Microwave Pyrolysis

Microwave pyrolysis

Closely allied to reactions carried out on supports is the pyrolysis of organic materials. Recent work has involved the use of zeolites or other catalyst beds, although early work showed that by rapidly heating solid organic materials with microwaves, it is possible to pyrolyse them in a manner analogous to that of conventional techniques. The earliest work involved the pyrolysis of cellulose to give predominantly furfural.¹⁴⁸ The pyrolysis of starch, or indeed any 1,4 glucan, may be carried out with an irradiation time of 5 - 15 minutes (depending upon the mass of starting material) to give the chiral synthon 1,6 - anhydro-[beta]-D-glucopyranose. It is suggested that the efficiency of the reaction may be dependent upon density of the starch.

starch pyrolysis scheme

By using zeolites to aid the pyrolysis, it has been found that differences are observed between conventional and microwave heating in the cracking reaction:¹⁴⁹

n-C₅H₁₂-------> CH₄ + iso-C₄H₈

There is an increase in the proportion of methane which is formed when microwave heating is employed. Whether or not this represents a specific microwave effect is debatable, since measurement of the temperature in microwave systems is subject to inaccuracy.

The cracking of methyl-2-pentane over a Pt-Alumina catalyst also shows differences in selectivity between the microwave and conventionally heated reactions (Table 1).¹⁵⁰ In particular, the proportion of benzene is dramatically increased.

Cracking^a

Isomerisation

3 x C₂

C₁+C₅

C₂+C₄

2xC₃

M3P^b

HEX^c

MCP^d

BEN^e

Microwave /300^oC

0.7

17.2

7.8

5.0

26.5

27.9

7.9

6.9

Classical / 300^oC

0.6

17.5

9.4

5.7

21.0

28.4

17.4

0.3

Classical / 325^oC

0.9

19.8

10.1

6.9

22.8

31.0

5.9

3.1

^a C_n where n=the number of carbon atoms, ^b Methyl-3-pentane, ^c n-Hexane, ^d Methylcyclopentane,^e Benzene

Table 1 Cracking of methyl-2-pentane over Pt-Alumina

In a more recent study, the cracking of benzene was performed over a Ni-Zeolite catalyst to give an unexpectedly high degree of selectivity and efficiency (Table 2).¹⁵¹ Conventionally, the catalysts used - Silica and alumina bonded molecular sieves, and Al-SR115 + Ni - show no benzene cracking abilities. A further zeolite - USHY - showed minor activity, but was only capable of converting 4.7% of the benzene, even at 545&#176C. The Microwave reaction was carried out by packing the zeolite into a 2cm diameter tube placed through a waveguide, and passing benzene vapour through under microwave irradiation. Microwave power was applied in 520ms pulses of ~2.5kW followed by 25 second rest periods. Although benzene is an expensive starting material for the formation of lower hydrocarbons, this selectivity is of interest to industrial processes where aromatic hydrocarbons are formed as a by-product, and in the disposal of toxic but unreactive industrial wastes

Thermal^a

Microwave^a

Product

Zeolite USHY / 545&#176C

Al-SR115-Ni

Unactivated

Al-SR115-Ni

Activated^b

Si-SR115-Ni

Unactivated

Methane

0.05

2.53

5.63

1.72

Acetylene

-

91.95

89.59

93.17

Ethylene

0.03

0.76

3.84

0.52

Ethane

-

0.11

0.11

-

C₃

0.04

0.30

0.27

-

C₄

0.01

4.26

0.11

2.06

C₅

-

0.05

0.41

2.52

Toluene

2.60

-

-

-

>= C₈

0.61

-

-

-

Coke

0.10

-

-

-

^aNumbers indicate the quantity of the product expressed as a percentage of the benzene introduced ^.b The zeolite was activated by heating to 400&#176C for 6 hours in a mixture of hydrogen and helium.

Table 2 Catalytic cracking of benzene using microwaves¹⁵¹

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