CA1199941A - Benzene synthesis - Google Patents

Abstract

ABSTRACT
"BENZENE SYNTHESIS"

A process for selectively producing benzene using intermediate pore size zeolites substantially free of acidity is disclosed.

Description

01 BE~NZENE SYNTHESIS

TECHNICAL FIELD
Benzene is one of the basic raw materials of 05 the chemical industry. It is used to synthesize rubbers, dyes, and detergents and is also used as a solvent and as an octane increasing gasoline additive. Benzene is usually produced from hydrocarbonaceous feed materials in a mixture with toluene, the xylenes, and higher aromatics through reforming reactions such as cyclization, dehydro-genation, and isomerization. A typical reforming process uses straight-run naphtha feeds and platinum-containing catalysts.
Recovery of purified benzene from the ben~ene-toluene-xylene mixture requires some further treatment~
for example some combination of fractionation, solvent extraction, and adsorptive extraction. The efficiency of these separation steps increases as th~ benzene content of the reformate increases~ Howe~ver, there has been no particularly efficient process for producing a high-benzene reformate which need not be fractionally ~istilled, solvent extract~d, or dealkylated to obtain a high ben2ene content feed suitable for subsequent purification steps.
The object of the present invention is to provide such a process.
We have discovered that intermediate pore size æeolitec can be used to convert light straight-run naph thas (and similar mixtures) to highly aromakic mixtures.
Most surprisingly and unlike the product of traditional reforming processes, the primary constit~ent of these aromatic mixtures is benzene. Benzene synthesis using our process becomes very much more eficient ~han processes known to the art~ The importance of this development can scarcely be overestimated in view of the increasing > ~i~,~,'', 01 demands for benzene by the chemical and the autom~tive industries, and in view of ~he decreasing amounts of petroleum feeds available to the world market.
BACKGROUND ART
05 A number of V.S. patents which r~late to the production of benzene/toluene/xylene (BTX) mixtures from various feeds have issued.
U.S. 3,756,942, Cattanach, September 4, 1973 dis~loses the preparation of BTX from a C5 to 250F feed using 2SM-5.
U.S. 3,760,024, Cattanach, September 18, 1973 discloses the preparation of C6+ aromatics from C2 to C~
para~fins or oleins using ZSM-5.
U.S. 3,77S,501, Kaeding, November 27, 1973 discloses BTX preparation from olefins using ZSM-5 in the presence of oxygen.
U.S. 3,813,330, Givens, May 28, 1974 discloses aromatizing olefins in the presence of easily cracked paraffins to produce BTX using ZSM-5.
U.S. 3,827,968, Givens, August 6, 1974 discloses a two step process for preparing 8TX from olefins using ZSM-5. C2-C5 olefins are oligomerized to C5-Cg olefins which are then aromatizedO
U.S. 3,843/7405 Mitchell, Octo~er 22, 1974 discloses the preparation of BTX using a two step process and ZSM-5~
U.50 3,945,913, Brennan, March 23~ 1976 discloses the preparation of BTX from alkylaromatics having nine or more carbon atoms.
U.S. 4,060,568, Rodewald, November 29, 1977 discloses the preparation of low molecular weight olefins and p-xylene from alcohols using ZSM-50 U.S. 4,097,367t Haag, June 27, 1978 discloses the preparation of BTX from olefinic naphthas and ~5 pyrolysis gasoline using ZSM-5.

.

01 U.S. 4,120,910, Chu, October 17l 1978 discloses the preparation of C5+ aromatics and BTX by aromatizing ethane~
U.S. 4,157,293~ Plank, June 5, 1979 discloses a 05 method for preventing the loss of zinc from Zn-ZSM-5 during the preparation of BTX from C~ to C10 paraffins and olefins.
A survey of the background art shows a failure to recognize the process of the present invention~ The benzene content o the BTX products is typically much less than 50%~
TECHNICAL DISCLOSURE
Our invention is embodied in a process for selectively preparing a product havin~ a substantial benzene content from normal and slightly branched chain hydrocarbons, comprising:
(a) contacting a hydrocarbonaceous eed, which comprises normal and slightly branched chain hydrocarbons and has a boiling range above about 40C and below about

2.00C with a conversion catalyst which comprises an intermediate pore si~e zeolite and a Group ~III metal compound, and wherein said zeolite is substantially free of acidity; and (b) recovering a benzene containing effluent.
Feeds appropriate for use in the process contain normal and slightly branched alkanes or olefins, or : both. The feeds can also contain naphthenes. Because the intermediate pore siæe zeolites used in the pro~ess are shape selective, the efficiency of the conversion will be greater the high~r the proportion in the fead of molecules which can it within or par~ially within ~he zeolites.
Typical hydrocarbonaceous feedstocks appropriate for use have a boiling range of above about 40C and below about 200C, preferably above about bOC and below about 120C. Normal feeds for refinery production of benzene .

~1 include light straight-run fractions and light naphthas.
Paraffinic feeds, which are not efficiently dehydro-cyclized by traditional reforming processes, can be efficiently processed using our invention. Whatever the S feed source, the higher the proportion of C6 and higher alkanes and olefins in the feed, the greater the effi-ciency of the process, and the higher the ben2ene content of the effluent~ The most preferred feeds consist essentially of hydrocarbons having from 6 to 8 carbon atoms.
A particularly use~ul application of the present process is in upgrading the effluent produced by reforming. In typical reforming processes operating on typical reforming feeds, the n-paraffins are unconvert-ed. By using the reformer effluent as the feed in our process for producing benzene, the aromatics content ofthe final reformate product can be substantially increased; the octane rating of the product increases as low octane n-paraffins are converted into high octane benzene.
The intermediate pore size zeolites used in the process are crystalline aluminosilicate zeolites having a silica to alumina mole ratio greater than about 10:1 and preferably greater than about 40:1. These zeolites have useful activity even at high silica:alumina mole ratios such as 1000 to 2000:1.
By "intermediate pore size" as used herein is meant an effective pore aperture in the range of about 5 to 6.5 Angstroms when the zeolite is in the ~-form.
Zeolites having pore apertures in this range tend to have unique molecular sieving characteristics. Unlike small pore zeolites such as erionite, they will allow hydro-carbons having some branching into the 2eolitic void spaces. Unlike large pore zeolites such as the faujasites, they can differentiate between n-alkanes and 01 slightly branched alXanes on the one hand and larger branched alkanes having, for example, quarternary carbon atoms.
The effective pore size of the zeolites can be 05 measured using standard adsorption techniques and hydro-carbonaceous compounds of known minimum kinetic diameters.
See Breck, Zeolite Molecular Sieves, 1974 (especially Chapter 8) and Anderson et al., J. Catalysis 58, 114 (1979).
Intermediate pore size zeolites in the E~-form will typically admit molecules having kinetic diameters of 5 to 6 Angstroms with little hindrance. Examples of such compounds (and their kinetic diameters in Angstroms) are: n-hexane (4.3), 3-methylpentane (5.5), benzene (5.85), and toluene ~5.8). Compounds baving kinetic diameters of about 6 to 6.5 Angstroms can be admitted into the pores, depending on the particular zeolite, but do not penetrate as quickly and in some cases, are effectively excluded (for example, 2,2-dimethylbutane is excluded from H-ZSM-5). Compounds having kinetic diameters in the range of 6 to 6.5 Angstroms include: cyclohexane (6.0), 2,3-dimethylbutane (6.1), ~,2-dimethylbutane (6.2), m-xylene ~6.1) and 1,2,3,4-tetramethylbenzene t6.4). Generally, compounds having kinetic diameters of greater than about ~5 6,5 Angstroms cannot penetrate the pore apertures and thus cannot be adsorbed in the interior of the zeolite.
Examples o such larger compounds include: o-xylene ~6.8), hexamethylbenzene (7.1), 1;3,5-trimethylbenzene ~7.5), and tributylamine (8~

3~ The preferred effective pore size range is from about 5.3 to about 6.2 Angstroms. ZSM-5, ZSM-ll, and silicalite, for example, fall within this range.
In performing adsorption measurements to deter-mine pore size, standard techniques are used. It is convenient to consider a particular molecule as excluded if it does not reach a-t least 95~ of its e~uilibrium adsorption value on the zPolite in less than about 10 minutes (P/Po = 0.5, 25C).
Examples of intermediate pore size zeolites include silicalite and members of the ZSM series such as ZSM-5, ZSM-ll, ZSM-12, ZSM-21, ZSM-23, ZS~-35, and ZSM-38.
ZS~-5 is described in United States Patents 3,702,886 and 3,770,614; ZSM-ll ls described in United States Patent 3,709,979; ZSM-12 is described in United States Patent 3,832,449; ZSM-21 is described in United States Patent 3,9'18,758; and silicalite is described in United States Patent

4,061,724. The preferred zeolites are silicalite, ZSM-5, and ZSM-ll.
The conversion catalyst must include a Group VIII
metal compound to have sufficient activity for commercial use.
By Group VIII metal compound as used herein is meant the metal itself or a compound -thereof. The Group VIII noble metals and -their compounds, platinum, palladium, and iridium, or combina-tions thereof can be used. The most preferred metal is platinum.
The Group VIII metal component can be impregnated into -the zeo-lite after it is formed, or the metal can be included in the reaction mixture from which the zeolite is hydrothermally crystallized. It is highly desirable for the metal component ~o be dispersed uniformly throughout the zeolite, by inclusion in the hydrothermal crystallization mixture for example~ The amount of Group VIII metal present in the conversion catalyst should be within the normal range of use in reforming catalysts, from about 0.1 to 1.0 weight percent, preferably 0.2 to 0.8 weight percent, and most preferably 0.2 to 0.6 weight percent.

- 6a -Reforming catalysts containing platinum are usually subjected to halogen or halide treatments to achieve or maintain a uniform metal dispersion and they also contain a halide component (especially a chlorine ~a 94:~L

01 compound~. The catalysts of our invention can be subjected to similar treatments without lessening the catalytic specifici~y for benzene synthesis. The halide treatment does not appear to have a significant effect on 05 the yield of benzene.
The intermediate pore siæe zeolite/Group VIII
metal conversion catalyst can be used without a binder or matrix. The preferred inorganic matrix, where one is used, is a silica-based binder such as Cab-O-Sil or Ludox.
Other matrices such as magnesia and titania can be used.
The preferred inorganic matrix is nonacidic.
It is critical to the selective production of benzene in useful quan~ities that the conversion catalyst be substantially free of acidity, for example by poisoning the zeolite with a basic metalt e.g., alkali metal, com-pound. Intermediate pore size zeolites are usually prepared from mixtures containing alkali metal hydroxides and thus have alkali metal contents of about 1-2 weiyht percent. These high levels of alkali metal, usually sodium or potassium, are unacceptable ~or most catalytic applications because they cause a high fouling rate.
Usually, the alkali metal is removed to low levels by ion-exchange with hydrogen or ammonium ions. By alkali metal compound as used herein is meant elemental or ionic alkali metals or their basic compounds. Surprisingly, unless the zeolite itself is substantially free of acidity, the basic compound is required in ~he present process to direct the synthetic reactions to benzene production.
The amount of alkali metal necessary to render the zeolite substantially free o acidity can be calcu-lated using standard techniques based on the aluminum conten~ of the zeolite. Under normal circumstances, the zeolite as prepared and without ion-exchange will contain sufficient alkali metal to neutralize ~he acidity of the ~atalyst. If a ~eoli~e free of alkali metal is the .

01 starting material, alkali metal ions can be ion exchanged into the zeolite to substantially eliminate the acidity of the zeolite. An alkali metal content of about 100%, or greater, of the acid sites calculated on a molar basis is 05 sufficient.
Where the basic metal content is less than 100%
of the acid sites on a molar basis~ the following test can be used to determine if the zeolite is substantially free of acidity. The test procedure is as follows: 0.1-0.59 of catalyst is mixed with lg of acid-washed and neutral-ized alundum and packed in a 3/16" stainless steel reactor tube with the remaining space filled with alundum. The reactor contents are calcined for one hour at 450C. The reactor is then placed in a clam shell furnace and the lS reactor outlet connected to the inlet of a gas chromatograph. The inlet is connected to the carrier yas line of the GC. Helium is pa~sed through the system at 30 cc/min. 0.04 Microliter pulses of n-decane are injected through a septum above the reactor and reaction products are determined by standard GC analysis. ~lank runs with alundum should show no conversion under the experimental conditions, nor should a 100% Catapal alumina catalyst.
~ pseudo-first-order, cracking rate constant, k, is calculated using the formula 5 k = 1- ln A l-x where A is the weight of zeolite in grams and x is the fractional conversion to products boiling ~elow decane.
The zeolite is substantially free of acidity when the value for ln k is less than about -3.8.
The preferred alkali metals are sodium and potassium. The zeolite itself can be substantially free of acidity only at very high silica:alumina mol ratios; by .

01 "zeolite consisting essentially of silica~ is meant a zeolite which is substantially free of acidity without base poisonlng.
The reaction conditions for the process typi-05 cally include pressures ranging from atmospheric to 10bar, and liquid hourly space velocities (LHSV) ranging from 0.1 to 15. If desired, hydrogen can be mixed with the feed to lessen the tendency of the catalyst to foul, but hydrogen need not be used. The reactions can take place at ~emperatures above 480C. Surprisingly, the process is most efficient at relatively high temperatures, above 510C and ranging up to about 595C.
By substantial amount of benzene is meant a benzene content of the C5+ aromatics produced which is greater than about 50% by weight of the C5~ aromatics, preferably greater than about 60% by weight, and most preferably greater than about 75% by weight.
The following examples illustrate the invention.
All percentages given are ~y weight, unless otherwise indicated.
Two light straight-run feeds were used in the tests, Feed 1 Gravity, API 79 ~5 Nitrogen, ppm C0.1 Sulfur~ ppm <0.2 Vol. ~ paraffin 83.6 ~istillation ~D86~ ~F:
Start/5 118/123 10/30 12~135 01 Feed 2 Gravity, API 75 Nitrogen, ppm <0.1 Sulfur, ppm <0.02 05 Vol. % paraffin 77.2 Distillation (D86) F:
Start~5 120/133 10/30 136/1~7 Example 1 A series of experiments were performed to illustrate the necessity of using a zeolite substantially free of acidity to produce benzene. In the first set of experiments a zeolite prepared according to U.S. 4,061,724 and having a silica:alumina mole ratio of 89~:1 was used. No inorganic matrix was used. Feed 2 was used as were the following conditions: L~SV=l, no hydrogen, 3.4 ~0 bar ~gau~e), and 538C~ Calculations show that 0.16%
sodium would saturate the acid (aluminum) sites.

Catalyst A B C _ Na level, ~ 0.017 0.99 4.12 Pt level, ~ 0.33 0.38 0.44 Product C5+, % of feed 36.B8 79~69 85.59 Aromatics, % of C5+gl.60 56O43 42.57 Benzene, % of aromatics 8.81 65.45 65.B9 These data $how that as the sodium level in-crea~ed taS the acidity of the zeolite decreased and was neutralized~ the yield of the C5~ fraction increased and 3~

3L~9~

01 the benzene content of the C5+ aromatics fraction in-creased substantially.
A second set of experiments also showed the surprising effect of an acid-free zeolite on benzene 05 specificity. Again using Feed 2, LHSV=l, H2/HC=1, 1.65 bar (gauge), and 538C together with zeolites exhibiting the ZSM-5 X-ray diffraction pattern and having a silica:alumina mole ratio of 121:1 (D and E), and, pre-pared according to U.S. 4,061,724 ~; silica:alumina mol ratio of 938:1), the following results were obtained:

Catalyst D E F
~Ja level ~ 2.D3 0.005 1.14 Pt % 0,44 0.34 0.36 Re % 0.42 0O37 0 Cl % 0.~9 <0.05 0 Product C5~r % of ~eed 52.67 37.5~ 48.55 ~ Aromatics, ~ of CS+ 88.63 92.05 92.82 Benzene, ~ of Aromatics 89.14 34.12 94.76 These data also illustrate the significant selectivity towards ben~ene production of the present invention.
Further experiments to show the selective production of benzene in the aromatics fraction and the effect of alkali metal poisoning were performed using a ; zeolite prepared according to U.S~ Patent 4,073~865, Flanigen, February 14, 197~ and having an aluminum content of 950 ppm (W/W). The reactions were performed on Feed 2 at LHSV=l, no hydrogen, 3O3 bar (gauge~, and 538C.

~9~

01 Catalyst G H
Na % 2.82 ~0.005 Pt % 0.49 0.24 05 Product C5+ % of feed 83 55 43.61 Aromatics % of C5~ 15.47 63.43 Benzene, ~ of aromatics62~44 7~91 Although the aromatics fraction of the C5+
product of G is low, it has a high ben~ene content, and, based on feed converted, G produced significantly more benæene ~ca. 8%) than H (ca. 2.5%).
Example 2 Experiments were performed to show the necessity of havin~ a Group VIII metal present to achieve not only benzene selectivity but also reasonable yields of aromatics. Feed 2 was used along with LHSV=l, 3.3 bar ~gauge), no hydrogen, and 538C. The zeolites of I
exhibited the X-ray dif~raction pattern o ZSM-5 while the zeolite of J was prepared according to U.S. 4,061,724.
The ~eolites were not composited with an inorganic matrix.

Catalyst I _ J
SiO2:A12O3 67~4 892 Na % 1.02 0.99 Pt ~ o 0.
.
Product C5+, % of eed 95.64 79.69 3 Aromatics~ ~ of ~5+ 3.24 66.43 Benzene, ~ aromatics 49.69 65.45 These data show that the ~roup VIIT metal, platinum in this case, is necessary not only to yield a 3~ .

g43~

Dl high benzene fraction of the aroma~ics formed, but also to yield practical amounts of an aromatic frac~ion.
Example 3 The preceding examples show ~hat benzene can be 05 selectively produced in high yields from zeolites having a wide range of silica:alumina mole ratios. Several further experiments were also performed to illustrate this activity. The catalyst of ~ was prepared according to U.S. 4,061,724 while that of L exhibited the ZSM-5 X-ray diffraction pattern~ Reaction conditions included 1.65 bar (gauge), H2/HC=l, LHSV=l, and 538C.

Feed #1 #2 Catalyst ~ L
Silica:Alumina 938 121 Na ~ 1.14 2.03 Pt % 0.35 0.44 Re ~ 0.54 0.42 Cl % 0.
Product C5~ ~ of feed 47.43 52.67 Aromatics, % of C5~ 100 88.63 Ben~ene~ ~ of aromatics 87~73 89.14 Example 4 Feed ~2 was processed with a platinum (0.3Ç%), sodium (1.14%) zeolite prepared according to U.S.
4,061,7~4 and having a silica:alumina mole ratio of 938:1. The reaction conditions included LHSV=l, 1.65 bar ~gauge), H~HC~l, and 538~C. The products were compared at the beginning and end of the run.

01 Beginning End ~after (after 1 hour) 20 hours) C5+, % oE feed 57.63 80.48 05 Aromatics~ % of C5+ 87.30 57.78 Benzene, % of aromatics 85.2 75.2 % of feed 42.86 35.89 C7+ aromatics~ % of feed 7.45 11.60 Unconverted feed 7.D5 33.97 These data illustrate the surprising result, that as the catalyst fouls, the yield of C5~ increases, benzene remains a major component of the C5~ aromatics, benzene remains a major product, and non-benzene aromatics remain a minor portion of the product. The sites which have undesirable cracking activity foul faster than ~he benzene synthesis sites.
Example 5 According to the dehydrocyclization mechanism through which conventional reforming proceeds, an n-heptane feed would yield a product which is primarily toluene, with only small amounts of ben2ene and other aromatics. An experiment was performed using an n-heptane . feed, a platinum, sodium ZSM-5 zeolite and L~SV-l, 1.65 bar ~gauge), H~/HC=lt and 538C. The data from the begin-2~ ning and end of the experiment show significant amounts of ben~ene are produced from n-heptane.

Beginning End ~after (after 1 hour~ ~0 hours) C5+, ~ of feed . 58.98 69.26 Aromatics~ % of C5~ 94.87 84.32 Benzene, % of feed 39.40 21.99 C7~ aromatic~ ~ of feed 16.56 36.41 Un~onverted feed 1.92 3~65 ~g~9~1 01 Example 6 An experiment was performed using Feed ~2 and a 2eolite exhibiting the ZSM-5 X-ray diffraction pattern.
The alkali metals were sodium (in M) and potassium ~in 05 N). Reaction conditions included 3.3 bar (gauge), no hydrogen, LHSV=l, and 538C.

Catalyst M N
Cation level, % 0.99 1.46 Pt, % 0.38 0.3~
C5+, % of feed 79.69 68.59 Aromatics, of C5+66.43 40.08 Benzene, % of aromatics 65.4S 76.67 Closure 98.77 89~50 These data indicate that an aromatics fraction with a high benzene content can be prepared from a zeolite whose acidity is neutrali2ed by different alkali metals.
Example_7 An experiment was performed using Feed #l and a zeolite prepared according to U.S. 4,061,724 to show the effect of halide on the content of the Cl to C4 gas. The catalysts were unbound.

Catalyst O P
~5 Cl, % 0.7~ 0 Pt, ~ ~.35 0.48 Re, ~ ~ 54 Na, ~ 1.14 1.14 01 Product Yield C5+, % 47.43 70.18 Aromatics, ~ of C5+ 100 52.22 Benzene, % of C5+
05 aromatics 87.73 77.10 Methane, wt % of feed55.62 11.98 C2 0 ~97 C3 5.35 C4 .7.24 The test shows the selective production of methane as opposed to the other light gases.

~5 : 3~

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for selectively preparing a product having a substantial benzene content from normal and slightly branched hydrocarbons, comprising (a) contacting a hydrocarbonaceous feed, which comprises normal and slightly branched hydrocarbons and has a boiling range above about 40°C and less than about 200°C with a conversion catalyst which comprises an intermediate pore size zeolite and a Group VIII metal compound, and wherein said zeolite is substantially free of acidity; and (b) recovering a benzene containing effluent.

2. A process according to Claim 1 wherein inter-mediate pore size zeolite has pore apertures in the range of about 5 Angstroms to about 6.5 Angstroms.

3. A process according to Claim 1 wherein said zeolite is selected from ZSM-5, ZSM-11, and silicalite.

4. A process according to Claim 1 wherein said zeolite contains a sufficient quantity of an alkali metal compound to be substantially free of acidity.

5. A process according to Claim 4 wherein said zeolite has an alkali metal content of about 100%, or greater, of the acid sites in said zeolite on a molar basis.

6. A process according to Claim 5 wherein said alkali metal is selected from potassium and sodium.

7. A process according to Claim 1 wherein said zeolite consists essentially of silica.

8. A process according to Claim 1 wherein said Group VIII metal is platinum.

9. A process according to Claim 1 wherein said feed has a boiling range above about 60°C and below about 120°C

10. A process according to Claim 9 wherein said feed consists essentially of hydrocarbons having from 6 to 8 carbon atoms.

11. A process according to Claim 1 wherein said con-tacting occurs at a temperature above about 480°C.

12. A process according to Claim 11 wherein said contacting occurs at a temperature above about 510°C.

13. A process according to Claim 1 wherein said conversion catalyst further comprises a halide component.

14. A reformer effluent upgrading process, comprising:
(a) reforming a reformer feed to produce a reformer effluent; and (b) producing an upgraded reformate having an increased benzene concentration by contacting said reformer effluent with a conversion catalyst which comprises an intermediate pore size zeolite and a Group VIII metal compound, and wherein said zeolite is substan-tially free of acidity.