METHOD FOR CONVERTING CARBON DIOXIDE INTO HIGH ADDED VALUE CHEMICAL COMPOUNDS THROUGH A MECHANOCHEMICAL PROCESS UNDER CONTINUOUS GAS FLOW CONDITIONS

Abstract

The present invention relates to a method for converting carbon dioxide (CO.sub.2) into high added value chemical compounds under continuous gas flow conditions. In particular, said process converts CO.sub.2 into a mixture of high added value chemical compounds comprising low molecular weight hydrocarbons, mainly methane, ethylene and ethane, along with products of mineral carbonation, mainly Mg and Fe carbonates. Such CO.sub.2 conversion is achieved through a mechanochemical process.

Claims

1. A continuous mechanochemical process under gas flow to carry out the conversion of CO.sub.2 into a mixture of high added value chemical compounds, said mixture comprising a mixture of low molecular weight hydrocarbons, mainly methane, ethylene and ethane, and solid products of mineral carbonation, mainly Mg and Fe carbonates, said process comprising at least the following steps: a) continuously passing a gas flow comprising, or consisting of, CO.sub.2 through a reactor equipped with milling means, and in the presence of Olivine powder and water, in liquid or gaseous form, at from room temperature to ?100? C.; said reactor being subjected to motion in such a way that the milling means trigger a mechanochemical reaction that, on one side, produces hydrogen from the present water, and then said hydrogen carries out the conversion of a portion of CO.sub.2 into said low molecular weight hydrocarbons, while, on the other side, the other portion of CO.sub.2 reacts with the transformation products of Olivine and non-reacted H.sub.2O to give said products of mineral carbonation; b) separating and recovering from the gas mixture exiting the reactor said low molecular weight hydrocarbons and non-reacted hydrogen obtained in step a); and c) at the end of the reaction, separating and recovering from the reactor said solid products of mineral carbonation of Olivine obtained in step a).

2. The process according to claim 1, wherein said reactor is a mill, or a jar, equipped with milling means consisting of rotating spherical bodies (balls); said mill and said balls being made of an abrasion resistant material, hardened stainless steel.

3. The process according to claim 1, wherein the motion to which the reactor is subjected is a rotary motion at a speed of 500 rpm to 1,500 rpm.

4. The process according to claim 3, wherein said rotary motion occurs at a speed of 600 rpm to 1,400 rpm.

5. The process according to claim 1, wherein the stoichiometric ratio of Olivine to H.sub.2O is in the range of 1:1 to 1:3.

6. The process according to claim 5, wherein the stoichiometric ratio of Olivine to H.sub.2O is 1:2.

7. The process according to claim 1, wherein the highest measured concentrations of the low molecular weight hydrocarbons produced in step a) are 680 ppm for methane, 280 ppm for ethane, and 100 ppm for ethylene.

8. The process according to claim 7, wherein methane is the main product with a selectivity of 60%, while the selectivity values for ethane and ethylene are 20-25% and 8-10%, respectively.

9. The process according to claim 7, wherein the global yield in methane production is 50%, while it is 10-15% for ethane and about 10% for ethylene.

10. Use of the process according to claim 1 to carry out the conversion of CO.sub.2 into a mixture of high added value chemical compounds; said mixture comprising a mixture of low molecular weight hydrocarbons, mainly methane, ethylene and ethane, and solid products of mineral carbonation, mainly Mg and Fe carbonates.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0034] FIG. 1 shows an illustrative embodiment of the experimental apparatus of the present invention.

[0035] FIG. 2 represents the relationship between conversion (X), selectivity (S) and yield (Y) when products B and C are obtained from the reagents, indicated as A, plus a certain quantity of non-reacted reagent A. At time t1 the process has not yet begun, and only the reagents (A) are present. At time t2, following the start of the process, the system will be composed of the products (B and C) and a part of reagents (A) not yet reacted.

[0036] FIGS. 3a) and 3b) show, respectively, the CO.sub.2 conversion rate and the concentration, expressed in ppm, of the products obtained, as recorded during the grinding process.

[0037] FIGS. 4a) and 4b) show, respectively, the selectivity of the products obtained during the grinding process and the estimated yield of the products.

[0038] FIG. 5 shows a comparison among tests conducted with, respectively, 0.15, 0.3 and 0.6 ml of H.sub.2O and 2 g of Olivine.

[0039] FIG. 6 shows the CO.sub.2 conversion rate, evaluated by using milling bodies having different masses.

[0040] FIG. 7 shows CO.sub.2 conversion as a function of grinding time. The two curves refer to tests conducted with different values of the revolution speed of the mill, as indicated in the box.

[0041] FIG. 8 represents a comparison that shows CO.sub.2 conversion as a function of time with Olivine at its first use (grey curve) and with Olivine at its second use (black curve).

[0042] FIG. 9 shows the diffraction patterns of unprocessed Olivine (bottom) and of the post-reaction materials treated under different conditions, as described in this specification.

[0043] FIG. 10 shows the diffraction patterns of unprocessed Olivine (bottom) and of a representative sample of processed Olivine (top). At its base, the graph shows the sequences of the characteristic signals of the main crystallographic phases involved.

[0044] FIGS. 11A and 11B show, respectively, an SEM micrography of a sample of Olivine pre-ground for 1 hour and an SEM micrography of a sample of Olivine processed in the presence of water and carbon dioxide for a time longer than one hour.

[0045] FIG. 12 shows the map of elements and the EDX microanalysis. The upper part shows the map of elements generated a sample of on Olivine processed by mechanochemical activation with water and carbon dioxide. The lower part shows the fluorescence spectrum, with the characteristic signals of the existing elements. The box shows the mean percentages of the detected elements.

[0046] FIG. 13 shows the FT-IR spectra of unprocessed and processed (i.e. subjected to mechanochemical 1 reaction) Olivine. Prior to recording the spectra, the samples were dried in a stove at 80? C. to remove all traces of humidity.

[0047] FIG. 14 shows the evolution of the non-reacted hydrogen produced during the CO.sub.2 activation process.

EXPERIMENTAL SECTION

Description of the Reactor and Reaction Conditions

[0048] The following experimental section will describe, by way of illustrative example, some of the characteristic aspects of the present invention, without however by no means limiting the broad application potential thereof. Based on the following explanations, those skilled in the art will have no difficulty in applying and modifying the teachings provided herein by adapting the dimensions, quantities and construction aspects to specific practical implementations. In one embodiment, the equipment used for the experimental activity is schematized in FIG. 1. It essentially consists of a cylindrical reactor (a grinding jar (mill) having an internal volume of approx. 65 ml), suitable for the execution of mechanical treatments by means of ball milling processes. The reactor and the milling bodies are made of hardened stainless steel (of course, the dimensions and materials of the reactor may be freely selected and/or defined for specific applications, depending on the type and rate of gas flow (e.g. an exhaust gas) to be treated. The same also applies to the number, size and material of the milling means). The cylindrical reactor is equipped with gas-tight covers and gas transfer lines. The gas lines, e.g. partly made of steel and partly consisting of flexible Teflon (PTFE) hoses, provide the connection between the reagent gas supply (or the exhaust gas flow) and the reactor, and between the latter and the two gas chromatographs for analyzing the reagent and the produced gases. Such a flow configuration of the reactor allows the execution of tests under a continuous supply of gaseous reagents. The gas chromatographs are both equipped with detectors suitable for a quantitative evaluation of the gaseous phases of interest. A first gas chromatograph, equipped with a GS-Q wide-bore capillary column and TCD-FID detectors, with He as a carrier gas, made it possible to evaluate the CO.sub.2 through the TCD detector and the low molecular weight hydrocarbons through the FID detector; a second gas chromatograph, equipped with a column packed with 13? (10 ?) molecular sieves and a TCD detector, with Ar as a carrier gas, made it possible to evaluate the amounts of H.sub.2, CO, O.sub.2 and N.sub.2.

[0049] The mechanochemical reactor was housed in the seat of a known Spex Mixer/Mill mod.8000, [Spex Certiprep., https://www.spexcertiprep.com], suitably modified and equipped with a three-phase electric motor controlled by a speed regulator, which provides control over the number of revolutions of the motor shaft and imparts to the mill a three-dimensional motion in space. The tests were conducted under two different conditions: at 875 and 1,000 revolutions per minute (rpm) of the motor shaft, respectively corresponding to 14.5 and 16.6 Hz.

[0050] In practice, the revolution speed of the reactor can be varied, depending on the type of application, from 500 rpm to 1,500 rpm; preferably, from 600 rpm to 1,400 rpm; more preferably, from 700 rpm to 1,300 rpm; even more preferably, from 800 rpm to 1,200 rpm.

[0051] The tests were conducted using solid samples having a mass of 2 g of commercial Olivine (produced by SATEF-HA, Italy). The latter was subjected to a mechanical pre-treatment in Air atmosphere for 1 h using 3 balls (made of stainless steel) having a mass of 4 g each, for the purpose of homogenizing the powder size. At the end of the pre-treatment, 0.3 ml of deionized H.sub.2O were added to the Olivine powder.

[0052] The above-described conditions correspond to an Olivine/H.sub.2O stoichiometric ratio of 1:2.

[0053] The reactor was connected to the gas lines and housed in the support of the Spex 8000 commercial mill, for the execution of the reactive tests.

[0054] The gaseous reaction mixture was composed of CO.sub.2 (Sapio, purity of 99.995), with abundance of 10% by volume in He (Sapio, BIP purity). After saturating the atmosphere of the mechanochemical reactor, the reactive mixture was fed in a flow of 10 ml/min, kept constant through a regulation valve and measured with a mass flowmeter (in the applications of the process of the invention, the flow rate may be changed freely according to the amount of exhaust fumes containing CO.sub.2 released per time unit, the chimney size, and other parameters related to the release of said fumes). The composition of the reactive mixture for the tests was selected in tight relation with the average composition data of the mixture containing CO.sub.2 exiting the above-mentioned industrial combustion processes. In these tests, the matrix was He, instead of N.sub.2, in order to avoid any side effects due to a possible interaction between N.sub.2 and Olivine and to avoid any overlapping effects in the analyses of the permanent gases. This will not limit the applicability of the invention, which may also be implemented by the operator with N.sub.2 as a gaseous matrix.

Description of the Results of the CO.SUB.2 .Conversion Tests and of the Analyses of the Solid and Gaseous Phases

[0055] The following will present the results of the CO.sub.2 conversion tests under the above-described conditions. Data will be provided about the analyses of the gaseous phases conducted by gas chromatography, the analyses of the solid phases conducted by X-ray diffraction techniques, and the morphological and elementary analyses conducted on the solid phases by, respectively, scanning electron microscopy (SEM) and X fluorescence.

[0056] The mechanical treatment in reactive atmosphere was conducted by using 3 steel balls having a mass of 4 g each and by setting the revolution speed of the motor to 875 rpm, unless otherwise specified. The kinetics of the CO.sub.2 conversion process was monitored by evaluating the degree of CO.sub.2 conversion via measurement of the concentration of CO.sub.2 eluted by the reactor after selected mechanical treatment times up to 180 minutes.

Definition of the Descriptor Parameters of the Process

[0057] The study of the process was conducted by monitoring, as parameters, the rate of CO.sub.2 conversion, the selectivity in giving the products, and the yield of a specific product. FIG. 2 illustrates, in graphic form, the meaning of these three descriptor parameters.

CO2 Percent Conversion

[0058] It indicates the rate of CO.sub.2 transformation compared to the initial concentration, and is expressed by the following mathematical relation:

[00001]$C{O}_2\mathrm{conversion},\%=\frac{{\left[C{O}_2\right]}_i-{\left[C{O}_2\right]}_t}{{\left[C{O}_2\right]}_i}.Math.100$ [0059] where [CO.sub.2].sub.i indicates the CO.sub.2 concentration at time i=initial, i.e. before the start of the process; [0060] where [CO.sub.2].sub.t indicates the CO.sub.2 concentration at time t, i.e. at a time instant after the start of the process.

[0061] In gas chromatography, the concentration of an analyte can be expressed as the product of a response factor and the area of the signal of the analyte under evaluation. From a mathematical viewpoint, this can be expressed as:

[CO.sub.2]=f.sub.f.Math.Area of CO.sub.2 signal [0062] where [CO.sub.2] is the concentration of the carbon dioxide analyte; and [0063] where f.sub.r is the response factor for the CO.sub.2 analyte and refers to a specific column and a specific method of analysis.

[0064] Therefore, with appropriate substitutions and simplifications, the equation for calculating the CO.sub.2 conversion rate can be rewritten as:

[00002]$\mathrm{CO}2\mathrm{conversion},\%=\frac{\left(f_r.Math.\mathrm{Area}{\mathrm{CO}}_{2_i}\right)-\left(f_r.Math.\mathrm{Area}{\mathrm{CO}}_{2_t}\right)}{\left(f_r.Math.\mathrm{Area}{\mathrm{CO}}_{2_i}\right)}.Math.100$$\mathrm{CO}2\mathrm{conversion},\%=\frac{f_r.Math.\left(\mathrm{Area}{\mathrm{CO}}_{2_i}-\mathrm{Area}{\mathrm{CO}}_{2_t}\right)}{f_r.Math.\mathrm{Area}{\mathrm{CO}}_{2_i}}.Math.100$$C{O}_2\mathrm{conversion},\%=\frac{\mathrm{Area}{\mathrm{CO}}_{2_i}-\mathrm{Area}{\mathrm{CO}}_{2_t}}{\mathrm{Area}{\mathrm{CO}}_{2_i}}.Math.100$

Selectivity

[0065] It expresses the ratio between the concentration of a specific product and the total product concentration. It can be calculated by means of the following mathematical relation (expressed for methane):

[00003]$C{H}_4\mathrm{Selectivity}\%=\frac{\left[C{H}_4\right]}{.Math.X.Math.}.Math.100;$$\left[X\right]=\mathrm{total}\mathrm{product}\mathrm{concentration}$

[0066] This relation is applied in order to obtain the selectivity of the detectable products, taking into account that, for each one of them, it will be necessary to draw a calibration straight line in order to be able to obtain the concentration.

Yield of the Reaction in Giving Methane (and/or Other Products)

[0067] It expresses the percent abundance of a product relative to the conversion rate of the process. From a mathematical viewpoint, it can be expressed as the product of selectivity and conversion:

CH.sub.4 Yield, %=CH.sub.4 Selectivity?CO.sub.2 Conversion/100

Results

[0068] The test results, expressed as percent values of CO.sub.2 conversion over time (FIG. 3a), highlighted very good reproducibility characteristics, with a variability percentage of less than 5% (attributable to the experimental/instrumental error), and FIG. 3a) shows a typical profile of the experimental data of the reaction kinetics. The pattern shows a sigmoidal CO.sub.2 conversion profile. In all tests carried out, conversion grows significantly only after an initial induction time, which in the test shown in FIG. 3a) was approximately 30 minutes. Conversion speed then increases, reaches a maximum value at an inflection point in the curve, and then decreases, corresponding to the asymptote the conversion tends to, which in the case represented in FIG. 3a) exceeds the value of 80%. As can be seen in FIG. 3a), 150 minutes after the start of the grinding process the CO.sub.2 conversion rate remained almost constant for 3 hours. The sigmoidal pattern of the kinetic curve is typical of many mechanochemical processes, and particularly of processes involving solid-gas heterogeneous systems. Such a pattern has often been interpreted as the result of nucleation-and-growth processes and analyzed on the basis of Avrami-Erofeev mechanisms. Without going into details of the mechanistic analysis, it is however useful to underline, in this context, that the shape of the kinetic curve profile comprises multiple elementary stages of the conversion process, including superficial H.sub.2O adsorption by Olivine, its dissociation, processes of H.sub.2 and O.sub.2 scattering in the bulk of the solid phase, oxidative processes undergone by Fe2+, adsorption and absorption of CO.sub.2, its reduction by H.sub.2, mineral carbonation processes, scattering and desorption of the formed hydrocarbons, etc. The exact correlation between such processes and the shape of the curve goes beyond the intention of the present invention, but they should be taken into consideration for a global evaluation of the process. A further aspect is the analysis of the hydrocarbon phases produced: the process highlights how a CO.sub.2 fraction undergoes a reduction process and hence can be transformed into light hydrocarbons such as methane, ethylene and ethane. The kinetic curves concerning the formation of such gaseous phases are shown in FIG. 3b): the profile of such curves seems to follow the same sigmoidal pattern as the one previously mentioned, and the concentration values of said phases different. The maximum concentrations measured for the three above-mentioned products are 680 ppm for methane, 280 ppm for ethane and 100 ppm for ethylene, measured after 330 minutes of grinding time (FIG. 3b)). FIG. 4a) shows the trend over time of the selectivity data for the above-mentioned products: methane is the main product, with a selectivity of 60%, while the selectivity values for ethane and ethylene are, respectively, 20-25% and 8-10%.

[0069] FIG. 4b shows global yield data, indicating a production of approximately 50% for methane, 10-15% for ethane and 10% for ethylene.

[0070] It must be pointed that the conversion, selectivity and yield data are very stable over time, with values reaching a maximum point after approximately 90 minutes and then remaining constant (except for the experimental error) throughout the observation interval, i.e. for up to 360 minutes of mechanical treatment.

[0071] In order to deepen the knowledge of the process mechanism and determine the experimental conditions that led to the best results in terms of conversion, selectivity and yield, the process was studied as a function of some experimental parameters, in particular the stoichiometric ratio between H.sub.2O and Olivine and the mechanical energy transferred to the reagents during the mechanical treatment.

Dependency of the Kinetics on the Olivine/H.SUB.2.O Stoichiometric Ratio

[0072] The H.sub.2O content in the reaction environment represents an important parameter in the reaction of methanation of CO.sub.2 mechanically activated in the presence of Olivine, because water is the hydrogen source. FIG. 5 shows the results of tests conducted with different H.sub.2O contents in the reaction environment.

[0073] The corresponding stoichiometric ratio values are as follows: [0074] 0.15 ml H.sub.2O, Olivine/H.sub.2O=0.01 mol Olivine/0.01 mol H.sub.2O=1:1* *Such stoichiometric ratios were calculated considering Olivine powders as consisting only of Forsterite Ferroan phase of formula (Fe.sub.0.184, Mg.sub.1.816)SiO.sub.4 and molar mass of 146.4693 g/mol. [0075] 0.3 ml H.sub.2O, Olivine/H.sub.2O=0.01 mol Olivine/0.02 mol H.sub.2O=1:2* [0076] 0.6 ml H.sub.2O, Olivine/H.sub.2O=0.01 mol Olivine/0.03 mol H.sub.2O=1:3*

[0077] The kinetic curves shown in FIG. 5 highlight the existence of an initial period during which CO.sub.2 conversion appears to be negligible. Such induction period seems to be influenced by the adopted experimental conditions, and its amplitude depends on the relative ratio between Olivine and H.sub.2O. In the three tests presented herein, therefore, the conversion data increase according to a sigmoidal kinetic profile, whose specific characteristics seem to differ (maximum conversion speed values and mechanical treatment time values at which measurements were taken). The maximum conversion value obtained during the test appears to be similar (approx. 60%) in all three cases. It should be noted that, in the test conducted with the lowest (1:1) Olivine/H.sub.2O ratio, the rate of conversion decreases in a monotonous manner after having reached the maximum value. This seems to suggest that, under these conditions, the H.sub.2O content of the reaction cannot keep constant the CO.sub.2 conversion value, although at this level it is not possible to discern whether the limiting factor is Fe2+ oxidation or CO.sub.2 reduction.

Dependency of the Kinetics on the Energy Transfer Conditions During the Mechanical Treatment

[0078] The present study is based on the activation of the process of conversion of carbon dioxide induced mechanochemically. With such mode of activation, the energy required for triggering the chemical process is administered in the form of kinetic energy (E.sub.k=? mv.sup.2): the dependency of the reaction kinetics on the mechanical energy transferred to the reagents was evaluated by means of tests conducted under different conditions in terms of the mass of the milling bodies and the revolution speed of the motor driving the Spex mill.

[0079] As far as the former parameter is concerned, i.e. the mass of the milling bodies, several tests were carried out using three steel balls having a mass of, respectively, 1 g each, 4 g each, and 8 g each, as highlighted by the three curves shown in FIG. 6.

[0080] FIG. 6 shows, again, sigmoidal patterns for each one of the three curves referring to different experimental conditions as concerns the masses of the milling bodies employed in the tests. The time of mechanical treatment being equal, the degree of CO.sub.2 conversion depends on the mass of the milling bodies in use, and conversion speed is also dependent on the same parameter. In general terms, it is important to point out that it is possible to determine the kinetic energy transferred during each collision, but such evaluation lies outside the present context, while still being important in view of an in-depth mechanistic survey.

[0081] Conversely, variations in the mass of the milling bodies do not seem to significantly affect the selectivity values for gaseous products like methane, ethylene and ethane, obtained during the process. In qualitative terms, these patterns follow those shown in FIG. 4a).

[0082] The energy transferred to the reagent species through collisions with the milling bodies can also be modulated by adjusting the revolution speed of the electric motor connected to the Spex 8000 mill. The rotation of the motor shaft of the mill imparts motion to the milling bodies and affects the frequency of the collisions between the balls and the jar walls. The tests were carried out using commercial Spex Mixer/Mill 8000 mills at revolution speeds of 875 rpm and 1,000 rpm. The results concerning the dependency of the CO.sub.2 conversion data on the revolution speed of the motor of the mill are shown in FIG. 7.

[0083] The revolution frequency of the motor of the mill affects induction time: with a revolution speed of 1,000 rpm, the CO.sub.2 conversion process is activated sooner than with a revolution speed of 875 rpm. In this case as well, the distribution of the final products obtained follows the pattern shown in FIG. 4a).

[0084] The study also tackled the behaviour of the systems used for the mechanically activated CO.sub.2 conversion process when re-processed in the presence of CO.sub.2 and H.sub.2O. FIG. 8 shows a comparison between the experimental data curves obtained with Olivine powders at their first use (grey curve) and at their second use (black curve).

[0085] It emerges that the system is more active at the second use: induction time is shorter, and the CO.sub.2 conversion value reaches 95%, i.e. it is higher than obtained in all tests carried out with Olivine at the first use.

[0086] This fact confirms the thesis that the proposed material (Olivine) is a very good candidate for use in the process of transforming CO.sub.2 into products of high commercial value. The distribution of the products thus obtained follows the same pattern as shown in FIG. 4a).

Morphological-Structural Characterization of the Solid Materials Before and After the Mechanochemical Reaction

[0087] Olivine powders before (pre-grounded for 1 hour) and after the reaction were characterized by X-ray diffraction using a Rigaku SmartLab diffractometer with a rotating anode source (?.sub.Cu=1.54178 ?) and equipped with a graphite monochromator on the diffracted beam, employing a Bragg-Brentano geometry. FIG. 9 shows a sequence of X diffraction patterns concerning unprocessed Olivine and samples obtained after several mechanochemical tests under the conditions specified in FIG. 9. From the sequence of diffraction patterns for the analyzed systems summarized in FIG. 9, one can see that in the processed materials the main phases of unprocessed Olivine (Forsterite, Enstatite e Clinochlore) are preserved, with the appearance of carbonate phases such as Nesquehonite (magnesium carbonate hydrate) and Iron. Other minority phases (<1% by mass) not yet attributed, but compatible with phases such as Magnesite (magnesium carbonate), Maghemite (Fe (III) oxide) were also detected.

[0088] The quantitative evaluation of the abundance of the crystallographic phases in the different samples, and of the microstructural parameters, was conducted using the Rietveld refinement on method all samples under examination. FIG. 10 shows, by way of example, the X diffraction patterns concerning two samples representative of the tests carried out: Olivine after pre-treatment (lower pattern) and Olivine after mechanochemical treatment with CO.sub.2 and H.sub.2O (upper pattern in FIG. 10). Along with the experimental data, the figure shows the profiles concerning the structural refinement made in accordance with the Rietveld method, and the following Table 1 and Table 2 show the results of the analysis of the abundance of the crystallographic phases and of the microstructural parameters.

[0089] The X-ray diffraction pattern of unprocessed Olivine can be described through the Forsterite (90% by mass), Enstatite (8%) and Clinochlore (2%) phases, whereas the samples processed in the reactive atmosphere show the Forsterite (70-86%), Enstatite (5-7%), Clinochlore (6-15%), Nesquehonite (1-3.6%) phases, and a minority fraction of elementary Iron (1-3%) can be recognized (the latter may be due to a phenomenon of precipitation of Fe particles from the walls of the mechanochemical reactor during the process).

TABLE-US-00001 TABLE 1 Crystallographic data concerning the phases in the unprocessed Olivine sample, subjected to pre-grinding for 1 hour. The quantitative evaluation was obtained by analyzing the X-ray diffraction data with the Rietveld method. AMCSD Grain Micro Name Chemical Formula code a/? b/? c/? sizes nm Strain Forsterite (Fe0.184, Mg1.816)SiO4 1696 4,763 10,225 5,994 103.25 2.33E?06 Enstatite (Fe0.155, Mg0.845)SiO3 1695 18,253 8,833 5,200 47.75 2.11E?04 Clinochlore [(Al1.84, Fe0.5, Mg4.5)Si3.16O18H8] 4250 5,162 9,562 14,418 153.25 3.74E?03

TABLE-US-00002 TABLE 2 Crystallographic data concerning the phases contained in the Olivine sample after mechanochemical treatment with CO.sub.2 and H.sub.2O. The quantitative evaluation was obtained by analyzing the X-ray diffraction data with the Rietveld method. AMCSD Grain Micro Name Chemical Formula code a/? b/? c/? sizes nm Strain Forsterite (Fe0.184, Mg1.816)SiO4 1696 4,762 10,223 5,993 106.59 4.40E?06 Enstatite (Fe0.155, Mg0.845)SiO3 1695 18,255 8,833 5,196 46.83 1.81E?04 Clinochlore [(Al1.84, Fe0.5, Mg4.5)Si3.16O18H8] 4250 5,193 9,552 14,398 156.78 1.46E?03 Nesquehonite MgCO33(H2O) 9432 7,767 5,375 1,213 119.23 1.93E?05 ?-Fe Fe 670 2,872 2,872 2,872 32 2.18E?03

[0090] The unprocessed materials and the processed materials (i.e. those subjected to mechanochemical reaction) were analyzed by scanning electron microscopy (SEM) using an FEI Quanta 200 microscope equipped with a Genesis XM2i Apollo 10SSD detector for EDS microanalyses.

[0091] FIG. 11 shows the images obtained by means of the ETD detector, concerning the secondary electrons emitted by the samples, and illustrates the morphology of 2 samples of Olivine, respectively pre-ground for 1 hour (FIG. 11A) and processed in the presence of CO.sub.2 and H.sub.2O (FIG. 11B). The samples show no substantial differences, except for a smaller grain size due to prolonged exposure to grinding. In the map of the distribution of elements in the processed Olivine samples, it is possible to observe the presence of Fe, Mg, Al, Cr, Si, O, that constitute the silicate, and carbon C, the presence of which is due to the formation of carbonates following the process of mechanochemical activation in the presence of CO.sub.2 (FIG. 12).

[0092] Pre-reaction and post-reaction samples were also characterized by infrared spectroscopy (FT-IR) using a Jasco FT-IR 4600 instrument in ATR mode. FIG. 13 shows the FT-IR spectra of unprocessed Olivine and Olivine subjected to the mechanochemical reaction.

[0093] The signals between 1,000 and 500 cm.sup.?1 can be attributed to stretching and bending vibrations of the SiOSi bonds that are characteristic of silicates; such bands are easily identifiable in both spectra. In the spectrum concerning the sample of mechanically processed Olivine, it is also possible to observe the presence of two new characteristic bands centered at 1,482 and 1,420 cm.sup.?1, which are compatible with the vibrations of the carbonate groups, whose appearance is due to the presence of CO.sub.2 during the grinding process. Such signals confirm the evidence obtained from the X-ray diffraction data.

Hydrogen Evolution

[0094] An experiment was also conducted in order to evaluate the process of evolution of hydrogen, which did not participate in the reaction with CO.sub.2, originated from the dissociation of water induced by the mechanochemical process in the presence of Olivine. This test was carried out under flow conditions, using a mixture of 10% CO.sub.2 balanced in Argon. The experimental setup can be summarized as follows: [0095] 2 g of Olivine pre-ground for one hour; [0096] H.sub.2O: 0.3 mL; [0097] Milling bodies: 3 balls weighing 4 g each; [0098] Revolution speed of the mill motor: 875 rpm.

[0099] The test was conducted in accordance with the same operating modes as described in the section entitled Description of the Invention. The experimental data concerning hydrogen evolution are shown in FIG. 14.

[0100] Hydrogen is produced starting from water by means of a dissociation process activated by the mechanochemical process in the presence of Olivine. By appropriately dosing the amount of H.sub.2O and the Olivine/H.sub.2O ratio in the reactor, it is possible to keep such production constant (which is a fundamental requisite) during the methanation process.

INDUSTRIAL APPLICABILITY OF THE INVENTION

[0101] The method of the present invention has made it possible to convert carbon dioxide (CO.sub.2) into high added value chemical compounds under continuous gas flow conditions. In particular, said method converts CO.sub.2 into a mixture of high added value chemical compounds comprising low molecular weight hydrocarbons, mainly methane, ethylene and ethane, and products of mineral carbonation, mainly Mg and Fe carbonates. Said CO.sub.2 conversion is achieved through a mechanochemical process and is advantageously applicable to any activity emitting into the atmosphere exhaust fumes comprising, among other possible gaseous substances, significant amounts of CO.sub.2.

LIST OF REFERENCES

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