Process for preparing a nickel or nickel-cobalt based catalyst for steam reforming

Abstract

There are described a process for producing a nickel or nickel-cobalt based catalyst for carrying out steam reforming reactions, the catalyst thus produced, and steam reforming processes using the catalyst.

Description

[0001] "PROCESS FOR PREPARING A NICKEL OR NICKEL-COBALT BASED CATALYST FOR STEAM REFORMING"

[0002] ************

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a method for preparing a supported metal catalyst for producing hydrogen by steam reforming natural gas or other hydrocarbons.

[0005] BACKGROUND ART

[0006] The steam reforming process is a process for producing hydrogen gas from steam and various hydrocarbons or lower alcohols, in particular from natural gas and / or bioethanol, and is based on a historically proven and established technology.

[0007] The steam reforming process is divided into two phases.

[0008] The first phase, referred to as primary reforming, includes an endothermic reaction in which the hydrocarbon reacts with water in the steam state, hence the name "steam reforming", to produce a gas mixture, referred to as "syngas", rich in hydrogen and carbon monoxide (CO).

[0009] The second phase, which involves an exothermic reaction, is referred to as "Water-Syngas Shift Reaction" and produces further hydrogen and carbon dioxide (CO2).

[0010] The general reaction of the hydrogen production process by steam reforming can therefore be diagrammatically described by the following equations (1 ) and (2) related to the steam reforming itself and to the "Water-Syngas Shift Reaction" phase, respectively, in which methane is reported as an example of a hydrocarbon subjected to reforming:

[0011] CH4+ H2O CO + 3 H2 (1 )

[0012] CO + H2O CO2 + H2 (2) thus obtaining the global reaction diagrammatically described by the following equation (3):

[0013] CH4 + 2 H2O CO2 + 4 H2 (3).

[0014] In practice, at an industrial level, from hydrogen production processes which exploit steam reforming, a hydrogen stream containing a percentage varying from 1 % to 10% of CO and from 15% to 20% of CO2 is obtained, due to the incomplete conversion of the "Water-Syngas Shift Reaction" (equation (2) above).

[0015] As mentioned, the steam reforming reaction is based on an endothermic reaction whereby it is necessary to provide heat to the reactants in order to be carried out. The heat necessary to support the steam reforming reaction is generally provided from the outside, typically by virtue of a burner which, depending on the type of industrial device, can heat the reactor where the process occurs directly or indirectly; when this burner uses hydrocarbons as fuel, further CO2 is also produced by the combustion reaction.

[0016] The most modem reactors, which meet high integration criteria, both fluidic and thermal, adopt some solutions which allow maximizing efficiency and simultaneously limiting CO2 emissions. In particular, in addition to the direct coupling of the burner with the steam reforming reaction chambers, the burner uses as fuel the tail gas from the hydrogen purification unit which is formed by a gas mixture (mainly H2, CO, CO2) with a calorific value sufficient to provide the necessary energy to the reactor. This feature allows utilizing a large part of the energy content of the exhaust gases which would otherwise be eliminated with emissions.

[0017] As a replacement for the burner, the use of plasma torches has recently been tested.

[0018] In this embodiment, the heat necessary in the endothermic reaction is provided precisely by a plasma torch which allows the temperature of the reactants to be raised locally well above that required in a traditional reforming process.

[0019] The use of a plasma torch in a steam reforming process allows limiting CO2 emissions and maximizing the consumption of the methane introduced. The high temperature generated not only allows triggering the transformation reactions of the hydrocarbon (for example, methane) into syngas, consisting of hydrogen, carbon dioxide and water, but also to have no air supply (i.e., oxygen) for completing the reforming reactions.

[0020] Advantageously, the secondary reforming reactions are facilitated by the use of a catalyst.

[0021] Nickel-based catalysts are generally used for this purpose.

[0022] Nickel-based catalysts, generally supported on a ceramic, are known from various patent documents.

[0023] Patent application WO 2019 / 015528 A1 describes a method for producing metal particles, containing nickel and a second metal, on alumina microspheres of diameter 50-500 pm. In the method of this document, a first aqueous solution containing a nickel salt and a salt of a second metal, and a second solution, referred to as a precipitant, of hexamethylenetetramine in an organic solvent are prepared; the two solutions are mixed and the alumina microspheres are added to the resulting mixture; a further aliquot of precipitant solution is added to the suspension thus obtained, after which the suspension is reflux heated, brought to neutral pH, and filtered to recover the solid which is then dried and calcined. However, this method is laborious, requires working in a heterogeneous phase, adding a precipitating solution at two distinct points in the process, and using an organic solvent which must then be recovered with further process complications, or disposed of safely.

[0024] Patent JP 4340892 B2 describes a catalyst for steam reforming reactions, obtained following a complex procedure and containing a mixture of nickel and ruthenium as active elements. The procedure of this document includes forming a layered material based on aluminum and magnesium hydroxides in which nickel and ruthenium deposits are present on the surface of the layers, and then subjecting this intermediate material to heat treatment to convert the hydroxides into the corresponding oxides. Patent JP 4340892 B2 also contains a comparison example related to a catalyst consisting of a nickel deposit on alumina supports, produced by spraying a solution of nickel nitrate hexahydrate on the support, and subsequent heat treatments first at 660 °C in an oxidizing atmosphere to form nickel oxide and then at 800 °C in a reducing atmosphere to obtain metallic nickel deposits. This catalyst is then tested in steam reforming with a molar excess of water, with water / hydrocarbon ratios of 1 .5 and 3; these ratios are used to shift the equilibrium towards the formation of CO2 and hydrogen, but this entails high energy costs in vaporizing the excess water, which does not take part in the reaction.

[0025] The present inventors have observed that the currently available catalysts are not entirely satisfactorily efficient, especially in the case of alcohol reforming.

[0026] It is the object of the present invention to provide a catalyst for steam reforming processes with improved efficiency, as well as to provide a process for its production.

[0027] SUMMARY OF THE INVENTION

[0028] These and other objects are achieved by the present invention, which in a first aspect thereof relates to a process for producing a nickel based or a nickel-cobalt mixture based catalyst supported on alumina, comprising the following steps: a) preparing a solution containing nickel nitrate at a concentration between 0.5 M and 1.0 M or a solution containing nickel nitrate and cobalt nitrate, at a concentration between 0.5 M and 0.95 M for the nickel salt and between 0.05 M and 0.5 M for the cobalt salt, in demineralized water; b) adding to the solution obtained in step a) polyvinylpyrrolidone with a molecular weight Mn between 10,000 and 50,000 Da evaluated by gel permeation chromatography, in a concentration between 0.045 M and 0.45 M referred to the monomeric unit, and leaving under stirring until complete dissolution; c) adding to the solution obtained in step b) alumina spheres with a diameter from 2 to 4 mm and a surface area between 150 and 300 m2 / g, in a quantity ranging from 400 g to 600 g per liter of solution, and leaving to rest until the solution is completely absorbed inside the spheres; d) separating the alumina spheres obtained in step c) from the liquid phase and dry them in an oven for 7-10 h at a temperature ranging from 70 °C to 110 °C; e) pyrolyzing the spheres separated and dried in step d) in an oven in an oxidizing atmosphere at a temperature ranging from 500 °C to 700 °C for 3-6 hours until obtaining alumina spheres covered with Ni oxide or a mixture of Ni and Co oxides; f) reducing the oxide or the mixture of oxides obtained in step e) by treatment with hydrogen at a temperature in the range from 450 °C to 600 °C for a time of not less than 3 hours.

[0029] In a second aspect, the present invention also relates to a catalyst obtained from said process.

[0030] In a third aspect, the present invention relates to the use of said catalyst in a steam reforming process.

[0031] BRIEF DESCRIPTION OF THE FIGURES

[0032] Figure 1 shows a diagrammatic depiction of the reforming process fed with methane and water.

[0033] Figure 2 shows a diagrammatic depiction of the reforming process fed with ethanol and water.

[0034] Figures 3 to 6 are reproductions of SEM micrographs, at different magnifications, of a catalyst of the invention.

[0035] DETAILED DESCRIPTION OF THE INVENTION

[0036] In a first aspect thereof, the present invention relates to a process for producing a nickel or nickel-cobalt based catalyst supported on alumina comprising steps a) to f) above.

[0037] In step a), the nickel nitrate solution, Ni(NOs)2, has a concentration between 0.5 M and 1 .0 M, while the nickel nitrate and cobalt nitrate solution, Ni(NOs)2 and Co(NOs)2, has a concentration between 0.5 M and 0.95 M for the nickel salt and 0.05 M and 0.5 M for the cobalt salt and, in a Ni:Co molar ratio between 95:5 and 50:50. Demineralized water is used as a solvent for the solution. The solubilization of salts can be promoted by heating to 50-90 °C and keeping the solution under stirring.

[0038] Preferably, the two nitrates are used in the hexahydrate form thereof, having the formula Ni(NOs)2-6H2O and Co(NO3)2-6H2O for the nickel and cobalt salt, respectively.

[0039] In step b), polyvinylpyrrolidone (commonly referred to in the chemical field by the abbreviation PVP) is added to the solution prepared in the previous step, in a quantity between 5 and 50 g, preferably between 8 and 15 g, more preferably about 10 g, per liter of solution. The PVP useful for the purposes of the invention has a weight average molecular weight between 10,000 and 50,000, preferably about 40,000 Da. Although PVP is highly soluble in water, this step is also preferably carried out at a temperature above room temperature, for example 50-90 °C, and under stirring, to accelerate the dissolution of the polymer and the homogenization of the solution.

[0040] In step c), alumina spheres with a diameter between 2 and 4 mm are added in a quantity ranging from 400 to 600 g per liter of the solution prepared in step b) and surface area between 150 and 300 m2 / g. The alumina must have a purity greater than 95%. Alumina spheres useful for the purposes of the invention are widely commercially available and are sold for example by the company Sasol Performance Chemicals of Hamburg (Germany) or by the company Zest of Florence (Italy) under the Disidry® brand.

[0041] In step d), the alumina spheres obtained in step c) are mechanically separated from the liquid phase and dried. The first separation from the liquid phase can be carried out by any known method, for example by filtration or preferably vacuum filtration, in which a depression is applied in the liquid collection vessel which accelerates the removal thereof. The spheres covered with Ni or Ni / Co salts and PVP are then dried in the oven at a T between 70 and 110 °C, in a static atmosphere or under a gas stream, to complete the removal of the liquid phase; the complete drying requires more than 6 h, preferably about 8 h.

[0042] In step e) the pyrolysis of the salts is carried out in an oven in an oxidizing atmosphere at a temperature between 500 and 700 °C, preferably about 600 °C, for a time between 3 and 6 h. The oxidizing atmosphere can simply be air, or oxygen or synthetic air (i.e. , a nitrogen / oxygen gas mixture reconstructed with a ratio of the two gases similar to that of air). In this step, the decomposition of the Ni or Ni / Co salts is carried out with the formation of the corresponding oxides. Finally, in the last step of the process, f), the oxides obtained in the previous step are reduced to the corresponding metals by treatment with hydrogen at a temperature in the range from 450 °C to 600 °C, preferably about 500 °C. Within this range, the duration of the reduction reaction is all the shorter the higher the temperature; typically this step requires between 3 and 6 hours.

[0043] In a preferred embodiment, when the catalyst is used in a reactor using a plasma torch, step f) is carried out inside the reactor itself as a preliminary step for starting the reaction. In this case, hydrogen is already produced by the reforming of the hydrocarbons or alcohols used before the reaction goes to full speed, which reduces the Ni or Ni / Co oxides in situ to the corresponding metal or mixture of metals forming the desired catalyst.

[0044] In a second aspect, the present invention relates to a catalyst obtained by the process described above. Such a catalyst is based on nanoparticles of metallic nickel or metallic nickel-cobalt mixtures, of size less than 5 pm, supported on porous alumina. The surface area of the catalyst is between 300 and 600 m2 / g.

[0045] Figures 3 to 6 show SEM micrographs obtained on various samples of catalyst of the invention consisting of nickel particles on alumina. In the images, the metal particles appear lighter than the ceramic support. From the SEM images it is apparent that the metal particles are all less than 5 pm in size.

[0046] In a third and final aspect, the present invention relates to the use of the nickel and / or cobalt based catalyst supported on alumina obtained from the process described above in a steam reforming process.

[0047] The steam reforming process uses water / methane mixtures or water / ethanol mixtures as starting raw materials. Figs. 1 and 2 diagrammatically show the structure of the complete reaction system and the diagram of the steam reforming process in the two cases. In the two figures, an equal number corresponds to an equal element.

[0048] Fig. 1 refers to the case of reaction starting from water / methane mixture. The system, 10, comprises a methane feeder, 11 , and a water tank, 12. The water is fed from the tank 12 to a vaporizer, 13, and the steam thus produced is sent to a mixer, 14, to which the methane is also fed from the element 11 ; a homogeneous gaseous mixture of methane and steam is formed in the mixer 14, which is sent to the reactor

[0049] 15 containing the catalyst described above; the interior of the reactor is maintained at the working temperature of the catalyst, between 400 and 600 °C. A plasma generator

[0050] 16 causes the formation in the reactor 15 of an electric arc having a voltage between 3 and 15 kV, which crosses the reactor causing the steam reforming reaction. The gases leaving reactor 15 are sent to a Water Gas Shift reactor, 17, for the transformation of the residual carbon monoxide. The reactor 17 is commercially available.

[0051] Fig. 2 refers to the case of reaction starting from water / ethanol mixture. The system, 20, is identical to the system 10 described above, except that it includes an ethanol tank 21 instead of the methane feeder 11 , and that the tank 21 is connected to the vaporizer 13 instead of directly to the mixer 14; in this case, since ethanol is liquid, this component is vaporized in the element 13 together with the water, and the mixture thus formed is fed to the mixer 14 where it is homogenized.

[0052] The invention is now shown below by means of some examples to be understood for illustrative and non-limiting purposes thereof.

[0053] EXPERIMENTAL TOOLS, METHODS AND CONDITIONS

[0054] Nickel nitrate hexahydrate (Ni(NO3)2-6H2O), cobalt nitrate hexahydrate (CO(N03)2-6H20) and polyvinylpyrrolidone of weight average molecular weight of 40,000 Da used in the examples are sold by Merck S.p.A., under catalog numbers 106721 , 102536 and PVP40, respectively.

[0055] The alumina spheres of the examples are sold by Disidry®, brand of Zest s.r.L, Florence (Italy).

[0056] The electron microscope used to obtain the micrographs is a Phenom Pharos desktop SEM tool from ThermoFisher.

[0057] EXAMPLE 1

[0058] This example relates to the production of a precursor of the catalyst of the invention.

[0059] 3 g of nickel nitrate hexahydrate were dissolved under stirring in 10 ml of water at room temperature, obtaining a 1 M nickel solution. Upon complete dissolution, always under stirring, 0.5 g of polyvinylpyrrolidone (PVP) of molecular weight 40,000 Da (concentration of PVP, in terms of monomer units, equal to 0.45 M) were added.

[0060] When the PVP was completely solubilized, 20 g of y-alumina as spheres of approximately 4 mm were added, always under stirring. After 12 hours the impregnated spheres were transferred into a porous ceramic capsule and heated in a muffle in air at 600 °C for 5 hours. In this step the nickel nitrate was decomposed to nickel oxide, which remains adherent to the alumina surface.

[0061] EXAMPLE 2 The same procedure as in Example 1 was followed, but first preparing a solution with a concentration of 0.5 M nickel nitrate and 0.5 M cobalt nitrate; in this case, a precursor of a catalyst consisting of an equimolar ratio of nickel and cobalt on alumina was obtained.

[0062] EXAMPLE 3

[0063] This example relates to a bioethanol reforming test using a catalyst of the invention.

[0064] The catalyst containing Ni and Co in molar ratio 1 :1 , prepared in Example 2, positioned inside a quartz reactor, was used for the test.

[0065] 162 g of demineralized water and 44 g of ethanol were fed to a thermostated boiler at a temperature of 130 °C (element 13 in Fig. 2). The output steam was sent to a homogenizer and from there blown into the reactor. The reactor was passed through by a high-temperature plasma. Inside the reactor, the gaseous mixture passed through a catalytic bed formed by the Ni / Co covered alumina spheres of Example 2, maintained at the temperature of 450 °C. The nickel and cobalt oxides produced in the procedure of Example 2 were reduced to the corresponding metals from the hydrogen produced in the first ethanol reforming steps at plasma temperatures, forming the catalyst of the invention in situ.

[0066] The gases output from the reactor were conveyed onto a WGS catalytic bed, which allows the carbon monoxide to be transformed into CO2 and H2.

[0067] The output gas was found to consist of 6 g H2 and 264 g CO2.

[0068] The amount of electricity required for the reforming reaction was 200 W.

[0069] EXAMPLE 4

[0070] This example relates to a methane reforming test using a catalyst of the invention.

[0071] An Ni-only catalyst was used, prepared as described in Example 1 , positioned inside a quartz reactor.

[0072] 4.5 kg of demineralized water were fed to a thermostated boiler at a temperature of 130 °C and the output steam was blown into the reactor together with 2 kg of methane, after passing through a homogenizer. The reactor was passed through by a high-temperature plasma. Inside the reactor, the gaseous mixture passed through a catalytic bed formed by the nickel oxide-covered alumina spheres of Example 1 , at a temperature of 450 °C. The nickel oxide was reduced to the metal by the hydrogen produced in the early ethanol reforming steps at plasma temperatures, forming the catalyst of the invention in situ. The gases output from the reactor were conveyed on a WGS catalytic bed, which allows the carbon monoxide to be transformed into CO2 and H2.

[0073] The output gas was found to consist of 1 kg H2 and 5.5 kg CO2.

[0074] The amount of electricity required for the reforming reaction was 15 kW. This result is much better than that obtained in comparative example 1 of patent JP 4340892 B2: as mentioned above, the tests of comparative example 1 of the Japanese patent are carried out in stoichiometric excess of water, a condition that promotes the desired reaction but at the expense of the energy efficiency of the system. Conversely, in the case of the present invention, there is a stoichiometric ratio between water and methane; as demonstrated by the data of the present example, the catalyst of the invention, while not requiring the use of excess water, allows obtaining higher conversion yields than those of the aforementioned Japanese patent.

Claims

CLAIMS1. Process for producing a nickel based catalyst or a nickel-cobalt mixture based catalyst supported on alumina, comprising the following steps: a) preparing a solution containing nickel nitrate at a concentration between 0.5 M and 1.0 M or a solution containing nickel nitrate and cobalt nitrate, at a concentration between 0.5 M and 0.95 M for the nickel salt and 0.05 M and 0.5 M for the cobalt salt, in a Ni:Co molar ratio between 95:5 and 50:50, in demineralized water; b) adding to the solution obtained in step a) polyvinylpyrrolidone with a molecular weight Mn between 10,000 and 50,000 Da evaluated by gel permeation chromatography, in a concentration between 0.045 and 0.45 M referred to the monomeric unit, and leaving under stirring until complete dissolution; c) adding to the solution obtained in step b) alumina spheres with a diameter from 2 to 4 mm and a surface area between 150 and 300 m2 / g, in a quantity ranging from 400 g to 600 g per liter of solution, and leaving to rest until the solution is completely absorbed inside the spheres; d) separating the alumina spheres obtained in step c) from the liquid phase and dry them in an oven for 7-10 h at a temperature ranging from 70 °C to 110 °C; e) pyrolyzing the spheres separated and dried in step d) in an oven in an oxidizing atmosphere at a temperature ranging from 500 °C to 700 °C for 3-6 hours until obtaining alumina spheres covered with Ni oxide or a mixture of Ni and Co oxides; f) reducing the oxide or the mixture of oxides obtained in step e) by treatment with hydrogen at a temperature in the range from 450 °C to 600 °C for a time of not less than 3 hours.

2. Process according to claim 1 wherein in step a) as nickel and cobalt nitrates nickel nitrate hexahydrate and cobalt nitrate hexahydrate are employed.

3. Process according to any one of claims 1 or 2, wherein in step b) polyvinylpyrrolidone with a molecular weight of about 40,000 Da is used.

4. Process according to any of the previous claims, in which at least one of the stepsa) and b) is carried out at 50-90 °C and / or under stirring.

5. Process according to any of the previous claims, wherein the alumina spheres of step c) have a purity greater than 95%.

6. Process according to any of the previous claims, wherein step f) is carried out in a dedicated reactor.

7. Process according to any one of claims 1 to 5, in which step f) is carried out through the use of a plasma torch, inside a steam reforming reactor, as a preliminary phase for starting the steam reforming reaction.

8. Catalyst for steam reforming reactions obtained according to any one of claims 1 to 7, consisting of metallic nickel nanoparticles or metallic nickel-cobalt mixtures nanoparticles, with size lower than 5 pm.

9. Steam reforming process of methane / water or ethanol / water mixtures, which uses a catalyst produced according to claim 6.

10. Steam reforming process of methane / water or ethanol / water mixtures, including a preliminary phase which consists in the in situ reduction of nickel oxide or a mixture of nickel and cobalt oxides supported on alumina spheres to the corresponding metals, through the hydrogen produced at the beginning of the steam reforming reaction.

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