J. Am. Chem. Soc. 1999, 121, 5587-5588
5587
Nanocrystal Metal Oxide-Chlorine Adducts:
Selective Catalysts for Chlorination of Alkanes
Naijian Sun and Kenneth J. Klabunde*
Department of Chemistry, Kansas State UniVersity
Manhattan, Kansas 66506
ReceiVed January 11, 1999
In recent reports the authors have described the preparation of
MgO and CaO ultrafine powders in nanocrystalline form, and
the smallest crystallite materials have been labeled AP-MgO and
AP-CaO (4- and 7-nm average crystallite sizes, respectively) as
designated by their aerogel preparation method.1 Somewhat larger
crystallites, but still in the nanometer size range have also been
prepared, and labeled CP-MgO and CP-CaO (7- and 15-nm
average crystallite sizes, respectively) designated “conventional
prepared”.
The surface reactivities and adsorption powers of these samples
are considerably higher than normal, commercially available
(CM-MgO and CM-CaO) samples. Furthermore, we have dis-
closed further reactivity enhancements of these materials by
depositing very thin layers of transition-metal oxides on the
crystallites of these MgO and CaO samples.2
It is believed that the enhanced surface reactivities of these
samples of MgO and CaO are due to the morphological features
of the small crystallites and, more specifically, are due to a
relatively higher population of reactive surface sites, for example,
edges, corners, and ion vacancies. The “smallness” and the shapes
of the crystallites allow much higher ratios of edge-corner ions
to total surface ions.3a,3b
Figure 1. Schematic drawing of an AP-MgO nanocrystal doped with
chlorine, which dissociative chemisorbs (exothermally) on strongly basic
surface sites. The Cl atoms are probably located at the most basic/reactive
edge/corner O2- sites.
The presence of these edge-corner sites and other reactive defect
sites (such as vacancies) allow these materials to possess sur-
prisingly high surface concentrations of reactive surface ions. For
example, an edge, or even more so, a corner O2- anion is coor-
dinately unsaturated and is “seeking” Lewis acids (electron-
deficient species) to help stabilize and delocalize its negative
charge. Conversely, a Mg2+ ion on an edge or corner is “seeking”
Lewis bases (electron-rich species) to stabilize and delocalize
its positive charge. Therefore, these coordinatively unsaturated
ably during this heating cycle). Knowing the surface area and
the amount of Cl2 released, the number of Cl atoms adsorbed per
nm2 could be calculated, and this ranges from 5 to 7 atoms. By
using a polyhedral nanocrystal model and filling in the necessary
chlorine atoms required for a 4-nm particle, it is evident that this
amount of chlorine adsorbed exceeds the number of edge/corner
sites available, and an illustration is shown in Figure 1.3b,c Thus,
chlorine atoms are located at not only the edge/corner sites but
also some adjacent face sites.4
Similar studies of normal MgO crystals have been carried out.
With common, commercially available MgO powder, less than 5
wt % Cl2 was adsorbed, and the process was not nearly as
exothermic.
This nanoparticle MgO-Cl2 adduct is extremely reactive. To
our surprise when 2,3-dimethylbutane came into contact with the
adduct in the presence of excess Cl2 at room temperature, an
explosion took place. However, by carrying out the adduct-2,3-
dimethylbutane contact at -78 °C followed by slow warming up
to room temperature in the dark, a smooth chlorination took place
to form mainly a monochlorinated product (CH3)2CClCH(CH3)2,
and smaller amounts of dichloro isomers. Control experiments
with Cl2 gas-2,3-dimethylbutane in the dark gave practically no
reaction, while photolysis for 1 h with a 450-W UV lamp (quartz
reactor) yielded about 95% of chlorinated organics with a large
O
2- and Mg2+ ions readily accept incoming reagents with Lewis
acid or Lewis base character.
This situation presents an opportunity to prepare new and
unusual materials where highly reactive Lewis base or Lewis acid
adsorbents could be stabilized by forming adducts with the
reactive-accepting surface sites on the MgO or CaO samples. In
the current context, nanocrystalline MgO and CaO were exposed
to chlorine gas at room temperature. During the adsorption, a
great deal of heat was generated, and the white powder samples
turned light yellow. Excess Cl2 gas was allowed to stand over
the powder for 30 min, followed by evacuation for 0.5 h. A light
yellow sample was transferred to a TGA instrument and heated
at 10 °C/minute under N2. Chlorine gas and a trace of O2 were
released over a broad temperature range 100-700 °C, and it was
determined that 13 wt % of Cl2 was released. In the case of AP-
MgO, the surface area was 383 m2/g (which decreased consider-
(1) Utamapanya, S.; Klabunde, K. J.; Schlup, J. R. Chem. Mater. 1991, 3,
175.
(4) A 4-nm polyhedral AP-MgO particle contains totally 4500 Mg2+
and 4500 O2-. The total number of edge and corner ions is about 6% of the
total number of ions. Thus, there are 270 Mg2+ and 270 O2- on a 4-nm
AP-MgO particle. From the mass of the particle, we can calculate that there
are 3.47 × 1018 particles in 1 g AP-MgO. We also know the surface area of
AP-MgO is about 380m2/g, and thus the surface area of one particle is
110 nm2. There are 5.8 Cl atoms per nm2 on the surface; thus, there are 5.8
× 110 ) 638 Cl per particle. The number of Cl atoms adsorbed is larger than
the number of edge/corner O2- of a particle; therefore, some Cl atoms are
located at the edge and corner sites, and the others are located at the adjacent
face sites.
(2) Klabunde, K. J.; Khaleel, A.; Park, D. High Temp. Mater. Sci. 1995,
33, 99.
(3) (a) Klabunde, K. J.; Stark, J.; Koper, O.; Mohs, C.; Park, D. G.; Decker,
S.; Jiang, Y.; Lagadic, I.; Zhang, D. J. Phys. Chem. 1996, 100, 12142. (b)
Koper, O.; Lagadic, I.; Klabunde, K. J. Chem. Mater. 1997, 9, 838. (c) The
polyhedral structure is supported by high-resolution TEM (ref 3b), and this
crystal shape is very different from microcrystals. However, the polyhedral
structure can only be considered an “average” since not all of the crystallites
are the same.
10.1021/ja990084f CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/29/1999