FR2683741A1 - Catalyst of the aluminosilicate type and its use in aromatisation of hydrocarbons containing 5 to 12 carbon atoms - Google Patents

Abstract

A composite catalyst which contains: - a zeolite of MFI structure in hydrogen form, containing in its structural framework at least the elements silicon, aluminium and/or gallium, - a matrix, - at least one noble metal from the platinum group, at least one additional metal chosen from the group consisting of tin, germanium, indium, copper, iron, molybdenum, gallium, thallium, gold, silver, ruthenium, chromium, tungsten and lead, - at least one halogen chosen from the group consisting of fluorine, chlorine, bromine and iodine, - optionally at least dope element chosen from the group consisting of gallium and zinc, - and optionally at least one element chosen from the group consisting of the alkali and alkaline-earth metals. The invention also relates to the use of this catalyst in the reactions of aromatisation of hydrocarbons containing from 5 to 12 carbon atoms per molecule.

Description

The present invention relates to a catalyst, referred to as a composite catalyst, which contains
a zeolite with structure MFI in hydrogen form, containing
in its framework the silicon and at least one element chosen in
the group formed by aluminum and gallium,
- a matrix,
at least one noble metal of the platinum family, at least one
additional metal selected from the group consisting of tin,
germanium, indium, lead, gallium and thallium, metals
Group I such as copper, gold, silver, metals of the group
VIII such as nickel, ruthenium, iron and metals of the group
VI such as chromium, molybdenum and tungsten,
at least one halogen selected from the group consisting of
fluorine, chlorine, bromine and iodine,
- possibly at least one other metal chosen in the group
consisting of thallium and zinc,
- and possibly at least one element chosen from the group
consisting of alkali and alkaline earth metals.

 The invention also relates to the preparation of the catalyst and its use in aromatization reactions of hydrocarbons having from 2 to 12 carbon atoms per molecule and more particularly from 5 to 12 carbon atoms per molecule.

 The alkali and alkaline earth metals, as well as the platinum and the so-called additional metals are hereinafter referred to as "metals", and do not refer to gallium or zinc when both of these elements are optionally used in the zeolite and / or the matrix as doping elements.

 Catalysts with zeolites doped with gallium, zinc or platinum are known to be active and selective in the aromatization of propane and butane. Conventionally, hydrocarbons containing more than 6 carbon atoms per molecule are converted into aromatics by catalytic reforming using platinum-containing acid alumina-type catalysts, to which tin or rhenium may be added, for example. These reforming catalysts are nevertheless very inefficient for the aromatization of hydrocarbons containing less than 6 carbon atoms per molecule. There is therefore a great practical interest in finding efficient catalysts for the aromatization of C5-C12 hydrocarbon-rich cuts.

The aromatization reaction of hydrocarbons containing less than 9 carbon atoms per molecule in the presence of zeolites has already been the subject of patents and publications. Several catalytic systems based on MFI zeolite are claimed, these systems being distinguishable by the additions they contain. Schematically, we can distinguish:
(i) gallium-doped systems (US-A-4175057), and
(ii) zinc-doped systems (US-A-4288645).

 These systems all suffer from a major defect, namely a high selectivity in methane. To improve the performance of these catalytic systems several solutions have been proposed including the addition of platinum (Jin Z., Y. Makino, A. Miyamoto, T. Inui, Chem.Express, 2, p.515, 1987). The use of a non-acidic MFI zeolite doped with various metal elements has also been claimed (Y Chen et al., WO 8904818).

 Recently (French patent application 91/10624 of the applicant), it has been discovered that the use of composite catalysts containing an MFI zeolite on the one hand, and on the other hand a generally amorphous support or matrix on which is deposited a noble metal of the platinum family and at least one additional metal such as tin, lead or indium, leads to catalytic performances in aromatization reactions of paraffins of 5 to 9 carbon atoms markedly improved compared to to the systems of the prior art.

 The use of such catalysts in particular makes it possible to limit the reactions which lead to the formation of methane, an undesired product.

 The research work carried out by the applicant led it to discover that, surprisingly, the use of a composite catalyst containing an MFI zeolite in hydrogen form, optionally thallium and / or zinc in oxide form and a matrix on which is deposited at least one noble metal of the platinum family (in particular palladium, platinum, nickel, iridium, rhodium), at least one additional metal selected from the group consisting of tin, germanium, lead, indium, lead, gallium and thallium, Group I metals such as copper, gold, silver, and Group VIII such as nickel, ruthenium, iron and Group VI metals such as that chromium, molybdenum, tungsten, said matrix also containing at least one halogen and optionally at least one alkali metal or alkaline earth metal (preferably lithium or potassium), leads to catalytic performances in the reactions of flavored paraffins containing from 5 to 12 carbon atoms per molecule significantly improved over the prior art catalysts.

 The MFI zeolite contained in the catalyst of the present invention can be prepared by any of the techniques described in the prior art.

Thus the synthesis of said zeolite can be carried out in a conventional medium
OH- in the presence or absence of organic agent and / or alcohol. The document "Synthesis of high silica zeolites, P. Jacobs and J. Martens,
Studies in Surface Science and Catalysis, Vol. 33, 1987 "describes the conventional synthesis of the MFI zeolite.The MFI zeolite used in the present invention may also have been synthesized in less conventional media such as the fluoride medium present (patent
EP-A-172068) or in the absence (French patent application 90/16529) of organic compound. The zeolite used in the present invention contains, in its crystallized framework, silicon and at least one element selected from the group formed by aluminum and gallium.

 After the synthesis step, the MFI zeolite is converted into a hydrogen form, denoted H-MFI, by virtually total elimination of the organic compounds and / or alkaline or alkaline earth cations that it contains optionally after synthesis. All the techniques described in the prior art can be used for the transition to the hydrogen form, such as, for example, ion exchange followed or not by calcination or various chemical treatments.

All zeolites synthesized in one of the following systems
Si-Al, Si-Al-Ga, Si-Ga, are suitable for the present invention. However, their Si / T ratio where T represents Al and / or Ga, is generally greater than 7, preferably greater than 10 and even more preferably between 13 and 500.

 The zeolite H-MFI, used in the present invention, may be optionally subjected as such to a deposition of gallium and / or zinc, or mixed with the other constituents of the catalyst, thallium and / or zinc can then optionally be subsequently introduced into said mixture (on the zeolite and / or the matrix).

 Many techniques for deposition of gallium and / or zinc can be used in the present invention, among which we can mention the ion exchange through the use of salts in aqueous solution, or impregnation with solutions of said salts.

 The cumulative content of these two doping elements optionally deposited on the composite catalyst is between 0.01 and 10% by weight, preferably between 0.03 and 4% by weight.

 The matrix comprises at least one refractory oxide, and in particular at least one oxide of a metal selected from the group consisting of magnesium, aluminum, titanium, zirconium, thorium, silicon and boron. In addition, it may also include coal.

 The preferred matrix is alumina, the surface area of which may advantageously be between 10 and 600 m 2 / g and preferably between 150 and 400 m 2 / g.

 The composite catalyst of the present invention can be prepared in two ways, the principle of which is given below, the practical embodiment being known to those skilled in the art.

First way
The zeolite H-MFI is mixed with the matrix. This mixture can be produced between two powders, between two previously shaped solids, between a powder and a solid previously shaped. It is also possible to formulate the two solids together by all the techniques described in the prior art: pelletizing, extrusion, coating, drop coagulation, spray drying. During these shaping operations, it is possible, if necessary, to add a shaping additive, selected from the group consisting of silica and alumina.

Thus the zeolite was mixed with the matrix and shaped. After mixing and shaping, the metal is deposited, the introduction of at least one halogen and optionally the doping element (gallium and / or zinc) on the assembly consisting of the matrix and the zeolite , the deposit order being of little importance. It is then considered that most of the metals, for example preferably from 60 to 100% by weight relative to the composite catalyst, are found on the matrix.

Second way
The metals and the halogen are first deposited on the matrix on the one hand, and possibly gallium and / or zinc on the zeolite H-MFI on the other hand. The order of introduction of the elements is indifferent. Then the H-MFI zeolite optionally containing gallium and / or zinc is mixed with the matrix containing the metals and the halogen and shaping them, the shaping being obtained under the same conditions as above. In one variant, the zeolite, on which thallium and / or zinc have been optionally deposited, may be mixed with the matrix at any of the metal deposition steps and the introduction of the halogen on the matrix. .

 The preferred method of preparation consists, optionally to deposit the gallium and / or zinc on the zeolite, to deposit the metals and to introduce the halogen on the matrix, then to introduce the zeolite possibly loaded with thallium and / or zinc in the matrix loaded with metals by shaping the two powders. The shaping is preferably carried out after micron milling, which can be carried out using the wet milling technique.

 The composite catalyst contains between 1 and 99% by weight of zeolite, the 100% complement consisting of matrix, metals and halogen. The respective proportion of zeolite and matrix varies over a wide range, as it depends on the one hand on the Si / T ratio of the zeolite, where T is Al and / or Ga, and on the other hand on the content of metals and in halogen of the matrix in the case of the preferred method of preparation.

 The matrix containing the metals and halogen, in the case of the preferred method of preparation, is generally prepared according to the procedures described in the French patent application 91/10624 of the applicant, a part of which is reproduced in the following. These procedures can be used, for example, on the matrix alone, or on the matrix / zeolite mixture, as has been described in the two preparation routes.

 For the impregnation of metals, either a common solution of the metals which it is desired to introduce, or distinct solutions for the noble metal of the platinum family and for the additional metal and optionally the element chosen from the group consisting of by alkali metals and alkaline earths. When several solutions are used, dryings and / or intermediate calcinations can be carried out. It is usually terminated by calcination, for example between 500 and 1000 ° C., preferably in the presence of molecular oxygen, for example by conducting an air sweep.

 The noble metal of the platinum family may be incorporated in the matrix by impregnation of said matrix with a solution, aqueous or not, containing a salt or a noble metal compound. Platinum is generally introduced into the matrix in the form of chloroplatinic acid, but any noble metal may also be used ammonia compounds or compounds such as for example ammonium chloroplatinate, platinum dicarbonyl chloride, hexahydroxyplatinic acid , palladium chloride, palladium nitrate.

The additional metal selected from the group consisting of tin, germanium, lead, indium, lead, gallium and thallium, group I metals such as copper, gold, silver, the metals of the group
VIII such as nickel, ruthenium, iron and Group VI metals such as chromium, molybdenum, tungsten can be introduced via compounds such as, for example, chlorides, bromides and nitrates. tin, halides, nitrate, acetate and lead carbonate, germanium chloride and oxalate, nitrate and indium chloride.

 The halogen may be introduced from at least one of the platinum family metal halides or at least one of the additional metal halides, in the case where these metals are introduced from halides. A complementary method may consist of impregnation of the matrix with an aqueous solution containing an acid or a halogenated salt. For example, chlorine can be deposited using a hydrochloric acid solution. Another method may consist of calcination at a temperature generally of between 400 and 900 OC, in the presence of an organic compound containing halogen, such as, for example, CCl 4, CH 2 Cl 2, CH 3 Cl, etc.).

 The element selected from the group consisting of alkali and alkaline earth metals, preferably lithium or potassium, can be introduced via compounds such as for example halide, nitrate, carbonate, cyanide. and the oxalate of said element.

A method of preparation, which we detail below, includes for example the following steps
a) introducing onto the matrix at least one element selected from the group consisting of fluorine, chlorine, bromine and iodine,
a ') Possible introduction on the matrix of at least one element selected from the group consisting of alkali and alkaline earth.

 b) Calcination of the product obtained in step a) or a ').

 c) Introduction on the matrix of at least one noble metal of the platinum family.

 d) calcination of the product obtained in step c).

 e) Introduction on the product obtained in step d) of at least one additional metal M.

 If no alkali or alkaline earth metal is used, only steps a), c), d) and e) of said preparation process are carried out.

 The use in the present invention of at least one noble metal of the platinum family can be exemplified by the use of ammonia compounds.

 In the case of platinum, there may be mentioned, for example, hexamines platinum IV salts of formula Pt (NH 3) 6X 4; platinum IV halopentamine salts of formula (PtX (NH 3) 5) X 3; platinum N tetrahalogenodiamine salts of formula PtX4 (NH3) 2; platinum complexes with halogen-polyketones and halogenated compounds of formula H (Pt (aca) 2X); X being a halogen selected from the group consisting of chlorine, fluorine, bromine and iodine, and preferably X being chlorine, and aca representing the remainder of formula C5H7O2 derived from acetylacetone.

 The introduction of the noble metal of the platinum family is preferably carried out by impregnation with an aqueous or organic solution of one of the organometallic compounds mentioned above. Among the organic solvents that can be used, mention may be made of paraffinic, naphthenic or aromatic hydrocarbons, and halogenated organic compounds having, for example, from 1 to 12 carbon atoms per molecule. For example, n-heptane, methylcyclohexane, toluene and chloroform may be mentioned.

Solvent mixtures can also be used.

 After introduction of the noble metal of the platinum family, the product obtained is optionally dried and is preferably calcined at a temperature of between 400 and 1000 OC.

 After this calcination, at least one additional metal is introduced, optionally preceded by a reduction with hydrogen at high temperature, for example between 300 and 500 OC. The additional metal called M below can be introduced in the form of at least one organic compound selected from the group consisting of the complexes of said metal, in particular the polyketal complexes of the metal M and hydrocarbylmetals such as alkyls, cycloalkyls aryls, alkylaryls and arylalkyls.

 The introduction of the metal M is advantageously carried out using a solution of the organometallic compound of said metal in an organic solvent. It is also possible to use organohalogen compounds of metal M. As compounds of metal M, mention may be made in particular of tetrabutyltin in the case where M is tin, tetraethylplomb in the case where M is lead and triphenylindium in the case where M is indium.

 The impregnating solvent is selected from the group consisting of paraffinic, naphthenic or aromatic hydrocarbons containing from 6 to 12 carbon atoms per molecule and halogenated organic compounds containing from 1 to 12 carbon atoms per molecule. For example, n-heptane, methylcyclohexane and chloroform may be mentioned. It is also possible to use mixtures of the solvents defined above.

 In the case where the method of preparation as described above is not used, it is also possible to envisage introducing at least one additional metal M before the introduction of at least one noble metal of the family of platinum. If the metal M is introduced before the noble metal, the compound of the metal M used is generally selected from the group consisting of the halide, the nitrate, the acetate, the tartrate, the carbonate and the oxalate of the metal M. The introduction is then advantageously carried out in aqueous solution. In this case, before proceeding to the introduction of at least one noble metal, it is calcined in air at a temperature between 400 and 1000 OC.

The composite catalyst contains 1) by weight relative to the matrix (but not necessarily on the matrix itself):
from 0.01 to 2% and preferably from 0.1 to 0.5% of at least one noble metal of the platinum family,
at least one additional metal whose concentration is from 0.005 to 2%, preferably 0.01 to 0.5%, of tin in the case where the additional metal is tin, from 0.005 to 0.7%, preferably 0.01 to 0.6%, of at least one additional metal other than tin (selected from the group consisting of germanium, indium, lead, gallium and thallium, the metals of the groups I and VIII such as copper, gold, silver, nickel, ruthenium, iron and Group VI metals such as chromium, molybdenum, tungsten); in the case where the catalyst contains two additional metals, the overall content of additional metals is between 0.02 and 1.20%, preferably between 0.02 and 1.0%, and even more preferably between 0, 03 and 0.80%.

from 0.1 to 15%, preferably from 0.2 to 10%, of a member selected from the group consisting of fluorine, chlorine, bromine and iodine, preferably chlorine,
optionally from 0.01 to 4%, preferably from 0.1 to 0.6%, of at least one metal selected from the group consisting of alkalis and alkaline earths, preferably selected from the group consisting of lithium and potassium.

2) between 1 and 99% by weight of zeolite MFI hydrogen form containing in its framework silicon and at least one element selected from aluminum and gallium, 3) and possibly by weight relative to the zeolite
between 0.01 and 10% by weight, preferably between 0.03 and 4%, of a doping element chosen from the group consisting of gallium and zinc, preferably gallium, this element being introduced into the zeolite and / or the matrix.

 In the process according to the invention, at the end of the preparation, the shaped catalyst contains an H-MFI zeolite, metals, at least one halogen and a matrix, and is calcined under air at a temperature between 450 and 1000 OC. The catalyst thus calcined may advantageously undergo an activation treatment under hydrogen at high temperature, for example between 300 and 500 OC. The hydrogen treatment procedure consists, for example, in a slow rise in temperature under a stream of hydrogen up to the maximum reduction temperature, generally between 300 and 500 ° C. and preferably between 350 and 450 ° C., followed by maintaining at this temperature for a period generally between 1 and 6 hours.

 The catalyst of the present invention described above is used for the aromatization of alkanes containing from 5 to 12 carbon atoms per molecule, in the presence or absence of olefins. This reaction is of particular interest because it can allow, for example, on the one hand to develop light fractions from refining operations by transforming them into products with higher added value (benzene, toluene and xylenes), on the other hand to transform paraffinic charges that may contain olefins into bases for high octane fuel, while in both cases contributing to the production of large quantities of hydrogen essential for hydrotreatment processes for example.

 The feed which contains compounds containing 5 to 12 carbon atoms per molecule is contacted with the catalyst of the present invention at a temperature of between 400 and 700 OC.

 The following examples specify the invention without limiting its scope.

Example I: Preparation of Alumina Containing Platinum, Tin and Chlorine (Catalyst A)

 The alumina used has a specific surface area of 220 m 2 / g and a pore volume of 0.52 cm 3 / g.

 To 100 g of alumina support is added 500 cm3 of an aqueous solution of hydrochloric acid. It is left in contact for 3 hours, filtered, dried 1 hour at 100-1200C.

The dried product containing the chlorine is then impregnated with platinum, adding to the solid 150 cm3 of an aqueous solution of hexachloroplatinic acid. The platinum concentration of this solution is 2.7 g / l. Leave 6 hours in contact, dry for 1 hour at 100-1200C and then calcine 2 hours at 5300C
On the calcined product containing chlorine and platinum, the tin is impregnated: an organic solution of tetrabutyltin is brought into contact with the alumina support at a rate of 100 cm 3 of solution per 100 g of support for 6 hours. The solid obtained is then drained and dried 1 hour at 100-1200C and then reduced under a stream of dry hydrogen for 2 hours at 4500C.

 The alumina then contains, by weight, 0.40% platinum, 0.3% tin and 1.0% chlorine.

Example 2: MFI zeolite form hydrogen (catalyst B)
A hydrogenated H-MFI zeolite is used which is obtained from the NaMFI form resulting from the synthesis as follows
- ammonium nitrate treatment
calcination at 5500C under air.

 The characteristics of the solid obtained are: an Si / Al ratio equal to 29 and a sodium content equal to 0.014% by weight. Its pore volume measured by nitrogen adsorption at 77K is 0.192 cm3 / g. The mesh size of the crystal lattice is 5339 cubic Angstroms.

Example 3 Preparation of the Blends (Catalysts D and E) Prenaration of Alumina Containing Platinum and Tin (Catalyst C)

 The alumina used is identical to that used in Example 1.

On 100 g of this alumina is impregnated with platinum, adding to the solid 150 cm3 of an aqueous solution of hexachloroplatinic acid. The platinum concentration of this solution is 2.7 g / l. Leave 6 hours in contact, dry for 1 hour at 100-1200C and then calcine 2 hours at 5300C
On the calcined product containing platinum, impregnation of tin is carried out: an organic solution of tetrabutyltin is brought into contact with the alumina support in a proportion of 100 cm3 of solution per 100 g of support for 6 hours. The solid obtained is then drained and dried 1 hour at 100-1200C and then reduced under a stream of dry hydrogen for 2 hours at 4500C.

 The alumina thus prepared (catalyst C) then contains, by weight, 0.40% platinum and 0.3% tin.

Preparation of the mixtures (catalysts D and E)
Catalysts D and E are prepared by mixing from catalysts A, B and C. These mixtures are produced in the following mass proportions
catalyst D = 60 g catalyst A + 40 g catalyst B
catalyst E = 60 g catalyst C + 40 g catalyst B
Each of the constituents of these mixtures is subjected beforehand to submicron milling. After mixing, the catalysts D and E are shaped by pelletization.

Example 4 Catalyst Performance
It is proposed to transform a charge consisting of a hydrocarbon mixture containing from 5 to 6 carbon atoms per molecule. This is done for the presence of one of the four catalysts A, B, D and E, the preparation of which has been described previously.

These catalysts were tested for conversion of a C5-C6 filler whose composition is as follows (expressed in% by weight)
Paraffins C5 .......... 90% C6 .......... 5.4% Naphthenes C5 3.7%
C6 0.9%
The operating conditions are as follows -temperature ... .470 0 C
- pressure 2.5 bar
- pph ............... 2.5 h-1
The results of the test are reported in Table 1.

Table 1

<tb><SEP> Selectivities <SEP> (% <SEP> Weight)
<tb> Catalyst <SEP> Conversion <SEP> CH4 <SEP> C2H6 <SEP> C3H8 <SEP> Butanes <SEP> Olefins <SEP> Aromatic
<tb> (% <SEP> weight) <SEP> + <SEP> + <SEP> + <SEP>
<tb><SEP> C2H4 <SEP> C3H6 <SEP> Butenes <SEP> C5 <SEP> + <SEP> C6
<tb><SEP> Catalyst <SEP> A
<tb><SEP> (comparative) <SEP> 55 <SEP> 4 <SEP> 11 <SEP> 19 <SEP> 17 <SEP> 39 <SEP> 10
<tb> Catalyst <SEP> B <SEP> 90 <SEP> 29 <SEP> 25 <SEP> 15 <SEP> 19 <SEP> 0 <SEP> 12
<tb><SEP> (comparative)
<tb><SEP> CatalystD <SEP> 1 <SEP><SEP> 85 <SEP> 8 <SEP> 14 <SEP> 17 <SEP> 12 <SEP> 1 <SEP><SEP> 48
<tb><SEP> Catalyst <SEP> E <SEP> 79 <SEP> 9 <SEP> 10 <SEP> 18 <SEP> 15 <SEP> 5 <SEP> 43
<tb><SEP> (comparative) <SEP> 1
<Tb>
It is therefore found that the catalyst D according to the invention leads to conversions and selectivities in aromatic products which are clearly superior to those of the comparative catalysts, in particular of the catalyst
E.

Claims (10)

1 - Composite catalyst containing:  from 1 to 99% by weight (relative to the catalyst matrix) of a  zeolite of structure MFI in hydrogen form, containing in its  framework silicon and at least one element selected from the group  formed by aluminum and thallium,  - a matrix,  deposited on the matrix and / or the zeolite at least one noble metal of the  Platinum family, at least one additional metal chosen from the group  consisting of tin, germanium, indium, lead, thallium and  thallium, or among Group I metals such as copper, gold,  Silver, Group VIII metals such as nickel, ruthenium,  iron or among the Group VI metals such as chromium,  molybdenum or tungsten,  deposited on the matrix and / or zeolite at least one halogen  selected from the group consisting of fluorine, chlorine, bromine and  iodine. 2 - Catalyst according to claim 1 and further containing at least one element selected from the group consisting of alkali metals and alkaline earth metals. 3 - Catalyst according to one of claims 1 and 2 further containing at least one doping element selected from the group consisting of gallium and zinc. 4 - Catalyst according to one of claims 1 to 3 wherein said catalyst contains, by weight relative to the matrix  from 0.01 to 2% of at least one noble metal of the platinum family,  from 0.005 to 2% of tin in the case where the catalyst contains  tin, or from 0.005 to 0.7% of at least one additional metal other than  than tin.  consisting of fluorine, chlorine, bromine and iodine.  from 0.1 to 15% of at least one halogen chosen from the group  hydrogen.  from 1 to 99% by weight of a zeolite of MFI structure in the form 5 - Catalyst according to claim 4 containing at least two additional metals, the concentration of additional metals being between 0.02 and 1.20% by weight relative to the matrix. 6 - Catalyst according to one of claims 1 to 5 and which further contains, by weight relative to the matrix, from 0.01 to 4% of at least one metal selected from the group consisting of alkalis and alkalinos earthy. 7 - Catalyst according to one of claims 1 to 5, the zeolite contains at least one doping element selected from gallium or zinc, the gallium or zinc content being between 0.01 and 10% by weight relative to the zeolite. 8 - Catalyst according to one of claims 1 to 7 and which contains from 0.01 to 4% by weight relative to the matrix of at least one metal selected from the group consisting of alkali and alkaline earth, and wherein the doping element content is from 0.01 to 10% by weight. 9 - Composite catalyst according to one of claims 1 to 8 wherein the matrix is alumina. 10 - Catalyst according to one of claims 7 to 9 wherein said driver element is thallium.  II - Use of the composite catalyst defined according to one of claims 1 to 10 or prepared according to one of claims 1 to 10 for the aromatization of hydrocarbons containing 5 to 12 carbon atoms per molecule.