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Steric-electronic model of templating effect
H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.
405
S t e r i c - E l e c t r o n i c Model of T e m p l a t i n g Effect Zaiqun Liu and Ruren Xu* KeyLaboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun 130023, P.R.China The function of organic amines during synthesis of A1PO4-21, A1PO4-11 and A1PO4-5 is studied by means of linear free-energy-relationship (LFER) analysis, and it is found that electronic effect of amines on crystallization-transition-state converts from strong nucleophilicity to electrophilicity with an increase of major channel from 8-ring to 12-ring. Furthemore, we observed steric effect of amines retards the crystallization rate of A1PO4-21 and accelerates the crystallization rate of A1PO4-5. Therefore, steric-electronic model of ~mplating effect in c~ystallization-transition-state is proposed, based on these experimental results. 1. INTRODUCTION It is well known that one type of template can be used to crystallize various aluminophosphate molecular sieves (A1PO4-n) whereas the same type of A1PO4-n can be crystallized by using different templates. From literature, we observed that researchers have paid more attention to the relationship between framework of A1PO4-n as host and amines as guest, but they neglected the dynamical factor[I,2,3] such as steric-electronic effect of amines on crystallization-transition-state. One of the important methods of researching template effect of amines on the crystallization of aluminophosphate molecular sieves is to detect the kinetic parameters of the reaction between initial gel of aluminophosphate and amines, because we couldnot see the intermediate st1~cmres along the path[4,5] in the crystallization. Recently, physical organic chemists have used many tools to gather evidences of reaction mechanism, and one of the most useful ideas is the concept that a given structural feature will affect reactions as described by a linear free-energy-relationship (Taft equation) in the following: ln(k/ko)=pZa*+sZEs
(1)
where Z(~* and ZEs are numbers characteristic of inductive and steric properties of substiuent of amines, and k stands for the crystallization rate, ko stands for the rate of model reaction, p and s represent the electronic and steric properties of the reaction, respectively. The formation of A1PO4-n with a special designed structure could be reasonably regarded as a chemical reaction process, which hydroxyl groups attaching toA1 and P atoms dehydrate to bind characteristic bond AI-O-P catalyzed by organic amine. The reactivity of synthesis of A1PO4-n is
405 affected by factors such as the electronic-steric effect of functional groups of organic amines. Therefore, the method of linear free-energy-relationship analysis may be also useful to solve the problem of templating effect of amines. A1PO4-n (n=21,11,5) w i t h similar substructural units are chosen to investigate stericelectronic effect of amines on synthesis of A1PO4-n (n=21,11,5)[6]. 2. EXPERIMENTAL 2.1 S a m p l e preparation A typical synthetic procedure was in accordance with a certain batch composition of starting mixture of (1.0~2.0) Amine/1.0A12Os/1.0P2Os/54HeO. Pseudoboehmite is firstly mixed with part of amount of water, then H3PO4 diluted by the other amount of water was dropped into above mixture. The aluminophosphate gel was formed after stirring 2 hours, followed by adding various organic amines to regulate the pH value range from 5.5 to 6.5. Finally, crystallization of the reaction mixture was carried out in a stainless steel autoclave at 202~ (475K) under autogenous pressure. The samples w e r e occasionally extracted, filtered, washed with distilled water and dried at ambient temperature. 2.2 S a m p l e c h a r a c t e r i z a t i o n The crystalline products were identified by X-ray powder diffractometer (Rigaku D/MAX with CuK~ radiation) and the crystallinities were calculated from the peak area range from 4 ~ to 40 ~ The crystallization rate (k) was calculated by the ratio of crystaUinity with reaction time. The chemical environments of A1 and P atoms in the crystal structure of ALP04-21 synthesized by using (CH3)~NH, C2HsNH2 and n-CsH7NH2 as templates respectively were identified by 27A1- and 3:P-MASNMR. 3. RESULTS AND DISCUSSIONS 3.1 The linear free energy r e l a t i o n s h i p analysis of AIPO4-21 The crystallization rates (refered k) of ALP04-21[7,8] synthesized by using Me2NH, EtNH2 and n-PrNH2 as template agents are determined respectively. The methyl (-CH3) is regarded conventionally as the basic substituent defined as either inductive constant (a*) or steric constant (E0 as 0.0. The rate of ALP04-21 by using Me2NH as template agent is chosen to be a model reaction whose crystallization rate is refered as ko. The other crystallization rate (k) campare with ko to establish the function of relative crystallization rate (refered as ln(k/ko)). The relationship between crystallization rate and substituent constants are shown in Table 1. With the increase of Z(~* and ZE. from propylamine to dimethylamine, the crystallization rate (k) is accelerated distinctly. This phenomeno~ indicates electronic and steric effect of organic amines stabilizes the transition state of this crystallization process. The negative charge of crystallization-transition-state is dispersed to more atoms than that in initial state. The reaction with this transition state is called nucleophilic mechanism. Moreover, ]n(k/ko) is regressed
407 Table 1 Relationship between crystallization rate of A1PO4-n (n=21,11,5) and substiment constants of organic amines at 475K A1PO4-n Template Inductive Steric Crystallization Structure constant(Za*) constant rate (xl03s ~)
(~E~) A1PO4-21
A1PO4-11
ALP04-5
(CH3)2NH C2H5 NH2 n-C3HTNH2 (n- C3H7)2NH (n- C4H9)2NH (i-C3HT)~NH (C2Hs)sN (n-C3H7)3N (HO CH2CH2)sN
0.0 -0.10 -0.115 -0.23 -0.26 -0.38 -0.30 -0.345 0.595
0.0 -0.07 -0.36 -0.72 -0.78 -0.94 -0.21 - 1.08 - 1.08
97.22 37.04 25.83 9.042 9.417 10.33 16.22 27.30 11.95
linearly by the substituent constants (Za* and EEl) to set up quantitative relationship in accordance with equation (1) as follows. ln(k/ko)=9.1lEa*+ 0.77ZEs
(2)
The equation (2) characterizes quantitatively the influence of steric-electronic effect of amine as "attacking reagent" on the initial aluminophosphate gel as substrate. In this equation, the magnitude of p is so high as 9.11, which indicate the transition state of crystalliza~on is stabilized most efficiently by electronattracting group attaching to N atom of amines. Also, since the value of p is about two times of typical organic nucleophilic reaction, it can be deduced that a complex is formed by coordination of amines to chemical species with positive charge in the initial aluminophosphate gel. On the other hand, the coefficient of ZEs is positive, so the coordination is limited by the increase of space of substituent of amines. In 1985, J.M.Bernett et al[8] reported the crystal structure of A1PO4-21 which contains 4AlsP3012OH'l.33N2CTH2x in a monoclinic cell. The framework contains three type of tetrahedral P, two trigonalbipyramidal A1 and one tetrahedral A1. All these types of A1 atoms and P atoms could be determined by MAS NMR method, so it could be compared with the difference in chemical environment of A1PO4-21 synthesized by using various amines as template agents. The crystallization mechanism of ALP04-21 is nucleophilicity and steric hindrance of amine in the transition state, which the electronic effect of amine improve the interaction between the framework of A1PO4-21 and amine, the space of amine decreases this kind of interaction. This proposal could be demonstrated by the 27A1-MAS NMR and 31P-MAS NMR, as shown in Figure 1. Because the most strong nucleophilicity during synthesis of AIPO~-21 by using Me2NH as template cause the most week shielding effect to
408
A1v, which the chemical shift (5=38.244ppm) locate more low field than that of the other samples synthesized by EtNH~. and n-PrNH~ as templates. 3.2 The linear free energy r e l a t i o n s h i p of A1PO4-11 and A1PO4-5
The c17r rates(k)of A1PO4-1119] are also detelunined respectively while (n-Pr),.lqH, (i-Pr)2NH and (n-Bu)2NH are used as template agents. The results similar to A1PO4-21 are also presented in Table 1. The linear free-energyrelationship equation of A1PO4-11 is presented as equation (3). ln(k/ko)=0.40Zt~*-0.88ZEs-2.91
(3)
43 960 ---1 r--- 38.244
44.511
/ Il
-16.759--1
]
-18.512----1
[
. -24.262
il
r-"--28.900
0.844
I 42.133--n~
=-41-.699
__ a.3,,
.
~l (~ i~ J jJl ~l i'i~-I J-liilil~jjl
l-ii i i i~-I
b3
/~-~~-'23.414
t
A -19.323-
i a lli
~I"
....J
~ j /~
. . . . 23.056
A/i/l v
\
til-
. .
80
4O 5 (ppm)
0
0
.-30 5(ppm)
Figure 1 MAS NMR spectra of (a) 27A1 and (b) sip on A1PO4-21 synthesized by using (1) MezNH,(2) EtNH2, (~) n-PrNH2 as template agents.
409 Table 2. Relationship between crystallization rate of AIPO4-n (n=21,11,5) and substiment constants of amine at 448K and AH~;AS~ of A1PO4-n (n=21,11,5) A1PO4-n Template Za* ZE, k (xl0-Ss -1) AH~ ASs Structure (kJ.mo1-1) (J.mol-l.K-1) APO-21 (CHs)2NH 0.0 0.0 18.45 105.1 -47.0 C2HsNH2 -0.10 -0.07 10.15 81.0 -105.7 n-CsH7NH2 -0.115 -0.36 8.921 65.8 -104.6 APO-11 (n-CsH7)2NH -0.23 -0.72 2.233 87.8 -103.1 (n-C4Hg).oNH -0.26 -0.78 2.040 96.4 -84.6 (i-C3H7)2NH -0.38 -0.94 2.524 88.5 - 100.5 APO-5 (C2Hs)sN -0.30 ,0.21 9.808 29.1 -221.7 (n-C3H7)3N -0.345 -1.08 14.49 37.7 -199.4 (HOCH2CH2)3N 0.595 -1.08 1.893 116.9 -39.5 The magnitude of p decrease from 9.11 of A1PO4-21 to 0.40 of AIPO4-11. Along with the increase of electron-donating effect of subsituent attaching to N atom, the nucleophilicity of amine decrease rapidly, which result in the decrease of nucleophilicity. In contrast, since the coefficient of ZEs is -0.88, a negative value, the steric factor of amine play an important role to support the formation of channel of A1PO4-11. As a result, it may be deduced that the steric-electronic effect of A1PO4-5, a large size channel with the similar structural subunit to A1PO4-21 and A1PO4-11 would get steric acceleration and electrophilicity, that is, the value of the coefficients of Z~* and ZE, are negative. Then, the crystallization rates of A1PO4-5 synthesized by using Et3N, (n-Pr)sN and (HOCH2CH2)sN as template agents are determined respectively and the results are also presented in Table 1. The linear free-energy-relationship equation of A1PO4-5 is presented as
(4).
ln(k/ko)=-0.88Za*-0.55ZEs-2.17
(4)
The inference of A1PO4-5 is demonstrated correct by the results of experimental equation (4). So, the electronic effect of A1PO4-n (n=21,11,5) could be concluded that the nucleophilicity of amines get to electrophilicity with the increase of size of channel; the steric effect of amine retard the crystallization of A1PO4-21 but accelerates the crystallization of AlPO4-11 and A1PO4-5. 3.3 The linear free e n e r g y r e l a t i o n s h i p of AIPO4-n (n=21,11,5) at 175~ (448K) All the experimental results as mentioned above are repeated at 175~ (448K) to obtain the correspond linear free-energy-relationship equation to discuss the influence of steric-electronic factor on the crystallization process at relative low temperature. Table 2 presents the crystallization rates of A1PO4-n (n=21,11,5) at 448K and active-enthalpy, active-entropy of A1PO4-n (n=21,11,5). The free-energy is related with reaction temperature. To investigate the influence of steric-
410 electronic effect of organic amines on crystallization transition state, the freeenergy is divided into active-enthalpy and active-entropy as follows: l~=kT/h.eAS'~.e-Arr*~w
(5)
where T stands for reaction temperature, l~ represents the crystallization rate, k is Boltzmann constant, and AH',AS" represent active-enthalpy and active-entropy respectively. The crystallization of A1PO4-21 by using Me2NH as template agent at 448K, whose the crystallization rate is refered as ko*, is also chosen to be model reaction. The linear free-energy -relationship equations of A1PO4-21 (equation (6)), A1PO4-11 (equation (7)), AIPO4-5 (equation (8). In (k/ko*)=5.88Z(~*+0.14ZEs ln(k/ko*)=- 11.4Z(~*+7.18ZEs+0.45 ln(k/ko*)=-2.17Z(~*-0.34ZEs- 1.35
(6) (7) (8)
Comparison equation (2,3,4) with equation (6,7,8), the corresponding linear free-energy-relationship equations indicates that all the nucleophilicity decrease. For instance, the magnitude of coefficient of Z(~* of A1PO4-21 changed fi-om 9.11 to 5. 88, which indicate that they keep nucleophilicity property. The value of coefficient of E(~* decrease from -0.88 to -2.17 for A1PO4-5 suggesting that the crystallization mechanism of AlPO4-5 keep electrophilicity property. In contrast, the crystallization mechanism of AlPO4-11 converts from week nucleophilicity (p=0.40) to strong electrophilicity (p=-11.4). Relatively, with the decrease of crystallization temperature, in the process of synthesis of A1PO4-21 the steric hindrance of amine is decreased, and in the process of crystallizing AIPO4-5 the steric acceleration of amine decrease but still keep the steric facilitation effect; in the process of synthesis of A1PO4-11 the steric acceleration of amines convert to retardation. So, the conclusion could be obtained that steric effect of amines only determine the crystallization rate according to the inductive effect of amines control the structural style of AIPO4-n (n=21,11,5). 3.4 Relationship of linear active enthalpy-entropy equations of A1PO4-n. The quantitative relationship between active enthalpy and substituent constants is established as linear active-enthalpy-relationship equations and linear active-entropy-relationship equations as presented as in Table 3. The inductive effect of amines lead to the decrease of active-enthalpy for A1PO4-n (n=21,11,5) and the steric effect results in the increase of the activeenthalpy of A1PO4-11 and AIPO4-5 and decrease of A1PO4-21. On the other hand, the inductive effect of amines lead to the increase of the absolute value of active entropy of all these three kinds of A1PO4-n, and steric effect of amines results in the decrease of that of A1PO4-11 and A1PO4-5, in contrast, increase that of A1PO4-21. Considering influence of inductive effect of substituent on either active-entropy or active-enthalpy, the amines took part in the whole crystallization process by means of electron-attracting or electron-donating effect of substituent on the structure of gel with positive or negative
411 Table 3 The linear active-enthalpy-relationship equations A1PO4-n The linear active enthalpy relationship ln(AH~/H%)=2.18Z~*+0.60ZEs AIPO4-21 ln(AH~/AH%)=8.37Z~*-5174ZEs-2.39 ALP04-11 ln(AH~/AH%)= 1.21Z~*-0.36ZEs- 1.00 ALP04-5 ln(AS~/AS%)=-7.69Z~*-0.592Es ALP04-21 ln(AS~/AS%)=- 17.52~*+ 12.1ZE~+5.44 ALP04-11 ln(AS~/AS%)=- 1.72Z~*+0.21ZE~+ 1.08 ALP04-5
(8) (9) (10) (11) (12) (13)
charge. The steric effect of substituent of amine showed different properties to different A1PO4-n. 4. CONCLUSION We concluded that steric effect of amines determined the crystallization rates and the electronic effects of amines directed remarkbaly the style of A1PO4-n. Nucleophilicity is related to small channel of A1PO4-n such as ALP04-21, and electrophilicity had the relationship with large channel of A1PO4-n such as A1PO4-5. Week nuc!eophilicity is related to middle channel of A1PO4-n such as ALP04-11, electrophilicity of amines can also direct this kind of aluminophosphate molecular sieve at relative low crystallization temperature. Moreover, all factors that can change charge distribution of initial gel strongly influence channel type of A1PO4-n. The steric factor of template agent play an important role in the crystallization rate. And this inference is demonstrated by the result that Et3N, a electrophile in synthesizing A1PO4-5 at 475K, shows nucleophlicity in transition state for synthesizing ZnAPO-34 with a small size channel, and (i-Pr)2NH a weak nucleophile of crystallizing A1PO4-11 at the same temperature as above showed strong electrophilicity for synthesizing MgAPO-5. The results were explained by addition of Zn 2§ Mg2§ which extensively changed charge distribution of initial aluminophosphate gel. ACKNOWLEDGEMENT
This work was supported by the National Natural Fundation of China and Key Laboratory of Inorganic Hydrothermal Synthesis in Jilin University of China. REFERENCES
[1] M. E. Davis and R. F. Labo; Chem.Mater.4(1992)756 [2] S. Minton, V. Vaitchev and I. Kanev; Zeolites 13(1993)102 [3] E. Narita, J. Crystal Growth,78(1986) 1 [4] X. Ren, S. Komarneni, D. M. Roy, Zeolites, ll(1991)142 [5] J. Shorter, Correlation Analysis in Organic Chemistry,Oxford, 1973 [6] B. Parlitz, U. Lohse, E. Schreier, Microporous Materials,2(1994)223 [7] J.M.Bennett, W.J.Dytrych,J.W.Richardson Jr, J.V.Smith, Zeolites,6(1986)349 [8] J. M. Bennett, J. M. Cohen, G. Artioli, J. J. Pluth, Inorg.Chem.,24(1985)188 [9] J. W. Richardson Jr, J. J. Pluth, J. V. Smith, Acta Cryst. B44(1988)367
405
S t e r i c - E l e c t r o n i c Model of T e m p l a t i n g Effect Zaiqun Liu and Ruren Xu* KeyLaboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun 130023, P.R.China The function of organic amines during synthesis of A1PO4-21, A1PO4-11 and A1PO4-5 is studied by means of linear free-energy-relationship (LFER) analysis, and it is found that electronic effect of amines on crystallization-transition-state converts from strong nucleophilicity to electrophilicity with an increase of major channel from 8-ring to 12-ring. Furthemore, we observed steric effect of amines retards the crystallization rate of A1PO4-21 and accelerates the crystallization rate of A1PO4-5. Therefore, steric-electronic model of ~mplating effect in c~ystallization-transition-state is proposed, based on these experimental results. 1. INTRODUCTION It is well known that one type of template can be used to crystallize various aluminophosphate molecular sieves (A1PO4-n) whereas the same type of A1PO4-n can be crystallized by using different templates. From literature, we observed that researchers have paid more attention to the relationship between framework of A1PO4-n as host and amines as guest, but they neglected the dynamical factor[I,2,3] such as steric-electronic effect of amines on crystallization-transition-state. One of the important methods of researching template effect of amines on the crystallization of aluminophosphate molecular sieves is to detect the kinetic parameters of the reaction between initial gel of aluminophosphate and amines, because we couldnot see the intermediate st1~cmres along the path[4,5] in the crystallization. Recently, physical organic chemists have used many tools to gather evidences of reaction mechanism, and one of the most useful ideas is the concept that a given structural feature will affect reactions as described by a linear free-energy-relationship (Taft equation) in the following: ln(k/ko)=pZa*+sZEs
(1)
where Z(~* and ZEs are numbers characteristic of inductive and steric properties of substiuent of amines, and k stands for the crystallization rate, ko stands for the rate of model reaction, p and s represent the electronic and steric properties of the reaction, respectively. The formation of A1PO4-n with a special designed structure could be reasonably regarded as a chemical reaction process, which hydroxyl groups attaching toA1 and P atoms dehydrate to bind characteristic bond AI-O-P catalyzed by organic amine. The reactivity of synthesis of A1PO4-n is
405 affected by factors such as the electronic-steric effect of functional groups of organic amines. Therefore, the method of linear free-energy-relationship analysis may be also useful to solve the problem of templating effect of amines. A1PO4-n (n=21,11,5) w i t h similar substructural units are chosen to investigate stericelectronic effect of amines on synthesis of A1PO4-n (n=21,11,5)[6]. 2. EXPERIMENTAL 2.1 S a m p l e preparation A typical synthetic procedure was in accordance with a certain batch composition of starting mixture of (1.0~2.0) Amine/1.0A12Os/1.0P2Os/54HeO. Pseudoboehmite is firstly mixed with part of amount of water, then H3PO4 diluted by the other amount of water was dropped into above mixture. The aluminophosphate gel was formed after stirring 2 hours, followed by adding various organic amines to regulate the pH value range from 5.5 to 6.5. Finally, crystallization of the reaction mixture was carried out in a stainless steel autoclave at 202~ (475K) under autogenous pressure. The samples w e r e occasionally extracted, filtered, washed with distilled water and dried at ambient temperature. 2.2 S a m p l e c h a r a c t e r i z a t i o n The crystalline products were identified by X-ray powder diffractometer (Rigaku D/MAX with CuK~ radiation) and the crystallinities were calculated from the peak area range from 4 ~ to 40 ~ The crystallization rate (k) was calculated by the ratio of crystaUinity with reaction time. The chemical environments of A1 and P atoms in the crystal structure of ALP04-21 synthesized by using (CH3)~NH, C2HsNH2 and n-CsH7NH2 as templates respectively were identified by 27A1- and 3:P-MASNMR. 3. RESULTS AND DISCUSSIONS 3.1 The linear free energy r e l a t i o n s h i p analysis of AIPO4-21 The crystallization rates (refered k) of ALP04-21[7,8] synthesized by using Me2NH, EtNH2 and n-PrNH2 as template agents are determined respectively. The methyl (-CH3) is regarded conventionally as the basic substituent defined as either inductive constant (a*) or steric constant (E0 as 0.0. The rate of ALP04-21 by using Me2NH as template agent is chosen to be a model reaction whose crystallization rate is refered as ko. The other crystallization rate (k) campare with ko to establish the function of relative crystallization rate (refered as ln(k/ko)). The relationship between crystallization rate and substituent constants are shown in Table 1. With the increase of Z(~* and ZE. from propylamine to dimethylamine, the crystallization rate (k) is accelerated distinctly. This phenomeno~ indicates electronic and steric effect of organic amines stabilizes the transition state of this crystallization process. The negative charge of crystallization-transition-state is dispersed to more atoms than that in initial state. The reaction with this transition state is called nucleophilic mechanism. Moreover, ]n(k/ko) is regressed
407 Table 1 Relationship between crystallization rate of A1PO4-n (n=21,11,5) and substiment constants of organic amines at 475K A1PO4-n Template Inductive Steric Crystallization Structure constant(Za*) constant rate (xl03s ~)
(~E~) A1PO4-21
A1PO4-11
ALP04-5
(CH3)2NH C2H5 NH2 n-C3HTNH2 (n- C3H7)2NH (n- C4H9)2NH (i-C3HT)~NH (C2Hs)sN (n-C3H7)3N (HO CH2CH2)sN
0.0 -0.10 -0.115 -0.23 -0.26 -0.38 -0.30 -0.345 0.595
0.0 -0.07 -0.36 -0.72 -0.78 -0.94 -0.21 - 1.08 - 1.08
97.22 37.04 25.83 9.042 9.417 10.33 16.22 27.30 11.95
linearly by the substituent constants (Za* and EEl) to set up quantitative relationship in accordance with equation (1) as follows. ln(k/ko)=9.1lEa*+ 0.77ZEs
(2)
The equation (2) characterizes quantitatively the influence of steric-electronic effect of amine as "attacking reagent" on the initial aluminophosphate gel as substrate. In this equation, the magnitude of p is so high as 9.11, which indicate the transition state of crystalliza~on is stabilized most efficiently by electronattracting group attaching to N atom of amines. Also, since the value of p is about two times of typical organic nucleophilic reaction, it can be deduced that a complex is formed by coordination of amines to chemical species with positive charge in the initial aluminophosphate gel. On the other hand, the coefficient of ZEs is positive, so the coordination is limited by the increase of space of substituent of amines. In 1985, J.M.Bernett et al[8] reported the crystal structure of A1PO4-21 which contains 4AlsP3012OH'l.33N2CTH2x in a monoclinic cell. The framework contains three type of tetrahedral P, two trigonalbipyramidal A1 and one tetrahedral A1. All these types of A1 atoms and P atoms could be determined by MAS NMR method, so it could be compared with the difference in chemical environment of A1PO4-21 synthesized by using various amines as template agents. The crystallization mechanism of ALP04-21 is nucleophilicity and steric hindrance of amine in the transition state, which the electronic effect of amine improve the interaction between the framework of A1PO4-21 and amine, the space of amine decreases this kind of interaction. This proposal could be demonstrated by the 27A1-MAS NMR and 31P-MAS NMR, as shown in Figure 1. Because the most strong nucleophilicity during synthesis of AIPO~-21 by using Me2NH as template cause the most week shielding effect to
408
A1v, which the chemical shift (5=38.244ppm) locate more low field than that of the other samples synthesized by EtNH~. and n-PrNH~ as templates. 3.2 The linear free energy r e l a t i o n s h i p of A1PO4-11 and A1PO4-5
The c17r rates(k)of A1PO4-1119] are also detelunined respectively while (n-Pr),.lqH, (i-Pr)2NH and (n-Bu)2NH are used as template agents. The results similar to A1PO4-21 are also presented in Table 1. The linear free-energyrelationship equation of A1PO4-11 is presented as equation (3). ln(k/ko)=0.40Zt~*-0.88ZEs-2.91
(3)
43 960 ---1 r--- 38.244
44.511
/ Il
-16.759--1
]
-18.512----1
[
. -24.262
il
r-"--28.900
0.844
I 42.133--n~
=-41-.699
__ a.3,,
.
~l (~ i~ J jJl ~l i'i~-I J-liilil~jjl
l-ii i i i~-I
b3
/~-~~-'23.414
t
A -19.323-
i a lli
~I"
....J
~ j /~
. . . . 23.056
A/i/l v
\
til-
. .
80
4O 5 (ppm)
0
0
.-30 5(ppm)
Figure 1 MAS NMR spectra of (a) 27A1 and (b) sip on A1PO4-21 synthesized by using (1) MezNH,(2) EtNH2, (~) n-PrNH2 as template agents.
409 Table 2. Relationship between crystallization rate of AIPO4-n (n=21,11,5) and substiment constants of amine at 448K and AH~;AS~ of A1PO4-n (n=21,11,5) A1PO4-n Template Za* ZE, k (xl0-Ss -1) AH~ ASs Structure (kJ.mo1-1) (J.mol-l.K-1) APO-21 (CHs)2NH 0.0 0.0 18.45 105.1 -47.0 C2HsNH2 -0.10 -0.07 10.15 81.0 -105.7 n-CsH7NH2 -0.115 -0.36 8.921 65.8 -104.6 APO-11 (n-CsH7)2NH -0.23 -0.72 2.233 87.8 -103.1 (n-C4Hg).oNH -0.26 -0.78 2.040 96.4 -84.6 (i-C3H7)2NH -0.38 -0.94 2.524 88.5 - 100.5 APO-5 (C2Hs)sN -0.30 ,0.21 9.808 29.1 -221.7 (n-C3H7)3N -0.345 -1.08 14.49 37.7 -199.4 (HOCH2CH2)3N 0.595 -1.08 1.893 116.9 -39.5 The magnitude of p decrease from 9.11 of A1PO4-21 to 0.40 of AIPO4-11. Along with the increase of electron-donating effect of subsituent attaching to N atom, the nucleophilicity of amine decrease rapidly, which result in the decrease of nucleophilicity. In contrast, since the coefficient of ZEs is -0.88, a negative value, the steric factor of amine play an important role to support the formation of channel of A1PO4-11. As a result, it may be deduced that the steric-electronic effect of A1PO4-5, a large size channel with the similar structural subunit to A1PO4-21 and A1PO4-11 would get steric acceleration and electrophilicity, that is, the value of the coefficients of Z~* and ZE, are negative. Then, the crystallization rates of A1PO4-5 synthesized by using Et3N, (n-Pr)sN and (HOCH2CH2)sN as template agents are determined respectively and the results are also presented in Table 1. The linear free-energy-relationship equation of A1PO4-5 is presented as
(4).
ln(k/ko)=-0.88Za*-0.55ZEs-2.17
(4)
The inference of A1PO4-5 is demonstrated correct by the results of experimental equation (4). So, the electronic effect of A1PO4-n (n=21,11,5) could be concluded that the nucleophilicity of amines get to electrophilicity with the increase of size of channel; the steric effect of amine retard the crystallization of A1PO4-21 but accelerates the crystallization of AlPO4-11 and A1PO4-5. 3.3 The linear free e n e r g y r e l a t i o n s h i p of AIPO4-n (n=21,11,5) at 175~ (448K) All the experimental results as mentioned above are repeated at 175~ (448K) to obtain the correspond linear free-energy-relationship equation to discuss the influence of steric-electronic factor on the crystallization process at relative low temperature. Table 2 presents the crystallization rates of A1PO4-n (n=21,11,5) at 448K and active-enthalpy, active-entropy of A1PO4-n (n=21,11,5). The free-energy is related with reaction temperature. To investigate the influence of steric-
410 electronic effect of organic amines on crystallization transition state, the freeenergy is divided into active-enthalpy and active-entropy as follows: l~=kT/h.eAS'~.e-Arr*~w
(5)
where T stands for reaction temperature, l~ represents the crystallization rate, k is Boltzmann constant, and AH',AS" represent active-enthalpy and active-entropy respectively. The crystallization of A1PO4-21 by using Me2NH as template agent at 448K, whose the crystallization rate is refered as ko*, is also chosen to be model reaction. The linear free-energy -relationship equations of A1PO4-21 (equation (6)), A1PO4-11 (equation (7)), AIPO4-5 (equation (8). In (k/ko*)=5.88Z(~*+0.14ZEs ln(k/ko*)=- 11.4Z(~*+7.18ZEs+0.45 ln(k/ko*)=-2.17Z(~*-0.34ZEs- 1.35
(6) (7) (8)
Comparison equation (2,3,4) with equation (6,7,8), the corresponding linear free-energy-relationship equations indicates that all the nucleophilicity decrease. For instance, the magnitude of coefficient of Z(~* of A1PO4-21 changed fi-om 9.11 to 5. 88, which indicate that they keep nucleophilicity property. The value of coefficient of E(~* decrease from -0.88 to -2.17 for A1PO4-5 suggesting that the crystallization mechanism of AlPO4-5 keep electrophilicity property. In contrast, the crystallization mechanism of AlPO4-11 converts from week nucleophilicity (p=0.40) to strong electrophilicity (p=-11.4). Relatively, with the decrease of crystallization temperature, in the process of synthesis of A1PO4-21 the steric hindrance of amine is decreased, and in the process of crystallizing AIPO4-5 the steric acceleration of amine decrease but still keep the steric facilitation effect; in the process of synthesis of A1PO4-11 the steric acceleration of amines convert to retardation. So, the conclusion could be obtained that steric effect of amines only determine the crystallization rate according to the inductive effect of amines control the structural style of AIPO4-n (n=21,11,5). 3.4 Relationship of linear active enthalpy-entropy equations of A1PO4-n. The quantitative relationship between active enthalpy and substituent constants is established as linear active-enthalpy-relationship equations and linear active-entropy-relationship equations as presented as in Table 3. The inductive effect of amines lead to the decrease of active-enthalpy for A1PO4-n (n=21,11,5) and the steric effect results in the increase of the activeenthalpy of A1PO4-11 and AIPO4-5 and decrease of A1PO4-21. On the other hand, the inductive effect of amines lead to the increase of the absolute value of active entropy of all these three kinds of A1PO4-n, and steric effect of amines results in the decrease of that of A1PO4-11 and A1PO4-5, in contrast, increase that of A1PO4-21. Considering influence of inductive effect of substituent on either active-entropy or active-enthalpy, the amines took part in the whole crystallization process by means of electron-attracting or electron-donating effect of substituent on the structure of gel with positive or negative
411 Table 3 The linear active-enthalpy-relationship equations A1PO4-n The linear active enthalpy relationship ln(AH~/H%)=2.18Z~*+0.60ZEs AIPO4-21 ln(AH~/AH%)=8.37Z~*-5174ZEs-2.39 ALP04-11 ln(AH~/AH%)= 1.21Z~*-0.36ZEs- 1.00 ALP04-5 ln(AS~/AS%)=-7.69Z~*-0.592Es ALP04-21 ln(AS~/AS%)=- 17.52~*+ 12.1ZE~+5.44 ALP04-11 ln(AS~/AS%)=- 1.72Z~*+0.21ZE~+ 1.08 ALP04-5
(8) (9) (10) (11) (12) (13)
charge. The steric effect of substituent of amine showed different properties to different A1PO4-n. 4. CONCLUSION We concluded that steric effect of amines determined the crystallization rates and the electronic effects of amines directed remarkbaly the style of A1PO4-n. Nucleophilicity is related to small channel of A1PO4-n such as ALP04-21, and electrophilicity had the relationship with large channel of A1PO4-n such as A1PO4-5. Week nuc!eophilicity is related to middle channel of A1PO4-n such as ALP04-11, electrophilicity of amines can also direct this kind of aluminophosphate molecular sieve at relative low crystallization temperature. Moreover, all factors that can change charge distribution of initial gel strongly influence channel type of A1PO4-n. The steric factor of template agent play an important role in the crystallization rate. And this inference is demonstrated by the result that Et3N, a electrophile in synthesizing A1PO4-5 at 475K, shows nucleophlicity in transition state for synthesizing ZnAPO-34 with a small size channel, and (i-Pr)2NH a weak nucleophile of crystallizing A1PO4-11 at the same temperature as above showed strong electrophilicity for synthesizing MgAPO-5. The results were explained by addition of Zn 2§ Mg2§ which extensively changed charge distribution of initial aluminophosphate gel. ACKNOWLEDGEMENT
This work was supported by the National Natural Fundation of China and Key Laboratory of Inorganic Hydrothermal Synthesis in Jilin University of China. REFERENCES
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