Elsevier

Applied Catalysis A: General

Volumes 417–418, 29 February 2012, Pages 236-242
Applied Catalysis A: General

Cu-doped zeolites for catalytic oxidative carbonylation: The role of Brønsted acids

https://doi.org/10.1016/j.apcata.2011.12.043Get rights and content

Abstract

The aim of this work was to establish the role of Brønsted acidic sites in Cu-doped zeolites for the oxidative carbonylation of ethanol to diethyl carbonate (DEC). In order to eliminate the influences of other factors such as channel structure, faujasite (FAU) with different SiO2/Al2O3 ratios and Beta before and after passivation were investigated. Fourier Transform infrared spectroscopy (FTIR) of pyridine adsorption indicated that all of Brønsted acidic sites were exchanged with CuCl to form effective active sites for oxidative carbonylation. Characterization data showed that passivation on Beta zeolites has little effect on surface area and pore volume distribution except Brønsted acidic sites. The relationship between Brønsted acidic sites and catalyst activity was built on different types of zeolite supports. Diffuse reflectance infrared Fourier transform spectroscopy (DIRFTS) of CO adsorption revealed that the location of Cu active sites was related to the distribution of Brønsted acidic sites, which also influenced catalytic performance of oxidative carbonylation.

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., Alsingle bondOHsingle bondSi) 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 Sisingle bondOsingle bondAl [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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