J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 919-929 FTIR Spectroscopic Study of the Zeolitic Adsorption of Hydrogen Cyanide on Acidic Sites Clive J. Blower and Thomas D. Smith* Chemistry Department, Monash University, Clayton, Victoria, Australia 3 168 The uptake of hydrogen cyanide by protonic forms of zeolites: Y, steam-treated Y, X, mordenite, L, beta and ZSM-5, has been studied using Fourier-transform infrared (FTIR) spectroscopic measurements of the -C=N stretch vibration to characterize the binding of hydrogen cyanide by the various Brsnsted acid sites of each zeolite. Diminished pressure and thermal desorption of zeolitically bound hydrogen cyanide, monitored by reduction in FTlR spectral band intensities, have been used to distinguish the strength of binding of hydrogen cyanide by the various acid sites.A relationship exists between the type of Brsnsted acid site, characterized by the -CEN stretch wavenumber of bound hydrogen cyanide, and the zeolite framework structural features in terms of the presence of sodalite units and the salient channel structures. A consideration of the acid sites of greatest strength for each zeolite indicates that the decreasing order of strength of hydrogen bonding of hydro-gen cyanide is ZSM-5 > mordenite > L x Y, X b beta. The IR spectral features of hydrogen cyanide bound by the Bransted acid sites on the protonic form of zeolite Y steamed at various temperatures have been interpreted in terms of acidic sites on extra-framework material and those associated with the zeolite framework.The extra-framework component has been interpreted to be a highly acidic, hydrated alumina-like material of much higher acidic strength than that of the framework acidic sites and whose formation is critically dependent on the temperature of steaming, and an aluminosilicate phase formed throughout the range of steaming temperatures. While there is no correlation between the strength of the various zeolitic Brsnsted acid sites and the IR spectral wavenumber of hydrogen cyanide bound to such sites, the wavenumber serves to characterize the sites and makes possible their identification in various zeolites and the elucidation of their relationship to zeolitic structur- al features. The zeolitic adsorption of gaseous material, whose molecules are capable of interaction with zeolitic functional sites, may serve to probe the internal structure of the zeolitic frame- work.FTIR has proved to be an invaluable investigative tool in the study of zeolitic functional groups and in the identifica- tion of materials within the zeolite.'-3 Therefore, a further requirement of the probe material is that it possesses an FTIR response, within the spectroscopic window of the zeolite, which could be used to monitor its tenancy of zeolitic acid sites. A chemical compound which meets these require- ments is hydrogen cyanide, as it diffuses easily into the zeolite, reacts with the acidic sites at room temperature and gives rise to an easily detected IR spectral response in the 2100 cm-' region. The present investigation deals with FTIR spectral mea- surements of hydrogen cyanide adsorbed by protonic forms of zeolites Y, steam-treated Y, X, L, beta and ZSM-5 and mordenite, which are designed to show the distribution of acidic sites, their relative concentrations and common fea- tures shared by the different zeolitic materials.Experimental Hydrogen cyanide (bp 298.7 K) was prepared by the method described in the literature? It was dried in the vapour phase by passage through anhydrous calcium chloride at 323 K and distillation from anhydrous calcium chloride after collection. The dried hydrogen cyanide was stored at low temperature and freshly distilled and dried before use. Starting with zeolite Y (Linde SK40, Si : A1 = 2.55 : 1)in its sodium form, it was converted to 90% ammonium form pH4(90)Y] by exchange with aqueous ammonium nitrate.5 Almost complete exchange of sodium ion by ammonium ion, NH4(98)Y, was achieved by hydrothermal exchange at 433 K while complete exchange to the ammonium form, NH4( lOO)Y, was accomplished by initial conversion of NaY zeolite to its silver ion form followed by treatment with aqueous ammonium thiocyanate.6 The sodium ion form of zeolite X (Si : A1 = 1.25 : 1) was prepared using the gel composition 1 rnol A1,o3-3.5 mol sio,-4.55 mol Na,0-200 mol H,O by the literature pro- ~edure.~The sodium form of zeolite X was converted to its 93% ammonium form [NH4(93)X] by treatment with a hot 353 K) 1 : 1 mixture of 0.5 mol dm-3 ammonium chloride and 0.5 mol dm-3 ammonium hydroxide.* The protonic form of zeolite ZSM-5, H(A1)ZSM-5, with Si : A1 = 38.8 : 1, was prepared as outlined previously.' The protonic forms of zeolite L (Si : A1 = 3.2 : 1) and mordenite (Si : A.= 6.0 :11) were kindly received from the Materials Science Division of CSIRO, Melbourne. Steam dealuminated samples (0.2-0.5 g) of NH4(98)Y were prepared by heating (393 K) the zeolite sample contained in a quartz tube in a stream of high-purity dinitrogen (30 cm3 min-') for 1 h before passage of steam through the zeolite at various temperatures (573, 623, 723, 823 and 1023 K). A similar procedure was used to prepare steam-treated alumina using chromatographic grade neutral material (Merck, 70-230 mesh) at 823 K.The analysis of the zeolitic material was carried out as described previo~sly.~ Sodium and aluminium contents were determined by atomic absorption spectroscopy, silicon by a combination of gravimetry and spectrophotometry. The zeo- lites were stored over saturated calcium nitrate solution. The X-ray powder diffraction patterns of the solid materials were recorded by a Rigaku Geigerflex instrument using nickel- filtered Cu-Ka radiation. Zeolite Adsorption of Hydrogen Cyanide and its IR Spectroscopy FTIR spectra were recorded using a Perkin-Elmer 1600 spec-trophotometer: resolution 2.0 cm-', range 4400-450 cm-' and four scans (16 s). Deconvolution, derivatization and mea- surements of the area under the curve on the IR bands of interest were carried out using the instrumental computer and software.Self-supporting wafers of the zeolites (about 15 mg, 15 mm diameter and 0.05 mm thick) were obtained by compression (1500 psi? for 15 min, 3000 psi for 15 min) of the zeolite and mounted in a cell with sodium chloride windows and a region for heat treatment of the wafer." The cell was con- nected to a gas-handling line which enabled gas transfer in and out of the cell to be carried out as well as provision for final drying of hydrogen cyanide and treatment of the wafer. Most of the treatment of zeolite wafers with hydrogen cyanide involved using enough of the reagent to saturate the zeolite at room temperature. This was initially heated to some desired temperature (usually 593 K), before recording the IR spectrum, it having been established that any free hydrogen cyanide in the gas phase within the cell did not contribute to the IR bands of interest.Hydrogen cyanide was removed from the cell by reducing the pressure to 0.0o01 Torr for 2 min (pressure desorption) and allowing the iso- lated cell to stand for 15 min. The IR spectrum was recorded once more by rolling the wafer assembly, after thermal or chemical treatment, into the light path. Spectral Analysis by Derivative Spectroscopy The analysis of IR spectral bands and other spectroscopic data by spectral deconvolution (resolution enhance-ment)' '-I4 and derivatization '5-18 has been described, while the limitations of such procedures have been o~tlined.''-~' Amongst a number of applications, enhanced resolution FTIR spectroscopy has been used to identify the components of the high-wavenumber (HW) band due to the hydroxy groups in zeolites X and Y." In another approach to analys- ing the components of the low-wavenumber (LW) IR spectral band due to the zeolite hydroxy groups, curve resolution was used and critically evaluated.23 To quantify, as far as pos- sible, the relative amounts of hydrogen cyanide bound at various sites by the zeolites, the rather sharp IR band due to zeolite-bound hydrogen cyanide was recorded with scale expansion to delineate the main peak constituents in the band.To identify the peaks further derivative spectra were obtained, the fourth derivative offering the best separation but with some distortion of relative intensities.The main purpose of taking the various derivatives was to establish the wavenumber range of each peak. With the additional aid of deconvolution of the experimental spectra, the areas under the curves of the experimental spectra for each wavenumber range appropriate to a particular peak in the fourth-derivative spectrum were computed instrumentally and used as a measure of the relative amounts of hydrogen cyanide in its various modes of binding. The procedure was used largely to monitor the relative changes in the binding of hydrogen cyanide when the zeolites were subjected to various initial thermal treatments, to sequential exposure to various amounts of hydrogen cyanide reaching saturation and finally in the partial removal of hydrogen cyanide from the zeolite at diminished pressure.A typical example of the segmentation of the experimental spectrum into areas whose wavenumber limits are defined by the fourth derivative of the experimental spectrum is shown in Fig. 1. An alternative approach used to establish the relative con- tributions of constituent bands to the area of the experimen- tal spectrum was to generate constituent band shapes such that their combined areas were close to that of the experi- 7 1 psi (pounds per square inch) z 6.894757 x lo3 Pa. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 mental spectrum and gave a good wavenumber fit. An example of such a procedure is illustrated in Fig.2 which involved the following steps. The fourth derivative of the experimental IR spectrum due to zeolite-adsorbed hydrogen cyanide was used to identify four peaks at 2121, 2116, 2107 and 2097 cm-'. A further peak at 2111 cm-'was identified from the fourth derivative of the experimental spectrum due to zeolite-adsorbed hydrogen cyanide after a short period of diminished pressure pumping. The band centred at 2097 cm-' was constructed first by using the second derivative of the experimental spectrum to measure the separation of the two positive lobes on both sides of the negative lobe at 2097 cm-'from which the half-width of the original bandIg was determined and found to be 9.0 cm-'. This value was used for the construction of the synthetic band centred at 2097 cm-', a value of 8.0 cm-' being finally chosen.Owing to the overlapping of the negative lobes of the second derivative of the experimental spectrum the height and half-width values of the remaining bands centred at 2121, 2116, 2107 and 2111 cm-' were constructed using Lorentzian lineshapes so that their combination gave a good fit of the experimental spec- trum which was judged by a comparison of the four deriv- atives of the experimental and the combined lineshape as shown in Fig. 2. Results Interaction of Hydrogen Cyanide witb Zeolite Y The Brransted acidity of zeolite Y is characterized by the (HW) IR spectral band at 3640 cm-' and (LW) band at 3549 cm-'. The variation in the intensity of these spectral bands as a result of heat treatment of the zeolite is a measure of the thermal stability of the Brransted acid sites.The IR spectral data collected for zeolite H(98)Y over the range 428 to 873 K show a decline in the existence of Brnrnsted acid sites over the temperature range 673-873 K. The IR spectral changes brought about by exposure at room temperature (298 K) of wafers of zeolite H(98)Y, ini-tially heated to 593 K, to increasing amounts (12.9 pmol to 450 pmol) of hydrogen cyanide show that the Br~rnsted acid sites, represented by the HW and LW IR spectral bands, take part in the binding of hydrogen cyanide while the reduction in the intensity of the HW spectral band is more marked compared to that of the LW spectral band when the expo- sures involve small amounts of hydrogen cyanide (77-450 pmol).The binding of hydrogen cyanide by the zeolite H(98)Y gives rise to bands on either side of the 2100 cm-' region which increase in intensity with greater amounts of hydrogen cyanide on the zeolite. Deconvolution of the IR spectral bands due to adsorbed hydrogen cyanide shows that the smaller additions (25 to 180 pmol) of hydrogen cyanide give rise largely to a band in the region 2116 cm-' and to a smaller extent at 2095 cm-' while larger additions (8 mmol) of hydrogen cyanide lead to additional bands in the 2120 cm-' region and a pronounced increase in the bind at 2095 cm-'. The component peaks of the original IR spectrum due to hydrogen cyanide adsorbed by zeolite H(98)Y, made clearer by deconvolution, are more easily discerned by derivatization with prominent peaks at 2097, 2120, 2116, 2108 cm-' and shoulders at 2112 and 2104 cm-'.Partial removal of hydro-gen cyanide by diminished pressure desorption effects a marked reduction in intensities of peak components at 2120 and 2097 cm-' and a diminution of intensities of com-ponents at 2116, 2112, 2108 and 2104 cm-' with the main form of hydrogen cyanide binding being represented by the peak at 2112 cm-'.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2150 2100 2050 2150 2100 2050 wavenumber/cm-' wavenumber/cm -' Fig. 1 Illustration of IR signals differentiation applied to the vCN region of hydrogen cyanide adsorbed onto H(98)Y zeolite prepared at 593 K, before pressure desorption.The shaded regions indicate the segment used as a measure of the area of bands at 2114 and 2096 cm-' in the experimental spectrum. The segments were determined from the fourth-derivative spectrum. (a) Experimental spectrum, (b) first-derivative spectrum, (c) second-derivative spectrum, (d) third-derivative spectrum, (e) fourth-derivative spectrum and (f)experi-mental spectrum including segments. The IR spectra at room temperature due to hydrogen cyanide adsorbed by wafers of zeolite H(98)Y, each pre- viously subjected to heating at a particular temperature and reduced pressure for 1 h, along with those after partial desorption of hydrogen cyanide by diminished pressure were recorded.Treatment of the IR data by deconvolution and derivatization to identify peak components followed by curve segmentation as described previously gives a measure of the variation of the modes of binding of hydrogen cyanide with initial temperature treatment of each zeolite wafer. The results are shown in Table 1. A similar sequence of IR mea-surements of hydrogen cyanide adsorption by heat-treated wafers of zeolite H( 1OO)Y and Y(90)Y were carried out and a similar treatment of the experimental data provided the infor- mation summarized in Table 2 for zeolite H( 1OO)Y and Table 3 for zeolite H(90)Y. Interaction of Hydrogen Cyanide with Steam-treated Zeolite H(98)Y Zeolite H(98)Y was heated in steam at 823 K, washed with water and finally washed with dilute (0.1 mol dm-3) hydro- chloric acid.After each treatment wafers of the zeolite material, after heating to 593 K, were exposed to hydrogen cyanide at room temperature, the IR spectra recorded and compared with those of wafers of the starting zeolite H(98)Y and chromatographic-grade neutral alumina heated in steam at 823 K (both heated to 593 K before being exposed to 921 I cu al C+ v).I) 0.30 21 50 2100 2150 2100 2150 2050 2150 wavenumber/cm -' wavenumber/cm-' Fig. 2 Comparison of vCEN regions of (a) H(98)Yprepared at 593 K before pressure desorption of hydrogen cyanide (total area 3.4152 A cm-' mg-') and (b) a synthetic peak system constructed on a back- ground of H(98)Y (593 K), before hydrogen cyanide adsorption, using the following Lorentzian peaks (total area 3.3212 A an-' mg-'): (i) 2097 cm-', abs 1.30, width 8.0 an-';(ii) 2121 cm-',abs 1.20, width 13.0 an-'; (iii) 2116 cm-' abs 0.70, width 12.0 cm-' an-'; (iv) 2107.5 cm-', abs 0.20, width 12.0 an-'; (v) 2111.5 an-', abs 0.225, width 10.5 cm-'.Fourth-derivative spectra of vCrN regions of (c) H(98)Y (593 K) before pressure desorption of hydrogen cyanide and (d) the total synthetic peak system described in (b).hydrogen cyanide at room temperature). The IR spectra after partial removal of hydrogen cyanide from the steamed, water-washed and dilute acid leached zeolite samples were also recorded and compared with those obtained after partial removal of hydrogen cyanide from the starting zeolite H(98)Y and steamed alumina. The effect of steaming the zeolite H(98)Y at various temperatures on the IR spectral character- istics of hydrogen cyanide adsorbed by wafers of steamed H(98)Y with the temperature of steaming of the zeolite, is shown in Fig. 3, from which it is evident that the peak area at 2226 cm- ' is little affected by diminished pressure pumping, while the peak area at 2147 cm-' is substantially reduced.The components of the IR bands due to hydrogen cyanide on the zeolite H(98)Y steamed at various temperatures were identified by derivatization. The variations in the peak areas at 2120 and 2097 cm-', both of which are substantially influ- enced by diminished pressure pumping, with temperature of steaming, as well as similar variations in peak areas with tem- perature and diminished pressure for peaks at 21 14,2108 and 2104 cm- ',are summarized in Table 4.Interaction of Hydrogen Cyanide with Zeolite HX The variation, with temperature, in the intensity of the com- bined peak areas of the IR bands at 3650 cm-'(HW) and 3572 cm-' (LW) due to the hydroxy groups responsible for the Brsnsted acidity of zeolite HX is given in Table 5, which J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Areas of constituent bands of the vCnN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite H(98)Y zeolites prepared at various temperatures (A,) after diminished pressure pumping for 2 min (A,) 2120 cm-' 2097 cm-' 2116 cm-' 2108 cm-' 2104 cm-' T/K A1 A2 A1 A2 A1 A2 A2 A2 A1 A2 ~ 430 0.65 0.10 1.37 0.57 0.34 0.11 0.4 1 0.15 0.45 0.21 535 1.03 0.26 0.90 0.20 0.44 0.28 0.55 0.12 0.37 0.15 595 0.95 0.16 0.80 0.15 0.44 0.28 0.46 0.12 0.33 0.10 680 0.85 0.15 0.82 0.15 0.48 0.30 0.38 0.14 0.33 0.12 870 0.10 0.05 0.05 0.05 0.12 0.10 0.26 0.06 0.18 0.08 ~ ~~~ Table 2 Areas of constituent bands of the vCsN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite H(100)Y zeolites prepared at various temperatures (Al), and after diminished pressure pumping for 2 min (A,) 2120 cm-' 2097 cm-' 2116 cm-' 2108 cm-' 2104 cm-' T/K A' A2 A1 A2 A' A2 A1 A2 A1 A2 428 0.61 0.06 1.10 0.42 0.34 0.18 0.26 0.10 0.38 0.10 532 0.78 0.12 0.70 0.10 0.45 0.28 0.28 0.08 0.25 0.08 595 1.05 0.13 0.82 0.13 0.62 0.32 0.33 0.08 0.25 0.08 675 0.67 0.10 0.72 0.13 0.43 0.30 0.3 0.08 0.18 0.08 Table 3 Areas of constituent bands of the vCZN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite H(90)Y zeolites prepared at various temperatures (Al), and after diminished pressure pumping for 2 min (A,) ~ ~~ ~ ~~ 420 0.40 0.04 0.87 0.22 0.27 0.12 0.45 0.12 0.43 0.3 1 540 0.70 0.07 0.97 0.12 0.42 0.15 0.58 0.18 0.44 0.2 1 598 0.65 0.07 0.96 0.16 0.45 0.20 0.56 0.20 0.42 0.26 670 0.35 0.07 0.97 0.2 1 0.38 0.25 0.72 0.25 0.50 0.37 870 0.07 0.02 0.36 0.02 0.13 0.07 0.28 0.07 0.19 0.08 also shows similar variations in terms of the combined peak particular temperature for 1 h at diminished pressure, along areas of the HW and LW bands for zeolite H(90)Y.with the effect of partial removal of' hydrogen cyanide by The IR bands due to hydrogen cyanide adsorbed at room diminished pressure pumping for a short time, were recorded. temperature on wafers of zeolite HX, previously heated at a Table 6 summarizes this variation in terms of the reductions of the total IR band areas due to adsorbed hydrogen cyanide with diminished pressure pumping and initial heat treatment 0.50 r of the zeolite wafer. With the aid of deconvolution and derivatization of the IR bands due to adsorbed hydrogen cyanide, the constituent bands were identified at 2120, 2114, 2107, 2104 and 2095 cm-'.The variation in the segmental peak areas derived from Table 6, where the wavenumber range of each segment is defined by the derivative peaks, with initial heat treatment of the zeolite wafer, is shown in Table 7, which illustrates that E 0.30< 0.40 (b) '\ the hydrogen cyanide represented by segments at 2120, 2114 and 2095 cm-' is easily removed by lowering the pressure, 2? I \ while that represented by segments at 2107 and 2104 cm- is 3 rather more firmly held. 0.20 -.-0 Thermal Desorption of Hydrogen Cyanide from Zeolite C H(98)Y and Zeolite HXi? Diminished pressure desorption of hydrogen cyanide from 0.10 -(a wafers of zeolites X and Y provides a means of identifying the more tenacious binding sites of hydrogen cyanide.Thermal desorption of hydrogen cyanide from wafers of the zeolite which have been subjected to a short period of reduced pres- """""""""""' sure pumping to remove the less firmly held hydrogen cyanide provides additional information on the relative strength of binding of hydrogen cyanide by the various stronger Brsnsted acid sites. The loss of constituent IR band areas due to hydrogen cyanide initially adsorbed at room temperature and after 2 min of reduced pressure on a wafer of zeolite H(90)Y, previously heated to 593 K for 1 h at J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 923 Table 4 Areas of constituent bands of the vCIN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite NH,(98)Y zeolites prepared at various temperatures (Al), after diminished pressure pumping for 2 min (A,) 2120 cm-' 2097 cm-' 2114cm-' 2108 cm-' 2104 cm-' T/K A1 A2 A1 A2 A1 A2 A1 A2 A1 A2 570 0.20 0.02 0.52 0.11 0.34 0.13 0.20 0.18 0.25 0.18 620 0.15 0.08 0.47 0.12 0.27 0.09 0.21 0.18 0.24 0.14 720 0.18 0.08 0.49 0.12 0.29 0.11 0.3 1 0.18 0.24 0.07 820 0.18 0.06 0.54 0.08 0.17 0.09 0.3 1 0.13 0.24 0.06 1020 0.09 0.02 0.47 0.04 0.14 0.05 0.22 0.06 0.19 0.05 diminished pressure, with increasing temperature of the tion enables constituent peaks of the spectra to be identified wafer, is summarized in Table 8.Similar information con-at 2144, 2108, 2101 and 2094 cm-'.The thermal desorption cerned with the loss of IR spectral peak areas due to hydro- of hydrogen cyanide from the wafer of zeolite HL after gen cyanide adsorbed, in the first instance at room reduced pressure pumping, in terms of the loss of spectral temperature followed by pumping for 2 min, by a wafer of segmental areas for the ranges 21 16-2106, 2106-2097 and zeolite HX initially heated to 593 K for 1 h at diminished 2097-2082 cm-', defined from the derivatized spectra at each pressure, with increasing wafer temperature is summarized in temperature, is summarized in Table 10.Table 9. Interaction of Hydrogen Cyanide with Mordenite Interaction of Hydrogen Cyanide with Zeolite L The IR spectrum due to hydrogen cyanide adsorbed at room The IR spectrum due to hydrogen cyanide adsorbed at room temperature by the protonic form of mordenite fabricated temperature by zeolite HL, initially heated to 593 K at into a wafer, which was initially heated at 593 K for 1 h at diminished pressure for 1 h, was recorded along with the loss diminished pressure, was recorded after partial removal of of IR spectral intensity after reduced pressure pumping for 2 hydrogen cyanide by reduced pressure pumping.A com-min. A combination of spectral deconvolution and derivatiza- bination of spectral deconvolution and derivatization results Table 5 Total peak areas of IR hydroxy-group region of H(90)Y and HX zeolites prepared at various temperatures Table 8 Decreases in areas of constituent bands in the vCEN envelope arising from thermal desorption of hydrogen cyanide from zeolite H(90)Y band area zeolite HX band area zeolite H(90)Y initially prepared at 593 K and pumped at diminished T/K (3675-3423 cm-') (3678-3423 cm-') pressure for 2 min 430 5.1 1.9 490 5.8 2.0 540 6.0 2.0 300 0.10 0.17 0.42 0.26 560 6.0 1.9 325 0.08 0.14 0.34 0.23 595 5.8 1.6 375 0.02 0.06 0.13 0.09 670 3.6 0.2 420 0.01 0.03 0.06 0.04 720 2.8 0 480 0 0 0.01 0.02 870 0.4 0 520 0 0 0 0.01 Table 6 Total peak area of the vCIN region after room-temperature Table 9 Decreases in areas of constituent bands in the vCEN adsorption of hydrogen cyanide onto HX zeolite prepared at various envelope arising from thermal desorption of hydrogen cyanide from temperatures zeolite HX initially prepared at 593 K and pumped at diminished pressure for 2 min A1 A2 T/K 2095 cm-' 2107 cm-' 2104 cm-' 430 1.40 0.67 540 1.40 0.62 300 0.24 0.09 0.12 590 1.25 0.61 325 0.20 0.06 0.10 680 0.94 0.45 370 0.09 0.02 0.04 870 0.05 0 430 0.03 0.01 0.01 480 0.01 0 0For A,, A,, see Table 1.Table 7 Areas of constituent bands of the vCIN envelppe due to room-temperature adsorption of hydrogen cyanide onto zeolite HX prepared at various temperatures (Al), and after diminished pressure pumping for 2 minutes (A,) 2120 cm-' 2095 cm-' 2114 cm-' 2107 cm-' 2104 cm-' T/K A1 A2 A1 A2 A1 A2 A1 A2 A1 4 -430 0.11 0.01 0.5 1 0.32 0.19 0.02 0.13 0.02 0.16 -535 0.10 0.01 0.60 0.23 0.19 0.02 0.18 0.07 0.19 595 0.05 0.01 0.56 0.23 0.13 0.03 0.16 0.09 0.20 -675 0.02 0.01 0.42 0.16 0.07 0.02 0.13 0.06 0.20 --870 0 0.04 0 0.01 0 0.01 0 0.01 924 Table 10 Decreases in areas of constituent bands in the vCpN envelope arising from thermal desorption of hydrogen cyanide from zeolite HL initially prepared at 593 K and pumped at diminished pressure for 2 min T/K 2111 an-' 2101 m-' 2093 an-' 300 0.13 0.18 0.09 320 0.11 0.15 0.07 370 0.05 0.08 0.04 420 0.0 1 0.2 0.01 in the identification of constituent peaks of the spectra, which indicates that the major band constituents, which represent the binding of hydrogen cyanide by the zeolite, occur at 2121, 2118, 2114, 2110, 2106, 2102 and 2098 cm-' with major binding sites being represented at 2118 and 2114 cm-'.The thermal desorption of hydrogen cyanide from the more tena- cious binding is summarized in Table 11, which shows the decreasing segmental areas of the IR bands with increasing temperature of the zeolite wafer exposed to hydrogen cyanide at room temperature followed by pumping for 2 min.Interaction of Hydrogen Cyanide with Zeolite Beta The adsorption, at room temperature, of hydrogen cyanide by a wafer of the protonic form of zeolite beta, initially heated to 573 K for 1 h at diminished pressure, give rise to the IR spectrum which shows a dramatic decrease in spectral intensity as a result of lowering the pressure for a short time. Spectral deconvolution and derivatization shows that the spectrum after pumping possesses constituent spectral bands Table 11 Decreases in areas of constituent bands in the v,--~ envelope arising from thermal desorption of hydrogen cyanide from the protonic form of mordenite, initially prepared at 593 K, pumped at diminished pressure for 2 min T/K 2118 cm-' 2114 an-' 2110 an-' 2106 an-' 298 0.15 0.10 0.07 0.07 322 0.13 0.09 0.06 0.06 372 0.08 0.06 0.03 0.04 420 0.04 0.03 0.02 0.02 443 0.0 1 0.02 0.01 0.0 1 Table 12 Decreases in areas of constituent bands in the vC-., envelope arising from the temperature desorption of hydrogen cyanide from the protonic form zeolite beta, initially prepared at 563 K, pumped at diminished pressure for 2 min T/K 2115 cm-' 2106 cm-' 300 0.024 0.010 3 20 0.019 0.007 350 0.007 0.003 375 0.003 0.002 Table 13 Decreases in areas of constituent bands in the vCpN envelope arising from the temperature desorption of hydrogen cyanide from the protonic form of zeolite ZSM-5,initially prepared at 563 K, pumped at diminished pressure for 2 min T/K 2122 m-' 2114 cm-' 300 0.070 0.029 325 0.058 0.018 378 0.040 0.0 12 440 0.018 0.004 470 0.006 0.002 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 at 2115 and 2106 cm-' with the latter band accounting for most of the adsorbed hydrogen cyanide, the thermal desorp- tion of which from the zeolite is summarized by Table 12. Interaction of Hydrogen Cyanide with Zeolite ZSM-5 The IR spectrum due to hydrogen cyanide adsorbed at room temperature by a wafer of the protonic form of zeolite ZSM-5, previously heated to 573 K for 1 h at diminished pressure, was recorded. The constituent bands of this spec- trum, identified by deconvolution and derivatization, occur at 2122, 2115, 2107 and 2098 cm-'.Diminished pressure pumping for a short time removes nearly all of the hydrogen cyanide represented by the peaks centred at 2098 and 2107 cm-'.The thermal desorption of the more firmly bound hydrogen cyanide from the zeolite, most of which may be accounted for by peaks at 2122 and 2114 cm-', is sum- marised in Table 13. Discussion Hydrogen-bonding Interactions by Brensted Acid Sites of Zeolite Y Gas-phase hydrogen cyanide possess three IR bands in the mid-range of wavenumbers typical of a linear (Cmv)triatomic molecule, namely: v3, C-H stretch, 331 1 cm-';vl, -CEN stretch, 2097 cm-' and v2, bond-angle deformation, 712 cm-l .24 Solid hydrogen cyanide exists in a linear hydrogen- bonded chain form crystallizing with an orthorhombic unit cell, 1 mm (C2v20with two molecules in the unit cell).25 In the solid form the wavenumber of the C-H stretch mode, v3, is reduced while that of the bending mode, v2, is increased. Even in the gas phase hydrogen cyanide shows a tendency towards the formation of hydrogen-bonded aggregates with about 10%in dimeric and 3% in trimeric form.26 The multi- mer forms, particularly that of the dimer, become increasingly important in low-temperature matrices. The effect of hydro- gen bonding in the dimeric aggregates is to shift the vl, v2 and v3 band wavenumbers, typically to values27 of v3, 3306 cm-'; vl, 2093 and 2112 cm-' and v2, 731 and 793 cm-' and, importantly, to enhance greatly the intensity of the v1 -CEN stretch band.27-34 The basicity of hydrogen cyanide is exemplified by its com- bination with hydrogen fluoride when in monomeric or superficially linear dimeric form3 to give the theoretically predi~table~~increase in the -CzN stretch wavenumber (vl) with intensity enhancement of the band.37-39 Hydrogen cyanide adsorbed by surface hydroxy groups on silicate glass surfaces gives rise to absorption bands due to single molecules as well as polymers.40i41 The adsorption of hydrogen cyanide on silica and alumina has been monitored by far-IR spectroscopy where hydrogen bonding between the surface hydroxy groups and hydrogen cyanide results in a hindered rotation of hydrogen cyanide.42 An important IR spectral band which occurs in all cases of zeolitic adsorption of hydrogen cyanide has a wavenumber in the 2095 cm-' region.On the grounds that the profile of band intensity with variation in temperature treatment of the zeolite parallels that of the spectral intensity changes in the hydroxy-group region, this band is attributable to Brmsted acid site binding of hydrogen cyanide rather than gas-phase hydrogen cyanide, which has a low spectral intensity in this wavenumber region, or a dimeric form of hydrogen cyanide which would give a band of high spectral intensity in concert with a weaker band at 21 12 cm-'. Although a band is obser- vable in the 2112 cm-' region, its spectral intensity is not coupled with that at 2095 cm-' in circumstances of partial removal of hydrogen cyanide from the zeolites.The remain- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ing IR spectral bands are attributable to hydrogen cyanide bound to Brsnsted acid sites whose relative strengths of binding have been differentiated by reductions in spectral band intensity brought about by diminished pressure or increasing zeolitic temperature desorption. However, there is no simple relationship between the strength of the hydrogen cyanide interaction with Bronsted acid sites and the IR spec- tral wavenumber shift from that of the free-gas value, some of the largest wavenumber shifts being associated with the less firmly held hydrogen cyanide. The hydrogen form of the near-faujasite Y zeolite, whose structural features are well known 43*44 and which contains various Brernsted acid sites, is formed as a result of heating to 593 K, with reversible evol- ution of ammonia and loss of physically bound water.A sig-nificant decrease in Bronsted acidity occurs as a result of slow water loss from neighbouring hydroxy groups at 593-823 K with little structural change. A faster dehydroxylation takes place at 823-903 K, which involves migration of framework aluminium into the intracrystalline pore system with com- plete disruption of the crystal The existence of several composite IR spectral bands due to zeolitically bound hydrogen cyanide is in keeping with the recognition of the various spectral bands which make up the HW and LW IR band due to the hydroxy gro~ps.~O-~~ The incremental addition of hydrogen cyanide to zeolite H(98)Y shows that the HW spectral band due to hydroxy groups is more affected by relatively small additions of hydrogen cyanide, this band being associated with the stronger acidity.A reduction in the intensity of the HW hydroxy-group is paralleled by the appearance of constituent bands due to hydrogen cyanide at 2116, 2108 and 2104 cm-' with a pre- dominance of the first two. Reductions in the intensity of the LW band, which requires the presence of larger amounts of hydrogen cyanide, coincide with the growth of constituent bands at 2120 and 2097 cm-'. Bearing in mind the uncer- tainties involved in comparing IR spectral band intensities, the greater intensities of the bands at 2120 and 2097 cm-' compared with those at 2116, 2108 and 2104 cm-' indicates that the former bands arise from a greater number of acid sites, albeit of lower strength than those of the latter.The variation of the intensities of the bands at 2120 and 2097 cm-' with initial thermal treatment of the zeolite indicates that both weaker acid sites become less numerous with increasing temperature of heat treatment of the zeolite. At lower temperatures of heat treatment the weaker acid sites, represented by the band at 2097 cm-', are present in greater amount while the full formation of the other weaker acid binding site (2120 cm-') requires zeolite temperatures of about 573 K.The variation of the spectral intensity of the constituent peaks at 2114, 2108 and 2104 cm-' due to the more firmly held hydrogen cyanide bound by the zeolitic Bronsted acid sites characterized by the HW hydroxy-group, with the tem- perature of prior thermal treatment of zeolite H(98)Y, shows a rise and fall which parallels that of the HW spectral band intensity with temperature. In conditions of diminished pres- sure desorption the greater part of the hydrogen cyanide is bound at a single site represented by the constituent band at 21 12 cm-'. A largely similar picture emerges for the binding of hydrogen cyanide by zeolite H(100)Y with some variations arising from the different synthetic route used to obtain the protonic form of the zeolite.However, the incomplete conver- sion of the protonic form of the zeolite, as in traditional zeolite H(90)Y, results in marked differences. For example, the more weakly held hydrogen cyanide, which is represented by the constituent peak at 2120 cm-', accounts for less of the bound hydrogen cyanide while amongst the more firmly held forms of hydrogen cyanrde, rather more is held by the site 925 Table 14 Percentage fall in vCrN IR segmental band areas with pumping for 2 min at 0.001 Torr for partial removal of adsorbed hydrogen cyanide wavenumber/cm -zeolite 2120 2097 2116 2108 2104 H(98)Y 80 64 36 76 69 H(100)Y 82 85 43 54 36 H(90)Y 85 82 56 64 36 HX 91 64 79 35 44 dealuminated HY 80 87 60 -30 represented by the constituent band at 2108 cm-' which remains the more dominant binding site throughout the tem- perature range of thermal treatment of zeolite Y.The amount of hydrogen cyanide on the other two sites (2116 and 2104 cm-') remains about the same as in zeolite H(98)Y and H(1OO)Y. Consideration of the percentage falls in IR segmental band areas as a result of diminished pressure pumping, which gives a measure of binding of hydrogen cyanide at the acid sites characterized by the IR spectral wavenumbers due to adsorbed hydrogen cyanide, the smallest fall indicating the highest degree of hydrogen bonding, are summarized in Table 14. It shows that zeolites H(90)Y and H(1OO)Y possess an abundance of sites of relatively low degree of hydrogen- bonding interaction (2120 and 2097 cm-'), sites of stronger hydrogen-bonding interaction (21 16 and 2108 cm-') and sites of a relatively high degree of hydrogen bonding (2104 cm-'). Assuming that the molar absorption coefficients at each wavenumber in Table 14 have comparable values, the amount of hydrogen cyanide bound at the acid site giving rise to the peak at 2104 cm-' is reasonably constant at 13.8% for H(90)Y, 11.5% for H(100)Y and 12% for H(98)Y of the total uptake of hydrogen cyanide.The interaction of hydrogen cyanide with silica gives rise to a band at 2105 cm-' which may indicate that the band at 2104 cm-',due to uptake of hydrogen cyanide on zeolite Y, is due to a distribu- tion of silica within the zeolitic structure whose hydrogen- bonding capacity for the most part shows a steady decline with increasing temperature of initial heat treatment of the zeolite.Influence of Steaming on the Uptake of Hydrogen Cyanide by Zeolite Y The dealumination of zeolite Y results in changes to unit-cell size, crystallinity, acid site strengths and spatial distribution, pore-size distribution and the occurrence of extra-framework material, all of which influence the zeolite in fluid hydrocar- bon cracking processe~.~~-~~High-temperature (1 173 K) steaming of zeolite HY results in zeolite crystal damage, a surface accumulation of an alumina-like material and a reduction of zeolitic acid sites6* which are important in hydrocarbon cra~king.~~?~'Framework dealumination results in a change of the Brsnsted acid ~ites.~' Steaming in the lower-temperature range (773-873 K) results in the for- mation of alumin~silicate,~~-~~which may play a role in hydrocarbon cracking.75 Other work has provided evidence for the formation of aluminium oxide in the sodalite as well as changes in zeolitic porosity.79 The aluminosilicate amorphous phase may reside largely in the supercages8' and possess Lewis acidity.81 The variation in the intensities of the IR spectral constitu- ent bands due to hydrogen cyanide adsorbed by steamed zeolite H(98)Y with the temperature of steaming may be used as a guide to the changes in zeolite structure which occur as a result of the steaming process with the aid of the following assignments of the bands.(i) The IR band at 2226 cm-'is in the same region as that at 2210 cm-' which characterizes hydrogen cyanide binding by similarly steamed hydrated alumina. It may be concluded that the band at 2226 cm-' represents easily removed, highly dispersed, water-solubilized material which is close in composition to hydrated alumina. The intensity of this band is critically dependent on the tem- perature of zeolite steaming. While small amounts of this material occur at lower steaming temperatures, there is a marked increase when the temperature of zeolite steaming reaches 723 K rising to a maximum at 923 K; thereafter the number of its acidic sites declines rapidly up to 923 K. By the yardstick of hydrogen cyanide binding, this material pos- sesses acid sites much stronger than those of the zeolite framework.(ii) The IR spectral band at 2147 cm-' represents material formed within the pore system of the zeolite, being removed by acid leaching and possessing an acidity compara- ble to that of the zeolite framework sites. The formation of this material is quite variable with temperature of steaming, the number of its acidic sites declining at higher steaming temperatures. (iii) The band at 2120 cm-' arises from the uptake of hydrogen cyanide by a zeolitic framework site, much reduced in intensity throughout the range of steaming temperatures compared with that due to the starting material. This band, whose growth is paralleled by a reduction in the intensity of the LW band of the hydroxy group, is most affected by the presence of sodium ion, being of lower intensity in H(90)Y compared with H(98)Y and H(100)Y, and is therefore thought to be associated with the migratory protons from the sodalite or prism cages.(iv) The band at 2097 cm-' which occurs due to hydrogen cyanide uptake by H(98)Y prior to steaming (though at an increased level), remains at a remarkably steady level throughout the temperature range of steaming while resisting the rapid decline in intensity in the case of H(98)Y at higher initial tem- perature of heat treatment. (v) The IR spectral band at 2104 cm-' is thought to be due to hydrogen cyanide bound to the acidic site of silica within the zeolite structure, its persistence throughout the temperature range of steaming being compa- rable with that observed with increasing initial temperature treatment of H(98)Y. Its most important features are its sur- vival with acid leaching and its assignment to the site of strong acidity.(vi) The band at 2114 cm-' is due to an acid site on the zeolite framework. The downward trend in its intensity with increasing steaming temperature beyond 573 K is comparable with the behaviour observed for the starting material. (vii) The band at 2108 cm-' represents the binding of hydrogen cyanide by a Brarnsted acid site on the zeolite structure. The intensities of this constituent band for H(98)Y and H(90Y) are similar at 523 K, but with increasing tem- perature the intensity falls for H(98)Y and increases for H(90)Y.The steep rise in the intensity of the 2108 cm-' band with temperature of H(90)Y in the range 573-673 K is paral- leled by an equally steep drop in the band at 2120 cm-', whereas the intensity of the 2108 cm-' band in this same temperature interval for H(98)Y falls by about 20% and the fall in the intensity of the 2120 cm-' band is rather less. The steaming process results in an increasing band intensity at 2108 cm-and depressed values of intensities of the band at 2120 cm-' throughout the temperature range. The growth of the band at 2108 cm-' due to the progressive uptake of hydrogen cyanide effects the demise of the HW hydroxy-group band of zeolite H(98)Y; the former band is assigned to hydrogen cyanide bound to a Brarnsted acid site located in the supercage.The temperature profiles of the IR spectral constituent band intensities which monitor the formation of extra-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 framework material and the structural changes of the zeolite framework may be used to form a picture of the changes in these components which occur as a result of the steaming process. For simplicity, the IR spectral band intensities are referred to by their wavenumbers. At the lowest temperature of steaming there is a breakdown of the zeolite framework involving sodalite cages and prisms (lowered 2120 cm-'), the structural fragments of aluminosilicate possessing acidic sites (2147 cm-') causing some blocking of the supercage (high 2147 cm-', lower 2097, 2116, 1208 and 2104 cm-').This process continues at higher temperatures with the initial surface of the supercage being extended or cleared of debris, thus exposing more acid sites (increasing 2108 cm-'). At steaming temperatures just in excess of 723 K there is a sudden and substantial formation of hydrated alumina-like phase, possibly from dealumination of the zeolite framework, highly dispersed and easily removed, collecting on the outer surface and pore openings so as not to impede gas flow into the microporous system and possessing very strong acid sites compared with those of the zeolitic framework (sharp increase at 2230 cm-'). At still higher steaming temperatures (1023 K) the numbers of Brarnsted acid sites of all kinds are substantially reduced, particularly those associated with non- framework material.The optimal steaming temperature of zeolite H(98)Y for a good mix of extra-framework and zeo- litic framework acidic binding sites is in the region of 823 K where the presence of the outstandingly strong acidic sites due to the hydrated alumina-like material could be expected to influence hydrocarbon cracking greatly, particularly in that part of the process involving relatively high molecular mass components which crack on the outside surface of the zeolite crystals just where this highly acidic material is thought to accumulate. Uptake of Hydrogen Cyanide by Zeolites X, L, Mordenite, Beta and ZSM-5 Compared with zeolite H(98)Y there are fewer Brarnsted acid sites in zeolite HX, since their viability at increased tem- perature is impaired, while the occurrence of hydroxy groups represented by the LW band is reduced.Since the IR spectral band positions representing the binding of hydrogen cyanide by zeolite HX are in the same wavenumber regions as those found in zeolite Y, clearly the nature of the acidic sites in both zeolites is closely similar. In making comparisons of the binding capacity for hydrogen cyanide at the various acidic sites it is appropriate to compare the interaction of hydrogen cyanide with zeolite HX and zeolite H(90)Y. Considering first the relatively weak acid sites represented by the bands at 2095 and 2120 cm-', the spectral contribution of the former is reduced by about one third while that due to the latter is much reduced compared with that found for zeolite H(90)Y in keeping with the reduced intensity of the LW hydroxy- group band with which these binding sites are associated.Comparison of the relatively stronger acid sites represented by spectral constituents centred at 2107, 2104 and 2114 cm-', reveals that their variations with temperature of initial thermal treatment are remarkably similar for both zeolites, but their contributions are reduced for zeolite HX. The simi- larity in the nature of the acid sites in zeolites X and Y is further illustrated by the thermal desorption of the more tenaciously bound hydrogen cyanide from the relatively strong acid sites which, for example, have an IR spectral con- tribution centred at 2108 cm-'.While the starting material, NH4(93)X is highly crystalline, there is an appreciable loss of crystallinity below 473 K and the zeolite is amorphous at 673 K.8 These earlier findings are in keeping with the results sum- marized in Tables 5-7. The loss in crystallinity influences J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 15 Percentage fall in vCZN IR segmental band areas for thermal desorption of hydrogen cyanide from various zeolites zeolite temperature range/K vc.&m -' band area decrease per K ("4) H(98)Y 125 2109 0.73 HX 125 2107 0.73 2093 0.78 HL 125 2111 0.74 2101 0.7 1 2093 0.79 H-mordenite 125 21 18 0.60 21 14 0.57 21 10 0.70 2106 0.72 H-beta 75 2115 1.37 2106 1.16 H-ZSM-5 175 2122 0.51 greatly the capacity of zeolite HX for hydrogen bonding of hydrogen cyanide.However, the value of the present results lies in establishing that both zeolites X and Y employ the same binding sites for hydrogen cyanide. The acidity, catalytic activity and thermal stability of various L zeolites have been studied and the effect of cation content on acidity and catalytic activity determined.82 The total amount of hydrogen cyanide bound by the protonic form of zeolite L by all its acidic sites is considerably less than that bound by zeolite H(98)Y or zeolite HX. The rela- tively weak acid binding site is represented by spectral con- stituent bands at 2095 cm-'.A major relatively stronger acid binding site for hydrogen cyanide is represented by the con- stituent spectral band centred at 2101 cm- ' whose wavenum- ber, persistence at diminished pressure and thermal desorption profile suggest that this site is distinctive. Rela- tively strong acid sites marked by spectral constituent bands centred at 2108 and 2116 cm-' are present, but in smaller amounts. The acidity and superacidity of mordenite have been The total amount of hydrogen cyanide taken up by the protonic form of mordenite is similar to that bound by zeolite HL. While the relatively weak acid sites for binding of hydrogen cyanide may be identified by the spectral constit- uent bands at 2094 and 2120 cm-' and the relatively strong- er acid sites by bands at 2114, 2110, 2106 and 2102 cm-' in descending order of relative intensity, the stronger binding of hydrogen cyanide is dominated by a distinctive spectral con- stituent band centred at 2118 cm-'.The structural features of zeolite beta have been described89 91 and its use in hydrocarbon conversions out- lined.92 The relatively strong acid site is characterized by the spectral constituent band centred at 2106 cm-' and zeolitic temperature desorption accounts for the greater part of this zeolitically adsorbed hydrogen cyanide. The remainder of the more tenaciously held hydrogen cyanide is represented by the spectral constituent band centred at 2115 cm-'. A very minor amount of hydrogen cyanide is held by the weak acid site monitored by the spectral constituent band centred at 2098 cm- '.The Brczrnsted acidity of zeolite ZSM-5 has been character- ized by ammonia adsorption93 95 and IR spectros-copy.96-'O4 Th e acidity depends on the conditions of deammoniation, 'O5 and temperature-programmed desorption of ammonia from zeolite H-ZSM-5 shows the existence of three types of acid site: weak, medium and strong.'06 In keeping with the lower occurrence of acidic sites on ZSM-5 zeolite compared with near-faujasite zeolites, much less hydrogen cyanide is retained by zeolite ZSM-5 than with zeolite H(98)Y. The IR spectral changes which occur due to partial removal of hydrogen cyanide by diminished pressure pumping points to adsorption in terms of weakly held and more bound hydrogen cyanide is represented by the IR spec-tral band centred at 2098 cm-'.The more strongly held hydrogen cyanide is accounted for by the spectral constituent band at 2122 cm-'. This acid site achieves distinction on the grounds of the IR spectral wavenumber of the bound hydro- gen cyanide and due to the retention of a significant amount of its hydrogen cyanide above 423 K, a temperature at which a proportionately larger amount of hydrogen cyanide would have been lost from all the other zeolites studied here. The percentage fall in IR band area with temperature, obtained from the thermal desorption data of hydrogen cyanide from the strongest acid site, as summarized in Table 15, decreases in the order ZSM-5 mordenite < L z X, Y $ beta which is in their order of decreasing acid strength.Conclusions While there is no correlation between the acid strength of the various zeolitic Brransted acid sites and the IR spectral wave- number of hydrogen cyanide bound to such sites, the wave- number serves to characterize the sites and makes possible their identification in various zeolites and the elicudation of their relationship to zeolitic structural features. Therefore, it would come as no surprise to find that the IR spectral wave- numbers of adsorbed hydrogen cyanide were the same for isostructural zeolites X and Y (2120, 2097, 21 14 and 2108 cm-'). The channel structure of mordenite, though con-structed differently, has hydrogen-bonding sites (21 18, 2 1 14, 2110 and 2106 cm-') which, by this criterion, are not very different from those found in the near-faujasites.In zeolite L the structural difference in channel construction becomes greater and the most important hydrogen-bonding site in the channel of ZSM-5 is obviously different (2122 crn-.') from that of the other zeolites, though the less important site (21 14 cm-') is similar to one found in the channel of mordenite. Despite differences in structure, the hydrogen-bonding site of relatively low strength (cu. 2095 cm- ') occurs in all of the zeolites, in the supercage of the near-faujasites and tubular channels of zeolite L, ZSM-5 and mordenite. The relatively weak hydrogen-bonding site (2120 cm-') occurs only in the near-faujasites which possess sodalite and prism cages.References I A. Janin, J. C. Lavalley, A. Macedo and F. 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