Promotional Effects of Cesium Promoter on Higher Alcohol Synthesis

Jul 25, 2016 - *E-mail for J.L.: [email protected]., *E-mail for D.M.: [email protected]., *E-mail for Y.W.: [email protected]. ... The promotional eff...
0 downloads 20 Views 2MB Size
Download PDF
Research Article pubs.acs.org/acscatalysis

Promotional Effects of Cesium Promoter on Higher Alcohol Synthesis from Syngas over Cesium-Promoted Cu/ZnO/Al2O3 Catalysts Jie Sun,†,‡ Qiuxia Cai,‡,§ Yan Wan,† Shaolong Wan,† Li Wang,∥ Jingdong Lin,*,† Donghai Mei,*,‡ and Yong Wang*,†,‡,⊥ †

National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China ‡ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of China ∥ Sinochem Quanzhou Petrochemical Co. Ltd., Quanzhou 362103, People’s Republic of China ⊥ Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164, United States S Supporting Information *

ABSTRACT: The promotional effects of a cesium promoter on higher alcohol (C2+OH) synthesis from syngas over Cs2O-Cu/ZnO/Al2O3 catalysts were investigated using a combined experimental and density functional theory (DFT) calculation method. In the presence of a cesium promoter, the C2+OH productivity increases from 77.1 to 157.3 g kgcat−1 h−1 at 583 K due to the enhancement of the initial C−C bond formation. A detailed analysis of chain growth probabilities (CGPs) confirms that initial C−C bond formation is the rate-determining step in the temperature range of 543−583 K. Addition of a cesium promoter significantly increases the productivities of 2-methyl-1-propanol, while the CGP values (C3* to 2methyl-C3*) are almost unaffected. With the assistance of a cesium promoter, the CGPs of the initial C−C bond formation step (C1* to C2*) increase from 0.13 to 0.25 at 583 K. DFT calculations indicate that the initial C−C bond formation during syngas synthesis over the ZnCu(211) model surface is mainly due to the HCO + HCO coupling. In the presence of Cs2O, the stabilities of key intermediates such as HCO and H2CO are enhanced, which facilitates both HCO + HCO and HCO + H2CO coupling steps with lower activation barriers. In addition, Bader charge analysis suggests that the presence of cesium ions could facilitate nucleophilic coupling between HCO and H2CO for the initial C−C bond formation. KEYWORDS: syngas, higher alcohol, Cu/ZnO/Al2O3, chain growth probability, cesium, DFT calculations

1. INTRODUCTION The increasing consumption of crude oil has caused a sharp decline in oil storage around the world.1 In recent years, to alleviate the strong dependence on oil, effective utilization of coal, natural gas or shale gas, and biomass has attracted great interest.2−4 Higher alcohols (C2+OH) have been considered as potential fuels, fuel additives, and hydrogen carriers.5 C2+OH could also be converted into other value-added chemicals (e.g., lower olefins, etc.) by more energy efficient and facile processes in comparison with processes for conversion of methanol to olefins.6,7 C2+OH can be synthesized directly from syngas (a mixture of H2 and CO), which is derived from abundant reserves of coal, natural gas, nonfood biomass, and even some solid and/or liquid carbonaceous wastes.8,9 In general, there are four types of synthesis catalysts: i.e., Rh-based catalysts, modified Fischer−Tropsch catalysts, Mo/MoS2-based catalysts, and modified Cu-based methanol synthesis catalysts.5 Cu-based © XXXX American Chemical Society

catalysts are usually modified with different promoters such as alkali (Li, Na, K, Rb, K, etc.) and/or transition metals (Fe, Co, Ru, etc.).10−12 With alkali-metal-promoted Cu-based catalysts, the C2+OH products mainly consist of ethanol, propanol, and some 2-methyl-branched alcohols.13 It was suggested that the effectiveness of alkali-metal promoters on C2+OH selectivity and productivity followed the trend Cs > Rb > K > Na > Li.14 The presence of alkali-metal promoters not only increases the selectivity of C2+OH but also enhances the conversion of CO.15 Thus, the productivity of C2+OH can be significantly improved. Although extensive studies have been previously reported, the promotional effects of alkali-metal promoters in C2+OH synthesis over Cu/ZnO/Al2O3 catalysts still not fully underReceived: March 31, 2016 Revised: July 19, 2016

5771

DOI: 10.1021/acscatal.6b00935 ACS Catal. 2016, 6, 5771−5785

Research Article

ACS Catalysis

retention of oxygen in C1*. Additionally, it was inferred that the retention of oxygen was promoted via a β-ketoalkoxide intermediate with its anionic oxygen bonded to the central Cs+ ion. It is worth noting that the chain growth mechanism of 2-propanol formation is quite different from that of 1-propanol. 2-Propanol could be derived from ethanol self-condensation followed by retro-aldol reactions.23 Interestingly, the productivity of 2-propanol was negligible unless a sufficient amount of extra ethanol was introduced into the feed gas.24,25 β addition between C1 and linear Cn (n ≥ 3) intermediates could yield 2methyl-branched alcohols such as 2-methyl-1-propanol and 2methyl-1-butanol, as proposed in previous literature.26 However, in the studies of Hilmen, Iglesia et al., it was demonstrated that 2-methyl-1-butanol and 2-methyl-1-pentanol were predominantly from aldol condensation between ethanol and 1-propanol and self-condensation of 1-propanol, respectively.23 These authors reported that the reaction rate of aldol condensation was as high as ∼22 times the linear chain growth rate when 1-propanol was introduced to syngas over Cs2Opromoted Cu/ZnO/Al2O3 catalysts. In addition, it was proposed that 1-butanol was predominantly derived from selfcondensation of ethanol and 1-pentanol was derived from aldol condensation between ethanol and 1-propanol, respectively. This was in consistent with previous studies of Breman et al.13,27 A few theoretical studies of the C2+OH synthesis over Cubased catalysts using density functional theory (DFT) calculations have been carried out. The Cu-based catalysts were modeled using Cu(211), Mn-doped Cu(211), and copper clusters.28−30 The initial C−C bond formation was attributed to a coupling of CO/HCO with CHx (x = 1−3), which was similar to the C−C coupling mechanism over Rh-based catalysts. To our best of our knowledge, DFT studies on the mechanism of C2+OH synthesis over alkali-metal-promoted Cu/ZnO/Al2O3 catalysts have not been reported. In the present work, chain growth probabilities are employed to investigate the promotional effects of a cesium promoter on the initial C−C bond formation and subsequent chain growth separately. Then, the combined influence of key reaction parameters on the initial C−C bond formation is examined in detail. To gain more fundamental insights into the promoting role of a cesium promoter in the initial C−C coupling steps, DFT calculations of several key reaction steps, such as HCO + HCO, HCO + H2CO, HCO + CO, etc. over the Cs2OZnCu(211) surface have been performed. Bader charge analysis has also been conducted to study the influence of a cesium promoter on the charge distribution of the catalyst surface and the adsorbed key intermediates.

stood.5,16 It has been generally accepted that the initial C−C bond formation is the rate-determining step for C2+OH production and subsequent chain growth is attributed to an aldol condensation reaction.17 It is worth noting that the reaction pathways over Cu/ZnO/Al2O3 catalysts are quite different from those over K-promoted CuMgCeOx catalysts. On CuMgCeOx catalysts, the initial C−C bond formation was via methyl acetate intermediates,18 while on Cu/ZnO/Al2O3 catalysts, the initial C−C bond formation was predominately attributed to the coupling of two C1 intermediates that could be derived from syngas and/or methanol.17,19 Using the isotopic tracers 12CO/H2 and 13CH3OH, Elliott et al. found that the fraction of 13C in ethanol was dependent on the partial pressure of 13CH3OH that had been introduced into the feed gas.19 The fraction of 13C-labeled ethanol decreased quickly with increasing residence time. At a shorter residence time (0.49 s), the percentage of 13C-labeled ethanol was 76.1%, including 51.5% of doubly labeled ethanol, at a 13CH3OH partial pressure of 0.29 MPa. However, at a longer residence time (5.8 s), the percentage of 13C-labeled ethanol was only 43.8%, including merely 14.3% of doubly labeled ethanol. On the basis of these observations, they argued that there was a common surface C1 intermediate shared by both methanol and ethanol.19 Using similar isotopic tracer experiments, Nunan et al. suggested that the initial C−C bond formation was through the nucleophilic attack between the nucleophile (adsorbed HCO species on Cs+) and the electropositive carbon of the adsorbed H2CO.20 The existence of surface HCO species has further been confirmed by CH3I trapping reactions.21 Xu and Iglesia reported that ethanol was mainly generated from a coupling reaction between two methanol-derived C1 intermediates over cesium-promoted Cu/ZnO/Al2O3 catalysts.18 They also suggested that the C1 intermediates could be stabilized by Cs+, and the presence of the Cs+−O2− cation−anion pair helped to activate C−H in formaldehyde for the formation of surface-bonded formyl species. On the other hand, Subramani and Gangwal proposed that the precursors of ethanol (i.e., acetyl species etc.) were formed from two adsorbed formyl intermediates.5 Subsequent hydrogenation of the acetyl intermediates led to ethanol formation. Until now, it has remained unclear whether the coupling reactions between two formyl species or between one formyl species and some other intermediate (e.g., formaldehyde, CHx, CO, etc.) are the key steps in the synthesis of C2+OH over alkali-metal-promoted Cu/ZnO/Al2O3 catalysts.16 Moreover, the influence of alkalimetal promoters on the adsorption of reactants and key reaction intermediates are also not fully understood. Previous studies suggested that the chain growth routes of C2+OH synthesis from syngas over Cu/ZnO/Al2O3 catalysts were different from the chain growth routes in the traditional Fischer−Tropsch synthesis using catalysts based on transition metals (e.g., Fe, Co, and Ru).22 As such, the distribution of C2+OH might not necessarily obey the Anderson−Schulz− Flory distribution. It was generally accepted that C1 species were formed through CO hydrogenation or methanol dehydrogenation.19,20 Coupling of two C1 species leads to the formation of C2-oxygenates. The chain growth step of C2* to C3* can then be achieved via both linear growth and aldol-type addition (β addition).23 Further study on the β addition by adding CH313CH2OH to 12CO/H2 over Cu/ZnO catalysts indicated that the cesium promoter enhanced the formation of 13 CH3CH2CH2OH.24 The position of labeled 13C in 1-propanol suggested that β addition of C1* to C2* occurred with the

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Catalyst Preparation and Catalytic Activity Measurements. All Cu/ZnO/Al2O3 catalysts were prepared with the coprecipitation method from a mixed aqueous solution of Cu(NO3)2·3H2O/Zn(NO3)2·6H2O/Al(NO3)3·9H2O (6/3/ 1 molar ratio).31,32 A stoichiometric amount of sodium carbonate was used as the precipitating agent. Coprecipitation was performed at 343 K and a pH of 7.0. The mixed metal nitrate solution and sodium carbonate solution were placed in a glass reactor simultaneously with stirring. The precipitate was aged at the same temperature for 5 h and washed with deionized water. The inductively coupled plasma (ICP) analysis of the Cu/ZnO/Al2O3 catalyst precursor showed that the sodium content in the catalyst precursor is below 0.01 mol %. 5772

DOI: 10.1021/acscatal.6b00935 ACS Catal. 2016, 6, 5771−5785

Research Article

ACS Catalysis

Figure 1. Scheme of simplified chain growth routes of C2+OH synthesis over alkali-metal-modified Cu/ZnO/Al2O3 catalysts.

This suggests that the residual sodium content is minimal and has a negligible influence on the performance of the catalyst. Then, the precursor was dried at 363 K overnight and calcined at 623 K for 4 h. Finally, the oxide precursor was modified with cesium promoter using the incipient wetness impregnation (IWI) method. The catalyst precursor was degassed within a vacuum drying oven at 393 K for at least 6 h before the IWI process. First, a stoichiometric amount of Cs2CO3 (Aladdin, 99.9%) was dissolved in deionized water and then it was added to the precursor followed by drying (393 K, 6 h) and recalcination (623 K, 4 h). H2 temperature-programmed reduction (H2-TPR), X-ray powder diffraction (XRD), the Brunauer−Emmett−Teller (BET) method, and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the obtained catalysts. The characterization results can be seen in our recent study.33 Catalytic activity measurements were carried out in a fixedbed reaction system. The quartz tubular reactor (i.d. 5 mm) was held within a three-zone heater to ensure a uniform axial heat distribution. A 0.1−0.6 g portion of the catalyst was diluted with silica sand (Vcat/VSiO2 = 1/2) before being transferred to the reactor. The syngas (from Linde Gas Ltd.) was purified by passing it through a heated 13X molecular sieve to remove the iron and/or nickel carbonyls prior to entering the catalyst bed. Before to the activity test, all of the samples were reduced in situ at 523 K for 4 h using 5% H2 diluted in argon. The products were analyzed with a gas chromatograph (GC 2060 from Shanghai Ruimin Instruments, Inc.) equipped with a packed column (3.0 m in length, filled with TDX-201, Tianjin Chem. Reagent Co. Ltd.) and two capillary columns (KB-PLOT Q, 30 m × 0.32 mm × 10.00 μm; DB-WAXETR, 50 m × 0.32 mm × 1.0 μm). The packed column was connected to a TCD for the analysis of CO, CO2, and N2 (internal standard gas). Two capillary columns worked in parallel for a better separation of alkanes and condensable oxygenates. Conversion (X) of CO, carbon-based selectivity (S), and productivity (P) of products were calculated using eqs 1−4. The selectivity of hydrocarbons, alcohols, and other oxygenates was calculated by an internal normalization method followed by a correction with CO2 selectivity, considering that CO2 is undetectable by FIDs. For example, the selectivity of ethanol (SC2H5OH) can be calculated with eq 3, where n is the relevant number of C atoms in each molecule. Then, the productivity of ethanol (PC2H5OH) was calculated with eq 4,

where MC2H5OH is the molecular weight of ethanol. Before the activity data were taken, each test was operated for more than 9 h until reaching the steady state. In addition, the liquid products were collected and analyzed with a 7890GC-5975MS system to further confirm the products.

XCO

⎡ ⎢ = ⎢1 − ⎢ ⎢⎣

SCO2 =

SC2H5OH

⎤ ⎥ product ⎥ × 100% CO ⎥ N2 ⎥ feed ⎦

( ) ( ) CO N2

(CO2 )product (CO)feed × XCO

(1)

× 100%

(2)

⎡ n ⎡ ⎤ = (C2H5OH) × 2/⎢ ∑ ⎢((CnH 2n + 2) × n)⎥ ⎢⎣ 1 ⎢⎣ ⎦⎥ n

+

⎡

∑ ⎢((CnH 2n+ 1OH) × n) + ((CH3OCH3) 1

⎣⎢

⎤⎤ × 2) + ((CH3OOCH) × 2)⎥⎥ ⎥⎦⎥⎦ × (1 − SCO2) × 100%

(3)

PC2H5OH = (CO)feed × XCO × SC2H5OH × (2)−1 × MC2H5OH (4)

2.2. Chain Growth Probabilities. The chain growth routes of C2+OH synthesis over Cu/ZnO/Al2O3 catalysts are shown in Figure 1. The overall productivity of carboxylic acid, esters, ethers, and some other 2-alcohols is negligible in this work, consistent with previous studies.10,34 Chain termination probabilities (CTPs) (βCn) have been used to describe the product distribution in the Fischer−Tropsch synthesis and C2+OH synthesis from syngas.23,35−37 Similar to the previous study,23 only the condensation routes for chain growth probabilities (CGPs) (αn) that are relative to 1-butanol and C5+OH are calculated using eq 7. For different chain lengths (n) of C2+OH, the CTPs are calculated using eqs 5 and 6. ϕCn is the mole fraction of different lengths of alcohols with number (n) of C atoms in the alcohol products. rp,Cn to Cn+1 (n ≥ 1) is the propagation reaction rate from Cn to Cn+1 species. rt,Cn (n ≥ 1) 5773

DOI: 10.1021/acscatal.6b00935 ACS Catal. 2016, 6, 5771−5785

Research Article

ACS Catalysis is the overall termination reaction rate toward the final Cn products. As indicated in eq 6, the overall termination probabilities equal a linear sum of individual steps toward the related final products with the number of carbons (n), where a, b, and c indicate different final products. All of the primary reaction routes are marked with rt,Cn or rp,Cn to Cn+1 shown in Figure 1. ∞

βC = rt, n/rp, n = ϕn / n

∑

ϕC

i

(5)

i=n+1

βT,C = βa,C + βb,C + βc,C + ... n

n

n

αn = 1/(1 + βT,C ) = n

(6)

n

∞

∞

∑

ϕC /∑ ϕC

i=n+1

i=n

i

i

(7)

2.3. Computational Methods. All DFT calculations were performed using the Vienna ab initio simulation package (VASP).38,39 The core and valence electrons were described by the projector augmented wave functions, and the kinetic cutoff energy was set to 400 eV.40 The exchange-correlation functional was described using a combination of the generalized gradient approximation (GGA) with the Perdew−Burke− Ernzerhof (PBE) functional.41 A self-consistent BEEF-vdW functional was used to calculate the van der Waals interaction.42,43 All of the structures were optimized until the maximum force on each atom was less than 0.05 eV Å−1. It is well-known that the Cu/ZnO/Al2O3 catalyst was first developed for methanol synthesis from syngas. Behrens et al. suggested that Cu could facilitate the reduction of adjacent ZnO under reducing conditions.43 The reduced Zn partially covered the Cu surface, leading to the formation of ZnCu surface alloy. The disordered metal surface evidenced by HRTEM was considered as the active centers. Therefore, the stepped Cu(211) partially decorated with Zn was used as a catalyst model in their work. This ZnCu(211) surface alloy model was further confirmed by other experimental characterization techniques.44 More recently, the rate of methanol synthesis has been quantitatively described by the coverage of Zn on the surface of Cu nanoparticles by Kuld et al.45 In the present work, the preparation and the activation procedures of the Cu/ZnO/Al2O3 catalysts are the same as in the previous studies.44,46 It is anticipated that the active centers in the present work should be similar to these previous studies, which is represented by the ZnCu(211) surface. The decomposition of the added Cs2CO3 promoter generally results in the formation of cesium oxides,47,48 which are highly dispersed on the catalyst surface, as shown in our recent study.33 Therefore, in our study, the Cu/ZnO/Al2O3 catalysts are represented by a ZnCu(211) surface with a (2 × 4) supercell and four atomic layers by replacing all the Cu atoms at the step sites with Zn,43,46,49 as shown in Figure 2a. The modification of the cesium promoter is represented by placing one Cs2O on the surface of ZnCu(211). A vacuum slab of 10 Å was used to separate the periodic slabs. The Brillouin zone integration was performed with a (4 × 4 × 1) Monkhorst−Pack grid. All the structures were put in a 12.71 Å × 10.38 Å × 18.24 Å simulation box. The top two layers and the adsorbates were allowed to relax during the optimization while the bottom two layers were fixed. The adsorption energies of all species (Ead) were obtained using eq 8, where Eadsorbate+surface, Esurface, and Eadsorbate are the energies of the combined system of the adsorbate and catalyst,

Figure 2. Top and side views of the ZnCu model catalyst as the ZnCu(211) surface (a) and the Cs2O-promoted ZnCu model catalyst as the Cs2O-ZnCu(211) surface (b). The synergistic effect between Cu and Zn is modeled by replacing all Cu atoms of the step rows with Zn atoms. The Cu, Zn, Cs, and O atoms are displayed in orange, gray, purple, and red, respectively.

the catalyst model only, and the adsorbate species under vacuum, respectively. A negative value of Ead meant that the adsorption was exothermic and favorable. The reaction energy (ΔH) was derived from the difference between the final state and the initial state of each reaction step, while the activation barrier (Ea) was calculated from the energy difference between the transition state and the initial state. The transition states were searched with the climbing image nudged elastic band method (CI-NEB).50,51 Each transition state has been further confirmed by vibrational frequency calculation whereby only one frequency value was generated at the saddle point. Ead = Eadsorbate + surface − Esurface − Eadsorbate

(8)

3. RESULTS AND DISCUSSION 3.1. Influence of Cesium Promoter on the Production of C2+OH, Methanol, and Alkanes. On the basis of our previous work,33 the optimized amount (1.64 mol %) of the cesium promoter was added to the Cu/ZnO/Al2O3 catalyst sample. The selectivities toward various products over the unpromoted and cesium-promoted Cu/ZnO/Al2O3 catalysts are shown in Figure 3a,b, respectively. Generally, the selectivity of C2+OH increases from 4.1% to 12.0% at a reaction temperature of 543−583 K over the unpromoted catalyst. With the addition of cesium promoter, the C2+OH selectivity increases from 9.3% to 23.1%. We note that the selectivity of methanol decreases with temperature for both the unpromoted and the cesium-promoted Cu/ZnO/Al2O3 catalysts. CO2 and alkanes (mainly methane) are the main byproducts in the C2+OH synthesis. The combined selectivities of CO2 and alkanes increase with temperature. It is noted that the selectivity of alkane is suppressed by the addition of a cesium promoter, especially at T ≤ 573 K, as shown in Figure 3b. The productivity of C2+OH from syngas is negligible when the reaction temperature is below 533 K. In the temperature range of 543−583 K, CO conversion of Cs2O-promoted Cu/ ZnO/Al2O3 is slightly higher than that of Cu/ZnO/Al2O3 catalyst, 35.8−42.6% versus 34.4−40.8%, respectively (Figure 5774

DOI: 10.1021/acscatal.6b00935 ACS Catal. 2016, 6, 5771−5785

Research Article

ACS Catalysis

Figure 3. Selectivity of the main products over Cu/ZnO/Al2O3 catalysts (a) and Cs2O-Cu/ZnO/Al2O3 catalysts (b). Productivity of methanol, C2+OH (c), and alkanes (d) over Cu/ZnO/Al2O3 (black lines) and Cs2O-Cu/ZnO/Al2O3 catalysts (red lines). Productivity of different C2+OH over Cu/ZnO/Al2O3 catalyst (e) and over Cs2O-Cu/ZnO/Al2O3 catalyst (f). Reaction conditions: H2/CO/N2 = 60/30/10, P = 5.4 MPa, GHSV = 3750 mL gcat−1 h−1. The Cs/(Cu + Zn + Al) mole ratio is 1.64%.

3c), in agreement with previous studies.15,33 The productivity of C2+OH increases significantly with the increasing reaction temperature, while the opposite trend is observed for methanol production. With the assistance of a Cs2O promoter, the productivity of C2+OH is dramatically improved, nearly 1 time higher than that of C2+OH over unpromoted catalyst. Under the reaction conditions of 583 K, 5.4 MPa and 3750 mL gcat−1 h−1, the productivity of C2+OH is 157.3 g kgcat−1 h−1, which is close to that of methanol (214.1 g kgcat−1 h−1). As shown in Figure 3e,f, ethanol, 1-propanol, 2-methyl-1-propanol, and 1butanol dominate in the C2+OH products over both promoted and unpromoted Cu/ZnO/Al2O3 catalysts, especially at relatively lower reaction temperature. Ethanol is a primary product in C2+OH at T < 563 K. However, the productivity of 2-methyl-1-propanol increases sharply and becomes a major contributor to the overall productivity of C2+OH when the reaction temperature is above 573 K. The productivities of linear 1-C4H9OH and 1-C5H11OH reach a maximum at 563 K, while the productivities of 2-methyl-branched C5+OH increase with increasing temperature, suggesting that chain growth

routes for these products are quite different from those of linear 1-C4H9OH and 1-C5H11OH. The addition of alkali-metal promoters, such as Cs2O, onto Cu/ZnO/Al2O3 catalysts not only enhances the productivity of C2+OH but also decreases the yield of byproducts (e.g., alkanes). Therefore, both the selectivity and productivity toward C2+OH are improved. Figure 3d shows that the productivity of alkanes increases sharply from 563 K. On the other hand, when Cu/ZnO/Al2O3 catalyst is modified with 1.64 mol % cesium promoter, the production of alkanes is significantly suppressed from 33.7 g kgcat−1 h−1 to 16.2 g kgcat−1 h−1 at 583 K. However, a fundamental understanding of this inhibiting effect of alkali-metal promoters on the alkane formation is still not clear. The inhibiting effect of the cesium promoter will be discussed later using DFT calculations in this study. 3.2. Effect of Cesium Promoter on the Distribution of C2+OH. The CGPs are introduced here to simplify and rationalize the influence of a cesium promoter on the complex distribution of C2+OH products. Although the CGPs have been 5775

DOI: 10.1021/acscatal.6b00935 ACS Catal. 2016, 6, 5771−5785

Research Article

ACS Catalysis discussed by Hilmen et al. in C2+OH synthesis,23 no comparison between the performances of Cu/ZnO/Al2O3 and the Cs2O-Cu/ZnO/Al2O3 catalysts was provided in their study. Thus, the promotional effect of cesium could not be illustrated in terms of CGPs. In addition, only one reaction temperature value (583 K) was investigated in their study. The reaction temperature is the most important reaction parameter for the C2+OH synthesis over the Cu/ZnO-based catalysts, and this reaction is favored in the temperature range of 543−583 K.16 Therefore, discussion of the CGPs in the full temperature range should provide an overall perspective and help understand the influence of temperature on the promotional effect of cesium. Accordingly, in the following study, we adopt CGPs to investigate the promotional effect of cesium promoter on the initial C−C bond formation (from C1* to C2*) and other primary chain growth steps (from C2* to C3* and C3* to 2-methyl-C3*) over both the cesium-promoted and unpromoted Cu/ZnO/Al2O3 catalysts at different reaction temperatures. Then, with the assistance of CGPs, two key reaction parameters, i.e., reaction temperature and gas space velocity, are systematically investigated to optimize the formation of the initial C−C bond for C2+OH synthesis. As shown in Figure 4, all CGPs increase with increasing temperature, suggesting that high temperature favors the chain

temperature, suggesting that the promotional effect of cesium on the initial chain growth is desirable only at relatively higher temperature (563−583 K). CGP values of C2* to C3* are improved by a cesium promoter at 543−563 K, but the improvement is quite weak at higher temperatures (563−583 K), indicating that the promotional effect of cesium on the chain growth of C2* to C3* could be suppressed when the reaction temperature is above 573 K. It appears that the reaction temperature has no effect on the CGPs of C3* to 2-methyl-C3* because the difference in the CGPs over the Cs2O-promoted and unpromoted Cu/ZnO/ Al2O3 catalysts is approximately 0 within the full temperature range, as shown in Figure 4c. In agreement with previous studies,24,52 the fraction (not the productivity) of 2-methyl-1propanol in C3+OH is mainly affected by reaction temperature, not by the cesium promoter. The improvement in the productivity of C3+OH is attributed to a large amount of C2* derived from the initial C−C bond formation over the Cu/ ZnO/Al2O3 catalysts modified with cesium promoter. Therefore, we conclude that the promotional effect of cesium promoter is predominantly on the initial C−C bond formation while it is very weak on the distribution of C3+OH over the Cs2O-promoted Cu/ZnO/Al2O3 catalyst. We note that the CGP values of C1* to C2* are much lower than that of C2* to C3* and of C3* to 2-methyl-C3* in the entire temperature range studied (Figure 4b,c), especially at lower temperatures. This indicates that it is more difficult for the initial C−C bond formation in comparison with the chain growth of C2* to C3* and C3* to 2-methyl-C3*, in agreement with previous studies, where the initial C−C bond formation was deemed as the rate-determining step.16,17 In addition, we suggest that the promotional effect of cesium on the initial C− C bond formation step can be substantially boosted only at higher reaction temperature (563−583 K). The chain growth step of C2* to C3* and C3* to 2-methyl-C3* are predominantly affected by the reaction temperature rather than the cesium promoters. 3.3. Effects of Reaction Temperature and Space Velocity on the Initial C−C Bond Formation. The productivity of C2+OH is largely dependent on the initial C− C bond formation, which can be described in terms of CGPs (C1* to C2*). Therefore, the catalytic performance can be well predicted with CGPs of C1* to C2* and some key reaction parameters can be systematically optimized. Previous studies have shown that various reaction parameters contribute to the selectivity of C2+OH, such as reaction pressure, space velocity/ contact time, H2/CO ratio and the amount of CO2 in feed gas, etc.5,16 It has been found that C2+OH synthesis over Cu/ZnO/ Al2O3 catalysts is favored at 543−583 K, 5.0−10.0 MPa, and 1800−7500 mL gcat−1 h−1 with/without a trace amount of CO2 in feed gas.53 The reaction temperature has been considered as the most important factor among these reaction parameters.16,54 In addition, under the reaction conditions used for mixed alcohol synthesis over Cu/ZnO/Al2O3 catalysts, methanol synthesis is limited by equilibrium.55 Therefore, increasing the residence time (decreasing space velocity) facilitates the chain growth in C2+OH synthesis. A recent study showed that the productivity of C2+OH remained relatively high even at a very low space velocity of 1875 mL gcat−1 h−1 because of the high CO conversion.33 Although those reaction parameters have been well studied previously, a systematic investigation on the effects of various key parameters on the production of C2+OH has been much less studied. Here,

Figure 4. Influence of cesium promoter on the CGPs of (a) C1* to C2*, (b) C2* to C3*, and (c) C3* to 2-methyl-C3* on the basis of the experimental study. Reaction conditions: H2/CO/N2 = 60/30/10, P = 5.4 MPa, GHSV = 3750 mL gcat−1 h−1. The Cs/(Cu + Zn + Al) mole ratio is 1.64%. Red lines indicate Cs2O-Cu/ZnO/Al2O3 catalysts and black lines the unpromoted catalysts.

growth of C2+OH. CGP values of C1* to C2* increase from 0.02 to 0.13 on the unpromoted catalyst, as shown in Figure 4a. In the presence of cesium promoter, CGP values of C1* to C2* increase from 0.05 to 0.25, about twice as much as the values for the unpromoted catalyst. This is consistent with the trend of overall C2+OH productivity shown in Figure 3c. The influence of cesium promoters on the CGPs can be represented by the difference between the corresponding CGPs over the promoted and unpromoted catalysts. Interestingly, the difference in the CGPs of C1* to C2* increases rapidly with reaction 5776

DOI: 10.1021/acscatal.6b00935 ACS Catal. 2016, 6, 5771−5785

Research Article

ACS Catalysis

Figure 5. Effects of reaction temperature and space velocity on CGPs of C1* to C2* over the Cs2O-promoted Cu/Zn/Al2O3 catalysts. Reaction conditions: 5.4 MPa, H2/CO = 2. The curved ribbon with CGP values of 0.21−0.25 is marked with hollow pentagrams. The Cs/(Cu + Zn + Al) mole ratio is 1.64%.

Figure 6. Schematic reaction pathways of the initial C−C bond formation in C2+OH and CH3OH synthesis from CO/H2 on the ZnCu(211) and Cs2O-ZnCu(211) model catalysts.

subject to severe sintering when they are operated at a higher temperature of ∼583 K, as indicated in the work of Boz et al., where the steady activity of alkali-metal-promoted Cu/ZnO/ Al2O3 catalysts was only 14% of the initial activity.15 However, it has been shown in our recent study that the cesiumpromoted counterparts exhibited much higher stability when they were tested at 563 K, where the loss of catalytic activity was less than 7% after 80 h.33 The minor deactivation in the catalytic performance was caused by the slight sintering of copper, as evidenced by the XRD results.33 It is also worth noting that the minor deactivation in catalytic performance is not due to the evaporation of the cesium promoter at higher temperature. The X-ray fluorescence analysis performed in this study showed that no detectable loss of the cesium promoter was observed after each test, implying that no evident evaporation of the cesium promoter occurred under the reaction conditions. 3.4. DFT Calculations of the Initial C−C Bond Formation. As mentioned above, the addition of cesium promoter to Cu/ZnO/Al2O3 catalysts predominately improves the initial C−C bond formation. It is worth noting that a significant amount of CO2 was also generated during C2+OH synthesis, as shown in Figure 3a,b. However, it is well accepted that the formation of CO2 is attributed to the water-gas shift reaction, which is not the focus of this study. To obtain a fundamental understanding of the promotional effects of a cesium promoter on C2+OH synthesis, the initial C−C bond formation step was investigated using DFT calculations over two stepped model catalyst surfaces, i.e., ZnCu(211) and Cs2OZnCu(211), focusing on the key reaction steps of the initial C− C bond formation leading to the synthesis of C2+OH, as shown

the H2/CO ratio was kept at 2 because a lower H2/CO ratio (