Parahydrogen conversion on 5A molecular sieve and 5A molecular sieve containing neodymium ions

JOURNAL OF CATALYSIS 16, 292-302 (1970)

Parahydrogen Conversion on 5A Molecular Sieve and 5A Molecular Sieve Containing Neodymium Ions

D. E. BROWN, D. D. ELEY, AND R. RUDHAM The Chemistry Department, The University of Nottingham,

Nottingham, England

Received June 2, 1969

Parahydrogen conversion on 5A molecular sieve and 5A molecular sieve containing neodymium ions has been investigated over the temperature range 77-605”K. Rates of orthodeuterium conversion at low temperatures and hydrogen-deuterium equilibration at high temperatures have been measured in order to interpret the mechanism of reaction.

At low temperatures, conversion proceeds by a physical mechanism in which the activity is determined by paramagnetic impurities and neodymium ions. At high temperatures, conversion proceeds by a chemical mechanism, and both catalysts exhibit a pronounced activation effect on exposure to hydrogen. The low temperature activity is unaffected by the hydrogen treatment.

Heterogeneous parahydrogen and orthodeuterium conversion can occur by both chemical and physical mechanisms. Chemi- cal conversion involves chemisorbed hydrogen atoms and is controlled by the same factors which determine activity for hydrogen-deuterium equilibration. At high temperatures, where adsorption is restricted to hydrogen atoms alone, the Bonhoeffer- Farkas (1) mechanism is the most probable. At lower temperatures, interactions between molecularly adsorbed hydrogen and adsorbed hydrogen atoms (2, S), or between the adsorbed molecular species themselves (4)) provide alternative chemical mechanisms. Physical conversion is fa- vored by low temperatures and results from the interaction of physically adsorbed molecules with paramagnetic sites in the catalyst surface (5-8). The paramagnetic sites arise from the presence of unpaired electrons on exposed lattice or impurity ions, surface-free radicals, or adsorbed species. The physical mechanism does not provide a path for hydrogen-deuterium equilibration, since the bonds within the

molecules remain unbroken during conversion.

Diagnostic tests of mechanism are mainly based on comparisons of reaction rates. For any chemical mechanism, the effect of zero-point energy on the activation energy (9, 10) results in the rate order pH, > H, + D, > oD,. Whereas for a physical mechanism, the rate order is pH, > oD, >> H, + D, = 0. Measure- ments of activation energy can assist, since physical conversion is frequently associated with a negative apparent activation energy, due to a fall in the coverage of physically adsorbed hydrogen with increasing temperature (7, 8, 11). Comparisons of surface coverages derived from pressure dependencies (1.2) with those from experimental isotherms give information on surface heterogeneity and the detailed mechanism of physical conversion (8, IO, IS).

Although molecular sieves are materials with exceptional adsorptive and catalytic properties, few investigations of hydrogen reactions have been made. Turkevich et al. (14) have investigated hydrogen-deuterium equilibration on NaY molecular sieves at

292

PARAHYDROGEN CONVERSION OS 5A MOLECULAR SIEVES 293

25°C. It was found that the catalytic activity was proportional to the extent of decationization and it was concluded that the active centers were decationated sites. Subsequently it was shown (15) that the number of active centers was considerably less than the tot’al number of decationated sites, and it was suggested that either impurities located near decationated sites, or a combination of decationated sites in close proximity, were responsible for the activity. It was also found (15) that parahydrogen conversion at 77°K was independent of the extent of decationization and was thus associated with paramagnetic impurities rather than with decationated sit’es. Molec- ular sieves at low temperatures will preferentially adsorb orthohydrogen from a mixture of the spin isomers, and this has been used as a basis for their gas-chromatographic separation (18). The separation is impaired when the sieves contain iron impurity, due to paramagnetic physical conversion on the chromatographic column.

The present contribution reports an in- vestigation of parahydrogen conversion on 5A molecular sieve and on 5A molecular sieve containing neodymium ions. Measure- ments of orthodeuterium conversion and hydrogen-deuterium equilibration are used to diagnose the mechanism of conversion, which is found to he physical at low temperatures and chemical at, high temperatures.

EXPERIMEKTAL

Apparatus

All measurcmcnts were made in conventional high vacuum systems, in which traps at 77°K protected the catalyst samples from contamination by water, grease, and mercury. Kinetic studies of parahydrogen conversion, orthodeuterium conversion? and hydrogen-deuterium equilibrat’ion were made in a constant volume reaction system (210-245 ml), where the gases were left in contact with the catalyst for a known time before being withdrawn and analyzed by either a micro-Pirani gauge, or an A.E.I. M.S.10 mass spectrometer.

Hydrogen and deuterium adsorption isotherms were determined in a constant volume system.

Gases

Pure normal hydrogen and normal deuterium were prepared by allowing purified cylinder gas to diffuse through a heated palladium-silver thimble. Enriched parahydrogen or orthodeuterium were prepared by contacting the appropriate normal gas wit,h an outgassed charcoal catalyst, at 21°K.

Catalysts

1. Linde 5A hlolecular sieve in powder form (Lot 949260), which had been con- tacted with distilled water for 16 hours at 25”C, filtered, washed and distilled water, and dried in an oven at 130°C for 24 hours.

2. Linde 5A Molecular sieve (Lot 949260)) in which 13.2 -+ 0.5 wt % of the sodium content and 0.45 -t 0.04 wt % of the calcium content had been exchanged for neodymium ions by contacting with an aqueous solution of neodymium chloride for 16 hours at 25’C. The ion-exchanged sieve was then washed and dried as with catalyst, 1, giving a final neodymium content of 2.04 X 10m4 g ion of Nd3+ per g of sieve.

3. Catalyst 2, which had been treated for 24 hours at 350°C with a current of air which had previously been saturated with hexamcthyldisiloxane at room temperature, followed by 24 hours at 3%400°C in a current of dried air.

4. Johnson Matthey Specpure neodymium oxide, with a specific surface area of 4.74 m”g-‘.

5. Catalyst 4, which had been treated with hexamcthyldisiloxane in the same manner as catalyst 3.

X-Ray powder photographs of catalysts 1 and 2, and of a sample of catalyst 2 outgassed for 48 hours at 350°C and treated with hydrogen at that temperature, were effectively identical. Calculated d-values for the more intense, Iow angle Iines were in very good agreement, with those quoted by Brcck et al. (17) for 5A molecular sieve.

294 BROWN, ELET, AND RUDHAM

1=4/- 0.2 0.4 ‘wa P

FIG. 1. Plot of log&, against log,@ for parahydrogen enrichment (-o-) and deplet’ion (-•-) on catalyst 2 at 77°K.

RESULTS

General

Parahydrogen conversion, orthodeuterium conversion, and hydrogen-deuterium equilibration obeyed first order kinetics at constant pressure on all catalysts studied. The rate constant k, being given by the equation

k e = 1 In ‘0, set-1, t x0 - xt (1)

where x0, xt, and xe are the fractions of parahydrogen, orthodeuterium, or hydrogen-deuteride present at times zero, t, and equilibrium, respectively. Because of the uncertainty in the effect.ive surface areas of molecular sieves, absolute rates of reaction k,,, are expressed on a mass basis rather than on the conventional basis of unit area. The value of k,, is given by

k, = k,cp/m, molecules mg-lsec-l, (2)

where c is the number of molecules in the reaction volume at unit pressure, p is the experimental pressure, and m is the mass of the catalyst in milligrams.

Molecular sieves preferentially adsorb orthohydrogen from a mixture of the spin isomers (16), and it has been shown that this can cause deviations from simple first order kinetics (18). To investigate possible kinetic deviations in the present work, a series of measurements of parahydrogen conversion at 77 and 90°K were made at constant pressure on a sample of catalyst 2 outgassed at 340°C. Plots of ln(xO - XJX, - xt) against t were linear and passed through the origin as demanded by equation 1. In addition, rates of both parahydrogen depletion and enrichment were measured as a function of pressure at 77°K on a further sample of catalyst 2 outgassed at 350°C. In these experiments the reactant gas is either enriched parahydrogen or normal hydrogen, so that if preferential adsorption is to influence the kinetics, a difference in reaction rate is to be antici- pated. The results are shown in Fig. 1, where log,&, is plotted against log,,,p and it is clear that the difference in reactant makes no systematic difference to the value of k,. It is concluded that preferential adsorption has no measurable effeot on the kinetics for parahydrogen conversion on the present catalysts and that meaningful data can be obtained by the application of equations 1 and 2 to measurements of the depletion reaction alone.

It was necessary to determine an outgassing temperature at which stable and reproducible activities were obtained. Rates of parahydrogen conversion were determined on samples of catalysts 1 and 2 after outgassing at ~10-~ torr for successive 14-

FIG. 2. The effect of outgassing temperature on the activity for parahydrogen conversion. --a- catalyst 1 at) 77X, p = 2.6 torr; -A- catalyst 1 at 473”K, p = 5.0 torr; -O- catalyst 2 at 77” K,p = 2.0 torr.

PARAHYDROGEN CONVERSION ON .?)A MOLECULAR SIEVES 295

hour periods at increasing temperatures. The results are shown in Fig. 2, where values of log,, (activity in arbitrary units) are plotted for the various outgassing temperatures.

The activity of both catalysts at 77°K rises with outgassing temperatures below 25Q”C, and the increase can be associated with the loss of zcolitic water from the sieve channels, facilitating the access of gas to the active sites. Between 250 and 55O”C, there is little change in activity with increasing temperature, and 350°C was selected as a convenient outgassing temperature. At temperatures above 550°C the activity of catalyst 1 rises, whereas that of catalyst 2 falls; this behavior is probably associated with the onset of the collapse of the sieve structure. The activity of catalyst 1 at 473°K shows that outgassing at’ 350°C is sufficient to generate an activity close to the apparent maximum obt’ained after outgassing at 375°C. To determine a suitable outgassing time, the activity of a sample of catalyst 2 was measured after outgassing at 350°C for 14 hours and again after outgassing for a further

I I

t I I I I I I

2 4 6 8 10 12

I+/ T ('K-'1

FIG. 3. Arrhenius plots of low temperature parahydrogen conversion at a pressure of 7.0 torr. -0- catalyst 1; -o- catalyst 2.

TABLE 1 .~!TI~ITIES FOR PAR.4HYDROGEN CONVERSIOK

AT 77°K AND 7 TORR

Mass of k, (molecules Kinetic Catalyst sample (mg) mg-‘see-I) order, n

2 16 6.9 x 10’5 0.0”

2 37 4.1 x 10’5 0.00

2 41 4.2 x 10’: 0.00 3 12 4.5 x 10’5 0.30

63 hours. The additional outgassing failed to generate any increase in activity, so that outgassing for 14 hours at 350°C and +10-G torr was accepted as the standard conditions for future samples. The repro- ducibility of catalysts outgassed in this way can be assessed from a comparison of the values of Ic,,, obtained at 77°K and 7 torr for three samples of catalyst 2 (Table 1).

Table 1 also contains the value of Ic, obtained for a sample of catalyst 3 under the same conditions. It is evident that the activities of catalysts 2 and 3 are similar, indicat’ing that treatment with hexamethyldisiloxane has not caused poisoning. The activities of samples of catalysts 4 and 5 :it 77°K and 7 torr, after the standard outgassing treatment, were 4.4 x 1O1” and 1.2 X 1O1* molecules mg-%ecl, respectively. With these neodymium oxide catalysts, where the active sites are in t,he surface, treatment with hexamethyldisiloxane has reduced the activity to 23% of its original value. We take this as evidence to show that the neodymium ions of t,he ion- exchanged 5A sieve are within the sieve structure.

The Effects of Temperature and Pressure on Parahydrogen Conversion

The effect of temperature on the rate of parahydrogen conversion has been studied on catalysts 1 and 2, first at increasing temperatures and then at decreasing temperatures on the same sample. At low temperatures, below 296°K for the untreated sieve (catalyst 1) and below 250°K for the sieve containing neodymium ions (catalyst 2), the activities at increasing and decreasing temperatures were identical. Arrhenius

296 BROWN, ELEY, AND RUDHAM

I I I I I I

2.0 30 4.0 2.0 3.0 4.0

1031 T (OK-')

FIG. 4. Arrhenius pIok of high temperature act,ivity at a pressure of 7.0 torr. -O- catalyst 1 parahydrogen, conversion at increasing temperatures; -+- catalyst 1, parahydrogen conversion at decreasing temperatures; +- catalyst 2, parahydrogen conversion at increasing temperatures; -a-- cat,alyat 2 parahydrogen conversion at decreasing temperatures; -A- hydrogen-deuterium equilibrat,ion. ’

plots for k,,, at a pressure of 7 torr are shown in Fig. 3, where it can be seen that conversion possesses a negative temperature coefficient over the low temperature range, and that the activity of catalyst 2 is greater than that of catalyst 1 by a fac- tor which varies between 5.0 and 19.0. The negative temperature coefficients and a higher activity for the catalyst, containing paramagnetic Nd3+ ions are indicative of a physical mechanism for conversion in this temperature range.

Both catalysts 1 and 2 increase in ac- t’ivity when exposed to hydrogen at, high temperatures, leading to a difference in activity for experiments at increasing and decreasing temperatures. Detailed Ar- rhenius plots for k, at a pressure of 7 torr are shown in Fig. 4 for temperatures greater than 250°K. Apart from the first +lOO” on the increasing temperature plot, apparent activation energies are positive for both catalysts. The effect of high temperature activation is to reduce both the activation energy and pre-exponential fac- tor for parahydrogen conversion. For catalyst 1 at increasing temperatures (426- 560”K), the activity at 7 torr can be expressed by k,, = 7.9 X 1031 exp (-38,400/ RT) , whereas at decreasing temperatures (471-403’K), k,, = 1.0 X lo’* exp (-S,SOO/

RT). Similarly, for catalyst 2 at increasing temperatures (400-552°K)) k, = 1.2 X 10zs exp (-19,30O/RT), whereas at decreasing temperatures (578-373”K)) k, = 6.0 X 1Oz6 exp (--4,200JRT). In the high temperature region, parahydrogen conversion will proceed predominantly by a chemical mechanism, although a contribution from a physical mechanism may still be present. It is unlikely that activation results from the further loss of molecular water, since kinetic measurements were restricted to temperatures below that of outgassing. It is more probable that it is associated with the activated adsorption of hydrogen in a way which does not effect the magnetic properties of the catalysts, since the low temperature activity remains unaltered.

The pressure dependence of the rate of conversion was measured over the range 130 torr at a number of temperatures. In all cases the effect could be described by the equation

hn = bpn, (3)

where n is the kinetic order with respect to pressure, pj and k, is a constant. The values of n, given in Tables 1 and 2, were obtained from the slope of plots of log&,, against log,,p, as exemplified by Fig. 1.

PARAHYDROGEN CONVERSION ON 5A MOLECULAR SIEVES

TABLE 2 KINETIC ORDERS FOR PARAHYDROGEN CONVERSION

297

Catalyst, 1 T(“K) 77 90 - 195 273 296 - 500 560 295 90 77 0.28 0.30 - 0.80 0.80 0.84 - 0.73 (I.!)6 OX0 0.30 0.30

Catalyst, 2 T()“Kj 7i 90 113 146 195 210 463 552 605 419 90 7; ,L 000 0.07 0.25 0.65 0.71 0.X% O.T!) 1.00 0.95 0.58 0.12 0.00

Hydrogen-Deuterium Equilibration

XIcasurements of the rate of equilibration on catalysts 1 and 2 were determined at a number of temperatures, in an increasing sequence, using 7 torr of an equimolar mixture of hydrogen and deuterium. The values for the absolute rates at 297°K and above are plotted in Fig. 4. Attempts to measure the rates of equilibration at 77°K showed that they were less than the lower limit of measurement, Ic,,, = 2 X 1O1l molecules mg-lsec-l. This lower limit, which is the same for b’oth cat’alysts and only applies for 77”K, is determined by the errors in the analysis of a H,, HD, and D, mixture when the preferential adsorption of HD and D, occurs. The values for the ratio of the rate of conversion to that of equilibration at 77”K, k,,, (pH,) /k, (H, + D,) , are thus >1.3 X 10” for catalyst 1 and >1.4 X lo4 for catalyst 2. These values are to be compared with those for 297°K and above, where the ratio lies between the limits 222 a,nd 40.05. The values for the ratio support a chemical mechanism for conversion at high temperatures and a physical mechanism at low temperatures.

Comparison of Parahydrogen Conversion with Orthodeuterium Conversion a,t 77 and 90°K

Rates of parahydrogen and orthodeuterium conversion were measured on single

samples of catalysts 2 and 4 over the pressure range 1-16 torr at both 77 and 90°K. The values of Ic,,, at 7 torr, the kinetic order n, and the ratio Ic, (pH,)/k,,,(oD,) are given in Table 3.

Adsorption Isotherms

Adsorption isotherms were determined for hydrogen and deuterium on catalysts 1 and 2 at both 77 and 90°K over the pressure range 0.3 to 20.0 torr. The time between t,he admission of an increment of gas and the measurement of the equilibrium pressure was sufficient to ensure that all the gas was equilibrated with respect to spin-isomer composition. Adsorption was found to be extremely rapid and fully reversible. The gas uptakes are best expressed in terms of the Freundlich isotherm, V = Ic,p”, where I’ is the volume of gas adsorbed (in ml at STP) per gram of sieve, p is the equilibrium pressure and lc, and m are constants for any one isotherm. Plots of log,,V against log,,p are shown in Fig. 5, where it can be seen that the Freundlich isotherm holds over the whole pressure range for catalysts 2, but over two separate pressure ranges for catalyst 1. Values of I’ at 7 torr and the pressure exponent m are given in Table 4. The value of m should be proportional to the absolute temperature, so that for any one adsorbent/ adsorbate system m at 90”K/m at 77°K

TABLE 3 P.IRAHYDROGEN AND ORTHODEUTERIVM CONVF:RSION .+T 77 AND 90°K

Parahydrogen conversion Orthoderlteri~un conversion L,(pHz)/km(oDz)

k, at 7 torr ?I k,, at 7 torr t1 (molecules mg-1 between (molecules mg-’ between

set-I) 1-16 torr set-‘) l-16 torr Experimental TheoreGcal

Catalyst 2 at 77°K 6.9 X lOI 0.02 4.0 x 10’5 0.20 1.7 3.4 Catalyst 2 at, 90°K 5.6 X lOI 0.05 3.4 x 10’5 0.20 1.6 4.0 Catalyst 4 at 77°K 4.4 x 10’6 0.16 3.6 X 1OlL 0.03 1.” 2.4 Cat,alyst 4 at 90°K 3.2 x 10’5 0.38 3.2 x 10’5 0.14 1 .o 2.5


1.6 -

0.2 0.4 0.6 0.8 1.0 1.2 0.2 0.4 0.6 0.8 1.0 1.2

~XllO P

FIG. 5. Freundlich plots for the adsorpt,ion of hydrogen and derlterium. Catalyst 1: (A) Dt at 77°K; (B)-Ht at 77°K; (C) DZ at 90°K; (D) HS at 90°K. Catalyst 2: (E) Dz at 77°K; (F) Hz at 77°K; (G) 1)~ at 90°K; (H) Hz at 90°K.

should be 1.17. For the present isotherms, values of the ratio are 1.25 + 1.51 for H, on catalyst 1, 1.47+ 1.48 for D, on catalyst 1, 1.18 for H, on catalyst 2, and 1.20 for D, on catalyst 2. Isosteric heats of adsorption have been calculated where pos-

=‘ i 2.2 - f ij u -

1 2.0

:: $ 1.8-

1.6 -

I I I I I I I 5 IO 15 20 25

Vods (ml at stp g-l)

FIG. 6. Heats of adsorption of hydrogen and deuterium. -.-. Hz on catalyst, 1; --. Dz on catalyst 1; --- Hz on cat.alyst 2; -- Dt on catalyst 2.

xible and t,hese are plotted as a function of uptake in Fig. 6.

The linear plots of log,,V against log,,p, the values of the ratio m at 9O”K/m at 77”K, and the exponential fall in the heat of adsorption with uptake show that the Freundlich isotherm is truly applicable to adsorption on catalyst 2. With catalyst 1, however, the applicability is less certain. It is evident that the exchange of sodium ions for neodymium ions has altered the adsorptive properties of 5A molecular sieve.

TABLE 4 VALUES OF THE UPTAKE AND PRESSURE EXPONENT FOR ADSORPTION ISOTHERMS

V at 7 t,orr (ml STP g-‘) m

Hz on catalyst 1 at 77°K 24.8 0.57-90.37 Hz on catalyst I at 90°K 9.7 0.71-+ 0.56 Ha on catalyst 2 at 77°K 24.9 0.38 Hz on catalyst, 2 at 90°K 13.2 0.45 Dt on catalyst 1 at 77°K 32.7 0.45-+0.33 Dz on catalyst 1 at 90°K 15.6 0.66-+0.49 Dz on catalyst 2 at 77°K 32.4 0.34 Dz on catalyst 2 at 90°K 17.5 0.42

General

It has been experimentally shown that the preferential adsorption of orthohydrogen from a mixture of the spin isomers (16) does not cause deviations from first order kinetics for parahydrogen conversion on molecular sieves at 77 and 90°K. With titanium dioxide at the same temperatures, deviat,ions have been observed with a catalyst sample of high surface area (18), but not wit#h a catalyst sample of low surface area (13: and unpublished results of Richardson, P. C., and Rudham, R.). The explanation for this does not lie in the existence of preferent,ial adsorption alone, but in the extent to which it influences the relative concentrations of the spin isomers remaining in the gas phase. This will de- pend upon the extent to which a given admission of gas is adsorbed, so that deviations are most pronounced with a high area catalyst in a low reaction volume. We calculate that 154070 of the gas intro- duced was adsorbed by the sieves during experiments at 77 and 90°K ; whereas, with titanium dioxide, 93% of the gas was adsorbed on the high area catalyst (18) and 5-20s adsorbed on the low area catalyst (13; and unpublished results of Richardson, P. C., and Rudham, R.).

Molecular sieves present a highly porous catalytic system, and it is necessary to show t,hat, the observed rates of parahydrogen conversion are not determined by diffusion in either the micropores of the sieve structure or the macropores between the sieve crystallites. Reaction rates determined by such diffusional processes have been discussed by Wheeler (19) and we have followed his method of calculation. For diffusion in micropores, it was assumed that transport was by Knudsen diffusion in pores of 5A in diameter, the pore volume was 0.3 c&g-l (20)) and the pellet diameter was that of the sieve crystallites, +lCV cm. For diffusion in macropores, it Teas assumed that the total mass of catalyst behaved as a single pellet with pore diameters equal t.o that of the sieve crys- tallitcs, transport was by bulk diffusion,

and the pore volume, calculated from the real and apparent densities (ZO), was 1.5 cm:(g-‘. First order rate constants calculated on the bases of these assumptions were several orders in magnitude greater than the highest values of k, observed in our experiments. We conclude that the reaction rates presently reported are free from diffusional effects.

High Temperatures

Restricting discussion to reaction at high temperatures (>297”K for catalyst 1 and >250”K for catalyst 2), the positive apparent activiation energies and the values of the ratio k,(pH2)/lC,,,(HZ + D,) indicate that parahydrogen conversion predominantly proceeds by a chemical mechanism. There is little evidence in the literature to suggest that hydrogen is either physically adsorbed or molecularly chemisorbed on oxide surfaces at the temperatures of the experiments, and it is usually accepted that both parahydrogen conversion and hydrogen-deuterium equilibration (13, 21, .%L?) proceed by the Bonhoeffer- Farkas mechanism (1). If this is so in the present case, then the chemisorbed hydrogen atoms must be mobile within the sieve, since the pressure dependencies are indicative of a low fractional coverage of the active surface or sites. Molecular sieves offer a special environment for mechanisms involving interactions between chemisorbed atoms and molecular species (a, S), since any hydrogen molecule within the sieve structure is, by necessity, in close proximity to the surface. This may permit reaction to proceed by such a mechanism, or per- haps by the Schwab molecular mixing mechanism (4)) under experimental conditions normally regarded as unfavorable. Any decision must, await further detailed kinetic studies.

The effect of neodymium ions on the activity for chemical conversion at high temperatures is unfortunately masked by the unknown contribution they will make to overall reaction from paramagnetic physical conversion. There is no doubt, however, that neodymium ions modify the catalytic properties, since the activity of


catalyst 2 for hydrogen-deuterium equilibration is higher than that of catalyst 1, except at the highest experimental temperatures.

Turkevich (15) found that hydrogen treatment of NaY molecular sieves de- stroyed their activity for both parahydrogen conversion and hydrogen-deuterium equilibration at room temperature, but left the activity for conversion at 77’K un- effected. With 5A sieves, we also find that hydrogen treatment has no effect on conversion at 77”K, but find that it has little effect at room temperature and causes activation at higher temperatures. Close par- allels between the two investigations are not to be expected, however, since the two sieve structures have different Si/Al ratios and the NaY sieves were extensively decationated. A mechanism for activation which does not effect the magnetic properties of the catalysts implies that high temperature activity, at least in part, is associated with the sieve itself rather than with the neodymium ions or paramagnetic impurities. Thus activation could result either from activated adsorption on exist- ing sieve sites, or from the generation of new sites. In the absence of high temperature adsorption isotherms and detailed information on pressure dependencies, it is not possible to say which effect is responsible for activation. If it is the latter, however, we tentatively suggest that at high temperatures hydrogen will react, with structural OH groups to give an active adsorbed hydrogen and water which is desorbed and condensed.

(sieve structure)-0H + Hz --) (sieve stnlrtrme)-H + Hz0

Low Temperatures

The rate orders which have been observed at 77”K, Ic ),(pH,) > > > Ic,(H, + D?) for catalyst 1 and Ic,, (pH,) > Ic,,, (oDJ > > > k,(H, + D?) for catalyst 2, indicate that parahydrogen conversion proceeds by a paramagnetic physical mechanism at low temperatures. This is further reinforced by the observation that the activity of catalyst 2, containing the paramagnetic

neodymium ions, is higher than that of catalyst 1 and that both catalyst 1 and 2 have negative apparent activation energies below 250°K.

Three detailed mechanisms for the heterogeneous paramagnetic conversion of parahydrogen have been developed from Wigner’s original theory (5). In these, conversion is considered to occur (a) during elastic collisions between hydrogen molecules and the paramagnetic surface, (b) when physically adsorbed molecules vibrate perpendicularly to the plane of the surface on strongly adsorbing paramagnetic sites (8, dS), and (c) during the translational motion of physically adsorbed molecules across the paramagnetic surface (7). De- cisions on the applicability of these mechanisms are based on comparisons of the pressure dependencies for conversion and derived coverages (12) with those from adsorption isotherms at the same temperatures, and from comparisons of calculated and experimental values of Ic, (8, 10, 19). With sieves we are restricted to the first of these tests due to the uncertainty in the effective surface area. The collisional mechanism (a) requires a first order pressure dependency, or the same as that of the adsorption isotherm if the molecules within the sieve behave as a compressed gas. The translational mechanism (c) also requires the same pressure dependency as the isotherm. A comparison of the equivalent values of n and vz in Tables 1, 2, and 4 clearly shows that neither of these mechanisms applies to the present results. We therefore conclude that low temperature coIlversion nrocceds by the vibrational mechanism (b) on strongly adsorb’ing sites. This is a realistic situation, since for catalyst 2 at, a pressure of 7 torr, the number of hydrogen molecules adsorbed for each neodymium ion in the sieve is five at 77°K and three at 90°K. The increase in pressure dependency of conversion on catalyst 2 over the temperature range 77 to 210°K (Table 2) reflects the fall in the extent of hydrogen adsorption as the temperature is raised. We assume that the activity of catalyst 1 arises from the presence of paramagnetic impurities, most probably iroll

PARAHYDROGEN CONVERSION ON 5A MOLECCLAH SIEVES 301

(la), in the sieves as supplied. The higher pressure dependencies at 77 and 90°K for this catalyst are then explicable in terms of t,he impurities possessing a lower adsorption potential than neodymium ions. This could arise if the impurities are incorpor- ated into the aluminosilicate framework of t,he sieve rather than in the positions normally occupied by cations. At 77°K and 7 torr the activity of catalyst 2 is 10.5 times greater than that of catalyst 1, and from the known extent of neodymium exchange we calculate an upper limit for ferric iron content of 7 X 10e6 g ion per g. It has been suggested that the nuclear magnetic moment of aluminum might be responsible for parahydrogen conversion on NaY sieves (15). We do not consider that this would make a significant contribution to conversion in t,he present experiments, since the aluminum content of catalyst 1 was 6 X 1O-3 g atom per g and the effectiveness of ,41z7 in the transition probability (5, 7, 8) would be zIO-~ that of any paramagnetic ion which may be present.

If both parahydrogen and orthodeuterium conversion proceed by the same vibrational mechanism, the theoretically pre- dicted value for the ratio of absolute rates is given by

where I*, G(T) , I, V, and 0 have their usual significance (7, 8, 10). Equation 4 differs in detail from that previously given (10, 24) in that, we have included v and have used subscripts to indicate atomic and molecular properties. Assuming that v a imolecular mass)-l/?, on the substitution of numerical values (10, 13, 23) we obtain k,,, (pH,)/k;,,(oD,j equal to 2.8 Q,,,/&,, at 77°K and 3.4 O,,lOnz at 90°K. Since n = (1 - 0) over a limited pressure range (12)) we can use experimental values of n to give values of Blr, and &,, and hence calculate theoretical values of Ic,(pH,)/lc,,(oD,) for catalysts 2 and 4. Such calculations can only be approximate, but for both catalysts the experimental values of the ratio are less than those from theory (Table 31, as

found for nickel (10) and gadolinium (24). It has been suggested (24) that such dis- crepancies may denote an effect of an inhomogeneous electric field operating on t,hc electric quadrupole moment of the tlcuteron, which cannot apply for the pro- ton since it’ has no quadrupole moment. If this is so in the present case, it is difficult to see why the experimental ratios for neodymium oxide are less than those for the molecular sieve, unless the electrostatic fields at surface anion vacancies (8, 13,25) are more effective than those within the sieve cages.

It is of interest to calculate the catalytic efficiency of an individual neodymium ion for parahydrogen conversion in the differ- ing environments of the sieve and the bulk oxide. From the values of k,,, at 77°K and 7 torr, assuming the concentration of active sites in Nd,O, to be 1Ol3 cm-* (25), the activit,ies are 0.03-0.06 molecule ion-lsec-l for the sieve and 3.0-9.0 molecules ion-l see-l for the bulk oxide. The difference in activity could arise from differences in the values of v and rs, the distance of the hydrogen molecule from the ion during interaction. However, with the bulk oxide, where the active sites are considered to be anion vacancies or anion vacancy clusters (25), interaction with more than one neodymium ion may enhance activity, so that the effective surface concentration of Nd3+ is greater than 1013 ioncm-2.

ACKNOWLEDGMENTS

The authors wish to thank Dr. D. R. Pearce for the measurements of hydrogen-deuterium equilibration and the Science Rrsearch Council for a studentship held by D. E. B.

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Documents

Parahydrogen conversion on 5A molecular sieve and 5A molecular sieve containing neodymium ions