Chunzheng Wang

Weisong Xu

Zhengxing Qin

Hailing Guo

Xinmei Liu

Svetlana Mintova
email: svetlana.mintova@ensicaen.fr

Highly active Pd containing EMT zeolite catalyst for indirect oxidative carbonylation of methanol to dimethyl carbonate

Keywords: EMT, Palladium, Stability, Carbonylation, Dimethyl carbonate

Palladium containing EMT zeolite catalyst (Pd/EMT) was prepared and used for the indirect oxidative carbonylation of methanol to dimethyl carbonate (DMC). The EMT zeolite was employed as a new catalyst support and compared with the conventional Pd containing FAU zeolite catalyst (Pd/FAU). The Pd/EMT in contrast to the Pd/FAU catalyst exhibited high intrinsic activity with the turnover frequency of 0.25 s 1 vs. 0.11 s 1 . The Pd/EMT catalyst showed high CO conversion of 82% and DMC selectivity of 79%, that maintained for at least 130 h, while the activity of the Pd/FAU catalyst rapidly deteriorated within 12 h. The enhanced interactions between Pd and EMT zeolite inhibited the sintering of palladium clusters and maintained the Pd 2+ active sites in the Pd/EMT catalyst.

The stabilization of the mono-dispersed Pd clusters within the EMT zeolite is paramount to the excellent performance of the catalyst for the indirect oxidative carbonylation of methanol to DMC.

Introduction

Dimethyl carbonate (DMC) is an environmentally sustainable compound with useful and versatile chemical properties [1,2]. DMC can serve as a green carbonylation and methylation reagent in place of toxic phosgene and dimethyl sulfate [3]. The high oxygen content (53 wt. %) of DMC and rapid biodegradability make it a promising gasoline additive [4]. DMC is also used as an effective solvent for organic synthesis, coating and electrolyte in lithium ion batteries, and as a common monomer for the production of polycarbonate [5]. The traditional production of DMC ((CH3O)2CO) is based on the reaction of phosgene (COCl2) with methanol (CH3OH) via methyl chloroformate; the extreme toxicity of COCl2 and serious equipment corrosion caused by hydrogen chloride byproduct are of major concern [6]. Recently, great efforts have been made to develop green chemical routes for DMC production including [79]: (1) the direct oxidative carbonylation of carbon monoxide (CO), oxygen (O2) and CH3OH; (2) the indirect oxidative carbonylation of CO, O2 and CH3OH; (3) the direct synthesis from carbon dioxide (CO2) and CH3OH; (4) the methanolysis of urea; and (5) the transesterification of ethylene/propylene carbonate with CH3OH. Among these processes, the indirect oxidative carbonylation of methanol to DMC is considered to be an ambitious route because of the mild reaction conditions, high atom efficiency and low cost [10]. This indirect route for DMC synthesis involves two separate reactions (Scheme S1 in Supporting Information (SI)): (1) non-catalytic synthesis of methyl nitrite (CH3ONO) with CH3OH and O2: 2 CH3OH + 2 NO + 1/2 O2  2 CH3ONO + H2O; (2) Pd-catalyzed oxidative carbonylation to DMC: CO + 2 CH3ONO  (CH3O)2CO +2 NO (The formed NO can be recycled for the synthesis of methyl nitrite). The overall reaction (CO + 2 CH3OH + 1/2 O2  (CH3O)2CO + H2O) results in an environmentally benign and highly efficient process.

The Pd-based catalysts used for the indirect oxidative carbonylation of methanol to DMC are either chlorine-containing or chlorine-free catalysts [1113]. The gas-phase process has been successfully commercialized by Ube Industries, Ltd. (15, 000 tons/year), using a PdCl2/active-carbon catalyst [14,15]. However, chlorine (e.g., 100 L L 1 hydrogen chloride) must be added into the reactants to maintain the activity of the chlorine-containing catalysts [13,15]. The introduction of chlorine leads to the corrosion of equipment, difficult regeneration of chlorine, and unsatisfactory purity of DMC product [13,15], which encounters problems related with the green chemistry concept. Therefore, significant work is dedicated to the development of chlorine-free, high-performance Pd catalysts. Tan et al. found that the activity of Pd/FAU catalyst was improved by increasing the Pd oxidation states [16]. They also revealed, by density functional theory (DFT) calculations, that the Pd 2+ species was responsible for the formation of DMC while the aggregation of Pd 0 species resulted in the generation of dimethyl oxalate (DMO) byproduct [10]. Dong et al. demonstrated that the potassium promoter improved the electron density of Pd clusters and thus enhanced the activation of CO reactant on the Pd/FAU catalyst; both theoretical and experimental proofs were provided [12]. Guo et al. reported that the Cu-doped Pd/FAU catalyst showed higher activity and selectivity than the Cu-free Pd/FAU catalyst, possibly due to the increased Pd 2+ /Pd 0 ratio of Pd clusters [17]. It is accepted that Brønsted acidity can accelerate the decomposition of CH3ONO reactant, and result in poor catalyst selectivity [1]. Dong et al. presented a nearly linear relationship between the turnover frequency (TOF) and the amount of Lewis acidity of the Pd/FAU catalyst [18]. They also suggested that Lewis acid sites could accept electrons from the Pd species and thus promote the formation of electron-deficient Pd x+ active sites [18]. The possible mechanism included the adsorption of CH3ONO on Pd 2+ actives sites forming Pd(OCH3)(NO) and the subsequent CO insertion generating the Pd(COOCH3)(NO); this is coupled with CH3ONO to produce DMC [19]. The catalyst stability is of crucial consideration, but as far as we know, the stability was only reported in a few articles [10,19]. Yamamoto et al. [19] found that the DMC yield of Pd/FAU catalyst declined remarkably within 100 h and decreased by about 50% after 300 h test, possibly due to the sintering of Pd clusters. Recently, Tan et al. [10] reported on a stable Pd/FAU catalyst for 100 h, but the selectivity of inevitable byproducts such as methyl formate and dimethoxymethane was not provided. The work reported up to now only focused on the use of FAU zeolite framework as a support [11,13]. Therefore, designing high-performance Pd catalysts by exploring alternative zeolite frameworks and properties is desirable for both fundamental research and commercial application.

Like FAU-type zeolite, the EMT-type zeolite is constituted of sodalite cages linked by double sixrings. The EMT framework structure results from a stacking of faujasite sheets through a mirror plane symmetry while the FAU framework structure is founded through an inversion center symmetry [20,21]. The EMT zeolite possesses two sets of cages, i.e. the hypocage (0.61 nm 3 ; channel: 0.75 nm  0.65 nm) and hypercage (1.24 nm 3 ; channel: 0.73 nm  0.73 nm) [22,23]. In contrast to the hexagonal EMT zeolite, the cubic FAU has only one type of supercage (1.15 nm 3 ; channel: 0.74 nm  0.74 nm).

The different topology of EMT and FAU also results in different cation locations and site energies [24,25]. The unique cage structure and cation locations in the EMT zeolite may bring benefits in catalysis [25,26]. Daou et al. [27] found that the EMT zeolite exhibited a higher performance (30%) in comparison to the USY zeolite in the nitrobenzene conversion. Feijen et al. [28] reported that the EMT zeolite showed molecular shape selectivity in decane isomerization and heptadecane hydrocracking compared to the FAU zeolite. Gao et al. [29] found that the intergrowth of EMT and FAU zeolite resulted in an appropriate pore structure and suitable Brønsted and Lewis acidity, and thus the CoMo catalyst supported on the EMT/FAU intergrowth zeolite, exhibited a good balance between hydrodesulfurization efficiency and research octane number.

Herein, the Pd-containing EMT zeolite catalyst (Pd/EMT) was prepared and used for the indirect oxidative carbonylation of methanol to dimethyl carbonate (DMC, (CH3O)2CO). The high activity, selectivity and stability of the Pd/EMT catalyst in comparison to conventional Pd/FAU catalyst for the indirect oxidative carbonylation of methanol to DMC are demonstrated.

Experimental

Catalyst preparation

The Na-form of EMT zeolite was synthesized according to the published procedure using a precursor mixture with the following molar composition: 1.0 SiO2 : 0.1 Al2O3 : 0.24 Na2O : 14.0 H2O : 0.1 18-crown-6 [30]. The initial materials sodium aluminate (41 wt. % Al2O3, Sinopharm Chemical Reagent Co., Ltd., China), colloidal silica (Ludox-HS, 30 wt. % SiO2, Aldrich), sodium hydroxide (NaOH, 96 wt. %, Xilong Scientific Co., Ltd., China) and 18-crown-6 (99 wt. %, Shanghai Macklin Biochemical Co., Ltd., China) were mixed in a polypropylene bottle and aged at room temperature (23 º C) for 24 h. The aged mixture was transferred into a Teflon-lined autoclave and subjected to heating at 110 º C for 15 days under static conditions. The crystalline product was filtered and thoroughly washed with deionized water. After drying at 80 º C for 6 h, the samples were calcined at 550 º C for 6 h in air to remove the crown ether template.

The Na-form FAU zeolite was synthesized from a precursor mixture with the following molar composition: 1.0 SiO2 : 0.1 Al2O3 : 0.8 Na2O : 40.0 H2O. The initial materials aluminium hydroxide (Sinopharm Chemical Reagent Co., Ltd., China), colloidal silica (Ludox-HS, 30 wt. % SiO2, Aldrich) and sodium hydroxide (NaOH, 96 wt. %, Xilong Scientific Co., Ltd., China) were mixed in a polypropylene bottle and aged at room temperature (23 º C) for 24 h. The aged mixture was transferred into a Teflon-lined autoclave and subjected to heating at 100 º C for 12 h under static conditions. The crystalline product was filtered, washed and dried at 80 º C for 6 h.

The introduction of palladium in both Na-form of EMT and FAU zeolites was performed according to the following procedure: palladium chloride (PdCl2, 59.0 wt. % Pd, Sinopharm Chemical Reagent Co., Ltd., China) was dissolved in a dilute ammonia solution (NH3H2O, 0.9 mmol L 1 ) and then the resulting Pd(NH3)4Cl2 solution was added dropwise into the as-prepared EMT or FAU zeolite slurry (liquid to solid mass ratio of 100). These mixtures were stirred (400 rpm) at 80 º C for 6 h, and then the solid samples were separated by vacuum filtration and thoroughly washed with deionized water. After drying at 80 º C for 6 h, the samples were calcined at 200 º C for 2 h in air to obtain the Pd/EMT or Pd/FAU catalysts. The Pd content of the two catalysts (Pd/EMT and Pd/FAU) was 1.0 wt. %, if not specified.

Catalysts characterization

Transmission electron microscopy (TEM) study of the catalysts was performed on a JEOL JEM-2100 instrument. In addition, the scanning electron microscopy (SEM) characterization of the catalysts was performed on a JEOL JSM-7900F equipped with an energy dispersive X-ray spectrometer (EDS).

X-ray diffraction (XRD) patterns of the materials were collected on a Bruker D8 Advance diffractometer, using Cu K radiation. The chemical compositions of the samples were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using a Thermo IRIS Intrepid II XSP. Additionally, X-ray photoelectron spectroscopy (XPS) characterization of samples was performed using an Escalab 250xi spectrometer with an Al Kα radiation (binding energy was referenced to the C 1s peak at 284.6 eV). N2 physisorption (77 K) experiments were performed on a Quantachrome Autosorb-IQ3 instrument. The specific surface area and pore size distribution were calculated using the Brunauer-Emmett-Teller (BET) theory and density functional theory (DFT) model based on the adsorption branch of the isotherms. Thermogravimetric-mass spectrometry (TG-MS) study was performed in a gas mixture of 20 vol. % O2 and 80 vol. % N2 on a Netzsch STA 449F5 instrument connected with QMS 403D mass spectrometer. H2-temperature programmed reduction (H2-TPR) was carried out using a Tianjin Xianquan TP-5080 apparatus. The H2 consumption was monitored by a thermal conductivity detector (TCD) or Tilon SR-D 200M mass spectrometer. Typically, 0.1000 g sample was pretreated in He flow (30 mL min 1 ) at 200 º C for 1 h and then cooled down to 20 º C. Subsequently, a gas mixture of 10 vol. % H2 in N2 was introduced into the reactor. The H2-TPR profiles were obtained in the range of 20700 º C with a heating rate of 10 º C min 1 at a constant flow rate of 30 mL min 1 . Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of pyridine adsorption were measured on a Nicolet-6700 spectrometer. The sample was evacuated at 200 º C for 5 h, followed by pyridine adsorption at room temperature (23 º C) for 24 h and pyridine desorption in a vacuum at 150 º C for 3 h. Subsequently, the spectra were recorded in absorption mode from 4000 cm 1 to 650 cm 1 at a resolution of 4 cm 1 over 64 scans. DRIFTS experiments of CO adsorption were conducted on a Bruker Vertex 70 spectrometer equipped with a liquid N2 cooling mercury-cadmium-telluride (MCT) detector and a Harrick Scientific HVC-DRP-4 reaction cell (ZnSe windows). The sample without diluents was placed into a reaction cell, prereduced in H2 (30 mL min 1 ) at 150 º C for 1 h and flushed with N2 (80 mL min 1 ) at 200 º C for 1 h. The sample was cooled down to 35 º C for taking a background spectrum and subsequently, pure CO gas was introduced into the reaction cell at 30 mL min 1 . After 30 min adsorption, the sample was flushed by N2 flow (80 mL min 1 ) at 35 º C for 1 h and then the DRIFTS spectra were collected in the range of 35200 º C. Each spectrum was recorded in absorption mode from 4000 cm 1 to 600 cm 1 at a resolution of 4 cm 1 over 64 scans.

Catalytic tests

The schematic diagram of the apparatus used for the catalyst evaluation is shown in Scheme S2 (SI). The catalysts were tested in a fixed-bed quartz tubular microreactor (8 mm i.d. and 350 mm length) at 0.1 MPa. A K-type thermocouple was placed in the middle of catalyst bed for measuring the reaction temperature. The methyl nitrite (CH3ONO) reactant was synthesized by dropping H2SO4 (9.3 mol L 1 ) into an aqueous solution of CH3OH and NaNO2 (2 NaNO2 + 2 CH3OH + H2SO4 → 2 CH3ONO  + Na2SO4 + 2 H2O), then collected into a steel cylinder and subsequently mixed with pure N2 to obtain a gas mixture of CH3ONO/N2 (1/4, volume). Three calibrated mass flow controllers maintained the flow rates of pure N2, gas mixtures of CO/N2 (1/9, volume) and CH3ONO/N2. The gas mixture of CO/CH3ONO/N2 (1/6/33, volume) was fed into the microreactor, which was loaded with 0.1000 catalyst or 0.1000 g mixture of catalyst and -Al2O3 (6080 mesh). The catalyst of 0.0200 g was mixed with 1.9800 g -Al2O3 (diluent) to obtain the mixture of catalyst and -Al2O3. The reaction temperature was varied in the range of 100120 ºC and the gas hourly space velocity (GHSV) was set at 8, 000 mL g 1 h 1 or 800, 000 mL g 1 h 1 . The on-line analysis was performed using a gas chromatography (GC; Agilent 7890B) equipped with thermal conductivity detector (TCD) connected to 5A and HayeSep Q packed columns (Agilent), and flame ionization detector (FID) connected to HP-Innowax capillary column (Agilent). The calculations of the turnover frequency (TOF), CO conversion, CH3ONO conversion, DMC, dimethyl oxalate (DMO), methyl formate (MF) and dimethoxymethane (DMM) selectivity were shown in Supporting Information (SI).

Results and discussion

General properties of Pd/EMT and Pd/FAU catalysts

The size and morphology of both Pd/EMT and Pd/FAU catalysts were evaluated by SEM (Fig. 1).

The typical hexagonal and cubic morphology for the EMT and FAU crystals are seen in the SEM images. The FAU crystals have a size of 24 m while the EMT crystals have a size of 35 m and a thickness of 0.50.8 m. The high crystallinity of both samples is confirmed by XRD (Fig. 2) [31,32].

The EMT zeolite has three characteristic Bragg peaks in the range of 57 º 2Theta (Fig. 2a), which are indexed as (100), ( 002) and ( 101) reflections [31,33]. The most intense Bragg peak of (111) at 6.2 º 2Theta is present in the XRD pattern of FAU zeolite (Fig. 2b). The Pd/EMT and Pd/FAU catalysts exhibit similar XRD patterns thus indicating that the Pd loading had negligible effect on the crystallinity of both EMT and FAU zeolite supports (Fig. 2). No Bragg peaks ascribed to PdO (JCPDS 88-2434) and Pd 0 (JCPDS 87-0638) were detected for the Pd/EMT and Pd/FAU catalysts, possibly due to the high dispersion and low content. The N2 adsorption isotherms of both Pd/EMT and Pd/FAU catalysts revealed the features for type Ⅰ Langmuir isotherm, typical of microporous materials (Fig. S1). Both Pd/EMT and Pd/FAU catalysts have almost the same BET surface area of 913916 m 2 g 1 and micropore volume of 0.33 cm 3 g 1 (Table 1). The chemical compositions of the Pd/EMT and Pd/FAU catalysts were presented in Table 1. The Si/Al ratios for Pd/EMT and Pd/FAU catalysts are 4.2 and 2.2, respectively (Table 1). The difference in the chemical compositions of the two zeolites is due to the fact that the EMT zeolite was synthesized with an organic template while the FAU zeolite was prepared with sodium hydroxide; similar results have been reported previously [30,34,35]. The Pd/EMT catalyst showed lower Na content than the Pd/FAU catalyst (7.6 wt. % vs. 10.4 wt. %), as the result of higher Si/Al ratio of EMT zeolite. Both Pd/EMT and Pd/FAU catalysts contain the same amount of Pd, i.e. 0.970.99 wt. % (Table 1). a BET surface area, and the micropore volume determined by the t-plot method.

b Si/Al molar ratio and the Pd content determined by ICP-AES.

c The Na content measured by EDS elemental mapping (an average of at least 6 measurements).

d Measured according to the CO conversion and Pd atoms (details provided in SI). Reaction conditions: 0.1000 g mixture of catalyst and -Al2O3 (diluent), CO/CH3ONO/N2 = 1/6/33 (volume), GHSV = 800, 000 mL g 1 h 1 , 110 º C, 0.1 MPa. The catalyst of 0.0200 g was mixed with 1.9800 g -Al2O3, and then 0.1000 g mixture of the catalyst and -Al2O3 was used for the catalyst testing.

e Apparent activation energy calculated from the Arrhenius plots in Fig. 4 (details provided in SI).

Pd/EMT and Pd/FAU catalysts in indirect oxidative carbonylation of methanol to dimethyl carbonate

The catalytic activity of both Pd/EMT and Pd/FAU catalysts in the indirect oxidative carbonylation of methanol to DMC as a function of time on stream is presented in Fig. 3. The Pd/EMT catalyst showed stable conversion and selectivity within 12 h (Fig. 3a), while the CO conversion decreased from 89% to 21% on the Pd/FAU catalyst (Fig. 3b). Both CO and CH3ONO conversions decreased remarkably within 12 h over the Pd/FAU catalyst, indicating the rapid deactivation (Fig. 3b). The selectivity of main product DMC was increased significantly from 56% to 87% within the first 6 h on the Pd/FAU catalyst, possibly due to the decrease of byproduct selectivity of dimethoxymethane (DMM) from 33% to 6%. Beyond 6 h reaction, the selectivity of the methyl formate (MF) and dimethyl oxalate (DMO) byproducts increased gradually while the DMC selectivity decayed (Fig. 3b). The appearance of DMO and obvious increase of MF likely suggested the generation of large Pd particles, since Pd particles are considered to be the active sites for the formation of DMO and MF byproducts [36,37]. Consequently, the deactivation behavior of the Pd/FAU catalyst was initially attributed to the sintering of Pd. It should be noted that no DMO byproduct was detected on the Pd/EMT catalyst within 12 h catalytic test (Fig. 3a). Reaction conditions: 0.1000 g catalyst, CO/CH3ONO/N2 = 1/6/33 (volume), GHSV = 8, 000 mL g 1 h 1 , 110 º C, 0.1 MPa. For each point an average of 3 measurements was conducted.

The apparent activation energies and TOF values for the indirect oxidative carbonylation of methanol to DMC were calculated and shown in Table 1 and Fig. 4 (details provided in SI). The Pd/EMT and Pd/FAU catalysts showed similar apparent activation energies of 72.7  1.9 kJ mol 1 and 65.6  1.6 kJ mol 1 , respectively under the identical reaction conditions (Fig. 4). This suggested that the reaction pathway and/or the rate-determining step were equivalent on the two catalysts [37,38].

However, the Pd/EMT catalyst showed a TOF (namely, intrinsic activity) of 0.25 s 1 , larger than that of 0.11 s 1 for the Pd/FAU catalyst (Table 1). The improved intrinsic activity was possibly assigned to the different properties of EMT and FAU zeolite framework. and Pd/FAU catalysts (Ea, apparent activation energy). Reaction conditions: 0.1000 g mixture of catalyst and -Al2O3 (diluent), CO/CH3ONO/N2 = 1/6/33 (volume), GHSV = 800, 000 mL g 1 h 1 , 0.1

MPa. The kinetic data were acquired within the differential regime while the CO conversion was controlled below 13% (details provided in SI). The catalyst of 0.0200 g was mixed with 1.9800 g -Al2O3, and then 0.1000 g mixture of the catalyst and -Al2O3 was used for the catalyst testing.

Features of stable Pd/EMT and deactivated Pd/FAU catalysts

The Pd/EMT and Pd/FAU catalysts after 12 h catalytic test of the indirect oxidative carbonylation of methanol to DMC were characterized by TG-MS analysis; the results are depicted in Fig. 5. The weight loss measured for the used Pd/EMT and Pd/FAU catalysts occurred in two stages (Fig. 5). The first stage is in the range of 35170 º C and corresponds to the removal of physically adsorbed water, which is evidenced by the MS curves [39,40]. In the second stage (170280 ºC ), a single peak at 226 º C for the Pd/EMT catalyst and two peaks at 179 ºC and 220 º C for the Pd/FAU catalyst were measured.

The DTG results are consistent with the MS, which is revealing the desorption of abundant amount of water as well as carbon dioxide at the same temperatures. The second stage is assigned to the loss of water strongly bound to the zeolite framework and to the thermal decomposition of residual reaction species such as (CH3O)2CO and CH3OCO [40,41]. Based on the peak area of the CO2 formation (170280 º C), the amount of the residual reaction species on the used Pd/EMT and Pd/FAU catalysts was estimated to be 0.032 g gcat. 1 and 0.036 g gcat. 1 , respectively. No oxidation peaks of CO2 formation were found above 300 º C (see TG-DTG and MS results), which suggested that the carbon deposits were not detectable on both Pd/EMT and Pd/FAU catalysts [42,43]. These results confirmed that the deactivation behavior of Pd/FAU catalyst cannot be attributed to the residual reaction species or carbon deposits. The DRIFTS spectra for both Pd/EMT and Pd/FAU catalysts using pyridine are presented in Fig. 6. The bands at 1442 cm 1 (pyridine coordinated to Na + ions), 1457 cm 1 (pyridine interacting with coordinately unsaturated Al 3+ ions), 1574 cm 1 (weak Lewis acidity), 1593 cm 1 (hydrogen-bound pyridine) and 1611 cm 1 (hydrogen-bound pyridine) are assigned to Lewis-bound pyridine; the bands at 1542 cm -1 and 1637 cm 1 are attributed to Brønsted-bound pyridine, and the band at 1487 cm 1 is assigned to pyridine associated with both Lewis and Brønsted acid sites [4447]. Brønsted acid sites on both Pd/EMT and Pd/FAU catalysts possibly resulted from the ion exchange between Na + and NH4 + (dissociation of NH3H2O) during the preparation of catalysts, since the Na-form of EMT and FAU zeolites should be in the absence of Brønsted acidity [45,48]. Brønsted acid sites could lead to the formation of methyl formate byproduct during the indirect oxidative carbonylation of methanol to DMC [18,49]. Using the band areas at 1542 cm 1 and 1442 cm 1 , the amount of Brønsted and Lewis acid sites was estimated, respectively (Fig. 6 and Table S1). Brønsted acid sites of 5.1 area mg 1 for the Pd/FAU catalyst were lower than 8.5 area mg 1 for the Pd/EMT catalyst after the catalytic test (Table S1). This is consistent with the catalytic results showing lower selectivity towards methyl formate (MF) byproduct with the Pd/FAU catalyst than with the Pd/EMT catalyst (Fig. 3). However, the Pd/EMT catalyst showed higher Lewis acid sites than the Pd/FAU catalyst after the catalytic test (47.4 area mg 1 vs. 38.1 area mg 1 , Table S1). This possibly contributed to the stable activity of the Pd/EMT catalyst (Fig. 3), because Lewis acid sites could accept electrons from the Pd species and thus promote the formation of the Pd 2+ active sites as previously stated [18]. TEM study of the Pd/EMT and Pd/FAU catalysts was performed to determine the size and distribution of palladium. As shown in Fig. 7ad, both fresh and used Pd/EMT catalysts showed the characteristic hexagonal platelike morphology and the crystalline fringes associated with EMT zeolite [50]. As described previously, the Pd/EMT and Pd/FAU catalysts were prepared by ion exchange (Pd precursor: Pd(NH3)4 2+ ), and the ion-exchanged samples were calcined at 200 º C. The TG-MS results suggested that the low-temperature calcination of the catalysts (200 º C) caused the Pd(NH3)4 2+ to lose partially the NH3 ligand and to generate the Pd(NH3)x 2+ (x < 2) sites that are anchored on the zeolite framework (Fig. S2) [51,52]. Neither Pd clusters nor nanoparticles were visible in the TEM images of fresh Pd/EMT and Pd/FAU catalysts (Fig. 7b,f). In contrast, mono-dispersed Pd clusters ( 1 nm) confined within the used Pd/EMT catalyst after 12 h catalytic test could be observed (Fig. 7d and Fig. S3). In comparison to the fresh Pd/FAU catalyst, the used catalyst contained large Pd nanoparticles located predominantly on the external surface of the FAU crystals (Fig. 7e,g). The presence of Pd nanoparticles was also identified by SEM and further confirmed by the EDS (Fig. S4). The aggregated Pd formed large nanoparticles with random distribution, i.e. 2.4 or 10.6 wt. % on the surface of the used Pd/FAU catalyst (Fig. 8), whereas the uniform Pd distribution on the used Pd/EMT catalyst (1.9 or 2.1 wt. %) is confirmed by EDS elemental mapping (Fig. 8). Furthermore, TEM revealed the Pd sintering with different particle sizes ranging from 3 nm to 25 nm deposited on the external surface and the edges of used Pd/FAU catalyst (Fig. 7h). The average size of the Pd nanoparticles on the Pd/FAU catalyst was estimated to be 8.7 nm based on measuring 200 nanoparticles (inset of Fig. 7h).

This finding is consistent with the catalytic results showing a remarkable increase of the DMO and MF byproducts beyond 6 h catalytic test (Fig. 3b). The large Pd nanoparticles have been shown to be the active sites for the formation of DMO and MF byproducts earlier [36,37]. In the case of Pd/EMT catalyst after 3 h and 12 h catalytic test, no Bragg peaks corresponding to PdO (JCPDS 88-2434) and Pd 0 (JCPDS 87-0638) were observed (Fig. 2a). The Bragg peak of Pd( 111)

is overlapped with that of FAU zeolite in the range of 39.840.8 º 2Theta. After the deconvolution, the peak of Pd(111) at 40.2 º 2Theta was clearly observed on the Pd/FAU catalyst after 12 h catalytic test (inset of Fig. 2b) indicating the Pd sintering. The size of Pd nanoparticles was 25 nm according to the calculation using the Scherrer-Debye equation; this size is larger than that of determined by TEM 8.7 nm (inset of Fig. 7h). Both TEM and XRD results demonstrated that palladium sintering occurred on the used Pd/FAU catalyst after 12 h catalytic test while no changes were observed on the used Pd/EMT catalyst.

As previously stated, the Pd 2+ species is the active sites for the formation of DMC [10,15].

Therefore, the state of Pd in the used Pd/EMT and Pd/FAU catalysts was measured by XPS. As shown in Fig. 9a, the peaks at 336.3 eV and 341.6 eV were attributed to the Pd 0 species while the peaks at 338.3 eV and 343.5 eV indicated the presence of Pd 2+ species [53,54]. These binding energies were 0.81.3 eV higher than the counterparts of conventional Pd 0 and Pd 2+ species due to Lewis acid sites of the Pd/EMT catalyst [18,37,55]. Lewis acid sites are considered to be electron acceptors and thus made the Pd species lose the electrons [18]. The Pd 2+ species may result from Pd clusters/nanoparticles charged by the zeolite framework through the formation of Pd-O ionic bonds [10,19]. Moreover, Pd 2+ species also could be generated during the synthesis of DMC, since the reaction follows a mechanistic pathway of oxidative addition and reductive elimination, and the Pd active sites undergo the Pd 0  Pd 2+  Pd 0 pathway [10,16]. As shown in Fig. 9b, the Pd/FAU catalyst exhibited four peaks corresponding to Pd 0 and Pd 2+ species, being lower by 0.50.7 eV in comparison to the Pd/EMT catalyst. This was attributed to the different properties of EMT and FAU zeolite framework. As shown by XPS, the peak area ratio of Pd 2+ /(Pd 2+ +Pd 0 ) for the Pd/FAU catalyst decreased remarkably from 79% to 36% with increasing the reaction time from 3 h to 12 h (Table 2). In contrast, the peak area ratio of Pd 2+ /(Pd 2+ +Pd 0 ) for the Pd/EMT catalyst declined from 69% to 54% (Table 2).

In order to oxidize Pd 0 into Pd 2+ species in the used Pd/FAU catalyst after 12 h catalytic test, a subsequent calcination was carried out at 200 º C for 2 h in air. After this calcination, the CO conversion was increased substantially from 21% to 76% (Fig. S5). However, with prolonged reaction time, the Pd 2+ species was gradually reduced by CO reactant, and the Pd/FAU catalyst showed similar deactivation behavior to the fresh catalyst (Fig. 3b and Fig. S5). The combined TEM and XPS results

indicate that the palladium sintering and reduction of Pd 2+ to Pd 0 cause the deactivation of Pd/FAU catalyst.

Stabilization of Pd in EMT and FAU zeolites

Carbon monoxide is a probe molecule widely employed to elucidate the nature of electron donor Pd species. The DRIFTS spectra of CO adsorbed on the reduced Pd/EMT and Pd/FAU catalysts in the range of 35200 º C are presented in Fig. 10. For the Pd/EMT catalyst after 3 h catalytic test (Fig. 10a), the band at 2135 cm 1 is attributed to linear CO interacting with Pd 2+ species; the bands at 2094 cm 1 and 2052 cm 1 are assigned to linear CO adsorbed on Pd 0 species; and the bands at 1977 cm 1 and 1906 cm 1 are associated with bridged CO adsorbed on Pd 0 species [41,56]. Both bands corresponding to linear and bridged CO were progressively decreased with increasing the temperature from 35 º C to 100 º C, and disappeared at 150 º C (Fig. 10a). The DRIFTS spectra of Pd/EMT catalyst after 3 h and 12 h catalytic test are almost identical in the whole temperature range of 35200 º C (Fig. 10a,b)

indicating the stability of the Pd species on the Pd/EMT catalyst. This result is in accordance with the stable catalytic performance of the Pd/EMT catalyst (Fig. 3a). As shown in Fig. 10b 10). This suggested the  back-donation was weak and thus the Pd-CO bond was weakened on the Pd/EMT catalyst [59,60]. This infers that the interactions between Pd and the EMT zeolite framework were stronger than within the FAU zeolite framework. As reported previously [24,25], the difference in the topology of EMT and FAU zeolites leads to the different locations of cations: Ⅰ, Ⅰ' and Ⅱ in the FAU vs. Ⅰa, Ⅰb, Ⅰ'a, Ⅰ'b, Ⅱa and Ⅱb in the EMT (Fig. S6). These different cation locations have a significant influence on the interactions between metal clusters and zeolites [25,61,62]. Therefore, the stabilization of mono-dispersed Pd clusters within the EMT zeolite was attributed to the enhanced interactions between Pd and EMT zeolite, as the result of the suitable negative charge distribution, abundant Lewis acid sites (Table S1) and unique cage To further understand the interactions between Pd and zeolite framework, H2-temperature programmed reduction (H2-TPR) experiments were conducted on both Pd/EMT and Pd/FAU catalysts.

The H2-TPR profiles of the two catalysts contain low-and high-temperature reduction peaks, which were assigned to the weak-and strong-interaction Pd species with zeolite framework, respectively (Fig. 11) [51,65,66]. Similar low-and high-temperature reduction peaks were also measured by mass spectrometer for a new Pd/EMT catalyst loaded with 5 wt. % Pd (Fig. S7). A negative peak (H2 desorption) below 100 º C, usually assigned to the decomposition of Pd -hydride, was not detected for Pd/EMT or Pd/FAU catalysts (Fig. 11). This result suggests that Pd was highly dispersed on the both fresh Pd/EMT and Pd/FAU catalysts [67,68]. About 82% of Pd 2+ in the EMT zeolite (Pd/EMT catalyst) was difficult to be reduced due to the enhanced metal-support interactions, while only 37% of Pd 2+ in the FAU zeolite was difficult (Pd/FAU catalyst) (Fig. 11). This indicates that the interactions between Pd and zeolite framework in the Pd/EMT catalyst were much stronger than that in the Pd/FAU catalyst.

The enhanced interactions between Pd and EMT zeolite framework stabilized the Pd/EMT catalyst as

shown by DRIFTS and H2-TPR results.

Stability of Pd/EMT catalyst

As previously stated, the stability of the catalysts for the indirect oxidative carbonylation of methanol to DMC is always a big challenge, especially when they are used in real reaction conditions [11,19]. A long-term catalytic test over the Pd/EMT catalyst was performed and the results are shown in Fig. S9). Moreover, in comparison with the catalysts reported in the literature, the Pd/EMT catalyst provided good activity and better stability (Table S2).

Fig. 1 .

 1 Fig. 1. SEM images of the (a) Pd/EMT and (b) Pd/FAU catalysts.

Fig. 2 .

 2 Fig. 2. (a) XRD patterns of EMT zeolite support, and Pd/EMT catalyst before and after catalytic tests

Fig. 3 .

 3 Fig. 3. Conversion of CO and methyl nitrite (CH3ONO), and selectivity of dimethyl carbonate (DMC),

Fig. 4 .

 4 Fig. 4. Arrhenius plot for the indirect oxidative carbonylation of methanol to DMC over the Pd/EMT

Fig. 5 .

 5 Fig. 5. TG-DTG (left) and MS (right) curves of the (a) Pd/EMT and (b) Pd/FAU catalysts after 12 h

Fig. 6 .

 6 Fig. 6. DRIFTS spectra of pyridine adsorbed on (a) Pd/EMT and (b) Pd/FAU catalysts before and after

Fig. 7 .

 7 Fig. 7. TEM images of (a,b) fresh and (c,d) used Pd/EMT catalysts after 12 h catalytic test (b,d, inset:

Fig. 8 .

 8 Fig. 8. SEM images and EDS elemental mapping of used (a) Pd/EMT and (b) Pd/FAU catalysts after

Fig. 9 .

 9 Fig. 9. XPS spectra of the used (a) Pd/EMT and (b) Pd/FAU catalysts after 3 h and 12 h catalytic test.

Fig. 10 .

 10 Fig. 10. DRIFTS spectra of CO adsorbed on (a) Pd/EMT and (b) Pd/FAU catalysts after 3 h and 12 h

Fig. 11 .

 11 Fig. 11. H2-TPR profiles of fresh (a) Pd/EMT and (b) Pd/FAU catalysts; the peak area ratios of low-

12 .

 12 The conversion and selectivity of the Pd/EMT catalyst remain constant for at least 130 h test, i.e., CO conversion of 82  1%, CH3ONO conversion of 36  2%, and DMC selectivity of 79  2%. No DMO byproduct was detected over the Pd/EMT catalyst during the 130 h test. None of Bragg peaks corresponding to Pd 0 (JCPDS 87-0638) and PdO (JCPDS 88-2434) were observed for the used Pd/EMT catalyst after 130 h reaction (Fig.S8), indicating the excellent resistance to Pd sintering. The TEM study showed the uniform Pd clusters (about 1 nm) confined within the used Pd/EMT catalyst after 130 h reaction; the size and distrubution of the Pd clusters were the same as in the catalyst after 12 h catalytic test (Fig.7dand Fig.

Fig. 12 .

 12 Fig. 12. Durability performance of the Pd/EMT catalyst in the indirect oxidative carbonylation of

Table 1

 1 Physicochemical properties of the Pd/EMT and Pd/FAU catalysts and their catalytic performance in the indirect oxidative carbonylation of methanol to DMC.

	Catalyst	SBET	Vmicro	Si/Al	Pd	Na	TOF	Ea
		(m 2 g 1 ) a	(cm 3 g 1 ) a	ratio b	(wt. %) b	(wt. %) c	(s 1 ) d	(kJ mol 1 ) e
	Pd/EMT	913	0.33	4.2	0.99	7.6	0.25	72.7  1.9
	Pd/FAU	916	0.33	2.2	0.97	10.4	0.11	65.6  1.6

Table 2

 2 Peak areas of Pd 2+ and Pd 0 , and the Pd 2+ /(Pd 2+ +Pd 0 ) area ratio determined by XPS. a Peak areas of Pd 2+ at 338.3 eV and Pd 0 at 336.3 eV based on XPS spectra of Pd/EMT catalysts. Peak areas of Pd 2+ at 337.8 eV and Pd 0 at 335.7 eV based on XPS spectra of Pd/FAU catalysts.

	Catalyst	Reaction time (h) Area of Pd 2+	Area of Pd 0 Area ratio of Pd 2+ /(Pd 2+ +Pd 0 )
	Pd/EMT	3	11067 b	4920 b	69%
		12	10518 b	8800 b	54%
	Pd/FAU	3	11077 c	2986 c	79%
		12	5974 c	10477 c	36%
	a XPS spectra of used Pd/EMT and Pd/FAU catalysts after 3 h and 12 h catalytic test (Fig. 9).

b c

  , linear CO (2135 cm 1 , 2086 cm 1 , 2029 cm 1 and 2034 cm 1 ) and bridged CO (1894 cm 1 and 1854 cm 1 ) were identified on the Pd/FAU catalysts after 3 h and 12 h catalytic test. Compared to the Pd/EMT catalysts,

the bands of bridged CO on the Pd/FAU catalysts were clearly increased (Fig.

10

), which is possibly due to the sintering of Pd

[57,58]

. The Pd/EMT catalysts exhibited a clear blue shift of both linear and bridged CO bands compared to the Pd/FAU catalysts (2086  2094, 2029 and 2034  2052, 1894  1977, 1854  1906 cm 1 ; see Fig.

Conclusions

The EMT zeolite was found to be a promising support for the stabilization of palladium clusters and preparation of Pd/EMT catalyst for the indirect oxidative carbonylation of methanol to dimethyl carbonate (DMC). The Pd/EMT catalyst exhibited higher intrinsic activity (i.e., turnover frequency) than conventional Pd/FAU catalyst. The conventional Pd/FAU catalyst was deactivated within 12 h due to the palladium sintering and decrease of Pd 2+ active sites. In contrast, the Pd/EMT catalyst (1.0 wt. % Pd) showed high CO conversion of 82% and DMC selectivity of 79%, and good stability during the continuous test of 130 h. It was found that the catalyst stability is strongly dependent on the interactions between the Pd and zeolite framework. The enhanced interactions between Pd and EMT zeolite inhibited the sintering of palladium clusters and maintained the Pd 2+ active sites in the catalyst. This work presents a newly designed Pd/EMT catalyst with great performance in the indirect oxidative carbonylation of methanol to DMC by understanding the benefit of the zeolite framework (EMT-type zeolite).

Acknowledgements

This work was supported by the Natural Science Foundation of China (21908246, 21975285, U1862118), China Postdoctoral Science Foundation (2017M622311), Fundamental Research Funds for the Central Universities (18CX02148A) and the Sino-French International Laboratory (LIA) "Zeolites".

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Yu Gu

Haijun Liu

Miaomiao Yang

Zhipeng Ma

Lianming Zhao

Wei Xing

Pingping Wu

Xinmei Liu

Svetlana Mintova

Peng Bai
email: baipeng@upc.edu.cn

Zifeng Yan

Highly stable phosphine modified VO x /Al 2 O 3 catalyst in propane dehydrogenation

Keywords: Phosphine modification, stability enhancement, propane dehydrogenation, coking, propylene desorption

The relatively low cost and mild toxicity of vanadium makes it an alternative to commercially applied Pt and Cr based catalysts in propane dehydrogenation (PDH). Conventional VOx/Al2O3 catalyst shows high initial activity, but deactivates fast due to severe coke formation, which requires frequent regeneration by coke combustion and causes massive CO2 emissions. In this context, we report on the phosphine (PH3) surface modification of VOx/Al2O3 catalyst leading to substantial improvement of its stability. The phosphorus considerably modifies the properties of the VOx/Al2O3 catalyst: (i) tunes surface acidity, (ii) separates polymerized vanadium oxide species and (iii) weakens interaction between vanadium species with alumina support. Synergy among all these factors facilitate desorption of produced propylene and inhibit the oligomerization of coke precursors, which contributed to the enhancement of the catalyst stability during propane dehydrogenation.

Introduction

Propylene is a fundamental chemical raw material that can be used to produce a variety of downstream derivatives such as polypropylene, acrylonitrile, isopropanol, acetone and epoxypropane [START_REF] Sattler | Catalytic dehydrogenation of light alkanes on metals and metal oxides[END_REF][START_REF] Yusuf | Manganese silicate based redox catalysts for greener ethylene production via chemical looping-oxidative dehydrogenation of ethane[END_REF]. Currently, propylene is mainly produced by steam cracking and fluid catalytic cracking processes [START_REF] Yusuf | Manganese silicate based redox catalysts for greener ethylene production via chemical looping-oxidative dehydrogenation of ethane[END_REF][START_REF] Han | The effect of ZrO2 crystallinity in CrZrOx/SiO2 on non-oxidative propane dehydrogenation[END_REF]. However, the foregoing two processes are high energy-consuming and could not meet the globally increasing demand for propylene due to crude oil dependence, thus are not economically and environmentally friendly. Propane dehydrogenation, as an alternative process which could produce highly value-added propylene using cheap propane from clean energy source such as shale gas and natural gas as a raw material, has attracted increasing attentions in past few decades [START_REF] James | Lower alkanes dehydrogenation: Strategies and reaction routes to corresponding alkenes[END_REF]. The development of propane dehydrogenation could be one of the most efficient and clean solutions for propylene supply.

Two different processes of propane dehydrogenation have been studied in recent years, which are oxidative dehydrogenation (ODH) of propane and non-oxidative dehydrogenation of propane (PDH). ODH is a fascinating technology with high conversion and energy saving due to the oxidation of hydrogen by oxidants such as oxygen, carbon dioxide and nitric oxide [START_REF] Cao | Graphitic carbon nitride catalyzes selective oxidative dehydrogenation of propane[END_REF][START_REF] Liu | CeO2 morphology-dependent NbOx-CeO2 interaction, structure and catalytic performance of NbOx/CeO2 catalysts in oxidative dehydrogenation of propane[END_REF][START_REF] Atanga | Oxidative dehydrogenation of propane to propylene with carbon dioxide[END_REF]. However, the obstacle of this technology, the low selectivity towards propylene caused by over-oxidation, still could not be overcome so far [START_REF] Atanga | Oxidative dehydrogenation of propane to propylene with carbon dioxide[END_REF]. PDH has been commercialized as Oleflex (Honeywell UOP) and Catofin (ABB Lummus) processes using Pt and Cr based catalysts respectively [START_REF] Xiong | Structure-Performance Relationships for Propane Dehydrogenation over Aluminum Supported Vanadium Oxide[END_REF][START_REF] Searles | Silica-supported isolated gallium sites as highly active, selective and stable propane dehydrogenation catalysts[END_REF]. Nevertheless, the high cost of platinum, toxicity of chromia and fast deactivation of both of them have posed economic and environmental issues [START_REF] Dai | γ-Al2O3 sheet-stabilized isolate Co 2+ for catalytic propane dehydrogenation[END_REF][START_REF] Searles | Highly Productive Propane Dehydrogenation Catalyst Using Silica-Supported Ga-Pt Nanoparticles Generated from Single-Sites[END_REF].

As an alternative, vanadium-based catalysts have been widely investigated since 1980s due to its high activity in oxidative dehydrogenation of lower alkanes [START_REF] Carrero | Critical Literature Review of the Kinetics for the Oxidative Dehydrogenation of Propane over Well-Defined Supported Vanadium Oxide Catalysts[END_REF]. Recently, supported VOx catalysts were found to be active in non-oxidative dehydrogenation of lower alkanes and successfully regenerated for several cycles [START_REF] Rodemerck | Influence of the kind of VOx structures in VOx/MCM-41 on activity, selectivity and stability in dehydrogenation of propane and isobutane[END_REF][START_REF] Hu | Vanadium-doped porous silica materials with high catalytic activity and stability for propane dehydrogenation reaction[END_REF]. But, in a single dehydrogenation run, the catalyst deactivates fast due to coke deposition and requires frequent regeneration by combusting coke, which causes massive CO2 emission. Modification of catalysts with non-metal elements, such as sulfur, is one of the solutions applied. Li et al. introduced sulfur into supported metal oxide catalysts, and the modified catalysts exhibited enhanced stability and selectivity toward olefins by deactivating the most active hydrogenolysis sites [START_REF] Sun | Studies on the promoting effect of sulfate species in catalytic dehydrogenation of propane over Fe2O3/Al2O3 catalysts[END_REF][START_REF] Wang | Promoting mechanism of sulfur addition in catalytic dehydrogenation of isobutane over Mo/MgAl2O4 catalysts[END_REF]. However, a gradual sulfur loss occurred during the reaction and the continuous co-feeding of H2S was needed to maintain a stable catalytic performance. Phosphorus has been used to modify industrial hydrotreating catalysts previously due to the strong interaction between phosphate and alumina. Recently, Tan et al. also reported the enhanced performance of Fe/P/Al catalyst in PDH by phosphorous doping, and phosphorus was found to induce the formation of active Fe3C sites and significantly improved C3H6 selectivity [START_REF] Tan | Propane Dehydrogenation over Alumina-Supported Iron/Phosphorus Catalysts: Structural Evolution of Iron Species Leading to High Activity and Propylene Selectivity[END_REF].

Nevertheless, as phosphorus was commonly introduced as phosphoric acid or phosphate by wetness impregnation, aluminum phosphate forms with a significant reduction in surface area and blocking of pore openings [START_REF] Sun | The effects of fluorine, phosphate and chelating agents on hydrotreating catalysts and catalysis[END_REF][START_REF] Pan | Pmodified microporous carbon nanospheres for direct propane dehydrogenation reactions[END_REF][START_REF] Horáček | Aluminasupported MoNx, MoCx and MoPx catalysts for the hydrotreatment of rapeseed oil[END_REF]. To circumvent these Liu et al. developed an elegant approach towards preparation of Ni2P/Al2O3 hydrodesulfurization catalyst using gaseous PH3 as the phosphiding agent, which eliminated the formation of aluminum phosphate [START_REF] Liu | Bulk and Al2O3-supported Ni2P HDS catalysts prepared by separating the nickel and hypophosphite sources[END_REF].

In this context, we modified VOx/Al2O3 catalysts in propane dehydrogenation with PH3 surface treatment, which leads to great improvement of catalyst stability by inhibiting coking deposition during propane dehydrogenation. Experimental results revealed that the synergy among tuned acidity, lower coking precursor intimacy and faster propylene desorption after phosphorus modification accounted for the enhancement in catalytic stability during propane dehydrogenation.

Experimental

Catalyst preparation

VOx/Al2O3 was synthesized via incipient wetness impregnation. In a typical procedure, 6.944 g of ammonium metavanadate (Alfa Aesar, USA) was dissolved together with 19.128 g oxalic acid dehydrate (Sinopharm, China) in 60 mL deionized water at 343 K. After stirring, the solution turned into dark blue. Then 40 g pseudo boehmite (Henghui Chemical, China) was added into the dark blue solution resulted in a muddy mixture. Then the mixture was fully stirred for 10 minutes and dried at 343 K for 4 h in air, followed by additional desiccation at 373K overnight to obtain a dried precursor. Calcination of the precursor was conducted in a muffle furnace (Nabertherm, Germany) at 823 K for 10 h with a ramp rate of 2 K/min. After cooling to room temperature, the VOx/Al2O3 catalyst with 10 wt% vanadium loading was prepared (PV-0).

Phosphine modification of the VOx/Al2O3 catalyst was carried out in a chemical vapor deposition system (Nabertherm, Germany). Sodium hypophosphite monohydrate (Sionpharm, China) was put upstream in a crucible placed in the quartz tube, and the VOx/Al2O3 was placed in the downstream.

Nitrogen was used as a carrier gas and flowed into the closed system continuously. Temperature was elevated to 393 K and hold for 45 minutes to remove the moisture. Then the tube was further heated to 573 K with a ramp rate of 2 K/min and hold for 1 h. During this period, sodium hypophosphite decomposed and released PH3 gas according to the eq. ( 1). 2NaH2PO2 → Na2HPO4 + PH3↑ [START_REF] Sattler | Catalytic dehydrogenation of light alkanes on metals and metal oxides[END_REF] After that, the heating was stopped and N2 was introduced to purge the container overnight. The exhaust gas was bubbled into a flask containing excessive amount of copper sulfate (Sinopharm, China) solution, which was used as an indicator of the phosphine formation. The exhaust phosphine reacted with copper sulfate and resulted in the formation of dark precipitates. After phosphine modification, resulting catalysts were denoted as PV-x, where x represents the molar ratio of phosphorus to vanadium (P/V) in the final catalysts.

Catalyst characterizations

Elemental composition of samples after phosphine modification was measured on a VISTA-MPX inductively coupled plasma optical emission spectrometer (ICP-OES, Varian, USA) equipped with a CCD detector.

Coking behavior of catalysts was analyzed by thermogravimetric-differential thermal analysis (TG-DTA) measured on a HCT-1 thermal differential analyzer (Beijing Hengjiu, China) under air flow. Below 320 K, the temperature ramp was set as low as 0.33 K/min to remove most of the moisture. Then the temperature was elevated to 930 K with a ramp rate of 10 K/min. The duration time for the two periods is 30 min and 60 min, respectively.

Energy dispersive X-ray (EDX) elemental mapping was recorded on a JSM-7500F scanning electron microscope (JEOL, Japan) equipped with an X-MAX N EDS detector (Oxford Instruments, UK). The electron accelerating voltage was set as 30 kV.

Surface acidic properties of catalysts were determined by both pyridine adsorption Fourier transform diffusing reflectance infrared spectroscopy (Py-DRIFTS) and NH3 temperature programmed desorption (NH3-TPD). Prior to Py-DRIFTS measurements, all samples with different P/V ratios were pretreated at 573 K for 3 h to remove moisture. Then they were placed in a desiccator with an open beaker filled with pyridine for 24 h to obtain adsorption saturation balance. After that all samples were heated at 423 K under vacuum for 3 h to remove physically absorbed pyridine. Additionally, the corresponding samples without pyridine adsorption were dehydrated at 573 K for 3 h as blank references. Ammonia temperature programmed desorption (NH3-TPD) was conducted on an AutoChem II 2920 chemisorption analyzer (Micromeritics, USA) equipped with a thermal conductivity detector (TCD). 0.1-0.2 g of catalyst (20-40 mesh) was packed in the middle of a U-shaped quartz tube. Prior to analysis, the samples were outgassed in an Ar flow (80 mL/min) at 573 K for 2 h. After cooling down to 353 K, the samples were saturated with NH3 under a flow of 20 vol% (Balanced with Helium, total flow:25 mL/min) for 30-40 min.

Then N2 gas was flowed over catalyst (50 mL/min) for 30 min to remove physically adsorbed NH3.

Temperature was increased from 313 K to 850 K under N2 flow to obtain the TPD profile.

X-ray powder diffraction (XRD) patterns were measured on an X'Pert Pro MPD diffractometer (PANalytical, Netherland) using a nickel filtered Cu Kα radiation (λ= 0.1542 nm) at 35 kV and 40 mA in the 2θ range of 5° to 75°.

Textual properties of catalysts were determined by N2 adsorption-desorption isotherms obtained on a TriStar 3000 (Micromeritics, USA). Prior to testing, samples were degassed in vacuum at 573 K for 4 h. The specific surface areas were calculated at the relative pressure P/P0 of 0.05-0.25 using Brunauer-Emmett-Teller (BET) equation. Pore diameters were derived from Barrett-Joyner-Halenda (BJH) equation using the adsorption branch of isotherms.

Ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) were acquired on a SPECORD 210 plus (Analytikjena, Germany) spectrometer in the range of 200-800 nm; BaSO4 (Aladdin, USA) was mixed with samples with a mass ratio of 4:1.

Raman spectra of powder samples were obtained on a DXR Raman spectrophotometer (Thermo Fisher Scientific, USA). Spectra were recorded at ambient temperature with an excitation source of 532 nm.

Fourier transform transmission infrared (FTIR) spectra were recorded on a Nicolet 6700 infrared spectrometer (Thermo Fisher Scientific, USA) equipped with a Mercury Cadmium Telluride (MCT) detector and a KBr beam splitter. Self-supporting wafers of samples were prepared with KBr.

X-ray Photoelectron Spectroscopy (XPS) data were acquired on an ESCALAB 250Xi instrument (Thermo Fisher Scientific, USA) with an 18.2 W laser monochromatic Al Kα radiation.

Total acquisition time of 5.25 min, pass energy of 30.0 eV and energy step size of 0.10 eV were applied. The reported binding energies were referenced to C 1S at 284.8 eV. Surface atomic ratio and vanadium species valences were determined by fitting peak XPSPEAK 41 program.

Evaluation of catalysts in propane dehydrogenation

The catalysts were tested in a continuous-flow fixed-bed micro-reactor (Tianda Beiyang Chemical Equipment Co., Ltd. China) at ambient pressure. Catalysts with different P/V ratios were pelleted, crushed and sieved into 20-60 mesh. 1.0 g catalyst was packed in the middle of a stainless steel tube (length of 500 mm, diameter of 10 mm and thickness of 3 mm), with a thermocouple inside and loaded in the middle of the catalyst bed. Thereafter, the reactor was heated to 863 K within 2 h in N2 with a flow rate of 30 mL/min. Then the gas flow was switched to H2 with a flow rate of 30 mL/min to conduct reduction for 2 h. Before evaluation, the reaction temperature was further increased to 883 K. Then the feed gas with a total flow rate of 30 mL/min containing propane and nitrogen with a ratio of 1:4 was pumped into the reactor. Products were analyzed by an online Agilent 7820A GC equipped with a flame ionization detector (FID) and a HP-PONA capillary column.

Conversion of propane, propylene yield and propylene selectivity were calculated based on the carbon atom balance according to the equations listed below:

X= N Propane Inlet -N Propane Outlet N Propane Inlet Y= N Propylene Outlet N Propane Inlet S=Y/X,
X, Y, S, N represent conversion of propane, yield of propylene, selectivity to propylene and molar amounts of gases, respectively.

Results and discussion

Catalytic performance

Fig. 1 shows the performance of different samples in PDH reaction. The parent catalyst (PV-0)

undergoes an induction period of 1.5 h, during which the conversion (initial reactivity) increases gradually to its maximum of around 73%, then decreases dramatically to only 25% at time on stream (TOS) of 8 hours, which is the lowest conversion among all tested samples. The rapid deactivation of the parent catalyst indicates a poor stability, though it has the highest initial reactivity. In contrast, after phosphine modification, the initial reactivity of the catalysts is reduced to different extents, while conversion curves become more and more flat with the increase of phosphorus amount. All phosphine modified samples have similar induction period. The difference between the samples is observed in the following 6.5 hours after induction period (1.5 h). Samples PV-0.59, PV-0.62 and PV-0.70 still retain a conversion of approximately 45% at TOS of 8 h, almost twice higher than the parent catalyst. Sample PV-0.86 exhibits the lowest initial reactivity, but with the highest stability. The propane conversion remains almost constant (about 28%). After phosphine modification, vanadium based catalyst lost part of its reactivity, while the stability was pronouncedly enhanced.

For the parent catalyst, the propylene selectivity increases and the maximum at TOS of 3.5 h is observed, and then decreased quickly due to catalysts deactivation. For the modified catalysts, selectivity is more steady than that of the parent catalyst. With increasing P/V ratio, the average propylene selectivity drops first and then rises above that measured for the parent catalyst. For phosphine modified catalysts (PV-0.59, 0.62 and 0.86), the selectivity shows interesting rising trend as a function of time. Among all samples, PV-0.86 exhibits the highest C3H6 selectivity at TOS of 8 h.

By-products are analyzed and their yield and selectivity are presented in Fig. S1. The byproducts are mainly C1-C2 hydrocarbons, and products with more than 3 carbon atoms are excluded due to their rather small quantity (<1%). The C1+C2 products show similar trend as that of propylene, which is caused by variations of propane conversion. With respect to C1+C2 selectivity, parent catalyst demonstrates a valley-shaped curve which is opposite to its propylene selectivity. This indicates that in the early stage of reaction, the parent catalyst tends to catalyze dehydrogenation after the induction period. After the first 4 h, deactivation happens and cracking dominates, which produces a significant amount of methane and C2. This phenomenon is consistent with the decrease in propylene selectivity of parent catalyst. In contrast, phosphine modified catalysts have better performance in resisting cracking. With increasing phosphorus amount, the C1+C2 selectivity decays. Analyses of by-products again confirm that after PH3treatment, the catalysts stability is greatly improved.

In summary, the stability of PH3-modified catalysts was improved at the expense of partial loss of initial reactivity. Nevertheless, with increasing the P/V ratio, the loss of initial reactivity reduced.

At TOS of 8 h, parent catalyst lost 66% of its initial reactivity, while the PV-0.86 catalyst keeps almost 91% of the initial reactivity. Furthermore, reactivity of catalysts in terms of conversion remaining percentage (Defined as conversion at TOS=8h divided by maximum conversion) has a well-fitted linear relation as a function of the P/V ratio (Fig. 2).

Coking of catalysts

Since coking has been well recognized as the main reason for catalyst deactivation [START_REF] Sattler | Catalytic dehydrogenation of light alkanes on metals and metal oxides[END_REF], the coke amount is closely related to catalyst stability. TG-DTA curves of used catalysts after 8 h of reactions are present in Fig. 3, and the coke amount is summarized in Table S1. Below 320 K, the temperature ramp was set as low as 0.33 K/min to remove the moisture from the used catalysts, which is demonstrated by steep weight loss at around 300 K. For all samples, weight loss bellow 400 K is ascribed to the removal of physically absorbed water; a small endothermal peaks in DTA curves correspond to dehydration of catalysts. The sharp decline in weight starting at 700 K is due to combustion of surface carbon species, which is also confirmed by the exothermal peak in DTA curves (Fig. 3). As shown, the amount of coke deposition decreases on the phosphine modified catalysts with increasing P/V ratio. Moreover, negligible weight loss and heat release of sample PV-0.86 suggest that almost no coke remains on the catalyst surface. This is also confirmed by the grey color of this catalyst after 8 h of reaction, while other catalysts display a black color.

Combining the results of carbon deposition with the performance of the catalysts in dehydrogenation reaction, it could be concluded that the carbon deposition may be the dominant factor for catalysts deactivation. The introduction of phosphorus could efficiently suppress coke formation during propane dehydrogenation process.

In addition, EDX mapping was performed to determine the distribution of surface elements.

Compared to parent catalyst, the amount of carbon deposits in phosphine modified catalysts is relatively lower, especially in the used catalyst PV-0.86 (Fig. S2c). The profiles of phosphorus highly coincide with those of aluminum and vanadium, which indicates strong intimacy between phosphorus, vanadium species and alumina support.

Acidic properties of phosphine modified catalysts

Surface acidic property is a key factor influencing the catalyst performance in PDH reaction and it is highly related to their coking behavior. To investigate variations in surface acid sites, NH3-TPD, transmission IR and Py-DRIFTS measurements were conducted.

NH3-TPD measurements were performed to evaluate total acid amounts of parent and phosphine modified catalysts. In general, as the amount of phosphorus increases, concentration of total acid sites decreases (Fig. S3). It has been widely accepted that VOx/Al2O3 is a bifunctional catalyst while acid sites facilitate adsorption of propane and vanadium species induce formation of active intermediate [START_REF] Sattler | Catalytic dehydrogenation of light alkanes on metals and metal oxides[END_REF][START_REF] Liu | Nature of the Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation[END_REF]. Decreasing acid amount weakens the adsorption of reactants on catalysts but favors the desorption of propylene, which inhibits deeper side reactions and lengthens catalyst life span, however at the expense of lowering their activity to some extent.

Py-DRIFTS characterization was conducted to understand the nature of acid sites of the catalysts.

A peak centered at 1540 cm -1 is attributed to Brønsted acid sites, while a peak at 1450 cm -1 is ascribed to Lewis acid sites (Fig. 4a). Results from the quantitative analysis are presented in Table S2a. Generally, amount of Brønsted acid sites increases after PH3 modification, which could also be confirmed by the P-OH vibration peak 1133 cm -1 in transmission IR spectra (Fig. S4). After phosphine modification, amount of Lewis acid sites decreases gradually and a small blue shift to higher wavenumber is indicating the strengthening of acidity. Similar variations of BAS and LAS in phosphoric acid treated alumina and zeolites were observed previously [START_REF] Mekhemer | Surface to bulk characterization of phosphate modified aluminas[END_REF][START_REF] Morterra | Surface Characterization of Modified Aluminas: III. Surface-Features of PO 4-Doped Al2O3[END_REF]. The increase in Lewis acid strength is due to the increase of aluminum unsaturation degree. This is verified by the decrease in the intensity of penta-coordinated aluminum sites (AlV) and increase in the intensity of tetra-coordinated aluminum sites (AlIV). Peaks at 1653, 1638, 1595 cm -1 are assigned to pyridine adsorbed on AlIII, AlIV and AlV, respectively [START_REF] Liu | DRFT-IR Studies of the Surface of γ-Alumina[END_REF][START_REF] Lundie | Improved Description of the Surface Acidity of η-Alumina[END_REF] exhibited unexpected high activity in isobutane dehydrogenation, and after hydrothermal treatment, significant amount of LAS changed to BAS and catalytic products switched to heavy coke [START_REF] Rodemerck | Unexpectedly high activity of bare alumina for non-oxidative isobutane dehydrogenation[END_REF],

indicating significant influence of LAS on activating alkane and olefin selectivity.

BAS and LAS amounts of used catalysts are calculated and presented in Fig. 4b and Table S2b.

After 8 hours of reactions, parent catalyst (PV-0) lost almost all BAS and LAS, which is consistent with its lowest activity among all tested catalysts. For phosphine modified catalysts, the more the phosphorus content is, the larger the amount of LAS remains. Besides, the intensity variations of all aluminum corresponding LAS (AlIII, AlIV and AlV) has same trends as fresh catalysts, which is also indicating better remaining of LAS with increasing the phosphorus content. The highest remaining amount of LAS on catalyst PV-0.86 (61% remaining) correlates well with the best catalytic stability (almost no deactivation). It is noteworthy that the activity is not solely related to acidity. For catalysts PV-0.62 and PV-0.70 with less LAS than that of PV-0.86 still have higher activity at TOS of 8 h. Hence, the special function of LAS in propane dehydrogenation could partially explain the decrease of initial activity and the enhancement of stability of the phosphine modified catalysts by the losses of surface LAS and increased Lewis acidic strength.

Evolution of surface vanadia species

Except for surface acidity variations, the PH3 treatment may also affect polymerization degree of surface vanadia species, which is another key factor influencing the reactivity of catalysts [START_REF] Xiong | Structure-Performance Relationships for Propane Dehydrogenation over Aluminum Supported Vanadium Oxide[END_REF][START_REF] Rodemerck | Influence of the kind of VOx structures in VOx/MCM-41 on activity, selectivity and stability in dehydrogenation of propane and isobutane[END_REF][START_REF] Rodemerck | Influence of support and kind of VOx species on isobutene selectivity and coke deposition in non-oxidative dehydrogenation of isobutane[END_REF]. Therefore, detailed characterizations of the evolution of surface vanadia species were conducted.

XRD patterns of different catalysts contain peaks at 37°, 45.9° and 66.9° (Fig. S5), which are attributed to γ-Al2O3 phase (JCPDS Card No. 10-0425) [START_REF] Dai | γ-Al2O3 sheet-stabilized isolate Co 2+ for catalytic propane dehydrogenation[END_REF]. Besides, no other diffraction peaks are present in the patterns, indicating the high dispersion of surface VOx species on alumina [START_REF] Gannoun | Elaboration and characterization of sulfated and unsulfated V2O5/TiO2 nanotubes catalysts for chlorobenzene total oxidation[END_REF].

Moreover, XRD patterns of samples modified by phosphine show no difference with that of parent sample, indicating that the modification did not change crystalline phase of the vanadium-based catalysts and/or the new phases formed by P-modification are in trace amounts.

Textural properties of all catalysts are listed in Table S3. All samples present typical type-Ⅳ isotherms with H-2 hysteresis loops (Fig. S6), implying an ink-bottle mesoporous structure [START_REF] Hu | Vanadium-doped porous silica materials with high catalytic activity and stability for propane dehydrogenation reaction[END_REF].

All modified catalysts exhibit slightly lower (18% in average) BET specific area in comparison to parent catalyst. BJH diameter remains almost unchanged and pore volume decreases slightly, indicating that most phosphorus atoms exist on the surface of catalysts instead of accumulating in pores. N2 physisorption data, together with the XRD analysis indicate that the introduction of PH3 only modified the catalyst surface without significant changes of their textural and crystalline structure, which exhibits great advantages over conventional phosphorus modifications by introducing phosphoric acid or phosphate.

Surface species on the catalysts were identified by Raman spectroscopy since alumina shows very weak absorption band in the range of 1100-100 cm -1 [START_REF] Mizukoshi | Local structure of vanadium in Ti-6Al-4V alloy anodized in acetic acid aqueous solution and its contribution to visible light response in photocatalysis[END_REF]. Fig. 5 shows the Raman spectra of different samples. For parent catalyst, a relatively sharp band in the range of 950-1050 cm -1 is observed, which is ascribed to V=O stretching vibration, indicating existence of isolated and polymerized vanadium species [START_REF] Bai | Relationship between Surface Chemistry and Catalytic Performance of Mesoporous gamma-Al2O3 Supported VOx Catalyst in Catalytic Dehydrogenation of Propane[END_REF][START_REF] Hu | Influence of gelation and calcination temperature on the structure-performance of porous VOx-SiO2 solids in non-oxidative propane dehydrogenation[END_REF][START_REF] Wu | MnOx-decorated VOx/CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3[END_REF]. Broad Raman band in the range 500-950 cm -1 can be attributed to V-O-V stretching and V-O-Al antisymmetric/symmetric stretching vibrations in polymerized VOx [START_REF] Liu | Nature of the Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation[END_REF][START_REF] Hu | Influence of gelation and calcination temperature on the structure-performance of porous VOx-SiO2 solids in non-oxidative propane dehydrogenation[END_REF]. The sharp but low intensity peak at 145 cm -1 represents crystalline vanadia [START_REF] Fukudome | Highly Selective Oxidative Dehydrogenation of Propane to Propylene over VOx-SiO2 Catalysts[END_REF]. Comparing with the parent sample, all modified samples contain similar but relatively weaker bands in the range of 500-950 cm -1 . Decrease in the intensity of V-O-V and V-O-Al bands confirms that the concentration of V-O-V and V-O-Al is lower than that in parent catalyst. Peak at 145 cm -1 of crystalline vanadia also disappears, indicating the decrease in vanadia polymerization degree. After phosphine modification of catalysts, newly formed phosphorus species replaced partially polymerized vanadia species anchored on alumina surface. This is also confirmed by the presence of a new band at 1015cm -1 , characteristic of V=O stretching mode in isolated tetrahedral-coordinated VO4 [START_REF] Hu | Vanadium-doped porous silica materials with high catalytic activity and stability for propane dehydrogenation reaction[END_REF][START_REF] Hu | Influence of gelation and calcination temperature on the structure-performance of porous VOx-SiO2 solids in non-oxidative propane dehydrogenation[END_REF][START_REF] Wu | MnOx-decorated VOx/CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3[END_REF][START_REF] Wu | On the Structure of Vanadium Oxide Supported on Aluminas: UV and Visible Raman Spectroscopy, UV-Visible Diffuse Reflectance Spectroscopy, and Temperature-Programmed Reduction Studies[END_REF][START_REF] Jeon | Propane dehydrogenation over vanadium-doped zirconium oxide catalysts[END_REF], indicating the presence of more isolated vanadium species. Besides, the strong band at 923 cm -1 could be ascribed to asymmetric P-O-P vibrations of P2O7 2-[40-42], while the rather weak band from 1140 cm -1 to 1180 cm -1 is attributed to the V-O-P stretching of (VO)2P2O7 as suggested by Wu et.al [START_REF] Wu | Enhancing catalytic performance of phosphorus-modified ceria supported VPO catalysts for n-butane oxidation[END_REF], indicating the existence of very small amount of VPO species. In summary, Raman study confirms the existence of POx and VOx species and ascertain that the phosphorus modification results in the formation of more isolated vanadium species by inserting phosphorus into polymerized vanadium oxides.

Further, the status of VOx species on different catalysts were characterized by UV-Vis DRS (Fig. 6). All spectra contain broad bands centered at 275, 310 and 350 nm due to O 2-→V 5+ ligand-metal charge transfer (LMCT). These are ascribed to surface isolated and low polymerized vanadium species [START_REF] Hu | Vanadium-doped porous silica materials with high catalytic activity and stability for propane dehydrogenation reaction[END_REF][START_REF] Wu | On the Structure of Vanadium Oxide Supported on Aluminas: UV and Visible Raman Spectroscopy, UV-Visible Diffuse Reflectance Spectroscopy, and Temperature-Programmed Reduction Studies[END_REF][START_REF] Weckhuysen | Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis[END_REF]. The relatively weak band at 350-400 nm is attributed to tetrahedralcoordinated and distorted octahedral-coordinated vanadium species, which indicate the existence of highly polymerized vanadium species [START_REF] Hu | Vanadium-doped porous silica materials with high catalytic activity and stability for propane dehydrogenation reaction[END_REF]. After PH3 surface modification of the catalysts, the bands in this range become weaker, and the band at 275 cm -1 becomes dominant, suggesting the decrease in concentration of highly polymerized vanadium species, which is in line with Raman results, evidencing that more isolated and oligomeric vanadia were produced due to the insertion of phosphorus. For modified samples, a new band at 208 nm emerges, which can be assigned to O 2-→V 4+ LMCT [START_REF] Rajan | Vapour phase dehydration of glycerol to acrolein over vanadium phosphorous oxide (VPO) catalyst[END_REF], implying that more V 4+ species were presented on the surface [START_REF] Jian | Supported Ni-Al-VPO/MCM-41 As Efficient and Stable Catalysts for Highly Selective Preparation of Adipic Acid from Cyclohexane with NO2[END_REF]. This may be caused by PH3 reduction of V 5+ to V 4+ during surface reaction. Besides, no band above 500 nm is observed, which is characteristic for the d-d transitions of V 4+ and V 3+ , implying the absence of trivalent vanadium species on the catalysts surface [START_REF] Martin | Ammoxidation of 3-picoline to nicotinonitrile over VPO catalysts[END_REF][START_REF] Cavani | Relationship between structural/surface characteristics and reactivity in n-butane oxidation to maleic anhydride[END_REF]. Thus the V 5+ and V 4+ vanadium species are dominating in all samples. After phosphine modification of catalysts, more reduced tetravalent vanadium, and more isolated and low-polymerized vanadium species are formed.

Band gap energy (Eg) could reflect surface coordination states of vanadium species (Table S4).

Eg is calculated according to the formula: Eg=1240/λg, where λg is determined by the intersectional point of the vertical and horizontal parts of the spectra presented in Fig. 6 [START_REF] Liu | Nature of the Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation[END_REF]. An obvious blue shift of Eg is observed after phosphine modification of catalysts and Eg becomes even larger with the increase of the P/V ratio of the catalysts. In our previous work, we have found that the increase of surface vanadium species will decrease band gap energy, evidencing more polymerized vanadium species [START_REF] Bai | Relationship between Surface Chemistry and Catalytic Performance of Mesoporous gamma-Al2O3 Supported VOx Catalyst in Catalytic Dehydrogenation of Propane[END_REF]. The blue shift of band gap energy after phosphine modification of catalysts shows a reverse trend that the polymerized vanadium species are separated, forming more isolated ones [START_REF] Liu | Nature of the Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation[END_REF]. This phenomenon was also reported before for VPO [START_REF] Rajan | Vapour phase dehydration of glycerol to acrolein over vanadium phosphorous oxide (VPO) catalyst[END_REF] and other phosphorus modified catalysts [START_REF] Kőrösi | Preparation and investigation of structural and photocatalytic properties of phosphate modified titanium dioxide[END_REF]. The variation of band gap energy observed is consistent with the Raman results discussed above.

Combining XRD, N2 sorption, Raman and UV-Vis results, we could conclude that during phosphine surface modification of catalysts, the phosphorous atoms interact with alumina support and separate polymerized vanadia species thus causing decrease in their polymerization. More isolated and low oligomeric vanadia species lead to lower initial reactivity. Nevertheless, this also inhibits the condensation of coking precursors, thus improving the catalysts stability.

Correlation between electronic effects and catalytic performance of phosphine modified samples

To further explore the nature of VOx on PH3 modified catalysts, XPS study on samples PV-0, PV-0.70 and PV-0.86 before and after reactions was conducted. Fig. 7 demonstrates binding energies of vanadium atoms with different valences. The characteristic V 2p3/2 peak was fitted and deconvoluted into 2 peaks for fresh catalysts ascribed to pentavalent and tetravalent vanadium oxidation states [START_REF] Hu | Vanadium-doped porous silica materials with high catalytic activity and stability for propane dehydrogenation reaction[END_REF][START_REF] Hu | Influence of gelation and calcination temperature on the structure-performance of porous VOx-SiO2 solids in non-oxidative propane dehydrogenation[END_REF][START_REF] Wu | MnOx-decorated VOx/CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3[END_REF]. Trivalent vanadium species were not taken into consideration since the reduction temperature of pentavalent vanadium to trivalent vanadium is usually above 773K under reduction atmosphere [START_REF] Li | Alkali-induced deactivation mechanism of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO by NH3 in aluminum hydrate calcining flue gas[END_REF] (PH3 surface modification of sample was conducted at 573K for 1 h). The absence of V 3+ is also verified by aforementioned UV-Vis study. For the used catalysts (8 h), the V 2p3/2 peak was deconvoluted into 3 peaks including V 3+ . As shown, after PDH reaction, binding energies of V 5+ and V 4+ are slightly higher than for the fresh catalysts due to the presence of carbonaceous deposit [START_REF] Tian | VOx-K2O/γ-Al2O3 catalyst for nonoxidative dehydrogenation of isobutane[END_REF]. It can also be observed that binding energies decrease with increasing P/V ratio for both fresh and used catalysts, indicating the weakened interaction between vanadium species with alumina support caused by phosphorus modification [START_REF] Haber | Fifty years of my romance with vanadium oxide catalysts[END_REF]. Previously it was reported that higher electron density leads to lower binding energy [START_REF] Wu | Electron density modulation of NiCo2S4 nanowires by nitrogen incorporation for highly efficient hydrogen evolution catalysis[END_REF]. The higher electron density of vanadium caused by weaker interaction with alumina support makes it more repulsive to olefin intermediates or propylene product, thus facilitating their desorption from catalyst surface and inhibiting deep side-reactions [START_REF] Aly | Effect of Boron Promotion on Coke Formation during Propane Dehydrogenation over Pt/γ-Al2O3 Catalysts[END_REF], resulting in the enhanced selectivity and better stability.

On the other hand, the state of vanadium cations also has significant influence on the catalytic performance of materials. After PH3 modification, the content of surface V 4+ on fresh catalysts increases (Fig. 7). After dehydrogenation reactions, the content of V 5+ drops dramatically and the V 4+ and V 3+ amount increases due to the reduction of pentavalent vanadium species. The amount of V 4+ andV 3+ species in the catalysts is directly related with the catalysts activity. It can be seen that the used PV-0.70 catalyst contains more vanadium cations with lower valence (+3 and +4) than the used PV-0 and PV-0.86 catalysts (Fig. 7); final C3H8 conversion for PV-0.70 catalyst is also much higher (Fig. 1). According to previous studies, both V 3+ and V 4+ are more active than V 5+ in dehydrogenation reactions [START_REF] Xiong | Structure-Performance Relationships for Propane Dehydrogenation over Aluminum Supported Vanadium Oxide[END_REF][START_REF] Liu | Nature of the Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation[END_REF][START_REF] Gong | Hydroxyl-mediated Nonoxidative Propane Dehydrogenation over VOx/Al2O3 Catalysts with Promoted Stability[END_REF]. Rajan et al. found that phosphorus could stabilize trivalent vanadium in the VPO catalyst [START_REF] Rajan | Vapour phase dehydration of glycerol to acrolein over vanadium phosphorous oxide (VPO) catalyst[END_REF]. Thus the higher content of lower valence vanadium species for used PV-0.70 and its higher final conversion may provide further explanation to the stability enhancement.

In summary, XPS data provide information about the electronic effects and catalytic performance of phosphine modified samples. Phosphine modification of catalysts weakened the interaction between vanadium species and alumina support, which makes it more repulsive to olefin intermediates or propylene product, thus facilitating their desorption from catalyst surface and inhibiting deep side-reactions. Besides, after phosphine treatment of catalysts, the vanadium species tend to exist in lower valences, which are more reactive in dehydrogenation and can retain a stable reactivity. However, higher phosphorus content could also lead to weaker vanadiumsupport interactions and causes drastic decline in initial activity of catalysts, so a balance between initial reactivity and stability has to be achieved at a proper low phosphorous doping amount.

Conclusions

Vanadium based alumina catalyst was prepared by incipient wetness impregnation and modified with phosphine at 573K. The parent catalyst showed good initial reactivity but deactivated quickly due to coke formation on active sites. As the content of phosphorus increased, the initial reactivity decreased but the stability of catalyst was improved. Sample with the highest phosphorus content showed negligible deactivation during 8 h of reaction.

The superior role of phosphorus in maintaining propane dehydrogenation reactivity was analyzed. Synergy of surface acidic properties, electronic and geometric effects after phosphine modification accounted for the stability enhancement. Compared to conventional phosphate impregnation, phosphine interacted extensively with alumina support and vanadium species without causing structural destructions. The insertion of phosphorus into polymerized vanadium species decreased polymerization degree and formed more isolated vanadium species, which has lower dehydrogenation reactivity. Furthermore, it weakened the interaction between vanadium species and alumina, and also reduced initial reactivity of the catalysts. However, weakened vanadium-support interaction made vanadium sites more repulsive to active intermediates and propylene products, which can desorb more easily from the catalysts surface. More isolated vanadium species also expanded the distance between active propylene intermediates and reduced the possibility of oligomerization, thus inhibited coking of the catalysts. Besides, phosphorus modification maintained more strong Lewis acid sites of alumina support and stabilized lower valence vanadium, which is also beneficial for retaining a stable reactivity during PDH reactions.

In summary, doping of phosphorus in catalyst leads to decrease of initial reactivity but enhanced stability of VOx/Al2O3 in PDH reactions. To obtain a balance between initial reactivity and catalytic stability, phosphorus with low amount has to be used so as to gain better propylene yield within a relatively long period.

FiguresFig. 1 .

 1 Figures

Fig. 2 .

 2 Fig. 2. Reactivity remaining and the maximum propane conversions of parent and phosphine

Fig. 3 .

 3 Fig. 3. TG-DTA profiles of used parent and phosphine modified catalysts: (a) weight loss and

Fig. 4 .

 4 Fig. 4. Py-DRIFTS spectra of (a) fresh and (b) used parent and phosphine modified catalysts.

Fig. 5 .

 5 Fig. 5. Raman spectra of parent and phosphine modified catalysts.

Fig. 6 .

 6 Fig. 6. UV-Vis spectra of parent and phosphine modified catalysts.

Fig. 7 .

 7 Fig. 7. V 2p3/2 XPS spectra of fresh and used parent and phosphine modified catalysts: (a) PV-0

  . According to Dong et al., the difference in acidic strength between AlIII and AlIV is not significant, while AlV has obviously weaker acidic strength than both of AlIII and AlIV[START_REF] Dong | Enhanced thermal stability of palladium oxidation catalysts using phosphate-modified alumina supports[END_REF]. These results indicate that phosphine tends to eliminate weaker LAS on the surface of alumina support. Previous works reported that LAS is critical for activating propane molecule and desorption of propylene[START_REF] Grzybowska-Świerkosz | Active centres on vanadia-based catalysts for selective oxidation of hydrocarbons[END_REF][START_REF] Choi | Propane dehydrogenation catalyzed by gallosilicate MFI zeolites with perturbed acidity[END_REF].Rodemerck et al. found that bare alumina (possessing only LAS formed by surface coordinatively unsaturated aluminum sites)

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (2017YFB0306600), the Joint Funds of the National Natural Science Foundation of China and China National Petroleum Corporation (U1362202), Natural Science Foundation of China (51601223, 21206195, U1510109), the Fundamental Research Funds for the Central Universities (17CX05018, 17CX02056, 14CX02050A, 14CX02123A), Shandong Provincial Natural Science Foundation (ZR2017MB003) and State Key Laboratory of Heavy Oil Processing Fund (SKLZZ-2017008).

Appendix A. Supplementary data

By-products analysis, TG-DTA, EDX-mapping, Acid properties, XRD, N2 adsorption.