J. Weitkamp, H.G. Karge, H. Pfeifer and W. HBlderich (Eds.) Zeolires and Related Microporous Marerials: &are of rhe A n 1994 Studies in Surface Science and Catalysis, Vol. 84 0 1994 Elsevier Science B.V. All rights reserved.
A Fourier-Transform Infra-Red Spectroscopic Study of the Adsorption of Hydrogen Cyanide by Zeolites and Pillared Clays J. Jamisa, T.D. Smitha, T.A.P. Kwakb and A. Dye+ Chemistry Department, Monash University, Clayton, Victoria, Australia, 3 168 bDepartment of Earth Science, La Trobe University, Bundoora, Victoria, Australia 3083 CDepartment of Chemistry and Applied Chemistry, University of Salford,
Salford, M5 4WT, United Kingdom
The uptake of hydrogen cyanide byprotonic forms of the synthetic zeolites Y, beta, mordenite, zeolites of natural occurrence, clinoptilolite, ferrierite, stilbite and alumina pillared clays derived from montmorillonite or from Kisi stone (termed rectorite) has been studied using Fourier-Transform infrared (F.T.I.R.) spectral measurements of the -C=N stretch vibration to characterise the binding of the hydrogen cyanide by the various Briansted acid sites of each zeolite or clay material. Diminished pressure desorption of zeolitically bound hydrogen cyanide, monitored by reductions in F.T.I.R. spectral band intensities, have been used to distinguish the strength of binding of hydrogen cyanide by the various Briansted acid sites. The I.R. spectral wavenumber of hydrogen cyanide bound to such sites has been correlated with structural features of the zeolites and pillared clays which give rise to their microporosity. The I.R. spectral wavenumbers due to hydrogen cyanide bound to alkali metal ion exchanged clinoptilolite has been shown to depend on the alkalimetal ion ionic radii and point to the more basic nature of the lattice oxygen atoms of zeolite Y compared with those of clinoptilolite. The treatment of silver ion-exchanged clinoptilolite with hydrogen cyanide leads to the reversible formation silver cyanide.
The adsorption of hydrogen cyanide, observed by Fourier-Transform infra-red (F.T.I.R.) spectroscopy, on self-supporting dehydrated wafers of the protonic forms of various synthetic zeolites results in an enhancement of the intensity of the I.R.spectral band due to -C=Nstretch along with a shift in band position to higher wavenumber compared with the free gas value. The i.r. band wavenumber shift is attributed to hydrogen bonding of hydrogen cyanide by Bransted acid sites within the microporous structure of the zeolite. The experimental evidence for the hydrogen bonding of hydrogen cyanide by the Bransted acid sites rests with the shift or extinguishment of i.r. spectral bands due to zeolitic hydroxyl groups. In keeping with the composite nature of the i.r. spectral bands due to zeolitic hydroxyl groups which is indicative of the presence of an array of different Bransted acid sites distributed throughout the zeolitic structure, instrumental derivatization of the i.r. spectral band due to hydrogen cyanide adsorbed by the zeolite reveals the wavenumber ranges of the component bands due to the tenancy by hydrogen cyanide of the different Bransted acid sites. The relative loss of spectral intensity of these component bands as a result of partial removal of hydrogen cyanide by diminished pressure or thermal desorption gives a measure of the relative strength of hydrogen bonding of hydrogen cyanide by the various Bransted acid sites. In general, a relationship exists between the type of Bransted acid site, characterized by the -CaN stretch wavenumber of bound hydrogen cyanide and the zeolite framework structural features which determine the salient microporous channel structure. A consideration of the acid sites of greatest strength for each zeolite indicates the decreasing order of strength of hydrogen bonding of hydrogen cyanide is ZSM5 > mordenite > zeolite L zeolite Y >> zeolite beta. However, there is no correlation between the strength of hydrogen bonding of hydrogen cyanide by the various Bransted acid sites and the shift of the i.r. spectral wavenumber due to adsorbed hydrogen cyanide from the free gas value since some of the largest spectral shifts are associated with the less tenaciously held hydrogen cyanide. The wavenumbers of the component bands of the observed i.r. spectral band due to adsorbed hydrogen cyanide serve to characterize the Bransted acid sites and makes possible their identification in various zeolites and their relationship to structural features.
Earlier F.T.I.R. measurements of the zeolitic adsorption of hydrogen cyanide involved the protonic forms of the synthetic zeolites Y,steam treated Y, X,mordenite, L, beta and ZSM-5 [ll. The present study is more concerned with similar measurements on the natural zeolites clinoptilolite, H-clinoptilolite (clinoptilolite exchanged with aqueous ammonium nitrate and heated), ferrierite, stilbite, and alumina pillared clays derived from montmorillonite (PILC) and a clay from Kisi stone (rectorite). More detailed measurements on the synthetic zeolites have been made for comparison with those concerned with zeolites of natural occurrence.
The synthetic zeolites were prepared by literature methods or obtained commercially. The naturally occurring zeolites were obtained as geological samples from various locations and the alumina pillared clays, derived fiom montmorillonite (PILC) and from Kisi stone (rectorite), were obtained from industrial sources. The synthetic zeolites, those of natural occurrence and pillared clays, characterized by elemental composition and X-ray powder diffraction were fabricated into self-supportingwafers and mounted in an LR. cell, which allowed for temperature control, gas exposure and gas removal by pressure reduction through a gas handling line. The zeolite wafers were heated initially to 593 K, the pillared clays to 473K and stilbite to 363K for six hours with diminished pressure (1.0x 10-2Tom) pumping and exposed to an atmosphere of hydrogen cyanide at room temperature followed by diminished pressure desorption of hydrogen cyanide with monitoring of each process by I.R. spectral measurements. Similar procedures were followed using alkali metal exchanged forms of the zeolites.
RESULTS AND DISCUSSION
The Bransted acid sites on zeolites HY, Beta and mordenite are characterized by well defined I.R. bands in the hydroxyl region ( H Y , 3640,3549 cm-1; beta 3731 cm-1; mordenite 3611 cm-1). Successive additions of hydrogen cyanide to these zeolites leads to a progressive reduction in the intensity of these bands and provides evidence for the participation of Brensted acid sites in the binding of hydrogen cyanide. Clinoptilolite and the pillared clay, PILC, have broader I.R. bands in this region which shift to lower wavenumbers on addition of hydrogen cyanide (clinoptilolite, 3596 3445 c m l ; PILC 3646 3627 cm-1). In the remaining zeolites the I.R. bands are too broad to allow further interpretation.
The wavenumbers of the component bands, obtained by fourth derivatization of the F.T.I.R. spectral bands due to hydrogen cyanide adsorbed by the various zeolites, are shown by the full lines of Figure 1. Subsequent pumping of the zeolite at diminished pressure (1.0x 10-2 Torr.) for two minutes to remove the less tenaciously held hydrogen cyanide fiom the weaker binding sites exposes the component bands due to the more tenaciously held hydrogen cyanide which are represented by the serrated additions to the lines of Figure 1. The component positions are sufficiently separated to show a characteristic array of weak and strong binding sites for hydrogen cyanide. The component band occurring at 2108 cm-1 is present for all the zeolites while the band at 2098 cm-1 is present in most cases.
Compared with both synthetic and other zeolites of natural occurrence, clinoptilolite, a member of the heulandite group of zeolite minerals with silicate and aluminate tetrahedra arranged in parallel layers whose interconnection by the silicate and aluminate tetrahedra establishes an interconnected lattice of channels and cavities, possesses a greater array of weaker and strong binding sites for hydrogen cyanide. Hydrogen cyanide binding sites characterized by F.T.I.R.spectral component bands at 2098 and 2108 cm-1, though associated with different binding strengths are common to most of the zeolites and therefore independent of the structural features of the zeolite. Apart from coincidences of the wavenumbers of the spectral component bands the arrays of strong binding sites for hydrogen cyanide serve to correlate the structural features of clinoptilolite and stilbite, both of which have the interconnected layer structures. The alumina pillared clays treated with hydrogen cyanide have spectral components whose wavenumbers are comparable with those of the zeolites and clearly arise from Br~nstedacid sites on the alumino-silicatelayer. In the case of PILC a further isolated band occurs a t 2222 cm-1whose intensity is hardly affected by diminished pressure pumping and therefore is associated with Br~nstedacid sites on the alumina pillars. For pillared rectorite these additional bands are not present. HY BETA MORDENITE FERRIERITE
CLlNOPTlLOLlTE STlLBlTE PlLC RECTORITE
B f l fl 9 ! I #
Wave nurnberkm-' Figure 1
Infra-red spectral component band positions for hydrogen cyanide adsorbed by various zeolites.
The reduction of the areas of the i.r. spectral bands due to zeolitically adsorbed hydrogen cyanide expressed as a percentage of the original area of the band with diminished pressure pumping for various times gives a measure of the strength of binding of hydrogen cyanide by the strongest binding sites represented by the serrated lines of Figure 1.
The results of such measurements are depicted by Figure 2 for the synthetic zeolites while those for the zeolites of natural occurrence are shown by Figure 3. In both figures the square root of the time of pumping is used for convenience of representation particularly for the longer times sometimes required for substantial desorption. Diminished pressure desorption from both the structurally similar clinoptilolite and stilbite is low compared with the other zeolites. The results for the desorption of hydrogen cyanide fiom clinoptilolite, illustrated by Figure 3, were obtained using a sample from a deposit in Indonesia. A sample of clinoptilolite from a deposit in Nevada gave pleasingly similar results. In keeping with the relatively fast hydrogen cyanide desorption from the protonic forms of synthetic zeolites desorption from the protonic form of clinoptilolite is equally rapid from a new array of Brgnsted acid sites which offer some advantage in its use as a catalyst. This result demonstrates the sensitivity of hydrogen cyanide desorptions to the presence of alkali metal ions which increase the strength of Brgnsted acidity. Indeed, using a sample of mordenite thoroughly decontaminated of sodium ions results in a more rapid desorption of hydrogen cyanide compared with mordenite containing some sodium ion so that, as shown by Figure 2, its Brgnsted acid binding strength for hydrogen cyanide becomes less than that of the HY zeolite. The slowest times desorption from the zeolites is largely attributable to the strongest hydrogen bonding of hydrogen cyanide to the Brgnsted acid sites being most marked in structurally similar stilbite and clinoptilolite. Steric effects may also play a role as considered in the desorption of hydrocarbon from zeolites [21.
i r 0
is 0 v) 0)
piio Figure 2
Diminished pressure desorption of HCN from synthetic zeolites at intervals of the square route of time.
Diminished pressure desorption of HCN from zeolites of natural zeolites at intervals of the square root of time.
The wavenumbers due to hydrogen cyanide adsorbed by alkali metal ion exchanged clinoptilolite as shown by Figure 4 depends on the alkali metal ion ionic radii. The desorption characteristics of hydrogen cyanide from alkali metal ion exchanged clinoptilolite is shown by Figure 5. The i.r. spectral wavenumber due to hydrogen cyanide adsorbed by zeolitic sodium ion depends on the nature of the zeolite being 2106 cm-1for clinoptilolite, 2100 cm.1 for zeolite A and 2101 cm-1 for zeolite Y [33. These small variations may arise from changes in the overall basicity of the zeolite frame work with zeolite Y being more basic than clinoptilolite. The desorption curves depicted by Fig. 5 show the expected easiest loss fiom the caesium exchanged form but the remaining results do not show a simple dependency of the alkali metal ion ionic radius. The adsorption band a t 2106 cm-1for hydrogen cyanide on sodium exchanged clinoptilolite was not observed in the zeolite in its natural form.
Crystal ionic radius (A)
Ionic radius dependence of adsorbed HCN wavenumber.
Diminished pressure desorption of HCN from alkali metal ion exchanged clinoptilolite.
It has been shown that treatment of the silver(1) exchanged sodium Y zeolite leads to the production of silver cyanide, characterized by an i.r. spectral band a t 2166 cm-1 and whose formation is reversed by diminished pressure pumping . Similarly, the treatment of silvedl) exchanged clinoptilolite with hydrogen cyanide gives rise to a band a t 2166 cm-1, whose intensity is reduced by diminished pressure pumping, and attributable to the reversible formation of silver cyanide which is independent of zeolite structure.
C. Blower and T.D. Smith, J. Chem. Soc., Faraday Trans., 90 (1994) to be published.
H.G. Karge and W. Niepn, Catal. Today 1991,8,541.
C. Blower and T.D. Smith, J. Chem. Soc., Faraday Trans., 90 (1994) to be published.