Highly Selective Epoxidation of Propylene in a Low-Pressure

Article pubs.acs.org/IECR

Highly Selective Epoxidation of Propylene in a Low-Pressure Continuous Slurry Reactor and the Regeneration of Catalyst Meng Liu, Xiaoxue Ye, Yangqing Liu, Xiangyu Wang,* Yiqiang Wen,* Haijie Sun, and Baojun Li College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan, 450001, P.R. China S Supporting Information *

ABSTRACT: The epoxidation of propylene to propylene oxide with H2O2 using a micron-sized TS-1 catalyst with hollow structure in a low-pressure continuous slurry reactor was investigated. Under the optimized reaction conditions, the process was successfully run for 247 h with superior selectivity (98.7%) and yield (96.4%) of propylene oxide. The reaction pressure was only 0.23 MPa which was much lower than that in the fixed-bed, and the catalyst stability was excellent. Though the deactivation of catalyst occurred after 247 h, the selectivity to propylene oxide was still kept above 97.5%, which was significantly better than the experimental results in the fixed-bed reactor. Washing deactivated catalyst with H2O2 can easily remove bulky byproducts deposited inside micropores and recover catalytic activity. This research verified the safety and high catalyst efficiency of epoxidation of propylene in a slurry reactor.

1. INTRODUCTION Propylene oxide (PO) is an important fine chemical intermediate in the manufacture of polyether polyols, propylene glycol, etc.1,2 Its worldwide production capacity in 2012 was around 9.69 million tons and continues to increase with the market demand. The dominant industrial technologies for PO manufacture are the chlorohydrin process and variations of the organic hydroperoxide process.3 These classic commercial technologies generate significant amounts of coproducts. For example, the chlorohydrin process produces chlorinated organic byproducts and CaCl2 and entails serious environmental pollution issues. The organic hydroperoxide process imposes fewer threats to the environment, but the process involves very complicated chemical installations for the recycle or separation of coproducts and requires very high capital investment.4 Owing to environmental concerns in recent years, an environmentally benign as well as economically practical production technology for PO is very desirable.5−9 Although much recent research has been focused on the direct epoxidation of propylene with molecular oxygen as oxidant,10−14 those routes have not yet be commercialized, because the current available catalysts are not sufficiently active, selective, or stable. Compared with classic commercial processes, direct epoxidation of propylene to PO with hydrogen peroxide (HPPO) is an ideal method due to its cleanness, high-efficiency, mild operating condition, and low investment.15,16 Since TS-1 zeolite has been shown to be an efficient catalyst for the epoxidation of propylene with H2O2, there are many researchers dedicated to this technology.17−24 Clerici et al.18,19 investigated the epoxidation of propylene to PO with H2O2 catalyzed by titanium silicalite in methanol under mild conditions. Thiele et al.20 studied the factors determining the catalytic activity and selectivity of titanium silicalite in propylene epoxidation. Wang and co-workers21,22 prepared various TS-1 catalysts for the propylene epoxidation and found that the TS-1/SiO2 catalyst treated with TPAOH solution © XXXX American Chemical Society

exhibited higher catalytic activity and stability. Chadwick, Di Serio, and Wang and co-workers systematically investigated the kinetics of propylene epoxidation, and these remarkable works were very useful for the design of the HPPO reactor and optimization of reaction conditions.23−25 In addition, there are many reports in the literature about bifunctional Au/TS-1 and Au/TiO2 catalysts for direct epoxidation of propylene with H2O2 formed in situ from H2 and O2.26−31 In 1998, Haruta and co-workers26 reported for the first time a nanogold catalyst supported on TiO2 that could selectively catalyze the epoxidation of propylene with H2 and O2 to produce PO. Nijhuis and co-workers27,28 had investigated the mode of operation of gold-on-titania catalysts for the epoxidation of propylene and studied the kinetics of propylene epoxidation with H2 and O2 over a Au/Ti-SiO2 catalyst in a wide range of reactant concentrations including the explosive region in a microreactor. Wu and co-workers29 prepared novel core/shellstructured composite materials of Au/TS-1@meso-SiO2, which were active and selective for the direct epoxidation of propylene to PO with H2 and O2. Li and co-workers30 had successfully prepared the new bioreduction Au/TS-1 catalysts by immobilizing the biosynthesized Au sol on TS-1 supports and systematically analyzed technological parameters for vapor phase propylene epoxidation with H2/O2 over bioreduction Au/TS-1 catalysts. The high and uniform dispersion of Au nanoparticles (4−6 nm) onto an as-synthesized TS-1 zeolite by employing a deposition-precipitation method using urea had been prepared by Lu and co-workers.31 The direct epoxidation of propylene with H2O2 formed in situ from H2 and O2 may greatly reduce the cost of production, which implies the HPPO process will possess a broader prospect in the future. Received: January 29, 2015 Revised: April 24, 2015 Accepted: May 4, 2015

A

DOI: 10.1021/acs.iecr.5b00410 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Diagram of Experimental Apparatus

is quite low, and the catalytic performance is unstable in these reports.25,49 The aim of the present work was to develop an efficient process for the highly selective epoxidation of propylene. Thus, a low-pressure continuous slurry reactor has been designed by employing suitable powdery TS-1 catalyst, and one stainless steel sintered filter was used as the separator for in situ separation of catalysts from the products. The various operating parameters (such as concentration of catalyst, residence time, pH value of reaction solution, reaction temperature, the molar ratio of propylene to H2O2, and stirring rate) were systematically optimized, and the stability of the catalyst and the safety of the reactor were tested under optimum reaction conditions for a long running time; the deactivated catalyst and regeneration methods were also investigated in detail.

Commercial scale plants of HPPO process had been installed, mostly by Evonik (former Degussa), BASF, Dow, and Uhde. In 2008, Evonik had launched the first commercialscale propylene oxide plant, based on the HPPO technology. The next year, a 300 kt/a PO industrial plant was built and put into production in cooperation between BASF and Dow.32,33 Besides those processes which had been or are coming close to commercialization, presently, there are more pilot plants being designed and operated for the process development of propylene epoxidation. For example, 1.0 and 1.5 kt/a pilot plants of the HPPO process were successfully run in SINOPEC and Tianjin Dagu Chemical Co., Ltd., respectively,34,35 and a 100 t/a pilot plant of propylene epoxidation was built by Dalian University of Technology.36 However, some challenges still remain in the current industrial processes. All of the installed HPPO processes were operated in fixed-bed reactors; thus, the TS-1 catalyst must be extruded with inorganic binder or immobilized onto carriers. First, powdery catalyst was shaped into bodies at the expense of reducing the effective surface area of catalyst, which led to the fact that catalytic activity decreased, compared with the counterparts highly dispersed in suspension.37,38 Second, the aluminates and transition metal oxides existing in inorganic binder can promote the H2O2 decomposition and PO solvolysis (hydrolysis and/or alcoholysis), which decreased the utilization of reactant and increased the risk of the process.39−41 Besides that, the on-stream pressure of the fixed bed is usually as high as 3 MPa to ensure propylene as liquid and overcome bed resistance, which resulted in an increase of subsequent energy consumption.22,36,39,41 Compared with the fixed-bed reactor, the slurry reactor has the advantages of simple construction, excellent heat transfer performance, online catalyst addition and withdrawal, and a reasonable interphase mass transfer rate with lower energy input.42 Furthermore, the catalyst in the slurry reactor is highly dispersed and does not require too much inorganic binder or catalyst carrier for shape. The application of slurry reactor in the catalytic reaction process can increase catalytic efficiency and reactant utilization and decrease the side reaction.43,44 As a result, there is a trend of shifting from the fixed-bed reactor to the slurry reactor in many chemical processes.45,46 There are lots of reports on the reaction conditions and catalyst deactivation/regeneration for ammoximation over TS-1 catalyst in the slurry reactor.47,48 However, there are only a few detailed studies on the epoxidation of propylene with H2O2 over TS-1 catalyst in a continuous slurry reactor; the average yield of PO

2. EXPERIMENTAL SECTION 2.1. Preparation of the Catalyst. 2.1.1. Preparation of Original Micron-Sized TS-1 Zeolite (Denoted as Sample MTS1). The row powder of TS-1 (Si/Ti = 33) was hydrothermally synthesized using tetrabutyl titanate (TBOT), colloidal silica, ethanolamine, and tetrapropylammonium bromide (TPABr). In a typical synthesis, ethanolamine and TPABr was dissolved in deionized water under stirring, followed by addition to colloidal silica, and then, TBOT-isopropanol solution was added dropwise with vigorous stirring. The obtained mixture was heated to remove isopropanol. Afterward, the composition of the final gel was maintained in the ratio of 1.0 SiO2/0.033 TiO2/0.1 TPABr/0.5 ethanolamine/30 H2O. The gel was transferred to a stirred autoclave and crystallized for 72 h at 175 °C. Finally, the product was filtered, washed with distilled water, dried at 120 °C, and calcined at 550 °C for 6 h. 2.1.2. Preparation of Micron-Sized TS-1 with Hollow Structure (Denoted as Sample MTS-1H). The MTS-1 sample was modified in the hydrothermal solution consisting of ethanolamine and TPABr at 175 °C for 48 h. The finally product was recovered by filtration, washed with distilled water, treated with dilute H2SO4 and H2O2 solution, dried at 120 °C, and calcined at 550 °C for 6 h. 2.1.3. Preparation of the Nanosized and the SubmicronSized TS-1 Catalysts. Nanosized TS-1 (denoted as sample NTS-1) and submicron-sized TS-1 (denoted as sample STS-1) were synthesized using the reported method50 and according to ref 51, respectively, for comparison. B


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Industrial & Engineering Chemistry Research

regeneration with methanol and 90 °C for the regeneration with H2O2 or NH3·H2O. Then, the catalyst was separated from the mixture by centrifugation, washed with distilled water, and dried at 60 °C in vacuum. For the regeneration by calcination, the deactivated catalyst was calcined at 550 °C under air for 6 h. 2.4. Collection and Analysis of Coke Composition. The coke compositions deposited on the surface of the deactivated catalyst were extracted first with trichloromethane. Then, the extracted catalyst was dissolved in 40 wt % hydrofluoric acid, and the obtained solution was extracted by trichloromethane to collect the coke compositions deposited in zeolite channels. Finally, the extracts were concentrated by evaporating the solvent and analyzed using a Thermo Fisher Scientific DSQ II series gas chromatograph and mass spectrometer system. 2.5. Characterizations. Powder X-ray diffraction (XRD) was performed on a Panalytical X’pert PRO Diffractometer with Cu Kα (λ = 1.5406 Å). The relative crystallinity (RC) was calculated from total integrated intensities of the reflections at 2θ = 7.9°, 8.9°, 23.1°, 23.9°, and 24.4° based on those of fresh TS-1 which was considered to be 100%. Fourier transform infrared (FT-IR) spectra were recorded in the region of 4000− 400 cm−1 on a Nicolet Nexus 470 FT-IR spectrometer, and the samples to be measured were ground with KBr and pressed into thin wafers. Ultraviolet−visible diffuse reflectance (UV− vis) spectra were obtained on an Agilent Cary 5000 spectrometer from 190 to 500 nm, and pure BaSO4 was spent as reference. The element compositions were determined by X-ray fluorescent spectrometry (XRF, Bruk S4 Pioneer, Germany). N2 sorption isotherms were measured on NOVA1000e (Quantachrome, USA) at 77 K, and pore size distribution was calculated using a desorption curve according to the Barrett−Joyner−Halenda (BJH) method. Thermogravimetric (TG) analysis was carried out in a Netzsch Sta 409 PC/ PG apparatus in air atmosphere with a heating rate of 10 °C/ min and temperature up to 800 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained with a S-4800 scanning microanalyser and JEM-2100 electron microscope, respectively.

2.1.4. Preparation of TS-1 Extrudate. TS-1 extrudate was prepared according to the procedure described in the literature.39 2.2. Epoxidation of Propylene. 2.2.1. Continuous Reaction. As shown in Scheme 1, the epoxidation of propylene to PO was carried out in a 250 mL continuous slurry reactor equipped with a water-jacket heater, and a stainless steel sintered filter was placed in the reactor and connected to the liquid outlet to separate catalyst from the reaction mixture. After the reactor was charged with TS-1 catalyst, methanol, and H2O2 (27.5 wt %), the propylene was then fed by a gas mass flow controller, and the continuous reaction was started. The controlled volume pumps were used to feed methanol and H2O2 with the volume ratio of 8:1 into the reactor, while another pump was used to discharge the mixture from the reactor continuously. The solution of the product was sampled and analyzed in a certain interval to monitor the constancy of the reaction. The epoxidation of propylene was carried out in fixed-bed reactor for comparison, and the reaction conditions were listed in the Supporting Information. 2.2.2. Batch Reaction. A 200 mL autoclave reactor was charged with 0.13 g of TS-1 catalyst, 3 mL of H2O2 (27.5 wt %), and 24 mL of methanol, and the pH of the reaction mixture was adjusted with dilute ammonia solution. The mixture was stirred and warmed to 45 °C, and then, propylene was charged at constant pressure (0.6 MPa); the reaction time was 60 min. The residual H2O2 was determined by iodometric titration. The products were analyzed on a GC 7890II gas chromatograph using a flame ionization detector and a capillary column (30 m × 0.32 mm × 0.5 μm). The conversion of H2O2 (X(H2O2)), selectivity to PO (S(PO)), utilization of H2O2 (U(H2O2)), and yield of PO (Y(PO)) were calculated as follows: The conversion of H2O2: X(H 2O2 ) =

n H0 2O2 − n H2O2 n H0 2O2

× 100%

The selectivity to PO: S(H 2O2 ) =

nPO

3. RESULTS AND DISCUSSION 3.1. Screening of Titanosilicate Catalysts. Four TS-1 zeolites (MTS-1, MTS-1H, NTS-1, and STS-1) were prepared, and the XRD patterns (Figure S1, Supporting Information) showed that all samples had the expected crystalline structure of MFI type. The FT-IR (Figure S2, Supporting Information) and UV−vis spectra (Figure S3, Supporting Information) of the samples showed characteristic adsorption bands at 960 cm−1 and 210 nm, respectively, which demonstrated that the tetrahedral Ti species existed in the zeolite framework.52,53 The bands at 240−270 and 300−310 nm indicated the existence of octahedral Ti and anatase TiO2, respectively.54,55 Only a limited amount of titanium could be inserted into the framework. The maximum was reported to be 2.5% (molar fraction), and the excess nonframework Ti existed as octahedrally coordinated Ti or anatase TiO2.56 As determined by XRF analysis, the Si/Ti of MTS-1, MTS-1H, NTS-1, and STS-1 catalysts were 32.56, 31.10, 32.43, and 32.78, respectively. Figure 1 showed the SEM and TEM images of the four samples. It could be seen that both the MTS-1 and MTS-1H had uniform shape and the approximate particle size range of 1.1 × 2.2 μm (Figure 1, M1 and M2). The particles of MTS-1H contained many voids consisting of mesopores and macropores with the average pore size of 110 nm (Figure 1,

nPO × 100% + nMME + nPG

The utilization of H2O2: U (H 2O2 ) =

nPO + nMME + nPG n H0 2O2 × X(H 2O2 )

× 100%

The yield of PO: n Y (PO) = 0PO × 100% n H2O2 MME and PG stand for propylene glycol monomethyl ethers and propylene glycol, respectively, which are the main byproducts. The n0H2O2 and nH2O2 represent the initial and final amount of H2O2 in molar, respectively. The nPO, nMME, and nPG stand for the molar content of PO, MME, and PG, respectively. 2.3. Catalyst Regeneration. After the continuous reaction, the used TS-1 catalyst was gathered by centrifugation and dried at 60 °C in vacuum and then regenerated. For the regeneration by washing, methanol, 10 wt % NH3·H2O, or 45 wt % H2O2 as detergent was added to a flask containing the deactivated catalyst. The mixture was stirred for 5 h at 65 °C for the C


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suitable catalyst for epoxidation of propylene in a continuous slurry reactor. 3.2. Reaction Condition. 3.2.1. Effect of Concentration of Catalyst. The concentration effect of catalyst was investigated by varying the concentration of MTS-1H catalyst between 5 and 33 g·L−1. As shown in Figure 2, the initial conversion of

Figure 1. SEM and TEM images of MTS-1 (M1,T1), MTS-1H (M2, T2), NTS-1 (M3, T3), and STS-1 (M4, T4).

T2). In comparison, the average particle size of NTS-1 and STS-1 was 260 and 710 nm, respectively. The specific surface area (BET) was 333.0, 428.8, 434.0, and 387.5 m2/g for MTS-1, MTS-1H, NTS-1, and STS-1, respectively (Table S1, Supporting Information). The N2 adsorption−desorption isotherm curve of MTS-1H exhibited one pronounced hysteresis loop with an abrupt closure on the desorption branch and differed from those of MTS-1, NTS-1, and STS-1 zeolites (Figure S4, Supporting Information), indicating the presence of mesopores in the particles of MTS-1H catalyst. The pore size distribution (Figure S5, Supporting Information) of the four samples all showed a peak at 3.8 nm, which was caused by the TSE (tensile strength effect) phenomenon.57,58 It can be clearly seen that mesopores and macropores were irregularly distributed in MTS-1H (Figure 1, T2); thus, there was no peak of the most probable pore diameter in the range from 2 to 60 nm. Table 1 showed the performance of these catalysts for epoxidation of propylene in the batch reactor. It can be seen

Figure 2. Effect of concentration of catalyst on the epoxidation of propylene. Reaction conditions: residence time: 1.5 h; pH: 6.5; temperature: 45 °C; n(C3H6)/n(H2O2): 1.4; stirring rate: 900 rpm.

H2O2 was significantly positively related to the catalyst concentration, and this positive correlation confirmed that the gas−liquid mass transfer resistance had been eliminated.23 When the concentration of catalyst was 26 g·L−1, the conversion of H2O2 and yield of PO reached 99.2% and 95.7%, respectively. Clerici et al.19 have reported that the key factor for the propylene epoxidation mechanism was the reversible splitting of a Ti−O−Si bond by H2O2 with the resulting formation of a Ti−OOH specie and the coadsorption of one methanol or water molecule stabilizing the hydroperoxide through a five-membered ring. Therefore, the conversion of H2O2 gradually increased with the increase in the amount of tetrahedral framework Ti species, which were the catalytic active sites for epoxidation. Due to the conversion of H2O2 being nearly 100%, the conversion and yield no longer increased with the further increase in the concentration of catalyst from 26 to 33 g·L−1. In all these cases, the selectivity to PO and the utilization of H2O2 reached above 96.4% and 98.1%, respectively, even when the catalyst concentration was 33 g·L−1. It demonstrated that, in this slurry reactor, the excessive catalyst did not cause significant side reactions, such as the solvolysis of PO and the nonproductive decomposition of H2O2. This result was very different from the epoxidation of propylene in the fixed-bed reactor. Inorganic binders or catalyst carriers are needed to increase the mechanical properties of catalysts used in the fixedbed reactor. These materials can cause the solvolysis of PO and the nonproductive decomposition of H2O2.39 By comparison, in this slurry reactor, the catalyst was pure TS-1 with mesopores and macropores. H2O2 reacted with propylene with very high selectivity, and PO existed stably in the reaction system even with excessive catalyst. 3.2.2. Effect of Residence Time. Figure 3 presented the effect of residence time in the slurry reactor on the epoxidation of propylene to PO. With increasing residence time from 1 h up to 1.5 h, the conversion of H2O2 increased considerably from 97.4% up to 99.2%, because the substrates could not get sufficient contact with the active centers in zeolite channels when the residence time was short. When residence time was 1.5 h, the conversion of H2O2 was nearly 100% and no longer increased with the further increase in residence time. The

Table 1. Catalytic Performance of MTS-1, MTS-1H, NTS-1, and STS-1 Samples samples

X(H2O2)/%

Y(PO)/%

S(PO)/%

U(H2O2)/%

MTS-1 MTS-1H NTS-1 STS-1

33.7 99.4 99.5 70.6

32.3 93.6 91.0 64.8

99.6 99.1 99.7 99.3

96.2 95.0 91.7 92.4

that the MTS-1H has a very good catalytic activity (PO yield of 93.6%), even better than that of NTS-1 (PO yield of 91.0%). This may be apparently explained because the high catalytic activity of MTS-1H is related to the elimination of molecular diffusion resistance in channels due to its hollow structure. The separation effect of different catalysts from reaction mixtures was examined by the Tyndall effect (Figure S6, Supporting Information). The filtrate of the reaction mixture with MTS-1 or MTS-1H exhibited a very weak Tyndall effect indicating little catalyst lost. The filtrate of the reaction mixture containing NTS-1 or STS-1 showed an obvious Tyndall effect. That was due to the extremely tiny particles compared with the pore size of the filter, and partially suspended particles could penetrate the filter layer to the filtrate. Therefore, complicated, expensive, and damageable equipment such as the membrane filter or ultracentrifuge is required to separate these superfine catalysts from the reaction mixture. The above results showed that MTS-1H not only exhibited good catalytic activity but also could be easily separated from the reaction mixture. Thus, this catalyst was selected as the D


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activity of MTS-1H.20 The above results showed that the acidity of the reaction solution greatly affected the product distribution of propylene epoxidation over MTS-1H catalyst in the continuous slurry reactor. The most appropriate acidity of the reaction solution for epoxidation was a pH of 6.5. 3.2.4. Effect of Reaction Temperature. As shown in Figure 5, the effect of reaction temperature on propylene epoxidation

Figure 3. Effect of residence time on the epoxidation of propylene. Reaction conditions: concentration of catalyst: 26 g·L−1; pH: 6.5; temperature: 45 °C; n(C3H6)/n(H2O2): 1.4; stirring rate: 900 rpm.

selectivity and yield of PO decreased with the residence time increase from 1.5 to 3 h, which was mainly caused by the subsequent solvolysis of PO. Because of the pure titanium silicalite catalyst in slurry reactor, the decomposition of H2O2 caused by transition metal impurities and acidic sites in the surface of catalyst hardly occurred; therefore, the utilization of H2O2 was nearly invariable with different residence times. The above results indicated that the epoxidation proceeded most effectively at an optimum residence time of 1.5 h in this slurry reactor. 3.2.3. Effect of pH. As the target product of epoxidation of propylene, PO can be easily further catalyzed by acid sites in the system and react with the solvent. An appropriate amount of basic additive can neutralize the excessive acid sites in solution and TS-1 catalyst, which restrains acid-catalyzed solvolysis of PO and improves the selectivity to PO.19 Thus, the acidity effect of reaction solution on the epoxidation of propylene was investigated by varying pH value between 6.1 and 6.9 using dilute ammonia solution. As shown in Figure 4,

Figure 5. Effect of reaction temperature on the epoxidation of propylene. Reaction conditions: concentration of catalyst: 26 g·L−1; residence time: 1.5 h; pH: 6.5; n(C3H6)/n(H2O2): 1.4; stirring rate: 900 rpm.

was investigated from 40 to 55 °C under otherwise similar reaction conditions. With the reaction temperature increased from 40 to 45 °C, the conversion of H2O2 and the yield of PO increased from 87.9% to 99.2% and 83.5% to 95.7%, respectively. The high yield of PO remained almost unchanged at 45 and 50 °C. When reaction temperature further increased, the utilization of H2O2 and the selectivity to PO decreased correspondingly, due to the accelerated side reactions such as decomposition of H2O2 and the solvolysis of PO. The apparent activation energy of the propylene epoxidation is much lower than that of the side reactions.23,24,60−62 Increasing temperature could accelerate both the main reaction and the side reactions, especially for side reactions.21 Therefore, more byproducts were formed at higher temperature, leading to the decrease of the selectivity to PO.24 All reaction steps in the epoxidation of propylene are exothermic. When the exothermic epoxidation process is carried out in a fixed-bed reactor, an important problem is the so-called “hot spot” which can decrease reaction selectivity, even leading to severe catalyst deactivation. By comparison, the slurry reactor has better temperature distribution and no hot spots; thus, epoxidation in the slurry reactor with MTS-1H possessed a relatively wide window of operating temperature from 45 to 50 °C and produced a very high selectivity to PO of over 96.3% at a conversion of 99.1%. 3.2.5. Effect of n(C3H6)/n(H2O2). As a basic reactant, the amount of propylene is a crucial factor in epoxidation of propylene. The molar ratio effect of C3H6 to H2O2 (n(C3H6)/ n(H2O2)) on propylene epoxidation was investigated from 1.2:1 to 1.6:1 with changing the pressure of propylene, and the results are presented in Figure 6. In all feeding ratios investigated, the pressure of propylene was much lower than that of the fixed-bed.26,27 Meanwhile, the selectivity to PO and the utilization of H2O2 were very high, even when the n(C3H6)/n(H2O2) was 1.2 (the pressure of propylene was 0.12 MPa). Although the epoxidation of propylene reacting with H2O2 is carried out in accordance with 1:1, when the propylene was fed in the reaction system according to n(C3H6)/n(H2O2) = 1.2, the average conversion of H2O2 and yield of PO were

Figure 4. Effect of pH on the epoxidation of propylene. Reaction conditions: concentration of catalyst: 26 g·L−1; residence time: 1.5 h; temperature: 45 °C; n(C3H6)/n(H2O2): 1.4; stirring rate: 900 rpm.

the highest yield of PO and utilization of H2O2 were obtained when pH was 6.5, below or beyond which the yield of PO and the utilization of H2O2 decreased. The selectivity to PO decreased in acidic solution, because acid sites were responsible for the desired reaction and increased the solvolysis of PO, and the increasing rate of the latter was higher than that of the former with increasing acidity.20,59 PO was further reacted with methanol or water catalyzed by proton in the reaction mixture to form propylene glycol monomethyl ethers or propylene glycol, respectively. When pH was greater than 6.5, the conversion of H2O2 decreased, owing to the fact that the deficiency of acidity in reaction solution can inhibit the catalytic E


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than 900 rpm. This was clear evidence for the elimination of gas−liquid mass transfer resistance at and above a stirring speed of 900 rpm.23 The utilization of H2O2 and yield of PO decreased slightly when the stirring rate increased to 1200 rpm, which indicated the H2O2 decomposition was accelerated at higher stirring rate. The epoxidation of propylene proceeded most effectively with a stirring rate of 900 rpm in this slurry reactor. 3.3. Stability of Catalyst in Slurry Reactor. The stability of MTS-1H was tested in this slurry reactor (Figure 8). The Figure 6. Effect of n(C3H6)/n(H2O2) on the epoxidation of propylene. Reaction conditions: concentration of catalyst: 26 g·L−1; residence time: 1.5 h; pH: 6.5; temperature: 45 °C; stirring rate: 900 rpm.

only 82.4% and 79.1%, respectively. Di Serio and co-workers24 reported that gas−liquid resistance to the mass transfer was much greater than the liquid−solid one on propylene epoxidation and the solubility of the propylene increased linearly with the pressure. Therefore, the increase of n(C3H6)/ n(H2O2) and pressure can improve the solubility of propylene and accelerate the main reaction. When the n(C3H6)/n(H2O2) increased to 1.4 (the pressure of propylene was 0.23 MPa), the average conversion of H2O2 and yield of PO significantly increased to 99.2% and 95.7%, respectively. When the n(C3H6)/n(H2O2) continued to increase to 1.6 (the pressure of propylene was 0.35 MPa), the conversion of H2O2, the selectivity, and yield of PO were nearly invariable. In consideration of the excessive propylene leads to the waste of reactant and the increase of energy consumption for the separation and recovery in industrial processes, the n(C3H6)/ n(H2O2) of 1.4 is the most effective feeding ratio for the epoxidation of propylene in this slurry reactor. 3.2.6. Effect of Stirring Rate. The stirring rate is a key factor affecting the efficiency of mass transfer in the slurry reactor, and the influence of stirring rate on the epoxidation of propylene is shown in Figure 7. When the stirring rate increased from 300 to

Figure 8. Results of epoxidation of propylene with H2O2 over the fresh MTS-1H catalyst in the continuous slurry reactor. Reaction conditions: concentration of catalyst: 26 g·L−1; residence time: 1.5 h; pH: 6.5; temperature: 45 °C; n(C3H6)/n(H2O2): 1.4; stirring rate: 900 rpm.

conversion of H2O2 changed insignificantly between 28 and 247 h and then decreased from 99.0% to 90.5% between 247 and 320 h, indicating the deactivation of the catalyst. The average yield of PO reached up to 96.4% between 28 and 247 h and then obviously decreased to 80.0% at 320 h. Remarkably, even when the catalytic activity had decreased, the selectivity to PO still maintained a very high level above 97.5% during the whole reaction period of 320 h, which was significantly better than the result in the fixed-bed reactor.41,63 The stable highselectivity in the slurry reactor was due to the lack of hot spot and acidic sites which can lead to solvolysis of PO. With the catalytic activity decreaseing, the conversion of H2O2 decreased, and the concentration of H2O2 in the reaction system increased. The nonproductive decomposition rate of H2O2 increased linearly with the concentration of H2O2,64 and as a consequence, the utilization of H2O2 decreased. The average decomposition ratio of H2O2 was 1.3% in the first 247 h, and the vent gas from the reaction system was monitored by an Albright gas analyzer, and the results showed that the average concentration of O2 in the reactor headspace was only 4.8%. The explosibility test of vent gas showed that the gas mixture in the reactor headspace was outside the explosion limit. Thus, this epoxidation process can be handled safely. The catalytic performances of MTS-1 and NTS-1 were also tested with propylene epoxidation in the continuous slurry reactor (Figures S7 and S8, Supporting Information). Before catalyst deactivation, the average conversion of H2O2 and yield of PO over MTS-1 were only 72.2% and 68.9%, respectively, and the stability was lower than that of MTS-1H. The conversion of H2O2 and yield of PO over NTS-1 rapidly decreased to 87.5% and 83.4% within 30 h, respectively. Tyndall effect tests and the frayed discharging pump

Figure 7. Effect of stirring rate on the epoxidation of propylene. Reaction conditions: concentration of catalyst: 26 g·L−1; residence time: 1.5 h; pH: 6.5; temperature: 45 °C; n(C3H6)/n(H2O2): 1.4.

900 rpm, the conversion of H2O2 increased from 74.9% to 99.2%, and the yield of PO increased from 71.8% to 95.7%, while the selectivity to PO and the utilization of H2O2 were nearly invariable. The increase of stirring rate can increase the gas−liquid mass transfer rate and make the system quickly reach the propylene solution saturation.24 Therefore, the increase of stirring rate accelerates the dissolution of propylene and increases conversion of H2O2. The conversion of H2O2 was independent of stirring rate when the stirring rate was greater F


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Figure 9. FT-IR spectra (A), UV−vis spectra (B), XRD pattern (C), nitrogen sorption isotherm (D), TG (E), and DTG (F) curves of MTS-1H catalysts: (a) fresh MTS-1H, (b) spent-180 h MTS-1H, (c) spent-320 h MTS-1H, (d) regenerated with methanol, (e) regenerated with NH3·H2O, (f) regenerated by calcinations, and (g) regenerated with H2O2.

demonstrated that there were suspended particles in the filtrate. The rest of the catalyst in the reactor was collected with a nylon membrane (0.22 μm). It was found that the average catalyst loss was 0.82%/h on stream (determined as the weight difference of the catalyst put in and recovered from the reactor divided by the initial catalyst weight and the time on stream). The suspended particles in the filtrate were also gathered with a nylon membrane (0.22 μm). The catalysts gathered from the reactor and filtrate were reused in the batch reaction of propylene epoxidation without regeneration, and the yields of PO were 86.4% and 87.8%, respectively. The results indicated that the decreased conversion of H2O2 in the continuous reaction was due to the NTS-1 catalyst loss instead of its deactivation. 3.4. Comparsion of Epoxidation in Slurry and FixedBed Reactor. As a comparison, the epoxidation of propylene was carried out in the fixed-bed reactor using MTS-1H extrudate as catalyst, and the results are shown in Figure S9, Supporting Information. When the average conversion of H2O2 was 94.1%, the average selectivity to PO and utilization of H2O2 was 90.8% and 92.5%, respectively. The results were obviously lower than that obtained from the continuous slurry reactor. The TOF values based on H2O2 and the productivity based on unit reactor volume are shown in Table S2, Supporting Information. The TOF value in this slurry reactor was obviously higher than that in the fixed bed,25,39,65 which indicated that the catalyst in the slurry reactor had higher efficiency. Besides the influence of backmixing, the catalyst was highly dispersed in the slurry reactor (0.026 g/mL), but tightly packed in fixed-bed reactor (0.4 g/mL); thus, the productivity based on unit reactor volume of slurry reactor was lower than that of fixed bed. It indicated that the main disadvantage of the slurry reactor was the low volume efficiency. A fairly comparative experiment was carried out by adding binder materials (silica gel) directly into the continuous slurry reaction of propylene epoxidation over MTS-1H (Figure S10, Supporting Information). In this reaction, the average conversion of H2O2 was as high as 99.2%, while the selectivity

to PO and utilization of H2O2 decreased obviously to 92.2% and 93.4%, respectively. Compared with the reaction in the slurry reactor without binders, the results indicated that the binder led to solvolysis of PO and decomposition of H2O2, which was consistent with the literature.39 3.5. Regenerated Catalyst with Different Methods. The FT-IR spectra of spent MTS-1H catalysts and regenerated MTS-1H catalysts with different methods are shown in Figure 9A. The main absorption bands are found at 550, 800, 960, 1100, and 1230 cm−1, in agreement with the typical FT-IR spectra of TS-1 reported in the literature.66,67 The bands at 550 and 800 cm−1 are assigned to δ(Si−O−Si) and ν(Si−O−Si), respectively. The absorption band near 960 cm−1 is due to the stretching vibration of [SiO4] units strongly effected by titanium ions in neighboring coordination positions, which is a proof of the introduction of Ti into the framework, and the intensity of this band is related to the amount of framework Ti. The intensity ratios of absorbance of the band at 960 cm−1 to that at 800 cm−1 (I960/I800) for all catalysts in the FT-IR spectra are summarized in Table 2. It could be observed that I960/I800 of the spent catalysts was slightly weaker than that of the fresh sample and decreased gradually with the reaction time. It can be ascribed to partially framework Ti being washed away in the long-time run. The I960/I800 of the catalyst regenerated with NH3·H2O treatment was higher than that of fresh catalyst, due Table 2. Data of FT-IR Spectra and XRF of Fresh, Spent, and Regenerated MTS-1H Samples samples fresh spent-180 h spent-320 h regenerated with regenerated with regenerated with regenerated with G

methanol NH3·H2O calcination H2O2

I960/ I800

SiO2/wt %

TiO2/wt %

n(Si/ Ti)

1.24 1.23 1.21 1.20 1.27 1.22 1.17

94.00 87.56 87.64 88.16 88.38 95.28 94.13

4.03 3.70 3.46 3.76 4.37 3.77 3.73

31.10 31.55 33.77 31.26 26.97 33.70 33.65


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Industrial & Engineering Chemistry Research Table 3. BET Surface Areas and Pore Volumes of Fresh, Spent, and Regenerated MTS-1H Samples samples fresh spent-180 h spent-320 h regenerated with regenerated with regenerated with regenerated with


surface area/m2·g−1

micropore surface area/m2·g−1

volume/cm3·g−1

micropore volume/cm3·g−1

428.8 284.9 250.1 271.1 306.9 432.6 405.8

404.4 228.6 217.7 214.1 246.7 404.4 364.3

0.2373 0.1778 0.1433 0.1941 0.2291 0.2424 0.2498

0.1717 0.0963 0.0886 0.0877 0.1028 0.1732 0.1557

to leaching of Si from framework caused by alkalic NH3·H2O washing, which was in accordance with the result of XRF in Table 2. Compared with the fresh MTS-1H, a few stretching peaks in the range of 2900−3000 cm−1 appeared on the spent catalysts, which can be assigned to the saturated C−H. This revealed that some bulky molecular substances with saturated C−H existed on the spent catalysts. After washing with H2O2 or calcination, these peaks disappeared, indicating that the bulky molecular substances can be destroyed by H2O2 or calcination. In the UV−vis spectra of the seven samples (Figure 9B), three major bands at 200−210, 240−270, and 300−310 nm, are assigned to tetrahedrally coordinated TiO4, extra-framework [HOTiO3] units, and anatase TiO2, respectively. Compared with fresh MTS-1H, the peak intensities at 210 nm of the spent and regenerated catalysts were somewhat lower, but the intensities at 240 nm were higher than the fresh sample. It further indicated that some framework Ti was leached from zeolite framework and changed to extra-framework Ti in this reaction, which agreed with the FT-IR spectra. The XRD patterns of the MTS-1H samples and relative crystallinity (RC) are shown in Figure 9C. The main diffraction peaks of all samples were the same as those of the typical TS-1 zeolite.68 The single peak at 2θ = 24.4° indicated the orthorhombic symmetry of TS-1. The results indicated that the deactivation and regeneration processes had an insignificant effect on the MFI structure. With the increasing reaction time, the crystallinity of spent catalysts decreased gradually, due to byproducts accumulating in zeolites. The crystallinity of the regenerated sample with H2O2 or calcination was higher than that of the deactivated catalyst and lower than that of the fresh sample. It indicated that there were some irreversible defects in TS-1 crystal of deactivated catalyst, which may be caused by the leaching of Ti from framework due to the dissolution of Ti in H2O2. The nitrogen adsorption−desorption isotherms of the catalysts are shown in Figure 9D, and the surface areas and pore volumes are summarized in Table 3. The fresh catalyst sample possessed much larger surface area and pore volume. When the catalyst had been used in epoxidation for 180 h, both the surface area and pore volume significantly decreased. Moreover, the surface area and pore volume of spent-320 h catalyst further decreased. Both the surface areas and pore volumes of the regenerated catalyst by H2O2 washing or calcination were almost completely recovered. Washing with methanol could not increase the surface area and pore volume of deactivated catalyst significantly. Washing with NH3·H2O partly recovered the surface area and pore volume. The results indicated that the deactivation of catalyst could be attributed to the blocking of micropores by organic byproducts, which hindered substrate

from reacting with H2O2 on Ti active sites, and such deactivation was reversible and regenerative. TG analysis of the fresh catalyst, spent catalysts, and regenerated catalysts were performed to investigate the thermal decomposition behavior of the organic byproducts deposited in catalyst. The TG and DTG curves are shown in Figure 9E,F, respectively. It can be seen that the DTG curve of fresh catalyst was almost linear. For the spent-180 h catalyst and the spent320 h catalyst, there was a major weight loss (8.5% and 9.1%, respectively) in TG curves with the significant peak in DTG curves at around 260−270 °C. It indicated that there were a mass of organic compounds deposited inside the microporous channels of the spent catalysts, and the amount of depositions increased with the increase in reaction time. For the catalyst regenerated with methanol and the catalyst regenerated with NH3·H2O, the peak of weight loss in DTG curves shifted to around 280−290 °C with a major weight loss of 7.9% and 7.0%, respectively. For catalysts regenerated by calcination or H2O2 washing, the peak around 260−290 °C almost disappeared. The major weight loss of the catalyst regenerated by calcination and the catalyst regenerated with H2O2 washing were only 1.0% and 2.2%, respectively. The SEM and TEM images of the catalysts are shown in Figures 10 and 11, respectively. The crystals of all samples were uniform with typical coffin-shaped morphology of the MFI zeolite and possessed similar crystal size. Moreover, the crystals of deactivated and regenerated MTS-1H samples also contained large intraparticle voids (mesopores and macropores). Compared with fresh catalyst, there was no significant

Figure 10. SEM micrograph of catalysts: (A) fresh MTS-1H, (B) spent-320 h MTS-1H, (C) regenerated with H2O2, and (D) regenerated by calcinations. H


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effect on regenerating the catalytic activity of the deactivated catalyst, due to most byproducts in the catalyst channels being removed. Catalytic performance was partly recovered when deactivated catalyst was washed by NH3·H2O, and methanol washing was noneffective on catalyst regeneration. Because the regeneration of catalyst by H2O2 washing can avoid energyextensive complicated calcination operations and it is easy to withdraw/replenish catalyst online from/to the slurry reactor without a shutdown,42 the online regeneration of catalyst by H2O2 washing is suitable to the scale-up of this continuous slurry process in the near future.

4. CONCLUSIONS In conclusion, the process for clean epoxidation of propylene was systematically investigated in a continuous slurry reactor using hollow structure micron-sized TS-1 as catalyst. The reaction pressure was only 0.23 MPa and much lower than that in the fixed-bed reactor. The reaction was successfully run for 247 h with very superior selectivity (98.7%) and yield (96.4%) of PO. This process exhibited excellent catalytic stability and handling safety. Even though the deactivation of catalyst occurred, the selectivity to PO was still kept above 97.5%, which was significantly better than the result in the fixed-bed reactor. Washing with H2O2 can almost completely remove the bulky organic byproducts deposited inside the micropores of catalysts and effectively recover catalytic activity. It enables the catalyst to be easily regenerated without a shutdown. The epoxidation of propylene in the continuous slurry reactor has the advantages of being clean, energy saving, and safe and having excellent selectivity and catalyst efficiency. This work provides an alternative process for the green industrial production of PO.

Figure 11. TEM micrograph of catalysts: (A) fresh MTS-1H, (B) spent-320 h MTS-1H, (C) regenerated with methanol, (D) regenerated with NH3·H2O, (E) regenerated by calcinations, and (F) regenerated with H2O2.

change in morphology of both the deactivated and regenerated catalysts. In order to investigate the organic compounds deposited in the deactivated catalyst, the depositions were collected by extraction and analyzed by GC-MS (presented in Table S3, Supporting Information). Except the reactant, solvent, and target product, there were four families of the organic compounds corresponding to their possible origin in the extract before hydrofluoric acid digestion of catalyst. Only the compounds in families 1 and 2 were found in the extract after digestion. From the above results, it can be seen that the dimeric compounds in families 3 and 4 were mostly formed and deposited on the external surface of the deactivated catalyst, due to the microporous channel constraints. The solvolysis byproducts in families 1 and 2 were formed in micropores of MTS-1H and could not diffuse out from the channels immediately. Most of them resided there, and the micropores were blocked. Because most of the active Ti-sites were located in channels of MTS-1H, the blockage of channels led to the catalyst deactivation.69,70 The catalytic performances of fresh catalyst, deactivated catalyst, and regenerated catalysts for the epoxidation of propylene were tested in the batch reactor, and the results are presented in Table 4. The MTS-1H regenerated by methanol washing and the spent-320 h catalyst gave similar conversion of H2O2. The conversion of H2O2 over the catalyst regenerated by NH3·H2O washing increased to 86.1%. The conversion of H2O2 obtained over the catalyst regenerated by H2O2 washing was 99.6%, which was similar to that obtained over the catalyst regenerated by calcination and even slightly higher than that obtained over the fresh catalyst. The results indicated that H2O2 washing or calcination had a significant

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ASSOCIATED CONTENT

* Supporting Information S

Some characterization results of MTS-1, MTS-1H, NTS-1, and STS-1, the Tyndall effect of filtrate of reaction mixtures, the results of epoxidation of propylene with H2O2 over the MTS-1 and NTS-1 in continuous slurry reactor, MTS-1H extrudate in fixed-bed reactor, and MTS-1H and binder in continuous slurry reactor. BET surface area and pore volume, the TOF values and productivity based on unit reactor volume of different reactors, and major organic compounds components deposited on the deactivated catalyst. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b00410.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Table 4. Catalytic Performance of the Fresh, Deactivated, and Regenerated MTS-1H Samples samples fresh spent-320 h regenerated with regenerated with regenerated with regenerated with


X(H2O2)/%

Y(PO)/%

S(PO)/%

U(H2O2)/%

99.4 52.5 59.6 86.1 99.8 99.6

93.6 51.5 57.8 83.6 93.8 94.1

99.1 98.4 98.6 98.9 98.9 98.9

95.0 99.7 98.4 98.1 95.1 95.7

I


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Industrial & Engineering Chemistry Research Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Innovation Fund for Elitists of Henan Province, China (Grants No. 0221001200), the Natural Science Foundation of China (No. 21401168), and the Joint Project of Zhengzhou University and Haohua-Junhua Group Co., Ltd. for the clean production of propylene oxide are acknowledged. The authors are highly indebted to teams of collaborators both from Zhengzhou University and from Haohua-Junhua Group Co., Ltd.

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Highly Selective Epoxidation of Propylene in a Low-Pressure

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