Synthesis, electron paramagnetic resonance and electron spin echomodulation studies on synthesized NiAPSO-41 molecular sieve andcomparison with ion-exchanged NiH-SAPO-41 molecular sieve

A. M. Prakash, Martin Hartmann¤ and Larry Kevan*Department of Chemistry, University of Houston, Houston, T exas 77204-5641, USA

Nickel-substituted silicoaluminophosphate type 41 (NiAPSO-41) molecular sieve has been synthesized hydrothermally using di-n-propylamine as the structure-directing agent. The reducibility of NiII in NiAPSO-41 by various reduction methods was comparedwith that of NiH-SAPO-41, where NiII was introduced into extraframework sites of SAPO-41 by partial ion exchange of HI byNiII. Dehydration at elevated temperatures or hydrogen treatment at moderate temperatures produces NiI species in NiH-SAPO-41. Two distinct NiI species, assigned as isolated NiI and as a complex, are observed by electron paramagnetic reso-NiIÈ(H2)nnance (EPR) in NiH-SAPO-41 after reduction in hydrogen at 573 K. By contrast, in NiAPSO-41 neither dehydration at hightemperatures nor hydrogen reduction was e†ective in producing NiI species. Isolated NiI species could, however, be obtained inNiAPSO-41 by c-irradiation of the materials at 77 K indicating the more stable nature of the NiII sites in NiAPSO-41. BothNiAPSO-41 and NiH-SAPO-41 show di†erences in their EPR characteristics after reduction and adsorption of various adsorb-ates suggesting that NiI in these two materials is in di†erent sites. Electron spin echo modulation studies of 31P modulation of NiIin NiAPSO-41 and NiH-SAPO-41 also showed signiÐcant di†erences in their modulation patterns. Simulation of the spectrumobserved for NiH-SAPO-41 shows two nearest-neighbour phosphorus atoms at a distance of 0.39 nm from NiI suggesting thatthe NiI ions are in the 10-membered ring channel near a 6-ring window. Simulation of the spectrum for NiAPSO-41 gives 12nearest-neighbour phosphorus atoms at a distance of 0.54 nm and can be rationalized in terms of NiI ions substituting into aframework phosphorus site.

Recent developments in the synthesis of microporousmaterials based on the aluminophosphate framework(AlPO4)have opened up new areas of interest in catalysis with molecu-lar sieves.1 Silicon-substituted aluminophosphate molecularsieves (SAPO) have evoked keen attention owing to the possi-bility of producing materials with controlled acidity. ThemodiÐcation of these materials by incorporation of transitionmetals either in framework or extraframework positions is ofpotential signiÐcance for speciÐc catalytic reactions.2 Severalstudies have been reported on the physical and chemicalnature of incorporated metal species in SAPO materials. Elec-tron paramagnetic resonance (EPR) spectroscopy in com-bination with electron spin echo modulation (ESEM)spectroscopy has been e†ectively employed to determine theenvironment of incorporated paramagnetic species.3 TheSAPO-5, SAPO-11 and SAPO-34 materials have been studiedfor the incorporation of various metals both in frameworkand extraframework positions.4h9 To a lesser extent, structuretypes such as SAPO-37, SAPO-42, SAPO-18 have been simi-larly investigated.10h12 However, other important large- andmedium-pore structure types such as SAPO-40, SAPO-41 andSAPO-46 have not been investigated for transition-metalincorporation, mainly because of the relative difficulties incrystallizing these structure types in pure form. The synthesisand characterization of pure SAPO-40, SAPO-41 andSAPO-46 have been reported recently.13h15

SAPO-41 is a novel medium pore molecular sieve.Transition-metal ion substitution into the framework ofSAPO-41 has not been reported. The structure of AlPO4-41,analogous to SAPO-41, has been reported.16 The frameworkstructure of SAPO-41 viewed along the [001] axis is shown inFig. 1. The framework topology consists of elliptical one-dimensional 10-ring channels that are slightly larger than the10-ring channels in AlPO4-11.

¤ Present address : Institut fu� r Chemische Technologie, Universita� tStuttgart, 70550 Stuttgart, Germany.

Substitution of silicon into frameworks can beAlPO4-nvisualized in terms of silicon substituting only aluminium(mechanism 1), only phosphorus (mechanism 2) or aphosphorusÈaluminium pair (mechanism 3). It has been gener-ally accepted that mechanisms 2 and 3 are responsible for the

Fig. 1 (a) Framework structure of SAPO-41 viewed along the [001]axis and (b) possible extraframework cation sites

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formation of SAPO structures.17 The mechanism of siliconsubstitution depends on the topology of the framework andon the method of synthesis. The mechanism of silicon substi-tution in the three known medium-pore SAPO structures,SAPO-11, SAPO-31 and SAPO-41, was studied recently by29Si MAS NMR and by temperature programmed desorptionof ammonia.14 While silicon substitution mostly proceeds viamechanism 3 in SAPO-11 and SAPO-31, mechanism 2 wasfound to be prominent in SAPO-41. It should be noted thatwhile mechanism 2 produces a net negative framework charge,mechanism 3 does not. Incorporation of a metal atom into abasic aluminophosphate framework can proceed either byreplacing an aluminium or a phosphorus in the framework.On the basis of various structural and elemental analysisresults, it has been shown that metal ions often replace alu-minium in the framework.18 However, recent ESEM studieson Ni-substituted SAPO-5 and SAPO-11 suggest that Ni ionsreplace phosphorus ions from the framework.19,20 Thus,depending on the type of substitution, incorporation of a diva-lent metal into a basic aluminophosphate framework can gen-erate either one or two framework negative charges. Theframework negative charge generated by the substitution ofsilicon and metal is normally balanced by protonated aminein the as-synthesized form and by H` ions in the calcinedform.

Incorporation of transition-metal ions into extraframeworksites in SAPO structures can also be achieved by exchangingwith H` ions. By analogy with the designation of extra-framework cation sites in SAPO-5,21 SAPO-1122 and inzeolite X,23 possible cation sites in SAPO-41 are designated inFig. 1. Site I (S I) is at the centre of a hexagonal prism (double6-ring straight channels). Site II (S II) is at the centre of asix-ring window that constitutes part of the side of a 10-ringstraight channel. The site II* (S II)* is displaced from site SIItowards the 10-ring channel and site II@ (S II@) corresponds todisplacement of site II away from the 10-ring channel towardsa double 6-ring. It should be noted that the 10-ring channel inSAPO-41 is surrounded by other 4-, 6- and 10-ring channelsrunning parallel to it. SAPO-41 di†ers in this respect fromother SAPO structures having a unidirectional channelsystem. In structure types such as SAPO-5, -11 and -31 themain 10- or 12-ring channel is surrounded by only 4- and6-ring channels and hence the S site is absent in theseII3structures. The adjacent nature of the 10-ring channels inSAPO-41 may be expected to inÑuence the interaction of theframework ions with external molecules and also the mobilityof extraframework cations through the channels.

It has been shown that NiI ions can be stabilized in zeo-lites,24 and SAPO19h22 materials. Supported NiI ions can beactive sites in catalytic reactions such as acetylenecyclomerization25 or ethene and propene oligomerization.26The catalytic activities of such materials are dependent uponthe valence state, location and dispersion of the NiI ions. Suc-cessful incorporation of NiII into both framework and extra-framework positions of SAPO-5 and SAPO-11 has beenreported recently.19h22 Ethene dimerization activity and selec-tivity on these materials has also been reported.27 Severalmethods have been adopted to reduce NiII ions in zeolites andother molecular sieves :28,29 dehydration at elevated tem-peratures, thermal reduction in at 473È573 K, photoreduc-H2tion by ultraviolet (UV) light irradiation at 77 K orc-irradiation at 77 K. The photoreduction method is very inef-Ðcient and normally requires long exposure times to reach NiIconcentrations sufficient for EPR studies. Dehydration andthermal reduction produces adequate NiI ions in certainmaterials, but the conditions must be carefully controlled tosuppress the formation of metallic nickel. Although c-irradiation at 77 K produces sufficient NiI ions, other specieslike trapped hydrogen and trapped holes on oxygen are alsoformed during this procedure.29

In this study we have hydrothermally synthesized NiAPSO-41 where nickel is believed to be incorporated into the frame-work. Spectroscopic evidence for Ni incorporation into theSAPO-41 framework is reported. The formation of mono-valent nickel in NiAPSO-41 and its interaction with severaladsorbates are compared to NiI species formed in NiH-SAPO-41 where NiII is incorporated by solid-state ion exchange intoknown extraframework sites.

Experimental

Synthesis

NiAPSO-41 was prepared hydrothermally using di-n-propyl-amine as the organic template. Samples of andAlPO4-41SAPO-41 were also prepared following the procedure report-ed elsewhere.14 The following chemicals were used withoutfurther puriÐcation : orthophosphoric acid (85%,Mallinckrodt), pseudoboehmite (Catapal-B, Vista), fumedsilica (Sigma), di-n-propylamine (98%, Aldrich), nickel acetatetetrahydrate (Aldrich) and (Aldrich). SynthesesNiCl2 ÉH2Owere carried out in a 100 cm3 stainless-steel reactor lined withTeÑon at autogenous pressure without agitation. In MAPO/MAPSO molecular sieves with M\ Co, Mn, Zn, Mg, etc., ithas often been assumed that the metal ion isomorphouslyreplaces Al from the framework. Assuming this initially fornickel (later shown to be wrong) the molar composition of thereaction mixture for the synthesis of NiAPSO-41 was chosenas : R : 55xNiO : (1.0 [ x/2)Al2O3 : 1.25 P2O5 : 0.1 SiO2 : 4.0

where R is di-n-propylamine. This speciÐc compositionH2O,was derived from the composition optimized for the synthesisof SAPO-41. Subsequent analysis (see text below) indicatesthat Ni substitutes for P in the framework. In a typical synthe-sis, 5.74 g of pseudoboehmite was slurried in 10 g of andH2Ostirred for 2 h. Then 0.30 g of nickel acetate dissolved in 8 g of

was mixed with 9.22 g of phosphoric acid and the solu-H2Otion added dropwise to the alumina slurry. After stirring themixture for about 1 h, 0.24 g of fumed silica mixed with 10 gof was added dropwise. The mixture was stirred for 30H2Omin. To this mixture 16.19 g of di-n-propylamine was addeddropwise followed by the addition of 5.4 g of The pH ofH2O.the gel was adjusted to 7.8 by adding 0.65 g of phosphoricacid diluted with 3 g of About 100 mg of Ðnely groundH2O.

crystals were then added to the gel as seed crystals.AlPO4-41The Ðnal mixture was stirred for another 1 h before charginginto the autoclave and heated to 473 K for 72 h. After crys-tallization the product was separated from the mother-liquor,washed with water and dried at 353 K overnight. The as-synthesized sample was calcined in by raising the tem-O2perature slowly to 823 K and held at this temperature for 12 hfor complete removal of the organic template. For EPR andESEM measurements a highly crystalline sample preparedwith 0.03 mol Ni in the synthesis mixture was used. Thechemical composition of this sample after calcination was

based on electron microprobe(Ni0.002Al0.510P0.464Si0.024)O2analysis.NiH-SAPO-41, where Ni ions exist in extraframework posi-

tions in the structure of SAPO-41, was prepared by solid-stateion exchange with and H-SAPO-41 at 823 K forNiCl2 É 6H2O16 h.22 Before and after solid-state ion exchange, the colour ofthe sample remains white. The chemical composition of thissample was based onNi0.002H0.028(Si0.030Al0.510P0.460)O2electron probe microanalysis.

Sample treatment and measurements

X-Ray powder di†raction patterns were recorded on a PhilipsPW1840 di†ractometer using Cu-Ka radiation. Thermogravi-metry (TG) was carried out in on a Dupont 957 thermalO2analyser at a heating rate of 10 K min~1. Chemical analyses

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of the samples were carried out by electron microprobeanalysis on a JEOL JXA-8600 spectrometer.

For EPR and ESEM measurements, calcined and hydratedsamples were loaded into 3 mm o.d. by 2 mm i.d. Suprasilquartz tubes and gradually heated in vacuum (\10~4 Torr)to 373 K and kept at this temperature for 16 h. To study thebehaviour of the nickel as a function of dehydration, thesamples were heated under vacuum from 373 to 773 K atintervals of 100 K. For each interval, the temperature wasraised slowly and held at for 16 h (thermal reduction). ThenEPR spectra were measured at 77 K to detect NiI species pro-duced by this thermal reduction. In another reduction methodthe samples were dehydrated as described above and thencontacted with 500 Torr of dry oxygen and subsequentlyheated to 823 K for 5 h. After oxidation with static oxygen thecolour of the samples was still white. The samples were thentreated with dry hydrogen (100 Torr) at room temperatureand subsequently heated to various temperatures from 373 to673 K for about 30 min (hydrogen reduction). It should benoted that for the success of these preparations the vacuumline and the hydrogen should be completely devoid of anywater since NiI is highly reactive to traces of In a thirdH2O.reduction method, the calcined dehydrated samples weretreated with oxygen and then with dry hydrogen as before.The samples were then exposed to c-radiation from a 60Cosource at 77 K to a total dose of 0.63 Mrad at a dose rate of0.25 Mrad h~1 (c-ray reduction).

In order to prepare nickel(I) complexes with various adsorb-ates the reduced samples were evacuated at room temperaturefor 10 min and then exposed to the room temperature vapourpressure of (Aldrich) and to 100 Torr (StohlerD2O ND3Isotope Chemicals). The samples with adsorbates were thenfrozen in liquid nitrogen and sealed. The colour of the sampleswas not changed during these sample treatments.

EPR spectra were recorded with a Bruker ESP 300 X-bandspectrometer at 77 K. The magnetic Ðeld was calibrated witha Varian E-500 gaussmeter. The microwave frequency wasmeasured by a Hewlett Packard HP 5342A frequency counter.ESEM spectra were measured at 4 K with a Bruker ESP 380pulsed EPR spectrometer. Three pulse echoes were measuredby using a n/2ÈqÈn/2ÈtÈn/2 pulse sequence as a function oftime t to obtain the time domain spectrum. To minimize 27Almodulation from framework aluminium in measurements ofphosphorus modulation, the q value was Ðxed accordinglydepending on the magnetic Ðeld position. The phosphorusmodulations were analysed by a spherical approximation forpowder samples in terms of N nuclei at distance R with anisotopic hyperÐne coupling The best Ðt simulation ofAiso .30an ESEM signal is found by varying the parameters until thesum of the squared residuals is minimized.

Results

Synthesis

In the synthesis of NiAPSO-41, crystals of wereAlPO4-41employed as seeds. A pure phase could be obtained with arelatively low concentration of Ni (0.01È0.05 mole) in the reac-tion gel while keeping the silica concentration at 0.1 mole. Assynthesized NiAPSO-41 was light violet in colour. At higherNi concentrations, a dense phase started crystallizing alongwith NiAPSO-41 and the colour of the sample changed fromviolet to green. Scanning electron microscopic analysis of thevarious products established the crystallinity and phase purityof these samples. A single phase of crystals with size \1 lmwas observed in samples prepared with a low concentration ofNi (\0.05 mole). In samples prepared with higher Ni concen-trations ([0.05 mole), particles of irregular shape, probablyamorphous nickel aluminophosphate, were also observed inincreasing proportions along with the major phase. Micro-

probe analysis of these irregular shaped particles gave a highconcentration on Ni as compared to the low nickel concentra-tion observed in crystals of the main phase.

X-Ray powder di†raction (XRD)

Samples were characterized by XRD. Fig. 2 shows the powderXRD pattern of as-synthesized SAPO-41 andAlPO4-41,NiAPSO-41 (prepared with 0.03 mole NiO in the gel). Thesepatterns, both in intensity and line position, matched wellwith the patterns reported for structure type 41.31 Slightreductions in the intensity of various lines of NiAPSO-41 wereobserved in comparison to that of and SAPO-41.AlPO4-41No extra peaks or peak broadening were observed inNiAPSO-41. Such a similarity of the XRD patterns has beendemonstrated previously for NiAPSO-519 and NiAPSO-11.32

Thermogravimetry (TG)

The TG curves of as-synthesized SAPO-41 andAlPO4-41,NiAPSO-41 are shown in Fig. 3. The initial small weight lossaround 373 K is due to the desorption of physically adsorbedwater. The desorption of the organic template takes place in asingle step in and in multisteps in SAPO-41 andAlPO4-41NiAPSO-41. The low-temperature weight losses around 473K in all the compositional variants are possibly due to decom-position of dipropylamine occluded inside the channels. Thehigh-temperature weight loss observed for SAPO-41 andNiAPSO-41 is probably due to the desorption and decompo-sition of protonated amines balancing the framework negative

Fig. 2 X-Ray powder di†raction pattern of as-synthesized AlPO-41(top), SAPO-41 (middle) and NiAPSO-41 (bottom)

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Fig. 3 Thermogravimetry curves of as-synthesized AlPO-41 (top),SAPO-41 (middle) and NiAPSO-41 (bottom)

charge.33 Being a neutral framework, does notAlPO4-41require any protonated amines in its channels which explainsthe absence of such a high-temperature weight loss in thismaterial. The high-temperature weight loss observed inSAPO-41 and NiAPSO-41 was higher than the correspondingweight loss reported for SAPO-11.32 This is possibly due to ahigher number of acid sites in these materials compared toSAPO-11.

Electron paramagnetic resonance

Calcined and oxidized samples of NiAPSO-41 and NiH-SAPO-41 did not show any EPR signal at 77 K. Thus, the Nispecies exist in the form of NiII. Fig. 4 shows the EPR spectraof NiH-SAPO-41 and NiAPSO-41 dehydrated at 673 K for 16h. Dehydration at this temperature leads to the formation ofthree paramagnetic species, denoted A, B and C, in NiH-SAPO-41. Species A with an axially symmetric g tensor isascribed to isolated NiI reduced by desorbing water andhydroxy groups as suggested earlier.32 The g values for speciesA and are similar to those reported(g

M\ 2.115 g

A\ 2.498)

for NiI in NiH-SAPO-5,21 NiH-SAPO-11,22 NiCa-Y34 andNiCa-X.35 In contrast, NiAPSO-41 does not show an EPRsignal corresponding to species A. Dehydration of this sampleeven at higher temperatures does not yield any signal due toisolated NiI species. Species B and C are both seen inNiAPSO-41 and NiH-SAPO-41. Similar species have alsobeen reported in and SAPO materials.21 ThereforeAlPO4species B and C are tentatively assigned to non-speciÐc frame-work defects arising due to water or hydroxy group desorp-tion.

Fig. 4 EPR spectra at 77 K of NiH-SAPO-41 and NiAPSO-41dehydrated at 673 K for 16 h : (a) NiH-SAPO-41 and (b) NiAPSO-41

Fig. 5 shows the EPR spectra of NiH-SAPO-41 andNiAPSO-41 after hydrogen reduction at 573 K. Two distinctNiI species are observed in NiH-SAPO-41. However, no signalascribable to NiI is observed in NiAPSO-41. Species A inNiH-SAPO-41 after hydrogen reduction is essentially thesame as that observed after thermal reduction. This speciesremains after hydrogen outgassing at room temperature [Fig.5(b)]. On the other hand, species E is lost after evacuation ofthe sample at room temperature but is readily regenerated bysubsequent exposure to dry hydrogen. Thus, species E isassigned to The behaviour of NiH-SAPO-41 onNiIÈ(H2)n .hydrogen reduction and subsequent evacuation is similar tothe behaviour observed earlier for NiH-SAPO-519 and NiH-SAPO-1132 upon the same treatments. Isolated NiI in NiH-SAPO-41 has comparable g values to NiH-SAPO-5 andslightly higher g values in comparison to NiH-SAPO-11. InNiAPSO-41 hydrogen treatment even at higher temperaturedid not produce any EPR signals due to isolated NiI.

Isolated NiI ions in both NiAPSO-41 and NiH-SAPO-41can be generated by c-ray irradiation at 77 K. Fig. 6 shows

Fig. 5 EPR spectra at 77 K of NiH-SAPO-41 and NiAPSO-41 aftertreatment : (a) NiH-SAPO-41 after treatment at 573 K for 30H2 H2min, (b) after 10 min evacuation of (a) at room temperature and (c)

NiAPSO-41 after treatment at 573 K for 30 minH2

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Fig. 6 EPR spectra at 77 K of (a) NiH-SAPO-41 and (b) NiAPSO-41 after c-ray irradiation at 77 K for 2.5 h

EPR spectra at 77 K after c-ray irradiation of NiH-SAPO-41and NiAPSO-41. Similar spectra are observed for bothsamples. Species A in NiH-SAPO-41 and in NiAPSO-41 issimilar to the species observed in hydrogen-reduced NiH-SAPO-41 where it was assigned to isolated NiI ions. c-Irradiation at 77 K of Y zeolite produces trapped hydrogenatoms (EPR doublet split by 500 G) and trapped holes onoxygen bonded to aluminium or silicon near g \ 2.00 (V1centre).29 These paramagnetic species are observed in bothNiH-SAPO-41 and NiAPSO-41. Another paramagneticspecies observed after c-ray reduction is species E with axiallysymmetric g values, and in NiH-SAPO-g

A\ 2.346 g

M\ 2.064

41 and and in NiAPSO-41. A similargA

\ 2.356 gM

\ 2.066species is also observed in hydrogen-reduced NiH-SAPO-41where it was assigned to The EPR signal intensityNiIÈ(H2)n .for species A produced by c-ray reduction is substantiallyhigher than the corresponding signal intensity for this speciesafter thermal or hydrogen reduction. Again the intensity ofspecies A in NiH-SAPO-41 is higher than in NiAPSO-41.After c-ray reduction about 2.9% of the Ni is EPR visible inNiAPSO-41, whereas in NiH-SAPO-41 this is about 5% ofthe total Ni content of the sample.

ESEM measurements of 31P modulation of NiI were per-formed on both NiAPSO-41 and NiH-SAPO-41 samples afterc-ray reduction at 77 K in The magnetic Ðeld was Ðxed atH2 .

(3318 G) of species A (see Fig. 6). At this magnetic Ðeld thegMvalue of q (0.27 ls) is selected so as to suppress both 27Al and

1H modulation. Furthermore, the sample is evacuated forabout 15 min prior to the ESEM measurements in order toavoid possible contributions from species E due to NiIÈ(H2)nto the 31P modulation of NiI species A. It is observed thatspecies E appears in the EPR spectrum only when hydrogen ispresent in the sample. ESEM measurements of 31P modula-tion of NiI species A produced by c-ray reduction in NiH-SAPO-41 and NiAPSO-41 show signiÐcant di†erencesbetween these two species although they have similar EPRparameters. Fig. 7 shows the experimental and simulatedthree-pulse 31P ESEM patterns recorded at q\ 0.27 ls tosuppress the zeolitic 27Al modulation. Table 1 summarizes theESEM parameters for NiI species in NiH-SAPO-41 andNiAPSO-41. Simulation of 31P modulation for NiI in NiH-

Fig. 7 Experimental (ÈÈ) and simulated (É É É É É) three-pulse ESEMspectra showing 31P modulation of NiI : (a) NiAPSO-41 and (b) NiH-SAPO-41

SAPO-41 shows two nearest-neighbour phosphorus atoms ata distance of 0.35 nm and three next nearest-neighbour phos-phorus atoms at a distance of 0.59 nm. The simulation param-eters obtained for NiH-SAPO-41 di†er from similarparameters obtained for NiH-SAPO-11 and NiH-SAPO-5.For both NiH-SAPO-5 and NiH-SAPO-11, 5.2 and 5.3nearest-neighbour phosphorus atoms are obtained at dis-tances of 0.33 and 0.34 nm respectively.20 In contrast, simula-tion of the 31P modulation for NiI in NiAPSO-41 showstwelve nearest-neighbour phosphorus atoms at a distance of0.54 nm and 10 next-nearest phosphorus atoms at a distanceof 0.78 nm. Somewhat similar parameters are reported inNiAPSO-519 and NiAPSO-1120 materials.

Before an adsorbate was added, hydrogen-reduced samplesof NiH-SAPO-41 and c-ray reduced samples of NiAPSO-41were evacuated for 10 min at room temperature so that onlyisolated NiI remains. The sample was then exposed to theadsorbate for 2 min at room temperature, and the reactionwas quenched at 77 K. Fig. 8 shows the EPR spectra obtainedafter adsorption on NiH-SAPO-41. The adsorptionD2O D2Oon NiH-SAPO-41 produces a single species with a rhombic gtensor and A signal with(g1\ 2.163, g2 \ 2.091 g3 \ 2.065).similar g values has been observed in NiH-SAPO-1132 andNiH-SAPO-5,21 where it was assigned to a complexNiIÈ(O2)nproduced by the decomposition of on NiI sites. ThisD2Oassignment was based on the observation that a species withsimilar EPR parameters is also found after adsorption ofoxygen on NiH-SAPO-11 and NiAPSO-11 as well as onNiCa-X zeolite.32,35 ESEM studies for species F at g \ 2.163and g \ 2.065 showed no deuterium modulation to assign thisspecies to a possible complex.21 This further sup-NiIÈ(D2O)

nports the present assignment. Species F in NiH-SAPO-41decays completely and species B with andg

A\ 2.022 g

M\

2.007 appears upon annealing the sample at room tem-perature for 1 h. Decomposition of water on NiI has been sug-gested for the formation of this radical species. On the other

Table 1 31P ESEM parameters for NiI species in NiH-SAPO-41 andNiAPSO-41

sample shell Na Rb/nm Aiso c/MHz

NiH-SAPO-41 1 2 0.35 0.102 3 0.59 0.05

NiAPSO-41 1 12 0.54 0.052 10 0.78 0

a Number of phosphorus atoms. b Distance between NiI and phos-phorus ; estimated uncertainty is ^0.01 nm. c Isotropic hyperÐnecoupling constant ; estimated uncertainty is ^10%.

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Fig. 8 EPR spectra at 77 K of NiH-SAPO-41 (a) after reductionH2at 573 K for 30 min and evacuation at room temperature, (b) afteradsorption for 2 min at room temperature and (c) after anneal-D2Oing (b) at room temperature for 1 h

hand, adsorption on NiAPSO-41 (Fig. 9) produces twoD2Ospecies F and G each characterized by three g components.The discrimination of the g components between species Fand G is based on the preferental thermal decay of the com-ponents of species G. Species F is also observed in NiH-SAPO-41 and is assigned to a complex. SimilarNiIÈ(O2)nspecies to F have also been reported on NiAPSO-519 andNiAPSO-1132 after adsorption of on these materials.D2OHowever, the second species G with g1 \ 2.257, g2 \ 2.136and was not observed in these other materials.g3 \ 2.122While species G disappears completely after annealing thesample at room temperature, species F reduces its intensity byabout 50%. Note that species F in NiH-SAPO-41 decayscompletely with the concomitant formation of an radicalO2~species upon the same treatment. It has been already reportedthat the radical species is observed on NiH-SAPO-5 andO2~-11 but not on NiAPSO-5 and -11 after adsorption andD2Osubsequent annealing at room temperature for 1 h.19,32However, the same comparison between NiH-SAPO-41 andNiAPSO-41 is not possible because of the very strong signalat g \ 2.00 already present in the spectrum of NiAPSO-41after c-ray irradiation.

Adsorption of at room temperature on NiH-SAPO-41ND3showing species A leads to the formation of species H (Fig. 10)with a rhombic g tensor with andg1\ 2.161, g2\ 2.088 g3\2.061. Hydrogen reduction in NiH-SAPO-41 is highly sensi-tive to the reduction conditions as can be seen from the broadbaseline due to Ni0 clusters observed after reduction. The gvalues of species H are almost the same as those of species Fobserved in this material after adsorption. However,D2Ospecies H appears to be more stable than species F as there isno intensity change observed after annealing the sample atroom temperature for 1 h. The intensity of a weak radicalspecies at g \ 2.009 observed after ammonia adsorptionremains practically the same even after 1 h of annealing thesample at room temperature. It should be noted that the

Fig. 9 EPR spectra at 77 K of NiAPSO-41 : (a) after c-ray reductionat 77 K for 2.5 h, (b) after evacuation at room temperature and sub-sequent adsorption for 2 min at room temperature and (c) afterD2Oannealing (b) at room temperature for 1 h

Fig. 10 EPR spectra at 77 K of NiH-SAPO-41 after (a) reductionH2at 573 K for 30 min and evacuation at room temperature, (b) ND3adsorption for 2 min at room temperature, and (c) annealing (b) atroom temperature for 1 h

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Fig. 11 EPR spectra at 77 K of NiAPSO-41 : (a) after c-rayreduction at 77 K for 2.5 h, (b) after evacuation at room temperatureand subsequent adsorption of for 2 min at room temperatureND3and (c) after annealing (b) at room temperature for 1 h

intensity of the radical species formed after adsorptionD2Oincreases substantially after 1 h of annealing at room tem-perature. On the other hand, the adsorption of ammonia onNiAPSO-41 produces two species H and I (Fig. 11) withrhombic g tensors. Species H with andg1\ 2.158, g2 \ 2.090

is similar to that observed in NiH-SAPO-41. Fur-g3\ 2.062thermore, the EPR parameters of species H and I observed inNiAPSO-41 are similar to those of species F and G observedin the same material after adsorption.D2OTable 2 summarizes the EPR parameters and speciesassignments for various NiI species after reduction andadsorption of various adsorbates on NiH-SAPO-41 andNiAPSO-41.

DiscussionSince dipropylamine templates at least nine structure types inthe synthesis of aluminophosphate-based molecular sieves, itis important to control the synthesis conditions in order toobtain crystals of a given structure type in pure form.33 Theinitial gel composition, pH, temperature and duration ofheating inÑuence the crystallinity and purity of these struc-tures. Some systematic studies have been reported on thepreparation of various structure types in the presence of di-propylamine by varying the synthesis conditions and gel com-position.18,33 The structure type 41 was reported to becrystallized from a gel having a high concentration of organictemplate at low silicon or metal concentration. Similar condi-tions were observed in the synthesis of NiAPSO-41. However,this material can be prepared only by adding seed crystals of

to the gel prior to heating. Our attempt to prepareAlPO4-41this material without seed crystals was not successful. On theother hand, both and SAPO-41 could be preparedAlPO4-41without seed crystals in the gel for longer periods of heating ata relatively lower temperature of 453 K. However, by addingseed crystals the period of crystallization of both AlPO4-41and SAPO-41 was reduced considerably. The e†ect of addingseed crystals to a synthesis batch is to increase the crys-tallization rate by providing more surface area for the assimi-lation of nutrient material from solution which shortens thetime required for the crystallization to be completed.36However, the exact mechanism of nuclei formation in a seededsystem is not yet fully understood.

The X-ray powder di†raction pattern of NiAPSO-41matches well with the patterns of and SAPO-41.AlPO4-41No peak broadening is observed between these compositionalvariants. Such a similarity of the XRD patterns has beendemonstrated previously for NiAPSO-5 and NiAPSO-11.19,32A signiÐcant colour di†erence depending on the location oftransition-metal ions in tetrahedral framework sites or in ion-exchanged sites has been demonstrated for MnAPO-115 andCoAPO6 samples. When NiII is added during synthesis toform NiAPSO-41, the product is light violet. After calcinationof the samples at 823 K, the colour changes to white.However, when NiII is incorporated by solid-state ionexchange to form NiH-SAPO-41, the product is white. Chemi-cal analysis gave the same nickel amount for these twosamples. Thus, the observed colour di†erence between the twosamples may be possibly due to a di†erence in the location ofnickel in these samples. Di†use reÑectance UVÈVIS spectro-scopic measurement on these samples was not successful dueto the very low concentration of nickel ions.

The TG data for compared with those forAlPO4-41SAPO-41 and NiAPSO-41 appear to be signiÐcantly di†erent.The extra step at high temperature assigned to the com-

Table 2 EPR g values of NiI species in NiH-SAPO-41 and NiAPSO-41 produced by various reduction methods and with various adsorbates

reductionmethod sample adsorbate species assignment g

Aor g1 g

Mor g2 g3

thermal NiH-SAPO-41 È A NiI 2.498 2.115NiAPSO-41 È È È È

H2 NiH-SAPO-41 È A NiI 2.489 2.109E NiIÈ(H2)n 2.339 2.067

NiAPSO-41 È È È È Èc-ray ] H2 NiH-SAPO-41 È A NiI 2.501 2.109

E NiIÈ(H2)n 2.346 2.064NiAPSO-41 È A NiI 2.493 2.108

E NiIÈ(H2)n 2.356 2.066H2 NiH-SAPO-41 D2O F NiIÈ(O2)n 2.163 2.091 2.065c-ray ] H2 NiAPSO-41 D2O F NiIÈ(O2)n 2.178 2.086 2.062

G NiIÈ(O2)n 2.257 2.136 2.112H2 NiH-SAPO-41 ND3 H NiIÈ(ND3)n 2.163 2.091 2.065c-ray ] H2 NiAPSO-41 ND3 H NiIÈ(ND3)n 2.178 2.086 2.062

I NiIÈ(ND3)n 2.229 2.121 25 2.119

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bustion of the templating agent in SAPO-41 and NiAPSO-41suggests that part of the dipropylamine is protonated.Depending on the mechanism of substitution of silicon andmetal ions, one or more net framework charges can be createdwhich can be balanced by protonated amine. Furthermore,the slightly higher weight loss observed for NiAPSO-41 (3.65wt.%) in comparison to SAPO-41 (3.23 wt.%) suggests thepresence of additional acid sites due to the incorporation ofmetal in the framework. Note that both the materials wereprepared with the same amount of silica in the synthesis gel.

The reducibility of NiII in zeolites and other molecularsieves depends on several factors such as the structure type,the location and the ion concentration.34 The reducibility ofNiII in SAPO-5 and SAPO-11 has been studied pre-viously.19h22,32 The location of the cation site and thereby theaccess to various reducing agents can also play a role in thesematerials. The synthesized NiAPSO-41 and ion-exchangedNiH-SAPO-41 di†er considerably in their behaviour towardsdehydration at elevated temperatures. NiH-SAPO-41 readilyproduces isolated NiI species when dehydrated at tem-peratures above 573 K as evidenced from the EPR spectra(Fig. 4). Water or hydroxy groups are suggested to beresponsible for this reduction. Kasai et al.37 suggested that thereduction of CuII ions in Cu-exchanged mordenite observedwhen heating in vacuum is due to reduction by residual water.NiAPSO-41 does not yield any EPR signal due to isolated NiIions when dehydrated at high temperatures. This di†erence inthe behaviour of the NiAPSO-41 compared to NiH-SAPO-41is also found by heating with hydrogen at moderate to hightemperatures. NiH-SAPO-41 produces several isolated NiIspecies depending on the reduction temperature.38 A singleisolated NiI species with rhombic g tensors is observed whenreduced at 373 K. At higher temperatures, this species trans-forms to another species with axially symmetric g values. Onthe other hand, hydrogen treatment was ine†ective in produc-ing monovalent nickel ions in NiAPSO-41 as shown by noEPR signal of NiI. The fact that both NiH-SAPO-41 andNiAPSO-41 behave di†erently on dehydration and hydrogentreatment suggests that NiII in NiAPSO-41 is in a di†erent sitethan that in NiH-SAPO-41.

Although thermal and hydrogen reduction are ine†ectivefor producing NiI ions in NiAPSO-41, c-ray irradiation at 77K is e†ective as shown by EPR (Fig. 6). This further suggeststhat NiII in NiAPSO-41 is in a more stable site than it is inNiH-SAPO-41.

The contrasting 31P modulations of NiI species A producedby c-ray reduction in NiH-SAPO-41 and NiAPSO-41 providegood evidence for framework substitution of nickel inNiAPSO-41. There are signiÐcant di†erences in the 31Pmodulation patterns for NiH-SAPO-41 and NiAPSO-41. Thetwo nearest-neighbour phosphorus atoms at a distance of 0.35nm and three next nearest-neighbour phosphorus atoms at adistance of 0.59 nm in NiH-SAPO-41 are consistent with NiIions at ion-exchange site II* in the main 10-ring channel (Fig.1). From the framework chemical composition of SAPO-41,there are two to three P nuclei and three Al nuclei and zero toone Si nuclei expected in a six-membered ring window of theframework. A 6-ring window consisting of three phosphorusand three aluminiums has no net framework charge for thatwindow. Thus, it is likely that the positively charged nickelion will locate near a negatively charged 6-ring window withat least one silicon substituted for phosphorus. The nickel ionsite observed in SAPO-41 di†ers from nickel ion sitesobserved in SAPO-11 and SAPO-5. On the basis of ESEMstudies, it has been shown that in NiH-SAPO-5 and NiH-SAPO-11, NiI ions situate at site I at the centre of a hexagonalprism.21,22

On the other hand, simulation of the 31P modulation forNiI in NiAPSO-41 shows twelve nearest-neighbour phos-phorus atoms at distance of 0.54 nm and ten next-nearest

phosphorus atoms at a distance of 0.78 nm. These observ-ations are consistent with nickel ions being substituted forphosphorus atoms in the original framework. This result isalso consistent with the chemical analysis. If Ni replaces Al,the nearest phosphorus distance would be much less than 0.54nm. Similar conclusions have also been drawn for nickel sub-stitution in the frameworks of SAPO-5 and SAPO-11.19,20

The behaviour of NiAPSO-41 and NiH-SAPO-41 afteradsorbing and also show some di†erences whichD2O ND3can be attributed to a di†erence in cation locations. Whileadsorption of produces a single species F in NiH-SAPO-D2O41, two di†erent species F and G (Fig. 9) are observed inNiAPSO-41. Species F has been reported earlier in both zeo-lites and SAPO materials as a complex. Decomposi-NiIÈ(O2)ntion of water on NiI is suggested for the formation of thisspecies.32 The presence of the second species G in NiAPSO-41suggests the possibility of more than one complexNiIÈ(O2)npossibly arising from nickel ions situated at two sites. Simi-larly, interaction of ammonia with NiI produces a singlespecies H (Fig. 10) in NiH-SAPO-41 and two species H and Iin NiAPSO-41 (Fig. 11). These species are likely NiIÈ(ND3)ncomplexes. As in the case of adsorption, ammoniaD2Oadsorption yields two species in NiAPSO-41. The similar EPRspectra for species F and H suggest that both species arelocalized at the same site in the molecular sieve structure.

The EPR signal observed in NiH-SAPO-41 after ammoniaadsorption di†ers considerably from that observed in NiH-SAPO-5.19 Ammonia adsorption on NiH-SAPO-5 producestwo species depending on the ammonia pressure. At a lowammonia pressure (20 hPa) a single species with an axiallysymmetric g tensor and is observed,(g

A\ 2.325 g

M\ 2.073)

whereas at high ammonia pressure (100 hPa) a second specieswith a rhombic g tensor and(g1 \ 2.635, g2\ 2.204 g3\

is observed together with the Ðrst species. HyperÐne2.034)splitting is also observed for these two species. For NiH-SAPO-41 no hyperÐne splitting is observed. The g values ofspecies H are di†erent from those of the ammonia speciesobserved in NiH-SAPO-5 which suggests that the coordi-nation of ammonia with NiI in these two materials is di†erent.

For nickel substituting into a framework site it is possibleto visualize more than one site for coordination with adsorbedmolecules. A nickel atom situated at a framework site whichconnects two 10-ring channels (site 1 in Fig. 1) may have dif-ferent coordination with or molecules than a siteD2O ND3which connects a 10-ring channel and a 4- or 6-ring channel(site 2). Thus, the observed two species after or inD2O ND3NiAPSO-41 can be explained on this basis.

ConclusionsHydrothermal synthesis of NiAPSO-41 molecular sieve hasbeen achieved using di-n-propylamine as the structure-directing agent. The material has been characterized byvarious physicochemical methods. Comparative EPR andESEM studies between synthesized NiAPSO-41 and ion-exchanged NiH-SAPO-41 show signiÐcant di†erences withrespect to NiI reduction, location and adsorbate interaction.Whereas thermal and hydrogen reduction produce isolatedNiI ions in NiH-SAPO-41, these methods are not e†ective inNiAPSO-41. In NiH-SAPO-41 a second NiI species assignedto a complex is also obtained after hydrogenNiIÈ(H2)nreduction at 373 K. This species is found to be stable only inthe presence of hydrogen. Although thermal and hydrogenreduction does not produce isolated NiI ions in NiAPSO-41,c-ray reduction at 77 K is e†ective which suggests that the NiIIions are more stable in NiAPSO-41 than in NiH-SAPO-41.Both NiAPSO-41 and NiH-SAPO-41 show di†erences in theirEPR spectra after and adsorption at room tem-D2O ND3perature. Upon adsorbing a complex with aD2O, NiIÈ(O2)nrhombic g tensor is produced in both NiAPSO-41 and NiH-

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SAPO-41. Decomposition of water on NiI is suggested for theformation of this species. A second species with aNiIÈ(O2)nrhombic g tensor due to NiI ions at a di†erent site is alsoobserved in NiAPSO-41. Two di†erent NiI ammonia complex-es are also observed in NiAPSO-41 after adsorption ofammonia at room temperature. NiH-SAPO-41 however,shows only a single complex. Electron spin echoNiIÈ(ND3)nmodulation studies of 31P modulation of NiI in NiAPSO-41and NiH-SAPO-41 show signiÐcant di†erences in their modu-lation patterns. Simulation parameters for NiH-SAPO-41suggest that NiI is situated at site II* in a 10-ring channel neara six-ring window. In contrast, simulation of the 31P modula-tion for NiAPSO-41 indicates that the NiI ions are situated ina framework phosphorus site.

research was supported by the National Science Founda-Thistion and the Robert A. Welch Foundation.

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Paper 6/06398E; Received 17th September, 1996

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