quantitative investigations of acidity, and transient acidity, in zeolites and molecular sieves ...

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Quantitative Investigations of Acidity, and Transient Acidity, in Zeolites and Molecular Sieves ² Xingwu Wang, Jeffrey Coleman, Xin Jia, and Jeffery L. White* Campus Box 8204, Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: December 18, 2001; In Final Form: February 21, 2002 To demonstrate that transient acidity exists in certain solid acid catalysts, a convenient and robust 1 H solid- state NMR spin-counting method is introduced for the accurate quantification of Bronsted acid sites in zeolites and molecular sieves. Poly(dimethylsiloxane) is used as an inert and easily handled spin-counting standard, allowing internal calibration of MAS NMR peaks arising from both acidic and nonacidic hydrogens in the catalyst. Results from example systems including H-ZSM5, H-ferrierite (H-FER), and SAPO-34 are presented, and the relevance of these measurements to zeolite synthesis conditions, postsynthetic treatments, and reaction mechanisms are discussed. Using this technique, we show the first direct spectroscopic proof that framework Bronsted acidity in SAPO-34 molecular sieves decreases with time following removal of the synthesis template molecules. This effect is similar to hydrothermally catalyzed dealumination in traditional zeolites, except that it occurs at ambient temperature and moisture levels. Our data indicates that these catalysts must be stored in a moisture-free atmosphere to preserve activity following template removal. These results are key to understanding hydrocarbon conversions in SAPO catalysts, particularly in the methanol-to-hydrocarbon chemistry area, where SAPO-34 is a leading candidate for commercialization. Introduction The specific characteristics of zeolites and molecular sieves that are responsible for their ability to catalyze chemical reactions are their regular crystalline structure and their acidity. In many cases, acid site density is not constant for a given catalyst but changes with experimental conditions. Although X-ray diffraction methods are routinely used for quantifying crystallinity and elucidating structure types, there does not exist a reliable, routinely accessible method to measure the simplest aspect of zeolite acidity, i.e., the number of Bronsted acid sites. The most common techniques involve adsorption or temperature programmed desorption of NH 3 and infrared spectroscopy. Each of these methods can present problems, as the NH 3 TPD methods may suffer from multiple adsorbates per acid site and extreme sensitivity to subtle changes in the experimental parameters (flow rates, catalyst bed depths, temperature gradi- ents, etc.). Furthermore, quantification by infrared methods is limited by a lack of knowledge of molar extinction coefficients for either direct observation of acidic hydroxyl groups or adsorbates such as pyridine. Several reports have highlighted the use of 31 P MAS NMR of trimethylphosphine 1 and trimeth- ylphosphine oxide 2 probe molecules for probing both Lewis and Brønsted acidity. Novel Na-poisoning experiments have also been effectively used to determine the number of catalytically active framework sites. 3 Farneth and Gorte have recently reviewed the variety of methods used to characterize both the strength and number of acid sites. 4 In this review, the preferred methods for quantifying Brønsted sites involve combined TPD and TGA analysis of reactive amines such as isopropylamine, because stoichiometric reaction with the catalyst produces one olefin and one ammonia molecule per acid site. 5 Even so, a convenient, direct, and noninVasiVe method for acidity deter- mination is necessary for the routine analysis of variations in framework acid site concentration with catalyst synthesis conditions, postsynthetic treatments, and catalyst modification methods (e.g., final Brønsted acid site concentration vs synthesis gel SiO 2 /Al 2 O 3 , synthesis pH, 6 template type and amount, 7,8 dealumination, 9,10 surface titrations, 11 and hydrothermal treat- ment 12 ). Such a technique is desirable because no assumptions about active site accessibility by a probe molecule are necessary. The ability to independently measure acid site concentrations in different zeolite types allows quantitative comparisons of acid site number vs acid site strength data in many hydrocarbon reactions. For example, one could imagine preparing a certain Si/Al ratio zeolite and comparing its activity/selectivity to a second catalyst that was synthesized with a lower Si/Al ratio but had been dealuminated to the same framework acidity as the original material. This would address postulates concerning creation of “unequal” or “enhanced” Brønsted acid sites via dealumination methods. 13 Many assumptions concerning zeolite chemistry could be tested if the experimentalist could easily and accurately determine the Brønsted acid site density in any zeolite using a routine noninvasive method. In this contribution, we demonstrate the first quantitative 1 H solid-state NMR data proving that Brønsted acid sites in SAPO- 34, a silicoaluminophosphate catalyst used for methanol-to- olefin hydrocarbon chemistries, are not stable in the presence of moisture, even at ambient temperatures. In our pursuit of an experimental strategy which provides unambiguous proof of this effect, we show that a combination of solid-state magic-angle (MAS) spinning at moderate spinning speeds and 1 H spin- counting NMR with an internal standard provides an easily accessible, noninvasive, and completely quantitative way to * To whom correspondence should be addressed. E-mail: Jeff_L_White@ ncsu.edu. ² Presented at the 43rd Rocky Mountain Conference on Analytical Spectroscopy, July 2001, Denver, CO. 4941 J. Phys. Chem. B 2002, 106, 4941-4946 10.1021/jp0145816 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002

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Quantitative Investigations of Acidity, and Transient Acidity, in Zeolites and MolecularSieves†

Xingwu Wang, Jeffrey Coleman, Xin Jia, and Jeffery L. White*Campus Box 8204, Department of Chemistry, North Carolina State UniVersity,Raleigh, North Carolina 27695-8204

ReceiVed: December 18, 2001; In Final Form: February 21, 2002

To demonstrate that transient acidity exists in certain solid acid catalysts, a convenient and robust1H solid-state NMR spin-counting method is introduced for the accurate quantification of Bronsted acid sites in zeolitesand molecular sieves. Poly(dimethylsiloxane) is used as an inert and easily handled spin-counting standard,allowing internal calibration of MAS NMR peaks arising from both acidic and nonacidic hydrogens in thecatalyst. Results from example systems including H-ZSM5, H-ferrierite (H-FER), and SAPO-34 are presented,and the relevance of these measurements to zeolite synthesis conditions, postsynthetic treatments, and reactionmechanisms are discussed. Using this technique, we show the first direct spectroscopic proof that frameworkBronsted acidity in SAPO-34 molecular sieves decreases with time following removal of the synthesis templatemolecules. This effect is similar to hydrothermally catalyzed dealumination in traditional zeolites, except thatit occurs at ambient temperature and moisture levels. Our data indicates that these catalysts must be storedin a moisture-free atmosphere to preserve activity following template removal. These results are key tounderstanding hydrocarbon conversions in SAPO catalysts, particularly in the methanol-to-hydrocarbonchemistry area, where SAPO-34 is a leading candidate for commercialization.

Introduction

The specific characteristics of zeolites and molecular sievesthat are responsible for their ability to catalyze chemicalreactions are their regular crystalline structure and their acidity.In many cases, acid site density is not constant for a givencatalyst but changes with experimental conditions. AlthoughX-ray diffraction methods are routinely used for quantifyingcrystallinity and elucidating structure types, there does not exista reliable, routinely accessible method to measure the simplestaspect of zeolite acidity, i.e., the number of Bronsted acid sites.The most common techniques involve adsorption or temperatureprogrammed desorption of NH3 and infrared spectroscopy. Eachof these methods can present problems, as the NH3 TPDmethods may suffer from multiple adsorbates per acid site andextreme sensitivity to subtle changes in the experimentalparameters (flow rates, catalyst bed depths, temperature gradi-ents, etc.). Furthermore, quantification by infrared methods islimited by a lack of knowledge of molar extinction coefficientsfor either direct observation of acidic hydroxyl groups oradsorbates such as pyridine. Several reports have highlightedthe use of31P MAS NMR of trimethylphosphine1 and trimeth-ylphosphine oxide2 probe molecules for probing both Lewis andBrønsted acidity. Novel Na-poisoning experiments have alsobeen effectively used to determine the number of catalyticallyactive framework sites.3 Farneth and Gorte have recentlyreviewed the variety of methods used to characterize both thestrength and number of acid sites.4 In this review, the preferredmethods for quantifying Brønsted sites involve combined TPDand TGA analysis of reactive amines such as isopropylamine,

because stoichiometric reaction with the catalyst produces oneolefin and one ammonia molecule per acid site.5 Even so, aconvenient, direct, andnoninVasiVe method for acidity deter-mination is necessary for the routine analysis of variations inframework acid site concentration with catalyst synthesisconditions, postsynthetic treatments, and catalyst modificationmethods (e.g., final Brønsted acid site concentration vs synthesisgel SiO2/Al2O3, synthesis pH,6 template type and amount,7,8

dealumination,9,10 surface titrations,11 and hydrothermal treat-ment12). Such a technique is desirable because no assumptionsabout active site accessibility by a probe molecule are necessary.The ability to independently measure acid site concentrationsin different zeolite types allows quantitative comparisons of acidsite number vs acid site strength data in many hydrocarbonreactions. For example, one could imagine preparing a certainSi/Al ratio zeolite and comparing its activity/selectivity to asecond catalyst that was synthesized with a lower Si/Al ratiobut had been dealuminated to the same framework acidity asthe original material. This would address postulates concerningcreation of “unequal” or “enhanced” Brønsted acid sites viadealumination methods.13 Many assumptions concerning zeolitechemistry could be tested if the experimentalist could easilyand accurately determine the Brønsted acid site density in anyzeolite using a routine noninvasive method.

In this contribution, we demonstrate the first quantitative1Hsolid-state NMR data proving that Brønsted acid sites in SAPO-34, a silicoaluminophosphate catalyst used for methanol-to-olefin hydrocarbon chemistries, are not stable in the presenceof moisture, even at ambient temperatures. In our pursuit of anexperimental strategy which provides unambiguous proof of thiseffect, we show that a combination of solid-state magic-angle(MAS) spinning at moderate spinning speeds and1H spin-counting NMR with aninternal standard provides an easilyaccessible, noninvasive, and completely quantitative way to

* To whom correspondence should be addressed. E-mail: [email protected].

† Presented at the 43rd Rocky Mountain Conference on AnalyticalSpectroscopy, July 2001, Denver, CO.

4941J. Phys. Chem. B2002,106,4941-4946

10.1021/jp0145816 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 04/17/2002

determine the number of acid sites in a zeolite or molecularsieve. We recognize that spin-counting methods are not new.Pfiefer and co-workers have reported early1H MAS NMRstudies in which the total hydrogen concentration was deter-mined via comparison to an external (i.e., a separate experiment)spin-counting standard or via adsorption of gaseous bases.14-16

The key advantages of our method are that (1) no probemolecules are required, (2) theinternalspin calibration standardis completely inert, (3) the standard may be handled as a solidalong with the catalyst, and (4) only one measurement isrequired, thereby ensuring identical experimental response forall species of interest. More importantly, no assumptionsregarding the accessibility of a specific probe molecule todifferent cage or channel sites in the catalyst are required.Finally, no assumptions about “nondetectable” spin behavior,as with 27Al NMR studies of framework aluminum sites, arepresent in this approach. All acidic and nonacidic hydroxylgroups are measured accurately and simultaneously. To be clear,this method does not provide any data on the relative strengthsof acid sites between zeolites or within the zeolite, only thenumber of acidic Brønsted sites. Our method involves theincorporation of an internal spin-counting standard, poly-(dimethylsiloxane) (PDMS), inside the MAS rotor with thecatalyst sample. Liu and Maciel have previously reportedcomparisons of the quantitative aspects of1H CRAMPS vsMAS-only experiments, in which a commercial silicone rubberwas used as the internal spin-counting standard.17 Although theirprimary focus was on the multiple-pulse CRAMPS experiment,the necessary details for quantitative1H spin-counting MAS-only experiments were outlined. We have used this method,with some modifications, to obtain the results reported in thispaper for zeolite and silicoaluminophosphate catalysts. Ourcriteria for selection of a suitable spin-counting standard weresimilar to the work by Liu and Maciel, and include thefollowing: (1) the standard must be a solid for ease of handling;(2) the standard must be inert, suitable even for air and moisturesensitive catalysts; (3) the standard must have a well-resolvedchemical shift to minimize interference with signals of interest;and (4) the standard must have a narrow1H MAS NMR signalfor any MAS spinning speeds>1 kHz. In cases where thereare signals of interest near 0 ppm, i.e., near the PDMS signal,a siloxane polymer like polydiphenylsiloxane (PDPS) may beused for the spin-counting standard because the signals for thisstandard are near 7 ppm. All data in this paper use PDMS asthe standard.

The experimental method is described in detail in thefollowing sections, with relevant examples from both silica andzeolite chemistry. A final example provides the first directspectroscopic proof of the hydrolytic destruction of active sitesin SAPO-34, a catalyst that is currently under investigation bymany research groups for methanol-to-olefin conversions.

Experimental Section

NMR data were obtained on a Bruker DSX 300 MHzspectrometer using MAS at speeds of 7-11 kHz with 4 mmzirconium oxide rotors. Silica samples were obtained fromGrace-Davidson, and the Ferrierite and ZSM-5 zeolites wereobtained from Zeolyst Corporation. The SAPO-34 sample wassynthesized by the method of Lok.25 The PDMS standard wasobtained from Rheometrics Scientific, Inc. Zeolite samples wereprepared from the ammonium form by a slow, stepwisecalcination in flowing air at final temperatures of 550°C on aglass vacuum line. Dehydrated zeolite samples were preparedvia stepwise temperature increase to 400°C under vacuum.

Silica samples were dehydrated under inert gas atmosphere atatmospheric pressure. All samples were loaded into MAS rotorsin the glovebox to prevent exposure to moisture, and the samplewas enclosed with a top and bottom press-fit Teflon spacer, aswell as the Kel-F cap. Dry nitrogen was used for spinning duringthe experiment, and the sample was stored in nitrogen untiltransferred immediately to the probe. In practice, we found thatthe sample could sit exposed to the atmosphere with this double-cap arrangement for several days before there was any detectablechange in the spectrum. The1H T1 values for the PDMS, silica,and zeolite protons were all less than 2.5 s. Therefore, the 30 srecycle time used in our experiments results in quantitative peakintensities. Typically, 16 scans were obtained for each spectrumshown.

Before the results on zeolite acidity are presented, it isimportant to describe the experimental requirements necessaryto achieve quantitative spin response in a MAS NMR experi-ment. As was discussed in great detail by Campbell and English,commercial solid-state NMR probes do not provide homoge-neous radio frequency excitation/detection over the entire samplevolume of the MAS NMR rotor.18 However, nonuniform r.f.field regions are minimized for smaller rotor volumes. Ourexperiments were performed using a 4 mmmagic-angle spinningrotor in which the sample region was confined to the middleregion of the rotor; that is, the sample volume element wascompletely contained within the middle1/5 element of the totalrotor length. For our 4 mm MAS rotors, this confined centralvolume was very nearly spherical, even though strictly speakingthe volume was cylindrical. In other words, the dimensions ofthe confined sample volume were a cylinder of length of 4 mmand 4 mm diameter. Sample confinement was achieved usinglower and upper Teflon spacers on either side of the samplevolume.Because each MAS probe has different radio frequencyhomogeneity, calibration prior to use is required by spin-counting a sample of known concentration, e.g., a hexameth-ylbenzene/PDMS or camphor/PDMS sample, andVarying therestricted sampleVolume until consistent and accurate massbalance is achieVed.This is necessary to ensure that each spinfrom the sample and the spin-counting standard are equallyexcited and detected in the measured signal response and cannotbe overlooked even for commercial probes that utilize sometype of standard spacer arrangement. Our results for thecalibration experiment, when run in triplicate, resulted in thedetection of 98, 101, and 99% of the expected signal based onthe known weights of the sample. In other words, a knownamount of hexamethylbenzene and a known amount of PDMSwere introduced into the confined sample volume in the centerof the rotor, as described above, and the spectrum wasdeconvoluted to determine their relative amounts. Of course,all sideband intensities were included in the calculation. Withinthe restricted volume region of the rotor, identical results wereobtained no matter where the PDMS standard was placed, e.g.,on the rotor wall at the top vs bottom vs in the center, therebyfurther confirming that the r.f. excitation and detection wasquantitative. In contrast, when the same experiment was runwithout sample volume reduction, i.e., using the standard rotorvolume, only 85% of the expected signal was measured. In thisway, the method is clearly shown to be detecting all spins,because the line width of HMB is much, much greater thaneither PDMS or any signals from the zeolite protons. If the ring-down time of the probe was too long to accurately detect all1H spins, as might be a concern, then clearly this would manifestitself in the calibration experiment, because the HMB wouldbe underestimated. This was not the case. Furthermore, even

4942 J. Phys. Chem. B, Vol. 106, No. 19, 2002 Wang et al.

though the experiments were run on the1H channel of a double-resonance probe, we routinely also run CRAMPS experimentswith 1.8 µs 90° pulse widths and 3.5µs sampling windows. Inour single-pulse experiments, it must be remembered that the1H T2 values of both the zeolitic and PDMS protons aresufficiently long to allow quantitative detection, and with a 4µs acquisition delay, all spins should be detected. Furthermore,comparitive studies of zeolites by CRAMPS and single-pulseexperiments provided identical results, thereby ensuring thatπ/2pulse, or Bloch decay, experiments are quantitative for zeolites.17

Our experimental procedure was as follows: (1) an emptyMAS rotor and volume-restricting spacers were weighed usinga Sartorius MC-5 microbalance with 1-2 µg accuracy andrepeatability; (2) a small amount of PDMS was weighed usingthe microbalance, and then the PDMS was placed on the insidewall of the rotor just above the bottom spacer; typically, 50-100 µg of PDMS was used; (3a) for the moisture-sensitivezeolite and dehydrated silica samples, the PDMS-containingrotor was filled with 4-8 mg of the sample of interest insidethe glovebox; (4) the total sample weight was determined usingthe microbalance; (5) the1H MAS NMR spectrum was acquiredusing a simpleπ/2 pulse; (6) the spectrum was deconvoluted,and spectral areas were integrated to provide quantitative PDMS:sample signal ratios for each signal in the spectrum. Spectrawere deconvoluted and integrated using the commercial softwarepackage PeakFit from Jandel Scientific. Deconvolution resultsto determine spectral areas for each species were obtained usingleast-squares fitting via minimization of residuals. MixedGaussian-Lorentzian line shapes were used in the analysis, andall correlation coefficient (R2) values exceeded 0.98. In everycase, only one line was used in the fit for each physicallyrelevant species in the sample, and sideband intensities, whenobserved, were included in the calculations. We have found thisprocedure to be highly reproducible for the determination ofzeolite acid site concentrations, with a standard deviation formultiple experiments of 3%.

Results and Discussion

As a control experiment, a silica sample containing known-OH group concentration was measured. The results are shownin Figure 1 as a function of dehydration temperature under N2

atmosphere. The reader should first recognize that the PDMSstandard does not affect the spectra in any way, other than theaddition of a single, narrow signal at 0.1 ppm. In addition toserving as an internal quantitation standard, the PDMS alsofunctions as an internal chemical shift reference. Figure 1 partsa and b are the MAS1H NMR spectra for silica gel containingsmall amounts of residual moisture because of the lowerdehydration temperatures of 150 and 400°C, respectively.Residual moisture exists at these dehydration temperaturesbecause the dehydration was done under atmospheric pressure;dehydration under vacuum would lead to removal of water atthese temperatures. The key point is that by comparison ofFigure 1a-c (150 vs 400 vs 800°C), we can quantitativelydetermine the amount of residual moisture and the concentrationof hydroxyl groups from the silica. Silica is an important supportfor many types of chemistry, and the critical factor is the numberof available surface hydroxyl groups. Spin-counting results forthe silica sample in Figure 1a-c reveal that the hydroxyl groupconcentration is 0.58, 0.61, and 0.60 mmol/g of bulk SiO2,respectively. Because this is the same SiO2 sample in each case,and only the dehydration temperature is varied, one shouldexpect to obtain the same [Si-OH] concentration. The residualmoisture in the 150 and 400°C samples of Figure 1a-b are

0.89 and 0.65 mmol/g SiO2, respectively. We note thathydrogen-bonded silanols, if they exist, do give rise to signalsin the same reagion as the physisorbed water. However, thehydroxyl group concentration is constant in each experiment,and no residual broadening of the silanol signal is observed inFigure 1c, suggesting that proximate, hydrogen-bonded hydroxylgroups are not present in this sample (in the absence ofmoisture). Also, these data agree very well with wet chemicaldetermination of [Si-OH] via methyl-magnesium halidetitrations; our experience has shown agreement within 10% inmost cases between the two methods for silicas dehydrated overa range of conditions. Of course, the ability to get data withoutresorting to wet or gas-phase titrations is appealing. The silicacontrol experiments, as well as the hexamethylbenzene/PDMScalibrations discussed above, indicate that all zeolitic hydrogensare detected in the data described below, because zeolites havehydrogen densities much less than hexamethylbenzene (andtherefore a longer T2) and comparable to the dehydrated silicas.

Figure 2 shows an example of the spectral deconvolution inwhich three Gaussian-Lorentzian peaks were used to fit eachspectrum, thereby revealing the quantities of water, silicahydroxyl groups, and PDMS. Of course for the 800°C sample,only two peaks were used in the fit, because all water wasremoved at this temperature. Excellent fits with correlation

Figure 1. 1H MAS spin-counting results on a sample of silicadehydrated at (a) 150, (b) 400, and (c) 800°C under nitrogenatmosphere at atmospheric pressure. The broad downfield feature seenin a and b results from residual H2O. The asterisk in c denotes a transientsignal unrelated to the sample. The quantities of silica hydroxyl groupsand residual water are reported in the text, and the deconvolution resultsare shown in Figure 2.

Figure 2. Example showing deconvolution results for the sample inFigure 1a. The experimental spectrum is overlaid with the total fit inthe top spectra; individual components are shown in the bottom framealong with their assignments. (The horizontal axis is expanded relativeto Figure 1.)

Quantitative Investigations of Acidity J. Phys. Chem. B, Vol. 106, No. 19, 20024943

coefficients in excess of 0.98 were obtained using only thesethree physically relevant components. We also note by compar-ing Figure 1a-c that line widths for the deconvolved silicahydroxyl (SiOH) peak at 1.9 ppm decrease with increasingdehydration temperature. The full-width at half-maximum linewidths are 0.62, 0.45, and 0.33 ppm in Figure 1-c, respectively.Such results are consistent with a decrease in the amount ofsilica hydroxyl groups that are hydrogen-bonded to residualwater molecules.

Spin-counting experiments on zeolite solid acid catalysts areparticularly useful because one can simultaneously determinethe concentration of acidic bridging hydroxyl groups, nonacidicterminal silanols, and residual template or ammonium ions.Shown in Figure 3 are1H MAS spin-counting spectra for theacidic zeolites H-ZSM5 (Figure 3a-b) and H-Ferrierite(Figure 3-d), each with different SiO2/Al2O3 ratios. SAPO-34, a synthetic chabazite analogue where each acid site isgenerated via silicon substitution for aluminum and phosphorus,is shown in Figure 3e. Although only three examples are shownin this report, we have conducted numerous experiments on avariety of zeolite types, including chabazites, Beta, and HY,and the results here are representative of the general applicabilityof this method. As has been reported many times in theliterature, Brønsted acid site protons in zeolites generally appearbetween 3.8 and 5.0 ppm, and nonacidic silanol groups appearnear 1.7-2.1 ppm.19-21 In addition, other signals between 2.5and 3.5 ppm are often detected in the1H MAS spectra ofzeolites. Such signals arise from nonframework or amorphousalumina species bearing hydroxyl groups, e.g., AlOH, and havebeen commonly observed in ZSM-5 and Y-type zeolite spectra.18

Finally, the presence of any residual ammonium ions from theexchange process or from incomplete removal of oxidizedtemplate molecules is revealed by signals in the 6-7 ppm range.For the SAPO-34 sample in Figure 3e, the Brønsted acid sitegives rise to a peak at 3.9 ppm, and the nonacidic Al-OH andP-OH groups may be seen in the 1-2 ppm range. As can beseen by inspection of the five example spectra in Figure 3, aswell as the deconvolution results in Figure 4, each of thesecomponents are observed and may be quantified by the spin-counting technique. Figure 3a-b shows HZSM-5 spectra for

samples with Si:Al of 15 and 25, respectively. The experimen-tally determined Brønsted acid site concentrations were 0.49and 0.34 mmol H+/g of zeolite, respectively, for the samplesin Figure 3a-b. The experimentally determined Brønsted acidsite concentration of the H-FER samples in Figure 3c-d were0.78 and 0.44 mmol H+/g of zeolite, respectively. Also, theH-FER sample in Figure 3d contains a small amount of residualNH4

+ ions as seen in the spectrum; the concentration determinedfrom spin counting was 0.025 mmol H+/g of zeolite. Finally,for the SAPO-34 sample in Figure 3e, the [H+] ) 1.10 mmol/gof molecular sieve. One particularly powerful benefit of the spin-counting method is the ability to compare the number ofmeasured Brønsted acid sites to that expected based on elementalSi/Al ratios of the final catalyst. For example, the H-FERsample in Figure 3c has Si:Al) 10, whereas the sample inFigure 3d has Si:Al) 27.5. On the basis of the structure offerrierite, one can calculate the number of Brønsted acid sitesthat would occur if all Al were incorporated in the framework.Such a calculation for the samples in Figure 3c indicates thatonly 78% of the elemental Al is in the framework, whereas95% of framework Al occurs for the lower Si:Al sample inFigure 3d (total Bronsted H+ + NH4

+). This is in everywayconsistent with the data, because a significant amount ofnonframework Al-OH species is observed in Figure 3c as thebroad signal near 2-3 ppm and in the detailed fit shown inFigure 4b for this sample. Also, comparison of the high AlZSM-5 in Figure 3a shows more nonframework Al-OH signalthan the low Al catalyst in Figure 3b. It is unlikely that everynonframework Al atom has an-OH group attached to it, andtherefore, one should not expect complete agreement based onthe elemental analysis data. Our experience to date has shown

Figure 3. 1H spin-counting MAS spectra of zeolite (a) HZSM-5, Si:Al ) 15, (b) HZSM-5, Si:Al) 25, (c) H-Ferrierite, Si:Al) 10, (d)H-Ferrierite, Si:Al) 27.5, and (e) HSAPO-34, Si/Al) 0.19 and Si/Si+Al+P) 0.095. Additional small signals seen in some of the spectranear 3 (a) and 6 ppm (d) are explained in the text. The narrow PDMSpeak is adjusted off-scale for clarity in the catalyst peak regions;however, the integrated intensity ratios of the H+ and PDMS peaksare near unity in each case.

Figure 4. (a) Fit of total spectrum deconvolution (top) and individualpeaks (bottom) for the SAPO-34 sample in Figure 3e. The peaks from1 to 2 ppm come from nonacidic terminal hydroxyl groups.(b) Similardata in the top and bottom frames are shown for the HFER samplefrom Figure 3c.

4944 J. Phys. Chem. B, Vol. 106, No. 19, 2002 Wang et al.

that as the Si:Al ratio decreases the percent of total Al that existsas framework Al decreases. In other words, the agreementbetween the Brønsted acid site concentration and total Al contentdecreases as the Al content increases. These results depend bothon the ability to incorporate more Al from the synthesis gelinto the framework during crystallization, as well as thehydrothermal stability of the framework Al during calcinationand dehydration.

For the SAPO-34 sample in Figure 4a, the experimentallymeasured value for the Brønsted [H+] concentration of 1.12mmol/g is less than the 1.5 mmol/g expected based on Sielemental analysis (i.e., assuming each Si leads to an acid site).The 1.12 mmol H+/g value is in agreement with previouslypublished values for this catalyst.26 As reported by Barthomeuf,there are clear inconsistencies for SAPO catalysts, in that severalmechanisms exist for Si substitution in the ALPO framework.Some of these incorporate multiple Si sites in close proximity,thereby reducing the number of acid sites from what would beexpected based on Si elemental analysis.22-24 An acid site iscreated when Si substitutesonly for P; if two Si atoms substitutefor an Al and a P, no acid site is formed. Previous work byBarthomeuf has shown that these acid-site generatingisolatedSi sites cannot exist in SAPO-34 above an atomic fraction of0.10-0.11.23 Also, Si can be incorporated in amorphous SiO2

phases, and Al may be incorporated as amorphous Al2O3 phases,thereby rendering comparisons of elemental Si+P/Al ratiossomewhat meaningless in relation to acidity. As such, there isa need to compare complete material balance for Si to theexperimentally measured Brønsted acid site concentrations. Thespin-counting method introduced here can address this problem,because the PDMS29Si signal can also serve as an internal spin-counting standard for the different Si sites, e.g., those withdiffering numbers of Al neighbors in the lattice, as measuredusing 29Si NMR.23 Therefore, in one sample preparation, wecan collect quantitative1H and29Si NMR data with an internalreference peak in each spectrum for calibration. Of course, thiswill be a time-consuming experiment in the29Si case, but theinformation will prove valuable for understanding Si incorpora-tion in SAPO catalyts.

Our experiments to date have clearly revealed that the acidsite density in SAPO-34 is not a constant. Figure 5 shows1H

MAS spin-counting NMR data on the SAPO-34 sample shownin Figure 4 but as a function of sample exposure to variousenvironmental conditions following removal of the tetraethyl-ammonium template used in the synthesis. As described in thefigure caption, we have acquired these data based on exposuretimes to either ambient atmosphere, i.e., moisture, or to a drynitrogen glovebox environment. Visual inspection shows thatthe Brønsted acid site peak near 4 ppm decreases its intensityin Figure 5a-d. Quantitative analysis of this acidity loss, usingthe spin-counting method, reveals acid site densities of 1.12,0.95, 0.87, and 0.42 mmol/g in Figure 5a-d, respectively. Theseresults directly address the stability questions raised by Briendand co-workers, in which they observed different crystallinitiesand porosities in similar SAPO catalysts as a function of catalystpreparation and exposure. They attributed this behavior to attackby water of the framework bonds around tetrahedral atoms inthe catalyst.22 Our results indicate unequivocally that this leadsto a loss of acidity. The fact that the acidity loss is severelyretarded by storing the catalyst in an inert atmosphere is strongevidence that framework hydrolysis is the active mechanism.The acidity loss appears to be irreversible, because subsequentvacuum heating/dehydration steps do not lead to any increasein acidity. Recall that immediately prior to analysis, the sampleis dehydrated stepwise up to temperatures of 450°C, all thewhile maintaining a vacuum at least at 10-3 Torr. Also, thenonacidic hydroxyl peaks between 1 and 1.6 ppm do not changein these experiments. Spin-counting of these species for eachof the spectra in Figure 5 indicates a constant concentration of0.45 ( 0.02 mmol total nonacidic hydroxyl groups per gramof catalyst, further showing that the spin-counting technique isvery reproducible from sample to sample. Although theseexperiments quantify the acidity loss specifically after templateremoval, the fact that the largest acid site density that we havemeasured is only 70-75% of the theoretical limit suggests thatacid site hydrolysis may also occur when the template is present,albeit at a much slower rate. The transient nature of the activesites in SAPO-34 must be considered when catalysis experi-ments involving this catalyst are carried out, particularly thoseinvolving methanol-to-hydrocarbon synthesis.26-28

In conclusion, we believe that this simple, accurate, and highlyreproducible method for experimental determination of Brønstedacidity in essentially any zeolite or solid acid molecular sievewill prove extremely useful in a variety of catalyst science areas.Because of the high sensitivity afforded by1H detection, thereader will recognize that the method is completely general andmay be applied to any sample in which resolved1H peaks areobserved in the MAS spectrum (e.g., metal oxides, functional-ized nanoparticles, thin films on surfaces, etc.). In particular,the ability to determine acidity as a function of catalyst synthesisconditions or postsynthetic modifications such as steaming, ionicstabilization, or acid site titrations will be most interesting. Forexample, we have done controlled steaming experiments onHZSM-5 in which the Bronsted acidity loss caused by dealu-mination is easily detected and quantified. The specific applica-tion of this method to SAPO-34 revealed, for the first time,that Brønsted acidity is lost when the calcined material isexposed to the moisture. Preliminary data also suggests that aciddensity decreases in SAPO-34 over time even prior to templateremoval, albeit at a much slower rate. We are conductingadditional experiments in which a full multinuclear NMRstrategy is being used in conjunction with the spin-counting workto reveal the mechanistic pathways for Si incorporation andacidity losses in SAPOs.

Figure 5. 1H MAS spin-counting spectra for the SAPO-34 sampleshown in Figure 4a as a function of elapsed time following removal ofthe tetraalkylammonium template molecule.(a) Immediately followingtemplate removal and dehydration, but with exposure to atmospherefor 3 h between the template removal and dehydration steps;(b) sameas in part a, but after 7 days of storage in a nitrogen glovebox;(c)same as in part b, but after 10 days of storage in a glovebox;(d) sameas in part c, but following reexposure to atmosphere for 3 h and asecond dehydration step.

Quantitative Investigations of Acidity J. Phys. Chem. B, Vol. 106, No. 19, 20024945

Acknowledgment. The authors gratefully acknowledgesupport from the National Science Foundation (DMR-0137968)and from the NSF-REU program (Grant 0097485).

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