고체 핵자기 공명법을 통한 격자 알루미늄과 OTP 고리화 반응성 관계 연구

추대현*, 김해진*, 김재창**, 이재성*

*포항공과대학교 화학공학과

**경북대학교 화학공학과

Relationship between Framework Al and OTP Cyclization Activity

:27Al MAS NMR study

Dae Hyun Choo*, Hae Jin Kim*, Jae Chang Kim** and Jae Sung Lee*

*Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH)

**Department of Chemical Engineering, Kyungpook National University

Introduction

1,5-Dimethyltetralin (1,5-DMT) is useful intermediate to synthesize 2,6-naphthalenedicarboxylic acid (2,6-NDCA) and is raw material for polyethylene naphthalate(PEN). 1,5-DMT can be obtained from 5-(o-tolyl)-pent-1-ene, 5-(o-tolyl)-pent-2-nen and mixture thereof (hereafter, are referred to OTP) through cyclization over acidic zeolite catalysts. After cyclization, 1,5-DMT is converted into 2,6-NDCA undergoing dehydrogenation to produce 1,5-dimethylnaphthalene (1,5-DMN), isomerization to produce 2,6-DMN and oxidation in due order. (1) Zeolite BEA is known to have highest activity among conventional large pore size zeolites, but there is no reports concerened with the relationship between OTP cyclization activity and Al configuratuions.

From the solid state NMR study (5) of BEA, the environment of Al atoms is known to be related with two acid sites, Lewis sites due to octahedrally connected Al and Br□nsted sites to tetrahedrally coordinated framework Al. As the Al atoms lose their symmetry, that is, the number of extra-framework Al and/or unsaturated framework Al increases, some portions of Al sites cannot be observed with simple 27Al MAS NMR due to the existence of invisible Al. Thus, the interpretation of 27Al NMR spectrum is interrupted by these invisible Al, but qualitative analysis of framework Al and the remnant species still gives us valuable information about environment of Al atoms.

From 27Al MQMAS study, M□ller et al (2) assigned peak at 60 ppm as tetrahedrally coordinated framework Al, peak at 0 ppm as octahedrally coordinated extra-framework Al and peak seen between 30 ppm and 50 ppm as penta-coordinated Al and/or distorted tetrahedrally coordinated extra-framework Al. On the other hand, Fajula et al (3) and Bekkum et al (4) noted Oh-Al as partially connected framework through tetrahedral-octahedral Al interconversion.

In the present work, we investigate how framework Al concentration affects cyclization activity with dealumination by means of 27Al MAS NMR techniques.

Experimental

Commercial BEA samples were purchased from TOSOH Co., Japan (denoted as BEA1) and PQ Co., U.S. (BEA2) and were NH4+ form when purchased. All the samples were calcined to H-form at 773 K for 4 hours in the air (100cc/min) before reaction.

The dealumination of BEA was carried out with nitric acid (Matsunoeu Chemicals. 30ml/g cat.) at 353 K for 4 hours under reflux condition. To remove residual acid and impurities, samples were repeatedly washed and filtered. After dealumination, samples were dried for 24 hours in the oven at 383 K and calcined 4 hours at 773 K (air 100cc/min) before reaction.

Liquid phase cyclization was carried out in a sealed batch reactor under nitrogen equipped with a magnetic stirrer. Reactions were carried out at 473 K under nitrogen pressure with 1 cat./OTP g.

FE-SEM, AAS, N2 adsoprtion and 27Al MAS NMR experiments were conducted to characterize catalyst samples.

27Al MAS NMR spectra were obtained by using Varian Unity Inova 300 spectrometer (7.4 T), equipped with 7.5 mm Chemagnetics MAS probe. The magic angle spinning (MAS) was carried out at a rotor speed of 5.5kHz and Al(H2O)63+ was referenced to 0 ppm. The 27Al MAS NMR spectra were recorded with a pulse sequence of a single □/2 pulse (2.5 □s), 400-1000 accumulated scans and a recycle-delay time of 0.1 second. Dehydrated samples for 27Al MAS NMR were prepared with modified shallow-bed CAVERN apparatus. Sample was heated to 773 K (1 K/min) and evacuated at 773 K for 12 hours under 10-4 torr vacuum, and then transferred to 7.5mm zirconia MAS rotor after cooled to room temperature.

Results and Discussion

To investigate relationship between acid sites configuration in the pore system and OTP cyclization reactivity, we tested dealuminated samples to the reaction. (Fig. 1)

Figure 1. Conversion (a) and selectivity (b) of OTP cyclo-isomerization over dealuminated BEA1 and BEA2.

As dealumination process continued, BEA2 showed reduced reactivity over cyclization. BEA2-D-0.5 showed maximum 80% conversion over OTP cyclization compared with BEA2 and portion of side product was below 5% in the final composition. For severe dealumination treatment with 10N nitric acid (BEA2-D-10), no activity was observed over cyclization, whereas dealuminated BEA1 maintained activity over OTP cyclization.

Through dealumination treatment, we could observe dealuminated samples had reduced activity over OTP cyclization, but could not explain why BEA2 and BEA1 with same Si/Al ratio had different reactivity and even why BEA1-D-10 with relatively high Si/Al ratio had higher activity than BEA2.

Figure 2 and 3 show 27Al MAS NMR spectra of dealuminated BEA1 and BEA2 samples. Tetrahedrally coordinated framework Al (Th-Al) at 54 ppm and octahedrally coordinated Al (Oh-Al) at 0 ppm appeared for calcined and slightly dealuminated BEA1 samples (BEA1-D-0.5), but Oh-Al peak disappeared as dealumination were continued. Dehydrated BEA1 (Figure 2 (c)) exhibited only trace of Oh-Al site and it appeared that Oh-Al at 0 ppm might be partially framework connected Al. We could not discriminate two Th-Al sites due to two different crystallographic sites of BEA near 54 ppm but could see two Si(0Al) sites in 29Si MAS spectra.

Contrary to BEA1, 27Al MAS NMR spectra of highly dealuminated BEA2 have much broadened line width due to considerable amount of randomly distributed Al species. We pretreated all the samples with deep bed calcination and BEA2 showed intense Oh-Al peak and broad peak between 0 and 50 ppm due to Al species with low symmetry. However only two narrow peaks at 0 and 54 ppm observed for BEA1-D groups, which implies the difference in line-shapes between BEA1-D and BEA2-D groups is not effect of hydration/dehydration by water or calcination methods but of different Al environment.Exceptional narrow single peak at 54 ppm for BEA2-D-0.5 might be interpreted that a point in BEA2 exist sufficiently remove partially connected Al and/or extra-framework, but does not attack Th-Al sites. 29Si MAS spectra of BEA2-D-0.5 and BEA2-D-1 exhibited remarkable reduction in 3Si(1Al) sites, but bulk Si/Al ratio was not changed much (73.1 and 81.1 respectively.

From these results, not ruling out any possibility of invisible Al sites,  27Al MAS study revealed that highly dealuminated BEA had considerable amount of non-tetrahedral Al species and the activity of OTP cyclization clearly depended on the amount of Br□nsted acid sites in the inner pore system.

Reference

1. Sikkenga, D. L., Zaenger, I. C., and Williams, G. S., U.S. Patent 5 118 892 (1992).

2. M□ller, M., Harvey, G., Prins, R., Microporous and Mesoporous Mater. 34, 281 (2000).2.

3. De M□norval, L. C., Buckermann, W., Figueras, F., and Fajula, F., J. Phys. Chem. 100, 465 (1996)

4. Kunkeler, P. J., Zuurdeeg, B. J., van der Waal, J. C., van Bokhoven, J. A., Koningsberger D. C., and van Bekkum, H., J. Catal. 180, 234 (1998).

5. Jansen, J. C., Creyghton, E., J., Njo, S. L., Henk van Koningsveld, H., and van Bekkum, H., Catal. Today 38, 205 (1997).