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Coupled hydrocarbon desorption in zeolite Beta-containing monolithic catalyst Po-Yu Peng, Zhiyu Zhou, Michael P Harold, and Dan Luss Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02844 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018
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Industrial & Engineering Chemistry Research
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Coupled hydrocarbon desorption in zeolite Beta-
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containing monolithic catalyst
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Po-Yu Peng, Zhiyu Zhou, Michael P. Harold*, Dan Luss*
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Department of Chemical & Biomolecular Engineering, University of Houston, Houston 77204-
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4004, United States
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EMAIL:
[email protected] (M.P. Harold),
[email protected] (D. Luss)
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Abstract
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The 3-D, large-pore zeolite Beta (BEA) is effective in trapping large hydrocarbons (HCs). We conducted temperature-programmed desorption (TPD) experiments of a BEA-containing monolithic catalyst pre-stored with various mixtures of propylene, hexane, and toluene. The addition of propylene to a hexane + toluene mixture results in an unusual interaction during TPD. A rather small concentration of propylene increases the desorption temperature of the hexane from a weakly held into stronger bound component. A variation in the propylene concentration leads to the nonmonotonic dependence of a high temperature hexane desorption peak. The data suggest that the blockage of hexane desorption by propylene depends on the uptake time. The findings complicate the analysis and design of HC traps used in applications such as vehicle aftertreatment systems.
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Keywords: Zeolite; Hydrocarbon Trap; Sorption; Beta; Diffusion; Hexane;
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Introduction
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Diesel vehicle exhaust contains HCs pollutants spanning low molecular weight alkanes
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and olefins, aromatics, to intermediate and high molecular weight paraffins. Hydrocarbon
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trapping by zeolites is used to mitigate the release of HCs from vehicular exhaust during 1 ACS Paragon Plus Environment
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warmup. The selectivity of a HC trap is determined by the shape and size of the channels. Large,
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linear HCs are readily trapped by a large pore zeolite such as Beta (BEA), a 3-D, 12-member
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ring zeolite. One of the channels is sinusoidal and orthogonal to a plane containing orthogonal,
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intersecting straight channels. The sinusoidal channels have dimensions of 5.6 x 5.6 Å while the
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straight channels are 6.6 x 6.7 Å. Bárcia et. al [1] examined the uptake of 6-carbon HC isomers
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on BEA and showed that the isomers exhibit varying storage capacities on different BEA storage
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sites. For example, the branched 2,3-dimethylbutane and 2,2-dimenthyl butane tend to adsorb in
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the straight channels, but the linear n-hexane and 1 branched 3-methylpentane are able to access
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any of the channels [1-3]. The site fractions of straight, sinusoidal, and intercept sites for n-
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hexane adsorption equilibrium on zeolite Beta are estimated as 56, 31, and 21% respectively [3].
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The coupled contributions of both zeolite topology and acidity make it difficult to
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characterize and understand hydrocarbon uptake and diffusion for a multicomponent HC
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mixture. To gain a deeper understanding of adsorption on zeolites it is important to use
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multicomponent mixtures representative of the application, such as in vehicle HC traps.
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Westermann et. al [4] studied the adsorption of propylene, toluene and decane on HY zeolites
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with a Si/Al ratio ranging from 2.5 to 100 under dry and wet conditions. On the less acidic sites
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in the high Si/Al zeolite, the hydrocarbons were trapped by the weaker van der Waal interaction.
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High Si/Al zeolites had a higher adsorption capacity for larger hydrocarbons, but have a low
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affinity for water because of their hydrophobicity. They also have a negligible hydrocarbon
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cracking activity. The influence of zeolite topology and acidity on ternary HCs adsorption was
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examined on different zeolite types (FAU, BEA, MOR, MFI, FER and LTA) [5]. The authors
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concluded that the 1-D zeolite is more suitable for cold-start application which is less selective to
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all types of HCs. 2 ACS Paragon Plus Environment
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In this study we report the coupled uptake and release features of a ternary HC mixture
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(propylene, hexane, and toluene) on a high Si/Al ratio (>150) BEA-containing-washcoated
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monolith catalyst via temperature-programmed desorption (TPD) experiments with pre-stored
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HCs. An unexpected, additional hexane desorption peak appeared at higher temperature when
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propylene was co-adsorbed. Experiments were conducted for various propylene concentrations
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to investigate the reason for the second hexane peak formation and the impact of propylene. The
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data suggest that this behavior originates from the blocking of the zeolite pores necessary for
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desorption of the other species.
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Experimental
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Materials
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The washcoated 400 cpsi cordierite monolith contained 0.7 g/in3 precious group metals
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(PGM), 0.5 g/in3 Beta-zeolite (BEA), and had a silica to alumina ratio (SAR) of 280 (provided
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by BASF; Iselin, NJ). The rather high SAR of this sample (D-0.5) suggests that the zeolite
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acidity is small. The Pt + Pd bimetallic mixture was 2:1 by mass (~1:1 atomic ratio) and
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supported on γ-Al2O3. The 1-inch diameter cylindrical sample was 3-inch long. A second sample
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(N-0.5) without precious group metals (PGM) was used. Detailed information (such as BET area)
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of the two samples is reported in the Supplementary Material section [6, 7]. Straight-chain
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isomer (n-) hexane (C6H14, >99%) and aromatic toluene (C7H8, >99%) were purchased from
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Sigma-Aldrich (St. Louis, MO). The certified 5% propylene balanced by Ar was purchased from
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Matheson Gas.
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Methods
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The uptake of HCs on the Beta zeolite-containing catalyst was determined in a bench flow
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reactor system. The liquid HCs (hexane and toluene) were generated by a custom vaporization 3 ACS Paragon Plus Environment
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system comprising a needle valve and a liquid syringe pump (Cole-Parmer, IL) with a 10 mL
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capacity glass gas-tight syringe (SGE, TX) [8]. Propylene, O2, and carrier gas Ar were metered
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by mass flow controllers (MKS). The effluent concentration of HCs and CO2 were measured by
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a FTIR (MKS MultiGas™ 2030 Gas Analyzer). The detection limits of propylene, hexane and
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toluene are 1.0, 1.0 and 6.1 ppm respectively, estimated as 50 times the standard deviation in
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pure Argon. The feed and outlet temperatures were measured by coherent optical frequency
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domain reflectometry (c-OFDR; model 4600), developed by Luna Innovation Inc. of Roanoke,
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Virginia [9]. Details and applications of c-OFDR are described elsewhere [8, 10-14]. Near
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adiabatic operation was achieved using a customized temperature control system. Additional
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details of the setup are reported elsewhere [8, 14].
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All experiments were conducted with a total gas flowrate of 8 L/min (GHSV of 12,630 L
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gas/L monolith h-1). The hexane and toluene concentrations were fixed at 320 ppm. The
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propylene feed varied from 350 to 4,000 ppm in several increments. The samples before the
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uptake of HC were pre-treated at 500 oC under a 10% O2 feed for an hour to remove any HCs
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residue and then cooled to the ambient temperature. Temperature programmed desorption (TPD)
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was conducted to evaluate the HC uptake and binding over a range of temperatures. HCs were
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stored on the BEA-containing catalyst at the initial ambient temperature before the start of the
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TPD. Several adsorption times between 20, 45, 60, 75 and 90 min were examined. The elution
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was initiated by pure Ar purging to remove the weakly bound HCs. Elution times of 20, 90 and
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240 min were conducted. The TPD was conducted at a constant ramp rate of 10 oC/min up to a
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terminal temperature of 300 oC.
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Results and discussion
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Temperature programmed desorption of ternary mixture of hydrocarbons
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A typical uptake, elution, and temperature-programmed experiment is shown in Figure 1.
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The vertical left axis indicates the hydrocarbons species concentration and feed temperature
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while the right vertical axis represents the propylene concentration. The horizontal axis is time
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over the course of the uptake, elution, and TPD; the process phases are separated by vertical
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dashed lines. The data show breakthroughs for each of the hydrocarbon species. While the
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toluene breakthrough monotonically increases after the toluene is turned off, the hexane shows
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an overshoot shortly after its breakthrough. This effect has been reported previously [15]. The
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faster diffusing hexane initially occupies the adsorption sites. However, the slower diffusing
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toluene causes the desorption of the hexane due to its stronger binding to the zeolite.
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Figure 2 (a) shows the hexane and toluene effluent concentrations during the TPD after
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their storage. The purple and red lines respectively correspond to the feed and outlet temperature
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under a linear temperature ramp. After the start of TPD, the hexane and toluene concentrations
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increase and reach their maxima at ~6 min and a feed temperature of 98 oC. The desorbed
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amounts of hexane and toluene, which desorb at the same temperature, are 4.4 and 10.0
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mg/g_absorbent, respectively. These desorption amounts closely match the measured HC uptake
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amounts, indicating that the intact HCs were completely released from the adsorbent.
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Figure 2 (b) exhibits the propylene, hexane and toluene effluent concentrations during
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TPD after their joint storage. The hexane and toluene release first at a similar time and
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temperature, as in Figure 2 (a). However, an unexpected additional peak of hexane is observed at
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~15 min at a feed temperature of 178 oC. The higher desorption temperature implies that this
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hexane fraction was more strongly bound to the adsorbent.
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The propylene effluent concentration, shown as the yellow line in the inset plot of Fig. 2
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(b), shows a maximum concentration of only 3 ppm. Despite this very small amount of adsorbed
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propylene compared to other co-feed species, a significant shift in the desorption behavior of
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hexane is observed. It is unlikely that the shift is a result of a chemical reaction involving
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propylene oligomerization (to hexane), which would be more likely for a much lower silica to
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alumina ratio (
