Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2001-09-13
2003-02-11
Davis, Brian J. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S473000, C568S474000
Reexamination Certificate
active
06518463
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a catalytic reactor bed arrangement comprising, in a specified distribution, a plurality of catalysts in one or more fixed bed reactors and a process using the same for conversion of methanol to formaldehyde. More particularly, the invention relates to (1) a catalytic reactor bed comprising, in a specified distribution, a first metal-molybdate catalyst and a second metal-molybdate catalyst, provided in one or more fixed bed reactors, and (2) a process using the same for oxidizing methanol to formaldehyde.
2. Description of the Related Art
The formation of formaldehyde involves the dehydrogenation and oxidation of methanol. One approach for converting methanol to formaldehyde involves oxidizing methanol over a silver catalyst. See, for example, U.S. Pat. Nos. 4,080,383; 3,994,977; 3,987,107; 4,584,412; 4,343,954 and 4,343,954. Typically, methanol oxidation to formaldehyde over a silver catalyst is carried out in an oxygen lean environment. One problem associated with silver catalyzed methanol oxidation is methanol leakage i.e., high amounts of unconverted methanol.
Accordingly, improved processes for oxidizing methanol to formaldehyde have been developed. These processes use a methanol/air mixture (e.g., a reactant gas feed stream of methanol, excess air and an inert carrier gas) introduced over an iron-molybdate/molybdenum trioxide catalyst. See, for example, U.S. Pat. Nos. 3,983,073 (conversion of methanol to formaldehyde using Fe
2
(MoO
4
)
3
and MoO
3
having a molar ratio of Mo/Fe from 1.5 to 1.7 and a degree of crystallinity of at least 90%); 3,978,136 (process for the conversion of methanol to formaldehyde with a MoO
3
/Fe
2
O
3
/TiO
2
catalyst wherein the MoO
3
:Fe
2
O
3
weight ratio is between 1:1 to 10:1 and TiO
2
is present between 1 to 90 weight % of total oxides); 3,975,302 (a supported iron oxide and molybdenum trioxide catalyst wherein the atomic ratio of Mo/Fe is from 1.5 to 5); 3,846,341 (a shaped and optionally supported iron molybdate type catalyst having high mechanical strength made by reacting ammonium molybdate and ferric molybdate); 3,716,497 (an optionally shaped iron molybdate type catalyst made by admixing with NH
4
+
A
−
); 4,829,042 (high mechanical strength catalyst of Fe
2
(MoO
4
)
3
and MoO
3
together with non-sintered Fe
2
O
3
); 4,024,074 (interaction product of Fe
2
(MoO
4
)
3
, MoO
3
and bismuth oxide for catalyzing oxidation of methanol to formaldehyde); 4,181,629 (supported catalyst of iron oxide and molybdenum oxide on silica, alumina and the like); 4,421,938 (a supported catalyst of at least two oxides of Mo, Ni, Fe and the like); and 5,217,936 (a catalyst of a monolithic, inert carrier and oxides of molybdenum, iron and the like).
In comparison to the silver catalyzed processes, iron-molybdatelmolybdenum trioxide catalyzed processes produce higher yields of formaldehyde. Iron-molybdate, Fe
2
(MoO
4
)
3
, in combination with molybdenum trioxide, MoO
3
, constitute the metal oxide phases of exemplary commercially available metal oxide catalysts suitable for oxidizing methanol to formaldehyde. During the oxidation of methanol to formaldehyde, the Fe
2
(MoO
4
)
3
MoO
3
catalyst can be generated in situ from physical mixtures of pure molybdenum trioxide, MoO
3
, and ferric oxide, Fe
2
O
3
. See copending patent application designated by attorney docket no. 00242.72876 and Provisional Application No. 60/081,950 entitled “In Situ Formation of Metal Molybdate Catalysts” of Wachs et al., filed on Apr. 15, 1998. The molar ratio MoO
3
/Fe
2
O
3
of these catalysts may be varied. Typically, such catalysts used in industrial and commercial applications contain an excess of MoO
3
. Thus, for example, the molar ratio MoO
3
/Fe
2
O
3
may vary from 1.5/1 to 12/1 or more. Excess MoO
3
is provided to ensure that sufficient amounts of Fe
2
(MoO
4
)
3
are formed in situ (from the mixture of Fe
2
O
3
and MoO
3
) for efficiently oxidizing methanol to formaldehyde in high yields.
Unfortunately, the use of excess MoO
3
in conjunction with Fe
2
O
3
or other metal oxides and/or metal molybdates is problematic. Oxidizing methanol to formaldehyde using a metal molybdate/molybdenum trioxide type catalyst, e.g., Fe
2
(MoO4)
3
/MoO
3
, is a highly exothermic process. The heat released during the oxidation reaction increases the catalyst and/or the fixed bed reactor temperature producing “hot spots” on the catalyst surface. These hot spots reach temperatures high enough to volatilize the Mo/MoO
3
species present within metal molybdate/molybdenum trioxide type catalysts. Thus, Mo/MoO
3
is sublimed from the hot spots so formed.
The sublimed Mo/MoO
3
species migrate downstream (e.g., within an exemplary fixed bed reactor housing the catalyst) towards cooler regions of the fixed bed reactor or the like. Typically, the downstream migration of sublimed Mo/MoO
3
species is facilitated by the incoming flow of the reactant gas feed stream containing, for example, methanol, air, and an optional inert carrier gas fed into the inlet end of a fixed bed reactor. The migrated Mo/MoO
3
species crystallize in the cooler downstream regions of the fixed bed reactor, for example, in the form of MoO
3
crystalline needles. Over time, the needle formation accumulates and ultimately obstructs the flow of the reactant gas feed stream through the fixed bed reactor. Thus, build up of MoO
3
crystals
eedles in the downstream region causes a substantial pressure drop in the reactant gas feed stream flow rate as the reactant gas feed stream is directed downstream. This pressure drop impedes the efficient oxidation of methanol to formaldehyde. See, for example, U.S. Pat. Nos. 3,983,073 (col. 1, lines 35-52); and 4,024,074 (col. 1, lines 60-68); and U.K. Patent No. 1,463,174 (page 1, col. 2, lines 49-59) describing the aforementioned volatility problem.
Often, the MoO
3
needle formation that occurs in the downstream region of the fixed bed reactor is so excessive that the reactor must be shut down, the needles cleaned out, and fresh catalyst charged therein. These steps unnecessarily increase the time, cost, inefficiency and/or complexity of operating a fixed bed reactor or the like for oxidizing methanol to formaldehyde.
Accordingly, there is a need to provide a catalytic reactor bed arrangement comprising, in a specified distribution, a plurality of catalysts within one or more fixed bed reactors and a process using the same that substantially alleviates, and/or eliminates the aforementioned crystallization problems associated with metal molybdate catalysts containing volatile Mo/MoO
3
species.
Further, (1) silver catalysts, (2) supported catalysts such as those containing silicon dioxide, titanium dioxide, non-sintered Fe
2
O
3
, bismuth interaction products, silica, and/or alumina, (3) high surface area solid supported catalysts, (4) catalysts containing zinc, zinc carbonates and/or indium, (5) catalysts on inert carriers of fibrous carrier material such as fibrous sheets of silica or monolithic inert carriers, (6) shaped catalysts, and (7) the like are often prohibitively expensive to use. Accordingly, there remains a need for a catalytic bed reactor arrangement (containing a specified distribution of a plurality of methanol oxidation catalysts) and a method using the same suitable for cost effectively oxidizing methanol to formaldehyde which is free or substantially free of one or more of (1) silicon dioxide, (2) titanium dioxide, (3) non-sintered Fe
2
O
3
, (4) interaction products of Fe
2
(MoO
4
)
3
, and MoO
3
, and bismuth, (5) silica, (6) alumina, (7) supported catalysts, (8) shaped catalysts for increasing mechanical strength, (9) catalysts containing Zn(CO
3
). 3Zn(OH),
2
In(NO
3
)
3
. 3H
2
O or one or more of the compounds listed in U.S. Pat. No. 4,421,938, (10) a fibrous carrier material such as silica, (11) monolithic inert materials or (12) the like.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a catalytic reactor bed arrangement of two o
Briand Laura
Wachs Israel E.
Davis Brian J.
Lehigh University
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