Surface reactivity of non-hydrolytic silicophosphate xerogels: A simple method to create Br?nsted or Lewis acid sites on porous supports

Article abstract of DOI:10.1039/c5nj02928g

Non-hydrolytic sol-gel reactions of silicon acetates with trimethylsilyl (TMS) esters of phosphoric and phosphonic acids produce cross-linked matrices containing homogeneous dispersions of silicate and phosphoryl groups connected together by networks of Si-O-P(=O) linkages. The condensation degrees reach 80 to 90%. Residual organic groups (10 to 20%) were reacted with a variety of compounds (H₂O, Me₃SiOSiMe₃, POCl₃, SiCl₄, AlMe₃, Al(NMe₂)₃, and AlCl₃) in order to enrich the surface of these porous matrices with Br?nsted (≡P-OH) and Lewis (tetracoordinated Al) acid functional groups. The differences in the reactivity of ≡Si-OAc and ≡P-OSiMe₃ groups were utilized for the selective modification at the silicon and phosphorus atoms. The reaction procedures were optimized and significantly porous silicophosphate materials with a high content of either hydroxyl groups or four-coordinated aluminium species were obtained. The activity and selectivity of prepared samples as catalysts for the dimerization of α-methylstyrene were tested. Excellent activities and moderate to very high selectivities were achieved suggesting the potential use of silicophosphate xerogels in heterogeneous catalysis.

Full text of DOI:10.1039/c5nj02928g

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Surface reactivity of non-hydrolytic
silicophosphate xerogels: a simple method to
create Brønsted or Lewis acid sites on porous
supports†
Cite this:DOI: 10.1039/c5nj02928g
ab
ab
a
c
Ales Styskalik, David Skoda, Zdenek Moravec, Craig E. Barnes and
Jiri Pinkas*
ab
Non-hydrolytic sol–gel reactions of silicon acetates with trimethylsilyl (TMS) esters of phosphoric and
phosphonic acids produce cross-linked matrices containing homogeneous dispersions of silicate and
phosphoryl groups connected together by networks of Si–O–P (Q O) linkages. The condensation
degrees reach 80 to 90%. Residual organic groups (10 to 20%) were reacted with a variety of
compounds (H
of these porous matrices with Brønsted (RP–OH) and Lewis (tetracoordinated Al) acid functional groups.
The differences in the reactivity of RSi–OAc and RP–OSiMe groups were utilized for the selective
2 3 3 3 4 3 2 3 3
O, Me SiOSiMe , POCl , SiCl , AlMe , Al(NMe ) , and AlCl ) in order to enrich the surface
3
Received (in Montpellier, France)
2
1st October 2015,
modification at the silicon and phosphorus atoms. The reaction procedures were optimized and signifi-
cantly porous silicophosphate materials with a high content of either hydroxyl groups or four-coordinated
aluminium species were obtained. The activity and selectivity of prepared samples as catalysts for the
dimerization of a-methylstyrene were tested. Excellent activities and moderate to very high selectivities
were achieved suggesting the potential use of silicophosphate xerogels in heterogeneous catalysis.
Accepted 17th February 2016
DOI: 10.1039/c5nj02928g
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contain a leaving group that forms a stable compound with the
hydroxyl proton. For instance the reaction of the Al–Cl moiety
Introduction
3
Chemical modification of the surfaces of inorganic supports is with surface Si–OH groups has been used to anchor AlCl on the
6
a common method for the fabrication of a broad spectrum of surface of silica (eqn (1)). Also Me SiCl reacts readily with
3
surface-engineered materials, such as chemical and biological hydroxyl groups eliminating HCl (eqn (2)) and making the silica
4
sensors, modified electrodes, separation membranes, selective surface hydrophobic. Alkoxysilanes were applied to introduce
adsorbents, sorbents for HPLC, heterogeneous metal-complex organic functional groups (eqn (3)) allowing for a wide variety
catalysts, biocompatible materials, and anti-adherent or antic- of organic reactions. As an example nitrile alkylalkoxysilane was
1
orrosion coatings. The most widely used materials are based attached to silica, the nitrile group transformed into the
on silica, which can be easily chemically modified with organo- carboxylic group, and the final material – silica modified with
silicon compounds. The purpose of the post-synthesis modifi- carboxylic groups – was utilized for the heterogenization of
2
,3
7
cations is often to introduce catalytically active groups, or to metal salts.
4
tailor the hydrophobicity of the material, or to change the
concentration and the strength of surface acidic centers.
RSi–OH + AlCl
3
- RSi–O–AlCl
2
+ HCl
+ HCl
(1)
(2)
(3)
5
Modification of the surface of silica and other oxide materials RSi–OH + Cl–SiMe
3
- RSi–O–SiMe
3
handled in air usually involves electrophilic substitution – a
RSi–OH + RO–SiR - RSi–O–SiR + ROH
3
3
surface hydroxyl proton is replaced by an anchoring group of the
modifier. From this it follows that the modifier molecule must Another possibility is to chlorinate silanol groups using CCl
4
or
2
to increase or modify the reactivity of the silica surface.
SOCl
a
Department of Chemistry, Masaryk University, Kotlarska 2, CZ-61137 Brno,
Czech Republic. E-mail: jpinkas@chemi.muni.cz; Fax: +420549492443;
Tel: +420549496493
This step extends the number of possible subsequent reactions
thus increasing the versatility of silica as a support for hetero-
8
geneous catalysts.
b
c
Masaryk University, CEITEC MU, Kamenice 5, CZ-62500 Brno, Czech Republic
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA
Electronic supplementary information (ESI) available. See DOI: 10.1039/
In the case of metal and metal oxide surfaces (alumina,
titania, zirconia, etc.), phosphoric and phosphonic acids are
suitable grafting agents for the improvement of hydrophobic,
†
c5nj02928g
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–12
anticorrosive, and other properties.
Grafting involves both In addition the differences in the reactivity of RSi–OAc and
3
the coordination of the phosphoryl oxygen to Lewis acidic RP–OSiMe groups were utilized for the selective modification
metal centres, and the condensation of the P–OH functions at the silicon and phosphorus atoms.
with surface M–OH groups. Accordingly the coupling agent may
The work described here is divided into two parts. In the
act as a mono-, bi- or tridentate ligand on the surface. The first part we focus on the detailed description of reaction
M–O–P bonds are in most cases very stable and therefore pathways between residual organic groups on the surface of
phosphoric and phosphonic acids provide stable monolayers silicophosphate xerogels and various modifiers (H
2
O, hexa-
1
1
with high grafting densities. In some cases (depending on the methyldisiloxane, POCl
3
, SiCl
4
, AlMe
3
, Al(NMe
2
)
3
and AlCl ).
3
pH and the metal oxide used) there are troubles with the The organic products of the reactions were identified by
dissolution/precipitation process which provides bulk metal GC-MS, while the changes on the xerogel surfaces were followed
phosphonates instead of metal oxides covered with a phosphonate mainly by IR and MAS NMR methods. The surface area of
layer. Grafting with esters of phosphonic acids in non-aqueous xerogels was measured by nitrogen adsorption techniques since
10
solvents solved this problem. The phosphonate layer provides it is the key feature of heterogeneous catalysts. The second part
better results than the siloxane layer for metal and metal oxide is devoted to the multi-step chemical modification based on
surfaces (stability, grafting density, etc.). Unfortunately the the reactivity of silicophosphate xerogels, affording solid acid
grafting of phosphonates onto the silica surface was shown catalysts with either Lewis (tetracoordinated Al) or Brønsted
to be problematic due to the hydrolytic instability of Si–O–P (RP–OH) acidic groups. The catalytic activities of these materials
1
3
bonds. However it was found recently that it proceeds are described for the dimerization of a-methylstyrene.
smoothly with exclusion of moisture and after heating to
1
4–17
temperatures of around 140 1C.
We recently described a new non-hydrolytic ester elimination Experimental
route to synthesize amorphous microporous silicophosphate
General
4
xerogels (SiP) by condensing silicon tetraacetate, Si(OAc) , and
1
8
2
All manipulations were performed under dry N using Schlenk
tris(trimethylsilyl)phosphate, OP(OSiMe
stitution of Si(OAc) by acetoxysilanes with bridging organic
groups, (AcO) Si–(CH –Si(OAc) , allowed us to synthesize
completely new hybrid mesoporous silicophosphate xerogels
3 3
) (TTP) (eqn (4)). Sub-
techniques or in an M. Braun dry box with both H
levels below 1 ppm. Si(OAc) was prepared according to the
(Merck, 99%) and acetic acid anhydride
2 2
O and O
4
4
3
2
)
n
3
2
0
literature. SiCl
4
1
9
(Riedel de Ha ¨e n, 98%) were distilled prior to use. Toluene
was freshly distilled from Na/benzophenone under N ; pyri-
dine, CH CN, CH Cl , and THF were dried by standard meth-
(
SiC2SiP, eqn (5)). Both xerogel families (parent and hybrid)
2
are built from Si–O–P bonds, exhibit large surface areas (230 to
2
À1
3
2
2
7
00 m g ) and high homogeneity of Si and P components on
ods and distilled before use. Tris(trimethylsilyl)phosphate
TTP) was prepared from H PO and Me SiCl and vacuum-
distilled. Benzene-d was dried over and distilled from Na/K
alloy and degassed prior to use. CDCl was dried by P and
vacuum-transferred to an ampoule. The parent silicophosphate
xerogels SiP and SiC2SiP were prepared from Si(OAc)4 and
the atomic scale. The degrees of condensation (DC, mol% of
eliminated trimethylsilylacetate in the course of the reaction)
reach 80 to 90%, and therefore xerogels possess residual organic
(
3
4
3
6
3
2 5
O
4 3 2 n
groups, both acetoxy (coming from Si(OAc) or (AcO) Si–(CH ) –
Si(OAc)3 precursors) and trimethylsilyloxy (coming from the
OP(OSiMe ) precursor) groups. With the knowledge of the DC
3
3
(
AcO) Si–(CH ) –Si(OAc) , respectively, in reactions with
3 2 2 3
we can rewrite eqn (4), which describes the complete condensa-
tion of organic groups (DC = 100%), and introduce eqn (6) for the
real reaction (DC = 80%). The situation is similar in the case of
hybrid xerogels with organic bridging groups.
1
8,19
OP(OSiMe3)3 as reported previously.
Their synthesis and
characterization are briefly described in the ESI.†
Characterization
4 3 3
3Si(OAc) + 4OP(OSiMe )
À1
The IR spectra (4000–400 cm ) were recorded on a Bruker
-
Si P O + 12CH C(O)OSiMe (DC = 100%)
(4)
3
4
16
3
3
Tensor T27 spectrometer. Samples were prepared as KBr
pellets. Surface areas (SA) and pore volumes were determined
by nitrogen adsorption at 77.4 K by a volumetric technique
(
AcO)
3
Si–(CH
OPO
+ 4OP(OSiMe
2
)
n
–Si(OAc)
3
3 3
+ 2OP(OSiMe )
2
1,22
-
3
Si(CH
2
n
) SiO
3
PO + 6AcOSiMe
3
, n = 1, 2, 3, 6
13.6(OAc)2.4(OSiMe
9.6CH C(O)OSiMe (DC = 80%)
(5)
on a Quantachrome Autosorb-1MP instrument. Prior to the
measurements, the samples were degassed at 25 1C for at least
3
Si(OAc)
4
3
)
3
- Si
3
P
4
O
3 2.4
)
À1
2
4 h until the outgas rate was less than 0.4 Pa min . The
+
3
3
adsorption–desorption isotherm was measured for each sample
at least three times. The specific surface area was determined
(6)
Here we report on the post-synthetic chemical modification of by the multipoint BET method with at least five data points
the silicophosphate xerogel surface. The reactivities of the with relative pressures between 0.05 and 0.3. Thermal analyses
RSi–OAc, RP–OSiMe , and PQO groups in our product, (TG/DSC) were performed on a Netzsch STA 449C Jupiter
3
3
À1
3 4
Si P O
13.6(OAc)2.4(OSiMe
3
)
2.4, are different than silanol groups apparatus from 25 to 1100 1C under flowing air (70 cm min
)
À1
on silica and allow us to use a wide variety of reactants leading with a heating rate of 5 K min . The samples for phosphorus
to novel materials with potential use in heterogeneous catalysis. and chlorine content estimation and alkalimetric titration with
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0
.01 M NaOH were prepared in the following way: 200 mg of the Table 1 Chemical modification of xerogels with water. Molar amounts of
3
residual organic groups and water used in the reactions. Relative amounts
of water
sample was hydrolyzed in 40 cm of distilled water; the suspen-
sion was shaken for 24 h. The white precipitate was filtered off
and the filtrate was diluted to 250 cm . The total amount of
3
a
mxerogel
[g]
nAcOSiMe3
[mmol]
n
H O
n
[%]
H O
2
Yield
[g]
2
Sample
[mmol]
hydrochloric, acetic, and phosphoric acid was estimated by
NaOH titration with a phenolphthalein indicator. The Si, Al and SiPH1
0.570
0.814
0.946
0.234
1.77
2.52
2.93
0.42
0.17
2.40
5.58
0.72
5
50
100
100
—
—
0.967
0.202
SiPH2
SiPH3
SiC2SiPH1
P contents were analyzed on an ICP OE spectrometer iCAP 6500
Duo (Thermo, generator 27.12 MHz, amplitude 1.15 kW,
3
À1
plasma gas flow 12 dm min ) using emission lines 212.412
and 251.611 nm for Si, 167.079 and 308.215 nm for Al, and
a
3
Relative to residual AcO–SiR and Me SiO–PR organic groups.
2
13.618 and 214.914 nm for P. The chlorine content was
analyzed by argentometric titration. Solution NMR spectra were
recorded on a Bruker Avance II 300 NMR spectrometer at (SiO
2
9
Si CPMAS NMR (SiPH2, ppm) d: 23.6 (POSiMe
), À214 (SiO ).
MAS NMR (SiPH2, ppm) d: À47 (P(OSi)
deuterated solvents as the external lock. The H and C{ H} (P(OH)(OSi) (OSi) ), À16 (P(OH) (OSi), À8
), À26 (P(OH)
NMR spectra were referenced to the residual proton signals or (OQP(OH) (OSi)).
3
), À115
4
6
1
13
31
frequencies 300.1 MHz for H and 75.5 MHz for C with
P
4
), À36
1
13
1
3
2
2
3
2
À1
carbon resonances of benzene-d
tively) and CDCl (7.26 and 77.0 ppm). Solid-state Al and
6
(7.15 and 128.0 ppm, respec-
TG/DSC (air, 5 K min ): weight loss at 1000 1C: 40.6%.
BET: nonporous
2
7
3
3
1
29
13
P MAS, and Si and C CPMAS NMR spectra were acquired
Data for SiPH1, SiPH3 and SiC2SiPH1 see ESI.†
on a wide-bore Varian INOVA 400 NMR spectrometer with a
broadband Chemagnetic 5 mm MAS probe. Inside a dry box,
Modification with hexamethyldisiloxane
samples were loaded into 5 mm pencil rotors, stoppered with Me
SiOSiMe (0.7146 g, 4.401 mmol) was added dropwise
3
3
teflon plugs and sealed with silicon grease and paraffin wax. through a syringe to the suspension of SiP (1.820 g, 4.680 mmol
2
9
13
3
Magic angle spinning rates were 5 kHz for Si and C CPMAS of residual AcOSiMe
3
) in 15 cm of toluene. The reaction
3
1
27
and 10 kHz for P and Al MAS spectra. Chemical shifts were mixture was kept at 80 1C for one week. Volatile organic
(85%)]: 0.0 ppm; Si d products were identified by GC-MS (trimethylsilylacetate, m/z
4
(Me SiO) Si O ]: 11.72 ppm; C d [adamantane]: 38.68 ppm; 45/61/75/117) and the inorganic product (SiPHMDSO, 2.013 g)
3
1
29
referenced externally to P d [H
3
PO
1
3
[
3
8
8 20
2
7
3+
Al d [[Al(H O) ] (aq. solution)]: 0.0 ppm. GC-MS spectra were was isolated by drying for 48 h at r.t. in vacuo. The same reaction
2
6
obtained on a Thermo Scientific Trace GC Ultra – TSQ Quan- was performed with Me
SiOSiMe (0.5241 g, 3.228 mmol)
3
3
tum XLS mass spectrometer. A TS-SQC column (15 m, 0.25 mm, and hybrid SiC2SiP xerogel (1.450 g, 2.970 mmol of residual
0
5
3
.25 mm) was heated with the following program: 50 1C (0 min), AcOSiMe ) affording 0.9128 g of white xerogel (SiC2SiPHMDSO).
À1
À1
1C min up to 80 1C, 15 1C min up to 120 1C, and 35 1C
Characterization and analyses of the SiPHMDSO xerogel.
À1
À1
min up to 200 1C (0.5 min); split mode, injector temperature
2
2
TG/DSC (air, 5 K min ): weight loss at 1000 1C: 53.5%.
À1
00 1C, interface temperature 200 1C, and detector temperature
IR (SiPHMDSO, KBr, cm ) n: 502 m, 606 vw, 654 vw (n SiC
3
),
00 1C. The column pressure was set to 31.5 kPa and ionization 692 vw, 762 m (r
s
SiCH
1100 s (n P–O–Si), 1256 s (d
3
), 849 vs (ras SiCH
3
), 1035 s (n Si–O–P),
energy to 70 eV.
s
SiCH ) 1300 sh (n PQO), 1418 w
3
(
das CH ), 1541 vw (n COO bidentate), 2904 w (n CH ), 2962 m
3
s
s
3
(
nas CH3).
1
3
Chemical modification of
silicophosphate xerogel surfaces
C CPMAS NMR (SiPHMDSO, ppm) d: 3.8 (POSiCH
SiOSiCH ).
Si CPMAS NMR (SiPHMDSO, ppm) d: 21.4 (POSiMe
SiOSiMe ), À214 (SiO ).
), À111 (SiO
P MAS NMR (SiPHMDSO, ppm) d: À45 (P(OSi)
i.e. xerogel contained 2.52 mmol AcO–SiR and 2.52 mmol (OQP(OSi) ).
3
+
3
2
9
3
), 13.6
Modification with H
2
O
(
3
4
6
3
1
A suspension of SiP (0.814 g, 2.52 mmol of residual AcOSiMe3,
4
), À29
3
3
2
À1
Me SiO–PR groups) in 40.7 cm of THF was reacted with
BET: 107 m g .
3
H
2
O (0.0433 g, 2.40 mmol, 50% relative to residual groups) at
Data for SiC2SiPHMDSO are provided in the ESI.†
r.t. for 24 h. The resulting xerogel (SiPH2) was isolated by
drying for 48 h at r.t. in vacuo. The reaction was performed
Modification with silicon tetrachloride
similarly with the H
2
4
O-to-residual group ratio of 100 and 5% SiCl (0.261 g, 1.53 mmol) was vacuum transferred to the
(
Table 1).
suspension of SiP (1.034 g, 3.26 mmol of residual AcOSiMe ,
1 Cl:1 –OAc or –OSiMe3 group) in 10 cm of toluene. The
3
3
Characterization and analyses of the SiPH2 xerogel.
À1
IR (SiPH2, KBr, cm ) n: 505 m, 606 vw, 689 vw, 762 m reaction temperature of 80 1C was maintained for 24 h. After
rs SiCH ), 852 vs (r SiCH ), 1074 vs (n Si–O–P), 1095 vs that, the solvent and volatile products (GC-MS: CH COCl, m/z
(
(
(
3
as
3
3
n P–O–Si), 1259 s (d
d OH), 2908 vw (n CH
C CPMAS NMR (SiPH2, ppm) d: À3.6 (POSiCH
s
SiCH
3
), 1424 w (das CH
3
), 1662 w 43/63/65, Me
3
SiCl, m/z 73/93/95/108) were removed in vacuo
s
3
), 2965 w (nas CH
3
).
affording white xerogel, which was dried for 48 h at r.t. in vacuo
(SiPSi, 0.913 g). The same reaction was performed with PhCH SiCl
1
3
3
).
2
3
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1
3
(
0.228 g, 1.02 mmol) and SiP xerogel (1.003 g, 1.495 mmol of
C CPMAS NMR (SiPAl1, ppm) d: À7.9 (AlCH
), 32.4 (CH COO).
Si CPMAS NMR (SiPAl1, ppm) d: 24.4 (POSiMe ), 16.6
3
), 1.8
residual AcOSiMe , 1 Cl : 1 –OAc or –OSiMe group) affording (POSiCH
3
3
3
3
2
9
1
.036 g of white xerogel (SiPPhCH Si).
2
3
Characterization and analyses of the SiPSi xerogel.
TG/DSC (air, 5 K min ): weight loss at 1000 1C: 24.6%.
(SiOSiMe ), À3.7 (Me SiO ), À64 (MeSiO ), À115 (SiO ), À214
3
2
2
3
4
À1
(SiO ).
6
À1
31
IR (SiPSi, KBr, cm ) n: 496 m, 646 w, 764 w (r
s
SiCH
), 1049 vs (n Si–O–P), 1113 vs
) 1316 sh (n PQO), 1375 w (d CH ), Al atom).
), 1541 m (n COO), 1724 vw (nas COO), 1773 m
as COO), 2912 vw (n CH ), 2964 w (nas CH ).
C CPMAS NMR (SiPSi, ppm) d: 1.8 (POSiCH ), 18.4
3
),
P MAS NMR (SiPAl1, ppm) d: À39 (OQP(OSi/Al)
3
).
2
7
[6]
8
(
1
53 m (ras SiCH
3
), 1022 vs (r CH
SiCH
3
Al MAS NMR (SiPAl1, ppm) d: À7 ( Al, hexacoordinated
n P–O–Si), 1261 w (d
s
3
s
3
418 vw (das CH
3
s
BET: nonporous.
(n
s
3
3
1
3
3
Modification with tris(dimethylamido)aluminum
(
CH COO bidentate), 23.6 (CH COO unidentate), 169.1 (CH COO
3
3
3
3
Dry toluene (10 cm ) was poured on the mixture of solid
Al(NMe
unidentate).
2 3
) (0.722 g, 4.53 mmol) and solid xerogel SiP (2.018 g,
.85 mmol of residual AcOSiMe ). The reaction mixture reacted
for 18 h at r.t. After drying in vacuo for 48 h at r.t., 2.534 g of
yellowish xerogel (SiPAl2) was obtained. GC-MS of volatile
organic products: dimethyl(trimethylsilyl)amine m/z 117/102/
2
9
Si CPMAS NMR (SiPSi, ppm) d: 29.0 (POSiMe
3
), À67.8
6
3
(
Cl
2
3
SiO
2
), À91.8 (ClSiO
3
), À111 (SiO
4
), À196 (SiO
6
), À215 (SiO
6
).
1
P MAS NMR (SiPSi, ppm) d: À42 (P(OSi)
4
).
2
À1
BET: 865 m g
.
Data for SiPPhCH
2
Si are provided in the ESI.†
7
3/59, dimethylacetamide m/z 87/72/44, hexamethyldisiloxane
m/z 147/131/73.
Modification with phosphoryl trichloride
POCl (0.302 g, 1.97 mmol) was added dropwise through a
syringe to the suspension of SiP (1.050 g, 3.30 mmol of residual
Characterization and analyses of the SiPAl2 xerogel.
TG/DSC (air, 5 K min ): weight loss at 1000 1C: 38.8%.
IR (SiPAl2, KBr, cm ) n: 495 m, 758 w (r SiCH ), 852 vs
À1
À1
3
s
3
3
AcOSiMe ) in 10 cm of toluene. The reaction mixture was kept (ras SiCH ), 1006 s (r CH ), 1052 vs (n Si–O–P), 1099 vs (n P–O–
3
3
3
at 80 1C for one week. Volatile products were identified by Si), 1257 s (d SiCH ) 1311 sh (n PQO), 1373 vw (d CH ), 1410
s
3
s
3
GC-MS (CH
POCl
3
COCl, m/z 43/63/65, Me
, m/z 47/82/117/119/152/156) and the inorganic product vw (n
3
SiCl, m/z 73/93/95/108, vw (das CH
3
), 1467 w (n
), 2960 w (nas CH
C CPMAS NMR (SiPAl2, ppm) d: À2.9 (POSiCH
COO bident), 16.9 (CH COO bident), 20.1 (CH COO uni-
), 39.0 (N(CH ), 166 (CH COO unident),
s
COO), 1627 w (nas COO), 2809 w, 2903
3
s
CH
3
3
).
1
3
(SiPP, 1.099 g) was isolated after drying for 48 h at r.t. in vacuo.
3
), 14.4
Characterization and analyses of the SiPP xerogel.
TG/DSC (air, 5 K min ): weight loss at 1000 1C: 21.8%.
IR (SiPP, KBr, cm ) n: 489 m, 759 w (r SiCH ), 853 m 174 (CH COO bident).
(CH
3
3
3
À1
dent), 32.7 (N(CH
3
)
2
3
)
2
3
À1
s
3
3
2
9
(
ras SiCH ), 1049 vs (n Si–O–P), 1093 vs (n P–O–Si), 1261 w
Si CPMAS NMR (SiPAl2, ppm) d: 17.7 (POSiMe ), 0.4
3
3
(
d SiCH ) 1302 sh (n PQO), 2918 vw (n CH ), 2965 w (n CH3). (AlOSiMe ), À68.5 ((Me N) SiO ), À90 (SiO + Me NSiO ),
s
3
s
3
as
3
2
2
2
4
2
3
1
3
C CPMAS NMR (SiPP, ppm) d: 2.8 (POSiCH
3
), 24.5 À110 (SiO
4
), À206 (SiO
6
), À214 (SiO
6
).
3
1
(CH
3
9
COO unidentate).
P MAS NMR (SiPAl2, ppm) d: À42 (P(OSi)
4
).
Al MAS NMR (SiPAl2, ppm) d: 35 ( Al), À17 ( Al).
2
27
[5]
[6]
Si CPMAS NMR (SiPP, ppm) d: 26.8 (POSiMe
).
P MAS NMR (SiPP, ppm) d: À40 (P(OSi)
OQP(OSi) ).
3
), À108 (SiO
4
),
2
À1
À214 (SiO
6
BET: 375 m g .
3
1
4
), À31 (ClP(OSi)
3
/
3
Modification with aluminum trichloride
2
À1
BET: 483 m g
.
3
Dry toluene (10 cm ) was poured on a mixture of solid AlCl
3
(
0.204 g, 1.53 mmol) and solid xerogel SiP (0.585 g, 1.99 mmol
Modification with trimethylaluminium
3
of residual AcOSiMe ). The reaction mixture reacted for 18 h at
AlMe3 (0.170 g, 1.18 mmol) was reacted with xerogel SiP r.t. After drying in vacuo for 48 h at r.t., 0.647 g of white xerogel
3
(
1.010 g, 3.35 mmol of residual AcOSiMe ) in toluene (10 cm ) (SiPAl3) was obtained. GC-MS of volatile organic products:
3
at r.t. for 18 h. GC-MS of volatile organic products: tetramethyl- trimethylchlorosilane m/z 95/93/73, hexamethyldisiloxane m/z
silane m/z 73/45, acetone m/z 58/43, hexamethyldisiloxane m/z 147/131/73.
1
47/131/73. The resulting xerogel SiPAl1 (1.019 g) was dried in
Characterization and analyses of the SiPAl3 xerogel.
À1
vacuo for 48 h.
TG/DSC (air, 5 K min ): weight loss at 1000 1C: 31.7%.
À1
Characterization and analyses of the SiPAl1 xerogel.
IR (SiPAl3, KBr, cm ) n: 502 m, 652 vw, 762 w (r
s
SiCH
3
),
À1
TG/DSC (air, 5 K min ): weight loss at 1000 1C: 26.2%.
3
855 m (ras SiCH ), 1051 s (n Si–O–P), 1148 vs (n P–O–Si), 1262 s
À1
IR (SiPAl1, KBr, cm ) n: 489 m, 689 m (nas AlC ), 766 w (d SiCH ) 1304 sh (n PQO), 1378 vw (d CH ), 1418 vw (d CH3),
3
s
3
s
3
as
(
r SiCH ), 856 s (r SiCH ), 1024 s (r CH ), 1052 vs (n Si–O–P), 1482 w (n COO), 1545 w (n COO), 1602 m (n COO), 1772 w
s 3 as 3 3 s s as
1
116 vs (n P–O–Si), 1193 vs (d AlCH ), 1260 s (d SiCH ) 1311 sh (nas COO), 2908 vw (n CH ), 2967 w (n CH3).
s
3
s
3
s
3
as
1
3
(n PQO), 1373 vw (d
s
CH
COO), 1604 w (nas COO), 1771 m (nas CQO), 2918 vw (CH
), 2937 m (nas AlCH ), 2965 w (nas CH ).
3
), 1420 vw (das CH
3
), 1477 w (n
s
COO),
C CPMAS NMR (SiPAl3, ppm) d: 1.1 (POSiCH
3
), 18.5
COO unident), 168.1 (CH COO
COO bident).
1
547 w (n
s
3
COO bident), 22.9 (CH
3
3
(n
s
CH
3
3
3
unident), 181.6 (CH
3
COO bident), 194.3 (CH
3
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2
9
Si CPMAS NMR (SiPAl3, ppm) d: 29.1 (POSiMe
), À113 (SiO ), À196 (SiO ), À214 (SiO ).
P MAS NMR (SiPAl3, ppm) d: À49 (P(OSi/Al) ).
3
), À91
(
ClSiO
3
4
6
6
3
1
4
[4]
2
7
[4]
[4]
Al MAS NMR (SiPAl3, ppm) d: 83 ( Al), 67 ( Al), 51 ( Al),
[
6]
À23 ( Al).
BET: 469 m g
2
À1
.
Preparation of solid acid catalysts
Lewis acidic catalysts (SiPL and SiC2SiPL). The silico-
phosphate xerogels were reacted with hexamethyldisiloxane
(
Me SiOSiMe : residual organic groups ratio = 1 : 2) in order
3 3
to deplete acetoxy groups by elimination of Me SiOAc. Sub-
3
sequent reaction with AlCl3 (Cl : residual organic group ratio =
1
3
: 1) resulted in the elimination of Me SiCl. Reactions were carried
out in toluene at temperatures and for times described in Part 1:
Chemical modification of silicophosphate xerogel surfaces.
Scheme 1 Methylstyrene dimerization.
À1
IR (SiPL, KBr, cm ) n: 525 m, 606 vw, 692 w, 763 m
(
s 3 3
r SiCH ), 853 vs (ras SiCH ), 1096 vs (n Si–O–P), 1186 s
1
3
(n P–O–Si), 1258 s (ds SiCH ), 1419 vw (d CH ), 1462 vw,
C CPMAS NMR (SiC2SiPB, ppm) d: 0.8 shoulder (POSiCH3),
3
as
3
2
909 vw (n CH ), 2966 m (n CH3).
5.6 (SiCH CH Si) 24.5 (CH COO unident).
2 2 3
s
3
as
1
3
29
C CPMAS NMR (SiPL, ppm) d: 4.0 (POSiCH3).
Si CPMAS NMR (SiPL, ppm) d: 25 (POSiMe
), À94 (ClSiO
), À196 (SiO ), À215 (SiO ).
P MAS NMR (SiPL, ppm) d: À49 (P(OSi/Al)
Si CPMAS NMR (SiC2SiPB, ppm) d: 29 (POSiMe
), (CSiO ).
3
), À66
), À23
2
9
3
3
3
3
1
À116 (SiO
4
6
6
P MAS NMR (SiC2SiPB, ppm) d: À35 (OQP(OSi)
(P(OH) (OSi) ).
BET: 172 m g
3
3
2
1
7
4
[4]
).
2
2
[4]
[4]
2
À1
Al MAS NMR (SiPL, ppm) d: 89 ( Al), 73 ( Al), 55 ( Al).
.
2
À1
BET: 434 m g
.
À1
IR (SiC2SiPL, KBr, cm ) n: 502 m, 701 vw, 763 w (r SiCH ), Catalytic test reactions
s
3
8
54 m (ras SiCH ), 1115 vs (n Si–O–P), 1258 s (d SiCH ), 1410 vw
3 s 3
Methylstyrene dimerization was recently utilized as a test
reaction for the Brønsted acid catalysts under laboratory con-
ditions in which superior results were achieved with phosphoric
acid impregnated silica. Protocol: 4À5 wt% of catalyst was
(das CH ), 2918 vw (n CH ), 2965 m (n CH3).
3 s 3 as
1
3
C CPMAS NMR (SiC2SiPL, ppm) d: 2.3 (SiCH
).
Si CPMAS NMR (SiC2SiPL, ppm) d: À71 (CSiO
P MAS NMR (SiC2SiPL, ppm) d: À52 (P(OSi/Al)
2 2
CH Si +
POSiCH
3
23
2
3
2
9
1
7
3
).
).
Al MAS NMR (SiC2SiPL, ppm) d: 73 ( Al), 57 ( Al),
mixed with dry a-methylstyrene and a stirred mixture was placed
in an oil bath preheated to 90 1C. After six hours the reaction
mixture was allowed to cool down, the heterogeneous catalyst
was filtered off and the mixture was diluted with toluene (1: 20).
GC-MS spectra of diluted samples were collected and the yields
of methylstyrene dimers (I, II, III, Scheme 1) were estimated
based on their intensities. The catalyst reusability was tested
after separating it from the reaction mixture by centrifugation,
4
[4]
[4]
[
5]
4
1 ( Al).
BET: 458 m g
Brønsted acidic catalysts (SiPB and SiC2SiPB). Xerogels were
reacted with POCl in toluene with the elimination of Me SiCl
and AcCl (Cl : residual organic group ratio = 3 : 2) and then with
water in THF with the elimination of HCl (H O : residual Cl
2
À1
.
3
3
3
2
washing three times with dry toluene (10 cm ), and drying in
ratio = 1 : 1). The surface was covered with the P–OH groups
after this process. All reactions were carried out (solvents,
times, temperatures) as described in Part 1.
vacuo at room temperature. The dried catalyst was used again for
the next a-methylstyrene dimerization.
À1
IR (SiPB, KBr, cm ) n: 497 m, 663 w, 763 w (r SiCH ), 854 m
s
3
(
ras SiCH ), 1105 vs (n Si–O–P), 1258 s (d SiCH ), 2913 vw
3
s
3
Results and discussion
Chemical modification of silicophosphate xerogel surfaces
(n
s
CH
3
3
), 2965 m (nas CH ).
1
3
3
C CPMAS NMR (SiPB, ppm) d: 3.3 (POSiCH ), 28.2
(
CH
3
9
COO unident).
Reactions with water. As already discussed, our parent
2
Si CPMAS NMR (SiPB, ppm) d: 28 (POSiMe
).
3
), À110 (SiO
4
), materials (SiP, SiC2SiP) possess 10À20% of reactive RSi–OAc
(AcO, acetoxy-) and RP–OSiMe (TMS, trimethylsilyl-) groups.
À213 (SiO
6
3
3
1
P
MAS NMR (SiPB, ppm) d: À41 (P(OSi)4), À30 There are two types of SiO units in the silicophosphate xerogels.
6
(
P(OH)(OSi) ), À19 (P(OH) (OSi) ), À10 (P(OH) (OSi)).
One is formed by coordination of phosphoryl oxygens (À213 ppm)
3
2
2
3
2
À1
18,19
BET: 117 m g
.
and the other by carboxylic oxygen of acetate (À194 ppm).
It is
À1
IR (SiC2SiPB, KBr, cm ) n: 497 m, 786 w, 858 w (ras SiCH
3
), clear from the comparison of IR spectra before and after hydro-
1
039 vs (n Si–O–P), 1262 s (d
s
SiCH
3
), 2913 vw (n
s
CH
3
), 2960 vw lysis that acetoxy groups are depleted (eqn (7)) in the course of
the reaction as indicated by the diminished intensity of the
(n
as CH
3
).
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2
6,27
to OQP(OH)
2
(OSi) moieties.
Based on these NMR data we
conclude that hydrolysis of the skeletal RSi–O–PR bonds
takes place (eqn (10)) to a large extent and the structure of
silicophosphate xerogels collapses. The more water used for the
3
1
reaction, the more intensive hydrolysis occurred – P MAS
NMR spectra showed more downfield signals with higher
intensities coming from the higher number of HO–PR groups
(
Fig. S1, ESI†). It is also noteworthy that while the skeletal
RSi–O–PR groups undergo facile hydrolysis, the surface
Me Si–O–PR bonds are stable (Fig. 1).
3
RSi–O–PR + H O - RSi–OH + HO–PR
2
(10)
The reaction in eqn (10) should yield strongly acidic RP–OH
groups. This was confirmed by IR spectroscopy of pyridine-
treated samples. Non-hydrolyzed SiP samples did not show any
uptake of pyridine. In contrast, absorption bands at 1488, 1537,
Fig. 1 Comparison of infrared spectra of parent xerogel (SiP), xerogel
after hydrolysis (SiPH2), and after reaction with hexamethyldisiloxane
À1
and 1640 cm were observed for all samples reacted with water
(
SiPHMDSO), KBr pellet.
(Fig. S2, ESI†). These wavenumbers are typical for pyridinium
cations suggesting that Brønsted acid sites were formed during
the hydrolysis in accordance with our previous observations.
The breakdown of the silicophosphate (SiP) structure by
absorption bands of COO stretches at 1768, 1721, and
1
evidenced also in the C and Si CPMAS NMR spectra. significant losses of SA (Table 4). Thus we obtain acidic but
Resonances of acetoxy groups are absent in the C spectra nonporous materials. Hybrid silicophosphates (SiC2SiP) dis-
and the SiO moieties with coordinated acetate oxygens dis- played better results in terms of porosity after the reaction with
appear from the Si spectra. It is also possible that the water. They lose acetoxy groups and a large amount of RSi–O–
formed RSi–OH groups react further with RSi–OAc groups PR bonds similar to parent SiP xerogels; however, they keep
and condense to RSi–O–SiR bridges (eqn (8)) or eliminate some surface area even if an equimolar amount of water (100%
À1
18
540 cm
(Fig. 1). The depletion of acetoxy groups was hydrolysis of skeletal RSi–O–PR linkages (eqn (10)) results in
1
3
29
1
3
6
2
9
18
water in homocondensation (eqn (9)).
relative to residual organic groups) is used in the hydrolysis
SiC2SiPH1, Table 4). Considering the possible application of
our materials, we have to be aware of their facile hydrolysis.
Therefore they should be used under anhydrous or better still
aprotic conditions. As we have shown recently, silicopho-
sphates show only limited resistance to methanol since this
(
RSi–OAc + H O - RSi–OH + AcOH
(7)
(8)
(9)
2
RSi–OH + AcO–SiR - RSi–O–SiR + AcOH
RSi–OH + HO–SiR - RSi–O–SiR + HOH
In contrast to the acetoxy groups, the RP–OSiMe groups are reaction produces acetic acid in the first step and methylacetate
3
stable under these reaction conditions. The intensities of absorp- and water in the second step. The esterification reaction is
2
4
tion bands characteristic for the SiMe moieties at 1258, 853, strongly shifted to the side of methylacetate, because water
3
À1
and 765 cm do not change and also the signals of trimethyl- is promptly consumed in the hydrolysis of the Si–O–P
silyloxy groups at À3.6 and 23.6 ppm remain unchanged in the backbone.
1
8
1
3
29
C and Si CPMAS NMR spectra, respectively.
If we consider the substitution of CH
Reactions with hexamethyldisiloxane. In the case of reac-
3
CO groups with tions with hexamethyldisiloxane, the acetoxy groups displayed
protons in terms of the mass of the samples, the weight should significant reactivity and were almost quantitatively substituted
decrease. However the yields of the products are slightly higher by trimethylsilyloxy groups (eqn (11)) if we used 50% of HMDSO
than the starting masses of the xerogels (Table 1). In accor- relative to residual organic groups (both acetoxy and trimethyl-
dance with increasing mass of the samples the mass losses silyloxy). The volatile product of the reaction – trimethylsilyl-
(
TGA) increase after hydrolysis (Table 4). It is consistent with acetate – was identified by GC-MS and the surface bound
the suggestion that water could either be adsorbed onto the RSi–OSiMe groups were evidenced by a new resonance at
surface of the xerogels or hydrolyze the RSi–O–PR bonds as 13.6 ppm in the Si CPMAS NMR spectrum. At the same time
3
2
9
2
5
these are not hydrolytically stable (eqn (10)). We were able to the intensity of vibrations of acetoxy groups diminished in the
distinguish between these two possibilities with the help of
3
1
13
P
IR spectra (Fig. 1), and their resonances were absent in the
C
MAS NMR spectra. The parent xerogel displayed only one signal CPMAS NMR spectra. Finally the signal of SiO6 moieties at
1
8
at À45 ppm (P(OSi) ) and this signal split after the reaction À196 ppm coming from the additional coordination of carboxyl
4
2
9
with water to several overlapping resonances with maxima groups disappeared in the Si CPMAS NMR spectrum.
at À47, À36, À26, and À16 ppm assigned to P(OSi)4Àn(OH) The substitution of SiO -coordinated bridging acetoxy
The least groups with terminal trimethylsilyloxy groups caused a signifi-
shielded resonance at À8 ppm of a low intensity was assigned cant decrease of surface area to half of its value. A similar effect
n
6
2
6,27
phosphorus environments, where n = 0À3.
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has been observed after the methanolysis of silicophosphate
3
xerogel, where the CH COO groups were exchanged with methoxy
18
t
groups. We also tried methyl butylether for the elimination of
acetoxy-groups. In all cases the resonance of SiO at À196 ppm
6
diminished, CH COO groups were effectively substituted and
3
samples lost from 45 to 65% of SA and pore volume in compar-
ison to original xerogels. Therefore we conclude that the bridging
nature of acetates is very important for the porous structure of
these silicophosphate xerogels (SiP samples).
The silicophosphate skeleton is also not resistant towards
HMDSO attack and new RSi–OSiMe and RP–OSiMe groups
3
3
are formed (eqn (12)) if excess of HMDSO is used for the
reaction. SiP heated in neat HMDSO completely decomposed
3 3 3 4
and provided a mixture of OP(OSiMe ) and Si(OSiMe ) . The
hybrid SiC2SiP xerogel showed similar reactivity.
29
Fig. 2
Si CPMAS NMR spectra of xerogels before surface modification
(
parent SiP) and after reaction with silicon tetrachloride (SiPSi) and phos-
RSi–OAc + Me SiOSiMe - RSi–OSiMe + AcOSiMe
3
3
3
3
phoryl trichloride (SiPP).
(11)
RSi–O–PR + Me SiOSiMe - RSi–OSiMe + Me SiO–PR
3
3
3
3
(12)
(SiPPhCH2Si) with similar surface areas and O SiCH Ph groups
3
29
2
1
3
bound to the surface as evidenced by C and Si CP MAS NMR
spectra (Fig. S3, ESI†). The SiÀC bonds are stable under the
reaction conditions. This reaction manifold provides us various
possibilities to modify the surface of porous inorganic support.
While phosphoryl trichloride reacted with both AcO and
Reactions with SiCl and POCl . Silicon tetrachloride reacted
with both acetoxy and trimethylsilyloxy groups (the ratio of Cl
4
3
to the sum of AcO and Me Si was 1 : 1), but with TMS to a larger
3
extent. The organic products were trimethylchlorosilane and
acetylchloride according to GC-MS (eqn (13) and (14)). The
4
TMS groups similar to SiCl (eqn (19) and (20)), the overall
reactivity of POCl3 towards SiP samples was lower. When
equimolar amounts of chlorine atoms of POCl3 with respect
to residual groups were used, unreacted phosphoryl trichloride
was observed among volatiles by GC-MS together with acetyl-
chloride and trimethylchlorosilane. The acetoxy groups were in
this case more effectively depleted in comparison to the reac-
tion with silicon tetrachloride (Fig. 2). We did not observe any
3
intensities of the signals of AcO and Me Si groups after the
1
3
29
reaction were significantly reduced in IR, C, and Si CPMAS
NMR spectra. At the same time, new signals of Cl SiO and
ClSiO moieties were observed in the Si CPMAS NMR spectra
2
2
2
9
3
(
Fig. 2), terminal Cl SiO groups were not observed. However,
3
these were observed at À43 ppm when the reaction was per-
formed at a 1.4 : 1 ratio of Cl : residual organic groups. Silicon
tetrachloride thus acted as a significant cross-linking agent and
this was reflected in a substantial and reproducible increase of
indication of ligand scrambling between RSi–OAc and POCl
3
2
9
in the Si CPMAS NMR spectra (Fig. 2). The surface area
2
À1
remained virtually unchanged (483 m g ).
As a proof of successful anchoring of POCl
2
À1
SA from 487 to 865 m g . Reactions that are consistent with
these observations are summarized in eqn (15)–(18).
3
onto the surface
of silicophosphate xerogels, a new signal at À31 ppm was
3
1
RSi–OAc + SiCl
4
- RSi–O–SiCl
3
+ AcOCl
(13)
(14)
observed in P MAS NMR spectra which could be assigned to
OQP(OSi/P) or ClP(OSi/P) moieties (Fig. 3). The first two steps
of condensation reactions leading to moieties with this shift are
3
3
RP–O–SiMe + SiCl - RP–O–SiCl + Me SiCl
3
4
3
3
RP–O–SiCl
3
+ Me
3
Si–O–PR - RP–O–Si(Cl
2
)–O–PR + Me
3
SiCl described in eqn (19)–(24). The condensation can proceed even
(
15) one step further to provide OQP(OSi/P) groups with a
3
3
1
19
chemical shift of ca. À30 ppm in P MAS NMR. The second
possible reaction providing phosphorus atoms with a similar
chemical shift is the coordination of free phosphoryl oxygen to
the Si atoms. This possible reaction would increase the coordi-
RSi–O–SiCl
3
+ AcO–SiR - RSi–O–Si(Cl
2
)–O–SiR + AcOCl
(
16)
)–O–SiR + AcOCl
17)
)–O–PR + Me SiCl
18)
The facile condensation between RP–OSiMe3 and Si–Cl
3 2
RP–O–SiCl + AcO–SiR - RP–O–Si(Cl
(
3
nation number of silicons and produce ClP(OSi/P) moieties.
3
1
The signals in the P MAS NMR spectrum of the SiPP sample
are however broad and overlapping which prevents a more
precise description of the new surface groups (Fig. 3).
RSi–O–SiCl
3
+ Me
3
Si–O–PR - RSi–O–Si(Cl
2
3
(
groups (eqn (14)) could be potentially used for anchoring of RSi–OAc + P(O)Cl
various organic functionalities through alkyl- or arylchloro-
3
- RSi–O–P(O)Cl
2
+ AcOCl
(19)
silanes. As an example, we reacted benzyltrichlorosilane,
PhCH SiCl , with silicophosphate xerogels providing samples
2 3
RP–O–SiMe
3
+ P(O)Cl
3
- RP–O–P(O)Cl
2
+ Me
3
SiCl (20)
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Table 4 Relative amounts of water used in the reactions, mass losses
during TGA, and surface areas of the products
a
b
2
À1
Sample
n
H O [%]
MLexp [%]
SA [m g
]
2
SiPH1
SiPH2
SiPH3
SiC2SiPH1
10
50
100
100
37.7
40.6
46.4
28.1
279
c
Nonporousc
Nonporous
42
a
b
Relative to residual organic groups. Mass loss at 1000 1C in air with
À1
c
2
À1
the heating ramp of 5 K min
. SA below 10 m g .
dimethylacetamide and dimethyl(trimethylsilyl)amine in the case
of Al(NMe ) , and trimethylchlorosilane for AlCl ). These com-
2
3
3
pounds are obviously the products of condensation of residual
RSi–OAc and RP–OSiMe groups with aluminium modifiers
3
3
1
and attest to the formation of new Al–O–Si and Al–O–P bonds on
the xerogel surfaces. The anchoring of Al atoms onto the surface
of xerogels was confirmed by elemental analysis of the samples,
where Si, P, and Al values were in good agreement with theoretical
values (Table 3). Although the reactions afforded expected organic
byproducts and Al atoms were grafted onto the surface, the
inorganic products displayed significant differences in the struc-
ture and coordination number of Al atoms. This was caused by
the different propensity of Al precursors towards ligand scram-
bling (see below).
Fig. 3
(
P MAS NMR spectra of xerogels before surface modification
parent SiP) and after reaction with phosphoryl trichloride (SiPP).
RP–O–P(O)Cl
2 3
+ Me Si–O–PR
-
RP–O–P(O)(Cl)–O–PR+ Me
3
SiCl
(21)
(22)
(23)
(24)
RSi–O–P(O)Cl + AcO–SiR
2
-
RSi–O–P(O)(Cl)–O–SiR+ AcOCl
RP–O–P(O)Cl
2
+ AcO–SiR
3
Al–Me moieties are present in samples treated with AlMe as
1
3
suggested by the IR and C CPMAS NMR spectra of the SiPAl1
-
RP–O–P(O)(Cl)–O–SiR+ AcOCl
sample, where we observed absorption bands at 689, 1193, and
À1
RSi–O–P(O)Cl
2
+ Me
3
Si–O–PR
2937 cm
assigned to nas AlC , d AlCH , and nas AlCH
3 s 3 3
2
8
13
vibrations and a C resonance of shielded methyl groups
-
RSi–O–P(O)(Cl)–O–PR+ Me SiCl
3
bound to Al at À7.9 ppm. However a significant amount of
The efficiency of anchoring Si and P compounds on the xerogel AlMe groups obviously exchanged with residual acetoxy groups
surfaces was proven by elemental analyses of phosphorus, total (eqn (25)). This reaction is consistent with Si CPMAS NMR
2
9
acidity, and argentometric analyses of Cl (Table 2). All experi- data, where a composite broad signal of Si(OP)4Àn(OAc) moi-
n
mental data are in reasonable agreement with theoretical eties (n = 0–3) in the SiP sample observed at À111 ppm split
values confirming the grafting of xerogel surfaces with SiCl4 into signals for OSiMe , O SiMe , O SiMe, and SiO groups at
3
2
2
3
4
and POCl3.
Reactions with AlMe
17, À4, À64, and À115 ppm, respectively, in the spectrum of
3
, Al(NMe
2
)
3
, and AlCl
3
. The chemical SiPAl1 (Fig. S4, ESI†). Simultaneously the SiO signal arising
6
modification of these silicophosphate gels with several aluminum from the additional coordination of carboxyl oxygens of the AcO
compounds was explored as a potential source of Lewis acidic groups at À196 ppm disappeared. Finally the substitution of
tetrahedral Al sites on the surface of silicophosphate supports. methyl groups at Al with bidentate acetoxy groups (eqn (25))
Volatile products of all three reactions contained expected organic leads to a high coordination number for Al as evidenced by a
2
7
compounds (acetone and tetramethylsilane in the case of AlMe3, signal at À7 ppm in the Al MAS NMR spectra assigned to Al
3 4
Table 2 Experimentally obtained and calculated P content from ICP, NaOH consumptions (HCl + HOAc + H PO ) and Cl content from argentometry of
SiPSi and SiPP samples
À1
À1
À1
À1
Sample
w(P)exp (%)
w(P)calc (%)
nexp NaOH (mmol g
)
ncalc NaOH (mmol g
)
nexp Cl (mmol g
)
ncalc Cl (mmol g
)
SiPSi
SiPP
15.7
19.5
17.7
nd
15.0
14.1
16.3
15.4
2.90
1.12
2.87
nd
Table 3 ICP-OES elemental analysis of P, Al, and Si contents. Comparison with theoretical values
Sample
w(P)exp (%)
w(P)calc (%)
w(Al)exp (%)
w(Al)calc (%)
w(Si)exp (%)
w(Si)calc (%)
SiPAl1
SiPAl2
SiPAl3
13.4
12.0
12.0
14.9
11.7
11.6
5.42
4.72
6.50
6.24
4.83
5.60
9.80
9.39
9.39
10.1
7.97
7.90
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after the grafting of Al compounds. This was accompanied also
1
3
by the changes in the C CPMAS NMR spectra. For example, a
new absorption band of the carboxyl group was observed at
À1
1
602 cm in the IR spectra of the SiPAl3 sample and at the
same time a new signal of carboxyl groups was identified at
13
1
81.6 ppm in the C CPMAS NMR spectra (in comparison with
parent SiP xerogel). The wavenumber of the absorption band as
well as the shift in the NMR spectrum suggest that the observed
3
5,36
carboxyl group is bidentate.
Acetoxy groups evidently coordinated
27
to aluminium atoms causing its hypercoordination ( Al MAS NMR,
Fig. 4) and shifting the parent signals in both IR and NMR spectra.
Therefore we conclude that the acetoxy groups have a negative
influence on the Lewis acidity of the products.
Preparation of solid acid catalysts. As already mentioned,
the reaction of silicophosphate xerogels with water provided
nonporous powders possessing Brønsted acidic RP–OH
groups. These, however, may not be accessible because of the
low surface area of samples. The only sample which retained
2
7
Fig. 4 Comparison of the Al MAS NMR spectra of xerogels after reaction
with aluminium compounds.
2
9
atoms in 6-fold coordination (Fig. 4). As a consequence of the porosity was the SiC2SiPH1 hybrid. Preliminary tests of silico-
total collapse of the structure the product was nonporous.
RSi–OAc + AlMe - RSi–Me + AlMe OAc (25)
2 3
Better results were achieved in reactions with Al(NMe ) .
phosphate samples in methylstyrene dimerization showed that
both Brønsted acidity and porosity are very important for
catalytic activity – the hybrid sample SiC2SiPH1 displayed a
conversion of 33% whereas the nonporous sample SiPH2 con-
verted less than 1% of methylstyrene (Table 5). The SiP sample
3
2
2
9
The Si CPMAS NMR spectra of the SiPAl2 sample displayed without modification with water did not show any catalytic
features of both ligand exchange (À68.5 ppm, (Me N) SiO activity as expected. Therefore we focused on the synthesis of
and condensation (À90.0 ppm, SiO ). porous samples with high numbers of RP–OH groups on the
2
2
2
,
3
0–32
0
.4 ppm, AlOSiMe )
3
4
The downfield shift of the SiO resonance with respect to silica surface. This was achieved by grafting of xerogels with phos-
4
(
À110.0 ppm, SiO ) points to the formation of Si–O–Al phoryl trichloride and subsequent hydrolysis of the P–Cl bonds
4
33,34
bonds.
The signal at À90.0 ppm could be increased in (samples SiPB and SiC2SiPB, Table 5).
30,32
intensity by ligand exchange (Me
2
NSiO
3
) as well.
The extent
The samples SiPB and SiC2SiPB synthesized by a two-step
of ligand exchange was significantly smaller than in the case of protocol (see Experimental) showed both high acidity and surface
the SiPAl1 sample and was comparable to the extent of con- areas (Table 5). It is noteworthy that even if the average number of
2
9
densation according to the Si CPMAS NMR spectra. This was OH groups on phosphorus is lower for these two samples, their
2
7
reflected in the Al MAS NMR spectra, where hexacoordinated overall acidity is higher as indicated by high pyridine uptakes. The
2
9
Al atoms were accompanied by pentacoordinated Al (Fig. 4).
acidity is unambiguously of Brønsted type as confirmed by the
presence of the absorption bands of the pyridinium cation in the
2
À1
The surface area decreased slightly to 375 m g
.
Aluminium trichloride provided the best results. We IR spectrum of the SiPB sample after pyridine adsorption (Fig. 5).
observed little evidence of ligand exchange by a signal of low This is caused by a large amount of phosphorus atoms on the
2
9
intensity at À91 ppm in Si CPMAS NMR spectra. This signal is surface of these samples after their grafting with POCl
3
.
probably not due to SiO centers associated with aluminium These two samples displayed a high catalytic activity in
4
because no acetylchloride is eliminated during the reaction and a-methylstyrene dimerization (Table 5) similar to silica impregnated
therefore no Si–O–Al bridges are formed. Five signals were
2
7
observed in the Al MAS NMR spectrum of the SiPAl3 sample:
two resonances of hexacoordinated Al atoms at À23 ppm and
À7 ppm and three peaks of tetracoordinated Al atoms at 51, 67,
and 83 ppm (Fig. 4). These three signals are typical for all our
Table 5 Acidity, porosity, and catalytic properties of silicophosphate
xerogels
Sample
SiPH2
1.16
SiC2SiPH1 SiPB SiC2SiPB
À1
samples grafted with AlCl
AlO , AlO Cl and AlO Cl moieties. The surface area increased
from 403 to 469 m g . For these reasons AlCl was identified as
3
and we suggest that they come from Pyridine uptake (mmol g
)
2.58
1.6
1.70 2.77
1.1 0.9
2
9
Average number of OH groups 1.6
4
3
2
À1
2
on P 2
À1
2
3
SA (m g
)
Nonporous 42
117 172
a
a promising functionalizing reagent for synthesizing Lewis Microporosity (%)
acidic and porous materials using silicophosphates as supports.
—
0
56
0
Conversion (%)
Selectivity (%) Isomer I
Selectivity (%) Isomer II
o1
33
1
82
17
99 99
14 o1
29 92
—
The residual acetoxy groups obviously play an important role
—
—
in the increasing coordination number of Al atoms. The region Selectivity (%) Isomer III
57
8
À1
between 1400 and 1800 cm , where absorption bands of COO
a
Vmicro/Vtotal, Vmicro determined by t-plot analysis, Vtotal estimated at
valence vibrations are located, was always significantly different p/p = 0.98.
0
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Fig. 5 IR spectra of the SiPB sample (a), SiPB after pyridine adsorption (b),
SiPL (c), and SiPL after pyridine adsorption (d).
2
7
Fig. 6
Al MAS NMR spectra of Lewis acidic xerogel (SiPL) before and
after pyridine adsorption.
23
with phosphoric acid. It is notable that selectivity differs signifi- of the signal assigned to hexacoordinated aluminium atoms in
2
7
cantly according to the pore sizes and structure of silicophosphate
Al MAS NMR spectra (Fig. 6) and absorption bands at 1450,
À1
samples – microporous xerogels based on the parent SiP silico- 1489, and 1615 cm in IR spectra typical for pyridine coordi-
phosphates reached moderate selectivity for 2-pentene (dimer nated to a Lewis acid center (Fig. 5). These samples were tested
III); however, mesoporous hybrid xerogels (SiC2SiP) showed a in a-methylstyrene dimerization and they displayed zero activity
high selectivity for 1-pentene (dimer II) (Table 5). Thus, together as expected. However this changed when moist methylstyrene
with our previous catalytic results achieved on mesoporous was used for the catalytic tests. In this case hydrochloric acid was
5 6
nanocrystalline Si P O25, we developed heterogeneous catalysts released in situ by the hydrolysis of residual RAl–Cl groups and
for a-methylstyrene dimerization with excellent conversions and catalyzed the dimerization similar to a published report where
2
3
moderate to excellent selectivities toward all possible methyl- AlCl was tested.
styrene dimers (I–III).
3
3
7
Both catalysts can be reused without regeneration and
maintain very high methylstyrene conversions in the second
and third catalytic run (98 and 96% for SiPB, 97 and 94% for
Conclusions
SiC2SiPB). The selectivity after two catalytic cycles decreased by The surface reactions of silicophosphate xerogels with various
% for 2-pentene by the SiPB sample and by 12% for 1-pentene reagents have been investigated and optimized to produce
8
by the SiC2SiPB catalyst. The heterogeneous nature of catalytic tailored Brønsted and Lewis acidic heterogeneous catalysts.
action is supported by the absence of phosphorus in the The residual organic (acetoxy and trimethylsilyloxy) groups
reaction solution (ICP-OES). The described two-step synthesis coming from the non-hydrolytic sol–gel synthesis were utilized
uses the knowledge of surface reactions on our silicophosphate in the reactions, however, the skeletal RSi–O–PR bonds have
xerogels to transform them into efficient and selective Brønsted also been found to participate in these reactions. Water caused
acidic catalysts.
the elimination of acetoxy groups and hydrolysis of RSi–O–
Finally we have also attempted to prepare porous Lewis PR bonds which provided Brønsted acidic HO–PR groups.
acidic xerogels. As mentioned above, we found that acetoxy The rupture of skeletal bonds leads to an abrupt decrease of
groups coordinate to Al atoms thus decreasing their Lewis surface area. Hexamethyldisiloxane was effective in the removal
acidity. However acetoxy groups can be depleted in the reaction of acetoxy groups from these matrices. SiCl and POCl reacted
4
3
with hexamethyldisiloxane. Therefore the first step of synthesis preferably with trimethylsilyloxy groups releasing trimethyl-
was the removal of acetoxy groups by hexamethyldisiloxane and chlorosilane and forming new RSi–O–PR and RP–O–PR
subsequent grafting with AlCl , which displayed the best results bridges while acetoxy groups reacted less readily. The surface of
3
among the tested aluminum precursors. This synthetic proto- silicophosphates was then covered by Si or P atoms with
col was performed on both parent and hybrid silicophosphate reactive residual chlorine atoms. Through stable Si–C and
xerogels and provided a high number of Lewis acidic Al atoms P–C bonds, various organic groups could be anchored to the
on the surface of porous silicophosphate support (samples SiPL surface adding new functionality to these xerogels. Finally,
2
À1
and SiC2SiPL, SA = 434 and 458 m g , respectively). The high AlMe , Al(NMe ) , and AlCl were investigated as potential
3
2 3
3
2
7
Lewis acidity was suggested on the basis of Al MAS NMR sources of tetrahedral aluminium atoms. The most promising
spectra where only resonances of tetracoordinated Al atoms results were obtained with aluminium trichloride which was
were observed (Fig. 6) and confirmed by pyridine adsorption. not subject to ligand exchange and condensed smoothly with
Samples after pyridine adsorption displayed an abrupt increase trimethylsilyloxy groups releasing trimethylchlorosilane and
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forming new RAl–O–PR bonds. The acetoxy groups coordi- 14 G. Guerrero, J. G. Alauzun, M. Granier, D. Laurencin and
nated readily to aluminium atoms in all cases and decreased
their Lewis acidity.
P. H. Mutin, Dalton Trans., 2013, 42, 12569.
15 E. L. Hanson, J. Schwartz, B. Nickel, N. Koch and M. F.
Danisman, J. Am. Chem. Soc., 2003, 125, 16074–16080.
The knowledge of studied reactions was utilized to synthe-
size Brønsted and Lewis acidic catalysts from silicophosphate 16 P. Thissen, A. Vega, T. Peixoto and Y. J. Chabal, Langmuir,
xerogels (supports). The two-step synthesis (POCl , H O) pro-
2012, 28, 17494–17505.
vided porous solid Brønsted acids which showed high catalytic 17 A. Vega, P. Thissen and Y. J. Chabal, Langmuir, 2012, 28,
activities for the dimerization of a-methylstyrene. The selectivity of
8046–8051.
the catalysts was dependent on their pore sizes. The second two- 18 A. Styskalik, D. Skoda, Z. Moravec, J. G. Abbott, C. E. Barnes
3
2
3
step protocol (HMDSO, AlCl ) resulted in the formation of porous
and J. Pinkas, Microporous Mesoporous Mater., 2014, 197,
and highly Lewis acidic xerogels. These are only two examples of
204–212.
procedures for tailoring the surface of silicophoshate xerogels and 19 A. Styskalik, D. Skoda, Z. Moravec, M. Babiak, C. E. Barnes
using them as potential supports for heterogeneous catalysts and
other functional materials.
and J. Pinkas, J. Mater. Chem. A, 2015, 3, 7477–7487.
20 J. H. Balthis, E. G. Rochow and D. G. White, in Inorganic
Syntheses, ed. J. C. Bailar, Jr., John Wiley & Sons Inc., 2007,
pp. 45–47.
Acknowledgements
2
2
2
2
1 J. Rouquerol, F. Rouquerol and K. S. W. Sing, Adsorption by
Powders and Porous Solids, Academic Press, 1998.
2 S. Lowell, Characterization of Porous Solids and Powders:
Surface Area, Pore Size and Density, Springer, 2004.
3 G. N. Kirichenko, N. G. Grigor’eva, V. I. Glazunova and
U. M. Dzhemilev, Pet. Chem., 2009, 49, 306–310.
4 Y. Song, Y. Huang, E. A. Havenga and I. S. Butler, Vib.
Spectrosc., 2001, 27, 127–134.
This research has been financially supported by the Ministry of
Education, Youth and Sports of the Czech Republic under the
project CEITEC 2020 (LQ1601).
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