Modification of Y Zeolite with Alkaline Treatment: Textural Properties

Article pubs.acs.org/IECR

Modification of Y Zeolite with Alkaline Treatment: Textural Properties and Catalytic Activity for Diethyl Carbonate Synthesis Shouying Huang, Pengzhen Chen, Bing Yan, Shengping Wang, Yongli Shen, and Xinbin Ma* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: In this work, we modified NaY zeolite (Si/Al = 5) with NaOH solutions of different concentrations followed by ion exchange with NH4NO3 to the H form of the zeolite. Treated NaY was used as a catalyst support for the preparation of CuY for diethyl carbonate (DEC) synthesis through the oxidative carbonylation of ethanol. The textural and acidic properties of NaY and the catalytic performance of the corresponding CuY materials were investigated. Compared with the untreated sample, CuY catalysts using modified NaY samples as supports exhibited higher conversions of ethanol and similar selectivities to DEC. Inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), N2 adsorption, Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM) were used to explore the origin of the improvement in activity. The experimental results showed that alkaline treatment induced defects in the zeolite framework and greatly promoted dealumination through ion exchange assisted by microwave radiation, which caused the generation of mesoand macropores in zeolite Y and contributed to the catalytic performance. Furthermore, the increased amount of hydroxyl species in supercages and extraframework aluminum species resulted in an increase in the number of Cu active sites and further DEC production.

1. INTRODUCTION Over the past few decades, the green chemistry initiative has gained momentum. This movement has drawn a great deal of attention to the design of environmentally benign compounds and clean and highly efficient synthesis processes, as well as the development of new catalysts. Recently, environmentally friendly dialkyl carbonates such as diethyl carbonate (DEC) have gained popularity because of their biodegradability and low toxicity. Because of its diversified chemical properties, DEC represents an alternative to hazardous chemical reagents and solvents for many applications in both extraction and reaction. DEC is increasingly being used as an excellent carbonylation and ethylation agent in place of hazardous, waste-generating compounds for the manufacture of agrochemicals, pharmaceuticals, paints, coatings, and fragrances.1 Compared to dimethyl carbonate (DMC) and ethanol, DEC has been proposed as a potential fuel oxygenate additive to replace methyl tert-butyl ether (MTBE) because of its high oxygen content (40.6 wt %) and favorable fuel/water partition coefficient.2,3 Because of the extreme toxicity and corrosiveness of phosgene processing, DEC is now manufactured by transesterification or oxidative carbonylation. In recent years, the oxidative carbonylation of ethanol catalyzed by Cu-based catalysts has attracted increased attention on account of its low pollution, high availability of raw materials, and atom economy. Although a good initial catalytic performance can be obtained over Wacker-type catalysts, these systems still suffer deactivation due to the loss of chloride. Microporous zeolites have been applied as catalyst supports for oxidative carbonylation because of their unique properties, such as high surface area, well-defined microporosity, high thermal stability, and intrinsic acidity.4 Most © 2013 American Chemical Society

importantly, the zeolite lattice provides a stable environment to confine Cu species. King first used a Cu/zeolite catalyst prepared by solid-state ion exchange (SSIE) in oxidative carbonylation to produce DMC, suggesting that the presence of chloride is not required.5 Anderson and co-workers reported that CuX showed better catalytic properties than CuZSM-5 for DMC production because of the weaker adsorption of CO and the fact that the residual Brønsted sites of the Cu-zeolite catalyst led to the decomposition of formed carbonates.6−8 The group of Bell investigated the effects of zeolite structure and composition as well as the local environment of Cu on catalytic performance.9−11 Richter et al. prepared chloride-free Cuimpregnated zeolite by starting with Cu(II) salts, which showed good performance for DMC synthesis.12,13 In a previous work, we investigated the role of zeolite structure in product selectivity, concluding that diffusion limitations lead to different product distributions.14 An appropriate porosity favors active-site accessibility and molecular transport for achieving optimal catalytic performance. We also demonstrated that the Brønsted acidic sites of zeolites have a quantitative relationship with active sites.15 Accordingly, zeolite properties should be tailored toward a suitable acidity combined with a proper porosity. Recently, treatment with alkaline medium has become a widely used postsynthesis method to induce mesoporosity in zeolites with multiple framework structures, including MFI, MOR, BEA, FER, and TON.16 However, the optimal Si/Al ratio for which intraReceived: Revised: Accepted: Published: 6349

November 22, 2012 April 3, 2013 April 20, 2013 April 20, 2013 dx.doi.org/10.1021/ie3032235 | Ind. Eng. Chem. Res. 2013, 52, 6349−6356

Industrial & Engineering Chemistry Research

Article

products DEC, acetal, and acetaldehyde according to the stoichiometry of the reactions

crystalline mesopores can be generated successfully is in the range of 25−50.16 For lower Si/Al ratios, the creation of mesopores by alkaline solution is hampered because of repulsion between OH− and the negatively charged framework. Here, mesopores were introduced successfully into NaY zeolite with a low Si/Al ratio by alkaline treatment followed by proton exchange. We gained deeper insight into the effects of modification with NaOH treatment on the properties of Y zeolite and its catalytic activity in the oxidative carbonylation of ethanol.

2CH3CH 2OH + CO +

1 O2 2

→ (CH3CH 2O)2 CO + H 2O

3CH3CH 2OH +

(1)

1 O2 → (CH3CH 2O)2 CH 2CH3 + 2H 2O 2 (2)

2. EXPERIMENTAL SECTION 2.1. Materials and Treatments. Commercial NaY zeolite (Si/Al = 5) was purchased from the Catalyst Plant of Nankai University (Tianjin, China). Alkaline treatment was performed by adding 5-g NaY samples to 50 mL of preheated NaOH solutions with varying concentrations (0.1, 0.3, and 0.5 M) at 343 K. After being stirred for 1 h, the slurry was cooled to room temperature, filtered, and washed with deionized water until the pH was ∼7. Subsequently, the filter cake was dried at 473 K for 8 h under a vacuum and calcined in air at 773 K for 3 h. Before exchange with CuCl to prepare CuY catalysts, the treated NaY samples were converted to protonic form through microwaveassisted NH4+ exchange (1-g sample in 10 mL of 0.5 M NH4NO3). The ion exchange was carried out in a Multiwave 3000 microwave reaction system (Anton Paar) equipped with a pressure/temperature sensor (in combination with an IR sensor) at 333 K for 45 min. Then, the samples were washed, filtered, and dried at 393 K for 4 h under a vacuum. This procedure was followed by calcination at 773 K for 3 h in air at a heating rate of 2 K/min. The final zeolite supports with alkaline treatment (e.g., 0.3 M NaOH solution) and proton exchange was designated as HY-0.3 M. 2.2. Catalyst Preparation and Activity Measurement. A mixture of alkaline-treated and protonated zeolites and purified CuCl with a mass ratio of 2 was ground in an agate mortar and then exposed to high-purity N2 (99.999%) flowing at 60 mL/min. The temperature was ramped at 2 K/min to 773 K, held for 6 h, and then decreased slowly to room temperature. All samples were stored in a drybox and then pressed and sieved to 20−40 mesh particles before catalytic tests. Catalytic measurements were performed using 1 mL (∼0.6 g) of catalysts loaded in a computer-controlled continuous microreactor system (WFS-3015) with a 4-mm-i.d. quartz tubular reactor. The reaction system was maintained at a total pressure of 0.7 MPa with a back-pressure regulator (26-1700 series, Tescom). The flow rates of CO, O2, and N2 were set by mass flow controllers, and ethanol was fed into the main stream by a syringe pump (Elite, P230) on an evaporator. Before being exposed to the reactants, each catalyst was pretreated at 413 K in a stream of high-purity N2. All lines were heated to avoid condensation of ethanol and products. The reaction products were analyzed by online gas chromatography using an Agilent 7890A GC instrument equipped with a DB-624 capillary column connected to a flame ionization detector (FID) and two Porapack Q columns and one molecular sieve 13X column connected to a thermal conductivity detector (TCD). Organic compounds (ethanol, DEC, acetal, and acetaldehyde) were detected with the FID, whereas gases (CO, O2, CO2) were detected with the TCD. Product analysis was completed at 30min intervals. Ethanol conversion was calculated from the

CH3CH 2OH +

1 O2 → CH3CHO + H 2O 2

(3)

The selectivity to DEC was determined by the equation SDEC = 2[(CH3CH 2O)2 CO] [CH3CHO] + 3[(CH3CH 2O)2 CH 2CH3] + 2[(CH3CH 2O)2 CO]

(4)

2.3. Characterization Methods. X-ray diffraction (XRD) measurements were carried out using Cu Kα radiation (λ = 1.54056 Å) on a Rigaku C/max-2500 diffractometer at 40 kV and 200 mA. The patterns were recorded in the 2θ range from 3° to 50° at a scanning rate of 8°/min. The concentrations of Si and Al in the treated samples were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) (VISTA-MPX, Varian). Before each measurement, the samples were digested in HCl, HNO3, and HF aqueous solutions assisted by microwave radiation (Multiwave 3000, Anton Paar). H3BO3 was used for the complexation of HF. The specific surface areas and pore volumes of the samples were determined by nitrogen adsorption−desorption isotherms at 77 K on a Micromeritics ASAP-2020 apparatus. Prior to analysis, all of the samples were outgassed at 573 K for 24 h. Surface areas were determined by the Brunauer−Emmett− Teller (BET) method, and the pore distribution was obtained from the desorption branch by applying the Barrett−Joyner− Halenda (BJH) method. Transmission electron microscopy (TEM) with energydispersive spectroscopy (EDS) was performed on a Philips Tecnai G2 F20 instrument. The samples were dispersed in ethanol by ultrasound, placed on a carbon-coated molybdenum grid, evaporated at ambient conditions. Fourier-transform infrared (FTIR) spectra were recorded on a Thermo Scientific Nicolet 6700 spectrometer equipped with a mercury cadmium telluride (MCT) detector. The samples were pressed into 13-mm-diameter self-supporting wafers and placed in a high-temperature cell with CaF2 windows. Before measurements, the samples were pretreated in a vacuum at 673 K for 1 h and then cooled to room temperature. Spectra were recorded using 32 scans with a resolution of 4 cm−1 in the range of 1100−4000 cm−1. Hydrogen temperature-programmed reduction (H2-TPR) was carried out on a Micromeritics ASAP 2920 instrument equipped with a TCD. For each run, ∼500 mg of fresh catalyst was loaded into a quartz tube and pretreated in an Ar stream at 473 K for 1 h before reduction. Subsequently, the catalyst was heated from 323 to 1323 K at a heating rate of 10 K/min in a flow of Ar containing 10 vol % H2. The overall flow rate was 30 mL/min. The consumption of H2 was determined from the integrated peak areas by calibration with argon pulses in the 6350

dx.doi.org/10.1021/ie3032235 | Ind. Eng. Chem. Res. 2013, 52, 6349−6356


Article

H2/Ar flow. The results were normalized to sample weights of 500 mg.

concentration of NaOH solution increased further, the framework Si/Al ratio became lower, and the unit-cell volumes decreased as a result of the extraction of Si from the zeolitic framework. Changes in unit-cell parameters are often associated with changes in the composition of zeolites.18 Because of the larger size of Al3+ and the weaker bonds of Al−O, a higher content of framework aluminum will typically lead to a larger unit-cell volume. As reported in Table 1, regardless of whether the NaY samples were treated with NaOH, a reduction of Al content was observed after ion exchange with NH4NO3, especially for the framework composition. This was caused by the dissolution of framework aluminum species in acidic solution. With increased concentration of NaOH, the differences in the Si/Al ratios between the bulk and the framework became relatively larger after ion exchange, and a severe loss of crystallinity was observed, which demonstrated the formation of extraframework aluminum species. This result is in accordance with the results from 27Al MAS NMR measurements (Figure S-1, Supporting Information). Both parent NaY and HY exhibited a typical peak of tetrahedrally coordinated aluminum at 60 ppm in the 27Al NMR spectra, whereas a peak between 0 and 3 ppm, which is ascribed to octahedrally coordinated extraframework aluminum species, became pronounced.19 These results indicated that NaOH treatment caused slight desilication and dramatically promoted the dealumination from the framework during successive ionexchange processes. In the N2 adsorption/desorption isotherm (Figure S-2, Supporting Information), HY zeolite exhibited typical type-I profiles that are characteristic of microporous structures. However, the isotherms for the NaOH-treated samples showed hysteresis loops, which are usually related to mesopores by capillary condensation. Table 2 summarizes the BET surface

3. RESULTS 3.1. Textural Properties. Y zeolite was treated in NaOH solution at different concentrations and then transformed to proton type by ion exchange in NH4NO3 solution followed by calcination. To quantify the structural modification, the composition and crystal texture were investigated by ICPOES and XRD. As shown in Figure 1, the structure of Y zeolite

Figure 1. XRD patterns of HY upon NaOH treatment at different concentrations.

was maintained after alkaline treatment and ion exchange with ammonium solution, despite some loss of crystallinity. The Si/ Al ratios, unit-cell volumes (Vcell), and degrees of crystallinity (Dxrd, %) are listed in Table 1. The relative crystallinity was evaluated by comparing the areas of all of the diffraction peaks of the NaOH-treated samples with those of the starting NaY. The unit-cell volumes (Vcell) were calculated using the nine peaks assigned to the (110), (220), (311), (331), (511), (440), (533), (642), and (555) reflections with UnitCell software. The bulk and framework Si/Al ratios were obtained by ICP-OES and XRD using the Fichtner-Schmittler relation. The NaOH treatment procedure resulted in a reduction in the bulk Si/Al ratio, which demonstrated the extraction of silanol defects and amorphous silica from the zeolite. During NaOH treatment, the hydroxide anions (OH−) attack the silanol defects and stabilize silicate anions,17 leading to an obvious decrease of the bulk Si/Al ratio. However, the corresponding Si/Al ratio of the zeolite framework underwent little change when the concentration of NaOH solution was lower than 0.5 M, indicating that the high content of negatively charged AlO4 tetrahedra hampered the hydrolysis of Si−O−Si bonds in low-concentration alkaline solution. When the

Table 2. N2 Sorption Data for Untreated and Treated Samples

a

concentration of NaOH (mol/L)

SBET (m2/g)

Sexta (m2/g)

Smicroa (m2/g)

Vmesob (cm3/g)

Vmicrob (cm3/g)

Vtotala (cm3/g)

0 0.1 0.3 0.5

506.1 668.8 441.5 507.9

9.6 33.3 43.3 61.4

496.5 635.5 398.2 446.5

0.023 0.119 0.175 0.158

0.237 0.303 0.186 0.208

0.253 0.426 0.363 0.373

:calculated by t-plot method. b:volume adsorbed at p/p0 = 0.99.

areas and t-plot data. Compared with HY without NaOH treatment and other samples, the sample treated with 0.1 M NaOH displayed the largest surface area (668.8 m2/g) and total pore volume (0.426 cm3/g), implying the extraction of amorphous species in the zeolite channel when exposed to mild alkaline solution. This explains why the crystallinity of the

Table 1. Changes in Composition and Relative Crystallinity of NaY after NaOH Treatment and NH4+ Ion Exhange NaY-NaOH

HY-NaOH

concentration of NaOH (mol/L)

bulk Si/Al ratioa

framework Si/Al ratiob

Vcellb

bulk Si/Al ratioa

framework Si/Al ratiob

Vcellb

Dxrdb (%)

0 0.1 0.3 0.5

4.83 4.48 4.39 4.19

3.53 3.63 3.55 2.83

14 901 14 890 14 904 15 030

4.81 5.16 6.91 6.57

4.77 4.70 9.85 12.11

14 758 14 765 14 510 14 461

100 103.8 94.0 71.6

a

Calculated from ICP-OES results. bCalculated from XRD patterns. 6351



Article

was performed to analyze the composition of the debris around the zeolite particles. The major constituents of the amorphous species are Al, O, and Si. As shown in Table 3, amorphous species of HY without alkaline treatment revealed a slightly higher content of aluminum than silicon. The content of Si in NaY-0.3 M was higher than that of Al, which is opposite to the behavior in HY. Hence, we propose that NaOH treatment can promote the intensity of dealumination during the subsequent proton-exchange step. 3.3. Acidity Characterization. Based on the mechanism of SSIE, the Brønsted acidity of the zeolites plays an important role in the formation of Cu active sites, so that it is necessary to investigate the acidic properties of zeolites after the alkaline treatment and ion-exchange procedure. The FTIR spectra of NaY with alkaline treatment and H-form Y were measured to probe the acidity of the samples. Figure 4 illustrates the influence of treatment with NaOH of different concentrations on the O−H stretching region of the IR spectra of HY. Two bands at 3640 and 3550 cm−1 were attributed to the Brønsted acid O−H stretching vibrations located in the supercage and sodalite cage, respectively.20,21 The band at 3740 cm−1 is assigned to the terminal silanol groups in zeolite Y. The shoulder peak centered at about 3680 cm−1 is attributed to hydroxyl groups in extraframework material, whereas the component at the lower-frequency side of the 3640 cm−1 band is attributed to supercage OH groups interacting with residual extraframework species.20,22 The intensity of the 3740 cm−1 band first declined and then increased with increasing severity of NaOH treatment, which suggested that the extraframework silica species were extracted at low alkalinity, whereas desilication from the zeolitic framework occurred at high alkalinity. This is in agreement with the results of composition analysis by ICP-OES and XRD. At the same time, the feature at 3650 cm−1 became stronger and sharper. 3.4. Formation of Cu Active Sites. No diffraction peaks of Cu species were detected in the XRD patterns of the catalysts (Figure S-3, Supporting Information), providing evidence for complete exchange of CuCl and high dispersion of excess Cu species. CO-adsorption DRIFTS was also used to determine the state of Cu, and the bands at 2146 and 2160 cm−1 are attributed to Cu(I) cations located at different sites in the supercages of Y zeolite,23 as shown in the Supporting Information (Figure S-4). The band of CO adsorbed on CuOx (2110 cm−1) cannot be observed clearly, which suggests that no agglomeration of Cu species occurred in the cages at this Cu loading. H2-TPR was carried out for the Cu-doped Y catalysts to determine the reducibility of the Cu species. The normalized H2-TPR profiles are shown in Figure 5, which illustrates the effect of the NaOH treatment on the reduction of Cu species. The CuY sample without NaOH treatment exhibited two H2 consumption peaks centered at 473 and 681 K. In Cu2+-exchanged zeolites, the overlapped two peaks with shoulder were assigned to the reduction of Cu2+ species in different lattice positions of Y, described as supercage, sodalite, and hexagonal positions.24 The peak observed at high temperature (>1000 K) was characteristic of reduction of Cu+ to Cu0. Drake et al. first proposed H2-TPR studies of CuY prepared by vapor-phase exchange with CuCl.10 They assigned the three H2 consumption peaks at 533, 783, and 1163 K to the reduction of Cu+ cations associated with sites III′, II, and I′, respectively. Among these sites, the most accessible position for reactant is site III′. Accordingly, the two peaks at 473 and 681 K for untreated CuY were assigned to Cu+ located on III′ and II

0.1 M sample increased. With increasing severity of treatment, the external surface area increased considerably from 9.6 to 61.4 m2/g as a result of the dissolution of the framework and the formation of mesopores. For severely treated samples (0.3 and 0.5 M), a decrease of the micropore area and volume was observed, suggesting that the micropore system was also affected during treatment. This result is in agreement with the destruction of the zeolite framework quantified by the XRD results. Simultaneously, the total pore volume increased, mainly because of a larger mesopore volume. The pore size distribution profiles obtained by applying the BJH model (Figure 2) provide details about the changes in mesopores. No

Figure 2. Mesopore size distribution of HY upon NaOH treatment at different concentrations.

mesopores existed for the protonated Y sample without alkaline treatment, indicating that considerable mesopores could not be created from the destruction of the framework solely through NH4+ ion exchange. We can also conclude that a wide range of meso- and macropores was obtained after NaOH treatment and subsequent ion exchange and calcination. Compared to the samples treated with high-concentration alkaline solution, the 0.1 M NaOH modified sample presented macropores but few mesopores, which indicates that treatment with mild alkaline solution had little effect on the promotion of dealumination, so that only a few intracrystalline mesopores formed. With increasing concentration of alkaline solution, 4−6-nm mesopores appeared and became more visible. This change possibly resulted from the extraction of aluminum atoms from the framework. 3.2. Morphology. TEM images of NaY before and after treatment, as well as protonated samples, were recorded to examine the morphology of the supports. As illustrated in Figure 3, the starting NaY sample revealed a typical octahedron morphology, with a well-crystallized lattice structure. The lattice structure remained unchanged during the protonexchange procedure, leaving only a little amorphous debris at the edge of the HY particles. The image of sample with 0.3 M NaOH treatment showed some small defects in structure, indicating the extraction of framework T atoms. After ion exchange with NH4NO3 assisted by microwave radiation, the zeolite grains partially collapsed, and string-like mesopores formed as interruptions of the lattice planes, with dimensions of 4−8 nm. The edges of the particles look melted, and the original shape can hardly be recognized. Much more amorphous debris was present around the particle than for the HY sample. This provides a reasonable explanation for the reduction of crystallinity calculated by the XRD pattern. EDS 6352



Article

Figure 3. TEM images of NaY and NaY after treatment, along with enlargements of particles of the corresponding zeolites (insets).

Table 3. Element Contents of Samples As Determined by EDS element (at. %) sample

Al

Si

O

HY NaY-0.3M HY-0.3M

23.724 6.552 32.154

19.816 29.051 13.650

56.460 64.396 54.196

Figure 5. H2-TPR spectra of CuY upon NaOH treatment at different concentrations.

sites in this work. No peak was detected at high temperature for untreated CuY. However, three peaks were observed for CuY treated with NaOH solution. The peak at about 1050 K became pronounced, indicating the presence of Cu+ located on I′ sites. As shown in Figure 5, the H2-consumption peak at low temperature (473 K) was sharp when the NaY was untreated. Post-treatment increased the intensity and width of this peak, suggesting that the amount of Cu+ located on III′ sites became larger. The area of the peak at about 681 K also improved. In

Figure 4. FTIR spectra in the OH stretching region of HY upon NaOH treatment at different concentrations. 6353



Article

4. DISCUSSION Postsynthesis techniques are widely used for tailoring the textural and chemical properties of zeolites, such as pore structure, acidity, crystallinity, and hydrothermal stability.26,27 For the synthesis of organic carbonate through oxidative carbonylation, the Brønsted acidity of the zeolite is a crucial factor in the formation of active sites during SSIE.15 Therefore, one must take into account the preservation of the Brønsted acidity of the zeolite framework during the modification of the pore structure by alkaline treatment of the zeolite support. It turned out that the NaOH treatment and subsequent NH4NO3 exchange procedure influenced the physicochemical properties of zeolites in favor of the catalyst activity. The discussion that follows focuses on relevant effects of the pretreatment of the support on the structure, acidity, active-site formation, and catalytic performance. 4.1. Influence on Textural Properties. The experiments carried out with a series of NaOH concentrations led to desilication of both the bulk and framework Si from parent NaY zeolite, and alkali treatment clearly aggravated the degree of dealumination during NH4NO3 ion exchange and calcination, as displayed in Figure 1 and Table 1. For the material treated with 0.1 M solution, the decrease in Si/Al ratio was attributed to the extraction of nonframework Si, so it had a limited effect on the further NH4+ ion-exchange procedure. As the concentration of NaOH increased, the change in the Si/Al ratio became more significant because of framework Si dissolution. NH4NO3 ion-exchange and calcination resulted in an increase of the Si/Al ratio. Moreover, N2 isothermal adsorption/desorption and TEM experiments provided evidence for differences in the structural properties and morphology of modified zeolite HY. After sequential treatment, the reductions of microporous areas and volumes were likely due to the partial destruction of the microporous structure (Table 2). In addition, mesopores were introduced at the same time. It is also noteworthy that, in addition to intracrystalline mesopores, the sequential pretreatment caused the creation of macropores because of the large amount of amorphous species formed, as shown in Figure 3. It has been reported that alkali treatment usually involves the selective extraction of framework Si atom, whereas treatment using NH4NO3 or other acid solution initiates dealumination, both of which can encourage the generation of mesoporosity.28,29 According to the literature,30 the extent of dealumination is related to the flexibility of the zeolite framework, and the accessibility of the Al atoms depends on the pore arrangement and sizes. After NaOH treatment and calcination, the amorphous species in the zeolite channels were removed and structural defects and mesoporosity were generated, so that ammonium ions could more easily to approach the Al atoms located in cages. Furthermore, we employed microwave irradiation to facilitate ion exchange, which would cause more intense dealumination and mesoporosity than the conventional method. Taking into account the negative contribution of diffusion limitations, the introduction of meso- and macropores in zeolite Y is beneficial to the diffusion of reactant and product molecules in the reaction system, which can contribute to the catalytic activity. 4.2. Influence on Acidity and Active Site Formation. In the oxidative carbonylation of alcohols catalyzed by Cu-doped zeolites, Brønsted acidity is a crucial factor based on the mechanism of SSIE for the formation of active sites. It has been

summary, pretreatment of the support was responsible for the accessibility of Cu+ to sites I′, and the total areas of the H2consumption peaks increased with increasing concentration of NaOH solution. 3.5. Catalytic Testing. Experiments were undertaken to determine the influence of the NaOH concentration on the activity and selectivity of CuY under the same measurement conditions. The results are presented in Figures 6 and 7. The

Figure 6. Activity of DEC synthesis on CuY catalysts treated at different NaOH concentrations.

Figure 7. Selectivity to products on CuY catalysts treated at different NaOH concentrations.

untreated sample displayed an initial DEC selectivity of 55% and a yield of 0.65%. Under the same reaction conditions, the activity of all catalyst samples exhibited a significant improvement after alkali treatment with a certain concentration of NaOH solution. Although the selectivity to DEC declined slightly, the increase in EtOH conversion dramatically improved the yield of DEC. The rate of DEC formation increased initially, reached a maximum value, and then declined. The optimal concentration of NaOH solution was found to be 0.3 M. Compared with that of the parent sample, the selectivity to diethyl ether (DEE) also increased after modification of the catalyst. When the concentration of NaOH solution was 0.5 M, the selectivity to DEE dropped. Because DEE is considered as a product from the condensation of acetaldehyde with ethanol,25 the variation in acetaldehyde selectivity followed a contrary trend to the selectivity to DEE. 6354



Article

conversion of ethanol increased significantly, with almost constant selectivity. Characterization indicated that alkaline treatment under appropriate conditions maintained the lattice structure of NaY and promoted dealumination through ion exchange assisted by microwave radiation. The sequential posttreatment and calcination led to the generation of 4−6-nm mesopores and a wide range of macropores, which would facilitate the accessibility of active sties and molecular transport. At lower NaOH concentration, the increase in crystallinity was most likely caused by the elimination of structural defects and extraframework species, whereas high concentration caused the obvious dissolution of the zeolite framework. Alkaline treatment caused limited effects in the protonic acidity of Y zeolite. The variation in OH groups located in zeolite cages provided evidence of the dissolution and reinsertion of Al atoms. The increased number of OH groups in supercages and extraframework Al species can contribute to the improvement in the amount of Cu active sites and further DEC production. Furthermore, I′ sites were probably more accessible for occupancy by Cu+ and accessibility by reactant molecules. With increasing concentration of NaOH solution, the total amount of Cu+ located at sites III′, II, and I′ increased, which caused an improvement in catalytic performance.

reported that alkaline treatment with appropriate conditions induces a limited effect in protonic acidity, whereas for dealumination caused by NH4+ exchange, a lower concentration of protonic sites is obtained but often with a higher acid strength.31 Herein, insights into the acidity changes of zeolite after pretreatment were gained from in situ IR spectra (Figure 4). The intensity of the band at 3640 cm−1 became stronger, which reflected the increase in Brønsted acidity in the supercages. For alkaline treatment, silicon and aluminum are dissolved at the same time, although silicon is dissolved in slightly larger amounts.32 On the basis of the literature,29,33 we attribute this behavior to the reinsertion of aluminum species into the vacancies during alkaline treatment and the migration of aluminum to the supercages during calcination. On the other hand, as shown in Figure 3, extraframework aluminum species located on the external surface, which had medium strong Brønsted acidity, probably also contributed to the formation of active sites and the further activity of DEC, as reported by Li et al.34 Therefore, despite the reduction of the aluminum content in the zeolite framework after treatment, the amount of Brønsted acidity located in the large cages was preserved and even increased. These changes in acidity supplied more opportunities to exchange Cu+ to form active sites for catalyzing ethanol conversion and DEC formation. The H2-TPR profiles (Figure 5) provide direct evidence of the different Cu active sites in CuY zeolite treated by NaOH solutions of various concentrations. For CuY using untreated zeolite as a support, there were only two H2-comsumption peaks at 473 and 681 K, ascribed to the Cu+ located on sites III′ and II. In comparison, one additional peak above 1000 K appeared after NaOH treatment and ion exchange with NH4NO3 due to the occurrence of Cu successfully doped in I′ sites. During ion exchange with NaY in ammonium nitrate solution, the exchange level would be limited and Na ions would partially remain, probably because NH4+ cannot approach the inside of the hexagonal prisms.29 It has been reported that Cu+ is exchanged with the protons of Brønsted acidic sites by SSIE. Therefore, I′ sites are not occupied first by Cu+ under our experimental conditions, which is different from the results of Bell and co-workers. After treatment with alkaline solution, lattice defects are formed, and mesoporosity is introduced into the NaY crystals. I′ sites are probably more accessible for exchanged with NH4+ and are protonated in the following calcination and then occupied by Cu+ after subsequent SSIE, which is in accordance with the TEM results. Drake et al. reported that only those Cu+ ions present at sites II and III′ are accessible to reactants and, hence, contribute to the activity of the catalyst.10 In our experiments, the areas of both peaks increased at low temperature, which means that more Cu+ was present at II and III′ sites. This explains the improvement in ethanol conversion and DEC yield. In addition, the high-temperature peak appeared and obtained its maximum value when the NaOH concentration was 0.3 M. The decrease in catalyst activity of the sample treated by 0.5 M solution was attributed to the deterioration of the zeolite framework and the formation of amorphous species in the zeolite channels, even though the total area of peak was the largest.

■

ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (NSFC) (Grant 20876112, 20936003), the Program for New Century Excellent Talents in University (NCET-04-0242), the Seed Foundation of Tianjin University (60303002), the Specialized Research Fund for the Doctoral Program of H igher Education (SRFDP) (Grant 20090032110021), and the Program of Introducing Talents of Discipline to Universities (B06006) is gratefully acknowledged.

■

REFERENCES

(1) Liu, T. C.; Chang, C. S. Vapor-phase oxidative carbonylation of ethanol over CuCl−PdCl2/C catalyst. Appl. Catal. A 2006, 304, 72− 77. (2) Dunn, B. C.; Guenneau, C.; Hilton, S. A.; Pahnke, J.; Eyring, E. M. Production of diethyl carbonate from ethanol and carbon monoxide over a heterogeneous catalyst. Energy Fuels 2002, 16, 177−181. (3) Roh, N. S.; Dunn, B. C.; Eyring, E. M.; Pugmire, R. J.; Meuzelaar, H. L. C. Production of diethyl carbonate from ethanol and carbon monoxide over a heterogeneous catalytic flow reactor. Fuel Process. Technol. 2003, 83, 27−38. (4) Groen, J. C.; Jansen, J. C.; Moulijn, J. A.; Perez-Ramirez, J. Optimal aluminum-assisted mesoporosity development in MFI zeolites by desilication. J. Phys. Chem. B. 2004, 108, 13062−13065. (5) King, S. T. Oxidative carbonylation of methanol to dimethyl carbonate by solid-state ion-exchanged CuY catalysts. Catal. Today 1997, 33, 173−182.

5. CONCLUSIONS The catalytic performance of alkaline-treated zeolite CuY in the oxidative carbonylation of ethanol for the synthesis of DEC has been studied. After treatment with 0.3 M NaOH, the 6355



Article

(26) Groen, J. C; Moulijn, J. A.; Ramírez, J. P. Decoupling mesoporosity formation and acidity modification in ZSM-5 zeolites by sequential desilication−dealumination. Microporous Mesoporous Mater. 2005, 87, 153−161. (27) Paixão, V.; Carvalho, A. P.; Rocha, J.; Fernandes, A.; Martins, A. Modification of MOR by desilication treatments: Structural, textural and acidic characterization. Microporous Mesoporous Mater. 2010, 131, 350−357. (28) Ž ilková, N.; Bejblová, M.; Gil, B.; Zones, S. I.; Burton, A. W.; Chen, C. Y.; Musilová-Pavlačková, Z.; Košová, G.; Č ejka, J. The role of the zeolite channel architecture and acidity on the activity and selectivity in aromatic transformations: The effect of zeolite cages in SSZ-35 zeolite. J. Catal. 2009, 266, 79−91. (29) Sato, K.; Nishimura, Y.; Matsubayashi, N.; Imamura, M.; Shimada, H. Structural changes of Y zeolites during ion exchange treatment: Effects of Si/Al ratio of the starting NaY. Micropor. Mesopor. Mat. 2003, 59, 133−146. (30) González, M. D.; Cesteros, Y.; Salagre, P. Comparison of dealumination of zeolites beta, mordenite and ZSM-5 by treatment with acid under microwave irradiation. Microporous Mesoporous Mater. 2011, 144, 162−170. (31) Matias, P.; Sá Couto, C.; Graça, I.; Lopes, J. M.; Carvalho, A. P.; Ramôa Ribeiro, F.; Guisnet, M. Desilication of a TON zeolite with NaOH: Influence on porosity, acidity and catalytic properties. Appl. Catal. A 2011, 399, 100−109. (32) van Laak, A. N. C.; Sagala, S. L.; Zečević, J.; Friedrich, H.; de Jongh, P. E.; de Jong, K. P. Mesoporous mordenites obtained by sequential acid and alkaline treatmentsCatalysts for cumene production with enhanced accessibility. J. Catal. 2010, 276, 170−180. (33) Ogura, M.; Shinomiya, S.; Tateno, J.; Nara, Y.; Nomura, M.; Kikuchi, E.; Matsukata, M. Alkali-treatment techniqueNew method for modification of structural and acid-catalytic properties of ZSM-5 zeolites. Appl. Catal. A 2001, 219, 33−43. (34) Xie, K. C.; Li, Z.; Slade, R. C. T. Studies of the interaction between CuCl and HY zeolite for preparing heterogeneous CuI catalyst. Appl. Catal. A 2001, 209, 107−115.

(6) Anderson, S. A.; Root, T. W. Kinetic studies of carbonylation of methanol to dimethyl carbonate over Cu+X zeolite catalyst. J. Catal. 2003, 217, 396−405. (7) Anderson, S. A.; Root, T. W. Investigation of the effect of carbon monoxide on the oxidative carbonylation of methanol to dimethyl carbonate over Cu+X and Cu+ZSM-5 zeolites. J. Mol. Catal. A: Chem. 2004, 220, 247−255. (8) Anderson, S. A.; Manthata, S.; Root, T. W. The decomposition of dimethyl carbonate over copper zeolite catalysts. Appl. Catal. A 2005, 280, 117−124. (9) Zhang, Y. H.; Briggs, D. N.; Desmit, E.; Bell, A. T. Effects of zeolite structure and composition on the synthesis of dimethyl carbonate by oxidative carbonylation of methanol on Cu-exchanged Y, ZSM-5, and mordenite. J. Catal. 2007, 251, 443−452. (10) Drake, I. J.; Zhang, Y. H.; Briggs, D.; Lim, B.; Chau, T.; Bell, A. T. The local environment of Cu+ in Cu-Y zeolite and its relationship to the synthesis of dimethyl carbonate. J. Phys. Chem. B 2006, 110, 11654−11664. (11) Zhang, Y. H.; Briggs, D. N.; Bell, A. T. Synthesis of dimethyl carbonate and dimethoxy methane over Cu-ZSM-5. J. Catal. 2006, 244, 219−229. (12) Richter, M.; Fait, M.; Eckelt, R.; Schneider, M.; Radnik, J.; Heidemann, D.; Fricke, R. Gas-phase carbonylation of methanol to dimethyl carbonate on chloride-free Cu-precipitated zeolite Y at normal pressure. J. Catal. 2007, 245, 11−24. (13) Richter, M.; Fait, M. J. G.; Eckelt, R.; Schreier, E.; Schneider, M.; Pohl, M. M.; Fricke, R. Oxidative gas phase carbonylation of methanol to dimethyl carbonate over chloride-free Cu-impregnated zeolite Y catalysts at elevated pressure. Appl. Catal. B 2007, 73, 269− 281. (14) Zhang, P. B.; Huang, S. Y.; Yang, Y.; Meng, Q. S.; Wang, S. P.; Ma, X. B. Effect of SSIE structure of Cu-exchanged β and Y on the selectivity for synthesis of diethyl carbonate by oxidative carbonylation of ethanol: A comparative investigation. Catal. Today 2010, 149, 202− 206. (15) Huang, S. Y.; Wang, Y.; Wang, Z. Z.; Yan, B.; Wang, S. P.; Gong, J. L.; Ma, X. B. Cu-doped zeolites for catalytic oxidative carbonylation: The role of Brønsted acids. Appl. Catal. A 2012, 417−418, 236−242. (16) Verboekend, D.; Ramírez, J. P. Design of hierarchical zeolite catalysts by desilication. Catal. Sci. Technol. 2011, 1, 879−890. (17) Svelle, S.; Sommer, L.; Barbera, K.; Vennestrøm, P. N. R.; Olsbye, U.; Lillerud, K. P.; Bordiga, S.; Pan, Y. H.; Beato, P. How defects and crystal morphology control the effects of desilication. Catal. Today 2011, 168, 38−47. (18) Bjørgen, M.; Joensen, F.; Spangsberg Holm, M.; Olsbye, U.; Lillerud, K. P.; Svelle, S. Methanol to gasoline over zeolite H-ZSM-5: Improved catalyst performance by treatment with NaOH. Appl. Catal. A 2008, 345, 43−50. (19) Yue, M. B.; Xue, T.; Jiao, W. Q.; Wang, Y. M.; He, M.-Y. Microporous Mesoporous Mater. 2012, 159, 50−56. (20) Tania Montanari, E. F.; Busca, G. Infrared spectroscopy of heterogeneous catalysts: Acidity and accessibility of acid sites of faujasite-type solid acids. J. Phys. Chem. C 2011, 115, 937−943. (21) Eichler, U.; Brandle, M.; Sauer, J. Predicting absolute and site specific acidities for zeolite catalysts by a combined quantum mechanics/interatomic potential function approach. J. Phys. Chem. B 1997, 101, 10035−10050. (22) Vimont, A.; Starzyk, F. T.; Daturi, M. Analysing and understanding the active site by IR spectroscopy. Chem. Soc. Rev. 2010, 39, 4928−4950. (23) Lamberti, C.; Zecchina, A.; Groppo, E.; Bordiga, S. Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy. Chem. Soc. Rev. 2010, 39, 4951−5001. (24) Kieger, S.; Delahay, G. Selective catalytic reduction of nitric oxide by ammonia over Cu-FAU catalysts in oxygen-rich atmosphere. J. Catal. 1999, 183, 267−280. (25) Chen, S.; Wang, S. P.; Ma, X. B.; Gong, J. L. Selective oxidation of methanol to dimethoxymethane over bifunctional VOx/TS-1 catalysts. Chem. Commun. 2011, 47, 9345−9347. 6356


Modification of Y Zeolite with Alkaline Treatment: Textural Properties

Recommend Documents