On the acidity characterization of pristine and zinc-modified H-ZSM-5 zeolites: Comparison of adsorption-based characterization methods

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Unlike temperature-programmed desorption (TPD) of ammonia (NH 3 -TPD) and Fourier-transform infrared spectroscopy of adsorbed pyridine (pyridine-FTIR), where ammonia and pyridine adsorb and desorb without reaction, TPD of isopropylamine (IPAm-TPD) relies on a Brønsted-acid-catalyzed decomposition of isopropylamine to propylene and ammonia to selectively probe Brønsted acidity. Prior studies have reported altered IPAm-TPD responses for metals-modified zeolites relative to pristine zeolites, yet hypotheses on the cause have diverged. This work examined the acidity characterization results of pristine and zinc-modified H-ZSM-5 zeolites from IPAm-TPD against those from NH 3 -TPD, and pyridine-FTIR. For pristine zeolites, the IPAm-TPD data exhibited the expected Brønsted-site-related propylene desorption feature, with propylene amounts proportional to framework aluminium, strongly adsorbed ammonia in NH 3 -TPD, and Brønsted-associated pyridine in pyridine-FTIR. However, the IPAm-TPD data of the zinc-modified zeolites was more complex, with multiple desorption features. The mass spectrometry data and comparison of the quantitative IPAm-TPD results to those of NH 3 -TPD and pyridine-FTIR suggested contributions from a Brønsted-site-related propylene release, an additional, likely Lewis-site-related propylene release, and unidentified side products overlapping with typical propylene responses, possibly from dehydrogenation and aromatization reactions. When the Brønsted-site-related propylene response was quantified and compared to quantitative pyridine-FTIR results, the results seemed representative of Brønsted acidity despite the side reactions.

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