Aromatic Hydrocarbon Formation in HSAPO-18 Catalysts: Cage Topology and Acid Site Density

Two samples of the silico-aluminophosphate HSAPO-18 with different acid site densities were prepared, characterized, and studied as methanol-to-olefin catalysts in comparison with the more widely studied alternate catalyst HSAPO-34. In particular, we used mild acid digestion and GC-MS analysis to profile the aromatic hydrocarbons forming in the cages as the catalysts deactivated during methanol conversion at 450 °C. The most notable difference between HSAPO-18 and HSAPO-34 was the accumulation of pyrene during the deactivation of the former as opposed to phenanthrene in the latter. Repetitive analyses of seven deactivated catalyst beds of each type revealed the following pyrene/phenanthrene ratios: HSAPO-18, 7.70 ± 4.96; HSAPO-34, 0.414 ± 0.040 (95% confidence). We attribute this to subtle differences in cage topology. The HSAPO-18 catalyst with the lowest acid site density also accumulated appreciable amounts of pyrenes with one to four methyl groups. We were able to synthesize pyrene as the predominant entrained organic compound in HSAPO-18 by converting methanol at 600 °C over the catalyst. In situ NMR studies using acetone-2- 13 C as a probe showed that the acidity of HSAPO-18 is very similar to that of HSAPO-34.