Al Multifunctional Catalyst for Steam

Jan 30, 2018 - A plate-type Cu/Ni/γ-Al2O3/Al catalyst exhibited a good stability in steam reforming of dimethyl ether (DME SR); however, a high CO co...
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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 2426−2433

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A Structured Cu-Based/γ-Al2O3/Al Multifunctional Catalyst for Steam Reforming of Dimethyl Ether: Investigation on in-Situ CO Reduction Strategy Feiyue Fan,†,‡ Qi Zhang,*,† and Hong Hou*,‡ †

Department of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China

‡

ABSTRACT: A plate-type Cu/Ni/γ-Al2O3/Al catalyst exhibited a good stability in steam reforming of dimethyl ether (DME SR); however, a high CO concentration (ca. 26%) was detected. As such, a multifunctional catalyst combined DME SR and high temperature water gas shift reaction (HT-WGSR) was developed in this work. It is found that the reaction temperature regions of Fe-based and Cu-based catalysts coupled perfectly, and thus resulted in an in situ CO reduction during DME SR process. Meanwhile, the CO reduction mechanism was proposed over the Fe-doped Cu-based multifunctional catalyst. Furthermore, the effects of iron loading on the physicochemical properties and performance of catalysts were extensively investigated. The results show that the proper amount of iron doping was helpful in improving the dispersion of Cu, and thus enhancing the catalytic performance and decreasing CO concentration. Finally, it is found that the optimized Cu/NiO/Fe3O4/γ-Al2O3/Al multifunctional catalyst has excellent stability during a 200 h test, and gives a 100% DME conversion at 400 °C. Various acidic catalysts such as zeolites, WO3/ZrO2 and γAl2O36−9 have been studied in DME hydrolysis (eq 2), which is the rate-limiting step of overall DME steam reforming.10 Among them, γ-Al2O3 with weak acid sites exhibits good stability along with the inhibition of side-reactions and is more suitable for DME hydrolysis, but a relatively high temperature above 300 °C is required to efficiently hydrolyze dimethyl ether.11 For steam reforming of methanol (eq 3), Cu-based catalysts are the common catalysts due to the high activity and low price, but they have poor thermal stability and can easily lose catalytic activity because of the sintering of metallic Cu above 300 °C.12−17 It was reported that the addition of nickel to Cu-based catalyst could improve the dispersion of metallic Cu, and thus enhance the thermal stability of catalyst significantly.3,18 However, a high CO concentration (ca. 26%) was detected over the Ni-doped Cu-based catalyst during DME SR because of the presence of methanol decomposition (eq 4).3

1. INTRODUCTION The depletion of fossil fuels and increasing energy demand has driven the research and exploration of new energy alternatives. Hydrogen is considered to be a clean source of energy and steam reforming of dimethyl ether (DME) is regarded as a promising hydrogen production process for fuel cell applications.1−3 Unlike many other fuels (e.g., methane, methanol, bioethanol), DME possesses high H/C ratio, high energy density, nontoxic and noncorrosive nature. Moreover, the well-developed infrastructure of LPG can readily be used for the distribution of DME due to their similar physical properties.4,5 Generally, steam reforming of dimethyl ether (DME SR, eq 1) consists of two successive reactions: DME hydrolysis (eq 2) to methanol over a solid acid catalyst, followed by steam reforming of methanol (MSR, eq 3) to hydrogen and carbon dioxide over a metallic catalyst. Thus, a bifunctional catalyst consisting of dual sites of acid sites and MSR sites is required for DME SR process. DME steam reforming: CH3OCH3 + 3H 2O ↔ 6H 2 +

CH3OH → CO + 2H 2

2CO2 ΔH 0r

‐1

Usually, a three-step process is used to reduce the CO content in the hydrogen fuel below 10 ppm in order to satisfy the requirement of proton exchange membrane fuel cells (PEMFC) system.19−21 High temperature water gas shift

(1)

= +135kJ·mol

DME hydrolysis: CH3OCH3 + H 2O ↔ 2CH3OHΔH 0r = +37kJ·mol−1

(2)

MeOH steam reforming: CH3OH + H 2O ↔ 3H 2 +

Received: Revised: Accepted: Published:

CO2 ΔΗ 0r

= +49kJ·mol−1

(3) © 2018 American Chemical Society

(4)

2426

November 28, 2017 January 10, 2018 January 19, 2018 January 30, 2018 DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433

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Industrial & Engineering Chemistry Research reaction (HT-WGSR, eq 5) was first performed at 320−450 °C using a Fe-based catalyst to reduce the CO concentration to 3− 5%. Then, further decrease in CO content (0.3−1%) was achieved over Cu-based catalyst at 190−250 °C by low temperature water gas shift reaction (LT-WGSR). Finally, the preferential oxidation of CO (CO-PROX) was employed at 80−140 °C to further decrease the CO concentration to less than 10 ppm. Thus, four stages consisting of DME SR and the following three-stage for CO removal is needed for the whole operation process. Nevertheless, this could increase the size and cost of the systems because of the technical complexity and multiple stages involved. As such, it is very essential to develop a multifunctional catalyst involving the active sites of DME SR and HT-WGSR for the in situ removal of CO to 3−5% (the inlet concentration of CO for LT-WGSR), which could save one stage and simplify the whole operating procedure. In addition, the coupling of endothermic reaction (DME SR, eq 1) and exothermic reaction (WGSR, eq 5) could improve energy efficiency and reduce energy consumption. It is worth noting that the temperatures for HT-WGSR should couple well with that of DME SR to ensure the synergic effects between these two active sites. However, the one-stage operation mode coupled the reactions of DME SR and HT-WGSR was rarely reported. CO + H 2O ↔ CO2 + H 2ΔH 0r = − 41kJ·mol‐1

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A commercial aluminum plate (1060, 4.5 cm × 15 cm × 0.0445 cm) was used to prepare structured γ-Al2O3/Al monolith. Each plate was pretreated in a sodium hydroxide solution (10 wt %) for 4 min and a nitric acid solution (10 wt %) for 2 min, followed by rinse with deionized water. Afterward, the pretreated plate was anodized in a 0.4 M oxalic acid solution for 12 h with a current density of 50 A/m2 at 20 °C to form porous alumina layers. The resulting plate was calcined in air at 350 °C for 1 h to remove the residual oxalic acid and then subjected to a hot water treatment (HWT) in deionized water at 80 °C for 1 h. Finally, the obtained monolith was dried and calcined in air at 500 °C for 4 h. With the anodic alumina support, a series of Cu/M/γ-Al2O3/ Al (M = Ni, Zn or Fe) catalysts were prepared through impregnation method. The monolithic γ-Al2O3/Al support was impregnated in an aqueous solution of Cu (II) nitrate or M nitrate (M = Ni, Zn or Fe) under ambient conditions. The resulting catalyst was then dried naturally, and calcined in air at 500 °C for 4 h. 2.2. Catalyst Characterization. The specific surface area and pore structure of catalysts were determined by N2 physisorption at 77 K in a Micromeritics ASAP 2020-M instrument, where the outgas operation was set at 300 °C for 10 h. The specific surface area was calculated using the BET method. The pore volume and average pore diameter were determined using the BJH method. The metal loading of catalysts was analyzed by an inductively coupled plasma-atomic emission spectrometry (ICP-AES, 725ES, Agilent) and is reported here based on the quantity of the surface alumina layers. The surface morphology and elemental composition of catalysts were observed by scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDS, JSM-6360LV, JEOL). Powder XRD patterns of the alumina layers by scraping from the plate were collected by an X-ray diffractometer (D/max 2550 VB/PC, Rigaku). The crystalline phases were identified using the Joint Committee on Powder Diffractions Standards (JCPDS) files. H2 temperature-programmed reduction (H2-TPR) analysis was performed in a Micromeritics ChemiSorb 2720 apparatus. A sample of 100 mg catalyst powder was outgassed at 300 °C for 1 h under He flow (25 mL/min), followed by cooling to room temperature. The temperature was then raised from room temperature to 920 °C with a ramping rate of 10 °C/min under a 10 vol % H2/Ar flow atmosphere. The copper surface area (SCu) was measured by N2O chemisorption method in a chemisorptions analyzer (ChemiSorb 2720, Micromeritics).19,25 Catalysts were first reduced in a 10 vol % H2/Ar stream at 400 °C for 1 h, followed by cooling to 60 °C under He stream. Then, surface copper species were oxidized in a 20 vol % N2O/N2 mixture for 1 h. Finally, samples were purged with He for 1 h to remove the residual N2O and then TPR was carried out in a 10 vol % H2/Ar stream. The total amount of chemisorbed O atoms was determined by the area of the TPR peak. The surface area of metallic copper was calculated based on the assumption that Cu:N2O is 2:1 and the copper surface density is 1.46 × 1019 copper atoms per square meter. The copper dispersion (DCu) was defined as the ratio of copper atoms on the surface of the catalysts to the total amount of copper atoms in the catalyst.

(5)

In addition, many studies3,5 have discussed the steam reforming process with a conventional packed bed reformer, which suffers from several drawbacks, such as poor heat transfer, high pressure drop, diffusion limitation, a large volume, long start-up time, and etc.22 Recently, increasing public interest in energy-related issues requires further efficiency in energy utilization. Moreover, downsizing of the systems followed by cost cuts is needed. As such, a structured platetype catalyst was fabricated. For the preparation of porous layer in the plate-type catalyst, various methods such as chemical vapor deposition, sol−gel coating and plasma sputter coating has been reported.23 Nevertheless, the coatings are easy to peel off from the base material at high temperatures due to their different thermal expansions. As an approach, a plate-type anodic alumina (γ-Al2O3/Al) support was developed through anodization technology in our previous works.9,18,24 The anodic γ-Al2O3/Al monolith was proven to have excellent catalytic performance in DME hydrolysis and was a promising support for DME SR reaction system. In this study, with the plate-type support, anodic alumina supported Cu-based catalysts were further prepared and applied in DME SR reaction system. The in situ CO reduction strategy during DME SR process was proposed. Two compositions (Zn and Fe) were first selected and added to Cu/Ni/γ-Al2O3/Al catalyst. Then, the CO reduction mechanism at high temperatures was proposed over Fe-doped Cu-based multifunctional catalyst. Meanwhile, the Fe-doped Cu-based catalysts were characterized using N2O chemisorption, XRD and H2-TPR to study the effects of iron loading on the variation of copper species. Furthermore, the effects of iron loading on the performance of catalysts were extensively investigated and a 200 h stability evaluation was carried out over the optimized Fe-doped Cu-based catalyst. 2427

DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433

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Industrial & Engineering Chemistry Research

Figure 1. Flowchart of experiment device for gas−solid catalytic reaction.

X-ray photoelectron spectroscopy (XPS) was performed using a Thermo ESCALAB 250 spectrometer with an Al Kα radiation. The binding energies (BEs) were calibrated based on the carbonaceous C 1s line at 284.6 eV. 2.3. Catalyst Activity Evaluation. The DME SR was performed in a fixed-bed reactor (I.D. Twelve mm) under atmospheric pressure as shown in Figure 1. The plate-type composite catalyst was cut into small pieces (ca. 6 mm2), and then packed into the reactor using Raschig ring (20−40 mesh) to dilute them. A thermocouple was placed in the center of the catalyst bed to measure the temperature. Prior to the evaluation of catalysts, no H2 prereduction treatment was conducted, unless stated otherwise. Water was supplied to a preheater at 150 °C, and then the vapor mixed with DME was fed into the reactor. The compositions of influent and effluent gases were analyzed by an online gas chromatograph equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The steam in the feed and reformate was trapped by a condenser before the gas analysis. A PORAPAK-Q column was used to separate the DME, CH3OH, and CH4 gas, and a TDX-01 column was used to separate the CO, CO2, and N2. DME conversion and selectivity to products are defined as follows: DME conversion(%) =

FDME,in − FDME,out

Selectivity of products(%) =

FDME,in Fi,out ∑ Fi,out

Figure 2. Catalytic performances of supported catalysts; (a) DME conversion, (b) CO selectivity and CO removal rate (inset); Reaction conditions: n(DME):n(H2O) = 1:4, Total flow rate = 3600 mL/(g·h).

CatCuNi, CatZnCuNi, and CatFeCuNi catalysts exhibited similar DME conversion. Whereas, CatFe sample had lowest DME conversion in the temperature range of 250−400 °C, which was the same as DME hydrolysis conversion over γ-Al2O3/Al support,24 indicating that CatFe could not catalyze MSR process at such temperature. As shown in Figure 2(b), CO selectivity was similar over CatCuNi and CatZnCuNi catalysts and the CO removal rate (inset) over CatZnCuNi was only about 1% in the whole reaction range, indicating that the presence of Zn could not promote WGSR at high temperatures. It is because the temperature for WGSR promoted by Cu/ZnO/Al2O3 is about 190−250 °C, which is much lower than that for CO formation over CatCuNi catalyst (above 300 °C). However, Fe-based catalyst was active for WGSR above 320 °C,26 so the coupling of Cu-based and Febased catalyst could decrease the CO content effectively due to the similar temperature regions. It can be seen that CO selectivity did decrease dramatically over CatFeCuNi catalyst above 325 °C. At 400 °C, the CO selectivity decreased from ca. 24−4% and CO removal rate (inset) was about 83%. This provides a possibility for the in situ removal of CO in one-stage by coupling DME SR and HT-WGSR. To investigate the effects of packing methods on the catalytic performance, three different packing methods of CatCuNi and CatFe, that is, supported (CatCuNiFe), mixed (mechanical

× 100% (6)

× 100% (7)

where FDME,in and FDME,out are the molar flow rates of DME at inlet and outlet, respectively, and Fi,out are the molar flow rates of gaseous products (H2, CO, CO2, CH4).

3. RESULTS AND DISCUSSION 3.1. In Situ CO Reduction Strategy of Cu-Based Catalyst in DME SR. Usually, Cu/Zn and Fe-based catalysts are widely used in WGSR (eq 5) due to their high activity and cost effectiveness. To reduce CO content by in situ method, these two compositions (Zn and Fe) were thus selected and added to the Cu/Ni/γ-Al2O3/Al catalyst in this work. For convenience, the term “Catxyz” represents the metal x, y, and z supported on the anodic alumina. Figure 2 compares the catalytic performances of supported catalysts. It was found that 2428

DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433

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Industrial & Engineering Chemistry Research mixture of CatFe and CatCuNi), and layered (CatFe (lower) + CatCuNi (upper)) were compared and the results were shown in Figure 3. It can be seen from Figure 3(a) that DME conversion was similar over these three catalysts in the whole range of reaction temperatures.

Figure 3. Catalytic performances of CatCuNi and CatFe catalysts with different packing methods in DME SR; (a) DME conversion, (b) CO selectivity; reaction conditions: n(DME):n(H2O)=1:4, total flow rate = 3600 mL/(g·h). Figure 5. XPS spectra of CatCuNiFe catalyst; (a) Cu 2p3/2, (b) Ni 2p3/2, (c) Fe 2p.

However, as shown in Figure 3(b), the CO selectivity was different over these three catalysts. The highest CO selectivity over Layered catalyst is mainly because that WGSR over CatFe could not proceed effectively after DME SR over CatCuNi due to the noncontact between the active components of Cu/Ni/γAl2O3 and Fe, and the synergetic effect of these two active components did not work. On the contrary, CO produced by DME SR could be reformed to CO2 and H2 instantly over the supported catalyst (CatCuNiFe), which is probably because of the close contact of Cu/Ni/γ-Al2O3 and Fe. To further study the distribution of metal components on the surface of supported catalyst (CatCuNiFe), SEM-EDS analysis was conducted and the results were shown in Figure 4. It can be

As shown in Figure 5(c), the CatCuNiFe sample exhibited two typical peaks at ca. 710.4 and 724.2 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 core level of Fe3O4, respectively.30,31 Usually, Fe3+ in Fe2O3 species shows a characteristic satellite at ca. 719.0 eV. In our case, the absence of the satellite peak confirms the sample own the pure Fe3O4 phase rather than Fe2O3.32 This is an important character to distinguish between Fe3O4 (magnetite) and Fe2O3 (hematite) since the two species have the same crystalline structure but differ only in the valence state of iron ions. Therefore, CatCuNiFe catalyst is composed of metallic Cu, NiO, and Fe3O4 species. Based on above analysis, a three-step reaction mechanism of DME SR over Cu/NiO/Fe3O4/γ-Al2O3/Al multifunctional catalyst was proposed and shown in Figure 6. It can be seen

Figure 4. SEM-EDS analysis of CatCuNiFe catalyst; (a) SEM image, (b) element mapping of Al, (c) element mapping of Cu, Ni, and Fe. Figure 6. Reaction mechanism of DME SR over Cu/NiO/Fe3O4/γAl2O3/Al multifunctional catalyst.

seen from Figure 4(b) that Al exists on the whole surface of the catalyst due to the support of anodic alumina (γ-Al2O3). As shown in Figure 4(c), the components of Cu, Ni, and Fe coexist closely and are well dispersed on the surface of the catalyst, which makes the reactions of DME SR and WGSR occurred sequentially and efficiently. To investigate the surface chemical states of copper, nickel and iron species during reaction, XPS measurement was carried out on CatCuNiFe catalyst pretreated by 10 vol % H2/N2 stream at 400 °C for 2 h and the results were shown in Figure 5. It can be seen from Figure 5(a) that the CatCuNiFe sample exhibited the presence of Cu0/Cu+ at ca. 932.5 eV.27 Whereas, only Ni2+ at ca. 856.0 eV and satellite at ca. 862.1 eV was observed in Figure 5(b), indicating that all the nickel species exhibited oxidation states on the surface of the catalyst,28,29 which is because NiO species could not be reduced below 420 °C.18

that the first step is DME hydrolysis to methanol over γ-Al2O3/ Al. Then, direct methanol decomposition (eq 4) occurred over Cu/NiO catalyst, resulting in CO and H2. Subsequently, the CO produced through methanol decomposition was further reacted with H2O to form CO2 and H2 over Fe3O4, which significantly decreased the CO concentration in the products, and thus increased the desired product H2 concentration. 3.2. Catalytic Performance of Cu/Ni/Fe/γ-Al2O3/Al multifunctional catalyst in DME SR. Since the addition of iron to the Cu-based catalyst could effectively reduce the CO concentration in the products during DME SR, the effects of Fe loading on the catalytic performances of Cu-based catalyst were further investigated. 2429

DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433

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Industrial & Engineering Chemistry Research Table 1. Physicochemical Properties of the Catalysts with Different Fe Loadings catalyst γ-Al2O3/Al CatF CatCN Cat2.5FCN Cat6.0FCN Cat7.7FCN Cat9.1FCN Cat10.9FCN a

Fe loadinga (wt %)

Cu loadinga (wt %)

Ni loadinga (wt %)

SBET (m2/g)

Vp (mL/g)

Dp (nm)

SCub (m2/gCat)

DCub (%)

2.1 2.0 2.2 2.0 2.2 2.1

84 80 72 71 69 68 65 62

0.15 0.14 0.14 0.12 0.11 0.11 0.10 0.09

5.6 6.0 6.2 6.3 6.4 6.4 6.3 6.3

16.8 17.3 17.9 18.5 19.2 17.6

7.4 7.6 7.9 8.1 8.5 7.7

9.0 2.5 6.0 7.7 9.1 10.9

9.3 9.5 9.3 9.4 9.5 9.2

Determined by ICP-AES analysis. bCu surface area (SCu) and Cu dispersion (DCu) determined by N2O titration method.

catalysts pretreated by 10 vol % H2/N2 stream at 400 °C for 2 h and the results were shown in Figure 7(b). For all the supported catalysts, CuO and hematite phases were reduced to metallic Cu and magnetite phase (Fe3O4, JCPDS 88-0866), respectively.33 This is important for the DME SR reaction system, because metallic Cu species worked as active sites for MSR (eq 3), while Fe3O4 was an active phase for HT-WGSR.25 In addition, the diffraction intensity of metallic Cu and Fe3O4 over Fe-doped Cu-based catalysts decreased as compared to CatCN and CatF catalysts, suggesting that the addition of a second component to the catalysts reduced the crystallite size of Cu and Fe3O4, and thus improved the dispersion of Cu and Fe3O4, which could inhibit the sintering of Cu effectively. In order to study the reducibility of the supported catalysts, H2-TPR analysis was performed and the results were shown in Figure 8. It can be seen that CatF shows the first peak at ca. 420

3.2.1. Characterization of Cu/Ni/Fe/γ-Al2O3/Al catalysts. Table 1 shows the characteristics of γ-Al2O3/Al monolith and supported catalysts. For convenience, the term “CatxFCN” represents Fe, Cu, and Ni supported on the anodic γ-Al2O3/ Al support with the iron loading of x. It can be seen that the anodic γ-Al2O3/Al support exhibited the highest surface area and pore volume. Both the specific surface area and pore volume of supported catalysts decreased with increasing metal loading, indicating that the impregnated metal component blocked some pores in the support. Meanwhile, the slight increment of the pore diameter could be attributed to the partial dissolution of the alumina film by the acidic solution when the metal phase was impregnated. Moreover, when the iron loading was lower than 9.1 wt %, the surface area and dispersion of metallic Cu increased with the increase of the Fe loading, and Cat9.1FCN exhibited the highest surface area and highest dispersion of metallic Cu. However, when iron loading was further increased to 10.9 wt %, the surface area and dispersion of metallic Cu decreased, indicating that excess addition of iron blocked the active sites of Cu. To examine the effects of iron loading on the crystal phases formed during catalyst preparation, XRD characterizations were performed on fresh Fe-doped Cu-based catalysts and the results were shown in Figure 7(a). It can be seen that the CatF catalyst

Figure 8. H2-TPR profiles of Cu/Ni/Fe/γ-Al2O3 catalysts with different Fe loadings; (a) CatF, (b) CatCN, (c) Cat2.5FCN, (d) Cat6.0FCN, (e) Cat7.7FCN, (f) Cat9.1FCN, (g) Cat10.9FCN.

°C and the second peak above 650 °C, which can be assigned to the reduction of Fe2O3 to Fe3O4 and then Fe3O4 to FeO and Fe species, respectively.19 For CatCN sample, a main reduction peak is observed at ca. 285 °C, corresponding to the reduction of CuO to Cu, while the minor broad peak observed in the temperature range of 420−700 °C was attributed to the reduction of nickel oxide. By adding iron to Cu-based catalysts, the reduction peak of CuO and Fe2O3 overlapped and shifted to lower temperature with increasing the loading of iron from 2.5 wt % to 9.1 wt % ((c)−(f)), suggesting that the reducibility of CuO and Fe2O3 species was markedly enhanced. This improvement of reducibility may be due to both of a synergistic interaction between the metal oxide phases and a decrease of their crystallite size, which was also confirmed by the XRD data.29 On the other hand, Cu could cause the spillover of hydrogen,

Figure 7. XRD patterns of Fe-doped Cu-based catalysts: (a) fresh catalysts, (b) reduced catalysts.

presents a diffraction pattern corresponding to the hematite phase (Fe2O3, JCPDS 33-0664).29 Moreover, only CuO phase (JCPDS 48-1548)33 was identified over the CatCN sample. The absence of nickel speies indicated the high dispersion of nickel species on the alumina support. For Fe-doped Cu-based catalysts, only diffraction peaks for Fe2O3 and CuO appeared. However, no peaks assigning to CuFe2O4 could be observed. This is mainly because the calcination temperature was too low to form the CuFe2O4 spinel.25 To further understand the nature of active sites during reaction, XRD measurements were carried out on different 2430

DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433

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Industrial & Engineering Chemistry Research the generating atomic hydrogen migrates to Fe2O3 sites and reduces them easily, which lowered the reduction of Fe2O3.19 However, when the loading of Fe was further increased from 9.1 wt % to 10.9 wt % ((f)−(g)), the reduction temperature of CuO and Fe2O3 increased slightly. This is mainly because excess introduction of iron formed an overlayer on the surface of the catalyst and decreased the reducibility of the catalyst. In addition, the minor peak at ca. 326 °C over Cat9.1FCN and Cat10.9FCN samples is attributed to the reduction of Fe2O3, which is not interacted with copper species, while the reduction peak above 420 °C is assigned to the reduction of Fe3O4 to FeO and Fe. 3.2.2. Effects of Fe Loading on the Catalytic Performance in DME SR. Figure 9 compares the catalytic behavior of Cu/Ni/

CH3OCH3 → CH4 + CO + H 2

(8)

CO + 3H 2 ↔ CH4 + H 2O

(9)

Figure 9(c), (d) and (e) show the C1 selectivity over different catalysts. It can be seen that the selectivity of CO2 increased, while the selectivity of CO decreased with increasing the loading of Fe from 2.5 wt % to 9.1 wt %. This is because WGSR (eq 5) was promoted by the higher iron loading, and produced some of H2. This also explained the fact that H2 selectivity increased with increasing Fe loading from 2.5 wt % to 9.1 wt %. Moreover, the selectivity of CO and CO2 over Cat10.9FCN was similar to that over Cat9.1FCN. The selectivity of CH4 was trace over all the supported catalysts below 400 °C, indicating a minor contribution of DME decomposition (eq 8). However, when the temperature was above 400 °C, CH4 selectivity increased with the reaction temperature, accompanied by the reduction of CO and H2, indicating the presence of CO methanation (eq 9), which may be due to the formation of a small amount of metallic Ni at high temperatures.29 3.3. Stability Evaluation of Cu/Ni/Fe/γ-Al2O3/Al Multifunctional Catalyst. As Cat9.1FCN catalyst showed excellent catalytic performance, that is, high H2 selectivity, low selectivity of CO and CH4, it was selected to evaluate the thermal stability and the results were shown in Figure 10. It can be seen that

Figure 10. Stability test over Cat9.1FCN catalyst; Reaction conditions: n(DME): n(H2O) = 1:4, Total flow rate = 3600 mL/(g·h), T = 400 °C.

Cat9.1FCN exhibited a good stability and no deactivation was shown during a 200 h test at 400 °C. During the whole process, DME conversion remained at 100%. The selectivity of H2, CO2 and CH4 maintained at ca. 71.6%, 23.5%, and 0.4%, whereas the selectivity of CO remained at ca. 4.5%, which approached to CO equilibrium value (4.0%). Moreover, the value of 4.5% is consistent with the literature value of ca. 4.3%,17,34 and also met the requirement of inlet CO concentration (3−5%) for LTWGSR. In addition, the addition of iron to Cu-based catalysts not only promoted the in situ CO removal, but also reduced the crystallite size of Cu, thus improved the thermal stability of Cu-based catalyst. This indicates that the added Ni and Fe modified Cu phase and acted like a barrier to suppress the sintering of copper.

Figure 9. Catalytic performance of Cu/Ni/Fe/γ-Al2O3/Al catalysts with different Fe loadings; (a) DME conversion, (b) H2 selectivity, (c) CO2 selectivity, (d) CO selectivity, and (e) CH4 selectivity; Reaction conditions: n(DME): n(H2O)=1:4, Total flow rate = 3600 mL/(g·h).

Fe/γ-Al2O3/Al catalysts with different Fe loadings in DME SR. It can be seen that all the supported catalysts had good thermal stability and exhibited similar DME conversion. As shown in Figure 9(b), the H2 selectivity increased with the increasing temperature and reached highest at 400 °C, while further increase in the temperature decreased the H2 selectivity. On the other hand, the H2 selectivity increased with increasing the loading of Fe from 2.5 wt % to 9.1 wt %, which may be due to the increase of surface area and dispersion of metallic Cu. Then, the H2 selectivity decreased with further increasing the iron loading from 9.1 wt % to 10.9 wt %, which resulted from the decrease of metallic Cu surface area.

4. CONCLUSIONS An anodized γ-Al2O3/Al monolith supported Cu-based composition catalyst was developed to investigate the in situ CO reduction performance in DME SR reaction system. The following conclusions can be drawn from this study: 2431

DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433

Article

Industrial & Engineering Chemistry Research

(3) Wang, X. L.; Pan, X. M.; Lin, R.; Kou, S. Y.; Zou, W. B.; Ma, J. X. Steam Reforming of Dimethyl Ether over Cu-Ni/γ-Al2O3 Bi-functional Catalyst Prepared by Deposition-precipitation Method. Int. J. Hydrogen Energy 2010, 35, 4060. (4) Oar-Arteta, L.; Remiro, A.; Aguayo, A. T.; Olazar, M.; Bilbao, J.; Gayubo, A. G. Development of a Bifunctional Catalyst for Dimethyl Ether Steam Reforming with CuFe2O4 Spinel as the Metallic Function. J. Ind. Eng. Chem. 2016, 36, 169. (5) Zang, Y. H.; Dong, X. F.; Wang, C. X. One-pot Synthesis of Mesoporous Cu-SiO2-Al2O3 Bifunctional Catalysts for Hydrogen Production by Dimethyl Ether Steam Reforming. Chem. Eng. J. 2017, 313, 1583. (6) Nishiguchi, T.; Oka, K.; Matsumoto, T.; Kanai, H.; Utani, K.; Imamura, S. Durability of WO3/ZrO2-CuO/CeO2 Catalysts for Steam Reforming of Dimethyl Ether. Appl. Catal., A 2006, 301, 66. (7) Ledesma, C.; Llorca, J. Hydrogen Production by Steam Reforming of Dimethyl Ether over Cu-Zn/CeO2-ZrO2 Catalytic Monoliths. Chem. Eng. J. 2009, 154, 281. (8) Oar-Arteta, L.; Remiro, A.; Vicente, J.; Aguayo, A. T.; Bilbao, J.; Gayubo, A. G. Stability of CuZnOAl2O3/HZSM-5 and CuFe2O4/ HZSM-5 Catalysts in Dimethyl Ether Steam Reforming Operating in Reaction-regeneration Cycles. Fuel Process. Technol. 2014, 126, 145. (9) Fan, F. Y.; Zhang, Q.; Xu, J. J.; Ye, Q.; Kameyama, H.; Zhu, Z. B. Catalytic Behavior Investigation of a Novel Anodized Al2O3/Al Monolith in Hydrolysis of Dimethyl Ether. Catal. Today 2013, 216, 194. (10) Faungnawakij, K.; Shimoda, N.; Viriya-empikul, N.; Kikuchic, R.; Eguchi, K. Limiting Mechanisms in Catalytic Steam Reforming of Dimethyl Ether. Appl. Catal., B 2010, 97, 21. (11) Oar-Arteta, L.; Aguayo, A. T.; Remiro, A.; Bilbao, J.; Gayubo, A. G. Behavior of a CuFe2O4/γ-Al2O3 Catalyst for the Steam Reforming of Dimethyl Ether in Reaction-regeneration Cycles. Ind. Eng. Chem. Res. 2015, 54, 11285. (12) Zhang, H.; Sun, J. M.; Dagle, V. L.; Halevi, B.; Datye, A. K.; Wang, Y. Influence of ZnO Facets on Pd/ZnO Catalysts for Methanol Steam Reforming. ACS Catal. 2014, 4, 2379. (13) Tang, Y.; Liu, Y.; Zhu, P.; Xue, Q. S.; Chen, L.; Lu, Y. Highperformance HTLcs-derived CuZnAl Catalysts for Hydrogen Production via Methanol Steam Reforming. AIChE J. 2009, 55, 1217. (14) Deshmane, V. G.; Abrokwah, R. Y.; Kuila, D. Synthesis of Stable Cu-MCM-41 Nanocatalysts for H2 Production with High Selectivity via Steam Reforming of Methanol. Int. J. Hydrogen Energy 2015, 40, 10439. (15) Zhang, B.; Zhao, D. D.; Wu, Y. H.; Liu, H. J.; Wang, T. H.; Qiu, J. H. Fabrication and Application of Catalytic Carbon Membranes for Hydrogen Production from Methanol Steam Reforming. Ind. Eng. Chem. Res. 2015, 54, 623. (16) Ledesma, C.; Llorca, J. CuZn/ZrO2 Catalytic Honeycombs for Dimethyl Ether Steam Reforming and Autothermal Reforming. Fuel 2013, 104, 711. (17) Oar-Arteta, L.; Remiro, A.; Epron, F.; Bion, N.; Aguayo, A. T.; Bilbao, J.; Gayubo, A. G. Comparison of Noble Metal- and Copperbased Catalysts for the Step of Methanol Steam Reforming in the Dimethyl Ether Steam Reforming Process. Ind. Eng. Chem. Res. 2016, 55, 3546. (18) Fan, F. Y.; Zhang, Q.; Wang, X.; Ni, Y. H.; Wu, Y. Q.; Zhu, Z. B. A Structured Cu-based/γ-Al2O3/Al Plate-type Catalyst for Steam Reforming of Dimethyl Ether: Self-activation Behavior Investigation and Stability Improvement. Fuel 2016, 186, 11. (19) Lin, X. Y.; Zhang, Y.; Yin, L.; Chen, C. Q.; Zhan, Y. Y.; Li, D. L. Characterization and Catalytic Performance of Copper-based WGS Catalysts Derived from Copper Ferrite. Int. J. Hydrogen Energy 2014, 39, 6424. (20) Meshkani, F.; Rezaei, M. A Highly Active and Stable Chromium Free Iron Based Catalyst for H2 Purification in High Temperature Water Gas Shift Reaction. Int. J. Hydrogen Energy 2014, 39, 18302. (21) Qiao, B. T.; Wang, A. Q.; Li, L.; Lin, Q. Q.; Wei, H. S.; Liu, J. Y.; Zhang, T. Ferric Oxide-supported Pt Subnano Clusters for Preferential

(1) By addition of iron to Cu/Ni/γ-Al2O3/Al catalyst, an in situ removal of CO at high temperatures was achieved in one-stage mode coupled DME SR and HT-WGSR due to the similar temperature regions. The supported catalyst (CatCuNiFe) showed lowest CO selectivity than the mixed and layered ones with CatCuNi and CatFe. SEMEDS analysis evidenced that the components of Cu, Ni, and Fe coexist closely and are well dispersed on the surface of CatCuNiFe, which makes the reactions of DME SR and WGSR occurred sequentially and efficiently. In addition, based on the endothermic (DME SR) and exothermic (WGSR) nature, the coupling of these two reactions could improve energy efficiency and reduce energy consumption. (2) The effects of iron loading on the physicochemical properties and performance of Fe-doped Cu-based catalyst were extensively studied. It is found that the proper amount of iron doping (9.1 wt %) not only improved the surface area and dispersion of Cu, but also reduced the Cu crystallite size, and thus enhanced the reducibility of catalysts, as evidenced by N2O-chemisorption, XRD and H 2-TPR analysis. Moreover, Cat9.1FCN catalyst has optimal catalytic performance, i.e. highest H2 selectivity and lowest CO selectivity. When the iron loading was lower than 9.1 wt %, the CO could not be effectively removed. Whereas, when the iron loading was higher than 9.1 wt %, the catalytic activity was decreased due to the lower surface area of metallic Cu. (3) Cat9.1FCN catalyst exhibits an excellent stability in a 200 h test at 400 °C, and gives a 100% DME conversion, which demonstrated that the novel γ-Al2O3/Al monolith supported Cu, Ni, and Fe composite catalyst was an excellent catalyst for DME SR reaction system. Furthermore, it would be very promising for the application of compact microchannel reformer as for its outstanding shape flexibility and catalytic performance.

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AUTHOR INFORMATION

Corresponding Authors

*(Q.Z.) E-mail: [email protected]. *(H.H.) E-mail: [email protected]. ORCID

Feiyue Fan: 0000-0002-9614-0299 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 20906023) and the Shanghai Rising-Star Program (B type) (Grant No. 13QB1401300).

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REFERENCES

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DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433

Article

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DOI: 10.1021/acs.iecr.7b04896 Ind. Eng. Chem. Res. 2018, 57, 2426−2433