Cu-doped zeolites for catalytic oxidative carbonylation: The role of Brønsted acids
Graphical abstract
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Highlights
► Nature of Brønsted acidic sites in Cu-doped zeolites was investigated. ► A quantitative relationship was drawn between Brønsted sites and reactivity. ► Effects of Brønsted acidic sites on anchoring Cu sites were examined.
Introduction
The rapid evolution of green chemistry has spurred tremendous interests on clean and safe synthetic routes and thus development of new catalysts. Production and applications of organic carbonates (e.g., dialkyl carbonates) are directly concerned within the frame of this principle. They are environmentally benign compounds with versatile applications in chemical synthesis [1]. Due to their low toxicity and bio-accumulation, they have been widely used as solvents [2] and oxygen-containing fuel additives. Oxidative carbonylation has been considered as one of most promising approaches to replace the conventional phosgene method to produce dialkyl carbonates (e.g., dimethyl carbonate (DMC) [3], [4], [5], [6], [7] and diethyl carbonate (DEC) [8], [9], [10]). Recently, DEC has been proposed as an attractive oxygen-containing fuel additive to replace methyl tert-butyl ether (MTBE) because of its high oxygen content (40.6 wt%) and favorable gasoline/water distribution coefficients, which are superior to alternatives such as DMC and ethanol. Thus, it has attracted increased attention to develop a gas-phase process for producing DEC (Eq. (1)).2CH3CH2OH + CO + 1/2O2 = (CH3CH2O)2CO + H2O
Zeolites have been extensively employed as catalytic materials for synthesis of a variety of chemicals due to their unique structural properties and tunable acidity [11]. Bridging hydroxyl groups (e.g., AlOHSi) on zeolites that function as Brønsted acidic sites play an important role in catalysis, ion exchange, adsorption, and other practical processes [12], [13], [14]. Recently, Cu-doped zeolites prepared by solid-state ion exchange (SSIE) with CuCl have received considerable interests in catalysis, particularly for oxidative carbonylation [15]. Since Cu-doped zeolites via a conventional ion exchange with aqueous solutions of cupric ions usually contain coexistence of copper ions in different aggregation and oxidation states, it is difficult to elucidate the structural properties of Cu active sites. By contrast, catalysts prepared via the SSIE have been considered as model solids, containing only isolated copper species in a single oxidation state [16]. Additionally, high exchange degree can be achieved in a single step via SSIE, and Cu ions can enter narrow pores and anchor at cavities more easily relative to exchange in aqueous solution. It has been generally recognized that the Brønsted acidic sites on zeolite can be exchanged by Cu+ ions during the SSIE process (Eqs. (2), (3)) [17], [18], [19]. The formed surface-bound Cu+ was proposed to be active species for the carbonylation reaction.
Therefore, the nature of Brønsted acidic sites in zeolites, which depends on the bond angle of SiOAl [13], composition of the framework (e.g., SiO2/Al2O3 ratio), and degree of dispersion of the Al ions [20], could influence the exchange degree and local environment of Cu species in the catalysts. We note that the local environment and concentration of Cu [21], residual Brønsted acidity [22], [23], and channel structure [24] of the zeolite supports are all crucial factors to determine catalytic performance. Therefore, architecture of channel and acidity are always considered in parallel. In order to solely determine the role of Brønsted acidity, it is necessary to design experiments for eliminating the influences of other factors such as channel structure. This paper, therefore, describes an investigation exploring the relationship between Brønsted acidity of zeolite supports and catalytic performance for oxidative carbonylation.
Section snippets
Chemical reagents
Na-FAU zeolites with different SiO2/Al2O3 molar ratios were purchased from XinNian Petrochemical Additives Company (Shanghai, China) and Na-β was provided by NanKai Catalyst (Tianjin, China). Reagents include cuprous chloride (AR, GuangFu Chemical Research Institute), NH4NO3 (≥99%, Standard Technology Company, Tianjin, China), ethanol, tetraethyl orthosilicate, pyridine and n-hexane (AR, Tianjin Kermel Chemical Co., Ltd.), potassium bromide (Spectrosol, VWR International, Inc.), and nitrogen,
Catalyst preparation
The ammonium form of zeolites were obtained via exchanging Na-zeolites (SiO2/Al2O3 = 2.5, 7.1, 10.1, 19) with a 0.5 M NH4NO3 solution twice at 333 K and dried at 393 K for 4 h under vacuum. The obtained NH4-zeolites were calcined at 773 K for 3 h in air at 2 K/min. The mixture of protonated zeolites and fresh CuCl with a mass ratio of 2 was exposed to N2 (99.999%) flowing at 60 ml/min, and the temperature was ramped at 2 K/min to 773 K and kept for 6 h. The sample was then cooled down to room temperature
Results and discussion
FAU zeolites offer good activity for oxidative carbonylation as supports among microporous materials, which possess characteristics such as a wide range ratio of SiO2/Al2O3, relative large pore diameter, and unique 3-dimensional channel structure. Therefore, to exclude the influence of crystal structure and geometry, we first prepared Cu-doped FAU zeolites with varied SiO2/Al2O3 molar ratios (i.e., tuning surface acidity) employed the SSIE method, on which catalytic behaviors were examined for
Conclusions
In summary, we have established a quantitative relationship between amount of Brønsted acidic sites and catalytic activities for oxidative carbonylation. Considering aggregation and accessibility of Cu species on zeolites, it is more reasonable to relate the Brønsted acidic sites to catalytic activity rather than Cu contents. Brønsted acidic sites on both internal surface and external surface of zeolite, which exchange with Cu+ during the SSIE process, form active sites for the reaction of
Acknowledgments
Financial support from the National Natural Science Foundation of China (20876112, 20936003), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (grant no. 20090032110021), the Program for New Century Excellent Talents in University (NCET-04-0242), Seed Foundation of Tianjin University (60303002), and the Program of Introducing Talents of Discipline to Universities (B06006) are gratefully acknowledged.
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