Synthesis and characterisation of aluminophosphate-based zeotype materials prepared with α,ω-bis(N-methylpyrrolidinium)alkane cations as structure-directing agents

Article abstract of DOI:10.1039/b706206k

New routes to the preparation of two families of small pore microporous solids are described. Flexible diquaternary alkyl ammonium cations (C ₄H₈)CH₃N⁺(CH₂) _nN⁺CH₃(C₂H₈), n = 3-5, have been found to act as structure directing agents in the synthesis of aluminophosphate-based materials of the ABC-6 group. The templates exhibit considerable selectivity in crystallising phases of the ERI or AFX-type (MeAPO-17 or MeAPO-56, respectively) from a wide variety of metalloaluminophosphate gels (Me = Mg²⁺, Co²⁺ or Si ⁴⁺). For example, from gels containing Mg/P = 0-0.25, 1,4-bis(N-methylpyrrolidinium)butane cations gave MgAPO-17, and 1,5-bis(N-methylpyrrolidinium)pentane cations gave MgAPO-56. Notably, materials could be prepared with levels of metal-substitution tailored to a high degree and distributed non-randomly within the framework. As-prepared solids have been characterised by powder XRD, multinuclear solid state NMR, FT-IR, SEM, and thermal and elemental analysis. The Royal Society of Chemistry.

Full text of DOI:10.1039/b706206k

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Synthesis and characterisation of aluminophosphate-based zeotype materials
prepared with a,x-bis(N-methylpyrrolidinium)alkane cations as
structure-directing agents†
Martin J. Maple* and Craig D. Williams
Received 24th April 2007, Accepted 11th July 2007
First published as an Advance Article on the web 31st July 2007
DOI: 10.1039/b706206k
New routes to the preparation of two families of small pore microporous solids are described. Flexible
+
+
diquaternary alkyl ammonium cations (C
4
H
8
)CH
3
N (CH
2
)
n
N CH
3
(C
2
H
8
), n = 3–5, have been found to
act as structure directing agents in the synthesis of aluminophosphate-based materials of the ABC-6
group. The templates exhibit considerable selectivity in crystallising phases of the ERI or AFX-type
(
(
MeAPO-17 or MeAPO-56, respectively) from a wide variety of metalloaluminophosphate gels
Me = Mg , Co or Si ). For example, from gels containing Mg/P = 0–0.25, 1,4-bis(N-methyl-
2
+
2+
4+
pyrrolidinium)butane cations gave MgAPO-17, and 1,5-bis(N-methylpyrrolidinium)pentane cations
gave MgAPO-56. Notably, materials could be prepared with levels of metal-substitution tailored to a
high degree and distributed non-randomly within the framework. As-prepared solids have been
characterised by powder XRD, multinuclear solid state NMR, FT-IR, SEM, and thermal and
elemental analysis.
Introduction
in which cations have been aliovalently substituted into framework
sites. Materials containing metals are designated MeAPOs, while
8
1
Aluminophosphates (AlPO -n) are zeotypes containing strictly
4
those containing Si are termed SAPOs. Combinations are also
possible. Solid acid catalysts are generated by substituting, for
alternating alumina and phosphate tetrahedra, which corner-
share to form open framework structures. The connectivity of
the tetrahedra define the topology, or framework type, which is
independent of composition. Novel topologies are assigned three
2+
3+
4+
5+ 9
example, Mg for Al or Si for P . The charge on the
framework is balanced, after the SDA is removed, by protons
which form Brønsted acid sites. Additionally, redox properties
2
letter codes by the International Zeolite Association and many are
2+
2+
may be conferred when cations such as Co or Mn are included
found as both aluminosilicates (zeolites) and aluminophosphates.
The preparation of such materials often requires the presence
of organic amines or quaternary ammonium salts which fill
the pores in the as-made materials. The inorganic framework
crystallises around the organic which remains within the channels
after synthesis. The precise role of such structure directing agents
3+ 10
in place of Al . Such substitutions invariably, but not exclusively,
result in families of isomorphous materials. A notable exception
are materials prepared in the presence of 1,4,8,11-tetramethyl-
3
1
,4,8,11-tetraazacyclotetradecane: STA-6 (SAS) crystallises in the
2+
4+
presence of Mg or Si (amongst others) while STA-7 (SAV) is
2+
2+
11
formed in Co or Zn containing gels. These materials exemplify
the need to examine closely details of the gel chemistry as part of
the search for novel materials and compositions.
(
SDAs), or ‘templates’ is, in many cases, poorly understood.
Particular organic moieties may or may not direct the synthesis of
materials having the same framework types when added to silicate
and phosphate-based preparations. For example, zeolite SSZ-16
Recently 1,4-bis(N-methylpyrrolidinium)butane bromide has
been found to template two new aluminosilicate zeolites, TNU-
4
shares the AFX topology and may be prepared with the same
12
13
9
and TNU-10. In the work we report here we have applied
1
5
,4-bis(1-azoniabicyclo[2.2.2]octane)butane cations as MgAPO-
6 (albeit the latter co-crystallises with STA-2). In contrast,
this template, and two further members of the homologous
series (one larger and one smaller), to the crystallisation of
aluminophosphate-based materials both pure and also substituted
5
6
decamethonium cations template EU-1 (EUO) zeolites, and the
magnesioaluminophosphate DAF-1 (DFO), each of which have
a different framework structure.
The search for novel topologies or compositions is driven
by the need to develop catalysts with new and different shape
selectivities and activities. Thus there is considerable interest in the
synthesis of microporous aluminophosphates, particularly those
7
2+
2+
4+
with Mg , Co , or Si cations. Here we describe the preparation
of materials with the ERI and AFX framework types, over a wider
14,15
range of compositions than previously reported.
The cavities
present within these solids (Fig. 1) match closely the shape of the
SDAs, which we suggest leads to the strong structure directing
effect.
School of Applied Sciences, University of Wolverhampton, Wulfruna Street,
Wolverhampton, UK WV1 1SB. E-mail: m.maple@wlv.ac.uk; Fax: +44
Experimental
(
0)1902 322714; Tel: +44 (0)1902 322737
Divalent a,x-bis(N-methylpyrrolidinium)alkane hydroxide tem-
plates (diquat-x) were synthesised via the Menschutkin reaction
and ion exchanged from the bromide forms with silver(I) oxide.
†
Electronic supplementary information (ESI) available: Structural stereo-
plots, FT-IR spectra, deconvoluted P MAS NMR spectra and simulated
XRD patterns. See DOI: 10.1039/b706206k
16
31
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Dalton Trans., 2007, 4175–4181 | 4175

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water until the pH of the filtrate leaving the filter approached 7.
The resultant aqueous solution contained the template and was
concentrated on a rotary evaporator before being added to the
synthesis gel.
Gels were made up by dissolving orthophosphoric acid (85%,
BDH) in distilled water and adding either magnesium acetate
tetrahydrate (Aldrich, 98%), cobalt acetate tetrahydrate (BDH,
97%), Ludox TMA colloidal silica (Aldrich) or fumed silica
(
(
Aldrich, 99.8%), followed by hydrated aluminium hydroxide
Aldrich) and the template. The mixture was stirred for 15 min, un-
til homogeneous, before being transferred to PTFE-lined stainless
◦
steel autoclaves and heated under autogeneous pressure at 190 C
for 48–72 h (for MeAPOs) or 96 h (for SAPOs prepared with
colloidal silica) or 168 h (for SAPOs prepared with fumed silica).
Initial gel pH were ca. 7. For comparison, aluminophosphate
gels were prepared without additional metal cations, and with or
without the addition of aqueous hydrofluoric acid (Fluka, 40%).
These gels were all treated in the same way as the MeAPOs. In
all cases the crystalline products were suction filtered, washed
with distilled water and dried before being characterised. Further
experimental details are given in Table 1.
Fig. 1 Illustrative diagrams showing the principal cavities present in the
framework types (a) ERI and (b) AFX, and the SDA molecules which direct
their formation. Only the tetrahedral framework connectivity is shown, the
oxygen positions being omitted for clarity.
A slight excess of 1-methylpyrrolidine was refluxed overnight in
acetone with either 1,3-dibromopropane, 1,4-dibromobutane, or
1
,5-dibromopentane. All organic reagents were obtained from
For phase identification, materials were analysed by XRD
on a Philips PW1050/81 diffractometer operating in Bragg–
Brentano geometry with Cu-Ka X-radiation and secondary
monochromation. Theta compensating divergence slits were used
giving a constant irradiated sample length of 12.5 mm. Materials
were placed on an aluminium plate sample holder and data
Aldrich (97–99%). The resultant white precipitates were suction
filtered and washed with acetone to remove unreacted amine.
Before addition to the aluminophosphate gels the required amount
of template was converted to the hydroxide form by stirring with
Ag
2
O (Alfa Aesar, 99%) in distilled water for 15 min. Solid material
◦
◦
was removed by filtration and the residue washed with distilled
collected in 40 min over the 2h range 3–50 , with a 0.02 step.
Table 1 Summary of synthesis gel compositions, product identification and selected materials’ framework compositions
Synthesis gel composition
Cation ratio
Inorganic composition
a
b
31
Run No.
Fluoride ratio
0.8
Template
Product solid
ICP-OES
P MAS NMR
1
2
3
4
5
6
7
8
9
1.0 Al : 1.0 P
1.0 Al : 1.0 P
0.1 Mg : 0.9 Al : 1.0 P
0.1 Co : 0.9 Al : 1.0 P
0.1 Si : 1.0 Al : 0.9 P
0.2 Si : 1.0 Al : 0.8 P
1.0 Al : 1.0 P
diquat-3
diquat-3
diquat-3
diquat-3
diquat-3
diquat-3
diquat-4
diquat-4
diquat-4
diquat-4
diquat-4
diquat-4
diquat-4
diquat-4
diquat-4
diquat-4
diquat-5
diquat-5
diquat-5
diquat-5
diquat-5
diquat-5
diquat-5
diquat-5
diquat-5
diquat-5
ERI + A
ERI + A
ERI + B
ERI
Co0.11Al0.89PO
4
ERI + B
ERI + B
ERI + A
ERI
1.0 Al : 1.0 P
0.8
AlPO
4
0.1 Mg : 0.9 Al : 1.0 P
0.15 Mg : 0.85 Al : 1.0 P
0.2 Mg : 0.8 Al : 1.0 P
0.25 Mg : 0.75 Al : 1.0 P
0.5 Mg : 0.5 Al : 1.0 P
0.1 Co : 0.9 Al : 1.0 P
0.1 Si : 1.0 Al : 0.9 P
0.2 Si : 1.0 Al : 0.8 P
1.0 Al : 1.0 P
ERI
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
ERI
Mg0.14Al0.86PO
4
Mg0.33Al0.67PO
4
ERI
ERI
Mg0.17Al0.83PO
Mg0.39Al0.61PO
4
4
Mg0.31Al0.69PO
Mg0.22Al0.78PO
4
4
ERI + C
ERI
Co0.11Al0.89PO
4
ERI
c
ERI
D
d
1.0 Al : 1.0 P
0.3
AFX + ERI
0.1 Mg : 0.9 Al : 1.0 P
0.15 Mg : 0.85 Al : 1.0 P
0.2 Mg : 0.8 Al : 1.0 P
0.25 Mg : 0.75 Al : 1.0 P
0.5 Mg : 0.5 Al : 1.0 P
0.1 Co : 0.9 Al : 1.0 P
0.1 Si : 1.0 Al : 0.9 P
0.2 Si : 1.0 Al : 0.8 P
AFX
AFX
Mg0.17Al0.83PO
Mg0.30Al0.70PO
Mg0.39Al0.61PO
4
4
4
Mg0.21Al0.79PO
Mg0.45Al0.55PO
4
4
AFX
AFX
AFX + E
AFX + F
d
AFX + ERI
AFX + F
a
Templates were added as concentrated aqueous hydroxide solutions at a mole ratio of 0.6, to give an initial gel pH of ca. 7; diquat-3 = 1,3-bis(N-
b
methylpyrrolidinium)propane; diquat-4 = 1,4-bis(N-methylpyrrolidinium)butane; diquat-5 = 1,5-bis(N-methylpyrrolidinium)pentane. Unidentified
c
d
impurity phases are labelled A–F. Poorly crystalline. Approximately 50 : 50 mixture.
4
176 | Dalton Trans., 2007, 4175–4181
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The crystal morphology in selected samples was examined by
scanning electron microscopy. Samples were mounted on carbon
stubs and gold coated before being examined on a Zeiss Evo 50
SEM. Infrared spectra were recorded on a Mattson Genesis II FT-
IR system equipped with an ATR accessory (32 scans at 1 cm
resolution).
content of the gel, or switching from fumed to colloidal silica, did
not change the products formed. The CoAPO is obtained pure,
as a blue solid, which indicates the presence of tetrahedral Co
included within the aluminophosphate framework.
2
+
−
1
Preparations using the 1,4-bis(N-methylpyrrolidinium)butane
(diquat-4) cation also yield materials of the ERI framework-
type. The crystals possess a hexagonal prismatic morphology,
however, they are of poor quality being composed of blades.
Aluminophosphates may be synthesised in the presence or absence
The inorganic composition of selected materials was determined
by inductively coupled plasma-optical emission spectroscopy
(
ICP-OES). Samples were dissolved in nitric acid (Aristar) and
−
analysed with a Spectro Ciros CCD spectrometer. The amount of
included organic and water was determined by thermogravimetric
analysis (TGA) on a Mettler Toledo TG 50 thermobalance. Finely
of F , phase pure material being obtained with a gel F/P ratio
of 0.8 (Fig. 2a–2b). The MgAPO, SAPO and CoAPO ERI-
type materials are prepared phase pure from gels containing
10 mol% Mg, Si or Co, respectively. Increasing the Si content above
15 mol%, using either fumed or colloidal silica sources, reduces the
crystallinity of the resultant material. This is presumably due either
to the accumulation of amorphous material, or to the presence of
faulting, as has been observed in other SAPO materials in the
3
−1
ground samples were heated in flowing air (200 cm min ) to
◦
◦
−1
900 C at 10 C min .
To determine whether the template molecules had been incor-
1
3
porated intact within the materials’ pores, C solid state NMR
spectroscopy was performed. Data were collected at ambient
temperature on a Varian UNITY Inova spectrometer equipped
17,18
ABC-6 family.
Notwithstanding this, the general reduction in
1
13
with a 7.05 T Oxford Instruments magnet. H– C CP MAS NMR
spectra were recorded at 75.40 MHz with an acquisition time of
impurity phases seen in these products of reaction (compared to
those prepared with diquat-3) is probably due to there being a
closer match between the size of the template and the ERI cavity.
1
◦
3
0 ms, relaxation delay of 1 s and contact time of 1 ms (the H 90.0
pulse was 4.5 ls). The samples were spun at 5 kHz and referenced
2
7
31
to TMS. Similarly Al and P NMR data were obtained, for
selected materials, to examine the local structure of the inorganic
2
7
framework. DP MAS NMR spectra were recorded for Al at
◦
7
8.12 MHz with a 1 ls (20.0 ) pulse, an acquisition time of 30 ms
3
1
and relaxation delay of 0.2 s, and for P at 121.37 MHz with a
.5 ls (90.0 ) pulse, an acquisition time of 20.2 ms and relaxation
◦
4
delay of 60 s. Samples were spun at 14 kHz and referenced to 1 M
1
29
AlCl
3
or 85% H
3
PO , for Al and P, respectively. A H– Si CP MAS
4
NMR spectrum was recorded at 59.56 MHz with an acquisition
time of 30 ms, relaxation delay of 1 s and contact time of 3 ms
1
◦
(
the H 90.0 pulse was 5 ls). The sample was spun at 5 kHz and
referenced to TMS. All relaxation delays were optimised.
Results and discussion
Synthesis
Materials were prepared with diquaternary templating cations,
the length of the methylene chain being varied systematically
to investigate the effect on product formation. Attempts were
2
+
4+
2+
made to introduce elements (Mg , Si , Co ) into the AlPO
4
framework, the level being adjusted (in selected cases) to try to
obtain phase pure materials. It is expected that the presence of
these cations will modify the catalytic behaviour of the solids
formed. The results of representative hydrothermal syntheses are
given in Table 1, including synthesis gel compositions, identified
phases and inorganic elemental analysis of selected samples.
Microporous materials having the framework-type code ERI
(
AlPO
4
-17) were formed as hexagonal prismatic crystals from gels
containing the 1,3-bis(N-methylpyrrolidinium)propane (diquat-
3
) template. Aluminophosphates crystallise with or without the
−
inclusion of F in the gel, although a trace amount of an
unidentified impurity is always present (denoted phase A, with the
Fig. 2 XRD patterns of as-prepared solids templated with diquat-4,
(
(
a) AlPO
4
4
–ERI + trace A, (b) AlPO (F)–ERI, (c) Mg0.1–ERI,
˚
longest d-spacing at ca 10.4 A). The MgAPO, SAPO and CoAPO
d) Mg0.15–ERI, (e) Mg0.2–ERI, (f) Mg0.25–ERI, (g) Mg0.5–ERI + C. As-
may also be prepared, although in the case of the Mg- and Si-
terisks mark reflections from impurity phases. The average refined unit cell
containing materials an additional impurity is observed (denoted
was found to be hexagonal with a = 13.23(3) A˚ , c = 14.76(9) A˚ , which is
˚
2
phase B, with longest d-spacing at ca 9.3 A). Increasing the Si
similar to literature values.
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To determine the range of gel compositions from which materials
could be obtained further Mg-containing gels were prepared
Doubling the Si ratio gave AFX, in the absence of ERI, but with a
small amount of an impurity phase present. This mixture (AFX +
F) formed again from the Co-containing gels. Including Mg in the
aluminophosphate gel led to the production of phase pure AFX-
type materials over the Mg/P range 0.1–0.25 (Fig. 3c–3f). Only
when Mg/P reaches 0.5 does an impure sample form, containing
materials with both the AFX and an unidentified framework-type
(Fig. 3g).
(
Table 1). Solids were obtained apparently pure (by XRD) with
Mg/P of 0.1–0.25. The XRD patterns (Fig. 2c–2f) change subtly,
◦
particularly in the 2h region 23–25 , as the Mg/P ratio in the gel
increases; this will be discussed further below. A gel with Mg/P
of 0.5 yields an impure sample containing materials with both the
ERI and an unidentified structure-type (Fig. 2g).
When syntheses were carried out with 1,5-bis(N-methyl-
pyrrolidinium)pentane (diquat-5) cations microporous solids were
again formed. SEM revealed twinned crystals, of a hexagonal
rosette-like appearance. From purely aluminophosphate gels, in
Framework characterisation
To characterise further the MgAPO materials synthesised from
gels with increasing Mg-loading, a spectroscopic analysis was
performed. FT-IR spectra, in the structural region, are given in
the ESI† for as-prepared materials synthesised with diquat-4 and
-5, respectively. The most important changes in the spectra to
note occur when Mg is included in the aluminophosphate gels.
There is significant broadening and a small red shift in the T–
O–T asymmetric stretching region (ca 1055 cm ) of the MgAPO
spectra when compared to the AlPO spectra, indicating that Mg
is substituting into the aluminophosphate lattice.
the absence of fluoride ions, an unidentified phase crystallised
−
(
Fig. 3a). If F was included in an identical gel, at an F/diquat-5
ratio of 0.5, a mixed phase product comprising ERI and AFX-
type aluminophosphates (Fig. 3b) was obtained. In a further
attempt to prepare pure AlPO
adjusted systematically. At ratios other than 0.5 AlPO
4
(F)-56 the F/diquat-5 ratio was
(F)-56 was
4
−
1
not obtained, instead a poorly crystalline material formed. A
mixed phase product, containing equal quantities of AFX and
ERI crystals, was formed if Si was included at a Si/Al ratio of 0.1.
4
Further characterisation was carried out on magnesioalu-
minophosphates prepared from gels with Mg/P = 0.15 and 0.25,
these being chosen to be representative samples. The mixed phase
2
7
formed with diquat-4 at Mg/P = 0.5 was also studied. The Al
MAS NMR spectra of as-prepared Mg0.15–ERI, Mg0.25–ERI and
Mg0.5–ERI (the subscript representing the Mg/P ratio in the
synthesis gel) all show a resonance at ca 35 ppm (Fig. 4a–4c)
which is attributed to Al in tetrahedral coordination. The sample
Mg0.15–ERI has an additional signal at 14.5 ppm which may be
attributed to a pentacoordinated species in which a water molecule
19
is additionally coordinated to the framework Al. The presence
of 5-coordinate Al leads to distortion of the framework structure,
which was indicated by changes in the powder XRD patterns
◦
discussed above (cf. Fig. 2d with 2f–g in the 2h region 23–25 ).
Mg0.15–AFX and Mg0.25–AFX both show two resonances in their
2
7
Al spectrum (Fig. 4d–4e). The signal at ca 36 ppm is assigned
to Al in tetrahedral coordination, while the signal at 7.0 ppm is
attributed to pentacoordinated Al, as before.
3
1
The P MAS NMR spectra (Fig. 5) of these samples comprise
a cluster of poorly resolved resonances between −5 and −32 ppm.
Additional signals (at ca 0.2 and −1.8 ppm) with narrow line-
widths, are seen in the samples Mg0.25–ERI and Mg0.5–ERI, and
these are ascribed to the unidentified phase C. It should be noted
that by XRD Mg0.25–ERI appears phase pure, however the greater
sensitivity of NMR has revealed the trace impurity. The relative
3
1
intensities of the other signals in each of the P spectra vary
between the samples and may be deconvoluted to yield four
3
1
or five resonances (see ESI†). If the changes in P chemical
shifts in the spectra are due primarily to changes in the next
nearest neighbour (i.e. Mg substitution), rather than differences in
20
crystallographic site, as several authors have stated, then the most
negative chemical shift may be assigned to a phosphorus with four
aluminium next-nearest neighbours, P(4Al), the next to P(3Al,
Mg), then P(2Al, 2Mg), then P(Al, 3Mg) and finally to P(4Mg)
species. Table 2 gives the relative contribution of each signal to the
spectrum, and also the expected contribution (calculated from the
Fig. 3 XRD patterns of as-prepared solids templated with diquat-5,
(
(
a) phase D, (b) AlPO
d) Mg0.15–AFX, (e) Mg0.2–AFX, (f) Mg0.25–AFX, (g) Mg0.5–AFX + E. As-
4
(F)–AFX and AlPO (F)–ERI, (c) Mg0.1–AFX,
4
terisks mark reflections from impurity phases. The average refined unit cell
21
was found to be trigonal with a = 13.78(2) A˚ , c = 19.93(9) A˚ , which is
binomial theorem ) if the Mg distribution is random. It can be
seen in all cases there is a discrepancy between the observed and
2
similar to literature values.
4
178 | Dalton Trans., 2007, 4175–4181
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View Article Online
27
31
Fig. 4
Al MAS NMR of as-prepared (a) Mg0.15–ERI (diquat-4),
Fig. 5
P MAS NMR of as-prepared (a) Mg0.15–ERI (diquat-4),
(
(
b) Mg0.25–ERI (diquat-4), (c) Mg0.5–ERI (diquat-4), (d) Mg0.15–AFX
diquat-5), and (e) Mg0.25–AFX (diquat-5).
(b) Mg0.25–ERI (diquat-4), (c) Mg0.5–ERI (diquat-4), (d) Mg0.15–AFX
(diquat-5), and (e) Mg0.25–AFX (diquat-5).
3
1
calculated populations which suggests there is some ordering of
the Mg within the aluminophosphate lattice. Such ordering has
To test whether the assignment of P resonances to P atoms with
differing numbers of Al next nearest neighbours is reasonable, the
framework composition many be calculated from the NMR data
and compared to that obtained by chemical analysis. Calculations
18
previously been noted in MgAPO-11, MAPO-36 and MgAPO-
21
2
0. In the spectra of Mg0.15–ERI, Mg0.25–ERI, Mg0.5–ERI and
21
Mg0.15–AFX the population of P(3Al, Mg) is greater than that
calculated for random distribution, but for Mg0.25–AFX is lower.
The lower level implies the presence of regions of high Mg
content which are predicted to be similar to the M–O–P–O–
were performed using the equations of Barrie and Klinowski,
and the results given in Table 1. The overall inorganic framework
composition determined by NMR and ICP-OES compare reason-
ably well, with the closeness in match being poorer for the ERI
materials. This is clearly, in the case of the material crystallised
from the gel of Mg/P = 0.5, due to the impurity C phase, which is
presumably Mg-rich. Thus the solid state NMR results distinguish
between the composition of the ERI material and the impurity,
whereas the ICP-OES gives the total bulk composition.
22
M units reported in CoAPO-50. The presence of such ordering is
attributed to the positive charge on the dumb-bell shaped organic
SDAs being concentrated towards each end of each molecule (see
Fig. 1), which may direct Mg-substitution towards the ends of the
large cavity in which the SDA resides.
31
Table 2 Comparison of P MAS NMR peak areas obtained by deconvolution of the spectra with those expected for materials with a random Mg
distribution within the framework (calculated from the binomial theorem)
31
P MAS NMR (%)
Binomial theorem (%)
Run no.
Mol. sieve
P(4Al)
P(3Al)
P(2Al)
P(1Al)
P(0Al)
P(4Al)
P(3Al)
P(2Al)
P(1Al)
P(0Al)
10
12
13
20
22
ERI
ERI
ERI
AFX
AFX
6.8
6.3
26.3
27.7
16.4
64.9
71.6
60.4
63.5
22.5
18.8
14.5
13.4
5.4
9.5
7.7
—
3.5
12.1
—
—
—
—
20.5
22.8
37.4
38.6
9.3
39.8
40.8
41.7
41.5
30.1
29.1
27.3
17.4
16.7
36.7
6.4
5.6
2.5
2.4
11.0
1.2
0.9
0.2
0.2
4.0
37.5
11.3
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To investigate the mode of Si substitution in the silicoalu-
minophosphate Si0.1–ERI MAS NMR spectra were measured.
Two possible modes of substitution are known in SAPO materials:
Si may substitute for P (Si → P, designated mechanism I) or two
Si may substitute for one Al and P (2Si → Al + P, designated
2
9
mechanism II). The Si NMR spectrum (Fig. 6) shows two signals
at −90.5 and −93.2 ppm. The first is assigned to isolated Si which
has substituted directly for P and so is surrounded by four Al
atoms as next-nearest neighbours (Si(4Al), mechanism I). The
resonance at −93.2 ppm is assigned to Si(3Al, Si), which results
from substitution by mechanism II. The absence of signals at lower
chemical shifts indicates that large silica ‘islands’ are not present
23
in this sample, as have been observed in, for example, SAPO-18.
Brønsted acidity will therefore be present once the organic SDA
is removed by calcination because of the requirement to maintain
charge neutrality.
13
Fig. 7
C MAS NMR of as-prepared (a) Mg0.15–ERI (diquat-4),
(
(
b) Mg0.25–ERI (diquat-4), (c) Mg0.5–ERI (diquat-4), (d) Mg0.15–AFX
diquat-5), and (e) Mg0.25–AFX (diquat-5) showing, in all cases, that the
template is included intact (see text for further details).
29
Fig. 6
Si MAS NMR of as-prepared Si0.1–ERI (diquat-4).
carbons is split into two due to differences in interaction between
each end of the template and the framework. The resonance
at 65 ppm is assigned to methylene carbons adjacent to the
quaternary nitrogens. In the MgAPO-56 materials additional
signals are present in the C MAS NMR spectra (Fig. 7d–
7e) which are likely to arise from fragments of SDA becoming
included in the smaller gmelinite-type cavity also present in the
framework.
Attempts to locate the SDA molecule within the large cavity
of Mg0.15–ERI and Mg0.15–AFX by Rietveld refinement of powder
XRD data collected on Station 2.3 of the Daresbury synchrotron
(not shown) have so far proved unsuccessful.
Template characterisation
The ERI and AFX framework types may be considered to be
constructed entirely of planar 6-rings (as rings composed of six
tetrahedral and six oxygen atoms are known). The rings may
have their centres at either the (0,0), (2/3, 1/3) or (1/3, 2/3)
positions (A, B or C, respectively) in the hexagonal ab plane,
and may link to other rings above and below them in the c
direction. Although materials containing randomly stacked 6-
rings have been synthesised, in ERI and AFX they are arranged
in the ordered stacking sequences AABAAC and AABBC-
CBB, respectively. The reason for this non-random arrangement
is necessarily due to the controlling presence of the organic
SDA.
1
3
24
To determine the amount of included structure-directing agent
and water, TGA was performed. For Co0.1–ERI templated with
◦
diquat-3 there is a small (1.8 wt%) loss below 175 C, followed by
◦
The diquat-4 and -5 template molecules were included intact
a larger two-stage loss (of 17.1 wt%) between 325 and 645 C. The
1
within the materials’ principal cavities, as evidenced by the H–
former is attributed to water, while the latter corresponds to the
template. Similarly, all preparations yielding pure ERI-type solids
with diquat-4 show an analogous pattern: 1.3 wt% loss below
1
3
C CP MAS NMR spectra (Fig. 7). Carbons not adjacent to
a nitrogen atom exhibit a resonance at ca 23 ppm, but are not
resolved, which leads to similar spectra for both templates. The
methyl carbons in diquat-4 appear as a single signal (51 ppm),
which indicates that both ends of the template are likely to be in
the same average environment. This contrasts the case of diquat-4
◦
115 C, attributed to water, followed by a 2.2 wt% loss between
◦
◦
365 and 450 C, and a 16.0 wt% loss between 450 and 665 C. These
latter two losses are more resolved and may be attributed either
to the removal of template molecules weakly bound to the outer
surface of the crystallites followed by those within the cavity, or to
13
in the STI zeolite channels in which the resonance of the methyl
4
180 | Dalton Trans., 2007, 4175–4181
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2
5
a two stage breakdown of the template included within the cavity.
Acknowledgements
No satisfactory explanation for the presence of a pentacoordinate
We are grateful to the Leverhulme Trust for funding and the
CCLRC for providing access to synchrotron radiation. We thank
Dr David Apperley (EPSRC Solid State NMR Service, University
of Durham) for collecting and deconvoluting the NMR spectra
and Mrs Diane Spencer (University of Wolverhampton) for the
ICP-OES analyses.
Al in Mg0.15–ERI and not in Mg0.25–ERI was obtained from the
TGA. In all MgAPO AFX samples TGA showed a weight loss of ca
◦
5
.4 wt% below 180 C which is attributed to loosely bound water.
The magnitude of the template mass loss corresponds well with the
expected change in mass with increasing methylene chain length.
The template (15.25 wt%) is removed in a two-stage loss between
◦
4
30 and 645 C. These are insufficiently resolved, however, to
permit calculation of the proportion of the template being removed
at each stage.
Combining the results of chemical and thermal analysis with
crystallographic data allows the calculation of the unit cell
contents. So, for example, the magnesioaluminophosphate pre-
Notes and References
1
S. T. Wilson, B. M. Lok, C. A. Messina, T. T. Cannan and E. M.
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2 Ch. Baerlocher, W. M. Meier and D. H. Olson, Atlas of Zeolite
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–
3
R. F. Lobo, S. I. Zones and M. E. Davis, J. Inclusion Phenom. Mol.
ERI) may be formulated as |(H O) P O72].
2
2
(C14
H
30
N
2
)
2.2|[Mg
3
Al15
18
Recognit. Chem., 1995, 21, 47.
The mass loss of 16 wt% observed by TGA equates to 1.93
diquat-4 molecules per cell. This suggests that the cavities are
likely to be completely occupied, the excess observed being
adsorbed onto outer surface sites. The analogous sample prepared
with diquat-5 (Mg0.25–AFX) has the overall unit cell contents
4 R. F. Lobo, S. I. Zones and R. C. Medrud, Chem. Mater., 1996, 8, 2409.
5
G. W. Noble, P. A. Wright and A˚ . Kvick, J. Chem. Soc., Dalton Trans.,
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6
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8
9
|
(H
2
O)11(C15
H
32
N
2
)
2.3|[Mg
9
Al15
P
24
O96]. The template excess may
be rationalised by the presence of organic fragments included
1
1
1
1
3
within the gmelinite-type cavity, as suggested by the C MAS
NMR spectra.
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1
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2
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2
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1
21 P. J. Barrie and J. Klinowski, J. Phys. Chem., 1989, 93, 5972.
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2
2
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This journal is © The Royal Society of Chemistry 2007
Dalton Trans., 2007, 4175–4181 | 4181