Reaction at Interfaces: The Silicoaluminophosphate Molecular Sieve

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J. Phys. Chem. C 2007, 111, 3116-3129

Reaction at Interfaces: The Silicoaluminophosphate Molecular Sieve CAL-1 Heloise O. Pastore,* EÄ rica C. de Oliveira, and Guilherme B. Superti Instituto de Quı´mica, UniVersidade Estadual de Campinas, CP 6154, CEP 13083-970, Campinas, SP, Brazil, and Centro Interdisciplinare Nano-SiSTeMI dell’UniVersita´ del Piemonte Orientale, 15100 Alessandria, Italy

Giorgio Gatti and Leonardo Marchese* Dipartimento di Scienze e Tecnologie AVanzate, UniVersita´ del Piemonte Orientale, Via Bellini 25G, 15100 Alessandria, Italy, and Centro Interdisciplinare Nano-SiSTeMI dell’UniVersita´ del Piemonte Orientale, 15100 Alessandria, Italy ReceiVed: NoVember 22, 2006

This work reports on the study to determine the best synthesis conditions for the preparation of CAL-1, a silicoaluminophosphate with a chabazite-like structure, prepared from a lamellar aluminophosphate, ALPOntu (herein called ALPO-kanemite), and using hexamethyleneimine (HMI) as the structure directing agent (SDA). These studies showed that optimum reaction conditions to obtain CAL-1 depend on the concentration of silicon desired in the structure. For low silicon concentrations, SiO2/Al2O3 e 0.4, no set of adequate conditions was found; the samples prepared were always contaminated with the starting lamellar materials. At SiO2/Al2O3 molar ratios of 0.8 and 1.2, crystalline samples were obtained after 24 h. For silicon concentrations still larger, an optimum set of reaction conditions could not be determined. The best sample with SiO2/Al2O3 ) 1.6 in the gel was obtained with HMI/Al2O3 ) 1.5 only after 48 h of reaction time, and even then, the crystallinity of the sample thus prepared was poor. The combination of 29Si magic-angle spinning NMR and in situ infrared spectroscopy indicated that, at CAL-1 samples with the lower amount of silicon (χSi ) 0.21), protons are found either at the borders of silica patches/islands or inside the aluminosilicate domains. Characterization showed that HMI was always accompanied by the n-butylammonium cation, the SDA for the ALPO-kanemite, and showed that larger amounts of HMI led to the contamination of the samples. A mechanism involving the full dissolution of ALPO-kanemite to serve as the reactant for the formation of CAL-1 would not explain the different characteristics of this material in relation to SAPO-34, a structurally similar microporous molecular sieve obtained by hydrothermal synthesis of a gel precursor. Therefore, a small range, molecular level rearrangement of the lamella of the ALPO-kanemite, brought up by the silicon, is proposed to explain the formation of this material.

Introduction Microporous zeolites and molecular sieves are largely synthesized through the formation of a gel prepared from sources of structure T atoms, water, a mineralizing agent, and a structure directing agent (SDA). The reaction for structure formation generally involves the dissolution of reactants and their reassembly into a noncrystalline network containing the structural atoms, entrapping water and SDA molecules. During the course of the reaction, at temperatures between 373 K and 573 K and autogenous pressure, this gel will either reorganize into a crystalline solid or redissolve to serve as a source of small fragments for the construction of the final solid material. Layered hydrated silicates were also used as sources of T atoms in the synthesis of zeolites by recrystallization processes. ZSM-5,1-5 -11,6 -12, -35, -39, and -485-7 were all prepared starting from magadiite, as well as EU-2, FU-1, SSZ-15,5-7 and mordenite.8 Kanemite was used to prepare silicalite-13,9-14 and -2.11 Schwieger et al.15 reported the transformation of kanemite into β-zeolite upon hydrothermal transformation in the presence of various concentrations of aluminum. The authors discovered * To whom correspondence should be addressed. E-mail: gpmmm@ iqm.unicamp.br (H.O.P.), [email protected] (L.M.).

that NaP1 was an intermediate phase in the process of transformation from kanemite to β-zeolite. Although SiO2/Al2O3 molar ratios in the product indicated high concentrations of aluminum in the solid product, pure β-zeolites were obtained only in high silica gels. Larger quantities of aluminum in the gels led to the formation of mordenite as a contaminating phase. Ferrierite was prepared by the recrystallization of magadiite in the presence of piperidine6,7,12 by a method that did not require the addition of water and was therefore called the dry method. Unfortunately, cation exchange capacity measurements indicated that part of the aluminum present in magadiite was not transformed into an ion exchange site on the ferrierite framework, although the analysis of bulk aluminum showed the presence of the totality of this element. Metal substituted zeolites can also be prepared from this method. Manganese exchanged magadiite was recrystallized into a tetrapropylammonium hydroxide solution into Mn-MFI metallosilicate. Scanning electron microscopy (SEM) showed that the samples were composed of very homogeneous elongated prismatic crystals, 20-30 µm in length, typical of ZSM-5.11 Since the discovery of the microporous aluminophosphate16 (ALPO) and silicoaluminophosphate17 (SAPO) molecular sieves, they were, as zeolites, synthesized hydrothermally from aqueous

10.1021/jp0677694 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/26/2007

Silicoaluminophosphate Molecular Sieve CAL-1 reactive mixtures of sources of aluminum, phosphorus, silicon when necessary, and an organic amine or quaternary ammonium ions as SDAs. Layered materials were suspected to be precursors in the synthesis of SAPO and ALPO structures such as CoSAPO-44 and CoAPO-44.18 Several aluminophosphate molecular sieves, ALPO-5, ALPO-22, ALPO-16, and SAPO-35, are formed via a common lamellar aluminophosphate material, ALPO4-L, during hydrothermal synthesis from a gel containing Al2O3, P2O5, SiO2, and hexamethyleneimine (HMI) in aqueous media or in ethylene glycol.19 The intermediate, ALPO4-L or SAPO4L, is a lamellar aluminophosphate or silicoaluminophosphate intercalating HMI and water and is stable up to 250 °C. SAPO4L, the precursor of SAPO-35, is stable up to 350 °C. Both are believed to be formed by the rearrangement of 3HMIH+Al3P4O16-3 sheets formed in the reaction medium. In the initial research on the chabazite (CHA) aluminophosphates templated by morpholine, lamellar phases or prephases were also invoked; however, a thorough examination of the SAPO-34 crystallization was only made by Vistad et al. from 1999 to 2003.20-22 In the first study, using powder X-ray diffraction (XRD), it was demonstrated that the presence or absence of HF causes significant spectral changes, even at room temperature. The addition of H3PO4, HF, or both to the source of aluminum showed that there is no direct interaction between fluoride and phosphate but that both reacted with the aluminum in acidic environments. Hydrothermal treatment of a gel containing fluoride, at 185 °C,21 led to the formation of a layered crystalline precursor phase, called the pre-phase, that was obtained after only 1 h, still during the heating of the autoclave, at 170 °C. After 4 h, only the pure triclinic SAPO-34 phase was present with no important changes at longer times. It was also discovered that the pre-phase could be synthesized separately. Its structure consists of AlPO4F layers separated by a double layer of morpholine molecules. Starting from the pre-phase, with addition of water, triclinic ALPO4-34 was obtained at 180 °C in 24 h, showing that the pre-phase already contained the elements necessary for the preparation of this molecular sieve. If the formation of the pre-phase was experimentally avoided by, for instance, rapid heating to the final temperature for the synthesis of SAPO-34, then the formation of SAPO-5 was observed. To describe the transformation of the pre-phase into the final CHA structure, the dissolution of the pre-phase followed by nucleation and growth of another intermediate phase was proposed. The solid-solid transition was considered less probable because of the large differences between the pre-phase itself and the chabazite topology for the triclinic SAPO-34. The final study in the series22 dealt with the in situ NMR examination of the SAPO-34 crystallization in the presence and absence of fluoride ions. The proposed mechanisms involve only 4R-type structures because they display the right charge density to match one of the morpholinium cations; larger rings or double rings would have a lower charge density. Silicon incorporation is proposed to occur on 4R-type units right in the first step from the gel transformation into a CHA structure, by substitution of aluminum or phosphorus, meaning a layered silicoaluminophosphate phase that reorganizes and condenses. The literature, thus, shows that zeolites can be prepared from reactants or from layered hydrated silicates and that aluminophosphates and silicoaluminophosphates are sometimes synthesized via layered intermediates. The question that remained unanswered concerned the possibility of preparing molecular

J. Phys. Chem. C, Vol. 111, No. 7, 2007 3117 sieves, ALPO and SAPO, from layered phosphates with structures analogous to the hydrated silicates. It would be of interest, as well, to determine the product’s physical and chemical properties altered by the change in the synthesis procedure. It is highly probable that the major problem in studying these aspects was the lack of good candidates as layered reactants, that is, layered aluminophosphates and silicoaluminophosphates with structures analogous to lamellar hydrated silicates. Cheng et al.23 reported the synthesis and a thorough characterization of the layered aluminophosphate ALPO-ntu, which has a kanemite-like structure. The material displayed the formulas AlPO2(OH)2[NH2(CH2)xCH3], x ) 3, 5, and 7, and was templated by these amines. The P and Al atoms show alternation and are all involved in groups of the type (O3Al)3PO--ammonium ion and (O3P)3Al-OH, indicating that the layers are very thin. As a matter of fact, they are represented as being composed of layers of connected 6MR only. We showed recently24 that ALPO-ntu, herein called ALPOkanemite, can be transformed into CAL-1 (CAL ) CampinasAlessandria), a CHA-type silicoaluminophosphate by the reaction with silica in the presence of HMI . In this work, we explore the best experimental conditions for the preparation of CAL-1 with different SiO2/Al2O3 molar ratios ranging from 0.4 to 1.6 and explore the characteristics of these materials. Experimental Section ALPO-Kanemite. The synthesis of ALPO-kanemite was performed according to the procedure already in the literature23 with slight modifications. In a polypropylene beaker, a suspension of 23 mL of water and 15.74 g of the pseudo-bohemite (Catapal-B, 72% Al2O3) was mechanically stirred until homogeneous. To this suspension was added dropwise 15.0 mL of phosphoric acid (Merck, 85%) which was followed by 30 mL of water. After 2 h of stirring, 22.4 mL of n-butylamine (n-BA, Riedel, 99%) was added dropwise. The molar composition of the gel was 1.0:1.0:2:30 Al2O3/P2O5/n-BA/H2O. The mixture was stirred for 2 h more and then loaded into Teflon lined stainless steel autoclaves and heated at 473 K for 48 h. The resultant solid was thoroughly washed with distilled water and dried at ambient temperature. CAL-1. In a one-neck round-bottomed flask containing 11.4 mL of water, 5.0 g of the as-synthesized ALPO-kanemite [AlPO3(OH)2(C4H9NH2)] was slowly added. After 3 h of stirring, 0.305 g of SiO2 (Aerosil 200, Degussa) was slowly added. The mixture was stirred for another 30 min after which 2.2 mL of HMI (Aldrich, 99%) was added dropwise. The molar composition of the gel was 2:x:y:z ALPO-kanemite/SiO2/HMI/ H2O; x, y, and z were varied to give the SiO2/Al2O3, HMI/Al2O3, and H2O/Al2O3 molar ratios displayed in Tables 1, 2, and 3. The final mixture was aged at room temperature for 48 h or else loaded after 30 min of homogenization into the autoclave and hydrothermally treated at 473 K for 24, 36, or 48 h. After that, the autoclaves were cooled under tap water, and the solid material was filtered, washed with copious amounts of water until the pH was close to neutral, and finally dried at ambient temperature. SAPO-34. The synthesis of SAPO-34 was made by employing the optimized synthesis procedure described in the literature25 using aluminum hydroxide hydrate (Aldrich, 55% A12O3), phosphoric acid (Aldrich, 85% H3PO4), and Aerosil 200 (Degussa) as aluminum, phosphorus, and silicon sources, respectively. The SAPO-34 sample was synthesized by mixing appropriate amounts of Al(OH)3, orthophosphoric acid, and

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TABLE 1: Synthesis Conditions for Preparation of Samples and Crystallinity of Productsa

sample

SiO2/Al2O3 (gel)

tTH, h

product

36B 38 37 43 41 39 58 42 40 44 49 45

0.4 0.4 0.4 0.8 0.8 0.8 1.2 1.2 1.2 1.6 1.6 1.6

48 36 24 48 36 24 48 36 24 48 36 24

CAL-1 + LEV CAL-1 + ALPO-kanemite CAL-1 + ALPO-kanemite CAL-1 CAL-1 CAL-1 CAL-1 CAL-1 + ALPO-kanemite (traces) CAL-1 CAL-1 ALPO-kanemite + CAL-1 (traces) ALPO-kanemite

% Cb

SiO2/Al2O3 (χSi) solid

100 76 100 67

0.73(0.18) 0.58(0.16 0.90(0.21)

93 59

1.06(0.24) 1.82(0.37)

a Hydrothermal treatment was made at T ) 190 °C, without aging of the gel, with dilution of 100, and HMI/Al2O3 ) 1.5. b % C = percentage crystallinity.

TABLE 2: Synthesis Conditions for Preparation of Samples with 48 h of Gel Aging and Phases Obtaineda sample SiO2/Al2O3 H2O/Al2O3 33 32 34 36A

1.6 1.6 1.6 0.4

25 50 100 100

products CAL-1 + ALPO-kanemite + LEV CAL-1 + ALPO-kanemite CAL-1 + ALPO-kanemite LEV (major) + CAL-1

a Hydrothermal treatment was made at T ) 190 °C for 48 h and HMI/Al2O3 ) 1.5.

TABLE 3: Synthesis Conditions, Product Identity, and Crystallinity for Samples Prepared at H2O/Al2O3 ) 100 with 24 h Hydrothermal Treatment at T ) 190 °C and without Gel Aging sample SiO2/Al2O3 HMI/Al2O3 52 37 51 50 55 39 54 53 59 40 60 56 48 45 47 46

0.4 0.4 0.4 0.4 0.8 0.8 0.8 0.8 1.2 1.2 1.2 1.2 1.6 1.6 1.6 1.6

2.0 1.5 1.0 0.5 2.0 1.5 1.0 0.5 2.0 1.5 1.0 0.5 2.0 1.5 1.0 0.5

products CAL-1 + ALPO-kanemite CAL-1 + ALPO-kanemite CAL-1 + ALPO-kanemite CAL-1 + ALPO-kanemite CAL-1 + ALPO-kanemite CAL-1 CAL-1 + ALPO-kanemite CAL-1 + ALPO-kanemite CAL-1 CAL-1 CAL-1 CAL-1 ALPO-kanemite + CAL-1 ALPO-kanemite ALPO-kanemite CAL-1 + ALPO-kanemite

C, %

840A. Samples were covered with a carbon conductive film by a Balzer metallizer, model MED 020. NMR in the solid state was performed in a Bruker AC 300/P for 29Si and 13C under cross-polarization (CP). Element concentrations were determined by solubilizing the samples by oxidizing alkaline fusion with lithium metaborate. The resulting material was dissolved, and the elements were determined by atomic absorption with a Varian A-5 spectrophotometer, using a nitrous oxide/acetylene flame. Argon adsorption isotherms were measured in the pressure range of 5 × 10-6 to 760 torr using an Autosorb-1MP (Quantachrome Instruments). Prior to the adsorption, the samples were outgassed for 1 h at 353 K, 2 h at 393 K, and finally 17 h at 623 K under high vacuum (final pressure 1 × 10-9 torr). Specific surface areas were determined by the Brunauer-Emmett-Teller approach and using 0.01 as the value of maximum relative pressure. Results and Discussion

100 66 93 84 66

distilled water and by homogenizing until the point of obtaining a uniform gel (∼3 h). SiO2 and morpholine were added in order to obtain the final gel with the composition 0.25:1:0.9:1.25:30 SiO2/A1(OH)3/H3PO4/morpholine/H2O. After vigorous stirring for 2 h, the resulting gel was moved into a Teflon lined stainless steel autoclave and heated at 463 K for 7 days. After that, the sample was isolated by filtering and was washed with plenty of water. HSAPO-34. HSAPO-34 was prepared by heating both the as-synthesized CAL-1 and the as-synthesized SAPO-34 slowly to 873 K under argon and by maintaining this temperature for 6 h under dry O2 for the removal of the organic template. Powder X-ray diffractograms were recorded in a Shimadzu XRD 6000 diffractometer with Cu KR radiation, 40 kV, 30 mA, at a rate of 2° 2θ min-1. The thermogravimetry was performed under argon or air (100 mL min-1) on a DuPont 2000 thermal analyzer, from 303 to 1273 K, at a heating rate of 10 °C min-1. SEM images were obtained in a JEOL microscope, model JVA

Synthesis of CAL-1. Table 1 shows the details of synthesis conditions, the final crystallinity, the SiO2/Al2O3 molar ratios, and the silicon molar fraction of the products obtained in reactions where CAL-1 was the sole product. Figure 1 displays the X-ray diffractograms of these samples: section A for SiO2/Al2O3 ) 0.4, section B for SiO2/Al2O3 ) 0.8, section C for SiO2/Al2O3 ) 1.2, and section D for SiO2/ Al2O3 ) 1.6. The samples were hydrothermally crystallized for 48 h (curves a), 36 h (curves b), and 24 h (curves c). The samples prepared with the smallest concentrations of silicon are contaminated either with ALPO-kanemite as indicated by the presence of the signal around 5° 2θ (Figure 1A, curves b and c) or with a levyne-type (LEV) silicoaluminophosphate (Figure 1A, curve a). The largest concentrations of silicon in the gel also produced CAL-1 after 36 h (Figure 1A, curve b) and 48 h (Figure 1A, curve a), the first one largely contaminated with reactant, the last one with a very low crystallinity. After only 24 h of reaction, the ALPO-kanemite is still the major phase (Figure 1A, curve c). When SiO2/Al2O3 ) 0.8 or 1.2 (Figure 1B,C), pure CAL-1 is formed in 24 h with the highest crystallinity and remains the only phase observed even after 48 h which indicates that this molecular sieve is stable in the reaction mixture for longer times. Thus, at HMI/Al2O3 ) 1.5, at a dilution of 100, and without gel aging, 24 h of thermal treatment is enough to produce pure phase CAL-1 with SiO2/Al2O3 molar ratios of 0.8 and 1.2. Elemental analyses have shown that the gel SiO2/Al2O3 molar ratio is reflected in the final solid (Table 1). The same reaction conditions with larger and smaller concentrations of silicon,

Silicoaluminophosphate Molecular Sieve CAL-1

J. Phys. Chem. C, Vol. 111, No. 7, 2007 3119

Figure 1. X-ray diffractograms of CAL-1 samples prepared at T ) 190 °C, without aging of the gel, with a dilution of 100, and HMI/Al2O3 ) 1.5 at SiO2/Al2O3 of (A) 0.4, (B) 0.8, (C) 1.2, and (D) 1.6 at different durations of hydrothermal crystallization, (a) 48 h, (b) 36 h, and (c) 24 h.

Figure 2. X-ray diffractograms of CAL-1 samples prepared (A) with 48 h of aging time at T ) 190 °C for 48 h of hydrothermal crystallization and HMI/Al2O3 ) 1.5 with SiO2/Al2O3 of 1.6 and H2O/ Al2O3 of (a) 25, (b) 50, and (c) 100 and (B) at T ) 190 °C for 48 h of hydrothermal crystallization and HMI/Al2O3 ) 1.5 (a) without an aging period and (b) with 48 h aging time.

SiO2/Al2O3 of 0.4 and 1.6, do not afford pure CAL-1 but a solid contaminated with the reactant or a LEV phase. In order to try to prepare samples with lower and higher silicon concentrations, a gel aging step and a variation in the dilution were introduced. The description of these reaction conditions are in Table 2. Figure 2 shows, in section A, the products of the reaction with SiO2/Al2O3 ) 1.6 and HMI/Al2O3 ) 1.5 with a gel aging time of 48 h before the hydrothermal treatment with an increasing dilution from curve a to curve c. Section B compares samples prepared at SiO2/Al2O3 of 0.4 and dilution of 100 and aged for 48 h (curve a) or non-aged (curve b).

Figure 2 section A shows that lower dilutions and aging increase the presence of contaminants, either LEV phase or ALPO-kanemite. The comparison of curve c of Figure 2A and curve a of Figure 1D shows that the aging step causes the appearance of a larger amount of amorphous material. The aging step at lower SiO2/Al2O3 molar ratios, Figure 2B, curves a and b, did not afford purer phase samples of CAL-1. As a matter of fact, the comparison of samples 36B and 36A shows that aging increased the contamination, as CAL-1 is the minor phase in the aged product. Since these two changes, dilution and aging, were not capable of producing samples in SiO2/Al2O3 of 0.4 and 1.6, the concentration of HMI in the gel was examined. Keeping in mind that for samples with SiO2/Al2O3 molar ratios of 0.8 and 1.2 CAL-1 was produced after 24 h of hydrothermal treatment (Figure 1B,C, curves c), we performed a study of the influence of the SDA for 24-h periods of hydrothermal crystallization. Table 3 shows the reaction conditions, the SiO2/Al2O3 molar ratio, and the crystallinities of the samples. Figure 3 displays the X-ray diffractograms of samples with SiO2/Al2O3 ) 0.4 (section A), 0.8 (section B), 1.2 (section C), and 1.6 (section D) in HMI/Al2O3 molar ratios of 2.0 (curves a), 1.5 (curves b), 1.0 (curves c), and 0.5 (curves d). The results obtained are of a very straightforward interpretation. The changes introduced in the reaction eliminated completely the formation of LEV-type SAPO; the only extra-phase found in these studies is the nonreacted ALPO-kanemite. For SiO2/Al2O3 ) 0.4, Figure 3A, the lowest concentration used in this work, no concentration of HMI afforded pure CAL1. All samples prepared with this lower concentration of silicon were a mixture of CAL-1 and ALPO-kanemite. Increasing the silicon concentration to SiO2/Al2O3 ) 0.8 makes it difficult to completely consume the ALPO-kanemite when the concentration of HMI is low. The HMI/Al2O3 molar ratio of 1.5 is ideal for the preparation of CAL-1 in 24 h of hydrothermal treatment with SiO2/Al2O3 ) 0.8; as a matter of fact, this was the best sample prepared in this work. At larger


Pastore et al.

Figure 3. X-ray diffractograms of CAL-1 samples prepared at H2O/Al2O3 ) 100 with 24 h hydrothermal treatment at T ) 190 °C, without gel aging, and at SiO2/Al2O3 of (A) 0.4, (B) 0.8, (C) 1.2, and (D) 1.6 at different HMI/Al2O3 of (a) 2.0, (b) 1.5, (c) 1.0, and (d) 0.5.

and smaller concentrations of SDA, a non-negligible concentration of reactant remains unreacted. In fact, at HMI/Al2O3 ) 0.5, the ALPO-kanemite does not react at all; no characteristic peaks at ∼30° 2θ are observed in the diffractogram (Figure 3B, curve d). The sample with the highest crystallinity in this work was prepared with SiO2/Al2O3 ) 0.8, H2O/Al2O3 ) 100, and HMI/Al2O3 ) 1.5. This sample represents SiO2/Al2O3 ) 0.9 (χSi ) 0.21). For SiO2/Al2O3 ) 1.2, Figure 3C, 24 h of hydrothermal treatment is enough to produce samples that are pure CAL-1 with varying crystallinities; the second best one is at HMI/Al2O3 of 1.5, Figure 3C, curve b (compare curve b of Figure 3B, sample 39, 100% crystalline, with curve b of Figure 3C, sample 40, 93% crystalline). The elemental analysis indicated that the SiO2/Al2O3 molar ratio of this sample as measured by atomic absorption spectroscopy is 1.06 (χSi ) 0.24). At still larger silicon concentrations, Figure 3D, SiO2/Al2O3 ) 1.6, only the sample with the smallest HMI concentration produced CAL-1 and, even in that case, was contaminated with ALPO-kanemite. Larger concentrations of HMI leave the reactant unchanged. In view of these results and the ones displayed in Figure 1D, the conclusion is that 48 h is necessary to produce pure CAL-1 at HMI/Al2O3 ) 1.5, however, with low crystallinity. In summary, these studies have shown that the optimum reaction conditions to obtain CAL-1 depend on the concentration of silicon desired in the structure. The general condition for a broad range of silicon concentrations is dilution of 100 without gel aging. For SiO2/Al2O3 ) 0.4, no reaction conditions afforded pure samples; the reactant was not completely consumed. At SiO2/Al2O3 molar ratios of 0.8 and 1.2 and HMI/Al2O3 ) 1.5, a reasonably crystalline sample was obtained after 24 h. Unfortunately, for larger silicon concentrations, an optimum set

of reaction conditions could not be determined. The best sample with SiO2/Al2O3 ) 1.6 in the gel was obtained with HMI/Al2O3 ) 1.5 only after 48 h of reaction, and even then, the crystallinity of the sample thus prepared was poor. The question that remains unanswered is related to the reason why a defined combination of silicon and HMI concentrations is needed to obtain CAL-1 without contamination with ALPOkanemite. Structure of CAL-1. CAL-1 presents a powder XRD pattern which does not find close correspondence with any of the zeolite patterns reported in the International Zeolite Association collected structures. It was possible to index26 the peaks of the CAL-1 phase, the structure of which will be described elsewhere.27 It is worth recalling that the obtained cell parameters are consistent with those of a trigonal chabazitic structure with a slightly larger cell volume with respect to known SAPO-34 precursors.27 This increase is certainly due to the SDA (HMI) present in the CAL-1 cages, which is different from that employed in the SAPO-34 precursors reported in the literature. Morphology and Porosity Properties of CAL-1. Scanning electron micrographs further show that ALPO-kanemite is a material typically composed of lamellae, some aggregated, others randomly distributed (Figure 4A). The as-synthesized CAL-1, Figure 4B, and the SAPO-34 derived upon calcination can both be described as randomly distributed lamellae as well, although more exfoliated than ALPO-kanemite. The aggregates are smaller, and the layers are more separated when compared to ALPO-kanemite. Some rhombohedral particles, less than approximately 10% of the total solid material, can also be seen. Figure 4C,D (enlargement of Figure 4B) shows that these are made of pilling, Figure 4C, and condensing, Figure 4D, of lamellae. Arrows in Figure 4E indicate lines where layers are condensed. In fact, these particles are smaller than the aggregates



Figure 4. Scanning electron micrographs of (A) ALPO-kanemite, (B), (C), (D), and (E) as-synthesized CAL-1 in different magnifications (see text).

found in ALPO-kanemite (compare Figure 4A,B). The differences in morphology are reflected in the density of these materials, as measured by helium picnometry. In these measurements, the samples are not degassed; therefore, the pores are filled with clusters of water molecules strongly adsorbed to framework protons,28 and thus the apparent density obtained for these materials reflects essentially the morphology of the crystals. ALPO-kanemite presents a density of 1.673 ( 0.002 g cm-3, and SAPO-34 (kanemite) presents a density of 2.211 ( 0.002 g cm-3. Other layered-type materials present densities around the value of 2, for example, magadiite,29 2.178 ( 0.002 g cm-3, and MCM-22 precursor,30 2.201 ( 0.002 g cm-3. SAPO-34 prepared from the reactants by the conventional gel method presents a density of 3.170 ( 0.002 g cm-3. The different ranges where density values are found certainly place SAPO-34 (kanemite) among the layered molecular sieves like MCM-22 precursor, for which transport phenomena and diffusion rates are modulated by interparticle rather than intraparticle transport. Figure 5 shows the isotherms of argon adsorption of sample 39 (filled squares) and of the SAPO-34 sample prepared by the

Figure 5. Isotherms of argon adsorption at low temperatures. Dark filled circles for SAPO-34 traditional and red filled squares for SAPO34 prepared from CAL-1.

traditional method (filled circles), in a logarithmic scale for P/P0. The surface areas of both samples are very close, 730 m2 g-1


Pastore et al.

Figure 6. Barrett-Joyner-Halenda (BJH) profile of SAPO-34 prepared by the traditional procedure (open circles) and of SAPO-34 from the calcination of CAL-1 (closed circles).

for SAPO-34 prepared from ALPO-kanemite and 718 m2 g-1 for SAPO-34 synthesized by the traditional method from the reactants. Despite the similarity of their surface areas, the complete description of their porosity shows subtle yet relevant differences. The microporosity of both samples is essentially the same; the curves are almost superimposed up to a value of P/P0 ) 0.001. From this pressure to approximately P/P0 ) 0.2, CAL-1 shows a slightly larger Ar adsorption in relation to SAPO-34 from individual reactants. This indicates that the larger micropores (P/P0 ) 0.01-0.2) are present in higher concentration in SAPO-34 from the calcination of CAL-1 in relation to traditional SAPO-34. On the other hand, from P/P0 ) 0.2 to P/P0 ) 1.0, secondary mesopores, typical of crystal aggregates, appear. The difference between these two materials in this region of relative pressure is another reflection of different crystal morphologies, as already shown by SEM and by apparent density measurements. The slope of the Ar isotherm at high P/P0 for the traditional SAPO34 is higher than that of SAPO-34 from CAL-1: the amount of secondary mesopores or aggregation pores is larger in traditional SAPO-34 than in SAPO-34 from CAL-1. Besides the slope of the curves, their shape is also indicative of differences in these silicoaluminophosphates. Hysteresis is present in the Ar isotherm of traditional SAPO-34 indicating that the pores formed by crystal aggregation are ink-bottle-type pores. This is not the case in SAPO-34 from the calcination of CAL-1: a smaller amount of aggregation pores is present, and they do not present constrictions. The difference in porosity in the region of P/P0 > 0.1, that is, the larger microporosity and the aperture of pores due to crystal aggregation of SAPO-34 from CAL-1 in comparison to that of traditional SAPO-34, explains the much faster calcination of SAPO-34 from CAL-1 in relation to traditional SAPO-34: the former calcines in a few hours while the latter may take days to calcine completely. In that case, with the larger pores closed, they can only be emptied through the micropores. These conclusions are fully supported by the curves in Figure 6. The volume due to pores from 15 to approximately 30 Å is larger in SAPO-34 prepared from CAL-1 while the volume of Ar adsorbed in pores larger than 30 Å is much more significant in traditional SAPO-34. These differences seem to compensate by yielding essentially equal surface areas. Formation of Silicon Islands in CAL-1: The 29Si MagicAngle Spinning (MAS) NMR. It has been shown before that

Figure 7. 29Si MAS NMR with CP of (A) as-synthesized and (B) calcined samples prepared with (a) χSi ) 0.21 and HMI/SiO2 ) 1.88 (sample 39), (b) χSi ) 0.24 and HMI/SiO2 ) 1.25 (sample 40), and (c) χSi ) 0.37 and HMI/SiO2 ) 0.94 (sample 44).

TABLE 4: Chemical Shift (in parts per million) Observed in 29Si CP-MAS NMR solid sample HMI/SiO2 SiO2/Al2O3 39 40

1.88 1.25

0.90 1.06

44

0.94

1.82

59

1.70

0.73

a

χSi

Q0a

Q1b

Q2c

Q3d

Q4e

0.21 90.2 94.9 0.24 90.8 95.1

99.9 99.7 104.8 109.5 114.0 0.37 90.9 94.7 100.0 103.8 109.4 114.6 0.20 90.1 94.5 99.8 109.0

b

Si(OAl)4. Si(OSi)(OAl)3. c Si(OSi)2(OAl)2. d Si(OSi)3(OAl). e Si(OSi)4.

the formation of silicon islands is influenced by the nature of the template used31 at a constant silicon content as well as the nature of the structure formed.31,32 In the case of SAPO-34 templated by morpholine, it was discovered that the amount of Si incorporated as isolated sites before the onset of the silicon island formation was higher than that in SAPO-18 and SAPO5, mostly because of the fact that two morpholine molecules fit in one chabazite cavity thus allowing a larger concentration of silicon in the structure, forming charged sites. In the case of CAL-1, a similar behavior was observed. Figure 7 shows the 29Si CP-MAS NMR of samples prepared with increasing χSi and decreasing HMI/SiO2 molar ratios in the gel (Table 4). In spectrum a, χSi ) 0.21, HMI/SiO2 ) 1.88, and one large signal was observed at -90.2 ppm because of the Si(OAl)4

Silicoaluminophosphate Molecular Sieve CAL-1 groups or isolated Si as in Si(4Al,9P)33 accompanied by a very weak shoulder at -94.9 ppm assigned to Si(OSi)(OAl)3 indicating the presence of dimers. An incipient formation of species with resonances in the range -95 to -115 ppm can be inferred, although these signals are exceedingly weak and almost buried in the instrumental noise. When the silicon content is increased to χSi ) 0.24 and HMI/SiO2 ) 1.25 (Figure 7A, curve b), the appearance of several signals in the range -95 to -114 ppm is clearer, as the related species are more abundant than in the sample with χSi ) 0.21. These are assigned to Si(OSi)(OAl)3 (-95.1 ppm), Si(OSi)2(OAl)2 (-99.7 ppm), Si(OSi)3(OAl) (-104.8 ppm), and Si(OSi)4 (-109.5 and -114.0 ppm). The signals become even more intense when the silicon concentration is increased to χSi ) 0.37 and HMI/SiO2 ) 0.94 (Figure 7A, curve c). In that case, contrary to what was reported in the literature,33 all signals are increased, not only the ones corresponding to Si(OSi)4 and Si(OSi)(OAl)3. This effect shows that the mechanism of silicon island formation is different when the material is prepared from the reactants in a gel synthesis or from a layered precursor, like ALPO-kanemite. It was observed that when the silicon concentration increases in samples prepared from the gel, the groups that increase the most are Si(OSi)4 and Si(OSi)(OAl)3, meaning that the growth of the island occurs homogeneously when they are in islands that have been nucleated at the beginning of the reaction.34,35 In CAL-1 however, as the amount of silicon increases, other than an increase in the silicon islands that already exist, it is possible that the nucleation of new islands occurs to a larger extent, as seen in Figure 7A, from curve a to curve c, as signals corresponding to Si(OSi)(OAl)3, Si(OSi)2(OAl)2, and Si(OSi)3(OAl) increase even more than that corresponding to Si(OSi)4. Another possibility is that on the increase of the silicon concentration in the gel, aluminum centers are trapped inside the islands, making their distribution more heterogeneous and increasing the concentration of groups containing one and two aluminum centers. Calcination of the samples (Figure 7, section B) makes the main signal move to around -88 ppm and widens it considerably. Although the chemical shift is in agreement with previous studies33-35 on this structure, the width of the signal indicates that silicon islands became even larger and more heterogeneous. To discover if the major factor in the formation of the silicon islands was the larger amount of silicon itself or the diminished concentration of HMI, another experiment was conducted where the solid was prepared with χSi ) 0.24, as in sample 40 (Figure 7A, curve b), but with HMI/SiO2 ) 1.70, very close to the molar ratio of the best sample, 39 (Figure 7A, curve a). The result is shown in Figure 8. It is clear that in the sample prepared in that way, the first peak corresponding to Si(OAl)4 is sharper than the same signal in sample 40 (compare Figure 8, curve a, and Figure 7A, curve b), but the spectroscopic features related to silicon islands of sample 59 (Figure 8, curve a) are more evident than the ones in sample 39 (Figure 7A, curve a); peaks at -99.9 ppm and at -95.0 ppm also seem more intense. When calcined (Figure 8b) sample 59 presents a profile much more like that of calcined sample 40 (Figure 7B, curve b), to which it is similar in silicon molar fraction, than like that of sample 39 (Figure 7B, curve a), to which it resembles the HMI/SiO2 molar ratio. Therefore, the conclusion is that if the amount of silicon is larger than χSi ) 0.21, then increasing the concentration of the SDA does not help in preventing silicon island formation.


Figure 8. 29Si MAS NMR with CP of (a) as-synthesized and (b) calcined samples prepared with χSi ) 0.20 and HMI/SiO2 ) 1.40 (sample 59 and its calcined form).

Figure 9 shows the 29Si CP-MAS NMR of samples 39, 41, and 43 prepared with SiO2/Al2O3 ) 0.8, HMI/Al2O3 ) 1.5, dilution of 100, and during different durations of hydrothermal crystallization. From curve a (24 h) to curve b (36 h), the signals corresponding to Si(OSi)4 (∼ -109 ppm) become clearer and do not change when the hydrothermal treatment is continued, as shown in curve c (48 h), showing that silicon island growth does not occur at this temperature and in this time frame. This indicates that, although the formation of silicon islands is favored by entropy,33 this effect is not seen in the reaction conditions used in this study. 27Al and 31P MAS NMR. Figure 10 displays the 27Al MAS NMR (A) and the 31P MAS NMR (B) of the as-synthesized sample 39, pure and crystalline CAL-1, and sample 37, CAL-1 still contaminated with reactant ALPO-kanemite. The 27Al MAS NMR spectra show a signal at 34.5 ppm assigned before to aluminum in a tetrahedral environment in aluminophosphates while the signal at 3.3 ppm indicates the presence of octahedral aluminum either bound to water or template molecules.36,37 Both sites are present either in samples that are pure CAL-1 (Figure 10A, curve a) or in samples where ALPO-kanemite is still present (Figure 10A, curve b), indicating that, for the quadrupolar nucleus, the structural change from ALPO-kanemite to CAL-1 does not significantly change its immediate environment or that the change is not clearly exposed by this technique. The 31P MAS NMR spectra of the same samples display the presence of one strong signal at -29.8 ppm and two very weak ones at -16.9 and -20.5 ppm. The first signal was attributed to the phosphorus nucleus in sites such as P(OAl)4 while the other weak peaks were assigned to P-OH groups on the surface of the crystals. The fact that sample 37 (Figure 10B, curve b) is still contaminated with ALPO-kanemite is even more evident by the intensity of the P-OH signals in the surface of the layered reactant. Status of the Organic SDAs in CAL-1: Thermogravimetry, Infrared Spectroscopy, and 13C MAS NMR. The role and situation of the organic molecules in the as-prepared CAL-1 were evaluated by three key techniques: thermogravimetry, infrared spectroscopy, and the nuclear magnetic resonance of 13C. Figure 11 displays the thermogravimetric analysis (TGA) curves (A) and the derivative thermogravimetry (DTG) ones (B) of ALPO-kanemite (curve a, thin solid line), sample 39 (curve b, thick solid line at the top of section A), sample 40 (curve c, dashed line), and sample 44 (curve d, dash-dotted


Pastore et al.

Figure 9. 29Si MAS NMR with CP of as-synthesized samples prepared at T ) 190 °C, without aging of the gel, with a dilution of 100, HMI/Al2O3 ) 1.5, with SiO2/Al2O3 ) 0.8, and different times of hydrothermal crystallization (a) 48 h (sample 43), (b) 36 h (sample 41), and (c) 24 h (sample 39).

TABLE 5: Temperature Ranges and Percentage Mass Loss in the Thermogravimetry %mass sample

T, °C 25-100

T, °C 200-350

T, °C 350-550

T, °C 550-1000

total

39 40 44 ALPO

1.3 2.2 3.8 2.8

a 1.1 3.9 37.6

10.2 9.8 8.7 a

5.7 6.6 5.6 a

17.2 29.7 22.0 40.4

a 27Al

Nonexistent.

31P

Figure 10. (A) MAS NMR and (B) MAS NMR of samples of (a) pure phase CAL-1 (sample 39) and (b) CAL-1 and ALPOkanemite (sample 37).

Figure 11. Thermogravimetry (A) and DTG (B) under inert atmosphere of samples (a) 39, (b) 40, (c) 44, and (d) ALPO-kanemite.

line). In the DTG, the ALPO-kanemite sample was multiplied by 0.4 to adjust to approximately the same intensity as the CAL-1 samples.

In all of the samples studied, water is released at temperatures below 100 °C under argon (Figure 11B, Table 5, second column). The temperature at which water is released by these samples increases with the crystallinity of samples and with the decrease in Si concentration. This is an indication that water is also held in noncrystalline regions of the CAL-1. The ALPO-kanemite shows one main mass loss, in the range of 200-350 °C, peaking at 259 °C, due to the thermal decomposition of n-butylammonium cation. There is a slight tailing in the high-temperature side of the signal that goes no further than 500 °C. The CAL-1 samples behave substantially differently from ALPO-kanemite. CAL-1 samples also show a little mass loss at the same position where n-butylammonium was found in ALPO-kanemite; the mass involved in this loss increases as the crystallinity of the sample diminishes and as the silicon concentration in the gel increases. It is also important to remember that the HMI/SiO2 molar ratio in the synthesis is 1.88 for sample 39, 1.25 for sample 40, and 0.94 for sample 44 (data in Table 4); therefore, one could conclude that n-butylammonium remains in the sample when the HMI concentration drops. However, this is not the case as will be shown by 13C MAS NMR results. The most probable reason for finding n-butylammonium decomposing at the same temperature as in ALPOkanemite is that, in less crystalline materials, there are areas

Silicoaluminophosphate Molecular Sieve CAL-1 (e.g., surface sites) where n-butylammonium plays the same role as it does in ALPO-kanemite, that is, to counterbalance P-Ogroups. The largest part of mass loss in the CAL-1 samples occurs around 450 °C. This signal is probably composed of the loss of fragments coming from hexamethyleneimmonium but, as suggested by 13C MAS NMR, also from n-butylammonium counterbalancing charges created by the silicon incorporation into the ALPO framework. This will be discussed in more detail in the part concerning NMR and thermal decomposition of organics by infrared spectroscopy. The heavier organic residue, for example, carbonaceous (coke) compounds formed during the thermal decomposition of the entrapped n-butylammonium and HMI, is decomposed at higher temperatures. In fact, the presence of silicon and the presence of a three-dimensional structure both cause the appearance of signals in temperatures higher than 550 °C, opposed to what was observed for ALPO-kanemite. Two signals were found for the CAL-1 samples: one at around 680-730 °C and the other at 850-900 °C. Sample 39 presented a higher temperature mass loss at essentially the last region, meaning that carbonaceous residues (coke) produced on samples with a smaller amount of silicon, higher crystallinity, and higher amounts of HMI are more condensed than in samples with larger amounts of silicon, where both n-butylammonium and hexamethyleneimmonium are present. This effect can be assigned to the different acidity and/or number of acid sites on these samples (see later). The disappearance of the mass loss at around 260 °C corresponding to the n-butylammonium with the simultaneous appearance of another one in the range 350-550 °C due to hexamethyleneiminium was shown by us before25 and was assigned to a possible ion exchange between the n-butylammonium cations between the layers of ALPO-kanemite with hexamethyleneiminium, the added SDA from solution. In that case, the temperature of HMI decomposition would be very close to the temperature observed for MCM-22 zeolite, for which HMI is the SDA.29 Table 5 shows that the total organics loss is smaller in the CAL-1 sample than it is in the reactant ALPO-kanemite. This is probably due to the fact that in the ALPO-kanemite each P-O- group is counterbalanced by a n-butylammonium cation,24 while when the layers condense, these groups bind to AlOH to form P-O-Al bonds that make the CAL-1 structure; in addition, n-butylammonium and hexamethyleneiminium counterbalance the charge, Si-O(-)-Al, created by the introduction of silicon into the framework of ALPO. Si-O(-)-Al groups in CAL-1 are in smaller concentration than P-O- ones in ALPO-kanemite. Figure 12 shows infrared spectra in the C-H stretching region of samples in KBr pellets (0.5 wt % concentration). Curve a of Figure 12 displays the spectrum of ALPO-kanemite. Bands assigned to antisymmetric and symmetric C-H stretching of CH2 in the middle of the chain are observed at 2934 and 2865 cm-1, respectively, while for the CH3 group, the antisymmetric C-H stretching appears at 2963 cm-1, and the symmetric one appears at 2875 cm-1. The intensity of the bands assigned to CH3 groups (2963 and 2875 cm-1), however, is higher than that of the CH2 groups (2934 and 2865 cm-1), and this does not relate to the fact that the number of CH2 groups is three times higher than CH3. This apparent contradiction can be explained considering that the stretching frequency of CH2 groups bound to the NH3+ groups is higher than that of the


Figure 12. Infrared spectra of as-synthesized samples of (a) ALPOkanemite, (b) MCM-49, (c) 39, (d) 40, and (e) 44.

CH2 in the middle of the butyl chain, and their absorptions overlap those of CH3 groups. The spectrum in curve b was obtained from a MCM-49 sample, a structure that is prepared with HMI but presents supercages in the as-synthesized form.30 This sample is used here as a model for hexamethyleneiminium in a confined environment, a closed cavity. This is characterized by the inphase and out-of-phase vibrations of the methylene groups at respectively 2938 and 2858 cm-1,38 the ones bound to the NH2+ group of the hexamethyleneiminium absorbing at higher wavenumbers (a shoulder at around 2970 cm-1 and a band at around 2875 cm-1 are in fact observed). The comparison of the spectra of HMIH+ in MCM-22 precursor (not shown), a lamellar material, with the assynthesized MCM-49 showed that, upon going from a layered material to a three-dimensional one, the in-phase vibration of the H atom on the CH2 group is displaced by 11 cm-1 to higher wavenumbers and the out-of-phase H vibration is displaced by 15 cm-1. Therefore, the confinement of HMI into cavities caused the shift of the CH vibrations of CH2 groups to higher frequencies. On the basis of these observations, the transformation of ALPO-kanemite from the layered material to another one containing cavities, where n-butylammonium would be confined, would as well make the C-H vibrations of n-butylammonium cation shift to higher wavenumbers. Keeping the above effect in mind, one can envision the spectra in curves c, d, and e of Figure 12 as weighted sums of ALPO-kanemite (curve a) displaced to higher wavenumbers and MCM-49 (curve b). Thus, the band at 2971 cm-1 corresponds mainly to CH2 groups bound to NH3+ and NH2+ of nbutylammonium and HMIH+, respectively, and, to a minor fraction, to CH3 groups of n-butylammonium. The band at 2945 cm-1 corresponds to CH2 groups which are more distant from NH3+ and NH2+ groups of n-butylammonium and in the hexamethyleneiminium cation. The presence of two bands due to the symmetric C-H stretching of CH2 groups in hexamethyleneiminium at around 2880-2860 cm-1 is less straightforward. In this case, although the effect described above is still present, the displacement of the band corresponding to CH2

3126 J. Phys. Chem. C, Vol. 111, No. 7, 2007 groups in hexamethyleneiminium seems to be more important than the one for the CH2 in n-butylammonium, causing the bands to coalesce into one signal with tailing to the lower wavenumber side. The vibrations corresponding to the CH3 group of nbutylammonium are naturally covered by the bands at 2971 cm-1 and at 2860 cm-1 and are not clearly distinguishable. However, the presence of n-butylammonium in CAL-1 samples was more clearly monitored by the presence of the CH3 bending mode at around 1395 cm-1 (spectra not shown for sake of brevity). These observations led us to believe that a complete ion exchange of n-butylammonium by hexamethyleneiminiun was not necessary and also that both molecules might work together to direct the crystallization of CAL-1. To examine the samples in more detail, the 13C MAS NMR was measured. Figure 13 shows the NMR spectra of as-synthesized ALPOkanemite (spectra a in sections A and B), samples 44, 60, 40, and 41 in section A, and samples 41, 39, 37, and 36B in section B. Table 6 indicates the positions of each 13C signal in the spectra. Peaks at approximately 48 ppm correspond to the carbon atom bound to the nitrogen atom in the HMI ring, and peaks at around 26 ppm correspond to the remaining carbon atoms of the ring. The signals at 42, 30, 20, and 13 ppm are assigned to the CH2 neighboring the N atom, the two central methylene groups, and the methyl group in the n-butylammonium carbon chain, by comparison with the n-butylammonium in the ALPO-kanemite. The samples displayed in Figure 13 all present the peaks corresponding to the carbon atoms in both molecules. As a matter of fact, the weighted ratio of the areas under the peaks at 48 ppm (HMI, two carbon atoms) and at 42 ppm (n-BA, one carbon atom) shows that the samples that were produced as the pure CAL-1 phase present a molar ratio of n-BA/HMI larger than one (last column of Table 6). If the amount of HMI relative to n-BA becomes too large (the ratio diminishes), then the samples are contaminated either with the reactant or with a LEV phase (HMI is a SDA of the SAPO-35 molecular sieve). It seems then that there is a compromise between the two SDAs: smaller or larger concentrations of HMI lead to the contamination of the product with the layered reactant. Smaller concentrations are deleterious because the best combination of n-BA and HMI is not formed in the necessary concentration and also probably because, at lower concentrations of HMI, the optimum pH for the synthesis is not attained. Larger concentrations of HMI also cause contamination of the product with the layered ALPOkanemite probably because the second SDA remains dissolved into the hydrophobic tails of n-BA between the layers. In this case, the interlamellar distance is larger, and condensation of one layer with the other through the bonding of a silicon atom becomes more difficult. Larger concentrations may also lead to the formation of SAPO-35 in the product, together with CAL1. In Situ Infrared Absorption Spectra: CAL-1 Acid Sites. Another indication of the mechanism by which CAL-1 is formed comes from an examination of the acidic sites in the solid: if the transformation of ALPO-kanemite into CAL-1 occurred by a long-range dissolution of the layered reactant and later crystallization into a chabazite-like structure, then acid sites with SAPO-34 characteristics (distribution and strength) should be expected. That was not observed; in fact, the acidity characteristics of CAL-1 samples are substantially different from those of the SAPO-34 prepared by traditional synthesis (vide infra). Figure 14 shows a fragment of the CHA structure where the crystallographically equivalent T positions (occupied by Si, Al,

Pastore et al.

Figure 13. 13C MAS NMR with CP of as-prepared samples. (A) ALPO-kanemite (a), 44 (b), 60 (c), 40 (d), and 41 (e). (B) ALPOkanemite (a), 43 (b), 39 (c), 37 (d), and 36B (e).

and P, in the case of SAPO materials) and four oxygens are highlighted adopting the labels proposed by Smith et al. for HSAPO-34.39 The HSAPO-34 structure presents three families of bridging OH groups [Si-O(H)-Al] having protons with different acidity. A combined neutron diffraction and Fourier transform IR (FTIR) study of CO adsorbed at 100 K showed that protons attached to O(4) absorb at 3630 cm-1 (species A) and are more acidic than those attached to O(2) that absorb at 3600 cm-1 (species C).38 A third family of acid sites absorbing at 3625 cm-1 (species B) was monitored by CO adsorption; however, their crystallographic position was not described.38 The downward wavenumber shifts (∆νOH) of the A, B, and C hydroxyls produced by CO adsorption, respectively 287, 349,



TABLE 6: Position of 13C CP-MAS NMR Signals for the Most Significant Samples sample

CR-HMIa

C1-BAb

C2-BAb

Cβ+γ-HMIa

C3-BAb

C4-BAb

BA/HMIc

ALPO 44 60 40 41 43 39 37 36B

48.88 48.61 47.63 49.24 47.89 48.09 48.99 49.46

39.58 42.94 42.82 41.84 43.50 42.15 42.30 43.20 43.20

30.58 30.25 30.33 29.68 31.10 29.63 29.40 30.87 31.00

26.89 26.64 25.76 27.40 26.01 26.17 26.89 27.11

20.52 20.72 20.64 19.75 21.49 20.01 20.12 21.06 21.39

14.34 14.08 14.12 13.28 14.80 13.45 13.61 14.76 14.80

1.98 1.53 1.47 1.44 1.44 1.18 0.92 0.54

a Cn-HMI ) carbon atoms in the HMI molecule, see Figure 13. b Cn-BA ) the atoms in the n-BA molecule, see Figure 10. c n-BA/HMI ) n-BA/HMI molar ratio in the final solid.

Figure 14. CHA fragment showing the four crystallographic oxygens of SAPO-34 and the only crystallographically equivalent T (Si, Al, P) atom.

and 190 cm-1, showed that the most acidic protons are those absorbing at 3625 cm-1. However, a shift of 349 cm-1 is too large a figure for isolated sites in silicoaluminophosphate structures, which are normally less acidic than sites in aluminosilicate with related structures.40 A recent FTIR study confirmed that protons of HSSZ-13 (a synthetic aluminosilicate zeolite with a CHA structure) located in similar crystallographic positions as those of HSAPO-34 show a larger downward shift by CO adsorption (316 vs 270 cm-1).41 A possible explanation for the controversial results found in the literature is that the very strong Brønsted acid sites of HSAPO-34, for example, those absorbing at 3625 cm-1, are located either at borders of silica islands or in Si-Al regions.40 This matter has been recently reviewed by us carrying out a careful FTIR study of CO adsorbed at 100 K on several HSAPO34 samples prepared by different synthetic routes which led to materials with variable relative concentrations of A, B, and C hydroxyls.42 For the purpose of the present work, it is sufficient to anticipate that the study mentioned above confirmed that three families of hydroxyls (OHA, OHB, and OHC) are present in HSAPO-34 and other chabazite-related SAPOs and that the B species display protons with the strongest acidity. The shift of the OH stretching of these hydroxyls produced by CO was in the range 310-330 cm-1, values very close to that found for the most acidic protons of HSSZ-13 zeolite. This result prompted us to assign the B species to hydroxyls located in silicon-rich regions. The hydroxyl’s bands of HSAPO-34 and of CAL-1 samples (Figure 15) can be resolved into three components by a curvefitting procedure, as displayed in Figure 15B. This study allowed estimating the relative concentration of the different hydroxyls (Table 7); however, a precaution has to be taken when the

Figure 15. Section A: FTIR in the hydroxyl region of (a) HSAPO34 derived from gel, (b-d) HSAPO-34 derived respectively from CAL-1 samples 39, 40, and 44. Section B: calculated spectra constituted by three components (OHA, OHB, and OHC) derived from a curvefitting procedure.

TABLE 7: Relative Concentration of OHA, OHB, and OHC Components Evaluated from Spectroscopic Parameters Derived from a Curve-Fitting Procedure of the Hydroxyl Bands of the CAL-1 Samples. Data of HSAPO-34 Derived from Gel Are Also Reported for Comparison. site SAPO-34 OHA νOH (cm-1) fwhm (cm-1) area‚10-3 (cm-1/g‚cm-2)a OH concn (%) OHB νOH (cm-1) fwhm (cm-1) area‚10-3 (cm-1/g‚cm-2)a OH concn (%) OHC νOH (cm-1) fwhm (cm-1) area‚10-3 (cm-1/g‚cm-2)a areacorr‚10-3 (cm-1/g‚cm-2)b OH concn (%) pellet density (g‚cm-2) χSi

39

40

44

3630 15.2 4.08

3629 23.5 4.02

3632 23.5 2.72

3632 23.4 2.90

47.4 3615 15,8 0.92

41.2 3610 26.0 1.67

41.0 3614 25.0 1.25

41.1 3612 26.7 1.36

10.7 3600 29.3 5.64

17.1 3593 47.0 6.78

18.9 3595 47.0 4.50

19.2 3591 47.9 4.92

3.61

4.07

2.66

2.80

41.9 3.6 × 0.14

41.7 10-3

5.5 × 0.21

40.1 10-3

3.2 × 0.24

39.7 10-3

5.0 × 10-3 0.37

a

Values obtained by the IR curve fitting and normalized for the pellet density for a direct comparison between the samples. b Values obtained by estimating the increment of the extinction coefficient according to ref 42 and assuming that these species are bound to lattice oxygens by H-bonding.

fraction of the C species is computed. These hydroxyls, in fact, should have a larger extinction coefficient because they interact with lattice oxygen atoms.39,41 For this reason, first, the increase


Pastore et al.

SCHEME 1

of the extinction coefficient of the C species has to be estimated assuming that they are H-bonded to the structure, and second, this effect has to be quantified using the approach proposed by Makarova et al.42,43 The first point to call attention to is the broadness of the OHrelated bands of CAL-1 in comparison with HSAPO-34. Even at 100 K, the signals corresponding to the individual sites are not resolved. This suggests that, instead of defined sites each of which presents homogeneous acidity all along the structure as in HSAPO-34, in CAL-1, a family of acid sites presenting varied acidity is formed in the synthesis. This is in agreement with the presence of Si-rich regions in CAL-1 samples as determined by 29Si MAS NMR (Figure 7A). It may be noticed, in fact, that the A, B, and C bands of CAL-1 are much larger than those of HSAPO-34 (compare the values of full width at half-maximum (fwhm) in Table 7). This is probably due to the fact that the attack of silicate in the ALPOkanemite may occur at different Al sites and that their development into a full chabazite-like structure proceeds probably by several different steps which lead to more heterogeneous (e.g., disordered) configurations of hydroxyls of CAL-1 in comparison to those of SAPO-34. As described above, the B species are hydroxyls related to the presence of Si islands. It is therefore evident that there is a drastic increase of Si islands on passing from HSAPO-34 (OHB ) 10.7%) to CAL-1 (OHB ) 17.1%) samples. There is a more gradual increase of the concentration of B species, from 17.1 to 19.2%, by increasing the silicon fraction along the series of CAL-1 samples (Table 1). It may be also noticed that the relative concentration of the A species is significantly affected on passing from HSAPO-34 to CAL-1 samples. This suggests that a fraction of A species might be involved in the formation of Si islands. Further evidence of the presence of Si-rich regions in CAL-1 is the fact that when the Si concentration increases by 24%, from the HSAPO-34 sample (χSi ) 0.14) to the CAL-1 sample (χSi ) 0.21), the overall intensity of the OH bands increases only approximately 15% (see Table 7). Considering that the Si/H+ ratio is approximately 1:1 in the case of the HSAPO-34

sample,44 it is evident that only a limited fraction of Si in CAL-1 generates OH groups. Finally, in agreement with 29Si MAS NMR results, it has to be noticed that the presence of either dimers or very small Si islands in CAL-1 with χSi ) 0.21 is sufficient to generate very strong Brønsted acid sites located at the OHB species. In fact, when the extension of Si islands increases significantly from the CAL-1 sample with χSi ) 0.21 to the one with χSi ) 0.35, the increases of OHB species are limited (around 12%). Formation of CAL-1. It seems that the formation of CAL-1 depends on the simultaneous action of n-butylammonium and hexamethyleneiminium cations as SDAs and silicon as a structure atom, possibly in a process such as the one displayed in Scheme 1. In composing this scheme, the structure of ALPOkanemite was taken as proposed by Cheng et al.,23 that is, a single layer of six-membered rings of alternating aluminum and phosphorus atoms. Each one of them has a bond toward the interlayer space; as proposed in the literature for the aluminum site, this is an OH bond, and for phosphorus, it is an O-R+ group. The possibility that the Al-OH groups also contribute to produce protonated amine groups cannot however be discarded. In any case, for the purpose of silicon insertion into the framework as shown in Scheme 1, the protonation of the Al site is not relevant. The first step in Scheme 1 is the attack of the silicate anion on the aluminum site in species I. The silicate anion was produced by the high pH in the reaction, brought about by the HMI. Either the aluminum site might transform into a pentacoordinated atom by the expansion of its coordination sphere as in II, or the six-membered ring might be opened by the reaction with silicon as in III. One P-O- group may attack the silicon atom forming, for instance, a four-membered ring that exists in the chabazitic structure of CAL-1 but not in the kanemite structure of ALPO. On the basis of the fact that no full dissolution was observed in the ALPO-kanemite during the course of the reaction by any of the physical techniques used to follow the process,24 this proposition implies that the transformation of ALPO-kanemite to CAL-1 occurs in the solid-solution interface by small range

Silicoaluminophosphate Molecular Sieve CAL-1 molecular rearrangements with the inclusion of silicon atoms in the newly formed framework. Conclusions The addition of HMI and silica to layered n-BA-ALPOkanemite produces CAL-1, a silicoaluminophosphate with a chabazite-type structure. This procedure allowed the formation of a less dense solid when compared to the one of SAPO-34 produced from the reactants in a traditional gel procedure. Besides that, other features of CAL-1 are different from the ones of traditional SAPO-34; the most clear ones are the different morphology of CAL-1 crystals and the particular acidity of HSAPO-34 produced from it. CAL-1 crystals keep the layered morphology of ALPOkanemite; an amount of about 10% of the sample is composed of 10-12 µm rhombohedra clearly made by layers stacking. Some rhombohedra also show lines and cracks on the surface that could indicate the sintering of layers. The process of CAL-1 formation is proposed to occur by a surface localized transformation based on silicate attack to the ALPO-kanemite layered structure. In that manner, the acid sites generated by the bonding of silica oligomers to the Al-OH sites are more heterogeneous in HSAPO-34 made from CAL-1 than the ones prepared from SAPO-34 from the gel as found by comparing the IR bands of the hydroxyls formed after calcination of the samples. A curve-fitting procedure of these bands showed that they are formed of three components, which have been assigned to the OHA, OHB, and OHC species already reported in literature. In HSAPO-34 derived from CAL-1 samples, the three components are much larger (show higher fwhm values) than in HSAPO-34 prepared traditionally, and this clearly indicates the presence of heterogeneities. A combined 29Si MAS NMR and FTIR study showed that the synthesis process proposed here also favors the formation of a larger concentration either of silicon islands/patches or of Si-rich regions where very strong acid sites are located. Acknowledgment. The authors are indebted to the Brazilian “Fundaç aõ de Amparo a` Pesquisa no Estado de Saõ Paulo” (FAPESP) and to the Italian MIUR “Ministero dell’Istruzione, dell’Universita` e della Ricerca” (in the frame of a FISR project, “Microcombustori ad Idrogeno”) for the financial support to this work. For the fellowships, E.C.O. acknowledges FAPESP, and G.B.S. is grateful to CAPES. The authors acknowledge Mr. Christian E. da Silva for the samples preparation, Prof. Jose´ S. Barone for the elemental analysis, and Dr. Kai Dallmann for the donation of pseudobohemite. Supporting Information Available: Intensity peaks for CAL-1, LEV, and ALPO-kanemite. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hogan, P. J. U.K. Patent Appl. 2,125,390A, 1983. (2) Zones, S. I. U.S. Patent 4,626,421, 1986. (3) Zones, S. I. U.S. Patent 4,676,958, 1987. (4) Zones, S. I. U.S. Patent 4,689,207, 1987. (5) Pal-Borbe´ly, G.; Beyer, H. K.; Kiyozumi, Y.; Mizukami, F. Microporous Mater. 1997, 11, 45. (6) Pal-Borbe´ly, G.; Beyer, H. K.; Kiyozumi, Y.; Mizukami, F. Microporous Mesoporous Mater. 1998, 22, 57. (7) Pa´l-Borbe´ly, G.; Beyer, H. K. Stud. Surf. Sci. Catal. 1999, 125, 383. (8) Araya, A.; Lowe, B. M. J. Chem. Res. 1985, 23, 192. (9) Salou, M.; Kiyozumi, Y.; Mizukami, F.; Nair, P.; Maeda, K.; Niwa, S. J. Mater. Chem. 1998, 8, 2125.

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Reaction at Interfaces: The Silicoaluminophosphate Molecular Sieve

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