Preparation and activity of NI MGO a al2o3 catalysts in the partical oxidation of methane

PREPARATION AND ACTIVITY OF Ni-MgO/α-Al2O3 CATALYSTS IN THE PARTIAL OXIDATION OF METHANE A Thesis Presented to The Faculty of Graduate School College of Engineering De La Salle Univers

PREPARATION AND ACTIVITY OF Ni-MgO/α-Al2O3 CATALYSTS IN THE PARTIAL OXIDATION

OF METHANE

A Thesis Presented to The Faculty of Graduate School

College of Engineering

De La Salle University

In Partial Fulfillment

of the Requirements for the Degree in

Master of Science in Chemical Engineering

by Doan, Long The Nam August 9, 2005

PREPARATION AND ACTIVITY OF Ni-MgO/α-Al 2 O 3 CATALYSTS IN THE

PARTIAL OXIDATION OF METHANE

A Thesis Presented to The Faculty of Graduate School College of Engineering

August 9, 2005

ACKNOWLEDGEMENT

It is very glad for me to express my deepest appreciation to everyone who gave me his

or her valuable support which help me finished my thesis on time

Special thanks to my advisors, Dr Luis Razon, Dr Carlito Salazar, Professor Hiroo Niiyama and Dr Leonila Abella for their guidance, support and contribution Without them, this thesis would not have been done

Gratitude and appreciation are for the following persons and institutions who have contributed to the accomplishment of my work

JICA/AUN/SEED-Net for financial support and great consideration

Ho Chi Minh City University of Technology, Vietnam; De La Salle University, The Philippines and Tokyo Institute of Technology, Japan

Professor Hirofumi Hinode, Professor Takashi Aida, Dr Susan Gallardo, Dr Servillano Olaño for giving their precious advice

Ms Gladys Cruz for consideration in my work and my life in the Philippines

Mr Benjie Cardoza, Mr Manny Burgos and Mr Peter Ascrate for technical advice and lending equipment

Engr Neil Macadaeg, Mr Sojo Akelardo, Mr Ismael Serrano for lending equipment

All of my Philippine and Vietnamese friends for their encouragement

To my family and my country, my success is always dedicated to you

ABSTRACT

Partial Oxidation is considered to be a promising method to convert natural gas

to synthesis gas (“syngas”) Because the reactions involved are difficult to control, it is necessary to investigate appropriate catalysts for this process in order to maximize

CH4 conversion and the yield of CO and H2. The catalyst should have high activity and high selectivity as well as high stability At the same time, it should be available at low cost

Effect of catalyst preparation methods (all slight variants of the precipitation method), Ni/Mg molar ratio and reaction temperature on the performance of Ni/MgO catalysts supported on α-Al2O3 in the partial oxidation of methane to syngas has been investigated A fixed bed flow system was used to conduct the reaction The ratio of

CH4: O2: N2: He equals to 12: 6: 14: 68 was used in the study

Three catalyst preparation methods produced comparable results in CH4conversion, CO and H2 Energy Dispersive X-Ray (EDX) analysis and the color of the catalysts after reaction showed that one method was superior due to its higher carbon deposition resistance

Ni/Mg molar ratio affected the reduction temperatures and the performance of the catalysts significantly When the ratio decreased, activity decreased and reduction temperature increased The catalyst that has Ni/Mg molar ratio of 1/2 was chosen since it gave the same results as the catalyst that has Ni/Mg molar ratio of 1 in similar conditions

At a Ni/Mg molar ratio of 1/2 and reduction temperature of 8500C, CH4conversion and CO selectivity increased (about 65-98% and 65-96%, respectively) when reaction temperature was increased from 600 to 8000C while H2 selectivity remained almost the same (about 90%)

This may be the first time when Ni-MgO/α-Al2O3 catalysts prepared by precipitation method were investigated The catalyst gave excellent activity and remained stable after 5 h time-on-stream

TABLE OF CONTENTS

Chapter 1 Introduction

Chapter 2 Review of Related Literature

2.1 Synthesis Gas Production and Catalytic Partial Oxidation 7

2.3 The development of catalysts contain Ni-MgO-Al 2 O 3 17

2.6 Reaction Temperature and Other Operating Conditions 24

Chapter 3 Theoretical Considerations

3.2 Partial Oxidation Catalysis 28

3.3.1 Scanning Electron Microscopy (SEM) method 38

Energy Dispersive X-ray unit

b) Gas Chromatography Unit 53

4.3.5 Effect of catalyst preparation procedures 55

Chapter 5 Results and Discussion

5.2.2 Catalyst color consideration and EDX results 62

Appendix B Flowmeter Calibration 89

Appendix D Catalyst Surface Analysis by SEM and EDX 106

LIST OF FIGURES

Figure 2-1: Suggested reaction scheme for partial oxidation of methane 19

Figure 2-2: Surface and gas phase temperature profiles measured 23 respectively with IR thermography with thermocouples

Figure 3-1: Thermodynamic equilibrium line of partial oxidation of 27 methane

Figure 3-2: Phases of nickel compounds present after calcination of 37 nickel alumina catalysts

Figure 3-3: Schematic diagram of the filament bridge arrangement 41

in a typical TCD

Figure 4-2: Schematic diagram of experimental set-up 49

Figure 5-1: CH4 conversion as a function of time on stream at different 59 catalyst preparation methods for CPOM

Figure 5-2: CO selectivity as a function of time on stream at different 60 catalyst preparation methods for CPOM

Figure 5-3: H2 selectivity as a function of time on stream at different 61 catalyst preparation methods for CPOM

Figure 5-5: Method 2’s fresh and used catalysts 62

Figure 5-7: Surface carbon deposition content of fresh and used 63 catalysts of three catalyst preparation methods (EDX analysis)

Figure 5-8: Surface Ni/Mg molar ratio of fresh and used catalysts 64

of three catalyst preparation methods (EDX analysis)

Figure 5-9: CH4 conversion as a function of time on stream at 65 different Ni/Mg molar ratio for CPOM

Figure 5-10: CO selectivity as a function of time on stream at 66 different Ni/Mg molar ratio for CPOM

Figure 5-11: H2 selectivity as a function of time on stream at 67 different Ni/Mg molar ratio for CPOM

Figure 5-12: Surface carbon deposition content of fresh and used 68 catalysts of three designed Ni/Mg molar ratios (EDX analysis)

Figure 5-13: Surface Ni/Mg molar ratio of fresh and used catalysts 68

of three designed Ni/Mg molar ratios (EDX analysis)

Figure 5-14: CH4 conversion as a function of time on stream at different 69 temperatures for CPOM

Figure 5-15: CO selectivity as a function of time on stream at different 70 temperatures for CPOM

Figure 5-16: H2 selectivity as a function of time on stream at different 71 temperatures for CPOM

Figure 5-17: Effect of reaction temperature on CH4 conversion 72

Figure 5-18: Effect of reaction temperature on CO selectivity 72

Figure 5-19: Effect of reaction temperature on H2 selectivity 73 Figure 5-20: Surface carbon deposition content of fresh and used 74 catalysts at different reaction temperature (EDX analysis)

Figure 5-21: Ni/Mg molar ratio of fresh and used catalysts at 75 different reaction temperature (EDX analysis)

NOMENCLATURE

CPOM Catalytic Partial Oxidation of Methane GHSV Gas Hourly Space Velocity

FID Flame Ionization Detector

TCD Thermal Conductivity Detector

SEM Scanning Electron Microscopy

EDX Energy Dispersive X-ray

PID Proportional Integral Derivative controller

LIST OF TABLES

Table 2-1: Thermodynamics of carbon formation reactions 15

Table 2-2: Catalysts contain Ni supported on MgO and/or α-Al2O3 18

Table 4-1: Comparison between three catalyst preparation methods 45

Table 5-1: Effect of temperature on the performance of CPOM over 71

Ni-MgO/α-Al2O3 catalyst

Chapter 1

INTRODUCTION

1.1 Background of the study

Natural gas is an abundant and relatively clean fuel Nowadays, natural gas satisfies about 25% of the energy demand all over the world (Albertazzi et al., 2003) Because methane is the main component of natural gas, the use of natural gas as a raw material is very economical Considering the same rate of energy consumption of the last decade; natural gas will still be used as one of the most potential energy supplies

in the future

Since the Philippines and Vietnam have significant natural gas reserves (around 3-4 trillion cubic feet in the Philippines (Philippine Department of Energy, 1996) and about 7.7 trillion cubic feet in Vietnam (Danish Embassy, 2005) with high quality, natural gas industry should be given more consideration and should be one of the driving forces for economic development

In 1992, Shell, Philippines discovered a rich deposit of natural gas in Malampaya, Palawan, Philippines The Malampaya gas field is located in the South China Sea, 80 kilometers northwest of Palawan, an island 400 kilometers southwest of Manila A total of 3-4 trillion cubic feet of natural gas, and about 120 million barrels

of condensate (Philippine Department of Energy, 1996) was estimated This reserve may contribute intensively to the economic development of the Philippines, mainly used for power stations

Vietnam ranks third in oil production among Asian countries, trailing only Indonesia and Malaysia Provided that the current rate of development continues, Vietnam will become the world's 30th largest oil-producing nation And perhaps the most interesting aspect about Vietnam, its gas reserves seems even more promising

than its oil reserves (Danish Embassy, 2005) Officially known as Vietnam Oil and Gas Corporation, Petrovietnam has developed rapidly since it was established in 1975, and its activities, through its various companies and wholly owned subsidiaries, now cover all the operations from oil and gas exploration and production to storage, processing, transportation, distribution and services Annual oil and gas production of Petrovietnam rapidly increased and reached 16.8 millions tons and 1.6 billions cubic meters respectively in 2005 (Petrovietnam, 2005)

Natural gas is a naturally occurring gaseous mixture of hydrocarbon components and consists mainly of methane Other constituents include ethane, propane, iso-butane, hexane, heptane which are referred to as liquefied petroleum gases and condensates which are heavier hydrocarbons The composition of the gas differs from one gas reservoir to another

Increasing concern about world dependence on petroleum oil has generated interest in the use of natural gas Economical uses of natural gas have attracted extensive attention all over the world Natural gas can be used in four main areas: heating, natural gas power plants, transportation and chemical feedstock However, the majority of these reserves are located in remote regions, which leads to high transportation costs Based on economical and technological point of view, it is believed that natural gas is better converted to useful liquid chemicals (Lyubovsky et al., 2003)

The first step in natural gas conversion to liquid chemicals is the production of synthesis gas, a mixture of gases composed of hydrogen and carbon monoxide Synthesis gas is used in the ammonia/urea production, methanol production, Fischer-Tropsch synthesis and in the steel industry as a reducing gas Hydrogen, a very important fuel for the future, may be attained from syngas by removing carbon monoxide (Basile and Paturzo, 2001)

For synthesis gas production, there are several choices such as (a) steam reforming, (b) catalytic partial oxidation, (c) CO2 reforming and (d) auto-thermal reforming method, which is the combination of both partial oxidation and steam reforming in one reactor Among them, steam reforming and catalytic partial oxidation have been commercialized CO2 reforming and auto-thermal reforming are still at their developing stages (Hui et al., 2000)

Among these methods, catalytic partial oxidation recently has been receiving more attention because of its mildly exothermic nature with a H2/CO ratio of about 2, which is appropriate for methanol synthesis and Fischer-Tropsch synthesis Because of the weak exothermic nature and simple reactor design, operation and energy costs are reduced significantly compared to other methods

1.2 Statement of the problem

In theory, although catalytic partial oxidation is very attractive in terms of economics, industrial application is still a big challenge There are only a few catalytic partial oxidation plants using air and natural gas which are now in operation (Albertazzi et al., 2003) There is a need to investigate and develop a new catalyst which can satisfy the strict condition of partial oxidation catalyst with low cost

The popular catalysts for methane reforming processes are Ni-containing catalysts, due to its availability, high activity and low cost (Ruckenstein and Hu, 1999) Ni must be dispersed in a support which was chosen as α-Al2O3, since the characteristics and the specific surface area of this carrier are rather suitable with selective oxidation reactions (Hagen, 1999) There has been a great interest about the strong interaction between MgO and NiO, namely “solid solution” MgO was used in this study as a promoter to enhance the stability and selectivity of the catalyst

For deeper understanding the performance of Ni-MgO/α-Al2O3 catalysts, some questions should be answered:

a) What is the suitable design of the catalytic partial oxidation set up to be used

to test these catalysts activity?

b) How should Ni-MgO/α-Al2O3 catalysts be produced?

c) What is the appropriate Ni/Mg molar ratio which gives high activity, selectivity and stability?

d) How do these catalysts behave when the reaction temperature is varied? 1.3 Objectives of the Study

1.3.1 General Objective

The general objective of this study is to investigate the performance of MgO catalysts supported on α-Al2O3 for partial oxidation of methane to synthesis gas, using different Ni/Mg molar ratios, reaction temperatures, and catalyst preparation methods

ii) CO and H2 selectivities

iii) Carbon deposition resistance

b) To investigate the effect of varying the Ni/Mg molar ratio on

i) CH4 conversion

ii) CO and H2 selectivities

c) To investigate the effect of varying the reaction temperature on

i) CH4 conversion

ii) CO and H2 selectivities

1.4 Significance of the Study

Partial oxidation is an important and challenging research area and is quite suitable to Philippine and Vietnam’s natural gases, since these gases have high percentage of methane Although utilizing natural gas for power plants seems to be the most preferable choice, production of synthesis gas should be conducted because of high value products In efforts to reduce the installation and operation cost for synthesis gas production, catalytic partial oxidation looks like the most attractive method

The future use of syngas and hydrogen will need more efficient syngas production systems, including cheaper large scale syngas plants for gas to liquid technology and the small scale applications of syngas technologies for fuel cells whether for stationary or for automotive use

Although catalysis is only one of several key factors for these developments, it plays an important role for advanced reactor designs, feedstock flexibility, and control

of carbon formation (Rostrup-Nielsen, 2000)

This study may contribute to synthesis gas manufacture knowledge by studying and identifying the suitable conditions for this specific process using Ni/MgO catalysts supported on α-Al2O3 Using catalytic partial oxidation widely will convert natural gas to more valuable liquid chemicals at a lower cost compared with the current steam reforming process

1.5 Scope and limitations of the Study

This study consists of three parts: firstly, the catalyst preparation Secondly, the design and fabrication of the laboratory-scale catalytic partial oxidation set up Finally, the catalysts performance in partial oxidation of methane was investigated on this set up

Generally, this study would be limited to the following:

a) The Ni-MgO/α-Al2O3 catalysts prepared would be in a powder form

b) Precipitation method was used for catalyst preparation

c) Flow through reactor was used No measurement of temperature or concentration gradients during the length of the catalyst bed were done

d) No kinetic or mechanism investigation was done in this study

e) H2O was not quantified in the product analysis due to the limitation of the Gas Chromatography Unit

f) No analysis of economic feasibility was done

Chapter 2

REVIEW OF RELATED LITERATURE

One of the big challenges in natural gas utilization is transportation from remote areas, where many large reserves are found To decrease the transportation cost, converting natural gas into higher hydrocarbons through gas-to-liquid (GTL) processes is considered to be an effective way of natural gas utilization (Lyubovsky et al., 2003) In theory, although the direct conversion of methane to valuable chemicals such as ethylene or methanol and formaldehyde is a most fascinating route, no feasible process or catalyst has yet been developed Until now, indirect transformation

of methane through synthesis gas production is still the most competitive process

(Wang and Ruckenstein, 1999)

2.1 Synthesis Gas Production and Catalytic Partial Oxidation

Nowadays, syngas is mainly produced by the well established steam reforming process (Basile and Paturzo, 2001; Zhu et al; 2001) However, it requires high investment costs and high energy inputs because it is highly endothermic In recent years, the catalytic partial oxidation of methane (CPOM) to syngas has been extensively investigated as a potential alternative to steam reforming (Basile and Paturzo, 2001; Zhu et al; 2001):

2 2

2

1

HCOO

298 =−Δ

In comparison to current technologies (steam reforming, CO2 reforming, thermal reforming), the CPOM shows the following advantageous process conditions

auto-a) Require lower energy costs because it is a weak exothermic reaction

b) Operate at low contact time (10-2 to 10-4 s), allowing the use of small reactors, and thus lowering the cost of the plant

c) The ratio H2/CO ≈ 2 obtained is suitable for the synthesis of methanol (main use of syngas)

On the other hand, a few disadvantages limit the expansion of this process:

a) The presence of hot spots along the catalytic bed leads to difficulties in control of the reaction temperature

b) The high stability required by the catalyst at operating temperature

For these reason, few CPOM plants using air and natural gas are now in operation (Albertazzi et al., 2003) Many efforts have been made to solve these problems, such as design appropriate reactors, coupling CPOM with steam reforming and/ or CO2 reforming of methane Above all, it is important to find a stable catalyst for CPOM The catalyst should have (a) high activity, (b) high coking resistance and (c) be available with low cost

2.2 Partial oxidation Catalysis

During partial oxidation operation, when the power demand rapidly decreases, the catalyst can overheat, causing sintering which is turn results in loss of activity Besides, for repeated start-up and shut-down cycles, carbon deposition phenomena easily occur Indeed, the most important factor to make a successful partial oxidation reformer for methane is the development of an active and stable catalyst (Pino et al., 2003)

Because of high activity, availability and low cost, Ni has been explored as a possible substitute for precious metals (Ruckenstein and Hu, 1999; Marnasidou, 1999; Liu et al, 2000; Zhu and Flytzani-Stephanopoulos, 2001; Olsbye et al., 2002) However, carbon deposition is easier to happen in Ni catalyst at elevated temperature than precious metals (Olsbye et al., 2002) During high temperature reactions, sintering and loss of Ni is difficult to avoid in industrial applications In spite of these disadvantages, Ni is still the most attracted active component due to economic reasons (Chu et al., 2002)

2.2.2 Catalyst Supports

Numerous effective Ni containing catalyst have been developed by

incorporation in suitable supports, such as MgO (Qin et al, 1996; Ruckenstein and

Hu, 1999), α-Al2O3 (Jin et al., 2000; Shishido, 2002; Pietruszka et al., 2004), γ-Al2O3(Liu et al, 2000; Pantu and Galavas, 2002; Albertazzi et al, 2003), CeO2 (Zhu and Flytzani-Stephanopoulos, 2001), SrTiO3 (Takehira, 2002), La2O3 (Nakagawa, 1999) and TiO2 (Yan et al., 2003)

Since the concepts of strong metal-support interaction become the key for developing new catalysts, different combinations between metal and support have been studied to attain the advantages of the interaction A number of researches about

CPOM over supported Ni show that the interaction between Ni and support may significantly affect activity and coking resistant of catalysts (Chu et al., 2002)

It was found by Ruckenstein and Hu (1999) that the reduced NiO-MgO catalyst provided a high activity and selectivity, as well as high stability in the composition range of 9.7-35 mol % NiO A CH4 conversion of about 86% and selectivities to CO and H2 of about 96 and 99%, respectively were achieved at 8500C and a Gas Hourly Space Velocity (GHSV) of 720,000 ml (g catalyst)-1 h-1 (CH4/O2 = 2/1)

NiO and MgO are completely miscible and form an ideal solid solution because of similar structure Indeed, in Ruckenstein and Hu (1999)’s experiments, the XRD indicates that NiO and MgO form a solid solution in the NiO/MgO catalysts prepared by impregnation and calcined at 800oC It has been suggested that the formation of a NiO/MgO solid solution in the catalyst precursor is responsible for the stability observed over MgO-supported Ni catalysts (Wang and Ruckenstein, 1999)

Ruckenstein and Hu (1999) reported that compared to MgO and NiO, in the MgO/NiO solid solution Mg(2p) has a lower binding energy than Ni(2p3/2) The binding energy of Ni(2p3/2) and Mg(2p) in 9.7 mol% NiO/MgO catalyst prepared by impregnation are 856 and 48.8, respectively This means that electron transfer from NiO to MgO takes place This phenomenon increases the interactions between two oxides, slows down the reduction of the NiO This leads to Ni atoms segregated over the surface of the catalyst, resulting in a high dispersion of Ni0, which is responsible for the high activity of the catalyst In addition, the segregated Ni atoms interact strongly with those remaining in the substrate and this inhibits their sintering, and the high dispersion of Ni inhibits the coke formation that requires large clusters of Ni (Ruckenstein and Hu, 1999)

2.2.3 Promoters

To promote the stability of Ni based catalysts, many effective promoter has been investigated, including La2O3 (Marnasidou, 1999; Olsbye et al.,2002), MgO (Choudhary et al.,1997), Ir (Nakagawa, 1999)

The loss and sintering of Ni, the deterioration of the support make the

NiO/γ-Al2O3 catalysts are usually unstable at high temperatures In addition, catalyst deactivation is accelerated by carbon deposition Liu et al (2000) modify the NiO/γ-

Al2O3 by adopting alkali and rare earth metal oxides (Li and La) They concluded that the modification could not only stabilize the support γ-Al2O3 phase, but also suppress

on the sintering and loss of Ni, and in addition, enhance the ability of suppressing carbon deposition over the NiO/ γ-Al2O3 during a high temperature reaction

Based on Choudhary et al (1997), although MgO, CaO and

NiO-Yb2O3, or other rare earth oxide catalysts show high activity and selectivity, these composite mixed metal oxide catalysts have poor mechanical strength, because of their hygroscopic nature But when improving their mechanical strength by depositing them on silica or alumina catalyst carrier, the activity of the catalysts was reduced significantly The main reason is the formation of NiAl2O4 (or NiSiO3), which is inactive and difficult to reduce To solve this problem, the researchers tried to precoat the support by MgO before depositing Ni The results show that MgO avoid the chemical interaction between NiO with Al2O3 or SiO2 by providing a stable protective layer of MgAl2O4 (or MgSiO3) on the support surface On the other hand, MgO stabilize Ni on the support surface against sintering by forming a NiO-MgO solution

For Al2O3 as a support, MgO could be used as a promoter to retard sintering of active component (Richardson, 1989; Hagen, 1999)

2.2.4 Reduction

The CPOM over Rh catalysts supported on MgO and SiO was investigated by Wang and Ruckenstein (1999) At 7500C and GHSV of 720,000 ml (g catalyst)-1 h-1, the 1% Rh/MgO catalyst exhibited a very high stability Its catalytic activity (80%

CH4 conversion) and selectivity (92% CO selectivity and 96% H2 selectivity) remained unchanged up to at least 100 h of reaction In contrast, the 1% Rh/SiO2catalyst deactivated rapidly after an induction time of 20 h The strong interaction between Rh2O3 and MgO (especially the formation of MgRh2O4 in the precursor catalyst) are responsible for the high stability This catalyst was reduced with difficulty to tiny Rh crystallites The strong interactions between Rh and MgO hinder the aggregation of metallic Rh and hence its sintering On the contrary, since no strong interaction between Rh and SiO2 exists, Rh2O3 can be easily reduced and the crystallites sinter, leading to the deactivation of the SiO2-supported Rh catalyst (Wang and Ruckenstein 1999)

In the case of NiO/MgO catalyst prepared by impregnation method, although the catalyst in the active form is Ni0/MgO, the NiO-MgO without prereduction can be used directly for the oxidative conversion of methane to syngas (Ruckenstein and Hu, 1999; Choudhary and Mamman, 2000)

The NiO from the catalyst is reduced by its reaction with CH4, i.e

NiO + CH4 → Ni0, CO, CO2 and H2O

during the initial short reaction period

A change in the catalyst reduction (by H2 for 1 h) temperature from 500 to

9000C had, however, a very little or no effect on the methane conversion and also on the selectivity for CO and H2 in the process (Choudhary and Mamman, 2000)

2.2.5 Thermal processes and Sintering

The easiest way to reduce sintering is to maintain low temperature Unfortunately, the CPOM has to proceed at high temperature (usually greater than

700oC) to get a meaningful conversion of methane (Wang and Ruckenstein, 1999)

The catalytic partial oxidation of methane to CO is exothermic, and even the low conversion to CO2 generates a large amount of heat, which leads to significant temperature gradients (hot spots) in the reactor Over the distance of only 1 mm from the hot spot, the temperature may differ by several hundred degrees Kelvin Since catalytic partial oxidation is a fast reaction (the remaining time can be 10-3 s or lesss) (Basini et al., 2001), it is difficult to remove the heat out of the reactor as the same rate

as it is generated, especially from the large scale reactor So, the process is potentially hazardous and explosion may occur (Hu and Ruckenstein, 2004) Hot spots may not only make the process become dangerous but will also deactivate the catalyst quickly due to active component sintering and coke deposition Indeed, hot spot is one of the main reasons leading to sintering (Basile et al., 2004) This also increases the reactor material cost

In order to eliminate hot spots, fluidized-bed reactors may be used This will minimize the thermal gradient (Olsbye et al., 2002) Also, the continuous catalyst circulation leads to smoother temperature gradient compared with fixed-bed reactors Another way is to combine exothermic partial oxidation with other endothermic reactions Choudhary et al., (1998) investigated the catalytic partial oxidation of methane in combination with CO2 and steam reformings over NiO supported on microporous aluminophospate (16.4 wt% NiO) The overall catalytic process can be made almost thermoneutral, reduce the hot spots significantly

Increasing the metal dispersion can decrease the intensity of hot spot due to the distribution on a large area of the oxidation reaction occurring in the first zone of the

reactor On the other hand, the hot spot due to the large particle size favors the fast consumption of the oxygen and produces very sharp peak of temperature (Basile et al., 2004), so reducing the particle size can suppress sintering phenomenon also

Above all, the catalyst should have high thermal resistance to stand for “hot spot” phenomenon The interaction between active component and support is the most important factor for eliminate sintering

2.2.6 Carbon Deposition on Ni-based Catalysts

Coke formation is a main problem in the operation of reforming reactions over Ni-based catalysts In methane-to-synthesis gas reactions, coke may be formed from two sources, CH4 and CO in the following manner:

2

4 C 2H

298 =ΔC

CO

298 =−Δ

Thermodynamic calculation of the first and second reactions is performed in Table 2-1 Table 2-1 shows that while the reverse Boudouard reaction is the dominating carbon source under 700oC, methane decomposition is a dominating carbon source at higher temperatures It is well known that by increasing the H: C and O: C ratios in the reactor feed, the coke formation will decrease due to a shift in the equilibrium of methane decomposition and Boudouard reaction (Olsbye et al., 2002)

Choosing a heterogeneous catalyst for partial oxidation should not only be based on high activity and selectivity of the catalyst, but also based on the stability of the catalyst during a long time under CPOM condition In CPOM, sintering of active component (always metal) and deposition of carbonaceous materials lead to the deactivation of the catalyst The rate of coke formation increases because of the aggregation of metal caused by sintering (Wang and Ruckenstein, 1999)

Table 2-1: Thermodynamics of carbon formation reactions (Tsang et al., 1986)

Temperature

(0C)

ΔG(CH4 = C + 2H2) (kJ/mol)

ΔG(2CO = C + CO2) (kJ/mol)

It is important to know that the acidity of the catalyst surface favors deposition, while the basicity of the catalyst surface prevents carbon-deposition (Liu et al., 2001) High dispersion of metal species over a catalyst or use of alkaline earth metal oxides in a catalyst may reduce coke formation (Shishido, et al., 2002) In 2001, Zhu and Flytzani-Stephanopoulos reported that only the 5 at % Ni-Ce(La)Ox catalyst with highly dispersed NiO in CeO2 showed excellent carbon deposition resistance at

carbon-6500C and a contact time of 0.18 g s/cm3

2.2.7 Catalyst Preparation Methods

Among studies about CPOM, it can be recognized that impregnation method is

a very popular method in catalyst preparation (Qin et al, 1996; Choudhary et al., 1998; Wang and Ruckenstein, 1999; Nakagawa et al., 1999; Ji et al., 2001; Olsbye et al, 2002; Albertazzi et al, 2003; Pino et al, 2003; Yan et al., 2003; etc.) because it is the easiest method to produce a catalyst (Satterfield, 1980) Like in the study of Liu et al (2000), the LiLaNiO/γ-Al2O3 catalyst was prepared by the impregnation method Appropriate amounts of LiNO3, Ni(NO3)2 and La(NO3)3 were impregnated on γ-Al2O3support for 24 h, dried at 393 K, and then calcined in air at 1073 K for 4 h

Otherwise, there are precipitation and co-precipitation methods (Pino et al., 2003; Tsyganok et al., 2004; Basile et al., 2004), glycothermal method (Takeguchi et al., 2003), citrate complexation technique (Pantu and Galavas, 2002) and combustion method (Pino et al., 2003; Zhu et al, 2004a)

Basile et al (2004) reported the Rh/Mg/Al catalysts were obtained the hydrotalcite-type (HT) precursors by calcination at 650 or 9000C for 14 h The HT precursors were prepared by precipitation of a solution containing nitrate salts of Rh3+ and Mg2+, Al3+ with a second solution containing a slight excess of Na2CO3 The pH was maintained at 10.0 by continuous addition of NaOH The precipitates were kept in suspension at 600C for 40 min and then filtered, washed (until the sodium content, as

Na2O, was lower than 0.02% w/w), and dried overnight at 900C

In the study of Pino et al (2003), 1 atom % Pt/CeO2 catalyst was obtained by the solution-combustion method The combustion mixture contained (NH4)2Ce(NO3)6,

H2PtCl6 and C2H6N4O2 (oxalyldihydrazide) in the molar ratio 0.99:0.01:2.33 Oxalyldihydrazide which derived from diethyl oxalate was used as the fuel Initially,

10 g of (NH4)2Ce(NO3)6, 0.095 g of H2PtCl6 and 5.175 g of C2H6N4O2 were dissolved

in a minimum volume of H2O in a borosilicate dish of 130 cm3 capacity The

borosilicate dish with this redox mixture was introduced into a muffle furnace maintained at 3500C The solution boiled with frothing and foaming with concomitant dehydration At the point of its complete dehydration, the fuel ignites the redox mixture with the flame temperature of 10000C, yielding a voluminous finely-dispersed solid product within about 5 min

It becomes clear that high dispersion of metal species over the catalyst surface may reduce coke formation New catalyst preparations have been developed Morioka and co workers (2001) have proposed and developed a “solid phase crystallization” method for preparing a stable and finely dispersed metal supported catalyst It was started from a crystalline precursor homogeneously containing active metal species in the structure It was reported that the catalyst showed high activity as well as high sustainability again coke formation Using this method, supported Ni catalysts on perovskite-type CaTiO3, SrTiO3 and BaTiO3 oxides (Ni loading of 5.9 wt%, Ni/Ti atomic ratio is 1/5) were prepared by Takehira et al (2002) The precursors were prepared as follows: an aqueous solution of reagent-grade nickel nitrate, alkaline earth carbonates, and titanium isopropoxide was treated with an excess amount of citric acid and ethylene glycol This mixture was evaporated at 353-363 K to make a sol of the organometallic complex This was followed by two-step decomposition by heating at

473 K for 5 h and 773 K for 5 h and finally by calcining at 1123 K in air for 5 h

Ni0.2/SrTiO3, followed by Ni0.2/BaTiO3 and Ni0.2/CaTiO3 showed the highest activity

as well as the highest sustainability against coking on the catalyst Takehira et al (2002) reported a CH4 conversion of about 98% and about 94% selectivity to CO were maintained at 1073 K, even at the high space velocity of 112,000 ml (g catalyst)-1 h-1

2.3 The development of catalysts contain Ni-MgO-Al 2 O 3

Since α-Al2O3 is a suitable supports for Ni based catalyst and MgO-NiO solid solution shows high anti-sintering effect, catalysts contain two or three of those

components were investigated extensively Table 2-2 shows the Ni based catalysts supported on α-Al2O3 or MgO, or both of them

Table 2-2: Catalysts contain Ni supported on MgO and/or α-Al2O3

Ni/α-Al2O3 Impregnation Jin et al., 2000; Olsbye et al.,

2002; Shishido et al, 2002; Pietruzka, 2004;

Shishido et al, 2002

calcination

Choudhary and Mamman, 2000

NiO/MgO/Al2O3-SiO2 Impregnation Choudhary et al., 1997

31, 61, 71)

Coprecipitation Basile et al., 1998

crystallization

Shishido et al., 2002

In this study, Ni-MgO/α-Al2O3 catalysts prepared by precipitation method were investigated

2.4 Catalytic Partial Oxidation Mechanism

It is widely accepted that metal catalysts first oxidize methane to CO2 and water in the first part of the catalyst bed until oxygen is exhausted, followed by reforming of the remaining methane with the CO2 and water formed initially (two-step mechanism) However, at extremely high temperatures and very short contact time in the order of milliseconds, it is still possible that syngas is formed directly (one-step

mechanism) (Zhu et al., 2003) The reaction scheme for both routes is shown in Fig

O

298 =−Δ

Steam Reforming:

2 2

4 H O CO 3H

298 =+Δ

CO2 Reforming:

2 2

4 CO 2CO 2H

298 =+Δ

4CO + 8H 2

A direct partial oxidation mechanism (one-step mechanism) was also proposed when some very high activities for CPOM were proved under high temperature and high-space velocity conditions (Qin et al., 1996)

2 2

2

1

HCOO

298 =−Δ

To check the mechanism of CPOM, Qin et al (1996) compared the reactivity

of mixed reforming (the combination between steam reforming and dry reforming) with CPOM over MgO-supported noble metals includes Pt, Rh, Ir, Ru (the amount of metal was 0.5 % weight of total catalyst) at high space velocity (5.5×105 h-1) using the molecular ratios indicated in Fig 2-1

All the results indicated that steam reforming and CO2 reforming in mixed reforming start simultaneously and have the same type of reaction intermediate, adsorbed atomic oxygen The mechanism of mixed reforming was suggested:

(a) CH4 + 2M → CH3-M + H-M

(b) CH3-M + 2M → CH-M + 2H-M Activation of CH4

(c) CH-M + M → C-M + H-M

(d) H2O + 3M → O-M + 2H-M Deposition of H2O and CO2

(e) CO2 + 2M → O-M + CO-M

(f) CHx-M + O-M + (x-1)M → CO-M + x H-M

(g) CO-M → CO + M Reaction of adsorbed species (h) 2H-M → H2 +2M and production of CO and H2

Mechanism of CPOM was suggested with activation of CH4, (a)-(c), is the same as in mixed reforming

(e) CHx-M + O-M + (x-1)M → CO-M + x H-M

(f) CO-M → CO + M Reaction of adsorbed species and (g) 2H-M → H2 +2M production of CO and H2

This process is very similar to that of mixed reforming except for using O2instead of H2O and CO to form O-M So, CPOM proceeds via both one-step and two-step mechanisms; the ratio of each mechanism is dependent on the concentration and kinetics of adsorbed atomic oxygen and gaseous atomic oxygen

Using 8 wt% Ni/α-Al2O3 catalyst on a pulse reactor, Jin et al (2000) studied the mechanism for CPOM syngas over this specific catalyst by temperature-programmed surface reaction (TPSR) techniques and proposed the mechanism of CPOM over Ni0 catalyst

O2 + Ni0 → (Niδ+…Oδ-: surface mobile oxygen species)

d) The activation of O2 over Ni0 sites forms a kind of mobile oxygen species

Niδ+…Oδ-, and CH4 activation generates surface active Ni…C species The Ni…C and Niδ+…Oδ- species are proposed to be the intermediates for the partial oxidation of methane to syngas The reaction of Ni…C and Niδ+…Oδ- generates the primary product of CO Therefore, the mechanism for the CPOM should follow the direct oxidation route (Jin et al., 2000)

The mechanism of methane partial oxidation to synthesis gas is still under discussion It may be suggested that the reaction pathway depends not only on the catalyst composition but also on the reaction condition

2.5 Heat transfer consideration

In research, in the case of the heterogeneous short residence time catalytic partial oxidation, reactions occur confined at the solid surfaces while the gas phase is chemically cool, since there was the large difference between gas and surface temperatures (see Fig 2-2) and the very large temperature gradients in the interlayer zones In addition, it is observed that even homogeneous phase reactions should remain confined in a thin boundary zone where collisions with the surfaces have a quenching effect on the propagation of radical reactions in the gaseous phase (Basini

et al., 2000)

Figure 2-2: Surface and gas phase temperature profiles measured

respectively with IR thermography with thermocouples (Basini et al., 2000)

In the study of Basini et al (2000), surface temperatures rise steeply to the highest values in the very initial portion of the catalytic bed and remain nearly constant, while gas temperatures gradually increase from the inlet to the exit zones

The steep temperature increase at the beginning of the catalytic bed and the flatness of the axial and radial temperature profiles of the solid, but not of the gas, could be explained by assuming that thermal energy is transferred from hotter to cooler surface zones

Radiative heat transfer, occurring through the confining catalyst particles, with

a minor contribution of heat conduction, can be considered responsible for this phenomenon Radiative heat transfer affects mainly surface temperature properties since the solid surfaces emit, absorb, and scatter the radiative heat much better than the gaseous phase The gas is gradually heated along the bed due to desorption of hot reaction products and due to collisions with the hot surfaces

The temperature difference between solid and gas clearly indicates the existence of a nonlocal thermal equilibrium (Basini et al., 2000)

In Choudhary and Mamman’s study (1999), it was said that the reaction is controlled by heat transfer, particularly at temperatures with low CO and H2selectivity At low temperatures, the heat produced is more because of the highly exothermic total combustion reaction as the CO selectivity is low

2.6 Reaction Temperature and Other Operating Conditions

According to the thermodynamics reported for the methane and oxygen system (CH4 : O2 = 2:1) (Tsang et al., 1996), the formations of CO2 and H2O are dominant at temperatures lower than about 823 K and the synthesis gas formation becomes favorable at higher temperatures Temperatures higher than 923 K are

thermodynamically required for a high methane conversion (>90%) and a high synthesis gas selectivity (>90%)

CPOM over Ni/SiO2 was conducted in the research of Li and Lu (2004) When the reaction temperature is under 5000C, the reaction products are mainly H2, H2O and

CO2; the products are mainly a mixture of H2, CO and CO2 when the reaction temperature is between 5000C and 7000C; the main products are CO and H2 when the reaction temperature is above 7000C It was found out that CH4 conversion, and H2and CO selectivities increase with an increase in reaction temperature The H2/CO molar ratios of the products remain between 2.00 and 2.10 with a CH4/O2 molar ratio

of 2 when the reaction temperature is between 7000C and 8000C

Effect of CO2 on the reaction performance of the partial oxidation of methane over a LiLaNiO/γ-Al2O3 catalyst was investigated by Liu et al (2000) These researchers concluded that adding CO2 to the partial oxidation of methane reaction not only alters the H2/CO ratio of the products, but also reduces the temperature of the hot spots in the catalyst bed significantly

In the study of Choudhary et al (1998), the methane-to-syngas conversion process involving simultaneous oxidative conversion and CO2 and steam reforming reactions occurs over the Ni/AlPO4 catalyst in an energy efficient and safer manner, requiring little or no external energy The process can be made almost thermoneutral, mildly exothermic, or mildly endothermic depend on the proportions of the reactions involve A desirable H2/CO ratio can be obtained conveniently by manipulating the process conditions (i.e with feed composition and reaction temperature)

Chapter 3

THEORETICAL CONSIDERATIONS

The basic principles and concepts of catalytic partial oxidation will be discussed in this chapter Other important considerations involved will also be discussed

3.1 Catalytic Partial Oxidation of Methane

Catalytic partial oxidation of methane is an attractive alternative to steam reforming for production of synthesis gas Because of the exothermic nature of the partial oxidation reaction, the process is less energy and capital cost intensive than the conventional endothermic steam reforming In addition, lower H2: CO ratio of about 2

is more favorable with respect to downstream processes such as methanol synthesis and Fischer-Tropsch synthesis of higher hydrocarbon

Partial oxidation involves the addition of a smaller amount of oxygen to methane, consists of these reactions:

Partial Oxidation:

2 2

2

1

HCOO

298 =−Δ

Combustion:

OHCO

O

CH4+2 2 → 2 +2 2 H0 801kJ/mol (3-2)

298 =−Δ

Steam Reforming:

2 2

4 H O CO 3H

298 =+Δ