Al(CH₃)₃-promoted Pt/MCM-41 catalysts for tetralin hydrogenation in the presence of benzothiophene and promotion mechanism of Al-promoted Pt/MCM-41 catalysts

Abstract

Al(CH₃)₃-promoted Pt–Al/MCM-41 catalysts with Al/Pt ratios from 0 to 20 were prepared for tetralin hydrogenation under sulfur-free and sulfur-containing conditions. NH₃-TPD and Py-FTIR results indicate that the amount of acid catalyst increases with increasing Al/Pt. The electron withdrawing effect of the Al-promoter decreases the electron density on the platinum particles and leads to the formation of electron deficient Pt^δ+. The isolation effect of the Al-promoter, which benefits platinum dispersion, plays a leading role at low Al/Pt, while the anchor effect, which leads to large platinum particles, dominates at high Al/Pt. Platinum dispersion increases at low Al/Pt and decreases at high Al/Pt. The catalyst with Al/Pt = 10 has the best platinum dispersion. All Al-promoted catalysts show much better tetralin hydrogenation activity and sulfur-tolerance than the Al-free catalyst, and the catalyst with Al/Pt = 10 is the best. The improvement in platinum dispersion is the primary factor that benefits both tetralin hydrogenation performance and sulfur tolerance. However, tetralin hydrogenation prefers less electron deficient platinum particles while sulfur tolerance favors more electron deficient Pt^δ+.

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1 Introduction

Reducing aromatic compounds in diesel fuel is an important process to increase the cetane number and to reduce the emission of particulates, NOx and polycyclic aromatic hydrocarbons (PAH).¹ Hydrodearomatization processes are typically used to reduce aromatic compounds. In contrast to hydrodesulfurization and hydrodenitrification, the reversible exothermic aromatic hydrogenation reaction is more favorable at mild reaction temperatures.^2,3 Generally, supported noble metal catalysts, which have a high aromatic hydrogenation activity at mild reaction conditions, are used in the production of low aromatic diesel.^2–4 However, the activity of noble metal catalysts is dramatically suppressed by the presence of sulfur-containing and nitride-containing compounds in the feedstock.

Efforts have been made to improve the sulfur tolerance of noble metal catalysts. It has been proved that supports with suitable acidity have positive effects on the hydrodearomatization activity and the sulfur tolerance of noble metal catalysts.^5–10 These effects have been attributed to the synergistic effect between the acid sites and the noble metal particles leading to the formation of electron-deficient metal centres (M^δ+),^9–11 the increase in the number of active sites provided by the acid sites, and the hydrogen spillover from the acid sites and the metal sites.^3,12–14 To better understand the interaction between the acid sites and noble metal particles, we have investigated the properties and performances of AlCl₃, Al(NO₃)₃ and Al(CH₃)₃ promoted Pt/MCM-41 catalysts by post-synthesis alumination and found that: (1) for the AlCl₃ promoted catalysts, the pre-grafting of AlCl₃ anchors the platinum around it and leads to the most electron-deficient Pt^δ+, while grafting AlCl₃ after the support of platinum offers an isolation effect which benefits platinum dispersion and leads to the formation of electron-deficient Pt^δ+;¹⁵ (2) the electron density of platinum particles decreases with increasing AlCl₃/Pt ratio, while the size of the platinum particles decreased first and then increased;¹⁶ (3) in contrast to Al(NO₃)₃ and AlCl₃, Al(CH₃)₃ has the best isolation effect and electron-donating methyl groups. Thus the Al(CH₃)₃-promoted catalyst has better platinum dispersion and is less electron-deficient than the Al(NO₃)₃- and AlCl₃-promoted catalysts;¹⁷ (4) tetralin hydrogenation is more favorable with a high platinum dispersion and less electron-deficient Pt particles, while sulfur tolerance is promoted by more electron-deficient Pt^δ+.^15–17

Due to their excellent promotion effect on the tetralin hydrogenation activity and the sulfur tolerance of the Pt/MCM-41 catalyst, further investigation on the effects of the Al/Pt ratio on the properties and performance of Al(CH₃)₃-promoted Pt/MCM-41 catalysts was performed in this work. The promotion mechanism of Pt/MCM-41 catalysts promoted by aluminium compounds is also summarized.

2 Experimental

The preparation, characterization and evaluation of the catalysts were similar to the procedure described previously.^15–17

2.1 Preparation of catalysts

MCM-41 (5 g) was placed in a three-neck flask equipped with mechanical stirrer. Hexane (50 mL) was added to disperse the MCM-41. An H₂PtCl₆–ethanol solution containing 0.05 g Pt (0.01 g Pt/g MCM-41) was added to the slurry under stirring. The slurry was maintained at 75 °C for 1 h, and then the hexane was vaporized under a nitrogen stream (200 mL min⁻¹). After this platinum support, another 50 mL hexane and the necessary amount of Al(CH₃)₃ solution was added and maintained at 75 °C for 1 h, then the solvent was vaporized under a nitrogen stream again. The obtained powder was pressed and sieved through a 16–20 mesh grain. Finally, the sample was dried at 110 °C for 2 h and calcined at 400 °C for 4 h. The obtained catalysts were labeled as Pt–xAl, where x = 0, 5, 10, 15 or 20, denoting the Al/Pt molar ratio.

2.2 Characterization of catalysts

The pore structure and specific surface area of the catalysts were measured by nitrogen adsorption–desorption isotherms on a Micromeritics TriStar 3000 analyzer at −196 °C. The multi-point Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area and the Barrett–Joyner–Halenda (BJH) model was used to calculate the average pore size and total pore volume.

The crystal structures of the catalysts were determined by powder XRD patterns on a Rigaku D/Max 2500 instrument with Cu Kα radiation (λ = 0.1541 nm) operated at 40 kV and 200 mA. Spectra were collected in the 2θ range 30–90°.

The morphology and size of the platinum particles were observed on a JEM-2100F transmission electron microscope (TEM) at 200 kV.

The NH₃-TPD profiles were obtained on a Quantachrome ChemBET TPR/TPD chemisorptions flow analyzer from 80 °C to 600 °C at a ramp rate of 15 °C min⁻¹ in a He atmosphere.

The Py-FTIR and CO-FTIR spectra were recorded on a Bruker Vertex 70 FTIR spectrometer in a transmission model with a resolution of 4 cm⁻¹. Self-supporting wafers of catalyst were placed in an IR cell with CaF₂ windows, in situ reduced with a H₂–N₂ mixture at 400 °C for 0.5 h (50 mL min⁻¹, 5% of H₂, ramp rate 2 °C min⁻¹), and then evacuated at 400 °C for 1 h. The sample was cooled to 100 °C and 30 °C before being exposed to pyridine vapor and CO, respectively. The Py-FTIR spectra were recorded after being degassed at 150 °C and 280 °C for 0.5 h. The CO-FTIR spectra were recorded after exposure to 2500 Pa CO for 30 min and after evacuation at 0.01 Pa for 20 min.

2.3 Catalyst evaluation

Catalytic activity and sulfur tolerance evaluation were performed on a continuous down-flow fixed bed reactor. The catalyst was loaded in the isothermal zone of the fixed bed reactor and reduced in situ with 120 mL min⁻¹ H₂ at 400 °C for 4 h (ramp rate: 2 °C min⁻¹). The reaction temperature and pressure were 280 °C and 5 MPa, respectively. Tetralin (20 wt%)–n-dodecane solution was supplied by a Series II piston pump at 0.3 mL min⁻¹ (0.26 g min⁻¹, WHSV = 52 h⁻¹). The hydrogen flow rate was 120 mL min⁻¹. Tetralin (20 wt%)–n-dodecane solution with 300 ppm benzothiophene (72 ppm sulfur) was used for the sulfur tolerance evaluation. The products were quantitatively analyzed on an Agilent 7890A GC equipped with an HP-PONA capillary column (50 m × 0.2 mm × 0.5 μm) and an FID detector.

3 Results

3.1 Textural properties of the catalysts

Table 1 lists the textural properties of the catalysts. The specific surface area, pore diameter and pore volume are similar to that of the MCM-41 support.

Table 1 Textural properties of the reduced catalysts^a

Catalyst	Al/Pt	Si/Al	S, m^2 g^−1	Dp, nm	Vp, cm^−3 g^−1	dPt,XRD, nm	dPt,TEM, nm
a S: specific surface area; Dp: pore diameter; Vp: pore volume; dPt,XRD: (111)/(200) face platinum particle diameter from the XRD line broadening using the Scherrer formula; dPt,TEM: average diameter of the platinum particles from the TEM micrographs.
MCM-41	—	—	912	3.89	1.04	—	—
Pt–0Al	0	∞	813	3.95	0.96	6.4/5.3	4.0
Pt–5Al	5	64	882.9	3.80	1.03	6.0/3.4	3.1
Pt–10Al	10	32	875.1	3.78	1.02	5.8/4.6	2.9
Pt–15Al	15	21	858.8	3.85	1.03	6.4/5.1	3.5
Pt–20Al	20	16	839.4	3.89	1.01	6.6/5.1	—

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Fig. 1 shows the XRD patterns of the catalysts in the 2θ = 30.0–90.0° range. The standard PDF card is drawn at the bottom of the figure. The face-centered cubic platinum (0) particle is detected in the catalyst. The good dispersion of platinum in the catalysts is indicated by the broadening of the diffraction peaks. The average sizes of nano-platinum particles are estimated from the (111) and (200) peaks using the Scherrer formula. The results are listed in Table 1. The nano-platinum particles can also be observed directly by TEM (Fig. 2). It is found that the size of the Pt particles firstly increases and then decreases as the Al/Pt ratio increases. The Pt–10Al catalyst possesses the smallest platinum particles.

Fig. 1 XRD patterns of the catalysts.

Fig. 2 TEM micrographs and platinum particle size distributions of the catalysts.

3.2 Acidity of the catalyst

The acidities of the catalysts were investigated by NH₃-TPD and Py-FTIR. Fig. 3 shows the NH₃-TPD profiles of the catalysts. The NH₃ desorption peak below 300 °C which is observed for all the catalysts and the MCM-41 support is ascribed to hydrogen-bonded NH₃ on silanol.^18,19 The NH₃-TPD profile of the Pt–0Al catalyst is similar to that of MCM-41, but has a slightly higher signal above 300 °C. This difference indicates that the platinum and residue chlorine create a small number of acid sites. As indicated by the signal intensity, the number of acid sites increases with increasing Al/Pt.

Fig. 3 The NH₃-TPD profiles of the support and catalysts.

Fig. 4A and C show the Py-FTIR spectra of the catalysts. The bands at about 1445 and 1595 cm⁻¹ indicate the silanol or hydroxyl group, those at 1453, 1575 and 1619 cm⁻¹ indicate the Lewis acid sites while 1542 and 1637 cm⁻¹ indicate the Brønsted acid sites. The peak at 1489 cm⁻¹ has contributions from three types of adsorption site.^19–21 The Al-free catalyst show some silanol bonded pyridine (1445 and 1595 cm⁻¹) and very few Lewis and Brønsted acid sites. When Al(CH₃)₃ is added, it reacts with the surface silanol and grafts onto the MCM-41. At low Al/Pt ratio (Al/Pt = 5), the Al–CH₃ bond may be hydrolysed which leads to more hydroxyl-aluminium, as indicated by the increase of the peaks at 1445 and 1595 cm⁻¹. As the Al/Pt ratio increases, the adjacent hydroxyl-aluminium dehydrates and results in the formation of Lewis and Brønsted acids. At the same time, the amount of hydroxyl decreases. This is indicated by the shift of the 1445 cm⁻¹ peak to 1453 cm⁻¹ and the increase in the peaks at 1489, 1542, 1619 and 1637 cm⁻¹.

Fig. 4 Py-FTIR spectra and amount of acid catalyst at (A and B) 150 °C and (C and D) 280 °C.

Fig. 4B and D shows the relative amount of acid sites. The values are calculated using the method provided by Emeis.²²

3.3 FTIR spectra of adsorbed CO

The FTIR spectra of adsorbed CO were collected to investigate the properties of the nano-platinum particles (Fig. 5). Two absorbance bands can be ascribed to CO adsorbed onto zero-ordered platinum atoms: the one at about 2080 cm⁻¹ is attributed to CO forming a linear bond to platinum (Pt⁰–CO) and the other one between 1800 and 1900 cm⁻¹ is attributed to CO bridging and adsorbing onto the platinum (Pt⁰–CO–Pt⁰).^13,23–26 The peak areas, which are related to the amount of adsorbed CO and roughly reflect the number of accessible platinum atoms,^27,28 are listed in Table 2. Peak positions are also listed. As the Al/Pt ratio increases, the area of the linearly-bonded CO band increases at low Al/Pt ratio and then decreases as the Al/Pt ratio is further raised. The catalyst with Al/Pt = 10 has the best Pt dispersion, which is consistent with the XRD and TEM results. There are more defect Pt atoms (corners, edges and kink sites) in the well dispersed catalyst and these defected platinum atoms bring a low vibration frequency to the linearly-bonded CO band (Table 2).^29,30 Hence, the peak position shifts from 2090.69 to 2084.98 and then to 2086.73 cm⁻¹ as the Al/Pt ratio increases.

Fig. 5 CO-FTIR spectra of the catalysts.

Table 2 Peak positions and integrated areas of linearly-bonded CO

Al/Pt	Saturated		After evacuation		Red shift, cm^−1
	Position, cm^−1	Integrated area	Position, cm^−1	Integrated area
0	2090.69	2.212	2079.31	0.331	−11.38
5	2085.25	3.484	2079.08	0.216	−6.17
10	2084.98	3.691	2078.36	0.217	−6.62
15	2086.43	2.804	2080.16	0.156	−6.27
20	2086.73	2.641	2079.18	0.173	−7.55

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Fig. 5B shows the FTIR spectra after CO evacuation. A very weak shoulder peak at about 2123 cm⁻¹ is observed for the Al-free catalyst. This peak is shifted to 2124, 2125, 2140 and 2165 cm⁻¹ and the integrated area increases with increasing Al/Pt. It is commonly agreed that the band from 2100 to 2170 cm⁻¹ can be assigned to CO bonded to electron-deficient Pt^δ+ atoms.^25,31–33 Moreover, high-frequency bands in the CO stretching region is characteristic of the high electron-deficient state of platinum. Thus it is reasonable to conclude that the electron density of platinum decreases with the increase in Al/Pt, and that the number of electron-deficient platinum atoms increases as well.

Additionally, a shoulder band can be observed at about 1960 cm⁻¹ for the catalysts with a high Al/Pt ratio. This band may be ascribed to that the interaction between platinum atoms and the acid sites leading to the formation of Pt-carbonyl-hydride ().³⁴

3.4 Catalytic performance

Fig. 6 shows the tetralin conversions in sulfur-free and sulfur-containing conditions. The pseudo-first-order rate constants (k = −WHSV ln(1 − x)) in the stable states (3 h, 11 h and 15 h, k₁–k₃, respectively) are listed in Table 3. The relative rate constants, relative to the reaction rate constants of the Al-free catalyst, were also calculated. The tetralin conversion increases with the increase of Al/Pt at low Al/Pt, and then decreases at high Al/Pt. The catalyst with Al/Pt = 10 has the best catalytic activity. The pseudo-first-order rate constant of this catalyst is 5.57 times higher than the Al-free one under sulfur-free conditions. This value rises to 14.27 under sulfur-containing conditions. The other Al-containing catalysts are much better than the Al-free one too. Furthermore, the relative rate constants of the Al-containing catalysts under sulfur-containing conditions (k₂′) and after the sulfur-containing conditions (k₃′) were much higher than under sulfur-free conditions (k₁′), which implies that the Al-containing catalysts have much better sulfur tolerance than the Al-free catalyst.

Fig. 6 Tetralin conversion in the presence and absence of 300 ppm benzothiophene (72 ppm sulfur). Reaction conditions: 280 °C, 5 MPa, 0.3 g catalyst, 120 mL min⁻¹ H₂, 0.3 mL min⁻¹ liquid (52 h⁻¹).

Table 3 Pseudo-first-order rate constants of the catalysts in the stable states^a

Al/Pt	Reaction rate constants, ×10^−3 s^−1			Relative rate constant
	k1	k2	k3	k1′	k2′	k3′
a Note: k = −WHSV ln(1 − x). k1: sulfur-free at 3 h; k2: 300 ppm benzothiophene (72 ppm sulfur) at 11 h; k3: sulfur-free at 15 h.
0	11.96	0.49	4.03	1.00	1.00	1.00
5	63.16	4.66	35.51	5.28	9.43	8.80
10	66.67	7.06	39.93	5.57	14.27	9.90
15	34.50	2.81	18.74	2.88	5.69	4.64
20	34.13	2.14	16.78	2.85	4.33	4.16

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4 Discussion

Our previous studies investigated the effects of the Al–Pt interaction sequence, the Al/Pt ratio using AlCl₃, and the aluminum promoter type on the performance of Pt–Al/MCM-41 catalysts. The promotion mechanism of aluminum promoters on the properties and performances of Pt/MCM-41 catalysts is discussed and summarized below.

4.1 Catalytic properties

As previously discussed, Al-promoters have an anchoring effect, an isolation effect, and an electron-withdrawing effect which affect the formation of platinum particles. Al-promoters also provide acid sites and hydroxyl groups and favour the spillover of hydrogen (Fig. 7).^15–17

Fig. 7 Interaction mechanism between the Al-promoter and Pt.

4.1.1 Isolation and anchoring effects and their role in platinum dispersion. Al-promoters can react with silanols which are on the surface of MCM-41 and anchor onto the support.^35–37 This anchored Al-promoter acts likes a barrier which surrounds the platinum compound and confines them to a definite region. Hence, the platinum is prevented from sintering and agglomerating and platinum dispersion is improved. At the same time, the homogeneous MCM-41 surface is destructed by the anchored Al-promoter. The property of the aluminum site is more similar to a platinum compound than a silicon site. Thus the platinum compound may be attracted to the aluminum sites and anchored onto them. In contrast to the isolation effect, the anchor effect leads to large platinum particles and slows down platinum dispersion. For specific samples, the effect of the Al-promoters can be discussed based on the XRD, TEM and CO-FTIR results. (1) The attraction effect dominates platinum particle formation in the sample in which AlCl₃ is grafted first, thus this catalyst has the lowest platinum dispersion. (2) The reactivity of the aluminum compounds decreases in the order Al(CH₃)₃ > AlCl₃ > Al(NO₃)₃, thus Al(CH₃)₃ has the best isolation effect while the Al(NO₃)₃ has the worst. Consequently, the platinum dispersion at the same Al/Pt ratio for the promoted catalysts is Al(CH₃)₃ > AlCl₃ > Al(NO₃)₃. (3) Both the isolation and anchor effects are strengthened when the Al/Pt ratio is increased. At high Al/Pt, AlCl₃ and Al(CH₃)₃ probably exist as the dimers Al₂Cl₆ and Al₂(CH₃)₆. These anchored aluminum dimers have a strong attraction effect on platinum which promotes the anchor effect. Thus, the isolation effect dominates at low Al/Pt while the anchor effect dominates at high Al/Pt. As a result, platinum dispersion increases at low Al/Pt and decreases when Al/Pt is further increased. (4) Al(CH₃)₃ is much more reactive with silanol than AlCl₃ is. For this reason, the isolation effect plays a leading role in a wider Al/Pt range for the Al(CH₃)₃-promoted catalysts. As a result, the optimal Al/Pt of the Al(CH₃)₃-promoted catalyst is 10 while the optimal Al/Pt of the AlCl₃-promoted catalyst is 2.

4.1.2 Electron-withdrawing effect and its role on the formation of Pt^δ+. The aluminum sites exist as Lewis acids, Brønsted acids or Al³⁺ ions. These sites can withdraw electrons from the adjacent platinum and lead to the formation of electron-deficient Pt^δ+. The residual –Cl, –NO₃ and –CH₃ in the catalyst may also affect the electron density of the platinum particles.^23,24 The electron density of the platinum particles is reflected in the band shift of the linearly-bonded CO and the band above 2100 cm⁻¹ in the CO-FTIR spectra. For the specific samples: (1) the pre-grafted AlCl₃ anchors the platinum particle onto it, thus the platinum particles are more electron-deficient in this catalyst than when AlCl₃ is grafted after the platinum support; (2) Al(NO₃)₃ is an electrovalently bonded compound in which aluminum behaves as an Al³⁺ ion; AlCl₃ is a covalently bonded compound with the electron-accepting element, chlorine; Al(CH₃)₃ is a covalently bonded compound with electron-donating methyl groups. The electron-withdrawing effect is Al(NO₃)₃ > AlCl₃ > Al(CH₃)₃, therefore the electron density of the Pt particles in the catalysts should increase in the sequence Al(NO₃)₃-promoted < AlCl₃-promoted < Al-free ≈ Al(CH₃)₃-promoted; (3) the electron-withdrawing effect is strengthened when the Al/Pt is increased.

4.1.3 Other effects. The Al-promoters provide Lewis and Brønsted acid sites for the catalyst.^35–37 The acid amount increases with the increase in Al/Pt. AlCl₃ increases the Lewis acidity greatly while Al(CH₃)₃ leads to more hydroxyl groups at low Al/Pt and a greater number of Lewis and Brønsted acid sites at high Al/Pt. Additionally, the dissociated hydrogen from platinum may migrate onto the acid sites. As a result, hydrogen spillover is enhanced.

4.2 Catalytic performances

Pseudo-first-order rate constants under sulfur-free and sulfur-containing conditions are plotted against the integrated area of the linearly-bonded CO band in Fig. 8.

Fig. 8 First-order reaction rate constants of tetralin hydrogenation vs. integrated area of the linearly-bonded CO band. (A) Sulfur-free conditions (solid), sulfur-free after sulfur-containing conditions (open); (B) with 300 ppm benzothiophene in the feedstock (solid: low WHSV, open: high WHSV).

4.2.1 Catalytic activity. A roughly linear relationship between the logarithmic pseudo-first-order rate constant and the integrated area of the linearly-bonded CO band is observed in Fig. 8. Since the area of linearly-bonded CO band relates to the platinum dispersion, it can undoubtedly be concluded that platinum dispersion is the primary factor which determines the activity of the catalyst.^17,38,39

By contrasting the catalysts with similar platinum dispersions, it can be found that: (1) the Al(CH₃)₃-promoted catalysts have much better catalytic activity than the AlCl₃-promoted ones; (2) the AlCl₃-promoted catalysts are better than the Al(NO₃)₃-promoted ones; (3) the Al-free catalyst is better than the Al(NO₃)₃-promoted one. Combining this with the electron-withdrawing effect of the Al-promoters discussed above, it can be concluded that tetralin hydrogenation is more favourable with less electron-deficient platinum particles.

Additionally, the acid sites and hydroxyl groups provide more active sites and favour hydrogen spillover which may also contribute to hydrogenation activity.^40,41

4.2.2 Sulfur tolerance. As previously discussed, benzothiophene is more adsorbable onto the platinum site than tetralin due to its benzene ring, thiophene ring and electronegative sulfur atom.^{15–17,42,43} The deactivation of the catalyst is mainly caused by the competitive adsorption of benzothiophene which hinders the adsorption and hydrogenation of tetralin. Therefore, good sulfur tolerance can be achieved if the hydrogen desulfurization activity can be improved. In other words, a low residual benzothiophene concentration in the product implies the catalyst should probably have good sulfur resistance. The residual benzothiophene concentrations under stable conditions are listed in Table 4.

Table 4 The residual benzothiophene (BT) concentrations under stable conditions

Promoter	Al/Pt, mol/mol	BT in product, ppm	Promoter	Al/Pt, mol/mol	BT in product, ppm
a For detailed reaction conditions see: ref. 16.b For detailed reaction conditions see: ref. 17.c For detailed reaction conditions see: ref. 18.d For detailed reaction conditions see: Fig. 6.e Platinum was supported after the grafting of AlCl3.f Platinum was supported before the grafting of AlCl3.
None^a	0	39.3	Al(NO3)3^c	2	13.2
AlCl3^a^,^e	0.8	15.4	AlCl3^c	2	8.7
AlCl3^a^,^f	0.8	6.3	Al(CH3)3^c	2	4.5
AlCl3^b	0	27.2	Al(CH3)3^d	0	39.8
	1	15.5		5	19.1
	2	8.7		10	12.8
	4	15.2		15	13.5
	8	16.6		20	16.3

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The primary factor that determines the sulfur tolerance of the catalyst is also the platinum dispersion. As can be observed in Fig. 8B, the pseudo-first-order rate constant increases accordingly with the increase in platinum dispersion under sulfur-containing conditions. The reason is that a high platinum dispersion provides more active sites for both tetralin hydrogenation and benzothiophene hydrogenation. As shown in Table 4, the residual benzothiophene concentration is 8.7 at Al/Pt = 2 and 12.8 at Al/Pt = 10 for the AlCl₃- and Al(CH₃)₃-promoted catalysts, respectively. These two catalysts are also the highest platinum dispersion catalysts for the AlCl₃- and Al(CH₃)₃-promoted catalysts, respectively.

The electron density of the platinum particles also affects the sulfur tolerance of the catalyst. As can be observed in Fig. 8, although the Al(NO₃)₃-promoted catalyst and the high Al/Pt ratio AlCl₃-promoted catalysts have low or similar pseudo-first-order rate constants as the Al-free catalyst under sulfur-free conditions, they have higher pseudo-first-order rate constants than the Al-free catalyst under sulfur-containing conditions. Table 3 also shows that the relative rate constants of all the Al-containing catalysts under sulfur-containing conditions (k₂′) and after the sulfur-containing conditions (k₃′) are much higher than under sulfur-free conditions (k₁′). The reasons are that (1) the platinum particles in the Al-containing catalyst are more electron-deficient than those in the Al-free one; (2) the electron-deficient Pt^δ+ strengthens the adsorption of electronegative benzothiophene; and (3) the Pt^δ+ pulls the electron density of benzene and thiophene rings, thereby destabilizing the rings and promoting the hydrogenation of the thiophene ring and the scission of the S–C bond. As a result, high benzothiophene hydrogenation activity is achieved for the electron-deficient Pt^δ+ catalyst. This is verified by the residual benzothiophene concentration listed in Table 4. All Al-containing catalysts have a much lower residual benzothiophene concentration than the Al-free one. When the benzothiophene concentration is brought down, the tetralin hydrogenation activity is improved. In summary, the sulfur tolerance of the catalyst is favoured by electron-deficient Pt^δ+.^{8–10,15–17}

Additionally, similar to tetralin hydrogenation, benzothiophene can be adsorbed onto the acid sites and the hydroxyl sites and then be hydrogenated by the spillover hydrogen. Therefore, acid sites might also contribute to the sulfur tolerance of the catalysts.^{5,7,12,14,40,41}

5 Conclusions

The Al(CH₃)₃ promoter affects the platinum dispersion, decreases the electron density of the platinum particles, provides additional acid sites and favors hydrogen spillover which benefits the tetralin hydrogenation activity and the sulfur tolerance of the Pt/MCM-41 catalysts. The pseudo-first-order rate constants of the catalyst with a Al/Pt = 10 are 5.57 and 14.27 times as high as that of the Al-free catalyst under sulfur-free and sulfur-containing conditions, respectively. The improvement in platinum dispersion, which is mainly attributed to the isolation and anchoring effects of the Al-promoters, is the primary factor that benefits both the tetralin hydrogenation performance and the sulfur tolerance of the Al-promoted catalysts. The formation of electron-deficient Pt^δ+, caused by the electron-withdrawing effects of the Al-promoters, enhances the sulfur tolerance while reducing the tetralin hydrogenation activity to some extent.

Acknowledgements

The authors acknowledge the support of the Analysis Center of Tianjin University for the characterization of the samples.

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