PATENT
Manganese
Oxides and Their Use in the Oxidation of Alkanes (Exxonmobil)
Publication number
US20130317272 A1
Publication type Application
Application number US 13/891,895
Publication date Nov 28, 2013
Also published as US8470289, US20090023972, WO2009012040A1
Inventors
Helge Jaensch, Wilfried J. Mortier
Original Assignee
Exxonmobil Chemical Patents Inc.
Abstract
Catalytic
structures are provided comprising octahedral tunnel lattice manganese oxides ion-exchanged with metal cations or mixtures
thereof. The structures are useful as catalysts for the oxidation of alkanes and may be prepared by treating layered
manganese oxide under highly acidic conditions, optionally drying the treated
product, and subjecting it to ion exchange.
FIELD OF THE INVENTION
The present invention is concerned with manganese oxide structures and their
synthesis. Furthermore, the invention is particularly but not exclusively
concerned with the oxidation of alkanes to produce oxygen-containing organic
compounds. Such compounds may be alcohols, and carbonyl-containing compounds
such as aldehydes, ketones and carboxylic acids. In particular the invention is
concerned with the oxidation of C1 to C5 alkanes and more
particularly methane, ethane and propane derived from natural gas.
BACKGROUND OF THE INVENTION
The availability of large quantities of natural gas has led to numerous
proposals to upgrade the alkanes in the gas to more valuable chemicals
including oxygen-containing organic compounds. The only commercial processes so
far developed have required the sequential steps of catalytic dehydrogenation
of the alkane to form an olefin followed by oxidation of the reactive olefinic
site to produce the oxygenate. Other processes have been proposed for the direct
oxidation of alkanes to oxygenates. However, these processes have low
conversion and low selectivity and usually require high temperatures.
The two stage vapour phase oxidation of propylene to produce acrylic acid is
known in the art. However, the production of acrylic acid from propane would be
more attractive than its production from propylene because of the significant
price difference between propane and propylene.
There are few references reported in the literature relating to the production
of acrylic acid from propane. U.S. Pat. No. 5,198,580 (Standard Oil) discloses
a process for partial oxidation of propane to yield acrylic acid, propylene,
acrolein, acetic acid and carbon oxides. The process involves the reaction of
propane in admixture with a molecular oxygen-containing gas in a reaction zone
with a catalyst containing Bib, Moe, Vv, Aa,
Dd, Ee, Ox; where A is one or more of K, Na,
Li, Cs and TI; D is one or more of Fe, Ni, Co, Zn, Ce and La; E is one or more
of W, Nb, Sb, Sn, P, Cu, Pb, B, Mg, Ca and Sr; a, d and e is each from 0 to 10;
b is from 0.1 to 10; c is from 0.1 to 20; v is from 0.1 to 10; c:b is from 2:1
to 30:1 and v:b is from 1.5:1 to 8:1. The acrylic acid yield achieved using the
bismuth molybdate type of catalyst at a pressure of 138 kPag (20 psig) and a
temperature of 400° C. is 5.4% at 19% conversion of propane.
EP-A-0608838 (Takashi et al/Mitsubishi) discloses a method of producing an
unsaturated carboxylic acid, mostly in the explosive regime of the propane, air
and water mixture, at 380° C. in the presence of a catalyst containing a mixed
metal oxide of MoVTeXO, wherein X is at least one element selected from
bismuth, cerium, indium, tantalum, niobium, aluminum, boron, tungsten,
titanium, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium,
nickel, palladium, platinum and antimony. The proportion of the respective
essential components is based on the total amount of the essential components
exclusive of oxygen and satisfies the following formulae: 0.25<rMo<0.98,
0.003<rV<0.5, 0.003<rTe<0.05, is 0.003<rX<0.5,
wherein rMo, rV, rTe and rX are
molar fractions of Mo, V, Te and X, respectively. Takashi et al also disclosed
in Patent Number JP1045643 (9845643—Feb 1998), the formation of acrylic acid
and acrolein in the presence of PaMobVcWdXeOn
(X=Nb, Ta, Ti, Zr, Sb; if a=1 then b=1-18, c=0-4, d=0-4 and e=0.05-20) at
380° C. achieving a yield of 0.9% to acrolein and 3.5% to acrylic acid at 12%
propane conversion
U.S. Pat. No. 6,646,158 (SABIC) suggests the use of a catalyst with a calcined
composition of Moa, Vb, Gac, Pdd,
Nbe, Xf, wherein X=at least one element selected from the
group consisting of La, Te, Ge, Zn, Si, In and W; a is 1; b is 0.01 to 0.9; c
is >0 to 0.2; d is 0.000000001 to 0.2; e is >0 to 0.2; and f is >0 to 0.5,
for the oxidation of propane to acrylic acid and acrolein. The numerical values
of a, b, c, d, e and f represent the relative gram-atom ratios of the elements
Mo, V, Ga, Pd, Nb and X respectively in the catalyst. The elements are
preferably present in combination with oxygen in the form of various oxides.
The above referenced catalysts disclosed in the literature result in low yields
of acrylic acid at relatively high temperatures and most produce propylene as
one of the significant by-products. Propylene can be expensive to separate,
especially in a recycling mode of operation.
Further examples are taught in the art for the mixed metal oxide component of
catalysts for the production of acrylic acid in one step by subjecting propane
to a vapour phase catalytic oxidation reaction. Such publications are a
Mo—Sb—P—O type catalyst (EP-A-0010902 [Rohm and Haas]); a V—P—Te—O type
catalyst (Journal of Catalysis, Col 101, p389 (1986), a Bi—Mo—O type catalyst
and a V—P—Te—O type catalyst (Japanese Unexamined Patent Publication No.
170445/1991). On the other hand, as an example of a catalyst for the production
of methacrylic acid in one step by subjecting isobutene to a vapour phase
catalytic oxidation reaction, a P—Mo—O type catalyst (Japanese Unexamined
Patent Publication No. 145249/1988) is known.
However, each of the methods using such catalysts has a drawback, for example
that the yield of the desired unsaturated carboxylic acid is not adequate or
the reaction system is complex.
We have now developed a process that enables alkanes to be oxidised under less
severe conditions and with a greater selectivity.
Manganese oxides having tunnel (4×4) structure are known and have been proposed
as oxidation catalysts, for example in EP-A-0710622 (Texaco). An article in Catalysis
Today 85 (2003) pages 225-233 describes the selective oxidation of alcohols
using octahedral molecular sieves (OMS). These tunnel shaped manganese oxides
contain significant quantities of lattice oxygen. It has been found that this
lattice oxygen can be activated to provide available oxygen which can be used
for oxidation of organic compounds such as benzyl alcohol to benzaldehyde,
2-butane and cyclohexane, as is described in Catalysis Today 85 (2003) pages
225-233.
It is also known, for example from U.S. Pat. No. 5,597,944 (Texaco) that a
transition metal cation can be substituted in the framework of OMS-3 by
co-dissolving a transition metal salt in the organic solvent used to dissolve
the manganese salt. These materials are proposed as catalysts for the
dehydrogenation of n-paraffins to n-olefins. The transition metal cation(s),
which can be designated as M<+n> (where M indicates the transition metal
and n indicates an oxidation state which is stable in the organic solvent
solution), can be any metal selected from Groups IIIA, IVA, VA, VIA, VIIA,
VIIIA, IB, IIB and VIIB of the Periodic Table of the Elements (Merck Catalogue
of 2001 page 1287). Preferably, the transition metal is a metal selected from
Groups IB, IIB and VIII of the Periodic Table of the Elements. Examples of
useful framework-substituting transition metals are said to include Mg, Fe, Co,
Ni, Cu, Ti, V, Cd, Mo, W, Cr, Zn, La, Ir, Rh, Pd and Pt. Preferred metals
include Co, Cu, Ni, Zn, La and Pd. Transition metal cation(s) M<+n> should
be present in the organic solvent in a concentration effective to introduce the
desired proportions of the transition metal(s) into the framework of OMS-3
structure during the course of the reaction. Therefore, any suitable salt
(inorganic or organic) of the selected transition metal(s) can be used which is
sufficiently soluble provided, of course, that the anion does not interfere
with the other reactants or the course of the reaction. For example, the
nitrates, sulfates, perchlorates, alkoxides and acetates can be used with
generally good results.
Manganese oxide octahedral molecular sieves possessing mono-directional tunnel
structures constitute a family of molecular sieves wherein chains of MnO6
octahedra share edges to form tunnel structures of varying sizes. Such
materials have been detected in samples of terrestrial origin and are also
found in manganese nodules recovered from the ocean floor. Manganese nodules
have been described as useful catalysts in the oxidation of carbon monoxide,
methane and butane (U.S. Pat. No. 3,214,236 [Mobil]), in the reduction of
nitric oxide with ammonia (Atmospheric Environment, Vol. 6, p.309 (1972)) and
in the demetallation of topped crude in the presence of hydrogen (Ind. Eng. Chem. Proc. Dev., Vol. 13, p.315 (1974)).
The hollandites are naturally occurring hydrous manganese oxides with tunnel
structures (also described as “framework hydrates”) in which Mn can be present
as Mn<+4> and other oxidation states. The tunnels can vary in size and
configuration and various mono- or di-valent cations can be present in the
tunnels. The hollandite structure consists of double chains of MnO6 octahedra
which share edges to form (2×2) tunnel structures. The average size of these
tunnels is about 4.6 A° square. Ba, K, Na and Pb ions are present in the
tunnels and coordinated to the oxygens of the double chains. The identity of
the tunnel cations determines the mineral species. Specific hollandite species
include hollandite (BaMn8O16), cryptomelane (KMn8O16), manjiroite (NaMn8O16)
and coronadite (PbMn8O16). The hydrothermal method of synthesizing a manganese
oxide octahedral molecular sieve possessing (2×2) tunnel structures such as
those possessed by the naturally-occurring hollandites is described in
“Hydrothermal Synthesis of Manganese Oxides with Tunnel Structures,” in
Synthesis of Microporous Materials, Vol. II, 333, M. L. Occelli, H. E. Robson
Eds. Van Nostrand Reinhold, N.Y., 1992. Such synthetic octahedral molecular
sieves having (2×2) tunnel structures are referred to in the art by the
designation OMS-2.
The hydrothermal method of producing OMS-2 involves autoclaving an aqueous
solution of manganese cation and permanganate anion under acidic conditions,
i.e., pH<3, at temperatures ranging from about 80 to about 140° C. in the
presence of counter cations having ionic diameters of between 2.3 and 4.6 A° .
The counter cations can serve as templates for the formation of OMS-2 product
and be retained in the tunnel structures thereof. Based on analytical tests,
OMS-2 produced via this method is thermally stable up to 600° C. Alternatively,
OMS-2 can be produced by the method disclosed in R. Giovanili and B. Balmer,
Chimica, 35 (1981) 53. Thus, when manganese cation and permanganate anion are
reacted under basic conditions, i.e., pH>12, a layered manganese oxide
precursor is produced. This precursor is ion exchanged and then calcined at
high temperatures, i.e., temperatures generally exceeding 600° C., to form
OMS-2 product. Analytical tests indicate that OMS-2 produced via this method is
thermally stable up to 800° C. and the average oxidation state of manganese ion
is lower.
The todorokites are naturally occurring manganese oxides with (3×3) tunnel
structures formed by triple chains of MnO6 edge-sharing octahedra. Todorokites
and related species are described by Turner et al. in “Todorokites: A New
Family of Naturally Occurring Manganese Oxides”, Science, Vol. 212, pp.
1024-1026 (1981). The authors speculate that since todorokites are often found
in deep-sea manganese nodules containing high concentrations of copper and
nickel, it is probable that such metals substitute for Mn<+2> in the
octahedral framework.
Todorokites have attracted particular interest because of their relatively
large tunnel dimension and their cation-exchange behaviour which is similar to
that of zeolites (Shen et is al., “Manganese Oxide Octahedral Molecular Sieves:
Preparation, Characterization, and Applications”, Science, Vol. 260, pp.
511-515 (1993)). The naturally occurring todorokites are poorly crystalline,
impure in composition and coexist with other manganese oxide minerals. Results
of high resolution transmission electron microscopy (HRTEM) show that
todorokite contains random intergrowth material of 3×2, 3×3, 3×4 and 3×5 tunnel
structure. Because of their disordered structure, the todorokites exhibit
variable and non-reproducible catalytic activity, a drawback which militates
against their commercial use.
A method of synthesizing a manganese oxide octahedral molecular sieve
possessing (3×3) tunnel structures such as those possessed by the
naturally-occurring todorokites is described in U.S. Pat. No. 5,340,562
(Texaco). Such synthetic octahedral molecular sieves having (3×3) tunnel
structures are referred to in the art by the designation OMS-1.
OMS-1 can be prepared by reacting manganese cation and permanganate anion under
strongly basic conditions to form a layered manganese oxide precursor;
thereafter aging the precursor at room temperature for at least 8 hours;
ion-exchanging the aged precursor; and then autoclaving the ion-exchanged
precursor at from 150 to 180° C. for several days. Analytical tests indicate
that OMS-1 produced via this method is thermally stable up to about 500° C.
We have now found that the framework of the 2×2 octahedral manganese compound
may be ion-exchanged with metal cations to provide a new material and that this
material may be used to oxidise alkanes. Our co-pending U.S. patent application
Ser. No. 60/950,008, filed concurrently herewith (Attorney Docket No.
2007EM184) discloses the use of manganese oxide molecular sieves as catalyst
for the oxidation of alkyaromatic compounds to the corresponding
hydroperoxides.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention there is provided a 2×2
octahedral manganese oxide structure ion-exchanged with one or more metal
cations.
In a second aspect of the present invention there is provided a method for
producing a 2×2 octahedral manganese oxide structure containing metal cations,
which process comprises treating a layered manganese oxide under highly acidic
conditions comprising a pH not greater than 5, to rearrange the oxide to
the 2×2 octahedral structure containing free protons, optionally drying the
proton-containing structure, and ion exchanging the proton-containing structure
with metal cations.
In a third aspect of the invention there is provided a process for the
oxidation of an is alkane comprising contacting the alkane with oxygen in the
presence of a catalyst system comprising a 2×2 octahedral manganese oxide
structure ion-exchanged with one or more metal cations.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is the X-ray diffraction (XRD) pattern of the layered manganese oxide
material made in Example 1.
FIG. 2 is the XRD pattern of the proton-containing 2×2 octahedral manganese
oxide material made in Example 2.
FIG. 3 is the XRD pattern of the vanadium cation exchanged 2×2 octahedral
manganese oxide material made in Example 4.
FIG. 4 is the Energy Dispersive X-ray spectrogram (vanadium-EDX) of the
material made in Example 4.
FIG. 5 is a plot of % selectivity versus time for the oxidation reaction
described in Example 6.
DETAILED DESCRIPTION
We have found that the 2×2 structure is particularly stable and has benefits
when ion-exchanged and used as a catalyst for the oxidation of alkanes. The
alkanes are preferably C1 to C5 alkanes, and may be oxidised according to the
invention as a single alkane or as a mixture of any two or more such alkanes.
The alkane feed to oxidation may contain other components that do not substantially
interfere with the target oxidation reaction.
In the oxidation process of the present invention the oxygen may be provided
solely by the lattice oxygen of the manganese oxide. Alternatively additional
oxygen such as molecular oxygen, as in air, may be provided.
In accordance with the present invention the octahedral porous manganese oxide
molecular sieve possesses a (2×2) tunnel structure and contains cations of one
or more metals as tunnel cations. The porous manganese oxide molecular sieve
preferably complies with the general formula (I):
[A16-aMaMn16-aO32]n (I)
in which
A represents a tunnel cation that may be in oxidation state +1, +2, +3, +4 or
+5, wherein the metal of the cation is selected from the transition metals
(Groups 3-12) and metals of Group 1 and Group 2 of the IUPAC Periodic Table of
the Elements (1 Nov. 2004);
M represents a lattice cation that may be in oxidation state +1, +2, +3, +4 or
+5, wherein the metal of the cation is a transition metal (Group 3-12);
Mn represents the transition metal, manganese;
a is a number equal to or greater than zero and less than 16; and
n is a number equal to or greater than 1.
Preferably, a is in the range of 0.1 to <16.
The porous manganese oxide may be hydrated, that is, it may have one or more H2O
molecules associated with the general formula (I).
In the general formula (I), where there is more than one A then the A's may be
the same or different with regard to oxidation state and/or metal.
Similarly, where general formula (I) has more than one lattice cation M, then
the M's may be the same or different with regard to oxidation state and/or
metal.
It will be recognized that A and M may be the same (transition) metal; but of
course A is a tunnel cation whereas M is a lattice cation.
It will also be recognized that the lattice cation M and/or the tunnel cation A
may be manganese.
Based on the preferred general formula (I), it will be recognized that the
porous manganese oxide molecular sieves of the invention may be, for example, K16
+1Mn16 +3O32, K8 +1Mn8
+3Mn8 +4O32, or Mn16 +4O32.
Many other combinations are possible depending on the values selected for a,
the oxidation states of A, M and Mn, and the metals of A and M.
The octahedral manganese oxide molecular sieves of the invention possess an
average pore diameter of approximately 4.6 Angstrom (Å). Preferably the manganese oxide octahedral molecular
sieve possesses a highly uniform and homogeneous structure, i.e., one made up
substantially entirely of (2×2) tunnel structure species without admixture of any
significant amount of other tunnel structure species.
The metal cations in the manganese oxide structure of the invention are not
necessarily homogenously dispersed throughout the structure. In one embodiment
they may be more concentrated in certain zones or regions of the structure than
others. Preferably the metal cations that have been ion-exchanged into the
structure are more concentrated in surface zones or regions than elsewhere in
the body of the oxide structure. It is believed to be beneficial for catalyst
function to have such enrichment of the metal cations at or near the surfaces
of the structure, and the ion exchange conditions employed may be adjusted to
is accomplish this configuration.
According to the second aspect of the invention there is provided a method for
producing the 2×2 octahedral manganese oxide structure containing metal
cations, as specified above. Preferably the highly acid conditions under which
the layered manganese oxide is treated comprise a pH of 4 or less, more preferably
3 or less and most preferably 2 or less. For example the pH range for the
treatment may be in the range 0.1 to 2, such as 0.5 to 1.5 or 0.1 to 1.1.
In one embodiment the method comprises mixing a solution of basic hydrogen peroxide and a solution of manganese
nitrate hydrate at a basic pH to form the layered manganese oxide, treating the
layered manganese oxide with acid to reduce the pH to the range of from 0.1 to
3 and to rearrange the layered oxide to form the octahedral 2×2 structure,
drying, and treating the resulting octahedral 2×2 manganese oxide structure
with a solution of metal ions to perform an ion exchange reaction with protons
in the 2×2 octahedral structure.
Preferably the layered manganese oxide is formed from the mixture at a basic pH
of from 11 to 14.
The metal cation-containing porous manganese oxide molecular sieves of the
invention may be prepared by the method specified above. Generally stated, this
method involves first producing the 2×2 tunnel octahedral molecular sieve
(OMS-2) that does not contain tunnel metal cations; then introducing the tunnel
metal cations.
A detailed description of a preferred embodiment of the synthesis method of the
invention is provided hereinafter. Thus, a preferred synthesis of OMS-2
initially involves preparing a solution (I) of basic hydrogen peroxide; and a solution (II) of manganese nitrate
hydrate. The hydrogen peroxide may be
made basic for example by use of an alkali metal hydroxide such as KOH.
Preferably the weight ratio KOH:H2O2 (35%):H2O
is 1:3.5:31.5). In solution (II) the weight ratio Mn(NO3)2*H2O:H2O
is 1:11.4 [* indicates water of crystallisation; and the Mw of Mn(NO3)2*H2O=287.04
g/mole]. The preferred synthesis involves mixing solution (I) and solution (II)
by slowly adding solution (I) to solution (II). The reaction is preferably
performed at a basic pH eg from 11 to 14. The mixed solutions are
mixed, eg by stirring, for some time, preferably for 1 to 10
hours and preferably at room temperature. Then the reaction products are
separated, eg centrifuged, in order to remove the solids. The solids are dried,
such as by heating eg to 110° C. in an air-ventilated oven for eg 3 hours, and
allowed to stand (material A). After grinding to a suitable particle size the
dry material is calcined, eg for 6 hours at 500° C. in dry air (material B).
Material B, which is a is layered manganese oxide, is then treated under highly
acidic conditions. Thus, it may be treated with diluted acid, preferably nitric
acid, to reduce the pH to 5 or below, preferably 2 or below. Preferably the
weight ratio HNO3 [65%]:H2O in the diluted nitric acid is
1:2. Typical treatment is for eg 24 hours at about eg 100° C. and autogeneous
pressure. It is notable that during this treatment the proton content increases
while the potassium content decreases. Solids are removed eg by centrifugation
and the recovered material is preferably washed with water. The material
rearranges in this reaction to produce the 2×2 structure which then is dried
(to produce material C). Drying may be for example for 3 hours at 110° C.
The metal cation-containing compounds of the present invention maybe prepared
by treating the octahedral 2×2 manganese oxide structure (material C), for
example in an autoclave at elevated temperature under autogeneous pressure, for
a suitable time with a solution of the metal with which it is to be ion
exchanged. The metal is preferably a transition metal. The compound in solution
is preferably a salt or complex of the metal. This is preferably a (mixed)
metal oxide precursor. The weight ratio of the metal compound eg transition
metal salt(s), and material C can for example range between 0.2 and 5,
depending on the metal source. For certain materials the addition of an
acidifying agent such as nitric acid may be desirable. Material C is then
treated as a slurry in the metal ion solution to effect cation exchange. For
example, the exchange may be at 200° C. for 24 hours under autogeneous
pressure. Thereafter the solids are removed, for example via centrifugation,
and dried, for example for 3 hours at 110° C., to remove substantially all
residual water. For certain materials it can be advantageous to then calcine,
for example for 300° C. and 24 hours, followed by repeated washing steps.
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