Cadmium Modified HZSM-5: A Highly Efficient Catalyst for Selective

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Cadmium modified HZSM-5: A highly efficient catalyst for selective transformation of methanol to aromatics Yongkun Zhang, Yixin Qu, Deliang Wang, Xiao Cheng Zeng, and Jidong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02908 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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Industrial & Engineering Chemistry Research

Cadmium modified HZSM-5: A highly efficient catalyst for selective transformation of methanol to aromatics

Yongkun Zhang a, Yixin Qu a, Deliang Wang b, Xiao Cheng Zeng c, d, Jidong Wang a*

a. Beijing Key Laboratory of Membrane Science and Technology, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing, 100029, P. R. China b. University of Chinese Academy of Sciences, Beijing, 100049, P. R. China c. Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, P. R. China d. Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE, 68588, USA

Corresponding author: Ass. Prof. Jidong Wang No.15 North Third Ring Road Chaoyang District Beijing city 100029 P. R. China Tel

+86 10 6445 4730

Fax

+86 10 6443 6781

E-Mail: [email protected]

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ACS Paragon Plus Environment


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Abstract Cadmium modified HZSM-5 catalyst (Cd/ZSM-5) prepared by incipient wetness impregnation shows markedly high selectivity of aromatics and low selectivity of light alkanes and C5-C9 non-aromatics in the methanol-to-aromatics (MTA) transformation. Majority of the introduced Cd cations can exchange with protons of the bridged hydroxyl groups of HZSM-5, leading to reduced number of Brønsted acid sites while generating new Lewis acid sites on the catalysts. The selectivity of aromatics with the Cd/ZSM-5 is found to be linearly correlated with the concentration of the strong Lewis acid sites. Compared to Zn/ZSM-5, the most extensively investigated catalyst thus far for the MTA, Cd/ZSM-5 is found to be more effective in promoting the aromatics formation. The different catalytic behavior observed for Cd/ZSM-5 and Zn/ZSM-5 catalysts is most likely due to the difference in ionic radius of Cd and Zn.

Keywords: HZSM-5 zeolite, Cadmium cations, Methanol, Aromatics, Promotional effect.

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1. Introduction Aromatics, especially benzene, toluene and xylene (BTX), are important intermediates for numerous chemical industry processes and are conventionally produced with feedstock originating from petroleum.

1-3

The high oil price in the past decade and

the continuously higher demand of aromatics worldwide

4

stimulate the research and

development for finding alternative processes of aromatics production. The methanol to aromatics (MTA) process is considered as a highly efficient and sustainable technique to meet the growing demand for aromatics since the feedstock methanol is relatively inexpensive and can be obtained from wide varieties of materials like coal, biomass, natural gas or shale gas.

5

The rapid development of coal chemistry and utilization of

natural gas in recent years have led to the methanol capacity in surplus in some countries. Although methanol can be used as fuel or fuel additive in automobile engines, its transformation to value added products is even more attractive. Methanol to hydrocarbon (MTH) was first discovered by Chang and Silvestri at Mobil Oil in the 1970s.

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Several processes have been developed since that discovery,

including methanol to gasoline (MTG), methanol to olefins (MTO) and methanol to aromatics (MTA) processes.

7, 8

The selective formation of target products can be

achieved by tailoring the catalysts and by appropriate selection of reaction conditions. ZSM-5 zeolite has been widely used in the MTH process due to its superior catalytic performance and shape selectivity, which is ascribed to its specific topology compared to other types of zeolites.

9-11

ZSM-5 zeolite is also suitable for the MTA process because

higher selectivity of aromatic can be achieved when the zeolite is modified with certain metals, e.g., Zn or Ag. 12-16 3



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With HZSM-5, the selectivity of aromatics is about 30-40% and typically large amount of undesired alkanes is produced.

12, 14-18

Also, it is difficult to increase the

aromatics selectivity through optimization of the reaction conditions because the formation of aromatics is accompanied by the formation of alkanes via hydrogen transfer reactions over HZSM-5.

16

In previous reports, Ag, La, Cu, Sn, Ni, Zn, Ga have been

used for the modification of HZSM-5 to increase the selectivity of aromatics in MTA. 1224

Silver exchanged ZSM-5 can improve the aromatics formation by providing active

dehydrogenation centers for alkane intermediates, but the Ag modified ZSM-5 deactivates rapidly due to the reduction of Ag+ cation to metallic silver. Ag, Cu and Ni were reported to improve the selectivity of C6-C8, C9-C11 and naphthalene by a factor of two or higher. And it is proposed that the enhanced performance of these catalysts stems from the contact interaction of the acid sites of zeolite with the metal oxides at the edge of zeolite crystals.

14

Ga modified HZSM-5 was found to increase the aromatics

selectivity, which was ascribed to the interaction between Ga2O3 and the Brønsted acid sites on the zeolite with the effect coined as contact synergy.

19, 20, 22

Introduction Zn to

HZSM-5 was reported to enhance the aromatics selectivity, regardless of its preparation with impregnation, ion-exchange or isomorphous substitution.

12, 15, 16, 21, 24, 25

The

enhanced aromatics formation was attributed to new active sites ZnOH+ formed by the interaction of Zn2+ cations with the bridged hydroxyl group of HZSM-5 zeolites, which could convert the alkane intermediates to aromatics by dehydroaromatization, and enhance the recombination of hydrogen atoms to H2 and the associated desorption of H2. 26, 27

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Like Zn, Cd is also an element of group 12 in the periodic table. Although Zn modified ZSM-5 shows excellent performance in the MTA, the performance of Cd modified ZSM-5 has rarely been investigated. In fact, catalysts with Cd as an active element show special activity for some reactions. An example is the hydration of acetylene. For this reaction, Cd modified ZSM-5 shows much higher activity than Zn modified ZSM-5.

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For ethane dehydrogenation, Cd/ZSM-5 is about four-times more

active than Ga/ZSM-5.

29

A theoretical study by Pidko et al.

30

indicated that Cd/ZSM-5

was also highly active in light alkanes activation. In view of the excellent performance of Zn modified HZSM-5 in the MTA, there could be a high potential in that the Cd modified HZSM-5 would show excellent performance as well. This hypothesis prompts us to conduct a detailed investigation concerning the modification effect of Cd on the catalytic selectivity of aromatics in MTA reaction. Our results clearly demonstrate that the Cd modified HZSM-5 zeolites offer higher selectivity of aromatics than Zn modified HZSM-5. New insights obtained from the cadmium modification can serve as a proof of principle for future design of more effective catalysts for the selective formation of aromatics in MTA reactions.

2. Experimental Section 2.1. Catalyst Preparation In this study, zeolite HZSM-5 with Si/Al2 = 38 (molar ratio) provided by Zibo Huayi Chemical Plant of China was used. Modification of the HZSM-5 was carried out by incipient wetness impregnation method using aqueous solution containing desired 5



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amounts of Cd(NO3)2·4H2O or Zn(NO3)2·6H2O. After impregnation, the precursors were kept overnight at room temperature and then dried at 120 °C for 12 h. Finally, they were calcined in static air at 550 °C for 5 h in a muffle furnace with a temperature program from ambient to 550 °C at a rate of 5 °C/min. For testing the catalytic performance, the samples were pelletized, crushed and sieved to obtain particles with an average size of 20-40 mesh. The modified samples were denoted as xwt%M/ZSM-5, where x represents the amount of metal loading and M represents Cd or Zn.

2.2. Catalyst Characterization The crystalline structure of samples was monitored by X-ray diffraction (XRD), which was recorded on a Rigaku D/max 2500 VB2 Diffractometer. The machine was operated at 40 kV and 40 mA using Cu Kα radiation (wave length = 1.54056 Å). Data were collected in continuous scan mode from 5° to 50° (2θ) with a 0.01 sampling interval and a 3°/min scan rate. The Brunauer-Emmett-Teller (BET) surface areas and pore volumes of the zeolite samples were measured by N2 adsorption-desorption at -196 °C using an ASAP 2020 instrument (Micromeritics Inc.). Before the measurement, the samples were degassed at 300°C for 5 h. Temperature-programmed desorption of NH3 (NH3-TPD) of the samples was carried out on a Micrometrics AutoChem II 2920 Analyzer. The samples (0.2 g) were pretreated at 350 °C for 2 h in a He flow of 20 ml/min before the adsorption of NH3. After the samples were cooled to 100 °C, a 10% NH3/90% He mixture at a rate of 20 ml/min was introduced for adsorption of NH3 for 90 min. Then, the NH3 containing flow was replaced by a helium flow at the same conditions for 1 h to remove the physisorbed NH3. 6


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Subsequently, the temperature of the samples was raised at a rate of 10 °C/min from 100 to 600 °C, and the desorbed NH3 was detected with a thermal conductivity detector. FT- IR and the FT-IR of pyridine adsorption (Py-IR) measurements were performed on a Thermo Nicolet Nexus 470 instrument. Each sample (16-17 mg) was pressed into a self-supported wafer and evacuated in an in situ IR cell. The sample was activated under a vacuum of 1×10−3 Pa at 500 °C for 2 h. After the sample was cooled to room temperature, a FT-IR spectrum was recorded. This spectrum was also used as a background for the measurement of Py-IR. Subsequently, the sample was heated to 100°C and pyridine was introduced into the IR cell for at least 30 min to ensure all acid sites on the catalyst were covered. Afterward, the sample was respectively heated to 200 °C and 350 °C at a rate of 10 °C/min. At each temperature, the sample was evacuated for at least 30 min to remove physically adsorbed pyridine and a Py-IR spectrum was collected. 16, 31 Ultraviolet-visible spectra (UV–Vis) were obtained using a shimadzu UV3600 spectrometer. The spectra were recorded at ambient conditions in the range of 220-800 nm with a scan speed of 100 nm min-1. Before the measurements, a background was recorded using BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo-Fisher ESCALAB 250 system with Al Kα radiation as the X-ray source. The measurements were carried out in an ultrahigh vacuum (UHV), and the sensitivity of ISS is 25 kcps/nA. The binding energy (BE) was calibrated with respect to the C 1s value of a contaminated carbon at 284.9 eV (estimated error ±0.05 eV).

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2.3. Catalytic tests The catalytic conversion of methanol was carried out in a fixed-bed continuous flow reactor, which is made of stainless steel tube (inner diameter 16 mm, length 500 mm). The reactor was equipped with a thermocouple well of 6 mm outer diameter, which was mounted coaxially into the tube. 6.0 g catalyst (20−40 mesh) was packed in the middle zone of the reactor between two layers of quartz sand. The quartz sand in the inlet section was also used as a preheating zone for vaporization of methanol. The reactor was heated by an electrical furnace with two heating zones which temperature were independently controlled by automatic feedback temperature controllers. A blank experiment was carried out to determine the catalytic effect of the reactor wall. No detectable conversion of methanol was observed. Prior to the introduce of methanol, the catalyst was treated at 400 °C for 0.5 h in a high-purity nitrogen stream (30 ml/min) to remove the possibly adsorbed H2O, CO2 and other materials on the samples. Then temperature of the catalyst was adjusted to the desired reaction temperature. The feed containing 94 wt% methanol and 6 wt% H2O, a simulated composition of industrial crude methanol, was injected into the reactor by a dual-syringe pump (Beijing satellite plant). To ensure a steady evaporation of methanol, a stream of high-purity nitrogen (about 30 ml/min) was fed at the inlet of the reactor. The reaction products first passed through a cooler, and then went into a gas-liquid separator, where they were separated into a gas phase, an oil phase and water phase. The cooler and the separator were maintained at 0-2 °C to minimize the loss of the light hydrocarbons in the liquid phase when it was discharged from the separator. The composition of the gas phase was analyzed via two online GCs (SP3420A and GC8600, Beijing Beifen Tianpu Instr. Technol. Co. Ltd.). The SP3420A was equipped 8


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with a TDX-01 packed column (3mm×2m) and a thermal conductivity detector (TCD) to detect H2, CO, CO2 and CH4. The GC8600 was equipped with a KB-Al2O3/Na2SO4 (0.32mm×30m×20µm) and a HP-Plot-Q (0.32mm×30m×40µm) capillary column and two flame ionization detector (FID) to analyze the light hydrocarbons and dimethyl ether (DME). Quantification of the gas products was conducted by internal standard method using the known flow rate N2 internal standard which was injected at the inlet of the reactor. The mass of the oil phase and the water phase were measured by an electronic balance. Their compositions were determined offline by another GC8600 equipped with a KB-Wax (0.25mm×30m×0.25µm) and a BP-1 (0.25mm×30m×0.25µm) capillary column and two FID detectors. The conversion of methanol (X%) was calculated using Eq (1):

X% =

min - mout ×100% min

(1)

where, min is the flow rate of methanol (g/h) pumped into the reactor; mout is the flow rate of unconverted methanol (g/h). The selectivity of products was calculated based on the amount of CH2 generated from the dehydration of methanol, which can be considered as an effective species for the formation of the hydrocarbons. The water product is not included in the calculations. The selectivity of hydrocarbon products (SHCi) was calculated using Eq (2):

SHCi ( wt %) =

mi × 100% mHC

(2)

where, mi is the flow rate of a hydrocarbon product i (g/h); mHC is the flow rate of CH2 (g/h) generated from the converted methanol. The selectivity of non-hydrocarbon product (SNHCi) was calculated using Eq (3): 9



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SNHC j (wt%) =

mj min − mout

× 100%

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(3)

where, mj is the flow rate of a non-hydrocarbon product j (g/h); min is the flow rate of methanol (g/h) pumped into the reactor and mout is the flow rate of unconverted methanol (g/h). The HC balance was calculated using Eq (4) without taking into account of CO and CO2 since the selectivity of CO + CO2 is generally less than 1%.

HC balance% =

mLH + mGH + mH2 ×100% mCH2

(4)

Where, mLH is the flow rate of hydrocarbons in the liquid phase (g/h), mGH is the flow rate of hydrocarbons in gas phase (g/h), mH2 is the flow rate of hydrogen and mCH2 (g/h) is the flow rate of CH2 generated from converted methanol. The HC balance was in the range of 95~105%. Considering the complicated product distribution of this process, the HC balance is quite reasonable.

3. Results and Discussion 3.1. Characterization of the catalysts Figure 1 shows the powder XRD patterns of HZSM-5, 1.0wt%Zn/ZSM-5 and Cd/ZSM-5 with different Cd loading. The diffraction patterns of all the samples are consistent with that of MFI crystal structure,

32

indicating that loading of Cd or Zn

species does not lead to a noticeable detrimental effect on the integrity of the crystalline structure of ZSM-5. The relative crystallinities of Cd and Zn modified ZSM-5 were estimated from the integrated areas of the peaks in the 2θ range of 22.5-25° using the parent HZSM-5 as a reference, which crystallinity was defined as 100%. 33 The estimated 10


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relative crystallinities are presented in Figure 1. Introduction of Cd or Zn into HZSM-5 results in a reduction of the crystallinity and the reduction extent becomes larger with increasing amount of Cd loading, implying that the interaction between metal species and zeolite framework become stronger with increasing amount of metal loading. From the absence of the diffraction peaks belonging to Zn and Cd oxides it can be concluded that the metal species is highly dispersed on the zeolite surface.14, 34 The textural properties of the samples as determined by N2-adsorption are given in Table 1. Addition of the metals results in reduced BET specific surface area and the reduction extent increases with increasing amount of Cd loading, as compared with the parent HZSM-5. In contrast, the pore volume shows no sign of change upon the loading of the metals, indicating that the loaded metals do not lead to noticeable blockage of pore channels. The FT-IR of the dehydrated catalyst samples are shown in Figure 2. There are three peaks appearing in the range of 3400-3800 cm-1, which are assigned to the stretching vibrations of OH groups. The peak at ca. 3740 cm-1 is assigned to the vibration of isolated silanol (Si-OH) present in the channels as well as on the external surface of the HZSM-5 framework.

35-37

The peak at ca. 3600 cm-1 is assigned to the vibration of framework

bridged OH groups, Si-OH-Al, which are responsible for Brønsted acid sites.

36-38

Another peak at ca. 3660 cm-1 is attributed to the vibration of the hydroxyl groups on the extra framework alumina and/or the perturbed framework alumina. 35, 38 Upon the loading of the metals, the intensity of the peak belonging to the bridged hydroxyl groups is distinctly reduced and the reduction increases with increasing amount of Cd loading. This implies that at least a part of the loaded Cd species interacts with the hydroxyl groups of 11



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the bridged OH groups. These results are consistent with the observations when Zn cations were loaded on HZSM-5.

16, 27

The consequence of this interaction is the

replacement of the protons of the Brønsted acid sites by the Cd cations, which leads to reduced number of Brønsted acid sites as demonstrated by the results of pyridine adsorption IR spectra in the following text. The NH3-TPD profiles of HZSM-5 and Cd modified HZSM-5 are shown in Figure 3. The concentration of total acid sites measured by NH3-TPD is presented in Table 2. As can be seen from Figure 3, there are two peaks present in the NH3-TPD profiles of all the samples, a low-temperature peak in the range of 183-186 °C, which is attributed to the desorption of NH3 adsorbed on weak acid sites, and a high-temperature peak in the range of 380~400 °C, which is attributed to the desorption of NH3 adsorbed on strong acid sites.

37, 39, 40

With increasing amount of Cd loading, the low temperature peaks of

Cd/ZSM-5 become broad, the high temperature peaks shift toward lower temperature and their intensities become weak, which indicate a reduced strength as well as a reduced concentration of the strong acid sites upon the loading of the Cd cations. The results of Py-IR can be used to estimate the concentration of Brønsted and Lewis acid sites in ZSM-5 zeolite catalysts. The Py-IR spectra of the samples were recorded in the range of 1400 - 1700 cm-1 at 200 and 350 °C and are shown in Figure 4. Four distinct IR adsorption peaks were detected for these samples. The peak at ca. 1455 cm-1 is related to the Lewis-bonded pyridine; the peak at ca. 1545 cm-1 is assigned to the pyridinium ions formed by the protonation of pyridine on the Brønsted acid sites; the peak at ca. 1490 cm-1 reflects the mix of two kinds of acid sites. 36, 37, 41 The peaks at ca. 1620 and 1635 cm-1 observed for HZSM-5 are respectively related to Lewis acid site and 12


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Brønsted acid site. 36 Incorporation of metals into the HZSM-5 zeolite results in increased intensity of the peak at 1455 cm-1 but decreased intensity of the peak at 1545 cm-1 as compared with that of the parent HZSM-5. This means that introducing Cd or Zn cations into the HZSM-5 removes some of the Brønsted acid sites while generates new Lewis acid sites on the catalysts. The removed Brønsted acid sites can be ascribed to the replacement of the H of the Si-OH-Al groups by metal cations. For Cd modified HZSM-5, the amount of the removed Brønsted acid sites and the newly generated Lewis acid sites increase with increasing Cd loading. In addition, the introduction of Cd cations results in presence of a new peak at 1612 cm-1, which is attributed to strong Lewis acid sites 36, 42 or new acid sites like ZnOH+ resulted from the exchange of Cd cations with the protons of Brønsted acid sites. 23 To investigate the specific changes in the Brønsted and the Lewis acid sites of the Cd-modified ZSM-5 zeolites as compared with the parent HZSM-5, quantitative analysis of the peaks at 1455 and 1545 cm-1, corresponding respectively to the pyridine adsorbed on the Lewis and the Brønsted acid sites was conducted according to the reports of Emeis 41

. The concentrations of the acid sites determined at 200 and 350 °C are respectively

used to represent the concentration of the total and the strong acid sites.

31, 43, 44

The

results are listed in Table 2. Upon the Cd loading, the total concentration of the Brønsted acid sites decreases by 23-56%. The concentration of the strong Brønsted acid sites decrease by 23-60%, which is more significant than the reduction of the weak and medium sites (14-43%). On the other hand, the concentration of the total Lewis acid sites increases by 170-320%, which is more significant than the reduction of the total Brønsted acid sites. Moreover, the 13



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concentration of the strong Lewis acid sites increases linearly with increasing the amount of Cd loading in the range of 0.4-1.2wt%; while that of the weak and medium keeps nearly constant. Figure 5 shows the UV-Vis spectra of Cd/ZSM-5 catalysts. An obvious change in the UV-Vis spectra is observed for the 1.4wt%Cd/ZSM-5 sample, where a weak peak at ~390 nm appears. This peak can be attributed to the presence of CdO crystals on the external surface of zeolite. 45 However, this peak is not observed for the samples with Cd loading less than 1.4 wt%. This may mean that when the Cd loading is less than 1.4 wt%, the loaded Cd is highly dispersed on the zeolite without forming CdO crystalline particles. The chemical state and distribution of Cd species on the catalyst surface were determined by XPS analysis. As shown in Figure 6, after deconvolution of the Cd3d5 XPS spectra, two types of Cd species can be discriminated. One has a binding energy at ca. 405.4 eV and the other at 406.4-407.6 eV. The peak at ca. 405.4 eV can be attributed to CdO since it is very close to the values of CdO (404.2 eV).

46

The shift of the binding

energy of CdO on the zeolite is caused by the interaction of the CdO particles with zeolite framework, as the framework of the zeolite has a much higher electronegativity as compared to that of the O2− group.

15, 47

Seyama et al.

48

studied the XPS of

montmorillonite containing exchangeable divalent cations and ascribed the peak at 406.2 eV to the bonding energy Cd3d5 of the exchanged Cd cations. Here a similar assignment of the peak at 406.2 eV to the exchanged Cd cations in ZSM-5 can be made as Seyama et al..

48

The results of XPS measurement for Cd/ZSM-5 indicate that major part of loaded

Cd exists in the exchanged state and only small part exists in CdO.

3.2. Catalytic performance 14


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The conversion of methanol to hydrocarbons over HZSM-5 is complicated and involves dozens of intermediates and a complex reaction networks, including dehydration, methylation, oligomerization, dehydrocyclization, alkylation/dealkylation, isomerization, cyclization, hydride transfer, and so on. 49 HZSM-5 zeolites are found to be highly active for methanol conversion to hydrocarbons, while the light hydrocarbons such as propane and butane isomers are the main products, and the selectivity of aromatics is not necessarily high (ca. 30-40wt%).

12, 14-18

Starting from methanol, aromatics are formed

through the methylation/oligomerization, dehydrocyclization and hydrogen transfer of intermediate alkenes. More specifically, the aromatics are mainly produced via dehydrocyclization of higher alkenes with simultaneous hydrogen transfer to light alkenes, giving light alkanes as the main products. Over the metal modified HSZM-5, the improved aromatic selectivity is achieved mainly with the expense of light alkanes. 15, 16, 24

3.2.1 Catalytic performance over Cd/ZSM-5 The product selectivity over Cd/ZSM-5 together with that over HZSM-5 is shown in Table 3. The methanol conversions is ca. 99.9 % over all of the catalysts in the 4 h on stream of time under the test conditions, indicating that modification of HZSM-5 with Cd cations do not significantly influence the initial activity of the catalysts. Over the parent HZSM-5, the selectivity of the total aromatic product is 36.3 wt% and a large amount of light alkanes such as propane (18.2 wt%) and butane (22.2 wt%) are produced. Compared with the parent HZSM-5, modification of HZSM-5 with Cd improves the selectivity of aromatics significantly. On the other hand, the selectivity of undesired alkanes is suppressed remarkably. The selectivity of aromatics increases with 15



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increasing Cd loading when the Cd loading is 0.4-1.2 wt%. While the selectivity of light alkanes such as propane, butane as well as the C5-C9 non-aromatics decreases. The highest aromatics selectivity of 63.0 wt% is obtained over 1.2wt%Cd/ZSM-5 catalyst. This means that the enhanced formation of aromatic over the Cd/ZSM-5 is mainly at the expense of light alkanes, particularly propane, butane isomers and C5-C9 non-aromatics, which is consistent with the observation for other metal modified ZSM-5. 15, 16, 23, 24 Over the Cd/ZSM-5, the selectivity of H2 is significantly higher than that of observed for HZSM-5 and increases smoothly with increasing amount of Cd loading to 1.2 wt%. Since the decomposition of methanol generates CO and also H2, 12 however, the comparison of the selectivity of COx with H2 clearly indicated that the enhanced formation of H2 is predominantly from the dehydroaromatization of hydrocarbons during the formation of aromatics. As can be seen in Table 3, further increase of Cd loading to 1.4 wt% leads to a rapidly reduction of the aromatics selectivity to 41.8 wt%. Besides, the product distribution is significantly different from that observed for the Cd/ZSM-5 with lower Cd loading. Over 1.4wt%Cd/ZSM-5, the selectivity of methane, ethylene and propylene and H2 is remarkably high. One of the reasons for the different product distribution over 1.4wt%Cd/ZSM-5 may be attributed to the promoted decomposition of methanol by CdO particles on this catalyst since an accompanied increase in the selectivity of COx is also observed. As indicated by the XPS results in Figure 6, on 1.4wt%Cd/ZSM-5 sample, a noticeable amount CdO is present. In study the MTA over Zn/ZSM-5, Ono et al.12 suggested that the ZnO particles on the Zn/ZSM-5 catalyst enhanced the decomposition of methanol. In addition, the lower concentration of acid sites on 1.4wt%Cd/ZSM-5 may 16


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cause a reduction of the oligomerization rate of the light alkenes, which also contributes to an increased production of ethylene and propylene. Previous studies

8, 49, 50

concerning the MTH over HZSM-5 indicate that aromatics

are mainly formed from the dehydrocyclization of higher alkenes, which is accompanied by hydrogen transfer to light alkenes, leading to the formation of light alkanes. Although the promotional effect of Zn on the aromatic formation over HZSM-5 has been confirmed by a number of investigations, 12, 15, 16, 24, 25 the detailed mechanism is still unclear due to the complicated reactions involved in MTA. The product distribution in MTH as well as in MTA is certainly affected by the acid properties of the catalysts. 51 In order to find the relationship between the acid properties of the catalysts and the aromatic selectivity, the acidity of catalysts has been characterized by NH3-TPD (Figure 3 and Table 2) and Py-IR (Figure 4 and Table 2). The correlations of the total aromatic selectivity with the amount of Cd loading, the concentration of the acid sites for the Cd modified HZSM-5 are presented in Figure 7. It is noticed that the aromatic selectivity is linearly correlated with the amount of Cd loading and with the concentration of strong Lewis acid sites when the amount of Cd is less than 1.2wt%. In contrast, the total aromatic selectivity is inversely correlated with both the concentration of the strong Brønsted acid sites and the total concentration of Brønsted acid sites. The reverse correlation can be attributed to the exchange of Cd cations with the Brønsted acid sites of HZSM-5, which reduces the amount of Brønsted acid sites. In a previous study concerning the selective conversion of bio-derived ethanol to renewable BTX over Ga-ZSM-5, a correlation between the BTX site time yield and exchanged Ga sites was found by Li et al.

52

The similar correlation

for Cd in this study suggests that the exchanged Cd cations in ZSM-5 channels play an 17



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important role in the formation of aromatics. One possible explanation for the promotional effect of Cd on the aromatic formation is the exchanged Cd cations promote the dehydroaromatization of higher alkenes. In addition, they also change the hydrogen transfer reactions that lead to the formation of light alkanes. In the presence of exchanged Cd cations, most of the hydrogen atoms formed in the dehydroaromatization of higher alkenes combine to H2 instead of being transferred to light alkenes. 3.2.2 Comparison of Cd/ZSM-5 and Zn/ZSM-5 Zn modified HZSM-5 zeolites have been intensively studied for both aromatization of alcohols and hydrocarbons due to their excellent promotional effect on the aromatic selectivity.

15, 16, 23, 24, 27, 53

The results presented in this study (Table 3) demonstrate that

Cd modified HZSM-5 provides even better effect. For instance, the 1.0wt%Cd/ZSM-5 gives a selectivity of aromatics of 59.9%, which is 12.8% higher than that obtained with 1.0wt%Zn/ZSM-5. Besides, some other differences between Zn and Cd can also be noticed. Over the Zn modified HZSM-5, the selectivity of aromatics increases monotonously with increasing Zn loading till 5.0%. For the Cd modified HZSM-5, the selectivity of aromatics is more sensitive to the amount of Cd loading. When the amount of loaded Cd is less than 1.2 wt%, the selectivity of aromatics increases with increasing the amount of Cd and reaches a maximum value of 63.0% at 1.2 wt% of Cd loading. Further increase of Cd to 1.4 wt% results in a sudden decrease of aromatic selectivity and significant decomposition of methanol. For Zn modified HZSM-5, no such significant decomposition of methanol occurs until the Zn loading reached 5.0 wt%. The exchange process of Cd and Zn with HZSM-5 is quantified by correlation of the Brønsted acid site concentration with the metal loading, which is given in Figure 8. Line 18


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A indicates the Brønsted acid site concentration if one metal atom would remove a Brønsted acid site; line B two sites; line C three sites and line D four sites. At low Zn loading, the decrease in Brønsted acid site concentration is close to 2 per Zn, suggesting that Zn is present as Zn2+. At higher Zn loading, the decrease in Brønsted acid site concentration is close to 1 per Zn, suggesting that Zn is present as Zn+1 or Zn(OH)+1. For Cd, the decrease in Brønsted acid site concentration is close to 4 per Cd at low Cd loading; at higher Cd loading it is close to 3. Although the maximum oxidation state of Cd is 2, the above correlations suggest that Cd is more efficient than Zn in removing the Brønsted acid sites of HZSM-5 zeolites. The ability of metal ions to remove the Brønsted acid sites of HZSM-5 can probably be scaled by their electronegativity. The Pauling scale electronegativity of Cd is 1.69, which is larger than that of Zn (1.65).

54

Therefore, the

higher ability of Cd in removing the Brønsted acid sites of HZSM-5 may not be related to their electronegativity. The difference observed for the Zn and Cd in promoting the aromatic selectivity in MTA most likely stems from the difference in their ionic radius. The ionic radius of Cd is 109 pm, which is 21 pm larger than that of Zn. It has been found that the alkaline-earth ions, Mg and Ca, located in the super cage of Y zeolite, show different activity in the activation of alkanes.

55

The activity of Y zeolite exchanged with Ca2+ is significantly

higher compared to that with Mg2+.

56

A later study concerning the conformation of

alkane adsorbed on zeolites cations using “atom in molecules” by Pidko et al.

55

reveals

that compared to Ca2+ cation, Mg2+ cation exchanged in faujasite is strongly shielded by the surrounding zeolitic oxygens due to its smaller radius and hence significant stabilization of the adsorbed complex comes from the weak hydrogen bonds between the 19



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H atoms of the alkane and O atom of the cation sites. The preferred conformation of the adsorbed alkane is to a significant extent controlled by the resulting steric constrains, which lead to longer hydrocarbon-cation bond. In contrast, the larger Ca2+ cation at the SII sites of faujasite is only slightly shielded and hence the interaction of the adsorbed alkane molecule with the non-directed electrostatic filed dominates. A theoretical study based on DFT cluster modeling about the activation of light alkanes over Cd2+ ions in ZSM-5 zeolite by Pidko et al. 30 revealed that ethane adsorbed on the exchanged Cd cation in HZSM-5 generates a rather stable σ–CH complex and subsequent heterolytic dissociation of adsorbed ethane is a favorable process. Based on this result, they predict that Cd exchanged HZSM-5 zeolite could be a highly active catalyst for the activation of alkanes. 3.2.3 Effect of reaction conditions To investigate the effect of reaction conditions on the catalytic performance of Cd/ZSM-5 in the MTA process, experiments at different reaction temperature (360480°C), pressure (0.1-0.9 MPa) and WHSV of methanol (0.5-8 h-1) have been conducted over 1.0wt%Cd/ZSM-5. In order to minimize the influence of catalyst deactivation, fresh catalysts were used for different reaction conditions. The product selectivity at different reaction temperature is given in Figure 9. Conversion of methanol is nearly complete at all the evaluated temperatures from 360 to 480°C. In the temperature range of 360-420°C, the selectivity of aromatics increases with increasing temperature. The temperature of 420 °C is a turning point, at which the highest aromatics selectivity of 63.0 wt% is obtained. Higher temperature than 420°C results in a decrease in the selectivity of aromatics. The major reason for the reduced selectivity of aromatics can be attributed to 20


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significant decomposition of methanol at temperature higher than 420 °C, as indicated by the high selectivity of CH4, and COx. Besides, higher temperature favors the cracking reactions leading to the decreased selectivity of C5-C9 non-aromatics, further resulting in the decrease of aromatic selectivity. With increasing temperature, the selectivity of propane, butane as well as C5-C9 non-aromatics decreases monotonically while that of light olefins increases. The significant increase of C2H4 and C3H6 is considered as the final products of cracking reactions due to their lower activity than butenes which show weak dependency of the reaction temperature. 57-59 Higher reaction temperature may also favor the dealkylation reactions of substituted aromatics, which are indicated by the slight increase of the selectivity of ethane and benzene and the slight decrease of the selectivity of ethylbenzene as well as A9+. Figure 10 shows the effect of reaction pressure on the MTA reactions observed on 1.0wt%Cd/ZSM-5 in the pressure range of 0.1 to 0.9 MPa at 420 °C and a WHSV of methanol of 2.1 h-1. Again, conversion of methanol is nearly complete at all the evaluated pressure. With increasing pressure from 0.1 to 0.5 MPa, the selectivity of total aromatics, H2 and light alkenes remarkably decreases, while the selectivity of C5-C9 non-aromatics rapidly increases. When the pressure is higher than 0.5 MPa, the selectivity of those components remains nearly constant. The changes of the above mentioned reaction products mean that higher pressure favors the oligomerization and methylation of light olefins but suppresses dehydroaromatization reactions of C5-C9 non-aromatics. Higher pressure favors the secondary reactions of intermediates due to the prolonging residence time and suppressing desorption of products on the active sites of catalysts. With the increasing reaction pressure, desorption of the light olefinic intermediates from the active 21



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centers of catalysts is difficult. The increased selectivity of final products C2H6 and C3H8 is attributed to the enhanced the hydrogen transfer to olefins other than desorption to form H2 with the high pressure. However, the selectivity of i-C4H10 shows slight decrease due to the higher activity of i-C4H10, which is converted to C2H6 and C3H8 via the metathesis with C2H4 and C3H6 at the high pressure.

60

Therefore, the lower reaction

pressure is benefit for the aromatics formation. Figure 11 shows the effect of WHSV of methanol on the MTA reactions observed over 1.0%wtCd/ZSM-5 in the range of 0.5 to 8 h-1 at 420 °C and 0.1MPa. Under these conditions, the conversion of methanol is also nearly complete and no DME is detected. The WHSV of 2.1 h-1 seems a turning point. When the WHSV of methanol is lower than this value, the selectivity of total aromatics and light alkenes remains practically constant. When the WHSV of methanol is higher than this value, the selectivity of total aromatics rapidly decreases, while the selectivity of light alkenes and C5-C9 non-aromatics rapidly increases. Higher WHSV means shorter contact time between the intermediates and catalysts. The insufficient interconversion of intermediates leads to an increase in the selectivity of light alkenes and C5-C9 non-aromatics and a decrease in the selectivity of aromatics. It is noticed that the selectivity of benzene and toluene decreases monotonously with increasing WHSV when WHSV is less than 2.1 h-1, while the selectivity of xylene and ethylbenzene is almost constant. This means that xylene and ethylbenzene are the primary products while benzene and toluene are the secondary products.

4. Conclusions 22


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The results presented in this paper demonstrate that the addition of Cd cations into HZSM-5 markedly improves the selectivity toward aromatics and H2 while lowers the selectivity to light alkanes. The catalytic reactivity of Cd/ZSM-5 catalysts is strongly dependent on the Cd loadings incorporated into HZSM-5. With appropriate Cd loading (0.8-1.2 wt%), the selectivity to aromatics is increased to 54.8-63.0 wt%. The introduction of Cd cations results in an increase of the amount of Lewis acid sites and a decrease of the amount of Brønsted acid sites due to the exchange between Cd cations with protons of bridged hydroxyl groups. The total aromatic selectivity over the Cd/ZSM-5 is linearly correlated to the amount of the Cd loading and to the concentration of the strong Lewis acid sites when the Cd loading is in the range of 0.4-1.2 wt%, which may indicate that the exchange Cd cations in HZSM-5 zeolite generate new active sites which significantly accelerate the formation of aromatics and promote the recombination of the hydrogen atom to H2; concomitantly resulting in a decrease in the selectivity of light alkanes, the major hydrogen transfer reaction products for the aromatic formation over the unmodified HZSM-5. Moreover, the promotional effect of Cd cations on the aromatic formation is more efficient than Zn cations and the aromatic selectivity and methanol decomposition over Cd/ZSM-5 are more sensitive to the amount of Cd loading than that over Zn/ZSM-5. The correlation of Brønsted acid sites with metal loading indicates that Cd cation is more efficient than Zn in removing the Brønsted acid sites on HZSM-5. The different behavior observed for Cd and Zn is most likely due to the difference of their ionic radius. The larger Cd2+ cation at the exchanged site of ZSM-5 is less shielded by the surrounding O

23



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atoms and its interaction with absorbed hydrocarbon molecules is stronger than that of the smaller Zn2+ cation.

Acknowledgements XCZ is supported by a fund from Beijing Advanced Innovation Center for Soft Matter Science & Engineering for summer visiting scholar.

24


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Figure 1. Powder XRD patterns of M/ZSM-5 catalysts used in the present work.

Figure 2. IR spectra of the HZSM-5 and M/ZSM-5 catalysts.

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Figure 3. NH3-TPD profiles of the HZSM-5 and M/ZSM-5 catalysts.

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Figure 4. Py-IR spectra of the HZSM-5 and M/ZSM-5 catalysts degassed at (a) 200 and (b) 350 °C.

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Figure 5. UV-Vis spectra of the HZSM-5 and M/ZSM-5 catalysts.

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Figure 6. XPS spectra of Cd (3d5) of Cd/ZSM-5 catalysts with different Cd loading.

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Figure 7. The correlation of aromatics selectivity with the concentration of the acid sites and the Cd loading on Cd/ZSM-5 catalysts.

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Figure 8. The correlation of Brønsted acid sites and metal contents over M/ZSM-5 samples. The dashed lines indicate the results that would be obtained if one Brønsted site were removed by one metal cation (A), two metal cations (B), three metal cations (C) and four metal cations (D), respectively.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Effect of the reaction temperatures on the product selectivity over 1.0wt%Cd/ZSM-5 catalysts. (a) the selectivity of H2, CO and CO2; (b) the selectivity of C1-C4 alkanes; (c) the selectivity of C2-C4 alkenes; (d) the selectivity of C5-C9 nonaromatics and aromatics. Reaction conditions: WHSV = 2.1 h-1, P = 0.1 MPa, flow rate of N2 = 30 ml/min, Time on stream = 4 h.

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Figure 10. Effect of the reaction pressure on the product selectivity over 1.0%wtCd/ZSM5 catalysts. (a) the selectivity of H2, CO and CO2; (b) the selectivity of C1-C4 alkanes; (c) the selectivity of C2-C4 alkenes; (d) the selectivity of C5-C9 non-aromatics and aromatics. Reaction conditions: WHSVMeOH = 2.1 h-1, Methanol: H2O = 9:1 (mol/mol), T = 420 °C, flow rate of N2 = 30 ml/min, Time on stream = 4 h.

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Figure 11. Effect of WHSVMethanol on the product selectivity over 1.0wt%Cd/ZSM-5 catalysts. (a) the selectivity of H2, CO and CO2; (b) the selectivity of C1-C4 alkanes; (c) the selectivity of C2-C4 alkenes; (d) the selectivity of C5-C9 non-aromatics and aromatics. Reaction conditions: Methanol: H2O = 9:1 (mol/mol), T = 420 °C, P = 0.1 MPa, flow rate of N2 = 30 ml/min, Time on stream = 4 h.

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Table 1. The texture of the HZSM-5 and M/ZSM-5 catalysts Specific surface area (m2/g) Catalysts SBET

a

Smicro

b

Smeso

c

Pore volume (cm3/g)

HZSM-5

365

224

141

0.20

0.4wt%Cd/ZSM-5

354

217

137

0.19

0.8wt%Cd/ZSM-5

342

214

128

0.19

1.0wt%Cd/ZSM-5

342

212

130

0.19

1.2wt%Cd/ZSM-5

335

211

124

0.19

1.4wt%Cd/ZSM-5

330

212

118

0.18

1.0wt%Zn/ZSM-5

352

217

135

0.20

a - BET method; b - t-plot method; c - Smeso = SBET - Smirco;

Table 2. Distributions of Brønsted and Lewis acid sites on HZSM-5 and M/ZSM-5 catalysts

Total (200 ºC )

Strong (350 ºC)

weak and medium

Total (200 ºC)

Strong (350 ºC)

weak and medium

Concentration of total acid sites measured by NH3-TPD (mmol/g)

HZSM-5

0.62 (-)*

0.47 (-)

0.15 (-)

0.10 (-)

0.03(-)

0.07(-)

0.96

0.4wt%Cd/ZSM-5

0.48 (-23%)

0.36 (-23%)

0.12(-14%)

0.27(170%)

0.07(133%)

0.20(186%)

0.91

0.8wt%Cd/ZSM-5

0.34 (-45%)

0.25 (-47%)

0.09 (-36%)

0.33(230%)

0.10(233%)

0.23(229%)

0.85

1.0wt%Cd/ZSM-5

0.32 (-48%)

0.21 (-55%)

0.11 (-21%)

0.36(260%)

0.13(333%)

0.24(243%)

0.85

1.2wt%Cd/ZSM-5

0.30 (-52%)

0.19 (-60%)

0.11 (-21%)

0.38(280%)

0.15(400%)

0.23(229%)

0.79

1.4wt%Cd/ZSM-5

0.27 (-56%)

0.19 (-60%)

0.08 (-43%)

0.42(320%)

0.13(333%)

0.29(314%)

0.74

1.0wt%Zn/ZSM-5

0.37 (-40%)

0.24 (-49%)

0.11 (21%)

0.32(220%)

0.12(300%)

0.20(186%)

0.86

Concentration of Brønsted sites (mmol/g)

Concentration of Lewis sites (mmol/g)

Catalysts

*-The number in parentheses are the percent change of acid site concentration

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Table 3. Product Selectivity of Methanol Transformation over HZSM-5 and M/ZSM-5 catalysts Product selectivity (wt%) Tc

EBd PXe MXf OXg TBh A9+i SBTXj ∑Ak

Catalysts

H2 CO CO2 CH4 C2H6 C2H4 C3H8 C3H6 i-C4H10 n-C4H10 t-C4H8 1-C4H8 i-C4H8 c-C4H8 C5-9a Bb

H-ZSM-5

0.1 0.2

0.1

2.4

0.9

0.9

18.2

1.2

17.2

5.0

0.2

0.1

0.4

0.1

15.8 2.3 12.0 0.8

3.3

7.3

3.3

2.7

4.6

28.3

36.3

0.4wt%Cd/ZSM-5 1.6 0.1

0.0

3.1

0.6

1.3

12.5

1.7

13.9

3.9

0.3

0.1

0.5

0.2

15.2 2.2 17.2 1.1

4.3

9.4

4.3

2.9

4.5

37.4

45.8

0.8wt%Cd/ZSM-5 1.9 0.2

0.0

3.1

0.4

1.1

8.3

1.6

11.5

2.8

0.3

0.1

0.7

0.2

14.7 2.2 17.7 1.6

5.0 11.2 4.9

3.9

8.3

40.9

54.8

1.0wt%Cd/ZSM-5 2.0 0.1

0.0

2.7

0.5

1.4

7.7

1.8

9.5

2.4

0.2

0.1

0.6

0.2

11.5 2.3 18.6 1.6

5.3 11.9 5.2

5.5

9.4

43.2

59.9

1.2wt%Cd/ZSM-5 2.2 0.2

0.0

3.2

0.5

2.1

5.2

2.5

6.8

1.7

0.4

0.2

0.8

0.3

11.8 2.3 18.1 1.8

6.4 13.4 6.0

5.8

9.2

46.1

63.0

1.4wt%Cd/ZSM-5 3.1 0.9 2.4 10.6

0.8

10.9

3.6

7.8

3.8

1.1

1.3

0.8

2.4

0.9

6.7

1.2

5.7

8.7

3.7

5.8

8.1

26.7 41.8

0.5wt%Zn/ZSM-5 0.7 0.2 0.0

2.1

0.5

1.1

13.4

1.7

15.9

4.2

0.3

0.1

0.6

0.2

16.9 1.9 12.7 1.1

3.9

8.6

3.9

3.4

5.2

31.0 40.7

1.0wt%Zn/ZSM-5 1.9 0.4

0.2

2.3

0.6

1.4

12.1

2.1

13.2

3.7

0.3

0.1

0.7

0.2

13.2 2.1 15.2 1.0

4.6 10.2 4.6

4.2

5.3

36.6

2.0wt%Zn/ZSM-5 2.1 0.5 0.8

2.1

0.5

1.8

9.0

2.5

10.4

2.9

0.4

0.2

0.9

0.3

13.7 1.8 15.5 1.3

5.2 11.4 5.0

4.0

6.6

39.0 50.8

5.0wt%Zn/ZSM-5 2.5 0.4 0.9

1.6

0.6

1.8

9.7

2.2

8.5

2.9

0.4

0.1

0.7

0.2

10.9 2.0 17.7 1.1

5.9 12.7 5.8

4.8

6.5

44.2 56.6

0.7 7.8

47.1

Reaction conditions: T = 420 °C, P = 0.1MPa, WHSV = 2.1 h-1, Flow rate of N2 = 30 ml/min, Time on stream = 4 h. a - C5-9 non-aromatics, b- Benzene, c – Toluene, d – Ethylbenzene, e - para-Xylene, f - meta-Xylene, g - ortho-Xylene, h – Trimethylbenzene, i - A9+ aromatics, j – Selelctivity of BTX, k-Total selectivity of aromatics.

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For Table of Contents Only

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Cadmium Modified HZSM-5: A Highly Efficient Catalyst for Selective

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