Preparation of N-alkyl and N,N-dialkylaniline

The present invention relates to a vapor phase process for the production of N-alkyl and N,N-dialkylaniline by alkylation of aniline with an alcohol, preferably methanol and ethanol, in the presence of a ZSM-5 catalyst. A modified ZSM-5 catalyst has the advantage of high selectivity for N-alkylation while suppressing the formation of undesired by-products, such as toluidines. The molar ratio of silica to alumina in the catalyst is from about 20:1 to 700:1 and preferably from about 30:1 to 200:1. The silica-alumina may be modified with alkali metal, alkali earth metal or transition metal ions, preferably cesium, potassium, magnesium and iron, to form the finished catalyst. The reactants are contacted in the presence of the catalyst at a temperature of from about 300.degree. to 500.degree. C., at a pressure of from about 1 to 3 atmospheres, and at a molar ratio of alcohol to aniline of from about 1 to 6. The feed rates expressed as weight hourly space velocity (g feed/g catalyst/hour) are broadly from about 0.2 to 4.

BACKGROUND OF THE INVENTION 
In the prior art, methylation of aniline with methanol was conducted in a 
batch reactor. Either sulfuric acid or phosphoric acid was used as the 
catalyst in the liquid-phase reaction that took place at a temperature of 
about 200.degree. C. under a pressure of from 30 to 50 kg/cm.sup.2. This 
traditional route suffers from the disadvantages of high capital cost, the 
corrosion of the reactor, and the need for waste acid treatment. The more 
recent vapor-phase technology has overcome corrosion problems and waste 
acid treatment but did not solve all the shortcomings associated with the 
liquid-phase reaction. 
U.S. Pat. No. 3,558,706 discloses a process for the preparation of 
N-methylaniline by the reaction of 1 mole of aniline with 6 moles of 
methanol at 500.degree..+-.50.degree. C. at 1 atmosphere over a catalyst 
consisting of 4MgCO.sub.3.Mg(OH).sub.2.4H.sub.2 O. The liquid hourly space 
velocity (LHSV) based on aniline was 0.3 to 1.0 hr.sup.-1, and the optimum 
yield was 68%. The reaction required high temperatures, wasted methanol, 
and produced unimpressive results. 
Japan Kokkai 74/81331 describes a process for making N,N-dimethylaniline by 
the liquid-phase reaction of aniline with methanol in the presence of a 
solid acid Al.sub.2 O.sub.3 -SiO.sub.2, Y-type zeolite catalyst at 
280.degree. C. to give 98.1% N,N-dimethylaniline. In order to obtain the 
end product, a three hour reaction time and a reaction pressure of 150 
kg/cm.sup.2 was required. In addition to the disadvantages previously 
mentioned, these processes have limited flexibility as far as the control 
of the N-alkyl to N,N-dialkylaniline ratio was concerned, and therefore 
could not meet market demand. 
The use of transition metal zeolites has also been described for the vapor 
phase catalytic N-methylation of aniline with methanol over a temperature 
range of 200.degree. to 300.degree. C. (Takamiya et al., "N-Methylation of 
Aniline with Methanol over Transition Metal Zeolite", Weseda University 
Report 21 (1975)). In this work, the catalysts were obtained by 
ion-exchanging HY zeolites with transition metal nitrate solution. The 
ion-exchanged Y zeolites, however, proved to be less active than the 
parent HY catalyst and gave poor control with regard to product selection. 
DESCRIPTION OF THE INVENTION 
In accordance with the present invention, a process for preparing N-alkyl 
and N,N-dialkylaniline by alkylation of aniline with a C.sub.1 to C.sub.3 
alcohol, preferably methanol or ethanol, is disclosed. The catalyst 
employed in this process is a crystalline aluminosilicate zeolite of high 
silica to alumina ratio, namely, from 20:1 to 700:1, preferably from 30:1 
to 700:1. Operative catalysts include ZSM-5 zeolites; particularly 
preferred are those modified with suitable ions of alkali metal, alkali 
earth metal and transition metal by impregnation or ion-exchange method. 
The modification regulates the acidity and pore size and provides 
flexibility in product selection. 
The process of the invention is highly selective to the N-alkylated 
products, suppressing the formation of such unwanted by-products as 
toluidines (e.g., o-, p- and m-alkyl anilines), and provides a means of 
controlling the ratio of the N-alkyl and N,N-dialkyl products. In 
addition, the instant process can be carried out continuously and in the 
vapor phase at a low temperature and gives better results than processes 
with other solid acid catalysts at lower temperatures and pressures. The 
advantages of the invention as compared with conventional processes are 
summarized in Table A. 
TABLE A 
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Catalyst 
Characteristics ZSM-5 H.sub.2 SO.sub.4, H.sub.3 PO.sub.4 
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Catalyst-products separation 
Available Unavailable 
Catalyst recovery 
Available Unavailable 
Corrosion problem 
Eliminated Severe 
Waste acid pollution 
Eliminated Severe 
Products selectivity 
Flexible Limited 
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The ratio of the N-alkyl to N,N-dialkylaniline can be varied widely, e.g., 
from 0.13:1 to 6:1. This ability is of significant commercial importance 
because it allows the output of the plant to be altered in response to the 
demand of the individual products. 
The zeolite ZSM-5 used in this invention is a crystalline aluminosilicate 
zeolite having a composition in terms of mole ratios of oxides as follows: 
EQU 0.9.+-.0.2 M.sub.2/n O:Al.sub.2 O.sub.3 :Y SiO.sub.2 :zH.sub.2 O 
wherein M is at least one cation having a valence n, Y is at least 5, z is 
between 0 and 40. This zeolite is further characterized by a specified 
X-ray diffraction pattern shown below in Table B: 
TABLE B 
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Interplanar Spacing d(A): 
Relative Intensity 
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11.1 .+-. 0.2 s. 
10.0 .+-. 0.2 s. 
7.4 .+-. 0.15 w. 
7.1 .+-. 0.15 w. 
6.3 .+-. 0.1 w. 
6.04 
.+-.0.1 w. 
5.97 
5.56 .+-. 0.1 w. 
5.01 - 0.1 w. 
4.60 .+-. 0.08 w. 
4.25 .+-. 0.08 w. 
3.85 .+-. 0.07 v.s. 
3.71 - 0.05 s. 
3.04 .+-. 0.03 w. 
2.99 .+-. 0.02 w. 
2.94 .+-. 0.02 w. 
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These values are determined by standard techniques. The radiation is the 
K-alpha doublet of copper, and a scintillation counter spectrometer with a 
strip chart pen recorder is used. The peak heights, I, and the positions 
as a function of 2 times theta, where theta is the Bragg angle, are read 
from the spectrometer chart. From these, the relative intensities, 100 
I/I.sub.o, where I.sub.o is the intensity of the strongest line or peak, 
and d (obs), the interplanar spacing in A.degree., corresponding to the 
recorded lines, are calculated. In Table B the relative intensities are 
given in terms of the symbols s.=strong, w.= weak and v.s.=very strong. It 
should be understood that this X-ray diffraction pattern is characteristic 
of all the species of ZSM-5 compositions. Ion-exchange of the sodium ion 
with cations reveals substantially the same pattern with some minor shifts 
in interplanar spacing and variation in relative intensity. Other minor 
variations can occur depending on the silicon to aluminum ratio of the 
particular sample, as well as if it had been subjected to thermal 
treatment. (See U.S. Pat. No. 4,082,085.) 
The zeolite ZSM-5, depending on the SiO.sub.2 to Al.sub.2 O.sub.3 ratio, 
has a surface area of from 250 to 450 m.sup.2 /g and a pore volume of from 
0.15 to 0.35 cm.sup.2 /g. Its Constraint Index is 8.3. (See U.S. Pat. No. 
4,350,835.) Zeolite ZSM-5 and its preparation are more particularly 
described in U.S. Pat. No. 3,702,886. 
In accordance with the present invention, the reactant mixture is pumped, 
vaporized, preheated, and introduced into a fixed-bed reactor and 
contacted with the specified catalyst at from 300.degree. to 500.degree. 
C., preferably from 300.degree. to 400.degree. C., and at a pressure of 
from 1 to 5 atm., preferably 1 atmosphere. The molar ratio of alcohol to 
aniline is from 1 to 6, preferably from 2 to 4, and the weight hourly 
space velocity (WHSV) is from 0.2 to 4 g feed/g catalyst/hour, preferably 
from 0.5 to 1.6 hr.sup.-1. 
The ZSM-5 zeolites of the invention are conventionally obtained in the 
sodium form. By ion-exchange processes, the sodium cations may be 
exchanged to form zeolites having a hydrogen ion as the cation. These 
catalysts are referred to herein as NaZSM-5 zeolites and HZSM-5 zeolites, 
respectively. For example, HZSM-5 may be ion-exchanged with 0.1 M 
magnesium or potassium or cesium nitrate solution at 80.degree. C. 
repeatedly until the maximum exchange capacity is reached. The product is 
then filtered, washed and dried. 
The two foregoing catalysts may be impregnated with alkali metal, alkali 
earth metal or transition metal ions. Generally, the impregnating solution 
contains water-soluble salts such as nitrates or acetates and the amount 
of impregnated cation, based on metal oxide, is generally in the range of 
from 0.2 to 50%, preferably from 4 to 24%. ZSM-5 zeolite (including both 
NaZSM-5 and HZSM-5) is first soaked in its impregnating solution 
overnight, then dried and calcined at 550.degree. C. for six hours. 
In the ion-exchange process, metal ions enter the zeolite pore channels and 
exchange with sodium or hydrogen ions (i.e., NaZSM-5 or HZSM-5). In the 
impregnation process, on the other hand, the metal ions remain on the 
zeolite outer surface and are converted to the metal oxide in the 
calcination step. Both of these modification methods change the intrinsic 
properties of ZSM-5. 
The NaZSM-5 zeolites are synthesized under conditions in which water is 
present in a considerable amount and frequently at elevated temperatures. 
This procedure is described in U.S. Pat. No. 3,702,886. 
The following examples are merely illustrative of preferred embodiments of 
the invention. Many variations thereon may be made without departing from 
the spirit of the disclosed invention, as will be evident to those skilled 
in the art, and such variations are intended to come within the scope of 
what is claimed.