An environmentally benign route to γ-butyrolactone through the coupling of hydrogenation and dehydrogenation

Applied Catalysis B: Environmental 57 (2005) 183–190 www.elsevier.com/locate/apcatb

An environmentally benign route to g-butyrolactone through the coupling of hydrogenation and dehydrogenation Yu-Lei Zhua,c, Jun Yangb, Gen-Quan Donga,c, Hong-Yan Zhenga,c, Hao-Hong Zhanga,c, Hong-Wei Xianga, Yong-Wang Lia,* a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, Shanxi, PR China b Department of Chemistry, Jinan University, Guangzhou 510632, PR China c Graduate School of the Chinese Academy of Sciences, PR China Received 22 July 2004; received in revised form 28 October 2004; accepted 8 November 2004 Available online 15 December 2004

Abstract The separated dehydrogenation cyclization of 1,4-butanediol, the hydrogenation of maleic anhydride, and the coupled process were carried out in a fixed-bed reactor over a Cu–Zn–Al catalyst, under different conditions of reaction temperatures and liquid hourly space velocity (LHSV). Compared to conventional processes, the coupled process has several advantages, e.g., improved g-butyrolactone yield, good energy efficiency, optimal hydrogen utilization, and environmentally benign process. In addition, the coupled process can dramatically inhibit the formation of de-carbonization compounds, namely n-propanol and CO, and facilitate the decrease of g-butyrolactone production cost in an industrial plant. The coupled operation leads to other advantages, such as easy temperature control in a tubular fixed-bed, due to its moderate heat release compared to the single maleic anhydride hydrogenation reaction, which can avoid the formation of apparent hotspots or coldspots in the practical process. Otherwise, the active hydrogen species released from the dehydrogenation cyclization of 1,4-butanediol is pretty suitable for the hydrogenation of maleic anhydride, and improves the selectivity of g-butyrolactone. The coupled operation shows the improved technology and presents the goal of green chemistry, namely atom economic way, in the view-points of material, energy utilization and environment. # 2004 Elsevier B.V. All rights reserved. Keywords: Maleic anhydride hydrogenation; 1,4-Butanediol dehydrogenation; g-Butyrolactone; Coupling reaction; Environmentally benign process

1. Introduction In this paper, we report a coupled catalytic reaction through hydrogen transfer between two reactants, 1,4butanediol (BDO) and maleic anhydride (MA) to produce one valuable product, g-butyrolactone (g-BL) in an atom economic way in the view points of material, energy utilization and environment. Catalytic hydrogenation transfer reactions have widely been investigated in the past years mostly for the reduction of organic compounds by using a hydrogen donor, which often leads to undesirable * Corresponding author. Tel.: +86 351 4130 337; fax: +86 351 4124 899. E-mail addresses: [email protected] (Y.-L. Zhu), [email protected] (Y.-W. Li). 0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.11.004

by-products [1,2]. In the coupled hydrogen transfer reaction, MA (hydrogen acceptor) is reduced by the hydrogen from BDO (hydrogen donor) to form the same desired product, gBL. g-BL is currently one of the most valuable alternatives to the environmentally harmful chlorinated solvents, and is one of the important intermediates in fine chemical industrial practices, typically for the synthesis of pyrrolidone, N-methylpyrrolidone, N-vinylpyrrolidone, herbicides, and rubber additives. There are two main routes for the production of g-BL: the catalytic dehydrogenation cyclization of BDO and the catalytic hydrogenation of MA. Both processes are mainly performed typically in multi-tubular fixed-bed reactors. Hydrogenation of MA is an important industrial reaction as all its products, viz. succinic anhydride (SA), g-BL and tetrahydrofuran (THF), are commodity

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chemicals of considerable industrial importance [3]. MA hydrogenation routes mainly include the following processes: (i) MA direct hydrogenation process based on vapor [4–8] or liquid [9–11] phase; (ii) Davy McKee process based on hydrogenation of diethyl or dimethyl maleates [3]. The routes of BDO dehydrogenation cyclization mainly include: (i) Reppe process based on acetylene–formaldehyde condensation [12]; (ii) Arco process based on isomerization of propylene oxide to allyl alcohol and subsequent hydroformylation; (iii) Mitsubishi Kasei process based on 1,3-butadiene diacetoxylation [13]. g-BL is currently being manufactured by the vaporphase hydrogenation of MA using reduced copper chromite catalysts [14] containing some physical and chemical promoters. Cr-containing catalysts are now limited in use owing to their toxicity. Hydrogenation of MA over different noble metal and Cu-based catalysts both in vapor and liquid phases has been reported [15–19]. Most of the knowledge of this important reaction is patented. Recent patent literature suggests that Al-containing catalysts may represent an interesting and promising alternative to the chromite catalysts [4,15,16]. Some patents report liquidphase hydrogenation of MA over supported Pd–Re and Ni– Co oxide catalysts [17,18]. A recent study reports a twostage liquid-phase hydrogenation of MA to g-BL over a ruthenium catalyst consisting of Ru(acac)3, P(octyl)3 and p-toluene sulfonic acid [19]. One drawback with the reported efficient catalytic hydrogenation of MA to g-BL is the use of expensive solvents like polyethylene glycol dimethyl ether [20]. Generally, liquid-phase hydrogenation requires either very severe operating conditions, or these homogeneous catalytic systems suffer from the problem of catalyst separation from the reaction mixture. Conveniently, the vapor-phase hydrogenation of MA is carried out in mild conditions such as normal pressure (below 0.1 MPa) [21]. MA hydrogenation is the most direct process to produce g-BL both in liquid and vapor phases, and it does not use the hazardous materials. Some researchers [22–25] have reported some important results about the hydrogenation of MA. The reaction equation of MA hydrogenation to g-BL can be represented [26] as follows:

Eq. (1) represents that producing 1 mol g-BL requires 1 mol MA and 3 mol H2, releasing 211 kJ heat. Due to the strong exothermic nature of reaction (1), the temperature control over this process is very difficult and this leads to apparent hotspots, typically in a tubular fixed-bed reactor, which very often causes thermal runaway and lowers the selectivity to desired product, g-BL. In addition, the supply of hydrogen to the system is needed.

The catalytic dehydrogenation of BDO to g-BL has also been described in literature [27–30], and generally the Cubased system catalysts are utilized. This reaction can be expressed as follows:

It is evident that producing 1 mol g-BL requires 1 mol BDO, releasing 2 mol H2 and requiring 61.6 kJ heat. It should be noted that the reaction is practically irreversible with typical industrial operation conditions (atmosphere, 200 8C, equilibrium constant Kp = 108) [31]. Due to the endothermic property of reaction (2), the increase of LHSV of BDO is relatively limited by low external heat supply in a practical reactor. In this reaction, the released hydrogen cannot be used effectively in a single dehydrogenation process. The new catalytic process combines reactions (1) and (2) into one catalytic system to significantly improve the yield of the catalytic hydrogenation of MA, apart from the better thermal balance and the effective usage of hydrogen through hydrogen transfer between the two reactants. In addition, this new catalytic process produces one desired product (gBL) with a substantially increased hydrogenation yield, and the ‘no-hydrogen’ (with hydrogen recycle but no hydrogen supply and release) operation also simplifies the technical procedure. The combined reaction can be expressed as follows:

The combined reaction in Eq. (3) shows that producing 2.5 mol g-BL requires 1 mol MA and 1.5 mol BDO, and is exothermic by 119.6 kJ/mol. With the perfect hydrogen mass balance, much easier temperature control in a practical reactor can also be expected than that in the single MA hydrogenation process due to much less heat release in the coupled system. Therefore, we decided to further investigate this coupled process and have been successful in forming g-BL with the coupled method [31]. This combined reaction can practically be realized on the basis of the fact that the hydrogen transfer from BDO (hydrogen donor) to MA (hydrogen acceptor) could be carried out over the same Cu–Zn–Al catalyst and under the similar reaction conditions. In industrial applications of both single MA hydrogenation and BDO dehydrogenation, Cr-containing catalysts are usually used, which are getting increasingly difficult due to

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their toxicity and pollution, while an environmentally friendly Cu–Zn–Al catalyst is used in this work. In addition, single MA hydrogenation consumes significant amount of hydrogen. Currently, hydrogen sources are dominantly manufactured with the carbonaceous substances, such as coal and natural gas. These processes result in many greenhouse gases and sulf-compounds, such as CO2, SO2, H2S, and cause serious environmental problems. The g-BL production process proposed in this work eliminates the separate hydrogen preparation procedures leading to wellarranged environmental-friendly clean process.

2. Experimental 2.1. Preparation of catalysts The Cu–Zn–Al catalyst used in this study was prepared via continuous precipitated method. A mixed solution of Cu(NO3)2
3H2O, Zn(NO3)2
6H2O and Al(NO3)3
9H2O salts (1 M of total metal ions) were used as metal precursors with atomic ratio of 4.5:3.2:1, and 1 M Na2CO3 solution was added as the precipitating agent. Precipitation was performed at ca. 75 8C, and the flow rates of the two solutions were adjusted to give a constant pH of ca. 7.5. The precipitate was aged for about 12 h at room temperature before filtering. The filtrated cake was washed with deionized water until no Na+ detected in the filtrate and subsequently dried at 110 8C for 24 h in air atmosphere. The dried catalyst was calcined at 450 8C for 5 h at a heating rate of 10 8C/h. The resulting catalyst was shaped into Ø 5 mm  5 mm particle by a tablet machine, crushed, and then sieved to 20–40 mesh for laboratory tests. 2.2. Catalyst characterization The surface areas of catalysts were measured by Tristar 3000 (Micrometrics) using N2 physisorption. The X-ray diffraction (XRD) spectra were determined on a XRD diffractmeter (D/max RB) by using Ni-filtered Cu Ka radiation in the range 158 < 2u < 708.

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reaction products had been condensed to keep the proper ratio of hydrogen to main reactants (MA, BDO, and (MA + BDO), respectively). The components in the ice trap were identified with a VG Quattro CG/MS (Fisons VG Biotech., Manchester, England), and the contents were determined by a GC-920 gas chromatograph (Shanghai Analyser Co., China) equipped with a flame ionic detector (FID) and a column (3.2 mm  2.0 m) filled with OV-101. The gaseous components (H2, O2, N2, CH4, and CO) were analyzed using an on-line GC equipped with a thermal conductivity detector (TCD) and a column filled with 13 molecular sieve. Because both the differences of temperature gradient in reactor and CO content in tail gas, between single MA hydrogenation, BDO dehydrogenation and coupled reactions are not significant at the micro-reactor level, a pilotplant-scale reactor (i.d. 40 mm, length 5000 mm) was devised to credibly measure these data. The tests for measuring both the hotspot distribution and tail gas were carried out in a pilot-plant-scale reactor, which consisted of a single tube reactor, in which 5.5 kg Cu–Zn–Al catalyst was packed. The reactor was maintained at a desired temperature by circulating hot oil through the jacket of the reactor, and there was a buffer tank for collecting the tail gas and a pump for recycling gas. At the beginning of all tests, N2 was introduced to purge the reaction system, and to replace the air in the reactor, and then replaced by mixed gas of H2 and N2 with 3% H2. The catalyst was pre-reduced in situ by increasing temperature from 25 to 130 8C at the rate of 15 8C/h and 130–280 8C at the rate of 5 8C/h, then keeping at 280 8C for 5 h. After the reduction, the mixed gas was replaced by pure H2, and the vaporized MA and/or BDO feeds in H2 stream were introduced into the reactor from the reactor top. The products coming out of the reactor were cooled and liquid products were collected in a knockout pot. The gases containing H2 and CO from the outlet of the reactor were further cooled with cold brine and recycled back to the reactor through the recycling compressor.

3. Results and discussion 2.3. Apparatus and procedures Generally, the reaction experiments were carried out in a fixed-bed micro-reactor (i.d. 12 mm, length 600 mm). The reaction system had a buffer tank for collecting the tail gas and a pump for cycling gas. Fifteen grams (20–40 mesh) of catalyst was packed. In the beginning of all tests, N2 was introduced to purge the reaction system for replacing the air in the reactor, and then replaced by the mixture of H2 and N2 with 3% H2. The catalyst was pre-reduced in situ by increasing temperature from 25 to 280 8C at the rate of 10 8C/h and then keeping at 280 8C for 2 h. After the reduction, the reactants were introduced into the reactor. The hydrogen in the tail stream was recycled after the

3.1. Effect of temperature and H2/BDO ratio on BDO dehydrogenation to g-BL The dehydrogenation cyclization of BDO over a Cu–Zn– Al catalyst at different reaction temperatures is shown in Table 1. It indicates that the conversion of BDO is nearly complete, and the selectivity of the dehydrogenation reaction to g-BL is very high (>97.5%), in the range of 190–250 8C and at a reaction pressure of 0.03 MPa, over copper-based catalyst. In order to check the thermal equilibrium status of the dehydrogenation cyclization of BDO, the experiment under different hydrogen to BDO ratios was conducted, and the results are summarized in

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Table 1 Reaction temperature influence on BDO dehydrogenation to g-BL T (8C)

190 200 210 230 240 250

BDO conversion (%)

99.2 99.6 100 100 100 100

Selectivity (%) g-BL

BuOHa

Otherb

98.8 98.7 98.6 98.3 98.1 97.5

0.4 0.7 0.9 1.2 1.3 1.7

0.3 0.3 0.5 0.5 0.6 0.8

Scheme 1.

The reaction conditions: 0.03 MPa; LHSV = 0.06 h 1; H2/BDO = 125/1 (molar ratio, recycling hydrogen). a n-Butanol. b Other products: mainly containing THF.

Table 2. It is clear that the conversion of BDO can reach over 99.8% under large hydrogen to BDO mole ratio in the reacting mixture. For all conditions listed in Table 2, it is estimated that the thermal equilibrium constant of BDO dehydrogenation is larger than 108 showing that this reaction can practically irreversibly occur.

of the desired product (g-BL) to an extent with even much lower yield than those in Table 3, for which experiments were conducted in a laboratory micro-reactor. The influence of LHSV on the MA hydrogenation is shown in Table 4. In a wide range of 0.01–0.1 h 1, the conversion of MA is 100% and the selectivity of g-BL is also over 90% at 0.03 h 1. 3.3. Effect of temperature, LHSV and H2/organic ratio on the coupled reaction

Table 3 shows that the almost complete conversion of MA can be achieved and the selectivity of g-BL is more than 90%, over the present Cu–Zn–Al catalyst at the range of 265–290 8C. However, the yield of g-BL substantially varies with the operating temperature, and the best is 93.6% at 275 8C. With reaction temperature of 250–265 8C, the conversion of MA is 95–99%, and the selectivity of g-BL is 82–90%. The SA is an intermediate in series MA hydrogenation, and is formed remarkably, while this reaction temperature retains below 2608C. Scheme 1 shows the complexity of the hydrogenation of MA. It is obvious that the MA hydrogenation can produce not only the desired product g-BL but also other by-products, depending on the reaction conditions. The non-uniform temperature profile in an industrial multi-tubular reactor generally lowers the yield

The coupled reaction was conducted using MA LHSV 0.04 h 1, and a BDO LHSV of 0.06 h 1 to approach the stoichiometry of the coupled reaction (3). It is observed that the complete conversions of MA and BDO are reached (Table 5), indicating that the conversion efficiency for hydrogenation of MA has been maintained at the same level of the single hydrogenation reaction (Tables 3 and 4) with co-production of the same important product, g-BL through BDO dehydrogenation under present operation conditions. Moreover, the coupled reaction has significantly improved the selectivity especially from MA to g-BL compared to those in separated reactions (Tables 3 and 4). It can be found that the yield to g-BL from MA hydrogenation optimum conditions is increased by about 4%, and that to g-BL from BDO dehydrogenation is increased by about 1% due to the coupling effect. For example, with MA single hydrogenation at 250 8C, the conversion to g-BL is raised by about 4%, and selectivity to g-BL is increased by about 14%. From Tables 5–7, it is obvious that the optimum reaction conditions of combined processes are

Table 2 Influence of H2 /BDO ratio on BDO dehydrogenation to g-BL

Table 3 Influence of temperature on the hydrogenation of MA to g-BL

3.2. Effect of temperature and LHSV on MA hydrogenation to g-BL

H2/BDO (mol/mol) 45 85 100 125 150 165

BDO conversion (%)

100 100 100 100 100 99.8

T (8C)

Selectivity (%) a

MA conversion (%)

b

g-BL

BuOH

Other

96.9 97.5 98.1 98.8 99.1 99.2

2.3 1.8 1.4 0.8 0.7 0.6

0.8 0.7 0.5 0.4 0.2 0.2

The reaction conditions—temperature: 245 8C; pressure: 0.03 MPa; LHSV: 0.06 h 1; H2/BDO: 125/1 (molar ratio, recycling hydrogen). a n-Butanol. b Other products: mainly containing THF.

250 260 265 275 290

95.2 98.5 99.7 100 100

Selectivity (%) g-BL

SA

BuOHa

PrOHb

Otherc

82.4 88.6 91.2 93.6 89.5

14.9 6.6 2.9 – –

1.7 2.8 3.2 3.8 5.7

0.8 1.7 2.1 2.8 4.1

0.2 0.3 0.4 0.6 0.7

MA vapor hydrogenation conditions—pressure: 0.03 MPa; LHSV: 0.04 h 1; H2/MA: 125/1 (molar ratio, recycling hydrogen). a n-Butanol. b n-Propanol. c Other products: mainly propionic acid, butanoic acid, etc.

Y.-L. Zhu et al. / Applied Catalysis B: Environmental 57 (2005) 183–190

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Table 4 Influence of LHSV on the hydrogenation of MA to GBL

Table 6 Influence of LHSV on the coupling of the BDO and MA reactions

LHSV (h 1) MA conversion (%) Selectivity (%)

LHSV (h 1) a

0.01 0.03 0.04 0.07 0.1

100 100 100 100 100

b

c

g-BL SA BuOH

PrOH

Other

83.1 87.8 94.1 92.9 92.5

6.2 4.3 2.3 2.2 1.9

0.8 0.5 0.4 0.3 0.1

– – – 0.6 2.1

9.9 7.4 4.1 4 3.4

0.05 0.1 0.13 0.16 0.2

Conversion (%)

Selectivity (%)

MA

BDO

g-BL

SA

BuOHa

THF

Otherb

100 100 100 100 100

100 100 100 100 100

96.5 98.3 98.5 98.3 97.4

– – – 0.5 1.5

2.1 0.9 0.9 0.9 0.7

0.9 0.5 0.4 0.3 0.4

0.5 0.3 0.2 – –

MA vapor hydrogenation conditions—pressure: 0.03 MPa; temperature: 270 8C; H2/MA = 125/1 (molar ratio, recycling hydrogen). a n-Butanol. b n-Propanol. c Other products: mainly propionic acid, butanoic acid, etc.

MA vapor hydrogenation conditions—pressure: 0.03 MPa; temperature: 245 8C; H2/MA + BDO: 125/1 (molar ratio, recycling hydrogen); g-BL selectivity = [products moles of g-BL produced by BDO and MA]/(moles of gross product)  100. a n-Butanol. b Other products: mainly propionic acid, butanoic acid etc.

summarized as reaction temperature 240 8C, LHSV 0.1 h 1, and H2/organic 80, and the selectivity of gBL is over 98% with complete conversion. When we compare the product distributions of single MA hydrogenation at 260 8C, 0.04 h 1 (Table 3) with that of the coupling reaction at 260 8C, 0.1 h 1 (Table 5) in detail, it is clear that total by-products dramatically decrease in the coupled process, from ca. 4.8% drop to ca. 1.4%, and especially the de-carbonization products of n-propanol can be remarkably suppressed. In addition, the coupled reactions improve the catalytic hydrogenation ability, for example, in single MA hydrogenation at 260 8C, 0.04 h 1, experimental result shows that 6.6% SA intermediate is not converted into products (Table 3), in contrast with this, in the coupled reaction at 260 8C, 0.04 h 1, it presents that SA intermediate can be hardly detected in product mixtures (Table 6). Comparing the product distribution (Tables 5–7) with that of single MA hydrogenation, it is found that this combined reaction is efficient and environmentally friendly. Furthermore, the coupled processes of both MA hydrogenation and BDO dehydrogenation only produce one desired product, gBL, leading to easy separation of the product mixture from the coupled system, and indicating less additional costs by the coupled process for industrial applications. These

advantages may impose great interests for industrialization of the coupled synthesis of the important product, g-BL.

Table 5 Influence of temperature on the coupling of the BDO and MA reactions

Table 7 Influence of H2/organic on the coupling of the BDO and MA reactions

T (8C)

H2/organic (mol/mol) Conversion (%) Selectivity (%)

220 230 240 250 260

Conversion (%)

Selectivity (%)

MA

BDO

g-BL

SA

BuOHa

THF

Otherb

100 100 100 100 100

100 100 100 100 100

97.2 97.7 98.5 98.6 98.1

2.3 1.3 0.2 – –

0.3 0.5 0.6 0.6 0.7

0.2 0.3 0.4 0.3 0.5

– 0.2 0.3 0.5 0.7

MA vapor hydrogenation conditions—pressure: 0.03 MPa; LHSV (MA + BDO): 0.1 h 1; H2/MA + BDO = 125/1 (molar ratio, recycling hydrogen); g-BL selectivity = [products moles of g-BL produced by BDO and MA]/(moles of gross product)  100. a n-Butanol. b Other products: mainly propionic acid, butanoic acid, etc.

3.4. Effect of CO content on MA hydrogenation and the coupled reaction ability As shown in Scheme 1, MA single hydrogenation produces not only the intermediate of SA and the desired product of g-BL, but also some other by-products of nbutanol, butanoic acid and so on. These compounds mainly including SA, and g-BL can be further hydrogenated and decarbonized to form C3 compounds, i.e. n-propanol and/or propionic acid; meanwhile, an almost equivalent amount of CO compound is always produced [32]. Although the CO compound is the one of the many by-products during MA hydrogenation process, however, it can be gradually accumulated in the recycling tail gas system. When its content reaches a certain value (e.g. more than 10%) in recycled tail gas of an industrial plant, the activity of catalyst will significantly be suppressed, which lead to low yield of the desired product of g-BL [32]. In order to keep the reaction systems maintaining sufficient effective H2 content, the CO formed during the reaction must be eliminated from this recycled system. This can be done by means of: (i)

45 85 100 125 150 165

MA

BDO

g-BL SA BuOHa THF Otherb

100 100 100 100 100 100

100 100 100 100 100 100

96.5 98.1 98.5 98.3 98.1 97.8

– – – – 0.2 0.4

2.1 0.9 0.9 0.9 1.3 1.4

0.9 0.5 0.4 0.3 0.4 0.4

0.5 0.3 0.2 – – –

MA vapor hydrogenation conditions—pressure: 0.03 MPa; temperature: 250 8C; LHSV (MA + BDO): 0.1 h 1, g-BL selectivity = [products moles of g-BL produced by BDO and MA]/(moles of gross product)  100. a n-Butanol. b Other products: mainly propionic acid, butanoic acid, etc.

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Table 8 Effect of duration time on MA hydrogenation to g-BL yield and the accumulated CO content

Table 9 Effect of reaction time on the coupled process to g-BL and the accumulated CO content

Duration (h)

Time (h) Conversion (%) Selectivity (%)

0.5 1.0 1.5 2.0 2.5

Conversion (%)

100 100 100 100 100

Yield (%)

CO (%)

g-BL

PrOHa

Othersb

93.2 92.9 92.8 92.5 91.2

2.2 2.2 2.2 2.1 2.1

4.6 4.6 4.6 5.3 6.6

1.2 2.4 3.6 4.8 5.9

MA vapor hydrogenation conditions: 275 8C, 0.03 MPa, LHSV: 0.04 h 1, H2/MA: 50/1 (molar ratio, recycling hydrogen). a n-Propanol. b Other products: mainly n-butanol, SA, THF butanoic acid, etc.

selective adsorption of tail gas; (ii) direct release of the tail gas. The H2 consumption will dramatically depend upon CO formation, which results in considerable rise of production cost of g-BL. In order to sufficiently observe the CO accumulation in recycle reaction system and its effect on MA hydrogenation, the experiments of MA vapor hydrogenation were carried out in a pilot-plant-scale reactor with recycled tail gas, and the pure H2 was continuously introduced to this system to keep the reaction pressure in the system, in which 5.5 kg Cu–Zn–Al catalyst was packed. In one case, the tail gas of the MA hydrogenation was completely recycled into the reactor for 2.5 h instead of partly released to the atmosphere, and the experimental results were listed in Table 8. The influences of CO content on MA hydrogenation to g-BL with completely recycled tail gas were shown in Fig. 1. In Table 8, a 100% MA conversion with approximately 92% yield to g-BL was obtained. Since the CO is produced by de-carbonization, it can gradually accumulate in the recycling tail system. If the CO content reaches a certain value in an industrial plant with recycled tail gas, the catalytic activity of MA hydrogenation will dramatically be

Fig. 1. Influence of CO content on MA hydrogenation to g-BL. [The MA vapor hydrogenation conditions: 275 8C, 0.03 MPa, LHSV 0.04 h 1; H2/ MA molar ratio ca. 50/1 (recycling hydrogen); by-products: n-propanol, nbutanol, propionic acid butanoic acid, etc.]

25 85 100 130 175 200

CO (%)

MA

BDO

g-BL SA BuOHa THF Otherb

100 100 100 100 100 100

100 100 100 100 100 100

97.5 97.8 98.2 98.1 97.5 97.2

– – – – 0.3 0.8

1.3 1.4 1.2 1.1 0.9 0.9

0.8 0.5 0.4 0.5 0.9 0.8

0.5 0.3 0.2 0.3 0.4 0.3

0.9 3.5 3.9 5.2 6.9 7.8

MA vapor hydrogenation conditions: pressure: 0.03 MPa; temperature: 250 8C; H2/MA + BDO: 125/1 (molar ratio, recycling hydrogen); LHSV 0.1 h 1; g-BL selectivity: [products moles of g-BL produced by BDO and MA]/(moles of gross product)  100. a n-Butanol. b Other products: mainly propionic acid, butanoic acid, etc.

inhibited, which lead to low yield of the desired product of g-BL. From Fig. 1, it can be seen that while the CO contents increase from 2 to 35% in the mixed gases of H2 and CO, MA hydrogenation activity significantly decreases from 100 to 88%, and the desired product yield of g-BL greatly drops from 93 to 67% with the increase of SA intermediate from 0.2 to ca. 9%. In a summary, the increase of CO content suppresses the activity of Cu–Zn–Al catalyst, which is a great disadvantage for the formation of desired g-BL. In a 200 h test for the combined process, the tail gas of the coupled reaction was totally recycled into the reactor instead of partly released to the atmosphere, and the experimental results were showed in Table 9. In contrast with the separated MA hydrogenation (Table 8 and Fig. 1), a 100% conversion of two reactants with approximately 97% yield to g-BL were obtained during the whole test period, and the CO accumulation in the recycled tail gas is significantly lowered. For example, the CO content of single MA hydrogenation reaches ca. 6% in the time of 2.5 h in recycled tail gas; however, in the coupled reaction, it takes 170 h to reach the similar CO content. In addition, at the same level of CO concentration, the activity of catalyst for single MA hydrogenation is suppressed more than that of the coupled reaction. The possible reason for this is that the donor activated hydrogen from BDO dehydrogenation favours the hydrogenation of MA in the coupled reaction, which is different from the single MA hydrogenation process in which the activity is remarkably influenced by the effective concentration of H2. As shown in Fig. 1, while the CO content increases up to ca. 7% in the mixed gases of H2 and CO, reaction activity can be suppressed in the single MA hydrogenation. In contrast with this, the coupled reaction greatly inhibits the formation of by-products, namely n-propanol and CO (Table 9), and can steadily perform for ca. 200 h without releasing tail gas. So this presents a great advantage for the formation of desired g-BL, and facilitates the decrease of production cost in industrial plant.

Y.-L. Zhu et al. / Applied Catalysis B: Environmental 57 (2005) 183–190 Table 10 The temperature gradient along pilot-scale fixed-bed reactor Bed height (mm)

BDOa dehydrogenation (8C)

MAb hydrogenation (8C)

Coupledc reaction (8C)

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

210 211 213 214 214 215 216 217 218 218 219 219 220 220 220

298 298 297 297 296 295 292 290 288 286 285 283 278 275 273

256 256 256 255 255 254 254 253 253 252 252 252 252 251 251

a BDO vapor dehydrogenation conditions: pressure: 0.03 MPa; temperature: 220 8C; H2/BDO: 30/1 (molar ratio, recycling hydrogen); LHSV 0.1 h 1. b MA vapor hydrogenation conditions: pressure: 0.03 MPa; temperature: 270 8C; H2/MA: 125/1 (molar ratio, recycling hydrogen); LHSV 0.05 h 1. c Coupled reaction conditions: pressure: 0.03 MPa; temperature: 250 8C; H2/MA + BDO: 125/1 (molar ratio, recycling hydrogen); LHSV 0.1 h 1.

3.5. Distribution of reaction hotspot along the fixed-bed reactor The separated hydrogenation of MA, the dehydrogenation of BDO, and the coupled reaction were carried out in a pilot-plant fixed-bed reactor over a Cu–Zn–Al catalyst so as to shed light on the difference between hot spots or cold spots. Since this industrial single tube (i.d. 40 mm) reactor height is 5000 mm, a length of about 1500 mm catalytic bed is chosen to measure the reaction temperature, and the catalytic bed top is set as starting point. As shown in Table 10, nearly at the same conversion (ca. 99.5%) of reactants, when conventional BDO dehydrogenation cyclization to gBL (0.03 MPa, 220 8C, LHSV 0.1 h 1), single MA hydrogenation to g-BL (0.03 MPa, 270 8C, LHSV 0.05 h 1), and the coupled reaction to g-BL (0.03 MPa, 250 8C, LHSV 0.1 h 1), are carried out in the reactor, respectively, the temperature gradients are ca. 10, 25, and 4 8C, respectively. It is well known that the high hot spots will accelerate the polymerization of MA or BDO not only between the particles leading to clogging of reactor tubes (pressure drop), but also in the pores leading to coke deposition on the catalyst (pore blocking). Since the coupling operation has the relatively smaller heat release compared with seperated hydrogenation reaction, it has the most uniform temperature profile, which results in good control of reaction temperature, and improves yield of desired g-BL. In order to explain the enhancement in the catalyst performance observed in the coupled reaction, we propose that the activated hydrogen species on the catalyst surface due to BDO dehydrogenation probably plays important roles

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in improving the yield of MA hydrogenation. In an industrial process, other factors like improved temperature profile along the reactor may further enlarge the effect substantially, suggesting a potential of using the combined reaction in practical applications. 3.6. Catalyst characterization The XRD patterns of the fresh Cu–Zn–Al catalyst without calcination and the calcined catalyst were measured. For fresh catalyst, three phases can be identified as (Cu, Zn)2CO3(OH)2, (Cu0.3, Zn0.7)5(CO3)2(OH)6 and CuZn(CO3)(OH)2. When the fresh Cu–Zn–Al catalyst were heated at 450 8C for 5 h, this calcined catalyst mainly contains three sorts of crystallite peaks: CuO, ZnO and Al2O3, and has a surface area of 55.5 m2/g. In our recent work, the similar Cu–Zn–Al catalyst had been studied in detail [33].

4. Conclusions In summary, we have demonstrated a coupled catalytic process, in which hydrogen transfer from BDO to MA can practically be realized to produce g-BL. Compared to conventional processes, the coupled process in this work has several advantages: improved yield, good energy efficiency, and optimal hydrogen utilization. In addition, compared with MA hydrogenation process, the coupled process can also be conducted at a low reaction temperature for getting nearly the same efficiency, otherwise, and can dramatically inhibit the formation of de-carbonation compounds, npropanol and CO. This shows that the rich, activated hydrogen species on the catalyst surface from BDO dehydrogenation might promote the MA hydrogenation reaction. The coupled operation leads to other advantages, such as the increase of total yield to the desired products of g-BL, and easy temperature control in a tubular fixed-bed due to its moderate heat release compared to the single MA hydrogenation reaction, which can avoid the formation of apparent hotspots or coldspots in the practical process. In addition, the simplified technical procedure by ‘no-hydrogen-supply’ operation exhibits the improved technology.

Acknowledgements We are indebted to the Natural Science Foundation of China (No. 20276077) and Shanxi Natural Science Foundation (No. 20021023) for financial support.

References [1] R.A.W. Johnstone, A.H. Wilby, Chem. Rev. 85 (1985) 129. [2] G. Zassinovich, G. Mestroni, Chem. Rev. 92 (1992) 1051. [3] N. Harris, M.W. Tuck, Hydrocarbon Process 69 (1990) 79.

190

Y.-L. Zhu et al. / Applied Catalysis B: Environmental 57 (2005) 183–190

[4] W. De Thomas, P.D. Taylor, H.F. Tomfohrde III, US Patent 5,149,836 (1992). [5] P.D. Taylor, W.D. Thomas, D.W. Buchanan Jr., US Patent 5,347,021 (1994). [6] M.J. Bergfeld, K. Uihlein, US Patent 6,075,153 (2000). [7] M. Bergfeld, G. Wiesgickl, US Patent 5,536,849 (1996). [8] G.L. Castiglioni, C. Fumagalli, US Patent 6,297,389 (2001). [9] S.H. Cho, T.H. Kim, K.I. Kim, S.C, Cho, J.K. Park, H.D. Jeong, H.C. Bajaj, US Patent 6,380,402 (2002). [10] U.R. Pillai, E. Sahle-Demessie, Chem. Commun. (2002) 422. [11] U. Herrmann, G. Eming, Ind. Eng. Chem. Res. 36 (1997) 2885. [12] R. Lancia, A.Vaccari, C. Fumagalli, E. Armbruster, US Patent 5,698,713 (1997). [13] A.M. Brownstein, Chem. Tech. 21 (1991) 506. [14] G.L. Castiglioni, A. Vaccari, G. Fierro, M. Inversi, M.L. Jacono, G. Minelli, I. Pettiti, P. Porta, M. Gazzano, Appl. Catal. A: Gen. 123 (1995) 123. [15] D.S. Thakur, B.R. Roberts, T.J. Sullivan, A.L. Vichek, US Patent 5,155,086 (1992). [16] R.W. Wegman, D.R. Bryant, US Patent 5,191,091 (1993). [17] K.A. McLachlan, P.S. Williams, European Patent 400,904 (1990). [18] G. Bjornson, J. Sturk, US Patent 5,086,030 (1992). [19] Y. Hara, H. Kusaka, H. Inagaki, K. Takahashi, K. Wada, J. Catal. 194 (2000) 188.

[20] M.W.M. Tick, M.A. Wood, A.G. Hiles, PCT Int. Patent, Appl. No. WO 9,743,234 (1997). [21] G.L. Castiglioni, M. Ferrari, A. Guercio, A. Vaccari, R. Lancia, C. Fumagalli, Catal. Today 27 (1996) 181. [22] A. Kuksal, E. Elemm, G. Emig, Appl. Catal. A: Gen. 228 (2002) 237. [23] U.R. Pillai, E. Sahle-Demessie, D. Young, Appl. Catal. B: Environ. 43 (2003) 131. [24] A. Cybulski, J. Chrzaszcz, M.V. Twigg, Catal. Today 69 (2001) 241. [25] S.M. Jung, E. Godard, S.Y. Jung, K.-C. Park, J.U. Choi, Catal. Today 87 (2003) 171. [26] M. Messori, A. Vaccari, J. Catal. 150 (1994) 177. [27] I. Tatsumi, K. Kenji, S. Sadakatsu, U. Hiroshi, European Patent 584,408 (1994). [28] I. Tatsumi, M. Shigesato, S. Sadakatsu, U. Hiroshi, Japan Patent 0,525,151 (1993). [29] S. Sadakatsu, F. Takeaki and U. Hiroshi, Japan Patent 03,232,825 (1991). [30] B.H. Edward, PCT Int. Patent, Appl. No. WO 9,200,973 (1992). [31] Y.L. Zhu, H.W. Xiang, G.S. Wu, L. Bai, Y.W. Li, Chem. Commun. (2002) 254. [32] Y.L. Zhu, G.-W. Zhao, J. Chang, J. Yang, H.Y. Zheng, H.W. Xiang, Y.W. Li, Catal. Lett. 96 (2004) 123. [33] J. Yang, H.Y. Zheng, Y.L. Zhu, G.-W. Zhao, C.-H. Zhang, B.-T. Teng, H.-W. Xiang, Y.W. Li, Catal. Commun. 5 (2004) 505.