Phosphate-based solid catalysts

Tanabe and Hdlderich (1999) have given an extensive statistical survey of industrial processes using solid acids/bases as catalysts. Over 300 solids and bases have been covered. A variety of reactions like alkylation, isomerization, amination, cracking, and etherification with catalysts like zeolites, oxides, complex oxides, phosphates and ion-exchange resins have been covered. Over 120 industrial processes are referred with 180 different catalysts. 

Zahouily, M Salah. M Bahlaouane. B Rayadh. A Iloumam. A Hamed. HA Sebti. S. Solid catalysts for the production of fine chemicals the use of natural phosphate alone and doped base catalysts for the synthesis of unsaiuraied ar>Jsultones. Tetrahedron. 2004 60, 1631-1635. 

Finally, calorimetric measurements can also be used to monitor adsorption phenomena on the surface of solid catalysts in contact with a liquid phase (in a solvent). For example, the so-called cal-ad method [30-33] has been used to measure the adsorption heats evolved upon addition of dilute solutions of pyridine in -hexane to a solid acid catalyst (TS, H-ZSM-5) in a slurry wifli -hexane. The amount of free base in solution is measured separately using a UV-Vis spectrophotometer [30,31]. A similar technique has been used to determine the acidic character of niobium oxide and niobium phosphate catalysts in different solvents [34,35], using aniline and 2-phenyl-ethylamine as probe molecules. 

Fig. 4.11 Water splitting with hematite and a cobalt-phosphate-based catalyst (a) Dark (dashed) and photocurrent (solid) densities for a-Fe203 (light) and Co-Pi/a-Fe203 (dark) photoanodes, collected using simulated AM 1.5 illumination (1 sun, backside illumination) at a scan rate of 50 mV/s. (b) Electronic absorption and (c) IPCE spectra for a-Fe203 and Co-Pi/ a-Fc203 (at 1.23 and 1 V vs. RHE, respectively). The absorption spectrum of Co-Pi on FTO without a-Fe203 is included in (b), but no photocurrent was detected for these anodes. From [116] used with permission...

Besides metal vanadates, other catalyst compositions were also tried in the past several decades. For instance, VPO based solids [101], FePO [102], MoVPO [103], heteropolyacids [104, 105], VPj/TiO [106, 107, 108], V-MOO [109] are widely used. Two isomorphous VOPO solids (a- and P ) were prepared and tested for the ammoxidation of 2MP to 2CPy [103, 110]. The reaction was carried out in the temperature range of 360 20°C. Between the two mono-phosphates tested, the a-VOPO showed better activity compared to P-VOP04. Interestingly, in situ synthesized 12-molybdophos-... 

A novel basic support and catalyst have been prepared by activation of aluminium phosphate with ammonia. Fine control of time and temperature allows to adjust the 0/N ratio of these oxynitride solids and thus to tune the acid-base properties. The aluminophosphate oxynitrides are active in Knoevenagel condensation, but a basicity range can not yet determined. Supporting Pt or Pt/Sn on AlPONs allows to prepare catalysts that are highly active and selective in dehydrogenation reactions. 

Solid metal sulphates and phosphates also exhibit acid—base properties their acid strength is lower than that of silica—alumina but they are stronger acids than some oxide catalysts [5]. Correlation of activity with electronegativity of cations has been obtained for several reactions [9, 50,51],... 

Patents and literature reviews reveal that a wide range of catalysts has been investigated for the production of pyridine bases [5,9-11,14-18]. Up to 1980, FCC-based catalysts, often in combination with metal promoters, featured heavily as supports. The list of early solid-acid supports includes AI2O3, clays (e. g. mont-morillonite), amorphous Si02-Al203, molecular sieves (i. e. LTA), zeolites MOR and FAU (i. e. H -Y) [19], and metal phosphates [20]. 

The gas-solid reaction between an amorphous precursor (of the phosphate-type in the present study) and flowing ammonia is a critical step in the preparation of oxynitride catalysts, active and selective for base-catalysed reactions [1]. 

Catalysts. - The catalysts and sources of HCHO that appeared in patents are listed in Table 6. Solid bases such as hydroxides of alkali and alkaline earth metal supported on a support such as silica gel or aluminosilicate have mainly been claimed in patents to be effective as the catalysts. In addition, another type of compounds, which possess acidic property as well as basic property, are also claimed in patents, for example, PbO, Mn02, AI2O3, metal phosphates, metal borates, multicomponent oxides containing V, Nb, W, and Mo. 

A number of solid base catalysts have been reported in the literature to be active for acetone condensation. These include alkali oxides (NajO, K O, CsjO) (7), alkaline earth oxides (MgO, CaO, BaO) (1,4-6), transition metal oxides (7) and phosphates (8-11), ion-exchange resins (12), zeolites (13) and clay minerals and hydrotalcites (HTC) (14,15). A suitable catalyst for the acetone-to-MIBK reaction must have several properties the condensation of acetone to DAA is catalyzed by either basic or acidic sites, the dehydration of DAA to MO is acid-catalyzed, and the selective hydrogenation of MO to MIBK requires appropriate metal sites (7,8). 

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