Chromium phosphate as an alkylation catalyst

An amorphous chromium (III) phosphate catalyst is used in alkylating aromatics and in the dehydration of alcohols to ethers.

This invention relates to metal phosphates. In particular it relates to the 
use of chromium (III) phosphates as catalysts in the alkylation of 
aromatic hydrocarbons. It also relates to the use of chromium (III) 
phosphates as catalysts in the dehydration of alcohols to ethers. In 
particular, it relates to the use of chromium (III) phosphate as a 
catalyst used in the methylation of toluene and in the dehydration of 
methanol to dimethyl ether. 
BACKGROUND OF THE INVENTION 
In order to alkylate aromatic hydrocarbons, it is necessary that an 
alkylating agent be present. Alcohols can serve as alkylating agents. 
Therefore, in a feed stream of alcohols and aromatic hydrocarbons, the 
dehydration reaction of the alcohol occurs at the same time the alkylation 
reaction occurs. Since xylene, an alkylated aromatic, is a preferred 
product, the alkylation reaction is the preferred reaction and catalysts 
are chosen because of their ability to hinder the dehydration reaction and 
advance the alkylation reaction. The dehydration reation will occur, 
however, using the same catalyst. 
The alkylation of aromatic hydrocarbons utilizing crystalline catalysts is 
well known in the art. The alkylation process is used to produce xylenes. 
Xylenes come in three isomeric forms: ortho-, meta- and para-xylene, the 
latter of which is of particular value in the manufacture of terephthalic 
acid which is an intermediate in the manufacture of synthetic fibers such 
as poly(ethylene terephthalate). 
Prior art has shown the use of crystalline aluminosilicate catalysts in the 
production of xylenes through aromatic alkylation. One problem with some 
of these catalysts is that they tend to lose their activity quickly. These 
catalysts must also be selective. The reaction must favor particularly 
desired compounds, for instance para-xylenes over ortho-or meta-xylenes. 
It must also give reasonably high conversion rates. 
Therefore, an object of this invention is to provide a catalyst that would 
not lose activity quickly. 
Another object of this invention is to provide a selective catalyst. A 
preferred object of this invention is to provide a catalyst selective to 
para-xylene. 
Another object of this invention is to provide a catalyst that provides 
high conversion rates. 
Another object of this invention is to provide a catalyst that increases 
the production of alkyl-substituted aromatic hydrocarbons. 
A specific object is to provide a catalyst for the production of xylene. 
Another object of this invention is to provide a catalyst for the 
dehydration reaction of alcohols. 
Other objects will become apparent from the following descriptions. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, metal phosphates have been 
discovered to be active catalysts for a variety of alkylation and 
dehydration processes. In particular, chromium (III) phosphates and 
combined chromium (III) aluminum phosphates are active for the alkylation 
of aromatic hydrocarbons and the dehydration of alcohols to ethers. In a 
specific embodiment, it has been discovered that chromium (III) and 
combined chromium (III) aluminum phosphates are active for the methylation 
of toluene with methanol and for the dehydration of methanol to dimethyl 
ether. 
DETAILED DESCRIPTION OF THE INVENTION 
Chromium (III) phosphates are prepared using conventional methods. One 
method for producing the chromium (III) phosphates of this invention 
includes precipitating from solutions of dissolved chromium (III) salts 
and phosphates or hydrogen phosphates, the solutions preferably being 
neutral or slightly basic. 
The metal phosphate catalyst can possess any ratio of chromium to 
phosphate. The atomic ratio of chromium to phosphorus will preferably be 
greater than unity. More preferably, the atomic ratio of chromium to 
phosphorus will be about 1.2:1 to about 1.7:1. 
The metal phosphate catalyst can be present with chromium (III) phosphate 
alone or in any combination of chromium (III) phosphate and aluminum 
phosphate. These metal phosphates can be present in any relative amounts, 
from entirely chromium phosphate to almost entirely aluminum phosphate. 
The preferred combined chromium (III) aluminum phosphate can be prepared in 
any conventional manner. Preferably, the combined phosphate is prepared by 
coprecipitation of CR.sup.+3 ions and Al.sup.+3 ions with PO.sub.4.sup.-3 
or HPO.sub.4.sup.-2 ions in aqueous solutions. 
The metal phosphate catalyst can also be attached to a hydroxide group. The 
following formula represents metal phosphate catalyst of this group: 
EQU Cr(PO.sub.4).sub.n.(OH).sub.m 
wherein n can range from about 0.5 to about 0.9 and m can range from about 
1.5 to about 0.3. 
The alkylation of aromatic hydrocarbons such as benzene or toluene to m-, 
o-, p-xylene can be carried out with linear or branched alcohols having 1 
to 10 and preferably 1 to 4 carbon atoms per molecule. The presently 
preferred aromatic hydrocarbon is toluene and the presently preferred 
alcohol is methanol. 
The alkylation of an aromatic hydrocarbon, in the presence of the metal 
phosphate catalyst, is effected by contact of the aromatic hydrocarbon 
with an alcohol, at a temperature between about 150.degree. C. and 
550.degree. C. and preferably between about 350.degree. C. and 500.degree. 
C. The reaction generally takes place at a pressure from about 50 psig to 
about 800 psig, but preferably the pressure will be within the approximate 
range of 150 psig to about 600 psig. 
The molar ratio of the alcohol to the aromatic hydrocarbon is generally 
between about 0.3:1 and about 3:1. When methanol is employed as an 
alkylating agent and toluene is used as the aromatic hydrocarbon, a 
suitable molar ratio of methanol to toluene has been found to be 
approximately 0.4:1 to 1.5:1 moles of methanol per mole of toluene. 
The metal phosphate catalyst can be present in any suitable amount 
necessary for the alkylation of aromatic hydrocarbons and for the 
dehydration of alcohol. The reaction is suitably accomplished utilizing a 
liquid hourly space velocity (volume of feed/hour/volume of catalyst) of 
between about 1 and about 10 preferably between about 2 and about 7. 
The condensed product of the reaction of toluene and methanol comprising 
dimethyl ether, p-xylene, o-xylene, and m-xylene can be separated by any 
suitable means such as recrystallization at low temperature or selective 
adsorption (e.g., in a chromatographic column). Generally, more o-xylene 
is formed than either p-xylene or m-xylene. 
The process of this invention may be carried out as a batch-type, 
semicontinuous, or continuous operation utilizing a fixed or moving bed 
catalyst system. The catalyst after use is conducted to a regeneration 
zone after which the regenerated catalyst is recycled to a conversion zone 
for further contacting with the toluene and methylating agent reactants. 
The metal phosphate catalyst can be partially regenerated by air oxidation 
of about 900.degree. F. for about 16 hours.

The following examples will serve to illustrate the process of this 
invention without limiting the same. 
EXAMPLE I 
Catalyst Preparation 
Control catalyst A, Al(PO.sub.4), was prepared by adding a solution of 115 
g (1 mol) of (NH.sub.4)H.sub.2 PO.sub.4 in 1 L of water to 237 g (1 mol) 
of NH.sub.4 Al(SO.sub.4).sub.2 in 1 L of water, giving a clear solution. 
Precipitation was attempted by urea hydrolysis. Thus, one mole of urea (60 
g) was added and the solution warmed to 60.degree. C. for 48 hours. 
Precipitation had still not occurred, so solid (NH.sub.4).sub.2 CO.sub.3 
was added over a 2 hour period to neutralize the solution. The resulting 
precipitate was collected by filtration, pulverized and washed with 1 L of 
dilute NH.sub.4 HCO.sub.3 (10 g/L), then oven-dried and finally calcined 
at 315.degree. C. (600.degree. F.) for 5 hours. The resulting catalyst had 
a surface area of 72.3 m.sup.2 /g and a pore volume of 2.16 ml/g. The 
catalyst was mostly amorphous, but x-ray diffraction analysis did indicate 
a small degree of crystallinity. 
Invention catalyst B, 1 mole percent Cr on Al(PO.sub.4), was prepared by 
mixing 1230.5 g (3.3 mol) Al(NO.sub.3).sub.3.9H.sub.2 O, 227.6 g (2.0 mol) 
(NH.sub.4)H.sub.2 PO.sub.4, and 15.6 g (0.04 mol) of 
Cr(NO.sub.3).sub.3.9H.sub.2 O with 75 mL of water, then heating the 
mixture until homogenous. An aliquot of 200 mL of the syrup thus obtained 
was blended with 90 mL of concentrated NH.sub.4 OH and stirred vigorously. 
The resulting mixture was heated to about 80.degree. C. and stirred while 
27.6 g (0.46 mol) of urea were added. After about 45 minutes, gelation 
occurred. The resulting gel was allowed to age in a forced air oven at 
80.degree. C. until liquid phase was no longer observable. The residue was 
then washed 3 times with equal volumes of dilute ammoniacal H.sub.2 O, 
then once with water and finally 2 times with equal volumes of acetone. 
The washed material was dried overnight in the vacuum oven to give about 
44 g of clear turquoise gelatinous beads. These were activated for 3 hours 
at 700.degree. C. in an air atmosphere. 
Invention catalyst C, Cr(PO.sub.4).sub.0.7.(OH).sub.0.9, was prepared by 
mixing a solution containing 123.5 g (0.5 mol) of Cr(OAc).sub.3 :H.sub.2 O 
in about 1 L of water with a solution of 39.6 g (0.3 mol) of 
(NH.sub.4).sub.2 HPO.sub.4 in about 300 mL of water in a 2 L round bottom 
flask equipped with a Dean-Stark trap. The solution was heated to boiling 
and about 650 mL of water collected, during which time a blue-green 
precipitate formed. The pot was cooled, about 450 mL of ethyl acetate was 
added, and the pot contents again heated to reflux. Azeotroped water was 
collected at the rate of about 30 mL per hour, over about 6 hours. The pot 
was again cooled, 300 mL of additional ethyl acetate were added, and the 
pot again warmed to reflux to remove remaining water. The majority of the 
added ethyl acetate was then distilled off, then vacuum was applied to aid 
removal of residual amounts of solvent. The residual blue-green solid was 
heated to about 480.degree. C. in stages under an air flow and held at 
480.degree. C. for about 1.5 hours. Catalyst turned gray-black when the 
calcination temperature reached about 250.degree. C. The resulting 
material was determined to have a surface area of 132 m.sup.2 /g and a 
pore volume of 1.08 ml/g. Elemental analysis revealed a chromium content 
of 42.1 weight percent and phosphorus content of 17.6 weight percent. This 
corresponds to an atomic ratio of Cr:P of about 1.4:1. 
Invention catalyst D, Cr(PO.sub.4).sub.0.6.(OH).sub.1.2, was prepared 
according to the procedure described above, except 46.2 g (0.35 mol) of 
(NH.sub.4).sub.2 HPO.sub.4 was employed. The resulting catalyst is gray, 
has a surface area of 142 m.sup.2 /g, a pore volume of 1.15 ml/g and 
elemental analyses of 37.8 weight percent chromium and 13.8 weight percent 
phosphorus. This corresponds to an atomic ratio of Cr:P of about 1.6 to 1. 
Invention Catalyst E, Cr(PO.sub.4).sub.0.6.(OH).sub.1.2, was prepared by 
blending a solution of 247 g (1 mol) of Cr(OAc).sub.3.H.sub.2 O in 300 mL 
water with a solution of 105.6 g (0.8 mol) of (NH.sub.4).sub.2 HPO.sub.4 
in 400 mL of water. The solution was boiled for about 30 minutes, when 
gelation occurred. The gel was placed in an evaporating dish and allowed 
to air dry in the hood over about 24 hours. The catalyst was further dried 
for about 8 hours in a forced draft oven at about 105.degree. C. 
(220.degree. F.). Finally, catalyst was calcined at about 480.degree. C. 
for 2 hours. Catalyst was analyzed and found to have a surface area of 155 
m.sup.2 /g, a pore volume of 0.25 ml/g. Elemental analysis revealed a 
chromium content of 39 weight percent and a phosphorus content of 13.9 
weight percent. This corresponds to an atomic ratio of Cr:P of about 
1.6:1. 
EXAMPLE II 
The various catalysts prepared as described above were tested for 
dehydration activity and activity for alkylation of toluene. In all cases, 
20 mL of catalyst was loaded into the center of a 1/2" diameter metal 
tubular reactor with a pre-heat and post-reaction zone each packed with 
about 6" of glass beads. The reactor was brought up to reaction 
temperature under hydrogen pressure, then hydrogen feed discontinued and 
methanol or methanol/toluene feed started. Both liquid and gas samples 
were collected and analyzed by gas liquid chromatography. Reaction 
conditions and results are summarized in the Table. 
TABLE 
__________________________________________________________________________ 
Feed 
Catalyst, 
mL/hr 
mL/hr 
Reaction Conditions 
Conversion, % 
% Selectivity to 
Run # 
g MeOH 
Toluene 
Temp, .degree.C. 
Press, psig 
MeOH 
Toluene 
Me.sub.2 O 
Xylenes 
__________________________________________________________________________ 
1 A, 15.9 
20 115 455 300 &gt;90 &lt;0.5 &gt;90 .about.85 
2 B, 10.2 
20 115 455 300 100 16 .about.90 
.sup. 81.sup.1 
3 C, 8.5 
140 -- 190 300 1.4 -- ND -- 
4 120 -- 370 300 40 -- .about.90 
-- 
5 40 -- 370 300 52 -- &gt;90 -- 
6 40 -- 370 300 58 -- &gt;95 -- 
7 D, 21 
30 60 315 150 low low ND ND 
8 20 50 425 150 &gt;90 26 &gt;90 83 
8A 20 110 540 50 ND &lt;0.5 ND ND 
9 E, 22 
20 90 425 300 ND 21 ND .sup. 78.sup.2 
10 30 95 425 600 ND 6.3 ND .sup. 87.sup.2 
11 30 100 410 300 ND 2.2 ND .sup. &gt;90.sup.2 
12 30 100 410 300 ND 1.2 ND .sup. &gt;90.sup.2 
__________________________________________________________________________ 
.sup.1 The composition of the xylenes was: about 29 weight%-p-xylene, 
about 20 weight% mxylene and about 51 weight% oxylene. 
.sup.2 The composition of the xylenes was: about 33 weight% pxylenes, 22 
weight% mxylenes and 44 weight% oxylenes. 
The results of these experiments demonstrate that phosphate deficient 
chromium phosphate catalysts are active dehydration catalysts for alcohols 
and are active for the alkylation of aromatics such as toluene in the 
presence of alkylating agents such as methanol.