Detection of the inhomogeneity of Brønsted acidity in H-mordenite and H-β zeolites: a comparative NMR study using trimethylphosphine and trimethylphosphine oxide as 31P NMR probes

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Abstract

The Brønsted acidity of H-mordenite (H-MOR) and H-β with similar Si/Al ratios has been characterized by conventional multinuclear solid-state NMR. 1H/27Al TRAPDOR NMR showed the presence of at least two different types of Brønsted acid sites in H-β. On the other hand, only a single 1H resonance due to Brønsted acid sites in H-MOR was observed. Solid-state 31P MAS NMR investigation of H-MOR and H-β zeolites loaded with trimethylphosphine oxide (TMPO) reveals multiple 31P resonances, resulting from the reaction of TMPO with different types of Brønsted acid sites. The results demonstrate that there is a wide distribution in the strength of the Brønsted acidity in H-MOR and H-β zeolites. When trimethylphosphine was used as a probe molecule, however, only a single 31P resonance was observed for both samples. Nevertheless, a larger 31P downfield shift associated with a larger JP–H value was observed for H-MOR as compared to that of H-β. This suggests that both 31P chemical shift and JP–H value might be used to correlate with their average Brønsted acidity, and thus their catalytic activity.

Introduction

An understanding of the type, concentration, and strength of acid sites in zeolites, and the role of the framework in controlling this, is fundamental to the understanding of their behavior as catalysts for several reactions. In addition to the Si/Al ratio, which describes the density of acids sites, information on the nature of Al sites, i.e., framework and extraframework species, and the strength distribution of acidity is needed. Methods of measuring acidity in zeolites have been recently reviewed [1], [2]. Spectroscopic techniques, for example, FT-IR and solid state NMR with and without adsorbed basic probe molecules, have been widely used to detect and characterize different types of hydroxyl groups in zeolites and related catalysts. A variety of solid-state 1H, 13C, and 15N NMR measurements were used to provide quantitative measurements of acidic sites in zeolites [3]. Unfortunately, the low sensitivity and/or limited chemical shift range of these nuclei usually limits their uses. In an attempt to overcome these limitations, NMR spectra of a probe containing a more sensitive nucleus, e.g., 31P are highly desirable. One advantage of phosphorus-containing probes over other NMR probes, such as pyridine or methylamines, is the higher sensitivity of the 31P nucleus, in comparison to 13C or 15N, due to its higher magnetogyric ratio and natural abundance of 100%. In addition, binding of 31P-containing probes, e.g., trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO), is expected to be less complicated than those of some other widely studied basic molecules such as water, methanol, and many amines, since extensive intermolecular hydrogen bonding networks are not formed. Several studies of the acid sites in zeolites and related oxides using TMP [4], [5], [6], [7] and TMPO [8], [9], [10] as probe molecules have been reported. These studies demonstrated that the 31P NMR chemical shift observed is very sensitive to the type of acid site to which the probe is adsorbed.

Both mordenite and β zeolites possess 12-membered ring pores with similar dimensions [11]. Mordenite exhibits a two-dimensional network of channels consisting of straight 12-membered ring pores (6.4×7.2 Å2) connected by twisted 8-membered pores of 2.6×5.7 Å2 side pockets. On the other hand, β zeolite processes a three-dimensional channel system with 12-membered ring apertures of 5.5×7.3 Å2. One should expect that there are at least two IR bands associated with the hydroxyl region of H-mordenite (H-MOR). The bridging hydroxyl bands of H-MOR, however, were not well-resolved in the IR spectrum, although the application of special techniques such as second derivative or adsorption–desorption of probe molecules permitted a separation of peaks [12]. Moreover, 1H NMR spectrum of H-MOR reported previously showed only a single resonance at 4.2 ppm for the Brønsted acid site [3]. On the other hand, Haw and co-workers first used variable temperature 1H MAS (magic angle spinning) NMR to reveal a complex acid function in zeolite H-β [13]. More recently, Paze et al. [14] further showed that by using the probe molecule d3-acetonitrile two families of Brønsted acid sites in H-β could be clearly distinguished in the IR and 1H MAS NMR spectra. In addition, Zhang et al. [15], [16] have used NH3-stepwise temperature programmed desorption and FT-IR to investigate the acidity characteristics of H-MOR and H-β, and revealed several different types of acid sites.

In this study TMP and TMPO combined with solid-state 31P NMR are employed, in view of the previous NMR studies using these two probe molecules on zeolites and related oxides, to address the nature, type, and strength of acid sites in H-MOR and H-β. Both zeolites exhibit 12-membered ring pores and similar Si/Al ratios. A major goal of the present study is to determine whether the presence of the inhomogeneity of the Brønsted acidity and of the similarity of acid characteristics in H-MOR and H-β that can be detected via 31P chemical shifts.

Section snippets

Sample preparation

The parent samples used in this study were NH4-MOR (Zeolyst, CBV-21A, Si/Al=10), and NH4-β zeolites (Zeolyst, CP814E, Si/Al=12.5). Dehydrated H-MOR and H-β were prepared from their ammonium forms by slowly ramping the temperature of approximately 0.5 g of samples to 400 °C under vacuum over a period of 12 h; the samples were kept at this temperature until the pressure dropped to below 10−3 Torr. The resulting material was characterized by using 1H MAS NMR. TMP (99%, Alfa) was introduced into

Results and discussion

To establish the effects of the thermal treatment on the framework composition, multinuclear NMR analyses were first performed on the parent zeolite samples and their corresponding H-form samples. 1H/27Al TRAPDOR NMR was used to characterize the nature of the hydroxyl groups, 27Al MAS NMR to investigate the coordination states of the aluminum atoms, and 29Si MAS NMR to determine the number of Si and Al atoms attached to the silicon unit considered.

Conclusion

The probe molecules TMP and TMPO were employed to detect the inhomogeneity of Brønsted acidity in H-mordenite and H-β zeolites. Solid-state 31P MAS NMR investigation of H-MOR and H-β zeolites loaded with TMPO reveals multiple 31P resonances, resulting from the reaction of TMPO and different types of Brønsted acid sites. Thus, the probe molecule TMPO can be used to gain valuable insight into the Brønsted acid strength distribution in H-mordenite and H-β zeolites. Our results demonstrate that

Acknowledgements

Support from the National Science Council of Taiwan, Republic of China is gratefully acknowledged.

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