TAP Study of Adsorption and Diffusion of 2,2,4 ... - ACS Publications

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TAP Study of Adsorption and Diffusion of 2,2,4-Trimenthylpentane and 2,5-Dimethylhexane on β and USY Zeolites Subramanya V. Nayak,*,†,‡ Palghat A. Ramachandran,† and Milorad P. Dudukovic† †

Chemical Reaction Engineering Laboratory (CREL), Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis (WUSTL), Missouri 63130, United States ‡ Artie McFerrin Department of Chemical Engineering, Texas A&M University, Jack E. Brown Engineering Bldg., 3122 TAMU Room 418 College Station, Texas 77843-3122, United States ABSTRACT: The dimensionless equilibrium constant, apparent heats of adsorption, intraparticle diffusivities, and activation energies are determined under high vacuum conditions for 2,2,4-trimethylpentane and 2,5-dimethylhexane. These are representative of the products formed in the alkylation process of isobutane
butene mixtures on solid acid catalysts such as β and USY zeolites. It is observed that in the straight and zigzag channels of β zeolites, dimethyl-branched C8 isoalkane is strongly adsorbed and diffuses at a slower rate in comparison to trimethyl-branched C8 isoalkane. In contrast, in the super cages of USY zeolites, both C8 isoalkanes demonstrate similar sorption and transport properties. On the basis of this understanding, it is concluded that the ideal solid acid catalyst for alkylation processes requires super cages of USY zeolite rather than zigzag channels of β-zeolites in order to increase product selectivity, decrease catalyst deactivation, and decrease the energy requirement for regeneration.

’ INTRODUCTION With today’s environmental concerns, the clean burning branched trimethylpentanes (C8 alkylates) are considered to be the premium standard for blending feed stocks in refineries. C8 alkylates have a high research octane number (RON ∼ 100) and motor octane number (MON ∼ 98); contain virtually no olefins, sulfur, or aromatics; and have a low Reid vapor pressure (RVP). Currently, 13
15% of the refined gasoline pool is made up of these C8 alkylates. From World War II until today, homogeneous catalysts such as hydrofluoric acid (HF) and sulfuric acid (H2SO4) have been successfully used in the alkylation of isobutane and C3
C5 olefins to produce C8 alkylates.1 From a safety standpoint, H2SO4 has some advantages over HF. However, given the inherent toxicity and environmental hazards associated with these acid catalysts, these World War II vintage processes should be replaced with “greener” ones. One promising alternative involves zeolites, specifically largepore β and USY zeolite.2
4 The remarkable ability of these zeolites to behave as solid acid catalysts enables them to replace mineral acids.4
7 Furthermore, owing to the molecular dimensions of their nanopores, these zeolites possess the peculiar property of molecular shape selectivity, which can increase C8 alkylate yield and selectivity.5 On the downside, zeolites deactivate with time on stream (TOS) and require periodic regeneration for continuous operation.6,8 To further enhance the science of zeolite-catalyzed alkylation processes, we need to examine the consequences of zeolite morphology on the diffusion and adsorption
desorption dynamics of the major products formed from alkylation reactions. The understanding gained will help in achieving better catalyst design and in determining optimal operating conditions. This study uses temporal analysis of products (TAP) pulse response experiments under high vacuum conditions to examine and quantify the transport and sorption of dimethyl-branched r 2011 American Chemical Society

and trimethyl-branched C8 isoalkanes, in β and USY zeolites. Although these experiments are conducted under high vacuum conditions which do not mimic the true industrial systems, they offer the following advantages: 1 They provide a direct estimate of transport and sorption processes at very low surface coverage (θi f 0). Consequently, the intraparticle diffusivity and adsorption
desorption constants estimated from the single pulse TAP experiments can be considered as values at zero surface coverage.9
11 2 The experiments are conducted in the absence of an inert carrier stream, with no external mass transfer resistance, and with a negligible thermal effect. This increases the reliability of estimated parameters12,13 as representing only zeolite isoalkane interactions. 3 The use of a thin zone TAP reactor configuration enables the use of small, commercially available zeolite particles without causing high bed resistance.2,9 TAP Pulse Response Experiments. The TAP experiments can be performed using single pulse or multipulse experimental protocols.14 In this study, single pulse TAP experiments are performed. The zeolite under investigation is packed in the microreactor using a thin zone reactor configuration.9 Here, β or USY zeolite particles (mass = 5 mg and mean diameter = 5 μm) are packed between two zones of nonporous quartz particles (mass = 800 mg and mean diameter = 200 μm). The packed microreactor is then housed in a high vacuum chamber. A Special Issue: Nigam Issue Received: March 19, 2011 Accepted: May 19, 2011 Revised: May 17, 2011 Published: May 19, 2011 1570

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Figure 1. Pictorial representation of single pulse TAP response experiments and sorption processes on zeolite boundary and transport in zeolite’s intraparticle space. The thin zeolite zone is sandwiched between two inert zones of nonporous quartz particles.

Table 1. Chemical and Physical Properties of β and USY Zeolites properties Si/Al ratio (mol/mol) average crystal size (μm) average agglomerate size (μm)

β 13.3 0.30 0.50

USY 2.90 0.25 0.50

total surface area (m2/ g) surface area with pores 0.2 nm (m2/ g)

64

55

total pore volume (cc/g)

0.286

0.312

pore volume with pores 0.2 nm (m2/ g) acidity (μmole NH3/g catalyst)

0.064 291

0.061 466

high-speed pulsing valve is used to inject a small number of probe molecules (1014 to 1018 molecules/pulse) at the entrance of the microreactor, and the probe molecules are evacuated at the other end (see Figure 1). A computer controlled quadruple mass spectrometer is used to record the mass exiting the microreactor as a function of time. Materials. β-zeolite with a Si/Al ratio of 13 and USY zeolite with a Si/Al ratio of 3 are chosen in this study, as they have in the past shown high product selectivity and longer catalyst activity for alkylation processes.15 The β zeolite has straight (pore diameter is 0.66  0.71 nm) and zigzag (pore diameter is 0.56  0.56 nm) channels.16 In contrast, USY zeolite has super cages (super cages size 1.24 nm) with a window opening of 0.74 nm.16 The chemical composition and basic physical properties of both zeolites are shown in Table 1. For the TAP experiments, the zeolites are calcined in situ in the microreactor at 673 K for 60 min. The major products formed from the alkylation reactions are highly branched trimethylpentanes with high octane numbers.7,15 However, dimethylhexanes having lower-octane blend values are also present in the product stream at varying levels.15 It is of interest to understand how the different zeolite morphologies

influence the intraparticle diffusivities and adsorption
desorption dynamics of these C8 isoalkanes. In this study, 2,2,4trimethylpentane (2,2,4-TMP) with 100 RON (Research Octane Number) and 2,5-dimethylhexane (2,5-DMH) with 56 RON are tested. Argon is used as an inert tracer to provide the information on transport in the interparticle voids. Thin Zone TAP Model. A detailed description of the thin zone TAP model and the assumptions used to develop this model can be found in the literature;2,9 only a brief description of the model equations is given here. The thin zone TAP reactor consists of three zones. The first and the last zone contain nonporous quartz, which are considered to be inert. Zeolite particles are placed between these two zones, as shown in Figure 1. In the inert zone, only interparticle diffusion occurs, and the dimensionless equation of continuity is Dci Dci ¼ 2 Dτ Dξ

ð1Þ

Here, ci is the pulse-normalized bulk concentration of species i, ξ is the dimensionless length of the reactor, and τ is dimensionless time. In the thin zeolite zone, in addition to interparticle diffusion, adsorption
desorption and diffusion inside the zeolite particle are considered. It is assumed that the probe molecule in the thin zeolite zone first adsorbs reversibly on the exterior surface of the particle (see Figure 1). The dimensionless equation of continuity in the thin zeolite zone then is Dci D2 ci  ¼ 2
ð1
εb Þkdi ½Keqi ci
θi jη ¼ 1 Dτ Dξ

ð2Þ

Here, θi is the pulse-normalized dimensionless surface coverage. It is the product of dimensionless surface coverage and the ratio of the maximum concentration of adsorption sites to the pulse intensity, defined as θi = θi(qmax)/(Ni/ALεb). To complete the model, we have to describe the behavior within the zeolite particles. It is assumed that only the molecules adsorbed at the zeolite particle exterior diffuse inside, as shown in Figure 1. Then, for a spherical geometry, the dimensionless mass 1571

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Industrial & Engineering Chemistry Research balance in the intraparticle space is " # Dθi D2 θi 2 Dθi ¼ τ pi þ Dτ Dη2 η Dη

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ð3Þ

dimensionless time constants

Equations 1
3 are the governing equations of the thin zone TAP model. The initial and boundary conditions required to solve these governing equations are explained next. At the inlet of the microreactor, it is considered that the flux is nonzero (Dirac pulse) for a very short time ( Keq i ð2, 2, 4-TMPÞ

ð11Þ

The lower value of Keqi in β zeolite for 2,2,4-TMP than for 2,5DMH indicates that the trimethyl-branched isoalkanes were weakly adsorbed in comparison to dimethyl-branched isoalkanes. This behavior of preferential adsorption of the dimethylbranched isoalkanes in β zeolite has been observed previously.23

17.5

9.204
13.97

567

Table 5. Apparent Heats of Adsorption (ΔH) Obtained from This Study and Reported in the Literature, kJ/mols C8 isoalkanes

19.5

542 569

518

USY

6.22

8.021
8.712

For USY zeolite, the estimated Keqi for 2,2,4-TMP was generally lower than for 2,5-DMH. However, the differences in the estimated Keqi values for 2,2,4-TMP and 2,5-DMH are minor in comparison to the ones observed in β zeolites. The temperature dependency of the obtained dimensionless equilibrium constant (Keqi ) is represented by van’t Hoff’s equation. Table 5 reports the estimated apparent heat of adsorption with standard deviation from this study and from the literature. It is found that the apparent heats of adsorption estimated here are lower than what is reported in the literature. The primary reason behind this discrepancy is because of the difference in the definition of Keqi used here and in the literature.9 This is discussed below. In the various experimental techniques reported in the literature for evaluation of the equilibrium constants, the values of the constants are obtained directly from the ratio of the adsorbed to bulk normalized concentrations when global equilibrium is established. In contrast, in TAP experiments, we cannot capture the equilibrium values directly, but we can obtain the ratios of the characteristic times for the adsorption and desorption and from 1575

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Table 6. The Values of the Dimensionless Parameter τpi (with the Standard Deviation and the 95% Confidence Interval), the Intraparticle Diffusivity (Dei /R2p), and Activation Energies (Ea) Obtained from the Arrhenius Equation C8 isoalkanes 2,2,4-TMP

2,5-DMH

2,2,4-TMP

2,5-DMH

a

zeolite β

β

USY

USY

T, K

τpi ( 10
5)

s.d. ( 10
6)

95% confidence interval ( 10
5)

(D0ei /R2p) ( 10
5) s
1

Ea kJ/mola

556

0.317

0.16

0.292
0.356

0.71

57

606

0.537

0.20

0.347
0.654

1.25

(s.d. 10.4)

624

0.914

0.27

0.841
1.025

2.16

634

1.690

0.95

0.967
2.239

4.03

586

0.132

0.13

0.091
0.198

0.30

72

592

0.138

0.11

0.935
0.237

0.53

(s.d. 6.48)

606 708

0.297 1.960

0.12 4.43

0.201
0.360 1.091
2.835

0.70 4.92

518

0.278

0.16

0.183
0.339

6.04

26.65

543

0.315

0.19

0.253
4.169

7.00

(s.d. 3.58)

570

0.375

0.13

0.230
0.501

8.50 9.48

591

0.411

0.12

0.303
5.855

470

0.201

0.16

0.119
0.257

4.47

24.5

546

0.294

0.13

0.255
0.521

7.03

(s.d. 1.67)

638

0.580

0.72

0.401
0.750

12.9

Note: s.d. is the standard deviation

them evaluate the equilibrium constant. The problem in obtaining direct values lies in the fact that no independent information is available on qmax, which represents the maximum sites available at zero coverage. As the temperature increases, zeolites have a tendency to convert Lewis sites to Brønsted sites,24,25 and at the low surface coverage, the adsorption of alkanes occurs only on the Brønsted sites.23,26 Furthermore, the accessible qmax also varies with temperature; for instance, as the temperature increases, the molecular mobility increases and more active sites can be reached. As a result of these effects, the qmax rises with the temperature. While the ratio of the adsorption and desorption constants decreases with the temperature, the rise in qmax with temperature reduces the apparent heats of adsorption estimated here. However, the trends observed here for heats of adsorption on various zeolites for different species are similar to what is reported in the literature. For example, we estimated that the heat of adsorption for 2,2,4-TMP is higher in β than USY zeolites; a similar observation is also reported by Denayer et al.23 Intraparticle Diffusion Time (Dei /R2p)
1. Table 6 reports the dimensionless parameter τpi (τpi = (L2/DKi )/(R2p/Dei )) estimated for 2,2,4-TMP and 2,5-DMH when pulsed over β and USY zeolites. The values reported in Table 6 fall inside the model sensitivity range. The characteristic time for intraparticle diffusion (R2p/Dei ) was then estimated at each temperature by multiplying the τpi obtained for the model match of experimental results by the characteristic diffusion time in the microreactor (L2/DKi ). It is observed that both the dimensionless parameter τpi and intraparticle diffusivities are much higher for trimethyl-branched isoalkane than dimethyl-branched isoalkane in β zeolite: τpi ð2, 2, 4-TMPÞ > τpi ð2, 5-DMHÞ

ð12Þ

and ðDei =Rp2 Þð2, 2, 4-TMPÞ > ðDei =Rp2 Þð2, 5-DMHÞ

ð13Þ

The strong adsorption of 2,5-DMH compared to that of 2,2,4TMP in β zeolite decreases its mobility from site to site, and this effect is manifested by a lower intraparticle diffusivity value. It is

concluded that in β zeolite, the molecular shape selective effects perturb the intraparticle transport behavior for C8 isoalkanes. In contrast, both the dimensionless parameter τpi and characteristic intraparticle diffusivity time of 2,2,4-TMP and 2,5DMH have the same order of magnitude in USY zeolite. This resemblance further verifies that in the large pore USY zeolite with super cages, C8 isoalkanes have similar mobility from one isolated site to another. It was concluded that due to the nonshape selective nature of USY zeolite toward C8 isoalkanes, the difference in estimated intraparticle diffusivity values is minor. Tables 6 also reports the activation energies for the C8 isoalkanes obtained using the Arrhenius equation. It was observed that in the β zeolite the activation energy for 2,2,4-TMP is lower than for 2,5-DMH, indicating lower temperature dependency. For USY zeolite, it was observed that both 2,2,4-TMP and 2,5-DMH have similar temperature dependencies. Furthermore, when the activation energies for the same molecules are compared for different zeolites, it was observed that Ea ðβ zeoliteÞ > Ea ðUSY zeoliteÞ

ð14Þ

This observation indicates that the stronger dispersive interaction caused by the relatively shorter distance between the adsorbed molecules and β zeolite framework results in increased activation energy.

’ DISCUSSION AND CONCLUDING REMARKS In the straight and zigzag channels of β zeolite, the adsorption of probe molecules takes place inside the channels. As a result, when the molecular dimension is similar to the pore dimension of the β zeolite, shape selectivity is observed in the pulse reponses. For example, in β zeolite of this study, the average straight channel pore diameter is 0.69 nm and the kinetic diameters of 2,2,4-TMP and 2,5-DMH are 0.6547 and 0.6511 nm, respectively. The slightly smaller and less branched 2,5-DM is more strongly adsorbed. Denayer et al.23 reported similar behavior, as they found higher estimates of Henry’s constants for C8 isoalkanes compared to C8 alkanes. 1576

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Industrial & Engineering Chemistry Research In contrast, the average pore diameter (0.74 nm) of USY zeolite allowing access to the super cage is considerably larger than the kinetic diameter of the 2,2,4-TMP and 2,5-DMH. Thus, these molecules can easily access the super cages. As a result, the mobility of the C8 isoalkanes with different molecular structure becomes to some extent comparable in the super cages of USY zeolite.23 This explains why there are only minor differences in the estimates of the dimensionless constants for 2,2,4-TMP and 2,5-DMH in USY zeolite. On the basis of the understanding gained so far from TAP experiments conducted under high vacuum conditions, we suggest that, to achieve higher selectivity for 2,2,4-TMP, the super cages of USY zeolite are more beneficial than the straight and zigzag channels of β zeolite. β zeolite has a stronger affinity for the undesired 2,5-DMH than for the desired 2,2,4-TMP. As a result, β zeolite is more likely to form 2,5-DMH than USY zeolite. In USY zeolite, the adsorption and transport characteristics of 2,2,4-TMP and 2,5-DMH are similar. This also explains the observation of Sarsani,15 who reported better selectivity to 2,2,4-TMP for the alkylation reaction catalyzed by USY zeolite than β zeolite, under otherwise identical experimental conditions. Further, on the basis of the estimated heats of adsorption and activation energies for diffusion, it appears that the USY zeolite will require less energy for regeneration than β zeolite.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ1 979 204 3462. Fax: þ1 979 845 6446. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. John Gleaves from Washington University in St. Louis for making the TAP instrument available for this research and the National Science Foundation (Grant EEC-0310689) and the Center for Environmentally Beneficial Catalysis (CEBC) for financial support ’ NOMENCLATURE A = cross-sectional area of microreactor, m2 Ci = bulk concentration of species i, mol/m3 ci = pulse-normalized bulk concentration, ci = Ci/(Ni/ALεb) Dei = effective diffusivity in the intraparticle space defined as Dei = εp/ςp DTi m2/s DKi = effective Knudsen diffusivity, m2/s DTi = Fickian diffusivity, m2/s Ea = activation energy, kJ/mol Ei(t) = time dependent intensity measured by the mass spectrometer, Mv/s = dimensionless exit flow F*(t) i ΔH = apparent heat of adsorption, kJ/mol kai = adsorption constant, m3/mol 3 s kdi = desorption constant, 1/s k*di = dimensionless desorption constant Keqi = dimensionless equilibrium adsorption constant L = microreactor length, m Mwi = molecular weight of species i, kg/mol Mj = jth moment Ni = number of moles qi = adsorbed phase concentration, mol/m3 qmax = maximum concentration of adsorption sites, mol/m3

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Rp = zeolite particle radius T = Temperature, K t = observation time, s Greek Letters

θi = dimensionless surface coverage or fraction surface coverage, θi = (qi/qmax) θi = pulse-normalized surface coverage, θi = θi (qmax/(Ni/ALεb)) δ(t) = delta function δ*(τ) = pulse-normalized delta function, δ*(τ) = (εbL2)/(DKi )δ(t) τ = dimensionless time, τ = (DKi t)/(εbL2) τpi = dimensionless particle time ξ = dimensionless spatial distance within the microreactor, ξ = (z)/(L) η = dimensionless spatial distance within the zeolite particle εb = solid holdup εp = zeolite particle porosity ζp = particle tortuosity

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