FeâCu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin

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Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-rich C2+ Hydrocarbons Wenjia Wang, Xiao Jiang, Xiaoxing Wang, and Chunshan Song Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00016 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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Industrial & Engineering Chemistry Research

ie-2018-000164

Fe-Cu Bimetallic Catalysts for Selective CO2 Hydrogenation to Olefin-rich C2+ Hydrocarbons Wenjia Wang,a,b Xiao Jiang,a,b Xiaoxing Wang,a and Chunshan Song,*,a,b,c a

Clean Fuels & Catalysis Program, EMS Energy Institute, and John and Willie Leone Family Department of Energy and Mineral Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA b

PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

c

Department of Chemical Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802, USA

* Corresponding author. E-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract This paper reports on Fe-Cu bimetallic catalysts supported on γ-alumina for selective CO2 hydrogenation to olefin-rich C2+ hydrocarbons. A strong synergetic promoting effect on C2+ hydrocarbon synthesis was observed over Fe-Cu bimetallic catalysts when the Cu/(Cu+Fe) atomic ratio was 0.17. Compared to mono-metallic Fe catalyst, the Fe-Cu bimetallic catalyst significantly enhances C2-C7 hydrocarbon formation and suppresses the formation of undesired CH4. The addition of K into Fe-Cu bimetallic catalysts inhibits the methanation and further enhances the formation of C2+ hydrocarbons in the meantime. The synthesis of olefin-rich C2-C4 hydrocarbons from CO2/H2 was achieved by using K-promoted Fe-Cu/Al2O3 catalysts with higher K/Fe atomic ratios (e.g., K/Fe ≥ 0.5), where the selectivity of C2-C4 olefin contents was even higher than K-promoted Fe-Co catalysts under the same reaction conditions.

Keywords: CO2 hydrogenation, Olefin-rich higher hydrocarbons, Fe-Cu bimetallic catalyst, K promotion


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1. Introduction Recently utilization of carbon dioxide (CO2) for synthesizing chemical feedstocks and liquid fuels has attracted great attention

1-4

. An energy-efficient catalytic CO2 conversion has

major advantages in reducing both greenhouse gas emissions and dependence on nonrenewable resources. If CO2 was used as a single reactant, the conversion would be highly energydemanding; however, it would become thermodynamically easier if another co-reactant with higher free Gibbs energy could be introduced, such as hydrogen 1. Thus, the synthesis of oxygenates

5, 6

and hydrocarbons

7-9

via CO2 hydrogenation using H2 produced with renewable

energy such as solar or wind 10 is one of promising approaches for the production of sustainable chemical feedstocks and fuels. Light olefins (C2-C4) are of great importance as building blocks for chemicals and polymers, thus the development of catalysts for selective CO2 hydrogenation to light olefins is highly desirable 11. Three reactions might occur in CO2 hydrogenation to hydrocarbons, as summarized below along with corresponding enthalpies (values of ∆H0(598K) per CH2 were calculated as 1/6 of those per n-C6H14 11), including direct CO2 hydrogenation (CO2 HYD) to hydrocarbons, reverse water-gas shift (RWGS), and CO hydrogenation (CO HYD) to hydrocarbons which is similar to what is known as Fischer-Tropsch synthesis (FTS). Thermodynamically, hydrocarbon synthesis from CO2/CO is favored at lower reaction temperatures. Apart from the direct reaction pathway to produce hydrocarbons from CO2, the indirect pathway, RWGS + FTS, is also notable. Thus, CO2 hydrogenation to hydrocarbons is also regarded as a modification of the FTS, which has a history of nearly one century

12

. Moreover, catalyst component for CO2 hydrogenation is

analogous to that for FTS but is amended to maximize the production of hydrocarbons. So far, traditional FTS catalysts, such as Fe, Co, Ni and Ru catalysts, have been studied for CO2



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hydrogenation to hydrocarbons. Ni and Ru catalysts mainly yield CH4, and only minor amounts of C2+ hydrocarbons were obtained 13-18. As traditional FTS catalysts, Co has been developed for around a century and shows excellent activity. However, when using CO2 instead of CO as the feed, Co-based FTS catalysts exhibit very little activity towards the production of C2+hydrocarbons, probably due to their low activity towards RWGS

19

. On the other hand, Fe-

based catalysts show activity for C2+ hydrocarbons formation from CO2 hydrogenation. This could originate from the formation of carbides and alkali surface coverage, thereof promoting carbon-carbon bond formation and chain growth

11, 20

. Efforts are still devoted to improving the

catalytic performance of Fe-based catalysts to form higher hydrocarbons, such as the incorporation of promoters and different supports12,

21-25

. We have recently reported on the

promoting effect of Fe-Co bimetallic catalysts on selective CO2 hydrogenation to higher hydrocarbons

26

, and the addition of K promoter significantly enhanced the formation of C2+

hydrocarbons 7. On K-promoted Fe-Co bimetallic catalysts, it was found that light olefins, apart from CO2, could also originate from the CO or CO-like intermediate, indicating the importance of CO formation produced via RWGS. The formation of C2+ hydrocarbons via the direct hydrogenation of CO2 has also been reported 27, 28.

CO2 hydrogenation

CO2 + 3H2 ↔ -(CH2)- + 2H2O

∆H0(598K) = -128 kJ mol-1

Reverse water-gas shift

CO2 + H2 ↔ CO + H2O

∆H0(598K) = 38 kJ mol-1

CO hydrogenation (FTS)

CO + 2H2 ↔ -(CH2)- + H2O

∆H0(598K) = -166 kJ mol-1

The competition between CO and CO2 is an important aspect to consider in indirect hydrogenation route

11

. Due to thermodynamic constraints, CO partial pressure would be

apparently low in CO2/H2 atmosphere, which suppresses the strong chemisorption of CO over catalyst surface, leading to a limited chance of carbon-carbon bond formation due to CO 26. Cu-


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based catalyst is well known for its activity in water-gas shift (WGS) as well as reverse watergas shift (RWGS) reaction

29-31

and does not lead to CO2 methanation compared with other

metals such as Co, Ni and Ru 2. The RWGS activity of Cu suggests a strategy for enhancing the carbon coverage by adding Cu into Fe. In bimetallic catalysts, Cu itself does not have methanation ability but can only catalyze RWGS to produce CO, while Fe component is responsible for chain growth in CO hydrogenation. On one hand, CO adsorption is stronger than CO2 on both Fe and Cu in regards of lower CO/Fe or CO/Cu chemisorption energy, which makes CO competitive in the case together with CO2, and CO desorption is expected to be less endothermic than the other surfaces due to the weak interaction between Cu and CO, which makes it possible for CO or CO-like intermediate to form on Cu and get into FTS on Fe

32, 33

.

Thus, an enhancement of activity and C2+ hydrocarbons formation could be expected due to the improved surface COx* and chain growth ability. On the other hand, a different reaction pathway without going through CO on Fe-Cu bimetallic alloy has been addressed in previous work and may lead to promotion of direct CO2 hydrogenation

34, 35

. Furthermore, the promoter K is

indispensable as it could significantly improve the stabilization of CO2 on the surface, thereof promoting the light olefin formation by increasing the surface coverage of carbon. This work focuses on the effect of combining Fe and Cu on CO2 hydrogenation to hydrocarbons and light olefins (C2=-C4=). For this purpose, a series of Fe-Cu/Al2O3 catalysts with a wide range of Cu/(Fe+Cu) atomic ratios (with 15 wt% total metal loading) were prepared. The effect of K addition was also studied by a comparative examination of the catalysts with and without K promoter.



2. Experimental 2.1 Catalyst preparation Gamma-alumina (Sasol PURALOX TH 100/150) with BET surface area of 139 m2 g-1, average pore diameter of 24 nm and pore volume 1.07 mL g-1 was used as the support. Fe-Cu bimetallic catalysts were prepared by a pore-filling incipient wetness impregnation method using an aqueous solution containing Fe(NO3)3 ·9H2O (Aldrich, 99.99%) and Cu(NO3)2 ·2.5H2O (Alfa Aesar ≥ 98%). Concentrations of Fe and Cu in the impregnating solution were adjusted to obtain desired Cu/(Fe+Cu) atomic ratios while maintaining total metal (Fe+Cu) loading at 15 wt% (support weight basis) when total volume of the added solution was equivalent to 90% of pore volume of the support. The impregnated sample was dried at 333 K in a rotary evaporator for 2 h, followed by drying in an electric oven at 383 K for 3 h in ambient air. The impregnated samples were then calcined in an electric furnace at 673 K for 2 h under flowing dry air (ca. 100 mL (NTP) min-1). The Cu/(Fe+Cu) atomic ratios were varied in the range of 0.0-1.0. For comparison, monometallic catalysts were prepared by using the same method. On the other hand, K-promoted catalysts were prepared by a two-step impregnation method. In the first step, K2CO3 (SigmaAldrich, ≥ 99.0%) aqueous solution was impregnated onto the alumina support by the porefilling method and dried at 333 K in a rotary evaporator for 2 h, followed by drying in the electric oven at 383 K overnight. This sample was then impregnated with the Fe or Cu or mixed solutions, dried and calcined under the same conditions as the unpromoted catalysts. The catalysts prepared in this work are denoted as Fe-Cu(X)/K(Y)/Al2O3, where X and Y represent the Cu/(Fe+Cu) and K/Fe atomic ratios, respectively. The hyphen “-” between two metals means co-impregnation of the two precursors, and the component beside support was impregnated first.


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The metal loadings for promoted and unpromoted monometallic catalysts, such as Fe/Al2O3, Cu/Al2O3, Fe/K(Y)/Al2O3 and Cu/K(Y)/Al2O3, are all fixed at 15 wt%. Also, Fe(12)/Al2O3, Fe(7)/Al2O3 and Fe(3.5)/Al2O3 referring to monometallic Fe/Al2O3 with metal loading of 12, 7 and 3.5 wt% (support weight basis), were prepared and tested as references. Corresponding loadings for different components are summarized in Table 1 for clarity. To make better comparison, Fe-Co(X)/K(Y)/Al2O3 catalysts were also prepared and tested following the same procedure, according to the literature 26. In addition, another catalyst, K(0.3)/Fe-Cu(0.17)/Al2O3, was prepared by first loading nitrates of Fe and Cu together and drying followed by loading K2CO3 by impregnation in order to allow comparison to examine the influence of K2CO3 impregnation sequence (either before or after Fe(NO3)3 and Cu(NO3)2). 2.2 Physical properties measurement The physical properties of calcined catalysts as well as Al2O3 support were measured using TriStar II analyzer (Micromeritics). Catalyst surface area, pore volume and average pore diameter were determined from the nitrogen (N2) adsorption/desorption isotherms measured at 77 K. All samples (ca. 0.25 g) were degassed under a N2 flow at 363 K for 1 h and 473 K for 12 h before analysis. The isotherms were elaborated according to the Brunauer-Emmett-Teller (BET) method for surface area calculation, while Barrett-Joyner-Halenda (BJH) model was used to obtain pore volume and average pore diameter. 2.3 Catalyst test The catalysts were tested in a high-pressure fixed-bed flow reactor system. About 0.20 g of the catalyst was charged into a stainless steel reactor (internal diameter, 6 mm) with amorphous SiO2 (around 0.48 g, Davisil Grade 62, particle size=75-250 × 10−6 m) as a diluent to



attain an aspect ratio of approximately 7.0. Prior to the activity test, the catalyst was reduced at 673 K for 2 h using a 50 mL (STP) min-1 H2 flow (purity > 99.995%) and then allowed to cool down to the reaction temperature of 573 K. The feed gas, 24 vol% CO2/ 72 vol% H2/ 4 vol% Ar (purity > 99.995 %), was employed to pressurize the system to 1.1 MPa (GHSV=3600 mL (STP) g-1 h-1). The flow rate and pressure of these gases were regulated with mass flow controllers and a backpressure regulator, respectively. Gaseous products were periodically sampled with computer-controlled gas samplers and analyzed by two online GCs, wherein an Agilent 3000 micro GC/TCD was used for online analyses of Ar, CO, CH4, and CO2, while gas-phase hydrocarbon products were analyzed by SRI 8610C GC/FID. CO2 conversion and space-time yields (STY) of gas-phase products were evaluated by the values obtained at 16-18 h on-stream. Activity data reported herein were based upon at least three-separated runs for each catalyst, and standard deviations for CO2 conversion, CH4 space time yield (STY), CO STY, and C2-C7 STY are < 0.9 %, 0.02 µmol g-1 s-1, 0.01 µmol g-1 s-1, and < 0.01 µmol g-1 s-1, respectively. 2.4 X-ray Diffraction (XRD) Reduced catalysts were cooled down under H2 flow to room temperature and then passivated with 0.99 vol% O2/He (purity > 99.999%) with a flow rate of 30 mL (STP) min-1 at ambient temperature for 5 h in order to preserve the reduced metallic phase after H2 reduction before exposure to air. . An area of O2 peak (effluent of O2) observed by Micro GC equipped with TCD was used as a check mark for the catalyst passivation and after 4~5 h, the O2 peak area became constant. The catalysts were then collected and hereafter denoted as reduced catalyst. XRD patterns of reduced catalysts were collected using a PANalytical Empyrean X-Ray Diffractometer with Cu Kα (λ=0.154059 nm) radiation, fixed slit incidence (0.25o divergence, 0.5o anti-scatter, specimen length 10 mm) and diffracted (0.25o anti-scatter, 0.02 mm nickel filter)


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optics. Samples were prepared by the front-loading method in which a powder sample is pressed into the cavity of a quartz zero-background support. Data was collected at 45 kV and 40 mA from 30-90o (2θ) using a PIXcel detector in scanning mode and 255 active channels for a duration time of 15 min. Resulting patterns were corrected for 2θ position using NIST (National Institute of Standards and Technology) 640c silicon and analyzed with Jade 2010 software by MDI of Livermore, CA. 3. Results and discussions 3.1 Physical properties of calcined catalysts Table 2 shows the textural parameters of the calcined Fe-Cu(X)/K(Y)/Al2O3 catalysts. The BET surface area of calcined Fe and Fe-Cu catalysts with small amount of Cu (X ≤ 0.17) were close to that of gamma-Al2O3 support (139 m2 g-1). More Cu leads to a decrease of BET surface area as a general trend. In the presence of metal oxides, the pore volume and average pore diameter were decreased from those of gamma-Al2O3 support, which suggested that small metal oxides were well dispersed inside the pores of support. As K content increased in FeCu(0.17)/K(Y)/Al2O3 catalysts, the BET surface area and pore volumes decreased, while the average pore volume barely changed. This decrease in surface area and pore volume results from the potassium covering some portions of gamma-Al2O3 pores.

3.2 Effect of Fe-Cu combination Table 3 summarizes the results for a series of Fe-Cu bimetallic catalysts with a wide range of Cu /(Fe+Cu) atomic ratios tested in CO2 hydrogenation at 573 K and 1.1 MPa. Figure 1



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illustrates the change of CO2 conversion as a function of Cu/(Fe+Cu) atomic ratio over FeCu/Al2O3 catalysts. The CO2 conversion with Fe alone (15 wt% Fe loading) is 22 %, which is consistent with the literature under the same reaction conditions

11

. Among the products, the

selectivity decreases in the following order: CH4 > C2+ > CO (see Table 3). CO2 conversion with Cu alone (i.e., 15 wt% Cu loading) exhibits a similar value (22.4 %) as Fe alone, but CO is the only product. Clearly, Cu is active for RWGS, as expected. Upon combining Cu and Fe together, the CO2 conversion increases with the increasing Cu content. The maximum was reached at atomic ratio Cu/(Cu+Fe)=0.17, but CO2 conversion decreases with a further increasing Cu content beyond this ratio. The star-marked points in the Figure 1 show the CO2 conversion with monometallic Fe catalysts with same Fe loading of Fe (12), Fe(7) and Fe(3.5) as in Fe-Cu(0.17), Fe-Cu(0.5) and Fe-Cu(0.75) bimetallic catalysts, respectively. In all the comparisons, Fe-Cu bimetallic catalysts give higher CO2 conversions than the corresponding monometallic Fe catalysts, which also demonstrates the synergetic enhancement of Fe-Cu bimetallic promotion on the activity. Figure 2 shows the time-on-stream (TOS) profiles for unpromoted and K-promoted Fe-Cu bimetallic catalysts. CO2 conversion exhibits a slight decrease for both catalysts with time on stream, but the rate of decrease is less than 0.1 % per hour from 16-50 h on stream. On the other hand, Both CH4 and CO yields are practically constant within ca. 5-50 h on stream. Figure 3 illustrates the effect of Cu content in Fe-Cu bimetallic catalysts on space-time yields (STY) of C2-C7 hydrocarbons, CH4, and CO. The STY of C2-C7 reveals an identical trend as that for CO2 conversion and maximizes at Cu/(Cu+Fe) atomic ratio of 0.17; however, CH4 and CO exhibit different composition-dependent behaviors: CH4 is significantly suppressed by the increase of Cu loading, while CO is promoted in the meantime. Compared with monometallic Fe


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catalysts (star points in the figure), Fe-Cu bimetallic catalysts (with the same Fe loading) give much higher C2-C7 hydrocarbon STYs. Considering Cu alone cannot produce hydrocarbons, FeCu bimetallic catalysts show strong synergetic promotion in the higher hydrocarbon synthesis. Similarly, the selectivities of C2+, CH4, and CO follow the same trends as corresponding STYs, and the combination of Cu and Fe considerably improves the selectivity of C2+ hydrocarbons formation in comparison to that with Fe alone (e.g., 53 vs 39 mol%, Table 3). The AndersonSchulz-Flory distribution 36 was also examined for the chain growth , and the resulting alpha (α) values are presented in Table 3. At the optimal composition (i.e., Cu/(Cu+Fe)=0.17), the alpha value also maximizes, indicating the promoting effect of Fe-Cu combination on carbon chain growth. Furthermore, at a similar CO2 conversion level, Fe-Cu(0.17) also exhibits higher C2+ selectivity and chain growth value than Fe-Co(0.17) under the same reaction conditions (see Table 3). Therefore, the combination of Fe and Cu leads to a significant promoting effect on C2+ hydrocarbon synthesis from CO2 hydrogenation. One possible reason for such advance is that the addition of Cu might enhance the formation of CO or CO-like intermediate via RWGS, thereof providing more possibility of C-C bond growth. On the other hand, potential formation of Fe-Cu alloy-like species might provide an alternate pathway to produce C2+ hydrocarbons. The mechanism of hydrocarbon synthesis from CO2/H2 is still in debate, and was considered by several studies as the modified version of Fischer-Tropsch synthesis (FTS)

12, 37

.

Some literature suggested that CO2 methanation proceeds via a direct hydrogenation process, and the formation of methane in this manner accounts for the formation of a majority of hydrocarbons

38-40

. The formation of C2+ hydrocarbons via the direct hydrogenation of CO2 has

also been considered in a few reports 27, 28. In a previous work, we considered the possibility for



CO2 hydrogenation to hydrocarbons over K-promoted Fe-Co catalysts via two-step reactions involving the formation of CO or CO-like intermediate as the first step, followed by the hydrocarbon formation via FTS 7. As discussed, the Cu catalyst can behave as a RWGS-active component to improve the conversion of CO2 to CO or CO-like intermediate, while Fe could function as active sites for C-C chain growth to form higher hydrocarbons. Another possibility is that the combination of Cu and Fe can create alloy-like species, and promote an alternate pathway without going through CO as an intermediate. Our recent computational work on Fe-Cu bimetallic catalysts suggests that a new reaction pathway becomes favored over Fe-Cu at a surface Cu coverage level of 4/9 monolayer which leads to a

significant bimetallic promoting effect on C-C coupling thus C2+ hydrocarbon

synthesis. The main finding from the computational work is that adding Cu to Fe can change the reaction pathway: such that formation of CO (on mono-metallic Cu and Fe) and subsequent hydrogenation of CO leading to CH* (on Fe(100)) is the main path which has been addressed 7,20, whereas bimetallic Fe-Cu catalyst leads to a completely different pathway without going through CO as the intermediate product, as shown below 34, 35. Main Path over Bimetallic Cu-Fe(100) Surface: CO2*HCOO*HCOOH*HCO*HCOH*CH* CH-CH*, in which both the HCOO*HCOOH* and HCO*HCOH* steps have substantial barriers. Main Path over Mono-metallic Fe(100) Surface: CO2*CO*HCO*HCOH*CH* CH-CH*, in which the hydrogenation of HCO* to HCOH* is the rate-limiting step. The significance of these prior findings 34, 35 relevant to the present experimental work is that (1) there are distinctly different reaction pathways between monometallic Fe catalyst and


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bimetallic Fe-Cu catalyst in CO2 hydrogenation and C-C coupling, and (2) Fe-Cu bimetallic catalysts may enable new reaction pathway for direct CO2 hydrogenation to higher hydrocarbons without going through CO or CO-like intermediates. 3.3 Crystallite structures of Fe-Cu catalysts XRD patterns of reduced catalysts are illustrated in Figures 4 and 5. In Figure 4, Fe-Cu bimetallic catalysts are compared with Fe and Cu monometallic catalysts and γ-Al2O3 support. The intensities of γ-Al2O3 peaks (Figures 4(a) and 5(a)) decrease significantly with the loading of Fe and Cu. In Figures 4(b) and 5(b), when spectra within 40o-50o of 2θ range are magnified, FeO is observed

41

for the reduced Fe/Al2O3 at 2θ = 42.2o with d-spacing = 2.14Å. Monoclinic

Fe4O5 with d-spacing = 2.04Å (combination of both iron II and III

42

) is observed at 44.4o,

implying the partial reduction of Fe2O3. In Figure 4(c), the FeO peak disappears in the reduced Fe-Cu(0.17)/Al2O3 catalyst, while the left shoulder of γ-Al2O3 peak becomes stronger. Multiple peaks superimpose in this shoulder. After deconvolution (Figure 5(c)), a new peak around 2θ = 45o with d-spacing = 2.01-2.02Å is observed, which is ascribed to Fe-Cu alloy species 43-45. The Bragg angles corresponding to Fe(0.75)Cu(0.25), Fe(0.7)Cu(0.3), Fe(0.6)Cu(0.4) and Fe(0.5)Cu(0.5) are so close that they cannot be distinguished at this stage. Furthermore, a small shoulder also appears in the reduced Fe-Cu(0.5)/Al2O3 at 44.4o (Figure 4(d)), but no distinct FeCu alloy peak shows up even after deconvolution (Figure 5(d)). For the reduced Cu/Al2O3, as shown in Figure 4(e) and Figure 5(e), typical Cu0 peaks are revealed, implying a larger Cu0 crystallite size in comparison to either Fe oxide or Fe-Cu alloy species.



3.4 Effect of K addition The effect of K addition on catalytic performance of Fe-Cu bimetallic catalysts was also studied. For comparison, all Fe-Cu bimetallic catalysts in Figure 1 were incorporated with promoter K, and the K/Fe atomic ratio was initially fixed at 0.3 26. The results in Figures 1 and 6 clearly show that K addition to Fe-Cu gives higher CO2 conversion at all Cu/(Cu+Fe) atomic ratios than the corresponding unpromoted Fe-Cu catalysts. Moreover, a similar promoting effect is observed for C2-C7 STY over the promoted catalysts in comparison to the unpromoted ones. As shown in Table 3, the addition of K suppresses CH4 formation while enhances the selectivity towards C2+ hydrocarbons, especially for light olefins (C2-C4) as evidenced from both olefin/paraffin ratios and alpha values. On the other hand, as shown in Table 3, we observed almost no variation in CO2 conversion and products selectivities between the catalysts prepared by different impregnation sequences of K2CO3 and Fe and Cu nitrites. The effect of K loading amount was also examined with the K/Fe atomic ratio varying within 0.0-1.0. Figure 7 illustrates the distribution of gas-phase hydrocarbons in terms of GCFID chromatograms as a function of K loading levels over Fe-Cu(0.17) catalysts. At lower loading level of K, namely Fe-Cu(0.17)/K(0.3), although paraffins still dominate, light olefins are observed evidently along with paraffins. C2-C4 light olefins are selectively produced among light hydrocarbons when the K/Fe atomic ratio was increased to 0.5. A further increase of K loading (i.e., K/Fe=1.0) dramatically enhanced the light olefin production to a level where C2-C4 olefins become dominant in the light hydrocarbons. Such K loading-dependent behavior becomes more distinct when plotting the STY of light olefins (C2-C4) and corresponding olefin/paraffin ratio (O/P) as a function of K/Fe atomic ratio in Figure 8. Clearly, the addition of K into Fe-Cu bimetallic catalysts significantly enhances the formation of light olefins as the STY


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increases with the increase of K/Fe atomic ratio. Moreover, the C2-C4 olefin production becomes dominant (O/P > 1) when K/Fe atomic ratio is greater than 0.5, the value of which even reaches as high as 5.20 at K/Fe=1.0. In addition, the corresponding alpha value of these K-promoted FeCu catalysts displays a similar K loading-dependent behavior as both C2-C4 olefin STY and O/P ratio. Thus, the correlation of light olefin (C2-C4) with K content indicates that the addition of K could enhance C2-C4 olefin yield while suppressing paraffin formation. As reported, K could tune the adsorption properties towards H2 and CO2 on the surface due to the partial coverage of active metal, which, thereof, restricts the hydrogenation of produced olefins

7, 22

. Thus, it would be of interest to investigate the effects of Fe-Cu

combination and K addition on adsorption properties towards different reactants. Table 3 also includes the activity performance of K-promoted Fe-Co catalysts with corresponding metal and promoter loadings for comparison. The Fe-Cu(0.17)/K catalysts exhibits higher selectivity to C2+ hydrocarbons and higher alpha value in comparison to FeCo(0.17)/K catalysts with a similar composition. Notably, the unpromoted or K(0.3)-promoted Fe-Co(0.17) bimetallic catalysts exhibit higher conversion than unpromoted or K-promoted monometallic Fe catalyst, but actually sacrifice C2+ hydrocarbon selectivity, which, again, demonstrates a remarkable advantage of Cu addition to Fe by increasing activity while maintaining or increasing the C2+ selectivity. On the other hand, when the K/Fe atomic ratio exceeds 0.5, the Fe-Cu-based catalysts exhibits superior selectivity and alpha value towards C2+ hydrocarbons and chain growth. This could be ascribed to the addition of K to Fe which provides a higher surface coverage of carbon species for C-C coupling and further suppresses undesired CH4 formation in the meantime. Compared to K-promoted Fe-Co catalysts in our previous work, the K promoted Fe-Cu bimetallic catalysts exhibit notably better selectivity towards C2+



hydrocarbon synthesis at higher K/Fe atomic ratios (e.g., ≥ 0.5) under the same reaction conditions. 4. Conclusion The combination of Fe and Cu at specific composition led to a strong bimetallic promotion in CO2 conversion and C2+ hydrocarbons formation. The addition of Cu to Fe can successfully suppress the undesired CH4 formation while enhancing CO and C2+ hydrocarbon formation. Furthermore, XRD results suggest the formation of Fe-Cu alloy in Fe-Cu(0.17)/Al2O3 catalyst. The addition of K into Fe-Cu dramatically enhances the production of olefin-rich C2-C4 hydrocarbons over Fe-Cu bimetallic catalysts. The Fe-Cu/K catalysts exhibit superior selectivity towards C2+ hydrocarbons synthesis than Fe-Co/K catalysts under the same reaction conditions. These results are crucial in developing new bimetallic catalysts and a fundamental understanding of the mechanism involving surface H/C ratio, C-C bond formation, and reaction pathways for higher hydrocarbon synthesis. This study also suggests the reaction pathways over Fe-Cu bimetallic catalysts are different from those over Cu alone and Fe alone. Acknowledgements This research is supported in part by the Pennsylvania State University through the Joint Center for Energy Research established between Penn State and Dalian University of Technology. Two of the authors (W. Wang and X. Jiang) also acknowledge the partial financial support from Chinese Scholarship Council (CSC). This contribution was identified as the Best Presentation by Dr. Maocong Hu (New Jersey Institute of Technology) as Session Chair in the ACS Symposium on “Carbon Management: Advances in Carbon Efficiency, Capture,


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Conversion, Utilization & Storage” as a part of the 2017 ACS Fall National Meeting in Washington, D.C.



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Caption to figures Figure 1. Changes of CO2 conversion over Fe-Cu(X)/Al2O3 and Fe-Cu(X)/K(0.3)/Al2O3 catalysts as a function of Cu/(Cu+Fe) atomic ratio. Figure 2. Time-on-stream profiles of CO2 conversion and product yields over FeCu(0.17)/Al2O3 (A) and Fe-Cu(0.17)/K(0.3)/Al2O3 (B). Figure 3. Effect of Cu/(Cu+Fe) atomic ratio on CH4, C2-C7 hydrocarbons, and CO STYs over Fe-Cu(X)/Al2O3 catalysts. Figure 4. XRD patterns of the reduced Fe-Cu(X)/Al2O3 catalysts (a) γ-Al2O3 support, (b) reduced Fe/Al2O3, (c) reduced Fe-Cu(0.17)/Al2O3, (d) reduced Fe-Cu(0.5)/Al2O3, (e) reduced Cu/Al2O3. Symbols assignment: ■:γ-Al2O3; ●: FeO; ▼: Fe4O5; ♦: Fe(x)Cu(1-x); ▲: Cu0. Figure 5. Magnified XRD patterns of the reduced Fe-Cu(X)/Al2O3 catalysts with thick lines indicating original patters and dashed lines indicating deconvoluted patterns (a) γ-Al2O3 support, (b) reduced Fe/Al2O3, (c) reduced Fe–Cu(0.17)/Al2O3, (d) reduced Fe–Cu(0.5)/Al2O3, (e) reduced Cu/Al2O3. Symbols assignment: ■:γ-Al2O3; ●: FeO; ▼: Fe4O5; ♦: Fe(x)Cu(1-x); ▲: Cu0. Figure 6. Effect of Cu/(Cu+Fe) atomic ratio on the C2-C7 hydrocarbons STY over FeCu(X)/Al2O3 and Fe-Cu(X)/K(0.3)/Al2O3 catalysts. Figure 7. GC-FID chromatograms of gas-phase hydrocarbons from CO2 hydrogenation on Kpromoted and unpromoted Fe-Cu(0.17)/Al2O3 catalysts. Figure 8. Effect of K/Fe atomic ratio (Y) on the C2-C4 olefin STY and O/P ratio from CO2 hydrogenation over Fe-Cu(0.17)/K(Y)/Al2O3 catalysts.


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Table 1 Metal loadings of K-promoted and unpromoted Fe-Cu/Al2O3 catalysts Loading / wt% (support weight basis)

Catalyst Fe Fe Fe Fe Fe-Cu(0.05) Fe-Cu(0.10) Fe-Cu(0.17) Fe-Cu(0.25) Fe-Cu(0.50) Fe-Cu(0.75) Cu

Fe 15 12 7 3.5 14.2 13.3 12.2 10.9 7.0 3.4 0

Fe/K(0.3) Fe-Cu(0.05)/K(0.3) Fe-Cu(0.10)/K(0.3) Fe-Cu(0.17)/K(0.3) Fe-Cu(0.25)/K(0.3) Fe-Cu(0.50)/K(0.3) Fe-Cu(0.75)/K(0.3) Fe-Cu(0.17)/K(0.5) Fe-Cu(0.17)/K(1.0) Cu/K(0.3) Fe-Co(0.17) a) Fe-Co(0.17)/K(0.3) a) Fe-Co(0.17)/K(1.0) a) K(0.3)/Fe-Cu(0.17) b)

Cu 0

K -

Co -

-

-

0.8 1.7 2.8 4.1 8.0 11.6 15

-

-

15 14.2 13.3 12.2 10.9 7.0 3.4 12.2 12.2 0

0 0.8 1.7 2.8 4.1 8.0 11.6 2.8 2.8 15

3.2 3.0 2.8 2.6 2.3 1.5 0.7 4.3 8.6 2.8

-

12.4 12.4 12.4 12.2

2.8

2.6 8.7 2.6

2.6 2.6 2.6 -

a)

-

As reference 26. b) K2CO3 was impregnated after Fe(NO3)3 and Cu(NO3)2, followed by drying and calcination in same conditions


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 26 of 36

Table 2 Physical properties of calcined Fe-Cu(X)/K(Y)/Al2O3 catalysts BET surface area (m2 g-1)

Pore volume (cm3 g-1)

Average pore diameter (nm)

Al2O3

139

1.07

24

Fe/Al2O3

146

0.72

19

Fe-Cu(0.10)/Al2O3

133

0.69

20

Fe-Cu(0.17)/Al2O3

141

0.73

20

Fe-Cu(0.25)/Al2O3

126

0.72

21

Fe-Cu(0.50)/Al2O3

116

0.76

22

Cu/Al2O3

110

0.81

24

Fe-Cu(0.17)/K(0.3)/Al2O3

117

0.66

19

Fe-Cu(0.17)/K(0.5)/Al2O3

108

0.61

19

Fe-Cu(0.17)/K(1.0)/Al2O3

78

0.51

20

Catalyst


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Table 3 CO2 conversion and product selectivities of K-promoted and unpromoted Fe-Cu/Al2O3 in comparison to monometallic catalysts a) Catalyst

CO2 conv. (%)

Prod. selec. (C-mol%) CH4 C2+ b) CO

O/P ratio c)

Alpha d)

Fe

22.0

43

39

18

0.02

0.41

Fe-Cu(0.05)

24.2

28

51

21

0.02

0.42

Fe-Cu(0.10)

26.2

26

51

23

0.02

0.42

Fe-Cu(0.17)

28.5

24

53

23

0.02

0.47

Fe-Cu(0.25)

24.6

24

50

27

0.03

0.44

Fe-Cu(0.50)

23.5

21

49

30

0.02

0.47

Fe-Cu(0.75)

22.8

18

37

45

0.00

0.42

Cu

22.4

0

0

100

-

-

Fe/K(0.3)

26.9

13

68

19

1.94

0.56

Fe-Cu(0.05)/K(0.3)

29.5

14

71

15

0.97

0.54

Fe-Cu(0.10)/K(0.3)

29.9

15

70

14

0.44

0.52

Fe-Cu(0.17)/K(0.3)

31.4

15

69

16

0.36

0.53

Fe-Cu(0.25)/K(0.3)

29.4

16

66

18

0.07

0.52

Fe-Cu(0.50)/K(0.3)

29.0

20

58

22

0.03

0.47

Fe-Cu(0.75)/K(0.3)

25.7

21

40

39

0.00

0.42

Fe-Cu(0.17)/K(0.5)

29.7

9

76

15

1.83

0.60

Fe-Cu(0.17)/K(1.0)

29.3

7

76

17

5.20

0.62

Cu/K(0.3)

22.2

0

0

100

-

-

Fe-Co(0.17) e)

26.5

49

39

12

0.00

0.42

Fe-Co(0.17)/K(0.3) e)

30.8

28

59

13

0.24

0.48

Fe-Co(0.17)/K(1.0) e)

30.9

15

64

21

5.19

0.53

K(0.3)/Fe-Cu(0.17) f)

30.6

18

62

20

0.35

0.52

a)

Reaction conditions: 573 K, 1.1 MPa, GHSV=3600 mL (STP) g-1 h-1; b) Including small amounts of alcohols; c) Olefin to paraffin atomic ratio (C2-C4); d) Alpha (chain growth probability) were calculated from molar fractions of C1-C7 hydrocarbons; e) Catalysts were prepared by same method from Fe(NO3)3·9H2O, Co(NO3)2·6H2O, and K2CO3 as precursors. Same combination as 26; f) K2CO3 was impregnated after Fe(NO3)3 and Cu(NO3)2, followed by drying and calcination in same conditions



Figure 1. Changes of CO2 conversion over Fe-Cu(X)/Al2O3 and Fe-Cu(X)/K(0.3)/Al2O3 catalysts as a function of Cu/(Cu+Fe) atomic ratio.


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Figure 2. Time-on-stream profiles of CO2 conversion and product yields over Fe-Cu(0.17)/Al2O3 (A) and Fe-Cu(0.17)/K(0.3)/Al2O3 (B).



Figure 3. Effect of Cu/(Cu+Fe) atomic ratio on CH4, C2-C7 hydrocarbons, and CO STYs over Fe-Cu(X)/Al2O3 catalysts.


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Figure 4. XRD patterns of the reduced Fe-Cu(X)/Al2O3 catalysts (a) γ-Al2O3 support, (b) reduced Fe/Al2O3, (c) reduced Fe–Cu(0.17)/Al2O3, (d) reduced Fe–Cu(0.5)/Al2O3, (e) reduced Cu/Al2O3. Symbols assignment: ■:γ-Al2O3; ●: FeO; ▼: Fe4O5; ♦: Fe(x)Cu(1-x); ▲: Cu0.



Figure 5. Magnified XRD patterns of the reduced Fe-Cu(X)/Al2O3 catalysts with thick lines indicating original patters and dashed lines indicating deconvoluted patterns (a) γ-Al2O3 support, (b) reduced Fe/Al2O3, (c) reduced Fe–Cu(0.17)/Al2O3, (d) reduced Fe–Cu(0.5)/Al2O3, (e) reduced Cu/Al2O3. Symbols assignment: ■:γ-Al2O3; ●: FeO; ▼: Fe4O5; ♦: Fe(x)Cu(1-x); ▲: Cu0.


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Figure 6. Effect of Cu/(Cu+Fe) atomic ratio on the C2-C7 hydrocarbons STY over FeCu(X)/Al2O3 and Fe-Cu(X)/K(0.3)/Al2O3 catalysts.



Figure 7. GC-FID chromatograms of gas-phase hydrocarbons from CO2 hydrogenation on Kpromoted and unpromoted Fe-Cu(0.17)/Al2O3 catalysts.


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Figure 8. Effect of K/Fe atomic ratio (Y) on the C2-C4 olefin STY and O/P ratio from CO2 hydrogenation over Fe-Cu(0.17)/K(Y)/Al2O3 catalysts.



TOC Graphic


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