Factors Affecting Selectivity in Catalytic Partial Oxidation and

Chapter 1

Factors Affecting Selectivity in Catalytic Partial Oxidation and Combustion Reactions Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch001

S. Ted Oyama Departments of Chemical Engineering and Chemistry, Virginia Polytechnic Institute and State University, 133 Randolph Hall, Blacksburg, VA 24061-0211 Control of selectivity is a dominant issue for both partial and total oxidation of hydrocarbons. This chapter provides a critical overview of various factors that affect selectivity. It is suggested that both kinetic and thermodynamic aspects are important in determining selectivity in oxidation, and that selectivity is intimately tied to reactivity. Discussion is made of the role of different types of oxygen, the mode of adsorbate bonding, the occurrence of branching steps, the reducibility of the catalyst, the oxygen-metal bond strength, and the effect of structure. Since oxidation is a complex process many contrary views exist, and, where possible, these are presented and contrasted.

Control of selectivity is one of the central problems in catalytic hydrocarbon oxidation (/). The problem arises naturally because reactants undergoing reaction can be oxidized to various extents. This paper discusses various factors that affect selectivity in both partial oxidation and complete combustion. The discussion of combustion is restricted to purely surface phenomena, and excludes surface-initiated gas-phase processes. Where possible, comparisons will be made between partial oxidation and combustion. The reactions of methane are diverse and provide a good illustration of the role of selectivity in oxidation. Table 1 Products of Methane Oxidation Reactant Species Oxidation state

Products CH3OH HCHO H C O O H

CH4

C2H6

C2H4

-4

-3

-2

-2

0

69

74

91

295

CO

C0

+2

+2

+4

503

579

801

kJ/mol CH4

0097-6156/96/0638-0002$15.00/0 © 1996 American Chemical Society In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

2

1. OYAMA

Selectivity in Catalytic Oxidation & Combustion Reactions 3

In the course of the various transformations the formal oxidation state of carbon increases to values that depend on the chemical species formed. The degree of oxidation required depends on the process. For oxidative coupling the desired products are C lh and C2H4, for chemicals production the end products are C H O H , HCHO and HCOOH, in syngas formation the target is CO, while in catalytic combustion the objective is full oxidation to C 0 . There has been limited success in the direct production of partial oxidation products from methane. Methane coupling to C 2 products has been studied over a number of catalysts and maximum yields are less than 40% (2,3,4). Oxygenates have been obtained over oxides of vanadium (5,6), molybdenum (7,8), and iron (9) as well as biological (P-450, and methane monooxygenase) (10,11) catalysts and yields have been even lower, less than 10%. Syngas (CO + H ) has been produced in high selectivity (> 90%) at low contact times with noble metals supported on monoliths (12,13,14) and at high contact times on perovskite (75) and pyrochlore (16) oxides. In the case of catalytic combustion (17,18,19,20) the problem of selectivity is different. In part the objective is to achieve full oxidation of methane to C O 2 and H 0 without the formation of partial oxidation products, but it is also essential to limit the formation of NO . One major application of catalytic combustion is in gas turbines (21), where catalysts are used to lower the temperature of reaction in the "precombustion region" (22), where most of the N O is generated. Another substantial application is in volatile organic compounds (VOC) abatement (23). Here the challenge is to eliminate small quantities of pollutants in an air stream. As in partial oxidation, both metals and oxides are used as catalysts in combustion. 2

3

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch001

2

2

2

x

x

Kinetics or Thermodynamics? The network of reactions involving methane is complex, with many parallel and consecutive reactions. The solid lines depict the main expected routes for consecutive reactions, while the dashed lines show other possible pathways.

Figure 1 Reaction Network of Methane Oxidation

In Heterogeneous Hydrocarbon Oxidation; Warren, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

4

HETEROGENEOUS HYDROCARBON OXIDATION

Any part of the network dealing with an intermediate can be simplified and described by a simple series of reactions: ki A +O

>B k

2


B +O

>C

In this sequence A is methane, O is oxygen, B is a partly oxidized species, C is the end member of the oxidation chain, C 0 , and ki and k are effective rate constants. Depending on whether the objective is partial or total oxidation, the desired product is either B or C. As seen from the Table 1 the standard free energy of formation is highly negative for C 0 . Thermodynamically C 0 formation is highly favored, and it is often stated that stopping at B requires kinetic control (7). In the simplest case the rate of formation of B on a surface undergoing reduction and oxidation steps may be given by 2

2

2

2

k K (A)K (P) x

1

K (A) A

A

0

+ K (B) + K (C) B

C

K (0)

+

0

The rate of formation of C is similarly given by r

k K (B)K (Q)

=

2

2

B

0

K (A) + K (B) + K (C) + A

B

c

K (0) 0

In these equations the small case k's represent rate constants while the upper case K's refer to equilibrium adsorption constants. The selectivity to B of the process is given by the relation giving the maximum value of (B).

n~r

2

= ^ j p = k K (A)K (0)-k K (B)K (0) x

A

0

2

B

=0

0

(3)

This results in (ff)max

(A)

K

hA

(4)

kK 2

B

Equation (3) shows that selectivity is intimately connected with the rates of the reactions involved in the sequence. Equation (4) demonstrates that both kinetic and thermodynamic factors are involved in determining the selectivity to B. Since in general the intermediate product is more functionalized than the reactant, it is more reactive and is also more strongly adsorbed, so k > kj and K > K . This is particularly true for alkane oxidations, and thus, obtaining a large selectivity to B is a challenge. For non-alkanes there are cases in which K » K , and high selectivity can be obtained, for example, in the oxidation of methanol to formaldehyde (24). One way to improve selectivity to the intermediate product is by reducing the rate of the second step, but this is usually accompanied by a decrease in the rate of the 2

B

A

A

B


1. OYAMA


first reaction. Although eventually a favorable selectivity may be achieved, this is usually at the cost of the overall rate. Table 2 shows an example. Table 2 Comparison of Activity and Selectivity on an Oxide and a Metal Catalyst

Turnover rate / s" 1.1 x lO 6.0 x 10 1

7.7% V 0 / S i 0 Pt


2

5

-3

2

5

CH CHO 12

Selectivity / % CH =CH 68

-

-

3

2

Reference C0 20 100

2

X

(25) (26)

Table 2 compares the performance of vanadium oxide and platinum in the oxidation of ethane. Turnover rates are based on sites titrated by selective chemisorption, using oxygen in the case of the vanadium oxide (27). It is seen that the metal is more active than the oxide by over 8 orders of magnitude, but produces mostly C 0 . Evidently, the price to pay for an increase in selectivity is a reduction in rate. This empirical observation has also been reported for a collection of methane oxidation catalysts where selectivity was found to decrease with conversion (28). 2

Electrophilic versus Nucleophilic Oxygen The role of adsorbed oxygen in controlling selectivity has been championed by Bielanski and Haber (29,30). Adsorbed oxygen is classified into two types: electrophilic and nucleophilic, which differ in their mode of reaction. Electrophilic oxygen includes species like, 0" (oxide), 0 * (superoxide), and 0 " (peroxide), which are believed to be responsible for deep oxidation. These species are electron deficient and are expected to react with the electron-rich regions of a hydrocarbon molecule, such as double bonds. Nucleophilic oxygen refers to the oxide ion, O *, often identified as lattice oxygen, which is believed to carry out partial oxidation. Because this oxygen has its full complement of electrons, it is expected to react with the portions of the molecule that are electron-poor. Evidence for this difference in reactivity between the two types of oxygen is provided by the simultaneous measurement of the mass and charge of surface oxygen species during catalytic reactions (3 J,32). On catalysts such as C o 0 , which favor total oxidation, the type of oxygen detected was electrophilic (O ", O"), while on catalysts like B i M o O i , which carry out partial oxidation, the type of oxygen found was nucleophilic (O "). Unfortunately, direct measurements like these have not been applied to a variety of catalysts and conditions. Such measurements are very desirable. Nevertheless, the concept of reactivity control by the type of oxygen appears to be useful for qualitatively explaining selectivity in a variety of oxide systems (33,34). Considerable care must be employed in the use of the above categorization. The concept does not hold in metallic systems (e.g. combustion catalysts), where ionic oxygen species are not present. Furthermore, in many oxide systems differing views have been proposed. For the case of selective oxidation, Tagawa, et al, have concluded that O" forms on the surface and abstracts a P-hydrogen from an adsorbed complex in the dehydrogenation of ethylbenzene (35). Szakas, et al, suggest that a mobile O" radical 2

2

2

2

3

4

2

2

3

2

2



6

is responsible for both partial and total oxidation of 1-butene and «-butene to maleic anhydride on V-P-O (36). Akimoto, et al., have given evidence that 0 " ions are involved in the oxidation of butadiene to maleic anhydride over supported molybdena catalysts (37). Yoshida, et al., have reported the reaction of 0 " on V 0 with propylene and benzene to form aldehydes while the lattice oxygen shows little reactivity below 423 K (38). Gleaves, et al., (39) suggest that adsorbed oxygen species are responsible for selective oxidation in vanadyl pyrophosphate, while Schi0tt, et al. (40) suggest that these are specifically peroxo oxygen species. In the methane coupling area many active forms of oxygen in the catalyst have been proposed, including O", 0 " and 0 ". These are reviewed by Lee and Oyama (41). In the case of combustion Arai and co-workers proved the participation of both adsorbed and lattice oxygen in the complete oxidation of methane (42). x

2


2

2

5

2

2

2

Selectivity Control by the Mode of Adsorbate Bonding So far not much has been said about the manner in which the hydrocarbon reactant is adsorbed on the surface. Yet, this is probably a crucial factor, since the stability of the surface intermediate will determine its lifetime on the surface, and its susceptibility to retrograde reactions leading to total oxidation. On a transition metal oxide surface an activated hydrocarbon molecule has the option of being bound to the metal or to oxygen. (See scheme below.) When the hydrocarbon is attached to the surface through an oxygen atom by an ether-type linkage it can undergo reaction by two well-known pathways. An a-H elimination produces an aldehyde, while a p-H elimination produces an olefin. On the other hand, if the hydrocarbon is bonded directly to the transition metal (M) by an M-C bond, there are no ready elimination reactions. The intermediate is relatively stable and remains on the surface for a long time, allowing the possibility of deep oxidation. There are several examples where differences in bonding result in different oxidation pathways. For methane oxidation on a variety of transition metal oxides Dowden, et al. (43) suggested that methoxy intermediates can be hydrated to methanol, whereas methylene species are oxidized to CO . In the case of ethane oxidation on supported vanadium oxide (44) it was found that the production rates of CH CHO and C H4 do not vary substantially with catalyst structure whereas the formation rate of C O varies by a factor of 30. This suggests that the products are not formed from a common intermediate, but rather that two independent paths for the formation of the two product types exist. These are suggested to be x

3

2

x

-CH CH 2

3

C H4 + CH3CHO 2

M O + C H6 2

CH CH 2

3

CO

x

The oxygen-bonded intermediate produces selective oxidation products, while the metal-bonded intermediate produces CO . Another example of this situation is x



1. OYAMA

found in the oxidation of propylene on supported molybdenum oxide (45). To test the effect of bonding, two model reactants are employed, allyl alcohol and allyl iodide, which are expected to form different intermediates. 0-CH -CH=CH 2

S M O + X-CH -CH=CH Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch001

2

2

2

OHC-CH=CH

2

M

. CH -CH=CH 2

2

CO

x

M The intermediates were decomposed in a temperature programmed experiment, and the products were detected by mass spectrometry. Again the oxygen-bonded intermediate produced a selective oxidation product, in this case acrolein, whereas the metal-bonded intermediate produced CO . Still another example of the importance of bonding is found in the oxidation of ethanol on silica-supported molybdenum oxide. Here the major products are the selective oxidation products, acetaldehyde and ethylene. Raman spectroscopy at in situ reaction conditions indicates that there are two different adsorbed ethoxide species associated with the formation of the two products (46). x

0 - C H

^ CH CH 0H 3

2

+ Mo=0

C H

3

C H

2

O H

+

Mo

C H

3

•

CH3CHO

•

CH =CH

> Mo Q - C H

Q /

2

/

\

>

Mo

*

/ \ Mo

2

C H

3

2

2

Mo

The ethoxide species bound to the terminal oxygen group produces acetaldehyde, while the ethoxide species bound to the bridging oxygen group produces ethylene. Selectivity Determining Step Kung (47) has suggested the existence of a selectivity-determining step which determines the fate of a surface intermediate. For the following sequence

A The selectivity-determining step is that involving the reaction of the intermediate, B*, and it includes the two irreversible reactions denoted by rate constants k\ and k . The intermediate is an adsorbed species, as otherwise the network would reduce to the trivial case of two independent reactions. For the two reactions the rates of formation 2


8


of C and D are *l(**)

2

where F is a common function of concentrations. The fractional selectivity to C can be defined as S = r /(r + r ) = ki/(ki + k ). Thus, provided that there are no other reactions, the selectivity can be related to the ratio of rate constants for the selectivitydetermining step. An example of where this concept arises is in the dehydrogenation of alkanes on vanadium oxide-containing catalysts (48). The intermediate B* in that case is an adsorbed alkyl or alkene. c


., k (B*)

c

M

D

2

M

Factors Affecting Selectivity in Catalytic Partial Oxidation and

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