Effects of Zr, Al, and Mordenite on Pt-MCM-48 catalyst in n-heptane isomerization: Preparation, characterization and catalytic performance

doi:10.21203/rs.3.rs-2604096/v1

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Effects of Zr, Al, and Mordenite on Pt-MCM-48 catalyst in n-heptane isomerization: Preparation, characterization and catalytic performance

https://doi.org/10.21203/rs.3.rs-2604096/v1

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published 02 May, 2023

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Herein, platinum loaded on various catalysts, MCM-48-Mordenite, Al-MCM-48, Al-MCM-48-Mordenite, Zr-MCM-48, and Zr-MCM-48-Mordenite has been synthesized and investigated for n-heptane isomerization reaction at four different temperatures. The XRD, FT-IR, UV-Vis DRS, NH₃-TPD, and BET analysis characterized the structural characterization and acid distribution of these catalysts. The Pt/Mordenite catalyst showed higher hydrogenation and cracking activity while the hybrid catalysts showed better isomerization selectivity. The best catalytic behavior was obtained by Pt/Al-MCM-48-Mordenite catalyst at 200°C, with suitable n-heptane conversion (78.8%) and the highest isomer selectivity (81.9%). The maximum isomerization selectivity as well as the maximum yield of multi branched isomers probably not only due to its suitable acidity and large pores but also due to higher metal dispersion. This result indicates that the Pt/Al-MCM-48-Mordenite catalyst can be a hopeful candidate for good n-heptane isomerization catalysts.

-Heptane isomerization

Catalyst

Catalytic performance

Al-MCM-48-Mordenite.

The environmental laws restrict the content of aromatics, benzene and other harmful fuel compounds that affect the quality of gasoline. A suitable alternative to aromatic compounds are branched paraffins with a high octane number. Accordingly, the isomerization of paraffins has been introduced as an important reaction for the production of high quality gasoline [1–4]. This process is performed at low temperatures to prevent cracking and aromatization. Hence, the reaction rate is slow and the catalyst used for this reaction must be very active. The use of strong acid catalysts reduces the reaction temperature and increases the tendency to form branched alkanes with a high octane number.

In order to prepare the acidic catalysts for achieve the best efficiency in the isomerization of paraffins, the modification treatment with various metals such as Co [5], Mn [6], Fe [7], Zr [8], Ti [9, 10] and Al [11] has been studied previously. Excellent ion-exchange capacity makes zirconia a good promoter or support in catalytic operations. In 2020, parsafard et al. have prepared collections of the Pt–Cr/Zr(x)-HMS catalysts with several molar ratios of Cr/Zr and used as solid acid catalysts for n-heptane isomerization. The best selectivity to isomer products was observed at Cr/Zr = 30 [12].

The mesoporous composite catalysts, when used in the isomerization reaction, the mesoporous structure can effectively increase the diffusion rate of multibranched hydrocarbons, allowing them to quickly pass through the zeolite micropore channels and reduce side reactions such as cracking [13]. The mass transfer yield in the 3D cubic mesoporous like MCM-48 and KIT-6 is higher than that in the 2D straight channels appropriate to the highly interpenetrating and interconnected 3D meso structure [14]. In 2021, Ghaderi et al. show that the Pt/MCM48-HZSM-5 catalyst has a good selectivity to multibranched isomers and also prove the catalytic stability during the isomerization reaction [15].

It is well known that the acidity and porosity of zeolite materials significantly affect the catalytic performance. Although platinum (Pt) metal supported on zeolites inhibits the coke formation, the catalytic conversion and selectivity to iso-products are still low [16, 17].

Mordenite zeolite is widely used for many catalytic reactions such as alkylation, isomerization and dehydrogenation because of its uniform, controlled chemistry, flexible framework, and high internal surface area [18]. This zeolite has two types of channel: larger and smaller channel. This zeolite also is a very strong acid catalyst [19].

In the present study, Pt/MCM48 catalyst modified by zirconium, aluminum, and Mordenite zeolite was studied and utilized as solid acid catalysts in reaction to produce multibranched compounds from n-heptane. The parameters such as activity, selectivity to different products, stability, coke deposition, and RON (research octane number) have been discussed at the temperature range of 200–350°C.

2.1. Catalyst preparation

Mordenite zeolite was prepared based procedures reported in literatures [20, 21]. 4.75 g sodium hydroxide was dissolved in 10 ml water, then 3.57 g of sodium aluminate was added, and the mixture was stirred until dissolution. Finally, 161.25 ml water and 24.55 g SiO₂ was added, and the mixture was stirred for 48 h. The molar composition of this mixture solution was 6 Na₂O: 1 Al₂O₃: 30 SiO₂: 780 H₂O. The obtained gel loaded in Teflon-lined steel autoclave and kept at 180°C during 24 h for crystallization. The product was filtered, washed and dried at 100°C for 10 h. The sample was ion-exchanged with NH₄NO₃ (1 M) solution for 1 h at 60°C. This work repeated three times, and then the obtained powder was calcined at 500°C.

The MCM-48-Mordenite was synthesized according to the method mentioned in the ref [15]. 2.4 g cetrimonium bromide (CTAB) was dissolved in 50 ml water, and then 0.5 g Mordenite zeolite was added to the mixture. Then, 12 ml ammonia (32 wt.%) and 50 ml ethanol were added to the solution and stirred for 15 min. Afterwards, 3.4 g tetra ethyl ortho silicate (TEOS) was added into the solution. The mixture was constantly stirred for 2 h at ambient temperature. The resulting mixture was recovered by filtration, and drying in the air. Finally, the sample was calcined at 500°C for 4 h.

In order to prepare Zr-MCM-48 and Al-MCM-48, 0.18 g zirconyl nitrate (ZrO(NO₃)₂.6H₂O) or 0.28 g aluminum nitrate (Al(NO₃)₃.9H₂O) was added to the solution instead of Mordenite zeolite. This method was used for synthesis of the catalyst with Si/Zr and Si/Al = 20. In the preparation of Zr-MCM-48-Mordenite and Al-MCM-48-Mordenite, after adding zirconyl nitrate or aluminum nitrate, 0.5 g Mordenite zeolite was also added.

Furthermore, catalysts containing platinum (0.6 wt.%) were also synthesized by impregnating the support with appropriate concentration of hexa chloro platinic acid. The catalysts were filtered and dried at 110 ^oC overnight. Afterwards, these solid samples calcined in air at 300 ^oC for 4 h.

2.2. Characterization tests

Powder X-ray diffraction (XRD) patterns, the Fourier transform infrared (FTIR) spectroscopy, the ultraviolet-visible diffuse reflectance spectra (UV‐Vis DRS), nitrogen adsorption isotherms, and temperature programmed desorption of ammonia (NH₃-TPD) were performed with an X-PERT diffractometer, a BOMEM FT-IR spectrophotometer, a Shimadzu UV‐2100 spectrophotometer, an ASAP-2010 Micromeritics instrument, and a TPD/TPR analyzer (2900 Micromeritics) instrument, respectively.

2.3 Catalytic evaluation

At the beginning of the experiment, 0.2 g of catalyst was loaded in a continuous fixed-bed reactor. Reaction temperature for all samples starts from 200°C and increases to 250, 300 and 350°C. The catalysts were reduced in H₂ gas flow (40 ml/min) at 400°C (2 h) prior to each experiment. The n-heptane (n-C₇) fed into the reactor by a syringe pump with a 2 ml/h flow rate and mixed with the H₂ stream. The efficiency of the calcined catalysts was tested after 1 h on stream (TOS) at each temperature …. The efficiency of all prepared catalysts was also studied at 300°C for 6 h on stream for investigating the amount of coke deposition. The analysis of the reactor products was carried out by GC-FID with an Agilent Technologies 7890A.

The catalytic activity is expressed by the n-C₇ conversion, which is calculated by the following equation.

Conv. (%) = percentage of n – C₇ transformed into products (1)

Also, the following equation was used for calculating the selectivity to various products:

$${\text{S}}_{\text{x}}\left(\%\right)=\frac{\text{n}-{\text{C}}_{7 }\text{t}\text{r}\text{a}\text{n}\text{s}\text{f}\text{o}\text{r}\text{m}\text{e}\text{d} \text{i}\text{n}\text{t}\text{o} \text{a} \text{c}\text{e}\text{r}\text{t}\text{a}\text{i}\text{n} \text{p}\text{r}\text{o}\text{d}\text{u}\text{c}\text{t}}{\text{t}\text{o}\text{t}\text{a}\text{l} \text{a}\text{m}\text{o}\text{u}\text{n}\text{t} \text{o}\text{f} \text{n}-{\text{C}}_{7} \text{c}\text{o}\text{n}\text{v}\text{e}\text{r}\text{t}\text{e}\text{d}}\times 100$$

3.1. Catalysts characterization

Figure 1 shows the XRD patterns of the prepared catalysts. The typical peaks of the MCM-48 are seen at 2θ = 2.9° is related to the d₂₁₁ diffraction of the MCM-48 cubic phase and peak at 2θ = 3.4° demonstrates the d₂₂₀ reflection [22, 23]. The Mordenite structure shows sharp peaks in the range of 9–36°, correspond to the reflections of Mordenite zeolite. The diffraction peaks at 9.77°, 13.46°, 19.62°, 22.20°, 23.17°, 25.64°, 26.25°, 27.67°, 27.85°, 30.89° and 35.61°, indicating that the Mordenite zeolite was successfully synthesized [24, 25]. Meanwhile, XRD patterns show no signals of zirconia, platinum and aluminum metals, probably because of homogeneous dispersion of these metallic phases in the framework of synthesized catalysts. Although the existence of these metals has been checked by XRF test.

The FTIR spectra for synthesized catalysts are presented in Fig. 2. The FTIR spectra of the MCM-48-Mordenite and other composites present the bands at 1639 and 3400 cm^− 1, indicating the presence of physisorbed water. The characterization bands of MCM-48 are seen at 1234 and 1080 cm^− 1, which are assigned to v_as(Si-O-Si). Also, the absorption bands at 460 cm^− 1 and 810 cm^− 1 are usually belonged to δ(Si–O–Si) and v_s(Si–O–Si) [26]. The FT-IR spectra of Mordenite zeolite exhibit the bands at 1080 cm^− 1 (asymmetric stretching vibration of Si-O bond), 810 cm^− 1 (symmetric stretching vibration of Al-O bond), 580 cm^− 1 (vibration of five-membered rings), and 450 cm^− 1 (T–O bending) [24]. In addition, no absorption band corresponding to aluminum and zirconium phase was found in FTIR spectra, this fact also confirms a good dispersion of aluminum and zirconium.

UV-vis diffuse reflection spectra of samples are shown in Fig. 3. This analysis was used to distinguish the chemical structure of Pt. The band near 250 nm is a charge transfer (CT) band (oxygen to the metal) and can be seen in all spectra. Also, the weak shoulder at above 350 nm must be due to a d-d transition band of Pt²⁺ species [27]. The observed shifts in the intensities of these peaks may be due to the diversity in the interaction strength of species and their population on the supports.

The acidity distributions on the surface of the prepared catalysts are listed in Table 1.

Table 1

Physicochemical properties of Pt synthesized catalysts denoted as: M: Mordenite, MM: MCM-48-Mordenite, ZM: Zr-MCM-48, ZMM: Zr-MCM-48-Mordenite, AM: Al-MCM-48, and AMM: Al-MCM-48-Mordenite.
Catalysts	M	MM	ZM	ZMM	AM	AMM
Acidity (µmol NH₃/g)
L	307.6	245.6	57.2	283.3	86.1	231.2
B	264.5	120.7	115.8	256.4	110.7	321.7
L + B	572.1	366.3	173.0	539.7	196.8	552.9
B/L	0.9	0.5	2.0	0.9	1.3	1.4
Surface properties
S_BET (m²/g)	341.1	398.2	458.4	158.8	771.9	378.2
V_p (cm³/g)	0.28	0.71	0.25	0.16	0.67	0.59
d_p (nm)	3.3	3.4	2.2	4.1	3.5	5.3

Figure 4 also shows the acid sites distributions for the calcined catalysts, which are determined by NH₃-TPD.

All samples have two desorption peaks in the temperature region of 100–800°C, interpreting as the weak and strong acids. The data in Table 1 show that the number of weak acid sites on all catalysts is smaller than Mordenite zeolite. The amounts of weak acid of these samples are in the range of 57 ~ 308 mmol g_cat⁻¹, and those of strong acid are in 111 ~ 322 mmol g_cat ⁻¹ range.

The density of total acid sites of the catalysts follows the sequence of Mordenite > Al-MCM-48-Mordenite > Zr-MCM-48-Mordenite > MCM-48-Mordenite > Al-MCM-48 > Zr-MCM-48. In other words, the strength of acidic sites in the catalyst of Al-MCM-48 and Zr-MCM-48 increases with the addition of Mordenite zeolite.

Figure 5 shows the adsorption/desorption isotherms of the all samples. The curves of samples exhibited typical Langmuir IV adsorption isotherms (ICPU), indicating that there was a certain amount of slit-shaped pores in the catalysts. For comparison, Table 1 summarizes the textural properties of the prepared catalysts (S_BET, V_P, and d_P). The results show that Al-MCM-48 catalyst has the highest BET surface area (S_BET). In addition, the specific surface area and pore volume of the Al-MCM-48 and Zr-MCM-48 decreased after composite formation with Mordenite zeolite. However, the pore diameter decreases. Furthermore, Al-MCM-48-Mordenite has a larger pore diameter (5.3 nm) than other catalysts.

3.2. Catalytic isomerization of n-C₇

The catalytic performance of the Pt-loaded Mordenite, MCM-48-Mordenite, Zr-MCM-48, Zr-MCM-48-Mordenite, Al-MCM-48, and Al-MCM-48-Mordenite was investigated over n-C₇ isomerization reaction in the range of 200–350°C. Table 2 lists the catalytic properties over synthesized catalysts. It is found that the n-heptane conversion increases with increasing temperature over all catalysts. The highest n-C₇ conversion is 96.7% achieved on the catalyst Pt/Al-MCM-48-Mordenite at 350°C. It is believed that the n-heptane conversion of catalysts often depend upon the acid strength of catalysts due to increased chance of interaction between the acidic sites and the olefinic intermediates [28–30].

Table 2

Catalytic activity (Conv.%), selectivity (S_x%), coke amount (C.%), and RON at various reaction temperatures over Pt synthesized catalysts, denoted as: M: Mordenite, MM: MCM-48-Mordenite, ZM: Zr-MCM-48, ZMM: Zr-MCM-48-Mordenite, AM: Al-MCM-48, and AMM: Al-MCM-48-Mordenite.
Catalyst	T/°C	C	Conv.	S_MOB	S_MUB	S_i−C7	S_C	S_A	S_H	RON
AMM	200		78.8	24.4	57.5	81.9	4.2	0.4	13.9	96.5
	250		86.7	15.2	38.4	53.6	10.7	0.7	34.9	84.6
	300	7.8	90.1	8.0	16.9	25.0	17.7	0.9	56.3	63.3
	350		96.7	3.2	8.9	12.2	20.3	1.4	66.3	58.5
AM	200		47.8	25.7	42.4	68.1	2.3	0.1	29.5	60.9
	250		50.7	26.3	28.2	54.5	4.2	0.2	41.1	59.6
	300	4.7	54.8	13.3	15.1	28.4	8.5	0.2	63.0	58.2
	350		64.8	5.1	7.2	12.3	14.0	0.5	73.1	56.3
ZMM	200		65.7	23.0	38.5	61.5	8.0	0.6	29.8	68.4
	250		74.7	21.7	24.6	46.3	12.0	0.9	40.8	64.1
	300	7.2	81.7	10.0	14.7	24.7	16.3	2.2	57.0	57.6
	350		89.4	3.3	7.8	11.1	19.0	3.2	66.7	52.8
ZM	200		42.7	22.6	35.0	57.6	5.7	0.1	36.5	49.0
	250		46.7	15.0	25.0	40.0	14.1	0.3	45.6	39.0
	300	4.1	49.6	10.6	16.8	27.4	17.1	0.4	54.5	35.0
	350		55.7	4.2	4.8	9.0	20.3	0.5	70.1	34.5
MM	200		59.8	34.6	42.2	76.8	5.3	0.2	17.7	65.6
	250		67.6	16.6	33.1	49.7	11.9	0.5	37.9	62.3
	300	6.3	75.8	8.3	15.5	23.8	18.0	1.1	57.1	51.9
	350		81.3	3.9	6.3	10.2	20.2	1.9	67.7	47.3
M	200		71.8	14.3	16.2	30.5	16.5	0.5	52.5	52.3
	250		84.7	8.2	9.0	17.3	20.0	0.6	62.1	51.6
	300	3.4	91.6	3.6	3.6	7.2	22.1	1.1	69.7	48.8
	350		95.7	0.7	0.8	1.5	23.4	1.2	73.8	46.8

The density of acid sites was identified qualitatively by the location of maximum peak temperature in the TPD profiles. In addition, the results of the selectivity to monobranched (S_MOB) and multibranched (S_MUB) isomers and total i-C₇ (S_i−C7) were shown in Table 2. These results show that at low reaction temperature, the selectivity toward isoheptanes for all catalysts is high. Because the isomerization reaction has a thermodynamic limit. In other words, temperature acts as a limiting factor in this reaction. In synthesized catalysts, the ratio of MUB isomers to MOB isomers (R) nearly ranges, between 1.0 and 3.0. The selectivity to MUB isomers is higher than MOB isomers in all catalysts. The selectivity toward MUB isomers depends on surface characteristics of catalysts, such as the pore volume (V_p) and diameter (d_p). The Pt/Al-MCM-48-Mordenite has large pore size (5.3 nm). This catalyst with proper pore size allow the good diffusion of MUB isomers through the pores before their cracking. Thus, Pt/Al-MCM-48-Mordenite catalyst has the best selectivity to MUB isomers and the R-value. MUB isomers are key product of the isomerization process due to their high octane number and great importance in the oil industry and provides a greater fuel resistance to knocking or pinging during combustion. In Table 2, we also expressed the selectivity of cracking, aromatization, and hydrogenolysis products of n-C₇ against the reaction temperatures.

For all tested catalysts, cracking (C) and hydrogenolysis (H) were the dominant side reaction at high temperature. Combination of the mesoporous silica (MCM-48) and aluminum in support decreased the diffusion limitations for transport and residence time of the carbocation intermediates on the acidic sites during the isomerization reaction. As a result, on the Pt/Al-MCM-48 catalyst, the occurrence of cracking and aromatization (A) reactions was limited. As can be seen in Table 2, the synthesized catalysts form a little aromatization product. However, this amount is not very low in Pt/Zr-MCM-48-Mordenite, indicating that aromatization is affected by geometry, acidity, type of acid location, and balance between acid and metal functions. Another important result that was reported in our work is effects of temperature and catalyst type on the research octane number (RON). To calculate this parameter, Eq. (3) was used [31, 32]:

RON= ${\sum }_{\text{i}=1}^{\text{k}}{\text{y}}_{\text{i}}{\text{R}\text{O}\text{N}}_{\text{i}}$ (3)

In this equation, RON_i represents the octane number of pure component (as i) and y_i is the volume fractions of molecule i. The results show that Pt/Al-MCM-48-Mordenite at 200°C compared to other catalysts, due to the production of molecules with higher RON_i, provides higher RON (RON = 96.5). Catalyst deactivation by coke is one of the problems of using catalysts in industrial processes. For this purpose, catalysts' stability was evaluated for n-heptane isomerization at 300 ^oC after 8 h on stream. As can be seen from Fig. 6, the catalysts have an almost constant performance in the course of the reaction, and are not deactivated significantly during this time on stream (TOS = 8 h). This verifies that the prepared catalysts process a very stable catalytic performance. However, the decrease in the Pt/Zr-MCM-48-Mordenite and Al-MCM-48-Mordenite catalyst is higher than other catalysts. One reason for this reduction is the rapid formation of coke on the catalytic surfaces. Therefore, coke formation on the catalysts was evaluated. The coke burning is a method to investigating the coke poisoning. The samples, which were tested for 8 hours at 300°C, were placed in an oven at 120°C to lose moisture, then weighed and placed in an oven at 300°C for one hour. Immediately after cooling, the tested catalyst was weighed again to obtain the weight difference. The weight difference is the amount of coke poisoning (Table 2). The low amount of coke deposited indicates the good stability of the prepared catalysts against deactivation during 8 h on stream. Overall, although the amount of coke (Table 2) over Pt/Al-MCM-48-Mordenite is larger than others, its catalytic activity and i-C₇ selectivity are both better than others.

Based on the results, although Pt/Mordenite catalyst with strong acid sites showed high conversion, its products were formed mainly through hydrogenation and cracking. On the other hand, Pt/Al-MCM-48-Mordenite catalyst showed higher isomerization selectivity. Notably showed the highest yield of multibranched isomers due to its appropriate acidity, large pores and high metal dispersion. The Pt/Al-MCM-48-Mordenite catalyst can be considered as a good candidate for isomerization catalysts. The results show that the hybrid catalysts have led to high octane number of gasoline. In addition, the stability of synthesized catalysts is suitable for this isomerization process. However, efforts to improve results will continue.

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Effects of Zr, Al, and Mordenite on Pt-MCM-48 catalyst in n-heptane isomerization: Preparation, characterization and catalytic performance

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Abstract

Figures

1. Introduction

2. Experimental

2.1. Catalyst preparation

2.2. Characterization tests

2.3 Catalytic evaluation

3. Results And Discussion

3.1. Catalysts characterization

3.2. Catalytic isomerization of n-C₇

4. Conclusions

References

Additional Declarations

Status:

Journal Publication

Version 1

Effects of Zr, Al, and Mordenite on Pt-MCM-48 catalyst in n-heptane isomerization: Preparation, characterization and catalytic performance

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Catalyst preparation

2.2. Characterization tests

2.3 Catalytic evaluation

3. Results And Discussion

3.1. Catalysts characterization

3.2. Catalytic isomerization of n-C7

4. Conclusions

References

Additional Declarations

Status:

Journal Publication

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3.2. Catalytic isomerization of n-C₇