The Clean Synthesis of Small-Particle TS-1 with High-Content Framework Ti by Using NH4HCO3 and Suspended Seeds as an Assistant.

The synthesis of a TS-1 zeolite with high-content framework Ti and small particles has been developed by adding NH4HCO3 and suspended seeds as an assistant. With the addition of NH4HCO3, the Hofmann decomposition of the tetrapropylammonium cation (TPA+) decreased, and the framework Ti content of the zeolite increased first and then decreased while the particle became larger. With the assistance of suspended seeds, the TS-1 synthesized under a low-alkalinity system possesses small particle size and high-content framework Ti, and it shows the best catalytic activity among the prepared catalysts. Because the decomposition of TPA+ decreased, the mother liquid could be reused in the next run of preparation. Even though the recycled mother liquid was reused five times, all obtained TS-1 samples exhibited similar catalytic performances in propylene epoxidation. This work provides an efficient process for preparing TS-1 with good catalytic performance and reduces the discharge of the waste liquid.

Introduction

Since the first discovery of titanium silicalite-1 (TS-1) by Tarramasso et al. in 1983, TS-1 has been widely used in selective catalytic oxidations by using H2O2 as an oxidant, such as aromatic hydroxylation,[1−3] oxidation of alcohols and alkanes,[4,5] ammoximation of ketones,[6−8] and epoxidation of alkenes.[9−14] Generally, organic template structure-directing agents (such as TPAOH or TPABr) are usually required to direct its structure in the synthesis of TS-1. However, the tetrapropylammonium cation (TPA+) used as the template is prone to thermal decomposition to generate tripropylamine and 1-propanol under higher alkalinity. Meanwhile, a large amount of an organic amine waste liquid is produced from the preparation of TS-1, which is harmful to the environment.

Many researchers have explored the method of the solvent-free synthesis of zeolites[15−17] to avoid environmental pollution, but the hydrothermal synthesis method is still the most commonly used method for the industrial synthesis of the TS-1 zeolite due to its simple operation and low cost. Therefore, a variety of synthetic routes have been developed for the reduction of discharge of the waste liquid on the basis of the traditional hydrothermal synthesis method.[18−20] For example, Shi et al.[20] had reported that the mother liquid produced from the synthesis of the TS-1 zeolite could be used to regenerate the catalyst of deactivated TS-1, and the regenerated TS-1 exhibited similar catalytic activity with fresh TS-1. Guo et al.[21] synthesized titanosilicate by using different templates (TPABr, TPACl, and TPAF), analyzed residual templates and organic bases in the mother liquid produced from the synthesis of titanosilicate, and reused the mother liquid in the next production of titanosilicate. Our group[22] prepared hollow TS-1 by using ethanolamine and TPABr as the desiliconization medium, and the mother liquid was reused eight times. However, bromide coming from the preparation process of TS-1 by using TPABr as a template may cause environmental pollution and equipment corrosion during calcination.[23]

To the best of our knowledge, the decomposition of TPA+ can be reduced under low alkalinity during the crystallization process of zeolites, and reuse of the mother liquid can reduce pollution of the environment. The addition of appropriate ammonium salts during the preparation of the TS-1 zeolite can decrease alkalinity of the synthesis system and facilitate the incorporation of Ti into the framework.[24−26] For example, Fan et al.[27] have synthesized a TS-1 zeolite with a high content of framework Ti (Si/Ti = 34) by using (NH4)2CO3 as a crystallization-mediating agent. The same group furthermore reported that several other ammonium salts are beneficial to Ti incorporation into the framework. Shakeri and Dehghanpour[28] synthesized TS-1 with minimum water and optimum pH by the addition of (NH4)2CO3 to maximize the framework Ti content (Si/Ti = 38). As known, the outstanding catalytic performance in selective oxidation reactions of the TS-1 zeolite results from the isomorphic substitution of Si in the zeolite framework by Ti. With the increasing addition of (NH4)2CO3 during the preparation of TS-1, the framework Ti content of the zeolite increases first and then decreases, while the particles of TS-1 become larger. So, the catalytic activity of the synthesized zeolite is poor with the addition of excessive ammonium salts. It has been reported the addition of seeds can promote the nucleation and decrease the crystal size of zeolites.[29−31] In addition, the addition of appropriate seeds during the preparation of the TS-1 zeolite is favorable for the incorporation of Ti into the framework.[32] For example, Cundy et al.[33] have synthesized size-controllable zeolites by using the nanocrystalline as seeds. Small-crystal TS-1 could be synthesized in a TPABr-ethylamine system by using the mother liquid of nanosized S-1 and TS-1 as seeds.[34−37] Our group also found that small-particle TS-1 could be synthesized by the addition of nanosized S-1 as the crystal seed because the nanosized S-1 could directly provide a crystal nucleus and enhance the crystallization rate significantly.[38] There are also some other methods to improve the catalytic activity, such as enhancing the external specific surface area of the TS-1 zeolite and creating mesopores into the TS-1 zeolite to bring about important activity enhancement.[39−41]

In this paper, NH4HCO3 was added during the synthesis of TS-1 to adjust the alkalinity of the synthesis system, and appropriate seeds were added to reduce the particle size of the zeolite. Meanwhile, the recycled mother liquid was reused as the media of hydrothermal crystallization and the supplement of raw materials in the next synthesis of TS-1, and the catalytic performances of the synthesized samples were investigated in propylene epoxidation.

Results and Discussion

The Synthesis of TS-1 Samples

XRD patterns of TS-1 samples are shown in Figure A. There are sharp peaks at 2θ = 7.8, 8.8, 23.0, 23.9, and 24.4° corresponding to the five characteristic peaks of MFI topology in all samples. The XRD pattern of dried seeds also possesses characteristic peaks of MFI topology (Figure S1). TS-1-0 is used as a reference sample, and its relative crystallinity (RC) is regarded as 100%. The crystallinity of TS-1-0.5 and TS-1-1 are 78 and 69%, respectively (Table ). The results indicate lower crystallinity for the TS-1 samples synthesized at lower alkalinity (pH = 10.2, shown in Table ). The relative crystallinity of TS-1-C (72%) is slightly higher than that of TS-1-1. It can be observed that the relative crystallinity of TS-1-S (85%) is obviously higher than that of TS-1-1, indicating that the addition of suspended seeds can significantly increase the crystallinity of zeolites.

Figure 1

XRD (A), FT-IR (B), UV–vis spectra (C), and N2 adsorption–desorption isotherms (D) of TS-1-0 (a), TS-1-0.5 (b), TS-1-1 (c), TS-1-C (d), and TS-1-S (e).

Table 1

Properties of the Synthesis Mixtures and TS-1 Samples

sample	RC (%)	pHa	n(Si/Ti)	I960/I800	SBET (m2/g)	Smic (m2/g)	Sext (m2/g)	Vtot (m3/g)	Vmeso (cm3/g)
TS-1-0	100	11.8	29.9	1.12	483	334	149	0.292	0.163
TS-1-0.5	78	10.8	30.2	1.27	450	357	93	0.199	0.052
TS-1-1	69	10.2	30.3	1.17	411	352	59	0.146	0.051
TS-1-C	72	10.2	30.3	1.18	428	346	82	0.180	0.062
TS-1-S	85	10.2	30.3	1.33	481	340	141	0.282	0.141

The pH of the synthesis mixture.

The molar ratios of Si/Ti in bulk TS-1 samples are shown in Table . The Si/Ti molar ratios of all TS-1 samples are similar.

FT-IR spectra of TS-1 samples are shown in Figure B. All TS-1 samples present obvious absorption peaks at 550, 800, 960, 1100, and 1230 cm–1, which are consistent with the typical FT-IR spectra of TS-1 reported in the literature.[43] The characteristic bands of the MFI zeolite are at 550 and 800 cm–1. The peak at 960 cm–1 is attributed to the stretching vibration of [SiO4] units strongly influenced by Ti in neighboring coordination sites, which is proof of the introduction of Ti into the framework. The ratio of absorption band intensity at 960 cm–1 to that at 800 cm–1 (I960/I800) can be used to evaluate the content of framework Ti species.[44,45] As shown in Table , the I960/I800 values of TS-1-0, TS-1-0.5, and TS-1-1 are 1.12, 1.27, and 1.17, respectively. It is clear that TS-1-0.5 has the largest amount of framework Ti among the three samples, and the reason is that the presynthesis mixture of TS-1-0.5 under suitable alkalinity (pH = 10.8) exhibits a suitable crystallization rate, which can benefit incorporation of Ti into the framework by harmonizing hydrolysis of metal alkoxides with the nucleation rate and the growth process.[27] The too strong or too weak alkalinity of the presynthesis mixture results in a too fast or too slow crystallization process, which makes the incorporation of Ti into the framework. The suspended seed is highly dispersed in the presynthesis mixture (Figure S2) and can provide a lot of crystal nuclei, which decrease the energy barrier of crystallization under weak alkalinity and promote incorporation of Ti into the framework via harmonizing the nucleation rate with the growth process. Therefore, the content of framework Ti in TS-1-S is higher than that in TS-1-1 (seen in Table ).

Figure C shows the UV–vis spectra of TS-1 samples. All TS-1 samples show major absorption bands at 200–230, 250–270, and 310–330 nm. The absorption band at 200–230 nm is the characteristic absorption of framework Ti species (tetracoordinated Ti).[46] The absorption bands at 250–270 and 310–330 nm are the characteristic absorption of nonframework Ti (five coordination Ti and six coordination Ti)[47] and anatase TiO2,[48] respectively. The absorption band at 310–330 nm in TS-1-0.5 is lower than that in TS-1-0 and TS-1-1, confirming that there is less anatase TiO2 in the TS-1-0.5 among the three samples. That is because TS-1-0.5 under suitable alkalinity (pH = 10.8) of the presynthesis mixture is beneficial to the content of framework Ti in zeolites, thereby reducing the formation of anatase TiO2, which is in agreement with the results of FT-IR spectra. The absorption bands at 250–270 and 310–330 nm in TS-1-S are significantly lower than those in TS-1-1, suggesting that there are less nonframework Ti and anatase TiO2 in TS-1-S. That is because the addition of suspended seeds can enhance the incorporation of Ti into the framework of zeolites, thus leading to less nonframework Ti and anatase TiO2 formation.

Nitrogen absorption isotherms of TS-1 samples are illustrated in Figure D. For the sake of clarity, the isotherms of TS-1-0.5 (b), TS-1-1 (c), TS-1-C (d), and TS-1-S (e) have been artificially shifted vertically upward to 75, 150, 225, and 300 cm3·g–1, respectively. All samples show remarkable transitions in a low relative pressure (P/P0 < 0.2), indicating the microporous structure existing.[49] There is a sharp increase at high relative pressure (P/P0 > 0.9) for TS-1-0 and TS-1-S, which is related to the interparticle voids formed by crystallites. The values of total and external surface areas decrease from the TS-1-0 to TS-1-1 sample (Table ) because of the increase in crystal sizes. The total and external surface areas of TS-1-C are slightly larger than those of TS-1-1, while the total and external surface areas of TS-1-S are significantly larger than those of TS-1-1.

Figure shows the SEM images of TS-1 samples. The morphologies of TS-1-0 and TS-1-0.5 are ball-like with average particles sizes of 0.19 and 0.49 μm, respectively. TS-1-1 was produced with round-boat morphology and an average particle size of 4.12 × 2.38 × 0.34 μm. The results indicate the addition of NH4HCO3 can enlarge the particle size of TS-1. The reason is the low nucleation rate caused by low alkalinity (pH = 10.2). The average particle size of suspended seeds measured using a laser particle analyzer was an average size of 0.10 μm (Figure S2), which is similar to the particle size observed in the SEM image (Figure S3). However, the average particle size of calcined seeds measured using a laser particle analyzer was 2.90 μm (Figure S2), which is remarkably larger than the particle size observed in the SEM image (Figure S3). TS-1-C shows the round-boat morphology, and the average particle size is 2.63 × 1.72 × 0.24 μm. The TS-1-S is ball-like with an average particle size of 0.50 μm, which is remarkably smaller than that of TS-1-1 and TS-1-C, indicating that the addition of suspended seeds can reduce the particle size of zeolites. The reason is that the suspended seed can provide more crystal nuclei than the calcined seed.[30]

Figure 2

SEM images of TS-1-0 (a), TS-1-0.5 (b), TS-1-1 (c), TS-1-C (d), and TS-1-S (e).

Table S1 shows the results of propylene epoxidation for 60 min over TS-1 samples, and it can be seen that the catalytic activities of the catalysts are already high. Table shows the results of propylene epoxidation for 30 min over the TS-1 samples. The catalytic activity of TS-1-0.5 is better than the sample of TS-1-0. The main reason is that TS-1-0.5 possesses a higher content of framework Ti. Due to the larger particle and smaller total and surface areas, the catalytic activity of TS-1-1 is lower than that of other TS-1 samples synthesized by adding NH4HCO3. The catalytic activity of TS-1-C is similar to that of TS-1-1. TS-1-S gives the best catalytic activity among all samples because of the smaller particle size and the highest content of framework Ti. These results indicate that TS-1 with small particles and a high content of framework Ti can be synthesized by adding NH4HCO3 and suspended seeds as an assistant, and the sample has good catalytic performance in propylene epoxidation.

Table 2

Catalytic Performances for Propylene Epoxidation over TS-1 Samplesa

sample	XH2O2 (%)	YPO (%)	SPO (%)	UH2O2 (%)
TS-1-0	84.3	77.6	95.6	96.3
TS-1-0.5	90.0	80.4	97.2	92.1
TS-1-1	66.7	54.9	94.6	86.9
TS-1-C	82.2	72.4	95.2	92.5
TS-1-S	91.9	83.3	99.5	91.0

Reaction conditions: propylene (0.7 MPa), catalyst (150 mg), solvent (595 mmol), H2O2 (97 mmol), 318 K, and 30 min.

In order to avoid experimental accidents, the same preparation procedure of TS-1-0 had been repeated 3 times independently, and the corresponding catalyst performance was tested independently. All the obtained TS-1-0 samples show similar catalytic activity in propylene epoxidation (Table S2).

Recycle of the Mother Liquid

The mother liquid produced from the synthesis of TS-1-0 was extracted by dichloromethane and analyzed by GC–MS (presented in Table S3). There were mainly tripropylamine and 1-propanol existed in the extract, indicating the thermal decomposition of TPA+. Mother liquids produced from the synthesis of TS-1 samples were extracted by dichloromethane, and the concentrations of tripropylamine in extracts were analyzed by gas chromatography. The amount of tripropylamine in extracts represents the decomposition of TPA+ during the crystallization process, and the results are illustrated in Table S4. The decomposition of TPA+ during the crystallization process of TS-1-0, TS-1-0.5, and TS-1-1 are 70.2, 21.9, and 1.9%, respectively. It shows that TPA+ decomposes less at lower alkalinity (pH = 10.2). The decomposition of TPA+ during the crystallization process of TS-1-C and TS-1-S are 2.0 and 2.1%, respectively, suggesting that the seeds have no significant effect on the decomposition of TPA+.

TS-1-S synthesized by adding suspended seeds exhibits the best catalytic performance among all of the TS-1 samples (Table ), and the decomposition ratio of TPA+ in that crystallization process is low. Therefore, the mother liquid produced from the synthesis of TS-1-S was recycled and reused as the media of hydrothermal crystallization and the supplement of template agents in the next synthesis process. We readjusted with small amounts of fresh reagents to fit to the initial conditions in terms of composition during the mother liquor circulation.

Figure A shows the XRD patterns of the TS-1 samples synthesized with the recycled mother liquid. It is clear that all TS-1 samples have characteristic peaks of the MFI structure. The relative crystallinity of TS-1 samples synthesized with the recycled mother liquid is similar to that of the TS-1-S sample synthesized by using fresh materials (Table ).

Figure 3

XRD (A), FT-IR (B), UV–vis spectra (C), and N2 adsorption–desorption isotherms (D) of TS-1-S (a), TS-1-R1 (b), TS-1-R2 (c), TS-1-R3 (d), TS-1-R4 (e), and TS-1-R5 (f).

Table 3

Physicochemical and Textural Properties of TS-1 Samples

sample	RC (%)	I960/I800	n(Si/Ti)	SBET (m2/g)	Smic (m2/g)	Sext (m2/g)	Vtot (m3/g)	Vmeso (cm3/g)
TS-1-S	85	1.33	30.3	481	340	141	0.282	0.141
TS-1-R1	83	1.34	30.3	481	339	142	0.285	0.144
TS-1-R2	82	1.32	30.2	483	320	163	0.287	0.152
TS-1-R3	87	1.33	30.3	486	346	140	0.274	0.135
TS-1-R4	89	1.31	30.2	490	353	137	0.268	0.134
TS-1-R5	88	1.30	30.2	481	336	144	0.283	0.145

The molar ratios of Si/Ti in bulk TS-1 samples are shown in Table . The Si/Ti molar ratios of all TS-1 samples are similar.

FT-IR spectra of the TS-1 samples are shown in Figure B, and the values of I960/I800 of all the samples are summarized in Table . There is no obvious difference in the value of I960/I800 between the samples synthesized with the recycled mother liquid and TS-1-S, indicating that the content of framework Ti of TS-1 samples synthesized with the recycled mother liquid is similar to that of TS-1-S. Figure C shows the UV–vis spectra of TS-1 samples. A weak absorption band located in the range of 310–330 nm appears on the UV–vis spectra of TS-1-R3, TS-1-R4, and TS-1-R5, confirming that anatase TiO2 exists in these samples. It is perhaps caused by some accumulated impurities such as sodium ions during the repeated recycle of the mother liquid. The concentration of sodium ions in the mother liquor of TS-1-S and TS-1-R5 had been extracted by ICP-OES, which increased from 7.71 to 21.9 mg/L.

The nitrogen absorption isotherms of TS-1 samples are illustrated in Figure D. For the sake of clarity, the isotherms in of TS-1-R1 (b), TS-1-R2 (c), TS-1-R3 (d), TS-1-R4 (e), and TS-1-R5 (f) have been artificially shifted vertically upward to 30, 60, 90, 120, and 150 cm3·g–1, respectively. The isotherms of the samples synthesized with the recycled mother liquid are similar to that of TS-1-S. As shown in Table , the surface area and the pore volume of synthesized samples do not change significantly with the repeated reuse of the recycled mother liquid. The morphologies of TS-1 samples synthesized with the recycled mother liquid are also quite similar with the sample of TS-1-S (Figure ). The average particles size of TS-1-R1, TS-1-R2, TS-1-R3, TS-1-R4, and TS-1-R5 are 0.48, 0.46, 0.50, 0.50, and 0.49 μm, respectively.

Figure 4

SEM images of TS-1-S (a), TS-1-R1 (b), TS-1-R2 (c), TS-1-R3 (d), TS-1-R4 (e), and TS-1-R5 (f).

Table S5 shows the results of propylene epoxidation for 60 min over TS-1 samples synthesized with the mother liquid. The catalytic activities of the catalysts are high and similar. The average H2O2 conversion, utilization, selectivity, and propylene oxide yield over the obtained samples reach to 98.4, 99.0, 94.5, and 97.2%, respectively. The results of propylene epoxidation for 30 min over TS-1 samples synthesized with the mother liquid are listed in Table . The catalytic activities of the catalysts are also similar. The average H2O2 conversion, utilization, selectivity and propylene oxide yield over the obtained samples reach to 91.7, 83.5, 99.2, and 91.8%, respectively. The results demonstrate that the mother liquid of TS-1 synthesized with the assistance of NH4HCO3 and suspended seeds is recyclable, and the TS-1 samples synthesized by using the recycled mother liquid have good catalytic performance.

Table 4

Catalytic Performances for Propylene Epoxidation over TS-1 Samplesa

sample	XH2O2 (%)	YPO (%)	SPO (%)	UH2O2 (%)
TS-1-S	91.9	83.3	99.5	91.0
TS-1-R1	91.6	83.8	99.0	92.4
TS-1-R2	91.4	83.0	98.4	92.3
TS-1-R3	91.8	83.6	99.6	91.4
TS-1-R4	92.0	83.5	99.1	91.6
TS-1-R5	91.3	83.6	99.4	92.1

Reaction conditions: propylene (0.7 MPa), catalyst (150 mg), solvent (595 mmol), H2O2 (97 mmol), 318 K, and 30 min.

Conclusions

In summary, the addition of NH4HCO3 can decrease the decomposition amount of the template (TPA+) during the crystallization process of zeolites. TS-1 synthesized by using NH4HCO3 and suspended seeds as an assistant presented small particle size and high-content framework Ti and exhibited good catalytic activity in propylene epoxidation. Also, the recycled mother liquid can be repeatedly reused as the media of hydrothermal crystallization and the supplement of templates. In the sustainable preparation by using the mother liquid, the amount of TPAOH added can be reduced significantly. TS-1 samples synthesized by using the recycled mother liquid exhibited similar morphology and textural characteristics to the sample of the initial synthesis with fresh materials and also exhibited good catalytic activities in propylene epoxidation. This work provides an alternative process that can effectively reduce the discharge of the waste liquid containing organic amines and economize on raw materials such as templates for the cleaner preparation of TS-1 with good catalytic performance.

Experimental Section

Preparation of TS-1

Synthesis of Seeds

Tetraethyl orthosilicate (TEOS, 28%) was hydrolyzed in a mixed solution of tetrapropylammonium hydroxide (TPAOH, 25%) and distilled water at 343 K, and a clear sol with a molar composition of 1 SiO2:0.256 TPAOH:30 H2O was obtained. Afterward, the sol was transferred to an autoclave and heated at 448 K for 24 h. The suspension seed was obtained. Then, the suspension seed was filtered and washed with distilled water, dried at 373 K for 12 h, and then calcined at 823 K for 6 h. Afterward, the calcined seed was obtained.[42]

Synthesis of TS-1

TEOS was mixed with deionized water and TPAOH solution, and the mixture was stirred at 343 K for 3 h. After that, tert-butyl titanate (TBOT) was mixed with isopropanol (IPA) and was added dropwise under stirring, and finally, a sol was obtained. The molar composition of the sol is 1 SiO2:0.033 TiO2:0.25 TPAOH:0.81 IPA:30 H2O. The above resulting mixture was added with NH4HCO3 and stirred at 353 K for 3 h and then transferred into a Teflon-lined stainless-steel autoclave. Afterward, the mixtures were hydrothermally crystallized at 453 K for 48 h. Subsequently, the product was filtered, washed with distilled water, dried at 373 K for 12 h, and then calcined at 823 K for 6 h. The samples with n(NH4HCO3)/n(TPAOH) = 0, 0.5, and 1 were denoted as TS-1-0, TS-1-0.5, and TS-1-1, respectively. The synthesis of TS-1 with assistance of seeds is the same as that of TS-1-1 except the extra addition of seeds while adding NH4HCO3, and the amount of seeds was 2 wt % of the total amount of silica in the precursor sol. The sample synthesized by adding NH4HCO3 and suspended seeds was denoted as TS-1-S. The sample synthesized by adding NH4HCO3 and calcined seeds was denoted as TS-1-C.

After the synthesis of TS-1, the mother liquid obtained from the previous batch synthesis was recycled and reused in the procedure of the next generation of samples. The experimental steps were the same with the initial synthesis with fresh materials. The obtained samples were denoted as TS-1-R1, TS-1-R2, TS-1-R3, TS-1-R4, and TS-1-R5 in turn.

Characterizations

Elemental compositions were determined by inductively coupled plasma (ICP) analyses carried out on a Thermo Scientific ICAP6000 instrument. Powder X-ray diffraction (XRD) was performed on a Panalytical X’Pert PRO diffractometer with Cu Kα (λ = 1.5406 Å) in the 2θ range of 5–40°. Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer FT-IR spectrometer from 4000 to 400 cm–1. Ultraviolet–visible diffuse reflectance (UV–vis) spectra were obtained on an Agilent Cary 5000 spectrometer in the region of 190 to 800 nm by using pure BaSO4 as the reference. The nitrogen adsorption and desorption isotherms were measured on an ASAP 2420 surface area analyzer (Micromeritics, USA) at 77 K; the samples were degassed under the condition of 150 °C for 3 h before N2 physisorption. After the test is completed, the specific surface area of the sample is calculated by the BET equation, and the pore size distribution is calculated by the Barrett–Joyner–Halenda (BJH) method. Scanning electron microscopy (SEM) images were obtained with an S-4800 scanning microanalyzer. All mother liquids produced from the synthesis with fresh materials were extracted by dichloromethane and analyzed by using a Thermo Fisher Scientific DSQ II series gas chromatograph and mass spectrometer system (GC–MS). In addition, the concentrations of the major substance in all extracts were analyzed using a GC 9790 plus gas chromatograph equipped with a flame ionization detector (FID) and a KB-624 capillary column (60 m × 0.25 mm × 0.33 μm).

Propylene Epoxidation

The epoxidation of propylene was carried out in a 200 mL stainless steel reactor. Briefly, 0.15 g of TS-1, 3 mL of 27.5 wt % H2O2, and 24 mL of methanol were fed into the reactor, and the mixed solution was adjusted to a stable pH (4.90) with ammonium hydroxide (0.01 M NH4OH). Then, the sealed stainless-steel reactor was placed in a water bath with magnetic stirring. When the temperature reached 318 K, propylene was charged to 0.7 MPa, and then, the reaction was kept for 30 and 60 min. Initial and residual H2O2 was checked by iodometric titration. The reaction products were analyzed on a PANNA A91 gas chromatography system with a flame ionization detector (FID) and an FFAP column (30 m × 0.32 mm × 0.25 μm). The main product was propylene oxide (PO), and the byproducts were mainly propylene glycol monomethyl ethers (MME) and propylene glycol (PG). The conversion of H2O2 (XH2O2), selectivity to PO (SPO), yield of PO (YPO), and utilization of H2O2 (UH2O2) were calculated as follows