Zeolites as Base Catalysts

ZEOLITES AS BASE CATALYSTS. PREPARATION OF CALCIUM ANTAGONISTS INTERMEDIATES BY CONDENSATION OF BENZALDEHYDE WITH ETHYL... 

Corma, A., Pomes, V., Martin-Aranda, R.M., Garcia, H., and Primo, J. 1990. Zeolites as base catalysts Condensation of aldehydes with derivatives of malonic esters. Applied Catalysis. 59, 237-248. 

C orma, A Martin-Aranda. RM Sanchez. F. Zeolites as base catalysts condensation of ben/aldchydc derivatives with activated methylcnic compinmds on germanium-substituted IauyjiS e. Journal of Catalysis, 1990 126. 192-198. 

Alkali metal ion-exchanged zeolites and occluded alkali metal oxide zeolites have been investigated extensively and applied as basic catalysts for a variety of organic transformations (1,41,221,222). Zeolites modified with alkaline earth compounds have been applied much less frequently as base catalysts for organic reactions. 

Microcrystalline solids such as zeolites and zeolite like structures have shown the utility of those properties in the domain of acid catalysis. However, little is known on their possibilities as base catalysts. It has been shown [ref. 1,2] that zeolites have basic sites which are able to catalyze reactions needing weak and medium basic strengths. Moreover, a correlation between the basicity and the Sanderson s average electronegativity Df the framework has been observed [ref. 3], Then, their activity as base catalysts can be modified by changing the countercation [ref. 4], the framework Si/Al ratio, or by introducing atoms other than Si and Al in the framework [ref. 5],... 

It is, of course, one thing to want to emplace the hydrogenation function in a controllable manner, and another to be able to do it in practice. Industrial zeolite-Y based catalysts contain a binder, usually alumina or an ASA/alumina mixture. In this case one can direct Pd to the alumina phase by employing PdCL as the metal... 

EBMax is a liquid phase ethylbenzene process that uses Mobil s proprietary MCM-22 zeolite as the catalyst. This process was first commercialized at the Chiba Styrene Monomer Co. in Chiba, Japan in 1995 (16-18). The MCM-22-based catalyst is very stable. Cycle lengths in excess of three years have been achieved commercially. The MCM-22 zeolite catalyst is more monoalkylate selective than large pore zeolites including zeolites beta and Y. This allows the process to use low feed ratios of benzene to ethylene. Typical benzene to ethylene ratios are in the range of 3 to 5. The lower benzene to ethylene ratios reduce the benzene circulation rate which, in turn, improves the efficiency and reduces the throughput of the benzene recovery column. Because the process operates with a reduced benzene circulation rate, plant capacity can be improved without adding distillation capacity. This is an important consideration, since distillation column capacity is a bottleneck in most ethylbenzene process units. The EBMax process operates at low temperatures, and therefore the level of xylenes in the ethylbenzene product is very low, typically less than 10 ppm. 

EF material free, alkali exchanged zeolites are used as quite mild basic catalysts. Light alkali and alkaline earth metal zeolites, such as Na-X, Na-Y [165], alkali-MOR, Na-A and Ca-A [166], have a mild Lewis acid behavior and do not appear to have strong basic character. The same occurs for Na-silica-alumina [167]. However, heavy alkali metal zeolites such as Cs-Y actually act as base catalysts, or rather as acid-base catalysts, for example for toluene side-chain alkylation. Stronger basic character arises from impregnation of alkali zeolites with alkali salts, later... 

There has been a large volume of data showing the benefit of having thin dense membranes (mostly Pd-based) on the hydrogen permeation rate and therefore the reaction conversion. An example is catalytic dehydrogenation of propane using a ZSM-5 based zeolite as the catalyst and a Pd-based membrane. Clayson et al. [1987] selected a membrane thickness of 100 m and achieved a yield of aromatics of 38% compared to approximately 80% when a 8.6 pm thick membrane is used [Uemiya et al., 1990]. 

The resistance of new zeolitic catalysts to temporary and permanent catalyst poisons is essential to the economic and commercial success of a zeolitic based cumene process. The following commercial data obtained using beta zeolite as a catalyst illustrates the outstanding ability of beta zeolite to cope with a wide range of feedstock contaminants ... 

The reaction pathway of benzene alkylation with propylene catalyzed by acids is very similar to that already reported for EB. The main difference is represented by the tendency of cumene to isomerize to n-propylbenzene, which is thermodynamically more stable at increased temperature. Also, cumene can undergo further alkylation to diisopropylbenzene (DIPB), which could be recovered by transalkylation with benzene to give cumene. The transalkylation reaction requires a higher temperature than the related alkylation. In addition, not all of the alkylation catalysts are suitable for transalkylation. Beta or dealuminated mordenite are suitable catalysts for transalkylation. The first industrial demonstrations of cumene technologies based on zeolite catalysts were started-up in 1996 by Mobil-Raytheon, EniChem and UOP, independently. In 2001, worldwide, 14 cumene units were already operating with zeolite catalysts. Around 98% ofcumene is used to produce phenol and expected world production of cumene in 2008 is around 9 million tons. For cumene, among the 40 units in the world (2004), 14 cumene plants were in operation with zeolite catalysts [222]. Today over 70% of cumene plants use a zeolite as the catalyst. 

Most of the previous work related to catalysts for chlorinated VOC abatement is focused on the development of two type of catalysts, namely those based on noble metals and on transition metal oxides. By contrast, the utility of zeolites as effective catalysts for the decomposition of chlorinated organics has not been explored in detail, when it is reported that metal loaded catalysts employed in commercial applications are susceptible to deactivation by the HCl and CI2 produced during reaction [4]. In our previous works [5,6] it was found that H-zeolites showed a high activity for chlorinated VOC destruction under dry and humid conditions, and that their activity was controlled by the presence of... 

It has been reported that zeolites can be used as base catalysts when exchanged with alkali metal ions. The base strength is inversely related to the charge to radius ratio of the compensating cation, i.e. the larger cation the stronger the basicity of the associated framework oxygen of the zeolite. 

Nitrided zeolites were prepared by thermal treatment of ultrastable zeolite Y (Si/Al ratio equal to 13) under ammonia flow for a prolonged time. The effects of the nitridation parameters on the extent of the nitrogen incorporation and on the structural and catalytic properties of the resulting zeolites were investigated. The amount of nitrogen incorporated in the zeolite increases with temperature and duration. Structural order and porous volume are preserved if the nitridation temperature and the nitridation time do not exceed 800°C and 72 h, respectively. Nitrided zeolites are base catalysts as revealed by the reaction of Knoevenagel condensation between benzaldehyde and malononitrile. 

Alkali and alkaline-earth exchanged zeolites or zeotypes (c.g. MCM-41) [15, 39, 130, 141-155] have been extensively used as base catalysts. However, they are w eak bases. Alternatively, either alkali metal or alkali and alkaline-metal oxides can be occluded into zeolites and zeotypes in order to increase their basicity [18,63, 154, 156-161]. This is usually referred to as the introduction of a basic guest into a zeolite host. Moreover, the loading of Yterbium or Europium metal on zeolites, thus yielding strong bases, has been reported [17]. 

The examples given below are mainly on zeolites and oxides, but the acid-base properties of all kinds of catalyst surfaces can be examined using adsorption calorimetry, including, for example, activated carbons [72], which are often used as supports of active phases due to their very high surface-area, oxynitrides [73] or hydrotalcites [74-76], which act as base catalysts [73], or heteropolyacids, which behave as strong acid catalysts [77],... 

All of the reactions discussed till now have been catalyzed by zeolites functioning as Bronsted or Lewis acids. In a major new development, zeolites have also been tailored to act as base catalysts (Hathaway and Davis, 1988a,b,c Tsuji et al., 1991). A superbase catalyst of this type has now found application in the synthesis of the key intermediate, 4-methylthiazole, used in the preparation of the anthelmintic, thiabendazole. The existing industrial route for 4 methylthiazole (18) involves using several hazardous chemicals such as chloroacetone and carbon disulfide. The new zeolite-based route uses the base-catalyzed reaction of SO2 with the imine from acetone (reaction 6.12). 

Zeolite membranes indicate inorganic membranes with a selective/cata-lytic layer composed of a zeolite which is crystalline aluminosilicate with the feature of a high ordered porous structure with size comparable to molecular dimension. An example of the use of zeolites as a catalyst in a multi-phase membrane reactor can be found in Shukla and Kumar (2004) who have immobilized a lipase on a zeolite-clay composite membrane by using glu-taraldehyde as a bifunctional ligand in order to carry out the hydrolysis of olive oil. An application of a zeolite-based membrane in a three-phase membrane reactor has been reported by Wu et al. (1998), where TS-1 zeoUte crystallites were embedded in a polydimethylsiloxane (PDMS) membrane in order to catalyse the oxyfunctionalization of n-hexane (from a gas phase) with hydrogen peroxide (from a liquid phase). 

Zeoliltes seem particularly suited to take over the job and in fact are doing so already for aromatic alkylation. Thus in ethylbenzene manufacture (from benzene and ethene) modern processes apply zeolites (H-ZSM-5, H-Y) as the catalyst, substituting conventional processes based on AICI3 or BF3-on-alumina catalysis. Substantial waste reductions are achieved. 

ZeoHte catalysts and adsorbents are widely accepted in industry. Commercial adsorbents based on synthetic aluminosihcates zeolite A and X became available in 1948 [4]. Zeolite Y as FCC catalyst became commercially available in 1964 [5]. 

Another recent new application of a microporous materials in oil refining is the use of zeolite beta as a solid acid system for paraffin alkylation [3]. This zeolite based catalyst, which is operated in a slurry phase reactor, also contains small amounts of Pt or Pd to facilitate catalyst regeneration. Although promising, this novel solid acid catalyst system, has not as yet been applied commercially. 

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