KR20050011383A - Process for the preparation of p-dioxanone from diethyleneglycol - Google Patents

Abstract

PURPOSE: A process for preparing p-dioxanone from diethyleneglycol is provided, thereby stably preparing p-dioxanone for a long period without reactivation of catalyst with improved selectivity and yield. CONSTITUTION: The process for preparing p-dioxanone from diethyleneglycol comprises dehydrocyclization of diethyleneglycol in the presence of a catalyst represented by the formula(1) of CuO(a)M(b)SiO2(c), in which M is one or more alkali earth metal oxides; and the (a), (b) and (c) represent percentage value based on weight, and are 30 to 85, 0.01 to 5 and 10 to 65, respectively, wherein the catalyst is prepared by (1) adding an alkali precipitating agent into a copper salt solution to produce the hydroxide type of precipitates, (2) adding nano-sized colloidal silica into the solution and maturing them to isolate and clean the precipitates, and (3) mixing the cleaned precipitates with an alkali earth metal compound in water, and drying and post-treating the resulting product.

Description

Process for producing X-dioxanone from diethylene glycol {PROCESS FOR THE PREPARATION OF P-DIOXANONE FROM DIETHYLENEGLYCOL}

The present invention relates to a method for preparing p-dioxanone from diethylene glycol, specifically, p-dioxanone from diethylene glycol under mild conditions by using a nano-sized copper oxide composite catalyst stabilized with silica particles. The present invention relates to a method for stably producing a long time without performing the reactivation operation of the catalyst with high selectivity and high yield.

P-dioxanone is used as a synthetic raw material, such as a monomer of a biodegradable polymer, or a morpholinone derivative. In particular, p-dioxanone is useful as a polymerization raw material of poly (p-dioxanone) which is used as a monofilament type surgical suture having bioabsorption properties, and poly (p-dioxanone) is easily heated under heating. It is attracting attention as a highly recyclable material because it can be decomposed to recover quantitatively the monomer p-dioxanone.

There are many ways to prepare p-dioxanone, but it is estimated that the preparation directly from diethylene glycol in the gas phase in the presence of a catalyst is the simplest and most economical. As a catalyst used to prepare p-dioxanone by dehydrogenation of diethylene glycol in the gas phase, US Pat. No. 2,900,395 discloses a copper-chromium catalyst system; U.S. Patent No. 3,119,840 discloses copper / pumice based catalysts; U. S. Patent No. 5,310, 845 discloses a catalyst system in which alumina supports alkaline components such as copper, zinc, and potassium; Japanese Patent Application Laid-Open No. 1998-120675 discloses a catalyst in which copper is supported on a carrier which defines a total amount of acid point where the acid strength (Ho) is stronger than 1.5; Japanese Patent Laid-Open No. 2000-351775 discloses a catalyst whose maximum particle size of supported copper is adjusted to 0.5 mu m or less based on the catalyst according to Japanese Patent Laid-Open No. 1998-120675.

However, the dehydrogenation cyclization catalyst prepared by simply supporting fine copper particles on a carrier such as silica or alumina is limited to 40 wt% or less of the active copper content. In addition, a common problem of the known catalysts is that the catalyst life is very short, for example, Japanese Patent Laid-Open No. 2000-351775 discloses p-dioxa from diethylene glycol in a catalyst system in which copper, zinc and potassium components are supported on alumina. Although the initial conversion to rice fields is 100%, the conversion rate is 88% after 12 hours of reaction, and the conversion rate drops to 22% after 100 hours of continuous reaction.

This rapid reduction in conversion is because the catalyst is deactivated due to carbon caulking, sintering, or the like. The reaction for producing p-dioxanone by dehydrogenation of diethylene glycol in the gas phase is carried out at 250 to 350 ° C. The Tamman temperature (0.33 Tm / K), which is a temperature at which sintering of the fine copper particles by heat proceeds, is performed. ) Is 180 ° C (453K) (Ref. Topics in Catalysis 8 (1999) 259), the inactivation by sintering of fine catalytically active particles whose main component is copper is inevitable.

In addition, the reaction product p-dioxanone is characterized by being easily polymerized in the presence of a compound having a hydroxy group, such as reactant (or unreacted) diethylene glycol, which is characterized by the separation and purification of crude p-dioxanone In addition to the complexity, the production yield is drastically reduced, and the activity of the catalyst proceeds rapidly due to the formation of coke by such a polymer in the catalyst layer.

In order to alleviate carbon coking, a method of adding hydrogen gas together with diethylene glycol has been proposed (US Patent No. 3,119,840, US Patent No. 5,310,945, Japanese Patent Application Laid-Open No. 1998-120675, Japanese Patent Application Laid-Open No. 2000-351775), In US Pat. No. 5,310,945, the conversion rate is reduced after 140 hours, and in Japanese Patent Laid-Open No. 2000-351775, the conversion rate is reduced after 100 hours. There is a problem to use as a catalyst.

In addition, the reaction may be carried out by a liquid phase method to prevent the rapid deactivation of the catalyst, but there is a problem that the catalyst component is eluted in the solution, and the deactivation of the catalyst is inevitable, and diethylene glycol and the product p-dioxanone are high temperature. Since the time to coexist in the reactor is long and the polymerization of p-dioxanone is achieved, p-dioxanone cannot be obtained with a high yield and it is disadvantageous in that it is inefficient for continuous process operation.

Accordingly, an object of the present invention is to provide a method for stably producing a long-term, high selectivity and high yield of p-dioxanone from diethylene glycol under mild conditions by using a catalyst having excellent thermal stability, reaction selectivity and activity. .

In order to achieve the above object, the present invention provides a method for preparing p-dioxanone comprising dehydrogenating diethylene glycol in the presence of a catalyst having a composition represented by the following formula (1):

Formula 1

CuO (a) M (b) SiO ₂ (c)

At this time,

M represents an oxide of at least one alkaline earth metal;

(a), (b) and (c) are percentages by weight and are numbers in the range of 30 to 85, 0.01 to 5 and 10 to 65, respectively.

Hereinafter, the present invention will be described in more detail.

In the dehydrogenated cyclization catalyst having the composition represented by Chemical Formula 1 according to the present invention, (1) an alkaline precipitant is added to an aqueous copper salt solution to produce a hydroxide form precipitate, and (2) nano-size colloidal silica is added thereto. After mixing and aging, the precipitate is separated and washed. (3) The washed precipitate and alkaline earth metal compound are mixed in water, followed by drying and post-treatment.

In the method for preparing a dehydrogenation cyclization catalyst according to the present invention, in step (1), an alkaline precipitant is added to an aqueous solution in which copper salt is dissolved to form a hydroxide-type precipitate, in which a water-soluble salt of a modifier component is added to the aqueous copper salt solution. It can be added to co-precipitate into particles in the form of a hydroxide mixed with copper and the modifier component. The concentration of the water-soluble salts of the copper and the modifier component may be adjusted in the range of 5 to 25% by weight, respectively, and the reaction may be performed at a temperature of 1 to 30 ° C. for 0.5 to 10 hours to maintain the hydrogel form.

The precursors of copper and modifier components usable in the present invention are not particularly limited so long as they do not impair the object of the present invention, and examples of the water-soluble salts that can be used include nitrates, hydrochlorides, acetates, sulfates, and the like. The case is most preferably used because the remaining anions after washing are effectively removed in the firing process.

As a modifier according to the present invention, zinc (Zn), chromium (Cr), manganese (Mn), silver (Ag), cobalt (Co), nickel (Ni), platinum (Pt), palladium (Pd), ruthenium (Ru) One or more components selected from elements such as) may be used to improve the performance of the catalyst. The catalyst of the present invention may comprise an oxide of this modifier component in an amount of 0.001 to 2% by weight, and a decrease in catalytic performance is observed when the amount exceeds 2% by weight.

As the alkaline precipitant according to the present invention, sodium hydroxide, potassium hydroxide, and carbonates and bicarbonates of alkali metals may be used, wherein the pH of the solution is 5-10, preferably after aging in step (2). It is necessary to adjust the amount to fall within the range of 6-9. When the pH of the solution exceeds 10 after aging, the added nano-sized silica may be dissolved, and when the pH is lower than 5, the precipitation of the components is not completely performed.

Subsequently, in step (2) according to the present invention, nano-size colloidal silica is added to the aqueous solution obtained in step (1), mixed and aged, and the precipitate is separated and washed, and the mixing is performed at a temperature range of 1 to 30 ° C. For 0.1 to 2 hours, the aging may be performed for 0.5 to 20 hours in the temperature range of 50 to 100 ℃.

The colloidal silica according to the present invention is stabilized with ammonium ions (NH ₄ ⁺ ) or sodium ions (Na ⁺ ), has a particle size of 4 to 60 nm, a surface area of 100 to 300 m ² / g, and a concentration of silica. Commercially available products (eg Ludox, Nalcoag, Snowtex) in the range of 5 to 60% by weight can be used.

In addition, in the post-aging washing process, it is important to control the residual amount of the cationic material such as sodium or potassium derived from the alkaline precipitant added in the step (1), the concentration of the cationic material is 1000ppm for the catalyst in the oxide state It should be as follows.

In steps (1) and (2), the alkaline precipitant and the colloidal silica can be added simultaneously to the aqueous solution containing the copper salt and the water-soluble salt of the optional modifier component while adjusting the pH to the range of 6-11.

Subsequently, in step (3) according to the present invention, the precipitate washed in step (2) and the alkaline earth metal compound are mixed in water, followed by drying and post-treatment. In the post-treatment, the dried catalyst is extruded, tableted. Or it is shape | molded by the method of carrying out on a refractory support, and it bakes at 300-1000 degreeC, Preferably it is 400-900 degreeC. If the firing temperature is too high, the size of the copper particles increases, the catalytic activity is lowered. If the firing temperature is too low, the selectivity is lowered due to an increase in the decomposition reaction of diethylene glycol.

Alkaline earth metal compounds according to the present invention used to improve the characteristics of the catalyst may be used nitrates, acetates or hydroxides of magnesium or calcium, sodium or potassium (K) instead of alkaline earth metals such as magnesium and / or calcium (K) In the case of using an alkali element such as), the selectivity is high at the beginning of the reaction, but it is not preferable because the activity decreases rapidly due to the surface movement of the catalyst particles.

As in the present invention, the size of the copper oxide particles can be adjusted to nano size as well as ultimately by mixing and aging copper oxide particles in the form of hydroxides, and then mixing and aging in the presence of colloidal silica particles having nano size. Is a nano-sized copper oxide composite catalyst stabilized with silica particles. The dehydrogenated cyclization catalyst according to the present invention thus prepared has a very high thermal stability, so that the acid strength of the catalyst can be controlled to some extent during the calcination process, and the catalyst active component can increase the fraction of copper to 50% by weight or more. The activity is high per unit volume (or unit volume), and the recovery rate is high after reactivation for inactive catalysts.

According to the present invention, the desired p-dioxanone is obtained by performing dehydrogenation of diethylene glycol by supplying diethylene glycol in the gaseous phase under the flow of nitrogen gas or hydrogen-containing nitrogen gas in the presence of the catalyst thus prepared. Manufacture.

In order to prepare p-dioxanone by dehydrogenating diethylene glycol on the catalyst according to the present invention, first, an oxide catalyst is used at about 150 to 300 ° C. for about 1 to 20 hours using hydrogen gas or hydrogen-containing gas. You have to go through the process of activation.

In the dehydrogenation of diethylene glycol using the activated catalyst of the present invention, it is preferable to use a fixed bed reactor. The reaction conditions are supplied so that the liquid hourly space velocity (LHSV) of diethylene glycol is in the range of 0.05 to 5.0 ^{hr −1} , preferably 0.1 to 2.0 hr ⁻¹ , and the reaction temperature is 200 to 400 ° C., Preferably 250 to 350 ℃, the reaction pressure is carried out at 0.3 to 2 atm (absolute pressure), preferably 0.3 to 1 atm (absolute pressure), the activated catalyst is charged to the reactor, and the reactant diethylene glycol is vaporized To the catalyst bed together with nitrogen gas or hydrogen-containing nitrogen gas.

In particular, the dehydrogenation reaction of diethylene glycol having a high boiling point is endothermic, and the product p-dioxanone has a polymerization characteristic as described above, and when the reaction pressure is high, the reactant and the product are condensed in the catalyst layer to produce a polymer. Deactivation of the catalyst proceeds rapidly. Therefore, the reaction pressure is advantageously as low as possible, it is preferable to operate at normal pressure to normal pressure, and more preferably at 1 atmosphere (absolute pressure) or less.

In order to suppress carbon coking, hydrogen is preferably contained in nitrogen used as a carrier gas.However, hydrogen is generated during dehydrogenation of diethylene glycol, and when the hydrogen concentration is high, the product p-dioxanone is hydrogenated on the catalyst. Since the reverse reaction, which is a decomposition reaction (hydrogenation reaction of p-dioxanone) proceeds, not only a high conversion rate is obtained, but also the reaction selectivity is poor, so that the hydrogen concentration is 0.1 to 50% by volume of the carrier gas used, preferably 0.1 It is preferred to use from 30 to 30% by volume.

As described above, when the dehydrogenation of diethylene glycol is carried out according to the method of the present invention, p-dioxanone can be stably produced for a long time under mild conditions without performing reactivation of the catalyst with high selectivity and high yield. Can be.

Hereinafter, the present invention will be described in more detail based on the following examples. However, the following examples are not intended to limit the invention only.

Example 1

(1-1) Dehydrogenation Cyclic Catalyst CuO (69.7) CaO (0.3) SiO ₂ (30) Preparation

200 g of copper nitrate [Cu (NO ₃ ) ₂ .3H ₂ O] was dissolved by dissolving 66.2 g of sodium hydroxide in 500 ml of water in a solution of 1500 ml of distilled water. At this time, the temperature of the slurry solution was adjusted so that it might not exceed 20 degreeC by the heat of neutralization. To this, 94.5 g of colloidal silica aqueous solution Ludox SM-30 (stable sodium, containing 30% by weight of silica, manufactured by Dupont) was added, and aged at 85 ° C for 6 hours. After aging the pH of the solution was 7.2. The precipitated slurry solution was washed with distilled water, filtered and the cake was recovered, dispersed in deionized water, and then mixed with 0.89 g of calcium acetate (Ca (CH ₃ COO) ₂ H ₂ O) in an aqueous solution. It dried by the drying method. The powder obtained was dried at 120 ° C. for 12 hours, and then calcined at 300 ° C., press-molded, crushed to 20-40 mesh, sorted, and calcined at 750 ° C. for 5 hours. The sodium content of the calcined catalyst oxide was 190 ppm as determined by inductively coupled plasma mass spectrometry (ICP-MS).

(1-2) Catalyst Activation

1.0 g of the calcined oxide catalyst was charged in a tubular reactor, and gradually heated up while flowing 5% H ₂ / N ₂ mixed gas, and activated at 300 ° C. for 6 hours.

(1-3) Dehydrogenation of Diethylene Glycol

After the reduction activation of the catalyst, the reaction was carried out under a flow of diethylene glycol LHSV (liquid feed rate) = 1.0 hr ⁻¹ and nitrogen at 35 ml / min under a reaction temperature of 290 ° C. and atmospheric pressure. The product was collected and analyzed by GC (gas chromatography). The reaction result was 99.9% conversion and 99.0% p-dioxanone selectivity after 24 hours, and 99.9% conversion and 99.0% p-dioxanone selectivity after 100 hours reaction.

Comparative Example 1

By not performing mixing with an aqueous calcium acetate solution, a catalyst CuO (70) SiO ₂ (30) consisting solely of copper and silica was prepared in the same manner as in Example (1-1), Example (1-2) and Activation of the catalyst and dehydrogenation of diethylene glycol were carried out in the same manner as in (1-3). The result of dehydrogenation reaction was 99.9% in conversion and 92.5% in p-dioxanone after 100 hours of reaction.

Comparative Example 2

The catalyst CuO (69.7) CaO (0.1) Na ₂ O (0.2) SiO ₂ (30) consisting of copper, calcium, sodium and silica was added by adding sodium acetate together when mixing with aqueous calcium acetate solution in Example (1-1). Prepared in the same manner as in, and the catalyst activation and dehydrogenation of diethylene glycol was carried out in the same manner as in Examples (1-2) and (1-3). The dehydrogenation reaction resulted in 99.9% conversion and 99.1% p-dioxanone selectivity after the reaction for 24 hours, but the performance drastically decreased to 97.4% conversion and 99.0% p-dioxanone selectivity after 100 hours of reaction.

Example 2

The catalyst CuO (70.6) ZnO (0.3) CaO (0.3) SiO ₂ (28.8) was prepared as in Example (1-1) by melting zinc nitrate [Zn (NO ₃ ) ₂ .6H ₂ O] together with copper nitrate. It was prepared by the method, and activation of the catalyst and dehydrogenation of diethylene glycol were carried out in the same manner as in Examples (1-2) and (1-3). The result of dehydrogenation cyclization reaction was 99.9% of conversion and 99.2% of p-dioxanone selectivity after 100 hours of reaction.

Comparative Example 3

In Example 2, catalyst CuO (68.7) ZnO (2.2) CaO (0.3) SiO ₂ (28.8) was prepared by increasing the content of zinc, and the catalyst was the same as in Examples (1-2) and (1-3). Activation and dehydrogenation of diethylene glycol were carried out. The result of the dehydrogenation cyclization reaction was 85.5% conversion and 97.3% p-dioxanone selectivity after 72 hours reaction.

Example 3

In 500 ml of distilled water, 200.0 g of copper nitrate, 1.04 g of zinc nitrate, and 5.17 g of cobalt nitrate [Co (NO ₃ ) ₂ · 6H ₂ O] were dissolved (solution A) and 56.3 g of colloidal silica rudox SM-30 was dissolved. 500 ml of an aqueous solution (solution B) and 500 ml of an aqueous solution of 68 g of sodium hydroxide (solution C) were prepared. 800 ml of deionized water was added to a 4 L beaker and co-precipitated by dropping the prepared solutions A, B and C simultaneously with vigorous stirring, and the pH of the mixed solution was adjusted to 9.3. The catalyst CuO (69.3) CoO (1.4) ZnO (0.3) MgO (0.3) SiO ₂ (28.7) was then carried out using 1.52 g of magnesium acetate (Mg (CH ₃ COO) ₂ .4H ₂ O) instead of calcium acetate. Prepared in the same manner as in Example (1-1), the catalyst was activated and the dehydrogenation of diethylene glycol was carried out in the same manner as in Examples (1-2) and (1-3). The dehydrocyclization reaction results are shown in Table 1 below.

Example 4

The same catalyst was prepared in the same manner as in Example 3, and then dehydrogenation of diethylene glycol was carried out using a mixed gas of 20% by volume of hydrogen and 80% by volume of nitrogen instead of nitrogen gas. The dehydrocyclization reaction results are shown in Table 1 below.

Reaction time (hr) Example 3 Example 4 % Conversion Selectivity (%) % Conversion Selectivity (%) 24 99.9 99.1 99.9 97.7 100 99.9 99.2 99.9 98.4 240 99.9 99.0 99.9 98.2 480 99.2 99.2 99.9 98.6 720 99.9 98.9 1000 99.9 99.0 1440 99.9 99.0

Example 5

In the same manner as in Example 3, a catalyst having a composition of CuO (69) CoO (0.72) ZnO (0.28) MgO (0.15) CaO (0.15) SiO ₂ (29.7) was prepared. At this time, 22.7% by weight of the silica component was used when the dropwise addition with Solution A and C using the Ludox SM-30 shown as Solution B in Example 3, the remaining 7% by weight Ludox AS-40 (NH ₄ ⁺ stable, sikyeotgo colloidal silica silica 40% containing, DuPont product weight) distributed into conjunction with magnesium nitrate, calcium nitrate was spray drying the mixture slurry. a dry powder for 12 hours at 120 ℃, at 300 ℃ First calcining and molding (5 mm x 2.4 mm: D x H) followed by secondary calcining for 5 hours at 750 ° C. 180 ml of the formed catalyst was charged into a 1 inch x 80 cm reactor, wherein the upper portion of the catalyst layer was filled with catalyst 50 ml was used with 50 ml of inert support to evenly fill in 5 ml increments, using the hydrogen / nitrogen mixed gas in the same manner as in Example (1-2), followed by LHSV = 0.5 of diethylene glycol. hr ^-1 , atmospheric pressure, The dehydrogenation cyclization reaction was carried out under a flow of H ₂ / N ₂ = 1/9 v / v The dehydrogenation cyclization reaction resulted in 99.9% conversion and 98.8% p-dioxanone selectivity after 1500 hours of operation.

Example 6

In the same manner as in Example 1, a catalyst of CuO (70.2) Cr 2 O 3 (0.5) ZnO (0.3) MgO (0.3) SiO ₂ (28.7) was prepared. At this time, chromium nitrate [Co (NO ₃ ) ₃ · H ₂ O] was used as the chromium precursor. In the same manner as in Examples (1-2) and (1-3), catalyst activation and dehydrogenation cyclization of diethylene glycol were performed. The dehydrocyclization reaction result was 99.9% conversion and 99.2% p-dioxanone selectivity after 100 hours of reaction, and the reaction continued for 720 hours was 95.6% conversion and 98.3% p-dioxanone selectivity.

Then, the reactant supply was stopped, and the decarbonization reaction was performed by heating up to 450 ° C. at a rate of 2 ° C./min under a flow of air / nitrogen mixed gas. After the temperature was lowered and reactivated using H ₂ / N ₂ mixed gas as in Example (1-2), dehydrogenation was carried out under the same conditions as in Example (1-3). After the reaction, the conversion was 99.9% and the p-dioxanone selectivity was 99.1%.

Example 7

In preparing a catalyst composed of CuO (70.3) PdO (0.1) ZnO (0.3) MgO (0.3) SiO ₂ (29.0), the slurry of the catalyst component except for the palladium component was prepared in the same manner as in Example 1, and then 120 Palladium nitrate [Pd (NO ₃ ) ₂ nH ₂ O,% Pd = 42.5] was dissolved in water and supported on a catalyst powder dried at 占 폚. Thereafter, the same treatment was performed as in Example (1-1), and activation of the catalyst and dehydrogenation of diethylene glycol were performed in the same manner as in Examples (1-2) and (1-3). The result of the dehydrogenation cyclization reaction was 100% conversion and 98.8% p-dioxanone selectivity after 100 hours reaction.

As described above, when the dehydrogenation of diethylene glycol is carried out according to the method of the present invention, p-dioxanone can be stably produced for a long time under mild conditions without performing reactivation of the catalyst with high selectivity and high yield. Can be.

Claims (11)

Method for preparing p-dioxanone comprising dehydrogenating diethylene glycol in the presence of a catalyst having a composition represented by Formula 1 below: Formula 1 CuO (a) M (b) SiO ₂ (c) At this time, M represents an oxide of at least one alkaline earth metal; (a), (b) and (c) are percentages by weight and are numbers in the range of 30 to 85, 0.01 to 5 and 10 to 65, respectively. The method of claim 1, The catalyst is prepared by (1) adding an alkaline precipitant to an aqueous copper salt solution to form a precipitate in the form of a hydroxide, and then (2) adding nano-sized colloidal silica to the mixture, ripening, and washing the precipitate by separating (3). A process prepared by mixing the washed precipitate and the alkaline earth metal compound in water, followed by drying and post-treatment. The method of claim 2, In step (1), copper (Zn), chromium (Cr), manganese (Mn), silver (Ag), cobalt (Co), nickel (Ni), platinum (Pt), palladium (Pd) and And further adding a water-soluble salt of at least one modifier component selected from ruthenium (Ru) such that the catalyst comprises an oxide of the modifier component in an amount of 0.001 to 2% by weight. The method of claim 2, The alkaline precipitant used in step (1) is selected from sodium hydroxide, potassium hydroxide, and carbonates and bicarbonates of alkali metals, which are used in an amount such that the pH of the solution is 5 to 10 after aging in step (2). How to feature. The method of claim 2, The colloidal silica used in step (2) is stabilized with ammonium ions (NH ₄ ⁺ ) or sodium ions (Na ⁺ ), has a particle size of 4 to 60 nm, a surface area of 100 to 300 m ² / g, and a concentration of silica Characterized in that it is in the range of 5 to 60% by weight. The method of claim 2, In step (2), the cleaning method is characterized in that the content of the cationic substance derived from the alkaline precipitant is 1000 ppm or less. The method of claim 2, In steps (1) and (2), an alkaline precipitant and colloidal silica are added simultaneously to an aqueous copper salt solution while adjusting the pH to a range of 6 to 11. The method of claim 2, In step (3), drying followed by shaping in post-treatment and firing at 300 to 1000 ° C. The method of claim 1, The catalyst is activated using hydrogen gas or hydrogen-containing gas at 150 to 300 ° C. for 1 to 20 hours and then used for the dehydrogenation cyclization reaction. The method of claim 1, The dehydrogenation cyclization reaction is carried out at 200 to 400 ° C. and 0.3 to 2 atmospheres (absolute pressure) under a flow of nitrogen gas or hydrogen-containing nitrogen gas. The method of claim 10, Wherein the hydrogen-containing nitrogen gas contains from 0.1 to 50% by volume of hydrogen gas.