Reactions and mechanisms in catalytic pyrolysis

There is no uniform acceptance for the mechanisms involved in catalytic pyrolysis. This is probably because of the complexity of the process and the range of products

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14.1 (A) simple fixed bed reactor. (a) is the carrier gas feed to remove products and (b) is the combined pyrolysis-catalysis reactor (tube).

(c) are either sinter discs or ceramic wool. (d) is the hydrocarbon source mixed with catalyst. (B) is a fixed bed pyrolysis chamber and reactor (d/b) combined with a separate catalytic compartment (f). (e) is an inlet for catalysis process gas e. g. hydrogen or water. (c) and (d) are fluidised bed reactors. (c) and (b) are the reactor chamber and the bed material recirculation chamber respectively. (a) and (e) are the process gas for product recovery etc. and gas stream for catalyst treatment. (d) is the fluidised bed. In (c) the catalyst and fluidised bed support are recirculated. In (d) only the bed support is recirculated and the separate catalyst bed can be fed with a separate process gas (f).

formed which make quantitative analysis difficult. It is difficult to experimentally resolve intermediates in the reaction because of the number of different products formed and the conditions within reactors are not always amenable to characterisation by either ex situ or in situ methods. Further, the reaction temperature and pressure as well as the nature of the catalyst may change the nature of the reaction. Two mechanisms have become widely suggested as being the basis of pyrolysis. These are a free-radical mechanism and a carbonium ion mechanism and both of these are now very well established in terms of catalytic assisted cracking of hydrocarbons145 and have been used to describe the mechanism of catalytic pyrolysis of biomass,146 heavy oils,147 natural oils149 as well as various polymers.150,151 Although these mechanistic models have been proposed since the 1940s and earlier, there has been little additional detail provided because of the experimental and analysis problems described earlier although Sakata et al. have extended the older models for the catalysed decomposition of polyethylene.152 The free-radical mechanism is based on a number of steps where high temperature homolytic reactions create free radicals; these free radicals are unstable and will have tendency to crack at b-bonds to form smaller hydrocarbons. Combination of free radicals allows recombination and, thus, some isomerisation. Water elimination, aromatisation, carbon oxide formation and hydrogen production follow from reaction of these radicals with each other or other hydrocarbon molecules. It is generally believed that the catalyst has a more profound effect on the initial free-radical generation step than the subsequent reactions since free-radical reactions are kinetically fast.

The carbonium ion reaction is similarly complex, involving carbocation formation and has been developed from early concepts into two distinctly different mechanisms. It should be noted that there is a confusion on the term carbonium ion. In the pyrolysis literature, few authors use the term correctly because the term is generally used to indicate any positively charged carbon atom. However, a carbonium ion is properly defined as penta — or tetracoordinated carbocation such as R5C+.153 The first carbocation mechanism is described as monomolecular cracking.154 Here, a penta-coordinated carbonium ion is formed from an alkane or alkane containing molecule group and this subsequently undergoes cracking and evolution of an alkene containing molecule or hydrogen. This reaction is considered relatively slow at lower pyrolysis temperatures and the second type of carbocation mediated reaction is probably more important; this is known as the bimolecular or b-cracking mechanism.155 This process is initiated by a carbenium ion (a trivalent carbocation of type R3C+) which subsequently undergoes hydrogen or hydride transfer followed by b-bond scission. Since the scission occurs with formation of an additional adsorbed carbenium ion, the mechanism is generally considered to be much faster than the monomolecular route.156

Separating the mechanism into two quite separate routes, free-radical and carbocation mediated, is probably not possible and both reaction mechanisms may contribute to the product formation although the relative importance of each is likely to vary with temperature. It has been found that the free-radical reaction will dominate at higher temperatures.147 A term RM has been used to describe the relative contributions of free-radical and carbocation mediated reactions in the cracking of heavy oils.147,148 RM can be estimated from the isobutane to normal butane ratio. When the RM term (greater than 1.5) is high, the reaction will be dominated by the carbocation mechanism and when low (below 0.5) by the free — radical reaction mechanism. Intermediate values indicate that both reaction mechanisms are important. This may be rationalised in terms of the shorter lifetime of free-radical species which mitigate more complex rearrangements.

Whatever the nature of the reaction mechanism, it is clear that the catalysed pyrolysis of complex organics must involve a number of different reactions.146 Following an early work by Chang and Wan in 1947157 for the decomposition of triglycerides (see later chapters in this book for a more detailed review of the catalysis of triglycerides) these will involve reaction steps similar to those below (not inclusive of all possible reactions):

1 Degradation of the complex reactants to yield acrolein (CH2=CHCHO) plus various complex fatty acids and ketenes as well as other similar compounds (e. g. RCOOH, RCH=CO where R is an alkyl group).

2 Degradation of fatty acids and acrolein into carbon dioxide, water and alkanes, e. g.

RCOOH — CO2 + RH

2RCOOH — CO2 + RCOR

3 Breakdown of ketenes, ketones and acrolein into carbon monoxide, light hydrocarbons and alkenes

2RCH=CO — 2CO + RHC=CHR

CH2=CHCHO — CO + C2H4

RCOCH2R — R2 + CH2CO

2RCOCH2R — 2 R2 + CO + C2H4

4 Decomposition of alkanes into hydrogen and carbon (principal char forming route)

CnH2n+2 — nC + (n+1)H2

5 Formation of alkenes from alkanes

Cm, — CH + H

n 2n+2 n 2n 2

6 Division of alkanes and alkenes into smaller alkane, alkene and di-alkene molecules, e. g.

C m, — C H„ ^ + C H,

n 2n+2 n-m 2(n-m)+2 m 2m

7 Growth of longer chain alkanes

C H ол + C H — C

n 2n+2 m 2m n+m 2(n+m)+2

8 Isomerisation of alkanes and alkenes

9 Aromatisation of alkanes and alkenes via reaction mechanisms such as the Diels-Alder reaction,149 e. g.

CnH2n+2 — Cn-6H2(n-6)+1C6H5 + 4H2

10 Formation of alkynes from alkenes

C H — C H + H

n 2n m 2m-2 2

11 Hydrogenation of alkenes and alkynes, e. g.

CnH2n+2 + H2 — CnH2n+2

More generally, the types of reactions occurring in catalytic pyrolysis that are directly affected by the presence of the catalyst can be described in terms of more general mechanisms based on combinations of cracking, reforming and other reactions. Some of these are described above. The water gas-shift and similar reactions also clearly affect the gas composition. This more general description is useful because the pyrolysis temperature (whether pyrolysis and catalytic reactions are separate or integrated) is the most critical process parameter because the products of the thermal pyrolysis reaction are strongly dependent on the temperature and these gas products will strongly affect these catalysed reactions by affecting the equilibrium and relative rate of the catalysed reactions. In this way, the product distribution can vary considerably with temperature. These reactions are:

1 Classic catalytic reforming reactions such as isomerisation, cracking and aromatisation as described above. The role of the catalyst is based on dissociative chemisorption of the alkanes and alkenes forming chemisorbed hydrocarbon fragments and hydrogen. Recombination of fragments leads to formation of smaller hydrocarbons, isomers and aromatics as well as hydrogen.

2 Hydro-cracking where hydrogen produced in other reactions is used in the fragmentation of long hydrocarbon chains into smaller units, e. g.

Подпись: C H + H n 2n+2 2 C H + C H

n-m 2(n-m)+2 m 2m+2

3 Hydrogen can also be important in terms of dehydration reactions with the products of the thermal pyrolysis,158 e. g.

C6H8O4 + 6H2 ^ 6CH2 + 4H2O

C6H8O4 + 4.5H2 ^ 6CH15 + 4H2O

C6H8O4 + 3.6H2 ^ 6CH12 + 4H2O

where CH2, CH15 and CH12 represent the average stoichiometry of the alkane, alkene and aromatic hydrocarbon products respectively and C6H8O4 is an indicative formula for the pyrolysis oil.

4 In order to maintain the highest amount of oil product the catalytic pyrolysis process must be carefully controlled to minimise processes such as steam reforming, partial oxidation and auto-thermal reforming (which combines steam reforming and partial oxidation158,159) as these reactions lead to H2, CO2 and CO formation.

CnH2n+2 + 2nH2O ^ nCO2 + (3n+1)H2 (steam reforming)

CnH2n+2 + n/2O ^ nCO + (n+1)H2 (partial oxidation)

The other important method of controlling product is by careful choice of the catalyst as the chemical nature of the catalyst will define which of the individual reaction steps is most strongly affected. The catalysts used in pyrolysis are described in some detail below.