CATALYSTS FOR THE REFORMING OF GASEOUS MIXTURES

Abstract

Pyrochlore-based solid mixed oxide materials suitable for use in catalysing a hydrocarbon reforming reaction are disclosed, as well as methods of preparing the materials, and their uses in hydrocarbon reforming processes. The materials contain a catalytic quantity of inexpensive nickel and exhibit catalytic properties in dry reforming reactions that are comparableif not betterthan those observed using expensive noble metal-containing catalysts. Moreover, the Pyrochlore-based solid mixed oxide materials can be used in low temperature dry reforming reactions, where other catalysts would become deactivated due to coking. Accordingly, the catalytic materials represent a sizeable development in the industrial-scale reforming of hydrocarbons.

Claims

1. A solid mixed oxide material suitable for use in catalysing a methane dry reforming reaction, wherein the solid mixed oxide material comprises a first crystalline phase, the first crystalline phase being attributable to a pyrochlore crystal structure, and wherein the solid mixed oxide material comprises 3.5-25.0% of nickel by weight relative to the total weight of the solid mixed oxide material.

2. The solid mixed oxide material of claim 1, wherein the solid mixed oxide material comprises 5.0-25.0% of nickel by weight relative to the total weight of the solid mixed oxide material.

3. The solid mixed oxide material of claim 1 or 2, wherein the solid mixed oxide material comprises 7.5-20.0% of nickel by weight relative to the total weight of the solid mixed oxide material.

4. The solid mixed oxide material of any one of claim 1, 2 or 3, wherein the solid mixed oxide material comprises 9.0-15.0% of nickel by weight relative to the total weight of the solid mixed oxide material.

5. The solid mixed oxide material of any preceding claim, wherein the first crystalline phase has a composition according to general formula (I) shown below
A.sub.2B.sub.2O.sub.7 (I) wherein A is at least one trivalent cation of an element selected from the group consisting of La, Ce, Pr, Nd, Sm, Sc, Y and Eu; and B is a mixture of i. at least one tetravalent or trivalent cation of an element selected from the group consisting of Zr, Ti, Cr, Mn and Mo, and ii. a divalent cation of Ni.

6. The solid mixed oxide material of claim 5, wherein A is a trivalent cation of La, and optionally one or more other trivalent cations of an element selected from the group consisting of Ce, Pr, Nd, Sm, Sc, Y and Eu; and B is a mixture of i. a tetravalent cation of Zr, and optionally one or more other tetravalent or trivalent cations of an element selected from the group consisting of Ti, Cr, Mn and Mo, and ii. a divalent cation of Ni

7. The solid mixed oxide material of claim 5 or 6, wherein A is a trivalent cation of La and B is a mixture of a divalent cation of Ni and a tetravalent cation of Zr.

8. The solid mixed oxide material of any preceding claim, wherein the solid mixed oxide material comprises a second crystalline phase, the second crystalline phase being attributable to a Ruddlesden-Popper crystal structure.

9. The solid mixed oxide material of claim 8, wherein the second crystalline phase has a composition according to general formula (II) shown below
A.sub.2BO.sub.4 (II) wherein A is at least one trivalent cation of an element selected from the group consisting of La, Ce, Pr, Nd, Sm, Sc, Y and Eu; and B is a divalent cation of Ni, and optionally one or more other divalent, trivalent or tetravalent cations of an element selected from the group consisting of Fe, Co, Cu, Ti and Zr.

10. The solid mixed oxide material of claim 9, wherein A is a trivalent cation of La and B is a divalent cation of Ni.

11. The solid mixed oxide material of any preceding claim, wherein the solid mixed oxide material further comprises 0.001-0.5% of at least one promoter by weight relative to the total weight of the solid mixed oxide material, and wherein the at least one promoter is selected from the group consisting of Sn, Ba, Ca, Mg, Ce, Sr, K, Pt, Rh, Pd, Mo, Ag, Au, Ru, Zn, Cu, Co and Ir.

12. The solid mixed oxide material of any preceding claim, wherein the solid mixed oxide material comprises 15.0-35.0% of zirconium by weight relative to the total weight of the solid mixed oxide material.

13. The solid mixed oxide material of any preceding claim, wherein the solid mixed oxide material comprises 48.0-60.0% of lanthanum by weight relative to the total weight of the solid mixed oxide material.

14. The solid mixed oxide material of any preceding claim, wherein the surface area of the solid mixed oxide material is 5-25 m.sup.2/g.

15. The solid mixed oxide material of any preceding claim, wherein the pore volume of the solid mixed oxide material is 0.02-0.2 cm.sup.3/g.

16. The solid mixed oxide material of any preceding claim, wherein the average pore size of the solid mixed oxide material is 2-10 nm.

17. The solid mixed oxide material of any preceding claim, wherein the solid mixed oxide material is in the form of a powder, pellet or foam.

18. The solid mixed oxide material of any preceding claim, wherein the solid mixed oxide material is self-supported.

19. A process for the preparation of a solid mixed oxide material as claimed in any preceding claim, said process comprising the steps of: a) providing a mixture comprising i. at least one solvent; ii. metal precursors, the respective amounts of the metal precursors being sufficient to form a pyrochlore crystalline phase in the solid mixed oxide material resulting from step c), and iii. at least one chelating agent; b) drying the mixture of step a); and c) thermally treating the solid material resulting from step b) at a temperature greater than 800 C., wherein at least one of the metal precursors mixed in step a) is a nickel precursor in an amount sufficient to provide a nickel content in the solid mixed oxide material resulting from step c) of 3.5-25.0% by weight relative to the total weight of the solid mixed oxide material.

20. The process of claim 19, wherein the at least one solvent is selected from the group consisting of water, methanol, ethanol and acetone.

21. The process of claim 19 or 20, wherein the at least one solvent is water.

22. The process of claim 19, 20 or 21, wherein the at least one chelating agent is selected from the group consisting of citric acid, EDTA, disodium EDTA, trisodium EDTA, EGTA and succinic acid.

23. The process of any one of claims 19 to 22, wherein the at least one chelating agent is citric acid.

24. The process of any one of claims 19 to 23, wherein the mixture of step a) comprises at least one chelating agent in an amount sufficient to give a molar ratio of total chelating agent to metal in the mixture of (0.3-1.0):1.

25. The process of any one of claims 19 to 24, wherein the mixture of step a) comprises at least one chelating agent in an amount sufficient to give a molar ratio of total chelating agent to metal in the mixture of (0.45-0.75):1.

26. The process of any one of claims 19 to 25 wherein step c) comprises thermally treating the solid material resulting from step b) at a temperature of 800-1500 C.

27. The process of any one of claims 19 to 26, wherein step c) comprises thermally treating the solid material resulting from step b) at a temperature of 900-1100 C.

28. The process of any one of claims 19 to 27, wherein step c) is performed for 4-24 hours.

29. The process of any one of claims 19 to 28, wherein step c) is performed for 6-10 hours.

30. The process of any one of claims 19 to 29, wherein prior to step c), the solid material resulting from step b) is milled or crushed.

31. The process of any one of claims 19 to 30, wherein step b) comprises drying the mixture of step a) at a temperature of 60-150 C. in air.

32. The process of any one of claims 19 to 31, wherein the metal precursors are selected from metal oxides and metal nitrates.

33. The process of any one of claims 19 to 32, wherein the mixture provided in step a) further comprises: iv. at least one Sn, Ba, Ca, Mg, Ce, Sr, K, Pt, Rh, Pd, Mo, Ag, Au, Ru, Zn, Cu, Co or Ir-based promoter precursor in an amount sufficient to provide a promoter content in the solid mixed oxide material resulting from step c) of 0.001-0.5% by weight relative to the total weight of the solid mixed oxide material.

34. A reduced or partially-reduced solid mixed oxide material, wherein the reduced or partially-reduced solid mixed oxide material is a reduced or partially-reduced form of the solid mixed oxide material as claimed in any one of claims 1 to 18.

35. A process for the preparation of a reduced or partially-reduced solid mixed oxide material as claimed in claim 34, the process comprising the step of: a) reducing or partially-reducing the solid mixed oxide material as claimed in any one of claims 1 to 18.

36. A solid mixed oxide material obtainable by the process of any one of claims 19 to 33.

37. A reduced or partially-reduced solid mixed oxide material obtainable by the process of claim 35.

38. A process for catalytically reforming a gaseous mixture, said process comprising the step of: a) contacting a gaseous mixture comprising CO.sub.2 and CH.sub.4 with either or both of: i. a solid mixed oxide material as claimed in any one of claims 1 to 18 and 36, and ii. a reduced or partially-reduced solid mixed oxide material as claimed in claim 34 or 37, wherein step a) is conducted at a temperature of 500-1000 C.

39. The process of claim 38, wherein step a) is conducted at a temperature of 550-850 C.

40. The process of claim 38 or 39, wherein step a) is conducted at a temperature of 550-800 C.

41. The process of any one of claims 38, 39 and 40, wherein step a) is conducted at a space velocity (WHSV) of 10-120 Lg.sup.1 h.sup.1.

42. The process of any one of claims 38 to 41, wherein step a) is conducted at a space velocity (WHSV) of 10-40 Lg.sup.1 h.sup.1.

43. The process of any one of claims 38 to 42, wherein step a) is conducted at a space velocity (WHSV) of 10-35 Lg.sup.1 h.sup.1.

44. The process of any one of claims 38 to 43, wherein the process is a dry reforming, bi-reforming or tri-reforming process, or a combination of two or more thereof.

45. The process of any one of claims 38 to 44, wherein the process is a dry reforming process.

46. The process of any one of claims 38 to 45, wherein the process is conducted according to a fixed bed regime.

Description

EXAMPLES

[0115] One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying figures, in which:

[0116] FIG. 1 shows the X-ray diffraction patterns of the as-prepared LaZrO, LNZ2, LNZ5 and LNZ10 samples.

[0117] FIG. 2 shows the X-ray diffraction pattern of the reduced LNZ10 sample.

[0118] FIG. 3 shows the Raman spectra of the LNZ2, LNZ5 and LNZ10 samples.

[0119] FIG. 4 shows SEM images of the LNZ2 (left), LNZ5 (middle) and LNZ10 (right) samples.

[0120] FIG. 5 shows the TPR patterns of the LaZrO, LNZ2, LNZ5 and LNZ10 samples.

[0121] FIG. 6 shows the N.sub.2 adsorption-desorption isotherms of the LaZrO, LNZ2, LNZ5 and LNZ10 samples.

[0122] FIG. 7 shows the influence of Ni metal loading (0, 2, 5, 10 wt. % Ni) on catalytic activity and stability. (a) CH.sub.4 conversion; (b) CO.sub.2 conversion; (c) H.sub.2/CO ratio. Reaction conditions: P=1 atm, CH.sub.4/CO.sub.2=1, T=650 C., WHSV=30000 mLg.sup.1 h.sup.1.

[0123] FIG. 8 shows the influence of temperature on catalytic activity and stability for the 10 wt. % Ni based catalyst LNZ10. (a) CH.sub.4 conversion; (b) CO.sub.2 conversion; (c) H.sub.2/CO ratio. Reaction conditions: P=1 atm; CH.sub.4/CO.sub.2=1; T=600 C., 650 C., 700 C.; WHSV=30000 mLg.sup.1 h.sup.1.

[0124] FIG. 9 shows the influence of space velocity on catalytic activity and stability for the 10 wt. % Ni based catalyst LNZ10. (a) CH.sub.4 conversion; (b) CO.sub.2 conversion; (c) H.sub.2/CO ratio. Reaction conditions: P=1 atm; CH.sub.4/CO.sub.2=1; T=700 C.; WHSV=15000, 30000 and 60000 mLg.sup.1 h.sup.1.

[0125] FIG. 10 shows the long-term stability test for the 10 wt. % Ni based catalyst LNZ10. Reaction conditions: P=1 atm, CH.sub.4/CO.sub.2=1, T=700 C., WHSV=30000 mLg.sup.1 h.sup.1.

EXAMPLE 1SYNTHESIS OF SOLID MIXED OXIDE MATERIALS

General Synthesis

[0126] The general protocol for preparing the pyrochlore-based solid mixed oxide materials is as follows: The precursors used for La, Ni, and Zr were lanthanum nitrate [La(NO.sub.3).sub.3-6H.sub.2O], nickel nitrate [Ni(NO.sub.3).sub.2-6H.sub.2O], and zirconyl nitrate [ZrO(NO.sub.3).sub.2-6H.sub.2O], respectively. The necessary amount of nitrate salts were separately dissolved in deionized water and then mixed with a citric acid (CA) solution in a molar ratio of CA:metal=0.6:1. The solution was stirred for 10 min and concentrated in the rotary evaporator. The resulting mixture was transferred into a petri dish and dried at 100 C. under air overnight. The nitrate precursors started to decompose which was evident by NOx release. The resulting material was then crushed into a fine powder and calcined at 1000 C. for 8 h.

[0127] The pure pyrochlore La.sub.2Zr.sub.2O.sub.7 was prepared and labeled LaZrO for simplicity. A series of Ni-containing pyrochlore-based solid mixed oxide materials were then prepared by substitution of Zr with Ni to give materials with 2, 5 and 10 wt % theoretical loading of Ni, labeled respectively as LNZ2, LNZ5 and LNZ10. The respective amounts of the metal precursors used in the preparation of 1 g of each sample are outlined below:

LaZrO (0 wt % Ni) (Reference Example)

[0128] [La(NO.sub.3).sub.3-6H.sub.2O]-1.51 g
[ZrO(NO.sub.3).sub.2-6H.sub.2O]-1.19 g

LNZ2 (2 wt % Ni) (Reference Example)

[0129] [La(NO.sub.3).sub.3-6H.sub.2O]-1.53 g
[ZrO(NO.sub.3).sub.2-6H.sub.2O]-1.08 g
[Ni(NO.sub.3).sub.2-6H.sub.2O]-0.10 g

LNZ5 (5 wt % Ni)

[0130] [La(NO.sub.3).sub.3-6H.sub.2O]-1.55 g
[ZrO(NO.sub.3).sub.2-6H.sub.2O]-0.93 g
[Ni(NO.sub.3).sub.2-6H.sub.2O]-0.25 g

LNZ10 (10 wt % Ni)

[0131] [La(NO.sub.3).sub.3-6H.sub.2O]-1.60 g
[ZrO(NO.sub.3).sub.2-6H.sub.2O]-0.68 g
[Ni(NO.sub.3).sub.2-6H.sub.2O]-0.49 g

EXAMPLE 2CHARACTERISATION OF SOLID MIXED OXIDE MATERIALS

X-Ray Diffraction Analysis

[0132] FIG. 1 shows the X-ray diffraction pattern for LaZrO, LNZ2, LNZ5 and LNZ10. The pyrochlore crystalline phase (La.sub.2Zr.sub.2O.sub.7) is clearly present in all samples. At higher loadings of Ni (e.g. LNZ10), a second crystalline phase attributable to the Ruddlesden-Popper crystal structure is present.

[0133] FIG. 2 shows the X-ray diffraction pattern for reduced LNZ10. It is clear that both the pyrochlore crystalline phase and the Ruddlesden-Popper crystalline phase remain in the reduced sample.

Raman Spectroscopy

[0134] FIG. 3 shows the Raman spectra of LNZ2, LNZ5 and LNZ10. It is clear that the Raman bands are sharp and ordered, indicating the presence of an ordered pyrochlore structure, rather than a disordered fluorite structure. The first intense peak corresponds to the E.sub.g internal LaO stretching mode, with the other two peaks corresponding to the T.sub.2g modes of pyrochlore.

Scanning Electron Microscopy

[0135] FIG. 4 shows SEM images of LNZ2, LNZ5 and LNZ10. The porous morphology of the samples is clear from the images.

Energy-Dispersive X-Ray Analysis

[0136] EDX analysis was carried out during SEM experiments. Table 1 below shows the chemical composition of the LNZ2, LNZ5 and LNZ10 samples obtained from EDX analysis. It is clear from the table that the actual Ni loadings are rather close to the nominal values, thereby corroborating the successful synthesis method.

TABLE-US-00001 TABLE 1 Chemical composition of LNZ2, LNZ5 and LNZ10 as determined by EDX analysis wt % O wt % Ni wt % Zr wt % La LNZ2 14.6 +/ 0.1 3.1 +/ 0.3 29.4 +/ 0.2 52.5 +/ 0.3 LNZ5 12.3 +/ 0.1 6.6 +/ 0.3 25.4 +/ 0.1 55.6 +/ 0.3 LNZ10 15.5 +/ 0.1 11.6 +/ 0.3 20.4 +/ 0.1 52.5 +/ 0.3

Temperature Programmed Reduction Analysis

[0137] TPR analysis was used to record the temperature at which the samples are reduced (consuming hydrogen). The analysis (see FIG. 5) shows that the support LaZrO is hardly reducible, therefore the peaks of hydrogen consumption for LNZ2, LNZ5 and LNZ10 are mainly attributed to the reduction of nickel only.

[0138] The TPR pattern in FIG. 5 shows that for LNZ2 and LNZ5, the TPR profile is very similar. However, for LNZ10, an extra peak is visible, which indicates the presence of a different structure containing nickel. This result appears to corroborate the XRD analysis indicating that LNZ10 contains both pyrochlore and Ruddlesden-Popper crystalline phases.

Textural Properties

[0139] FIG. 6 shows the nitrogen adsorption-desorption isotherms for the as-prepared LNZ2, LNZ5 and LNZ10 samples. The isotherms can be categorised as type IV, which is characteristic of mesoporous materials.

[0140] Table 2 below outlines the textural properties of the LaZrO, LNZ2, LNZ5 and LNZ10 samples.

TABLE-US-00002 TABLE 2 Textural properties of LaZrO, LNZ2, LNZ5 and LNZ10 Surface area Pore volume Average pore size Sample (m.sup.2/g) (cm.sup.3/g) (nm) LaZrO 13 0.110 3.05 LNZ2 7 0.057 3.41 LNZ5 9 0.046 4.31 LNZ10 11 0.091 4.31

EXAMPLE 3CATALYTIC ACTIVITY TESTS

General Protocol

[0141] The catalytic activity tests were carried out in a inch continuous flow quartz reactor, at a pressure of 1 atmosphere and a CH.sub.4/CO.sub.2 ratio of 1. Prior to the reaction the samples were activated in H.sub.2/He during 1 h at 700 C.

Nickel Loading Effect

[0142] FIG. 7 shows the influence of metal loading (0-10 wt. %) on the catalytic activity and the stability of the reduced samples at 650 C. The activity is expressed in terms of CH.sub.4 conversion (FIG. 7(a)) and CO.sub.2 conversion (FIG. 7(b)). FIG. 7(c) displays the H.sub.2/CO ratio, which gives an indication of the products distribution. It is observed that increasing the nickel loading of the catalyst results in an increase in both catalytic activity and stability. The lanthanum zirconate pyrochlore alone (LaZrO) and the 2% Ni sample (LNZ2) show no activity for dry reforming. In contrast, the 5% Ni sample (LNZ5) shows good catalytic activity. The 10% Ni sample (LNZ10) shows outstanding performance, being comparable or even superior to the activity levels achieved using expensive noble metal-based catalysts. This sample (LNZ10) was then taken forward to be tested under different reaction conditions.

Temperature Effect

[0143] The catalytic properties of the reduced LNZ10 sample were tested at various temperatures. FIG. 8 shows that remarkable conversions levels can be achieved using LNZ10 even when working at temperatures as low at 600 C., which would otherwise result in the poisoning of certain other catalysts due to carbon formation. The results at 700 C. in terms of CO.sub.2 and CH.sub.4 conversion are exceptional, with the H.sub.2/CO ratio of the produced syngas being close to 1 (the maximum imposed by thermodynamics), thereby illustrating the usefulness of the material for chemical CO.sub.2 recycling.

Space Velocity Effect

[0144] Space velocity is directly related to the volume of the reactor needed to perform the experiment and hence to the cost of the process. As a consequence, it is important to find the optimum condition to run the process to minimise capital cost in a real application for fuel processing.

[0145] The catalytic performance of the reduced LNZ10 sample was tested at different space velocities. FIG. 9 shows that whilst there is no significant difference between WHSV of 15 and 30 Lg.sup.1 h.sup.1, the use of 60 Lg.sup.1 h.sup.1 does have an effect on the performance of the catalyst.

Stability Testing

[0146] The long-term stability of hydrocarbon reforming catalysts is a key factor for industrial scale-up. Many catalysts become deactivated over time due a process of coking (carbon formation) or metal sintering.

[0147] The catalytic properties of the reduced LNZ10 sample were tested over an extended period of time to investigate the stability of the material. FIG. 10 shows that when tested over a period of 350 hours, the reduced LNZ10 sample exhibited only a 6% decrease in activity, thereby underlining the exceptional long-term stability properties of the material.

[0148] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.