Reactions and Mechanisms of Xylene Isomerization and Related Processes

para -Xylene is a large volume chemical intermediate of polyester that is used to make fiber for clothing, bottles, and film and trays for food packaging. The dominant feedstock for xylene isomerization units is an A8 (aromatics with 8 carbon numbers) distillation cut of refinery reformate. It contains not only the three xylene isomers (XYL), but also ethylbenzene (EB) a structural isomer. pX cannot be recovered from this mixture by distillation. It can be recovered by low temperature crystallization or selective adsorption on a molecular sieve. The equilibrium distribution of the xylenes is about 25% ortho -xylene (oX), 50% meta -xylene (mX), and 25% para -xylene (pX). After removal of some pX from the process, the xylenes are isomerized to near equilibrium. This establishes a recycle loop. EB must be converted to XYL or to lower or higher boiling byproducts that can be readily separated to prevent its buildup in the loop. Several types of catalysts have been developed that convert EB in different ways: (i) EB isomerization catalysts can convert some EB to xylenes; (ii) EB transalkylation catalysts convert EB by transfer of the ethyl to another A8 (EB or XYL); and (iii) EB dealkylation catalysts convert EB primarily by its reaction with hydrogen to form benzene (Bz) and ethane (C2). All three types can be competitive depending on the local markets and pricing of the byproducts. The mechanisms of EB conversion and the advantages and disadvantages of each type of catalyst are reviewed. Successful development of the latter two types was revolutionized by the development of medium pore size molecular sieves. Ethyl transfer occurs readily via a bulky C16 alkylbiphenylalkane intermediate in large pore sieves where there is ample room for its formation. Methyl transfer can occur via a similar intermediate, which leads to excessive xylene loss. This intermediate can only form, with great difficulty, at channel intersections in medium pore sieves, such as ZSM-5. This shifts the mechanism of C2 transfer to dealkylation/realkylation with ethylene as a stable intermediate and opens a route for EB dealkylation when a mild hydrogenation catalyst is added to hydrogenate the ethylene to ethane. This shift was also important to the development of selective toluene/A9+ transalkylation (TOL/A9+ TA) catalysts that convert TOL and A9+ to XYL to make the most of reformate to produce pX in dedicated reformer aromatics complexes. The A9 fraction of reformate contains about 50% trimethylbenzenes (TMB) that can transfer a methyl cleanly to TOL to make 2 XYL. It also contains about 40% methylethylbenzenes (MEB) that must first be dealkylated to form TOL and C2. Thus, good TOL/A9+ catalysts must contain both medium and large pore sieves admixed or in separate beds. There is no mechanistic pathway for converting oX directly to pX. It must pass through mX. pX diffuses much faster than oX and mX in ZSM-5. This leads to diffusion-disguised kinetics for xylene isomerization. This was exploited to very accurately determine the true equilibrium distribution of the xylenes. Old equilibrium data in a standard reference by Stull, Westrum, and Sinke that is still used by many were shown to be in error. Xylene isomerization cannot lead to xylenes with pX above equilibrium. However, the high diffusivity of pX compared to mX and oX can be exploited in S elective T oluene D is P roportionation (STDP) and S elective T oluene A lkylation (STA) by methanol to produce XYL with pX/XYL of greater than 90%. Comments and insights regarding the reaction mechanisms and the advantages and disadvantages of these processes are provided.