J. Phys. Chem. B 1998, 102, 10879-10884
10879
Modification of Potassium-Containing Zeolites Loaded with Mo3S44+ Clusters by Solid-State Reaction between Zeolites and KCl Ming Jiang Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, Anhui 230026, Peoples Republic of China
Takashi Tatsumi*,† Department of Applied Chemistry, Graduate School of Engineering, The UniVersity of Tokyo, Hongo, Tokyo 113-8656, Japan ReceiVed: July 9, 1998; In Final Form: October 20, 1998
By aqueous ion exchange, cationic molybdenum sulfide Mo3S44+ clusters have been successfully introduced into KY and KL without destruction of the zeolite structures as verified by the measurements of X-ray diffraction and low-temperature nitrogen adsorption. Infrared and mass spectra indicate that a large quantity of acid sites is produced in the zeolites during the preparation and activation of the samples. However, through solid-state ion exchange of the zeolites with KCl, the produced acid sites are largely removed upon activation of the samples in vacuo or in helium flow at high temperatures. During sulfidation of the KCl-added samples, the solid-state reaction occurs to a greater extent without loss of zeolite crystallinity, eliminating nearly all the acidic protons and partly suppressing the production of Lewis acid sites in the zeolites. The KCl-added catalysts produce a significant amount of alcohols in CO hydrogenation. This product distribution is considered to reflect the decrease in the acid sites in the zeolites by the solid-state ion exchange.
1. Introduction Molybdenum-based catalysts have been widely studied because of their excellent properties in a variety of reactions, e.g., hydrodesulfurization, hydrodenitrogenation, methanation syntheses of light hydrocarbons and mixed alcohols from CO hydrogenation, and dehydroaromatization of methane. As a result of the well-known specific properties of zeolites, numerous methods have been attempted to introduce molybdenum species into zeolites.1-5 Impregnation of ammonia heptamolybdate on zeolites has been applied to the preparation of molybdenum-loaded zeolites but often gives rise to a poor dispersion of molybdenum species on zeolites.1 Aqueous ion exchange is an effective method for introducing transition metal cations into zeolites. Molybdenum-loaded Y zeolites have been prepared with Mo2(ethylenediamine)4Cl4 and MoO2Cl2 solutions.2,3 So far however, only a few studies have succeeded in incorporating molybdenum into zeolites by means of aqueous ion exchange since cationic molybdenum species usually exist in high oxidation states in strongly acidic solutions in which the structures of many zeolites undergo destruction. Alternatively, Dai and Lunsford succeeded in introducing molybdenum species into zeolites by solid-state reactions of HY and ultrastable HY with MoCl5.4 Thermal decomposition of molybdenum hexacarbonyl adsorbed into zeolites has also been frequently used for preparation of molybdenum-loaded zeolites.5 Molybdenum sulfide compound, [Mo3S4(H2O)9]Cl4, is stable in air and soluble in aqueous solution. 6-8 The cationic Mo3S44+ * To whom correspondence should be addressed. † Present address: Division of Materials Science & Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. Phone, fax: +81-45-3393943. E-mail:
[email protected].
clusters with incomplete cubane-type structures can be introduced as sulfide species into zeolites (e.g., NaY, HUSY, KL, NaMOR, NaHβ) by conventional aqueous ion exchange.9,10 The performance of the zeolites loaded with Mo3S44+ clusters has been tested for CO hydrogenation. The products deviate from the typical Schulz-Flory distribution with the selectivity of C2 hydrocarbons as high as about 50% based upon the calculation of carbon efficiency. Besides the hydrocarbons, however, no oxygenate products, e.g., alcohols, have been detected.10 Alkali-promoted, especially potassium-promoted molybdenum catalysts, have been extensively investigated for synthesis of mixed alcohols from CO hydrogenation.11 It has been recognized that properties of catalysts for yielding alcohols are influenced by the basicity of promoters and/or catalysts themselves.12 The acidity/basicity of zeolites can be adjusted, to some extent, by varying the cations in zeolites. It has been shown that K+, Rb+ and Cs+ cation-exchanged zeolites exhibit higher basicity than NaY.13,14 In aqueous solution, however, due to strong hydration of the cations, Rb+ and Cs+ are too large to penetrate into the small cages of zeolites (e.g., β cages in Y zeolites). It is therefore difficult to obtain Rb+ and Cs+ zeolites with high exchange degrees by aqueous ion exchange, although they are more basic than K+-containing zeolites.15,16 In this work, KY and KL were used as supports for loading the cationic Mo3S44+ clusters. However, the test reaction of CO hydrogenation over the samples indicates that no oxygenate products can be obtained (see, e.g., ref 10). The restriction of zeolite pore structures where active phases of molybdenum sulfide clusters stay seems not crucial for producing oxygenates with low carbon numbers (e.g., methanol and ethanol). Thus, it is implied that the basicity of potassium-cation-containing zeolites is not high enough for promoting the formation of oxygenates, on one hand,
10.1021/jp982931r CCC: $15.00 © 1998 American Chemical Society Published on Web 12/04/1998
10880 J. Phys. Chem. B, Vol. 102, No. 52, 1998
Jiang and Tatsumi
and that the acid sites may be produced in the zeolites during activation of the incorporated Mo3S44+ species, on the other hand. During ion exchange of the zeolites with Mo3S44+, the solution is rather acidic with a pH value of 3-4.5, which may also bring about the production of acid sites in the zeolites. These findings prompt us to investigate in detail the acid properties of the zeolites loaded with Mo3S44+ clusters. To suppress the production of the acid sites in the zeolites, KCl was added to the samples and allowed to react with the zeolite protons by solid-state ion exchange at high temperatures. The production and elimination of the acid sites in the zeolites after loading Mo3S44+ species were verified by means of techniques of infrared and mass spectroscopies, X-ray diffraction, and lowtemperature nitrogen adsorption. 2. Experimental Section Zeolite KY was prepared by 5-fold aqueous ion exchange of NaY (FAU, SK-40, Nikka Seiko; Si/Al ) 2.3) in 0.1 mol dm-3 KCl solution. The exchange degree was 98% based upon chemical analysis of the zeolite. KL (LTL, Tosoh, TSZ500KOA; Si/Al ) 3.1) was used as supplied. The molybdenum sulfide compound, [Mo3S4(H2O)9]Cl4, was synthesized according to the procedures described by Shibahara et al.8 The zeolites loaded with Mo3S44+ clusters, denoted as Mo3S4-KY and Mo3S4-KL, were prepared by aqueous ion exchange as described before.9,10 The molybdenum contents in the two samples were 5.0 and 2.5 wt %, respectively. The mixtures of Mo3S4-KY and Mo3S4-KL with KCl with an atomic K/Mo ratio of 1.2 were prepared by a suspension method in order to avoid possible destruction of the zeolite structure and oxidation of Mo3S44+ clusters caused by usual grinding in an agate mortar in the air. KCl was ground in a mortar and mixed with Mo3S4-KY and Mo3S4-KL in hexane solvent. The resulting suspension was stirred magnetically for 3 h and then allowed to stand for about 0.5 h. The solvent was decanted, and the residual hexane in the mixtures was evaporated to dryness at room temperature. The resulting samples were denoted as Mo3S4-KY-KCl and Mo3S4-KL-KCl. The activation of the samples was performed in vacuo (
