SYNTHESIS OF 2,6-DIMETHYLNAPHTHALENE OVER SAPO-11, SAPO-5 AND MORDENITE MOLECULAR SIEVES

Abstract

Shape-selective methylation of naphthalene over SAPO-11, SAPO-5 and mordenite molecular sieves were carried out in a fixed-bed flow reactor under atmospheric pressure. Methanol and mesitylene were used as methylation and solvent agents. The experiment results showed that SAPO-11 exhibited higher stability, higher selectivity of 2,6-dimethylnaphthalene (2,6-DMN) and higher 2,6-/2,7-DMN ratio than SAPO-5 and mordenite molecular sieves. The catalytic performances for the methylation of naphthalene were mainly related to the pore structure of the catalysts. The comparison of the spent SAPO- 11 with fresh SAPO-11 suggested that structure collapse of the SAPO-11 by dealumination was occurred during the methylation of naphthalene with methanol, which may have been caused by high temperature steam from water produced in the reaction or by high temperature methanol vapor.

INTRODUCTION

2,6-Dialkylnaphthalene (2,6-DKN) is an important product in industry, which can be oxidized to 2,6-naphthalene dicarboxylic acid (2,6-NDA) and then to prepare the commercially valuable polymer of poly(ethylene naphthalene dicarboxylate) (PEN). Compared with poly(ethylene terephthalate) (PET), in addition to its excellent properties as a gas barrier, PEN has good thermal and chemical stability as well as mechanical properties, leading to its wide application in electronics components, insulation material, food containers, aviation, and so on (Song et al., 1993; Tsutsui et al., 2004; Wu et al., 2010; Wu et al., 2015). Highly regioselective synthesis of 2,6-DKN can be achieved over different molecular sieve catalysts using various alkylating agents and solvents (Brzozowski et al., 2012; Liu et al., 1997; Smith et al., 2000; Smith et al., 2003; Smith et al., 2012; Song et al., 2000; Sugi et al., 2008). Liu et al. (1997) reported that the alkylation of naphthalene with t-butanol should be both practical and attractive not only because of the high activity, but also due to the easy separation of the desired product from the reaction mixtures by crystallization. Smith et al. (2000) surveyed the dialkylation of naphthalene over a dealuminated mordenite molecular sieve using tert-butanol as an alkylating agent. The experimental results showed that a 60% yield of 2,6-di-tert-butylnaphthalene was obtained with a 2,6/2,7-ratio of over 50. Sugi et al. (2008) reported that the channels of molecular sieves with three dimensional pore-system were too large to differentiate 2,6-DAN from other β,β-DAN by restriction of their transition states in the channels even with bulky alkylating agents of 1-butene and 2-methylpropene.

Among 2,6-DKNs, 2,6-dimethylnaphthalene (2,6-DMN) is regarded as the most suitable raw material for the synthesis of PEN because no carbon atoms of 2,6-DMN are lost in the oxidation reaction and 2,6-DMN can be oxidized easily compared with other 2,6-DKNs (Pu et al., 1996). At present, 2,6-DMN is mainly produced by BP-Amoco through a four-step process from o-xylene and butadiene, but the process is quite complex (Lillwitz et al., 2001). In order to establish a simple and “green” synthesis route of 2,6-DMN with fewer reaction steps and less reaction waste, it is desirable to methylate naphthalene to 2,6-DMN directly over a shape-selective molecular sieve catalyst.

In recent years, methylation of naphthalene over molecular sieve catalysts to synthesize 2,6-DMN was reported by several research groups. As a result, there were great differences in the catalytic activity and selectivity among these molecular sieves (Fraenkel et al., 1986; Park et al., 2002; Jin et al., 2006; Yoo et al., 2002; Jin et al., 2006; Wu et al., 2010). For example, Fraenkel et al. (1986) first investigated the methylation of naphthalene with methanol over ZSM-5, mordenite and HY molecular sieves, and found that the medium pore size ZSM-5 showed a high product selectivity and moderate activity, while the large pore size molecular sieves, HY and mordenite, providing enough space for multiple reactions, would lead to a lower product selectivity and a higher activity. Park et al. (2002) reported that the selectivity for 2,6+2,7-DMN was only 16% using MCM-22 as catalyst after 1 hour time-on-stream (TOS). Yoo et al. (2002) reported that the high stability of ZSM-12 molecular sieves during the methylation was because ZSM-12 possessed one dimensional non-interpenetrating channels which behaved as “perfect tubes” and did not lead to the accumulation of coke precursors. Jin et al. (2006) reported that Zr/(Al)ZSM-5 could improve the selectivity of 2,6-DMN and the stability of the catalyst. They attributed it to the weakening of the acid strength and the enlargement of the pore dimensions because of the part incorporation of Zr in the framework instead of Al. Wu et al. (2010) concluded that the small dimensions of zeolites crystals and the advanced mesoporosity were the general reasons for the remarkable catalytic action of MTW zeolites in naphthalene methylation. Among these molecular sieves, SAPO-11 molecular sieve has been proved to show high catalytic performances in the methylation of naphthalene with methanol (Liu et al., 2013; Wang et al., 2012; Wang et al., 2013; Wang et al., 2015). First of all, SAPO-11 is a one-dimensional pore zeolite with pore opening of 0.39 nm × 0.64 nm (Liu et al., 2015; Wu et al., 2015; Zhang et al., 2007), which is between the pore size of large-pore molecular sieves and the pore size of medium-pore molecular sieves, making it an interesting material for catalytic cracking, reforming and alkylation (Subramanian et al., 1997; Prakash et al., 1996; Han et al., 2014; Liu et al., 1991; Rabaev et al., 2015; Tian et al., 2014; Wang et al., 2015). Similar to ZSM-12, SAPO-11 also possesses one dimensional and non-intersecting channels (Zhang et al., 2010), which shows excellent resistance to deactivation by carbonaceous deposition.

Based on these reports, it is concluded that the catalytic activity for the methylation of naphthalene has relation with the acidity and the structure of molecular sieves. However, the effect of the acidity and the structure of molecular sieves on the naphthalene methylation with methanol have not been understood clearly, which hinders the development of a commercial catalyst for naphthalene methylation. In addition, the research on deactivation of SAPO-11 molecular sieve during methylation of naphthalene with methanol has not been reported. Meanwhile, there are few studies about the methylation of naphthalene with methanol over SAPO-5 molecular sieve so far.

In the present work, SAPO-11, SAPO-5 and mordenite were evaluated for the methylation of naphthalene with methanol, respectively. And they were investigated in detail by using ammonia temperature programmed desorption (NH₃-TPD), infrared spectroscopy with pyridine adsorption (Py-IR) and N₂ adsorption techniques in order to study the impact of the pore structure and acidity of molecular sieves on catalytic performance in the methylation of naphthalene with methanol. In addition, the fresh and the spent SAPO-11 molecular sieves have been characterized by different characterization methods to analyze the cause of deactivation of the SAPO-11.

EXPERIMENTAL

Catalyst Preparation

Mordenite was obtained from the Catalyst Plant of Nankai University. The acidic form of the molecular sieve was prepared by ion-exchange with aqueous NH₄NO₃ solution and then calcination at 550 ºC for 2 h in air.

SAPO-5 and SAPO-11 were synthesized hydrothermally. Pseudoboehmite (75 wt% Al₂O₃), orthophosphoric acid (85% H₃PO₄) and silica sol (30 wt% SiO₂) were used as source of Al, P and Si. Di-n-propylamine (DPA) was used as the template.

SAPO-11 was synthesized by hydrothermally crystallizing a sol-gel mixture with a composition of 1.0Al₂O₃:1.0P₂O₅:0.6SiO₂:1.2DPA:49H₂O (Blasco et al., 2006). The final crystallization temperature of 180 ºC and crystallization time of 20-28 h were employed. The products were washed with distilled water, then dried at 120 ºC for 5 h and calcined at 600 ºC for 4 h.

SAPO-5 was synthesized by a hydrothermal method from a gel molar composition of 1.0Al₂O₃: 1.1P₂O₅:0.66SiO₂:1.6DPA:40H₂O. The gel mixture was transferred into a stainless steel autoclave, heated in an oven at 200 ºC for 6 h, and subsequently at 300 ºC for 9 h. The product thus obtained was washed, dried at 120 ºC for 6 h, and calcined in air at 550 ºC for 24 h to remove the template completely.

Catalyst Characterization

X-ray powder diffraction (XRD) analysis was performed on RigakuD/maxrB X-ray diffractometer. Diffraction patterns were recorded with Cu Ka radiation at 40 kV and 100 mA in the scan range between 5º and 50º to identify the phase structure of the catalyst.

The morphologies of the samples were examined by Hitachi S-4800 scanning electron microscopy (SEM).

The textural properties of the samples were derived from N₂ adsorption-desorption measurement on Micromeritics Tristar 3000. In each case, the sample was outgassed under vacuum at 300 ºC for 3 h before N₂ adsorption.

The acidity of the samples was examined by temperature-programmed desorption of ammonia (NH₃-TPD) techniques carried out by a flow system with a thermal conductivity detector. All samples were preheated from room temperature to 500 ºC in an argon flow and kept at 500 ºC for 1 h, which was followed by NH₃ saturation in a flowing NH₃/Ar stream at 40 ºC for 5 min. Evacuation at 40 ºC for 40 min was carried out to remove physically adsorbed NH₃ then the catalyst was heated to 600 ºC at a linear rate of 10 ºC·min^-1, and the detector signal of NH₃ was recorded.

The acid sites of the catalyst samples were characterized by FT-IR (EQUINOX55) spectroscopy with chemisorbed pyridine.

The thermogravimetric analysis and differential thermal analysis (TG-DTA) were recorded on a Rigaku Thermo plus Evo TG 8120 instrument at a heating rate of 10 ºC·min^-1 in air with a flow rate of 30 ml·min^-1.

The total (bulk) Si and Al contents in the catalysts were determined by an Inductive Couple Plasma (ICP) emisson spectrometer (Thermo ICAP6300). The SAPO-11 catalyst was subjected to themogravimetric analysis both before reaction (fresh) and after reaction (spent).

Catalytic Evaluation

The experiments were performed in a fixed-bed continuous-flow reactor equipped with 20 mm diameter and 600 mm length stainless steel tube. 2.5 g of 20-40 mesh molecular sieve catalysts were loaded in the reaction tube. After preheating, the reaction mixture including naphthalene, methanol and mesitylene (solvent) in a molar ratio of 1:5:3.5 was fed into the reactor by a quantity measuring pump under the pressure kept by N₂. The weight hourly space velocity (WHSV) of naphthalene was 0.19 h^-1 in all experiments, and the reactive temperature was 350 ºC. Reaction products were analyzed by gas chromatography (GC9560) with a Beta-Dex120 capillary column.

The naphthalene conversion was calculated as follows:

$\mathrm{naphthalene}\mathrm{conv}.\left(\%\right)=\left(\frac{{\mathit{n}}_{\mathit{N},0}-{\mathit{n}}_{\mathit{N}}}{{\mathit{n}}_{\mathit{N},0}}\right)\times 100\%$

where n_N,0 and n_N are the molar percentage of naphthalene before and after the reaction. Product distribution includes the corresponding molar percentages of ethylnaphthalene (EN), methylnaphthalene (MN), dimethylnaphthalene (DMN) and trimethylnaphthalene (TMN) in the total product mixture. The selectivity of 2,6-DMN and 2,7-DMN is the corresponding molar percentage in the sum of all DMN isomers, respectively. 2,6-/2,7-DMN stands for the molar ratio of 2,6-DMN to 2,7-DMN. The 2,6-DMN yield was calculated by the following equation.

2,6-DMN yield (%) = (naphthalene conversion × 2,6-DMN distribution) /100%.

RESULTS AND DISCUSSION

Characterization of Catalysts

XRD patterns of as-synthesized SAPO-5 and SAPO-11 samples presented in Figure 1 show the presence of highly crystalline SAPO phases and no impurity phases are detected (see Figure 1). The characteristic peaks of the SAPO-11 phase (i.e. 2θ = 8. 15º, 9. 40º, 13. 20º, 15. 57º) are observed and are identical to those reported for SAPO-11 in the literature (Lok et al., 1984). Also the positions of the lines of the SAPO-5 are identical to those reported for SAPO-5 (Wei et al., 2015; Zhu et al., 2016). It is a well-crystallized sphere-shaped material as determined by the SEM studies (see Figure 2).

According to the reference materials (Zhang et al., 2007; Chao et al., 2000; Bandyopadhyay et al., 2002), the physicochemical properties of the investigated molecular sieves are summarized in Table 1. The BET surface area and the pore volume of SAPO-11, SAPO-5 and Mordenite molecular sieves are determined by N₂ adsorption-desorption measurement. As observed in Table 1, the surface area and the pore volume of the investigated molecular sieves decrease in the order of Mordenite > SAPO-5 > SAPO-11.

Figure 1
XRD Patterns of SAPO-11 sample and SAPO-5 sample.

Figure 2
SEM images of SAPO-11 sample (a) and SAPO-5 sample (b).

Table 1
Physicochemical properties of different molecular sieves.

Molecular sieves	SiO2/Al2O3	Topology①	Channelstructure②	Pore Opening /nm③	BET surfacearea/ (m2/g)	Porevolume/(cm3/g)
Mordenite	25	MOR	Unidimensional	0.65×0.70 (001)	318	0.176
SAPO-5	-	AFI	Unidimensional	0.73×0.73 (001)	238	0.160
SAPO-11	-	AEL	Unidimensional	0.39×0.64 (001)	216	0.145

• et al;
• ^②
  Chao et al., 2000;
• ^③
  et al.

The acidic properties are usually evaluated by NH₃-TPD, and the profiles can be differentiated both in the integral area of the profiles and in the shift of peak temperature. The former corresponds to the amount of acid sites, and the latter indicates the strength of the acid sites. The NH₃-TPD profiles of different catalyst samples illustrated in Figure 3 present three ammonia desorption peaks at ca. 210 ºC, 300 ºC and 510 ºC, corresponding to a weak acid site, a medium strong acid site and a strong acid site. The acidity values of different molecular sieve catalysts are shown in Table 2. From Figure 3 and Table 2, it can been seen that the total acidity of the molecular sieves and the acid strength of their acidic sites both decrease in the order of Mordenite > SAPO-11 > SAPO-5.

Table 2
Acidity values of different molecular sieves.

Sample	weak acid sites		Medium strong acid sites		strong acid sites
	Peak temperature / ºC	The acid amount/ (mmol g-1)	Peak temperature/ ºC	The acid amount/ (mmol g-1)	Peak temperature/ ºC	The acid amount/ (mmol g-1)	The total acid amount/ (mmol g-1)
SAPO-5	210	0.55	0	0	0	0	0.55
SAPO-11	210	0.47	288	0.28	0	0	0.76
Mordenite	213	0.68	0	0	516	11.2	11.88

Pyridine-IR is used to further evaluate the type (Lewis and Bronsted) of acid sites for all the samples. The Bronsted acid sites of the investigated molecular sieves will protonate pyridine, forming the pyridinium ion, which has a characteristic ring vibrational frequency at 1540 cm^-1. Lewis aluminum, being an electron-pair acceptor, can bind pyridine in a covalent fashion, giving rise to a vibrational band at 1450 cm^-1. The relative amounts of Bronsted and Lewis acid sites can be estimated by integration of these two bands after correction for differences in oscillator strengths (Harris et al., 2016; Nieminen et al., 2004). The concentration of Bronsted and Lewis acid sites after pyridine desorption at 200 ºC and 400 ºC are presented in Table 3. At any desorption temperature, the density of Bronsted acid sites of these samples decreases in the order of Mordenite > SAPO-11 > SAPO-5, indicating that the strong acid sites of these samples decreases in the same order, which is in agreement with the results of NH₃-TPD shown in Figure 3. Meanwhile, Bronsted acid sites are predominant for all the investigated molecular sieves, but the ratio of Bronsted acid sites to Lewis acid sites varies among different molecular sieves. This may be due mainly to the fact that the acidic sites of molecular sieves, i.e., aluminum species of molecular sieves-framework Al and external-framework Al, change with the synthetic methods and conditions (Zhu et al., 2006).

Table 3
Bronsted acid sites and Lewis acid sites of molecular sieves spectra of absorbed pyridine^a.

Samples	Bronsted acid sites		Lewis acid sites		Total acid sites
	200 (ºC)	400 (ºC)	200 (ºC)	400 (ºC)	200 (ºC)	400 (ºC)
Mordenite	23.25	18.12	10.2	6.56	33.45	24.68
SAPO-11	11.76	11.46	2.20	0.58	13.96	12.04
SAPO-5	11.2	10.2	1.90	0.43	13.1	10.63

Figure 3
NH₃-TPD profiles of samples SAPO-5 (a), SAPO-11(b), Mordenite (c).

Catalytic Activity in the Methylation of Naphthalene

The catalytic performances of the investigated molecular sieves with 1h of TOS are listed in Table 4. It can be seen that 1,8-DMN is not detected for all the molecular sieves, probably due to steric repulsion. Among all the molecular sieves in the present study, Mordenite exhibits relatively high naphthalene conversion compared with the other molecular sieves. SAPO-11 exhibits the highest selectivity for DMN, i.e. 52.8% with the very high 2,6-/2,7-DMN ratio of 1.46 for 1 h of TOS. The 2,6-DMN yield decreases in the order of SAPO-11 > Mordenite > SAPO-5.

Table 4
Comparison of catalytic performance of different molecular sieves in the methylation of naphthalene.

	Mordenite	SAPO-11	SAPO-5
Naphthalene onv(%)	60.2	55.2	26.1
Product distribution (mol%)
EN	3.50	0	1.07
MN	40.1	43.7	51.8
DMN	36.3	52.8	39.7
TMN	20.1	3.5	7.43
DMN distribution (mol%)
2,6-DMN	22.6	23.9	21.30
2,7-DMN	20.5	16.4	21.4
1,7-DMN	20.0	13.4	20.1
1,3-DMN	6.0	7.93	6.81
1,6-DMN	18.1	12.5	15.89
2,3-DMN	5.6	8.33	2.90
1,4-DMN	2.4	4.51	1.80
1,5-DMN	1.7	9.43	9.8
1,2-DMN	3.1	3.60	0
1,8-DMN	0	0	0
2,6-/2,7-DMN	1.10	1.46	0.99
2,6-DMN yield	4.94	6.96	2.73

• Reaction conditions: Temperature = 350 ºC, Pressure = 0.1MPa, WHSV= 0.19h^-1, naphthalene:methanol:mesitylene = 1:5:3.5 (molar ratio), TOS = 1h. Product distribution includes the corresponding molar percentages of EN (ethylnaphthalene), MN (methylnaphthalene), DMN (dimethylnaphthalene) and TMN (trimethylnaphthalene) in the total product mixture.

The catalytic conversion of the investigated molecular sieves is compared in Figure 4 shows that mordenite presents the highest conversion of naphthalene (60.2%) at 1h of TOS. In contrast, SAPO-11 shows the highest conversion of naphthalene (60.2%) at 6 h of TOS, and the conversion of naphthalene for SAPO-5 is the lowest in the reaction time. Meanwhile, mordenite and SAPO-5 show initially the highest conversion of naphthalene and the lowest conversion of naphthalene, respectively, which may be related to their acidity. As shown in Figure 3 and Table 2, mordenite and SAPO-5 exhibit the highest acidity and lowest acidity, respectively, leading to the initial activity of mordenite and SAPO-5 molecular sieves. In additon, Table 3 shows that mordenite has the most Bronsted acid sites and SAPO-5 possesses the least Bronsted acid sites, implying that the Bronsted acid sites probably be active sites in the methylation of naphthalene with methanol. However, their relatively low stability may be caused by the deposition of carbonaceous material (“coke”) in the pores or the breakdown of the structure of the molecular sieves. The smaller pores of SAPO-11 may restrain the formation of coke, leading to somewhat greater stability.

Figure 4
The conversion of naphthalene comparison of different molecular sieves.

The 2,6-/2,7-DMN ratios for the three molecular sieves are shown in Figure 5. The 2,6-/2,7-DMN ratio is very important in the purification of 2,6-DMN. It is very hard to separate them when the 2,6-/2,7-DMN ratio is less than or equal to 0.7 due to the formation of the eutectic mixture. A higher 2,6-/2,7-DMN ratio can facilitate the subsequent separation of 2,6- and 2,7-DMN mixture. When the 2,6-/2,7-DMN is more than 1.4, 2,6-DMN can be separated more easily from the eutectic mixture. As shown in Figure 5, the ratio of 2,6-/2,7-DMN on the SAPO-11 is always the highest among the investigated molecular sieves (see Table 4). However, the 2,6-/2,7-DMN ratio for the investigated molecular sieves are not associated with the acidity of the molecular sieves shown in Figure 3 and Table 2. Therefore, the 2,6-/2,7-DMN ratio is probably related to the differences in pore structure of the molecular sieves. Fang et al. (2006) calculated that 2,6-DMN is somewhat larger than 2,7-DMN in molecular dimension and suffers more diffusion resistance than 2,7-DMN does during the diffusion process. Mordenite and SAPO-5, with nominal pore openings of 0.70 nm and 0.73 nm, respectively, are not expected to be very selective in the methylation of naphthalene because their channels are wide enough to accommodate the formation and diffusion of both 2,6- and 2,7-DMN molecules. SAPO-11, however, possesses a pore opening (0.39 nm×0.64 nm), which is comparable with the kinetic diameter of the naphthalene molecule (0.62 nm) and its β-methylation products (at least 0.62 nm), making it more discerning for the formation and diffusion of the 2,6- and 2,7-DMN products. Therefore, the 2,6-/2,7- DMN ratio over SAPO-11 is mainly ascribed to its specific pore structure.

Figure 5
Effect of different molecular sieves on molar ratio of 2, 6- to 2, 7-DMN

Figure 6 shows the 2,6-DMN yield with 1-6 h of TOS on different molecular sieve catalysts. It can be seen that SAPO-11 exhibits the highest 2,6-DMN yield compared with the other molecular sieve catalysts.

All these findings prove that the relatively good catalytic performance of SAPO-11 for the methylation of naphthalene with methanol is mainly related to the catalyst’s pore structure and SAPO-11 is the most promising of the catalysts tried for the methylation of naphthalene because of its suitable pore structure, which favors the formation and/or diffusion of the 2,6-DMN product over the 2,7-isomer.

Characterization of the Spent Catalyst

To further analyze the cause of deactivation of SAPO-11, the spent SAPO-11 catalyst (6 hour reaction time) was investigated in comparison with the fresh SAPO-11.

Figure 6
The 2,6-DMN yield over the investigated molecular sieves.

XRD Characterization

The XRD patterns of the fresh and the spent SAPO-11 are shown in Figure 7. The spent SAPO-11 still retains the structure of AEL (aluminophosphate eleven) type material. However, the peak strength of the spent SAPO-11 obviously decreases compared with the fresh SAPO-11 catalyst, which indicates that the crystal structure of the catalyst is severely damaged.

Figure 7:
XRD patterns of the fresh SAPO-11 (a) and the spent SAPO-11(b).

SEM Characterization

The SEM micrographs of the fresh and the spent SAPO-11 catalysts are presented in Figure 8. Compared with the fresh SAPO-11, the SEM micrograph of the spent SAPO-11 changes dramatically. It shows that the spherical particles disappear and are replaced by irregular particles after the reaction, which is in agreement with the results of the XRD shown in Figure 7.

Figure 8
SEM images of the fresh and the spent SAPO-11.

ICP Characterization

The SiO₂/Al₂O₃ ratio of the fresh and the spent SAPO-11 detected by ICP are shown in Table 5. It shows that the SiO₂/Al₂O₃ ratio of the spent SAPO-11 increases compared with that of the fresh SAPO-11. That would indicate that the content of Al₂O₃ of SAPO-11 decreases after the end of reaction.

Table 5
ICP results of the fresh and the spent SAPO-11.

Sample	n(SiO2/Al2O3)
fresh SAPO-11	0.53
spent SAPO-11	0.68

Pore Structure Characterization

The pore characteristics of the fresh and the spent SAPO-11 catalysts were studied by the N₂ adsorption-desorption technique, and the results of S_BET and V_pore results are listed in Table 6. The S_BET of 215 m² g^-1 and the V_pore of 0.145 cm³ g^-1 on the fresh SAPO-11 sample decrease to 121 m² g^-1 and 0.091 cm³ g^-1 on the spent SAPO-11 sample after the reaction, respectively.

Table 6
BET Surface area and pore volume of the fresh and the spent SAPO-11.

Sample	BET Surface area / (m2 g-1)	Pore volume / (cm3 g-1)
fresh SAPO-11	215	0.145
spent SAPO-11	121	0.091

The N₂ adsorption-desorption isotherms of the fresh and the spent SAPO-11 catalysts are shown in Figure 9. The typical N₂ adsorption-desorption isotherms of the SAPO-11 samples are of the type IV isotherm according to the IUPAC classification (Sing et al., 1985). A hysteresis between adsorption and desorption branches can be observed at medium relative pressure (0.45-1.0) for all the samples, which demonstrates the presence of a large number of secondary mesopores. However, the N₂ adsorption-desorption isotherms of the spent SAPO-11 catalyst is smaller than that of the fresh SAPO-11 catalyst.

Figure 9
N₂ adsorption-desorption isotherms of the fresh (a) and the spent SAPO-11(b).

Figure 10 shows the micropores and secondary mesopores size distributions of the fresh and the spent SAPO-11 catalysts. Generally speaking, the secondary mesopores probably result from SAPO-11 microcrystals piled up during the synthesis process. As seen in Figure 10, the fresh SAPO-11 catalyst has broader micropores size distributions and secondary mesopores size distributions than the spent SAPO-11 catalyst. From the above characterization results, we speculate that deactivation of the SAPO-11 is mainly due to the breakdown of the structure caused by dealumination (Lucas et al., 2009).

Figure 10
Pore size distributions of the fresh and the spent SAPO-11.

Thermogravimetric (TG) Analysis

The TG analysis was performed to investigate the coke deposition on the fresh and the spent SAPO-11 catalyst. The weight loss in 50-250 ºC shown in Figure 11 and Figure 12 is attributed to the physically adsorbed water in the porous materials (Garces et al., 1988). The weight loss in 250-480 ºC shown in Figure 11 is attributed to the volatilization of residual template. The weight loss in 480-800 ºC in Figure 11 and the weight loss in 250-800 ºC in Figure 12 are attributed to the coke deposition, which may block the active sites in the catalyst (Wang et al., 2011). The amount of coke deposition over the fresh SAPO-11 is 0.613% (see Figure 12), and the amount of coke deposition over the spent SAPO-11 only slightly increases to 1.64% (see Figure 12), which indicates that there is not much coke deposition caused by the acidity of SAPO-11 catalyst during the reaction process.

Figure 11
TG curve of weight loss of the fresh SAPO-11.

Figure 12
TG curve of weight loss of the spent SAPO-11.

Acidity Characterization

The differences of the acid amount and the acid strength between the fresh and the spent SAPO-11 catalysts can be shown in Figure 13 by the NH₃-TPD profiles. Compared with the fresh SAPO-11 catalyst, the acid amount and the acid strength of the spent SAPO-11 catalyst decrease slightly.

Figure 13
NH₃-TPD results of the fresh and the spent SAPO-11.

Based on the thermal gravimetric analysis and the acidity characterization, it can be inferred that the deactivation of SAPO-11 molecular sieves is not because of the coke deposition. According to the value of n(SiO₂/Al₂O₃) by ICP characterization, the deactivation of SAPO-11 is possibly because water is generated during the methylation of naphthalene with methanol, leading to the structure collapse by dealumination of the catalyst by high temperature steam or even by the methanol itself.

CONCLUSION

In summary, the catalytic activities for the methylation of naphthalene were mainly related to the pore structure of the molecular sieves catalysts and the experimental results showed that SAPO-11 exhibited higher catalytic performance than Mordenite and SAPO-5. In addition, the spent SAPO-11 was investigated in comparison with the fresh SAPO-11, which indicated that the structure of SAPO-11 had collapsed due to dealumination, probably by high temperature steam resulting from water formed in the reaction or by high temperature methanol vapor, thus resulting in the deactivation of the catalyst.

ACKNOWLEDGMENTS

This work was supported by Startup Project of Dr. of School of Chemical and Biological Engineering, Taiyuan University of Science and Technology.

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Datas de Publicação

• Publicação nesta coleção
  Jan-Mar 2017

Histórico

• Recebido
  22 Fev 2016
• Revisado
  26 Abr 2016
• Aceito
  19 Jun 2016