US3755493A - Normal paraffin isomerization with liquid phase asf5/hf catalyst - Google Patents

Abstract

NORMAL PARAFFINS ARE ISOMERIZED TO SKELETAL ISOMERS CONTAINING THE SAME NUMBER OF CARBON ATOMS BY CONTACTING PARAFFINS WITH A LIQUID MIXTURE OF ARSENIC PENTAFLUORIDE (ASF5) AND HF. FOR EXTENDED REACTION PERIODS, HYDROGEN IS EMPLOYED TO SUPPRESS CRACKING.

Description

United States Patent Office 3,755,493 Patented Aug. as, 1973 ABSTRACT OF THE DISCLOSURE Normal parafiins are isomerized to skeletal isomers containing the same number of carbon atoms by contacting paraflins with a liquid mixture of arsenic pentafluoride (AsF and HF. For extended reaction periods, hydrogen is employed to suppress cracking.

This invention relates to hydrocarbon conversion. In accordance with another aspect, normal paraflins are isomerized to skeletal isomers containing the same number.

of carbon atoms by contacting the parafiins with a liquid mixture of AsF and HF. In accordance with a further aspect, cracking of hydrocarbon feed is suppressed during isomerization of normal paraflins in the presence of a liquid phase AsF /HF catalyst by the addition of hydrogen. In accordance with a further aspect, an isomerization catalyst comprising liquid phase AsF HF is provided. In accordance with a further aspect, this invention relates to an improved continuous process for the skeletal isomerization of normal paraffins in the presence of a liquid phase AsF /HF catalyst by spraying the reactant parafiin into a column of the liquid catalyst.

It is well known that the more highly branched isomers of the paraflinic hydrocarbons occurring in petroleum gasoline fractions are more valuable than the corresponding slightly branched or straight chain hydrocarbons because of their higher octane ratings. The demand for motor fuels of greater octane number has increased markedly as the automotive industry has provided gasoline engines with increasingly higher compression ratios to attain greater efficiency. One of the economically important ways in which the increased demands for high octane fuels can be met is through the isomerization of the light naphtha components of such fuels.

It may be generally stated that the isoparaflinic and branched chain paraifin hydrocarbons are of greater commercial value to the petroleum industry than the corresponding straight chain hydrocarbons. Thus, for example, 2,2-dimethylbutane has a higher octane rating than the isomeric normal hexane. Isobutane is more valuable than normal butane since the former can be used as a basis for the preparation of 8-carbon-atom, branched-chain-hydrocarbons by alkylation with butylene. v

The isomerization of normal paraflin hydrocarbons into the corresponding branched chain homolo gs is well known. For effecting the isomerization, it is customary to employ certain metal halides, particularly aluminum chloride or aluminum bromide, in conjunction with certain promoters such as hydrogen chloride, hydrogen bromide, or boron fluoride. Recently, strong acid systems such as solutions of fluorosulfonic acid and antimony pentafluoride have tion in the presence of a liquidphase AsF /HF catalyst. It has been further found that the use of hydrogempressure during isomerization inhibits cracking and favors isomerization, especially during extended reaction periods.

Accordingly, an object of this invention is to provide an improved isomerization process.

Another object of this invention is to provide an isomerization process whereby formation of undesirable side products during isomerization is mnimzed. f

A further object of ths invention is to provide an improved catalyst for isomerization.

Other objects and aspects as well as the several advantages of the invention will be apparent to those skilled in the art upon further consideration of the specification and the appended claims.

In accordance with the invention, it has been found that a liquid phase mixture of arsenic pentafluoride and anhydrous hydrogen fluoride (AsF /HF) isomerizes normal alkanes to branched isomers containing the same number of carbon atoms with a minimum formation of undesirable side products during isomerization.

Further according to the invention, it has been found that the use of hydrogen pressure in the range 30-200 p.s.i.g. inhibits cracking and favors isomerization.

In accordance with one specific embodiment, n-hexane isomerizes to predominantly neohexane at room temperature in the presence of AsF and liquid HF.

In accordance with another embodiment, normal butane is readily isomerized in high conversion with high selectivity to isobutane.

Arsenic pentafluoride presents advantages over other superacid catalysts in that it provides a reaction rate significantly higher than those previously known. Additionally, normally n-butane is difiicult to isomerize and requires higher temperatures and pressures but utilizing the catalysts of the invention high conversion and high selectivity are obtained at ambient conditions.

Suitable feeds for the purposes of the present invention will contain normal paraffinic hydrocarbons having from 4 to 7 carbon atoms in the molecule and may be exemplified by normal butane, normal pentane, normal hexane, and the like. While the feed may substantially comprise a purified normal parafiinic hydrocarbon stream, it is also contemplated that mixtures of various paraflinic hydrocarbons can be employed.

Arsenic pentafluoride, AsF (M.P.79.8 C., B.=P. 532 C., M.W. 169.9), is a known compound and is commercially available and is readily dissolvable in liquid HF at room temperature. Any other procedure for combining arsenic pentafluoride and liquid HF to form the catalysts of the invention can be employed, as desired.

The mole ratio of liquid HF/AsF forming the catalysts of the invention will generally be in the range 400:1 to 1:1, preferably 20:1 to :1. The mole ratio of normal paraffinic hydrocarbon to AsF will generally be in the range 50:1 to 0.2:], preferably 1:1 to 20:1.

The reaction conditions for isomerization of the feed can be in the range of 0 to 100 C., usually 15 to 65 C., and pressure sufficient to maintain the hydrocarbon reactants and catalysts as liquids in the reaction zone and the temperatures and pressures should be chosen accordingly. The time of contact is subject to wide variation, the length of residence time dependent in part upon the temperature and catalyst concentration employed. In general, contact times ranging from about 5 minutes to 48 hours, preferably 15 minutes to about 2 hours, are employed.

As indicated above, the formation of undesirable side products can be suppressed by carrying out the isomerization in the presence of at least 30 p.s.i.g. of hydrogen pressure. The hydrogen pressure can vary from 0 to 400 p.s.i.g., but when employed will preferably range from 30 to 200 p.s.i.g. For reaction periods exceeding about 60 the preferred range in the catalyst system to suppress cracking reaction.

The process of the invention is conducted as a batch or a continuous operation. The apparatus employed can be of a conventional nature and can comprise a single reactor equipped with suflicient stirring devices. Unreacted reactants, catalysts and other products of the reaction can be separated from the desired isomeric product and from one another such as by distillation and returned in whole or in part to the isomerization zone. The resultant product can be further processed as by alkylation and the like or be employed directly as a high octane gasoline blending agent. The reaction zone is preferably constructed of materials which are resistant to corrosion by the catalysts; for example, the reactor can be IMonel reactor. In a continuous system, the parafiin The results of Table I show that n-butane conversions of about 50 percent are obtained with a butane/AsF mole ratio of about 10:1 (8.2:1 to 10.9:1) over the temperature range 25 to 70 C. It is to be noted that a higher conversion (70%) was achieved-by decreasing the butane/AsF mole ratio to about 5/1 (4.8:1) at a temperature of 50 C.

EXAMPLE II Hexane was isomerized to neohexane using a liquid AsF /HF system in the presence and absence of hydrogen in a 300 ml. Monel reactor as set forth above and described in Example I. The mole ratio of normal hexane/ASF in the runs varied from 3.111 to 63:1.

Results of the isomerization of hexane are given below in Table II. The molar ratios of HF/AsF used in runs 1-7, respectively, were the following 42.4, 86.1, 47.1, 42.4, 42.4, 42.4 and 38.5.

TABLE II [Hexane isomerization in ASFs/HF] Selectivitles a Molar Hexane Selectivity DIP H2, Time, ratio conv., to cracked 2,2 an p.s.i.g. min. Ct/AsFt percent products DMB 2-MP 3-MP Heavies 30 i 3.3 88. 0 0.4 49. 9 38.3 11. 3 Trace 0 60 6. 9 62. 0 Trace 21. 9 69. 4 18. 6 Trace 0 120 3. 8 98.8 69. 20. 0 8. 3 2. 4 10. 0 120 3. 4 98.4 52. 6 35. 7 9. 0 2. 7 9. 5 120 3. 4 93. 5 3. 5 46. 3 38 12. 1 0. 7 40 120 3. 4 92. 2 1. 2 42. 8 42. 6 13. 4 0. 2 200 90 3. 1 92. 8 0. 6 49. 1 38. 2 12. 1 0

'2,2-Dl\1l]3 is 2,2-dimethyloutane (neohexane); DIP is 2,3'dimethylbutane (diisopropyl); 2-MP and S-MP represent Z-methylpentane and 3-methylpentane, respectively.

reactant can be sprayed into a column of the liquid catalyst.

EXAMPLE I Normal butane was isomerized to isobutane with a liquid phase AsF /HF catalyst utilizing a Monel reactor.

The procedure for carrying out the isomerization in a Monel reactor is set forth below.

A 300 ml. Monel reactor under dry nitrogen flow was cooled in ice and charged with liquid HF and paraffin before metering in the desired weight of gaseous arsenic pentafluoride. The capped reactor was equipped with a pressure gage, valves and thermocouple. The reactor was maintained in a thermostated Eberbach reciprocating shaker-water bath under specified reaction conditions.

Workup involved cooling the Monel reactor in Dry Iceacetone and then rapidly pouring the cold mixture into a polyethylene separatory funnel. The upper hydrocarbon layer was drained under nitrogen into a Fischer- Porter Aerosol compatibility bottle containing about five grams of potassium carbonate and cooled in a Dry Ice- TABLE I The above results show that in reaction periods up to minutes that isomerization is favored over cracking even in the absence of hydrogen (Runs 1 and 2). Longer runs (60 minutes) require hydrogen pressures of 30-200 p.s.i.g. to suppress cracking (Runs 7, 3; 6, 4 and 5).

EXAMPLE HI A spray nozzle reactor system was filled with n-hexane and the reactor was charged with AsF (23.5 g.) and HF (120 ml.). The reaction was carried out on total recycle for 80minutes.

The procedure employed for the spray nozzle reactor is set forth below:

The spray nozzle reactor consisted of a carbon steel Jorgenson gage with Kel-F lined windows. The paraffin hydrocarbon feed tank consisted of a 3300 ml. stainless steel tank which was connected by stainless steel tubing to a Jorgenson gage calibrated to determine feed rate. The bottom of the reactor was fitted with-a stainless steel hydraulic atomizing nozzle. Acid, for example, AsF HF, was blown into the reactor under nitrogen pressure via a port in the lower side of the reactor. A pump was placed in parallel with the reactor so that the product from the reactor could be recycled. The reactor functioned by introducing the paraffin hydrocarbon as a fine spray intothe acid phase. The less dense immiscible hy- (Reaction of n-butane with AsF in HF Selectivltles Mole ratio Butant 2-MP butane] Time, Temp., percent 2,2- and Run number As 5 hrs. 0. ccnv Ca [-04 l-C m0; DMB DIP 3-MP n-Cu Heavies 50 ml. HF used in each experiment in a 300 ml. Monel reactor, andthe molar ratios of HF/AsFt} used in runs 16, respectively, were the following: 56.8, 50, 60, 44.6, 22.3, and 50.

ydroons are contacted in liquid phase under isomeriza- F ranging from paraflin/AsF ranging 3. A process according to claim 2 wherein the parafiin is normal butane.

4. A process according to claim 2 wherein the paraflin n is normal hexane. ying 5. A process according to claim 2 wherein the parafiin was is normal heptane.

6. A process according to claim 2 wherein the isomerization is carried out in the presence of hydrogen and with 7. A process according to claim 2 wherein the isomeri- DIP plus 2, 2-DMB 2-MP 3-MP Heavies 2. A process according to claim 1 wherein the h carb tion conditions with a mole ratio HF/As p fig 40011 to 1:1 and a mole ratio of Se 5 fro 5 syringe fitted with a 6-inch needle. Samples for glc analm to ysis were transferred to vials at Dry Ice temperature containing potassium carbonate.

a hydrogen pressure of -200 p.s.i.g. when the reaction exceeds about minutes to supress cracking reaction.

Selectivities i-Cs H- s TABLE III [Isomerlzation of hexane in spray nozzle reactor with AsFs/HF at 25 C.]

Immediately following the reactor was a valving sys- The total hydrocarbon phase was freed of acids by passing the liquid through a sodium fluoride tube and the bubbling through a 20 percent KOH scrubber and dr Results of the normal hexane isomerization using the spray nozzle reactor system are tabulated below in Table III:

Hexane Time, con., min. percent drocarbon phase was thus circulated through the dense stationary acid phase.

tern for sample withdrawal as the reaction Samples were withdrawn by an ordinary the efliuent over 4A molecular sieves. The effiuent collected in a bomb. All valves and tubing which were in contact with HF were made of Monel and the others were stainless steel.

Sample number:

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