INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 3 doc

Using hydrocarbons, Cu-ion exchanged PILC yielded higher than those of Cu-ZSM-5-based catalysts; and their activity was only tested different ion-exchanged pillared clays as catalysts fo

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Comparison of Catalytic Reduction of

NO by Propene on Zeolite-Based and Clay-Based Catalysts Ion-Exchanged

by Cu

JOSE L VALVERDE, FERNANDO DORADO, PAULA SA ´ NCHEZ, ISAAC ASENCIO, and AMAYA ROMERO University of Castilla–La Mancha, Ciudad Real, Spain

Selective catalytic reduction (SCR) of NO with hydrocarbons has been a subject

of extensive study due to its potential for the effective control of NO emission

in oxidant environments [1–11] Hydrocarbons would be the preferred reducing

and slippage through the reactor Although many types of catalysts have been studied for this purpose, only a few copper-loaded zeolites have been demon-strated as adequate, and among them Cu-ZSM-5 gives good yields and seems to

be one of the most active ones [3,5,12–15] The majority of these catalysts are

have also been studied Shimuzu et al [16,17] recently reported that Cu-aluminate

high de-NOx activity comparable to Cu-ZSM-5 and higher hydrothermal stabil-ity The activity of copper-loaded zeolites was found to depend on the Cu content Iwamoto et al [18] observed that the activity of Cu-ZSM-5 increased with the

in the NO-containing stream plays an important role in the reaction rate and product selectivity of the SCR reaction It has been suggested that the roles of

carbona-ceous deposits on Cu-ZSM-5 [20,22]

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overexchanged Cu-ZSM-5 zeolites exists as small clusters of Cu-O or as isolated

Corma et al [27/38] observed that the most active Cu-beta zeolites for SCR of

produced under reaction conditions This conversion was easier in overexchanged Cu-beta samples

Pillared clays (PILCs) are two-dimensional materials prepared by exchanging charge-compensating cations between the clay layers with large inorganic metal hydroxycations that are oligomeric and are formed by hydrolysis of metal oxides

or salts After calcination, the metal hydroxycations are decomposed into oxide pillars that keep the clay layers apart and create interlayer and interpillar spaces, thereby exposing the internal surfaces of the clay layers The size of these oligo-mers appears to control the size of the pore opening in the pillared clays It is

or salt forming polynuclear species upon hydrolysis can be inserted as a pillar Intercalated clays are usually natural smectites clays Properties such as acidity, surface area, pore size distribution, and both thermal and hydrothermal stability depend on the method of synthesis as well as on the nature of the host clay Most common ions used as pillaring agents prepared by hydrolysis of the correspond-ing salts in solution are polycationic species of Al, Zr, Fe, Cr, etc Can˜izares et

al [28] recently reported a comparative study in which different PILCs with single-oxide pillers of Fe, Cr, and Zr and mixed-oxide pillars of these metals and

Al were prepared from two different bentonites

One of the ﬁelds of applications of pillared clays is catalysis More

and hydrocarbons [29–31] Using hydrocarbons, Cu-ion exchanged PILC yielded higher than those of Cu-ZSM-5-based catalysts; and their activity was only

tested different ion-exchanged pillared clays as catalysts for selective catalytic reduction of NO by ethylene Cu-Ti-PILCs showed the highest activities at tem-peratures below 643 K, whereas Cu-Al-PILC was the most active at temtem-peratures above 673 K Ti-PILC was obtained using a procedure in which the pillaring

into a HCl solution

In spite of these relevant results, Ti-PILCs have received considerably less attention than other pillared clays As a result, few preparation methods for these

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materials have been reported [34,35] In our case, Ti metoxide was used as the source of Ti in the preparation of Ti-PILCs

The aims of this work are:

Ti-PILC-based catalysts ion-exchanged by Cu

chemical properties to the catalytic behavior of the two sets of catalysts

A Preparation of Catalysts

NaZSM-5 zeolite (Si/Al ratio of 20) was synthesized according to the method described elsewhere [36] using ethanol as the template X-ray diffraction (XRD) conﬁrmed that the product was 100% crystalline [37] Cu was introduced by

solutions per gram of zeolite The mixture was kept under agitation at the desired ion exchange temperature (303, 328, or 353K) for 14 h Next, the suspension was ﬁltered and thoroughly washed with deionized water in order to completely remove the occluded salt, and the solid was then air-dried at 393 K for 14 h The whole procedure was repeated twice for some catalysts Finally, the samples were

here prepared These catalysts were referred to as a function of the copper load-ing For instance, CuZ-2.9 corresponds to a Cu-ZSM5 with a copper content of 2.9% by weight

Ti-PILC was prepared as follows The starting clay was a puriﬁed montmoril-lonite (puriﬁed-grade bentonite power from Fisher Company), which has a

Ti-polycations was prepared by ﬁrst adding titanium metoxide to a 5 M HCl solution The solution was aged for 3 h at room temperature Then 1 gram of starting clay was dispersed in 1 L of deionized water for 3 h under stirring The pillaring solution was slowly added with vigorous stirring into the clay suspen-sion until the amount of pillaring solution reached that required to obtain a Ti/ Clay ratio of 15 mM of Ti/g clay The intercalation step took about 16 h Subse-quently, the mixture was separated by vacuum ﬁltration or centrifugation and washed with deionized water until the liquid phase was chloride free The sample was dried at 393 K for 12 h and calcined at 773 K for 2 h The basal spacing

One gram of the Ti-pillared bentonite was added to 200 mL of 0.05 M copper acetate solution The mixture was stirred for 6 h at room temperature The ion

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TABLE 1 Composition and Characterization of Zeolite-Based Catalystsa

Ion exchange steps Cu Weak acid site Strong acid site

and temperature content Ion exchange density density Surface area Micropore area Micropore Catalyst (K) (wt %) level (%) (mmol NH3/g) (mmol NH3/g) (m2/g) (m2/g) volume (m3/g) NaZSM-5 — 0 0 0.950 (579 K) Not detected 369.3 (100%) 360.0 (100%) 0.158 (100%) CuZ-2.4 1—303 2.4 94 0.881 (565 K) Not detected 345.4 (94%) 338.8 (94%) 0.140 (89%) CuZ-2.6 1—328 2.6 103 0.822 (573 K) Not detected — — —

CuZ-2.9 2—303 2.9 116 0.907 (555 K) Not detected 326.4 (88%) 309.6 (86%) 0.130 (82%) CuZ-3.7 2—328 3.7 148 0.781 (573 K) Not detected — — —

CuZ-4.4 1—353 4.4 175 0.477 (581 K) 0.440 (911 K) 321.0 (83%) 298.7 (83%) 0.126 (79%)

aTemperatures corresponding to the maximum of the desorption peak are included in parentheses together with the acid sites density value.

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exchange product was collected by filtration or centrifugation, followed by wash-ing five times with deionized water The obtained solid sample was first dried

at 393 K in air for 12 h and then calcined at 773 K for 2 h After this pretreatment,

PILC-based catalysts here prepared Pillared clay-PILC-based catalysts were referred to as a function of the copper loading For instance, CuTi-7.4 corresponds to a Cu-Ti-PILC with a copper content of 7.4% by weight

B Characterization Methods

X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710

diffractometer using Ni-ﬁltered CuKα radiation To summarize the (001)

reﬂec-tion intensity in PILC samples, oriented clay-aggregate specimens were prepared

by drying clay suspensions on a glass slide

Surface area and pore size distributions were determined by using nitrogen

as the sorbate at 77 K in a static volumetric apparatus by using a micromeritics ASAP 2010 sorptometer For this analysis, samples were outgassed at 453 K for

by using the Brunauer, Emmett, and Teller (BET) equation The Horvath– Kawazoe method was used to determine microporous surface area and volume Total acid site density of the samples was measured by a temperature pro-grammed desorption (TPD) of ammonia, by using a Micromeritics TPD-TPR analyzer Samples were housed in a quartz tubular reactor and pretreated in

773 K, the samples were cooled to 453 K and saturated for 0.25 h in an ammonia (99.999%) stream The sample was then allowed to equilibrate in a helium ﬂow

at 453 K for 1 h Finally, ammonia was desorbed using a linear heating rate of

average relative error in the acidity determination was lower than 3%

Temperature programmed reduction (TPR) measurements were carried out with the same apparatus previously described After loading, the sample was

was constant for 30 min Next, it was cooled to 298 K and stabilized under an argon/hydrogen (99.999%, 83/17 volumetric ratio) ﬂow The temperature and

cool-ing trap placed between the sample and the detector retained the liquids formed during the reduction process The TPR proﬁles were reproducible with an average relative error in the determination of the reduction maximum temperatures lower than 2%

The metallic content (wt%) was determined by atomic absorption measure-ments by using a SpectrAA 220 FS analyzer In all cases, calibrations from the corresponding patron solutions were performed

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TABLE 2 Composition and Characterization of PILC-Based Catalystsa

Weak acid site

Cu content Ion exchange level density Strong acid site Surface area Micropore area Micropore Catalyst (wt%) (%) (mmol NH3/g) density (mmol NH3/g) (m2/g) (m2/g) volume (m3/g) Ti-PILC 0 0 0.437 (579 K) 0.092 (691 K) 273.2 (100%) 224.5 (100%) 0.181 (100%) CuTi-4.6 4.6 149 0.108 (548 K) 0.362 (628 K) 241.7 (88%) 202.2 (90%) 0.153 (85%) CuTi-7.4 7.4 240 0.136 (543 K) 0.594 (623 K) 234.3 (86%) 189.1 (84%) 0.143 (79%) CuTi-9.0 9.0 292 0.156 (533 K) 0.738 (613 K) 201.8 (74%) 153.7 (68%) 0.126 (69%)

aTemperatures corresponding to the maximum of the desorption peak are included in parentheses together with the acid sites density value.

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C Reaction Studies

The catalytic tests were carried out in a ﬁxed-bed ﬂow reactor The standard

(5%), and balance He at ambient pressure The flow rates were controlled by calibrated Brooks flowmeters The total flow rate was 125 mL/min The space

ana-lyzer)

III RESULTS

A Characterization of the Catalysts

Tables 1and2list, for all the catalysts, the speciﬁc surface area, micropore area, and micropore volume, the weak and strong acid site density, and the copper content The same tables also summarize the Cu ion exchange levels that were determined taking as a reference, in the case of zeolite-based catalysts, the num-ber of aluminum atoms contained in the structure and, in the case of PILC-based catalysts, the cation exchange capacity (CEC) of the clay [34] It can be observed that, except for the CuZ-2.4 sample, all the catalysts presented more Cu content than that corresponding to 100% ion exchange In fact, all the zeolite-based cata-lysts contained sodium ions As expected, the loading of these decreased with increasing copper content in zeolite (0.16 wt% of Na for the CuZ-2.4 sample and 0.07 wt% of Na for the CuZ-4.4 sample It is also observed for these catalysts that for the same number of ion exchange steps the copper loading increased with increasing ion exchange temperatures and the acid site density progressively diminished from the value corresponding to the parent Na/ZSM5 (0.95 mmol

respectively In this case, it is clear that acidity is a combination of two effects: the presence of Cu and Na on the catalyst The ﬁrst Cu ions incorporated to zeolites would occupy hidden sites (small zeolite cages) [38] These sites may

needed to achieve the complete ﬁlling of small cages, the increase of copper content would lead Cu species to incorporate at more accessible positions As a consequence the acidity increases Strong acid site density was observed only for zeolite-based catalysts with copper content higher than 4.4% by weight In contrast, all the PILC-based catalysts presented strong acid sites

An increase of Cu content was accompanied in both sets of catalysts by a decrease of BET surface area and micropore volume, indicating that Cu intro-duced into the pillared matrix would preferentially occupy the interlayer area [39], whereas the Cu present in zeolite would cause the partial blocking of the

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Cu2 ⫹⫹1

Some authors showed that, depending on the copper content, the reduction of

would occur at higher temperatures [41–43] When the copper content in the sample is higher, the excess copper may be found as oxygenated clusters more easily reduced than the isolated copper species [24,34]

indi-cating that Cu species are hard to reduce to lower valence In the case of zeolite-based catalysts, this fact is justiﬁed considering the existence of metal ions in

coordinated to the framework oxygens This bonding is generally much stronger for multivalent than for monovalents ions Each ion would not be readily

ions would be rather high These results are in good agreement with the

TABLE 3 Ratios of H2Consumption to Cu (H2/Cu, mol/mol, Measured by TPR Experiments) of Zeolite-and PILC-Based Catalystsa

H2/Cu (mol/mol) H2/Cu (mol/mol) H2/Cu (mol/mol) H2/Cu (mol/mol) Catalyst CuO to Cu0 Cu2⫹to Cu⫹ Cu⫹to Cu0 Total NaZSM-5 — — — — CuZ-2.4 Not detected 0.169 (517 K) 0.322 (643 K) 0.491 CuZ-2.6 Not detected 0.181 (503 K) 0.271 (676 K) 0.452 CuZ-2.9 Not detected 0.249 (503 K) 0.204 (693 K) 0.453 CuZ-3.7 Not detected 0.177 (450 K) 0.260 (653 K) 0.437 CuZ-4.4 0.251 (450 K) 0.134 (461 K) 0.216 (663 K) 0.601 Ti-PILC — — — — CuTi-4.6 0.405 (473 K) 0.172 (543 K) 0.046 (663 K) 0.623 CuTi-7.4 0.321 (431 K) 0.170 (507 K) 0.051 (685 K) 0.542 CuTi-9.0 0.395 (426 K) 0.107 (510 K) 0.036 (693 K) 0.538

aTemperatures corresponding to the maximum of the reduction peak are included in parentheses.

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higher than that of the zeolite, also showing that in these samples an important

For the zeolite-based catalysts with a copper content lower than 4.4 wt%, the

sample CuZ-4.4, a peak existing at 450 K would be related to the presence of CuO aggregates Due to the absence of a diffraction line of CuO species in XRD patterns of this sample, the occurrence of CuO aggregates larger than 3 mm can

be ruled out [40] According to the measurements of acidity, the presence of CuO aggregates would be related to the occurrence of strong acid sites On the other

In the case of PILC-based catalysts, the peaks corresponding to the three reac-tions involved in the copper reduction are present Again, there is a clear shift

This fact would indicate that the higher the Cu content is, the lower CuO species dispersion that is observed In a similar way as observed in zeolite-based

the TPR-proﬁles of the CuZ-2.9 and CuTi-7.4 samples

Figure 2shows, for all the catalysts, the H2consumption for the Cu2 ⫹to Cu⫹

consumption increased, passing through a maximum, and then decreased at higher loadings It can be veriﬁed that the maximum in the case of zeolite-based catalysts corresponds to the CuZ-2.9 sample and in the case of PILC-based cata-lysts to the CuTi-7.4 sample

B NOx Reduction Activity

The catalytic performance of the catalysts for the SCR reaction of NOx with

presence of copper in the catalysts enhanced the catalytic activity With an in-crease in reaction temperature, NOx conversion inin-creased, passing through a maximum, and then decreased at higher temperatures According to Yang et al [34], the decrease in NOx conversion at higher temperatures was due to the com-bustion of propene In general, all the Cu-zeolites samples presented the maxi-mum NOx conversion at the same temperature (623 K) Similar observations can

be derived for all PILC-based catalysts, but in this case the corresponding maxi-mum appeared at 523 K It can be observed that increasing the copper loading increased NOx conversion until the copper loading reached 116% ion exchange

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FIG 1 TPR proﬁles of CuTi-7.4 and CuZ-2.9.

FIG 2 Hydrogen consumption for the Cu2⫹to Cu⫹reduction processes as a function

of Cu loading

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