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1H and 31P MAS NMR studies of solid heteropolyacids and H3PW12O40 supported on SiO2
Journal of Molecular
Catalysis,
60 (1990) 65-70
65
‘H AND s’PMASNMR STUDIES OF SOLID RRTRROPOLYACIDS AND HsPWlaOlo SUPPORTED ON SiO, V. M. MASTIKHIN, S. M. KULIKOV, A. V. NOSOV, MUDRAKOVSKY and M. N. TIMOFEEVA
I. V. KOZHEVNIKOV,
Institute of Catalysis, Siberian Bmnch of the USSR Academy of Sciences, (U.S.S.R. ) (Received June 15,1989;
revised November
I. L.
630090Novosibirsk
7,1989)
lH and 31PMAS NMR spectra of solid heteropolyacids (HPA) HmXY12040 (X= P, Si; Y = W, MO; m = 3, 4) as well as H3PW12040 HPA supported on SiOZ were studied. NMR data show the interaction of supported HPA with surface OH groups. Different types of supported species were found to depend on the surface HPA concentration.
1Iltroducti0n Polyoxocompounds of the H,XY,0p.xH20 type (where X = P, Si, Y = MO, V, W), usually called heteropolyacids (HPA), possess a number of interesting features; it was found recently they can be used as catalysts for a variety of homogeneous as well as heterogeneous chemical reactions [l-3]. Their strong Brijnsted acidity is of special interest for catalysis. The acidic properties of HPA in solution have been studied in detail [ 23, but there are sparse data on the proton structure of solid HPA. The broad line ‘H NMR data indicate that the most probable sites of the localization of acidic protons are near the bridged oxygen atoms of the Keggin structure 141. No data are available on the proton structure of the supported HPA used in heterogeneous catalytic reactions. It was found recently that the chemical shift in lH NMK high resolution spectra of the surface OH groups in oxides and zeolites [5,61 correlates with the proton acidity, thus providing valuable information on their catalytic properties. In this work we have used magic angle spinning (lHMAS) NMR to elucidate the state of acidic protons of the HPA H,PW,,O,, supported on SiOa. The 31PMAS spectra were also used for the characterization of the supported HPA. Experimental The HPAs used were commercial products. The samples of HPA supported on SiOZ (commercial KSK type) were obtained by impregnation of 0304-5102/90/$3.50
(Q Elsevier Sequoia/Printed
in The Netherlands
SiOz with an aqueous solution of HPA, with subsequent drying at 373 K and calcination at 473 K for 4 h. Prior to NMR experiments, the samples were placed in special NMR sample tubes of 7 mm o.d. and 12 mm length and then evacuated at 473 K at 10e3 Pa for 3 h. NMR spectra were obtained on a CXP-300 Bruker spectrometer at 3OOMHz, using frequency range 5OKHz, jt/2 pulse duration 5 ,US,pulse repetition frequency 1 Hz. Chemical shifts were measured relative to TMS as external reference. The MAS of samples was performed in quartz rotors at a frequency of 3-3.5 KHz, using a special probe head with the minimum background signal. Prior to measurements the probe head, rotor and sample tubes were dried to remove traces of water from their outside surfaces. The 31PMAS NMR spectra were recorded at 121.47 MHz in the frequency range 50 KHz, x/2 pulse duration 10 ps and pulse repetition rate 0.1 Hz. The chemical shifts were measured relative to 85% H,PO, as an external reference. Polymethylmethacrylate rotors were used, with a rotation frequency of 3 KHz. The number of accumulations amounted to 1000 for lH spectra and 500-1000 for 31P spectra.
Results and discussion The starting point of our studies was the measurement of the ‘H MAS spectra of evacuated solid HPA. The chemical composition of the samples under study and the isotropic ‘H chemical shifts are listed in Table 1. Representative spectra of H,PW,,O, evacuated at 473 K are illustrated in Fig. 1. The spectrum of this compound obtained without MAS (Fig. l-l) has an asymmetric shape due to the chemical shift anisotropy. For H3PW1z040 u,, = 4.2 f 0.3 ppm, U* = 12.3 f 0.3 ppm with oi, = l/3(20,, + a,> = 9.6 f 0.3 ppm. The MAS spectrum (Fig. 1-2) consists of one single line with isotropic chemical shift 9.3 f 0.1 ppm. This value coincides with that calculated from the anisotropic spectrum and with the value reported recently in 171. Inspection of the isotropic chemical shift values presented in Table 1 shows their dependence on the type of central atom (Si or P) and on the nature of the metal atom (MO or W). The chemical shift increases on replacing P with Si and MO with W. Thus the largest chemical shift occurs for H.,SiW1z04,, (ai, = 9.7 ppm). TABLE 1 The chemical composition of HPA and isotropic ‘H chemical shifts after dehydration Sample
W’W,,O,,
H,SiW,,O, H,SiMo,,O, HJ’Mo,,O,
ffM0 (ppm f0.1)
9.3 9.7 8.5 7.4
12a34.2
67
J-L 2 *
I
t
30
5
20
*
t
10
I
I
0
-10
L
-20
-30
PPM
Pig. 1. ‘HMAS spectra of H3PW120,c evacuated at 573 K; (1) spectrum recorded without rotation; (2) spectrum obtained with MA& all chemical shifts here and below are repoti relative to external TMS.
1
30
20
10
0
-10
-20
-30
PPM
spectra of H,PW120,0 supported on SiO, (KSK type, specific area 396 m2 g-l); (1) SiOz; (2) 5wt.8 HsPW,,O, on SiO,; (3) 37 wt.% HsPWI,OM on SiO,; (4) SO*.% H,PW,,O, on SiO,. fig.
2.
‘HMM
-16.7
1
2
-15.8
in,i 1
20
70
I
0
1
-10
-20
,
-30
I
PPM
Fig. 3. 31Ph&IS spectra of HPA H,PW,&,; Cf.1 wtalline HP&, (2) sample 1 evacuated at 473 K, low3 Pa, 4 h; 3-20 wt.% HPA supported on SiO,; (4) sample 3 evacuated at 473 K, lop3 Pa, 4 h.
The ‘H NME spectra of H3PW120r10 supported on SK& are shown in Fig. 2. The most intense line, with 6 = 13 ppm, belongs to the silica surface OH groups. Its intensity drops as the quantity of supported HPA increases (Fig. 4). No line with 6 = 9.3 ppm, typical for crystalline H3PW12040rcan be seen in the spectra up to 50 wt.% content of HPA (Fig. 2). At the same time, at 320% HPA content a line with 6 = 5 ppm appears in the spectra, indicating the formation of a new type of proton on the SiOz surface. Its surface concentration, estimkted from the intensity of the line with 6 = 5 ppm, approximately corresponds ta the quantity calculated from the concentration of the supported HPA. We can conclude therefore that at a concentration of supported HPA <5Owt.% its proton structure is different from that typical for crystalline
69
HPA, due to HPA interaction with the support. At 50 wt.% HPA, a line with 6 = 9.3 ppm typical for crystalline HPA appears in the spectrum (Fig. 2-4). 31PMAS spectra are shown in Fig. 3. The spectrum of crystalline H3PW1z04,,consists of a single line with 6 = -16.7 ppm. This value is close to that reported in 171. Evacuation of water from this sample results in a shift of the 31PIVMR line and its broadening, most probably due to localization of acidic protons near the bridged W - 0 - W oxygens (Fig. 3-2). Supporting H3PW12040 on SiOz, even without evacuation, results in a shift of the 31PNMR spectrum, indicating the interaction of HPA with SiOa (Fig. 3-3). Evacuation of the supported sample considerably broadens the 31P spectrum (Fig. 3-41, indicating a distortion of the HPA structure compared with that in crystalline form and therefore its interaction with SiOa. lH MAS spectra show the decrease in the Si-OH line intensity upon SiOz loading with increasing HPA (Fig. 4). This phenomenon can be explained as a result of Si-OH groups interacting with HPA. It seems however that a reaction of type (1): H3PW1z04,,+ mOH -
H3--mPW12040+ mH,O
(1)
I
I
cannot be the single reason for this effect. Reaction (1) at m = 3 and 10 wt.% HPA would result in a lo-15% decrease in initial SiOZ concentration. This is considerably smaller than that observed in experiment (Fig. 4). Another source of surface Si-OH ‘elimination’ may be its interaction with acidic protons formed in solution due to HPA heterolytic dissociation. In addition, the interaction of SiOz with an acid such as HPA can result in a decrease in its specific area and therefore a decrease in Si-OH group concentration [8]. At this moment we cannot conclude which of these effects prevails.
0
I 10
20
30
40
50 WA
4. The quantity of surface SiaH Hd’W,,O,. Fig.
wt%
groups of SiO, as a function of its loading with
70
Thus we can conclude that at least three types of supported HPA are present on the SiOa surface. At concentrations up to 20 wt.% of supported HPA, the surface species are the isolated surface HPAs formed due to reaction (1). Their ‘H NMR signal has not been found, most probably due to its proximity to the intense Si-OH signal or its absence if m = 3 in eqn. (1). At HPA contents >20 wt.%, clusters of supported HPA exist, with chemical shift S = 5 ppm. Finally, at 50 wt.% content, crystals of supported HPA are formed on the SiOz surface. The bonding of heteropolyanions in the bulk structure involves formation of hydrogen bonds 131. The latter affect both the H1 chemical shift and proton acidity. If the correlation between the chemical shift and Brijnsted acidity reported for oxides and zeolites [5,61 is valid also for hydrogen-bonded protons in the HPA structure we can compare the acidity of supported forms of HPA. The most acidic is the crystalline form of HPA having 6 = 9.3 ppm; clusters contain less acidic protons (6 = 5 ppm); we have no information on the chemical shift of the isolated HPA species due to the low intensity of their signal and its probable overlapping with the intense signal from S&OH groups. (If m = 3 in eqn. (1) supported HPA has no attached protons at all). Thus the results presented here indicate that MAS ‘HNMR and 31P&MR is a valuable tool for the investigation of solid as well as supported catalysts based on heteropolyacids. References 1 K. I. Matveev, Kin&. KataZ., 18 (1977) 862 (Russian). 2 I. V. Kozhevnikov and K. I. Matveev, Usp. Z&m., 51 (1982) 1875 (Russian). 3 M. Misono, K. Sakata, Y. Yoneda and W. Y. Lee, 7th Znt. Congr. Catalysis, Tokyo, 1980, Preprints, B27. 4 T. Wada, C. R., 259 (1964) 553. 5 H. F’feifer, D. Freude and M. Hunger, Zeolites, 5 (1984) 274. 6 V. M. Mastikhiu and K. I. Zamaraev, CoUoi& Surf. Sci., 12 (1984) 401. 7 Y. Kanda, K. Y. Lee, S. Nakata, S. Asaoka and M. Misano, Chem. L&t., (1988) 139. 8 B. C. Linsen (ed.) Physical and Chemical Aspects of Aakorbents and Catalysts, Academic Press, London, New York, 1970.
Catalysis,
60 (1990) 65-70
65
‘H AND s’PMASNMR STUDIES OF SOLID RRTRROPOLYACIDS AND HsPWlaOlo SUPPORTED ON SiO, V. M. MASTIKHIN, S. M. KULIKOV, A. V. NOSOV, MUDRAKOVSKY and M. N. TIMOFEEVA
I. V. KOZHEVNIKOV,
Institute of Catalysis, Siberian Bmnch of the USSR Academy of Sciences, (U.S.S.R. ) (Received June 15,1989;
revised November
I. L.
630090Novosibirsk
7,1989)
lH and 31PMAS NMR spectra of solid heteropolyacids (HPA) HmXY12040 (X= P, Si; Y = W, MO; m = 3, 4) as well as H3PW12040 HPA supported on SiOZ were studied. NMR data show the interaction of supported HPA with surface OH groups. Different types of supported species were found to depend on the surface HPA concentration.
1Iltroducti0n Polyoxocompounds of the H,XY,0p.xH20 type (where X = P, Si, Y = MO, V, W), usually called heteropolyacids (HPA), possess a number of interesting features; it was found recently they can be used as catalysts for a variety of homogeneous as well as heterogeneous chemical reactions [l-3]. Their strong Brijnsted acidity is of special interest for catalysis. The acidic properties of HPA in solution have been studied in detail [ 23, but there are sparse data on the proton structure of solid HPA. The broad line ‘H NMR data indicate that the most probable sites of the localization of acidic protons are near the bridged oxygen atoms of the Keggin structure 141. No data are available on the proton structure of the supported HPA used in heterogeneous catalytic reactions. It was found recently that the chemical shift in lH NMK high resolution spectra of the surface OH groups in oxides and zeolites [5,61 correlates with the proton acidity, thus providing valuable information on their catalytic properties. In this work we have used magic angle spinning (lHMAS) NMR to elucidate the state of acidic protons of the HPA H,PW,,O,, supported on SiOa. The 31PMAS spectra were also used for the characterization of the supported HPA. Experimental The HPAs used were commercial products. The samples of HPA supported on SiOZ (commercial KSK type) were obtained by impregnation of 0304-5102/90/$3.50
(Q Elsevier Sequoia/Printed
in The Netherlands
SiOz with an aqueous solution of HPA, with subsequent drying at 373 K and calcination at 473 K for 4 h. Prior to NMR experiments, the samples were placed in special NMR sample tubes of 7 mm o.d. and 12 mm length and then evacuated at 473 K at 10e3 Pa for 3 h. NMR spectra were obtained on a CXP-300 Bruker spectrometer at 3OOMHz, using frequency range 5OKHz, jt/2 pulse duration 5 ,US,pulse repetition frequency 1 Hz. Chemical shifts were measured relative to TMS as external reference. The MAS of samples was performed in quartz rotors at a frequency of 3-3.5 KHz, using a special probe head with the minimum background signal. Prior to measurements the probe head, rotor and sample tubes were dried to remove traces of water from their outside surfaces. The 31PMAS NMR spectra were recorded at 121.47 MHz in the frequency range 50 KHz, x/2 pulse duration 10 ps and pulse repetition rate 0.1 Hz. The chemical shifts were measured relative to 85% H,PO, as an external reference. Polymethylmethacrylate rotors were used, with a rotation frequency of 3 KHz. The number of accumulations amounted to 1000 for lH spectra and 500-1000 for 31P spectra.
Results and discussion The starting point of our studies was the measurement of the ‘H MAS spectra of evacuated solid HPA. The chemical composition of the samples under study and the isotropic ‘H chemical shifts are listed in Table 1. Representative spectra of H,PW,,O, evacuated at 473 K are illustrated in Fig. 1. The spectrum of this compound obtained without MAS (Fig. l-l) has an asymmetric shape due to the chemical shift anisotropy. For H3PW1z040 u,, = 4.2 f 0.3 ppm, U* = 12.3 f 0.3 ppm with oi, = l/3(20,, + a,> = 9.6 f 0.3 ppm. The MAS spectrum (Fig. 1-2) consists of one single line with isotropic chemical shift 9.3 f 0.1 ppm. This value coincides with that calculated from the anisotropic spectrum and with the value reported recently in 171. Inspection of the isotropic chemical shift values presented in Table 1 shows their dependence on the type of central atom (Si or P) and on the nature of the metal atom (MO or W). The chemical shift increases on replacing P with Si and MO with W. Thus the largest chemical shift occurs for H.,SiW1z04,, (ai, = 9.7 ppm). TABLE 1 The chemical composition of HPA and isotropic ‘H chemical shifts after dehydration Sample
W’W,,O,,
H,SiW,,O, H,SiMo,,O, HJ’Mo,,O,
ffM0 (ppm f0.1)
9.3 9.7 8.5 7.4
12a34.2
67
J-L 2 *
I
t
30
5
20
*
t
10
I
I
0
-10
L
-20
-30
PPM
Pig. 1. ‘HMAS spectra of H3PW120,c evacuated at 573 K; (1) spectrum recorded without rotation; (2) spectrum obtained with MA& all chemical shifts here and below are repoti relative to external TMS.
1
30
20
10
0
-10
-20
-30
PPM
spectra of H,PW120,0 supported on SiO, (KSK type, specific area 396 m2 g-l); (1) SiOz; (2) 5wt.8 HsPW,,O, on SiO,; (3) 37 wt.% HsPWI,OM on SiO,; (4) SO*.% H,PW,,O, on SiO,. fig.
2.
‘HMM
-16.7
1
2
-15.8
in,i 1
20
70
I
0
1
-10
-20
,
-30
I
PPM
Fig. 3. 31Ph&IS spectra of HPA H,PW,&,; Cf.1 wtalline HP&, (2) sample 1 evacuated at 473 K, low3 Pa, 4 h; 3-20 wt.% HPA supported on SiO,; (4) sample 3 evacuated at 473 K, lop3 Pa, 4 h.
The ‘H NME spectra of H3PW120r10 supported on SK& are shown in Fig. 2. The most intense line, with 6 = 13 ppm, belongs to the silica surface OH groups. Its intensity drops as the quantity of supported HPA increases (Fig. 4). No line with 6 = 9.3 ppm, typical for crystalline H3PW12040rcan be seen in the spectra up to 50 wt.% content of HPA (Fig. 2). At the same time, at 320% HPA content a line with 6 = 5 ppm appears in the spectra, indicating the formation of a new type of proton on the SiOz surface. Its surface concentration, estimkted from the intensity of the line with 6 = 5 ppm, approximately corresponds ta the quantity calculated from the concentration of the supported HPA. We can conclude therefore that at a concentration of supported HPA <5Owt.% its proton structure is different from that typical for crystalline
69
HPA, due to HPA interaction with the support. At 50 wt.% HPA, a line with 6 = 9.3 ppm typical for crystalline HPA appears in the spectrum (Fig. 2-4). 31PMAS spectra are shown in Fig. 3. The spectrum of crystalline H3PW1z04,,consists of a single line with 6 = -16.7 ppm. This value is close to that reported in 171. Evacuation of water from this sample results in a shift of the 31PIVMR line and its broadening, most probably due to localization of acidic protons near the bridged W - 0 - W oxygens (Fig. 3-2). Supporting H3PW12040 on SiOz, even without evacuation, results in a shift of the 31PNMR spectrum, indicating the interaction of HPA with SiOa (Fig. 3-3). Evacuation of the supported sample considerably broadens the 31P spectrum (Fig. 3-41, indicating a distortion of the HPA structure compared with that in crystalline form and therefore its interaction with SiOa. lH MAS spectra show the decrease in the Si-OH line intensity upon SiOz loading with increasing HPA (Fig. 4). This phenomenon can be explained as a result of Si-OH groups interacting with HPA. It seems however that a reaction of type (1): H3PW1z04,,+ mOH -
H3--mPW12040+ mH,O
(1)
I
I
cannot be the single reason for this effect. Reaction (1) at m = 3 and 10 wt.% HPA would result in a lo-15% decrease in initial SiOZ concentration. This is considerably smaller than that observed in experiment (Fig. 4). Another source of surface Si-OH ‘elimination’ may be its interaction with acidic protons formed in solution due to HPA heterolytic dissociation. In addition, the interaction of SiOz with an acid such as HPA can result in a decrease in its specific area and therefore a decrease in Si-OH group concentration [8]. At this moment we cannot conclude which of these effects prevails.
0
I 10
20
30
40
50 WA
4. The quantity of surface SiaH Hd’W,,O,. Fig.
wt%
groups of SiO, as a function of its loading with
70
Thus we can conclude that at least three types of supported HPA are present on the SiOa surface. At concentrations up to 20 wt.% of supported HPA, the surface species are the isolated surface HPAs formed due to reaction (1). Their ‘H NMR signal has not been found, most probably due to its proximity to the intense Si-OH signal or its absence if m = 3 in eqn. (1). At HPA contents >20 wt.%, clusters of supported HPA exist, with chemical shift S = 5 ppm. Finally, at 50 wt.% content, crystals of supported HPA are formed on the SiOz surface. The bonding of heteropolyanions in the bulk structure involves formation of hydrogen bonds 131. The latter affect both the H1 chemical shift and proton acidity. If the correlation between the chemical shift and Brijnsted acidity reported for oxides and zeolites [5,61 is valid also for hydrogen-bonded protons in the HPA structure we can compare the acidity of supported forms of HPA. The most acidic is the crystalline form of HPA having 6 = 9.3 ppm; clusters contain less acidic protons (6 = 5 ppm); we have no information on the chemical shift of the isolated HPA species due to the low intensity of their signal and its probable overlapping with the intense signal from S&OH groups. (If m = 3 in eqn. (1) supported HPA has no attached protons at all). Thus the results presented here indicate that MAS ‘HNMR and 31P&MR is a valuable tool for the investigation of solid as well as supported catalysts based on heteropolyacids. References 1 K. I. Matveev, Kin&. KataZ., 18 (1977) 862 (Russian). 2 I. V. Kozhevnikov and K. I. Matveev, Usp. Z&m., 51 (1982) 1875 (Russian). 3 M. Misono, K. Sakata, Y. Yoneda and W. Y. Lee, 7th Znt. Congr. Catalysis, Tokyo, 1980, Preprints, B27. 4 T. Wada, C. R., 259 (1964) 553. 5 H. F’feifer, D. Freude and M. Hunger, Zeolites, 5 (1984) 274. 6 V. M. Mastikhiu and K. I. Zamaraev, CoUoi& Surf. Sci., 12 (1984) 401. 7 Y. Kanda, K. Y. Lee, S. Nakata, S. Asaoka and M. Misano, Chem. L&t., (1988) 139. 8 B. C. Linsen (ed.) Physical and Chemical Aspects of Aakorbents and Catalysts, Academic Press, London, New York, 1970.