Investigation of the stability of Zn-based HZSM-5 catalysts for methane dehydroaromatization

Applied Catalysis A: General 505 (2015) 365–374

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Investigation of the stability of Zn-based HZSM-5 catalysts for methane dehydroaromatization Victor Abdelsayed a,b,∗ , Mark W. Smith a,b , Dushyant Shekhawat a a b

National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Rd., Morgantown, WV 26507, USA AECOM, 3610 Collins Ferry Rd., Morgantown, WV 26507, USA

a r t i c l e

i n f o

Article history: Received 24 March 2015 Received in revised form 7 August 2015 Accepted 8 August 2015 Available online 11 August 2015 Keywords: Methane utilization Dehydroaromatization reactions Zn catalyst Benzene ZSM-5 Non-oxidative methane conversion

a b s t r a c t Non-oxidative methane conversion into aromatic compounds was studied over Zn/HZSM-5 catalysts at 700 ◦ C, 3000 scc/gcat /h and atmospheric pressure. In addition to reaction studies, the stability of Zn at different loadings (1, 2, 3, and 8 wt%) was investigated by XRD, ICP-OES, EDS, TGA, BET, and NH3 -TPD characterization techniques. The results suggest the presence of two Zn species during reaction: (1) loosely bound and easily reduced ZnO particles; (2) anchored and thermally stable [Zn(OH)]+ . At low loading (1 and 2 wt%) anchored Zn is the dominant, thermally stable specie on the catalyst surfaces showing the most retained Zn after the reaction. At high loading (3 and 8 wt%) most of the Zn is in the form of ZnO particles susceptible to reduction to Zn metal, which slowly vaporized under reaction conditions. The catalyst with 3 wt% Zn produced the highest benzene yield; however, it decreased rapidly, due to coke formation, compared to the 1 wt%, which showed more yield stability. Small amounts of CO2 (0.5–2%) were added to the reaction stream to help stabilize ZnO and reduce coke formation during the reaction over 3 wt% Zn/HZSM-5. Results showed that the addition of CO2 resulted in retaining more Zn on the spent catalyst and improved the catalytic performance stability, but it significantly decreased the aromatic yield, indicating that the ZnO particles are not the active Zn species. Instead, the reactive specie was concluded to be the anchored [Zn(OH)]+ acting as a strong Lewis acid. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Methane, a greenhouse gas and the main constituent of natural gas, can be converted into valuable chemicals by either an oxidative, indirect route, or a non-oxidative, direct route [1–5]. Catalytic methane dehydroaromatization (MDA) is a promising direct route to convert methane into aromatic feedstocks to be used in various petrochemical industries. However, excessive coking, which rapidly deactivates catalysts, along with the equilibrium-limited nature of the reaction, are significant obstacles for commercialization [1–5]. Many researchers have focused on finding ways to improve catalyst performance and reduce deactivation from coke. These approaches include modifying the surface of the zeolite support [6–8], adding promoters [9–12], adding trace amounts of oxidants [13–15], periodic methane/H2 switching [13,16], optimizing the number and density of acid sites on HZSM-5 by changing

∗ Corresponding author at: National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Rd., Morgantown, WV 26507, USA. Tel.: +1 3042855273; fax: +1 3042854850. E-mail address: [email protected] (V. Abdelsayed). http://dx.doi.org/10.1016/j.apcata.2015.08.017 0926-860X/© 2015 Elsevier B.V. All rights reserved.

the silica to alumina ratio (SAR) [17], and shifting the reaction equilibrium by removing hydrogen from the reaction using membrane reactors [18,19]. Nevertheless, the search for a more robust catalyst is still required. Generally, MDA requires a bi-functional catalyst [2,5,20,21]. Baba et al. [22] demonstrated through 13 C isotope-labeling studies that the bi-functionality of acidic protons from zeolite supports, and supported metal cations are essential for MDA. Mo-based catalysts are the most studied catalysts for this reaction due to high activity and selectivity to aromatic products [5,20,23]. However, the carbidic nature of their active sites (Mo oxycarbide [5,20,24]) has disadvantages, such as preferential coking and deactivation of these sites, as well as challenges to catalyst regeneration since deactivated Mo carbide must be oxidized and re-carburized [25]. Zn supported HZSM-5 is a non-carbidic, more easily regenerable catalyst. Most of the existing Zn-based HZSM-5 catalysts are reported for dehydroaromatization of light alkanes (C3+ ) with a reaction temperature ranging between 500 and 600 ◦ C [26–28]. The main drawback of light alkanes dehydroaromatization over Zn/HZSM-5 include the high yield of methane, which is a side product formed during reactant cracking. This indeed will lower

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the economic value of the process. A second issue is the rapid coke formation from these light alkanes compared to MDA, which deactivate the catalyst rapidly, even at lower reaction temperatures [26–28]. Finally, the stability of Zn-based catalysts represents another challenge, where Zn oxide can get reduced into Zn metal and vaporized under the reductive environment of the reaction [29,30]. The nature of the active Zn species during dehydroaromatization of light alkanes is still unclear due to the complex nature of the reaction, different proposed mechanisms, and the presence of many intermediates. Berndt et al. [30,31] studied the nature of the active Zn sites on HZSM-5 during propane conversion to aromatics. Three Zn species were proposed on the HZSM-5 surface: Zn anchored to Bronsted acid sites (BAS) inside the micropore channels [Zn(OH)]c + , Zn anchored to BAS on the external surface [Zn(OH)]s + , and unreactive residual ZnO crystals outside the HZSM-5 pores. Under activation with hydrogen the ZnO species was converted to the first, reactive Zn species, while the second Zn species remained constant. This transformation from ZnO to anchoring Zn inside the pores resulted in an increase in the active Lewis acid sites (LAS). Recently, three types of Zn species were identified for MDA using in-situ solid state NMR spectroscopy supported by DFT calculations [32]: isolated Zn2+ ions, isolated Zn+ ions, and Zn+ -O− -Zn2+ clusters. The isolated Zn2+ was responsible for the formation of zinc methyl species via heterolytic dissociation of C–H bond, while dizinc cluster was associated with the formation of methoxy species via hemolytic cleavage of C–H bond. Finally, the Zn+ ion acted as a spectator due to the significantly high activation energy required for C–H bond cleavage. Different methane activation mechanisms have been reported in literature for Zn catalysts. Liu et al. [33] proposed a reaction mechanism for MDA at 700 ◦ C that involved an active [ZnO-CH3 + ···− HZnO] intermediate for methane dissociation and dehydrogenation. Two Zn species were observed using the Zn LMM Auger signal analysis, which were H3 C-OZn+ and HZnO− each in a different coordination environment. Similarly, Zn methoxy species were observed using 13 C solid state NMR by the dissociative adsorption of methane on ZnO species located inside the zeolite channels [27]. A carbenium ion mechanism has also been proposed [23,34] where Zn2+ cations act as a hydride acceptor to give [Zn-H]+ and a methyl group. Santen et al. [35] used DFT calculations to propose that Zn ions can polarize the C–H bond of methane leading to heterolytic cleavage and the formation of CH3 − anions, which rapidly attach to Zn2+ ions to produce zinc methyl species [ZnCH3 ]+ . Ivanova et al. [36] showed that the formation of these surface zinc methyl species did not lead to any further reaction, up to 500 ◦ C. Similar results were reported showing that even at 600 ◦ C Zn was inactive for producing aromatics [37]. Our approach was to study indirectly the active sites for noncarbidic zinc oxides during MDA. Previously we reported the presence of two Zn species, a loose and a bound form of Zn oxide, over Zn-promoted Mo/HZSM-5 catalysts during the MDA reaction [38]. In the present work, we further investigate these Zn species in terms of reactivity and thermal stability. Our first objective was to identify these Zn species by investigating the effect of Zn loading and to correlate this with their catalytic activity. Our second objective was to study the effect of CO2 addition on the catalyst activity and stability as well as the thermal stability of these Zn species during MDA reaction. The CO2 addition indirectly helps determine the active species during the reaction. The CO2 is an oxidant molecule that stabilizes the oxide forms of Zn against reduction and vaporization. To the best of our knowledge, no data have been reported on the effect of CO2 addition during methane dehydroaromatization over Zn/HZSM-5. Additionally, scarce data have been reported on MDA using Zn/HZSM-5 catalysts.

2. Experimental 2.1. Catalyst synthesis The ammonium ZSM-5 zeolite with SiO2 /Al2 O3 mole ratio (SAR) of 55 was supplied by Zeolyst Inc. The NH4 -ZSM-5 powder was calcined at 500 ◦ C in air for 3 h to convert the powder from the ammonium form to its protonated form (HZSM-5). Conventional incipient wetness impregnation was used to prepare Zn catalysts. Typically, zinc nitrate hexahydrate salt (Zn(NO3 )2 ·6H2 O), purchased from Alfa, corresponding to 1, 2, 3, or 8 wt% Zn was dissolved in deionized water and was added drop wise to HZSM5. The powder was mixed using a mortar and pestle for 15 min before drying it at 75 ◦ C overnight. The powder was then calcined in air at 550 ◦ C for 4 h. For simplicity, abbreviations for each catalyst are given here as Zn followed by its weight loading percentage on HZSM-5 (i.e. Zn3 catalyst would have 3 wt% Zn on HZSM-5). 2.2. Catalyst characterization 2.2.1. X-ray diffraction (XRD) Powder X-ray diffraction analysis was performed on a Panalytical X’pert Pro (PW3040) X-ray diffraction system utilizing Cu K␣ radiation. Samples were placed on a zero diffraction Si holder and were scanned from 5◦ to 30◦ (2). Analysis was carried out with Highscore Plus Analysis software equipped with a standard ICDD X-ray diffraction database supplied by Panalytical. 2.2.2. Surface area and micropore analysis Nitrogen adsorption experiments were performed at 77 K using Micromeritics ASAP-2020 unit. Catalyst samples were vacuumdegassed at 300 ◦ C for 10 h to remove the surface humidity and pre-adsorbed gases before exposure to adsorption gas. The surface area was calculated from the N2 isotherm data using the Brunauer-Emmett-Teller (BET) model [39]. The micropore volumes and micropore areas were measured using t-plot analysis [40]. 2.2.3. Ammonia temperature programmed desorption (NH3 -TPD) NH3 -TPD experiments were carried out using a flow system with a thermal conductivity detector (Micromeritics Autochem 2910). Prior to each TPD run, the catalyst (∼0.1 g) was heated to 500 ◦ C for 60 min under pure He (50 sccm, ramp 5 ◦ C/min) to remove all moisture. The catalyst was then cooled to 150 ◦ C and exposed to 30 sccm 15% NH3 in He for 30 min. The catalyst was purged with pure He to remove excess ammonia before the temperature was ramped up to 750 ◦ C at 5 ◦ C/min ramp rate to obtain NH3 -TPD data profile. 2.2.4. Thermo-gravimetric analysis (TGA) Catalyst reduction was conducted in a thermo-gravimetric analyzer (TA Model 2050). Approximately 0.5 g of the fresh catalyst was heated in a quartz bowl from ambient to 700 ◦ C at a heating rate of 5 ◦ C/min in 10% H2 /N2 with a flow rate of 25 sccm. The sample temperature was then maintained isothermally for 200 min at 700 ◦ C. 2.2.5. ICP-OES and EDS analysis Inductively coupled plasma optical emission spectrometry (ICPOES) from PerkinElmer (Optima 7300 DV) was used to analyze the bulk composition. The samples were fused with LiBO2 at 1100 ◦ C for 4 min. The ratio of sample to flux was 50 mg sample and 400 mg LiBO2 flux. Fused samples were then digested in 5% nitric acid on low heat with vigorous stirring and further diluted 20fold using a mixture of high purity 2% nitric and 1% hydrochloric acids prior to analysis. The elemental surface composition was

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obtained from energy dispersive spectroscopic (EDS) analysis utilizing a JEOL FE-7600 scanning electron microscope interfaced to a Thermo-Electron Noran System Seven (NSS) X-ray microanalysis system. The EDS detector utilized in the X-ray microanalysis was a Thermo-Electron Ultradry Energy dispersive spectrometer, which was calibrated utilizing the Cu K␣ line at 8.041 kV. 2.3. Catalyst evaluation 2.3.1. Experimental setup and reaction conditions All reactions were carried out in a continuous fixed-bed flow reactor (6.25 mm ID) system from Autoclave Engineers, Inc. (model # 5010-2334) at 700 ◦ C and at atmospheric pressure. The catalyst powders were pressed and sieved between −20/+60 mesh particle size. Typically, 1.0 g catalyst was heated in the reactor under 50 sccm N2 to 700 ◦ C at a ramp rate of 5 ◦ C/min. Once the catalyst temperature reached 700 ◦ C the reaction was conducted for 12 h under methane with 10% Ar (used as internal standard for GC) resulting in a WHSV of 3000 scc/g/h. The CO2 addition experiments were done by flowing a fixed methane/Ar flow at 40 sccm and changing the CO2 /N2 ratio to maintain the desired CO2 concentrations in the total reaction gas stream. The total flow of CO2 /N2 was kept at 10 sccm. The CO2 concentrations tested in this work were 0, 0.5, 1.0, and 2.0%. All gases used were purchased from Butler Gas Products Co. with UHP grade. 2.3.2. Product analysis All reactants and products were detected on the same GC (Perkin Elmer-Claurs 500) equipped with dual detectors (flame ionization (FID) and thermal conductivity (TCD) detectors). The transfer line between the reactor and the GC was heat traced to prevent product condensation that might cause plugging. The product gas stream (H2 , methane, Ar, ethane, ethylene, N2 , CO2 , and CO) was detected using the TCD equipped with HayeSep N 60/80, HayeSep T 60/80, Molecular Sieve 5A 45/60, and Molecular Sieve 13x 45/60 packed columns. Benzene and toluene were detected with the FID equipped with GS-GASPRO capillary column (30m, 0.32 mm ID) from Agilent Technologies. Insignificant amounts of naphthalene were observed on the exit lines of the reactor but could not be quantitatively measured by GC due to its high boiling point. The GC data were processed using TotalChrom Workstation software. The GC was calibrated with the appropriate standard gas mixtures before each run. The product yield and conversion were calculated based on carbon moles. 2.3.3. Temperature programmed oxidation (TPO) In-situ TPO runs were measured directly after methane dehydroaromatization reaction. The reactor was first cooled from 700 to 500 ◦ C under N2 for 30 min before ramping again to 700 ◦ C at 5 ◦ C/min under 50 sccm of air and 50 sccm of N2 , and the temperature was kept at 700 ◦ C for 60 min. The TPO data were recorded using Pfeiffer Omnistar mass spectrometer (MS) coupled with the reactor. The spent catalyst was defined as the catalyst collected from the reactor after the MDA run followed by TPO. 3. Results and discussion 3.1. Pre-reaction catalyst characterization 3.1.1. X-ray diffraction (XRD) analysis Fig. 1a shows the X-ray diffraction patterns of HZSM-5 before and after loading with 1, 2, 3, and 8 wt% of Zn. The similarity in the spectra before and after Zn loading indicates that the MFI crystal framework of ZSM-5 is preserved after the chemical and thermal treatments during the impregnation and calcination processes. Moreover, no diffraction peaks for Zn oxide crystallites were

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observed, even at 8 wt%, indicating either good Zn dispersion, or the formation of amorphous ZnO on the zeolite surface. In order to compare the diffraction patterns of these catalysts, all XRD data were normalized at 2 ≈ 23.2◦ . At Zn loading of 3 wt% and higher, the diffraction peak intensity decreased significantly compared to pure HZSM-5, which suggests a lower crystallinity for Zn3 and Zn8 catalysts [41,42]. For both Zn1 and Zn2 catalysts, the diffraction peak intensities are very similar to pure HZSM-5, consistent with the literature on similar Zn-loaded HZSM-5 catalysts [26]. This could indicate that at 3 wt% and higher the ZnO particle concentration on the external surface of the HZSM-5 increased significantly, while at 1 and 2 wt% most of the Zn is anchored to the zeolite acid sites. During the impregnation method used in this study some of the Zn ions were presumably exchanged with the acidic groups of the HZSM-5, some might have been incorporated into the HZSM-5 framework [43–47], and some likely remained on the external surface as ZnO particles, depending on the level of Zn loading. As the Zn loading increased the likelihood of ZnO particles depositing on the external surface increased. It is generally accepted that low angle diffraction peaks are sensitive to any species inside the HZSM-5 channels [47]. Fig. 1b shows the corresponding small angle diffraction patterns between 2 = 7.5 and 9.5◦ for these catalysts. A shift in diffraction peaks was observed from 2 ≈7.96◦ in fresh HZSM-5 to 2 ≈ 8.11◦ in 8 wt% Zn/HZSM5. This shift indicates that the micropore channels of HZSM-5 decreased with loading, possibly due to pore blockage from Zn deposition on both the external surface, as well as inside the HZSM5 channels [42,48]. It is also possible that the Zn was incorporated into the framework of the HZSM-5 [43–47], which would have an impact on the alumina framework of HZSM-5 associated with the Bronsted acid sites [49,50]. The slow ramping rate (2 ◦ C/min) used during the calcination of these catalyst could have played a role in incorporating Zn into the HZSM-5 framework. In another study [43] a low ramp rate of 1 ◦ C/min the zeolite was reported to produce a greater number of Bronsted acid sites and fewer Lewis acid sites compared to when a higher ramp rate (30 ◦ C/min) was used. 3.1.2. Surface area and micropore analysis All catalysts exhibited a typical type I isotherm with a small hysteresis loop in the range of P/Po = 0.4–1.0 revealing some mesoporosity due to the adsorption of N2 on the external surface of their crystallites and capillary condensation in spaces between crystallites [51,52]. The BET surface areas (SBET ), microporous surface areas (Smicro ), microporous volume (Vmicro ), and total pore volume (Vtotal ) of fresh and Zn-loaded HZSM-5 materials are listed in Table 1. The BET surface area and total pore volume decreased as the Zn loading increased, possibly due to partial pore blockage. Increasing the Zn loading on zeolite was reported to partially block some of the pore structures of HZSM-5 [31,53]. Similarly, the microporous textural properties (Smicro and Vmicro ) of HZSM-5 decreased with Zn loading possibly due to Zn deposited inside the microporous channels. Zn1 and Zn2 catalysts shared relatively similar surface properties suggesting that most of the Zn was well dispersed on the surface and anchored to the BAS of the HZSM-5 framework. At Zn loading above 2 wt% the relative dispersion of Zn decreased. A decrease in the measured value for these microporous surface properties was first observed with Zn2 catalyst and became more obvious with Zn8 catalyst. This suggests that Zn2+ not only anchored on the surface or inside the pores as [Zn(OH)]+ , but also a considerable portion of it is present on the external surface of the HZSM-5 as ZnO particles. These results suggest that the proportional ratio between Zn present as ZnO particles compared to that incorporated or anchored to the Al acid groups (ZnO/[Zn(OH)]+ ) increased with the Zn loading. For example Zn1 would have a greater percentage of its Zn content incorporated into the HZSM-5 structure compared to Zn8 ([Zn(OH)]+ > > ZnO).

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Fig. 1. (a) X-ray diffraction patterns for freshly calcined HZSM-5 and Zn-loaded HZSM-5 with 1, 2, 3, 8 wt%, (b) small angle X-ray diffraction (SAXRD) patterns for these catalysts.

Table 1 Surface properties and elemental composition of fresh and spent Zn/HZSM-5 catalysts with different Zn loadings. Catalyst

SBET (m2 /g)a

Smicro (m2 /g)b

Vmicro (cm3 /g)b

Vtotal (cm3 /g)c

HZSM-5 Zn1 Zn2 Zn3 Zn8

403 392 391 381 328

284 272 251 250 236

0.114 0.110 0.102 0.101 0.095

0.251 0.248 0.246 0.237 0.193

a b c d e

ICP-OES (Zn wt%)

EDSd (Zn wt%)

Fresh

Retainede

Fresh

Retainede

1.1 2.3 3.3 8.8

87.5 47.6 34.0 21.2

1.2 2.4 4.4 9.3

78.0 52.5 43.2 14.0

Calculated using BET method in the range up to P/Po = 0.1. Calculated using t-plot method. Total pore volume up to P/Po = 0.985. Average metal wt% concentrations and errors were calculated from at least 10 EDS point measurements. Retained Zn is the Zn % remained in the catalyst after reaction for 12 h at 700 ◦ C.

3.1.3. Thermal gravimetric analysis (TGA) under reductive atmosphere The thermal stability of Zn oxide species were tested against reduction and vaporization under reductive environment similar to MDA reaction conditions. Zn loss was confirmed by TGA over Zn-loaded HZSM-5 catalysts under 10% H2 (balance nitrogen) as the reducing gas. All sample weights were normalized at 300 ◦ C in order to avoid any weight change due to water adsorbed on the surface or inside the pores of HZSM-5. Fig. 2 shows the percent catalyst weight loss as a function of temperature. During the temperature ramp from 300 to 700 ◦ C (5 ◦ C/min) the weight loss varied depending on the Zn loading amount. As the Zn loading increased the percent Zn loss increased, with a maximum loss of 0.7% observed in 8 wt% Zn catalyst. In general the weight loss observed during the ramping is relatively small, which agrees with literature [51] where ZnO reduction was slow due to its high lattice energy. Zn1 catalyst showed a slightly larger weight loss compared to pure HZSM-5 during temperature ramping. However, during isothermal reduction at 700 ◦ C the weight loss observed for Zn1 and HZSM-5 were very similar. This indicates that all the loosely bound Zn that could be reduced and vaporized was removed from the Zn1 catalyst surface and the residual Zn was thermally stable and bound to HZSM-5. Under

Fig. 2. (a) TGA profiles of different Zn-loaded HZSM-5 (0–8 wt% Zn) measured from 300 to 700 ◦ C at 5 ◦ C/min and (b) isothermal TGA profiles measured at 700 ◦ C for 200 min under reductive atmosphere of 5% H2 /N2 (3000 scc/g/h and 0.1 MPa).

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isothermal reduction of Zn samples at 700 ◦ C, an abrupt weight loss was observed indicating more Zn oxide was reduced to Zn metal and vaporized, which depended on the Zn loading, thermal stability, and the vapor pressure of Zn metal at 700 ◦ C. The reduction step of ZnO was observed to be a relatively slow step [54], which suggests a shrinking core-shell reduction model with ZnO as the core and Zn metal as the shell, making it diffusion limited. The observed weight loss was significant during the isothermal reduction of Zn loaded catalysts with the highest loss observed with Zn8. The percent weight loss was found to be in the following order: Zn8 (2.67%) > Zn3 (1.44%) > Zn2 (0.91%) > Zn1 (0.48%) after 200 min at 700 ◦ C. 3.1.4. Ammonia-temperature programmed desorption (NH3 -TPD) analysis The total number of acid sites on the catalysts studied was measured by the NH3 -TPD technique. This characterization can provide information about acid site strength and Zn distribution on the catalyst surface, as well as its interaction with the HZSM-5 surface. Fig. 3a shows the NH3 -TPD plot for fresh and Zn-loaded HZSM-5. Two distinct desorption peaks were observed for fresh HZSM-5. The low temperature desorption peak at about 220 ◦ C corresponds mainly to Lewis acid sites (LAS), and the high temperature peak at about 400 ◦ C corresponds to Bronsted acid sites (BAS) [55–57]. As Zn loading increased the acidity of the catalyst changed, as well as the ratio of BAS to LAS. With Zn1 the LAS peak increased due to the presence of ionic Zn, which acts as a new, strong LAS. The BAS peak decreased due to Zn anchoring on some of the OH groups inside the channels and on the external surfaces of HZSM-5. Similar results were reported [31] for addition of Zn to HZSM-5 where the BAS peak decreased and the LAS peak widened. Both peaks were shifted to higher temperatures upon Zn addition. The area of the LAS peak increased significantly from Zn1 to Zn2, but increased only slightly from Zn2 to Zn8. Increasing the Zn loading to 2 wt% (Zn2) resulted in a decrease in the BAS peak even more. Xiong et al. [58] and Ni et al. [43] both observed that the addition of Zn+2 resulted in the elimination of most of the strong surface BAS. Zn loading between 1 and 2 wt% is enough to interact significantly with BAS of HZSM-5, consistent with reported results from IR-spectroscopy [59]. The concentration of BAS was calculated from Fig. 3a for fresh HZSM-5 to be 0.42 mmol/g, while the Zn concentration in Zn1, Zn2, Zn3 and Zn8 catalysts was calculated to be 0.15, 0.30, 0.45 and 1.22 mmol/g, respectively. 3 wt% Zn loading was enough to use most of the BAS on HZSM-5, while the 8 wt% was overloaded with Zn. Similar NH3 -TPD profiles were observed for 3 and 8 wt% Zn-loaded catalysts. However, due to the continuous loss of Zn from the HZSM5 surface, it is difficult to correlate Zn concentration with its surface acid properties during the reaction. 3.2. Reaction tests 3.2.1. Effect of Zn loading Fig. 4 displays the effect of time on stream (TOS) on methane conversion and benzene yield with fresh and Zn-loaded HZSM-5 catalysts. Very low levels of toluene and naphthalene were also produced during reaction. Toluene yield trended with and was consistently ca. 10% of the benzene yield. Naphthalene could not be measured with the analytical system used since it was condensed out before analysis; however, it was observed to be deposited on the exit lines of the reactor. A negligible amount of methane conversion and benzene yield were observed over HZSM-5 demonstrating the necessity of Zn in benzene formation due to its dehydrogenation activity and the bifunctional nature of Zn/HZSM-5 catalyst [22,26,55,57,58,60]. As the Zn loading increased from 1 to 8 wt%, the methane conversion

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increased from 0.4 to 1.7%, consistent with literature [31,61]. Guo et al. [37] also studied the effect of Zn loading during the codehydroaromatization of methane and propane and showed that below 3 wt% Zn loading the conversion of methane is relatively low. The presence of higher Zn loading on the HZSM-5 increases the methane activation resulting in higher conversion. The dependence of benzene yield on Zn content showed an optimum Zn loading believed to be related to a balance between BAS on the zeolite surface and Zn content that can activate the methane [31]. Fig. 4b shows a slow deactivation rate for Zn1 and Zn2. Comparing these results with the literature results for Mo-based catalysts [57], which are prone to coking, a faster deactivation rate was observed over Mo/HZSM-5 catalyst. Zn-based catalysts showed a lower formation rate of benzene, but a relatively longer yield stability than Mo-based catalysts. For all Zn catalysts methane conversion was relatively stable, while the benzene yield decreased with TOS as a result of reduced selectivity. Zn3 suffered the greatest decrease in selectivity, which is typically a result of pore blocking where aromatization occurs on the BAS. It is not obvious from the characterization why this occurs more for Zn3 than the other catalysts. Further study beyond the scope of this work, such as FTIR and Raman spectroscopy, would provide additional insight into this trend. An induction period was observed over all Zn/HZSM-5 catalysts in Fig. 4b, which could be due to the reduction of loosely bound Zn species [38]. These species are likely ZnO on the external surface of the catalysts. This induction period increased with Zn loading suggesting that Zn8 had the highest ZnO on its surface. As the reaction proceeded, the quantity of unreactive ZnO started to decrease by reduction [31] and subsequent vaporization leaving only the reactive and stable anchored Zn species. Our earlier work on Znpromoted Mo/HZSM-5 catalysts [38] showed the presence of at least two types of Zn species: unreactive species, loosely-bound, but easily reducible, and active species, strongly-bound, but not easily reducible. These results agree with the literature [30,31] where anchored Zn species were hardly reducible at low Zn loading (3 wt%) on HZSM-5. Berndt et al. [30,31] suggested that during activation with hydrogen some of the unreactive ZnO is reduced and some of this reduced Zn is converted to reactive anchored Zn2+ inside the pore system of HZSM-5. Similarly, under the reducing MDA reaction conditions some of the Zn vapor could enter inside the micropores of HZSM-5 and anchor to acid sites. These newly-formed, anchored [Zn(OH)]+ , or those which were previously covered by unreactive ZnO, started to play a role in conjunction with the acid sites of HZSM-5 towards MDA reaction. As the reaction proceeded further, active sites started to deactivate due to carbon formation. The initial benzene yield observed after 16 min of reaction was in the order Zn3 > Zn8 > Zn2 > Zn1. After 12 h Zn catalysts with 1, 2 or 3 wt% showed almost the same benzene yield, which could indicate that most of the free ZnO on HZSM-5 was reduced to metal and vaporized, and that the remaining bounded Zn species in these three catalysts are very similar as they share the same support properties. However, it is not fully clear why Zn3 had a higher methane conversion than Zn1 and Zn2. Extent of dispersion of the anchored Zn species and metallic Zn on the catalyst surface likely played a role. In comparison, after 12 h Zn8 produced the highest yield, which is probably because it had the highest Zn content (1.9 wt%), as confirmed by ICP-OES results (Table 1). With Zn1 and Zn2, the concentration of active Zn sites might not be the optimum loading to produce higher benzene yields, as observed for Zn3 catalysts, while with Zn8 catalyst initial Zn overloading resulted in the blocking of BAS needed for the MDA reaction. However, as unreactive ZnO reduced and vaporized, anchored [Zn(OH)]+ started to play a role in the reaction. The highest deactivation rate for benzene yield was observed over Zn3 catalyst. The

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Fig. 3. NH3 -TPD profiles for freshly prepared (a) and for spent catalysts (b) for Zn-loaded HZSM-5 (0–8 wt%).

Fig. 4. (a) methane conversion and (b) benzene yield over different Zn-loaded HZSM-5 (0, 1, 2, 3, 8 wt%) 700 ◦ C, 0.1 MPa, 3000 scc/h/g. (a) and (b) used the same legends.

number of active Zn sites responsible for methane activation and the number of Bronsted acid sites responsible for aromatization has to be balanced during the reaction on a stable and reactive catalyst [37]. 3.2.2. Effect of CO2 addition To confirm that ZnO particles that are not anchored to the zeolite structure have no catalytic role in benzene formation, the effect of CO2 was studied. CO2 was used as an oxidant probe molecule in this study to stabilize ZnO particles against reduction and vaporization during the MDA reaction. Zn3 catalyst was chosen to study the effect of CO2 addition since it had the highest benzene yield and modest Zn loss. Small amounts of CO2 were used (0.5–2.0 vol%) producing CO2 /CH4 ratio between 0.007 and 0.028, respectively. The EDS and ICP-OES results in Table 3 show that the Zn retained in the spent catalyst increased with CO2 level, which suggests that the CO2 played a role in stabilizing the loosely bound ZnO. Fig. 5a shows the methane conversion over Zn3 under different CO2 concentrations. In the absence of CO2 , the conversion

increased rapidly to reach a maximum at about 1%, and then slowly decreased with TOS to about 0.7% after 12 h. In the presence of CO2 the methane conversion decreased as the CO2 concentration increased. These results are consistent with Ohnishi et al. [62] where the addition of CO2 reduced the accumulation of coke on the active catalyst sites by reverse Boudouard reaction. In general, the addition of CO2 stabilized the methane conversion, especially at 0.5% CO2 . Although the addition of CO2 decreased the benzene yield (Fig. 5b) it was stable throughout each run. The benzene yield under no CO2 addition increased slowly to a maximum, and then started to decrease. The decrease in methane conversion and benzene yield for Zn3 catalyst under no CO2 addition observed in Fig. 5 compared to Fig. 4 is due to the addition of CO2 /N2 to the gas stream which slightly changed the WHSV of methane. Under all CO2 concentrations, the benzene yield increased slowly then became stable for the entire reaction time. An induction period was observed in Fig. 5b, which increased as the CO2 concentration increased indicating that the CO2 is slowing down the rate at which zinc is lost by vaporization. Because of this, it takes longer for the Bronsted acid sites to

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371

Fig. 5. Effect of CO2 addition on (a) the methane conversion and (b) benzene (BZ) yield during methane dehydroaromatization reaction: 3000 scc/g/h, 700 ◦ C, and 0.1 MPa.

be uncovered by the excess ZnO, which leaves them unable to participate in the aromatization step of the reaction. Conversion was relatively constant with TOS, this increase in yield over the course of the run is due to an increase in the selectivity to benzene. Since aromatization occurs on the BAS in the zeolite pores, this supports further that the loss of Zn corresponds to an increase in the accessibility of these sites, likely due to the unblocking of the pores from excess ZnO particles. Two factors contributed to the deactivation of the Zn catalyst in the absence of CO2 : coke formation and Zn loss mainly from unreactive ZnO [38]. ZnO species can block some of the micropores of HZSM-5, but once reduced and vaporized during the MDA coke can form on these acid sites previously covered by ZnO. Also, Zn metal has more dehydrogenation activity to decompose methane into coke and H2 . Increasing CO2 concentration resulted in stabilization of loosely bound ZnO from reduction and loss. The CO2 concentration correlated positively with the stability of the ZnO species on the external surface, and to lower benzene yield. This suggests that the active Zn species for MDA is not the ZnO or the Zn metal, but is the stable LAS of [Zn(OH)]+ anchored to the acidic Al on the catalyst surface. The formation of ZnCO3 during reaction of ZnO and CO2 as an active Zn species is excluded due to its lower decomposition temperature 255–500 ◦ C [63,64]. 3.3. Post reaction catalyst characterization 3.3.1. ICP-OES and EDS analysis 3.3.1.1. Catalysts with different Zn loading. The surface and bulk elemental compositions of fresh and spent catalysts were measured using EDS and ICP-OES, respectively (Table 1). The spent catalysts were collected after reaction and burning off of the carbon in 1:1 air to N2 ratio at 700 ◦ C for 60 min. The concentration of Zn measured by ICP-OES and EDS analysis for fresh samples was close to the nominal Zn concentration from synthesis. Spent Zn1 catalyst had the highest retained Zn of 87.5% indicating the highest interaction with the HZSM-5. This catalyst also had the highest Zn dispersion and the highest performance stability under reaction. This could suggest that the Zn is mostly in the anchoring oxide form rather than the ZnO form on the zeolite surface. As the Zn loading increased the percentage of Zn retained decreased, possibly from the zeolite surface being overloaded. This was most pronounced for Zn8. At a loading of 8 wt% the Zn would be deposited to a greater degree on the external surfaces of the catalyst. This Zn would be easier to reduce and vaporize under reaction conditions. Only 21.2% of Zn in Zn8 was retained in spent Zn8 as measured from the ICP analysis. TPR results from Berndt et al. [30] showed that the amount of Zn oxide species reduced at 550 ◦ C was trivial under low Zn

loading of 1.5 wt% (less than 5% of the total Zn content on HZSM-5), which is consistent with our TGA and ICP-OES/EDS results. Higher reaction temperature and Zn loading results in significant loss of Zn content, up to 78.8 and 86 wt%, as shown from ICP-OES and EDS analysis, respectively, indicating that it is from unstable ZnO species on the surface rather than anchored or incorporated Zn cations in the HZSM-5 structure. Similarly, Wan et al. [54] studied the reaction kinetics and mechanism of propane dehydrogenation over partially reduced ZnO (2 wt%) deposited on silicalite. Since hydrogen is one of the main products from propane dehydroaromatization, Zn oxide species will be partially or nearly completely reduced under reaction conditions. Their H2 -TPR results showed that the reduction of ZnO species was not complete, even under 50% H2 -N2 flow at 550 ◦ C for 4 h. The reduction of ZnO species started at 200 ◦ C and extended to 800 ◦ C, which agrees well with our results. During activation with H2 some ZnO on the external surfaces of HZSM-5 was reported to be reduced to metallic Zn [30] and migrate as a vapor into the zeolite channels, reacting with Bronsted acid sites to form anchored Zn ions. The mobility of Zn in Zn/HZSM-5 catalysts has been reported to occur at high reaction temperature [29]. Seddon et al. [29] studied the conversion of light alkanes (C3–4 ) into aromatics, which is a strongly reducing environment, since H2 is produced in the first step of the reaction, along with the formation of coke. These strong reducing agents (H2 and carbon) can convert ZnO into metal. Because of the low melting point and boiling point of Zn (m.p. 419 ◦ C, b.p. 907 ◦ C) the Zn can volatilize from the catalyst if it is not strongly bound or anchored to the surface. The vapor pressure of Zn at 727 ◦ C (2.5 mmHg), which is close to the reaction temperature in this work, is much higher than the vapor pressure of its oxide form (6 × 10−12 mmHg) [29]. 3.3.1.2. Zn3 with different CO2 additions. Addition of CO2 stabilized the Zn in its oxide form either on the HZSM-5 as ZnO particles or anchored to the acidic Al sites. ICP-OES and EDS both confirmed the highest Zn retention was observed with the highest CO2 addition (2%); 90% of the bulk Zn was retained in Zn3 (ICP-OES), and 100% of the surface Zn was retained (EDS). The bulk Zn retained (ICPOES) in Zn3 after the reaction with 0, 0.5, 1.0, and 2.0% CO2 added is 34.0, 68.1, 74.9, and 90.4% respectively. A similar trend was also observed for surface Zn (EDS) confirming the CO2 stabilizing effect, even under the reductive atmosphere of the reaction. Higher Zn retention measured in the spent catalysts correlated with higher Zn stability experienced by the addition of more CO2 where the Zn interacts more with the HZSM-5. However, as the reaction proceeded ZnO decreased and anchored [Zn(OH)]+ increased with CO2 concentration.

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2% CO2

0.5 0.0

1% CO2

TCD Signal (a.u.)

0.5 0.0

0.5% CO2

0.5 0.0

0% CO2

0.5 0.0

Fresh

0.5 0.0 200

400

o

600

Temperature ( C)

Fig. 6. NH3 -TPD profiles for fresh and spent Zn3 catalyst exposed to different CO2 concentration during the MDA reaction (0, 0.5, 1, and 2% CO2 ).

3.3.2. Ammonia-temperature programmed desorption (NH3 -TPD) analysis 3.3.2.1. Catalysts with different Zn loading. After the MDA and subsequent carbon burn off, the NH3 -TPD profiles of the spent catalysts were obtained and are shown in Fig. 3b. The NH3 -TPD for these spent catalysts confirmed the Zn loss associated with some changes in acidic properties on the catalysts’ surfaces. No significant change was observed in surface acidity between fresh and spent HZSM-5. The BAS peak on the spent catalysts (Zn2, Zn3, and Zn8) are partially restored indicating the reduction and vaporization of inactive ZnO that partially blocked the micropores and/or covered some of the BAS on the external surfaces leaving behind more free Al-OH groups. The decrease in the LAS peak intensity and area indicates Zn loss from the spent catalysts. The broadening in the LAS peaks of Zn-loaded spent catalysts relative to spent HZSM-5 suggests the presence of anchored Zn2+ even after 12 h TOS. The similarity in the restored BAS peaks on the spent catalysts also suggests that most of the unreactive ZnO species were reduced and vaporized during the reaction and only anchored [Zn(OH)]+ remained. Fresh Zn1 catalyst had almost no ZnO particles to cover BAS or partially block micropores, but mainly anchored, stable Zn2+ species, as shown in Fig. 3a. This could explain the similarity in the NH3 -TPD profiles between fresh and spent Zn1. Also ICP-OES results showed that Zn1 had the highest % retained Zn, indicating that Zn species in this catalyst are highly coordinated to the HZSM-5 structure. 3.3.2.2. Zn3 with different CO2 additions. Fig. 6 shows the NH3 -TPD for fresh and spent Zn3 catalysts tested for MDA with different CO2 concentrations. Table 2 shows the total mmols of ammonia desorbed from all acid sites on the catalyst surfaces during the NH3 -TPD. The fresh Zn3 catalyst has a broader low temperature desorption peak corresponding to additional LAS created by the

Zn oxide loaded on the HZSM-5 surface. The BAS are almost completely removed due to [Zn(OH)]+ anchored to acidic Al-OH sites and to external ZnO particles partially blocking microporous channels of HZSM-5. The amount of ammonia desorbed increased from 0.48 to 0.64 mmol/gcat suggesting the formation of new Lewis acid sites upon Zn loading [38]. After 12 h of reaction time in the absence of any CO2 in the feed gas, the spent Zn3 catalyst had a total ammonia desorption of 0.49 mmol/gcat , similar to fresh HZSM-5 (0.48 mmol/gcat ). This indicates that most of the loosely bound ZnO species is lost and the BAS peak is partially restored, while the LAS peak area decreased, consistent with the ICP-OES and EDS results. A Bronsted acid shoulder peak was observed on spent Zn3 catalysts after MDA reaction with 0.5, 1 and 2% CO2 additions. The restored Bronsted acid sites decreased with the amount of CO2 added. The shoulder peak completely disappeared for high CO2 levels. At 2% CO2 most of the Zn was retained and the ammonia desorbed is 0.62 mmol/gcat , similar to fresh Zn3 catalyst (0.64 mmol/gcat ). The amount of ammonia desorbed increased with CO2 addition (Table 2) indicating more ZnO was stabilized and retained on the external surfaces. Similarly, the Lewis acid sites increased with CO2 addition, which could indicate that the amount of ZnO retained was increased and stabilized by CO2 . The Bronsted acid peak was shifted to a lower temperature and its area decreased as the CO2 concentration increased. These results correlate well with the benzene yield observed in Fig. 5b, which decreased with the CO2 addition, further supporting that the ZnO is not the active Zn species. 3.3.3. Surface properties after CO2 addition The BET results of the spent Zn3 catalysts with the addition of different CO2 concentrations are listed in Table 3. The results show a decrease in surface properties compared to the fresh catalyst, possibly due to structure changes and Zn metal redistribution during reaction [29]. In general, the results show that the BET and microporous surface area of Zn3 catalyst decreased with CO2 addition. This could be from a possible increase in the water formed during the reaction (via reverse water gas shift), which would result in partial structure damage to the internal pores of HZSM-5. This would also create some secondary pore structure (mesopores) from dealumination caused by the steam production [65]. This could also affect the total number of acid sites and decrease the BAS to LAS ratio. 3.3.4. Temperature programmed oxidation (TPO) analysis In situ TPOs were performed on all spent Zn3 catalysts after MDA reaction with different CO2 concentrations. Only high temperature carbon with oxidation temperature from 500 to 700 ◦ C was studied since it is the main cause for catalyst deactivation [20,66–69]. The CO2 signal intensities monitored by the mass spectrometer during the TPOs are shown in Fig. 7. The high peak area and the peak broadening for the sample with no CO2 addition indicated a large amount of carbon was formed, as well as the presence of different types of carbon on the spent catalyst surface. The fast ramping rate used during the TPO experiments could possibly cause overlap of the oxidation temperature peaks from different carbon types on the catalyst surface [14,62,70]. For the samples run with CO2 addition the TPO peaks narrowed and shifted to lower oxidation temperature. Two major types of relatively high temperature carbon were observed in the TPO profiles. The lower temperature peak did not change significantly with the addition of CO2 to the reaction stream. The higher temperature peak, associated with carbon that usually causes catalyst deactivation [38], decreased significantly with the amount of CO2 and reduced catalyst deactivation caused by this type of carbon and enhanced its performance stability. The oxidation peak area is proportional to the amount of carbon on the catalyst surface, which decreased with higher CO2

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373

Table 2 Quantitative analysis of NH3 -TPD profiles for fresh and spent Zn catalysts exposed to different CO2 concentration.

HZSM-5 Zn3 Zn3 Zn3 Zn3 Zn3 a

Desorption temp. (◦ C)

CO2 (%)a

Catalyst

Fresh Fresh Spent Spent Spent Spent

– – 0 0.5 1.0 2.0

MNH3 (mmolNH3 /gcat )

T1

T2

210 221 216 218 215 212

407 398 356 345

0.48 0.64 0.49 0.52 0.59 0.62

Amount of CO2 added to reaction gas stream.

Table 3 Surface properties and elemental composition of spent Zn3 catalysts reacted with different CO2 concentration during MDA. CO2 (%)f

SBET (m2 /g)a

Smicro (m2 /g)b

Vmicro (cm3 /g)b

Vtotal (cm3 /g)c

ICP-OES (Zn wt%) Retainede

EDSd (Zn wt%) Retainede

0.0 0.5 1.0 2.0

349 344 330 294

244 236 228 199

0.100 0.096 0.093 0.082

0.243 0.235 0.229 0.221

34.0 68.1 74.9 90.4

43.2 65.9 81.8 100.0

a–e

See Table 1 footnotes. Concentration of CO2 added to the total gas feed during the reaction.

f

4. Conclusion

CO2 Intensity (a.u.)

2% CO2

1% CO2

0.5% CO2

0% CO2 500

550

600

o

650

700

Temperature ( C) Fig. 7. CO2 mass spectrometer profiles obtained during the TPO experiments of spent Zn3 catalyst treated with different CO2 concentration (0, 0.5, 1, and 2%) during methane dehydroaromatization reaction.

concentration. Addition of CO2 to the reaction preferentially reduced the formation of the higher temperature carbon and increased the catalyst stability. These results are consistent with the methane conversion data shown in Fig. 5, which decreased with CO2 addition. At higher CO2 concentration more unreactive ZnO was stabilized and retained on the catalyst surface decreasing the catalyst activity and reducing carbon formation on the surface. A balance between catalyst activity and performance stability is very crucial. The most stable benzene yield was observed with Zn3 catalyst under 0.5% CO2 addition, which also had the lowest amount of higher temperature carbon on its surface (Fig. 5b) This is also consistent with the TPO results reported by Ichikawa group [62] on Mo/HZSM-5 where CO2 addition resulted in a significant reduction in the amount of carbon formed on the catalyst surface. At CO2 concentrations higher than 4%, the TPO of the spent catalyst (Mo/HZSM-5) showed a reduction in not only inert coke, but also reactive coke, which can be converted into aromatic compounds [62].

The effect of Zn loading (1–8 wt%) and the effect of CO2 addition (0.5–2%) were both studied over Zn/HZSM-5 catalysts to investigate the active Zn species and examine their thermal stability during methane dehydroaromatization (MDA) reaction at 700 o C and atmospheric pressure. The methane conversion increased from 0.4 to 1.7% as the Zn loading was increased from 1 to 8 wt%, and is believed to be due to methane dehydrogenation and decomposition to carbon over Zn. Zn3 catalyst showed the highest benzene yield, but it rapidly deactivated over 12 h of reaction. Zn1 and Zn2 produced lower benzene yields, but were more stable over the entire reaction tests. The yield stability was inversely proportional to the Zn loading up to 3 wt%. Catalyst characterization before and after reaction revealed two Zn oxide species on the HZSM-5 surface: (1) anchored, stable, and reactive [Zn(OH)]+ species, which are strongly bound to Bronsted sites, and (2) unreactive ZnO, which is easily reduced and vaporized, and is mostly located on the external support surfaces. The optimum loading on HZSM-5 for Zn retention, and benzene yield stability, was found to be between 1 and 2 wt% according to XRD, surface analysis, TGA, EDS and ICP-OES, and NH3 -TPD results. The XRD results showed a significant drop in crystallinity at Zn loading higher than 2 wt%, while the microporous properties of the catalysts decreased significantly at Zn loading of 2 wt% and higher. The TGA results showed that the Zn loss was significant during isothermal reduction at 700 o C except for Zn1. Similarly, the bulk and surface concentrations of Zn revealed that almost all of the Zn was retained for Zn1, but retention decreased as the Zn loading increased due to ZnO loss by reduction and vaporization. NH3 -TPD results showed that the high temperature Bronsted acid desorption peak was partially restored after reaction for all Zn catalysts indicating the loss of unreactive ZnO species during reaction. Only Zn1 catalyst had almost no change before and after reaction indicating that the Zn is retained as anchored [Zn(OH)]+ species on the surface. These results suggest that the ratio between [Zn(OH)]+ /ZnO, which reflect the state of Zn dispersion on HZSM-5, decrease with Zn loading. Addition of CO2 to the reactant stream resulted in stabilizing ZnO against reduction and vaporization. It also reduced the high temperature carbon formed on the catalyst surface and prevented rapid catalyst deactivation as supported by the TPO results. The most stable benzene yield was observed when 0.5% CO2 was added to the reaction gas stream. However, a lower benzene yield was

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observed due to the presence of unreactive ZnO on the external surface. The EDS and ICP-OES for Zn3 reacted with CO2 addition correlated well with the performance data and showed that as the CO2 increased the total Zn retained increased. Similarly, as the CO2 increased the Bronsted acid desorption peak in the NH3-TPD on spent Zn3 catalyst was not restored due to the increased amounts of ZnO retained on the external surface. Addition of CO2 gave additional evidence that the ZnO is not the active Zn species for the MDA, but it is instead the strong Lewis acid sites formed by anchored zinc [Zn(OH)]+ , which are responsible for methane activation. Disclaimer This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with AECOM. Neither the United States Government nor any agency thereof, nor any of their employees, nor AECOM, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Acknowledgements This work was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES contract DE-FE0004000. We gratefully acknowledge Dr. Dirk Link and Dr. Bryan D. Morreale for their technical guidance throughout this project. We also acknowledge Donald Floyd and James Poston for reactor setup and electron microscopy, respectively. References [1] K. Skutil, M. Taniewski, Fuel Process. Technol. 88 (2007) 877–882. [2] S. Majhi, P. Mohanty, H. Wang, K.K. Pant, J. Energy Chem. 22 (2013) 543–554. [3] M. Gharibi, F.T. Zangeneh, F. Yaripour, S. Sahebdelfar, Appl. Catal. A 443–444 (2012) 8–26. [4] T.V. Choudhary, E. Aksoylu, D. Wayne Goodman, Catal. Rev. 45 (2003) 151–203. [5] M.C. Alvarez-Galvan, N. Mota, M. Ojeda, S. Rojas, R.M. Navarro, J.L.G. Fierro, Catal. Today 171 (2011) 15–23. [6] W. Ding, G.D. Meitzner, E. Iglesia, J. Catal. 206 (2002) 14–22. [7] S. Kikuchi, R. Kojima, H. Ma, J. Bai, M. Ichikawa, J. Catal. 242 (2006) 349–356. [8] Y. Li, L. Liu, X. Huang, X. Liu, W. Shen, Y. Xu, X. Bao, Catal. Commun. 8 (2007) 1567–1572. [9] S. Burns, J.S.J. Hargreaves, P. Pal, K.M. Parida, S. Parija, Catal. Today 114 (2006) 383–387. [10] S. Li, C. Zhang, Q. Kan, D. Wang, T. Wu, L. Lin, Appl. Catal. A 187 (1999) 199–206. [11] I. Chul Hwang, D. Heui Kim, S. Ihl Woo, Catal. Today 44 (1998) 47–55. [12] B.S. Liu, L. Jiang, H. Sun, C.T. Au, Appl. Surf. Sci. 253 (2007) 5092–5100. [13] H. Ma, R. Kojima, S. Kikuchi, M. Ichikawa, Catal. Lett. 104 (2005) 63–66. [14] J. Bai, S. Liu, S. Xie, L. Xu, L. Lin, Catal. Lett. 90 (2003) 123–130. [15] M.C.J. Bradford, M. Te, M. Konduru, D.X. Fuentes, Appl. Catal. A 266 (2004) 55–66.

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