Molecular sieve abstract
The present invention relates to the selectivity of production
of methylamines from methanol and/or dimethylether and ammonia using
at catalytic amount of acidic molecular sieve.
Molecular sieve claims
We claim:
1. A method for the production of monomethylamine, dimethylamine,
and trimethylamine comprising contacting methanol, and/or dimethylether,
and ammonia in amounts sufficient to provide a carbon:nitrogen (C:N)
ratio from about 0.2 to about 1.5 and at a reaction temperature
from about 250.degree. C. to about 450.degree. C., in the presence
of a catalytic amount of an acidic molecular sieve which has an
AFX crystalline structure, wherein the ratio of silicon to aluminum
(Si:Al) in said molecular sieve is greater than about 2.5:1 wherein
methanol and/or dimethylether is converted to monomethylamine, dimethylamine,
and trimethylamine.
2. The method of claim 1 wherein the pressure is from 1 kPa to
7000 kPa.
3. The method of claim 1 wherein space time of methanol and/or
dimethylether is about 0.01 hours to 80 hours.
4. The method of claim 1 wherein the conversion of methanol and/or
dimethylether to monomethylamine, dimethylamine, and trimethylamine
is greater than 75%, on a carbon basis.
5. The method of claim 1 wherein the selectivity of conversion,
on a carbon basis, to dimethylamine is greater than 60%.
6. The method of claim 1 wherein the selectivity of conversion,
on a carbon basis, to trimethylamine is less than 10%.
7. The method of claim 1 wherein the reaction temperature is from
about 270.degree. C. to about 370.degree. C. and the reaction pressure
is from about 170 kPa-3500 kPa.
8. The method of claim 1 wherein the ratio of silicon to aluminum
(Si:Al) in said molecular sieve is about 5.0:1.
9. The method of claim 1 wherein said acidic molecular sieve is
SSZ-16.
10. The method of claim 1 wherein said acidic molecular sieve has
been modified by treatment with one or more compounds containing
at least one element selected from the group consisting of silicon,
aluminum, boron and phosphorus, to deposit thereon at least 0.05
weight percent of said element.
11. The method of claim 7 wherein said molecular sieve is SSZ-16.
Molecular sieve description
FIELD OF THE INVENTION
This invention generally relates to a process for preparation of
monomethylamine, dimethylamine and trimethylamine in which methanol
and/or dimethylether and ammonia are contacted in the presence of
an acidic molecular sieve catalyst with the AFX structure (e.g.,
SSZ-16). In particular, the reactants are contacted in the presence
of an acidic SSZ-16 catalyst, wherein the ratio of silicon to aluminum
(Si:Al) in said catalyst is greater than about 2.5:1.
BACKGROUND OF THE INVENTION
Methylamines are generally prepared commercially by continuous
reaction of methanol and ammonia in the presence of a dehydration
catalyst such as silica-alumina. The reactants are typically combined
in the vapor phase, at temperatures in the range of 300.degree.
C. to 500.degree. C., and at elevated pressures. Trimethylamine
is the principal component of the resulting product stream accompanied
by lesser amounts of monomethylamine and dimethylamine. The methylamines
are used in processes for pesticides, solvents and water treatment.
From a commercial perspective, the most valued product of the reaction
is dimethylamine in view of its widespread industrial use as a chemical
intermediate (e.g., for the production of dimethylformamide). Thus,
a major objective of those seeking to enhance the commercial efficiency
of this process has been to improve overall yields of dimethylamine
and monomethylamine, relative to trimethylamine. Among the approaches
taken to meet this goal are recycling of trimethylamine, adjustment
of the ratio of methanol to ammonia reactants and use of selected
dehydrating or aminating catalyst species. Many patents and technical
contributions are available because of the commercial importance
of the process. A summary of some of the relevant art for methylamine
synthesis using a variety of catalysts is disclosed in U.S. Pat.
No. 5344989 (Corbin et al.).
Molecular sieves are important catalysts in the production of methylamines.
Molecular sieves have proven useful in catalysis applications because
some of them have appreciable acid activity with shape selective
features that are not available in the compositionally equivalent
amorphous catalyst. Molecular sieves are classified based on their
elemental composition and are able to separate components of a mixture
based on sizes and shapes of the components. The structure of molecular
sieves are based on a three dimensional network of oxygen ions.
Within the tetrahedra sites are cations. The AlO.sub.2.sup.- tetrahedra
in the structure determine the framework charge which is balanced
by cations that occupy non-framework positions.
Molecular sieves that have only Al.sup.+3 or Si.sup.+4 cations
within their tetrahedra sites are advantageous in methylamine production.
These aluminosilicate molecular sieves often are referred to as
zeolites. Chabazite and rho zeolites have a common structural framework
of eight-member rings of tetrahedral atoms that are often associated
with catalytic selectivity for production of dimethylamine from
methanol and ammonia. Chabazite zeolites, where the zeolite is derived
from mineral sources and the silicon to aluminum ratios in said
zeolites is less than about 2:1 as well as rho zeolites are known
to be useful as catalysts for methylamines. See U.S. Pat. No. 5569785
(Kourtakis et al.) and references cited therein. The catalysts have
geometric selectivity which permits the release of dimethylamine
and monomethylamine from the zeolite pores. The use of natural H-exchanged
and M-exchanged chabazites, where M is one or more alkali metal
cations selected from the group consisting of Na, K, Rb and Cs,
is disclosed in U.S. Pat. No. 4737592 (Abrams et al.).
U.S. Pat. No. 5399769 (Wilhelm et al.) discloses an improved
methylamines process using synthetic chabazites as catalysts. Runs
3-5 in Table 5 show the methylamines distribution for different
synthetic chabazites with a Si:Al ratio of about 2.5:1. The molar
ratio of ammonia to methanol was 3.5:1. Such an excess of ammonia
is known to decrease trimethylamine formation. The percentage of
dimethylamine for each run was 26 48.7 and 51.5 respectively.
What are needed and are of significant interest to the chemical
industry are process improvements which suppress production of trimethylamine
and optimize dimethylamine and monomethylamine yields.
SUMMARY OF THE INVENTION
The invention provides a method for the production of dimethylamine
(i.e., (CH.sub.3).sub.2 NH or DMA), monomethylamine (i.e., CH.sub.3
NH.sub.2 or MMA) and trimethylamine (i.e., (CH.sub.3).sub.3 N or
TMA), comprising contacting methanol, and/or dimethylether, and
ammonia in amounts sufficient to provide a carbon/nitrogen (C:N)
ratio from about 0.2 to about 1.5 at a reaction temperature from
about 250.degree. C. to about 450.degree. C., in the presence of
a catalytic amount of an acidic molecular sieve which has an AFX
crystalline structure (e.g., SSZ-16), and wherein the ratio of silicon
to aluminum (Si:Al) in said molecular sieve is greater than about
2.5:1 wherein methanol and/or dimethylether is converted to monomethylamine,
dimethylamine, and trimethylamine
DETAILED DESCRIPTION OF THE INVENTION
Molecular sieves are well known in the art and are defined in R.
Szostak, "Molecular Sieves--Principles of synthesis and Identification,"
Van Nostrand Reinhold (1989) page 2. The molecular sieves useful
for the present invention are those with an AFX structure (e.g.,
SSZ-16 and SAPO-56). For a description of the AFX structure see
R. Lobo et al., Chem. Mater., (1996), 8 pages 2409-2411. The structure
of SSZ-16 a synthetic zeolite, is described in this reference.
Synthetic SSZ-16 has a composition, as synthesized, and in the anhydrous
state in terms of oxide mole ratios of:
(0.5 to 1.4)R.sub.2 O:(0 to 0.50)M.sub.2 O:W.sub.2 O.sub.3 (greater
than 5)YO.sub.2 where R is an organic cation, M is an alkali metal,
W is selected from aluminum, gallium and mixtures thereof and Y
is selected from silicon, germanium and mixtures thereof. Further
details about the preparation of SSZ-16 can be found in U.S. Pat.
Nos. 4508837 and 5194235 both of which are incorporated herein
in their entirety by reference. The alkali metal can be exchanged
for H.sup.+ using mineral acids, by ion exchange or by conversion
to an ammoniated form which can then be converted to the acid form
by calcination at elevated temperatures, generally ranging from
about 400.degree. C. to about 600.degree. C.
Molecular sieves which have the AFX crystal structure, and acidic
zeolites wherein the ratio of silicon to aluminum in said zeolites
is at least about 2.5:1 preferably greater than about 5:1 can
be prepared by heating an aqueous mixture containing an organic
nitrogen-containing compound, a silicon oxide source and an aluminum
oxide source to a temperature of at least 100.degree. C. The heating
is continued until crystals of the desired AFX structure zeolite
or riolecular sieve are formed. The preferred organic nitrogen-containing
cations for SSZ-16 preparation are described in the two U.S. patents
cited immediately above. For SAPO-56 preparation, the preferred
organic nitrogen-containing cation is N,N,N',N'-tetramethyl-16-hexadiamine.
If desired, the silicon to aluminum ratio of SSZ-16 zeolites can
be increased by procedures known in the art such as leaching with
chelating agents, e.g., EDTA, or dilute acids.
The process of the present invention comprises contacting methanol
and/or dimethylether (DME) and ammonia, in amounts sufficient to
provide a carbon:nitrogen (C:N) ratio from about 0.2 to about 1.5
in the presence of a catalytic amount of an acidic molecular sieve,
preferably the zeolite SSZ-16 wherein the acidic molecular sieve
has a ratio of silicon to aluminum of at least about 2.5:1 preferably
5:1 at a temperature from about 250.degree. C. to about 450.degree.
C. Reaction pressures can be varied from about 1 psig-1000 psig
(110 kPa-7000 kPa) with a methanol/DME space time of 0.01 hours
to 80 hours. The resulting conversion of methanol and/or DME to
methylamines is generally in excess of 75% (on a carbon basis) and
selectivity (on a carbon basis) to dimethylamine is generally greater
than 60%. In addition, selectivity to and yield of trimethylamine
is suppressed. Thus, carbon yields of dimethylamine generally exceed
60% and carbon yields of trimethylamine are generally less than
10% under the process conditions of the present invention.
The molar equilibrium conversion of methanol and ammonia to a mixture
of the methylamines at 400.degree. C. and a C:N ratio of 1.0 is
17:21:62 (MMA:DMA:TMA).
The variables to be monitored in practicing the process of the
present invention include C:N ratio, temperature, pressure, and
methanol/DME space time. Space time is calculated as the mass of
catalyst divided by the mass flow rate of methanol and DME introduced
to a process reactor (mass catalyst/mass methanol+DME fed per hour.)
Generally, if process temperatures are too low, low conversion
of reactants to dimethylamine and monomethylamine will result. Increases
in process temperatures will ordinarily increase catalytic activity,
however, if temperatures are excessively high, equilibrium conversions
and catalyst deactivation can occur. Preferably, reaction temperatures
are maintained between about 270.degree. C. and about 370.degree.
C. more preferably 290.degree. C. to 350.degree. C. with lower temperatures
within the ranges essentially preferred in order to minimize catalyst
deactivation.
At relatively low pressures, products must be refrigerated to condense
them for further purification. Refrigeration adds costs to the overall
process. However, excessively high pressures require thick-walled
reaction vessels which are also costly. Preferably, pressures are
maintained at 10 psig-500 psig (170 kPa-3500 kPa). Short methanol/DME
space times result in low conversions and tend to favor the production
of monomethylamine.
Long methanol space times may result either in inefficient use
of catalyst or production of an equilibrium distribution of the
products at very high methanol/DME conversions. Generally, methanol/DME
space times of 0.01 hours to 80 hours are satisfactory, with methanol/DME
space times of 0.10 hours to 1.5 hours being preferred. These space
times correspond to methanol/DME space velocities of 0.013-100 g
methanol+DME/g of catalyst/hour, preferably 0.67-10 g of methanol+DME/g
of catalyst/hour, respectively.
The molar reactant ratio of methanol and/or dimethylether to ammonia,
herein expressed as the C:N ratio (g atoms C/g atoms N), is critical
to the process of the present invention. As the C:N ratio is decreased,
production of monomethylamine is increased. As the C:N ratio is
increased, production of trimethylamine increases. Catalyst deactivation
is also greater at high C:N ratios. Accordingly, for best results,
C:N ratios should be maintained between 0.2:1 and 1.5: 1 and preferably
from 0.5:1 to 1.2:1 in conducting the process of the present invention.
The efficiency of the process of the invention is measured by overall
conversion of methanol and/or DME to methylamines, and by selectivity
of dimethylamine production. For example, if methanol (MeOH) is
used as the sole reactant, overall conversion is determined by comparison
of the amount (in moles) of methanol in the product mixture, which
is considered to be unconverted, to the amount in the reactant feed.
Thus, overall conversion in carbon percent is given by:
Selectivity of methanol to monomethylamine (MMA) in carbon percent,
is given by:
Similarly, selectivity of methanol to trimethylamine (TMA), in
carbon percent, is given by:
Selectivity to dimethylamine (DMA) is calculated by analysis of
product composition. Thus, selectivity to DMA, in carbon percent,
is provided by the following expression:
Finally, selectivity to dimethylether (DME) in mole percent is
given by:
where X in the above equations is the number of moles of the relevant
compounds.
For efficient operation, the catalyst must be selective at high
conversions (75% to 98%) and a C:N ratio of 0.5:1 to 1.2:1.
In practicing the process of this invention, the molecular sieve
catalyst cam be combined with another material resistant to the
temperature and other conditions employed in the process. Such matrix
materials include synthetic or natural substances such as clays,
silicas and metal oxides.
Comparison of selectivities for different samples should be made
at similar conversions since selectivity varies with conversion.
At low conversions, MMA production is favored, at very high conversions,
the reaction will approach an equilibrium distribution and thus
result in increased TMA production.
Selectivities can be further improved by modifying the catalyst
with a coating, an example of which is described in Bergna et al.,
U.S. Pat. Nos. 4683334 and 4752596 the entire contents of which
are incorporated by reference herein. Specifically, to improve selectivity,
coating of an acidic zeolite which has a SSZ-16 crystalline structure,
wherein the ratio of silicon to aluminum (Si:Al) in said zeolite
is at least about 5:1 can be accomplished in the following manner:
(1) a sample of the catalyst is exposed to the ambient atmosphere
and is immersed in tetraethylorthosilicate (TEOS) for 2 hours; (2)
the sample is filtered and dried at room temperature overnight;
and (3) the sample is then heated in flowing nitrogen at 550.degree.
C. for 3 hours. The preceding treatment can be performed with one
or more compounds containing at least one element selected from
the group consisting of silicon, aluminum, boron and phosphorus,
to deposit substantially on the external surfaces of the acidic
molecular sieve with the SSZ-16 crystalline structure at least 0.05
weight % of the element.
EXAMPLE 1
Preparation of SSZ-16 and Its Use in the Preparation of Methylamines
14-bis(1-azoniabicyclo[2.2.2]octane)butyl dibromide (1.89 g) was
dissolved in H.sub.2 O (6 mL) and mixed with a sodium silicate solution
(5 g, 38.3% solids, SiO.sub.2 /Na.sub.2 O=3.22). The solution was
poured into a TEFLON.RTM. (polytetrafluoroethylene) lined autoclave
followed by the addition of a solution of Al.sub.2 (SO.sub.4).sub.3
.cndot.18H.sub.2 O (0.25 g) and NaOH (0.67 g) in H.sub.2 O (6 mL).
After mixing thoroughly, the autoclave was sealed and the mixture
heated, with no stirring, for 6 days at 140.degree. C. under autogeneous
pressure. The product was recovered by filtration, then washed with
water and dried.
The recovered product was calcined stepwise in a N.sub.2 /air mixture
as follows: brought from room temperature to 93.degree. C. over
2 hours, from 93.degree. C. to 204.degree. C. over 2 hours, from
204.degree. C. to 316.degree. C. over 2 hours, from 316.degree.
C. to 427.degree. C. over 2 hours, and from 427.degree. C. to 528.degree.
C. over 3 hours. X-ray powder diffraction of this material showed
it to be the molecular sieve of the subcategory, zeolite SSZ-16.
This material was ion-exchanged from the Na.sup.+ into the NH.sub.4.sup.+
form by three exchanges with a 10% NH.sub.4 NO.sub.3 solution (10
mL/g molecular sieve) at 90.degree. C. for one hour per exchange,
and then calcined stepwise again into the H.sup.+ form as follows:
room temperature to 110.degree. C. at 10.degree. C./minute, 110.degree.
C. to 400.degree. C. at 5.degree. C./minute, 400.degree. C. to 450.degree.
C. at 1.degree. C./minute and then held at the latter temperature
for 8 hours to obtain the catalytically active acidic H-SSZ-16.
Before use in the reactor, the molecular sieve catalyst was pressed
into pellets and crushed and sieved to 20 to 40 mesh (0.84 to 0.42
mm). One gram of the resulting catalyst was placed in a stainless
steel U-tube reactor, 0.25 in (0.64 mm) in diameter and 18 to 20
inches length (45.7 to 50.8 cm). First, the reactor was heated to
reaction temperature in a fluidized sand bath. The reacticn pressure
was maintained at 200 psig (1480 kPa) to resemble commercial production
conditions. Reactants methanol and ammonia were fed to a pre-heater,
which consisted of a 2.03 m length by 0.32 mm diameter stainless
steel coil at a molar ratio of about 1 vaporized and then passed
through the reactor into contact with the catalyst sample. The reactor
effluent was continuously measured by gas chromatography for ammonia,
dimethylether (DME), methanol, water, and mono-, di- and trimethylamine.
The percentage selectivities of conversion to each methylamine species
are given in Table 1 below, for conversion of methanol of 79%,
which was the maximum conversion obtained. |