EP0739237A1

EP0739237A1 - Catalytic materials

Info

Publication number: EP0739237A1
Application number: EP95900824A
Authority: EP
Inventors: Janet Bovey; Edmund Fowles; William Jones; Robert Mokaya; Mary Elizabeth Davies
Original assignee: Laporte Industries Ltd
Current assignee: Evonik LIL Ltd
Priority date: 1993-11-24
Filing date: 1994-11-11
Publication date: 1996-10-30
Also published as: GB9324170D0; WO1995014530A1; CA2185798A1; ZA949286B; AU8148794A

Abstract

A catalytic material consists essentially of a pillared layered clay mineral, the clay mineral being acid-leached and having an increased Bronsted acidity, due to the introduction of lattice protons, which may be at least 0.3. The pillaring material may be aluminium, zirconium or chromium compounds. The catalytic material has an increased the pore volume, surface area and basal spacing which also help to enhance the catalytic properties of the final pillared product. The pillared clay mineral is found to have inherent catalytic activity in a wide variety of organic transformations for example in the alkylation of benzene, the dehydration of pentanol or the cracking of cumene.

Description

Catalytic Materials

This invention relates to catalytic materials and a method for preparing such materials and to the use of such materials. More particularly, the invention relates to catalytic materials comprising layered clay minerals.

Layered clay minerals, in suitable metal cation exchanged form, may act as catalysts for a variety of organic reactions but their practical applicability has been severely restricted by a lack of thermal stability. This problem has, to some extent, been alleviated by pillaring the clay minerals, that is, by introducing a solution of a material into the interlayer space to achieve expansion of the basal spacing of the clay mineral and fixing the material in place to prevent the basal spacing shrinking back to its original value when the clay mineral is dried. Suitable materials for pillaring purposes are oligomeric metal cations, for example hydroxymetal cations, of metals such as aluminium, chromium, zirconium or titanium. These cations may be introduced into the layered clay mineral in aqueous solution, deposited therein as pillar precursors by drying and converted 'in situ' to the corresponding oxides by a suitable calcination regime. The resulting pillared clay mineral is relatively thermally stable and is regarded as being very suitable for use as a support for catalytically active metals. Pillared clay minerals may also show some degree of inherent catalytic activity due to their high surface area and to acidity associated with the pillaring material itself. However they are, generally, less efficient catalysts than metallic-cation-exchanged, for example aluminium-exchanged, but unpillared, clay minerals.

The present invention relates to a new or improved catalytic material consisting essentially of a specially modified pillared layered clay mineral, to a process for the preparation of such a modified pillared layered clay mineral and to the use of the modified pillared layered clay mineral as a catalyst in organic reactions. The modified pillared layered clay mineral is found to have inherent catalytic activity.

The present invention provides a catalytic material consisting essentially of a pillared layered clay mineral and characterised in that the layered clay mineral is an acid-leached layered clay mineral having increased Bronsted acidity in comparison with the corresponding pillared material based on a layered clay mineral which has not been acid leached. The invention includes both the dried layered clay mineral containing deposited pillar precursor compounds and the final product containing the pillaring materials in oxide form. The terminology "consisting essentially" is used to indicate that catalytic activity resides in the modified pillared layered clay mineral itself without reliance on any other catalytic materials. The use of the catalytic material of the present invention in combination with other catalytic materials is not excluded from the scope of the invention.

Bronsted acidity may be detected using the cyclohexylamine method (C Breen (1991), Clay Minerals, 26.473) which, for example at 240°C, gives a good indication of the number of available protons and is more satisfactory for the purposes of this invention than other methods, for example the Hammett method. Preferably the catalytic material of the present invention that is, the final pillared product, has a surface acidity, measured by the Breen method, of at least 0.3, particularly preferably at least 0.45 for example, very suitably, at least 0.55 m mole H⁺/g. The said surface acidity may be, for example, up to 0.8 m mole H⁺/g. The increased acidity required by the present invention may be attained by acid-leaching the layered clay mineral prior to pillaring. The basal spacing of the pillared product is preferably at least 17 Angstroms.

Acid-leached or activated clays are readily available as articles of commerce for example from Laporte Absorbents, Widnes, England under the Trade Name Fulmont XMP. Alternatively these may be produced as part of the production of the catalytic materials of the present invention. Besides increasing the Bronsted acidity of the clay mineral the acid-leaching process also modifies the ιχ. clay mineral by increasing the pore volume_surface area and average pore diameter which properties also enhance the catalytic properties of the final pillared product. The pore volume of that product is preferably at least 0.2, particularly preferably at least 0.25, for example at least 0.3 cc/g and may be up to 0.6 cc/g or above. The surface area of the final pillared product, measured by the BET method using a Micromeretics ASAP 2400 instrument using 5 points up to a value of P/Po (relative pressure) of 0.1, is preferably at least 300 m^/g. The average pore diameter may be derived from the above figures by the use of a standard formula and is preferably at least 20 Angstroms and up to 80 Angstroms or above.

The layered clay mineral which is the basis of the catalytic material of the present invention is preferably a swellable layered clay mineral capable of expanding in water to give a basal spacing which is increased with respect to an original spacing of at least 5 Angstroms. Such a clay may be a naturally occurring mineral or a synthetic analogue thereof. It may be a two-layer mineral such as a kaolinite or a ribboned clay mineral containing a layered structure such as attapulgite or sepiolite but is preferably a three-layer clay mineral such as a smectite. Amongst the smectites the montmorillonites or the beidellites particuarly, for example, the calcium/magnesium forms of such clays, are particularly preferred. The saponites, sauconites or hectorites also come into particular consideration. It is understood that the invention may be applied to a range of layered clay minerals with appropriate modification, if required to take account of the particular characteristics of a particular clay mineral .

The layered clay mineral is preferably prepared for acid-leaching by blunging, sieving and milling, as necessary, to a particle size below about 100 microns. The acid-leached layered clay mineral required according to the invention may be produced by digesting the clay mineral in a strongly acidic aqueous slurry. The digestion may be controlled, for example, by controlling the duration, temperature, pressure or acid concentration utilised. The acid is suitably a strong mineral acid, such as an acid having a pKa value below 3.0, for example a mineral acid such as sulphuric acid, nitric acid or hydrochloric acid or a suitable organic acid. Any alumina layer in the clay mineral is attacked at the platelet edges to leach out some aluminium and/or other octahedral constituents and to generate pores having a diameter in excess of 15 Angstroms, usually from 20 to 50 Angstroms and up to or above 80 Angstroms, in the platelets. The acid may have an initial concentration of, for example, 77 to 100% by weight and a concentration in the aqueous slurry of about 5% to 40%, preferably 15 to 30%, by weight. The acid-clay ratio is preferably optimised for a particular pillaring metal and may be in the range 0.05 to 1.5 by weight calculated as 100% acid. Preferably the acid-clay ratio is in the range 0.25 to 0.5 where the pillaring metal is aluminium and in the range 0.05 to 0.3 where the pillaring metal is chromium or zirconium. The acid digestion may be conducted for from about 1 to 24 hours, preferably 10 to 16 hours, if atmospheric pressure digestion is used or from 1 to 8 hours when pressure digestion is used. Pressure digestion may suitably be conducted at a pressure of up to about 200 psig (about 13.5 bars) but preferably of up to about 150 psig (about 10 bars) and is preferably conducted in a closed vessel at a temperature suitable to generate the required pressure. Atmospheric pressure digestion may suitably be conducted at a temperature of about 70 to 100°C, preferably about 85 to <100°C. The digestion may be terminated at the desired point by quenching with cold water after which the slurry of acid-treated clay mineral may be pumped to a suitable filter press where it may be water-washed and dried as desired.

Any source of inorganic pillars may be utilised according to the present invention. Preferably, however, the pillaring material comprises water soluble aluminium, zirconium or chromium cations or mixtures of two or more of these. Such cations may be produced by forming a metal hydroxide or halohydroxide solution and allowing it to age. United States Patent No. 4 216 188 describes the preparation and use of aluminium hydroxide and chromium hydroxide cations to 'cross-link' or pillar montmorillonite clays although these clays are in, or are first converted to, the monosodium or monolithium form. United States Patent No 4 176 090 describes the preparation and similar use of aluminium, zirconium or titanium cations. The disclosure of these two patents in respect of the manufacture of these cations is incorporated herein by reference. Alternatively such cations, particularly but without limitation zirconium or chromium, may be produced from aqueous solutions of other water soluble salts such as the nitrates or the oxyhalides.

A suitable solution of a pillaring cation may preferably be prepared from an aqueous solution of a metal halohydrate, preferably a metal chlorohydrate. Preferably the solution of the halohydrate or other compound used to form the pillaring cation may be aged for, for example, from 20 minutes to 30 hours at temperature which may suitably be from 20°C, but preferably at least 40°C, up to the boiling point. Preferably the ageing is at superambient temperature. Examples of suitable ageing conditions are heating at 80°C for 1 hour or storing at room temperature for up to 24 hours or intermediate temperature/time combinations. Where the pillaring metal is aluminium the pH of the halohydrate solution is suitably maintained in the acidic range, suitably below 6.0, by the addition of an acidic compound if necessary. In the case of the chromium cation the addition of base appreciably enhances the formation of suitable pillaring cations. A suitable quantity of base is that which gives a base to metal ratio greater than 1, for example from 1 to 3. It is found to be helpful, to maximise the oligomerisation of the pillaring cation, to stabilise the solution by the addition of a stabilising agent such as sodium acetate, sugars such as the aldohexoses, aldonic acid or aldonolactones, for example glucose, mannose or galactose, the corresponding acids or lactones, or the dibasic hydroxy acids such as citric acid, tartaric acid or malic acid. A suitable quantity of stabiliser would be at least 0.5% and up to 5% or more by weight of the metal calculated as the oxide. The above procedures and conditions may have to be varied appropriately depending on the particular pillaring cation used.

The metal cation may be modified by the inclusion in the aqueous solution during the preparation thereof, of suitable precursors of one or more cations of the alternative metals identified above. Alternatively an analogous method may be employed to separately produce suitable cations of these other metals or of mixtures of them, and these cations may be included. Any of these cations may be further modified by including ions of other metals, in the form of soluble salts. The other metal, or metals, may be selected from, but are not limited to the transition elements, silicon, gallium, germanium, phosphorus or arsenic.

The treatment of the layered clay mineral with the solution of the pillaring cation may be conducted by forming a dispersion of the clay in the solution and maintaining that dispersion until intercalation has occurred to a sufficient degree, for example for a duration of from 30 minutes to 5 hours. This may be accomplished either by first dispersing the layered clay mineral and adding the pillaring solution to it or by dispersing the layered clay mineral directly in the pillaring solution. The contact of the acid-treated clay with the solution of pillaring species may be conducted at any practicable temperature, even above the boiling point of the solution if pressure is used. Where the pillaring cation is aluminium the contact is preferably maintained between 50°C and the boiling point but particularly preferably at not more than 90°C. Where the pillaring cation is zirconium the preferred temperature may be down to 30°C but is otherwise as for aluminium. Where the pillaring cation is chromium the preferred temperature is from 60°C to the boiling point.

The slurry of the acid-leached (activated) layered clay mineral, after treatment with pillaring material, may suitably be filtered and repeatedly washed in deionised water until free of soluble salts. The washed solids so obtained may be redispersed and dried by, for example, spray drying or ring drying.

To convert the pillar precursors to the oxide form the dried product may be heated at, for example, at least about 350°, preferably at least about 400°C and up to about 650°C, for example from 400° to 550°C. Where the pillaring metal is aluminium or zirconium the heating is preferably conducted in air but where it is chromium the heating is preferably conducted in an inert atmosphere. The control of the heating conditions as above described can give an appreciable improvement in the properties of the pillared product. The efficacy of the pillared clay may also be optionally improved by ion-exchange with an acidic cation such as, for example, Al⁺³, Cr³⁺, Fe³⁺, Ti⁴⁺ or H⁺. This has the effect of atieast partially redressing some loss of surface acidity of the original acid-leached clay resulting from the blocking or removal of acidic sites by the presence of pillars.

It is preferred not to include either too great a quantity of pillaring species in the layered clay mineral, since this may reduce the pore volume, or too small a quantity since this may detract from the stability of the pillared product. Preferably the quantity of the pillaring species is from 0.5 to 3 m.mole metal/g dry clay.

The pillared acid-leached layered clay mineral produced as above described are effective catalysts, used by themselves, for a wide variety of organic transformations. They may be applied, particularly, to alkylations, dehydrations, isomerisations, esterifications, etherifications, dehydrogenations and cracking reactions.

The invention will now be illustrated by means of the following Examples which are not intended to limit the scope of the invention described herein.

Figure 1 attached hereto represents x-ray diffraction patterns of products or intermediate species prepared in the course of the Examples. Figure 2 is a histogram representing the % conversion of 1-dodecene obtained by the use of these products or species. Figure 3 is a histogram similarly representing the % conversion of pentanol and the % selectivity to pentene, to 1,1-dipentyl ether and to 1,2- dipentyl ether. In Figures 1, 2 and 3 the indicators as to the identity of the pillaring metal and the acid/clay ratio are omitted. These are apparent from the corresponding text. In the Examples the terminology used to identify the layered clay mineral, whether acid-leached or not and whether pillared or not, is as follows.

M Montmorillonite layered clay mineral AM(0.1, 0.15 etc.) Acid-leached, washed and dried M(acid/clay ratio) CAM Calcined AM

PM/PAM/CPAM(A1/Zr/Cr) Pillared M/AM/CAM (Pillaring metal) Thus, CPAM(A1)0.35 denotes calcined aluminium-pillared montmorillonite which had been acid-leached at an acid/clay ratio of 0.35.

The following abbreviations and units are also used: SA - Nitrogen surface area measured on a

Micromeritics ASAP 2400 instrument with outgassing at 280°C and using a five point BET analysis up to a maximum relative pressure P/P₀ of 0.1. - XΆ-/~

PV - Pore Volume (measured on the same instrument- P/Po = approx. 0.98 - cc/g

MPV - Micropore Volume (measured on the same instrument- calculated using Micromeritics Key Plot Analysis Software with Harkins and Jura equation for T between T = 3.5 and 5 Angstroms - cc/g

APD - Average Pore Diameter = (4PV/SA).10⁴ - Angstroms d(100) - d(100) XRD Spacing - Angstroms

Al/Cr/Zr - Pillaring Metal Content - by XRF - m.mole/g of A1203,Cr or Zr.

Acidity - Measured by the Breen cyclohexylamine procedure at 240°C - m.mole H⁺/g Example 1 A calcium/magnesium montmorillonite having the structural formula ^c*0.24[Si₃.93AI0.073 [Al1.42^0.15^90.43l°10(°^H)2 was treated with sulphuric acid, having a concentration of 98% w/w, at an acid/clay weight ratio of 0.35 calculated as 100% acid, and a slurry concentration of 20% by weight, at 90-100°C for 16 hours. The treated clay was removed from the acid, washed in demineralised water to remove soluble metal salts and air-dried at 110°C. The acid-treated clay had a content of octahedral ions (Al, Fe, Mg) reduced by 23.5% from their original value. This acid-leached material and also a sample of the original montmorillonite were separately added to a stirred solution of aluminium chlorohydrate (0.06M) at 80°C at a solution/clay ratio of 50 ml/g and the stirring was continued for 1 hour. The resulting slurries were repeatedly centrifuged in de- ionised water and reslurried until free of chloride ions. The damp products so obtained were redispersed in a minimum amount of de-ionised water and air dried to obtain the respective precursor pillared species. Samples of these materials were calcined by heating in air for 4 hours at 500°C to convert the pillaring precursor salts to the oxide. The acid-leached clay product was also calcined under the same conditions to produce a further product. Figure 1 attached hereto shows the powder x-ray diffraction patterns of the materials so produced.

The following Table sets out the basal spacings for some of these species and Table 1 sets out the respective basal spacings in Angstroms. Table 1

Species Angstroms

M 15.8

AM 0.35 15.1

PAM(A1)0.35 ~19.6

PM(A1) "19.6

CPM(Al) 18.8

CPAM(A1)0.35 19.3

The basal spacing of the acid-leached montmorillonite was slightly decreased as a result of the exchange of H⁺ for Ca⁺2 in the interlayer region. The XRD pattern indicates that the montmorillonitic structure was maintained despite acid treatment. The XRD patterns of the pillared clays indicate that there has been an expansion of the basal spacing, the intensity of the patterns indicating that the pillared materials were well ordered and had comparable thermal stability.

The amount of aluminium incorporated (as m.mole Al2θ3/g), the surface area, pore volume and surface acidity are set out in the following Table for certain of the above products.

Al SA PV Acidity

It is seen that the calcined pillared acid-leached material had a higher surface area, pore volume and surface acidity compared to the corresponding material based on a montmorillonite which had not been acid-leached despite incorporating less alumina. It appears that, while calcination may render some of the acid sites inaccessible, pillaring exposes those sites.

Example 2

The species M, Al⁺³ exchanged M, CPM(Al), AM 0.35, CAM 0.35, CPAM(Al) 0.35 and a commercially available acid- activated but unpillared montmorillonite were used to catalyse (a) the alkylation of benzene by 1-dodecene and (b) the dehydration of 1-pentanol. The reactions were carried out in stainless steel pressure vessels of approximately 20 cm³ capacity. The charge for the alkylation reaction was 0.15 g of catalyst and 10 ml of a mixture of benzene and 1-dodecene in a 10:1 molar ratio. The charge for the dehydration reaction was 0.3 g of catalyst and 3.0 ml of reactant. The pressure vessels were heated in an oven and, at the end of the reaction time of 2 hours at 175°C or 3 hours at 200°C for the alkylation and 4 hours at 200°C for the dehydration, were immersed in ice. The products were analysed using gas chromatography.

The main products of the alkylation reaction were mono- alkylated benzenes (a mixture of 2-6 phenyl dodecane) with no dialkylated products being detected. For the three calcined catalysts the order of activity was CPAM(Al) (0.35) > CAM(0.35) > CPM(Al). This indicates that both acid leaching and pillaring have beneficial effects. When these processing features were combined however the resulting material was an excellent catalyst with higher activity even when compared to other clay materials such as Al³⁺ exchanged M and the competitive material. The results are shown in Figure 2. For the dehydration reaction the trends in catalytic activity were similar to those observed for the alkylation reaction. The main products were the alkene, produced by the proton-catalysed dehydration, and a mixture of 1,1 and 1,2 dipentyl ether with the 1,1 species preponderating. The selectivity of the catalyst was very dependent on the activity of the catalyst with higher activity accompanying higher selectivity to the alkene. The results are shown in Figure 3. Example 3

A series of calcined alumina-pillared acid activated montmorillonite samples were prepared as in Example 1 but using a range of acid/clay weight ratios from 0.30 to 0.60, These products and certain comparative products were characterised and the data is set out in Table 3 below.

Table 3

Cumene cracking and dehydrogenation was performed at temperatures between 300° and 500°C in a pulse microreactor with a helium flow of 25 ml/min. The catalyst bed of 0.05g 30/60 mesh was activated for 1.5 hours under helium before pulsing 2 ul (14.4)umole of cumene. Products were separated and analysed by means of an in-line gas chromatograph. Cumene cracking conversions and, in parenthasis, the ratio of benzene to alpha-methylstyrene are given in Table 4 below. Table 4 Clay % Conversion

Sample 300°C 400°C 500°C

CPM(Al) 3.2(7.1) 33.0(18.0) 50.1(7.7)

CPAM(A1)0.3 14.2(36.0) 42.0(31.8) 60.3(7.9)

CPAM(A1)0.4 13.3(44.0) 46.2(29.1) 61.3(7.6)

CPAM(A1)0.45 10.0(34.8) 40.7(28.2) 57.8(7.8)

CPAM(A1)0.6 7.3(37.5) 35.6(31.4) 53.2(11.0)

CAM 0.3 1.2 (2.4) 9.4 (2.6) 31.4 (1.8)

CAM 0.45 2.2 (0.9) 7.1 (1.2) 35.6 (1.2)

CM - - 3.1 (0.35) 24.4 (0.25)

Example 4

Samples of a calcium montmorillonite having the structural formula ^Ca0.05l[^Si3.44^A10.56H^A10.55^Fe3+1.05^Mg0.39_°10(°^H)2 were treated with 98% w/w sulphuric acid at a slurry concentration of 20% wt and at a variety of acid/clay weight ratios for 16 hours at 95°C. The samples of acid- leached clay were removed from the acid by filtration and were washed and dried at 50 - 60°C.

The samples of the dried acid-leached clay were pillared with zirconium or chromium materials as follows.

Zirconium.

A pillaring solution was prepared by ageing a 0.24 Molar solution of ZrOCl2-8H2θ for 1.5 hours at a temperature of 80°C under conditions of agitation. Samples, of the clay, both acid-leached and not acid-leached, were added, at a solution/dried clay ratio of 50 ml/g, to portions of the pillaring solution and were maintained at a temperature of 50°C and under agitation for a further 1.5 hours. The mixtures were then centrifuged and washed until the supernatent was free of Cl^~ ions. The resulting clay samples were air-dried at 50°C and at this stage were analysed for zirconium content. The samples were then calcined at a temperature of 500°C for 3 hours.

Chromium

The pillaring solution was prepared by ageing a 1.0 Molar solution of Cr(N<_>3)3.9I_2θ, containing Na2CC_3 at a base/chromium ratio of 2,for 24 hours at a temperature of 95°C under conditions of agitation. Samples of the clay, both acid-leached and not acid-leached, were added, at a solution/dried clay ratio of 500 ml/g, to portions of the pillaring solution and were maintained at a temperature of 95°C and under agitation for a further 1.5 hours. The clay samples were then removed from the pillaring solution and washed until the supernatent was free of Nθ3~ ions. The resulting clay samples were air-dried at 50°C and at this stage were analysed for chromium content content. The samples were then calcined under argon at a temperature of 500°C for 3 hours.

Certain of the samples were used to catalyse the dehydrogenation of 1-pentanol. The identification of the samples and the % Pentanol converted are set out in Table 5 below.

The characterisation of the zirconium and chromium containing clays is set out in Table 6 below.

Table 5

Table 6

Clay

Sample SA PV MPV APD d(100) Cr/Zr

PM(Cr) 19.7 2.21

CPM(Cr) 297 0.19 0.08 25.6 21.2

PAM(Cr)0.25 20.7 1.54

CPAM(Cr)0.25 347 0.23 0.04 26.5 21.7

PM(Zr) 18.21 2.65

CPM(Zr) 330 0.21 0.08 25.5 20.49

PAM(Zr)0.1 18.21 2.44

CPAM(Zr)0.1 358 0.25 0.07 27.9 19.12

PAM(Zr)0.15 17.96 1.94

CPAM(Zr)0.15 368 0.28 0.04 30.4 21.39

PAM(Zr)0.2 17.39 1.50

CPAM(Zr)0.2 370 0.34 0.04 36.8 21.28

Claims

Claims :

1. A catalytic material consisting essentially of a pillared layered clay mineral and characterised in that the layered clay mineral is an acid-leached layered clay mineral having increased Bronsted acidity.

2. A catalytic material as claimed in claim 1 having a surface acidity of at least 0.3 m.mole H⁺/g measured by the Breen method.

3. A catalytic material as claimed in claim 2 having a surface acidity of up to 0.8 m.mole H⁺/g.

4. A catalytic material as claimed in any preceding claim having a pore volume greater than 0.2 cc/g.

5. A catalytic material as claimed in any preceding claim having a surface area of at least 300 m^/g.

6. A catalytic material as claimed in any preceding claim wherein the pillars comprise compounds of one or more of aluminium, zirconium and chromium.

7. A catalytic material as claimed in any preceding claim having a d-100 basal spacing of at least 17 angstroms.

8. A catalytic material as claimed in any preceding claim in the calcined form.

9. A catalytic material as claimed in any preceding claim exchanged with an acidic cation.

10. A process for the production of a catalytic material as claimed in any one qf claims 1 to 9 comprising acid-leaching a layered clay mineral to increase the Bronsted acidity thereof, pilaring the increased acidity clay mineral and calcining the pillared clay mineral.

11. A process as claimed in claim 10 wherein the clay mineral is pillared by treating it with a solution of a compound of aluminium, zirconium or chromium.

12. A process as claimed in claim 11 wherein the clay mineral is treated with a compound or compounds of aluminium at a pH below 6.0.

13 A process as claimed in claim 11 wherein the clay mineral is treated with a compound or compounds of chromium in the presence of added base and the clay mineral, containing the chromium compound, is calcined under an inert atmosphere.

14. A catalytic material or a process for producing it as claimed in any preceding claim and substantially as described herein with reference to any one of the Examples.

15. A process for the transformation of an organic material through the agency of a catalytic material as claimed in any preceding claim.

16. A process as claimed in claim 11 wherein the organic transformation is an alkylation, a dehydration, an isomerisation, an eεterification, an etherification, a dehydrogenation or a cracking reaction.