Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Organic component
Reexamination Certificate
1997-09-30
2001-05-01
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Organic component
C423S253000, C502S300000, C502S305000, C502S314000
Reexamination Certificate
active
06224841
ABSTRACT:
The present invention relates to catalytic oxidation, especially of organic compounds to provide destructive decomposition.
Environmental factors have recently come to the fore in relation to the effective operation of industrial chemical plants. It is now essential that emissions from such plants of so-called volatile organic compounds or VOCs which are produced in the waste streams arising from such plants are minimised. Catalytic oxidation in air of VOCs offers a procedure for destroying such compounds. Such a process offers for example advantages over thermal incineration in that operating temperatures and therefore operating costs are lower. Also, using a catalyst imparts greater control over the products of the process.
Catalysts must be found which can suitably destroy VOCs in an efficient manner in destructive oxidation processes. A number of catalysts have in the prior art been proposed for use in this application. These include noble metal catalysts, mainly Pd based and single metal oxide catalysts eg V
2
O
5
, CuO, Co
3
O
4
, Cr
2
O
3
and MnO
2
. Mixed metal oxides have also been evaluated in the prior art and some success has been recorded for example with hopcalite (MnO
2
/CuO), copper chromite, cobalt molybdate and cobalt manganese oxides.
At present it is recognised that catalysts with improved activity and with more resistance to deactivation need to be found.
According to the present invention there is provided a process for the destructive oxidation of one or more organic compounds which contain one or more optionally substituted hydrocarbon groups which comprises the step of carrying out the oxidation in the presence of a catalyst comprising uranium oxide in which the average oxidation state of the uranium is greater than four. The oxide may for example be UO
3
, U
3
O
8
or a material having mixed oxidation states. The term “average oxidation state” refers to the average oxidation state of the “active catalyst”, that is to say, the catalyst as it exists under the conditions of the reactions.
The process according to the present invention is especially suitable for the destruction of VOCs which as noted above are the volatile usually unwanted by-products of chemical reactions, eg compounds present in the waste streams of industrial chemical processes. VOCs include for example straight and branched chain and cyclic alkanes, alkenes and alkynes, aromatic, polyaromatic and fused aromatic compounds and derivatives of all of these compounds which are partially or fully substituted, eg by halogen or other substituent groups, or which contain heteroatoms, bridging groups and the like.
Where the present invention is applied to the oxidative destruction of VOCs several VOCs may be treated together and these may have similar or different functional groups.
In the present invention the oxidation reaction is likely to be heterogeneous using a uranium oxide catalyst in the solid phase, oxygen in the gas phase and the organic compound(s) to be decomposed in either the liquid and/or the gas phase.
It may be desirable in some reactions to carry out the oxidation using oxygen in the presence of steam (eg in concentrations of up to 40 per cent steam by volume). Where air or oxygen is used as the oxidant, the air or oxygen may in any case be present in a mixture with an inert diluent gas such as nitrogen.
The uranium oxide catalyst employed in the destruction of VOCs by the invention can be in a number of different forms, preferably those forms that give the optimum surface area for the reaction. Suitable forms of the catalyst include oxide, eg U
3
O
8
, powder, supported uranium oxide, eg U
3
O
8
(eg silica, alumina or monolith support). Suitable methods of preparation include decomposition of uranium nitrate to the oxide, decomposition of the nitrate to the oxide in the presence of a support (commonly referred to as incipient wetness), oxidation of a thin film of uranium metal, sol-gel precipitation, chemical vapour deposition and plasma deposition. The uranium may comprise natural or depleted material, where depleted material has a
235
U content less than natural uranium.
The uranium oxide employed in the present invention may be in the form of a particulate-containing aerosol, a particulate bed, a collection of pellets or a supported catalyst or other suitable form. Where a supported catalyst is used, the uranium oxide may comprise a thin surface layer formed or absorbed on or embedded in a suitable substrate. The substrate may for example comprise a solid material which is stable at the maximum operational temperatures of the process which will depend upon the particular organic compound(s) to be oxidised but will generally be less than 800° C.
The catalyst may also comprise uranium oxide, preferably U
3
O
8
, and another metal or metal oxide system, examples being sodium and potassium metals and an oxide of one or more of cobalt, molybdenum, vanadium, nickel, copper, chromium, manganese and iron. These dopants may be used to tailor the catalyst for specific compositions of the VOC system and to control the product selectivity.
A mixed oxide catalyst may include two or more oxides in a single lattice (one-phase) mixed oxide and for two or more separate oxides which may be physically rather than chemically combined. Suitable methods of preparation for the mixed oxide catalyst include, decomposition of uranium nitrate and an appropriate soluble metal salt, decomposition of the nitrates in the presence of a suitable support, sol-gel precipitation, chemical vapour deposition and plasma deposition.
The substrate may conveniently comprise a microporous or mesoporous material, where uranium oxide may be held within the pores of the material, distributed across the internal surface area and/or incorporated into the support framework. The substrate material in this example may comprise a zeolite type material. For example, a soluble uranium salt such as the nitrate, may be added to the synthesis gel of suitable zeolite type preparation. One possible outcome of this method of preparation is to form nanoparticles of the oxide supported in a microporous environment. The method may also be used to form the mixed oxide form of the catalyst.
Other suitable solid materials which may or may not be microporous for use as the substrate to support the uranium oxide include ceramic, metal or mixed ceramic/metal materials. Alumina or silica are suitable as oxide substrates. Stainless steel is suitable as a metal substrate.
The uranium oxide may be doped with a known stabiliser such as yttria to stabilise the material in an oxidising environment.
As exemplified below we have found that uranium oxide with uranium of high oxidation state, especially U
3
O
8
, provides a higher activity catalyst which shows good selectivity allowing greater catalyst volumes to be employed at lower cost than prior art catalysts, especially for VOC destruction. Uranium oxide such as U
3
O
8
also shows high resistance to deactivation and can be readily reactivated.
In particular, we have found surprisingly and beneficially that a high yield can be obtained from oxidation reactions even at relatively low temperatures compared with prior art catalysts under otherwise similar conditions. Initial heating to a temperature above a threshold activation temperature may be required to initiate the oxidation reaction but after the reaction has been initiated and sustained the applied environmental temperature may be significantly reduced, without significant reduction in the oxidation conversion yield.
Where the process according to the present invention is a gaseous phase reaction employed to oxidise a VOC, a flow of an appropriate oxidant (source of oxygen) and a flow of a vaporised form of the VOC obtained from heating the compound may be combined prior to or at a reactor containing the uranium oxide catalyst. Examples of appropriate oxidants include air, oxygen, ozone, nitrous oxide, nitric oxide, nitrogen dioxide and hydrogen peroxide. The reactor may be held in a enclosure, eg a furnace, whose temperature can be adj
Hudson Ian
Hutchings Graham John
Taylor Stuart Hamilton
British Nuclear Fuels LC
Dunn Tom
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Nguyen Cam N.
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