Tuning the selectivity of methanol-to-hydrocarbons conversion on H-ZSM-5 by co-processing olefin or aromatic compounds

The product selectivity of dimethyl ether (DME) conversion to hydrocarbons on H-ZSM-5 was systematically tuned by co-feeding small amounts of ¹³C-propene and ¹³C-toluene (4 kPa) with ¹²C-DME (70 kPa) under isoconversion conditions (20.8-22.7 C%) at 548 K. The selectivity to ethene (14.5-18 C%) and aromatics (7.1-33.7 C%) increased while selectivity to C ₄-C ₇ aliphatics (42.8-16.9 C%) decreased with increasing amounts of toluene (0-4 kPa) in the co-feed. Similar trends were also observed at lower conversions (4.6-5.1 C%) at 548 K and at higher temperatures (623 K), showing that the olefin-to-aromatic ratio can be used as a parameter to propagate the olefin- and aromatic-based carbon pools to varying extents within the range of conditions studied in this work. The co-reaction of ¹³C-propene with ¹²C-DME showed that C ₅-C ₇ olefins are formed almost exclusively from methylation reactions while butenes are formed from both olefin cracking and methylation reactions. The high fraction of propene (55.1%) with at least one ¹²C indicated that a large fraction of propene is a product of olefin cracking reactions. Under conditions in which the aromatic-based cycle is dominant (increasing amounts of toluene in the co-feed), both ethene and propene contained approximately 10% ¹³C atoms, showing that when the olefin-based cycle is suppressed, these light olefins primarily originate from the aromatic-based cycle. The ¹³C content of toluene in the effluent was unchanged compared to that in the ¹³C-toluene feed, implying that toluene is not formed as a significant product. Additionally, at least 9.8% of p-xylene, 1,2,4-trimethylbenzene, and 1,2,4,5-tetramethylbenzene isotopomers were entirely ¹²C-labeled, while less than 2% of toluene and o-xylene isotopomers were entirely ¹²C-labeled, showing that under the conditions studied in this work, cyclization reactions occur predominantly for C ₈₊ aliphatics to form p-xylene and larger aromatics. Because the olefin- and aromatic-based cycles are not isolated from one another, understanding communication between the two cycles is an important step in controlling selectivity of MTH on H-ZSM-5.