Features of the dehydrogenation process of butane

How dehydrogenation reaction of butane takes place

[Deposit Photos]

The de­hy­dro­gena­tion re­ac­tion is the re­ac­tion of hy­dro­gen atoms split­ting off from an or­gan­ic mol­e­cule. This is a cat­alyt­ic process – i.e. the pres­ence of a cat­a­lyst is re­quired to car­ry it out. Dif­fer­ent un­sat­u­rat­ed com­pounds may be prod­ucts of de­hy­dro­gena­tion: for ex­am­ple, alkenes and alka­di­enes. In the de­hy­dro­gena­tion of low­er alka­nes (with a car­bon chain of a length of C₂-C₄), mol­e­cules do not form a cy­cle. This is char­ac­ter­is­tic for alka­nes with a car­bon chain con­tain­ing more than 4 car­bon atoms. The fea­tures of the de­hy­dro­gena­tion re­ac­tion of low­er alka­nes can be seen clear­ly based on the ex­am­ple of hy­dro­gen split­ting off from the bu­tane mol­e­cule.


Re­ac­tion con­di­tions

For hy­dro­gen to split off from a sat­u­rat­ed aliphat­ic (non-cycli­cal) hy­dro­car­bon, heat­ing of up to 500-600 °C (or 932-1112 °F). Var­i­ous met­als are used as cat­a­lysts – for ex­am­ple, Ni, Pt, Pd and Fe are suit­able, and also the ox­ides ZnO, Cr₂O₃ and Fe₂O₃.

In cer­tain con­di­tions, an alka­di­ene rather than an alkene may form from bu­tane of nor­mal struc­ture (CH₃-CH₂-CH₂-CH₃) dur­ing de­hy­dro­gena­tion. The re­ac­tion must be con­duct­ed in the pres­ence of the cat­a­lyst Cr₂O₃/Al₂O₃ at a tem­per­a­ture of 450-620 °C, or 842-1118 °F.

How the de­hy­dro­gena­tion re­ac­tion takes place

In de­hy­dro­gena­tion, C-H bonds are torn, hy­dro­gen atoms split off from neigh­bor­ing car­bon atoms, and in this place a dou­ble bond forms.

In the de­hy­dro­gena­tion of n-bu­tane, a mix­ture of iso­mers may form (sub­stances with an iden­ti­cal atom­ic com­po­si­tion and molec­u­lar mass, but with dif­fer­ent struc­ture or spa­tial po­si­tion of atoms). 2 prod­ucts of re­ac­tion form – butene-1 and butene-2:

2CH₃-CH₂-CH₂-CH₃ = CH₃-CH=CH-CH₃ + CH₂=CH-CH₂-CH₃ + 2Н₂

butene-2 and butene-1 form re­spec­tive­ly, in the pres­ence of Ni at a tem­per­a­ture of 500 de­grees °C (932 °F)

Ball and stick model of isobutane [Wikimedia]

From isobu­tane, isobuty­lene may be ob­tained:

(CH₃)₂CH-CH₃ = (CH₃)₂C=CH₂ + H₂

re­ac­tion takes place with heat­ing to 550-600 °C (1022-1112 °F): the chro­mia-alu­mi­na cat­a­lyst Cr₂O₃/Al₂O₃ is used

With the chro­mia-alu­mi­na cat­a­lyst, at a tem­per­a­ture of 450-650 °C (842-1202 °F), bu­ta­di­ene-1,3 forms from bu­tane.

CH₃-CH₂-CH₂-CH₃ = CH₂=CH-CH=CH₂ + 2H₂

In de­hy­dro­gena­tion, bu­tane does not form in a cy­cle and does not form cy­clobutene be­cause cy­clobutene has an un­sta­ble struc­ture; at the re­ac­tion tem­per­a­ture it is also ca­pa­ble of ther­mal­ly break­ing down to eth­yl­ene СН₂=СН₂.

Pro­ce­dure for ob­tain­ing butylenes in a re­ac­tor

A re­ac­tor for ob­tain­ing butylenes from n-bu­tane is a tubu­lar de­vice. In the de­hy­dro­gena­tion of bu­tane, hy­dro­car­bon goes to the de­vice in com­pressed form (in nor­mal con­di­tions, bu­tane is a gas which eas­i­ly changes to liq­uid at -0.5 °C or 31.1 °F). With a pis­ton, bu­tane is moved to the heat ex­chang­er, where it is heat­ed, evap­o­rat­ed, and thus changes to gaseous form. Then in the tubu­lar heater the bu­tane is heat­ed to the tem­per­a­ture re­quired for the re­ac­tion (around 500 °C, or 932 °F).

Heat­ed bu­tane va­por pass­es into the tubu­lar re­ac­tor (a heat­ed cat­a­lyst is con­tained in the pipes). On mak­ing con­tact with the cat­a­lyst, the bu­tane va­por de­hy­drates, form­ing a mix­ture from un­re­act­ed n-bu­tane, butylenes (butene-1 and butene-2), hy­dro­gen and sec­ondary prod­ucts. The mix­ture cools, and then breaks down. The cat­a­lyst should not make con­tact with the buty­lene for more than 2-3 sec­onds, oth­er­wise a large quan­ti­ty of sec­ondary prod­ucts and soot may form, which af­fects the yield of the prod­uct of re­ac­tion – buty­lene.

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Process for ob­tain­ing bu­ta­di­ene in a re­ac­tor

Af­ter the sep­a­ra­tion of sec­ondary prod­ucts, bu­tane and buty­lene in the mix­ture, buty­lene may be sub­ject­ed to fur­ther de­hy­dro­gena­tion – in this way bu­ta­di­ene-1,3 can be ob­tained.

Ball-and-stick model of 1,3-butadiene [Wikimedia]

On the whole this process is tech­no­log­i­cal­ly sim­i­lar to the process used in the de­hy­dro­gena­tion of bu­tane, but in­cludes a num­ber of spe­cial fea­tures:

  1. the for­ma­tion of di­vinyl (bu­ta­di­ene-1,3) is also pos­si­ble with­out a cat­a­lyst (only with heat­ing), but the yield of the prod­uct does not ex­ceed 40%, while when cat­a­lysts are used the yield is around 77%;
  2. the yield of bu­ta­di­ene-1,3 can be in­creased by de­creas­ing its par­tial pres­sure in the re­ac­tor – this can be achieved by adding wa­ter va­por to the buty­lene va­por;
  3. af­ter de­hy­dro­gena­tion, the re­ac­tive mix­ture must be swift­ly cooled (“chilled”) in a spe­cial plate col­umn in the de­vice, as at a high tem­per­a­ture bu­ta­di­ene may break down.

Di­vinyl (bu­ta­di­ene-1,3) can be ob­tained in one stage di­rect­ly from bu­tane (a mix­ture of pure bu­tane with the un­re­act­ed bu­tane-buty­lene mix­ture must be put into the re­ac­tor), but the yield of the prod­uct will be low­er than in the de­hy­dro­gena­tion of bu­tane in two stages.

Cat­alyt­ic de­hy­dro­gena­tion of bu­tane is used in in­dus­try for ob­tain­ing un­sat­u­rat­ed hy­dro­car­bons. The re­ac­tion is of­ten car­ried out in prac­tice, as it takes place with high prod­uct yields – up to 80-90%.