Note: Descriptions are shown in the official language in which they were submitted.
~2~L615353
TITLE
IMPROVED CATALYST
BACKGROI~ND OF THE INVENTION
Field of the Invention
The present invention relates to the
production of isocyanates, pre~erably methyl
isocyanate. At present methyl isocyanate is produced
in the U.S. by phosgenation of monomethylamine
followed by decomposition of the resulting carbamyl
chloride to methyl isocyanate and hydrogen chloride.
Prior Art
U.S. 4,207,251 discloses the oxidation of
N-alkylformamides to the corresponding isocyanates
over precious metal catalysts including silver.
Summary of the Invention
The present invention relates to an improved
process of converting formamides of the formula
R(NHCHO)n where R is an organic group and n is 1 or
2 to the corresponding isocyanate over a silver or
silver/gold catalyst wherein the silver or
silver/gold is sputtered or ion plated on an inert
support. Such catalysts give a higher productivity
of isocyanate. When measured as g of product/g of
silver/hr, productivity increases by more than a
factor of 50 as compared with conventional silver
crystal or silver wool catalysts.
Detailed Description
The catalyst used in the present invention
is sputtered or ion plated silver or silver/gold on
an inert support. Generally the metal or metal alloy
will comprise from .05-50 weiyht percent of the total
catalyst composition. Generally the catalyst support
will be a hard, nonporous refractory particulate
material having a mean particle diameter in the range
CH 1218 35 of from 0.1 micron to 0.5 centimeter. Generally the
~%:~L6~3
support should have a surface area below about 20
sguare meters per gram and preferably less than 3
square meters per gram. Generally the support will
be a ceramic material. Alumina and silica are tne
preferred catalyst supports although other oxides
such as ceria, yttria, zirconia or titania can be
used.
The catalysts of the invention were ~repared
by physical vapor deposition where evaporated metal
was allowed to deposit on a silica support which was
biased electrically or by RF sputtering. In the
latter method, fused silica particles which pass a 30
mesh (U.S. Sieve Series) screen and are retained on a
50 mesh ~U.S. Sieve Series) screen were distributed
in several Pyrex* dishes on a rotating substrate table
beneath a silver target or gold/silver target and
coated with the metal or metals. The particulate
substrates were periodically mixed outside the
sputtering chamber to ensure uniformity.
Generally tne process of the present
invention is carried out at from 300-600~C with
from 400-500C being the preferred range. ~nder
comparable conditions when using silver wool where it
is possible to have hot spots, the catalysts of the
invention offer a more uniform distribution of small
amounts o~ the metal or metal alloy on the support
which results in a higher selectivity to methyl
isocyanate.
The reaction is carried out in the gas phase
in the presence of an inert gas P.g. nitrogen,
carbon dioxide, helium, neon, argon or xenon.
Nitrogen is the preferred carrier gas because it is
inexpensive, but helium is used in some of the
Examples to facilitate product analysis by gas
*denotes trade mark.
68S;~
chromatography. Generally the proportion of
methylformamide in the reaction mixture at the start
should be from 0.1-40 volume percent. There should
be oxygen present in the reaction inixture ~or
carrying out the reaction. The amount of oxygen
present in the feed to the reactor is generally from
0.1-20 volume percent.
The pressure used is not particularly
critical and may be varied from <1 X 105 Pa to 1 X
106 Pa or higher. For operational reasons it is
most preferred that the reaction be carried out at an
absolute pressure of 1 atmosphere.
The present invention is applicable to
N-monosubstituted for~amides of the formula
H O
il
R(N- CH)n
where R is an unsubstituted hydrocarbon group or
substituted hydrocarbon group, generally containing
not more than 18 carbon atoms, including substitute~
or unsubstituted alkyl groups, cycloalkyl groups,
aryl groups, preferably phenyl, aralkyl groups or
alkaryl groups, where the substituents may be, for
example, chlorine, fluorine, cyanogen and alkyl
carbonyl or alkoxyl carbonyl, preferably containing
not more than 10 carbon atoms in the alkyl or alkoxy
groups, and where n is 1 or 2.
Methyl isocyanate is used in the production
of certain insecticides and nematicides including
s-methyl-N-[(methylcarbamoyl)oxy~ thioacetimidate
(methomyl), an insecticide.
EXAMPLES
In each of the Examples the catalyst was
charged to a "U" shaped ~uartz tube reactor which had
a 12 mm outside diameter. Tne reactor was brouyht to
temperature in a sand bath under a flow of inert
;853
carrier gas. ~hell the desired temperature is
reached, oxygen flow is initiated and after a~out 2
minutes the monomethylformamide is introduced witn
sufficient carrier gas to acnieve the ~esired
composition. Total flow is such that the nominal
residence time in the catalyst ~ed is about 0.2
second. After allowing one hour for e~uilibrium to
be reached, samples are periodically withdrawn for
analysis. In Examples 1-4 the gas flows reported are
as measured at 0C and atmospheric pressure. Inert
carrier gas and residual oxygen are excluded from the
analysis and results are reported as molar
percentages.
Example 1
Particulate silica of irregular shape whicn
passes a 30 mesh (U.S. Sieve Series) screen and is
retained on a 50 mesh (U.S. Sieve Series) screen lS
coated with O.g weight percent silver by ion plating
to prepare the catalyst. The reactor lS chargea ~ith
3.2 9 of the catalyst and the bed heate~ to 470C.
Flows to the reactor are adjusted to 500 cc/minute o~
nitrogen, 20 cc/minute of oxygen and 5.76 ml/hour of
liquid monomethylformamide. After reaching
equilibrium, analysis of the effluent stream
indicates a 43% conversion of monometAylformami~e anu
a 78~ selectivity for methyl isocyanate. This is
equivalent to a production rate of 58.3 g methyl
isocyanate per gram of silver per hour.
Example 2
Particulate silica of irregular sllape whicn
passes a 30 mesh (U.S. Sieve Series) screen and is
retained on a 50 mesh (U.S. Sieve Series) screen lS
sputter coated with ~ weight percent silver to
prepare the catalyst. Tne reactor is charyed witn
3.2 g of the catalyst and the bed heated to 470~.
~2~ 53
Flows to the reactor are adjusted to 500 cc/minute of
helium, 25 cc/minute of oxygen and 5.76 ml/hour o~
monomethylformamide. After reaching equilibrium,
analysis of the effluent stream indicates dn ~5
conversion of monomethylformamide and a 75~
selectivity for methyl isocynate. After 14 hours,
both numbers begin to drop but then rise. After 30
hours of exposure, a conversion of 90% of
monomethylformamide was achieved wit~ 81~ selectivity
for methyl isocyanate. No further chanye in catalyst
performance is noted during additional exposure.
This is equivalent to a production rate of 25.3 ~ o~
methyl isocyanate per g of silver per hour.
ExamPle 3
Tnis Example is a comparison usin~ sllver
crystals as the catalyst.
The reactor is charged with 8.4 g of silver
crystals and heated to 470C Flows to the reactor
are adjusted to 500 cc/minute of nitrogen, 20
cc/rninute of oxygen and 5.76 ml/hour of liquid
monomethylformamide. After reaching equilibrium,
analysis indicates an 80~ conversion of
monomethylformamide and a 47% selectlvity for metnyl
isocyanate. This is equivalent to a production rate
of 0.2 g of methyl isocyanate per g of silver per
hour.
Example ~
This Example is a comparison ~sing silver
wool as the catalyst.
The reactor is charged with 3.5 9 of 0.03 mm
O.D. silver wool and heated to 470C. Flows to the
reactor were adjusted to 500 cc/minute of heiium, 25
cc/minute of oxygen and 5.76 ml/hour of liquid
monomethylforrnamide. After reaching equilibrium,
analysis indicated an 80% conversion of monomethyl-
~%~ 3
formamide and a 77~ selectivity for methyl isocyanate.
This is eq~ivalent to a production rate of 1.0 g of
methyl isocyanate per g of silver per hour.
Example 5
Example 1 was repeated with 37 cc/minute of
monomethylformamide vapor, 25 cc/minute of oxygen
from air and 475 cc/minute of nitrogen, fed as a
mixture of air and nitrogen. Under these flow
conditions two different sputter coated catalysts
were evaluated at 470C and 500C. After
reaching equilibrium the product from the reactor was
bubbled into an aqueous solution of monomethylamine
to trap the isocyanate as 1,3-dimethylurea which was
isolated after removal of excess of the trapping
1~ medium. The trapping period in all cases was 120
minutes and the conversion of the formamide ~90~.
Results are summarized in Table I. The Ag/Au on
SiO2 catalyst used in Table I was cosputtered with
silica ont~ a silica support.
Table I
Catalyst Wt. ~. Temp. C ~t o~ Urea, q.
4% Ag on SiO2 2.6 470 13.5
4~ Ag on SiO2 2.6 500 13.9
1% Ag/Au on
SiO2 (87/13
atomic ratio) 2.5 470 14.~
" 2.5 500 12.5
The theoretical weight of urea for 100% conversion
and 100% yield is 17.3 9.
