Effect of post treatment on the local structure of hierarchical Beta prepared by desilication and the catalytic performance in Friedel–Crafts alkylation

Microporous and Mesoporous Materials 206 (2015) 42–51

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Effect of post treatment on the local structure of hierarchical Beta prepared by desilication and the catalytic performance in Friedel–Crafts alkylation Yi Wang a,b, Yinyong Sun a,⇑, Christine Lancelot b, Carole Lamonier b, Jean-Charles Morin b, Bertrand Revel b, Laurent Delevoye b, Alain Rives b a b

School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China Université Lille Nord de France, Unité de Catalyse et de Chimie du Solide, UMR8181, F-59655 Villeneuve d’Ascq, France

a r t i c l e

i n f o

Article history: Received 20 November 2014 Received in revised form 11 December 2014 Accepted 12 December 2014 Available online 23 December 2014 Keywords: Hierarchical Beta Benzyl alcohol Benzylation Mesitylene Post treatment

a b s t r a c t Desilication method has been developed to prepare hierarchical zeolites. However, the hierarchical Beta prepared by desilication generally exhibited much poorer catalytic activity than parent one in some acidcatalyzed reactions mainly due to the great loss of acidity. Here, we report that subsequent acid treatment after desilication can greatly recover the acidity of zeolite Beta. As a result, the hierarchical Beta with acid treatment exhibited enhanced catalytic performance in the benzylation of benzene or mesitylene with relatively large molecular size. Additionally, the local structural change in zeolite Beta before and after post treatment was deeply studied by the NMR technique. The results indicated that during the desilication partial tetrahedral coordination Al was transformed into the distorted tetrahedral and pentahedral coordination Al and subsequent acid treatment led to the increase of the proportion of tetrahedral coordination Al. Thus, the number of total Brönsted acid sites in zeolite Beta was decreased after the desilication and subsequent acid treatment may increase the number of total and accessible Brönsted acid sites. These results suggested that acid treatment is helpful for improving the catalytic performance of the desilicated Beta in some acid-catalytic reactions, especially toward large molecules. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Friedel–Crafts alkylation is an important class of reactions in organic chemistry [1,2]. Among these alkylations, the liquid phase benzylation of aromatic compounds by benzyl chloride or benzyl alcohol is of significance for the production of diphenylmethane and substituted diphenylmethanes which are key industrial compounds used as pharmaceutical intermediates and fine chemicals. Generally, such reactions were carried out in the presence of homogeneous acid catalysts such as AlCl3 and H2SO4 [3,4]. However, these catalysts pose several serious problems such as corrosion, handling, toxicity, and separation and recovery of the catalyst. Hence, the replacement of the homogeneous catalysts by heterogeneous solid catalysts has been pursued [5–8]. Several types of zeolites (ZSM-5, Mordenite, Beta, Y) as solid acid catalysts have been widely studied in benzylation reactions because of their strong acidity, large surface area, regular pore array, and excellent hydrothermal/chemical stability [9–16]. The ⇑ Corresponding author. Tel.: +86 451 86413708; fax: +86 4006358835 00379. E-mail address: [email protected] (Y. Sun). http://dx.doi.org/10.1016/j.micromeso.2014.12.017 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

results indicated that zeolite Beta is an excellent catalyst in the benzylation of benzene with benzyl alcohol [11–16]. However, in the benzylation of relatively large molecular reactants like toluene and xylene, microporous Beta was less active due to its small micropore size (ca. 0.7 nm) [16]. The development of hierarchical zeolites can be a good route to solve the above problems because it may not only increase the accessibility of acid sites but also overcome the diffusion limitation [17–32]. Currently, hierarchical zeolites were mainly prepared by direct (soft or hard template) or indirect (desilication, dealumination) method. Thereinto, desilication method is of great interest because of its simple operation conditions and low preparative cost [21–28]. Earlier works showed that hierarchical zeolites including ZSM-5, mordenite and Y prepared by desilication can exhibit enhanced catalytic performance compared with parent ones in many reactions [21,33]. However, zeolite Beta is a special example. Some works have reported that hierarchical Beta obtained by desilication would lead to lower catalytic activity than parent Beta in some reactions, which was attributed to the great loss of acidity in desilicated Beta. It was proposed that aluminum species in Beta framework had relatively low stability and cannot act as a silicon

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51

extraction moderator and the desilication would negatively affect the acidity [34]. Recently, Tian et al. reported that the Si/Al molar ratio in parent Beta had a big influence on the properties of hierarchical Beta prepared by desilication. The zeolite Beta with a low Si/ Al molar ratio less than 30 could well preserve its microporosity and acidic properties after desilication [25,26]. Nevertheless, the catalytic performance of hierarchical Beta has not been evaluated in Friedel–Crafts alkylation involving the reactants with large molecular size. On the other hand, it was suggested that desilication could result in part of Al species deposited on the external surface of zeolite framework and thus disadvantage the improvement of catalytic reactivity [35]. Acid wash after desilication may remove the deposited Al species to improve the catalytic performance. Some experimental results seemed to support this viewpoint. For example, acid wash after desilication on zeolites like ZSM-5, mordenite, and Y exhibited better catalytic activities than desilicated ones [33,36,37]. However, the effect of such treatment on desilicated Beta has been not presented. Additionally, the study about the local structural change in zeolite Beta before and after post treatment is rare for clarifying this process. In the present work, we firstly prepared hierarchical Beta by desilication based on a commercial Beta with a low Si/Al molar ratio of 12.5. Then, acid treatment was employed to study its effect on the structure and properties of the obtained hierarchical Beta. The catalytic performance of the catalysts was first evaluated in the benzylation of benzene with benzyl alcohol, then in the benzylation of mesitylene with relatively large molecular size. The change of local structure and acidity in zeolite Beta before and after post-treatment was deeply studied. The relationship between the structural properties of catalysts and catalytic performance was discussed. 2. Experimental 2.1. Materials Commercial Beta was purchased from the Catalyst Plant of Nankai University. HNO3, NaOH, NH4NO3, benzene, mesitylene, and benzyl alcohol were purchased from Sinopharm Chemical Reagent Co. Pyridine and pivalonitrile were purchased from Sigma company. Both were transferred in glass cylinder in the presence of molecular sieves 3A. Other reagents were used without further purification. 2.2. Preparation of catalyst The commercial Beta calcined at 823 K for 5 h was labeled as HB. The sample HB was treated with a solution of 0.2 M NaOH with a liquid-to-solid ratio of 30 ml/g at 338 K for 0.5 h. Then, the product was filtered, washed with deionized water, dried, ionexchanged into NH4-form, and calcined at 823 K for 5 h. The resultant sample was named as HB-B. It was then treated with a solution of 0.1 M HNO3 at 338 K for 6 h with a liquid-to-solid ratio of 30 ml/g. The product was ion-exchanged as described above, followed by calcination at 823 K for 3 h to obtain the sample labeled as HB-BA. 2.3. Catalyst characterization X-ray powder diffraction (XRD) patterns were recorded using a Siemens D-5000 diffractometer equipped with the Cu Ka radiation (wavelength of k = 1.5418 Å) and at BM01B-SNBL beam line, ESRF, France (k = 0.5 A, Si (1 1 1) channel cut monochromatic) using the two cycle diffractometer equipped by six detectors having Si

43

(1 1 1) analyzer crystals and Na-I scintillation counters. The determination of the relative crystallinity value was based on the intensity of the characteristic peaks in the range between 6 and 9°. The parent Beta was assigned a crystallinity of 100%. Nitrogen sorption isotherms were obtained at 77 K on a Micromeritics TriStar II 3020 Gas Sorption and Porosimetry system. Prior to the experiments, the samples were outgassed at 423 K under vacuum for 3 h. SEM images were recorded on Hitachi S-4700 Scanning Electron Microscope system operated with an acceleration voltage of 10 kV. TEM images were recorded on Tecnai microscope operating at 200 kV with a LaB6 crystal. 29Si NMR experiments were performed at room temperature on a 400 MHz Bruker Advance Avance II 400 spectrometer at a spinning rate of 12 kHz using a 4 mm probe and the 29Si MAS NMR chemical shift was referenced to 0 ppm relatively to tetramethylsilane (TMS). The 27Al MAS NMR spectra [38] were recorded on a 800 MHz Bruker Avance III 800 spectrometer by using single pulse length of p/6 and a relaxation delay of 1 s. The 27Al chemical shift was referenced to [Al(H2O)6]3+ (Chemical shift = 0 ppm). The 27Al MQMAS spectra were collected using the Z-filter sequence [39], which consists of two hard pulses of 6 and 1.4 ls at an RF field of 90 kHz, for triple-quantum excitation and reconversion, respectively, followed by a central transition selective pulse of 7 ls at an RF field of 12 kHz. The IR studies of pyridine and pivalonitrile adsorption were carried out to measure the number of acid sites and accessible acid sites, respectively, recorded with a Nicolet Protege System 460 equipped with a DTGS detector. All samples were ground in an agate mortar and were pressed into the form of self-supporting wafers (5 mg/cm2), then heated at 723 K under high vacuum (10 6 mbar) for 2 h before probe molecule adsorption. All recorded spectra were recalculated to a normalized wafer of 10 mg. The concentration of Brönsted and Lewis acid sites was determined by quantitative IR studies of pyridine adsorption experiments. In order to quantify Lewis and Brönsted acid sites, the following bands and absorption coefficients were used: Pyridinium (PyH+) band at 1545 cm 1, eB = 1.23 cm/ lmol and pyridine (PyL) 1454 cm 1, eL = 1.73 cm/lmol [40]. The concentration of accessible Brönsted and Lewis acid sites was determined by quantitative IR studies of pivalonitrile adsorption experiments. In order to quantify Lewis and Brönsted sites, the following bands and absorption coefficients were used: pivalonitrile (PiH+) band at 2274 cm 1, eB = 0.11/f cm/lmol and pivalonitrile (PiL) 2302 cm 1, eL = 0.15/f cm/lmol [41], where f was a constant depending on the IR system. XPS analyses were performed on an Axis ultra DLD (Kratos analytical) using a monochromatic Al Ka X-ray source (hm = 1486.6 eV). The emission voltage and the current of this source were set to 15 kV and 10 mA, respectively. Data treatment and peak-fitting procedures were performed using Casa XPS software. Obtained spectra were calibrated in respect of C 1s (C–C bond) at 285 eV. The peaks were decomposed using Gaussian–Lorentzian peak shapes. 2.4. Catalytic tests The liquid phase benzylation of benzene or mesitylene with benzyl alcohol (BA) (Scheme 1) was carried out in a three-necked round-bottom flask equipped with a reflux condenser and heated in a temperature-controlled oil bath under atmospheric pressure. The reaction temperature for the benzylation of benzene was at 353 K while the benzylation of mesitylene was carried out at 373 K. 160 mmol of benzene (or mesitylene) was added to 100 mg catalyst. The reaction mixture was maintained for 30 min at the required reaction temperature and then 2 mmol of benzyl alcohol was added. This moment was regarded as the initial reaction time. Liquid samples were withdrawn at regular intervals and analyzed by gas chromatography on an Agilent 6890 GC with an FID detector using a 50 m packed HP5 column. The products were

44

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51

Scheme 1. Benzylation reaction pathway of benzene with benzyl alcohol.

3. Results and discussion 3.1. Characterization of catalyst The powder XRD patterns of HB, HB-B, HB-BA are shown in Fig. 1. All the samples exhibited well-resolved diffraction peaks, which are characteristic of the Beta framework structure. This result indicated that the structure of zeolite Beta can be well preserved after base and acid treatment. However, the crystallinity of the sample decreased to 60% after base treatment, perhaps due to the formation of debris. After acid treatment, the crystallinity was recovered to 73%. Fig. 2A exhibits N2 sorption isotherms of HB, HB-B and HB-BA. It can be seen that all the samples gave a type I isotherm at low relative pressure and a type IV/II isotherm with a hysteresis loop at high relative pressure, indicating the existence of hierarchical pores. The detailed sorption data are shown in Table 1. Obviously, BET surface area, external surface area, mesopore volume of zeolite Beta increased after base treatment from 550 m2/ g, 134 m2/g, 0.34 cm3/g to 676 m2/g, 329 m2/g, 0.70 cm3/g, respectively. The external surface area and mesopore volume of HB-B is twice more than that of HB. These results indicated that the base treatment had a great influence on the pore structure of zeolite Beta. The mesoporosity in zeolite Beta can be greatly improved by desilication. Further, subsequent acid treatment slightly

increases those values to 717 m2/g, 348 m2/g, 0.74 cm3/g for HBBA. This means that acid treatment had a minor effect on the pore structure. Fig. 2B showed the curves of pore distribution obtained by BJH method for various samples. All the samples exhibited the existence of mesopores (average pore size around 4 nm) and macropores (average pore size around 75 nm). Differently, HB-B and HB-BA had richer mesopores and macropores than HB. However, the pore distribution curve of HB-B is very similar to that of HBBA. Additionally, it can be noted that micropore volume could be mostly maintained after base and acid treatment, suggesting the persistence of Beta micropore structure. The SEM and TEM images of HB, HB-B and HB-BA were shown in Fig. 3. From SEM images, the particle size of all the samples

800 3 Volume adsorbed (cm /g)

also identified by GC–MS (HP-5975C) analysis. Since benzene or mesitylene was in excess, conversion was calculated based on the benzylating reagent, i.e. BA. The selectivity to the product diphenylmethane (DPM) or 2-benzyl-1,3,5-trimethylbenzene (BTB) was expressed as the amount of particular product divided by the amount of total products and multiplied by 100.

A HB-BA

600

HB-B

+200

400

+100

HB

200

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po) 1.0

Beta Beta-B Beta-BA

B

0.9

HB-BA

0.8

dV/dlog(D)

Intensity (a.u.)

0.7

HB-B

0.6 0.5 0.4 0.3 0.2 0.1

HB

0.0 2 10

20

30

40

50

3

4

5

6

25

50

75

100 125 150

Pore diameter (nm)

2 Theta (degree) Fig. 1. XRD patterns of HB, HB-B and HB-BA.

Fig. 2. N2 sorption isotherms (A) and pore distribution curve (B) of HB, HB-B and HB-BA.

45

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51 Table 1 Textural properties and chemical composition of various samples.

a b c d e

Samples

BET surface area (m2/g)

External surface area (m2/g)a

Micropore volume (cm3/g)a

Mesopore volume (cm3/g)b

Crystallinity (%)c

Si/Ald

Si/Ale

HB HB-B HB-BA

550 676 717

134 329 348

0.20 0.17 0.18

0.34 0.70 0.74

100 60 73

16.5 13.4 15.3

15.9 10.0 17.8

t-Plot method. BJH method (adsorption branch). XRD. 27 Si MAS NMR. XPS.

Fig. 3. SEM and TEM images of HB, HB-B and HB-BA.

was similar between 100 and 200 nm. Some particle aggregates can be observed, which may explain the reason of mesopores formation in HB. Further, TEM images revealed the presence of porosity between the particles formed for HB-B and HB-BA (see the circled images in Fig. 3) compared with HB. Meanwhile, the crystal fringe characteristic of Beta structure in HB-B and HB-BA can be clearly seen, suggesting again that micropore structure of zeolite Beta was well kept after post treatment. These results are in accordance with N2 sorption data concerning the creation of mesopores and macropores and XRD analysis concerning the preservation of the zeolite structure. The IR studies of pyridine adsorption were performed to measure the number of acid sites in HB, HB-B and HB-BA. The data are listed in Table 2. Obviously, the number of total and Brönsted acid sites (582 lmol/g, 269 lmol/g) in HB is twice more than that (254 lmol/g, 129 lmol/g) in HB-B. Certainly, the number of acid sites was greatly decreased after desilication, which is in agreement with some reported results [22,23,25,27,34]. Comparatively, the number of total and Brönsted acid sites (445 lmol/g, 209 lmol/g) for HB-BA is much larger than that of HB-B (nearly twice). This result indicated that the number of total acid sites can be greatly recovered by subsequent acid treatment after desilication.

Due to its larger size than that of pyridine, pivalonitrile with steric hindrance has been proven to be a good probe molecule to characterize the number of acid sites in zeolites accessible to larger molecules [41]. The band of 2274 cm 1 and 2302 cm 1 corresponds to the adsorption of pivalonitrile on Brönsted acid sites and Lewis acid sites, respectively. The data of the corresponding acid sites in HB, HB-B and HB-BA are listed in Table 2. As shown, HB-B and HB-BA had more acid sites accessible to larger pivalonitrile than HB. The number of accessible acid sites in HB-B is three times larger than that in HB. This means that desilication of zeolite Beta can increase the number of accessible acid sites, which should be attributed to the improvement of external surface area due to the introduction of additional mesopores. After acid treatment, the number of total accessible acid sites was slightly increased. Apparently, HB-BA had larger number of accessible Brönsted acid sites than HB-B. 3.2. Catalytic evaluation 3.2.1. Benzylation of benzene with benzyl alcohol In the beginning, the benzylation of benzene with benzyl alcohol as a probe reaction for Friedel–Crafts alkylation was chosen to evaluate the catalytic performance of catalysts. The products in

Table 2 The number of acidic sites and accessible acid sites of various samples. Samples

Bpy (lmol/g)

Lpy (lmol/g)

Totpy (lmol/g)

Bpi (  f lmol/g)

Lpi (  f lmol/g)

Totpi (  f lmol/g)

HB HB-B HB-BA

269 129 209

313 125 236

582 254 445

2.0 5.9 9.5

3.4 10.5 7.5

5.4 16.4 17.0

46

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51

100

A

Conversion (%)

80

60

40

20

HB HB-B HB-BA

0 0

10

20

30

40

50

Reaction time (min)

100

B

Yield of DPM (%)

80

60

40

20

HB HB-B HB-BA

0 0

10

20

30

40

50

Reaction time (min) Fig. 4. Conversion of BA (A) and yield of DPM (B) over various catalysts in the benzylation of benzene with benzyl alcohol.

Table 3 Catalytic reaction data about various catalysts in the benzylation of benzene. Samples

Conversion of BA (%)a

Yield of DPM (%)a

Reaction rate constant, ka (  10 3 min 1)

HB HB-B HB-BA

58 37 52

49 31 45

87 46 73

a Reaction time = 10 min; reaction conditions: amount of catalyst = 0.1 g; benzene = 14 ml; benzyl alcohol = 0.2 ml; reaction temperature = 353 K.

this reaction are mainly diphenylmethane (DPM) and dibenzyl ether (DBE) (Scheme 1). The conversion of BA and yield of DPM with reaction time over various catalysts are shown in Fig. 4. Obviously, the conversion of BA was increased with reaction time and reached nearly 100% at a reaction time of 30 min over all the catalysts (Fig. 4A). Meantime, the yield of DPM was increased with reaction time until the formed DBE was completely converted (Fig. 4B). Considering that the product distribution in the liquidphase reaction system might be not homogeneous during the earlier reaction time (5 min), the catalytic activity of various catalysts at a relatively long reaction time (10 min) was selected for comparison. Furthermore, the apparent reaction rate constant was calculated to avoid the errors as far as possible from the catalytic data

of single site. As seen in Table 3, HB gave higher catalytic activity than HB-B. After a reaction time of 10 min, the conversion of BA and yield of DPM for HB reached 58% and 49%, respectively. Comparatively, HB-B showed the BA conversion of 37% and DPM yield of 31%. The reaction rate constant of HB is 1.9 times that of HB-B. This means that hierarchical Beta prepared by desilication had poorer catalytic activity than parent one in the benzylation of benzene. It needs be noted that HB-BA had better catalytic activity than HB-B. After a reaction time of 10 min, the conversion of BA and yield of DPM for HB-BA reached 52% and 45%, respectively. The reaction rate constant of HB-BA is 1.6 times that of HB-B. This result indicates that acid treatment may improve the catalytic performance of desilicated hierarchical Beta in this reaction. To clarify the above catalytic results, the factors influencing the catalytic performance in this reaction need to be considered. Because the benzylation of benzene with benzyl alcohol is an acid-catalyzed reaction, the acidity of catalysts should be an important factor. Based on the proposed reaction mechanism [42], it is undoubted that the number of Brönsted acid sites should play a key role in the reaction. In fact, as shown in Table 2, HB possessed more number of Brönsted acid sites characterized by pyridine adsorption than HB-B and HB-BA. The number of Brönsted acid sites in HB-B is the lowest. Apparently, the catalytic reactivity of the catalysts had a good relationship with the number of Brönsted acid sites. Additionally, the ability of mass transfer should be also an important factor influencing the catalytic performance. It has been demonstrated that the catalyst with larger external surface area and mesopore volume had better ability of mass transfer [33,35,36]. Obviously, HB-B and HB-BA possessed larger external surface area and mesoporous volume than HB but exhibited lower catalytic activity (Table 1). The catalytic reactivity seemed to have no good correlation with the ability of mass transfer. Based on these analyses, it can be concluded that the number of Brönsted acid sites played a more vital role than the ability of mass transfer in the reaction. The reason why the desilicated hierarchical Beta exhibited relatively low catalytic activity in the reaction may be mainly attributed to the great loss of acid sites during base treatment. Subsequent acid treatment is helpful for improving the catalytic performance of hierarchical Beta due to the partial recovery of acid sites. 3.2.2. Benzylation of mesitylene with benzyl alcohol The benzylation of mesitylene with a larger molecular size than benzene was carried out to evaluate their catalytic reactivity involving large molecules. The products in this reaction are mainly 2-benzyl-1,3,5-trimethylbenzene (BTB) and dibenzyl ether (DBE) (Scheme 2). The conversion of benzyl alcohol and yield of BTB with reaction time over various catalysts is shown in Fig. 5. As it can be seen, the conversion of BA was rapidly increased with reaction time and reached nearly 100% after a reaction time of 120 min over HB-BA and HB-B except for HB (Fig. 5A). Meantime, the yield of BTB was increased with reaction time until the formed DBE was completely converted (Fig. 5B). HB showed lower catalytic activity than HB-B. After a reaction time of 30 min, the conversion of BA and yield of BTB for HB reached 45% and 30%, respectively (Table 4). In comparison, HB-B gave the BA conversion of 77% and BTB yield of 50%. The reaction rate constant of HB-B is twice as much as that of HB. Further, HB-BA exhibited better catalytic activity than HB-B. After a reaction time of 30 min, the conversion of BA and yield of BTB for HB reached 92% and 73%, respectively. The reaction rate constant of HB-BA is 1.2 times that of HB-B and 2.5 times that of HB. To explain the above catalytic results, the molecular size of reactant and product has to be considered. In the benzylation of benzene, the molecular size of benzene and main product is around 0.5 nm, which is smaller than micropore size of zeolite Beta

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51

47

Scheme 2. Benzylation reaction pathway of mesitylene with benzyl alcohol.

100

A

Conversion (%)

80

60

40

20

HB HB-B HB-BA

0 0

30

60

90

120

Reaction time (min)

100

B

Yield of BTB (%)

80

60

40

(0.67 nm). However, in the benzylation of mesitylene, the molecular size of mesitylene and main product is around 0.8 nm [43], which is larger than micropore size of zeolite Beta. This means that the reactant cannot reach acid sites in micropores of zeolite Beta and the reaction has to be carried out on its external surface. Obviously, some acid sites characterized by pyridine (molecular size around 0.5 nm) adsorption cannot effectively be utilized in this reaction. Comparatively, the data of the accessible acid sites characterized by pivalonitrile adsorption should supply some correlated information. As seen in Table 2, HB-BA and HB-B possessed more Brönsted acid sites accessible to pivalonitrile than HB. The number of accessible Brönsted acid sites in HB-BA is the highest and that in HB is lowest. The catalytic reactivity is well correlated with the number of accessible Brönsted acid sites. In earlier reports, it is often displayed that the desilicated zeolites had better catalytic performance in the reactions involving whether small or large molecules than parent ones. However, such behavior that the desilicated zeolite showed poorer catalytic activity in the small molecular reactions and better catalytic performance in the large molecular reactions than parent one is rarely observed. This means that the desilicated Beta may be used in the benzylation of the reactant with large molecular size although the number of total acid sites characterized by pyridine adsorption was greatly decreased. Further, these results demonstrated again that subsequent acid treatment is helpful for improving the catalytic activity of desilicated Beta in the benzylation reaction. 3.3. Change of local structure in zeolite Beta and its effect on acidity: NMR study

20

HB HB-B HB-BA

0 0

30

60

90

120

Reaction time (min) Fig. 5. Conversion of BA (A) and yield of BTB (B) over various catalysts in the benzlyation of mesitylene with benzyl alcohol.

Table 4 Catalytic reaction data about various catalysts in the benzylation of mesitylene. Samples

Conversion of BA (%)a

Yield of BTB (%)a

Reaction rate constant, ka (  10 3 min 1)

HB HB-B HB-BA

45 77 92

30 50 73

38 77 96

a Reaction time = 30 min; reaction conditions: amount of catalyst = 0.1 g; mesitylene = 14 ml; benzyl alcohol = 0.2 ml; reaction temperature = 373 K.

To understand the change of local structure in zeolite Beta before and after post treatment, the samples HB, HB-B and HB-BA were firstly studied by 29Si MAS NMR and the recorded spectra with their corresponding best fit simulations are displayed in Fig. 6. All the samples showed six resonance peaks centered at 94 to 100 ppm (SiOH), 103 to 108 ppm (Si(3Si,1Al)) [44,45], and 111 to 116 ppm (Si(4Si)) [33,46], respectively. The data about the proportion of different Si types are listed in Table 5. Obviously, the base treatment removed part of Si(4Si) species in the framework because the proportion of Si(4Si) decreased from 71.8% to 63%. The proportion of SiOH was increased from 4% to 7% because the created mesopores extended into the microporous crystalline zeolite framework and would be terminated by silanol groups on the mesopore walls [47]. Further, it could be observed that the proportion of Si(3Si,1Al) was increased from 24.2% to 30% after the desilication. Generally, it was thought that the Si species such as Si(4Si) can be easily leached out during desilication but it is difficult to remove the Si species such as Si(3Si,1Al) because the negatively charged AlO4 tetrahedral structure prevented the hydrolysis of the Si–O–Al bond in the presence of OH [48]. Therefore, it is reasonable to think that the proportion of Si(3Si,1Al) will increase after

48

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51

Fig. 6. Experimental (blue) and simulated (red) 29Si NMR spectra of HB, HB-B and HB-BA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 5 Relative intensity of different Si types from

Fig. 7. Experimental (blue) and simulated (red) 27Al NMR spectra of HB, HB-B and HB-BA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 29

Si MAS NMR.

Si types

HB I (%)

HB-B I (%)

HB-BA I (%)

SiOH Si(3Si,1Al) Si(4Si)

4.0 24.2 71.8

7.0 30.0 63.0

7.1 26.1 66.8

the desilication process that removes some Si(4Si) and SiOH species. The Si/Al molar ratio from NMR calculation was decreased from 16.5 to 13.4 after alkaline treatment (Table 1), indicating that part of silicon species in zeolite Beta has been successfully removed. After acid treatment, the proportion of Si–OH (7.1%) almost had no change. The proportion of Si(3Si,1Al) and Si(4Si) was changed from 30% to 26.1% and from 63% to 66.8%, respectively. These results suggested that a small proportion of Al species should be removed during the acid treatment. As shown in Table 1, the Si/Al molar ratio after acid treatment was increased from 13.4 to 15.3. Si/Al molar ratios have also been calculated from XPS analysis (Table 1). The obtained values are in accordance with those determined by NMR, indicating that the surface species evolve as the bulk species with a decrease of Si surface species after base treatment and a decrease of aluminum surface species after the acid one. In this case, the Si/Al surface ratio could be different to the NMR Si/Al ratio. Secondly, the 27Al MAS NMR experiments were performed to detect the coordination state of aluminum species in HB, HB-B and HB-BA (Fig. 7). It can be seen that the recorded spectra exhibited a strong peak centered at around 54 ppm corresponding to tetrahedral coordination aluminum and a small peak centered at around 0 ppm corresponding to octahedral coordination aluminum, indicating that Al coordination in all the samples mainly consisted of tetrahedral coordination with a small amount of octahedral coordination. Differently, a trace of pentahedral coordination aluminum can be observed in HB-B and HB-BA.

To deeply uncover the type of aluminum coordination state in the samples, the experiment of two-dimensional 27Al MQ MAS NMR was done. Considering the resonance peaks of 27Al MAS NMR in HB-B and HB-BA are similar, here we just showed the results from HB and HB-B for comparison. Moreover, d2 projections of the spectra are shown to clearly indicate the position and shape of resonance peaks (Fig. 8). As seen, they displayed two resonance peaks with a low quadrupolar effect presented at nearly 58 ppm and 54 ppm and one resonance peak affected by a large quadrupolar interaction at nearly 60 ppm, and two resonance peaks at nearly 0 ppm. All resonance peaks between 54 ppm and 65 ppm could be regarded as tetrahedral coordination aluminum. The two resonance peaks affected by low quadrupolar interaction were assigned to aluminum atoms occupying different T-sites. It has been proposed that there are nine different T-sites in zeolite Beta based on the difference of the averaged Al–O–Si angles [49,50]. They are T1 at 155.3°, T2 at 155.9°, T3 at 148.0°, T4 at 148.2°, T5 at 151.8°, T6 at 152.1°, T7 at 152.8°, T8 at 151.4° and T9 at 149.7°, respectively. According to the theoretical calculation and experimental results, aluminum atoms on positions T1 and T2 in the framework of Beta would give a resonance peak at nearly 54 ppm (Tetra Al(2)) and aluminum atoms positioned in T3–T9 would show a resonance peak between 56 ppm and 58 ppm (Tetra Al(1)) [51,52]. The resonance peak with large quadrupolar effect visible in the isotropic dimension was from a locally distorted aluminum atom associated with a defective site in the Beta framework (labelled as DTetra Al), which may be regarded as a transition from the crystalline framework lattice state to the amorphous silica-alumina state [53]. In addition, two resonance peaks at nearly 0 ppm were located in the octahedral coordination region. The octahedral coordination aluminum close to 0 ppm (Octa Al(2)) showed a weak quadrupolar

49

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51

Fig. 8. Two dimensional

27

Al MAS NMR spectra and their respective projections of HB and HB-B correspond to the anisotropic d2 and isotropic d1 dimensions, respectively.

interaction and a small distribution in the isotropic chemical shifts. One explanation is that the octahedral coordination aluminum is from the aluminum connected to the framework via one or two oxygen atoms [51,52]. Another is that it is from the aluminum connected via 4 oxygen atoms to the framework and coordinated by one water molecule and one hydronium ion [54]. The resonance peak with a strong quadrupolar effect centered between 0 ppm and 5 ppm was considered as distorted octahedral coordination aluminum [52]. Based on the results from two-dimensional 27Al MAS NMR, the deconvolution of 1D 27Al MAS NMR was done to understand the change of Al sites in zeolite Beta (Fig. 7). The detailed data are collected in Table 6. As seen, the simulated resonance peaks were identical in shape and position shown in d2 projections. HB contained five types of Al sites: DTetra Al, Tetra Al(1), Tetra Al(2), Octa Al(1) and Octa Al(2). Their proportion was 35.7% for DTetra Al, 18.7% for Tetra Al(1), 23.2% for Tetra Al(2), 18% for Octa Al(1) and 4.4% for Octa Al(2), respectively. No pentahedral coordination aluminum (Pent Al) was observed. After desilication, a small amount of Pent Al in HB-B was formed with a proportion of 12.6%. Meanwhile, the proportion of

Table 6 NMR parameters and relative intensity from Al sites

DTetra Al Tetra Al(1) Tetra Al(2) Pent Al Octa Al(1) Octa Al(2)

HB

27

Al (MQ) MAS NMR.

HB-B

HB-BA

Res. (ppm)

I (%)

Res. (ppm)

I (%)

Res. (ppm)

I (%)

59.49 57.71 53.97 – 0.10 0.04

35.7 18.7 23.2 – 18.0 4.4

60.48 57.33 54.05 34.53 1.57 0.31

47.2 2.8 14.1 12.6 19.6 3.7

59.59 57.72 54.20 32.02 3.66 0.26

42.4 6.6 23.8 5.7 16.0 5.5

DTetra Al was increased from 35.7% to 47.2% and the proportion of Tetra Al(1) and Tetra Al(2) was decreased from 18.7% to 2.8% and from 23.2% to 14.1%, respectively. The total proportion of Octa Al was kept around 23%. This result indicates that the Si species connected with Al by oxygen have to be removed. Otherwise, the proportion of different Al sites should be kept constant. Considering that the proportion of Octa Al did not change significantly, Pent Al and DTetra Al with the increased proportion should mainly result from the transformation of partial Tetra Al(1) and Tetra Al(2). Additionally, it can be noted that the decreased proportion

50

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51

of Tetra Al(2) is less than that of Tetra Al(1) (Table 6), suggesting that the Si species of Si–O–Al in Tetra Al(2) should be more difficult to remove than those in Tetra Al(1). It is well known that the acidity of zeolites mainly depended on the tetrahedral coordination aluminum sites. The proportion of Al sites from Tetra Al in zeolites is higher, the number of Brönsted acid sites could be larger. Thus, the decreased acidity in HB-B should mainly be attributed to the greatly decreased proportion of Tetra Al from 42% to 17% after base treatment. By calculation, the concentration of Tetra Al was decreased from 403 lmol/g to 198 lmol/g. After acid treatment, the proportion of Pent Al and DTetra Al was decreased from 12.6% to 5.7% and from 47.2% to 42.4%, respectively. The proportion of Octa Al was changed from 23.23% to 21.53%. Notably, the proportion of Tetra Al was increased from 17% to 30.4%. By calculation, the concentration of Tetra Al was increased from 198 lmol/g to 313 lmol/g. This suggested that HB-BA could possess more Brönsted acid sites than HB-B. These results may well explain the reason why the Brönsted acid sites can be greatly recovered after acid treatment. Considering that the amount of aluminum species in zeolite Beta is low, it is difficult that the concentration of Tetra Al was greatly enhanced just by the removal of partial Al species on the surface of catalyst. Possibly, such change occurred in the presence of dissolution of the partial Pent Al or/and DTetra Al and realumination under mild acid condition because the aluminum species in acid solution may be easily reinserted into the framework of zeolite Beta [55,56]. Certainly, it is also possible that the number of acid sites was decreased because partial pores might be blocked after base treatment. After subsequent acid treatment, the pores might have been cleared, leading to high accessibility to acid sites in and superior catalytic performance of HB-BA. The further study to clarify the process is underway.

4. Conclusions Hierarchical Beta was firstly prepared by base treatment based on a commercial Beta with a low Si/Al molar ratio of 12.5. After desilication, the number of total acid sites in zeolite Beta was greatly decreased, which should be mainly attributed to the transformation of partial tetrahedral coordination Al into the distorted tetrahedral and pentahedral coordination Al. Subsequent acid treatment may greatly increase the proportion of tetrahedral coordination Al while decreasing the proportion of the distorted tetrahedral and pentahedral coordination Al in zeolite Beta. As a result, after acid treatment the number of acid sites in zeolite Beta was greatly recovered. Catalytic results in the benzylation of benzene with benzyl alcohol indicated that the catalytic reactivity of the catalysts had a good relationship with the number of Brönsted acid sites. Further, in the benzylation of mesitylene with relatively large molecular size, hierarchical Beta exhibited much better catalytic performance than parent one, which should be attributed to the existence of more accessible Brönsted acid sites due to the introduction of additional mesopores and macropores. Based on this reason, HB-BA gave highest catalytic activity. The more accessible Brönsted acid sites in HB-BA than in HB-B may be attributed to the increased tetrahedral coordinated Al sites. These results revealed clearly that hierarchical Beta obtained by post-treatment method can find applications in Friedel–Crafts alkylations although the number of total acid sites is decreased, especially toward the reactions involving large molecules. Additionally, acid treatment is helpful for improving the catalytic performance of the desilicated hierarchical Beta in some acid-catalytic reactions.

Acknowledgments The authors acknowledge the financial support from the Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2015046), the Scientific Research Starting Funding from Harbin Institute of Technology, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the Program Caiyuanpei (project N°30323NF) between China (CSC)–France (Campus-France). The TEM facility in Lille (France) is supported by the Conseil Regional du Nord-Pas de Calais, and the European Regional Development Fund (ERDF).

References [1] G.A. Olah, Friedel–Crafts and Related Reactions, Wiley-Interscience, New York, 1963. [2] G.A. Olah, Friedel–Crafts Chemistry, Wiley-Interscience, New York, 1973. [3] R. Commandeur, N. Berger, P. Jay, J. Kervenal, European Patent 0422 986 1991. [4] I. Iovel, K. Mertins, J. Kischel, A. Zapf, M. Beller, Angew. Chem. Int. Ed. 44 (2005) 1917–3913. [5] J.J. Chiu, D.J. Pine, S.T. Bishop, B.F. Chmelka, J. Catal. 221 (2004) 400–412. [6] K. Bachari, J.M.M. Millet, B. Benaïchouba, O. Cherifi, F. Figueras, J. Catal. 221 (2004) 55–61. [7] N.B. Shrigadi, A.B. Shinde, S.D. Samant, Appl. Catal. A 252 (2003) 23–35. [8] S.K. Jana, Catal. Surv. Asia 9 (2005) 25–34. [9] V.R. Choudhary, S.K. Jana, B.P. Kiran, Catal. Lett. 59 (1999) 217–219. [10] G. Winé, J. Matta, J.-P. Tessonnier, C. Pham-Huu, M.-J. Ledoux, Chem. Commun. (2003) 530–531. [11] U. Sridevi, V.V. Bokade, C.V.V. Satyanarayana, B.S. Rao, N.C. Pradhan, B.K.B. Rao, J. Mol. Catal. A 181 (2002) 257–262. [12] G. Bellussi, G. Pazzuconi, C. Perego, G. Girotti, G. Terzoni, J. Catal. 157 (1995) 227–234. [13] N. Candu, M. Florea, S.M. Coman, V.I. Parvulescu, Appl. Catal. A 393 (2011) 206–214. [14] V.D. Chaube, Catal. Commun. 5 (2004) 321–326. [15] B. Coq, V. Gourves, F. Figueras, Appl. Catal. A 100 (1993) 69–75. [16] S. Jun, R. Ryoo, J. Catal. 195 (2000) 237–243. [17] H. Yang, Z. Liu, H. Gao, Z. Xie, Appl. Catal. A 379 (2010) 166–171. [18] M. Guidotti, C. Canaff, J.-M. Coustard, P. Magnoux, M. Guisnet, J. Catal. 230 (2005) 375–383. [19] W. Kim, J.-C. Kim, J. Kim, Y. Seo, R. Ryoo, ACS Catal. 3 (2013) 192–195. [20] J.-C. Kim, K. Cho, R. Ryoo, Appl. Catal. A 470 (2014) 420–426. [21] J.C. Groen, L.A.A. Peffer, J.A. Moulijn, J. Perez-Ramirez, Micropor. Mesopor. Mater. 69 (2004) 29–34. [22] J. Perez-Ramirez, S. Abello, A. Bonilla, J.C. Groen, Adv. Funct. Mater. 19 (2009) 164–172. [23] M.S. Holm, M.K. Hansen, C.H. Christensen, Eur. J. Inorg. Chem. (2009) 1194– 1198. [24] D. Verboekend, G. Vile, J. Perez-Ramirez, Cryst. Growth Des. 12 (2012) 3123– 3132. [25] Y. Wu, F. Tian, J. Liu, D. Song, C. Jia, Y. Chen, Micropor. Mesopor. Mater. 162 (2012) 168–174. [26] F. Tian, Y. Wu, Q. Shen, X. Li, Y. Chen, C. Meng, Micropor. Mesopor. Mater. 173 (2013) 129–138. [27] S.K. Saxena, N. Viswanadham, T.J. Sharma, Mater. Chem. A 2 (2014) 2487– 2490. [28] K. Tarach, K. Gora-Marek, J. Tekla, K. Bryewska, J. Datka, K. Mlekodaj, W. Makowski, M.C. Igualada, J. Catal. 312 (2014) 46–57. [29] X. Li, R. Prins, J.A. van Bokhoven, J. Catal. 262 (2009) 257–265. [30] Y. Sun, R. Prins, Appl. Catal. A 336 (2008) 11–16. [31] H. Jin, M.B. Bismillah, E.-Y. Jeong, S.-E. Park, J. Catal. 291 (2012) 55–62. [32] K. Na, C. Jo, J. Kim, K. Cho, J. Jung, Y. Seo, R.J. Messinger, B.F. Chmelka, R. Ryoo, Science 333 (2011) 328–332. [33] K. Leng, Y. Wang, C. Hou, C. Lancelot, C. Lamonier, A. Rives, Y. Sun, J. Catal. 306 (2013) 100–108. [34] J.C. Groen, S. Abello, L.A. Villaescusa, J. Perez-Ramirez, Micropor. Mesopor. Mater. 114 (2008) 93–102. [35] A.N.C. van Laak, S.L. Sagala, J. Zecevic, H. Friedrich, P.E. de Jongh, K.P. de Jong, J. Catal. 276 (2010) 170–180. [36] D. Verboekend, S. Mitchell, M. Milina, J.C. Groen, J. Perez-Ramirez, J. Phys. Chem. C 115 (2011) 14193–14203. [37] D. Verboekend, G. Vile, J. Perez-Ramirez, Adv. Funct. Mater. 22 (2012) 916– 928. [38] L. Frydman, J. Hardwood, J. Am. Chem. Soc. 117 (1995) 5367–5368. [39] J.-P. Amoureux, C. Fernandez, S. Steuernagel, J. Magn. Reson. 123 (1996) 116– 118. [40] M. Tamura, K.-I. Shimizu, A. Satsuma, Appl. Catal. A 433–434 (2012) 135–145. [41] K. Sadowska, K. Gora-Marek, J. Datka, J. Phys. Chem. C 117 (2013) 9237–9244. [42] N. Narender, K.V.V. Krishna, Catal. Commun. 7 (2006) 583–588. [43] S. Nakashima, Y. Takahashi, M. Kiguchi, Beilstein J. Nanotechnol. 2 (2011) 755– 759.

Y. Wang et al. / Microporous and Mesoporous Materials 206 (2015) 42–51 [44] M. Mcmillan, J.S. Brinen, J.D. Carruthes, G.L. Haller, Colloids Surf. 38 (1989) 133–148. [45] J.M. Thomas, J. Klinowski, Adv. Catal. 33 (1985) 199–374. [46] P. Sazama, Z. Sobalik, J. Dedecek, I. Jakubec, V. Parvulescu, Z. Bastl, J. Rathousky, H. Jirglova, Angew. Chem. Int. Ed. 52 (2013) 2038–2041. [47] K.T. Hojholt, P.N.R. Vennestrom, R. Tiruvalam, P. Beato, Chem. Commun. 47 (2011) 12864–12866. [48] A. Bonila, D. Baudouin, J. Perez-Ramirez, J. Catal. 265 (2009) 170–180. [49] C.A. Fyfe, H. Strobl, G.T. Kokotailo, C.T. Pastzto, G.E. Barlow, S. Bradley, Zeolites 8 (1988) 132–136. [50] J. Stelzer, M. Paulus, M. Hunger, J. Weitkamp, Micropor. Mesopor. Mater. 22 (1998) 1–8.

51

[51] J.A. van Bokhoven, D.C. Koningsberger, P. Kunkeler, H. van Bekkum, A.P.M. Kentgens, J. Am. Chem. Soc. 122 (2000) 12842–12847. [52] A. Omega, M. Vasic, J.A. van Bokhoven, G. Pirngruber, R. Prins, Phys. Chem. Chem. Phys. 6 (2004) 447–452. [53] T.-H. Chen, K. Houthoofd, P.J. Grobet, Micropor. Mesopor. Mater. 86 (2005) 31– 37. [54] L.C. de Menorval, W. Buckermann, F. Figueras, F. Fajula, J. Phys. Chem. 100 (1996) 465–467. [55] Y. Oumi, R. Mizuno, K. Azuma, S. Nawata, T. Fukashima, T. Uozumi, T. Sano, Micropor. Mesopor. Mater. 49 (2001) 103–109. [56] Y. Oumi, S. Nemoto, S. Nawata, T. Fukushima, T. Teranishi, T. Sano, Mater. Chem. Phys. 78 (2003) 551–557.