Role of Supports in the Tetrapropylammonium Hydroxide Treated

Article pubs.acs.org/IECR

Role of Supports in the Tetrapropylammonium Hydroxide Treated Titanium Silicalite‑1 Extrudates Yi Zuo,† Min Liu,† Luwei Hong,† Mengtong Wu,† Ting Zhang,† Mengtong Ma,† Chunshan Song,‡ and Xinwen Guo*,† †

State Key Laboratory of Fine Chemicals, PSU−DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ EMS Energy Institute, PSU−DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, State College, Pennsylvania 16802, United States ABSTRACT: Titanium silicalite-1 (TS-1) was extruded with silica and alumina supports, and then the extrudates were treated with dilute tetrapropylammonium hydroxide (TPAOH) solution. The treated TS-1 extrudates were characterized and evaluated in the epoxidation of propene. The two supports exhibited different properties after the treatment. The dissolution of silicon, forming hollows in the TS-1 crystals, preferably occurred when alumina support was used. The inhibition of generation of hollows in the silica supported TS-1 arose from the dissolution and recrystallization of silica support. The crystallization of support also led to the unblocking of inherent micropores, which was beneficial for improving the catalytic performance. Alumina support showed a different result. A very small amount of aluminum was inserted into the framework of TS-1, but the small amount of framework Al caused a sharp increase of the acid amount, and thus a low selectivity of propene oxide and poor catalyst stability in propene epoxidation. basicity and introduction of Br− usually make the crystal size larger, and this will further lead to a poorer catalytic performance. Therefore, TPAOH was still irreplaceable for TS-1 preparation. The epoxidation of propene to produce PO in industry often employs a fixed-bed reactor, which needs shaped catalysts. There are mainly two methods for shaping the powdery TS-1. One is spraying or in situ synthesizing TS-1 on inert supports, of which the active component loading is small, so that the output of PO is quite low. The other method is extruding powdery TS-1 with supports, producing TS-1 extrudate.18 The TS-1 extrudate has a much more active component loading. Nevertheless, the diffusion pathway in the shaped catalyst is too long for products and reaction exotherm to diffuse out of the catalyst;19 thus, PO may react with solvents and H2O2 may decompose. Moreover, supports can block the micropores of TS-1, which will show a negative effect on the diffusion of substrates.20 Therefore, eliminating the diffusion limitation is the key point of this shaping method. In many previous works, researchers found that the treatment of powdery TS-1 with dilute TPAOH solution could improve the diffusion property by generating some mesopores in the TS-1 crystals.21,22 The treatment involves the dissolution and recrystallization of silica in TS-1. The silica inside the crystals was dissolved, transferred to the outside of channels, and recrystallized on the external surface. Song et al. found that treatment with TPAOH could improve the catalytic activity and stability of silica supported TS-1.23

1. INTRODUCTION Propene oxide (PO) is an important chemical intermediate for producing polyether polyol polymers. The commercial routes for PO manufacture are the chlorohydrin and Halcon routes, which produce much pollution and coproducts, respectively.1,2 The titanium silicalite-1 (TS-1) catalyzed propene epoxidation with hydrogen peroxide to produce PO (HPPO) route is the most possible environmentally friendly alternative for the current routes. In recent years, the HPPO route became a mature technology and BASF/Dow Chemical and Evonik/ Uhde in Belgium and South Korea, respectively, tried to commercialize it.3,4 However, many problems still exist, such as the low output and short lifetime of the catalyst. TS-1 was first hydrothermally synthesized by Taramasso et al. in 1983.5 The unique catalytic performance of TS-1/H2O2 in selective oxidation reactions, such as oxidation of alkanes, epoxidation of alkenes, and hydroxylation of aromatics, attracts much attention.6−8 The classical synthesis method provided by Taramasso et al. using tetrapropylammonium hydroxide (TPAOH) as the template can produce TS-1 with very a small crystal size (∼150 nm). However, the synthesis process must be operated in a glovebox in order to prevent tetraethyl titanate (TEOT) from getting into contact with water and/or CO2. An improved synthesis method was reported by Thangaraj et al. using tetrabutyl titanate (TBOT) as the titanium source, instead of TEOT.9 This method saves time (12 h) and provides a relatively mild condition. From then on, many works focused on improving the synthesis procedure and catalytic performance of TS-1 by changing titanium and silicon sources,10−12 tuning templates,13,14 and adding additives.15,16 For cutting down the cost of TS-1, many researchers tried to use tetrapropylammonium bromide (TPABr) as the template, due to the high cost of TPAOH.17 However, the decrease of © 2015 American Chemical Society

Received: Revised: Accepted: Published: 1513

November 18, 2014 January 15, 2015 January 21, 2015 January 21, 2015 DOI: 10.1021/ie504531v Ind. Eng. Chem. Res. 2015, 54, 1513−1519

Article

Industrial & Engineering Chemistry Research

extrudates at 25 °C. The weight increments of the samples with prolonging of adsorption time were measured. 2.3. Epoxidation of Propene. The epoxidation of propene was carried out in a fixed-bed reactor. The loading of TS-1 extrudates was 7.0 g and typical reaction conditions were as follows: methanol as solvent; temperature 40 °C; concentration of H2O2 3.0 mol/L; molar ratio of C3H6/H2O2 3/1; pressure 3.2 MPa; weight hourly space velocity (WHSV) of propene 1.4 h−1. The residual H2O2 was checked by iodometric titration. The products were analyzed on a Tianmei 7890F gas chromatograph with an FID and a PEG-20 M capillary column (30 m × 0.25 mm × 0.4 μm). PO was the main product, and propylene glycol (PG) and its monomethyl ethers (MME) were the byproducts. The conversion of H2O2 (X(H2O2)), selectivity of PO (S(PO)), and utilization of H2O2 (U(H2O2)) were calculated with eqs 1, 2, and 3, respectively:

In this work, we extruded TS-1 with different supports (silica and alumina) and treated the TS-1 extrudates with dilute TPAOH solution with the aim of improving their diffusion properties. The effects of different supports in the treatment were studied. It was found that the crystallization of supports (insertion into the TS-1 framework) influenced the catalytic performances significantly.

2. EXPERIMENTAL SECTION 2.1. Preparation of TS-1 Extrudates. Nanosized TS-1 was synthesized in TPAOH hydrothermal system according to ref 24. Tetraethyl orthosilicate and TBOT were used as silicon and titanium sources, respectively. The molar composition was SiO2:TiO2:TPA+:H2O = 1:0.025:0.33:10. The synthesis gel was crystallized at 170 °C for 48 h. The obtained solid was separated from the mother liquor, washed, dried, and calcined at 540 °C for 6 h. The nanosized TS-1 powder was extruded with silica sol or γalumina,20 getting TS-1/SiO2 or TS-1/Al2O3, respectively. The mass ratio of TS-1/support was 4/1. TS-1/SiO2 and TS-1/ Al2O3 were treated with 0.06 mol/L TPAOH solution at 170 °C for 48 h, and then washed, dried, and calcined at 540 °C for 6 h. The treated samples were denoted as TPA/TS-1/SiO2 and TPA/TS-1/Al2O3, respectively. Classic TS-1 was also synthesized by using an improved conventional method for comparison, according to ref 9. 2.2. Characterization of TS-1. X-ray powder diffraction (XRD) patterns were generated on a Rigaku Corp. SmartLab 9 X-ray diffractometer using Cu Kα radiation. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker EQUINOX55 spectrometer from 4000 to 400 cm−1, and the KBr pellet technique was adopted. Ultraviolet−visible diffuse reflectance (UV−vis) spectra with wavelengths from 190 to 500 nm were obtained on a Jasco UV-550 spectrometer, and pure BaSO4 was used as reference. The appearances of the crystals was detected on a Tecnai G220 S-Twin transmission electron microscope (TEM). The elemental analysis of TS-1 was carried out on a Thermo VG ESCALAB250 instrument using Al Kα radiation and operating at a constant power of 260 W, and a PerkinElmer OPTIMA 2000DV ICP optical emission spectrometer. Nitrogen physisorption measurements were performed at liquid nitrogen temperature (−196 °C) on a Quantachrome Autosorb iQ2 physical sorption apparatus. Surface area and pore volume were calculated according to the BET and t-plot method, respectively. NH3 temperature programmed desorption (NH3-TPD) was performed on a Quantachrome CHEMBET-3000 apparatus at a temperature ramp rate of 10 °C/min from 120 to 650 °C. Before the test, samples were pretreated as follows: He flow for 20 min at room temperature, heating to 500 °C at 10 °C/min and then holding for 60 min, decreasing the temperature to 120 °C in 1 min and followed by holding for 30 min. Thermogravimetry (TG) and differential thermogravimetry (DTG) were performed on a Mettler-Toledo TGA/SDT851e instrument with a nitrogen flow rate of 60 mL/min. The samples were heated from room temperature to 800 °C at 10 °C/min. n-Hexane and cyclohexane physisorption were carried out on a homemade physical adsorption apparatus. The samples were dried at 120 °C for 1 h in nitrogen before the test. The adsorption was performed as follows: 0.2 g of TS-1 extrudates were settled in a glass U-tube. The saturated n-hexane or cyclohexane vapor was taken into the U-tube by nitrogen and adsorbed by TS-1

X(H 2O2 ) = (n0(H 2O2 ) − n(H 2O2 ))/n0(H 2O2 )

(1)

S(PO) = n(PO)/(n(PO) + n(MME) + n(PG))

(2)

U (H 2O2 ) = (n(PO) + n(MME) + n(PG)) /(n0(H 2O2 ) X(H 2O2 ))

(3)

The n0(H2O2) and n(H2O2) represent the initial and final molar contents of H2O2, respectively. The n(PO), n(MME), and n(PG) stand for the molar contents of PO, MME, and PG, respectively. The epoxidation of propene also carried out in a stainless steel batch reactor. Powdered TS-1, methanol, and H2O2 were fed into the reactor; then propene was charged to 0.4 MPa. The reaction was kept at 40 °C for 1 h. The residual H2O2 and the products were analyzed with the same method with those in the fixed-bed reactor.

3. RESULTS AND DISCUSSION The nanosized TS-1 powder and extrudates before and after TPAOH treatment were characterized by various tools to determine their physicochemical properties. XRD patterns of the samples are shown in Figure 1. Five characteristic peaks sited at 2θ of 7.8, 8.8, 23.0, 23.9, and 24.4° were observed in each sample, indicating that extrusion and treatment did not destroy the framework of TS-1. However, the relative crystallinities (RCs) of the samples, which were calculated by comparing the total intensity of the five characteristic peaks of the samples with that of nanosized TS-1, are quite different.

Figure 1. XRD patterns of nanosized TS-1 powder and extrudates. RC is short for relative crystallinity. 1514

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by isolated [TiO4] or [HOTiO3] with two water molecules,31 but some thought it was caused by the condensation of hexahedrally coordinated Ti involving a Ti−O−Ti bond.32 We found that there at least two Ti species exist in this region.33 One is active pentahedrally coordinated Ti (230−250 nm); the other is inert octahedrally coordinated Ti (250−290 nm). The state of Ti at 240−280 nm in the three samples is primarily octahedral coordination. The content of this state Ti in TPA/ TS-1/SiO2 is the largest; next is TPA/TS-1/Al2O3. Powdery TS-1 contains the least octahedrally coordinated Ti. The treatment with TPAOH involves the dissolution and recrystallization of silica. The crystallization process leads to the rearrangement of [TiO4] and [SiO4], which will saturate the tetrahedrally coordinated Ti and increase the content of the octahedral one. TPA/TS-1/SiO2 has the most content of silicon, due to the introduction of silica support. Therefore, more silicon can coordinate with titanium, forming octahedrally coordinated Ti. Although there was no silica on the external surface of TPA/TS-1/Al2O3, the dissolved silica from the inside of crystals also showed the same effect, increasing the amount of octahedrally coordinated Ti. Furthermore, the bathochromic shift of this band in TPA/TS-1/SiO2 indicates that the coordination number is more than those of the other two samples. The FT-IR spectra are shown in Figure 3. The characteristic bands of MFI topology are sited at 450, 550, 800, 1110, and

The RC of nanosized TS-1 was considered as 100%. The RCs of TS-1/SiO2 and TS-1/Al2O3 decreased to different extents, mainly due to the introduction of amorphous supports and the blocking of channels by supports. The peaks at 7.8 and 8.8° were assigned to planes (011) and (020) of TS-1, which could be considered as the characteristic peaks of TS-1 channels.25 Extrusion makes the intensity of the two peaks decrease sharply, suggesting that extrusion can block the channels of TS1, which may show a negative effect on the catalytic performance. Furthermore, the percentage of TS-1 in the extrudate was reduced, which also decreased the intensity of the XRD patterns. Silica particles are bigger than γ-Al2O3 (cf. TEM images), leading to more γ-Al2O3 diffusing into the TS-1 channels and more serious blocking of channels. Therefore, the RC of TS-1/Al2O3 is lower than that of TS-1/SiO2. After the treatment, the RCs increased obviously, which may be accounted for by the dissolution and crystallization of supports and other amorphous substrates in TS-1. The RC of TPA/TS1/SiO2 was even higher than that of nanosized TS-1, indicating that a large amount of amorphous substrates crystallized in the treatment. It also can be seen that the intensity of the first two peaks recovered in the treated samples, demonstrating that the channels were unblocked partly. Figure 2 shows the UV−vis spectra of nanosized TS-1 and TPAOH treated TS-1 extrudates. This spectroscopy is quite

Figure 3. FT-IR spectra of nanosized TS-1 powder and extrudates. Figure 2. UV−vis spectra of nanosized TS-1 powder and extrudates.

1220 cm−1.34 The band at 960 cm−1 usually appears in the spectrum of TS-1, and is thought to be related to the insertion of Ti into the TS-1 framework.35 The more framework Ti content there is, the stronger the intensity of this band is. From the spectra of samples, it can be seen that the intensity of this band decreases after extrusion and treatment with TPAOH. The decrease was considered to mainly occur during extrusion, and was caused by the blocking of channels and covering of tetrahedrally coordinated Ti. The treatment with TPAOH makes silica support crystallize, form new frameworks, and unblock the channels. Therefore, the band at 960 cm−1 in TPA/TS-1/SiO2 recovered partly. The weak band at ∼870 cm−1 was assigned to the Si−OH group,36 the intensity of which can be considered as a reflectance of the intact extent of the surface. A stronger intensity indicates more defects exist on the surface of TPA/TS-1/Al2O3. The nitrogen physisorption curves and thus obtained surface areas and pore volumes of nanosized TS-1 powder and extrudates are shown in Figure 4 and Table 1. Nanosized TS-1 powder shows a type I adsorption−desorption curve, which is

sensitive for the Ti coordination state.26 The multibands were deconvoluted with the PeakFit program using the Gaussian fitting method.27 Three major bands were observed in all the samples. The band at 210−220 nm was due to the transition of the 2p electron of oxygen to the Ti4+ 3d orbit.28 This band suggests the existence of tetrahedrally coordinated Ti, which is also called framework Ti. The framework Ti is the main active center for selective oxidation, but its content cannot be higher than 2.5 wt %, because of the lattice expansion when Ti is inserted into the framework.29 The contents of framework Ti are almost the same except TPA/TS-1/Al2O3, may be accounted for by the alumina support exhibiting a negative effect on the recrystallization of Ti. The band at 310−330 nm was the characteristic band of anatase TiO2,30 which could decompose H2O2. The increase of anatase TiO2 might be harmful for the utilization of H2O2. The contribution of the band at 240−280 nm is controversial. Some researchers believed it belonged to octahedrally coordinated Ti formed 1515

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To further study the influence of treatment on the TS-1 channels, we examined the saturation adsorption amount of nhexane and cyclohexane on extruded samples (cf. Figure 5). n-

Figure 4. Nitrogen physisorption curves of nanosized TS-1 powder and extrudates.

Table 1. Surface Area and Pore Volume of Nanosized TS-1 Powder and Extrudatesa catalyst

ST (m2/g)

VT (cm3/g)

Smicro (m2/g)

Vmicro (cm3/g)

Sexter (m2/g)

nanosized TS-1 TS-1/Al2O3 TS-1/SiO2 TPA/TS-1/Al2O3 TPA/TS-1/SiO2

503 432 424 510 499

0.43 0.45 0.41 0.75 0.54

448 317 312 332 454

0.19 0.13 0.13 0.14 0.20

55 115 112 178 45

Figure 5. n-Hexane (a) and cyclohexane (b) physical adsorption curves of TPAOH treated TS-1 extrudates.

Hexane can access the pores of TS-1, while cyclohexane can only adsorb on the external surface and orifices.37 Therefore, the two adsorbates characterize different parts of the catalysts. The saturation adsorption amount of n-hexane increased after the treatment; furthermore, the adsorption rates of the treated samples were both faster than those of the untreated ones, indicating that the channels of TS-1 extrudates were unblocked in the treatment. The saturation adsorption amount of the samples using silica as support is obviously larger than for those using alumina, because the blocking of pores by alumina is more serious than that by silica. The trend of cyclohexane adsorption was different from that of n-hexane. After the treatment, the saturation adsorption amount over TPA/TS-1/SiO2 is lower than that over TS-1/ SiO2. The extrusion caused much disordered mesopores by the support, which is proved by N2 physisorption. These mesopores, which could be adsorbed by cyclohexane, were destroyed, and new micropores were formed (cf. TEM) by treatment with TPAOH. Therefore, the saturation adsorption amount decreased after the treatment. On the other hand, the amount over TPA/TS-1/Al2O3 was higher than that over TS1/Al2O3. The rareness of crystallization of alumina, the generation of mesopores in the crystals, and the preservation of mesopores formed by extrusion were considered as the primary reasons for this phenomenon. The TEM images (Figure 6) provide a distinct observation of the appearances of samples. It is clear that the crystals of extruded samples were surrounded by much amorphous support. After treatment with TPAOH solution, many hollows

a

ST and Smicro stand for the total and microporous surface areas, respectively. VT and Vmicro represent the total and microporous pore volumes, respectively. Sexter stands for the sum of external and mesoporous surface areas.

the typical curve of microporous materials. When the TS-1 powder was extruded with supports, the curves have a type H3 hysteresis loop, which demonstrates the disordered or lamellar pore structures. These pores were generated by the removal of pore former (Sesbania powder) in extrusion. However, the total surface areas of the two samples decreased obviously, regardless of the newly formed disordered pores. This is due to the blocking of inherent channels by supports, resulting in the sharp decrease of microporous surface area. After treatment with TPAOH, the type of hysteresis loops changed to H4, indicating the hollows appeared in the crystals. The total surface areas likewise recovered to that before extrusion, but the reasons for the recovery of the two treated samples are not same. The formation of many hollows in the crystals increases the mesoporous surface area of TPA/TS-1/Al2O3, but the crystallization of alumina is small. In TPA/TS-1/SiO2, much silica crystallized in the treatment, leading to a small mesoporous surface area but a large microporous area. The comprehensive result of micropores and mesopores was the similar total surface areas of the two samples. The absolutely larger total pore volume of TPA/TS-1/Al2O3 than those of the other samples was also resulted by the formation of mesopores in the crystals of TPA/TS-1/Al2O3. 1516

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when alumina was used. This also proves that silica support can prevent the leached Ti remaining in the liquid phase, but alumina cannot. The strengths of the four samples are also shown in Table 2. The radial and the axial strengths of the samples using silica support increased obviously after the treatment, demonstrating that the crystallization of the support does not affect the strength negatively. The catalytic performance for propene epoxidation over nanosized TS-1 compared with that over classic TS-1 is exhibited in Table 3. The former shows a higher H2O2 Table 3. Catalytic Performances in Propene Epoxidation over TS-1 Powdera catalyst

X(H2O2)/%

S(PO)/%

U(H2O2)/%

nanosized TS-1 classic TS-1

81.3 77.0

93.6 95.9

95.2 90.3

a

Reaction conditions: catalyst 0.2 g, methanol 32 mL, H2O2 concentration 1.1 mol/L, propene pressure 0.4 MPa, 40 °C, and 1 h.

conversion and utilization. The quite high H2O2 utilization indicates that the very small amount of anatase TiO2 hardly led to the decomposition of H2O2. The catalytic performances of TPAOH treated TS-1 extrudates in a fixed-bed reactor are shown in Figure 7. The initial conversions of H2O2 were both

Figure 6. TEM images of TS-1 extrudates before and after treatment with TPAOH.

(about 40 nm) appeared in TPA/TS-1/Al2O3, and the amount of amorphous support outside the crystals changed unobviously. We examined the elemental composition on the surface and in the bulk of the samples by XPS and ICP, respectively (Table 2). The composition on the surface of Al2O3 supported TS-1 changed a little after the treatment. The content of Si increased slightly, due to the recrystallization of silica dissolved from the inside of the crystals. On the other hand, there is hardly any hollow in TPA/TS-1/SiO2 . Moreover, the amorphous support seemed to disappear. We believe that most of the support was crystallized in the TPAOH solution, but not washed away, because of the increased relative crystallinity (XRD), the larger crystal size, and the constant Si content. We can also infer that the crystallization of amorphous silica support restrained the generation of hollows in the crystals by occupying TPA+ as the template. The content of Ti on the surface of the silica supported samples increased slightly after the treatment, but that of alumina supported samples unchanged. This is accounted for by the fact that Ti can be leached from the inside of crystals. The crystallization of silica support makes Ti insert into the framework again on the external surface, but the weak crystallization in TPA/TS-1/ Al2O3 has little effect on the insertion of Ti. Therefore, the leached Ti will stay in the liquid phase. The variation of the Si/ Ti molar ratio (n(Si/Ti)) in the bulk was also distinct when different supports were used. The n(Si/Ti) remained nearly constant when silica was used as the support, but increased

Figure 7. Catalytic performances in propene epoxidation over TPAOH treated TS-1 extrudates: solid symbols, TPA/TS-1/SiO2; hollow symbols, TPA/TS-1/Al2O3.

higher than 95%, but the stability of TPA/TS-1/Al2O3 was poorer than that of TPA/TS-1/SiO2. The excellent stability of the latter (the conversion of H2O2 was still higher than 90% after 300 h reaction) is considered to be related to a lower acid amount and more unobstructed channels. The acid amount can also be reflected indirectly by the selectivity of PO. The utilization of H2O2 over TPA/TS-1/SiO2 is higher than that over TPA/TS-1/Al2O3, which is due to the lower content of anatase TiO2 in the former.

Table 2. Elemental Composition and Strength Analysis of TS-1 Extrudates in the bulk

on the surface

strength

catalyst

Si/wt %

Ti/wt %

Al/wt %

Si/wt %

Ti/wt %

Al/wt %

radial/(N/cm)

axial/(N/cm2)

TS-1/SiO2 TS-1/Al2O3 TPA/TS-1/SiO2 TPA/TS-1/Al2O3

45.7 34.3 45.7 35.0

1.5 1.5 1.5 1.3

null 12.7 null 12.1

46.0 26.1 45.9 27.4

0.9 0.8 1.0 0.7

null 0.2 null 0.2

80.8 87.0 151.6 101.0

1024 1084 1483 959

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There are mainly three peaks in the DTG curves of the samples (Figure 9b). The peak at lower than 100 °C is the physically adsorbed solvents, and that at 100−200 °C is assigned to MME and PG. The oligomers show a desorption peak at ∼250 °C. The weight losses in different temperature regions are shown in Table 4. The total weight loss of TPA/TS-1/Al2O3 was less

The NH3-TPD curves of the nanosized TS-1 powder and extrudates are shown in Figure 8. The powdery TS-1 and TPA/

Table 4. Weight Loss of the Used TS-1 Extrudates in Different Temperature Regions weight loss/% catalyst