Selective methylation of toluene using CO ₂ and H ₂ to para -xylene

Abstract

Toluene methylation with methanol to produce xylene has been widely investigated. A simultaneous side reaction of methanol-to-olefin over zeolites is hard to avoid, resulting in an unsatisfactory methylation efficiency. Here, CO2 and H2 replace methanol in toluene methylation over a class of ZnZrOx–ZSM-5 (ZZO-Z5) dual-functional catalysts. Results demonstrate that the reactive methylation species (H3CO*; * represents a surface species) are generated more easily by CO2 hydrogenation than by methanol dehydrogenation. Catalytic performance tests on a fixed-bed reactor show that 92.4% xylene selectivity in CO-free products and 70.8% para-xylene selectivity in xylene are obtained on each optimized catalyst. Isotope effects of H2/D2 and CO2/13CO2 indicate that xylene product is substantially generated from toluene methylation rather than disproportionation. A mechanism involving generation of reactive methylation species on ZZO by CO2 hydrogenation and migration of the methylation species to Z5 pore for the toluene methylation to form xylene is proposed.

Homogeneous (De)hydrogenative Catalysis for Circular Chemistry – Using Waste as a Resource

“… [112] Several (hetero)arenes, such as indoles, pyrroles, and electron‐rich arenes were C‐methylated using CO 2 and H 2 (Scheme 4). Notably, this is the only example of a homogeneous catalyst reported to date for this type of transformation although some heterogeneous catalysts have also been reported recently [113,114] . A recent perspective report by Beller discusses the utilization of CO 2 for catalytic methylation reaction in more detail [100] …”

Recent Trends in Upgrading of CO2 as a C1 Reactant in N‐ and C‐Methylation Reactions

“…Toluene conversion and para ‐xylene selectivity in xylene do not vary considerably in the stability test of ZZO‐4SZ5 (Figure 18B ) . The proposed mechanism is that CO 2 hydrogenation generates reactive methylation species, which move to the pore structure of Z5 and methylate toluene to xylene (Figure 18C) [46] …”

“…[45] Next, the group of Yuan in 2020 explained the tremendous role of CO 2 and H 2 as methylation reagents for selectively methylating toluene to xylene over ZZOÀ Z5 dual-functional catalysts. [46] The CO 2 hydrogenation technique is significantly easier to get the reactive methylation species of the H 3 CO* entity than the methanol route. In CO-free products, optimising the mass ratio of ZZO to modified Z5 resulted in a high selectivity to xylene of 92.4%.…”

“…The proposed mechanism is that CO 2 hydrogenation generates reactive methylation species, which move to the pore structure of Z5 and methylate toluene to xylene (Figure 18C). [46] In the same year, Shimizu along with his co-workers created a heterogeneous catalytic system for the methylation of benzene using CO 2 and H 2 . [47] The combination of Re(1)/TiO 2 and HMOR(90) was discovered to boost the methylation of benzene and generate a high yield of C-methylated products at a reaction temperature of 250 °C after a rigorous catalyst screening procedure (Figure 19).…”

Tandem catalytic methylation of naphthalene using CO2 and H2

“…In this regard, recent attempts have been made towards reductive methylation of toluene with CO 2 and H 2 in the presence of dual-functional ZnZrO x -ZSM-5 or ZnCrO x -ZSM-5 catalysts, giving selectivity up to 65% for industrially vital para-xylene. 17,18 Methylnaphthalenes, essential raw materials for manufacturing sophisticated polymers and specialty drugs, represent another important target for reductive methylation reactions (Scheme 1a). [19][20][21][22] Methylation of naphthalene (NAP) with methanol over aluminum-containing MFI zeolites is a proven route to access methylnaphthalenes (Scheme 1b).…”