Zeolites fine chemicals synthesis

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

However, some characteristics of the reactant and product (and solvent) molecules used in the reactions of fine chemical synthesis have to be considered their large size, their low thermal stability, and their polarity. The small size of the micropores limits the use of zeolites to the synthesis of relatively small molecules solutions to overstep this limitation have been found with the development of larger (mesoporous) molecular sieves and of nanocrystallite zeolites, the reaction then occurring on the large external surface. The low thermal stability of the molecules leads to operation at a low ternperamre, often in the liquid phase. Last, as will be shown in the first example (Section 12.2.3.1), the differences in polarity between reactant(s), product(s), and solvent molecules have to be considered for optimizing both the zeolite catalyst and the operating conditions. 

The three themes of the symposium selective hydrogenation, selective oxidation and acid-base catalysis were introduced by four plenary lectures and two invited communications. A panel concerned with the future of zeolites and other shape-selective materials for fine chemical synthesis was conducted by specialists in the field D. Barthomeuf (University of Paris 6), E. Derouane (University of Namur), L. Forni (University of Milan), M. Gubelmann (Rhone-Poulenc, St Fons), W. Hoelderich (BASF, Ludwigshafen) and G. Perot (University of Poitiers). An exhibition of equipment was held during the symposium on October 3 and 4. Over 20 firms exhibited equipment, chemicals and catalysts which were of interest to researchers involved with the synthesis of functional compounds by heterogeneous catalysis. 

Although zeolites have been known for their adsorption properties for over a century, it was not until 1952, when the first synthetic zeolite was prepared, that their utility in chemical transformations was explored. Since that time, zeolites have been used for a multitude of purposes, and to this day, they are essential catalysts in the petroleum industry, converting large and small hydrocarbons into high-octane compounds. As an outgrowth of this work, zeolites have found utility in industrial fine chemical synthesis for the construction of aromatics, heterocycles, aliphatic amines, and ethers, and the photochemistry within zeolites has already grown out of its infancy. 

Applications of Zeolites and other Microporous and Mesoporous Solids to Catalytic Fine Chemical Synthesis... 

Therefore the association in the same team of experts in organic chemistry and in catalysis on zeolites is ideal to develop zeolite catalysts for Fine Chemicals synthesis. The organic chemists bring the necessary knowledge about reaction mechanisms (with therefore the possible prediction of secondary products, etc.) and advanced methods in organic analysis and product purification the specialists in catalysis on zeolites orient the choice of the zeolite catalysts and of activation procedures as well as the selection of the reactor and operating conditions. Moreover, the researchers should have a two-fold culture with, in addition to a... 

The use of solid catalysts and especially zeolites in Fine Chemical synthesis introduces another complication with respect to homogeneous reactions. There is always a progressive decrease of the catalyst activity with increasing reaction time.1191 In some reactions, this deactivation can be due to irreversible chemical transformation of the zeolite catalyst, e.g. reactions with acid reactants causing dealumination and sometimes collapse of the framework. However, in most cases, deactivation results from poisoning of the active sites by the desired reaction... 

Figure 2.5 Relative occupancy (%) of the intracrystalline volume of a H-BEA zeolite during the transformation of a 2 1 molar anisole - acetic anhydride mixture in a batch reactor, assuming no adsorption of acetic acid and full occupancy of the micropores. Anisole ( ), acetic anhydride (o) and 4-methoxyacetophenone (x). Reprinted from Journal of Catalysis, Vol. 187, Derouane et al., Zeolite catalysts as solid solvents in Fine Chemicals synthesis 1. Catalyst deactivation in the Friedel-Crafts acetylation of anisole, pp. 209-218, copyright (1999), with permission from Elsevier...

Competition between reactant, solvent and product molecules for adsorption within the zeolite micropores is demonstrated directly (adsorption experiments) and indirectly (effect of the framework Si/Al ratio on the activity, kinetic studies) to occur during Fine Chemical synthesis over molecular sieve catalysts. This competition, which is specific for molecular sieves (because of confinement effects within their micropores), adds up to the competition which exists over any catalyst for the chemisorption of reactant, solvent and product molecules on the active sites. Both types of competition could affect significantly the activity, stability and selectivity of the zeolite catalysts. Although the relative contributions of these two types of competition cannot be estimated, the large change in the activity of the acidic sites (TOF) with the zeolite polarity seems to indicate that the competition for adsorption within the zeolite micropores often plays the major role. 

There are different reasons leading to deactivation of zeolite and mesoporous molecular sieves during Fine Chemical synthesis ... 

Derouane, E. G., Dillon, C. J., Bethell, D., Derouane Abd-Hamid, S. B. Zeolite catalysts as solid solvents in fine chemicals synthesis-1. Catalyst deactivation in the Friedel-Crafts acetylation of anisole, J. Catal., 1999, 187, 209-218. 

The unique properties of zeolites and other micro- or mesoporous solids that may favour their application to fine chemical synthesis are (1) the compatibility between the size and shape of their channels or cavities with the size of the reactants and/or products (generally referred to as molecular shape selectivity) that may direct the reaction away from the thermodynamically favoured route (2) the occurrence of confinement effects increasing the concentration of reactants near the catalytic sites and (3) the ability to tune their catalytic properties (acidic, basic, or other) via various treatments as described in this Volume. 

Chapter 1 is a general overview of zeolite, zeotype and mesoporous solids chemistry, including their design, synthesis and general catalytic properties. Chapter 2 deals with the problems and pitfalls that may occur in the applications of zeolites and other microporous and mesoporous solids to fine chemical synthesis. The remaining chapters deal with specific applications of these catalysts to fine chemical synthesis. 

All these aspects were thoroughly discussed by lecturers and participants during the round table organized during the Poitiers School on The Future Trends in Zeolite Applications . Special emphasis was placed on the role played by the sites at the external surface (pockets, etc.) or at the pore mouth, by mesopores, extraframework aluminum species, as well as by the polarity of reactant and product molecules. Other important topics dealt with the remarkable catalytic properties of BEA zeolites for fine chemical synthesis, the potential of mesoporous molecular sieves, zeolitic membranes and the role of combinatorial catalysis in the development of zeolite catalysts. It is our hope that the fruits of these discussions will appear in the literature or even better as new and environmentally friendly products or processes. 

Metal oxides are widely used as catalyst supports but can also be catalytically active and useful in their own right. Alumina, for example, is used to manufacture ethene from ethanol by dehydration. Very many mixed metal oxide catalysts are now used in commercial processes. The best understood and most interesting of these are zeolites that offer the particular advantage of shape selectivity resulting from their narrow microporous pore structure. Zeolites are now used in a number of large-scale catalytic processes. Their use in fine chemical synthesis is discussed in Chapter 2. 

Catalytic behaviors of solid base catalysts for fine chemicals synthesis as well as the fundamental reactions are described. The reactions included are double bond isomerization of olefins, addition of hydrogen and amines to conjugated dienes, dehydration, dehydrogenation, reduction, alkylation, aldol addition and condensation, Wittig-Horner and Knoevenagel reactions, dehydrocyclodimerization, and ring transformation. The characteristic features of different types of solid base catalysts, zeolites, metal oxides, solid superbases and non metal-oxides, are summarized. 

In 1999, Tanabe and Hoelderich have reviewed 74 industrial processes ranging from cracking to fine chemical synthesis in which zeolites are used as a catalyst. 

Last but not least, as easily recoverable and reusable catalysts, zeolites also allow to perform acid-mediated transformations with a clear environmental benefit, since it avoids the formation of huge amounts of salts which is typically encountered in acid catalyzed processes. Noteworthy, zeolites can also host active catalytic species such as metals or organometallic complexes for fine chemical synthesis. 

The discovery that the protonated forms of zeolites could be used as active, stable and shape selective catalysts in hydrocarbon transformations has been of immense benefit to the refining and petrochemicals industry. The need for optimised microporous acid catalysts will continue as fuel specifications change and the requirements of the chemicals market shift. The likely growth in demand for synthetic fuels, including diesels, is one expected trend that could involve zeolite catalysts. Diverse feedstock chemicals and fine chemicals synthesis involving zeolite catalysts are also being developed. 

In the following section, methods for the fabrication and deposition of Pd-based and zeolite MMs are discussed, as well as applications in (de) hydrogenation, SR, WGS, partial oxidation (POx) reactions and fine chemical synthesis. The research on Pd-based MMRs for hydrogen separation, purification and production (by dehydrogenation, SR and WGS reactions) has been selected as a case study, as significant research and, therefore, much information can be found in the literature on this field. 

Since 2004, many articles on preparation of zeolite MMs have been published, on such areas as MFI or Sil-1 zeolite etched on the Si substrate for gas separation applications, and MMRs for KCR and fine chemical synthesis (Coronas and Santamaria, 2004 Kwan et al, 2010 Wan et al., 2001 Yeung et al, 2005). Coronas and Santamaria (2004) have reported on the use of zeolite films and interfaces in micro-scale and portable applications, including the removal of volatile organic compounds from indoor air, recovery of catalysts in homogeneous reactions, zeolitic microreactors and microseparators, for example. Moreover, zeolite coated microreactors and microseparators exhibit high surface-to-volume ratio, and are capable of high productivity as a result of the good contact between reactants and catalyst wall. 

These kinds of MMs are useful for MMRs and/or incorporation into microfluidic devices with thin-fllms. The potential applications of such zeolite MMs include separations in pharmaceutical and fine chemical synthesis, as well as in lab-on-chip devices, sensors, zeolite micro-FCs and adsorption screening tools (Pina et al, 2011). 

Lau, W.N., Zhang, X.F., Yeung, K.L., and Martin-Aranda, R. (2005) Zeolite membrane microreactor for fine chemical synthesis. Stud. Surf. Sci. Catal., 158, 1335-1342. 

---