Obtaining butadiene by oxidative dehydrogenation of n-butenes on a solid catalyst

OBTAINING BUTADIENE BY OXIDATIVE DEHYDROGENATION OF n-BUTENES ON A SOLID CATALYST* I. I. YUKEL'SON and E. A. :BOGUSLAVSKII Voronezh Technological Institute, Technological Faculty of Fundamental Organic Synthesis and Synthetic Rubber (Received 4 April 1964)

SCIENTISTS have given increasing attention to oxidative dehydrogenation reactions which make it possible to obtain olefin and diene monomers from the respective, more saturated hydrocarbons. The main feature of oxidative dehydrogenation is the partial oxidation of hydrogen of the molecule of an organic substance without disrupting the carbon skeleton. Being an irreversible reaction i~). practice, oxidative dehydrogehation makes it possible to obtain high yields of special products. The oxidative methods m a y acquire considerable importance in the synthesis of butadiene and isoprene from petroleum hydrocarbons. Thermodynamically, equilibrium butadiene yields do not exceed 70-80?/oo i~1 the industrial dehydrogenation of butene. On raising temperature, equilibrium yields increase; however, at the same time, due to the increased role of secondary reactions, the extent to which yields obtained in practice approximate to thermodynamically possible yields markedly decreases. Unlike convetional dehydrogenation, the oxidative method opens up possibilities of almost quantitative conversion of butene into butadiene. Literature references on heterogeneous oxidative dehydrogenation of hydrocarbons are extremely infrequent. A few English and American patents [1-4] are k:lown which propose catalysts for this process. The contacts recommended mainly consist of oxides of metals of groups V and VI. Indigenous studies ou the oxidation of butene and butadiene are reported in only three papers [5-7]. This paper studies the effect of temperature and the butene : air ratio on the technical features of oxidative dehydrogenation. A catalyst consisting of metal oxides of group V I I I obtained b y the authors was used in the experiments; elements of group IV of the periodic system were used as modifiers. Oxidation of butene was effected in the presence of steam at 300-400 °. An industrial butene fraction of the following composition (moles-%) was used *Neftekhimiya 4, No. 6, 834-838, 1964. 311

312

I . I . YUKEL'SON and E. A. BOGUSLAVSKII

as raw material: H~--0.1; C~H4--0.3; C3H6--0"6; C4H10--1.5; a-C4H8--32'6; fl-C4H8--63.4; C4H6--1.5. The space velocity of the butene-air mixture in the experiments was 1150 hour -~. The initial mixture was diluted with steam in the proportion of 1 : 7. EXPERIMENTAL

The investigations were carried out in a continuous laboratory apparatus. A quartz tube (8 mm diameter) equipped with an electrical heating coil was used as reactor. The reactor tube was filled with 1 ml catalyst of 2-2.5 mm particle size. Temperature was measured with a chromel-alumel thermocouple placed in the middle of the catalyst layer. The water, fed b y a measuring pump, was evaporated in the pre-contact zone of the reactor. Dilution with steam of butene was controlled b y the amount of condensate collected. Air and butene consumption was measured b y rheometers. The composition of the initial butene-air mixture was controlled ehromatographieallly; 2-3 determinations were made during the experiment. Contact gas was analysed chromatographically and in a VTI-2 device. Composition of reaction products. Table shows that in the range of 300-400 ° the reaction products consisted of butadiene, unreacted butene, C02, CO and a small amount of ethylene. Saturated hydrocarbons were not formed during the process. CONTACT GAS COMPOSITIOn" IN OXIDATIVE DEHYDROGENATION OF n-BUTENES ~i

. !

Dilution, tool.

Contact gas composition, moles- %

Io '-~1 °~ z~ 311 317 318 319 324 325

40~ 400 380 360 400 380

1150 1170 1100 1100 1110 1115

~

~

1. 688 il. 7.3017.65i 0.30 1.09 76.01 3.32! 0.13 0.801 71.63i 2.24! 0.08 1:4 6511:7.0018.0310 1: 48011: 700[ 9.23 0.40 1'38[ 70-97 2.10 0.08 1 . 4 . 7 4 1 : 6 . 8 0 ! 8 . 6 4 0.20 3.14j 70.36 1.461 0.05 1 6 . 0 0 ] 1 : 6 . 7 2 7.55 0.20 1 : 5 . 4 5 1 : 6 . 7 0 ! 7.35 0.60 !

75.33 2.36

0.07 o

0.25 0.35 0.33 0.31 0.26 0.28

4.96[ 6.2 9.63i 7.2

7.92 '! 7.5 9.00i 6.8 6.03! 6.9 6.80 7-2

I

At 360-400 ° hydrogen was found in contact gases in a proportion not exceeding 2-3 moles- ~ . Traces of acids were found in the aqueous condensate. Carbonyl compounds, tested b y formation of oximes hydroxylamine, were not detected. Effect of temperature. Investigation showed a marked effect of process temperature on the yield of main and by-products of the reaction. As shown in

Oxidative dehydrogenation of n-butenes

313

Fig. 1, on changing temperature from 300 at 400 °, butadiene yield, calculated on butene passed through, increases from 1 4 - 1 8 ~ to 3 5 - 4 4 ~ . The position of the maximum on the curve depends on the temperature and concentration of butene in the butene-air mixture. A reduction of butene concentration in the initial mixture within the range studied causes a displacement of the maximum yield towards high temperatures; the absolute value of the m a x i m u m increases. The conversion curves (Fig. 2) have a similar nature. The dependence of selectivity on temperature is show~ in Fig. 3. As can be seen, a moderate dependence of selectivity on reaction temperature is retained for three fixed dilutions. The value of 02 stoich C°~-- 02used is an interesting characteristic for the process studied, where 02 stoich is the amount of oxygen required for butadiene formation according to the reaction: 1 C4H s + -~ 02 = C4H 6+ H20, 02 used is the total amount of oxygen used during the process. Value Co, can represent the selectivity of the process for oxygen. As can be seen in Fig. 4, at temperatures corresponding to maximum butadiene yields, selectivity for oxygen does not exceed 16-17~. Since in the process %

a -7

c/

%

o-I

35

5b25 25

15

230×

I

310 FIG. 1.

o~

15290

I

I

1

~0

370

°C

FIG. 2.

Fin. 1. Dependence of butadiene yield calculated on butene passed through on the reaction temperature for various C4Hs: air ratios: 1--C4Hs: air = 1:7.2; 2-1:6.0; 3--1:4.7. These symbols also apply to the other Figures. FIG. 2. Effect of reaction temperature on butene conversion for various C4Hs : air ratios.

314

I.I. YUKEL'SON and E. A. BOOUSLAVSKII

studied no carbonyl and carboxyl compounds were formed, the low selectivity for oxygen m a y confirm the intense oxidation reaction of hydrocarbons. Figure 5 shows the dependence of uncombined hydrogen yield on process temperature. As can be seen, hydrogen yield calculated on decomposed butene decreases almost linearly with the reduction of temperature. o-7

70~- ~

..

50

x

)'

×

A_

3

o-1 ×-2 I ~,-3

I

280

×~

330 '

370

°C

230

330

FIG. 3.

])0

oC

FTG. 4.

FIG. 3. Dependence of butadiene yield calculated on butene decomposed on the reaction temperature for various C~Hs: air ratios. FIG. 4. Effect of temperature on selectivity for oxygen for various C4Hs: air ratios.

Effect of butene : air ratio. The characteristic displacement of maxim~ ou curves showing the dependence of butadiene yield on temperature (Fig. 1) confirms the effect of temperature and the factor of butene concentration in the initial mixture. A particularly marked effect of butene concentratio~l c,n the degree of its conversion and butadiene yield is observed over the range of dilutions from 1 : 4 to 1 : 7 (Fig. 6). As in the case of temperature curves, the curves showing the dependence of butadiene yield (calculated on butene passed) on the butene : air ratio pass through a maximum. Figure 6 shows that the higher the temperature, the greater the displacement of the maximum towards increased dilution.

290

330

370

°C

1.4

/G

1.8

I.IO

1:12

C4H8 AlP

FIG. 5.

FT~;. 6.

FIG. 5. Dependence of hydrogen yield calculated on butene decomposed on the reaction temperature for various C4Hs: air ratios. FIG. 6. Dependence of butadiene yield calculated on butene passed through on butene: air (mole) ratio at various temperatures.

Oxidative dehydrogenation of n-butenes

315

RESULTS

From experimental data obtained it can be assumed t h a t the rate of oxidative dehydrogenation and the rate deep oxidation rections increase with an increase of temperature and dilution of butene with air. Probably, both the initial butene and the butadiene formed are partially subjected to intense oxidation. The maximum on the yield curves corresponds to the moment when the anticipated rate increase of the main reaction is offset by the increased rate of secondary and side reactions. At increased temperatures the rate increase of oxidative dehydrogenation is high and only under conditions of marked dilution of butene with air do secondary reactions develop to such an extent t h a t they offset the anticipated rate increment of the main reaction. Paper [8] shows t h a t during oxidation of ~- or fl-butene to butadicne, the primary phenomenon of hydrocarbon conversion is cleavage of hydrogen and ~dsorption of crotyl 0~1 the catalyst surface: I-IzC= CH--CH~--CH~ -~ H2C.~.CH.-~.CH--CH3÷ H H3C--CH = CH--CH3

~ $1

$2

where S~ and S 2 are the active surface centres. Subsequent combination of hydrogen with oxygen to form water, apparently, takes place on the catalyst surface. It can be assumed t h a t the hydrogen in the reaction products was formed during the non-catalytic processes in zones of local over-heating, is spaces between grains. For further elucidation (.f the mechanism of the process, detailed kinetic investigations are required. Comparing the results Gf this study and the results reported in references [5-7], it should firstly be noted that the operating temperatures of the process iu our experiments were 120-150 ° lower. Under these conditions approximately the same values were obtained for butadiene yields calculated on butene passed through und.:r somewhat poorer conditions of selectivity. SUMMARY

1. An active catalyst was obtained for oxidative dehydrogenation of n-butenes into butadiene. The butadiene yields obtained in the experiments, calculated on butene passed through, were 4 0 - 4 4 ~ ~ i t h a selectivity of 7 0 ~ . 2. A study was made of the effect c,f temperature and concentration of butene in the initial butene-air mixture on the technical efficiency of the process. I t was sho~n t h a t the curves of the dependence of butadiene yield on temperature and dilution of butene with air pass through a maximum. Translated by E. SEI~IERE

316

I . I . YUKEL'SONand E. A. BOGUSLAVSKII

REFERENCES 1. 2. 3. 4. 5.

Brit. Pat. 902952; 9, 2, 1962; Ref. Zh. Khim., 16 N1OP, 1963 Brit. Pat. 915590; 16,1, 1963; Ref. Zh. Khim., 21N2P, 1963 U.S.A. Pat. 3028440; 3, 4, 1962; ReL Zh. Khim., 13N15P, 1963 U.S.A. Pat. 2991321; 4, 7, 1961; Chem. Abstr. 56, No. 7, 6706a V.A. KOLOBIKHIN, I. Ya. TYURYAYEV, V. M. SOBOLEV and YE. N. YEMEL'YANOVA, Dokh Akad. Nauk SSSR 144, 5, 1053, 1962 6. M. S. BELEN'KII and T. G. ALKHAZOV, NeW i gaz 9, 57, 1963 7. T. G. ALKHAZOV, M. S. BELEN'KII, R. I. MOTYAKOVA and V. M. KHITEYEVA, Neft' i gaz 2, 49, 1964 8. W. M. H. SACHTLER, Recueil. tray. chim. 82, No. 2, 243, 1963