Chapter 2. Production of Xylenes by Naphtha Reforming

M.C. RANGEL; C.L. PIECK; L.S. CARVALHO; M.R. SAD

Xylenes: Synthesis, Characterization and Physicochemical Properties

Nova Science Publishers

Lugar: New York; Año: 2013; p. 13 - 32

Naphtha reforming is one of the most important routes to produce high value chemicals for petrochemical industry such as benzene, toluene and xylenes. Among them, xylenes are especially desirable due to their use in several applications such as solvent in printing, rubber and leather industries and components of ink, rubber, adhesive and leather industries. They also can be used as cleaning agents for steel, silicon wafers and integrated circuits. In addition, p-xylene is the principal precursor to terephthalic acid and dimethyl terephthalate, both monomers used in the production of polyethylene terephthalate for plastic bottles and polyester clothing. On the other hand, o-xylene is an important precursor to phthalic anhydride. The xylene contents in the reformate stream largely depend on the composition of feedstock, the type of reformer, the operating conditions and on the catalysts. The catalysts are especially important, since by changing their properties, one can increase the yield of xylenes or even one of their isomers, by keeping the process conditions. The commercial catalysts are chloride alumina-supported platinum, which have metal and acidic sites. Both catalytic functions act cooperatively for most reactions of naphtha reforming, such as skeletal isomerization and dehydrocyclization of n-alkanes and aromatization of alkyl cyclopentanes. Other metals (germanium, iridium and others) are often added to the monometallic catalysts generating new systems able to work under different conditions making possible to obtain tailored reformates. In this chapter, we compare the activity, selectivity and stability of Pt/Al2O3 catalysts modified by tin, rhenium and germanium prepared by coimpregnation and successive impregnation for the production of xylenes at atmospheric pressure. As most heterogeneous catalysts, the properties of these catalysts largely depend on the preparation method (successive impregnation or coimpregnation). Also, they depend on the kind of the second and/or third metal (rhenium, germanium, tin), the replacement of water by acetone or ethanol in the impregnation step and the order of precursor addition, during impregnation. It was found that it is possible to design a catalyst able to increase the xylenes yield as well as of some of their isomer.