b. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China;
c. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
In view of the increasing concerns over the environmental pollution and shortage of fossil fuel resources, the catalytic conversion of syngas into higher alcohols has drawn much attention for both industrial application and fundamental research in the past few decades [1-3]. Generally, the syngas, mainly composed of CO and H2, can be derived from various carbon sources, such as natural gas, coal, and renewable biomass, via gasification. Higher alcohols have high octane numbers, and this fact makes the most potential application of them as an additive for gasoline or replaces the methyl tert-buthyl ether for the reduction of exhaust emission [4, 5].
Up to now, several catalyst systems for the conversion of syngas into higher alcohols have been developed. In general, the potential catalysts can be classified into two broad categories: noble metal-based and non-noble metal-based catalysts [6, 7]. Reportedly, as a significant noble metal-based catalyst, Rh-based catalysts have been found to be the most selective catalyst for the synthesis of higher alcohols from CO hydrogenation and show high selectivity to C2+OH (mainly ethanol). Nevertheless, rhodium is too expensive and sensitive to sulfur poisoning, which makes it difficult to realize the large-scale application in industry . The non-noble catalysts for higher alcohol synthesis mainly include Cu-based catalysts and Mo-based catalysts. Cu-based catalysts exhibited a high selectivity to total alcohol, however the selectivity to C2+OH was fairly low and the dominant product was methanol . Mo-based catalysts have shown high selectivity to C2+OH, and in some cases low selectivity to hydrocarbons. In particular, it has some advantages, such as low cost, excellent resistance to sulfur poisoning and coke deposition, etc. [10, 11].
Increasing attempts have been made to optimize the higher alcohol synthesis performances of Mo-based catalyst and to enhance the potential capability for application [10, 12, 13]. Previous studies have shown that both the activity and selectivity for higher alcohol synthesis over Mo-based catalysts are strongly dependent on the type of promoters, the interaction between promoters and Mo species, the chemical state and dispersion of surface Mo species, and surface acidity of catalysts, etc. [13-17]. Unpromoted Mo catalysts produced mainly light hydrocarbons . With the addition of alkali metal, such as potassium, the catalysts selectivity was greatly shifted from hydrocarbons to alcohols [19, 20]. The alkali metal promoters can neutralize the surface acidity of catalyst, enhance the adsorption and insertion of CO, as well as inhibit the hydrogenation rate to promote the growth of carbon chain [12, 21, 22]. Different alkali metals have different influences on the catalyst activity. The generally accepted order is Cs+ > Rb+ > K+ > Na+ > Li+, consistent with the sequence of decreasing basicity . Some 3d transition metals, in particular Fischer-Tropsch (FT) elements such as Co or Ni, are effective promoters for improving the C2+OH selectivity as well as alcohol yield of Mo-based catalysts. The Co or Ni promoters acted as a synergistic system with the Mo species and promoted the growth of carbon chain. The structure, morphology of promoters, and their interaction with Mo species have significant impact on the promotional effects [24, 25].
As a potential element, Mn has been widely used as a promoter for CO hydrogenation. Morales et al.  investigated the electronic state and location of Mn in Co-based FT catalysts with a combination of EXAFS and STEM-EELS. They found that the presence of MnO phase in the catalysts can improve the percentage of higher hydrocarbons and increase the selectivity in the FT synthesis. For the silica-supported Co-Mn catalysts, the Mn promotion increased the rate of CO consumption, decreased the selectivity to methane, and increased the selectivity to C5+ products, and rationalized the promotion effects in terms of H availability and CO coverage . More recently, Johnson et al.  reported that MnO would act as a Lewis acid to assist in CO dissociation as well as to increase the ratio of CO to H adsorbed on the Co catalyst surface. They demonstrated that active sites near the Co-MnO interface were responsible for the improved activity and C5+ selectivity. Over the K/Ni/MoS2 catalysts, it was found that the addition of Mn significantly affected the dispersion of active components on the catalyst surface, improving the selectivity to C2+OH . For Mn-Fe-Cu catalysts, there existed a strong interaction between Mn and Fe components. The addition of Mn could improve the surface concentration of Fe and Cu species, leading to the increase of active sites for higher alcohol synthesis . Zhang et al.  have employed density functional theory calculations to investigate the effect of Mn on ethanol formation on a Mn-promoted MnCu (211) surface. They suggested that with the addition of Mn, CH3 species were maximized and CH3OH formation was suppressed. The C2 oxygenates formation via CHO insertion into CH3 was more favorable in both kinetics and thermodynamics when compared with the Cu (211) surface. Ojeda et al.  also compared the CO hydrogenation over Rh/Al2O3 and Rh-Mn/Al2O3 catalysts and found that higher activity of Rh-Mn/Al2O3 catalyst was attributed to the decrease of the relative surface carbon coverage over the Rh particles.
So far, there are rare reports concerning the effects of Mn on the reduced Mo-oxide catalyst. Considering the unique structural and electronic effects of Mn oxide, it was proposed that the addition of Mn can play a positive impact on the catalytic performance of Mo-oxide catalyst for higher alcohols synthesis. Herein, a series of Mn-doped K-Co-Mo catalysts were prepared by a sol-gel method. The activity of catalysts for the synthesis of higher alcohols from syngas was evaluated and their structures were characterized by means of X-ray diffraction (XRD), N2 physisorption, NH3 temperature programmed adsorption (NH3-TPD), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray absorption fine structure spectroscopy (XAFS). Based on the characterization results, the promotional effects of Mn on the structure and performance of the Mo-based catalysts were discussed.Ⅱ. EXPERIMENTS A. Catalyst preparation
The Mn-K-Co-Mo catalysts were prepared by a sol-gel method with citric acid as a complexant. A typical procedure is as follows: Firstly, an aqueous solution of Co (NO3)2 was added slowly to an aqueous solution of (NH4)6Mo7O
The phase composition and crystalline structure of fresh catalysts were measured on a Rigaku TTR-Ⅲ X-ray power diffractometer equipped with a Cu Kα (
The BET surface area, pore volume and average pore diameter of the catalysts were determined by nitrogen adsorption at -196 ℃ by means of a micromeritics TriStarII 3020 instrument. Prior to the measurement, all the samples were degassed under vacuum at 150 ℃ for 2 h to remove volatile adsorbates on the surface.
NH3-TPD was performed for determining the surface acid properties of the catalysts. Approximately 80 mg of the sample was loaded into a U-type quartz tube, which was mounted on the instrument. Then the sample was pretreated in He at 200 ℃ for 0.5 h (heating rate 10 ℃/min). After cooling to room temperature, 0.5% NH3 in He was passed through the sample for 1 h, with subsequent flushing with helium at 100 ℃ for 1 h. The TPD analysis was carried out in flowing He from 100 ℃ to 850 ℃ at a heating rate of 10 ℃/min.
In situ DRIFTS experiments were performed on a Bruker Vertex 70v FT-IR spectrometer equipped with a mercury cadmium telluride detector. A high temperature chamber fitted with CaF2 windows was utilized as the sample cell. The high-purity carbon monoxide was used as probe gas. Nitrogen and hydrogen were used as the flushing gas and reducing gas, respectively. All steps were carried out at atmospheric pressure. The catalysts were ex situ reduced in flowing pure H2 at 400 ℃ for 12 h. After cooling to room temperature under pure H2, the catalysts were passivated with 1%O2 in N2 for 2 h. The catalysts were then placed into the IR sample cell and reduced again in a H2 flow at 400 ℃ for 3 h to avoid the presence of any oxidized species produced by passivation or re-oxidation during sample handling. After the reductive pretreatment, the samples were then cooled down to room temperature in flowing pure H2. At this temperature, a background spectrum was collected. Next, CO gas was fed to the cell and the infrared spectra were taken at 4 cm-1 resolution with 32 scans. CO adsorption continued for about 40 min, with spectra being recorded about once every two minutes. After adsorption, the samples were flushed with nitrogen for 20 min (30 mL/min) to remove the gaseous carbon monoxide from the chamber and the IR spectra were collected again.
The X-ray absorption spectra at the Mo K-edge on catalysts and standard references were measured at the beamline of 1W1B of Beijing synchrotron radiation facility, China. The electron storage ring was operated at 2.5 GeV with an average current of 200 mA. A Si (111) double crystal was used as monochromater and the data of absorption samples were collected in transmission mode. The energy of the absorption spectra was calibrated by measuring the X-ray absorption near-edge spectroscopy (XANES) of a Mo metal foil. For each measurement, sample was ground into fine powders and brushed onto adhesive tapes. Thickness and homogeneity of the samples were optimized to obtain the best signal-to-noise ratio. The obtained data were processed by established methods with the ATHENA software package . The normalized Extended X-ray absorption fine-structure spectroscopy (EXAFS) was converted from energy to k-space and weighted by k3. These data were then Fourier transformed to R-space. Due to the uncorrection for photoelectron phase shifts, distances in R-space are 0.3-0.5 Å shorter than actual bond distances.C. Catalytic activity measurement
The catalytic performance testing was performed in a fixed-bed pressurized flow reaction system with a conventionally designed microreactor (600 mm length and 8 mm internal diameter stainless-steel tube). Feed gases were composed of 60% H2, 30% CO, and 10% N2 as an internal standard. The feed gas was purged into the reactor at a desired rate using high-pressure mass flow controllers. Temperature was monitored with a K-type thermocouple inserted in the catalyst bed. The samples (0.5 g, 40-60 mesh particle size) were diluted with a certain amount of quartz sand in order to maintain isothermal conditions and then charged into the fixed-bed reactor, which was housed in an electric furnace controlled by a temperature controller. Prior to the reaction, the catalysts were in situ reduced in pure H2 at a flow rate of 40 mL/min under atmospheric pressure for 12 h. Then the reactor was cooled down. When the reaction temperature rose to a desirable temperature, feed gas was introduced into the reactor. The effluent gas was cooled in an ice-water bath to separate into gas and liquid phases. Details on the product analytical procedure are described in our previous work . For Mo-based catalysts, the higher alcohols synthesis reaction requires an introduction period to reach steady state. The activity data were measured after the reaction was performed for 24 h.Ⅲ. RESULTS AND DISCUSSION A. XRD results
The XRD patterns of the catalysts with varying Mn amounts are shown in Fig. 1. For the catalyst free of Mn, the XRD pattern exhibited three diffraction peaks at 2
The nitrogen adsorption-desorption isotherms as well as pore size distribution curves for all samples are displayed in Fig. 2. The unpromoted catalyst exhibited a type IV isotherm with a hysteresis loop of type H3 according to the Brunauer-Deming-Deming-Teller (BDDT) classification. The capillary condensation in pores occurred at a relative pressure (P/
Table Ⅰ lists the textural properties of the samples. The BET surface area, pore volume, and average pore diameter of the unpromoted catalyst were 21.2 m2/g, 0.07 cm3/g, and 13.2 nm, respectively. The catalyst with Mn/Mo ratio of 0.05 exhibited higher BET surface area and pore volume as comparison to the unpromoted catalyst, which may be caused by the destruction of MoO2 structure and formation of poorly crystalline CoMoO3 phases. However, with further increasing Mn content, the BET surface area and pore volume of the catalysts began to decrease. This result may be ascribed to the formation of well crystallized Co-Mo-O species, leading to the decrease of BET surface area and blockage of the pores of the particle networks.C. NH3-TPD results
The NH3 desorption profiles of all catalysts are presented in Fig. 3. It was generally accepted that the types of acid sites were related to the corresponding desorption temperature, whereas the area of desorption peak can reflect the amount of acid sites [34, 35]. Figure 3 the catalysts exhibited several NH3 desorption peaks in the range of 200-850 ℃. The low-temperature peak located at 200-400 ℃ corresponded to the desorption of NH3 on the weak acid sites, whereas the high-temperature desorption between 400 and 850 ℃ were attributed to the NH3 coordinated to the strong acid sites. For the unpromoted K-Co-Mo catalysts, the low-temperature desorption peak was very weak and three strong peaks at high temperature were observed, indicating that the strong acid sites were formed on the surface of unpromoted catalyst. After the addition of Mn, it was clearly observed that the high-temperature desorption peaks decreased sharply, while that of the low-temperature peaks inversely increased with the increment of Mn content. These results confirmed that the addition of Mn had a significant impact on the surface acidity of K-Co-Mo catalyst. The strong acid sites were reduced significantly and weak acid sites were predominant. This effect may be due to the presence of a large amount of Co-Mo-O species in the Mn-promoted catalyst, as evidenced by XRD results.D. In situ DRIFTS Results
Figure 4 shows the in situ DRIFTS spectra of CO adsorbed on the reduced catalysts. From the figure, two broad peaks centered at around 2173 and 2115 cm-1 have been attributed to the rotational/vibrational transitions of the residual gaseous CO . For the unpromoted catalyst, the peak at around 2010 cm-1 can be assigned to linearly coordinated CO to Co0 species on the surface . This low frequency band of CO linearly adsorbed on Co0 was attributed to low-index surface crystallographic planes or corners and step sites with low-coordinated surface sites . The addition of Mn promoter induced a shift of this peak toward higher wavenumber so that the catalyst with Mn/Mo ratio of 0.25 had a peak position of 2035 cm-1. Blue shifting of the linearly adsorbed CO peak indicated that the binding of CO to the Co surface was weaker, which reflected a decrease in
The bands in the range of 1950-1700 cm-1 were commonly assigned to two-fold, three-fold, and possibly four-fold bridge-bonded CO to Co0 species according to the literatures . It was apparent that with the Mn content increase, these peaks intensities of CO adsorption both in the linear and bridge types increased, indicating that the presence of Mn created more active sites for the linear and bridge adsorption of CO. The spectral region between 1700 and 1200 cm-1 was the characteristic frequencies of carbonate species . Specifically, the peak at around 1675 cm-1 was assigned to the bidentate-bonded bicarbonate species. The peak at 1590 cm-1 and the shoulder at 1310 cm-1 corresponded to the bidentate-bonded carbonate species, and the peaks at 1540 and 1340 cm-1 can be attributed to monodentate-bonded carbonate species. For the unpromoted K-Co-Mo catalyst, the peaks related to carbonate species were very weak. When the Mn was added, the bands intensities of carbonate species increased significantly with increasing the Mn content, indicating that the addition of Mn promoted the formation of carbonate species, in accordance with the previous report .E. XAFS Results
XANES spectra are sensitive to the local symmetry around center atom and useful in revealing the local coordination structure of center atom. The near-edge can provide much detailed information on the average valence and coordination geometry around center atom and the shift in the absorption edge can be used to identify the changes of valence state. The normalized Mo K-edge XANES spectra of the catalyst samples and two reference compounds MoO3 and MoO2 are shown in Fig. 5. A characteristic feature of pre-edge peak were observed for all the samples except for MoO2, which can be attributed to the so-called "1s-4d bound state transition". The transition probability of the formally forbidden excitation depends on the local symmetry around the Mo atom. If a center of inversion symmetry in the molybdenum site is lost, the intensity of the transition probability from the 1s to 4d state will be increased. In the case of tetrahedral Na2MoO4 structure, the p character in the final state becomes dominant by the effective mixing of metal d-states with ligand p-orbitals, resulting in an intense pre-edge feature . Thermodynamic MoO3 crystallizes in a unique orthorhombic crystal structure, showing the layered structure. The Mo atom is located at the off-center position in a heavily distorted MoO6 octahedron that gives three crystal-lographically inequivalent oxygen sites . This non-perfect octahedral surrounding of the central Mo atom slightly allows the 1s-4d transition to occur. As a result, a weak pre-edge peak was observed in the XANES spectra of MoO3. MoO2 shares a monoclinic structure closely related to that of rutile. Each Mo atom in MoO2 is coordinated by six oxygen atoms which have two distinct metal-oxygen bond lengths . So MoO2 has a more regular octahedron in comparison with MoO3, and the pre-edge peak becomes nearly invisible in the XANES spectra of MoO2.
As shown in Fig. 5, the XANES spectra of the unpromoted catalyst possessed the highest pre-edge peak and the absorption edge was close to that of MoO3. These results suggested that a less centro-symmetric surrounding was present around the Mo atom and not only octahedral coordinated Mo6+ species but also tetrahedral Mo6+ species were formed on the catalyst. It should be mentioned that no MoO3 diffraction peaks were observed in XRD pattern of the unpromoted sample, which may be attributed that the specie in this sample existed in the form of amorphous state. For the Mn-doped catalyst, at a low Mn content, the intensity of the pre-edge peak decreased to similar level to that of MoO3, and the absorption edge shifted to lower energy, indicating that there existed a fraction of octahedral coordinated Mo4+ species in the catalyst. With further increasing the Mn content, the pre-edge peak became weaker and the absorption edge shifted to lower energy, close to that of MoO2. The results indicated that the addition of Mn promoter promoted the transformation of the coordination environment of the Mo atom from tetrahedral to octahedral structure and the reduction of Mo6+ to Mo4+ species.
The Fourier transforms of the
The catalytic performance of the catalysts with different Mn/Mo ratio for higher alcohol synthesis were measured under the reaction conditions of 5.0 MPa, 300 ℃, 4800 h-1, and H2 to CO molar ratio of 2. The measured CO conversion, total alcohols selectivity, total alcohols STY, alcohols distribution and MeOH/C2+OH ratio after reaction for 24 h are summarized in Table Ⅱ. The unpromoted catalyst exhibited a very high CO conversion, but the selectivity and STY toward alcohols were extremely low. The predominant alcohol product was methanol. When the Mn promoter was added, the alcohol formation was enhanced significantly although the CO conversion decreased. The alcohol STY and selectivity increased with the Mn content increase and reached the highest level on the catalyst with Mn/Mo ratio of 0.15. Over the catalyst, the alcohol STY was 91.4 g/(kg·h), about 6.2 times more than that of the unpromoted sample. The alcohol selectivity also increased significantly from 4.2% to 51.6%. More interestingly, the addition of Mn promoted the formation of C
Over the Mo-based catalysts for alcohol synthesis, the Co promoter is known to act synergistically along with Mo species, rather than acting independently. It was reported that in some similar catalysts, the presence of Co3Mo3C, Ni6Mo6C, Co-Mo-S, and Ni-Mo-S phases were responsible for the higher alcohol synthesis [44, 47, 48]. Liakakou et al.  studied CO hydrogenation reaction over K-Ni-Mo/AC catalysts. The formation of oxygenates was attributed to the Ni-Mo synergetic interaction and the Ni-Mo-O sites were suggested to be the active phase for CO activation. Over the Mo2C catalyst, the interaction between Co and Mo species can enhance the overall reactivity and promote the formation of higher alcohols over methanol . A density functional theory study also indicated that Co atoms can be incorporated in the S-edge sites of MoS2, which may generate the active Co-Mo-S species and increase the activity of catalyst . For the present catalysts, as revealed by the XRD and XAFS results, the incorporation of Mn promoter enhanced the interaction between Co and Mo and thus led to the formation of Co-Mo-O species, which was suggested to be active for the alcohol synthesis. Furthermore, the good synergetic effect between Co and Mo was also conducive to enhance the promotional effect of Co.
The acidity of catalysts plays an important role in the alcohol formation. The surface acidity was favorable for the CO activation , but strong acid sites were also the active site for alcohol dehydration and thus decreased the alcohol selectivity . Furthermore, reduced acidity can facilitate the aldol-type condensation reactions, which was regarded as the intermediate step for the production of higher alcohols . A correlation between acid site concentration and CO conversion further revealed that decreasing the surface acidity of catalyst was conducive to the formation of higher alcohols [9, 34]. The NH3-TPD results shown in Fig. 3 clearly indicated that the presence of Mn reduced the amount of strong acid sites significantly and meanwhile promoted the formation of weak acid sites. The change trend was suggested to have a positive effect on the synthesis of higher alcohols from syngas according to the above discussion.
The CO adsorption property has a crucial impact on the CO hydrogenation reaction. The in situ DRIFTS results indicated that the incorporation of Mn can enhance the linear and bridge adsorption of CO on the surface of catalysts. Moreover, the formation of carbonate species was also enhanced. The bridge-type adsorbed CO was easily dissociated to carbon and oxygen . The C-O bond cleavage led to the more frequent appearance of C1 species that can scavenge H to promote the growth of carbon chain . The linearly adsorbed CO was beneficial for the formation of oxygenated compounds, because a linearly-adsorbed CO molecule had a stronger C-O bond and thus can be less easily dissociated . For the reduced Mo-based catalysts, the higher alcohol synthesis consists of several steps including the CO dissociation, hydrogenation and CO insertion, etc. The CO dissociation and hydrogenation promoted the formation of alkyl group and growth of carbon chain. The alkyl group was further transformed to the acyl species by the insertion of non-dissociative CO. Finally, the acyl species was hydrogenated to form the corresponding alcohol product. From these perspectives, the enhanced adsorption of linear-and bridge-type CO due to the incorporation of Mn contributed to the growth of carbon chain and formation of alcohol, leading to the increase of the selectivity to C2+OH. This work developed a highly active Mo-based catalyst for the synthesis of higher alcohols and provided some new insight into the promotional effect of Mn.Ⅳ. CONCLUSION
The Mn-K-Co-Mo catalysts with different Mn content were prepared by a sol-gel method. It was found that the addition of Mn enhanced the interaction between Co and Mo, leading to the formation of active Co-Mo-O species. Meanwhile, the surface strong acidity of catalyst was reduced significantly and the amount of weak acid sites increased. Furthermore, the incorporation of Mn promoted the adsorption of linear and bridge-type CO significantly. Benefit from these features, the Mn-doped catalysts exhibited an excellent catalytic activity for the alcohols synthesis from syngas. In particular, the selectivity to C2+ alcohol increased greatly. The alcohol products distribution deviated from the ASF law. The formation of methanol was suppressed significantly and the ethanol became the predominant product.Ⅴ. ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (No.11179034 and No.11205159) and National Basic Research Program of China (No.2012CB922004).
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b. 北京化工大学, 有机-无机复合材料国家重点实验室, 北京 100029;
c. 中国科学技术大学材料科学与工程系, 中国科学院能源转换材料重点实验室, 合肥 230029