Keggin-type heteropolyacid

Herve et al. (57) investigated the thermal changes of structures by means of XRD and TG-DTA for Keggin-type heteropolyacids and proposed Scheme 2. Infrared spectroscopy of H4PMo, VO40 showed the release of vanadium atoms to form H3PM012O40 and vanadium phosphate species (55). Exposure to water vapor induces the decomposition of the latter (indicated by the disappearance of a band at ca. 1037-1030 cm -1) (58). 

Figure 23 shows a correlation between the catalytic activity and the Hammett acidity function (H0) of Keggin-type heteropolyacids in CH3CN. The catalytic activity increases with the acid strength (63). 

Table XVII is a comparison of the catalytic activities for liquid-phase MTBE synthesis from isobutylene and methanol (179). The catalyst structure and composition have a strong effect on the activity. The highest activity per proton was obtained with a Dawson-type heteropolyacid, H6P2W 8062, although the acid strength of H WigO is lower than that of the Keggin-type H3PW12O40 (Section HI). Water added to the mixture has little effect on the reaction rate at water concentrations less that 2 wt%, but at 5 wt% the rate is less by a factor of 2.5. At the same time the selectivity is less due to the formation of (erf-butyl alcohol.

Figure 3.11 FTIR skeletal spectra of a-Zr(HPOi)2 H2O, CdMo04 (scheelite type) and the Keggin-type heteropolyacid salt K3PM012O40.

Keggin-type heteropoly compounds have attractive and important characteristics in terms of catalysis. They consist of heteropolyanions and counter-cations such as H, Cs or NHT When the counter-cations are protons, they are called heteropolyacids (HPA). An important characteristic of HPAs, such as 12-tungstophos-phoric acid (H3PW12O40), is the presence of very strong Bronsted acid sites. But the characteristics of HPAs strongly depend on temperature and relative humidity. When they are used in heterogeneous catalysis, it is often necessary to support them on high-surface-area oxides or activated carbons, in order to increase the surface contact with the reactants. 

The acid strength of Keggin-type heteropolyacids is stronger than such conventional solid acids such as Si02-Al203 and H-Y zeolites. The acid strength of crystalline heteropolyacids decreases in the series. ... 

The development of catalysts obtained by means of heteropolyacids (HPA) and related compounds is a very important growing field. Since HPA are less corrosive and produce lower amount of wastes than conventional acid catalysts, they can be used as replacement in ecofriendly processes. Within the HPA, there is a special interest in those which present a Keggin type structure. 

The Keggin-type heteropolyacid (hereafter abbreviated HPA) is a unique catalyst material because it has the dual catalytic functions of strong acidity and high oxidizing capacity [1-5]. HPA has been applied commercially as an efficient catalyst in several petrochemical processes, including the direct hydration of propene (1972) [6,7], isobutene (1984) [8] and n-butenes (1989) [9], the oxidation of metha-crolein to methacrylic acid (1982) [10], the oligomerization of tetrahydrofuran to polymeric diols (1985) [11], and the oxidation of ethene to acetic acid (1997) [12]. 

Izumi, Y., Ogawa, M., and Urabe, K. 1995. Alkali metal salts and ammonium salts of Keggin-type heteropolyacids as solid acid catalysts for liquid-phase Friedel-Crafts reactions. Appl. Catal. A Gen. 132 127-140. 

Figure 2 a,b 1-butene conversion and selectivity towards cis 2-butene for the Dawson-type heteropolyacid. C,d 1-butene conversion and selectivity towards cis 2-butene for the Keggin-type heteropolyacid. Total flow 10ml min , 20% 1-butene in Helium, 0.3ml catalyst. 

The behaviour of Dawson and Keggin-type heteropolycids has been studied over surface-type and bulk-type reactions. Both heteropolyacids have shown similar activity for all the reactions investigated, with the biggest differences found for t-BuOH dehydration and isobutene polymerisation. It is possible for t-BuOH dehydration that the pseudo-liquid phase is responsible for the higher peformance of the Dawson, as suggested by Misono for MTBE synthesis. However this does not explain why the Dawson, which is the most selective catalyst at low temperature for 1-BuOH and 2-BuOH dehydration, suddenly becomes the less selective for f-BuOH dehydration. 

Mukai and co-workers [105] used Keggin-type heteropolyacids immobilized in the network structure of resorcinol-formaldehyde carbon gels as catalysts for the synthesis of methyl tert-butyl ether from methyl alcohol and tert-butyl alcohol. Large amounts of 12-tungstophosphoric and 12-molybdophosphoric acids were immobilized into the support by two methods, pore shrinkage and the ship-in-the-bottle method, which are essentially impregnation methods. The authors reported that these catalysts showed activity in the reaction studied and could be of practical utility as solid acid catalysts in various reactions. 

Here the active component in the catalyst has a finite vapor pressure and is carried out of the reactor. Examples are the loss of M0O3 from Keggin-type heteropolyacids (vanadomolybdatophosphates) and the loss of HgClj as the catalytically active species in the production of vinyl chloride from ethylene and HCl. A possible countermeasure is saturation of the feed with the active component. 

MOFs. An exceptional feature of HKUST-1 is its controlled dimensionality and high porosity with potential applications in gas storage, catalysis and drug delivery. Sun et al. [31] have reported the incorporation of Keggin-type heteropolyacids (HPA) into the HKUST-1 pores by hydrothermal synthesis with the objective of using this hybrid material in acid catalysis. 

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