Reaction Mechanism for Ammonia Activation in the ... - ACS Publications

Subscriber access provided by University of Manitoba Libraries

Article

The Reaction Mechanism for Ammonia Activation in the Selective Ammoxidation of Propene on Bismuth Molybdates Sanja Pudar, and William A. Goddard III J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06224 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on November 24, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

The Reaction Mechanism for Ammonia Activation in the Selective Ammoxidation of Propene on Bismuth Molybdates Sanja Pudar and William A. Goddard, III* Materials and Process Simulation Center (139-74) California Institute of Technology, Pasadena, California 91125 USA * Email: [email protected] Abstract In this paper we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for various pathways of ammonia activation on bismuth molybdates, a process required for ammoxidation of propene to acrylonitrile. Using a Mo3O9 cluster to model the bulk surface, we examined the activation of ammonia by both fully oxidized (MoVI) and reduced (MoIV) molybdenum sites. Our results show that ammonia activation does not take place on the fully oxidized Mo(VI) sites. Here the net barriers for the first hydrogen transfer (∆E‡ = 44.6 kcal/mol, ∆G‡673K = 44.2 kcal/mol) and the second hydrogen transfer (∆E‡ = 54.5 kcal/mol, ∆G‡673K = 51.7 kcal/mol) are prohibitively high for the reaction temperature of 400° C. Instead, our calculations show that the reduced Mo(IV) surface sites are far more suitable for this process. Here, the calculated barrier for the first hydrogen transfer from a Mo(IV)-NH3 to an adjacent Mo(VI)=O is 18.2 kcal/mol (∆G‡673K = 15.4 kcal/mol). For the second hydrogen transfer step, we explored three pathways, and found that the H transfer from a Mo-NH2 to an adjacent Mo(V)OH to form water is more favorable (∆E‡ = 26.2 kcal/mol (∆G‡673K = 24.0 kcal/mol) than transfer to an adjacent Mo(VI)=O or Mo(V)=O group. These studies complement previous studies for activation and reaction of propene on these surfaces, completing the QM study into the fundamental mechanism. 1.0 Introduction Catalytic selective oxidation of lower olefins to unsaturated aldehydes and ammoxidation to nitriles are of major commercial importance, representing 25% of the chemicals used in the manufacture of industrial and consumer products. Among these catalytic processes, the ammoxidation of propene to acrylonitrile (Eq. 1) is one of the most important. Worldwide, about ten billion pounds of acrylonitrile are produced in this fashion every year. o

(1)

CH3CH=CH2 + 3/2O2 + NH3

400-460 C cat.

CH2 =CHCN (acrylonitrile) + 3H2O

In the early stages of this industry, acrylonitrile was produced from propene on simple bismuth and molybdenum oxide catalysts, but incorporation of other metals has significantly increased yield (to +80%). Even so, small improvements in the efficiency of the catalyst can have a major effect on environmental impact and energy requirements, making it important to understand the mechanism. The prototypical catalyst for this process is BiMoOx, where experiments suggest that the propene is activated by a BiOx site to produce allyl, which subsequently interacts with Mo=O sites to produce acrolein (CH2=CH-CH=O), or in the case of ammoxidation interacts with Mo=NH sites produced by exposure to NH3 to form acrylonitrile (CH2=CH-CN).

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

While the selective oxidation of propene to form acrolein is carried out at 320 °C, ammoxidation requires much higher temperature of 400-460 °C. The presence of ammonia is known to affect the oxidation of propene over Bi/Mo catalysts in several ways. The oxidation of propene to acrolein is high at 320 °C, but upon the addition of ammonia to the feed, the oxidation activity declines rapidly and selectivity reduces slightly. However, upon increasing the temperature to 400 °C, the addition of ammonia does not inhibit oxidation.1 It has been suggested that these results indicate that ammonia blocks the active site(s) via formation of coordinately saturated molybdenum centers at lower temperatures, while at temperatures higher than 400° C, ammonia can be activated or desorbed to form catalytically active, coordinately unsaturated, molybdenum sites. The first quantum mechanical calculations (Generalized Valence Bond, GVB) of the mechanism (Allison and Goddard2) using model metal oxide clusters identified that particularly important sites are Mo with two Mo=O bonds or in the case of ammoxidation, either two Mo=NH bonds or one each of Mo=O and Mo=NH. Allison and Goddard (1985) showed that reactions at these sites to form Mo-O-CH2CH=CH2 or Mo-NHCH2CH=CH2 were 10 to 15 kcal/mol more exothermic because the second (spectator) Mo=O or Mo=NH site could utilize the orbital previously in the double bond to form a partial triple bond (the spectator oxo or imido effect). This built on earlier ideas from Rappe and Goddard3,4 on the role of such sites in transition metal oxidations. These studies suggested that at least two sites each with two oxo groups were essential for producing acrolein and that three or four were required to produce acrylonitrile. These results were extended significantly in 2001 and 2002 by Jang and Goddard 5,6 using advances in Density Functional Theory (DFT) for much more accurate calculations on metal oxide clusters. These calculations reaffirmed the role of the spectator oxo and imido effect in promoting these reactions and provided good estimates for the barriers of the various reactionsteps. The overall mechanism was quite consistent with experiment7, showing how product formation depends on whether the catalytic sites were promoted to two Mo=NH groups or just one Mo=NH combined with a Mo=O, which explained the dependence on NH3 concentration. A third generation of studies was initiated by Pudar, Goddard, and coworkers8 in 2007 aimed at very systematic studies of the propene oxidation reaction barriers by applying more recent advances in DFT theory. These studies used the –MoO2-O-MoO2-O- cyclic cluster to obtain more accurate mechanistic results on acrolein formation. This work was extended to clusters suitable for ammoxidation in 2010, providing a detailed examination of the mechanism.9 Again the results remained consistent with the role of spectator effects in promoting the reactions. In this paper we report a detailed study of various pathways of ammonia activation over molybdenum oxide, which occurs prior to the ammoxidation of propene. We considered only Mo dioxo sites for ammonia activation (rather than bismuth sites) because the experimental evidence1,10,11,12 and our previous calculations8 show that the oxygen associated with a bismuth carries the task of activating propene to form the allyl intermediate, while the subsequent conversion of allyl to acrylonitrile occurs at molybdenum oxide sites. More recently13 we found in ReaxFF reactive dynamics studies of BiMoOx surfaces exposed to propene gas that the propene is activated by a M=O bond on a MoVI site coupled through an O to a Bi site. Similar


2

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


simulations on MoO3 do not see such propene activation, showing that Bi is essential but suggesting that it has an indirect effect on the oxo bonds of the adjacent Mo. 2. Technical Details 2.1 Theoretical methodology: All calculations were performed using the B3LYP flavor of the density functional theory (DFT), which combines exact HF exchange with the Becke generalized gradient exchange function14, and the Lee, Yang, and Parr correlation functional15 (LYP). Molybdenum and bismuth were described using the LACVP relativistic angular momentum projected effective core potentials (pseudopotentials) and basis sets of Hay and Wadt,16 which treat explicitly 14 valence electrons on molybdenum (the 4s,4p,4d,5s,5p shells). The O, C, N, and H atoms were described using the Pople 6-31G** basis set, including core and valence electrons. Energetics for open shell species were calculated using spin unrestricted DFT (UDFT) whereas restricted DFT (RDFT) was used for closed shell species. Many of the states involved have unpaired spins, in which case we optimized the Nα up-spin (α) and Nβ down-spin (β) orbitals independently (UDFT). The net spin projection MS = (NαNβ )/2 is loosely referred to as the spin so that Ms = 0 is called a singlet and Ms=1 is called a triplet. This is not strictly true since a determinant wavefunction with unrestricted orbitals is contaminated by the presence of higher spin states. A pure doublet state should have value of 0.75 and a quartet should have = 3.75. Geometries were fully optimized for each structure. The minima and saddle points were confirmed by diagonalizing the Hessian matrix and computing the vibrational frequencies. Each minimum had no imaginary frequencies, and each transition state was confirmed to be a firstorder saddle point (one imaginary frequency). The vibrational frequencies were used to calculate zero-point energy (ZPE) and enthalpies at 0K for each structure, and to calculate the enthalpy and entropy corrections to QM energy at the ammoxidation temperature, 673K. All calculations were performed using the Jaguar 6.5 program.17 2.2 Cluster models: In this work we use the cyclic Mo3O9 cluster (Figure 1) as a model to represent the molybdenum oxide part of the catalyst. In this cluster model, each molybdenum is coordinated to two terminal oxo oxygens and two bridging or ether oxygens, corresponding to oxo and ether bridging oxygens in the MoO3 crystal. We prefer the cyclic cluster models over open clusters since it avoids terminating the clusters with OH groups. O O

O

Mo 1.91 103.8 O

O

1.70

Mo

O

Mo

113.7

O

O

O Mo3O9


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

Figure 1. Cluster model used to represent the MoVI site in Mo3Ox-related clusters. The geometry was fully optimized. Selected bond lengths (Angstroms) and angles (degrees) are shown. The rationalization for using the Mo3O9 cluster to represent the surface Mo species is given in detail in our previous papers, where it was employed in mechanistic studies of propene oxidation8 and ammoxidation9 on a BiMoOx catalyst. We consider the Mo3O9 cluster to be the appropriate representation of active Mo sites since it has reactivity, stoichiometry and coordination similar to that found in both pure MoO3 and α-Bi2Mo3O12 catalysts. This cluster model was also used by Goddard and Jang in mechanistic studies of (amm)-oxidation of propene,5,6 and by Fu et al. to study methane and propane activation.18,19 3. Results This paper reports our investigations of the activation of ammonia on both MoVI and Mo surface sites. In the oxidized form of the catalyst, molybdenum is mostly in the 6+ oxidation state, but the reduced molybdenum sites (Mo IV) might be present due to possible defects on the catalyst surface, or as the result of the oxidation of propene to acrolein prior to the ammoxidation. In addition, reduced sites are formed in the process of forming acrylonitrile products. IV

3.1 Ammonia Activation on a Mo(VI) site The potential energy surface for activation of ammonia on a MoVI site of the Mo3O9 cluster is shown in Figure 2. This process involves the coordination of ammonia to a coordinately unsaturated Mo site, followed by transfer of two hydrogen atoms from ammonia to an adjacent oxo group on the same molybdenum site. The energy cost for activating one molecule of NH3 and releasing one molecule of H2O, to convert a Mo=O group to a Mo=NH, is 18.4 kcal/mol (∆G673K = 18.5 kcal/mol). VI

The first step in this process is the coordination of ammonia to a coordinately unsaturated molybdenum center to form a coordination complex 2. We calculate that this process is exothermic by ∆E =-27.5 kcal/mol (the electronic binding energy from the DFT, without correcting for zero point energy), reflecting the strong binding of ammonia with the acidic MoVI. At 673K ammonia adsorption is still favorable on the free energy surface (∆G673K = -1.5 kcal/mol) despite the high entropic cost of associating ammonia with the surface. This site has a Mo-NH3 donor acceptor bond of 2.32Å, which causes the bridging oxygen trans to the NH3 to elongate from 1.91 Å to 2.00 Å, (a bond order change of ~1/2), in response to the ammonia coordination. The next step in ammonia activation is the first hydrogen transfer from the NH3 to an Mo=O group on the same molybdenum site to give 3. The net ∆E barrier for this step through the transition state TS1 is 44.6 kcal/mol (∆G‡ 673K = 44.2 kcal/mol), which we consider prohibitively high at the ammoxidation temperature of 400 °C. The TS1 involves a four-member ring, where all four atoms (Mo, N, H, and O) lie almost in the same plane (dihedral angle is 171 degrees) (see


4

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3). O

60 O O O Mo

NH3

45

O

+ O

30

O

O

Mo

15

H

HO

O Mo

O

H O

NH2

O

O Mo O O

NH

O Mo O

H2O +

TS2 27.0 (25.4) [50.5]

17.1 (16.5) [42.6]

O Mo

1

Mo O

TS1

O Mo O O

O

O

O O Mo

O

O Mo O O

O Mo HN

18.4 (16.8)

[0.0]

[18.5]

4

3 -15

-30

-45

0.6

-2.9 (-4.6) [22.2] O

O

5

0.0 (0.0)

0

O

O Mo

(1.6) [25.2] O O O Mo O

O

O

2 O O Mo O O Mo -27.5 Mo O O O (-24.5) NH3

O O Mo HO

NH2

O Mo O O

O Mo O

O Mo H2O

NH

O

O Mo O

[-1.5]

-60

Figure 2. Singlet potential energy surface for ammonia activation on a MoVI site. The top energy parameter is the ∆E from QM, the middle is ∆H0K = ∆E+∆ZPE, and the bottom is ∆G673K. All reported values are in kcal/mol. Note that NH3 is placed below the Mo-O-Mo plane for the purpose of clearer representation of all atoms. Here the O-H distance is 1.11 Å and the respective N-H distance is 1.44 Å, indicating that the hydrogen transfer is almost complete. The Mo-NH3 distance decreases from 2.32 Å to 2.19 Å, and the Mo=O distance elongates from 1.71 Å to 1.83 Å (typical terminal Mo-OX bond is ~2.0 Å). The transition state eigenvector corresponding to the imaginary frequency ν=1090i cm-1 represents the hydrogen transfer from nitrogen to oxygen in the four-member ring. The value obtained using the UDFT method is 0.000.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.44

Page 6 of 23

1.13

1.11 1.29

1.22

1.11

1.39

1.33

1.83

2.19

2.08

2.15

1.87

1.85

TS1

TS2

TS3

Figure 3. Structural parameters for transition states of ammonia activation on a Mo(VI) site. Bond lengths shown in angstroms. Our calculated barrier for the first hydrogen transfer suggests that ammonia is not activated on fully oxidized MoVI sites. Therefore, we consider that the presence of a surface defect or reduced molybdenum sites might play an important role in the process of ammonia activation. Later in this section we explore NH3 activation on reduced MoIV sites (which could be generated by propene conversion to acrolein) and find this process quite favorable. The first hydrogen transfer results in the closed shell intermediate 3 at ∆E = -2.9 kcal/mol (G673K = 22.2 kcal/mol). The active Mo center is now penta-coordinated with one terminal oxo, two bridging oxygens, one OH, and one NH2 group in a trigonal bipyramidal conformation. The Mo-NH2 bond distance is 1.96Å, and the Mo-OH distance is 1.91 Å. The two bridging oxygen distances to the penta-coordinated Mo are both 1.97 Å, which is 0.06 Å longer than in the Mo 3O9 structure. The next step in the ammonia activation is the transfer of second hydrogen from NH2 to OH on the same molybdenum center to give complex 4. The net ∆E‡ barrier through the transition state TS2 is 54.5 kcal/mol (∆G‡673K = 51.7 kcal/mol), which is even higher than the barrier for the first H transfer (TS1). The TS2 is a four-center transition state, where all four atoms, Mo, N, H, and O lie almost in the same plane (dihedral angle is 172 degrees). The O-H distance is 1.22 Å and the corresponding N-H distance is 1.29 Å, indicating that TS2 is a symmetrical transition state, with the transferring hydrogen positioned almost exactly at the half of N-O distance. The Mo-OH distance elongates from 1.91 Å to 2.15 Å, and the Mo-NH2 distance decreases from 1.96Å to 1.87 Å. The transition state eigenvector corresponding to the imaginary frequency ν=1549i cm-1 represents the hydrogen transfer from NH2 to OH in the four-member ring. Calculated is 0.000, confirming that the TS2 is a closed shell species. The second hydrogen transfer results in the closed shell complex 4, where ammonia is converted to the Mo=NH (imido) group, while the terminal oxygen is condensed into H2O, which coordinates to molybdenum with the oxygen lone pair. This complex is 24.6 kcal/mol less stable than the relevant complex 2 due to the stronger effect of a spectator oxo group (found in 2) than that of a spectator imido group (4), and due to a stronger donor-acceptor coordination of


6

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ammonia with the molybdenum in 2 as compared to the Mo-O2H interaction in 4. This spectator oxo stabilization was discovered by Allison and Goddard2 in earlier studies on these systems showing that the presence of M=X (X = O or NH) significantly promotes a reaction at the adjacent double-bonded group. Allison and Goddard estimated that the stabilizing effect of a spectator oxo group is 15 kcal/mol stronger than that of a spectator imido group. The same effect was observed in our recent study of ammoxidation of propene where spectator imido groups are indirectly involved in decreasing hydrogen abstraction barriers via destabilization of a respective reactant as compared to the spectator oxo group.9 These more accurate calculations indicate that the spectator oxo effect is 14.8 kcal/mol, corresponding closely to the older values (15 kcal/mol). All molybdenum atoms in 4 are in the oxidation state MoVI. The Mo=NH distance is 1.75 Å, and the Mo-OH2 donor-acceptor bond distance is 2.34 Å. Binding of H2O to ammonia-activating MoVI center is significant, where the cost of desorption of water is 17.8 kcal/mol relative to 4, resulting in Mo3O8(NH) (5) and free H2O. The same water desorption is exothermic by 6.7 kcal/mol on the free energy surface due to the entropic effects. Finding that the ∆G barrier for the TS2 is prohibitively high even at the ammoxidation temperature, prompted us to consider other pathways for this step, such as a role of a single water molecule in the TS2, as well as the H transfer to another oxo group on the same molybdenum site. Energetics for both pathways are shown in Scheme 1. 3.11 Alternative pathways for the second H transfer Water assisted 2nd H transfer. We find that presence of an additional water molecule makes the second hydrogen transfer slightly more favorable, but does not lower the barrier enough to make this step feasible at the reaction temperature (Scheme 1). Introducing water to 3 resulted in H2O coordination to molybdenum (6) with ∆E = -26.6 kcal/mol (∆G673K = 29.0 kcal/mol). The Mo-OH2 distance is 2.45 Å. We could not locate an intermediate that does not involve Mo-OH2 interaction, since all such attempts resulted in formation of 6. The H2O-assisted second hydrogen transfer to the Mo-OH group occurs through the six-member transition state TS3, with net calculated barrier ∆E‡ = 23.4 kcal/mol (∆G‡673K = 47.6 kcal/mol). This step seems plausible on the ∆E surface, however the entropic cost of associating the ammonia and the water with the surface leads to a prohibitively high net barrier on the free energy surface (∆G‡673K = 47.6 kcal/mol).


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

Scheme 1. Alternative pathways for the second hydrogen transfer. The lower pathway involves water-assisted second H transfer to the Mo-OH group, while the upper pathway involves hydrogen transfer to the Mo=O group. Neither pathway is energetically favorable. The top energy parameter is the ∆E from QM, the middle is ∆H0K = ∆E+∆ZPE, and the bottom is ∆G673K. All reported values are in kcal/mol. In TS3, all atoms in the six-member ring lay almost in the same plane . The H2O oxygen is coordinated to three hydrogens, one points out of the plane of the ring with R(O-H) = 0.97 Å , and the other two are in the plane of the ring with O-H distances of 1.13 Å and 1.11 Å. The MoOH and Mo-NH distances are 2.08 Å and 1.85 Å respectively, and the corresponding O-H and N-H distances with hydrogen atoms on water moiety are 1.33 Å and 1.39 Å, respectively. The transition state eigenvector corresponding to the imaginary frequency ν=980i cm-1 represents the hydrogen transfer between Mo-NH2 and Mo-OH groups through H2O oxygen. TS3 leads to the intermediate 7 at ∆E = -14.8 kcal/mol (∆G673K = 32.5 kcal/mol), with Mo=NH and two water molecules, one coordinated to molybdenum with Mo-OH2 distance of 2.70 Å, while the other hydrogen-bonds to the Mo-OH2 water with the H2O-(H2O-Mo) distance of 1.63 Å. The energy cost for dissociation of loosely-bound water to give back 4 is ∆∆E = 15.4 kcal/mol (∆∆G673K = 7.3 kcal/mol). We located another isomer of 7 with ∆E = -13.4 kcal/mol


8

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(∆G673K = 36.7 kcal/mol), where both water molecules are coordinated to the molybdenum center. Second H transfer to the Mo=O. We also explored an alternative pathway for the 2nd H transfer that involves H transfer from the Mo-NH2 to an oxo group on the same molybdenum site, yielding an intermediate 8 with MoVI(OH)2 (Scheme 1). We find this pathway even less energetically accessible than the pathway involving TS2, as the calculated barrier through the transition state TS4 is 78.6 kcal/mol (∆G‡673K = 79.9 kcal/mol). Here, the higher barrier might be due to a loss of yet another stabilizing Mo=O bond. Likewise, the product of this step (8) is 10.8 kcal/mol (∆∆G673K = 12.8 kcal/mol) higher than the corresponding intermediate 4. Our calculations show that the fully oxidized surface does not activate ammonia, since the barriers for hydrogen transfers in this process are prohibitively high (∆E‡ = 44.6 kcal/mol and ∆E‡ = 54.5 kcal/mol for first and second hydrogen transfer, respectively). Thus, we suggest that surface defects or presence of reduced active sites might play an important role in the activation of ammonia. Below we describe several pathways for ammonia activation on a reduced MoIV site. 3.2

Ammonia Activation on a Mo(IV) site

We suspect that a number of Mo IV sites will be present on the catalyst surface due to possible surface defects and as a result of the reduction of catalyst in the process of oxidation and ammoxidation. It is also likely that reduced Mo sites will be present on the surface as a result of oxidation that might occur prior to ammonia activation at 400 °C, as the experiment shows that oxidative activity of catalyst is restored once temperature reaches 400 °C. Jang and Goddard previously reported that the ammonia activation on reduced molybdenum sites is energetically more favorable due to the reduced energy cost of individual steps compared to activation on fully oxidized sites.5 In the current study, we used the Mo3O8 cluster model which has one vacant MoIV and two MoVI sites. This process involves the coordination of ammonia to a vacant Mo IV site, and transfer of two hydrogen atoms from ammonia to an oxo group(s) of an adjacent Mo VI site. We explored three different pathways for NH3 activation on MoIV leading to formation of Mo=NH group and different terminal oxygen-based surface species depending on the nature of hydrogen accepting group. The potential energy surface for the most favorable pathway is shown in Figure 4, while the energetics for the three possible pathways are shown in Scheme 3. Ammonia activation was studied on a triplet potential energy surface, since the lowest electronic state for the starting species Mo3O8 is a triplet, while open shell singlet is calculated to be 5.9 kcal/mol less stable. Triplet and open shell singlet states are similar in energy for most of the remaining species on the reaction pathway. Path 1. The Path 1 (Figure 4) involves condensation of Mo=O to H2O as a result of transferring two hydrogens from ammonia to the same oxo group of an adjacent molybdenum site. We find the Path 1 to be the most energetically favorable pathway among the three different mechanisms studied here. The energy cost for activating one NH3, to create one Mo=NH group and release one molecule of H2O is 20.0 kcal/mol (∆G673K = 20.3 kcal/mol). However, by including the energy of NH3 or O2 assisted water desorption, the overall reaction energy becomes


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

∆E = -15.6 kcal/mol (∆G673K = 16.2 kcal/mol) or ∆E = -28.2 kcal/mol (∆G673K = 2.4 kcal/mol) respectively, which suggests that the reoxidation plays an important role in the process. Coordination of ammonia to a vacant MoIV site of Mo3O8 forms a coordination complex 10, which is strongly exothermic by ∆E = -38.6 kcal/mol (∆G673K = -4.8 kcal/mol), as expected since the Mo IV is strongly acidic. The Mo-NH3 distance is 2.28Å, and there is a hydrogen bonding between one hydrogen on NH3 and an oxo group on the adjacent molybdenum, with NH3-O distance of 2.13 Å. The next step in the ammonia activation is the first hydrogen transfer from the NH3 to an oxo group on the adjacent Mo site, resulting in intermediate 11. The net barrier for this step through the transition state TS5 is 18.2 kcal/mol (∆G‡673K = 15.4 kcal/mol), and thus we expect this step to be very fast. This indicates that the ammonia activation is significantly faster in the presence of reduced surface sites. In the TS5, the breaking N-H bond distance is 1.36 Å, and the corresponding O-H distance is 1.18 Å. The Mo=O bond elongates from 1.73 Å to 1.82 Å, and the Mo-NH3 shortens from 2.28 Å to 2.09 Å. This structure forces the respective Mo-Mo distance to shorten from 3.46 Å to 3.36 Å, in order for hydrogen transfer to take place. The transition state eigenvector corresponding to the imaginary frequency ν=1079i cm-1 represents the hydrogen transfer from nitrogen to an oxygen on neighboring Mo site. The first hydrogen transfer results in 11, which has MoV-NH2 and MoV-OH centers. This step is exothermic with ∆E= -22.1 kcal/mol (∆G673K = 6.7 kcal/mol) with respect to 9. The structure of 11 is very similar to that of TS5, but with the Mo-NH2 distance decreased to 1.99Å and MoOH distance increased to 1.88 Å. The Mo-Mo distance relaxes to 3.40 Å, and there is a hydrogen bonding between hydrogen on the OH group and a lone pair on NH2 nitrogen, with the H-N distance of 2.11 Å.


10

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

H2O

13 30 O

20

Mo O

O Mo O O

O Mo O

+ NH3

0.0 (0.0) [0.0]

H2 N Mo O

0

O

H Mo

O O

O

Mo

+

-10.6 (-11.6) [19.2]

O

TS5

O O HN Mo

-20 11

-40 H3 N

-50

Mo O

O O Mo O O O

10 O Mo O

-36.8 (-33.4) [-4.8]

H2 N Mo O

O

O O Mo O O

HN

OH

Mo O

Mo O

O

O O Mo O O

O

-15.6 (-13.8) [16.2]

OH2 Mo O

14 O O -40.2 H O HN O Mo O2 NH3 (-35.7) Mo Mo O [16.0] O

Figure 4. Triplet potential energy surface for the most energetically favorable pathway (Path 1) for ammonia activation on Mo(IV). The highest barrier is ∆E‡ = 26.2 kcal/mol (∆G‡673K = 24.0 kcal/mol), making this process rapid. The top energy parameter is the ∆E from QM, the middle is ∆H0K = ∆E+∆ZPE, and the bottom is ∆G673K. All reported values are in kcal/mol. The next step in the Path 1 mechanism is a second hydrogen transfer from the MoV-NH2 to the MoV-OH, which results in formation of a Mo=NH group and one H2O molecule (12). The net barrier for this step through the transition state TS6 is 26.2 kcal/mol (∆G‡673K = 24.0 kcal/mol) with respect to 10, which is the highest barrier on this pathway, but similarly to the first hydrogen transfer, this step is expected to be very fast. The broken N-H distance is 1.37 Å, and the corresponding O-H distance is 1.18 Å. The Mo-OH elongates from 1.88 Å to 2.11 Å, and Mo-NH2 shortens from 1.99 Å to 1.84 Å, indicating a late transition state. There is a hydrogen bond between the OH hydrogen and an oxo group on the neighboring Mo VI site, with the distance of 2.28 Å. The transition state Eigenvector corresponding to the imaginary frequency ν=928i cm-1 represents the hydrogen transfer from NH2 to the OH group on the neighboring Mo site. This transition state leads to the intermediate 12 at ∆E= -12.3 kcal/mol (∆G673K = 19.2 kcal/mol), where ammonia is converted to an imido group, MoVI=NH, and the resulting H2O coordinates to the unsaturated MoIV site. This intermediate has similar structure to that of TS6, except that the Mo-OH2 distance increases to 2.21 Å, and Mo=NH decreases to 1.80 Å. There are two hydrogen bonds involving H2O hydrogens; one with the Mo=NH nitrogen lone pair at the distance of 1.78 Å, and the other with an oxo group from another adjacent Mo site, with the 1.92 Å.


Mo O

O

-60

NH3

15

-12.3 (-10.7) [19.2]

-22.1 (-20.6) [6.7]

O Mo O

O

12

-30

O

H2O

TS6

-18.6 (-19.1) [10.6]

-10

O

O H OH HN Mo O O Mo Mo O O O

9 10

+

20.0 O O (18.3) HN Mo O O [20.3] Mo Mo

O

11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

1.18 1.19 1.36 6

1.18 6

2.09

1.82

2.11

1.84

1.85

TS7

TS6

TS5

1.82

1.33

1.37

1.26 1.85

1.25 1.81

1.82 77.8 ̊ 1.82

1.27

1.28

TS8a

TS9

1.46 1.77

1.78 1.46

TS8b

Figure 5. Structural parameters for the transition states for ammonia activation on a Mo(IV) site. Bond lengths shown in angstroms.


12

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dissociation of H2O from 12, leading to the intermediate Mo3O7(NH) (13) with one vacant Mo and two MoVI sites, is endothermic by ∆E = 32.3 kcal/mol (∆G673K = 1.1 kcal/mol), pointing to the strong donor-acceptor coordination of oxygen lone pair with highly acidic MoIV. Desorption of H2O from the true catalyst surface is likely to be more feasible since it might couple with a second ammonia activation, or a re-oxidation process via lattice oxygen migration and vacancy filling with gaseous O2. These processes would prevent formation of the unfavorable MoIV vacant site. IV

Given that the calculated energy cost for this step from the Mo3O7(OH2)(NH) is significantly higher than the activation barriers for the conversion of ammonia to an imido group, we explored the associative mechanisms where water dissociation is assisted by coordination of an additional NH3 molecule or a gaseous O2 to the reduced site. Although it is generally assumed that a dioxygen dissociates at a site other than the catalytically active site, we considered the consequence of allowing an O2 to coordinate with the reduced MoIV in 12 (prior to the desorption of water). We previously used this concept in a study of propene oxidation on bismuth molybdates, and found that coordination of an O2 significantly assisted desorption of final product (acrolein)8. Also, Cheng and Goddard,20 found a similar effect in our previous study of O2-assisted propane conversion to propene over the cyclic V4O10 cluster model. A similar idea was used in a recent study of CH4 activation on Si-supported Mo=O by Chempath and Bell.21 We find that the coordination of NH3 or O2 promotes water desorption quite significantly, by destabilizing the binding of water and avoiding formation of a vacant Mo(IV) site. The coordination of NH3 to 12 to form intermediate 14 has a calculated ∆E = -40.2 kcal/mol (∆G673K = 16.0 kcal/mol, Figure 4). The MoIV-NH3 distance is 2.33Å, which is 0.05Å longer than the MoIV-NH3 in 10, and the Mo IV-H2O distance elongates from 2.21Å to 2.31Å, indicating weakened donor-acceptor coordination. Desorption of H2O from 14 is endothermic by 24.6 kcal/mol (∆G673K = 0.2 kcal/mol), which is 7.7 kcal/mol lower than the desorption from 12. However, the H2O desorption from 14 is exothermic by ∆E = -15.6 kcal/mol (∆G673K = 16.2 kcal/mol) with respect to reactants (9), therefore we expect this step to be more favorable in presence of free ammonia. These results suggest that re-oxidation of the reduced sites prior to the H2O desorption improves ammonia activation process. Coordination of O2 to the MoIV-OH2 site has even more dramatic effect on the water desorption energetics (Scheme 2). Coordination of 3O2 to 12 to form a triplet cyclic peroxyl species (21t), has a calculated ∆E= -37.4 kcal/mol (∆G673K = 19.4 kcal/mol). The Mo-OH2 distance in 21t is 2.50 Å, which is 0.29 Å longer than in 12, indicating significantly weakened donor-acceptor coordination (Figure 6). Both Mo-O distances in the cyclic MoO2 moiety are 2.11 Å, which is 0.20 Å longer than the typical single covalent Mo-O bond in the Mo3Ox-type cluster, indicating more of a donor -acceptor coordination rather than formation of single covalent bonds. The O-O bond length of 1.31 Å represents a bond order of 3/2, as in the case of HO2 radical where O-O distance is 1.33 Å. The closed shell singlet state for this species (21s) is 11.2 kcal/mol more stable than the triplet state (21t). In 21s, H2O is no longer coordinated to molybdenum but sits in the middle of the ring, with a distance to either molybdenum of approximately 2.7 Å. Two Mo-O distances in the cyclic MoO2 moiety of 21s are 1.92 Å and 1.96 Å, and the O-O distance of 1.46 Å, suggesting a single covalent O-O bond and two single covalent Mo-O bonds.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

Scheme 2. Dioxygen-assisted H2O desorption in ammonia activation on MoIV (Path 1). The top energy parameter is the ∆E from QM, the middle is ∆H0K = ∆E+∆ZPE, and the bottom is ∆G673K. All reported values are in kcal/mol. Desorption of water from 21s to form closed shell cyclic peroxyl species 22s is endothermic by 20.4 kcal/mol (∆∆G673K = -8.2 kcal/mol), but slightly more favorable than the desorption from the Mo-NH3 site in 14 (∆E = 24.6 kcal/mol), and ∆E = 11.9 kcal/mol more favorable than the desorption from 12. In addition, the H2O desorption from 21s is 28.2 kcal/mol (∆G673K = 2.4 kcal/mol) exothermic with respect to reactants (9), making this process energetically feasible in presence of a re-oxidant such as lattice oxygens. A triplet state of this cyclic peroxyl species


14

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22t) is 13.2 kcal/mol (∆∆G673K = 9.3 kcal/mol) higher in energy than the closed shell analog 22s. The species 22s can be described as a product of binding triplet O2 to the triplet state of the Mo3O7(NH) (13) formed by removing one terminal oxygen from Mo3O8(NH). The two Mo-O distances in MoO2 ring are 1.90 Å and 1.94 Å and the O-O bond distance is 1.45 Å, suggesting a single covalent O-O bond and two single covalent Mo-O bonds. We also located another isomer of species 22s that has an “open” peroxyl structure, with a shorter Mo-O distance of 1.83 Å and the O-O bond distance of 1.29 Å. The closed shell state of this isomer is 26.6 kcal/mol (∆∆G673K = 24.4 kcal/mol) higher than the closed shell 22s, while the triplet version of this conformer is not stable and converts to the cyclic peroxyl structure upon optimization.

H HO

O

O O

2.11

H N

2.50 2.11

Mo

Mo

O O

O

Mo

O O

O

21t Figure 6. Structure of the intermediate species (21t) in the di-oxygen assisted H2O desorption in ammonia activation on MoIV. Bond lengths shown in angstroms. 3.21 Alternative pathways for the 2nd H transfer We also explored two other pathways for ammonia activation, Path 2 and Path 3, which start from the intermediate 11 in the Path 1 and lead to different surface O-based species depending on the nature of the second hydrogen accepting group. The energetics for both pathways are shown in Scheme 3. Path 2. The Path 2 is the second most energetically favorable pathway of the three mechanisms of ammonia activation on MoIV studied here. It involves the transfer of second hydrogen from the NH2 to an oxo group on the neighboring MoVI site, to produce the intermediate 14 with two MoV-OH sites and one MoVI=NH site. The net ∆E barrier for this step through the transition state TS7 is 33.7 kcal/mol (∆G‡673K = 27.3 kcal/mol), which is 7.5 kcal/mol higher than the corresponding TS6 barrier on the Path 1. However, we expect that this step is still feasible at the ammoxidation temperature. In the TS7, the broken N-H distance is 1.33 Å and the corresponding O-H distance is 1.19 Å. The Mo-NH2 shortens from 1.99Å to 1.85 Å and the Mo=O elongates from 1.70 Å to 1.82 Å, and the Mo-Mo distance of the two active centers shortens from 3.40 Å to 3.29 Å. The transition


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

state eigenvector corresponding to the imaginary frequency ν=1403i cm-1 represents the hydrogen transfer from NH2 to an oxo group on the neighboring MoVI site.

Scheme 3. Various pathways for ammonia activation on Mo IV site. Path 1 is energetically most favorable. The top energy parameter is the ∆E from QM, the middle is ∆H0K = ∆E+∆ZPE, and the bottom is ∆G673K. All reported values are in kcal/mol. This transition state leads to the intermediate 16 at ∆E= -9.3 kcal/mol (∆G673K = 15.5 kcal/mol), where ammonia activation is completed with formation of MoVI=NH and two MoVOH groups. The Mo=NH, and two Mo-OH distances in 16 are 1.79 Å and 1.90 Å respectively, and there is a hydrogen bonding between hydrogen on one of the Mo-OH groups and the lone pair on Mo=NH nitrogen with the distance of 2.27 Å. The MoV-OH groups can further participate in an activation of another ammonia coordinated to the neighboring surface Mo site, or re-oxidized to give MoVI=O sites. Both of processes have a good chance of taking place,


16

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


considering that the activation energy for reoxidation is in 8-27 kcal/mol range22, and the calculated barrier for the first hydrogen transfer from a MoIV-NH3 is 18.2 kcal/mol. Path 3. The Path 3 is the least energetically favorable pathway of the three mechanisms of ammonia activation on MoIV studied here. It involves the transfer of the Mo-NH2 second hydrogen to an oxo group on the neighboring MoV-OH site, to produce the intermediate 20 with MoVI=NH and MoIV-(OH)2 sites. As the NH2 hydrogen and the oxo group on the neighboring MoV-OH site are positioned at a large distance from each other (5.82 Å) for the hydrogen transfer to take place, we considered a mechanism where the H from the MoV-OH first migrates to the oxo group on the same Mo site to form 19. This step is then followed by the second H transfer to a newly created oxo on the MoV-OH. The net ∆E barrier for H migration from MoV-OH to the oxo on the same Mo site through TS8a is 60.6 kcal/mol (∆G‡673K = 53.1 kcal/mol). This step is prohibitively high at 400 °C. The TS8a involves a symmetrical four-member ring, where the two Mo-O distances are 1.82 Å and the respective O-H distances are 1.27Å and 1.28 Å.Below we explore an H2O-assisted hydrogen migration to determine an effect of H2O on the barrier of this step. Water-assisted H migration from Mo-OH to Mo=O on the same Mo site. We find that the role of H2O in the second H transfer on the Path 3 is very important (Scheme 4). The ∆E barrier for hydrogen migration from the Mo-OH to the Mo=O through the transition state TS8b decreases to 16.2 kcal/mol (∆G‡673K = 34.4 kcal/mol with respect to 19). We expect that this step will occur in the presence of water molecules on the catalyst surface. Water is likely to be present on the surface as it is a by-product of ammonia activation on MoVI or MoIV sites, oxidation and ammoxidation processes, and is a part of the feed mixture.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

Scheme 4. Water-assisted hydrogen migration from a Mo-OH to an oxo group on the same Mo center prior to the second hydrogen transfer in the Path 3 of ammonia activation on a Mo(IV) site. The top energy parameter is the ∆E from QM, the middle is ∆H0K = ∆E+∆ZPE, and the bottom is ∆G673K. All reported values are in kcal/mol. We did not explore a potential catalytic role of H2O in earlier steps of this mechanism, since it was not required, however since it appears to be crucial in the second hydrogen transfer of Path 3 mechanism, we included water only in this step. We introduced water molecule to 11 to give 19, which is found to be exothermic by ∆E= 15.5 kcal/mol (∆G673K = 1.6 kcal/mol). In 19, lone pair on H2O oxygen hydrogen-bonds to the hydrogen of MoV-OH group, with a distance of 1.71Å, and the OH group is no longer interacting with MoV-NH2 as in 11. The TS8b has a six-member structure, where all six atoms (Mo, O, H, O, H, and O) lie in the same plane. The six-member ring is almost symmetrical, where H2O oxygen is coordinated to three hydrogens, one is pointing out of the plane of the ring with R(OH)=0.97 Å , and the other two are in the plane of the ring with O-H distance of 1.08 Å. The two Mo-O distances are 1.78 Å and 1.77 Å, and the corresponding O-H distances with hydrogen atoms on water moiety are both 1.46 Å. The transition state Eigenvector corresponding to the imaginary frequency ν=634i cm-1 represents the hydrogen transfer between two Mo=O groups through H2O oxygen.


18

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This transition state leads to the intermediate 20 at ∆E = -37.4 kcal/mol (∆G673K = 7.5 kcal/mol). This structure involves hydrogen bond between an oxygen lone pair on the H2O and the hydrogen on the new MoV-OH, with the distance of 1.68Å. Since we do not study a role of water in later steps of the Path 3, the rest of the mechanism continues from the intermediate 17. This species is a result of a removal of the H2O from 20, with the energy cost of ∆E = 12.5 kcal/mol (∆G673K = 7.5 kcal/mol). The next step in Path 3 is the second hydrogen transfer from the MoV-NH2 to the terminal oxo on the MoV-OH site. The net ∆E barrier for this step through the transition state TS9 is 35.0 kcal/mol (∆G‡673K = 31.2 kcal/mol). The TS9 is slightly more symmetric than other transition states for this step, where the distance of breaking N-H bond is 1.26 Å and the corresponding OH distance is 1.25 Å. The Mo-NH2 and Mo=O distances are 1.85 Å and 1.81 Å respectively, and the Mo-Mo reduces from 3.64 Å to 3.29 Å. The transition state Eigenvector corresponding to the imaginary frequency ν=1291i cm-1 represents the hydrogen transfer from NH2 to the MoV=O group. This step leads to the product (20) at ∆E = -8.9 kcal/mol (∆G673K = 18.7 kcal/mol), with MoVI=NH and MoIV-(OH)2 sites. The Mo=NH and the two Mo-OH bond distances are 1.78 Å, and 1.88 Å, and 1.90 Å, respectively. 4. Discussion Ammonia activation on a Mo(VI) site. According to our calculations, ammonia activation does not occur on a fully oxidized Mo(VI) site. Although the adsorption of ammonia onto a Mo(VI) site is quite favorable (∆E = -27.5 kcal/mol, ∆G673K = -1.5 kcal/mol), the subsequent steps are not as likely to take place. We find that net barriers for the first hydrogen transfer (∆E‡ = 44.6 kcal/mol, ∆G‡673K = 44.2 kcal/mol) and the second hydrogen transfer (∆E‡ = 54.5 kcal/mol, ∆G‡673K = 51.7 kcal/mol) are prohibitively high at the reaction temperature. The presence of water does not seem to facilitate this process, as the entropic cost of binding ammonia and water molecules to the surface Mo site is considerable. Despite the reduction of the net ∆E barrier for the water-assisted second hydrogen transfer (∆E‡ = 23.4 kcal/mol), this step is not plausible on the free energy surface (∆G‡673K = 47.6 kcal/mol). Ammonia activation on a Mo(IV) site. Since high barriers for the first and the second hydrogen transfers on a Mo(VI) site make this pathway of ammonia activation inaccessible, an alternative pathway might involve an activated Mo(IV) center. The first step would be desorption of ammonia from Mo(VI), at ∆E = 27.5 kcal/mol, ∆G673K = 1.5 kcal/mol. This would open a site for adsorption of an allyl radical on a Mo=O to form a σ-O allyl molybdate (allyl being produced upon the activation of propene by an oxygen associated with a bismuth site). The net barrier for oxidation of σ-allylic intermediate to acrolein is ∆E‡ = 35.6 kcal/mol (∆G‡593K = 37.7 kcal/mol), as discussed in our recent paper on propene oxidation over BiMoOx catalyst.8 Desorption of final product, acrolein, from the molybdenum oxide surface leaves a vacant Mo(IV) site that has a potential to be involved in ammonia activation. In fact, the acrolein desorption would be greatly assisted by simultaneous adsorption of ammonia on the same site, which would yield Mo(IV)NH3, the starting point for favorable ammonia activation. In an attempt to understand how re-oxidation affects the acrolein dissociation, we found in our previous calculations that the coordination of O2 promotes acrolein desorption quite


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

significantly, destabilizing the binding of acrolein and avoiding formation of a vacant Mo(IV).8 A similar effect would be observed in a case of ammonia assisted dissociation of acrolein. Both the dissociation of ammonia from a Mo(VI) site and the oxidation of allyl seem plausible at the ammoxidation temperature (400 °C - 460 °C). In fact, this agrees with experiments showing a rapid decrease in the oxidation activity of bismuth molybdates catalyst upon the addition of ammonia to the feed, while the oxidation process is restored upon increasing temperature to 400 °C.1 Grasselli et al. suggested that this might be due to ammonia blocking the active surface sites required for oxidation via formation of coordinately saturated molybdenum centers at lower temperatures, while at temperatures higher than 400° C, ammonia can be desorbed or activated to form catalytically active, coordinately unsaturated, molybdenum sites. We suggest that the presence of reduced active sites play an important role in the activation of ammonia. We suspect that a number of Mo(IV) sites will be present on the catalyst surface due to possible surface defects and as a result of the reduction of catalyst in the process of oxidation of propene prior to ammonia activation. We expect ammonia activation on a reduced Mo(IV) site to be favorable. The first H transfer from the Mo(IV)-NH3 to a neighboring Mo=O (TS5) is rapid, with the barrier of 18.2 kcal/mol (∆G‡673K = 15.4 kcal/mol). The second H transfer can occur through three different pathways, where the most favorable pathway involves the H transfer from a Mo-NH2 to a neighboring MoOH site to make Mo=NH and Mo-OH2 (Path 1, TS6, Scheme 3). The net barrier for this step is 26.2 kcal/mol (∆G‡673K = 24.0 kcal/mol), and similarly to the first hydrogen transfer, this step is expected to be fast at the reaction temperature. Another plausible pathway (Path 2) involves H transfer from the Mo-NH2 to a neighboring fully oxidized Mo=O site (TS7), with net barrier of 33.7 kcal/mol (∆G‡673K = 27.3 kcal/mol). Although the energy cost for this step is 7.5 kcal/mol higher than the cost for the corresponding step on the Path 1 (TS6), it is still feasible at the reaction conditions. The least favorable pathway for the second H transfer presented here involves the H transfer to an oxo group on the neighboring reduced Mo-OH site. Due to geometric considerations, the second H transfer was preceded by the H migration from the Mo-OH to the oxo group on the same Mo site, in order to create a receiving Mo=O group close enough for the second H transfer to occur. This H migration is costly (60.6 kcal/mol), but water greatly assists this step, reducing the H migration barrier to 16.2 kcal/mol (∆G‡673K = 34.4 kcal/mol, TS8b). Finally, the cost for the second H transfer on the Path 3 is 35.0 kcal/mol (∆G‡673K = 31.2 kcal/mol), which is higher than the corresponding cost on the other pathways, yet still feasible at the reaction temperature. Path 1 has the lowest barrier for the second hydrogen transfer, and consequently is the most favorable pathway for ammonia activation on a Mo(IV) site. As discussed earlier, our results also show that the NH3 activation is more favorable on a reduced Mo(IV) site than on a fully oxidized Mo(VI). This is because ammonia adsorbs more easily on a vacant Mo(IV), and hydrogen transfers do not occur through tight, four-member rings typical of H transfers on a Mo(VI), but rather hydrogen is transferred to a Mo=O or Mo-OH group of an adjacent molybdenum site. Also, all steps on ∆E surface for ammonia activation on Mo(IV) are exothermic with respect to reactants, making this process significantly more favorable than on a Mo(VI) site. These results


20

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


suggest that once the activation has initiated, and the reduced Mo(IV) sites start appearing in higher ratios, ammonia will also be activated more rapidly. Summary Our calculations show that the ammonia activation is not favorable on the fully oxidized Mo(VI) sites. We find that net barriers for the first hydrogen transfer (∆E‡ = 44.6 kcal/mol, ∆G‡673K = 44.2 kcal/mol) and the second hydrogen transfer (∆E‡ = 54.5 kcal/mol, ∆G‡673K = 51.7 kcal/mol) are prohibitively high at the reaction temperature. However, we find that reduced Mo(IV) surface sites are well suited for this process. The first hydrogen transfer from Mo(IV)-NH3 to an adjacent Mo(VI)=O occurs rapidly. The calculated barrier for this step is 18.2 kcal/mol (∆G‡673K = 15.4 kcal/mol). Similarly, the second hydrogen transfer is quite favorable. We explored three different pathways for this step, and found that the H transfer from Mo-NH2 to an adjacent Mo(V)-OH is more favorable (∆E‡ = 26.2 kcal/mol (∆G‡673K = 24.0 kcal/mol) than transfer to an adjacent Mo(VI)=O or Mo(V)=O group. Our results point to the crucial role of reduced Mo sites in the ammonia activation process. Reduced surface sites could be present due to a surface defect, or as a result of propene oxidation occurring prior to the ammonia activation.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org/. It includes Cartesian coordinates for all reported compounds. AUTHOR INFORMATION Corresponding Author *(W.G.) E-mail: johannes.niskanen@helsinki.ﬁ. Present Address: Mail Code (139-74) (400 South Wilson Ave.), California Institute of Technology, 1200 East California Blvd., Pasadena, California 91125 USA

AKNOWLEDGEMENTS The authors thank Jonas Oxgaard for invaluable discussions.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

1

Burrington, J. D.; Grasselli, R. K. Aspects of Selective Oxidation and Ammoxidation Mechanisms over Bismuth Molybdate Catalysts. J. Catal. 1979, 59, 79-99. 2 Allison, J. N.; Goddard, W. A. Active Sites on Molybdenum Surfaces, Mechanistic Considerations for Selective Oxidation and Ammoxidation of Propene. American Chemical Society; Washington, DC, 1985, Vol. 279, pp 23. 3

Rappé, A.K; Goddard III, W.A. Bivalent Spectator Oxo Bonds in Metathesis and Epoxidation Alkenes. Nature 1980, 285, 311-312. 4 Rappé, A.K; Goddard III, W.A. Mechanism of Metathesis and Epoxidation in Chromium and Molybdenum Complexes Containing Methyl-Oxo Bonds. J. Am. Chem. Soc. 1980, 102, 5114-5115. 5 Jang, Y. H.; Goddard, W. A. Selective Oxidation and Ammoxidation of Propene on Bismuth Molybdates, Ab Initio Calculations. Top. Catal. 2001, 15, 273-289. 6 Jang, Y. H.; Goddard, W. A. Mechanism of Selective Oxidation and Ammoxidation of Propene on Bismuth Molybdates from DFT Calculations on Model Clusters. J. Phys. Chem. B 2002, 106, 5997-6013. 7 Burrington, J. D.; Kartisek, C. T.; Grasselli, R. K. Surface Intermediates in Selective Propylene Oxidation and Ammoxidation over Heterogeneous Molybdate and Antimonate Catalysts. J. Catal. 1984, 87, 363-380. 8 Pudar, S.; Oxgaard, J.; Chenoweth, K.; van Duin, A.; Goddard, W. A. Mechanism of Selective Oxidation of Propene to Acrolein on Bismuth Molybdates from Quantum Mechanical Calculations. J. Phys. Chem. C, 2007, 111, 16405-16415. 9 Pudar, S.; Oxgaard, J.; Goddard, W. A. Mechanism of Selective Ammoxidation of Propene to Acrylonitrile on Bismuth Molybdates from Quantum Mechanical Calculations. J. Phys. Chem. C, 2010, 114, 15678-15694. 10 Grzybowska, B; Haber, J.; Janas, J. Interaction of Allyl Iodide with Molybdate Catalysts for the Selective Oxidation of Hydrocarbons. J. Catal. 1977, 49, 150-163. 11 Swift, H. E.; Bozik, J. E.; Ondrey, J. A. Dehydrodimerization of Propylene Using Bismuth Oxide as the Oxidant. J. Catal. 1971, 21, 212-224. 12 Massoth, F. E.; Scarpiello, D. J. Kinetics of Bismuth Oxide Reduction with Propylene. J. Catal. 1971, 21, 225238. 13 Goddard, W.A.; van Duin, A.; Chenoweth, K.; Cheng, M.J.; Pudar, S.; Oxgaard, J.; Merinov, B.; Jang, Y.H.; Persson, P. Development of the ReaxFF Reactive Force Field for Mechanistic Studies of Catalytic Selective Oxidation Processes on BiMoO x. Topics in Catalysis, 2006, 38, 93-103. 14 Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098- 3100. 15 Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle-Salvetti Correlation Energy Formula into a Functional of the Electron Density. Phys. Rev. B, 1988, 37, 785-789. 16 Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299-310. 17 Jaguar 6.5; Schrodinger, LLC: Portland, OR, 2005. 18 Gu, G.; Xu, X.; Lu, X., Wan, H. Mechanisms of Methane Activation and Transformation on Molybdenum Oxide Based Catalysts. J. Am. Chem. Soc. 2005, 127, 3989 –3996. 19 Gu, G.; Xu, X.; Lu, X., Wan, H. Mechanisms of Initial Propane Activation on Molybdenum Oxides: A Density Functional Theory Study. J. Phys. Chem. B, 2005, 109, 6416. 20 Cheng, M.; Chenoweth, K.; Oxgaard, J.; van Duin, A.; Goddard, W. A. Single-Site Vanadyl Activation, Functionalization, and Reoxidation Reaction Mechanism for Propane Oxidative Dehydrogenation on the Cubic V4O10 Cluster. J. Phys. Chem. C, 2007, 111, 5115. 21 Chempath, S; Bell, A. T. A DFT study of the mechanism and kinetics of methane oxidation to formaldehyde occurring on silica-supported molybdena. J. Catal. 2007, 247, 119. 22 Brazdil, J. F.; Suresh, D. D.; Grasselli, R. K. Redox Kinetics of Bismuth Molybdate Ammoxidation Catalysts. J. Catal. 1980, 66, 347.


22

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents QM based Mechanism for Ammonia Activation on Bismuth Molybdates 1.13 1.44

1.29 2.19

1.11

1.11 1.22

1.39

1.33

1.83 2.08

2.15

1.87

1.85

TS1

TS2

TS3


23

Reaction Mechanism for Ammonia Activation in the ... - ACS Publications

Recommend Documents