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Microporous and Mesoporous Materials 213 (2015) 192e196

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate/micromeso

Short communication

Microwave synthesis of AFI-type aluminophosphate molecular sieveunder solvent-free conditions

Xinhong Zhao a, *, Jiangbo Zhao a, Juanjuan Wen a, An Li a, Guixian Li a, Xiaolai Wang b

a School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, Chinab State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,China

a r t i c l e i n f o

Article history:Received 13 February 2015Received in revised form18 March 2015Accepted 29 March 2015Available online 3 April 2015

Keywords:AlPO4-5Solvent-freeHierarchical porousMicrowave

* Corresponding author. Tel.: þ86 931 7823126.E-mail address: Licpzhaoxh@lut.cn (X. Zhao).

http://dx.doi.org/10.1016/j.micromeso.2015.03.0311387-1811/© 2015 Elsevier Inc. All rights reserved.

a b s t r a c t

AlPO4-5 aluminophosphate molecular sieve with AFI topology has been successfully synthesized bymicrowave irradiation under solvent-free conditions. The key influential factors controlling the crys-tallization of AFI structure were thoroughly investigated. The optimum synthetic conditions of thismaterial are as follows: the initial composition is 1.0Al2O3: 3.0P2O5: 2.0HF: 8.0TEABr(te-traethylammonium bromide): 125C, aluminum isopropoxide is the best aluminum source, and activatedcarbon needs to be pretreated before used as the reaction medium and hard template. The resultantAlPO4-5 molecular sieves were characterized by X-ray diffraction (XRD), N2 physisorption, scanningelectron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that theobtained samples exhibit hierarchically porous characteristic.

© 2015 Elsevier Inc. All rights reserved.

1. Introduction

AFI-type molecular sieve, composed of straight 12 MR one-dimensional channels along the c crystallographic direction, is aprominent one among the aluminophosphate molecular sievefamily. Recently, AFI molecular sieves have receivedmuch attentiondue to their extensive use in catalysis, separation and nonlinearoptics [1]. Generally, aluminophosphate molecular sieves are syn-thesized by conventional hydrothermal or solvothermal methodwith water or organic solvents as the reaction medium. The use ofthese solvents inevitably results in many drawbacks such as wastegeneration, low synthesis efficiency and high autogenous pressure[2]. Presently, considerable efforts have been devoted to solve theseproblems.

Ionothermal synthesis, reported by Morris' group [3,4], caneliminate safety concerns associated with high hydrothermalpressure because it can be carried out under ambient pressure. Butthis synthetic method involves the use of expensive ionic liquid,and its recycling is not convenient [5]. Most recently, Xiao and co-workers developed a novel solvent-free synthesis, which exhibitsmany advantages, such as the low waste production and high

zeolite yield. Additionally, similar to ionothermal synthesis, thesolvent-free synthesis also eliminates high pressure [2,6e9].However, this route is energy intensive due to employing conven-tional heating.

As an efficient and selective heating method, microwave irra-diation has been used to the synthesis of many new materials[10e12]. In our previous work, FeAPO-16 [13], FeAPO-5 [14] andSAPO-5 [15] molecular sieves were ionothermally synthesized bymicrowave heating with cheap deep eutectic mixture as both sol-vent and template. This route is an energy-saving one comparedwith conventional synthesis. Unfortunately, it is still not environ-mental benign owing to the heavy use of eutectic mixture. Inspiredby Xiao and co-workers’ results and considering the versatility ofhierarchically structured aluminophosphate molecular sieves, inthe present workwe aim to develop a novelmethod of synthesizingAFI molecular sieve with micro- and meso-porous structure, inwhich activated carbon replaces the eutectic mixture solvent as thereaction medium and hard template.

2. Experimental section

2.1. Synthesis of AlPO4-5

For the synthesis of AlPO4-5 molecular sieve, activated carbon(Sinopharm Chemical Reagent Co. Ltd) was pretreated by nitric acid

X. Zhao et al. / Microporous and Mesoporous Materials 213 (2015) 192e196 193

(65 wt%, Yantai Shuangshuang Co. Ltd) before used as the reactionmedium. All other reagents were of reagent grade and usedwithoutfurther purifications: tetraethylammonium bromide (TEABr, Sino-pharm Chemical Reagent Co. Ltd), aluminum hydroxyacetate(Sinopharm Chemical Reagent Co. Ltd), aluminum isopropoxide(Sinopharm Chemical Reagent Co. Ltd), pseudo-boehmite (Shan-dong Zibo Senchi Chemical Co., Ltd), phosphoric acid (85 wt% inwater, Tianjin De'en Chemical Reagent Co. Ltd), hydrofluoric acid(40 wt% in water, Sinopharm Chemical Reagent Co. Ltd), acetone(Beijing Chemical Plant) and deionized water.

General synthesis procedure of AlPO4-5 molecular sieve was asfollows: the raw or pretreated activated carbon, TEABr, aluminumsource, phosphoric acid, hydrofluoric acid and additional deionizedwater if required were measured out in the molar ratio of 1.0Al2O3:(2e6)P2O5: (1e4)HF: (4e8)TEABr: (0e20)H2O: (62.5e125)C andground in a mortar or ball mill (XQM-04L, speed of rotation300 rpm, Nanjing Kexi Institute of Experimental Instruments Co.,China) for 20 min. The resulting solid mixture was transferred to around-bottomed flask equipped with a condenser and placed in amicrowave reaction system (model XH-MC-1 from Beijing XianghuScience and Technology Development Co., Ltd, Beijing, China). Thenthemixturewas heated to a designated temperature at about 20 �C/min and kept there for appropriate time (1e2 h). After crystalli-zation, the synthesis mixture was cooled to room temperature. Theresultant product was washed thoroughly with acetone anddistilled water, dried in air, and calcined at 550 �C for 5 h to removethe template. Detailed synthesis conditions were listed in Table 1.

2.2. Characterization

X-ray powder diffraction (XRD) patterns of the synthesizedmaterials were conducted on a D/Max-2400 Rigaku diffractometerwith Cu Ka radiation operated at 40 kV and 150 mA. Nitrogenadsorption/desorption studies were conducted on a MicromeriticsASAP 2020 surface area and pore size analyzer at �196 �C. Sampleswere outgassed at 200 �C for 4 h prior to measurements. Specificsurface areas of materials were calculated from the adsorption dataobtained at p/p0 between 0.07 and 0.20, using the Bru-nauereEmmetteTeller (BET) equation. The micropore volumes

Table 1Initial compositions and crystallization conditions for the synthesis of AFI.

Sample Al2O3:P2O5:HF:TEABr:H2Oa:C(molar ratio)

Temperature(�C)

Time (h)

P-2 1.0:2.0:2.0:8.0:10.0:125 180 1P-3 1.0:3.0:2.0:8.0:10.0:125 180 1P-6 1.0:6.0:2.0:8.0:10.0:125 180 1C-0 1.0:3.0:2.0:8.0:10.0:0.0 180 1C-62.5 1.0:3.0:2.0:8.0:10.0:62.5 180 1C-raw 1.0:3.0:2.0:8.0:10.0:125b 180 1TEABr-4 1.0:3.0:2.0:4.0:10.0: 125 180 1HF-1 1.0:3.0:1.0:8.0:10.0:125 180 1HF-4 1.0:3.0:4.0:8.0:10.0:125 180 1H2O-0 1.0:3.0:2.0:8.0:0.0:125 180 1H2O-20 1.0:3.0:2.0:8.0:20.0:125 180 1Al-IP 1.0c:3.0:2.0:8.0:0.0:125 180 1Al-PB 1.0d:3.0:2.0:8.0:0.0:125 180 1T180-2h 1.0c:3.0:2.0:8.0:0.0:125 180 2T200-1h 1.0c:3.0:2.0:8.0:0.0:125 200 1T200-2h 1.0c:3.0:2.0:8.0:0.0:125 200 2Mill-20e 1.0c:3.0:2.0:8.0:0.0:125 180 2

a Additional water, not include the water contained in HF and H3PO4.b Raw activated carbon is used in the synthesis.c Aluminum isopropoxide.d Pseudo-boehmite.e 20-min mechanical treatment by ball mill. Unless otherwise specified,

aluminum hydroxyacetate is used as the aluminum source.

were determined by the t-plot method. The mesopore volumeswere calculated from the difference between the total pore volumeand the micropore volume. The pore size distributions weredetermined from desorption branches by BJH method. Thermog-ravimetric (TG) and differential thermal analysis (DTA) (Netzsch,STA409) was performed in air at a heating rate of 10 �C/min.Scanning electron microscope images were obtained from a SEM(JSM-6701F) instrument. Transmission electron micrographs (TEM)were obtained on a JEOL JEM-2010 microscope. For TEM, thesample was ground, dispersed in ethanol, and deposited on a holeycarbon film supported on a copper grid.

3. Results and discussion

3.1. Optimization of synthesis conditions

For the synthesis of aluminophosphate molecular sieves, theresulting structure types are commonly controlled by the P2O5/Al2O3 ratio in the initial gel [16]. As shown in Fig. 1, when P2O5/Al2O3 ratio is 2.0 (sample P-2), five main diffraction peaks ascribedto AFI phase can be clearly observed. Meanwhile, large amounts ofdense phases, i.e. tridymite, cristobalite and berlinite, are alsodetected in this sample. As P2O5/Al2O3 ratio is increased to 3.0(sample P-3), dense phases still exist, but the intensity of thosediffraction peaks attributed to tridymite and berlinite significantlydeclines. On the other hand, the intensity of five diffraction peaks ofAFI phase increases in varying degrees. It is interesting to note thatimperfect AEL phase (sample P-6) can be produced when P2O5/Al2O3 ratio arrives at 6.0. To the best of our knowledge, this may bethe first case in which tetraethylammonium bromide is used as thestructure directing agent to synthesize AEL phase. Based on aboveanalysis, it can be found that the appropriate P2O5/Al2O3 ratio is 3.0for the synthesis of AFI phase.

To synthesize AFI phase in a cost-effective manner, the effect ofthe amount of activated carbon and the role of pretreatment werestudied in detail. As seen in Fig. 2, the product is pure berlinite(sample C-0) when no activated carbon is added to the synthesismixture. As the C/Al2O3 is decreased from 125 to 62.5 (sample C-62.5), the major product is dense cristobalite accompanied by mi-nor AFI phase. It is worth mentioning that dense cristobalite phaseinstead of AFI is produced when raw activated carbon is introducedto the synthesis mixture. In Xiao and co-workers’ research, AFIphase was successfully prepared in the presence of TEABr and di-n-

Fig. 1. XRD patterns of three samples under the conditions of different P2O5/Al2O3

ratios.

Fig. 2. XRD patterns of four samples under the conditions of low C/Al2O3 and TEABr/Al2O3 ratios.

Fig. 3. XRD patterns of five samples under the conditions of different H2O/Al2O3 andHF/Al2O3 ratios.

Fig. 4. XRD patterns of seven samples under the conditions of different aluminumsource, crystallization time, temperature and mechanochemical treatment.

X. Zhao et al. / Microporous and Mesoporous Materials 213 (2015) 192e196194

propylamine [2]. If there was only one template in the synthesis,AFI phase cannot be obtained. These results indicate that in our casethe interaction between pretreated activated carbon and TEABrmay play a key role in synthesizing AFI phase. To corroborate therole of pretreated activated carbon, we performed the TG-DTA ex-periments of two mixtures prepared in mortar. One is the mixtureof pretreated activated carbon and TEABr (mixture A), and the otheris raw activated carbon and TEABr (mixture B). As shown in Fig. S1,the initial exothermic effect accompanied by large weight loss,which is associated with a Hoffmann elimination reaction of tet-raethylammonium cations [17], can be clearly observed in the210e275 �C temperature range of DTA curves. The endothermicpeak centered at about 290 �C with no obvious weight loss can beascribed to the melt of free TEABr. It should be noted that not onlyHoffmann elimination temperature but also the melting tempera-ture of TEABr for mixture A is higher than those for mixture B. Suchresults fully confirm that strong interactions occur between pre-treated activated carbon and TEABr.

From an economic point of view, the high use of organic tem-plate is unfavorable in zeolite synthesis. Therefore, a syntheticexperiment was performed under a TEABr/Al2O3 ratio of 4.0. Theresult shows that the product is dominated by dense cristobalitephase (Fig. 2, sample TEABr-4). Obviously, the suitable ratios of C/Al2O3 and TEABr/Al2O3 are 125 and 8.0, respectively.

Fluoride has recently been an extremely useful mineralizer foraluminophosphate [18] and silicate synthesis [19], in which it mayhelp to solubilize the starting materials under the reaction condi-tions and in some cases play a structure-directing role. In thepresent study, the influence of hydrofluoric acid on the crystalli-zation was investigated by varying the initial HF/Al2O3 ratio. Theresults demonstrate that among the three samples (Fig. 3, P-3, HF-1and HF-4), P-3 has the highest crystallinity in terms of AFI phase.High hydrofluoric acid dosage is favorable for the generation ofdense berlinite. Thus, the optimum HF/Al2O3 ratio is 2.0 in thisstudy.

The most important factor affecting ionothermal synthesis wasconsidered to be water content in the original research of Morris'group [3]. A very small amount of water was advantageous forionothermal synthesis [20]. However, the addition of largeramounts of water to the synthesis mixture seemed to disrupt thetemplating ability of the ionic liquid, leading to dense phase beingformed [21]. The synthetic method reported by us has something incommon with ionothermal synthesis, namely, water is not the

dominant species in both methods. Thus, the added water in ourcase may play a similar role as that in ionothermal synthesis. Asdisplayed in Fig. 3, the sample H2O-0 has the least dense phaseamong the three samples (P3, H2O-20, H2O-0) synthesized underthe conditions of different H2O/Al2O3 ratios, again verifying theeffect of water. In fact, large amounts of water will compete withactivated carbon to interact with TEABr, thus disturbing the tem-plate ability of TEABr. In the subsequent study, additional water wasno longer introduced to the synthesis system.

Aluminum source also has a large impact on the synthesis ofaluminophosphate molecular sieves because different aluminumsources have distinct reactivity with phosphoric acid. Comparingthe XRD profiles of the three samples (Fig. 4, H2O-0, Al-AIP and Al-PB), it is found that the diffraction peak intensity of AFI phase is thehighest in the sample Al-AIP. Therefore, aluminum isopropoxide ischosen as the best aluminum source in the following optimization.

Crystallization temperature and time are two key factors incontrolling the purity of zeolite phase. As shown in Fig. 4, in com-parison to the sample Al-AIP synthesized within 1 h microwaveheating, the diffraction peak intensity of dense phases in thesample T180-2h is prominently decreased when the crystallizationtime is extended to 2 h. This result suggests that longer crystalli-zation time seems to favor the transformation of dense phases to

Fig. 5. N2 adsorption/desorption isotherms and pore size distributions (inset) of T180-2h and Mill-20. The isotherms are shifted upwards with 40 cm3/g interval betweenthem.

Table 2N2 physical adsorption-desorption data of representative samples.

Sample SBET(m2/g)a

SExt(m2/g)b

Vmicro

(cm3/g)cVmeso

(cm3/g)dPSDs(nm)e

T180-2h 166 64 0.047 0.18 5e100Mill-20 172 53 0.055 0.16 5e100

a BET surface area.b External surface area.c Micropore volume, t-plot method.d Mesopore volume ¼ total pore volume e micropore volume.e Pore size distributions.

X. Zhao et al. / Microporous and Mesoporous Materials 213 (2015) 192e196 195

other phase. Such phenomenon is quite unusual. In other words,these so-called dense phases are not really stable. On the otherhand, higher crystallization temperature results in the AFI phaseamorphize because the curved baseline and minor AFI phaseconfirm that the major phase is amorphous in the sample T200-1h.Moreover, by comparing the XRD profiles of the samples T200-1hand T200-2h, it can be seen that amorphous phase is graduallyconverted to AFI phase or dense phases with prolonged crystalli-zation time. In Xiao and co-workers’ study, all AFI phases areexclusively synthesized at 200 �C [2]. But in the present work, theoptimum crystallization temperature and time are 180 �C and 2 h.The varying template types and their interaction with aluminumand phosphor sources may account for the different crystallizationconditions.

Javier Perez-Ramirez and co-workers recently reviewed thebenefits of (seed) milling as a tool for preparing and “activating”precursor mixtures prior to their hydrothermal treatment forzeolite synthesis [22]. A lot of advantages of this mechanochemicalprocess had been demonstrated in a series of studies. The mainones are summarized as follows: faster crystallization kinetics [23],reduced amount of the organic structure-directing agent, easyattainment of a pure zeolite phase [24]. Motivated by these excitingresults, we performed further synthesis optimization throughreplacing mortar with ball mill to pretreat the precursor mixtures.As shown in Fig. 4, cristobalite disappears almost completely in thesample Mill-20, at the same time the amount of berlinite issignificantly decreased when compared with the sample T180-2h.This result again verifies the beneficial role of high-energy millingtreatment, which can be attributed, in one hand, to the peculiarmechanochemical effect, and in other hand, possibly to thereduction of water. The ball milling process can lead to highinstantaneous local temperature, which may exceed 1000 �C [25].Thus, water in the initial synthesis mixtures will inevitably evap-orate during this process, while low amount of water can restrictthe formation of dense phases, which had been discussed in pre-vious section.

3.2. Structure and morphology analysis

After experiencing a series of optimization, the appropriateconditions for the synthesis of aluminophosphate molecular sievewith AFI topology are obtained, and showed as follows: the initialcomposition is 1.0Al2O3: 3.0P2O5: 2.0HF: 8.0TEABr: 125C,aluminum isopropoxide is the best aluminum source and activatedcarbon needs to be pretreated before used as the reaction mediumand hard template. Among all synthesized samples listed in Table 1,the samples T180-2h and Mill-20 reveal a relatively pure AFI phaseand high crystallinity. Thus, the two samples are selected and usedfor further characterization.

Fig. 5 exhibits the nitrogen sorption isotherms and pore sizedistributions (PSDs) of the samples T180-2h and Mill-20. Bothisotherms exhibit a hysteresis loop generated by capillarycondensation in the meso- and macropores, implying that thesematerials have a mesopore characteristic [26]. This result is inagreement with the findings reported in the literature [26e32].However, the hysteresises do not have a steep step, and theirshapes indicate that the samples do not contain ordered meso-pores. The PSDs (Fig. 5, inset) of these samples determined from thedesorption branch of the isotherm, show twomaxima, one at about3.8 nm, which is probably attributed to the so-called tensilestrength effect [33], and a relatively broad one usually centered atabout 35 nm, representing the mesopores. The pore texture pa-rameters summarized in Table 2 show that the BET surface area andmesopore volume of the two samples are comparable. Xiao and co-workers reported that the hierarchical porosity of SAPO-5 can be

formed in the conditions of solvent-free and absence of anymesoscale organic templates [2], and the mesopore volume ofSAPO-5 reported by them (0.18 cm3/g) is comparable to the datapresented here (0.16e0.18 cm3/g). On the other hand, the micro-pore volume of the sample Mill-20 is a little higher than that in thesample T180-2h, which can be explained that the former has lessdense phases.

Fig. 6 shows the scanning electron micrographs of calcinedT180-2h and Mill-20. It can be seen from the two images thatboth samples are composed of the mixtures of nanoparticle ag-gregates and typical large hexagonal crystals. Minor irregularcrystals with high aspect ratio can also be observed (not shown),which may belong to the dense phases. In addition, the largehexagonal crystals are found to be covered by few amounts ofnanoparticles. The electron diffraction patterns shown in Fig. S2indicate that these nanoparticles are crystalline in nature. Forthe two samples, the heterogeneity regarding the morphologymay be due to the local over-heating effect related with micro-wave heating [34]. The restricted mass transfer occurred in solid-state reaction are also responsible for this result. Based on theSEM images, the size of nanoparticles is estimated to be 60 nm inboth samples. According to the Scherrer equation, the averageparticle sizes of T180-2h and Mill-20 are calculated to be 66 nmand 62 nm, respectively, which are in agreement with the SEMobservation. As expected, the presence of mesoporosity in the twosamples can be ascribed to the intercrystalline void space be-tween nanoparticles. It is the heterogeneity that results in thewide pore size distribution, as has been revealed in previous N2physisorption analysis.

Fig. 6. SEM images of the typical samples T180-2h (left) and Mill-20 (right) (Inset is the image of large hexagonal crystals).

X. Zhao et al. / Microporous and Mesoporous Materials 213 (2015) 192e196196

4. Conclusions

In conclusion, aluminophosphate molecular sieve with AFI to-pology has been synthesized by microwave heating under solvent-free conditions. The appropriate initial composition for AFI struc-ture is 1.0Al2O3: 3.0P2O5: 2.0HF: 8.0TEABr: 125C. Aluminum iso-propoxide is the optimum aluminum source and activated carbonneeds to experience pretreatment. The characterization resultsindicate that the resulting AFI molecular sieves have hierarchicalmicro- and meso-porous structure.

The synthetic strategy reported herewell integrates the featuresof carbon template, microwave synthesis and solvent-free synthe-sis. It can be conveniently extended to the synthesis of other hier-archically structured aluminophosphate molecular sieves byutilizing the interaction between activated carbon and organictemplate. Such work is still under way.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant No. 21306072, 51263012) and Devel-opment Program of Lanzhou University of Technology for excellentteachers (Grant No. Q201113). We would like to thank Professor W.Zhou for helpful discussion. We cordially thank the Reviewers forproviding us with valuable comments and suggestions.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.micromeso.2015.03.031.

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