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Controllable synthesis of aluminosilica monoliths with hierarchical pore structure and their catalytic performance
Microporous and Mesoporous Materials 127 (2010) 213–218
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Controllable synthesis of aluminosilica monoliths with hierarchical pore structure and their catalytic performance Heqin Yang a, Qian Liu b, Zhicheng Liu b, Huanxin Gao b, Zaiku Xie a,b,* a b
Research Institute of Industrial Catalysis, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Shanghai Research Institute of Petrochemical Technology, SINOPEC, 1658 Pudong Beilu, Shanghai 201208, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2009 Received in revised form 4 June 2009 Accepted 18 July 2009 Available online 23 July 2009 Keywords: Aluminosilica Monolith Hierarchical Acidity 1,3,5-Triisopropylbenzene
a b s t r a c t Aluminosilica monoliths with bicontinuous macropores and interconnected mesopores were successfully prepared by a double-template technique. The influence on monolithic pore structures and acidic properties based on aluminum variety was studied in detail, which demonstrated that aluminum addition and water amount in starting compositions had great effects on the formation of hierarchical pore system with bicontinuous macropores and interconnected mesopores. In addition to bicontinuous macropores and plenty of mesopores, the obtained aluminosilica monoliths also possessed considerable acid sites for catalysis. And such aluminosilica monolith with proper acidity and good diffusion showed best performance on catalytic reaction of large molecules such as the cracking 1,3,5-triisopropylbenzene compared with other aluminosilica monolith. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Recently, many efforts have been made to prepare bimodal pore silica monoliths based on polymerization-induced phase separation process [1–4]. The monoliths exhibit bicontinuous macropores and interconnected mesopores distribution in the skeleton walls [5–7]. Such distinct hierarchical pore structure will favor mass transfer and reduce transport limitation, which makes the silica monolith attractive in solid-phase microextraction, separation column, encapsulation of biomolecules and etc. [8]. In terms of catalytic reactions, however, silica monolith has poor chemical activity and could only be used as catalytic supports. It is well known that the incorporation of aluminum into porous silica framework can form active acidic sites, which will greatly increase their catalytic applications in petrochemical industry [9]. Nevertheless, the addition of aluminum salt as electrolyte will change the timing of the onset of phase separation relative to the sol–gel transition, which directly affects the formation of bicontinuous marcropores [10]. Therefore, though several reports have been centered on the preparation of aluminosilica monolith, it still remains a challenge to adjust framework Si/Al molar ratio and bimodal pore size distribution [10–12]. * Corresponding author. Address: Shanghai Research Institute of Petrochemical Technology, SINOPEC, 1658 Pudong Beilu, Shanghai 201208, PR China. Tel.: +86 21 68461279; fax: +86 21 68462283. E-mail addresses: [email protected], [email protected] (Z. Xie). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.07.016
Takahashi et al. first reported the synthesis of macroporous aluminosilica monolith using water-soluble organic polymers as templates [11]. They found that the macropore size could be adjusted by altering the starting compositions. However, the control of nanometer mesopore size and the effect of aluminum addition on macroporous morphology were not discussed. Till now, the mesopores of the silica monoliths have mostly been formed by base treatment, which would lead to wide pore size distribution and small pore volume [3,13]. Alternately, Chmelka et al. reported a double-templating route using emulsion droplets and triblock copolymers to control the bimodal pore size distribution independently [12]. Yet the obtained droplet-like macropore structure may be not good for mass transportation compared with bicontinuous macropore structure. In our previous work, we ever prepared meso/macroporous silica monoliths using an effective double-templating approach [14]. Water-soluble organic polymers were used to induce spinodal decomposition during the gelation process and finally form bicontinuous macropores; and triblock copolymers were applied as supermolecular templates to direct the formation of ordered hexagonal mesostructure. Here in this work, we researched the preparation of aluminosilica monolith with bicontinuous macropores using the double-templating route. The effects of aluminum addition on pore structures and acidic properties were discussed and its catalytic performance on cracking 1,3,5-triisopropylbenzene (1,3,5-TIPB) was also investigated.
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2. Experimental section 2.1. Synthesis procedures Samples were prepared by mixing 4.0 g of 1 mol/L nitric acid, 7.0 g of water, 0.7 g of poly ethylene glycol (PEG, average molecular weight of 10,000), 1.0 g of triblock copolymer (Pluronic P123), 5.1 g of tetramethoxysilane (TMOS) and appropriate amount of Al(NO3)39H2O to give Si/Al molar ratios of 100–10. After the mixture was stirred for 20 min in an ice-bath and became homogeneous, it was sealed in a plastic container, and kept at 333 K for 24 h for gelation. The resultant wet gel was dried at room temperature and then heated at 873 K for 8 h to get the monolith. 2.2. Characterization Nitrogen adsorption–desorption isotherms were measured on a Micromeritics ASAP 2010 apparatus. Before measurement, the samples were first degassed at 383 K for 3 h and then at 623 K for 5 h under vacuum. The specific surface areas of the samples were calculated with the BET equation. The pore volumes were determined at a P/P0 value of 0.995. The mean mesopore diameters were calculated with the BJH equation using the desorption isotherms. The mean macropore diameters were measured by mercury porosimetry (Micromeritics Autopore 9500). Scanning electron microscope (SEM) experiments were performed with a Philips XL30E electron microscope. The samples were vapor-deposited with gold before analysis. Transmission electron microscope (TEM) study was carried out on a Tecnai 20 S-TWIN instrument operating at 200 kV and using CeB6 filament as an electron beam. Sample for TEM analysis was prepared by dipping a carbon-coated copper grid into a suspension of samples in ethanol that was pre-sonicated. Fourier transform infrared spectra (FTIR) were obtained with a Nicolet380 spectrometer. The spectra were collected after 32 scans for the wavenumber range of 1400–1600 cm 1, with a resolution of 4 cm 1. NH3 temperature-programmed desorption (NH3-TPD) profiles were measured on an apparatus equipped with a thermal conductivity detector (TCD). About 100 mg of sample were activated in flowing He at 873 K for 1 h, then cooled to room temperature and equilibrated with NH3 gas for 30 min. A fraction of adsorbed NH3 was removed by He flow for about 1 h at 373 K. Then the reactor temperature was ramped up from 373 to 873 K at a rate of 10 K min 1, meanwhile, TCD curve for NH3 desorption was recorded, with a He flow of 20 cm3 min 1. The temperature was kept constant at 873 K until the TCD curve returned to the baseline. Solid-state 27Al and 29Si NMR experiments were carried out on a Bruker DMX Avance 500 spectrometer. The 29Si NMR spectra were recorded at 59.62 MHz with a pulse length of 5 ls and a spinning rate of 4 kHz, while the 27Al NMR spectra were recorded at 78.20 MHz with a pulse length of 0.31 ls and a spinning rate of 12 kHz. Si/Al* molar ratios were determined by inductively coupled plasma (ICP) elemental analyses from Optial Emission Spectrometer, Varian 725-ES.
weight hourly space velocity (WHSV) of 2 h 1. The molar ratio of hydrogen to 1,3,5-TIPB was 5: 1. Reaction products were collected and analyzed with Agilent 6820 gas chromatograph. Three different aluminosilica monoliths were examined in this study, namely the bicontinuous macroporous sample with Si/ Al = 10 (denoted as BM10), the bicontinuous macroporous sample with Si/Al = 100 (denoted as BM100) and the dense gelatinous mesoporous sample with Si/Al = 10 (denoted as DG10). 3. Results and discussion 3.1. Effect of aluminum addition on macroporous structure Silica monoliths with bicontinuous macroporous structure were prepared by introducing phase separation into the sol–gel process [2,14], during which alkoxysilanes hydrolyzed and polycondensed within a double-copolymers solution (scheme illustrated in Fig. 1). For this process, the timing of the onset of phase separation relative to gelation is a key factor to determine the final pore structures. If the phase separation takes place much faster than the gelation, particle aggregate morphology tends to occur; on the contrary, if gelation is much faster than phase separation, the dense gelatinous mesoporous morphology will be formed. The aimed bicontinuous macroporous structure can be obtained only if the phase separation and the sol–gel transition are triggered almost simultaneously. Fig. 2 shows SEM images of samples prepared with different aluminum additions under the condition of H2O/SiO2 = 3.5. It is shown that the macropore diameter of sample increases from 1.4 to 7.3 lm as Si/Al molar ratio of samples decreases from 100 to 50. However, when Si/Al molar ratio of samples reach 10, the morphology of sample becomes spherical particle aggregates and no bicontinuous macropore could be observed. Such a phenomenon might be caused by salting out effect of aluminum precursor [15–18]. As we know, aluminum nitrate has higher solvency in water than PEG. So the interaction of PEG and water is destroyed as the increase of aluminum addition, resulting in PEG molecules breaking away from solvent. In addition, part of water forms hydrated ions with aluminum, which will lead to the solubility of PEG lowered. Thereby the addition of aluminum reduces the effective concentration of PEG in water, which accelerates the phase separation. Moreover, the more aluminum addition is, the faster the timing of the onset of phase separation is. When aluminum addition makes the timing of the onset of phase separation change slightly, macropore diameter increases with aluminum addition (as can be seen in Fig. 2 from Si/Al = 100 to 50). Once the aluminum amount is increased high enough, the phase separation will occur much faster than the sol–gel transition, then the macropore morphology would turn from bicontinuous domains to spherical particle aggregates (as can be seen in Fig. 2 from Si/Al = 50 to 10).
2.3. Cracking reaction The 1,3,5-TIPB cracking reaction was performed in a continuous fixed-bed micro-reactor with H2 as the carry gas and reduction atmosphere. The catalyst sample was crushed and sieved to 20– 40 mesh. In a typical experiment, prior to the reaction, 1 g sieved catalyst was activated for a few hours at 773 K in air flow. The reaction was carried out at 573 K. The feed was pure 1,3,5-TIPB with
Fig. 1. Schematic representation of phase separation process during sol–gel transition.
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Fig. 2. SEM images of samples prepared with different aluminum amount under the condition of H2O/SiO2 = 3.5.
With the findings discussed above, it is clear that the increase of aluminum amount will destroy the formation of bicontinuous macroporous structure without changing the other synthesis conditions. In order to eliminate the effects of aluminum addition on macroporous structure and prepare bicontinuous aluminosilica monoliths with adjustable Si/Al molar ratios, we found that changing water amount in starting composition is the most convenient and effective way. Fig. 3 displays the effect of H2O/SiO2 and Si/Al molar ratios on the morphologies of resulted aluminosilica samples. It can be seen that the forming of bicontinuous macroporous structure needs water amount reducing with the increase of aluminum addition. There may be several reasons leading to above changes. Water amount decrease will lead to an increase of methanol concentration brought forth by hydrolysis of TMOS, which would delay the triggering of phase separation by hindering PEG to be adsorbed on the surface of silicon species. On the other hand, lower amount of water also enhances the concentration of acid and increases the viscosity, which can accelerate the polymerization reaction and
shorten the timing of onset of the sol–gel transition to make it match with that of phase separation. Hence changing water amount in starting compositions can eliminate the effects of aluminum addition on macroporous structure and obtain the bicontinuous macroporous monoliths with adjustable Si/Al molar ratio. 3.2. Effect of aluminum addition on mesoporous structure Fig. 4 shows N2 adsorption–desorption isotherms and Barrett– Joyner–Halenda (BJH) pore size distribution curves for samples with hierarchical pores and different Si/Al molar ratios. All samples show similar type IV isotherms with hysteresis loops at P/P0 = 0.4– 0.8, indicating the existence of mesoporous structures [19–20]. The textural properties of the samples are summarized in Table 1. With the increase of aluminum amount, the specific surface area reduces from 750 m2 g 1 to 625 m2 g 1, while the mesopore volume and mean pore size slightly increases. Fig. 5 displays TEM image of sample with Si/Al = 10. It can be seen that the mesoporous structure of the samples transits from the hexagonally ordered mesopores to worm-like orderless mesopores after incorporation of aluminum into silica monolith [14]. Similar images can also be observed for aluminosilica samples with Si/Al = 100 and 50 (not shown). These may be caused by the substituting of aluminum for silicon due to different size of aluminum and silicon atoms. 3.3. Effect of aluminum addition on acidic properties
Fig. 3. Effect of H2O/SiO2 and Si/Al molar ratios on the morphologies of resulted aluminosilica samples.
Fig. 6 shows the NH3-TPD profiles of samples with hierarchical pores and Si/Al = 10, 50 and 100, respectively. All samples show two NH3 desorption peaks centered at low and high temperature, which should be attributed to weak and medium strong acid sites respectively. It is seen that with the increase of aluminum addition, the peak area for both acid sites are increased simultaneously, which indicates the amount of acid sites also increase. Besides, the peak center for medium strong acid sites shifts dramatically to lower temperature with the increase of aluminum addition, which infers that acid strength becomes weaker as the aluminum content enhances. However, the peak center at low temperature attributed to weak acid sites change little. These changes of amount and strength of acid sites may have great effect on the
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Fig. 5. TEM image of samples with Si/Al = 10.
Fig. 4. N2 physisorption isotherms and BJH pore size distributions for samples with Si/Al = 100 (square), Si/Al = 50 (cycle) and Si/Al = 10 (triangle).
catalytic properties for some reactions, which we will discuss in later part. To investigate the reasons of the change of acid type and acid amount, three samples with different aluminum addition are characterized by 29Si MAS NMR and 27Al MAS NMR. Fig. 7 shows the 29Si MAS NMR spectra of samples with Si/ Al = 10, 50 and 100 after calcination. All samples have three peaks centered at 107, 101 and 92 ppm, which could be assigned to Si(0Al) (Si Q4), Si(1Al) (Si Q3) and Si(2Al) (Si Q2), respectively [21]. As can be seen from Fig. 7, with the aluminum addition, the Si atoms in Q3 environments and in Q4 environments decrease, whereas those in Q2 environments increase. Apparently, it may be caused by the introducing of Al atoms into skeleton. Fig. 8 gives corresponding 27Al MAS NMR spectra of the above three samples. Every spectrum is composed of a sharp resonance at 52 ppm which is indexed to structural, tetrahedral aluminum
and another small resonance at 0 ppm which is due to nonstructural, octahedral aluminum [22]. By considering the relative intensities of the 27Al MAS NMR signals arising from the octahedrally and tetrahedrally coordinated Al species, it can be concluded a majority of aluminum atoms are incorporated into the monolith framework for all samples and the framework aluminum increases with the increase of aluminum amount. It is well known that framework aluminum and its distribution have important influence on the amount and strength of acid sites. Since the acidity of framework Si(2Al) is weaker than that of the framework Si(1Al) site [23], the increase of framework Si(2Al) will lead to acid strength of medium strong acid falling as the aluminum addition. On the other hand, the increase of framework aluminum makes an increase of acid amount, which is consistent with NH3-TPD results (Fig. 6). The FT-IR spectra of pyridine absorbed on samples with different Si/Al molar ratios is shown in Fig. 9. There are three sharp bands due to C–C stretching vibrations of pyridine. The strong band at 1491 cm 1 is due to the pyridine adsorbed on both Brönsted and Lewis acid sites, while bands at 1545 and 1456 cm 1 are due to protonation of the pyridine molecule by Brönsted acid sites and pyridine adsorbed on Lewis acid sites, respectively. The Brönsted acid sites, Lewis acid sites, and B/L ratios of samples with different Si/Al molar ratios are shown in Table 2. It can be seen that all samples possess the Brönsted acid sites and Lewis acid sites which increase with the increase of aluminum addition. Combining with the results of ICP (Table 1) and 27Al MAS NMR, it can be confirmed that the more aluminum is added in starting compositions the
Table 1 Textural properties of the samples with different Si/Al mol ratios. Si/Al
Si/Al*
H2O/SiO2
SBET (m2/g)
VMesop (cm3/g)
DMesopa (nm)
DMesopb (nm)
DMacrop (lm)
100 50 10
144 86 16
3.5 3.5 2.5
750 704 625
0.48 0.51 0.52
3.3 3.8 4.3
3.2 3.7 4.1
1.4 7.3 2.7
*
Determined by inductively coupled plasma (ICP). Calculated with the BJH equation using the absorption isotherms. b Calculated with the BJH equation using the desorption isotherms. a
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Fig. 6. NH3-TPD images of samples prepared with different Si/Al molar ratio.
Fig. 9. IR spectra of pyridine absorbed on samples prepared with different Si/Al molar ratio.
Table 2 Comparison of acid sites and B/L ratios determined by pyridine-IR spectra for the samples with different Si/Al mol ratios. Sample
a
B acid sites L acid sitesa B/Lb
Si/Al = 100
Si/Al = 50
Si/Al = 10
573 k
573 k
473 k
573 k
673 k
0.02 0.02 1.50
0.04 0.04 1.50
0.10 0.14 1.1
0.07 0.11 0.95
0.03 0.11 0.41
a Calculated by the ratio of the integrated area of IR adsorption peak to sample weight. b Calculated by the equation of 1.5 (AB/AL), where AB/AL is the absorbance ratio [24].
Fig. 7.
29
Si MAS NMR spectra of samples with different Si/Al molar ratio.
acid band is also shown in Table 2. It is obvious that the relative intensity of the Brönsted acid band gradually decreased with the increase of desorption temperature, and the intensity of the Lewis acid band also decreased with the increase of desorption temperature from 473 to 573 K, but when the temperature was higher than 573 K the Lewis band intensity had hardly any change and corresponding B/L ratios of the sample with Si/Al = 10 decreased. This indicates that the strength of Brönsted acid is widely distributed (from weak to strong), and that of Lewis acid is narrowly distributed [25]. 3.4. Catalytic performance on craking 1,3,5-TIPB
Fig. 8.
27
Al MAS NMR spectra of samples with different Si/Al molar ratio.
more aluminum atoms enter into frameworks to form abundant acid sites. For the sample with Si/Al = 10, the effect of desorption temperature on the relative intensity of the pyridine Brönsted and Lewis
Cracking 1,3,5-TIPB is an effective probe reaction to test diffusion property of materials [26]. Three aluminosilica monoliths with different macroporous structures and aluminum amounts were used as catalysts for cracking 1,3,5-TIPB. As shown in Fig. 10, BM10 demonstrates the highest 1,3,5-TIPB conversion and best stability compared with the others. As has been discussed, the acidity and mass transport property of a catalyst are decisive of the acid catalytic performance. While the former is closely related to the distribution and concentration of the framework Al species, the latter is generally manipulated by the aperture and pore structure [27]. Compared with BM10, BM100 possesses similar bicontinuous macropores. But as it has stronger medium strong acid sites (see discussion in Section 3.3), and it is more active and easily results in an intense deactivation due to strong interaction with by-product. Hence BM100 shows faster deactivation. For DG10, it has similar initial conversion of 1,3,5-TIPB compared with BM10. However, the conversion reduces fast from 63% to 46% after 5 h.
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4. Conclusions Aluminosilica monoliths with adjustable Si/Al molar ratio as well as bicontinuous macropores were prepared using a doubletemplating approach. Aluminum addition has an important effect on pore structures and acidic properties through accelerating the triggering of phase separation. Changing water amount in starting compositions can adjust the timing of onset of the sol–gel transition and make it match with that of phase separation to obtain the bicontinuous macroporous aluminosilica monolith with adjustable Si/Al molar ratio. Furthermore, the catalytic conversion of 1,3,5-TIPB on bicontinuous macroporous aluminosilica monoliths with Si/Al = 10 was higher due to its proper acidity and open channel than that of the samples with different Al contents or with dense gelatinous mesopore structure. Besides, acidity of monolith plays a key role on the product distribution and selectivity. Acknowledgment Fig. 10. 1,3,5-TIPB conversion over different aluminosilica samples.
We are grateful to the Major State Basic Research Development Program of People’s Republic of China (2009CB23506) for financial support. And we also appreciate the help of Dr. Yang xueping on paper writing. Great thanks also to professor He heyong (Fudan university) for solid state NMR analysis.
Table 3 Selectivity to products in TIPB cracking reaction over various samples. Sample
BM10 DG10 BM100
Selectivity (%)
References
Benzene
IPB
m-DIPB
p-DIPB
21.4 22.1 22.5
12.5 12.2 5.8
51.7 51.4 58.2
10.9 10.5 8.6
Since BM10 and DG10 have same aluminum amount, diffusion limitation might play a key role in the reaction process. Thus it implies that an aluminosilica monolith combined with bicontinuous macroporous structure and proper acid sites may lead to substantial performance improvement in catalytic reactions involving large molecules. The main products of 1,3,5-TIPB cracking over these catalysts are benzene, isopropylbenzene (IPB), m-diisopropylbenzene (m-DIPB) and p-diisopropylbenzene (p-DIPB). Product selectivity is given in Table 3. It can be noticed that BM100 differs with BM10 and DG10 in selectivity to products except for benzene. BM10 and DG10 catalysts have shown much higher selectivity to IPB and p-DIPB compared to BM100, whereas the selectivity to m-DIPB is lower. However, BM10 and DG10 show similar selectivity to all products. This is can be explained that acidity plays an important effect on the selectivity and differences of pore structure have hardly any influence on it for these samples. 1,3,5-TIPB craking may take place through a four-step mechanism, namely (a) dealkylation of TIPB to 1,3-DIPB, (b) isomerization of 1,3-DIPB to 1,4-DIPB, (c) dealkylation of DIPBs to IPB, and (d) dealkylation of IPB to benzene [28]. Acid sites strength required for dealkylation may increase in the order of TIPB < DIPBs < IPB [29]. Besides, IPB cracking is catalyzed by strong Brønsted acid sites [30], while only weak acid sites are required for dealkylation of TIPB [31]. Compared with BM100, BM10 and DG10 possess more and weaker medium strong acid sites which facilitate isomerization and dealkylation of DIPBs to produce p-DIPB and IPB. Moreover, p-DIPB is more stable than m-DIPB. Thereby the selectivity to IPB and p-DIPB are high and selectivity to m-DIPB is low on BM10 and DG10.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Controllable synthesis of aluminosilica monoliths with hierarchical pore structure and their catalytic performance Heqin Yang a, Qian Liu b, Zhicheng Liu b, Huanxin Gao b, Zaiku Xie a,b,* a b
Research Institute of Industrial Catalysis, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Shanghai Research Institute of Petrochemical Technology, SINOPEC, 1658 Pudong Beilu, Shanghai 201208, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2009 Received in revised form 4 June 2009 Accepted 18 July 2009 Available online 23 July 2009 Keywords: Aluminosilica Monolith Hierarchical Acidity 1,3,5-Triisopropylbenzene
a b s t r a c t Aluminosilica monoliths with bicontinuous macropores and interconnected mesopores were successfully prepared by a double-template technique. The influence on monolithic pore structures and acidic properties based on aluminum variety was studied in detail, which demonstrated that aluminum addition and water amount in starting compositions had great effects on the formation of hierarchical pore system with bicontinuous macropores and interconnected mesopores. In addition to bicontinuous macropores and plenty of mesopores, the obtained aluminosilica monoliths also possessed considerable acid sites for catalysis. And such aluminosilica monolith with proper acidity and good diffusion showed best performance on catalytic reaction of large molecules such as the cracking 1,3,5-triisopropylbenzene compared with other aluminosilica monolith. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Recently, many efforts have been made to prepare bimodal pore silica monoliths based on polymerization-induced phase separation process [1–4]. The monoliths exhibit bicontinuous macropores and interconnected mesopores distribution in the skeleton walls [5–7]. Such distinct hierarchical pore structure will favor mass transfer and reduce transport limitation, which makes the silica monolith attractive in solid-phase microextraction, separation column, encapsulation of biomolecules and etc. [8]. In terms of catalytic reactions, however, silica monolith has poor chemical activity and could only be used as catalytic supports. It is well known that the incorporation of aluminum into porous silica framework can form active acidic sites, which will greatly increase their catalytic applications in petrochemical industry [9]. Nevertheless, the addition of aluminum salt as electrolyte will change the timing of the onset of phase separation relative to the sol–gel transition, which directly affects the formation of bicontinuous marcropores [10]. Therefore, though several reports have been centered on the preparation of aluminosilica monolith, it still remains a challenge to adjust framework Si/Al molar ratio and bimodal pore size distribution [10–12]. * Corresponding author. Address: Shanghai Research Institute of Petrochemical Technology, SINOPEC, 1658 Pudong Beilu, Shanghai 201208, PR China. Tel.: +86 21 68461279; fax: +86 21 68462283. E-mail addresses: [email protected], [email protected] (Z. Xie). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.07.016
Takahashi et al. first reported the synthesis of macroporous aluminosilica monolith using water-soluble organic polymers as templates [11]. They found that the macropore size could be adjusted by altering the starting compositions. However, the control of nanometer mesopore size and the effect of aluminum addition on macroporous morphology were not discussed. Till now, the mesopores of the silica monoliths have mostly been formed by base treatment, which would lead to wide pore size distribution and small pore volume [3,13]. Alternately, Chmelka et al. reported a double-templating route using emulsion droplets and triblock copolymers to control the bimodal pore size distribution independently [12]. Yet the obtained droplet-like macropore structure may be not good for mass transportation compared with bicontinuous macropore structure. In our previous work, we ever prepared meso/macroporous silica monoliths using an effective double-templating approach [14]. Water-soluble organic polymers were used to induce spinodal decomposition during the gelation process and finally form bicontinuous macropores; and triblock copolymers were applied as supermolecular templates to direct the formation of ordered hexagonal mesostructure. Here in this work, we researched the preparation of aluminosilica monolith with bicontinuous macropores using the double-templating route. The effects of aluminum addition on pore structures and acidic properties were discussed and its catalytic performance on cracking 1,3,5-triisopropylbenzene (1,3,5-TIPB) was also investigated.
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2. Experimental section 2.1. Synthesis procedures Samples were prepared by mixing 4.0 g of 1 mol/L nitric acid, 7.0 g of water, 0.7 g of poly ethylene glycol (PEG, average molecular weight of 10,000), 1.0 g of triblock copolymer (Pluronic P123), 5.1 g of tetramethoxysilane (TMOS) and appropriate amount of Al(NO3)39H2O to give Si/Al molar ratios of 100–10. After the mixture was stirred for 20 min in an ice-bath and became homogeneous, it was sealed in a plastic container, and kept at 333 K for 24 h for gelation. The resultant wet gel was dried at room temperature and then heated at 873 K for 8 h to get the monolith. 2.2. Characterization Nitrogen adsorption–desorption isotherms were measured on a Micromeritics ASAP 2010 apparatus. Before measurement, the samples were first degassed at 383 K for 3 h and then at 623 K for 5 h under vacuum. The specific surface areas of the samples were calculated with the BET equation. The pore volumes were determined at a P/P0 value of 0.995. The mean mesopore diameters were calculated with the BJH equation using the desorption isotherms. The mean macropore diameters were measured by mercury porosimetry (Micromeritics Autopore 9500). Scanning electron microscope (SEM) experiments were performed with a Philips XL30E electron microscope. The samples were vapor-deposited with gold before analysis. Transmission electron microscope (TEM) study was carried out on a Tecnai 20 S-TWIN instrument operating at 200 kV and using CeB6 filament as an electron beam. Sample for TEM analysis was prepared by dipping a carbon-coated copper grid into a suspension of samples in ethanol that was pre-sonicated. Fourier transform infrared spectra (FTIR) were obtained with a Nicolet380 spectrometer. The spectra were collected after 32 scans for the wavenumber range of 1400–1600 cm 1, with a resolution of 4 cm 1. NH3 temperature-programmed desorption (NH3-TPD) profiles were measured on an apparatus equipped with a thermal conductivity detector (TCD). About 100 mg of sample were activated in flowing He at 873 K for 1 h, then cooled to room temperature and equilibrated with NH3 gas for 30 min. A fraction of adsorbed NH3 was removed by He flow for about 1 h at 373 K. Then the reactor temperature was ramped up from 373 to 873 K at a rate of 10 K min 1, meanwhile, TCD curve for NH3 desorption was recorded, with a He flow of 20 cm3 min 1. The temperature was kept constant at 873 K until the TCD curve returned to the baseline. Solid-state 27Al and 29Si NMR experiments were carried out on a Bruker DMX Avance 500 spectrometer. The 29Si NMR spectra were recorded at 59.62 MHz with a pulse length of 5 ls and a spinning rate of 4 kHz, while the 27Al NMR spectra were recorded at 78.20 MHz with a pulse length of 0.31 ls and a spinning rate of 12 kHz. Si/Al* molar ratios were determined by inductively coupled plasma (ICP) elemental analyses from Optial Emission Spectrometer, Varian 725-ES.
weight hourly space velocity (WHSV) of 2 h 1. The molar ratio of hydrogen to 1,3,5-TIPB was 5: 1. Reaction products were collected and analyzed with Agilent 6820 gas chromatograph. Three different aluminosilica monoliths were examined in this study, namely the bicontinuous macroporous sample with Si/ Al = 10 (denoted as BM10), the bicontinuous macroporous sample with Si/Al = 100 (denoted as BM100) and the dense gelatinous mesoporous sample with Si/Al = 10 (denoted as DG10). 3. Results and discussion 3.1. Effect of aluminum addition on macroporous structure Silica monoliths with bicontinuous macroporous structure were prepared by introducing phase separation into the sol–gel process [2,14], during which alkoxysilanes hydrolyzed and polycondensed within a double-copolymers solution (scheme illustrated in Fig. 1). For this process, the timing of the onset of phase separation relative to gelation is a key factor to determine the final pore structures. If the phase separation takes place much faster than the gelation, particle aggregate morphology tends to occur; on the contrary, if gelation is much faster than phase separation, the dense gelatinous mesoporous morphology will be formed. The aimed bicontinuous macroporous structure can be obtained only if the phase separation and the sol–gel transition are triggered almost simultaneously. Fig. 2 shows SEM images of samples prepared with different aluminum additions under the condition of H2O/SiO2 = 3.5. It is shown that the macropore diameter of sample increases from 1.4 to 7.3 lm as Si/Al molar ratio of samples decreases from 100 to 50. However, when Si/Al molar ratio of samples reach 10, the morphology of sample becomes spherical particle aggregates and no bicontinuous macropore could be observed. Such a phenomenon might be caused by salting out effect of aluminum precursor [15–18]. As we know, aluminum nitrate has higher solvency in water than PEG. So the interaction of PEG and water is destroyed as the increase of aluminum addition, resulting in PEG molecules breaking away from solvent. In addition, part of water forms hydrated ions with aluminum, which will lead to the solubility of PEG lowered. Thereby the addition of aluminum reduces the effective concentration of PEG in water, which accelerates the phase separation. Moreover, the more aluminum addition is, the faster the timing of the onset of phase separation is. When aluminum addition makes the timing of the onset of phase separation change slightly, macropore diameter increases with aluminum addition (as can be seen in Fig. 2 from Si/Al = 100 to 50). Once the aluminum amount is increased high enough, the phase separation will occur much faster than the sol–gel transition, then the macropore morphology would turn from bicontinuous domains to spherical particle aggregates (as can be seen in Fig. 2 from Si/Al = 50 to 10).
2.3. Cracking reaction The 1,3,5-TIPB cracking reaction was performed in a continuous fixed-bed micro-reactor with H2 as the carry gas and reduction atmosphere. The catalyst sample was crushed and sieved to 20– 40 mesh. In a typical experiment, prior to the reaction, 1 g sieved catalyst was activated for a few hours at 773 K in air flow. The reaction was carried out at 573 K. The feed was pure 1,3,5-TIPB with
Fig. 1. Schematic representation of phase separation process during sol–gel transition.
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Fig. 2. SEM images of samples prepared with different aluminum amount under the condition of H2O/SiO2 = 3.5.
With the findings discussed above, it is clear that the increase of aluminum amount will destroy the formation of bicontinuous macroporous structure without changing the other synthesis conditions. In order to eliminate the effects of aluminum addition on macroporous structure and prepare bicontinuous aluminosilica monoliths with adjustable Si/Al molar ratios, we found that changing water amount in starting composition is the most convenient and effective way. Fig. 3 displays the effect of H2O/SiO2 and Si/Al molar ratios on the morphologies of resulted aluminosilica samples. It can be seen that the forming of bicontinuous macroporous structure needs water amount reducing with the increase of aluminum addition. There may be several reasons leading to above changes. Water amount decrease will lead to an increase of methanol concentration brought forth by hydrolysis of TMOS, which would delay the triggering of phase separation by hindering PEG to be adsorbed on the surface of silicon species. On the other hand, lower amount of water also enhances the concentration of acid and increases the viscosity, which can accelerate the polymerization reaction and
shorten the timing of onset of the sol–gel transition to make it match with that of phase separation. Hence changing water amount in starting compositions can eliminate the effects of aluminum addition on macroporous structure and obtain the bicontinuous macroporous monoliths with adjustable Si/Al molar ratio. 3.2. Effect of aluminum addition on mesoporous structure Fig. 4 shows N2 adsorption–desorption isotherms and Barrett– Joyner–Halenda (BJH) pore size distribution curves for samples with hierarchical pores and different Si/Al molar ratios. All samples show similar type IV isotherms with hysteresis loops at P/P0 = 0.4– 0.8, indicating the existence of mesoporous structures [19–20]. The textural properties of the samples are summarized in Table 1. With the increase of aluminum amount, the specific surface area reduces from 750 m2 g 1 to 625 m2 g 1, while the mesopore volume and mean pore size slightly increases. Fig. 5 displays TEM image of sample with Si/Al = 10. It can be seen that the mesoporous structure of the samples transits from the hexagonally ordered mesopores to worm-like orderless mesopores after incorporation of aluminum into silica monolith [14]. Similar images can also be observed for aluminosilica samples with Si/Al = 100 and 50 (not shown). These may be caused by the substituting of aluminum for silicon due to different size of aluminum and silicon atoms. 3.3. Effect of aluminum addition on acidic properties
Fig. 3. Effect of H2O/SiO2 and Si/Al molar ratios on the morphologies of resulted aluminosilica samples.
Fig. 6 shows the NH3-TPD profiles of samples with hierarchical pores and Si/Al = 10, 50 and 100, respectively. All samples show two NH3 desorption peaks centered at low and high temperature, which should be attributed to weak and medium strong acid sites respectively. It is seen that with the increase of aluminum addition, the peak area for both acid sites are increased simultaneously, which indicates the amount of acid sites also increase. Besides, the peak center for medium strong acid sites shifts dramatically to lower temperature with the increase of aluminum addition, which infers that acid strength becomes weaker as the aluminum content enhances. However, the peak center at low temperature attributed to weak acid sites change little. These changes of amount and strength of acid sites may have great effect on the
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Fig. 5. TEM image of samples with Si/Al = 10.
Fig. 4. N2 physisorption isotherms and BJH pore size distributions for samples with Si/Al = 100 (square), Si/Al = 50 (cycle) and Si/Al = 10 (triangle).
catalytic properties for some reactions, which we will discuss in later part. To investigate the reasons of the change of acid type and acid amount, three samples with different aluminum addition are characterized by 29Si MAS NMR and 27Al MAS NMR. Fig. 7 shows the 29Si MAS NMR spectra of samples with Si/ Al = 10, 50 and 100 after calcination. All samples have three peaks centered at 107, 101 and 92 ppm, which could be assigned to Si(0Al) (Si Q4), Si(1Al) (Si Q3) and Si(2Al) (Si Q2), respectively [21]. As can be seen from Fig. 7, with the aluminum addition, the Si atoms in Q3 environments and in Q4 environments decrease, whereas those in Q2 environments increase. Apparently, it may be caused by the introducing of Al atoms into skeleton. Fig. 8 gives corresponding 27Al MAS NMR spectra of the above three samples. Every spectrum is composed of a sharp resonance at 52 ppm which is indexed to structural, tetrahedral aluminum
and another small resonance at 0 ppm which is due to nonstructural, octahedral aluminum [22]. By considering the relative intensities of the 27Al MAS NMR signals arising from the octahedrally and tetrahedrally coordinated Al species, it can be concluded a majority of aluminum atoms are incorporated into the monolith framework for all samples and the framework aluminum increases with the increase of aluminum amount. It is well known that framework aluminum and its distribution have important influence on the amount and strength of acid sites. Since the acidity of framework Si(2Al) is weaker than that of the framework Si(1Al) site [23], the increase of framework Si(2Al) will lead to acid strength of medium strong acid falling as the aluminum addition. On the other hand, the increase of framework aluminum makes an increase of acid amount, which is consistent with NH3-TPD results (Fig. 6). The FT-IR spectra of pyridine absorbed on samples with different Si/Al molar ratios is shown in Fig. 9. There are three sharp bands due to C–C stretching vibrations of pyridine. The strong band at 1491 cm 1 is due to the pyridine adsorbed on both Brönsted and Lewis acid sites, while bands at 1545 and 1456 cm 1 are due to protonation of the pyridine molecule by Brönsted acid sites and pyridine adsorbed on Lewis acid sites, respectively. The Brönsted acid sites, Lewis acid sites, and B/L ratios of samples with different Si/Al molar ratios are shown in Table 2. It can be seen that all samples possess the Brönsted acid sites and Lewis acid sites which increase with the increase of aluminum addition. Combining with the results of ICP (Table 1) and 27Al MAS NMR, it can be confirmed that the more aluminum is added in starting compositions the
Table 1 Textural properties of the samples with different Si/Al mol ratios. Si/Al
Si/Al*
H2O/SiO2
SBET (m2/g)
VMesop (cm3/g)
DMesopa (nm)
DMesopb (nm)
DMacrop (lm)
100 50 10
144 86 16
3.5 3.5 2.5
750 704 625
0.48 0.51 0.52
3.3 3.8 4.3
3.2 3.7 4.1
1.4 7.3 2.7
*
Determined by inductively coupled plasma (ICP). Calculated with the BJH equation using the absorption isotherms. b Calculated with the BJH equation using the desorption isotherms. a
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Fig. 6. NH3-TPD images of samples prepared with different Si/Al molar ratio.
Fig. 9. IR spectra of pyridine absorbed on samples prepared with different Si/Al molar ratio.
Table 2 Comparison of acid sites and B/L ratios determined by pyridine-IR spectra for the samples with different Si/Al mol ratios. Sample
a
B acid sites L acid sitesa B/Lb
Si/Al = 100
Si/Al = 50
Si/Al = 10
573 k
573 k
473 k
573 k
673 k
0.02 0.02 1.50
0.04 0.04 1.50
0.10 0.14 1.1
0.07 0.11 0.95
0.03 0.11 0.41
a Calculated by the ratio of the integrated area of IR adsorption peak to sample weight. b Calculated by the equation of 1.5 (AB/AL), where AB/AL is the absorbance ratio [24].
Fig. 7.
29
Si MAS NMR spectra of samples with different Si/Al molar ratio.
acid band is also shown in Table 2. It is obvious that the relative intensity of the Brönsted acid band gradually decreased with the increase of desorption temperature, and the intensity of the Lewis acid band also decreased with the increase of desorption temperature from 473 to 573 K, but when the temperature was higher than 573 K the Lewis band intensity had hardly any change and corresponding B/L ratios of the sample with Si/Al = 10 decreased. This indicates that the strength of Brönsted acid is widely distributed (from weak to strong), and that of Lewis acid is narrowly distributed [25]. 3.4. Catalytic performance on craking 1,3,5-TIPB
Fig. 8.
27
Al MAS NMR spectra of samples with different Si/Al molar ratio.
more aluminum atoms enter into frameworks to form abundant acid sites. For the sample with Si/Al = 10, the effect of desorption temperature on the relative intensity of the pyridine Brönsted and Lewis
Cracking 1,3,5-TIPB is an effective probe reaction to test diffusion property of materials [26]. Three aluminosilica monoliths with different macroporous structures and aluminum amounts were used as catalysts for cracking 1,3,5-TIPB. As shown in Fig. 10, BM10 demonstrates the highest 1,3,5-TIPB conversion and best stability compared with the others. As has been discussed, the acidity and mass transport property of a catalyst are decisive of the acid catalytic performance. While the former is closely related to the distribution and concentration of the framework Al species, the latter is generally manipulated by the aperture and pore structure [27]. Compared with BM10, BM100 possesses similar bicontinuous macropores. But as it has stronger medium strong acid sites (see discussion in Section 3.3), and it is more active and easily results in an intense deactivation due to strong interaction with by-product. Hence BM100 shows faster deactivation. For DG10, it has similar initial conversion of 1,3,5-TIPB compared with BM10. However, the conversion reduces fast from 63% to 46% after 5 h.
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4. Conclusions Aluminosilica monoliths with adjustable Si/Al molar ratio as well as bicontinuous macropores were prepared using a doubletemplating approach. Aluminum addition has an important effect on pore structures and acidic properties through accelerating the triggering of phase separation. Changing water amount in starting compositions can adjust the timing of onset of the sol–gel transition and make it match with that of phase separation to obtain the bicontinuous macroporous aluminosilica monolith with adjustable Si/Al molar ratio. Furthermore, the catalytic conversion of 1,3,5-TIPB on bicontinuous macroporous aluminosilica monoliths with Si/Al = 10 was higher due to its proper acidity and open channel than that of the samples with different Al contents or with dense gelatinous mesopore structure. Besides, acidity of monolith plays a key role on the product distribution and selectivity. Acknowledgment Fig. 10. 1,3,5-TIPB conversion over different aluminosilica samples.
We are grateful to the Major State Basic Research Development Program of People’s Republic of China (2009CB23506) for financial support. And we also appreciate the help of Dr. Yang xueping on paper writing. Great thanks also to professor He heyong (Fudan university) for solid state NMR analysis.
Table 3 Selectivity to products in TIPB cracking reaction over various samples. Sample
BM10 DG10 BM100
Selectivity (%)
References
Benzene
IPB
m-DIPB
p-DIPB
21.4 22.1 22.5
12.5 12.2 5.8
51.7 51.4 58.2
10.9 10.5 8.6
Since BM10 and DG10 have same aluminum amount, diffusion limitation might play a key role in the reaction process. Thus it implies that an aluminosilica monolith combined with bicontinuous macroporous structure and proper acid sites may lead to substantial performance improvement in catalytic reactions involving large molecules. The main products of 1,3,5-TIPB cracking over these catalysts are benzene, isopropylbenzene (IPB), m-diisopropylbenzene (m-DIPB) and p-diisopropylbenzene (p-DIPB). Product selectivity is given in Table 3. It can be noticed that BM100 differs with BM10 and DG10 in selectivity to products except for benzene. BM10 and DG10 catalysts have shown much higher selectivity to IPB and p-DIPB compared to BM100, whereas the selectivity to m-DIPB is lower. However, BM10 and DG10 show similar selectivity to all products. This is can be explained that acidity plays an important effect on the selectivity and differences of pore structure have hardly any influence on it for these samples. 1,3,5-TIPB craking may take place through a four-step mechanism, namely (a) dealkylation of TIPB to 1,3-DIPB, (b) isomerization of 1,3-DIPB to 1,4-DIPB, (c) dealkylation of DIPBs to IPB, and (d) dealkylation of IPB to benzene [28]. Acid sites strength required for dealkylation may increase in the order of TIPB < DIPBs < IPB [29]. Besides, IPB cracking is catalyzed by strong Brønsted acid sites [30], while only weak acid sites are required for dealkylation of TIPB [31]. Compared with BM100, BM10 and DG10 possess more and weaker medium strong acid sites which facilitate isomerization and dealkylation of DIPBs to produce p-DIPB and IPB. Moreover, p-DIPB is more stable than m-DIPB. Thereby the selectivity to IPB and p-DIPB are high and selectivity to m-DIPB is low on BM10 and DG10.
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