Method of adsorbing a target substance

Abstract

The present invention relates to a method of adsorbing a target substance comprising the following steps: (I) passing a gaseous stream containing said target substance through a solid adsorbent; said solid adsorbent delimiting pores having BET pore widths ≤ 50 nm, at least part of said pores containing organic receptors covalently bonded to said solid adsorbent; (II) adsorbing at least part of said target substance on said organic receptors to obtain a loaded solid adsorbent and a target substance-depleted stream; (III) applying plasma conditions to said loaded solid adsorbent to desorb at least part of said target substance; (IV) optionally recycling said target substance-depleted stream of step (II) to step (I) to further reduce an amount of said target substance.

Description

[0001] “Method of adsorbing a target substance’

[0002] DESCRIPTION

[0003] Field of the invention

[0004] The present invention relates to a method of adsorbing a target substance in a gaseous stream, in particular in a selective manner.

[0005] Prior art

[0006] WO 2022 / 010346 A1 - also published as US 2023 / 219031 A1 - discloses a system for CO2 capture.

[0007] Such system is operated with pellets that, during CO2 capture, undergo structural and morphological changes due to a plasma treatment. Such changes lead to alterations of adsorption and / or desorption behaviour.

[0008] Without being bound by theory, these changes may be caused by a bombardment of energetic electrons, interaction with reactive species, and / or formation of local hot spots.

[0009] Based on BET analysis of spent pellets it has been evaluated that changes already occur after an exposure to plasma of 50 min and lead to a decreased in CO2 capacity. These pellets are therefore required to be replaced frequently because such decrease is not reversible by regeneration.

[0010] Besides, also it would be desirable if the energy efficiency and selectivity of the process disclosed in WO 2022 / 010346 A1 were increased. Besides, US 2006 / 0165574 A1 belongs to the known art.

[0011] After long and intensive research and development the Applicant has developed a method that provide an adequate response to the existing limitations, drawbacks, and problems. Therefore, the subject matter of the present invention is method of adsorbing a target substance. of the invention

[0012] The purpose of the invention is to solve the aforementioned drawbacks of the known art.

[0013] The above aims are achieved by a method of adsorbing a target substance comprising the following steps:

[0014] (I) passing a gaseous stream containing said target substance through a solid adsorbent; said solid adsorbent delimiting pores having BET pore widths < 50 nm (nanometres), at least part of said pores containing organic receptors covalently bonded to said solid adsorbent, in particular to inner surfaces of such pores;

[0015] (II) adsorbing at least part of said target substance on said organic receptors to obtain a loaded solid adsorbent and a target substance-depleted stream;

[0016] (III) applying plasma conditions to said loaded solid adsorbent to desorb at least part of said target substance;

[0017] (IV) optionally recycling said target substance-depleted stream of step (II) to step (I) to further reduce an amount of said target substance.

[0018] In the present description, “pore width” means pore size of the pores delimited by the solid adsorbent. “BET pore widths” means that the pore widths / sizes, and more precisely a pore size distribution, is / are measured with BET (Brunauer- Emmett-Teller) method, in particular with a standard nitrogen physisorption BET method. E.g. a Micromeritics 3Flex analyser can be used for such measure.

[0019] In the present description the expression “covalently bonded” is used as a synonym of grafted.

[0020] Furthermore, “organic receptor” in this description is an organic chemical group or molecule on a surface and / or in an interior of the solid adsorbent that has an affinity for the specific target substance. In the present description “affinity” means that said organic chemical group or molecule exerts an attractive force towards the target substance that causes the organic chemical group or molecule and the target substance to enter into contact and to remain in chemical combination.

[0021] Advantageous effects of the invention

[0022] The invention has the following advantages.

[0023] Advantageously, the method of the present invention makes use of electric energy produced by at least a renewable source, so that such method proposes as a main driver for the future decarbonisation path to achieve net zero emissions.

[0024] Advantageously, the application of plasma conditions is flexible: it can be switched on / off quickly, and / or it can follow fluctuations of a time-variable energy supply.

[0025] Advantageously, the application of plasma conditions can bring synergistic effects because both plasma generation and adsorption are nanoscale phenomena, so that process intensification is possible.

[0026] Advantageously, the application of plasma conditions is particularly suitable for molecules for which activation is typically difficult, such as thermodynamically stable molecules.

[0027] Advantageously, the application of non-thermal plasma allows to operate at milder conditions, so that limits related to catalysis-related equipment and / or chemical equilibrium can be overcome.

[0028] Advantageously, plasma-based desorption and conversion allow a direct utilization of CO2 as a syngas without further steps such as compression, storage, transportation, reverse-WGS. CAPEX and OPEX are thus lowered.

[0029] Advantageously, the present method can be implemented to retrofit existing processes and plants (e.g. ammonia or methanol synthesis) to carry out CO2 capture and allow production of blue chemicals. Advantageously, the method of the present invention allows a faster chemical desorption of the adsorbed chemical species with respect to the known art.

[0030] Advantageously, the method of the present invention promotes a desorption that may be accompanied by a conversion / reaction of the previously adsorbed species. Hence, the present method provides for a capture of chemical species, and for a desorption of other at least partially converted chemical species.

[0031] Preferred embodiments

[0032] According to a preferred embodiment, said pores are mesopores having pore widths comprised from 2 nm to 50 nm, preferably comprised from 3 nm to 30 nm, more preferably comprised from 4 nm to 20 nm, even more preferably comprised from 5 nm to 15 nm, still more preferably comprised from 6 nm to 10 nm.

[0033] “Pore width” intends width of a slit-shaped pore or a diameter of a cylindrical pore to avoid a misleading change in scale when comparing cylindrical and slit-shaped pores. As a reference see Pure & Appl. Chem., Vol. 66, No. 8, pp. 1739-1758, 1994.

[0034] According to another embodiment, said pores are micropores having pore widths < 2 nm. In case of micropores pore widths should be sufficiently large to allow one or more organic receptor(s) to be contained in each pore.

[0035] Step (III) preferably comprises applying non-thermal plasma conditions wherein no external heat is provided.

[0036] According to an advantageous embodiment, a plasma-induced decomposition of said organic receptors is substantially prevented in the plasma conditions of step (III) by said pore widths. In other words, the pore widths of the solid adsorbent are dimensioned so as to avoid plasma to enter the pores, and consequently to decompose the organic receptors.

[0037] The plasma conditions of step (III) are preferably applied by a dielectric-barrier discharge (DBD) between two electrodes distanced from each other to delimit an interspace, said solid adsorbent being at least partially arranged in said interspace.

[0038] The two electrodes may be planar. As an alternative, an electrode may be tubular and coaxially arranged to enclose at least part of the other electrode.

[0039] According to different embodiments, the plasma conditions of step (III) comprise any one of the following:

[0040] - a frequency comprised from 20 kHz to 1 MHz; and / or

[0041] - a discharge power comprised from 10 W to 2 kW; and / or

[0042] - an electric potential difference comprised from 10 V to 500 kV.

[0043] Said plasma conditions of step (III) are at least partially provided by a renewable energy source, e.g., a solar panel and / or a wind turbine.

[0044] Said target substance is preferably selected from the group consisting of: carbon dioxide (CO2), ammonia (NH3), nitrous oxide (N2O), nitrogen oxides (NOx; i.e. NO and NO2), and mixtures thereof.

[0045] More preferably, said target substance is CO2. According to such embodiment, step (III) comprises a simultaneous conversion of at least part of said CO2 into carbon monoxide (CO).

[0046] According to another embodiment, the method further comprises feeding at least part of said CO and of any unconverted CO2 as a synthesis gas (syngas) to produce oxygenates (e.g. alcohols or ethers) or hydrocarbons, e.g., any one from among: acetic acid, formaldehyde, sustainable aviation fuels (SAF), methane, and / or methanol.

[0047] Preferably, said solid adsorbent is selected from any of: aluminium oxide (alumina), silicon dioxide (silica), silicate or aluminosilicate (e.g., a zeolite or MCM-41 ), titanium dioxide (titania), magnesium-aluminium oxide, magnesiumaluminium carbonate (e.g., hydrotalcite), cerium dioxide (ceria), etched derivatives thereof (e.g., etched zeolite), and any combination thereof. Etching of the etched derivatives may be performed with ammonium fluoride (NH4F). More preferably, said solid adsorbent is alumina or silica, even more preferably y-alumina.

[0048] Said solid adsorbent may be in the physical form of a granulate, a pellet, or a powder.

[0049] According to an embodiment, said organic receptors are metal-organic frameworks (MOFs). MOFs are a class of porous polymers comprising or consisting of metal clusters (also known as Secondary Building Units; SBUs) coordinated to organic ligands to form one-, two- or three-dimensional structures.

[0050] According to further different embodiments, said organic receptors are primary amines, and / or secondary amines, and / or amine derivatives; said amines or amine derivatives are preferably selected from the group consisting of:

[0051] - (3-aminopropyl)trimethoxysilane;

[0052] - (3-aminopropyl)triethoxysilane;

[0053] - (3-aminoethyl)trimethoxysilane;

[0054] - N-[3-(trimethoxysilyl)propyl]ethylenediamine;

[0055] - N1 -(3-trimethoxysilylpropyl)diethylenetriamine; and any mixtures thereof.

[0056] According to a still further embodiment, said organic receptors are (3- mercaptopropyl)trimethoxysilane.

[0057] Preferably, said amine or amine derivative is (3-aminopropyl)trimethoxysilane or (3-aminoethyl)trimethoxysilane.

[0058] The advantages of the invention will be even more evident from the following detailed description made on the basis of the enclosed figures, which are provided as a non-limiting example. Description of the figures

[0059] Fig. 1 : desorbed CO and CO2 (“COx”), comparison among different materials - i.e. bare y-alumina, functionalized y-alumina, bare a-alumina, functionalized a- alumina - tested for four cycles;

[0060] Fig. 2: amount of COx desorbed of plasma-treated and thermally treated am inefunctionalized Y-AI2O3 and the capacity of bare alumina [P = 18.7 W, U = 4.9 kV],

[0061] Fig. 3: Attenuated Total Reflection (ATR) spectra of fresh and spent amine functionalized Y-AI2O3;

[0062] Fig. 4: Scanning Electron Microscopy (SEM) pictures of amine-functionalized alumina before and after plasma treatment [P= 18.7 W, U = 4.9 kV],

[0063] Fig. 5: desorption test of APTMS-functionalized amorphous silica and bare amorphous silica for four cycles.

[0064] Fig. 6: desorption test of APTMS-functionalized alumina and bare alumina for eight cycles.

[0065] The present invention will now be illustrated on the basis of the examples, which are provided for illustrative purpose only.

[0066] EXAMPLE

[0067] Example 1 : Preparation of a solid adsorbent with organic receptors according to the present invention

[0068] Y-alumina (4g; from Sigma Aldrich) was dried overnight at 110°C under vacuum. The dried Y-alumina was added to a solution comprising 200 ml of toluene, 1.2 ml of water and 2.1 ml of (3-aminopropyl)trimethoxysilane to obtain a mixture. Such mixture was heated at 85°C and stirred overnight for at least 12 h until the grafting reaction had taken place. The resulting reacted mixture was filtered, washed with toluene and ethanol, and dried at room temperature overnight under vacuum to obtain a functionalized solid adsorbent with organic receptors.

[0069] Example 2: Tests performed on functionalized solid adsorbents (invention) and bare solid adsorbents (reference) C02and CO (globally indicated as “COx”) were desorbed from solid adsorbents comprising organic receptors according to step (III) of the present invention and from bare solid adsorbents (reference). For brevity, such solid adsorbents with organic receptors are named “functionalized”. Solid adsorbents used a reference that do not comprise organic receptors are named “bare” (non-functionalized). The functionalized solid adsorbent and the bare solid adsorbent were otherwise identical. y-alumina (Y-AI2O3; invention) with an average pore width of 7.6 nm and a- alumina (a-A^Os; reference) with an average pore width above 50 nm were used as solid adsorbents. The organic receptor was an amine, in particular (3- aminopropyl)trimethoxysilane (APTMS).

[0070] In the tests of Fig. 1 , bare y-alumina and bare a-alumina were confronted with y- alumina and a-alumina functionalized with APTMS as to desorption of COx in solid adsorbents to whom plasma conditions were applied four times (cycles). Conditions of applying plasma conditions were a discharge power P = 18.7 W and an electric potential difference U = 4.9 kV.

[0071] From Fig. 1 it can be seen that the functionalized solid adsorbents have better performances with respect to corresponding bare solid adsorbents.

[0072] Furthermore, from the same figure it is possible to assess that Y-AI2O3 has at least two advantages over a-Al2O3 because: (1 ) the increase of capacity is almost double in the cycle 1 ; and (2) it is subjected to a minor reduction of desorbed COx between the cycle 1 and the cycle 2. This is due to the fact that a-Al2O3 has a larger pore width (> 50 nm) that does not prevent plasma-induced decomposition of the organic receptors.

[0073] As to Fig. 2, desorption of step (III) was performed by applying plasma conditions (invention) or by applying a thermal treatment (reference) where the sample was heated to 125 °C for 30 minutes with a ramp of 10°C / min. For all cycles, the functionalized alumina outperforms the bare alumina with a higher amount of desorbed COx. This shows that the functionalization with APTMS indeed increases the CO2 adsorption capacity.

[0074] In the cycle 1 , 24.8 ml of COx are desorbed from 2.48 g of functionalized alumina, corresponding to 0.40 mmol / g. Then, the desorbed volume is reduced by 30% in cycle 2, followed by a decrease of 6% and 4% in the third and fourth cycles respectively. On the contrary, the amount of desorbed COx from bare y-alumina and in the case of thermally treated functionalized y-alumina are stable over the four cycles and are 0.09 mmol / g and 0.38 mmol / g respectively.

[0075] In the case of plasma treatment with functionalized y-alumina, the amount of COx desorbed is reduced significantly by the treatment of cycle 1 , but for subsequent cycles 2-4, the reduction is limited. Despite the sharp reduction of capacity, the plasma-treated functionalized y-alumina still significantly outperforms the bare solid adsorbent, indicating that the functionality of the amine groups is partially preserved over the cycles.

[0076] Plasma can decompose organic molecules such as amines. Amine groups grafted on an outer surface of the y-alumina that are directly exposed to plasma and can therefore be decomposed. The above sharp reduction of capacity observed between cycle 1 and cycle 2 of Fig. 2 can be explained with a decomposition of the amine organic receptors on the outer surface of the y- alumina.

[0077] However, plasma can only penetrate pores with a width that is large enough, in particular larger than 50 nm. Inside pores having a pore width equal to or smaller than 50 nm there is no direct exposure of the amine organic receptors to plasma. Amine groups functionalizing the mesopores of y-alumina are therefore protected against direct plasma exposure and any decomposition is therefore substantially prevented. This circumstance is evident from the substantially constant capacity observed in cycle 2, cycle 3 and cycle 4 of Fig. 2 for Y-AI2O3.

[0078] Attenuated Total Reflection (ATR) spectroscopy (instrument: Thermo Fischer Nicolet iS50) was used to characterize the functional groups on the surface of solid adsorbents in the form of pellets, and afterwards of powdered pellets. ATR spectra of fresh and plasma-treated amine-functionalized v-alumina are shown in Fig. 3.

[0079] Peaks at 1330 cm-1, 1385 cm-1, 1490 cm-1, 1560 cm-1, and 1633 cm-1are assigned to the CCh-amine complexes, the peak at 2930 cm-1is assigned to the C-H bound, and the broad peak above 3000 cm-1is assigned to the O-H bound.

[0080] Peaks belonging to CCh-amine complexes are present in the fresh solid adsorbent in the form of pellets and relate to complexes formed on the outer surface of the v-alumina. “Fresh” solid adsorbent is synonym of loaded solid adsorbent after step (II).

[0081] However, after applying four cycles of plasma conditions (same conditions as above), the spectra of the spent specimen do not, or only barely show peaks belonging to this complex because the amine organic receptors on the outer surface of the v-alumina are decomposed in plasma conditions, thus explaining the reduction of the CO2 capacity.

[0082] The expression “spent” reported in Fig. 3 refers to the solid adsorbent subjected to four cycles of plasma conditions.

[0083] Afterwards the pellets of solid adsorbent were ground with a mortar and pestle for at least one minute to obtain a powder, and new ATR spectra were taken on the powder so obtained.

[0084] When measuring ATR spectra of the spent powder, the organic receptors that were originally located in the pores of the solid adsorbent, within the bulk of the pellets, were also detected.

[0085] The peaks belonging to the CCh-amine complex are again visible. This means that the organic receptors within the pores of the solid adsorbent substantially do not undergo a plasma-induced decomposition and that amine organic receptors in the bulk of the solid adsorbent remain even after plasma conditions have been applied. When looking at Scanning Electron Microscopy (SEM) pictures in Fig. 4 no apparent differences can be seen between the fresh and the spent functionalized solid adsorbent. SEM was performed with a SEM Quanta with a Field Electron Emission (FEG) electron source and a backscattered electron detector using the following parameters: Magnification: 10.000 times, acceleration rate: 5.0 kV. This means that the plasma conditions do not significantly change the morphology of the y-alumina.

[0086] In below Table 1 the BET surface area is shown. After functionalization with APTMS, the surface area of y-alumina is reduced significantly because APTMS that binds covalently to the solid adsorbent blocks the pores of the y-alumina.

[0087] After thermal treatment, the surface area of the tested materials does not change significantly. On the contrary, after applying plasma conditions, the surface area is increased. Plasma conditions decompose some of the APTMS groups blocking the pores of the y-alumina, whereas the thermal treatment does not decompose such amine groups. This explains why the BET surface of functionalized APTMS- Y-AI2O3 increases up to 84.5 m2 / g after plasma treatment, but remains substantially constant (242 m2 / g I 65.4 m2 / g) after thermal treatment of the bare and of the functionalized y-alumina.

[0088] Table 1

[0089] APTMS grafted on amorphous silica have also been tested and have shown similar results. See Fig. 5. Longer cycle test confirmed better performance and durability of amine- functionalized y-alumina. See Fig. 6.

[0090] Example 3: Other tested solid adsorbents

[0091] Following Table 2 lists other solid adsorbents that were tested together with desorption conditions, amounts of desorbed COx in cycle 1 , solid adsorbent stability, conversion in cycle 1 and efficiency in converting CO2 into CO.

[0092] Table 2

[0093] Example 4: Comparison of desorption speed performed by applying plasma conditions (invention) versus thermal desorption (reference) and versus applying vacuum pressure swing adsorption (VPSA; reference).

[0094] A solid adsorbent with organic receptors was obtained according to the procedure of Example 1 for grafting (3-aminopropyl)trimethoxysilane (APTMS) to an MCM-

[0095] 41 silicate. Three samples (5 grams each) were prepared with such solid adsorbent, each of which was subjected to adsorption, flushing and desorption in a reactor having a reactor inner volume of 2.6 cm3.

[0096] Adsorption was performed with an inert gas (argon or nitrogen) comprising 33% molar of carbon dioxide for 15 minutes. The flow rate of the inert gas was 40 ml / min. Flushing was carried out with the same inert gas alone (without carbon dioxide therein) by feeding a flow rate of 40 ml / min for the time needed for feeding an inert gas volume that is at least three times the reactor inner volume.

[0097] Desorption was performed in presence of the same inert gas alone (without CO2) with the same flow rate as during adsorption (40 ml / min).

[0098] A first sample was subjected - according to the invention - to plasma conditions (DBD; 10 W), a second sample (reference) to thermal heating (10 W with a heating rod; up to a temperature of 125 °C), a third sample (reference) to vacuum pressure swing adsorption (VPSA) in presence of a negative pressure of 20 mbar.

[0099] Desorption was considered to be completed when COx concentration - i.e. the sum of the concentrations of CO and of CO2 - dropped below 0.5% molar concentration.

[0100] These experimental tests show that, unexpectedly, the time for a complete desorption of the sample desorbed by applying plasma conditions (732 seconds) was at least 29% smaller than the desorption times of the samples desorbed thermally or by VPSA (1015 seconds).

[0101] Moreover, only the samples treated under plasma desorption showed an unexpected change in composition of the desorbed species. In particular, approximately 50% molar of the adsorbed carbon dioxide was desorbed as carbon monoxide (CO).

[0102] Other preliminary desorption speed tests carried out according to the same procedure as above on target substances different from CO2 - in particular, on ammonia (NH3), nitrous oxide (N2O), and nitrogen oxides (NOx) - showed a reduction of the desorption times by applying plasma conditions with respect to desorption performed with the reference thermal heating and VPSA.

[0103] Even if not specified above, a person skilled in the art may envisage, using the expertise typical of this technological field, varying, or replacing some of the above features with other technically equivalent elements. These variations or replacements also fall within the scope defined by the following claims.

[0104] Furthermore, each alternative illustrated in connection with a particular embodiment can be realised independently of the other embodiments here described.

Claims

CLAIMS1 . A method of adsorbing a target substance comprising the following steps:(I) passing a gaseous stream containing said target substance through a solid adsorbent; said solid adsorbent delimiting pores having BET pore widths - i.e. , a BET pore size distribution - < 50 nm, at least part of said pores containing organic receptors covalently bonded to said solid adsorbent;(II) adsorbing at least part of said target substance on said organic receptors to obtain a loaded solid adsorbent and a target substance-depleted stream;(III) applying plasma conditions to said loaded solid adsorbent to desorb at least part of said target substance;(IV) optionally recycling said target substance-depleted stream of step (II) to step (I) to further reduce an amount of said target substance.

2. The method according to claim 1 , wherein said pores are mesopores having pore widths comprised from 2 nm to 50 nm, preferably comprised from 3 nm to 30 nm, more preferably comprised from 4 nm to 20 nm, even more preferably comprised from 5 nm to 15 nm, still more preferably comprised from 6 nm to 10 nm.

3. The method according to claim 2, wherein step (III) comprises applying non-thermal plasma conditions.

4. The method according to any of the previous claims, wherein a plasma- induced decomposition of said organic receptors is substantially prevented in the plasma conditions of step (III) by said pore widths.

5. The method according to any of the previous claims, wherein the plasma conditions of step (III) are applied by a dielectric-barrier discharge (DBD)between two electrodes distanced from each other to delimit an interspace, said solid adsorbent being at least partially arranged in said interspace.

6. The method according to any of the previous claims, wherein the plasma conditions of step (III) comprise a frequency comprised from 20 kHz to 1 MHz, a discharge power comprised from 10 W to 2 kW and an electric potential difference comprised from 10 V to 500 kV.

7. The method according to any of the previous claims, wherein said plasma conditions of step (III) are at least partially provided by a renewable energy source, e.g., a solar panel and / or a wind turbine.

8. The method according to any of the previous claims, wherein said target substance is selected from the group consisting of: carbon dioxide (CO2), ammonia (NH3), nitrous oxide (N2O), nitrogen oxides (NOx), and mixtures thereof.

9. The method according to claim 8, wherein said target substance is CO2, and wherein step (III) comprises a simultaneous conversion of at least part of said CO2 into carbon monoxide (CO).

10. The method according to claim 9, further comprising feeding at least part of said CO and of any unconverted CO2 as a synthesis gas to produce oxygenates or hydrocarbons, e.g., any one from among: acetic acid, formaldehyde, sustainable aviation fuels (SAF), methane, and / or methanol.11 . The method according to any of the previous claims, wherein said solid adsorbent is selected from any of: aluminium oxide, silicon dioxide, silicate or aluminosilicate, titanium dioxide, magnesium-aluminium oxide, magnesium-aluminium carbonate, cerium dioxide, NH4F-etched derivatives thereof, and any combination thereof.

12. The method according to claim 11 , wherein said solid adsorbent is alumina or silica, preferably y-alumina.

13. The method according to any of the previous claims, wherein said solid adsorbent is in the physical form of a granulate, a pellet, or a powder.

14. The method according to any of the previous claims, wherein said organic receptors are primary amines, and / or secondary amines, and / or amine derivatives; said amines or amine derivatives being selected from the group consisting of:- (3-aminopropyl)trimethoxysilane; - (3-aminopropyl)triethoxysilane;- (3-aminoethyl)trimethoxysilane;- N-[3-(trimethoxysilyl)propyl]ethylenediamine;- N1 -(3-trimethoxysilylpropyl)diethylenetriamine; and any mixtures thereof.

15. The method according to claim 14, wherein said amine or amine derivative is (3-aminopropyl)trimethoxysilane or (3- aminoethyl)trimethoxysilane.

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