Quasi-Catalytic Identification of Intermediates in the Oxidation of

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Quasi-catalytic identification of intermediates in the oxidation of propene to acrolein over a multicomponent Bi-Mo catalyst Gennady I. Panov, Eugeny V. Starokon, Mikhail V. Parfenov, Beichen Wei, Vladimir I. Sobolev, and Larisa V. Pirutko ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03833 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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Quasi-Catalytic Identification of Intermediates in the Oxidation of Propene to Acrolein over a Multicomponent Bi-Mo Catalyst Gennady I. Panov*†, Eugeny V. Starokon*†, Mikhail V. Parfenov†, Beichen Wei‡, Vladimir I. Sobolev†, Larisa V. Pirutko† †

Department of Heterogeneous Catalysis, Boreskov Institute of Catalysis, Pr. Lavrentieva 5,

Novosibirsk 630090, Russian Federation ‡

Department of Natural Science, National Research University - Novosibirsk State University, Pirogov

Avenue 2, 630090, Novosibirsk, Russian Federation

ABSTRACT The generally accepted 3-step mechanism of propene oxidation to acrolein includes abstraction of two hydrogen atoms and addition of one oxygen atom. The first step is a well-known H-abstraction from the methyl group. The sequence of two other steps is unclear. We investigated the reaction in quasi-catalytic mode at 100-200°C with extraction and analysis of accumulated surface products. The reaction intermediate, surface ether of allyl alcohol, was identified. This strongly proves that O-addition precedes the second H-abstraction. Activation energy and kinetic isotope effect of the quasi-catalytic reaction correlate with appropriate parameters of the conventional catalytic process.

Keywords: propene oxidation; acrolein; allyl alcohol; intermediate; bismuth molybdate; quasicatalytic approach; product extraction, kinetic isotope effect.

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INTRODUCTION Gas-phase catalytic reactions commonly proceed at 300-450°C. With decreasing temperature, the reaction slows down, and, starting from a certain limit, its rate becomes immeasurably small. A visible manifestation of the reaction in the form of product desorption into the gas phase disappears. Experiments in this temperature range are non-informative and usually not performed. However, this does not mean that the reaction stops completely. For a certain time, it can proceed in a latent way providing products accumulation on the catalyst surface. The activation energy of the surface diffusion of adsorbed molecules is much lower than the activation energy of their desorption.1,2 Therefore, the products (both final and intermediate) can migrate, without desorption, from catalytically active sites to other surface centers, thus opening the way for new reaction cycles. The accumulated products can be extracted from the surface and analyzed by various analytical methods. Such reaction mode, which we called quasi-catalytic (QC), was first described in our studies on the methane oxidation to methanol over FeZSM-5 catalyst where the reaction runs on the so-called αsites, the concentration of which can easily be measured.3,4 At 200°C the α-sites can make up to seven turnovers with no product desorption. The QC approach makes it possible to obtain new information on the reaction mechanism and, in particular, identify intermediate substances that are invisible under catalytic conditions. In this work, the QC approach was applied to study the oxidation of propene to acrolein (ACR): CH2=CH-CH3 + O2 →

CH2=CH-CHO + H2O

(1)

This reaction underlies one of the most important processes in modern catalytic chemistry, which is conducted on multicomponent Bi-Mo catalysts with an annual ACR production of 109 kg.5 The mechanism of reaction (1) has been studied for many decades, with the main results being reviewed in papers.6-13 It is a universal agreement that the reaction includes three main steps: abstraction of two hydrogen atoms and addition of one oxygen atom. However, the sequence of these steps is not quite clear.


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Scheme 1 illustrates the mechanism proposed in the classical works by Adams and Jennings,14,15 which was generally recognized for a long time. According to the Scheme, abstraction of the allyl hydrogen to form a symmetric π-allyl radical (AR) is the first, rate-determining, step.

H

H

H

H CH3

H

-H

H

1 (slow)

H H

H

-H 2

H

H H

[O]

H

3

H

H H

O

Scheme 1. The reaction mechanism with successive abstraction of H atoms (Adapted from.16 Copyright 1980, Elsevier)

Step 2 is the abstraction of the second H atom, which occurs equiprobably at any end of AR leading to carbenoid biradical CH2=CH2-CH:. Addition of oxygen (step 3) completes the formation of ACR. This mechanism, which is accepted by many authors, well describes the distribution of ACR isotopomers at the oxidation of labeled propene. However, a thermodynamic consideration showed that abstraction of the second hydrogen from AR should be more difficult than abstraction of the first one from propene molecule,10 which is a clear disagreement with Scheme 1. To resolve the problem, Grasselli et al.11,16 suggested changing the sequence of steps 2 and 3. So, oxygen addition should precede the second hydrogen abstraction, thus making the latter step faster than step 1. The idea of Grasselli et al. was widely accepted.17-26 However, as was noted by Bell et al.,18 “there is no decisive experimental evidence to support either view”. As one can see further, such evidence can be successfully obtained using the QC study of the reaction.

EXPERIMENTAL The QC experiments were carried out with a SiO2 supported multicomponent Bi-Mo catalyst, mBiMo.27 The procedure for m-BiMo synthesis as well as that for testing its catalytic properties are described in the Supporting Information (Sections 1S and 2S). At 350 °C, the selectivity for ACR is equal


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to 91 % (together with some amount of acrylic acid) at a propene conversion 95 %. Such parameters are typical of this type catalysts.5 The QC experiments on propene oxidation were conducted in a static vacuum setup,28 the scheme of which is given on Figure S1 of the Supporting Information. The setup was equipped with two absolute pressure gauges and a mass-spectrometer to control the gas phase pressure and composition. The initial mixture of reactants, C3H6 + O2, was prepared in the reaction volume of the setup (750 cm3). Then, for a short time (10 s), the volume was connected with the quartz reactor (25 cm3) charged with 0.25 g of the catalyst sample to feed the mixture. This moment was the beginning of the run that proceeded in the closed reactor. After a predetermined time, the reaction was terminated by cooling the reactor to approximately minus 100 °C to prevent secondary transformations of the surface compounds. After disconnecting the reactor from the setup, the sample was poured into the vial with an extractant represented by methanol with the addition of 10% H2O. Extraction of surface products was performed by the procedure developed by Starokon et al.29 In more detail, the QC technique is given in the Supporting Information (Section S3).

RESULTS AND DISCUSSION General description of data in Table 1 Table 1 lists the results of QC runs carried out at 100-200°C with the reaction time 5-120 min. In most cases the reaction mixture contained 10 Torr C3H6 + 2 Torr O2, the weight of the catalyst was always 0.25 g. The total amount of extracted products, Nextr, in QC runs changed from 0.37 to 4.8 µmol/g. Assuming a complete extraction and taking into account the initial C3H6 amount in the reactor (NС3Н6 = 13.0 µmol), one can estimate the propene conversion: ХС3Н6 = 0.25 Nextr/ NС3Н6

(2)

which equals 0.7-9.2 %. However, the true degree of extraction is unknown. Data of our recent studies on stoichiometric and QC oxidation of methane to methanol,3,4 and ethene30 and propene31,32 to


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epoxides on FeZSM-5 zeolite can be used as reference points. In those studies, the extraction degree was 50-78 %. The m-BiMo catalyst certainly has a weaker adsorption capacity in comparison with the zeolite. So in this case, one may expect a much higher (if not complete) degree of extraction, without its significant influence on the results obtained. The reaction rate, r, presented in Table 1 was calculated by eq. (3): r = Nextr/t

(3)

where t is the reaction time. Figure 1 displays the temperature dependence of the rate, which corresponds to the activation energy E = 18 kcal/mol. Note that this value refers only to the formation of surface compounds, without desorption into the gas phase. Nevertheless, the obtained E value is well consistent with the literature data for the catalytic process, where E value is 16-22 kcal/mol.24,33,34

o

Temperature, C 175 125

200 100 6

150 150

125 175

100 200

E = 18 kcal/mol

5

ln r + 6

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

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4 3 2 1 0 2.1

2.2

2.3

2.4

2.5

2.6

2.7

1000/T

Figure 1. Arrhenius plot for QC oxidation of propene C3H6.

The composition of products extracted after QC reaction (Table 1) is of particular interest. These products strongly differ from those of the catalytic process, which consist of ACR, acrylic acid, acetic acid, CO and CO2 (Table S1 in the Supporting Information). Unlike that, the QC products, beside ACR, include allyl alcohol (AAlc), a minor part of which transforms to propanal and acetone.


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The isomerization of AAlc to propanal and acetone (Scheme 2) is accompanied by a pronounced decrease in the free energy, 14.5 and 19.6 kcal/mol, respectively.

O

-14.5

-5.4

OH -19.6

O

Scheme 2. Isomerization of AAlc to propanal and acetone (∆rG0298 in kcal/mol).

The amount of oxygen in the gas phase (PO2) only slightly affects the QC reaction. It is seen from runs nos. 2 and 3 at 170 °C, in which a decrease in PO2 from 5 to 2 Torr at a constant propene pressure PC3H6 = 10 Torr produces a minor decrease in ХС3Н6 (from 3.6 to 3.3 %), while the composition of products remains virtually unchanged. The QC reaction proceeds even after a complete O2 removal providing ХС3Н6 = 1.1 % (run no. 4). Of course, in this case the reaction proceeds due to the surface oxygen of the catalyst, as is observed in conventional catalytic experiments at 350-450 °С.9,35,36 The increased conversion of propene in the O2 presence testifies that the catalyst active sites are reoxidized during the run and make several turnovers during QC reaction.

Identification of the intermediate. The reaction mechanism The presence of AAlc in the extract is quite suggestive. Since AAlc is the result of monooxygen oxidation, it can be considered as a probable intermediate whose further oxidation gives ACR. However, an alternative hypothesis, according to which ACR and AAlc are formed by parallel routes, is also plausible. Such a situation was described by Wang et al. for the oxidation of propene on CuO/SiO2 via two independent routes leading to ACR and propylene oxide, respectively.37,38


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To resolve this dilemma, it is reasonable to analyze selectivities for ACR and AAlc as functions of propene conversion. Such dependences are displayed on Figure 2. (Here, acetone and propanal are added to AAlc; run no. 4 is not included). One can see that the selectivity changes are opposite to each other. With decreasing ХС3Н6, the selectivity for ACR tends to zero, whereas the selectivity for AAlc tends to 100 %. Such a picture is a convincing argument in favor of the consecutive mechanism, in which AAlc is an intermediate in the formation of ACR.

ACR

100 80

Selectivity, %

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

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60 40 20

AA

0 0

2

4

6

8

10

XC3H6, %

Figure 2. Selectivity for ACR and AAlc as a function of propene conversion. (Shadowed points show results with C3D6).

The result obtained gives an unambiguous answer concerning the sequence of steps discussed at the beginning of the paper. It is evident that oxygen addition precedes the second hydrogen abstraction resulting in C3H4: biradical (Scheme 1). In the latter case, AAlc could not form because the addition of oxygen to C3H4: would immediately produce ACR. Note that, although we are talking about AAlc, most probably its surface ether should be considered the actual intermediate. The latter is formed by the interaction of AR with the catalyst oxygen, which is usually assumed to be oxygen atom bound to the Mo.11,18,39 This correlates with results of Concepcion et al.40 Using infrared spectroscopy, the authors came to the conclusion that the interaction of propene with the catalyst oxygen leads to the formation of surface AAlc ether having


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absorption bands at 1450-1490 cm-1. The transformation of the ether into AAlc should occur via the hydrolysis at the step of product extraction:

H2O

OMo

OH

+

MoOH

(4)

The results of our study make it possible to suggest the mechanism shown on Scheme 3.

H

CH3

H

-H

1

H

H

H

H

H

H

H

H

MoO

2

H

CH2

H

OMo

-H

H

3

C H

H

O

Scheme 3. The reaction mechanism with the intermediate formation of AAlc surface ether.

According to the Scheme, the first step is the generally accepted H abstraction from propene molecule to form AR. The second step is the interaction of AR with the surface oxygen to produce AAlc ether. And finally, the second H atom is abstracted to yield ACR in the third step. In general, our results are consistent with the mechanism proposed by Bell et al.18,20,24 Based on theoretical and spectral data, the authors assumed molybdenum ether of AAlc to be the intermediate (called “allyl alkoxy intermediate”) in the propene oxidation to ACR. The present study provides experimental evidence of the existence of such an intermediate. As for propanal and acetone, small amounts of which are observed in QC experiments but not observed in the catalytic process, they are possible intermediates in the formation of side products, such as acetic acid and COx.

Kinetic isotope effect (KIE) The catalytic oxidation of propene to ACR is accompanied by a significant KIE (kH/kD = 1.72.0) related to the abstraction of the first H atom from C3H6.14,41 A similar effect is observed also in the


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QC oxidation of propene. As seen from runs nos. 6, 7 at 140°C and nos. 9, 10 at 120°C (Table 1), the replacement of C3H6 by C3D6 substantially decreases the reaction rate, which corresponds to kH/kD = 2.1 and 1.9, respectively. Simultaneously, quite a remarkable phenomenon takes place, i.e. nearly a twofold decrease in selectivity for ACR and a similar increase in selectivity for AAlc. Nevertheless, this phenomenon, even being unexpected at first glance, is quite consistent with the mechanism displayed on Scheme 3. Indeed, it is natural to assume that KIE takes place not only at the first but also at the third step, where cleavage of the C-H bond also occurs. Although the rate constant of this step is insignificant for the overall reaction rate, its decrease in the case of C3D6 should exactly result in a decrease of the selectivity for ACR and an increase of the selectivity for AAlc due to a slower overoxidation of the intermediate ether. However, this explanation is difficult to reconcile with the selectivity dependences on ХС3Н6, shown in Figure 2. One can see that the results for both C3D6 and С3Н6 satisfactorily fall on the general dependence. This means that at the same value of ХС3Н6 the selectivity is approximately the same, regardless of the isotopic composition of propene. An additional analysis is required to solve this riddle of selectivity.

CONCLUSION A detailed mechanism of catalytic reactions is one of the most difficult problems in catalytic chemistry. This is caused primarily by difficult identification of the surface intermediates whose successive formation and conversion constitute a catalytic cycle. In recent years, an important role here is played by quantum-chemical calculations and spectroscopy data. However, their results in many cases are ambiguous and allow alternative interpretations. This work demonstrates that QC method suggests a new efficient way for identification of intermediates.

ASSOCIATED CONTENT


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

Supporting Information Catalytic properties of m-BiMo; preparation procedure of m-BiMo; procedure for catalyst testing; scheme of vacuum setup; GC calibration (PDF). These materials are available free of charge on the ACS Publications website.

ACKNOWLEDGEMENTS We thank G.A. Zenkovets for providing the catalyst, V.A. Utkin and M.V. Shashkov for GC-MS analysis. The work was performed as part of a State Assignment for the Boreskov Institute of Catalysis (Siberian Branch, Russian Academy of Sciences), project no. 0303-2016-0006.

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(3)

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(10) Snyder, T. P.; Hill, C. G., Jr. Catal. Rev.-Sci. Eng. 1989, 31, 43-45. (11) Grasselli, R. K. Top. Catal. 2001, 21, 79-88. (12) Moro-Oka, Y.; Ueda, W. Adv. Catal. 1994, 40, 233-273. (13) Hanna, T. A. Coord. Chem. Rev. 2004, 248, 429−440. (14) Adams, C.R. ; Jennings, T.J.; J. Catal. 1963, 2, 63-68. (15) Adams, C. R.; Jennings, T. J. J. Catal. 1964, 3, 549-558 . (16) Burrington, J. D.; Kartisek, C. T.; Grasselli, R. K. J. Catal. 1980, 63, 235-254. (17) Pudar, S.; Oxgaard, J.; Chenoweth, K.; van Duin, A. C. T.; Goddard, W. A., III J. Phys. Chem. C 2007, 111, 16405-16415. (18) Getsoian, A.; Shapovalov, V.; Bell, A. T. J. Phys. Chem. C 2013, 117, 7123−7137. (19) Getsoian, A.; Zhai, Z.; Bell, A.T. J. Amer. Chem. Soc. 2014, 136, 13684-13697. (20) Licht, L. B.; Getsoian, A.; Bell, A.T. .J. Phys. Chem. C 2016, 120, 29233-29247. (21) Licht, R.B.; Bell, A.T. ACS Catal. 2017, 7, 161−176. (22) Concepcion, P.; Botella, P.; Lopez Nieto, J.M. J. Mol. Catal. A: Chem. 2006, 256, 1–8. (23) Yi, X.D.; Zhang, X.B.; Weng, W.Z.; Wan, H.L. J. Mol. Catal. A: Chem. 2007, 277, 202–209. (24) Zhai, Z.; Getsoian, A.; Bell, A.T. J. Catal. 2013, 308, 25–36. (25) Schuh, K.; Kleist, W.; Høj, M.; Trouillet, V.; Beato, P.; Jensen, A. D.; Patzke, G. R.; Grunwaldt, J.-D. Appl. Catal., A: Gen. 2014, 482, 145–156. (26) Bui, L.; Chakrabarti, R.; Bhan, A. ACS Catal. 2016, 6, 6567−6580. (27) Sobolev, V. I.; Koltunov, K. Yu.; Zenkovets, G. A. Catal. Lett. 2017, 147, 310-317. (28) Starokon, E. V.; Dubkov, K. A.; Pirutko, L. V.; Panov, G. I. Topics in Catal., 2003, 23 (1-4), 137-143.


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(29) Starokon, E. V.; Parfenov, M. V.; Pirutko, L. V.; Abornev, S. I.; Panov, G. I. J. Phys. Chem. C 2011, 115, 2155-2161. (30) Starokon, E. V.; Parfenov, M. V.; Pirutko, L. V.; Soshnikov, I. E.; Panov, G. I. J. Catal. 2014, 309, 453–459. (31) Panov, G. I.; Starokon, E. V.; Parfenov, M. V.; Pirutko, L. V. ACS Catal. 2016, 6, 3875−3879. (32) Starokon, E. V.; Malykhin, S. E.; Parfenov, M. V.; Zhidomirov, G. M.; Kharitonov, A. S. Mol. Catal. 2017, 433, 43-51. (33) Ueda, W.; Asakawa, K.; Chen, Ch-L.; Moro-Oka, Y.; Ikawa, T. J. Catal. 1986, 101, 360-368. (34) Peacock, J.M.; Parker, A.J.; Ashmore, P.J.; Hockey, J.A. J. Catal. 1969, 15, 398-406. (35) Ueda, W.; Chen, Ch-L.; Asakawa, K.; Moro-Oka, Y.; Ikawa, T.; J. Catal. 1986, 101, 369375. (36) Popova, G.Ya.; Andrushkevich, T.V.; Zenkovets, G.A.; Plyasova, L.M.; Grebenuk, N.V. React. Kinet. Catal. Lett., 1990, 41, 21-27. (37) Long, W.; Zhai, Q.; He, J.; Zhang, Q.; Deng, W.; Wang, Y. ChemPlusChem. 2012, 77, 27-30. (38) He, J.; Zhai, Q.; Zhang, Q.; Deng, W.; Wang, Y. J. Catal. 2013, 299, 53-66. (39) Schuh, K.; Kleist, W.; Hoj, M.; Jensen, A.D.; Beato, P.; Patzke, J.R.; Grunwaldt, J.-D. J. Solid State Chem. 2015, 228, 42-52. (40) Concepcion, P.; Botella, P.; Lopez Nieto, J.M. Appl. Catal. A: General, 2004, 278, 45-56. (41) Krenske, L.D.; Keulks, G.W. J. Catal. 1980, 61, 316-325.


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Table 1 Quasi-catalytic oxidation of propene on m-BiMo catalyst (0.25 g)

Run no.

Reaction condition

Reaction parameters

Reaction selectivity of product extracted, mol. %

%

r 102, µmol/g min

АCR

АAlc

propanal

acetone

4.8

9.2

96

93

6.0

0

1.0

10+5 10+2 10+0

1.9 1.7 0.57

3.6 3.3 1.1

13 11 3.5

85 85 84

10 9.5 16

2.0 1.5 0

3.0 4.0 0

15 30 30

10+2 10+2 10 (C3D6)+2

0.67 1.2 0.54

1.3 2.3 1.1

4.5 4.0 1.8

61 78 36

38 16 56

0 2.5 3.0

1.0 3.5 5.0

120 120 120

30 60 60

10+2 10+2 10 (C3D6)+2

0.40 0.67 0.37

0.8 1.3 0.7

1.3 1.1 0.6

40 52 28

51 39 66

4.5 5.0 3.0

4.5 4.0 3.0

100

120

10+2

0.61

1.2

0.5

41

50

5.5

3.5

Temperature, ºC

Time, min

PС3Н6+РО2, Тоrr

Nextr., µmol/g

ХС3Н6,

1

200

5

10+10

2 3 4

170 170 170

15 15 15

5 6 7

140 140 140

8 9 10 11

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TABLE OF CONTENTS

-H

H

[O]

O

-H MoO

H

-H

OMo

O


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Quasi-Catalytic Identification of Intermediates in the Oxidation of

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