Patent Application
20250170562
The present invention relates to the field of catalysts, and more particularly to a process for preparing a catalyst useful in the production of syngas (or synthesis gas), to the catalyst thus obtained and to its use.
Syngas is a gas mixture consisting essentially of hydrogen and carbon monoxide, and is considered one of the most important intermediates in the production of a wide range of high-value industrial molecules, such as methanol, ethylene and propylene, and in energy production. The hydrogen contained in syngas is also a ‘clean’ fuel source from which energy can be produced without polluting emissions.
To date, syngas is mainly produced by reforming methane or by partial oxidation of fossil fuels, but other processes have also been developed to produce it from CO2 and water by supplying thermal energy. Among these processes, two-phase thermochemical processes, based on the use of metal oxides, are a viable option for producing syngas and consist of a first phase (1) of endothermic reduction of metal oxide (MOox) to metal, or to metal oxide with a lower valence than the MOred metal;
MOox→MOred+½O2 (1)
MOred+H2O→MOox+H2 (2)
MOred+CO2→MOox+CO (2′)
The re-oxidised metal oxide can then be recycled back to the first phase. The overall reaction of the two-stage process illustrated above can be written as the following reaction (3):
H2O→H2+½O2 (3)
CO2→CO+½O2 (4)
To improve the thermochemical processes described above, maximising syngas production, various metal oxides, such as ferrites, zinc oxide, cerium oxide, etc., have been studied and tested. Among these, processes using ferrites have slow kinetics and suffer from the sintering process, just as processes using zinc oxide which require a rapid hardening process due to the volatility of the material. Cerium oxide (CeO2), instead, has performed well in the thermochemical production of CO by CO2 dissociation. In fact, it is an active material with a high melting temperature of around 2800 K and high catalytic activity towards carbon-containing gases; moreover, it retains its properties, even after several thermal processes.
In one of the first works using cerium oxide to achieve H2O splitting by means of a solar reactor, a considerable reduction of pure cerium oxide was achieved at 2000 K, accompanied by a partial vaporisation of the powders. The high operating temperature also led to other negative consequences, such as practical problems in reactor design and sintering phenomena that lowered the reactive surface of the metal catalyst (Furler P. et al., Energy Environ. Sci. (2012) 5, 6098-6103).
In order to lower the temperature required for the reduction reaction, various researches have therefore been carried out in recent years, leading to the development of doped catalysts. For example, Kaneko H. et al. in Energy (2007) 32:5, 656-663, proposed solid solutions containing cerium oxide and various transition metal oxides such as Mn, Fe, Ni and Cu as catalysts for the production of H2 at 1273 K (approximately 1000° C.) in a tubular quartz reactor heated by an infrared furnace. The H2 produced by the CeO2-MOx catalysts was reported to be in greater quantities than that produced with pure cerium oxide, except in the case of doping with Cu, where the quantities of H2 obtained were almost equal.
Again with copper as dopant, an even worse result was obtained by Abanades S. et al. J. Mater Sci. (2010) 45, 4163-4173, who synthesised CeO2—CuO samples via a polymeric route derived from Pechini's method, and reduced them to a temperature of 1500° C. In this case, X-ray diffraction (XRD) analysis showed the coexistence of CeO2 and Cu2O, which was responsible for the mass losses of CuO during thermogravimetric analysis (TGA), while production data showed that these reduced Cu-based powders were not reactive with water to generate H2 at a temperature of 1000° C.
Although other studies (see e.g. Le Gal et al. Energy Fuels (2011) 25, 4836-4845) have shown that cerium oxide powders synthesised with the Pechini method have a better catalytic capacity in H2 production than the same powders obtained with different synthesis techniques, the results obtained with Cu doping have not been encouraging so far, as seen above.
The high reduction and conversion temperatures, highlighted in the aforementioned literature, are a major impediment to the dissemination of H2O and CO2 splitting technology to obtain syngas, due to the practical problem in designing a reactor suitable for such temperatures and the phenomena of structural modification (sintering, etc.) of the catalysts when exposed to temperatures above 1000° C. In the case of cerium oxide, which has been proposed as a catalyst for syngas production in several studies to date, the reduction temperature is 1673 K (approximately 1400° C.). In addition to the aforementioned problems of the reactor and sintering phenomena, a low thermal-to-fuel energy conversion efficiency (less than 3.5%), defined as the ratio between the calorific value of the syngas produced and the corresponding energy input required to produce it, has also been noted.
Commercially available cerium oxide catalysts have a further disadvantage; since they are designed to maximise the catalytic surface area, they are marketed in the form of extremely small particles (tens of nanometres) or, with a view to cost containment, immobilised on inert substrates such as ceramics. Catalysts in the form of particles of this size or immobilised on inert substrates are not suitable for use in fluidised bed reactors. Indeed, in order to fluidise the bed, the particles must be at least 20 microns in size (see Geldart diagram). When using commercially available catalysts in the form of smaller particle sizes, it has been noted that preferential paths are created, resulting in insufficient movement of the fluidised bed and poor reactor efficiency. One solution could be to encase the catalyst particles in larger particles, but this is excessively time-consuming and costly and has a negative impact on the efficiency of the catalyst.
At present, therefore, there is still a strong need for a suitably doped catalyst that, inserted into an appropriate reactor, overcomes the disadvantages described above for known catalytic systems, reducing process temperatures and also increasing conversion efficiency in the production of syngas from water and CO2.
The Applicant has now devised a process for preparing a cerium oxide catalyst doped with copper, manganese or nickel, having a reduced catalytic reduction temperature in a syngas production process from water and CO2, and a reactive catalytic surface area increased by about six orders of magnitude over cerium oxide catalysts of the known art.
It is therefore an object of the present invention to provide a process for preparing an M/CeO2 catalyst in which M is a dopant metal selected from the group consisting of copper, manganese and nickel, in the form of nano- and microparticles, the essential characteristics of which are defined in the first of the appended claims.
The catalyst obtainable by the aforementioned process, in particular in the form of a porous macrostructure, the reactor comprising this catalyst, and their use in the production of syngas from water and CO2, the essential characteristics of which are defined in the respective independent claims, constitute further objects of this invention.
Furthermore, the catalyst according to any of the embodiments described herein is also found to be suitable in the iron and steel industry for the production of directly reduced iron (or DRI), in particular as a reducing agent in the conversion from CO2, to CO, allowing for the reduction of climate-changing emissions in a hard-to-abate sector.
Further important features of the preparation process, the obtainable catalyst, the reactor comprising it, and the related use of this invention are defined in the dependent claims appended hereto.
The characteristics and advantages of the preparation process, of the catalyst, of the reactor comprising it and of their uses according to the invention, will result more clearly from the following description of their embodiments made by way of example and not by way of limitation with reference to the annexed drawings in which:
As mentioned above, the inventors have devised a process for the preparation of an M/CeO2 catalyst in which M is a metal selected from the group consisting of copper, manganese and nickel, and preferably is copper, in the form of a porous macrostructure, comprising the following steps:
In the context of the present invention, water preferably means deionised water.
In one aspect of the present process, the mixing in step iii) is carried out under vigorous agitation, for example for a time between 20 and 40 minutes.
In step iv) of this process, in which a precipitate is formed in a mother liquid, the liquid is preferably kept under stirring, for example for about 3 hours, at a temperature between about 70° C. and about 90° C., before the precipitate is separated from the mother liquid, for example by filtration. Precipitation in this step (iv) is caused by raising the pH of the solution to at least 11 (resulting in a zeta potential value of less than-40 mV) with an aqueous ammonia solution, e.g. of a concentration between 26% and 32% v/v.
Controlling the pH at this stage is important in order to give the catalyst the desired morphological characteristics, i.e. a porous macrostructure. In fact, as will be evident from the examples, working at pHs other than 11-12 does not give the same result.
Without wishing to be bound to any theory, the inventors hypothesise that pH, by influencing the zeta potential (ζ) of the mother liquor, also affects the deposition of the precipitate and thus its final morphological characteristics. In fact, it has been noted that at a pH lower than 11 (resulting in a zeta potential value greater than −40 mV), a different compactness of the precipitate is obtained, which negatively affects the performance of the catalyst.
The sedimentation step under controlled pH conditions thus makes it possible to obtain a precipitate in which the nanoparticles, previously dispersed in the mother liquid, aggregate to the correct degree of compactness to give aggregates of particles (or clusters) greater in size. A porous macrostructure, better defined below, is thus obtained, which is stabilised thanks to a subsequent drying phase.
Compared with catalysts known in the state of the art, therefore, it will not be necessary to immobilise the catalyst nanoparticles on an inert porous material because with the process of the invention, an equally porous structure is already obtained but entirely made up of catalyst.
The process of the invention thus makes it possible to obtain a catalyst with improved performance since the macroporous structure increases the catalytic surface that comes into contact with the gases.
Co-precipitation processes of the type of the present invention are traditionally considered disadvantageous in that they involve a grinding step of the precipitate downstream of the precipitation/dessication step in order to obtain nanoparticles. However, with the process according to any of the embodiments herewith disclosed which, as mentioned above, envisages a precipitation phase under controlled pH conditions, the precipitate after the drying and calcination phases is itself already effectively employable as a catalyst, precisely by virtue of the macroporous structure that can be obtained by controlling the deposition of the nanoparticles in the precipitation phase.
Therefore, the process of the invention does not provide for a micronisation or pulverisation phase of the dried precipitate, let alone a micronisation phase upstream of the precipitation, phases which would be necessary to prepare a catalyst in nanoparticle form such as those commercially available and described in the state of the art, and therefore to disaggregate the clusters of particles eventually formed with the precipitation. The process of the invention therefore does not comprise a micronisation or pulverisation step of the dried precipitate and/or a micronisation step upstream of the precipitation.
Instead, a coarse milling phase (up to a particle size greater than 20-30 microns) downstream of the drying phase may be envisaged so as to have macroporous structures of a size suitable to be fluidised in fluidised bed reactors. In any case, bed movement will result in a further reduction in catalyst particle size with a consequent increase in catalytic surface.
The precipitate can be washed with water and ethyl alcohol, then subjected to drying, e.g. by heating to about 110° C. for 9-12 hours.
According to a particular embodiment of the present process, the calcination step (v) is conducted by gradually increasing the temperature up to a value of 850° C., for example by initially increasing the temperature up to 500° C. at a rate of 1° C./min, maintaining this temperature for about 1 hour, then increasing to 850° C. at a rate of 3° C./min and maintaining this temperature for a further 3 hours.
In one aspect of the present process, the calcining temperature is between 850° C. and 1000° C.
Typically, the process of this invention was carried out entirely under ambient pressure.
In other words, the invention relates to a process for the preparation of an M/CeO2 catalyst in the form of a porous macrostructure, in which M is a dopant metal chosen from the group consisting of copper, manganese and nickel, said process comprising the following steps:
The above-described process, found by the inventors, enabled the preparation of a metal doped cerium oxide catalyst, also referred to herein as an M/CeO2 catalyst, which exhibited improved properties compared to catalysts known in the art, particularly in terms of reactive catalytic surface, which was found to be increased by about 6 orders of magnitude, and in terms of catalytic reduction temperature in the process of producing syngas from water and CO2, which was found to be reduced to 850° C.
Therefore, the present invention also relates to an M/CeO2 catalyst obtained or obtainable by the process according to any of the embodiments described herein.
The catalyst of this invention has been characterised from a point of view of chemical composition, structure and morphology resulting in the following features:
Furthermore, this catalyst was found to be a porous macrostructure, with pores between 3.6 and 3.8 nm in diameter as measured by TEM electron microscopy and an average volume between 0.009 and 0.013 cm3/g as measured by BET. XRD analysis shows that this porous macrostructure consists of nanostructured particles smaller than 20 nm in size and aggregated together in clusters (or precisely aggregates of particles) with an average diameter of between 20 and 30 μm.
In other words, the present invention also relates to a porous macrostructure comprising or consisting of an M/CeO2 catalyst in which M is a dopant metal selected from the group consisting of copper, manganese and nickel, with atomic % of metal M between 20 and 22% and atomic % of Ce between 78 and 80%, said macrostructure being characterised by pores having a diameter between 3. 6 to 3.8 nm and an average volume between 0.009 and 0.013 cm3/g, as measured by BET analysis, and by a homogeneous distribution of said dopant metal M throughout the structure.
In the context of the present description, the terms “catalyst in the form of a porous macrostructure” and “porous macrostructure comprising or consisting of a catalyst” are therefore used interchangeably.
The M/CeO2 catalyst in the form of a porous macrostructure of the invention has been found to be particularly suitable for use in both a fixed bed and a fluidised bed reactor.
In particular, a fixed-bed reactor typically comprises a cylinder, suitably heated, containing a bed of catalyst particles, which are passed through by a mixture of different gases reacting due to the action of the catalyst. For the reactor to function properly, the bed must be sufficiently porous to allow gases to pass through it with low pressure losses.
As is generally known in the art, in a fluidised bed reactor, a flow of gases passes through the solid bed, filling the inter-particle space. At a certain flow rate, the solid begins to expand and the upper surface of the bed assumes a moving profile. This ensures excellent mixing of the reactants with the catalyst. There is in fact contact between the gaseous molecules and the catalyst itself with a high reaction rate. An example of a fluidised bed reactor is shown schematically in
As is generally known in the art, dual fluidised bed reactors are used when the catalyst periodically requires a regeneration process. In this type of apparatus, the first fluidised bed serves as the actual reactor; in the second, catalyst regeneration takes place. An example of a fluidised bed reactor is shown schematically in
The catalyst according to any of the embodiments of the present invention is found to be suitable for catalysing the syngas preparation process in each of these reactors. In particular, the conversion of CO2 and water to syngas was achieved by fluxing the gases through a catalyst bed prepared according to the invention suitably heated to the correct reaction temperature.
The inventors of this invention have also developed a reactor, an exemplary embodiment of which is illustrated in
In an aspect of this invention, the energy supply of the reactor is provided by a linear parabolic concentration system of solar radiation, which is capable of heating the reactor tube 1 and enabling the catalytic reaction for syngas production. One embodiment of such a system, illustrated in
In addition to the aforementioned advantages related to the advantageous characteristics of the catalyst compared to catalysts known in the state of the art, this invention also has the advantage of being able to use the clean energy of solar radiation to power the catalysed syngas production reaction. This aspect, coupled with the possibility of using water and CO2 from combustion plants as a starting material, makes the present invention particularly appreciable from the point of view of environmental sustainability.
As mentioned above, the catalyst of the invention can also be used in the iron and steel industry in particular in directly reduced iron or DRI processes, specifically to catalyse the conversion of CO2 into CO, preferably where CO is then used in the same DRI process as the reducing agent instead of hydrogen. This makes it possible to significantly reduce the operating pressure of the Midrex or Teneva reactors commonly used and generally known to a person skilled in the art from 8 bar (according to the present technology based on using hydrogen as the reducing agent) to 1 bar (using CO as the reducing agent). Accordingly, the present invention also relates to the use of the porous macrostructure comprising catalyst according to any embodiment of the invention or the reactor according to any embodiment of the invention in the preparation of directly reduced iron (or DRI), preferably to catalyse the conversion from CO2 to CO, more preferably wherein CO is then reused as a reducing agent in said DRI preparation process.
The following examples are given for illustrative and non-limiting purposes of the present invention.
Cu(NO3)2 3H2O and Ce(NO3)3 6H2O were dissolved in deionised water at molar ratios of Cu/(Cu+Ce) of 0.2. The resulting solution was added to a solution of CTAB in water in such an amount as to realise a 1:1 ratio of CTAB to Cu+Ce metal ions. The two solutions were mixed under vigorous stirring for 30 minutes, resulting in a single, clear and homogeneous solution. Subsequently, a 28% aqueous ammonia solution was added to this solution drop by drop and mixed under vigorous stirring until the pH reached a value of 11 (resulting in a zeta potential value of less than −40 mV) and a precipitate began to form in the mother liquor. After stirring the mother liquor for 3 hours at 80° C., the precipitate obtained was filtered and washed repeatedly with sufficient deionised water and ethyl alcohol. Finally, the recovered solid was dried at 110° C. for 12 hours in an electric oven and then underwent calcination in the same oven. Specifically, for calcination, the electric oven was programmed to reach 500° C. at a rate of 1° C./min, maintain this temperature for 1 hour, and then increase it to 850° C. at a rate of 3° C./min. The temperature of 850° C. was maintained for a further 3 hours. The entire process was carried out at ambient pressure.
The catalyst, in the form of a porous macrostructure, was characterised by various analyses:
The process of Example 1 was repeated with different pH values in the precipitation phase. It was noted that at pH values<11 and zeta potential>−40 mV, there was a degradation of the porosity of the porous macrostructure with a reduction in the average diameter of the clusters, making them unsuitable for use in fixed-bed or fluidised reactors.
The present invention has been so far described with reference to a preferred embodiment. It is to be understood that there may be other embodiments pertaining to the same inventive core, as defined by the scope of the claims reported below.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102021000030047 | Nov 2021 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2022/061532 | 11/29/2022 | WO |
