Alkylation of phenols to alkyl aryl ethers using phosphate catalysts

Acid Phosphates are used, in either the gas or liquid phase, to catalyze the alkylation of phenols by an alcohol to selectively form alkyl aryl ethers.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to catalytic alkylation reactions. More 
particularly, it is concerned with the selective alkylation of phenols to 
yield alkyl aryl ethers. 
BACKGROUND OF THE INVENTION 
With the development of direct liquefaction of coal, and since phenolic 
compounds are a product of this direct liquefaction, the supply of phenols 
present in tar acids will increase to the point where the tar acids will 
present a disposal problem. One possible solution is using these phenols 
as fuel extenders. Because of the toxicity and corrosiveness of phenols, 
however, they must be converted to less hazardous compounds for most fuel 
applications. Major alternatives to the disposal of these phenolic tar 
acids are hydrotreating and conversion to methyl aryl ethers. The claimed 
invention provides a method for converting these phenols to alkyl aryl 
ethers which are good octane boosters and anti-knock agents. 
The model reaction for the formation of methyl aryl ethers is the formation 
of anisole from phenol and methanol. 
U.S. Pat. No. 2,487,832 teaches a process for preparing anisole by the 
alkylation of phenols using dimethylether and a solid dehydrating 
catalyst. An example of such a solid dehydrating catalyst is alumina. 
Although alumina is a good catalyst for phenol alkylation, it has some 
selectivity for c-alkylation, producing cresols which are as toxic and 
corrosive as the phenols and therefore is not a suitable catalyst for 
phenol alkylation. 
Another solid dehydrating catalyst, thoria, is also described in this 
patent. Thoria is highly selective for O-alkylation of phenol but its 
radioactivity makes it an unattractive alternative to other catalysts. 
U.S. Pat. No. 3,642,912 also discloses a process for the catalytic 
alkylation of phenols. The catalysts described in this patent are titanium 
dioxide and derivatives thereof. These catalysts also prove to be good 
alkylating agents of phenols but, as with alumina, some selectivity to 
c-alkylation takes place producing unwanted cresols. 
Anisole can also be selectively synthesized from sodium phenoxide and 
dimethylsulfate in the presence of NaOH. This process is known as the 
Williamson synthesis; see The Merck Index. Eighth Edition, 1968, page 
1226. Because stoichiometric quantities of sodium hydroxide are required 
to produce the sodium phenoxide. and because of the large quantities of 
salt by-product, this type of process has little commercial appeal. 
U.S. Pat. No. 4,405,784 discloses the use of strontium phosphate catalysts 
for organic condensation reactions. The reactions disclosed using 
strontium phosphate catalysts were concerned with condensation reactions 
for the synthesis of amines. 
SUMMARY OF THE INVENTION 
It has now been found that alkylation of phenols with alcohols when carried 
out using an acid phosphate catalyst in either the gas or liquid phase can 
result in high alkyl aryl ether selectivity with little cresol formation. 
Specific acid phosphate catalysts which can be used for this selective 
alkylation include the pyrophosphate, monohydrogen phosphate, and 
dihydrogen phosphate of strontium, copper, magnesium, calcium, barium, 
zinc, lanthanum, aluminum, cobalt, nickel, cerium and neodymium, and 
mixtures thereof. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention consists of a process for etherification of phenols 
and alcohols to alkyl aryl ethers using acid phosphate catalysts. The acid 
phosphate catalysts which can be used for this reaction are those 
catalysts disclosed in parent application Ser. No. 381,232 and include the 
pyrophosphate, monohydrogen phosphate and dihydrogen phosphate of copper, 
magnesium, calcium, barium, zinc, lanthanum, aluminum, cobalt, nickel, 
cerium and neodymium, mixture thereof, and mixtures thereof with one or 
more strontium phosphates. Although not disclosed in the parent case, the 
strontium phosphate catalysts, alone or mixtures thereof, are also 
suitable for this reaction. 
The catalysts disclosed in the presently claimed process, have up to 99% 
selectivity for O-alkylation resulting in the production of alkyl aryl 
ethers, with very little cresol formation. The claimed process can be 
carried out using these catalysts in either the gas phase or liquid phase. 
The preferred temperature range of operation of this process is from 
250.degree. C. to 350.degree. C. Above 350.degree. C. selectivity to 
O-alkylation decreases. In a liquid phase operation the preferred reaction 
time is in the range of about 3-5 hours. Longer reaction times give higher 
conversion, but lower alkyl aryl ether selectivity. It is also preferred 
to run this reaction with alcohol in excess. 
The phenol feed used in the claimed reaction can include phenol, cresols, 
xylenols, naphthols, the substituted phenols found in coal liquids, and 
crude coal liquids. While any alcohol can be used which is capable of 
alkylating the particular phenol feed used, only C-1 to C-12 alcohols have 
practical application with C-1 to C-4 alcohols being preferred. 
The reaction product is an alkyl aryl ether generally having the structural 
formula R--O--R' wherein R is the monovalent radical of phenol, cresol, 
xylenol, naphthol or the substituted phenols found in coal liquids and R' 
is any C-1 to C-12 aliphatic hydrocarbyl group. A hydrocarbyl group is 
defined as the monovalent radical of any hydrocarbon. 
When carried out in the gas phase, this process can be operated at a 
pressure range of about 1 to 83 atmospheres with 1 to 8 atmospheres being 
preferred, and at a gas hourly space velocity (GHSV) of the phenolic feed 
stock per volume of catalyst in the range of about 500 to 700. An inert 
carrier gas such as nitrogen, argon or helium may also be used but is not 
required. 
When carried out in a stirred reactor in the liquid phase, the reaction can 
be carried out at a considerably higher pressure range. This pressure 
range is anywhere between about 1 to 137 atmospheres, with 35 to 83 
atmospheres being preferred. 
Producing methyl aryl ethers using the type of catalysts of the present 
invention under the conditions applied is superior to the processes of the 
prior art which use alkali metal phenoxides because there is not the 
problem of disposal of inorganic salts nor the cost of alkali metal 
hydroxides. This method is also superior to previous applications of 
heterogeneous catalysts other than thoria because selectivity to anisole 
is higher. It is also superior to the use of thoria because it does not 
involve radioactive materials. The use of the catalysts of this invention 
provides reduced toxicity in the process. 
When phenol and methanol are used in the claimed process, the catalyst 
system also has the advantage of minimizing the extent of isomerization of 
anisole, once formed, to cresols.

CATALYST PREATION 
The La.sub.2 (HPO.sub.4).sub.3 catalyst is prepared by mixing an aqueous 
solution of La(NO.sub.3).sub.3 with a solution of (NH.sub.4).sub.2 
HPO.sub.4. The mixture is stirred and La.sub.2 (HPO.sub.4).sub.3 
precipitates out. 
EXAMPLE 1 
The La.sub.2 (HPO.sub.4).sub.3 catalyst was prepared by mixing together a 
2.0M aqueous solution of La(NO.sub.3).sub.3.5H.sub.2 O (415 g, dilute with 
H.sub.2 O to 500 cc) and a 3.00M solution of (NH.sub.4).sub.2 HPO.sub.4 
(198 g, dilute with H.sub.2 O to 500 cc). The mixture was stirred a few 
minutes and the La.sub.2 (HPO.sub.4).sub.3 precipitate was filtered, 
washed and dried. In this catalyst preparation it is inherent that some 
La.sub.2 (PO.sub.4).sub.3 will also be found. 
USE OF CATALYSTS 
The following are examples of etherification reactions in which the above 
catalyst was employed. These examples are meant to be illustrative and not 
limiting. 
EXAMPLE 2 
8.65 g (5 cc) of La.sub.2 (HPO.sub.4).sub.3 was loosely packed as a fixed 
bed in a vertical down-flow tube reactor (inside diameter=1.1 cm). The 
system was purged with helium and heated to 300.degree. C. A feed of 
phenol, methanol, and helium in molar ratios of 14.1:70.6:15.3 at 1 atm. 
was passed over the catalyst with GHSV=538. The resulting product was 
trapped at approximately -25.degree. C. and the liquid analyzed for phenol 
derivatives. The components of the stripped exit gas were also analyzed. 
The results of the analysis of both the liquid and the exit gas are 
reported in Table 1. 
TABLE 1 
______________________________________ 
Exit Gas Liquid 
Rate: 521 SCCH Recovered: 4.45 g/hr 
Component Mole % Component Wt. % 
______________________________________ 
MeOH trace Anisole 5.33 
H.sub.2 O trace o-Cresol 0.257 
CH.sub.3 OCH.sub.3 
5.66 m,p-Cresol 0.053 
H.sub.2 -- Methylanisoles 
-- 
CO.sub.2 0.13 Other aromatics 
0.019 
CH.sub.4 trace 
C.sub.2 H.sub.4 
trace 
C.sub.2 H.sub.6 
-- 
______________________________________ 
As shown in Table 1, the liquid product contained 5.33% anisole, 0.257% 
o-cresol and 0.053% meta and para cresol with a selectivity of 94.2% to 
anisole. 
EXAMPLE 3 
La.sub.2 (HPO.sub.4).sub.3 was tested in the same system used in Example 4 
at 401.degree. C. A feed of phenol, methanol, and helium in molar ratios 
of 14.1:70.5:15.3 at 1 atm. was passed over the catalyst with a GHSV=652. 
The analytical results of this test are shown in Table 2. 
TABLE 2 
______________________________________ 
Exit Gas Liquid 
Rate: 652 SCCH Recovered: 4.75 g/hr 
Component Mole % Component Wt. % 
______________________________________ 
MeOH -- Anisole 20.5 
H.sub.2 O trace o-Cresol 5.10 
CH.sub.3 OCH.sub.3 
9.4 m,p-Cresol 1.88 
H.sub.2 -- Methylanisoles 
4.36 
CO -- Dimethylphenols 
2.15 
CO.sub.2 0.092 Other aromatics 
1.08 
CH.sub.4 25.7 
C.sub.2 H.sub.4 
0.55 
C.sub.2 H.sub.6 
0.53 
______________________________________ 
The product contained 20.5% anisole, 5.10% o-cresol and 9.47% other 
alkylation products. The selectivity was 57.2% to anisole. 
EXAMPLE 4 
La.sub.2 (HPO.sub.4).sub.3 was tested in the same system used in Example 4 
at 349.degree. C. A feed of phenol, methanol, and helium in molar ratios 
of 14.1:70.6:15.2 was passed over the catalyst with a GHSV=657. The 
results of this analysis are contained in Table 3. 
TABLE 3 
______________________________________ 
Exit Gas Liquid 
Rate: 657 SCCH Recovered: 4.91 g/hr 
Component Mole % Component Wt. % 
______________________________________ 
MeOH -- Anisole 15.7 
H.sub.2 O trace o-Cresol 1.94 
CH.sub.3 OCH.sub.3 
7.89 m,p-Cresol 0.58 
H.sub.2 -- Methylanisoles 
0.84 
CO -- Dimethylphenols 
0.28 
CO.sub.2 trace Other aromatics 
0.09 
CH.sub.4 4.0 
C.sub.2 H.sub.4 
trace 
C.sub.2 H.sub.6 
trace 
______________________________________ 
The selectivity was 81% to anisole. 
EXAMPLE 5 
This example shows that the reaction can be conducted in a stirred reactor 
in the liquid phase. 5.0 g of La.sub.2 (HPO.sub.4).sub.3 was loaded into a 
stirred 300 ml reactor with 50 g of a 37 wt.% phenol in methanol solution. 
The reactor was purged with helium at 1 atmosphere and sealed. The 
temperature was increased to 300.degree. C., which pressurizes the system 
to 83 atm., and the reactor operated at 1,000 RPM. After 4 hours the 
reactor was cooled and a sample taken. The results of this reaction are 
reported in Table 4. 
TABLE 4 
______________________________________ 
Liquid Product 
wt % @ 4 hr. 
wt % @ 19 hr. 
______________________________________ 
Anisole 20.6 31.2 
o-cresol 0.50 2.07 
m,p-cresol 0.08 0.35 
methylanisoles 
0.14 1.22 
dimethylphenols 
0.005 0.38 
other aromatics 
0.03 1.03 
Conversion 50.2% 85.3% 
Selectivity 96.6% 86.% 
______________________________________ 
This table shows that, while reactant conversion increases with time, 
selectivity for the desired product decreases. 
EXAMPLE 6 
Liquid phase alkylation was also conducted as in Example 4 using 75% phenol 
in methanol, with the system pressurized to 42 atm. and run for 4 hours. 
The results of this run are shown in Table 5. 
TABLE 5 
______________________________________ 
Liquid Product wt % @ 4 hr. 
______________________________________ 
Anisole 38.0 
o-cresol 0.27 
m,p-cresol 0.02 
methylanisoles 0.0258 
dimethylphenol 0.03 
other aromatics 
0.09 
Conversion 45.0% 
Selectivity 99.0% 
______________________________________ 
The selectivity was 91.1% to anisole with a yield of 0.20 g anisole/g 
catalyst hour. 
Control 1 
For a comparison, 9.23 g (10 cc) of Alcoa alumina F/20 was tested in the 
system described in Example 4 at 303.degree. C. Alumina is an acidic 
catalyst which will promote alkylation of aromatics. A feed of phenol, 
methanol and helium was passed over the catalyst in molar ratios of 
13.8:68.9:17.3 at GHSV=521. The results of this test are shown in Table 6. 
TABLE 6 
______________________________________ 
Exit Gas Liquid 
Rate: 1038 SCCH Recovered: 7.73 g/hr 
Component Mole % Component Wt. % 
______________________________________ 
MeOH -- Anisole 8.2 
H.sub.2 O trace o-Cresol 1.8 
CH.sub.3 OCH.sub.3 
2.31 m,p-Cresol 0.092 
H.sub.2 -- Methylanisoles 
0.060 
CO -- Dimethylphenols 
-- 
CO.sub.2 trace Other aromatics 
0.60 
CH.sub.4 -- 
C.sub.2 H.sub.4 
-- 
C.sub.2 H.sub.6 
-- 
______________________________________ 
The selectivity to anisole was 76.3% with a yield of 0.069 anisole/g 
catalyst hour. Although this catalyst resulted in good selectivity to 
anisole, it also resulted in a 16.7% selectivity for cresol. It is this 
relatively large cresol formation which demonstrates the need for the type 
of catalysts defined in this invention. 
Control 2 
Also for comparison 5.60 g (10 cc) of synthetic zeolite catalyst HZSM5 was 
tested in the system described in Example 4 at 299.degree. C. A feed of 
phenol, methanol and helium was passed over the catalyst in molar ratios 
of 13.9:69.5:16.7 at GHSV=540. The results of this test are shown in Table 
9. 
TABLE 7 
______________________________________ 
Exit Gas Liquid 
Rate: 1000 SCCH Recovered: 7.71 g/hr 
Component Mole % Component Wt. % 
______________________________________ 
MeOH 0.82 Anisole 6.0 
H.sub.2 O trace o-Cresol 2.4 
CH.sub.3 OCH.sub.3 
11.8 m,p-Cresol 1.4 
H.sub.2 0.24 Methylanisoles 
0.20 
CO 0.15 Dimethylphenols 
0.24 
CO.sub.2 -- Other aromatics 
0.16 
CH.sub.4 0.34 
C.sub.2 H.sub.4 
0.86 
C.sub.2 H.sub.6 
0.057 
______________________________________ 
The selectivity to anisole was 57.7% with a yield of 0.083 g anisole/g 
catalyst hour. This poor selectivity also demonstrates the need for the 
catalysts defined in this invention.