Catalytic activity of Brønsted acid sites in zeolites: Intrinsic activity, rate-limiting step, and influence of the local structure of the acid sites
Introduction
Acid-catalyzed hydrocarbon transformations, such as cracking, isomerization, and alkylation, are very important reactions in the petrochemical industry. Zeolites are used in several of these reactions, as well as in many reactions in the production of fine chemicals. The zeolitic Brønsted acid sites are the active species in many of these reactions [1], [2]. Therefore, characterizing the Brønsted acid sites in zeolites and determining the structure–activity relationship are of fundamental importance in understanding and improving zeolites for use as catalysts.
Alkanes can be activated by two mechanisms, depending on the reaction conditions [3], [4], [5]: a monomolecular mechanism and a bimolecular mechanism. In the monomolecular mechanism, an alkane is protonated by a Brønsted acid site [6], [7], [8], [9], [10] to form a five-coordinated carbon atom. This carbonium ion may undergo cracking to yield an alkane and an alkene, regenerating the acid site, or it may dehydrogenate to yield H2 and an alkoxide species [11]. Desorption of the alkoxide yields an olefin and regenerates the acid site. Alternatively, alkoxides may initiate bimolecular reactions. Scheme 1 shows the monomolecular reactions of propane. Reaction (1) represents cracking, with a rate-determining step of protonation [12]; reaction (2) represents dehydrogenation, with a rate-determining step of olefin desorption [13]. Low reactant pressure, low conversion, and high temperature favor the monomolecular reaction, and the resulting product distribution is simple. In the bimolecular mechanism, however, an alkane is activated by hydride transfer between the alkane and the adsorbed alkoxide species [9], [14], [15], [16], [17], [18], [19], [20], [21]. This reaction may be followed by β-scission or isomerization [3], [4], [22]. The alkoxide species can be formed in the monomolecular dehydrogenation reaction or by adsorption of olefins present in the reactant feed or formed early in the reactor. The rate-limiting step in the bimolecular mechanism is hydride transfer after initial formation of the alkoxide species [11]. High reactant pressure, high conversion, and low temperature favor the reactions in the bimolecular mechanism, and, due to the dimerization, oligomerization, isomerization, and β-scission reactions, the product distribution is complicated.
Monomolecular conversion of small alkanes is a good reaction for characterizing zeolites and establishing the relationship between their structure and catalytic activity. The reactants are physisorbed in the pores of the zeolites and at high temperature are activated through proton transfer from the Brønsted acid sites. Because the rate-limiting step of monomolecular cracking is the protonation of the alkane, this reaction is an acid–base reaction between the zeolite and the alkane. Therefore, its intrinsic rate is a measure of zeolitic acidity. The intrinsic activation energy () equals the apparent activation energy () minus the heat of reactant adsorption () [23] as follows: In contrast, the rate-limiting step in the dehydrogenation reaction is desorption of the olefins [13]. The stability of the alkoxide species depends strongly on the local neighborhood of the Brønsted acid site [24], and the intrinsic reaction parameters of the dehydrogenation reflect the stability of the alkoxide species.
Several studies have used the monomolecular cracking of hexane to characterize the Brønsted acid sites of different zeolites [25], [26], [27]. The results show that different zeolites have different activities. These differences were interpreted as being due to the different heats of reactant adsorption () [25], [27] and to the different acid strengths of the Brønsted acid sites in various zeolites [26]. Here we report the intrinsic activity of the Brønsted acid sites and the influence of the local structure of the Brønsted acid sites by determining the cracking and dehydrogenation pathways of the conversion of propane. The influence of zeolite type and framework Si/Al ratio were studied.
Section snippets
Catalyst preparation
Four different structure types (MFI, MOR, Beta, and FAU) of zeolites were used. Table 1 lists the samples and their physical properties. ZSM5 was prepared by hydrothermal synthesis [28]. As-made ZSM5 was calcined in air at 823 K for 5 h to remove the structure-directing agent and then converted to the NH4 form by triple ion exchange with 1 M NH4NO3 solutions at 353 K. MOR samples with Si/Al = 4.9, 9.9, and 16.7 in the Na form were obtained from Tosoh. Three Beta samples with Si/Al ratios of 10.5,
Results
Fig. 1 shows the 27Al MAS NMR spectra of the zeolite samples. For all of the samples, only a single peak at around 60 ppm was observed, assigned to a tetrahedrally coordinated framework Al. The positions of the peaks of the samples differed because of the varying average AlOSi angles in the structures. None of the samples showed an octahedral coordination typical of nonframework Al.
The Arrhenius plots of the monomolecular conversion of propane over the MOR and Beta samples with different Si/Al
Intrinsic activity per site: cracking
In protolytic cracking, alkane protonation is the rate-limiting step [13]. Because this involves the transfer of a proton to a weak base, the intrinsic activity of cracking is a means of measuring the acid strength of the Brønsted acid sites [1], [31], [32], [33], [34]. All zeolite samples used in this study have only tetrahedrally coordinated aluminium, as shown by 27Al MAS NMR (Fig. 1), which is the framework Al that generates the Brønsted acid site. Thus, the catalytic results are not
Conclusion
Different reactions depend differently on the strength and local structure of the Brønsted acid sites in zeolites. The activity of Brønsted acid sites in zeolites is strongly affected by the rate-limiting step of the reaction. The monomolecular cracking of alkanes proceeds via protonation of the alkane as the rate-limiting step. This reaction is affected by the size and shape of the pores that affect the heat of adsorption; the intrinsic reaction parameters are much more similar than the
Acknowledgements
The authors thank Anuji Abraham for assisting with the 27Al MAS NMR measurements and Hye Ja Jung for preparing the Beta samples.
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