Research Article pubs.acs.org/journal/ascecg
Conversion of Ethanol and Acetaldehyde to Butadiene over MgO− SiO2 Catalysts: Effect of Reaction Parameters and Interaction between MgO and SiO2 on Catalytic Performance Qiangqiang Zhu, Bin Wang,* and Tianwei Tan* Beijing Key Laboratory of Bioprocess, National Energy R&D Center for Biorefinery, College of Life Science and Technology, Beijing University of Chemical Technology, No. 15 of North Three-Ring East Road, Chaoyang District, Beijing 100029, P. R. China
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ABSTRACT: For the effect of structural features on the catalytic performance of the conversion of ethanol and acetaldehyde to butadiene to be investigated, a series of MgO−SiO2 catalysts with different structural properties were synthesized by tuning the calcination temperature, investigated, and characterized. The best butadiene selectivity of 80.7% appears for the MgO−SiO2 catalyst calcined at 500 °C using a mixture of acetaldehyde/ethanol/water (22.5:67.5:10 wt %) as feed. Addition of the appropriate amount of water (10 wt %) improved butadiene selectivity by inhibiting the formation of 1-butanol and C6 compounds. Results from XRD, FT-IR, and 29Si MAS NMR indicate the generation of a significant amount of amorphous magnesium silicates along with few crystalline magnesium silicates for the catalyst calcined at 500 °C. XPS results indicate that it contains the lowest binding energies of both Si−O and Mg−O from Si−O−Mg bonds. For the catalysts calcined at low temperature (350 and 400 °C), more 1-butanol and C6 compounds formed, which are considered to be related to residual Mg(NO3)2. Additionally, more ethylene, diethyl ether, and butylene isomers were produced over the MgO−SiO2 catalyst calcined at 700 °C with the formation of forsterite Mg2SiO4. Further results from Fourier transform infrared spectroscopy after pyridine adsorption and CO2 temperatureprogrammed desorption show that the high catalytic performance is related to the presence of Lewis acidic sites and an intermediate number of basic sites. KEYWORDS: Ethanol, Butadiene, Si−O−Mg bonds, Two-step process, Amorphous magnesium silicates
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INTRODUCTION Butadiene is an important building block for the production of a wide variety of synthetic rubbers, elastomers, and resins upon self-polymerization or in conjunction with other polymerizable monomers. Currently, butadiene is mainly obtained via extractive distillation of the C4 fraction from naphtha steam crackers.1 Additionally, with increased availability and decreased cost, ethanol, especially bioethanol from nonfood biomass feedstocks, is potentially a promising platform molecule for the production of a variety of value-added chemicals.2 Thus, the conversion of ethanol to butadiene has resulted in increasing attention over the past few years due to both environmental concerns connected with petrochemical processes and new economic opportunities seen in biobased feedstocks. The conversion of ethanol to butadiene is divided into two processes: a one-step process developed by Sergey Lebedev3 and a two-step process.4 The two-step process includes partial ethanol dehydrogenation to acetaldehyde as the first step, followed by the transformation of the mixture of ethanol and acetaldehyde to butadiene as the second step. The mechanism of the ethanol conversion to butadiene is complicated and has © 2016 American Chemical Society
not yet been fully elucidated. Nevertheless, there is a consensus on several key steps5−7 (Scheme 1): (1) the dehydrogenation of ethanol to acetaldehyde, (2) aldol-crotonic condensation, which is the integration reaction of acetaldehyde aldol condensation followed by dehydration, (3) a Meerwein− Ponndorf−Verley (MPV)-type reduction by ethanol resulting in crotyl alcohol, and (4) a final dehydration step to butadiene. Scheme 1. Main Reaction Pathway of Ethanol Conversion to Butadiene
Received: August 28, 2016 Revised: November 15, 2016 Published: November 24, 2016 722
DOI: 10.1021/acssuschemeng.6b02060 ACS Sustainable Chem. Eng. 2017, 5, 722−733
ACS Sustainable Chemistry & Engineering
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The most studied catalysts in ethanol to butadiene conversion are MgO−SiO28−18 and ZrO2−SiO219−22 based materials in the one-step process and Ta−SiO223−25 catalysts in the two-step process. Moreover, Hf−SiO226 and ZrO2−SiO227 based catalysts were also reported in recent literature in oneand two-step processes, respectively. Among these catalysts, MgO−SiO2-containing materials are the most widely studied and are considered as a potential basis for the development of industrial catalysts of the process. A significant amount of research on the effect of catalyst components, catalyst preparation methods (wet kneading, mechanical mixing, sol− gel, and mechanochemical), and reaction conditions has been carried out. Recent studies on MgO−SiO2 catalysts have focused on the nature of active sites, especially the interaction between MgO and SiO2 in the conversion of ethanol to butadiene. Kvisle et al. conducted studies on individual oxides of MgO and SiO2, the MgO−SiO2 catalyst prepared by wet kneading, and MgO−SiO2 prepared by mechanical mixing in this process.28 They concluded the synergic effect in catalytic performance is unlikely to be due to the sole presence of MgO in SiO2 or vice versa. Conversely, they mentioned the possibility that this result is related to new but unclear Mg− O−Si interactions. Natta et al. reported the requirement of a high dispersion of magnesia on silica and the presence of a limited amount of amorphous magnesia hydrosilicate phase shown by XRD results for high performance, resulting from the interaction of dissolved Mg2+ with the silanols of the silica surface.29 Additionally, Cavani et al. studied MgO−SiO2 catalysts with a high Mg/Si ratio (9:15) and mentioned the importance of Mg−O−Si pairs characterized by Lewis-type acidity.13 Notably, Sels et al. recently observed the presence of an amorphous magnesium silicate phase demonstrated by 29Si NMR and the OH stretch region FT−IR analysis.11 Larina et al. also show that the formation of the Mg−O−Si structural fragments detected by 29Si MAS NMR analysis leads to an increase of its activity and selectivity in the conversion of ethanol to butadiene.17 These research studies address the importance of SiO2−MgO interactions in ethanol conversion to butadiene; however, an accurate active phase between SiO2 and MgO remains unclear. This difficulty is at least partly related to the difficulty in characterizing amorphous magnesium silicate structures. The most used catalyst characterization methods have inherent drawbacks for such characterization; for example, XRD analysis is blind to the amorphous silicates and IR spectra of the OH stretch region, and though it is useful for detecting the presence of magnesium silicates, is still difficult to accurately distinguish different magnesium silicates. Recently, 29Si MAS NMR analysis has been used to detect the presence of amorphous magnesium silicates, providing meaningful information.11,17 Hence, a comprehensive characterization of catalysts with different characterization methods is necessary to locate the active sites and understand the SiO2−MgO interaction. To date, the MgO−SiO2 catalyst system is mostly used in the one-step ethanol to butadiene process. It is thus of significant interest to test such a catalyst in the two-step process, which has the potential to improve butadiene selectivity. Hence, in this paper, MgO−SiO2 catalysts with different structural features were synthesized and tested in the two-step ethanol to butadiene process. The effect of reaction parameters and interaction between MgO and SiO2 on catalytic performance are reported after detailed evaluation and comprehensive characterization.
Research Article
EXPERIMENTAL SECTION
Materials and Chemicals. All chemicals were used as received without further purification. Silica gel (50−100 μm) and Mg(NO3)2· 6H2O were supplied by Aladdin and Sigma-Aldrich, respectively. Ethanol, acetaldehyde (99.5%), and magnesium oxide (50 nm, 99.9% metals basis) were purchased from Cleman Chemical, Adamas Reagent Co., and Aladdin, respectively. Deionized water was used in all reactions. Catalyst Preparation. The MgO−SiO2 catalysts were prepared by impregnation using ethanol as solvent. Preparation of the MgO−SiO2 catalyst (1:1 MgO/SiO2 molar ratio) was shown as an example. First, 25.6 g of Mg(NO3)2·6H2O was dissolved in 100 g of ethanol with stirring, and then 6.0 g of silica gel was added to the resultant solution. The mixture was stirred at 60 °C until ethanol was totally evaporated and a gel was formed. Afterward, the gel was calcined at a selected temperature in air for 3 h with a heating ramp of 5 °C min−1. The same procedure was used to prepare the MgO−SiO2 catalysts with different MgO/SiO2 ratios. Catalyst Characterization. Nitrogen adsorption−desorption measurements were performed on a Micromeritics ASAP 2020 HD 88 surface area and porosity analyzer. The catalysts were degassed at 180 °C in a vacuum of 1.33 × 10−3 Pa for 10 h and then switched to the analysis station for adsorption−desorption analysis. The morphology of the obtained solids was observed via scanning electron microscope (SEM) (HITACHI S-4800, HITACHI Ltd., Japan). Transmission electron microscopy (TEM) was recorded with a JEOL JEM-2010 high-resolution transmission electron microscope equipped with an energy-dispersive X-ray (EDX) detector. X-ray diffraction (XRD) measurements were carried out using a Bruker diffractometer with Cu radiation (40 kV, 120 mA). Data were recorded in the 2θ range of 5−90° with an angular step size of 0.2° and a counting time of 8 s per step. 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker AV300 spectrometer (7.05 T). At this field, the resonance frequency of 29Si is 59.6 MHz; the recycle time is 30 s, and the pulse length is 5.0 μs. The spinning frequency of the rotor is 5 kHz. The catalysts were packed in 4 mm Zirconia rotors. Tetramethylsilane was used as chemical shift reference. X-ray photoelectron spectroscopy (XPS) was acquired on a Thermo Scientific Kα spectrometer using an Al Kα (hν = 1486.6 eV) monochromatic small-spot X-ray source. All binding energies were referenced to the C 1s line at 284.8 eV, and the XPS spectra were fitted by a Lorentzian−Gaussian function assuming a Shirley background by XPSPEAK Version 4.1 software package. Fourier transform infrared resonance (FT-IR) of MgO−SiO2 catalysts in the OH region of 3800−3600 cm−1 were recorded on a thermo Nicolet 6700 FTIR spectrometer equipped with an MCT detector and a high temperature environmental cell fitted with a KBr window. In each IR test, approximately 0.020 g of catalyst was pressed into a pellet and placed into the cell. The catalyst was first dried in situ at 350 °C under a vacuum for 30 min. Then, a spectrum was collected at this temperature to investigate the OH signals of the various MgO− SiO2 catalysts. FT-IR spectra in the wavenumber range of 4000−400 cm−1 were also recorded with a BRUKER TENSOR 27 spectrometer at room temperature by diluting samples in KBr. The acidic properties of the samples were evaluated by temperatureprogrammed desorption of CO2 (CO2-TPD). The TPD profiles were normalized by sample weight. The sample (0.1 g) was preheated in a glass U-tube at 350 °C for 1 h in He flow (20 mL min−1), and then the catalyst was cooled from 350 °C to room temperature. CO2 (20 kPa) was introduced into the glass tube at room temperature for 30 min. And then, the catalyst was evacuated at 100 °C for 30 min. The TPD measurements were conducted from 100 to 400 °C at a heating rate of 10 °C min−1, and the signals were recorded. Catalyst Evaluation. The conversion of ethanol (EtOH) and acetaldehyde (AA) to butadiene was performed in a downstream fixedbed quartz reactor with an internal diameter of 8 mm and a length of 300 mm under atmospheric pressure. Nitrogen was the carrier gas, and the flow rate was fixed at 5 mL min−1. Before the introduction of 723
DOI: 10.1021/acssuschemeng.6b02060 ACS Sustainable Chem. Eng. 2017, 5, 722−733
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Effect of AA Content in Feed on Catalytic Performancea carbon selectivity (C mol %) AA content (wt %)b
WHSV (g g−1 h−1)
EtOH/AA conversion (%)
ethylene
propylene
butylene isomers
butadiene
diethyl ether
1-butanol
carbon balance (%)
0 10 20 25 30
0.06 0.20 0.28 0.36 0.38
27.7 28.6 32.0 31.5 30.2
35.6 14.8 4.1 3.2 2.9
3.7 2.5 2.4 2.6 2.5
1.3 1.9 4.1 2.9 2.8
22.8 62.6 73.2 75.9 71.8
29.4 9.4 3.7 1.7 1.6
3.6 5.9 8.3 9.2 10.7
96.4 97.1 95.8 95.5 92.3
a Conditions: 1 g of MgO−SiO2 (1:1 Mg/Si) catalyst calcined at 500 °C at a reaction temperature of 350 °C. bAA content (wt %) = massAA/ massAA+EtOH × 100.
Table 2. Effect of Water Content in Feed on Catalytic Performancea carbon selectivity (C mol %) water content (wt %)b
WHSV (g g−1 h−1)
EtOH/AA conversion (%)
ethylene
propylene
butylene isomers
butadiene
diethyl ether
1-butanol
carbon balance (%)
