High Zn/Al ratios enhance dehydrogenation vs hydrogen transfer reactions of Zn-ZSM-5 catalytic systems in methanol conversion to aromatics

Abstract

Two series of Zn-ZSM-5 catalysts were prepared by ion exchanging two commercial zeolites with different Si/Al ratios (40 and 15) with increasing Zn loadings. The nature of the Zn sites in the zeolite was studied by spectroscopy using laboratory and synchrotron techniques. All the evidences suggest that catalytic activity is associated with [Zn(H2O)n(OH)] + species located in the exchange positions of the materials with little or no contribution of ZnO or metallic Zn. The effect of Zn/Al ratio on their catalytic performance in methanol conversion to aromatics has been investigated. In all cases, higher Zn content causes an increase in the yield of aromatics while keeping the production of alkanes low. For similar Zn contents, high densities of Al sites favour the hydrogen transfer reactions and alkane formation whereas in samples with low Al contents, and thus higher Zn/Al ratio, the dehydrogenation reactions in which molecular hydrogen is released are favoured.

Evolution of Zn Species on Zn/HZSM‐5 Catalyst under H2 Pretreated and its Effect on Ethylene Aromatization

“…However, ZnO or metallic Zn hardly contribute to the catalytic activity. [30] It is interesting to note that the production of aromatics increased at the expense of C1À C4 hydrocarbons on the H 2 -treated catalysts compared with the parent HZSM-5 catalyst. This means that the active ZnOH + species helped accelerate the aromatization by acting as dehydrogenation sites for ethylene and thus promoted the production of aromatics.…”

“…All the evidence suggest that catalytic activity is associated with ZnOH + species located in the exchange positions of the HZSM‐5. However, ZnO or metallic Zn hardly contribute to the catalytic activity . It is interesting to note that the production of aromatics increased at the expense of C1−C4 hydrocarbons on the H 2 ‐treated catalysts compared with the parent HZSM‐5 catalyst.…”

C−C Bond Formation in Syngas Conversion over Zinc Sites Grafted on ZSM‐5 Zeolite

“…Pinilla‐Herrero et al. reported that ZnOH + over ZSM‐5 facilitated aromatization in the methanol‐to‐aromatics (MTA) process . Almutairi et al.…”

“…Note that 13 C‐enriched CO was used in the feed. It is interesting that both the 13 C single pulse (SP) and CP/MAS spectra (Figure S17a) show no signal in the region of 120–155 ppm, indicating the absence of (methylated) aromatic and olefinic groups, which are characteristic species of the hydrocarbon pool observed in MTH processes . Furthermore, thermal gravimetric analysis of the used Zn 0.34 /ZSM‐5 catalyst indicates that almost no carbon residues are formed (Figure S18).…”

Insight into the Formation of COX By‐Products in Methanol‐to‐Aromatics Reaction over Zn/HZSM‐5: Significantly Affected by the Chemical State of Surface Zn Species

“…[1][2][3] HZSM-5 zeolite is regarded as the most appropriate catalytic-material for this process, because of its adjustable acidic properties, high hydrothermal stability and regular three-dimensional 10-ring pore matching with the light aromatic molecules. [4][5][6] Generally, the significantly enhanced selectivity of aromatics can be obtained over the metal modified HZSM-5 (in particular, the catalysts containing Zn, Ag or Ga), [1,4,5,[7][8][9][10][11][12][13][14][15][16][17][18][19] owing to the interaction-dehydrogenation [5,13,[20][21][22][23] of metal species with acid sites. For instance, Bi et al obtained an aromatic selectivity of up to 77.9 % at the time-on-stream of 10 min over a Zn-modified HZSM-5 (SiO 2 /Al 2 O 3 = 38) with the retention of more medium-strength Brønsted acid sites at 753 K and 2.0 h À 1 WHSV.…”