December 12, 2011
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Obtain xylenes efficiently by disproportionating toluene. For the past 15 years, poly(ethylene terephthalate) (PET) has been the fastest-growing polymer in the petrochemicals industry . PET is produced from two monomers, ethylene glycol and purified terephthalic acid (TA). TA is prepared by oxidizing p-xylene, which is therefore in great demand.
More than 90% of the world’s p-xylene is made in petroleum refinery reformers, but these units cannot satisfy the p-xylene demand. To make up this shortfall, toluene, which has relatively little use in the chemical industry, is converted by disproportionation technologies to mixtures of xylenes and benzene.
J. R. Butler, R. Hall, and X. Xiao disclose an improved catalyst for disproportionating toluene. Their catalyst is based on mordenite molecular sieves that contain niobium; it is an improvement over conventional nickel mordenite catalysts because it forms very low levels of nonaromatic byproducts.
In the patent’s sole example, Zeolyst Mordenite Extrudate (Zeolyst International, Valley Forge, PA) was impregnated with 1.9 wt% niobium, and 30 mL of the catalyst was loaded into a tubular reactor. The initial reaction conditions were
- toluene feed liquid hourly space velocity, 3 h–1;
- hydrogen/hydrocarbon mol ratio, 1:1;
- inlet pressure, 600 psig; and
- reactor temperature, 350 °C.
The temperature was adjusted throughout the run to give constant toluene conversion; feed rate and inlet pressure were held constant. On day 1, toluene conversion was 25.6%, xylene selectivity 54.1%, benzene selectivity 38.1%, and the yield of nonaromatics 0.5%. Because the nonaromatics yield was <1%, the hydrogen/toluene mol ratio was raised to 3:1. On days 2–3, toluene conversion leveled out to ≈47%.
By 30 days into the run, the temperature had increased to 408 °C to maintain the toluene conversion rate. Xylene and benzene selectivity decreased slightly to 50.7% and 36.5%, respectively. Nonaromatics yield decreased to 0.31%. (Fina Technology [Houston]. US Patent 8,063,259, Nov. 22, 2011; Jeffrey S. Plotkin)