Kinetic Effects of Chemical Modifications of PMo12 Catalysts for the Selective Oxidation of Isobutane

Reaction Kinetics and the Development of Catalytic Processes G.F. Froment and K.C. Waugh (Editors) 91999 Elsevier Science B.V. All rights reserved.

283

K i n e t i c E f f e c t s of C h e m i c a l M o d i f i c a t i o n s of PMo12 C a t a l y s t s for t h e S e l e c t i v e O x i d a t i o n of I s o b u t a n e M. Sultan, S. Paul and D. Vanhove ~ Laboratoire de G~nie Chimique et d'Automatique, Ecole Centrale de Lille et Ecole Nationale Sup~rieure de Chimie de Lille, BP 48, 59651 Villeneuve d'Ascq, France

Abstract A kinetic approach of the screening of HPA-catalysts for the direct transformation of isobutane into methacrylic acid is carried out. PMo12 HPA have been modified by substitution of the countercations by NH4 § and/or Cs § and insertion of V in the Keggin structure. The rate of consumption of isobutane is well represented by a Mars & Van Krevelen model and a single simplified reaction scheme can be proposed for all the catalysts tested. The study of the rate constants obtained underlines quantitatively the role of each modification on the catalytic performance. Hence, V stabilises the catalysts and enhances the selectivity into desired products but the main effects are encountered w h e n NH4 § and Cs* countercations are added to the formulation. NH4* lowers the direct degradation of isobutane whereas Cs § strengthens the activation of isobutane while reducing in parallel the degradation of products.

1. INTRODUCTION

Because of the global abundance of liquefied petroleum gas (LPG), interest in the potential use of propane and butanes as sources of corresponding alkenes or their derivatives is increasing [1,2]. In the last decade much progress has been made and various kinds of catalytic reactions and processes have been proposed, particularly for the selective oxidation of light alkanes with molecular oxygen in gas phase. The most successful process for the oxidation of n-butane has been industrialised [3-5]. In a same approach, isobutane could be used in a near future to produce methyl methacrylate, an important monomer of resins. Industrial production of this methacrylate is traditionally achieved by the acetocyanohydrin process [6-8]. However, this process uses the dangerous hydrogen cyanide and overproduces solid ammonium bisulphate. Recently, alternative methods - the methylation of propionaldehyde and the oxidation of isobutene - have been developed but these processes still have problems using high-price feedstocks and consist in two steps synthesis [6-9]. Therefore, direct synthesis of methacrylic acid via the oxidation of isobutane looks more promising. Obviously, this reaction needs a multifunctional catalyst since the reaction is a multielectron oxidation. Many patents and papers have already been published concerning this reaction [10-15] and the catalysts used are mostly Keggin-type heteropolycompounds (HPA) containing phosphorus as central element and molybdenum as peripheral atom and modified by the addition of v a n a d i u m in the primary structure and of different metal ions in cationic position. At present, however, the achievements reported are not good enough to be industrialised reflecting the high difficulty of the effective activation of isobutane over solid surfaces. Corresponding author- E-mail : [email protected]

284 In most of the works, the catalysts were evaluated by direct comparison of the conversions and selectivities obtained in standard operating conditions (contact time, temperature, partial pressures). This approach makes it difficult to understand the effects of formulation modifications due to the strong dependence of selectivities on isobutane conversions and temperature. Moreover, the important thermal effects observed at high conversions cause a drastic decrease of selectivity and activity [16]. The purpose of this study is, therefore, to trace a new route towards formulation of more active, selective and stable catalysts for the reaction. To achieve this objective, a quantitative evaluation of the effects of each modification in composition on the isobutane activation, the reoxidation of the catalyst but also on the selectivities obtained has been carried out.

2. EXPERIMENTAL 2.1. P r e p a r a t i o n of the catalysts (NH4)3PMo120,0 synthesis has been described in [17]. The desired quantities of ammonium molybdate and phosphoric acid were dissolved in hot water, and then 10ml of conc. HNO 3 were poured into the solution in order to precipitate a yellow compound. Dried solid was calcinated at 350~ for 5h and used as such for the catalytic tests. H4PMol~VO,0 was prepared by a method derived from Courtin works [13]. It consists in the preparation of three solutions: i) Sol.[A] : 0.1 mole of NaVO 3 was dissolved to 500 ml of boiling water, then 0.1 mole of Na2HPO4.2H20 was added and the solution obtained was cooled at room temperature; ii) Sol.[B] : 1.1 mole of NaMoO4.2H20 was dissolved to 500 ml of water at ambient temperature; iii) Sol. [C] : 410 ml of conc. HC1 (37%). Sol.[A] was acidified rapidly by a fraction of Sol. [C]. Then Sol. [B] was added dropwise and finally the remaining of Sol. [C]. A red orange solution was obtained and cooled to ambient temperature. H4PMo~VO40 was extracted by diethyl ether, then a quantity of w a t e r equivalent to half of the volume of the organic phase was mixed to it. After the evaporation of ether, the remaining aqueous solution was placed at 4~ to crystallise. Ammonium and caesium salts of H4PMo~IVO,0 were precipitated from mixing 50ml of 0.2M chloride salt solution with 20 ml of 0.005M H4PMollVO40 solution. The precipitates were washed four times by centrifugation to eliminate the unreacted compounds and then dried at 50~ Mixed salts were prepared by dispersing required ratio of the insoluble salts in 20 ml of water. After stirring, the suspension was dried at 50~ The synthesis of the caesium, ammonium coprecipitate catalyst (catalyst F in Table 1) is described in detail in [18]. All the catalysts tested (Table 1) were dried at 120~ overnight in an oven to evaporate the w a t e r of crystallisation. Moreover, as heteropolyanions are very sensitive to temperature, a thermal pre-treatment has been done at 360~ for 5h under a nitrogen flow in a view to stabilise their catalytic performance.

285 Table 1 List of the catalysts tested . . . . . . . . . . . Catalyst ......Reference ......Preparation . Method H4PMollVO40 A Crystallization (NH4),PMol~VO40 B Precipitation (NH4)3PMo 12~ 40 C Precipitation Cs1.2Ho.35(NH4)2.45PMollVO40 D Mixture Cs175Ho.6(NH4)l.65PM~ E Mixture CSl .~(NH,)~.~PM011VO,~ F Coprecipitation

2.2. A p p a r a t u s

The experimental investigations were conducted in a t u b u l a r fixed-bed reactor described in [18]. In order to have an isothermal catalytic bed, differential conditions of conversion were m a i n t a i n e d and dilution with SiC powder (250 l~m) was used. The fixed bed consists in three 5 cm high layers : the catalytic bed made of 3 ml of catalyst (3.7 g) diluted in SiC (1:1 by volume) was sandwiched between identical pure SiC layers. The reactor was fed with a mixture of isobutane (0.09-0.26 atm), oxygen (0.060.20 atm), w a t e r (0.12 atm) and nitrogen at a total flow rate of 3N1/h. All the experiments were carried out at 340~ and i atm. 2.3. A n a l y s i s o f t h e r e a c t a n t s a n d p r o d u c t s

The concentrations of each component (except water) at the inlet and outlet of the reactor were determined by on-line gas chromatography. Two i n s t a n t a n e o u s mass balances, based on the conversions of the reactants (isobutane and oxygen) and the yields in oxidised products, were calculated (see [18] for detail). 2.4. C a l c u l a t i o n s m e t h o d

The carbon and oxygen balances are usually close to 100%. However, the sum of the selectivities is very sensitive to the fluctuations of C balance especially at low isobutane conversion. This is essentially caused by the lack of accuracy in the estimation of isobutane conversion in differential conditions. Thus, in order to avoid erratic results during the determination of kinetic parameters, the isobutane conversion used for modelling was t a k e n as the sum of the products yields. The reaction rate for each reactant or product can be evaluated by the global balance on the catalytic bed : (p~ - Pi)V~a,. r~ =

t cmRT

-

Fi~

pjVcata F/BuYj

rj - t ~ m R T -

m

m

i = iBu or 0~. j = MACO or MAA

286 3. R E S U L T S A N D D I S C U S S I O N We have recently proposed a more rational method for catalyst screening based on a kinetic study [18]. Actually, the rate of disappearance of isobutane on heteropolyanionic type catalysts has well followed the redox kinetic model of Mars and Van Krevelen (MVK). This model is based on the redox dynamics of the catalyst sites, reduced by reaction with hydrocarbons coming from the gaseous phase and further oxidised by the gaseous oxygen, as follows : iBu + Cata-O

~- Products + C a t a

Cata + 02

,-"

iBu + 0 2

Cata-O

~ Products

The balance on catalytic sites at steady state leads to the following equation" r ~

kr "ko " PiB, " Po2 N s kr " PiB, + k o P o 2

where N s cannot be easily determined. Consequently the values of the e s t i m a t e d kinetic p a r a m e t e r s are the specific rate constants : kr.N s and ko.N s. Since the p a r t i a l pressure of isobutane is very high in comparison to t h a t of i n t e r m e d i a t e s at low isobutane conversions, their reaction rates have not been included in the sites balance in order to avoid the excess of kinetic parameters. Therefore, the corresponding t e r m s for products do not appear in the denominator. This a p p r o x i m a t i o n has been verified a posteriori as being founded and it permits us to write a r a t e equation i n d e p e n d e n t of the products kinetic terms. These kinetic p a r a m e t e r s were d e t e r m i n e d by a non-linear regression method (Marquardt's m e t h o d [19]) based on the sum of the squared differences between e x p e r i m e n t a l and calculated values of outlet isobutane partial pressure expressed by

o.f .- ~(PiBu -/3iBu) 2 i=1

o ~) iBu = P iBu - -

22.4-tc. Vcata

where

m. k o

k9 r -Po2 " -PiBu

"3.6" (k,.-Pibu - ~ k o "Po 2 )

N s

In order to d e t e r m i n e well-defined rate constants, large ranges of isobutane and oxygen feed concentrations have been used.

287

j

0.30 0.25

. . . . . oA

aB

+D

0.20

~0.15 ~0.10

0.05 0.00 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Experimental partial pressure of iBu (atm)

Figure 1. Calculated vs. experimental values of isobutane partial pressure. Figure 1 shows the excellent agreement obtained between experimental and calculated values of isobutane partial pressure. This means t h a t the MVK model remains applicable for all the HPA tested in this work. Table 2 Redox rate constants Catalyst

Reference

H4PMo11VO40 A (NH4)4PMo11VO40 B (NH4)3PMo~2040 C Cs 1.2Ho.35(NH4)2.45PMo11V040 D C s 1.75Ho.6(NH4)1.65PMO 11VO40 E .......................Cs:,:~(NH,)2.~PMo,:V0,n ................................................F ............

ko.N "10 .3 3.3 5.1 3.3 16.3 21.9 11.0

kr.N ~ "10 .3 2.6 1.3 O.8 3.4 7.6 2.4

ko/kr 1.3 3.9 4.1 4.8 2.9 4.6

Preparation Method C P P M M CoP

W h a t e v e r the catalyst, the results in the Table 2 show t h a t the value of ko.N s is higher t h a n kr.N, the activation of isobutane is therefore always the limiting step. Moreover, a strong influence of the composition modifications is observed. The total substitution of the protons by NH4 § ions increases twice k o.N~ values whereas kr.N ~ is decreased in the same proportion (catalysts A & B). This behaviour can be a t t r i b u t e d to the reduction of the acidity of the HPA. When a V atom is introduced in the Keggin structure of (NHt)4PM01204o (catalysts B & C), the stability of the catalyst is significantly improved. Moreover, the rates of activation of isobutane and reoxidation are identically enhanced (constant ko/kr). Pure Cs3HPMo11VO40 and KtPMo11VO40have proved to be completely inactive for the reaction studied. The partial substitution of NH4 § with Cs § during the coprecipitation of the salt (catalyst F) leads to a further twice increase of both rates of reoxidation and reduction. The mixtures of catalysts (D & E) showed very interesting results. They are actually the most active of the set of catalysts tested. Both values of ko.Ns and kr.N ~ are increased in the same proportion for the catalyst D whereas the best results are obtained for the Cs175Ho.6(NH4)1.65PMo~1040 formulation (E) for which the rate of isobutane activation is more enhanced t h a n the reoxidation one (lower ko/kr). This result shows the i m p o r t a n t role of the Cs ratio in the catalyst. It is proposed t h a t the Cs ratio allows to control the acidity of the HPA. If it's too weak the activation is

288 impossible, on the contrary, if it's too strong the degradation is important. A good balance have then to be found to keep both activity and selectivity. The catalyst E gives a significant increase in MACO and MAA yields under standard conditions (Table 3). In varying operating conditions, this yields reaches up to 5.3%. It has been noted t h a t stability is relatively long to be attained with this kind of catalysts. It has to be underlined at this point that the study of the Table 3 on the only basis of selectivities and conversions would place the catalyst E in a bad position because of the apparent low selectivity in desired products. The total yield is a better feature of its performance. Table 3 Catalysts performance - reaction conditions: isobutane 0.26 atm, O 5 0.13 atm, H~O 0.12 atm, T-340~ tc=3.6S ~P = l atm. Catalyst Ref. ~su Xo~ SMAA S MACO S C O Total (%) (%) (%) (%) (%) Yield (%) A 2.5 2.3 25.0 38.9 26.8 1.6 H,PMo11VO,o B 2.3 11.4 49.4 32.2 12.5 1.9 (NH,),PMox~VO,o C 1.5 7.9 31.9 43.8 18.4 0.7 (NH,)3PMo~.O,o D 6.1 36.3 45.6 14.8 29.7 3.7 Cs1.~Ho.35(NH,)~.,sPMol~VO,o E 10.3 72.4 32.1 8.1 45.5 4.1 Cs~ ~sH06(NH,)~6~PM~ ,0 Cs, .~(NH,)2.~PMo,1VO,o F 4.1 26.0 44.6 23.3 ......25,6 . 2.8 ............ Furthermore, the following simplified reaction scheme is proposed for studying the effects of catalyst's composition on the various steps of the reaction: KI

Isobutan!

Methacrylic Acid + Methacrolein ~K3 ~ DegradationProducts

The rate of product formation can be derived from this scheme as dP iBu

k ok r -ffiBuff o,

dt

k o P o~ + k r P iBu

~ m

dPAMACO dt

NS

klkoPiBuPo2 - k3koPo, PAuaCO koPo2 + krP~u

N s with k 1 = Kl.k~ ,k 3 = IZ~.k~and Kx+ K~ =1

After integration and simplification, these equations lead to

o

PiBu P AMACO =

9

9

(K,-l)

K 3 -1

The kinetic parameters were determined by the above discussed non-linear

289 regression method.

Table 4 Relative rate constants Catalyst H4PMollVO40 (NH4)4PMollVO40 (NH4)3PMo12040 Cs, 2H035(NH4)~45PMo,~VO40 Cs175H06(NH4)~.~sPMo~VO40 .......... Cs,.~(NH,)~:~PMo,,VO,o

Reference A B C D E F

K~

K3

0.75 1.0 0.9 0.73 0.6 ~ 0.76

13.0 19.0 25.0 5.6 7.8 9.0

Preparation Method C P P M M CoP

The results in the Table 4 show t h a t I~ is always much greater t h a n K1, i.e. the desirable products react so fastly compared to isobutane t h a t a good yield could not be obtained at high conversions. As compared to the other HPA tested, the pure ammonium salts give a negligible initial degradation of isobutane (I~ ~ 0) but the further degradation of required products is rapid (K 3 - 20-25). The introduction of a V atom in the Keggin structure has slightly increased the selectivity in valuable products by reducing K 3 (catalysts B & C) but the more significant effects in this way is the presence of Cs in the formulation. All the catalysts containing Cs (D, E, F) have actually much lower K 3 constants t h a n the others. The exact role of Cs is not totally elucidated but it is expected to play a role in the enhancement of the rate of transformation of MACO into MAA avoiding hence the degradation of the intermediate aldehyde.

3. C O N C L U S I O N This kinetic approach of the formulation of HPA catalysts for the selective oxidation of isobutane into MACO and MAA seems to be promising. Actually, it allows to underline the importance of V in the Keggin structure, stabilising the solid and leading to more selective catalysts. Moreover, NH4 § cations presence leads to very selective catalysts at low conversion but their weak acidity prevents a good isobutane activation. In this way their coexistence in the formulation with Cs § cations is very important because it gives more active catalysts leading to good selectivity and therefore to higher yields. The role of Cs § cations is not determined precisely but an effect on the rate of transformation of MACO into MAA is expected.

Abbreviations and Notations MAA : methacrylic acid MACO

: methacrolein

isobutane 9 F"i : inlet molar flow rate of reactant i (mol/h) k o : turnover frequency for the reoxidation step (mol/atndsite/h) k r : turnover frequency for the reduction step (mol/atm/site/h) m : weight of the catalyst sample (g) iBu

290 N " concentrations of sites in the catalyst (sites/g) Pi: inlet partial pressure for component i (atm) Pi " outlet partial pressure for component i (atm) Pi" mean partial pressure in component inside the reactor (atm) /~i,/~j" calculated outlet partial pressure for reactants or products (atm) o

ri, rj" mean reaction rate for reactant i or product j (mol/h/g) S selectivity 9 for products t c contact 9 time (s) Vcata" volume of catalyst (ml) X~" conversion of reactant i Y yield 9 in products

References

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