Silicoaluminophosphate Molecular Sieves STA-7 and STA-14 and

J. Phys. Chem. C 2009, 113, 15731–15741

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Silicoaluminophosphate Molecular Sieves STA-7 and STA-14 and Their Structure-Dependent Catalytic Performance in the Conversion of Methanol to Olefins Maria Castro,† Stewart J. Warrender,† Paul A. Wright,*,† David C. Apperley,‡ Youssef Belmabkhout,§ Gerhard Pirngruber,§ Hyung-Ki Min,⊥ Min Bum Park,⊥ and Suk Bong Hong*,⊥ School of Chemistry, UniVersity of St. Andrews, Purdie Building, North Haugh, St. Andrews, KY16 9ST, United Kingdom, Department of Chemistry, Durham UniVersity, South Road, Durham, DH1 3LE, United Kingdom, IFP-Lyon, BP. 3, 69390, Vernaison, France, and Department of Chemical Engineering and School of EnVironmental Science and Engineering, POSTECH, Pohang 790-784, Korea ReceiVed: May 12, 2009; ReVised Manuscript ReceiVed: July 5, 2009

Details of the synthesis of the small-pore silicoaluminophosphate (SAPO) molecular sieves STA-7 (SAV) and STA-14 (KFI) prepared via a co-templating approach are described. STA-7 includes Si in the framework with Si/(Si+Al+P) ratios varying from 0.04 to 0.17, whereas STA-14 crystallizes from gels with a narrower compositional range (Si/(Si+Al+P) ) 0.2-0.3). The main route to Si incorporation is by substitution for P, but in the presence of fluoride, aluminosilicate regions are formed in STA-7. 27Al 3Q MAS NMR studies enable resolution of tetrahedral Al in Al(OP)4 and Al(OP3,OSi) environments in both as-prepared and calcined materials. For STA-7 with low Si content the presence of five- and six-fold Al coordinated with water molecules or fluoride or hydroxide ions has also been confirmed. Upon calcination, all Al adopts tetrahedral coordination. High pressure CO2, CO, and CH4 adsorption at 100 °C indicates that the proton forms of the SAPO D6R zeotypes present effective polarities intermediate between cationic zeolites and pure silica chabazite. The methanol-to-olefin (MTO) performance of the SAPOs H-SAPO-34, H-STA-7, and H-STA-14 was found to depend on both crystallite size and cage connectivity and topology. H-STA-7 with a crystal size of 2-3 µm has MTO stability comparable to that observed for H-SAPO-34, whereas the same materials with larger crystal sizes (g10 µm) deactivate rapidly. Ex situ GC-MS analyses of the SAPO catalysts after MTO reaction demonstrate that the uniformity in cage shape and size in cage-based, small-pore molecular sieves is a critical factor governing the type of the accumulated aromatic hydrocarbons and, hence, their MTO activity and deactivation behavior. 1. Introduction Aluminophosphate (AlPO4) molecular sieves and related microporous solids with tetrahedral frameworks similar to those of aluminosilicate zeolites were first discovered by Flanigen and co-workers.1,2 Of these, SAPO-34 (framework type CHA), a small pore silicoaluminophosphate (SAPO) material, is one of the most widely studied because of its promising performance as a selective acid catalyst in the methanol-to-olefin (MTO) conversion.3-6 The structure of SAPO-34, which is isotypic with the zeolite chabazite (CHA), is built up from double sixmembered rings (D6Rs) based on [Al6P6O30] units, sharing oxygen atoms with other D6Rs via four-membered rings (Figure 1S, Supporting Information). In the CHA framework, each D6R has the same orientation. The resulting framework contains one kind of cage (Figure 1), and the entire pore space is accessible three-dimensionally via eight-membered rings (Table 1).7,8 Synthesis in the presence of silica can lead to the substitution of P by Si in the framework, giving a negatively charged framework. Subsequent calcination to remove charged organic molecules occluded during synthesis results in the proton * To whom correspondence should be addressed. E-mail: paw2@ st-andrews.ac.uk (P.A.W.); [email protected] (S.B.H.). † University of St. Andrews. ‡ Durham University. § IFP-Lyon. ⊥ POSTECH.

form of the solid, with the formation of bridging hydroxyl groups, Si-OH-Al. A combination of the pore geometry and the Brønsted acidity associated with these bridging hydroxyls makes SAPO-34 an efficient catalyst for the conversion of methanol to light olefins (mainly ethylene and propylene). Mechanistic studies indicate that an active ‘hydrocarbon pool’ builds up within the cages of the solid, built up from alkylated ring compounds, particularly methylated benzenes, and that this is able to generate small hydrocarbons by rearrangement and scission steps.3-6,9-11 SAPO-34 is not the only SAPO molecular sieve built from D6Rs. SAPO-18 (AEI), for example, is similar, but instead of layers of D6Rs being related by translation, they are related by a mirror plane, giving a three-dimensionally connected small pore structure with one kind of cage (slightly different in shape to that of CHA). Like SAPO-34, SAPO-18 is a selective and active catalyst for the MTO reaction.12 In addition to SAPO materials with the CHA- and AEI-type structures, we have recently synthesized two other SAPO materials with structures built only from D6Rs, STA-7 (SAV), and STA-14 (KFI).13 The STA-7 structure has no aluminosilicate analogue, whereas STA14 has the same structure type as the zeolite ZK-5 (KFI).14 Each of these structures has two different cage types (Figure 1 and Table 1), and so the materials can be prepared via a cotemplating approach. The co-template pair for STA-7, 1,4,7,11tetraazacyclotetradecane (cyclam) and tetraethylammonium

10.1021/jp904623a CCC: $40.75  2009 American Chemical Society Published on Web 08/11/2009

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Figure 1. Structural diagrams showing the cages and their connectivity in (a) SAPO-34, (b) STA-7, and (c,d) STA-14. In each case Al atoms are represented by blue spheres, P (and Si) atoms by green spheres and bridging O atoms by red spheres. (a) In SAPO-34 there is only one kind of cage, each of which is connected to six others via an 8MR opening (one of which is illustrated in yellow). (b) In H-STA-7 there are two cages, small (left of image, type A) and large (right of image, type B). A window between A-type cages is illustrated in cyan, one between B-type cages in orange, and one between A- and B-type cages in yellow. (c, d) In H-STA-14 there are small pau and large lta cages. Each pau cage is connected to two lta and four pau cages. In (c) an example of the window between pau cages is outlined in yellow. Each lta cage is connected to six pau cages. In (d) an example of the window from an lta cage to a pau cage is illustrated in yellow.

TABLE 1: Crystallographic Dimensions of Eight-Ring Windows and Cages in SAPO-34, STA-7, and STA-14 cages/48 T-sites

approximate cage dimensions (Å) and volumec (Å3)

eight-ring window size (Å) and areaa (Å2) 3.8 × 3.8, 11.3 A cage [100] 3.6 × 3.9, 11 (A-B) [001] 3.2 × 3.2, 8.0 (A-A) B cage [100] 3.6 × 3.9, 11 (A-B) [001] 4.1 × 4.1, 13.2 (B-B) 3.1 × 3.1, 7.5 (pau-pau) 4.2 × 4.2, 13.9 (lta-pau-lta)

material

IZA code

cage types containing eight-ring windowsb

SAPO-34d

CHA

20-hedral ([4126286]) cha-cage

4

7.5 × 7.5 × 8.1, 240

STA-7d

SAV

18-hedral ([41286]) A-cage 22-hedral ([4126486]) B-cage

2 2

3.7 × 6.7 × 6.7, 90 6.7 × 9.9 × 9.9, 340

STA-14d

KFI

18-hedral ([41286]) pau-cage 26-hedral ([4126886]) lta-cage

3 1

3.5 × 4.2 × 4.2, 30 10.1 × 10.1 × 10.1, 540

a Calculated using the equation A ) πab/4, where A, a, and b are the pore area and the shortest and longest eight-ring pore diameters, respectively. The eight-ring pores in each material are assumed to be ideally circular or elliptical in shape. b In the notation [mn...], polyhedra are defined by the n number of faces with m T-O-T edges. c Calculated using Marler’s equation V ) πabc/6,7 where V, a, b, and c are the pore volume and the width, length, and height of the cage, respectively. All the cages are assumed to be ideally ellipsoidal or spherical in shape. d The dimensions of pores and cages in these materials were determined from the refined structures of their calcined forms.8,13

(TEA+), was discovered empirically, but a designed synthesis was achieved for STA-14, involving the combined use of the azaoxacryptand 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (K222) and TEA+ to template its large lta and small pau cages, respectively. This co-templating strategy permitted the syntheses of the SAPO versions of STA-7 and STA-14 for the first time, and has recently been communicated.13 29 Si MAS NMR revealed that Si can substitute for P in the framework in these two materials and the presence of Brønsted acid sites in calcined STA-7 (i.e., H-STA-7) was shown by IR spectroscopy.15

In this paper we discuss the syntheses of STA-7 and STA14 in more depth, detailing the compositional range that has been achieved and the possible mechanisms of Si substitution in these SAPO materials. Si substitution into the framework can theoretically proceed via three different mechanisms. The most common mechanism is where P is replaced by Si to produce isolated Si(OAl)4 units (written Si(4Al)) or larger aluminosilicate domains, as previously observed, for example, for SAPO-3416 and also for STA-7.13 A coupled substitution could proceed either via replacement of many adjacent Al and P atoms to give large silica islands (terminated at Al sites) or

Silicoaluminophosphate Molecular Sieves STA-7 and STA-14 by a combined mechanism of substitution of P to produce Al(4Si) units and then the central Al by Si to give Si(4Si) units. In each of these coupled substitution mechanisms unfavorable P-O-Si bonds are avoided. This latter situation has been observed, for example, in SAPO-18.12 Solid-state MAS NMR, both single-quantum (1Q) MAS NMR (27Al, 31P, 29Si) and triplequantum (3Q) 27Al MAS NMR, has been of particular importance in determining the distribution of Si atoms and the presence of additional species coordinated to framework Al in both as-prepared and calcined materials. The SAPO versions of STA-7 and STA-14 are found to be stable to template removal without loss of crystallinity,13 giving highly porous zeotypes with appreciable solid acidity. For convenience, we will refer to these two SAPO materials simply as STA-7 and STA-14 from now on. Adsorption of CO2 on STA-7 has been shown to achieve uptakes approaching 7 mmol g-1 with heat of adsorption of 30-35 kJ mol-1 over most of this range.17 These heats are lower than those observed on the cationic form of zeolites like NaX (35-45 kJ mol-1)18 but higher than that observed on AlPO4-18 (25-30 kJ mol-1), which is structurally similar to STA-7 but possesses a neutral framework. In this work, CO2, CO, and CH4 adsorption isotherms on SAPO34, STA-7, and STA-14 are measured gravimetrically at 100 °C and elevated pressures and compared with those on a pure silica chabazite and zeolite NaX (FAU). Furthermore, the catalytic performance of STA-7 and STA14 in the MTO reaction has been compared with that of SAPO34 to determine the effect of cage size of catalytic performance in materials with similar composition. The deactivation rate in this reaction is known to be highly sensitive to the particle size of SAPO-34, even below 2 µm,19 as well as the pore geometry and the framework composition, and so three STA-7 materials with different particle sizes were prepared and tested as catalysts in this reaction. 2. Experimental Section 2.1. Synthesis. For STA-7 most synthesis gels were prepared without the addition of fluoride as a mineralizer. Al(OH)3 · 0.5H2O (Aldrich) was added to o-H3PO4 (85 wt % in water, Prolabo) prior to the addition of fumed silica (97%, Fluka). The gel composition was varied with the aim of introducing different amounts of Si into the lattice via different hypothetical mechanisms. Three different strategies were adopted, (i) replacement of H3PO4 by SiO2 in the gel, with the aim of allowing different amounts of Si in ‘P positions’ in the AlPO4 lattice (Table 2, expts A1-A5), (ii) replacement of AlPO4 by 2 SiO2, to see if a silica-rich neutral framework could be obtained (expts A6-A9), and (iii) a combination of a replacement of 20% of P sites by Si (which was found to occur readily during the course of this work) followed by addition of a variable amount of SiO2 with the aim of replacing AlPO4 (expts A10-A12). Cyclam (98%, ABCR) was then added to the gel, followed by adjustment of the pH to 7 by the addition of TEAOH solution (34 wt % aqueous, Aldrich). Some preparations were also performed in the presence of fluoride in the synthesis (HF, 48% aqueous, Aldrich) as an additional mineralizer for the silica. These experiments are labeled as A1F, etc. In addition, some preparations of STA-7 were performed on a larger scale. All gels were stirred at room temperature until homogeneous then loaded in Teflon-lined stainless steel autoclaves and heated at 190 °C for 144 h. After removing the autoclave from the oven and allowing it to cool, the reaction mixture was suspended in water, and if necessary, the suspensions were sonicated to force separation from amorphous material, which was decanted.

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15733 TABLE 2: Synthesis Details of As-made STA-7 Materialsa and Framework Compositions expt no.b

Al

Si

P

crystalline product

unit cell framework compositionc

A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A1-F A2-F A4-F A5-F A00-F A04-F A6-F A7-F A8-F A9-F A10-F A11-F STA-7(10) STA-7(20) STA-7(30)

50 50 50 50 50 40 30 20 10 45 40 35 50 50 50 50 50 48 40 30 20 10 45 40 50 50 50

10 20 25 30 40 20 40 60 80 25 40 55 10 20 30 40 0 4 20 40 60 80 25 40 5 10 15

40 30 25 20 10 40 30 20 10 30 20 10 40 30 20 10 50 48 40 30 20 10 30 20 45 40 35

STA-7 STA-7 STA-7 STA-7 amorphous STA-7 STA-7 STA-7 amorphous STA-7 STA-7 amorphous STA-7 STA-7 STA-7 amorphous AlPO4-5 STA-7 STA-7 STA-7 + UT-6 UT-6 amorphous STA-7 STA-7 STA-7 STA-7 STA-7

Al23.8Si4.2P20.0O96 Al23.8Si6.7P17.4O96 Al23.7Si8.7P15.6O96 Al25.0Si8.0P15.0O96 Al22.7Si6.6P18.6O96 Al21.1Si12.0P14.9O96 Al23.5Si5.4P19.4O96 Al23.0Si7.7P17.3O96 Al24.0Si6.7P17.3O96 Al22.0Si6.4P19.6O96 Al24.9Si7.9P15.2O96 Al23.8Si1.9P22.3O96 Al22.6Si7.4P18.2O96 Al21.6Si11.0P15.4O96d Al24.0Si4.3P19.7O96d Al24.0Si6.5P17.5O96d Al24.0Si9.2P14.8O96d

a The ratio of reactants in mixtures A1-A12 is 0.125 cyclam: 2(Al(OH)3•0.5H2O, H3PO4, SiO2):40H2O:xTEAOH, and the total amount of water was 9 mL. For STA- ) 7(10), (20), (30) the amounts of reactants were scaled up ×6. The initial pH of all the synthesis mixtures was adjusted to 7 with TEAOH, and crystallization was performed at 190 °C for 144 h. b AN-F denotes syntheses conducted in the presence of fluoride ions. For these fluoride-containing preparations, 0.5 HF was added as an aqueous solution and the amount of cobase modified to maintain pHo 7. c From ICP analysis. d From EDX analysis.

The product was filtered from the reaction mixture, washed, and allowed to dry. In the case where small crystal sizes were required for catalytic experiments, the preparations were seeded by the addition of 2% mass/mass of finely ground STA-7 crystals. For syntheses aiming to produce STA-14, the same procedure was followed, with K222 (ABCR, 97%) being added in place of cyclam before the pH was adjusted to 7 by addition of TEAOH. For comparison, two SAPO-34 samples with larger and smaller crystal sizes for high-pressure gas adsorption experiments and MTO catalysis, respectively, were prepared according to the procedures given elsewhere.20,21 Purely siliceous chabazite was prepared according to the literature.22 N,N,N-Trimethyladamantammonium (TMADA) hydroxide was used as a structuredirecting agent. A gel of the molar composition 1 SiO2:0.5 TMADAOH:0.5 HF:3 H2O was heated to 150 °C for 90 h in the presence of seeds of purely siliceous chabazite (2% mass/ mass). Also, NaX with Si/Al ) 1.2 was obtained from IFP.23 For use in adsorption and catalysis, SAPO materials were calcined in flowing O2. The samples were heated at 5 °C min-1 to 550 °C, and held at this temperature for 12 h before being cooled to room temperature. Thermogravimetric and differential thermal analyses (TGA/DTA) suggested that under these conditions all the templates would have been removed. Calcination gave white solids without any carbon content. 2.2. Characterization. Product phase identification was performed by powder X-ray diffraction (XRD) on a Stoe STAD

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i/p diffractometer, using monochromated Cu KR X-radiation. Samples were identified on the basis of their powder diffraction patterns (given in the Supporting Information) which were compared with published diffraction patterns of SAPO-34, STA7, and STA-14.13,20 Chemical analysis of the framework composition in terms of Si, Al, and P was performed either by energy dispersive X-ray (EDX) analysis in a JEOL JSM-5600 SEM or using an Agilent 7500 Series ICP-MS instrument working on a New Wave Research Mechantek Products UP 213 laser. Scanning electron microscopy (SEM) was also used to determine morphology and particle size. The 27Al and 31P MAS spectra were obtained using a Varian VNMRS spectrometer operating at 104.20 and 161.88 MHz for 27 Al and 31P, respectively. The 29Si MAS spectra were recorded at a frequency of 59.56 MHz using a Varian Unity Inova spectrometer. Spectral referencing is with respect to a 1 M Al(H2O)63+ solution, an 85% H3PO4 solution, or neat tetramethylsilane. One-dimensional 27Al spectra were obtained using direct-polarization from 10 000 repetitions with a recycle delay of 0.2 s, ∼30° pulse angle, and spin-rate of 14 kHz. 31P and 29 Si spectra were obtained in a similar fashion but with a 90° pulse, 120 s recycle delay, 16 repetitions, and 14 kHz spin-rate for 31P and 350 repetitions and a 5.1 kHz spin-rate for 29Si. Two-dimensional 27Al MQMAS were obtained using an amplitude-modulated split-t1 experiment. Between 900 and 2400 repetitions were acquired for each of 90-128 increments in t1. The recycle delay was 0.1 s, and the spinning rate was 14 kHz. The indirectly detected 3Q axis is labeled according to the procedure of Amoureux.24 The micropore volume of the calcined samples was measured by N2 adsorption at -196 °C using a Hiden IGA gravimetric analyzer, and converting the mass of N2 adsorbed in the micropores at P/Po ) 0.1 into a volume by assuming the adsorbed nitrogen has the density of liquid nitrogen at that temperature. High-pressure gas adsorption isotherms were recorded gravimetrically on a Rubotherm magnetic suspension balance. Between 150 and 500 mg of sample were used for each measurement. Prior to measurement the samples were outgassed at 400 °C for 4 h under vacuum to remove adsorbed water. All adsorption isotherms were recorded at 100 °C. The use of this higher temperature (compared to that used previously for CO2 adsorption on STA-7 at 30 °C) shifts the adsorption isotherm to higher pressures and accentuates the differences between samples. After recording an adsorption isotherm, the samples were regenerated under the same conditions as those used for the initial activation and the isotherm with next gas was performed. Ammonia temperature-programmed desorption (TPD) profiles were recorded on a fixed bed, flow-type apparatus linked to a Hewlett-Packard 5890 series II gas chromatograph with a thermal conductivity detector (TCD). About 1.0 g of sample was activated in flowing He (50 cm3 min-1) at 550 °C for 2 h: 10 wt % ammonia was passed over the sample at 150 °C for 0.5 h. The treated sample was subsequently purged with He at the same temperature for 1 h to remove physisorbed ammonia. Finally, the TPD profiles were obtained in flowing He (30 cm3 min-1) from 150 to 650 °C at a temperature ramp of 10 °C min-1. TGA on coked samples were performed in air on an EXSTAR 6000 thermal analyzer, where the weight loss related to the combustion of coke deposits formed during MTO was further confirmed by DTA using the same analyzer. The coke deposits formed on the SAPO catalysts after MTO at 350 °C for 90 min on stream were characterized by GC-MS total ion chromatog-

Castro et al. raphy. The used catalysts were completely dissolved by 10% HF solutions and neutralized with K2CO3. CCl4 (Aldrich, 99.5%) was employed to extract the organic species from the resulting solutions. Water in the organic phase was removed by adding a small amount of Na2SO4 that was subsequently recovered using an Advantec DISMIC-13JP syringe filter. The GC-MS total ion chromatograms of extracted organic phases were recorded on a Varian CP 3800 gas chromatograph equipped with a Varian 320-MSD mass selective detector, using electron impact ionization at 70 eV. The split ratio was 100:1, and the column used was a VF-5 capillary column (30 m × 0.25 mm) with flowing He (0.3 cm3 min-1). The temperature program ramped the column from 70 to 280 °C at a rate of 4 °C min-1. 2.3. Catalysis. A conventional continuous-flow microreactor was used to carry out the MTO reaction over various SAPO molecular sieves at atmospheric pressure. Prior to the experiments, the catalyst was routinely activated under flowing N2 (130 cm3 min-1) at 550 °C for 2 h and kept at 350 °C to establish a standard operating procedure, allowing time for the reactant/ carrier gas distribution to stabilized. Then, methanol vapor was fed at a rate of 0.085 cm3 h-1 (WHSV ) 0.67 h-1) into the reactor containing 0.1 g of catalyst at the same temperature. The total gas flow at the reactor inlet was kept constant at 30 cm3 min-1. The reaction products were analyzed online in a Varian CP-3800 gas chromatograph equipped with a CPPoraPLOT Q capillary column (0.25 mm × 25 m) and a flame ionization detector, with the first analysis carried out after 5 min on stream. CO2 was separated using a packed Carbosphere column and analyzed with a thermal conductivity detector. The conversion of methanol was defined as the percentage of methanol consumed during the MTO reaction. Dimethylether was not considered as a product. The yield of each product was calculated as the percentage of the amount (in mol) of methanol converted to hydrocarbons. 3. Results and Discussion 3.1. Synthesis. Attempts to synthesize STA-7 with different Si contents were made according to the rationale and methods described in the Experimental Section, and the anhydrous unit cell compositions of the resulting products determined from inductively coupled plasma (ICP) analysis are given in Table 2. Gel series (i) gradually replaces P in the synthesis gel with Si (A1-A5); series (ii) gradually replaces Al and P in equal amounts in the gel by Si (A6-A9); and series (iii), starting from a typical composition of STA-7, replaces both Al and P in different amounts in the gel by Si to target a combined mechanism of Si substitution (A10-A12). Preparations were also performed in the presence of fluoride in the synthesis. For the first series of preparations, some crystalline STA-7 product is obtained until the gel composition reaches 30% Si, 20% P, 50% Al. Above this composition, only amorphous solid was produced (ESI). Although synthesis A1 (10% Si, 40% P, 50% Al) gave a framework composition similar to that of the gel (Table 2), the preparations with higher Si contents yielded products with a Si content that converges to 17% Si of the total tetrahedral atoms (T-atoms), replacing P. Repeating these preparations in the presence of added hydrofluoric acid had little effect on A2 and A4, but elemental analysis suggests that the level of Si incorporation has been increased in A1F from 9% to 14% of the total T sites, due to the effect of fluoride on mineralizing the silica. A second approach traced gel compositions from pure AlPO4 toward pure SiO2 by maintaining a 1:1 ratio of Al/P and increasing the proportion of Si in the synthesis gel. Although

Silicoaluminophosphate Molecular Sieves STA-7 and STA-14

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15735

Figure 2. SEM images of (a) STA-7(20), (b) STA-14(20), and (c) SAPO-34 with smaller (right) and larger (left) crystal sizes.

all these preparations had some amorphous silica in the products, selected area EDX analysis of well-defined crystals indicates that Si can make up a maximum of around 23% of the total T-atom content, the highest observed in STA-7. It thus appears that silica islands exist in this material, via a coupled substitution measurement. A third series of syntheses was conducted (syntheses A10-A12) in an alternative approach that aimed to form silica islands within the structure. Gels with compositions 30% P, 45% Al, 25% Si and 20% P, 40% Al, 40% Si yielded STA-7, and ICP analysis of these samples suggests that STA-7 compositions with Si levels reaching a maximum of ∼16% of the T-atoms. As with syntheses of the first series, elemental analysis suggests that Si predominately substitutes for P, although a small proportion of Al substitution via a coupled mechanism might also occur. Similar preparations to the second and third sets, in both of which coupled substitutions were targeted, were also conducted in the presence of fluoride. Attempts to prepare pure AlPO4 STA-7 were unsuccessful, resulting instead in AlPO4-5. For preparations in which Al and P were replaced equally by Si in the gel, there is a transition from pure STA-7 with a low level of Si, to SAPO(F) UT-6, a fluorinated version of SAPO-34.25 The low silica STA-7 (A04-F: composition, from ICP, TGA, and CHN: (cyclam)1.96(TEA)1.96F1.96Al23.8Si1.90 P22.3O96) is labeled below as STA-7(F,8) where 8 represents the percentage inclusion of Si in framework ‘P’ sites. A series of syntheses targeting mixed substitution mechanisms for Si gave STA-7 in the presence of fluoride, with a maximum Si level estimated at 21%

of the total T-atom content (A12(F)). One of these preparations, A10F (framework Si/(Al+P+Si) ) 0.15), labeled STA7(A10F), was examined further by solid-state NMR. Once the synthesis behavior of STA-7 was established, some syntheses were performed on a larger scale (6×). In these experiments the ratios in the gel were those assuming direct replacement of P by Si and Si/Al gel ratios of 0.1, 0.2, and 0.3 were chosen, and the subsequent products denoted STA-7(10), STA-7(20), and STA-7(30) (Table 2). Powder XRD patterns indicate that the products are highly crystalline STA-7. STA7(20) gave the highest yield of crystals (particle size ca. 30 µm). The SEM images in Figure 2 reveal that most of the crystals of this sample have tetragonal prism morphologies and EDX analysis indicates a Si/Al ratio of 0.27. Seeding the preparation gave STA-7(20) with a smaller particle size, ca. 3 µm. STA7(10) and STA-7(30) samples possess crystals with more surface overgrowths, and with Si/Al ratios (determined by EDX) of 0.18 and 0.38, respectively. STA-7(20) material was chosen for studies of adsorption and catalytic activity. Another series of SAPO syntheses was performed using the co-template pair K222/TEA+, which has been reported to give the SAPO and MgAPO versions of STA-14.13 Gel compositions were used that replaced some o-H3PO4 with the same molar amount of silica. Powder XRD experiments indicate that AlPO442 or SAPO-42 (LTA framework topology)2 were the dominant phases at lower contents of Si (Si/Al ) 0 or 0.1) and only poorly crystalline product resulted from gels with Si/Al ) 0.4. STA14 was obtained at Si/Al ratios of 0.2 and 0.3 (STA-14(20) and

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Figure 3. 27Al MAS NMR spectra of (a) as-prepared STA-7(20), (b) as-prepared STA-7(F,8) (c) calcined STA-7(20), and (d) STA-7(A10F). Sample nomenclature as in text.

STA-14(30)): EDX analysis gives Si/Al ratios of 0.21 and 0.27, respectively. The SAPO(20) STA-14 preparation was successfully scaled up (2×) and subsequently examined as an adsorbent and catalyst. As seen in Figure 2, the morphology of STA-14(20) prepared on a small scale gave cuboctahedra of ca. 10 µm in size with well-developed 〈111〉 and 〈100〉 faces. Scaled-up syntheses gave a similar morphology but with a smaller particle size (5 µm). 3.2. Solid-State NMR. The 31P MAS NMR spectra of all samples prepared here show a main resonance of tetrahedral P(4Al) at -28.4 ppm. A representative 31P MAS NMR spectrum can be found in the Supporting Information, Figure 3S. 29Si MAS NMR spectra of as-made STA-7(10), STA-7(20), and STA-7(30) samples are dominated by a resonance at -91.6 ppm assigned to tetrahedral Si(4Al), indicating Si substituting at P positions (Figure 4S). A different distribution of Si environments is observed for the STA-7(A10F) sample prepared in the presence of fluoride (Si/(Si+Al+P) ) 0.15, Figure 4S). The 29 Si MAS NMR spectrum of this sample has a strong signal at -92.1 ppm (attributed to Si(4Al)) accompanied by four other resonances at -96.2, -100.5, -106.3, and -110.6 ppm, attributed to Si(3Al,Si), Si(2Al,2Si), Si(Al,3Si), and Si(4Si), respectively, as found in SAPO-18.12 A small concentration of silica islands can therefore be formed within the STA-7 framework when the fluoride route is employed. This is attributed to the mineralizing effect of fluoride ions, giving more reactive silicate species in solution. All the 27Al MAS NMR spectra of STA-7 samples synthesized without fluoride are characterized by a sharp resonance at 36.7 ppm assigned to tetrahedral Al(4P) with an additional broad signal upfield (with a maximum around 9 ppm in the spectrum of as-made STA-7(20), Figure 3a). In the SAPO material formed in the presence of fluoride, STA-7(A10F), there is also at least one broader resonance upfield from the Al(4P) signal and the low silica STA-7(F,8) sample exhibits a well

Castro et al. resolved series of broad resonances upfield from the Al(4P) signal. Upfield resonances observed in the 27Al MAS NMR spectra of various SAPO and AlPO4 molecular sieves have variously been attributed to tetrahedral Al(3P,Si), Al(2P,2Si), or 5- or 6-fold coordinated Al species.26-28 To investigate this further, 27Al 3Q MAS NMR experiments were performed on these samples.29 An MQMAS spectrum correlates single- and triple-quantum resonances. The secondorder quadrupolar broadening that results in signal overlap in the 1Q dimension is absent in the 3Q dimension, resulting in increased resolution in the resulting 2D plot. The width of the resonances in the 1Q dimension increases with increasing quadrupolar coupling, which in turn derives from site asymmetry. The 27Al 3Q MASNMR spectrum of STA-7(20) in Figure 4a shows three signals at 10, 40, and 60 ppm on the 3Q axis. The most intense line around 40 ppm is from tetrahedral Al(4P), whereas the broader signal around 60 ppm can be assigned to tetrahedral Al with a higher degree of asymmetry. In SAPO molecular sieves where Si substitutes for P, this is expected to be Al(3P,Si), as suggested by Chen et al. for the 27Al MQMAS data of SAPO-37 (FAU).26 Assuming a random Si distribution (Si/Al ) 0.3), the probability that one Si atom is in the coordination shell of Al is 59%, but the Al(3P,Si) peak is less intense than the Al(4P) resonance in the 27Al 3Q MAS NMR spectrum from STA-7(20) because intensities in MQMAS spectra are not quantitative. No resolved Al(2P,2Si) environment is detected in the 3Q MAS NMR experiment. There is also a minor peak at around 10 ppm on the 1Q and 3Q axes, which can be attributed to a small amount of Al in 5-fold coordination. The three different crystallographic T-sites in the STA-7 framework are not resolved in the 3Q MAS NMR experiment. To confirm the 27Al NMR line assignments given above, the STA-7(F,8) sample prepared in the presence of fluoride and with a very low Si content was also examined. The 27Al MAS NMR spectrum in figure 3b is different from that of STA-7(20), with a well-defined tetrahedral Al(4P) resonance at 39.1 ppm and an additional broad envelope of peaks with maxima at 8.2 and 7.0 ppm. 27Al 3Q MAS NMR resolves the spectrum further (Figure 4b). There is little evidence of any broad peak associated with tetrahedral Al(3P,Si), which is consistent with the low Si content of the sample. The broad envelope upfield is resolved into three peaks with maxima in the 3Q axis of 8, 0, and -8 ppm. These can be attributed to 5-fold and octahedrally coordinated Al species, where the extra coordination is made up by water molecules or hydroxide or fluoride ions, the latter two performing the additional function of balancing the positive charge on the templates. Therefore, it is most likely that Al(4P) and Al(3P,Si) environments give very different lineshapes and that only the symmetrical Al(4P) environment shows a sharp line, with a chemical shift of 36-39 ppm. Tetrahedral Al(3P,Si) yields a broad resonance that is readily distinguished by 27Al MQ MAS NMR, both from Al(4P) and from Al with higher coordination, which is observed at lower chemical shifts (close to 0 ppm), and the presence of this resonance in a SAPO is therefore strong evidence for the substitution of Si for P in the lattice. The 31P and 27Al MAS NMR spectra of as-made STA-14 were found to be very similar to those of STA-7 samples. Its 31P MAS NMR spectrum shows a single sharp peak at -28.9 ppm. The 27Al MAS NMR spectra of STA-14 materials give two overlapping signals, a narrow line at 36.4 ppm, and a strong broad signal centered at 8 ppm. The 29Si MAS NMR spectrum of SAPO(20) STA-14 has a single narrow peak at -92.2 ppm (Supporting Information).


Figure 4. 27Al 3Q MASNMR spectra of (a) as-prepared STA-7(20), (b) STA-7(F,8), and (c) calcined STA-7(20).

The 29Si MAS NMR spectra of calcined STA-7(10), STA7(20), and STA-7(30) samples (Figure 6S) are considerably broader than those of the corresponding as-prepared materials, although powder XRD experiments show that upon calcination all the STA-7 and STA-14 samples remain highly crystalline. The 29Si NMR spectrum of STA-7(20) with a Si/Al ratio of 0.27, for example, shows a broadened line shape comprising more than one line, which may be due to different T-sites in STA-7 framework. At a higher Si content the spectrum gives additional poorly defined resonances (for example, at -108.6 ppm) that may be from silica islands. The 27Al MAS NMR spectrum of calcined STA-7(20) in Figure 3c shows a signal from tetrahedral Al with a peak at 33 ppm. Its 3Q 27Al MAS NMR spectrum (Figure 4c) resolves two peaks, with a resonance from the Al(4P) environment and another which can be assigned

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15737 to the Al(3P,Si) environment observed in the as-prepared material. Notably, the Al(3P,Si) peak is less asymmetric in this calcined material than in the as-prepared SAPO STA-7, so that removal of the template decreases the asymmetry. Similar results are observed for the 3Q MAS NMR spectra of calcined STA14(20) (Figure 7S). 3.3. CO2, CO, and CH4 Adsorption Studies on H-STA-7 and H-STA-14. The chemical compositions and N2 micropore volumes of a series of molecular sieves used in CO2, CO, and CH4 adsorption studies, including SAPO-34, STA-7, and STA14 can be found in Table 3. The theoretical pore volumes and measured N2 uptakes of the three SAPO materials are very similar, revealing their highly crystalline nature. Figure 5 shows the adsorption isotherms at 100 °C of CO2 on H-SAPO-34, H-STA-7(20), and H-STA-14(20). While these three SAPO materials are characterized by very similar isotherms, their CO2 uptake decreases slightly in the order H-SAPO-34 > H-STA-7 > H-STA-14. At 40 bar the adsorption uptake for each material approaches 4.5 mmol g-1, which is below the maximum uptake observed for H-STA-7 at 30 °C (7 mmol g-1).17 The Si content of the framework gives rise to the formation of bridging hydroxyl sites, which are likely to be the main adsorption sites, particularly at low coverages. EDX analysis indicates that this H-SAPO-34 contains around 40% of the ‘P’ site as Si, whereas for H-STA-7(20) and H-STA14(20) this is 27% and 21%, respectively. This might explain in the higher adsorption uptake for H-SAPO-34(40), although the presence of silica islands in its framework would result in the framework charge being lower than the Si content. The pore geometry may also play a role. As seen in Table 1, all these D6R-containing materials have 4 cages per 48 T-sites. However, SAPO-34 has only one cage type, whereas STA-7 and STA-14 have two different types. STA-7 possesses two small (A) and two large (B) cages, and STA-14 has three small pau cages and one much larger lta cage. In the case of STA-7 and, particularly STA-14, the size of the larger cages could result in a weaker adsorption of CO2 molecules in their center, decreasing the adsorption uptake at comparable pressures in these solids. The stronger effect of framework polarity in determining the uptake of CO2 is emphasized when the isotherm of CO2 on H-SAPO-34 is compared to that on pure silica chabazite. Both samples have very similar micropore volumes (Table 3). As seen in Figure 5, however, adsorption uptake over the pressure range studied was found to be much higher for H-SAPO-34. A similar trend can also be observed for the CH4 adsorption isotherms at 100 °C on the three SAPOs (Figure 5). This indicates that in each material all of the space accessible to CO2 is also accessible to the slightly larger CH4. Figure 6 compares the adsorption isotherms of CO2, CO, and CH4 on H-STA-7(20) with the isotherms of the same gases measured on NaX. H-STA-14(20) shows similar behavior to H-STA-7(20). The uptake of CO2 is higher at the same pressure on NaX than on H-STA-7(20) because in the former material the strongest adsorption sites are the Na+ cations (0.45 Na+ per T-atom). By contrast, the main adsorption sites in H-STA7(20) are thought to be the bridging hydroxyl groups resulting from replacement of framework P by Si. (Harlick and Tezel have shown experimentally that the Henry constant of CO2 on H-ZSM-5 zeolites increases with decreasing Si/Al ratio, i.e., with increasing concentration of bridging hydroxyl groups30). We therefore presume that the hydroxyl groups are the preferred adsorption sites for CO2, so for 27% Si substitution into the P site there should be a maximum of 0.135 bridging hydroxyl sites per T-atom. Furthermore, bridging hydroxyl groups are

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TABLE 3: Chemical Compositions and N2 Micropore Volumes of Molecular Sieves Used for High Pressure Gas Adsorption Measurements T-atom composition (mol %) sample SAPO-34 STA-7(20) STA-14(20) pure-silica chabazite NaX

IZA code

cation form

Al

P

Si

micropore volume, cm3 g-1

CHA SAV KFI CHA FAU

+

0.96 1 1 0.45

0.64 0.73 0.79 -

0.41 0.27 0.21 1 0.55

0.29 0.29 0.33 0.32 0.28

H H+ H+ Na+

expected to show weaker Coulombic effects than Na+ cations because although protons are smaller than sodium cations, their electrostatic effect is reduced by polarization of the electron density of the adjacent oxygen atom. As expected, all three SAPO materials show preferential uptake of CO2, the most polar gas molecule used in this study. In addition, whereas NaX adsorbs slightly more CO than CH4, H-STA-7 adsorbs more CH4 than CO. Generally, the interactions of CH4 and CO (adsorption enthalpy determined from the isosteric heat) are similar for zeolites but the interaction is not the same for the two molecules. CH4 is only attracted by van der Waals and induced dipole interactions, while in the case of CO there is a permanent dipole. van der Waals forces do not depend strongly on the composition (the molecules interact with the lattice oxygen atoms and only weakly with the T-atoms), but cation-dipole interactions depend on the electric field in the adsorbent. The electric field in NaX is stronger than that in H-STA-7, which may explain the difference in relative uptakes of CH4 and CO in the two solids. From the high pressure gas

Figure 5. (Above) Adsorption isotherms at 100 °C of CO2 on H-SAPO-34 (b), H-STA-7 (2), and H-STA-14 (9). The inset shows the isotherms of H-SAPO-34 (b) and pure-silica chabazite ((). (Below) Adsorption isotherms at 100 °C of CH4 on H-SAPO-34 (O), H-STA-7 (∆), and H-STA-14 (0).

adsorption isotherms in Figures 5 and 6, it can therefore be concluded that the proton forms of the SAPO D6R zeotypes have specific micropore volumes comparable to the value of a commercial NaX and present effective polarities intermediate between highly polar cationic zeolites and pure-silica polymorphs with low polarity. 3.4. MTO Catalysis. Table 4 lists the characteristics of the samples examined for the MTO reaction. Each of the three H-STA-7 SAPO materials has a similar silicon content, and chemical analysis and MASNMR suggests that all Si replaces P in each, but each has a different particle size. STA-7(III) refers to the scaled up STA-7(20), STA-7(I) to a seeded preparation of the same gel composition, and STA-7(II) to preparation A1 of Table 2. All three SAPO structure types are expected to be threedimensional to methanol transport. SAPO-34 (pore size 3.8 Å) and STA-14 (pore size 4.2 Å) both act as catalysts and are known to take up methanol and catalyze the MTO reaction. Diffusional measurements on a single crystal of H-STA-7 indicate 3-D diffusion for methanol on this structure at room temperature31 so that methanol is able to pass through windows between B cages (along [001]) and between A and B cages along [100] and [010] (Table 2). Also, the CH4 uptake at 100 °C for H-SAPO-34, H-STA-7, and H-STA-14 decreases in the same order as CO2 uptake for the three structures (Figure 5) indicating that CH4 can gain access to all of the pore space in each structure. The Lennard-Jones diameter of CH4 molecules (3.7 Å)32 is similar to the Lennard-Jones distance between methyl groups in methanol (3.6 Å)33 and close to the crystallographically determined B-B and A-B window sizes in STA-7 (Table 2). The NH3 TPD profiles of these SAPO catalysts are compared in Figure 7. All the TPD profiles are characterized by two desorption peaks with maxima in the temperature regions 250-270 and 400-450 °C, corresponding to weak and strong acid sites, respectively. As expected from their similarity in Si contents, the total amount of NH3 desorption (i.e., the density of acid sites, as represented by the areas under the thermal

Figure 6. Adsorption isotherms at 100 °C of CO2 (b, O), CH4 (2, ∆), and CO (9, 0) on H-STA-7 (closed symbols) and NaX (open symbols).



TABLE 4: Sample Details and Amounts of Coke Deposits Formed on SAPO Catalysts during MTO Reaction at 350 °C and 0.67 h-1 WHSV for 90 min on Stream

catalyst

Si/(Si+Al+P)

crystal shape and size (µm)a

H-SAPO-34 H-STA-7(I) H-STA-7(II) H-STA-7(III) H-STA-14

0.08 0.12 0.09 0.13 0.11

cuboids, 2-6 tetragonal prisms, 2-3 tetragonal prisms, 10 tetragonal prisms, 70-80 cuboctahedra, 5

a

amount (wt %) coke depositedb 5.5 5.9 11.0 7.4 10.4

Determined by SEM. b Determined by TGA/DTA up to 800 °C.

Figure 9. Yields in lower olefins and C3)/C2) ratios as a function of TOS in MTO reaction over (a) H-SAPO-34, (b) H-STA-7(I), (c) H-STA-7(II), (d) H-STA-7(III), and (e) H-STA-14 at 350 °C and 0.67 h-1 WHSV. Figure 7. NH3 TPD profiles from (a) H-SAPO-34, (b) H-STA-7(I), (c) H-STA-7(II), (d) H-STA-7(III), and (e) H-STA-14.

Figure 8. Methanol conversion as a function of TOS in MTO reaction over H-SAPO-34 (b), H-STA-7(I) (2), H-STA-7(II) (left-pointing triangle), H-STA-7(III) (right-pointing triangle), and H-STA-14 (9) at 350 °C and 0.67 h-1 WHSV.

desorption curves) was found to be comparable for all five SAPO materials, especially at higher temperatures. Although the temperature of the maxima of high temperature desorption peaks from H-STA-7 catalysts is slightly higher than that from H-SAPO-34 or H-STA-14, it is likely that there are no significant differences in the strength of both weak and strong acid sites among these catalysts. In this case, comparison of their MTO activities and deactivation patterns would then allow us to illustrate the effects of the geometrical constraints imposed by differences in the shape and size of the particular cages of each SAPO catalyst, as well as those of the crystal size. Figure 8 shows the methanol conversion as a function of time on stream (TOS) in the MTO reaction over the proton form of the five SAPO molecular sieves with different framework topologies and crystal sizes but with similar Si contents (Table 4). All samples retained crystallinity during catalysis, as shown by the XRD patterns (Supporting Information, Figure 8S). The composition of the hydrocarbon product from each catalyst as a function of TOS is plotted in Figure 9. The reactions were measured at 350 °C and 0.67 h-1 WHSV of methanol. Under

the reaction condition studied, all catalysts except H-STA-14 show initial methanol conversions of 95% or higher. Good stability during the first 240 min on stream is only achieved for H-SAPO-34 and H-STA-7 with small particle size. The two H-STA-7 samples with particle sizes of 10 µm and of 70-80 µm is rapidly deactivated, while H-STA-14 (5 µm) shows intermediate behavior. Particle size therefore plays a crucial role in determining deactivation rate, as expected. This is consistent with olefin formation in MTO catalysis over cage-based molecular sieves proceeding via the so-called hydrocarbon pool mechanism, in which organic reaction centers, e.g., methylbenzenes on H-SAPO-34, act as scaffolds for producing olefins within the pores of molecular sieves.3-6,9-11 Catalyst deactivation occurs as the methylbenzenes are replaced by polyaromatic hydrocarbons which inhibit intracrystalline diffusion. As seen in Table 4, the amount of coke formation is not a simple function of particle size since H-STA-7(II) has a much higher coke content that H-STA-7(III) but has a smaller particle size. Furthermore, the coke content does not by itself control the activity: H-STA-7(II) and H-STA-14 have similar coke contents after 90 min on stream (11% and 10.4%, respectively) but whereas H-STA-7(II) is almost fully deactivated at this time the H-STA-14 material achieves its highest conversion then. The activity is therefore a complex function of particle size, coke content, and pore topology. Figure 9 shows the yields of light olefins and the propylene/ ethylene C3)/C2) ratio as a function of TOS in MTO over H-SAPO-34, H-STA-7, and H-STA-14 measured under reaction condition described above. These data reveal that ethylene and propylene are the two dominant products, regardless of the framework topology. There are no detectable aromatic compounds because they are too large to diffuse out of the eightring pore windows in each catalyst. While the propylene yield over H-SAPO-34 at the beginning of the reaction is high, it becomes notably smaller as TOS increases, together with a simultaneous increase in ethylene yield. Thus, the C3)/C2) ratio was found to decrease to 0.7 after 240 min on stream. This has previously been attributed to transition state selectivity4 and most recently to product diffusion selectivity.6 A similar trend was

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Figure 10. GC-MS total ion chromatograms of the CCl4 extracts from H-SAPO-34 (bottom), H-STA-7(I) (middle), and H-STA-14 (top) catalysts obtained by dissolving in HF after MTO reaction at 350 °C and 0.67 h-1 WHSV for (a) 5, (b) 90, (c) 240, and (d) 600 min on stream. The asterisk represents the mass signal of C2Cl6 produced by pyrolysis of CCl4.

also observed for H-STA-14. All three H-STA-7 catalysts show C3)/C2) ratios higher than 1.0 over the period of TOS studied here, regardless of notable differences in their crystal size (Table 4). The C4 fraction is also higher in STA-7 than in the other samples. Therefore, it can be concluded that the olefin distribution from MTO over cage-based, small-pore molecular sieves can differ significantly between materials with cages of different size and shape and are more dependent on the cage topology than on the crystal size of the catalyst. This has been reported in previous studies.34-36 Figure 10 shows the GC-MS total ion chromatograms of CCl4 extracts obtained by dissolving H-SAPO-34, H-STA-7(I) and H-STA-14 catalysts in HF after MTO reaction at 350 °C and 0.67 h-1 WHSV as a function of TOS. The identification of coke deposits formed on MTO catalysts gives useful information not only on the reaction intermediates governing major olefins, but also on the aromatic hydrocarbons leading to deactivation.4,6 The structures annotated onto the total ion chromatograms in Figure 10 are selected peak identifications made by comparison of the mass spectra with those in the NIST database.37 The distribution of aromatic hydrocarbons occluded in the cages is found to depend strongly on the cage topology.

Castro et al. In the H-SAPO-34 catalyst (which remains active during 90 min on stream), the major aromatic hydrocarbons formed are tetramethylbenzenes that are reported to preferentially produce ethylene.38 As seen in Figure 10, however, the amounts of polymethylated naphthalenes and methylphenanthrene increase significantly with prolonged TOS, suggesting that these di- and tricyclic aromatic compounds may be related to the deactivation of H-SAPO-34. The hydrocarbon species formed on H-STA-7(I) after 90 min on stream show greater variety than on H-SAPO-34. They include tetra-, penta-, and hexamethylbenzenes, the latter two compounds of which are known to form propylene with high selectivity and naphthalenes at short TOS. Also of particular interest is the observation that after 240 min on stream, when H-STA-7(I) is still active, its major occluded products are methylated pyrenes. This is also the case with H-STA-7 samples of larger crystallite sizes that are almost fully deactivated after 90 min on stream (Figure 9S). It thus appears that the selective accumulation of methylpyrenes in H-STA-7 occurs within its B cage, rather than in the much smaller A cage (Table 1), and may be responsible for the catalyst deactivation. Methylphenanthrenes, which are smaller than methylpyrenes, were hardly detected in H-STA-7(I) even when fully deactivated, unlike for H-SAPO-34, which has smaller cages than the B cage of STA-7 and shows phenanthrenes but not pyrenes. The similarity in shape and size of pyrenes and methylated pyrenes to those of the aliphatic azamacrocycles cyclam and tetramethylcyclam that are known to act as templates for the B cage of STA-7 during its synthesis suggests that the empty inorganic cages themselves act as reVerse templates for the formation of these polyaromatics during the MTO reaction over H-STA-7. Unlike the case of H-SAPO-34 and H-STA-7(I), penta- and hexamethylbenzenes are the two dominant products observed for H-STA-14 after 90 min on stream (Figure 10). This remains true over the period (600 min) of TOS studied here. Furthermore, the formation of methylphenanthrenes and methylpyrenes even in deactivated H-STA-14 was found to be negligible. As described above, the STA-14 structure is built by a strict alternation of large lta cages and much smaller pau cages. Thus, one possible explanation on the GC-MS results from H-STA14 is the selective encapsulation of species, including methylbenzenes, within pau cages which have an internal cavity with free diameter of ca. 10.7 Å. If so, further diffusion of methanol molecules to the acid sites within adjacent lta cages would then be stopped, leading to continuous deactivation but minimizing the intracrystalline formation of polycyclic aromatic compounds such as methylnaphthalenes, methylphenanthrenes or methylpyrenes. Regardless of the exact interpretation of the GCMS data in Figure 10, however, it is clear that the uniformity in cage shape and size in cage-based, small-pore molecular sieves is a critical factor governing the type of the accumulated aromatic hydrocarbons, and hence their MTO activity and deactivation pattern. 4. Conclusions The synthesis of the SAPO version of STA-7 (SAV) and STA-14 (KFI) molecular sieves via a co-templating approach from gels with different compositions has been investigated. In the absence of F- ions, 29Si MAS NMR indicates that STA-7 crystallizes from a wide range of gel compositions and includes Si in the framework almost entirely by substitution for P, with Si/(Si+Al+P) ratios varying from 0.04 to 0.17. In the presence of F-, greater solubilization of the silica enables aluminosilicate islands to form at higher Si contents. Low Si content (STA-



7(F,8)) has also been prepared, where coordinated F- has a charge-balancing role. By contrast, SAPO STA-14 crystallizes from gels with a narrow compositional range (Si/(Si+Al+P) ) 0.2-0.3) from hydrothermal syntheses where SAPO-42 (LTA) is observed to form at lower Si contents or if F- is present. 27 Al 3Q MAS NMR studies of these materials have been particularly helpful in resolving signals that overlap in the singlepulse spectra. In 27Al 3Q MAS NMR spectra of as-prepared STA-7 samples, for example, several resonances can be resolved. The sharp peak at 39 ppm is attributed to tetrahedral Al(4P), with no evidence of resolved peaks from crystallographically distinct sites. Overlapping this in the 1-D 27Al MAS NMR spectra, but clearly resolved as a peak with strong quadrupolar broadening in the MQ spectrum at an isotropic chemical shift of 60 ppm, is a signal assigned to the Al(3P,Si) environment. This assignment is supported by the following observations: (i) it is almost absent in samples that contain little Si and (ii) it remains upon calcination, although it does become narrower. This reduction in broadening is attributed to the removal of the template molecules. These two peaks from tetrahedral framework Al are also observed in SAPO-34 and SAPO STA-14 samples. In addition to these peaks, additional resonances upfield are assigned as 5- or 6-fold Al species (with coordinated water molecules or with coordinated hydroxide or fluoride ions that are removed upon calcination). High-pressure adsorption isotherms of CO2 and CH4 on molecular sieves studied here reveal that their adsorption capacities are high (0.29-0.33 cm3 g-1) and show a slight increase in uptake of CO2 and CH4 on going from H-STA-14 < H-STA-7 < H-SAPO-34. Each H-SAPO shows a reduced ratio of adsorbed CO/CH4 compared to cationic zeolites, but enhanced CO2 uptake compared to pure silica molecular sieves of similar structure, in line with polarity increasing in the chemical composition order SiO2 < SAPO < cationic zeolite. The adsorption measurements also indicate that all cages in the D6R containing zeolites can take up CH4, which has a van der Waals diameter of 3.8 Å and are all expected to be accessible to methanol. The methanol-to-olefins performance of H-SAPO-34, H-STA7, and H-STA-14 materials with similar acidic properties was found to depend on both crystallite size and cage connectivity and topology. H-STA-7 with a small crystal size of 2-3 µm has a MTO stability on line comparable to that observed for H-SAPO-34, whereas the same materials with larger crystal sizes (g10 µm) deactivate rapidly. H-STA-14 also shows reasonable stability with TOS, even though the particle size was a little larger (ca. 5 µm). The type of aromatics formed within the cages of the three solids depends on the pore topology, and for H-STA-7 the distribution includes a high concentration of methylated pyrenes that are probably formed within its larger B cages.

Paillaud, and P. Caullet of the LMPC, Mulhouse are thanked for providing the pure silica chabazite.

Acknowledgment. Funding for this work was provided by the European Commission FP6 Marie Curie Research Training Network ‘INDENS’ (MRTN-CT-2004-005503), the University of St. Andrews and the Korea Science and Engineering Foundation (through the National Research Laboratory Program R0A-2007-000-20050-0). Solid-state NMR spectra were obtained at the EPSRC facility in Durham. A. Darwiche, J.-L.

Supporting Information Available: Additional XRD patterns, NMR and GC-MS spectra are supplied as described in the text (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (2) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, T. R.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1984, 106, 6092. (3) Sto¨cker, M. Microporous Mesoporous Mater. 1999, 29, 3. (4) Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. Acc. Chem. Res. 2003, 36, 317. (5) Haw, J. F.; Marcus, D. M. Top. Catal. 2005, 34, 41. (6) Hereijgers, B. P. C.; Bleken, F.; Nilsen, M. H.; Svelle, S.; Lillerud, K.-P.; Bjørgen, M.; Weckhuysen, B. M.; Olsbye, U. J. Catal. 2009, 254, 77. (7) Marler, B. PhD. Thesis, Universitat Kiel, 1989. (8) Ito, M.; Shimoyama, Y.; Saito, Y. Acta Crystallogr. C 1985, 41, 1698. (9) Dahl, I. M.; Kolboe, S. Catal. Lett. 1993, 20, 329. (10) Dahl, I. M.; Kolboe, S. J. Catal. 1994, 149, 458. (11) Dahl, I. M.; Kolboe, S. J. Catal. 1996, 161, 304. (12) Chen, J.; Wright, P. A.; Thomas, J. M.; Natarajan, S.; Marchese, L.; Bradley, S. M.; Sankar, G.; Catlow, C. R. A.; Gai-Boyes, P. L.; Townsend, R. P.; Lok, C. M. J. Phys. Chem. 1994, 98, 10216. (13) Castro, M.; Garcia, R.; Warrender, S. J.; Wright, P. A.; Cox, P. A.; Fecant, A.; Mellot-Draznieks, C.; Bats, N. Chem. Commun. 2007, 3470. (14) Meier, W. M.; Kokotailo, G. T. Z. Kristallogr. 1965, 121, 211. (15) Deroche, I,; Maurin, G.; Llewellyn, P.; Castro, M.; Wright, P. A.; Bejblova, M.; Cejka, J. Stud. Surf. Sci. Catal. 2007, 170, 1660. (16) Ashtekar, S.; Chilukuri, S. V. V.; Chakrabarty, D. K. J. Phys. Chem. 1994, 98, 4878. (17) Deroche, I.; Gaberova, L.; Maurin, G.; Castro, M.; Wright, P. A.; Llewellyn, P. L. J. Phys. Chem. C 2008, 112, 5048. (18) Maurin, G.; Llewellyn, P. L.; Bell, R. G. J. Phys. Chem. B 2005, 109, 16084. (19) Chen, D.; Moljord, K.; Fuglerud, T.; Holmen, A. Microporous Mesoporous Mater. 1999, 29, 191. (20) Prakash, A. M.; Unnikrishnan, S. J. Chem. Soc., Farady Trans. 1994, 90, 2291. (21) Mertens, M.; Strohmaier, K. G. U.S. Patent 6,773,688, 2004. (22) Diaz-Cabana, M. J.; Barrett, P. A.; Camblor, M. A. Chem. Commun. 1998, 1881. (23) Dee, D. P.; Chiang, R. L. U.S. Patent 6,824,590, 2004. (24) Amoureux, J.-P.; Fernandez, C. Solid State NMR 1998, 10, 211. (25) Oliver, S.; Kuperman, A.; Lough, A.; Ozin, G. A. J. Mater. Chem. 1997, 7, 807. (26) Chen, T.-H.; Wouters, B. H.; Grobet, P. J. Colloids Surf. A 1999, 158, 145. (27) Mali, G.; Meden, A.; Risti, A.; Tuar, N. N.; Kaucic, V. J. Phys. Chem. B 2002, 106, 63. (28) Gougeon, R. D.; Brouwer, E. B.; Bodart, P. R.; Delmotte, L.; Marichal, C.Chzeau; J, M.; Harris, R. K. J. Phys. Chem. B, 2001, 105, 12249. (29) Brown, S. P.; Wimperis, S. J. Magn. Reson. 1997, 128, 42. (30) Harlick, P. J. E.; Tezel, F. H. Microporous Mesoporous Mater. 2004, 76, 71. (31) Heinke, L.; Kortunov, P.; Tzoulaki, D.; Castro, M.; Wright, P. A.; Kaerger, J. EPL, 2008, 81, Article 26002. (32) Vrabec, J.; Stoll, J.; Hasse, H. J. Phys. Chem. B 2001, 105, 12126. (33) Evans, M. W. J. Chem. Soc., Farady Trans. 2 1986, 82, 1967. (34) Chen, D.; Moljord, K.; Fuglerud, T.; Holmen, A. Microporous Mesoporous Mater. 1999, 29, 191. (35) Wilson, S.; Barger, P. Microporous Mesoporous Mater. 1999, 29, 117. (36) Park, J. W.; Lee, Y. Y.; Kim, K. S.; Hong, S. B.; Seo, G. Appl. Catal., A 2008, 339, 36. (37) NIST Chemistry Web book, http://webbook.nist.gov/chemistry. (38) Song, W.; Fu, H.; Haw, J. F. J. Am. Chem. Soc. 2001, 123, 4749.

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