Pseudoliquid phase

The reaction of tert-butyl alcohol and methanol to form MTBE is also catalyzed by heteropoly compounds (221-223). A relationship was found between the amount of pyridine sorbed in or on heteropoly compounds and tert-butyl alcohol conversion (221). The dependence of the rate on methanol partial pressure resembles that for the absorption of methanol in the bulk, suggesting pseudoliquid phase behavior (223). 

A wide variety of acid-catalyzed reactions besides those described above have been investigated with heteropoly compounds as catalysts. Al203-supported H3PW12O40 (probably decomposed) catalyzed propylene-ethylene codimerization at 573 K to form pentenes with a selectivity of 56% (butenes 17%, hexenes 27%) (224). Propylene oligomerization proceeded on various kinds of salts of H3PWl204o (225). The activities of the salts decrease in the order A1 Co Ni, NH4 H, Cu Fe, Ce K. The A1 salt gave trimers with 90% conversion at 503 K. The selectivities to trimer are about 40% for Al, Ce, Co, and Cu, while that of the acid form is 25%. 

In ordinary heterogeneous catalysis of gas-solid and liquid-solid reactions, the reactions take place on the two-dimensional surfaces of solid catalysts (both on the outer surface and on the surfaces of pore walls). In contrast, the reactions of polar molecules in the presence of heteropoly catalysts often proceed not only on the surface but also in the bulk phase. We call this pseudoliquid phase behavior. The pseudoliquid phase is a unique reaction medium consisting of the three-dimensional solid bulk, as was first proposed in 1979 (17, 233, 234). 

Because of the flexible and hydrophilic nature of the secondary structures of the acid forms and group A salts (Section II), polar molecules like alcohols and amines are readily absorbed into the solid bulk by substituting for water molecules and/or by expanding the distance between polyanions. The number of absorbed molecules is 10—102 times greater than the amount of monolayer 

A salts (e.g., Na) Pseudoliquid (bulk type I) Surface type 

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Pseudoliquid-phase catalysis (bulk type I catalysis) was reported in 1979, and bulk type II behavior in 1983. In the 1980s, several new large-scale industrial processes started in Japan based on applications of heteropoly catalysts that had been described before (5, 6, 72) namely, oxidation of methacro-lein (1982), hydration of isobutylene (1984), hydration of n-butene (1985), and polymerization of tetrahydrofuran (1987). In addition, there are a few small- to medium-scale processes (9, 10). Thus the level of research activity in heteropoly catalysis is very high and growing rapidly. 

In this section, these influences will be described. Besides the acidic properties, the absorption properties of solid heteropolyacids for polar molecules are often critical in determining the catalytic function in pseudoliquid phase behavior. This is a new concept in heterogeneous catalysis by inorganic materials and is described separately in Section VI. With this behavior, reactions catalyzed by solid heteropoly compounds can be classified into three types surface type, bulk type I, and bulk type II (Sections VII and IX). Softness of the heteropolyanion is important for high catalytic activity, although the concept has not yet been sufficiently clarified. 

In catalytic dehydration of alcohols, pseudoliquid phase behavior (bulk type I reaction) of heteropolyacids has been demonstrated (Section VI). The high catalytic activity is associated with this behavior and the strong acidity. Unique pressure dependences of the catalytic activity and selectivity are found for H3PW1204o due to the pseudoliquid phase (Fig. 40). 

It was confirmed by the same methods that dehydration of ethanol also proceeds in the pseudoliquid phase of H3PW12O40 (240). 

There is evidence that at least two different pseudoliquid phases may be present, even during catalytic reactions, and these may change reversibly with changes in the reactant partial pressures, as shown for dehydration of 2-propanol in Fig. 39 (242). A small change in reactant partial pressure led to an abrupt... 

Since ethylene is formed from one molecule of ethanol and ether from two molecules, it is understandable that ethylene is preferentially formed when the ratio of ethanol to protons in the pseudoliquid phase is low and ether is favored as this ratio increases. Equations (23)—(25) represent a possible mechanism that explains the essential trend in Fig. 40. 

Pseudoliquid phase behavior facilitates the spectroscopic investigation of the catalysts as the phenomena occur nearly uniformly in the bulk. 

As described above, heterogeneous catalytic reactions on heteropoly compounds are classified into three different types, surface, bulk type I (pseudoliquid phase), and bulk type II (Fig. 1). The surface reactions are typical of... 

There are three prototypes of heterogeneous catalysis with heteropoly compounds as shown in Fig. 2 [4, 5]. Actual cases could be intermediate and vary by the kind of heteropoly compounds, reacting molecules, and reaction conditions. Ordinary heterogeneous catalysis is the surface type, where the catalytic reaction takes place on a two-dimensional surface. Bulk type I is the reaction in the pseudoliquid phase. The secondary structure (Fig. lb) of certain HPAs is flexible and polar molecules are readily absorbed in interstitial positions of the solid bulk to form the pseudoliquid phase. Bulk type II has been demonstrated for several catalytic oxidations at relatively high temperatures. The reaction fields for the bulk types are three-dimensional. 

The heteropolyanion stabilizes protonated intermediates by coordination in solution and the pseudoliquid phase as well as on the surface, thus lowering the activation energy and accelerating reactions. Several protonated intermediates including the protonated ethanol dimer and monomer [18], the protonated pyridine dimer [12, 19], and protonated methanol [20] have been detected in the pseudoliquid phase directly by use of X-ray diffraction (XRD), IR or solid-state NMR. In solid-state H NMR, the chemical shift for the protonated ethanol dimer, (C2HsOH)2H+ is 9.5 ppm down-field from tetramethylsilane, which lies in the range of supcracids reported by Olah et al. [18]. This fact also supports the strong acidity of heteropolyacids. 

Acid Form - Pseudoliquid Phase Behavior. Owing to a high affinity for polar molecules, large quantities of molecules such as alcohols and ether are absorbed within the bulk phase of crystalline heteropolyacids. The amounts of pyridine, methanol, and 2-propanol absorbed correspond to 50-100 times that which can be adsorbed on the surface, while nonpolar molecules like ethylene and benzene are adsorbed at the surface only. Catalytic reactions of polar molecules occiu both on the surface and in the bulk, so that the solid heteropolyacid behaves as a highly concentrated solution, called a pseudoliquid phase . The dehydration of alcohols, various conversions of methanol and dimethyl ether to hydrocarbons in gas-solid systems, and the alkylation of phenol and pinacol rearrangements can all occur in the pseudoliquid. The transient response using isotopically labeled 2-propanol provides evidence for the pseudoliquid phase behavior of H3PW12O40. This behavior influences the selectivity, for example, the aUcene/aUcane ratio, in the conversion of dimethyl ether. 

In order to find the behaviour of protons in pseudoliquid phase, the effect... 

Primary alkyl halides do react in superacids, but ordinarily the products are 2° or 3° carbocations arising from rearrangements. Nevertheless, there is evidence for the ethyl cation in mixtures of CH3CH2F and SbFs at low temperature in SO2 solution as well as in a H3PW12O40 pseudoliquid phase. There are also indications for formation of the ethyl cation in a reaction of ethene with methane and a HF-TaFs catalyst and in the solvolysis of ethyl tosylate in concentrated H2S04. In addition, a 1° carbocation-brosylate ion pair was proposed as an intermediate in the El... 

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