Detection of the inhomogeneity of Brønsted acidity in H-mordenite and H-β zeolites: a comparative NMR study using trimethylphosphine and trimethylphosphine oxide as P NMR probes
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 H, C, and N 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., P 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 C or N, due to its higher magnetogyric ratio and natural abundance of 100%. In addition, binding of P-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 P 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, H 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 H 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 H 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 P 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 H 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. H/Al TRAPDOR NMR was used to characterize the nature of the hydroxyl groups, Al MAS NMR to investigate the coordination states of the aluminum atoms, and Si 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 P MAS NMR investigation of H-MOR and H-β zeolites loaded with TMPO reveals multiple P 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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