Catalytic Selective Oxidation - American Chemical Society

Chapter 4

Silica-Supported MoO and V O Catalysts in Partial Oxidation of Methane to Formaldehyde

Catalytic Selective Oxidation Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/26/15. For personal use only.

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Factors Controlling Reactivity 1

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A. Parmaliana , V. Sokolovskii , D. Miceli , F. Arena , and N. Giordano 1

3

Dipartimento di Chimica Industrial, Universitàdegli Studi di Messina, Salita Sperone 31, c.p. 29, I-98168 Messina, Italy Department of Chemistry, University of Witwatersrand, Johannesburg, P.O. Wits 2050, South Africa Istituto CNR-TAE, Salita Santa Lucia 39, I-98126 Santa Lucia, Messina, Italy 2

3

The partial oxidation of methane to formaldehyde with molecular O has been investigated on bare SiO , 4%MoO /SiO and 5%V O /SiO catalysts at 550-650°C in batch, pulse and continuous flow reactors at 1.7·10 kPa. The HCHO productivity (g ·kg -1·h ) r e s u l t s i n the o r d e r 4%MoO /SiO 400°C and can be oxidized to CO more easily than C H itself. Hence, a suitable reactor device and appropriate operating conditions result to be of fundamental importance in order to attain reliable data unaffected by experimental artefacts. The batch reactor, above described, permits both to operate at quasi-zero conversion per pass and to evaluate the catalytic activity at finite values of the reagents conversion. A typical test performed on S i 0 catalyst at 600°C is presented in Figure 1. It is remarkable how in our approach the product selectivity is unaffected by the methane conversion. A special care was taken to avoid oxygen-limiting conditions and, hence, methane conversion data obtained for oxygen conversions below 20% only have been used for the calculation of reaction rates. The product selectivity is strongly affected by the flow rate, reactor geometry (i.e., internal diameter and "heated" zone) and weight of catalyst. On this account, the space time yield to HCHO - or HCHO productivity feHCHO'kScat" "* " ) b ^ fi parameter to evaluate the reactivity of the partial oxidation catalysts. In an earlier paper (12) we have observed that the S i 0 surface presents a significant activity in the partial oxidation of methane, whereas from the literature it emerges that M o 0 (3) and V 0 (5) are effective promoters of its activity. Then, in order to carefully evaluate such a promoting effect, a comparative investigation of the catalytic pattern of unpromoted S i 0 and 4% M o 0 and 5% V 0 promoted silica systems in the methane partial oxidation with molecular oxygen has been performed. The reactivity data of such catalytic systems, in the T range 550-650°C, are summarized in Table I in terms of overall reaction rate, product selectivity and HCHO productivity. The contribution of the gas-phase reactions has been also quantified by performing a series of experiments with the empty reactor. The related results are also included in Table I. On the whole these data indicate that M o 0 significantly depresses the reactivity of the bare S i 0 catalyst, while the V 0 greatly enhances the functionality of the S i 0 surface towards the production of HCHO. x

4

2

1

1

1

t 0

e m

e

m o r e

e

m t e

2

3

2

5

2

3

2

R

3

2

5

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2

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4. PARMALIANA ET AL.

Silica-Supported Mo0 & V 0 Catalysts 3

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The opposite influence exerted by the M o 0 and V 0 on the S i 0 reactivity has been fully confirmed by evaluating the catalytic behaviours of 0.2-10.0 wt% M o 0 and V 0 loaded S i 0 catalysts. In particular, by raising the M 0 O 3 g fr° ° -0 the HCHO productivity gradually decreased, while for vanadia dopant, the HCHO yield increased with loading in the range 0.2-5.0 wt%. A further increase in the loading of the oxides did not modify the reactivities of the medium loaded (4% M 0 O 3 , 5% V 0 ) supported oxide catalysts (25). 3

3

l o a d i n

2

m

5

2 t 0

2

5

2

2

4

2


47

s

5

Table I. Activity and Selectivity of S i 0 , 4% M o 0 / S i 0 and 5% V 0 / S i 0 Catalysts in Methane Partial Oxidation. Batch reactor data 2

3

2

2

5

2

a

Catalyst

T

R

CO

Reaction Rate

Selectivity HCHO Productivity

1

d^mol^g^'S' )

(mol%)

1

(g'kg^-h' )

CO CO HCHO 2

550 Empty Reactor 600 650

0.02 0.04 0.15

550 600 650

Si0

2

4%Mo0 /Si0 3

5%V 0 /Si0 2

a

5

2

2

2

36 46

63 52

0.3 0.8

4.0 14.4 44.3

10 7 14

11 18 23

79 75 63

34.0 116.0 304.0

550 600 650

1.0 3.1 10.5

30 30 28

2

70 70 70

7.6 22.7 76.1

550 600 650

15.3 60.8 210.6

37 38 14

2 14 51

61 48 35

101.0 318.0 793.0

1

_

For reaction conditions see Figure 1.

The validity of the data obtained with the batch reactor has been confirmed by continuous flow reactor tests. In Table II are shown comparative tests in the reactivity of the unpromoted S i 0 catalyst by using batch and flow reactors at 600 and 650° C. The good agreement of these tests confirms the adequacy of our batch reactor approach. On the basis of the above data obtained with the empty reactor and of the results of other series of experiments performed with differently sized empty and SiC or quartz filled reactors as well as with an empty stainless-steel reactor, we observe that the contribution of the gas-phase reaction in the Τ range investigated is ab solutely negligible. 96002 0 -Q 'uoîfiiMifse/n "ΛΙ 'N -is mi ssn 2

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48

CATALYTIC SELECTIVE OXIDATION

Table II. Methane Partial Oxidation over S i 0 Catalyst in Batch and Continuous Flow Reactors 2

Experiment

T CO

Reaction Rate (itfmol^g^-s )

R

Selectivity HCHO Productivity (mol%) (g-kg^-h' ) C0 CO HCHO

1

1


2

600

13.2

13

50

37

52

650

38.4

15

59

26

108

600

10.4

1

61

38

43

650

41.6

13

57

30

166

Batch Reactor

Flow Reactor

b

Reactor size: , 10 mm; 1, 100 mm. Reaction conditions: P , 1.1· 10 kPa; flow rate, 1000 Ncm^min" ; W ^ , 1.0 g. Reaction mixture composition (mol%):CH , 18.4; 0 , 9.2; N , 18.4 and He,54.0. id

2

1

R

4

2

2

Moreover, as neither the concept of surface initiated homogeneous-heterogeneous reaction (11) can be invoked to explain our results, it can be stated that the methane partial oxidation reaction proceeds via a surface catalysed process which likely involves specific catalyst requirements. However, by comparing the HCHO productivity of the different catalytic systems previously proposed (9) with that of our 5% V 0 / S i 0 catalyst, it emerges that our findings constitute a relevant advancement in this area (23). 2

5

2

Catalyst Properties and Reaction Mechanism. Apart from the generally accepted theory of the pure homogeneous gas phase oxidation of methane, based on a chain reaction of free radicals (26), three distinct theoretical approaches have been adopted to formulate the mechanism of the partial oxidation of methane to formaldehyde on oxide catalysts. One consists in a classical Langmuir-Hinshelwood model, where all the reactions take place on the surface (27). It is assumed that the reacting molecules interact simultaneously with the catalyst surface and the reaction between such activated species gives rise to the formation of the reaction products without the participation of oxygen from the oxide lattice (28). Dowden et al. (27) pointed out that the dissociation of methane and the activation (i.e., interaction with surface activated oxygen species) require quite different sites. This led to the suggestion of using afunctional catalysts (29,30). By contrast, Lunsford's group (31) claimed a stepwise mechanism involving the reduction of the oxide surface by the reacting C H and its reoxidation by the oxidant N 0 . This cyclic mechanism invokes the formation of active O" ions on the surface and their interaction with C H molecules to form methyl radicals which rapidly react with the surface yielding methoxyde complexes and then HCHO and/or CH OH. Therefore, the oxide 4

2

4

3


4.

PARMALIANA ET AL.

Silica-Supported Mo0 & V 0 Catalysts 3

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lattice seems to supply the oxygen atoms of the oxygenated molecules. A third approach deals with the proposition of an heterogeneous-homogeneous mechanism (11). Even if the HCHO is assumed to be formed on the surface, it essentially entails the generation of radicals on the surface and formation of products in gas phase. This last theory still presents some undefined points providing on the whole semiquantitative analyses of such processes (11). Therefore, two fundamental issues still remain under debate: i) the direct participation of lattice oxygen in the formation of selective oxidation products and ii) the role of the acidic properties of the catalysts on their reactivities (32). In order either to answer the above questions or to understand the antithetic behaviours of the silica supported M o 0 and V 0 catalysts, basic insights into the reaction mechanism have been gained by comparing the catalytic behaviour of pure Si0 , 4%Mo0 /Si0 and 5%V 0 /Si0 catalysts in the presence and absence of molecular 0 . These studies have been performed in a pulse microreactor by continuous scanning of the reaction mixture with QMS technique. In Figures 2-4 are shown the rates of formation of HCHO, CO, C 0 in presence of 0 in the T range 550-650°C. These data further support the superior activity and efficiency in HCHO production of the 5%V 0 /Si0 catalyst and the apparent poisoning effect exerted by the M o 0 on the reactivity of the S i 0 surface. The amount of the products formed over the studied catalysts, in the presence and absence of molecular 0 , are listed in Table III. It is evident that the formation of the oxidation products is associated with the gas phase oxygen supply. Then, as the reaction rates in the mixture of reactant and in separate steps differ (19), these data exclude the participation of lattice oxygen in the partial oxidation of methane via a two step redox mechanism as main reaction pathway proving the occurrence of a "concerted mechanism". Since the acid-base interactions of reagents (or products) with the catalyst could in principle affect the reaction pathway (32), the acidic properties of bare Si0 , 4%Mo0 /Si0 and 5%V 0 /Si0 have been comparatively evaluated by ZPC and NH -TPD measurements. The results are reported in Table IV in terms of ZPC value, amount of N H uptake and temperature of N H desorption peak maximum (T ). The bare S i 0 surface, after treatment at 600°C either in 0 /He or CH /0 /He atmosphere, is unable to adsorb N H . However, it is evident that both 4%Mo0 /Si0 and 5%V 0 /Si0 are considerably more acidic that the bare S i 0 sample, -with 4%Mo0 /Si0 resulting the most acidic system. These evidences do not find any correspondence in the reactivity scale, hence the change in the acidic properties cannot be the reason for the difference in the catalytic activity of the studied systems. On the other hand, it is generally accepted that the redox properties of the selective oxidation catalysts control the oxygen activation as well as the surface stabilization of the oxygen activated species and their reactivity (19). In particular, the stabilization of active oxygen forms requires the presence of reduced sites on the surface. In fact, the peculiar behaviour of Mo, V and Fe oxides in selective oxidation reactions is strictly linked with the stabilization of reduced states (19). This point has stimulated a growing interest in providing correlation between the degree of reduction (32) or the extent of reduced sites (20) and the reactivity in 3

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5

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R

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M

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25

100

20

80

ο

• CH

ο £15 Catalytic Selective Oxidation Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/26/15. For personal use only.

Ε 60

Conversion • 0 Conversion Α HCHO Selectivity

c ο

4

2

- 40

.

ω 10

c ο υ

> ο ω Φ

(/) Ο I

-

20

0

I

I

I

I

1

2

3

4

ϋ I

5

Reaction Time [h]

Figure 1. Batch reactor test of methane partial oxidation on S i 0 catalyst at 600°C. Reaction conditions: P , 1.7· 10 kPa; recycle flow rate, 1000 Ncm^min' ; W ^ , 0.05 g; reaction mixture composition (mmol): C H , 18; 0 , 9; N , 18 and He, 53. 2

2

1

R

4

2

2

7 A 4% Mo0 /SiO • 5% V O /SiO • SiO 3

6

2

w

s

s5 ο

b) ο4 Φ

ο Ε Φ 2

ce

DC 1

550

530

570

590

610

630

650

670

Reaction Temperature [°C]

Figure 2. Methane Partial Oxidation. Rate of HCHO formation on unpromoted Si0 , 5 % V 0 / S i 0 and 4%Mo03/Si0 catalysts. Reaction conditions: P , 1.2-10^ kPa; He carrier flow, 50 Ncm^min" ; W , 0.05 g; pulse, 5.5 Ncm of C H , 0 and He mixture (CH /0 /He=2/l/7). 2

2

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2

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1

R

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c a t

4

4

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Silica-Supported Mo0 & V O Catalysts

PARMALIANA ET AL.


4.

3

2

s

20 • 4% Mo0 /Si0 3

~16

_

• 5% V 0 / S i 0 • Si0 2

CO

5

2

2

2

υ 12 ω ο Ε

DC

4

. , 530

I 550

I 570

I 590

I I 630

I 610

• 650

670

Reaction Temperature [°C]

Figure 4. Methane Partial Oxidation. Rate of C 0 formation on unpromoted Si0 , 5%V 0 /Si0 and 4%Mo0 /Si0 and catalysts. For reaction conditions see Figure 2. 2

2

2

5

2

3

2

51

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Table III. Influence of the Gas Phase Oxygen on the Products Formation in Methane Partial Oxidation over S i 0 , 4% M o 0 / S i 0 and 5% V 0 / S i 0 Catalysts. Pulse Reactor Data * 2

Catalyst

T (°C) R

3

Reagents

2

2

5

2

Product Amount (10 molec/pulse) CO co HCHO xl


2

550

CH + 0 CH CH + 0 CH CH + 0 CH 4

2

4

Si0

600

2

4

2

4

650

4

2

4

550

CH + 0 CH CH + 0 CH CH + 0 CH 4

2

4

600

4

2

4

650

4

2

4

550

CH + 0 CH CH + 0 CH CH + 0 CH 4

2

4

600

4

2

4

650

4

2

4

a

0.21 0.56 1.18 -

0.87 2.30 5.60 0.25

0.41 0.87 2.11 -

0.10 0.30 0.85 -

0.16 0.35 1.20 -

0.36 0.65

0.56 1.20 1.92 0.10

0.82 6.68 0.45 25.50 2.40

0.46 2.76

1

0.75 -

5.60 1.10

3

W ^ , 0.05g; He carrier flow, 50 Ncm^min" ; pulse, 5.5 Ncm of (i) C H and He mixture (CH /He=2/7) and (ii) C H , 0 and He mixture (CH /0 /He=2/l/7). 4

4

4

2

4

2

selective oxidation of light alkanes. On this account, we have estimated the density of reduced sites (number of reduced sites per g of catalyst, s / g^) by oxygen chemisorption after treating the catalyst sample under reaction conditions. It is worth noting that this approach allows the probing of the surface redox properties of "working catalysts" under steady state conditions. Care was taken in avoiding a bulk reduction of the oxide catalyst (21). Separate experiments have indicated that the extent of 0 uptake depends upon the chemisorption temperature. Therefore the estimation of reiduced active sites after treating the catalyst in atmospheres different from the reaction mixture as well as the evaluation of the 0 chemisorption at temperatures different from that of reaction could lead to a very erratic assessment of the amount of reduced sites. As shown in Figure 5, the density of reduced sites on 5 % V O / S i 0 catalyst is 4-10 times higher than that of bare S i 0 and 4%Mo0 /Si0 catalysts. Then, according to Le Bars et al. (32), we have observed r

2

2

2

s

2

3

2

2

4.

53

Silica-Supported Mo0 & V O Catalysts

PARMALIANA ET AL.

3

2

s

that the reaction mixture is effective in reducing to some extent the catalytic sur face. The presence of reduced sites signals the capability of the catalyst surface in activating the gas phase oxygen (19).

Table IV. Acidity Characterization of S i 0 , 4% M o 0 / S i 0 and 5% V 0 / S i 0 Catalysts 2


2

Sample

5

ZPC

0

4

0 4%Mo0 /Si0 3

M

2

2

61.6

198

59.3

194

20.1

185

10.3

188

3.0

2

CH + 0 4

0 5

T rC)

2

CH + 0

2

1

fmol^g^

4.7

2

5%V 0 /Si0

2

NHyTPD Treatment at600°C

Si0

3

2

2

2

3.1

2

C H 4- 0 4

2

The rates of oxygen conversion on the studied catalysts in the Τ range 550650°C are shown in Figure 6. This parameter corresponds to the sum of the rates of products formation or in other words to the overall rate of methane conversion. These values, even if obtained with a transient technique (pulse reactor data), are in satisfactory agreement with those obtained with the batch and continuous reac tors (see Tables I and II). However, by comparing the data presented in Figure 5 and 6, it emerges that the trends of oxygen conversion (Fig. 6) are similar to those of the density of the reduced sites (Fig. 5). This indicates that the reactivity of Si0 , 4%Mo0 and 5 % V 0 promoted S i 0 catalysts is mainly controlled by the amount of reduced sites. Assuming that the methane partial oxidation occurs according to a concerted mechanism, it follows that an active catalyst must expli cate concomitantly two actions: i) the activation of the C H molecule and ii) the activation of the gas phase 0 . Then, taking into account the above experimental evidences and the literature data (33) dealing with the suitability of the S i 0 sur face in activating C H molecules, the following reaction scheme seems adequate to describe the partial oxidation of methane on the bare S i 0 surface: 2

3

2

5

2

4

2

2

4

2

CH

4

+

L

(CH )* 4

(1)


54 CATALYTIC SELECTIVE OXIDATION

4.

3

+ CH * + Ρ* 0

R ^ Ο* — —

2

4

2

s

2(0)*

(2)

(3) (4)

P* Ρ+ L+ R

where (CH^* and (O)*, indicate C H and 0 surface activated species respec tively, L and R represent specific S i 0 centres for C H activation and reduced site respectively, while Ρ refers to HCHO and C 0 . It can be also envisaged that the acidic properties of silica are quite sufficient in promoting the C H activation, while its redox properties are rather weak. Therefore, for the bare S i 0 catalyst it can be argued that the rate determining step is the oxygen activation. This state ment has been confirmed by performing additional experiments on the catalytic be haviour of several commercial S i 0 samples (12) prepared by different methods (pyrolysis, sol-gel, precipitation and extrusion). In fact, a linear relationship be tween the density of reduced sites and the reactivity of such S i 0 samples has been found (34). The higher activity of the 5%V 0 /Si0 catalyst can be explained in terms of higher density of reduced site and of stabilization of reduced state of vanadium ions (8, 14-15, 19, 21, 32). Such features enhance the rate of oxygen activation and likely favour the stabilization of active oxygen forms providing the formation of selective oxidation products (15, 18-19, 27). On the contrary, M o 0 lowers the "degree of reduction" of the S i 0 surface and correspondingly the rate of oxygen activation (Figure 6) and the activity of the catalyst (Table I). In order to explain this evidence, it can be inferred that on the silica surface molybdenum ions exist in an octahedral coordination (13) which assists the stabilization of the highest oxida tion state ( M o ) . 4


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Silica-Supported Mo0 & V 0 Catalysts

PARMALIANA ET AL.

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2

4

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4

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2

2

2

5

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3

2

6+

Conclusions - The partial oxidation of methane to formaldehyde with molecular 0 over unpromoted S i 0 and 4%Mo0 or 5 % V 0 promoted S i 0 catalysts, in the Τ range 550-650°C, proceeds via a concerted mechanism. - The functionality of the S i 0 surface towards the formation of HCHO is sig nificantly promoted by V 0 , while it is depressed by the M o 0 . -The stronger acidity of 4%Mo0 /Si0 and 5%V 0 /Si0 catalysts with respect to the unpromoted S i 0 support seems to exert no direct influence on the reaction pathway of the methane partial oxidation. - A direct relationship exists between the density of reduced sites and the reactivity of such oxide catalysts in the partial oxidation of methane to formaldehyde. 2

2

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Acknowledgments The financial support of this work by the Consiglio Nazionale delle Ricerche (Roma) "Progetto Finalizzato Chimica Fine II" is gratefully acknowledged.

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Silica-Supported MoO & V O Catalysts 3

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30 Otsuka, K.; Hatano, M. J. Catal. 1987, 108, 252 31 Liu, H.F.; Liu, R.S.; Liew, K.Y.; Johnson, R.E.; Lunsford, J.H. J.Am. Chem.Soc. 1984, 106, 4117 32 Le Bars, J.; Vedrine, J.C.; Auroux, Α.; Pommier, B.; Pajonk, G.M. J. Phys. Chem. 1992, 96, 2217 33 Low, M.J.D. J. Catal. 1974, 32, 103 34 Parmaliana, Α.; Sokolovskii, V.; Miceli, D.; Arena, F.; Giordano, N. in preparation. RECEIVED October 30, 1992

Catalytic Selective Oxidation - American Chemical Society

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