close

Вход

Забыли?

вход по аккаунту

?

Hf2NiP The Planned Modification of an Intermetallic Phase by (Formal) Substitution of Nickel by Phosphorus.

код для вставкиСкачать
COMMUNICATIONS
nation process proceeds in the cytosine complex for which no
hexaaqua complex could be isolated. The coordination of cytosine or its derivatives solely through O(2) was found, until
now, only in Ni" and Mn" c ~ m p l e x e s . Finally,
[ ~ ~ ~ ~ our structural data show that Mg" ions form M g - 0 covalent bonds
similar to those of Mn" or Ni" ions. and this could mimic thc
behavior of the transition metals. In conclusion, these studies
show that the complex formation between Mg" and 1-Mecyt in
aqueous solutions passes through a precursor supramolecular
assembly that contains hexaaquamagnesium ions, which is
linked to the ligands through hydrogen bonds."'] To check the
affinity of Mg" and transition metal ions, we tested the reactivity of I-Mecyt towards Mn" and Co" ions. In the case of Mn"
only one complex similar to 1 was found, while for Co", besides
the hexaaqua complex, also a compound with methylcytosine
coordinated to the cation through its nitrogen atoms was observed. This validates the hypothesis that 1 is really a precursor.
Further studies are required to establish whether this behavior
also occurs for other metal-nucleobase systems.
Expevbnental Section
1-3: Compounds 1 and 2 were obtained, as a mixture of parallelepiped and
polyhedral crystals, respectively. by slow evaporation at room temperature of
aqueous wlutions containing equimolar amounts of magnesium perchloratc
hexahydrate and I-Mecyt. Compound 3 was obtained in the same way, as needles
togcther with platelike crystals of cyt, from a solution of magneaium perchlorale
hexahydrate and cyt. Perchlorate was chosen because it forms very stable salts or
complexes and is relatively inert. The yields were 15% and 70% for 1 and 2,
respectively. and 750'5 for 3. All the compounds provided correct elemental analyse,
( C ,H, N ) .
Crystal structure analyses of 1-3: Siemens R3m!V automatic diffractometer, Mo,,,
1. = 0.71073 A. graphite monochromator, 295 K. Data collection, solution, and
refinement: 01-20 scan (1. 3). w scan (2), direct methods with subsequent Fourier
recycling, SHELXTL PLUS [ I l l . The perchlorate group of 3 is disordered. This
disorder has been treated by assuming a population factor 0.7 for three oxygen
atoms. 1 : C,,H,,CI,MgN,,O,,,
monoclinic, space group P2,/n, a =7.592(2), b =
32.017(7), c = 10.427(3) A. B = 104.30(2), v = 2456.0(11) Az, z= 2, PEr,fd =
1.474 g ~ m - 3<28<52".
~ ,
crystal dimensions 0.64 x 0.36 x 0.40 mm3, 4848 unique
reflections and 2863 assumed as observed with I> 3 ~ ( 1 )Refinement
.
of 346 parameters with anisotropic thermal parameters for non-hydrogen atoms gave R = 0.054,
Rii- = 0.063, and S =1.77. 2 : C2,H,,C1,MgN,,O,,,
triclinic, space group PT,
LI =7.386(2),
b = 8.938(2), c =14.331(3) A. IX = 108.09(2). /I = 101.15(2),
7 = 92.32(2) , V = 8 7 7 . 2 ( 4 ) A 3 , Z = l , ~ i d l=r d1 . 5 0 6 g c m ~ 3 . 3 < 2 0 < 5 4 , c r y s t a l d i mensioiis 0.42 x 0.39 x 0.25 mm', 3866 unique reflections and 2916 assumed as observed with I > 3 ~ ( 1 ) Refinement
.
of 229 parameters for non-hydrogen atoms (except the oxygen atoms of disordered perchlorate group) gave R = 0.046, RW =
0.056, a n d 3 = 1.79.3: C,,H,,Cl,MgN,,O,,
triclinic, space group P7, (7 = 6.776(5),
b = 9.981(4), c =12.896(5).&,
c( =71.86(3),
B = 87.40(5), y =71.86(5)",
~,
g c t K 3 , 3<2y<54', crystal dimensions
V=786.3(7) A3, Z = I . P ~ . , ~=1.638
0.60 x 0.15 x 0.05 mm3. 3456 unique reflections and 2005 assumed as observed uitb
I > 3u(1).Refinement of 221 parameters with anisotropic thermal parameters for
non-hydrogen atoms gave R = 0.063, Riv = 0.071, and S =1.88. Crystallographic
data (excluding structure factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-179.151. Copies of the data can be obtained free of charge on
application to The Director, CCDC, 12 Union Road, Cambridge CB2 IEZ, U K
(fax: Int. code +(1223) 336-033; e-mail deposit@chemcrys.cam.ac.uk).
Received September 2, 1996 [Z9511IE]
German version A n g e ~ Chrm 1997, fOY, 528-530
Keywords: amino acids * magnesium . nucleobases
structures * supramolecular chemistry
*
[6] K. Aoki, 1 Clirni. Soc. Chrm. Commun. 1976, 748.
[7] G. De Munno, S . Mauro, T. Pizzino, D. Viterbo, J Chrrrr. So<. Dirlran Tmzs.
1993, 1113.
[XI R . Cini, M. C. Burla, A. Nunzi, G. P. Polidori. P. F. Zanazzi, J Ckrm. Soc.
Dulton Truns. 1984, 2467.
[9] J.-M. Lehn, Angeiv. Chem. 1990, 102, 1347; Angew Chen7. h i t . Ed Engl. 1990,
2Y, 1304.
[lo] J. T. Theophanides, J.-E. Angiboust. M. Polissiou, J. Anastassopoulou, M.
Manfait, Mugnesinm Res. 1990, 3, 5.
[ l l ] SHELXTL PLUS. Version 4.211V. Siemens Analytical X-Ray Instruments
Inc., Madison, WI, 1990.
Hf,NiP: The Planned Modification
of an Intermetallic Phase by (Formal)
Substitution of Nickel by Phosphorus**
Holger Kleinke and Hugo F. Franzen"
In the last few decades the metal-rich mictides and chalcogenides of the early transition elements have attracted wide interest.[1'21Expanding these systems by adding a late transition
metal atom has led to the synthesis of new high-temperature
materials with new structural types that are stabilized by strong
early plate transition metal bonds and correspond to the Lewis
acid/base concept. Examples are Nb,Ni, -xS, +x,[31 Ta,M',S,
(M' = Fe, Co, Ni),I4I Ta,,M',Se, (M' = Fe, Co, Ni),['I
Td,NiSe,,[61 Hf,M'Te, (M' = Mn, Fe),17]Hf,M'Te, (M' = Fe,
Co),[*l Hf5Col+,P,-,,[91 and Zr,M',P4 (M' = Co, Ni),"'] and
all of their structural types contain extended regions of the early
transition element with interstitial late transition elements, located in one-, two-, or three-capped trigonal prisms. The chalcogen atoms usually sheathe either channels or layers, whereas the
P atoms of both phosphides, Zr,M',P4 and Hf,Col +*P3-,, are
situated in trigonal prismatic arrangements comparablc to those
of the late transition metal. Only the structure of Zr,M',P4
consists in part of bcc fragments, which is a common feature
among binary sulfides, selenides, and phosphides. Onc striking
feature is that in two cases, Nb9Ni,-,S3+, and Hf5Col+xP3-x,
the nonmetal and late transition metal atoms are statistically
disordered on one atomic position. Although Zr and Hf are very
similar, which is also true of Nb and Ta, for none of these
compounds has the analogue containing the corresponding
metal from the other period been synthesized so far.
The striking structural motif of six-membered Hf channels
with the Hf,CoP, structure, filled with infinite zigzag Co-P
chains, is closely related to that observed in the structure of
HfNi (CrB type), which contains an infinite zigzag Ni- Ni chain
(Figure 1) .[I' I In contrast, HfCo crystallizes in the CsCl structural type.["] It was anticipated that formal substitution of
every other Ni atom in the Ni-Ni chain by phosphorus would
lead to HfNi,,,P,,,=Hf,NiP, forming a new superstructure of
the CrB structural type.
solid-state
['I
[l] G . L. Eichhorn, If70rg. Btochrm. 1975, 3, 1207, 1210.
[2] T. Theophanidcs. Int. .
I
Quunrum Clzem. 1984. 26, 933; H. Sigel, Chem. Sor..
Rev. 1993, 22, 255.
[3] D. Tran. E. Palacios, Actu Crj,.stal/ogr. Srct C 1990. 46, 1220; L. G . Marzilli,
Biochenz. Bioplrys. Rrs. Co~~miuii.
1975, 63, 601; K. Aoki, J. Chrm. Soc. Chem.
Commuri. 1979, 589.
[4] D. T. Qui, M. Bagieu, Aclu Crpfullogr. Sect. C 1990, 46, 1645.
[5] G. Cervantes. J. J. Fiol, A. Terron, V. Moreno, J. R. Alabart, M. Aguilo. M.
Gomez, X. Solans, Inorg. Chem. 1990, 2Y, 5168.
Angpw. Chem. Inl. Ed. Engi. 1997. 36. No. 5
(
;
)
[**I
Prof. Dr. H. F. Franzen, Dr. H. Kleinke
Ames Laboratory-DOE
Iowa State University
Ames, IA 50011 (USA)
Fax: lnt. code +(515)294-5718
e-mail: franzen(2ameslab.gov
H. K. thanks the Deutsche Forschungsgemeinschaft for financial support of
this work. The Ames Laboratory is operated for the US Department of Energy
by Iowa State University under Contract No. W-7405-Eng-82. Ihis research
was also supported by the Office of the Basic Energy Science\, Materials
Science Division. Department of Energy.
VCH Vrrlu~.sgrsr//schnft
mbH, D-6Y45f Weinlzeim,iYY7
0570-0833/Y7i3605-(~513$ 15.00+ 2 5 11
513
COMMUNICATIONS
Therefore, we arc-melted a
pressed mixture of 1.25 mmol
HfP, 1.25mmol Hf, and
1.25 mmol Ni (ratio Hf/Ni/
this
P = 2/1/1). Annealing
sample in a welded Ta tube at
1400°C for one week gave homogeneous Hf,NiP, as shown by
the powder diffraction pattern.
The alternation of Ni and P in
the structure of Hf,NiP[l2] is asFigure 1. Structure of HfNi in a
sociated with a lower (monoclinprojection along [OOI]. Vertical: u
ic) symmetry. The unit cell is a 2c
axis. Small, black circles: Ni;
large, white circles. Hf.
superstructure of the primitive
cell of HfNi. Figure 2 emphasizes the condensed Hf channels, which surround the Ni-P
chain in such a way that each Ni and each P atom is located in
a trigonal Hf, prism, in which the rectangular faces are capped
by one Hf and two P or two Ni atoms (Figure 3).
’
Figure 2. Structure of Hf,NiP in a projection along [OIO]. Vertical: c axis. Small,
black circles: Ni; medium, white circles: P; large, white circles: Hf.
Figure 3. Structure of Hf,NiP in a projection along [OOI]. Vertical: h axis. Small,
black circles: Ni: medium, white circles: P: large, white circles: Hf.
As in the structures of Zr,Ni,P, and Hf,Co,,xP,-,, the
Hf-P bonds are slightly longer ( d H H=
f ~2.733
~ P A) than the
Hf-M’ bonds (dHf-Ni
= 2.704 A), although the unit cell volume
of HfNi is slightly larger than that of Hf,NiP (129.7 A3 vs.
129.05(8) A3).The corresponding distances within similar coordination spheres are
dZr-Ni
= 2.732 8, and &-,, = 2.759 8, for
Zr,Ni,P,, and dHf-Co
= 2.677 8, and dHf-p= 2.691 8, for
Hf,Col+,P,-x. In order to compare the lattice constants of
Hf,NiP and HfNi, we transformed the unit cell of HfNi to the
setting of Hf,NiP (transformation matrix: 0.5,0.5,0/0,0,1/
z,0,0). The resulting lattice constants of the transformed cell of
HfNi are a=5.1528,, h = 4 . 1 1 7 & c = 6 . 4 3 6 & 8=108.2”,
and therefore do,,, = 6.114 A. The u axes and do,, of Hf,NiP are
longer (by ca. 1.4% and 5 YO,respectively), whereas the b axis is
about 11% shorter.
Accordingly, a significant difference occurs between the
Ni-Ni and Ni-P chains of HfNi and Hf,NiP running parallel
to [OlO]: the Ni-P distances of2.337(7) 8, are much shorter than
the Ni-Ni distances of2.607 A,and the bond angles vary from
103.1” (Ni-P-Ni) to 104.3”(Ni-Ni-Ni). (The atomic positions for
HfNi were taken from refinements of the isostructural ZrNi,
with dNi+Ni
= 2.616
Although these M’-P bonds are
longer than those observed in Hf,Co,+,P,_, (dco-p= 2.32(2) A,
whereby rc,>rNi) and HfNiNi,P(dNi+p
= 2.1578(7) 8,)[lS1
in
which Co, Ni, and P are also located in trigonal Hf, prisms,
bonding character is assumed, as shown by the bond order of
n = 0.73, calculated with Pauling’s equation: <(n) =
d(1) -0.6Ign ( d = bond length, rNi =1.154& rp =1.10A).i’61
The formation of strong bonding Ni-P interactions is most
likely the main driving force for this deformation. On the other
hand, the Ni-Ni contacts in HfNi have only weak bonding
character. Probably as a consequence of the structure deformation, the mean Hf-Ni distance of2.687 A in HfNi is significant= 2.704 A).
ly smaller than in Hf,NiP (dHf-Ni
The substitution of Ni by P leads to the occurrence of two
crystallographically different Hf atoms in the structure of
Hf,NiP: one is coordinated by three Ni and four P atoms, and
the other by four Ni and three P atoms, with similar Hfenvironments for both types of Hfatom. The shortest Hf-Hf distances
in Hf,NiP of about 3.2 8, (corresponding distances in HfNi
about 3.4 A) occur within puckered Hflayers parallel to (Oll),
as emphasized with bold lines in Figure 2, which are interconnected parallel to [I 101 by longer Hf-Hf bonds of about 3.4 8,
(HfNi: 3.4 8,). Hf-Hf distances of 3.4 8, are still to be considered as bonding (Pauling bond order of 0.14, calculated with
rHf =1.4428,).i’61 The Hf-Hf interactions along the b and c
axes, which complete the trigonal prisms surrounding the interstitial atoms, are significantly different from those in HfNi
(parallel to c axis of Hf,NiP: alternating 3.473(5) 8, and
3.719(5) Avs. 4.117 8,inHfNi; baxis: 3.6567(8) 8,vs. 3.218 A).
Altogether, the sums of the Hf-Hf (Pauling) bond orders
(PBOs) do not significantly differ from Hf,NiP (mean total
PBO,,-.,
=1.43 per Hf atom) to HfNi (total PBOH,-H, = 1.40
per Hf atom).
Delocalized electrons within the three-dimensional framework of Hf atoms should lead to metallic behavior. To explore
this expectation and our considerations of the bonding character of the different interactions further, we calculated the band
structure of Hf,NiP, using the Extended Hiickel approximation.[”] The Hf and Ni parameters were obtained by solid-state
charge iteration on Hf7P411 and Zr,Ni,P, ,[Io1 respectively, and
the P parameters were taken from standard sources.[’9JFigure 4
shows the calculated DOS (density of states) curve and the Hf
and Ni contributions for the total DOS for Hf,NiP, highlighting
the Fermi levels for the different valence electron counts of
Hf,NiP (46 e-), and (theoretical) “Hf,CoP” (44 e-) and
“Hf,FeP” (42 e-).
As expected, because of the nonsaturated character of
Hf,NiP, a significant density of states occurs at the Fermi level
of -9.2 eV, which consists mainly of the Hf 5d states. The 3d
states of Ni are located in a small region around - 11 eV, which
is well below the Fermi level. This leads to the assumption that
the d states are completely filled, and thus, since the s states are
not significantly filled, to a formal oxidation state of Ni’. Since
the 3p block of P is found between - 13 eV and - 15.5 eV, a
formal oxidation state of -111 can be assumed for the P atoms,
and therefore Hfcan be formally regarded as Hf””. This leads
f)833,’97/3605-0514 $15.00+ .25111
Angew. Clwm. I n t . Ed. Engl. 1997. 36, N o . 5
COMMUNICATIONS
-4.0
-6.0
-6.0
t-
-10.0
EIeV
r
-12.0
I
the early and the late transition metals are confirmed by our
calculations, in accordance with the Lewis acid/base concept. Of
course, the metal-nonmetal interactions also have strong bonding character, with averaged Mullikan overlap populations for
the Hf-P bonds of 0.23 and for the Ni-P bonds of 0.14.
The expectation of metallic behavior of Hf,NiP, in accordance
with the calculations of the electronic structure, were confirmed
by the experimentally determined weak Pauli paramagnetism,
which is temperature independent between 6 and 300 K.
For a more detailed consideration of the differences between
HfNi and Hf,NiP, we calculated the band structures for the
isolated Ni-Ni chain of HfNi and for the Ni-P chain of
Hf,NiP. As can be seen in Figure 6, the DOS of the Ni-P chain
Figure 4. Density of states of Hf,NiP. Left: Hf contributions; right: Ni contributions emphasized.
to the formal ionic formula (Hf’.5+),Ni0P3- and indicates that
2.5 delectrons per Hf atom remain for the formation of metalmetal bonds. The mixing of Hf and Ni states with thep block of
P, however, indicates covalent interactions with the p orbitals of
P. Thus, the discussion of oxidation states is a formal one and
is not meant to imply ionic interactions.
The crystal orbital overlap populations (COOP curve, see
Figure 5) do indeed show the bonding character of the Hf-Hf
and Hf- Ni interactions, which are the only interactions with
t
EleV
Figure 6. DOS curves of the Ni-P chain in Hf,NiP (left) and of the Ni-Ni chain
in H f N (right).
-8 0
-8 5
-9 0
-9 5
-10 0
EIeV
-10 5
12 0
il-22
-0 2
00
02
Figure 5. COOP curve of Hf,NiP. Solid line Hf--Hf, dashed line Hf-Ni interactions. The left half of the diagram covers the antibonding, the right half the bonding
interactions.
significant overlap populations at the Fermi level. Decreasing
the valence electron concentration, which corresponds to a substitution of Ni by Co or Fe, would result in loss of bonding
Hf-Hf and HT-M’ interactions (M’ = Fe, Co, Ni). From this
point of view, the possibility of the formation of isostructural
“Hf,CoP” and “Hf,FeP” is unlikely but not impossible.
The sum of the overlap populations for the Hf-Hf bonds per
Hf atom decreases from HfNi (0.99) to Hf,NiP (0.83); this
indicates that fewer electrons are available in Hf2NiPfor Hf-Hf
bonding. This is in accordance with the higher oxidation state of
the P atom. The mean Mullikan overlap population (MOP) for
the Hf-Ni bonds in Hf,NiP (0.10) is only slightly smaller than
that for the HfNi structure (0.12). Both values can be directly
compared to the Zr-Ni interactions in Zr,Ni,P, (MOP =
0.13). In all of these cases, strong bonding interactions between
An,qeiv. Chein. h r . Ed. Engl. 1997, 36. No. 5
,C;
contains a band gap of about 3 eV in the region of the Fermi
level of Hf,NiP ( - 9.2 eV). Therefore, the Ni-P chain does not
contribute to the metallic character of Hf2NiP; in contrast the
Ni-Ni chain does contribute to the metallic properties of HfNi,
since it does not have a band gap in the region of the Fermi level
of HfNi ( - 9.34 eV). The energy window shown between - 15
and -5 eV consists in the case of the Ni-P chain o f three wellseparated parts: the p block of P is found below - 12 eV, the d
states of Ni around - 11 eV and the s states of Ni at approximately - 6 eV, well above the Fermi level. Thus, the orbitals of
the Ni-P chain are filled with exactly 18 electrons below the
large band gap, which corresponds to the above-mentioned electron count, in which Ni is formally regarded as Ni’, and P as
P3-. The states of the isolated Ni-Ni chain can be filled with
approximately 21.5 electrons up to the Fermi level of -9.34 eV,
which indicates a negative charge of the Ni-Ni chain. As a
consequence, the s orbitals of Ni are assumed to be partially
filled only in the case of HfNi, since they are spread over the
range of-11.5 to -6.5eV.
In summary, the formation of Hf,NiP clearly shows that new
structures are predictable in some cases, and that in general
small late transition metal atoms in intermetallic phases can be
substituted by nonmetal atoms such as phosphorus. In this particular case, a negatively charged Ni-Ni chain with metallic
character was replaced by a more negatively charged Ni-P
chain that does not contribute to the metallic properties. Further attempts to modify other refractory intermetallic phases,
for example ZrNi, which is isostructural with HfNi, by substitution of the late transition metal by P or S are currently in progress.
VCH I~eerlug.~ge.c.ellscliu~
nzbH, 0.69451 Weinheinz,1997
Received: September 30,1996 [Z95021E]
German version: Angew. Chem. 1997, 109, 530-533
0570-0#33:97/3605-05i5$ i5.00+ .25/0
515
COMMUNICATIONS
Keywords: hafnium intermetallic phases
rus solid-state structures
-
nickel
. phospho-
[I] H . F. Franzen, Prog. Solid State Chenz. 1978. 12. 1-39.
121 G . A. Marking, H. F. Franzen, J. A1loy.s Conzpd. 1994, 204, L17-L20.
L3J B. Elarbrecht, Z. Kristdlogr. 1988, 182. 118-120.
[4] B. Harbrecht, H. F. Franzen, J Less-Common Met. 1985, 113, 349-360; B.
Harbrecht. ibid. 1986, 124, 125-134.
[5] 8 . Harbrecht, J. Less-Common M e / . 1988, 141, 59-71.
161 M. Conrad, B. Harbrecht, J Al1oy.s Compd. 1993, 197. 57 - 64.
171 K. L. Abdon. T. Hughbanks, Chrn?.M u t e r . 1994. 6. 424-428.
[8] T. Hughbanks, J Alloys Conzpd. 1995, 229, 40-53.
[Y] H. Kleinke, H. F. Franzen, J M o J s Conzpd. 1996, 229, 40-53.
[lo] H. Kleinke, H. F. Franzen, Inorg. Chem. 1996, 3.5, 5272-5277.
[I I] K. M. ran Essen. K. H. J. Buschow, J. Less Common Met. 1979, 64, 277-284.
[I?] Crystal structure analysis of Hf,NiP: lattice constants for the ideal monoclinic
cell (space group P2,;m) were obtained from Guinier powder patterns under
vacuum, with S i as an internal standard: a = 5.2259(9) A,h = 3.6567(8)
c =7.192(1)
/{ =110.12(1)r, V = 129.05(8) A3 (12 reflections, Cu,, =
1.5406
An automatic four-circle diffractometer with a rotating anode was
used for the data collection (AFC6R, Rigaku, 23 "C, Mo,,). Experimental
details of data collection: Z = 2, p =11.49 gcm-3, 1( = 870.7cm-',
20md,= 7 0 , min./inax. transmission: 0.75- 1.26. Refinements and absorption
correction (Y scan, followed by DIFABS [13]) were carried out with the
TEXSAN program package [14]. Final residuals are R(P2)= 0.065,
R , ( F 2 ) = 0.070. G O F =1.08 with 184 independent observed reflections
(T> 3n(I)) and 23 refined parameters. Further details of the crystal structure
investigations may be obtained from the Fachinfonnationszentrum Karlsruhe.
D-76344 Eggenstein-Leopoldahafen (Germany), on quoting the depository
number CSD-405616.
[13] N. Walker, D. Stuart, Acla Crystallogr. Se<,t.A 1983, 39, 159-166.
1141 'IEXSAN: Single Crystal Structure Analjsir Soffwurr, Version 5.0, Molecular
Structure Corporation, The Woodlands, TX, 1989.
[ I S ] H. Kleinke. H F. Franzen, Z. Anorg. Allg. Chem. 1996, 622, 1342-1348.
[16] L. Pauling, The Nature o/ the Chemical Bond, 3rd ed., Cornell University Press.
Ithaca. NY. 1948.
[17] R. Hoffmann,J. Chem. P h j , ~1963,39,1397-1412;
.
M.-H. Whangbo, R. Hoffniann, J. Am. Chrm. Sw. 1978,100,6093-6098; R. Hoffmann, Angrw. Chrm.
1987. YY, 871 -906; Angeu. Chem. 1m.Ed. Engl. 1987. 26, 846-878.
[I81 t3. Kleinke, H. F. Franzen, Angew. Chem. 1996. 108, 2062-2064; Angew.
Chrm Int. Ed. Engl. 1996, 35, 1934-1936.
[19] E. Clementi, C. Roetti, Aloniic Dutu andNnclear Data Tahles 1974, 14. 177478.
A).
A,
A.
Assembly of Mesoporous Molecular Sieves
Containing Wormhole Motifs by a
Nonionic Surfactant Pathway:
Control of Pore Size by Synthesis Temperature**
Eric Prouzet and Thomas J. Piiinavaia"
The Mobil M41S family of mesoporous molecular
has expanded greatly the range of materials available for hetero**~
geneous catalysis[3 61 and supramolecular a ~ s e m b l y . [ ~These
mesostructures are formed through structure-directing interaclions between surfactant micelles and inorganic precursors. Assembly can occur by electrostatic charge-matsching mechanisms".'- I 3 l or by electrically neutral pathways that depend on
hydrogen bonding""
or complexation['9~20]
at the organic/
-
-
[*I
Prof. T. J. Pinnavaia*
Department of Chemistry and Center for 1,undainental Materials
Research
Michigan State University
bast Lansing, MI 48824 (USA)
Fax: Int. code +(517)432-1225
e-inail : pinnavaia(0 cemvax.cem,msu.edu
Dr. E. Prouzet
liistitut des Materiaux de Nantes. CNRS (France)
[**I This research was supported by the National Science Foundation through
Chemistry Research Group Grants CHE-9224102 and CHE-9633798.
516
<-) VCH VerlrrggeseNschuftmbH, 0.69451 Weinheim,1997
inorganic interface. Whereas M41S structures characteristically
exhibit long-range hexagonal or cubic symmetry, less ordered
structures with wormhole motifs have been obtained recently
through the NoIo assembly of nonionic polyethylene oxide
(PEO) based surfactants (NO) and neutral inorganic precursors
(Io) .[17,211 These wormhole structures, which have been denoted
MSU-X, lack regular channel packing order. Nevertheless, they
exhibit uniform channel diameters over a range comparable to
M41S materials and offer attractive advantages for processing,
owing in part to the low cost and biodegradability of N o surfactants.
Here we report an unprecedented property of No surfactants
for mesostructure assembly, namely, the ability to control pore
size rationally simply by regulating the synthesis temperature in
the presence of a single surfactant. Previous approaches to mediating the pore sizes of mesostructures have dependcd on the
use of surfactants of different chain lengths, organic cosurfactants,"] and post-synthesis hydrothermal t r e a t m e n t ~ [ ~for
~.~~I
modifying pore structure. However, only the Nolo pathway is
capable of tailoring pore sizc by choice of the assembly temperature. For instance. the average pore diameter of MSU-X silicas
can be altered by as much as 2.4 nm simply by controlling the
assembly temperature over a relatively narrow range (25
65 "C).
Our approach to the pore-size-selective NoIo assembly of
MSU-X silicas utilizes a homogeneous reaction medium. This
"solution" method differs from the gel-based procedure developed earlier for NoIo assembly.[' 'I The inorganic alkoxide precursor is added to an aqueous No suspension, and then the
mixture is allowed to age at room temperature to obtain a homogeneous solution. Mesostructure assembly is initiated in a
second step by the addition of fluoride ion, which catalyzes
alkoxide hydrolysisr241and mesostructure cross-linking. This
second step is the key to facilitating mesostructure formation
from solution and to varying the pore size through the synthesis
temperature.
NoIo assembly of MSU-X silicas from solution has been accomplished by using several commercial PEO surfactants. including the Tergitol 15-S-n series of the type Cll-15HZ3.310(CH,CH,O),H with 7 < n < 20 (Union Carbide, Danbury,
Connecticut), Triton X-100 (CH,),CCH,C(CH,),C,H,(CH,CH,O),,H
(Union Carbide), and Igepal RC-760
CH,(CH,), ,C,H,(CH,CH,O),,H
(Rhhe-Poulenc, Cranbury,
New Jersey). Because the mesostructures obtained with the
Tergitol family of surfactants are representative of the pore size
properties generally observed for solution NoIo assembly, only
these derivatives will be considered in detail.
X-ray patterns of calcined MSU-1 silicas prepared from Tergitol 15-S-12 at 35°C and at different NaFiTEOS molar ratios
are displayed in Figure 1. To facilitate comparison, the diffraction peaks have been separated from the background scattering
by fitting the background to the exponential Ae'-R/24)+C.
The
background-corrected peaks are shown in the insert in Figure 1.
Fluorine not only improves the scattering intensity, but also
increases the basal spacing from about 4.2 to about 4.7 nm; this
indicates the probable presence of fluorine in the as-synthesized
structure. Magic angle spinning (MAS) 29Si NMR spectra of
MSU-1 silicas prepared at molar NaF/TEOS ratios up to 10%
all exhibited Q 3 and Q" signals with relative intensity ratios
close to 0.4. Although somc of the silicon centers in the assynthesized materials are bonded to fluorine, all fluorine is
removed upon calcination at 600°C, as shown by 19F NMR
spectroscopy.
Figure 2 shows the XRD patterns for MSU-1 silicas assembled from Tergitol15-s-I 2 at two temperatures. The d spac-
0570-0833~97/36f/5-~~516
$ 1.5.00f ,2510
~
Angric. Chem. I n r . Ed. En$/. 1997. 36. .Yo. 5
Документ
Категория
Без категории
Просмотров
1
Размер файла
532 Кб
Теги
nickell, forma, intermetallic, hf2nip, substitution, modification, phase, phosphorus, planned
1/--страниц
Пожаловаться на содержимое документа