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A Protons Exchanged Montmorillonite Clay as an Efficient Catalyst for the Reaction of Isobutylene Polymerization

Laboratoire de Chimie des Polymères, Département de Chimie, Faculté des Sciences, Université d’Oran Es-Senia BP N° 1524 El M’Naouar, 31000 Oran Algerie
*
Author to whom correspondence should be addressed.
Int. J. Mol. Sci. 2002, 3(7), 790-800; https://0-doi-org.brum.beds.ac.uk/10.3390/i3070790
Received: 27 April 2002 / Accepted: 13 May 2002 / Published: 31 July 2002

Abstract

“Maghnite” a montmorillonite sheet silicate clay, exchanged with protons to produce “H-Maghnite” is an efficient catalyst for cationic polymerization of many vinylic and heterocyclic monomers (Belbachir, M. U.S. Patent. 066969.0101 –2001). The structure compositions of both “Maghnite” and “H-Maghnite” have been developed. Isobutylene monomer, wich is polymerizable only by cationic process (Odian,G. La Polymerisation :principes et Applications; Ed.Technica: New York, 1994; pp 222-226), was used to elucidate the cationic character of polymerization. The polymerization was performed under suitable conditions at isobutylene vaporization temperature (–7°C). Experiments revealed that polymerization induced by “H-Maghnite” proceed in bulk and in solution. In contrast to findings with methylene chloride CH2Cl2 as a polar solvent, polymerization yields with hexane C6H14 non-polar solvent is very significant. In bulk polymerization, Isobutylene conversion increases with increasing “H-Maghnite” proportion.
Keywords: Maghnite; Montmorillonite; Catalyst; Isobutylene; Polyisobutylene Maghnite; Montmorillonite; Catalyst; Isobutylene; Polyisobutylene

Introduction

The use of acid treated clays as a solid source of protons in a number of industrial significant reactions continue because they constitute a widely available, inexpensive solid source of protons, e.g they were employed as cracking catalysts until the 1960s [1]. And are still currently used in industrial processes such as the alkylation of phenols [2] and the dimerization and polymerization of unsaturated hydrocarbons [3].
Montmorillonites have both brönsted and Lewis acid sites and when exchanged with cations having a high charges density, as protons, produce highly-active catalysts for acid-catalysed reactions [4]. Inercalated organic molecules are mobile and can be highly polarized when situated in the space between the charged clay layers.These exchanged montmorillonites have been successfully used as catalysts for the reactions of polymerization [3].
The present study is also concerned with polymerization and examines the catalytic activity of an Algerian proton exchanged montmorillonite clay called “Maghnite” via isobutylene polymerization to exhibit the cationic character of the reation [5]. The aim of this research is to extend the scope of other promising new field of polymer synthesis by the use of another catalyst system that has been shown to exhibit higher efficiency.
This paper concerns detailed analyses of “raw-Maghnite”, “H-Maghnite” and polymerization products, effect of catalyst ration and solvent polarity on Isobutylene conversion.

Experimental

A. Materials

1)
n-C6H14 was refluxed with fuming sulfuric acid, washed neutral with distilled water, dried over molecular sieves, refluxed and distilled from CaH2 under a nitrogen atmosphere on the day of the experiment.
2)
CH2Cl2 was dried over molecular sieves, distilled over CaH2, refluxed with triethylaluminium under nitrogen atmosphere overnight and freshly distilled on the day of the experiment.
3)
“H-Maghnite xM”: The acid forms of “raw-Maghnite” is prepared by shaking the material raw (raw-Maghnite) with solution of sulfuric acid until saturation achieved over two days at room temperature, washing the mineral with water until sulfate-free and drying. The concentrations 0.05M, 0.10M, 0.15M, 0.20M, 0.25M, 0.30M and 0.35M of sulfuric acid treatment solutions were used to prepare “H-Maghnite0.05M”, “H-Maghnite0.10M”, “H-Maghnite0.15M” , “H-Maghnite0.20M”, “H-Maghnite0.25M”,”H-Maghnite0.30M” and “H-Maghnite0.35M” respectively.
4)
Isobutylene (IB) was dried by passing the gas through a column packed with BaO, and condensing it under a nitrogen atmosphere.
5)
Methanol was used as received

B. “Maghnite” and “H-Maghnite” characterization:

1)
Samples for XRF analysis were prepared using the LiB4O7 fusion method. The resulting beads were analyzed on a Philips PW 2400XRF spectrometer in Laboratory of Inorganic Chemistry, Granada University, Spain
2)
XRD profiles for pressed powder samples were recorded on a Philips PW 1710 diffractometer using Cu-Kα radiation (λ=1.5418A°)
3)
IR absorption spectra were recorded on a ATI Matson FTIR N°9501165 spectrometer using the KBr pressed-disc technique, 0.5mg of sample was added to 300mg KBr and mixed for 3min in a vibratory grinder prior to pressing a 13mm disc.
4)
High-resolution solid-state 29Si and 27Al MAS NMR spectra of untreated (raw-Maghnite) and acid treated (H-Maghnite0.25M) samples were recorded on a Brüker ASX 500 spectrometer at 59.6 and 130.3 MHz respectively. The sample spinning frequency was 4 KHz for 29Si and 11.5 KHz for 27Al.

C. Procedure and Polymer characterization

Polymerizations were carried out in stirred flasks at –7 °C. The catalyst was dried in a muffle at 120°C over night and then transferred to a vacuum desiccator containing P2O5. After cooling to room temperature under vacum, the mineral was added to the Isobutylene (10g) or IB (10g)-solvent (90ml) mixture preliminary keeped in stirred flask at Isobutylene boiling point temperature (-7°C). At the required time, an aliquot of the reaction mixture was then taken in such manner as to exclude any clay mineral and slowly added to methanol with stirring. The precipitated polymer was filtered off and dried at 35-40°C under vacuum and weighed. The polymers were redissolved in CH2Cl2 and precipitated into methanol for characterization and molecular weight measurement.
Molecular weights were determined by a Waters high pressure GPC instrument (Model 6000A Pump) having a serie of ultra-Styragel columns (100, 500, 103, 104, 105 A°), a Differential Refractometer 2401 and a UV absorbance Detector Model 440. The flow rate of tetrahydrofuran (THF) was 1 ml/min. The calibration curve was made with well-fractionated polyisobutylene standards.
1H NMR spectra were recorded on an AM 300 FT Bruker instrument using CDCl3 as solvent, and tetramethylsilane (TMS) as internal standard.

Results and Discussion

Catalyst Structure

Various methods of analysis, such as 27Al and 29Si MAS NMR, show that “Maghnit” is a montmorillonite sheet silicate clay. The elementary analysis of the selected samples obtained using XRF and monomer conversions obtained from the reaction of 1g of each sample with 10g of Isobutylene during 3 hours in a bulk polymerization at –7°C, are as settled in the following Table 1.
It is necessary to report that the best value of isobutylene conversion was obtained with “H-Maghnite0.25M”, for this reason we kept this sample to study the effect of catalyst proportions and solvent on isobutylene polymerization. Acid treatment of ‘’Raw-Maghnit’’ cause reduction in octahedral content (Al2O3) resulted in an increase in the proportion of silica (SiO2) observed.
The X-Ray powder diffraction profiles (Figure 1 and Table 2) exhibited the presence of other crystalline phases as quartz, feldspath and calcite in “raw-Maghnit” By treatment acid, all trace of calcite was removed in “H-Maghnite”.
Table 1. Elementary compositions of Protons exchanged samples “H-Maghnite”
Table 1. Elementary compositions of Protons exchanged samples “H-Maghnite”
sampleCompositions wt%IB conversion %
SiO2Al2O3Fe2O3CaOMgONa2OK2OTiO2SO3AsPF*
Raw-Maghnite69.3914.671.160.301.070.500.790.160.910.051100
H-Mag0.05M70.7514.671.050.301.010.490.780.160.750.04102
H-Mag0.10M71.0014.601.000.300.980.390.780.160.550.0410.23.21
H-Mag0.15M71.5814.450.950.290.910.350.770.150.420.0310.110
H-Mag0.20M71.6514.200.800.280.850.300.770.150.390.0110.613.7
H-Mag0.25M71.7014.030.710.280.800.210.770.150.340.011165
H-Mag0.30M73.2013.850.700.270.780.200.760.130.310.029.7830
H-Mag0.35M75.3113.520.710.260.780.180.750.130.320.018.0327
*PF : Pert in Fire
The increase in basal spacing from 12.5 Å in “raw-Maghnite’’, characteristic of one water layer between the sheets, to value of 15.02 Å in ‘’H-Maghnite’’, where there are two interlamellar water layers, subsquent to acid treatment reflects the change in interlayer cation and its associated hydration state [6].
Figure 1. X-ray Power Diffraction of (a) “Raw-Maghnite” and (b) “H-Maghnite0.25M”
Figure 1. X-ray Power Diffraction of (a) “Raw-Maghnite” and (b) “H-Maghnite0.25M”
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Table 2. RX characteristic of Raw-Maghnite and H-Maghnite0.25M.
Table 2. RX characteristic of Raw-Maghnite and H-Maghnite0.25M.
samplesdhkl (Å)hklNature of sample
Raw-Maghnite12.50001Montmorillonite
4.47110Montmorillonite
4.16,,Quartz
3.35,,Quartz
3.21,,Feldspath
3.03,,Calcite
2.55200Montmorillonite
1.68009Montmorillonite
1.49060Montmorillonite
H-Maghnite0.25M15.02001Montmorillonite
4.47110Montmorillonite
4.16,,Quartz
3.35,,Quartz
3.21,,Feldspath
3.03,,Calcite
2.55200Montmorillonite
1.68009Montmorillonite
1.49060Montmorillonite
The IR Spectra of Raw-Maghnit and H-Maghnite0.25M (Fig. 2) are in good agrement with literature [7]. The characteristic vibrations of hydroxy groups, the silicate anion and the octahedral cations were observed in the IR spectra of both montmorillonites. The most intense band near 1040 Cm-1 is attributed to the Si-O stretching vibrtations of the tetrahedral layer and the bands near 522 and 466 Cm-1 are assigned to Si-O bending vibrations of smectites. A low intensity band near 800 Cm-1, assigned to four-coordinated silica [8], indicates the presence of amorphous silica. Compareson of Raw-Maghnite and H-Maghnite0.25M spectra show a small increasing in the SiO2 band intensity near 800 Cm-1 in H-Maghnite0.25M, reflects alterations in the amount of amorphous silica [9].
The 27Al MAS NMR spectra of both Raw–Maghnite and H-Maghnite-0.25M (Fig. 3) show Aluminium in two tetrahedral environments with resonances centered at 60 and 68 ppm, in addition to the main contribution from the octahedral aluminium at 2.9ppm. These values and assignments concur with those published in literature [6,10,11].
The 29Si MAS NMR spectra for the Rw-Maghnite and H-Maghnite0.25M are shown in Fig 4. The dominent resonance at – 93,5 ppm corresponds to Q3(OAl) units,i.e SiO4 groups crosslinked in the tetrahedral sheets with no aluminium in the neighbouring tetrahedral [12]. The resonance at –112 ppm corresponds to three-dimensial (3D) silica with no aluminium present, designed Q4 (OAl) [6,13].
Figure 2. IR Spectra of (a) untreated Clay “Raw-Maghnite” and (b) Acid treated Clay “H-Maghnite0.25M”
Figure 2. IR Spectra of (a) untreated Clay “Raw-Maghnite” and (b) Acid treated Clay “H-Maghnite0.25M”
Ijms 03 00790 g002
Figure 3. 27Al MAS NMR spectra of (a) “Raw-Maghnite” and (b) “H-Maghnite0.25M”
Figure 3. 27Al MAS NMR spectra of (a) “Raw-Maghnite” and (b) “H-Maghnite0.25M”
Ijms 03 00790 g003
Figure 4. 29Si MAS NMR spectra of (a) “Raw-Maghnite” and (b) “H-Maghnite0.25M”
Figure 4. 29Si MAS NMR spectra of (a) “Raw-Maghnite” and (b) “H-Maghnite0.25M”
Ijms 03 00790 g004

Polymerization and Products characterization

The results of experiments of Isobutylene polymerization induced by “H-Maghnite0.25M” are reported in Table 3. For all these experiments the temperature was kept constant at –7°C for 3 hours.
Table 3. Polymerization of IB induced by the “H-Maghnite0.25M”
Table 3. Polymerization of IB induced by the “H-Maghnite0.25M”
Experiment SolventIB g[IB] mol/l“H-Maghnite0.25M”(g)Yield %MnMwMw/Mn
1Bulk10-0.54772832694.49
2Bulk10-1.06571330204.24
3Bulk10-1.57252024104.63
4CH2Cl21021.037327038391.17
5C6H141021.050369042911.16

Effect of “H-Maghnite0.25M” proportion

We can see from Table 3 that, the conversion of monomer increases with increasing “H-Maghnite0.25M” proportion (experiments 1,2,3). This lead was followed up by a serie of deeper experiments in which the conversion of Isobutylene with the time has been observed at various amounts of catalyst. Table 4 and Figure 5 show that increasing the “H-Maghnite0.25M” amounts yielded higher Isobutylene conversions. This phenomena is probably the result of number of “initiating active sites ” responsible of inducing polymerization, this number is prorata to the catalyst amount used in reaction.
Table 4. Isobutylene conversions with time : for 10g of Isobutylene, the amounts of H-Maghnite0.25M were: a)0.5g b)1g c) 1.5g
Table 4. Isobutylene conversions with time : for 10g of Isobutylene, the amounts of H-Maghnite0.25M were: a)0.5g b)1g c) 1.5g
Time (minutes)0306090120150180240300
Yield%a01525353843475456
b03040475560657583
c03548556368728081
Figure 5. Effect of catalyst proportion upon the conversion of Isobutylene
Figure 5. Effect of catalyst proportion upon the conversion of Isobutylene
Ijms 03 00790 g005

Effect of solvent:

Data in Table 3 show that polymerization carried out in solution leaded to higher molecular weights and narrower MWDs (Mw/Mn) . However conversions in solution were smaller than the ones obtained in bulk polymerization. Low conversions in solution polymerization may be explained by the difficult contact in heterogeneous phase, between monomer particles and the “initiating active sites” of “catalyst” surface.
Conversion of monomer obtained in non-polar solvent (C6H14) was more important than the ones obtained in polar solvent (CH2Cl2), this result is due to “catalyst” structure. As all minerals clay, it shows an affinity over polar particles, in this case, adsorbed molecules of polar solvent (CH2Cl2) occupied “initiating active sites”, and led to low Isobutylene conversions.

Characterization of products

An investigation was devoted to the analysis of the polymer from experiment 2 in Table 3 by 1H NMR spectroscopy at 300MHz (Table 5 and Fig. 6).
Table 5. Results of product analysis by 1H NMR spectroscopy Ijms 03 00790 i001
Table 5. Results of product analysis by 1H NMR spectroscopy Ijms 03 00790 i001
Proton type(a)(b)(c)(d)(e)(f)
δ in ppm0.991.111.414.644.845.14
Figure 6. 1H NMR spectra of polyisobutylene product of experiment 3 in Table 3.
Figure 6. 1H NMR spectra of polyisobutylene product of experiment 3 in Table 3.
Ijms 03 00790 g006
Chains of Polyisobutylene present two types of unsaturation. Spontaneous termination and transfer involving proton abstraction lead to the expected exo/endo terminal double bonds (Structures A, B). The integration of 1H NMR peaks of protons carried by terminal double bonds is necessary to determinate proportions of these structures (A, B).
Ijms 03 00790 i002
According to the work published by Francis and Archer [14], and Stanly et al. [15], 1H NMR spectroscopy at 300 MHz (Solvent CDCl3) (Fig. 6) showed different peaks, the methyl groups of the main chain at 1.11ppm, the methylene groups at 1.41ppm, the terminal tert-butyl groups at 0.99ppm. Besides these well-known resonances, analysis shows at 5.14ppm the characteristic resonance of the proton borne by the double bond of the terminal trisubstituted unsaturation PIB-CH=C(CH3)2, at 4.64ppm the proton at a cis position of the methyl group belonging to the function PIB-C(CH3)=CH2 and at 4.84ppm the multiplet of the proton at a trans position of the same function.

Mechanism of the reaction

Isobutylene is an monomer witch is polymerizable only by cationic way. According to the foregoing discussion and the results of product analysis, we may suggest a cationic mechanism for the resulting reaction of polymerization induced by “H-Maghnite0.25M”.
Protons carried by montmorillonite sheets of “H-Maghnite0.25M” induced the cationic polymerization, these montmorillonite sheets take place as counter-anions. Propagation and termination then take place by conventional cationic mechanism. Termination by proton transfer to monomer and/or to initiator produced unsaturations onto polyisobutylene chains.
Initiation:
Ijms 03 00790 i003
Propagation:
Ijms 03 00790 i004
Termination:
  • Transfer to monomer:
    Ijms 03 00790 i005
  • Transfer to initiator:
    Ijms 03 00790 i006

Conclusion

The present work shows that Isobutylene polymerization can be induced in heterogeneous phase by a proton exchanged montmorillonite clay called “H-Maghnite”. Polyisobutylenes were produced accordingly by an easy-to-handle procedure in one batch process. This new chemistry for cationic polymerization will be explored deeply in forthcoming work.

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