MESOPOROUS CARBONS

Abstract: An adsorbant wide pore (preferably mesoporous) carbon having a very regular arrangement of pores and consequently pathways defined by the pores have low tortuosities. The carbon comprises a form-stable assembly of carbon particles in which the particles are closed packed arrangements based on cubic and/or hexagonal close packing. Also a method for making these carbons which comprises forming a colloidal dispersion of particles of carbonisable polymer and depositing the particles under conditions such that their mutual repulsive forces guide them into a regularly packed deposit. The deposit is at least partially degraded to render it form-stable and then it is fully carbonised to produce the adsorbant carbon. Preferably the deposit is heated to 700 to 3000`C to close micropores and sinter contiguous particles.

Description: Mesoporous Carbons This invention relates to mesoporous and wider pore carbons which are useful as supports for catalysts or as acsorbants for larger organic molecules, for example aromatic molecules. It also relates to a method for making mesoporous carbons. Hitherto adsorbant carbons such as activated charcoals have been made by carbonising for example coal, peat, wood or coconut shell to produce a predominantly carbon substrate and then treating the substrate with an oxidising agent to create adsorbant pores in the carbon. Pores created in this way have a wide variety of sizes ranging from micropores (ie having diameters well below 2 nm) through mesopores (ie having diameters of from 2 to 80 nm . or more conventionally 2 to 50 nm) to macropores (ie having diameters well above 100 nm) . Neither the size of pores created nor the distribution of pore sizes can be pre- eύermined accurately because a continuing treatment by the oxidising agent not only enlarges existing pores but it also creates new micropores. Accurate pre-deter inaticn of pore size and pcre size distribution are important because both influence the performance of supported catalysts or adsorb * cion reaction. British patent specification 1 535 7 -5 ce≤cribes an attempt to produce mesoporous carbons having a pre-detεrminable narrow range of pore sizes. This is dene by packing particles of furnace black or channel black in a polymeric binder so that the spaces between adjacent packed particles are of me≤cpcrcus size. The binder then carbonised to render the packed assembly form-stable. However, the pore size and pore size distribution obtained in this way are not accurately pre-determinable partly because the particle sizes of carbon blacks vary over wide ranges even after sieving and partly because G3 1 536 715 does not proviαe means for packing the carbon particles in a regular assembly- A further consequence of irregular packing is that the pathways created by interconnected pores are highly tortuous owing to the irregular location of the adjacent particles which define the pores. High tortuosity inhibits the flow of molecules through the pathways. Also in practice high tortuosity restricts the size of molecules which can flow usefully to below the maximum size of molecule which would theoretically make * a clearence fit in a pathway. An object of this invention is to provide an adsorbant wide pore (especially a mesoporous) carbon comprising a more regular assembly of -carbon particles-so * allowing more- accurate pre-dεtermination of the size and size distribution of the pores and creating less tortuous pathways of interconnected pores. A further object is to provide an adsorbant wide pore carbon having a prε-cε .εrmined degree of regularity. Another object is to provide a method for making such adsorbant wide pore carbons and to provide a method for prε-dεt.ermir.ing the degrεε of rεguiarity in the assembly of carbon ^articles. Accordingly this invention provides an adsorbant wide pcre carbon comprising a form-stable assembly of a family of contiguous spheroidal (preferably spherical or oblate spherical) carbon particles wherein the surfaces of adjacent contiguous particles define interstitial pores characterised in that a) the maximum diameters of at least 90? (preferably at least 95?) by number cf that particles of the family lie in the range 8 to 3000 (preferably 50 to -400) n , b) the root mean square deviation of the maximum diameters of the particles of the family is less than 15? (preferably less than 5?) and c) in at least one zone of the adsorbant carbon, particles are close packed to an extent that the volume of interstitial pores in the zone is from 18 to 30? (preferably 23 to 28i). Often when such a carbon is provided with a smooth surface it is very lustrous which is indicative of the regularity of t.-.e carbon particles in the assembly which in turn implies a low tortuosity cf pathways created by interconnected interstitial pores. The size cf the pcres can be changed in a pre-determinable w by selective variation cf the size of the particles. Close packing cf perfect spheres is defined by A.F. Wells in the cZmm "Structural Inorganic Chemistry" fourth edition ,'cages 13C y • and 133) published by the Clarendon Press of Oxford in 1975- The contents of these pages are herein incorporated by reference. Briefly spheres pack closely in two basic arrangements- namely cubic close packed or hexagonal close packed. The surfaces of 75 adjacent contiguous spheres define two types of inter-connected interstitial pores, namely cuspidate tetrahedral or cuspidate octahedral pores. The size of the pores is characterised by the diameter of the largest sphere which can just be accommodated in the pore and for a tetrahedral pore this diameter is 0.255 d 80 and for an octahedral pore it is Q Λ l d where d is the diameter of the close packed spheres. Access from one pore to an interconnected pore is via a cuspidate triangular window or foramen defined by the surfaces of three contiguous close packed spheres. The useful size of a foramen is characterised by the 85 diameter of the largest sphere which could make a notional clearance fit in the foramen. This diameter is 0.155 d. The useful size of a foramen is a dimension of major importance in .that * : , it " has-.a major- ' af.fect--on: ■ the" " abi-lrty of- ' material " to di'ffύse into and out from the interstitial pores. Fully regular close 90 packing may comprise essentially all cubic or- all hexagonal clos packing or a mixture of the two. The volume of the interstitial pores in a cubic or hexagonal clos packed assembly of perfect spheres of equal size is approximately 26? of the total volume of the assembly. If contiguous pεrfεct 95 spheres of equal size are packed with increasing randomness, then the volume of the interstitial pores increases. When they are packed totally randomly, the volume of the pores reaches a theoretical maximum of 37?. Hence the volume of the interstitial pores is indicative of the regularity of the packing. Volumes 100 greater than 37? can occur if the contiguity of the packed spheres is disrupted or if the shapeyof packed particles varies from spherical to spheroidal. Likewise volumes much less than 26? can occur if the shape of the packed particles varies from spherical to spheroidal as may occur if the particles are deformed 105 at their points of contiguity by interactions which keεp the assembly form-stable. For example sintering together of adjacent particles may cause them to become oblate and in a three dimension- ally sintered assembly, particles may be oblate about mutually ' orthogonal pairs of poles. Preferably any reduction in the volume 110 of the assembly and hence of the pores brought about by such sintering should be less than 25? of the volume achieved with perfect sirperes. Adjacent particles may also be held " together by chemical bending including van der Waals' forces. Defects may occur in the structure of an assembly and so pore volume 115 may only be characteristic of one or more zones of an adsorbant carbon. In modification of the invention, the regularity of the assembly is disrupted to an extent such that the volume cf the interstitial pcrεs is from above 30 to -40? of the volume of the zone. One way 120 which a pre-determinable disruption can be achieved is to introduce one or more extra families of spheroidal carbon particle into the assembly, each family comprising particles having the same range of maximum diameters and the same range of root mean square deviations of maximum diameters as specified for the 125 unmodified invention but wherein the number average maximum dia etεr of the particles of one family differs from that of any other family. Preferably the number average maximum diameter cf the largest family (ie the family comprising the largεst number of particles) should not differ from that of any other family by 130 more than - 30? (especially - 10?). Preferably the numerically largεst family should provide at least 85? , (more preferably 95?) by volume of the carbon in a zone of the adsorbent carbon. By introducing particles of one or more pre-selected different ranges of size into the assembly, the regularity of the assembly can be 135 disrupted in a way which is to a large extent pre-determinable and in ' particular * the-. ' degree--of- .tortuosity " of-pathways * in * a- * zone -can b varied. Also the introduction of dissimilar particles complicates the close packing arrangements by allowing hybrid close packing arrangements as described on pages 131 to 133 of the took by wells 1-40 This invention also provides a method for making a wide pore adsorbant carbon characterised in that a) a colloidal dispersion of a family of spheroidal (preferably spherical) particles of carbonisable organic polymer is formed in a liquid wherein the maximum diameters of at least 90? 145 (preferably at least 95?) by number of the particles of the family lie in the range 10 to 3500 (preferably 58 to 230) nm and the root mean square deviation of those maximum diameters is less than 15? (preferably less than 5?) , b) the particles are caused to deposit from the liquid and 150 a packing force is exerted on them, c) the deposited particles are heated to a temperature which is below the softening point of the polymer but which is high enough to cause at least partial degradation of the polymer and thereby create a form-stable assembly of 155 particles and d) the form-stable assembly is heated to ensure complete carbonisation ' , of- the- polymer. The particles may be caused to deposit from the liquid by for example centrifuging the dispersion in which case the packing 160 force is a centripetal force or by evaporation of the liquid in which case the packing force is exerted by the surface tension of the residual quantities of liquid just prior to completion of the evaporation. Yet a further method for causing the particles to deposit comprises forcing the liquid through a porous membrane 165 by means of direct hydrostatic pressure. The particles are deposited and packed on the membrane. It seems probable that provided the particles are speroidal with maximum diameters of less than 3500 nm, then Brσwnian motion confers enough mobility on them while in dispersion to enable their strong mutual 170 repulsions to guide them towards regular assembly under the influence of the packing force. The packing force may be any compression which does not destroy the particles. For example it may be as low as that generated by their own weight but typical cc pressive forces used when generated by surface tension 175 ≥re from 20 to 40 bar and typical compressive forces used when generated hydrostatically are from 1 to 100 (preferably 5 to 65) bar. Provided at least the partial degradation of the polymer is performed at a temperature below that at which the polymer soften 180 so that its particles are substantially deformed, then the deposi car_;-be ' ;i__ade. • suf-fie-iehtl -form-stable-to permit'-full -carbonisation of the particles without major unpredictable disruption of the regularit of the assembly. This form-stability of the degraded . particles is enhanced if the polymer is capable of cress-bonding 185 even if cross-bonding enly occurs as a result of the degradation. Generally cross-bonding will be chemical cross-linking but it may also be for example by ionic attraction or by crystallite formati By centrifuging carefully (for example using speeds cf 800 to 140 revolutions per minute and a radius cf revolution of 5 cms) or by 190 slow evaporation (for example evaporation carried out at least 9°C and preferably at least 40 C below the normal boiling point of the liquid) it is possible to achieve a cubic and/or hexagonal close packed assembly. By depositing under progressively more disturbed conditions it is possible to introduce increasing 195 pre-determinable amounts of disruption into the deposited assembly which can be retained in the fully carbonised assembly. Controlled disturbance can be achieved by for examplε centrifuging a partially flocculated colloidal dispersion such as occurs when small amounts of electrolyte are added to a charge-stabilised colloid or whεn, 200 in the case of a stεrically-stabilised colloid, the composition of the dispersing liquid is changed. Alternatively controlled disturbance can be achieved by evaporation at temperatures progressively nearer the boiling point of the liquid. A further method cf introducing pre-determinablε disruption into 205 -the- ' . ' depos * it'εdv ' a * ssembly * and ''* hence ' "into the corresponding carbonised assembly is to use a poiymodal (preferably a bimodal) colloidal dispersion. A poiymodal dispersion comprises two or more families of spheroidal particles each family having a number average maximum spheroid diameter which is different from that of the other 2ι0 families. Deposition from a poiymodal dispersion produces a deposited assembly which is disrupted in a pre-determinable way by the presence of particles of different ranges of sizes. In this way an even wider range of pore sizes can be produced. ?rPεf-~εrably the carbonised assembly is heated to a temperature of 215 from 700 to 3000°C (especially 1000 to 2000°C) to close micropor The colloidal dispersions may be prepared by conventional techniques including ultra-sonic dispersion or polymerisation in the dispersing liquid (which may be aqueous or non-aqueous) unti colloidal particles of the required size have been grown. Where 220 necessary the dispersion may be stabilised by the use of colloid stabilisers (sometimes known as protective colloids), for exampl a block copolymer of polystyrene with terminal blocks of poiy imethylsilcxane. • Suitable carbonisable organic polymers include any polymers 225 (including copolymers) which can be at least partially degraded if not fully carbonised at temperatures below the softening poin cf the polymer. Examples of suitable thermosetting polymers include pol (furfuryi alcohol), phenol-formaldehyde and phεnol- benzaidehyde resins. Thermosetting resins have the advantage of 230 being highly cross-linked. Examples of suitable thermoplastic polymers include coal tar pitch, poly(vinylicene chloride) and polyacrylonitrile. ?oly(vinyliαene chloride) cross-links during degradation and polyacrylcnitrile can form so called ladder poly in which adjacent chains cross-bond. Frequently a particle ' 235 assembly shrinks by from 12 to 20? as a result of carbonisation. The invention is illustrated by the following example which describes the preparation of a mesoporous carbon and which refers to the drawings of which Figure 1 is a graph showing the adsorpticn/desorption hysteresis 240 isotherm at 25 C obtained when benzene is adsorbed onto and desorbed from a mesoporous carbon according to the Figure 2 is a graph derived from figure 1 using a conventional pore size analysis technique and showing (a) how the 245 volume attributable to all tetrahedral pores and foramina of a given size varies with variation of the given size and (b) how the useful volume attributable to all foramina of a given useful size varies with the variaticn of the useful size. ■ C Figure 3 is a graph which shows by way of comparison three adsorption/desorption hysteresis isotherms at 25 C obtained using three different known activated carbons. ■ .XAMPLE An adsorbant mesoporous carbon having pore size characteristics 255 as shown in figures 1 and 2 of the drawings was made as follows: Firstly a polymeric colloid-stabiliser was prepared by polymerising styrene in tetrahydrofuran under an inert, atmospher at 20 C. The polymerisation was initiated by dilithium naphthalene. The growing polymer was then reacted with hexam- . 260 ethylcyclotrisiloxane to produce a block copoly er comprising a central polystryene block flanked by blocks or polydimethylsilcx each terminated by a silanol end group. Next a colloidal dispersion of polyacrylonitrile in hexane was produced as follows: 265 Acrylonitrile and benzoyl peroxide initiator were charged to pur hexane containing a small amount of the stabiliser as made above The hexane was heated under reflux and nuclei of poiyacrylonitil began to appear. Extra feeds of acrylonitrile were added drop w and the particles grew to 249 - 6 n diameter whereupon the reac 270 mixture was cooled to room temperature and further polymerisatio prevented. Electron microscopy and chemical icroanalysis showe the product to be a colloidal dispersion having a monodispersε particle size in which the particles were virtually spherical. The root mean square deviation of the maximum diameters of the 275 spheroidal particles was between 2 and 3?- The hexane was evaporated very slowly from the colloidal dispers by maintaining the dispersion at 26 ^ C over a pεriod cf £0 hours. The deposit of particles of polyacrylcnitrilε which was obtained was highly irridescent which indicated the presence of very 280 regular close packing of the particles. To cause degradation, the deposit was heated from 30 C to 170 C over a period of 3 hours, maintained at 170 C for a further 3 hours and then heated slowly to 250 C and maintained at that temper turε overnight. The particles degraded to a fcrm-stable assembly. 285 Finally the degraded assembly was heated to 1600 C to close micropores, to ensure completε carbonisation, and to sintεr lightly togethεr adjacεnt carbonised particles to produce a mere form-stati; adsorbant carbon. The carbon produced was a pseudemorph of the ' - deposited assembly of particles of polyacrylonitrile and it was 290 resistant to conversion to graphite. The assembly shrank by • - about 16? during carbonisation. Figure 1 shows the adsσrption/desorpticn hysteresis isotherm at 25- C"for- --carbon -made " ' according * -to " 'the-- example ' as'-described above and obtained by subjecting the carbon to a increasing pressure ? '295 of benzene (expressεd as a fraction cf the saturated vapour pressure Po of benzene at 25 C, ie ?/?o) so as to cause increasing amounts of benzene (expressed as g of adsorbed benzene per gram of carbon) to adsorb onto the carbon and then decreasing the pre≤surε of benzene to cause the benzene to dε≤crb from the carbon. Part AΞ 30C of the hysteresis loop relates to the adsorption process and part CD to the dε≤orption process. It is conventionally supposed that the adsorption part AB of tne loop is indicative of the volume o the tetrahedral pores and the foramina whereas the desorption pa CD of the loop is indicative of the useful size of the foramina. 305 It should be explained that for reasons which are not properly -- understood, the hysterεsis isothεrm fails to dεtεct consεquεnces of the presence of the octahedral pores. It should also be explained that the relationship betwεen the desorption part cf t loop and the useful size of the foramina derives from the fact t 310 desorbing benzene is preceeded by a he isphεrical meniscus which must of course be small enough to pass through foramina in order to escape from the porous carbon. By using a conventional pore size analysis technique and the relationship between the adsorption part of the hysteresis loop 315 and the volume of the tetrahedral pores, it is possible to deriv a curve which shows how the combined volume of tetrahedral pores attributable.J.o..pcres..of a given.so-called "diameter".- (ie .the diameter of the largest sphere which could notionally be accommo ated within the pore) variεs as thε "diameter" .varies. Such a ■320 curve is shown in figure 2 marked "Curve Ξ" . In figure 2, this variation in volume per gram of carbon with variation of pore diameter is expressed in ml/g of carbon/n and pore diameter is εxpressεd in nm. Similarly using thε relationship between the desorption part of the hysteresis isothεrm and the useful size o 325 foramina it is possible to derive a curve which shows how the combined useful volume attributable to oramina of a given "usefu sizε varies as the useful size varies. Such a curve is shown in figure 2 as "Curve F". Figure 2 indicates that the tetrahedral pores have pore "diametεrs" of around 52 nm and that the foramina 330 have useful sizes of around 31 nm. Figure 3 shows for the purposes of comparison three adsorption/ desorption hysterisis isotherms at 25 C obtained in the same way as the isotherm shown in figure 1 except that three known activated carbons were used instead of the carbon made according to this 335 invention. More particularly, isotherms G, H and I shown in figure 3 relate respectively to lightly, moderately and heavily steam activated charcoals prepared from anthracite. The predo in- _. ance of adsorption at low benzene pressures indicates that most of the porosity of the charcoals is provided by micropores. This 340 contrasts with the substantial absence of adscrbtion at low pressures in figure 1 which indicates an absence of micropores. The height . of the . .hysteresis loop shown in figure 1. indicates that most of the carbon of the invention is provided by mesoporεs whereas in figure 3 the hysteresis loops indicate the presence of only a 345 few (if any) mesopores. Two samples of colloidal dispersion made according to the example were caused to deposit frc the hexane under more disturbing conditions. In the first case, the hexane was evaporated off very rapidly by heating the dispersion to 80 C. The deposit showed a 350 slight yellowness which is indicative of less regular packing. In the second case the dispersion was heated slowly to 30 C and evaporation completed over a period of 30 hours. The deposit was white and exhibited a pattern of coloured rings when viewed in sunlight.' This indicates more regular packing. 355 The adsorbant carbons find uses as supports for catalysts, particularly precious metal catalysts such as palladium. Their pre-determinable wide pore structure and pre-determinable degrεε of pathway tortuosity enable catalysts to be located deep inside the carbon away from poisons, harmful solvents or other deleteri 360 substances yet the pre-determinability of the pore structure all the pore size and/or pathway tortuosity to be tailored to favour entry of wanted molecules even when such molecules are quite lar The carbons may be used to support catalysts used in the hydrogεnation of fatty oils as described more fully in British 365 patent,specifications .1 57.8-..122. and- 578 123 ' .the .coatents of which are herein incorporated by refεrence. Thεy ma;/ also be us in the structure of porous electrodes or as adsorbants, expeciai for larger molecules.

Claims: CLAIMS 370 1. An adsorbant wide pore carbon comprising a form-stable assembly of a family of contiguous spheroidal carbon particles wherein the surfaces of adjacent contiguous particles define interstitial pores characterised in that a) the maximum diameters of at least 90? by number of the 375 particles of the family lie in the range 8 to 5000 nm, b) the root mean square deviation of the maximum diameters of the particles of the family is less than 15? and c) in at least one zone cf the adsorbant carbon, particles are cubic or hexagonal close packed to an extent that the 380 volume of interstitial pores in the zone is from 18 to 3-0? of the volume of the zone. ' 2. A : - ' modi-fication" ' of * -the wide pore- ' -carb ' on claimed in ' claim "' !' • characterised in that the assembly "of particles is disrupted to an extent that the volume of the interstitial pores in a 385 zone of thε wide pore carbon is from above 50 to 40? of the volume of thε zone. 3. A wide pore carbon as claimed in claim 1 or a modified wide pore carbon as claimed in claim 2 characterised in that the assemble of carbon particles comprises one or more extra 390 families of spheroidal carbon particles wherein K a) the maximum diameters of at least 90? by number of the particles of a family lie in the range 3 to 1000 n . b) the root mean square deviation of the maximum diameters of the particles of a family is lεss than 15? and 395 c) thε number average maximum diamεter of the particles cf any one family differs from that of any other family. 4. A wide pore carbon as claimed in any one of claims 1 to 3 charactεrised in that the wide pore carbon is substantially free from micropores in the carbon particles. 400 5. A method for making a wide pore adsorbant carbon characteris in that a) a colloidal dispersion of a family of spheroidal partic of carbonisable organic polymer is formed in a liquid wherεin the maximum diameters of at least 90? by number "405 Of • the' * ' * -pa * rti'ci : es- " o ' f ' tfie- am ' ily "* lie iri " the *' - * r ngε " l " 0"to -p ' S nm and thε root mean square deviation of those maximum diameters within a family is less than 15?. b) the particles are caused to deposit from thε liquid and a packing force is exerted on them, 410 c) the deposited particles are heated to a tempεrature which is below the softening point of the polymer but which Is high enough to cause at least partial degrad¬ ation of the polymer and thereby create a form-stable assembly of " particles and 415 d) the form-stable assembly is heated to ensure complete carbonisation of the polymer. 6. A method as claimed in claim 5 characterised in that the colloidal dispersion .comprises one or more extra families of spheroidal particles of carbonisable organic polymer wherein 420 a) the- ■ maximum ciametεrs of at least 90? by number of the particles of a- family lie in the range 10 to 3500 nm, b) the root mean square deviation of those maximum diameters within a family is less than 15? and c) the number average maximum diameter of the particlεs of 425 any onε family differs from that of any other family. 7. A method as claimεd in claim 5 or claim 6 charactεrised in that thε packing forcε is provided by centrifuging. Q . ' '' -me • thόσa ■ s-c-ϊai_he ^ --- ■ i '' clai "' 5 ' -• • or : . ' -clai-__- -6 ■• characterised ' .iπ-- that the particles are caused to deposit from the liquid by 30 evaporating the liquid and the packing force is exerted on the particles by means of the surface tension of the liquid prior to completion of evaporation. 9. A method according to claim .5 or claim 6 charactεrised in that the particles are caused to deposit from the liquid by forcing ^35 the liquid under hydrostatic pressure through a membrane which is imperveous to thε particles wherεby thε particlεs dεposit ^ o _. ^ - on the membrane . ϋ^ Λ* -' '1 "■ * 10. A method as claimed in claim 5 or claim 6 characterised in thai the form-stable assembly of deposited particles is heated to a 440 tempεraturε of from 700 to 2500 C to closε micropores in the carbon particles and to sinter together contiguous particles. 11. A catalyst characterised in that the catalyst is supported on an adsorbant wide pore carbon as claimed in claim 1 or claim 2 or on the product obtained by the method of claim 5.

Inventors: BLIGH S, EVERETT D, ROJAS F
Applicants: BLIGH S, EVERETT D, JOHNSON MATTHEY CO LTD, ROJAS F, JOHNSON MATTHEY & CO LTD
Application year: 1981
Main classification: C01B31
IPC codes: B01J20, B01J21, B01J37, C01B31