Influence of Nitrate and Phosphate on Silica Fibrous Beta Zeolite Framework for Enhanced Cyclic and Noncyclic Alkane Isomerization

Framework for Enhanced Cyclic and Noncyclic Alkane Isomerization

Article

Inﬂuence of Nitrate and Phosphate on Silica Fibrous Beta Zeolite

Siti Maryam Izan, Aishah Abdul Jalil,* Che Ku Nor Liana Che Ku Hitam, and Walid Nabgan

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b02914

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sı Supporting Information

ABSTRACT: Phosphate and nitrate were loaded on silica BEA

P/HSi@BEA and N/HSi@BEA), which is ﬁbrously protonated

(

by the impregnation method for n-hexane and cyclohexane

isomerization. The characterization analysis speciﬁed the removal

of tetrahedral aluminum atoms in the framework, which was

triggered by the existence of phosphate and nitrate groups in the

catalyst. The exchanged role of Si(OH)Al to P−OH as active

acidic sites in the P/HSi@BEA catalyst reduced its acidic strength,

which was conﬁrmed by the FTIR results. Lewis acidic sites of P/

HSi@BEA performance are a signiﬁcant part in the generation of

high protonic acid sites, as proven by the in situ ESR study.

However, FTIR evacuation and ²⁷Al NMR revealed that the

reduction in the amount of extraframework Al (EFAl) is due to its interaction with the nitrate group on the outside of the catalyst

surface. The N/HSi@BEA catalyst exhibited high acidic strength because of the existence of more Si(OH)Al, which was initiated

during the nitrate-incorporation process. Of signiﬁcance is that the catalytic performance of n-hexane isomerization in the presence

of hydrogen reached 50.3% product isomer yield at 250 °C, which might be ascribed to the presence of P−OH active sites that are

responsible for accepting electrons, forming active protonic acid sites. NO −EFAl interaction induced the formation of Brønsted

acid sites, and higher mesopore volume favors the production of cyclohexane isomers up to 48.4% at 250 °C. This fundamental study

exhibits that signiﬁcant interactions given by such phosphate and nitrate groups with the unique silica ﬁbrous BEA support could

enhance isomerization, which contributes to the high quality of fuel.

. INTRODUCTION

been explored by a lot of researchers in order to study in detail

9,10

the eﬀects of metal sites toward hydroisomerization.

Currently, worldwide energy utilization increases every year,

especially for transportation, which causes environment

pollution and incredible energy pressure due to a lot of

formation of unwanted emissions. Hydroisomerization has

captured extensive consideration in producing a high quality

fuel that can reduce pollution.

produce green fuel, an eﬀective strategy such as increasing the

lifetime of catalysts and the yield of isomer products can

improve hydroisomerization performance.

Physical and chemical properties such as strong acidity, high

surface area, shape selectivity, and high performance provided

by microporous zeolite catalysts are extensively used in the

4,11,12

chemical and petrochemical industries.

These properties

of zeolites strongly rely upon the amount of aluminum present

in the zeolite framework. Thus, dealumination that occurred

from the framework of the catalyst had a signiﬁcant eﬀect on

the acidity and catalytic performance of zeolites. So, there are

burgeoning studies discussed about the dealumination of

Therefore, in order to

Bifunctional

13−16

catalysts that have metal sites and acid sites were operative

catalysts for the hydroisomerization of n-alkane. During the

zeolite.

Generally, extraction of aluminum from the

diﬀerent types of zeolite frameworks such as mordenite

(MOR), beta (BEA), and ZSM-5 (MFI) could gave diﬀerent

reaction, noble metals usually provide metal sites to alkane and

its alkene intermediates for promptly undergoing a process of

equilibrium between dehydrogenation and hydrogenation.

Subsequently, the intermediate of alkenes experiences a

protonation on Brønsted acid sites of acid supports before

undergoing the carbenium ion mechanism to produce isomer

17−19

behaviors of zeolites.

Beta zeolite has a high degree of

dealumination followed by mordenite and ZSM-5 for the same

number of T-sides in four rings. Aluminum in the structure

of beta zeolite is easier to remove due to its higher ﬂexibility

and crack products. Therefore, isomer product yield can be

Received: October 2, 2019

enhanced by good equilibrium among the functions of acid

sites and metals, which is one of the signiﬁcant factors needed

to raise the catalytic performance in isomerization. To date, the

introduction of alteration metal and metal load varying have

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

compared to mordenite and ZSM-5. There are a lot of factors

that inﬂuence the dealumination behavior of zeolite such as the

silicon aluminum ratio, the zeolite structure, the size of the

crystal, the amount of Brønsted acidic or defect sites, and also

the method used for abstraction of Al from the catalyst

structure. Earlier studies revealed that acidic properties and

catalytic activity of catalysts were pretentious by dealumination

of the tetrahedral Al framework via acid treatment. For that

reason, the dealumination of zeolite has to be a focus of most

2. EXPERIMENTAL SECTION

.1. Catalyst Preparation. A modiﬁed microwave supported by

the hydrothermal treatment technique was used to synthesize the

34,35

ﬁbrous silica beta.

Toluene (28 mol) and 1-butanol (1.62 mol)

were mixed before the addition of 1 mol of tetraethyl orthosilicate in

the mixture. A solution of urea (0.9 mol) and cetyltrimethylammo-

nium bromide (0.27 mol) was dissolved in distilled water by string for

15 min at ambient temperature. Subsequently, the BEA seed was then

added and magnetically stirred for 30 min. The resultant solution was

irradiated with 400 W microwave radiation at 120 °C intermittently

for 6 h. The catalyst was centrifuged, washed with acetone and

distilled water, and then dried in an oven at 100 °C overnight. The

dried solid was then calcined using a furnace at 550 °C for 6 h. The

sample was labeled as the Si@BEA catalyst. Then, ammonium

nitrate(V) solution was used in two cycles of the protonation process

of the Si@BEA catalyst before being dried at 120 °C overnight and

calcined at 550 °C for 3 h to give HSi@BEA.

7,18,21

work.

In the past two decades, phosphorus has been one of the

promoters of interest to improve hydrothermal stability,

22−24

especially for ZSM-5.

Furthermore, phosphorus gave

positive inﬂuences on catalyst acidity, especially on the

25,26

Brønsted acidity of the catalyst.

Initial research by Mobil

stated that phosphorus in oxide form was incorporated in

zeolite pores analyzed from their prepared alteration of

The phosphate and nitrogen ion treated HSi@BEAs were

27,28

zeolites. Incorporation of phosphorus with Y,

and

synthesized via impregnation of the HSi@BEA with 0.5 N H

(aq) and HNO (aq) solution, respectively, followed by drying at 120

C overnight and calcination at 550 °C in air. The catalysts were then

3 4

3,24,29

especially ZSM-5,

has been studied by numerous

researchers. Acidic properties of zeolite Y were changed due

denoted as P/HSi@BEA and N/HSi@BEA, respectively.

to the formation of diﬀerent surface aluminophosphates after

2.2. Catalyst Characterization and Reaction Study. The

catalysts’ diﬀraction plots were collected on a Bruker Advance D8 X-

ray powder diﬀractometer (Cu Kα radiation source, 40 mA, 40 kV) in

the range of 2θ = 5−70° to conﬁrm the crystallinity of catalysts. Field

emission scanning electron microscopy (JSM-6710 F) and trans-

mission electron microscopy (Philips EM420) were used to examine

the surface morphology of catalyst. Catalysts were subjected to an

the addition of phosphorus in the form of H PO . In zeolite

ZSM-5,

2,24,29,30

the phosphorus compounds are preferred to

interact with bridge hydroxyl groups, leading to aﬀecting the

catalytic activity and modifying shape selectivity due to the

decrease in zeolite acidity. Caro et al. described the reduction

of acidic properties of the catalyst by the formation of

aluminum phosphate upon framework dealumination; how-

outgassing step at 300 °C for 1 h and then underwent N adsorption

ever, Seo et al. observed no removal of the Al framework

after introduction of phosphorus, and they ascribed the

interaction of the catalyst with phosphorus to a reduction of

Brønsted acidity due to the formation of octahedral aluminum.

at −196 °C via Beckman Coulter SA 3100. Fourier transform infrared

(FTIR) analyses were performed with an Agilent Carry 640 FTIR

Spectrometer. FTIR with the KBr method was used to analyze

chemical functional groups of samples within a scan rate of 400−4000

−

cm . Concisely, acidity measurement was performed in an in situ

process using a basic probe molecule, 2,6-lutidine. The catalyst was

evacuated at 400 °C for 1 h. Then, the evacuated samples experienced

adsorption with 4 Torr of 2,6-lutidine (15 min) before desorption at

Lischke et al. revealed that hydrothermal treatment of the

phosphate group loaded on ZSM-5 by impregnation preserves

the high Brønsted acid site. However, Blasco et al. found that

bridge OH located in the ZSM-5 channels preferred to react

0−200 °C. A Bruker Advance 400 MHz 9.4 T spectrometer was

with H PO and NH H PO . They also observed that the

used to perform Al MAS NMR by means of a spin rate of 7 kHz, a

pulse length of 1.9 μs, and a relaxation time delay of 2 s. The

formation of unpaired electrons was evaluated by a JEOL JES-FA100

ESR spectrometer. The catalysts were introduced with hydrogen gas

(100 Torr) at RT after being outgassed at 400 °C for 1 h.

properties of phosphorus incorporated in the ZSM-5 catalyst

showed no signiﬁcant changes when using diﬀerent sources of

phosphorus treatment. In addition, nitric acid treatment over

BEA zeolite was studied by Baran et al., who demonstrated the

dealumination occurred at a faster rate when BEA was treated

up to 0.08 h. The concentration of Lewis and Brønsted acid

sites present in the dealuminated BEA zeolite was inﬂuenced

by the technique used to remove Al atoms and the amount of

A microcatalytic pulse reactor was used to perform isomerization of

n-hexane under atmospheric pressure at 423−523 K. The gas feed

(hydrogen) ﬂow was 100 mL/min. A total of 0.2 g of the catalyst

underwent activation at 400 °C (3 h) and then cooled to the favored

reaction temperature. Once 1 μL of n-hexane was injected, the liquid

product was kept at −196 °C, and the products were detected using a

Al atoms dislodged from the catalyst structure. However, it

seems that the details of the interaction between nitrate and

phosphate with BEA zeolite and the nature of the acid sites in

the hydroisomerization of cyclic and noncyclic hydrocarbon

are worthy of further clariﬁcation.

6090N Agilent GC FID detector equipped with an HP-5 capillary

column. Then, the process was repeated using cyclohexane as a feed.

Equation 1 and eq 2 were used to consider the isomers’ selectivity (S_i)

and yield (Y_i).

In this work, we attempt to introduce phosphate and nitrate

groups on highly accessible-active-site catalysts, bicontinuos

silica lamellar HBEA, for comparison of n-hexane and

cyclohexane hydroisomerization activity. For this purpose, we

provide a better understanding of the physicochemical

processes that take place during impregnation of phosphoric

or nitric acid over the bicontinuous silica lamellar HBEA

catalyst (HSi@BEA), and we obtain qualitative insights into

phosphate−catalyst or nitrate−catalyst interactions that give

diﬀerent routes during n-hexane and cyclohexane hydro-

isomerization. A new plausible structure mechanism was

proposed in accordance with catalyst characterization and

the catalyst activity test.

^Ci

S_i=

× 100%

∑ C − C

res_reactant

(1)

∑

^Ci ⁻^C

res_reactant

X_r_e_a_c_t_a_n_t

× 100

∑

^Ci

(2)

(3)

X_r_e_a_c_t_a_n_t× S_i

^Yi ⁼

where C is the isomer mole fraction in the stream of the product and

C_r_e_s_{_}_r_e_a_c_t_a_n_tis the residual reactant.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

. RESULTS AND DISCUSSION

.1. Crystallinity and Morphological and Textural

Studies. Figure 1 A shows the XRD patterns of HSi@BEA, P/

Article

silicon aluminum ratio of the parent HSi@BEA catalyst used

(

32). Furthermore, P atoms have a higher distribution

compared to N atoms. This might be associated with the lower

electronegativity of P associated with N atoms, which can react

with hydroxyl groups of the catalyst framework. Meanwhile,

the TEM image of HSi@BEA (Figure 1C) illustrates the

dendrimeric ﬁbrous structure covering the catalyst core, which

could contribute to the high surface area, besides giving more

active sites for subsequence reactions.

Figure 2A demonstrates the UV-DRS spectra of all catalysts

in the area of 200 −400 nm. Peaks at 223, 245, 300, and 340

Figure 1. (A) XRD patterns of HSi@BEA, P/HSi@BEA, and N/

HSi@BEA. (B) FESEM and (C) TEM images of HSi@BEA.

HSi@BEA, and N/HSi@BEA catalysts. The diﬀraction peaks

of all catalysts appeared at 2θ = 7.8°, 22.47°, 27.1°, 29.6°, and

3.6°, which are similar to the usual diﬀraction patterns of the

BEA zeolite (JCPDS No. 48-0074). The peak at 2θ = 22.47°

is magniﬁed in the inset ﬁgures to show the diﬀerences in

intensity and probable shift of the peak in the catalysts. It was

observed that the loading of phosphate and nitrate into the

HSi@BEA support decreased the peak intensity by 0.6% and

Figure 2. (A) UV-DRS spectra for all catalysts. (B) Gaussian peak

area of deconvoluted peaks shown in Figure 2A.

nm, in the Gaussian ﬁtting, reveal the presence of tetra-, penta-,

tri-, and hexa-coordinated elements in the HSi@BEA, P/HSi@

43−46

.4%, respectively. The shift in the corresponding peak

BEA, and N/HSi@BEA catalysts.

For a better view, the

37−39

speciﬁed the dealumination of the catalyst framework.

Gaussian peak area of the above-mentioned peaks is

summarized in Figure 2B. Generally, in comparison with the

parent HSi@BEA catalyst, P/HSi@BEA shows the lowest area

of all corresponding peaks, demonstrating the greatest

disturbance of the catalyst framework. The N/HSi@BEA

catalyst exhibits a higher peak area of tri- and penta-

coordination compared to parent HSi@BEA and P/HSi@

BEA catalysts, which could be due to more dealumination

occurring with respect to the additional formation of EFAl

Shifting of 2θ = 22.46° for the parent beta to 2θ = 22.50° for

hydrochloric acid treatment, followed by steaming of the beta

catalyst, was claimed due to dealumination of the catalyst

framework, as conveyed by Wang et al. The disturbance of

the parent framework was most probably due to the acid

treatment during the impregnation of phosphate and nitrate

groups on the catalyst, which could aﬀect the extra-framework

14,17,40

aluminum species (EFAl).

Zhao et al. revealed that after

47,48

modiﬁcation of HZSM-5 with phosphoric acid, the structure

and acidity of the catalyst were aﬀected due to the extraction of

tetrahedral aluminum in the catalyst framework to the extra-

species.

The decrease in hexa-coordination in the N/HSi@

BEA catalyst as compared to the parent HSi@BEA suggested

an interaction between the nitrate group and both frameworks.

framework.

Figure 3 illustrates the N physisorption isotherms and

FESEM and TEM were used to analyze the morphological

properties of the HSi@BEA catalyst, as demonstrated in Figure

NLDFT analysis of the catalysts. As can be seen, all catalysts

exhibited a characteristic isotherm of microporous materials at

B and C, respectively. It was clearly observed that the HSi@

a lower P/P , while mesoporous materials of type IV with an

BEA catalyst contained typical microspheres, with a

bicontinuous concentric lamellar morphology. Similarly, an

identical morphology was observed despite the addition of

phosphate and nitrate groups by the impregnation method

H1 hysteresis loop contain intraparticle and interparticle

,49

pores.

The textural properties of the catalysts were possibly

changed after the addition of phosphate and nitrate groups,

which was indicated by the change in the isotherm of the

catalysts. The BET speciﬁc surface areas for HSi@BEA, P/

HSi@BEA, and N/HSi@BEA catalysts were 619, 361, and 486

(Figure S1 ). These correlated with the XRD results, which

showed the preservation in crystallinity of the catalysts after

the introduction of phosphate and nitrate groups. Elemental

mapping showed that the silica amount was more highly

dispersed than that of Al, which was most likely because of the

−1

m g , respectively (Table 1). After the addition of phosphate

and nitrate groups, the decrease in surface area conﬁrmed the

successful loading of both species on the HSi@BEA catalyst.

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 3. N adsorption (white-circle symbol)desorption (black-circle symbol) isotherms and NLDFT pore size distribution (colored-circle

symbol) of HSi@BEA, P/HSi@BEA, and N/HSi@BEA catalysts.

Table 1. Physicochemical Properties of All Catalysts and Presence of Hydrogen

Weize−Prater value

activation energy

−1

−

(

)

(kJ mol )

BET surface area

micropore volume

mesopore volume

average pore

TOT

−1

catalysts

(m g

)

(cm g

)

(cm g

)

(nm)

(cm

)

hexane cyclohexane hexane cyclohexane

HSi@BEA

N/HSi@

BEA

P/HSi@

BEA

619

486

0.0647

0.0521

0.748

0.774

5.75

6.52

1084

1.08

5.20

1.51

4.18

21.6

16.9

15.8

10.6

1090

1086

361

0.0305

0.592

6.35

3.22

4.38

8.65

10.7

Average pore was calculated was calculated from NLDFT.

The N/HSi@BEA catalyst displayed a rise in mesopore

volume; this could be due to the dealumination, which is in

agreement with UV-DRS results. The obvious decrease in

micropore and mesopore volumes, as well as surface area of the

P/HSi@BEA catalyst, is most likely correlated with the same

phenomenon as above, but with replacement of Al with the

of phosphate-impregnated HZSM-5 was lower than that of

nonimpregnated catalysts, demonstrating that Al ions at

substitutional positions in the SiO framework were partially

protected by phosphorus from being removed from its original

position. The catalyst impregnated with nitric acid gave

similar results, decreasing and shifting the tetrahedrally

coordinated Al of the N/HSi@BEA catalyst because of the

detachment of Al from the catalyst framework (Figure 4C).

Nevertheless, the peak intensity at 54.3 ppm obviously

decreased with a simultaneous increase in the peak at 49.6

ppm, attributed to the transition from the tetrahedrally

coordinated Al to either penta-coordinated Al or distorted

1−53

larger P atoms.

The ²⁷Al MAS NMR spectra of HSi@BEA, P/HSi@BEA,

and N/HSi@BEA catalysts in high (A,B,C) and low (D,E,F)

ranges, respectively, are shown in Figure 4. According to the

deconvoluted peaks of the parent HSi@BEA catalyst (Figure

A), there are four peaks belonging to a tetrahedral

coordination of the aluminum which are located at 60, 55,

tetrahedral Al.

3,54

0.5, and 46 ppm.

The ﬁrst peak is associated with the

In addition, the octahedrally coordinated framework and

extra-framework Al for the parent HSi@BEA catalyst were

manifested at 0 and −1.5 ppm, respectively, as displayed in

Figure 4D. The peak at −1.5 was shifted to −1.8 ppm after the

incorporation of phosphate and nitrate groups, indicating the

preferable Al atoms’ exclusion from the structure (Figure 4E

and F). This clariﬁcation is consistent with previous studies

which exhibited preferential removal of Al atoms placed in

particular tetrahedral sites of the acid leaching MOR

tetrahedrally coordinated extra-framework Al. However, the

peak at 46 ppm is attributed to penta-coordinated Al species or

distorted tetrahedral Al in the framework or extra-framework.

After impregnation with phosphoric acid (Figure 4B), the

peaks at 55 and 50.5 ppm decreased markedly and shifted to

4.5 and 49.6 ppm due to dealumination of the catalyst

framework. In addition, the peak intensity of the shifted peaks

decreased markedly with the concomitant increased peak

intensity at 43.2 ppm, suggesting that the phosphate group

interacts with Si−O−Al in the catalyst framework. This is in

agreement with the result of the BET and UV-DRS. Liu et al.

reported similar results, in which dealumination resulted in a

drop in peak intensity of tetrahedral Al and the appearance of

,21

zeolites.

However, a new peak appeared at −5.8 ppm for

the N/HSi@BEA catalyst, which is attributed to the distorted

octahedral Al species. This was probably due to the

interaction of the nitrate group with some extra-framework

Al. Thus, we can summarize that the amount of EFAl species

present in all catalysts is in the following order: HSi@BEA >

N/HSi@BEA > P/HSi@BEA.

distorted tetrahedral Al peaks. However, diﬀerent results

were reported by Lischke et al., which claimed that the degree

of dislodgement of some aluminum atoms from the framework

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 4. ²⁷Al-NMR spectra for HSi@BEA, P/HSi@BEA, and N/HSi@BEA catalysts at high and low ranges.

Article

Figure 5. FTIR spectra of (a) HSi@BEA, (b) P/HSi@BEA, (c) N/HSi@BEA. (B) Deconvolution range of 990−1300 cm⁻¹and (C) Gaussian

peak area of the deconvoluted peaks.

−

−1

3.2. Vibrational Spectroscopy. FTIR spectra show the

cm , respectively, while the band at 460 cm was because of

internal and external bending and stretching vibrations found

in zeolites, which are depicted in Figure 5A. All catalysts

exhibited characteristic bands identiﬁed at 1230, 1170, 1150,

the bending mode vibration of T−O−T in the framework and

5,56

corresponds to Si−O−Al materials.

Meanwhile, the

external silanol groups (Si−O−H) produce a characteristic

−

−1

084, 1050, 960, 796, 565, and 460 cm . Deconvolution peaks

band at 960 cm . There are two bands centered at 565 and

−

−1

of the FTIR patter in the 1300−990 cm range are shown in

Figure 5B. External and internal asymmetric stretching and

symmetric stretching appeared at bands 1230, 1084, and 796

525 cm , which are representative of ﬁve- and six-membered

rings of T−O−T (T = Si or Al) vibrations of beta zeolite,

implying that beta zeolite is preserved in the samples. The

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

−

Figure 6. FTIR spectra for (a) HSi@BEA, (b) P/HSi@BEA, and (c) N/HSi@BEA in the range of (A) 3800−3400 cm . (B) Gaussian peak area

−

of evacuated peaks and (C) 2200−1500 cm .

appearance of bands at 1150 and 1050 cm⁻relate to PO

nests, respectively. The latter band is attributed to Si−OH

−

and NO , respectively, signifying the probable contribution of

defect bonds hydrogen bonded to the oxygen in the

7−59

those species on the HSi@BEA support.

The band at

framework. From the summary of the deconvoluted band

−

170 cm was ascribed to asymmetrical stretching vibrations

areas shown in Figure 6B, it is obvious that areas of the bands

changed signiﬁcantly, indicating that those species interacted

with both acidic and nonacidic hydroxyl groups after the

of Si−O−Si or Si−O−P bonds. To determine the desilication

and/or dealumination of the zeolite framework for determining

the accurate the catalysts’ structure, the Gaussian peak areas of

the above-mentioned bands (1230, 1170, 1150, 1084, and

introduction of phosphate and nitrate groups. Notably, the

perturbation of external OH groups for the formation of Si−

O−P and/or new Si−O−Si bonds, as well as dealumination,

diminished the amount of terminal silanol groups (3740

−

050 cm ) are summarized in Figure 5C. The asymmetrical

−

stretching vibration of the T−O−T framework at 1084 cm

for the parent catalyst (HSi@BEA) shifted to 1090 cm for

the N/HSi@BEA catalyst, suggesting framework dealumina-

−

−1

cm ), which were also caused by the presence of phosphate

and nitrate groups. In addition, a broad shoulder band

7,60

−1

tion.

The P/HSi@BEA catalyst showed a slight shift in the

attributed to silanol nests (3480 cm ), resulting from

−

v_T_O_Tvalue (1086 cm ) from the parent catalyst, which

indicates that some aluminum atoms are hard to remove due to

high stability, and only a few Al atoms were removed to give

dealumination and/or desilication, was also increased after

1,63,64

impregnation of phosphoric and nitric acid.

However,

−

the larger increase in the 3480 cm band after incorporation

of nitrate compared to that of the phosphate group is most

likely due to the higher tendency for the dealumination and/or

EFAl.

Deconvolution of the FTIR spectrum of HSi@BEA, P/

HSi@BEA, and N/HSi@BEA catalysts in the 3800−3400

cm range by Gaussian curve-ﬁtting (Figure 6A) showed that

14,18

desilication process to occur.

From Figure 6A,B, it could

−

be observed that the Brønsted acid sites greatly decreased after

the spectrum consisted of eight bands at 3780, 3740, 3720,

incorporation of phosphate groups, indicating a weakening in

−

15,53

690, 3665, 3610, 3550, and 3480 cm . For a better view, the

acid sites by the existence of that species.

In previous

deconvoluted peak areas are presented in Figure 6B. Two

major bands were observed at 3740 (isolated terminal silanol

studies, it was reported that the acid site numbers increased

signiﬁcantly after impregnation of H PO on the catalyst and

−

group (Si−OH)) and 3610 cm (bridging hydroxyl groups

that they are much weaker than unmodiﬁed catalysts in terms

2,46

−1

25,31,65,66

[

Si(OH)Al]).

Furthermore, the 3780 and 3665 cm

of strength.

Some studies have claimed that the

bands are related to the atoms of the tricoordinated aluminum

that partly connected to the framework and extra-structure

aluminum types, respectively, while the peaks at 3720 and

dealumination, which was tempted by thermal and acid

treatment, caused the loss of Brønsted acid sites. The higher

−

appearance of EFAl species at 3665 cm in P/HSi@BEA

compared to the N/HSi@BEA catalyst was evident from the

results in Figure 6A,B, indicating that the phosphate group did

not interact with EFAl. The results also suggested that the

nitrate group was probably bonded with EFAl species at the

external surface of the catalyst. After introduction of the nitrate

−

690 cm correspond to the oxygen in the silanol group that

1,42,46,55

interacts with a side electron pair acceptor site.

Moreover, the weak broad bands that appeared at 3550 and

−

480 cm are attributed to connecting hydroxyls agitated by

H-bond interaction with the zeolitic framework and hydroxyl

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 7. (A) IR spectra of 2,6-lutidine adsorbed on (a) HSi@BEA, (b) P/HSi@BEA, and (c) N/HSi@BEA at room temperature. Variations of

concentration of (B) Brønsted and (C) Lewis acid sites for all the catalysts as a function of outgassing temperature from 50 to 200 °C after 2,6-

lutidine adsorption.

−

group (Figure 6A,C), the reduction in the peak at 3665 cm

acid sites because of the amount of escalation of the latter acid

sites after modiﬁcation. This was conspicuously ascribed to

−

and the growth of the 3610 cm peak speciﬁed the formation

of EFAl−NO via the NO coordinatively interacted with

acidic hydroxyl groups corresponding to extra-framework

aluminum.

Figure 6C displayed the catalysts’ stretching frequency of the

−

development of the P−OH groups that are presented in the

framework-incorporated phosphate groups. The lutidinium ion

is strongly attached on the N/HSi@BEA catalyst, which shows

a slight reduction in the concentration of both Brønsted and

Lewis acid sites throughout the outgassing temperature. This

can be dictated that the loading of nitrate groups, which aﬀords

an essential concentration increase of strongly acidic Si(OH)Al

−

vibrational lattice in the range of 2200−1500 cm . All

−

catalysts exhibited bands at 1856 and 1620 cm , which

corresponded to the association of Si atoms with four other −

O−Si− atoms, and the latter band relates to the Si−O−Si or

−

groups (3610 cm ), in agreement with the FTIR evacuation

results.

Si−O−H bonding. No signiﬁcant change in the former band

was observed for N/HSi@BEA, with a marked increase in the

latter band compared to the P/HSi@BEA catalyst. These

results were due to the formation of more Si−O−H (1620

3.4. Proposed Structure of P/HSi@BEA and N/HSi@

BEA. According to the considered and experimental results, a

model of the phosphate group or nitrate group incorporated

into the HSi@BEA catalyst is proposed in Scheme 1. Extra-

−

cm ) in N/HSi@BEA than that of the P/HSi@BEA catalyst

as a consequence of dealumination of the catalyst framework.

framework Al species (Al , AlO , Al(OH) , or Al(OH) , or

4,13

.3. Acidity Studies. Deconvolution of the bands shown in

Al O_y) also act as Lewis acid sites in the catalysts.

It is

Figure 7A by Gaussian curve ﬁtting was caused in six major

proposed that the inclusion of a phosphate group in HSi@BEA

was formed by replacing OH with a H PO group via

2 4

hydrolysis during the calcination process. The Brønsted acid

−

absorption bands at 1705, 1675, 1650, 1627, 1605, and 1580

−

cm . The lutidinium cation was coordinately bonded to the

protonic sites linked to the formation of bands at 1705, 1675,

sites’ strength was signiﬁcantly aﬀected by the replacement of

−

650, and 1627 cm , although the bands at 1605 and 1580

the Brønsted acidic hydroxyl group with a H PO group. The

−

cm appeared due to the unbrokenness of lutidine with Lewis

existence of P−OH reduced the Brønsted acid sites’ strength

acid sites. The strengths of the Brønsted and Lewis acid sites

as an outgassing temperature function for HSi@BEA, P/HSi@

BEA, and N/HSi@BEA are demonstrated in Figure 7B and C.

The spectra of all catalysts are shown in Figure S3 . The results

show important changes above 150 °C because the isomer-

izations of n-hexane and cyclohexane were active above 150

due to the terminal position of its hydroxyl group, which has a

7,69

lower acidic strength than the bridging hydroxyl.

Furthermore, this also resulted in an increase in the acid

sites number. After introduction of a nitrate group, some

framework Al was transformed to octahedral extra-framework

Al and interacted with the nitrate ion to form EFAl−NO as a

C. It is observable that the parent HSi@BEA catalyst

result of dealumination. This led to an escalation in the

strength of Brønsted acid sites, resulting from the formation of

a new bridge hydroxyl group. For the catalytic mechanism,

generally, hydrogen molecules undergo dissociation to hydro-

gen atom formation, which then diﬀuses to Lewis acid sites of

the catalysts. Protonic acid sites were generated when the

atomic hydrogen lost an electron to Lewis acid sites. The Lewis

acid sites that accept the electron react with another hydrogen

atom to cause a hydride formation which associated to the

Lewis acid site. This phenomenon gives rise to the formation

of protonic acid sites and hydride on the surface of P/HSi@

possesses the highest Brønsted acid sites, followed by N/

HSi@BEA and P/HSi@BEA catalysts. However, the HSi@

BEA catalyst had the weakest Lewis acid site nature, with acidic

strength enhanced by the addition of nitrate and phosphate

groups. The presence of phosphate in the catalyst framework

caused weaker Brønsted and Lewis acid sites compared to the

nitrate-loaded catalyst. This was veriﬁed by the higher

desorption of the lutidinium ion from the P/HSi@BEA

catalyst above 100 °C of the elevated temperature. The

phosphatation aﬀects the weak acid site less than the strong

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Possible Structure of (A) Phosphate and (B)

Nitrate on HSi@BEA

Table 2. Product Distributions of n-Hexane and

Cyclohexane Isomerization in the Presence of Hydrogen

reaction temperature [°C]

200

250

300

350

n-hexane

HSi@BEA

conversion (%) 5.8

selectivity of isomers (%) 100

12.5

100

19.8

80.0

19.9

27.1 22.0

75.2 48.0

23.8 52.0

selectivity of cracking

products (%)

yield of isomers products 5.8

P/HSi@BEA

conversion (%) 26.1

selectivity of isomers (%) 100

12.5

18.6

26.8 22.0

32.7

100

50.3

100

58.1 65.2 (53)

70.5 57.0

(44.8)

selectivity of cracking

products (%)

29.5 43.0

(54.2)

yield of isomers products 26.1

32.7

50.3

41.0 37.2

(23.7)

N/HSi@BEA

conversion (%) 20.8

selectivity of isomers (%) 100

24.0

100

27.2

100

47.2 51.9

45.5 26.0

54.5 74.0

selectivity of cracking

products (%)

yield of isomers products 20.8

24.0

27.2

21.5 13.5

cyclohexane

HSi@BEA

conversion (%) 4.7

selectivity of isomers (%) 100

10.4

100

18.8

70.2

28.8

26.8 32.9

24.8 10.8

74.2 88.2

selectivity of cracking

products (%)

yield of isomers products 4.7

P/HSi@BEA

conversion (%) 21.4

selectivity of isomers (%) 100

10.4

13.2

6.65 3.55

BEA and N/HSi@BEA catalysts. The model of acid sites and

catalyst structure was proposed depending on the results

obtained in this study and the previous report as presented in

Scheme 1. The hydrogen adsorption during isomerization

29.1

100

43.3

85.1

14.9

54.0 39.3

63.1 15.3

36.9 84.7

selectivity of cracking

products (%)

−

resulted in H and H formation, where H is placed on the O

atom near the cus (coordinating unsaturated) metal cation to

form OH (active sites). However, for the P/HSi@BEA

yield of isomers products 21.4

N/HSi@BEA

29.1

34.6

36.8

48.4

30.3 6.02

conversion (%) 18.8

70.3 89.8

catalyst, H is placed on the O atom near the P atom resulting

(62.6)

in P−OH formation, which is the active site for the catalyst.

When 2,6-lutidine was adsorbed on the catalyst, 2,6-lutidine

was corresponding to the cus Al. The hydrogen atom provided

an electron to the cus Al and became the acidic OH after the

hydrogen was adsorbed on the lutidine-preadsorbed catalyst.

selectivity of isomers (%) 100

100

49.8 14.6 (6.8)

selectivity of cracking

products (%)

50.2 85.4

(92.2)

yield of isomers products 18.8

33.3

48.4

35.0 13.1

(4.26)

−

The cus Al became Al and decreased the Lewis acidity. The

In the experiment conducted, 10 pulses were measured at each

,6-lutidine adsorbed on the cus Al is eﬀectively desorbed and

reaction temperature. The activity in Table 2 was taken from the

average of the sixth, seventh and eighth pulses, in which the activity

was stable.

moves to the acidic OH to form a lutidinium ion. The cus Al

responds with other hydrogen atoms to form cus-Al clinging to

hydride. Clearly, the model suggested for the acidic center

structure is theoretical. Nevertheless, the model might have the

option to decipher the outcomes acquired in the present

investigation and those revealed in the literature.

(HSi@BEA) at a reaction temperature of 150 °C. The gradual

increase in the conversion from P/HSi@BEA at the elevated

temperature reached 65.2% due to the moderate acidity of the

catalyst, which was triggered by the structure of phosphate in

the framework. At 150−250 °C, the nitrate-loaded catalyst

slightly converted n-hexane to its products and started to

increase apparently at 300 °C, probably assisted by thermal

behavior, to form isomers and cracking products. In contrast,

the conversion of cyclohexane using the N/HSi@BEA catalyst

increased markedly up to 89% at 350 °C. Nevertheless, the P/

HSi@BEA catalyst gave 54% cyclohexane conversion at 300

°C and continuous falloﬀ by 14% at high temperatures.

Insigniﬁcantly, both acid-loaded catalysts demonstrated diﬀer-

ent cyclohexane conversions at reaction temperatures below

3.5. Catalytic Testing. The activities of HSi@BEA, P/

HSi@BEA, and N/HSi@BEA catalysts were performed on n-

hexane and cyclohexane at temperatures of 100−350 °C, and

the outcomes are listed in Table 2. The results show that the

percentage of n-hexane and cyclohexane conversion increased

signiﬁcantly with increasing reaction temperature. As summar-

ized in Figure 8A and B, higher conversion of n-hexane was

dominated by P/HSi@BEA, while the N/HSi@BEA catalyst

was highly favorable for cyclohexane conversion. For n-hexane

conversion (Figure 8A), the acid-impregnated catalyst

increased nearly 4 times more than the parent catalyst

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 8. Percentage of conversion of (A) n-hexane and (B) cyclohexane as a function of temperature. (C) Relationship between yield of isomers

and pore volumes of all the catalysts. (D) Dependence of the percentage yield of all the catalysts on Brønsted and Lewis acid sites

50 °C. The properties displayed in Figure 8C and D could

the production of more cracking products, which favor the

high acidity of the catalyst. Thus, 2,3-dimethylbutane, 2-

methylpentane, and 3-methylpentane, which are the isomers of

n-hexane, are preferable for production from a catalyst with

moderate acidity and high Lewis acid sites. Notably, the

tetrahedral form of the phosphate ion that was attached to the

framework reduced the acidity of the HSi@BEA catalyst,

which was evident in the FTIR results.

justify these consequences. Figure 8C shows the relationship

between the yield of isomers and the mesopore volume for

each catalyst. From this ﬁgure, it is clear to see that the yield of

isomers seems linearly related with the pore volumes. The

highest mesopore volume possessed by the N/HSi@BEA

catalyst led to a maximum in bulkier alkane production such as

that of cyclohexane. This is due to the fact that the cyclohexane

molecules were comparable with the pore diameters of the N/

HSi@BEA catalyst, with a diameter of 6.5 nm, making it

In addition, the yield of branched isomers was approximately

3 times that (250 °C) over the nitrate ion incorporation

catalyst than the parent HSi@BEA, while the branched isomer

selectivity was 100% at 150 °C, which was then reduced to

∼14% at higher temperatures (350 °C). When compared to

the P/HSi@BEA catalyst, an abundant production of branched

isomers was identiﬁed at higher temperatures compared to the

production of isomers using the N/HSi@BEA catalyst. It is

noted that the performance of the N/HSi@BEA catalyst was

comparable to its acidity. A one-third increase in the

concentration of the acid sites (Lewis acid sites) conspicuously

resulted in the desired increase in activity and selectivity. The

cyclohexane molecules were contented to generate isomers on

N/HSi@BEA, which has a high acid strength (Figure 7) in

appropriate for the molecules to pass through the catalyst.

This result was parallel with the pore size distribution from the

N physisorption results. It is also clearly observed that the

isomer yield from both reactants was not dependent on the

micropores of P/HSi@BEA or N/HSi@BEA catalysts. For the

N/HSi@BEA catalyst, the mesoporosity volume had signiﬁ-

cant roles in the production of cyclohexane isomers. Figure 8D

shows the plot of the dependence of yield of isomers of all the

catalysts upon acid site concentration after adsorption of 2,6-

lutidine. There was no signiﬁcant change in the yield ratio to

Brønsted acid site concentration, which could be attributed to

the clear correlation between the concentration of Brønsted

acid sites and the production of isomers. The Lewis acid site

showed a signiﬁcant contribution to the yield of n-hexane and

cyclohexane isomers as evidenced over P/HSi@BEA. This is in

agreement with our previous statement that claimed that Lewis

acid sites play an essential part in n-hexane and cyclohexane

71,72

order to assist the reactant for skeleton rearrangement.

Isomerization of n-hexane was prompted by the n-hexane and

protonic acid site interaction to result in n-hexyl carbenium ion

formation. After that, branched hexyl carbenium ions interact

with hydride ions to form iso-hexane and subsequently desorb

from the catalyst surface. Concurrently, n-hexyl or iso-hexyl

carbenium ions undergo β-scission to form cracking products.

It is important to emphasize that the existence of protonic acid

sites and hydride ions increases the isomerization activity.

However, the cracking and β-scission of n-hexyl and iso-hexyl

carbenium ion was triggered by an excessive quantity of

hydride ions and protonic acid sites that led to suppression of

the isomers’ selectivity.

isomerization. However, the P/HSi@BEA catalyst exhibited

a signiﬁcantly higher contribution of Lewis acid sites for the

production of n-hexane isomers compared to the N/HSi@BEA

catalyst.

The formation of isomers in n-hexane isomerization

remarkably upsurged when using the P/HSi@BEA catalyst as

compared to N/HSi@BEA, with a maximum yield (2,3-

dimethylbutane, 2-methylpentane, and 3-methylpentane) of

approximately 50% at 250 °C. The isomer selectivity for the

particular catalyst was decreased to 57% at 350 °C, suggesting

the formation of cracking products, which was aﬀected by the

hydrothermal conditions. The cracking products took place in

parallel with isomerization at high temperatures, which might

also lead to deposition of coke on the catalyst. The N/HSi@

BEA catalyst contributed only 27% of the maximum isomer

yield at 250 °C, near the maximum yield of isomers of the

parent HSi@BEA catalyst (26%). This result may be due to

The Arrhenius equation allows us to calculate apparent

activation energies (E ) for all the catalysts over the range of

150 to 350 °C (Table 1). Related to the Arrhenius assumption,

lower activation energy provides a higher amount of branched

isomers from n-hexane or cyclohexane isomerization. The

values of E for n-hexane isomerization using P/HSi@BEA

were 2 times lower than those for the N/HSi@BEA catalyst.

However, when using cyclohexane as the reactant, N/HSi@

BEA exhibited a slight reduction in the E value compared to

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 9. ESR spectra of P/HSi@BEA and N/HSi@BEA activated at 400 °C for 1 h, 50 Torr of preadsorbed n-hexane/cyclohexane at (a) room

temperature (RT)-black color line, 100 Torr of hydrogen was adsorbed (b) room temperature (RT)-blue color dotted line, (c) 50 °C, green color

dotted line; (d) 100 °C, red color dotted line; (e) 150 °C, orange color dotted line; (f) 200 °C, purple color dotted line. The preadsorbed n-hexane

and cyclohexane were labeled as H and C, respectively.

the P/HSi@BEA catalyst. This diﬀerent trend implied that the

involvement of mesoporosity and the acidic sites concentration

of the catalysts take a signiﬁcant role in the reaction.

environment. ESR results can indirectly certify the ability of

phosphate or nitrate groups in the generation of an electron

from the formation of atomic hydrogen through dissociative

−

The eﬀect of internal diﬀusional limitation of catalysts can

adsorption of molecular hydrogen. Overall, the (PO ) (H )

x y

−

be evaluated by applying the Weisz−Prater (NWP) criterion

as shown in Table 1. In summary, the presence of mesopores

provides no diﬀusion limitation in the catalysts. The silica

lamellar structure of HSi@BEA enhances the diﬀusion and

accessibility of the reactants and products throughout the

catalyst. It is also noted that the Weisz−Prater values are

parallel with the above-mentioned results, where the phosphate

group-type catalyst is favored for the straight chain reactant,

while the nitrate group-type catalyst is more appropriate for

the cyclic reactant.

or (NO ) (H ) was formed from the subsequent bonding of a

x y

proton with a P or N atom, respectively. The proton was

generated by the loss of an electron from atomic hydrogen,

followed by trapping the electron in the electron-deﬁcient

metal cations, which resulted in a reduction in the ESR signal

at g = 1.97. The signal at g = 1.97 shows a signiﬁcant decrease

after being heated up to 200 °C in the presence of hydrogen

for the preadsorbed n-hexane or cyclohexane of the P/HSi@

BEA catalyst, indicating faster formation of an electron from

hydrogen and being trapped in the electron-deﬁcient metal

cations. The N/HSi@BEA catalyst showed no signiﬁcant

changes in the signal intensity, demonstrating a lower electron

formation from hydrogen and the electrons stuck in the

electron-deﬁcient metal cations. Higher generation of Lewis

acid sites in the P/HSi@BEA catalyst, which is evident from

Figure 8D, generates a large protonic acid site amount by

alteration of the role of electron acceptors after dissociation of

In summary, the HSi@BEA catalyst loaded with the

phosphate group provided much higher yields of branched

isomers in n-hexane isomerization, while the nitrate group

loaded on the HSi@BEA catalyst aﬀorded greater yield in

cyclohexane isomerization. The improvement in the active site

accessibility and molecular transport in an environment of

nonrestricted mesoporous channels clearly inﬂuences the

enhancement of catalytic activity. Nevertheless, the fact that

the augmentation in the yield of branched isomers is also

associated with contributions from the strength of acidity of

the catalysts could not be excluded. The moderate acidity with

a high amount of Lewis acid sites from the P/HSi@BEA

catalyst has provided a signiﬁcant eﬀect on increasing the yield

or activity of straight chain hydrocarbon isomerization.

Diversely, the high acidity character of the N/HSi@BEA

catalyst was essential to contributing prominently in the cyclic

H . As a results, a larger amount of protonic acid sites triggered

the production of n-hexane isomers. Moreover, all catalysts

also exhibited an additional two signals at g = 1.91 which relate

to the vibration structure of nonbridging oxygen hole centers

in silica or lattice defects present in both catalysts. Both

catalysts showed similar changes in the signal, which suggests

that the lattice defects also play an insigniﬁcant role in the

isomer formation of n-hexane and cyclohexane.

The promotive result of hydrogen can be distinct from the

formation of protonic acid sites from the spillover of hydrogen

from metal sites onto the catalyst support, which was reported

hydrocarbon isomerization. Furthermore, the presence of

electron acceptor Lewis acid sites in the catalysts was also

accountable for the catalytic activity enhancement, as

79,80

by Hattori and co-workers.

In addition, Iglesia and co-

42,74−76

workers also have studied the hydrogen spillover eﬀect on the

conﬁrmed in the literature for numerous hydrocarbons.

It was shown that the higher Lewis acid sites give greater

zeolite as a support catalyst.

.0. CONCLUSION

42,77

generation of protonic acid sites during isomerization.

.6. Dissociative Adsorption of Hydrogen Gas. Figure

displays the in situ ESR spectra of activated P/HSi@BEA

P/HSi@BEA and N/HSi@BEA catalysts were prepared by

impregnation of HSi@BEA with H PO and HNO₃,

and N/HSi@BEA catalysts with preadsorbed n-hexane or

cyclohexane at room temperature up to 200 °C in a hydrogen

respectively. The catalysts’ physical and chemical properties

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

1) Hengsawad, T.; Srimingkwanchai, C.; Butnark, S.; Resasco, D.

Article

were observed by XRD, N physisorption, FESEM, TEM, ²⁷Al

Research Grant (No. 06G52 and 06G53) and Ministry of

Higher Education Malaysia through MyBrainSc Scholarship

(Siti Maryam binti Izan).

NMR, UV-DRS, FTIR, and ESR. The FESEM images revealed

the preservation of the bicontinuous concentric lamellar

morphology of HSi@BEA after the incorporation of phosphate

and nitrate. XRD and Al NMR results demonstrated that the

impregnation of phosphate and nitrate led to extraction of Al

from the catalyst framework. The N/HSi@BEA catalyst

possessed the highest average pore diameter of 6.52 nm,

followed by P/HSi@BEA and HSi@BEA catalysts. FTIR

analysis conﬁrmed that the existence of the P−OH species in

the P/HSi@BEA catalyst reduced its acidity due to the weak

acidic character of the species. However, the introduction of

nitrate groups increases the catalyst acidity attributed to the

Si(OH)Al formation as the Brønsted acid sites of the catalyst.

Both catalysts displayed high interaction with hydrogen, as

evident by the ESR analysis. However, ESR analysis showed

that the dissociation of hydrogen to form protonic acid sites

was faster for the P/HSi@BEA catalyst, and its Lewis acid sites

inﬂuence the production of n-hexane isomers by playing the

role of electron acceptor in generating the protonic acid sites.

The mesoporosity of the N/HSi@BEA catalyst was found to

play an essential role in enhancing the selective production of

cyclohexane isomers. The phosphate group catalyst gave a

higher yield of n-hexane isomers (50.3%), while the nitrate

group catalyst contributed 48.4% of the cyclohexane isomers at

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Aishah Abdul Jalil − Universiti Teknologi Malaysia,

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