEM images of the samples were recorded on a JEOL JEM-2100 (Japan) operating at 200 kV. Temperature-programmed reduction (TPR) of the catalysts with hydrogen (5 vol.% H2 in helium) was performed from 50°C to 600°C with heating rate of 10°C/min in a conventional flow system equipped with a TCD detector for monitoring of the H2 consumption.

Results and Discussion

Screening the catalytic performance different temperature and the molar ratios of methane and CO2

Figure 1 depicts the rates of acetic acid formation depending on temperature: the initial activity within 21 min. (a); till 1 hr after 21 min. (b); and hourly activities at 500°C (c); and dependency of CH4 to CO2 ratios over Cu0-Na-ZSM-5 catalyst (d). The catalytic activity was tested between 425°C to 525°C. The activities towards the formation of acetic acid increased with temperatures. The highest activity was observed at 525°C about 50 mmole gcat-1h-1 at the first run but rapidly decayed compared to that at 500°C. Interestingly steady formation of acetic acid was observed with the highest at 500°C for 10 h of time-on-stream. Since acetic acid formation is directly dependent on the molar ratios of methane and CO2, thus the flow optimization was carried out as shown in Figure 1d and the equimolar ratio 1:1 of methane and CO2 showed the highest rate of formation of acetic acid at 500°C.

chemical-technology-reaction-acetic-acid

Figure 1: Acetic acid formation rates on the reaction time over °0-Na-ZSM-5; initial activities for 21 min (a); and activities from 24 min till 1 h (b) at different temperatures; acetic acid formation rates for 10 h at 500°C (c); and effect of CH4/CO2 molar ratios on the formation of acetic acid at 500°C (d).

Effect of basic cations: In order to know the role of basic cations in Cu0-M-ZSM-5 catalysts (M=Li+, Na+, K+ and Ca++), their activities were compared with Cu0-H-ZSM-5 (Figure 2) at the initial 9th min (Figure 2a), 1 h (Figure 2c) and 10 h (Figure 2d) time-on-stream after the initial 1 h acetic acid was the only product obtained. So, Figure 2b showed the product distributions which gave formic acid and methanol only at the initial period of 9th min. For the whole range of reaction temperatures, Cu0-H-ZSM-5 catalyst gave poor activities than any other basic cation exchanged Cu0-M-ZSM-5 catalysts. Cu-K-ZSM-5 catalyst gave the highest activity (395 μ mol. g-1 h-1) for 10 h after one hour of drastic decrease. Even the Cu-Na-ZSM-5 catalyst gave the highest formation rate at the initial period but showed more drastic decline after 24 min than Cu-K-ZSM-5. This indicates that the activation of CO2 is closely related to the alkali cations, which helps to insert onto C-H activated species from methane. The higher CO2 adsorption capacity in K-ZSM5 is attributed to the acid-base interaction between framework and CO2 molecules. The CO2-TPD of the catalysts which Cu-K-ZSM5 exhibited the highest concentration of adsorbed CO2 among all of the prepared catalysts (Figure S1). So, the effective charge density of the cations inside the zeolite pores is assumed to be the reason for higher CO2 adsorption. The CO2 adsorption capacity slightly increases with the cation size as follow Li<Na <K. The population of divalent cation at the pore entrance of zeolite framework is less and has an only half number of cation than that of monovalent exchanged ZSM-5, which explains the low activity in Ca-exchanged ZSM-5 catalyst [26]. The experimental results were observed to match reasonably with the above explanation (Figure 2).

chemical-technology-Methane-acetic-cation

Figure 2: Methane conversion and acetic acid formation rate over different cation exchanged Cu-Na-ZSM-5 catalysts at 500°C; methane conversion and acetic acid yields (a); and product distribution in liquid products (b); at 9th min of time-on-stream; methane conversions and acetic acid yields at 1 h (c); and after 10 h of time-on-stream (d).

Effect of catalyst regeneration: The regeneration studies were performed in the cases of Cu-Na-ZSM-5 and Cu-K-ZSM-5 catalysts. The used catalysts after 10 h of reaction were calcined at 550°C for 5 h in the muffle furnace and tested for another 10 h as shown in TABLES. S1-S3. The regenerated catalysts showed comparable activities to those of fresh catalysts (~70% in terms of formation rate of acetic acid). The activities of both fresh and regenerated catalysts (Figure S2) showed a steady decline in the catalytic activity, aggregation of the Cu0 nanoparticles was suspected to be the reason for deactivation. The aggregation, in turn, increased the Cu sizes and thus diminishing the effective Cu0 nanoparticles of appropriate sizes for methane activation [27].

Figure 3a shows plots of the methane conversions and acetic acid selectivity over Cu-M-ZSM-5 catalysts with the different exchanged cations. Methane conversion decreased significantly for all of the catalysts after 20 min by the rapid agglomeration of Cu under the reductive environment due to methane activation as CH3H.

chemical-technology-Methane-acetic-selectivity

Figure 3: Methane conversion and acetic acid selectivity over different cations exchanger Cu0-M-ZSM5.

XPS analysis: XPS analysis of calcined Cu-Na-ZSM-5 showed the presence of peak corresponding to Cu(2p3/2) at 933.9 eV, the evidence of shakeup peak at 943.9 eV indicating the presence of Cu (II) and at 932.4 eV which could be due to Cu+ or Cu0 in FIG 4a [28]. Practically in the calcined catalyst, the existence of Cu0 species could be hardly possible. So, it’s better to consider that the calcined Cu-Na-ZSM-5 consisted of Cu2+ and Cu+. XPS of the reduced Cu-Na-ZSM-5 in Figure 4c also showed two peaks at 933.9 eV and at 932.4 eV. The peak at 932.4 eV is comparatively higher compared to the freshly calcined catalyst since the catalyst was reduced by hydrogen so the major peak could be assigned as Cu1+ or Cu0 species and it is assumed that the reduced catalyst was rich in Cu0 species. Also, XPS of the used Cu-Na-ZSM-5 in FIG 4b catalyst showed a decrease in the intensity of the peak at 932.4 eV.

chemical-technology-calcined-reduced-ZSM

Figure 4: XPS and XRD results of Cu-ZSM-5: XPS of fresh calcined Cu-ZSM-5 (a); used Cu-ZSM-5 (b); and reduced Cu-ZSM5 (c); XRD of calcined Cu-ZSM-5, reduced Cu-ZSM-5 and used Cu-ZSM-5 (e&f).

XRD pattern: XRD analysis was carried out on the three catalysts: calcined, reduced and used Cu-Na-ZSM-5. XRD shows the typical diffraction peaks at 2ϴ=7.94°, 8.85°, 14.90°, 23.33°, 23.90° and 24.50° which are characteristic to the parent ZSM-5 [29].Figure 4f shows the XRD patterns of the fresh reduced and used catalyst. The reduced catalyst showed the existence of Cu0 species confirmed at 2ϴ, 43° and 51° which corresponds to the lattice planes of the cubic copper phase (111) and (200), respectively [30]. These peaks were also observed in the case of used catalyst. Thus, based on the XRD and XPS analyses the certain sized Cu0 species would be responsible or direct formation of acetic acid from CH4 and CO2 together with basic cation in ZSM-5.

EPR and UV-vis-NIR analysis: EPR and UV-vis-NIR spectra of the calcined Cu-Na-ZSM-5 catalyst are telling the presence of the isolated Cu2+ ions with tetragonally distorted octahedral coordination. In EPR spectrum, typical Cu2+ ion species having axial anisotropy of g factors with parameters g‖=2.33, A=137 G, g⊥=2.054 (Figure 5a) and large absorption band in the range of 500 nm to 1400 nm centred at 800 nm was observed in (Figure S3). The broad transition band at ca. 600 nm ~ 850 nm in UV, which corresponds to d-d transition 2T2g2Eg, which is inherent of Cu2+ ions in zeolites [31]. An intense peak at 250 nm can be assigned to the O2– → Cu2+ charge transfer transition. And additional peak closed to 365 nm was observed. They are related to d-d transitions of Cu (II) species. The spectrum of these conditions is in agreement with the reported in the literature for comparative specimens and relates to Cu (II) in zeolitic cavities [32]. As shown in the EPR (Figure 5b) spectroscopy, the adsorption of methane over calcined catalysts have comparably small amounts of isolated Cu2+ Oh ions. The EPR spectrum having axial anisotropy of g factors with parameters g‖=2.31, A=150 G, g⊥=2.051, EPR signal intensity and area shows that the amounts of isolated copper ions Cu2+ Oh decreased with methane adsorption, which means the methane reduces Cu2+ species to Cu1+ and Cu0 by the hydrogen species which were induced through the dissociative activation of methane. This is also reflected in the case of UV-vis DR after adsorption of methane. When the calcined sample was reduced with hydrogen at 450°C, the “d-d” transitions of Cu2+ ions in the range of 500 nm to 1400 nm was not visible on the spectrum, as a result of this treatment, the reduction of Cu2+ ions happened. The H2 treated Cu-Na-ZSM-5 exhibiteddifferent line shapes at 230 nm, 314 nm, and 560 nm. The first band showed at 230 nm corresponding to Cu1+, and plain to see the band at 560 nm in the reduced catalyst is assigned to Cu° and this peak was also observed to be increased after introducing CH4/CO2. The transition appearing at 314 nm upon reduction at 450°C is assigned to the internally allowed transition 3d104s → 3d104f of isolated copper atoms of Cu0 [33]. These phenomena can be assigned to the aggregation of Cu0[34]. This drastic decrease in the intensity of the hyperfine splitting of Cu2+ was also confirmed by the in-situ EPR by the intrusion of methane over calcined Cu-Na-ZSM-5 catalyst.

chemical-technology-calcined-methane-intrusion

Figure 5: In-situ EPR of calcined, reduced, and after introducing of methane and CO2 over Cu-Na-ZSM-5 (a); calcined and methane intrusion at 450°C onto the calcined Cu-Na-ZSM-5 (b).

chemical-technology-catalyst-hydrogen-methane

Figure 6: In-situ IR spectra for Cu-Na-ZSM-5 at 450°C. The catalyst reduced with hydrogen from room temperature to 450°C then methane and CO2 introduced simultaneously for 15 min at the same temperature then the IR measured at room temperature.

In-situ FT-IR spectra: In-situ FT-IR studies were carried out to elucidate the reaction mechanism over reduced Cu-Na-ZSM-5 catalyst. In-situ FT-IR experiment was carried over a self-supported pellet of Cu-Na-ZSM-5; the catalyst was dosed with H2 several times, using the same program used for the reduction mentioned in the experimental. After complete reduction, the catalyst was exposed to a pulse injection of an equimolar mixture of methane and carbon dioxide simultaneously. The in-situ IR spectrum is presented in FIG 6. and showed a peak at 1742 cm-1 which can be attributed to the C=O stretch of acetic acid monomer [35]. An additional peak at 1420 cm-1 confirms the existence of O-H deformation in a dimer of acetic acid, [36] and peaks at 3650 and 2340 cm-1 corresponds to O=C=O stretch of carbon dioxide. The band at 1568 cm-1 is attributed to va(OCO) [37]. Along with the O=C=O, peaks corresponding to strong C-H peaks at 2960, 2922, and 2854 cm-1, as well as peaks corresponding to CO2 at 2360 and 2340 cm-1 were also observed [22]. This study confirms the formation of acetic acid over reduced Cu-Na-ZSM-5 catalyst.

TEM analysis: More profound evidence on the catalyst deactivation and regeneration, the fresh calcined, used and regenerated catalysts were obtained by TEM as shown in Figure 7. The TEM micrograph of the synthesized Cu-Na-ZSM-5 and Cu-K-ZSM-5 revealed the formation of the small size of Cu nanoparticles in the range of 1~2 nm and had a good dispersion on ZSM-5. But after the reaction, the TEM image reveals the formation of small Cu aggregates with different size in the range of 4 nm~9 nm. This indicates that the Cu nanoparticles are aggregated resulting in the loss in catalytic activity. The used catalyst was regenerated by calcining in air at 550°C, which shows the re-dispersion of the aggregated Cu particles size into the smaller size of 2 nm~3 nm again. The particle sizes of the catalysts are comparable with the histogram (Figure S4).

chemical-technology-HR-TEM-calcined-Regenerated

Figure 7: HR-TEM images of fresh calcined Cu-Na-ZSM5 (a); Used Cu-Na-ZSM5 (b); Regenerated Cu-Na-ZSM5; fresh calcined Cu-K-ZSM5 (d); Used Cu-K-ZSM5 (e); Regenerated Cu-K-ZSM5 (f).

Proposed mechanism

A plausible mechanism towards the formation of acetic acid through co-activation of CH4 and CO2 on Cu/M+-ZSM-5 zeolite is represented in (Scheme 1). In the initial step, CH4 activation on Cu/M+-ZSM-5 zeolite creates surface a (-Cu-CH3) species (Step 1). Simultaneously, CO2 activation on M+-ZSM-5 zeolite helps in the formation of surface carbonate species where the basic cations help in surface CO2 adsorption (Steps 1 and 2). These carbonate species acts like a reservoir which in turn promotes insertion into Cu-CH3 bond of copper methyl species (Step 3). These activated surface species transforms into giving a surface acetate intermediate (-Cu-OOCCH3) (Step 4). Which then abstracts a proton from the dissociative cleavage of methane and finally the acetic acid product is desorbed from the surface recovering the active site for the continuation of the catalytic cycle (Step 5). The proton transfer step is the vital step in the acetic acid synthesis. The Cu-exchanged zeolite helps in stabilizing these protons (Figure S5-S7).

chemical-technology-ostensible-mechanism-acetic

Scheme 1: An ostensible mechanism for the Co-Conversion of CH4 and CO2 into acetic acid over Cu-M+-ZSM-5 Zeolite: (Steps 1 to 3) activation of CH4 leads to the formation of (-Cu-CH3), while the activation of CO2 helps in the formation of surface carbonate species over cations site. (Steps 4 to 5): insertion of CO2 into the (-Cu-CH3), produces surface acetate species (-Cu-OOCCH3) as a reaction intermediate which abstracts the proton to form acetic acid.

The simultaneous activation of CH4 and CO2 is of great significance as both contributes towards global warming and can be converted into fuels which are of particular interest in the chemical industry. The C-H activation over transition metals with fcc(111) or hcp(0001) surfaces is reported by Gong et al. where he collected some of the reaction energies calculated previously which hinders methane decomposition [38]. The direct formation of acetic acid in one step with simultaneous activation of methane and carbon dioxide is a thermodynamically unfavorable process and has an atom economy of 100% thus making it important in academic and industrial research. To the best of our knowledge, this is the first report where the continuous formation of acetic acid has been demonstrated experimentally using a unique bifunctional zeolite catalyst. The near-perfect selectivity of acetic acid at equilibrium conditions is attributed to the Cu-exchanged zeolite catalyst which was found to be capable of simultaneous activation of both the reactant species.

Conclusion

In summary, we have developed a new direct method for synthesis of acetic acid by co-activation of methane and carbon dioxide in the one-step process. Cu nanoparticle loaded basic cation ZSM-5 catalysts were proven to activate both methane and carbon dioxide simultaneously in concurrent feed, which enabled to the formation of acetic acid in a continuous flow reactor. The activation of CO2 was observed to be closely related to the alkali cation in the MFI structure. The formation rate of acetic acid was observed to be in the following order with respect to the cationic species K > Na > Ca > Li, this indicates that basic cation contributes to the activated CO2 on the surface and thus resulted in high insertion activity. The Cu-M+-ZSM-5 catalyst exhibited high starting activity which decreased within an hour but sustained state-state yield on acetic acid of 395 μ mole gcat-1h-1 for 10 h. The deactivation was caused mainly due to the aggregation of Cu° species. Upon calcination, the catalyst activity was recovered up to more than 70% which was mainly due to re-dispersion of Cu nanoparticle over the catalyst surface.

References