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Remediation and Control Technologies

Microwave-assisted catalytic combustion for the efficient continuous cleaning of VOC-containing air streams Hakan Nigar, Ignacio Julian, Reyes Mallada, and Jesús Santamaría Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00191 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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Environmental Science & Technology

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Microwave-assisted catalytic combustion for the

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efficient continuous cleaning of VOC-containing air

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streams

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Hakan Nigar,† Ignacio Julián,† Reyes Mallada,*,†,‡ Jesús Santamaría*,†,‡

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† Nanoscience Institute of Aragon and Chemical and Environmental Engineering Department,

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University of Zaragoza, 50018 Zaragoza, Spain

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‡ Networking Research Centre CIBER-BBN, 28029 Madrid, Spain

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Adsorption, Catalytic oxidation, Volatile organic compounds, Microwave heating

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ABSTRACT: A microwave-heated adsorbent-reactor system has been used for the continuous

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cleaning of air streams containing n-hexane at low concentrations. Both a single catalytic bed

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(PtY zeolite) and a double (adsorptive DAY zeolite + catalytic PtY zeolite) fixed-bed reactor

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configurations were studied under dry and humid conditions. The zeolites were selectively

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heated by short periodic microwave pulses that caused the desorption of n-hexane and its

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subsequent catalytic combustion. The double bed configuration was attractive because it allowed

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nearly the same performance with only half of the catalyst load. The operation was especially

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efficient under realistic humid gas conditions that favored more intense microwave absorption,

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producing a faster heating of the adsorptive and catalytic beds. Under these conditions,

ACS Paragon Plus Environment

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continuous gas cleaning could be achieved with short (3 min, 30 W) microwave heating pulses

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every 5 min.

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INTRODUCTION

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Among air pollutants, Volatile Organic Compounds, VOCs, are significant contributors to

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poor air quality in the cities, both indoor and outdoor air. These low vapor pressure pollutants are

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released to the atmosphere by both biogenic and anthropogenic sources, e.g., vehicles, processes

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using solvents and industry. In 2014, according to the European Environment Agency, EEA, 8

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billion tons of VOCs were emitted only in Europe. Although the emissions of many air pollutants

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have decreased substantially in Europe over the last decades, this is still a major concern due to

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their participation in atmospheric photochemical reactions and their contribution to ozone

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formation and potential toxicity.1

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The indoor emissions of VOCs are of even higher concern since we spend almost 90% of our

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time in indoor environments such as houses, offices, shops, public buildings, and vehicles. Inside

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buildings, VOCs are continuously released to indoor air from different sources, i.e., tobacco

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smoke, cleaning products, furniture (e.g., varnish and glues) and office equipment (e.g., printers

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and computers).2-4 For large buildings where a substantial proportion of air is recirculated to save

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energy in air conditioning, the concentration of VOCs can reach high levels in comparison to the

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outdoor values. In particular, high VOCs concentrations present inside buildings could lead to

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the so-called “Sick Building Syndrome” a medical condition in which building occupants suffer

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from some symptoms of illness or feeling unwell gratuitously,5 which can be linked to time spent

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inside a building.

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The concentration of VOCs in air can be lowered to acceptable levels using either recovery

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technologies (condensation, membrane separation, adsorption, absorption) or destruction


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technologies, mainly thermal or catalytic oxidation.6-9 The choice of technology is case-specific

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and depends not only on the type of pollutant but also on its concentration, the main factor

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regarding the economics of the VOC removal process.10 Thus, when VOCs concentrations are

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high, as is often the case of industrial emissions, thermal oxidation at high temperatures, i.e.,

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1000 – 1600 K,11 can be considered since most or all of the energy required to achieve

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combustion temperatures can be obtained from VOCs combustion. In addition, the thermal

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energy of process gases can be recovered either by regenerative or recuperative systems.

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Regenerative thermal oxidation system is recommended for streams between 2000 and 100,000

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Nm3/h, and concentration of pollutants in the range 0.3 - 10 g/Nm3. Since a high (>95%) thermal

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efficiency can be achieved in heat recovery, the consumption of fuel is minimized.12

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However, for dilute concentrations (6 hours) sample 1.8 ± 0.2 nm (150 particles measured in both cases). Therefore, there is

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no appreciable change in particle size after microwave heating. TEM results are in accordance

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with XRD data of non-microwave-heated sample, in which the Pt peak was not distinguishable

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in XRD diffractogram due to the small size of Pt particles.

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Figure 3. TEM images of a PtY sample a) before and b) after microwave heating (30 W, >6

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hours), and statistical analysis of the Pt particle size distribution.

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Nitrogen adsorption isotherms and DFT pore size distributions of NaY and ion-exchanged PtY

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zeolites are shown in Figure 4. They correspond to a Type I isotherm, which is characteristic of


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microporous materials according to the classification of IUPAC. This technique allows to assess

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any changes in the porosity of the crystalline structure that could have been produced during the

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ion exchange and also after several catalytic cycles, see Table 1. Obtained BET surface areas

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were 948 m2/g zeolite and 909 m2/g zeolite and micropore volumes were 0.36 cm3/g and 0.33

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cm3/g for NaY and PtY zeolites, respectively. Both surface area and the microporous volume

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were slightly reduced after the ion-exchange. This could be due to the effect of some Pt

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aggregates, formed during the ion-exchange process.

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Table 1. Textural properties of NaY and PtY zeolites BET Surface Area

Total Pore Volume

(m2/g)

(cm3/g)

NaY

948

0.36

PtY, after ion-exchange

909

0.33

PtY, after combustion

810

0.29

Sample

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In Table 1, it could be observed that surface area and pore volume of zeolite were reduced after

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several (more than 100) catalytic cycles of combustion. This could be to the hydrothermal

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stability of the zeolite Y, the cubic FAU lattices (12.7 T/nm3) undergo dramatic changes of their

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framework beginning at 423 K, i.e., at relatively mild conditions. In our case, during the

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combustion process these temperatures were achieved under humidity conditions. The deep

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analysis of XRD for the sample after combustion, see Figure 2 zoom in, also showed that there is

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a slight loss in crystallinity showed by a broadening of the peak at 23.5º. However, these loss of

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crystallinity and reduced pore size are not significant.


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Figure 4. Nitrogen adsorption isotherms of NaY and PtY zeolite before and after catalytic tests,

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inset: pore size distribution by DFT analysis.

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The amount of Pt incorporated onto the zeolite catalysts was determined by atomic absorption

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emission spectroscopy (AAS). Five PtY samples prepared at the same conditions were analyzed

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after being digested with aqua regia under MW heating. Pt content is 1.53 ± 0.21 wt.%.

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Catalytic bed, cyclic operation: Adsorption followed by catalytic combustion assisted by MW

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The initial purpose was to study in detail the capacity of the catalytic bed for the combustion of

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desorbed n-hexane by microwave heating starting from a “cold” bed where n-hexane had been

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adsorbed during the loading stage before MW activation (see Figure 1 c). The PtY catalyst bed

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load was 400 mg. The inlet gas flow, 100 mL STP/min, which contains 500 ppm of n-hexane,

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was fed into the fixed-bed for 5 min. All the n-hexane fed during the adsorption stage remained

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on the zeolite bed since no breakthrough of hexane was observed during this period (Figure 5),

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i.e., the total n-hexane load was 2.2 mg/g catalyst. After the predetermined loading period, the


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bed was swept by synthetic air (100 mL STP/min) for 5 min and then the microwave source at 30

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W input power, was switched on for 10 min.

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The evolution of n-hexane, carbon dioxide concentrations at the outlet flow and the catalyst

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bed temperature are presented in Figure 5. After MW activation, the temperature increases

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sharply accompanied by the combustion of the desorbed pulse of n-hexane. From the evolution

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of the carbon dioxide peak in Figure 5, it can be concluded that n-hexane combustion takes place

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in approximately 70 seconds, and no breakthrough of unconverted n-hexane is observed. After

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10 min of MW irradiation the bed is allowed to cool down and then a new cycle of microwave

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irradiation takes place while passing N2 as a purge gas to make sure that no adsorbed n-hexane

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remains on the bed, and indeed Figure 5 shows that the bed has been successfully regenerated in

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the combustion step since there is no n-hexane detected at the outlet stream. This completes the

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cycle and after cooling the system is ready for the following cycle. The cycle was repeated three

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times, and 100% conversion of n-hexane was achieved in each of them under these conditions.

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Figure 5. Evolution of n-hexane, carbon dioxide concentration and the catalytic bed temperature

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at the different stages of the cycle: adsorption, flushing with air and microwave activation

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(power: 30 W). Conditions: single-component fixed bed; load: 400 mg PtY zeolite, total flow:

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100 mL/min, 500 ppm n-hexane, 5 min loading time, 0% relative humidity.

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The temperature evolution presented in Figure 5 corresponds to the measurement of the fiber-

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optic located at the bottom of the catalytic bed. Due to the fiber-optic sensor temperature

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limitations, the fiber-optic was manually removed from the bed when the temperature

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approached its operational limit, and therefore no fiber optic data are available above 533 K.

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However, the temperature evolution could also be tracked using an infrared (IR) camera, giving

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temperature readings that correspond to the outer quartz wall. This temperature correlates with

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the inner temperature of the bed measured with the optical fiber, although the differences can be

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substantial. Thus, the temperature difference measured at the steady-state in between the quartz

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wall and the inner part of the bed under MW heating (in the absence of reaction, under a power

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of 30 W) was around 333 K. Under reaction the gradient is expected to be higher since the

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combustion of n-hexane is exothermic the heat released adds microwave heating (the adiabatic

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temperature increment, assuming that all the heat released during n-hexane combustion remained

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in the catalyst bed, i.e., neglecting the heat losses and the heat carried away by the gas, was

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estimated around 400 K).

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Figure 6 a illustrates the transient average surface temperature of the quartz tube, measured by

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the IR camera, together with the inner bed temperature measured by the optical fiber at the end

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of the bed, and corresponds to the first cycle of the experiment in Figure 5. The inset shows an

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enlarged IR image of the outer surface of the quartz tube and the area of interest, occupied by the

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catalyst, which has been used to calculate the corresponding average surface temperature data.


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The initial sharp temperature increment from 0 to around 75 seconds observed in the curve of the

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fiber-optic readings corresponds to the heating caused only by microwaves. During this time, n-

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hexane is desorbed from the bed. Then, above a certain temperature combustion starts and the

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slope changes abruptly because of the contribution from the exothermic combustion of n-hexane.

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The same change of slope can be observed with some delay in the IR camera measurements

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presented in Figure 6 a. The evolution of the thermal images with time starting at 72 seconds,

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just before the combustion initiation are presented in Figure 6 b. It can be observed that the

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ignition of n-hexane starts at the top of the fixed-bed, which is expected since the saturated

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zeolite is on the top part. After ignition, the bed is heated by both, the microwave energy

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absorbed by the zeolite and the heat produced in the reaction, which is carried downstream by

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the hot gases exiting the reaction zone. The combustion is completed in a short time (see CO2

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evolution in Figure 5) and then the temperature (as measured by the IR camera at the external

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surface) goes through a maximum and starts decreasing. Since the temperature inside the fixed

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bed above 523 K cannot be measured by a fiber-optic (as the limit temperature approaches the

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fiber is removed from the bed to avoid damage), the quartz wall temperature was followed

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instead and is reported in the subsequent experiments.


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Figure 6. a) Evolution of temperature at the end of the catalyst bed (measured by optical fiber)

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and of the average external surface of the quartz wall in the region occupied by the catalyst bed

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measured by IR camera (the inset shows the IR image of the outer surface of the quartz tube, and

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the area of interest) b) Thermal IR images of the external quartz tube wall at different times to

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follow the heating of the bed during the combustion after MW irradiation (MW power 30 W,

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single catalytic bed; load: 400 mg PtY zeolite, total flow: 100 mL/min, 500 ppm n-hexane, 5 min

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loading time, 0% relative humidity).

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Continuous operation in catalytic bed, under periodic microwave pulses

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Dry feed operation

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After performing full combustion and regeneration of the bed in cyclic operation, the

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possibility of continuous operation with cyclic combustion was also investigated. The aim was to


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study opportunities for process intensification by avoiding a separate regeneration stage, i.e., the

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polluted air stream would be continuously fed to the catalyst bed while microwave pulses would

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be activated periodically to cause desorption and combustion of the trapped n-hexane. Therefore,

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the system acts as follows: when the microwave is off the catalyst bed acts as a sorbent and

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accumulates n-hexane. Under MW heating, n-hexane is rapidly desorbed in a high concentration

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pulse. Combustion of the desorbed pollutant on the catalytic surface rapidly increases the

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temperature, giving rise to a hot wave that travels down the catalytic bed (similar to the

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experiment presented in the previous section). Matching of the heating, desorption, combustion

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and transport rates is critical here. If the pollutant is desorbed and entrained out of the reactor

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before the catalyst bed downstream has had time to heat up to a sufficiently high temperature,

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then combustion will be incomplete and a breakthrough of the pollutant will occur.

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The experiments were carried out by feeding continuously a stream (100 mL/min) containing

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400 ppm of n-hexane in dry or humid air to the catalytic bed, while the MW was switched off for

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different periods of time (5, 10, 15 min), followed by a 10 min period of MW activation. Unlike

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the experiments in the previous section, the feed of polluted air was not interrupted during MW

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activation, allowing continuous operation. Table 1 shows the n-hexane loads in the bed, and the

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conversions achieved following MW activation for different loading (Microwave OFF) times. It

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can be seen that the conversion decreased from 1 to 0.78 as the adsorption (loading) period

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increased from 5 to 15 min. Interestingly, the load could be doubled (from 2.2 to 4.4 mg

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hexane/g of catalyst), and essentially full conversion could still be achieved. A similar behavior

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was also observed by Roland et al.27 in radio-frequency heated beds when a high loading of

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toluene was used. The results in Table 1 indicate that there is a limitation of the catalytic capacity

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to completely remove n-hexane as the hydrocarbon loading increases. This behavior can be


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explained as a consequence of the desorption/combustion dynamics. As the loading time

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increases, the region of the bed containing adsorbed n-hexane extends further into the reactor.

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Once microwave power is supplied, the whole bed is heated within seconds, promoting fast n-

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hexane desorption (n-hexane desorbs at temperatures lower than those required for

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combustion13). The n-hexane desorbed in regions closer to the end of the bed does not have

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enough contact time for a complete combustion and a breakthrough of unconverted n-hexane is

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observed.

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Table 1. Effect of different n-hexane loading (PtY) on its conversion by catalytic oxidation via

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microwave heating Microwave OFF Microwave ON Experiment

(Adsorption)

Time (min)

mg n-hexane

(% total cycle)

/g catalyst

5

10 (67%)

2.2

10

10 (50%)

15

10 (40%)

Time (min) 1 2 3 4 5 6

(Desorption+Combustion)

n-hexane loading

xnhexane

1.00 1.00 0.98 4.4 0.98 0.76 6.6 0.78

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Humid feed operation

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Designing realistic processes for adsorption of VOCs from ambient air necessarily has to

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address the effect of environmental moisture, often present in concentrations that are orders of

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magnitude higher than the target VOC. This is especially important when zeolites are used, since


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the hydrophilic nature of most zeolites may lead to a displacement of the organic pollutant by

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water in the adsorption sites of the zeolite. Furthermore, the addition of water, a well-known

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MW absorber, is also going to affect the heating behavior and the final temperature increment of

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the catalyst bed. In the following experiments, the effect of environmental moisture was

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investigated during continuous operation of a MW-assisted air purification scheme.

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Figure 7 a presents the average transient temperature profiles at the quartz wall and the carbon

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dioxide evolution during the combustion in dry and humid conditions. There is a striking

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difference in the evolution of temperature profiles following MW activation for beds that had

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been exposed to dry and humid air conditions. It can be seen that, under the same applied MW

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power, the addition of water results in a faster microwave heating and higher final temperatures.

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This is because, the water molecules are excellent absorbers of MW irradiation 13 and because of

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this, a larger fraction of the applied MW power is efficiently absorbed by the humid zeolite bed.

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Indeed, for the bed exposed to humid air the temperature rise is immediate (after only 18 seconds

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a hot spot is already visible on the outer wall compared to 75 seconds for the bed exposed to dry

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air, see Figure 7 c) causing a rapid combustion of the adsorbed hexane (combustion is essentially

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complete in 1.5 min compared to 2.5 min for the bed exposed to dry air).

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Figure 7. Average transient temperature profiles at quartz wall, and evolution of carbon dioxide

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concentration and Thermal IR images of the external quartz tube wall in b) humid and c) dry

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conditions (input power: 30 W, single catalytic bed load: 400 mg, total flow: 100 mL/min, 400

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ppm n-hexane in air, and 0 (dry) or 50% relative humidity).

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Given the efficient MW absorption by the bed exposed to humid air, the duration of the MW

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heating pulses was decreased, first to 5 min and then to 3 min, allowing considerable energy

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savings compared to the dry air case. Figure 8 a shows that during continuous cleaning these 3

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min MW pulses were enough to achieve complete combustion of the n-hexane fed continuously

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to the bed, as there was no n-hexane detected at the outlet. This system was operated

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continuously for 11 cycles with a stable performance (see Figure S1 a in the supporting

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information).


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Figure 8. n-hexane, carbon dioxide, water and reactor wall temperature evolution during

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continuous operation in the a) catalytic bed (input power: 30 W, catalytic bed load: 400 mg PtY

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zeolite, total flow: 100 mL/min-1, 400 ppm n-hexane in air, 50% relative humidity), and b)

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double bed (input power: 30 W, adsorptive bed load: 200 mg DAY zeolite, catalytic bed load:

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200 mg PtY zeolite, total flow: 100 m/Lmin, 400 ppm n-hexane in air, 50% relative humidity. In

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the double bed configuration, the temperatures reported correspond to the average surface

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temperature of the wall in the catalyst bed.


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Continuous operation in double fixed-bed, under periodic microwave pulses

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In a previous work,13 it was found that the DAY (Si/Al=40, H+) zeolite has higher n-hexane

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selectivity in a binary mixture of n-hexane and water than NaY (Si/Al=2.5, Na+) zeolite. Taking

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this into account, a new concept of double fixed-bed, see Figure 1 d, was investigated in the

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following experiments. In this double fixed-bed configuration, the upstream half of the PtY

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catalyst bed was replaced with the hydrophobic DAY zeolite (same weight respect to PtY

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zeolite) in an attempt to favor n-hexane adsorption in the first half of the bed in the presence of

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water, while concentrating the catalytic function in the second part of the bed.

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Figure 8 b shows the results obtained under the most severe conditions, i.e., a double bed

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configuration (containing half of PtY catalyst compared to Figure 8 a) and a MW activation

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period of only 3 min. It can be seen that even under these conditions, almost complete

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combustion of n-hexane is achieved, with only a small amount of unreacted n-hexane detected in

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each combustion pulse at the outlet of the bed, corresponding roughly to 1% of the adsorbed

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hexane, i.e., the conversion is still around 99%. This shows that the amount of catalyst can be

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reduced by 50% provided that pollutant adsorption is concentrated preferentially in the first half

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of the bed. This system operated stably for 11 cycles, giving high n-hexane conversions higher

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than 99 % (see Figure S1 b, supporting information).

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The temperature distribution in this double bed configuration is presented in Figure 9. In the

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thermal images the areas corresponding to the adsorptive, (DAY zeolite), and catalytic, (PtY

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zeolite), beds walls were evaluated individually in order to calculate the average temperature of

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each bed separately. The adsorptive bed started heating before the catalytic bed. Even though the

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adsorptive bed is more hydrophobic compared to the catalytic bed, it can still adsorb water

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and being upstream is able to contact first with the water containing feed, adsorbing it

13

,


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preferentially. For the experimental conditions of this test, the combustion start is made visible

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by the temperature of the wall after 45 seconds, beginning from the top of the catalytic bed, and

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moving downstream, see Figure 9 b.

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Figure 9. a) Average transient temperature profiles at quartz wall (the inset shows the IR image

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of the outer surface of the quartz tube, and the area of interests, which are occupied by the

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adsorptive and catalytic bed, respectively), and b) its corresponding thermal IR images during

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the combustion (Input power: 30 W, adsorptive bed load: 200 mg DAY zeolite, catalytic bed

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load: 200 mg PtY zeolite, total flow: 100 mL/min, 400 ppm n-hexane in air, 50% relative

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humidity).

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The MW-assisted desorption-combustion scheme presented in this paper constitutes a highly

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attractive technology for a continuous destruction of pollutants present in humid-air streams at


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low concentrations, where other available technologies may not be feasible because of the energy

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costs involved.

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Supporting Information

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The evolution of n-hexane, carbon dioxide, water and temperature during continuous operation

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in the catalytic bed and double bed and corresponding n-hexane conversions regarding the

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double bed configuration supplied as Supporting Information in Figure S1 (PDF).

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Corresponding Author

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*Phone: (+34) 876761153; e-mail: [email protected]

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*Phone: (+34) 876555440; e-mail: [email protected]

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ACKNOWLEDGMENTS

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Financial support from the European Research Council ERC-Advanced Grant HECTOR

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Project (ID:267626) is gratefully acknowledged. Hakan Nigar also acknowledges financial

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support from the Spanish Ministry of Education for the FPU grant (Formación del Profesorado

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Universitario – FPU12/06864).

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REFERENCES 1. Air pollution harms human health and the environment; www.eea.europa.eu/themes/air/intro. 2. Mamaghani, A. H.; Haghighat, F.; Lee, C. S., Photocatalytic oxidation technology for indoor environment air purification: The state-of-the-art. Appl. Catal., B 2017, 203, 247-269. 3. Steinemann, A.; Wargocki, P.; Rismanchi, B., Ten questions concerning green buildings and indoor air quality. Build. Environ. 2017, 112, 351-358. 4. Campagnolo, D.; Saraga, D. E.; Cattaneo, A.; Spinazzè, A.; Mandin, C.; Mabilia, R.; Perreca, E.; Sakellaris, I.; Canha, N.; Mihucz, V. G.; Szigeti, T.; Ventura, G.; Madureira, J.; de


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