Selenium Grid 3 node startup ignoring -nodeconfig node.json - selenium-grid

Question

I'm just setting up a new Selenium Grid, and have just one node to register to my hub at the moment. Both hub and node running on CentOS 7. I have two different versions of Firefox on my node, and no other browser, as I will be setting up different nodes for different browser types.
If I start the node completely from the command line, specifying the different Firefox versions and locations, it all registers and starts properly. On the Grid console page I see my node with the two versions of Firefox, 3 instances of each, correctly detailed with the version numbers I'm specifying. This is the command line I use:
java -jar selenium-server-standalone.jar -role node -hub http://xx.xx.xx.xx:4444/grid/register -browser browserName=firefox,version=50.1.0,maxInsfirefox_binary=/opt/firefox-50.1.0/firefox,maxInstances=3,platform=LINUX -browser browserName=firefox,version=51.0.1,firefox_binary=/opt/firefox-51.0.1/firefox,maxInstances=3,platform=LINUX
If however I try using a .json config file to specify my parameters, the java startup completely ignores the config file and starts up using defaults; on the Grid console page it shows 5 Firefox sessions, 1 IE and 5 Chrome, and none of the Firefox sessions show the versions I'm specifying. This is the command line invoking the json file:
java -jar selenium-server-standalone.jar -role node -hub http://xx.xx.xx.xx:4444/grid/register -nodeConfig /opt/selenium/node.json
... and this is the node.json file itself:
{
“capabilities”:
[
{
"browserName": firefox,
"firefox_binary": "/opt/firefox-50.1.0/firefox",
"maxInstances": 3,
"platform": LINUX,
"version": 50,
"seleniumProtocol": WebDriver
},
{
"browserName": firefox,
"firefox_binary": "/opt/firefox-51.0.1/firefox",
"maxInstances": 3,
"platform": LINUX,
"version": 51,
"seleniumProtocol": WebDriver
},
],
“maxSession”: 5,
“port”: 5555,
"hub": "http://xx.xx.xx.xx:4444",
“register”: true,
"registerCycle": 5000,
"nodeStatusCheckTimeout": 5000,
"role": node,
"cleanUpCycle": 2000
}
I have tried putting all parameters encased in double quotation marks, in case that was the issue; made no difference.
Any clues why the parameters in my node.json file are being ignored, please?