Improvement of Activity and Stability of CuGa Promoted Sulfated

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Improvement of Activity and Stability of CuGa Promoted Sulfated Zirconia Catalyst for n-Butane Isomerization Kang Yang, Honglin Li, Siqi Zhao, Shengsong Lai, Weikun Lai, Yixin Lian, and Weiping Fang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04590 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Industrial & Engineering Chemistry Research

Improvement of Activity and Stability of CuGa Promoted Sulfated Zirconia Catalyst for n-Butane Isomerization Kang Yang, Honglin Li, Siqi Zhao, Shengsong Lai, Weikun Lai*, Yixin Lian*, and Weiping Fang National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, PR China *

E-mail addresses: [email protected] (Y. Lian), [email protected] (W. Lai).

ABSTRACT The use of CuGa promoted sulfated zirconia (CuGa/SZ) catalyst for the isomerization of n-butane to isobutane has potential value as a replacement for the current catalyst for this important reaction (chlorinated alumina). Ga promoted SZ work well but deactivate rapidly. Several related catalysts (SZ, Ga/SZ, Cu/SZ and CuGa/SZ) were compared and only CuGa/SZ shows the most interesting properties that the stable conversion is 55 % and the selectivity to isobutane is 82 % for 200 h. This is mainly attributed to Cu promoting a better incorporation of GaOx into the support, higher Bronsted/Lewis ratio, smaller ZrO2 crystallite, retard in the transformation from tetragonal (active for isomerization) to monoclinic (inactive) ZrO2 and higher sulfate stability. This effect is finally compared to the effect of Pd and Pt, demonstrating potential benefits of CuGa/SZ for commercial application due to their similar yields of isobutane (40~45 %). Keywords: n-Butane; Ga-promoted sulfated zirconia; Cu; Stability; Isomerization

INTRODUCTION An increasing proportion of heavy and sour (high sulfur) crude oil is being processed globally, resulting in deteriorating crude oil quality. Based on an analysis of available crude resources, refineries of all countries will continue processing a growing amount of inferior crude oil in the future.1 At the same time, increasingly stringent environmental regulations are accelerating improvements in petroleum product quality to decrease air pollution, for 1

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example, a haze under the blue sky. Fortunately, alkylation of isobutane with C3-C5 olefins results in the production of alkylate, which is a valuable blending component for the gasoline. It has a low vapor pressure and a high-octane level which improves fuel combustibility.2,3 Thus, the isomerization of n-butane to isobutane is of important academic value and has broad applications. Sulfated zirconia-based catalysts (SZ) are considered as alternatives to the industrial platinum on chlorinated alumina catalysts for n-butane isomerization which requires a continuous supply of chlorine compound to maintain the activity.4 Over the past 30 years, SZ has been extensively investigated and holds considerable promise for being the next generation of catalyst for alkane isomerization due to not only high catalytic activity companying with high selectivity to isobutane at low reaction temperature, but also its environmental friendliness. However, the rapid deactivation of the catalyst limits its application.5-7 Despite many attempts have focused on this,8-11 no satisfactory promotion effect has been reached. Moreno et al.4 systematically studied Al- and Ga-promoted SZ and the results indicate that Ga was a more efficient promoter than Al. Platinum addition significantly increased the overall n-butane conversion (52 %) at expense of the selectivity to isobutane (83 %). Latterly, the addition of La/Ni has a remarkable influence on the catalytic properties of alumina-promoted sulfated zirconia (SZA) for n-butane isomerization, while the highest conversion is 45 % between 200 and 240oC.12 Moreover, it has been found that Cu is able to enhance the stability of Ni-Mg-Al catalysts during hydrogen production by catalytic decomposition of methane, which also suffers deactivation by coke formation.13 The main goal of this contribution is to determine the effect of Cu addition on catalytic and physicochemical properties of Ga-promoted SZ catalysts for n-butane isomerization. The catalytic features (activity and selectivity) of several systems with SZ, Ga/SZ, Cu/SZ and CuGa/SZ have been compared and the modifier roles in the carbon tolerance and acidity of SZ has been tentatively proposed. At the end of this work, the performance of optimized CuGa/SZ was compared with those of Pt and Pd promoted SZ catalysts.

EXPERIMENTAL SECTION Catalysts Preparation. The first step was the preparation of zirconia hydroxide by 2


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precipitation. Zr(NO3)4·5H2O was dissolved in deionized water and ammonia solution (25-28 wt.%) was added dropwise up to pH 8-10. The resulting hydroxide suspension was aged for 24 h at room temperature. The precipitate was then filtered, washed with deionized water and dried at 110oC for 12 h. Subsequently, the 50 g dry Zr(OH)4 powder was immersed in 16 mL (NH4)2SO4 solution (0.44 g/mL). After dried at 110oC, 16 mL Ga(NO3)3 solution (0.25 g/mL) was added to the as-prepared SZ. After dried at 110oC, 16 mL Cu(NO3)2 solution (0.05 g/mL) was added to the as-prepared Ga/SZ. After dried at 110oC, the sample was calcinated at 700oC for 3 h to obtain CuGa promoted sulfated zirconia (CuGa/SZ). Cu/SZ (0.6 wt.%) and Ga/SZ with 3 wt.% Ga2O3 loading were prepared by the similar incipient wetness technique. Catalysts Characterization. N2-adsorption studies were used to examine the porous properties. The measurements were carried out on Micromeritics TriStar 3020 adsorptive and desorptive apparatus after the samples were pretreated in vacuum at 350oC for 15 h. The specific surface area was obtained using the Brunauer-Emmett-Teller (BET) method. Powder X-ray diffraction (XRD) patterns of the prepared catalysts were obtained on a Rigaku Ultima-IV X-ray powder diffractometer coupled to a copper anode tube. An angular range 2θ from 10-80° was recorded. Raman spectra were obtained on a Renishaw inVia Raman spectrometer with an incident laser light excited at a wavelength of 532 nm. Temperature-programmed desorption of ammonia and sulfur dioxide (NH3(SO2)-TPD) experiment was measured with a Micromeritics AutoChem 2920 apparatus. Prior to the measurement, ca. 0.2 g samples were reduced in H2 (99.99%, 50 mL/min) at 350oC for 1 h. After cooling to 100oC in He, the gas flow was switched to 10 % NH3 in He for 50 min. The desorption of NH3 and SO2 were carried out in He with a temperature ramp of 10oC/min to 900oC. Simultaneously, mass spectrometer (OMNIStar) was employed to monitor both NH3 (m/e=17) and SO2 (m/e=64) signal. Temperature-programmed reduction (H2-TPR) experiment was measured with the same apparatus and 5 % H2/Ar was used as the reducing gas at a flow rate of 50 mL/min. The rate of temperature rise was 10oC/min up to 800oC. The H2-O2 titration was carried out to calculate the Cu dispersion. After reduction at 350oC in 5 % H2/Ar for 60 min, the sample was subjected to N2O (99.99 %) at 40 mL/min for 60 min at 90oC. The pure H2 was injected to sample tube every two minutes at 300oC to reduce surface 3



Cu atoms until the TCD signal is stable. The volume of loop is 1 mL. The in situ IR spectra of adsorbed pyridine (Py-IR) were recorded by using a Bruker Vertex 70 infrared spectrometer. The samples were pressed into thin wafers and placed in a low temperature reaction chamber with two ZnSe windows. The samples were reduced in 99.99 % hydrogen at 350oC for 1 h and then degassed in vacuum before pyridine adsorption at room temperature. Afterward, the pyridine was passed over the sample for 30 min and the pyridine adsorption spectra were recorded after degassing at 250oC for 1 h. Two metal (Ga & Cu) loadings were determined by using a PANalytical Axios Petro X‐ray fluorescence (XRF) instrument. X-ray photoelectron spectrum (XPS) was obtained with an X probe spectrometer (ESCALAB 250 XI). The binding energies of the Ga 2p, Zr 3d and Cu 2p peaks were referenced to that of the C-(C, H) component of the C 1s peak of adventitious carbon at 284.8 eV. Peak decomposition was performed with a least-squares fitting routine, using a Gauss/Lorentz ratio of 85/15 and after nonlinear baseline subtraction. Transmission electron microscopy (TEM) studies were conducted by using JEOL JEM-2100 instruments equipped with an energy dispersive X-ray (EDX mapping). Before being placed on the support, the samples were dispersed in excess ethanol by sonicating for 30 min. The sulfur and carbon contents were measured by a high frequency infrared carbon sulfur analyzer (varioEL III Elementar). The simultaneous thermogravimetric analysis and differential thermal analysis (TGA-DTA) were performed on a TG 209 F1 thermogravimetric analyzer (NETZSCH). After drying overnight at 110oC, about a 7 mg fresh sample before calcination was heated in a stream of air of 20 mL/min from 50 to 800oC at a rate 10oC /min to obtain the weight changes and the DTA profile.7,14,15 Catalysts Evaluation. n-Butane isomerization reactions were conducted in a fixed-bed downflow micro-reactor at 250oC, 2.0 MPa and weight hourly space velocity (WHSV) of 0.4 h-1. The catalyst particles (40-60 mesh) in 1 mL was placed in the middle of the stainless-steel reactor (12 mm i.d.) and supported by quartz sand. Prior to the reaction, the catalyst was activated in situ under flowing H2 at 350oC for 2 h. During the test, n-butane was fed to the reactor with hydrogen as diluent gas (H2/butane molar ratio = 3/1). The composition of feed and product was analyzed by a Shanghai Wuhao GC 9560 gas chromatograph (KB-PLOTQ capillary column, 30 m × 0.53 mm × 20.00 um) equipped with a FID detector. Evaluation 4


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data were online recorded after reaction for 3 h. The n-butane used in this study was purchased from Dalian Special Gases Co., Ltd. and had a purity higher than 99 wt.%.

RESULTS AND DISCUSSION Catalytic Performance of n-Butane Isomerization. To investigate the effect of metal addition on the catalytic activity of n-butane isomerization, the conversion-time curves obtained at 250°C over nonpromoted and promoted catalysts are compared in Figure 1a. All the promoted catalysts exhibit much higher initial conversions than SZ, which suffers from fast deactivation within only 16 h. For the promoted catalysts, two Cu-promoted catalysts (Cu/SZ & CuGa/SZ) show two steady conversions (46 % & 55 %) after 150 h except for the Ga/SZ catalyst, which shows a continuous decrease in the conversion of n-butane. Thus the addition of Cu has a positive effect on the long-term stability of catalysts. In addition, the CuGa/SZ catalyst shows the most stable as well as highest activity (55 %) from 20 to 200 h. At steady conversion, the selectivities to isobutane are in the range 80-83 % for the most active catalyst, and 85-92 % for the less active Cu/SZ and Ga/SZ catalysts in Figure 1b. For the nonpromoted SZ catalyst, the decline of selectivity to isobutane with time on stream (TOS) is resulted from the fact that the side reactions, such as disproportionation and cracking,6,10,12,16 become severer initially. Table 1. Conversion, selectivity and yields at steady regime over promoted and nonpromoted SZ catalysts Yield (%)

Conv. (%)

Select. (%)

CH4

C2H6

C3H8

i-C4H10

C5H12

SZa

1.5

87

0

0

0.1

1.3

0.1

Ga/SZ

20

90

0.1

0.4

0.3

18

1.2

Cu/SZ

46

87

0.5

2

2

40

1.5

CuGa/SZ

55

82

1

3

4

45

2

Catalyst

a

Catalytic performance determined with TOS of 16 h.

5



90

a

Conversion of n-butane/(%)

80

SZ Ga/SZ Cu/SZ CuGa/SZ

70 60 50 40 30 20 10 0 3

23

43

63

83

103

123

143

163

183

203

Time on stream/(h) 100

b Selectivity to isobutane/(%)

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

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90

80


70

60 3

23

43

63

83

103

123

143

163

183

203

Time on stream/(h) Figure 1. Comparison of conversion of n-butane (a) and selectivity to isobutane (b) with TOS over promoted and nonpromoted SZ catalysts at 250oC.

The main products for n-butane isomerization are isobutane and propane in addition to very small amounts of methane, ethane and pentanes as secondary products in Table 1. The presence of C5H12 among the reaction products implies a scission of part of the C8 intermediates at steady regime, namely, where the very strong acid sites operate.4,10,17 The 6


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greater abundance of C3H8 over C5H12 for CuGa/SZ suggests that the reaction is probably more complex, since simple cleavage of C8 intermediate should produce equal amounts of C3 and C5 alkanes throughout the reaction. Further cracking of C5 in C2 and C3 alkanes cannot be excluded because the C3/C2 molar ratio (0.09/0.10) is almost equal on CuGa/SZ catalyst. This product distribution may predict there is strong and stable Brønsted acidity on CuGa/SZ catalyst surface. In order to optimize the advantage of Cu promoter, an effect of the Cu content on the isomerization performance of CuGa/SZ was further investigated. Table 2. Effect of the Cu content on the conversion, selectivity and yields over CuGa/SZ catalystsa

a

Yield (%)

Cu content (wt.%)

Conv. (%)

Select. (%)

CH4

C2H6

C3H8

i-C4H10

C5H12

0.4

53

84

0.8

2.7

3.3

44

2.2

0.6

55

82

1

3

4

45

2

0.8

56

82

1

3.3

3.9

46

1.8

1.0

52

85

0.8

2.8

3.1

44

1.3

Catalytic performance determined with TOS of 18 h.

As shown in Table 2, all the catalysts at various Cu content (0.4, 0.6, 0.8 and 1.0 wt.%) demonstrate the similar conversion and selectivity at 250°C. It is obvious that Cu content has little effect on selectivities to C1-C3 alkanes. Besides, both of yields to isobutane on Cu0.6 and Cu0.8Ga/SZ are almost the same at the same activity as well as selectivity. Furthermore, introduction of more Cu in porous structure of SZ leads to first an increase and then a decrease in both BET surface area and pore volume, but the changes are not very noticeable (Table S1). It may be possible that the incipient wetness process (followed by calcination) implies quite a high degree of chemical reaction of Cu with the matrix. While it is obvious that the BET surface area and pore volume within the ranges have no significant effect on the evaluation results in this work. Last but not least, the more Cu content the higher catalyst preparation costs. Thus the suitable Cu content is 0.4-0.6 wt.% and the nominal Cu content of CuGa/SZ is 0.6 wt.%. Based on this CuGa/SZ catalyst, the suitable reaction temperature is 250°C after optimization (Figure S1 & Table S2).

7



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Table 3. Textural properties of fresh (left) and used (right) samples Sample

a

SBET (m2•g-1)

Vpore (cm3•g-1)

Dpore (nm)

SZ

50

52a

0.087

0.086a

6.8

6.7a

Ga/SZ

55

49

0.098

0.085

6.6

6.5

Cu/SZ

44

41

0.068

0.062

6.0

5.9

CuGa/SZ

64

66

0.094

0.095

5.5

5.4

Three physical parameters of SZ determined after reaction for 16 h.

Catalysts Characterization. In order to explore the unexpected catalytic behavior of the CuGa/SZ catalysts, the textural structure of catalysts before and after reaction for 200 h was first determined by nitrogen adsorption-desorption method. As listed in Table 3, the fresh Cu/SZ shows a lower surface area and pore volume compared with the Ga/SZ. On the contrary, the surface area increases obviously after addition both Ga and Cu by impregnation, while the pore size decreases. It has been reported that a final calcination step, which led to a drastic decrease of specific surface area, was essential for the formation of active sulfate sites on the catalyst surface.12 From this point of view, these values of textural properties after 700oC calcination are much lower than those in other reports.7,11,18 Besides the incorporation of Ga into bulk zirconia probably alleviates the sintering of catalyst, increases the specific surface areas and the pore volumes. However, some of pores are blocked by Cu promoter during impregnation. Furthermore, the physical properties of used catalysts are also illustrated in Table 3. A slight increase in the surface area of spent catalysts is observed. It is inevitable that a little coke and/or oligomer are formed over CuGa/SZ (Table 4). It is well accepted that some types of coke also have topological structure.19,20 It is also obvious that the surface area and pore structure did not change significantly before and after reaction in this work. Thus loss of surface area and change in pore structure can be ruled out as possible reasons for catalyst deactivation.

8


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a

T T

Intensity/(a.u.)

M

T

T

(4)

(3)

(2) (1) 10

20

30

40

50

60

70

80

2Theta/(degree) T

b

T M

Intensity/(a.u.)

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


T

T (4)

(3) (2) (1)

10

20

30

40

50

60

70

80

2Theta/(degree) Figure 2. XRD patterns of (1) SZ, (2) Ga/SZ, (3) Cu/SZ and (4) CuGa/SZ catalysts before (a) and after reaction for 200 h (b): (T) tetragonal ZrO2; (M) monoclinic ZrO2. Used SZ was only evaluated for 16 h. All the fresh samples were reduced at 350oC for 1 h.

XRD patterns of all catalysts before and after reaction for 200 h are shown in Figure 2. All diffraction peaks of fresh samples are ascribed to the tetragonal and monoclinic zirconia. No peaks assigned to Ga and/or Cu or related compounds are observed, which implies that the promoters are well dispersed that no or very little crystal size of them are formed on the surface or in the network of supports in Figure 2a. Despite above, the addition of promoters 9



results in some perceivable changes. Almost only monoclinic ZrO2 with a little tetragonal phase is observed for SZ after 700oC calcination without any modification. Furthermore, the diffraction peaks of tetragonal phase are more obvious on Cu/SZ and Ga/SZ catalyst. That is, the addition of Cu or Ga has a positive effect on the tetragonal phase. However, this advantage for Cu is inferior to Ga. Surprisingly, for the CuGa/SZ catalyst, the monoclinic phase is mostly disappeared, indicating that the addition of Cu has positive effect on the incorporation of GaOx into bulk supports, which retards the phase transformation of zirconia from tetragonal to monoclinic. The similar trend observed above can also be evidenced by the striking differences in the Raman spectra between two phases of ZrO2 in Figure 3. The Raman bands at 149, 269 and 312 cm-1 are exclusively assigned to the tetragonal phase of ZrO2 and the bands at 181 and 337 cm-1 are assigned to the monoclinic phase. The bands at 472 and 631 cm-1 are common for both phases.21 Raman spectrum clearly shows that the tetragonal phase becomes more and more predominant when GaOx and Cu-GaOx are incorporated. Moreno et al.4 also confirmed that both Al and Ga promoters enhanced the stability of the tetragonal structure. Since that the crystallite size of sulfated zirconia can strongly affect its surface properties and catalytic activity,12 it was determined by Scherrer equation using (111) orientation at 30.1° in the scale 2θ for tetragonal zirconia.21,22 As shown in Table 4, with the addition of Cu or GaOx, both crystallite sizes of tetragonal ZrO2 are smaller than that of unmodified SZ catalyst (around 214 Å), which implies that addition only one promoter is able to retard transformation of small tetragonal ZrO2 crystallites. Specially, the crystallite size of CuGa/SZ catalyst (125 Å) is much smaller than those of other samples, which reveals that the well dispersed Cu-GaOx along with the bulk zirconia prevents the agglomeration of tetragonal ZrO2 grains. The smaller crystallite size leads to a higher specific surface area and pore volume. (Table 3).

10


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149

269

181

312

472

CuGa/SZ

337

Intensity/(a.u.)

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


631

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Ga/SZ

Cu/SZ SZ 900

800

700

600

500

400

300

200

100

Wavenumber/(cm-1) Figure 3. Raman spectra of fresh SZ, Ga/SZ, Cu/SZ and CuGa/SZ sample. All the fresh samples were calcinated at 700oC for 1 h.

XRD patterns of used SZ for 16 h, Cu/SZ, Ga/SZ and CuGa/SZ catalysts from 200 h evaluation are shown in Figure 2b. No difference with respect to the fresh catalyst was observed, therefore excluding detectable phase modification. Many studies have shown that only a tetragonal ZrO2 based catalyst shows good activity for alkane isomerization, while a monoclinic zirconia based catalyst, which is thermodynamically more favorable, is completely inactive for alkane isomerization.7 Because the monoclinic ZrO2 is the main structure for pure SZ, the declines in activity and selectivity to isobutane (Figure 1) are fast and significant as a result of very weak interaction between sulfate species and support during reaction. Detailed deactivation mechanism will be discussed later. Although Ga promoter has detrimental effect on catalytic stability as shown in Figure 1a, GaOx as a structure promoter can retard the phase transformation of tetragonal to monoclinic zirconia before and after reaction for 200 h. After adding Cu to Ga/SZ further, the stability of tetragonal structure is not destroyed. Instead smaller tetragonal ZrO2 crystallites are achieved and reserved during reaction because its physical properties are not changed at all (Table 3). It was proposed that the oxygen vacancy on the surface of tetragonal zirconia plays an important role in stabilizing the sulfate species even after a high temperature calcination process.23 So it can be deduced 11



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that it is the strong interaction between Cu and GaOx that makes contribution to the higher fraction of tetragonal ZrO2. As a bifunctional catalyst, the acidity of CuGa/SZ is another important factor for the high and stable activity. Table 4. Chemical composition and comparison of sulfur/carbon content and Cu dispersion of fresh and used SZ samples XRF analysis (wt.%)

Sulfur/Carbon (wt.%)

Sample

DCua (%)

Dcb B/L

Ga

Cu

Fresh

30 h

200 h

Fresh | 200 h

(Å)

SZ

--

--

1.38/0.003

1.22/0.004c

--

--

214

0.17

Ga/SZ

1.26

--

1.15/0.004

1.14/0.012

0.90/0.572

--

136

0.38

Cu/SZ

--

0.49

1.12/0.003

1.10/0.007

0.85/0.008

42 | 20

167

0.11

CuGa/SZ

1.25

0.49

1.18/0.004

1.17/0.013

1.17/0.014

31 | 25

125

0.53

a

Cu dispersion determined from H2-O2 titration based on the molar of consumed H2.

b

Crystallite size determined from Scherrer’s equation based on the tetragonal zirconia.

c

The sulfur content determined after reaction for 16 h.

Because the sulfur on the surface determines the super acidity of the catalyst, we measured the sulfur contents for all samples before and after reaction. As shown in Table 4, the sulfur contents of all modified samples are slightly lower than that of unmodified SZ catalyst before reaction. Some sulfate are destroyed or covered by Cu and/or GaOx anchored on the surface, resulting in that a fraction of sulfur removes from surface. However, the more tetragonal zirconia, the more sulfur for the promoted SZ samples. So more SO2 are produced over CuGa/SZ than those over Cu/SZ and Ga/SZ samples. It is well known that the gas production of CO2 or SO2 is the reason for detection result of carbon or sulfur analyzer. This process for sulfur loss is shown in Scheme 1.

Scheme 1. The process for sulfur loss from sulfate species oxidation reaction.

It is reasonable that the Zr3+ cations of tetragonal phase can chemisorb SO42- to form SZ, 12


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while the interaction between Zr4+ cations of monoclinic phase and SO42- belongs to the weak static adsorption during calcination. The presence of Zr3+ will be discussed later. For the pure SZ, oxidative dehydrogenation of n-butane is the only reaction path for butenes formation. In this way, the amount of sulfate species becomes less and less due to reduction of SO42- to SO32-, which is catalytically inactive. After reaction for 16 h, sulfur content of unmodified SZ decreases to 1.22 % since more SO2 are produced and removed from SZ surface. It is deduced that more and more SO32- cannot be oxidized into SO42- over inactive monoclinic surface and some of them decompose into SO2, leading to the loss of sulfur content of used SZ. Thus, reduction of sulfate species is the main reason for deactivation of SZ rather than the complete removal of sulfur. Ng and Horvát reported that the sulfate species on the catalyst surface underwent gradual loss during reaction in the form of H2S,24 while Wang et al.7 ascribed the deactivation to reduction of S6+ to a lower oxidation state, consistent with this work. Although sulfur loss for Ga/SZ (0.01 %) and Cu/SZ (0.02 %) are close to that for CuGa/SZ (0.01 %) with TOS of 30 h, this loss becomes much less significant for CuGa/SZ (0.00 %) than those of Ga/SZ (0.24 %) and Cu/SZ (0.25 %) with TOS of 200 h. It means less and less sulfite species are produced during reaction, so the sulfate retention is unavoidably favored over CuGa/SZ catalyst. The mechanism for this super stability will be discussed later. Considering two equal Ga contents of Ga/SZ and CuGa/SZ as well as two equal Cu contents of Cu/SZ and CuGa/SZ, this sulfate retention should be related to the interaction between Cu and GaOx over CuGa/SZ catalyst. Although Ga/SZ and Cu/SZ show the similar sulfur loss with TOS of 200 h, the activity of Cu/SZ (46 %) is twice more than that of Ga/SZ (20 %) in Table 1. Thus, there must be another reason for the deactivation of Ga/SZ. It is noteworthy that the carbon deposition is faster over Ga/SZ than that of Cu/SZ catalyst during the first 30 h of reaction and the formation of coke over Cu/SZ is almost ignored with TOS to 200 h in Table 4, consistent with what Li and Gonzalez et al.25 reported. After reaction for 200 h, the carbon content increases to 0.572 % for Ga/SZ, leading to a continuous loss in activity (Figure 1a). It is reasonable that some types of coke with topological structure can compensate the loss of BET surface area and pore volume, as mentioned above.19,20 In this case, Ga species are proved to be able to catalyze not only the dehydrogenating steps for n-butane but also cracking and successive dehydrogenation of 13



cyclic olefins to aromatics, some of which can be regarded as coke precursors.26,27 Consequently, fast carbon deposition and gradual deactivation are observed. It has been applied in industry that aromatization of light alkanes (mainly propane and butane) on zeolite-supported gallium catalysts, which is known as the Cyclar process in industry.28 Many studies show that gallium is involved in the dehydrogenation of the reactant alkane to produce alkenes that undergo oligomerization, followed by cracking and cyclization reactions on the zeolite’s Brønsted acid sites.29-31 If the rate of cyclization of butenes from oxidative dehydrogenation of n-butane and/or deep dehydrogenation of the cyclic hydrocarbons to aromatics is faster than that of isomerization, more and more heavy products are depositing on the active sites. So only Ga promoter is not enough to optimize SZ. Fortunately, the Cu favors a higher hydrogen mobility inhibiting the formation of encapsulating coke.32 It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke.13 It is noteworthy that the isomerization reaction was carried out in the reducing atmosphere (H2). Recently, Götsch et al. found that Cu-rich facets inhibit carbon growth.33 In this work, the presence of Cu enhances carbon resistance and thus promotes the catalytic activity. Besides the addition of Cu to Ga/SZ may have detrimental effect on the carbon filament formation over Ga species even if the carbon deposition is also obvious over CuGa/SZ catalyst during the first 30 h, but the formation of coke can be ignored with TOS to 200 h in Table 4. Although Cu has inferior effect to stabilize the sulfur content (due to less tetragonal phase), the carbon content (0.008 %) of Cu/SZ without deactivation trend is much lower than that of Ga/SZ catalyst (0.572 %) after 200 h. Thus the carbon tolerance is an very important factor for the long catalytic lifetime during isomerization. The acidity of fresh samples was tested by NH3(SO2)-TPD measurements with a mass spectrometer. As shown in Figure 4A, two adsorption ranges can be differentiated. The region above 400°C was due to the desorption of NH3 from strong acid sites. The region below 400°C in the TPD profile was the contribution of hydrogen bond formed by NH4+ cation or was assigned to weak acid sites.34 Compared to Ga/SZ, the Cu/SZ shows another desorption peak at ca. 500°C (Figure 4A), which was mainly assigned to the interaction between NH3 and metallic Cu.35 In spite of the loss in sulfur content of CuGa/SZ, both amounts of weak 14


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and strong acid sites increase. Because its specific surface area also increases obviously (Table 3), some of pores are created by the aid of Cu-GaOx promoters during calcination process and become accessible for the probe molecules (NH3). Actually, the SO2 can be formed by a redox reaction between sulfate species and strong adsorbed ammonia molecules. In addition, SO2 can also be produced by the decomposition of sulfate species in anaerobic atmosphere.6 As shown in Figure 4B, the increased intensity at T > 700°C should be mainly assigned to the latter reason, which has been proved in previous study by a mass spectrometer.7 The difference in the evolution of produced SO2 is consistent with that in the sulfur contents (Table 4). A SO2 MS signal/(a.u.)

B

NH3 MS signal/(a.u.)

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



100

200

300

400

500

600

700

800

900


100

200

300

400

500

600

700

800

900

Temperature/(oC)

Temperature/(oC)

Figure 4. NH3-TPD (A) and SO2-TPD (B) profiles of promoted and nonpromoted SZ samples.

In order to clarify this strong acid sites on CuGa/SZ, in situ IR of adsorbed pyridine (Py-IR) introduced the variation of the amount of medium strong Lewis acid and Brønsted acid, measured after degassing at 250°C (reaction temperature). The absorption bands around 1450 and 1540 cm-1 are assigned to the vibration modes of pyridine adsorbed on Lewis sites and Brønsted sites, respectively.34 As shown in Figure 5, the introduction of GaOx and Cu-GaOx indeed has a great effect on the acidity of SZ samples except for Cu/SZ sample. First, the Brønsted acidity perceptibly increases with the addition of GaOx and Cu-GaOx, while the Lewis acidity decreases in a certain extent. As a result, the ratio between Brønsted and Lewis acid sites increases significantly (Table 4). This trend demonstrates that the introduction of GaOx and Cu-GaOx consumes Lewis acid sites and brings out new as well as strong Brønsted acid sites on the catalyst surface. Moreno et al.4 also evidenced that that promotion with Ga brought about an important increase of the strong Brønsted acidity with 15



respect to nonpromoted SZ. However, the increase of Lewis acid sites is more significant on Cu/SZ sample because the monometallic Cu acted as a new type of Lewis acid site with a strong acid strength.36 On the contrary, the strong Lewis acid strength is not observed on CuGa/SZ sample in Figure 5. Thus the interaction between Cu and GaOx should be strong so that less Cu-rich facets are exposed on the catalyst surface to adsorb pyridine. The Brønsted sites of unmodified SZ catalysts are mainly derived from sulfate species and Lewis sites from unsaturated Zr atoms on the surface.37 In this case, some Lewis sites are destroyed or covered by Cu and/or GaOx anchored on the surface of unsaturated Zr atoms. Because catalytical active sites are generated by the sulfate species adsorbed on the stepped-edges of tetragonal ZrO2.18 An increase in total surface area would provide more surface and/or edge sites for adsorption of sulfate species. Therefore, it seems that the observed increase in strong Brønsted sites content is very much likely due to the increase in the total surface area and stable smaller tetragonal ZrO2 crystallites as a result of Cu-GaOx addition. Although Cu has inferior effect to stabilize the tetragonal ZrO2, leading to the lower ratio between Brønsted and Lewis acid sites, adding Cu to Ga/SZ increases the amount of active tetragonal ZrO2 and thus achieves the strong Brønsted sites. Thus both isomerization activity and reaction stability are improved effectively. As shown in Table 1, a bimolecular mechanism is strongly supported by experimental evidence that propane and pentanes are produced from C8+ cracking. So the Bronsted sites should be the main factor for isomerization. Although required acidity to trigger the reaction can be related to the Lewis acid site near the S-OH-Zr bond in SZ, Lewis acid sites of ZrO2 are not enough acidic to abstract a hydride from n-butane.37 As shown in Tables 1 and 4, the higher B/L ratio on CuGa/SZ shows the higher isobutane yield compared to Cu/SZ with lower B/L ratio. Therefore, as a bifunctional catalyst, the Bronsted acidity of CuGa/SZ is another important factor for the high and stable activity.

16


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L B Absorbance/(a.u.)

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


(1) (2)

(3) (4)

1700 1675 1650 1625 1600 1575 1550 1525 1500 1475 1450 1425

Wavenumber/(cm-1) Figure 5. Py-IR spectra of samples degassing at 250oC: (1) SZ, (2) Ga/SZ, (3) Cu/SZ and (4) CuGa/SZ. All the catalysts were reduced at 350oC for 1 h.

To further explore the interaction between Cu and GaOx, the redox properties of four fresh samples were investigated by H2-TPR measurement (Figure 6). It has been reported that the sulfate species on the catalyst surface is usually reduced to SO2 and/or H2S.12 The coefficients of thermal conductivities of SO2 and H2S are much lower than that of H2. Therefore, we can assume that all peaks above 400°C are attributed to hydrogen consumption derived from the reduction of sulfate species, while the reduction peaks of CuO and/or GaOx are hardly observable considering low contents of promoters. However, an obvious first reduction peak accompanies with a shoulder in the temperature range of 300-350°C centered at ca. 275°C of CuGa/SZ after amplification (an insert in Figure 6). The first peak is attributed to the reduction of copper oxide to metal copper because the Cu/SZ sample shows the similar and only one reduction peak in this range. The second peak of CuGa/SZ appeared at 350°C could be assigned to the reduction of copper oxide that has strong interaction with GaOx promoter, like what proposed in Figure 5. Above 400°C, only one narrow reduction peak with high intensity is observed for SZ around 640°C. Obviously, the peak areas decrease after addition of Cu and/or GaOx, which is in good agreement with the lower sulfur contents of modified catalysts than that of the unmodified sample (Table 4).

17



(1) (4)

TCD Signal/(a.u.)

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

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(3)

(4)

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o

Temperature/ C

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(3)

(2)

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Temperature/(°C) Figure 6. H2-TPR profiles of (1) SZ, (2) Ga/SZ, (3) Cu/SZ and (4) CuGa/SZ.

As to Ga/SZ sample, gallium oxide being hardly reduced below 600°C (an insert in Figure 6) and the reduced oxide being not restored after O2 treatment.31 So the reduction peak in the temperature range of 400-600°C is only related to the loss of sulfate species. Furthermore, the reduction peaks of sulfate species shift towards low temperature and become broader. And the reduction temperature for all samples increases in the order: Ga/SZ < CuGa/SZ < Cu/SZ < SZ, indicating that the addition of Ga species accelerates the reduction of sulfate species. In addition, the addition of Cu changes the interaction between SO42− and supports. One potential explanation for this maybe that there exists a hydrogen spillover effect between GaOx and sulfur species. It was demonstrated that dihydrogen molecules were dissociated over Ga2O3 into H+ and H− species in the reducing atmosphere although the gallium oxide are still existing.28 So it is reasonable that the gallium oxide can promote the reduction of sulfur species ca. 500°C, similar to what proposed over Ni2SZA sample.12 Again, this peak at ca. 524°C of CuGa/SZ reflects that the interaction between Cu and GaOx is strong. In addition, the sulfate species cannot be reduced below 350°C for all samples (Figure 6) and the n-butane isomerization reaction in this work was carried out at 250°C. Therefore, removal of sulfur as H2S should not be the deactivation mechanism for SZ and Ga/SZ samples. After integration, the peak areas of SZ, Ga/SZ, Cu/SZ and CuCa/SZ are 249, 215, 18


Page 19 of 30

216 and 216, respectively. The sulfur contents (Table 4) of fresh samples are 1.38, 1.15, 1.12 and 1.18 wt.%, respectively. According to quantitative analysis, the calculated peak areas of SZ, Ga/SZ, Cu/SZ and CuCa/SZ are 243, 208, 202 and 213, respectively. So the difference in the hydrogen consumption should be consistent with that in the sulfur contents.

100

3.5

105

3.0

100

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2.0

DTA 561oC

349oC

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658 C 715oC

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678oC 90

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309 C

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SZ 75

Ga/SZ

0.0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

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Temperature/(oC) 2.0

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DTA 336oC

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86

TGA

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DTA

-1.5 -2.0

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TGA 325oC

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Cu/SZ 78

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102

∆T/(oC) TG Weight/(%)

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∆T/(oC)

95

544oC ∆T/(oC) TG Weight/(%)

TG Weight/(%)

2.5

∆T/(oC)

105

TG Weight/(%)

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-2.5 -3.0

CuGa/SZ

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74

-3.5 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

o

Temperature/(oC)

Temperature/( C)

Figure 7. TGA-DTA profiles in flowing air: SZ, Ga/SZ, Cu/SZ and CuGa/SZ samples before calcination.

The H2-O2 titration is an accurate measurement of the copper dispersion because only surface Cu atoms (Cus) are oxidized by N2O (N2O + 2Cus → (Cus)2O + N2).38 Then the pure H2 was injected to reduce Cus at 300oC ((Cus)2O + H2 → 2Cus + H2O). So the twice molar of consumed H2 is equivalent to the molar of Cus. In this way, Cu dispersions of fresh and used Cu/SZ and CuGa/SZ samples were calculated in terms of the ratio of the total number of outer surface Cu atoms of crystallites to the total number of Cu atoms present.38 As shown in Table 4, because of no strong interaction with GaOx, the Cu dispersion of Cu/SZ decreased after 200 h of reaction from 42 % to 20 %, while this difference for CuGa/SZ in two dispersions (31 % → 25 %) is less than that for Cu/SZ. As shown in Figure 1a, Cu/SZ work as well as CuGa/SZ initially and deactivate very slowly compared to Ga/SZ. The decrease in 19



Cu dispersion should be the main reason for the lower stability of Cu/SZ. Because two Cu contents are relatively lower (

Improvement of Activity and Stability of CuGa Promoted Sulfated

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