Tyres hydrocracking

A number of patents have been filed claiming the processing of waste rubber, either vulcanized or unvulcanized, by hydrocracking.10-13 The degradation usually proceeds in the presence of catalysts CoNi/Al203, FeMo/Cr203, Mo acetates, Cp2ZrCl2, etc. In some cases, a certain positive effect has been derived from the presence of sulfur compounds, which may either be the sulfur contained in the used tyre or H2S added to the reaction mixture. 

Anderson et al,15 have studied the hydrocracking of scrap automotive tyres after removal of the steel thread and the textile netting present in the starting material. Figure 6.8 shows the results obtained in non-catalysed experiments carried out at 400 °C at different cold-hydrogen pressures. A maximum conversion into gas and oils of about 70% was obtained, confirming that the carbon black, which accounts for 31% of the tyre, is not reactive under these... 

Mastral et al.23,24 have also investigated the effect of the main components present in tyres (carbon black, styrene-butadiene copolymer and polybutadiene) on the liquefaction of coal. Coprocessing of coal and carbon black confirmed the catalytic role of the latter, as it promotes hydrocracking reactions leading mainly to the formation of gaseous products. The addition of SBR to coal improves the yield of gases, oil and asphalt fractions, even at relatively low temperatures (350-375 °C). It is proposed that SBR favours the stabilization of the radicals involved in the process through alkylation reactions... 

In a further work, Tang and Curtis26 have studied the influence of the tyre components by replacing coal with model compounds 4-(l-naphthylmethyl)-bibenzyl (NMBB), dibenzothiophene and 5-methyl-8-(l-methylethyl)dibenzo-thiophen-4-ol (MMDH). Carbon black was active for the NMBB hydrocracking, whereas little activity was found with SBR, waste tyres and waste tyre liquefaction residues. However, when the latter residues were previously heat treated to remove organic coatings and recover the carbon black component, significant NMBB conversions were observed. Moreover, both carbon black and heat-treated residues were active for the hydrodesulfurization of dibenzothiophene and MMDH, particularly when combined with Mo naphthenate and S. These results confirm the catalytic properties of carbon black in coal-tyre coliquefaction, provided that its surface area is accessible. 

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