Effects of Cesium Ions and Cesium Oxide in Side ... - ACS Publications

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Effects of Cesium Ions and Cesium Oxide in Side-Chain Alkylation of Toluene with Methanol over Cesium-Modified Zeolite X He Han,† Min Liu,† Fanshu Ding,† Yiren Wang,† Xinwen Guo,*,† and Chunshan Song*,†,‡ †

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Centre for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P.R. China ‡ Department of Energy and Mineral Engineering, EMS Energy Institute, PSU-DUT Joint Centre for Energy Research, Pennsylvania State University, University Park 16802, Pennsylvania, United States S Supporting Information *

ABSTRACT: The side-chain alkylation of toluene with methanol was studied on cesium-modified zeolite X prepared by ionexchange and impregnation with different cesium precursors. The catalyst was characterized by X-ray diffraction, scanning electron microscopy, X-ray fluorescence spectroscopy, Ar physical adsorption−desorption, NH3 temperature-programmed desorption (TPD), CO2-TPD, thermogravimetric/differential thermal analysis, ultraviolet-Raman spectroscopy, energy-dispersive spectrometry, and X-ray photoelectron spectroscopy. Cesium hydroxide was found to be the most effective precursor for modifying zeolite X. Cesium ions bonded on the framework and cesium oxide formed in the pore were found to take different roles in this reaction. Cesium ions could adsorb and activate toluene or modify the basicity of framework oxygen, whereas cesium oxide could ensure the effective conversion of methanol to formaldehyde and inhibit the process of coke deposition. A possible reaction pathway for this reaction over cesium-modified samples was described. The synergistic effect of cesium ions and cesium oxide was proven to be important for the formation of styrene and ethylbenzene.

1. INTRODUCTION Styrene is a commodity chemical sold and utilized in extremely large volume for synthesizing and manufacturing polystyrene, copolymers, and other industrial resins. One of the potential new processes for making styrene monomer with low cost is toluene side-chain alkylation with methanol. A number of researchers have attempted to synthesize styrene via the toluene−methanol route during the past 40 years.1−5 Alkali metal-modified zeolites have been recognized as suitable catalysts for the side-chain alkylation of toluene to styrene and ethylbenzene. Especially for cesium-modified zeolite X or Y, it has been studied extensively and deeply. Ion-exchange and impregnation are two conventional methods for adding cesium into zeolites. Because of the differences of these two methods, cesium normally exists in various forms. Cesium ion-exchanged zeolite X or Y has been accepted to be the most effective solid base catalyst for producing styrene from toluene and methanol. According to Unland and Baker6 and Lin and Sphon,7 cesium ion-exchanged zeolite X or Y further modified with boron or phosphorus could favor the formation of styrene. Borgna et al.8 found that formation of styrene and ethylbenzene was significant only on CsNaY zeolites of exchange degree higher than about 40%. Bimolecular side-chain alkylation reaction would require a proper surface geometric configuration of Oδ‑−Cs+ pairs that is achieved only on Cs-rich zeolites. Jiang et al.9 found that the zeolites X exchanged by both cesium and other alkali metals exhibited higher catalytic performance than the one modified by single alkali metal ions. By cesium ion-exchange, sodium ions of zeolite X or Y could be partially or almost completely substituted by cesium ions. Cesium species of ion-exchanged zeolites could exist as cesium cations. Palomares et al.10 © XXXX American Chemical Society

indicated that cesium ions bonded on the framework could not only improve the catalyst base strength but also stabilize adsorbed toluene. Compared with cesium ion-exchange, the impregnation method seems to be researched infrequently. Lacroix et al.11 reported that cesium ion-exchanged zeolite without washing with water demonstrated higher reactivity than the washed catalyst. Engelhardt et al.12 found that high selectivity for ethylbenzene formation was achieved on cesium ion-exchanged zeolite X containing excess alkali hydroxides. Hunger et al.13 indicated that the reaction of toluene/methanol mixtures on cesium-exchanged zeolites Y further impregnated with cesium hydroxide leads to a direct conversion of formaldehyde and toluene to styrene and ethylbenzene. Alabi et al.14 prepared a series of Cs2O modified Cs-X (Cs-X was prepared by ionexchange) by impregnation for side-chain alkylation of toluene. They found that the toluene conversion to side-chain product increased, but the selectivity to styrene decreased upon modification of Cs-X with Cs2O. Apparently, by cesium impregnation of zeolites, cesium species may exist as a cesium compound, which has been accepted by researchers.13,14 Despite the above-mentioned work, the different effects of cesium ions and cesium compounds on the performance of the catalysts remain to be clarified. In particular, the promotion effects of Cs2O by adding extra cesium compound into zeolite still need to be further studied. Received: November 3, 2015 Revised: January 21, 2016 Accepted: January 29, 2016

A

DOI: 10.1021/acs.iecr.5b04174 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

exposed to ammonia−helium mixture (8% NH3−92% He) for 30 min. The physically adsorbed NH3 was removed by helium at 393 K for 1 h. The TPD curves were obtained at a heating rate of 10 K min−1 from 393 to 923 K. The desorbed ammonia was detected by gas chromatography with a thermal conductivity detector. CO2-TPD measurements were conducted the same as NH3-TPD, and the absorption gas was pure CO2. It is worth noting that the purified samples were cooled to 303 K for absorption and desorption of CO2. Thermogravimetric/differential thermal analysis (TG/DTA) curves were measured with a TGA/SDTA851e thermobalance (Mettler Toledo). The thermal analysis data were collected in the range of 308−1173 K in air flow to investigate the coke deposition of catalysts. The heating rate was 10 K min−1, and the sample weight was 10 mg. Ultraviolet (UV) Raman spectroscopy measurements were recorded on a homemade DL-2 Raman spectrometer. A 244 nm line of LEXEL LASER was used as the excitation source. X-ray photoelectron spectroscopy (XPS) measurements were performed on Escalab 250 (Thermo Fisher VG). The based pressure was 2.4 × 10−8 Pa. Monochromatized Al Kα 1486.6 eV radiation was used as the X-ray source. The binding energies were referenced to the C 1s band at 284.6 eV. The powders of the as-prepared samples were taken and kept in a valve bag to protect the samples from being deliquesced before XPS analysis. Cleaning of the sample surface was performed by Ar+ ion sputtering followed by annealing in vacuum. The depth profiling was performed over an area of 2.0 × 2.0 mm2 under Ar+ sputtering with 3 keV and 2 μA. The etching rate was calculated to be 0.1 nm s−1 on standard Ta2O5. The sample was eroded by ion sputtering for analyzing the species in the depth of 50 and 100 nm; the vacuum was then 6.0 × 10−6 Pa. The binding energy values and the area of XPS peaks were determined after analysis of the line shapes by curve fitting of the experimental data using Gaussian−Lorentzian functions and Shirley background subtraction. 2.4. Catalytic Testing. Toluene (T) side-chain alkylation with methanol (M) was carried out at atmospheric pressure in a fixed bed, vertical, down-flow stainless steel reactor placed inside a tube furnace. N2 was used as carrier gas with molar ratio of N2/(T + M) = 2. About 1.6 g of the catalyst was employed in the test. All of the catalysts were pressed into thin wafers without any binder and sieved to retain particle size with 10−20 mesh for catalytic measurements and pretreated in situ at 773 K for 1 h under a flow of N2 prior to reaction; they were then cooled to the reaction temperature, which was 698 K. The mixture of toluene and methanol with a molar ratio of T/M = 2 was fed into the reactor using an injection pump with a weight hourly space velocity (WHSV) of 2.0 h−1. The gaseous effluent from the reactor was collected in a cold trap (273 K). Liquid product was analyzed in a gas chromatograph (Agilent Technologies GC6890) equipped with an INNOWAX capillary column (60 m length, inner diameter 0.25 mm, stationary phase thickness 0.25 μm) and a flame ionization detector (FID). Gaseous product was analyzed in another gas chromatograph (Tianmei GC 7890F) equipped with a HayeSep Q packed column (2 m length, inner diameter 2 mm) and a FID. The gaseous product passed through a methanization converter filled with Ni catalyst, and all the carbon oxide product was converted to methane before they went to the FID to ensure a high intensity of signal.

In the present study, cesium ion-exchanged zeolite X with different precursors were examined; in addition, this work focus on investigating the different performances of cesium hydroxide modified zeolite X by ion-exchange and impregnation. When different characterization results are combined, both cesium ions bonded on the framework and Cs2O crystal phase covered on the particle surface or highly distributed in the pore were found on the catalysts prepared by ion-exchange and impregnation. Different catalytic performance of the catalysts can be correlated with the different ratios of cesium ions and Cs2O. The synergistic effect between cesium ions and Cs2O was found by correlating the catalytic performance with the preparation methods of the catalysts.

2. EXPERIMENTAL SECTION 2.1. Materials. Zeolite NaX (SiO2/Al2O3 = 2.69) was obtained from the Catalyst Plant of Nankai University. Cesium hydroxide monohydrate (CsOH·H2O, 99.9%), cesium nitrate (CsNO3, 99.0%), and cesium carbonate (Cs2CO3, 99.9%) were obtained from Aladdin Industrial Corporation. 2.2. Catalyst Preparation. Zeolite NaX was ion-exchanged with aqueous solution of different cesium precursors according to conventional exchange procedures. Typically, a catalyst was prepared as follows: 10 g og NaX was exchanged three times at 343 K for 2 h with a 0.2 M solution of cesium ion (solid/liquid ratio, 10 g/50 mL). The solid was separated from the slurry by centrifugation and washed with excess pure water each time. The material was then dried in flowing air at 373 K overnight and calcined at 813 K for 3 h. The catalysts obtained from different precursors (CsOH·H2O, CsNO3, and Cs2CO3) were denoted as CsX-Hex, CsX-Nex, and CsX-Cex, respectively. For comparison, zeolite NaX was modified with cesium hydroxide aqueous solution by incipient wetness impregnation. The loading of Cs2O was the same as the amount of CsX-Hex, which was determined by X-ray fluorescence spectroscopy. After impregnation, the catalyst was dried and calcined in the same conditions as CsX-Hex, and the obtained catalyst was denoted as CsX-Him. (In this work, we assumed that all the mentioned cesium oxide was in the form of Cs2O for convenience.) 2.3. Catalyst Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2400 diffractometer with a Cu Kα radiation (λ = 1.542 Å) source operating at 40 kV and 100 mA. The spectra were recorded from 5° to 50° with a scanning rate of 8° min−1. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) images were obtained on a Hitachi S-5500 instrument with an acceleration voltage of 3 kV. All the samples were sputtered with a thin film of chromium. The chemical compositions of the samples were analyzed by X-ray fluorescence (XRF) spectroscopy on a SRS-3400 X-ray fluorometer. Argon isotherms at 87 K were measured using a Quantachrome AUTOSURB-1 gas adsorption analyzer to determine the Brunauer−Emmett−Teller (BET) surface area and pore volume of the parent and modified samples. Temperature-programmed desorption (TPD) of NH3 and CO2 was performed on a CHEMBET 3000 chemical adsorber (Quantachrome) to analyze the nature of surface acid and basic sites. NH3-TPD measurements were conducted by the following procedures: About 0.1 g of the catalyst sample was purified in helium at 773 K for 1 h, cooled to 393 K, then B


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crystallite because the estimated average crystallite sizes of the Cs2O phase were much bigger than the pore size of zeolite X. Table 1 summarizes the main physicochemical characteristics of NaX and different cesium-modified samples. The SiO2/ Al2O3 of ion-exchanged samples with different cesium precursors were almost the same as the parent; only a slight increase in CsX-Him was observed. In addition, the ionexchange degrees of different samples were almost the same because the identical experimental parameters (Cs+ concentration, contact time, and number of consecutive exchange operations) were employed. Thus, the facts confirmed that by employing the uniform exchange condition the species of anions of different cesium precursors did not influence the exchange degree significantly. By cesium modification, the BET surface area (SBET), micropore volume (Vmicro), and total pore volume (Vtotal) were all decreased to a different extent. Through cesium ionexchange, the BET surface area of CsX-Hex decreased about 23% compared with NaX; the micropore volume and total pore volume also decreased about 18% and 19%, respectively. This result clearly indicated that partial sodium cations were substituted by cesium cations because the radius of the cesium cation was much larger than that of the sodium cation. The textural properties of CsX-Nex and CsX-Cex were similar to those of CsX-Hex. By cesium impregnation, the BET surface area of CsX-Him decreased more dramatically (about 50% compared with NaX) than by cesium ion-exchange. Similar results were observed for the micropore volume and total pore volume of CsX-Him (reduced about 48% and 40% compared with NaX, respectively). Due to the SiO2/Al2O3 of CsX-Him being similar to CsX-Hex, the losses of porous structure should be attributed to the occluding of cesium species inside the cavities of zeolite. Considering that the sodium cation could not be removed and that it was hard to be exchanged by impregnation, most cesium species of CsX-Him probably existed in the state of cesium oxide. It could be inferred that the occlusion of partial Cs2O crystal phase inside the micropores resulted in the decreases of surface area and pore volume of CsX-Him. Figure 2 shows the NH3-TPD curves of NaX and cesiummodified zeolite X. NaX presented two overlapping peaks centered at 500 and 530 K, respectively. It was indicated that these two peaks probably resulted from alkali cations located at different sites in NaX zeolite.18 Total desorbed amounts of NH3 represented by the integral areas of NH3-TPD curves of cesium-modified samples decreased about 90% compared with the NaX. The desorbed amount of NH3 is a function of the number of acid sites.16 Thus, it can be inferred that the amounts of acid sites in the cesium-modified samples decreased markedly. Only small quantities of acid sites remained for ion-

The conversion of toluene (CT), the selectivity of styrene (SST), and the total selectivity of styrene and ehtylbenzene (SST+EB) are defined in the following equations: ⎛ [toluene outlet] ⎞ C T(%) = ⎜1 − ⎟ × 100% ⎝ [toluene inlet] ⎠ ⎛ ⎞ [styrene] SST(%) = ⎜ ⎟ × 100% ⎝ [tolune inlet] − [toluene outlet] ⎠ ⎛ [styrene] + [ethylbenzene] ⎞ SST + EB(%) = ⎜ ⎟ × 100% ⎝ [tolune inlet] − [toluene outlet] ⎠

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. Figure 1 exhibits the XRD patterns of different samples. It can be seen that all the

Figure 1. XRD patterns of NaX and cesium-modified zeolite X.

samples showed typical diffraction patterns corresponding to faujasite framework. However, significant decreases in the relative diffraction peak intensities were observed by comparing the XRD patterns of cesium-modified samples with NaX. The same phenomenon has been observed by many researchers.15−17 On the one hand, the incorporation of cesium probably changed the structure factors or X-ray absorption coefficients of the parent because the radius of cesium was much bigger than that of sodium.8 On the other hand, the framework of zeolites was partially damaged in the process of ion-exchange or impregnation. It is worth noting that a new diffraction peak at 25.6° was observed for both cesium ionexchanged and impregnated samples, which was attributed to Cs2O phase (PDF No. 09-0104). The average crystallite sizes of Cs2O were determined by the Scherrer equation and are summarized in Table 1. It could be inferred that partial Cs2O crystal phase may distribute on the surface of zeolite X

Table 1. Physicochemical Characteristics of NaX and Cesium-Modified Zeolite X chemical composition (wt %)a catalyst

SiO2

Al2O3

Na2O

Cs2O

SiO2/Al2O3

EDb (%)

SBET (m2·g−1)

Vmicroc (cm3·g−1)

Vtotal (cm3·g−1)

DCs2Od (nm)

NaX CsX-Hex CsX-Nex CsX-Cex CsX-Him

49.4 39.2 39.9 40.1 38.5

31.2 24.8 25.2 25.3 22.3

19.3 9.2 8.8 9.2 14.0

0.0 26.7 26.1 25.5 25.2

2.7 2.7 2.7 2.7 2.9

− 52.3 54.4 52.3 −

729 564 554 547 364

0.33 0.27 0.27 0.26 0.17

0.42 0.34 0.33 0.32 0.25

− 71 73 72 49

Given by XRF. bExchange degree: [(% Nainitial -% Nafinal)/% Nainitial] × 100 cMicropore volume was calculated by NLDFT method. dCrystallite size of Cs2O (DCs2O)was determined from the Scherrer equation. a

C


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the decomposition temperature of cesium carbonate is 885 K, partial CO2 may react with Cs2O to form cesium carbonate over CsX-Him. The above-mentioned results proved that the basicities of the catalysts were strengthened dramatically by cesium modification, which was indispensible for side-chain alkylation of toluene with methanol. The same phenomenon was observed by infrared spectra of adsorbed pyridine and CO2.9,14 3.2. Catalytic Performance. Figure 4 shows the variations in toluene conversion (CT) as a function of time on stream.

Figure 2. NH3-TPD curves of NaX and cesium-modified zeolite X.

exchanged samples. In addition, most acid sites of NaX were diminished in the process of cesium impregnation. On the one hand, the acidity of NaX was weakened observably when the sodium ions were exchanged with more electropositive cesium ions.16,19 On the other hand, cesium oxide which was tentatively formed in the zeolite influenced the acidity of the parent.14 Of course it should be noted that the total absence of acid sites over CsOH·H2O modified samples could also be justified by the fact that the strong basicity of CsOH·H2O aqueous solution led to the neutralization of acid sites. CO2-TPD curves of different samples are presented in Figure 3. It can be seen that NaX exhibited three isolated peaks at 358,

Figure 4. Conversion of toluene as a function of time on stream over different catalysts. T = 698 K; WHSV = 2 h−1; n (toluene)/n (methanol) = 2.

Figure 3. CO2-TPD curves of NaX and cesium-modified zeolite X.

473, and 634 K. The first peak was attributed to physically adsorbed CO2 desorption. The other two peaks were both caused by chemically adsorbed CO2 desorption.16 Because the area of the first peak was much greater than the area of the second or the third peak, it can be inferred that most of the adsorbed CO2 over NaX was physically adsorbed CO2. Each curve of the cesium-modified samples also exhibited three peaks in the range of 350−650 K. By cesium modification, the area of the first peak decreased obviously while the area of the second peak increased dramatically. This fact indicated that partial physically adsorbed CO2 was bonded with basic sites and subsequently turned to chemically adsorbed CO2. Although total peak areas of cesium-modified samples reduced about 40% compared with NaX, it can be inferred that the amounts of chemically adsorbed CO2 were added over cesium-modified samples. Thus, more basic sites were formed by cesium modification. It is worth noting that the temperature of the second peak of CsX-Him was higher than those of ionexchanged samples, which indicated that the basic strength of CsX-Him was stronger than that of ion-exchanged samples. However, the curve of CsX-Him raised up over 800 K. Because

Figure 5. Selectivity of styrene and total selectivity of styrene and ethylbenzene as functions of time on stream over different catalysts. T = 698 K; WHSV = 2 h−1; n (toluene)/n (methanol) = 2.

Figure 5 shows the variations in the selectivity of styrene (SST) and total selectivity of styrene and ethylbenzene (SST+EB) as a function of time on stream. It can be seen that NaX demonstrated the highest initial toluene conversion in 1 h and decreased rapidly for 25 h. However, no side-chain alkylation was observed over NaX; xylenes and polyalkylates were the main product. Conversely, all the samples modified by cesium exhibited high activity for side-chain alkylation. For ionexchanged samples with different cesium precursors, the initial toluene conversions were 3.7% for CsX-Hex, 2.9% for CsXNex, and 2.1% for CsX-Cex. Each of them deactivated gradually with time on stream for 50 h. The initial styrene selectivity for different ion-exchanged samples were 58.1% for CsX-Hex, 54.2% for CsX-Nex, and 66.6% for CsX-Cex. The styrene selectivity of different samples also decreased with time on stream. Although CsX-Cex got the highest initial styrene selectivity, CsX-Hex surpassed the former after reaction for 4 h. D


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Table 2. Gas Product Distribution and Coke Information of Side-Chain Alkylation of Toluene with Methanol over Different Catalysts gas product distribution (mol %) catalyst

Sintb

CT (%)

SST (%)

SST+EB (%)

CO

CH4

CO2

C2−C4

CH3OH

DME

aromatic

cokec (%)

IG/IDd

a

3.29 3.14 3.19 3.18 2.99

4.8 1.6 1.5 1.1 4.3

0 39.8 23.9 32.1 6.9

0.6 65.1 45.1 49.1 97.6

2.4 16.1 12.4 15.2 57.6

21.1 1.0 1.9 1.5 0.7

1.6 0.2 0.4 0.2 0.4

5.1 0.1 0.2 0.6 0.3

33.2 63.3 66.0 61.2 34

26.7 10.6 11.6 11.3 2.4

9.9 8.7 7.5 10.0 4.6

31.0 22.6 19.3 23.1 3.0

1.47 1.54 1.49 1.55 1.50

NaX CsX-Hex CsX-Nex CsX-Cex CsX-Him a

Data for NaX was collected for 25 h. bIntermediate Sanderson electronegativity. cCoke amount was calculated by the equation coke amount =

(

[Masss of 573 K] − [Mass of 923 K] [Mass of 923 K]

) × 100%

d

IG/ID was calculated by the equation IG/ID =

area of G band area of D band

It was reported that light hydrocarbons and DME were the main gas byproduct for toluene ring alkylation with methanol or conversion of methanol over acid catalysts such as HZSM-5 due to the methanol dehydration over acid sites.20,22 On the other hand, CO and H2 were the main byproducts in the gas phase for side-chain alkylation of toluene with methanol over base catalysts such as alkali metal-modified zeolite X or Y due to the methanol decomposition over strong basic sites.21 Thus, it can be summarized that for NaX, toluene alkylation with methanol mainly took place on the benzene ring over acid sites, as indicated by the NH3-TPD results. However, NaX deactivated rapidly due to the coke formation accompanied by the conversion of methanol to CH4 and DME. Over cesium ion-exchanged samples, toluene alkylation with methanol took place both on the benzene ring and the methyl group because there were both acid sites and basic sites over these ionexchanged samples. It was consistent with the NH3-TPD and CO2-TPD results. Consequently, CH4, DME, and CO were formed simultaneously. The activity of side-chain alkylation decreased gradually with time on stream due to the continuous coke deposition. Over cesium impregnated samples, styrene and ethylbenzene were the main alkylation products, indicating that side-chain alkylation of toluene was the predominant reaction in the process. Moreover, CsX-Him demonstrated excellent stability with time on stream, and a huge amount of methanol decomposed to CO and H2 over this catalyst. It could be inferred that the process of coke deposition over CsX-Him was inhibited and that it was related to the production of large amounts of CO and H2. As previously investigated,23,24 alkylation of toluene with methanol could take place either on the benzene ring over acid catalysts or on the methyl group of toluene over base catalysts depending on the intermediate Sanderson electronegativity (Sint) of the catalysts. The Sint has been used to rank the basicity of zeolites; as the value of Sint decreases, the overall basicity of a material increases.25 We calculated the Sint of different catalysts (summarized in Table 2), which decreased in the sequence NaX > CsX-Nex > CsX-Cex > CsX-Hex > CsX-Him. The average selectivities of styrene, ethylbenzene, and xylenes are plotted against Sint in Figure 6; in addition, the distribution of partial gas product is also plotted versus Sint. As show in Figure 6, the selectivity of ethylbenzene and the fraction of CO increased monotonously with the decreasing of Sint. In contrast, both of the xylenes selectivity and the total fraction of CH4 and DME decreased monotonously with the decreasing of Sint. Apparently different from the former two types, the styrene selectivity and the fraction of methanol both appeared to be a maximum at the medium value of Sint. It was consistent with the report that the activity of toluene side-chain alkylation

The changes in total selectivity of styrene and ethylbenzene over ion-exchanged samples appeared to have the same trends as styrene selectivity. By impregnation of NaX with cesium hydroxide aqueous solution, interesting reaction results was observed. CsX-Him demonstrated toluene conversion varied from 5.5% to 3.4% during 50 h and almost stable styrene selectivity/total selectivity of styrene and ethylbenzene, which were 6.9% and 97.6%, respectively. To our surprise, the catalytic performance of CsX-Him was totally different from that of CsX-Hex, although the cesium amount of these two samples were controlled to be the same. The average toluene conversion, styrene selectivity, and total selectivity of styrene and ethylbenzene for 50 h are shown in Table 2. Among the different precursors, cesium hydroxide seemed to be the most effective precursor to modify zeolite X for side-chain alkylation of toluene with methanol. It was found that the SiO2/Al2O3 ratios, ion-exchange degrees, pore structure properties, and acid−base properties of different ion-exchanged samples were similar because identical preparation methods were employed. Small quantities of cesium precursors might be residual over zeolite samples after ionexchange procedures. CsX-Hex demonstrated slightly higher side-chain alkylation activity compared with CsX-Nex and CsXCex. This phenomenon might be related with the fact that CsOH·H2O was more likely to decompose to Cs2O than CsNO3 and Cs2CO3 by calcination in 813 K. By using CsOH· H2O as precursor, ion-exchanged zeolite X demonstrated the highest styrene selectivity with a relative appropriate toluene conversion. However, impregnated zeolite X exhibited the lowest styrene selectivity and the highest ethylbenzene selectivity. It is worth noting that stable activity was achieved over impregnated sample simultaneously. To the best of our knowledge, the gas product distribution of toluene alkylation with methanol over both acid20 and basic21 catalysts are very important for understanding the reaction process; however, data for this part is often neglected. We list the average gas product distribution for 50 h over different catalysts in Table 2. The average carbon mass balances of sidechain alkylation of toluene with methanol over different catalysts are exhibited in Table S2. From Table 2, it can be seen that CH4 and dimethyl ether (DME) were the main byproduct in the gas phase over NaX. The fractions of CH4 and DME were 21.1% and 26.7%, respectively. By cesium ionexchange of zeolite X, the fractions of CH4 and DME decreased dramatically, while CO became the main byproduct. The fraction of CO, CH4, and DME over CsX-Hex was 16.1%, 1.0%, and 10.6%, respectively. CsX-Him exhibited the greatest fraction of CO, which was 57.6%, and the least fractions of CH4 and DME, which were 0.7% and 2.4%, respectively. E


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Figure 8. UV Raman spectra of coke species formed in different catalysts after reaction for toluene side-chain alkylation with methanol.

Figure 6. Changes in partial liquid product selectivity and partial gas product distribution related to Sanderson electronegativity.

increased dramatically with the decreasing of Sint.23,25 It should be noted that the maximum styrene selectivity was achieved at the medium Sint value, which indicated that neither too strong basicity nor too weak basicity was beneficial to the formation of styrene. Unfortunately, the conversion of methanol seemed to be low in these conditions. TG curves of the spent catalysts are exhibited in Figure 7. All the spent samples showed slight initial weight loss in the range

coke species deposited over the spent samples were polyaromatic species.22 Because the relative intensity of G band to D band (IG/ID) can be used as a qualitative measurement for the order extent of carbon species,27 we calculated IG/ID of different spent samples (summarized in Table 2). It is interesting that the IG/ID of different samples ranged from 1.47 for NaX to 1.55 for CsX-Cex. This fact indicated that although the coke amount of CsX-Him was only 3.0%, coke species of cesium-impregnated samples was the same as those of cesium ion-exchanged samples. It can be inferred that the processes of coke formation over different samples were similar. However, CsX-Him was more coke resistant than other samples due to the decomposition of methanol to CO and H2. It was reported that the coke deposition was mainly caused by the toluene ring-alkylation with methanol or conversion of methanol to light hydrocarbons over acid sites.20,22 Intermolecular dehydration of methanol to DME over acid sites had close relationship with the coke formation. However, methanol dehydrogenated to CO and H2 over basic sites, which could be seen from the gas product distribution. The amount of basic sites over catalyst CsX-Him increased greatly with the modification of cesium species. Thus, methanol underwent dehydrogenation reaction to form CO and H2 preferentially. Then the amount of methanol contributed to the formation of coke would reduce simultaneously. This point of view was strongly supported by the gas product distribution because the CO fraction of CsXHim was 57.6%, which was the highest among different samples. 3.3. States of Cesium Species and Oxygen Species. Because CsX-Hex and CsX-Him demonstrated very different catalytic performance in the reaction of side-chain alkylation of toluene with methanol, the essential reason for this phenomenon was investigated in further experiments. To investigate the chemical nature of cesium species on the surface and in the inner section of CsX-Hex and CsX-Him, Figures 9 and 10 depict the XPS spectra in the Cs 3d5/2 and O 1s region for both of the samples, respectively. It can be seen that the Cs 3d5/2 binding energies (BE) of CsX-Hex decreased from 722.7 to 722.1 eV by analyzing the species on the surface to the depth of 100 nm. On the other hand, those of CsX-Him remained 723.1 eV from the surface to the inner section. This fact clearly indicated that the chemical states of cesium species of CsX-Hex were different from those of CsX-Him. Each spectrum corresponding to cesium was decomposed into two contributions, thus suggesting the presence of cesium in at least two different chemical

Figure 7. TG curves of different catalysts after reaction for toluene side-chain alkylation with methanol.

of 300 to 573 K, which was attributed to desorption of physically adsorbed water and organics.26 Among different samples, obvious weight losses above 573 K were observed, which was ascribed to decomposition of coke species deposited or accumulated over the catalysts;9 however, CsX-Him showed a gradual weight loss with increasing temperature. The coke amounts of different catalysts were calculated and are summarized in Table 2. It can be seen that the coke amounts decreased in the sequence NaX > CsX-Cex > CsX-Hex > CsXNex ≫ CsX-Him. These results indicated that the deactivation processes of cesium ion-exchanged samples were the same as that of NaX. On the other hand, it seemed that only a small quantity of coke species were formed on the CsX-Him, which explained why CsX-Him demonstrated the highest stability for toluene side-chain alkylation with methanol. UV Raman spectroscopy has been demonstrated to be a powerful technique for characterization of coke species formed in zeolites. The UV Raman spectra of coke species deposited on the spent catalysts are presented in Figure 8. It can be seen that two bands at 1400 and 1615 cm−1 were presented in the spectra of all the spent samples. They were attributed to D band and G band, respectively. These signals indicated that the F


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Table 3. Cs 3d5/2 and O 1s XPS Analyses of CsX-Hex and CsX-Him Catalysts XPS spectra binding energy (eV) assignment

O 1s

723.3

529.8−530.4

528.8

Cs of Cs+

Cs of Cs2O

O of framework

O of Cs2O

catalyst

depth

ηCsion (%)

ηCsoxi (%)

ηOfra (%)

ηOoxi (%)

CsX-Hex

surface 50 nm 100 nm surface 50 nm 100 nm

33.6 52.5 73.9 30.7 28.2 31.7

66.4 47.5 26.1 69.3 71.8 68.3

24.2 71.7 73.5 30.3 79.2 79.7

75.8 28.3 26.5 69.7 20.8 20.3

CsX-Him

Figure 9. Cs 3d5/2 spectra in different depth of CsX-Hex and CsXHim.

Cs 3d5/2 721.9

of 50 nm. This depth was approximate to the crystal size of Cs2O calculated by XRD results, indicating that the particle surface of zeolite X of CsX-Hex was covered by a layer of Cs2O at the thickness of 50 nm. Most of the internal cesium species of the particles were in the form of cesium ion. However, the fraction of Cs2O of CsX-Him remained at about 70% from the surface to the depth of 100 nm, which indicated that most of the cesium species of CsX-Him were in the form of Cs2O. Based on the analyses of O 1s spectra, the fractions of framework oxygen on the surface of CsX-Hex and CsX-Him were only 24.2% and 30.3%, respectively. However, when the species at the depth of 50 nm were analyzed, the fractions of framework oxygen for CsX-Hex and CsX-Him were found to be 71.7% and 79.2%, respectively. These proportions did not change by continuing to erode the samples. This result indicated that the particle surfaces of zeolite X of CsX-Hex and CsX-Him were both covered by a layer of Cs2O phase, which was consistent with the analyses of Cs 3d5/2 spectra. Most of the oxygen species under the surface of 50 nm were attributed to framework oxygen. The other oxygen species should be assigned to the highly dispersive Cs2O phase distributed in the intracrystalline cavities or the pores of zeolite X. It was suggested that the O 1s BE of the zeolite is directly correlated to the basicity of the framework oxygen, whose basic strength increases with decreasing of O 1s BE.33 Because the O 1s BE of Cs2O and framework oxygen of CsX-Hex and CsXHim were 528.8 eV, 529.8, and 530.4 eV, respectively, it can be inferred that the basicity of oxygen of these three types were in the sequence Cs2O > CsX-Hex > CsX-Him. This result indicated that the basicity of framework oxygen of NaX can be increased more dramatically by cesium ion-exchange than by impregnation. 3.4. Catalytic Mechanism. According to the mechanism reported for side chain alkylation of toluene with methanol over alkali metal modified zeolite,10,34,35 the first elementary step is the dehydrogenation of methanol to produce formaldehyde. Then, formaldehyde as active alkylating reagent may attack the methyl group of toluene retained on the large cation to produce phenyl ethanol; finally, phenyl ethanol intramolecular dehydration would generate styrene and water. However, CO and ethylbenzene are inevitable products in this process. CO is formed mainly by further decomposition of formaldehyde over strong basic sites.21,25 Ethylbenzene is formed by undesirable hydrogenation of styrene with hydrogen2,4,34 or transfer hydrogenation between styrene and methanol.14,36 To investigate the catalytic mechanism of side-chain alkylation of toluene with methanol, the effects of WHSV

Figure 10. O 1s spectra in different depth of CsX-Hex and CsX-Him.

environments. Yagi et al.28 found that when zeolitie X contains cesium in excess of the ion-exchange capacity the cesium species would be located in the intracrystalline cavities of zeolite or on the outer surface of zeolite crystallites in a highly dispersive state. Laspéras et al.15 observed that when cesium ion-exchanged NaX by impregnation with cesium acetate after thermal decomposition, cesium oxide (Cs2O) would form inside the cages of the CsNaX zeolite. Based on the XRD results, Cs2O crystal formed on all of the cesium-modified samples in these experiments. It can be inferred that cesium species of CsX-Hex and CsX-Him would include cesium ions as both exchanged cation and Cs2O crystal phase. According to the research,29,30 the Cs 3d5/2 signal at 721.9 eV could be assigned to cesium ion as exchanged cation in the zeolite framework. The other signal centered at 723.3 eV was attributed to Cs2O.31,32 On the other hand, the O 1s BE of CsX-Hex increased from 529.0 to 529.7 eV based on analyzing the species on the surface to the depth of 100 nm. The same phenomenon was observed on the CsX-Him; the O 1s peak at 528.9 eV shifted to 530.0 eV from the surface to the inner section. The O 1s peaks were also decomposed into two contributions; the O 1s signal at 528.8 eV was attributed to oxygen atoms of Cs2O crystal phase,31,32 and the other signal, ranging from 529.8 to 530.4 eV, can be ascribed to framework oxygen.33 Table 3 summarizes the quantitative analyses of cesium and oxygen species for the CsX-Hex and CsX-Him catalysts based on XPS. It can be seen that the fraction of cesium ions of CsXHex increased from 33.6% to 73.9% by analyzing the species on the surface to the depth of 100 nm. Meanwhile, the fraction of Cs2O exhibited a reverse trend. The amount of cesium ions bonded on the framework surpassed those of Cs2O at the depth G


Article


styrene selectivity over CsX-Him ranged from 5.8% to 7.7%. The decreasing of styrene selectivity over CsX-Hex should be attributed to transfer hydrogenation of styrene with methanol to ethylbenzene. However, it was found that the styrene selectivity over CsX-Hex remained at 50.7% even though the methanol was much more superfluous. Although the efficiency of transfer hydrogenation probably related with the active components of cesium ions or Cs2O crystallites, the extremely low styrene selectivity over CsX-Him should not be completely attributed to transfer hydrogenation of styrene with methanol. Direct hydrogenation of styrene to ethylbenzene may also take important roles in the process. Once a methanol molecule decomposed to CO, two molecules of hydrogen would be released. Thus, the formation of excessive ethylbenzene over CsX-Him should also be attributed to the environment of high hydrogen partial pressure, which could be inferred from the high CO proportion in the gas phase. Based on the abovementioned results, both transfer hydrogenation of styrene with methanol and direct hydrogenation of styrene with hydrogen made important contributions to the formation of ethylbenzene. Finding the true active center for the transition state formation was important for improving the catalytic performance. Table 1 indicates that CsX-Him exhibited the lowest SBET (364 m2 g−1), Vmicro (0.17 cm3 g−1), and Vtotal (0.25 cm3 g−1) among the cesium-modified samples. However, CsX-Him demonstrated the highest activity for side-chain alkylation of toluene with methanol. It should be indicated that the SBET (364 m2 g−1) of CsX-Him included micropore surface area (320 m2 g−1) and external surface area (44 m2 g−1). (Data for other samples are provided in Table S1.) Thus, micropore surface area was still dominant for CsX-Him. Cesium cations exchanged within the micropores and cesium oxide distributed in the micropores were important for the formation of the transition state because the specific pore structure of faujasite was the most effective for this reaction, as indicated by Wieland et al.25 If the active centers were distributed in internal micropores, the decreasing of surface area and pore volume may inhibit the formation of transition state because the diffusion of toluene and methanol was hindered. However, the side-chain alkylation activity of CsX-Him was the highest. One of the most possible explanations was that the true active center for transition state formation was distributed in external or subexternal micropores. The contributions of internal cesium cations or cesium oxide for this reaction were limited. As indicated in Table 2, methanol could either dehydrate to DME by etherification reaction or dehydrogenate to CO by oxidation reaction. Because the kinetic diameter of methanol is much smaller than that of toluene, the active sites in internal micropores would be more accessible for methanol than for toluene. Thus, the diffusion of toluene into active sites in internal micropores was hindered by the modification of cesium species. In this condition, the active sites in internal micropores could not be utilized for side-chain alkylation of toluene. Besides, methanol would take place in either etherification reaction or oxidation reaction. It was reported that dehydrogenation of methanol on alkali metal-exchanged zeolite X proceeded via the hydroxyl oxygen interacting with the alkali metal cation of zeolite.10,37 Additionally, the methyl group or the hydroxyl group would interact with basic framework oxygen, polarizing one of the methyl C−H bonds or the hydroxyl O−H bond.10 In the end, one hydrogen atom of the methyl group and another hydrogen

and molar ratio of toluene to methanol on the catalytic performance of CsX-Hex and CsX-Him were further studied, and the results are listed in Table 4. We investigated the Table 4. Effects of Space Velocity and Molar Ratio of Toluene to Methanol on the Catalytic Performance of CsXHex and CsX-Him Catalystsa CsX-Hexb

reaction conditions

CsX-Himb

WHSV (h−1)

nT/ nM

CT (%)

SST (%)

SEB (%)

CT (%)

SST (%)

SEB (%)

1 2 4 8 2 2 2

6/1 6/1 6/1 6/1 2/1 1/1 1/2

2.0 1.5 1.2 0.6 3.7 4.8 4.7

52.8 62.0 66.2 78.9 58.1 53.1 50.7

41.9 32.3 28.1 17.0 36.4 40.8 44.9

2.2 2.2 2.0 1.7 5.5 6.8 8.6

4.3 5.8 7.3 10.2 6.6 7.1 7.7

92.3 91.6 90.7 88.6 87.3 86.8 85.5

a

Reaction temperature: T = 698 K. bThe toluene conversion (CT), styrene selectivity (SST), and ethylbenzene selectivity (SEB) over CsXHex and CsX-Him in different reaction conditions were obtained from the product in the first 1 h.

catalytic performance of CsX-Hex and CsX-Him in different WHSV (1, 2, 4, and 8 h−1) by employing the same molar ratio of toluene to methanol (6/1). It was found that toluene conversion over CsX-Hex decreased from 2.0% to 0.6% with the increase of WHSV from 1 to 8 h−1 while styrene selectivity increased from 52.8% to 78.9%. The same phenomenon was observed over CsX-Him catalyst. Both of these catalysts demonstrated that toluene conversion decreased with increasing WHSV. Simultaneously, styrene selectivity increased and ethylbenzene selectivity decreased. It could be inferred that the residence time of the reactants and products on the catalyst surface would be shortened. Thus, the formation of transition state of side-chain alkylation of toluene with formaldehyde would be difficult. As a result, toluene conversion decreased with the increasing of WHSV and selectivity of styrene improved because the hydrogenation of styrene to ethylbenzene was inhibited. Consequently, styrene should be seen as the primary product, while ethylbenzene ought to be formed by subsequent hydrogenation of styrene on the catalyst surface. This is consistent with the literature.2,4,9 The pathway of ethylbenzene formation in side-chain alkylation of toluene with methanol is an important aspect to be investigated for improving the styrene selectivity because styrene is more profitable than ethylbenzene. A series of contributions indicated that ethylbenzene was mainly formed from styrene hydrogenation.2,4,11,34 Recently, Hattori and coworkers14,36 discussed this problem and pointed out that transfer hydrogenation of styrene with methanol to ethylbenzene was much faster than hydrogenation of styrene to ethylbenzene. Which reaction contributed more to the ethylbenzene formation seemed to be controversial. To investigate this problem, the catalytic performance of CsXHex and CsX-Him in different molar ratios of toluene to methanol (6/1, 2/1, 1/1, and 1/2) was studied by employing the same WHSV (2 h−1), and the results are listed in Table 4. It was found that the toluene conversions of CsX-Hex and CsXHim were both increased with the decreasing of the molar ratio of toluene to methanol. However, the styrene selectivity over CsX-Hex decreased from 62% to 50.7% by decreasing the molar ratio of toluene to methanol from 6/1 to 1/2 while the H


Article

Industrial & Engineering Chemistry Research atom of the hydroxyl would coordinate to form a hydrogen molecule getting away from methanol; meanwhile, formaldehyde would be produced. Unfortunately, formaldehyde would continue dehydrogenation via the same decomposition pathway to form CO. As mentioned above, Cs2O was detected by both XRD and XPS on CsX-Hex and CsX-Him. The basicity of lattice oxygen of Cs2O was stronger than that of framework oxygen, as proven by lower binding energies of O 1s. It can be inferred that methanol would dehydrogenate to formaldehyde or further decompose to CO more easily on Cs2O than on cesium ions bonded on the framework. This can be proved by the fractions of CO on CsX-Hex and CsX-Him catalysts, which were 16.1% and 57.6%, respectively. It is worth noting that formaldehyde was not detected in the gas phase in this experiment probably because the amount of formaldehyde was only a trace. Besides, formaldehyde was easy to decompose over base catalysts in the reaction conditions. Considering the XPS results, most cesium species of CsX-Hex were cesium ion-bonded on the framework, while most cesium species of CsX-Him were Cs2O. This also explained why the toluene conversion over CsX-Him was higher than that over CsX-Hex: formaldehyde formed more easily on Cs2O. Thus, the synergistic effect of cesium ions and Cs2O was important for the styrene formation. It should be noted that Cs2O was distributed not only on the particle surface of zeolite X but also inside the micropores of the particle. The influence of Cs2O located inside the zeolite framework could be attributed to two aspects: (1) the pore structure and (2) the acid−base properties of the framework. On the one hand, the BET surface area and micropore volume of the host framework decreased with the addition of the guest Cs2O inside the framework, which can be seen from Table 1. On the other hand, the containing of the guest Cs2O inside the framework dramatically changed the acid−base properties of the framework, which can be seen from Figures 2 and 3. The amount of acid sites decreased greatly while the amount of basic sites increased significantly. In this condition, the acid sites catalyzed side-reactions (like coking) would be inhibited. In addition, the dehydrogenation of methanol would be promoted. Although the side-chain alkylation of toluene with formaldehyde was enhanced, a great amount of methanol would decompose to CO and H2. Then the hydrogenation of styrene to ethylbenzene was inevitable, especially for CsX-Him catalyst because the high hydrogen partial pressure in the gas phase might contribute to the high ethylbenzene selectivity. The vinyl group of styrene retained on the cesium cation or not yet removed from the catalyst surface was active to be hydrogenated by hydrogen. Figure 11 shows the reaction pathway of side-chain alkylation of toluene with methanol over cesium-modified zeolite X. Methanol was more likely to dehydrogenate into formaldehyde over Cs2O crystal phase due to the strong basicity of lattice oxygen. Formaldehyde would attack the methyl group of toluene retained on the cesium cation bonded on the framework to form styrene and water. Synergistic effect of cesium ions and Cs2O would favor the styrene formation. However, formaldehyde would continue decomposing into CO and H2 over Cs2O crystal phase inevitably. Along with methanol decomposition, a huge amount of hydrogen would be released. Styrene would be undesirably hydrogenated by hydrogen on the catalyst surface. Based on the discussion above, it seems that an ideal cesiummodified zeolite X catalyst for side-chain alkylation of toluene

Figure 11. Reaction pathway of side-chain alkylation of toluene with methanol over cesium-modified zeolite X.

with methanol to styrene needs to fulfill three requirements. (1) The catalyst should have enough cesium ions bonded on the framework to adsorb and activate toluene or modify the basicity of framework oxygen. (2) An appropriate amount of cesium oxide should be added into the catalyst to ensure methanol could be converted to formaldehyde effectively and inhibit the process of coke deposition. (3) To pursue the highest yield of styrene, the synergistic effect between cesium ions and cesium oxide should be utilized in appropriate conditions by adjusting the amounts of these two kinds of cesium species. The optimized amounts or ratio of cesium ions to cesium oxide still need to be further studied.

4. CONCLUSIONS In summary, for cesium ion-exchanged zeolite X, most internal cesium species were in the form of cesium ions bonded on the framework. Over this catalyst, styrene was the main reaction product. However, for cesium-impregnated zeolite X, most cesium species were in the form of Cs2O. Under this condition, although the catalyst had stable activity, ethylbenzene was the main product. Cesium ions and Cs2O probably take different roles in this reaction. The synergistic effect of these two species was important for the formation of styrene. The basicity of the overall catalyst should be strong enough for the conversion of methanol to formaldehyde and further side-chain alkylation of toluene. However, overly strong basicity should be avoided to pursue the highest yield of styrene and reasonable methanol utilization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04174. Description of (1) SEM images of NaX and cesiummodified zeolite X; (2) Ar physical adsorption− desorption isotherms at 77 K, pore size distribution, and pore structure properties of different samples; (3) EDS mapping distribution of Al, Na, and Cs of CsX-Hex and CsX-Him; and (4) carbon mass balances of sidechain alkylation of toluene with methanol over different catalysts (PDF) I


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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 0411 84986134. E-mail: [email protected]. *Fax: +1 814 863 4466. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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