Radical polymerization of acrylonitrile within the pores of zeolites Y and mordenite produces intrazeolite polyacrylonitrile that can be pyrolysed to form conducting carbon filaments

Bein, Thomas

Enzel, Patricia

Zoller, Joseph J.

English

Open Access LMU ( Ludwig-Maximilians-Univ. München)

J .  C H E M .  SOC. ,  C H E M .  C O M M U N . ,  1992 633 lntrazeolite Assembly and Pyrolysis of Polyacrylonitrile Patricia Enzel,t Joseph J.  Zoller and Thomas Bein*t Department of Chemistry, Purdue University, West 1 afa yette, IN 47907, USA Radical polymerization of acrylonitrile within the pores of zeolites Y and mordenite produces intrazeolite polyacrylonitrile that can be pyrolysed to form conducting carbon filaments. The understanding and control of electronic conductivity at nanometer dimensions is a subject of current research activity. The influence of microstructure on the conduction mechanism in conducting organic polymers is of particular importance in this context. We have recently demonstrated the encapsula- tion of charged conjugated polymers such as polypyrrole, polyaniline and polythiophene in the crystalline channel systems1 of zeolites.? In this communication, we report the first successful polymerization of acrylonitrile as a precursor Department of Chemistry, University of New Mexico, Albuquerque, is an Ope11 framework alUminosikate with 1.3 nm Size NM 87131. USA. supercages formed from interconnected sodalite cages, with Pre\*iorls acidresJ (at which part of this work was carried out): for intraZeOlite, conducting graphite-like filaments. Zeolite Y 634 J. CHEM. SOC., C H E M .  COMMUN.,  1992 0.75 nm pore openings. Mordenite (MOR) is characterized by a pseudo-one-dimensional twelve-ring channel system with similar window size, while silicalite offers intersecting zigzag and straight channels with pore openings of about 0.55 nm diameter. The polymerization of acrylonitrile in montmoril- lonite3 and of aminoacetonitrile in layered metal phosphates4 has been reported, but these systems contain sheets of macroscopic dimensions different from the nanometer chan- nels of the zeolite hosts. The degassed (670 K, 10-5 Torr) zeolite crystals were loaded with acrylonitrile vapour on a vacuum line for 60 min at 298 K. Acrylonitrile-containing zeolite (1 g) was then suspended in deaerated water (20 cm3) at 313 K. To that suspension were added aqueous solutions of potassium peroxodisulfate (0.01 mol dm-3) and sodium hydrogen sulfite (0.025 mol dm-3) as radical initiators,$ and the suspensions were stirred for 12 h. The zeolite frameworks were dissolved with a 25% aqueous solution of HF to recover the intrazeolite polyacrylonitrile (PAN). IR and NMR data show no damage to the polymers after this treatment. For pyrolysis, the zeolite-PAN adducts were heated under nitrogen or vacuum to 920 and 970 K with a heating rate of 2 K per minute, for 12 and 24 h. After vapour saturation, zeolite Y contains 46 molecules of acrylonitrile per unit cell, mordenite 6, and silicalite 9 molecules. However, even though the monomers adsorb into silicalite, no polymerization is observed within the latter host. The diameter of its channel system of approximately 5.5 A may be too small for the diffusion of radical initiator into the zeolite, or for polymerization to proceed. IR spectra of the zeolite-polymer inclusions and of PAN extracted from the zeolites show peaks characteristic of the bulk polymer.sJj Bands at 2940 and 2869 cm-1 are assigned to the methylenic C-H stretching vibrations of the backbone, while a band at 2240 cm-1 is assigned to the pendant nitrile group. The spectra of the extracted intrazeolite polymers are indistinguishable from the spectrum of the bulk polymer. It can be concluded that the polymer formed in the zeolites is polyacrylonitrile. IH and 13C NMR spectra of the recovered polyacrylonitrile extracted from zeolite Y,  and from Na-mordenite in (CD3)*S0 and DCON(CD3)2 are also identical to that of PAN bulk.' The 'H NMR spectra show two broad bands in a ratio of 2 : 1, one at 6 2.0 corresponding to the methylenic group in the polymer, and the other at 6 3.1 in agreement with a methine group. The 13C NMR data of the zeolite-extracted PAN are identical to those of the bulk material: 6 27.5 (CH2), 32.7 (CH) and 120.1 (-CN). The molecular weight of the polymer extracted from the zeolites was determined with gel permeation chromatography relative to a PAN broad standard (Mw = 86000; M ,  = 23 000). The complex molecular weight distribution curves are shown in Fig. 1. For PAN extracted from Nay ,  a peak molecular weight of ca. 19 000 is observed, corresponding to 360 monomer units or about 0.2 pm for an extended chain. An additional shoulder appears at lower molecular weight. In Na-mordenite, two groups of two peaks are observed; a smaller fraction with a peak molecular weight of ca 19 000 and a larger fraction with M ,  close to 1000, or about 0.01 pm long in a fully extended form. For both hosts, the distributions at the high molecular weight end are quite similar. This could originate from similar pore diameters of the hosts, but details of the polymerization mechanism are not known in these systems. Crystal defects or diffusional constraints might cause the shorter chain length fraction in the one-dimensional mordenite. $ For molar ratios of acrylonitrile : peroxodisulfate : hydrogen sulfite of 1 : 0.0027 : 0.0035. C I I I I I I I I  16 18 20 22 11 12 13 14 tR I min Fig. 1 Molecular weight distribution curves for PAN samples obtained from gel permeation chromatography: ( A )  PAN broad standard ( M ,  = 86 000, M ,  = 23 000); ( B )  PAN extracted from Nay-PAN; (C) PAN extracted from NaMOR-PAN On pyrolysis, the PAN-containing zeolites changed d o u r  from white to grey-black.§ The pyrolysed PAN extracted from NaY is insoluble in organic solvents; this suggests that the chain length is substantial even after pyrolysis. The methylenic stretching vibrations of the intrazeolite PAN main chain disappear, together with the characteristic nitrile band. New bands appear in the 1400-1600 cm-1 region which have been assigned to C=C and C=N double bonds in the pyrolysed bulk PAN,S as well as the tail of the electronic excitation corresponding to free carrier absorption. A sample of bulk PAN heated to 800 K instead of 920 K shows more defined features in the 1400-1600 cm-1 region, similar to the case of pyrolysed PAN (920 K) extracted from the zeolite, suggesting an early stage of graphitization. All the zeolite-PAN samples pyrolysed at different temperatures (920 or 970 K), times (12 or 24 hours) and atmospheres (nitrogen or vacuum) show spectra similar to the spectrum of bulk PAN pyrolysed to 800 K. This result suggests the ability of the zeolite channels to limit formation of an extended graphite- like structure. The electronic absorption spectrum of pyrolysed PAN extracted from NaY shows the appearance of a feature at about 350nm, as in the bulk, probably due to conjugated carbon-nitrogen double bonds in the ladder polymer .s The rest of the spectrum is practically structureless and resembles the absorption spectrum of graphite. Thermal analyses under nitrogen of the zeolite-poly- acrylonitrile inclusions show that the zeolite host drastically changes the pyrolysis reactions. Bulk PAN loses weight rapidly above 533 K, accompanied by a sharp exotherm at that temperature. In contrast, the zeolite inclusions show a much slower onset of decomposition with a broad exotherm (Nay-PAN: 518-588 K; NaMOR-PAN: 513-553 K). These observations show that the zeolite host imposes kinetic constraints on the graphitization reaction of intrazeolite PAN. After dissolution of the NaY host, 30% polymer was recovered based on the amount of monomer loaded into the zeolite. This yield corresponds to an average of 14 poly- merized acrylonitrile units per zeolite unit cell. The balance is (i X-Ray powder diffraction data demonstrate that the zeolite framework remains intact even after pyrolysis treatments at 970 K for N a y .  Scanning electron micrographs show no apparent external bulk polymer coating the zeolite crystals indicating that most of the polymer chains reside in the interior of the zeolite crystals. J .  C H E M .  S O C . ,  C H E M .  C O M M U N . ,  1992 635 assumed to be unreacted monomer. The =CH bands of the monomer disappear after polymerization; thus the unreacted monomer apparently desorbs from the host during the polymerization. For mordenite, the polymer recovery was 18%. This value corresponds to approximately one molecule of acrylonitrile per unit cell of mordenite. The NaY host containing pyrolysed PAN showed d.c. conductivityfl smaller than 10-8 S cm-1, in contrast to 1.2 S cm-1 with bulk pyrolysed PAN (920 K). The absence of measurable d.c. conductivity in the Nay-pyrolysed PAN samples is not surprising because the polymer is encapsulated completely within the insulating zeolite, and because no external polymer coats the zeolite crystal surfaces. However, the conductivity of the pyrolysed PAN extracted from the zeolite, 2 x 10-5 S cm-1, is almost identical with that of bulk PAN pyrolysed at 800 K, and five orders of magnitude smaller than that of the bulk sample pyrolysed at 920 K. In contrast to the bulk polymer, pyrolysis treatment conditions above about 870 K have little effect on the resulting conductivity of the extracted intrazeolite samples. The same conductivity value of about 2 x 10-5 S cm-1 is obtained for extracted pyrolysed PAN, pyrolysed at different temperatures, atmospheres and times. It can be concluded that the spatial limitations within the zeolite channels prevent the formation of more extended, graphitized structures with higher conductivity. In summary, this study demonstrates the inclusion synthesis of polyacrylonitrile in the channel systems of NaY and Na-mordenite zeolites, and its pyrolysis to yield a conducting f[ Four-point conductivity measurements of pressed pellets. material consisting of nanometer size carbon filaments. The effect of larger host pore sizes on polymerization and graphitization of acrylonitrile inclusions and their associated electronic conductivity are presently being explored. These systems are promising candidates for low-field conductivity at nanometer scale dimensions. The authors acknowledge partial funding for this work from Sprague Electric Company, and from the NSF-REU Program. We thank Bill Ackerman (UNM) for thermal analyses. Received, 10th December 1991; Com. 11062200 References 1 D. W. Breck, Zeolite Molecular Sieves, Krieger, Malabar, FL, 1984. 2 P. Enzel and T. Bein, J .  Phys. Chem.,  1989, 93, 6270; J .  Chem. SOC., Chem. Commun. ,  1989, 1326; T. Bein and P. Enzel, Angew. Chem., Int. Ed. Engl., 1989, 28, 1692; Mol. Cryst. Liq.  Cryst., 1990, 181, 315. 3 N. Sonobe, T. Kyotani, Y. Hishiyama. M. Shiraishi and A. Tomita, J .  Phys. Chem.,  1988, 92, 7029. 4 J .  E .  Pillion and M. E .  Thompson, Chem. Muter., 1991,3, 777. 5 M. M. Coleman and R. J. Petcavich, J .  Polym. Sci. Phys. Ed. ,  1978, 6 T.-C. Chung, Y. Schlesinger, S. Etemad, A. G. MacDiarmid and 7 G. Svegliado, G. Talamini and G. J. Vidotto, J .  Polym. Sci., Part 8 C. L. Renschler, A. P. Sylwester and L. V. Salgado, J .  Muter. Res., 16, 821. A. J. Heeger, J .  Polym. Sci. Phys. Ed. ,  1984, 22, 1239. A-I ,  1967, 5 ,  2875. 1989, 4, 452. 

Zeolite Molecular Sieves,

Intrazeolite assembly and pyrolysis of polyacrylonitrile

Patricia Enzel

Joseph J. Zoller

Thomas Bein

Crossref

Universität München: Elektronischen Publikationen

J .  C H E M .  SOC. ,  C H E M .  C O M M U N . ,  1992 633 
lntrazeolite Assembly and Pyrolysis of Polyacrylonitrile 
Patricia Enzel,t Joseph J.  Zoller and Thomas Bein*t 
Department of Chemistry, Purdue University, West 1 afa yette, IN 47907, USA 
Radical polymerization of acrylonitrile within the pores of zeolites Y and mordenite produces intrazeolite 
polyacrylonitrile that can be pyrolysed to form conducting carbon filaments. 
The understanding and control of electronic conductivity at 
nanometer dimensions is a subject of current research activity. 
The influence of microstructure on the conduction mechanism 
in conducting organic polymers is of particular importance in 
this context. We have recently demonstrated the encapsula- 
tion of charged conjugated polymers such as polypyrrole, 
polyaniline and polythiophene in the crystalline channel 
systems1 of zeolites.? In this communication, we report the 
first successful polymerization of acrylonitrile as a precursor 
Department of Chemistry, University of New Mexico, Albuquerque, is an Ope11 framework alUminosikate with 1.3 nm Size 
NM 87131. USA. supercages formed from interconnected sodalite cages, with 
Pre\*iorls acidresJ (at which part of this work was carried out): for intraZeOlite, conducting graphite-like filaments. Zeolite Y 
634 J. CHEM. SOC., C H E M .  COMMUN.,  1992 
0.75 nm pore openings. Mordenite (MOR) is characterized by 
a pseudo-one-dimensional twelve-ring channel system with 
similar window size, while silicalite offers intersecting zigzag 
and straight channels with pore openings of about 0.55 nm 
diameter. The polymerization of acrylonitrile in montmoril- 
lonite3 and of aminoacetonitrile in layered metal phosphates4 
has been reported, but these systems contain sheets of 
macroscopic dimensions different from the nanometer chan- 
nels of the zeolite hosts. 
The degassed (670 K, 10-5 Torr) zeolite crystals were 
loaded with acrylonitrile vapour on a vacuum line for 60 min 
at 298 K. Acrylonitrile-containing zeolite (1 g) was then 
suspended in deaerated water (20 cm3) at 313 K. To that 
suspension were added aqueous solutions of potassium 
peroxodisulfate (0.01 mol dm-3) and sodium hydrogen sulfite 
(0.025 mol dm-3) as radical initiators,$ and the suspensions 
were stirred for 12 h. The zeolite frameworks were dissolved 
with a 25% aqueous solution of HF to recover the intrazeolite 
polyacrylonitrile (PAN). IR and NMR data show no damage 
to the polymers after this treatment. For pyrolysis, the 
zeolite-PAN adducts were heated under nitrogen or vacuum 
to 920 and 970 K with a heating rate of 2 K per minute, for 12 
and 24 h. 
After vapour saturation, zeolite Y contains 46 molecules of 
acrylonitrile per unit cell, mordenite 6, and silicalite 9 
molecules. However, even though the monomers adsorb into 
silicalite, no polymerization is observed within the latter host. 
The diameter of its channel system of approximately 5.5 A 
may be too small for the diffusion of radical initiator into the 
zeolite, or for polymerization to proceed. 
IR spectra of the zeolite-polymer inclusions and of PAN 
extracted from the zeolites show peaks characteristic of the 
bulk polymer.sJj Bands at 2940 and 2869 cm-1 are assigned to 
the methylenic C-H stretching vibrations of the backbone, 
while a band at 2240 cm-1 is assigned to the pendant nitrile 
group. The spectra of the extracted intrazeolite polymers are 
indistinguishable from the spectrum of the bulk polymer. It 
can be concluded that the polymer formed in the zeolites is 
polyacrylonitrile. 
IH and 13C NMR spectra of the recovered polyacrylonitrile 
extracted from zeolite Y,  and from Na-mordenite in 
(CD3)*S0 and DCON(CD3)2 are also identical to that of PAN 
bulk.' The 'H NMR spectra show two broad bands in a ratio of 
2 : 1, one at 6 2.0 corresponding to the methylenic group in the 
polymer, and the other at 6 3.1 in agreement with a methine 
group. The 13C NMR data of the zeolite-extracted PAN are 
identical to those of the bulk material: 6 27.5 (CH2), 32.7 
(CH) and 120.1 (-CN). 
The molecular weight of the polymer extracted from the 
zeolites was determined with gel permeation chromatography 
relative to a PAN broad standard (Mw = 86000; M ,  = 
23 000). The complex molecular weight distribution curves are 
shown in Fig. 1. For PAN extracted from Nay ,  a peak 
molecular weight of ca. 19 000 is observed, corresponding to 
360 monomer units or about 0.2 pm for an extended chain. An 
additional shoulder appears at lower molecular weight. In 
Na-mordenite, two groups of two peaks are observed; a 
smaller fraction with a peak molecular weight of ca 19 000 and 
a larger fraction with M ,  close to 1000, or about 0.01 pm long 
in a fully extended form. For both hosts, the distributions at 
the high molecular weight end are quite similar. This could 
originate from similar pore diameters of the hosts, but details 
of the polymerization mechanism are not known in these 
systems. Crystal defects or diffusional constraints might cause 
the shorter chain length fraction in the one-dimensional 
mordenite. 
$ For molar ratios of acrylonitrile : peroxodisulfate : hydrogen sulfite 
of 1 : 0.0027 : 0.0035. 
C 
I I I I I I I I  
16 18 20 22 11 12 13 14 
tR I min 
Fig. 1 Molecular weight distribution curves for PAN samples obtained 
from gel permeation chromatography: ( A )  PAN broad standard 
( M ,  = 86 000, M ,  = 23 000); ( B )  PAN extracted from Nay-PAN; 
(C) PAN extracted from NaMOR-PAN 
On pyrolysis, the PAN-containing zeolites changed d o u r  
from white to grey-black.§ The pyrolysed PAN extracted 
from NaY is insoluble in organic solvents; this suggests that 
the chain length is substantial even after pyrolysis. The 
methylenic stretching vibrations of the intrazeolite PAN main 
chain disappear, together with the characteristic nitrile band. 
New bands appear in the 1400-1600 cm-1 region which have 
been assigned to C=C and C=N double bonds in the pyrolysed 
bulk PAN,S as well as the tail of the electronic excitation 
corresponding to free carrier absorption. 
A sample of bulk PAN heated to 800 K instead of 920 K 
shows more defined features in the 1400-1600 cm-1 region, 
similar to the case of pyrolysed PAN (920 K) extracted from 
the zeolite, suggesting an early stage of graphitization. All the 
zeolite-PAN samples pyrolysed at different temperatures (920 
or 970 K), times (12 or 24 hours) and atmospheres (nitrogen 
or vacuum) show spectra similar to the spectrum of bulk PAN 
pyrolysed to 800 K. This result suggests the ability of the 
zeolite channels to limit formation of an extended graphite- 
like structure. 
The electronic absorption spectrum of pyrolysed PAN 
extracted from NaY shows the appearance of a feature at 
about 350nm, as in the bulk, probably due to conjugated 
carbon-nitrogen double bonds in the ladder polymer .s The 
rest of the spectrum is practically structureless and resembles 
the absorption spectrum of graphite. 
Thermal analyses under nitrogen of the zeolite-poly- 
acrylonitrile inclusions show that the zeolite host drastically 
changes the pyrolysis reactions. Bulk PAN loses weight 
rapidly above 533 K, accompanied by a sharp exotherm at that 
temperature. In contrast, the zeolite inclusions show a much 
slower onset of decomposition with a broad exotherm 
(Nay-PAN: 518-588 K; NaMOR-PAN: 513-553 K). These 
observations show that the zeolite host imposes kinetic 
constraints on the graphitization reaction of intrazeolite PAN. 
After dissolution of the NaY host, 30% polymer was 
recovered based on the amount of monomer loaded into the 
zeolite. This yield corresponds to an average of 14 poly- 
merized acrylonitrile units per zeolite unit cell. The balance is 
(i X-Ray powder diffraction data demonstrate that the zeolite 
framework remains intact even after pyrolysis treatments at 970 K for 
N a y .  Scanning electron micrographs show no apparent external bulk 
polymer coating the zeolite crystals indicating that most of the 
polymer chains reside in the interior of the zeolite crystals. 
J .  C H E M .  S O C . ,  C H E M .  C O M M U N . ,  1992 635 
assumed to be unreacted monomer. The =CH bands of the 
monomer disappear after polymerization; thus the unreacted 
monomer apparently desorbs from the host during the 
polymerization. For mordenite, the polymer recovery was 
18%. This value corresponds to approximately one molecule 
of acrylonitrile per unit cell of mordenite. 
The NaY host containing pyrolysed PAN showed d.c. 
conductivityfl smaller than 10-8 S cm-1, in contrast to 
1.2 S cm-1 with bulk pyrolysed PAN (920 K). The absence of 
measurable d.c. conductivity in the Nay-pyrolysed PAN 
samples is not surprising because the polymer is encapsulated 
completely within the insulating zeolite, and because no 
external polymer coats the zeolite crystal surfaces. However, 
the conductivity of the pyrolysed PAN extracted from the 
zeolite, 2 x 10-5 S cm-1, is almost identical with that of bulk 
PAN pyrolysed at 800 K, and five orders of magnitude smaller 
than that of the bulk sample pyrolysed at 920 K. In contrast to 
the bulk polymer, pyrolysis treatment conditions above about 
870 K have little effect on the resulting conductivity of the 
extracted intrazeolite samples. The same conductivity value of 
about 2 x 10-5 S cm-1 is obtained for extracted pyrolysed 
PAN, pyrolysed at different temperatures, atmospheres and 
times. It can be concluded that the spatial limitations within 
the zeolite channels prevent the formation of more extended, 
graphitized structures with higher conductivity. 
In summary, this study demonstrates the inclusion synthesis 
of polyacrylonitrile in the channel systems of NaY and 
Na-mordenite zeolites, and its pyrolysis to yield a conducting 
f[ Four-point conductivity measurements of pressed pellets. 
material consisting of nanometer size carbon filaments. The 
effect of larger host pore sizes on polymerization and 
graphitization of acrylonitrile inclusions and their associated 
electronic conductivity are presently being explored. These 
systems are promising candidates for low-field conductivity at 
nanometer scale dimensions. 
The authors acknowledge partial funding for this work from 
Sprague Electric Company, and from the NSF-REU Program. 
We thank Bill Ackerman (UNM) for thermal analyses. 
Received, 10th December 1991; Com. 11062200 
References 
1 D. W. Breck, Zeolite Molecular Sieves, Krieger, Malabar, FL, 
1984. 
2 P. Enzel and T. Bein, J .  Phys. Chem.,  1989, 93, 6270; J .  Chem. 
SOC., Chem. Commun. ,  1989, 1326; T. Bein and P. Enzel, Angew. 
Chem., Int. Ed. Engl., 1989, 28, 1692; Mol. Cryst. Liq.  Cryst., 
1990, 181, 315. 
3 N. Sonobe, T. Kyotani, Y. Hishiyama. M. Shiraishi and A. Tomita, 
J .  Phys. Chem.,  1988, 92, 7029. 
4 J .  E .  Pillion and M. E .  Thompson, Chem. Muter., 1991,3, 777. 
5 M. M. Coleman and R. J. Petcavich, J .  Polym. Sci. Phys. Ed. ,  1978, 
6 T.-C. Chung, Y. Schlesinger, S. Etemad, A. G. MacDiarmid and 
7 G. Svegliado, G. Talamini and G. J. Vidotto, J .  Polym. Sci., Part 
8 C. L. Renschler, A. P. Sylwester and L. V. Salgado, J .  Muter. Res., 
16, 821. 
A. J. Heeger, J .  Polym. Sci. Phys. Ed. ,  1984, 22, 1239. 
A-I ,  1967, 5 ,  2875. 
1989, 4, 452. 


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Intrazeolite assembly and pyrolysis of polyacrylonitrile

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