Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
CATALYST AND PROCESS FOR PREPARING AMMONIA
The research leading to the invention described herein
was supported in part by funds from Grant ~CTS-9257306 from
the National Science Foundation. As such, the United States
Government may have certain rights in the present invention.
BACXGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a novel supported
catalyst comprising a Group VIII transition metal supported on
a basic molecular sieve, for providing i~uved synthesis of
ammonia from nitrogen and hydrogen gases and a process for
using the same in preparing ammonia.
Discussion of the Backqround
Conventional industrial ammonia synthesis processes use a
triply promoted iron catalyst operating at temperatures of
400-700~C and pressures in excess of 300 atm. However, at
such e~L ~- ? temperatures, the equilibrium reaction of N2 and
H2 to give ~ on; a is not especially favored, hence the need
for the extreme pressures.
In U.S. Patent No. 3,770,658, Ozaki et al disclosed a
~ transition metal based catalyst, preferably of ruthenium,
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which contained alkali metal, for the preparation of ammonia
from nitrogen and hydrogen under temperatures less than 400~C.
In U.S. Patent No. 4,142,993, Elofson et al also disclose
a Group VIII transition metal based catalyst cont~in;ng alkali
metal, which is supported on an activated carbon support for
synthesis of ammonia at temperatures of 375~C or higher and
pressures of 27-67 atm.
Such traditional ruthenium-based ammonia synthesis
catalysts consist of ruthenium clusters supported on carriers
like carbon and magnesium oxide. In addition, as shown by
Ozaki et al, alkali metal promotors such as potassium or
cesium are often added to enhance the catalytic activity of
the ruthenium.
In U.S. Patent 4,600,571, McCarroll et al disclose the
use of ruthenium based ammonia synthesis catalyst which
contain an alkali metal and barium, all supported on a carbon
~L.
R~ntly published work by Cicn~os and Lunsford [J.
Catal. 141 (1993) 191-205] and Wellenbuscher et al (Catal.
Letters 25(1994) 61-74) shows that ruthenium clusters
supported on alkali-cont~; ni ng zeolites also catalyze the
synthesis of ammonia from nitrogen and hydrogen at atmospheric
pressure. From the results of Cisneros and Lunsford, ammonia
synthesis at 650 K and atmospheric pressure over ruthenium
clusters supported on potassium-loaded zeolite X occurs at a
rate of 1.7 x 10-5 mol NH3/g Ru/sec.
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However, each of the prior art catalysts still do not
provide the desired level of activity and reaction rate.
Accordingly, an improved ammonia synthesis catalyst is
desired.
SUMMARY OF THE INVENTION
Accordingly, one object of this invention is to provide a
catalyst which provides improved ammonia synthesis rates under
industrially useful conditions.
A further object of the present invention is to provide
an i~ o~ed process for the preparation of ammonia from
nitrogen and hydrogen gases.
These and other objects of the present invention have
been satisfied by the discovery of a catalyst for ammonia
synthesis, comprising Group VIII transition metal clusters
supported on a basic zeolite, which further comprises alkali
metal ions and divalent metal ions, which provides markedly
improved rates of reaction of N2 and H2 to give ammonia.
DETATT~n DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a Group VIII transition
metal/basic zeolite catalyst for the synthesis of ammonia from
N2 and H2. The catalyst of the present invention comprises
Group VIII transition metal clusters supported on a basic
zeolitic support. The Group VIII transition metal cluster is
preferably made of Fe, Ru or Os, with Ru being most preferred.
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The Group VIII transition metal cluster can be prepared using
conventional methods (see "Catalyst Manufacture", 2nd Ed.,
A.B. Stiles and T.A. Xoch, Marcel Dekker, New York, 1995).
For example, ruthenium clusters can be formed from various
ruthenium compounds, such as Ru (NH3 )6Cl3. In particular, the
Group VIII metal compounds are ion exchanged or impregnated
onto the zeolite, following which the resulting material is
reduced,.for example, by hydrogenation, thus providing the
resulting clusters in an oxidation state of the corresponding
metal (such as Ru~).
The zeolite used in the present catalyst as a support for
the Group VIII metal clusters is preferably a Faujasite-type
zeolite (or Faujasitic zeolite), such as Zeolite X, Zeolite Y,
EMT, ZSM-3, ZSM-20, Zincophosphate X or SAPO-37. Preferably
the zeolite is a microporous cryst~ll;ne aluminosilicate,
preferably having a Si:Al ratio of from 1:1 to 6:1, more
preferably 1:1 to 2.5:1.
The catalyst of the present invention further contains
Group I alkali cations and clivalent metal cations. The
divalent metal cations can be alkAl;ne earth ions or divalent
transition metal ions. Preferably the A 1 kA l; ne earth ions are
used, with Ba~2 being most preferred. The divalent metal ions
can be incorporated into the Group VIII metal/zeolite catalyst
by conventional processes, such as ion exchange or
impregnation.
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C_ ~~cially available zeolites often already contain
alkali metal ions, such as Na. For example, Zeolite X is
available containing Na ions and is conventionally called NaX
zeolite. These zeolites can be used as supplied, or can first
be subjected to modification, such as ion exchange or
impregnation, to replace Na with another alkali metal, such as
K, with the preferred alkali metals being K, Rb and Cs. The
alkali metal con~; n ing zeolite is then subjected to
modification with divalent cations, such as alkaline earth
metal ions, by ion exchange or a combination of ion exchange
and impregnation.
Prior to or after this divalent cation modification, the
Group VIII metal can be incorporated into the zeolite using
conventional t~chn;ques to form the metal clusters and provide
the Group VIII/M~2/basic zeolite catalyst. While the above
sequence of steps can be used to prepare the present catalyst,
the steps can be performed in any order, to provide
incorporation of the divalent metal ions and the Group VIII
metal clusters, to provide the catalyst of the present
invention which is active for the production of ammonia from N2
and H2.
The Group VIII metal based catalyst of the present
-~ invention provides its advantages in reaction rate upon
incorporation of even minute quantities of Group VIII metal
into the basic zeolite. However, it is preferred that the
loading be in the range of 0.1 to 10%, most preferably in the
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range of 1-5% by weight, based on the amount of zeolite. In
the case of the more expensive Group VIII metals, the
preferred loading is in the range of 0.1 to 2.0%.
The divalent metal ions also provide their advantages
even upon incorporation of very small quantities into the
catalyst. Preferably, the molar ratio of divalent metal ions
to alkali metal ions is in the range from 0.01 to 100, most
pre~erably from 10 to 100.
Once the divalent ions and Group VIII metals have been
incorporated into the zeolite, it is important to render the
zeoiite basic in nature. This is preferably done by
impregnating the composition with a basic compound such as
divalent or alkali metal hyclroxides, A lk;~l; alkoxides, ~lk;~l;
oxides, alkali metals, etc.
In using the catalyst o~ the present invention, the
catalyst is contacted with ~T2 and H2 gas in a N2:H2 molar ratio
of ~rom 10:1 to 1:10, preferably from 1:3 to 1:6. The
reaction is performed at a temperature and pressure su~icient
to provide ~c~llent yield per unit time. Pre~erably the
tr ~ature of ammonia synthesis is from 200-600~C and the
pressure is from atmospheric to 400 atmospheres. These
parameters can be adjusted to provide the optimum reaction
rate, depending primarily on the conditions achievable in the
reaction vessels used. The contact time, or weight hourly
space velocity (g feed/g ca~alyst/hour), is adjusted to
achieve the desired yield o~ ammonia (i.e., longer contact
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gives more ammonia and vice versa). Further, the reaction can
be performed either batchwise or continuously.
By using the catalyst of the present invention, it is
possible to obtain surprisingly improved rates of ammonia
synthesis when compared to the catalysts of the prior art.
These improved rates of reaction allow the use of milder
conditions, which can prove industrially advantageous
financially, as well as in the area of industrial safety.
Having generally described this invention, a further
understanding can be obtained by reference to certain specific
examples which are provided herein for purposes of
illustration only and are not intended to be limiting unless
otherwise specified.
EXAMPLES
CatalYst SYnthesis Exam~les
Example 1. Ru/NaX
NaX zeolite (lS g) ~rom Union Carbide (elemental
analysis: Si - 20.48%, Al - 16.55%, Na - 12.6%) was ion-
~h~nged with 0.936 g o~ Ru(NH3)6C13 in 300 mL of distilled,
deionized water. The resulting solids were filtered, dried
and reduced in flowing H2 at 723 K. The final solid cont~;n~
1.8S% of Ru by weight.
ExamPle 2. Ru/KX
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First, NaX zeolite ~rom Union Carbide (elemental
analysis: Si -- 20.48~6, Al - 16.55%, Na -- 12.67%) was ion--
exchanged three times with 1 molL~l aqueous KNO3 (7S, 230 and
350 mL, respectively) and dried to produce KX zeolite. 15 g
of KX zeolite was ion o~r~h~rlged with 0.936 g of Ru(NH3)6Cl3 in
l.Z L of water. The resulting solids were filtered, dried and
reduced in ~lowing H2 at 723 K. The reduced solid was then
impregnated with 310 mL oi~ a 0.2 molal aqueous solution o~ KOH
and dried. Elemental analysis: Ru -- 2.04%, Si -- 19.70%, Al
13.03%, Na -- 0.58%, K -- 16.08%.
Exam~le 3. Ru/CsX
1.24 g o~ sample in example 2 (before impregnation with
KOH) were ion-exchanged three times with l molL~l aqueous
cesium acetate solutions (12.5, 12.5 and 30 mL, respectively),
~iltered and dried. The resulting solids were impregnated
with 0.2 molal aqueous solution o~ Cs(OH). Elemental
analysis: Ru - 2.01%, Si - 13.88%, Al -- 7.9%, Na - 0.93%, K --
2.50%, Cs -- 24.S3%.
Example 4. Ru/BaX
1.22 g of sample in example 2 (be~ore; - ey-~ation with
KOH) were ion--exchanged two times with 1 molL~1 aqueous barium
acetate solutions (10 and 15 mL), ~iltered and dried. The
resulting solids were impreynated with 50 mL o~ a 0.2 molal
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aqueous solution of Ba(OH) 2~ Elemental analysis: Ru -- 2.05%,
Si -- 13.12%, Al -- 9.48%, K -- 1.32%, Ba -- 24.4%.
ExamPle 5. Ru/BaX(2)
First, NaX zeolite from Union Carbide (elemental
analysis: Si -- 20.48%, Al - 16.55%, Na -- 12.67%) was ion--
exchanged three times with 1 molL~l of a~ueous KNO3 and dried
to produce KX zeolite. 17.68 g of KX zeolite were ion
exchanged with 1.1048 g of Ru(NH3)6C13 in water. The resulting
solids were filtered, dried and reduced in flowing H2 at 723 K.
3.285 g of the solids were ion-~cch~nged two times with 1 molL~
aqueous barium acetate solutions (10 and 10 mL), filtered and
dried. The resulting solids were impregnated with 30 mL of a
0.2 molal aqueous solution of Ba(OH) 2 ~ Elemental analysis: Ru
-- 2.10%, Si - 14.38%, Al - 9.04%, K - 1.47%, Ba -- 21.6%.
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Reactivity Examples
All samples were sieved to 170 mesh (90 ~m) before
reaction. Catalysts were loaded into a constant volume
recirculation reactor and reduced in-situ at 723 K before
reaction. A 3:1 molar ratio of H2:N2 reactant mixture at a
total pressure of 1 atmosphere was introduced into the system.
The product ammonia was conclensed into a liquid nitrogen trap
thus preventing the product from inhibiting the reaction.
Rates were calculated from the total pressure drop in the
system as a function o~ time and are reported as moles of
ammonia produced per gram of Ru per time. The fraction of Ru
exposed to the surface (dispersion) was evaluated by a
st~n~d hydrogen chemisorption measurement and results were
used to calculate the specific rate of reaction per surface Ru
atom.
Reaction Results~ Synthesis over ~--th~nium Catalysts
Fraction of Temperature Reaction Rate 5pe~ific Rate
ExampleSampleRu exposed /R(+5 R)/10-C molN~3 /10--s-~
(gRu)~ls~l
1 Ru/NaX 0.57 650 4.48 7.94
2a Ru/RX 0.93 650 2.84 3.09
20 2b Ru/XX 0.93 700 14.2 15.4
3a RutCsX 0.72 650 7.62 10.7
3b Ru~CsX 0.72 700 25.9 36.4
4a Ru/8aX 0.92 650 34.5 37.9
4b Ru/BaX 0.92 700 105 115
25 SaRu/BaX(2)0.79 650 33.6 43.0
5bRu/BaX(2)0.79 700 127 163
At a given temperature, the Ba-c~nt~i"ing Ru catalyst was more active for i~
synthesis than catalysts cont~in;ng alkali cations (Na, R, Cs) without Ba.
=. . . =
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Obviously, numerous modifications and variations of the
present invention are possible in light of the above
teachings. It is therefore to be understood that within the
scope of the appended claims, the invention may be practiced
otherwise than as specifically described herein.
