Chemical News

Monday, February 11, 2008

New process for p-Xylene without Benzene as Co-product

A new method for making p-xylene eliminates benzene coproduct. p-Xylene is the main raw material for the manufacture of terephthalic acid, one of the major ingredients for making the polyester poly(ethylene terephthalate) (PET). Because PET represents the fastest-growing polymer production in the petrochemical industry, p-xylene is in high demand. The major source of p-xylene is oil refinery–based reformate; but refineries cannot produce all the needed p-xylene, and secondary production sources are required.

The predominant secondary source is the toluene disproportionation (TDP) process. In TDP, 2 equiv toluene is converted into 1equiv benzene and 1 equiv xylenes (mixed xylenes or mainly p-xylene, depending on the TDP catalyst). This process, however, produces benzene whether it is wanted or not.

J. P. Breen and co-inventors at Johnson Matthey disclose catalysts and operating conditions for converting toluene and MeOH to p-xylene in high yield. This approach could have an advantage over TDP in that no coproduct (other than water) is produced. One of the patent’s best examples is described here.

The catalyst was prepared by dry-milling NH4 ZSM-5 (silica/alumina ratio 80:1) with B2O3 to give a 10 wt% boron loading. The catalyst was pressed into discs, crushed, and sieved to give particles in the range of 250–850 ┬Ám. The particles were loaded into a reactor and heated under nitrogen at 440 °C for 30 min. A feed consisting of MeOH, toluene, steam, and hydrogen was directed through the catalyst bed at 440 °C. The ratio of toluene to MeOH was 8:1, and the contact time was 0.36 sec. After 15 min, the catalyst reached a stable performance level, and toluene methylation reached the expected maximum of 12.5%. The selectivity to p-xylene was 99.9%, and MeOH conversion was ~100%.

Source: CAS