Isolated Sn on mesoporous silica as a highly stable and selective catalyst for the propane dehydrogenation

Journal Pre-proof Isolated Sn on mesoporous silica as a highly stable and selective catalyst for the Propane dehydrogenation Haoren Wang, Huiwen Huang, Kashan Bashir, Chunyi Li

PII:

S0926-860X(19)30446-6

DOI:

https://doi.org/10.1016/j.apcata.2019.117291

Reference:

APCATA 117291

To appear in:

Applied Catalysis A, General

Received Date:

2 August 2019

Revised Date:

4 October 2019

Accepted Date:

4 October 2019

Please cite this article as: Wang H, Huang H, Bashir K, Li C, Isolated Sn on mesoporous silica as a highly stable and selective catalyst for the Propane dehydrogenation, Applied Catalysis A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117291

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Isolated Sn on mesoporous silica as a highly stable and selective catalyst for the Propane dehydrogenation

Haoren Wanga, Huiwen Huanga, Kashan Bashira and Chunyi Li*, a

a. State Key Laboratory of Heavy Oil Processing, China University of Petroleum,

ro of

Qingdao 266580, PR China

Jo

ur

na

lP

re

-p

Graphical abstract

Highlights: ●

SiO2 supported SnOx catalysts with different Sn loadings were prepared and tested in dehydrogenation of propane.

●

The Sn2+ species shows better stability than previously reported Sn0 species.

●

The enhanced stability is due to strong interaction between Sn2+ species and SiO2

ro of

support.

Abstract: Tin oxide supported on silica (SnO2/SiO2) were prepared by

-p

incipient-wetness impregnation method and examined for propane dehydrogenation at 600 °C. The physicochemical properties of SnO2/SiO2 were characterized by

re

N2-physisorption, XRD, UV-vis-DRS and H2-TPR. The results indicate that, at Sn

lP

loading ≤ 1.78 wt%, SiO2 contains a uniform distribution of Sn as isolated species in tetrahedral coordination, while crystalline SnO2 together with isolated Sn species are found for the higher Sn loadings. The reducibility of various Sn species was studied

na

by a combination of XPS, H2-TPR, and reduction-reoxidation-TPR techniques. The results reveal that after high-temperature H2 reduction, crystalline SnO2 is fully

ur

reduced to metallic Sn, whereas isolated Sn are stabilized in the Sn2+ state with high

Jo

dispersion due to a strong interaction between isolated Sn and support. Catalytic activity tests showed that these Sn2+ species exhibits both high activity and stability for propane dehydrogenation. Keywords: Propane dehydrogenation; Silica supported isolated Sn; long-term stability;

1. Introduction

In recent years, the increasing availability of inexpensive propane from natural gas and the rapidly growing demand for propylene has led to an increased interest in the propane dehydrogenation (PDH), which has the potential to make up the shortfall of propylene supply left by conventional crackers [1,2]. Nevertheless, PDH is a highly endothermic process with equilibrium limitations, and both kinetic and thermodynamic considerations make this reaction particularly challenging [3].

ro of

Catalyst deactivation due to coke deposition, sintering, and restructuring of the active species is often severe [4]. Hence, the operating conditions put a difficult demand on catalyst stability, and it is, therefore, desirable to be able to design and develop new

-p

catalysts with high stability and selectivity [5,6]. In this context, much effort has been

re

dedicated to the improvement of commercial CrOx and Pt based catalysts [7,8] or the

[13] and ZnOx [14,15].

lP

development of novel and potential catalysts, such as VOx [9,10], GaOx [11,12], CoOx

na

Tin (Sn) is known as an efficient promoter of Pt catalyst, but is generally considered to be an inactive metal as a dehydrogenation catalyst [16]. However, in our

ur

previous work, metallic Sn supported on silica (Sn0/SiO2) has been proved to be an efficient PDH catalyst. The catalyst gives good selectivity for propylene (>85%) with

Jo

little cracking or coking, making this an excellent candidate for the dehydrogenation of propane. [17]. Unfortunately, Sn0 is not completely stable under the reaction condition due to its low melting point (232 oC). The high reaction temperature triggers the sintering and loss of active Sn0 species, ultimately resulting in irreversible catalyst deactivation. Thus, the stability of Sn0/SiO2 catalyst is not entirely satisfactory.

ions (Fe[3], Co[18], Zn[19], Ga[20], etc.) isolated on silica surface have also been suggested as highly selective and stable catalysts for PDH. The authors believe that generating isolated sites on silica could diminish reduction of active metal oxide and allow for the development of a catalyst with enhanced stability under PDH conditions. In fact, isolated Sn on silica prepared via incipient wetness impregnation has been reported [21], and it has been successfully tested in several reactions, such as the

ro of

Baeyer-Villager oxidation of ketones to lactones 3-5, and the Meerwein-Ponndorf reduction of aldehydes and ketones [22]. However, to the best of our knowledge,

isolated Sn on silica has rarely been studied as a catalyst for the PDH. This current

-p

situation invites us to carry out a systematic study in this field.

re

Therefore, the aim of the present study is to explore the potential of isolated Sn on SiO2 for PDH with a particular focus on a long-term stability. Meanwhile, the effect

lP

of tin loadings on the distribution of SnOx species on the silica as well as the valence

na

state of various Sn species during the PDH reaction are also elucidated with the application of different methods (XRD, BET, UV-vis DRS, H2-TPR, XPS). The

ur

structure-activity relationships of the prepared catalysts are analyzed and elucidated.

2. Experimental section

Jo

2.1 Catalyst preparation

A commercial silica with a particle size of 0.18-0.25 mm and specific surface area

of 330 m2/g was used as support. Prior to its use, it was washed with 1 M HNO3 aqueous solution for 6 h at room temperature, and then filtered, rinsed copiously with deionized water, dried at 100 oC, and calcined at 300 oC for 3 h. Tin (Ⅳ) chloride

pentahydrate (SnCl4∙5H2O, 99.0 %) obtained from Sinopharm Chemical Reagent Company was used as tin salt precursor. Bulk SnO2 (purity, 99.9 %) was purchased from Aladdin Chemical Reagent Co., Ltd, and used as received. The silica supported tin oxide (SnO2/SiO2 ) catalysts were prepared, respectively, by the incipient wetness impregnation method and water-washing of impregnated samples. The impregnation of silica with tin was performed by incipient wetness

ro of

impregnation with an aqueous solution of SnCl4∙5H2O. The mixture were stored in partially covered containers at ambient conditions for 6 h. Then, the obtained solid was quantitatively divided into two parts: (1) One part was dried at 30 oC under

-p

vacuum for 24 h and then calcined at 600 oC for 3 h (incipient wetness impregnation

re

method); (2) The other part was thoroughly washed with distilled water at room temperature through a Büchner until the filtrate was neutral and free from chloride

lP

ions (negative AgNO3 test). The resulting catalyst precursor was subsequently

na

subjected to drying at 30 oC for 24 h and calcination at 600 oC for 3 h. The (1) and (2) catalysts are hereafter referred to as x-Sn/Si(n) and y-Sn/Si(n)-(ws), respectively, where

ur

x and y in the catalyst label express tin content in wt.% of Sn (determined by X-ray fluorescence), suffix n is numeric identifier (n = 1, 2…the same n value in the

Jo

x-Sn/Si(n) and y-Sn/Si(n)-(ws) means that two catalysts are derived from the same impregnated solid) and ws in the bracket represents water-washed impregnated samples. 2.2 Catalyst characterization X-ray diffraction measurements. XRD patterns of all samples were obtained using a

Rigaku D/Max RB diffractometer with Cu Kα radiation at 40 kV and 40 mA. The data were collected in the 2θ range between 15° and 75° with a scan rate of 5°/min. Nitrogen adsorption and desorption analysis. Nitrogen physisorption experiments at -196 oC were carried out using a Quantachrome BET instrument to examine the textural property of each sample. About 0.1 g of the sample was pretreated in vacuum

using the BET (Brunauer-Emmett-Teller) equation.

ro of

at 300 oC for 6 h before the measurement. Specific surface area values were obtained

Diffusion reflectance UV-vis. Diffuse reflectance measurements in the UV-vis region were recorded on a Shimadzu UV-2700 spectrophotometer at room

-p

temperature. All the samples were dried at 300 oC before measurement. The spectra

re

were recorded within the wavelength range of 200-600 nm using BaSO4 as a standard. Temperature-programmed reduction. The reducibility of catalysts was assessed

lP

with H2 temperature programmed reduction. 100 mg sample was loaded in a

na

U-shaped quartz tube and pretreated in a flow of He at 600 °C for 1 h. After the sample had cooled to room temperature in He atmosphere, TPR was carried out up to

ur

900 oC in 10 % H2/Ar with a heating rate of 10 °C/min. The profile was registered with a thermal conductivity detector. The amount of hydrogen consumed by the

Jo

sample was obtained by integrating the TPR profile. For H2 consumption quantification, CuO (99.999 %, Aldrich) was used as the calibration standard sample. About 10, 20, 30, 50 and 75 mg CuO were employed to do H2-TPR experiments to achieve a plot of H2 consumption amounts versus TCD integration areas. The average oxidation state of tin species was calculated on the basis of H2 consumption by

assuming that the fresh catalyst was totally in the oxidation state Sn4+ and that 1 mol of hydrogen consumed would mean the reduction of 1 mol of Sn4+ to Sn2+. Reduction-reoxidation-TPR experiment. The experiment was carried out in the setup described above for TPR measurements. 300 mg sample was reduced in H2 at 600°C for 1 h. After cooling to room temperature in H2 flow, the sample was flushed with He for 15 min and then reoxidized in the air flow for 30 min at room temperature,

ro of

after which the H2-TPR was performed with 10 % H2/Ar. The heating rate was 10 o

C/min in the range 50-700 oC.

X-ray photoelectron spectroscopy. XPS studies were carried out on an ESCALAB

-p

250Xi system (Thermo Scientific, USA) equipped with an Al Kα X-ray radiation

re

source. Before the measurements, all samples were reduced under a flow of hydrogen at 600 °C for 1 h in the catalyst cell attached to the analysis chamber so that the

lP

transfer was free of reoxidation. The residual pressure inside the analysis chamber

284.8 eV.

na

was below 1×10-7 Pa. The binding energies were calibrated by setting the C 1s peak to

ur

2.3 Catalytic tests

The catalytic performance in the PDH reaction of catalysts was measured in a

Jo

quartz fixed-bed reactor under atmospheric pressure at 600 oC. The sieved catalyst (60-80 mesh) was held in the reactor by quartz wool and preheated in a stream of dry N2 (purity, 99.999 %) for 15 min at 600 oC before each test. Finally, N2 was replaced by a reactant stream consisting of propane (99.9 %) without dilution, and the PDH reaction started. The weight hourly space velocity (WHSV) of propane was 0.65 h-1.

The product gas was analyzed by gas chromatography (Bruker GC-450). The propane conversion and the propylene selectivity were calculated using the following equation: Conversion(C3H8) =

Selectivity(C3H6) =

outlet ninlet C3H8 -nC3H8

ninlet C3H8 noutlet C3H6 outlet ninlet C3H8 -nC3H8

ro of

where n with superscripts ‘‘inlet” and ‘‘outlet” stand for molar flow of components at the reactor inlet and outlet, respectively.

3. Results and discussion

-p

3.1 Characterization of calcined SnO2/SiO2 catalysts

re

The following four sections describe the results of physicochemical

preparation method.

lP

characterization of the calcined SnO2/SiO2 catalysts as a function of Sn loading and

na

3.1.1 N2 adsorption-desorption

Catalyst

SBET/m2∙g-1

Vtotal/cm3∙g-1

ur

Table 1 Textural properties of the calcined SnO2/SiO2 catalysts

SiO2

330.5

0.96

328.8

0.96

1.36-Sn/Si(2)

326.6

0.96

1.96-Sn/Si(3)

321.9

0.95

2.57-Sn/Si(4)

315.2

0.92

3.82-Sn/Si(5)

310.7

0.90

0.85-Sn/Si(1)-(ws)

330.1

0.96

1.32-Sn/Si(2)-(ws)

329.7

0.96

Jo

0.83-Sn/Si(1)

1.73-Sn/Si(3)-(ws)

328.4

0.95

1.78-Sn/Si(4)-(ws)

327.3

0.95

The textural properties of the freshly prepared SnO2/SiO2 samples, as well as SiO2 support, are characterized by low-temperature adsorption of N2. The calculated specific surface areas and pore volume are presented in Table 1. For both the x-Sn/Si(n) and x-Sn/Si(n)-(ws) samples, the surface area and pore volume decrease gradually with

ro of

an increase in the Sn loadings, indicating that at higher Sn loadings some of the pores are partly occupied by tin oxide. However, this decrease is moderate for all the

samples. Even in the case of 3.82-Sn/Si(5) with the highest Sn loading, the SBET value

-p

is only 6 % lower than that of bare SiO2 support, ruling out the presence of large tin oxide particles that may cause plugging of the SiO2 pores.

ur

na

lP

re

3.1.2 X-ray diffraction

Jo

Fig. 1. XRD patterns of the (a) x-Sn/Si(n) as well as bulk SnO2, and (b) x-Sn/Si(n)-(ws). Fig. 1 depicts the XRD patterns obtained for calcined SnO2/SiO2 samples, along

with that of bulk SnO2 as a reference. The 2θ angles at 26.6°, 33.9°, 38.0°, 51.8°, 54.8°, 57.8°, 61.9°, 64.7°, 65.9° and 71.3° are characteristic of the tetragonal rutile phase of SnO2 (JCPDS No. 41-1445). The broad diffraction peak at ca. 22°, which is observed in the XRD patterns of all the SnO2/SiO2 samples, is typical for amorphous

silica. For the x-Sn/Si(n) samples, the diffraction peaks corresponding to SnO2 are detected in the XRD patterns of the samples with Sn loading ≥ 1.96 wt% (Fig. 1a). It can be seen that the peaks due to crystalline SnO2 become sharper and narrower as the Sn loadings increase, indicating an increase of SnO2 mean particle size. Estimated from the Debye-Scherrer formula based on the (110) reflection of XRD, the average

ro of

particle sizes of SnO2 are 5, 18 and 35 nm for the 1.96-Sn/Si(3), 2.57-Sn/Si(4) and

3.82-Sn/Si(5) samples, respectively. By contrast, no diffraction peaks other than that of the silica support are identified in the patterns of the x-Sn/Si(n) samples with Sn

-p

loading ≤ 1.36 wt%, and all the x-Sn/Si(n)-(ws) samples (Fig. 1b). The absence of the

re

diffraction peaks characteristic for SnO2 phase in the XRD patterns of these samples may be explained either by the presence of a high dispersion of SnO2 phase on the

lP

SiO2 support or by a detection limit of XRD technique. Hence, UV-vis spectroscopy,

na

which has been successfully used to distinguish the SnOx species in silica supported tin systems, is also utilized to obtain more definitive information about the possible

ur

presence of XRD-undetectable surface SnOx species for SnO2/SiO2 samples.

Jo

3.1.3 UV-vis diffuse reflectance spectroscopy

ro of

Fig. 2. Diffuse reflectance UV-vis spectra of (a) x-Sn/Si(n), and (b) x-Sn/Si(n)-(ws). A comparison of the UV-vis spectra of the SnO2/SiO2 samples investigated is

-p

shown in Fig. 2. For the x-Sn/Si(n) samples with Sn loading ≤ 1.36 wt% and all the x-Sn/Si(n)-(ws) samples, the spectra are essentially the same, revealing a main

re

absorption band with a maximum absorbance below 200 nm. According to [23-24,25],

lP

this band is assigned to the O→Sn4+ charge transfer of isolated Sn4+ species in tetrahedral coordination. The intensity of this absorption increases with the Sn loading,

na

indicating an increase in the concentration of these isolated Sn species. In the case of the x-Sn/Si(n) samples with Sn loading ≥1.96 wt%, in addition to the band

ur

characteristic of isolated Sn4+ species, a broad UV-vis band at around 280 nm is

Jo

observed. According to [26-27], the band around 280 nm is ascribed to the electron transition from valence band to conduction band of crystalline SnO2. Such a result indicates that crystalline SnO2 is formed on these samples, in agreement with the XRD observations as shown in Fig. 1a. In a previous report we notice absence of SnO2-related signals in UV-vis considering a detection limit of 0.08 wt% Sn for crystalline SnO2 [31]. Since no signs of the presence of crystalline SnO2 is found in

x-Sn/Si(n) samples with Sn loading ≤ 1.36 wt%, and all the x-Sn/Si(n)-(ws) samples, it is concluded that these samples are free of crystalline SnO2.

ro of

3.1.4 Temperature-programmed reduction

-p

Fig. 3. H2-TPR profiles for differently prepared SnO2/SiO2 catalytic materials as a function of Sn loading: (a) x-Sn/Si(n) and (b) x-Sn/Si(n)-(ws).

re

The reducibility of various SnOx species in the SnO2/SiO2 catalysts is investigated

lP

by H2-TPR experiments and the profiles are displayed in Fig. 3. Quartz wool and SiO2 are examined as control experiments, and these are found to show negligible reduction

na

signals. For comparison and calibration purposes, the TPR profile of bulk SnO2 is first obtained (Fig. S1). This shows a broad reduction peak at ca. 620 oC, attributable to the

ur

characteristic reduction of Sn4+ to Sn0 [28-29].

Jo

The TPR profiles of the x-Sn/Si(n) samples are shown in Fig. 3a. It can be seen that the reduction effects for the samples depend on the Sn loading. Only one sharp and intense peak at ca. 500 oC is observed for the samples with Sn loadings lower than 1.36 wt%. According to the above UV-vis observations and literature [30-31, 34], this peak could be ascribed to the reduction of isolated Sn4+ to Sn2+. An increase in the Sn content above 1.36 wt% of Sn results in the appearance of additional peaks above 500

o

C. These peaks are only seen for these high-loaded samples which have been shown

to contain crystalline SnO2 (XRD, Fig. 1 and UV-vis-DRS, Fig. 2) and thus are assigned to the reduction of these bulk-like SnO2 crystallites species [32]. Besides, the Tmax of the peaks due to crystalline SnO2 gradually shifts to the higher temperature with the increase of Sn loading. A former study proves that for bulk tin oxide, the larger the particle size, the harder for it to be reduced [33]. As the particle

ro of

size increases tin oxide become more difficult to reduce due to bulk diffusion

limitation, which will result in a shift of the reduction peaks to higher temperatures

[34]. It has been reported that large SnO2 particles (27 nm) display a reduction peak at

-p

650 oC [40], whereas reduction Tmax of small SnO2 particles (10 nm) locates at 580 oC

re

[35]. Thus, these shifts to higher Tmax imply that the average sizes of crystallite SnO2 tend to increase with the Sn loading. This trend is in accord with the variation

lP

tendency of crystallite SnO2 sizes calculated by Debye-Scherrer formula.

na

The TPR profiles of all the x-Sn/Si(n)-(ws) samples are almost the same as those of low-loaded x-Sn/Si(n) samples (≤1.36 wt%) and also characterized by one peak of H2

ur

consumption related to the reduction of isolated Sn4+ species (Fig. 3b ). In addition, the high-temperature reduction peaks originated from crystalline SnO2 are not

Jo

observed for all the x-Sn/Si(n)-(ws) samples, indicating that they are free of crystalline SnO2, which agrees well with the XRD and UV-vis DRS results. On the basis of the results obtained from above XRD, H2-TPR and UV-vis-DRS work, it is determined that in the calcined SnO2/SiO2 samples an initial coverage of the silica support occurs, constituted of isolated Sn4+ species in tetrahedral

coordination anchored to the support, whereas crystalline SnO2 and isolated species are found for the higher Sn loading. Besides, from a comparison of characterization results for x-Sn/Si(n) and x-Sn/Si(n)-(ws), it can be found that rinsing the impregnated SnO2/SiO2 samples with water prior to drying and calcination process can effectively reduce the amount of crystalline SnO2 in the calcined catalysts, whereas isolated Sn4+ species are almost unaffected by such treatment. For x-Sn/Si(n) samples with Sn

ro of

loading ≤ 1.36 wt%, the Sn loading and the types of Sn speices are basically

unchanged after water-washing. This means that during the initial impregnation, some tin chloride complex in the impregnating solution adsorbs strongly on the silica

-p

support, which resist being removed by treatment with water. After calcination, these

re

strongly adsorbing species are converted into isolated Sn species. These results suggest the existence of a strong interaction between isolated Sn4+ and support, which

lP

leads to the anchoring of partial tin chloride complex to support, causing a buildup of

na

isolated Sn4+ in the early stages of impregnation.

Jo

ur

3.2 Propane dehydrogenation activity

Fig. 4. Propane dehydrogenation over the calcined SnO2/SiO2 catalysts: (a) propane conversion and (b) selectivity to propylene. Reaction conditions as described in

section 2.3 The dehydrogenation of propane to propylene over the calcined SnO2/SiO2 catalysts is investigated in a fixed-bed reactor at 600 °C under 1 atmosphere total pressure. Prior to the catalytic tests, a control experiment is performed in the absence of catalyst, with the catalyst replaced by SiO2 of equal volume. It is found that under the conditions used for the tests, the background propane conversion is negligible.

ro of

The propane conversion and propylene selectivity of the investigated catalysts as a function of time-on-stream are depicted in Fig. 4.

-p

It can be noted that all of the freshly prepared SnO2/SiO2 catalysts exhibit relatively

low C3H8 conversion and extremely poor C3H6 selectivity at the first data point (5 min

re

on stream). Subsequently, conversion of propane and selectivity to propylene increase

lP

simultaneously and reach their maximum at 60 min. After this period, stable activities with pronounced selectivity to propylene (>85 %) can be maintained for all the

na

samples. Meanwhile, a relatively large amount of COx is detected during the initial period of reaction. Such results indicate that an induction period is required to develop

ur

the active site for the production of propylene, that is, a period for the in situ creation

Jo

of active and selective Sn species.

ro of

Fig. 5. Propane dehydrogenation over the H2 pre-reduced SnO2/SiO2 catalysts: (a) Propane conversion and propylene selectivity; (b) The rate normalized per unit surface area of propane conversion.

-p

To understand better the behavior of the catalysts during the induction period, the activities of the SnO2/SiO2 catalysts are measured after hydrogen reduction (10%

re

H2/Ar, 600 oC and 1 h), and the results are presented in Fig. 5a. It is of interest to note

lP

that the H2-pretreatment has a distinct influence on the catalytic performance of all SnO2/SiO2 catalysts at the initial stage of the reaction, with essentially no influence on

na

the steady-state activity. Pre-reduced SnO2/SiO2 samples show immediate high initial activity without experiencing the induction period. More importantly, the activities of

ur

the pre-reduced samples are at the same level as the steady-state activities of their

Jo

calcined counterparts. These findings, together with the H2-TPR revealing that extensive reduction of tin oxide on the SnO2/SiO2 catalysts takes place below 600 oC (Fig. 4), suggest that a partial reduction of highly oxidized Sn4+ species in the calcined catalysts is beneficial for the extended conversion of C3H8, and particularly for the formation of C3H6. The C3H8 conversion increase with an increase of the tin loading and reach the

maximum for the catalyst containing 1.78 wt % of Sn. A further increase in the Sn loading affects the catalytic activity slightly, and no sharp drop in activity is observed. The rate normalized per unit surface area of propane conversion is proportional to the Sn content of 0.83-1.78%. With further increasing Sn content, the rate normalized per unit surface area does not increase. The characterization results show that in low loaded (≤1.78 wt%) SnO2/SiO2 samples, isolated Sn4+ species predominate, and the

ro of

amount of these well-dispersed Sn species increase as the Sn loading increases, similar as the catalytic activity increases. This confirms the involvement of the

3.3 Oxidation states of tin oxide after reduction

-p

isolated Sn species in the PDH reaction.

re

Considering the fact that the PDH reaction over the calcined SnO2/SiO2 catalysts is characterized by an induction period, and pre-reduction of the catalysts at 600 oC with

lP

hydrogen can fully eliminate the induction period, it is believed that Sn species with

na

lower oxidation state is more active for the PDH reaction than Sn4+ species in calcined catalysts. Thus, it is essential to find out the oxidation state of Sn after

ur

high-temperature H2 reduction.

Thus, the amounts of H2 consumed during the H2-TPR experiments were measured,

Jo

and the average oxidation state of tin species was calculated. For comparison purpose, a quantitative chemisorption experiment of the consumed H2 is also performed by using calibrated pulses of pure H2 at 650 oC to quantify the reduction extent of tin speices (detailed information can be obtained in section S2 of the supporting information).

Table 2 Average oxidation state of Sn calculated from TPR and quantitative chemisorption experiments. Degree of reduction (%) Catalyst

Average oxidation state of Sn

q. c.a

H2-TPR

q. c.a

3.82-Sn/Si(5)

81.6

-

+0.7

-

2.57-Sn/Si(4)

69.8

-

+1.2

-

1.96-Sn/Si(3)

57.6

54.8

+1.7

+1.8

1.36-Sn/Si(2)

54.7

51.5

+1.8

+1.9

0.83-Sn/Si(1)

52.2

48.3

+1.9

+2.1

0.85-Sn/Si(1)-(ws)

45.7

48.8

+2.2

1.31-Sn/Si(2)-(ws)

53.4

52.6

+1.9

1.73-Sn/Si(3)-(ws)

54.7

57.2

+1.8

1.78-Sn/Si(4)-(ws)

54.9

56.6

+1.8

Bulk SnO2

99.6

-

-p

ro of

H2-TPR

+0.0

+2.0

+1.9 +1.7

+1.7 -

re

a. quantitative chemisorption experiment (Fig. S2)

lP

Table 2 shows the average oxidation states of Sn obtained with two techniques. For the bulk SnO2, the degree of reduction of Sn4+ is 99.6 %, indicating that bulk SnO2

na

can be completely reduced to Sn0 [36]. For the samples with Sn loading less than 1.78 wt%, on which isolated species are dominant, the degree of reduction of Sn4+ varies

ur

slightly with the increase of Sn loading and is 50 ± 5 % based on a Sn4+ to Sn0

Jo

reaction. Thus, the average oxidation state of isolated tin speices after H2 reduction should therefore be Sn2+. These results indicate that isolated Sn4+ species are only partially reduced, which agrees with results of Aguirre et al. [37]. However, for samples with Sn loading ≥ 1.96 wt%, the degree of reduction of Sn increases with the increase of Sn loading, and it gradually exceeds 50% but is still less than 100%, suggesting that some of Sn4+ in these samples are reduced to the metallic state. To

probe this hypothesis, XRD studies of 2.57-Sn/Si(4) and 3.82-Sn/Si(5) samples after H2-TPR experiments are carried out, and the results are shown in Fig. S3. It can be seen that the diffraction peaks owing to crystalline SnO2 in the XRD patterns of calcined samples entirely disappear, and the diffraction peaks due to metallic Sn emerge after H2-TPR. Since the above characterization results suggest that these two samples contain both isolated Sn4+ species and crystalline SnO2, and isolated Sn4+

ro of

species are only partially reduced after TPR, the metallic Sn species are more likely

ur

na

lP

re

-p

produced by the complete reduction of “bulk-like” crystalline SnO2.

Fig. 6. Sn 3d spectra of the catalysts reduced at 600 oC for 1 h: (a) 2.57-Sn/Si(4), (b)

Jo

1.96-Sn/Si(3), (c) 1.78-Sn/Si(4)-(ws), and (d) 1.36-Sn/Si(2). To gain further insight into the oxidation state of the supported Sn species after H2

reduction, XPS studies on reduced SnO2/SiO2 samples (10 vol.% H2/Ar, 600 oC and 1 h) are performed, and the results are shown in Fig. 6. In the case of the reduced 1.96-Sn/Si(3) and 2.57-Sn/Si(4) samples, the shape of the Sn 3d5/2 peak indicates the presence of a doublet. After curve fitting, two peaks are obtained with peak positions

at 486.5-487.0 and 484.5 eV corresponding to oxidized Sn (Sn2+ or Sn4+) and metallic Sn (Sn0), respectively [38-39,40,41]. For the reduced 1.78-Sn/Si(4)-(ws) and 1.36-Sn/Si(2) samples, Sn 3d5/2 peaks lie in the range of 487-488 eV, and peaks due to Sn0 is not detected, which tell us that the majority of the Sn is in the oxidized Sn (+2, +4) state. Unfortunately, XPS cannot discriminate between Sn2+ and Sn4+ oxidized states [50]. However, the results in Table 2 regarding an assessment of the extent of

ro of

Sn reduction indicate that the reduction of isolated Sn4+ species does not proceed

below an average oxidation state of Sn2+. Hereby, we propose that isolated Sn4+ is reduced only to Sn2+ state.

-p

Besides, a noticeable shift of Sn 3d5/2 peak to the higher binding energy is observed

re

when the loading of Sn decreases. By XPS and Auger spectroscopy, Jiménez et al. find that at very low coverages of SnO on SiO2, XPS shows a positive binding energy

lP

shift of around 1 eV in the BE of the Sn3d5/2 peak with respect to the values found for

na

the bulk SnO. Electron energy loss spectroscopy shows that this shift is due to interaction effects among SnO and SiO2 since excitation energy of Sn-O-Si tends to

ur

be higher than Sn-O-Sn [42]. Thus, the shifts in the Sn3d5/2 peak observed in Fig. 7

Jo

suggests the existence of interactions between Sn2+ species and SiO2 support.

ro of

Fig. 7. H2-TPR profiles of selected samples obtained in the reoxidation treatment at room temperature.

It has been reported that metallic Sn could react with oxygen to form oxide

-p

overlayer even at room temperature (RT), whereas Sn2+ do not adsorb oxygen at all

re

[43]. Based on this, a reduction-reoxidation-TPR experiment is carried out to further clarify whether isolated Sn4+ species are reduced to the Sn0 during the

lP

high-temperature reduction. After H2-reduction, the reduced catalyst is cooled to room

na

temperature (RT) in hydrogen and then reoxidized in air flow at RT. The re-oxidized catalyst is again subjected to the second TPR. For comparison, a physical mixture of

ur

silica and metallic Sn powder (0.6 wt% Sn) are prepared by manually grinding the silica and Sn powder (>99.9%, Aladdin) in a mortar, and are also subjected to the

Jo

experiment. The TPR profiles of the reoxidized samples are shown in Fig. 7. The TPR profile of a physical mixture of metallic Sn and SiO2 shows a symmetric

peak at 400 oC. Since the SiO2 shows negligible reduction signals, it can be concluded that the detected H2 consumption peak is the result of the reduction of tin oxide layer formed on the metallic Sn during the reoxidation treatment. Similar reduction peaks

are observed for 1.96-Sn/Si(3) and 2.57-Sn/Si(4) samples, demonstrating that metallic Sn is formed on the two samples, in agreement with the XPS observations. Instead, there is no distinct reduction peak for 0.83-Sn/Si(1), 1.36-Sn/Si(2) and 1.78-Sn/Si(4)-(ws) samples, implying that the reduced Sn species present on the catalysts cannot be reoxidized at room temperature. Combined with the results of XPS and TPR, it is concluded that isolated Sn4+ species are reduced only to Sn2+ state.

ro of

It is known that bulk SnO is thermodynamically unstable, and can disproportionate to Sn0 and SnO2 above 550 oC [44]. However, the above results demonstrate that

isolated tin species can be stabilized in the Sn2+ state after hydrogen reduction. This

-p

must indicate that there is a strong interaction between the Sn2+ and the silica support,

re

which hinder the disproportion and complete reduction of Sn2+. Besides, the XRD pattern of 1.78-Sn/Si(4)-(ws) samples taken after reduction in hydrogen shows only a

lP

broad diffraction peak due to the amorphous silica, and no obvious diffraction peaks

na

corresponding to SnO (Fig. S3), suggesting that these Sn2+ species are not present as bulk SnO, but are well-dispersed on the silica support.

Jo

ur

3.4 Long-term stability test of well-dispersed Sn2+ species

Fig. 8. On-stream propane conversion and propylene selectivity in PDH reaction over pre-reduced 1.78-Sn/Si(4)-(ws) catalyst. Reduction condition: T = 600 oC; 10 vol.% H2/Ar; 1 hour. Reaction conditions: T = 600 oC; C3H8 molar ratio = 100 %; WHSV = 0.65 h-1. One of the biggest challenges of the PDH is the deactivation of catalysts with time-on-stream. Hence, the catalysts must be regenerated frequently to maintain

ro of

sufficient activity. Nevertheless, such treatment results in a gradual loss of initial activity and reduced on-stream stability on account of the restructuring of the

catalytically active species [45]. In our previous work, Sn0 has been observed to be

-p

active for the PDH reaction and shows stable performance during 60 h reaction [26].

re

Unfortunately, Sn0 is not completely stable because of low melting point. The high

deactivation of the catalyst.

lP

temperature of reaction triggers the loss and sintering of Sn0, resulting in irreversible

na

Thus, an extended 120 h on-stream operation for PDH is carried out on the reduced 1.78-Sn/Si(4)-(ws) catalyst with the aim of examining the likely long-term behavior of

ur

well-dispersed Sn2+ species under the reaction conditions. Fig. 8 shows the variation of propane conversion and selectivity to propylene as a function of time-on-stream. It

Jo

can be seen that the catalyst exhibits exceptionally stable activity during evaluation period. After 120 h on-stream, only a 3.5 % loss of initial activity is detected, stressing the high stability of these Sn2+ species. To gain insights into the recyclability of these well-dispersed Sn2+ species, the 1.78-Sn/Si(4)-(ws) catalyst after 120 h time-on-stream is regenerated with air at 600 oC

for 3 h followed by the reduction in hydrogen. From Fig. S4, it is clear that the initial activity of the catalyst can be fully recovered upon oxidative regeneration, and the regenerated catalyst exhibit stable performance without apparent deactivation. To check whether a change in Sn constitution occurs during the reaction-regeneration process. After reaction, the catalyst is treated with air again to burn of any carbon deposits and cooled down to room temperature in pure N2, and then taken out for later

ro of

XRD, UV-vis-DRS and XRF analysis. As shown in Fig. S5, the XRD pattern of the

regenerated catalyst exhibits no peaks except for those of the silica support, and the

UV-vis spectrum reveals only one absorption bands below 200 nm which is assigned

-p

to isolated Sn4+. Our previous work shows that the volatilization of metallic Sn over

re

the course of long-term PDH reaction leads to the decrease of Sn content in the catalyst since the melting point of metallic Sn is much lower than the reaction

lP

temperature. Nevertheless, XRF results in Table S1 show that no significant change in

na

Sn loading is observed for 1.78-Sn/Si(4)-(ws) catalyst after 120 h reaction-regeneration. Sn loading slightly decreases to 1.71 % from 1.69 %. These results indicate that these

ur

dispersed Sn2+ species are highly stable during the long-term reaction.

Jo

4. Conclusion

In this study, silica-supported tin oxide catalysts with the Sn contents varying from

0.8 to 3.8 wt % are prepared. Combination of the characterization results obtained by different techniques for SnO2/SiO2 catalysts reveals that the nature and distribution of SnOx species depend on the Sn loading. SnOx surface species are highly dispersed as

isolated species in the samples with Sn loading up to 1.78 wt%. Crystalline SnO2, as well as isolated species is present on the samples with higher Sn loading. Catalytic activity tests in propane dehydrogenation show that Sn species with a lower oxidation state favor selective conversion of propane to propylene than highly oxidized Sn4+ in the calcined catalysts. A detailed investigation of the valence state of various Sn species on the support surface is therefore carried out by using several

ro of

characteration techniques. From the results of these studies, it reveals that after

reduction, isolated Sn4+ species are stabilized in Sn2+ state by interaction with the silica support. The results of

long-term stability test show that these Sn2+ species

-p

exhibit extremely stable catalytic performance for 120 h reaction on-stream without

re

apparent deactivation, which could be explained by the strong interaction between

na

Acknowledgement

lP

Sn2+ and support.

This work was financially supported by the National Natural Science Foundation of

Jo

ur

China (No. U1362201).

References

[1] J.J. Sattler, J. Ruiz-Martinez, E. Santillan-Jimenez and B.M. Weckhuysen, Chem. Rev., 2014, 114, 10613-10653. [2] J. Li, J. Li, Z. Zhao, X. Fan, J. Liu, Y. Wei, A. Duan, Z. Xie and Q. Liu, J. Catal., 2017, 352, 361-370.

Miller, A.S. Hock, ACS Catal., 2015, 5, 3494-3503.

ro of

[3] B. Hu, N.M. Schweitzer, G. Zhang, S.J. Kraft, D.J. Childers, M.P. Lanci, J.T.

[4] J.A. Moulijn, A.E. Van Diepen and F. Kapteijn, Appl. Catal. A, 2001, 212, 3-16.

[5] S. Tan, B. Hu, W. Kim, S.H. Pang, J.S. Moore, Y. Liu, R.S. Dixit, J.G. Pendergast,

-p

D.S. Sholl, S. Nair, C.W. Jones, ACS Catal., 2016, 6, 5673-5683.

[6] K. Searles, K.W. Chan, J. A. Mendes Burak, D. Zemlyanov, O. Safonova and C.

re

Copéret, J. Am. Chem. Soc., 2018, 140, 11674-11679.

lP

[7] H.F. Xiong, S. Lin, J. Goetze, P. Pletcher, H. Guo, L. Kovarik, K. Artyushkova, B.M. Weckhuysen and A. K. Datye, Angew. Chem., 2017, 56, 8986-8991.

na

[8] T.P. Otroshchenko, U. Rodemerck, D. Linke and E.V. Kondratenko, J. Catal., 2017, 356, 197-205.

ur

[9] P. Hu, W. Lang, X. Yan, L. Chu and Y. Guo, J. Catal., 2018, 358, 108-117. [10] Z. Zhao, T. Wu, C. Xiong, G. Sun, R. Mu, L. Zeng and J. Gong, Angew. Chem.,

Jo

2018, 57, 6791-6795.

[11] S.W. Choi, W.G. Kim, J.S. So, J.S. Moore, Y. Liu, R.S. Dixit, J.G. Pendergast, C. Sievers, D.S. Sholl, S. Nair and C.W. Jones, J. Catal., 2017, 345, 113-123.

[12] K.C. Szeto, Z.R. Jones, N. Merle, C. Rios, A. Gallo, F. Le Quemener, L. Delevoye, R.M. Gauvin, S.L. Scott and M. Taoufik, ACS Catal., 2018, 8, 7566-7577.

[13] B. Hu, A. Getsoian, N.M. Schweitzer, U. Das, H. Kim, J. Niklas, O. Poluektov, L.A. Curtiss, P.C. Stair, J.T. Miller and A.S. Hock, J. Catal., 2015, 322, 24-37. [14] G. Liu, L. Zeng, Z.J. Zhao, H. Tian, T. Wu and J. Gong, ACS Catal., 2016, 6, 2158-2162. [15] N.M. Schweitzer, B. Hu, U. Das, H. Kim, J. Greeley, L.A. Curtiss, P.C. Stair, J.T. Miller and A.S. Hock, ACS Catal., 2014, 4, 1091-1098. [16]H.N. Pham, J.J.H.B. Sattler, B.M. Weckhuysen and A.K. Datye, ACS Catal., 2016,

ro of

6, 2257-2264. [17] G. Wang, H. Zhang, H. Wang, Q. Zhu, C. Li and H. Shan, J. Catal., 2016, 344, 606-608.

-p

[18] D.P. Estes, G. Siddiqi, F. Allouche, K.V. Kovtunov, O.V. Safonova, A.L. Trigub, I.V. Koptyug, C. Coperet, J. Am. Chem. Soc., 2016, 138, 14987-14997.

re

[19] N.M. Schweitzer, B. Hu, U. Das, H. Kim, J. Greeley, L.A. Curtiss, P.C. Stair, J.T.

lP

Miller, A.S. Hock, ACS Catal. 2014, 4, 1091-1098.

[20] K. Searles, G. Siddiqi, O.V. Safonova, C. Coperet, Chem. Sci., 2017, 8, 2661-2666.

na

[21] D. Casas-Orozco, E. Alarcón, C.A. Carrero, J.M. Venegas, W. McDermott, E. Klosterman, I. Hermans, A.L. Villa, Ind. Eng. Chem. Res., 2017, 56, 6590-6598.

ur

[22] J. Dijkmans, J. Demol, K. Houthoofd, S. Huang, Y. Pontikes, B. Sels, J. Catal.,

Jo

2015, 330, 545-557

[23] T.R. Gaydhankar, P.N. Joshi, P. Kalita and R. Kumar, J. Mol. Catal. A: Chem., 2007, 265, 306-315.

[24] J. Dijkmans, D. Gabriëls, M. Dusselier, K. Houthoofd, P.C.M.M. Magusin, S. Huang, Y. Pontikes, M. Trekels, A. Vantomme, L. Giebeler, S. Oswald and B.F. Sels, ACS Catal., 2015, 5, 928-940.

[25] J.C. Vega-Vila, J.W. Harris and R. Gounder, J. Catal., 2016, 344, 108-120. [26] S.Y. Chen, H.D. Tsai, W.T. Chuang, J.J. Lee, C.Y. Tang, C.Y. Lin and S. Cheng, J. Phys. Chem. C, 2019, 113, 15226-15238. [27] X. Fan, J. Li, Z. Zhao, Y. Wei, J. Liu, A. Duan and G. Jiang, RSC Adv., 2015, 5, 28305-28315. [28] P.W. Park, H.H. Kung, D.W. Kim and M.C. Kung, J. Catal., 1999, 184, 440-454. [29] J. Zhang, Y. Liu, Y. Sun, H. Peng, X. Xu, X. Fang, W. Liu, J. Liu and X. Wang,

ro of

Ind. Eng. Chem. Res., 2018, 57, 10315-10326. [30] L. Sun, Y. Chai, W. Dai, G. Wu, N. Guan and L. Li, Catal. Sci. Technol., 2018, 8, 3044-3051.

-p

[31] A. Vicente, G. Lafaye, C. Especel, P. Marécot and C.T. Williams, J. Catal., 2011, 283, 133-142.

re

[32] R. Burch, V. Caps, D. Gleeson, S. Nishiyama and S.C. Tsang, Appl. Catal. A,

lP

2000, 194, 297-307.

[33] F. Lan, X. Wang, X. Xu, R. Zhang and N. Zhang, React. Kinet. Mech. Catal., 2012, 106, 113-125.

na

[34] A. Auroux, D. Sprinceana and A. Gervasini, J. Catal., 2000, 195, 140-150. [35] A.V. Marikutsa, M.N. Rumyantseva, L.V. Yashina and A.M. Gaskov, J. Solid

ur

State Chem., 2010, 183, 2389-2399.

Jo

[36] X. Wang and Y. Xie, Chem. Lett., 2001, 30, 216-217 [37] A.D. Aguirre, P. Reyes, M. Oportus, I. Melian-Cabrera and J.L.G. Fierro, Appl. Catal. A: Gen., 2002, 233, 183-196.

[38] E. Merlen, P. Beccat, J.C. Bertolini, P. Delich`ere, N. Zanier and B. Didillon, J. Catal., 1996, 159, 178-188. [39] B.A. Riguetto, C.E.C. Rodrigues, M.A. Morales, E. Baggio-Saitovitch, L.

Gengembre, E. Payen, C.M.P. Marques and J.M.C. Bueno, Appl. Catal. A, 2007, 318, 70-78. [40] Y. Zhang, Y. Zhou, L. Huang, M. Xue and S. Zhang, Ind. Eng. Chem. Res., 2011, 50, 7896-7902. [41] Y. Zhu, Z. An, H. Song, X. Xiang, W. Yan and J. He, ACS Catal., 2017, 7, 6973-6978. [42] V. Jiménez, A. Fernández, J.P. Espinós and A.R. González-Elipe, Surf. Sci., 1996,

[43] H. Lieske and J. Völter, J. Catal., 1984, 90, 96-105.

ro of

350, 123-135.

[44] F. Gauzzi, B. Verdini, A. Maddalena and G. Principi, Inorganica Chimica Acta,

-p

1985, 104, 1-7.

Jo

ur

na

lP

Catal., 2012, 293, 67-75.

re

[45] S. Sokolov, M. Stoyanova, U. Rodemerck, D. Linke and E.V. Kondratenko, J.