A.I. Biaglow

The effect of alumina structure on surface sites for alcohol dehydration

We have studied the effect of alumina form(-/, 5, and a), and impurities, on 2-propanol dehydration activity. In a steady-state flow reactor experiment, α-alumina was approximately 1000 times less reactive (in terms of moles propene formed/s/m2) than reforming grade γ-alumina. The structure of the transitional alumina (γ vs δ) played a smaller role than Na contamination in determining alcohol dehydration activity. For instance, δ-alumina was ≈3–6 times less reactive than reforming grade γ-alumina in its activity for alcohol conversion, while Na-poisoned γ-alumina was ≈180 times less reactive. The number of surface sites on these aluminas were determined using temperature-programmed reaction (TPR) of 2-propanol. It was found that ≈1–2 × 1018 alcohol molecules/m2 reacted to form propene on all of the aluminas independent of alumina form or impurity concentration. The temperature at which the alcohol reacted to form propene in the TPR experiment correlated inversely with steady-state alcohol dehydration activity. The peak desorption temperature (TP) during TPR ranged from 440 K for reforming grade γ-alumina to 600 K for α-alumina. These results are discussed in terms of a model for the nature of reactive sites on the surface of alumina.

2-Propanamine adsorption on a fluid catalytic cracking catalyst

Abstract We have examined temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of 2-propanamine on a fluid catalytic cracking catalyst as a function of steam deactivation time and compared the results to the activity for gas-oil cracking. The TPD-TGA results for the FCC catalyst are similar to results obtained on high-silica zeolites and show ammonia and propene desorbing in a well-defined feature between 575 and 650 K. The sites responsible for this desorption feature appear to be acid sites associated with framework Al in the zeolite phase. The concentration of these sites decreases steadily with steam deactivation and correlates very well with the activities for gas-oil cracking.

The role of sodium and structure on the catalytic behavior of alumina: I. Isopropanol dehydration activity

Abstract Three different transitional aluminas were doped with controlled and measured amounts of Na and were tested for 2-propanol dehydration activity. These alumina samples were activated by heating at 773 K in situ before use. In order to examine the sites responsible for the reaction, simultaneous temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) measurements of 2-propanol were performed on the samples. A typical γ-alumina sample adsorbed ca. 2·10 18 molecules of 2-propanol per m 2 . When 400 ppm (0.04 wt.-%) of Na was added to this sample, the reactivity fell by 75%. Since the Na level corresponds to 0.07·10 18 atoms/m 2 , this suggests that the active sites are much fewer in number than the number of adsorption sites. Besides affecting the active sites, Na also modifies the adsorption sites causing a fraction of the adsorbed 2-propanol to desorb before reaction in the TPD experiment. The effect of added Na was contrasted with the effect of alumina form (γ, δ and α). These alumina samples were also characterized by IR spectroscopy in a companion study (part II of this paper, Appl. Catal. A, 000 (1995) 000) where we found that the added Na impurities affected the hydroxyl groups and adsorbed pyridine. This study provides a comprehensive picture of how Na impurities modify the catalytic behavior of transitional alumina.

A method for measuring the titania surface area on mixed oxides of titania and silica

We have used temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of 2-propanol to characterize several silica, titania, and silica-supported titania samples. Upon evacuation, most of the 2-propanol desorbed intact from the silica samples. This is in contrast to the results on titania and on silica-supported titania, where a significant amount remained on the surface following evacuation, with a fraction of this reacting to propene and water. The coverages of 2-propanol are approximately proportional to the titania surface area, corresponding to between 2.4 and 6.1×1018 molecules/m2 of titania, depending on the form of the titania. The results suggest that selective adsorption of 2-propanol may be useful for determining the surface area of titania in titania-silicates.

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.

Quantification of acidity in H-ZSM-5*

Calorimetry measurements of simple amines and 13C NMR measurements of adsorbed allyl alcohol have been used to characterize the acidity of H-ZSM-5. It is shown that H-ZSM-5 is made up of identical sites in a coverage of one/Al and that the differential heats for a series of bases adsorbed at the Al sites correlate very well with gas-phase, proton affinities. In contrast, there is no correlation between the differential heats of adsorption and aqueous-phase heats of protonation. A simple model, based on a thermochemical formalism and the calorimetry data, is developed which allows one to calculate the relative energies for various intermediates which may be formed in simple unimolecular reactions. This model was tested by examining the initial products formed by the reaction of allyl alcohol using 13C NMR and was shown to predict the observed chemistry very well.

Evidence for the allyl and secondary cation intermediate on adsorption of allyl alcohol on H-ZSM-5 zeolite

13 C NMR spectroscopy is used to show that for surface coverages corresponding to less than one allyl alcohol molecule per Bronsted site in H-ZSM-5, there is evidence for the formation of an allyl carbocation.