Laser ablation Fourier transform mass spectrometric study of zeolites

MICROPOROUS MATERIALS ELSEVIER

Microporous Materials 4 (1995) 467 473

Laser ablation Fourier transform mass spectrometric study of zeolites Sejin Jeong, Keith J. Fisher, Russell F. Howe, Gary D. Willett * School of'Chemistry, The UniversiO, of New South Wales, Sydney, N S W 2052. Australia Received 31 October 1994; accepted 1 April 1995

Abstract

Laser ablation (1064 nm) Fourier transform mass spectrometry has been used to study the sodium ion-exchanged LTA (Na-A) and MOR (Na-M) zeolites and the potassium ion-exchanged LTL (K-L) zeolite. When the ionexchanged element is taken into account, the positive-ion and negative-ion laser ablation Fourier transform mass spectra from all three zeolites are similar and apparently independent of the zeolite structure. With the exception of small atomic and diatomic ions, the anions detected from the laser ablation of the zeolites are of the form M (SiO2).cO (where M = H, Na, K, AIO and x = 1-5). The laser ablation mass spectra show a strong dependence on laser power, with the higher Si/AI ratio zeolite, mordenite, exhibiting the highest stability under 1064 nm laser irradiation. Keywords." Zeolite; Laser ablation; Fourier transform; Mass spectrometry

1. Introduction

There are few reports in the literature of mass spectrometric ( M S ) studies of zeolites. Fast atom b o m b a r d m e n t MS [1,2] and secondary ion MS [3,4] have been used to study the surface composition of zeolites but the use of laser ablation MS to examine the bulk composition of zeolites has not to our knowledge been attempted. Fourier transform mass spectrometry ( F T M S ) using an ion cyclotron resonance ( I C R ) cell is a powerful analytical technique capable of ultrahigh resolution mass-to-charge measurements and multi-stage tandem MS [5,6]. As with time-offlight MS [7], the pulsed nature of the F T M S experiment facilitates the use of high-power pulsed lasers ( M W c m -2) in the study of various thermally labile and involatile organic and biological * Corresponding author. 0927-6513/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-6513(95)00025-9

compounds [8-14] and gas-phase ion molecule chemistry of various bare and mixed elemental cluster systems [15-19]. A typical F T M S experiment is based on a timeresolved sequential process from ion generation through to ion detection in a single I C R cell. Under the influence of a strong magnetic field (typically 3 7 T), ions undergo cyclotron motion with an angular frequency expressed by the equation ~)= qB/m, where q represents the charge of the ion, B the magnetic field and m the mass of the ion. A resonant radiofrequency excitation pulse is used to induce an ion image current on a pair of detection electrodes which is subsequently converted into a time domain-digitised signal. This signal is then Fourier-transformed into a frequency domain spectrum, from which a mass spectrum is obtained after suitable mass linearisation and calibration. In this study we examine the effect of the laser

468

S. Jeong et al./Microporous Materials 4 (1995) 467-473

power, zeolite structure and Si/A1 ratio on the FT mass spectra of the selected aluminosilicate zeolites NaLTA, NaMOR and KLTL. Here the term laser ablation is used to indicate laser-induced photodecomposition-ionisation of the zeolite lattice [8,20,21] as distinct from laser desorption-ionisation [9] of species adsorbed on the external surface of the zeolite or in its lattice cages. The distinction between these two regimes of ionisation is often unclear but is pertinent to laser-based MS investigations of intrazeolite molecular species, especially the so-called "ship-in-a-bottle" organic and organometallic clusters [22-25]. We may anticipate that zeolites will behave like other wide bandgap insulating metal oxide materials such as sapphire and quartz, where the lasersurface interaction is that of the electromagnetic field with the electrons in the zeolite. Laser energy is absorbed by multiphoton processes [26], and its subsequent relaxation to the lattice determines the final result of the interaction. Recent experiments show that laser ablation of such oxides is often strongly influenced by electronic mechanisms rather than by the thermal rapid heating of the surface [27]. Petite et al. [28] have measured electron and ion product energy distributions from the laser ablation of sapphire and s-quartz, and their results emphasise the role of defect states present in the sample from impurities, vacancies or laser-induced defects. Haglund and Itoh [27] have proposed a model to examine the kinetic and dynamic factors of laser-induced desorption and ablation from nonmetallic crystalline surfaces. In solids which have a finite band gap they propose a succession of electronic processes: production of electron-hole pairs, followed by lattice-localised relaxation, a transition to a relaxed excited state and, finally, a non-radiative transition to an anti-bonding potential hypersurface on which one or more species move away from the surface. Generally, to produce ionic clusters a buffer gas is necessary to provide third-body stabilising conditions and so, in our instrument, the ultra-high vacuum limits the range of clusters that will form [29]. A variety of factors influence the laser ablation FT mass spectra including: laser power density, photon wavelength, electronic absorption by the

zeolite, background gas pressure, orientation of laser beam to sample and the zeolite composition. Even the pulse width of the laser can have a significant effect [26]. Long-pulse (gs) irradiation produces deeper ablation craters than when Q-switch (ns) radiation is used because the intense electric field associated with the latter causes multiphoton ionisation of the ablated material and creates a micro plasma at the surface which ultimately limits the amount of laser radiation reaching the surface. Gas-phase ion-molecule reactions in the resultant plasma will also produce a range of ions of which only the most stable will survive to be detected in the FT mass spectrometer approximately 100 ms after the laser pulse. In negative-ion mode, photoablated electrons as well as anions ablated directly from the zeolite surface or formed in the plasma above it will be trapped in the ICR cell. Neutral clusters formed from the ablation process will not be trapped in the cell although their high electron affinities [30] (e.g., EA=3.7eV for AIO and EA=4.1 eV for A102) will promote electron attachment or charge exchange reactions to form a range of anions. The anion distribution will relate indirectly to the ablated neutral product distribution provided other reaction mechanisms such as resonant dissociative attachment [31,32] are insignificant.

2. Experimental The NaLTA, KLTL and NaMOR zeolites with framework Si/A1 ratios of 1.0, 3.0 and 5.0, respectively, were purchased from Toyo and used without further purification. Each powdered zeolite was compressed to a thickness of ~ 2 mm in a stainlesssteel probe and inserted into a cylindrical ICR cell (r=30 mm, h=60 mm) through a magnetic propelled solid insertion probe. More complete descriptions of the ICR cell, the FT mass spectrometer which is equipped with a 4.7-T superconducting magnet (Spectrospin CMS-47) and the laser (Spectra Physics DCR-11 Nd-YAG) have been presented previously, and only brief operational procedures are described here [13,14]. Each zeolite sample was heated in the ICR cell vacuum chamber for 10 h at a temperature and pressure of ~ 500 K and ~ 10 -5 Pa, respectively,

S. Jeong et al./Microporous Materials 4 (1995) 467-473

with the laser ablation F T M S experiments undertaken at pressures of ~ 10 -8 Pa. The Nd-YAG laser was used in the Q-switch pulse mode (8 ns pulse width) and the 1064-nm laser beam was focussed ( f = 102 mm) to a spot size of diameter of ~ 0.2 mm. Laser energies of up to ~ 0.275 J per pulse (typically 1 2000 M W c m -2) were used as measured with a Scientech power meter. Reproducible fluence variations of ~ 1 0 % were obtained by using uncoated neutral density filters. By careful regulation of the laser fluence near the ionisation threshold, it was possible to monitor the efl'ect of the laser fluence on the resulting laser ablation FT mass spectra. A typical event-sequence for the laser ablation F T M S experiment is shown in Fig. 1. Initially, a quench pulse P1 = 5 ms (by inversion of the voltage on one of the trapping plates), followed by a delay D1 =0.3 ~as, was used to remove ions generated in the previous experiment from the ICR cell. The leading edge of a second pulse, P2= 10 ms, was used to trigger a single focussed laser pulse onto the surface of zeolite. Before mass analysis, a delay of D 2 =0.1 s then allowed for most of the evaporated neutrals to be pumped away and the ICR cell chamber pressure to return to a pressure of 10 s Pa. The ions generated by laser ablation of the zeolite sample and the associated ionmolecule reactions were trapped in the ICR cell electromagnetically by potentials of 1-4 V applied to two trapping plates which were aligned perpendicular to the magnetic field. After the delay O2, a 3-ms radiofrequency chirp pulse P 3 = 3 m s (135V and 180 ° out of phase applied to each of two excitation plates) was used to bring all the

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469

ions in the ICR cell into coherent motion. Next, a prescan delay D E = 1 ms followed by an acquisition time of 13 ms was employed to detect all the laser-desorbed ions. Typically, a single experimental cycle of 32K data points was used in taking the wide-band FT to obtain the magnitude-mode spectra [5] displayed below. Exact masses measured in the narrow-band (high-resolution) FT mode [5] were within 20 ppm of the theoretical values and were used to assign the ions listed in Table 1. 3. Results and discussion

3. l. Positive ions

The positive-ion laser ablation F T mass spectra of partially dehydrated NaMOR, K L T L and NaLTA zeolites obtained at threshold laser irradiances of 2000, 900 and 200 MW cm- 2 respectively, are shown in Fig. 2. The threshold ionisation laser power is defined in this case as the minimum fluence which enables a reproducible observation of positive or negative ions which arise from the photodecomposition of the zeolite lattice. For the N a M O R zeolite, Fig. 2a indicates that both the ion-exchanged sodium and lattice-element ions of aluminium and silicon are observed. The hydrated aluminium cation, AI(H20) +, is also observed in this case. For the NaLTA and K L T L zeolites, Fig. 2b and c reveal that the ion-exchanged cations form the base peaks in the spectra. For the KLTL zeolite, a K(A102) + ion peak at m/z 98 is observed. It is not possible to assign the origin of this latter ion as it may arise from gas-phase ion-molecule reactions of the photoablated fragments or from intact components photoablated directly from the K L T L zeolite lattice. It appears that at low laser powers (kW cm 2) only sodium and potassium cations are photodesorbed from the internal cage or surface of the zeolites. At higher laser powers (200-2000 MW cm--2), the zeolites photodissociate and give rise to lattice element cations such as Si + and A1 + The differences observed in the fragment ion distributions of the three zeolites are related to a combination of the different threshold laser irradiances used to photoablate and ionise the zeolites as well as to the varied alkali ion content. The

S. Jeong et al./Microporous Materials 4 (1995) 467-473

470

Table 1 Negative ion formed by laser ablation (1064nm, 200 2000 MW cm -2) of NaLTA and NaMOR and KLTL zeolites observed by FT-MS

m/z

Anion

m/z

Anion

m/z

Anion

m/z

Anion

16 43 59 60 76 77 99

O A10 A1Oz SiO2 SiO3 HSiO 3 NaSiO 3

102 103 115 119 136 137 159

A1203 A1SiO3 KSiO3 A1SiO4 SizOs HSi205 NaSi205

175 179 196 197 219 235 239

KSi20 s A1Si206 Si307 HSi307 NaSi307 KSi307 AISi308

257 279 295 299 317 339 359

HSi409 NaSi409 KSi409 A1Si4Olo HSisO 11 NaSisO11 A1SisOI/

Structural assignments are based on high-resolution measurements.

alkali metal ion content is effectively determined by the aluminium content of the zeolite lattice with NaMOR (Si/AI=5)
3.2. Negative ions The negative-ion laser ablation FT mass spectra of partially dehydrated NaMOR, KLTL and NaLTA zeolites, which were obtained at threshold ionisation laser irradiances of 2000, 900 and 200 MW cm-2, respectively, are shown in Fig. 3. The mass spectra show many ions in common to all three zeolite, although relative intensities vary widely. Table 1 lists the observed ion masses and corresponding high-resolution mass assignments. With the exception of some small atomic and diatomic anions with m/z<59 and A1203, all anions for the three zeolites are clusters based on SiO 2 oligomers and are assigned as follows: (SiO2)~O (x= 1-3), A10(SiOz)xO- (x=0-5), M(SiOz)xO- ( M = H , Na, K and x = 1-5). At a laser power of 2000 MW cm 2, the negative-ion photoablation spectrum of the NaMOR zeolite (see Fig. 3a) reveals the widest distribution of anions and confirms extensive photoablation of this material. It is not clear at this point if the wide range of photoablated silicate-based negativeion clusters listed in Table 1 arise as part of the negatively charged zeolite structure which is photoablated directly with the local structure remaining intact. At high laser fluence (>200 MWcm-2), various ion-molecule reactions including associative aggregation among smaller fragments and

further fragmentation-ionisation of larger fragments will also occur in the laser-induced plasma plume above the surface of the zeolite. The neutral counterparts of the anions listed in Table 1 are expected to have high electron affinities and could undergo electron attachment reactions of photoejected electrons which are also trapped in the ICR cell in this experiment. Further experiments are in progress to determine if the cluster anions arise from reactions in the laser plume, or if they desorb intact from the zeolite lattice. McDaniel and Maher [25] have examined the thermal stability of sodium- and other cationexchanged faujasites. They report that the highest thermal stabilities are observed for materials containing the highest Si/A1 ratios. This trend is apparent in our results, where we observe a correlation between the zeolite Si/A1 ratios and the photoablation-ionisation threshold laser powers: NaMOR (2000 M W c m 2 Si/AI=5.0)>KLTL (900 MWcm 2, Si/AI=3.0)>NaLTA (200 MW cm 2, Si/AI= 1.0). Near-infrared spectra of the zeolites (measured under ambient conditions) show no absorption bands at 1064 nm. The photoablation of the zeolite structures nevertheless appears to be related to composition of the zeolite, which may undergo multiphoton absorption processes under high laser fluence. Alternatively, zeolite decomposition may be initiated by the thermal heating of residual adsorbed water. Experiments in progress with FAU zeolites indicate that the laser ablation threshold does depend strongly on the adsorbed water content. These experiments have shown that laser abla-

471

S. Jeong et al./Microporous Materials 4 (1995) 46 7-473

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tion F T M S can p r o d u c e a range o f positive a n d negative ions from zeolites which are related to the c o m p o s i t i o n of the zeolites. O u r o b s e r v a t i o n o f negative cluster a n i o n s in p a r t i c u l a r suggests

that the technique m a y be useful for studying at least qualitatively the c o m p o s i t i o n of zeolite lattices. F u r t h e r investigations of this p o i n t a n d of the m e c h a n i s m s of cluster a n i o n f o r m a t i o n are in

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progress. We also note the potential o f the techn i q u e for observing a d s o r b e d or occluded species in zeolites a n d will report later o n studies of such species.

Acknowledgements F i n a n c i a l s u p p o r t from A u s t r a l i a n Research C o u n c i l is acknowledged. Helpful discussions with

S. Jeong et al./Microporous Materials 4 (1995) 467-473

Dr. Stephen McElvany and Prof. Ben Freiser are also gratefully acknowledged.

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