Acid/basic bifunctional properties

The activity and selectivity are therefore governed by the balance of acidic and basic properties of a catalyst. When the basic property is too strong, the maximum yield of acrolein becomes low. Possibly polymerization of the produced acrolein is promoted by the strongly basic sites. However, when the basic property is suppressed too much by the incorporation of an acidic oxide, the catalyst loses the activity. On the other hand, the degradation of HCHO is promoted also by acid-base bifunctional properties. The control of the acid-base property of catalyst is performed by the combination of an acidic oxide with a more basic oxide. Therefore, it is considered that the acid-base properties of the Ni-P, Mn-P, Co-P, Fe-P, and V-P oxide systems are best fit for promoting solely the condensation reaction. 

Tests for Probing Acid and Basic Bifunctional Properties... 

The surface of Zr02 is considered to have bifunctional acid-base catalytic properties. A pair of weakly acidic and basic sites on Zr02 might possibly be important for the catalytic properties, because the catalytic performance of non-acidic ZrOj is superior. 

ZrOa and Th02 show similar catali ic behaviors. Although their basic properties are not so strong, they possess weakly acidic properties, and, therefore, show the acid-base bifunctional catalysis. ZrOa and Th02 as well as rare earth oxides showed dehydration activity for alcohols. 

As described above, most industrial catalysts are mixed oxides of at least two oxide phases, because catalyst performance is greatly improved by mixing different oxides, according to be following mechanisms (i) stabilization, (ii) control of redox properties, (iii) creation of acidity and basicity, (iv) control of electronic and coordination state of the metal ion, and (v) combination of more than two functions to evolve acid-oxidation and acid-base bifunctional catalysts. Examples are provided in the following sections. 

The selectivity of the metal sulfate catalyst is influenced by many factors besides its acidic property, such as geometric structure involving a pore structure, arrangement of basic sites, polarity of the surface, etc. For example, the relative values of the first-order rate constants (per imit acidity at pK — 3) of the depolymerization catalyzed by nickel sulfate, cupric sulfate, and silica-alumina were found to be 1100 300 1. The difference may be attributed to the differences in acid-base bi-functional catalysis of these catalysts. This view may be said to have originated in 1948 when Turkevich and Smith (45) showed that the isomerization of 1-butene to 2-butene is catalyzed by metal sulfates, sulfuric acid, phosphoric acid, etc., but little by acetic acid, hydrogen chloride, etc. The high catalytic activity of the catalysts of the former group is considered as due to acid-base bifunctional catalysis as illustrated by Fig. 14. Independently, Horiuti (45a) advanced the same idea... 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

Nineteen years have passed since the monograph Solid Acids and Bases was published in 1970. During this period many new kinds of solid acids and bases have been found and synthesized. The surface properties (in particular, acidic and basic properties) and the structures of the new solids have been clarified by newly developed measurement methods using modern instruments and techniques. The characterized solid acids and bases have been applied as catalysts for diversified reactions, many good correlations being obtained between the acid-base properties and the catalytic activities or selectivities. Recently, acid-base bifunctional catalysis on solid surfaces is becoming an ever more important and intriguing field of study. 

One-Pot Multistep Synthesis of Ketones on Bifunctional Zeolite Catalysts. One-pot multistep reactions constitute an elegant and efficient way to decrease the number of chemical and separation steps, hence, to develop greener synthesis processes. Bifunctional metal-acidic or metal-basic zeolite catalysts, which can be prepared easily with the desired properties (e.g., distribution of the... 

The alumina or silica-alumina supports used in bifunctional catalysts have been shown to be acidic in nature. The acidic properties are readily demonstrated by the affinity of these solids for adsorption of basic compounds such as ammonia, trimethylamine, re-butylamine, pyridine, and quinoline (01, R5). Furthermore, adsorption of certain acid-base indicators such as butter yellow gives a coloration similar to that observed in acid media (B3, B4). With regard to the origin of the acidity, Tamele (Tl) has suggested in the case of silica-alumina that aluminum atoms replace silicon atoms in the surface of the silica structure, giving rise to surface sites of the form... 

At the basis of the application of zeolites in fine chemicals reactions is the rich variety of catalytic functions with which zeolites can be endowed. Bronsted acidity, Lewis acidity and metallic functions are well known from classical bifunctional chemistry but for specific reactions, unusual sites, e.g. Lewis acid Ti4+ centres, have been introduced into zeolites. Moreover, zeolites can acquire more or less weakly basic properties metal complexes can be entrapped in zeolite pores or cavities, and enantioselective reactions have been performed by decorating the zeolite surface with chiral modifiers. 

Heterogeneous catalysts, in particular zeolites, with their various properties, contribute extensively to the environmental protection in the synthesis of fine chemicals. For that a broad and very impressive range of acidic and basic catalysts is already available, having all levels of properties between super acidity and super basicity. Also the possibility preparing bifunctional catalysts will gain in importance. 

Various kinds of oxide materials, including single oxides, mixed oxides, molybdates, heteropoly-ions, clays, and zeolites, are used in catalysis they can be amorphous or crystalline, acid or basic. Furthermore the oxides can be the actual catalysts or they can act as supports on which the active catalysts have been deposited. Silica and alumina are commonly used to support both metals and other metal oxide species. Amorphous silica/alumina is a solid acid catalyst, it is also used as a support for metals, when bifunctional (acid and metal) catalysis is required, e.g., in the cracking of hydrocarbons. Other acid catalysts are those obtained by the deposition of a soluble acid on an inert support, such as phosphoric acid on silica (SPA, used in the alkylation of benzene to cumene. Section 5.2.3). They show similar properties to those of the soluble parent acids, while allowing easier handling and fixed bed operation in commercial units. 

The surface acid-base properties of bulk oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. Acidic and basic sites are often present simultaneously on solid surfaces. The two centers may work independently or in a concerted way, and the occurrence of bifunctional reaction pathways requiring a cooperative action of acidic and basic centers has also received considerable attention [39]. The acid-base properties of numerous amorphous metal oxides investigated by mrcrocalorime-try have been summarized in an extensive review by Cardona-Martinez and Dumesic [11]. 

A wide variety of catalysts have been found effective in promoting the decomposition of plastic materials Friedel-Crafts catalysts, acidic and basic solids, bifunctional solids, etc. Friedel-Crafts systems, mainly A1C13/HC1, were initially used as acid catalysts but they have now been replaced in most processes by solids with acid properties due to the corrosion and environmental problems they cause. 

For the first, third, and fourth catalysts in (84) the two tautomers are chemically identical, and the same is true for ions such as HCOJ, HPO4", H2PO2, and H2ASO4, which have been reported to have an abnormally high catalytic activity in some reactions.It is clear that the effectiveness of this kind of catalyst is related to its particular electronic structure rather than to its acid-base properties, and the process is more appropriately described as tautomeric catalysis than as bifunctional or concerted acid-base catalysis. It is of interest that a theoretical treatment of some molecules in which acidic and basic groups form part of the same TT-electron system shows some parallelism between catalytic activity and the coupling constants of the molecular orbital theory moreover, a very general treatment of concerted proton transfers indicates that simple bifunctional acid-base catalysis is likely to be of importance only under very restricted conditions. ... 

Rare earth oxides have been studied to a lesser extent than alkaline earth oxides. However, they show characteristic selectivity in the dehydration of alcohols. Secondary alcohols form 1-olefins, whereas the same reaction over an acid catalyst produces the thermodynamically more stable 2-olefin (312). An example of an industrially important rare earth oxide catalyst is Zr02. It has several applications, including the reduction of aromatic carboxylic acids with hydrogen to aldehydes (314), the dehydration of 1-cyclohexyl ethanol to vinyl cyclohexane (315), and the production of diisobutyl ketone from isobutyraldehyde (316). The extensive use of Zr02 is mainly due to its resistance to poisoning by H2O and CO2, and its inherent catalytic activity is a result of its bifunctional acid-base properties. It contains both weakly acidic and basic sites, neither of which is susceptible to poisoning. The acid-base functionality of Zr02 is displayed in the reaction of alkylamine to nitrile (278) (Fig. 33). To form nitriles from both secondary and tertiary amines, both acid and base sites are required. 

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

In Sect. 2.1, heterobimetallic transition metal (Cu)/rare earth metal (Sm) system with Lewis acid/Brpnsted basic properties was introduced. By suitably selecting metal combinations depending on the targeted reactions, variety of chiral bimetallic Lewis acid/Br0nsted base bifunctional catalysts could be created, such as aPd/La/la... 

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