Mixed Alcohol Synthesis over Sulfided Molybdenum-Based Catalysts

Jun 3, 2013 - Research and Development Center, Sekisui Chemical Company, Limited, 32, ... The CO conversion and selectivity of the catalysts changed ...
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Mixed Alcohol Synthesis over Sulfided Molybdenum-Based Catalysts Takashi Toyoda,† Takayuki Minami,‡ and Eika W. Qian*,† †

The Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16, Nakacho, Koganei, Tokyo 184-8588, Japan ‡ Research and Development Center, Sekisui Chemical Company, Limited, 32, Wadai, Tsukuba, Ibaraki 300-4292, Japan ABSTRACT: Several alumina-supported molybdenum-based catalysts were prepared by impregnation. The prepared catalysts were characterized using N2 adsorption, X-ray diffraction, temperature-programmed desorption of ammonia, and temperatureprogrammed reduction with hydrogen. The catalytic activity in mixed alcohol synthesis was evaluated using a fixed-bed pressurized flow reaction system under conditions of 250−320 °C, 3.5−5 MPa, gas hourly space velocity of 280−5000 h−1, and H2/CO ratio of 0.5−1. Effects of reaction conditions, alumina supports, and the addition of potassium were elucidated. The CO conversion and selectivity of the catalysts changed depending upon the calcination temperature of alumina supports that influenced the acid amount of the catalysts. The selectivity of C2+ alcohols eventually reached ca. 40% in the case of calcination temperatures higher than 1000 °C. The selectivity of C2+ alcohols was almost proportional to the addition of potassium that promoted the formation of C2+ alcohols. The selectivity of C2+ alcohols and the chain growth probability of alcohols with potassium carbonate were higher than those with nitrate.

1. INTRODUCTION A mixture of hydrogen and carbon monoxide, i.e., synthesis gas or syngas, is made via steam reforming of various carbon sources, such as biomass, natural gas, coal, and steel exhaust gas. The main utilization technique of syngas is gas-to-liquid (GTL), and Fischer−Tropsch (F−T) synthesis is a wellknown process.1−3 In this process, alcohols with more than two carbon numbers, described as higher alcohols, are considered as byproducts. However, the novel process of transforming syngas to higher alcohols becomes more important and has been studied and reviewed4−6 recently because higher alcohols are available as octane-number improvers, substitute fuels, chemical intermediates, etc. Reportedly, rhodium-based catalysts have high selectivity on C2+OH, and the selectivity of C2+OH (mainly ethanol) was 37−56%.7,8 However, rhodium is much more expensive than non-noble metals and is sensitive to sulfur poisoning.9 As a catalyst with non-noble metal, the modified methanol synthesis catalysts (i.e., CuZn-based catalyst) showed high selectivity of total alcohols (60−89%); however, methanol was also its main product, and C2+OH selectivity was 11−25%.10,11 A molybdenum-based catalyst as another catalyst of nonnoble metal was recently reviewed in terms of activity and with Mo catalysts.12 Only the MoS2 catalyst produces mainly hydrocarbons at the conventional condition of alcohol synthesis.13 From the literature with the presence or absence of potassium over reduced Mo/Al2O3,14 CO conversion, CO2 yield, and selectivity to hydrocarbons decreased from 9.7 to 3.5%, from 4.7 to 1.6%, and from 98.8 to 65.5%, respectively. The selectivity to C2+ alcohols and methanol increased from 0 to 24.7% and from 0.8 to 9.9%, respectively. The alkali−MoS2based catalyst was highly selective of alcohol synthesis relatively and resistant to sulfur poisoning, 15 and with K−MoS 2 catalysts,16−18 although the selectivity of total alcohols was 55−68%, excluding CO2, and did not have low selectivity, the main constituent of alcohols was methanol (44−63%). In © XXXX American Chemical Society

several studies, the addition of alkali metals increased the selectivity to alcohols and space time yield (STY) with an oxide or a hydroxide as a precursor.19−21 Usually, the addition of cobalt as a promoter to Mo/Al2O3 or MoS2 catalyst is well-known to increase hydrodesulfurization (HDS) activity22 and would also exhibit the activity of CO hydrogenation. The addition of a transition metal in a MoS2 system has the potential to promote activity of higher alcohol synthesis and to improve propagation of carbon chains because transition metals, such as Fe, Co, and Ni, are available for F−T synthesis catalysts. Reportedly, an unsupported alkali-promoted VIII group metal (Co, Ni, and Fe) MoS2 catalyst shifted the product yield toward ethanol or higher alcohol and reduced the selectivity to hydrocarbons. Especially the addition of Co caused the highest selectivity to alcohols and the lowest hydrocarbon selectivity.23 It is essential to explore non-noble catalysts for the practical applications. An alkali-promoted cobalt molybdenum sulfide catalyst can be expected to produce C2+ alcohols via mixed alcohol synthesis. The study about the effect of supports on the catalyst is more necessary for the new development of catalysts. K−CoMoS-type catalysts for syngas conversions have also been rather well-studied in several supported or unsupported catalysts. 24−29 The C 2+ OH selectivity with unsupported and activated-carbon-supported K−CoMoS2 was 71−72%24,25 and 84−90%,26−29 respectively, but excluded CO2. CO conversion with either unsupported or supported catalyst was 13−15%. This study was focused on the influence of the combination of the acid property and textual structure of alumina as one support with the amount of alkali metal on higher alcohol synthesis and the side reactions with a K−CoMo-based catalyst. Then, a series of Co−Mo supported on alumina that had Received: February 14, 2013 Revised: June 3, 2013

A

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microreactor of a conventional design (8 mm inner diameter stainlesssteel tube packed with 1.0 g of catalyst). The flow rate of feed gas was controlled by high-pressure mass flow controllers (EL-FLOW, OVAL Co.). The volumetric flow rate of the reactor outlet gas is measured using a bubble flow meter and a dry-system gas meter. The reaction conditions were 250−300 °C, 3.5−5.0 MPa, gas hourly space velocity (GHSV) of 600−5000 h−1, and H2/CO ratio of 1.0. Most of the activity tests for higher alcohol synthesis from syngas were performed during a period of below 15 h. In all experiments on the various catalysts, no significant deactivation was observed during the reaction. When the reactor temperature rose to a desirable temperature, feed gas was introduced to the reaction apparatus. After the activity and selectivity became constant after starting the reaction for about 3−5 h, the outlet gas, heated at around 70−80 °C at atmospheric pressure, was sampled every 30 min and injected into an online gas chromatograph (GC), which was equipped with a flame ionization detector (FID, model GC-14B, Shimadzu, CP-Sill 5CB, 0.25 mm × 60 m length) to analyze oxygenated compounds produced, such as alcohols and hydrocarbons, and another online GC with a thermal conductivity detector (TCD, Unibeads C, 1/8 in. × 2.0 m length, GC323, GL Science, Inc.) to analyze CO as a reactant and CO2 and C1−4 light hydrocarbons produced after passing the cooling line by a dumping-styled cooler (model BE200F, Yamato Co.) at −30 to −40 °C. At least three samples were collected under every reaction condition, and three stabilized data were averaged. A relative error of ±5% for stabilized data occurred. After alteration to the next reaction condition, the activity tests were stabilized after ca. 3 h. The molar flow rates of inlet and outlet CO were denoted as F0,CO and FCO. The activity of catalysts in alcohol synthesis was mainly assessed with CO conversion (XCO), the carbon-based selectivity of product i with a carbon number of n (Sproduct,i), and the probability of carbon chain growth in hydrocarbons and alcohols (P). They were defined with eqs 1−3. The selectivity defined the molar flow rate of a product based on the moles of carbon for all products (CO2, methanol, C2+ alcohols, and C1−4 hydrocarbons).

different textual or crystalline structures and acidities was prepared to prevent influences on the sublimation and sintering of metal oxides. The effects of supports and alkali metal for higher alcohol synthesis were elucidated in the present study.

2. EXPERIMENTAL SECTION 2.1. Materials. γ-Al2O3 (Nippon Ketjen Co., Ltd., NK31925) used as a support was supplied as 1/32 in. extrudates. Hexaammonium heptamolybdate 4-hydrate [(NH4)6Mo7O24·4H2O], cobalt nitrate hexahydrate [Co(NO3)2·6H2O], potassium carbonate (K2CO3), and potassium nitrate (KNO3) were commercial guaranteed reagent (GR) grade (Wako Pure Chemicals) and used as catalyst precursors. The feed gas was a mixture of carbon monoxide (purity of 99.9 vol %) and hydrogen (purity of 99.999 vol %) or was a standard gas of H2/CO/ N2 = 45:45:10 (vol %). 2.2. Preparation of Catalysts. All catalysts were abbreviated as K(C)xCoyMoAln [where x is the molar ratio of K/Mo, y is the molar ratio of Co/Mo, and n is the calcination temperature of alumina support (°C), with abbreviations to parenthetic C in the case of potassium carbonate as a precursor and only “K” in the case of potassium nitrate] and were prepared by a conventional and sequential impregnation procedure as described below. γ-Al2O3 support, which was calcinated at different temperatures of 600−1100 °C to prevent influences on the sublimation and sintering of metal oxides, was impregnated successively with aqueous solutions of (NH4)6Mo7O24·4H2O and Co(NO3)2·6H2O as precursors (the former was introduced first) and calcinated at 450 °C for 12 h in air. After calcination, cobalt−molybdenum oxide/Al2O3 was impregnated into a solution of K2CO3 or KNO3 and calcinated in air at 450 °C for 12 h in the same way. The loading amounts of metals in MoO3, CoO, K2CO3, and K2O equivalent were 11, 3, 0−3, and 2 wt %, respectively. 2.3. Characterization of Catalysts. Specific surface area (SA), pore size, etc. of all samples were determined using N2 adsorption with a automatic specific surface area and pore size distribution measurement device (BELSORP-mini II, BEL Japan). X-ray diffraction (XRD) measurements were conducted using a Xray diffractometer (RAD-IIC, Rigaku Co.) with Cu Kα radiation. A continuous scan mode was used to collect 2θ data from 5° to 80° at the rate of 1.5°/min. The voltage and current were 40 kV and 30 mA, respectively. The acidity of samples was measured using the temperatureprogrammed desorption of ammonia (NH3-TPD) with a ChemBET Pulsar TPR/TPD instrument (Quantachrome Instruments). NH3TPD was performed at temperatures of 100−800 °C. Before NH3TPD measurement, all catalysts (0.2 g) were sulfided in situ at 400 °C for 3 h with a 5% H2S/95% H2 gaseous mixture at a flow rate of 30 mL/min. After this, all samples were set up and treated for 2 h in helium at a flow rate of 30 mL/min, whereas the temperature was raised from room temperature to 450 °C at a heating rate of 20 °C/ min. After pretreatment, the sample was cooled to 30 °C in a helium atmosphere. Then, the flow of helium was switched to 10% NH3/90% He flow and kept for 40 min at a flow rate of 30 mL/min to adsorb NH3. The sample was purged at 100 °C in a helium flow for 2 h to remove physically adsorbed NH3. Then, the sample temperature was raised at a heating rate of 10 °C/min to 800 °C in helium flow, after which the sample was cooled to room temperature. Similarly, temperature-programmed reduction with hydrogen (H2TPR) measurements were conducted. A sample of 0.1 g was loaded, and all samples were pretreated in helium at 450 °C for 2 h. After pretreatment, 5% H2/Ar stream with a flow rate of 30 mL/min was introduced. The sample was heated from 30 to 1000 °C at a heating rate of 5 °C/min. 2.4. CO Hydrogenation and Analysis. Prior to activity tests, all catalysts were presulfided with a mixed gas of 5% H2S and 95% H2 in situ at a flow rate of 30 mL/min at 400 °C and atmospheric pressure for 3 h. Feed gases were high-pressure carbon monoxide (purity of 99.9 vol %) and hydrogen (purity of 99.999 vol %) or a mixed gas of H2/CO/N2 = 45:45:10. Synthesis of alcohols from syngas was performed with a fixed-bed pressurized flow reaction system with a

XCO =

F0,CO − FCO

Sproduct, i =

F0,CO

(1)

nFproduct, i ∑ nFproduct, i

Yn = P n − 1(1 − P)

(2) (3)

The probability of chain growth was determined using Anderson− Schultz−Flory (ASF) distribution, where Yn denoted the molar fraction of alcohol or hydrocarbon with a carbon number of n.30 The catalysis performance was also evaluated with C2+OH STY [the yield per a weight of catalyst (mol h−1 kg−1 of catalyst)].

3. RESULTS AND DISCUSSION 3.1. Characterization of Supports and Catalysts. Alumina supports were calcinated at temperatures of 600− 1100 °C for 6 h. The surface area and pore volume of calcinated supports were listed in Table 1. The surface area and pore volume of the supports decreased concomitantly with an increasing calcination temperature. The average pore diameter of the support was larger at a high calcinated temperature. Physical properties of metal-supported catalysts were also tabulated in Table 1. The surface area of the catalysts declined by about 10% in comparison to that of supports. Although three kinds of metals were impregnated sequentially, the loss of the surface area of the catalysts was not remarkable. The surface area of catalysts with supports calcinated at higher temperatures decreased along with the surface area of supports. With almost no influence on the K/Mo ratio and potassium precursor, the surface area, pore volume, and mean pore diameter were 105− 108 m2/g, 0.52−0.56 cm3/g, and 19.9−20.8 nm, respectively. B

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types of acid sites were determined via curve fitting of NH3TPD profiles with Gaussian function. The NH3-TPD profiles of various supports were presented in Figure 2. The acid amount

Table 1. Physical Properties of Various Supports and Catalysts support or catalyst

calcination temperature (°C)

SBETa (m2/g)

Vpb (cm3/g)

dac (nm)

Al600 Al800 Al1000 Al1100 K(C)06Co05MoAl600 K(C)06Co05MoAl800 K(C)06Co05MoAl1000 K(C)06Co05MoAl1100 Co05MoAl1000 K(C)01Co05MoAl1000 K(C)02Co05MoAl1000 K06Co05MoAl1000

600 800 1000 1100 600 800 1000 1100 1000 1000 1000 1000

269 196 121 76.3 244 165 111 70.1 107 107 108 105

1.33 0.89 0.72 0.46 0.69 0.66 0.57 0.36 0.56 0.55 0.56 0.52

19.8 18.2 23.7 23.9 11.3 16.1 20.6 20.3 20.8 20.6 20.6 19.9

a

SBET = Brunauer−Emmett−Teller (BET) surface area. bVp = pore volume. cda = average pore diameter.

Figure 2. NH3-TPD profiles for alumina supports: (a) Al600, (b) Al800, (c) Al1000, and (d) Al1100.

XRD patterns of the supports and oxided catalysts were measured and presented in Figure 1. After calcination at 1000

of various supports and sulfided catalysts were calculated with curve-fitting and presented in Table 2. When the calcination Table 2. NH3 Adsorption Amounts of Several Supports and Sulfided Catalysts on Different Acid Sites amount of NH3 (μmol/g)

a

support or catalyst

weaka

moderateb

strongc

total

Al600 Al800 Al1000 Al1100 K(C)06Co05MoAl600 K(C)06Co05MoAl800 K(C)06Co05MoAl1000 K(C)06Co05MoAl1100 Co05MoAl1000 K(C)01Co05MoAl1000 K(C)02Co05MoAl1000 K06Co05MoAl1000

879 502 344 206 52.0 314 299 349 221 231 197 249

661 417 299 196 792 314 356 192 387 231 290 215

862 886 668 494 519 276 128 130 277 398 355 343

2402 1805 1311 896 1363 904 783 671 885 860 841 807

Weak < 300 °C. bModerate = 300−400 °C. cStrong > 400 °C.

temperature increased, all three types of acid amount on alumina supports decreased because the hydroxyl group on the support was removed via dehydration by calcination. Those results were consistent with the transition of a part of the γ phase on alumina to the α phase as measured with XRD. The total acid amount of catalysts was also decreased at a higher calcination temperature of alumina. The acid sites of the moderate and strong acid strength (moderate and strong) decreased at a high calcination temperature, while those of the weak acid strength (weak) inversely increased. With the addition of alkali metal, potassium was inversely proportional to the total acid amount on catalysts. The acid sites of strong acid strength were smallest at a K/Mo ratio of 0.6, while those of weak acid strength were largest at the same K/Mo ratio. The effect of alkali addition on the basicity of a catalyst has been reported by several studies.37−39 In a report with K/Al2O3 catalyst,36 the amount of desorbed CO2 increased by increasing the amount of potassium. In our study, it would also be considered that the basicity of the K-doped catalyst could be proportional to K addition. When potassium carbonate was changed to nitrate, the total acidity increased slightly.

Figure 1. XRD pattern of (I) alumina supports calcinated at (a) 1100 °C, (b) 1000 °C, (c) 800 °C, and (d) 600 °C and (II) oxided catalysts with alumina support calcinated at (e) 1100 °C, (f) 1000 °C, (g) 800 °C, and (h) 600 °C: (▲) α-Al2O3, (●) γ-Al2O3, and (■) MoO3.

and 1100 °C, several peaks with stronger intensity occurred at 28.3°, 45.3°, 61.4°, 62.9°, and 66.7° and were attributed to the α-Al2O3 phase. At the calcination temperature of 600 °C, the peaks were broad, except for the peaks of 34.4°, 43.8°, 58.5°, and 66.0°. The phases of α-Al2O3 and γ-Al2O3 were detected for all samples. At a high calcination temperature, γ-Al2O3 phases were transmitted to α-Al2O3 phases. This result agreed with other references.31,32 Acid properties of supports and catalysts were determined using NH3-TPD. The types of acid sites were related to the corresponding desorption temperature. Generally, the acid sites were classified as weak (400 °C) acid sites.33−36 The acid amount of three C

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ratio of 0.1 and 0.2 (curves e and f in Figure 3). When the potassium precursor was changed to carbonate, two peaks of low temperature overlapped and reduction temperatures of Mo became lower than nitrate from 575 to 535 °C and from 640 to 570 °C, respectively (cruves c and h in Figure 3). The reducibility of the catalyst with nitrate was higher than that of carbonate. In this way, the catalysts impregnated with potassium nitrate might be promoted to a product of hydrocarbons by the reducibility of catalysts. 3.2. CO Hydrogenation. 3.2.1. Effects of Reaction Conditions. The effect of the reaction temperature on the activity of alcohol synthesis was investigated under the conditions at K(C)06Co05MoAl1000, 260−320 °C, 7 MPa, 5000 h−1, and H2/CO molar ratio of 1.0 (Table 3). With an

H2-TPR profiles of all oxided catalysts and simple metal oxide on alumina calcinated at 1000 °C were shown in Figure 3.

Table 3. CO Conversion, Selectivity in Various Products, and Chain Growth Probability at Different Reaction Temperaturesa

Figure 3. H 2 -TPR profiles of oxidized catalysts: (a) K(C)06Co05MoAl600, (b) K(C)06Co05MoAl800, (c) K(C)06Co05MoAl1000, (d) K(C) 06 Co 05 MoAl 1100 , (e) K(C) 02 Co 05 MoAl 1000 , (f) K(C)01Co05MoAl1000, (g) Co05MoAl1000, (h) K06Co05MoAl1000, (i) 11MoAl1000, (j) 3CoAl1000, (k) 3K(C)Al1000, and (l) 2KAl1000.

temperature (°C) CO conversion (%) selectivity (%) MeOH C2+OH EtOH 2-PrOH PrOH 2-BuOH BuOH hydrocarbons CH4 C2H6 C3H8 iso-C4H10 n-C4H10 CO2 chain growth probability C2+OH STY (mol h−1 kg−1 of catalyst)

11MoAl1000 catalyst (11 wt % MoO3-supported Al1000) showed two main reduction peaks (curve i in Figure 3), which were the peaks at a low temperature of 502 °C and at a high temperature of 902 °C. The former peak was the partial reduction of Mo6+ to Mo4+ of amorphous, highly defective, multilayered Mo oxides or heteropolymolybdates and octahedral Mo species.40−44 The latter peak at 800−950 °C was generally related to the deep reduction from Mo4+ to Mo, and all Mo species strongly bound to the support, including the highly dispersed tetrahedral Mo species.33,44,45 From the result of 3 wt % CoOsupported Al1000, denoted as 3CoAl1000 (curve j in Figure 3), three main small peaks were detected. The first peak at 375 °C was assigned to the reduction of Co3O4 to CoO. The second peak at 636 °C was attributed to the reduction of CoO to Co0. The third peak at 912 °C was attributed to the reduction of very small metal particles and mixed metal-support oxides.46 3K(C)Al1000 (3 wt % K2CO3-supported Al1000) and 2KAl1000 (2 wt % K2O-supported Al1000) showed almost no reduction peak in curves k and l in Figure 3. At the alumina calcination temperature of 600 °C, two main reduction peaks were observed at low and high temperature ranges (curve a in Figure 3). The low-temperature peak was almost consistent with the reduction peak of Mo6+. The hightemperature peak was nearly assigned to the reduction peak of Mo4+ to Mo or mixed metal-support oxides over 11MoAl1000. When alumina was calcinated at a higher temperature (curves b−d in Figure 3), the reduction peak at a low temperature was weak and shifted from 529 to ca. 580 °C, whereas the other peak around 640 °C revealed a calcination temperature higher than 1000 °C. This peak could be attributed to the reduction of CoO to Co0. This suggested that cobalt oxide was reduced easily by increasing the alumina calcination temperature and that the alumina support calcinated at a higher temperature had lower dispersion because the surface area was depressed when the alumina calcination temperature increased (Table 1). Another peak of high temperature around 880 °C was split into two peaks at around 840 and 900 °C when alumina was calcinated at a temperature higher than 800 °C. With the addition of potassium, the shoulder peak appeared around ca. 570 °C besides the broad peak at a low temperature at a K/Mo

a

260

300

320

4.4 8.9 36.8 31.6 0.5 3.6 0.2 0.9 21.1 13.4 4.7 2.5 0.1 0.5 33.2 0.19 1.8

6.3 6.7 34.0 29.3 0.2 3.8 0.0 0.6 27.1 13.2 8.0 4.8 0.2 0.9 32.2 0.21 2.4

7.1 5.4 26.2 21.3 0.3 3.5 0.2 0.9 33.9 13.4 11.3 7.4 0.4 1.5 34.5 0.28 2.0

K(C)06Co05MoAl1000, H2/CO = 1, 7 MPa, and GHSV = 5000 h−1.

increasing temperature, CO conversion and selectivity of hydrocarbons and CO2 as byproducts increased, while the selectivity of higher alcohols and methanol decreased. Those results were consistent with thermodynamic calculation results, which concluded that the concentration of ethanol decreased when the temperature increased, by AspenPlus software, as Spivey et al. reported.5 However, at higher temperatures, the distribution in C2+ products of both alcohols and hydrocarbons increased and the probability of chain growth increased as well. As shown in Table 3, the higher reaction temperature and lower pressure favor F−T synthesis, i.e., the formation of hydrocarbons, although the formation of CH4 had hardly been influenced. The one possible reason was that the catalysis performance of sulfided CoMo catalysts in our study was slightly different from the conventional F−T catalysts: reduced CoMo catalyst. As another reason, the loading amount of CoO was only 3 wt % and the dominant supported metal component was MoO2 (11%). With a maximum at 300 °C, C2+OH STY was reduced because C2+OH selectivity decreased despite the increment of CO conversion and the chain growth probability. Those suggested that a formation of C2+OH from CO hydrogenation should be conducted at a lower temperature (less than ca. 300 °C). D

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The effect of a reaction pressure on the activity of alcohol synthesis was investigated under the conditions of K(C)06Co05MoAl1000, 300 °C, 3.5−7.0 MPa, 5000 h−1, and H2/ CO molar ratio of 1.0 (Table 4). When the pressure was high,

Table 5. Effects of GHSV on CO Conversion, Selectivity, Carbon Distribution in Various Products, and Chain Growth Probabilitya GHSV (h−1) CO conversion (%) selectivity (%) MeOH C2+OH

Table 4. CO Conversion, Selectivity, Carbon Distribution in Various Products, and Chain Growth Probability under Different Pressuresa pressure (MPa) CO conversion (%) selectivity (%) MeOH C2+OH EtOH 2-PrOH PrOH 2-BuOH BuOH hydrocarbons CH4 C2H6 C3H8 iso-C4H10 n-C4H10 CO2 chain growth probability C2+OH STY (mol h−1 kg−1 of catalyst) a

3.5

5

7

6.1 5.7 28.8 24.4 0.2 3.2 0.1 0.9 34.4 12.2 14.2 6.9 0.3 0.7 31.1 0.19 1.8

6.0 6.7 33.2 28.2 0.3 3.7 0.1 0.8 29.8 11.2 11.6 5.9 0.2 0.9 30.3 0.21 2.4

5.9 7.7 34.3 29.3 0.5 3.7 0.1 0.6 27.8 10.5 10.5 5.5 0.2 1.1 30.2 0.28 2.0

EtOH 2-PrOH PrOH 2-BuOH BuOH hydrocarbons CH4 C2H6 C3H8 n-C4H10 CO2 chain growth probability C2+OH STY (mol h−1 kg−1 of catalyst) a

280

600

11.7 0.5 4.5 2.0 1.7 0.2 0.6 0.0 44.4 6.1 18.3 13.9 6.2 50.6 0.65 3.3 × 10−2

6.1 7.0 36.8 17.2 8.3 5.8 4.2 1.3 18.5 4.3 6.7 7.3 0.0 37.7 0.64 3.0 × 10−1

K(C)06Co05MoAl1000, 260 °C, H2/CO = 0.5, and 3.5 MPa.

adsorption sites might be depressed. As shown in Table 2, the acidity of the moderate and strong acid strength decreased but that of the weak acid strength increased at a higher calcination temperature. The change in the acid sites of strong acid strength corresponded with the behavior in the selectivity of hydrocarbons. Ferreira-Aparicio et al. reported that the hydroxyl group over alumina support dehydrated oxygenated intermediates.48 This report might suggest that the selectivity of hydrocarbons decreased with decreasing hydroxyl groups over alumina support because hydrocarbons could be formed via dehydration of alcohols. When hydroxyl groups over alumina decreased, the acid amount of stronger acid strength also descended. Therefore, the stronger acid sites on the catalyst favored the formation of hydrocarbon. When alumina was calcinated up to 800 °C, methanol selectivity was increased and the selectivity of higher alcohols diminished. In contrast, the selectivity of CO2 was maximized at this temperature. As discuss in section 3.3, the reaction products of higher alcohols, methanol, and CO2 involve associative CO adsorption deeply. In Figure 4A, CO2 selectivity reached a maximum at the calcination temperature of 800 °C. Conversely, C2+OH selectivity was minimal at this temperature, which suggested that the reaction product via associative CO adsorption shifted from CO2 to higher alcohols at a higher calcination temperature than 800 °C. The selectivity of C2+ alcohols was increased when alumina was calcinated at temperatures higher than 800 °C and eventually became constant at 40% when the calcination temperature was higher than 1000 °C. In view of CO conversion, the catalyst with alumina calcinated at 1000 °C was more suitable for the activity of higher alcohol synthesis among this range of temperatures of alumina calcination. The effect of the calcination temperature on the distribution of alcohols was shown in Figure 4B. At a calcination temperature over 800 °C, the percentage of methanol decreased and ethanol increased inversely. With calcination at 1100 °C, secondary alcohols (2-propanol and 2-butanol) decreased and n-C2+ alcohols (n-propanol and n-butanol) increased. Table 6 showed the effects of the calcination

K(C)06Co05MoAl1000, H2/CO = 1, 300 °C, and GHSV = 5000 h−1.

the selectivity of hydrocarbons decreased and CO2 selectivity slightly decreased. The selectivity of higher alcohols and methanol was increased in a higher pressure. Those results were also consistent with thermodynamic calculation results, which concluded that an equilibrium ethanol concentration increases with pressure.5 In Table 4, a higher reaction pressure suppressed the formation of methane slightly. This result agreed with a result of conventional F−T synthesis.47 The distribution in C2+ products and the probability of chain growth were little changed at higher pressures. C2+OH STY reached its maximum at 2.2 mol h−1 kg−1 at more than 5 MPa. Those suggested that a formation of C2+OH from CO hydrogenation should be conducted at a higher pressure. The effect of GHSV was investigated under the conditions of K(C)06Co05MoAl1000, 260 °C, 3.5 MPa, 280−600 h−1, and H2/ CO molar ratio of 0.5 (Table 5). With an increasing GHSV, CO conversion and the selectivity of hydrocarbons and CO2 decreased, while the selectivity of higher alcohols and methanol was notably increased. The probability of chain growth was little changed at higher space velocity. C2+OH STY at 600 h−1 was about 10 times higher than at 280 h−1. Therefore, higher GHSV presents advantages in C2+OH STY. 3.2.2. Effect of Support. The effect of a calcination temperature of a support on the activity of higher alcohol synthesis was investigated. Alumina supports were calcinated from 600 to 1100 °C. The higher alcohol synthesis reaction conditions were K(C)06Co05MoAln (n = 600, 800, 1000, and 1100), 260 °C, 3.5 MPa, 600 h−1, and H2/CO molar ratio of 1.0. Figure 4A showed the effect of the calcination temperature of alumina on activity. The selectivity of hydrocarbons as byproducts decreased from 46 to 21% as the calcination temperature increased, while CO conversion decreased. This is why the surface area of the catalysts declined with an increasing calcination temperature, as shown in Table 1, and CO E

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Figure 4. Effects of the calcination temperature of the support on (A) CO conversion and selectivity: (◆) C2+ alcohols, (◇) methanol, (■) hydrocarbons, (▲) CO2, and (○) CO conversion and (B) alcohol distribution: (■) MeOH, (●) EtOH, (▼) 2-PrOH, (○) PrOH, (▲) 2-BuOH, and (◆) BuOH.

the reaction pathway, as shown afterward in Scheme 1, nondissociative CO adsorption was favored by increasing the

Table 6. Effects of the Calcination Temperature of the Support on the Probability of Chain Growth and C2+OH STY

Scheme 1. Formation Pathway of Higher Alcohols and Byproducts

chain growth probability calcination temperature (°C)

alcohols

hydrocarbons

C2+OH STY (mol h−1 kg−1)

600 800 1000 1100

0.36 0.29 0.50 0.61

0.40 0.29 0.31 0.59

0.26 0.16 0.16 0.08

temperature of alumina on the probability of chain growth and C2+OH STY. Chain propagation with alumina that was calcinated at 800 °C was minimized, along with a change of C2+OH selectivity. C2+OH STY was decreased at higher calcination temperatures. 3.2.3. Effect of Alkali Metal. The influence of the addition of potassium as an alkali metal was investigated. Activity tests were performed under the conditions of 260−300 °C, 5 MPa, 5000 h−1, H2/CO ratio of 1.0, and K/Mo molar ratio of 0−0.6. The alumina-support calcinated temperature at 1000 °C was used in all catalysts. The effects of the K/Mo ratio on CO conversion and selectivity were shown in Figure 5A. The selectivity of C2+ alcohols increased almost linearly with the ratio of K/Mo. Regarded as byproducts, the hydrocarbon selectivity decreased by increasing the K/Mo ratio of 0.6. CO2 selectivity tended to be dropped as well. Regarding the effect of the K/Mo ratio on

amount of potassium because of the increment in the formation of alcohols and CO2. Non-dissociative CO adsorption had relevance to the product of alcohols and CO2. Then, with the addition of a small amount of potassium (K/Mo ratio of 0.1− 0.2), CO2 was preferred, but with the addition of a larger

Figure 5. Effects of the ratio of potassium/molybdenum on (A) CO conversion and selectivity: (◆) C2+ alcohols, (◇) methanol, (■) hydrocarbons, (▲) CO2, and (○) CO conversion and (B) alcohol STY: (■) MeOH, (●) EtOH, (▼) 2-PrOH, (○) PrOH, (▲) 2-BuOH, and (◆) BuOH at 260 °C. F

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°C in the course of catalyst preparation. The capture of produced CO2 on a part of K2O over a catalyst could transform from K2O to K2CO3. The difference in the potassium precursor affected the chain propagations of products. The probability of the chain propagation of alcohols with carbonate was higher than that for nitrate salt. Those results suggested that the activity of higher alcohol synthesis with K(C)06Co05MoAl1000 had advantages over K06Co05MoAl1000. 3.3. Hydrogenation Mechanism of CO. The role of alkali promotion in syngas conversion to alcohols over CuZn-based catalysts is well-known to be a consequence of an increased basicity and, hence, increased aldol reaction rates.10,49 In Co− MoS2 catalyst, where aldol condensation does not occur, the possibility of CO adsorption on Co−MoS2 has been in dispute. In a hypothetical mechanism over a VIII group metal modified Mo-based catalyst, it could be considered that at least two forms of transition metal simultaneously existed over transitionmetal-doped MoS2-based catalysts, namely, separated phases of transition metal sulfides, CoxSy, and the phases of mixed metal sulfide, such as so-called “Co−KMoS” and “Ni−KMoS”.50,51 The non-dissociation of CO on only MoS2 is known to be energetically favorable.52 Huang et al. reported that the barrier energy for dissociation of adsorbed CO (2.6 eV) was much higher than the non-dissociative CO adsorption energy (−2.24 eV). The formation pathway of higher alcohols and byproducts in hydrogenation of CO was proposed (Scheme 1). First, carbon monoxide adsorbed dissociatively on site II like the phase of transition metal sulfides (Co9S8, Co3S4, CoS2, etc.) and adsorbed associatively on site I like the MoS2 phase or “KMoS phase or Co−KMoS phase”.6,34,53 CO2 was produced by a part of dissociative O* adsorption sites and associative CO adsorption or CO gas. Methanol was produced if an associativeadsorbed CO species was simply hydrogenated. Hydrocarbons, such as methane and ethane, were produced by hydrogenation and conjugation of dissociative-adsorbed CO species or dehydration of an acyl intermediate. In our study, the hydrocarbon selectivity was suppressed at a higher calcination temperature, i.e., the decline of the hydroxyl group by dehydration on an alumina support, at section 3.2.2. Then, in the case that, in a part of the dehydration, the hydroxyl group on the support affected the formation of alkyl species, it suggested that production of hydrocarbons was facilitated on the acid sites of strong acid strength. C2+ alcohols, such as ethanol, were produced by hydrogenation of an acyl

amount of potassium (K/Mo ratio of 0.2−0.6), the generation of higher alcohols was preferred. The effect of the K/Mo ratio on STY of alcohols was shown in Figure 5B. n-C2+OH STY also increased because both C2+OH selectivity and CO conversion rose. 2-Propanol and 2butanol were reduced at a K/Mo ratio of 0.6. This might be because those secondary alcohols were intermediate and transformed to n-alcohols. Eventually C2+OH STY was 2.9 mol h−1 kg−1 of catalyst, with a K/Mo ratio of 0.6 and 300 °C. The changes of conversion, selectivity, and STY tended to be similar to those at other temperatures (280 and 300 °C). The chain growth of products with a carbon chain (i.e., alcohols and hydrocarbons) was shown in Table 7. The range of the chain Table 7. Effects of the Addition of Potassium on the Probability of Chain Growth at Different Temperatures alcohols

hydrocarbons

K/Mo ratio

260 °C

280 °C

300 °C

260 °C

280 °C

300 °C

0 0.1 0.2 0.6

0.23 0.19 0.26 0.21

0.19 0.20 0.25 0.27

0.27 0.22 0.25 0.30

0.22 0.17 0.21 0.22

0.22 0.18 0.22 0.21

0.24 0.19 0.25 0.23

growth probability was ca. 0.21 ± 0.03. In comparison to the activity for the effect of the support calcination temperature (Table 6), the range of those values was 0.29−0.61, which was wider than that for the effect of the K/Mo ratio. There was not regularly any difference of a chain growth probability on the effect of K/Mo. 3.2.4. Effect of the Potassium Precursor. The influence of the potassium precursor was investigated through a comparison of potassium carbonate to potassium nitrate as precursors with K06Co05MoAl1000 or K(C)06Co05MoAl1000 under the same conditions as those described in section 3.2.3. Table 8 showed the effects of potassium precursors on conversion, selectivity, C2+OH STY, and the chain growth probability. For carbonate, the selectivity of higher alcohols was higher at several temperatures. Both methanol and methane selectivities decreased. This indicated that hydrogenation of associative or dissociative adsorbed CO species was more inhibited than nitrate. The selectivity of hydrocarbons with K06Co05MoAl1000 was higher than with K(C)06Co05MoAl1000. The CO2 selectivity was lower in the case of nitrate salt. Potassium nitrate as a precursor changed to potassium oxide by a calcination at 450

Table 8. CO Conversion, Selectivity, C2+OH STY, and Chain Growth Probability on Catalysts with Different Potassium Precursorsa catalyst

K(C)06Co05MoAl1000

precursor temperature (°C) conversion (%) selectivity (%)

C2+OH STY (mol h−1 kg−1 chain growth probability

a

C2+OH MeOH CO2 HC2+ CH4 of catalyst) alcohols hydrocarbons

K06Co05MoAl1000

K2CO3

KNO3

260

280

300

260

280

300

5.2 35.6 8.7 18.8 10.4 26.5 2.1 0.21 0.22

4.8 34.2 4.0 30.4 14.5 16.8 1.8 0.27 0.21

8.5 30.6 3.9 33.5 18.2 13.9 2.9 0.30 0.23

3.5 29.2 11.5 14.6 17.9 26.7 1.1 0.19 0.39

4.5 25.8 9.6 15.9 22.1 26.6 1.3 0.20 0.37

5.3 19.1 6.2 18.7 27.0 29.0 1.1 0.21 0.33

GHSV = 5000 h−1, H2/CO = 1, and 5 MPa. G

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intermediate, which was generated with insertion of adsorbed CO species in an adsorbed alkyl species. For Scheme 1, if non-dissociative CO adsorption on site I is increased, the product of C2+OH or methanol would be promoted. Now, in section 3.2.3, C2+OH and methanol selectivities were increased by the addition of K (Figure 4A). Those might agree with the mechanism of non-dissociative CO adsorption in Scheme 1.

4. CONCLUSION The favorite conditions about the formation of higher alcohols were a more moderate temperature under ca. 300 °C and a higher pressure around 5 MPa. Those results were consistent with the thermodynamic consequences. The higher GHSV presented advantages in C2+OH STY. When the alumina support was calcinated from 600 to 1000 °C, the selectivity of hydrocarbons as byproducts decreased from 46 to 21%, although CO conversion decreased. On the basis of the results of NH3-TPD measurement, the acid amount on the alumina support and acid sites (especially acid sites of strong acid strength) were decreased with an increasing calcination temperature. The acid sites of the strong acid strength might be in favor of the production of hydrocarbons. Depression of the acid amount on the support at calcination temperatures of more than 800 °C resulted in an increment of the selectivity for C2+ alcohols. It eventually reached ca. 40%, including CO2, when the calcination temperature became higher than 1000 °C. The addition of potassium improved the activity for higher alcohol synthesis because of the increasing proportion of selectivity for higher alcohols to the ratio of K/Mo at a maximum of higher alcohol selectivity with a K/Mo ratio of 0.6. The addition of potassium could favor the associative CO adsorption by the proposed formation pathway for the hydrogenation of CO. The higher alcohol selectivity and the chain growth probability of alcohols with potassium carbonate were higher than those with nitrate.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +81-42-388-7410. E-mail: [email protected]. jp. Notes

The authors declare no competing financial interest.

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