Process for producing xylene

An improved process for producing xylene from feedstock containing C.sub.9 alkyl aromatic hydrocarbons with the aid of a catalyst capable of disproportionation, rearrangement, and dealkylation, wherein said improvement comprises performing the reaction in the presence of an aromatic hydrocarbon having one or more ethyl groups in an amount of 5 to 50 wt %.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for efficient production of 
xylene from feedstock containing C.sub.9 alkyl aromatic hydrocarbons 
(which are generally regarded as useless) by disproportionation, 
transalkylation, and dealkylation, said process being carried out in the 
presence of a specific aromatic hydrocarbon whose concentration is within 
a certain range. 
2. Description of the Prior Art 
Xylene as a feedstock for p-xylene and o-xylene is usually produced from 
naphtha by reforming, followed by extraction and fractionation, or by 
extraction and fractionation of cracked gasoline as a by-product of 
thermal cracking of naphtha. Xylene is also produced on an industrial 
scale from toluene or a mixture of toluene and C.sub.9 aromatic 
hydrocarbons by disproportionation and transalkylation of alkyl groups. 
However, toluene itself is an industrially important raw material for the 
production of benzene by dealkylation. 
On the other hand, there has been disclosed in Japanese Patent Publication 
Nos. 48413/1974 and 16782/1975 a process for producing C.sub.10 aromatic 
hydrocarbons (such as durene) from C.sub.9 aromatic hydrocarbons 
(including propylbenzene isomers, methylethylbenzene isomers, and 
trimethylbenzene isomers) by disproportionation and transalkylation. 
However, nothing is known about the efficient production of xylene from 
feedstock composed mainly of C.sub.9 aromatic hydrocarbons. 
There is a known process for industrially producing xylene from toluene and 
C.sub.9 aromatic hydrocarbons with the aid of amorphous silica-alumina 
catalyst. (PETROTECH, 2 (12) 1160, 1970) This process suffers the 
disadvantage that the catalyst has to be continuously regenerated by using 
a moving bed so as to maintain a certain level of yield and activity. 
There has been reported a process for producing xylene from C.sub.9 
aromatic hydrocarbons alone or in combination with toluene with the aid of 
a zeolite catalyst. (J. Das et al., Catalysis Letter 23 (1994), I. Wang et 
al., Ind. Chem. Res. 29 (1990) 2005) This process is not necessarily 
satisfactory in yields. 
So far, there has been no efficient process for producing xylene from 
C.sub.9 aromatic hydrocarbons. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for 
efficiently producing xylene by disproportionation, transalkylation, and 
dealkylation from a feedstock composed mainly of substantially 
toluene-free C.sub.9 aromatic hydrocarbons generally regarded as useless. 
The present inventors found that it is possible to produce xylene 
efficiently from a feedstock composed mainly of substantially toluene-free 
C.sub.9 aromatic hydrocarbons by disproportionation, transalkylation, and 
dealkylation if an aromatic hydrocarbon having one or more ethyl groups is 
present in a certain amount. 
The gist of the present invention resides in an improved process for 
producing xylene from a feedstock containing C.sub.9 alkyl aromatic 
hydrocarbons with the aid of a catalyst capable of disproportionation, 
transalkylation, and dealkylation, wherein said improvement comprises 
performing the reaction in the presence of an aromatic hydrocarbon having 
one or more ethyl groups in an amount of 5 to 50 wt %. 
DETAILED DESCRIPTION OF THE INVENTION 
The process of the present invention employs a feedstock composed mainly of 
C.sub.9 alkyl aromatic hydrocarbons. It also employs an aromatic 
hydrocarbon having one or more ethyl groups, which is exemplified by 
ethylbenzene, methylethylbenzene, dimethylethylbenzene, and diethyl 
benzene. 
According to the present invention, xylene is produced efficiently from a 
feedstock composed mainly of C.sub.9 alkyl aromatic hydrocarbons with the 
aid of a catalyst capable of disproportionation, transalkylation, and 
dealkylation, in the presence of an aromatic hydrocarbon having one or 
more ethyl groups in an amount of 5 to 50 wt %, preferably 15 to 50 wt %. 
The catalyst is not specifically restricted so long as it is capable of 
disproportionation, transalkylation, and dealkylation. It should 
preferably be one which contains zeolite. A preferred zeolite is 
mordenite. 
The zeolite should contain at least one member selected from the metals 
belonging to the VIB, VIIB, and VIII Groups, in an amount of 0.001-5 wt %, 
preferably 0.02-1 wt % (as an element). A preferred example of the metal 
is rhenium. 
The reaction involving the above-mentioned catalyst should be carried out 
in the presence of hydrogen at 1-6 MPa and 300.degree.-550.degree. C., 
with the WHSV (weight hourly space velocity) being 0.1-10/hr.

EXAMPLES 
The invention will be described with reference to the following examples. 
Example 1 
A pasty mixture was prepared by mixing 105 g of powdery synthetic 
mordenite(sodium form), 45 g of .alpha.-alumina, 12 g of alumina sol 
(containing 10 wt % alumina), 10.5 g of alumina gel (containing 70 wt % 
alumina), and an adequate amount of deionized water. After kneading for 
about 2 hours, the pasty mixture was molded into cylindrical pellets, each 
measuring 1.0 mm long and 1.2 mm in diameter. The pellets were dried at 
120.degree. C. for 16 hours. The dried pellets (50 g in absolute dry 
condition at 520.degree. C.) were baked at 400.degree. C. for 5 hours in 
an atmosphere of air. After cooling, the baked pellets were treated with 
100 g of 10 wt % aqueous solution of ammonium chloride at 
80.degree.-85.degree. C. for 1 hour. The treated pellets were strained off 
the solution and thoroughly washed with water. The pellets were treated 
with 100 g of 5 wt % aqueous solution of tartaric acid at 80.degree. to 
85.degree. C. for 3 hours. The treated pollets were strained off the 
solution and thoroughly washed with water. The washed pellets were dipped 
in 6.5 g of 5 wt % aqueous solution of rhenium(VII) oxide (Re.sub.2 
0.sub.7) at room temperature for impregnation with rhenium. The pellets 
were dried again at 120.degree. C. for 16 hours and then baked at 
540.degree. C. for 8 hours in an atmosphere of air. Thus, there was 
obtained hydrogen ion exchanged mordenite catalyst (A). This catalyst (A) 
contained 0.25 wt % of rhenium (in absolute dry condition at 520.degree. 
C.). 
Using this catalyst (A) in a fixed-bed catalytic reactor, xylene was 
produced from a feedstock composed of trimethylbenzene (TMB for short) as 
a C.sub.9 alkyl aromatic hydrocarbon and methylethylbenzene (ET for short) 
as an aromatic hydrocarbon having an ethyl group in varied ratios. The 
reaction conditions were as follows: 
Temperature: 400.degree. C. 
Pressure: 4 MPa 
WHSV: 2.5 h-1 
H/2 feedstock: 4.0 mol/mol 
The results are shown in Table 1. It is noted that the yield of xylene 
increases as the amount of ET increases up to 50 wt %. However, beyond 
this limit, the yield of xylene decreases. 
TABLE 1 
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Run Ratio (by weight) of 
Amount (g) of xylene produced 
No. ET/(TMB + ET) in feedstock 
per 100 g of feedstock 
______________________________________ 
1 0 20 
2 0.25 31 
3 0.45 34 
4 0.65 28 
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Example 2 
Using the catalyst (A) in a fixed-bed catalytic reactor, xylene was 
produced in the same manner as in Example 1 from a feedstock in which ET 
was replaced by ethylbenzene (EB for short) or diethylbenzene (DEB for 
short). 
The results are shown in Table 2. It is noted that the yield of xylene is 
favorably affected by both EB and DEB. 
TABLE 2 
______________________________________ 
Composition (by weight) 
Amount (g) of Xylene produced 
Run No. or feedstock per 100 g of feedstock 
______________________________________ 
1 TMB + EB 34 
(EB/TMB = 30/70) 
2 TMB + DEB 34 
(DEB/TMB = 35/65) 
______________________________________ 
Example 3 
Catalysts were prepared in the same manner as in Example 1 except that the 
amount of rhenium was varied. Using the catalysts in a fixed-bed catalytic 
reactor, xylene was produced in the same manner as in Example 1 from the 
same feedstock as used in Run No. 3 in Example 1. 
The results are shown in Table 3. It is noted that the yield of xylene 
increases with the increasing amount of rhenium in the range of 0.01 wt % 
to 0.02 wt %. The effect of rhenium levels off beyond 0.10 wt %. 
TABLE 3 
______________________________________ 
Content of rhenium 
Amount (g) of xylene produced 
Run No. as element (wt %) 
per 100 g of feedstock 
______________________________________ 
1 0 20 
2 0.01 23 
3 0.02 32 
4 0.10 34 
5 0.20 34 
______________________________________ 
Example 4 
Six catalysts (B to G) were prepared, each containing rhenium, nickel, 
cobalt, molybdenum, chromium, or tungsten. The first four catalysts (B to 
E) were prepared in the same manner as in Example 1 by impregnation with 
an aqueous solution containing each metal element. The last two catalysts 
(F and G) were also prepared in the same manner as in Example 1 except 
that the compound shown in Table 4 was incorporated into the catalyst 
components at the time of mixing. 
TABLE 4 
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Catatyst 
Metal Compound Incorporated by 
______________________________________ 
B Re Re.sub.2 O.sub.7 
Dipping and impregnation 
C Ni Ni(NO.sub.3).sub.2 6H.sub.2 O 
Dipping and impregnation 
D Co Co(NO.sub.3).sub.2 6H.sub.2 O 
Dipping and impregnation 
E Mo (NH.sub.4).sub.6 Mo.sub.7 O4H.sub.2 O 
Dipping and impregnation 
F Cr CrO.sub.3 Mixing 
G W WO.sub.3 Mixing 
______________________________________ 
Using each catalyst (B to G) in a fixed-bed catalytic reactor, xylene was 
produced under the same condition as in Example 1 from the same feedstock 
as used in Run No. 3 in Example 1. The results are shown in Table 5. It is 
noted that the catalyst containing rhenium is most active with the minimal 
content. 
TABLE 5 
______________________________________ 
Content (wt %) of 
metal (as element) in 
Amount (g) of xylene produced 
Catalyst 
Metal catalyst from 100 g of feedstock 
______________________________________ 
B Re 0.15 34 
C Ni 0.40 30 
D Co 0.40 26 
E Mo 0.40 32 
F Cr 0.40 28 
G W 0.24 22 
______________________________________ 
Example 5 
Three catalysts, each containing a different amount of rhenium, were 
prepared in the same manner as in Example 1. Using each catalyst in a 
fixed-bed catalytic reactor, xylene was produced under the same condition 
as in Example 1 from the same feedstock as used in Run No. 3 in Example 1. 
The rate of decrease in yield was recorded. The results are shown in Table 
6. It is noted that the catalyst becomes less liable to deterioration in 
proportion to the amount of rhenium contained therein. 
TABLE 6 
______________________________________ 
Content (wt %) of rhenium 
Decrease in yield of xylene 
(as element) in catalyst 
(wt % per day) 
______________________________________ 
0 1.50 
0.01 0.96 
0.20 less than 0.04 
______________________________________