Luận Án tiến sĩ synthesis of cu fesapo 34 catalysts for the selective catalytic reduction scr of nox with nh3

RESULTS AND DISCUSSION ‘The influsnee of OSDAs on the formation of SAPO-34 structure ‘The influence of silicon sources for SAPO-34 formation Copper-iron bimetal ion-exchanged SAPO-34 fo

MINISTRY OF EDUCATION AND TRANING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

DOAN ANIT TUAN

Synthesis of Cu-Ke/SAPO-34 catalysts for the sclective catalytic reduction

(SCR) of NO, with NHs

CHEMICAL ENGINEERING DOCTORAL DISSERTATION

Hanoi — 2022

MINISTRY OF EDUCATION AND TRANING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

DOAN ANIT TUAN

Synthesis of Cu-Fe/SAPO-34 catalysts for the selective catalytic reduction

1 Assoc Prof: Pham Thanh Huyen,

2 Prof Le Minh Thang

Hanoi 2022

STATUTORY DECLARATION

Thereby declare that 1 myself have writicn this thesis book The data and

resulis presented in the dissertation are truc and have net been published by other authors

1ianoi, 21% March 2022

1, Assoc Prof Pham Thanh Huyen Doan Anh Tuan

2 Prof Le Minh Thang

ACKNOWLEDGEMENT

First and foremost, urilonbtedly, T must give gratitude to my advisor, Assoc Prof Pham Thanh Huycn, for giving me the opportunity to work under her supervision for the last four years She provided patience, encouragement, and advice, which is necessary for me to proceed through the PhD program and complete the thesis would like to thank Prof Le Minh ‘Thang for her ftom-time-to-time encouragement, She has been a strong supervisor lo me throughout my school years

at HUST, but she has always given me sufficient ficedom to carry onl independent work, Af the same time, T also wart to thank Dr Vuong Tharih Huyen for her support, great contribution and feedbacks on the publications and disscrtation

1 would like to acknowledge Prof, Dr Angelika Brickner and Dr Jabor Rabeah for helpful guidance, the experience shared, and discussions dumng my

research at Leibniz Institute for Catalysis (University of Rostock, Germany) Further

thank goes to Dr Stephan Bartling for the XPS measurements and useful ideas, Dr llenrik Lund for the XRD measurements and valuable comments Additionally, [ would like to send appreciations to Mr Reinhard fickelt for BET measurements, Mrs

Anja Simmula for the ICP-OES me: nents,

Last bút not least, [ would like to thank my friends at Hanoi University of Science and ‘Technology and at Leibniz Institute for Catalysis for all assistances and for the enjoyable time, friendly events we shared together

Finally, I would like to express my decpest appreciation to my family and my love for all their love, patience, encouragement, and unconditional support throughout ny life including the years of PhD studying

THE NEW CONTRIBUTION OF THE DESSERTATION

CHAPTER 1 STATE OF THE ART

1.4 Nitrogen oxides emission and abatement

1.2 Selective catalytic reduction of NO; with NH:

1.2.1 Overview of the selvclive catalytic reduction Icolmology

1.2.2 The mechanism of NHs-SCR

1.2.3, Catalysts for NHs-SCR of NOx

1.2.4, Effect of other components in NH:-SCR of NOx

1.24.1 Inhibition of water vapor

1243 Porsoning hy sulfur dioxide

1.24.3 Possoning by alkali metals

1.3 - 7colile and silicoaluminophosphale materials

1.3.1 Overview of zeolite materials

1.3.2, Overview of silicoaluminophosphuale maicrials

1.4 Calalysts selection for NH3-SCR of NO

1.4.1, Supports selection for NTI-SCR of NOx

Copper species as active sites

14.23 Multimetallic species as active sites

eB

gs BR

1.5 Scope of the dissertation

Synthesis of SAPO-34 support

Preparation of metal/zeolite catalysts

NLUG5-SCR activity test of catalysts

Catalyst characterization methods

‘The X-ray diffraction spectroscopy

Inductively coupled plasma - optical emission spectrometry

Klame atomic absorption spectrometry

Field emission scanning electron microscopy and energy dispersive

Xeray spectroscopy

Brunaucr — Exunclt— Teller sunface arca analysis

Fourier transformed infrared spectroscopy

Chemisorption temperature-programmed

Solid-state nuclear iagnelic resonarive spectroscopy

UV-Vis diffise reflectanes spectroscopy

2.3.10 X-ray photoelectron spectrascopy

2.3.11 Electron paramagnetic resonance

CILAPTER 3 RESULTS AND DISCUSSION

‘The influsnee of OSDAs on the formation of SAPO-34 structure

‘The influence of silicon sources for SAPO-34 formation

Copper-iron bimetal ion-exchanged SAPO-34 for NIT:-SCR of NOx

Structure and texture of catalysts

Redox and acid properties results

Cu and Fe species onto SAPO-34

Catalyst performance

A comparison catalysts performance between metals-based SAPO-

34 and metals-based ZSM-5

‘The stubility of SAPO-34 bascd catalysts

Influcnee of hydrothomal aging on activity

89

18

9

Water vapor and SO; poisoning resistance

3.6 Structure-reactivity relationships and active sites

3.6.1, in-sinc EPR investigations

3.6.2, in-sitc FY-LR investigations

3.7 Proposal NIIs-SCR mechanism over Cu-Fe/SAPO-M catalyst

GENERAL CONCLUSIONS AND OUTLOOK

PUBLICATIONS OF THE DISSERTATION

AI

AI8

Brunauer Emmett Teller Chabavite

Double 6-membered rings Dicthylamine

Diesel oxidation calalyst

Diesel particulate filter

Energy-dispersive X-ray spectroscopy

Flectron paramagnetic resonance Elsy-Rideal

European Union

Flame atomic absorption spectrometry

Ficld cinission scanning electron mnicrosenpe Kourier transformed infrared spectroscopy Gas hourly space velocity

Selective catalytic reduction by hydrogen

‘The temperature-programmed reduction with H»

Selective catalytic reduction by hydrocarbons

Inductively coupled plasma optical cimission spectrometry Internationat Union of Pure and Applied Chemistry

Langmuir-Hinshelwood

Morpholine

Membered rings

Methanol to light olefins

Selectve catalytic reduction by ammonia or urea Temperature-programmed desorption with ammonia Nuclear magnitic resonances

Nitrogen oxides

Organic structurz-directing agents

Parts per million

Silicoaltuminophosphates

Secondary building units

Selective catalytic reduction

‘Triethylamine

Weight by weight

Weight percentage X-ray photoelectron spectroscopy X-ray difraction

Zeolite Socony Mobil 5

LIST OF FIGURES

Ligure 1.1 Schematics of atmospheric NO, reactions

Figure 1.2, The entission of NOc in the EU from different sector groups

Figure 1.3 Concept of installing urea tanks in heavy duty vehicles

Figure I.4.A schematic diagram of SCR reaction following E-R mechanism

Alef), L-H mechanism (right)

Figure 1.8 NiTeSCR reactian process over iron-exchanged zeolites

according to Brandenherger et ai

Figure 1.6 The developed zeolite-based catalysts with various topology

structures for NH-SCR

Figure 1.7 Three steps of the sulfate deposition and the corresponding

methods for the restriction of the negative affects af the SO> paisoning

Figure 1.8, Schematic of zevlites Bronsted acid site

Figure 1.9 Schematic formation of AIPO4

Figure 1.10 A planar schematic af silicon incorporation mechanisms in an

AIPOs framework

Figure 1.11 Browsted acidity in zeolite and SAPOs

Figure 1.12 Kramework of MEI projected along [010] and aa illustration

of the molecular chummely and cages for the 10MR opening

Figure 1.13 Framework of CHA projected along [010] and illustration of

the molecular channels and cages for SMR pore opening

Figure 1.14 Possible iron species present as active sites Fe-seolites for

NH+SCR

Figure 1.15 Possible cation positions in the CHA structure

Pigure 1.16 Proposed reaction mechanism of NUs-SCR reaction over Cu-

zeolites

Figure 2.1, Experimental diagram for preparation of SAPO-34 support

Figure 2.2 Experimental diegram for preparation catalysts

}gure 2.3 Schematic dingram of NHj-SCR experimental apparanis

Figure 2.4.4 scheme set up of the water evaporation Jor the experiments of

the effect of the water

Figure 25 a! The diffraction of the X-ray bean on the planes of the

crystalline of the solid b) the principle of the X-ray powder diffraction

Figure 2.6 a} BET isatherm ired) compares to Langmuir isotherm (blue)

Figure 2.7 The scheme of insite FT-IR experiments set up

Figure 2.8 Mustration of the Zeeman splitting for aS — 1/2 system with one

unpaired eleciron in an external magnetic field Bc

Figure 2.9 2) CW EPR spectrum for an axially elongated copper complex;

4i same spectrum drawn as absorption spectrum with a 2D plot

Figure 2.10 Scheme of the in-situ EPR experiment sei up

Figure 3.1 XRD diffraction pattern of as-synthesized samples

Figure 3.2 PU-SHM images of as-synthesized samples

Figure 3.3 FT-IR spectra of all samples with a fiull range of wavelength

Figure 3.4 Ne adsorption and desorption isvtherms of as-synthesized

samples

Figure 3.5 NU;-EPD profiles of as-synthesized samples

Figure 3.6.2?Si MAS NMR spectrum of all samples

Figure 3.7 X-ray diffraction patterns of as-synthesi

+igure 3.8 FE-SEM images of as-synthesi:

Figure 3.9 No adsorption and desorption isotherms of S08 and S12 sanples

Figure 3.10 NH:-TPD pattern of samples

Figure 3.11 “Si MAS NMR spectrum of S08 and S12 samples

Higure 3.12, XRD patterns of as-synthesized SAPO-34 and catalysts

Figure 3.13 Nz adsorption-desorption isotherms of as-synthesized sumples

Figure 3.14 RE-SEM images of all catalysis

Figure 3.16 {TPR profiles of catalyst samples

Figure 3.17 UV-Vis DRS spectra of the catalyst samples

Figure 3.18 XPS results of © Is of ail catalysts

Figure 3.19 XPS results of Fe 2p of 1Fe/SAPO-34 and 3Cu-1Fe/SAPO-34

Figure 3.20 XPS results of Cu 2p of 3Cw/SAPO-34 and 3Cu- If e/SAPO-34

Figure 3.21 EPR spectra of catalysts measured at roon: temperature

Figure 3.22 a} Conversion of NOx and b) selectivity of Nz and N:O

concentration during standard NH3-SCR of Cu/SAPQ-34 catalysts

Pigure 3.23 a) Conversion of NOx and 6) selectivity of Nz and N;O

concentration during standard NH:-SCR of Fe/SAPO-34 catalysts

Figure 3.24, a) Conversion of NOx and b) selectivity of Nz: and N20

concentration during standard NIU3-SCR of Cu-Fe/SAPO-34 catalysts

Figure 3.25 a) Comparison conversion of NO; during the standard NHj-

SCR and bj NH: conversion during the NHs oxidation experiment of

3CWwiSAPO-34, 3F e/SAPO-34 and 3Cu-LFe/SAPO-34

Figure 3.26 XRD patterns of 75M-S and as-synthesized catalysis

igure 3.27, PE-SEM images of all catalysts

Figure 3.28 Nz adsorption and desorption isotherms of catalysts

Figure 3.29 EPR spectra of catalysts measured at room temperate

Figure 3.30 a) Conversion of NOx and b) selectivity of Nz and N;O

concentration during standard NH+-SCR of CwZSM-S catalysts

Figure 3.31 a) Conversion of NOx and b) selectivity of Nz and N2O

concentration during standard NH3-SCR of Fe28M-5 catalysis

Figure 3.32 a) Conversion of NOx and b) selectivity of Nz and N;O

concentration during standard NH3-SCR of Cu-Fe/ZSM-3 catalysts

Figure 3.33 NOx conversians versus temperatures aver metals-hased ZSM-

5 fdash line) and metals-based SAPU-34 (straight line)

Figure 3.34 a) NOx conversion of Cu/SAPO-34 aud Cu-Fe/SAPO-34 afier

hydrothermal aging with GHSV of 120000 h" and (bi XRD panerns of fresh

and hydrothermal aging catalysts

Figure 3.33 NO conversion ever Cu/SAPO-34, Fe/SAPO-34 and Cục

FeiSAPO-34 catalysts at 200 °C under GHSV of 70000 ir in the co-

presence of Hi + 80:

Figure 3.36 NO, conversion aver CwSAPO-34, Pe/SAPO-34 and Cu-

FeiSAPO-34 catalysts at 300 °C under GHSV of 70000 ir! in the co-

presence of TiO \ SQ:

Higure 3.37, in-situ BPR spectra of a) Cu/SAPO-34 and bj C

afier NHi/He/NO+O> adsorption at 200 °C

Fiyure 3.38 Skeleton structure diagram of the unt cell of SAPO-34 Fach

solid circle represents an Al, P, or Si and the open circles represent oxygen

Figure 3.39 insite FT-IR spectra of all catalysts obtained after pre-

adsorption of 0.2 vol NH:/He and subsequent exposure to 0.2 vol4 NÓ —

3 tai %4 O2He for 45 min at 200 °C

Figure 3.40, In-situ 'T-I spectra of all catalysts recorded at 200 °C after

J pre-adsorption of 0.2 vol.% NO/He for 30 min, then 2) 0.2 vel.% ppm NO

+ S vol O:/He for 30 min, followed by dosing of 3) 0.1 vol.6 NH:/He for

45 min with aj 4000 — 1300 env! and b) 2000 — 1300 on?

Sigure 3.41 Propose NHs-SCR mechanism over Cu-Fe/SAPU-34 catalyst

HB efSAPO-34

Figure 3.42 The NH:-SCR reaction state over Cu/S4PO-34 and Cu-

Fe/SAPO-34 catalyst samples under different temperatures a) and b}

catalyst at 200-350 °C, c} and d catalyst above 350°C 114

LIST OF TABLES

Table 1.1 Properties of selected nitrogen oxides

Table 1.2 Name, abbreviation, and structure of sume OSDAs used to

synthesize SAPQ-34

Lable 2.1 The composition of OSDAs for SAPO-44 preparation

Table 2.2 Different silicon ratio by TEOS and LUDOX AS-30 of as-

pnthesized samples

Yable 3.1 Physicochentical properties and crystallinity af the all samples

Table 3.2 Elemenial compositions and relative ervsiallmity of the producis

Table 3.3 Acid properties of the as-synthesized samples

Table 3

comparation between TEOS and AS-30 precursor

Physico-chemical properties of synthesized samples and

Table 3.5 Acid properties of the as-synthesized samples

Table 3.6 Physico-chemical properties and crystallinity of as.ynthesised

saniples

Table 3.7 Acid properties of the as-synthesized catalyst

Table 3.8 XPS quantitative analysis of afl catalysts

Yable 3.9 #e, Cu, and Na amounts obtained by ICP-OES and textural

INTRODUCTION

‘Nitrogen oxades exist in the environment in various species such as N-O, NO, NO», Nz, N2Os, and N:Os, By definition, the abbreviation NOs is used for nitie oxide (NO) and nitrogen dioxide (NO2) They are considered to be toxic and chemical precursors that lead to ground-level ozone, a ubiquitous air pollutant in urban areas

A major source of NOs is generated during the combustion of fossil fuels trom stationary sources such as coal-fired power plants and mobile sources such as diescl~ powered vehicles, Over the past years, many technologies including fuel control, combustion control and post-combustion control have becom developed, and are

commercially available for the control of NO; emissions Among these technologies, the selective catalytic reduction of nitrogen oxides by ammonia (NHs-SCR) is ane of the most popular post-combustion techmques for NOx emission control and is worldwide applicd in stationay sources and dicsel vehicles duc to its high eflicicney, high selectivity and low cost,

Since the early 1970s, various catalytic mateials have been developed for the SCR of NOx to mcol the stringent regulation of NO, reduction The most popular NIIy-SCR catalysts used for cleaning flue gases from power plants are V0,- WOsTiO» oxides which, however, operate only in a slightly high and narrow temperature range of 300 - $00 °C Also, the toxicity of vanadium species is an issue Additionally, VinO; has been attracted significant interest in the development of low-

temperature SCR catalysts With Mn(,-based catalysts, almost total NO, conversion

has been obtained already at temperatures well below 150 °C Although significant elferts have been given to the investigation of MnOx for low-temperature NIIs-SCR, poor resistance to SO: and H+O as well as large N«O formation are serious problems for practical applications In recent years, the ion-exchanged zeolites have been

reported as promising catalysts for diesel vehicles duc to their high adaptability to high space velocity Furthermore, zeolites have the advantage of inexpensive cost, nontoxicity and good thermal stability, Several types of metal-ions exchanged zeolites meluding ZSM-5, femerits, mordenite, and Beta-zeolite have been studied

for WH3-SCR of NOs Medium pors

have been considered to be more active compared to larger pore zeolites (zeolites Y,

outiles such as ZSM-S, ferrierils or mordumte

Beta-zeolites) Recently, zeolites with even smaller pore window size with the chabavite structure altracted much attention in NI5-SCR for NOx elimination due to their excellent low-lemperature activity and high hyebothermal stability Moreover, small pores have been considered decreasing the net dealumination rate, which will

1

1oad to cuhaneing the BÊ: span 6Ÿ he gafalysls Ít any gase, The proscuce of a transition metal ion is necessary since this ion acts directly as a redox catalytic center

Many metals such as Cu, Fe, Ce, Co, Mn, ote, have already been investigated

for NHs-SCR of NOx Among them, Cuand Fe have been attracted significant interest

due to their availability and high activity and suitable catalysts for the wide operating

lemperature window F

(above 400 °C), whereas Cu-zeolites have been reported to reach higher activity at

low to medium temperature (200 - 400 °C) ‘Ihe temperature of NO,-containing

liles ave shawn boller activity at high temperature

exhaust gases from other sources such as diese! or lean-bum gasoline engines is much lower around 150 - 300 °C, while the presence pf tioisture al the temperature of the soot and ash filter regeneration of a diesel particulate filters often over 450 °C: Furthermore, due of the limited volume of catalytic converters in these engines, catalysts that arc active at high gas howly space velocity circumstances yet at

activity

The metal loading on the catalysts is primarily responsible for the catalytic performance, whorvas the eartict is woslly responsible for the creation of stability and selectivity Zeolites have lately gained the interest of numerous researchers due

to their ubiquitous appheation in related catalysis, their ease of availability, and their

frame stability at a range of working temperatures It has been demonstrated that the sinaller the pore sive of the veulite, the more aclive the catalyst SAPO-34 with a chabazite structure was shown to have superior hydrothermal stability than SSZ-13

or ZSM-5 Hydrothermal stability is requized when the exhaust temperature exceeds

680 °C in amoistnized environment, as occurs during the regeneration of the diesel particulate filters section, which is typically located in front of the SCR section

SAPO-34 molecular sieves with a low silicon concentration and a homogeneous

distribution are critical for maximaizing nitrogen selvetivity SAPO-34 is synthesi

by hydrothermal method in the presence of organic structure-directing agents (alkylamines and morpholine) Various factors atfect the proparties of synthesized

re

SAPO-34 including the chasor templates, the Al and Si sources, the molar ratios of Si/AV'templates of the gel, the gel aging time and temperature, and the reachon time and tempcraturs Among them, organic structure-directing agents and silicon souress

interaction affects both dispersion and redox behavior of the active phase and also work fumction properties the use of bumetal ion-exchanged zeolites as catalysts for wide-temperature windows of NHsSCR of NOs is limited and the available

information on the structure and the redox behavior of aclive siles in Iimelal ion-

optimized in a suitable way

‘The objective of the study

‘The main objective of the thesis is “Synthesis of Cu-Ne/SAPU-34 catalysts for the selective catalytic reduction (SCR) of NO with NH” To wake this approach efficient oparando studies, exhaustive catalysts characterization and mechanistic studies will be also considered

THE NEW CONTRIBUTION OF THE DESSERTATION

‘The impact of various mixes of organic structure-directing agents on the creation of the SAPO-34 structure was explored, SAPO-34 was synthesized through

4 hydrothermal process using three different templates: tricilrylamnine, tettaethylammonium hydroxide, and morpholine The concentration of the templates was adjusted by excluding competing phases in order to achieve the purest form of the SAPO-34 phase Applying combincd onganie structure-dirccting agents including

‘triethylamine, tetraethylammonium hydroxide, and morpholine has demonstrated to

be an effective and cost-down method to synthesize SAPO-34 catalysts Also, the SAPO-34 molecular sicve has been synthesized under hydrothermal conditions by using a combination of tri-emplatss with different siica sources, such as TCOS and colloidal silica LULOX AS-30 ‘The extent and effects of silicon substitution on these materials have been investigated Using TEOS as silica source may result in uniform

wilh a relatively stall s

‘The SAPO-34 based and ZSM-S commercial based with Cu-, Ke-, and Cu-e-

were prepared through the ion-exchanged method in an aqueous solution The investigated catalysts have been applied in the selective catalytic reduction of NOz

with NII; at the normal reaction condition and the presence of SO: and [0 conditions The results suggest that suitable metal content could promote NOx

conversion, while the excessive Cu or Fe loading could block the “channel” of veolites The synergistic effect belween iron and copper in the Cu-Fe/based catalyst

prompted higher catalytic performance in mote extensive temperature as well as hydrothermal stability and poisoning stability after iron incorporation Meanwhile,

SAPO-34 based eatalysts showed higher catalytic performance in more extensive

temperature compared with ZSM-5 commercial based catalysts The Cu-Fe/SAPO-

34 showed superior H»O and SO> resistance, compared to Cu/SAPO-34, maybe because Fe supported onto the surface of catalysts, resulted the reaction with SƠ: and

120 became much more efficiently, and then the Cn active sites were protected

These results showed their potential application in industrial catalysts

Spectroscopic in-site studies are performed with a series of

alematially ion- exchanged SAPO-34 calaysls im [he selective calalytLe teduclion of NÓ; with NIỊ;

since the few known results promise a high potential in this important application

4

field whon the catalysts aro optimized in a suitable way The lacation of Cu and Fe ion in SAPO-34, the nature and role of active sites, particularly the valence states of

Cu ion spscics under reaction conditions arc controversially discussed, The well-

obtained valu:

in the in-stéz analysis would reconunend the potertial application of

these bimetallic Cu-Fe/SAPO-34 for NO, removal in the operation of the wide-

temperature window,

CHAPTER 1 STATE OF THE ART

1.1 Nitrogen oxides emission and abatement

One of the mosl serious concems in urban arcas is air pollution, which is caused by the high concentration of sousces of airborne contaminants Air pollutants include a wide variety of compounds that negatively influence the environment,

animals, plants, people, ecosystems, and thmgs humans own, such as agricultural

crops or man-made structures The major sonrees of air pollution are power plarts (20%), automobiles (34%), and incineration operations (26%) that use the combustion of fossil fucls [1] Sulfur oxides, particulate matter, carbon monoxide, unburned hydrocarbons, and nitrogen oxides are the primary contributors to urban air pollution due to the products generated during the combustion process in internal

Properties

NO, compomnd Color Solubility in von fee | Density (giảm) Ambicut

inthe environment in various species which was shown in Table 1.1 hi atmospheric

chemistry, as a result of its stability and environmental impact, NO: is commonly referred to as the genetic term for nitrogen oxide (NO) and nitrogen dioxide (NO+}

|2] NO is unstable and quickly interacts with O: in the atmosphere to create NO: by photochernical oxidation, which has a direct cfTect on health and is a sourec of oxone

production as well as a significant cause of fog formation [3] Moreover, dinitrogen oxide (N-0) is also known as one of the greenhouse gases, which can absorb infrared

6

radialion at a 270 times higher inlensily than carbon choxide (CO;) [3] and NzO ha

a long half-life around 126 to 160 years since itis not highly reactive |2]

Figure 1.1 Schematics of atmospheric NO;, reactions [2]

‘NOy is emitted into the atmosphere by both natural and anthropogenic sources Tỉisalso created nalumally by lightning, volcaniv activities, and biornass burning from forest fires, as well as to a lesser amount by microbialogical processes in soils [3] Some reactions which NO, can undergo in the atmosphere are presented briefly in Figure 1.1, There are three major anthropogenic sources of nitrogen oxide,

© Thermal NO;: Thermal NO, refers to NOs formed through high temperature oxidation of the diatomic nitrogen found in combustion air [2] The formation rate is primarily a fimotion of temperature and the residence time of nitrogen

at that temperature

Ne | O22 NO, APE = 150.6 kilfinole (fig 1h This reaction is exposed above 1300 K and followed by the Zeldovich mechanism |4] of chain reactions involving N and O> activated atoms,

NO formation is dominated by the Zeldovich mechanism [4] The production

of NO is affected by the fucl-air ratio, and it is inercascd on the fucl-Ican side

in the stoichiometric ratio

«_ Fuel NO: Transportaon fuels are estimated to be responsible for 54% of all anthropogenic NOx [2] During combustion, the nitrogen bonded to the fuel is

Ni or NO NÓ; production is generally independent of temperature in fuel, contrary to thermal NOx

© Prompt NO Prompt NO, is produced when molecular nitrogen in the air

released as a free radical, resulting in the formation of fr

China contributes the greales anthropogenic NO; emissions (approximately

21546 Gg/year), which is over twice the emissions generated by the United States (14687 Gp/year) and the Ewopean Union (EU, 10074 Gg/vear) [5] In the United Stles suả the Euopem Union, approximately 30% of total anthropogenic NOx emissions are emitted from mobile sources In the emerging countries such as Vietnam, power production and industrial processes are the primary NO sources, for example, in 2020, about 655899 tons, 816 tons, 32342 tons were discharged tiom industrial avlivily, steel production and power production, respectively [5] Aldiough

NO, levels remain high, Western nations and the United States have achieved a steady reduction in NO; emissions However, decreasing NO; emussions is one of the major issues confronting growing countries such as China, Brazil, India, and Victnam Cunently in Vietnam, there are only very few industrial plants that have a NOx control system in exhaust tumes The rest, other factories still do not have treatment equipment, bul ruainly rely on the ability lo diffuse pollutants by chimneys

Anthropogenic NOx emissions are primarily caused by energy burning in stationary, mobile sources, and industrial processes An inspection of NOx sources is

useful for determining where the largest reductions im NOx ctnissions may be accomplished Vascellari et al [5] has described the distribution of NOx emission sources in Europe ftom different sector groups in Figure 1.2 Road transportation accounts for the majority of NOx emissions (40%), and since it accounts for such a big proportion of total atniss

Around 80% of the NO, released by automobiles is produced by diesel-powered vehicles, which have a significantly greater proportion of hazardous NO: than

ons, Tednetious hers will have a sigmificenl impact

gasoline-powered vehicles, In recent years, more stringent car emission regulations (Euro standards) have been implemented in Europe in order to reduce the extent of air pollution caused by automobiles The Euro | standard was approved as a final rule

in 1992, requiring the installation of catalytic converters in gasoline vehicles to

minimize carbon monoxide (CO), hydrocarbons (HC), and NO; emissions The most recent standard, Euro VI, became applicable to all new cars in September 2015, which

requires a further dramatic reduction in NO; produced by diesel engines to a limit of

80 mg/km This number is significantly lower than the threshold of 180 mg/km

necessary for diesel vehicles to fulfill the previous Euro V standard The NOs limit for gasoline vehicles stays at 60 mg/km, which is equivalent to the Euro V standard

By 2018, all newly registered commercial cars in Vietnam will have to meet the minimum standard of Euro I'V instead of Euro II as before Therefore, this dissertation

focuses on one such technology to control and curb the amount of NO; released from

a heavy-duty diesel engine

14%

Energy used in industrial [Commercial insttutional and households

Figure 1.2 The emission of NO, in the EU from different sector groups [5]

In the face of increasing NO; emission limitations set by the Gothenburg and Kyoto Protocols, the development of new technologies and improvements to already employed procedures are required In principle, there are several ways for reducing

NO, emissions from energy combustion [2, 6], such as:

© The fuel control approaches attempt to reduce the nitrogen content of fuels

pnior to combustion by utilizing ultra-low nitrogen fuels such as ethanol,

natural gas instead of diesel oil, or pure oxygen instead of air The efficacy of

combustion control methods, however, is highly dependent on the kind of

combustion system

© By adjusting or modifying the firing conditions, combustion control

technologies can minimize NO; production throughout the combustion

process Therefore, the major goals of these approaches are to generate a fuel-

rich state at maximum flame temperature, lower flame temperature, or alter

rasidence duration within different regions of the combustion vone However, the efficacy of combustion control methods is highly dependent on the kind of combustion system,

© Post-combustion methods or after-treatment methods comprise the selective

ison is thal three-way calalysls arc oplitriza:

emissions management under net stoichiometric exhaust conditions, which occur when a diesel engine operates in lean-bum mode AAs a result, the base difficulty in controlling NOz in Jean-bumn engine

an O:-containing enviroment, rll now, past-heatinen! techuiques have

aust is reducing NO; in

been classified into two categories based on the type of control strategy used sclective calalyic reduction and lean-NOx trapping [2] In the first approach, NOx is removed using adsorption/reduction processes, taking advantage of the better solubility of NO, at high concentration As a result, this technique is

mostly used to minimize the amount of NO, emitted by industrial operations

However, sulfur and high temperature exposure cau have a substantial negative effect on NO, reduction efficiency, and this approach requires

expensive equipment, The second technique avoids these disadvantages, as

NÓ; is often reduced to N: by selective catalytic reduction with ammonia,

urea, hydrocarbon, or hydrogen [7]

Of couse, other techniques like absorption, adsorption or electrical discharge anc also widely uscd [2, 6] Nevertheless, all of these tcolmiquos have limits and drawbacks, such as being expensive and requiring a consistent sonrce of electricity

or hydrogen, and they are not likely to play a major role anytime soon Furthermore, NOx emission regulations imposed on certain industrialized counties are quite sbingent, As a result, traditional vehicles with catalytic converters thal omploy NOx storage and reduction as well as selective catalye reduction to mimmize NOx emissions remain the most appealing duc to their low cost and high cflicicncy

1.2 Selective catalytic reduction of NOx with NH3

4.2.1 Overview of the selective catalytic reduction technology

The sclective catalytic reduction (SCR) iechnique may be used to combustion gases to substantially reduce the residual NO; concentrations in the exhaust ‘The

10

fundamental idea of the SCR of NOx is to transform the nitroxides into harmless components, notably nitrogen gas and water, by contacting the combustion gases with

a reducing agent on an intelligently designed catalyst [1] At present, the main reducing agents being examined are hydrocarbons (HCs-SCR) and ammonia or urea

(commonly referred to as NHs-SCR) and hydrogen (H:-SCR) However, HCs and Hs

have some drawbacks which prevent their practical application H2-SCR on supported noble metals (Pd, Pt) can decrease NO; at low temperatures (below 200 °C), but these

catalysts are costly and not resistant to H:O and SO: [8] Meanwhile, because

pattially bumt or unburned hydrocarbons are already present in the exhaust stream,

using hydrocarbons as reducing agents for the HCs-SCR process appears to be a

highly appealing alternative However, at high temperatures (over 400 °C), HCs are

oxidized, and their NO; reduction effectiveness is decreased [8] Therefore, NHs-

SCR is the most applied technology for the NO; abatement

e NOx = ePM/HC

`

Figure 1.3 Concept of installing urea tanks in heavy duty vehicles [9]

Ammonia assisted SCR was already in use during the 1970s [1, 7, 8], although

mainly for stationary applications Because of the toxicity of ammonia, storage of

ammonia in pressure containers within the vehicle is not feasible for mobile

applications As demonstrated in Figure 1.3, urea is the ideal selective reducing agent

for mobile SCR applications due to its low toxicity and safety To meet emission

regulations, the exhaust gas treatment unit includes a diesel oxidation catalyst (DOC),

a diesel particulate filter (DPF), and a SCR catalyst

The basic đea behind Urea-SCK ïs to docompose squoous urca in water to

NHb via the following reactions |1, 10]:

(NHj:CO — NH: + HNCO (Eq 1.4}

HNCO + HO — NXH: + CÓ: (Eg 1.5)

The NHs-SCR process is described by the following reactions:

4 NH: — 4NO+ OQ; > 4 No + 6800 (Eg 1.6)

2NH; + NO + NÓ 2N + 8 HO Œg L7)

A NH3 — 6 NO2 > 7 Nz + 12.420 (Eg 1.4)

Eq 1.6 is the so-called “standard SCR” reaction which typically works well at high temperatures about 300 — $00 °C in the presenve of oxygen and constitutes the overall stoichiometry of the reaction (NHs/NO = 1/1), This reaction can proceed faster when a 1:1 mixture of NO and XO: reacts with NHs (Eq 1.7), whichis referred

to as “fast SCR”, duc lo the slrongor oxidizing ability of NO-; compared to that of O2 [11] When the feed contains only NOs, the reaction with NUs is called “NO-SCR” (Eq 1.6) Since the investigations in this thesis are focused on standard SCR only, the following description of state of the art is restricted to this reaction

Frequently, the N2 selectivity in NH3-SCR is limited by undesired XH; oxidation giving rise to N-O and/or NO (Eg LI 1.13), which depends on the catalyst and is usually most pronounced at high tarmperalure [12]

2NHs + 207 NIỚ+ 3 HhÓ (Bạ 111) 4NH,+ 5Q; — #NG + 6 HịO (Ra 112)

ANH + £NÓO — 3Ða — 4 MO — 6 HạO (Eụ 111)

Moreover, NO can arise from NO alone (Rg 1.14 - 1.13):

$NO2N,G | O: (Bg 1.14) 3NO > N:O + NO: (Eq 1.15:

1.2.2 The mechanism of NH3-SCR

In general, the heterogeneous catalytic reaction of NH: and NO, comprises four rain steps (1, 8]

* The adserplion of N13 on the surface of the catalyst to form either NTIa*as on

‘the Bronsted acid sites or NHasgs on the Lewis cites

12

© The reaction of NHvNHs* with either NO/O2 gases according to the Eley- Ridcal (E-R) mechanism or with nitiites/nitrates according to the Langmuir Tiinshelwood (1-11) mechanistm lo form the intermediate species NT-NOy

© The decomposition of NH,-NOy lo form Nz and H20

© The cxadation of metal fiom low valence state to high valence statz by oxygen

Figure 1.4 A schematic diagram of SCR reaction following E-R mechanism Vefii,

My +280 Figure 1.5, NHSCR reaction process over iron-exchanged zeulitey according, to

Arandenberger etal {7, 23]

However, up to now, the reacton mechanism of NHs-SCR remains

controversial Onc of the most controversial issues is the Brensted and Lewis acid sites! role in the SCR reaction ‘Topsee et al has proposed the SCR mechanism on

Bronsted acid sites, including two cvoles: the acid circle and the redox circle (14, 15]

Ramis cl al has proposed the SCR reaction on the Lewis cites The reaction of NH3ads

with NO forms NII:-NO as the intermediate, which further decomposes to N; and

13

H;O [16] hupinoipie, đopznding ơn he Iype of catalysts suổ thơ roaetion conditions, the involvement of the Lewis and Bronsted sites 1s different and still under debate

‘The reaction of NHz (NHy/NHs') with NO is also a matter of discussion in NHs-SCR

studies, however, there is no evidence for the existence of the complex NILA-NO by

IR spectroscopy because of either the low concentration or the fast decomposition [12] NH can react with gaseous NO according to the E-R mechanism or with the adsorbales of NO on the surface (Witriles/nitratus) aveording to the 1-H mechanism

to form the intermediate species NLL-NOy, as shown in Figure 1.4 llowever, some

studies have reached the consensus that this reaction’s pathway depends on the temperature At low temperatures, the SCR reaction follows the E-R mechanism, while the T.-H tnochavism dominates al high tomperatures [1], Branckanbergor ob al have summarized the SCR reaction process over metal-exchanged zeolites according

to the scheme in Figure 1,5 [13]

1.2.3 Catalysts for NH3-SCR of NOx

To approach the practical appheation, the SCR catalysts need to perform the

high aclivity al wide tcumperatures and tolerate the negative effects of the other components in the flue gas The enhancement of the redox property and the surface

acidity are the targets for preparing the SCR catalysts

Supported varadiun-bascd catalysis have been well-known NHy-SCR catalysts Three critical variables influence the catalytic activity of these catalysts in NH+SCR: the structure of the VO, surface species, the acid-base characteristics of

of VOs surface sites and suppor's

‘During a few decades, various materials, such as ‘TiO [17], AlOs [18], CNTs [19],

CeO, [20], and mixed metal oxides have been investigated as the supports of vanadium-bascd oxides catalysts However, because the catalysts operate only in a

sublisnes al high (emperntizes above 580 °C Currently, metal oxide catalysts and zeolite-based catalysts are receiving a lot of attention as vanadium-free catalysts for the low-temperature NHs-SCR process in diesel engines, and some of them have great potential in practical applications

Manganese-containing oxide catalysts have been shown to be the most active among a range of oxide catalysts evaluated for low-temperature NH:-SCR [21], therefore MnO has attracted the interest of rescarchers working on low-temperature

14

SCR catalysts At temperatures far below 150 ®Ơ, slmosi lolal NÓ cơnvergiort Was achieved using MnOx-based catalysts, Because various stable oxides exist under ambicnt scttings, the catalytic efficacy is highly dependent on the kind of MnOs used

‘The greatest activity was found in amorphous MnO: with a large suface wea, followed by MnsOs, MnO, MmOu, and MnO in decreasing order [22] Although substantial attempts to investigate MnQx for low-temperature SCR, poor resistance

10 SO» and HLO, as well as high N:O production, are sovere issues for practical applications 'lo address these problems MnO has been mixed or doped with other metal elements such as Ce, Zr, Fe, and others [1, 23], The low-temperature deNO, performance of MnO;-CeO: catalysts can be further improved by a third metal co- component such as $n [24] and Ti [25] Despite significant prograss with Mucba:

Aside trom MnO.-based catalysts, the ion-exchanged zeolites have been

described as potential diesel vehicle catalysts due to their great adaptability to gas hourly space velocity (GHSV) [1, 21, 22] Furthermore, zeolites offer the benefits of

low cost, nontoxicity, and high thermal stability Because of their outstanding low-

temperature avtivily and greal Irydrotlermal stibilily, zcotites with ever smaller pore window sizes with the chabazite (C1LA) structure, nowadays, have gamered a lot of

attention in NH:-SCR for NO, removal, as illustrated in Figure 1.6 [1, 9, 26]

Furthermore, small pores have been explored to reduce the net dealumination rate

and jroblems conmected with unburned hydrocarbons, which will lead to an increase

in the life duration of the catalysts In every instance, the presence of a transition

metal ion is required since this ion functions directly as a redox catalytic center Many

inelals, including Cu, Fe, Ce, Co, Mn, and others, have already been studied for NH3-

SCR of NO Among them, Cu and Fe have been attracted significant interest due to their availability and high activity and suitable catalysts for the wide operating

temperature witilaw Fe-zcolites lave shown betler aelivily al high temperature (above 400 °C), whereas Cu-zeolites have been reported to reach higher activity at low to medium temperature (200 400 °C) [1, 8, 27] CwZSM-5 catalysts have been

intensively investigated as SCR catalysts since their discovery in 1986 by Iwamoto

etal [28], but are restricted by their relatively low activily im the presence of water

vapor and by dealumination at high temperatures, resulting in.a loss of activity ‘These

18

issues arise because ZSM-5 is sensitive to adsorbing hydrocarbons at low temperatures, which generates heat as the temperature rises, causing damage to the

zeolite structure This is especially troublesome in automobile emissions since

substantial amounts of hydrocarbons can be adsorbed on the catalysts during cold

start [8] To solve these problems, other CHA zeolites studied by Dustin et al [29],

including as Cu/SSZ-13 and CwSAPO-34, exhibit enhanced thermal stability and

superior SCR performance both before and after high-temperature hydrothermal

treatment Wang et al [30] discovered that the SCR performance of ion-exchanged

CwSAPO-34 is tightly connected to zeolite structure and acidity, Cu loading and

Cu” species position Furthermore, the ammonia adsorption capability of the catalyst influences NO conversion over it Although CwSSZ-13 and CwSAPO-34 can

effectively remove NO, with high stability [30, 31], these catalysts, however, can

quickly adsorb sulfur species, leading to a severe decrease of the SCR performance

Many researches have been carried out in order to create NH3-SCR catalysts

with high activity and stability in the low temperature range in order to address the

problem of NO, emissions from both stationary sources such as coal-fired power

stations and mobile sources such as diesel cars The optimal catalyst for broad

temperature NH3-SCR of NO; should have high activity, N2 selectivity, sulfur-water

resistance, and hydrothermal aging, which results from a correct balance of acidity and redox characteristics of active sites, as well as appropriate pore widths As a result, in addition to zeolite selection, the metal used for ion exchange and its

16

proportics such as pore si⁄e, sơidHly, surface arca, and acid strongih dis

have a significant impact on catalyst performance,

1.2.4 Effect of other components in NHs-SCR of NOx

1.2.4.4 Inhibition of water vapor

The presence of water vapor or moisture is unavoidable while using NH+-SCR, whether on stalieniy sources or dics] engines Al low (omperatur: and high temperatures, different effects of HO on the activity of the NHo-SCR catalyst were reported [1, 8, 35] [he H-O might prevent the unselective oxidation of NH; at high

enhancing the high-tamperatias aotivily The

temperatun

can have a significant role in limiting SCR reaction and thereby decreasing activity

at low temperatures Its commonly described as the result of adsorption competition

20, on the other hand,

‘between HO and NH3/NO; molecules on the reaction sites, or as a change in the

struclure of active sites, the transformation of Lewis acid sites into Brerrsted acid sites [12, 35]

1.2.4.2 Poisoning by suifur dioxide

When fossil fuels, such as coal and diesel, are bumed, sulfur dioxide (80>) is released into the air The catalyst's NH:-SCR aetivity can be greatly affected by the

m

the SO, oxidation to SOs, and iit) the deposition of (NIL)›§Oy/XTUNSO¡ on (he surface of the catalyst [32, 33] ‘The latter blocks the active sites for SCR reaction, enormously decreasing the catalytic activity The pathways of restiictmg SO:

¢ Of SOs Three steps can describe the cffect of SOx i) the SO, adsorption, ii)

poisoning ara based on the sulfite formation mechanism and its offecls on the active sites of the catalyst, as shown in Figure 1.7

The transfonnation of SO› tơ SO4” on the surface of the catalyst increases the surface avidity, which enhances the adsorption of NH, consaquently inaproving the high-temperature NIIs-SCR activity considerably [12, 36] Additionally, certain particular catalysts, such as V-Os/AC, exhibit an increase in low-temperature activity [37] However, for vanadium-fiee catalysts, the active motal night be progressively sulfated, resulting in permanent poisoning and a significant loss in low-temperature activity Additionally, the formation of ammonium sulphites ar sulphates as a result

of the interaction af NH; with SO or SOs might result im the covering of active s

` and plugping of catalvst pores Thiz poisoning efifsct occurs actoss the board for all low temperature NHs-SCR catalysts [36, 38] Thermal decomposition of ammonium sulfatas is a highly successful method for reactivating deactivated NHs-SCR catalysts

17

0

Ê((NHJ,SO/NHỤHSO, —ˆ Esplerngtae Factesting the

{olerance componads Secompoitie stan

Figure 1.7 Three steps of the sulfate deposition and the corresponding methods for

the restriction of the negative effects of the SO2 poisoning 1.2.4.3 Poisoning by alkali metals

Alkali metals derived from fly ash are another important problem for the use

of SCR catalyst in coal-fired power plants The sources of alkali metals in diesel cars

include fuel and lubricating oil additives, as well as urea solutions [8, 10] A small

quantity of these metals accumulated will have a significant poisoning impact on the catalytic activity by reducing the number and strength of the Bronsted acid sites [33,

39] Periodic washing is also an efficient method of regenerating the alkali metal-

polluted catalyst, particularly for stationary sources

1.3 Zeolite and silicoaluminophosphate materials

1.3.1 Overview of zeolite materials

Pure-silica materials represent one molecular sieve subset, which consist of [SiO,] tetrahedra joined together to create a three-dimensional SiO» framework The

framework is neutral in the absence of lattice substitution The introduction of

tetrahedral aluminum into the framework leads in a negative framework charge,

necessitating the use of a cation to achieve neutrality Aluminosilicates, or zeolites,

are materials consisting only of [SiOs] and [AlO.] units These tetrahedra are the

fiamework's fundamental construction blocks Secondary building units (SBUs) are

extended configurations of connected tetrahedra with [Si-O-Si] or [Si-O-Al] sequence [40] Inorganic cations (e.g., alkali or alkaline earth cations), organic cations, ammonium, or protons can balance the framework charge The framework structure of the zeolite generates a pores structure within the zeolite The amount of

oxygen atoms in the ring determines the varies diameter of zeolite pores Zeolites are

classified into three types depending on pore size: small, medium and large pore [41].

The quantity of ahoninum in the zeolils framework is one of the most significant factors controlling the zeolite's material characteristics, SiOz has a neutral charge in a tetrahedral framework, but the equivalent alumina structure (AI: ) is negatively charged and must be bulanced by a counter ion, Alkali metal ions, hydrogen ions (protons), and larger metal ions can all perform the role of counter ion,

characteristics ‘The negative charge was given by aluminum in the framework also contributes to the hydrophilic characteristics of the zeolite, Water molecules, since they are polar and have a high dipole, tend to attract to the negative sites in the zeolite

coumer ion causes changes in the zeotite's phy

framework, As a rosull, vcolilos with a high alumingn content arc hydrophike, whereas zeolites witha high silica content are liydrophobic The higher the alumimm concentration of the zeolite, the higher the acidity [42], and this is umpertant to consider when-using zcolites as solid acid catalysts, When protons arc used as charge- balancing species, Bronsted acid sites are formed, and the resullant material ean be used in acid-catalyzed processes, Furthermore, the specified pore structure allows for shape-selective catalysis Figure 1.8 depicts the integration of aluminum into a silicate lattice, which is just one example of potential replacement, the total charge of

he strength of the

a basic fame in zeolite by Al atomic number in the frame

Bronsted avid force in zeolite is determined by this charge

Figure 1.8 Schematic of zeolites Bronsted acid site [42]

‘The transformation of an amorphous aluminosilicate gel into a crystalline zeolite product is the process through which zeolites are formed, At the moment, two major mechanisms for the production of zeolites have been postulated [43] One method is referred to as solution-mediated transport, and it involves the redissolution

of the aluminosilicate gel and the rebuilding of silicate and aluminate ions in the solving to form the zeolite structure The alternative method is the solid hydrogel transformation, in which the structiwe of a zzotite is generaled by reamangament of the solid alumincsilicate gel created by silicate and aluminate condensation ‘The real mechanism in the creation of a particular zeolite structure may lie halfway between

th two extreme Furthermore, depending on the synthetic

the formation process

19

circumstances, different molcuular sicves ean crystallize by different methods, and

evena single molecular sieve might be produced via distinct procedures

Despite the fact that zcolites were discovered in the middle of the XVIII century, their large-scale commercial application did not begin until two centuries later, in the 1950s Catalysts based on zeolites are now superior in many catalytic pre

around 27% of the worldwide catalyst production business [44] This is mostly due

to the fact that zeolite catalysts have several distinguishing features, such as a regular pore system and a high imer micropore surface area, include both Bronsted and

lo environmental issues and legal pressure for NOx emission reduction (1,8, 45]

1.3.2 Overview of silicoaluminophosphate materials

Aside from zeoliles, aluminophasphates (AIPO4) phases have beew found, which constitute the first family of framework oxide molecular steves produced without silica and were discovered in 1982 by Wilson et al [46] AIPOs are another class of microporous materials with tetrahedra of [ATO,] and [POg] This farnily of

materials has a structure similar to zeolites, however, the SBUs are composed of a

[Al-O-P] connection rather than the [Si-O-Si] or [Si-O-Al] bridges seen in zeolites [17] The changes to Al and P linnil structural flexibility to SBUs with an even uumber

of ‘T-atoms and introduce a greater charge distribution in the neutral framework

Figure 1.9 shows schematic formation ot AlPOx The striking ditterence of AlPO4

compared to conventional zcolitcs is the composition of compenents in the network frame The P aterns al the frameworks of the AIPO, structure correspond to the

location of the Si atoms in the zeolite structure ‘his leads to the dissociation of H*

in the acidic centers, or in other words, the acid force of the weak acid centers, so AIPOs is a tnotccular sieve with low acidity aud low activity Furlicrtmore, because

of the neutral frameworks of AIPO,, the lack of Bronsted acidity has severely

restricted their employment as acidic catalysts ‘I'o solve these drawbacks, AIPOs was

changed by replacing additional atoms into the lattice node of frameworks, such as atkali metals, alkali carth metals, aud fansilion metals, lo improve ils activity The substitution of transition metals in crystals is highly complicated and depends on

several parameters, namely the replacement ion charge statc, metal ion charge and

20

radius, thermodynamic stability of the wictoporons crystal AIPO, framework, transition metal link coordination, and synthesis conditions

Figure 1.9 Schematic formation of AIPOs [47]

Despite the fact that the well-known metal substitution in the crystal frame generates a thermally unstable structure as compared to AIPOs, the silicon potential

in the AIPO, framework produces a stable structwe The incorporation of silicon atoms into the neutral framework of ALPO, leads in the formation of a novel family

of compounds known as silicoaluminophosphates (SAPOs) These novel materials exhibit characteristics of both zeolites and AIPO but are also distinct ina number of ways Thoy should find use as adsorbents for motccular 5 separation and

purification, as catalysts or catalytic supports, and as ion-exchange agents (in the

other hand, due to the coexistence of silicon and transition metals, the material shows dual-function catalytic propertics: acidity and redox, cnabling materials to be used in

4 broader range of applications SAPOs molzcular sieves have istrahedral oxide frameworks containing silicon, aluminum, and phosphorus ‘The common linkages in SAPOs are [Si-O-All, [Si-O-Si], and [P-O-Al], while [Si-O-P] linkages have been proven to be energetically unfavorable [47-49] We may think about their composition in terms of silicon substitution into hypothetical AlPOs frameworks mechanistically, These changes result in the creation of acidic centers of varying strengths, depending on the degree of silicon potential and its distribution throughout the network ftame [49] ‘'hree different mechanisms have been proposed for silicon substitution into the AIPOs framework, as shown in Figure 1.10 [41, 49]

© The first mechanism (SMD), consists of the substtution of an aluminum atom

by siticon

© The second mechanism ($M2), is a silicon substitution for phospharous

« The last mechanism (§M3) is the simultaneous replacement of a pair of aluminum-phosphorous by two silicon atoms

‘The SM1 mechanism is unlikely to happen since it results in both a positive framework charge and the creation of an unstable [$i-O-P] link [50], Combinung the

2

SM2 and SM3 processes results in the development of silicon aggregates or "silicon islands" [51] which are produced when silicon occupies at least two neighboring T sites Silicon islands are hypothesized to occur when the silicon concentration approaches a threshold that is specific to each topology [52] Based on topological

constraints, a greater number of isolated heteroatoms may be incorporated into

systems with a lower topological density It appears that silicon aggregates form islands as a result of the movement of silicon T-atoms in the presence of vacancies, which is facilitated by additional framework components such as water [53, 54]

Silicon islands may occur during synthesis or during post-synthesis modifications

such as calcination, which occur at elevated temperatures and in samples with a relatively high silicon content in the synthesis gel The benefit of synthesizing SAPOs

for acid catalytic processes is the increase in the number of acidity sites These acidic

centers are dispersed silicon atoms or silicon atom groups However, when the silicon concentration increases, the crystallinity decreases, resulting in the buildup of silicon

atoms As a result, the silicon concentration of SAPOs must be restricted

Apart from the acidity induced by bridging hydroxyl groups incorporated into

the AIPO, framework by silicon inclusion, SAPOs exhibit additional kinds of acidity

Protonic (Bronsted) or aprotonic (Lewis) sites may also contribute to the surface acidity of SAPOs The proton required to balance the charge of the framework creates

a Bronsted acid site on the surface bridge Bronsted acidity is a property of SAPOs due to silicon inclusion in the framework, but it is a property of zeolites due to aluminum, as seen in Figure 1.11 While the local structure of Bronsted acid sites in

SAPOs and zeolites ([Si-OH-AI] umits) is similar, SAPOs are known to have a gentler acidity than zeolites, even when their topologies and acid concentrations are identical

[55] Acidity is determined by numerous factors, including the angle of the bond, the

length of the bond, the electrostatic potential around the acid centers, and the initial

coordination sphere of T-atoms surrounding the acid centers [56] Bronsted acidity

could potentially arise from hydroxyl groups, classified as:

¢ Lattice termination silanol groups [Si-OH], [Al-OH] and [P-OH] [57]

© OH- groups occurring at vacancy defect sites (hydroxyl nests) [58]

© OH- group attached to extra framework species [57, 58]

® bridging OH groups [= Si-(OH)-Al =] [57]

When Bronsted acid sites are discussed in SAPOs, they are often referring to

the bridging OH group [42] Silicon islands have a direct effect on the acidity of SAPOs materials due to their formation As a result of the formation of silicon islands,

the number of acid sites reduces and stronger acid sites (equivalent to the acid strength

in zeolite) occur along at the border of the island or within the island if it is large enough to contain isolated Al-atoms [48, 49, 51] Within the SAPOs, a huge silicon

island is analogous to a nano-zeolitic region

hoof fof

Zeolite Figure 1.11 Bronsted acidity in zeolite and SAPOs [55]

Lewis's acidity is another type of acidity found in SAPOs It is produced from sites that receive electron pairs Lewis's acidity is generated in SAPOs by tr-

coordinated aluminum sites on the framework and non-framework (extra lattice)

aluminum sites Lewis sites can be produced during either high temperature treatment

or post-synthesis modification [59] There is a connection between crystallinity loss

and the amount of Lewis sites in a sample, as demonstrated by experiments [60]

In comparison to zeolites, SAPOs' crystallization processes have not been extensively

explored There is no general pathway for the crystallization of SAPOs molecular

sieves It has been hypothesized that the two extreme processes for the production of

zeolites coexist during the crystallization of SAPOs under hydrothermal conditions

23

[61, 62] TLis genorally rogandod that, AIPO is widely belicved to be formed in two steps [43] Due to the structural similarities between SAPOs and the equivalent AIPOs, the production processes of SAPOs may also include two stages The first stage involves the inleraction of an aluminum-containing starting material with a supply of phosphorous to create an amorphous AlPO: layer, ‘The second stage invelves the transformation of the layer to the final crystalline product, This procedwe, on the other and, is less transparent The process begins with the formation of an amorphous or crystalline precursor ‘The precursor is then converted into the final crystalline product, However, due to the rapid crystallization that occurs under hydrothermal conditions, such a layered precursor is uncommon and extremely

difficull te extract during hydrothermal synthesis, Another unresolved issue is the incorporation of Si into the frameworks of AlM),-based molecular sieves At an early stage of the formation process, Si atoms are expected to participate directly in caystallization with Al and P aloms, resulting in a variety of Si species in the

frameworks (52, 61, 62], ‘They are also probably incorporated into an AlPO, intermediate at a later stage [63] Al these possibilities highly depend on synthesis

conditions

1.4 Catalysts selection for NHs-SCR of NOx

‘This section includes further information on how to choose an optimum SCR

catalyst based on a variety of specific criteria To begin, a basic review of the catalyst

types that have been studied or tested in practice is provided The kinds are compared

with respect lo their porformmmuce, availabiity, and case of synth

s As soon in this review, the combination of high performance, defined as activity and selectivity, with hydrothermal stability makes metal-exchanged zeolites an excellent choice for SCR

catalysts

41.4.1 Supports selection for NHs-SCR of NO,

In terms of zeolite frameworks for NH:-SCR deNOx, the most thoroughly

8 are ZSM-S (MIT framework) and, more recenlly, SAPO-34 (CHA framework) In practice, the tendency is toward zeolites with narrower pore windows (mostly CHA structure), which provide greater hydrothermal stability When subjected to Inigh heat, larger holes ean hold more hydrocarbons, which can thermally destroy the catalyst [1, 27, 34] Additionally, small-pore retard the rate of net deakunination, thereby extending the life of the catalyst Thus, this dissertauon

ite and the SAPO-34 veolype fo

It is worth noting here that SAPO-34 is a zeotype material with a structure extremely

investigated allernali

investigales the 78-5 72 upporl-hased catalysts

24

similar to that of zeolite, thus, we regard it throughout this dissertation as a "zeolite" type material

1.4.1.1 ZSM-5 zeolite

‘The MFI framework is comprised of 3-dimensional pore network consisting

of intersecting straight and sinusoidal channels (= 5.6 A diameter), as shown in Figure

1.12 Zeolite ZSM-5 (Zeolite Socony Mobil—5), a MFI family member, was first

synthesized by Argauer and Landolt in medium pore size with channels defined by

10-membered rings (10MR) and the chemical formula is Naa*AlaSi—os- 2)Oi92.16H20, n> 27 [41] The ZSM-S structure is composed of several pentasil units which consist

of eight 5-membered rings (SMR) with Al or Si atoms as the vertices Oxygen bridges

connect the pentasil units to create pentasil chains Corrugated sheets with 1OMR

holes are formed by joining pentasil chains together through oxygen bridges Each 10MR hole is composed of Al or Si vertices connected by an O Each corrugated

sheet is linked to the next by oxygen bridges, resulting in a structure with straight

10MR channels running laterally to the corrugations and sinusoidal LOMR channels running perpendicular to the sheets The pore size of the parallel channels running parallel to the corrugations is estimated to be around 5.5 A, and the Si/Al ratio ranges

from 10 to infinity

I al `

Cages = Opening 10-ring opening: 5-6 A

Figure 1.12 Framework of MFI projected along [010] and an illustration of the

molecular channels and cages for the 10MR opening [64]

In the catalyst area, ZSM-5 is referred to as a heterogeneous catalyst that

contains both Bronsted and Lewis sites As a result, ZSM-5 is one of the most

commonly utilized zeolites in the industry for catalysis, adsorption, and separations

The ZSM-5 molecular sieve has a sub-nanometer porous structure with a high

25

coneenlralion of aeidie cơntors that can roact with SH: to fonn imlennediale NHị vía

a fast adsorption and activation process The mmterior spaee can be thought of as a

miniature "nanorcactor," providing an advantageous reaction space and cleetronie enviroment for the formation of highly dispersed transition metal oxides clusters,

showing unique catalytic activity in SCR reaction for removal NO, [65, 66]

1.4.1.2 SAPO-34 zealife

SAPO-34 is likely the most interesting material in the SAPOs family of molecular sieves with small pores It is isomorphic to the naturally occurring tiny pore zeolite CHA, which has a structure resembling layer of double 6-membered rings (6K) linked by units of 4MR ‘Ihe D6R layers are stacked in an ABC fashion This results in a framework composed of a regular array of barel-shaped cages with

a diameter of 9.4 A, connected by SMR windows (3.8 A), with a cage dimension of 8x 12 A in addition to the channels, and inchuding huge cavities accessible (rough

three-dimensional SMR SAPO-34 has the chemical formula (SiQ2),(P2Os)(ALOs)

[41, 63] In the SAPO-34 topology, there is just one symmetrically independent Ietrahcdral site and four distinct oxygen sites Figure 1.13 depicis he SMR chamet and cages [64] The protons supplied to maintain charge neutrality following silicon incorporation in AIPO4-34 may coordinate with one of the four oxygen atoms Indeed, silicon has been reported to funetion as a replacement for phosphorus in the alternating Al-P framework locations of ALPO,-34, forming isolated centers or constitute silicalite-like islands [67]

SAPO-34 is most often used as a catalyst in the conversion of methanol to light otefins (MTO) [68, 69] and in the ze moval of NOx from diesel cars [1, 34, 69], both of which are newly commercialized processes, SAPO-34 has made significant progress in the NH>-SCR of NO, owing toils cxedlert sclevtivity and stability, high activity, and wide working temperature range With the same CIIA topology, SAPO-

34 was shown to be more resistant to hydrothermal degradation than SSZ-13 [29] The hydrothermal stability was required when exhaust temperatures exceeded 630 °C

in a moistuived cuvironment during the regeneration of the DPF section, which is

typically located in front of the SCR section SAPO-34 molecular sieves with a low

silicon concentration and a homogeneous distribution are critical for maximizing nitrogen selectivity [70] With regards to the acidic characteristics of the catalysts,

by which Bronsted acid siles are formed in 897-13 amd SAPO-34 are distinct, While the Bronsted acid sites in SSZ-13 are entirely dependent on the Al content, the SAPO-34 acid sites are mostly duc fo the substitution of Sĩ atorns for P

the mechani:

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