Adsorption (2008) 14: 241–246DOI 10.1007/s10450-008-9106-0

Adsorption and diffusion of propane and propylenein Ag+-impregnated MCM-41

F. Iucolano · P. Aprea · D. Caputo · C. Colella · M. Eic ·Q. Huang

Received: 2 May 2007 / Revised: 27 November 2007 / Accepted: 29 January 2008 / Published online: 20 February 2008© Springer Science+Business Media, LLC 2008

Abstract Equilibrium data and diffusion characteristics ofpropane and propylene were determined on mesoporous ad-sorbents modified with an organic molecule (APTES) and/orimpregnated with AgNO3, in order to obtain a separation byadsorption via π-complexation.

Adsorption capacities were determined by a gravimet-ric technique, while diffusion characteristics were evalu-ated by the ZLC technique. The equilibrium isotherms datashowed that the modification with an organic molecule willweaken the π-interaction between Ag+ and double C=Cbond. On the other hand slightly higher adsorption capac-ities for propylene (about 1.5 mol/kg) were obtained forthe sample prepared by a direct impregnation with largeramounts of AgNO3 (M4 sample).

Diffusion runs confirmed that the propane desorption rateon M4 sample was much higher compared to propylene.This evidence leads to a potential application of that adsor-bent material for a kinetic separation.

Keywords Propane · Propylene · MCM-41 · Adsorption ·Diffusion · Separation

1 Introduction

Propane-propylene separation is one of most important andmost expensive operations in the chemical and petrochemi-

F. Iucolano (�) · P. Aprea · D. Caputo · C. ColellaDipartimento di Ingegneria dei Materiali e della Produzione,Università Federico II, Napoli, Italye-mail: iucolano@unina.it

M. Eic · Q. HuangDepartment of Chemical Engineering, University of NewBrunswick, Fredericton, Canada

cal industry. Propylene feedstock, used for the production ofpolypropylene, requires a purity of at least 99.5%, normallyachieved by distillation in large plants. The high cost of thisoperation, due to the small separation factor of the two mole-cules, having approximately the same size and similar boil-ing points, prompted a number of investigations searchingfor unconventional techniques in the recent years (Padin etal. 1999; Padin and Yang 2000; Grande et al. 2004). At thepresent time adsorption appears to offer a suitable alternativeto distillation, as it makes possible to increase the separationfactor and therefore reduce the cost of the process (Ruthven2000).

Among the many commercial adsorbents tested, meso-porous materials—characterized by large specific surface ar-eas, narrow pore size distribution, adaptable pore size andpossibility to be modified and functionalized—seem to bepromising candidates for this kind of separation.

In order to improve the olefins/paraffins separation effi-ciency, an useful procedure consists of spreading out metalcations, such as Ag+ or Cu+, over the mesoporous internalsurface, making it possible the formation of π -complexesbetween a cation of adsorbent and a double C=C bond ofa sorbate (Padin and Yang 2000; Takahashi et al. 2002;Grande et al. 2004; Basaldella et al. 2006). Separation byadsorption via π -complexation is a sub-group of adsorptionreactions, characterized by bonds stronger than the Van derWaals forces alone, and therefore achieving high selectivityregarding olefins separation from the mixtures. On the otherhand, this type of bond is still sufficiently weak, thus al-lowing regeneration to be performed by raising temperatureand/or decreasing pressure.

The aim of this work was to investigate the adsorptionfeatures of suitably modified MCM-41 mesoporous silica, inorder to evaluate possible application in propylene/propaneseparation process.

242 Adsorption (2008) 14: 241–246

Table 1 Textural properties andAg content (%) of MCM-41samples

Sample SBET Vpores at P/P0 = 0.8 Dpores Ag content

(m2/g) (cm3/g) (Å) (%)

M1 423 0.39 32 9.26

M2 382 0.43 30 8.92

M3 757 0.45 35 2.98

M4 793 0.49 36 6.03

2 Experimental

2.1 Materials

MCM-41 mesoporous silica impregnated with AgNO3 atdifferent amounts—with or without a preliminary modifi-cation with 3-aminopropyltriethoxysilane (APTES)—weresynthesized as powders, with a procedure described in oneof our earlier studies (Aiello et al. 2006).

M1 and M2 samples are MCM-41 mesoporous samplesimpregnated with different amounts of AgNO3 after a pre-liminary modification with APTES, while M3 and M4 wereobtained with a direct impregnation with different amountsof AgNO3 (Aiello et al. 2006).

The spreading of Ag+ on mesoporous substrate was ac-complished using the wet impregnation technique. Accord-ingly, a sample of MCM-41 was contacted with an aqueoussolution of the salt to be dispersed along a time sufficient forits mesopores imbibition. Then the sample was recoveredand heated for 4 h at 105 °C in air to remove water.

All the samples were characterized by nitrogen adsorp-tion-desorption isotherm analysis (Micromeritics ASAP2010 apparatus) and X-ray dispersion analysis (JSM-T330AScanning Microscope).

Main textural properties and Ag amounts of all the adsor-bents tested are reported in Table 1.

2.2 Adsorption equilibrium measurements

The adsorption properties of the mesoporous materials wereinvestigated from the equilibrium isotherms of propane andpropylene at 20 °C. Experimental adsorption data were col-lected by means of a gravimetric technique, using a McBain-type balance (detailed description has been reported inCaputo et al. 2004).

2.3 Diffusion measurements

Diffusion measurements for propane and propylene wereperformed using the ZLC (Zero Length Column) chromato-graphic technique (Eic and Ruthven 1988). A very smallamount (1.5–2.0 mg) of adsorbent was placed in the ZLCcolumn and equilibrated with sorbate diluted in a heliumflow, to obtain a low concentration required by the ZLC

theory. The desorption was performed by purging with he-lium at a flow rate high enough to minimize external massand heat transfers. Essentially the same diffusion parame-ters were extracted from the experimental runs under variouspurging flow rates, thus confirming kinetically controlledprocess. A purge flow rate of 30 cm3(STP)/min helium wasselected as a standard flow rate for all experiments in thisstudy. The effluent sorbate concentrations from the ZLC col-umn were measured using a gas chromatograph (HP 5900)equipped with a FID detector. Details of the experimentalmethod have been described elsewhere (Eic and Ruthven1988; Jiang and Eic 2003).

3 Theory

The analysis of the ZLC desorption curve involves solv-ing Fickian diffusion equation with appropriate initial andboundary condition, i.e., zero concentration of sorbatespecies on the surface of the adsorbent particle (Eic andRuthven 1988). For a linear equilibrium system with uni-form spherical particles the normalized effluent gas concen-tration is given by:

c

co

=∞∑

n=1

2L

[β2n + L(L − 1)] exp(−β2

nDt/R2) (1)

where βn are eigenvalues given by the root of the equation:

βn cotβn + L − 1 = 0 (2)

and

L = 1

3

εν

(1 − ε)z

R2

KH D= 1

3

purge flow rate

crystal volume

R2

KH D(3)

In (3) ε is the voidage of the ZLC bed, ν the interstitial gasvelocity, z the ZLC bed depth, t the time of purging, KH

the dimensionless Henry’s Law constant and D/R2 the dif-fusion time constant.

For the long time case, (1) can be reduced to a simple ex-ponential decay curve, since only the first term of the sum-mation is significant:

c

co

= 2Lexp(−β1Dt/R2)

[β21 + L(L − 1)] (4)

Adsorption (2008) 14: 241–246 243

Fig. 1 Propylene (!) and propane (") adsorption isotherms on M1–M4 samples at 20 °C

A plot of ln(c/co) versus t should produce a linear asymp-tote in the long time region, and from the slope and interceptthe parameters D/R2 and L can be extracted.

4 Results and discussion

4.1 Adsorption equilibrium isotherms

The adsorption isotherms at 20 °C showed that all the sam-ples exhibited a greater adsorption capacity for propylenecompared to propane (Fig. 1), and such a behaviour wasmore evident for the samples M3 and M4, impregnated withAgNO3 without a preliminary modification with APTES.Adsorption capacities at 1 atm (760 Torr) for propylenein M1–M4 samples were 1.25, 0.70, 1.23, 1.54 mol/kg,respectively, and those for propane were 1.00, 0.55, 0.69and 1.02 mol/kg, respectively. The increased adsorption ca-pacity for propylene is attributed to the presence of Ag+

ions for π -complexation. Although M3 was loaded approx-imately with only a half amount of Ag+ content comparedto M4, its adsorption capacity at 1 atm for propylene was80% of M4. This suggests that there is an optimum load-ing of Ag+ for propylene adsorption, which is between 3%and 6%.

Nevertheless, in general, the adsorption capacities forpropylene were lower than expected. They were comparableor even smaller than on a silver-based MCM-41 measuredat 70 °C (Padin and Yang 2000). This may be related, eitherto the inactivation of Ag+ by reduction to Ag0 or formationof Ag2O (also the formation of this oxide reduces the possi-bility of π -complexation between Ag+ and the olefin dou-ble bond, resulting in a lower adsorption capacity for propy-lene), or to the degradation of the organic functional group(M1 and M2 samples), with consequent deactivation of theadsorbent materials.

The equilibrium selectivities (expressed as the propylene-to-propane adsorption capacity ratio) versus the pressure,

244 Adsorption (2008) 14: 241–246

Fig. 2 Propylene/propane equilibrium selectivity at 20 °C for M1 (!),M2 ("), M3 (1) and M4 (2) samples

are reported in Fig. 2. M3 and M4 samples show highervalues than M1 and M2, revealing that the modification ofAPTES doesn’t enhance the separation factor of propyleneover propane. All the adsorbents appear to be rather selec-tive at low pressure values, with a quick decrease of selec-tivity at higher pressures. Such a dependence on the drivingforce indicates that, together with the formation of Ag+−π

bonds, the steric effect plays an important role in the sepa-ration mechanism.

4.2 Diffusion results

The ZLC measurements were performed for propane andpropylene in a temperature range of 35 to 75 °C. The con-centration of the sorbate in helium was set at 0.2 vol. %,which is low enough to ensure that the ZLC experimentswere carried out in the linear region of isotherms as required.

Figure 3 shows the desorption curves for all the sam-ples, while diffusion time constants for propane and propy-lene obtained from the long time region of ZLC curves andthe corresponding activation energies are summarized in Ta-ble 2. It is clear that propane has higher values of diffusiontime constants than propylene.

Table 2 also shows that M2 sample has an activation en-ergy for diffusion higher for propane than propylene, whichmay appear unusual remembering that π electrons from un-saturated HC’s should provide stronger interaction with theactive sites of an adsorbent surface (π complexation), thusincreasing activation energy.

Nevertheless, as discussed in the previous paragraph, thecomparable adsorption capacities exhibited by M2 sampletoward propane and propylene (see Fig. 1) suggest a pos-sible deactivation of the adsorbent material. Therefore the

unexpected values of activation energies are probably dueto the fact that the adsorption mechanism of propylene onM2 sample is no more strongly related to a π -complexationwith Ag+ cations but (as for propane) essentially based onvan der Waals-type interactions with mesopores walls.

From the ZLC curves, it appears that the propane desorp-tion rate is strongly dependent on the sample type, e.g., bothM1 and M4 samples (obtained using a higher Ag amount inthe reaction batch) show a very fast decrease in the propaneconcentration, whereas the desorption is much slower forM2 and M3 samples. The desorption rates for the M4 samplewere faster than those for M1 sample, and all of them wereso fast that no ZLC analysis was possible. Furthermore, forall the samples the desorption rate increases with the tem-perature.

The adsorption behaviour involving propylene was some-how different, e.g., the ZLC curves showed that the diffusionrates were almost the same for M1, M2, M3 samples, indi-cating that the modification carried out by APTES had notchanged the adsorption kinetics of propylene.

The dependence of the diffusion rate on the tempera-ture is quite modest for M1–M3 samples. On the contrary,M4 sample exhibited a desorption rate for propylene sig-nificantly lower and more dependent on the temperature,i.e., higher activation energy (around 30 kJ/mol). The ac-tivation energy for diffusion of propylene in M4 sample iscomparable with the heat of adsorption of same species in asilver-based π -complexation mesoporous silica with cylin-drical pores, which was found to be 34 kJ/mol (Grande et al.2004). This suggests that the transport of propylene in M4sample appears to be mainly controlled by mesopore diffu-sion (Vinh-Thang et al. 2006).

Concerning the ability to separate propane from propy-lene by means of a diffusion process (kinetic separation),the M2 and M3 samples do not seem to be the good can-didates. As can be seen from the collected data, the behav-iour of those samples towards both sorbates is essentiallythe same. On the other hand, M4 sample shows the maxi-mum difference with regard to mass transfer rates of propaneand propylene, thus suggesting the possibility of using thissample for the kinetic separation of these sorbates. Relatedto this, M1 sample exhibits the second largest difference inmass transport behaviours for the sorbates, and as such couldalso be considered for the kinetic separation.

The above mentioned results indicate that the presence ofan organic molecule within the mesopores can weaken theπ -interaction between Ag+ and double C=C bond, whilethe more promising behavior of M4 sample compared to M3and M2 may be related to a larger amount of Ag+ present inits mesopores.

Adsorption (2008) 14: 241–246 245

Fig. 3 Experimental ZLC curves for propane (left) and propylene (right) in M1, M2, M3, M4 samples

5 Conclusions

The experimental data provide a good overview of the equi-librium and diffusion behaviour of MCM-41 impregnatedwith AgNO3, either modified with APTES or not. Equilib-rium results are not conclusive with respect to a use of inves-tigated samples for thermodynamic separation of propaneand propylene, due to the modest adsorption capacity forboth sorbates and the varied selectivity, which is high at verylow pressure, but quickly diminishes at the higher pressures.However, kinetic separation appears to be more promisingbased on the different diffusion behaviour of the sorbates inthe material: in particular, propane demonstrated a very highdiffusion rate in the M4 sample (MCM-41 impregnated onlywith AgNO3 at higher content), while propylene was shownto be slow, indicating a possibility for the successful appli-cation.

Nomenclature

c = sorbate concentration, mol/m3

c0 = initial sorbate concentration, mol/m3

L = constant defined in (3), dimensionlessβn = eigenvalue: roots of (2), dimensionlessD = intracrystalline diffusivity coefficient, m2/st = time, s

R = particle radius, mT = temperature, Kε = voidage of the ZLC bed, dimensionlessν = interstitial gas velocity, m/sz = bed depth, m

KH = Henry’s law constant, dimensionless

246 Adsorption (2008) 14: 241–246

Table 2 Diffusivity data and activation energy for propane and propylene in M1, M2, M3, M4 samples

Propane Propylene

T (°C) D/R2 (s−1) × 103 E (kJ/mol) T (°C) D/R2 (s−1) × 103 E (kJ/mol)

M1 35 – – 35 2.05 6.19

45 – 55 2.39

60 – 75 2.71

M2 35 1.55 14.43 35 1.49 10.96

55 2.16 55 1.76

75 2.96 75 2.44

M3 35 3.12 10.12 35 1.85 11.56

55 4.25 55 2.41

75 4.91 75 3.11

M4 35 – – 35 0.43 28.96

55 – 55 0.83

75 – 75 1.59

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