Direct alkenylation of aromatics with phenylacetylene over supported H3PW12O40 catalysts as a clean and highly efficient approach to producing α-arylstyrenes

Journal of Catalysis 288 (2012) 44–53

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Direct alkenylation of aromatics with phenylacetylene over supported H3PW12O40 catalysts as a clean and highly efficient approach to producing a-arylstyrenes Zhongkui Zhao ⇑, Yitao Dai, Ting Bao, Renzhi Li, Guiru Wang State Key Laboratory of Fine Chemicals, Department of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 4 December 2011 Revised 27 December 2011 Accepted 28 December 2011 Available online 2 March 2012 Keywords: Alkenylation Phosphotungstic acids a-Arylstyrenes Impregnation methods Clean synthesis Heterogeneous catalysis

a b s t r a c t Phosphotungstic acid (PTA) catalysts supported on MCM-41 prepared via a wet impregnation method assisted by vacuum with heating (IMPVH) were first employed for direct alkenylation of different aromatics with phenylacetylene to synthesize a-arylstyrenes. N2 adsorption–desorption, FT-IR, X-ray diffraction (XRD), and NH3 temperature-programmed desorption (NH3 TPD) characterization techniques were used to reveal the relationship between the catalyst’s nature and properties. The results demonstrate that the fabricated 25 wt.% PTA/MCM-41 catalyst exhibits outstanding catalytic performance, remarkably better than that on HY zeolite. It is also found that the catalytic properties of the catalysts are strongly dependent on PTA dispersity, the nature of the acid sites, the preservability of PTA Keggin structure, and the mesopore architecture, notably affected by PTA loading and calcination temperature. The results for catalytic stability illustrate that more than 99% of maximum conversion can be obtained, and more than 92% conversion can be maintained for up to 540 min time on stream. We find that the decrease in catalytic activity, along with the long reaction time, is mainly ascribable to deactivation by coke deposition. The spent catalyst can be refreshed, and 97.1% conversion can be obtained over the regenerated catalyst. This approach is also highly efficient for extra-substituted benzene, polycyclic aromatics, and even heteroaromatics, suggesting that the method presented in this paper can be a green and highly efficient synthesis protocol for a-arylstyrenes. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Alkenylation of aromatic compounds, also known as hydroarylation of alkynes, has undoubtedly become one of the most important strategies for synthesizing alkenylaromatics, extensively applied in fields such as pharmaceuticals, agrochemicals, natural products, flavors, and dyes [1]. In contrast to well-established alkylation, alkenylation still remains a significant challenge to be resolved. The biggest difficulty is to efficiently avoid the oligomerization of alkynes due to the poor stability of vinyl cation species [2]. Traditional Lewis acid catalysts for alkylation, such as AlCl3, could catalyze alkenylation, but only an extremely low 6% yield was obtained with heavy pollution. Using metal triflates M(OTf)x, a breakthrough was made, and a good yield for alkenylated products was achieved [3]. FeCl3 [4], HSO3F [5], [BMIM][Sb2F11] [6], etc. were also employed to catalyze this reaction, but there still are separation and equipment corrosion issues. Heterogeneous catalysis provides a new approach to organic synthesis in terms of clean, easy separation, reusability, ⇑ Corresponding author. Fax: +86 411 84986231. E-mail address: [email protected] (Z. Zhao). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2011.12.024

and high selectivity. Recently, zeolite has been regarded more and more as an environmentally benign solid acid catalyst for Friedel–Crafts alkylation [7–10], and its acidity and pore size decisively affect the catalytic activation, selectivity, and coke resistance [11–14]. Due to the diffusion confinement of micropores, mesoporous solid acids have been extensively used for alkylation [15–17]. However, rare reports on alkenylation over solid acid catalysts can be found. Sartori did some pioneering work on alkenylation of aromatics over HSZ-360 zeolite; unfortunately the results are not very satisfactory, and there exits an irreconcilable contradiction between the selectivity and the catalytic activity. The zeolites calcined at lower temperature exhibited high catalytic activity, but a considerable amount of acetophenone (5–20%) was detected; the higher calcination temperature could efficiently decrease the formation of acetophenone, but led to a remarkable decrease in catalytic activity. Moreover, low catalytic efficiency (1.0 g catalyst for 1.0 g phenylacetylene) could be observed when the HY was used as a catalyst for the alkenylation, ascribed to the reaction taking place only on the external surface of the catalyst because of the narrow pore channels within the HY zeolite [18]. Much attention should be paid to developing novel and effectual solid acid catalysts for alkenylation reaction. It is known that

Z. Zhao et al. / Journal of Catalysis 288 (2012) 44–53

heteropolyacid (HPA) can act as an efficient catalyst for alkylation, due to its strong acidity and Brønsted type fundamentally, which are comparable to those of HF and H2SO4. Among them, PTA is usually employed as a good catalyst for its high acidic strength and relatively high thermal stability [19,20]. But its lower surface area (about 5–8 m2/g) is a serious drawback as a heterogeneous catalyst [21]. The immobilization of HPA was considered an effectual approach to increasing the surface area. The silica [22,23], active carbon and acidic ion-exchange resin [24] have been found to be suitable carriers to support HPA. Furthermore, the silica-supported HPA usually present superior catalytic performance [25–32]. Herein, the mesoporous supported PTA catalyst on MCM-41 was first employed to catalyze direct alkenylation for synthesizing a series of a-arylstyrenes in a steel fixed-bed continuous-flow reactor. The effect of PTA loading, calcination temperature (Tc), and reaction parameters such as molar ratio of aromatics to phenylacetylene (nAr/Phen), reaction temperature (Tr), system pressure (Ps), and volume hourly space velocity (VHSV) on the catalytic properties was investigated by using the alkenylation of p-xylene with phenylacetylene as model reaction. HY zeolite was also involved for comparison. N2 adsorption–desorption, FT-IR, XRD, and NH3 TPD characterization techniques were employed to reveal the relationship between the nature and catalytic properties of the catalysts. We found that the supported PTA catalysts with good dispersion of PTA, appropriate acid concentration and strength, good maintainability of Keggin structure of PTA units, and a well-ordered mesopore architecture of support are essential for the alkenylation reaction. 2. Experimental 2.1. Preparation of catalyst Pure silica, MCM-41, was synthesized with a surfactant, hexadecyltrimethylammonium bromide (CTAB), as structure-directing agent by the use of a hydrothermal method according to the typical procedure in the previous reports [33–35]. After the hydrothermal step, the samples were thoroughly washed with deionized water and dried at 105 °C for 12 h, and then calcined in air at 550 °C for 5 h by a step-by-step temperature rise method. The obtained MCM-41 support powder was sieved into 20–40 mesh particles after extrudation. Then the PTA was dispersed on the MCM-41 support via the IMPVH method. The method exploited in our case was carried out by impregnating MCM-41 (1.0 g) with an aqueous solution of PTA (the concentrations depended on the desired loadings: 10, 20, 25, 30, 50, and 70 wt.% PTA on MCM-41) in a vacuum environment with heating at 100 °C. Then the impregnated samples were dried at 105 °C in air overnight, followed by calcination in air at 300 °C for 5 h, and PTA/MCM-41 catalysts with a loading of 10–70 wt.% were obtained.

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pulse injection of ammonia in an Ar stream. Finally the desorption step was carried out from 100 and 700 °C at a heating rate of 10 °C min 1 and with an Ar flow of 30 ml min 1. The NH3 TPD profiles were obtained via monitoring the desorbed ammonia with a thermal conductivity detector. The PTA contents of the fresh/spent catalysts and the reaction mixture were measured by the inductively coupled plasma (ICP) technique on the ICP spectrometer (Optima 2000DV, Perkin Elmer). 2.3. Catalytic reactions The experiments on the alkenylation of aromatics with phenylacetylene were performed in a stainless steel fixed-bed continuous-flow reactor. A sample of 1.0 g catalyst with 20–40 mesh was loaded into the reactor for all the reaction tests, and the remaining space of the reactor tube was filled with 20–40 mesh quartz granules. Before the introduction of feedstock, the catalyst was preactivated for 1 h online in N2 flow. The liquid stream was introduced into the fixed-bed reactor by a syringe pump. N2 (99.999% purity) was used to maintain system pressure. In all cases, a time on stream of 8 h was used. The HY (provided by the Chinese Fushun Institute of Petroleum and Chemicals) was used for comparison. Quantitative analysis of the collected products was performed on a FULI 9790 II GC equipped with an HP-5 column, 30 m  0.32 mm  0.25 lm, and an FID detector. GC/MS and 1H NMR were performed for the structure identification of samples (qualitative analysis; see the Supporting information). As an evaluation index of the alkenylation reaction, the phenylacetylene conversion was calculated by weight percent of the consumed phenylacetylene in the total phenylacetylene amount in the feed; the selectivity to a-arylstyrene was calculated by weight percent of desired a-arylstyrene in total products. The yield corresponding to a-arylstyrene was the GC yield, which was calculated based on the conversion of phenylacetylene and the selectivity of the desired products. 3. Results and discussion 3.1. Products analysis and reaction mechanism The as-prepared 20 wt.% PTA/MCM-41 catalyst was characterized by N2 adsorption–desorption and XRD. Figs. 1 and 2 present

2.2. Characterization of catalyst XRD patterns of the samples were recorded using a Rigaku D/ max-2400 apparatus using Cu Ka radiation. The FT-IR spectra of the samples were collected on a Nexus Euro infrared spectrometer using the KBr pallet method. Nitrogen adsorption experiments at 196 °C were carried out on a Quantachrome Autosorb instrument to measure the surface area, pore volume, and pore size distribution. The samples were degassed at 200 °C for 2 h prior to the N2 adsorption experiments. NH3 TPD measurements were performed to characterize the acidity of the samples. After pretreatment of 50-mg samples in Ar (up to 300 °C with a ramp rate of 10 °C min 1, and then kept for 0.5 h under 30 ml min 1 Ar flow), the samples were saturated with ammonia (10% NH3–90% Ar) at 10 °C via the

Fig. 1. Nitrogen adsorption (open symbols)–desorption (solid symbols) isotherms and corresponding Barrett–Joyner–Halenda (BJH) pore size distribution profiles (inset) determined from the sorption branches of the isotherms of 20 wt.% PTA/ MCM-41 catalysts prepared via IMPVH method.

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product was also characterized by 1H NMR (see the Supporting information). Based on the MS data and the related Refs. [18,38– 40], it can be seen that, besides the main product (a-arylstyrene), a series of byproducts such as acetophenone, a-(2,5-dimethylphenyl)ethylbenzene, b-(2,5-dimethylphenyl)styrene, and oligomers can be detected, indicating that the mesoporous solid-acid-catalyzed alkenylation of p-xylene with phenylacetylene is a complex competition process with several possible side reactions, such as hydration of phenylacetylene, hydrogenation of a-arylstyrene, thermodynamic control alkenylation, and oligomerization reactions. Depending on the interaction between the acidic site and the triple bond, the alkenylation of aromatics with phenylacetylene is proceeding smoothly via alkenyl cationic intermediates for homogeneous catalytic reaction [5,6]. Based on the above references and reaction results, it can be proposed that the alkenylation of p-xylene with phenylacetylene over solid acid catalyst also takes place through alkenyl cationic intermediate, and Scheme 1 presents a possible reaction mechanism. The isomer (IV) of the main product is formed via a similar alkenyl cation mechanism. The oligomers are hypothesized to be generated from the cyclopolymerization of phenylacetyleneand the codimerization of a-(2,5-dimethylphenyl) styrene with phenylacetylene and acetophenone respectively. The trace amount of acetophenone (II) is produced by the possible hydration of phenylacetylene with traces of water present in the reaction mixture. The a-(2,5-dimethylphenyl) ethylbenzene (III) is produced by consequent hydrogenation of the main product with the generated H atoms from oligomerization as H sources.

3.2. Effect of PTA loading

Fig. 2. Low (A) and wide (B) angle XRD patterns of 20 wt.% PTA/MCM-41 and bare MCM-41 support samples (the wide angle of bulk PTA is included for comparison).

the nitrogen adsorption–desorption isotherms/pore size distributions and the XRD patterns, respectively. From Fig. 1, according to the IUPAC classification, the isotherms with type IV are a typical feature of mesoporous materials [33,34], suggesting the preservation of mesoporous architecture in 20 wt.% PTA/MCM-41 catalyst prepared by the IMPVH method. A pore size of 2.7 nm for the mesoporous channels in the supported PTA sample can be observed, although smaller than that of the bare support MCM-41 ascribed to the immobilized PTA units inside the channels occupying some space [35,36]. From Fig. 2A, it can be observed that the bare MCM-41 exhibits three well-resolved diffraction peaks respectively indexed as the [1 0 0], [1 1 0], and [2 0 0] planes, implying long-range ordered hexagonal mesoporous silica [34,35,37]. The well-resolved diffraction peak assigned as the [1 0 0] plane can also be observed on the as-prepared 20 wt.% PTA/MCM-41 catalyst, demonstrating the preserved well-ordered mesoporous structure, although it suffers from the impregnation process with ultrasonic assistance. From the wide-angle XRD patterns (Fig. 2B), no feature peaks corresponding to the PTA phase are observed on 20 wt.% HPW/MCM-41, suggesting the high dispersity of PTA on MCM-41 support. The alkenylation of p-xylene with phenylacetylene was performed over the as-prepared 20 wt.% PTA/MCM-41 catalyst. The reaction mixture from the fixed-bed reactor used for the alkenylation of p-xylene with phenylacetylene was analyzed by GC and GC/MS, and the separated

The previous report has shown that the type and number of acid sites on PTA/MCM-41 catalysts have a close relationship with the PTA loading [33]. It was proposed that the low PTA loading could produce more Lewis acid sites than Brønsted acid sites; nevertheless, when the PTA loading increases, the Brønsted acidity becomes dominant. Here, we investigate the effect of PTA loading on the catalytic properties of PTA/MCM-41 catalysts for the alkenylation of p-xylene with phenylacetylene. The results are presented in Table 1. The bare support is included for comparison. From Table 1, the MCM-41 support is almost inactive for alkenylation, with conversion as low as 38.3%, due to the lack of acid sites [33]. For the supported PTA mesoporous catalysts, the conversion increases with the rise of PTA loading, and while the loading approaches 25 wt.%, the conversion reaches a maximum (94.0%). Further increase will lead to a decrease in conversion. On the other hand, the increase of PTA loading can lead to a slight drop in the percentage of the main product and a rise of the amount of (E)-b-(2,5-dimethylphenyl)styrene and oligomers. It can be noted that when loading exceeds 30 wt.%, the amount of isomer (E)-b-(2,5dimethylphenyl) styrene begins gradually dropping, and the spent catalysts with loadings of 50 and 70 wt.% become brown, which implies the occurrence of deeper polymerization, leading to a decrease in conversion. As the loading increases the acetophenone percentage first drops, at 25 wt.% it reaches the minimum value, and a further rise in loading leads to an increase in the acetophenone percentage. In addition, the PTA loading does not apparently affect the amount of hydrogenated product. From above, the optimum PTA loading is 25 wt.%. As the form and microcosmic chemical environment of the materials have a close relationship with the framework of the FT-IR spectrum due to the bond vibrations, FT-IR can be adopted as an important method for the structural characterization of polyanions. We have investigated the FT-IR spectrum of the PTA/MCM41 catalysts with different PTA loadings, with bulk PTA and bare MCM-41 included for comparison to determine the state of PTA

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Scheme 1. Proposed reaction mechanism for the alkenylation of p-xylene with phenylacetylene over the PTA/MCM-41 solid acid catalyst.

Table 1 Reaction results of the alkenylation of p-xylene with phenylacetylene over PTA/MCM41 catalysts with different PTA loadings.a Loading (wt.%)

0 10 20 25 30 50 70

Con. (%)

38.3 79.7 92.5 94 93.7 91.8 78.2

Product distribution (%) I

II

III

IV

Oligomers

89.5 91.7 91.5 90.8 89.3 89.1 88.5

5.8 2.1 1.6 1.4 1.6 1.8 2.4

0.3 0.4 0.3 0.3 0.4 0.3 0.4

0.7 0.5 1.2 2.1 2.9 2.7 1.4

3.7 5.3 5.4 5.4 5.8 6.1 7.3

a Reaction conditions: catalyst 1.0 g, nAr/Phen 20:1, Tr 160 °C, Ps 1.0 MPa, VHSV 6 ml h 1 g 1 cat, TOS 8 h.

species impregnated into the pores of MCM-41. The FT-IR spectra are illustrated in Fig. 3. From Fig. 3, a broad band around 1000–1300 cm 1 is observed on the FT-IR spectrum of bare MCM-41, which can be assigned to the asymmetric stretching mode of SiAOASi. The band at 809 cm 1 is associated with symmetric stretching vibration of the rocking mode of SiAOASi bond. There exist four main adsorption peaks for the bulk PTA, which appear at the 1080 cm 1 band (a) for PAO, the 982 cm 1 band (b) for W@O, and both the 894 cm 1 (c) and 800 cm 1 (d) bands for WAOAW due to asymmetric bond stretching vibration assigned to the PTA Keggin structure [41]. The IR spectra of 10–25 wt.% PTA/MCM-41 are similar to that of the bare support, and no apparent adsorption peaks for the typical Keggin structure of PTA are observed at the 982 and 894 cm 1 bands, probably because of the low loading and the highly efficient dispersion of PTA. As the PTA loading is further increased,

Fig. 3. FT-IR spectra of PTA/MCM-41 samples with different PTA loadings (bare MCM-41 and bulk PTA included for comparison). (a) Bare MCM-41; (b) 10 wt.%; (c) 20 wt.%; (d) 25 wt.%; (e) 30 wt.%; (f) 50 wt.%; (g) 70 wt.%; (h) bulk PTA.

gradually resolved adsorption peaks corresponding to the PTA Keggin structure can be observed, and similar results were presented in the previous report [41]. In addition, the characteristic adsorption peaks for PAO (1080 cm 1) and WAOAW (800 cm 1) on the supported PTA catalysts are even stronger than those on the bulk PTA, for the reason that the strong adsorption peaks of support MCM-41, also around 1080 and 800 cm 1, have overlapped with the PAO and WAOAW adsorption peak, respectively [42]. The FT-IR spectra of PTA/MCM-41 catalysts confirm that the PTA

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Fig. 4. NH3 TPD profiles of PTA/MCM-41 samples with different PTA loadings (bare MCM-41 included for comparison). (a) Bare MCM-41; (b) 10 wt.%; (c) 20 wt.%; (d) 25 wt.%; (e) 30 wt.%; (f) 50 wt.%; (g) 70 wt.%.

Keggin structure can be maintained after being impregnated into MCM-41. The catalytic performance of solid acid catalysts is strongly dependent on the acidic properties. The NH3 TPD technique was used to reveal the relationship between the acidic nature and the catalytic properties of supported PTA catalysts with different PTA loadings. The NH3 TPD profiles are shown in Fig. 4. From Fig. 4, a monotonic increase in the numbers of both weak and medium acid sites can be observed as the PTA loading rises. However, the concentration of strong acid sites increases with the PTA loading up to 30 wt.%, and then decreases. It is suggested that the much higher loading leads to the PTA units not spreading well on the silica surface and their serious agglomeration, thus lowering the number of accessible strong acid sites. The high concentration of acid sites is favorable to the alkenylation reaction, but the strong acid sites also promote polymerization. Moreover, it is noteworthy that while the loading is as high as 50 wt.%, a small but definitely visible desorption peak around 600 °C related to the much stronger acid sites is observed. When the loading is further increased up to 70 wt.%, the desorption peak assigned to the strong acid sites is shifted to higher temperature, showing that the strength of acid sites is further intensified, which strongly boosts the oligomerization and leads to the deactivation of the solid acid catalyst by deeper polymerization and coke deposition. Therefore, the appropriate PTA loading is required. Previous reports show crystallite formation by the PTA phase on the supported PTA catalysts, as the loading is not less than 50 wt.%, implying poor dispersion at the too high loading [43]. The XRD experiments were performed to investigate the PTA dispersity and the pore structure of the PTA/MCM-41 with the different loadings, and Fig. 5 illustrates the XRD patterns. From Fig. 5A, the bare MCM-41 exhibits three well-resolved diffraction peaks, which could respectively be indexed as the [1 0 0], [1 1 0], and [2 0 0] planes, associated with long-range ordered hexagonal mesoporous silica [34,35,37]. Only the diffraction peak assigned as the [1 0 0] plane can be observed on the PTA/MCM-41 catalysts, suggesting a decrease in the order degree of the mesoporous materials. Moreover, the supported PTA catalysts on MCM-41 show a shift of the [1 0 0] peak to a higher 2h value, implying a compressed pore size after PTA impregnation. Gradually weaker and broader [1 0 0] peaks can be observed as the PTA loading increases, indicating a loss of structural order [34,44]. Too much higher loading of PTA can give rise to a less ordered mesostructure of the support MCM-41, which deteriorates the catalytic

Fig. 5. Low-angle (A) and wide-angle (B) XRD patterns of the PTA/MCM-41 samples with different PTA loadings (bare MCM-41 and bulk PTA included for comparison). (a) Bare MCM-41; (b) 10 wt.%; (c) 20 wt.%; (d) 25 wt.%; (e) 30 wt.%; (f) 50 wt.%; (g) 70 wt.%; (h) bulk PTA.

properties of the supported PTA catalysts. From Fig. 5B, no characteristic peaks for crystalline PTA are displayed while the loading is not more than 30 wt.%, suggesting the good dispersion of PTA. As the loading is increased to 50 wt.%, distinctly visible peaks corresponding to the PTA crystalline phase can be observed, showing the poor PTA dispersity. Compared with the 50 wt.% PTA/MCM41, the one with 70 wt.% loading exhibits sharper and stronger PTA peaks, demonstrating that too high loading results in the serious agglomeration of PTA, affirmed by TEM in previous reports [44,45]. This is responsible for the existence of much stronger acid sites at the 50–70 wt.% high loading. Besides, it has been reported that the BET surface area, pore diameter, and volume of PTA/MCM41 catalysts decrease with the increase of the PTA loadings [33,43,44], which is also a reason for the worse catalytic performance of PTA/MCM-41 with too high loading. In summary, the PTA loading has a striking effect on the acid properties, mesostructure, and dispersity of the supported catalysts, and consequently affects the catalytic performance in alkenylation. The high loading can increase the concentration of acid sites, which benefits the alkenylation reaction. However, the too high PTA loading would lead to the destruction of the highly ordered mesoporous structure and the serious PTA agglomeration, as well as the generation of strong acid sites with too high acid strength. Too large a number of the strong acid sites gives rise to

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more serious oligomerzation and even deeper polymerization, which are disadvantageous to the catalytic activity. It can be claimed that for a catalyst for the alkenylation of p-xylene with phenylacetylene the appropriate PTA loading is essential. 3.3. Effect of calcination temperature Generally speaking, the calcination temperature (Tc) of the solid acid catalysts remarkably affects the catalytic performance. Table 2 presents the reaction results of alkenylation on the 25 wt.% PTA/ MCM-41 catalysts calcined at different Tc ranging from 200 to 450 °C. From Table 2, the decreased conversion of phenylacetylene can be seen with the increase in Tc, and it reaches the minimum (85.1%) at a Tc of 350 °C. Surprisingly, as the Tc is further increased to 450 °C, the conversion rebounds dramatically (93.3%). On the other hand, the percentage of a-(2,5-dimethylphenyl)styrene (the desired product) in the product distribution increases rapidly with Tc going up, and reaches a maximum (90.8%) when Tc is increased to 300 °C, but the further increased Tc results in a dramatic decrease in the percentage of the desired product. Moreover, it can be observed that when the Tc is as low as 200 °C, large amounts of the isomer (E)-b-(2,5-dimethylphenyl)styrene and oligomers are detected in the reaction mixture, and the amount decreases distinctly with the increase of Tc, but a dramatic increase in oligomers surprisingly appears as the Tc exceeds 300 °C (67.8% at 450 °C). Furthermore, the acetophenone percentage increases with Tc up to 350 °C, and then drops when Tc further increases to 450 °C. No obvious change in the amount of hydrogenated compound can be observed. From above, 300 °C is the appropriate Tc for the alkenylation reaction. NH3 TPD and XRD techniques were utilized to investigate the nature of surface acid sites and structure of catalysts calcined at different Tc. Fig. 6 describes the NH3 TPD profiles of 25 wt.% PTA/ MCM-41 calcined at different temperatures. From Fig. 6, as Tc rises from 200 to 450 °C, the numbers of medium and strong acid sites of the catalysts decline notably, and even the medium acid sites almost disappear at a Tc of 450 °C. Moreover, an increase in the number of weak acid sites can be observed as Tc is increased. It can be found that the increase in Tc leads to an obvious increase in the acid strength of both the medium and strong acid sites. Especially, the sample calcined at 450 °C exhibits remarkably stronger acid sites, which probably belong to Lewis acid W6+ ions [34], resulting in high conversion and heavy oligomerization. Considering the reaction results, the conversion falls progressively because of the gradual decrease in the concentration of the medium and strong acid sites as Tc is increased from 200 to 350 °C. On the other hand, we can see that the variation of the acetophenone depends on the concentration of the weak acid sites. The medium acid sites with relatively weaker acid strength on the catalyst promote isomer production. The enhanced acid strength and reduced number of medium and strong acid sites with the increase in Tc compress the amount of isomer, but improves the Table 2 Reaction results of the alkenylation of p-xylene with phenylacetylene over 25 wt.% PTA/MCM-41 catalysts at different temperatures.a Tc (°C)

200 250 300 350 450

Con. (%)

98 97.5 94 85.1 93.3

Fig. 6. NH3 TPD profiles of 25 wt.% PTA/MCM-41 samples calcined at different temperatures: (a) Tc = 450 °C; (b) Tc = 350 °C; (c) Tc = 300 °C; (d) Tc = 250 °C; (e) Tc = 200 °C.

desired product generation. After Tc exceeds 300 °C, the number of medium acid sites decreases dramatically, leading to a distinct reduction of the percentage of the desired product, a-(2,5-dimethylphenyl) styrene, which further confirms that the medium acid sites are the main active catalytic sites for the alkenylation. Furthermore, a decrease in the percentage of oligomers can be observed as the Tc is increased to 300 °C, which is in accord with the concentration of strong acid. However, a further increase in Tc results in a leap in the percentage of oligomers, which can be attributed to the generation of sites with higher acid strength. In addition, it has been reported that the Brønsted-type acidity of the catalysts decreases as Tc increases [34]. Thus from the view of their acidic nature, on the basis of the catalytic reaction results of the catalysts with different loadings and calcined at different temperatures, it can be suggested that too many Brønsted-type acid sites and Lewis-type acid sites both can lead to the promotion of oligomerization. From the above, it is believed that the alkenylation of p-xylene with phenylacetylene requires a suitable concentration and strength distribution of the acid sites. The low-angle XRD patterns (S18 in the supporting information) of the 25 wt.% PTA/MCM-41 calcined at different temperatures indicate that no obvious change in pore architecture occurs on the catalysts. From the wide-angle XRD patterns (S18 in the supporting information), the absence of characteristic peaks corresponding to the PTA crystalline phase suggests that the PTA units are highly dispersed inside the pores, which was also confirmed by the TEM technique [34]. Moreover, on all catalysts calcined at 200–450 °C, no WO3 crystalline phase peaks appear, which is consistent with the previous reports [34], suggesting no decomposition of PTA units even at the higher Tc of 450 °C. Therefore, we can safely say that the effect of Tc on the catalytic properties for the alkenylation reaction can be ascribed to a change in the nature of the acid sites, but not in the structure of the catalyst.

Product distribution (%) I

II

III

IV

Oligomers

65.5 78.4 90.8 47.6 29.2

0.7 1.3 1.4 2.2 1.6

0.2 0.2 0.3 0.2 0.3

23.5 11.8 2.1 1.1 1.1

10.1 8.3 5.4 48.9 67.8

a Reaction conditions: catalyst 1.0 g, nAr/Phen 20:1, Tr 160 °C, Ps 1.0 MPa, VHSV 6 ml h 1 g 1 cat, TOS 8 h.

3.4. Comparison of PTA/MCM-41 with HY zeolite HY zeolites as ecofriendly and general solid acid catalysts have already been tentatively exploited for the alkenylation of p-xylene with phenylacetylene, and good reaction results was obtained in the batch reactor [18]. Herein, we compared the catalytic performance of the developed PTA/MCM-41 catalyst with that of the HY zeolite under the same reaction conditions in the fixed-bed

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continuous reactor. Considering that the higher Tc resulted in a remarkable decrease in catalytic activity [12], the HY zeolite calcined at 160 °C (denoted as HY-160) at the same reaction temperature was involved for comparison. Table 3 illustrates reaction results. From Table 3, the inferior catalytic properties of HY zeolites can be observed, although satisfactory results were reported [18]. However, the developed PTA/MCM-41 catalyst exhibits remarkably better catalytic properties for the alkenylation reaction than HY zeolites, regardless of the Tc. From further analysis of reaction results, although the lower Tc can increase the catalytic activity (conversion varying from 17.8% to 28.2%), it is still unsatisfactory. Regrettably, the lower Tc can lead to the generation of more acetophenone due to the water linked to the zeolites instead of the sole trace water in the reaction mixtures [18]. Moreover, the narrow porous channels are the intrinsic nature of HY zeolites, which can discourage the diffusion of relatively large molecules, such as the transition state for ortho-alkenylation, consisting of the aromatic substrate and phenylacetylene. The narrow pore structure also prevents the generated oligomers from floating out of the pores, resulting in further deep oligomerization and coke deposition that would cover the acid sites inside the zeolites and occlude the pores. As a result, the HY would easily be deactivated. This is also a reason that the very poor catalytic properties in our studies (continuous reaction mode) can be obtained, although 91% conversion and 88% selectivity of the desired product were achieved in batch mode due to the large usage of HY catalyst (low catalytic efficiency, 1.0 g catalyst for 1.0 g phenylacetylene). 3.5. Effect of reaction parameters From these results, it is claimed that the developed PTA/MCM41 catalyst exhibits outstanding catalytic performance for the alkenylation of p-xylene with phenylacetylene, strongly depending on the nature of the acid sites, PTA dispersity, and pore architecture, which are significantly affected by impregnation method, PTA loading, and Tc. This has shown the limitation of HY zeolite from its intrinsic nature. Herein, the effect of reaction parameters such molar ratio of p-xylene to phenylacetylene (nAr/Alk), reaction temperature (Tr), system pressure (Ps), and volume hourly space velocity (VHSV) on the alkenylation reaction over the 25 wt.% PTA/MCM-41 catalyst calcined at 300 °C was investigated.

Table 4 Effect of nAr/Alk on the alkenylation of p-xylene with phenylacetylene over the PTA/ MCM-41 solid acid catalyst.a nAr/Alk

Con. (%)

Product distribution (%) I

II

III

IV

Oligomers

5:01 10:01 15:01 20:01 25:1

63.2 68 90.8 94 >99.0

86.2 88.6 88.7 90.8 89.8

2.1 1.7 1.5 1.4 0.5

0.3 0.3 0.3 0.3 0.3

2.5 1.2 3.3 2.1 4.7

8.9 8.2 6.2 5.4 4.7

a Reaction conditions: catalyst 1.0 g, Tr 160 °C, Ps 1.0 MPa, VHSV 6 ml h TOS 8 h.

1

g

1

cat,

of the desired product can be observed, which is ascribed to the solvent effect of p-xylene inhibiting the self-polymerization of alkynes and other side reactions. In addition, as was shown before, the increasing conversion may be the reason for the reduced percentage of acetophenone with the increasing nAr/Alk. Moreover, the nAr/Alk has no obvious influence on the hydrogenation side reaction. 3.5.2. Reaction temperature The effect of Tr on the catalytic performance of MCM-41 supported PTA catalysts for alkenylation was studied. Table 5 illustrates the catalytic reaction results. From Table 5 the conversion of phenylacetylene increases with the rise of Tr up to 150 °C, but no obvious change in catalytic performance takes place as Tr is further increased from 150 to 160 °C. Moreover, no obvious change in the percentage of the desired product can be observed when the Tr is not more than 160 °C. It can be observed that in the range of 110 to 160 °C, the increase of Tr can result in a progressive rise in the quantity of isomer, suggesting that the high Tr is favorable to the production of thermally more stable isomer. Furthermore, no distinct variation in the amount of acetophenone and hydrogenated compound is detected over the whole range of Tr. The temperature range 150–160 °C is the optimum Tr for the alkenylation reaction over PTA/MCM-41 catalyst.

3.5.1. Molar ratio of p-xylene to phenylacetylene In general, the mixture ratio of feedstock plays a significant role in the alkenylation reaction. Table 4 provides the reaction results. From Table 4, due to the improvement of the collision probability of alkenyl cations with aromatic rings, a continuous increase in conversion can be observed with the rise of nAr/Alk, and more than 99% conversion and 89.8% product percentage can be obtained at an nAr/Alk of 25:1. The large numbers of p-xylene molecules can be used as both reagent and solvent, and no extra solvent is required. As the nAr/Alk is increased, an increase in the percentage

3.5.3. System pressure The reaction pressure has a certain effect on the diffusion of the reactants and products in the heterogeneous catalysis. Table 6 shows the catalytic reaction results as a function of Ps for alkenylation over PTA/MCM-41 catalyst. From Table 6, the significantly low conversion (37.7%) and percentage of the desired product (59.7%) can be obtained at ambient pressure, which can be ascribed to the partial gasification of the liquid feedstock, since the Tr (150 °C) is higher than the boiling point of the reaction mixture. The gas-phase reaction atmosphere might result in the generation of more oligomers. More than 99% conversion can be obtained if the Ps is not less than 0.6 MPa, implying that the alkenylation reaction just proceeds smoothly under

Table 3 Reaction results of the alkenylation of p-xylene with phenylacetylene over PTA/MCM41 and HY zeolites.a

Table 5 Effect of Tr on the alkenylation of p-xylene with phenylacetylene over the PTA/MCM41 solid acid catalyst.a

Catalyst

PTA/MCM-41b HY-300 HY-160

Con. (%)

94 17.8 28.2

Product distribution (%)

Tr (°C)

I

II

III

IV

Oligomers

90.8 69 59.5

1.4 12.9 21.8

0.3 0.4 0.2

2.1 7 6.1

5.4 10.7 12.4

a Reaction conditions: catalyst 1.0 g, nAr/Phen 20:1, Tr 160 °C, Ps 1.0 MPa, VHSV 6 ml h 1 g 1cat, TOS 8 h. b 25 wt.% PTA loading.

110 130 150 160

Con. (%)

86.4 95.5 >99.0 >99.0

a Reaction conditions: 6 ml h 1 g 1 cat, TOS 8 h.

Product distribution (%) I

II

III

IV

Oligomers

91 89.9 90.4 89.8

0.7 0.6 0.5 0.5

0.2 0.2 0.2 0.3

2 2.6 4.1 4.7

6.1 6.7 4.8 4.7

catalyst

1.0 g,

nAr/Phen

25:1,

Ps

1.0 MPa,

VHSV

51

Z. Zhao et al. / Journal of Catalysis 288 (2012) 44–53 Table 6 Effect of system pressure on the alkenylation of p-xylene with phenylacetylene over the PTA/MCM-41 solid acid catalyst.a Ps (MPa)b

Con. (%)

0 0.6 1 1.5

37.7 >99.0 >99.0 >99.0

a

Reaction conditions: 6 ml h 1 g 1 cat, TOS 8 h. b Gauge pressure.

Table 8 Regeneration of the spent 25 wt.% PTA/MCM-41 catalyst for alkenylation of p-xylene with phenylacetylene.a

Product distribution (%)

Catalyst

I

II

III

IV

Oligomers

59.7 86.8 90.4 89.6

6.2 0.6 0.5 0.5

0.2 0.3 0.2 0.3

7.6 5.9 4.1 3.6

26.3 6.4 4.8 5.9

catalyst

1.0 g,

nAr/Phen

25:1,

Tr

150 °C,

Fresh RCAT-1b RCAT-2c

Con. (%)

>99.0 75.6 97.1

Product distribution (%) I

II

III

IV

Oligomers

90.4 92.1 91

0.5 1.1 1.6

0.2 0.2 0.2

4.1 0.9 0.7

4.8 5.7 6.5

a

VHSV

Reaction conditions: catalyst 1.0 g, nAr/Phen 25:1, Tr 150 °C, Ps 1.0 MPa, TOS 8 h. Regenerated catalyst through washing with acetone. Regenerated catalyst through washing with acetone and adding the leached PTA in the process of washing with acetone. b

c

Table 7 Effect of VHSV on the alkenylation of p-xylene with phenylacetylene over the PTA/ MCM-41 solid acid catalyst.a VHSV (ml h

4 6 8 10 12 a

1

g

1

cat)

Con. (%)

92.5 >99.0 92 88.2 58.5

Product distribution (%) I

II

III

IV

Oligomers

68.6 90.4 89.4 90.5 88.1

0.7 0.5 1 1.6 2.7

0.2 0.2 0.3 0.3 0.2

24 4.1 3.4 1.7 1.9

6.5 4.8 5.9 5.9 7.1

Reaction conditions: catalyst 1.0 g, nAr/Phen 25:1, Tr 150 °C, Ps 1.0 MPa, TOS 8 h.

the liquid-phase condition. In addition, a slight increase in the percentage of the desired product can be observed with increasing Ps from 0.6 to 1.0 MPa. No obvious change takes place if Ps is further increased from 1.0 to 1.5 MPa. From above, a Ps of 1.0 MPa is optimum for this reaction. 3.5.4. Volume hourly space velocity For the reaction conducted in the continuous-flow fixed bed reactor, the effect of VHSV on the reaction is usually significant. Table 7 presents the reaction results for the alkenylation of pxylene over the PTA/MCM-41 catalyst. From Table 8, the conversion is enhanced with the increase of VHSV, and reaches its maximum value as VHSV rises to 6 ml h 1 g 1 cat, but further increase in VHSV lead to a decrease. The lower VHSV means a longer time that the reactant and product molecules stay in the catalyst layer, which more easily leads to more serious oligomerizaiton and even the generation of high polymers for coke deposition. In addition, the long-time stay of the reactant benefits isomer production. As a result, the 24.0% IV percentage can be detected as the VHSV of 4 ml h 1 g 1 cat is used. On the other hand, too high VHSV stands for a large number of reactants flowing through the catalyst layer in the unit time. The catalyst cannot provide adequate active sites for so many reaction substrates. In addition, it can also be speculated that at high VHSV part of the reactants did not have enough time to access the acid sites inside the pores [46]. From above, a VHSV of 6 ml h 1 g 1 cat is appropriate. In summary, the PTA/MCM-41 catalyst prepared by the IMPVH method with appropriate PTA loading and Tc exhibits excellent catalytic properties for the alkenylation reaction of p-xylene with phenylacetylene. The optimum reaction conditions are nAr/Phen 25:1, Tr 150–160 °C, Ps 1.0 MPa, and VHSV 6 ml h 1 g 1 cat. 3.6. Catalytic stability and regeneration Fig. 7 presents the conversion and selectivity as a function of time on stream for the alkenylation of p-xylene with phenylacetylene over the developed PTA/MCM-41 solid acid catalyst. From

Fig. 7. Conversion and selectivity as a function of time on stream for the alkenylation of p-xylene with phenylacetylene.

Fig. 7, more than 92% conversion can be maintained for up to 540 min of time on stream, but a decrease in catalytic activity takes place with time. From the Refs. [47,48], coke deposition and PTA leaching are possible reasons for the loss of catalytic activity. Through ICP analysis, we found that the PTA content for the refreshed and spent catalyst is 26.0 and 25.5 wt.%, respectively. Further ICP analysis was performed on the total reaction mixture (all reaction mixtures at various times on stream collected together). From ICP analysis results on the reaction mixture, 2.6 ppm PTA was detected, which corresponds to 0.3 mg PTA. From this analysis, no obvious leaching can be observed, now that the reaction system is not strongly polarized. The thermal gravity experiments were performed on the fresh and spent catalysts after they were pretreated by calcination at 300 °C for 3 h to eliminate the crystal water and the absorbed reaction mixture. The results show that 7.3 and 13.3 wt.% weight loss were observed for fresh and spent catalysts, respectively, implying 6 wt.% weight loss from the combustion of deposited coke (7.3 wt.% weight loss resulted from the decomposition of PTA). From above, the coke deposition is the main reason for the loss of catalytic activity; the slight leaching of PTA is another factor in the deactivation of catalyst. The developed catalyst exhibits outstanding catalytic activity and selectivity for the alkenylation reaction, but the improvement in stability will be addressed in future work. The regeneration behavior of the spent 25 wt.% PTA/MCM-41 was also investigated, and Table 8 presents the reaction results. From Table 8, 75.6% conversion can be observed on the RCAT-1 (regeneration through washing with acetone to eliminate the deposited coke). The unsatisfactory catalytic performance of RCAT-1 can be observed. It is well known that PTA can dissolve in a polar solvent, and therefore the regeneration process through

52

Z. Zhao et al. / Journal of Catalysis 288 (2012) 44–53

Table 9 Catalytic properties of the developed catalyst for the alkenylation reaction of different aromatics with phenylacetylene.a Conversion (%)

Selectivity (%)

Yieldc (%)

1

66.6

79.6

53.0

2

99.0

90.9

90.0

3

99.0

94.1

93.2

4

94.6

90.6

85.7

5d

97.2

86.2

83.8

6d

98.9

82.1

81.2

7d

98.7

90.7

89.5

8d

98.7

93.7

92.5

9e

99.0

90.3

89.4

10

99.0

93.0

92.1

Entry

a

Productb

Ar

Reaction conditions: catalyst 1.0 g, nAr/Phen 25:1, Tr 150 °C, Ps 1.0 MPa, VHSV 6 ml h 1 g 1 cat, TOS 8 h. H NMR presented in Supporting information. GC yield. The cyclohexane or dioxane was introduced into the feed for easy material transfer in the fixed-bed reactor. The Ps was increased to 3.0 MPa to maintain the liquid phase of the feed.

b 1 c d e

washing with acetone may lead to the leaching of PTA, which is the reason for the nonideal catalytic properties of RCAT-1. We tried our best to use nonpolar solvents (benzene, cyclohexane, p-xylene, etc.) to wash the spent catalyst for catalyst regeneration. Unfortunately, this process failed to refresh them. Therefore, 7.5 wt.% PTA was added into the RCAT-1 to further improve the regeneration behavior of the spent catalyst, and RCAT-2 was obtained. From Table 8, 97.1% conversion and 91.0% main production percentage (selectivity) can be obtained on the RCAT-2, suggesting that the spent catalyst can be well refreshed by combining the process of washing with acetone to eliminate the deposited coke with adding an appropriate amount of PTA. 3.8. Catalytic performance for extra aromatics From

above,

more

than

99%

conversion

and

90.4%

a-(2,5-dimethylphenyl)styrene percentage has been obtained via the alkenylation of p-xylene with phenylacetylene over the developed PTA/MCM-41 solid acid catalyst under optimum conditions. Finally, the catalytic performance of the developed catalyst was investigated using various aromatics as substrates. Table 9 presents the reaction results. The molecular structures of products were identified by 1H NMR and Refs. [6,49–54]. From Table 9, except for benzene as substrate, more than 80% yield can be obtained. With some substrates, the yield of a-arylstyrenes can reach more than 90%. Reaction results demonstrate that the developed direct alkenylation of aromatics with phenylacetylene over the improved PTA/MCM-41 solid acid catalysts can be considered a green and highly efficient synthesis protocol to produce a-arylstyrenes, which can be extensively used with various substrates such as substituted benzenes,

polycyclic arenes, and even heterocyclic aromatics. Therefore, we can say that the as-prepared PTA/MCM-41 may be considered a promising catalyst for the alkenylation of various aromatics with phenylacetylene, but improving catalytic stability will be addressed in future work. 4. Conclusions We have developed a clean and highly efficient protocol for the synthesis of a-arylstyrenes with different substrates via the direct alkenylation of various aromatics over a novel mesoporous MCM41 supported PTA catalyst in a fixed-bed continuous-flow reactor. The developed solid acid catalyst has displayed excellent catalytic performance in the alkenylation in comparison with HY zeolites. Using the alkenylation of p-xylene with phenylacetylene as a model reaction, we have found that the catalytic properties of the PTA/ MCM-41 catalyst strongly depend on the concentration and strength of acid sites, the dispersity of PTA units, and the mesostructure of the supports, which in turn are affected by the impregnation method, PTA loading, and calcination temperature. The optimum reaction conditions are nAr/Phen 25:1, Tr 150–160 °C, Ps 1.0 MPa, VHSV 6 ml h 1 g 1 cat. Moreover, it was found that the optimum PTA loading (25 wt.%) could bring about acid sites with appropriate concentration and strength, as well as good preservation of the PTA Keggin structure and well-ordered mesoporous architecture. It can be proposed that the medium acid sites are the main active sites for the alkenylation reaction to produce both the desired a-arylstyrene and its isomer, and only if the medium acid sites are weakened to a certain degree can the generation of isomer be mainly promoted; otherwise the dominant alkenylated product turns out to be the desired a-arylstyrene. In addition,

Z. Zhao et al. / Journal of Catalysis 288 (2012) 44–53

the weak acid sites may promote the hydration of phenylacetylene with the trace water, while the strong acid sites may not only facilitate the desired alkenylation but also accelerate the oligomerization reaction. The much stronger acid sites (desorption peak at about 600 °C) are suggested to be favorable for the oligomerization and also the deeper polymerization, which is responsible for the deactivation of solid acid catalyst by coke deposition. The deactivated catalyst can be regenerated, and 97.1% conversion (TOS, 8 h) on the refreshed catalyst can be observed, suggesting that the developed PTA/MCM-41 sample could be a promising catalyst for alkenylation. The catalytic stability will be addressed in future work. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (DLUT). We are also grateful to the State Key Laboratory of Fine Chemicals, Dalian University of Technology, China, for sample analysis. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcat.2011.12.024. References [1] C. Jia, D. Piao, J. Oyamada, W. Lu, T. Kitamura, Y. Fujiwara, Science 287 (2000) 1992. [2] J.A. Reilly, J.A. Nieuwland, J. Am. Chem. Soc. 50 (1928) 2564. [3] T. Tsuchimoto, T. Maeda, E. Shirakawa, Y. Kawakami, Chem. Commun. (2000) 1573. [4] R. Li, S.R. Wang, W. Lu, Org. Lett. 9 (2007) 2219. [5] M.Y. Yoon, J.H. Kim, D.S. Choi, U.S. Shin, J.Y. Lee, C.E. Song, Adv. Synth. Catal. 349 (2007) 1725. [6] D.S. Choi, J.H. Kim, U.S. Shin, R.R. Deshmukh, C.E. Song, Chem. Commun. (2007) 3482. [7] S. Barman, N.C. Pradhan, J.K. Basu, Ind. Eng. Chem. Res. 44 (2005) 7313. [8] Z.K. Zhao, W.H. Qiao, G.R. Wang, Z.S. Li, L.B. Cheng, J. Mol. Catal. A: Chem. 250 (2006) 50. [9] N. Lucas, A. Bordoloi, P. Amrute, P. Kasinathan, A. Vinu, W. Bohringer, J.C.Q. Fletcher, S.B. Halligudi, Appl. Catal. A: Gen. 352 (2010) 74. [10] G. Kostrab, M. Lovic, I. Janotka, M. Bajus, D. Mravec, Appl. Catal. A: Gen. 335 (2008) 74. [11] S. Pai, U. Gupha, S. Chilukuri, J. Mol. Catal. A: Chem. 265 (2007) 109. [12] Z.K. Zhao, W.H. Qiao, X.N. Wang, G.R. Wang, Z.S. Li, L.B. Cheng, Micropor. Mesopor. Mater. 94 (2006) 105. [13] Z.K. Zhao, W.L. Wang, W.H. Qiao, G.R. Wang, Z.S. Li, L.B. Cheng, Micropor. Mesopor. Mater. 93 (2006) 164. [14] Z.K. Zhao, W.H. Qiao, X.N. Wang, G.R. Wang, Z.S. Li, L.B. Cheng, J. Mol. Catal. A: Chem. 241 (2005) 194.

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