MFI 구조를 갖는 ferrisilicate의 제조와

시클로헥산 산화반응에서 촉매로서의 반응성

염 영훈, 남 상성, 김 성보, 이 규완*

한국화학연구소 화학기술연구단 1 팀*

Synthesis, characterization, and catalytic activity of cyclohexane oxidation of MFI-type ferrisilicates

Young-Hoon Yeom, Sang-Sung Nam, Seong-Bo Kim, Kyu-Wan Lee*

Chemical Technology Division 1 Team, KRICT*

Introduction

Ferrisilicate can be directly synthesized and iron cation can be located in the tetrahedral framework sites of a molecular sieve structure.  However, the amount of iron incorporated is dependent on the conditions of synthesis.1  A major problem in iron incorporated materials is that the iron does not necessarily occupy exclusively framework sites.  It can also exist as extraframework cations balancing the negative charge of the framework or as an interstitial phase of small particles located either within the molecular sieve cavities or in between the crystallites.2,3

The degree of iron substitution is generally very low, and one usually ends up with a material including the iron species in several environments.  Therefore, overlapping spectra are obtained, which are difficult to unravel.  Some of techniques commonly used to characterize the iron sites are M□ssbauer,4 EPR,5 EXAFS6.

Inspite of the diverse methods of characterization, the assignment and the identification of Fe3+ containing molecular sieves are still controversial.

Adipic acid is an important industrial material which is used mainly to produce nylon 66, synthetic resins and lubricants, etc.  Almost all adipic acid is produced by a two-step cyclohexane oxidation process with air and nitric acid as oxidants.  The oxidation of cyclohexane, under mild conditions is an industrially important subject in recent years.  Various methods have been reported for cyclohexane-to-cycohexanol and cyclohexanone oxidation on zeolite catalysts.7  In this presentation, we describes the pH effect on the optimum snythetic conditions for the synthesis of MFI type ferrisilicates and their use in the catalysts for the cyclohexane oxidation.

Experimental

Four samples of MFI type ferrisilicates (Si/Fe = 300) molecular sieves were synthesized hydrothermally at autogeneous pressure.  In a typical preparation, 0.91 g of Fe(NO3)3·9H2O (98% Aldrich) was dissolved in a solution containing 1.0 g H2SO4 (96%) in 60 g H2O, and 42.5 g tetraethylorthosilicate (TEOS) (98% Aldrich) was added slowly to this fresh solution under vigorous stirring.  The resulting yellow solution was stirred at 298 K for 3 hrs.  Then, to the above resulting gel,  25.5 ml of tetrapropylammoniumhydroxide (1.0 M in methanol Ardrich) as a template. was added.  Finally, the required amount of NaOH dissolved in 100 g of H2O was added to the above mixture under vigorous stirring to adjust pH of the gel mixture.  The final pH used in this synthesis varied from 8 to 11. The resulting white gel was then stirred for 1 h before transferring it to a stainless-steel autoclave.  The crystallization was carried out at 443 K for 6 days.  The resultant material was then calcined in nitrogen flow at 773 K for 8 h followed by in air flow at 773 K for 4 hrs.  Four MFI type ferrisilicate samples prepared by varing pH of final gel mixture in the synthesis will be hereinafter denoted as FeMFI-1 (pH = 7.92), FeMFI-2 (pH = 8.92), FeMFI-3 (pH = 9.92), and FeMFI-4 (pH = 11.26), respectively.

The reactions were performed in a Teflon-lined 450 ml Parr Autoclave equipped with a mechanical stirrer.  The ratio of acetone (Mallinckrodt, 99.6%)/ cyclohexane (Merk, 99.5%)/ H2O2 (30% soln., Junsei) was 30/4/4 (v/v/v) giving a total of 38 ml; 0.20 g of catalyst.  The autoclave was closed and maintained at 343 K for 4 hrs.  After the reactions, the reacted samples were analyzed by a gas chromatograph (Shimadzu GC-8A).

Results and Discussion

We observed that the catalysts with the same Si/Fe ratio in the synthesis did not always display identical catalytic activity in the cyclohexane oxidation reaction.  This was due to structural differences in the crystalline solids resulting from the different synthesis conditions employed.  Previous workers did not consider a systematic study between the pH of gel mixture in the synthesis and the reactivity of the catalysts produced in the cyclohexane oxidation reaction.  The pH of the gel mixture during the synthesis affects the reaction kinetics, the yield and the size, morphology and chemical composition of the crystals.

In this work, the final pH of the gel mixture during the synthesis is varied from 7.92 to 11.26, respectively.  The initial ferrisilicate complex have been prepared under the acidic condition to avoid precipitation of rust-red iron hydroxide.  It is well known that iron tends to precipitate as a rust-red colloidal ferric hydroxide at pH > 4. Once formed, ferric hydroxide is almost completely insoluble, thereby limiting availability of FeO2- species for incorporation into the silicate units during crystal growth.  Once the ferrisilicate complex is formed, it might be avoided the formation of ferric hydroxides at elevated pH.  The pH is then adjusted with NaOH to the basic conditions necessary to promote molecular sieve crystallization.  After addition of NaOH solution to the acidic ferrisilicate complex, the white gel was produced.  After crystallization, all synthesized materials were white but after calcination, distinct color changes are observed with depending on pH.  FeMFI-1 and FeMFI-2 are white and FeMFI-3 and FeMFI-4 are rust-red respectively.  For the FeMFI-3 and FeMFI-4, it is due to a loss of crystallinity  causes from the removal of Fe from the zeolite framework and the consequence generation of structural defects during the calcination of samples.

The EPR spectra recorded at liquid nitrogen temperature were shown in Fig. 1.  The EPR spectra of FeMFI samples were examined as a function of pH of gel mixture in the synthesis, in order to establish whether the EPR signature of the Fe were the same at low and high pH.

The broad line at g = 2.0 and sharp line at g = 4.3 are the major.  Similar spectra were reported previously from other works.8,9  There are minor signals components at g = 2.1 and 2.2-2.3.  The EPR spectrum of zeolites containing Fe3+ usually consists of three major signals at g = 4.3, g = 2.2-2.3, and g =2.0.

Figure 1. Electron paramagnetic resonance spectra (liquid nitrogen temperature) of various MFI type ferrisilicates.

The commonly accepted assignment of these signals has been as follows: framework iron, iron in interstitial oxide or hydroxide phases, and iron in cation-exchange sites, respectively.  Howerver, the assignment of the EPR spectra of Fe3+ containing molecular sieves is still controversial.

Adopting Inui's interpretation8 to our samples, we consider that the individual signals are due to the following species: framework Fe3+ in tetrahedral site for g = 4.3, ion exchange site Fe3+ for g = 2.0, and precipitated Fe3+ species on the surface and the channel for g = 2.1, respectively.  In addition, we consider around g = 2.3 signal as the extraframework clustered ferric ions.  The relative intensity of the spectra at g = 4.3 increased with decreasing the pH of gel mixture (Fig. 1).  Additionally, the relative intensity of the g = 4.3 compared to the g = 2.0 is smaller in the high pH samples. Therefore two samples of FeMFI-3 and FeMFI-4 contains significant amounts of extraframework Fe3+.

FeMFI samples were used as catalysts for the oxidation of cyclohexane under mild conditions.  The results obtained in the oxidation of cyclohexane were shown in Table 1.  FeMFI-1 sample with the higher amount of tetrahedral site of Fe3+ shows higher conversion of cyclohexane oxidation.  The relative concentration of the framework Fe3+ in the FeMFI-1 and FeMFI-2 is  much higher than the remaining two samples thus the tetrahedral Fe3+ of the framework is responsible for the active species in the cyclohexane oxidation.   This interpretation is good agrement with the result of EPR signal.

Table 1. The pH Effect of the Gel Mixture in the Oxidation of Cyclohexane

Catalyst	pH of Gel Mixture	Cyclohexanol (mole%)	Cyclohexanone (mole%)	Total (mole%)	ol/one
FeMFI-1	7.92	2.811	3.098	5.909	0.907
FeMFI-2	8.92	2.604	2.829	5.431	0.921
FeMFI-3	9.92	2.121	2.433	4.554	0.872
FeMFI-4	11.26	1.706	2.065	3.771	0.826
Reaction conditions: Cyclohexane 4 ml, Acetone 30 ml, H2O2 4 ml, Catalyst 0.2 g, Rxn. Time 4 h, Rx. Temp. 70 oC

References

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