International Journal of Inorganic Materials 3 (2001) 461–466

Hydrothermal synthesis and thermal characterization ofqniobium-aluminophosphate with AEL structure

*Antonio O.S. Silva, Marcelo J.B. Souza, Antonio S. AraujoFederal University of Rio Grande do Norte, Department of Chemistry, CP 1662, 59078-970 Natal RN, Brazil

Abstract

Niobium-aluminophosphate materials with AEL structure (NbALPO-11) were prepared by hydrothermal method, using pseudo-boehmite, phosphoric acid, hydrated niobium oxalate, water and di-isopropylamine. The obtained materials were characterized by X-rayfluorescence, X-ray diffraction, infrared spectroscopy, scanning electron microscopy, thermogravimetry and measurements of acidity.From the X-ray diffraction it was confirmed that niobium was incorporated in the framework at low Nb concentration. The obtainedNbALPO-11 presented acidity of weak and medium strength, as verified by n-butylamine desorption. The characterization of the catalytic

21active sites was carried out by isopropanol dehydration at a temperature of 4008C, and space velocities of 1.6, 3.2, 4.8, 6.4 and 8.0 h .The materials were active for this process where the main products were propene and di-isopropyl ether. 2001 Published by ElsevierScience Ltd.

Keywords: Hydrothermal synthesis; Aluminophosphate; Niobium; Molecular sieves

1. Introduction providing acid–base functions. On the other hand, theincorporation of transition metal elements (viz. Co, Mn, V)

In the early 1980s, the discovery of a new class of into the framework of aluminophosphates generates activemolecular sieves was reported by researchers from Union sites for redox reaction [7].Carbide [1,2]. These new materials comprise a series of The aluminophosphate with AEL structure (ALPO-11),crystalline, microporous aluminophosphates (ALPOs) hy- exhibits a unidimensional channel system with 10-mem-drothermally prepared from reaction mixtures containing bered ring pores. The pore opening is elliptical in shape

˚inorganic sources of Al and P and an organic template with diameters of 3.936.3 A [8]. This kind of channel is(such as amine or a quaternary ammonium salt). The interesting for application in several reactions of industrialALPOs represent the first family of molecular sieves free interest. However, the ALPO-11 is a very weakly acidicof silica on its structure. solid due to terminal hydroxyl groups and structural

The structures of the aluminophosphate molecular sieves defects or weak Lewis acid sites [9]. Accordingly, ALPO-are three-dimensional networks in which AlO and PO 11 does not promote methylation of toluene or m-xylene4 4

tetrahedra are connected via oxygen atoms. The global isomerization. Therefore, this aluminophosphate molecularcharge of the frameworks of the aluminophosphate molec- sieve has been found to be active for isobutanol dehydra-ular sieves is zero and consequently these compounds have tion [10].no ion-exchange capacity. For application of these materi- In this work was studied the influence of the niobiumals in catalysis, negative charges have been introduced into concentration on catalytic and acidic properties of a seriesthe network by isomorphic substitution of a portion of of niobium-containing aluminophosphates with AEL struc-phosphorus and/or aluminum in the framework by silicon ture synthesized by hydrothermal method.or other metals of lower oxidation states, typically 11, 12and 13 [3–6]. Consequently, positive counter-ions inextra-framework positions counterbalance these charges

2. ExperimentalqThis paper was presented at the Second International Conference on

2.1. Synthesis of materialsInorganic Materials, UCSB, 2000.*Corresponding author. Fax: 155-84-211-9224.E-mail address: asa-ufrn@usa.net (A.S. Araujo). The following chemicals were purchased commercially

1466-6049/01/$ – see front matter 2001 Published by Elsevier Science Ltd.PI I : S1466-6049( 01 )00043-5

462 A.O.S. Silva et al. / International Journal of Inorganic Materials 3 (2001) 461 –466

Fig. 1. X-ray powder diffraction pattern of the as-synthesized materials, where (a) x50.025, (b) x50.050, (c) x50.075 and (d) x50.100.

and used without purification: hydrated aluminum oxide crystallinity was estimated by the measurement of the area(pseudoboehmite, catapal B, Vista Chemical) as the Al of the peaks at 2u ranging from ca. 20.7 to 24.1 degrees.source, phosphoric acid (85%, Merck) as the P source, The sample that presented larger area was considered asniobium oxalate hydrated (CBMM, AD-463) as the Nb 100% crystalline and the areas of the other samples weresource and di-isopropylamine, DIPA (98%, Riedel) as the normalized in relation to this. For the unit cell volume, thetemplate agent. interplanar d-spacing obtained from the XRD curve were

The samples of niobium-aluminophosphate were syn- corrected with respect to that of silicon (internal standard),thesized by the hydrothermal method from the gel with and then used for the determination of the unit cellmolar compositions that can be represented by: 1.0DIPA: parameters; and these were further refined by least square1.0Al O : (12x)P O : xNb O : 80H O where x50.025, fitting programs. The morphology and size of the crystals2 3 2 5 2 5 2

0.050, 0.075 and 0.100. were determined by scanning electron microscopy, using aThe gel synthesis was prepared according to a procedure Carl Zeiss microscope (DSM-960 model). Infrared mea-

described in the literature [11,12]. The gel was transferred surements were performed in a Fourier transform infraredto a teflon-lined stainless-steel autoclave and heated in an spectrometer, FT-IR Prospec (from Midac) using the KBroven at a temperature of 1708C for a period of 72 h. The wafer technique. The wafers were prepared by mixing ca.obtained products were recovered by filtration, washed 1 mg of the sample with 100 mg of KBr powder andwith deionized water and dried at 1008C for 6 h. The pressing to obtain transparent disks. The spectra were

21calcination of the samples was performed at 5508C in dry attained in the region of 1400–400 cm . The chemical3 21air dynamic atmosphere with flow of 100 cm min for a composition of the materials were determined by X-ray

period of 20 h. fluorescence using a Philips BW 2400. Thermogravimetricanalysis (TG) was performed in a DuPont TA-2950

212.2. Characterization thermobalance, at a heating rate of 108C min , in thetemperature range from ambient to 9008C using nitrogen

3 21The unit cell volume and the degree of crystallinity of flow of 60 cm min .the materials were determined in a Philips powder diffrac- Prior to the determination of the acidity, ca. 0.1 g of thetometer using Ni-filtered Cu K radiation, with the diffrac- previously calcined sample, was activated at 4008C ina

3 21tion angle (2u ) at a range of 5–408. The degree of nitrogen flow (60 cm min ) for 2 h. Then, the tempera-

A.O.S. Silva et al. / International Journal of Inorganic Materials 3 (2001) 461 –466 463

ture was reduced to 958C and then saturated with vapors ofn-butylamine for a period of 1 h. Afterwards, the solidsaturated with amine was purged with nitrogen for 0.5 h atthe same adsorption temperature, to remove the physicallyadsorbed base. The n-butylamine desorption was per-formed by heating ca. 10 mg of the saturated sample in aDuPont TA-2950 thermobalance, at a heating rate of 58C

21 3 21min up to 6008C in the nitrogen flow of 60 cm min .The acidity was determined by means of the amount, inmmol, of n-butylamine desorbed, normalized by mass of

21sample, being expressed in mmol g . This thermogravi-metric method of acidity measurement has been previouslydescribed [13].

The reactions of isopropanol dehydration were con-ducted in a glass micro-reactor with diameter of 1 cm andlength of 15 cm. The experiments were carried out using150 mg of sample at a temperature of 4008C and weighthourly space velocity (WHSV) of 1.6, 3.2, 4.8, 6.4 and 8.0

21h , with nitrogen as gas carrier. The separation andidentification of the reaction products were performed by agas chromatograph CG-25S, with thermal conductivitydetector and a Carbowax column, which was coupledon-line to the reactor outlet by an injection valve.

3. Results and discussion Fig. 2. FT-IR spectra of the synthesized materials, where (a) x50.025,(b) x50.050, (c) x50.075 and (d) x50.100.

The X-ray diffractogram (Fig. 1) of the niobium-aluminophosphate samples are characteristic of the pureAEL phase, as shown in the literature [14], except for the [15], however, these spectra exhibited an additional band

21sample with x 5 0.100 that present a contaminant not- centered at ca. 800 cm which can be assigned to theclearly identified phase not identified as indicated by vibration due to the presence of Nb–O bond.arrows on the X-ray diffraction curve. This contaminant From the scanning electron micrographs of the samplesphase is probably formed for extra framework niobium (Fig. 3) was observed the formation of crystals of size ca.oxide. 2–5 mm, with orthorhombic morphology which is typical

The degree of crystallinity values of the samples are of ALPO-11 [11,12].shown in Table 1. These results indicate that the syn- The thermogravimetric curves of the uncalcined materi-thesized materials present excellent degree of crystallinity als are shown in Fig. 4, in which were observed three(.87%). The values of the unit cell volume, presented in weight loss steps in temperatures below 4008C, whichTable 1, were practically identical for all samples in- were identified through derivative of curve thermogravime-dependently of the niobium concentration in the material. ty (DTG). Thus the temperature used for calcination of theMoreover, these unit cell volumes are greater than that NbALPO-11 samples was ca. 4508C.determined for ALPO-11 [8], evidencing an expansion in In Table 2 are indicated the temperature range and thethe unit cell, occasioned by the niobium incorporation in respective weight losses for the steps shown in Fig. 4. Thethe framework. first event (i) of weight loss was attributed to desorption of

The infrared spectra (Fig. 2) of the synthesized materials intracrystalline water; the second event (ii) was associatedshow absorption bands similar to those found for ALPO-11 to the liberation of di-isopropylamine molecules weakly

Table 1Crystallinity, unit cell volume and chemical composition of the synthesized samples

3˚Sample Crystallinity (%) Unit cell volume (A ) Chemical composition

x50.025 93 2139 (Al P Nb )O0.520 0.470 0.011 2

x50.050 87 2136 (Al P Nb )O0.496 0.480 0.023 2

x50.075 100 2139 (Al P Nb )O0.486 0.476 0.038 2

x50.100 88 2140 (Al P Nb )O0.495 0.461 0.045 2

464 A.O.S. Silva et al. / International Journal of Inorganic Materials 3 (2001) 461 –466

Fig. 3. Micrographs of the samples of NbALPO-11, where (a) x50.025, (b) x50.050, (c) x50.075 and (d) x50.100.

bonded to the surface (without chemical reaction); the third associated not only with the liberation of the di-iso-event (iii) was associated to the liberation of the molecules propylamine molecules, but also with the desorption ofof propene and ammonia from the reaction of di-iso- other substances associated to the amorphous regions (inpropylamine with an active site of the material surface case of the sample x50.050) or the other crystalline phase(known as Hoffman elimination). These attributions were (in case of the sample x50.100).realized according with studies performed by Biaglow et The thermogravimetric curves of n-butylamine desorp-al. [16] and Parrillo et al. [17], where the desorption of tion (Fig. 5) showed three stages of weight loss. The firstamine steps in SAPO-5 and MeAPO-5 (Me5Mn and Mg) stage (i), was associated to desorption of water; the secondwere identified, respectively, by mass spectrometry. stage (ii), was attributed to the desorption of n-butylamine

It is interesting to observe that the samples with x5 bonded to the weak acidic sites; the third event (iii), was0.050 and x50.100 (Table 3) presented a large weight loss associated to the liberation of butene and ammonia (due toin the temperature range from 300 to 4008C, and it is reaction decomposition of the n-butylamine molecules onapproximately twice those observed in the samples with medium strength acidic sites).x50.025 and x50.075. These weight loss steps can be The concentration of acidic sites obtained through

A.O.S. Silva et al. / International Journal of Inorganic Materials 3 (2001) 461 –466 465

Fig. 4. Thermogravimetric analysis of the as-synthesized materials, where (a) x50.025, (b) x50.050, (c) x50.075 and (d) x50.100.

Table 2 thermogravimetry (Table 3) showed that niobium-Temperature range and respective weight loss of the as-synthesized aluminophosphates present two types of acid sites withmaterials obtained by thermogravimetry

weak and medium relative strength. The quantity of acidSample Temperature range (8C) Weight loss (%) sites is inversely proportional to the niobium concen-Steps: (I) (II) (III) (I) (II) (III) tration, indicating that an increase in the niobium con-

centration in the gel generates extra-framework niobiumx50.025 25–146 146–306 306–401 5.95 8.77 1.49oxide species decreasing, by the way, the acid sites.x50.050 25–156 156–306 306–401 5.75 7.06 3.83

x50.075 25–153 153–313 313–402 8.72 7.07 2.07 The reaction of isopropanol dehydration presented highx50.100 25–145 145–283 283–401 6.08 8.09 5.81 conversion, in the range of ca. 90–95%, as indicated in

Table 4. From these data, it is evident that all the niobium-aluminophosphate samples exhibit a similar catalytic per-formance, showing low dependence of conversion inrelation to the WHSV. The main reaction product waspropene, with selectivity higher than 90% and di-isopropylether as a secondary product.

Through the data conversion in function of WHSVTable 3

(Table 4) was determined the initial velocity of theMeasurements of acidity of the samples of niobium-aluminophosphatereaction using the plot of a curve of space time versus

Sample Acidity (mmol /g )cat conversion /space time. The intersection value of this curveWeak Medium Total with the Y-axis corresponds to the initial velocity of the

x50.025 0.312 0.097 0.409 reaction. The initial velocities determined for this reactionx50.050 0.282 0.092 0.374 present values practically equal for all samples, indicatingx50.075 0.223 0.080 0.303 that the studied materials exhibit similar catalytic prop-x50.100 0.145 0.057 0.202

erties.

466 A.O.S. Silva et al. / International Journal of Inorganic Materials 3 (2001) 461 –466

Fig. 5. Thermogravimetric analysis of the desorption of n-butylamine of the samples of NbALPO-11, where (a) x50.025, (b) x50.050, (c) x50.075 and(d) x50.100.

[5] Flanigen EM, Lok BM, Patton RI, Wilson ST. In: Proceedings 7thTable 4International Zeolite Conference, Tokyo/Amsterdam: Kodansha /Data of conversion and initial velocity for the dehydration reaction ofElsevier, 1986, p. 120.isopropanol

[6] Flanigen EM, Lok BM, Patton RI, Wilson ST. Pure Appl Chem21Sample WHSV (h ) Initial velocity 1986;58(10):1351.

(mmol /h g )reag cat [7] Singh PS, Kosuge K, Ramaswamy V, Rao BS. Appl Catal A:1.6 3.2 4.8 6.4 8.0General 1999;177:149.Conversion (%)

[8] Meier VIM, Olson DH, Baerlocher Ch. In: 4th ed, Atlas of zeolitex50.025 94.9 94.4 94.3 93.2 92.5 170 structure types, New York: Elsevier, 1996.x50.050 93.3 92.1 91.6 91.1 89.9 165 [9] Singh PS, Bandyopadhyay R, Shaikh RS, Rao BS. Stud Surf Scix50.075 90.0 89.7 89.0 88.8 88.0 161 Catal 1995;97:343.x50.100 93.1 91.6 90.9 90.5 89.5 164 [10] Corma A, Hermenegildo G. Catal Today 1997;38:257.

[11] Araujo AS, Diniz JC, Silva AOS, Melo RAA. J Alloys Comp1997;250:532.

[12] Melo RAA. In: Hydrothermal crystallization of aluminophosphateAcknowledgementsmodified by rare earth and application in adsorption and catalysis,Natal, RN, Brazil: Federal University of Rio Grande do Norte, 1995,Financial support for this work was provided by CNPqp. 68, MSc dissertation.

´(Conselho Nacional de Desenvolvimento Cientıfico e [13] Araujo AS, Fernandes Jr. VJ, Giolito I, Zinner LB. Thermochimˆ´ ´Tecnologico) and ANP (Agencia Nacional do Petroleo). Acta 1993;223:129.

[14] Treacy MMJ, Higgins JB, von Ballmoos R. In: 3rd ed, Collection ofsimulated XRD powder patterns for zeolites, New York: Elsevier,1996.References

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