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Oxide supported metal catalysts for the aldehyde water shift reaction: Elucidating roles of the admetal, support, and synergies
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Review
Oxide supported metal catalysts for the aldehyde water shift reaction: Elucidating roles of the admetal, support, and synergies Wei-Chung Wen, Shawn C. Eady, Levi T. Thompson
⁎
Department of Chemical Engineering and Hydrogen Energy Technology Laboratory, University of Michigan, Ann Arbor, MI, 48109-2136, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Aldehyde oxidation Hydrogen production Heterogeneous catalysts Bifunctional mechanism
There is significant interest in the use of H2O as a reactant for production of commodity chemicals and fuels, in particular from water-laden biomass-derived feedstocks. In this paper, we describe results for the aldehyde water shift (AWS), a reaction where aldehydes are partially oxidized by water to produce the corresponding carboxylic acid and H2, over a series of CeO2-, Al2O3-, and SiO2-supported Cu, Pt, and Au catalysts. The effects of support were further investigated by evaluating the performances of a Cu-Zn-Al water gas shift catalyst and bulk Cu nanoparticles. The supported Cu catalysts were more active than the supported Pt and Au catalysts, and the reducible oxide (CeO2) supported Cu catalyst had the highest AWS activity. The high activity of the Cu/CeO2 catalyst is believed to derive from coupling H2O dissociation and aldehyde oxidation. The two major side reactions, aldol condensation and aldehyde disproportionation, appeared to be catalyzed by acid and Cu sites, respectively. The results indicate that both the admetal and support play critical roles in catalyzing the AWS reaction and provide guidance for the design of highly active AWS catalysts.
CH3CHO + H2O → CH3COOH + H2
1. Introduction The conversion of biomass is considered as an environmentally sustainable approach for the production of a variety of chemicals and fuels [1,2]. Given the high water content in biomass derivatives, there is considerable interest in using H2O as a reactant during its conversion [3]. The steam reforming reaction (Eq. (1)) is perhaps the most important reaction that involves H2O, and it is used to convert the hydrocarbon or oxygenate constituents in biomass into a mixture of H2, CO, and CO2 [2,4]. As hydrogen is often the desired product, the reforming reaction is typically coupled with the water gas shift (WGS, Eq. (2)) reaction [5], another reaction involving H2O, to increase the H2 yield. The aldehyde water shift (AWS) reaction is analogous to the WGS and can be used to convert aldehydes and H2O into the corresponding carboxylic acids and H2 (Eq. (3)) [6]. Aldehydes are key constituents in biomass-derivatives [7], and their partial oxidation using H2O produces H2 as a valuable by-product and presents an attractive alternative to the use of conventional oxidants like O2, where it could be difficult to avoid complete oxidation to CO2. For research described in this paper, acetaldehyde was used as the model reactant. CH3CH2OH + H2O → 2CO + 4H2
(1)
CO + H2O → CO2 + H2
(2)
⁎
(3)
To date, research on AWS catalysis has focused primarily on the development of homogeneous catalysts. Murahashi et al. first reported the use of dihydridotetrakis(triphenylphosphine)ruthenium(II), with benzalacetone as the hydrogen acceptor [8]. Stanley et al. reported that heptanoic acid was formed from heptaldehyde during the hydroformylation of 1-hexene when using a dirhodium tetraphosphine catalyst [9]. More recently, Brewster and co-workers developed a series of half-sandwich cyclopentadienyl Ir, Rh, and Ru complexes [6] and evaluated their activities for the AWS of alkyl and aromatic aldehydes. They reported acetaldehyde to acetic acid selectivities (via AWS and disproportionation reactions) in excess of 85% for the Ru diamine complexes at 105 °C [10]. Despite their high selectivities, the use of homogeneous catalysts for commercial processes is often cost prohibitive [11]. Heterogeneous catalysts, which offer a number of advantages including facile product separation from the catalyst and improved thermal stabilities, represent a more economical option for industrial applications [11,12]. Recently, Orozco et al. reported the conversion of aldehydes to carboxylic acids as a step during the ketonization of heptanal over heterogeneous catalysts [13]. Based on isotopic and kinetic studies, they confirmed the occurrence of the AWS reaction as part of the overall ketonization mechanism over a monoclinic ZrO2 catalyst.
Corresponding author. E-mail address: [email protected] (L.T. Thompson).
https://doi.org/10.1016/j.cattod.2019.03.064 Received 1 January 2019; Received in revised form 28 February 2019; Accepted 25 March 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Wei-Chung Wen, Shawn C. Eady and Levi T. Thompson, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.064
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and cooled to 40 °C. The material was then saturated with NH3 (Cryogenic Gases) flowing at 30 ml/min for 1 h. The physisorbed/excess NH3 was removed by purging the sample in flowing He for 30 min. The sample was then heated from 40 to 800 °C in flowing He at a heating rate of 10 °C/min. The desorbed gases were monitored using a mass spectrometer. The active site densities for the Cu-based materials were determined via N2O chemisorption [29]. Before chemisorption, the Cu-based catalysts were pretreated with 4% H2/N2 at 200 °C for 4 h. Following pretreatment, the materials were degassed with He at 210 °C for 0.5 h, then cooled to 40 °C and exposed to a flowing mixture of 10% N2O/He (Cryogenic Gases) for 2 h. After the N2O treatment, the material was purged with He to remove excess N2O, then a H2 temperature-programmed reduction (TPR) experiment was conducted while the temperature was increased from 40 to 500 °C in 4% H2/Ar (Cryogenic Gases) at 10 °C/min. The active site densities were calculated based on the H2 consumed during the TPR [29]. Active site densities for the Ptbased materials were determined via CO chemisorption. Before CO chemisorption, the catalysts were pretreated under the same conditions used prior to the NH3 TPD experiments. Following the pretreatment, degassing, and cooling steps, the materials were cooled to 40 °C. The catalysts were then dosed with pulses of 5% CO/He (Cryogenic Gases, calibrated volumes); exposure was repeated until reaching saturation, at which point the active site densities were determined.
Orozco et al. also investigated ketonization over a CeO2 catalyst and proposed that the AWS reaction involved oxygen vacancies on the CeO2 [14]. Xiang et al. reported AWS as a side reaction during ethanol dehydrogenation over a series of CuCr catalysts. By varying the Cu:Cr ratio, they attributed the AWS activity to the presence of surface Cu species [15]. However, beyond these reports, there lacks an understanding of effective catalyst design strategies and reaction kinetics; factors influencing selectivity also remain undetermined. In this paper, we describe an exploration of the AWS activities and selectivities of a series of oxide-supported metal catalysts with the goal of understanding the roles of the admetal and support, and defining key structure-function relationships. A key hypothesis driving the research was that high AWS activities could be achieved for catalysts with intimately dispersed sites for H2O dissociation and aldehyde oxidation; this derives from our understanding of the WGS reaction, where the close proximity of distinct sites for H2O dissociation and CO oxidation resulted in very high WGS activities [16–18]. For our research, the catalysts examined include CeO2-, Al2O3-, and SiO2-supported Cu, Pt, and Au. CeO2 is a reducible oxide that is capable of H2O dissociation [14,17,19] and could provide oxygen for oxidation of the aldehyde adsorbed on the admetal [15,20–23]. Al2O3, an irreducible and acidic oxide [24,25], was selected to assess the importance of support reducibility and the effect of acidity on AWS. SiO2 is known to be relatively inert compared to CeO2 and Al2O3 [26]. We also evaluated the performance of bulk Cu nanoparticles and a commercial Cu-Zn-Al WGS catalyst. In addition to the AWS activities, selectivities for side reactions and the potential site requirements were investigated.
2.3. Reaction rate and selectivity measurements Prior to the activity measurements, the Pt-based catalysts were pretreated with 10% H2/N2 at 300 °C for 1 h, and Cu- and Au-based catalysts were pretreated with 4% H2/N2 at 200 °C for 4 h [25,28,29]. The measurements were carried out in a packed bed quartz reactor at 200–300 °C and 2.5 psig using a gas hourly space velocity of 5600 h−1. The temperature range that was used is typical of that for the low temperature WGS [5] and much lower than those reported for other heterogeneous AWS catalysts [13–15]. The reactant stream consisted of 9% acetaldehyde, 15% H2O, and 76% N2. A N2 stream was saturated with acetaldehyde using a bubbler at 9 °C and 2.5 psig, and the acetaldehyde-saturated stream was diluted with N2 to obtain the reactant composition. Ultra-pure H2O was then injected and vaporized in the system. The H2O was continuously purged with N2 during the entire period of the experiment to keep the system deaerated. The acetaldehyde and H2O streams were mixed and passed through the reactor in the furnace. The reactor effluent was passed through a condenser to separate the gas and liquid phases. Rate data was collected during the deactivation and subsequent pseudo-steady-state regimes. The production of H2 was monitored using a SRI 8610C gas chromatograph with a thermal conductivity detector (GC-TCD). Compositions of the liquid products were analyzed post reaction using a Varian 450-GC gas chromatograph equipped with a flame ionization detector (FID). Between 25 and 75 mg of catalysts were used for the reaction measurements. The amounts were chosen to maintain similar conversions, while maintaining a manageable catalyst bed height. The catalyst beds were diluted with low surface area SiO2 (Alfa Aesar) to maintain the same bed height for all the experiments.
2. Material and methods 2.1. Catalyst preparation Cu, Pt, or Au admetals were deposited onto CeO2 (Sigma-Aldrich), γ-Al2O3 (Alfa Aesar), and SiO2 (AEROSIL®) supports using the incipient wetness impregnation method [25]. The target metal loading was equivalent to 0.1 monolayers (ML) based on 1019 sites/m2. Copper(II) nitrate hydrate (Cu(NO3)2•3H2O, Sigma-Aldrich), chloroplatinic acid hexahydrate (H2PtCl6•6H2O, Sigma-Aldrich), and gold(III) chloride trihydrate (HAuCl4•3H2O, Sigma-Aldrich) were used as precursors for Cu, Pt, and Au deposition, respectively. The metal precursor was dissolved in a quantity of water sufficient to fill the pore volume of the support as determined by N2 physisorption. The solution was added to the support drop-wise using a pipet until the solution was fully absorbed by the support. The material was subsequently dried in a vacuum oven at 110 °C overnight, and finally calcined in air for 5 h. Commercially available Cu-Zn-Al (Süd-Chemie/Clariant) and Nano-Cu (QuantumSphere) were also acquired for evaluation. 2.2. Catalyst characterization The crystalline phases and average crystallite sizes of the catalysts were determined by X-ray diffraction (XRD) using a Rigaku MiniFlex equipped with a Cu Kα radiation source and a Ni filter. All of the data was collected over a 2θ range of 10 to 90° at a scan rate of 5°/min. The surface areas were determined based on N2 physisorption and the BET method, using a Micromeritics ASAP 2020 analyzer. We used NH3 temperature-programmed desorption (NH3-TPD) to characterize the acid sites [24,27] with a Micromeritics ASAP 2920 analyzer. Before exposing the materials to NH3, the supported Pt catalysts were pretreated with 10% H2/N2 (Cryogenic Gases) at 300 °C for 1 h, and the supported Cu and Au catalysts were pretreated with 4% H2/N2 (Cryogenic Gases) at 200 °C for 4 h [25,28,29]. These conditions were identical to the pretreatment conditions used prior to evaluation of their AWS activities and selectivities (described below). Following pretreatment, the materials were degassed with He (Cryogenic Gases) at a temperature 10 °C higher than the pretreatment temperature for 0.5 h
3. Results and discussions 3.1. Characterization of catalysts The surface areas, metal contents, site densities, and NH3 chemisorption uptakes for all of the catalysts are listed in Table 1. The surface areas did not change significantly from the deposition of the metal, indicating that the admetals did not significantly block the support pores. Diffraction patterns for all the materials are shown in Fig. 1. For the CeO2- and Al2O3- supported Cu and Pt catalysts, the absence of peaks 2
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Table 1 Select catalyst surface and physical properties for the catalysts. Catalysts
Surface Area (m2/g)
Metal Contenta (wt%)
Active Site Density (μmol/g)
NH3 Uptake (μmol/g)
CeO2 Cu/CeO2 Pt/CeO2 Au/CeO2 Al2O3 Cu/Al2O3 Pt/Al2O3 Au/Al2O3 SiO2 Cu/SiO2 Cu-Zn-Al Nano-Cu
40 41 37 36 79 76 68 72 94 95 60 8
0 0.4 1.0 1.0 0 0.8 1.9 2.2 0 1.0 33 94.2
16b 471b 27c 0c 0b 48b 7c 0c 0b 16b 494b —
39 20 57 98 67 87 133 154 7 22 178 —
a
Determined by the amount of precursor used. Cu contents for the Cu − Zn − Al catalysts were obtained from vendor specification. Cu content for nano-Cu was obtained by phase composition analysis in XRD. b Site density probed by N2O chemisorption. c Site density probed by CO chemisorption.
for Cu and Pt suggested that the metal domains were small and well dispersed. In contrast, peaks for Au are clearly observed in patterns for the Au/CeO2 and Au/Al2O3 catalysts, with average crystallite sizes of 18 and 31 nm, respectively, based on line broadening analysis [30]. For the Cu/SiO2 catalyst, small CuO peaks were observed, indicating the formation of crystalline domains, albeit with dimensions that were too small to be quantified using XRD. The NH3 TPD profiles for the CeO2-, Al2O3-, and SiO2-supported catalysts are shown in Fig. 2. The majority of the NH3 desorbed at 100–200 °C, consistent with the presence of weak acid sites on all of the materials [24]. The NH3 uptakes for all of the catalysts are provided in Table 1. Note that the NH3 uptakes decreased in the following order: Al2O3-based catalysts > CeO2-based catalysts > SiO2-based catalysts. It was not unexpected given the known acidity of Al2O3 [24,31]. Overall, the uptakes and desorption temperatures are consistent with prior reports, in which the majority of the acid sites for these oxides were characterized as weak to intermediate [24,32]. The N2O uptake-based Cu dispersions for the Cu/Al2O3 and Cu/SiO2 catalysts were 40% and 10% respectively; these values are typical of those reported in the literature [33]. Results for the Cu/CeO2 catalyst implied a N2O:Cu surface site ratio greater than the theoretical maximum stoichiometry of 1:2. This is likely a consequence of the oxygen from N2O spilling over to oxygen vacancies on the CeO2 support, and the deposited Cu significantly enhanced the reducibility of CeO2 [34,35]. Similar observations have been reported by Sun et al. and Maciel et al. [36,37]. Given that N2O chemisorption was not suitable for determining the Cu surface site density for the Cu/CeO2 catalyst and the Cu particles were x-ray amorphous, we estimated the turnover
Fig. 2. Surface area normalized NH3 desorption profiles for the oxide-supported metal catalysts and bulk supports.
frequencies (TOF) for the Cu/CeO2 catalyst assuming 100% Cu dispersion. This would underestimate the TOF but provide a useful metric for comparison with the other Cu-based catalysts.
3.2. Activities and stabilities For all the measurements, the aldehyde conversions were limited to 5% to maintain differential conditions (see Table S1 for aldehyde consumption rates and single-pass conversions) and minimize the reverse reaction (see Table S2 for the equilibrium calculation). In control experiments, the aldehyde conversion at 240 °C was inversely proportional to the flow rate, indicating that the rates were not limited by mass transport. The dominant gas phase product was H2. In addition to the AWS, ethanol dehydrogenation [38,39] and acetaldehyde steam reforming are potential sources of H2 known to occur under conditions used in our work. The product distributions did not change on varying the flow rate, suggesting that ethanol dehydrogenation was not a significant source of H2 for our catalysts. For most of the catalysts, neither CO or CO2 were observed in the product stream, indicating that steam reforming was not a significant source of H2. For the Pt/CeO2 and Pt/ Al2O3 catalysts, CO and CH4 were initially produced, perhaps as a Fig. 1. Diffraction patterns for the (a) CeO2-, (b) Al2O3-, and (c) SiO2-supported Cu, Cu-ZnAl, and Nano-Cu catalysts. Relevant standards were included: CuO (JCPDF 98-00-0429), Cu2O (JCPDF 98-000-0186), Cu (JCPDF 00004-0836), Pt (JCPDF 00-004-0802), Au (JCPDF 00-004-0787), ZnO (JCPDF 00-0050664), CeO2 (JCPDF 00-034-0394), and Al2O3 (JCPDF 00-047-1308). The SiO2 and SiO2supported catalysts were amorphous.
3
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Table 2 The AWS rates and Cu site-normalized rates at 240 °C, and the associated activation energies. Catalysts
H2 Production Rate (nmol/s/m2)
AWS Rate (mmol H2/s/ molCu)
Activation Energy (kcal/mol)
CeO2 Cu/CeO2 Pt/CeO2 Au/CeO2 Cu/Al2O3 Cu/SiO2 Cu-Zn-Al Nano-Cu
1.3 13.6 3.3 2.1 0.5 < 0.1d 27.8 13.8
– 8.8a – – 0.8b < 0.6b 3.4b 0.8c
– 19 19 18 23 – 22 –
a b
Fig. 3. Normalized deactivation profiles (relative to initial rates) for AWS (H2 production) at 240 °C for the CeO2- and Al2O3- supported Cu and Pt, Cu-Zn-Al, and Nano-Cu catalysts.
c d
Assumed 100% dispersion for Cu on CeO2. Cu site density was determined by N2O chemisorption. Calculated based on the surface area, assuming 1019 Cu sites/m2. Calculated based on the detection limitation of the GC-TCD.
aldehyde decomposition was catalyzed instead of AWS, likely due to the strong interactions between acetaldehyde and Pt [40]. The strong bonding could also be the principal cause for deactivation of the Pt/ CeO2 and Pt/Al2O3 catalysts. For the Au/CeO2 catalyst, the aldehydeadmetal interaction may not be strong enough to facilitate reaction turnover. The relatively large Au particle sizes may also explain the lower activities; for reactions including CO oxidation and the WGS reaction, Au particles larger than a few nanometers are inactive [45,46]. Compared with the CeO2-supported metal catalysts, the Al2O3supported metal catalysts were relatively inactive; the rate for the Cu/ CeO2 catalyst was nearly 30-fold of that for the Cu/Al2O3. Recall that Al2O3 is irreducible while CeO2 is reducible. This highlighted the importance of oxide selection. Of the Al2O3-supported metal catalysts, only the Cu/Al2O3 catalyst produced small amounts of H2 after deactivation, with a rate 0.5 nmol/m2⋅s of H2 at pseudo-steady state. The bulk Al2O3 was inactive for AWS. The activation energies were similar for each support group (see Table 2 and Fig. 4), which is consistent with the rate limiting step occurring on the supports. Given that the activation energies for the AWS were similar to those reported for WGS (e.g., 19 and 17 kcal/mol for Pt/ CeO2 and Cu-Zn-Al catalysts, respectively [25,47]), it is possible that water dissociation was the rate limiting step, which parallels many reports for WGS over oxide-supported metal catalysts [17,48,49]. The effect of the support was further investigated by measuring activities for the Nano-Cu, Cu-Zn-Al, and Cu/SiO2 catalysts. Considering the Cu contents and site densities, the Cu site-normalized rates (mol H2/s/molCu) were a strong function of the support type and decreased in the following order: Cu/CeO2 > Cu-Zn-Al > Cu/Al2O3 ˜
consequence of acetaldehyde decarbonylation [40], but the amounts decayed to levels below the detection limit after ˜8 h on stream. After this initial deactivation, no CO and CH4 were detected. Based on these observations, AWS was concluded to be the primary source of H2 production after deactivation of the catalysts; therefore the H2 production rate was used as a direct measure of the pseudo-steady AWS activity. In general, the AWS rates determined from the H2 production rates were consistent with the hydrocarbon production results. In addition to acetic acid and H2, crotonaldehyde, ethanol, and in some cases small amounts of paraldehyde and acetone were produced, suggesting that reactions other than AWS were facilitated by the catalysts. The source of these additional products will be detailed later in the paper. The AWS rates for all of the catalysts decayed during the first few hours on stream. The AWS rates for the Cu/CeO2 and Cu-Zn-Al catalysts reached pseudo-steady state after ˜8 h on stream, deactivating by ˜20% (see Fig. 3). The Cu/Al2O3 and Nano-Cu deactivated by ˜70% during the first 10 h on stream. The H2 production rates for the Pt/CeO2 and Pt/ Al2O3 catalysts decayed by more than 90%. Recall that these catalysts initially produced some CO and CH4, which could participate in the WGS or reforming reactions. Consequently, it was not possible to accurately assess the AWS deactivation rates for these catalysts. The Au/ CeO2 catalysts produced small and unstable amounts of H2; the Al2O3 and SiO2 supports were inactive for the AWS. In general, the supported Cu catalysts including reducible oxides (CeO2 or ZnO [41,42]) were the most stable. Deactivation profiles for the catalysts were fit to model decay laws (see Table S3) to interrogate the deactivation mechanisms. Results for the Cu/CeO2 and Cu-Zn-Al catalysts were reasonably well fit by exponential and hyperbolic models, while data for the Nano-Cu and Pt/ CeO2 catalyst was best fit by exponential models. The exponential decay model is typically associated with deactivation by surface poisoning whereas the hyperbolic model suggests sintering. Results for the Cu/Al2O3 and Pt/Al2O3 catalysts were best fitted by the reciprocal power model, which often indicates deactivation via site fouling or coking [43,44]. The basis for these differences is not currently known and is the subject of future research. The pseudo-steady AWS rates at 240 °C after deactivation are listed in Table 2. Overall, both the admetal and support appeared to have a significant impact on the AWS reaction rate. CeO2 had a low AWS activity (1.3 nmol/m2⋅s), which was not surprising considering previous evidence of AWS catalysis with CeO2 reported by Orozco et al. [13,14]. After the deposition of metal onto CeO2, the activity increased dramatically. The stable rate for the Cu/CeO2 catalyst was 13.6 nmol/m2⋅s, which was 4 times higher than those for the CeO2-supported Pt and Au catalysts, and an order of magnitude higher than the rate for bulk CeO2. The activity enhancement associated with Cu highlighted the importance of judicious admetal selection. For the Pt/CeO2 catalyst,
Fig. 4. Arrhenius plots for AWS rates over the Cu-Zn-Al, CeO2-based, and Cu/ Al2O3 catalysts. 4
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Fig. 5. Schematic representation of the proposed AWS reaction mechanism for the Cu/CeO2 catalyst.
Nano-Cu. Recall that we assumed 100% dispersion for Cu on CeO2. Therefore, the Cu site-normalized rate for the Cu/CeO2 catalyst can be considered as a lower limit for its activity. The intrinsic activity for the Cu/CeO2 catalyst was approximately an order of magnitude higher than those for the Cu/Al2O3, Cu/SiO2, and bulk Cu catalysts. The results described thus far are consistent with a bifunctional mechanism for AWS over the Cu/CeO2 catalyst, in which the sites on CeO2 are responsible for H2O dissociation and aldehyde oxidation occurs at sites on or at the interface of the Cu domains, with oxygen directly or indirectly (via, for example, a Mars-Van Krevelentype mechanism) sourced from H2O (see Fig. 5). Intimacy between these two distinct catalytic sites would facilitate high AWS rates. As discussed earlier in connection with the N2O uptakes, Cu could stabilize and increase the oxygen vacancies on CeO2 [34,35], which could also enhance the AWS activity. A similar type of bifunctional mechanism could explain results for the Cu-Zn-Al catalyst, with ZnO responsible for H2O dissociation and Cu for acetaldehyde oxidation. Given that ZnO is considered less reducible than CeO2 [17], it is not unexpected that the AWS activity for the Cu-Zn-Al catalyst was moderate in comparison to that for the Cu/CeO2 catalyst. For the Cu/Al2O3 catalyst, it is likely that the AWS activity was due solely to the Cu admetal. Note that the Cu site-normalized rates and deactivation profiles for the Cu/Al2O3 and bulk Nano-Cu catalysts were nearly identical. In comparing results for the supported Cu catalysts described in this paper with those reported for homogeneous catalysts, the heterogeneous catalysts appeared to be less active and, in some cases, less selective for the AWS reaction. Brewster reported results that are consistent with an AWS TOF of 3.6•10−3 1/s at 105 °C for a [(p-cymene) RuCl2]2 catalyst [10]. Extrapolating AWS rates determined in this research using the measured activation energies, the TOF for the Cu/CeO2 catalyst would be two orders of magnitude lower, or 1.1•10-5 1/s at 105 °C, assuming atomic dispersion of the Cu. The assumption of 100% dispersion overestimates the active site densities, but we do not believe that the dispersion is two orders of magnitude lower (i.e., 1%). While the Cu/CeO2 catalyst is probably less active than the homogeneous catalysts, it appears to be more active than other heterogeneous catalysts that have been described in the literature. Xiang et al. did not report AWS rates directly, but the results indicate an AWS rate of 0.55 μmol/gCu/s at 350 °C [15]. Extrapolating data for our Cu/CeO2 catalyst to 350 °C yields a rate of 3596 μmol/gCu/s. Dispersions for these materials are dissimilar, although it is unlikely they differ by four orders of magnitude. Similarly, it is unlikely that differences in the reaction conditions can reconcile the significantly different rates.
Fig. 6. Carbon selectivities for the catalysts. The selectivities are defined as the moles of acetaldehyde reacted to form a specific product divided by the total amount of acetaldehyde converted.
homogeneous catalysts [10]. In addition to the acid, the Nano-Cu favored ethanol formation with a selectivity of approximately 35%. To determine the source of the ethanol, a control experiment was conducted in which only H2 and acetaldehyde were fed to the Cu/CeO2 catalyst. Under these conditions, very little ethanol was detected and there was no evidence of H2 consumption. This result suggests that aldehyde hydrogenation was not a significant source of ethanol under the conditions employed in this study. Instead, we believe that the ethanol was produced via a Cannizzaro-type disproportionation reaction of acetaldehyde (Equation 4) [6,13]. The crotonaldehyde was likely a product of aldol condensation (Equation 5) [50,51]. Paraldehyde was a minor product for some catalysts, which may have been formed via acetaldehyde trimerization [52]. CO and CH4 were only produced on Pt-based catalysts and likely were products of acetaldehyde decarbonylation [40]. 2CH3CHO + H2O → CH3CH2OH + CH3COOH
(4)
2CH3CHO → CH3CHCHCHO + H2O
(5)
The disproportionation (ethanol production) and aldol condensation (crotonaldehyde production) reaction rates at 240 °C are provided in Table 3. Given the high disproportionation reaction rate for the bulk Cu catalyst and the low rates observed for the bulk metal oxide supports, ethanol production over the oxide-supported Cu catalysts appears to have been a consequence primarily of the Cu admetal. As metallic Cu is capable of activating H2O [48], it is not surprising that Cu was observed to catalyze the disproportionation reaction. For aldol condensation, the other major side reaction, reaction rates for most of the Table 3 Average ethanol and crotonaldehyde production rates and turnover frequencies for metal oxide-supported catalysts and bulk supports at 240 °C during 14 h on stream.
3.3. Selectivities In addition to acetic acid and H2, crotonaldehyde was produced for all of the catalysts; ethanol was produced for the Cu/CeO2, Cu-Zn-Al, and Nano-Cu catalysts; and CO, CH4, and small amounts of paraldehyde and acetone were produced for some of the catalysts. Selectivities to the carbon-containing products are shown in Fig. 6. The Cu-based catalysts were the most selective to acetic acid, the AWS product. Notably, selectivities for the Cu/CeO2, Cu-Zn-Al, and Nano-Cu catalysts were proximal to those reported for the
Catalysts
Ethanol Production Rate (nmol/s/m2)
Crotonaldehyde Production Rate (nmol/s/m2)
Aldol Condensation TOFs (1/s * 103)a
CeO2 Cu/CeO2 Au/CeO2 Al2O3 Cu/Al2O3 Au/Al2O3 Cu/SiO2 Pt/CeO2 Pt/Al2O3 Cu-Zn-Al Nano-Cu
0.12 2.54 0.63 0.13 0.23 0.27 – – 0.15 16.4 13.1
0.76 0.38 4.03 0.80 1.01 1.80 0.16 0.53 0.92 0.64 1.42
0.78 0.80 1.50 0.95 0.89 0.85 0.67 0.34 0.47 – –
a
5
Normalized by the acid sites.
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catalysts correlated with the weak acid site densities with the exception of the Pt-based catalysts (see Table 3 and site densities in Table 1). These results are consistent with reports in the literature that aldol condensation can be acid-catalyzed [53]. The Pt-based catalysts had lower TOFs, a possible consequence of strong interactions between acetaldehyde and Pt and subsequent decomposition of acetaldehyde. For completeness, we also mention that acid-base pairs could be responsible for the condensation reaction [54]; this was not confirmed in our work. Mechanisms for the disproportionation and aldol condensation reactions will be investigated and described in a future paper.
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4. Conclusions The results presented in this paper explore the use of CeO2-, Al2O3-, and SiO2-supported Cu, Pt, and Au catalysts for the AWS reaction and demonstrate that the type of admetal and support have significant impacts on the activities. The supported Cu catalysts had higher AWS activities than the supported Pt and Au catalysts, while the Pt-based catalysts favored acetaldehyde decomposition and the Au-based catalysts were relatively inactive for AWS. The combination of Cu sites and sites from a reducible oxide (i.e., CeO2 or ZnO) yielded materials with high AWS activities, stabilities, and selectivities. We posited that the reducible oxide catalyzed H2O dissociation, while the Cu domains catalyzed aldehyde oxidation using oxygen from H2O. The materials also catalyzed other reactions producing crotonaldehyde, ethanol, and very small amounts of paraldehyde. The aldol condensation (crotonaldehyde formation) rates correlated with the acid site densities, and the disproportionation reaction (ethanol formation) appeared to be catalyzed by surface Cu. Future work will focus on further characterizing of the support reducibility and the admetal valence state, determining the overall AWS mechanism for the Cu/CeO2 catalyst, and designing other materials that couple H2O dissociation and aldehyde oxidation. Acknowledgements This work was supported by the National Science Foundation, under the CCI Center for Enabling New Technologies through Catalysis (CENTC) Phase II Renewal [CHE-1205189]. The authors would also like to acknowledge assistance and support from Drs. Saemin Choi, Yuan Chen, Jason Gaudet, and Brian Wyvratt, and Professors Michael Heinekey and Karen Goldberg. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.03.064. References [1] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411–2502. [2] R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B 56 (2005) 171–186. [3] A. Kruse, A. Gawlik, Ind. Eng. Chem. Res. 42 (2003) 267–279. [4] L. Lin, W. Zhou, R. Gao, S. Yao, X. Zhang, W. Xu, S. Zheng, Z. Jiang, Q. Yu, Y.-W. Li, C. Shi, X.-D. Wen, D. Ma, Nature 544 (2017) 80–83. [5] C. Ratnasamy, J.P. Wagner, Catal. Rev. 51 (2009) 325–440.
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Review
Oxide supported metal catalysts for the aldehyde water shift reaction: Elucidating roles of the admetal, support, and synergies Wei-Chung Wen, Shawn C. Eady, Levi T. Thompson
⁎
Department of Chemical Engineering and Hydrogen Energy Technology Laboratory, University of Michigan, Ann Arbor, MI, 48109-2136, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Aldehyde oxidation Hydrogen production Heterogeneous catalysts Bifunctional mechanism
There is significant interest in the use of H2O as a reactant for production of commodity chemicals and fuels, in particular from water-laden biomass-derived feedstocks. In this paper, we describe results for the aldehyde water shift (AWS), a reaction where aldehydes are partially oxidized by water to produce the corresponding carboxylic acid and H2, over a series of CeO2-, Al2O3-, and SiO2-supported Cu, Pt, and Au catalysts. The effects of support were further investigated by evaluating the performances of a Cu-Zn-Al water gas shift catalyst and bulk Cu nanoparticles. The supported Cu catalysts were more active than the supported Pt and Au catalysts, and the reducible oxide (CeO2) supported Cu catalyst had the highest AWS activity. The high activity of the Cu/CeO2 catalyst is believed to derive from coupling H2O dissociation and aldehyde oxidation. The two major side reactions, aldol condensation and aldehyde disproportionation, appeared to be catalyzed by acid and Cu sites, respectively. The results indicate that both the admetal and support play critical roles in catalyzing the AWS reaction and provide guidance for the design of highly active AWS catalysts.
CH3CHO + H2O → CH3COOH + H2
1. Introduction The conversion of biomass is considered as an environmentally sustainable approach for the production of a variety of chemicals and fuels [1,2]. Given the high water content in biomass derivatives, there is considerable interest in using H2O as a reactant during its conversion [3]. The steam reforming reaction (Eq. (1)) is perhaps the most important reaction that involves H2O, and it is used to convert the hydrocarbon or oxygenate constituents in biomass into a mixture of H2, CO, and CO2 [2,4]. As hydrogen is often the desired product, the reforming reaction is typically coupled with the water gas shift (WGS, Eq. (2)) reaction [5], another reaction involving H2O, to increase the H2 yield. The aldehyde water shift (AWS) reaction is analogous to the WGS and can be used to convert aldehydes and H2O into the corresponding carboxylic acids and H2 (Eq. (3)) [6]. Aldehydes are key constituents in biomass-derivatives [7], and their partial oxidation using H2O produces H2 as a valuable by-product and presents an attractive alternative to the use of conventional oxidants like O2, where it could be difficult to avoid complete oxidation to CO2. For research described in this paper, acetaldehyde was used as the model reactant. CH3CH2OH + H2O → 2CO + 4H2
(1)
CO + H2O → CO2 + H2
(2)
⁎
(3)
To date, research on AWS catalysis has focused primarily on the development of homogeneous catalysts. Murahashi et al. first reported the use of dihydridotetrakis(triphenylphosphine)ruthenium(II), with benzalacetone as the hydrogen acceptor [8]. Stanley et al. reported that heptanoic acid was formed from heptaldehyde during the hydroformylation of 1-hexene when using a dirhodium tetraphosphine catalyst [9]. More recently, Brewster and co-workers developed a series of half-sandwich cyclopentadienyl Ir, Rh, and Ru complexes [6] and evaluated their activities for the AWS of alkyl and aromatic aldehydes. They reported acetaldehyde to acetic acid selectivities (via AWS and disproportionation reactions) in excess of 85% for the Ru diamine complexes at 105 °C [10]. Despite their high selectivities, the use of homogeneous catalysts for commercial processes is often cost prohibitive [11]. Heterogeneous catalysts, which offer a number of advantages including facile product separation from the catalyst and improved thermal stabilities, represent a more economical option for industrial applications [11,12]. Recently, Orozco et al. reported the conversion of aldehydes to carboxylic acids as a step during the ketonization of heptanal over heterogeneous catalysts [13]. Based on isotopic and kinetic studies, they confirmed the occurrence of the AWS reaction as part of the overall ketonization mechanism over a monoclinic ZrO2 catalyst.
Corresponding author. E-mail address: [email protected] (L.T. Thompson).
https://doi.org/10.1016/j.cattod.2019.03.064 Received 1 January 2019; Received in revised form 28 February 2019; Accepted 25 March 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Wei-Chung Wen, Shawn C. Eady and Levi T. Thompson, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.064
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and cooled to 40 °C. The material was then saturated with NH3 (Cryogenic Gases) flowing at 30 ml/min for 1 h. The physisorbed/excess NH3 was removed by purging the sample in flowing He for 30 min. The sample was then heated from 40 to 800 °C in flowing He at a heating rate of 10 °C/min. The desorbed gases were monitored using a mass spectrometer. The active site densities for the Cu-based materials were determined via N2O chemisorption [29]. Before chemisorption, the Cu-based catalysts were pretreated with 4% H2/N2 at 200 °C for 4 h. Following pretreatment, the materials were degassed with He at 210 °C for 0.5 h, then cooled to 40 °C and exposed to a flowing mixture of 10% N2O/He (Cryogenic Gases) for 2 h. After the N2O treatment, the material was purged with He to remove excess N2O, then a H2 temperature-programmed reduction (TPR) experiment was conducted while the temperature was increased from 40 to 500 °C in 4% H2/Ar (Cryogenic Gases) at 10 °C/min. The active site densities were calculated based on the H2 consumed during the TPR [29]. Active site densities for the Ptbased materials were determined via CO chemisorption. Before CO chemisorption, the catalysts were pretreated under the same conditions used prior to the NH3 TPD experiments. Following the pretreatment, degassing, and cooling steps, the materials were cooled to 40 °C. The catalysts were then dosed with pulses of 5% CO/He (Cryogenic Gases, calibrated volumes); exposure was repeated until reaching saturation, at which point the active site densities were determined.
Orozco et al. also investigated ketonization over a CeO2 catalyst and proposed that the AWS reaction involved oxygen vacancies on the CeO2 [14]. Xiang et al. reported AWS as a side reaction during ethanol dehydrogenation over a series of CuCr catalysts. By varying the Cu:Cr ratio, they attributed the AWS activity to the presence of surface Cu species [15]. However, beyond these reports, there lacks an understanding of effective catalyst design strategies and reaction kinetics; factors influencing selectivity also remain undetermined. In this paper, we describe an exploration of the AWS activities and selectivities of a series of oxide-supported metal catalysts with the goal of understanding the roles of the admetal and support, and defining key structure-function relationships. A key hypothesis driving the research was that high AWS activities could be achieved for catalysts with intimately dispersed sites for H2O dissociation and aldehyde oxidation; this derives from our understanding of the WGS reaction, where the close proximity of distinct sites for H2O dissociation and CO oxidation resulted in very high WGS activities [16–18]. For our research, the catalysts examined include CeO2-, Al2O3-, and SiO2-supported Cu, Pt, and Au. CeO2 is a reducible oxide that is capable of H2O dissociation [14,17,19] and could provide oxygen for oxidation of the aldehyde adsorbed on the admetal [15,20–23]. Al2O3, an irreducible and acidic oxide [24,25], was selected to assess the importance of support reducibility and the effect of acidity on AWS. SiO2 is known to be relatively inert compared to CeO2 and Al2O3 [26]. We also evaluated the performance of bulk Cu nanoparticles and a commercial Cu-Zn-Al WGS catalyst. In addition to the AWS activities, selectivities for side reactions and the potential site requirements were investigated.
2.3. Reaction rate and selectivity measurements Prior to the activity measurements, the Pt-based catalysts were pretreated with 10% H2/N2 at 300 °C for 1 h, and Cu- and Au-based catalysts were pretreated with 4% H2/N2 at 200 °C for 4 h [25,28,29]. The measurements were carried out in a packed bed quartz reactor at 200–300 °C and 2.5 psig using a gas hourly space velocity of 5600 h−1. The temperature range that was used is typical of that for the low temperature WGS [5] and much lower than those reported for other heterogeneous AWS catalysts [13–15]. The reactant stream consisted of 9% acetaldehyde, 15% H2O, and 76% N2. A N2 stream was saturated with acetaldehyde using a bubbler at 9 °C and 2.5 psig, and the acetaldehyde-saturated stream was diluted with N2 to obtain the reactant composition. Ultra-pure H2O was then injected and vaporized in the system. The H2O was continuously purged with N2 during the entire period of the experiment to keep the system deaerated. The acetaldehyde and H2O streams were mixed and passed through the reactor in the furnace. The reactor effluent was passed through a condenser to separate the gas and liquid phases. Rate data was collected during the deactivation and subsequent pseudo-steady-state regimes. The production of H2 was monitored using a SRI 8610C gas chromatograph with a thermal conductivity detector (GC-TCD). Compositions of the liquid products were analyzed post reaction using a Varian 450-GC gas chromatograph equipped with a flame ionization detector (FID). Between 25 and 75 mg of catalysts were used for the reaction measurements. The amounts were chosen to maintain similar conversions, while maintaining a manageable catalyst bed height. The catalyst beds were diluted with low surface area SiO2 (Alfa Aesar) to maintain the same bed height for all the experiments.
2. Material and methods 2.1. Catalyst preparation Cu, Pt, or Au admetals were deposited onto CeO2 (Sigma-Aldrich), γ-Al2O3 (Alfa Aesar), and SiO2 (AEROSIL®) supports using the incipient wetness impregnation method [25]. The target metal loading was equivalent to 0.1 monolayers (ML) based on 1019 sites/m2. Copper(II) nitrate hydrate (Cu(NO3)2•3H2O, Sigma-Aldrich), chloroplatinic acid hexahydrate (H2PtCl6•6H2O, Sigma-Aldrich), and gold(III) chloride trihydrate (HAuCl4•3H2O, Sigma-Aldrich) were used as precursors for Cu, Pt, and Au deposition, respectively. The metal precursor was dissolved in a quantity of water sufficient to fill the pore volume of the support as determined by N2 physisorption. The solution was added to the support drop-wise using a pipet until the solution was fully absorbed by the support. The material was subsequently dried in a vacuum oven at 110 °C overnight, and finally calcined in air for 5 h. Commercially available Cu-Zn-Al (Süd-Chemie/Clariant) and Nano-Cu (QuantumSphere) were also acquired for evaluation. 2.2. Catalyst characterization The crystalline phases and average crystallite sizes of the catalysts were determined by X-ray diffraction (XRD) using a Rigaku MiniFlex equipped with a Cu Kα radiation source and a Ni filter. All of the data was collected over a 2θ range of 10 to 90° at a scan rate of 5°/min. The surface areas were determined based on N2 physisorption and the BET method, using a Micromeritics ASAP 2020 analyzer. We used NH3 temperature-programmed desorption (NH3-TPD) to characterize the acid sites [24,27] with a Micromeritics ASAP 2920 analyzer. Before exposing the materials to NH3, the supported Pt catalysts were pretreated with 10% H2/N2 (Cryogenic Gases) at 300 °C for 1 h, and the supported Cu and Au catalysts were pretreated with 4% H2/N2 (Cryogenic Gases) at 200 °C for 4 h [25,28,29]. These conditions were identical to the pretreatment conditions used prior to evaluation of their AWS activities and selectivities (described below). Following pretreatment, the materials were degassed with He (Cryogenic Gases) at a temperature 10 °C higher than the pretreatment temperature for 0.5 h
3. Results and discussions 3.1. Characterization of catalysts The surface areas, metal contents, site densities, and NH3 chemisorption uptakes for all of the catalysts are listed in Table 1. The surface areas did not change significantly from the deposition of the metal, indicating that the admetals did not significantly block the support pores. Diffraction patterns for all the materials are shown in Fig. 1. For the CeO2- and Al2O3- supported Cu and Pt catalysts, the absence of peaks 2
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Table 1 Select catalyst surface and physical properties for the catalysts. Catalysts
Surface Area (m2/g)
Metal Contenta (wt%)
Active Site Density (μmol/g)
NH3 Uptake (μmol/g)
CeO2 Cu/CeO2 Pt/CeO2 Au/CeO2 Al2O3 Cu/Al2O3 Pt/Al2O3 Au/Al2O3 SiO2 Cu/SiO2 Cu-Zn-Al Nano-Cu
40 41 37 36 79 76 68 72 94 95 60 8
0 0.4 1.0 1.0 0 0.8 1.9 2.2 0 1.0 33 94.2
16b 471b 27c 0c 0b 48b 7c 0c 0b 16b 494b —
39 20 57 98 67 87 133 154 7 22 178 —
a
Determined by the amount of precursor used. Cu contents for the Cu − Zn − Al catalysts were obtained from vendor specification. Cu content for nano-Cu was obtained by phase composition analysis in XRD. b Site density probed by N2O chemisorption. c Site density probed by CO chemisorption.
for Cu and Pt suggested that the metal domains were small and well dispersed. In contrast, peaks for Au are clearly observed in patterns for the Au/CeO2 and Au/Al2O3 catalysts, with average crystallite sizes of 18 and 31 nm, respectively, based on line broadening analysis [30]. For the Cu/SiO2 catalyst, small CuO peaks were observed, indicating the formation of crystalline domains, albeit with dimensions that were too small to be quantified using XRD. The NH3 TPD profiles for the CeO2-, Al2O3-, and SiO2-supported catalysts are shown in Fig. 2. The majority of the NH3 desorbed at 100–200 °C, consistent with the presence of weak acid sites on all of the materials [24]. The NH3 uptakes for all of the catalysts are provided in Table 1. Note that the NH3 uptakes decreased in the following order: Al2O3-based catalysts > CeO2-based catalysts > SiO2-based catalysts. It was not unexpected given the known acidity of Al2O3 [24,31]. Overall, the uptakes and desorption temperatures are consistent with prior reports, in which the majority of the acid sites for these oxides were characterized as weak to intermediate [24,32]. The N2O uptake-based Cu dispersions for the Cu/Al2O3 and Cu/SiO2 catalysts were 40% and 10% respectively; these values are typical of those reported in the literature [33]. Results for the Cu/CeO2 catalyst implied a N2O:Cu surface site ratio greater than the theoretical maximum stoichiometry of 1:2. This is likely a consequence of the oxygen from N2O spilling over to oxygen vacancies on the CeO2 support, and the deposited Cu significantly enhanced the reducibility of CeO2 [34,35]. Similar observations have been reported by Sun et al. and Maciel et al. [36,37]. Given that N2O chemisorption was not suitable for determining the Cu surface site density for the Cu/CeO2 catalyst and the Cu particles were x-ray amorphous, we estimated the turnover
Fig. 2. Surface area normalized NH3 desorption profiles for the oxide-supported metal catalysts and bulk supports.
frequencies (TOF) for the Cu/CeO2 catalyst assuming 100% Cu dispersion. This would underestimate the TOF but provide a useful metric for comparison with the other Cu-based catalysts.
3.2. Activities and stabilities For all the measurements, the aldehyde conversions were limited to 5% to maintain differential conditions (see Table S1 for aldehyde consumption rates and single-pass conversions) and minimize the reverse reaction (see Table S2 for the equilibrium calculation). In control experiments, the aldehyde conversion at 240 °C was inversely proportional to the flow rate, indicating that the rates were not limited by mass transport. The dominant gas phase product was H2. In addition to the AWS, ethanol dehydrogenation [38,39] and acetaldehyde steam reforming are potential sources of H2 known to occur under conditions used in our work. The product distributions did not change on varying the flow rate, suggesting that ethanol dehydrogenation was not a significant source of H2 for our catalysts. For most of the catalysts, neither CO or CO2 were observed in the product stream, indicating that steam reforming was not a significant source of H2. For the Pt/CeO2 and Pt/ Al2O3 catalysts, CO and CH4 were initially produced, perhaps as a Fig. 1. Diffraction patterns for the (a) CeO2-, (b) Al2O3-, and (c) SiO2-supported Cu, Cu-ZnAl, and Nano-Cu catalysts. Relevant standards were included: CuO (JCPDF 98-00-0429), Cu2O (JCPDF 98-000-0186), Cu (JCPDF 00004-0836), Pt (JCPDF 00-004-0802), Au (JCPDF 00-004-0787), ZnO (JCPDF 00-0050664), CeO2 (JCPDF 00-034-0394), and Al2O3 (JCPDF 00-047-1308). The SiO2 and SiO2supported catalysts were amorphous.
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Table 2 The AWS rates and Cu site-normalized rates at 240 °C, and the associated activation energies. Catalysts
H2 Production Rate (nmol/s/m2)
AWS Rate (mmol H2/s/ molCu)
Activation Energy (kcal/mol)
CeO2 Cu/CeO2 Pt/CeO2 Au/CeO2 Cu/Al2O3 Cu/SiO2 Cu-Zn-Al Nano-Cu
1.3 13.6 3.3 2.1 0.5 < 0.1d 27.8 13.8
– 8.8a – – 0.8b < 0.6b 3.4b 0.8c
– 19 19 18 23 – 22 –
a b
Fig. 3. Normalized deactivation profiles (relative to initial rates) for AWS (H2 production) at 240 °C for the CeO2- and Al2O3- supported Cu and Pt, Cu-Zn-Al, and Nano-Cu catalysts.
c d
Assumed 100% dispersion for Cu on CeO2. Cu site density was determined by N2O chemisorption. Calculated based on the surface area, assuming 1019 Cu sites/m2. Calculated based on the detection limitation of the GC-TCD.
aldehyde decomposition was catalyzed instead of AWS, likely due to the strong interactions between acetaldehyde and Pt [40]. The strong bonding could also be the principal cause for deactivation of the Pt/ CeO2 and Pt/Al2O3 catalysts. For the Au/CeO2 catalyst, the aldehydeadmetal interaction may not be strong enough to facilitate reaction turnover. The relatively large Au particle sizes may also explain the lower activities; for reactions including CO oxidation and the WGS reaction, Au particles larger than a few nanometers are inactive [45,46]. Compared with the CeO2-supported metal catalysts, the Al2O3supported metal catalysts were relatively inactive; the rate for the Cu/ CeO2 catalyst was nearly 30-fold of that for the Cu/Al2O3. Recall that Al2O3 is irreducible while CeO2 is reducible. This highlighted the importance of oxide selection. Of the Al2O3-supported metal catalysts, only the Cu/Al2O3 catalyst produced small amounts of H2 after deactivation, with a rate 0.5 nmol/m2⋅s of H2 at pseudo-steady state. The bulk Al2O3 was inactive for AWS. The activation energies were similar for each support group (see Table 2 and Fig. 4), which is consistent with the rate limiting step occurring on the supports. Given that the activation energies for the AWS were similar to those reported for WGS (e.g., 19 and 17 kcal/mol for Pt/ CeO2 and Cu-Zn-Al catalysts, respectively [25,47]), it is possible that water dissociation was the rate limiting step, which parallels many reports for WGS over oxide-supported metal catalysts [17,48,49]. The effect of the support was further investigated by measuring activities for the Nano-Cu, Cu-Zn-Al, and Cu/SiO2 catalysts. Considering the Cu contents and site densities, the Cu site-normalized rates (mol H2/s/molCu) were a strong function of the support type and decreased in the following order: Cu/CeO2 > Cu-Zn-Al > Cu/Al2O3 ˜
consequence of acetaldehyde decarbonylation [40], but the amounts decayed to levels below the detection limit after ˜8 h on stream. After this initial deactivation, no CO and CH4 were detected. Based on these observations, AWS was concluded to be the primary source of H2 production after deactivation of the catalysts; therefore the H2 production rate was used as a direct measure of the pseudo-steady AWS activity. In general, the AWS rates determined from the H2 production rates were consistent with the hydrocarbon production results. In addition to acetic acid and H2, crotonaldehyde, ethanol, and in some cases small amounts of paraldehyde and acetone were produced, suggesting that reactions other than AWS were facilitated by the catalysts. The source of these additional products will be detailed later in the paper. The AWS rates for all of the catalysts decayed during the first few hours on stream. The AWS rates for the Cu/CeO2 and Cu-Zn-Al catalysts reached pseudo-steady state after ˜8 h on stream, deactivating by ˜20% (see Fig. 3). The Cu/Al2O3 and Nano-Cu deactivated by ˜70% during the first 10 h on stream. The H2 production rates for the Pt/CeO2 and Pt/ Al2O3 catalysts decayed by more than 90%. Recall that these catalysts initially produced some CO and CH4, which could participate in the WGS or reforming reactions. Consequently, it was not possible to accurately assess the AWS deactivation rates for these catalysts. The Au/ CeO2 catalysts produced small and unstable amounts of H2; the Al2O3 and SiO2 supports were inactive for the AWS. In general, the supported Cu catalysts including reducible oxides (CeO2 or ZnO [41,42]) were the most stable. Deactivation profiles for the catalysts were fit to model decay laws (see Table S3) to interrogate the deactivation mechanisms. Results for the Cu/CeO2 and Cu-Zn-Al catalysts were reasonably well fit by exponential and hyperbolic models, while data for the Nano-Cu and Pt/ CeO2 catalyst was best fit by exponential models. The exponential decay model is typically associated with deactivation by surface poisoning whereas the hyperbolic model suggests sintering. Results for the Cu/Al2O3 and Pt/Al2O3 catalysts were best fitted by the reciprocal power model, which often indicates deactivation via site fouling or coking [43,44]. The basis for these differences is not currently known and is the subject of future research. The pseudo-steady AWS rates at 240 °C after deactivation are listed in Table 2. Overall, both the admetal and support appeared to have a significant impact on the AWS reaction rate. CeO2 had a low AWS activity (1.3 nmol/m2⋅s), which was not surprising considering previous evidence of AWS catalysis with CeO2 reported by Orozco et al. [13,14]. After the deposition of metal onto CeO2, the activity increased dramatically. The stable rate for the Cu/CeO2 catalyst was 13.6 nmol/m2⋅s, which was 4 times higher than those for the CeO2-supported Pt and Au catalysts, and an order of magnitude higher than the rate for bulk CeO2. The activity enhancement associated with Cu highlighted the importance of judicious admetal selection. For the Pt/CeO2 catalyst,
Fig. 4. Arrhenius plots for AWS rates over the Cu-Zn-Al, CeO2-based, and Cu/ Al2O3 catalysts. 4
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Fig. 5. Schematic representation of the proposed AWS reaction mechanism for the Cu/CeO2 catalyst.
Nano-Cu. Recall that we assumed 100% dispersion for Cu on CeO2. Therefore, the Cu site-normalized rate for the Cu/CeO2 catalyst can be considered as a lower limit for its activity. The intrinsic activity for the Cu/CeO2 catalyst was approximately an order of magnitude higher than those for the Cu/Al2O3, Cu/SiO2, and bulk Cu catalysts. The results described thus far are consistent with a bifunctional mechanism for AWS over the Cu/CeO2 catalyst, in which the sites on CeO2 are responsible for H2O dissociation and aldehyde oxidation occurs at sites on or at the interface of the Cu domains, with oxygen directly or indirectly (via, for example, a Mars-Van Krevelentype mechanism) sourced from H2O (see Fig. 5). Intimacy between these two distinct catalytic sites would facilitate high AWS rates. As discussed earlier in connection with the N2O uptakes, Cu could stabilize and increase the oxygen vacancies on CeO2 [34,35], which could also enhance the AWS activity. A similar type of bifunctional mechanism could explain results for the Cu-Zn-Al catalyst, with ZnO responsible for H2O dissociation and Cu for acetaldehyde oxidation. Given that ZnO is considered less reducible than CeO2 [17], it is not unexpected that the AWS activity for the Cu-Zn-Al catalyst was moderate in comparison to that for the Cu/CeO2 catalyst. For the Cu/Al2O3 catalyst, it is likely that the AWS activity was due solely to the Cu admetal. Note that the Cu site-normalized rates and deactivation profiles for the Cu/Al2O3 and bulk Nano-Cu catalysts were nearly identical. In comparing results for the supported Cu catalysts described in this paper with those reported for homogeneous catalysts, the heterogeneous catalysts appeared to be less active and, in some cases, less selective for the AWS reaction. Brewster reported results that are consistent with an AWS TOF of 3.6•10−3 1/s at 105 °C for a [(p-cymene) RuCl2]2 catalyst [10]. Extrapolating AWS rates determined in this research using the measured activation energies, the TOF for the Cu/CeO2 catalyst would be two orders of magnitude lower, or 1.1•10-5 1/s at 105 °C, assuming atomic dispersion of the Cu. The assumption of 100% dispersion overestimates the active site densities, but we do not believe that the dispersion is two orders of magnitude lower (i.e., 1%). While the Cu/CeO2 catalyst is probably less active than the homogeneous catalysts, it appears to be more active than other heterogeneous catalysts that have been described in the literature. Xiang et al. did not report AWS rates directly, but the results indicate an AWS rate of 0.55 μmol/gCu/s at 350 °C [15]. Extrapolating data for our Cu/CeO2 catalyst to 350 °C yields a rate of 3596 μmol/gCu/s. Dispersions for these materials are dissimilar, although it is unlikely they differ by four orders of magnitude. Similarly, it is unlikely that differences in the reaction conditions can reconcile the significantly different rates.
Fig. 6. Carbon selectivities for the catalysts. The selectivities are defined as the moles of acetaldehyde reacted to form a specific product divided by the total amount of acetaldehyde converted.
homogeneous catalysts [10]. In addition to the acid, the Nano-Cu favored ethanol formation with a selectivity of approximately 35%. To determine the source of the ethanol, a control experiment was conducted in which only H2 and acetaldehyde were fed to the Cu/CeO2 catalyst. Under these conditions, very little ethanol was detected and there was no evidence of H2 consumption. This result suggests that aldehyde hydrogenation was not a significant source of ethanol under the conditions employed in this study. Instead, we believe that the ethanol was produced via a Cannizzaro-type disproportionation reaction of acetaldehyde (Equation 4) [6,13]. The crotonaldehyde was likely a product of aldol condensation (Equation 5) [50,51]. Paraldehyde was a minor product for some catalysts, which may have been formed via acetaldehyde trimerization [52]. CO and CH4 were only produced on Pt-based catalysts and likely were products of acetaldehyde decarbonylation [40]. 2CH3CHO + H2O → CH3CH2OH + CH3COOH
(4)
2CH3CHO → CH3CHCHCHO + H2O
(5)
The disproportionation (ethanol production) and aldol condensation (crotonaldehyde production) reaction rates at 240 °C are provided in Table 3. Given the high disproportionation reaction rate for the bulk Cu catalyst and the low rates observed for the bulk metal oxide supports, ethanol production over the oxide-supported Cu catalysts appears to have been a consequence primarily of the Cu admetal. As metallic Cu is capable of activating H2O [48], it is not surprising that Cu was observed to catalyze the disproportionation reaction. For aldol condensation, the other major side reaction, reaction rates for most of the Table 3 Average ethanol and crotonaldehyde production rates and turnover frequencies for metal oxide-supported catalysts and bulk supports at 240 °C during 14 h on stream.
3.3. Selectivities In addition to acetic acid and H2, crotonaldehyde was produced for all of the catalysts; ethanol was produced for the Cu/CeO2, Cu-Zn-Al, and Nano-Cu catalysts; and CO, CH4, and small amounts of paraldehyde and acetone were produced for some of the catalysts. Selectivities to the carbon-containing products are shown in Fig. 6. The Cu-based catalysts were the most selective to acetic acid, the AWS product. Notably, selectivities for the Cu/CeO2, Cu-Zn-Al, and Nano-Cu catalysts were proximal to those reported for the
Catalysts
Ethanol Production Rate (nmol/s/m2)
Crotonaldehyde Production Rate (nmol/s/m2)
Aldol Condensation TOFs (1/s * 103)a
CeO2 Cu/CeO2 Au/CeO2 Al2O3 Cu/Al2O3 Au/Al2O3 Cu/SiO2 Pt/CeO2 Pt/Al2O3 Cu-Zn-Al Nano-Cu
0.12 2.54 0.63 0.13 0.23 0.27 – – 0.15 16.4 13.1
0.76 0.38 4.03 0.80 1.01 1.80 0.16 0.53 0.92 0.64 1.42
0.78 0.80 1.50 0.95 0.89 0.85 0.67 0.34 0.47 – –
a
5
Normalized by the acid sites.
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catalysts correlated with the weak acid site densities with the exception of the Pt-based catalysts (see Table 3 and site densities in Table 1). These results are consistent with reports in the literature that aldol condensation can be acid-catalyzed [53]. The Pt-based catalysts had lower TOFs, a possible consequence of strong interactions between acetaldehyde and Pt and subsequent decomposition of acetaldehyde. For completeness, we also mention that acid-base pairs could be responsible for the condensation reaction [54]; this was not confirmed in our work. Mechanisms for the disproportionation and aldol condensation reactions will be investigated and described in a future paper.
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4. Conclusions The results presented in this paper explore the use of CeO2-, Al2O3-, and SiO2-supported Cu, Pt, and Au catalysts for the AWS reaction and demonstrate that the type of admetal and support have significant impacts on the activities. The supported Cu catalysts had higher AWS activities than the supported Pt and Au catalysts, while the Pt-based catalysts favored acetaldehyde decomposition and the Au-based catalysts were relatively inactive for AWS. The combination of Cu sites and sites from a reducible oxide (i.e., CeO2 or ZnO) yielded materials with high AWS activities, stabilities, and selectivities. We posited that the reducible oxide catalyzed H2O dissociation, while the Cu domains catalyzed aldehyde oxidation using oxygen from H2O. The materials also catalyzed other reactions producing crotonaldehyde, ethanol, and very small amounts of paraldehyde. The aldol condensation (crotonaldehyde formation) rates correlated with the acid site densities, and the disproportionation reaction (ethanol formation) appeared to be catalyzed by surface Cu. Future work will focus on further characterizing of the support reducibility and the admetal valence state, determining the overall AWS mechanism for the Cu/CeO2 catalyst, and designing other materials that couple H2O dissociation and aldehyde oxidation. Acknowledgements This work was supported by the National Science Foundation, under the CCI Center for Enabling New Technologies through Catalysis (CENTC) Phase II Renewal [CHE-1205189]. The authors would also like to acknowledge assistance and support from Drs. Saemin Choi, Yuan Chen, Jason Gaudet, and Brian Wyvratt, and Professors Michael Heinekey and Karen Goldberg. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.03.064. References [1] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411–2502. [2] R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A. Dumesic, Appl. Catal. B 56 (2005) 171–186. [3] A. Kruse, A. Gawlik, Ind. Eng. Chem. Res. 42 (2003) 267–279. [4] L. Lin, W. Zhou, R. Gao, S. Yao, X. Zhang, W. Xu, S. Zheng, Z. Jiang, Q. Yu, Y.-W. Li, C. Shi, X.-D. Wen, D. Ma, Nature 544 (2017) 80–83. [5] C. Ratnasamy, J.P. Wagner, Catal. Rev. 51 (2009) 325–440.
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