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AC (M = Cu, Fe, Co, Ni and Pd) catalysts
Accepted Manuscript Catalytic upgrading of ethanol to n-butanol over M-CeO2/AC (M=Cu, Fe, Co, Ni and Pd) catalysts
Xianyuan Wu, Geqian Fang, Zhe Liang, Wenhua Leng, Kaiyue Xu, Dahao Jiang, Jun Ni, Xiaonian Li PII: DOI: Reference:
S1566-7367(17)30247-9 doi: 10.1016/j.catcom.2017.06.016 CATCOM 5082
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
27 January 2017 13 June 2017 14 June 2017
Please cite this article as: Xianyuan Wu, Geqian Fang, Zhe Liang, Wenhua Leng, Kaiyue Xu, Dahao Jiang, Jun Ni, Xiaonian Li , Catalytic upgrading of ethanol to n-butanol over M-CeO2/AC (M=Cu, Fe, Co, Ni and Pd) catalysts. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.06.016
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ACCEPTED MANUSCRIPT Catalytic upgrading of ethanol to n-butanol over M-CeO2/AC (M = Cu, Fe, Co, Ni and Pd) catalysts Xianyuan Wu, Geqian Fang, Zhe Liang, Wenhua Leng, Kaiyue, Xu, Dahao Jiang* , Jun Ni* and Xiaonian Li
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Institute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou 310014, China
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Corresponding authors
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* E-mail addresses: [email protected] (D.H. Jiang), [email protected] (J. Ni).
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ACCEPTED MANUSCRIPT ABSTRACT M-CeO2 /AC (M = Cu, Fe, Co, Ni and Pd) catalysts were prepared and evaluated in the
continuous catalytic upgrading of ethanol to n-butanol under mild reaction conditions. The highest selectivity to n-butanol (67.6%) was achieved over Pd-CeO2 /AC catalyst, while Cu-CeO 2 /AC catalyst exhibited the highest ethanol conversion (46.2%) with moderate selectivity to n-butanol (41.3%). The catalytic performance of these catalysts was shown for the first time to largely depend on the dehydrogenation, hydrogenation and C-C bond cracking capabilities of each individual supported metal.
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KEYWORDS：M-CeO2 /AC catalysts; ethanol; catalytic upgrading; n-butanol.
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ACCEPTED MANUSCRIPT 1. Introduction The interest in biofuels has increased considerably over the recent years, particularly in view of concerns over energy security and climate change. In this context, n-butanol is considered as a highly desirable biofuel in the future if derived from bio-ethanol, due to its lower miscibility with water, higher
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energy density and limited destruction to engine parts [1]. The upgrading of ethanol to n-butanol is
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suggested to proceed via the Guerbert pathway at temperatures lower than 673 K [2]. Three tandem
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reaction steps are involved in this route, namely: ethanol dehydrogenation, aldol condensation of acetaldehyde and hydrogenation of crotonaldehyde (Scheme S1) [3]. Basic zeolites [4], basic metal
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oxides [5,6], Mg-Al metal oxides [7-9] and hydroxyapatites (HAP) [10-13] have been comprehensively
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investigated in the last decade. However, these catalysts normally show low yields of n-butanol (<10%) even in harsh reaction conditions (reaction T > 573 K and P > 6.0 MPa). Incorporation of metals into
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these catalysts could significantly improve the conversion of ethanol and decrease the reaction
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temperature, mainly due to the enhanced ethanol dehydrogenation on metals. For example, a Ni/Al2 O3 catalyst exhibited up to 80% selectivity to n-butanol with 25% ethanol conversion at 523 K and 7 MPa
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(Ar) after reaction of 72 h in a batch reactor [14]. However, due to the strong cracking capability of C-C
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bonds over Ni metal, considerable amounts of gaseous carbon products such as CO, CO2 and CH4 , were also detected. Zhang et al. [15] reported highly dispersed Ni-MgAlO catalysts exhibiting 55.2%
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selectivity to n-butanol and 85% selectivity to C 4 -C8 alcohols with 18.7% ethanol conversion at 523 K and 3 MPa. The characterization results showed that Ni metal promoted ethanol dehydrogenation, where the released hydrogen spilled over the catalyst, leading to in situ hydrogenation of reaction intermediates. However, the activities of the above metal-based catalysts were low and high reaction pressure was therefore applied. We have demonstrated [16] that Cu-CeO 2 /AC catalysts exhibited nearly 20% n-butanol yield under mild reaction conditions of 523 K, 2 MPa (N2 ), LHSV=4 ml/(h·gcat ) and N2 /ethanol (v/v) = 500:1. It was proposed that the high yield to n-butanol was mainly ascribed to the 3
ACCEPTED MANUSCRIPT high activities of metals for both ethanol dehydrogenation and hydrogenation of crotonaldehyde [17]. In general, ethanol dehydrogenation requires metals having a strong affinity with hydrogen. However, such a strong interaction leads to lower activity for the hydrogenation of crotonaldehyde. Therefore, a balance of interaction between hydrogen and metals is needed for the design of promising catalysts for ethanol upgrading to n-butanol. In the present study, for the first time a range of activated carbon (AC)
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supported M-CeO 2 catalysts (M = Cu, Co, Ni, Pd and Fe) were prepared and evaluated in the continuous
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catalytic upgrading of ethanol to n-butanol with special emphasis on the effect of metal’s inherent capabilities for dehydrogenation, hydrogenation and C-C bond cracking on the catalytic performance of
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M-CeO2 /AC catalysts. These catalysts exhibit competitive activity and/or selectivity in ethanol upgrading to n-butanol when compared with catalysts reported in the open literature under similar or
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milder reaction conditions.
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2. Experimental
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2.1. Catalyst preparation
All catalysts were prepared by the simple wetness impregnation method, including impregnation,
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drying, calcination and reduction steps at desired temperatures. Typical details for catalyst preparation
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are provided in the Supporting Information (SI). 2.2. Catalyst characterization
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The synthesized catalysts were characterized by means of powder XRD, H2 -TPR, H2 -TPD and CO2 TPD techniques. Details of the general procedures are described in the Supporting Information. 2.3. Catalyst performance evaluation The catalytic tests were performed using a fixed-bed reactor under the following reaction conditions: T=523 K, P=2 MPa (N2 ), LHSV=4 ml/(h·gcat ) and N 2 /ethanol = 500:1 (v/v). Detailed information is presented in the Supporting Information.
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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. XRD patterns of M-CeO2 /AC catalysts
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CeO2 /AC; (d) Ni-CeO2 /AC; (e) Pd-CeO2 /AC.
XRD patterns of reduced catalysts before and after reaction are presented in Fig. 1 (A) and (B),
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respectively, while the textural parameters of M-CeO 2 /AC catalysts are shown in Table S1. The
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diffraction peaks at 2θ=28.5, 33.1, 47.5 and 56.4o are characteristic of pure CeO 2 crystal phase. It can be found that fresh and spent M-CeO 2 /AC catalysts all exhibit weak and diffuse CeO 2 diffraction peaks
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with an average CeO 2 nanoparticles size of less than 5 nm, indicating that CeO 2 is highly dispersed over
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the AC support surface. In addition, the mean diameters of Cu, Co, Ni and Pd metal nanoparticles before and after reaction are all less than 10 nm, despite the fact that an increase in the size of metal nanoparticles to different extents did occur after 12 h of reaction. This result suggests that metal particles are also highly dispersed over the AC support. On the other hand, remarkably large Fe nanoparticles (77 and ~ 100 nm) are observed in both the fresh and spent Fe-CeO 2 /AC catalysts and changes in the valence state of Fe species (such as FeO→Fe3 O4 ) took place in the ethanol upgrading reaction, which probably account for the low catalytic activity of Fe-CeO 2 /AC catalyst (Section 3.5). 5
ACCEPTED MANUSCRIPT 3.2. H2 -TPR over M-CeO2 /AC catalysts H2 -TPR traces recorded over the various calcined M-CeO 2 /AC catalysts are shown in Fig. 2. Five H2 consumption peaks in the TPR trace of Cu-CeO 2 /AC catalyst (Fig. 2a) are observed. The hightemperature peak centered at 935 K is ascribed to the reduction of oxygen-containing groups present on the surface of AC support, while the peak at 826 K is assigned to the reduction of CeO 2 (Fig. S2) [16].
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The main and shoulder peaks centered at 464 K and 547 K can be attributed to the reduction of surface and bulk CuO species, respectively [18, 19]. Reduction of CeO 2 is usually promoted in the presence of
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metals due to the hydrogen spillover effect [20, 21]. Thus, the H2 consumption peak centered at 715 K
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can be ascribed to CeO 2 that has an intimate interaction with Cu metal. Ni-CeO 2 /AC catalyst exhibits similar reduction behavior as that of Cu-CeO 2 /AC catalyst (Fig. 2d). Two reduction peaks centered at
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530 K and 581 K can be assigned to NiO with small and large particle size, respectively [22], whereas
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two peaks centered at 698 K and 792 K can be assigned to the reduction of CeO 2 that is strongly and weakly interacting with Ni metal, respectively [20]. For the Co-CeO 2 /AC catalyst (Fig. 2c), the H2
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consumption peaks centered at 537 K and 822 K should be assigned to a two-step reduction of cobalt
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oxide (Co3 O 4 →CoO→Co) [23]. The H2 consumption peak centered at 705 K is ascribed to the reduction of CeO 2 intimately interacting with Co metal, while the reduction peak of CeO 2 that weakly
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interacts with Co metal overlaps with the reduction peak of CoO to Co [23]. Similarly, the H2
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consumption peaks centered at 618 K and 958 K over the Fe-CeO 2 /AC catalyst can be assigned to the two-step reduction of iron oxide (Fe2 O3 →Fe3O4 →Fe) [22, 24, 25], while the H2 consumption peak centered at 750 K can be ascribed to the combined reduction of Fe3 O4 and CeO 2 (Fig. 2b). The presence of Pd metal decreases the reduction temperature of CeO 2 (ca. 789 K), suggesting a strong ability of Pd for hydrogen activation and spillover (Fig. 2e) [21]. In summary, the reduction peaks of CeO 2 in close contact with metals are: 715, 750, 705, 698 and 705 K for Cu-, Fe-, Co-, Ni- and Pd-CeO 2 /AC catalysts, respectively, implying that the capability of hydrogen activation and subsequent spillover can be Fe < 6
ACCEPTED MANUSCRIPT Cu < Co ~Pd < Ni. Considering the lower loading of Pd metal (2 wt%) relative to those of other metals (10 wt%), the trend should increase as follows: Fe < Cu < Co < Ni < Pd. Generally, the stronger the ability of metals for molecular hydrogen dissociation and surface diffusion is, the lower the reduction temperature of CeO 2 will be.
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3.3. H2 -TPD over M-CeO2 /AC catalysts
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Pd-CeO2 /AC.
H2 -TPD profiles of the reduced M-CeO 2 /AC catalysts are presented in Fig. 3. The H2 desorption peak
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at 394 K is observed for all samples, which is attributed to the desorption of H2 adsorbed on metals [26].
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There are also several desorption peaks at temperatures higher than 700 K, which can be attributed to hydrogen species spilled over ceria and likely AC, because these peaks only exist in the presence of
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metals but not on ceria or AC alone (Figs. S3-S8) [26-29]. The amount of H2 desorbed from metals (Table S1) increases in the order: Fe-CeO 2 /AC (0.096 mmol/g) < Cu-CeO 2 /AC (0.102 mmol/g) < CoCeO 2 /AC (0.170 mmol/g) < Ni-CeO 2 /AC (0.232 mmol/g) < Pd-CeO 2 /AC (0.253 mmol/g), implying that the amount (mmol/g) of active metals increases in the same order, which is consistent with the capability of metals for hydrogen activation.
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3.4. CO2 -TPD over M-CeO2 /AC catalysts
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Fig. 4 presents the CO 2 -TPD profiles of reduced M-CeO 2 /AC catalysts. The CO 2 desorption peaks
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observed at 419 K are attributed to CO 2 adsorbed on surface basic sites of CeO 2 [16], while those peaks
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at temperatures higher than 700 K are due to hydrogen spilled over ceria and likely AC, as illustrated in the H2 -TPDs (Fig. 3). The amount of CO 2 desorbed (Table S1) also follows the same order as that of
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hydrogen activation and surface diffusion over the metals, which can be explained by the fact that reduced CeO 2 is more basic than pristine CeO 2 [30]. Because basic sites are essential for the aldol
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condensation of acetaldehyde (Scheme S1), the amount of basic sites must be directly related to the
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aldehyde coupling products. The latter is indeed confirmed by the trend of selectivity to C4 + alcohols and aldehydes (including crotonaldehyde, butyraldehyde, butanol and C 6 products) from acetaldehyde coupling (Table 1).
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(e) Pd-CeO2 /AC.
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Fig. 4. CO2 -TPD profiles of reduced catalysts: (a) Cu-CeO2 /AC; (b) Fe-CeO2 /AC; (c) Co-CeO2 /AC; (d) Ni-CeO2 /AC;
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The catalytic performance of M-CeO 2 /AC and M/AC catalysts (M = Cu, Fe, Co, Ni and Pd) for the
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upgrading of ethanol to n-butanol is presented in Table 1 and Table S2, respectively. As shown in Table 1, only 3.1% of ethanol conversion and 1.6% of n-butanol selectivity but 50.0% selectivity to
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crotonaldehyde are observed for the CeO2 /AC catalyst, suggesting low activities for ethanol
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dehydrogenation and hydrogenation of crotonaldehyde without metals but high activity for acetaldehyde condensation. M/AC catalysts also showed poor catalytic activity and n-butanol selectivity (Table S2).
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However, the incorporation of CeO 2 into the M/AC catalysts dramatically improved the catalytic
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performance of these solids. It appears that the introduction of CeO 2 effectively promotes metal dispersion and provides basic sites for acetaldehyde condensation, therefore, improving the whole Guerbert reaction as discussed in Sections 3.1-3.4. Cu-CeO 2 /AC catalyst exhibits the highest ethanol conversion (46.3%) and moderate selectivity to n-butanol (41.3%), which are mainly ascribed to the strong capability of ethanol dehydrogenation but relatively low hydrogenation activity of Cu metal. In contrast, Co-, Ni- and Pd-CeO 2 /AC catalysts exhibit higher selectivities to n-butanol (47.6, 50.6 and 67.6%, respectively), but lower ethanol conversions. Because the selectivity to n-butanol mainly depends on the formation and hydrogenation of crotonaldehyde steps in the Guerbert pathway, the trend 9
ACCEPTED MANUSCRIPT of selectivity to n-butanol (Fe < Cu < Co < Ni < Pd) should be ascribed to the difference in the capability of hydrogen activation over active metals, as well as the capability of acetaldehyde condensation over basic sites, as discussed in Sections 3.2-3.4. In addition, ethanol conversion displays the opposite trend (Cu > Co > Ni > Fe > Pd), which can be understood when considering that
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dehydrogenation and hydrogenation is a pair of reverse catalytic processes.
Conversion (%)
Selectivity (%) Acetaldehyde
Ethyl acetate
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Table 1. Catalytic performance of M -CeO 2/AC catalysts for the upgrading ethanol to n-butanol[a]
Butyraldehyde
Butanol
C6 products [b]
Others [c]
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CeO 2/AC 3.1 13.5 2.2 50.0 4.4 1.6 4.9 23.4 Cu-CeO 2/AC 46.2 7.2 10.9 2.8 7.8 41.3 19.7 10.3 Fe-CeO 2/AC 4.7 21.5 20.6 2.5 2.9 28.5 2.1 21.9 Co-CeO 2/AC 34.1 7.4 5.3 2.4 5.6 47.6 18.4 13.3 Ni-CeO 2/AC 31.6 4.7 3.2 1.1 3.9 50.6 17.8 18.7 Pd-CeO 2/AC 11.9 6.6 4.8 0.3 2.9 67.6 5.8 12.0 [a] Convers ion and s electivity are obt ained at st eady -stat e; reaction conditions: mass of cat alyst, 1.0 g; T =523 K, P= 2 M P a (N 2), LH SV= 4 ml/(h·gcat), N 2 /ethanol(v/v)=500:1; [b] C6 products include: 2-ethylbutyraldehyde, hexaldehyde, 2-ethylbut anol, and 1hexanol; [c] Other products include: diethyl ether, 1,1 -diethoxy ethane, butyl acet ate, etc. Specifically, the diethyl ether for CeO 2/AC = 12.3%; Fe-CeO 2/AC = 6.1%; Ni-CeO 2/AC =4.6%.
Additionally, gaseous products formed ca. via ethanol cracking and/or decarbonylation of
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acetaldehyde [31], such as CH4 , CO and CO2 , were not detected in the outlet gas over the Cu-CeO 2 /AC catalyst, indicating that the cleavage of C-C bonds was not favorable over Cu metal. On the contrary, Fe,
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Co, Ni, and Pd all showed different degrees of ability to break the C-C bond in ethanol or acetaldehyde.
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As shown in Fig. S9, Ni-CeO 2 /AC catalyst shows 77.3% yield of liquid products, followed by 85, 93.6 and 97.9% for Co-, Fe- and Pd-CeO2 /AC, respectively. In this context, Cu exhibited the highest activity for ethanol dehydrogenation, while Pd showed the highest ability to hydrogenate the intermediate crotonaldehyde; moreover, Cu and Pd possessed negligible C-C bond cracking capability. Therefore, for the production of liquid products (C4 + alcohols and aldehydes), a combination of Cu and Pd might be considered as a promising strategy for the design of more efficient catalysts and corresponding research work is underway. Table S3 compares the catalytic performance of M-CeO 2 /AC catalysts with that of 10
ACCEPTED MANUSCRIPT other catalytic systems reported in the open literature. It is apparent that Cu-CeO 2 /AC strongly competes with all catalysts reported with respect to the n-butanol yield.
4. Conclusions
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M-CeO2 /AC (M= Cu, Fe, Co, Ni and Pd) catalysts were prepared by the wetness impregnation
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method and evaluated in the continuous catalytic upgrading of ethanol to n-butanol under mild reaction
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conditions. Different ethanol conversions (5-50%), selectivities toward n-butanol (30-70%) and yields
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of total liquid products (77.3-100%) were obtained for these catalysts, among which the highest selectivity toward n-butanol (67.6%) was achieved over the Pd-CeO2 /AC catalyst, while the highest
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ethanol conversion (46.2%) was achieved over the Cu-CeO 2 /AC catalyst with moderate selectivity toward n-butanol (41.3%). The catalytic performances of these catalysts depended largely on the
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dehydrogenation, hydrogenation and C-C bond cracking capabilities of each individual supported metal.
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The metal with high capabilities of hydrogen activation and spillover does not only favor hydrogenation
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of crotonaldehyde but also aldol condensation of acetaldehyde due to the enhanced basicity of CeO 2 . The combination of two metals in the same catalyst composition, one being active in ethanol
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dehydrogenation (such as Cu) and the other in hydrogenation steps (such as Pd, Ni, Co etc.), might be considered as a promising approach to designing efficient catalysts for the production of liquid products
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(C4 + alcohols and aldehydes) from the conversion of ethanol.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC Grant No. 21303163), the Natural Science Foundation of Zhejiang Province (LY17B060006) and the Scientific Research Fund of Zhejiang Provincial Education Department (Y201328681). 11
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ACCEPTED MANUSCRIPT Graphical abstract SYNOPSIS TOC
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M-CeO2 /AC (M = Cu, Fe, Co, Ni and Pd) catalysts were synthesized and compared in the continuous catalytic upgrading of ethanol to n-butanol under mild reaction conditions. The catalytic performance of catalysts was mainly depended on their dehydrogenation, hydrogenation and C-C bond cracking capabilities.
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ACCEPTED MANUSCRIPT Highlights High selectivity toward n-butanol (67.6%) was achieved over Pd-CeO 2 /AC catalyst
Metals with high capability of hydrogen activation and spillover have high n-butanol selectivity
Performance of catalysts was depended on their de/hydrogenation and C-C cracking capabilities
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Xianyuan Wu, Geqian Fang, Zhe Liang, Wenhua Leng, Kaiyue Xu, Dahao Jiang, Jun Ni, Xiaonian Li PII: DOI: Reference:
S1566-7367(17)30247-9 doi: 10.1016/j.catcom.2017.06.016 CATCOM 5082
To appear in:
Catalysis Communications
Received date: Revised date: Accepted date:
27 January 2017 13 June 2017 14 June 2017
Please cite this article as: Xianyuan Wu, Geqian Fang, Zhe Liang, Wenhua Leng, Kaiyue Xu, Dahao Jiang, Jun Ni, Xiaonian Li , Catalytic upgrading of ethanol to n-butanol over M-CeO2/AC (M=Cu, Fe, Co, Ni and Pd) catalysts. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.06.016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Catalytic upgrading of ethanol to n-butanol over M-CeO2/AC (M = Cu, Fe, Co, Ni and Pd) catalysts Xianyuan Wu, Geqian Fang, Zhe Liang, Wenhua Leng, Kaiyue, Xu, Dahao Jiang* , Jun Ni* and Xiaonian Li
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Institute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou 310014, China
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Corresponding authors
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* E-mail addresses: [email protected] (D.H. Jiang), [email protected] (J. Ni).
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ACCEPTED MANUSCRIPT ABSTRACT M-CeO2 /AC (M = Cu, Fe, Co, Ni and Pd) catalysts were prepared and evaluated in the
continuous catalytic upgrading of ethanol to n-butanol under mild reaction conditions. The highest selectivity to n-butanol (67.6%) was achieved over Pd-CeO2 /AC catalyst, while Cu-CeO 2 /AC catalyst exhibited the highest ethanol conversion (46.2%) with moderate selectivity to n-butanol (41.3%). The catalytic performance of these catalysts was shown for the first time to largely depend on the dehydrogenation, hydrogenation and C-C bond cracking capabilities of each individual supported metal.
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KEYWORDS：M-CeO2 /AC catalysts; ethanol; catalytic upgrading; n-butanol.
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ACCEPTED MANUSCRIPT 1. Introduction The interest in biofuels has increased considerably over the recent years, particularly in view of concerns over energy security and climate change. In this context, n-butanol is considered as a highly desirable biofuel in the future if derived from bio-ethanol, due to its lower miscibility with water, higher
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energy density and limited destruction to engine parts [1]. The upgrading of ethanol to n-butanol is
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suggested to proceed via the Guerbert pathway at temperatures lower than 673 K [2]. Three tandem
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reaction steps are involved in this route, namely: ethanol dehydrogenation, aldol condensation of acetaldehyde and hydrogenation of crotonaldehyde (Scheme S1) [3]. Basic zeolites [4], basic metal
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oxides [5,6], Mg-Al metal oxides [7-9] and hydroxyapatites (HAP) [10-13] have been comprehensively
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investigated in the last decade. However, these catalysts normally show low yields of n-butanol (<10%) even in harsh reaction conditions (reaction T > 573 K and P > 6.0 MPa). Incorporation of metals into
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these catalysts could significantly improve the conversion of ethanol and decrease the reaction
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temperature, mainly due to the enhanced ethanol dehydrogenation on metals. For example, a Ni/Al2 O3 catalyst exhibited up to 80% selectivity to n-butanol with 25% ethanol conversion at 523 K and 7 MPa
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(Ar) after reaction of 72 h in a batch reactor [14]. However, due to the strong cracking capability of C-C
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bonds over Ni metal, considerable amounts of gaseous carbon products such as CO, CO2 and CH4 , were also detected. Zhang et al. [15] reported highly dispersed Ni-MgAlO catalysts exhibiting 55.2%
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selectivity to n-butanol and 85% selectivity to C 4 -C8 alcohols with 18.7% ethanol conversion at 523 K and 3 MPa. The characterization results showed that Ni metal promoted ethanol dehydrogenation, where the released hydrogen spilled over the catalyst, leading to in situ hydrogenation of reaction intermediates. However, the activities of the above metal-based catalysts were low and high reaction pressure was therefore applied. We have demonstrated [16] that Cu-CeO 2 /AC catalysts exhibited nearly 20% n-butanol yield under mild reaction conditions of 523 K, 2 MPa (N2 ), LHSV=4 ml/(h·gcat ) and N2 /ethanol (v/v) = 500:1. It was proposed that the high yield to n-butanol was mainly ascribed to the 3
ACCEPTED MANUSCRIPT high activities of metals for both ethanol dehydrogenation and hydrogenation of crotonaldehyde [17]. In general, ethanol dehydrogenation requires metals having a strong affinity with hydrogen. However, such a strong interaction leads to lower activity for the hydrogenation of crotonaldehyde. Therefore, a balance of interaction between hydrogen and metals is needed for the design of promising catalysts for ethanol upgrading to n-butanol. In the present study, for the first time a range of activated carbon (AC)
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supported M-CeO 2 catalysts (M = Cu, Co, Ni, Pd and Fe) were prepared and evaluated in the continuous
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catalytic upgrading of ethanol to n-butanol with special emphasis on the effect of metal’s inherent capabilities for dehydrogenation, hydrogenation and C-C bond cracking on the catalytic performance of
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M-CeO2 /AC catalysts. These catalysts exhibit competitive activity and/or selectivity in ethanol upgrading to n-butanol when compared with catalysts reported in the open literature under similar or
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milder reaction conditions.
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2. Experimental
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2.1. Catalyst preparation
All catalysts were prepared by the simple wetness impregnation method, including impregnation,
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drying, calcination and reduction steps at desired temperatures. Typical details for catalyst preparation
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are provided in the Supporting Information (SI). 2.2. Catalyst characterization
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The synthesized catalysts were characterized by means of powder XRD, H2 -TPR, H2 -TPD and CO2 TPD techniques. Details of the general procedures are described in the Supporting Information. 2.3. Catalyst performance evaluation The catalytic tests were performed using a fixed-bed reactor under the following reaction conditions: T=523 K, P=2 MPa (N2 ), LHSV=4 ml/(h·gcat ) and N 2 /ethanol = 500:1 (v/v). Detailed information is presented in the Supporting Information.
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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1. XRD patterns of M-CeO2 /AC catalysts
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Fig. 1. XRD patterns of reduced catalysts (A) before and (B) after reaction: (a) Cu-CeO2 /AC; (b) Fe-CeO2 /AC; (c) Co-
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CeO2 /AC; (d) Ni-CeO2 /AC; (e) Pd-CeO2 /AC.
XRD patterns of reduced catalysts before and after reaction are presented in Fig. 1 (A) and (B),
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respectively, while the textural parameters of M-CeO 2 /AC catalysts are shown in Table S1. The
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diffraction peaks at 2θ=28.5, 33.1, 47.5 and 56.4o are characteristic of pure CeO 2 crystal phase. It can be found that fresh and spent M-CeO 2 /AC catalysts all exhibit weak and diffuse CeO 2 diffraction peaks
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with an average CeO 2 nanoparticles size of less than 5 nm, indicating that CeO 2 is highly dispersed over
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the AC support surface. In addition, the mean diameters of Cu, Co, Ni and Pd metal nanoparticles before and after reaction are all less than 10 nm, despite the fact that an increase in the size of metal nanoparticles to different extents did occur after 12 h of reaction. This result suggests that metal particles are also highly dispersed over the AC support. On the other hand, remarkably large Fe nanoparticles (77 and ~ 100 nm) are observed in both the fresh and spent Fe-CeO 2 /AC catalysts and changes in the valence state of Fe species (such as FeO→Fe3 O4 ) took place in the ethanol upgrading reaction, which probably account for the low catalytic activity of Fe-CeO 2 /AC catalyst (Section 3.5). 5
ACCEPTED MANUSCRIPT 3.2. H2 -TPR over M-CeO2 /AC catalysts H2 -TPR traces recorded over the various calcined M-CeO 2 /AC catalysts are shown in Fig. 2. Five H2 consumption peaks in the TPR trace of Cu-CeO 2 /AC catalyst (Fig. 2a) are observed. The hightemperature peak centered at 935 K is ascribed to the reduction of oxygen-containing groups present on the surface of AC support, while the peak at 826 K is assigned to the reduction of CeO 2 (Fig. S2) [16].
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The main and shoulder peaks centered at 464 K and 547 K can be attributed to the reduction of surface and bulk CuO species, respectively [18, 19]. Reduction of CeO 2 is usually promoted in the presence of
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metals due to the hydrogen spillover effect [20, 21]. Thus, the H2 consumption peak centered at 715 K
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can be ascribed to CeO 2 that has an intimate interaction with Cu metal. Ni-CeO 2 /AC catalyst exhibits similar reduction behavior as that of Cu-CeO 2 /AC catalyst (Fig. 2d). Two reduction peaks centered at
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530 K and 581 K can be assigned to NiO with small and large particle size, respectively [22], whereas
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two peaks centered at 698 K and 792 K can be assigned to the reduction of CeO 2 that is strongly and weakly interacting with Ni metal, respectively [20]. For the Co-CeO 2 /AC catalyst (Fig. 2c), the H2
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consumption peaks centered at 537 K and 822 K should be assigned to a two-step reduction of cobalt
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oxide (Co3 O 4 →CoO→Co) [23]. The H2 consumption peak centered at 705 K is ascribed to the reduction of CeO 2 intimately interacting with Co metal, while the reduction peak of CeO 2 that weakly
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interacts with Co metal overlaps with the reduction peak of CoO to Co [23]. Similarly, the H2
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consumption peaks centered at 618 K and 958 K over the Fe-CeO 2 /AC catalyst can be assigned to the two-step reduction of iron oxide (Fe2 O3 →Fe3O4 →Fe) [22, 24, 25], while the H2 consumption peak centered at 750 K can be ascribed to the combined reduction of Fe3 O4 and CeO 2 (Fig. 2b). The presence of Pd metal decreases the reduction temperature of CeO 2 (ca. 789 K), suggesting a strong ability of Pd for hydrogen activation and spillover (Fig. 2e) [21]. In summary, the reduction peaks of CeO 2 in close contact with metals are: 715, 750, 705, 698 and 705 K for Cu-, Fe-, Co-, Ni- and Pd-CeO 2 /AC catalysts, respectively, implying that the capability of hydrogen activation and subsequent spillover can be Fe < 6
ACCEPTED MANUSCRIPT Cu < Co ~Pd < Ni. Considering the lower loading of Pd metal (2 wt%) relative to those of other metals (10 wt%), the trend should increase as follows: Fe < Cu < Co < Ni < Pd. Generally, the stronger the ability of metals for molecular hydrogen dissociation and surface diffusion is, the lower the reduction temperature of CeO 2 will be.
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3.3. H2 -TPD over M-CeO2 /AC catalysts
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Pd-CeO2 /AC.
H2 -TPD profiles of the reduced M-CeO 2 /AC catalysts are presented in Fig. 3. The H2 desorption peak
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at 394 K is observed for all samples, which is attributed to the desorption of H2 adsorbed on metals [26].
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There are also several desorption peaks at temperatures higher than 700 K, which can be attributed to hydrogen species spilled over ceria and likely AC, because these peaks only exist in the presence of
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metals but not on ceria or AC alone (Figs. S3-S8) [26-29]. The amount of H2 desorbed from metals (Table S1) increases in the order: Fe-CeO 2 /AC (0.096 mmol/g) < Cu-CeO 2 /AC (0.102 mmol/g) < CoCeO 2 /AC (0.170 mmol/g) < Ni-CeO 2 /AC (0.232 mmol/g) < Pd-CeO 2 /AC (0.253 mmol/g), implying that the amount (mmol/g) of active metals increases in the same order, which is consistent with the capability of metals for hydrogen activation.
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Fig. 3. H2 -TPD profiles of reduced catalysts: (a) Cu-CeO2 /AC; (b) Fe-CeO2 /AC; (c) Co-CeO2 /AC; (d) Ni-CeO2 /AC; (e)
3.4. CO2 -TPD over M-CeO2 /AC catalysts
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Fig. 4 presents the CO 2 -TPD profiles of reduced M-CeO 2 /AC catalysts. The CO 2 desorption peaks
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observed at 419 K are attributed to CO 2 adsorbed on surface basic sites of CeO 2 [16], while those peaks
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at temperatures higher than 700 K are due to hydrogen spilled over ceria and likely AC, as illustrated in the H2 -TPDs (Fig. 3). The amount of CO 2 desorbed (Table S1) also follows the same order as that of
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hydrogen activation and surface diffusion over the metals, which can be explained by the fact that reduced CeO 2 is more basic than pristine CeO 2 [30]. Because basic sites are essential for the aldol
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condensation of acetaldehyde (Scheme S1), the amount of basic sites must be directly related to the
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aldehyde coupling products. The latter is indeed confirmed by the trend of selectivity to C4 + alcohols and aldehydes (including crotonaldehyde, butyraldehyde, butanol and C 6 products) from acetaldehyde coupling (Table 1).
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3.5. Catalytic performance of M-CeO2 /AC catalysts
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(e) Pd-CeO2 /AC.
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Fig. 4. CO2 -TPD profiles of reduced catalysts: (a) Cu-CeO2 /AC; (b) Fe-CeO2 /AC; (c) Co-CeO2 /AC; (d) Ni-CeO2 /AC;
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The catalytic performance of M-CeO 2 /AC and M/AC catalysts (M = Cu, Fe, Co, Ni and Pd) for the
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upgrading of ethanol to n-butanol is presented in Table 1 and Table S2, respectively. As shown in Table 1, only 3.1% of ethanol conversion and 1.6% of n-butanol selectivity but 50.0% selectivity to
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crotonaldehyde are observed for the CeO2 /AC catalyst, suggesting low activities for ethanol
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dehydrogenation and hydrogenation of crotonaldehyde without metals but high activity for acetaldehyde condensation. M/AC catalysts also showed poor catalytic activity and n-butanol selectivity (Table S2).
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However, the incorporation of CeO 2 into the M/AC catalysts dramatically improved the catalytic
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performance of these solids. It appears that the introduction of CeO 2 effectively promotes metal dispersion and provides basic sites for acetaldehyde condensation, therefore, improving the whole Guerbert reaction as discussed in Sections 3.1-3.4. Cu-CeO 2 /AC catalyst exhibits the highest ethanol conversion (46.3%) and moderate selectivity to n-butanol (41.3%), which are mainly ascribed to the strong capability of ethanol dehydrogenation but relatively low hydrogenation activity of Cu metal. In contrast, Co-, Ni- and Pd-CeO 2 /AC catalysts exhibit higher selectivities to n-butanol (47.6, 50.6 and 67.6%, respectively), but lower ethanol conversions. Because the selectivity to n-butanol mainly depends on the formation and hydrogenation of crotonaldehyde steps in the Guerbert pathway, the trend 9
ACCEPTED MANUSCRIPT of selectivity to n-butanol (Fe < Cu < Co < Ni < Pd) should be ascribed to the difference in the capability of hydrogen activation over active metals, as well as the capability of acetaldehyde condensation over basic sites, as discussed in Sections 3.2-3.4. In addition, ethanol conversion displays the opposite trend (Cu > Co > Ni > Fe > Pd), which can be understood when considering that
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dehydrogenation and hydrogenation is a pair of reverse catalytic processes.
Conversion (%)
Selectivity (%) Acetaldehyde
Ethyl acetate
Crotonaldehyde
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Table 1. Catalytic performance of M -CeO 2/AC catalysts for the upgrading ethanol to n-butanol[a]
Butyraldehyde
Butanol
C6 products [b]
Others [c]
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CeO 2/AC 3.1 13.5 2.2 50.0 4.4 1.6 4.9 23.4 Cu-CeO 2/AC 46.2 7.2 10.9 2.8 7.8 41.3 19.7 10.3 Fe-CeO 2/AC 4.7 21.5 20.6 2.5 2.9 28.5 2.1 21.9 Co-CeO 2/AC 34.1 7.4 5.3 2.4 5.6 47.6 18.4 13.3 Ni-CeO 2/AC 31.6 4.7 3.2 1.1 3.9 50.6 17.8 18.7 Pd-CeO 2/AC 11.9 6.6 4.8 0.3 2.9 67.6 5.8 12.0 [a] Convers ion and s electivity are obt ained at st eady -stat e; reaction conditions: mass of cat alyst, 1.0 g; T =523 K, P= 2 M P a (N 2), LH SV= 4 ml/(h·gcat), N 2 /ethanol(v/v)=500:1; [b] C6 products include: 2-ethylbutyraldehyde, hexaldehyde, 2-ethylbut anol, and 1hexanol; [c] Other products include: diethyl ether, 1,1 -diethoxy ethane, butyl acet ate, etc. Specifically, the diethyl ether for CeO 2/AC = 12.3%; Fe-CeO 2/AC = 6.1%; Ni-CeO 2/AC =4.6%.
Additionally, gaseous products formed ca. via ethanol cracking and/or decarbonylation of
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acetaldehyde [31], such as CH4 , CO and CO2 , were not detected in the outlet gas over the Cu-CeO 2 /AC catalyst, indicating that the cleavage of C-C bonds was not favorable over Cu metal. On the contrary, Fe,
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Co, Ni, and Pd all showed different degrees of ability to break the C-C bond in ethanol or acetaldehyde.
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As shown in Fig. S9, Ni-CeO 2 /AC catalyst shows 77.3% yield of liquid products, followed by 85, 93.6 and 97.9% for Co-, Fe- and Pd-CeO2 /AC, respectively. In this context, Cu exhibited the highest activity for ethanol dehydrogenation, while Pd showed the highest ability to hydrogenate the intermediate crotonaldehyde; moreover, Cu and Pd possessed negligible C-C bond cracking capability. Therefore, for the production of liquid products (C4 + alcohols and aldehydes), a combination of Cu and Pd might be considered as a promising strategy for the design of more efficient catalysts and corresponding research work is underway. Table S3 compares the catalytic performance of M-CeO 2 /AC catalysts with that of 10
ACCEPTED MANUSCRIPT other catalytic systems reported in the open literature. It is apparent that Cu-CeO 2 /AC strongly competes with all catalysts reported with respect to the n-butanol yield.
4. Conclusions
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M-CeO2 /AC (M= Cu, Fe, Co, Ni and Pd) catalysts were prepared by the wetness impregnation
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method and evaluated in the continuous catalytic upgrading of ethanol to n-butanol under mild reaction
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conditions. Different ethanol conversions (5-50%), selectivities toward n-butanol (30-70%) and yields
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of total liquid products (77.3-100%) were obtained for these catalysts, among which the highest selectivity toward n-butanol (67.6%) was achieved over the Pd-CeO2 /AC catalyst, while the highest
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ethanol conversion (46.2%) was achieved over the Cu-CeO 2 /AC catalyst with moderate selectivity toward n-butanol (41.3%). The catalytic performances of these catalysts depended largely on the
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dehydrogenation, hydrogenation and C-C bond cracking capabilities of each individual supported metal.
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The metal with high capabilities of hydrogen activation and spillover does not only favor hydrogenation
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of crotonaldehyde but also aldol condensation of acetaldehyde due to the enhanced basicity of CeO 2 . The combination of two metals in the same catalyst composition, one being active in ethanol
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dehydrogenation (such as Cu) and the other in hydrogenation steps (such as Pd, Ni, Co etc.), might be considered as a promising approach to designing efficient catalysts for the production of liquid products
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(C4 + alcohols and aldehydes) from the conversion of ethanol.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC Grant No. 21303163), the Natural Science Foundation of Zhejiang Province (LY17B060006) and the Scientific Research Fund of Zhejiang Provincial Education Department (Y201328681). 11
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ACCEPTED MANUSCRIPT Graphical abstract SYNOPSIS TOC
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M-CeO2 /AC (M = Cu, Fe, Co, Ni and Pd) catalysts were synthesized and compared in the continuous catalytic upgrading of ethanol to n-butanol under mild reaction conditions. The catalytic performance of catalysts was mainly depended on their dehydrogenation, hydrogenation and C-C bond cracking capabilities.
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ACCEPTED MANUSCRIPT Highlights High selectivity toward n-butanol (67.6%) was achieved over Pd-CeO 2 /AC catalyst
Metals with high capability of hydrogen activation and spillover have high n-butanol selectivity
Performance of catalysts was depended on their de/hydrogenation and C-C cracking capabilities
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