Influence of Si/Al ratio on hexane isomers adsorption equilibria

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Microporous and Mesoporous Materials 111 (2008) 171–177

Influence of Si/Al ratio on hexane isomers adsorption equilibria

Alexandre F.P. Ferreira a,*, Marjo C. Mittelmeijer-Hazeleger a,Alfred Bliek a, Jacob A. Moulijn b

a University of Amsterdam, HIMS - Van ‘t Hoff Institute for Molecular Sciences, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlandsb Delft University of Technology, DelftChemTech, Nieuwe Julianalaan 136, 2628 BL Delft, The Netherlands

Received 23 April 2007; received in revised form 10 July 2007; accepted 11 July 2007Available online 25 September 2007

Abstract

In this work we aim to have a better knowledge of the influence of silica/alumina ratio (SAR) on the adsorption equilibrium of hexaneisomers on MFI zeolites. With a manometric set-up coupled with a micro-calorimeter we did address adsorption properties and heats ofadsorption of n-hexane, 2-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane on three samples with different Si/Al ratio. Thebranched isomers all show a type I isotherm on the three samples, while n-hexane presents a dual site for SAR 1600 and 100, and typeI for SAR 50. The Si/Al ratio has a large influence on the adsorption equilibrium properties, in addition the Henry’s constant and initialheats of adsorption increase with aluminium content. The saturation loading decreases with the increasing aluminium content. The dif-ferential heat of adsorption of n-hexane increases slightly with loading, whereas the heat of adsorption of branched hexanes presents adecrease with loading. The three MFI zeolite samples present equilibrium selectivity towards n-hexane.� 2007 Published by Elsevier Inc.

Keywords: Zeolites; Hexane isomers; Isotherms; Differential heat of adsorption

1. Introduction

MFI type zeolites are the most frequently studied zeo-litic materials. They have been investigated in numerouscatalytic, adsorption and diffusion studies. Moleculeshosted in the narrow pores of MFI have a strong interac-tion with the force field exerted by the zeolite pore wallsleading to pronounced differences in the adsorption of mol-ecules with different size, polarity, shape, etc [1]. Evidently,a good knowledge of these adsorption properties will helpto understand zeolite catalysis, explaining the efforts putinto the determination of adsorption equilibria on MFItype zeolites. Particularly, the adsorption of n-alkanes onMFI type zeolites has been studied extensively by severalauthors, whereas adsorption of branched alkanes isomershas received far less attention [2–7]. A complete set of data

1387-1811/$ - see front matter � 2007 Published by Elsevier Inc.

doi:10.1016/j.micromeso.2007.07.043

* Corresponding author.E-mail address: afpferreira@yahoo.com (A.F.P. Ferreira).

for all isomers at a certain temperature is hard to find in lit-erature, majority of studies found in the literature focustheir attention on n-hexane and one of its branchedisomers.

Protonic zeolites, like MFI, find industrial applicationsas acid catalysts in several hydrocarbon conversion reac-tions. The activity of these zeolitic materials is related totwo main properties. The first is shape selectivity effectsdue to the molecular sieving properties associated to thewell defined crystal pore sizes, where at least a part of thecatalytic active sites are located. The second property is astrong Brønsted acidity of bridging Si–(OH)–Al sites gener-ated by the presence of aluminium inside the silicate frame-work [8]. The catalytic behaviour and adsorptionproperties of zeolite depend not only on the aluminiumcontent (acid sites) but also on its distribution in the zeoliteframework [9]. The aluminium zoning on MFI crystals hasbeen reported on literature by several authors, the work ofvon Ballmoos and Meier [10] and Dessau et al. [11] are ref-erence studies on this subject. They provided evidences for

172 A.F.P. Ferreira et al. / Microporous and Mesoporous Materials 111 (2008) 171–177

aluminium zoning in crystals larger than 1 lm, with alu-minium concentrated near the exterior surface of the crys-tals. In addition, the Si/Al ratio has also effects on thesurface properties of MFI zeolites [8], their framework flex-ibility [12] and phase transition from orthorhombic tomonoclinic structures [13,14].

Branched hydrocarbons are preferred to linear hydro-carbons as ingredients in petrol, as they enhance the fueloctane number. By catalytic isomerisation linear hydrocar-bons are converted into branched hydrocarbons, then itbecomes necessary to separate the mixture. Gasolineoctane number enhancement is a possible application ofthis study. In this paper we present a study on the influenceof Si/Al ratio in the adsorption equilibria properties andheats of adsorption of hexane isomers on MFI type zeo-lites. The adsorption experiments have been performedon a manometric set-up connected to a micro-calorimeter.The adsorption of n-hexane, 2-methylpentane, 2,3-dim-ethylbutane and 2,2-dimethylbutane, on three samples withdifferent Si/Al ratio (50, 100 and 1600) has been measuredat 423 K. The experimental isotherms were modeled withLangmuir and Dual Site Langmuir models. Additionally,Henry’s constants and initial heats of adsorption were cal-culated for the four components on the three differentsamples.

2. Experimental section

The MFI samples have three different ratios: 50, 100 and1600. Adsorption isotherms and differential heats ofadsorption are accessed directly by a manometric set-upcombined with a micro-calorimeter (Calvet C80, Setaram).Fig. 1 shows a scheme of the experimental setup.

The calorimeter used for these experiments is of the Cal-vet type, it measures the heat flux in and/or out of the sam-ple cell, and can be operated isothermally at a fixedtemperature. Gas or vapor can be fed into the system by apiston, that can introduce a full, ½ or ¼ stroke. The intro-

Fig. 1. Scheme of the e

duction pressure cannot be higher than 100 kPA. Two pres-sure transducers with different sensitivities allow anaccurate measurement of pressure, of the gas phase in con-tact with the sample, from 0 kPa to 10 kPa and from 0 kPato 1000 kPa. The system has two independent data acquisi-tion systems, one for the manometric (isotherm) data, theother for the calorimetric data. A detailed description ofthe experimental system is given in previous work [15].

The experimental procedure includes measuring blanks,at 423 K, for the different vapors. The aim is to correct forthe non-ideality of the vapors. The blank measurementswere carried out by introducing a known amount of gasinto the empty sample holder and register the obtainedpressure. The next step on experimental procedure is toaccess the amount adsorbed of each vapor on the three dif-ferent samples. The zeolite samples were out gassed at573 K during 6 h under a vacuum higher than5 · 10�7 mbar. Isotherms and heat fluxes have been mea-sured in a continuous way for the four gases at 423 K. Amore detailed description of experimental procedure isgiven on previous work [15].

MFI zeolite samples from Zeolist (Zeolist International)were used as adsorbent material, with a Si/Al ratio (SAR)of 50, 100 and 1600. In Fig. 2 scanning electron micrograph(SEM) images of the three different samples of MFI crys-tals are presented. The samples were calcined for 6 h at873 K.

The adsorbates used were n-hexane 99+% from Merck,2-methylpentane 99+%, 2,3-dimethylbutane 98+% and2,2-dimethylbutane 98%, the last three components werefrom Acros Organics. These liquids were vaporized, andthen fed into the system without any further treatment.

2.1. Isotherm models

The well-known Langmuir isotherm model reproducesthe general shape of a Type I isotherm, and can be givenby the following equation:

xperimental set-up.

Fig. 2. SEM pictures of MFI samples. SAR: (a) 50, (b) 100 and (c) 1600.

q(c

m3

g-1

)

0

50

100

150

200

0.0 0.2 0.4 0.6 0.8 1.0p / p0

Fig. 3. N2 adsorption isotherms on MFI at 77 K. SAR: (s) 50, (D) 100,and (d) 1600.

Table 1Dubinin–Radushkevich micropores volume, Vmic

SAR Vmic (cm3 g�1)

50 0.18100 0.151600 0.16

A.F.P. Ferreira et al. / Microporous and Mesoporous Materials 111 (2008) 171–177 173

q ¼ qsat

Kp1þ Kp

ð1Þ

However, the topology of MFI type zeolites consists ofintersecting straight and sinusoidal channels. The straightchannels are elliptical and have diameters equal to0.52 · 0.58 nm. The sinusoidal channels are nearly circularwith a diameter of 0.54 nm [16]. The intersections have adiameter of approximately 0.9 nm [17]. This means thatthere are two different possible adsorption sites on MFI.Therefore, a two-site Langmuir model can be required todescribe the adsorption on MFI type structures

q ¼ qsat;C

KCp1þ KCp

þ qsat;I

KIp1þ KIp

ð2Þ

where q is loading (adsorbate concentration in the parti-cles), p is pressure (adsorbate concentration in the gas

phase), and the different saturation loadings are indicatedby qsat,C and qsat,I, referring to the channels and the inter-sections, respectively. KC and KI are the adsorption equilib-rium constants for the two sites.

In the Henry’s Law region (low pressures) adsorptionloading is directly proportional to the pressure, and canbe described as in the following equation

q ¼ KHp ð3Þ

The KH is known as Henry’s Law constant.

3. Results and discussion

3.1. Nitrogen equilibrium adsorption data

The N2 adsorption isotherms are presented in Fig. 3. InTable 1 is presented the micropores volume (Vmic) for threesamples. The volume of micropores was obtained by apply-ing the Dubinin–Radushkevich (D–R) equation to the N2

equilibrium data, see Table 1.From Fig. 3 and Table 1 it can be observed that only

small difference exists in the microporous volume of thethree samples. Furthermore, the small difference does notshow a systematic trend with the SAR value.

174 A.F.P. Ferreira et al. / Microporous and Mesoporous Materials 111 (2008) 171–177

3.2. Hexane Isomers equilibrium adsorption data

A systematic set of experimental data on single compo-nent adsorption equilibrium and differential heats ofadsorption, of n-hexane, 2-methylpentane, 2,3-dimethylbu-tane and 2,2-dimethylbutane, on MFI zeolite, at 423 K, fora pressure range from 0.01 kPa to 250 kPa, was obtained.

The equilibrium experimental results and the Langmuirmodel isotherms are presented in Fig. 4. The adsorption ofthe hexane isomers depends on the component itself and onthe Si/Al ratio of the sample. This can be explained by dif-ferent adsorbent–adsorbate interactions. These interactionsdepend principally on the chemical composition of theadsorbent, on the adsorbate molecular geometry, and onthe pore structure. Tables 2–6 present some of the equilib-rium adsorption parameters. On previous work [18], datafrom other sample with SAR of 100 was compared with lit-erature data. On that comparison it was stated that the sat-uration loadings were slightly lower than the ones found inthe literature. If one compares the same literature data withthe results presented on this work, one can observe that thesample with SAR of 1600 presents saturation loadings nearthe ones found in the literature. Therefore, the silica to alu-mina ratio is an important factor (besides temperature) tohave in consideration when comparing results with litera-ture. The heats of adsorption presented on this work arein accordance with ones available in the literature [18].

3.3. Henry’s constants

In Table 2 the Henry’s constants value for the four com-ponents in function of the SAR value are presented. It canbe clearly observed that this constant increases withdecreasing SAR value (increasing aluminium content).The presence of aluminium in the zeolite frameworkincreases the adsorbent–adsorbate interactions, increasingaccordingly the adsorbed amount at low pressures. A clearrelationship with branching degree and SAR value isobserved: the higher the SAR value and the degree ofbranching the lower the Henry’s constant. This suggeststhat the adsorbent–adsorbate interactions are higher whenaluminium is present within zeolite structure, and decreasesupon branching. This phenomenon might be related to thelocation of aluminium in the framework; however it is nota direct proof of it. Linear alkanes are reported in the liter-ature to adsorb preferentially in the straight channels,whereas the bulky branched isomers, due to their sidegroups prefer to sit on the channel intersections [19]. How-ever, there is no agreement in the literature over the loca-tion of the aluminium T atoms in the MFI zeolite structure.

3.4. Limiting heat of adsorption at low coverage

The limiting heat of adsorption at low coverage data,presented in Table 3, presents the same trends as theHenry’s constant. Consequently, the calorimetric data cor-roborates the relation presented previously for the adsor-

bent–adsorbate interaction strength with branchingdegree and Si/Al ratio.

3.5. Langmuir model

Tables 4 and 5 give the Langmuir isotherm parametersfor the adsorption of the four components on the three dif-ferent samples. The inflection point observed on the equi-librium isotherm of n-hexane on MFI with SAR 100 and1600 is clearly indicative of a dual site adsorption. There-fore, a dual site Langmuir model was used to fit the data,and the obtained parameters are reported in Table 6.

One can directly observe from Fig. 4 that the saturationloading for the four components decreases with aluminiumcontent in the zeolite. In view of the fact that there is nosignificant difference on the micropores volumes given bythe N2 adsorption data, one may conclude that aluminiumcontent has a direct influence on the total adsorptioncapacity of the zeolite for alkanes.

A non-uniform distribution of aluminium within thezeolite crystal (aluminium zoning) has been reported[10,11]. The aluminium concentrates near the exterior sur-face, which implies lower values than 25 for the SAR on theouter layers of the crystals. However, it has been reportedthat such phenomenon happens for primary crystals biggerthan 1 lm [11]. From Fig. 2a–c it can be observed that thesamples primary crystals tend to be on the nanometersscale only. Therefore, due to the small primary crystal size,one can assume that aluminium zoning cannot play amajor role in our results. Structure flexibility and phasetransition from orthorhombic to monoclinic could be otherfactors that play a role; both are influenced by the presenceof aluminium in the zeolite structure. The flexibility of theMFI structure is reduced when aluminium is introduced onzeolite [12], and phase transition depends on the tempera-ture and Si/Al ratio. For lower Si/Al ratios (high alumin-ium content) the temperature where the phase transitionoccurs is lower [14]. Although these factors could explainthe observed decrease for the more bulky molecules (mono-and di-branched isomers), they barely would explain the50% decrease for the linear hexane. In the case of n-hexanethe saturation loading drops from around 1.4 mmol/g(8 m.u.c.) to around 0.66 (slightly less than 4 m.u.c.). Theprevious values suggest that one of the sites becomesinaccessible for the n-hexane under the studied conditions(temperature and pressure). In addition, Fig. 5 presentsthe X-ray diffraction profiles for the three samples at423 K; the observed peaks for all the samples correspondto the orthorhombic phase [12].

In Fig. 4, the heats of adsorption for the four compo-nents on the three samples can be observed. n-Hexaneexhibits a small decrease up to loadings of 0.7 mmol/gfor the SAR 100 and 1600. From loadings higher than0.7 mmol/g the heats of adsorption present a small increasewith loading, this can be attributed to adsorbate–adsorbateinteractions that gradually increase with the packing ofmolecules in the zeolite pore structure. For SAR 50 the

q (m

mol

g-1

)q

(mm

ol g

-1)

0

0.4

0.8

1.2

1.6

0

0.2

0.4

0.6

0.8

q (m

mol

g-1

)

0

0.2

0.4

0.6

0.8

q (m

mol

g-1

)

0

0.2

0.4

0.6

0.8

0.01 0.1 1 10 1000

25

50

75

100

0 0.2 0.4 0.6 0.8

)

0

25

50

75

100

0

25

50

75

100

0

25

50

75

100

Q (kJm

ol -1)Q

(kJmol -1)

Q (kJm

ol -1)Q

(kJmol -1)

p (kPa) q (mmol g-1)

0.01 0.1 1 10 100 0 0.2 0.4 0.6 0.8p (kPa) q (mmol g-1)

0.01 0.1 1 10 100 0 0.2 0.4 0.6 0.8p (kPa) q (mmol g-1)

0.01 0.1 1 10 100 0 0.2 1.40.4 0.6 0.8 1.21p (kPa) q (mmol g-1)

Fig. 4. Adsorption isotherms and heats of adsorption on MFI at 423 K. (a) n-Hexane, (b) 2-methylpentane, (c) 2,3-dimethylbutane and (d) 2,2-dimethylbutane. SAR: (s) 50, (D) 100, and (d) 1600. Lines are the isotherm model fits by Eq. (1) for n-hexane and Eq (2) for branched isomers.

A.F.P. Ferreira et al. / Microporous and Mesoporous Materials 111 (2008) 171–177 175

heats of adsorption present a different behaviour; initiallythe heat of adsorption is constant and then as a rather pro-nounced peak that achieves its maximum at a loading of

0.7 mmol/g. This can be explained by sudden increase onthe adsorbate–adsorbate interactions, this might be dueto a structuring of the previously disordered adsorbate

Table 2Henry’s constants on MFI, KH (mmol g�1 kPa�1), at 423 K

Sorbate SAR

50 100 1600

n-Hexane 0.42 0.31 0.112-Methylpentane 0.36 0.23 0.152,3-Dimethylbutane 0.18 0.12 0.112,2-Dimethylbutane 0.30 0.11 0.11

Table 3Limiting heat of adsorption at low coverage on MFI, Q0 (kJ mol�1) at423 K

Sorbate SAR

50 100 1600

n-Hexane 75.74 71.43 69.322-Methylpentane 73.69 71.05 67.362,3-Dimethylbutane 69.74 70.20 67.122,2-Dimethylbutane 68.50 69.22 65.77

Table 4Langmuir adsorption equilibrium constant on MFI, K (kPa�1), at 423 K

Sorbate SAR

50 100 1600

n-Hexane 0.29 n.a. n.a.2-Methylpentane 0.33 0.55 0.362,3-Dimethylbutane 0.52 0.30 0.292,2-Dimethylbutane 0.84 0.33 0.32

n.a. – Not available, the single site Langmuir model was not used to fit thedata.

Table 5Langmuir saturation loading on MFI, qsat (mmol g�1), at 423 K

Sorbate SAR

50 100 1600

n-Hexane 0.66 n.a. n.a.2-Methylpentane 0.55 0.64 0.722,3-Dimethylbutane 0.40 0.60 0.692,2-Dimethylbutane 0.41 0.60 0.68

n.a. – Not available, the single site Langmuir model was not used to fit thedata.

Table 6Dual site Langmuir parameters on MFI for n-hexane, K and qsat, at 423 K

Parameters SAR

100 1600

qsat,c 0.69 0.70Kc 0.55 0.32qsat,i 0.70 0.69Ki 0.01 0.02

Inte

nsity

(a.u

.)

5 10 15 20 25 30 352θ (º)

Fig. 5. Influence of Si/Al ration on selected X-ray diffraction profile (—)Si/Al = 50, (—) Si/Al = 100, (—) Si/Al = 1600, at 423 K.

176 A.F.P. Ferreira et al. / Microporous and Mesoporous Materials 111 (2008) 171–177

phase (it may be thought of as being from a disorderedfluid phase to a lattice fluiddlike phase [20,21]. For themono- and di-branched isomers the heat of adsorptiontends to monotonously decrease with loading. This

decrease is clearer for the 2,2-dimethylbutane. However,for 2-methylpentane on MFI with SAR 50 a peak similarto the one exhibited by n-hexane on the same sample canbe observed. This might indicate that the two moleculesadsorb on the same site and interact with the zeolite struc-ture on similar ways.

4. Conclusions

The aluminium content influences the adsorption behav-iour for the hexane isomers that have been studied on thiswork. The Henry’s constant and limiting heat of adsorp-tion at low coverage increases with the amount of alumin-ium on the zeolite framework. Such phenomena can beinterpreted by the adsorbate–adsorbent interactions; thesewill be higher when aluminium is included in the zeolitestructure. On the other hand the total loading decreaseswith increasing aluminium content, particularly for the n-hexane case. For this isomer the saturation loading dropsto almost half of the value presented by the almost pure sil-icalite sample (SAR = 1600). Furthermore, the heats ofadsorption are in agreement with the adsorption equilib-rium data. The SAR value is an important factor whencomparing results with the literature, only data from sam-ples with similar SAR value can be compared.

Acknowledgment

This research was carried out within the project CW/STW 349-5203. The authors acknowledge the StichtingTechnische Wetenschappen for their financial support.

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