G.T. Kokotailo

Amine adsorption in H-ZSM-5

Abstract We have examined the adsorption of simple amines, including methylamine, ethylamine, n-propylamine, isopropylamine, t-butylamine and pyridine on H-ZSM-5 using temperature-programmed desorption (TPD), thermogravimetric analysis (TGA) and transmission infrared spectroscopy. For each adsorbate, a well-defined adsorption state corresponding to a coverage of one molecule per aluminum atom could be easily identified in TPD-TGA. With the exceptions of methylamine and pyridine, the 1:1 complexes formed by each amine reacted completely to desorb as ammonia and alkene products. Amine molecules adsorbed in excess of one per aluminum atom desorbed unreacted at much lower temperatures. Approximately half of the 1:1 species formed by methylamine also reacted to form a complex mixture of products, while the 1:1 species formed by pyridine desorbed unreacted above 700 K. Infrared spectroscopy was used to demonstrate that the stoichiometric complexes for each amine were protonated and associated with the hydroxyls at the aluminum sites in the zeolite. These results imply that the acid sites in H-ZSM-5 are simple Bronsted sites and that TPD-TGA measurements of amine adsorption can be used to quantify the acid site density.

Stoichiometric adsorption complexes in [B]- and [Fe]-ZSM-5 zeolites

Abstract The adsorption of ammonia, 2-propanamine, and 2-propanol on H-[B]-ZSM-5 and H-[Fe]-ZSM-5 has been examined using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) to investigate the effect of the incorporation of trivalent ions into the zeolite framework. For each of the adsorbates on H-[Fe]-ZSM-5, clearly defined adsorption complexes corresponding to one molecule/Fe atom could be identified. An adsorption complex with a stoichiometry of one molecule/B atom could also be observed for 2-propanamine on H-[B]-ZSM-5 but complexes were less clearly defined for ammonia and 2-propanol. The reactivity of these 1: 1 complexes indicates that the acid sites formed by the hydroxyls associated with B incorporation are much weaker than those associated with Fe or Al. We also examined a sample of ZSM-5 which had extraframework Fe using 2-propanamine adsorption and found that there was negligible adsorption due to these sites. These results indicate that TPDTGA measurements of selected adsorbates are valuable in characterizing the materials created by the incorporation of trivalent cations into the zeolite framework.

Characterization of Mg and Mn Substitution in AIPO4-5

We have examined the framework substitution of Mg and Mn into the A1PO4-5 structure by studying the adsorption of reactive probe molecules, including ethylamine, isopropylamine, 2-propanol, and propene. Lattice parameters from X-ray diffraction suggests that there is a maximum framework substitution of 5 mol% for both metals. However, samples with 5 mol% Mg or Mn appear to contain large concentrations of defects in the structure, based on the observation of slow adsorption of branched alkanes. In simultaneous temperature-programmed-desorption (TPD)/ thermogravimetric-analysis (TGA) measurements, both amines reacted to an olefin and ammonia in a well-defined decomposition feature between 575 and 650 K for isopropylamine and between 625 and 675 K for ethylamine. By comparison wth similar results on high-silica zeolites, the sites which decompose the amines are known to be Bronsted acid sites and decomposition occurs through an alkylammonium ion intermediate. For low concentrations of Mn and Mg (<1 mol%), the moles of isopropylamine and ethylamine which decompose in TPD-TGA are the same and equal to the moles of substituted metal atoms, indicating that the metals substitute for the A1 in the framework to form Bronsted-acid sites. These same sites also catalyze the dehydration of 2-propanol above 400 K and the oligomerization of propene at 300 K.

Determination of framework concentrations of gallium in [Ga]-ZSM-5

The adsorption of ammonia, 2-propanol, and 2-propanamine on H- [Ga]-ZSM-5 has been examined using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA). For each of the adsorbates, a clearly defined adsorption complex with a stoichiometry of one per Ga could be identified. For 2-propanol and 2-propanamine, these 1:1 adsorption complexes react to desorb as propene and either water or ammonia. Adsorption complexes were not observed when a high-silica ZSM-5 was impregnated with Ga2O3, implying that the adsorption complexes are associated with the hydroxyls at framework Ga sites in the zeolite and are not observed for Ga which is not in the framework. The results suggest that adsorption can be used to determine the framework concentrations of Ga in H-[Ga]-ZSM-5.

An examination of the acid sites in SAPO-5

We have examined a series of SAPO-5 materials using temperature-programmed desorption and thermogravimetric analysis (TPD-TGA) of isopropylamine, n-propylamine, ammonia, propene, and 2-propanol and compared the results to those for AIPO-5. For isopropylamine and n-propylamine, desorption of propene and ammonia was observed from well-defined features in the TPD-TGA curves, features which were not observed on AIPO-5. These desorption features appear to be due to simple Bronsted-acid sites, and the concentration of sites determined by both amines was the same. For low Si concentrations, there appears to be one acid site per framework Si. The differences between AIPO-5 and SAPO-5 materials could also be observed with propene, which oligomerized at 300 K in SAPO-5 but not in AIPO-5. While differences were observed between AIPO-5 and the SAPO-5s for TPD-TGA of ammonia and 2-propanol, desorption from sites present in AIPO-5 were difficult to distinguish from the stronger sites present in the SAPO-5s. The results suggest that adsorption of simple amines may be very useful for characterization of acid sites in silica aluminophosphate molecular sieves.

A Study of Acid Sites in Substituted Aipo-5

Abstract We have examined temperatured-programmed desorption (TPD) and thermogravimetric analysis (TGA) of isopropylamine on a series of Si-, Co-, and Mg-substituted AIPO-5 samples. The TPD-TGA results on the substituted samples show ammonia and propene desorbing in a well-defined feature between 575 and 650K, a feature not observed on pure AIPO-5. The results suggest that TPD-TGA measurements of isopropylamine may be useful in determining the framework concentrations and acid site densities for SAPO-5, CoAPO-5, and MAPO-5.

THE FORMATION OF STOICHIOMETRIC ADSORPTION COMPLEXES FOR SIMPLE THIOLS IN H-ZSM-5 AND H-ZSM-12

ABSTRACT We have examined the adsorption of ethanethiol, 1-propanethiol, 2-propanethiol, and 2-methyl-2-propanethiol on H-ZSM-5 and H-ZSM-12 zeolites. With the exception of 2-methyl-2-propanethiol on H-ZSM-5, each of the thiols forms stoichiometric adsorption complexes with the Al sites. While the 1:1 complexes formed by the primary and secondary thiols decompose reversibly back to the thiols, 2-methyl-2-propanethiol desorbs as H2S and olefin products. A potential energy model based on the proton affinities of the various species along the reaction coordinate is able to explain the reactivity of the thiols, suggesting that the chemistry is dominated by proton transfer from the acid sites to the adsorbed species.