Chemistry Letters 2001
535
sample. Both the Ag 3d5/2 chemical shift (368.6 eV as referenced
to Ag0: 368.2 eV) in the XPS spectra and the presence of a unique
absorption band at 250 nm typical of Ag+ in the UV–vis. DRS
revealed that all silver was present as ionic state in the catalysts
before the dehydrogenation. Figure 2 shows the XRD patterns of
silver containing ceramics catalysts before and after reaction. As
shown in Figure 2(a), the XRD pattern of catalysts before reaction
was not significantly different from that of the blank. No diffrac-
tion peaks corresponding to crystalline silver or any other silver-
containing crystalline phases were observed even at high silver
content up to 20%, indicating that all silver species were present
in ionic state. After treatment with reaction mixture for 2 h, four
small diffraction peaks at 2θ of 38.1, 44.5, 64.6, and 77.5° attrib-
utable to metallic Ag (111), (200), (220) and (311) appeared, as
shown in Figure 2(c). The result indicated that some of the Ag+
was reduced to metallic silver after the reaction, as has also been
confirmed by chemical analysis.
microscopy (SEM). All the results mentioned above showed that
the novel silver containing ceramics catalyst not only exhibited
excellent selectivity and activity at a temperature of 600 °C, but
also can be regenerated by high temperature treatment under oxy-
gen atmosphere. Hence, this novel catalyst showed promising
potential application in industrial manufacture of water-free
formaldehyde.
Figure 2(b) shows the XRD pattern of the resulting catalysts
after treatment by oxygen at high temperature. The peaks corre-
sponding to metallic silver disappeared, indicating that metallic
silver was oxidized to Ag+ again. Chemical analysis and XPS
also verified the transformation of Ag0 → Ag+. Thus, an Ag+ ↔
Ag0 cycle was present in this silver-containing ceramics catalyst
under different atmosphere.
The direct dehydrogenation experiments were also carried
out over the blank ceramics and over electrolytic silver powders.
The results showed that pure ceramics exhibited no dehydrogena-
tion activity, and electrolytic silver treated in oxygen atmosphere
at 650 °C gave initial yield of formaldehyde lower than 5% and
was deactivated in 0.5 h. As mentioned above, ionic silver pre-
dominated in the active catalyst and the deactivation of catalyst
was accompanied by the decrease of ionic silver amount.
Therefore, Ag+ or Ag+-related structure is supposed to be the
active center of this silver containing ceramics in methanol dehy-
drogenation. The extraordinary stability of Ag+ in the ceramics
catalyst under the reducing ambient can be explained by the
magic angle spinning nuclear magnetic resonance (MAS NMR).
In the 27Al spectra, there is a peak at 44.5 ppm due to the exis-
tence of [AlO4] tetrahedral unit.5 The [AlO4] tetrahedron and sil-
ver cation formed ion pairs, and the strong static interaction
between them could stabilize the ionic state of silver, and, as a
result, maintained the high catalytic activity of the catalyst.
As a conclusion, the present study is the first report on a
novel silver-containing catalyst used in the direct dehydrogena-
tion of methanol to formaldehyde without any side-products.
The catalyst can be easily regenerated with oxygen at high tem-
perature, which leads to longer lifetime. The dehydrogenation
process can be performed in a riser reactor in which deactivated
catalysts could be regenerated while not influencing the continu-
ity of the reaction. Moreover, considering the cheapness of the
natural kaolin support and the simpleness of the preparation pro-
cedure, this novel catalyst is a promising potential candidate for
water-free formaldehyde manufacture. Further study on its
detailed mechanism is under way.
It should be noted that the conversion of methanol decreased
along with the process of the direct dehydrogenation, suggesting
that the activity of the catalyst dropped with reaction time, which
can be ascribed to partial consumption of the Ag+ active sites dur-
ing the reaction as mentioned above. It is also interesting to find
that the lifetime of the catalyst with a steady high activity could
be more than 1 h and the yield of another product—hydrogen
was almost the same as formaldehyde. The molar ratio of the
resulting HCHO to Ag+ in the novel Ag/ceramics catalyst is more
than 500, meaning that 1 g 20% Ag/ceramics can catalyze more
than 25 g CH3OH to HCHO. However, the deactivated catalyst
could be easily regenerated after oxygen treatment at 650 °C for
1 h. Therefore, the lifetime of the present catalyst could be great-
ly extended by discontinuous operations. As shown in a discon-
tinuous lifetime test, the catalytic activity remained virtually
invariable in a 100-h activity test. On the other hand, there was
no increase of carbonaceous species on the surface after the life-
time test as determined by means of XPS and scanning electron
Project 20073009 supported by NSFC.
References and Notes
1 M. V. Twigg, “Catalyst Handbook”, Wolfe Publishing Ltd.,
London(1989), p.490.
2 C. A. Bazilio, W. J. Thomas, U. Ullah and K. E. Hayes,
Proc. Roy. Soc. London, Ser. A, 399, 181 (1985).
3 S. Brandani, V. Brandani, and G. Di Giacomo, Ind. Eng.
Chem. Res., 31,1792(1992).
4 S. Su, P. Zaza, A. Renken, Chem. Eng. Technol., 17,34
(1994).
5 S. Hayashi, T. Uda, K. Hayamizu, E. Akiba, J. Phys. Chem.,
96, 10922 (1992).