Characterization of hydrogen-[iron]ZSM-5 and an investigation of adsorption complexes formed in acylation reactions over zeolite acids

The adsorption and reaction properties of an Al-free H-[Fe]ZSM-5 were compared to an H-[Al]ZSM-5 sample with the same acid site density. Differential heats of adsorption of common probe molecules such as pyridine and ammonia are not capable of distinguishing Brønsted sites in the materials. The Brønsted-acid sites in H-[Fe]ZSM-5 are capable of performing oligomerization of 1-butene; however, the sites are much less active than those in H-[Al]ZSM-5 and do not readily catalyze hydride-transfer reactions. Due to this difference, H-[Fe]ZSM-5 appears to exhibit a much lower tendency to form coke during reactions with olefins. The adsorption of typical acylating agents (acetic acid, acetic anhydride and acetyl chloride) on the Brønsted-acid sites in H-[Al]ZSM-5 has been investigated. Acetic acid forms a hydrogen-bonded complex with the Brønsted sites, while acetyl chloride and acetic anhydride react readily with Brønsted sites to form an acetyl-ester intermediate. The acetyl ester is highly reactive and is likely to be an important intermediate in acylation reactions. At high temperature the acetyl-ester intermediate decomposes to products that likely lead to coke formation. Differential heats of adsorption measurements suggest the importance of non-specific, van der Waals interactions on stabilizing the adsorption complex formed in the reaction. A novel method of removing the tetramethylammonium (TMA+) template from zeolite α has been developed. The treatment, involving reaction of the TMA+ with ammonia, led to less framework damage during template removal. The results from this work also suggest that TMA + is an intermediate involved during formation of mono- and di-methyl amine during reactions of methanol and ammonia. The Brønsted-acid site densities of the treated samples are characterized via alkylamine chemistry. n-Propylamine was found to be an effective probe of the total Brønsted-acid site density, while the external site density can be probed with tri-iso-butylamine. n-Hexane cracking was used as final test of acidity and the rates were found to decrease with crystallite size, suggesting that the reaction is diffusion-limited