Catalyst beds for non-steady state processes

Details Catalyst beds for non-steady state processes Catalyst beds for non-steady state processes

1996-09-10

2001-04-03

Bell, Mark L. (Department: 1755)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Nitrogen compound containing

C502S224000, C502S232000, C502S243000, C502S258000, C502S439000, C502S527110, C502S527120, C502S527130, C502S527140, C502S527150, C428S403000, C428S404000, C428S570000

Reexamination Certificate

active

06211113

ABSTRACT:

The present invention relates to coated catalysts for non-steady state, heterogeneously catalyzed processes which are composed of a support, an intermediate layer and generally at least one active layer.
Non-steady state process procedures have been described for a series of reactions, for example for the oxidation of alkanes to give aldehydes, carboxylic acids or carboxylic anhydrides, for the oxidative dehydrogenation of alkanes or alkylaromatics or for the oxidative coupling of methane. In these reactions, the catalyst functions as an oxygen reservoir which is reduced during the actual oxidation reaction and is oxidized in a separate step. Non-steady state processes are also known for the preparation of Cl
2
from HCl (Deacon process) in which the catalyst is chlorinated in a first step with liberation of water (“loading”) and then is oxidized in a second step with release of Cl
2
(“stripping”). The individual reaction stages can be carried out separated in time (e.g. by switching the material flows in a fixed-bed reactor) or in space (e.g. by discharging the catalyst in a circulating fluidized-bed reactor or riser reactor).
EP-A-283 198 discloses a process for preparing chlorine by reaction of HCl and oxygen in the presence of a chromium oxide catalyst, where the reactor is lined with a material based on an oxide, nitride or carbide of the elements B, Al, Si, Ti, Zr or Cr. The lining prevents corrosion of the reactor and poisoning of the catalyst by wall material.
WO-A-85/02557 discloses an inert catalyst support having a porous intermediate layer, its porosity being in the micrometer range, which is applied by physical gas-phase deposition and comprises an oxide, nitride or carbide of the elements B, Si or Al. The intermediate layer serves to improve the adhesion of the active layer to the support. Owing to the described porosity of the intermediate layer, no advantages of these catalysts are to be expected for non-steady state reaction procedures.
Known, commercially available catalyst supports, for example those based on Al
2
O
3
or SiO
2
, have a strong interaction with starting materials or products because of their acid surface. Therefore, even in the case of micropore-free supports, undesired adsorption of the corresponding molecules on the surface of the support can take place to an increased extent. Thus, the support materials mentioned show, for example in the non-steady state Deacon process, a high HCl uptake which makes uneconomically long flushing phases between the individual cycles necessary. Shortening of this flushing time leads to a less pure product stream and necessitates more complicated work-up. Besides the uptake of the starting materials on the catalyst support, the uptake on the inner wall of the reactor or the reactor pipes can also lead to undesired contamination of the product stream.
It is an object of the present invention to develop a supported catalyst suitable for non-steady state procedures and to provide a solution to the abovementioned disadvantages, in particular with low uptake capacity for the starting materials or products.
We have found that this object is achieved by means of coated catalysts for non-steady state, heterogeneously catalyzed processes comprising a catalyst support, an intermediate layer and optionally at least one active layer, wherein between the support and optionally one or more active layers there is located an intermediate layer comprising a nitride, oxide, carbide or chloride of a metal or a nonmetal or a mixture thereof, and having a dense, pore-free microstructure and a nonpolar surface having a very low density of acid centers.
The coated catalysts of the present invention have a core (support) and a protective or intermediate layer, preferably an intermediate layer, which has a high barrier action to diffusion and a low adsorption of the starting materials or products. The intermediate layer of the present invention comprises at least one nitride, oxide, carbide or chloride of a metal such as Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rn, Pd, Ag, Cd, In, Sn, Sb, Tc, Hf, Ta, W, Re, Os, Ir, Pt, Au, Pb, Bi, preferably Al, Ti, Cr, Zr, Ag, Ta, or a nonmetal such as boron, silicon, germanium, preferably silicon.
The more precise selection of the intermediate layer is determined by the boundary conditions of the respective non-steady state process. Examples of processes which can be carried out in a non-steady state manner are the Deacon process and oxidation reactions. The non-steady state processes can be carried out at from 50 to 600° C., preferably from 100 to 550° C., particularly preferably from 150 to 520° C., and at a pressure of from 0.1 to 100 bar, preferably from 0.5 to 80 bar, particularly preferably from 1 to 70 bar.
In the non-steady state Deacon process, suitable intermediate layers are those comprising nitrides, highly calcined oxides (e.g. SiO
2
), silver chloride or mixtures thereof, preferably Si
3
N
4
, silver chloride or mixtures thereof, particularly preferably Si
3
N
4
.
The thickness of the intermediate layer is generally from 1 to 10,000 nm, preferably from 5 to 100 nm. The minimum thickness depends on the desirability of the support being covered completely and without pores by the intermediate layer.
The intermediate layers of the present invention can be prepared by PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition). Possible PVD processes are evaporation, cathode atomization or electric arc coating (cf. R. F. Bhunshah et al., “Deposition technologies for Films and Coatings”, Noyes Publications, 1982). A possible CVD process is plasma-assisted gas-phase deposition (cf. H. Yasuda, “Plasma Polymerization”, Academic Press, 1985). The intermediate layer can preferably be deposited by plasma-assisted production processes, since these generally lead to dense, pore-free layers having a high barrier action to diffusion.
The deposition processes can be carried out by reactive or nonreactive means in a manner known per se. In the case of reactive preparation, the metal or nonmetal is deposited in conjunction with a reactive gas (e.g. O
2
, N
2
, hydrocarbons, Cl
2
), with the corresponding oxides, nitrides, carbides or chlorides being obtained directly. In the case of nonreactive deposition, the metal or nonmetal is deposited as a thin layer in a first step and is, in a separate subsequent step, oxidized, nitrided, carburized or chlorinated.
Suitable supports for the catalysts of the present invention are, for example, shaped bodies or monolithic packings comprising ceramics, metals or plastics. Possible materials are, for example, glass, fused quartz, aluminum oxide, silicon nitride, titanium dioxide, aluminosilicates, magnesium silicates, metals or carbon. Suitable shaped bodies are, inter alia, pellets, extrudates or granules, with the support being able to have a spherical, cylindrical, ring-shaped, star-shaped, wagon wheel shaped or other suitable geometry. Suitable packings are the reactor packings known per se having static mixing properties (Chemical Engineering, June 1994, pages 76-82). It is possible to use either pore-free or pore-containing supports. Preference is given to using pore-free supports or micropore-free supports having a pore size which can be set in the mesopore and macropore range. Other suitable supports are, in particular, low-cost standard catalyst supports whose adsorption behavior without coating would be unsuitable for a non-steady state production process for the abovementioned reasons, for example supports based on Al
2
O
3
or SiO
2
, preferably supports of glass.
The application of the catalytically active layer to the support provided with the intermediate layer can generally be carried out by known wet chemical or powder metallurgical coating processes such as impregnation, spray impregnation, high-coating, flame or plasma spray, or a PVD or CVD process.
When using the coated catalysts of the present invention in non-steady state processes it is advantageous to also coat the walls and the feed lines of the reactor with a low-

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