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Proton affinity in heterogeneous acid-based catalysis. Measurements and use for analysis of catalytic reaction mechanism
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Proton affinity in heterogeneous M e a s u r e m e n t s a n d u s e for a n a l y s i s mechanism
3231
acid-base catalysis. of c a t a l y t i c r e a c t i o n
E.A.Paukshtis Boreskov Institute of Catalysis, Prospekt Akademika Lavrentieva, 5, 630090 Novosibirsk, Russia, fax: 7(3832)343766, e-mail: [email protected] In the present paper we review possible ways for measuring the strength of surface acid sites and for studying acid-base reactions using the proton affinity scale. PA measurements are done with the IR spectroscopy of hydrogenbonded complexes. The proton affinity of acid sites and reagent molecules is found to correlate with the heats of ions formations on the surface, and thus with the rates of catalytic reactions. Difference in the PA of acid sites and PA of bases (APA= pAb-pA a) determines the threshold for the existence of H-complexes and ion pairs on the surface. Suggested is empirical method for calculating the activation energies of reactions catalyzed by the Broensted regarding the proton affinity of acid sites, bases and reaction products. 1.INTRODUCTION Measurement of acid and base sites strength occupies a very important place in the acid-base heterogeneous catalysis. Historically, surface sites strength was measured with indicator method, characterizing the site strength with acidity function Ho [1]. However, later this method was shown to be incorrect, since adsorption processes altered the main principle of its application. According to this principle there must be an equilibrium between the acid sites and base probes [2]. In 1979 a new method for determining the strength of proton sites regarding proton affinity scale was designed at the Boreskov Institute of Catalysts [3]. In the present paper we review the 20 years experience in the application of proton affinity approach in heterogeneous acid-base catalysis. 2.EXPERIMENTAL. In acid-base catalysis proton affinity scale (PA) is determined by the enthalpies of the following gas phase reactions [4] taken with reversal signs (PA a for acids and PA b for bases) : for acids (AH) H § + A-= AH for bases (B) H § + B = BH §
3232 40 -
[-_
o
HY + CO
O t'Xl
30 r
O
"
11) O
'- 20 ..o !,.. O
<
10 l_ 8vOH=265 cm ~ -
I
3200
,
I
3400
I-
I
,
3600
Wavenumber, cm
"3800
-1
Fig.l.IR spectra of OH groups ofHY zeolites before (1) and after (2) CO adsorption. In recent years proton affinity scale, originated from the gas phase chemistry became r a t h e r popular for measuring the strength of Broensted sites on the surface of heterogeneous catalysts. The reason is that regarding thermodynamics PA is maximum correct, and has a clear physical essence. It is also easy to measure. Measurement methods use the shifts of stretching IR absorption bands from OH groups (AvOH) caused by the hydrogen bonds interaction with weak bases. Figure 1 shows how the IR spectra of zeolite OH groups, differing by strength, change during the low temperature adsorption of CO. Apparently, hydrogen bonds with two type OH groups. On type encompasses bridge OH groups in large cavities (band 3630 cml). Another type OH groups are characterized by band 3730 cm 1 and also correspond to the bridge OH groups, but located on the zeolite surface. For the first type groups the shift of band from OH groups giving complexes with CO is 350 cm 1. The second type groups are less acid, and their bands shift by 265 cm 1. Therefore, it is easy to see the difference between the strength of these groups regarding the spectra of hydrogen bonded OH ~oups. PA values are calculated with formula [3]: PAoH=PAsio H - Iog(AVoH8/ AVOHBsion)/0.00226
Silica is used as an standard with PA of SiOH groups 1390 kJ/mol, the shift of OH groups on CO (B=CO) adsorption (hvsioHc~ being equal to 90 cm 1, coefficient 0.00226 is used for recalculation of the OH groups shifts into PA values.
3233 Similar methods are designed for measuring the strength of basic sites. Here, CDC13 is used as a probe [5]. In the present paper we shall focus on the PA scale application for the acid sites only. 3. RESULTS AND DISCUSSION At present PA values are measured for many various solid acids (see Fig. 2).. For various type catalysts from heteropoly acids to basic oxides PA values range from 1120 to 1500 kJ/mol, reflecting the well known periodic regularities of properties of chemical elements. Li
Na
K
Be
B
C
N
1440
1440
1400
1320
Mg
AI
Si
P
S
1560
1440
1390
1300
1200
Ca 1740
Sc
V 1140
Ge
Ga
Zn
Cu
Ti 1380
Cr
Mn
Fe,Co,Ni
M(element )
As
1495(PA, kJ/mol for MOH groups)
1445
1550
Rb
Sr
Y 1470
Tc
Zr Nb
Mo
1245
1220
Ru, Rh, Pt
1495
Fig.2 Proton affinity oh MOH groups for different metals oxides.
160-
13,, 10
0
E 120 & 80
6
z 0 iIl m
0c !
0
40 0
1 -4()0
-3 o
' -3bo
BASE
PA
ACID
-PA
' -25o
, kJ/mol
_
Fig.3. Dependency of protonation heat (QH+-nQH-bo,a) of N-bases from value pAbase-pAa~ on zeolites HNaY (3,5-13),H-ZSM-5 (4), HNaX(1) and zirconium phosphate(2). 1,3,4-ammonia, 2,6-pyridine, 5-2Cl-pyridine, 7- n-butilamine, 8-tert-butilamine, 9-2,5- dimethylpyridine, 10- 3,5-dimethylpyridine, 11-2,3-dimethylpyridine, 12-trymethyamine, 13-2,4,6-trym ethylp yridine.
3234 PA scale allows one to predict the properties of surface proton sites[6]. For example, there is a correlation between PA and protonation heats for bases on OH groups different solid acids (Fig.3). The activity of Broensted acid sites (BAS) is a function of PA ~ in the double bond migration, alcohol dehydration and other reactions. We have designed [7] simple methods to analyze the mechanisms of catalytic reactions by measuring PAa of Broensted sites and PAb of reagents, allowing the revelation of possible limiting stages and estimation of expected activation energies. Catalytic transformations of bases on Broensted sites of catalysts proceed through hydrogen bonds and ion pairs formation. The designed method bases on the suggested empirical equations allowing to find the heats of hydrogen bond formation and base protonation using values PAb, PA ~. For APA (APA=pAb-pA~) value a demarcation line between the region of stable existence of H-complexes and ion pairs lies at 356 kJ/mol. For various groups of bases we have found a different correlation hydrogen bond formation heat between versus PAb. At the same PAb values the strongest H-bond forms for bases with N and O hetero-atoms. We observed [8] the following sequence of Hbond energy: N(O)-bases > olefins > benzenes>naphtalenes.(Fig.4). There are different rules for ion formation in the case of olefins and bases with N and O hetero-atoms. The heats of protonation of all bases on the catalyst surface are composed by three constituents Q1 + Q~ + Q3. Constituent Q1 is determined by the enthalpy of proton transfer from acid site to base, producing ion in the gas phase (Q1= pAb-pA"). Q~ reflects the energy of electrostatic attraction of ions into a pair. This constituent is approximately the same for various bases and acid sites of different strength. Constituent Q3 for oxygen and nitrogen bases is responsible for the energy of hydrogen bonds between a cation
-0,6 Protonated n-Bases H-bonded n-bases H-bonded olefins Protonated arenes H-bonded arenes H-bonded naffalenes Protonated olefins
-0,8 -1,0 ~,~ -1,2 O __1
-1,4 -1,6 -1,8 [ Ion pairs I 200
3()0
I H-bonded bases /
46o
560
660
760
800
pAAClD-pABASE,kJ/mol
Fig.4 Correlation between Log(F), F=AvoH/Voa and PAacid-pAbase for H-complexes of n-bases, olefins (ethene, propene and different butenes), arenes (benzene and its methyl derivatives) and naphtalenes with OH surface groups and ion pairs on surface and in solution.
3235
and acid residue on the surface. Q3 decreases, when the strength of acid site increases, and thus compensates the growth of Q1 in the same series. Somewhat different situation occurs with carbenium ions. Carbenium ions, forming during the protonation of olefins, use their free p-orbital to interact with the base oxygen of acid residue. This bond is donor-accepting, and thus is stronger than hydrogen bond that formed in the case N or O bases. Therefore, Q3 for carbenium ion provides a better compensation of Q1 than for bases with N an O heteroatoms. Basing on our experience and literature data related to the protonation of various bases, we have found that hydrogen bonded complexes and ion pairs
may coexist in equilibrium only if their geometry essentially differs[7].
I H-bond
ION pair
H /O\
/O-\
According to this rule surface alkyl carbenium ions, possessing less than 6 carbon atoms, may exist only as transition states. We suggest the following algorithm to calculate the energy profile of organic reactions involving Broensted catalytic sites. At first it is necessary to calculate H-complex formation heat for reagents and all possible products. Then we estimate theirs protonation heats. At the third stage we calculate the relative energies for transitions of various intermediates and reaction pathways. Energy diagram is plotted using the value of reaction enthalpy as reference. The algorithm allows the estimation of energy for individual stages of catalytic process. This is shown on fig.5. tert-C4Hg §
NH3 + C4H8 + OH
tert-C4HgNH 2 OH O
E
.-j
O3
'-
-50 ,,
-100
+ 8
C
uJ
"E calculated = E experimental
-150 tert-C4HgNH3*
Reaction way
Fig.5 Calculated energy diagram for tert-butilamine deammination on HNaY zeolite.
3236 12
150 O
9
E 125
.--j
5
11
10
-d 100
.Z oi
= 75
Ifwaterdesorbs into gas phase in the dehydration process
6 m m..~.....
0
9
0
m- 50
1 =, '
5'0
7 7'5
100
If water adsorbed on surface in the dehydration process
'
1:25 '
150
E experimental, k J/tool
Fig.6 Calculated energy activation (E) versus experimental one for the butene-1 isomerization(1-3), n-butanol (4-6), sec- butanol (7-9) dehydration, secbutylamine (11) and tert-butylamine(12) deamination Thus one finds the limiting stage without quantum chemistry calculations. Estimated and experimental activation energies coincide with an accuracy of 10 kJ/mol[7]. According to the Fig.6 here we meet some uncertainty related to the estimated heat of water adsorption in the dehydration process. If at the limiting stage water desorbs into the gas phase, then estimated activation energies are larger than experimentally measured ones. In case, when water stays for time on the surface with forming carbenium ions, estimated activation energy values are close to experimental ones. Therefore, we assume that molecular water plays a very important role in the processes of alcohols dehydration on the proton sites of catalysts.
REFERENCES 1. C. Walling, J.Amer.Chem.Soc. 72 (1950) 1164. 2. M.Deeba, W.K. Hall, J.Catalysis. 60 (1979) 417. 3. E.A. Paukshtis, E.N.Yurchenko, React.Kinet.Catall.Lett. V.16(1981) 131. 4. P. Kebarle, Ann.Rev. Phys.Chem. 25 (1977) 445. 5. Paukshtis E.A., Yurchenko E.N. Uspekhi Khimii, 52 (1983) 426. 6. Yu.D.Pankratiev, E.A.Paukshtis, V.M.Turkov, E.N.Yurchenko, Acta Phys. Et Chem. Szeged ,31 (1984) 55. 7. E.A. Paukshtis, IR spectroscopy for heterogeneous acid-base catalysis. Nauka, Novosibirsk. 1992 (in russian). 8. E.A.Paukshtis, L.V.Malysheva, V.G. Stepanov, React.Kinet.Catall.Lett., 65 (1998)145.
Proton affinity in heterogeneous M e a s u r e m e n t s a n d u s e for a n a l y s i s mechanism
3231
acid-base catalysis. of c a t a l y t i c r e a c t i o n
E.A.Paukshtis Boreskov Institute of Catalysis, Prospekt Akademika Lavrentieva, 5, 630090 Novosibirsk, Russia, fax: 7(3832)343766, e-mail: [email protected] In the present paper we review possible ways for measuring the strength of surface acid sites and for studying acid-base reactions using the proton affinity scale. PA measurements are done with the IR spectroscopy of hydrogenbonded complexes. The proton affinity of acid sites and reagent molecules is found to correlate with the heats of ions formations on the surface, and thus with the rates of catalytic reactions. Difference in the PA of acid sites and PA of bases (APA= pAb-pA a) determines the threshold for the existence of H-complexes and ion pairs on the surface. Suggested is empirical method for calculating the activation energies of reactions catalyzed by the Broensted regarding the proton affinity of acid sites, bases and reaction products. 1.INTRODUCTION Measurement of acid and base sites strength occupies a very important place in the acid-base heterogeneous catalysis. Historically, surface sites strength was measured with indicator method, characterizing the site strength with acidity function Ho [1]. However, later this method was shown to be incorrect, since adsorption processes altered the main principle of its application. According to this principle there must be an equilibrium between the acid sites and base probes [2]. In 1979 a new method for determining the strength of proton sites regarding proton affinity scale was designed at the Boreskov Institute of Catalysts [3]. In the present paper we review the 20 years experience in the application of proton affinity approach in heterogeneous acid-base catalysis. 2.EXPERIMENTAL. In acid-base catalysis proton affinity scale (PA) is determined by the enthalpies of the following gas phase reactions [4] taken with reversal signs (PA a for acids and PA b for bases) : for acids (AH) H § + A-= AH for bases (B) H § + B = BH §
3232 40 -
[-_
o
HY + CO
O t'Xl
30 r
O
"
11) O
'- 20 ..o !,.. O
<
10 l_ 8vOH=265 cm ~ -
I
3200
,
I
3400
I-
I
,
3600
Wavenumber, cm
"3800
-1
Fig.l.IR spectra of OH groups ofHY zeolites before (1) and after (2) CO adsorption. In recent years proton affinity scale, originated from the gas phase chemistry became r a t h e r popular for measuring the strength of Broensted sites on the surface of heterogeneous catalysts. The reason is that regarding thermodynamics PA is maximum correct, and has a clear physical essence. It is also easy to measure. Measurement methods use the shifts of stretching IR absorption bands from OH groups (AvOH) caused by the hydrogen bonds interaction with weak bases. Figure 1 shows how the IR spectra of zeolite OH groups, differing by strength, change during the low temperature adsorption of CO. Apparently, hydrogen bonds with two type OH groups. On type encompasses bridge OH groups in large cavities (band 3630 cml). Another type OH groups are characterized by band 3730 cm 1 and also correspond to the bridge OH groups, but located on the zeolite surface. For the first type groups the shift of band from OH groups giving complexes with CO is 350 cm 1. The second type groups are less acid, and their bands shift by 265 cm 1. Therefore, it is easy to see the difference between the strength of these groups regarding the spectra of hydrogen bonded OH ~oups. PA values are calculated with formula [3]: PAoH=PAsio H - Iog(AVoH8/ AVOHBsion)/0.00226
Silica is used as an standard with PA of SiOH groups 1390 kJ/mol, the shift of OH groups on CO (B=CO) adsorption (hvsioHc~ being equal to 90 cm 1, coefficient 0.00226 is used for recalculation of the OH groups shifts into PA values.
3233 Similar methods are designed for measuring the strength of basic sites. Here, CDC13 is used as a probe [5]. In the present paper we shall focus on the PA scale application for the acid sites only. 3. RESULTS AND DISCUSSION At present PA values are measured for many various solid acids (see Fig. 2).. For various type catalysts from heteropoly acids to basic oxides PA values range from 1120 to 1500 kJ/mol, reflecting the well known periodic regularities of properties of chemical elements. Li
Na
K
Be
B
C
N
1440
1440
1400
1320
Mg
AI
Si
P
S
1560
1440
1390
1300
1200
Ca 1740
Sc
V 1140
Ge
Ga
Zn
Cu
Ti 1380
Cr
Mn
Fe,Co,Ni
M(element )
As
1495(PA, kJ/mol for MOH groups)
1445
1550
Rb
Sr
Y 1470
Tc
Zr Nb
Mo
1245
1220
Ru, Rh, Pt
1495
Fig.2 Proton affinity oh MOH groups for different metals oxides.
160-
13,, 10
0
E 120 & 80
6
z 0 iIl m
0c !
0
40 0
1 -4()0
-3 o
' -3bo
BASE
PA
ACID
-PA
' -25o
, kJ/mol
_
Fig.3. Dependency of protonation heat (QH+-nQH-bo,a) of N-bases from value pAbase-pAa~ on zeolites HNaY (3,5-13),H-ZSM-5 (4), HNaX(1) and zirconium phosphate(2). 1,3,4-ammonia, 2,6-pyridine, 5-2Cl-pyridine, 7- n-butilamine, 8-tert-butilamine, 9-2,5- dimethylpyridine, 10- 3,5-dimethylpyridine, 11-2,3-dimethylpyridine, 12-trymethyamine, 13-2,4,6-trym ethylp yridine.
3234 PA scale allows one to predict the properties of surface proton sites[6]. For example, there is a correlation between PA and protonation heats for bases on OH groups different solid acids (Fig.3). The activity of Broensted acid sites (BAS) is a function of PA ~ in the double bond migration, alcohol dehydration and other reactions. We have designed [7] simple methods to analyze the mechanisms of catalytic reactions by measuring PAa of Broensted sites and PAb of reagents, allowing the revelation of possible limiting stages and estimation of expected activation energies. Catalytic transformations of bases on Broensted sites of catalysts proceed through hydrogen bonds and ion pairs formation. The designed method bases on the suggested empirical equations allowing to find the heats of hydrogen bond formation and base protonation using values PAb, PA ~. For APA (APA=pAb-pA~) value a demarcation line between the region of stable existence of H-complexes and ion pairs lies at 356 kJ/mol. For various groups of bases we have found a different correlation hydrogen bond formation heat between versus PAb. At the same PAb values the strongest H-bond forms for bases with N and O hetero-atoms. We observed [8] the following sequence of Hbond energy: N(O)-bases > olefins > benzenes>naphtalenes.(Fig.4). There are different rules for ion formation in the case of olefins and bases with N and O hetero-atoms. The heats of protonation of all bases on the catalyst surface are composed by three constituents Q1 + Q~ + Q3. Constituent Q1 is determined by the enthalpy of proton transfer from acid site to base, producing ion in the gas phase (Q1= pAb-pA"). Q~ reflects the energy of electrostatic attraction of ions into a pair. This constituent is approximately the same for various bases and acid sites of different strength. Constituent Q3 for oxygen and nitrogen bases is responsible for the energy of hydrogen bonds between a cation
-0,6 Protonated n-Bases H-bonded n-bases H-bonded olefins Protonated arenes H-bonded arenes H-bonded naffalenes Protonated olefins
-0,8 -1,0 ~,~ -1,2 O __1
-1,4 -1,6 -1,8 [ Ion pairs I 200
3()0
I H-bonded bases /
46o
560
660
760
800
pAAClD-pABASE,kJ/mol
Fig.4 Correlation between Log(F), F=AvoH/Voa and PAacid-pAbase for H-complexes of n-bases, olefins (ethene, propene and different butenes), arenes (benzene and its methyl derivatives) and naphtalenes with OH surface groups and ion pairs on surface and in solution.
3235
and acid residue on the surface. Q3 decreases, when the strength of acid site increases, and thus compensates the growth of Q1 in the same series. Somewhat different situation occurs with carbenium ions. Carbenium ions, forming during the protonation of olefins, use their free p-orbital to interact with the base oxygen of acid residue. This bond is donor-accepting, and thus is stronger than hydrogen bond that formed in the case N or O bases. Therefore, Q3 for carbenium ion provides a better compensation of Q1 than for bases with N an O heteroatoms. Basing on our experience and literature data related to the protonation of various bases, we have found that hydrogen bonded complexes and ion pairs
may coexist in equilibrium only if their geometry essentially differs[7].
I H-bond
ION pair
H /O\
/O-\
According to this rule surface alkyl carbenium ions, possessing less than 6 carbon atoms, may exist only as transition states. We suggest the following algorithm to calculate the energy profile of organic reactions involving Broensted catalytic sites. At first it is necessary to calculate H-complex formation heat for reagents and all possible products. Then we estimate theirs protonation heats. At the third stage we calculate the relative energies for transitions of various intermediates and reaction pathways. Energy diagram is plotted using the value of reaction enthalpy as reference. The algorithm allows the estimation of energy for individual stages of catalytic process. This is shown on fig.5. tert-C4Hg §
NH3 + C4H8 + OH
tert-C4HgNH 2 OH O
E
.-j
O3
'-
-50 ,,
-100
+ 8
C
uJ
"E calculated = E experimental
-150 tert-C4HgNH3*
Reaction way
Fig.5 Calculated energy diagram for tert-butilamine deammination on HNaY zeolite.
3236 12
150 O
9
E 125
.--j
5
11
10
-d 100
.Z oi
= 75
Ifwaterdesorbs into gas phase in the dehydration process
6 m m..~.....
0
9
0
m- 50
1 =, '
5'0
7 7'5
100
If water adsorbed on surface in the dehydration process
'
1:25 '
150
E experimental, k J/tool
Fig.6 Calculated energy activation (E) versus experimental one for the butene-1 isomerization(1-3), n-butanol (4-6), sec- butanol (7-9) dehydration, secbutylamine (11) and tert-butylamine(12) deamination Thus one finds the limiting stage without quantum chemistry calculations. Estimated and experimental activation energies coincide with an accuracy of 10 kJ/mol[7]. According to the Fig.6 here we meet some uncertainty related to the estimated heat of water adsorption in the dehydration process. If at the limiting stage water desorbs into the gas phase, then estimated activation energies are larger than experimentally measured ones. In case, when water stays for time on the surface with forming carbenium ions, estimated activation energy values are close to experimental ones. Therefore, we assume that molecular water plays a very important role in the processes of alcohols dehydration on the proton sites of catalysts.
REFERENCES 1. C. Walling, J.Amer.Chem.Soc. 72 (1950) 1164. 2. M.Deeba, W.K. Hall, J.Catalysis. 60 (1979) 417. 3. E.A. Paukshtis, E.N.Yurchenko, React.Kinet.Catall.Lett. V.16(1981) 131. 4. P. Kebarle, Ann.Rev. Phys.Chem. 25 (1977) 445. 5. Paukshtis E.A., Yurchenko E.N. Uspekhi Khimii, 52 (1983) 426. 6. Yu.D.Pankratiev, E.A.Paukshtis, V.M.Turkov, E.N.Yurchenko, Acta Phys. Et Chem. Szeged ,31 (1984) 55. 7. E.A. Paukshtis, IR spectroscopy for heterogeneous acid-base catalysis. Nauka, Novosibirsk. 1992 (in russian). 8. E.A.Paukshtis, L.V.Malysheva, V.G. Stepanov, React.Kinet.Catall.Lett., 65 (1998)145.