18001
2007, 111, 18001-18003 Published on Web 11/09/2007
13C
Isotope Labeling Study of Propane Ammoxidation over M1 Phase Mo-V-Te-Nb-O Mixed Oxide Catalyst N. Raveendran Shiju,† Ramesh R. Kale,‡ Suri S. Iyer,‡ and Vadim V. Guliants*,† Department of Chemical and Materials Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0012, and Department of Chemistry, UniVersity of Cincinnati, Cincinnati, Ohio 45221 ReceiVed: September 19, 2007; In Final Form: October 25, 2007
Direct ammoxidation of propane to acrylonitrile has been investigated recently as an alternate method for the current process based on propene. The Mo-V-Te-Nb-O mixed oxide is the most promising catalyst at present for this reaction. The reaction mechanism of propane ammoxidation over this catalyst is still not investigated experimentally. In this work, we have tested the presence of C6 intermediates during the course of the reaction using 13C-labeled propane. The results show that a direct pathway for the ammoxidation of propane to acrylonitrile without skeletal rearrangement of the carbon backbone operates under realistic, steadystate conditions.
Introduction The development of highly active and selective catalysts for the one-step direct conversion of light alkanes to valuable intermediates is of prime importance in the area of heterogeneous catalysis. One of the important targets in this field is direct ammoxidation of propane for the production of acrylonitrile, a valuable intermediate for the manufacture of synthetic fibers, resins, and rubber. The current process is based on catalytic ammoxidation of propene.1 A process based on the direct ammoxidation of propane is highly attractive since propane is much cheaper and more abundant than propene.2-11 The most promising catalyst system reported thus far for direct propane ammoxidation is the Mo-V-Te-Nb-O mixed metal oxide system.12 This catalyst system is made up of two main phases, the so-called M1 and M2 possessing an orthorhombic and pseudohexagonal structures, respectively, with the M1 phase being the active and selective phase in propane ammoxidation.13-21 However, the mechanism of propane ammoxidation over this catalyst is yet to be understood, and this information is crucial for the development of this novel catalytic process. Previous studies of a related reaction, selective oxidation of propane, suggested a mechanism involving oxidative dehydrogenation of propane to propylene, followed by dimerization to C6 hydrocarbons.22-24 Selectivities up to 64% for 1,5- hexadiene have been reported in propene oxidation over the Bi-Zn-O catalyst.22 Hexadiene is reported as the dominant product also in propylene oxidation using polymer-supported heteropolyacids as catalysts.23 Several reaction mechanisms are proposed for the analogous partial oxidation reaction of n-butane to maleic acid, catalyzed by vanadium phosphates.25 Butenes, butadiene, and furan are reported as intermediates for this reaction. * To whom correspondence should be addressed. Phone: (513) 5560203. Fax: (513) 556-3473. E-mail:
[email protected]. † Department of Chemical and Materials Engineering, University of Cincinnati. ‡ Department of Chemistry, University of Cincinnati.
10.1021/jp0775496 CCC: $37.00
However, these compounds were detected under high vacuum in the temporal analysis of products (TAP) reactor or at low oxygen and high n-butane concentrations and at very low contact times, all of them far from typical fixed bed conditions. The use of 13C-labeled probe molecules in these previous studies was critical in elucidating the nature of hydrocarbon backbone transformations in these reactions. However, the mechanism of propane ammoxidation over the Mo-V-Te-Nb-O catalysts has not been yet probed with 13Clabeled propane in order to establish whether the propene intermediate undergoes dimerization or another rearrangement similar to that found in other catalytic mixed metal oxide systems. In light of the previous investigations,22-25 it is essential to study the mechanism of the direct ammoxidation of propane to acrylonitrile under realistic conditions to elucidate the nature of intermediates present during the reaction. It may be anticipated that the reaction may proceed either by conversion of propene to acrylonitrile directly or via oxidative dimerization of propene to hexadiene, -NH insertion, followed by its fragmentation to acrylonitrile (Scheme 1). We have investigated for the first time propane ammoxidation over an M1 phase MoV-Te-Nb-O catalyst employing 13C isotopically labeled propane and examined the reaction products by NMR spectroscopy. Experimental Section The orthorhombic (M1 phase) Mo-V-Te-Nb-O catalyst with the molar ratio of Mo/V/Te/Nb ) 1:0.3:0.17:0.12 was synthesized hydrothermally at 448 K for 48h. Ammonium molybdate (Alfa Aesar, U.S.A., 81-83% as MoO3), vanadyl sulfate (Alfa Aesar, 99.9%), telluric acid (Alfa Aesar, 99%), and niobium(V) oxalate hexahydrate (Alfa Aesar) were used as the sources of the respective elements. After hydrothermal synthesis, the materials obtained were filtered, washed, and dried at 353 K overnight. The catalyst was calcined under prepurified nitrogen flow (20 mL min-1) for 2 h at 873 K before use. © 2007 American Chemical Society
18002 J. Phys. Chem. C, Vol. 111, No. 49, 2007
Letters
SCHEME 1: Reaction Pathways Investigated for Propane Ammoxidation to Acrylonitrile
The phase purity and crystallinity of the catalyst were confirmed by powder X-ray diffraction (Siemens D500, Cu KR radiation), and the morphology was confirmed by SEM (Philips XL-30 Field Emission ESEM). The catalytic reaction was carried out in a fixed bed tubular flow reactor at atmospheric pressure using 0.2 g of the M1 catalyst diluted with 0.5 g of SiC. The feed was composed of C3H8, NH3, O2, and He in the molar ratio of 6:7:17:70 at the total flow rate of 20 mL/min. The products and unreacted reactants were collected in deuterated chloroform using a liquid N2 trap and analyzed using a Bruker 400 NMR. The 1-13Clabeled n-propane with 99% purity was obtained from Cambridge Isotope Laboratories. Results and Discussion The direct ammoxidation of propane to acrylonitrile was investigated in a fixed bed reactor over a Mo-V-Te-Nb-O M1 phase catalyst as described in detail in the Experimental Section. The catalyst, with the molar ratio of Mo/V/Te/Nb ) 1:0.3:0.17:0.12, was synthesized hydrothermally at 448 K for 48 h and calcined under prepurified nitrogen flow (20 mL min-1) for 2 h at 873 K. This procedure yielded a highly crystalline, pure M1 phase, as confirmed by powder X-ray diffraction, characterized by the peaks at 2θ ) 6.7, 7.8, 9.0, 22, 45, and 27° (Supporting Information, Figure S1). We found no evidence of the M2 phase, which is characterized by a major XRD peak at 2θ ) 28.2°, or other crystalline impurity phases. The M1 phase characteristically exhibited the presence of aggregates of cylinder-shaped, 250-300 nm long crystallites (SEM, Figure S2). The 1H NMR spectrum of the product collected after ammoxidation of unlabeled propane at 693 K revealed the presence of propene, acetonitrile, acetic acid, acrylonitrile, and unreacted propane (Figure 1b). Acrylonitrile was observed as the major product, along with acetonitrile and acetic acid as the minor products. The -CHd and dCH2 groups in acrylonitrile are characterized by the presence of peaks between 5.6 and 5.8 ppm and 6 and 6.3 ppm, respectively. The presence of propene in the collected product allowed us to assume that the first step was the oxidative dehydrogenation of propane. The 13C NMR spectrum of the product mixture showed the signals due to the -CH3 group of propane and the dCH2, dCH-, and -CN groups of acrylonitrile, the main product (Figure 2b).
To investigate the mechanism, the reaction was then conducted using selectively labeled propane (13CH3-12CH2-12CH3) as the oxidation substrate, under the same reaction conditions. The 13C NMR shows an intense peak of the -CH3 group of propane (chemical shift: 16.43 ppm) due to the labeling at the terminal carbon (Figure 2a). The peak at ∼77 ppm is due to that of the solvent, CDCl3. The signal of the dCH2 group of acrylonitrile is the most intense (chemical shift: 137.2 ppm), followed by that of -CN (116.9 ppm) and dCH- (107.7 ppm), as shown in Figure 2a. The ratio of the intensities of dCH2 and -CN was similar to that observed for acrylonitrile obtained after an ammoxidation reaction of unlabeled propane; however, the ratio of dCH2 and dCH- was much higher. The difference in intensities of these carbons is clearly visible from Figure 2, which shows that the 13C label is present at the terminal positions of the acrylonitrile product and not at the middle dCH- carbon. The presence of the 13C label at the dCH2 carbon was confirmed by the 1H NMR spectrum, in which the signal of this carbon exhibited splitting due to the interaction of 13C with the protons attached to it, while the signal of dCH- carbon was largely unaffected (Figure 1a). The dCH2 and -CH3 of propene were observed at 115.5 and 19.2 ppm (Figure 2a), respectively, while the signal due to the dCH- carbon (expected at 133 ppm) was absent, confirming the lack of any structural rearrangement during oxidative dehydrogenation of 1-labeled propane to propene. If the ammoxidation mechanism involved the coupling of the propene intermediate to form the C6 intermediate, such as hexadiene, at any of the reaction steps, its fragmentation to C3 products would have resulted in the presence of the 13C label at the middle carbon (Scheme 1). The low NMR signal intensities from the dCH- groups of acrylonitrile and propene indicated that the former is formed directly from the latter, without any involvement of a C6 intermediate. The experiments conducted at a lower temperature (653 K) confirmed these conclusions (Figures S3 and S4). Although the selectivity to propene was higher at this temperature, the pattern of 13C NMR signals from different propene carbons remained virtually unchanged, confirming the direct conversion of propene to acrylonitrile under the steady-state conditions of our experiment. The results indicate the absence of propene intermediate dimerization and skeletal rearrangements of the carbon backbone that result in 13C label scrambling during the conversion of propane to acrylonitrile. The proposed active and selective
Letters
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18003 In summary, the above results provided strong evidence, for the first time, for a direct ammoxidation pathway of propane to acrylonitrile over an M1 phase Mo-V-Te-Nb-O mixed metal oxide catalyst under realistic, steady-state conditions. Conclusions Selective ammoxidation of propane to acrylonitrile over a Mo-V-Te-Nb-O M1 phase mixed oxide was investigated using 13C-labeled propane as the reactant. The labeling pattern in the final products revealed that the presence of C6 intermediates such as hexadiene during the course of the reaction can be ruled out. Furthermore, a direct pathway without skeletal rearrangement of the carbon backbone operates under realistic, steady-state conditions.
Figure 1. 1H NMR spectra of the product collected after the ammoxidation reaction of 1-13C-labeled propane (a) and unlabeled propane (b). The expanded spectra of the acrylonitrile regions are given in insets e and f. The propene CH3 signals are expanded in c and d. Signals from protons of propane CH3 (H1), CH2 (H2), propene CH3 (H3), acetonitrile CH3 (H4), and acetic acid CH3 (H5) are also indicated.
Acknowledgment. The authors acknowledge the financial support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG0204ER15604. Supporting Information Available: Details of synthesis, characterization of catalyst, reaction procedure, and NMR results at 653 K. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 2. 13C NMR spectra of products collected after the ammoxidation reaction of 1-13C-labeled propane (a) and unlabeled propane (b). The expanded spectra of the acrylonitrile regions are given in the insets. The peak at ∼77 ppm is of the solvent, CDCl3.
surface of the M1 phase contains multivalent active sites characterized by their ordered arrangement within bonding distances to one another.4,5 A vanadium site can activate propane by the abstraction of a methylene hydrogen, as suggested for V-Sb-Ox and V-Mo-Ox catalysts previously.4 An adjacent Te site is proposed to abstract methyl-H of the propyl radical to form a propene molecule. This molecule is coordinated to a Mo site and loses an R-H to an adjacent Te site to form an allyl radical. The same Mo site is proposed to insert an NH in the allyl radical.4,5 The presence of these active sites on the surface of this catalyst is confirmed by X-ray photoelectron spectroscopy (Figure S5).
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