PROCESS FOR PREPARING A POROUS CARBON MATERIAL AND A POROUS CARBON MATERIAL OBTAINABLE BY THIS PROCESS

Abstract

Known processes for preparing a porous carbon material with a hierarchical porosity comprise the steps of a) providing at least one carbon source and at least one amphiphilic species, b) combining the carbon source and the amphiphilic species to obtain a precursor material, and c) heating the precursor material to obtain the porous carbon material having a modal pore size and a pore volume. In order to avoid a lengthy hydrothermal treatment and to allow tunability of the pore size, pore size distribution and pore volume in carbon material, it is proposed that the heating step c) comprises a low temperature treatment in which the precursor material is heated to a first temperature in the range between 300° C. and 600° C. to obtain a self-assembled porous carbonaceous material, and wherein heating to the first temperature comprises a first average heating rate in the range of 0.5° C./min to 5° C./min.

Claims

1. Process for preparing a porous carbon material comprising the steps: a) providing at least one carbon source and at least one amphiphilic species, b) combining the carbon source and the amphiphilic species to obtain a precursor material, and c) heating the precursor material to obtain the porous carbon material having a modal pore size and a pore volume, wherein the heating step c) comprises a low temperature treatment in which the precursor material is heated to a first temperature in the range between 300° C. and 600° C. to obtain a self-assembled porous carbonaceous material, and wherein heating to the first temperature comprises a first average heating rate in the range of 0.5° C./min to 5° C./min.

2. The process according to claim 1, wherein the first average heating rate is set to a value in the range 0.6 to 2.5° C./min.

3. The process according to claim 1, wherein the first average heating rate is set in dependence of a pre-determined modal pore size and a pre-determined pore volume of the porous carbon material, wherein setting the average heating rate comprises a step of establishing a calibration curve to the dependency of the pore size and/or the pore volume on the average heating rate.

4. The process according to claim 1, wherein heating according to step c) is started within 1 hour of the combining step b).

5. The process according to claim 1, wherein the low temperature treatment comprises a temperature dwell time of 15 to 240 minutes at a holding temperature lower than the first temperature, wherein the holding temperature is less than 450° C. and the dwell time is in the range of from 15 to 60 min.

6. The process according to claim 1, characterized in that the heating of the precursor material in heating step c) comprises an oxidation phase in which the precursor material is treated in an atmosphere containing an oxidizing agent.

7. The process according to claim 6, characterized in that the atmosphere containing an oxidizing agent during heating the precursor material during the oxidation phase is an atmosphere containing oxygen in molecular form.

8. The process according to claim 7, characterized in that the heating of the precursor material during the oxidation phase is carried out in a temperature range between 150° C. and 520° C.

9. The process according to claim 8, characterized in that the oxidation phase has a duration in the range of 60 to 360 minutes.

10. The process according to claim 1, wherein the heating step c) comprises a high temperature treatment, during which the self-assembled porous carbonaceous material is subjected to a second temperature of at least 700° C. and not more than 3000° C.

11. The process according to claim 1, wherein the precursor material is such that when subjected to a low temperature treatment at an average heating rate of 5° C./min it has more than twice the modal pore size than at an average heating rate of 2° C./min.

12. The process according to claim 1, wherein the amphiphilic species comprising a first amphiphilic compound, the first amphiphilic compound comprising two or more adjacent ethylene oxide based repeating units.

13. The process according to claim 1, wherein the precursor material comprises a block copolymer of propylene oxide and ethylene oxide, containing 15 wt. % to 25 wt. % ethylene oxide.

14. The process according to claim 1, wherein the precursor material comprises a surfactant containing 50 to 80 wt. % of ethylene oxide.

15. The process according to claim 1, wherein the carbon source is selected from the group consisting of Novolac type phenolic-formaldehyde resins, hydrolysable tannic acid, lignin, and cellulose resins.

16. The process according to claim 1, wherein the precursor material is free of solvents.

17. A porous carbon material obtainable by the process according to claim 1, said porous carbon material having pores defined by a modal pore size in the range between 50 and 280 nm and, for a sample number of at least three, a standard deviation in the modal pore size of less than 50 nm.

Description

SHORT DESCRIPTION OF THE FIGURES

[0077] The invention is now further elucidated with reference to the figures. The figures and figure descriptions are exemplary and are not to be considered as limiting the scope of the invention.

[0078] FIG. 1 shows a diagram of a thermal gravimetric analysis of a specific amphiphile material,

[0079] FIG. 2 shows an SEM image of the surface of a material prepared according to the invention prepared by using a first recipe and a heating rate of 0.67° C./min,

[0080] FIG. 3 shows an SEM image of the surface of a material prepared according to the invention prepared by using a first recipe and a heating rate of 1° C./min,

[0081] FIG. 4 shows an SEM image of the surface of a material prepared according to the invention prepared by using a first recipe and a heating rate of 2° C./min,

[0082] FIG. 5 shows an SEM image of the surface of a material prepared according to the invention prepared by using a first recipe and a heating rate of 5° C./min,

[0083] FIG. 6 shows an SEM image of the surface of a material prepared according to the invention prepared by using a second recipe and a heating rate of 0.67° C./min,

[0084] FIG. 7 shows an SEM image of the surface of a material prepared according to the invention prepared by using a second recipe and a heating rate of 1° C./min,

[0085] FIG. 8 shows an SEM image of the surface of a material prepared according to the invention prepared by using a second recipe and a heating rate of 2° C./min,

[0086] FIG. 9 shows an SEM image of the surface of a material prepared according to the invention prepared by using a second recipe and a heating rate of 5° C./min,

[0087] FIG. 10 shows a diagram of the dependence of the pore volume on the heating rate for four different recipes,

[0088] FIG. 11 shows a diagram of the dependence of the modal pore size on the heating rate for four different recipes,

[0089] FIG. 12 shows a diagram of the dependence of the total BET surface on the heating rate for four different recipes,

[0090] FIG. 13 shows a diagram of the dependence of the extern BET surface on the heating rate for four different recipes,

[0091] FIG. 14 shows a plot about the standard deviation in the modal pore size in dependence of heating rates for four different recipes, and

[0092] FIG. 15 shows the result of a thermogravimetric analysis (TGA) of a specific precursor material heated up to 1000° C. in both argon and in synthetic air.

EXAMPLES

[0093] In the preferred embodiment the porous carbon material is produced by combining an aqueous resorcinol-formaldehyde resin (Novolac resin) or solid pellets of resorcinol-formaldehyde/phenolic-formaldehyde resin (Novolac types) and an amphiphilic molecule (either a block copolymer or a surfactant or a (non-ionic) emulsifier or a combination of amphiphilic molecules). The components are mixed to obtain a homogenous precursor material.

[0094] The heat treatment of this is performed in one-step in inert atmosphere (nitrogen or argon). The heating rate from room temperature to 400° C. ranges from 0.67° C./min to 5° C./min. Thereafter the heating begins again to the desired final temperature from 600 to 3000° C., preferably above 900° C. The thus obtained porous carbon material is cooled down, removed from the oven, and mechanical crushed/milled to the desired particle size.

[0095] The recipe of the constituents of the specific precursor material sets the initial parameters for the resulting porous carbon material. Additional tuning of the modal pore size and of the pore volume can be achieved by the heating ramp during the critical self-assembly step.

[0096] To understand the heating rate dependence for the carbon materials, a series of heating ramps (nine in total) were performed on four different precursor materials (recipes). The experimental data are summarized in Table 1.

TABLE-US-00001 TABLE 1 Ramp r.sub.H1 T1 t1 r.sub.H2 T2 t2 number [° C./min] [° C.] [min] [° C./min] [° C.] [min] R1 1 250 30 1.4 500 30 R2 1 120 30 1 900 180 R3 1 325 30 1 900 180 R4 1 200 30 1 900 180 R5 1 900 180 — — R6 5 400 60 1 900 180 R7 1 400 30 2 900 180 R8 2 250 30 1 400 30 R9 0.67 250 30 1 400 30 r.sub.H1 is an average heating rate starting from room temperature, T1 is a first holding temperature t1 is a first dwell time at first holding temperature r.sub.H2 is a heating rate from first holding temperature T2 is a second holding temperature t2 is a second dwell time at second holding temperature

[0097] In the low temperature region (here until 400° C.) the heating ramp 9 provides an average heating rate of 0.67° C./min, the heating ramp 8 provides an average heating rate of 2° C./min, the heating ramp R6 provides an average heating rate of 5° C./min and the remaining heating ramps have an average heating rate of 1° C./min.

[0098] Various dwell times were tested to see whether potential processes were critical for the final product, especially in temperature ranges in which a transformation of the precursor material may be expected.

[0099] It was found that dwell times did not have as much influence on pore sizes and pore volumes as the initial heating ramp up to 400° C. In addition, it was proved that the heating rate dependence of the modal pore size and pore volume occurs only during the low temperature heat treatment up to about 500° C. and prior than start of the carbonization of the precursor material.

[0100] Table 2 lists the tested recipes No. 1 to No. 4:

TABLE-US-00002 TABLE 2 Weight ratio Carbon source/ Recipe Amphiphilic No Carbon source Amphiphilic species species 1 Askofen ® 779 W 50 Genapol ® PF20 10:3 2 Askofen ® 779 W 50 Pluronic ® P123  5:3 3 Askofen ® 779 W 50 Genapol ® PF20  5:3 4 Askofen ® 779 W 50 Genapol ® X-100 10:3

[0101] The ‘weight ratio’ indicated in the fourth column refers to the ratio of the total masses of the respective substances. The 779 W 50 Askofen resin, for example, is an aqueous resorcinol-formaldehyde resin of the Novolac type, and contains 50 wt. % solid resin and 50 wt. % liquid phase. Therefore, 5 parts by weight of this substance correspond to 2.5 parts by weight of the resin

[0102] For each recipe, four crucibles were filled in each run to see the temperature homogeneity of the oven and the reproducibility of the recipes.

[0103] The diagram of FIG. 1 show results of a thermal gravimetric analysis in argon up to 1000° C. of an amphiphile material. The remaining mass compared to the original sample mass Δm (in %) of the sample is plotted on the ordinate as a function of the heating temperature T (in ° C.). The heating temperature is a linear function of the heating rate which remains constant at 5° C./min until the end temperature of 1000° C. is reached.

[0104] Curve A in FIG. 1 shows the evolution of the mass of the sample as a function of temperature, and curve B the mass flow of argon flushing (which is constant at 20 ml/min). The weight loss can be explained by continuing thermal decomposition of the amphiphilic species. Accordingly, until a temperature of about 200° C. the mass loss of the amphiphile material is small. At a temperature above 500° C. the mass loss is almost completed, showing the nearly full decomposition of the amphiphilic soft template material. The amphiphile material loses about 98.85% of its initial weight upon treatment up to 1000° C.

[0105] At a temperature of about 380° C. the amphiphilic species has lost 60% of its initial weight. In this specific case, the temperature of 380° C. represents a “first temperature” of the low temperature treatment process. Most of the amphiphilic species is decomposed so that the porosity of the remaining materials is set during the low temperature treatment until the “first temperature” is reached. At higher temperature, the porosity will not change dramatically so that further temperature treatments do not need have such a slow heating ramp.

[0106] The scanning electron images in FIGS. 2 to 5 show impressively that the porous nature of the carbon material produced from the precursor material of recipe No. 1 changes with the heating rate during the low temperature treatment. At a heating rate of 0.67° C./min (FIG. 2), the porosity is significantly lower than at a heating rate of 5° C./min (FIG. 5). The scale bar has a length of 10 μm in each of the SEM photos.

[0107] The scanning electron images in FIGS. 6 to 9 show the same dependency for recipe No. 2. Since the recipe itself generates smaller pores, the effect of the pore size dependency is less visible as for recipe No. 1. Again the scale bar has a length of 10 μm in each of the photos.

[0108] In the diagram of FIG. 10, the mean pore volume V.sub.p (in cm.sup.3/g) of each of the four recipes is plotted versus the initial heating rate r.sub.H (in ° C./min) during the low temperature treatment in the temperature interval between 25° C. and 400° C. The pore volume increases slightly as the rate of the heating is increased.

[0109] In the diagram of FIG. 11, the modal pore size D.sub.p (in nm) of each of the four recipes is plotted versus the initial heating rate r.sub.H (in ° C./min) during the low temperature treatment in the temperature interval between 25° C. and 400° C. The modal pore size shows a dramatic increase when the heating rate is increased above 1° C./min. The recipes 1 and 3 show the clearest dependence between pore size and heating rate. A heating rate of 5° C./min causes more than twice the pore size than the heating rate of 2° C./min, and the heating rate of 2° C./min causes more than twice the pore size than the heating rate of 1° C./min.

[0110] The values plotted in FIGS. 10 and 11 are the mean values of four reproductions for each recipe.

[0111] The diagrams of FIGS. 12 and 13 show the dependence of the BET surface area, both total (BET.sub.total in m.sup.2/g) and external (BET.sub.ext in m.sup.2/g), on the initial heating rate r.sub.H (in ° C./min) for all four recipes tested. The values plotted are the mean of the four reproductions for each recipe. The total BET.sub.total does not show a consistent change as the heating rate is changed, however the external BET.sub.ext., for most recipes, shows a decrease when the heating rate is increased above 1° C./min.

[0112] The external specific surface area BET.sub.ext is defined by subtracting the micropore specific surface area from the total specific surface area, BET.sub.ext=BET.sub.total−BET.sub.micro.

[0113] FIG. 14 shows the sample standard deviations of the modal pore size (in nm) obtained for the four crucibles of each recipe as a function of the ramp number (see Table 1).

[0114] The formula for the sample standard deviation is

[00001]$s=\sqrt{\frac{{{\Sigma }_{i=1}^N\left(x_i-\overline{x}\right)}^2}{N-1}}.$

where {x.sub.1, x.sub.2, . . . , x.sub.N} are the measured values of the modal pore diameters of each sample. x is the mean value of the modal pore sizes (the sum of the sampled values divided by the number N of measurements, wherein N=4.

[0115] The standard deviation increases as the initial heating rate increases (ramp of recipe 6 with 5° C./min and ramp of recipe 8 with 2° C./min). Ramp of recipe 5 has no holds and goes from room temperature direct to 900° C. with 1° C./min. The ramp with lowest standard deviation is the ramp of recipe 3 which has a hold at 325° C. for 30 minutes. The low values of standard deviation resulting from the claimed low temperature treatment process represent a narrow pore size distribution.

[0116] The precursor material mixture typically contains a novolac resin and an amphiphilic surfactant. An example for the novolac resin is Alnovol® PN320 (Allnex) and for the surfactant Genapol® PF20 (Clariant) or Synperonic® PE/L64 (Croda). The ratio of the resin to surfactant is in general 5:(1.5-9).

[0117] FIG. 15 shows the result of a thermogravimetric analysis (TGA) carried out with a mixture of Alnovol PN320 with the Genapol® PF20 Synperonic PE/L64 with the ratio of 5:5, where said mixture was separately heated up to 1000° C. in both argon (curve 151) and synthetic air (curve 152) atmospheres. Similar as in FIG. 1, the remaining mass compared to the original sample mass Δm (in %) of the sample is plotted on the ordinate as a function of the heating temperature T (in ° C.). The heating temperature is a function of the heating rate which is 3° C./min until a temperature of 600° C. is reached and 5° C./min until the end temperature of 1000° C. Until about 400° C. the TGA curves 151, 152 of the samples heated in different atmospheres have rather similar profiles. At 400° C. the argon pyrolyzed sample (151) shows a larger mass loss and its slope decreases afterwards. The air pyrolyzed sample (152) has a plateau in the temperature range from about 400 to 450° C., where the mass loss of carbon is about 10 wt. % lower than the mass loss of the argon pyrolyzed sample (151). The mass loss however strongly increases when oxidation takes place at temperatures higher than 450° C. The difference in the carbon yields at 450° C. is indicated by the distance bar 153.

[0118] Similar thermogravimetric analysis results were revealed for a precursor material made from Alnovol® PN445 and Genapol® PF20 (5:5) mixture. At a temperature of 450° C., the mass loss of the sample crosslinked in argon atmosphere was almost 15 wt. % larger compared to the sample crosslinked in synthetic air.

[0119] Based on these thermogravimetric results, an experiment was designed to prove that the increase of the carbon yield (the reduction of the carbon mass loss) during the pyrolysis can be transferred to the synthesis of the porous carbon material by the process of the invention. A mixture of Alnovol® PN320 and Genapor® PF20 with the ratio of 5:5 was crosslinked and pyrolyzed up to 600° C. with the following heating ramp profile: 20-350° C.: on average 0.5° C./min.fwdarw.350-450° C.: 1° C./min.fwdarw.450-600° C.: 2° C./min. At a temperature of 450° C. the decomposition of the amphiphile is sufficiently complete and the atmosphere is changed from containing the oxidizing agent to one with inert gas only. This temperature (450° C.) is at the same time the maximal temperature of the oxidation phase in which the precursor material is heated in an atmosphere containing an oxidizing agent and the “first temperature” of the low temperature treatment.

[0120] In the first trial the mixture was pyrolyzed in nitrogen atmosphere. In the second trial the mixture was heated in an open retort in order to ensure an atmosphere containing an oxidizing agent (air) during pyrolysis until 450° C. As soon as that temperature was reached, the retort was closed, and the nitrogen flow was turned on in order to protect the carbonaceous material from further oxidation by air at higher temperatures (450-600° C.). Table 3 shows a comparison of the yields of the porous carbon material after the pyrolysis in nitrogen and in air (until 450° C.).

TABLE-US-00003 TABLE 3 Yield at Yield at 600° C. 900° C. Atmosphere Precursor mixture in % in % Nitrogen Alnovol ® PN320 30.3 26.3 Genapol ® PF20 Air until 450° C. Alnovol ® PN320 32.7 29.7 (and afterwards N.sub.2) Genapol ® PF20

[0121] The sample treated in nitrogen has a yield of 30.3 wt. % at 600° C., while the air crosslinked sample has a higher yield of 32.7 wt. %. The statistical error of the yield is typically 0.5 wt. %. In fact, the yield of the carbonized air crosslinked sample is 3.4 wt. % larger compared to the nitrogen crosslinked sample. This improvement is even larger than the yield gain of 2.4 wt. % found at 600° C. This implies that oxygen stabilizes the polymeric network and this improvement can be preserved. In this first experiment the air was replaced by nitrogen at a temperature of 470° C. By other experiments it could be shown that even a higher carbon yield might be obtained by process optimization, e.g. by switching to an inert atmosphere at a temperature of 400° C. and lower. The crosslinking and pyrolysis result in a macroporous carbon material.

[0122] The crosslinking and pyrolysis of the precursor material mixtures result in a macroporous carbon with a cumulative pore volume above 0.4 cm.sup.3/g and with a modal pore size in the range between 50-280 nm.