FR3094239A1 - CARBON AND OXYGEN BASED MATERIAL USABLE AS A SUPPORT FOR CATALYSIS - Google Patents

Abstract

Material based on carbon and oxygen with a specific surface area greater than 100 m² / g, in which the mass ratio of the elemental content of Oxygen (O) to that of Carbon (C) is greater than 0.15.

Description

CARBON AND OXYGEN BASED MATERIAL USABLE AS SUPPORT FOR CATALYSIS

The present invention relates to a material based on porous carbon, that is to say with a high specific surface, in particular greater than 100 m ² /g, which can be more particularly used in the field of the purification of a fluid. , in particular air or another gas charged with gaseous pollutants, such as in particular carbon monoxide CO, VOCs (volatile organic compounds, formaldehyde or its derivatives), unburnt hydrocarbons "HC", or even SOx or NOx. Such a material could also be used for the purification of a gas or a liquid, but also for any application involving the hydrolysis or electrolysis of water.

More particularly, the invention relates to systems for treating fluids, in particular gases, at low temperature, typically preferably between 100° C. and less than 350° C., in particular catalytic systems, in particular for the treatment of gases, and/or adsorbent systems, in particular for the absorption of gaseous species.

Different VOC treatment techniques are thus presented in the engineering techniques Doc. J 3 928 in particular depending on the concentration, the gas flow and the number of pollutants to be treated. A well-known technique consists in particular of oxidation of the VOCs into CO ₂ and H ₂ O, the most convincing techniques integrating treatment by thermal means.

Porous carbonaceous products and their application as catalytic support are known in particular from the publications “Recent Progress in the synthesis of Porous Carbon Materials” in Advanced Materials, 2006, 18 page 2073-2094 and “Mesoporous Carbon Materials synthesis and modification” in Angewandte Chemie Int. Ed. 2008, 47 pages 3696-3717. The catalytic supports can be micro- or/and meso-porous carbon supports obtained from a deposit of a polymer or a resin on an impression which serves as a model (“template” in English) followed by drying and carbonization. Said model can be, for example, a zeolite or a silica gel. The structure obtained can be respectively of ordered or disordered porosity.

The contribution of heteroatoms such as nitrogen N to the surface of a porous carbon support is also known from the publication "Nitrogen Doping Effects on the Physical and Chemical Properties of Mesoporous Carbons" in "The Journal of Physical Chemistry C » 2013, 117, pages 8318-8328. Doping with heteroatoms is generally carried out on a presynthesized carbon support, for example by heating in a nitrogenous atmosphere or by attack with ammonia in the case of nitrogen doping. The authors of the previous publication propose to do an in-situ nitrogen doping of the carbonaceous support by adding a precursor such as melamine, before drying and then carbonization. Such supports have improved thermal stability but insufficient catalytic activity, in particular the conversion of CO to CO ₂ , at low temperature.

One of the aims of the present invention is to remedy such shortcomings, in particular by allowing the conversion of a substantial quantity of polluting species, in particular of the carbon monoxide CO type, in an oxidizing atmosphere, in a temperature range comprised between 100 and 300° C., and in particular below 250° C., without substantial degradation of the catalyst and in particular of its carbon-based support.

In other words, there currently exists a need for a catalytic and/or absorbent system comprising a porous carbon support, autonomous and stable in an oxidizing atmosphere at a temperature between 100 and 300° C., and in particular less than 250° C., leading to improved catalytic performance, including better direct removal of pollutants.

According to a first aspect, the present invention relates to a material, which can in particular serve as a support, based on carbon and oxygen, in particular a catalyst support, in particular usable in particular for the elimination of CO, volatile organic compounds (VOC) , unburned hydrocarbons "HC", formaldehyde or its derivatives, or even SOx or NOx, whose specific surface is greater than 100 m²/g and in which the O/C mass ratio of the elementary content of Oxygen (O) over that of Carbon (C) is greater than 0.15, preferably greater than or equal to 0.16, or even greater than 0.20, or even greater than 0.25.

More preferably, the mass ratio of the elementary content of Oxygen (O) to that of Carbon (C) is less than 0.53, more preferably is less than 0.52, and very preferably less than 0.51, or even less than 0.50.

By elemental content, we mean the content of the chemical element considered in said material (for example elemental oxygen O or elemental carbon C).

The specific surface of the porous carbonaceous material according to the invention is determined according to the BET method. This specific surface measurement method by adsorption of inert gas, developed by S. Brunauer, P.H. Emmet and J. Teller, has long been known to those skilled in the art, in particular from the publication “Journal of American Chemical Society 60 (1938), pp. 309-316”.

The elementary mass contents of O and N can be determined after pyrolysis under inert gas, for example by means of an analyzer marketed under the reference TC-436DR by the company LECO Corporation.

The elementary (total) mass content of C can be determined after pyrolysis under inert gas, for example by means of an analyzer marketed under the reference LECO CS 300 by the company LECO Corporation.

Such a proportion of oxygen in a material according to the invention is characteristic of a material in which a significant part of the oxygen is present not only at the surface of the material but also within the carbon network, independently of the high specific surface. of said material. The presence of such a level of oxygen in a carbon network according to the invention makes it possible to obtain a greatly improved catalytic efficiency while maintaining temperature stability, in particular between 100°C and 300°C.

Within the meaning of this description, VOCs include in particular formaldehyde, acetaldehyde, acetone, acrolein, hexacholorobutadiene, aromatic compounds BTEX (such as benzene, toluene, xylene, ethylbenzene, styrene, trimethylbenzene) , 1,2-dichlorobenzene, 1,3-butadiene, benzyl chloride, carbon tetrachloride, vinyl chloride, acrylonitrile, dodecane, ethers, glycol, aliphatic compounds of hydrocarbons, ethyl acetate, terpenes.

According to preferred but non-limiting embodiments of the material according to the invention, which can be combined with each other if necessary:

- The mass content of the material in oxygen element (O) is greater than 12%, preferably greater than 15%, or even greater than 20% or even more preferably greater than 22%.

- The mass content of the material in oxygen element (O) is less than 35%, more preferably still less than 33%.

- The carbon element content by weight of the material is greater than 50%, preferably greater than 60%, more preferably greater than 65%.

- the carbon element mass content of the material is less than 77.5%.

- The mass content of the material in oxygen element (O) is less than 87%, preferably less than 85% or even less than 80%.

- The cumulative mass content of the material in oxygen element and in carbon element is greater than 75%, preferably greater than 80%, very preferably greater than 90% or even greater than 95%.

- The mass content of the material in nitrogen element (N) is less than or equal to 5%, preferably less than 3%, or even less than 4% or even less than 1%.

- The mass ratio (O/N) of the elementary content of oxygen (O) to that of nitrogen (N) of said material is greater than 2, preferably greater than 3, or greater than 10, or even greater than 30 , preferably greater than 50, greater than 100, or even greater than 200.

- The mass ratio of the elementary content of oxygen (O) to that of nitrogen (N) of said material is less than 345, preferably less than 330.

- The specific surface of the material is greater than 300 m²/g, or greater than 500 m²/g. More preferably it is greater than 1000 m²/g, preferably greater than 1500 m²/g. Preferably, it is less than 3000 m²/g, in particular in order to allow sufficient stability at a temperature of between 100 and 350°C.

- the material has a degree of graphitization characterized by an I _D /I _G ratio of between 0.8 and 1.35, as determined by Raman spectroscopy, I _D and I _G corresponding respectively to the intensities of the peak at 1360 cm ^{- 1} and 1575 cm ^-1 , measured on a spectrum between 1000 and 1800 cm ^-1 . The determination of such peaks being for example described in the publication “Cuesta et al Carbon, Vol. 32, No. 8, p. 1523-1532, 1994)”. Preferably, the I _D /I _G ratio of the material according to the invention is less than 1.15, or even less than 1.1.

The invention also relates to a catalytic system comprising a support comprising or consisting essentially of a material based on carbon and oxygen as described above.

According to one possible mode, in the catalytic system according to the invention, the support can be coated with a catalyst, preferably a catalytic metal or an oxide comprising an element chosen from Pt, Pd, Rh, Ru, Cu, Mn, Ag , Ni, Fe and Au.

In particular one or more elements of the type of those known to promote the oxidation of CO, VOC or HC, in particular among Pt, Pd, Rh, Au.

In the catalytic system according to the invention, according to one possible mode, the metals or their oxides, in particular the precious metals, are deposited on the surface of the system by impregnation, in particular of precious metals, of the type of those known to date for promote the oxidation of CO, VOC or HC, in particular those based on Pt, Pd, Rh, Au.

The catalytic system as described above can advantageously be used for the synthesis or depollution of gases, operating at an average temperature above 90° C., preferably above 100° C., and below 350°, preferably below 300°C, or even less than 250°C.

The catalytic system as described above can advantageously be used for the purification of indoor and/or outdoor air, in particular in dwellings, passenger compartments, in particular motorized vehicles, or confined spaces, in particular aircraft.

The present invention also relates to the use of the catalytic system as described above for the pollution control of a gas resulting from:

-combustion systems for energy production;

-combustion systems in the tertiary and residential sectors, in particular heating systems producing gases to be treated such as CO, HC, VOCs and even NOx;

-combustion systems in industry, in particular the glass, steel and cement industries;

- land, sea and air transport;

- waste treatment, in particular incinerators.

Preferably, the catalytic system is placed in an environment at a temperature of between 90°C and 350°C.

A catalytic system according to the invention can therefore be used for the synthesis or depollution of gases, operating at an average temperature above 90° C. and below 350° C. or alternatively for the purification of indoor and/or outdoor air, especially in homes, passenger compartments or confined spaces.

The invention relates in particular to the use of the catalytic system as described above for producing a catalytic gas synthesis reactor, in particular to a membrane reactor.

The invention also relates to a process for manufacturing said carbonaceous material described above.

The process comprises the following essential steps:

-preparation of a mixture of at least one carbon precursor and an oxygen source in the form of a polymer chosen from monosaccharides, disaccharides; polyols; the polysaccharides said polymer being added gradually in a basic solution maintained at a temperature in order to start the polymerization reaction,

- adding an impression material or replication model, preferably a silica gel precursor, to the mixture at a controlled temperature in order to continue the polymerization reaction,

- cooling then drying,

-carbonization, preferably by pyrolysis, of said powder,

- mixing said carbonized powder with a basic solution, preferably with stirring and controlled temperature, in order to eliminate the impression material or the replication model.

Preferably and in more detail, the process for manufacturing said carbonaceous material comprises the following steps:

i- preparation of a mixture of at least a) a carbon precursor, preferably a phenolic and/or formaldehyde resin and b) an oxygen source in the form of a polymer chosen from monosaccharides, such as preferably glucose, fructose, galactose, xylose, disaccharides such as lactose, maltose, sucrose, trehalose, cellobiose, chitobiose; polyols such as sorbitol, mannitol; polysaccharides such as preferably starch, amylose, amylopectin, glycogen, cellulose, hemicellulose, pectins, chitin. Said polymer is added gradually to a basic solution, preferably sodium hydroxide (NaOH), in order to reach a pH of between 8.5 and 12, which is maintained at a temperature, preferably greater than or equal to 70°C, preferably with stirring. in order to promote the homogenization of the mixture, for example at a speed of 250 rpm (revolution per minute) for at least 20 minutes, with the aim of allowing the polymerization reaction to start without or with a very weak vaporization reaction.

ii- addition of an inorganic borrowing material or rigid replication model ("hard template"), in particular a silicate or an aluminosilicate, for example a zeolite, silica nanoparticles or mesoporous silica, in particular in the form of a precursor of silica gel, for example Ludox SM-30, to this mixture, the mixture then being brought to a temperature allowing the continuation of the polymerization reaction, under a confined atmosphere so as to avoid evaporation of the mixture, for example at a temperature of 80° C. in a closed atmosphere, preferably in a confined atmosphere under a pressure of less than 1.5 bars, for 3 days, without stirring.

iii- cooling, optionally grinding and drying, this last step preferably allowing the residual water to be eliminated, for example at 80°C for at least 12 hours. The brown powder obtained is typically sieved at 250 μm.

iv-carbonization, in particular pyrolysis, of said sieved powder according to a heat treatment under a neutral atmosphere (in particular of argon) between 800°C and 1000°C, preferably for 3 hours according to a ramp rate of 5°C / minute.

v-mixing of said carbonized powder thus obtained, with an acid solution, for example of hydrofluoric acid, for example at room temperature; or with a basic solution, preferably a sodium hydroxide solution, preferably at a pH of around 12, typically at a concentration of 2M, with stirring and controlled temperature at 80°C preferably for 15 hours in order to remove the silica from the borrowing or replicating material. After filtration and washing to neutral pH, said filtered and washed powder is dried, preferably at 100° C. for 15 hours, in order to form a carbon- and oxygen-based material according to the invention.

The invention also relates to a process for depositing a catalyst on the support formed by the carbonaceous material as previously manufactured. Preferably, the deposition process comprises the following steps:

-the deposition of the catalyst by impregnation of the support. The support is placed in deionized water for 10 to 20 minutes under sonication, in particular under ultrasound. The appropriate quantity of catalyst is dissolved in water, the mixture is subjected to sonication for 20 minutes followed by heating at 80°C (under air) with constant stirring for 6 hours. The water is then vaporized in an evaporator and the catalyzed support is dried at 80°C for 12 hours.

Figure 1 shows a Raman spectroscopy diagram produced on several examples described below using a ThermoScientific DXRxi spectroscope equipped with a 532 nm laser, with a power of 5 mW. The laser beam is focused on the sample through a microscope offering a high level of magnification (for example at least 50x). The diagrams obtained are characteristic of a highly graphitized carbon support.

The invention and its advantages will be better understood on reading the following non-limiting examples:

a) Preparation of supports:

Example 1 (comparative)

20.93 grams of a mixture comprising 7.34 g of phenol (Merck, 99%) and 12.66 g of an aqueous solution of formaldehyde (Sigma-Aldrich, mass concentration 37% in water, stabilized with methanol) are added gradually to 100 mL of a 0.2 M sodium hydroxide solution (NaOH) in order to reach a pH of 9, heated to a temperature of 70° C. with stirring at a speed of 250 rpm for at least 20 minutes.

100 grams of a silica gel precursor, Ludox SM-30®, is added to this mixture, the mixture then being brought to a temperature of 80°C in an air atmosphere in a closed tank for 3 days without stirring. After cooling, then grinding, the mixture is dried at 80° C. for at least 12 hours. The powder obtained, brown in color, is sieved at 250 μm. The sieved powder is pyrolyzed under a neutral atmosphere (nitrogen) at 800° C. for 3 hours according to a ramp rate of 5° C./minute. The carbonized powder obtained is mixed with a 2M concentration sodium hydroxide solution with stirring and at a controlled temperature of 80° C. for 15 hours. After filtration and washing to neutral pH, the filtered and washed powder is dried at 100°C for 15 hours, in order to form a porous carbon support.

Example 2 (comparative):

A support is made using the same process as Example 1. Its surface is then functionalized with “O” species by oxidizing the surface. 4.02 g of a support sample is placed in 100 ml of deionized water. 2.5 g of nitric acid (mass concentration 65%) are added to the mixture which is stirred at ambient temperature for 4 hours. The support thus functionalized is washed with deionized water to eliminate the excess acid then dried at 100° C. for 15 h.

Example 3 (according to invention):

The support is made using the same process as example 1 but 7.02 grams of glucose supplied by AnalaR NORMAPUR® are added to the resin.

Example 4 (according to the invention):

The support is produced according to the same process as example 1 but 28.15 grams of glucose are added to the resin.

Example 5 (according to the invention):

The support is produced according to the same process as example 1 but 50.02 grams of glucose are added to the resin.

Example 6 (comparative):

The support is produced according to the same process as example 1 but 19.69 grams of melamine (99% purity) supplied by Sigma-Aldrich are added to the resin.

Example 7 (comparative):

The support is made using the same process as example 1 but 70.52 grams of glucose is added to the resin.

The characterization results (by LECO analysis as described above) of the supports obtained (before catalyst deposition) are set out in Table 1 below:

b) Preparation of the catalyzed supports:

The deposition of the catalyst on the porous carbonaceous supports of the previous examples is carried out by known impregnation techniques. Each support is placed in deionized water for 10-20 minutes under ultrasonic sonication. Pt nitrate is dissolved in deionized water to obtain a solution with a concentration of 0.21 g/litre of water. This solution is added drop by drop to the suspension of the support in water in order to fix 1% weight of Platinum on the support.

The aqueous mixture is then subjected to ultrasound sonication for 20 minutes, followed by heating at 80°C in air and under constant stirring for 6 hours. The water is then vaporized in an evaporator then the catalyzed support is dried at 80°C for 12 hours.

c) Description of the tests carried out

CO conversion test

A sample of catalyzed support powder from the examples according to the invention and from those comparative ones previously described, is placed in a fixed-bed quartz reactor. More specifically, approximately 100 mg of the catalytic system is introduced into the reactor, equipped with a porous quartz frit supporting the catalyzed support powder. The volume of the sample catalyst powder is of the order of 0.30 cm ³ . The sample powder is placed under a stream of helium (10 L/h) at atmospheric pressure and at ambient temperature in order to eliminate traces of oxygen. The sample is reduced under a mixture of helium and hydrogen gas (40% by volume H ₂ ) at a temperature of 200° C. and a flow rate of 10NL/h for 77 minutes. Then a reaction mixture, composed of a CO/O ₂ /He mixture, is introduced onto the catalyst at a total flow rate of 10 NL/h. The gas mixture initially comprises 3000 ppm of O ₂ for 2000 ppm of CO. The temperature of the reactor in which the powder bed is placed is gradually increased from ambient to 300° C. at a ramp rate of 2° C./min.

On-line gas analysis is provided by a microchromatograph (Agilent, R-3000) equipped with two parallel analysis channels, each with a katharometric micro-detector (TCD). The first channel, 5A molecular sieve, 14 m long and 10.32 mm in diameter, is equipped with a pre-column (Back-flush) allowing the separation of small molecules (in particular O ₂ and CO). The second channel is equipped with a PLOT U column (6 mx 0.32 mm), used to separate larger molecules (carbon dioxide).

Comparative values T ₂₀ and T _{50 are thus determined,} corresponding to the temperatures for which percentages of 20% and 50% respectively by volume of the CO initially present are converted into CO ₂ .

Stability test:

The support powders not impregnated with catalyst are placed in a reactor under a helium flow of 10NL/h, the oxygen concentration being controlled and maintained at 3000 ppm, the temperature being increased according to a ramp of 10°C/min up to at 200°C. The formation of CO ₂ is monitored by micro-chromatography. The quantity of CO ₂ released at 200°C is measured by the microchromatograph. A correspondingly higher proportion of CO ₂ signifies decomposition and therefore greater thermal instability of the support. A CO ₂ content of less than 80 ppm and preferably less than 70 ppm, on the contrary, demonstrates an acceptable stability of the material constituting the substrate.

Table 2 below brings together the results of the various tests for the various examples previously described.

The results obtained set out in Table 2 above demonstrate the advantages of the materials according to the present invention:

The examples according to the invention have a much better thermal stability than comparative example 1 (whose support material consists of an undoped carbon) and a comparable catalytic performance, or even significantly improved in particular compared to comparative example 6 according to the prior art, the support material of which consists of a carbon comprising nitrogen.

The results reported in Table 2 show that Examples 3 to 5 according to the invention in particular have the best compromise between thermal stability and catalytic activity.

The catalytic performance due to the supply of oxygen is further improved when the oxygen/carbon mass ratio is close to 0.50 as shown in Example 5. This improvement is notable from the lowest temperatures (lower T ₂₀ ) .

In addition, an oxygen content in the material that is too high, and in particular an O/C mass ratio greater than or equal to 0.53, leads to too low a thermal stability, as demonstrated by the results observed for comparative example 7. In particular, too high a proportion of oxygen leads to a drastic reduction in the specific surface of the material and therefore its catalytic interest.

Claims (15)

Material based on carbon and oxygen whose specific surface is greater than 100 m²/g, in which the O/C mass ratio of the elementary content of Oxygen (O) to the elementary content of Carbon (C) is greater than 0.15. Material according to Claim 1, in which the mass ratio of the elemental content of oxygen (O) to that of carbon (C) is less than 0.53. Material according to one of the preceding claims, in which the mass ratio of the elementary content of Oxygen (O) to that of Carbon (C) is between 0.25 and 0.50. Material according to one of the preceding claims, in which the mass content of oxygen element is greater than 12%, and/or less than 35%. Material according to one of the preceding claims, in which the mass content of carbon element is greater than 60%, and/or less than 87%. Material according to one of the preceding claims, in which the cumulative mass content of the material in oxygen (O) and in carbon (C) is greater than 75%. Material according to one of the preceding claims, further comprising nitrogen, in which the mass content of nitrogen element (N) is less than or equal to 5%. Material according to one of the preceding claims, further comprising nitrogen, in which the mass ratio of the elemental oxygen content to that of nitrogen is greater than 10. Material according to one of the preceding claims, in which the specific surface is greater than 500 m²/g, preferably it is greater than 1000 m²/g. Material according to one of the preceding claims, having a degree of graphitization characterized by an I _D /I _G ratio between 0.8 and 1.35, determined by Raman spectroscopy, I _D and I _G corresponding respectively to the intensities of the peak at 1360 cm ^-1 and 1575 cm ^-1 , measured on a spectrum between 1000 and 1800 cm ^-1 . Process for manufacturing the material according to one of the preceding claims, comprising the following steps:
-preparation of a mixture of at least one carbon precursor and an oxygen source in the form of a polymer chosen from monosaccharides, disaccharides; polyols; the polysaccharides said polymer being added gradually in a basic solution maintained at a temperature in order to start the polymerization reaction,
- adding an impression material or replication model, preferably a silica gel precursor, to the mixture at a controlled temperature in order to continue the polymerization reaction,
- cooling then drying,
-carbonization, preferably pyrolysis, of said powder,
-mixing said carbonized powder with a basic solution, preferably with stirring and controlled temperature, in order to eliminate the impression material or the replication model. Catalyst support comprising or consisting of a material according to one of Claims 1 to 10. Catalyst system comprising a support according to claim 12 or comprising a material according to at least one of claims 1 to 10, said support or material being coated with a catalyst comprising a metal or a metal oxide, said metal or said metal oxide metal being preferably chosen from: Pt, Pd, Rh, Ru, Cu, Mn, Ag, Ni, Fe and Au. Use of the catalytic system according to the preceding claim, for the synthesis or depollution of gases, operating at an average temperature above 90°C and below 350°C. Use of the catalytic system according to claim 13, for the purification of indoor and/or outdoor air, in particular in dwellings, passenger compartments or confined spaces.