Copper–Manganese Mixed Metal Oxide Catalysts for the Direct

Article pubs.acs.org/IECR

Copper−Manganese Mixed Metal Oxide Catalysts for the Direct Epoxidation of Propylene by Molecular Oxygen Anusorn Seubsai,*,† Michael Kahn,‡ Bahman Zohour,‡ Daniel Noon,‡ Metta Charoenpanich,† and Selim Senkan‡ †

Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok, 10900, Thailand Department of Chemical and Biomolecular Engineering, University of CaliforniaLos Angeles, Los Angeles, California 90095, United States

‡

ABSTRACT: Cu−Mn mixed metal oxide (MMO) catalysts doped with NaCl were synthesized using combustion synthesis technique with a variety of fuels for the direct gas-phase epoxidation of propylene with molecular oxygen. The catalyst structure consisted of Mn2O3 and Cu1+xMn2−xO4 crystals. The catalyst crystallinity, primarily influenced by the combustion synthesis fuel choice and calcination temperature, has been found to be of particular importance in affecting the selectivity of the epoxidation reaction. The optimum catalyst composition was 10−20 mol % Cu in Mn doped with 0.1−0.2 mol % NaCl. It exhibited 30−37% propylene oxide selectivity and 1.3−1.5% propylene conversion. 4818,19 as support allowing for the reaction temperature to be increased. Copper-modified catalysts have also received attention because of the need to eliminate the coreactant H2, increased selectivity and durability compared to Au-modified catalysts.20−30 In this regard we reported a series of Cu-modified catalysts, including (i) coimpregnated RuO 2 −CuO− NaCl,27,31−34(ii) coimpregnated SnO2−CuO−NaCl/SiO2,28 (iii) Cu-on-Mn/SiO2 catalysts prepared by pulsed laser ablation (PLA).35 Here we report the results of an investigation of bimetallic Cu−Mn propylene epoxidation catalysts synthesized by combustion method with the aim to establish the effects of different fuels used in the initial catalyst preparation step, the Cu/Mn ratio of the catalyst, the amount of NaCl dopant used, and the combustion/calcination protocol used on the selectivity and yields of direct PO synthesis from propylene and oxygen.

1. INTRODUCTION Propylene oxide (PO) is one of the most valuable and widely produced industrial chemical intermediates used in industrial chemistry.1−3 Some noteworthy PO derivatives are polyether polyols (polyurethane industry), propylene glycol (food, drug, and automotive industries), and glycol ethers (protective coatings, cleaners).1 Thus, it is consistently in demand, driving the chemical process industry to search for more cost-effective and environmentally friendly technologies for its production.3 Direct gas-phase epoxidation of propylene to PO by molecular oxygen using heterogeneous catalysts is considered to be the most desirable route to resolve the challenges of the established processes: C3H6 +

1 O2 → C3H6O 2

The major side product is CO2, while other byproducts include acrolein (AC), acetone (AT), and acetaldehyde (AD). No industrial scale process for direct propylene epoxidation currently exists, since the required breakthrough in catalysis has yet to be achieved. Several research groups have been working intensively to discover new robust catalysts that can deliver high propylene conversions with superior PO selectivities under economically feasible conditions. Catalysts composed of Ag modified with various types of promoters and supports were first studied because they were commercially used for ethylene oxide production.4−9 However, for propylene epoxidation, these catalysts suffer from low PO selectivities and low propylene conversions because of the high reactivity of the allylic hydrogens in propylene, the abstraction of which leads to deep combustion products.2,10 In 1998, catalysts comprising 2− 5 nm Au particles on titania with H2 (as a coreactant) were discovered to have >90% PO selectivity with only 1−2% propylene conversion due to a low reaction temperature of ∼100 °C.11,12 The propylene conversion of this catalytic system was later improved to 6−10% by using mesoporous Ti-silicate (Ti-SiO2),13,14 microporous TS-1, MCM-41,15−17 or MCM© 2015 American Chemical Society

2. EXPERIMENTAL SECTION The Cu−Mn catalysts were prepared by the following combustion synthesis process. An aqueous solution comprising 1 M Cu2+ [Cu(NO3)2·2.5H2O), Alfar-Aesar, ACS] was thoroughly mixed with 1 M Mn3+ [(Mn(NO3)2·4H2O, AfarAesar, ACS] at various molar ratios (Cu/Mn = 0.0−1.0). Then this resulting solution was mixed with 2 M fuel (i.e., glycerol, maleic acid, citric acid, ethyl acetoacetate, or nitric acid) in a vial at a [NO3−]/[fuel] molar ratio of 2.0. The total concentration of Cu2+ and Mn3+ ions in each batch was kept constant at 0.8 mmol. Aqueous solutions of 1 M NaCl (Alfar-Aesar, ACS) were added into each mixture at several different concentrations (0.0−1.0 mol % of Cu2+ plus Mn3+ ions). The vial was then nested in a steel block and placed in a programmable oven. The Received: Revised: Accepted: Published: 2638

November 4, 2014 February 25, 2015 February 26, 2015 February 26, 2015 DOI: 10.1021/ie5043598 Ind. Eng. Chem. Res. 2015, 54, 2638−2645

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Figure 1. PO selectivity (left) and propylene conversion (right) at 250, 275, and 300 °C for Cu−Mn MMO catalysts synthesized using the glycerol method with a Cu/Mn ratio of 1:4 and calcined at 450 °C. Catalysts were prepared as undiluted samples (100% catalyst), 10% catalyst in 90% SiO2, and 1% catalyst in 99% SiO2.

Figure 2. Product selectivities (left) and the propylene conversions (right) of various Cu−Mn MMO catalysts synthesized using different fuels for combustion synthesis and calcined at 500 °C.

oven temperature was increased from 20 to 150 °C with a ramp rate of 10 °C/min and then held at 150 °C for 12 h, during which the solution underwent boiling and drying until a thick, viscous solution formed. The temperature was then increased at 1 °C/min for calcination/combustion to 400−700 °C and held for 6 h. While the temperature rose, the solution foamed and spontaneously underwent a self-propagating combustion reaction and a porous oxide network appeared inside the material. The synthesized catalysts were then uniformly ground and mixed with a diluent (silica power, Alfar-Aesar, −325 mesh). The catalysts were tested using a high-throughput multichannel reactor36 at 250−300 °C and 1 atm. For the initial catalyst screening results of Figures 1 and 2, the reactant gases contained C3H6 (Matheson, 99%)/O2 (Matheson, 99.9%)/He (Matheson, 99.99%) at 20/20/60 by volume. For the catalytic performance results of Figures 5, 7 and 10, the reactant gas ratio was changed to 10/30/60. Each channel held 5 mg of catalyst, and the reactant gas flow rate was set using mass flow controllers (MKS) such that the standard gas hourly space velocity (GHSV) was 20 000 h−1. Gas chromatography (Varian CP-4900 Micro GC with 5 Å molecular sieves and Porapak U columns) was used to analyze the reaction products. Product selectivities and propylene conversions were calculated on a carbon balance basis. The PO selectivity, propylene conversion, and PO yield were calculated using eqs 1 − 3, respectively:

% SPO =

x PO + xAC

% XC3H6 =

x PO × 100 + xAT + xAC + xCO2 /3

x PO + xAC + xAT + xAD + xCO2 /3 xC3H6,feed

% YPO = (% SPO)(XC3H6)

(1)

× 100 (2) (3)

where each x denotes a corresponding species mole fraction in the reactor effluent and xC3H6,feed is the mole fraction of C3H6 in the feed. The repeatability of all experiments was within ±10%. In general, a variety of catalysts in the same set were prepared in parallel to minimize uncertainty. GC calibrations for gaseous species (propylene, O2, CO2) were conducted with He as the carrier gas, and calibrations for liquid species (PO, AC, AT, AD) were performed by vaporizing the injected liquids of these species in a heated, evacuated 2250 cm3 stainless steel tank (He as carrier gas). By use of peak area as the basis for GC calculations, all calibrations yielded linear five-point curves with R2 ≥ 0.995.28 For imaging the catalytic materials, a JEOL JSM-6700F scanning electron microscope (SEM) was used with field emission gun using backscattered electron detector, operated at 10.0 kV and with a working distance of 2.5 mm to achieve adequate resolution. Powder X-ray diffraction (PXRD) was used to determine the effect of combustion synthesis fuels on catalyst crystallinity (PANalytical X’Pert PRO, Cu Kα emission 2639

DOI: 10.1021/ie5043598 Ind. Eng. Chem. Res. 2015, 54, 2638−2645

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Industrial & Engineering Chemistry Research 40 mA and 45 kV). To further characterize the material, transmission electron microscopy (TEM) was conducted by FEI CM120 operating at 120 kV. Moreover, BET (Brunauer− Emmett−Teller nitrogen sorption analysis) using a Micromeritics ASAP 2020 operating at 77 K (liquid nitrogen conditions) was also used to determine the surface areas of the samples.

3. RESULTS AND DISCUSSION The catalysts used for the initial screenings were synthesized with glycerol as fuel. The Cu/Mn atomic ratio of 1:4 (known as

Figure 5. Effect of using NaCl as the dopant on Cu−Mn MMO catalysts: 1:4 Cu/Mn ratio, calcined at 500 °C, with 0.0−1.0 mol % NaCl doping.

Figure 3. PXRD spectra for Cu−Mn MMO catalysts synthesized using various fuels and calcined at 500 °C. The fuel types are glycerol, maleic acid, citric acid, ethyl acetoacetate, and nitrate decomposition.

hopcalite) was previously found to be the most effective for combustion when the MMO catalyst is calcined at 450 °C.37 Therefore, this composition and calcination temperature were the first to be investigated because of the attractive propylene conversion. Moreover, to minimize the extent of the surface combustion and the resulting temperature increase, catalysts were diluted by mixing with SiO2 powder. The catalysts were tested as undiluted samples (100% catalyst), 10% catalyst in 90% SiO2, and 1% catalyst in 99% SiO2 by weight at reaction temperatures of 250, 275, and 300 °C. The total amount of the undiluted or diluted sample was 5 mg for each test. From

Figure 6. PXRD spectra of Cu−Mn MMO catalysts with 1:4 Cu/Mn ratio, calcined at 500 °C with 0.0−1.0 mol % NaCl doping.

Figure 1 the undiluted catalyst unsurprisingly shows substantial combustion activity, with a PO selectivity of 0.2 mol %), however, the PO selectivity decreases gradually. This is potentially because after all of the catalyst’s acidic sites are captured by Na (as NaCl), the remaining NaCl starts to form crystals, leading to a blockage of the active sites for creating PO molecules.31 It should be noted that the crystal sizes of Mn2O3 were unchanged at about 70 nm, indicating that the addition of

combustion, and because Cl has beneficial effects on PO selectivity due to geometric/ensemble and electronic effects, as well as gas-phase kinetic effects.4,31,32 Hence, in order to enhance PO yields, 0.1−1.0 mol % NaCl was added to the Cu−Mn MMO system with a Cu/Mn ratio of 1:4 and calcined at 500 °C. Figure 5 indicates that catalysts doped with 0.1−0.2 mol % NaCl have improved PO selectivities with slight decreases in propylene conversion (28−30% PO selectivity at 1.9−2.0% propylene conversion) compared to the undoped Cu−Mn MMO catalyst (21% PO selectivity at 2.4% propylene conversion). In contrast, doping with 0.4−1.0 mol % NaCl decreases both the PO selectivity and propylene conversion to 17−11% and 2.1−1.5%, respectively. The PXRD investigations of these materials, as shown in Figure 6, reveal corresponding crystallite sizes of 75, 69, 72, 72, 72, 75, and 69 nm for Mn2O3 and 32, 37, 38, 45, 70, 73, and 79 nm for Cu1+xMn2−xO4 at 0.0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 mol % NaCl, 2642

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shown, increasing the calcination temperature adversely affects both. Some of the samples were also investigated by TEM, as shown in Figure 9. In Figure 9A, the sample was composed of amorphous and crystalline domains. Fast Fourier transform (FFT) analysis (inset) confirmed that all the visible crystal lattice fringes in the image corresponded to the same planes in CuMn2O4 nanocrystals (d = 0.48 nm) with a BET average pore diameter of 19.2 nm. For the catalyst calcined at 700 °C in Figure 9B, it is apparent that higher calcination temperatures led to sintering, i.e., the particle size and pore size were larger than those of the sample in Figure 9A. Figure 9C shows a TEM image of the best performing catalyst in this set, Cu−Mn MMO catalyst doped with 0.2% NaCl using glycerol fuel and calcined at 550 °C. The BET surface area was 6.2 m2/g, and the pore diameter was 39.4 nm. Subsequently, a full spectrum of Cu/Mn ratios were reexplored using glycerol as the fuel and calcined at 550 °C, ranging from pure Mn2O3 to pure CuO and doped with 0.2 mol % NaCl. The results are shown in Figure 10 and the corresponding PXRD spectra are shown in Figure 11. Unimetallic catalysts of Mn2O3 (0% Cu) exhibited poor performance, consistent with the previous report.40 Remarkably, bimetallic samples in the 10−20% Cu range exhibited a large increase in PO selectivity (30−37%) and conversion (1.3−1.5%), corresponding to the best Cu−Mn ratio for Cu− Mn MMO synergy for PO production. The PXRD spectra at 10% Cu and 20% Cu show that the crystallite sizes of Mn2O3 are identical at 87 nm, and Cu1+xMn2−xO4 are 36 and 39 nm, respectively. The effect of the difference in Cu1+xMn2−xO4 crystallite size at 10% and 20% Cu on PO production, however, is relatively small compared to the effect of the difference in peak intensity for Mn2O3. This shows that the number of Mn2O3 crystals in the material of the 10% Cu sample is higher than it is in the 20% Cu sample, suggesting that the Mn2O3 crystals also play an important role for producing PO molecules. When Cu loading is increased to 30−50% Cu, PO selectivity decreases to ∼16% while propylene conversion gradually increases to 2.6%. Their PXRD spectra reveal that the peak intensities of Mn2O3 are lowered, whereas the peak intensities of Cu1+xMn2−xO4 become higher, while the crystallite sizes of Mn2O3 remains unchanged at 87 nm with slightly larger Cu1+xMn2−xO4 crystals (42, 46, and 55 nm, respectively). This indicates that the reduction of the Mn2O3 has a strong influence on the PO creation. With further increase of Cu loading (50−80% Cu), the CuO crystals start to appear at 2θ of 32.5°, 35.5°, and 38.5°. The higher Cu loading shows the more dominant peaks of CuO crystals and the lesser amount of Cu1+xMn2−xO4. This results in gradual decreases in PO selectivity and slight increases in propylene conversion. In this case, the clusters of CuO presumably disrupt the Cu−Mn MMO synergy and thus enhance the combustion of reactants and intermediates. At very high Cu loadings (90−100%), a small amount of PO is detected, similar to the individual CuO/ SiO2 catalyst previously reported.27,40

Figure 10. Performance results from testing various ratios of Cu−Mn MMO catalysts with 0.2% NaCl doping, synthesized using glycerol as fuel and calcined at 550 °C.

Figure 11. PXRD spectra of various Cu−Mn MMO catalysts with 0.2% NaCl doping, synthesized using glycerol as fuel and calcined at 550 °C.

NaCl has a significant influence on the growth of the Cu−Mn MMO crystals but not of the Mn2O3 crystals. The effects of calcination temperature on PO selectivity and propylene conversion are shown in Figure 7 (left) together with the BET surface area of the catalyst (Figure 7 right). The corresponding PXRD spectra are shown in Figure 8. The corresponding crystallite sizes of Mn2O3 are 69, 75, 83, and 94 nm and Cu1+xMn2−xO4 are 32, 36, 55, and 70 nm at calcination temperatures of 500, 550, 600, and 700 °C, respectively. Again, there was no evidence of NaCl crystals, which is anticipated because only 0.2 mol % NaCl was added. Higher calcination temperatures (600−700 °C), which created larger crystallite sizes for the Cu−Mn MMO catalysts (55−70 nm), resulted in slightly higher PO selectivities (35−46%). However, because of lower surface areas (6−4 m2/g), the propylene conversions (0.5−0.1%) were significantly lower. The highest PO yield in this set was obtained when calcining at 550 °C (32% PO selectivity, 1.5% propylene conversion). This phenomenon suggested that in order to increase propylene conversion at high PO selectivity, larger crystallite sizes of Cu−Mn MMO catalysts as well as larger surface area are required. However, as

4. CONCLUSION Propylene epoxidation catalysis was studied using Cu and Mn mixed metal oxide catalysts doped with NaCl. The catalytic materials were prepared by combustion synthesis using a variety of fuels. Catalysts characterized using TEM, SEM, PXRD, and BET indicated that the Cu−Mn MMO catalysts were essentially biphasic, consisting of Mn 2 O 3 and 2643

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Cu1+xMn2−xO4 crystal phases (0 < x < 1). Larger Cu−Mn MMO crystallites were determined to be better with regard to propylene epoxidation with glycerol as the combustion fuel. The optimal PO selectivity of 30−37% PO was achieved at 1.3−1.5% propylene conversion using a MMO catalyst with the following composition: Cu content between 10−20 mol % and NaCl loading at 0.1−0.6 mol % Na. The Mn2O3 and Cu1+xMn2−xO4 crystallite sizes and overall catalyst surface area were found to increase and decrease, respectively, as the calcination temperature was increased, with 550 °C providing an optimum for epoxidation performance. However, a larger crystallite size resulted in lower surface area for the overall catalyst and thus lower catalyst activity (i.e., lower propylene conversion).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+66) 02-561-4621. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.S. acknowledges the Kasetsart University Research and Development Institute (KURDI), the Thailand Research Fund (TRF), and the Commission on Higher Education (Grant TRG5780257) for financial support. M.K., B.Z., and D.N. acknowledge Department of Chemical and Bimolecular Engineering at UCLA for financial support.

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