WO2002055196A1

WO2002055196A1 - Catalyst capable of selectively oxidizing hydrogen, use thereof and a method of preparing said catalyst

Info

Publication number: WO2002055196A1
Application number: PCT/NL2002/000014
Authority: WO
Inventors: Elbert Arjan De Graaf; Alfred Bliek
Original assignee: Universiteit Van Amsterdam
Priority date: 2001-01-12
Filing date: 2002-01-10
Publication date: 2002-07-18
Also published as: EP1349656A1

Abstract

A catalyst capable of selectively oxidizing hydrogen comprises a solid oxygen carrier that is partially coated with a metal oxide. The metal oxide comprises a metal that is a catalyst for selectively oxidizing hydrogen. The catalyst can be used in the selective oxidation of hydrogen from mixtures of hydrogen, alkanes and alkenes, such as e.g. in the dehydrogenation of alkanes to alkenes. A method of preparing said catalyst, for example via homogeneous deposition precipitation is also disclosed.

Description

Catalyst capable of selectively oxidizing hydrogen, use thereof and a method of preparing said catalyst.

The invention pertains to a catalyst capable of selectively oxidizing hydrogen, comprising a solid oxygen carrier, the outer surface of which comprises a coating of a metal oxide.

Such a catalyst is known in the art, e.g. it has been presented by the applicant in the past at the Netherlands Catalysis & Chemistry Conference 2000 (NCCC) at 17-19 January 2000. With regard to the industrial production of light olefins by the process of steam cracking of alkanes, there is a general and continuous search for catalysts that can provide an alternative route to said process. The latest developments relate to the use of two types of catalysts that both have a distinctive function: i) a dehydrogenation catalyst that accompanies reaction I below, and ii) an oxidation catalyst which takes part in reaction II below. Both reactions are given by the following schemes :

C_nH_2n+2 <-> C„H_2n + H₂ (Reaction I) H₂ ^•+ MO_x -> MO_x__! + H₂0 (Reaction II)

wherein n is an integer, x is real, and

M is a metal, the oxide of which is capable to react with elemental hydrogen according to reaction II.

Typically, a temperature is maintained between 350-850°C at atmospheric pressure for both reactions. Both reactions are carried out either simultaneously in one reactor or consecutively in different reactors .

Reaction I represents the equilibrium limited conversion of alkanes to olefins over a dehydrogenation catalyst (not indicated in the scheme) . It is a main objective to find solutions to drive the equilibrium of this reaction further towards the side of olefins. Such can, according to thermodynamics, be accomplished by removing any of the products of reaction I, from the reaction mixture. A removal of hydrogen by oxidation, also referred to as combustion, offers a solution of high interest and is hereinafter discussed.

Firstly, the removal of hydrogen can be accomplished simply by introducing gaseous oxygen upstream or downstream reaction I, often referred to as the co-fed mode. However, the presence of gaseous oxygen demands extra measures. These measures generally relate to controlling thermal runaway effects as well as the selectivity towards hydrogen oxidation (as opposed to the unwanted oxidation of olefins or alkanes) . In the art it is generally recognized that said selectivity is over-all less controllable when higher conversion levels are to be reached.

As a second way of removing hydrogen, reaction II represents the removal of hydrogen using a metal oxide. Such is accomplished by applying a solid oxygen carrier (SOC) in reaction II, and implementing a cyclic mode of operation, where this SOC is cyclically reduced and reoxidized. In this specification a SOC is defined as a solid compound that has both a high storage and a high release capacity with regard to oxygen. Thus, a SOC can be used for oxidizing hydrogen, thereby excluding the supply of gaseous oxygen. An additional requirement to the SOC when used as a catalyst, is that it can be regenerated by oxidation. Such a regeneration is, for example, carried out in a parallel recycling process. An example of a SOC that is suitable to be used as a catalyst in reaction II is a mixed cerium/zirconium oxide. A drawback of the mixed cerium/zirconium oxide is however, that its selectivity towards the oxidation of hydrogen is low, leading to the oxidation of other constituents of the gas mixture involved in reaction I. Especially, the formation of any oxidation products of light olefins or alkanes is to be avoided. Therefore, the applicant has proposed a SOC with a coating of a metal oxide that is a more selective oxidation catalyst than the SOC itself. A coated SOC as such can thus be construed as a supported catalyst, with the additional feature that the support also contributes to the reaction.

A generally known deleterious effect in the art of supported catalysts is sintering of the supported active metal. Sintering comprises the formation of a metal conglomerate during reduction through clustering of a part of the atoms of the supported active metal, as well as of the support. As a result of this sintering the catalyst is harder to reduce. A loss of active surface area is inferred which makes the catalyst less attractive. This effect of sintering is observed regardless of the inertness or reactivity of the support. The sintering effect has also been observed by the applicant in a SOC with a coating of a more selective oxidation catalyst. It is the object of the invention to provide a SOC capable of selectively oxidizing hydrogen, the outer surface of the SOC comprising a coating of a metal oxide, wherein the above mentioned drawbacks have been reduced or even eliminated.

The solid oxygen carrier according to the invention is characterized in that the outer surface of the solid oxygen carrier is partially coated with the metal oxide.

Surprisingly, it has been found that a high selectivity towards oxidation of hydrogen does not require a complete coating of the outer surface of the SOC. In this context, by complete coating is meant the application of an amount of metal oxide sufficient to form a mono-layer of the metal oxide upon the SOC, i.e. a layer that has a thickness that approximately corresponds to the dimensions of one atom of the deposited metal, which layer fully covers the outer surface of the SOC. Such a complete coating can be calculated a priori based on the weight specific surface area of the SOC and the weight of the metal oxide, which will be further explained hereinafter. A main advantage as a consequence of the partial coating of the outer surface of the SOC is that the stability of the supported metal oxide is increased, resulting in an improved durability and reliability of the catalyst according to the invention when it is employed in the redox mode. Such has been observed by measuring the activity of the catalyst according to the invention after numerous consecutive cycles of oxidation of hydrogen (resulting in a concomitant reduction of the catalyst itself) and a subsequent regeneration by oxidation of the catalyst itself. Data relating to the activity of the catalyst are presented hereinafter. The invention thus addresses the problem of the non-selectivity of a SOC itself, combined with the sintering problems that are related to supported metal oxides. Furthermore, the invention increases the economical value of an oxidation catalyst with respect to the amount of metal oxide necessary and the reliability of the catalyst with respect to its conversion rates.

The outer surface of the solid oxygen carrier is coated with the metal oxide by deposition techniques known in the art, e.g. through impregnation or homogeneous deposition precipitation (HDP) . The deposition technique is chosen so that a physical or chemical bonding is obtained between the metal oxide and the solid oxygen carrier, said bonding being capable of withstanding the working conditions of at least the combustion reaction II, and preferably the dehydrogenation reaction 1 as well.

Numerous metal oxides are known per se which are capable of selectively oxidizing hydrogen, viz. WO 96/33152. These metal oxides comprise Bi, In, Sb, Zn, TI, Pb, and Te. A drawback of these metal oxides is that they are vulnerable towards the reaction temperatures: bismuth oxide, for instance, will melt above 300°C. To establish a more stable oxidation catalyst these metal oxides are supported onto inert supports such as silica, in a ratio of approx. 50% in weight. The inert supports de facto are not capable to function as a SOC that provides oxygen atoms to the oxidation reaction, and do therefore not contribute towards the oxidation process.

Preferably, the catalyst according to the invention comprises a metal oxide that comprises at least one metal selected from the group consisting of Bi, In, Sb, Zn, TI, Pb, and Te. The metals from this group have shown, with regard to their respective oxides, a high activity and selectivity towards the oxidation of hydrogen. With respect to their activity and selectivity, the more preferred metals from this group are Bi, In, Sb, and most preferred is Bi .

Preferably, the catalyst according to the invention comprises a SOC comprising a mixed cerium zirconium oxide. This specific oxide has a particularly high storage and release capacity with regard to atomic oxygen, and stability. Other suitable materials for a SOC are for example, cerium oxide and vanadium (optionally mixed with magnesium) oxide. More preferably, the mixed cerium zirconium oxide has a formula Ce_nZr_(ι-_n)O_x, wherein n ranges from 0.1 to 1, and x ranges from 1.5 to 2. In a particularly preferred embodiment, the mixed cerium zirconium oxide has the formula Ce_nZr(i- _n)O_x wherein n = 0.6 and x = 2.

In a further preferred embodiment of the catalyst according to the invention, the solid oxygen carrier comprises particulate material. For use in a reactor it is beneficial to provide a heterogeneous catalyst in a form so that a high surface area is established. This can be accomplished by using a SOC that comprises particulate material, and more specifically, a high specific surface. More preferably, the solid oxygen carrier has a specific surface ranging from 4 to 200 m²/g. In a particularly preferred embodiment, the specific surface ranges from 100 to 120 m²/g.

In another preferred embodiment of the catalyst according to the invention, the solid oxygen carrier is coated by 0.6 to less than 2.6 mg of the metal oxide per square meter of the carrier surface in the case of bismuth oxide. Within this range, the catalyst shows on the one hand its capability to selectively oxidize hydrogen whilst on the other hand the stability over consecutive redox cycles is high with regard to the oxygen capacity towards storage and release. In this respect an even more preferred embodiment of the catalyst comprises a coating of 0,6 to 1,0 mg of the metal oxide per square meter of the carrier surface.

Preferably, the catalyst according to the invention has less than 50% of the surface of the solid oxygen carrier coated by the metal oxide. Within this range of coated surface of the SOC, the catalyst shows on the one hand its capability to selectively oxidize hydrogen whilst on the other hand the stability over consecutive redox cycles is high with regard to the oxygen capacity. Even more preferably, the surface of the solid oxygen carrier coated by the metal oxide ranges from 20-30%.

The invention also relates to the use of the catalyst according to the invention for selectively oxidizing hydrogen from a hydrogen containing gaseous medium. When the catalyst is applied in this way, an alternative to the co-fed mode is provided that includes a catalyst having a high oxygen capacity, stability and selectivity towards the oxidation of hydrogen, whilst the loading of the metal oxide coating is relatively small. With respect to its use, a more durable, more reliable and more viable process is provided.

In a particularly preferred embodiment of the use of the catalyst according to the invention, the catalyst is used for selectively oxidizing hydrogen from a hydrogen containing gaseous medium that comprises an alkane-alkene mixture. When the catalyst is applied in this way, the conversion of alkanes to alkenes will be shifted considerably towards the side of formation of alkenes, which is beneficial to the industrial production of alkenes or light olefins .

Preferably, the use of the catalyst according to the invention relates to selectively oxidizing hydrogen at temperatures around 350- 850°C. Within this temperature range the catalyst has shown to be both stable and active. Moreover, this temperature range is also typical to the catalyzed conversion of alkanes to alkenes, so that both dehydrogenation and hydrogen oxidation (reactions I and II vide supra) can be performed in one reactor, or can be performed consecutively without the need of taking measures to adjust temperatures.

The invention also relates to a method of preparing a catalyst, capable of selectively oxidizing hydrogen, said method comprising a step of depositing a metal oxide or precursor thereof on the outer surface of a solid oxygen carrier, wherein the deposition step is carried out to an extent such that the outer surface of the solid oxygen carrier is partially coated by the metal oxide or a precursor thereof. By depositing a metal oxide partially on the outer surface of the SOC, a process of preparing a catalyst is provided, wherein the consumption of metal oxide or a precursor thereof, is relatively low.

Preferably, the method according to the invention comprises a deposition step that comprises a- homogeneous deposition precipitation (HDP) . By using HDP a high degree of spreading of the metal oxide over the outer surface of the SOC can be accomplished, which is beneficial with regard to the amount of metal oxide needed for the catalyst with regard to its stability and selectivity. In this way, a process of preparing a catalyst is provided that further reduces the consumption of metal oxide or a precursor thereof.

The following examples are provided as a means of illustrating the oxidation catalyst according to the present invention. These examples are not be construed as imposing a limitation to the present invention. Additionally, the present invention will be further elucidated by the accompanying drawing, wherein: Fig. 1 shows the results referred to in Example 2 of measurements of catalytic activity of some catalysts used according to the invention.

Example 1

Preparation of a mixed cerium zirconium oxide Ce_0.6Zr₀.₄θ₂ coated with Bi₂0₃

On a cerium zirconium mixed oxide, Ce_0.6 ro./i0, various coatings have been applied. The fresh mixed oxides were coated with appropriate amounts of bismuth (Bi (N0₃) ₃.5H₂0, Aldrich, 98% pure, A.C.S. reagent) via homogeneous deposition precipitation (HDP). Two grams of cerium zirconium oxide and the appropriate amount of salts are added together with 5 grams of urea to 1 litre of water which is heated to 90 °C and kept for 24 hours at this temperature. The precipitate is filtered from the slurry and subsequently calcined in air at 600°C during 5 hours.

BET areas of the prepared materials as determined by nitrogen physisorption using a Sorptomatic 1990 (CE Instruments) are listed in table l.The following compositions listed in table 1 below were prepared.

Table 1

Composition, BET surface area and pore volume of the synthesized materials

Material composition BET area pore volume

( Vgram) (ml/c [ram

CeZr Ceo .6Z o . O₂ 114 0.21

CeZrBi(6.5) -t-Bi 6 . 5 wt% 107 0.19 CeZrBi(12) +Bi 12 wt% 92 0.18 CeZrBI (21) +Bi 21 wt% 76 0.14

It is hereby further explained that a mono-layer is calculated a priori assuming that approximately 25% in weight of bismuth oxide is at least needed to establish a complete coating of a mixed cerium/zirconium oxide having a specific surface of 100 m²/g. From this calculation it can be derived that, for example, CeZrBi(6.5) has 25% of the surface of the solid oxygen carrier coated by the bismuth oxide.

Example 2

Cyclic reduction/oxidation

All samples are tested under cyclic reducing/oxidizing atmospheres at a fixed temperature of 600°C and atmospheric pressure. To this end in a reactor tube 100 mg sample was mounted. The sample was oxidized for 1 minute in an oxygen (Praxair 2.5, dried over a molsieve) - helium (Praxair 4.6) gas flow (30 mln/min with an oxygen content of 33.3 vol%) . After this oxidation step the reactor was purged with helium for 2 minutes (flow 20 mln/min) . The gas composition in the reduction step was made up of 20 vol% hydrogen (Praxair 5.0, purified by a BTS column followed by a molsieve), 20 vol% ethane (Ucar 2.3), 20 vol% ethene (Praxair 2.7) and balance helium (flow 50 mln/min) . The sample was exposed to this reducing atmosphere for 2 minutes. Following the reduction step of the redox cycle the reactor was again purged with helium. The product gas was monitored continuously using both a MS and GC. The mixture was analyzed by MS at a frequency of. 0.33 s^"1 by measuring the signals corresponding to m/e = 2, 4, 15, 16, 18, 26, 28, 30, 40 and 44 (in an experiment with pure cerium zirconium oxide all signals m/e ranging from 1 to 100 were scanned, demonstrating that no signals with m/e > 44 were present) . In the oxygen flow a trace of argon appeared to be present. The constant ratio between argon and oxygen was used to calculate the oxygen uptakes. After the sampling point of the MS 1 mln/min N₂ (Hoekloos 2.^""5) is added to the product gas stream as internal standard for the GC analysis. The gas stream is dried using a membrane prior to being sent to the GC column. The GC is equipped with a 16-way valve to enable fast sampling rates. The gas samples are analyzed on an Unibead IS column (Alltech) at 50°C. Hydrogen, nitrogen/oxygen, carbon monoxide, methane, ethane, carbon dioxide and ethene can thus be determined (in this order) .

By cyclic reduction (by a hydrogen, ethane, ethene and helium mixture) and reoxidation (by a helium oxygen mixture) the conditions in a cyclic oxidative dehydrogenation process are mimicked. Following samples have been investigated under cyclic reducing and oxidizing conditions: CeZr, CeZrBi(6.5) and CeZrBi (21) .

The CeZrBi (6.5) exhibits good selectivity in the hydrogen oxidation in a mixture with ethene/ethane. The oxygen uptake of this sample has reached a stable value after a limited number of cycles as depicted in Fig. 1. The selectivity (ratio between the amount of oxidized hydrogen and the total amount of oxidized feedstock (hydrogen and hydrocarbons) in the hydrogen combustion is still remarkably good, i.e. > 99%. Figure 1 shows the respective oxygen uptake values per cycle for three different compositions that were prepared according to Example 1.

Claims

C L A I S

I. Catalyst capable of selectively oxidizing hydrogen, comprising a solid oxygen carrier, the outer surface of which comprises a coating of a metal oxide,- characterized in that the outer surface of the solid oxygen carrier is partially coated with the metal oxide. 2. Catalyst according to claim 1, characterized in that the metal oxide comprises at least one metal selected from the group consisting of Bi, In, Sb, Zn, TI, Pb, and Te .

3. Catalyst according to any of the preceding claims, characterized in that the metal oxide comprises Bi as a metal. 4. Catalyst according to any of the preceding claims, characterized in that the solid oxygen carrier comprises a mixed cerium zirconium oxide.

5. Catalyst according to any of the preceding claims, characterized in that the solid oxygen carrier comprises a mixed cerium zirconium oxide of the formula Ce_nZr₍ι__n)O_x, wherein n ranges from 0.1 to 1, and x ranges from 1.5 to 2.

6. Catalyst according to any of the preceding claims, characterized in that the solid oxygen carrier comprises particulate material . 7. Catalyst according to any of the preceding claims, characterized in that the solid oxygen carrier has a specific surface ranging from 4 to 200 m²/g.

8. Catalyst according to any of the preceding claims, characterized in that the solid oxygen carrier is coated by 0.6 to less than 2,6 mg of bismuth oxide per square meter of the carrier surface.

9. Catalyst according to any of the preceding claims, characterized in that less than 50% of the surface of the solid oxygen carrier is coated by the metal oxide. 10. Use of the catalyst according to any of the preceding claims for selectively oxidizing hydrogen from a hydrogen containing gaseous medium.

II. Use of the catalyst according to claim 10, characterized in that the gaseous medium comprises an alkane-alkene mixture. 12. Use of the catalyst according to claim 10 or 11, characterized in that the oxidation is carried out at 350-850°C. 13. Method of preparing a catalyst, capable of selectively oxidizing hydrogen, said method comprising a step of depositing a metal oxide or precursor thereof on the outer surface of a solid oxygen carrier, characterized in that the deposition step is carried out to an extent such that the outer surface of the solid oxygen carrier is partially coated by the metal oxide or a precursor thereof.

14. Method according to claim 13, characterized in that the deposition step comprises a homogeneous deposition precipitation.