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,
Received June 2, 1969
Parahydrogen conversion on 5A molecular sieve and 5A molecular sieve contain- ing neodymium ions has been investigated over the temperature range 77-605K. 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 temper- ature activity is unaffected by the hydrogen treatment.
Heterogeneous parahydrogen and ortho- deuterium conversion can occur by both chemical and physical mechanisms. Chemi- cal conversion involves chemisorbed hy- drogen atoms and is controlled by the same factors which determine activity for hy- drogen-deuterium equilibration. At high temperatures, where adsorption is restricted to hydrogen atoms alone, the Bonhoeffer- Farkas (1) mechanism is the most proba- ble. At lower temperatures, interactions between molecularly adsorbed hydrogen and adsorbed hydrogen atoms (2, S), or between the adsorbed molecular species themselves (4)) provide alternative chemi- cal 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 con- version.
Diagnostic tests of mechanism are mainly based on comparisons of reaction rates. For any chemical mechanism, the effect of zero-point energy on the activa- tion 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 tem- perature (7, 8, 11). Comparisons of surface coverages derived from pressure dependen- cies (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
PARAHYDROGEN CONVERSION OS 5A MOLECULAR SIEVES 293
25C. It was found that the catalytic ac- tivity 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 total number of decationated sites, and it was suggested that either im- purities 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 77K was independent of the extent of decationization and was thus as- sociated with paramagnetic impurities rather than with decationated sites. Molec- ular sieves at low temperatures will prefer- entially adsorb orthohydrogen from a mix- ture of the spin isomers, and this has been used as a basis for their gas-chromato- graphic separation (18). The separation is impaired when the sieves contain iron im- purity, due to paramagnetic physical con- version 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.
All measurcmcnts were made in conven- tional high vacuum systems, in which traps at 77K protected the catalyst samples from contamination by water, grease, and mercury. Kinetic studies of parahydrogen conversion, orthodeuterium conversion? and hydrogen-deuterium equilibration were made in a constant volume reaction sys- tem (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 iso- therms were determined in a constant vol- ume system.
Pure normal hydrogen and normal deu- terium were prepared by allowing purified cylinder gas to diffuse through a heated palladium-silver thimble. Enriched para- hydrogen or orthodeuterium were prepared by contacting the appropriate normal gas wit,h an outgassed charcoal catalyst, at 21K.
1. Linde 5A hlolecular sieve in powder form (Lot 949260), which had been con- tacted with distilled water for 16 hours at 25C, filtered, washed and distilled water, and dried in an oven at 130C for 24 hours.
2. Linde 5A Molecular sieve (Lot 949260)) in which 13.2 -+ 0.5 wt % of the so- dium 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 25C. 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 350C with a current of air which had previously been saturated with hexamcthyldisiloxane at room temperature, followed by 24 hours at 3%400C in a current of dried air.
4. Johnson Matthey Specpure neodym- ium oxide, with a specific surface area of 4.74 mg-.
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 out- gassed for 48 hours at 350C 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 para- hydrogen enrichment (-o-) and depletion (--) on catalyst 2 at 77K.
Parahydrogen conversion, orthodeute- rium 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 hydro- gen-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 reac- tion 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 90K were made at constant pressure on a sample of catalyst 2 outgassed at 340C. 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 para- hydrogen depletion and enrichment were measured as a function of pressure at 77K on a further sample of catalyst 2 outgassed at 350C. 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 ad- sorption 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 out- gassing temperature at which stable and reproducible activities were obtained. Rates of parahydrogen conversion were deter- mined 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 473K, 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 77K rises with outgassing temperatures below 25QC, 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 55OC, there is little change in activity with increasing temperature, and 350C was selected as a convenient outgassing temperature. At temperatures above 550C 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 473K shows that out- gassing at 350C is sufficient to generate an activity close to the apparent maximum obtained after outgassing at 375C. To de- termine a suitable outgassing time, the ac- tivity of a sample of catalyst 2 was meas- ured after outgassing at 350C for 14 hours and again after outgassing for a further
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 77K AND 7 TORR
Mass of k, (molecules Kinetic Catalyst sample (mg) mg-see-I) order, n
2 16 6.9 x 105 0.0 2 37 4.1 x 105 0.00 2 41 4.2 x 10: 0.00 3 12 4.5 x 105 0.30
63 hours. The additional outgassing failed to generate any increase in activity, so that outgassing for 14 hours at 350C 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 77K 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, indicating that treatment with hexamethyl- disiloxane has not caused poisoning. The activities of samples of catalysts 4 and 5 :it 77K and 7 torr, after the standard out- gassing 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 sur- face, 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 tem- peratures on the same sample. At low tem- peratures, below 296K for the untreated sieve (catalyst 1) and below 250K for the sieve containing neodymium ions (catalyst 2), the activities at increasing and decreas- ing 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 para- hydrogen, 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 tempera- ture 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- tivity 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 250K. Apart from the first +lOO on the increasing temperature plot, apparent activation energies are positive for both catalysts. The effect of high tem- perature activation is to reduce both the activation energy and pre-exponential fac- tor for parahydrogen conversion. For cata- lyst 1 at increasing temperatures (426- 560K), the activity at 7 torr can be expressed by k,, = 7.9 X 1031 exp (-38,400/ RT) , whereas at decreasing temperatures (471-403K), k,, = 1.0 X lo* exp (-S,SOO/
RT). Similarly, for catalyst 2 at increasing temperatures (400-552K)) k, = 1.2 X 10zs exp (-19,30O/RT), whereas at decreasing temperatures (578-373K)) k, = 6.0 X 1Oz6 exp (--4,200JRT). In the high temper- ature 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...