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Improved performance of mesoporous HZSM-5 designed with selective deactivation of external surface acidity in the isomerization of styrene oxide
Microporous and Mesoporous Materials 297 (2020) 110037
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Improved performance of mesoporous HZSM-5 designed with selective deactivation of external surface acidity in the isomerization of styrene oxide Ming-Lei Gou *, Junqing Cai, Wensheng Song, Yonghua Duan, Zhen Liu, Qingshan Niu School of Chemical Engineering and Pharmaceutics, Henan University of Science and Technology, Luoyang, Henan, 471023, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous HZSM-5 External surface deactivation Isomerization Styrene oxide Phenylacetaldehyde
A series of mesoporous HZSM-5 designed by phosphorous modification with or without hexadecyl trimethylammonium bromide (CTAB) pretreatment were prepared and tested in the isomerization of styrene oxide. CTAB pretreatment changed the external surface properties of mesoporous HZSM-5 with positive charged, which could selectively adsorb the phosphate anions and deactivate the external Brønsted acid sites caused by a framework dealumination during calcinations. The phosphate species on the external surface could not migrate into the micropores. There were only a few phosphate species formed in the micropores and most of the internal Brønsted acid sites were preserved. External surface deactivation whilst retaining the hierarchical pore structure of mesoporous HZSM-5 was achieved by phosphorous modification with CTAB pretreatment, thus providing a potential industrial application catalyst which remarkably improves the catalytic stability (lifetime 59.8 h) while preserving the high activity (>99%) and selectivity (>96%).
1. Introduction Epoxidation of olefins and subsequently ring opening of epoxides catalyzed by acidic or basic catalysts is widely used for production of aldehydes, ketones, alcohols, ethers, etc. For example, the isomerization of styrene oxides over acid catalysts provides an environmentally benign route to phenylacetaldehyde and its derivatives used for preparation of fine chemicals such as pharmaceuticals, insecticides, herbicides, fungi cides, and so on [1]. However, these reactions usually generate some high-boiling molecules via aldolization or polymerization of aldehydes, leading to rapid deactivation of catalysts by coke deposition. Being a problem at the industrial scale, the isomerization of styrene oxides has been investigated under different conditions. Liquid-phase isomerization of styrene oxides over a series of solid acid catalysts, such as natural silicates [2], silica-alumina gels [3,4], Nafion-H [5], heteropoly acids [6] and zeolites [7,8], has been extensively studied. In order to suppress the side-products and catalyst deactivation, a large amount of solvents, usually over 90 wt percent, must be employed. But the subsequent purification of the products is difficult and high energy consumption. Different styrene oxides can also be isomerized in a fixed-bed reactor under gas-phase conditions using inert gas (such as N2, He) as carrier
gas. Hoelderich et al. [9–11] have found that MFI-Type zeolites are superior to other catalysts for this process. The side-reactions can be efficiently suppressed with phenylacetaldehyde yields up to 90% at 200 � C and WHSV ¼ 3.0 h 1 because of the steric constraints of the frame work, particularly, the product can be separated in a simple manner without additional purification. However the activity of zeolites began to decrease after running for 6 h time on stream (TOS). Our previous research [12] has revealed that increasing the acid concentration while decreasing the acid strength of HZSM-5 zeolites can improve the catalyst lifetimes for this gas-phase process. Several approaches have been employed to adjust the acidity of zeolites, such as ion-exchange with Csþ or Ni2þ [9,13], dealumination [14], fluorination [15], modification with phosphorus [16], isomorphous substitution of boron [17,18] or titanium [19] into the zeolite frameworks, etc. Unfortunately, the catalyst sta bility is still not satisfactory. In view of the prior findings [9,10], the residence time over the catalyst bed should be less than 4 s, preferably less than 1 s in order to decrease probabilities of the side-reactions and prevent the catalyst deactivation. Mesoporous zeolites which possess the advantages both of zeolite crystals (good hydrothermal stability and strong acidity) and mesoporous materials (good mass transfer) can reduce the diffusion path length and enhance transport of products out of the zeolite crystals, thus
* Corresponding author. E-mail address: [email protected] (M.-L. Gou). https://doi.org/10.1016/j.micromeso.2020.110037 Received 16 October 2019; Received in revised form 14 January 2020; Accepted 16 January 2020 Available online 25 January 2020 1387-1811/© 2020 Elsevier Inc. All rights reserved.
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exhibiting higher stability than the conventional zeolites. However, the acid sites located at the external surface are readily accessed with nonselective, resulting in an apparent decline of the target product selectivity [20]. Hence, external surface deactivation whilst retaining the hierarchical pore structure of mesoporous zeolites is expected to obtain both good selectivity and stability. The main methods for surface deactivation of zeolites, such as chemical vapor or liquid deposition (CVD or CLD) with silicon alkoxides [21–23], modification with B [17, 24] and P [16,25] compounds, mechanochemical approach [26], and so on, have been well studied. However, these methods are always accompanied by decline of activity and/or stability due to the pore blockage of zeolites. Surfactant modified zeolites have been widely used to remove pollutant anions from contaminated water [27,28]. Cationic surfactants (i.e. CTAB) can be selectively exchanged with native inorganic cations contained within zeolites, forming a relatively stable organic coating on the zeolite surface. The stages and structures of CTAB loading on the zeolite surface have been studied in the literatures [29–31]. When the CTAB concentration of the solution in which zeolite is placed is main tained above the critical micelle concentration of the surfactant, a bilayer structure of surfactant forms, which can efficiently adsorb an ions. In this study, CTAB pretreatment was in order to change the external surface properties of mesoporous HZSM-5 with positive charged, which could selectively adsorb the phosphate anions and deactivate the external Brønsted acid sites. The physic-chemical and acid properties of samples were analyzed by XRF, N2 adsorption, XRD, 27 Al/31P MAS NMR, NH3-TPD and Py/Coll-FTIR. The catalytic perfor mances were evaluated in the isomerization of styrene oxide under gas-phase free of solvents.
ratios of samples on a Bruker S4 pioneer X-ray fluorescence spectrom eter. X-ray powder diffraction (XRD) was carried out on a Bruker D8 Advance diffractometer(Cu Kα radiation, λ ¼ 0.154056 nm, 40 kV, 40 mA) with a step size of 0.01� at a scanning rate of 5� /min in the range of 2θ ¼ 5–50� . N2 adsorption-desorption isotherms were obtained at 77K using a Quantachrome Autosorb-1 instrument. The total surface area (Stotal) and pore volume (Vtotal) were obtained according to the BET and BJH methods. The micropore surface area (Smicro) and volume (Vmicro) were determined by the t method. The external surface area (Sexter) and mesoporous volume (Vmeso) were calculated by subtracting the Smicro and Vmicro from the Stotal and Vtotal, respectively. The 27Al and 31P MAS NMR spectra were measured with a Varian Infinity-Plus 300 NMR spectrometer with resonance frequencies of 78.13 and 121.37 MHz. The acid properties of samples were analyzed by temperatureprogrammed desorption of ammonia (NH3-TPD) and FTIR spectra of adsorbed pyridine (Py-FTIR) and 2,4,6-collidine (Coll-FTIR). NH3-TPD was performed in quartz U-tube reactor equipped with a thermal con ductivity detector (TCD). The sample (0.1 g, 20–30 mush) was pre treated at 500 � C for 2 h in a N2 flow. After saturation with NH3, the reactor temperature was maintained at 150 � C for 1 h to remove the physically adsorbed NH3. Then, the TPD profile was recorded at a � heating rate of 15 C/min from 150 to 500 � C. Py-FTIR and Coll-FTIR were measured on a Nicolet 380 spectrometer with a resolution of 2 cm 1. The catalyst was pressed into a thin selfsupporting wafer (weight: 15 mg, diameter: 13 mm) and then acti vated at 500 � C for 2 h under vacuum (10 3 Pa). Adsorption of pyridine or 2,4,6-collidine in situ proceeded at room temperature, followed by desorption at 200 � C for 1 h. Then, an FTIR spectrum was recorded and its difference spectrum was calculated by subtracting the spectrum of the sample before probe adsorption.
2. Experimental
2.3. Catalytic measurements
2.1. Catalyst preparation
The performances of catalysts were tested in the isomerization of styrene oxide on a fixed-bed reactor (stainless steel, 9 mm inner diam eter). Typically, the catalyst (0.5 g, 20–30 mush) was packed in the constant-temperature zone and pretreated at 500 � C for 2 h in a N2 flow. Then, styrene oxide (>98 wt%, TCI Development Co., Ltd) free of any solvents was fed into the reactor at 200 � C and WHSV ¼ 3.0 h 1 with N2 flow of 120 mL/min. The reactor effluent was collected in an ice-water trap and analyzed by GC (Agilent 6820, FID detector) and GC/MS (Agilent 6890/5873, MSD detector) equipped with a VF-5ms capillary column (5%phenyl-95% dimethylpolysiloxane, 30 m � 0.25 mm id � 0.25 μm film thickness).
Conventional HZSM-5 (Si/Al ¼ 50) zeolite was purchased from Nakai University Catalyst Factory (Tianjin, PR China) used as the parent sample. The Mesoporous HZSM-5 sample (denoted as HZ-AT, where AT represents alkaline treatment) was prepared via controlled desilication in alkaline medium according to the literature [32]. The parent HZSM-5 (5.0 g) was treated in 100 mL 0.2 mol/L NaOH solution at 65 � C for 60 min. The resulting slurry was filtered, washed with demineralized water and dried at 80 � C. Prior to further treatment, the sample was ion-exchanged with a 0.1 mol/L NH4NO3 solution for three times and finally calcined at 550 � C for 5 h. Selective deactivation of external surface acid sites of HZ-AT was synthesized as follows: First, HZ-AT (1.0 g) was positive charged with 20 mL 0.05 mol/L CTAB solution at 60 � C for 24 h. The CTAB concen tration of the solution was above the critical micelle concentration of the surfactant, a bilayer structure of the surfactant formed, which could efficiently adsorbed anions. Then, the sample was recovered by filtra tion, washed and dried at 80 � C. Second, phosphorus modified samples pretreated by CTAB were prepared by impregnating with an aqueous solution containing the desired amount of phosphoric acid. The phos phorus contents of the samples were 0.5, 1.0, 1.5 and 2.0 wt%, which were named HZ-AT-SM-0.5P, HZ-AT-SM-1.0P, HZ-AT-SM-1.5P, HZ-ATSM-2.0P (where SM represents surfactant modification, P represents phosphorous modification and the figure represents the impregnation amount of phosphoric acid), respectively. Meanwhile, a sample with 1.0 wt% phosphorus content on HZ-AT without CTAB pretreatment was also prepared and named HZ-AT-1.0P. Finally, all the phosphorus impreg nated samples were obtained after drying at 110 � C for 4 h and calcining at 550 � C for 5 h.
3. Results and discussion 3.1. Physic-chemical properties of catalysts The XRD patterns of all samples in Fig. 1 exhibited the typical MFI topology and no other phases of phosphorous species had been found. The relative crystallinity (RC) was calculated from the intensity of main peaks between 2θ ¼ 22–25� using the parent HZSM-5 as the reference. HZ-AT successfully preserved the lattice structure of zeolite, although the RC slightly decreased to 98%. The RC of phosphorus modified samples with CTAB treatment slightly decreased from 97% (HZ-AT-SM0.5P) to 85% (HZ-AT-SM-2.0P) with phosphorus addition, which can be attributed to lattice defects caused by dealumination upon phosphorus modification [33,34]. While the RC of HZ-AT-1.0P (75%) without CTAB pretreatment was much less than that of HZ-AT-SM-1.0P (92%), sug gesting that CTAB pretreatment for positive charged of the external suface of mesoporous HZSM-5 can protect the zeolite framewotk from attack by phosphorus species. Fig. 2(A) shows the N2 adsorption-desorption isotherms of samples and their texture properties are summarized in Table 1. The parent HZSM-5 exhibited a type I isotherm, characteristic of microporous
2.2. Catalyst characterization X-ray fluorescence (XRF) was used to determine the Si/Al and P/Al 2
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external surface can facilitate adsorption of phosphate anions. That is to say, the phosphate species more easily enter the micropores and block them without surface modification by CTAB. 27 Al and 31P MAS NMR analyses have also been conducted to monitor the chemical states of Al and P species. As exhibited in Fig. 3(A), each sample showed an intense peak at around 53 ppm corresponding to the tetrahedral-coordinated framework aluminum (FAL) species with strong Brønsted acidity [36]. The HZSM-5 and HZ-AT samples presented similar spectra, although a negligible peak at 0 ppm related to the octahedral extra-framework aluminum (EFAL) species was observed for HZ-AT, which meaning that the chemical state of Al was little changed after alkaline treatment. After introduction of phosphorus, the peak of FAL species decreased dramatically and the peak of EFAL species shifted from 0 to 14 ppm due to attaching of the EFAL species to phosphorus species, indicating that some FAL species turned into extra-framework aluminum phosphates via dealumination upon phosphorus modifica tion. The higher peak at 14 ppm of HZ-AT-1.0P than that of HZ-AT-SM-1.0P demonstrated again that CTAB pretreatment can protect the zeolite framewotk from attack by phosphorus species, which was consistent with the XRD results. In addition, a small peak at around 34 ppm appeared on the samples with high phosphorus content (e.g., HZ-AT-SM-1.5P, HZ-AT-SM-2.0P) could be assigned to the intermediate aluminum species in a distorted environment or the penta-coordinated species at either framework or non-framenwork positions [37]. From above, the deactivation of Brønsted acid sites by phosphate species is mostly caused by a framework dealumination via the reaction between phosphorous acid and FAL species during calcinations. Different EFAL species (i.e., penta-coordinated aluminum, distorted tetrahedral coor dinated aluminum or octahedral coordinated aluminum) mostly con nected to phosphate species are formed. Penta-coordinated aluminum or distorted tetrahedral coordinated aluminum may be intermediate spe cies in the dealumination process. As shown in the 31P MAS NMR spectra (Fig. 3(B)), the small peaks at 1 and -6 ppm were generally associated with free monomeric ortho phosphate (PO4) groups and terminal phosphate groups of pyrophos phate or linear polyphosphate not connected to aluminum [38], whereas the peaks between 12 and 19 ppm were related to the terminal or intermediate groups in polyphosphates connected to aluminum [37,39]. The broad signal at 26 ppm was caused by polymeric phosphates attached to aluminum, and the signals at 32 and 40 ppm were respectively assigned to amorphous aluminum phosphate and highly condensed polyphosphate [33,37]. It can be seen in Fig. 3(B) that larger amount of phosphorous existed as free monomeric orthophosphate and
Fig. 1. XRD patterns of HZSM-5 and modified samples.
material with limited external surface area (17.9 m2/g) and meso porosity volume (0.014 cm3/g). While the samples with alkaline-treated (HZ-AT) and subsequent phosphorous modification (HZ-AT-SM-0.5P/ 1.0P/1.5P/2.0P and HZ-AT-1.0P) all presented a mixture of type I and IV isotherms, in which the enhanced N2 uptake at p/p0 ¼ 0.4–0.9 accom panied by a hysteresis loop for each sample confirmed the presence of mesoporosity with pore size at approximately 10–20 nm (Fig. 2(C)). The size of micropores (centered at about 0.55 nm) remained almost un changed for all the samples as shown in Fig. 2(B), indicating that the microporous structures were well preserved upon alkaline treatment and phosphorus modification. As shown in Table 1, HZ-AT had the maximum external surface area (108.2 m2/g) and mesoporosity volume (0.086 cm3/g) owing to the controlled desilication in the alkaline me dium. Comparing HZ-AT (Smicro ¼ 233.4 m2/g, Vmicro ¼ 0.125 cm3/g), the micropore surface areas (233.1–220.2 m2/g) and volumes (0.124–0.118 cm3/g) of phosphorus modified samples with CTAB pre treatment (HZ-AT-SM-0.5P/1.0P/1.5P/2.0P) showed slightly decline with increasing of phosphorus impregnation amount, while the phos phorus modified samples without CTAB pretreatment (HZ-AT-1.0P) showed the minimum micropore surface area (205.1 m2/g) and volume (0.089 cm3/g). This can be explained that the cationic surfactant (CTAB) may be selectively coated on the external surface of mesoporous HZSM-5 (HZ-AT) because of its large size [35], and the resulting positive charged
Fig. 2. N2 adsorption-desorption isotherms (A) and the corresponding H–K micropore size (B) and BJH mesopore size (C) distribution of HZSM-5 and modified samples: (a) HZSM-5, (b) HZ-AT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM-1.5p, (f) HZ-AT-SM-2.0P, (g) HZ-AT-1.0P. 3
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Table 1 Chemical composition and textural properties of the parent and modified HZSM-5 zeolites. Sample
Si/Al
P/Al
Stotal (m2/g)
Smicro (m2/g)
Sexter (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
HZSM-5 HZ-AT HZ-AT-SM-0.5P HZ-AT-SM-1.0P HZ-AT-SM-1.5P HZ-AT-SM-2.0P HZ-AT-1.0P HZ-AT-SM-1.0P (after six run)
50.3 33.7 33.6 33.8 33.7 33.8 33.9 34.1
– – 0.16 0.27 0.41 0.58 0.28 0.29
292.7 341.6 339.4 333.9 328.2 321.8 310.4 332.5
274.8 233.4 233.1 230.9 225.7 220.2 205.1 230.8
17.9 108.2 106.3 103.0 102.5 101.6 105.3 101.7
0.144 0.211 0.207 0.201 0.192 0.181 0.174 0.200
0.130 0.125 0.124 0.123 0.120 0.118 0.089 0.123
0.014 0.086 0.083 0.078 0.072 0.063 0.085 0.077
Fig. 3. 27Al (A) and 31P (B) MAS NMR spectra of HZSM-5 and modified samples: (a) HZSM-5, (b) HZ-AT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM1.5P, (f) HZ-AT-SM-2.0P, (g) HZ-AT-1.0P.
pyrophosphate or linear polyphosphate not connected to aluminum on HZ-AT-SM-1.0P, which is due to the positive charged external surface after CTAB modification can facilitate adsorption of phosphate anions, thus preventing the phosphate species from entering the micropores of samples and combining with aluminum. 3.2. Acid properties of catalysts The NH3-TPD was used to analyze the acid strength and concentra tion of samples, as shown in Fig. 4 and Table 2. There were three different desorption peaks at <200, 250–279 and 371–435 � C, which were corresponding to physisorbed NH3 or NH3 adsorbed on external silanol groups, weak and strong acid sites, respectively [40]. Compared with HZSM-5, HZ-AT had similar acid strength and concentration, meaning that the alkaline treatment had little influence on the FAL and EFAL species (well known as strong and weak acid sites). However, after modification with phosphorus, the high-temperature peak slightly shifted to lower temperature and lost its intensity, implying the strength (from 402 to 384 � C) and concentration (from 0.36 to 0.13 mmol/g) of strong acid sites reduced. This is consistent with the results of 27Al MAS NMR and XRD that some FAL species turned into extra-framework aluminum phosphates via dealumination upon phosphorus modifica tion. Furthermore, the strength and concentration of strong acid sites (371 � C, 0.07 mmol/g) on HZ-AT-1.0P were significantly lower than those on HZ-AT-SM-1.0P (398 � C, 0.35 mmol/g), which may be ascribed to the phosphate species more easily diffuse into the micropores and deactivate strong acid sites without surface modification by CTAB sup ported by the texture analysis. The type and accessibility of acid sites were analyzed by FTIR spectra of adsorbed pyridine (5.7 Å in diameter) and 2,4,6-collidine (7.4 Å in diameter), in which Py-FTIR can be used to determine the acidity type without distinguishing the internal or external acid sites, while CollFTIR can only probe the external acid sites [41,42]. As illustrated in
Fig. 4. NH3-TPD curves of HZSM-5 and modified samples: (a) HZSM-5, (b) HZAT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM-1.5p, (f) HZ-AT-SM2.0P, (g) HZ-AT-1.0P.
Fig. 5(A), the characteristic peaks at 1445 cm 1 and 1545 cm 1 were related to the pyridine molecules interacting with Lewis and Brønsted acid sites, respectively [43]. The percentages of Lewis and Brønsted acid sites based on HZSM-5 were calculated by the integrated area of each characteristic peak and summarized in Table 2. Likewise to the NH3-TPD results, the alkaline treatment caused a slight decrease (2%) of Brønsted acid sites and increase (4%) of Lewis acid sites, and an increase of phosphorus content brought a decrease of Brønsted acid sites and weakening of Lewis acid sites by the formation of aluminum phosphate species [16,33]. The remaining percentage of Brønsted acid sites on 4
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compared to HZ-AT. The above results indicated that selective deacti vation of external surface acid sites of mesoporous HZSM-5 could be achieved by phosphorous with CTAB pretreatment. In summary, CTAB pretreatment is in order to change the external surface properties of mesoporous HZSM-5 with positive charged, which can selectively adsorb the phosphate anions. Only a few phosphate an ions can enter into the micropores if the impregnation amount is limited. After drying at 110 � C and calcining at 550 � C, CTAB is completely decomposed and the phosphate species are formed on the external surface of HZSM-5. Therefore, these phosphate species can deactivate the external Brønsted acid sites caused by a framework dealumination via the reaction between phosphorous and FAL species during calcina tions after thermally decomposing of CTAB. No matter before or after decomposing of CTAB, the phosphate species on the external surface apparently cannot migrate into the micropores. There are only a few phosphate species formed in the micropores and most of the internal Brønsted acid sites are preserved.
Table 2 Acid properties of the parent and modified HZSM-5 zeolites. Sample
HZSM-5 HZ-AT HZ-AT-SM0.5P HZ-AT-SM1.0P HZ-AT-SM1.5P HZ-AT-SM2.0P HZ-AT1.0P HZ-AT-SM1.0P (after six run)
Amounts of acid sites by NH3-TPD (mmol/g)
Percentage of acid sites by Py/CollFTIR (%)
Weak (250–279 � C)
Strong (371–435 � C)
Brønsted acid sites
Lewis acid sites
External acid sites
0.11 0.09 0.14
0.56 0.55 0.36
100% 98% 90%
100% 104% –
trace 100% 15%
0.16
0.35
85%
–
trace
0.19
0.27
79%
–
trace
0.29
0.13
55%
–
trace
0.28
0.07
45%
–
36%
0.14
0.35
85%
–
trace
3.3. Performances of catalysts Styrene oxide is a substance with high reactivity to phenyl acetaldehyde and other by-products over Lewis or Brønsted acid sites at 70–200 � C and WHSV ¼ 0.5–30 h 1 in a fixed bed or fluidized bed [10]. In general, the styrene oxide conversion and phenylacetaldehyde selectivity remain constant at increasing temperature, while the selec tivity and catalytic stability decrease with increasing WHSV and resi dence time over the catalyst [45]. In order to explore the performances of catalysts, the appropriate reaction condition was estimated at 200 � C and WHSV ¼ 3.0 h 1 with N2 flow of 120 mL/min, under which the reaction could reach steady within 1 h TOS. Fig. 6 exhibits the conversion of styrene oxide with TOS over HZSM-5 and modified samples. As a catalysis process implies, no significant uptake of styrene oxide was observed in a blank test without catalyst addition (not shown here), while the initial conversions (TOS ¼ 1.0 h) on HZSM-5 and modified samples were all more than 99%. To obtain a quantitative description of catalytic stability, the catalyst lifetime was defined as the reaction duration with the conversion higher than 96%. As can be seen in Fig. 6, HZ-AT could dramatically prolong its lifetime from 7.4 h to 26.6 h. According to the textural (as shown in Table 1) and acid (as shown in Table 2) properties of samples, it can be concluded that the mesopores formed via desilication in the alkaline medium can reduce the diffusion path length and enhance transport of products out of the zeolite crystals, thus inhibiting coke deposition and exhibiting longer lifetime than the conventional zeolite. Further modified with phosphorous, the lifetime continuously increased to the maximum of
HZ-AT-1.0P (45%) was much lower than that on HZ-AT-SM-1.0P (85%), indicating that the Brønsted acid sites can be more preserved after CTAB modification, which is in agreement with the 27Al MAS NMR measure ments. When the calcinations time of HZ-AT-SM-1.0P was prolonged to 24 h at 550 � C, the remaining percentage of Brønsted acid sites was still about 85%, implying that the phosphate species at the external surface cannot migrate into the micropores during calcinations. Fig. 5(B) presents the Coll-FTIR spectra of HZSM-5 and modified samples. The characteristic peak at 1638 cm 1 was assigned to adsorption of 2,4,6-collidine on the external Brønsted acid sites [42,44]. HZSM-5 only had trace amount of external acid sites, while the alkaline treatment caused some acid sites derived from the micropores exposing to the external surface. An increase of phohphorous impregnation amount leaded to decrease of the peak intensity at 1638 cm 1, sug gesting that the external acid sites were gradually deactivated by phosphorous species. The percentages of external acid sites based on HZ-AT were obtained by the integrated area of the characteristic peak at 1638 cm 1 and listed in Table 2. A small content of phosphorous (e.g., 0.5 wt%) could reduce the external acid sites by 85%, and further increasing the phosphorous content (e.g., 1.0, 1.5, 2.0 wt%) resulted in almost complete deactivation of external acid sites. However, HZ-AT-1.0P still remained about 36% of the external acid sites
Fig. 5. Difference FTIR spectra of adsorbed pyridine (A) and 2,4,6-Collidine (B) on HZSM-5 and modified samples: (a) HZSM-5, (b) HZ-AT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM-1.5p, (f) HZ-AT-SM-2.0P, (g) HZ-AT-1.0P. 5
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and trimer (7) were formed through aldol condensation and polymeri zation, respectively [46]. Some other side-products, such as phenyl ethanediol (2), styrene (3), phenylethanol (4), and so on, were also detected by GC/MS with their overall selectivity less than 1% (Scheme 1). Fig. 7 shows the initial product distribution (TOS ¼ 1.0 h) and se lectivities of phenylacetaldehyde and timer with TOS over HZSM-5 and modified samples. As shown in Fig. 7 (A), the initial selectivity of dimer remained at a similar level (1–3%) on each sample, suggesting that the aldol condensation of phenylacetaldehyde is not sensitive to the textural and acid properties and may be a spontaneous process. There was no trimer formation on HZSM-5 and the initial selectivity of phenyl acetaldehyde reached more than 96%. However, the trimer formed on HZ-AT with initial selectivity of 7%, leading to a decline of phenyl acetaldehyde selectivity. From the above characterization analysis, the differences between HZSM-5 and HZ-AT were the mesoporosity and the resulting external acid sites, e.g., HZSM-5 had limited external surface area (17.9 m2/g), mesoporosity volume (0.014 cm3/g) and external acid sites (trace amount), while HZ-AT had the maximum external surface area (108.2 m2/g), mesoporosity volume (0.086 cm3/g) and external acid sites (percentage of 100%). It can be inferred that the trimer probably formed on the external surface without spatial constraints, where three adsorbed phenylacetaldehyde molecules in adjacent external acid sites could link together through an acid-catalyzed process. According to the Coll-FTIR results, 0.5 wt% phosphorous could reduce the external acid sites by 85%, and further increasing the phosphorous content (e.g., 1.0, 1.5, 2.0 wt%) resulted in almost complete deactiva tion of the external acid sites except HZ-AT-1.0P. So the initial selec tivity of phenylacetaldehyde over HZ-AT-SM-1.0P, HZ-AT-SM-1.5P and HZ-AT-SM-2.0P (all over 96%) was improved significantly than that of HZ-AT-0.5P (92%) by suppressing the formation of trimer at the external acid sites. The external acid sites without confinement effects always exhibit very high catalytic activity [47,48], while the acid sites on the external surface were preferentially deactivated by coke deposition than those in micropores [49,50], therefore the trimer disappeared with TOS and simultaneously the selectivity of phenylacetaldehyde reached over
Fig. 6. Conversion of styrene oxide with time on stream over HZSM-5 and modified samples.
59.8 h over HZ-AT-SM-1.0P due to decrease of the strong acid strength. Nevertheless, the lifetimes degraded with phosphorous content increasing to 1.5 wt% (54.5 h) and 2.0 wt% (46.2 h), suggesting that excess phosphorous would slightly block the pore entrances or reduce the strong acid sites, so that resulted in decrease of the catalytic stability. The lifetime of HZ-AT-1.0P (36.5 h) was much shorter than that of HZAT-SM-1.0P (59.8 h), confirming the effectiveness of the CTAB pre treatment, which is consistent with the N2 adsorption and NH3-TPD results that the phosphate species more easily enter and block the mi cropores and deactivate strong acid sites without pretreating by CTAB. It is generally accepted that the isomerization of styrene oxide (1) to phenylacetaldehyde (5) proceeds through a carbocation mechanism [1, 5]. The neighboring aryl group favors the formation of α-carboncation, giving rise to high regioselectivity over the acid catalysts. In view of the high activity of phenylacetaldehyde, two major side-products dimer (6)
Scheme 1. Reaction pathways in this process. 6
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Fig. 7. Initial product distribution (A) and selectivities of phenylacetaldehyde and trimer with time on stream over HZSM-5 and modified samples.
96% as shown in Fig. 7(B). HZ-AT-SM-1.0P was also reusable up to six times at the optimum temperature (300 � C) [45] after regeneration by burning off the coke deposition in an air flow at 550 � C for 10 h for each run. As illustrated in Fig. 8, no significant decreases in the conversion of styrene oxide (>99%) and yield of phenylacetaldehyde (>95%) were noted in the catalyst recycling experiments. This suggested that the textural and acid properties of catalyst did not change after six repeated experiments, which are listed in Tables 1 and 2. Therefore, HZ-AT-SM-1.0P has po tential industrial application in the gas-phase isomerization of styrene oxide to phenylacetaldehyde and other similar reactions. 4. Conclutions Mesoporous HZSM-5 (HZ-AT) with the micropore structure and acidity preserved is prepared via controlled desilication in an alkaline medium, which exhibiting a longer lifetime (26.6 h) than the conven tional HZSM-5 (7.4 h) in the isomerization of styrene oxide to phenyl acetaldehyde. But the resulting external acid sites lead to a worse selectivity of phenylacetaldehyde due to formation of the trimer. CTAB can be selectively coated on the external surface of HZ-AT because of its large size, and the resulting positive charged external surface can facilitate adsorption of phosphate anions, which deactivate the external Brønsted acid sites caused by a framework dealumination during calci nations. The phosphate species on the external surface cannot migrate into the micropores. There are only a few phosphate species formed in the micropores and most of the internal Brønsted acid sites are pre served. External surface deactivation whilst retaining the hierarchical pore structure of mesoporous zeolites is achieved by phosphorous modification with CTAB pretreatment, thus providing a potential in dustrial application catalyst (HZ-AT-SM-1.0P) which remarkably im proves the catalytic stability (lifetime 59.8 h) while preserving the high activity (>99%) and selectivity (>96%).
Fig. 8. Performances of HZ-AT-SM-1.0P in the recycling experiments (T ¼ 300 � C, flow-rate of N2 ¼ 120 mL/min, WHSV ¼ 3.0 h 1, TOS ¼ 100 h).
the work reported in this paper. Acknowledgment We acknowledge financial supports from the Doctoral Scientific Research Start-up Foundation from Henan University of Science and Technology, China (No. 4008/13480049). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2020.110037.
Author contribution statement Ming-lei Gou: Formulation of overarching research goals and aims, writing original draft preparation, reviewing and editing. Junqing Cai: Development of methodology, analyze study data. Wensheng Song: Experiments and other research outputs. Yonghua Duan: investigation and Data curation. Zhen Liu: Supervision. Qingshan Niu: Funding acquisition.
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Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 7
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Improved performance of mesoporous HZSM-5 designed with selective deactivation of external surface acidity in the isomerization of styrene oxide Ming-Lei Gou *, Junqing Cai, Wensheng Song, Yonghua Duan, Zhen Liu, Qingshan Niu School of Chemical Engineering and Pharmaceutics, Henan University of Science and Technology, Luoyang, Henan, 471023, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous HZSM-5 External surface deactivation Isomerization Styrene oxide Phenylacetaldehyde
A series of mesoporous HZSM-5 designed by phosphorous modification with or without hexadecyl trimethylammonium bromide (CTAB) pretreatment were prepared and tested in the isomerization of styrene oxide. CTAB pretreatment changed the external surface properties of mesoporous HZSM-5 with positive charged, which could selectively adsorb the phosphate anions and deactivate the external Brønsted acid sites caused by a framework dealumination during calcinations. The phosphate species on the external surface could not migrate into the micropores. There were only a few phosphate species formed in the micropores and most of the internal Brønsted acid sites were preserved. External surface deactivation whilst retaining the hierarchical pore structure of mesoporous HZSM-5 was achieved by phosphorous modification with CTAB pretreatment, thus providing a potential industrial application catalyst which remarkably improves the catalytic stability (lifetime 59.8 h) while preserving the high activity (>99%) and selectivity (>96%).
1. Introduction Epoxidation of olefins and subsequently ring opening of epoxides catalyzed by acidic or basic catalysts is widely used for production of aldehydes, ketones, alcohols, ethers, etc. For example, the isomerization of styrene oxides over acid catalysts provides an environmentally benign route to phenylacetaldehyde and its derivatives used for preparation of fine chemicals such as pharmaceuticals, insecticides, herbicides, fungi cides, and so on [1]. However, these reactions usually generate some high-boiling molecules via aldolization or polymerization of aldehydes, leading to rapid deactivation of catalysts by coke deposition. Being a problem at the industrial scale, the isomerization of styrene oxides has been investigated under different conditions. Liquid-phase isomerization of styrene oxides over a series of solid acid catalysts, such as natural silicates [2], silica-alumina gels [3,4], Nafion-H [5], heteropoly acids [6] and zeolites [7,8], has been extensively studied. In order to suppress the side-products and catalyst deactivation, a large amount of solvents, usually over 90 wt percent, must be employed. But the subsequent purification of the products is difficult and high energy consumption. Different styrene oxides can also be isomerized in a fixed-bed reactor under gas-phase conditions using inert gas (such as N2, He) as carrier
gas. Hoelderich et al. [9–11] have found that MFI-Type zeolites are superior to other catalysts for this process. The side-reactions can be efficiently suppressed with phenylacetaldehyde yields up to 90% at 200 � C and WHSV ¼ 3.0 h 1 because of the steric constraints of the frame work, particularly, the product can be separated in a simple manner without additional purification. However the activity of zeolites began to decrease after running for 6 h time on stream (TOS). Our previous research [12] has revealed that increasing the acid concentration while decreasing the acid strength of HZSM-5 zeolites can improve the catalyst lifetimes for this gas-phase process. Several approaches have been employed to adjust the acidity of zeolites, such as ion-exchange with Csþ or Ni2þ [9,13], dealumination [14], fluorination [15], modification with phosphorus [16], isomorphous substitution of boron [17,18] or titanium [19] into the zeolite frameworks, etc. Unfortunately, the catalyst sta bility is still not satisfactory. In view of the prior findings [9,10], the residence time over the catalyst bed should be less than 4 s, preferably less than 1 s in order to decrease probabilities of the side-reactions and prevent the catalyst deactivation. Mesoporous zeolites which possess the advantages both of zeolite crystals (good hydrothermal stability and strong acidity) and mesoporous materials (good mass transfer) can reduce the diffusion path length and enhance transport of products out of the zeolite crystals, thus
* Corresponding author. E-mail address: [email protected] (M.-L. Gou). https://doi.org/10.1016/j.micromeso.2020.110037 Received 16 October 2019; Received in revised form 14 January 2020; Accepted 16 January 2020 Available online 25 January 2020 1387-1811/© 2020 Elsevier Inc. All rights reserved.
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exhibiting higher stability than the conventional zeolites. However, the acid sites located at the external surface are readily accessed with nonselective, resulting in an apparent decline of the target product selectivity [20]. Hence, external surface deactivation whilst retaining the hierarchical pore structure of mesoporous zeolites is expected to obtain both good selectivity and stability. The main methods for surface deactivation of zeolites, such as chemical vapor or liquid deposition (CVD or CLD) with silicon alkoxides [21–23], modification with B [17, 24] and P [16,25] compounds, mechanochemical approach [26], and so on, have been well studied. However, these methods are always accompanied by decline of activity and/or stability due to the pore blockage of zeolites. Surfactant modified zeolites have been widely used to remove pollutant anions from contaminated water [27,28]. Cationic surfactants (i.e. CTAB) can be selectively exchanged with native inorganic cations contained within zeolites, forming a relatively stable organic coating on the zeolite surface. The stages and structures of CTAB loading on the zeolite surface have been studied in the literatures [29–31]. When the CTAB concentration of the solution in which zeolite is placed is main tained above the critical micelle concentration of the surfactant, a bilayer structure of surfactant forms, which can efficiently adsorb an ions. In this study, CTAB pretreatment was in order to change the external surface properties of mesoporous HZSM-5 with positive charged, which could selectively adsorb the phosphate anions and deactivate the external Brønsted acid sites. The physic-chemical and acid properties of samples were analyzed by XRF, N2 adsorption, XRD, 27 Al/31P MAS NMR, NH3-TPD and Py/Coll-FTIR. The catalytic perfor mances were evaluated in the isomerization of styrene oxide under gas-phase free of solvents.
ratios of samples on a Bruker S4 pioneer X-ray fluorescence spectrom eter. X-ray powder diffraction (XRD) was carried out on a Bruker D8 Advance diffractometer(Cu Kα radiation, λ ¼ 0.154056 nm, 40 kV, 40 mA) with a step size of 0.01� at a scanning rate of 5� /min in the range of 2θ ¼ 5–50� . N2 adsorption-desorption isotherms were obtained at 77K using a Quantachrome Autosorb-1 instrument. The total surface area (Stotal) and pore volume (Vtotal) were obtained according to the BET and BJH methods. The micropore surface area (Smicro) and volume (Vmicro) were determined by the t method. The external surface area (Sexter) and mesoporous volume (Vmeso) were calculated by subtracting the Smicro and Vmicro from the Stotal and Vtotal, respectively. The 27Al and 31P MAS NMR spectra were measured with a Varian Infinity-Plus 300 NMR spectrometer with resonance frequencies of 78.13 and 121.37 MHz. The acid properties of samples were analyzed by temperatureprogrammed desorption of ammonia (NH3-TPD) and FTIR spectra of adsorbed pyridine (Py-FTIR) and 2,4,6-collidine (Coll-FTIR). NH3-TPD was performed in quartz U-tube reactor equipped with a thermal con ductivity detector (TCD). The sample (0.1 g, 20–30 mush) was pre treated at 500 � C for 2 h in a N2 flow. After saturation with NH3, the reactor temperature was maintained at 150 � C for 1 h to remove the physically adsorbed NH3. Then, the TPD profile was recorded at a � heating rate of 15 C/min from 150 to 500 � C. Py-FTIR and Coll-FTIR were measured on a Nicolet 380 spectrometer with a resolution of 2 cm 1. The catalyst was pressed into a thin selfsupporting wafer (weight: 15 mg, diameter: 13 mm) and then acti vated at 500 � C for 2 h under vacuum (10 3 Pa). Adsorption of pyridine or 2,4,6-collidine in situ proceeded at room temperature, followed by desorption at 200 � C for 1 h. Then, an FTIR spectrum was recorded and its difference spectrum was calculated by subtracting the spectrum of the sample before probe adsorption.
2. Experimental
2.3. Catalytic measurements
2.1. Catalyst preparation
The performances of catalysts were tested in the isomerization of styrene oxide on a fixed-bed reactor (stainless steel, 9 mm inner diam eter). Typically, the catalyst (0.5 g, 20–30 mush) was packed in the constant-temperature zone and pretreated at 500 � C for 2 h in a N2 flow. Then, styrene oxide (>98 wt%, TCI Development Co., Ltd) free of any solvents was fed into the reactor at 200 � C and WHSV ¼ 3.0 h 1 with N2 flow of 120 mL/min. The reactor effluent was collected in an ice-water trap and analyzed by GC (Agilent 6820, FID detector) and GC/MS (Agilent 6890/5873, MSD detector) equipped with a VF-5ms capillary column (5%phenyl-95% dimethylpolysiloxane, 30 m � 0.25 mm id � 0.25 μm film thickness).
Conventional HZSM-5 (Si/Al ¼ 50) zeolite was purchased from Nakai University Catalyst Factory (Tianjin, PR China) used as the parent sample. The Mesoporous HZSM-5 sample (denoted as HZ-AT, where AT represents alkaline treatment) was prepared via controlled desilication in alkaline medium according to the literature [32]. The parent HZSM-5 (5.0 g) was treated in 100 mL 0.2 mol/L NaOH solution at 65 � C for 60 min. The resulting slurry was filtered, washed with demineralized water and dried at 80 � C. Prior to further treatment, the sample was ion-exchanged with a 0.1 mol/L NH4NO3 solution for three times and finally calcined at 550 � C for 5 h. Selective deactivation of external surface acid sites of HZ-AT was synthesized as follows: First, HZ-AT (1.0 g) was positive charged with 20 mL 0.05 mol/L CTAB solution at 60 � C for 24 h. The CTAB concen tration of the solution was above the critical micelle concentration of the surfactant, a bilayer structure of the surfactant formed, which could efficiently adsorbed anions. Then, the sample was recovered by filtra tion, washed and dried at 80 � C. Second, phosphorus modified samples pretreated by CTAB were prepared by impregnating with an aqueous solution containing the desired amount of phosphoric acid. The phos phorus contents of the samples were 0.5, 1.0, 1.5 and 2.0 wt%, which were named HZ-AT-SM-0.5P, HZ-AT-SM-1.0P, HZ-AT-SM-1.5P, HZ-ATSM-2.0P (where SM represents surfactant modification, P represents phosphorous modification and the figure represents the impregnation amount of phosphoric acid), respectively. Meanwhile, a sample with 1.0 wt% phosphorus content on HZ-AT without CTAB pretreatment was also prepared and named HZ-AT-1.0P. Finally, all the phosphorus impreg nated samples were obtained after drying at 110 � C for 4 h and calcining at 550 � C for 5 h.
3. Results and discussion 3.1. Physic-chemical properties of catalysts The XRD patterns of all samples in Fig. 1 exhibited the typical MFI topology and no other phases of phosphorous species had been found. The relative crystallinity (RC) was calculated from the intensity of main peaks between 2θ ¼ 22–25� using the parent HZSM-5 as the reference. HZ-AT successfully preserved the lattice structure of zeolite, although the RC slightly decreased to 98%. The RC of phosphorus modified samples with CTAB treatment slightly decreased from 97% (HZ-AT-SM0.5P) to 85% (HZ-AT-SM-2.0P) with phosphorus addition, which can be attributed to lattice defects caused by dealumination upon phosphorus modification [33,34]. While the RC of HZ-AT-1.0P (75%) without CTAB pretreatment was much less than that of HZ-AT-SM-1.0P (92%), sug gesting that CTAB pretreatment for positive charged of the external suface of mesoporous HZSM-5 can protect the zeolite framewotk from attack by phosphorus species. Fig. 2(A) shows the N2 adsorption-desorption isotherms of samples and their texture properties are summarized in Table 1. The parent HZSM-5 exhibited a type I isotherm, characteristic of microporous
2.2. Catalyst characterization X-ray fluorescence (XRF) was used to determine the Si/Al and P/Al 2
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external surface can facilitate adsorption of phosphate anions. That is to say, the phosphate species more easily enter the micropores and block them without surface modification by CTAB. 27 Al and 31P MAS NMR analyses have also been conducted to monitor the chemical states of Al and P species. As exhibited in Fig. 3(A), each sample showed an intense peak at around 53 ppm corresponding to the tetrahedral-coordinated framework aluminum (FAL) species with strong Brønsted acidity [36]. The HZSM-5 and HZ-AT samples presented similar spectra, although a negligible peak at 0 ppm related to the octahedral extra-framework aluminum (EFAL) species was observed for HZ-AT, which meaning that the chemical state of Al was little changed after alkaline treatment. After introduction of phosphorus, the peak of FAL species decreased dramatically and the peak of EFAL species shifted from 0 to 14 ppm due to attaching of the EFAL species to phosphorus species, indicating that some FAL species turned into extra-framework aluminum phosphates via dealumination upon phosphorus modifica tion. The higher peak at 14 ppm of HZ-AT-1.0P than that of HZ-AT-SM-1.0P demonstrated again that CTAB pretreatment can protect the zeolite framewotk from attack by phosphorus species, which was consistent with the XRD results. In addition, a small peak at around 34 ppm appeared on the samples with high phosphorus content (e.g., HZ-AT-SM-1.5P, HZ-AT-SM-2.0P) could be assigned to the intermediate aluminum species in a distorted environment or the penta-coordinated species at either framework or non-framenwork positions [37]. From above, the deactivation of Brønsted acid sites by phosphate species is mostly caused by a framework dealumination via the reaction between phosphorous acid and FAL species during calcinations. Different EFAL species (i.e., penta-coordinated aluminum, distorted tetrahedral coor dinated aluminum or octahedral coordinated aluminum) mostly con nected to phosphate species are formed. Penta-coordinated aluminum or distorted tetrahedral coordinated aluminum may be intermediate spe cies in the dealumination process. As shown in the 31P MAS NMR spectra (Fig. 3(B)), the small peaks at 1 and -6 ppm were generally associated with free monomeric ortho phosphate (PO4) groups and terminal phosphate groups of pyrophos phate or linear polyphosphate not connected to aluminum [38], whereas the peaks between 12 and 19 ppm were related to the terminal or intermediate groups in polyphosphates connected to aluminum [37,39]. The broad signal at 26 ppm was caused by polymeric phosphates attached to aluminum, and the signals at 32 and 40 ppm were respectively assigned to amorphous aluminum phosphate and highly condensed polyphosphate [33,37]. It can be seen in Fig. 3(B) that larger amount of phosphorous existed as free monomeric orthophosphate and
Fig. 1. XRD patterns of HZSM-5 and modified samples.
material with limited external surface area (17.9 m2/g) and meso porosity volume (0.014 cm3/g). While the samples with alkaline-treated (HZ-AT) and subsequent phosphorous modification (HZ-AT-SM-0.5P/ 1.0P/1.5P/2.0P and HZ-AT-1.0P) all presented a mixture of type I and IV isotherms, in which the enhanced N2 uptake at p/p0 ¼ 0.4–0.9 accom panied by a hysteresis loop for each sample confirmed the presence of mesoporosity with pore size at approximately 10–20 nm (Fig. 2(C)). The size of micropores (centered at about 0.55 nm) remained almost un changed for all the samples as shown in Fig. 2(B), indicating that the microporous structures were well preserved upon alkaline treatment and phosphorus modification. As shown in Table 1, HZ-AT had the maximum external surface area (108.2 m2/g) and mesoporosity volume (0.086 cm3/g) owing to the controlled desilication in the alkaline me dium. Comparing HZ-AT (Smicro ¼ 233.4 m2/g, Vmicro ¼ 0.125 cm3/g), the micropore surface areas (233.1–220.2 m2/g) and volumes (0.124–0.118 cm3/g) of phosphorus modified samples with CTAB pre treatment (HZ-AT-SM-0.5P/1.0P/1.5P/2.0P) showed slightly decline with increasing of phosphorus impregnation amount, while the phos phorus modified samples without CTAB pretreatment (HZ-AT-1.0P) showed the minimum micropore surface area (205.1 m2/g) and volume (0.089 cm3/g). This can be explained that the cationic surfactant (CTAB) may be selectively coated on the external surface of mesoporous HZSM-5 (HZ-AT) because of its large size [35], and the resulting positive charged
Fig. 2. N2 adsorption-desorption isotherms (A) and the corresponding H–K micropore size (B) and BJH mesopore size (C) distribution of HZSM-5 and modified samples: (a) HZSM-5, (b) HZ-AT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM-1.5p, (f) HZ-AT-SM-2.0P, (g) HZ-AT-1.0P. 3
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Table 1 Chemical composition and textural properties of the parent and modified HZSM-5 zeolites. Sample
Si/Al
P/Al
Stotal (m2/g)
Smicro (m2/g)
Sexter (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
HZSM-5 HZ-AT HZ-AT-SM-0.5P HZ-AT-SM-1.0P HZ-AT-SM-1.5P HZ-AT-SM-2.0P HZ-AT-1.0P HZ-AT-SM-1.0P (after six run)
50.3 33.7 33.6 33.8 33.7 33.8 33.9 34.1
– – 0.16 0.27 0.41 0.58 0.28 0.29
292.7 341.6 339.4 333.9 328.2 321.8 310.4 332.5
274.8 233.4 233.1 230.9 225.7 220.2 205.1 230.8
17.9 108.2 106.3 103.0 102.5 101.6 105.3 101.7
0.144 0.211 0.207 0.201 0.192 0.181 0.174 0.200
0.130 0.125 0.124 0.123 0.120 0.118 0.089 0.123
0.014 0.086 0.083 0.078 0.072 0.063 0.085 0.077
Fig. 3. 27Al (A) and 31P (B) MAS NMR spectra of HZSM-5 and modified samples: (a) HZSM-5, (b) HZ-AT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM1.5P, (f) HZ-AT-SM-2.0P, (g) HZ-AT-1.0P.
pyrophosphate or linear polyphosphate not connected to aluminum on HZ-AT-SM-1.0P, which is due to the positive charged external surface after CTAB modification can facilitate adsorption of phosphate anions, thus preventing the phosphate species from entering the micropores of samples and combining with aluminum. 3.2. Acid properties of catalysts The NH3-TPD was used to analyze the acid strength and concentra tion of samples, as shown in Fig. 4 and Table 2. There were three different desorption peaks at <200, 250–279 and 371–435 � C, which were corresponding to physisorbed NH3 or NH3 adsorbed on external silanol groups, weak and strong acid sites, respectively [40]. Compared with HZSM-5, HZ-AT had similar acid strength and concentration, meaning that the alkaline treatment had little influence on the FAL and EFAL species (well known as strong and weak acid sites). However, after modification with phosphorus, the high-temperature peak slightly shifted to lower temperature and lost its intensity, implying the strength (from 402 to 384 � C) and concentration (from 0.36 to 0.13 mmol/g) of strong acid sites reduced. This is consistent with the results of 27Al MAS NMR and XRD that some FAL species turned into extra-framework aluminum phosphates via dealumination upon phosphorus modifica tion. Furthermore, the strength and concentration of strong acid sites (371 � C, 0.07 mmol/g) on HZ-AT-1.0P were significantly lower than those on HZ-AT-SM-1.0P (398 � C, 0.35 mmol/g), which may be ascribed to the phosphate species more easily diffuse into the micropores and deactivate strong acid sites without surface modification by CTAB sup ported by the texture analysis. The type and accessibility of acid sites were analyzed by FTIR spectra of adsorbed pyridine (5.7 Å in diameter) and 2,4,6-collidine (7.4 Å in diameter), in which Py-FTIR can be used to determine the acidity type without distinguishing the internal or external acid sites, while CollFTIR can only probe the external acid sites [41,42]. As illustrated in
Fig. 4. NH3-TPD curves of HZSM-5 and modified samples: (a) HZSM-5, (b) HZAT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM-1.5p, (f) HZ-AT-SM2.0P, (g) HZ-AT-1.0P.
Fig. 5(A), the characteristic peaks at 1445 cm 1 and 1545 cm 1 were related to the pyridine molecules interacting with Lewis and Brønsted acid sites, respectively [43]. The percentages of Lewis and Brønsted acid sites based on HZSM-5 were calculated by the integrated area of each characteristic peak and summarized in Table 2. Likewise to the NH3-TPD results, the alkaline treatment caused a slight decrease (2%) of Brønsted acid sites and increase (4%) of Lewis acid sites, and an increase of phosphorus content brought a decrease of Brønsted acid sites and weakening of Lewis acid sites by the formation of aluminum phosphate species [16,33]. The remaining percentage of Brønsted acid sites on 4
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Microporous and Mesoporous Materials 297 (2020) 110037
compared to HZ-AT. The above results indicated that selective deacti vation of external surface acid sites of mesoporous HZSM-5 could be achieved by phosphorous with CTAB pretreatment. In summary, CTAB pretreatment is in order to change the external surface properties of mesoporous HZSM-5 with positive charged, which can selectively adsorb the phosphate anions. Only a few phosphate an ions can enter into the micropores if the impregnation amount is limited. After drying at 110 � C and calcining at 550 � C, CTAB is completely decomposed and the phosphate species are formed on the external surface of HZSM-5. Therefore, these phosphate species can deactivate the external Brønsted acid sites caused by a framework dealumination via the reaction between phosphorous and FAL species during calcina tions after thermally decomposing of CTAB. No matter before or after decomposing of CTAB, the phosphate species on the external surface apparently cannot migrate into the micropores. There are only a few phosphate species formed in the micropores and most of the internal Brønsted acid sites are preserved.
Table 2 Acid properties of the parent and modified HZSM-5 zeolites. Sample
HZSM-5 HZ-AT HZ-AT-SM0.5P HZ-AT-SM1.0P HZ-AT-SM1.5P HZ-AT-SM2.0P HZ-AT1.0P HZ-AT-SM1.0P (after six run)
Amounts of acid sites by NH3-TPD (mmol/g)
Percentage of acid sites by Py/CollFTIR (%)
Weak (250–279 � C)
Strong (371–435 � C)
Brønsted acid sites
Lewis acid sites
External acid sites
0.11 0.09 0.14
0.56 0.55 0.36
100% 98% 90%
100% 104% –
trace 100% 15%
0.16
0.35
85%
–
trace
0.19
0.27
79%
–
trace
0.29
0.13
55%
–
trace
0.28
0.07
45%
–
36%
0.14
0.35
85%
–
trace
3.3. Performances of catalysts Styrene oxide is a substance with high reactivity to phenyl acetaldehyde and other by-products over Lewis or Brønsted acid sites at 70–200 � C and WHSV ¼ 0.5–30 h 1 in a fixed bed or fluidized bed [10]. In general, the styrene oxide conversion and phenylacetaldehyde selectivity remain constant at increasing temperature, while the selec tivity and catalytic stability decrease with increasing WHSV and resi dence time over the catalyst [45]. In order to explore the performances of catalysts, the appropriate reaction condition was estimated at 200 � C and WHSV ¼ 3.0 h 1 with N2 flow of 120 mL/min, under which the reaction could reach steady within 1 h TOS. Fig. 6 exhibits the conversion of styrene oxide with TOS over HZSM-5 and modified samples. As a catalysis process implies, no significant uptake of styrene oxide was observed in a blank test without catalyst addition (not shown here), while the initial conversions (TOS ¼ 1.0 h) on HZSM-5 and modified samples were all more than 99%. To obtain a quantitative description of catalytic stability, the catalyst lifetime was defined as the reaction duration with the conversion higher than 96%. As can be seen in Fig. 6, HZ-AT could dramatically prolong its lifetime from 7.4 h to 26.6 h. According to the textural (as shown in Table 1) and acid (as shown in Table 2) properties of samples, it can be concluded that the mesopores formed via desilication in the alkaline medium can reduce the diffusion path length and enhance transport of products out of the zeolite crystals, thus inhibiting coke deposition and exhibiting longer lifetime than the conventional zeolite. Further modified with phosphorous, the lifetime continuously increased to the maximum of
HZ-AT-1.0P (45%) was much lower than that on HZ-AT-SM-1.0P (85%), indicating that the Brønsted acid sites can be more preserved after CTAB modification, which is in agreement with the 27Al MAS NMR measure ments. When the calcinations time of HZ-AT-SM-1.0P was prolonged to 24 h at 550 � C, the remaining percentage of Brønsted acid sites was still about 85%, implying that the phosphate species at the external surface cannot migrate into the micropores during calcinations. Fig. 5(B) presents the Coll-FTIR spectra of HZSM-5 and modified samples. The characteristic peak at 1638 cm 1 was assigned to adsorption of 2,4,6-collidine on the external Brønsted acid sites [42,44]. HZSM-5 only had trace amount of external acid sites, while the alkaline treatment caused some acid sites derived from the micropores exposing to the external surface. An increase of phohphorous impregnation amount leaded to decrease of the peak intensity at 1638 cm 1, sug gesting that the external acid sites were gradually deactivated by phosphorous species. The percentages of external acid sites based on HZ-AT were obtained by the integrated area of the characteristic peak at 1638 cm 1 and listed in Table 2. A small content of phosphorous (e.g., 0.5 wt%) could reduce the external acid sites by 85%, and further increasing the phosphorous content (e.g., 1.0, 1.5, 2.0 wt%) resulted in almost complete deactivation of external acid sites. However, HZ-AT-1.0P still remained about 36% of the external acid sites
Fig. 5. Difference FTIR spectra of adsorbed pyridine (A) and 2,4,6-Collidine (B) on HZSM-5 and modified samples: (a) HZSM-5, (b) HZ-AT, (c) HZ-AT-SM-0.5P, (d) HZ-AT-SM-1.0P, (e) HZ-AT-SM-1.5p, (f) HZ-AT-SM-2.0P, (g) HZ-AT-1.0P. 5
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and trimer (7) were formed through aldol condensation and polymeri zation, respectively [46]. Some other side-products, such as phenyl ethanediol (2), styrene (3), phenylethanol (4), and so on, were also detected by GC/MS with their overall selectivity less than 1% (Scheme 1). Fig. 7 shows the initial product distribution (TOS ¼ 1.0 h) and se lectivities of phenylacetaldehyde and timer with TOS over HZSM-5 and modified samples. As shown in Fig. 7 (A), the initial selectivity of dimer remained at a similar level (1–3%) on each sample, suggesting that the aldol condensation of phenylacetaldehyde is not sensitive to the textural and acid properties and may be a spontaneous process. There was no trimer formation on HZSM-5 and the initial selectivity of phenyl acetaldehyde reached more than 96%. However, the trimer formed on HZ-AT with initial selectivity of 7%, leading to a decline of phenyl acetaldehyde selectivity. From the above characterization analysis, the differences between HZSM-5 and HZ-AT were the mesoporosity and the resulting external acid sites, e.g., HZSM-5 had limited external surface area (17.9 m2/g), mesoporosity volume (0.014 cm3/g) and external acid sites (trace amount), while HZ-AT had the maximum external surface area (108.2 m2/g), mesoporosity volume (0.086 cm3/g) and external acid sites (percentage of 100%). It can be inferred that the trimer probably formed on the external surface without spatial constraints, where three adsorbed phenylacetaldehyde molecules in adjacent external acid sites could link together through an acid-catalyzed process. According to the Coll-FTIR results, 0.5 wt% phosphorous could reduce the external acid sites by 85%, and further increasing the phosphorous content (e.g., 1.0, 1.5, 2.0 wt%) resulted in almost complete deactiva tion of the external acid sites except HZ-AT-1.0P. So the initial selec tivity of phenylacetaldehyde over HZ-AT-SM-1.0P, HZ-AT-SM-1.5P and HZ-AT-SM-2.0P (all over 96%) was improved significantly than that of HZ-AT-0.5P (92%) by suppressing the formation of trimer at the external acid sites. The external acid sites without confinement effects always exhibit very high catalytic activity [47,48], while the acid sites on the external surface were preferentially deactivated by coke deposition than those in micropores [49,50], therefore the trimer disappeared with TOS and simultaneously the selectivity of phenylacetaldehyde reached over
Fig. 6. Conversion of styrene oxide with time on stream over HZSM-5 and modified samples.
59.8 h over HZ-AT-SM-1.0P due to decrease of the strong acid strength. Nevertheless, the lifetimes degraded with phosphorous content increasing to 1.5 wt% (54.5 h) and 2.0 wt% (46.2 h), suggesting that excess phosphorous would slightly block the pore entrances or reduce the strong acid sites, so that resulted in decrease of the catalytic stability. The lifetime of HZ-AT-1.0P (36.5 h) was much shorter than that of HZAT-SM-1.0P (59.8 h), confirming the effectiveness of the CTAB pre treatment, which is consistent with the N2 adsorption and NH3-TPD results that the phosphate species more easily enter and block the mi cropores and deactivate strong acid sites without pretreating by CTAB. It is generally accepted that the isomerization of styrene oxide (1) to phenylacetaldehyde (5) proceeds through a carbocation mechanism [1, 5]. The neighboring aryl group favors the formation of α-carboncation, giving rise to high regioselectivity over the acid catalysts. In view of the high activity of phenylacetaldehyde, two major side-products dimer (6)
Scheme 1. Reaction pathways in this process. 6
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Microporous and Mesoporous Materials 297 (2020) 110037
Fig. 7. Initial product distribution (A) and selectivities of phenylacetaldehyde and trimer with time on stream over HZSM-5 and modified samples.
96% as shown in Fig. 7(B). HZ-AT-SM-1.0P was also reusable up to six times at the optimum temperature (300 � C) [45] after regeneration by burning off the coke deposition in an air flow at 550 � C for 10 h for each run. As illustrated in Fig. 8, no significant decreases in the conversion of styrene oxide (>99%) and yield of phenylacetaldehyde (>95%) were noted in the catalyst recycling experiments. This suggested that the textural and acid properties of catalyst did not change after six repeated experiments, which are listed in Tables 1 and 2. Therefore, HZ-AT-SM-1.0P has po tential industrial application in the gas-phase isomerization of styrene oxide to phenylacetaldehyde and other similar reactions. 4. Conclutions Mesoporous HZSM-5 (HZ-AT) with the micropore structure and acidity preserved is prepared via controlled desilication in an alkaline medium, which exhibiting a longer lifetime (26.6 h) than the conven tional HZSM-5 (7.4 h) in the isomerization of styrene oxide to phenyl acetaldehyde. But the resulting external acid sites lead to a worse selectivity of phenylacetaldehyde due to formation of the trimer. CTAB can be selectively coated on the external surface of HZ-AT because of its large size, and the resulting positive charged external surface can facilitate adsorption of phosphate anions, which deactivate the external Brønsted acid sites caused by a framework dealumination during calci nations. The phosphate species on the external surface cannot migrate into the micropores. There are only a few phosphate species formed in the micropores and most of the internal Brønsted acid sites are pre served. External surface deactivation whilst retaining the hierarchical pore structure of mesoporous zeolites is achieved by phosphorous modification with CTAB pretreatment, thus providing a potential in dustrial application catalyst (HZ-AT-SM-1.0P) which remarkably im proves the catalytic stability (lifetime 59.8 h) while preserving the high activity (>99%) and selectivity (>96%).
Fig. 8. Performances of HZ-AT-SM-1.0P in the recycling experiments (T ¼ 300 � C, flow-rate of N2 ¼ 120 mL/min, WHSV ¼ 3.0 h 1, TOS ¼ 100 h).
the work reported in this paper. Acknowledgment We acknowledge financial supports from the Doctoral Scientific Research Start-up Foundation from Henan University of Science and Technology, China (No. 4008/13480049). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2020.110037.
Author contribution statement Ming-lei Gou: Formulation of overarching research goals and aims, writing original draft preparation, reviewing and editing. Junqing Cai: Development of methodology, analyze study data. Wensheng Song: Experiments and other research outputs. Yonghua Duan: investigation and Data curation. Zhen Liu: Supervision. Qingshan Niu: Funding acquisition.
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Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 7
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