Supramolecular Architecture - American Chemical Society

Chapter 12

Thermal Analysis of Porphyrin—Clay Complexes 1

Downloaded by STANFORD UNIV GREEN LIBR on August 6, 2012 | http://pubs.acs.org Publication Date: July 14, 1992 | doi: 10.1021/bk-1992-0499.ch012

K. A. Carrado, Κ. Β. Anderson , and P. S. Grutkoski Chemistry Division 200, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439

Aluminosilicate smectite clays have been ion-exchanged with water– soluble, cationic porphyrins and metalloporphyrins. Characteristics of their thermal stability were measured by thermal gravimetric analysis in an inert atmosphere, which yielded approximately 60% weight loss of organics. Detailed structural information about the decomposition products was obtained by performing pyrolysis-gas chromatography-mass spectrometry on the clay-organic complexes.

The ability of clay structures to provide supramolecular organization in terms of catalysis, chiral reactions, colloid science, electron transfer and pillaring has been well recognized (1). The supramolecular architecture of clays is therefore of critical importance, and structural design can be finely tuned to suit specific applications. One approach is to modify natural clays (2) while another tech nique of potential is the preparation of tailor-made synthetic clays. Although synthetic zeolites can be routinely made in the presence of organic templating molecules (3), few references currently exist wherein the use of organics is employed during the synthesis of clays (4-6). Recently the magnesium silicate hectorite clay system was modified to incorporate water-soluble cationic porphyrins and metalloporphyrins by direct hydrothermal crystallization (6). The thermal characteristics of synthetic organo-clay complexes are of interest for several reasons. First, since clay synthesis takes place at elevated temperatures (100-300°C), the stability of each constituent must be assured. The degree to which the thermal characteristics of a template are affected by a support is also of importance. In addition, the porphyrin-clay complexes are of interest as advanced materials in such areas as electrochemistry and catalysis (7), and as highly organized molecular assemblies (#). Since thermal stability 1

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156

SUPRAMOLECULAR ARCHITECTURE

may be an important factor in these applications, thermal gravimetric analysis ( T G A ) was applied to characterize in detail the stability of the organic portion in porphyrin ion-exchanged montmorillonite clays. Further information about the degradation of porphyrin-clay complexes was obtained using the technique of pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS).


Experimental Synthesis. The cationic water-soluble porphyrins teirafe(A^methyl-4-pyridyl) porphyrin (TMPyP), ie/rato(A^A/,Ar-trimethyl-4-amlinium) porphyrin (TAP), and the metalloporphyrin Fe(III)TAP were purchased as chloride salts from Midcentury Chemicals, Posen, IL. Ion-exchanged clays were prepared by stirring 1-2 wt% clay in lxlO' M aqueous solutions of porphyrin overnight with subsequent isolation, washing and air-drying of products. Final porphyrinexchanged clays contain about 10% organic by weight (see Table I). The natural montmorillonite used for these experiments, Bentolite L , is a C a bentonite treated to remove excess iron, and was obtained from Southern Clay Products, Gonzales, T X . Detailed characterization of this clay has been published elsewhere (9). A l l starting materials were used without further purification. 3

2+

TMPyPCl

TAPC1

Characterization. X-ray powder diffraction ( X R D ) was done on a Scintag P A D - V instrument using C u K radiation. Scans were collected at either 0.5° or 1.0° 2 θ / π ύ η . Oriented films were made by air-drying clay slurries from water on glass slides. Data was collected on a D G Desktop computer system. e



12. CARRADO ET AL.

Thermal Analysis of Porphyrin-Clay Complexes 157

Thermal gravimetric analysis was performed on a Cahn 121 electrobalance from 25-800°C at a rate of 10°C/min with data collection every two seconds, under a nitrogen atmosphere; Cahn software allowed processing of the data to yield first-derivative thermograms. Pyrolysis-GC-MS analyses were carried out on an HP-5890 gas chromatograph coupled to an HP-5970 M S D (mass-selective detector), using a C.D.S. "pyroprobe" coil-type pryrolyzer made from Pt wire. Samples were held in a specially-designed quartz boat, and the pyrolysis occured with a temperature rise time of 20 seconds. A 60m DB-1701 chromatographic column allowed excellent separation of components; the oven was ramped from 40-280°C at a rate of 8°C/min under a helium atmosphere.

Results and Discussion Table I contains microanalysis and X-ray diffraction data for the porphyrinexchanged complexes. The weight percentage of organics is typical for that expected considering the cation exchange capacity of this clay (80 meq/100gm). Comparison of the theoretically expected C / N ratios to the actual experimental values is excellent, considering experimental error, and the porphyrins are assumed to be incorporated fully intact. The results of UV-visible absorption spectroscopy which have been published elsewhere (7) confirm this assumption. The basal spacing or d ^ value, which is the c-dimension of the unit cell, includes the clay layer (-9.6 Â for montmorillonites) and the height of the clay gallery. A complete description of the d ^ values given in Table I, which are primarily based on orientation of the porphyrin macrocycles within the clay gallery, has already been provided (7).

Table I. Characterization of Porphyrin-Clays

Weight % %N %C

Sample

TMPyP-bentonite TAP-bentonite FeTAP-bentonite

7.5 9.3 8.5

1.5 1.6 1.4

C / N Ratio Theor. Expt.

5.5 7.0 7.0

XRD dooi>Â

5.8 6.8 7.1

14.5 15.7 18.0

Thermal Analysis. Clays, porphyrins, and porphyrin-clay complexes were analyzed by thermal gravimetric analysis under an inert nitrogen atmosphere. One previous study of the thermal stability of porphyrins reports that weight loss is greater and less complex in oxygen compared to nitrogen (10). Weiss



158


and Roloff (11) observed that hemin was stable on the surface of montmorillo nite at 300°C with air exclusion, with an implication that this was an enhanced thermal stability caused by the presence of clay. Results obtained under the more reactive, combustive oxygen environment will be reported later (72). First, the characteristics of both the clay and porphyrin constituents were observed separately. Then, ρ ο φ η ν τ π ^ loaded onto natural clays (e.g. bentonite) by ion-exchange were measured and compared to the separate comp onents. The data will be used as a foundation for characterization of the synthetic porphyrin-hectorite systems in later studies (12). Weight loss and first-derivative curves from thermal gravimetric analysis in nitrogen of pure ρ ο φ ί ι ν π η chloride salts and ion-exchanged ροφ1ινπη-€ΐαν8 were collected. Figure l a displays the T G curve for pure bentonite, which consists of three major transitions: a loss of surface water from 25-107°C (12.8%), interlayer water is lost from 110-167°C (1.8%), and dehydroxylation occurs from 610760°C (2.5%). Figures lb-c, 2a-b and 2c-d correspond to the TMPyP, T A P and Fe(III)TAP systems, respectively. The occurrence of two peaks in the 25-100°C temperature range is probably not real, but an artifact of the furnace heating at low temperatures, and is taken together as Transition A . Data is also tabulated in Table II. The T G curve of T M P y P C l in Figure l b has three main transitions indicated, with transition A being due to loss of waters of hydration. Except for determining the temperatures of maximum weight loss in the remaining trans itions, in conjunction with the information in Table II, no other information can be gleaned. In other words, one cannot obtain with certainty any structural information about what is being lost upon decomposition from the technique of T G A alone. Consequently, as in Figure l c for T M P y P loaded onto bent onite, the peak at 475°C cannot be assigned to either of the two main trans itions shown from T M P y P C l in Figure l b without further information. For both the anilinium-poφhyrins TAPC1 and F e T A P C l , there is a major transition at 145°C and 215°C, respectively, which does not appear to occur when ion-exchanged into the corresponding clays. The total weight loss observed for the pure ρ ο φ ί ι ν π η chloride salts in this inert atmosphere is only about 60%. Note also that all the ρ ο φ Ι ι ν Γ ΐ η - ΰ ^ samples show a weight loss at roughly 650°C, which occurs about 60°C less than the pure clay dehydrox ylation peak. Py-GC-MS. The technique of pyrolysis-gas chromatography-mass spectrometry was used to definitively assign the weight loss peaks in T G A by identifying the decomposition products under an inert atmosphere. A t least two different pyrolysis temperatures were used for each material in Table II (except for bentonite and TAP-bentonite). In this fashion, isolation and identification of the products evolved during at least two major T G A transitions was possible. Thus, T M P y P C l was pyrolyzed at 420°C and 600°C to determine transitions Β and C (by subtraction), respectively; transition A is known to be due to water. The temperatures of pyrolysis for all samples are summarized in Table III, along with product assignments. The terms "pyridines" and "anilines" in



Figure 1. T G A curves of (a) bentonite, (b) T M P y P C l and (c) ionexchanged TMPyP-bentonite. Weight curve is dashed, derivative curve is solid; transitions A , Β and C are indicated by vertical dashed lines.


160


(a)

0.05

; C;

ι

AA

V

fi - -0.05

500°


\jl43° -0.1

\

(b)

\

A

!

B ;

C

-0.02 -0 --0.02 --0.04 S 0.05

. A"

C:

(c) 0

Φ Ο

-0.05 -0.1 215° -0.15 0.01

h-0.01

200

eoo 400 Temperature ( C)

-0.02

Figure 2. T G A curves of (a) TAPC1, (b) ion-exchanged TAP-bentonite, (c) Fe(III)TAPCl and (d) ion-exchanged Fe(III)TAP-bentonite. Weight curves are dashed, derivative curves are solid; transitions A , Β and C are indicated by vertical dashed lines.


12. CARRADO ET AL.


Table III refer to the many derivatives and isomers observed, e.g. methylsubstituted pyridines or bipyridines. A n example of these is indicated in Figure 3 for T M P y P C l pyrolyzed at 600°C. For this particular sample, two small peaks of m/e 249 are observed but are as yet not assigned, although they may be due to isomers of tripyridines. A small amount of pyrrole is also seen at this temperature (but not at 420°C, and therefore cannot be attributed to impurity), indicating a slight degree of decomposition of the porphyrin core.


Table II. T G A Data of Porphyrin Systems in Nitrogen*

Major Transitions Temp Range (°C) and W t % Loss Trans A Trans Β Trans C

Sample

Total Trans

Bentonite

25-107 12.8%

110-167 1.8%

610-760 2.5%

25-800 18.8%

TMPyPCl

20-162 14.4%

211-392 19.7%

505-584 17.4%

20-800 60.2%

TMPyP-bentonite

19-148 3.3%

420-567 2.1%

570-800 4.8%

19-800 11.6%

TAPC1

20-104 13.8%

105-198 19.9%

468-520 5.3%

20-800 58.2%

TAP-bentonite

17-174 8.5%

558-680 3.1%

680-801 2.9%

17-801 16.8%

FeTAPCl

20-151 18.5%

151-236 15.5%

399-522 7.1%

20-810 55.7%

FeTAP-bentonite

20-154 6.0%

381-484 1.6%

584-710 2.2%

20-801 13.1%

*data represent an average of several runs.

Transition Β in all of the pure porphyrins has been determined by PyG C - M S to be due to release of chloromethane (see Table III). After ionexchange of the porphyrin chloride salts into the clays, chloride ion is displaced into the aqueous solution; clay exchange sites now balance the charge on the porphyrin ring. As a result, the porphyrin-exchanged clays can no longer show


162


chloromethane as a possible decomposition product. This information, which could be verified only by a supporting technique such as Py-GC-MS, greatly aids in the assignment of the remaining T G A transitions.

Table III. Py-GC-MS Data for Porphyrin Systems

Pyrolysis Temp (°C)


Sample

Products Observed

TMPyPCl

420 600

chloromethane chloromethane, pyridines

TMPyP-bentonite

540

pyridines

TAPC1

420 600

chloromethane chloromethane, anilines, some pyrrole

—

T G A inconclusive

Fe(III)TAPCl

300 600

chloromethane chloromethane, anilines, some pyrrole

FeTAP-bentonite

540

anilines, some pyrrole

TAP-bentonite

The % H 0 of each porphyrin as determined by T G A is listed in Table IV, and was used to calculate their molecular weights and formulas. From this data the expected %CH C1 was determined and then compared with the actual wt% loss seen by T G A . The results, also in Table IV, are in very good agreement. The same analysis can be applied to the pyridinium and anilinium substituents. For example, T M P y P C l loses 17.4 wt% from 505-584°C, which Py-GC-MS shows to be due primarily to a variety of pyridine analogs (see Figure 3). Since the calculated wt% of pyridine in T M P y P C l is roughly 32%, it is apparent that after partial decomposition and release of pyridine, the porphyrin rearranges in some unknown fashion because only about half of the pyridine is lost. Nearly 40% of the sample is not volatile under this nitrogen atmosphere at temperatures of 800°C. For the porphyrins with anilinium substituents, even less wt% loss is seen in Transition C. TAPC1 and F e T A P C l lose just 5.3% and 7.1%, respectively, in the region that Py-GC-MS reveals is primarily anilines (with some pyrroles). The reason for this difference between pyridinium and anilinium substituents is not quite clear. In addition, since porphyrins are loaded onto clays at only 2

3



12. CARRADO ET AU

Thermal Analysis of Porphyrin—Clay Complexes163

100

a 80

60 H

e

40

Ζ

20

Ί

10

20

1

30

"Ί

40

I 50

60

Time imin) Figure 3. G C trace of Py-GC-MS analysis of T M P y P C l pyrolyzed at 600°C: a = chloromethane, b = pyridine, c = pyrrole, d = methyl-pyridine, e = bipyridine isomers, and f = m/e 249 isomers (possible tripyridines).


164


about 10 wt%, this small weight loss is even further reduced. As a result, peaks due to porphyrin decomposition for TAP-bentonite could not be discerned by thermal gravimetric analysis.


Table IV. Calculation of Decomposition Products from Pure Porphyrins

MW (g/mol)

%CH C1 Expt." Calc." 3

Porphyrin

%H O

TMPyPCl

14.4

CJiJijO,-

8H 0

964

19.7

21.0

TAPC1

13.8

C H«N C1 - 9 H 0

1150

19.9

17.6

FeTAPCl

15.7

FeQAoHA-llHp

1275

15.5

15.8

2

a

Formula

56

8

2

4

2

'determined by T G A ; Calculated from formulas.

Conclusions The thermal stability of porphyrin-clay systems has been examined in detail. Results from T G A and Py-GC-MS indicate that the porphyrin nucleus is extremely stable in the presence of clay minerals, especially in an inert atmosphere like nitrogen. Substituents on the nucleus such as pyridinium or anilinium are, on the other hand, slightly destabilized, which can be attributed to the acidic nature of the clay surface. The use of pyrolysis-gas chromatography-mass spectrometry in conjunction with thermal gravimetric analysis greatly clarifies the assignment of weight loss peaks. Analysis of the resulting solids after thermolysis is currently underway.

Acknowledgments Microanalyses were carried out by Mr. S. Newnam of the Analytical Chemistry Division of A N L . Helpful discussions with Dr. R . Hayatsu of A N L about M S data analysis are greatly appreciated. M r . J. S. Gregar is acknowledged for fabrication of the quartz samples boats used for Py-GC-MS. The technical assistance of Mr. A . G . Stellpflug and his financial support from the Division of Educational Programs at A N L are also recognized. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract number W-31109-ENG-38.


12.

CARRADO ET AL.


Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12.

Fripiat, J. J. Clays Clay Miner. 1986, 34, 501. Pinnavaia, T. J. Science 1983, 220, 365. Szostak, R. Molecular Sieves: Principles of Synthesis and Identification Van Nostrand Reinhold Catalysis Series; Van Nostrand Reinhold: NY, 1989. Barrer, R. M.; Dicks, L. W. R. J. Chem. Soc. (A) 1967, 1523. Barrer, R. M.; Denny, P. J. J. Chem. Soc. 1961, 971. Carrado, Κ. Α.; Thiyagarajan, P.; Winans, R. E.; Botto, R. E. Inorg. Chem. 1991, 30, 794. Carrado, Κ. Α.; Winans, R. E.; Chem. Mater. 1990, 2, 328. Giannelis, E. P. Chem. Mater. 1990, 2, 627. Carrado, Κ. Α.; Kostapapas, Α.; Suib, S. L.; Coughlin, R. W. Solid State Ionics 1986, 22, 117. Said, Ε. Z.; Al-Sammerrai, D.J.Analy. Appl. Pyrolysis 1985, 9, 35. Weiss, Α.; Roloff, G. Z. Naturforsch. 1964, 19b, 533. Carrado, Κ. Α.; Winans, R. E.; Grutkoski, P. S.; Melnicoff, P. to be published.

RECEIVED January 16, 1992


Supramolecular Architecture - American Chemical Society

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