Modified Zeolites Part 2.-Sorption by Dealuminated, Silanated Mordenites BY RICHARD M. BARRER* AND JEAN-CHRISTIAN TROMBE~ Physical Chemistry Laboratories, Chemistry Department, Imperial College, London SW7 2AY Received 23rd March, 1978 A comparison has been made of sorption isotherms and kinetics of 02, N2 and Ar in each of a series of partially dealuminated mordenites before and after different extents of silanation. Before silanation partially dealuminated mordenites sorbed each gas freely but after silanation some very selective sorbents were obtained which at 77 K imbibed 0 2 much more copiously than either N2 or Ar. This selectivity was further improved when acidic H was replaced by Na in the dealuminated H-mordenites before silanation. In many of the sorbents a rapid initial uptake was followed by a slow diffusion at rates in the sequence O2 > N2 > Ar, which is the inverse order of their van der Waals cross-sectional diameters.The results provide evidence that after silanation the intracrystalline channels are not uniformly accessible to a given sorbate. A number of partially dealuminated H- and Na-mordenites were prepared (Part 1) in which the ratios Si02/A1,03 were in the range 11 .& to 61.6,. These zeolites were then variously silanated, to provide a range of crystals in which the mordenite channels had different contents of chemisorbed fragments. It had previously been shown that silanation of outgassed H-mordenite sorbents greatly altered their behaviour. * Also, while reactions of SiH4 with zeolitic water inside zeolites Na-Y and Na- mordenite produced little change in uptake of O2 and N, in the three-dimensional channel system of Na-Y, these reactions greatly and selectively altered sorption of the gases in the one-dimensional channel system of Na-m~rdenite.~ It was therefore of interest to investigate and compare the sorption behaviour of the partially de- aluminated and silanated mordenites inter se and with the modified mordenites examined previously, EXPERIMENTAL The sorbents employed are listed in table 1.The numbers in brackets are the percentages by weight of Si added as a result of chemisorption of SiH4 (Part l).4 Also Na*-M3 is Na-M3 after further extraction with distilled water, which reduced the Na content from 0.83 to 0.66 mmol g-l of zeolite (Part 1, table 2). The Na-M forms were each produced from the corresponding partly dealuminated H-form, M, by treatment with dilute NaOH, prior to silanation (Part 1).The sorbates were 02, N2 and Ar chosen for their increasing equilibrium distance cross- sectional diameters in the order given. Uptakes are all expressed as cm3 of sorbate g1 of zeolite outgassed at 360°C. Volumetric and gravimetric procedures were used for measuring uptakes and kinetics. In the kinetic runs the pressures were allowed to decline as the gases were sorbed. For isotherms, up to 30min was normally allowed per point. Because slow processes were sometimes present desorption points could lie a little above 7 Present address : Institut National Polytechnique de Toulouse, Laboratorie de Physico-Chimie des Solides et des Hautes Temperatures, 38 rue des 36 Ponts, Toulouse, France.2798R . M. BARRER AND J.-C. TROMBE 2799 adsorption points [e.g. fig. l(u), curve 4 ; fig. 2(6), curves 3 and 41. However in these situations a mean curve has been drawn. RESULTS AND DISCUSSION DEALUMINATED, SILANATED H-MORDENITES Some isotherms at 77K of 02, N2 and Ar in certain of the H-mordenites of table 1 are shown in fig. 1 and 2, and rates of sorption in fig. 3. Table 2 gives apparent sorption capacities at 50 Torr. Before silanation the H-mordenites gave capacities independent of equilibration times allowed ; after silanation gradual drifts were, as noted in the previous section, to be considered even though sorption and desorption points were usually close to a common curve.Accordingly for silanated sorbents TABLE 1 .-ZEOLITES USED* SiOz!A1203 in SiOz/.A1203 in unsilanated unsilanated zeolite form zeolite form M2 Na-M2 Na-M2 (3.33) H-mordenite (2.46) H-mordeni t e H-mordenite (2.5 6 ) H-mordenite (2.S6) M1 M1 (3.60) Na-M1 Na-M, (1.26) Na-M1 (1.g3) Na-M1 (2.5,) M3 M3 (3.02) . 13.24 Na-M3 Na-M3 (2.73) Na*-M3 (2.6,) .26.02 * " M " denotes " dealuminated H-mordenite " as in Part 1 .4 " Na-M " denotes these mordenites treated with 0.1 mol dm-3 NaOH (Part 1). PlmmHg FIG. 1.-Some isotherms at 77 K for (a) 02, (b) Ar and (c) N2 in H-mordenites and silanated H- mordenites. Curves 1, H-mordenite ; curves 2, H-mordenite (2.46 % Si) ; curves 3, H-mordenite (2.56 % Si) ; curves 4, H-mordenite (2.g6 % Si). + and open symbols denote adsorption points and x and filled symbols desorption points.Scale of ordinate the same in (a), (b) and (c).2800 MODIFIED ZEOLITES I 4 I 150c 1 100 150 20 0 50R . M. BARRER AND J . - C . TROMBE 2801 capacities are apparent ones appropriate for our conditions. Slow diffusion following initial rapid sorption is seen for example in fig. 3(a), and for Ar in M4 in fig. 3(b). The slow process sometimes gave linear plots of amount sorbed, Qt, against $ (t = time ; the ,/? law of diffusion). The line slope should then be proportional to D3/a, where D is the diffusivity and a is the mean crystal diameter in the powder. If, as expected, a is the same for the H-mordenites having 2.56 and 2A6 % by weight of Si added through chemisorption of SiH4 then for these samples the ratio of the two diffusivities for N2 [from curves 5 and 7 of fig.3(a)] is -6.8. Thus D appears to decline sharply as the extent of prior silanation at 200°C increases. The amount of initial rapid sorption is also a function of the extent of prior silanation [e.g. curves 5 and 7 of fig. 3(a)] and of molecular dimensions [e.g. curves 3 and 7, or 2 and 4, of fig. 3(a)]. The equilibrium separation cross-sectional diameters of 02, N2 and Ar are respectively 2.8, 3.0 and 3 . S 3 k 5 Effects of molecular dimensions upon isotherms can be seen by comparing curves 2 in fig. l(a), (6) and (c). At 77 K silanated H-mordenite can be very selective for 02. In the case of the 02, TABLE 2.-uPTAKES AT 50Torr AND 77K IN DEALUMINATED H-MORDENITES OUTGASSED AT 360°C, BEFORE AND AFTER SILANATION capacity of sorption at 77 K in cm3 g-1 dry, at 50 Torr weight loss under vacuum at 36OOC % N2 0 2 % Si added before after silanation before after before after material by wt silanation first run second run silanation silanation silanation silanation H-mordenite 2.S6 12.53 5.65 112 10 134 83 2.56 12.53 6.30 10.8 112 19 134 95 2.46' 12.53 8.86 9.9 112 17 134 97 M1 3.60 14.22 6.86 10.5 118 27 142 76 M2 3.31 13.62 9.41 10.1 113 40 143 92 M3 3.0, 14.47 7.16 10.2 110 55 148 92 M4 1.85 9.33 6.20 7.6 107 83 135 115 * In this case the silanation was performed at 150 "C, instead of 200 "C, for all the other runs.N2 pair selectivity is best for the most heavily silanated H-mordenite with SO2/ A1,03 = 11 -84 (table 2), and for comparable degrees of silanation tends to diminish as the extent of dealumination is increased [cf.H-mordenite (2.86) with M3 (3.0,) in table 21. For pairs of bigger molecules of graded and differing dimensions good selectivities might reappear in M3 (3.02), the larger molecule being more fully excluded. In M2 prior to silanation isotherms for 0, and N, at 77 K were measured after outgassing first at 200 and then at 360°C. Sorption was virtually unaltered for each gas [fig. 2(b), curve 1 and curve 21 so that any residual zeolitic water remaining at 200°C is insufficient to change the isotherms. Silanation of the H-mordenite of table 1 to give at 200°C 2S6 % of added Si, and at 150°C 2.46 % of extra Si, gave sorbents very much alike in uptakes of both O2 and N2 (table 2). Thus within this range, for almost equal extents of silanation, the modification was not sensitive to the silanation temperature.FIG. 2.-Isotherms at 77K in dealuminated, and in dealuminated and silanated H-mordenites Symbols filled and unfilled and + and x have the same sirncance as in fig, 1. (a) Curve 1, O2 in MI ; curve 2, N2 in Mi ; curve 3, O2 in MI (3.60 % Si); curve 4, N2 in MI (3.60 % Si). (b) Curve 1, O2 in M2 outgassed at 360 (0 and a), and at 200°C (V and V) ; curve 2, NZ in Ma outgassed at 360 (+ and x ), and at 200°C (0 and +) ; curve 3, O2 in M2 (3.31 % Si) ; curve 4, Nz in M2 (3.31 % Si). Outgassing before both runs was at 360°C. (c) Curve 1, O2 in M4 ; curve 2, Ar in Mq ; curve 3, O2 in M4 (1.85 % Si) ; curve 4, N2 in M4 ; curve 5, Nz in M e (1.85 Si) curve 6, Ar in Mq (1.85 % Si).Outgassing before runs was at 360°C.2802 MODIFIED ZEOLITES '""t- 1 1 5 10 15 qT/min+ FIG. 3.-Rates of sorption measured at 77 K in (a) silanated H-mordenite and (b) silanated de- aluminated H-mordenite Ma. Prior outgassing was at 360°C. (a) Curve 1, O2 in H-mordenite (2.56 % si, silanated at 200°C; at t = 116 min, p = 35.5 Torr) ; curve 2, O2 in H-mordenite (2.46 % Si, silanated at 100°C ; at t = 427 min, p = 21.5 Torr) ; curve 4,02 in H-mordenite (U6 % Si, silanated at 200°C; at f = 100min p = 9.0 Tom); curve 4, N2 in H-mordenite (2.46 % Si; at t = 121 min, p = 65.1 Tom) ; curve 5, N2 in H-mordenite (2.56 % Si ; at t = 180 min, p = 57.4 Tom) ; curve 6, Ar in H-mordenite (2.46 % Si ; at t = 100 mh,g = 53 Torr) ; curve 7, N2 in H-mordenite (2.86 % Si; at t = 196 min, p = 63.6 Tom).(b) Sorption in M4 (I& % Si). Curve 1, 0 2 (at t = 25 min, p = 6.8 Torr) ; curve 2, N2 (at t = 64 min, p = 8.7 Torr) ; curve 3, Ar (at t = 152 min, p = 46 Torr).R . M. BARRER A N D J . - C . TROMBE 2803 Table 2 records weight losses at 360°C under vacuum before and after silanation. With the exception of M2 (3.31) in the first runs (column 4) the samples were not oxidised after silanation but were evacuated at the silanation temperature, cooled in uacuo and only then contacted with air, prior to evacuation at 360°C. Weight losses in the first run following silanation are considerably less than the losses before silanation. After these first runs the silanated zeolites were stored for one day over saturated Ca(N03)2, and the weight losses obtained as before by outgassing at 360°C.The losses in these second runs were increased but remained significantly less than TABLE 3.-uPTAKES OF 0 2 AND Nz AT 77 K AND 50 TOIT IN DEALUMINATED Na-MORDENITES BEFORE AND AFTER SILANATION sorption at 77 K and p = 50 Torr in cm3 at s.t.p./g N2 0 2 % Si added before after before after material by wt silanation silanation silanation silanation N a - m o r d e n i t e 93 1 1 5 Na-M1 1 . 2 6 1 0 8 75 125 1 1 9 1.93 1 0 8 38 1 2 5 105 2 . 5 8 1 0 8 8 1 2 5 80 Na-Mz 3.33 1 0 9 2 1 1 2 9 88 Na-M3 2.73 1 0 0 1 8 121 90 Na*-M3 2.63 1 0 3 46 1 2 5 1 0 0 the losses from the unsilanated forms. The weight losses are largely due to evolution of zeolitic water, but in the first runs of column 4 of table 2 some of this zeolitic water may be trapped during heating by converting chemisorbed -SiH, and -SHY- to -Si(OH), and -Si(OH),,- within the zeolite and so may not all escape.Increased polarity due to these new -OH groups should also increase the selectivity for water. DEAL U MI N A T E D A N D S I LAN ATE D Na-M 0 R D E N I T E S Isotherms at 77 K for 02, Nz and Ar are shown in fig. 4 with Na-M1, Na-M1 (1.2& Na-MI (1.g3) and Na-Ml (2.5,) sorbents. As with the H-mordenites of the previous section the uptakes at 50 Torr depend strongly upon the extent of silanation and upon the dimensions of the sorbed molecules. The effect of molecular dimen- sions can be seen, for example, by comparing curves 2 in fig. 4(a), (b) and (c) (4 (4 -4----.-;t---+- - 106 bo 4 h x u M 3 50 100 50 100 50 100 1% 200 Plmm H g FIG.4.--Isotherms at 77K for 02, Ar and Nz in Na-M1 and Na-MI variously silanated and outgassed at 360°C. Symbols + and x , and filled and unfilled, have the same significance as in fig. 1. (a) O2 ; (b) Ar ; (c) Nz. Curves 1 refer to Na-Ml ; 2 to Na-M1 (1.26 % Si); 3 to Na-M1 (1 .g3 % Si) ; and 4 to Na-Ml (2& % Si). Silanation was effected at 200°C. Scale of ordinate is the same in (a), (b) and (c).2804 MODIFIED ZEOLITES ++-- lo 4 Lrr x l/T/min+ FIG. 5 . 4 0 ) Rates of sorption of 02, Ar and N2 in Na-Mi and variously silanated Na-M1 at 77 and at 195 K. Outgassing was at 360°C in (a) and (6); curve 1, O2 at 77 K in Na-Mi (1.26 % Si, p= 16.2 Torr at t = 36 min) ; curve 2, Oz at 77 K in Na-Mi (1.93 % Si,p = 37.9 Torr at t = 114 min) ; curve 3, NZ at 77 K in Na-Mi (1.26 % Si, p = 24.0 Torr at t = 121 rnin) ; curve 4, O2 at 77 K in Na-MI (2.58 % Si, p = 54 Torr at t = 640 min) ; curve 5, N2 at 77 K in Na-MI (1.93 % Si, p = 54.1 Torr at t = 121 min) ; curve 6, Ar at 77 K in Na-Mi (1.26 % Si,p = 64 Torr at t = 79 mm) ; curve 7, NZ at 195 K in Na-MI (2& % Si, p = 54.3 Torr at t = 165 min); curve 8, O2 at 195 K in Na-MI (2.58 % Si, p = 48 Torr at t = 121 min); curve 9, N2 at 77 K in Na-Mi (2.58 % Si, p = 49.1 Torr at t = 64 min) ; curve 10, Ar at 77 K in Na-M1 (2.58 % Si, p = 49.5 Torr at t = 60 min) ; curve 11, Ar at 195 K in Na-Ml (2.58 % Si, p = 52.9 Tom at t = 214 min) ; (b) rates of sorption at 77 K of Oz and Nz in Na-M3 (2.73) and in Na*-M3 (2.63) : curve 1, O2 in Na*-M3 (2.63 % Si, p = 57.6 Torr at t = 169 min) ; curve 2, O2 in Na-M3 (2.73 % Si, p = 23 Tom at t = 240 min); curve 3, NZ in Na*-M3 (2.63 % Si, p = 59.5 Torr at t = 196 min) ; curve 4, Nz in Na-M3 (2.73 % Si, p = 103 Torr at t = 570 min).R .M. BARRER AND J . - C . TROMBE 2805 respectively, the uptakes in Na-MI (l.26) being in the order 0, > N, > Ar, the inverse order of the cross-sectional diameters. Table 3 gives uptakes of 0, and N2 measured at 50Torr and 77K in parent Na-mordenite and in various modified mordenites, and table 4 gives these uptakes for O,, N2 and Ar at several temperatures. These tables show Na-M, (2.58) to be particularly effective in differentiating O2 from N2 and from Ar. As the extent of dealumination increased the selectivity for 0, over N, diminished, although it remains good for Na-M, (3.3,) and Na-M3 (2.7,) (table 3).When Na-M, (2.7,) was given extra washing which removed some Na to give Na*-M3 (2.6,) (table 1) the uptake of N, at 77 K and 50Torr increased 2S6-fold but that of 0, increased by only 11 %. Thus Na+ in the crystals, as well as dealumination and silanation are involved in differentiating between O2 and N,. When one compares the dealuminated and silanated H-mordenites of the previous section with analogous Na-mordenites it is seen that the Na-forms differentiate more strongly [e.g. M2 (3.3,) in table 2 and Na-M2 (3.3,) in table 31. TABLE UPTAKES OF Oz, N2 AND Ar AT SEVERAL TEMPERATURES IN Na-M1 (2.5,) IN cm3 AT S.T.P. PER g OF OUTGASSED ZEOLITE temperature /K sorption at p = 50f5 Tom NZ 0 2 Ar 77 8 80 5 195 30 13 8 273 0.7 0.7 0.3 Rates of sorption at 77 and 195 K are compared in fig.5(a) for O,, N2 and Ar in Na-M, with 1.26, 1.93 and 2S8 % Si added by silanation, and in fig. 5(b) for 0, and N2 at 77 K in Na-M, (2.7,) and Na*-M, (2.6,). After an initial rapid sorption, the extent of which, for a given gas and temperature, is seen from fig. 5(a) to decrease as the Si added increases, there may be a slow further uptake the rate of which is also a function of the degree of silanation. This is illustrated in fig. 5(a) by curves 1, 2 and 4 for 0, and 3, 5 and 9 for N,, at 77 K in Na-M, (l.26), Na-M, (1.9,) and Na-M, (2.58), respectively. A role of Na+ in such effects is seen from fig. 5(b) for 0, and N, for 77 K in Na-M, (2.73) and Na*-M, (2.6,).Removal of some Na+ has increased the extent of the initial rapid uptake and has changed the rate of the slow process. The slow process in the uptake of N, obeys the ,/Flaw [curves 3 and 4 of fig. 5(b)] and from the slopes of the plots of uptake against & as described in the previous section, one obtains -19 for the ratio of diffusivity, D, in Na*-M, (2.6,) to D in Na-M, (2.7,). Further evidence of slow, activated diffusion processes was obtained by deter- mining uptakes of 02, N, and Ar at -50 Torr at each of several temperatures in Na-M, (2.58). The isobars, as shown by the figures in table 4, exhibit maxima for N, and Ar at different intermediate temperatures because, although uptakes at 50 Torr at first increase as the temperature falls, eventually activated diffusion becomes so slow that these increased equilibrium uptakes cannot be realised on the time scale of the experiments, and so the amounts sorbed must pass through maxima.For 0, the maximum is at a temperature below 77 K and so is not observed in our experiments. At 195 K true equilibrium was established for N, and 0, and table 4 shows that N, is then selectively sorbed. This behaviour is attributed to the molecular quadrupole of N2 which interacts exothermally with intracrystalline electrostatic field gradients. The resultant contribution to the energy and free energy of sorption can be considerable.62806 MODIFIED ZEOLITES CONCLUDING REMARKS The kinetic measurements suggest that the intracrystalline channels in the silanated crystals are not uniformly accessible to the sorbates.The partial blocking associated with silanation may not have occurred equally among all crystallites in the powder, or even throughout one crystal. Nevertheless, it has been established that silanation of H-mordenites, dealuminated H-mordenites and dealuminated Na-mordenites can strongly and selectively influence both amounts and rates of uptake of 02, N2 and Ar, the effects observed at 77 K following the sequence of the equilibrium cross- sectional diameters. For H- and Na-mordenites, each dealuminated and silanated to the same extent, the Na-form differentiates more sharply than the H-form between O2 and N2 or between O2 and Ar. Before silanation and at 77 K the gases penetrate all the dealuminated mordenites to give true equilibria.At 50 Torr the uptakes in cm3 at s.t.p. per g of zeolite are given below : 0 2 Ar N2 OZ/NZ Oz/Ar Na-mordenite 115 - 93 1.24 - H-mordenite 134 121 112 1.20 1.11 Ml 142 - 119 1.19 - M2 143 - 113 1.27 - M3 149 - 111 1.34 - M4 134 124 107 1.25 1.08 Na-Ml 125 114 - 1.10 Na-M2 129 - 109 1.18 - Na-M3 121 - 100 1.21 - Na*-M3 125 - 103 1.21 - The ratios of the numbers of molecules sorbed, 02/N2 and OJAr as given in the last two columns of the above tabulation, vary only a little for most of the sorbents, suggesting that the intracrystalline pore volumes are approximately equally accessible in each, and that sorptions are nearly in accordance with Gurwitsch's rule? For this rule to be strictly valid the above ratios would be constant and in the inverse ratio of the molecular volumes of the sorbed fluids at 77 K. The experiences reported in this paper for mordenites are expected to be repeated in modified ways for other silanated zeolites having one dimensional channel systems and stable H-forms, examples being offretite and zeolites L and Q. We wish to acknowledge grants from the Royal Society and from the C.N.R.S. which made possible the participation of J.-C. T. in this work. R. M. Barrer, R. G. Jenkins and G. Peeters, in MoZecuIur Sieves I1 (Amer. Chem. SOC. Symp. Ser., No. 40, 1977), p. 258. R. M. Barrer, E. F. Vansant and G. Peeters, J.C.S. Faraday I, 1978, 74, 1871. R. M. Barrer and J.-C. Trombe, J.C.S. Faraday I, in press. R. M. Barrer and J.-C. Trombe, J.C.S. Furuday I, 1978, 74, 2786. L. Pauling, The Nature of the Chemical Bond (Cornell, 1940), p. 187 et seq. R. M. Barrer, J. Colloid Interface Sci., 1966, 21, 415. ' L. G. Gurwitsch, 2. phys. Chem., 1914,87, 323. (PAPER 8/557)