METHOD OF REGENERATING ZEOLITE CATALYST FOR AROMATIZATION OF ACETYLENE BY PLASMA TREATMENT

Abstract

The present invention relates to a method of preparing an aromatic compound from acetylene, which includes synthesizing an aromatic compound from an acetylene-containing reactant gas in the presence of a zeolite catalyst for the aromatization of acetylene, and subjecting the zeolite catalyst deactivated by the coke formed in the aromatization of acetylene, to plasma treatment at ambient temperature and pressure so as to selectively remove the external cokes and partial internal coke, thereby regenerating the zeolite catalyst; a method of regenerating the zeolite catalyst used in the aromatization of acetylene by plasma treatment; and a regenerated zeolite catalyst for the aromatization of acetylene, prepared thereof.

Claims

1. A method of preparing an aromatic compound from acetylene, comprising: a first step of synthesizing an aromatic compound from an acetylene-containing reactant gas in the presence of a zeolite catalyst for the aromatization of acetylene, thereby forming internal coke inside the micropores of the zeolite catalyst and external coke in the space between catalyst particles, outer surface of the particles, and/or empty space other than the micropores; a second step of (i) locating the zeolite catalyst used in the first step between a high voltage electrode and a ground electrode into a plasma reactor, in which the high voltage electrode is provided inside of the reactor and the ground electrode is provided outside of the reactor, and (ii) treating with the plasma generated by applying a voltage at the kV level to the high voltage electrode, so as to selectively remove 25 wt % to 50 wt % of the total amount of the external coke and 0.5 wt % to 10 wt % of the total amount of the internal coke from the zeolite catalyst; and a third step of repeatedly performing the first step using, as a zeolite catalyst for the aromatization of acetylene, the plasma-treated zeolite catalyst of the second step.

2. The method of claim 1, wherein the second step is performed by applying a voltage in the range of 7 kV to 50 kV to the high voltage electrode.

3. The method of claim 1, wherein, after the third step, a 2-n step and a 3-n step, which are the same as the second step and the third step, respectively, are repeatedly performed n times, in which n is a positive integer.

4. The method of claim 1, wherein the cokes removed by plasma treatment are graphitic carbonaceous materials.

5. The method of claim 1, wherein the plasma-treated zeolite catalyst comprises cokes that remain at a ratio of 0.5 to 0.7 (the amount of external coke/the amount of internal coke).

6. The method of claim 1, wherein the plasma-treated zeolite catalyst shows 20% to 45% of acetylene conversion for the first 10 minutes of a reaction in the third step.

7. The method of claim 1, wherein the zeolite catalyst regenerated by plasma treatment in the second step shows 40% to 55% of a benzene, toluene, and xylene (BTX) selectivity in an aromatization of acetylene of the third step.

8. The method of claim 1, wherein the second step is achieved by dielectic barrier discharge (DBD).

9. The method of claim 1, wherein the acetylene-containing reactant gas is prepared by supplying methane as a raw material to a plasma reactor and generating plasma by dielectric barrier discharge.

10. The method of claim 9, wherein the reaction of the second step is achieved in the plasma reactor, which has been used in preparing the acetylene-containing reactant gas.

11. A method of regenerating a zeolite catalyst for the aromatization of acetylene used in the synthesis of an aromatic compound from an acetylene-containing reactant gas, in which internal coke is formed inside the micropores of the zeolite catalyst and external coke is formed in the space between catalyst particles, outer surface of the particles, and/or empty space other than the micropores, wherein the method comprises: (i) locating the used zeolite catalyst between a high voltage electrode and a ground electrode into a plasma reactor, in which the high voltage electrode is provided inside of the reactor and the ground electrode is provided outside of the reactor, and (ii) treating with the plasma generated by applying a voltage at the kV level to the high voltage electrode, so as to selectively remove 25 wt % to 50 wt % of the total amount of the external coke and 0.5 wt % to 10 wt % of the total amount of the internal coke from the zeolite catalyst.

12. The method of claim 11, wherein a zeolite catalyst, which comprises cokes that remain at a ratio of 0.5 to 0.7 (the amount of external coke/the amount of internal coke), is provided.

13. The method of claim 11, wherein the regenerated catalyst maintains the crystal structure of the zeolite catalyst itself before it is used in the reaction for the synthesis of an aromatic compound from an acetylene-containing reactant gas.

14. A zeolite catalyst for the aromatization of acetylene, regenerated by the method of claim 11.

15. The method of claim 14, wherein the rate of the aromatization is adjusted so that the acetylene conversion for the first 10 minutes of the reaction is in the range of 20% to 45%.

16. The method of claim 14, wherein the zeolite catalyst shows a benzene, toluene, and xylene (BTX) selectivity in the range of 40% to 55% in the aromatization of acetylene.

17. A dielectic barrier discharge (DBD) plasma reactor for regenerating a zeolite catalyst for the aromatization of acetylene used in the synthesis of an aromatic compound from an acetylene-containing reactant gas, in which internal coke is formed inside the micropores of the zeolite catalyst and external coke is formed in the space between catalyst particles, outer surface of the particles, and/or empty space other than the micropores, wherein the dielectic barrier discharge plasma reactor comprises: a channel-type container made of a dielectric material, which is able to receive the zeolite catalyst used in the aromatization of acetylene; a ground electrode, which is located on the outer wall of the channel-type container; a high voltage electrode, which is inserted into the zeolite catalyst received inside the channel-type container to be spatially separated in parallel from the channel-type container made of a dielectric material, and which has a higher voltage than the ground voltage; a fixing part, which fixes the used zeolite catalyst, that is received inside the channel-type container, in a predetermined area; and a power supply part, which provides a controlled voltage to the high voltage electrode, so as to selectively remove 25 wt % to 50 wt % of the total amount of the external coke and 0.5 wt % to 10 wt % of the total amount of the internal coke from the used zeolite catalyst.

18. The dielectic barrier discharge plasma reactor of claim 17, wherein the zeolite catalyst, used in the aromatization of acetylene, to be regenerated, is supported by the fixing part to be received within the area covered with the ground electrode within the channel-type container.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0053] FIG. 1 shows a schematic diagram of the dielectric barrier discharge (DBD) plasma system for the regeneration of the spent H-ZSM5 catalyst.

[0054] FIG. 2 shows TCD signal areas of CO and CO.sub.2 of the spent H-ZSM-5 after regeneration by plasma at (a) 17.5 kV and (b) 20.0 kV.

[0055] FIG. 3 shows transmission electron microscopy (TEM) images of (a) the fresh H-ZSM-5, (b) the spent H-ZSM-5, (c) the plasma-regenerated H-ZSM-5 at 17.5 kV, and (d) the plasma-regenerated H-ZSM-5 at 20.0 kV.

[0056] FIG. 4 shows TEM images of (a) the thermally treated H-ZSM-5 at 800 C., (b) the spent H-ZSM-5 after regeneration by plasma at 17.5 kV, (c) the spent H-ZSM-5 after regeneration by plasma at 20.0 kV, and (d) the spent H-ZSM-5 after thermal treatment at 800 C.

[0057] FIG. 5 shows (A) Ar physisorption isotherm; (B) pore size distribution obtained by means of the H-K method; and (C) BJH pore size distribution obtained from adsorption branches of Ar physisorption for (a) the fresh H-ZSM-5, (b) the spent H-ZSM-5, (c) the plasma-regenerated H-ZSM-5 at 17.5 kV, (d) the plasma-regenerated H-ZSM-5 at 20.0 kV, and (e) the thermally treated H-ZSM-5 at 800 C.

[0058] FIG. 6 shows wide angle X-ray diffraction (XRD) patterns of (a) the fresh H-ZSM-5, (b) the spent H-ZSM-5, (c) the plasma-regenerated H-ZSM-5 at 17.5 kV, (d) the plasma-regenerated H-ZSM-5 at 20.0 kV, (e) the spent H-ZSM-5 after regeneration by plasma at 17.5 kV, and (f) the spent H-ZSM-5 after regeneration by plasma at 20.0 kV.

[0059] FIG. 7 shows thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results of the spent H-ZSM-5, the plasma-regenerated H-ZSM-5 at 17.5 kV, the plasma-regenerated H-ZSM-5 at 20.0 kV, and the thermally treated H-ZSM-5 at 800 C.

[0060] FIG. 8 shows wide angle XRD patterns of (a) the thermally treated H-ZSM-5 at 800 C. and (b) the spent H-ZSM-5 after thermal treatment at 800 C.

[0061] FIG. 9 shows Fourier transform infrared spectroscopy (FT-IR) spectra of (a) the fresh H-ZSM-5, (b) the spent H-ZSM-5, (c) the plasma-regenerated H-ZSM-5 at 17.5 kV, (d) the plasma-regenerated H-ZSM-5 at 20.0 kV, and (e) the thermally treated H-ZSM-5 at 800 C. Asterisks designate a peak around 1580 cm.sup.1 to 1590 cm.sup.1, which can be ascribed to the CC stretching band derived from the presence of the graphitic coke on the samples.

[0062] FIG. 10 shows FT-IR spectra of (a) the spent H-ZSM-5 after regeneration by plasma at 17.5 kV, (b) the spent H-ZSM-5 after regeneration by plasma at 20.0 kV, and (c) the spent H-ZSM-5 after thermal treatment at 800 C. Asterisks designate a peak around 1580 cm.sup.1 to 1590 cm.sup.1, which can be ascribed to the CC stretching band derived from the presence of the graphitic coke on the samples.

[0063] FIG. 11 shows catalytic performances of the fresh H-ZSM-5, the plasma-regenerated H-ZSM-5 at 17.5 kV, the plasma-regenerated H-ZSM-5 at 20.0 kV, and the thermally treated H-ZSM-5 at 800 C. (from left to right).

DETAILED DESCRIPTION OF THE INVENTION

[0064] Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, these Examples are for illustrative purposes only and the scope of the invention is not limited by these Examples.

Example 1: Preparation and Characterization Analysis of the Catalyst for the Aromatization of Acetylene

[0065] Commercialized conventional zeolite H-ZSM-5 (Zeolyst international Inc., USA) with Si/Al ratio of 15 was purchased for the catalyst in the aromatization of acetylene. Transmission electron microscopy (TEM) images of fresh H-ZSM-5, spent H-ZSM-5, and regenerated H-ZSM-5 were taken on the JEM-2100F field emission transmission electron microscope (FE-TEM, USA) under working voltage of 200 kV. X-ray diffraction (XRD) analysis was performed in the /2 configuration using a Rigaku D/Max-2500V/PC diffractometer (Japan) with a Cu K radiation (40 kV, 200 mA, =0.154 nm) from 5 to 50. Argon physisorption measurements were conducted at 87 K with an ASAP 2020 analyzer (Micromeritics, Inc., USA). Before the measurements, the samples were degassed at 623 K for 12 hours under vacuum condition. Fourier transform infrared spectroscopy (FT-IR) analysis were performed by using Nicolet iS50 (Thermo Fisher Scientific, Inc., USA) with potassium bromide pellet.

[0066] After the aromatization or the dielectric barrier discharge (DBD) plasma regeneration, the total weight of coke formed at the catalysts were measured by conducting thermogravimetric analysis (TGA). Specifically, all catalysts was heated from 40 C. to 800 C. with a ramping speed of 40 C..Math.min-1 under air (Q50, TA Instruments, USA; TGA 1, Mettler Toledo, USA). The weights of the internal and external cokes in the spent catalysts were calculated. With the estimated total cokes of all four samples from TGA data, the inventors further measured the Ar physisorption isotherms of the spent and regenerated catalysts, and determined their microporous volumes by the H-K method. By comparing micropore volumes of the spent and regenerated catalysts with that of the fresh catalyst, the inventors approximated the amount of coke formed inside the micropores by assuming that the decreased micropore volumes were occupied solely by coke. The internal coke amount was equal to the product of the decreased microporous volume and coke's density (1.22 g.Math.cm.sup.3, please refer to Nature, 2009, 406: 246-249)

Example 2: Aromatization Reactor and Activity Test

[0067] The aromatization of acetylene was performed in a lab-made 2-channel micro fixed-bed reactor system. Each channel was constructed with 540 mm length and 10 mm inner diameter quartz tube. A spacer for loading catalysts was placed in the middle of quartz tube. The zeolite catalysts were loaded in the middle of the reactor and the zeolite catalysts were diluted (20 wt %) with ball-type alumina (-Al2O3, =1 mm). The aromatization was performed at 650 C. and atmospheric pressure. The composition of feed was C.sub.2H.sub.2:H.sub.2:N.sub.2=1:2:7 with gas hourly space velocity (GHSV) of 20,000 mL.Math.g.sub.cat.sup.1.Math.h.sup.1.

[0068] On-line Gas Chromatography (6500GC Young Lin Instrument Co., Korea) was used to analyze the products from the reaction. The on-line GC employs Porapak-Q columns and a Molecular Sieve 5A connected with a thermal conductivity detector (TCD), and a Gas-pro column connected with a flame ionization detector (FID). H.sub.2, N.sub.2, and CH.sub.4 in the products were detected by using the TCD, and hydrocarbons such as CH.sub.4, C.sub.2H.sub.6, C.sub.2H.sub.4, C.sub.2H.sub.6, C.sub.3H.sub.8, C.sub.3H.sub.6, benzene, toluene, and xylene (ortho-, meta-, para-) were detected by using the FID. An acetylene conversion and a benzene, toluene and xylene (BTX) selectivity according to the reaction time were shown in Table 1 below. As shown in Table 1, from 90 minutes after the initiation of the reaction, the changes in the acetylene conversion and BTX selectivity were slow, and no further changes were observed after 130 minutes.

TABLE-US-00001 TABLE 1 Acetylene TOS conversion Selectivity (%) (min) (%) Benzene Toluene Xylene BTX H-ZSM-5 10 99.95 32.65 21.1 4.79 58.54 (Si/Al = 15) 50 32.43 19.81 4.51 5.98 30.3 at 650 C. 90 22.46 26.61 3.19 7.62 40.42 130 20.8 31.97 3.1 8.03 43.1 170 20.27 32.81 2.89 7.85 43.55

Example 3: DBD Plasma Bed for Regeneration

[0069] A DBD plasma regeneration was performed at atmospheric pressure with aeration and near room temperature. The schematic diagram for this device was illustrated in FIG. 1. GHSV was 1500 mL.Math.mL.Math.g.sub.cat.sup.1.Math.h.sup.1 with feed of air and time for the regeneration was 720 min. FIG. 1 shows the lab-made DBD plasma system. An alumina tube of 6 mm internal diameter and 2 mm thickness was used as a dielectric barrier for the plasma bed. A 3 mm diameter stainless steel rod was used as a high voltage electrode, and a steel wire was used as a ground electrode. The 150 mm length discharge zone was covered with the ground electrode. The discharge gap between inner surface of the alumina tube and the high voltage electrode was 1.5 mm, and the spent H-ZSM-5 catalyst was densely packed in this region for the regeneration. A sinusoidal AC power supply (0 V to 220 V, 60 Hz to 1,000 Hz) was connected to a transformer (0-20 kV, 1,000 Hz), and this electrical system continuously applied high voltage to the plasma bed. The applied voltage to the plasma bed was varied from 17.5 kV to 20.0 kV, and the frequency was fixed as 1 kHz. A capacitor with 1 F capacitance was connected in series between the plasma bed and the ground. A high voltage probe (1,000:1, P6015A, Tektronix) for the high voltage electrode and a voltage probe (10:1, P6100, Tektronix) for the ground electrode were connected to a digital oscilloscope (TDS 3012C, Tektronix), and each voltage was measured.

[0070] The effluent gas from the plasma bed was analyzed by on-line gas chromatography (6500GC Young Lin Instrument Co., Korea) employing a Porapak-N and a Molecular Sieve 13X columns connected with a thermal conductivity detector (TCD). O.sub.2, N.sub.2, CO and CO.sub.2 in the effluent were detected by using the TCD. The elution profiles of CO and CO.sub.2 at 17.5 kV and 20.0 kV were shown in FIG. 2.

[0071] For comparison, some amount of the spent H-ZSM-5 sample was placed to a muffle furnace. A thermal treatment for the sample was then conducted at 800 C. for 5 h at the ramping speed of 10 C. min.sup.1.

Experimental Example 1: TEM Image Analysis and Physisorption Methods

[0072] TEM imaging analysis was conducted with the fresh, the spent, and the DBD plasma-treated H-ZSM-5 after aromatization, and the results are shown in FIG. 3. As seen in FIG. 3, the amount of coke was slightly removed inside the micropores at 17.5 kV, and the removed amount was further increased at 20.0 kV. FIG. 4 additionally shows the TEM images of the thermally treated H-ZSM-5 at 800 C., the spent H-ZSM-5 after regeneration at 17.5 kV, the spent H-ZSM-5 after regeneration at 20.0 kV, and the spent H-ZSM-5 after thermal treatment at 800 C. From the viewpoint of coke removal only, the thermal treatment was found a more efficient method because there remained a very small amount of coke after this treatment (FIG. 4a vs. FIGS. 3c and 3d). Even though the spent catalyst treated by either plasma or heat, the carbonaceous materials were spotted inside pores and around the particles after the 2.sup.nd run of aromatization (FIGS. 4b to 4d).

[0073] Table 2 shows the results of Ar physisorption. The BET surface area, the outer surface area, and the volume of micropores are shown. After the aromatization from acetylene at elevated temperature, a lot of carbon deposition formed and the BET surface area decreased significantly by 96.4%, from 402 m.sup.2/g to 14.4 m.sup.2/g. Consequently, the pore volume of 0.132 cm.sup.3/g decreased down to 0.00317 cm.sup.3/g. The difference in volume could be ascribed to the volume of carbon deposited inside the micropores. The spent catalyst was first treated with the DBD plasma at 17.5 kV and 1 kHz and S.sub.BET, S.sub.ex and V.sub.micro were slightly changed. When the plasma power increased to 20.0 kV at the same frequency level, S.sub.BET, S.sub.ex, and V.sub.micro increased 2.63 times, 1.85 times, and 3.31 times compared with the spent catalyst, respectively. For comparison, the inventors have conducted a thermal treatment with the spent catalyst at 800 C. with aeration. The remaining amount of carbon deposition at thermally regenerated catalyst seemed to be negligible by comparing V.sub.micro volumes between the treated and fresh H-ZSM-5 ones. The thermal treatment apparently seemed to be a very effective way to remove carbon deposition, however, more closely looking into the decrease in V.sub.micro and the increase in S.sub.ex revealed compared with those of the fresh catalyst that the changes were believed to be the result of structural modification or damage. According to Catalysis today (2011, 178: 72-88), a similar kind of thermal treatment at elevated temperature for regeneration could damage the aluminum framework, elucidated by employing .sup.27Al MAS NMR spectroscopy. They pointed out that the peak intensity at around 54 ppm was decreased and a new peak near 0 ppm appeared, indicating the tetrahedrally coordinated alumina framework was damaged, and octahedrally coordinated Al atoms were formed out of the zeolite framework, respectively. Therefore, the inventors believe that the damaged framework and the dealuminated fraction due to thermal treatment made extra empty space other than the micropores in the original framework, leading to a significant increase in S.sub.ex (outer surface area of particles and induced surface area due to newly formed empty space inside the particles) nearly 1.56 times. Because of this, the microporous structure in the framework also seemed slightly damaged, and the V.sub.micro and S.sub.BET was decreased by 0.009 cm.sup.3/g and 15 m.sup.2/g, respectively.

TABLE-US-00002 TABLE 2 S.sub.BET S.sub.ex.sup.a V.sub.micro.sup.b Samples (m.sup.2 .Math. g.sup.1) (m.sup.2 .Math. g.sup.1) (cm.sup.3 .Math. g.sup.1) Fresh H-ZSM-5 402 39.1 0.132 Spent H-ZSM-5 14.4 13.7 0.00317 Regenerated H-ZSM-5 at 17.5 kV 15.7 11.8 0.00374 Regenerated H-ZSM-5 at 20.0 kV 37.9 25.4 0.0105 Thermally treated H-ZSM-5 at 387 60.9 0.123 800 C. .sup.at-calculated by t-plot method. .sup.bcalculated by H-K method.

[0074] FIG. 5 shows the results of physisorption isotherms (A), pore size distribution of micropores (B), and mesopores (C). As shown in FIG. 5A, all the samples seemed to have mostly micropores. FIG. 5B shows the size distribution ranging from 0.4 to 0.6 nm, which is close to the channel size of H-ZSM-5 (about 0.5 nm). It was found that the micropores of the spent and the regenerated at 17.5 kV seemed entirely filled with carbonaceous material (FIGS. 5B(b and c)). When the plasma power was increased to 20.0 kV, the coke inside the micropores was removed to some degree (FIG. 5B(d)). In the case of the thermal treatment (FIG. 5B(e)), almost all the coke seemed eliminated, however, the microporous volume was slightly decreased.

[0075] In addition, the inventors have further investigated the mesoscale pores and showed the result in FIG. 5C. The amount of this kind of pores was relatively small, but the inventors could detect the changes after the reaction and each treatment. As expected, the pore volume of the spent catalyst was greatly decreased (FIG. 5C(b)). When the DBD plasma at 17.5 kV was applied, the amount of coke removal was negligible at this scale (FIG. 5C(c)). When the power was increased to 20.0 kV, a slight amount of coke was removed (FIG. 5C(d)). It was very interesting that the thermally treated one (FIG. 5C(e)) was found to have more pore space than that of the original fresh catalyst (FIG. 5C(a)). This seemed to be the result of similar structural change as seen in the microporous region. Simultaneously considering the additional observations discussed in later sections (specifically, XRD, FT-IR, and TGA), the decrease in microporous volume and the increase in mesoporous volume were possibly due to the structural modification or damage in the thermally treated sample (FIG. 5e).

[0076] Table 3 summarizes the amounts of internal coke formed inside the micropores and the external coke formed elsewhere at such spaces between particles, the space close to the outer surface of particles, and the induced void fraction due to chemical reaction and regeneration processes. By using this volume difference and the density of carbon, the amount of internal coke could be calculated, and the amount of external coke corresponded to the amount difference between the total coke and the internal coke. In the case of the spent H-ZSM-S, V.sub.micro was very small compared to V.sub.micro fresh, indicating that almost all micropores seemed filled with carbonaceous material. The amount of external coke was estimated to be 0.84 times the amount of internal coke. When treated with the DBD plasma at 17.5 kV, the amount of total coke was decreased from 0.290 g.Math.zeolites g.sup.1 to 0.248 g.Math.zeolites g.sup.1, mostly due to the decrease in external coke. The removed amount of internal coke seemed very small. In contrast, when treated at 20.0 kV, V.sub.micro increased and the amount of coke was significantly removed. The effect on the removal of external coke was shown to be more pronounced. At the comparative run, 92.4% of internal coke was burnt off and the amount of total coke decreased down to 0.012 g.Math.zeolites g.sup.1. Actually, no trace of external coke was observed.

TABLE-US-00003 TABLE 3 V.sub.micro.sup.a/ Total coke External External V.sub.micro.sup.b (g .Math. zeolites Internal coke coke coke/internal Samples (%) g.sup.1) (g .Math. zeolites g.sup.1) (g .Math. zeolites g.sup.1) coke Spent H-ZSM-5 2.39 0.290 0.158 0.132 0.84 Regenerated H-ZSM-5 at 2.82 0.248 0.157 0.091 0.58 17.5 kV Regenerated H-ZSM-5 at 7.96 0.234 0.149 0.085 0.57 20.0 kV Thermally treated H-ZSM-5 92.6 0.012 0.012 0.000 0.00 at 800 C. .sup.aMicropore volume of zeolites after catalytic activity test or regenerated treatment obtained by H-K method. .sup.bMicropore volume of fresh zeolites obtained by H-K method.

[0077] Considering the results of Tables 1 and 2, the inventors believe that this DBD plasma treatment could be more successfully applied when carbon depositions located on the outer surfaces and in the space between particles, rather than inside the micropores whose dimension seems far below the optimum range for microdischarge and streamer generated by the DBD plasma technique.

Experimental Example 2: X-Ray Diffraction Method

[0078] In FIG. 6, the results of fresh, spent, regenerated, and spent after regenerated H-ZSM-5 catalysts are shown. As shown in FIG. 6a, the characteristic peaks of the fresh H-ZSM-5 were clearly seen and easily identified compared with the reference spectrum generated by the single crystal software Mercury 3.7. After the aromatization at elevated temperature in FIG. 6b, the structure of the spent H-ZSM-5 appeared well maintained and unchanged except for the peak at 26.4, which is known to be a peak of crystalline graphite carbon. The carbon peak could be ascribed to the coke deposited around the particles and inside pores of the zeolite catalyst. Considering the results of TGA in FIG. 7, the carbonaceous materials must have been a mixture of crystalline and amorphous carbons since the decomposition temperature ranged from 600 C. to 800 C. This XRD spectrogram well explained the existence of crystalline graphite components formed during the aromatization. In order to regenerate the spent H-ZSM-5 with carbon depositions, the inventors applied the DBD plasma by adjusting the power supply. At 17.5 kV of supply power (FIG. 6c), the peak intensity for the coke near 26.4 seemed unchanged, however, the intensity in the case of 20.0 kV did decrease (FIG. 6d). It is believed that the increased energy delivered by plasma helped remove the crystalline coke to some extent. With this regenerated zeolite, the aromatization was carried out again at the same reaction condition for the same reaction time. The XRD results of the spent H-ZSM-5 after regeneration at 20.0 kV was shown in FIG. 6e. As shown in FIG. 6e, the intensity of graphite carbon did not seem to change, indicating no further crystalline graphite formed during the 2nd run of the aromatization. The additional amount of coke formed at the 2.sup.nd run was found quite small as shown in Table 4. In contrast, nearly the same amount of coke was formed again (22.80 wt % vs. 22.49 wt %) in the case of the thermally treated sample in comparison with the spent H-ZSM-5 after the 1st aromatization run (first sample in Table 4). It was also clearly verified through the XRD method, and the result was shown in FIG. 8. This indicates that the remaining coke after the plasma treatment blocks further formation of graphite-like coke materials. It was believed that this phenomenon prevented the significant wasting of acetylene towards graphite-like coke and maintained the BTX selectivity although the remaining carbon deposition might have compromised catalytic activity by partially preoccupying or covering acid sites.

TABLE-US-00004 TABLE 4 Additional amount of coke formed in the 2.sup.nd Weight loss run after each Samples (%) treatment (wt %)* Spent H-ZSM-5 22.49 Regenerated H-ZSM-5 at 17.5 kV 19.85 7.54 Regenerated H-ZSM-5 at 20.0 kV 18.93 6.57 Thermally treated H-ZSM-5 at 800 C. 1.19 22.80 *Amount of coke generated = amount of coke formed amount of coke remaining in the treated sample

[0079] For comparison, the inventors regenerated the spent H-ZSM-5 by conducting a traditional thermal treatment of the spent H-ZSM-5 by supplying thermal heat and air flows. The XRD results after the treatment showed that there existed nearly no crystalline coke. For the purpose of graphite-like coke removal only, this thermal treatment definitely was an improve method. However, when a 2.sup.nd aromatization over this catalyst regenerated by the thermal treatment was conducted, practically the similar amount of graphite-like coke was also generated, which caused a waste of acetylene, a raw material for the aromatization, and deactivation of the catalyst as well. Looking into the results of the regenerated catalysts treated with the DBD plasma, the selectivity towards aromatics still remained at similar ranges, and the abrupt initial decrease in conversion could be avoided. Reportedly, a better structural stability was achieved when the DBD plasma was chosen in comparison with the thermal treatment at elevated temperature.

Experimental Example 3: Analysis of Carbonaceous Residue Materials by TGA, DTA & FT-IR Methods

[0080] As shown in the result of TGA in FIG. 7, the amount of coke in samples in decreasing order was found: the spent H-ZSM-5, the regenerated H-ZSM-5 at 17.5 kV DBD plasma, and the regenerated H-ZSM-5 at 20.0 kV DBD plasma. It seemed obvious that higher power applied to the bed could eliminate more carbon materials. Regarding the result of DTA, the most intense peak of DTA curve in the case of the regenerated H-ZSM-5 at 20.0 kV DBD plasma was observed around 688 C., which was lower than the peak temperature (ca. 701 C.) of different samples by 13 C. We believe that more intense power could eliminate more coke materials, and partially turn solid crystalline carbons into more easily decomposable and removable carbon species. Furthermore, the inventors also conducted the 2.sup.nd aromatization runs at the same reaction condition after each treatment described. The result of TGA was shown in Table 4. As shown in Table 4, the amount of additional coke by the 2.sup.nd run seemed quite small compared with the amount of coke from the thermally treated sample at 800 C. The remaining carbon material partially covering the acidic surface seemed to have suppressed excessive side reactions by keeping moderate level of conversion, especially at the very early stage.

[0081] FIG. 9 shows the FT-IR spectra of the fresh, the spent, the plasma-treated, and the thermally treated catalyst samples. Different from the fresh catalyst, the peak at 1580 cm.sup.1 to 1590 cm.sup.1 appeared in all the spent catalysts, regardless of the number of runs. The FT-IR results after the 2.sup.nd aromatization run were additionally shown in FIG. 10. This peak, reportedly, can be ascribed to the CC stretching band, whose source was definitely the graphitic coke materials in the spent catalysts. This peak seems different from the peak at the slightly higher band of ca. 1640 cm.sup.1, which seemed to be caused by water adsorption. Regarding the regenerated samples by the DBD plasma, the CC stretching bands were observed, while the thermally treated sample at 800 C. did not show any trace of this stretching.

[0082] It was found that the existence of in-plane aromatic CH deformation mode was vaguely observed at 1,440 cm.sup.1 except for the fresh and the thermally treated samples, whereas aromatic CH group (3,000 cm.sup.1 to 3,100 cm.sup.1) and out-of-plane aromatic CH deformation mode (870 cm.sup.1, 820 cm.sup.1, and 755 cm.sup.1) were barely seen. Considering the stretching mode and the results of the thermogravimetric analysis, the species of coke seemed to be pseudo-graphite cokes.

Experimental Example 4: Catalytic Performance

[0083] A series of aromatization runs were conducted and 4 data sets are shown in FIG. 11. The reactions were conducted using the catalysts, which were regenerated by plasma treatment at 17.5 kV and 20.0 kV, respectively, according to a method of regeneration of the present invention, and the thermally regenerated H-ZSM-5 at 800 C. as another Comparative Example. The specific numbers of acetylene conversion, selectivity for an individual BTX compound, and total BTX selectivity specifically measured for these 3 kinds of samples are shown in Tables 5 to 7, respectively. Furthermore, the inventors compared the results with Table 1 which shows the results of the aromatization of acetylene using the fresh H-ZSM-5 conducted as a control group. In the case of fresh catalyst, the acetylene conversion was very high initially and abruptly dropped down to the level of about 20% within 2 h. The acetylene was so reactive at 650 C., and the consumption rate was very fast. In the case of severe deactivation of catalysts, many researchers tried to overcome the problem by incorporating mesoporosity and modifying surface characteristics physically and chemically. The other reasonable option could be a regeneration of catalyst by eliminating carbonaceous materials at a certain interval. The easiest way is to burn all the coke formed in the zeolite catalyst by heating and aeration at elevated temperature, normally over 800 C. However, this heat treatment may lead to the structural damage of a catalyst and requires a lot of energy. Even worse, the lifetimes of a catalyst and reaction system devices are subject to be shortened. In the aromatization over regenerated H-ZSM-5 by thermal treatment at 800 C., the conversion dropped remarkably at very early stage, which seemed similar to the test run with the fresh catalyst. This indicates that a lot of acetylene was wasted due to side reactions to form coke even though the valuable BTX were simultaneously synthesized from acetylene. Useless wasting of acetylene especially at the early stage could be avoided by using mildly regenerated catalyst, which could be achieved by applying the DBD plasma at ambient temperature and pressure. In addition, this mild technique hardly harmed the structure of the original catalyst as shown in the results of physisorption and XRD.

TABLE-US-00005 TABLE 5 Acetylene Selectivity (%) TOS Conversion Ben- Tol- Xy- (min) (%) zene uene lene BTX Regenerated 10 29.52 22.31 3.57 7.48 33.36 H-ZSM-5 at 50 19.66 34.37 3.76 10.84 48.97 17.5 kV 90 19.06 35.62 3.84 10.98 50.44 (Si/Al = 15) 130 18.6 36.67 3.89 10.98 51.54 at 650 C. 170 18.74 36.09 3.78 10.18 50.05

TABLE-US-00006 TABLE 6 Acetylene Selectivity (%) TOS Conversion Ben- Tol- Xy- (min) (%) zene uene lene BTX Regenerated 10 35.77 19.3 5.27 5.56 30.13 H-ZSM-5 at 50 21.52 32.46 3.54 7.7 43.7 20.0 kV 90 21.13 33.02 3.2 7.66 43.88 (Si/Al = 15) 130 19.47 36.55 3.52 8.03 48.1 at 650 C. 170 19.2 36.96 3.64 8.24 48.84

TABLE-US-00007 TABLE 7 Acetylene Selectivity (%) TOS Conversion Ben- Tol- Xy- (min) (%) zene uene lene BTX Thermally treated 10 65.08 18.99 17.86 10.57 47.42 H-ZSM-5 at 50 33.65 22.15 5.21 6.48 33.84 800 C. 90 27.37 25.11 4.05 6.55 35.71 (Si/Al = 15) 130 23.89 27.48 3.56 7.24 38.28 at 650 C. 170 21.53 30.01 3.26 7.97 41.24

[0084] With respect to the selectivities of aromatics, the selectivity levels of benzene, toluene, and xylene were found slightly different from each other, but as shown in FIG. 11, the difference was not quite big in the cases of all catalyst samples after a few sampling points. At very early stage before 1 h, the BTX selectivity of fresh catalyst was very high, and that of thermally treated H-ZSM-5 at 800 C. also showed comparably high selectivity. In contrast, the initial BTX selectivity was the lowest in the cases of regenerated zeolites treated with plasma at 17.5 kV and 20.0 kV. After the first sampling period, the BTX selectivity was increased to the level of 45% to 50% for both cases. The abrupt drop of conversion was successfully avoided, the initial conversion changes (difference between the first and the second conversions) were 9 percentage points and 12 percentage points for the second and the third samples, respectively. In the cases of fresh and thermally treated samples, the initial conversion changes were 65 percentage points and 31 percentage points. For the plasma-regenerated samples, the BTX were produced slightly more after the first sampling point, since the excessive side reactions were successfully avoided. Among the BTX products, the benzene selectivity was shown relatively high. We believe that the narrowed micropores of H-ZSM-5 caused by the remaining coke possibly exerted an influence on increasing the benzene selectivity, relatively limiting the formation of larger aromatics due to the geometrically confined environment.

[0085] The relative compositions of BTX and their profiles with time in the case of thermally treated catalyst at 800 C. appeared very similar to the results of its original one, i.e., fresh catalysts.

[0086] A non-thermal, dielectric barrier discharge (DBD) plasma was successfully applied to the deactivated H-ZSM-5 catalyst bed for regeneration. By adjusting the strength of discharge power, the amount of coke eliminated was accordingly controlled. Unlike the conventional thermal treatment, this plasma treatment barely harmed the original crystalline and nanoporous structure of H-ZSM-5 catalyst. In addition, the mildly treated and regenerated catalyst by plasma showed comparable BTX selectivity to the fresh one. Due to the more restricted microporous environment, the formation of relatively smaller benzene seemed more favored at internal acid sites. It should be noted that the plasma-regenerated catalyst suppressed the excessive wasting of acetylene caused by enormous coke especially at very early stage of aromatization, keeping the comparable BTX selectivity while compromising the level of conversion. Furthermore, the plasma-regenerated catalysts showed a better structural stability and higher BTX selectivity than the thermally regenerated catalyst at elevated temperature.