lithium iodide melts

Applied Catalysis, 77 (1991) 163-174 EIsevier Science Publishers B.V., Amsterdam

163

Oxidative dehydrogenation of propane in lithium hydroxide/lithium iodide melts Ivar M. Dahl’, Knut Grande2,Klaus-J. Jensl, Erlmg RytteP, &e Slagtern**’ ‘Senter for Zndustriforskning, P. Box 124, Blindem, N-0314 Oslo 3 (Norway), tel. (+47-2)452010, fax. (+47-2)452040 ?Statoil F&U, Postuttak, N-7004 Trondkeim (Norway), tel. (+47-7)584011, fax. (+47-7)!?67286 (Received 1 June 1991)

Abstract

The reactions of propane with iodine have been evaluated theoreticaiiy in the temperature range 700900 K. These calculations have been compared with test runs with propane/air in lithium hydroxide/ lithium iodide melts. Thermodynamic and kinetic conkderations show that low temperaturea give higher propene selectivities. The experimental rem&a confirm this and indicate that the lea&ion pm by a radicai mechanism. A major factor governing propene r&rctivity is the ratio of normal/isopropyl radicals. At the relatively high radical concentrationa encountered in the system, benzene ie the ultimate product. Optimizing the iodine/hydrogen iodide-catalyzed oxidative dehydrogenation of propane with oxygen for propene or benzene production is feasible. Keywords: benzene, iodine, lithium hydroxide, lithium iodide, oxidative dehydrogenation, propane, propene, salt melt.

INTRODUCTION

The objective of this work has been the study of an alternativeroute for the dehydrogenationof alkanes, especiallypropane. Direct dehydrogenation(i.e. the Star or the Oleflex process) suffers from the disadvantagesof thermodynamic limitationsand high reactiontemperatures.Oxidativedehydrogenation may give a higher yield at lower temperatures.Typically, solid catalysts like vanadium/magnesia catalysts give ca. 20% conversion with 60% selectivity [ 11. However, complete combustion to CO, may easily occur. Oxidative dehydrogenationof light alkanes using halogens, especially iodine, has previouslybeen explored by Shell and Petro-Tex [ 21. Iodine is produced from a salt melt (lithium iodide) and oxygen. Hydrogen iodide is removed by reaction with lithium hydroxide.The reactionsare assumedto be: 0166-9634/91/$03.50

0 1991 EIsevier Science Publishem B.V. AII ri&te reserved.

164

WL + Iz

+ CsH6+ 2HI

(I)

2HI + 2LiOH

+ 2LiI + 2H20

(2)

2LiI + 1/202 + HzO+ BLiOH+ IZ

(3)

C3H8+ 1/202

(4)

-+C,H,+H,O

If the reactions (l)-(3) are performed in one reactor, the total reaction will be oxidative dehydrogenation of propane (reaction (4) ). This reaction is thermodynamically very favourable and 100% conversion is expected. However, the scheme above is an ideal case; i.e. some CO, will be produced since oxygen and propane are present at the same time. Another concept is to separate the reactions into several reactors. Iodine may be produced in one reactor (reaction (3) ). In the next reactor propane can be dehydrogenated by iodine and the hydrogen iodide produced can be adsorbed by lithium hydroxide (reaction ( 1) and (2 ) ) . Shell [ 3 ] has patented two different single-reactor systems, where the reactions (l)-(3) take place in different zones in the reactor, and a solid acceptor capable of binding hydrogen iodide as a halide is circulated in the system. The high temperature reaction of propane with iodine to form propene and hydrogen iodide has been investigated earlier. The radical nature of the reactions seems to be well established [4,5]. In the present work the oxidative dehydrogenation of propane is accomplished in the presence of iodine and air. The mechanism of the reaction and the process as a whole is further elucidated on the basis of experiments as well as from thermodynamic and kinetic considerations. EXPERIMENTAL

Oxidative dehydrogenation of propane is performed in an alumina tube reactor as shown in Fig. 1. The height from the quartz sinter to the top lock is 20 cm. The inner diameter of the alumina tube is 3.4 cm. The experiments are performed in a lithium hydroxide/lithium iodide salt melt (starting content of LiOH = 2-5% ) . Propane and air are introduced continuously from the bottom of the reactor. The total gas flow at any time was 400 ml/min. A new gaspermeable sinter had to be used for each experiment since the salt melt reacted with quartz. During the heating period nitrogen is fed through the reactor in order to avoid production of iodine from oxygen and lithium iodide in the reactor. Analysis of the product stream is performed after increasing time on stream. The contact time of the feed with the melt is estimated to be in the range 0.1-l 8. The gas mixture remains in the gas phase in the reactor for another 10-20 8. The analytical set up is shown in Fig. 2. Analysis of the product stream is

165 Gas out

Stainless

Radiation

sinless

Steel top lock

bracket

steel lower

lock

Fig. 1.Schematic presentation of the reactor.

Air

HC

N2

Fig. 2. Schematic presentation of the experimental set up.

performed by use of a Hewlett Packard 5890 GC/5970 MS. The analytical system is calibrated with a known mixture of hydrocarbons from C, to Ce as well as air, carbon monoxide and carbon dioxide. The calibration factors for the iodinated hydrocarbons is related to the corresponding hydrocarbon. RESULTS

Dehydrogenation of propane is performed in 5 wt.-% LiOH*H,O/LiI salt melt at 6OO”C, with a propane& mixture of 1: 2 (melt height 5.1 cm). The results are given in Table 1. Conversion and selectivities are given in C, units.

166 TABLE 1 Dehydrogenation of propane in 5 wt.-% LiOH*H,O/LiI, 1:2 Time on stream (min) Conversion GH, (W) 02 (%) Selectivities (% of C,H,) GH, CzH, CzH, CO, CHI C,H, co 2-Iodopropane Iodoethene

melt height 5.1 cm, 6OO”C,propane:air

3

42

73

109

150

50.6 97.9

58.2 98.0

63.9 99.4

62.8 99.7

62.7 99.6

73.9

76.3 0.9 7.5 3.1 3.9 6.9 1.4

74.9 0.6 7.2 4.0 3.9 8.2 1.3

75.0 0.7 7.6 3.1 4.2 7.9 1.6

75.4 0.7 7.8 3.2 4.6 7.3 0.9 0.1

7.2 0.7 2.8 14.2 1.0 0.1

_

As can be seen from the table some benzene is produced, but very low amounts of iodinated products are detected under these conditions. Dehydrogenation was performed in 2.8 wt.-% LiOH.H,O/LiI at 666°C with propane : air mixtures varying from 1: 1 to 1: 4. Even though a propane : air mixture of 1:3 and 1: 4 corresponds to an excess of oxygen, assuming 100% conversion of propane and 100% selectivity to propene, all oxygen is consumed during the reaction. As the air content is increased, more CO,, benzene and iodinated hydrocarbons are produced. The residence time was changed by varying the height of the salt melt. It was generally observed that less salt melt gave a higher conversion of propene. This is due to the construction of the reactor (see Fig. 1) in that a smaller melt height gives a shorter contact time with the melt, but a longer time in the gas phase above the melt, and a longer total residence time in the reactor. Thus pure gas phase reactions play an important role in our experimental setup. Dehydrogenation of propane in 5 wt.-% LiOH*H,O/LiI melt at 566°C with a propane : air mixture of 1:2 to 1:2.5 has been performed (melt height 5.1 cm). The results are given in Table 2. We see that the main effect of increasing the air/propane ratio is to produce more CO,. The large amount of iodine-containing products found at 506°C points to post-reactor reactions, as these iodated compounds are not stable at 560 ’ C. They are probably mainly produced during cooling down of the reactor effluent. Most of these products could probably be taken out as propene by a suitable process modification.

167 TABLE 2 Dehydrogenation of propane in 5 wt.-% LiOH.H,O/LiI,

melt height 5.1 cm

Temp( “C) Propane : air Time(min)”

500 1:2 3

500 1:2 40

500 1:2.5 5

500 1:2.5 46

500 1:2.5 84

600 1:2.5 16

600 1:2 24

Conversion CsHs (%) 02 (%)

28.5 74.2

29.4 73.0

41.3 88.7

50.8 98.0

49.8 95.0

54.5 99.6

51.7 97.5

Selectivities % of C&H, :::

79.9

79.7

71.2

60.6

66.1

59.1

65.3

1.4 0.4 0.3 4.0

1.7 2.0 0.6 4.6 3.3 2.9 3.1 0.4 0.2 1.6

2.0 13.0 2.0 4.3 4.5 2.2 7.3 2.5 0.1 0.4 1.0

2.1 7.7 1.2 4.1 2.9 2.3 8.9 3.4 0.2 0.3 0.9

6.9 9.1 2.2 20.1 2.3

4.9 2.5 4.8 0.5

1.8 6.2 0.8 4.4 1.4 3.1 5.0 4.0 0.3 0.5 1.3

7.3 5.9 2.4 17.6 1.0 0.1 0.4

GH, CO, CH, GH, co 1-Iodopropane 2-Iodopropane Iodopropenes Iodoethane Iodoethene Methyliodide

1.2

0.3

“Time in the table is the time from which new conditions (temperature, mixing ratio) are established. The same salt melt is used for one series of experiments.

A variation in the conversion and selectivitiesis observed when comparing the resultsat 600°C in the last series of experimentswith those in Table 1. A major differencebetweenthe experimentin Table 2 and that in Table 1 is that the melt in the former case probably contains much less iodine. A blank experiment in an empty reactor was performed (SOO’C, propane : air 1:2). The conversion was almost as high as in a salt melt reactor (100% 0, conv., 53% C3HBconv.). The selectivities,however, were very different: &He: 34.5%, C&He:3.9%, (&HI: 36.0%, CH,: 15.8%, COz: 1.7%, CO: ca. 8%. The reaction in a blank reactor gives much less propene and more ethene comparedto the salt melt reactor. Surprisingly,no benzene was observed. DISCUSSION

Catalyticexperiments

In all experimentsthe conversion of propane initially increasessomewhat, but levels off to a stable value later on. This may have to do with the transport of iodine from the melt into the post catalytic zone (gas phase in the reactor

168

above the melt), thereby enhancing the gas phase reactions. It is evident from the presence of iodated products that such reactions play an important role in our experiments. Increasing the temperature increases the oxygen and propane conversions. However, the selectivity to propene decreases at the same time. At higher conversions, coupling of C3 species to benzene becomes and important side reaction. Thermodynamics The equilibria for thermodynamic dehydrogenation of propane compared to dehydrogenation of propane with iodine is shown in Fig. 3. The maximum theoretical conversion of propane with 190% selectivity to propene is calculated as a function of the temperature. The effect of reducing the pressure is also shown for the direct dehydrogenation. Equilibrium pressure is assumed as 1 bar. At 900 K, 90% conversion of propane is possible by using iodine. The reaction HI+LiOH-+LiI+H20

(5)

has a rather favourable equilibrium constant, AG= - 70 to - 75 kJ/mol over the temperature range 700-1900 K. This means that the total reaction of C&H8 and I2 in the presence of LiOH gives close to 100% conversion. The production of iodine from lithium iodide, reaction (3), may be a limiting factor. Assuming a ratio of LiI : LiOH of 1: 1, and a mixture containing 1 part CS, two parts N2 and one part HzO, the conversion of oxygen is calculated at different temperatures and different oxygen initial pressures. The results are given in Fig. 4. As we see, almost total conversion of oxygen is feasible for p02 = 0. l-

700

800

900

1000

TK

Fig. 3. Equilibria for different propane dehydrogenation conditiona as a function of temperature. ( 0 ) Thermal dehydrogenation at 1 bar total pressure, ( q) thermal dehydrogenation at 0.5 bar total pressure, ( 0 ) propane + iodine 1: 1.

169

80 0,o

0,l

0,2

0,3 pO2

0,4

0,5

c 6

bar

Fig. 4. Conversion of oxygen as a function of the starting pressure of oxygen for the reaction 2 LiI + 0.5 O2+ H,0-+2 LiOH + Iz in the temperature interval 800-1000 K. ( 0 ) 800 K, (a) 900 K, (0) lOOOK.

0.5 bar and at temperatureslower than 900 K. Thermodynamically,this reaction will not be the limitingstep. However,side reactionsmustbe taken into account. One or more of the following reactions may occur: C3HB+ I+ CzH4+ HI + CHJ

(61

C,H,-C,H,+CH,

(71

2C3H6-C6H,+3H2

(3)

Thermodynamically, all these reactions give more than 95% conversion at temperaturesas low as 700 K. At equilibriumthese reactionswill be favoured over propene formation. The kinetics of the different reactions are therefore of great importanceto the process. Kinetics The elementary reactions participating are partly known, partly open for estimation,and partly unknown. The reaction steps and kinetics of the gross reaction (3) have not, to our knowledge,been investigated It should be a fairly fast reaction, since lithium iodide is unstable in air even at 2OO”C,showing visible evolution of iodine. Aside from this reaction,which is responsiblefor iodine production, one must also consider the gross reaction: 2HI + 0.50,-+ H,O + IS

(91

Haleyet al. [ 41 claim that this reaction is also ratherselectivein the presence

170

of hydrocarbons, but these authors obtained a somewhat lower propene selectivity with oxygen/iodine than with pure iodine. If oxygen is present in the gas phase as the reaction products from (1) are evolved, this rather complicated reaction would contribute, releasing hydroxyl-, peroxy- and other oxygen-containing radicals into the gas phase. These radicals would be expected to be much less selective towards propene formation. This can be confirmed from the results of the blank experiment, where the selectivities to unwanted products are much higher. Ignoring these iodine-producing reactions, the relevant reaction steps involved in the dehydrogenation proper are: 1+21’

(10)

C&H8+ I’ -+nC,H; + HI

(11)

C&H8+ I’ + i&H; + HI

(12)

nC&H;+ I,+ nC&HJ + I’

(13)

i&H; + 12-+i&H71 + I’

(14)

n&H; + I, + CH&H; + CHJ

(15)

nC,H;-+ C&H4+ CH;

(16)

nC,H;+C,Hs

+ H’

(16a)

iC,H;-+C,H,+H.

(17)

n&H; + O+ C&H6+ HO;

(16)

Calculations [ 61 of the rate constants of (16) and (16a) show that reaction (16) is about 100 times faster than reaction (16a) at 990 K. This indicates that the n-propyl radical will give mainly cracked products by pure thermal reaction, and only a little propene. In our case we can assume that the radicals produced in (16)-( 17) react rapidly with iodine or with iodine radicals. At 800 K and 0.5 atm iodine the equilibrium pressure of iodine radical from reaction (10) is 3 mm Hg [ 71. This gives 3.5 mm Hg of iodine radical at 823 K. In the temperature interval 790-999 K the iodine radical equilibrium pressure is between 2 and 5 mm Hg. The creation and fate of the n-propyl radicals seems to be a main factor in determining the selectivity of the oxidative dehydrogenation. In Fig. 5 we present estimates for the relative rates of n- and i-propyl radical formation (for details see the appendix). In the temperature interval 700-900 K reaction (16) is the main reaction, in preference to (16a). Reaction (13) can therefore ‘save’ the n-propyl radical from cracking, if reaction (15) is not too fast. The following possible reaction pattern for the n-propyl radical may be suggested.

171 0.3

02

0.1

0.0 500

600

700

600 T

900

1000

K

Fig. 5. The eetimated relative rates of reaction (11) and (12). For details see Appendix.

--I2-->

C?-V

C3H7I

(-43&j)

q2

-02~>

c3Hg

‘02

->

c2+c1

‘ClC2

Fig. 6. Possible reaction routee of the n-propyl radical.

From estimates given in the Appendix (see also Fig. 6), the relative rates 1: 0.15 : 0.7 at 823 K, so at this temperature approximately 40% of the n-propyl radicals can be expected to crack. As reaction (16) has the highest activation energy, this fraction will be lower at low temperature, and higher at high temperatures. All this points to the use of low temperatures to obtain high selectivity to propene. Our experimental results reflect these theoretical deductions. r1, : ‘-0’02 : ~~~~~are

Aromatization The conditions that give high conversion of propane will also usually give further reaction of propene to benzene (see Tables 1 and 2). We can expect that propene can easily be transformed to ally1 radicals, these can combine to Ce species that, in the next step, quickly generate benzene. Of course, a bimolecular coupling of ally1 radicals is not the only way to generate C6 species. A diagram for relative selectivity to benzene as a function of propane conversion is shown in Fig. 7. This plot has a greater similarity to what one would expect from an aromatization that is first-order in propene concentration than one that is second-order in propene concentration. This would also indicate that the propyl radicals participate in coupling reactions, in accordance with earlier suggestions in the literature [ 51. More specific kinetic experiments should, however, be performed to eluci-

10

20

30 propane

40

50

60

conversion

70

60

90

El

run1

.

run2

100

%

Fig. 7. Relative selectivity (area % in total ion chromat~gram) as a function of conversion (600” C, varying air/propane ratios, two different runs).

date the mechanismof benzene formation. Severalreactionpathwaysare possible: primaryproduction of C, (from propyl radicals), from propene or from mixed propyl/propen (yl ) reactions. Furthermore,both free radical and cationic pathways (hydrogeniodide-catalyzed) could be involved. CONCLUSIONS

Iodine/hydrogen iodide catalyzed ozidative dehydrogenationof propane in the presence of oxygen might give interestingconversions and selectivities. These reactions are assumed to proceed by radical mechanismsas suggested earlier [ 41. Thermodynamicand kinetic considerationsindicate that low temperatures give higherpropene selectivities.The experimentalresultsconfirm this. A major factor governingpropene selectivityis the ratio of normal/isopropyl radicals. At the relativelyhigh radical concentrations encounteredin the system propane/iodine, coupling reactions to benzene are observed as the ultimate product. Optimizingthis systemfor propene or benzeneproduction is feasible. APPENDIX

Rate constants for n-propyl reactions paths (Fig. 6) In this Appendix we use publishedrate constants of relevantand analogous reactions to make rough estimatesof the reaction path. All rates are in units of mol cme3 s-l. All rate constants and frequencyfactors are in units of cm3 mol-1*-l . Assuming823 K, [ n-C,H,] = 1 and [ IZ]= 0.5 atm r1,=k1,~[n-C3H,]~[I,]=1~10”~1~6.5~10-6=6.5~106

See Ref. 7

173 TABLE Al Constanta for calculation of rate constant for reaction (16) and (16a): C3Hs+14/n-&H,+HI

A"

ab

1.1 x 10’3 0.9 x 10’3

Primary Seconh

0.97 0.97

Gb

D(C-H)

E

(kcal/mol )

(kcaI/mol)

(kcal/mol)

69 69

98 95

28.13 25.22

“Estimate from reaction of primary and secondary bonds of 0 and H, taking into account the mass OfI. bFbf. 8. “Ref. 9.

Assuming0.15 atm oxygen unreactedz Fo+Zo,+Z-C,H,]~[0+4.65x1010x1x2x10-6=9.3x1~ Fclcz

=&CP*

See Ref. 6

[n-C3H,] =4.5X105

Rate constants

See Ref. 6

forn-pFopylandi-pmpy~FadicdfoFmution

C3H8+I’+nC3H;+HI

k,

=AI.,tiexp(

C,H,+I*-+iC,H;+HI

It,

=AI_,exp(

(Fig.5)

-E/RT) -E/RT)

Reasonable estimates for A0 for I from reactions of primary and secondary bondsofOandH [6]: AOI_pti: 2 x 1013 AOI-set: 5x1013 Taking into account the mass of iodine and numbersof C-H primaryand secondary bonds in propane: AI+*=

(2x10’3/(127)“2)

x6=1.1~10’~

AI-%3c = (5~10’~/(127)“~)x2=0.9x10’~ The activation energyE, is givenby [ 81: E=a-

[D(C-H)-C,]

where a! and C, are empirical constants and D(C-H) is the strength of the bond broken. Valuesare given in Table Al. The resultscan be comparedto resultspreviouslyobtained on butane at 823 K [ 41. Assumingthat reaction of C3H8with I’ is the dominant reaction, the reaction rate is given by:

114

dC,H,/dt=

- (k,+kz)* [Il. [G&,1

At 823 K kl + k2 = k3 is equal to 2.18~ 10’. Kate constants for reaction of i- and M&Hi,, and CsHs with HO;, H’ and OH’ give kcl=1.55*kos [S]. We assume that the same is valid for I’. For butane this gives /+rc4=1.55x2.18x106=3.38x106 dC,H,,/dt= 3.38 x 10’. [I] - [ C4H10] Assuming 823 K, [ C4H10]=0.4 atm, [I] =ca. 3.5 mm Hg=4.6 x 10e3 atm, [&Hi,] = 5.2 x lo-‘mol cmV3, [I] = 5.98 x lo-8mol cmm3 dC,H,,/dt=

1 x 10-6mol cm-3s-1

i.e. l/5.2, which corresponds to 19% conversion after 1 s reaction time. Raley et al. [4] obtained 51 and 76% conversion after 0.7 and 1.3 a. Our estimate gives a somewhat low reaction rate.

REFERENCES 1 2 3 4 5 6 7 8 9

K.T. Ngyyen and H.H. Kung, J. Catal, 122 (1990) 415. R.W. King, Hydrocarbon Process., 45 (1966) 189-194. British Patent 978.181 (1962). J.H. Raley,R.D.MullineauxandC.W. Bittner, J.Am.Chem. Sot., 85 (1963) 3174. G. F61di& end R.H. Schuler, Z. Naturforach, 38a (1983) 1154. C.T. Adams, S.G. Brandeberger, J.B. DuBois, G.S. Miller, M. Neger end D.B. Richardson, J. Org. Chem., 42 (1977) 1. William C. Gardiner (Editor), Combustion Chemistry, Springer-V&g, New York, 1964. G. F&ldU and R.H. Schuler, Z. Naturforsch. 38n (1983) 1154. J.T. Herron and R.E. Huie, J. Phys. Chem., 73 (1969) 3327. Handbook of Chemistry and Physics, 55th edition, 1974.