Preparative effects on the surface area and pore structure of microporous heteropoly oxometalates

Lapham, Darren P.; Moffat, J.B.

doi:10.1021/la00058a049

Cited by 53 publications

(15 citation statements)

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“…Earlier work has shown that the ammonium salts have higher surface areas than the potassium salts. (11,12) The surface areas decreased with increasing calcination temperature, as previously reported, (13) and, with the exception of K 3 PMo 12 O 40 which had a surface area of 30 m 2 g À1 after heating at 753 K, reached negligible values at 693 K.…”

Section: Chaptersupporting

confidence: 83%

“…(4) Increases in pretreatment temperature up to 400 C have been shown to reduce the surface area of NH 4 PMo from values greater than 200 m 2 =g À1 to less than 10 m 2 =g À1 . (13) The micropore volumes decrease to negligible values by 400± 500 C. In contrast, the surface areas of NH 4 PW, KPW, and CsPW, although decreasing with temperature, retain signi®cant values up to 400±500 C. In particular, the surface area and micropore volume of KPW are relatively unchanged up to 400 C, whereas the micropore volumes of NH 4 PW and CsPW vanish by 500 C.…”

Section: Chaptermentioning

confidence: 97%

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Stability

2002

Metal-Oxygen Clusters

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A large number of publications have reported studies of the thermal stability of heteropoly oxometalates as part of investigations of other aspects of these materials. Although these reports may have been discussed elsewhere in this monograph where relevant, they have also been included here. The present discussion of thermal stability is, for convenience, restricted to unsupported heteropoly oxometalates with that pertaining to the supported materials included in the section on the properties of the latter. A cautionary note should be added here. The results from studies of stability are dependent not only on the method employed for such evaluations, but in addition are frequently dependent on the conditions under which the experiment was performed.One of the earliest studies of the stability of the heteropoly acids employed differential thermal analysis and X-ray diffraction. (1) With each of the four acids examined, an endotherm and an exotherm are observed at 300 C or less and 340 C or more, respectively (Table 5.1). The low-temperature endotherm is attributed to the removal of water whereas the high-temperature exotherm signals the decomposition of the anion. The authors note that metatungstic acid H 8 [W 12 O 40 ]ÁxH 2 O decomposes at 50 C. When the void in the center of the [W 12 O 40 ] 8À anion is occupied by Si 4 or P 5 , the decomposition temperature increases by approximately 400 or 500 C, respectively. The presence of a positively charged species reduces the effect of thermal vibrations and thus of decomposition.Thermogravimetric analyses of H 3 XW 12 O 40 with X being zinc, iron, cobalt, boron, silicon, or phosphorus, their potassium salts, and their ether-addition compounds have been reported. (2) With the ®rst three acids, three peaks were observed whereas only two were obtained with the latter three acids (Table 5.2). The third peak was attributed to decomposition of the anion. 42CHAPTER 5 STABILITY 43

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Section: Chaptersupporting

confidence: 83%

Section: Chaptermentioning

confidence: 97%

Stability

2002

Metal-Oxygen Clusters

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“…Later, Moffat et al measured the pore diameter distribution of (NH 4 ) 3 PW 12 O 40 and other salts prepared at ambient temperature [39,40], and from the corresponding X-ray powder diffraction patterns, claimed that (NH 4 ) 3 PW 12 O 40 and other porous salts give relative peak intensities much different from those of non-porous salts. They then went on to propose that the transformation of the crystal structure was responsible for the generation of micropore channels in (NH 4 ) 3 PW 12 O 40 and other porous salts.…”

Section: Porous Partially Salified Csmentioning

confidence: 99%

“…The relationship among the concepts of ''mesocrystals,'' usual microporous crystals (like zeolites), and the new category of ''sponge crystals'' will be discussed. 40 . These catalysts were applied to liquid-solid systems as well as to gas-solid systems for reactions of various organic compounds [9,10].…”

Section: Introductionmentioning

confidence: 99%

“Sponge Crystal”: a novel class of microporous single crystals formed by self-assembly of polyoxometalate (NH4)3PW12O40 nanocrystallites

Inumaru

2006

Catal Surv Asia

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In this account, a new concept of ''sponge crystals'' is presented on the basis of new interpretation of our previous results of porous heteropolyacids, that is, porous aggregates of self-assembled (NH 4 ) 3 PW 12 O 40 nanocrystallites (Ito, Inumaru, and Misono, J. Phys. Chem. B 101 (1997) 9958; Chem. Mater. 13 (2001) 824) ''Sponge crystals'' are defined as single crystals having continuous voids within them, but unlike zeolites, no intrinsic structural pores. This new category includes molecular single crystals having continuous voids originating from series of neighboring vacancies ( ‡1 nm) of the constituent large molecules, affording nanospaces in the crystals. A typical example of ''sponge crystals'' is (NH 4 ) 3 PW 12 O 40 , which is formed via the dropwise addition of ammonium hydrogen carbonate into an H 3 PW 12 O 40 aqueous solution (titration method) at 368 K. The resulting (NH 4 ) 3 PW 12 O 40 nanocrystallites (ca. 6-8 nm) then self-assemble with the same crystal orientation to form porous dodecahedral aggregates in the solution. During the formation process, necks grow epitaxially between the surfaces of the nanocrystallites (''Epitaxial SelfAssembly'') to form aggregates of which each aggregate has an ordered structure as a whole single crystal. Although the crystal structure of (NH 4 ) 3 PW 12 O 40 has no intrinsic structural(''built-in'') pores, X-ray diffraction, electron diffraction and gas adsorption experiments all reveal that each (NH 4 ) 3 PW 12 O 40 aggregate is comprised of a single crystal bearing many micropores. These pores are considered to be continuous spaces as neighboring vacancies of the molecules (anions and cations) originating from the residual spaces between the self-assembled nanocrystallites. The porous (NH 4 ) 3 PW 12 O 40 single crystals are considered a special case of ''mesocrystals,'' as was recently discussed by Co¨lfen and Antonietti (Angew. Chem. Int. Ed. 44 (2005) 5576). In contrast to most ''mesocrystals,'' which are generally polycrystalline in nature, each aggregate of (NH 4 ) 3 PW 12 O 40 is a characteristic porous single crystal. Furthermore, the micropores of (NH 4 ) 3 PW 12 O 40 are totally different from those found in other microporous crystals: zeolites have ''built-in'' pores defined by their crystal structure, while the (NH 4 ) 3 PW 12 O 40 nanocrystallites have none. Since (NH 4 ) 3 PW 12 O 40 micropores are continuous spaces as neighboring vacancies of the molecules, the shapes of the (NH 4 ) 3 PW 12 O 40 pores can in principle, assume various connectivities or networks within the crystal, and as such, are subsequently termed: ''sponge crystals.''

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“…The structure and the catalytic performance of such HPC-supported catalysts are very sensitive to the preparation method and process parameters, as noticed by Volpe et al [19], Lapham et al [20], and also by us in previous work [21]. In the case of Cs 3 PMo 12 O 40 supports, for example, the reported values of the specific surface area and pore volumes varied from 2 to 150 m 2 /g [12,[21][22][23][24][25][26][27][28][29][30][31][32][33][34] and from 0.020 to 0.175 mL/g [21,22], respectively.…”

Section: Introductionmentioning

confidence: 99%

Keggin-type H4PVMo11O40-based catalysts for the isobutane selective oxidation

Paul

Chu²,

Sultan

et al. 2010

Sci. China Chem.

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Cesium heteropolysalts Cs 3 PMo 12 O 40 and HCs 3 PVMo 11 O 40 were synthesized by modifying the preparation conditions in order to get materials with a much higher surface area than the original Keggin-type heteropolyacids (H 3 PMo 12 O 40 and H 4 PVMo 11 O 40 ). These solids were used as carriers for the dispersion of H 4 PVMo 11 O 40 heteropolyacid by the incipient wetness impregnation technique. The textural and structural properties of supports and catalysts were examined by scanning electron microscopy, N 2 adsorption-desorption isotherms and Raman spectroscopy. The supported catalysts were studied before and after red/ox pretreatments by X-ray photoelectron spectroscopy, which showed that both the surface composition and oxidized to reduced species ratio depend on the used carrier. The catalytic performances of these novel supported catalysts in the selective oxidation of isobutane to methacrylic acid and methacrolein were studied. The best catalytic properties were obtained when H 4 PVMo 11 O 40 was supported on HCs 3 PVMo 11 O 40 . The isobutane conversion and yield of the desired oxygenates increased along the unsupported H 4 PVMo 11 O 40 < H 4 PVMo 11 O 40 /Cs 3 PMo 12 O 40 < H 4 PVMo 11 O 40 /HCs 3 PVMo 11 O 40 series. cesium heteropolysalts supports, supported H 4 PVMo 11 O 40 , red/ox XPS, isobutane selective oxidation, Keggin-type heteropolycompounds

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Preparative effects on the surface area and pore structure of microporous heteropoly oxometalates

Cited by 53 publications

References 0 publications

Stability

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“Sponge Crystal”: a novel class of microporous single crystals formed by self-assembly of polyoxometalate (NH4)3PW12O40 nanocrystallites

Keggin-type H4PVMo11O40-based catalysts for the isobutane selective oxidation

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