SAPO-18 and SAPO-34 catalysts for propylene production from the oligomerization-cracking of ethylene or 1-butene

Accepted Manuscript Title: SAPO-18 and SAPO-34 catalysts for propylene production from the oligomerization-cracking of ethylene or 1-butene Authors: Eva Epelde, Mar´ıa Ib´an˜ ez, Jos´e Valecillos, Andr´es T. Aguayo, Ana G. Gayubo, Javier Bilbao, Pedro Casta˜no PII: DOI: Reference:

S0926-860X(17)30417-9 http://dx.doi.org/10.1016/j.apcata.2017.08.036 APCATA 16391

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

29-3-2017 5-7-2017 22-8-2017

Please cite this article as: Eva Epelde, Mar´ıa Ib´an˜ ez, Jos´e Valecillos, Andr´es T.Aguayo, Ana G.Gayubo, Javier Bilbao, Pedro Casta˜no, SAPO-18 and SAPO-34 catalysts for propylene production from the oligomerization-cracking of ethylene or 1-butene, Applied Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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SAPO-18 and SAPO-34 catalysts for propylene production from the oligomerization-cracking of ethylene or 1-butene Eva Epelde, María Ibáñez, José Valecillos, Andrés T. Aguayo, Ana G. Gayubo, Javier Bilbao, Pedro Castaño* Department of Chemical Engineering, University of the Basque Country UPV/EHU, PO Box 644, 48080 Bilbao, Spain. *Telephone: +34 946 018 435, E-mail: [email protected]

Graphical abstract

Highlights    

Comparison between SAPO-34 and SAPO-18 catalysts for the conversion of ethylene or 1butene into propylene SAPO-18 catalyst suffers less deactivation by coke due to the higher accessibility of acid sites, while this coke is more aliphatic in nature SAPO-34 catalyst has higher selectivity of propylene but deactivates completely after 100 min The deactivation is faster in the conversion of ethylene compared with that of 1-butene for both catalyst

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Abstract The performance of SAPO-18 and SAPO-34 catalysts has been compared during the conversion of ethylene or 1-butene to propylene. This comparison has been made in terms of activity (conversion), selectivity and stability against coke deposition. The SAPOs were synthesized, agglomerated, calcined, characterized and tested in a fixed-bed reactor at 500 ºC. The spent catalysts (after 5 h on stream) were characterized to assign the location and nature of coke. The results point to the higher activity and stability of SAPO-18 catalyst in the conversions of each reactant (ethylene or 1-butene), which has been explained on the basis of its acidity, pore topology and above all, the faster diffusion of aromatics causing deactivation. Thus, the SAPO-18 catalyst suffers slower coke deposition, while this coke is of lighter nature (more aliphatic and less aromatic). The advantages of SAPO-18 over SAPO-34 catalyst are more relevant for the transformation of 1-butene, where the propylene selectivity and yield increase over time, as secondary reactions are selectively neglected and coke condensation is slowed down. Keywords: light olefins; AEI structure; CHA structure; oligomerization-cracking; coke deactivation; propene

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1. Introduction Propylene is a key raw material for the production of many petrochemicals such as polypropylene, propylene oxide, acrylonitrile, among others. Nevertheless, the petrochemical market in the last years has set out a deficit of propylene, which is currently produced by processes like Lurgi methanol to propylene (MTP) [1], steam cracking [2], fluid catalytic cracking (FCC) [3,4], methanol to olefins (MTO) [5] and by propane dehydrogenation [6]. Besides, there is an increasing interest in the selective production of propylene from other sustainable sources, such as bio-oil (product of lignocellulosic biomass pyrolysis) [7], bioethanol [8] or wastes like polyolefins [9] and from olefins of lower demand. The transformation of ethylene into propylene has caught great attention via metathesis [10,11] or oligomerization-cracking reactions [12-14]. The metathesis uses severer operational conditions than the oligomerization-cracking, while requiring much more expensive catalyst based on WO3/SiO2 or Ru [15]. On the other hand, conversion of 1-butene (whose demand is lower than that of ethylene) into propylene is an alternative to the oligomerization aiming middle distillates [16]. This interconversion of olefins is an interesting route whose main challenges are the rational design of the catalyst, operating conditions and reactor. In this sense, several advances should be pointed in the conversion of ethylene [12,13,17-20], butenes [20-22] and C5+ olefins [23,24] to propylene. The most studied catalyst for the interconversion of olefins is the HZSM-5 zeolite with pore opening of 10-MR (membered rings), which has been widely modified by different methods in order to enhance propylene yield and selectivity, and to attenuate coke deactivation [2527]. Other catalysts with different shape selectivity have also been studied and reported in the literature: 12-MR, as HY, Hβ, HEU-1, HMOR, HMCM-49; another 10-MR, as HMCM-22, HZSM-11, -22, -23, -35, or -48; or 8-MR, as SAPO-34, -18, -5 or -35 [12,13,17,19,21,28-31]. Among all of them, SAPO-34 catalyst, which has been industrially implemented in the MTO

4

process with UOP/Norsk Hydro technology [32], shows the best perspectives for the selective production of propylene; however, it is rapidly deactivated by coke. In order to improve the properties of SAPO-34, the following alternatives have been proposed: i) synthesis of SAPO34 nanocrystals by conventional (i.e. dry gel conversion) and unconventional (i.e. sonochemical synthesis) methods [33,34]; ii) modification of acidity by oxalic acid treatment [35] or by doping with Ni, Mn, Co or Ce [36]. In addition, the synthesis of hybrid catalysts (i.e. HZSM-5/SAPO-34, SAPO-34/ZrO2; SAPO-34/SAPO-18) [37], hierarchical pore systems or core-shell structures [38] are interesting alternatives in order to obtain intermediate properties between the individual materials as well as a synergetic effect to increase propylene selectivity and to attenuate coke deactivation. SAPO-18 catalyst (AEI structure) is an alternative to its isomorphic SAPO-34 (CHA structure), showing a slower deactivation rate in the MTO process [39]. The topology of CHA, consists of cavities of 1.27 × 0.94 nm channels interconnected by small pore size 0.38 × 038 nm formed by 8 MR. In the case of AEI the cavities are slightly higher, 1.27 × 1.16 nm, the pore sizes are like those in CHA [40,41]. The main difference between AEI and CHA structures is the orientation of the double 6-rings (D6R): the successive layers of double rings conforming the crystalline AEI structure have different orientations among them, while they have the same orientation in the CHA structure [42]. Wragg et al. [40] have determined that this difference results in a higher rigidity of the SAPO-18 crystalline structure, which explains the less capability to hold coke precursors in its cages and this justifies that the coke deposition rate on SAPO-18 is slower than that on SAPO-34. SAPO-34 and SAPO-18 have been mostly studied for the MTO process [5,31,34,36,38] whereas the literature is scarce for ethylene or 1-butene conversion, which have been studied only on SAPO-34 and confirming that it is less active than in the MTO reaction [12,13,21].

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This work compares the performance of SAPO-34 and SAPO-18 catalysts for intensifying propylene production from the conversion of ethylene or 1-butene, as raw materials. To this aim, two SAPOs were synthesized, agglomerated with a matrix material (bentonite and

-

alumina), calcined, characterized (XRD, N2 adsorption-desorption isotherms, NH3 adsorption isotherms and temperature programed desorption, and FTIR spectroscopy) and tested in each conversion (ethylene or 1-butene) in a fixed-bed reactor at 500 ºC. The location and nature of the deactivating species (coke) has been studied by characterizing the spent catalyst obtained after 5 h on stream runs (N2 adsorption-desorption isotherms) and the coke deposited (FTIR spectroscopy and temperature programmed oxidation). Finally, the catalytic performance, in terms of conversion, selectivity and deactivation, has been correlated with the catalytic properties.

2. Experimental 2.1. Catalysts preparation SAPO materials were prepared using the reagents and crystallization conditions summarized in Table S1 in the supporting information. The resulting molar composition of the gel has been

reported

in

a

previous

work

[39].

The

composition

of

SAPO-34

is

(SiO2)0.08(Al2O3)1(P2O5)0.96 (C8H19N)2·89H2O) and it was prepared following the method of Lok et al. [43], using tetraethyl ammonium hydroxide (TEAOH) as template, whereas the composition of SAPO-18 is (SiO2)0.51(Al2O3)1(P2O5)0.76(C8H19N)1.34·39H2O), and it was prepared following the method of Chen et al. [44], using N,N-diisopropyl ethyl amine (iPr2EN) as template. The amount of the latter template is lower than that of TEAOH used for the SAPO-34 synthesis [42]. The removal of the organic template was carried out by calcination in a muffle furnace at 575 ºC for 6 h with a heating rate of 5 ºC min-1.

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The catalysts were obtained by agglomerating the SAPO (-18 or -34, 25 wt%) by wet extrusion with a binder (bentonite, Exaloid, 30 wt%) and an inert charge (α-alumina, Martinswek, 45 wt%). The extrudates were first dried (110 ºC, 24 h) and then sieved to a particle diameter between 0.15 and 0.30 mm. This agglomeration confers a higher mechanical resistance to the particle, which is required for the industrial practice [45]. Moreover, with the suitable preparation, the presence of a mesoporous matrix, as the one generated by bentonite [46], could help attenuating coke deposition [47] and avoiding irreversible activity loss (by zeolite dehydroxilation) in the catalyst regeneration by coke combustion [48]. The effect of attenuation of coke deposition is similar to the one obtained by creating mesopores within the crystal particles. These mesopores favor the diffusion to the exterior of coke precursors and mitigate the blockage of micropore mouths [49]. Finally, the catalysts were calcined at 570 ºC for 2 h. This treatment is required in order to achieve a balance of the acid sites, which allows keeping the catalyst hydrothermally stable and maintaining its kinetic performance throughout reaction-regeneration cycles. 2.2. Fresh and spent catalysts characterization The structural properties of the SAPOs were determined by X-ray diffraction (XRD) in a Phillips PW1710 diffractometer with automatic slit, graphite monochromator and Cu anode operated at 40 kV and 20 mA. The physical properties were determined by N2 adsorptiondesorption (Micromeritics ASAP 2010) using 100 mg of sample (SAPO or final catalyst) at 77 K and relative pressures between 0.01 and 1. The specific surface and pore volume were calculated by the Brunauer-Emmett-Teller (BET) simplified equation and by the t-method based on Harkins-Jura equation, respectively. The acidity and acid strength of the catalysts were measured by monitoring the adsorptiondesorption of NH3 by combining the techniques of thermo-gravimetric analysis and the differential scanning calorimetry, using a Setaram TG-DSC calorimeter connected on line

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with a Thermostar mass spectrometer (Balzers Instruments) [39]. The presence of Brönsted (BAS) and Lewis (LAS) acid sites was studied by adsorption of NH3 or pyridine on the SAPOs (20 mg in a pressed disc) at 150 ºC. The samples were placed in a Specac high temperature-high pressure cell and the spectra were recorded using a Nicolet 6700 FTIR spectrometer. The sample was exposed to pulses of NH3 or pyridine and FTIR spectra were recorded after each pulse until saturation. The coke content deposited on the catalyst was determined by temperature-programmed oxidation (TPO) in a TGA Q5000 thermobalance (TA Instruments), with a ramp of 8 ºC min1

from 250 ºC to 600 ºC. FTIR spectra of the spent catalysts were collected in a Nicolet 6700

(Thermo) spectrometer using a transmission cell (60 scans, and resolution of 4 cm-1). 2.3. Reaction equipment and product analysis The reaction runs were carried out in an automated device (Microactivity of PID Eng&Tech) equipped with an isothermal fixed bed reactor which has been described in detail in a previous work [27]. The operating variables are controlled by bespoke software (Process@ from PID Eng&Tech). The product stream was analyzed in a micro-gas chromatograph (MicroGC 3000A), which is provided with four analytical modules and columns for the separation of each compound. Product stream was analyzed every 4 min. The balance of atoms (C, H) was closed in all runs above 99.5%. The transformation of ethylene or 1-butene was performed under the following operating conditions: total pressure, 1.5 bar (small overpressure to account for the pressure drop in the fixed bed reactor and the device for continuous sampling); 500 ºC; feed flow rate (1-butene or ethylene), 35 cm3 min-1 diluted in He (10 vol. %); partial pressure in the feed, 1.35 bar; mass of catalyst, 0.55 g; space time, 6.4 g h mol-1; time on stream, 5 h. Blank runs (without

8

catalyst) were carried out to confirm that thermal cracking of ethylene or 1-butene was insignificant below 550 ºC. The conversion of ethylene and 1-butene with time on stream is defined as follows: X 

F0  F  100 F0

(1)

where Fo and F are the molar flow rates of the reactant (1-butene or ethylene) in the feed and outlet stream, respectively, expressed in C atoms contained. It should be noted that 1-butene rapidly reaches (at the reactor entrance) the thermodynamic equilibrium of the isomerization with the rest of butenes (iso-, cis- and trans-butene). Consequently, when 1-butene is used as feed F corresponds to the total flow rate of butenes. The selectivity of each gaseous product i is calculated as follows: Si 

Fi  100 F0  F

(2)

where Fi is the molar flow rate of the gaseous product i. Propylene yield is defined as: YP 

FP  100 F0

(3)

where Fp is the molar flow rate of C3H6 in the product stream expressed as number of C atoms. 3. Results and discussion 3.1. Fresh catalysts properties The crystalline structures synthesized were checked by XRD. No diffraction peaks other than those of SAPO-34 (2θ = 2.4, 9.6, 16, 20.6, 25.2, 30.6, 31.2º) and SAPO-18 (9.6, 17, 21, 26.2º) were found. Table 1 summarizes the physical and acid properties of the SAPOs and the

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catalysts. The BET surface area of SAPO-18 is slightly higher than that of SAPO-34. It should be noted that the micropore volume in the catalyst corresponds to the SAPO, whereas the agglomeration with bentonite and alumina provides most part of the meso- and macropores to the final catalyst.

Table 1. Properties of the as-synthesized SAPOs and the fresh catalysts (agglomerated). Property SBET (m2 (gcatalyst)-1) Sm (m2 (gcatalyst)-1) Vp (m3 (gcatalyst)-1) Vm (m3 (gcatalyst)-1) dp (Å) Acidity (mmolNH3 (gcatalyst)-1) Acid strength (kJ (molNH3)-1)

As-synthesized SAPO-18 SAPO-34 798 611 326 288 0.28 0.24 0.080 0.060 0.37 0.64 142 153

Agglomerated SAPO-18 SAPO-34 236 215 150 103 0.23 0.20 0.072 0.050 38.4 37.6 0.10 0.17 142 153

Fig. S1a in the supporting information shows the TPD-NH3 profiles and the acid strength distribution for SAPO-18 and SAPO-34 and agglomerated catalysts. Two desorption peaks are identified in the TPD profiles: the first peak at 243-258 ºC corresponding to weak acid sites, whereas the second peak (335-348 ºC) corresponds to strong acid sites. The intensity of the second peak for SAPO-34 suggests the presence of a higher amount of strong acid sites in comparison to SAPO-18, and the shift of this peak to higher temperatures implies a higher acid strength of the sites. As observed in Fig. S1b, both SAPOs have a homogeneous and similar level of acid strength (153 kJ (molNH3)-1 for SAPO-34 and 142 kJ (molNH3)-1 for SAPO-18), whereas the acidity of SAPO-34 (0.64 mmolNH3 g-1) is significantly higher than that of the SAPO-18 (0.37 mmolNH3 g-1). Although both SAPOs have isomorphic structures, the differences found in the number of acid sites could be explained by the different mechanism of Si incorporation into the framework of the corresponding AlPO-n during the synthesis as reported in the literature by MAS NMR spectroscopy [40,50]. In the SAPO-34, P

10

is substituted by the incorporated Si, whereas in the SAPO-18 both Al and P are substituted by Si. In the last case, some of the Brönsted acidity is eliminated, and this leads to lowering the amount of strong acid sites for SAPO-18 in comparison to SAPO-34 for the same level of Si substitution. Fig. S1 shows that the acidity per catalyst mass unit is approximately a quarter of that corresponding to the SAPO and maintaining the same level of acid strength. Thus, the contribution of the catalyst matrix (bentonite and alumina) to the acidity is insignificant. Fig. 1 shows the FTIR spectra of pyridine and ammonia adsorbed on SAPO catalysts in the 1300-1800 cm-1 region. The adsorbed-pyridine spectrum shows bands at 1450 cm-1 and 1550 cm-1 that are ascribed to Lewis acid sites (LAS, SPy) and Brönsted acid sites (BAS, S···H– Py+), respectively. As observed, the BAS band is almost negligible for SAPO-34 catalyst due to the severe steric restriction to the diffusion of pyridine through their micropores. However, SAPO-18 catalyst shows a significant absorbance at 1550 cm-1, which is indicative of BAS accessible to pyridine indicating the presence of external acid sites. In the ammonia adsorbed spectra, the absorption band ranging 1375-1450 cm-1 is associated with BAS (S···NH4+), whereas the band at 1620 cm-1 is ascribed to LAS (SNH3). The calculation of BAS/LAS ratio by using molar extinction coefficients for this system would lead to an inconsistent value due to two facts: i) the Lewis molar extinction coefficient is too small in comparison to that of Brönsted [51]; and, ii) the Brönsted absorption band continues increasing after saturation in the presence of NH3 by changing its position and shape, which brings uncertainty about the neutralization of acid sites. Nevertheless, NH3 adsorbed spectra clearly evidenced the presence of BAS.

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BAS BAS

LAS

1375

1620 Ammonia

Absorbance (a.u.)

SAPO-18

SAPO-34 BAS

1550

1450 LAS

0.1

Pyridine SAPO-18

SAPO-34 1800

1700

1600

1500

1400

1300

-1

Wavenumber (cm )

Fig. 1. FTIR spectra of pyridine and ammonia adsorbed on SAPO-18 and SAPO-34 catalysts at 150 °C.

3.2. Catalytic performance Fig. 2 shows the evolution with time on stream (TOS) of conversion for the transformation of ethylene or 1-butene for the two catalysts. The conversions at zero time on stream of 1butene (77 %) or ethylene (75 %) are significantly higher for SAPO-18 than those for SAPO34 (67 % and 40 %, respectively). SAPO-18 catalyst also has a higher stability in each reaction. Besides, the deactivation is faster during the conversion of ethylene in comparison with that of 1-butene for both catalysts. In the conversion of ethylene on SAPO-34 catalyst, the catalyst was completely deactivated after 100 min time on stream.

12

100

Conversion (%)

80

60 Ethylene 1-Butene SAPO-18 SAPO-34 40

20

0 0

50

100

150

200

250

300

Time on stream (min)

Fig. 2. Evolution with time on stream of ethylene or 1-butene conversions for SAPO-18 and SAPO-34 catalysts. Reaction conditions: T = 500 ºC, PA = 1.35 bar, W/F0 = 6.4 g h mol-1.

Ethylene and 1-butene are transformed throughout oligomerization-cracking reactions [17,28], ethylene oligometization being slower than that of 1-butene [52], which explains the lower conversion observed in the conversion of ethylene. Zhao et al. [53] determined that the rate of alkene conversion on SAPO-34 catalyst decreased in this sequence: butene > propylene >> ethylene. The higher activity and stability of SAPO-18 catalyst compared with that of SAPO-34 could be explained by the differences in their acid and micropore properties. The acidity and acid strength of SAPO-18 catalyst are ca. 41.2% and 7.2% lower than those of SAPO-34 catalyst (Table 1), respectively, while BAS accessibility is significantly higher for SAPO-18 compared with SAPO-34 catalyst (Fig. 1). Thus, a combination of lower acid site density, of lower acid strength together with the higher accessibility (and higher diffusion rate of coke precursors) are the main responsible parameters of the lower deactivation rate of

13

SAPO-18 compared with that of SAPO-34 catalysts. In fact, the highest stability of SAPO-18 catalyst has also been reported in the MTO reaction [39,40,54-56]. Aguayo et al. [39] suggested that the higher stability of SAPO-18 is due to the lower acid strength of the sites and thus, having a lower fraction of the BAS that are prone to coke formation and condensation [57]. Furthermore, in a comparative study with other molecular sieves, Zhao et al. [28] explained that the high severity of shape selectivity occurring in the SAPO-34 restricts the internal diffusion of reactants and products, limiting the progress of the reaction towards the cracking of C4+ olefins. The gaseous product distribution (selectivity) at zero time on stream for ethylene or 1-butene conversion on SAPO-18 and SAPO-34 catalysts is shown in Fig. 3. The products were grouped into the following lumps: i) methane (CH4); ii) ethane and propane (C2-C3 light paraffins); iii) ethylene (C2H4); iv) propylene (C3H6); v) butenes (C4H8); vi) butanes (C4H10); vii) C5+ aliphatics, which include all the olefins and paraffin with more than five carbon atoms, and; viii) aromatics BTX (benzene, toluene and xylenes). In all cases, propylene was the main product, reaching the highest selectivity for SAPO-34 catalyst compared to SAPO-18. Propylene selectivity was remarkably high for ethylene conversion on SAPO-34 catalyst (90%). On the other hand, C4H8 and C5+ hydrocarbons are also important reaction products, particularly for 1-butene conversion on both catalysts and for ethylene conversion on SAPO-34, which is consistent with a mechanism of oligomerization-cracking reactions [17,28,53]. Alkanes (C2-C3 and C4H10) are also present in the product stream, which may be identified as secondary products resulting from hydrogen transfer reactions of ethylene, propylene and butenes [53]. The presence of BTX in the products distribution on SAPO-18 catalyst suggests that aromatization reactions also take place in the conversion of ethylene and 1-butene. Aromatics have also been identified as reaction byproducts in the methanol into olefins conversion on modified SAPO-18 catalyst

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[58] and on SAPO-34 [21], and they have been formed on the BAS that are not constrained in the micropores of the catalyst. These acid sites with better accessibility catalyze bimolecular reactions in particular, such as hydrogen transfer or oligomerization, that are implicated in the catalytic deactivation by forming coke precursors [59]. These precursors are not constrained within the micropores of the SAPO-18 and escape toward the exterior of the catalyst, causing less severe deactivation.

100

(a) Ethylene

SAPO-18 SAPO-34

CH4 C2-C3 C3H6 C4H8 C4H10

C5+

Selectivity (%)

80 60 40 20

Selectivity (%)

0 BTX

100

Product distribution SAPO-18 (b) 1-Butene

80

SAPO-34

60 40 20 0 CH4 C2-C3 C2H4 C3H6 C4H10

C5+

BTX

Product distribution

Fig. 3. Selectivity of product lumps at zero time on stream in the transformation of ethylene (a) or 1-butene (b). Reaction conditions: T = 500 ºC, PA = 1.35 bar, W/F0 = 6.4 g h mol-1.

The evolution with time on stream of the different lumps of products reveals that propylene and C5+ (of olefinic nature) selectivities are enhanced with TOS during the conversion of 1-

15

butene or ethylene on SAPO-18, whereas the selectivity of the rest of lumps of products decreases with TOS, being light paraffins the most affected. This effect is more pronounced for the performance of SAPO-34 and when ethylene is used as feed. Fig. 4 shows a correlation between propylene yield and selectivity for the conversion of ethylene or 1-butene on both SAPO-18 and -34 catalysts, for different values of conversion (whose decay has been shown in Fig. 2). Both catalysts deactivates severely when used in the conversion of ethylene, so that the propylene yield decreases although its selectivity increases, which is a result of the blocking of the strongest acid sites (those converting propylene in secondary reactions) [60]. This effect is also observed for 1-butene conversion on SAPO-34 catalyst, but unobservable for the 1-butene conversion on SAPO-18 catalyst, where both yield and selectivity of propylene increase as the reaction proceeds. This trend is indicative of the selective blockage of those acid sites participating in the secondary reactions of converting propylene and hydrogen transfer, having a lesser impact on the oligomerization-cracking pathways. Consequently, it is noteworthy that the transformation of 1-butene on SAPO-18 catalyst approaches to a conversion isoquant line (between 70 and 80 %), in steady-state conditions, which is a promising feature for industrial implementation. Mazoyer et al. [11] achieved a propylene yield of 15% for a conversion of 1-butene of 30% in the tungsten hydride metathesis system, which is higher than that obtained with SAPO-34 but significantly lower than that obtained for the SAPO-18 studied.

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Propylene selectivity (%)

100

1%

5%

10%

20%

80

Conversion = 30% 40% 50%

60 60% 70%

40

80%

20

TOS

Ethylene 1-Butene SAPO-18 SAPO-34

0 0

5

10

15

20

25

30

35

Propylene yield (%)

Fig. 4. Correlation of propylene selectivity and yield in the transformation of ethylene or 1-butene. Reaction conditions: T = 500 ºC, PA = 1.35 bar, W/F0 = 6.4 g h mol-1.

For explaining the differences observed in the 1-butene conversion among the catalysts studied, it should be pointed that both SAPO catalysts have a relatively similar porous structure. The SAPO-18 cages are slightly higher than those of SAPO-34 as a result of the different spatial arrangement within their structures [40,56]. Besides, the unit cell of SAPO18 shows a 0.9 % volume expansion compared with a 3 % for SAPO-34 in the MTO process, which is explained by the larger cages and more rigid double 6-membered ring arrangement in the SAPO-18 and therefore delaying the pore blockage by coke [40]. The smaller space of SAPO-34 cages could be rapidly occupied with coke which may have a drastic effect on activity. Nevertheless, it is more likely that the lower acidity and the higher accessibility of the acid sites of SAPO-18 catalyst (Table 1) are responsible of a higher diffusivity of coke precursors, which has been recurrently proved to be a key element of a slower deactivation [61-64]. Despite of the higher concentration of aromatics in the reaction medium for SAPO18 catalyst (Fig. 3) and considering the participation of these as coke precursors, the fact that

17

this catalyst shows slower deactivation should be ascribed to the diffusion of the coke precursors outside the SAPO-18 framework. This diffusion is much limited in the SAPO-34 catalyst, and hence, it deactivates faster. 3.3. Deterioration of catalyst properties Table 2 summarizes the physical properties of the spent catalysts after 5 h on stream and the change respect to the properties of the fresh catalysts (Table 1). The surface area significantly decreased for both used catalysts after each reaction. In particular, the micropore area is negligible for SAPO-34 catalyst after 5 h time on stream for each reaction as well as the micropore average volume. This suggests that coke is preferably deposited in the micropores of SAPO-34 catalyst, thus causing its rapid deactivation by blocking the access to acid sites. On the other hand, the previously explained diffusion of coke precursors for the SAPO-18 catalyst justifies the lower degradation of the spent catalyst and the lower change as compared with its fresh counterpart. In turn, this lower degradation of SAPO-18 catalyst is responsible of its higher remaining activity during 1-butene or ethylene conversions observed in Fig. 2. Table 2. Physical properties of spent catalysts after 5 h time on stream. Catalyst Ethylene SAPO-18 (Change, %) SAPO-34 (Change, %) 1-Butene SAPO-18 (Change, %) SAPO-34 (Change, %)

3.4. Coke properties

SBET (m2 g-1)

Sm (m2 g-1)

Vp (m3 g-1)

Vm (m3 g-1)

dp (Å)

135 (-43) 89 (-59)

68 (-55) 0 (-100)

0.17 (-26) 0.14 (-30)

0.033 (-54) 0 (-100)

124 (+222) 62 (+66)

118 (-50) 76 (-65)

54 (-64) 0 (-100)

0.16 (-30) 0.12 (-40)

0.027 (-63) 0 (-100)

119 (+208) 63 (+67)

18

Fig. 5 shows the FTIR spectra of the spent catalysts. The following absorbance bands of the coke deposited were identified and assigned to [65,66]: 1460 cm-1, highly branched aliphatics; 1590 cm-1, polyaromatic hydrocarbons (condensed coke); 1610 cm-1, conjugated double bonds (dienes) in hydrocarbon chains; 2930 cm-1, aliphatics -CH2 and -CH; 2960 cm1

, aliphatics -CH3; 3040 cm-1, alkylated mono-aromatics. FTIR spectra were deconvoluted

and the fractions of the most representative bands –associated to aromatics (1590 and 1610 cm-1) and aliphatics (2930 and 2960 cm-1)– were calculated as well as the ratio of aromatics to aliphatics (Table 4). These results are qualitative and comparable among samples. Results indicate that the nature of the coke formed during the ethylene conversion is more aromatic than that from 1-butene conversion. On the other hand, the coke deposited on the SAPO-18 catalyst has less condensed aromatic nature, which is consistent with its lower deactivation (particularly for 1-butene conversion).

Absorbance (a.u.)

Ethylene

SAPO-18

SAPO-34 0.02

1-Butene

SAPO-18

SAPO-34 3000

2800

1600

1400 -1

Wavenumber (cm )

Fig. 5. FTIR spectra of the catalysts spent in the transformation of ethylene and 1-butene.

19

Table 3. Fraction of FTIR absorption bands for spent catalysts. Catalyst Ethylene SAPO-18 SAPO-34 1-Butene SAPO-18 SAPO-34

1590 cm

-1

FTIR absorption bands 1610 cm-1 2930 cm-1

2960 cm

-1

Aromatics/aliphatics ratio

0.40 0.38

0.40 0.42

0.08 0.07

0.12 0.12

4.04 4.13

0.34 0.39

0.35 0.41

0.14 0.08

0.18 0.13

2.19 3.92

Fig. 6 shows the TPO profiles of the coke deposited on SAPO-18 and SAPO-34 catalysts after 5 h on stream during the transformation of ethylene or 1-butene. The combustion of coke showed two fractions which burn at different temperature: i) coke I, distinguished as a shoulder, that burns at ca. 460 ºC, and ii) coke II, that burns between 520 and 550 ºC. Cokes I and II could be respectively associated with different carbonaceous species with different H/C ratio, being the one burning at lower temperature (coke I) that with the highest H/C ratio [7]. Besides, the delay observed in the combustion of coke II on the SAPO-34 catalyst is coherent with the preferable location of coke within the micropores. Assuming the FTIR spectroscopy results (Fig. 5), coke I should be assigned to species with more aliphatic nature whereas coke II to aromatic species with different condensation degree. Coke content (CC, g of coke per g of catalyst) was calculated from TG-TPO data, and profiles were deconvoluted in order to determine each fraction of coke (Table 4). The fraction of coke I is higher for SAPO-18 than for SAPO-34, which indicates that the coke deposited on SAPO-18 catalyst has more aliphatic nature and/or a higher proportion of it is deposited on the outside of the SAPO crystals, as it happened for HZSM-5 compared to SAPO-34 catalyst in similar reaction conditions [20]. The larger cages of SAPO-18 enable to form relatively larger molecules trapped within the microporous structure [55], However its crystalline

20

structure is more rigid than the one of SAPO-34, which can hold more coke II in its cages than the SAPO-18 catalysts [40]. On the other hand, the aforementioned results (Figs. 1 and 3) have proved that the SAPO-18 catalyst has less mass transfer limitations to allow the diffusion of coke precursors, normally identified as aromatics, leading to a slower deactivation [58].

600

460 ºC

20

500

450

550 ºC

520 ºC

-1

Ethylene SAPO-18

SAPO-34 400

350 1-Butene SAPO-18

Temperature (ºC)

-1

Derivative weigh loss (µg s g )

550

300

250 SAPO-34 0

10

20

30

40

50

60

70

80

Time (min)

Fig. 6. TPO-TGA profiles for the coke deposited on SAPO-18 and SAPO-34 catalysts used in the transformation of ethylene and 1-butene.

Although, the amounts of coke are quite similar for each reaction, the coke formation rate is quite different. Thus, we have calculated the average coke formation rate (rC) in each reaction as follows:

21

W C C    F0 rC  t   t  X i  dt

(4)

0

where CC is the total amount of coke in the catalyst (in g of coke per g of catalyst) Xi is the conversion of ethylene or 1-butene at t time. The calculation is extended to the total reaction time (300 min). Then, rC has units of mg of coke per moles of 1-butene or ethylene converted, per h. The calculated values of rC (Table 4) suggest that the average coke formation rate on the SAPO-18 catalyst is 44 and 6 times lower than that for SAPO-34 catalyst, when feeding ethylene or 1-butene, respectively. This lower coking rate observed for SAPO-18 is related to the fewer amount of acid sites present in this catalyst in comparison to SAPO-34. These values of rC are consistent with the trend of catalytic decay observed in Fig. 2. Table 4. Coke content of the spent catalysts after 5 h time on stream, fraction of coke I and average coke formation rate. Catalyst Ethylene SAPO-18 SAPO-34 1-Butene SAPO-18 SAPO-34

CC (gcoke (gcatalyst)-1)

Coke I (%)

rC (mgcoke molconverted-1 h-1)

0.039 0.046

22.0 10.7

41.8 1844

0.044 0.041

15.7 12.2

15.2 88.4

4. Conclusions We have compared the performance of SAPO-18 and SAPO-34 catalysts in the transformation of ethylene or 1-butene for intensifying propylene production. The synthesized SAPO-18 catalyst has ca. 31% more micropore area and ca. 41 % lower total acidity than SAPO-34 catalyst. The slight difference in shape selectivity and the lower acid

22

site density-strength, justifies partially the higher conversion of ethylene and 1-butene while the higher stability using the SAPO-18 catalyst. However, the much slower coke deactivation of this catalyst (SAPO-18) compared with the SAPO-34 should be attributed to the faster diffusion of aromatics within the former, escaping the micropores, leading to slower coke formation. Besides, the coke deposited on the SAPO-18 has a more aliphatic nature, so it is easier to be burned off, and less condensed towards aromatic structures that blocks the micropores and the acid sites. The differences between the SAPO-18 and -34 catalysts are more pronounced in the 1-butene conversion due to the fact that the remaining oligomerization-cracking activity of both catalysts is higher than for the ethylene conversion. These results encourage for preparing microporous SAPO-18 catalysts with hierarchical architectures for controlling coke deposition on the catalyst.

Nomenclature CC

Coke content on the catalyst, gcoke (gcatalyst)-1

dp

Average pore diameter, Å

F, F0 Molar flow rate in the outlet and inlet of the reactor, respectively, mol h-1 FP

Propylene molar flow rate, mol h-1

PA

Reactant partial pressure, bar

rC

Average coke formation rate, mgcoke molconverted-1 h-1

SBET

BET specific surface area, m2 (gcatalyst)-1

Si

Product selectivity, where is the product, %

T

Temperature, ºC

t

Time on stream, min

23

Vm

Micropore volume, m3 (gcatalyst)-1

Vp

Pore volume, m3 (gcatalyst)-1

W/F0 Space time, g h mol-1 X

Conversion, %

YP

Propylene yield, %

Acknowledgements This work was carried out with the financial support from the Ministry of Economy and Competitiveness (MINECO) of the Spanish Government (CTQ2013-46172-P, CTQ201019623 and CT1-19188 projects, partially cofounded by ERDF) and the University of the Basque Country (UFI 11/39). E. Epelde, M. Ibáñez and J. Valecillos are grateful for their Ph.D. grants: BFI08.122, BFI-2012-203 and BES-2014-069980, respectively. M. Ibáñez also is grateful for the postgraduate grant from the University of the Basque Country (No. UPV/EHU 2016). The technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, ESF) is gratefully acknowledged.

References [1] H. Koempel, W. Liebner, Fábio Bellot Noronha, M. Schmal, E. Falabella Sousa-Aguiar, Stud. Surf. Sci. Catal. 167 (2007) 261-267. [2] S.M. Sadrameli, Fuel. 140 (2015) 102-115. [3] S.M. Sadrameli, Fuel. 173 (2016) 285-297. [4] A.I. Hussain, A.M. Aitani, M. Kubů, J. Čejka, S. Al-Khattaf, Fuel. 167 (2016) 226-239. [5] P. Tian, Y. Wei, M. Ye, Z. Liu, ACS Catal. 5 (2015) 1922-1938. [6] D. Sanfilippo, I. Miracca, Catal. Today. 111 (2006) 133-139.

24

[7] M. Ibáñez, B. Valle, J. Bilbao, A.G. Gayubo, P. Castaño, Catal. Today. 195 (2012) 106113. [8] A.G. Gayubo, A. Alonso, B. Valle, A.T. Aguayo, M. Olazar, J. Bilbao, Fuel. 89 (2010) 3365-3372. [9] M. Ibáñez, M. Artetxe, G. López, G. Elordi, J. Bilbao, M. Olazar, P. Castaño, Appl. Catal. B. 148 (2014) 436-445. [10] E. Mazoyer, K.C. Szeto, N. Merle, S. Norsic, O. Boyron, J.M. Basset, M. Taoufik, C.P. Nicholas, J. Catal. 301 (2013) 1-7. [11] E. Mazoyer, K.C. Szeto, S. Norsic, A. Garron, J.M. Basset, C.P. Nicholas, M. Taoufik, ACS Catal. 1 (2011) 1643-1646. [12] H. Oikawa, Y. Shibata, K. Inazu, Y. Iwase, K. Murai, S. Hyodo, G. Kobayashi, T. Baba, Appl. Catal. A. 312 (2006) 181-185. [13] Y. Iwase, K. Motokura, T.R. Koyama, A. Miyaji, T. Baba, Phys. Chem. Chem. Phys. 11 (2009) 9268-9277. [14] X. Ding, C. Li, C. Yang, Energy Fuels. 24 (2010) 3760-3763. [15] W.H. Meyer, M.M.D. Radebe, D.W. Serfontein, U. Ramdhani, M.D. Toit, C.P. Nicolaides, Appl. Catal. A. 340 (2008) 236-241. [16] A. Coelho, G. Caeiro, M.A.N.D.A. Lemos, F. Lemos, F.R. Ribeiro, Fuel. 111 (2013) 449-460. [17] B. Lin, Q. Zhang, Y. Wang, Ind. Eng. Chem. Res. 48 (2009) 10788-10795. [18] J.K. Reddy, K. Motokura, T.R. Koyama, A. Miyaji, T. Baba, J. Catal. 289 (2012) 53-61. [19] S. Follmann, S. Ernst, New J. Chem. 40 (2016) 4414-4419. [20] E. Epelde, M. Ibañez, A.T. Aguayo, A.G. Gayubo, J. Bilbao, P. Castaño, Microporous Mesoporous Mater. 195 (2014) 284-293. [21] X. Zhu, S. Liu, Y. Song, L. Xu, Appl. Catal. A. 288 (2005) 134-142. [22] L. Lin, C. Qiu, Z. Zhuo, D. Zhang, S. Zhao, H. Wu, Y. Liu, M. He, J. Catal. 309 (2014) 136-145. [23] O. Bortnovsky, P. Sazama, B. Wichterlova, Appl. Catal. A. 287 (2005) 203-213. [24] Y.H. Guo, M. Pu, B.H. Chen, F. Cao, Appl. Catal. A: Gen. 455 (2013) 65-70. [25] N. Rahimi, R. Karimzadeh, Appl. Catal. A: Gen. 398 (2011) 1-17. [26] E. Epelde, A.T. Aguayo, M. Olazar, J. Bilbao, A.G. Gayubo, Appl. Catal. A: Gen. 479 (2014) 17-25. [27] E. Epelde, A.G. Gayubo, M. Olazar, J. Bilbao, A.T. Aguayo, Chem. Eng. J. 251 (2014) 80-91.

25

[28] G.L. Zhao, J.W. Teng, Z. Xie, W.M. Yang, Q.L. Chen, Y. Tang, Stud. Surf. Sci. Catal., 2007, pp. 1307-1312. [29] G. Zhao, J. Teng, Y. Zhang, Z. Xie, Y. Yue, Q. Chen, Y. Tang, Appl. Catal. A. 299 (2006) 167-174. [30] T.R. Koyama, Y. Hayashi, H. Horie, S. Kawauchi, A. Matsumoto, Y. Iwase, Y. Sakamoto, A. Miyaji, K. Motokura, T. Baba, Phys. Chem. Chem. Phys. 12 (2010) 25412554. [31] D. Chen, K. Moljord, A. Holmen, Microporous Mesoporous Mater. 164 (2012) 239-250. [32] J.Q. Chen, A. Bozzano, B. Glover, T. Fuglerud, S. Kvisle, Catal. Today. 106 (2005) 103-107. [33] M. Razavian, R. Halladj, S. Askari, Rev. Adv. Mater. Sci. 29 (2011) 83-99. [34] S. Askari, R. Halladj, M. Sohrabi, Microporous Mesoporous Mater. 163 (2012) 334-342. [35] G. Liu, P. Tian, Q. Xia, Z. Liu, J. Nat. Gas Chem. 21 (2012) 431-434. [36] S. Tian, S. Ji, D. Lü, B. Bai, Q. Sun, J. Energy Chem. 22 (2013) 605-609. [37] C. Duan, X. Zhang, R. Zhou, Y. Hua, L. Zhang, J. Chen, Fuel Process. Technol. 108 (2013) 31-40. [38] F. Schmidt, S. Paasch, E. Brunner, S. Kaskel, Microporous. Mater. 164 (2012) 214-221. [39] A.T. Aguayo, A.G. Gayubo, R. Vivanco, M. Olazar, J. Bilbao, Appl. Catal. A: Gen. 283 (2005) 197-207. [40] D.S. Wragg, D. Akporiaye, H. Fjellvåg, J. Catal. 279 (2011) 397-402. [41] J. Chen, J. Li, Y. Wei, C. Yuan, B. Li, S. Xu, Y. Zhou, J. Wang, M. Zhang, Z. Liu, Catal. Commun. 46 (2014) 36-40. [42] R. Wendelbo, D. Akporiaye, A. Andersen, I.M. Dahl, H.B. Mostad, Appl. Catal. A. 142 (1996) L197-L207. [43] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, J. Am. Chem. Soc. 106 (1984) 6092-6093. [44] J. Chen, J.M. Thomas, P.A. Wright, R.P. Townsend, Catal. Lett. 28 (1994) 241-248. [45] S. Mitchell, N.L. Michels, J. Pérez-Ramírez, Chem. Soc. Rev. 42 (2013) 6094-6112. [46] P. Pérez-Uriarte, M. Gamero, A. Ateka, M. Díaz, A.T. Aguayo, J. Bilbao, Ind. Eng. Chem. Res. 55 (2016) 1513-1521. [47] P. Castaño, J. Ruiz-Martinez, E. Epelde, A.G. Gayubo, B.M. Weckhuysen, ChemCatChem. 5 (2013) 2827–2831. [48] N.L. Michels, S. Mitchell, J. Pérez-Ramírez, ACS Catal. 4 (2014) 2409-2417.

26

[49] A.G. Gayubo, A. Alonso, B. Valle, A.T. Aguayo, J. Bilbao, Appl. Catal. B: Environ. 97 (2010) 299-306. [50] A. Buchholz, W. Wang, M. Xu, A. Arnold, M. Hunger, Microporous. Mater. 56 (2002) 267-278. [51] J. Datka, K. Góra-Marek, Catal. Today. 114 (2006) 205-210. [52] R.J. Quann, L.A. Green, S.A. Tabak, F.J. Krambeck, Ind. Eng. Chem. Res. 27 (1988) 565-570. [53] H. Zhou, Y. Wang, F. Wei, D. Wang, Z. Wang, Appl. Catal. A: Gen. 348 (2008) 135141. [54] M. Hunger, M. Seiler, A. Buchholz, Catal. Lett. 74 (2001) 61-68. [55] D.M. Marcus, W. Song, L.L. Ng, J.F. Haw, Langmuir. 18 (2002) 8386-8391. [56] M.A. Djieugoue, A.M. Prakash, L. Kevan, J. Phys. Chem. B. 104 (2000) 6452-6461. [57] M. Guisnet, L. Costa, F.R. Ribeiro, J. Mol. Catal. A: Chem. 305 (2009) 69-83. [58] T. Álvaro-Muñoz, C. Márquez-Álvarez, E. Sastre, Topics in Catalysis. 59 (2016) 278291. [59] M. Ibáñez, E. Epelde, A.T. Aguayo, A.G. Gayubo, J. Bilbao, P. Castaño, Appl. Catal. A: Gen. 543 (2017) 1-9. [60] C.H. Collett, J. McGregor, Cat. Sci. Technol. 6 (2016) 363-378. [61] M. Ibáñez, M. Artetxe, G. Lopez, G. Elordi, J. Bilbao, M. Olazar, P. Castaño, Appl. Catal. B: Environ. 148–149 (2014) 436-445. [62] J.M. Arandes, I. Torre, M.J. Azkoiti, P. Castaño, J. Bilbao, H. de Lasa, Catal. Today. 133 (2008) 413-419. [63] G. Elordi, M. Olazar, M. Artetxe, P. Castaño, J. Bilbao, Appl. Catal. A: Gen. 415-416 (2012) 89-95. [64] P. Castaño, A. Gutierrez, I. Villanueva, B. Pawelec, J. Bilbao, J.M. Arandes, Catal. Today. 143 (2009) 115-119. [65] A. Marcilla, A. Gómez-Siurana, F.J. Valdés, Appl. Catal. A: Gen. 352 (2009) 152-158. [66] J.W. Park, G. Seo, Appl. Catal. A. 356 (2009) 180-188.