Volume 55, Number 8, 2001 APPLIED SPECTROSCOPY 10990003-7028 / 01 / 5508-1099$2.00 / 0q 2001 Society for Applied Spectroscopy

Elemental Analysis Using NMR: SimultaneousDetermination of Aluminum and Sodium in Zeolite A UsingLow-Field NMR

THIERRY M. GUIHENEUF and LARRY S. SIMERAL*Resonance Instruments Ltd., Unit 13, Thorney Leys Business Park, Witney 0X8 7GE, United Kingdom (T.M.G.); and AlbemarleCorporation, Technical Center, P.O. Box 14799, Baton Rouge, Louisiana 70898 (L.S.S.)

The standard methods for elemental analysis of aluminum and so-dium in zeolites typically require signi� cant sample and reagentpreparation before the � nal titrimetric or atomic spectroscopicmeasurement. Low-resolution, bench top nuclear magnetic reso-nance (NMR) has shown important applications for rapid quanti-tative analysis of hydrogen and/or � uorine containing materialswithout sample preparation in the food, chemical, and polymer in-dustries. Here we demonstrate the determination of aluminum andsodium in zeolite A without sample preparation by using multinu-clear, low-� eld NMR. The simultaneous determination of aluminumand sodium can be performed in under � ve minutes with a precisionof ;1% relative. The method, calibration, data collection, and sig-ni� cant other applications are discussed.

Index Headings: Low-� eld NMR; Low-resolution NMR; Bench topNMR; Zeolite A.

INTRODUCTION

The aluminum and sodium contents of zeolites are im-portant parameters in zeolite properties and in qualitycontrol speci� cations.1,2 Zeolite A has important uses asa detergent builder.1 The typical aluminum and sodiumcontents of zeolite A are both in the 14–15 wt % rangeon a dry basis. The standard method of determination foraluminum and sodium in zeolites is acid digestion, fol-lowed by potentiometric or colorimetric titration or byatomic spectroscopy. Although the precision of suchmethods is very high, considerable sample and reagentpreparation are necessary. Interference from other ele-ments can also present problems. The sample and reagentpreparation, plus possible interferences, can lead to sig-ni� cant turnaround times in product analysis. In qualitycontrol and process analysis environments, some sacri� ceof precision is often allowed in order to increase samplethroughput and reduce turnaround times.

Bench top, low-resolution NMR has found importantapplications using 1H NMR for water and fat analysis infood and chemical products and using 19F NMR to de-termine � uoride content in toothpaste and � uorine con-tent in � ber � nishes.3,4 There are other 1H NMR appli-cations for determination of polymer properties, such asxylene extractables and crystallinity in polyethylene.3,4

Rapid elemental analysis without sample preparation us-ing multinuclear, low-� eld, bench top NMR is a signi� -cant new application. We demonstrate this analysis forzeolite A and discuss calibration, data collection and pro-cessing, precision, speed of analysis, and other possibleapplications.

Received 24 February 2001; accepted 10 April 2001.* Author to whom correspondence should be sent.

EXPERIMENTAL

The NMR data were collected using a bench top Res-onance Instruments Maran Ultra instrument (23 MHz for1H). The system was equipped with broad band, multi-nuclear electronics, and an 18 mm low Q, match adjust-able probe operating at 6.04 MHz for 27Al and 23Na.Eighteen mm diameter Pyrex sample tubes were � lled toa constant volume taller than the NMR probe detectioncoil and tamped 5–10 times for reproducible density. Twominute and seven minute data collections (2000 and 8000acquisitions, respectively) were performed using 5.5 ms908 pulses, a 15 ms probe dead time, and a 50 ms relax-ation delay. The relaxation delay appeared to be near op-timum for these samples, but extensive studies were notperformed. No signi� cant � ltering was used. Under theseconditions, 27Al and 23Na are observed in the same 500kHz spectral window (see Fig. 1). The resolved peaks for23Na and 27Al were digitally integrated from Fouriertransformed data without treatment with line broadeningfunctions. Integral data point areas were chosen for max-imum reproducibility of the data. The integrals obtainedfor sample without zeolite A present were subtracted toremove small background signals from the probe. Preci-sion was assessed using thirteen replicates of two sam-ples, one at the high end and one at the low end of thecalibration. Two minute data collection was fully ade-quate for a precision of ;1% relative standard deviation.

Calibration standards were prepared from highly crys-talline EZAt zeolite A (Albemarle Corporation) by ac-curate weighed dilution and thorough mixing with re-agent grade anhydrous magnesium sulfate (J. T. Baker).The sodium and aluminum contents of the original zeolitewere determined using acid digestion followed by induc-tively coupled plasma atomic emission spectroscopicanalysis. Each calibration sample was analyzed � ve timesusing the NMR method.

RESULTS AND DISCUSSION

The range of calibration tested here was 0–15 wt % ofboth elements. Calibration standards for the low-resolu-tion NMR analysis for both sodium and aluminum wereprepared by mixing pure zeolite A having known sodiumand aluminum contents with pure magnesium sulfate.Pure magnesium sulfate was used as the 0 wt % alumi-num and sodium blank sample. In practice, for analysisof a zeolite A product having a narrow range of sodiumand aluminum contents (e.g., 14–16 wt % each), only asingle calibration standard may be used. The speed of the

1100 Volume 55, Number 8, 2001

FIG. 1. Low-� eld NMR spectrum (2 min data collection) for zeolitecalibration mixture containing ;9 wt % each of sodium and aluminum.The center frequency, ‘‘0 Hertz’’, is 6.04 MHz at 23 MHz for protons.The background signal is marked with an asterisk, *.

TABLE I. Precision of aluminum determination in zeolite A.

Measuremen ttime

Sample wt % aluminum

15.17% (actual) 7.56% (actual)

2 min7 min

15.37% 6 0.11% (found)15.53% 6 0.10% (found)

7.51% 6 0.09% (found)7.68% 6 0.06% (found)

FIG. 2. Calibration for wt % Al in zeolite A (2 min data collection). FIG. 3. Calibration for wt % Na in zeolite A (2 min data collection).

analysis also allows this calibration standard to be run asa validation sample between actual determinations.

Figure 2 shows the calibration plot for 27Al using atwo minute data collection (2000 acquisitions). The lin-earity of the plot is very good. The correlation coef� cientfrom linear regression is 0.996. A seven minute data col-lection (8000 acquisitions) improved the correlation co-ef� cient only slightly to 0.998. Table I shows the preci-sion data for the aluminum analysis.

Figure 3 shows the calibration for 23Na using the twominute data collection. Again the linearity is very good.The correlation coef� cient is 0.994, slightly lower thanfor aluminum due to the lower signal-to-noise ratio of the23Na resonance. Table II presents the precision data forthe sodium analysis.

Improvements in precision for the analysis may be an-ticipated through increases in the signal-to-noise ratio andin reduction of probe 27Al background signal. The signal-to-noise ratio in NMR increases as the square root of theincrease in the number of acquisitions. When we in-creased the collected acquisitions by a factor of four wedid see a slight improvement in the precision, but prob-ably not enough to justify the longer data collection timesin actual practice. Application of empirically tailored line

broadening functions might provide some additional im-provement. Incorporation of digital � ltering to eliminateany sources of aliased noise may also provide furtherimprovements. Weighing each sample for analysis to de-termine a signal per gram would add an extra step in theanalysis, but may be justi� ed to further improve the pre-cision. Lastly, optimizing the probe design and materialsof construction to increase the signal-to-noise ratio andto decrease the 27Al background should lead to additionalimprovements.

As noted above, the method is dependent on calibra-tion using well characterized materials for setting the rel-ative signal intensities, as is true of most low-resolutionNMR quantitative analyses. The method is not an abso-lute spin counting method but is referenced to samplesthat are already characterized. Application of the ap-proach reported here to other zeolites or other materials� rst requires calibration with materials of known com-position.

The traditional use of NMR spectroscopy in zeolitematerials is structural analysis. Typical ‘‘quantitative’’applications of either 27Al or 23Na NMR to solid zeolitesdetermine the ratios or distributions of coordination sitetypes and use high-� eld, solid-state NMR techniques,such as magic angle spinning (MAS).5 For example, con-siderable attention has been focused on distinguishingsix- and four-coordinate aluminum in zeolites and ac-counting for ‘‘missing’’ aluminum of other or distortedcoordination environments.5 For quadrupolar nuclei, suchas 27Al and 23Na, deviations from octahedral and tetra-hedral symmetry around the site typically cause fasttransverse relaxation leading to very broad resonancesthat are dif� cult to detect. In the case studied here, onlyfour-coordinate aluminum exists in high quality, crystal-line zeolite A. The focus is on determination of the totalcontent of this aluminum rather than on distinguishing

APPLIED SPECTROSCOPY 1101

TABLE II. Precision of sodium determination in zeolite A.

Measurementtime

Sample wt % sodium

14.00% (actual) 6.98% (actual)

2 min7 min

13.96% 6 0.15% (found)14.14% 6 0.10% (found)

6.89% 6 0.11% (found)7.04% 6 0.07% (found)

other environments. High quality, crystalline zeolite Acontaining four-coordinate aluminum is used in the cali-bration. Deviations from this environment would be ex-pected to adversely affect the zeolite properties. At thatpoint, signi� cant deviations from the expected amount ofaluminum due to ‘‘missing’’ aluminum in distorted en-vironments in a product sample could indicate poor prod-uct quality and manufacturing process problems. A totalaluminum analysis using the standard methods would notdetect these kinds of quality problems. Hence, the methoddescribed here presents an additional feature for qualitycontrol in the zeolite A products. In setting up the methodfor other zeolites, the presence of aluminum in other thanfour-coordinate environments will need to be carefullyconsidered.

It should be noted that the zeolite A product used inthis analysis is fully hydrated (about 18 wt % water).Hydration has been shown to be important in structuralanalysis and in determining ratios of six-coordinate tofour-coordinate aluminum sites using 27Al NMR5,6 and in23Na studies of zeolites.5 Narrowing of 23Na NMR reso-nances has been attributed to hydration and may be oneof the reasons that the 23Na is easily observed here. Theeffect of hydration levels was not studied here, but it maybe important in applying this method to other zeolite ma-terials.

Low-resolution 1H NMR determination of water in ze-olite A has been demonstrated previously.7 The analysisof water, aluminum, and sodium content could be com-bined into one low-� eld NMR instrument by appropriateconstruction of an NMR probe with separate coils and

channels for 1H and 27Al/23Na detection, as is typical inhigh-resolution broad band NMR probes. The determi-nation of all three species could be performed in underten minutes without sample preparation using a singleinstrument.

Although the method demonstrated here was devel-oped for a solid material, applications of this technologyto liquids should be straightforward. It will be particu-larly useful where reductions in sample preparation andanalysis time and in ease of use are desired. Two inter-esting examples are analysis of pyrophoric organosodiumand organoaluminum species, where particularly carefuland time consuming hydrolysis must be performed beforeelemental analysis can be attempted. Use of low-resolu-tion 27Al or 23Na NMR for elemental analysis in thesecases would require little handling and no hydrolysis.Further, the concentrations of aluminum in neat orga-noaluminums would be higher than that in zeolite, pro-viding a larger NMR signal and, hence, a higher signal-to-noise ratio for optimum precision of the analysis.

Extension of elemental analysis using NMR to othernuclei is most easily and straightforwardly done with theelements of highest natural abundance, sensitivity, andfrequency for NMR detection. The most obvious exam-ples are 7Li, 11B, and 31P. Analysis of a wide variety ofsample types containing these nuclei can be envisionedwhere concentrations are high and sample preparationand turnaround times are important.

1. Zeolite Technology and Applications. Recent Advances, J. Scott, Ed.(Noyes Data Corporation, Park Ridge, New Jersey, 1980).

2. D. W. Beck, Zeolite Molecular Sieves (Robert Craggier Publishing,Malabo, Florida, 1984).

3. S. Bank, Concepts Magnet. Res. 9, 83 (1997).4. R. M. Pearson, L. R. Ream, C. Job, and J. Adams, Cereal Food

World 32, 822 (1987).5. G. Engelhardt and D. Michel, High Resolution Solid-State NMR of

Silicates and Zeolites (John Wiley and Sons, Chichester, UK, 1987).6. G. J. Ray, B. L. Meyers, and C. L. Marshall, Zeolites 7, 307 (1987).7. L. S. Simeral and P. H. Krygsman, Appl. Spectrosc. 53, 1009 (1999).