IMPROVED VAPOR PHASE BROMINATION OF AROMATIC COMPOUNDS

BACKGROUND OF THE INVENTION

Field of the Invention:

The present invention relates to a process for the vapor phase oxidative bromination of aromatic compounds using a metal catalyst on a support such as a zeolite.

Discussion of the Background:

The bromination of aromatic compounds is tradi¬ tionally carried out by electrophilic substitution reactions using aromatic substrates such as benzene or naphthalene. The most common catalysts are the bromides of iron or aluminum although many other catalysts including iodine are effective. These reactions are generally known as Friedel-Crafts reactions.

Halogenation of aromatic compounds can also be carried out by an oxidative halogenation process. Vapor phase oxidative chlorination of aromatics is a commercial process and has been described in U.S. Patents 1.963,761; 3.303.223; 3.389.186 and

3.644.542. In contrast, the vapor phase oxybromi- nation of aromatics has received little attention, although liquid phase oxybromination is known. See for example U.S. 4.380.682 which discloses the oxybromination of aliphatic hydrocarbons by first performing a non-selective oxyhalogenation given intermediate partiallyhalogenated product which is then reacted with bromine gas in the absence of oxygen over a silica-alumina catalyst to give the final product

U.S. 3.591,645 discloses the oxybromination of aromatic compounds, preferably benzene and toluene by heating the substrates in an inert solvent in the presence of bromides and a compound containing nitrate ions. The oxybromination is carried out over a catalyst which is suspended in the solvent and is preferably an oxidizing metal such as copper, mangan¬ ese, cobalt, vanadium, etc.

Other liquid phase oxybrominations of aromatic compounds are known which use iron or copper salts to effect bromination. See Japanese 49/18832 and , 49/18831. Additionally, the use of molecular sieves to increase para-selectivity in liquid activity phase bro inations is described in J. Catal., 60 _. , 110 (1979).

A need still exists, however, for an efficient and selective method for the vapor phase oxybromi¬ nation of aromatic compounds.

SUMMARY OF THE INVENTION

Accordingly, one object of the present invention is to provide a method for the vapor phase oxybromi¬ nation of aromatic compounds which can be carried out over readily available catalysts. Another object of the invention is to provide a method for the vapor phase oxybromination of aromatic compounds which is selective for para-bro inated products.

Still a further object of the invention is to provide a method which has a selectivity similar to liquid phase bromination.

These objects and other objects of the present invention which will become apparent from the follow¬ ing specification have been achieved by the present method for brominating an aromatic compound which is

reacting a source of bromine in the presence of a source of oxygen over a metal catalyst supported on a solid support.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The aromatic compounds which can be utilized in practice of the present invention are essentially any aromatic compound including substituted and unsub- stituted aromatics. Suitable aromatic compounds include hydrocarbon aromatics, nitrogen containing aromatics and sulfur containing aromatics. Typical hydrocarbon aromatics include benzene and biphenyl, condensed ring aromatics such as naphthalene and anthracene, sulfur containing aromatics including thiophene, and benzothiophene, nitrogen containing aromatics such as pyridine and benzopyridine and substituted aromatics such as εulfones, diaryl ethers, diaryl carbonyls, diaryl εulfides and the like. Aromatic compounds substituted by alkyl groups are generally not preferred for utilization in the present technique. It has been found that alkyl substituted aromatics are not only brominated by the present catalyst system but also oxidized as well and further that the brominization reaction is not limited to the aromatic ring but that one obtains a mixture of products. That is. the products are brominated not only on the ring but also on the side chains. Further, the product obtained will also contain oxidized side chains and the like. Thus, while alkyl substituted aromatics can be utilized in the present technique their use is not preferred. Substituentε which may be present include phenyl. fluoro, bromo. iodo, chloro, cyano, and hydroxyl εubstituents.

Specific aromatic compoundε which can be utilized in the present invention include benzene, biphenyl, terphenyl, naphthalene, benzophenone. diphenyl εulfone, diphenyl ether, chlorobenzene, fluoro- benzene, chloronaphthalene, benzonitrile, phenol, l-naphthol _r 2-naphthol. pyridine, chloropyridine, and iodobenzene.

The catalyεt uεed in the preεent method is an oxidizing metal and a solid support. The oxidizing metal may be a wide variety of transition metals. Preferred metals include copper, iron, molybdenum, manganeεe, chromium, vanadium, nickel, cobalt, and ixtureε thereof. More preferred metals are copper and iron, with iron being the most preferred metal catalyst.

The solid support may be acidic or non-acidic and may contain a wide variety of solid inert support materials. Typical examples of support materials include alumina, silica, and titania. Other examples include the zeolites such as the alkali and alkaline-earth zeolites. Within the zeoliteε, the X-zeoliteε and Y-zeoliteε are preferred. In general, the zeoliteε are preferred over other inert support materialε εince catalyεts made from zeoliteε are more active and exhibit more εelectivity in the bromination reaction. The zeoliteε εhould have a pore εize at leaεt equal to the apparent εize of the molecule of the aromatic ring compound being reacted. Benzene aε well aε naphthalene have apparent ring εizeε of 6 A and thiε is the lower limit on the pore εize of the zeolite catalyst which is useful. If the aromatic compound cannot enter into the pore of the zeolite catalyst then only very little conversion of the aromatic compound will occur. Hence, the preferred zeolites have a pore εize of 6 A or larger.

The catalyεt containing both the oxidizing metal and the inert εolid εupport is generally produced by impregnating the εolid support with the oxidizing metal. Thiε impregnation process can be achieved by any method which incorporates an adequate amount of the oxidizing metal into the εolid support. A preferred method iε the introduction of the oxidizing metal into the εolid εupport in the form of an aqueous salt solution of the oxidizing metal cation. In thiε process, the inert εupport and the aqueous solution of the metal salt are mixed together in an ion exchange type process. The period time over which the contact between the aqueous metal salt εolution and the inert εupport is conducted and the number of times the ion exchange process is performed is dependent upon the degree of replacement desired. Thus, if one begins with a zeolite in the sodium form, one may ion exchange this material with another counterion to partially or εubstantially completely replace the sodium ion with a different counterion such as iron. Alternatively, the desired metals can be impregnated onto the support by vapor phase depoεition or by contacting non-aqueouε εolutionε of the deεired metal with the εupport. The counterion for the metal εalt iε of no particular importance and any appropriate counterion may be uεed. Suitable counterions include chloride, bromide, nitrate, hydroxide, carbonate, perchlorate. sulfate, or acetate. The temperature at which the reaction is to be conducted iε not critical and can be any temperature at which the aromatic compound is in the vapor phase. The maximum temperature at which the process can be carried out iε that at which combustion of the aromatic compound occurs. Generally, temperatures of from 100°C to 500°C have been found εatiεfactory with

temperatures of from 200-400°C being preferred, more preferably from 300-400°C. In operating at the lower ranges, the catalyεtε exhibit their greateεt selectivity. The pressure at which the procesε iε conducted iε not critical and can range from εubatmospheric to superatmoεpheric. The utilization of elevated preεεureε in the gaε phaεe proceεε may be preferred so aε to minimize equipment εize. In general. preεεures from atmospheric to 42 kg/cm (600 psig) have proven εatiεfactory although higher or lower preεεures can be utilized.

The source of oxygen for the oxybromination reaction can be pure oxygen, air or air diluted with any other inert material, such as carbon dioxide or water vapor. Esεentially, oxygen from any convenient source may be utilized. The purpoεe of the oxygen iε to regenerate the active εite of the zeolite catalyst to its active form once the bromination reaction haε occurred. Thuε the amount of oxygen preεent during the reaction iε not critical. However, it iε preferred that at leaεt one-half mole of oxygen be uεed for every mole of bromine measured as Br or HBr. Greater or lesεer quantitieε of bromine can be utilized aε one deεireε. The utilization of large excesses of bromine result in a product which iε contaminated with unreacted bromine. When all the bromine iε reacted, a colorleεε product iε obtained. In general, it iε deεired to run the proceεε to obtain aε cloεe to 100% converεion of the bromine aε practical so aε to simplify the purification steps in the recovery of any unreacted bromine. Suggested molar ratioε of εtarting materialε material to bromine to oxygen are from 1:0.5:0.25 to 1:2:3. However, other ratioε may be utilized aε deεired. The molar ratio of bromine to starting material is not critical.

Essentially, any source of bromine may be employed including Br , HBr. and alkyl bromides. The alkyl bromides are generally lower alkyl bromides. By "lower alkyl" is meant alkyl groups having 1-8 carbon ato ε. preferably 1-4 carbon atoms. The alkyl group may be straight chain, branched or cycloaliphatic. Specific examples include methyl bromide, ethyl bromide, propyl bromide, n-butyl bromide, pentyl bromide, hexyl bromide and cyclohexyl bromide. Methyl bromide iε the preferred alkyl bromide employed. The HBr may be utilized aε an aqueouε solution, preferably 48%. although solutions having other concentrations of HBr may be used εatiεfactorily. It iε necessary to vaporize the aqueous HBr before it contacts the catalys .

It iε anticipated by the preεent process would be carried out continuously by the continuous addition of bromine, oxygen and starting materials to the reactor, however, the procesε can be carried out on as batch or semi-batch processes deεired. Further, εtarting aterialε and bromine can be reacted over the catalyεt to produce the brominated product, the addition of εtarting materialε and bromine then being terminated and oxygen then added to the reactor to regenerate the catalyεt to itε active form and then the proceεε commenced again. Alternatively, in a continuouε proceεε it iε poεsible to utilize two reactants circulating the catalyεt between them. In the first reactor the bromine and aromatic compound would be added and reacted to form the brominated compound. The catalyεt would then be circulated to the second reactor where it would be contacted with oxygen to be regenerated and then recycled to the first reactor to catalyze additional reactions of aromatic compound with bromine.

The space velocity of the procesε iε not critical and may be readily selected by the artisan. Gas hourly space velocity iε between 10 to 50,000, preferably between 100 and 20,000 literε per hour of reagentε per liter of active zeolite have proven εatiεfactory.

The catalyεt iε proven to have an extremely long life and degrades only slowly with time. The degra¬ dation of the catalyst is believed to be cauεed by the combuεtion of very εmall quantitieε of the aromatic compound which depoεitε εmall quantitieε of carbon on the active sites thereby degrading the catalyst activity. When the reaction conditions are selected such that none of the aromatic εtarting material iε oxidized, the life of the catalyεt iε eεεentially indefinite. However, when the catalyεt becomeε deactivated reactivation iε εimple. An excellent regeneration technique is pasεing air or oxygen over the catalyεt for εeveral hourε at elevated temperatureε. Typically the temperature iε above 400°C although higher or lower temperatures are proven equally satisfactory. The temperature need only be high enough so aε to enεure combuεtion of the carbon depoεit on the catalyεt. When pure oxygen iε employed lower temperatureε can be utilized, while when air iε employed temperatureε on the order of 400°C have proven εatiεfactory.

Other featureε of the invention will become apparent in the course of the following deεcriptionε of exemplary embodimentε which are given for illuε- tration of the invention and are not intended to be limiting thereof.

EXAMPLES In the following exampleε, 25 cc of the εtated catalyεt waε placed in a vycor reactor tube with an internal thermowell. The tube waε heated with an

electric furnace while the reactants were added dropwiεe over the catalyst bed at the specified rate. Air was fed cocurrently at 300 ml/min. Productε were collected by condenεing against cold water and identified by gas chromotography-roaεs εpectrometry and quantified by gas chromotography (reported as mole %).

EXAMPLE 1 Catalyεt: Fe-Silica-alumina

Furnace temp: 300°C

Reaction temp: 351°C

Aromatic feed: benzene at 0.5 ml/min

Bromine feed: 48% aq HBr at 0.5 ml/min

The reaction product contained 59.0% benzene, 31.7% bromobenzene, and 8.8% dibromobenzeneε (o:m:p=31:3:66) . The bromine converεion waε 60%.

EXAMPLE 2

Catalyst: Cu-Silica-alumina

Furnace temp: 300°C

Reaction temp: 342°C

Aromatic feed: benzene at 0.5 ml/min Bromine feed: 48% aq HBr at 0.5 ml/min

The reaction product contained 65.2% benzene, 27.0% bromobenzene, and 6.6% dibromobenzenes (o:m:p=32:4:66) . The bromine conversion waε 49%.

EXAMPLE 3

Catalyst: Fe-NaX

Furnace temp: 300°c

Reaction temp: 305°C Aromatic feed: 1-bromonaphthalene at 0.5 ml/min

Bromine feed: liquid bromine at 0.1 ml/min

The reaction product contained 24.7% bromonaphthal- ene, 57.4% dibromonaphthaleneε, and 17.9% tribromo- naphthaleneε. Bromine converεion waε 93%.

In the following exampleε, benzene, aqueous HBr, and air are fed at the specified ratios over 25 cc of an Fe-NaX catalyst.

Benzene Oxybromination Yield Data

-1 -1

Example Feed Holar Ratio Bed Contact . t Conversion Space Time Yield, ql hr m/p/o,

No. 1/2.0-3 Hbr H Temp.°C Time, sec $H HBr monobromo meta para ortho tribromo % ___ _? __ Q _? __»

4 1.0 1.0 1.0 362 .65 56 87 716 17 184 59 14 6/71/23 .191 4.150

5 1.0 1.0 .5 348 .70 68 82 410 13 148 49 16 6/71/23 .111 6.340

6 .5 .5 .5 345 1.34 78 98 410 13 157 45 11 6/73/21 .247 3.313

7 .5 1.0 1.0 345 .77 26 50 4% 5 44 17 1 8/67/25 .240 1.390

8 .5 1.0 .5 344 .83 54 50 241 4 35 14 2 8/66/26 .167 1.205

9 1.0 .5 .5 349 1.05 88 99.8 331 14 212 52 24 5/76/19 .184 7.620

1

10 .5 .5 1.0 351 1.19 48 99.7 585 8 92 26 3 6/73/21 .396 1.806

11 1.0 .5 1.0 349 .96 60 99.9 530 7 101 25 4 5/76/19 .257 7.421 1

Obviously, numerous modificationε and variationε of the preεent invention are poεεible in light of the above teachingε. It iε therefore to understood that within the scope of the appended claims, the inven¬ tion may be practiced otherwise than as specifically described herein.