5364
J. Am. Chem. Soc. 2001, 123, 5364-5365
Table 1. Average Volume Per Al Atom, Specific Surface Areas,
and Phases Identified by Powder X-ray Diffraction
Plasma-Fluorination Synthesis of High Surface Area
Aluminum Trifluoride from a Zeolite Precursor
surface area
vol/Al surface of fluorinated
atom
(Å3/Al) (m2/g)
James L. Delattre,‡ Peter J. Chupas,† Clare P. Grey,† and
Angelica M. Stacy*,‡
areaa
productsa
(m2/g)
phases
precursor
identifiedb
R-Al2O3
21.2
23.0
82.4
6
90
18
6
15
32
R-Al2O3, R-AlF3
R-AlF3, γ-Al2O3
R-AlF3, â-AlF3
n/a
Department of Chemistry, UniVersity of
California at Berkeley, Berkeley, California 94720
Department of Chemistry, State UniVersity of
γ-Al2O3
Al2Si2O5(OH)4
H-SSZ-32
4254
100
190
New York at Stony Brook, Stony Brook, New York 11794
a BET isotherm (N2 Adsorption) using Quantachrome Autosorb-1.
b Compared to JCPDS-ICDD (1999). Siemens D5000, Cu KR.
ReceiVed February 7, 2001
ReVised Manuscript ReceiVed April 13, 2001
metathesis reaction, forming AlF3 and NOx(g), and the silicate
fraction would readily decompose and volatilize as SiFx(g) and
NOx(g). NF3 was used as a fluorinating agent because carbon-
containing PFCs produced residual fluorocarbon polymers on the
solid products.
We were intrigued by the possibility that the density of
aluminum cations in the precursors would affect the properties
and phase of AlF3 products, so we chose a series of aluminas
and aluminosilicates that encompasses a wide range of aluminum
cation densities: from 21.2 Å3/Al to 4254 Å3/Al. Table 1 lists
the average volume per aluminum atom, based on crystallographic
data (JCPDS-ICDD, 1999), for each of the precursors. We also
anticipated that under sufficiently mild reaction conditions, high
surface areas could be transferred from precursors to their fluoride
products. Therefore, γ-Al2O3 aerogel and high surface area zeolites
seemed to be logical choices for precursors to high surface area
AlF3.
The metastable phases of aluminum trifluoride (â-, η-, θ-,
κ-AlF3) are well-known halogen exchange catalysts.1-3 These
polymorphs are typically synthesized using chimie douce tech-
niques, but the surface areas are low (<65 m2/g) relative to oxide
catalysts.3,4 Active AlF3 catalysts have also been prepared via the
low-temperature fluorination of high surface area γ-Al2O3 with
HF, NH4F, SF4, hydrofluorocarbons, or chlorofluorocarbons.2,5-11
However, as fluorination proceeds, the specific surface areas of
treated γ-Al2O3 drop significantly, eventually reaching dimensions
that are equivalent to or smaller than traditionally prepared
AlF3.4,12 Considering that heterogeneous reaction rates scale with
surface area, new synthetic routes to high surface area AlF3 are
currently being sought. In this communication, we describe a
novel method for converting zeolites to high surface area AlF3
via a plasma-assisted fluorination reaction with nitrogen tri-
fluoride.
The low-temperature, perfluorocompound (PFC) plasma fluo-
rination of oxides is routine in semiconductor processing, but the
scope of this technique remains generally unexplored, especially
in the context of materials synthesis. We have investigated the
heterogeneous reaction of a cold NF3 plasma with various
aluminum-containing powders, including R-Al2O3, γ-Al2O3,
Al2Si2O5(OH)4 (kaolinite), and H1.3[Al1.3Si22.7O48] (zeolite H-SSZ-
32; structure type: MTT) for the preparation of AlF3. The main
goals of this investigation were to determine if a metastable form
of AlF3 could be prepared in a PFC plasma, and to correlate the
structural characteristics of the precursors with the properties of
the fluoride products.
Alumina precursors were selected because there is precedent
in the literature for the conversion of alumina to aluminum
fluoride in a PFC plasma,13 but no metastable phases of AlF3
were observed. While there was no preexisting evidence that
plasma fluorination of aluminosilicates would yield AlF3, we
speculated that the aluminum oxide fraction would undergo a
Plasma reactions were carried out in an inductively coupled
tubular reactor that has been described previously.14 The tem-
perature of the reactor exterior, measured using a thermocouple,
did not exceed 180 °C. The gaseous products were monitored
continuously at the reactor exit by a Spectra Satellite quadrupole
mass spectrometer.
For each fluorination reaction, 200 mg of powdered precursor
was weighed into an alumina boat, which was placed inside the
reaction chamber prior to evacuation. The power was set at 150-
250 W and the chamber pressure was maintained at 250 mTorr.
The NF3 (Advanced Specialty Gases, 99.9%) flow rate was 10
sccm for all reactions. The reactions ran until an endpoint was
detected with the mass spectrometer, typically 10-20 min.
Endpoints were marked by the disappearance of oxygen-contain-
ing and/or silicon-containing species in the effluent. The identities
of the products were determined by powder X-ray diffraction and
the surface areas were determined from N2 adsorption isotherms.
The results are given in Table 1.
Plasma fluorination of R- and γ-Al2O3 gave R-AlF3, the
thermodynamically stable and catalytically inactive phase of
aluminum trifluoride,2 and unreacted starting material. Fluorina-
tion of R-Al2O3 did not affect the specific surface area, while
fluorination of γ-Al2O3 decreased the measured surface area from
90 m2/g to 15 m2/g. The plasma fluorination of R- and γ-Al2O3
did not yield the desired metastable phases with high surface areas.
The plasma fluorination of Al2Si2O5(OH)4 gave a material that
is comparable to traditionally prepared AlF3. The powder X-ray
diffraction pattern showed reflections from both R-AlF3, which
has a distorted ReO3 structure,15 and â-AlF3, which is metastable
and structurally related to the hexagonal tungsten bronzes.16
‡ University of California at Berkeley.
† State University of New York at Stony Brook.
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(14) Delattre, J. L.; Freidman, T. L.; Stacy, A. M. J. Vac. Sci. Technol. B
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10.1021/ja015645t CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/10/2001