CA1046481A - Dehydrogenation of alkyl aromatic hydrocarbons - Google Patents

Abstract

A B S T R A C T
Method for activating self-regenerative catalysts used for dehydrogenating alkyl aromatic hydrocarbons at 600-700°C., having specified number of carbon atoms char-acterized by interrupting the feed of hydrocarbon, while continuing the feed of steam, for 7-30 minutes each 24-48 hours.
Suitable catalysts are those having a major pro-portion of at least one oxide of iron, zinc, chromium, or magnesium and an alkali metal oxide, hydroxide or carbonate.
By this method, conversion to the desired alkenyl derivatives is increased, selectivity remains high and the dehydrogenation can be run under high severity conditions without excessive coking of the catalyst.

Description

~046481 The present invention is a method for activating self-regenerative dehydrogenation catalysts used in dehydro-genating alkyl aromatic hydrocarbons.
Dehydrogenation of alkyl aromatic hydrocarbons, such as ethyl benzene to styrene, ethyl toluene t~ viny1 toluene, diethyl benzene to a mixture of divinyl benzene and vinyl ethyl benzene, isopropyl benzene to isopropenyl benzene, and ethyl naphthalene to vinyl naphthalene, by ~assing a mixture of steam and such alkyl aromatic hydrocarbon over a "self regenerative" catalyst is known. The "self-regenerative" catalysts are usually described as those con-taining one or more oxides of iron, zinc, chromium or magnesium, as the major ingredient, and an alkali metal -oxide, hydroxide or carbonate, particularly potassium or rubidium oxides, hydroxides or carbonates, as water gas reaction-promoting ingredients. This reaction tends to mitigate carbon build-up on the catalyst surface, thereby permitting long periods of continuous dehydrogenation cycles ` for converting alkyl aromatic hydrocarbons having at least one alkyl group of 2 to 3 C at~ms to the corresponding alkenyl aromatic hydrocarbon. Representative "self-regenerative" catalysts are disclosed in U.S. Patents

2,370,797, issued to Kearby, March 6, 1945, 2~414J585~
issued to Eggertsen et al., January 21, 1945, 3,703,593, issued to Turley et al., November 21~ 1972, and 3,205,179, '` issued to Soderquist et al., September 7, 1965. These catalysts usually contain one or more of the oxides of iron, zinc, chromium or magnesium as the major ingredient i and an alkali metal oxide, hydroxide or carbonate, pre-ferably the potassium compound, as a water gas-promoting i~ ~
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ingredientO The catalysts may also contain other additives such as stabilizers, binders and porosity control agents.
These additives are known in the art and are described in the above cited patents.
` 5 It has been found that a self-regenerative cata- .
lyst which consists essentlally of a major proportion of at least one oxide of iron, zinc7 chromium or magnesiumg and - which also contains an amount of an alkali metal oxide, hydroxide, or carbonate, preferably the potassium compounds, to promote the water gas reaction, a chromium compound as a promoter, and optionally, a stabilizer, binders and/or porosity control agents, can be activated by short, periodic steaming cycles ~without presence of alkyl aromatic hydro~ ~: .
carbon).
: 15 The present invention is a method of activating self-regenerative dehydrogenation catalysts used for dehydro-~ genating alkyl aromatic hydrocarbons having from 1 to 2 six-- -membered rings and from 1 to 2 alkyl groups of 2-3 carbon atoms to the corresponding alkenyl aromatic hydrocarbon, by .
passing a mixture of steam and said alkyl aromatic hydrocarbon over said catalyst at a temperature of 600-700C., said method comprising interrupting the feed of said alkyl aromatic hydro-carbon and continuing the feed of steam at said temperature range for a period of 7-30 minutes each 24-48 hours: resuming :~
the feed of said alkyl aromatic hydrocarbon under high severity .
rejaction conditions; and repeating said activation cycles and said alkyl aromatic hydrocarbon dehydrogenation cycles.
The steaming cycle of about 7-30 minutes every 24-48 hours at a temperature of from about 600 to about i. - ~. .
700C., is sufficient to activate the catalysts~ Most -... . .
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` ~IL046481 conveniently the previous operating temperature is used for activation.
Activation of the catalyst apparently is not due to decoking, because little or no carbon oxides are found in the effluent during the activation cycle.
After activation, the conversion of alkyl aromatic hydrocarbons to alkenyl derivatives is increased/ the selectivity remains high and the dehydrogenation cycle ` can be run under high severity conditions, e.g. low - 10 steam to hydrocarbon weight ratios or higher temperatures, or both~ without excessive coking of the catalyst. It is thus possible to attain increased throughput per reactor and aIso to obtain higher yields of alkenyl aromatic hydro-carbon at lower reaction temperatures.
;15 The catalytic dehydrogenation of alkyl aromatic ; hydrocarbon having at least one alkyl group with 2 to 3 C
- atoms is known to be an e~uilibrium reaction~ It is also ~^ known that as the hydrogen partial pressure in the dehydro-~i genation step is decreased, the ratio of styrene to ethyl benæene is increased at any given temperature. Similarly, an increase in temperature at any given partial pressure of hydrogen, also results in an increase in the styrene/ethyl benzene ratio. It is also known that as ~emperature increases coking is also increased at any given sp ce velocity9 and that if space velocity is reduced (reaction time increased) at any given temperature, the degree of coking increases.
In addition, the activity of the particular catalyst must be taken into consideration. An overactive ; catalyst tends to convert a high percentage of the alkyl benzene, but its specificity to simple dehyrogenation to .~
~ alkenyl benzenes is comparatively low.
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In commercial single-stage adiabatic reactors the available self-regenerative catalyst have functioned satis-factorily at comparatively high steam to alkyl aromatic hydrocarbon ratios and at conversions of about 38~/~ or slightly higher. The steam ratio will vary somewhat depending on the particular alkyl aromatic hydrocarbon undergoing dehydrogenation.
; Representative alkyl aromatic hydrocarbons and ranges of steam to hydrocarbon ratios of the processes of the prior art are tabulated below.
TABLE I, ; Hydrocarbon Steam/Hydrocarbon Ratio Ethylbenzene 2.6 to 1 to 2.0 to 1 Ethyltoluene 5 to 1 to 3 to 1 Diethylbenzene 8 to 1 to 3 to 1 Isopropylbenzene 3 to 1 to 2 ~o 1 When the catalysts are activated by steaming for 7 to 30 minutes ever 24-48 hours, the ratios are as follows.
TABLE II
Hydrocarbon Steam/Hydrocarbon Ratio Ethylbenzene 1 to 1 to 0.4 to 1 Ethyltoluene 3 to 1 to 1.5 to 1 Diethylbenzene 6 to 1 to 1.5 to 1 Isopropylben.~ene 1.5 to 1 to 0.4 to 1 Ethyl naphthalene 1.5 to 1 to .4 to 1 Ethyl biphenyl 2 to 1 to .8 to 1 Usually, the effect of the above defined steam activation of catalyst is not apparent if the steam-hydrocarbon ratios are increased appreciably above the maximum values given in Table 2.

17,092-F -4-6~
Although all the self-regenerative catalysts tested have responded to the activation procedure, the degree of re~pon~e is not uniform for each such catalyst.
In general, the greatest degree of desirable activation was found with catalysts containing approximately equal amounts - by weight of ferric and zinc oxides containing from about 5 to about 3~/O by weight of potassium oxide, hydroxide or carbonate, from about 5 to about 10 weight percent of a copper, cadmium, thorium or silver oxides, from about 5 to 10 weight percent of alkali metal chromate, a minor amount 1 to 5 percent of a refractory type cement J and a minor amount of carbonaceous material such as methyl cellulose ethers, graphite or other ingredients defined in U.S. Patent 3,205,179, previously identified.
A representative catalyst made by the procedure of U.S. Patent 3~205,179 contained the following.
Inqredient Weiqht Percent F O 24.81 - 30.0 ZnO 24.81 - 30.0 - 20 K2CO3 9 - 22.5 Cu2O 7.44 - 9.0 2 r27 7.44 - g.o Alumina (low silica) Cement 3.90 Methyl Cellulose 4.00 Graphite 5.10 Other catalysts which are especially responsiYe to the activation procedure are thoqe which are made from a mixture of hydrated (yellow) iron oxide and anhydrous (red) iron oxide in a weight ratio of 1:4 to 17:20 and about 13:7 to 4:1, respectively. These catalysts and their .~
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methods of preparation are described in U.S. Patent 3,703,593, previously identified. A representative cataly~t had the following composition. Fe203 + Fe203 H20 58~/oJ K2C03 16.7%, K2Cr207 2.5%J V205 2.5%, methyl cellulose 8.3%j7 graphite 8.3%, -cement 2.8%.
Other catalysts which are activated so that they perform better under high severity reaction conditions ; (e.g~ low steam:hydrocarbon ratio or high temperature or both) includa those having the following compositions.
Weight Percent Component I II III IV _V
Fe203 74-5 83.0 87.9 71.9 62.5 Cr203 2.0 2.2 2.5 2.3 2.2 K20 - - 9.6 - -2 3 20.0 14.8 - 25.8 35.3 V2o5 3.5 i The steam to hydrocarbon ratio during the dehydro-genation cycle is within the ranges defined in Table 2 above.
All the tests reported below were run in a stain-less steel tubular reactor 36 inches (0.914 m.) long, with an I.D. of 824 inches (2.2 cm.). The reactor was electri- -cally heated and was equipped with metered steam and hydrocar-bon feed systems, temperature controls and recovery apparatus.
In each instance the reactor was loaded with 70 ml. o catalyst pellets.
Conversion is the percentage of the original alkyl aromatic hydrocarbon converted to the alkenyl aro-matic derivative per pass. Selectivity is the percentage of the consumed alkyl aromatic hydrocarbon that resulted in the formation of a desired alkenyl aromatic derivative per pass.
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Example 1 The alkyl aromatic compound used was ethyl toluene~
The steam to hydrocarbon weight ratio was 1.7 to 1. The catalyst contained 24~81 weight percent Fe203, 24.81 ZnO, 22~5~/o K2CO3J 7~44% Cu2O~ 7~44% Na2Cr207, 3~90 alumina cement, 4.00~/0 methyl cellulose and 5~1~/o graphite, before calcining. The mixture was formed into a paste with water~
extruded, pelletized and dried at 110C. The catalyst was thereafter steamed at 200 to 600C. for 3 hours and then roasted for 6 hours at 810-845C.
Reaction conditions were adjusted to obtain a steady state of about 4~/0 conversion of the ethyl toluene.
The ~emperatures ranged between 638 and 645C~ At 642C~
the conversion was 40.6 with a ~electivity to vinyl toluene of 91.7%. The catalyst was then steame~ fox 15 minutes at 642C~ and~ thereafter the ethyl toluene feed was resumed.
The reaction temperature was maintained at 640-645C~ A
steaming cycle of about 15 minutes duration every 24 hours ~ethyl toluene feed was cut off) was provided to activate ; 20 the catalyst. Samples were taken for analysis about 1/2 hour before steaming and about 1 1/2 hours after steaming.
Data taken during the test are given below.
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Days TemP~ C. % Conv. % SelectivitY
1 642 41.0 92.3 2 645 40.3 91.9

3 645 42.1 92.9

4 640 39.4 91.9 6 640 39.6 91.8 7 643 40.6 91.7 8 638 38 o8 92~2 9 642 40.6 91.7 - 10 Steam Cycle 9 64247.7 91.5 64246.~ 91.7 Steam Cycle 64~~8.3 91.9 11 64046.7 91.9 Steam Cycle 11 64048.4 92.3 13 64046~8 92.0 :
- Steam Cycle 13 64248~9 92.1 ~ -. . .
14 64547.6 91.8 Steam Cycle ; 14 64549.7 92.0 64347.7 91.9 Steam Cycle 64350.0 , 16 64247.7 91.8 -, Steam Cycle ~ 16 642 50.1 92.0 ; 30 17 642 48.8 91.9 Steam Cycle 17 642 50.1 92.0 . . .
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For comparative purposes a non self-regenerative catalyst having all the ingredients above, in the same proportions, except that K2C03 was omitted, was tested.
The catalyst was steamed for 15 minutes every 24 hour~ at the reaction temperature. The steam to ethyl toluene ratio ~ was 1.7 to 1. Data taken during this test are given below.
- Days Temp C. % Conv. % 5electivi~y ~ 2 687 44.0 84.1 -~ 3 690 41.9 83.~
~ 10 4 686 39.3 84.3 - 8 681 36.4 84.1 9 694 40.3 82.5 Steam Cycle 9 696 40.6 81.8 Steam Cycle ; 10 694 42~2 82.3 Steam Cycle 693 39.3 81.6 Steam Cycle 11 693 39.5 81.9 Steam Cycle ; 11 695 39.8 81.3 From the above it is apparent that the steaming cycle had no benefit for this catalyst.
In another series of runs, the self-regenera-tive catalyst de~cribed above in this example was tested at a steam to ethyl toluene ratio of 3 weight parts to 1.
The temperature for 40/O conversion was 621C. Thereafter the temperature was maintained at 620-626C. and a 15 min-ute steaming cycle was given to the catalyst each 24 hour ~ period. During these tests the following data were taken.
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Days TemP. C. % Conv % Selectivitv ; 7 B 626 A 625 44.9 94.0 . 8 B 623 42.2 93.4 A 623 42.7 94.0 9 B 620 40.9 93.8 A . 621 42.4 93.9 :: .
B 622 41.4 93.9 A 623 42.4 93.9 11 B 623 41.8 93.5 A 623 43.4 93.8 .
12 B 624 42.4 93.8 ~ 624 43.7 g3.7 13 B 623 - 42.6 93.6 A 623 43.8 93.5 14 B 625 43.1 93.3 A 623 43.1 93.6 B 624 43.1 93.3 A 624 44.6 93.5 A - After steaming of catalyst B - Before steaming The data show that the convex~ion increase i9 perceptible .:
but is considerably lower than that at a 1.7 to 1 steam to hydrocarbon ratio.
Exam~le 2 :
In a series of tests using the self-regenerative ~.
catalyst of Example 1 the degradation pattern of the catalyst activity after a 15 minute steaming cycle was compared with a :
pattern after a 1 hour steaming cycle. It was found that a 1 hour cycle does not improve catalyst activity to any greater degree than the 15 minute cycle.

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Using the same ~team-ethyl toluene ratio and a 24 hour teaming cycle showed substantially no greater increase in conver~ion to vinyl toluene than the 15 minute cycle and selectivity had decreased slightly. A 24 hour

- 5 steaming cycle, however, increases the negative effect of by-product formation.
Ex~am~le 3 In this series, the cataly t was brought to steady state of about 40/O conver ion, using a steam-ethyl toluene ; . .
ratio of 1.7 to l. The temperature was 641-648C. There-after the catalyst wa~ subjected to 15 minute steaming cycles each 24 hours fox 21 days and then to 7 minute steaming ~ cycles each 24 hours for 8 days. It was found t~at a 40O/o - conversion level could be maintained at temperatures considerably lower than 640C. and about a 2% higher ~elect-ivity wa~ realized. Data taken during the~e test~ are tabulated below.

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;. Days Tem~. C. ~ nv. % SelectivitY

6 648 39O9 91.0 ~ 7 646 39.2 91.8 .~: 11 641 3~.~ 92.0 512 643 39.3 92.0 13 647 41.3 92.0 14 647 41.7 91.8 17 642 37.0 92.1 - Steam Cycle (15 min.) ~~ 10 18 629 43.1 93.7 Steam Cycle : 19 628 43.1 94.0 Steam Cyele 623 39~8 94.2 Steam Cycle 21 622 38.9 94.
Steam Cyele 24 621 36.g 94.6 - Steam Cycle 629 42.2 93.6 Steam Cycle . 27 630 42.2 94.4 Steam Cycle 28 625 38.7 94.1 Steam Cycle 31 626 40.5 94.3 : Steam Cyele 32 627 40.8 94.2 ' Steam Cycle : 30 33 627 ~0.9 93.
Steam Cyele . 34 l~28 40.7 93.9 Steam Cyele ~` 35 628 41.6 94.3 Steam Cyele : 38 627 39.5 94.3 : Steam Cyele . 39 625 39-7 94 4 ;~.

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cont.
Days Temp. C. % Conv. % Selectivity Steam Cycle 627 42.3 93.6 41 626 41.1 93.8 . 5 Steam Cycle (7 min.) 43 628 42~7 93.6 Steam Cycle : 44 628 42.5 94.3 Steam Cycle 628 41.9 94.2 : Steam Cycle 46 628 42.1 93.9 Steam Cycle 47 628 42.2 94.1 Steam Cycle 48 626 40O9 94.4 ~ Steam Cycle 49 628 41.0 94.1 ExamPle 4 Ethyl benzene was dehydrogenated to styrene by the procedure described in Example 1 using a catalyst con-taining 8709 weight percent Fe203, 2.5% Cr203, and 9.6%
K20. When a steam to hydrocarbon ratio of 1 to 1 or lower :~
was fed to the reactor, at temperature of 590 to about 600C., with 15 minute steaming cycles each 24 hours, there was a . perceptible change in conversion but no change if steam-ethyl benzene ratios greater than 1 to 1 were used. However, .I when the severity was increa~ed by lowering the steam to ethyl benzene ratio to 0.7 to 1 or as low as 0.4 to 1 and temperature was increased con~iderably, the activation ef~ect ~ .

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of 15 minute ~teaming cycles each 24 hour~ was readily appar~nt. Tabulated below are data taken at 0.7 and 0.4 part~ of steam per part of hydrocarbon.
Steam - Ethyl Benzene Ratio 0.7 to 1 DaYs ~emP. C. % Conv. % Sel~ctivit~
36 628 37.2 ~39.6 , : Steam Cycle i 36 628 40.0 91.4 ; 37 631 38.5 92.0 :
.' 10 Steam Cycle `

. 37 630 42.7 91.6 39 629 38.1 90.9 Steam Cycle 39 630 42.6 90.3 :

- 15 40 629 39.1 91.6 -.

Steam Cycle 630 44.9 91.1 41 631 3~.7 90.7 ~.

Steam Cycle -~ 20 41 632 47.0 91.0 42 630 39.1 91.1 , . Steam Cycle ., 42 631 46.4 90.3 43 631 39.4 90.7 Steam Cycle 43 629 43~6 91.2 '' 44 631 40.2 ~ Steam Cycle ?i ts 44 631 45.4 - .

~ 30 45 628 39.6 .1, ;
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Cont.
Days Temp. C. % Conv. % Selecti_ ty Steam Cycle 628 45.4 46 627 38.3 91.0 Steam Cycle 46 627 41.6 47 627 38.5 $team Cycle 47 626 46.8 Steam - Ethyl Benzene Ratio 0.4 to 1 - Days Tem~. C! % Conv. % SelectivitY
- 6 680 30.3 75.7 Steam Cycle '~ 15 6 681 38.0 79.9

7 682 30.6 75.9 Steam Cycle ; 7 682 38.6 79.g 11 681 35.1 77.2 Steam Cycle 11 ~81 40.1 80.7 12 6~31 33.8 77.8 Steam Cycle 12 681 45.9 82.1 13 681 33.7 78.3 Steam Cycle 13 681 43.7 82.1 14 681 33.1 78.0 Steam Cycle 1~ 681 45.9 83.5 682 32.8 78.3 :.
17,092-F -15-: 1046~81 ~- Cont.
Days Temp. C. % Conv. % Selectiv-tY
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Steam Cycle 682 43.8 83.0 17 680 36O7 79.6 Steam Cycle 17 680 46.9 83.3 18 680 34J7 77.8 Steam Cycle 18 681 43.7 80.4 ; -~ Similar results are obtained with a catalyst ,~; analyzing 74.5 weight percent Fe203, 2.0/~ Cr2O3 3 20~0%
K2CO3, and 3.5% V2O5. Tests were run with this catalyst : ~o determine whether excess coki~g of catalyst at the end of a 24 hour period at 630C. and a steam to ethyl benzene ratio of 1 to 1 caused the lower activity. During a 15 minute steam cycle the noncondensible vent gases were ,~ collected and analyzed. The carbon content of the catalyst ranged from 2.2 to 2.5 weight percent. This compares with a typical range o~ 2.4 to 2.6% carbon on a catalyst during ,j; iow severity operations e.g. 3 to 1 steam to ethyl benzena , ratio and a temperature of about 590C. These tests show ;..................................................................... ..
excessive coking is not the cause of lowering catalyst activity.
ExamPle 5 '1j Isopropyl benzene was dehydrogenated to alpha methyl styrene with the catalyst described in Example 1.
The procedure used was similar to that described above, in `t that, after attaining about 40/O conversion, the steaming ~ 30 cycle of about 15 minutes each 24 hours was begun. Data ... .
taken during the tests are tabulated below.

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1~46~1 Steam - Isopropyl Benzene Ratio 1 to 1 - Days Temp. C. % Conv. % SelectivitY
32 620 41.5 96.6 Steam Cycle 32 619 51.~ 97.4 33 619 40.2 97.4 Steam Cycle `~ 33 619 51.8 97.4 34 621 40.6 96.2 Steam Cycle 34 621 56.0 97.7 621 39.5 97.4 ~ i Steam Cycle 621 51.4 98.0 `.15 38 625 42.0 96.0 Steam Cycle 38 623 57.5 97.4 39 623 42.9 95.3 Steam Cycle 39 620 58.1 97.5 620 40.2 95.9 , Steam Cycle ``
, 619 59.7 97.
41 62~ 40.~ 95.8 Steam Cycle ..
41 625 5g.9 97.3 ;;:~
622 38.5 g5.5 Steam Cycle 619 54~0 97.
46 618 37.3 96.0 `
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Days TemP. CO % Conv. % Selectivity Steam Cycle 46 618 43.9 96.9 ~;- 5 47 617 35.7 96.6 Steam Cycle 47 622 45.6 98.6 When tests were run at a steam to isopropyl benzene ration of 2 to 1 at 586-596C., the activation of catalyst was perceptible, but not of the magnitude shown at the 1 to 1 ratio.
~ ExamPle 6 ; The catalyst described in Example 4 wa~ used to , dehydrogenate diethyl benzene (DEB). A mixture of ethyl vinyl benzene (EVB) and divinyl benzene (DVB) was produced.
At a steam to hydrocarbon ratio of 6 to 1 the improvement in catalyst activity is perceptible, but not as great as that o~tained at a ratio o~ 3 to 1. Tabulated below are data obtained whila dehydrogenating diethyl benzene at a steam to hydrocarbon ratio of 3 to 1. The steam cycle was of 15 minutes duration each 24 hours.
Crude Product Anal.
- Temp. % % % EVB.
C. DEB. EVB. DVB. DVB.
645 B 44.1 26.3 23.3 49.5 645 A 35.6 28.1 28.1 56.2 645 B 46.5 24.7 22.9 47.6 644 A 36.0 27.6 28.1 55.7 644 B 44.8 26.7 22.8 49.5 644 A 36.2 27.7 27.5 55.2 644 B 45.7 26.2 22.1 48.3 17,092-F -18-:: ~ - . . . . , . :
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Crude Product Anal.
Temp. % % % EVB. +
C. DEB. EVB . DVB . DVB . _ 643 A 36.2 27.4 27.2 54.6 644 B 45.3 26.6 22.5 49.1 643 A 36.0 27~9 29.5 57.4 A - after steaming B - before steaming These data show that the major increase was in the amount .
of divinyl benzene produced.
The conversion of ethyl naphthalene to vinyl .~
naphthalene or ethyl biphenyl to vinyl biphenyl can be effected with the self-regenerative catalysts ~recited above~ The dehydrogenation of the alkyl bicyclic com-pounds takes place under about the same conditions as those used for dehydrogenating the alkyl monocyclic hydrocarbons with comparable conversions and selectivities for the dehydrogenation of ethyl naphthalene. However, when ethyl biphenyl is the hydrocarbon undergoing dehydrogena-tionJ the conversion and selectivity is generally slightly i~ lower than those for ethyl naphthalene at identical operating conditions.
;, All the run~ above were made at a luqiud hourly space velocity o~ about 0.5. However, the spase velocity can range fr~m about 0.1 to about l. In a large scale test for converting ethyl toluene to vinyl toluene a 3 to l steam to ethyl toluene 9 a liquid hourly space velocity of ,, 0.3 and a temperature of 620-640C. J with 15 minute steaming cyclec each 24 hours, was found to improve conversions and selectivities of about the same magnitude as shown in the examples.

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The process of this invention is applicable to adiabatic or heated case reactor procedures. The catalyst can be a fixed bed, either radial or packedJ or a fluidized bed. In addition, the process is applicable to single or multi reactor systems, especially those multi reactor ~ystems in which interstage steam injection is possible, although multi reactor systems with provision for indirect heat =xchang= b=tw=en =tag=s i= also operabl=.

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Claims (12)

1. A method of activating self-regerative dehydrogenation catalysts used for dehydrogenating alkyl aromatic hydrocarbons having from 1 to 2 six-membered rings and from 1 to 2 alkyl groups of 2-3 carbon atoms to the corresponding alkenyl aromatic hydrocarbon, by passing a mixture of steam and said alkyl aromatic hydrocarbon over said catalyst at a temperature of 600-700°C. said method comprising interrupting the feed of said alkyl aromatic hydrocarbon and continuing the feed of steam at said temperature range for a period of 7-30 minutes each 24-48 hours; resuming the feed of said alkyl aromatic hydrocarbon under high severity reaction conditions; and repeating said activation cycles and said alkyl aromatic hydrocarbon dehydrogenation cycles.

2. The method of Claim 1 in which the catalyst has a weight ratio of hydrated iron oxide and anhydrous iron oxide of 1:4 to 17:20 and 13:7 to 4:1, respectively.

3. The method of Claim 1 in which the catalyst contains from 62.5 to 87.9% by weight of Fe2O3, from 2 to 2.5% by weight of Cr2O3, and from 9.6 to 35.3% by weight of K2O or K2CO3.

4. The method of Claim 3 in which the catalyst contains about 3.5% by weight of V2O5.

5. The method of Claim 1 in which the catalyst contains from 24.8 to 30% by weight each of Fe2O3 and ZnO, from 5 to 30% by weight of potassium oxide, hydroxide or carbonate, from 5 to 10% by weight of an oxide of copper, cadmium, thorium or silver, from 5 to 10% by weight of an alkali metal chromate, from 1 to 5% by weight of a refractory cement and a small amount of a carbonaceous material.

6. A method of dehydrogenating an alkyl aromatic hydrocarbon having from 1 to 2 six membered rings and from 1 to 2 alkyl groups of from 2 to 3 C atoms each and a total of 8 to 14 C atoms in said hydrocarbon, in the presence of steam, at a temperature of from 600 to 700°C., by passing a mixture of the steam and said hydrocarbon over a self-regenerative catalyst, comprising, interrupting the flow of the alkyl aromatic hydrocarbon while continuing the flow of steam at the said temperature for a period of from 7 to 30 minutes during each 24-48 hours 7 then resuming the flow of alkyl aromatic hydrocarbon and repeating the steps of interrupting the flow of said alkyl aromatic hydrocarbon and continuing the flow of steam for said 7-30 minutes each 24-48 hours.

7. The method of Claim 6 in which the alkyl aromatic hydrocarbon is ethyl toluene, and the feed during dehydrogenation contains from 1.5 to 3 parts by weight of steam per part of said hydrocarbon.

8. The method of Claims 6 or 7 in which the steam to hydrocarbon ratio is 1.7 to 1.

9. The method of Claim 6 in which the alkyl aromatic hydrocarbon is ethyl benzene and feed during dehydrogenation contains from 0.4 to 1 part of steam per part of hydrocarbon.

10. The method of Claim 6 in which the alkyl aromatic hydrocarbon is diethyl benzene and the feed during dehydrogenation contains from 1.5 to 6 parts of steam per part of hydrocarbon.

11. The method of Claim 6 in which the alkyl aromatic hydrocarbon is isopropyl benzene and the feed during the dehydrogenation step contains 0.4 to 1.5 parts of steam per part of hydrocarbon.

12. The method of Claim 6 in which the alkyl aromatic hydrocarbon is ethyl biphenyl and the feed during the dehydrogenation step contains from 0.8 to 2 parts of steam per part of hydrocarbon.