J. Phys. Chem. 1991, 95, 5910-5914
5910
Intercalation and Pillaring of Zlrconlum Bls(monohydr0genphosphate) with
Liansheng Li: Xinsheng Liu,+,i Ying Ge> Liyun Li,t and Jacek Klinowski**l Department of Chemistry, Jilin University, Changchun, People's Republic of China, Wuhan Institute of Physics, Wuhan, People's Republic of China, and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, U.K. (Received: November 20, 1990) Intercalation and pillaring of zirconium bis(monohydr0genphosphate) (a-ZrP) with 3 4 (triethoxy)silyl]-1-propylamine, NH2(CH&3Si(OC2Hs)3, was investigated by using XRD, FTIR, MAS NMR, chemical analysis, and adsorption measurements. The organosilicon compound undergoes polymerization and the cationic dimers and trimers formed are intercalated into the interlayer space of a-ZrP. Further polymerization occurs by dehydroxylation of the OH groups attached to the Si atoms during the intercalation. Pillaring with a silica-like species occurs at about 400 OC after decomposition of the organic moiety of the intercalate. Pillared a-ZrP is stable to over 700 OC and can sorb species such as n-hexane. Introduction Layered compounds such as clays, oxides, and phosphates may be intercalated with polar organic molecules and inorganic cations. The layered compound becomes porous upon pillaring and has a significant surface area as well as sorptive and catalytic properties. Since the resulting structures can be finely 'tuned" by changing the size, shape, and concentration of the intercalating species, these materials have received much attention. Layered clays and phosphates have been particularly extensively investigated.'V2 The latter are mainly four-valent acid-form metal phosphates and five-valent metal oxyph~sphates.~*~ Zirconium bis(monohydrogenphaphate), Zr(HP04)2-H20(known as a-ZrP), is one of these materials. It was first synthesised by Clearfield and Stynes,' who also studied its thermal stability and ion-exchange properties. Detailed studies of the intercalation of a-ZrP with a number of organic amines, alcohols, and organometallic compounds have been carried out,s10 and catalytic properties of the intercalated materials examined."*'2 However, since the intercalated organics are inherently thermally unstable, the usefulness of the products is severely limited. As a result, studies of intercalated a-ZrP have recently been extended to inorganic intercalate^.'^-'^ Lewis et intercalated the octosilicate ion into a-ZrP by attaching amino groups to the silicate and subsequently pillaring by thermal removal of the organic. 13PNMR spectra of a-ZrP were first obtained by Clayden.I6 We have studied the intercalation and pillaring of a-ZrP with NH2(CH2)3Si(OC2HS)3, using powder X-ray diffraction, Fourier-transform infrared spectroscopy, magic-angle-spinning (MAS) NMR spectroscopy, sorption measurements, and chemical analysis. We have found that (i) the organosilicon compound undergoes polymerization through OH groups (formed via hydrolysis) between the molecules and forms the NH3+(CH2)3(0H)2Si-OSi(OH)2(CH2)3NH3+dimer and the NH3+(CH2),(0H)Si-[OSi(OH)2(CH2)3NH3+]2 trimer; (ii) intercalated dimers and trimers undergo further polymerization through their OH groups; (iii) because they contain -NH3+ groups, the dimer or trimer moleculess are positively charged and interact with the PO, sheets of a-ZrP (As a consequence, they orient with their chains along the c direction); (iv) decomposition of the organic groups upon heating takes place at about 400 OC, and during calcination silica-like clusters with OH groups on their surfaces are formed; (v) the crystalline pillared products are porous, thermally stable above 700 OC, and are accessible to the adsorption of species such as n-hexane.
Preparation of u-ZrP. A 0.56 M solution of ZrOC12(solution 1) was prepared by dissolving 9.0 g of ZrOC12.8H20 in 50 mL of 3 M HC1. Mixing 18.7 mL of 85% H3P04with 50 mL of 6 M HCI and diluting to 100 mL with distilled water gives a solution 2.8 M in H3P04and 3 M in HCl (solution 2). Solution 1 was slowly added to solution 2 with stirring. The mixture was stirred until homogeneous and then aged for 12 h. The solid gel was separated by filtration and washed with 2% H3P04 until no C1was detected in the filtrate. The gel was then mixed with an extra amount of concentrated H3P04with stirring. Finally, the mixture was transferred into autoclaves lined with Teflon and heated at 100 OC for 48 h. Crystalline a-ZrP was separated, washed with distilled water, and dried in air. Intercalation snd Pillaring. 1.5 g of a-ZrP powder was added, under stirring, to a 10 wt% aqueous solution of NH2(CH2)$i(OC2Hs)3 (corresponding to Si/Zr = 10 in the mixture). The suspension was refluxed for 72 h, and then the solid was separated by filtration, washed with distilled water, and dried in air. The intercalated a-ZrP was calcined at 450 or 600 OC for 2 h. Powder XRD patterns were collected on a Rigaku D/MAXIIIA diffractometer operated with Cu Ka radiation (A = 1.5418 A). Infrared spectra were recorded on a Nicolet 5DX FT-IR instrument by using KBr wafer techniques. High-temperature IR spectra were recorded after keeping the sample for 5 min at the desired temperature. 31P,*)C,and %i MAS NMR spectra were recorded at 161.98, 100.61, 79.49 MHz, respectively, on a Bruker MSL-400 multinuclear NMR spectrometer using a dou-
Experimental Section Chemicals. ZrOCI2.8H20, 85% aqueous H3P04,concentrated HCI, and NH2(CH2)3Si(OC2Hs)3 were used as received.
and Electrocaralysis; ACS Symposium Series; American Chemical Society: Washington, DC, 1982; p 192. (13) Lewis, R. M.;van Saten, R. A,; Ott, K. C. Eur. Pat. No. 0159756
Jilin University.
(1) Thomas, J. M. In Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Press: London, 1982, p 55. (2) Alberti, G.; Costantino, U. Ibld., p 147 (and references therein). (3) (a) Alberti, G.; Costantino, U.; Marmottini, F.;Vivani, R.;Zappelli, P. In Pillared Luyered Structures: Current Trends and Applications; Mitchell, I. V., Ed.; Elsevier Applied Science: London, 1990, p 119. (b) Tomlinson, A. A. G. Ibid. p 91. (4) Clearfield, A.; Stynes, J. A. J . Inorg. Nucl. Chem. 1964, 117, 26. (5) Costantino, U. J . Chem. Soc., Dalton Trans. 1979,402. ( 6 ) Tindwa, R. M.; Ellis, D. K.; Peng, G.; Clearfield, A. J. Chem. Soc., Faraday Trans. I 1985,81, 545. (7) Kijima, T.; Ueno, S.;Goto, M. J . Chem. Soc., Dalton Trans. 1982,
2499. (8) Johnson, J. W. J . Chem. Soc., Chem. Commun. 1980,263. (9) (a) Ferra ha, C.; LaGinettra, A.; Mmucci, M. A.; Patrono, P.; T O " , A. 0. Phys. Ch" 1905,89,4762. (b) Ferrrginr, C.; Mmucd, M. A.; Patrono, P.; LaGincstra, A.; Tomlinm, A. A. G. J. Chem. Soc., Dalton Trans. 1988, 851. (c) Ibld. 1%, 265. (10) MacLachlan, D. J.; Morgan, K. R. J. Phys. Chem. 1990,91,7656. (11) Alberti, G.; Costantino, U. 1. Mol. Caral. 1984, 27, 235. (12) Dines, M. B.; DeGiacomo, D. M.; Callahan, K. P.; GriMith, P. C.; Lane, R. H.;Coolteey, R. E. In Chemistry Modifled SurfonJ in Catalysis
f
A2, 1985. (14) Clearfield, A.; Roberts, B. D. Inorg. Chem. 1988, 27, 3237. (1 5) Caravajal, G. S.; Leyden, D. E.;Maciel, 0. E. In Silanes, SurJaces
and Interfaces; Leyden, D. E., Ed.; Gordon and Breach Science Publishen: New York, 1986; p 283. (16) Clayden, N . J. J . Chem. Soc., Dalton Trans. 1987, 1877.
I University of Cambridge.
* Wuhan Institute of Physics. OO22-3654/91/2095-5910$02.50/0
0 1991 American Chemical Society
Intercalation and Pillaring of a-ZrP
!-
5.3 A
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5911
--
Figure I. Schematic representation of the structure of a-ZrP.
ble-bearing probehead and zirconia rotors spinning at 3-5 kHz. X-ray fluorescence analysis was performed on a Zeiss VRA-20 instrument.
Results and Discussion a-ZrP (see Figure 1 ) is composed of ZrOs octahedral sheets each sandwiched between two sheets of PO4 tetrahedra. Each Zr atom is octahedrally coordinated to oxygens belonging to six PO4 tetrahedra of which three are in the upper PO4 sheet and three in the lower sheet. Each P atom is tetrahedrally coordinated to three oxygens shared with three ZrOs octahedra and to one hydroxyl oxygen. H+ (not shown) can be readily ion-exchanged for other monovalent and divalent cations, and interacts with water molecules via hydrogen bonds.17 The interaction between layers is so weak that intercalation of a variety of molecules can easily take place.z3 The structure is monoclinic with space group P2,/c, two crystallographically inequivalent P sites, and unit cell dimensions a = 9.060 A, b = 5.297 A, c = 15.414 A, and 0 = 101.71°.'7 Figure 2 shows partial XRD patterns of the as-synthesized a-ZrP, intercalated a-ZrP, and intercalated a-ZrP calcined at 600 OC. The as-synthesized sample is highly ~rystalline.~~J* 31P MAS NMR spectra show the presence of some free H3P04(P/Zr = 2.6) (see below). Upon treatment with the aqueous solution of NH2(CH2)3Si(OC2Hs)3, the interlayer spacing (dOo2)of the sample increases from 7.5 to 22.07 A, as seen from the shift of the (002) peak toward a low 28 angle in Figure 2. The increase of the interlayer spacing by as much as 14.57 A upon treatment clearly shows that intercalation has indeed taken place. Chemical analysis by XRF gives P/Zr = 2.04 and Si/P = 0.47 for the intercalated sample. This implies the following: (i) The extra phosphate is removed during the process (P/Zr ratio decreases from 2.6 to 2.04, the latter value being close to the ideal composition expected from the structure". (ii) The Si content of the intercalated sample corresponds to two Si atoms for every four P atoms, with two Si atoms located in the interlayer space of dimensions a X b X 22.07 A (c/2) (where a, b, and c are the unit cell parameters). The Si content together with the magnitude of interlayer spacing suggests that the chain of the molecule is perpendicular to the layers in a-ZrP. (iii) The number of phosphate sites in a-ZrP is twice the number of amine molecules, which means that only half of the sites are covered by the intercalate, rather than each site. We believe this to be a size effect: the amine molecule is so large as to occupy two phosphate sites. In order to understand what happens during the intercalation, the liquid phase and the solid product were examined by NMR spectroscopy. The 2%i NMR spectrum of the liquid, after being refluxed for 72 h (Figure 3), contains four signals at -39.3,-47.4, -47.6, and -55.8 ppm. As the 29Sichemical shift of the pure (17) (a) Troup,,J. M.;Clearfield, A. Inorg. Chcm. 1977, 16, 3311. (b) Clearfield, A.; Smith, G. D.Inorg. Chcm. 1%9,8,431. (18) Amphlett, C. B. fnorganfcIon Exchangers; Elacevier: Amsterdam,
1964.
.. 5
25
15
35
2 e (degrees) Figure 2. XRD patterns of (a) as-synthesized,(b) intercalated, and (c)
intercalated and calcined a-ZrP. 2QSl Q0
-39.3
i
0' _v_
-47.6
Q' -55.8
..
do
. 4-5
-
1
sb
..
.
55
ppm from TMS
Figure 3. 29SiNMR spectrum of supernatant liquid after reluxing for 72 h for intercalation of a-ZrPwith NHz(CHz)3Si(OCzH,)3.
organosilicon compound (spectrum not shown) is -42 ppm, it is clear that hydrolysis and polymerization took place: the first signal corresponds to silicon linked to three hydroxyl groups instead of
5912 The Journal of Physical Chemistry, Vol. 95, No. 15, 1991
Li et al.
I Zirconium phosphate layer]
-67.52
CH,
CH,
“‘v
I \
-18.24
*
I
1
I Si -0 -Si-
HO-
CH,
7I H i Q 3 + H t Q 2
I
CH,
0 -Si-
OH
v - --101.90
(b)
Zirconium phosphate layer
I
Figwe 5. Schematic representation of intercalatesin the interlayer space of a-ZrP.
450OC
-.. - - - SI E
NH3+ CH2 CH2 CH2 1%
io
8 . 0
*
a
:
II
i
i0
I
i 21.7
re’
!
;
!
0
20
-io
-i o
ppm from TMS
Figure 6. I3CMAS NMR spectrum of intercalated a-ZrP.
*
-i
-20
-40
-60
- 8 0 -100 -120 -140 -180
ppm from TMS Figure 4. %i MAS NMR spectra of a-ZrP (a) intercalated; (b) calcined at 450 O C for 2 h; (c) calcined at 600 O C for 2 h. Spectrum (d) is a IH-% CP/MAS spectrum of sample (b).
three methoxy groups, and the remaining three signals to silicons in polymeric species. Following Madel et aI.,l9we assign the signal at -39.3 ppm to QO Si units in the monomer NH3+(CH2)$i(OH)3, those at -47.4 and -47.6 ppm to Q‘ units in the NH3+(CH,)3s(OH)20&i(OH)2(CH2)3NH3+ dimer and the NH3+(CH,),Si(OH) [OSi(OH)2(CH2)3NH3+]2 trimer, and the signal at -55.8 ppm to Q2units in the NH3+(CH2)&(OH)[OSi(OH)2(CH2)3NH3+]2 trimer. (Silicon atoms comsponding to each (19) Maciel, 0. E.; Sindorf, P. W.; Bartwka, V. J. J . Chromarogr. 1981, 205, 438.
signal are underlined.) The slight difference in the chemical shift of the monomer in comparison with the starting organosilicon compound is due to the replacement of the ethoxy groups by OH groupg through hydrolysis. Figure 4a shows the 29SiMAS NMR spectrum of the solid product. There are three signals at -49.3, -59.2, and -67.5 ppm. It is clear from the chemical shifts that further polymerization of the intercalated species occurs. The signals at -49.3 and -59.2 ppm are due to Ql and q,respectively, which are similar to those observed for dimers and trimers in the liquid, and that at -67.5 ppm to Cy, which forms through further polymerization, as shown in Figure 5. I3C MAS NMR spectrum of the intercalated solid (see Figure 6)shows four signals with chemical shifts at 42.5,21.7, 16.6, and 10.3 ppm assigned as20 1
2
3
NH~+-CH~€H~-CH~-S~IS 42.5 21.7 10.3 (20) Boyer, E.; Albert, K.; Rciners, J.; Nieder, M.; Mlller, D. J . C h m
malogr. 1983, 264, 197.
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 5913
Intercalation and Pillaring of a-ZrP LI.2r.p
2-
320(1
1300
1wO
1100
050
650
400
Wawnumbon (cm- 1)
Figure 7. FTIR spectrum of intercalated a-ZrP.
- 33.0I
31P
100
ZOO
300
400
500
600
'
0
Temperature ( OC )
Intercalated a-ZIP Calcined at 800OC
Figure 9. Changes of dool spacing of intercalated a-ZrP as a function of temperature.
Calcined
40
20
0
-
20
-
40
-SO
80
ppm from 85% H3 PO4 Figure 8. ''P MAS NMR spectra of a-ZrP: (a) as-synthesized; (b) intercalated, (c) intercalated and calcined at 450 O C ; (d) intercalated and calcined at 600 O C .
The three signals have equal intensities, which is consistent with the structure. The intensity of the signal at 16.6 ppm (marked with an asterisk) is smaller than that of the others. We believe this signal comes from impurities and, possibly, from unhydrolyzed ethoxy groups of the intercalate. The formation of the NH3+ groups was confirmed by the presence of NH3+vibrations in the F H R spectrum of the solid product, as shown in Figure 7.
*
um
400
WlV8nUmbW8
(cm'
1)
Figure 10. FTIR spectra of intercalated a-ZrP at different temperatures.
Figure 8 gives the 3'P MAS NMR spectra of the as-synthesized a-ZrP, the intercalated sample, and the sample intercalated and calcined at two temperatures. The as-synthesizedsample (Figure 8a) gives two signals with chemical shifts at 0.2 and -19.7 ppm,
5914 The Journal of Physical Chemistry, Vol. 95, No. 15, 1991
'"I/ 0
Y
0.1
0.2
0.3
0.4
0.6
0.5
P I Po
Figure 11. Adsorption isotherm of n-hexane on intercalated a-ZrP calcined at 600 O C .
due to phosphorus in the occluded H3P04and in framework PO4 tetrahedra, respectively. The fact that there is only one signal for two crystallographically inequivalent P sites in the structure" is due to the similar average P-0-P angles corresponding to the two P sites. Upon intercalation, the free H3P04molecules are removed, as seen from the disappearance of the 0.2 ppm signal, and the shift of the -19.7 ppm signal to -17.3 ppm (Figure 8b). MacLachlan et a1.I0 investigated intercalation of a-ZrP with a number of amines and found that the chemical environment of P is significantly and inhomogeneously distorted upon such intercalation. As a result, more than one signal in the MAS NMR spectra was found. However, for our sample, the observation of only one signal from the 31Pspectrum of the intercalated a-ZrP implies that the effects of the intercalates on the P sites in its structure are similar. Figure 9 shows the changes in doo2interlayer spacing of the intercalated a-ZrP as a function of temperature. The spacing decreases with increasing temperature, and a significant change occurs a t ca. 400 OC, corresponding to decomposition of the organic moiety of the intercalate. After heating to 600 OC, the d spacing reaches 13.18 A and remains constant as temperature
Li et al. is raised to 700 OC, indicating that a stable pillared material has formed. The change of d spacing below 300 OC is due to removal of water adsorbed during the intercalation and to distortion of the intercalated species during calcination. Variable temperature IR studies (see Figure 10) confirm this interpretation. The 3430-cm-' band observed in IR spectra of intercalated a-ZrP disappeared below 150 O C and the band at 1637 cm-' decreased in intensity, showing the removal of water molecules. In the range of 150-300 O C , no significant changes of the spectra were observed except for the shifts of the bands around 1200-900 an-',indicating distortion of the structure of the intercalated a-ZrP. Above 400 OC, the IR spectra change significantly. The bands assigned to organics disappear, and this is accompanied by significant changes in the region of 1200-900 cm-I and shifts of the 625- and 554-an-' bands. ,IP MAS NMR spectra of samples calcined a t 450 and 600 OC (Figure 8c,d) show splittings and shifts of the signal. Two signals with chemical shifts at about -22 and -33 ppm indicate that the two P sites in the intercalated a-ZrP are no longer identical. From their intensity ratio we concluded that the interaction of some P atoms with the siliceous species is stronger than those of the others. 29Si MAS N M R spectra of the same samples (Figure 4) show dramatic shifts of the signals of the intercalated sample toward low frequency (more negative chemical shift) after calcination. Three signals at -91.8, -101.9, and -1 12.2 ppm are now found. The chemical shifts of the signals of the calcined sample indicate that they are attributable to silica-like clusters. The enhancement of the signals at -92 and -102 ppm in the 'HJ9Si CP/MAS spectrum shows that these Si atoms are attached to O H groups*l (see Figure 4b-d). Chemical analysis detects no loss of silicon from the sample upon calcination. Adsorption of n-hexane of the final product clearly shows that the calcined intercalated a-ZrP (600 OC, 1.5 h) is porous. This means that the lateral spacing of the silicate pillars is larger than the size of the n-hexane molecule. The uptake of n-hexane at p / p o = 0.5 is as high as 5.6%by weight, as compared with 0.7% for the calcined a-ZrP ( 1 80 OC, 1.5 h) (higher temperature treatment converts a-ZrP to a pyrophosphate). Figure 1 1 gives the adsorption isotherm of n-hexane by the sample. Acknowledgment. We are grateful to the National Natural Science Committee of China, Wuhan Institute of Physics, China and Shell Research, Amsterdam, for support, and to Dr.JoHo Rocha and Dr.P. J. Barrie for experimental help. (21) Engelhardt, G.; Lohse, U.;Samoson, A.; Magi, M.; Tarmak, M.; Lippmaa, E. Zeolites 1982, 2, 59.
