Enhancement of the catalytic performance of H-clinoptilolite in propane–SCR–NOx process through controlled dealumination

Accepted Manuscript Enhancement of the Catalytic Performance of H-Clinoptilolite in Propane-SCRNOx Process through Controlled Dealumination Naser Ghasemian, Cavus Falamaki, Mansour Kalbasi, Monireh Khosravi PII: DOI: Reference:

S1385-8947(14)00471-9 http://dx.doi.org/10.1016/j.cej.2014.04.039 CEJ 12018

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

19 February 2014 7 April 2014 10 April 2014

Please cite this article as: N. Ghasemian, C. Falamaki, M. Kalbasi, M. Khosravi, Enhancement of the Catalytic Performance of H-Clinoptilolite in Propane-SCR-NOx Process through Controlled Dealumination, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.04.039

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Enhancement of the Catalytic Performance of H-Clinoptilolite in Propane-SCR-NOx Process through Controlled Dealumination

Naser Ghasemiana, Cavus Falamaki1,a,b, Mansour Kalbasia, Monireh Khosravic

a

Chemical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran b

Petrochemical Center of Excellence, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran

c

Mining and Metallurgical Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran

Abstract We report for the first time the effect of clinoptilolite dealumination on the propane-SCR-NOx process. This has been accomplished using a mild acid like oxalic acid to avoid excess catalyst crystallinity deterioration. It had been shown that dealumination may result in a significant enhancement of NOx conversion to N2 when an optimum acid concentration of 0.050 M is used for a treatment period of 2 h. Dealumination substantially affects the distribution of the concentration of acid sites of different strength. The effect of dealumination on the HC-SCR activity of the zeolite samples is discussed in terms of Si/Al ratio, crystallinity, distribution of acid site strength, extra framework species concentration and textural characteristics of the samples. Keywords: HC-SCR; clinoptilolite; oxalic acid; propane; dealumination 1

Corresponding author. Tel: +98 21 64543160, Fax: +98 21 66405847, E-mail: [email protected]

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1. Introduction The abatement of NOx emissions is definitely one of the most important current environmental challenges. One of the relevant proposed technologies is the selective catalytic reduction of NOx. Since the discovery of the selective catalytic reduction process (SCR) of nitrogen oxides by hydrocarbons (HCSCR), various materials such as transition metal-loaded zeolites [1, 2], H-form zeolites [3, 4] and alumina-based metal oxides [5] have been reported to act as effective catalysts. Feeley et al. [6] performed a thorough study on the effect of hydrocarbon kind (C3H8, C3H6, CH4) on the HC-SCR process using Ga/S-ZrOx and S-GaZr/ZSM-5 as catalyst. Methane has been considered as reductant by Li et al. using Ce and Ag exchanged ZSM-5 as catalyst [7] and Ohtsuka et al. using Pd-Mordenite zeolite [8]. Capek et al. [9] reported the use of propane as reductant using Co exchanged beta zeolite. NO or NO2 has been the main chemical form of NOx used in the cited studies. Mishima et al. [10] proposed the use of natural zeolites for the abatement of NOx in stationary diesel exhaust gas streams. However, they considered NH3 as reductant, and this turns out to be costly and problematic due to safety requirements of ammonia for such installations. Using the same reductant, Moreno-Tost et al. [11] used Cu-exchanged clinoptilolite for the NH3-SCR of NOx. Recently Ghasemian et al. reported that H-clinoptilolite zeolite shows high activity for NOx reduction using propane as a reductant [12]. In continuation of that work, our group has focused on the dealumination process of natural clinoptilolite zeolites using oxalic acid to modify the catalytic properties of the natural zeolite. Acidic groups affect the yield of nitrogen in the selective reduction process of NOx [13, 14]. In addition, it has been found that saturated hydrocarbons like propane can be activated for NOx-SCR on the catalyst surface if the support material is modified to contain strong acidic sites [15]. The present study concentrates on the catalytic behavior of the dealuminated clinoptilolite zeolite produced by treatment with oxalic acid for the selective catalytic reduction of NOx with propane as reductant. Oxalic acid has been used successfully for the dealumination and production of lattice defects in mordenite zeolite in the past [16]. This work investigates the possible enhancement of catalytic

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properties in the propane-SCR process, i.e., pointing at increased selectivity versus nitrogen and decreased CO production.

2. Experimental The natural zeolite used in this study was purchased from Afrazand Co. (Iran) and belongs originally to the Ab Garm mine located in the south west of Semnan region. The zeolite is about 60 wt. % pure in clinoptilolite. The total cation exchange capacity of the natural zeolite is 0.247 mole g-1. The composition of the raw material (based on XRF analysis) is as follows: 61.90 wt. % SiO2, 7.20 wt. % Al2O3, 0.11 wt. % Na2O, 1.62 wt. % K2O, 2.88 wt. % CaO, 1.13 wt. % MgO and 4.15 wt. % Fe2O3. The dealumination process consisted of the following steps: a) The material was ion-exchanged with a 2 M aqueous NH4Cl solution at 70 °C for 8 h b) After isolation, the samples were calcined at 500 °C for 3 h c) the resulting solid sample was acid treated with oxalic acid with different initial molarities at 70 °C for 2 h. The labeling of the samples is described in table 1. The catalytic behavior of the different samples was investigated using a special setup reported elsewhere [12]. In brief, a gas stream with a constant composition of NO 30 ppm, NO2 460 ppm, O2 2.5 vol. %, C3H8 1000 ppm and Ar (balance) at near atmospheric pressure with an GHSV of 45067 h-1 is preheated and guided into an integral fixed bed reactor loaded with 0.45 g catalyst. The reactor inner diameter is ½ inch (stainless steel) and is heated with an outer electrical furnace mantel. Three thermocouples at the feed entrance, middle of the bed and outlet stream provide the required feedback for an on/off temperature controlling system. This system allows the control of the fixed bed reactor in the temperature range of 200-600 °C with ± 1 °C temperature fluctuation. The catalyst is pretreated for 1 h at 500 °C before each experiment. In each catalytic test, the zeolite was first isothermally heated for 20-30 min at the reaction temperature. Afterwards, the gas mixture was allowed to pass through the reactor bed and the sampling was performed after 20 min. Experimental tests showed that the system reached steady state already after 10 min. The

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increase of reactor temperature (each time 50 °C), the heating was performed with a heating rate of approximately 2 °C min-1. Gas analysis of the outlet stream is carried out on-line using a sensor probe. The analyzer is a KANE 940 apparatus with detectors for NO (0-5000 ppm range), NO2 (0-1000 ppm range), O2 (0-21%) and CO (0-10000 ppm range). For the catalytic experiments, the solid sample was crushed to a size smaller than500 µm, to avoid internal diffusion effects. In addition, using a GSHV of 45067 h-1 ascertains the absence of bulk gas film mass transfer resistance. Ghasemian et al. [12] reported that propane-SCR process over clinoptilolite zeolite results in very low concentrations or nil of N2O species. This holds also for the samples subject of study in the present work. Hence, N2 in the product stream could be calculated by atomic nitrogen mass balance. The catalyst samples have been characterized by SEM technique using a XL30 (Philips) instrument. The assessment of samples acidity was performed by NH3-TPD analysis using a Micromeritics Chemisorb 2750 apparatus. For this means, the degasification was performed by purging helium gas at a flow rate of 20 cm3 min-1at 150 °C for 2 h. Then, keeping the same flow rate, the sample was cooled to 100 °C. Ammonia adsorption was performed by purging a He/NH3 gas mixture (5 mole % ammonia) at a flow rate of 40 cm3 min-1 at 100 °C for 40 min over the sample. Desorption was performed by heating initially the sample at 100 °C for 30 min for desorption of physisorbed ammonia and afterwards heating the sample at a rate of 10 °C min-1 up to 1000 °C with the same gas stream flow rate. Crystallinity was assessed using X-ray diffractometry, using a PW 1140 (Philips) apparatus. Crystallinity was estimated by discerning the amorphous and crystalline area of the XRD diagrams. Si/Al molar ratios were measured based on XRF analysis using an X’Unique II (Philips) instrument. Nitrogen adsorption isotherms have been obtained using a Belsorp (BEL) apparatus. 3. Result and Discussion The SEM picture of the sample produced by 2 h treatment using 0.05 M oxalic acid is shown in figure 1a. This is a typical picture showing the existence of well formed clinoptilolite zeolite crystals with sizes larger than 20 µm. The width of the crystals is generally smaller than 5 µm. This is an appropriate

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size limit for catalytic purposes, considering the strong diffusional resistance for the reactants and products within the channels of the zeolite (4.7 x 2.8 Å2 along (100), 4.6 x 3.6 Å2and 3.1 x 7.5 Å2 along (001)) [17]. The SEM picture of the calcined zeolite prior to chemical treatment has been shown in figure 1b. Generally, it is difficult to relate any morphological change to the kind of chemical treatment as we are dealing with a natural zeolite sample in the form of very heterogeneous agglomerate. However, it is highly probable that acid treatment may result in the partial dissolution and elimination of amorphous binder material initially present between the zeolite crystals. Accordingly, it might be stated that the voids observable between the zeolite crystals in figure 1a are mainly due to the extraction of such material. We could not find such clear detachments between the zeolite crystals in the CLP000 sample. Besides the regular particles observed in figure 1a, other phases are present which may be attributed to the impurity phases present like SiO2 polymorphs and metal oxides (such as iron oxides). It would be informative to consider the change in the nitrogen adsorption characteristics of the samples as a function of chemical treatment. Figure 2a and 2b show the nitrogen adsorption isotherms of the raw zeolite (CLP000) and the one treated with 0.500 M acid solution (CLP500). It is clearly observed that the adsorption capacity increases substantially after treatment. In addition, mesoporosity increases significantly as a result of dealumination. This phenomenon is accompanied with the creation of a clearly observed hysteresis in the adsorption/desorption isotherms. The mesopore volume increases from 0.067 to 0.094 cm3 g-1 upon acid treatment. This is accompanied with an increase of the specific surface area from 13.7 to 63.9 m2 g-1. It should be added that performing an extra acid treatment for a time period of 6 h instead of 2 h using 0.5 M oxalic acid resulted in a specific surface area of 128 .0 m2 g-1. Accordingly, it is clear that both increasing the acid morality or treatment time result in an increased specific surface area. A better insight in the effect of chemical treatment may be demonstrated referring to the BJH analysis of the pore size distribution in the mesopore rage (adsorption branch). Figure 2c and 2-d show the pore size distribution of the raw and CLP500 samples calculated by the BJH theory based on the adsorption branch of the isotherms. It is observed that the raw zeolite and treated sample have a wide monomodal pore size

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distribution in the 2-50 nm range. The general size distribution profile does not change significantly upon dealumination. However, the average pore size undergoes a clear increase from 12.24 to 22.24 nm after treatment with oxalic acid. Accordingly, it may be stated that the dealumination process results in pore enlargement or creation of new mesopores. These new pores are considered to reduce diffusion mass transfer resistance of the reactant and product molecules through the agglomerate. Change in porosity in the meso range might be attributed to the partial dissolution of the amorphous phase (the binder). At this stage, it is noteworthy to consider an important experimental observation pertaining to the adsorption/desorption isotherms. None of the isotherms shown in figures 2a and 2b show a closure of the hysteresis loop at a relative pressure in the range of 0.4-0.5 (the so-called tensile strength effect). Referring to the IUPAC classification of hysteresis loops [18], it might be seen that all the H1,H2, H3 and H4 isotherms may generally not close around a relative pressure of 0.4-0.45 and continue to near zero relative pressure. The reason is generally attributed to the change in the volume of the adsorbent (like swelling of non-rigid pores). This is completely possible in our case, as we are dealing with composite materials (zeolite crystals, amorphous binders) and there exist a high degree of probability for an irreversible swelling upon adsorption. Figure 3 shows the XRD patterns of the different samples. Based on this figure, the crystallinity of each sample has been estimated and listed together with the pertaining Si/Al ratio in table 1. As mentioned before, the raw zeolite showed 60 wt. % crystallinity. The crystallinity is therefore retained after protonation without dealumination. The CLP000 sample (see table 1 for code definition) has a Si/Al molar ratio of 7.30. The exact Si/Al ratio of the zeolite crystals of CLP000 is difficult to measure, but is certainly less than 7.30 due to the presence of SiO2 polymorphs and amorphous SiO2 within the agglomerate. Table 2 shows the effect of acid treatment on the chemical composition of the samples. It is observed that acid treatment generally expels Na+ cations out of the framework. The concentrations of K , Ca and Mg atoms undergo a slight change upon acid treatment and does not follow a distinct trend.

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Instead, Fe atoms concentration undergoes a continuous and significant decrease with increasing acid concentration. Considering the total equivalents based on Al and metal atoms (Na, K, Ca and Mg) shown in the same table, it may be deduced that all the samples have some extra framework cations. Fe atoms, perhaps in the form of Fe2O3, constitute the majority of the extra framework metal atoms. Upon mild dealumination with 0.005 M oxalic acid, the crystallinity reduces only 5 % but the Si/Al ratio increases more than one unit. Dealumination with 0.050 M acid increases the Si/Al ratio by approximately 1 unit with respect to CLP005. This is while the crystallinity is approximately preserved (ca. 52 %). The CLP500 sample shows a high degree of dealumination (Si/Al= 11.46), but the crystallinity is about 20 % less than the CLP000 sample. At this stage, the effect of acid treatment on the acidity of the catalysts is described. Figure 4a shows the NH3-TPD curves of the different samples. It may be observed that all the samples show approximately three distinct peaks in the 100-370, 370-670 and 670-900 °C temperature ranges. We attribute these peaks to weak, medium and strong acid sites, respectively. It should be noted that the mentioned peaks (especially the weak and strong ones) do overlap. Therefore, we have deconvoluted them to obtain their pertaining integrated intensity (proportional to the acid site concentration) and this has been shown in figure 4b. Referring to the latter figure, it is observed that there exists a clear trend for each of the integrated peak intensity versus acid concentration for all the samples. The concentration of the strong and medium acid sites increase upon treatment with oxalic acid concentrations up to 0.050 M and further decrease with increasing the acid concentration to 0.500 M. The weak acid sites undergo a slight decrease in concentration upon treatment with 0.005 M oxalic acid. However, these acid sites show a trend similar to the strong and medium acid sites after treatment with acid solutions with concentrations higher than 0.005 M. It is observed that generally the weak acid sites have a higher concentration with respect to the medium and strong acid sites. Summing up, it may be stated that the maximal concentration of each kind of acid sites is obtained at an optimum acid concentration of 0.050 M. It should be stressed that the peaks attributed to the medium and strong acid sites observed in the TPD diagram are not due to

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any dehydroxilation of the zeolite structure. This has been previously demonstrated by our group considering the thermal gravimetric analysis of the parent zeolite sample. The zeolite subject of this study shows a somewhat different NH3-TPD diagram with respect to most natural clinoptilolite zeolites. The strong acid sites release chemically adsorbed NH3 at a temperature near 700 °C. This is while existing reports show smaller temperatures normally ranged between 500 and 550 °C [19, 20]. Nonetheless, dealuminated zeolites have been reported to result in the desorption of ammonia even at temperatures around 700 °C [21,22]. Such very strong acidic sites have been denominated as super-acids [21] and are considered to be created from the strong interaction between framework Broensted and extra-framework Lewis acid sites.

Actually, we presume that the dealumination process in our case results in the

production of oxoaluminum species ( (AlO)n)+ within the zeolite channels. In resemblance with inorganic super-acids, the oxoaluminum species interact strongly with the remaining Broensted acid sites within the zeolite channels, eventually resulting in what we denominated as “strong active sites”. The relevance of each type of acid types may then envisage to be as follows: Mechanistic studies of the HC-SCR process over zeolites in the past converge on the determining role of Broensted acid sites in the course of the reaction [7, 14]. As discussed in our previous study, the Broensted acid sites result in the production of nitrosonium ions which are responsible for the production of nitrate species [12]. The nitrate species are then reduced by propane into nitrogen gas. It is well known that the TPD technique is not able to distinguish Broensted and Lewis sites. A recent study by Jin and Li [23] shows that for a HZSM-5 zeolite both Broensted and Lewis sites do exist in the TPD temperature range of 100-200 °C, mostly Lewis sites in the temperature range of 200-300 °C and mostly Broensted sites at higher temperatures. Accordingly, we presume that the medium acid sites and strong acid sites (super-acids) are of the Broensted type and are the major responsible for the propane-SCR process. It should be recalled that a previous FTIR study using pyridine as probe molecule on the clinoptilolite zeolite of the same region showed that the concentration of the Broensted sites is approximately two-fold that of the Lewis acids. This supports the previous discussion that attributes the main catalytic activity to the presence of Broensted acid sites.

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Based on figure 4a, the temperature corresponding to the maximum peak intensity of the strong acid sites is around 705 °C, irrespective of the acid treatment concentration. This temperature is ca. 750 °C for the parent H-form zeolite (CLP000). The temperature corresponding to the maximum peak intensity of the medium acid sites is ca. 580 °C for all the samples under consideration. The weak acid sites, however, show a rather complicated behavior upon acid treatment. It is observed that the corresponding peak tends to bifurcate after treatment with oxalic acid. The latter behavior is most pronounced for the highest acid concentration. Figure 5 shows the total conversion of NOx into N2 as a function of reaction temperature and zeolite treatment procedure. It is observed that all the zeolite samples follow a similar trend: The conversion increases from 200 °C up to 450 °C, and afterwards decreases. The parent zeolite (not acid treated) shows the least activity throughout the temperature span of 200-500 °C and results in a maximum conversion of 52 % at 450 °C. Treatment with 0.005 M oxalic acid solution results in an abrupt increase in the conversion of NOx to N2 over the whole temperature range understudy and a maximum conversion of 61 % is obtained. The conversion enhancement is rather sharp for temperatures equal or larger than 300 °C. Treatment with 0.050 M acid solution generally does not result in significant conversion improvement in the temperature range of 200-400 °C, when compared to the sample treated with 0.005 M oxalic acid solution. However, the conversion increases more than 6 % in the temperature range 400-450 °C and reaches 72 % in the optimum temperature of 450 °C. Treatment with 0.500 M acid solution results in the deterioration of the catalytic properties. The conversion versus temperature variation diagram falls down and we get the weakest catalytic activity among the acid treated catalysts. It is noteworthy that this sample clearly shows a better activity with respect to the untreated catalyst, with a maximum conversion of 64 % at 450 °C. It should be mentioned that although the non-zeolitic portion of the zeolite agglomerate may be catalytically active, its contribution is supposed to be minimal. Based on the NH3-TPD results and the data presented in tables 1 and 2, the NOx to N2 conversion variations as a function of kind of treatment may be explained as follows. Dealumination with 0.005 M

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oxalic acid solution results in a mild increase (ca. one unit) of Si/Al ratio along with a small decrease of crystallinity (about 5 %). This is accompanied with the increase of medium and strong acid sites concentration of the zeolite samples. At this stage it might be advantageous to allude to the presence of extra framework metal oxides like Fe2O3 which are well known to be active as NOx-abatement catalysts [24]. Based on table 2, it is observed that the content of iron atoms (framework and extra framework) undergoes a continuous and significant reduction upon acid treatment. The 6.5 wt. % reduction of the iron oxide content of the zeolite upon treatment with 0.005 M acid solution may have a detrimental effect on the zeolite HC-SCR activity. It seems that the effect of increase in the concentration of the medium and strong acid sites is dominant and eventually results in the increase in the activity of the zeolite sample. It should be noted that the exact nature of iron species in our zeolite agglomerates is unclear and very difficult to determine. Chavez-Rivas et al. [25] intentionally deprived a raw Cuban clinoptilolite from iron species to reinsert it in a controlled manner, in order to be able locating the Fe3+ cations in the zeolite framework. In our case, the structure and distribution of the iron species (coordination number, ferric or ferrous type) is unclear. Iron oxide species in zeolites may exist both as Broensted or Lewis type acid types, depending on the oxidation state of iron and its coordination number [26]. Accordingly, the TPD desorption peaks shown in figure 4 may be also to such the distribution of such acidic sites. Discrimination between Lewis/Broensted sites belonging to iron species and the zeolite framework is subject of a separate and detailed study. Treatment with 0.050 M oxalic acid increases again the Al/Si ratio by one unit, while the crystallinity decreases just slightly. This is accompanied with the increase of the concentration of weak, medium and strong acid sites. All these phenomena affect positively the activity of the catalyst for propane-SCR. The iron oxide content of the sample decreases ca. 28.5 wt. % with respect to the parent sample. Again, we postulate that the increase of the acid sites concentration is dominant and results in the increase in the activity and selectivity of the zeolite sample.

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Treatment with 0.500 M acid solution, however does not result in activity increase and, instead, is accompanied with a clear decrease in the activity throughout the temperature span of 200-500 °C. This is while the concentration of the weak, medium and strong acid sites is significantly smaller with respect to the sample treated with 0.050 M oxalic acid solution. Accordingly, a loss of catalytic activity is expected due to the reduction of total acid site concentration. The decrease of crystallinity may also contribute to the activity attenuation. The minimal content of iron belongs to the CLP500 sample (43.4 wt. % reduction of iron oxide content). Therefore, if extra framework iron oxide species also do play a role in the HC-SCR activity of the samples, their contribution should be minimal for the CLP500 sample. Based on these experimental evidences, it is expected that this sample shows the least catalytic activity for the HC-SCR process. An additional point is worth mentioning. The significant increase of surface area in the case of the CLP500 sample (from 13.7 to 63.9 m2 g-1) also may affect positively the catalytic activity. In other words, it is supposed that up to an acid concentration of 0.500 M, the increase of specific surface area may have a determining and positive effect on the zeolite catalytic performance. Figure 6 and 7 show the outlet NO and NO2 concentrations, respectively. Referring to figure 6, the outlet NO concentration increases steadily through the 200-500 °C temperature span. The parent Hform zeolite results in the largest outlet NO concentration, reaching 145 ppm at 500 °C. Treatment with 0.005 M oxalic acid solution decreases the outlet NO concentration in the range of 400-500 °C. However, the maximum NO concentration is still high (128 ppm). Treatment with 0.050 M acid solution results in the least NO formation throughout the temperature range of 200-500 °C. Maximum outlet NO concentration is 99 ppm at 500 °C. Treatment with 0.500 M acid solution increases slightly the outlet NO concentration and the maximum concentration reaches a level of 105 ppm. It should be noted beforehand that the increase in outlet NO concentration with increasing reaction temperature is expected from a thermodynamic point of view. Ghasemian et al. [12] have shown that in an equilibrium system consisting of NO, NO2 and O2 species, NO concentration increases steadily with increasing temperature.

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Specific attention should be paid to the outlet CO concentration. Figure 8 shows the outlet CO concentration as a function of reaction temperature and type of treatment. It is observed that generally CO concentration increases with increasing the reaction temperature. At this stage, it is noteworthy to consider the main reactions describing the system [12]: 10 + 2  → 5 + 6  + 8  1 2 +  + 4 →  + 3  + 4  2  + 5 → 3  + 4  3 7  +  → 3  + 4  4 2 1  ↔  +  5 2 CO may act as a reductant in the SCR process. However, Ghasemian et al. [12] excluded this possibility due the relative low CO concentration and presence of excess oxygen which may inhibit reduction by CO. Reactions (3) and (4) are undesired side reactions that consume the hydrocarbon reductant producing CO2 and CO instead of nitrogen. As propane and oxygen exist in stoichiometric excess, the latter reactions are unavoidable. Figure 9 shows the Gibbs free energy change of reactions (3) and (4) as a function of temperature. Reaction (4) is un-favored with respect to reaction (3) throughout the temperature span of 250-500 °C. However, the slope of the free energy change of reaction (4) is larger (more negative). Accordingly, based on purely thermodynamic reasons, an increase of selectivity towards CO is expected as the reaction temperature is increased. This is in agreement with our experimental results (figure 8). Production of CO is undesired, however is inherent when using HC-SCR processes. It is observed that the parent H-form zeolite results in a 110 ppm outlet concentration. Treatment with 0.005 M acid solution increases substantially the outlet CO concentration in the temperature range of 250-500 °C, with the highest level being 160 ppm. However, treatment with 0.050 M acid solution damps

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substantially the outlet CO concentration at 500 °C, reducing it to 78 ppm. Treatment with 0.500 M acid solution reduces slightly the outlet CO concentration. The observed behavior is a complex function of the zeolite chemical and physical structure.

4. Conclusion We report for the first time the effect of clinoptilolite dealumination on the propane-SCR-NOx process. This has been accomplished using a mild acid like oxalic acid to avoid excess catalyst crystallinity deterioration. It had been shown that dealumination may result in a significant enhancement of NOx conversion to N2 when an optimum acid concentration of 0.050 M is used for a treatment period of 2 h. Dealumination substantially affects the concentration distribution of acid sites of different strength. The effect of extent of dealumination on the HC-SCR activity of the zeolite samples is discussed in terms of Si/Al ratio, crystallinity, distribution of acid site strength, extra framework species concentration and textural characteristics of the samples. CO production is strongly dependent on the extent of dealumination and is significantly suppressed using high acid concentrations.

Acknowledgements This work had been totally funded by the Iran National Science Foundation under the grant number 89000540. The authors express their gratitude for the support provided. The authors would like to acknowledge engineer Majid Mollavali and Dr. Amir Faramarzi for their great help for the preparation of various equipment. In addition, we wish to thank Dr. Sohrab Fathi for his collaboration.

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[21] C. Mirodatos, D. Barthomeuf, Superacid sites in zeolites, J. C. S. Chem. Comm., 1981 (1) 39-40. [22] A. Corma, V. Fornes, F. V. Melo, J. Herrero, Comparison of the information given by ammonia t.p.d. and pyridine adsorption-desorption on the acidity of the dealuminated HY and LaHY zeolite cracking catalysts, Zeolites, 7 (1987) 559-563. [23] F. Jin, Y. Li, A FTIR examination of the distributive properties of acid sites on ZSM-5 zeolite with pyridine as a probe molecule, Catal. Today, 145 (2009) 101-107. [24] X. Wang, K. Gui, Fe2O3 particles as superior catalysts for low temperature selective catalytic reduction of NO with NH3, J. Environ. Sci. 25(2013) 2469–2475. [25]. F. Chavez-Rivas,n G. Rodriguez-Fuentes, G. Berlier, I. Rodriguez-Iznaga, V. Petranovskii, R. Zamorano-Ulloa, S. Coluccia, Evidence for controlled insertion of Fe ions in the framework of clinoptilolite natural zeolites, Mic. Mes. Mat., 167 (2013) 76-81. [26] L. Kiwi-Minsker, D. A. Bulushev, A. Renken, Active sites in HZSM-5 with low Fe content for the formation of surface oxygen by decomposing N2O: is every deposited oxygen active?, J. Catal., 219 (2003) 273-285.

Captions Figure 1.SEM picture of a) the sample produced by 2 h treatment using 0.050 M oxalic acid and b) calcined H-form natural zeolite after calcination.

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Figure 2.The nitrogen adsorption isotherms of a) the raw zeolite (CLP000) and b) the zeolite treated with 0.500 M acid solution (CLP500). Figure 3.The XRD pattern of the different samples. Figure 4.The NH3-TPD diagram of different samples in the original (a) and deconvoluted form (b). Figure 5.The total conversion of NOx into N2 as a function of reaction temperature and zeolite treatment procedure. Figure 6.The outlet NO concentration as a function of reaction temperature and zeolite treatment procedure. Figure 7.The outlet NO2 concentration as a function of reaction temperature and zeolite treatment procedure. Figure 8. The outlet CO concentration as a function of reaction temperature and zeolite treatment procedure. Figure 9.The Gibbs free energy change of reactions (3) and (4) as a function of temperature.

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Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Table 1. Effect of oxalic acid treatment on crystallinity and Si/Al molar ratio.

sample code

sample description

CLP000

H-form, not dealuminated dealuminated dealuminated dealuminated

CLP005 CLP050 CLP500

oxalic acid concentration (M) --

treatment duration (h) --

crystallinity (%) 58.8

Si/Al molar ratio 7.30

surface area (m2 g-1) 14

0.005 0.050 0.500

2 2 2

53.5 52.0 42.6

8.37 9.19 11.46

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Table 2. Effect of oxalic acid treatment on the chemical composition of the samples.

sample code

Na2O (wt. %)

K2O (wt. %)

CaO (wt. %)

MgO (wt. %)

Fe2O3 (wt. %)

CLP000 CLP005 CLP050 CLP500

0.11 0.00 0.00 0.00

1.62 1.80 1.92 1.58

2.88 2.21 2.60 2.29

1.13 0.64 0.99 0.85

4.15 3.88 2.97 2.35

total equivalent (Al based, mole/100 g zeolite) 0.14 0.14 0.13 0.10

total equivalent (alkali and transition metal based, mole/100 g zeolite) 0.35 0.29 0.29 0.25

Graphical abstract

The effect of zeolite treatment on the total conversion of NOx into N2 as a function of reaction temperature and zeolite treatment procedure.

Highlights > The effect of clinoptilolite dealumination on the propane-SCR-NOx process is reported> Dealumination was performed using oxalic acid> NOx conversion to N2 is enhanced significantly> Dealumination effect on the zeolite chemical/physical structure is assessed.