ADVANCED POROUS CARBONACEOUS MATERIALS AND METHODS TO PREPARE THEM

Abstract

The present invention concerns porous carbonaceous particles having pores including micropores and macropores, having a mean diameter, determined by laser diffraction, ranging from 15 to 100 m and porous carbonaceous monoliths comprising aggregates of said carbonaceous particles.

Claims

1. Porous carbonaceous particles (I) having pores including micropores and macropores, said porous carbonaceous particles (I) having a mean diameter, determined by laser diffraction, ranging from 15 to 100 m.

2. The particles (I) according to claim 1 having a span of at most 2.5.

3. The particles (I) according to claim 1, wherein the particles have a macropore volume, measured by Hg porosimetry, of at least 0.05 cm.sup.3/g and a BET surface area of at least 800 m.sup.2/g.

4. A process for reducing the size of porous carbonaceous particles (II) having pores including micropores and macropores, said particles (II) having a mean diameter, determined by laser diffraction analysis, ranging from 150 m to 800 m, said process comprising the steps of: cryogenically freezing the particles (II), grinding the frozen particles thereby obtaining particles of reduced size.

5. Porous particles (III) comprising at least one vinylidene chloride polymer having a melting point, said particles having macropores, a mean diameter, determined by laser diffraction analysis, ranging from 20 to 140 m and a span of at most 2.

6. The particles (III) according to claim 5, wherein the vinylidene chloride polymer is a homopolymer.

7. A process for reducing the size of porous particles (IV) having macropores, said particles (IV) comprising at least one vinylidene chloride polymer having a melting point and a mean diameter, determined by laser diffraction analysis, ranging from 170 m to 800 m, said process comprising the steps of: cryogenically freezing the particles (IV) of the vinylidene chloride polymer, grinding the frozen particles (IV) thereby obtaining particles of reduced size.

8. A process for manufacturing porous particles having macropores, said particles comprising a vinylidene chloride polymer having a melting point, said process comprising a step of free radical suspension polymerization of at least one monomer comprising vinylidene chloride in the presence of from 0.13 wt. % to 1.50 wt. %, based on the total weight of said monomer, of at least one cellulosic dispersing agent wherein the manufactured porous particles comprising a vinylidene chloride polymer are the particles (III) of claim 5.

9. A process for manufacturing porous carbonaceous particles by causing the pyrolysis of the vinylidene chloride polymer comprised in the porous particles (III) according to claim 5.

10. A process for manufacturing porous carbonaceous particles, said process comprising: manufacturing porous particles by the process of claim 7, causing the pyrolysis of the vinylidene chloride polymer comprised in the so-manufactured porous particles.

11. A process for manufacturing a porous carbonaceous monolith comprising the steps of: ipreparing a precursor material comprising the particles (III) according to claim 5, iiforming a shaped body (S) comprising aggregates of particles (III), iiiintroducing the shaped body in a furnace, ivcausing the pyrolysis of the vinylidene chloride polymer in the furnace until the porous carbonaceous monolith is obtained.

12. The process according to claim 11, wherein step ii consists of forming a shaped body, by concurrently applying to the precursor material, previously introduced in a mold, at a pressure P ranging from 10 to 300 bars, and a temperature T.sub.1 ranging from T.sub.1,min=20 C. to T.sub.1,max=T.sub.m50 C. wherein T.sub.m is the melting point of the vinylidene chloride polymer.

13. The process according to claim 11, wherein step iv comprises: iv.sub.abringing the temperature of the shaped body up to a temperature T.sub.2 strictly above T.sub.2,min=T.sub.m50 C. and strictly below T.sub.2,max=T.sub.m, wherein T.sub.m is as previously defined, iv.sub.bmaintaining the shaped body, at the temperature T.sub.2 under inert gas flow, to cause the pyrolysis of the vinylidene chloride polymer and the formation of an infusible char, iv.sub.cbringing the temperature of the infusible char up to a temperature T.sub.3 strictly above T.sub.3,m n=Tm and strictly below T.sub.3,max=1300 C., wherein T.sub.m is as previously defined, iv.sub.dmaintaining the infusible char, at the temperature T.sub.3 under inert gas flow, to cause the pyrolysis of the vinylidene chloride polymer, thereby obtaining the porous carbonaceous monolith structure.

14. A shaped body (S) comprising aggregates of particles (III) according to claim 5, said shaped body having macropores.

15. A porous carbonaceous monolith comprising aggregates of carbonaceous particles (I) according to claim 1.

16. The porous carbonaceous monolith according to claim 15, wherein the porous carbonaceous monolith has a total macropore volume measured by Hg porosimetry of at least 0.12 cm.sup.3/g and it has a BET surface area of at least 800 m.sup.2/g.

17. The porous carbonaceous monolith according to claim 15, characterized in that it has a honeycomb structure.

18. A method, comprising extracting CO.sub.2 from a gas composition by selectively adsorbing CO.sub.2 gas using the porous carbonaceous monolith according to claim 15.

19. The process according to claim 4, wherein the obtained particles of reduced size are porous carbonaceous particles (I) having pores including micropores and macropores, said porous carbonaceous particles (I) having a mean diameter, determined by laser diffraction, ranging from 15 to 100 m.

20. The process according to claim 7, wherein the obtained porous particles are porous particles (III) comprising at least one vinylidene chloride polymer having a melting point, said particles having macropores, a mean diameter, determined by laser diffraction analysis, ranging from 20 to 140 m and a span of at most 2.

21. The process according to claim 10, wherein said porous carbonaceous particles are porous carbonaceous particles (I) having pores including micropores and macropores, said porous carbonaceous particles (I) having a mean diameter, determined by laser diffraction, ranging from 15 to 100 m.

Description

EXPERIMENTAL

[0213] Small Porous Particles Comprising Vinylidene Chloride Polymer According to the Invention Obtained by Cryogenic Milling (FP1).

[0214] IXAN PV925 vinylidene chloride copolymer commercialized by Solvay Specialty Polymers was composed of particles having a mean diameter of 201.2 m and a span of 0.6.sub.5. The particle size and the span were determined with the same equipment and procedures as previously described. The particle size was strictly comprised between 70 m and 480 m (see FIG. 2). The melting point of the copolymer determined as previously described was 158 C.

[0215] The said copolymer commercialized by Solvay Specialty Polymers was milled in a cryogenic milling process. The cryogenic milling process was carried out by using liquid nitrogen as the refrigerant and a ball mill. The resulting powder contained fragmented particles (FP1) having a mean diameter of 55.1 m and a span of 1.4.sub.5. The particle diameter was strictly comprised between 4 m and 175 m, and at least 98 vol. % of said particles had a diameter, determined by laser diffraction analysis, ranging from 5 to 150 m (see FIG. 3). The total (high pressure) pore volume and the average pore size, both measured by Hg porosity, were respectively 0.22 cm.sup.3/g and 2.9.sub.5 m. 90 vol. % of the macropores of particles (III) had a diameter, measured by Hg porosimetry, ranging from 0.01 m to 10 m.

[0216] Small Porous Particles Comprising Vinylidene Chloride Polymer and Obtained by Suspension Polymerization According to the Invention (PVDC 1)

[0217] Vinylidene chloride homopolymer was produced in a suspension process. A 65-liter, glassed line autoclave was loaded with 36.9 kg of demineralized water and mechanical stirring was started at 130 rpm (revolutions per minute). Then, 230 g of dimyristyl peroxydicarbonate (DPDC) and 4600 cm.sup.3 of a 10 g/L aqueous solution of methyl hydroxypropyl cellulose (MHPC) as suspending agent were introduced under stirring. Afterwards, mechanical stirring was stopped and the reactor was put under vacuum. Then, 23000 g of vinylidene chloride (VDC) were loaded in the reactor and mechanical stirring was started at 235 rpm. The autoclave was then heated at 60 C. and the polymerization was allowed to proceed until the desired degree of conversion was reached. Monomer consumption was followed by monitoring the pressure drop in the autoclave. After the conversion of monomer to polymer was approximately 85%, the residual monomer was removed by stripping of the slurry formed. For this purpose, the stripping was carried out under vacuum (0.5 bar) at a temperature of 80 C. for 8 hours. The autoclave was then cooled and drained. A slurry purified of the residual monomers was recovered, washed, filtered and dried in a fluidized bed. The resultant product (PVDC 1), was a powder, comprising particles having a mean diameter D50 of 96 m and a span of 0.8, wherein at least 90 vol. % of said particles has a diameter, determined by laser diffraction analysis, ranging from 5 to 150 m. The resultant product has a melt temperature of 170.6 C., a porosity measured by absorption of a DINP of 15.3%. The total pore volume and the average pore size, both measured by Hg porosity, were respectively 0.20 cm.sup.3/g and 1.3 m. 90 vol. % of the macropores of particles (PVDC 1) had a diameter, measured by Hg porosimetry, ranging from 0.01 m to 10 m.

The powder had 20 ppm (part per million) residual vinylidene chloride.

[0218] Transformation of Particles Comprising a Vinylidene Chloride Polymer into Porous Carbon Particles According to the Invention (CP1 and CP2).

[0219] Particles comprising vinylidene chloride polymer obtained by cryogenic milling (FP) or by suspension polymerization (PVDC 1) according to the invention were pyrolysed in a horizontal tubular furnace under a flow of inert gas (Ar) in 3 steps: [0220] About 20 g of particles, in a ceramic crucible, were placed in a quartz tube, fitted in a 3 zone horizontal tubular furnace (carbolite HZS 12/900). The temperature was ramped from room temperature to 130 C. at a rate of 10 C./min and was held at that temperature for 1 h. [0221] In a second step the temperature was further increased with a heating rate of 1 C./min to an intermediate temperature (150 C.) and held at this temperature for 17 h. [0222] In the final step, the temperature was increased to the carbonization temperature (600 C.) at a heating rate of 10 C./min and held at this temperature for 1 h.

[0223] The principal features of the resulting particles are reported in table 3 (see CP1 made from FP1 and CP2 made from PVDC 1).

[0224] Small Porous Carbonaceous Particles According to the Invention Obtained by Cryogenic Milling of Commercially Available Materials (CP3).

[0225] Carbonaceous particles ATMI Brightblack commercialized by Entegris GmbH were milled via a cryogenic milling process. The cryogenic milling process was carried out by using liquid nitrogen as the refrigerant and a ball mill.

[0226] ATMI BB porous carbon microbeads were cryogenically milled, sieved and 3 different fractions were collected: particles larger than 100 m, particles between 20 and 100 m and particles below 20 m.

[0227] The fraction of particles having a diameter comprised between 20 m and 100 m, CP3, represented 53 wt. % of the total weight of cryogenically milled ATMI BB particles. The mean diameter of these particles was 67.8 m and their principal features are reported in table 1 (see CP3).

[0228] Principal Features of the Porous Carbon Particles According to the Invention (CP1 to CP3) and of Comparative Porous Carbon Particles(CP4 to CP7).

[0229] As previously described, the size of porous carbon particles articles were measured by laser diffraction, macroporosity was evaluated by Hg adsorption and microporosity was evaluated by N.sub.2 adsorption.

[0230] Porous carbon particles according to the invention were obtained, as previously described, respectively by pyrolysis (CP1 and CP2) and by cryogenic milling of commercially available carbon particles (CP3). It is clear from the results reported in table 1 that carbonaceous particles combining low diameter, high macroporosity and high microporosity were obtained by the processes disclosed in the present document.

TABLE-US-00001 TABLE 1 Principal features of the porous carbon particles according to the Invention. Carbonaceous Particles CP1 CP2 CP3 Origin of the particle FP1 PVDC 1 ATMI BB pyrolysis pyrolysis Cryogenic milling Mean diameter D50 (m) 24.5 77.8 67.8 Span 2.2 1.2 1.1.sub.5 5 m < particles < 150 m (vol. %) 86 98 99 5 m < particles < 120 m (vol. %) 82 94 90 Macropores mean diameter (m) 7.2 1.1 1.4 0.1 m < macropores < 10 m (vol. 80 80 90 %) Macropores volume (cm.sup.3/g) 0.83* 0.22 0.24 Specific surface area BET (m.sup.2/g) 937 1069 1127 Micropores volume 0.37 0.41 0.4 (cm.sup.3/g) *Intra-particle and inter-particles macroporosities taken together, resulting in overestimated volume.

[0231] Commercially available carbonaceous particles and particles prepared by pyrolysis of commercially available PVDC particles CP4, CP5, CP6 and CP7 were evaluated as comparative examples. CP4 was obtained by pyrolysis of IXAN PV925 composed of particles having a mean diameter of 201.2 m in the same experimental conditions as for the pyrolysis of FP1 or PVDC 1. CP5 were commercially available carbon particles ATMI Brightblack commercialized by Entegris GmbH and CP7 were commercially available carbon particles YP50F commercialized by Kuraray Chemical Co., Japan. CP5 was prepared by pyrolysis of vinylidene chloride polymer comprising particles while CP7 was a steam activated carbon powder prepared from coconut. Finally, CP6 was obtained by jet milling of CP5. The principal features of these comparative examples are reported in table 2.

TABLE-US-00002 TABLE 2 Principal features of comparative porous carbon particles Carbonaceous Particles CP4 CP5 CP6 CP7 Origin of the particle IXAN ATMI ATMI BB YP50F PV925 BB jet milled pyrolysis Mean diameter D50 (m) 160 204.6 5.1 5.5 Span 0.8 0.8 1.2.sub.5 n.a. 5 m < particles < 150 m 38 15 49 n.a. (vol. %) 5 m < particles < 120 m 17 4.5 49 n.a. (vol. %) Macropores mean 3.2 1.0 n.a. n.a. diameter (m) 0.1 m < macropores < 10 m 80 >85 n.a. n.a. (vol. %) Macropores volume 0.207 0.25 n.a. n.a. (cm.sup.3/g) Specific surface area BET 955 1060 1114 1692 (m.sup.2/g) Micropores volume 0.37 0.4 0.43 0.68 (cm.sup.3/g)

[0232] For CP1, CP6 and CP7, Hg porosimetry was not suitable to characterize the macroporosity. Indeed, during measurements, for particles having low mean diameter, typically 22 m for CP1 and, a fortiori, 5.1 m or 5.5 m for CP6 and CP7, Hg filled the macropores inside the particles and voids between the particles in the same pressure range. Consequently, the macropores volume was over estimated for CP1 (0.83 cm.sup.3/g for CP1, see table 2) while it was even not possible to carry out any measurement for CP6 and CP7 (not available results, n.a., in table 2). However, for the first sample (CP1), the macropores were clearly observed by microscopy.

[0233] Preparation of Shaped Bodies According to the Invention.

[0234] A binderless shaped body (SB1) was produced by compressing the particles FP1 below their melting point. A rectangular frame of 3 mm thickness was filled with the resin and pressed at a temperature of 40 C. and a pressure of 28.8 bars. The pressure was maintained for 6 minutes at 28.8 bars at a constant temperature of 40 C. The resulting shaped body was composed of individual polymer particles agglomerated together. After cooling the shaped body was unmolded. A binderless shaped body (SB2) was produced similarly by compressing the particles PVDC 1 at a temperature of 120 C. and a pressure of 28.8 bars.

[0235] Preparation of Shaped Bodies for Comparative Examples.

[0236] As comparative examples, a binderless shaped body (SB3) was prepared in a similar way, at a temperature of 85 C. and a pressure of 19.2 bars, using particles of IXAN PV925 which are particles of large diameter (mean diameter=201.2 m).

[0237] Porosity of Shaped Bodies.

[0238] The overall porosity was determined by DINP adsorption at room temperature as previously described in the core of the specification. The results reported in table 3 illustrate that the porosity of shaped body according to the invention is similar to the porosity of the shaped body of the comparative example. Accordingly it is possible to obtain shaped body with high porosity when using smaller particles.

TABLE-US-00003 TABLE 3 Porosity measurements onto shaped body Shaped body SB2 SB3 Porosity DINP (wt. %) 12.2 11.3

Preparation of a Porous Carbonaceous Monolith According to the Invention.

[0239] The shaped body made from cryogenically milled IXAN PV925 (SB1) was placed in a quartz tube disposed in a 3 zone horizontal tubular furnace (carbolite HZS 12/900). The temperature was then ramped from room temperature to 130 C. at a rate of 10 C./min and was held at that temperature for 1 h in order to dry the shaped body. Then the temperature was further increased with a heating rate of 1 C./min to an intermediate temperature (i.e. 150 C.) and held at this temperature for 17 h. Afterwards, the temperature was further increased to 300 C. with a heating rate of 1 C./min and held at this temperature for 1 h. Finally, the temperature was increased to the carbonization temperature (i.e. 600 C.) at a heating rate of 10 C./min and held at this temperature for 1 h.

[0240] The monolith M1 was recovered after cooling. A monolith M2 made from the shaped body SB2 was prepared in the same way except that the intermediate temperature was set at 160 C. instead of 150 C.

[0241] Preparation of Porous Carbonaceous Monolith for Comparative Examples.

[0242] The shaped body made from IXAN PV925 (SB3) was placed in a quartz tube disposed in a 3 zone horizontal tubular furnace (carbolite HZS 12/900). The temperature was then ramped from room temperature to 130 C. at a rate of 10 C./min and was held at that temperature for 1 h in order to dry the shaped body. Then the temperature was further increased with a heating rate of 1 C./min to an intermediate temperature (i.e. 150 C.) and held at this temperature for 17 h. Afterwards, the temperature was further increased to 300 C. with a heating rate of 1 C./min and held at this temperature for 1 h. Finally, the temperature was increased to the carbonization temperature (i.e. 600 C.) at a heating rate of 10 C./min and held at this temperature for 1 h. The monolith M3 was recovered after cooling.

[0243] Preparation of Honeycomb Monoliths

[0244] Several straight channels, of 0.8 mm wide and 0.8 mm deep, were sculpted along the length of the porous carbon monoliths using a CNC Milling Machine (Datron Electronics, CAT 3D-M5). Individual monoliths of about 1.2 cm wide and 8-12 cm long, each containing 5-7 channels, were cut out from this grooved porous carbon monolith. Afterwards, the individual monoliths were superimposed and assembled giving porous carbon honeycomb monoliths with 5 or 6 levels.

[0245] Assembling of the monoliths was made by wrapping up with a polymer film. Finally, the grooved monoliths assembled in such way resulted into honeycomb monoliths with square channels and a cell density of 200 cpsi (cells per square inch).

[0246] The wall thickness of the individual monolith was measured using a digital micrometer and the wall thickness of the grooved monolith, thus of the honeycomb monoliths, was deduced by subtracting the 0.8 mm deep of the milling cutter. The wall thickness value of the individual monolith was an average of 5 measurements made at 5 different places of said monolith, distanced from each other of 1 cm.

[0247] FIG. 4 represents a picture of grooved monolith of low wall thicknesses prepared from PVDC 1 (mean diameter 96 m).

[0248] FIG. 5 represents a picture of grooved monolith of large wall thicknesses prepared from IXAN PV925 (mean diameter 201 m).

[0249] Comparing the pictures of FIG. 4 and FIG. 5 reveals that the monolith made from the thinner particles had a smoother surface than the other. These pictures illustrate that the small particles were better suited than larger ones for the manufacture of grooved monoliths, having good mechanical properties and low wall thicknesses, by machining of individual monoliths.

[0250] Attempts to prepare monoliths having a wall thickness below 750 m starting from particles having a mean diameter of 160 m resulted in monoliths having poor cohesion and thus very low mechanical properties. These monoliths were unsuitable for any gas adsorption application.

[0251] Porosities of Porous Carbonaceous Monoliths.

[0252] Microporosity of the carbonaceous monoliths was measured by N.sub.2 porosimetry while macroporosity was evaluated by Hg porosimetry, both techniques previously described in the specification. The results confirmed that the monoliths presented 3 kinds of porosity: a microporosity and a macroporosity, which was intra-particle porosities, and a macroporosity which was located between the particles, thus qualified as inter-particles macroporosity.

[0253] The results collected in table 4 clearly show that the micropore volume and the specific surface area of the particles and of monoliths according to the invention were similar to the micropore volume and the specific surface area of the microbeads and of monoliths of comparative examples. Accordingly, forming a carbonaceous monolith comprising aggregates of small carbonaceous particles is not detrimental to the microporosity.

TABLE-US-00004 TABLE 4 Microporosities and specific surface area of porous carbonaceous particles and monoliths Particles Monoliths Micropore Micropore volume S.sub.BET volume Particles (cm.sup.3/g) (m.sup.2/g) Monoliths (cm.sup.3/g) S.sub.BET (m.sup.2/g) CP1 0.37 937 M1 0.38 964 CP2 0.41 1069 M2* 0.37 971 CP4 0.37 955 M3** 0.38 940 *Monoliths manufactured from shaped bodies prepared at 28.8 bars and 40 C. **Monoliths manufactured from shaped bodies prepared at 19.2 bars and 85 C.

[0254] Moreover, the results collected in table 5 show that the total macropore volume of the monoliths according to the invention was close to the total macropore volume of the monoliths from comparative examples.

[0255] In the case of M1 which is a monolith according to the invention, it was impossible to discriminate the intra-particle macropores from the inter-particle macropores by Hg porosity measurements. However, both of them were present as revealed by the high total macropore volume measured for this monolith M1.

TABLE-US-00005 TABLE 5 Macroporosities of porous carbonaceous microbeads and monoliths Particles Intra-particle Monoliths macropore Macropores Macropores mean Total mean volume (cm.sup.3/g) diameter (m) macropore volume size Intra- Inter- Intra- Inter- volume Particles (cm.sup.3/g) (m) Monoliths particle particle particle particle (cm.sup.3/g) CP1 0.83 7.2 M1 0.60 2.7 0.63 CP2 0.22 1.1 M2 0.15 0.277 0.55 9.7 0.45 CP4 0.21 3.2 M3 0.17 0.57 1 33.5 0.75 comparative examples

Selective CO.SUB.2 .Capacity of Porous Carbonaceous Particles and Monoliths

[0256] Experiments on particles were performed using a column with a length of about 10 cm and an internal diameter of 0.6 cm, packed with 0.5 to 0.9 g of adsorbent. Experiments on monoliths were performed on monoliths with a length of 8-12 cm and a cross section of about 1 cm.sup.2. Monoliths were directly connected to the in- and outlet tubing.

[0257] The selective CO.sub.2 capacities, Q.sub.CO2, are reported in table 6.

TABLE-US-00006 TABLE 6 Selective CO.sub.2 capacities Q.sub.CO2 of porous carbonaceous particles and monoliths Monoliths Particles Wall thickness Particles Q.sub.CO2 (mg/g) Monoliths Q.sub.CO2 (mg/g) (m) CP1 29.4 M1 30.9 700 CP2 42.2 M2 34.7 400 CP3 40.3 CP4 28.4 M3 28.2 1000 CP5 41.2 CarboTech 11.4 350 comparative examples

[0258] Surprisingly, the selective CO.sub.2 capacity, Q.sub.CO2, of the particles obtained by the processes according to the invention was similar to the capacity of commercially available carbonaceous particles or to the capacity of carbonaceous particles resulting from the pyrolysis of commercially available vinylidene chloride copolymer both having higher mean diameter.

[0259] Moreover, the honeycomb monoliths of the present invention had improved performances with regard to the performances of honeycomb monolith from CarboTech AC GmbH at equivalent wall thickness and equivalent cell density. This is due to the fact that the CarboTech monolith contained, next to a porous carbon adsorbent, an inert inorganic binder which actively decreased the overall adsorption capacity.

[0260] It was not possible to measure Q.sub.CO2 neither in the case of CP6 nor in the case of CP7 because the experimental setup required a nitrogen gas flow through a column packed with the studied particles. When the size of the particles was very low, as it was the case for CP6 or CP7, the inventors noticed a plugging of the column and thus the impossibility to carry out any measurement. Finally, this observation rendered the question of the presence of macroporosity, either intra-particle or inter-particles, within those packed fine particles questionable.

[0261] Generally, the selective CO.sub.2 capacities of the monoliths were slightly lower than the selective capacities of the corresponding particles (compare respectively CP1 with M1, CP2 with M2 and CP4 with M3). It is worth noting that the capacity of the monolith M1 was similar to the capacity of comparative example M3 while the capacity of the monolith M2 was higher.

[0262] These results gave evidence that the processes according to the invention allowed the manufacture of porous carbonaceous particles having low mean diameter and of monoliths having low wall thickness, typically below 1000 m, while maintaining a high selective CO.sub.2 capacity.

[0263] Effective Particles Diffusivity

[0264] The effective particle diffusivity was measured while ensuring that the system is under kinetic control as previously described. Results for the different particles are reported in table 7.

TABLE-US-00007 TABLE 7 Effective particles diffusivity Carbonaceous particles Carbonaceous according to the invention Comparative examples Particles CP1 CP2 CP3 CP4 CP5 CP6 CP7 Mean diameter 24.5 72 67.8 160 204.6 5.1 5.5 D50 (m) Span 2.2 1.2 1.15 0.8 0.8 1.2.sub.5 n.a. CO.sub.2 diffusion 7.3 .Math. 10.sup.12 9.7 .Math. 10.sup.11 1.1 .Math. 10.sup.10 6.7 .Math. 10.sup.10 6.9 .Math. 10.sup.10 4.7 .Math. 10.sup.13 7.9 .Math. 10.sup.13 constant (m.sup.2/s)

[0265] The results reveal that the effective particle diffusivity was strongly affected by the macropore structure of the carbonaceous particles. On one hand, one can see from comparative examples that, for particles having an optimized macropore structure (mean diameter larger than 150 m), the CO.sub.2 diffusion constant was in the range above of 5.10.sup.10 m.sup.2/s (see CP4 and CP5) while for particles for which the macropore structure has been reduced (particles having a diameter close to 5 m (see CP6 and CP7)), the CO.sub.2 diffusion constant was 3 orders of magnitude lower. On the other hand, the examples according to the invention (CP1 to CP3) reveal that particles having an intermediate mean diameter had surprisingly a CO.sub.2 diffusion constant which was only 1 or 2 orders of magnitude lower than the CO.sub.2 diffusion constant of particles having a mean diameter larger than 150 m and that, consequently, they represent a good trade-off between size and CO.sub.2 diffusion.

[0266] It is also interesting to note that particles having similar particles size manufactured starting from different raw materials, PVDC (CP6) or coconut (CP7: Kuraray YP50F), had similar CO.sub.2 diffusion constant.

[0267] With diffusion experiments, the applicant has shown that the carbonaceous particles according to the invention had a mean diameter which was compatible with the manufacture of carbonaceous monolith having low wall thickness and a high CO.sub.2 diffusion constant.

[0268] Moreover, the applicant has shown that the processes according to the invention were particularly well adapted for manufacturing said carbonaceous particles having high CO.sub.2 diffusion constant.

[0269] Additionally, the applicant has shown that the porous particles comprising vinylidene chloride polymer according to the invention were particularly suitable for the manufacture of carbonaceous particles having high CO.sub.2 diffusion constant and small diameter and, for the manufacture of monoliths having low wall thickness and high CO.sub.2 diffusion constant.

[0270] The applicant has shown that the processes according to the invention were particularly well adapted for manufacturing said porous particles comprising vinylidene chloride polymer.

[0271] Among these processes, the cryogenic milling of the particles comprising vinylidene chloride polymer had the advantage over the milling in the presence of ice described in the prior art in that the span of the resulting particles was kept below 2.5. Consequently, the particles distribution was free of fine particles which might be responsible for some plugging and free of large particles which might be responsible for monoliths having poor cohesion and thus low mechanical properties. Finally the applicant has shown that the cryogenic milling of carbonaceous particles had the advantage over the jet milling of carbonaceous particles that the energy applied was better controlled and that consequently the particles were not sprayed as objects of very small size (compare CP3 with CP6). This better control of the milling process allowed the easy manufacture of particles according to the invention.