MCF Catalyst for Production of 1,3

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Highly active and selective Zr/MCF catalyst for production of 1,3-butadiene from ethanol in a dual fixed bed reactor system Jian Liang Cheong, Yaling Shao, Sherman J.R. Tan, Xiukai Li, Yugen Zhang, and Su Seong Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01193 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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ACS Sustainable Chemistry & Engineering

Highly active and selective Zr/MCF catalyst for production of 1,3-butadiene from ethanol in a dual fixed bed reactor system Jian Liang Cheong, Yaling Shao, Sherman J.R. Tan, Xiukai Li, Yugen Zhang and Su Seong Lee* Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, #04-01, Singapore 138669 * E-mail: [email protected]

Abstract: Copper and zirconium oxide clusters were highly dispersed on mesocellular siliceous foam (MCF), a mesoporous silica support with ultra large, interconnected nanopores. These catalysts (denoted as Cu/MCF and Zr/MCF) were seperately loaded into two fixed bed reactors as catalysts for the conversion of ethanol (EtOH) to 1,3-butadiene (BD). Under optimal conditions, high BD selectivity (up to 73%) and ethanol conversion (up to 96%) were achieved at weight hourly space velocities of 1.5 and 3.7 h-1. This translates to an unprecedented productivity of 1.4 gBD/gcatalyst·h-1(208 gBD/lcatalyst.h-1). The high catalytic performance is attributed to the highly selective and active catalysts. The EtOH dehydrogenation activity of Cu/MCF could be accurately controlled in the first reactor, which delivers a fixed ratio of acetaldehyde/EtOH mixture to Zr/MCF in the second reactor. The optimal ratio minimizes EtOH dehydration to ethylene by Zr/MCF, while maximizing the selectivity to BD. MCF was found to be superior

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over commercial porous silica in terms of EtOH conversion, BD selectivity and tolerance to coking. High BD selectivity was maintained with slight decrease of EtOH conversion over 42 h, which was readily restored upon regeneration by thermal treatment in air.

Keywords: Biomass Conversion, Heterogeneous catalysis, Mesoporous materials, Ethanol, 1,3butadiene

INTRODUCTION The decreasing supply of fossil fuels and their negative impacts on the environment leads to a growing interest in the using biomass as a renewable source for bio-based chemicals.1 Unlike fossil fuels, biomass is a carbon-neutral and environmentally-friendly feedstock. Bioethanol, which can be obtained from biomass conversion, is already the largest biofuel in the market. As such, the chemical conversion of bioethanol to bulk chemicals were actively being explored and investigated.2-4 1,3-Butadiene (BD) is an example of a bulk chemical that is used to produce synthetic rubber, elastomers and polymer resins. Currently, BD is obtained as a by-product from ethylene plants via naphtha cracking. Alternatively, BD can be obtained from bioethanol. It is noteworthy that the recent shift towards the use of shale gas for ethylene production, will result in shortage of aromatics and olefins like BD.5 Thus, there is both economic and environmental need to develop efficient catalytic systems for the conversion of bioethanol to BD. The conversion of ethanol to BD has been well documented in the literature and patents, mainly using silica and alumina-supported single, binary, or ternary metal oxides such as copper, magnesium, silver, tantalum, zinc and zirconium.6-28 The reaction was first reported in the early 20th century by Lebedev and Ostromislensky. Lebedev used a tantalum-on-silica catalyst for one-step conversion. Ostromislensky employed a two-step process: the


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dehydrogenation of ethanol to acetaldehyde for the first step and then a mixture of ethanol and acetaldehyde to BD. The general consensus for the mechanism involves conversion of ethanol to acetaldehyde via dehydrogenation path, followed by aldol condensation of acetaldehyde and then dehydration to crotonaldehyde, which subsequently undergoes a Meerwein-Ponndorf-Verley (MPV) reduction by ethanol to crotyl alcohol, and then dehydration to yield BD (Figure 1a).6, 1315

The ultimate yields obtained by the Lebedev and the Ostromislensky processes were 56% and

64%, respectively, at a liquid hourly space velocity of 0.6 h-1.11 Although both the processes were used in the first half of the 1900s, they became economically unattractive due to the emergence of oil-based technologies, i.e. naphtha cracking.29 Overall butadiene yield could be improved by using a mixture of acetaldehyde (or crotonaldehyde) and ethanol as a feedstock.6-11, 30. According to a 1947 patent, a mixture of acetaldehyde and ethanol was produced from denatured ethanol using a reduced copper catalyst on alumina, and fed directly to another reactor containing tantalum oxide supported on silica to produce BD with 23% yield and low productivity over 20 h.12 It was calculated that the Ostromislensky reaction, at an ethanol conversion of 44% and BD selectivity of 55%, was industrially viable.11, 31 The type of catalyst support was found to have a strong influence on the catalyst performance.8, 22-24 Using a mixture of acetaldehyde (AcH) and ethanol (EtOH), a study on a series of silica-supported tantalum oxide catalysts with pore sizes ranging from 2.5 nm to 13.4 nm revealed a positive relationship between pore size and BD selectivity or productivity. They also showed that mesoporous silica (SBA-15)-supported catalyst had much better longevity and reactivity than commercial porous silica-supported ones because larger pore SBA-15 is less sensitive to coking by deactivation.24


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In single fixed bed reactor systems using Zr on a silica support, additional metal (e.g Ag, Zn, Cu) is required as a dopant to catalyze the EtOH-to-BD reaction.8, 13 These dopant metals enable the dehydrogenation of ethanol to acetaldehyde, while Zr is supposed to catalyze the subsequent steps of aldol condensation and MPV to produce BD. Sushkevich et al. found that an increase in Ag content from 0.3 to 1.0 wt% in a Zr/silica catalyst increases EtOH conversion. A further increase to 2 wt% only slightly improves EtOH conversion. The increase of Ag loading led to increased production of heavy byproducts via subsequent aldol condensation and slightly decreased BD selectivity due to increased acetaldehyde selectivity.13 This demonstrates the importance of controlling the EtOH-to-AcH ratio to enhance EtOH conversion and BD selectivity. However, it is challenging to control the intricate reactions occurring inside the single bed reactor by varying the metal ratios in a bimetallic catalyst, to obtain an optimal amount of AcH for high BD yield. Therefore, by separating the dehydrogenation step from the reaction, it would be possible to control EtOH/AcH ratio. Recently, a two-stage process of producing butadiene from ethanol was reported by Klein et. al.32 They employed a Cu/SiO2 and MgO/K/zeolite-β system. Although the selectivity to butadiene was as high as 72%, the overall yield was only 33% at a WHSV of less than 0.24 h-1. As elaborated by Makshina et. al., the productivity (gBD/gcatalyst·h-1) value is a key parameter in evaluating catalytic performance.20 To the best of our knowledge, the best result so far in the direct conversion of EtOH to BD was achieved by Larina et. al., which was 0.71 gBD/gcatalyst·h-1, complete ethanol conversion and a BD selectivity of 60.2%, using a quaternary Zn-La-Zr-Si catalyst operating at 400 °C and a WHSV of 2 h-1.33 No data on the catalyst stability was reported. Despite the improved conversion and BD selectivity reported in the literature, it is still challenging to achieve high BD productivity with long-term catalyst stability.


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To address the issues of low productivity due to low EtOH conversion, BD selectivity and catalyst stability for the direct conversion of EtOH to BD, we designed a dual fixed bed system with two different metal-on-support catalysts (Figure 1b). These catalysts were made up of earth abundant metals, Cu and Zr, that are highly-dispersed on a mesoporous silica material with ultralarge nanopores, mesocellular siliceous foam (MCF).

Figure 1. a) A catalytic cycle that involves generation of AcH by transfer hydrogenation between crotonaldehyde and EtOH, b) Dual fixed bed reactor system. At the optimal reaction conditions, our dual catalyst system gives an average EtOH conversion of 96% and an average BD selectivity of 73% over 15 h at a WHSV of 1.5 h-1. At a higher WHSV of 3.7 h-1, a mass productivity value of 1.4 gBD/gcatalyst·h-1 was achieved based on high conversion (92%) and high selectivity (70%). The corresponding LHSV and volume productivity value were 0.7 h-1 and 208 gBD/lcatalyst.h-1, respectively. Operating temperatures of our catalysts were below 400 °C. Clearly, our catalytic system performs much better in terms of BD yield, productivity and stability. In addition, the metals used for the catalysts are earth abundant metals.


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Our system also showed similar high performance with a wet ethanol with 10% water content. Lastly, we also showed that the catalysts could be regenerated with nearly 100% recovery of the original reactivity.

EXPERIMENTAL SECTION Materials and characterization methods All chemicals were purchased from Fisher Scientific, Sigma Aldrich, Alfa Aesar or Merck Millipore Singapore. Gas standards were procured from Singapore Oxygen Air Liquid Pte Ltd. Nitrogen adsorption–desorption isotherms at -196 °C were collected using Micromeritics ASAP 2020 sorption analyzer. Samples (~100 mg) were degassed at 150 °C for 12 h before measurement. Specific surface areas were calculated using the BET (Brunauer–Emmet–Teller) method. Pore size and pore size distribution (PSD) were obtained by the BJH method using the cylindrical pore model. Data obtained were analyzed using the BJH model and pore volume was taken at P/P0 = 0.988 single point. X-ray powder diffraction (XRD) experiments were carried out using a Bruker D8 Advance instrument equipped with a LynxEye detector and Cu anode (CuKα radiation), operating at 30 kV and 50 mA. Samples were scanned from a diffraction angle (2θ) of 15° to 90° at a step size of 0.020°. The loading amount of copper or zirconium on MCF was obtained by ARL Quant'X Energy-Dispersive X-Ray Fluorescence Spectrometer, using known amounts of copper or zirconium on silica standards. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were taken on a FEI Tecnai F20 electron microscope equipped with a Gatan CCD camera, a HAADF detector and an energy dispersive X-ray (EDX) spectrometer operating at 200kV. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer Pyris-1. Samples were placed on a platinum crucible and


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heated from 50 °C to 800 °C at a rate of 10 °C/min under air flow. Weight change was monitored with an internal balance during heating. In situ Diffuse Reflectance Fourier transform Infrared spectroscopy (DRIFTS) were performed on a Shimadzu FTIR Affinity-IS equipped with a DLATGS detector and Harrick Praying Mantis Diffuse reflectance accessory. A high temperature reaction sample cell and anhydrous pyridine were used for the DRIFTS experiments.

Catalyst preparation MCF was synthesized according to a literature method.34 A modified ion-exchange method was used to prepare Cu/MCF.35, 36 Briefly, 30 mL of DI water was added to 1 g of MCF. An appropriate amount of soluble copper precursor (e.g. copper nitrate or copper chloride) was added into the mixture under rapid stirring. A solution of aqueous ammonia (4 M) was added dropwise until pH reached ~9. The mixture was further stirred for 10 min, and then filtered and washed several times with water. The obtained particles were dried under vacuum for 12 h. The resulting powder was heated at 500 °C for 3 h to obtain the final green-coloured Cu/MCF catalyst. For Zr/silica, Zr was loaded onto MCF/commercial silica via an urea hydrolysis method.37 20 mL of DI water was added to 1 g of MCF. An appropriate amount of zirconium precursor (ZrOCl2·8H2O or ZrONO3·xH2O) and urea in a mole ratio of 1:10 were added to the mixture under rapid stirring. The resulting mixture was heated up to 90 °C under stirring for 6 h. After cooling down to room temperature, the mixture was filtered, washed several times with water and then dried under vacuum for 12 h. The resulting powder was heated at 500 °C for 3 hrs to obtain the final white Zr/MCF catalyst. Preparation of other Zr/support catalysts was identical to that of Zr/MCF except that Merck silica gel 60, Davisil 60 Å grade 635 or Davisil 150 Å grade 645 were used as a support material.


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Catalytic reaction procedure A reaction system with two fixed beds was constructed as shown in Figure 1b. Reactor 1 was directly connected to reactor 2 under the same flow and each reactor was heated independently. The fixed bed reactors were prepared by packing 20 mg of Cu/MCF catalyst and 60 mg of Zr/MCF catalyst, respectively, into a quartz tube (OD 8 mm X ID 6 mm X 203 mm), held in place by quartz wool. For quantitative analysis of products, Agilent Gas Chromatography machine (7890A), equipped with a flame ionization detector, was calibrated by injecting standard ethanol, acetaldehyde, crotonaldehyde or crotyl alcohol of known amount. For quantitative analysis of olefins, GC-FID was calibrated using certified gas blends in nitrogen balance. An ultra-precise high performance liquid chromatography (HPLC) pump (Agilent 1260 infinity capillary pump) was used to deliver ethanol into the dual fixed bed reaction system at a fixed flow rate. A mass flow controller (MFC) from Brooks Instruments was used to control the flow rate of nitrogen (20 ml/min) for delivering vaporized ethanol through the first fixed bed reactor packed with Cu/MCF (reactor 1). Initial EtOH amount in the flow was determined via sampling using the gas valve system at 150 °C in both reactors. Weight hourly space velocities (WHSV) of 1.5 and 3.7 gEtOH/gcatalyst·h-1 were used. All the gas lines were heated to 110 °C. The outlet stream was analyzed using GC-FID with a 30 m Poraplot Q column via a gas valve system periodically. Output gas was bubbled into CDCl3 for 1H NMR spectroscopy and analysed for qualification purposes (SI, Figure S1). All experimental values of conversion and selectivity were calculated by simple average. Carbon balances were determined to be 95 ± 5%.


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RESULTS AND DISCUSSION Preparation and characterization of catalysts MCF was chosen as the catalyst silica support due to its large pore size (cell = 29 nm, window = 14 nm), high surface area (> 400 m2/g) and unique three dimensional interconnected network of pores.34 Its ultralarge nanopores and highly porous structure are expected to facilitate substrate mass transfer and offer strong resistance to the coke formation. To obtain small copper and zirconium oxide nanoparticles highly dispersed on MCF for high catalytic activity, we adopted the ion-exchange method using an aqueous ammonia solution for the preparation of Cu/MCF, and the urea hydrolysis method for the preparation of Zr/MCF. Nitrogen sorption data showed that MCF and the supported catalysts exhibit type IV isotherms and H2 hysteresis loops, which were typical for mesoporous materials with interconnected pores (Figure 2). These catalysts retained their high surface area while their pore volumes were lower as compared to the bare MCF after considering their weight increase due to metal incorporation, indicating that these metals were deposited within the pores of the support (Table 1). However, the pore size of the catalysts did not change implying the formation of very small metal oxide nanoparticles.


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Figure 2. N2 absorption-desorption isotherm and pore size distribution plot (inset) of bare MCF, Cu/MCF and Zr/MCF of various Zr loadings. Table 1. BET surface area and pore volume of the catalysts

a

Material

BET surface area (m2/g)

Pore volume (cm3/g)a

MCF

409

2.44

Cu/MCF (4.7 wt%)

377

1.78

Zr/MCF (1.4 wt%)

381

2.20

Zr/MCF (2.7 wt%)

375

2.15

Zr/MCF (4.3 wt%)

354

1.97

Estimated at the relative pressure of P/P0 = 0.988.


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Figure 3. X-ray powder diffraction of Cu/MCF and Zr/MCF (offset for clarity). The absence of X-ray patterns attributed to Cu and Zr oxides for Cu/MCF and Zr/MCF confirmed the high dispersion of these oxide nanoparticles (Figure 3). HAADF-STEM imaging for Cu/MCF revealed small Cu oxide nanoparticles that are well-spaced between each other (Figure 4a). The size is about 2 nm. For Zr/MCF, Zr oxide particles were not observed by HAADF-STEM (Figure 4b), indicating that the Zr oxide might be present as clusters instead of nanoparticles. EDX mapping by TEM showed that Zr was present and highly dispersed on the support (Figure 4c). These results confirmed that copper oxide nanoparticles and zirconium oxide nanoparticles were formed on the support.


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Figure 4. HAADF-STEM image of a) Cu/MCF, b) Zr/MCF, c) EDX mapping of Zr/MCF Dehydrogenation over Cu/MCF catalyst It was previously reported that copper-on-silica catalyst synthesized using the ion-exchange method has a greater dehydrogenation activity as compared to the copper catalyst synthesized using the traditional impregnation method.36 The ion-exchange method enabled the formation of small copper oxide particles that were highly dispersed on the silica support, which was found to be positively correlated with dehydrogenation activity.36 Therefore, Cu/MCF was synthesized using the ion-exchange method. The maximum loading of Cu on MCF by the ion exchange method was found to be around 6 wt%. For this study, we fixed the Cu loading to 4.7 wt%. Cu/MCF showed high catalytic activity without reduction with H2, possibly due to rapid selfactivation via reduction with EtOH.38 By adjusting the reaction temperature of Cu/MCF (Figure 5a), the EtOH/AcH ratio could be varied for the subsequent tandem reaction to BD by Zr/MCF in reactor 2 (Figure 1) could be achieved. The EtOH/AcH ratio could also be maintained over 17 h (Figure 5b). This is distinctively different from a two-stage system by Klein et. al., where the


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ratio of EtOH to AcH was determined to be 4.32 We found that varying this ratio is essential in maximising the selectivity towards BD, and minimizing the formation of other side products, particularly ethylene.

Figure 5. a) Ratio of EtOH/AcH ratio at different reaction temperatures of Cu/MCF and different WHSV of EtOH. b) EtOH-to-AcH ratio by reactor 1 (Cu/MCF) at the optimal temperature of 235 °C and a WHSV of 1.5 h-1 for 17 h Effect of the Zr loading amount The catalytic activity for EtOH to BD reaction was found to be dependent on the acid strength of Zr on support, which could be optimized by varying the Zr loading.39 Different loadings of Zr


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were synthesized using the deposition-precipitation method via the urea hydrolysis, resulting in a high dispersion of Zr on MCF.37 Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) data revealed that Zr/MCF contained only Lewis acid sites (1606 cm-1), while Brønsted acid sites were not observed (Figure 6). The relative strength and proportion of Lewis acid sites on Zr/MCF could be quantified by monitoring the peak area at 1606 cm-1 at various temperatures (SI, Figure S3). We found that a higher proportion of medium and strong Lewis acid sites coincided with the Zr loading of 2.7 wt%, corresponding to the highest EtOH conversion and BD selectivity (Table 2). Zr/MCF with the Zr loading of 2.7 wt% was used for the optimization of the dual fixed bed reactor system.

Figure 6. DRIFTS spectra of Zr/MCF (Zr loading: 2.7 wt%), ZrO2 and MCF, the pyridine absorption temperature of 100 °C, with Y offsets offset for clarity.


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Table 2. Proportion of Lewis acid vs. acid strength Distribution of Lewis acid sites (%)a Lewis acid strength

Zr/MCF

Zr/MCF

Zr/MCF

(1.4 wt%)

(2.7 wt%)

(4.3 wt%)

Weak (100 – 200 °C)

55

39

60

Medium (200 – 300 °C)

18

27

21

Strong (> 300 °C)

27

34

19

EtOH Conversion (%)b

99 (98)

96

98 (98)

BD Selectivity (%)b

62 (69)

73

60 (67)

a

Based on the peak area at 1606 cm-1. Detailed calculation in SI. bThe conversion and selectivity values of each catalyst were obtained by fixing the temperature of reactor 1 (Cu/MCF) and reactor 2 (Zr/MCF) to 235°C and 400 °C respectively. Values in parentheses were obtained at the optimum Cu/MCF temperatures of 225°C and 220°C at Zr loading of 1.4 wt% and 4.3 wt% respectively Optimizing the reaction temperature Zr/MCF selectively dehydrated ethanol to ethylene with increasing temperature by feeding only EtOH (SI, Figure S2). However, Zr/MCF could selectively produce BD by feeding a mixture of EtOH and AcH via partial conversion of EtOH to acetaldehyde by Cu/MCF. BD selectivity was found to be strongly dependent on the EtOH/AcH ratio. Our optimization experiments were based on the generally accepted mechanism of EtOH to BD.29 According to stoichiometry in the catalytic scheme, two molecules of EtOH are converted to two molecules of AcH in the first step. These two molecules of AcH, together with one molecule of EtOH, were then converted to one molecule of BD and one molecule of AcH via two more steps (Figure 1a). One molecule of AcH from the last step is back involved in the second step. Although a fixed amount of EtOH and AcH from reactor 1 (Cu/MCF) enters reactor 2 (Zr/MCF), a dynamic


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catalytic loop works in reactor 2 via an in situ generation of AcH. Based on this mechanism, we reasoned that the ideal ratio of EtOH-to-AcH is 1. Accordingly, we adjusted the EtOH/AcH ratio to 1 by controlling the temperature of reactor 1. While the EtOH conversion was high (99%), selectivity to BD was only 66%. Our experimental findings revealed that EtOH-to-AcH ratio of slightly more than 1 was required for optimal BD production because formation of ethylene by Zr/MCF

is

a

competing

side

reaction

(SI,

Figure

S2).

Figure 7. Optimization of reaction temperatures for both Cu/MCF (top left, temperature of Zr/MCF = 400 °C) and Zr/MCF (top right, temperature of Cu/MCF = 235 °C) AcH/EtOH = 1.18) at a WHSV of 1.5 h-1. Optimization of reaction temperature for Cu/MCF (bottom left, temperature of Zr/MCF = 400 °C) and Zr/MCF (bottom right, Cu/MCF temperature = 250°C, EtOH/AcH = 1.38), at a WHSV of 3.7 h-1. Complete product distribution can be found in SI (Table S1-S3), including that for 90 v/v% aqueous EtOH. TOS = 1 h.


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The optimal EtOH/AcH ratio was obtained by fixing the temperature of reactor 2 at 400 °C and varying that of reactor 1 (Figure 7a and 7c). Lowering the EtOH/AcH ratio (by increasing the temperature of Cu/MCF) decreased the selectivity towards ethylene due to the reduced availability of EtOH. However, selectivity to BD was also lowered because EtOH was required for a MPV reduction of crotonaldehyde to crotyl alcohol (Figure 1a, Step 3). The reaction temperature of each reactor that led to the highest EtOH conversion and BD selectivity, while minimizing ethylene selectivity, was selected for stability studies. At a WHSV of 1.5 h-1, Cu/MCF (reactor 1) produced a range of optimal EtOH/AcH ratio (1.18-1.60) at 230 - 235 °C and Zr/MCF (reactor 2) achieved the highest conversion and BD selectivity at 400 °C, resulting in an EtOH conversion of 98% and BD selectivity of 73% (see ESI, Table S1). At a higher WHSV of 3.7 h-1, Cu/MCF (reactor 1) produced a range of optimal EtOH/AcH ratio (1.17-1.64) at 245 - 255 °C and Zr/MCF (reactor 2) achieved the highest conversion and BD selectivity at 400 °C, resulting in an EtOH conversion of 95% and BD selectivity of 70% (see ESI, Table S2). The presence of water (10 vol%) slightly decreased the optimal ratio of EtOH-toAcH (0.86-1.16) to achieve high conversion (98%) and BD selectivity (69%) (SI, Table S3).


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Catalytic reactions at the optimal reaction temperatures

Figure 8. Conversion and product distribution in the ethanol to butadiene reaction over 15 h TOS at a WHSV of 1.5 h-1. At a WHSV of 1.5 h-1 over 15 h TOS) the reactor system operating at the optimal temperatures of 235 °C and 400 °C over Cu/MCF and Zr/MCF, respectively, resulted in an average BD selectivity of 73% and an average ethanol conversion of 96% (Figure 8). In addition, the reaction system achieved BD selectivity of 70% and ethanol conversion of 92% at a much higher WHSV (3.7 h-1) over 15h TOS, which was an unprecedentedly high productivity of 1.4 gBD/gcatalyst·h-1 (SI, Figure S4). The side products observed at the WHSV of 1.5 h-1 were ethylene (6%), propylene (3%), butenes (6%), ether (3%) and other compounds such as crotyl alcohol/crotonaldehyde (4%) (Figure 8). The formation mechanism of these side products have been reported in the literature. 2, 29


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As crude bioethanol is more cost-effective and contains more water as compared to refined bioethanol, we have attempted to use a mixture of ethanol/water (90:10, v/v) as reaction feed. Temperature optimization was performed, and the reaction was monitored over a period of 15 h. Remarkably, the reaction system was stable in the presence of water, and an average ethanol conversion of 94% and BD selectivity of 68% were achieved at a WHSV of 1.5 h-1 (SI, Figure S6). The lower BD yield was likely to be due to equilibrium effects as water is a by-product of the EtOH-to-BD reaction. Comparison of MCF and commercial porous silica as support Table 3. Surface area and average pore sizes of Zr/support. Support

BET Surface Areaa (m2/g)

Pore Size

MCF

375

29.4c

Merck silica gel 60

386

8.9

Davisil 60 Å grade 635

411

11

Davisil 150 Å grade 645

292

16.3

(nm)b

a

BET surface area was calculated using the Brunauer–Emmet–Teller method. bPore sizes were derived from the adsorption branch of the N2 sorption isotherm, using a Broekhoff-de Boer method (BdB-FHH)cWindow pore size = 14.0 nm, derived from the desorption branch of the N2 sorption isotherm, using a Broekhoff-de Boer method (BdB-FHH)


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Figure 9. Conversion of ethanol (solid line) and BD selectivity (short dashed line) by catalysts supported on several porous silica materials. Comparison experiments were performed under the same conditions except Zr/silica catalysts, so the same Cu/MCF was used for all the catalytic reactions. Reaction conditions: temperature of reactor 1 = 235 °C, temperature of reactor 2 = 400 °C, WHSV = 1.5 h-1. EtOH conversion and BD selectivity is highest when MCF was used as a support (Figure 9). Although conversion by Zr/Davisil 60 Å was similar to that over Zr/MCF at a WHSV of 1.5-1, Zr/MCF led to ~17% higher conversion than Zr/Davisil 60 Å at a WHSV of 3.7 h-1 (SI, Figure S4 and S5). The pore sizes and its distributions of the commercial porous silica supports were found to be smaller and broader as compared to MCF (SI, Figure S9). Although the pore size of Davisil 150 Å is bigger than that of Davisil 60 Å, it showed slightly lower conversion that Davisil 60 Å possibly due to lower surface area or presence of more micropores (Table 3 and SI, t-plot pore volume in Table S4). Jones et. al. reported that catalyst deactivation was strongly correlated to pore blocking by reactants or products.8 Here we found that the decrease in surface


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area and pore volume of MCF as a result of coking is not as pronounced as the other support materials (SI, Table S4, Figure S8 and S9). We hypothesize that smaller pores have a higher tendency of pore blockage, due to coking, as compared to larger pores, resulting in less accessible active catalytic sites with time. This pore size effect was consistent with findings by Jones et. al. and others.8, 24 A catalyst with a large pore size and an interconnected pore structure allowed for efficient mass transfer and demonstrated a greater resistance to coking.40 Long-term catalytic studies with catalyst regeneration

Figure 10. Long-term studies with catalyst regeneration. Reaction conditions: Temperature of reactor 1 (Cu/MCF) = 235 °C, temperature of reactor 2 (Zr/MCF) = 400 °C,WHSV = 1.5 h-1. We regenerated the catalyst twice over a period of more than 110 h. The conversion of EtOH decreased by about 15% over 42 h from 99% to 84%, possibly due to blockage of micropores due to coking. Performance remained very good even after two cycles of catalyst regeneration


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through simple heat treatment in air suggesting that the decrease of the catalyst activity with time was mainly due to coking (Figure 10). Finally, the scalability of the catalyst system was examined by increasing the total catalyst mass from 80 mg to 500 mg. Under the same reaction conditions, i.e. reactor temperature and the aspect ratio of the beds, the larger scale reaction showed similar performance. Higher average BD selectivity (77%) and lower ethylene selectivity (4%) could be achieved by lowering the reaction temperature of reactor 2 from 400 °C to 385 °C, but the average conversion slightly decreased to 93%. (SI, Figure S10). CONCLUSIONS To conclude, we have demonstrated an efficient catalytic system for the conversion of the bioethanol to 1,3-butadiene, an important commodity chemical that is obtained primarily from crude oil. The catalysts were synthesized using inexpensive and abundant metals: copper and zirconium, supported on mesoporous silica. The exceptional performance of the dual fixed bed reactor system, in terms of EtOH conversion and BD selectivity, could be attributed to a combination of three factors: 1) large and uniform pore size of the support material, MCF, and its 3-dimensional pore structure, which allowed efficient mass transfer and excellent resistance to coking and, 2) highly active and selective catalysts due to the small size and high dispersion of the catalytic active metal oxide nanoparticles. With this well-designed system, an unprecedentedly high EtOH conversion of 96% and BD selectivity of 73% were attained at a WHSV of 1.5 h-1 and 15 h TOS. In addition, the BD mass productivities at WHSV of 1.5 and 3.7 h-1 were 0.64 and 1.4 gBD/gcatalyst·h-1, respectively, which were much higher than those previously reported in the literature. The corresponding volume productivities were 95 and 208 gBD/lcatalyst.h-


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1

. The catalysts were also very robust and could be regenerated under simple heat treatment in air

without loss of reactivity. ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures, XRD, TGA, BET and reaction profile. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research (A*STAR), Singapore), Biomass-to-Chemicals Program (Science and Engineering Research Council, A*STAR, Singapore). REFERENCES 1.

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Angelici, C.; Weckhuysen, B. M.; Bruijnincx, P. C. A. Chemocatalytic conversion of

ethanol into butadiene and other bulk chemicals. ChemSusChem 2013, 6, 1595-1614.


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TOC

Highly active and selective Zr/MCF catalyst for production of 1,3-butadiene from ethanol in a dual fixed bed reactor system Jian Liang Cheong, Yaling Shao, Sherman J.R. Tan, Xiukai Li, Yugen Zhang and Su Seong Lee Synopsis: An efficient catalytic system was developed for the conversion of bioethanol to 1,3butadiene with unprecedentedly high mass productivity


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MCF Catalyst for Production of 1,3

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